AN ESTROGENICALLY REGULATED POTENTIAL TUMOR SUPPRESSOR
GENE, PROTEIN TYROSINE PHOSPHATASE γ (PTPγ), IN HUMAN BREAST
DISSERTATION
Presented in Partial Fulfillment of the Requirements for
the Degree Doctor of Philosophy in the Graduate
School of The Ohio State University
By
Suling Liu, B.S.
* * * * *
The Ohio State University
2003
Dissertation Committee: Approved by Dr. Young C. Lin, Adviser Dr. Robert W. Brueggemeier ______Dr. Yasuko Rikihisa Adviser Dr. Pui-kai Li Veterinary Biosciences Graduate Program
ABSTRACT
Except for skin cancer, breast cancer is the most common cancer among women
and second only to lung cancer as the primary cause of cancer deaths in women. Among
the endocrine factors associated with breast cancer, estrogens are considered to play a
central role in human breast carcinogenesis. In 1988, Henderson et al. showed that breast
cancer risks are increased by long-term exposure to estrogens, such as estradiol-17β (E2).
Zeranol (Z) (Ralgro®) is a nonsteroidal agent with estrogenic activities and used as a growth promoter in the U.S. beef, veal and lamb industries. We showed E2 and Z induced
human breast epithelial cell neoplastic transformation with the similar potency in the
long-term exposure through the redox-pathway, in which estrogen metabolites undergo
redox-cycling and produce free radicals which directly induce DNA damage leading to
tumor initiation, and/or ERβ-mediated pathway and they have the similar potency in
stimulating and inhibiting some target gene expressions in human breast cancer cells.
Protein tyrosine phosphatase γ (PTPγ) is a member of the receptor-like family of
tyrosine-specific phosphatases and has been implicated as a tumor suppressor gene in
kidney and lung cancers. Little is known about PTPγ expression in human breast. In our
study, we found that PTPγ was mainly immunolocalized to the epithelium and PTPγ
mRNA expression was lower in cancerous than in normal breast tissues, both E2 and Z suppressed PTPγ levels to a much greater degree in cultured normal human breast tissues
ii or epithelial cells co-cultured with stromal cells in comparison with the cultured epithelial
cells alone. The results indicated that both E2 and Z downregulate PTPγ expression in
human breast and that epithelial-stromal cell interaction is important in the regulation of
PTPγ expression by estrogenically active agents. Furthermore, we showed that lower
PTPγ was associated with higher ERα in cancerous human breast tissues, and the
estrogenic downregulation of PTPγ expression by E2 and Z in human breast is associated
with estrogen receptor α (ERα). Based on these findings, we hypothesize that PTPγ is a potential estrogen-regulated tumor suppressor gene in human breast cancer that may be involved in neoplastic processes of human breast epithelium, which lead us to examine the action of PTPγ on in vitro growth of MCF-7 human breast cancer cells and compare the estrogenic responses of human breast cancer cells with different expression levels of
PTPγ. Our results showed that PTPγ overexpression decreased the doubling time of
human breast cancer cell line MCF-7 and led to a decrease in the anchorage-independent
growth of MCF-7 cells in soft agar. Furthermore, we described that PTPγ overexpression
could reduce the estrogenic responses of MCF-7 cell proliferation to E2 and Z. Our data
suggested that PTPγ is able to inhibit proliferation and anchorage-independent growth of
human breast cancer cells in vitro and has anti-estrogenic activities in human breast
cancer cells. The PTPγ may function as a potential tumor suppressor gene in regulating
the process of tumorigenesis in human breast.
Collectively, these studies provide new insight into the role of an endocrine
disruptor, Zeranol, in the neoplastic transformation of human breast and the importance
of PTPγ in the suppression of human breast carcinomas, and shed a light on the tight
relationship among estrogen, PTPγ and breast cancer. The investigations not only further
iii our understanding of human breast cancer biology but also provide rationales for the development of therapeutic approaches targeting on PTPγ as a breast cancer suppressor.
iv
Dedicated to my parents
v ACKNOWLEDGMENTS
I must first thank my advisor Dr. Young C. Lin, whose scientific guidance, support and mentorship has been invaluable. Dr. Lin, over the past four and a half years you have been a constant source of encouragement, advice and financial support for my study and research. I am especially grateful to my PhD committee members Drs. Robert W. Brueggemeier, Yasuko Rikihisa, and Pui-kai Li who provide me advice and direction throughout my graduate studies. I also show my appreciation to Drs. Yasuro Sugimoto and Samuel K. Kulp for their technical support and suggestions. Thanks also go to all members in Dr. Lin’s laboratory for their sincere friendship. To my parents, brother and sister, thank you for your continued support. Finally, to Xi, thank you for your encouragement, support, and love.
vi VITA
October 17, 1974...... Born – Liuyang, Hunan, China
1997...... B.S., Biology University of Science and Technology of China Hefei, Anhui, China
1997 – 1999 ...... Research Associate University of Science and Technology of China Hefei, Anhui, China
1999-present...... Graduate Research Associate, Department of Veterinary Biosciences The Ohio State University Columbus, Ohio
PUBLICATIONS
Research Publications
S. Liu, Y. Sugimoto, S.K. Kulp, J. Jiang, H.L. Chang, K.Y. Park, Y. Kashida, and Y.C. Lin. Estrogenic down-regulation of Protein Tyrosine Phosphatase γ (PTPγ) in human breast is associated with Estrogen Receptor α. Anticancer Research 22: 3917-3924, 2002.
S. Liu, S.K. Kulp, Y. Sugimoto, J. Jiang, H.L. Chang, and Y.C. Lin. Involvement of breast epithelial stromal interactions in the regulation of Protein Tyrosine Phosphatase γ (PTPγ) mRNA expression by estrogenically active agents. Breast Cancer Research and Treatment 71: 21-35, 2002.
S. Liu, S.K. Kulp, Y. Sugimoto, J. Jiang, H.L. Chang, and Y.C. Lin. The (-)-enantiomer of gossypol possesses higher anticancer potency than racemic gossypol in human breast cancer. Anticancer Research 22: 33-38, 2002.
vii S. Liu, Y. Sugimoto, C. Sorio, P. Melotti, and Y.C. Lin. Function analysis of estrogenically regulated Protein Tyrosine Phosphatase γ (PTPγ) in human breast cancer cell line MCF-7. Oncogene (submitted), 2003.
S. Liu, and Y.C. Lin. Transformation of human breast epithelial cells MCF-10A by environmental disruptor, zeranol (Z), and estradiol-17β (E2). The Breast Journal (accepted, in press), 2003.
S. Liu, Y. Sugimoto, H.L. Chang, J. Jiang, K.Y. Park, C. Sorio, P. Melotti, K. Huebner, and Y. C. Lin. Function analysis of estrogenically regulated Protein Tyrosine Phosphatase γ (PTPγ) in vitro in human breast cancer. American Association for Cancer Research 94th Annual Meeting: A4962, 2003.
S. Liu, Y. Sugimoto, S.K. Kulp, J. Jiang, H.L. Chang, K.Y. Park, and Y.C. Lin. Estrogenically active agents downregulate Protein Tyrosine Phosphatase γ (PTPγ) expression in human breast through estrogen receptor α not estrogen receptor β. American Association for Cancer Research 93rd Annual Meeting: A5237, 2002.
S. Liu, S.K. Kulp, J. Jiang, Y. Sugimoto, and Y.C. Lin. Importance of breast epithelial stromal interactions in the regulation of Protein Tyrosine Phosphatase γ (PTPγ) mRNA expression by estrogenically active agents. American Association for Cancer Research 92nd Annual Meeting Proceedings: A4737, 2001.
Y.C. Lin, S. Liu, S.K. Kulp, J. Jiang, Y. Sugimoto, and M.K. Dowd, P. Wan. The (-) – enantiomer of gossypol (GP) is a potent inhibitor of normal and cancerous breast cell growth. American Association for Cancer Research 92nd Annual Meeting Proceedings 42: A387, 2001.
S.K. Kulp, S. Liu, Y. Sugimoto, R.W. Brueggemeier, Y.C. Lin. Localization of Protein Tyrosine Phosphatase γ (PTPγ) to the mammary epithelium of ACI rats” Biol Reprod, 62(suppl 1): A191, 2000.
H.L. Chang, Y. Sugimoto, S. Liu, J. Jiang, S.K. Kulp, K.Y. Park, Y. Kashida, and Y.C. Lin. Regulation of Protein Tyrosine Phosphatase γ (PTPγ) mRNA expression by estrogenically active agents in canine prostate. Biol Reprod, 64 (suppl 1), 2002.
J. Jiang, S.K. Kulp, S. Liu, H.L. Chang, Y. Sugimoto, and Y.C. Lin. Estrogenic action elevates cyclin D1 mRNA expression in canine prostate. Biol Reprod, 63 (suppl 1), 2001.
H.L. Chang, Y. Sugimoto, S. Liu, J. Jiang, S.K. Kulp, R.W. Brueggemeier, and Y.C. Lin. Development of an in vitro model for the screening of biologically active keratinocyte growth factor (KGF/FGF-7) receptor antagonists. Biol Reprod, 63 (suppl 1), 2001.
M.P. Wick, Y. Sugimoto, S. Liu, and Y.C. Lin. Cellular and molecular functions of cultured myogenic cells derived from a day old calf. In vitro Cellular and Developmental viii Biology-Animal (Submitted), 2003.
Y. Kashida, Y. Sugimoto, S. Liu, and Y.C. Lin. Protein Tyrosine Phosphatase γ (PTPγ) mRNA expression in the non-tumorous or tumorous uterus tissues. Biol Reprod, 68(suppl 1): A167, 2003.
K.Y. Park, Y. Sugimoto, S. Liu, H.L. Chang, W. Ye, L.S. Wang, and Y.C. Lin. Involvement of adipocytes on local estrogen level. Biol Reprod, 68(suppl 1): A444, 2003. H.L. Chang, Y. Sugimoto, K.Y. Park, S. Liu, W. Ye, L.S. Wang, Y.W. Huang, and Y.C. Lin. Regulation of estrogen receptor-α mRNA expression by keratinocyte growth factor (KGF) in MCF-7 cells. Biol Reprod, 68(suppl 1): A659, 2003.
J. Jiang, S.K. Kulp, Y. Sugimoto, S. Liu, H.L. Chang, and Y.C. Lin. Effects of estrogens and age on the growth of canine prostate cells. Biol Reprod, 68(suppl 1): A176, 2003.
H.L Chang, Y. Sugimoto, K.Y. Park, S. Liu, W. Ye, L.S. Wang, Y.W. Huang, and Y.C. Lin. Regulation of estrogen receptor α mRNA by Keratinocyte growth factor (KGF) in MCF-7 cells. Biol Reprod, 68(suppl 1): A659, 2003.
J. Jiang, Y. Sugimoto, S.K. Kulp, S. Liu, H.L. Chang, K.Y. Park, and Y.C. Lin. Inhibitory effects of gossypol on human prostate cancer cells-PC3 are associated with transforming growth factor β signal transduction pathway. Anticancer research. 2003 (submitted).
W. Ye, A. Murakami, J. Jiang, S. Liu, H.L. Chang, K.Y. Park, Y. Sugimoto, and Y.C. Lin. Anti-proliferative effects of 1’-acetoxychavicol acetate and auraptene in human breast cancer cells. American Association for Cancer Research 94th Annual Meeting: A2704, 2003.
J. Jiang, S.K. Kulp, Y. Sugimoto, S. Liu, and Y.C. Lin. Cell-specific and age-dependent changes in canine prostatic cell function and growth. The FASEB Journal 15 (5): 22A, 2001.
J. Jiang, S.K. Kulp, Y. Sugimoto, S. Liu, Y.C. Lin. The effects of Gossypol on the invasiveness of MAT-Lylu Cells and MAT-Lylu Cells from the metastasized longs of MAT-Lylu-bearing copenhagen rats. Anticancer research 20: 4591-4598, 2000.
J. Jiang, S. Kulp, Y. Sugimoto, S. Liu, and Y.C. Lin. Antimetastatic effects of gossypol in prostate cancer. 91st AOCS Annual Meeting & Expo. April 25-28. San Diego, California, 2000.
D. Ruan, J. Chen, W. Zhao, S. Liu, W. Gu. Effects of Lead on L-LTP and PPF in Hippocampal Dentate Gyrus. Guangdong Weiliang Yuansu Kexue 5: 25-29, 1998.
J. Jiang, P.K. Gosh, S.K. Kulp, Y. Sugimoto, S. Liu, J. Czekajewski, H.L. Chang, and
ix Y.C. Lin. Effects of gossypol an O2 consumption and CO2 production in human prostate cancer cells. Anticancer research 22: 1491-1496, 2002.
FIELDS OF STUDY
Major Field: Veterinary Biosciences
Studies in Reproductive and Molecular Endocrinology.
x TABLE OF CONTENTS
Page Abstract...... ii
Dedication...... v
Acknowledgments...... vi
Vita...... vii
List of Tables ...... xiii
List of Figures...... xiv
Abbreviations...... xvii
CHAPTERS
1. Introduction
Breast cancer...... 1 Estrogen and breast cancer...... 2 Environmental estrogens and breast cancer...... 3 Zeranol ...... 4 Estrogen receptors (ERs) ...... 6 Protein tyrosine phosphatase γ (PTPγ) ...... 7 Overview of chapters 2 through 5...... 9
2. Transformation of a normal human breast epithelial cell line MCF-10A by an environmental disruptor, Zeranol (Z), and estradiol-17β (E2)
Abstract...... 16 Introduction...... 17 Materials and Methods...... 20 Results...... 25 Discussions ...... 28
xi 3. Regulation of Protein Tyrosine Phosphatase γ (PTPγ) by estrogenically active agents, estradiol-17β (E2) and Zeranol (Z), in human breast
Abstract...... 38 Introduction...... 39 Materials and Methods...... 43 Results ...... 49 Discussions ...... 53
4. Estrogenic downregulation of Protein Tyrosine Phosphatase γ (PTPγ) in human breast is associated with estrogen receptor α (ERα)
Abstract...... 76 Introduction...... 77 Materials and Methods...... 79 Results ...... 85 Discussions ...... 88
5. Functional analysis of estrogenically regulated Protein Tyrosine Phosphatase γ (PTPγ) in human breast cancer cell line MCF-7
Abstract...... 105 Introduction...... 107 Materials and Methods...... 109 Results ...... 115 Discussions ...... 119
6. Conclusions
Perspectives...... 141 Future works and clinical implications...... 145 Concluding remarks...... 147
Literature cited...... 148
xii LIST OF TABLES
Table Page
2.1 Comparison of MCF-10A cell proliferation by doubling time assay ...... 32
xiii LIST OF FIGURES
Figure Page
1.1 Summary of factors influencing breast carcinogenesis ...... 10
1.2 Zeranol (α-zearalanol) and related compounds ...... 11
1.3 The diagram of structure homology between ERα and ERβ ...... 12
1.4 The diagram for E2 action in human breast cells through ER-mediated pathway...... 13
1.5 The family of receptor-like protein tyrosine phosphatases...... 14
1.6 The diagram for PTPγ structure...... 15
2.1 Effects of E2 or Z on anchorage-independent growth of MCF-10A cells in soft agar...... 33
2.2 Effects of E2 and Z on ERβ expression in transformed MCF-10A cells ...... 35
2.3 Potency comparison of E2 and Z on pS2 mRNA expression in human breast cancerous cell line MCF-7 ...... 36
2.4 Potency comparison of E2 and Z on PTPγ mRNA expression in human breast cancerous cell line MCF-7 ...... 37
3.1 Isolation of epithelial cells and stromal cells from normal human breast tissues and phase contrast photomicrograph of cultured human breast epithelial cells and stromal cells...... 60
3.2 Modified in vitro co-culture assay system...... 61
3.3 Comparison of PTPγ mRNA expression by RT-PCR in normal human breast tissues and cancerous human breast tissues ...... 62
xiv 3.4 Comparison of PTPγ mRNA expression by RT-PCR in normal human breast tissues, breast epithelial cells and stromal cells ...... 64
3.5 Immunocytochemical staining of primary cultured normal human breast epithelial cells and stromal cells ...... 66
3.6 Regulation of PTPγ mRNA expression in normal human breast tissues by E2 and Z...... 68
3.7 Regulation of PTPγ mRNA expression in normal human breast epithelial cells by E2 and Z ...... 70
3.8 Immunohistochemical localization and reactivity of PTPγ in normal and cancerous human breast tissues, and the regulation of PTPγ immunohistochemical reactivity by E2 and Z in normal human breast tissue ...... 72
3.9 Regulation of PTPγ mRNA expression E2 and Z in normal human breast epithelial cells and stromal cells in co-culture assay system ...... 74
4.1 Comparison of estrogen receptors (α and β) mRNA expression in MCF-7 cells and MDA-MB-231 cells as determined by RT-PCR...... 92
4.2 Comparison of estrogen receptors (α and β) protein expression in MDA- MB-231 cells, MDA-MB-231-ERα-1000 cells and MCF-7 cells as determined by western blotting assay...... 94
4.3 Effects of E2, Z and ICI 182,780 on PTPγ mRNA expression in MCF-7 cells as determined by RT-PCR...... 95
4.4 Effects of E2, Z and ICI 182,780 on PTPγ mRNA expression in MDA- MB-231 cells as determined by RT-PCR ...... 97
4.5 Effects of E2, Z and ICI 182,780 on PTPγ mRNA expression in MDA- MB-231-ERα-1000 cells as determined by RT-PCR...... 99
4.6 Comparison of ERα mRNA expression in human breast as determined by RT-PCR...... 101
4.7 Comparison of ERβ mRNA expression in human breast as determined by RT-PCR...... 102
4.8 Comparison of immunoreactivities of ERα, ERβ and PTPγ in normal and cancerous human breast tissues as determined by immunohistochemical staining...... 103 xv 5.1 The procedures for the establishment of stably transfected cell lines ...... 122
5.2 The sequences for the inserted PTPγ cDNAs ...... 124
5.3 Identification and confirmation of successfully transfected cell lines by RT-PCR...... 125
5.4 Identification and confirmation of successfully transfected cell lines by RT-PCR...... 127
5.5 The expression of PTPγ mRNA in transfected MCF-7 cells...... 129
5.6 Immunohistochemical staining for PTPγ in transfected MCF-7 cells ...... 131
5.7 Effects of PTPγ on the proliferation of MCF-7 cells...... 133
5.8 Optimization of cell numbers for anchorage-independent growth of MCF- 7 cells ...... 135
5.9 Effects of PTPγ on anchorage-independent growth of MCF-7 cells ...... 137
5.10 Effects of PTPγ on estrogenic activities of E2 and Z on MCF-7 cell proliferation...... 139
xvi ABBREVIATIONS
CA: Carbonic anhydrase-like domain
CD: Cell doubling
CoR: Co-repressor
CYP: Cytochrome P450
DMEM/F12: Dulbecco’s Modified Eagle Medium/Ham’s F12
DBD: DNA binding domain
DCC: Dextran-coated charcoal
DES: Diethylstilbestrol
DMSO: Dimethylsulphoxide
DNA: Deoxyribonucleic acid
E2: 17β-Estradiol
EC: Epithelial Compartment
EDTA: Ethylenediaminetetraacetic acid
ER: Estrogen receptor
ERE: Estrogen-responsive element
FBS: Fetal Bovine Serum
Fn-III: Fibronectin type III-like repeat
HBEC: Human breast epithelial cell
xvii HS: Horse serum
HSP: Heat-shock protein
LBD: Ligand binding domain
LOH: Loss of heterozigosity
NLS: Nucleus localization sequence
PBS: Phosphate buffered saline
PTPases: Protein tyrosine phosphatases
PTPγ: Protein Tyrosine Phosphatase γ
RPTPases: Receptor-like protein tyrosine phosphatases
RT-PCR: Reverse Transcriptase-Polymerase Chain Reaction
SC: Stromal Compartment
SD: Standard deviation
Z: Zeranol
ZL: Zearalenone
xviii CHAPTER 1
INTRODUCTION
Breast cancer
Excluding cancers of the skin, breast cancer is the most common cancer among women, accounting for nearly one of every three cancers diagnosed in American women.
[American Cancer Society, 2001]. Currently, one out of nine American women will develop breast cancer in her lifetime. The rate of breast cancer incidence has increased 2-
3% per year over the past decade in both premenopausal and postmenopausal women.
Every year approximately 200,000 American women are diagnosed with breast cancer, of which 40,000 will die from the disease. The mortality rate from breast cancer has not risen as dramatically due to early detection and treatment of breast cancer patients.
Breast cancer results from a combination of many factors including inherited mutations or polymorphism of cancer susceptibility genes, environmental agents that influence the acquisition of somatic genetic changes and several other systemic and local factors (Figure 1.1). Like most cancers, breast cancer arises from alterations in expression
1 and/or regulation of genes that control normal cellular proliferation and differentiation.
Two major classes of genes that are altered in cancers are proto-oncogenes and tumor suppressor genes. Proto-oncogenes control normal growth functions including cell signaling and cell cycle progression. Upon mutation, proto-oncogenes become oncogenes that may act in a dominant-negative manner to permit uncontrolled cellular proliferation.
To maintain controlled cellular proliferation, the cell contains innate growth suppressors encoded by tumor suppressor genes. Like proto-oncogenes, tumor suppressors also maintain cellular homeostasis at many different levels. Alteration of tumor suppressor expression and/or regulation results in uncontrolled proliferation, which can lead to tumor formation. A major obstacle in the cure for cancer is elucidation of the mechanisms by which proto-oncogenes and tumor suppressors are functionally regulated and how they control cellular proliferation.
Estrogen and breast cancer
Among the endocrine factors associated with breast cancer, estrogens are considered to play a central role in human breast carcinogenesis. Support for the critical role of ovarian estrogens in breast tumorigenesis include the associations between increased risk of breast cancer and the early onset of menarche, late onset of menopause, and higher age at first birth [Bernstein and Rose, 1993; Colditz et al., 1996]; as well as the significantly lower frequency at which breast cancer occurs in men and in women without functional ovaries during her lifetime than in women with intact ovaries
[Henderson et al., 1988; Lippman and Dickson, 1989]. Approximately 70% of all breast cancer patients have hormone-dependent breast cancer, which contains estrogen receptors
2 and requires estrogen for tumor growth. This importance of estrogens in breast cancer is a
basis for concern about exposure to environmental estrogens.
Environmental estrogens and breast cancer
Endogenous estrogens are not the only sources of estrogen suspected of
promoting breast cancer. Many compounds found in the environment, such as pesticides,
herbicides, and industrial byproducts, are able to bind to the receptor and have estrogenic
activities in vitro. Exposure to environmental estrogens has been attributed as one
possible cause for the observed increase in breast cancer incidence since 1940.
Phytoestrogens are plant-derived compounds (Flavonoids, coumestans, chalcones, and
lignans) that also activate the estrogen receptor. These compounds display a range of
binding affinities and agonistic activities with the estrogen receptor [Collins et al., 1997;
Miksicek et al., 1995; Kuiper et al., 1998; Zava et al., 1997]. Compounds with the highest
affinities for ERα, coumestrol, zearalenone, genistein, and daidzein have relative binding
affinities of 20, 7, 4, and 0.1%, respectively, as compared to estradiol at 100% [Kuiper et
al., 1998]. In addition, genistein and coumestrol were shown to bind to ERβ with a 7-fold
and 2-fold higher affinity than ERα, respectively [Kuiper et al., 1997]. Phytoestrogens act as estrogen agonists in most tissues. Soy products and coumestrol alone have been shown to lower serum cholesterol and prevent bone loss [Dodge et al., 1996; Carroll et al.,
1991]. Recent research suggests that the consumption of soy products, which contain large amount of phytoestrogens, may be linked to the lower rates of breast cancer incidence seen in women living in China and Japan. Women in these countries have a four- to six-fold lower risk of breast cancer than women in western countries [Ziegler et
3 al., 1993; Lee et al., 1991]. The antitumor and chemopreventive activity of these compounds may be derived from non-estrogen receptor activity of these compounds. In vitro, genistein has been shown to inhibit tyrosine kinase activity, DNA topoisomerase II activity, and angiogenesis [Fotsis et al., 1993; Akiyama et al., 1987; Peterson et al.,
1995]. In addition, some flavonoid compounds inhibit the steroid metabolizing enzymes, aromatase and 17β-hydroxysteroid dehydrogenase, which regulate local estrogen concentrations [Makela et al., 1995; Kellis et al., 1984].
Zeranol (Z)
Zeranol (Z) (Ralgro) (Figure 1.2), which is marketed as Ralgro (Schering-
Plough Corp, Kenilworth, NJ 07033), is synthesized from the mycotoxin Zearalenone
(ZL, a nonsteroidal, resorcyclic acid lactone compound produced by fungi of the genus
Fusarium) and is a nonsteroidal agent with estrogenic activities that is used as a growth promoter in the U.S. beef, veal and lamb industries to accelerate weight gain, improve feed conversion efficiency and increase the lean meat-to-fat ratio. Thus, Z is not an environmental contaminant per se. Rather, people are exposed to Z as a result of the introduction of the compound into food animals by veterinary professionals on behalf of beef industry farmers. Given the documented estrogenicity of ZL and its potential hazard to human health, it is interesting that a commercially produced derivative of ZL is currently utilized to great advantage as a growth promotant in beef cattle, veal calves and sheep in the U.S.. Like its precursor ZL, Z has been shown to have estrogenic activities.
This evidence includes: (1) Z-induced increases in uterine weight and synthesis of uterine induced protein [Katzenellenbogen et al., 1979]; (2) stimulation of beef heifer [Moran et
4 al., 1991] and mouse mammary gland growth by Z with, in the latter case, a potency
similar to that of estradiol [Sheffield and Welsch, 1985]; (3) specific binding of Z to
uterine cytosolic and nuclear estrogen receptors [Katzenellenbogen et al., 1979]; (4) Z-
stimulated proliferation of cultured normal human breast cells [Lin et al., 1996] and the
estrogen-sensitive human breast cancer cell line, MCF-7 [Welshons et al, 1990]; (5) Z-
induced stimulation of estrogen-inducible gene expression (KGF, cathepsin D) in
cultured normal human breast cells and human breast cancer cell line, MCF-7 [Zhang et
al., 1999; Zheng et al., 2000] and primary cultured human breast cancer cells [Lin et al.,
1996]; (6) Z-induced reduction of estrogen-suppressible gene expression (protein tyrosine
phosphatase-γ) in cultured normal and cancerous human breast cells and human breast
cancer cell lines [Zheng et al., 2000] and (7) induction by Z of hepatotoxicity and
subsequent hepatic neoplasia in the Armenian hamster, an animal that is especially
susceptible to exogenous estrogen-induced liver damage, and prevention of this
carcinogenic process by the antiestrogen, tamoxifen [Coe et al., 1992]. Furthermore, in
studies that examined the estrogenic activities of both ZL and Z, the synthetic product, Z,
was found to have the higher affinity for the estrogen receptor and, not coincidentally, to
be the more active compound [Katzenellenbogen et al., 1979; Welshons et al, 1990].
Furthermore, we have shown that meat and serum from Z-implanted cattle possess heat-
stable mitogenicity for cultured breast cells, and that both normal and cancerous human breast cells exhibit estrogenic responses to Z [Lin et al., 1996; Lin et al., 2000; Irshaid et al., 1999].
5 Estrogen receptors (ERs)
About 10 years after the cloning of the estrogen receptor (ER), a novel member of
ERs, termed ERβ, has been identified in cDNA libraries from rat prostate. The rat ERβ is a protein of 485 amino acids residues with a calculated molecular weight of 54.2 kDa (rat
ERα: 595 a.a., 66 kDa). ERβ is highly homologous to ERα, particularly in the DNA- binding domain and in the C-terminal ligand-binding domain. ERα and ERβ are functionally homologous and both bind to estrogen with high affinity. Some differences exist between ERα and ERβ such as aspects of regulation and they may response to some antagonists in different ways due to the lack of homology in the amino-terminal domains of these proteins where the activation function-1 (AF-1) resides (Figure 1.3).
As we have known, the mechanisms responsible for estrogen-stimulated carcinogenesis are still unclear. One possible mechanism is that E2 acts through an ER-
mediated pathway (Figure 1.4). E2 enters the ER-positive cells just by simple diffusion
and binds to the ERs in the cells. Binding of E2 to ERs results in the conformational
changes in structure that convert the receptor from an inactive to an active conformation
and the subsequent release of the associated heat-shock proteins. The changes result in
the formation of “activated” or “transformed” ER-estradiol complex, dimerization and
phosphorylation of the complex that has a high affinity for the specific estrogen response
elements (EREs) in the estrogen targeted genes in nucleus. The binding of E2-ER complex to the regulatory elements usually involves a variety of other proteins such as other DNA binding proteins (coactivators) or transcription factors and results in gene activation, i.e., transcription of the gene by RNA polymerase to produce mRNA. The transcribed mRNA in nucleus is translocated to the cytoplasm and translated on
6 cytoplasmic ribosomes to produce the appropriate protein, which alters cell function, growth, or differentiation and result in estrogenic effects in breast cells. Except for ER- mediated pathway, evidence indicates potential tumorigenic mechanisms of estrogen, such as direct genotoxic effects of estrogen metabolites and estrogen-induced expression of genes encoding growth and transcription factors [Li and Li, 1987; Lippman and
Dickson, 1989].
Protein tyrosine phosphatase γ (PTPγ)
Previous reports showed that the protein superfamily of PTPases comprises some
50 members which display a bewildering variety of amino acid motifs fused to the catalytic PTPase domains [Barford et al., 1995; Brady-Kalnay et al., 1995]. The PTPase family can be divided in two main classes based on their subcellular localization, namely
(i) intracellular, cytosolic or nuclear PTPases that contain only one PTPase domain, and
(ii) receptor-like, transmembrane PTPases that have one or two tandemly repeated catalytic domains. The receptor-like PTPases (RPTPases) can be classified in seven subtypes based on their extacellular motifs (Figure 1.5). Two other protein families are as well capable of dephosphorylating tyrosine-phosphorylated proteins: the structurally distinct low molecular-weight PTPases and the structurally related dual specificity phosphatases which catalyze the dephosphorylation of phospho-tyrosine, threonine and serine residues [Barford et al., 1995;].
PTPγ is a member of the receptor-like family of tyrosine-specific phosphatases originally cloned from human brain stem and placental cDNA libraries using probes derived from the intracellular domain of CD45 [Kaplan et al., 1990] or Drosophila
7 PTPase cDNA clone, DPTP12 [Kaplan et al., 1990], respectively. The structure of the receptor-like PTPs (RPTPs) includes an extracellular, a transmembrane, and one or two tandemly repeated catalytic domains (Figure 1.6). This structure implies ligand-binding capability which may modulate enzymatic activity. The putative ligands for most of the
PTPs with receptor-like structures are yet to be identified. One isoform of leucocyte common antigen (LCA) [CD45RO], a type I RPTP, has been shown to functionally interact with CD22β [Stamenkovic et al., 1991]. Two type II RPTPs, RPTPκ and RPTPµ, participate in homophilic binding, although this self-binding has not been shown to affect the PTP activity of either enzyme [Zondag et al., 1995]. Two type V PTPs have been reported to date: PTPγ and PTPζ [Barnea et al., 1993; Levy et al., 1993; Krueger et al.,
1992]. These receptor-like PTPs each have a carbonic anhydrase-homologous amino terminus followed by a fibronectin type three domain, a variable length cysteine-free domain, a transmembrane domain and tandem intracellular PTPase catalytic domains
[Barnea et al., 1993; Levy et al., 1993; Krueger et al., 1992]. According to mRNA analysis [LaForgia et al., 1991; Barnea et al., 1993], PTPγ is a broadly expressed enzyme that exists in many tissues, including human lung, stomach, esophagus, colon, liver, spleen, and kidney [Tsukamoto et al., 1992]. In contrast, the closely related PTPζ/RPTPβ is expressed solely in specific regions of the central nervous system and can bind to the extracellular matrix protein tenascin [Levy et al., 1993; Krueger et al., 1992]. Based on the chromosomal location of the PTPγ gene (3p14.2) [LaForgia et al., 1991; LaForgia et al., 1993] and studies showing loss of heterozygosity of the gene in kidney tumors
[Lubinski et al., 1994], PTPγ has been implicated as a candidate tumor suppressor gene.
8 Overview of Chapters 2 through 5
The next four chapters will investigate the estrogenic regulation and the function
of PTPγ in human breast. Chapter 2 looks at the estrogenic potency of Z in comparison
with E2 in human breast and provides evidences that Z has the ability to induce human
breast carcinogenesis, which is just as potent as E2. Chapters 3 and 4 focus primarily on the estrogenic regulation of PTPγ expression in human breast. Chapter 3 elucidates that breast carcinogenesis can reduce PTPγ expression and both E2 and Z can downregulate
PTPγ expression in human breast. Then, chapter 4 defines the mechanism of the
downregulation of PTPγ expression by E2 and Z, which is associated with ERα. Until
now, we have known that there are tight relationships between estrogen and breast
cancer, estrogen and PTPγ, breast cancer and PTPγ, which indicate that PTPγ might play
an important role in human breast carcinogenesis. Finally, chapter 5 addresses the critical
issue of PTPγ’s anti-tumorigenesis in human breast, which is examined using an in vitro
system. Together, these studies provide a better understanding of the estrogenic
regulation and function of PTPγ in human breast and shed a light on the tight relationship
among estrogen, PTPγ and breast cancer, which suggest that PTPγ is a potential tumor
suppressor gene in human breast.
9
…… Chemicals, radiation, … Genetic alterations
Family history Risk Factors Diets
Systemic factors Endocrine disruptors (hormones, GFs,… ) (Zeranol,… )
Figure 1.1. Summary of factors influencing breast Carcinogenesis.
10
Figure 1.2. Zeranol (α-zearalanol) and related compounds.
All are beta resorcyclic acid lactones and each can be metabolized/converted into all the other compounds, albeit with different efficiencies (Leffers et al., 2001).
11
Homology rERß 16.5% 95.5% 28.9% 59.7% 16.7%
rERα 100% 100% 100% 100% 100%
A/B C D E F Modulator DBD Hinge LBD
NH2 - - COOH
AF-1 Zn++ CTE CoR-box AF-2 NLS
Transactivation DNA binding Ligand binding Dimerization Dimerization , transactivation Hsp binding, NLS Hsp binding, NLS Coactivator and Corepressor binding
Figure 1.3. The diagram of structure homology between ERα and ERβ.
The figure shows the comparison of rat (r) ERα and ERβ proteins and percent amino acid homology in the functional regions. The ERs consist of six functional domains (A-F).
DBD: DNA binding domain; LBD: ligand binding domain; AF: activation function; HSP: heat-shock protein; NLS: nucleus localization sequence; CoR: co-repressor. Adapted with modification from reference [Macgregor et al., 1998].
12
Cell membrane Nuclear membrane
E2 Diffusion Diffusion Diffusion
E2 E2
ER E2-ER dimerization
ERE DNA
Processing
AUG UAA 5’ 3’ mRNA
Altered cell function
Export New protein Translation
Figure 1.4. The diagram for E2 action in human breast cells through ER-mediated pathway.
ERE: estrogen response element
13
Figure 1.5. The family of receptor-like protein tyrosine phosphatases.
The figure shows the distinct subtypes of RPTPases based on their extracellular motifs.
The classification is according to Brady-Kalnay and Tonks [1995], except for ChPTPλ, which can be regarded as a CD45-type RPTPase. Only the largest isoform of each member is shown. The identified structural features are indicated in the open box. Fn-III: fibronectin type III-like repeat; Ig: immunoglobulin-like domain; MAM: domain with homology to meprin, the A5 glycoprotein, and RPTPµ; CA: carbonic anhydrase-like domain; PTP: protein tyrosine phosphatase domain. Adapted with modification from reference [Schaapveld et al, 1997].
14
CA-like CA: Carbonic anhydrase Extracellular Fn III: Fibronectin type III Fn III-like
Membrane Transmembrane
PTP1 PTP: protein tyrosine Intracellular phosphatase PTP2
Figure 1.6. The diagram for PTPγ structure.
The figure shows that PTPγ is a member of RPTPases and it has an extracellular domain, a transmembrane domain and an intracellular domain. In the intracellular domain, PTP1 is active, whereas PTP2 is inactive due to the replacement of cysteine by aspartic acid.
Adapted with modification from reference [Schaapveld et al, 1997].
15 CHAPTER 2
TRANSFORMATION OF A NORMAL HUMAN BREAST EPITHELIAL CELL LINE
MCF-10A BY AN ENVIRONMENTAL DISRUPTOR, ZERANOL (Z), AND
ESTRADIOL-17β (E2)
ABSTRACT
Among the endocrine factors associated with breast cancer, estrogens are
considered to play a central role in human breast carcinogenesis. Breast cancer risks are
increased by long-term exposure to estrogens. Zeranol (Z) (Ralgro) is a nonsteroidal agent with estrogenic activities that is used as a growth promoter in the U.S. beef, veal and lamb industries. To determine whether Z and estradiol-17β (E2) play a role in the
neoplastic transformation of human breast and to compare the estrogenic potency of Z to
that of E2 in human breast, we treated the immortalized human breast epithelial cell MCF-
10A with different doses of Z or E2 for 10 repeated treatment cycles (three days per
cycle). By utilizing doubling time assay, soft agar assay and Reverse Transcriptase-
Polymerase Chain Reaction (RT-PCR) assay, we showed that ten repeated E2- or Z-
16 treatment cycles to MCF-10A cells decrease the doubling time of the cells by 30% ~
40%, and stimulate colony formation in soft agar and induce estrogen receptor β (ERβ)
mRNA expression, all of which are not dose-related in our tested dose range.
Furthermore, we described that Z and E2 have the similar potency in the stimulation and
inhibition of gene expressions in human breast cancer cell line MCF-7 by RT-PCR.
These results indicate that both Z and E2 can induce human breast epithelial cell neoplastic transformation with the similar potency in the long-term exposure through the redox-pathway, in which estrogen metabolites undergo redox-cycling and produce free radicals which directly induce DNA damage leading to tumor initiation, and/or ERβ- mediated pathway.
INTRODUCTION
Except for skin cancer, breast cancer is the most common cancer among women and is second only to lung cancer as the primary cause of cancer deaths in women
[American Cancer Society, 2001]. Among the endocrine factors associated with breast cancer, estrogens are considered to play a central role in human breast carcinogenesis.
This importance of estrogens in breast cancer is a basis for concern about exposure to environmental estrogens.
Endogenous estrogens are not the only sources of estrogen suspected of promoting breast cancer. Zeranol (Z) (Ralgro), which is marketed as Ralgro (Schering-
17 Plough Corp, Kenilworth, NJ 07033), is synthesized from the mycotoxin Zearalenone
(ZL, a nonsteroidal, resorcyclic acid lactone compound produced by fungi of the genus
Fusarium) and is a nonsteroidal agent with estrogenic activities that is used as a growth
promoter in the U.S. beef, veal and lamb industries as described in Chapter 1. Thus, Z is
not an environmental contaminant per se. Rather, people are exposed to Z as a result of
the introduction of the compound into food-producing animals by veterinary
professionals on behalf of beef industry farmers.
Evidence indicates potential tumorigenic mechanisms of estrogen, such as direct
genotoxic effects of estrogen metabolites and estrogen-induced expression of genes
encoding growth and transcription factors [Li and Li, 1987; Lippman and Dickson,
1989]. However, despite the clear importance of estrogens in the etiology of breast
cancer, the mechanisms responsible for estrogen-stimulated carcinogenesis remain undefined. In the receptor-mediated pathway, estradiol and its metabolites bind to the estrogen receptor and activate gene expression to promote cell proliferation, which makes estrogens excellent tumor promoters after the initial cellular damage is induced. In the redox-mediated pathway, estrogen metabolites undergo redox-cycling and produce free radicals which directly induce DNA damage leading to tumor initiation. E2, under the
effect of 17β-oxidoreductase is continuously interconverted to estrone (E1), and both are
hydroxylated at C-2, C-4, or C-16 positions by cytochrome P450 isoenzymes, i.e.
CYP1A1, CYP1A2, or CYP1B1, to form catechol estrogens [Liehr et al., 1986; Roy and
Liehr, 1988; Yan and Roy, 1997; Ball and Knuppen, 1980; Zhu et al., 1994; Ashburn et
al., 1993]. The metabolic activation of estrogens can be mediated by various cytochrome
P450 (CYP) complexes, generating through this pathway reactive intermediates that elicit
18 direct genotoxic effects by increasing mutation rates. Estrogen and estrogen metabolites exert direct genotoxic effects that might increase mutation rates, or compromise the DNA repair system, leading to the accumulation of genomic alterations essential to tumorigenesis [Liehr et al., 1986; Roy and Liehr, 1988; Yan and Roy, 1997; Ball and
Knuppen, 1980; Zhu et al., 1994; Ashburn et al., 1993]. Previous work showed that short-term treatment of MCF-10F cells with physiological doses of E2 induces anchorage
independent growth and colony formation in agar methocel, which indicate neoplastic
transformation of human breast [Russo et al., 2002]. The fact that the MCF-10F cells are
both ERα- and ERβ-negative, are in favor of a metabolic activation of estrogens
mediated by various cytochrome P450 (CYP) complexes, generating through this
pathway reactive intermediates that elicit direct genotoxic effects by increasing mutation
rates [Russo et al., 2002].
Two estrogen receptor types, named ERα and ERβ, have been found to be the
major mediators of a variety of biological functions of estrogens [Warner et al., 1999;
Gupta et al., 2001; Ogawa et al., 2000; Hilakivi, 2000]. ERβ is similar to ERα with
approximately 96% and 60% homology in the DNA-binding domains and ligand-binding
domains, respectively. However, their exact roles are still poorly elucidated, especially in
the case of the recently discovered ERβ [Warner et al., 1999]. On the other hand, it is
becoming increasingly clear that both receptor types are responsible for different
biological functions, as indicated by their specific expression patterns and various
consequences in gene knockouts [Lubahn et al., 1993; Krege et al, 1998; Couse et al.,
1999; Dupont et al., 2000]. It is thought that ERβ may also have distinct functions in the
biology of breast cancer. Moreover, both ERs work as either homo- or heterodimers, 19 when inducing transcription from gene promoters equipped with estrogen response
elements (ERE) [Kumar and Chambon, 1988; Cowley et al., 1997; Pace et al., 1997;
Ogawa et al., 1998]. In addition, estrogens and anti-estrogens can induce differential
activation of ERα and ERβ to control transcription of genes that are under the control of Activating Protein 1 (AP1) sites [Paech et al., 1997].
In the present study, we have shown that MCF-10A cells treated with either E2 or
Z have acquired tumor cell properties including anchorage-independent cell growth and increased cell proliferation rate, and ERβ mRNA expression is increased in the transformed MCF-10A cells treated with E2 or Z. In addition, E2 and Z have the same
potency in the neoplastic transformation of MCF-10A cells.
MATERIALS AND METHODS
Cell culture. Both MCF-10A and MCF-7 cell lines were purchased from
American Type Culture Collection (ATCC, Manassas, VA). MCF-10A cells were
cultured in phenol red-free low calcium DMEM/F12 (0.04 mM CaCl2) supplemented
with Chelex-100 (Bio-Rad Laboratories, Richmond, CA) – treated Fetal Bovine Serum
(FBS) (10%, Atlanta Biologicals, Norcross, GA). MCF-7 cells were cultured in phenol
red-free high-calcium DMEM/F12 (1.05 mM CaCl2) supplemented with 5% FBS. Both
cell lines were plated separately in 75-cm2 culture flasks in a humidified incubator (5%
CO2: 95% air, 37°C). The media of both human breast cell lines were changed every two
20 days. When the cells grew to 85-90% confluence, cells were washed twice with calcium-
and magnesium-free Phosphate Buffered Saline (PBS, pH7.3), and then trypsinized with
0.5% trypsin - 5.3 mM EDTA (GibcoBRL) in PBS for 10 minutes at 37°C. The
trypsinization was stopped by addition of culture medium with 5% or 10% serum. After
centrifugation, The dissociated cell viability (about 99%) were determined by using a
hemacytometer and the cells were resuspended in the same medium and subcultured into
75-cm2 culture flasks at a ratio of 1 flask to 5 flasks.
Cell treatment and RNA extraction. Human breast epithelial cell line MCF-10A
was plated in 75-cm2 culture flasks (1 × 105 viable cells/flask) and treated with 0.1, 1, 10,
100 nM of E2, Z, or vehicle as a control in phenol red-free low-calcium DMEM/F12
supplemented with Dextran-Coated Charcoal (DCC) (Dextran T-70; Pharmacia; activated
charcoal; Sigma)-stripped Chelex-100-treated HS (10%) for 3 days. Treatments were
repeated for 10 times. The cells were passaged at the end of every two treatment cycles.
At the end of both sixth treatment cycle and tenth treatment cycle, partial of the cells
were collected for reverse transcription-polymerase chain reaction assay and cell
proliferation assay. In addition, human breast cancer cell line MCF-7 was plated in 25-
cm2 culture flasks (2 × 105 viable cells/flask) and cultured overnight. The media was
changed to phenol red-free high-calcium DMEM/F12 supplemented with Dextran-Coated
Charcoal (DCC) (Dextran T-70; Pharmacia; activated charcoal; Sigma)-stripped FBS
(5%). After 24 hours, cells were treated with 7.5, 15, 30, 60, 120 nM of E2 or Z or vehicle
as controls in phenol-red-free high-calcium DMEM/F12 supplemented with 5% DCC-
treated FBS for 24 hours. Total RNA was isolated in 2.5 ml TRIZOL Reagent
(GibcoBRL) according to manufacturer’s instructions.
21 Reverse transcription-polymerase chain reaction (RT-PCR). RT-PCR was
performed in a gradient mastercycler (Eppendorf ®). PCR conditions were optimized for
MgCl2 concentration, annealing temperature and cycle number for the amplification of
each PCR product [ERα, ERβ, pS2, protein tyrosine phosphatase γ (PTPγ) and 36B4].
Under optimal conditions, the amounts of PCR products generated fell within the linear
portion of the PCR amplification curve between twenty-six and thirty-nine amplification
cycles. Briefly, 1 µg of total RNA from cultured cells or tissues was reverse transcribed
with 200 U M-MLV Reverse Transcriptase (GibcoBRL) at 42°C for 1 hour in the
presence of 5 mM each of dATP, dCTP, dGTP and dTTP, 4 µl 5X 1st strand buffer
(GibcoBRL), 0.01M DDT, 1 U RNA Guard RNase inhibitor (Pharmacia Biotech,
Uppsala, Sweden), and 2.5 mM random hexamers in a total volume of 20 µl. The reaction
was terminated by heating to 95°C for 3 minutes. The newly synthesized cDNAs were
used as templates for PCR after adjusting reagent concentrations to 1.5 mM (pS2), 1.0
mM (ERα), 2.5 mM (ERβ) or 3.5 mM (PTPγ and 36B4) MgCl2, 2.5 µl 10X PCR Buffer
(GibcoBRL), 1 U Platinum® Taq DNA polymerase (GibcoBRL), and 0.24 µM primers.
The reactant was incubated at 95 °C for 5 minutes. Then, thirty-five cycles (ERα and
ERβ) or thirty cycles (pS2, PTPγ and 36B4) of amplification were performed with each
cycle consisting of denaturation at 95°C for 1 minute, annealing at 63°C for 1 minute,
and extension at 72°C for 1 minute. For pS2, the primer sequences were 5′_TTT GGA
GCA GAG AGG AGG CAA TGG_3′ (sense) and 5′_TGG TAT TAG GAT AGA AGC
ACC AGG G_3′ (antisense); for ERα, they were 5′_TAC TGC ATC AGA TCC AAG
GG_3′ (sense) and 5′_ATC AAT GGT GCA CTG GTT GG_3′ (antisense); for ERß,
22 they were 5′_TGA AAA GGA AGG TTA GTG GGA ACC_3′ (sense) and 5′-TGG TCA
GGG ACA TCA TCA TGG_3′ (antisense); for PTPγ, they were 5′_GCG CAG CGA
CTT TAG CCA GAC GA _3′ (sense) and 5′ _GCT CCC GCT CCC CAT CCT CAC TC
_3′ (antisense); for 36B4, they were 5′ _AAA CTG CTG CCT CAT ATC CG _3′ (sense)
and 5′ _TTG ATG ATA GAA TGG GGT ACT GAT G_3′ (antisense). The final PCR
products (10 µl) mixed with 1 µl of 10 × loading buffer were separated on a 1.0 ~ 1.5%
agarose gel containing ethidium bromide. The specific bands were quantified by
ImageQuaNT software (Molecular Dynamics, Sunnyvale, CA). The results are presented
as the ratio of PTPγ to 36B4. 36B4 is a cDNA clone for human acidic ribosomal
phosphoprotein PO [Masiakowski et al, 1982], for which mRNA levels have been shown
to be unmodified by estradiol treatment [Laborda, 1991].
Doubling time Assay. MCF-10A cells collected at the end of both sixth treatment
and tenth treatment with 0.1, 1, 10, 100 nM of E2, Z or vehicle and wild type MCF-10A cells were plated separately at a density of 5000 in 24-well plates in a volume of 1 ml/well. After cells are attached to the wells, the medium was replaced with 2 ml of fresh low-calcium DMEM/F12 with 10% HS. At the same time (time 0 hour), a group of cells were counted. Cells were grown for 2 days and counted every 4 hours. Adherent cells were detached by rapid trypsinization (about 5 minutes incubation in 1 ml of 0.5% trypsin - 5.3 mM EDTA). An adequate volume of medium containing 50% trypan blue was added. Then cells were counted by use of a hemacytometer. Experiments were performed in 4 replicate culture wells for each group, and each experiment was repeated twice. Based on the counted cell numbers at different time points, a cell proliferation curve was generated. Cell doubling (CD) was calculated using the formula ln (Nj-Ni)/ln 2 23 where Nj or Ni are the cell numbers at different time points Tj or Ti (Tj > Ti) in the growth log phase of the cells. Doubling time (DT) was consequently obtained by dividing the time interval (Tj > Ti) by CD [Poliseno et al., 2002].
Soft agar assay for colony formation. MCF-10A cells collected at the end of both sixth treatment and tenth treatment with 0.1, 1, 10, 100 nM of E2, Z or vehicle and wild type MCF-10A cells were cultured in 6-well plates first covered with an agar layer (1.0 ml of phenol red-free low-calcium DMEM/F12 with 0.5% agar and 10% HS). The middle layer contained 8000 cells in 1.0 ml of phenol red-free low-calcium DMEM/F12 with 0.35% agar and 10% HS. The top layer, consisting of 1 ml of medium, was added to prevent drying of the agar in the plates. The plates were incubated for 30 days and another 1 ml of medium will be added to the top layer at day 15. After 30 days’ incubation, the plates were stained in 0.5 ml of 0.005% crystal violet for >1 hour and the cultures were inspected and photographed.
Statistical analysis. The results for doubling time assay were presented as the mean ± standard deviation (SD) for 4 replicate culture wells as one cell group. Analysis was performed using Minitab statistical software for Windows (Minitab Inc., State
College, PA, USA). Statistical differences were determined by using one-way ANOVA for independent groups. P-values of less than 0.05 were considered statistically significant.
24
RESULTS
Effects of long-term exposure to E2 or Z on the proliferation of MCF-10A cells.
Exposure to estrogen is known to be a contributing factor in the development of breast
and endometrial cancers [Henderson et al., 1988]. Pre-natal exposure to estrogens has
been linked to the development of vaginal, ovarian, testicular, and prostate cancers.
Several risk factors associated with an increased risk of breast cancer reflect an increased
exposure to estrogens. For example, both early age at menarche and old age at
menopause increase a woman’s risk of developing breast cancer [Kelsey et al., 1993]. We
have shown that Z-treatment of cultured human breast cancer cells and human breast
cancer cell lines results in the elevation of mRNA levels of estrogen-regulated genes
involved in the control of normal and cancerous human breast cell proliferation such as
keratinocyte growth factor (KGF/ FGF –7), cyclin D1 and cathepsin [Lin et al., 2000]. To
examine whether long-term exposure to E2 or Z has a mitogenic effect on MCF-10A
cells, a proliferation study was performed by using doubling time assay. Our results
showed that six repeated E2- or Z-treatment cycles have no effects on the doubling time of MCF-10A cells (data not shown), but ten repeated E2- or Z-treatment cycles decrease
the doubling time of MCF-10A cells by 30% ~ 40%, which is not dose-related in our
tested dose range (Table 2.1). These results suggested that long-term exposure to E2 or Z
can stimulate MCF-10A breast epithelial cell growth, and both E2 and Z play important
roles in neoplastic transformation of breast cancer.
25
Effects of E2 or Z on anchorage-independent growth of MCF-10A cells. Short- term treatment of MCF-10F cells with physiological doses of E2 induces anchorage
independent growth and colony formation in agar methocel, which is indicative of
neoplastic transformation [Russo et al., 2002]. In the present work, evaluation of colony
formation at the end of the tenth treatment cycle of E2 or Z reveals that MCF-10A cells
formed colonies in soft agar, and there is no significant difference in the size and colony
efficiency in different doses of E2 or Z-treatment groups and the potency of E2 and Z in
the stimulation of colony formation in MCF-10A cells are similar (Figure 2.1). Whereas,
there are no formed colonies found in the end of sixth treatment of any doses of E2 or Z
(data not shown).
Effects of E2 and Z on ERβ expression in MCF-10A cells. Previous work showed
that ERβ expression can be induced in chemical carcinogen-transformed human breast epithelial cell MCF-10F, and the more transformed cells showed higher levels of ERβ expression, regardless of which chemical carcinogens were initially used for cell transformation [Hu et al., 1998]. As shown in Figure 2.2, both wild type MCF-10A and the transformed MCF-10A do not express ERα and pS2. ERβ is expressed at a very low
level in both wild type MCF-10A and controls, and an elevated level of ERβ expression
is observed in the transformed MCF-10A cells at the end of the tenth different doses of
E2- or Z-treatment and the degree of ERβ expression induction is similar in treatments
with different doses of E2 and Z in our tested range (Figure 2.2). Whereas, ERβ
expression is not changed in the MCF-10A cells in the end of the sixth different doses of
E2- or Z-treatment (data not shown). Furthermore, the expression of protein tyrosine 26 phosphatase γ (PTPγ), an estrogenically regulated tumor suppressor gene, is not changed
by the E2 or Z transformation of MCF-10A cells (Figure 2.2).
Potency comparison of E2 and Z in human breast cell lines. The data presented above showed that E2 and Z had the similar potency in the stimulation of MCF-10A cell
proliferation, anchorage-independent growth of MCF-10A cells and the induction of ERβ
expression in MCF-10A cells. In order to determine whether Z has the similar potency to
E2 in human breast cancer cell lines, we treat estrogen receptor (ER)-positive human
breast cancer cell line MCF-7 with different doses of E2 and Z to determine the dose-
response curve of some gene expression to E2 and Z in MCF-7 cells. As we know, the
pS2 promoter region contains an estrogen response element and pS2 is an estrogen-
regulated gene through estrogen receptor in human breast. Previously, we suggested that
both E2 and Z regulate PTPγ expression in human breast [Liu et al., 2002a], which is
associated with ERα [Liu et al., 2002b].
Our current results show that E2 and Z can upregulate pS2 mRNA expression and
downregulate PTPγ mRNA expression in a dose-response manner, as low as 15 nM of E2 and Z can upregulate pS2 mRNA expression by ~ 30% and downregulate PTPγ mRNA expression by ~ 26% (Figure 2.3, Figure 2.4). These results indicate that E2 and Z have
the similar potency in the regulation of some target gene expressions in ER-positive
MCF-7 cells.
27
DISCUSSIONS
Epidemiological evidence of human breast cancer indicates that white American women have a 5-fold greater risk for breast cancer than Asian women in China and
Japan. Furthermore, the risk of acquiring breast cancer among Asian women immigrants in the U.S. approaches to that of American women after 1 to 2 generations [Kelsey and
Berkowitz, 1988]. These observations indicate a strong environmental, rather than genetic, component in the etiology of this disease [Buell, 1973]. It has been speculated that dietary factors may contribute to this ethnic difference in human breast cancer incidence [Committee on Diet, 1982]. The growth promoter, Z, is a federally approved agent used primarily in the beef, veal and lamb industries in the U.S. Based on our published data generated from the use of cultured normal and cancerous human breast cells and human breast cancer cell lines, the estrogenicity of Z in terms of the induction of estrogen-regulated genes is comparable to the natural estrogen, estradiol-17β (E2), and the synthetic estrogen, diethylstilbestrol (DES).
In our present work, we have demonstrated the transformation of human breast epithelial cell (HBEC) line MCF-10A by an environmental disruptor, Z, for the comparison with some characteristics induced by the natural E2. Long-term treatment of these cells with Z or E2 induces post-confluent foci formation (data not shown), anchorage-independent growth and colony formation in soft agar. The immortalized
HBEC MCF-10A has been claimed as an ER-negative cell line. However, we found that
28 there is low level of ERβ mRNA expression but no ERα mRNA expression in these
cells. In our previous work, we described that both Z and E2 downregulate PTPγ
expression is associated with ERα [Liu et al., 2002b]. All of these findings suggested
that both Z and E2 might transform MCF-10A cells through the redox-mediated pathway
not estrogen receptor-mediated pathway. Some reactive intermediates are generated
through this pathway, which elicit direct genotoxic effects by increasing mutation rates.
Genomic analysis revealed that short-term E2-treated cells exhibited loss of
heterozigosity (LOH) in chromosome 11 [Russo et al, 2002]. In the future, genomic
analysis to the cells treated with Z or E2 is necessary to investigate whether long term and
low doses of Z or E2 can damage the genomic DNA of the treated cells and cause HBEC transformation through the redox-mediated pathway.
Furthermore, our results indicated that expression of ERβ could be induced in both estrogen- and environmental disruptor-transformed MCF-10A cells, which suggested that expression of ERβ might contribute to the initiation and progression of Z- or E2-induced neoplastic transformation. The regulatory mechanisms of inducing ERβ
expression need to be investigated in the future. ERβ can mediate estrogen-induced
biological responses by forming heterodimers with ERα as well as homodimers in a
manner similar to ERα [Cowley et al., 1997]. Since we could not find any ERα
expression in MCF-10A cells, we could speculate that the transformation of MCF-10A
cells by Z and E2 might also be mediated through ERβ homodimer signaling pathway.
However, the role of ERβ-mediated estrogen signaling pathways in the pathogenesis of
malignant diseases is still not quite clear. To provide a quantitative and more functionally
29 relevant evaluation of changes in ERβ expression levels, we have purchased several
commercial ERβ antibodies and conducted both immunohistochemical staining assay and
western blotting assay for ERβ protein. Unfortunately, none of these antibodies have
been found suitable for our experiments.
In our study, we have noticed that there is no dose effect on those tested
biological markers, and we think that this can be due to the following two possibilities.
The dose range we used in our experiment is from 0.1 nM to 100 nM. There is no dose
effect of these biological markers in our tested dose range, but we cannot exclude the
possibility that there will be dose effect if we lower the treatment dose in our experiment.
For example, we could choose 10-4 nM to 0.1 nM as our test dose range in a future
experiment. The other possibility is that ten treatment cycles might have been too long,
so that the effect for each dose has reached the maximum and thus, we did not observe a
dose effect. If this is the case, we might be able to see a dose effect if we terminated the
treatment at the seventh, eighth or ninth treatment cycle.
As we have known, pS2 was cloned from a poly (A) RNA library from the
hormone-dependent, breast cancer cell line, MCF-7 [Masiakowski et al., 1982] and the
pS2 promoter region contains an estrogen response element that varies from the
consensus sequence by one base pair [Berry et al., 1989]. pS2 gene expression is
regulated by the estrogen in breast tissues. Here, we compared the estrogenic potency of
E2 and Z on the regulation of pS2 or PTPγ expression. Surprisingly, we found that the
potency of Z on these targeted gene expression is almost the same as that of E2. One possible explanation is the potential potency of Z is much larger than its actual concentrations suggest (up to 50 times) since most exogenous hormone-like chemicals,
30 including Z and the other synthetic growth promoting hormones, exhibit limited or no
binding to carrier proteins, such as sex hormone binding globulin (SHBG) [Mastri et al.,
1985; Shrimanker et al., 1985; Nagel et al., 1998]. In our study, we showed that MCF-
10A doesn’t express pS2 and we couldn’t examine the estrogenic effects of E2 and Z on
this estrogenic targeted gene expression. In the future, we can use another estrogen
targeted gene PR as the biomarker in our experiments.
In conclusion, we have demonstrated that both Z and E2 can induce human breast epithelial cell line transformation and can induce ERβ expression in human breast epithelial cells by long term and low dose exposure, which might be mediated through the redox-pathway and/or ERβ-mediated pathway needed to be confirmed in the future
experiments. Most interestingly, Z and E2 showed the similar potency in these experiments.
31
Treatments DT (hours) Wild type MCF-10A 17.8 ± 1.2 Control (0.1% DMSO) 18.0 ± 1.0
0.1 nM E2 12.1 ± 0.8
1 nM E2 11.4 ± 0.9
10 nM E2 11.8 ± 0.6
100 nM E2 12.0 ± 0.7 0.1 nM Z 11.6 ± 0.8
1 nM Z 12.0 ± 0.5
10 nM Z 12.0 ± 0.6
100 nM Z 11.4 ± 0.4
Table 2.1. Comparison of MCF-10A cell proliferation by doubling time assay.
MCF-10A cells were treated with different doses of E2 or Z (0.1 nM, 1 nM, 10 nM or 100 nM) or vehicle (0.1% DMSO) as controls for 10 repeated treatment cycles and 0.5 × 104
cells were plated into 24-well plates in low-calcium DMEM/F12 with 10% HS. Cells
were grown for 2 days and counted every 4 hours. The results represented the mean ± SD
of 4 replicate wells. Each experiment was repeated twice. DT: Doubling Time. Statistical
differences were determined by using one-way ANOVA for independent groups. P-
values of less than 0.05 were considered statistically significant.
32
Figure 2.1. Effects of E2 or Z on anchorage-independent growth of MCF-10A cells in
soft agar.
Cells were cultured in 6-well plates first covered with an agar layer [phenol red-free low-
calcium DMEM/F12 with 0.5% agar and 10% Horse Serum (HS)]; the middle layer
contained 8000 cells (wild type MCF-10A or MCF-10A treated with DMSO, E2 or Z for ten repeated treatment cycles) in phenol red-free low-calcium DMEM/F12 with 0.35% agar and 10% HS; the top layer, consisting of cell culture medium, was added to prevent drying of the agarose gels. The plates were incubated for 30 days and another 1 ml of medium was added to the top layer at day 15. After 30 days’ incubation, the plates were stained in 0.5 ml of 0.005% crystal violet for >1 hour and the cultures were inspected and photographed. Bar: 500µm.
A: Wild type MCF-10A; B: 0.1% DMSO-treated MCF-10A; C: 0.1 nM E2-treated MCF-
10A; D: 1 nM E2-treated MCF-10A; E: 10 nM E2-treated MCF-10A; F: 100 nM E2-
treated MCF-10A; G: 0.1 nM Z-treated MCF-10A; H: 1 nM Z-treated MCF-10A; I: 10
nM Z-treated MCF-10A; J: 100 nM Z-treated MCF-10A.
Results showed that E2 or Z-treated MCF-10A cells formed colonies in soft agar, and
there is no significant difference in the size and colony efficiency in our tested different
doses of E2 or Z-treatment groups and the potency of E2 and Z in the stimulation of colony formation in MCF-10A cells are similar.
33
Figure 2.1
34 pS2 (220 bp) ERα (650 bp) ERβ (528 bp) PTPγ (492 bp) 36B4 (563 bp) 2 2 2 2 l Z Z Z Z o E E E E pe y MCF-7 1 nM Marker Contr 1 nM 10 nM 0.1 nM 10 nM 100 nM Wild t 0.1 nM 100 nM
MCF-10A
Figure 2.2. Effects of E2 and Z on ERβ expression in transformed MCF-10A cells.
MCF-10A cells were treated with 0.1, 1, 10, 100 nM of E2, Z or vehicle as controls for
ten repeated treatment cycles. At the end of the tenth treatment cycle, partial of the cells
were collected for RT-PCR assay. Ethidium bromide-stained PCR products were
separated in a 1.5% agarose gel.
Our results showed that both wild type MCF-10A and the transformed MCF-10A does
not express ERα and pS2. ERβ is expressed at a very low level in both wild type MCF-
10A and controls, and an elevated level of ERβ expression is observed in the transformed
MCF-10A cells at the end of the tenth different doses of E2- or Z-treatment and the degree of ERβ expression induction is similar in treatments with different doses of E2 and
Z in our tested range. Furthermore, the expression of protein tyrosine phosphatase γ
(PTPγ), an estrogenically regulated tumor suppressor gene, is not changed by the E2 or Z
transformation of MCF-10A cells. 35 A. M
pS2 (220 bp)
36B4 (563 bp)
B. 2.50
estradiol-17β 2.25 Zeranol
2.00 pression 4)
1.75 /36B 2 (pS mRNA ex
e 1.50 tiv a 1.25 Rel
1.00 0.0 7.5 15 30 60 120 Concentrations (nM)
Figure 2.3. Potency comparison of E2 and Z on pS2 mRNA expression in human
breast cancerous cell line MCF-7.
MCF-7 cells were treated with 7.5, 15, 30, 60, 120 nM of E2 or Z or vehicle as controls
for 24 hours. Then, cells were collected for RT-PCR assay. A. Ethidium bromide-stained
PCR products were separated in a 1.5% agarose gel. B. The mRNA ratios of pS2 to 36B4
as measured by densitometry.
Our results showed that E2 and Z can upregulate pS2 mRNA expression in a dose-
response manner, as low as 15 nM of E2 and Z can upregulate pS2 mRNA expression by
~ 30%.
36 A. M PTPγ (492 bp) 36B4 (563 bp) B. 0.4
estradiol-17β Zeranol
0.3 ssion e r p 4) x
/36B 0.2 γ NA e P R T m (P ve i 0.1 lat Re
0.0 0.0 7.5 15 30 60 120 Concentrations (nM)
Figure 2.4. Potency comparison of E2 and Z on PTPγ mRNA expression in human breast cancerous cell line MCF-7.
MCF-7 cells were treated with 7.5, 15, 30, 60, 120 nM of E2 or Z or vehicle as controls for 24 hours. Then, cells were collected for RT-PCR assay. A. Ethidium bromide-stained
PCR products were separated in a 1.5% agarose gel. B. The mRNA ratios of PTPγ to
36B4 as measured by densitometry.
Our results showed that E2 and Z can downregulate PTPγ mRNA expression in a dose- response manner, as low as 15 nM of E2 and Z can downregulate PTPγ mRNA expression by ~26%.
37 CHAPTER 3
REGULATION OF PROTEIN TYROSINE PHOSPHATASE γ (PTPγ) BY
ESTROGENICALLY ACTIVE AGENTS, ESTRADIOL-17β (E2) AND ZERANOL (Z),
IN HUMAN BREAST
ABSTRACT
Protein tyrosine phosphatase γ (PTPγ) has been implicated as a tumor suppressor gene in kidney and lung cancers. Our previous results indicate that estradiol-17β (E2)-
induced suppression of PTPγ may play a role in mammary tumorigenesis. Zeranol (Z), a
nonsteroidal growth promoter with estrogenic activities that is used by the U.S. meat
industries, induces estrogenic responses in primary cultured breast cells and breast cancer
cell lines. PTPγ mRNA expression in human breast tissues and cells isolated from
surgical specimens of mammoplasty and breast cancer patients were detected and
quantified by RT-PCR. Immunohistochemical staining was used to localize PTPγ in
human breast tissues. Breast epithelial and stromal cells were isolated and co-cultured to
determine the involvement of cell-cell interactions in the regulation of PTPγ mRNA
38 expression by E2 and Z. PTPγ mRNA expression was lower in cancerous than in normal
breast tissues. Both E2 and Z suppressed PTPγ mRNA levels in cultured normal breast tissues by ~80%, but had a lesser effect in the cultured epithelial cells isolated from normal breast tissues. In the co-culture system, both E2 and Z suppressed PTPγ mRNA to
a greater degree in epithelial cells than in stromal cells. In whole breast tissues, PTPγ was
immunolocalized to the epithelium. Treatment with E2 or Z diminished PTPγ staining
indicating reductions in PTPγ at the protein level. The results indicate that both E2 and Z
downregulate PTPγ expression in human breast and that epithelial-stromal cell
interactions are important in the regulation of PTPγ expression by estrogenically active
agents.
INTRODUCTION
Protein tyrosine phosphatases (PTPases) are a family of proteins of which the first
one, PTP1B, was discovered in 1988 by Fischer and co-workers [Tonks et al., 1988].
PTPases get their names from the enzymatic roles they perform inside cells, namely the
removal of phosphate groups from phosphotyrosine residues of specific target proteins.
The phosphorylation of these tyrosine residues is catalyzed by the protein tyrosine
kinases (PTKs) and regulates important cellular processes like metabolism, gene
expression, cell division and differentiation, development, transport, and locomotion.
Since PTPases act on the same kind of biochemical switches as the PTKs, they are 39 thought to play an equally important biological role [Barford et al., 1995; Sun et al.,
1994]. In support with this contention, Klarlund, already in 1985, showed that addition of the PTPase-inhibitor vanadate to cells in culture leads to increased amounts of phosphotyrosine-containing proteins and cellular transformation [Klarlund et al., 1985].
Thus, a delicate balance between PTK and PTPase action is essential for normal functioning of cells. The V-Src, a transforming principle of the chicken Rous sarcoma virus, was determined to have tyrosine kinase activity as it phosphorylates both itself and other proteins on tyrosine residues [Hunter et al., 1980]. Several other viral oncogenes have been shown to be tyrosine kinases [Bishop, 1985]. In addition, the epidermal growth factor receptor was also shown to have tyrosine kinase activity [Ushiro et al., 1980].
Protein tyrosine phosphatases (PTPs) play an essential role in the regulation of cell activation, proliferation and differentiation, since they counterbalance the growth- promoting effects of protein tyrosine kinases (PTKs) [Shock et al., 1995]. Therefore, alterations in PTPs activity might affect cell growth, neoplastic processes and transformation [Gaits et al., 1995].
PTPγ is a member of the receptor-like tyrosine-specific phosphatase family originally cloned from human brain stem and placental cDNA libraries using probes derived from the intracellular domain of CD45 [Kaplan et al., 1990] or Drosophila
PTPase cDNA clone, DPTP12 [Kaplan et al., 1990], respectively. According to mRNA analysis [LaForgia et al., 1991; Barnea et al., 1993], PTPγ is a broadly expressed enzyme that exists in many tissues, including human lung, stomach, esophagus, colon, liver, spleen, and kidney [Tsukamoto et al., 1992]. Based on the chromosomal location of the
PTPγ gene (3p14.2) [LaForgia et al., 1991; LaForgia et al., 1993] and studies showing
40 loss of heterozygosity of the gene in kidney tumors [Lubinski et al., 1994], PTPγ has
been implicated as a candidate tumor suppressor gene.
Human breast tumors exhibit enhanced tyrosine kinase activity relative to benign
breast disease or normal breast tissues [Hennipman et al., 1989]. In breast cancer,
HER2/neu overexpression is an important prognostic indicator, and constitutes a
therapeutic target [Ross et al., 1999], but other receptor tyrosine kinases are also
overexpressed [Ghoussoub et al., 1998]. The growth rate of a large proportion of breast
cancers is influenced by sex steroid hormones, and both steroid hormones and protein tyrosine phosphorylation are demonstrated to play important roles in cell proliferation
[van Biesen et al., 1995].
Zeranol (Z) (described in Chapter 1) is a nonsteroidal agent with estrogenic activities and used as a growth promoter in the U.S. beef, veal and lamb industries.
The estrogen receptor (ER)-positive MCF-7 human breast cancer cell line shows estrogen-dependent growth in vitro, as well as estrogen-dependent tumorigenicity in vivo
[Katzenellenbogen et al., 1987]. Previous work suggests that PTPs may also be important in the growth of breast cancer and may be affected by estrogenic agonists and antagonists. PTP activity can be increased with O-phospho-L-tyrosine with a consequent
S-phase block of breast cancer cell line growth and an enhancement of the effects of chemotherapy on cell killing [Mishra et al., 1993]. Anti-estrogens such as 4-hydroxy- tamoxifen increase the activity of membrane associated PTPs in ER positive breast cancer cell lines, but not in those that are ER negative. This increased PTP activity correlates with decreased growth rates, and the tamoxifen effect on growth rates can be blocked with the PTP antagonist sodium vanadate [Freiss et al., 1994].
41 Consistent with these findings are our previous results that show lower PTPγ mRNA expression levels in diethylstilbestrol (DES)-induced kidney tumors in hamsters than in normal hamster kidney [Lin et al., 1994]. Also, we have shown that, in ACI rats,
PTPγ localizes to the mammary epithelium and Z can suppress PTPγ mRNA levels in mammary glands [Kulp et al., 2000]. Furthermore, we have reported that PTPγ is expressed in normal and malignant human breast epithelium, and that PTPγ mRNA levels can be suppressed by estrogens through an estrogen receptor-mediated mechanism
[Zheng et al., 2000]. These findings suggest that PTPγ is a potential estrogen-regulated tumor suppressor gene in human breast cancer which may play an important role in neoplastic processes of human breast epithelium.
Complex interactions between epithelium and mesenchyme play an essential role in epithelial cell proliferation and differentiation during normal breast development [Van
Roozendaal et al., 1996]. Stromal influences upon epithelia are part of a continuum of cellular interactions that begins at fertilization and extends into adulthood [Donjacour et al., 1991]. A considerable amount of evidence supports the role of stromal cells and their factors in the development and growth of breast cancer. Conditioned media of stromal cells derived from breast tumors has been shown to stimulate the proliferation of several human breast cell lines and primary cultured human breast cancer epithelial cells via the secretion of not-yet-defined factors [Van Roozendaal et al., 1992; Hofland et al., 1995].
In the current study, we demonstrate that the nonsteroidal agent, Z, induces estrogenic effects in human breast tissues, that both E2 and Z regulate PTPγ expression in human breast and that epithelial-stromal cell interactions are important in the regulation of PTPγ expression by estrogenically active agents. 42
MATERIALS AND METHODS
Human breast tissues. Normal human breast tissues and breast cancer tissues were obtained through the Tissue Procurement Program of The Ohio State University Hospital and Riverside Methodist Hospital in Columbus, Ohio. At the time of procurement, the tissue samples were placed in a mixture of Dulbecco's Modified Eagle's Medium and
Ham's F12 Medium (1:1) (DMEM/F12) without phenol red (Sigma Chemical Co., St.
Louis, MO) and stored at 4°C before transfer to the laboratory.
Tissue dissociation. Tissues were sterilized in 70% ethanol for 30 seconds, and then washed three times with fresh DMEM/F12. Tissue samples were minced, and then dissociated overnight at 37°C with 0.1% collagenase (GibcoBRL, Bethesda, MD) in phenol red-free DMEM/F12 medium (1 gram tissue/ml) supplemented with 5% fetal bovine serum (FBS) (Atlanta Biologicals, Norcross, GA) and antibiotic-antimycotic (100 unit/ml penicillin G sodium, 100 mg/ml streptomycin sulfate and 0.25 mg/ml amphotericin B) (GibcoBRL, Bethesda, MD).
Cell culture. The digested mixture was centrifuged at 200 × g (~ 3200 rpm) for 5 min at 25°C. The cell pellet was re-suspended and allowed to settle by gravity for about
10 minutes. The supernatant (containing mostly stromal cells) was then centrifuged at
200 × g for 5 min at 25°C and the pelleted stromal cells were re-suspended in phenol red- free high-calcium DMEM/F12 (1.05 mM CaCl2) supplemented with 5% FBS. The initial
43 sedimented cells (containing mostly epithelial cells) were washed three times with 20 ml
of phenol red-free DMEM/F12 medium and allowed again to settle by gravity for about
10 minutes. Then, the sedimented epithelial cells were re-suspended in phenol red-free
low calcium DMEM/F12 (0.04 mM CaCl2) supplemented with Chelex-100 (Bio-Rad
Laboratories, Richmond, CA) – treated FBS (10%) (Figure 3.1). Sometimes, adipose
stromal cells were also isolated from normal human breast tissues. After initial
centrifugation of the digested tissue, yellow adipose fraction was collected and suspended
in phenol red-free high-calcium DMEM/F12 (1.05 mM CaCl2) supplemented with 5%
FBS. All three isolated specific cell types were plated separately in 75-cm2 culture flasks in a humidified incubator (5% CO2: 95% air, 37°C). The media of all primary cultured
human breast cells and human breast cancer cells were changed every two days. When
the cells grew to ~ 85-90% confluence, cells were washed twice with 10 ml of calcium-
and magnesium-free Phosphate Buffered Saline (PBS, pH7.3), and then trypsinized with
1 ml of 0.5% trypsin - 5.3 mM EDTA (GibcoBRL) in PBS for 10 minutes at 37°C. The
trypsinization was stopped by addition of 10 ml of culture medium with 5% FBS. After
centrifugation, the dissociated cells were resuspended in the culture medium with 5%
FBS and subcultured into 75 cm2 culture flasks at a ratio of 1 flask to 5 flasks.
Immunocytochemistry was used to determine the purity of the isolated primary cultured human breast epithelial cells and stromal cells. Stromal cells were distinguished from epithelial cells based on not only their characteristic morphology but also their specific gene expression, which was confirmed by immunocytochemical staining for multi- cytokeratin and vimentin. Epithelial cells should be positive for the presence of cytokeratin and stromal cells should be positive for the presence of vimentin. Human
44 breast epithelial cells and stromal cells were cultured in multichamber slides (Nunc Inc.,
Naperville, IL). After -10°C methanol fixation for 5 minutes, cells were stained for multi-
cytokeratin (4/5/6/8/10/13/18) (Cat. No. NCL-C11, Vector Laboratories, Inc.,
Burlingame, CA) or vimentin (Cat. No. NCL-VIM, Vector Laboratories, Inc.,
Burlingame, CA) following the manufacturer’s instructions (VECTASTAIN Universal
Quick Kit, Cat. No. PK-8800, and DAB Substrate Kit, Cat. No. SK-4100, Vector
Laboratories, Inc., Burlingame, CA). The optimal primary antibody dilution was 1:10 and
1:100 for multi-cytokeratin and vimentin, respectively. Omission of primary antibody
served as a negative control.
Cell treatment and total RNA extraction. Treatments and total RNA extractions
were performed on cells not propagated beyond the third passage, and the viabilities of
each cell type were greater than 95% as determined by the trypan blue dye exclusion
method [Tennant et al., 1964]. Epithelial cells were plated in 25-cm2 culture flasks (2 ×
105 viable cells/flask) and cultured overnight. The media was changed to phenol red-free
low-calcium DMEM/F12 supplemented with Dextran-Coated Charcoal (DCC) (Dextran
T-70; Pharmacia; activated charcoal; Sigma) -stripped Chelex-100-treated FBS (5%).
After 24 hours, cells were treated with 30 nM E2, 30 nM Z or vehicle as controls in
phenol-red-free low-calcium DMEM/F12 supplemented with 5% DCC-treated FBS for
24 hours. The treatment dose was chosen based on the dose-response curve and our
previous experiments. Total RNA was isolated in 2.5 ml TRIZOL Reagent (GibcoBRL) according to manufacturer’s instructions.
Tissue culture. Normal human breast tissues were sterilized in 70% ethanol for 30 seconds, and then were washed three times with DMEM/F12. Tissue samples were cut
45 into ~ 1mm3/piece pieces which were cultured (5-6 pieces per group, about 60 mg total)
overnight on collagen hemostatic sponges (Integra LifeSciences, Plainsboro, NJ) in 6-
well plates (Corning Costar, Corning, NY). Then, the media was changed to phenol red-
free high-calcium DMEM/F12 supplemented with 5% DCC-treated FBS. After 24 hours,
the tissues were treated with 30 nM E2, 30 nM Z or vehicle in phenol red-free high-
calcium DMEM/F12 supplemented with 5% DCC-treated FBS for 24 hours. All tissues
within a treatment group were pooled and total RNA was then isolated. Tissues were
rapidly frozen in liquid nitrogen and then homogenized with a mortar and pestle in the
presence of TRIZOL Reagent (1 ml/group). Subsequent steps for RNA isolation
followed the manufacturer’s instructions. In our condition, we got about 5-8 µg RNA per
group.
Co-culture system. Co-cultures of epithelial and stromal cells were performed by
using flat-bottomed cell culture plates (Figure 3.2). Stromal cells (0.5 × 106 cells/well) were plated on the nucleopore polycarbonate membrane (0.4 mm pore size) of the cell culture inserts (upper chamber). Epithelial cells (1.0 × 106 cells/well) were seeded on the
bottom plates (lower chamber). The cells were cultured in their specific culture media
overnight, which were replaced with phenol red-free high-calcium DMEM/F12
supplemented with 5% DCC-treated FBS. After 24 hours, the cells were treated with 30
nM E2, 30 nM Z or vehicle in the same medium for 24 hours. Total RNA was then
isolated using 5 ml TRIZOL Reagent following manufacturer’s instructions.
Reverse transcription-polymerase chain reaction (RT-PCR). RT-PCR was
performed in a gradient mastercycler (Eppendorf ®). PCR conditions were optimized for
MgCl2 concentration, annealing temperature and cycle number for the amplification of 46 each of the PCR products (PTPγ and 36B4). Under optimal conditions, the amounts of
PCR products generated fell within the linear portion of the PCR amplification curve
between twenty-six and thirty-nine amplification cycles. Briefly, 1 µg of total RNA from
cultured cells or tissues was reverse transcribed with 200 U M-MLV Reverse
Transcriptase (GibcoBRL) at 42°C for 1 hour in the presence of 5 mM each of dATP,
dCTP, dGTP and dTTP, 4 µl 5× 1st strand buffer (GibcoBRL), 0.01M DDT, 1 U RNA
Guard RNase inhibitor (Pharmacia Biotech, Uppsala, Sweden), and 2.5 mM random
hexamers in a total volume of 20 µl. The reaction was terminated by heating to 95°C for
3 minutes. The newly synthesized cDNAs were used as templates for PCR after adjusting
reagent concentrations to 3.5 mM MgCl2, 2.5 µl 10X PCR Buffer (GibcoBRL), 1 U
Platinum® Taq DNA polymerase (GibcoBRL), and 0.24 µM primers. The reactant was incubated at 95 °C for 5 minutes. Then, thirty cycles of amplification were performed with each cycle consisting of denaturation at 95°C for 1 minute, annealing at 63°C for 1 minute, and extension at 72°C for 1 minute. The primer sequences for PTPγ were
5´_GCG CAG CGA CTT TAG CCA GAC GA _3´ (sense @ 51 to 73) and 5´_GCT
CCC GCT CCC CAT CCT CAC TC _3´ (antisense @ 542 to 519). The primer sequences for 36B4 were 5´_AAA CTG CTG CCT CAT ATC CG _3´ (sense @ 306 to
325) and 5´_TTG ATG ATA GAA TGG GGT ACT GAT G_3´ (antisense @ 868 to
848). The final PCR products (10 µl) mixed with 1 µl 10 × loading buffer were separated on a 1.5% agarose gel containing ethidium bromide. The lengths of the PCR products were 492 bp for PTPγ and 563 bp for 36B4. The specific bands were quantified by
ImageQuaNT software (Molecular Dynamics, Sunnyvale, CA). The results are presented as the ratio of PTP γ to 36B4. 47 Immunohistochemical staining. Normal human breast tissues were cultured and treated as described above. All tissues within a treatment group were fixed, dehydrated, embedded in paraffin, and sectioned at 5-micron thickness for immunohistochemical staining of PTP γ. As a reference antibody, we used an affinity-purified goat polyclonal antibody raised against a peptide corresponding to amino acids 1421-1438 mapping at the carboxy terminus of the PTPγ precursor of human origin (C-18; Cat. No. sc-1111, Santa
Cruz Biotechnology, Santa Cruz, CA). According to the manufacturer’s instructions, the
C-18 anti-PTPγ serum reacts with PTPγ of mouse, rat and human origin by Western blotting and immunohistochemistry. The goat ABC staining system (Cat. No. sc-2023,
Santa Cruz Biotechnology, Santa Cruz, CA) was used to stain PTPγ in paraffin- embedded tissue sections following the manufacturer’s instructions. An optimal primary antibody concentration of 1.60 µg/ml was determined by titration in this system, and omission of the primary antibody served as a negative control. Furthermore, the specificity of the PTPγ antibody was confirmed by elimination of specific binding after preincubation of the antibody with the PTPγ blocking peptide. Briefly, following the manufacture’s instructions, the PTPγ primary antibody was combined with five-fold (by weight) excess of PTPγ blocking peptide (Cat. No. sc-1111p, Santa Cruz Biotechnology,
Santa Cruz, CA) in a small volume of PBS and incubated for 2 hours at room temperature. The antibody/peptide mixture was then diluted to the predetermined optimal primary antibody concentration (1.60 µg/ml) and the immunohisto-chemistry procedure described above was performed.
48 RESULTS
Comparison of PTPγ mRNA expression levels in normal and cancerous human breast tissues. PTPγ expression has been previously documented in human lung, stomach, esophagus, colon, liver, spleen, and kidney tissues [Tsukamoto et al., 1992].
Also, previous work in our laboratory showed that PTPγ mRNA expression was lower in mixed cell populations from cancerous human breast tissues than in mixed cell populations from normal human breast tissues [Zheng et al., 2000]. To compare mRNA expression of PTPγ in normal and cancerous human breast tissues, PTPγ mRNA levels were determined by RT-PCR in breast tissues from 3 reduction mammoplasty patients
(20-30 years of age) and 3 breast cancer patients (50-70 years of age). Normal breast tissues had 50 – 60% higher levels of PTPγ mRNA than the breast cancer tissues (Figure
3.3). Also, PTPγ mRNA expression was greater in isolated epithelial cells than in stromal cells from the same patient (Figure 3.4) suggesting a cell-specific expression pattern.
The purity of the human breast stromal and epithelial cell preparations were indicated by morphology and confirmed by immunohistochemistry. Stromal cells were distinguished from epithelial cells based on not only their characteristic morphology but also their specific gene expression, which was confirmed by immunocytochemical staining (Figure 3.5). Epithelial cells tended to grow in characteristic rounded shapes, while stromal cells exhibited typical spindle-shaped morphology. Immunocytochemical staining revealed that more than 95% of the cultured epithelial cells were positive for the presence of cytokeratin (Figure 3.5B), while almost no expression of vimentin was 49 detected (Figure 3.5C). Similarly, the majority of stromal cells (>95%) were
immunopositive for the presence of vimentin (Figure 3.5F) and no expression of
cytokeratin was detected (Figure 3.5E), which confirmed the fibroblastic nature of the
stromal cells.
Regulation of PTPγ mRNA expression in normal human breast tissues and
epithelial cells by E2 and Z. We have shown that PTPγ mRNA expression is inhibited by
E2 in a dose-dependent manner in primary cultured human breast cells [Zheng et al.,
2000], and both normal and cancerous human breast cell exhibit estrogenic responses to
Z [Irshaid et al., 1999]. To investigate the effects of E2 and Z on PTPγ mRNA expression
in normal human breast tissues, RT-PCR was used to determine PTPγ mRNA levels in
cultured breast tissue after treatment with E2 or Z. Levels of PTPγ mRNA in the E2- and
Z-treated tissues were significantly suppressed in comparison to levels in the control
tissues (approximately an 80% reduction) (Figure 3.6). Since epithelial cells were shown
to express higher levels of PTPγ than stromal cells (Figure 3.4 and 3.8), the sensitivity of
PTPγ mRNA expression level to estrogenic action in epithelial cells was examined and
compared to that in cultured breast tissues. After treatment of normal breast epithelial
cells with 30 nM E2 or Z, PTPγ mRNA levels were reduced by 30% (Figure 3.7), as
opposed to the 80% reduction observed in E2 or Z-treated cultured breast tissues (Figure
3.6). Thus, E2 and Z suppressed PTPγ mRNA levels to a greater degree in cultured breast
tissues than in epithelial cells. Also, under the in vitro conditions described, Z was
capable of suppressing PTPγ mRNA levels to a degree identical to that of E2.
Immunohistochemical localization and reactivity of PTPγ in normal and
50 cancerous human breast tissues, and the regulation of PTPγ immunohistochemical reactivity by E2 and Z in normal human breast tissue. The expression of PTPγ is severely reduced (>50%) in lung tumors and ovarian tumors in comparison to normal tissues [van
Niekerk et al., 1999]. We have shown PTPγ is localized to the mammary epithelium of
ACI rats [Kulp et al., 2000]. The present results show that PTPγ was primarily localized to the glandular epithelium while staining was almost absent from the stromal compartment in both normal and cancerous breast tissues; however, the degree of staining was observably diminished in cancerous human breast tissue (Figure 3.8E) when compared to normal human breast tissue (Figure 3.8D). In Z-treated cultured human breast tissue (Figure 3.8H), PTPγ was also immunolocalized to the epithelium; however, the degree of staining was observably diminished in comparison to the control tissue
(Figure 3.8F). Comparable results were obtained for the E2-treated breast tissues (Figure
3.8G) in which PTPγ staining was diminished in most of the epithelium observed.
Densely immunopositive glandular components were occasionally observed, but these were sparsely distributed throughout the normal tissues. Thus, these results indicate that in human breast tissues, immunoreactive PTPγ is localized to the glandular epithelium and is not detected in the stromal compartment. The expression of PTPγ is reduced in cancerous human breast tissue compared to normal tissue. Furthermore, treatment with
E2 or Z apparently does not alter the epithelial localization of PTPγ, but the degree of immunoreactivity is diminished by these treatments. These results are consistent with those presented in Figure 3.6 above in which treatment of cultured normal human breast tissues with 30 nM of E2 or Z resulted in the suppression of PTPγ mRNA expression as
51 determined by RT-PCR.
Effects of E2 and Z on PTPγ mRNA expression on normal human breast epithelial
cells and stromal cells in co-culture systems. It is known that stromal cells and their
factors play a very important role in the development and growth of breast cancer [Van
Roozendaal et al., 1996; Donjacour et al., 1991; Van Roozendaal et al., 1992; Hofland et
al., 1995]. Based on the results above showing that E2 and Z suppressed PTPγ mRNA expression to a greater degree in breast tissues than in breast epithelial cells, we hypothesized that stromal cells were important for the greater suppression of PTPγ mRNA expression by E2 and Z in normal human breast tissues. To test this hypothesis,
co-culture systems containing epithelial and stromal cells were used to determine effects
of the presence of stromal cells on E2- or Z-induced suppression of PTPγ mRNA in
epithelial cells. The results showed that both E2 and Z (30 nM; 24 h) suppressed PTPγ
mRNA expression to a greater degree in epithelial cells (80-90% reduction) than in
stromal cells (20-30% reduction) (Figure 3.9). Also, the degree of suppression observed
in the co-cultured epithelial cells was greater than that seen in epithelial cells cultured
alone (80-90% vs 30% reductions) (Figures 3.9 and 3.7), and comparable to that observed
in cultured breast tissues (Figure 3.6). Furthermore, we found that the conditioned
medium from the co-culture model also increased the suppression of PTPγ expression by
E2 and Z in the epithelial cells cultured alone, and the degree of suppression is similar to that in the co-culture model. Combined together, these results suggested that stromal cells might be able to produce some factors to enhance the estrogenic effects of E2 and Z on
PTPγ expression in human breast.
52
DISCUSSIONS
Previous work has indicated an intriguing link between PTPγ and cancer. In renal and lung cancers, PTPγ has been implicated as a tumor suppressor gene [LaForgia et al.,
1991] and its expression is reduced in lung and ovarian tumors [van Niekerk et al., 1999].
Our own findings have shown that PTPγ mRNA levels are lower in breast tissues from breast cancer patients than from normal patients (Figure 3.3; [Zheng et al., 2000]). We also showed that E2 suppresses PTPγ mRNA expression via an ER-mediated mechanism
[Zheng et al., 2000]. The findings reported here extend our previous work by showing that the epithelial compartment is the primary site of PTPγ expression in human breast, and demonstrating the importance of stromal cells in the suppression of epithelial PTPγ expression by estrogenic agents. We also demonstrate that a nonsteroidal agent with estrogenic activities, Z, suppresses PTPγ mRNA expression to a degree comparable to that of E2.
Using cultured epithelial and stromal cells isolated from normal human breast tissues acquired from 3 different reduction mastectomy patients, we showed that
PTPγ mRNA levels are approximately 2-fold greater in epithelial cells than in stromal cells as determined by RT-PCR (Figure 3.4). This finding is supported by the results of immunohistochemistry that reveal PTPγ immunoreactivity to be localized exclusively to the epithelium in cultured normal human breast tissues (Figure 3.8). Similarly, our
53 previous results immunolocalized PTPγ to the glandular epithelium in mammary glands
of ACI rats, an animal model used for the study of estrogen-induced mammary
tumorigenesis [Kulp et al., 2000]. The absence of observable PTPγ immunoreactivity in
the stroma of cultured breast tissues (Figure 3.8), despite detection of PTPγ mRNA in
isolated stromal cells by RT-PCR (Figure 3.4), may indicate the presence of epithelial
contamination in the stromal cell isolated. This possibility seems unlikely since
immunohistochemistry failed to reveal the presence of cytokeratin-positive cells in the
isolated stromal cell population (Figure 3.5). Indeed, the isolated epithelial and stromal
cell populations were shown to be greater than 95% pure based on cytokeratin and
vimentin immunoreactivity (Figure 3.5). Alternatively, the results may suggest that
immunohistochemistry was not sufficiently sensitive to detect low expression levels of
PTPγ protein in the cultured breast tissues, or indicate the presence of a suppressive
epithelial influence in stromal PTPγ expression that is active in cultured tissues, but
absent in the culture of isolated stromal cells. These possibilities are speculative and
require further work to clarify.
Previous work from our laboratory demonstrated substantial suppression of PTPγ
mRNA expression by E2 treatment in mixed breast cell populations that contained both
epithelial and stromal components [Zheng et al., 2000]. Utilizing cultured normal human
breast tissues, the present study produced similar results. Treatment with 30 nM E2 resulted in an approximately 80% reduction in PTPγ mRNA expression (Figure 3.6).
Immunohistochemical staining of these cultured breast tissues supported this finding by revealing a noticeable reduction in the intensity of PTPγ immunoreactivity in the epithelial compartment (Figure 3.8G). However, identical treatment of cultured epithelial 54 cells isolated from normal human breast tissues suppressed PTPγ mRNA levels to a much
smaller degree (only a 30% reduction; Figure 3.7). These findings led to the hypothesis
that stromal cells are important for full suppression of PTPγ mRNA expression by E2.
The results of subsequent experiments using co-culture systems of isolated breast epithelial and stromal cells supported this hypothesis. Treatment of epithelial cells co- cultured with stromal cells from normal human breast tissues with 30 nM E2 resulted in a
level of suppression of PTPγ mRNA expression (80 – 90% reduction) that was nearly
identical to that observed in cultured breast tissues (Figures 3.6 and 3.9). Because the co-
culture system utilized for this study eliminated the influence of cell-to-cell contact
(Figure 3.2), the findings suggest the presence of a soluble factor(s) of stromal cell origin
that affects the response of epithelial cells to E2-mediated suppression of PTPγ mRNA
expression. Paracrine interactions between stromal and epithelial cells are known to play
an essential role in epithelial and stromal cell proliferation and differentiation during
normal breast development [Clark et al., 1992]. A disruption or perturbation of stromal-
epithelial interaction of normal cells might lead to abnormal cell growth and tumor
development [Donjacour et al., 1991]. Therefore, understanding the regulation and
mechanisms of stromal-epithelial interactions with respect to proliferation of normal
breast cells is critically important. It has been reported that normal breast epithelial cells
significantly inhibited, while normal breast stromal cells significantly stimulated, breast
cancer cell growth [Dong-LeBourhis et al., 1997; Hu et al., 1995]. Furthermore, it was
shown that primary cultured normal breast epithelial cells significantly inhibited the
proliferation of normal breast stromal cells and adipose stromal cells in co-culture, and
that TGF-β1 may play an important role in mediating normal human breast stromal- 55 epithelial interactions [Zhang et al., 1999]. Growth factors have been implicated as autocrine/paracrine mediators of epithelial-stromal interactions [Aaronson et al., 1991].
Human breast stromal cells secret peptide growth factors including insulin-like growth factor-I and -II [Cullen et al., 1991] and transforming growth factor-α [Cunha et al.,
1994] to regulate breast epithelial cell functions. Perhaps some of these known growth factors and/or other not-yet-defined growth factors from stromal cells play an important role in the regulation of PTPγ expression by estrogenically active agents in human breast.
Our previous work demonstrating estrogen-induced suppression of PTPγ in ER- positive breast cancer cell lines and primary cultured human breast cells was one of only two reports describing the pharmacological modulation of PTPγ expression [Zheng et al.,
2000; Schumann et al., 1998]. The demonstrated estrogenic regulation of PTPγ expression in human breast tissues is consistent with our previous work using the diethylstilbestrol (DES)-induced hamster kidney tumor model of estrogen-dependent tumorigenesis [Lin et al., 1994]. Kidney tumors in DES-treated hamsters contained lower levels of PTPγ mRNA than kidneys from control hamsters. These findings are intriguing in light of previous studies that showed changes in PTPs and PTP activity in human breast cancer. Hydroxy-tamoxifen increases the activity of a membrane-associated
PTP(s) in ER-positive breast cancer cell lines, but not in those that are ER-negative
[Freiss et al., 1994], a pattern that is complementary to that which we find for PTPγ, a transmembrane PTP. Nonetheless, a definitive role for PTPγ in breast cancer has not yet been established. Based on our results, however, it appears that estrogen might promote breast cell growth at least in part by manipulating the expression of PTPγ. This notion is supported by findings of our further work using MCF-7 cells transfected with an 56 expression vector containing a full-length PTPγ cDNA insert. As determined by RT-PCR,
the PTPγ-transfectants express PTPγ mRNA levels that are approximately 150% higher
than those in the mock-transfected and wild-type MCF-7 cells. Also, the 3H-thymidine
incorporation assay indicated that the proliferation rate of the PTPγ-transfectants is
approximately 70% lower than the mock-transfected and wild-type MCF-7 cells (data
shown in Chapter 5).
If the suppression of PTPγ expression plays a role in estrogen-induced breast cell
growth and/or tumorigenesis, the question arises as to whether other estrogenic agents,
such as environmental estrogens/xenoestrogens, induce similar changes in PTPγ. One
such compound is Z, a nonsteroidal anabolic growth promoter with estrogenic activities.
Human exposure to Z occurs via the consumption of food products, particularly beef,
veal and lambs, derived from food animals treated with Z. Our results show that, in
cultured normal human breast tissues, isolated human breast epithelial cells and breast
epithelial cells in co-culture with stromal cells, Z at 30 nM significantly reduced PTPγ
mRNA levels (Figures 3.6, 3.7 and 3.9). This reduction is reflected in the
immunohistochemical results, which show markedly reduced PTPγ immunoreactivity in
the epithelium of Z-treated breast tissues (Figure 3.8H). The ability of Z treatment to
stimulate pS2 mRNA expression levels (data not shown) confirms its estrogenic activities
and indicates the absence of a generalized toxic effect of Z on the cultured breast tissues.
Of particular significance is the finding that the degree of Z-induced reduction of PTPγ
mRNA levels in these in vitro systems was equivalent to the reductions induced by E2.
Thus, exposure to equimolar amounts of E2 or Z results in identical effects in estrogen-
sensitive tissues, at least with respect to PTPγ expression in human breast under the 57 conditions described. These findings may have implications for human health,
particularly in regards to breast cell growth and perhaps breast cancer, by suggesting the
presence of a health risk from the consumption of beef or other food products derived
from Z-treated animals. The relevance of this potential relationship to human health is
strengthened by our preliminary findings, which revealed that Z levels, as measured by
High Performance Liquid Chromatography (HPLC), in edible tissues from Z-implanted
beef cattle were much lower than the permissible limits of free Z established by the FDA
[Code of Federal Regulations, 1991]. In particular, muscle tissue (meat) was shown to
contain an HPLC-detectable Z level that was approximately 29-times lower than the FDA
established limit (5.16 ± 0.46 ng/gm vs 150 ppb). Despite this low level of HPLC-
detectable Z, extracts of this meat containing 0.34, 1.70 and 8.50 ng Z/ml, stimulated 3H- thymidine incorporation by cultured human breast cells [Lin et al., 2000]. Effects of the
long-term consumption of low levels of Z in foods derived from Z-implanted animals are
unknown. Exploration of the potential risk to human health posed by this mode of
exposure is an active area of our research.
The experiments described here yield new and potentially important information
regarding the regulation of PTPγ, a potential tumor suppressor, in human breast by
estrogenic agents. The results show that the breast epithelium is the primary site of PTPγ
expression and that full estrogenic suppression of PTPγ mRNA expression involves
epithelial-stromal interactions. Furthermore, Z, a nonsteroidal agent with estrogenic
activities and a potential source of environmental estrogen exposure, reduces PTPγ
expression to a degree identical to that induced by the natural estrogen, E2. The
identification of PTPγ’s ligands, intracellular substrates and function, which are yet 58 unknown, and the complete elucidation of the mechanisms by which its expression and activity are regulated are essential for establishing the importance of PTPγ in the processes of estrogen-induced breast cell growth and/or breast tumorigenesis.
59
Tissue
Mince tissue Digest tissue in collagenase
Cultured in high Ca2+ Cultured (1.05 mM) DMEM/F12 human breast stromal cells Centrifuge
Cultured Cultured in low Ca2+ Wash (5 times, human breast (0.04 mM) DMEM/F12 epithelial cells allow settle by gravity)
Figure 3.1. Isolation of epithelial cells and stromal cells from normal human breast tissues and phase contrast photomicrograph of cultured human breast epithelial cells and stromal cells.
Epithelial cells and stromal cells from normal human breast specimens were isolated as described in Materials and Methods section. Normal human breast epithelial cells and stromal cells were cultured for 5 days. Epithelial cells grow in sheets and have round prominent nuclei; stromal cells have a typical spindle shape.
60
Medium level
Upper Chamber (0.5x106) Stromal cells
Polycarbonate membrane Epithelial cells 6 (pore size: 0.4µm) (1.0x10 )
Lower Chamber
Figure 3.2. Modified in vitro co-culture assay system.
The 70 mm TranswellTM system was used for co-culture assay. The diameters of the upper chamber and lower chambers were 7.5 cm and 8.5 cm, respectively. The bottom of upper chamber is a polycarbonate membrane with a 0.4 µm pore size. Stromal (0.5×106) cells were seeded in the upper chamber and epithelial cells (1.0×106) were seeded in the lower chamber. Since the size of human breast stromal cell is larger than that of human breast epithelial cell, we seeded less number of stromal cells in the upper chamber.
61
Figure 3.3. Comparison of PTPγ mRNA expression by RT-PCR in normal human breast tissues and cancerous human breast tissues.
Normal human breast tissues and breast cancer tissues were obtained through the Tissue
Procurement Program of The Ohio State University Hospital and Riverside Methodist
Hospital in Columbus, Ohio. A. Ethidium bromide-stained PCR products separated in a
1.5% agarose gel. Tissues were not treated and total RNA was isolated from each tissue sample separately. Lane 1-3: normal human breast tissues from 3 different patients. Lane
4-6: cancerous human breast tissues from 3 different patients. 36B4 was used as internal standard. B. The mRNA ratios of PTPγ to 36B4 as measured by densitometry.
The result showed that normal breast tissues had 50 – 60% higher levels of PTPγ mRNA
than the breast cancer tissues.
62
A 123 456 36B4 (563 bp)
PTPγ (492 bp)
B
0.8
0.6 Expression /36B4) γ 0.4 mRNA (PTP e
0.2 Relativ
0.0 123 4 56 Normal Tissue Cancerous Tissue
Figure 3.3
63 Figure 3.4. Comparison of PTPγ mRNA expression by RT-PCR in normal human breast tissues, epithelial cells and stromal cells.
As described in Materials and Methods, normal human breast epithelial cells and stromal cells were isolated from the normal human breast tissue, which were obtained through the
Tissue Procurement Program of The Ohio State University Hospital and Riverside
Methodist Hospital in Columbus, Ohio. Epithelial cells were cultured in phenol red-free low calcium DMEM/F12 (0.04 mM CaCl2) supplemented with Chelex-100 (Bio-Rad
Laboratories, Richmond, CA) – treated FBS (10%). Stromal cells were cultured in phenol red-free high-calcium DMEM/F12 (1.05 mM CaCl2) supplemented with 5% FBS. A.
Ethidium bromide-stained PCR products separated in a 1.5% agarose gel. 1, 2 and 3 represent patient 1, patient 2 and patient 3, respectively. Tissues and specific cell types were not treated and total RNA was isolated from each group of tissues or cells separately. 36B4 was used as internal standard. B. The mRNA ratios of PTPγ to 36B4 as measured by densitometry.
The result showed that PTPγ mRNA expression was greater in isolated epithelial cells than in stromal cells from the same patient.
64
A B Relative mRNA Expression (PTPγ/36B4) 0. 0. 0. 0. 0. 0. 0. 0. 0. 0 1 2 3 4 5 6 7 8 1
2 Tissues
Tis 31 s ue
Epi 23 Figure 3.4 Epit t h h
65 e lial C el ia l C e lls e
* lls 1 2 S S tr tr o o ma ma 3 l l C C el ell ls s 36B4 (563bp) PT Patient 3 Patient 2 Patient 1 P γ (492 bp)
Figure 3.5. Immunocytochemical staining of primary cultured human breast epithelial cells and stromal cells.
Human breast epithelial cells and stromal cells were cultured in multichamber slides
(Nunc Inc., Naperville, IL). After -10°C methanol fixation for 5 minutes, cells were stained for multi-cytokeratin (4/5/6/8/10/13/18) (Cat. No. NCL-C11, Vector Laboratories,
Inc., Burlingame, CA) or vimentin (Cat. No. NCL-VIM, Vector Laboratories, Inc.,
Burlingame, CA) following the manufacturer’s instructions (VECTASTAIN Universal
Quick Kit, Cat. No. PK-8800, and DAB Substrate Kit, Cat. No. SK-4100, Vector
Laboratories, Inc., Burlingame, CA). The optimal primary antibody dilution was 1:10 and
1:100 for multi cytokeratin and vimentin, respectively. Omission of primary antibody served as a negative control.
Brown staining represents cytokeratin or vimentin immunoreactivity; A. Negative control in epithelial cells; B. Cytokeratin staining in epithelial cells; C. Vimentin staining in epithelial cells; D. Negative control in stromal cells; E. Cytokeratin staining in stromal cells; F. Vimentin staining in stromal cells.
These staining results showed that more than 95% of the cultured epithelial cells were positive for the presence of cytokeratin, and the majority of stromal cells (>95%) were immunopositive for the presence of vimentin, which indicated the high purity in the isolated specific cells.
66
A D
5µm 5µm
B E
5µm 5µm
C F
5µm 5µm
Figure 3.5
67
Figure 3.6. Regulation of PTPγ mRNA expression in normal human breast tissues by E2 and Z.
Normal human breast tissues were sterilized in 70% ethanol for 30 seconds, and then were washed three times with DMEM/F12. Tissue samples were cut into ~ 1 mm3 /piece pieces which were cultured (5-6 pieces per group) overnight on collagen hemostatic sponges (Integra LifeSciences, Plainsboro, NJ) in 6-well plates (Corning Costar, Corning,
NY). Then, the media was changed to phenol red-free high-calcium DMEM/F12 supplemented with 5% DCC-treated FBS. After 24 hours, the tissues were treated with 30 nM E2, 30 nM Z or vehicle in phenol red-free high-calcium DMEM/F12 supplemented
with 5% DCC-treated FBS for 24 hours. All tissues within a treatment group were
pooled, and total RNA was then isolated and RT-PCR was performed.
A. Ethidium bromide-stained PCR products separated in a 1.5% agarose gel. 1 and 2
represent patient 1 and patient 2, respectively. Normal human breast tissues were treated
with 30 nM E2, 30 nM Z or vehicle in phenol red-free high-calcium DMEM/F12
supplemented with 5% DCC-treated FBS for 24 hours. Total RNA was isolated from
each treatment group separately. 36B4 was used as internal standard. B. The mRNA
ratios of PTPγ to 36B4 as measured by densitometry.
The result showed that 30 nM of E2 or Z can downregulate PTPγ mRNA expression by
~80% in normal human breast tissues.
68
A ß 17 l- ol SO radio an M st er D E Z
0.03% 30 nM 30 nM 112212 36B4 (563 bp)
PTPγ (492 bp) B
0.8 Patient 1
on Patient 2 essi 0.6 4) B Expr /36 γ 0.4 mRNA (PTP e
tiv 0.2 a Rel
0.0
ol SO 17ß M l- an D radio 0.03% st 30 nM Zer M E
30 n
Figure 3.6
69
Figure 3.7. Regulation of PTPγ mRNA expression in normal human breast epithelial
cells by E2 and Z.
Epithelial cells were plated in 25-cm2 culture flasks (2 × 105 viable cells/flask) and cultured overnight. The media was changed to phenol red-free low-calcium DMEM/F12 supplemented with Dextran-Coated Charcoal (DCC) (Dextran T-70; Pharmacia; activated charcoal; Sigma) -stripped Chelex-100-treated FBS (5%). After 24 hours, cells were treated with 30 nM E2, 30 nM Z or vehicle as controls in phenol-red-free low-calcium
DMEM/F12 supplemented with 5% DCC-treated FBS for 24 hours. Total RNA was isolated and RT-PCR was performed.
A. Ethidium bromide-stained PCR products separated in a 1.5% agarose gel. 1 and 2 represent patient 1 and patient 2, respectively. Normal human breast epithelial cells were treated with 30 nM E2, 30 nM Z or vehicle in phenol red-free low-calcium DMEM/F12
supplemented with 5% DCC-treated FBS for 24 hours. Total RNA was isolated from
each treatment group separately. 36B4 was used as internal standard. B. The mRNA
ratios of PTPγ to 36B4 as measured by densitometry.
The result showed that 30 nM of E2 or Z can downregulate PTPγ mRNA expression by
~30% in normal human breast epithelial cells.
70
B Relative mRNA Expression A (PTPγ/36B4) 0. 0. 0. 0. 0 2 4 6 11
0.03% 0.03% 22 DM D SO M SO
30 n
M Figure 3.7 Est 30 nM
71 r adio E st l- rad 17ß io l 1 -17ß
30 nM 30 n
M 2 Z Zer er ano a nol l PT 36 P P B4 (563 P a a γ ti ti (492 bp) e e n n t 2 t 1 bp) Figure 3.8. Immunohistochemical localization and reactivity of PTPγ in normal and cancerous human breast tissues, and the regulation of PTPγ immunohistochemical reactivity by E2 and Z in normal human breast tissue.
Normal human breast tissues were cultured and treated as described in figure 3.6. All
tissues within a treatment group were fixed, dehydrated, embedded in paraffin, and
sectioned for immunohistochemical staining of PTPγ. As a reference antibody, we used
an affinity-purified goat polyclonal antibody raised against a peptide corresponding to
amino acids 1421-1438 mapping at the carboxy terminus of the PTPγ precursor of human
origin (C-18; Cat. No. sc-1111, Santa Cruz Biotechnology, Santa Cruz, CA). The goat
ABC staining system (Cat. No. sc-2023, Santa Cruz Biotechnology, Santa Cruz, CA) was
used to stain PTPγ in paraffin-embedded tissue sections.
Brown staining represents PTPγ immunoreactivity; Blue staining represents nuclei. EC:
Epithelial compartment; SC: Stromal compartment. A. Negative control; B. Peptide
neutralization in normal human breast tissue without treatment; C. Peptide neutralization
in cancerous human breast tissue without treatment; D. Normal human breast tissue
without treatment; E. Cancerous human breast tissue without treatment; F. Normal
human breast tissue treated with 0.03% DMSO for 24 hours; G. Normal human breast
tissue treated with 30 nM E2 for 24 hours; H. Normal human breast tissue treated with Z
for 24 hours.
These results indicated that in human breast tissues, immunoreactive PTPγ is localized to
the glandular epithelium. The PTPγ level is reduced in cancerous human breast tissue
compared to normal tissue. Furthermore, the degree of immunoreactivity is diminished
by the treatment with E2 or Z. 72 A E
SC EC
EC
10µm 10µm
B F
SC EC EC SC 10µm 10µm
C G
EC
EC SC
10µm 10µm
D H
SC
EC EC SC
10µm 10µm
Figure 3.8
73
Figure 3.9. Regulation of PTPγ mRNA expression E2 and Z in normal human breast
epithelial cells and stromal cells in co-culture assay system.
Stromal (0.5×106) cells were seeded in the upper chamber and epithelial cells (1.0×106)
were seeded in the lower chamber in the co-culture system. The cells were cultured in
their culture media specific for epithelial cells or stromal cells overnight, which were
replaced with phenol red-free DMEM/F12 supplemented with 5% DCC-treated serum.
After 24 hours, the cells were treated with 30 nM E2, 30 nM Z or vehicle in the same medium for 24 hours. Total RNA was then isolated and RT-PCR was performed.
A. Ethidium bromide-stained PCR products separated in a 1.5% agarose gel. 1, 2 and 3 represent patient 1, patient 2 and patient 3, respectively. Cells were treated with 30 nM
E2, 30 nM Z or vehicle in phenol red-free low-calcium DMEM/F12 supplemented with
5% DCC-treated FBS for 24 hours. Total RNA was isolated from epithelial cells and
stromal cells in each treatment group separately. 36B4 was used as internal standard. B.
The mRNA ratios of PTPγ to 36B4 as measured by densitometry.
The results showed that the degree of suppression by both E2 and Z observed in the co-
cultured epithelial cells was greater than that seen in epithelial cells cultured alone (80-
90% vs 30% reductions) .
74
A lls s e ll e l C C lia l e th ma i ro Ep St 112233 36B4 (563 bp)
PTPγ (492 bp) B 0.6
0.3% DMSO n 30 nM Estradiol-17ß
io 0.5 30 nM Zeranol * = P<0.05 ess r 0.4 Exp
/36B4) 0.3 γ mRNA
(PTP 0.2 e * * v i t
la *
e 0.1 * R
0.0 s ll lls e e C l C al a li m the o Str Epi
Figure 3.9
75 CHAPTER 4
ESTROGENIC DOWNREGULATION OF PROTEIN TYROSINE PHOSPHATASE γ
(PTPγ) IN HUMAN BREAST IS ASSOCIATED WITH ESTROGEN RECEPTOR α
ABSTRACT
We have reported PTPγ expression was downregulated by 17β-estradiol (E2) and
Zeranol (Z), and PTPγ may function as an estrogen-regulated cancer suppressor in human breast. We utilized RT-PCR to examine expression of estrogen receptor α (ERα) and
β (ERβ) mRNA in MCF-7 and MDA-MB-231 cells and to investigate the regulation of
PTPγ expression by E2 and Z in the absence or presence of ICI 182,780 (ICI) in both cells, and immunohistochemistry to examine ERα and ERβ protein in normal and cancerous human breast. Results show that MCF-7 express both ERα and ERβ, and
MDA-MB-231 express only ERβ. Both E2 and Z (30nM; 24 h) suppressed PTPγ by
~56% in MCF-7 cells, and these effects were completely blocked by 1 µM of ICI. In contrast, E2, Z and ICI had no effects on PTPγ expression in MDA-MB-231 cells.
Interestingly, both E2 and Z suppressed PTPγ by ~45% in MDA-MB-231 cells 76 transfected with ERα, and these effects were completely blocked by 100 nM of ICI. Both
RT-PCR and immunohistochemical staining showed that ERα expression was significantly higher in cancerous human breast than in normal breast, while ERβ was higher in normal human breast than in cancerous breast. In combination with our previous findings of greater PTPγ expression levels in normal human breast than cancerous breast, current results show that there are lower ERα expression levels in normal human breast than cancerous breast, which indicate that lower PTPγ was associated with higher ERα in cancerous human breast tissues. In conclusion, our results indicate that Z induces estrogenic effects in human breast relative to PTPγ expression and the estrogenic downregulation of PTPγ expression in human breast is associated with
ERα.
INTRODUCTION
Protein tyrosine phosphatases (PTPs) play an essential role in the regulation of cell activation, proliferation and differentiation, since they counterbalance the growth- promoting effects of protein tyrosine kinases (PTKs) [Shock et al., 1995]. Therefore, alterations in PTPs activity might affect cell growth, neoplastic processes and transformation [Gaits et al., 1995]. Based on the chromosomal location of the PTPγ gene
(3p14.2) [LaForgia et al., 1991; LaForgia et al., 1993] and studies showing loss of
77 heterozygosity of the gene in kidney tumors [Lubinski et al., 1994], PTPγ has been
implicated as a candidate tumor suppressor gene.
The growth rate of a large proportion of breast cancer cells is influenced by sex
steroid hormones, and both steroid hormones and protein tyrosine phosphorylation are
demonstrated to play important roles in cell proliferation [van Biesen et al., 1995].
Zeranol (Z) (Ralgro) (described in Chapter 1) is a nonsteroidal agent with estrogenic activities that is used as a growth promoter in the U.S. beef, veal and lamb industries. Our previous results demonstrated lower PTPγ mRNA expression levels in the diethylstilbestrol (DES)-induced kidney tumors in hamsters than in normal hamster kidneys [Lin et al., 1994]. Also, we have shown that, in ACI rats, PTPγ localizes to the
mammary epithelium and Z can suppress PTPγ mRNA levels in mammary glands [Kulp
et al., 2002]. Recently, we have shown that both E2 and Z can downregulate PTPγ
expression in both human breast tissues and human breast cells [Liu et al., 2002a]. These
findings implicate that PTPγ may be considered as a potential estrogen-regulated tumor
suppressor gene in human breast cancer which may play an important role in neoplastic
processes of human breast epithelium.
The estrogen receptors (ERs) are member of a superfamily of nuclear receptors
which are ligand-regulated and sequence-specific transcription factors. Until now, two
ER subtypes have been identified: ERα and ERβ. Both of ERα and ERβ have five major
well-conserved structural domains (A/B, C, D, E and F). The C-domain (DNA binding
domain, DBD) is the most conserved region between ERα and ERβ, which suggests that
they can both bind to estrogen response elements (EREs); and conservation in regions
within the DBD required for dimmer formation suggests that the two receptors may 78 heterodimerize. Therefore, ERα and ERβ may influence or affect each other’s transcriptional activity if expressed in the same cells. The E-domain (Ligand binding domain, LBD) is also highly conserved between ERα and ERβ, indicating that they might bind similar ligands. In contrast, the A/B- and D-domains are not as conserved. It has been reported that both ERα and ERβ have high affinity to E2, and Z can interact with both subtypes with similar affinities [Kuiper et al., 1997].
In the present study, we investigated the distributions of ERα, ERβ and PTPγ expression in human breast; and we examined whether estrogenically active agent, such as Z, downregulates PTPγ expression via ERα or ERβ.
MATERIALS AND METHODS
Human breast tissues. Normal human breast tissues and cancerous human breast tissues were obtained through the Tissue Procurement Program of The Ohio State
University Hospital and Riverside Methodist Hospital in Columbus, Ohio. At the time of procurement, the tissue samples were placed in a mixture of Dulbecco's Modified Eagle's
Medium and Ham's F12 Medium (1:1) (DMEM/F12) without phenol red (Sigma
Chemical Co., St. Louis, MO) and stored at 4°C before transfer to the laboratory for process.
Tissue dissociation. Tissues were sterilized in 70% ethanol for 30 seconds, and 79 then washed three times with fresh DMEM/F12. Then, tissues were dissociated as
described previously in Chapter 3.
Cell culture. Specific cell types (epithelial cells and stromal cells) were separated
from the digested mixture and cultured as described previously in Chapter 3. To
determine the purity of the isolated human breast epithelial cells and stromal cells,
immunocytochemical staining was used for multi-cytokeratin (4/5/6/8/10/13/18) or
vimentin [Liu et al., 2002a]. Both MCF-7 cells and MDA-MB-231 cells were purchased
from American Type Culture Collection (ATCC, Manassas, VA). MDA-MB-231 cells
stably transfected with ERα (MDA-MB-231-ERα-1000) was a gift from Dr. Robert
Brueggemeier at College of Pharmacy, The Ohio State University. All of those cell lines
were cultured in phenol red-free high-calcium DMEM/F12 (1.05 mM CaCl2)
supplemented with 5% FBS and were plated separately in 75-cm2 culture flasks in a
humidified incubator (5% CO2: 95% air, 37°C). The media of all human breast cell lines and primary cultured human breast cells were changed every two days. When the cells grew to 85-90% confluence, cells were washed twice with calcium- and magnesium-free
Phosphate Buffered Saline (PBS, pH7.3), and then trypsinized with 0.5% trypsin - 5.3 mM EDTA (GibcoBRL) in PBS for 10 minutes at 37°C. The trypsinization was stopped by addition of culture medium with 5% FBS. After centrifugation, the dissociated cells were resuspended in the same medium and subcultured into 75-cm2 culture flasks at a
ratio of 1 flask to 5 flasks. The concentration of geneticin used for maintenance of MDA-
MB-231-ERα-1000 cells was only one half (500 µg/ml) of the concentration (1000
µg/ml) which was used for the selection every 2-3 passages.
Cell treatment and total RNA extraction. Each human breast cancer cell line was
80 plated in 25-cm2 culture flasks (2 × 105 viable cells/flask) and cultured overnight. The
media was changed to phenol red-free high-calcium DMEM/F12 supplemented with
Dextran-Coated Charcoal (DCC) (Dextran T-70; Pharmacia; activated charcoal; Sigma) -
stripped FBS (5%). After 24 hours, cells were treated with 30 nM of E2 or Z, different
doses of ICI 182,780 (10 nM, 100 nM, or 1 µM) in the presence or absence of 30 nM of
E2, Z or vehicle as controls in phenol-red-free high-calcium DMEM/F12 supplemented with 5% DCC-treated FBS for 24 hours. Total RNA was isolated in 2.5 ml TRIZOL
Reagent (GibcoBRL) according to manufacturer’s instructions.
Reverse transcription-polymerase chain reaction (RT-PCR). RT-PCR was performed in a gradient mastercycler (Eppendorf®, Eppendorf Scientific, Inc., Westbury,
NY). PCR conditions were optimized for MgCl2 concentration, annealing temperature
and cycle number for the amplification of each of the PCR products (ERα, ERβ, PTPγ
and 36B4). Under optimal conditions, the amounts of PCR products generated from the
experimental samples were fallen within the linear portion of the PCR amplification
curve between twenty and thirty-nine amplification cycles. Briefly, 1 µg of total RNA
from cultured normal and cancerous human breast cells or tissues was reverse transcribed
with 200 U M-MLV Reverse Transcriptase (GibcoBRL) at 42°C for 1 hour in the
presence of 5 mM each of dATP, dCTP, dGTP and dTTP, 4µl 5× 1st strand buffer
(GibcoBRL), 0.01M DDT, 1 U RNA Guard RNase inhibitor (Pharmacia Biotech,
Uppsala, Sweden), and 2.5 µM random hexamers in a total volume of 20 µl. The reaction
was terminated by heating to 95°C for 3 minutes. For ERα, the newly synthesized
cDNAs were used as templates for PCR after adjusting reagent concentrations to 1.0 mM
® MgCl2, 2.5µl 10X PCR Buffer (GibcoBRL), 1 U Platinum Taq DNA polymerase 81 (GibcoBRL), and 0.24 µM primers. The reactant was incubated at 95 °C for 5 minutes.
Then, thirty-five cycles of amplification were performed with each cycle consisting of denaturation at 95°C for 1 minute, annealing at 60°C for 45 seconds, and extension at
72°C for 1 minute. For ERβ, the newly synthesized cDNAs were used as templates for
PCR after adjusting reagent concentrations to 2.5 mM MgCl2, 2.5µl 10X PCR Buffer
(GibcoBRL), 1 U Platinum® Taq DNA polymerase (GibcoBRL), and 0.24 µM primers.
The reactant was incubated at 95 °C for 5 minutes. Then, thirty-five cycles were performed with each cycle consisted of denaturation at 95°C for 1 min, annealing at 56°C for 45 seconds, and extension at 72°C for 1 minute. For PTPγ and the housekeeping gene,
36B4, the conditions were described before [Liu et al., 2002a]. The primer sequences for
ERα were 5′-TAC TGC ATC AGA TCC AAG GG-3′ (sense) and 5′-ATC AAT GGT
GCA CTG GTT GG-3′ (antisense), for ERβ, they were 5′-TGA AAA GGA AGG TTA
GTG GGA ACC-3′ (sense) and 5′-TGG TCA GGG ACA TCA TCA TGG-3′ (antisense), and the primer sequences for PTPγ and 36B4 were listed previously [Liu et al., 2002a].
The final RT-PCR products (10 µl) were run on a 1.5 % agarose gel containing ethidium bromide. The specific bands were quantified by ImageQuant software (Molecular
Dynamics, Sunnyvale, CA). The results are presented as the ratio of targeted genes
(ERα, ERβ, or PTPγ) to 36B4 or cyclin D1 to 36B4.
Western blotting assay. MDA-MB-231 cells, MDA-MB-231-ERα-1000 cells and
MCF-7 cells were plated in separated 75-cm2 flasks (1.5×106 cells/flask) and cultured overnight before cell lysis. Cells were rinsed with cold 1× PBS and 0.5 ml of M-PERTM
Mammalian Protein Extraction Reagent (Cat. No: 78501, PIERCE, Rockford, IL) was added to each flask and flasks were kept gently shaking at 4 °C for 5 minutes. Then, the 82 cell lysate was collected into a microcentrifuge tube and samples were centrifuged at
27,000 x g, 4 °C for 10 minutes to pellet the cell debris. Finally, the supernatant was transfer to a clean tube for further analysis. The concentration of total protein was determined by using Micro BCATM Protein Assay (Cat. No: 23232, PIERCE, Rockford,
IL) according to manufacturer’s instructions. 50 µg of protein from each sample was denatured in Laemmli sample buffer (62.5 mM Tris-HCl, pH 6.8, 2% SDS, 25% glycerol,
0.01% Bromophenol Blue) (Cat. No: 161-0737, Bio-Rad Laboratories, Hercules, CA) at
99 °C for 5 minutes and loaded onto ready gel 4-15% Tris-HCl Gel for SDS-PAGE (Cat.
No: 161-1158, Bio-Rad Laboratories, Hercules, CA) and run at 100 volts for 1.5 hours in
1× Tris/Glycine/SDS buffer (Cat. No: 161-0732, Bio-Rad Laboratories, Hercules, CA).
The proteins were then transferred to a PVDF membrane in Semi-Dry Blotting Unit (Cat.
No: FB-SDB-2020, Fisher Scientific, Pittsburgh, PA) for 2.5 hours according to the manufacturer’s instructions. The membrane was detected by using ECL+Plus Western blotting detection reagents (Cat. No: RPN 2132, Amersham pharmacia biotech,
Piscataway, NJ) according to the manufacturer’s instructions. Briefly, the membrane was rinsed with PBS-T (PBS + 0.1% Tween-20) and blocked in a PBS-T solution containing
10% dried milk. After 1 hour, the membrane were rinsed with PBS-T and probed with an affinity-purified rabbit polyclonal antibody raised against a peptide mapping at the carboxy terminus of the ERα of human origin (HC-20; Cat. No. sc-543, Santa Cruz
Biotechnology, Santa Cruz, CA) at 1:500 dilution for 1 hour. Then, the membrane was rinsed with PBS-T and probed with a secondary anti-rabbit antibody linked to horseradish peroxidase (NA 934, Amersham pharmacia biotech, Piscataway, NJ) at a 1:5000 dilution for 1 hour. The membrane was rinsed with PBS-T, detected with ECL+Plus Western
83 blotting detection reagents and exposed to Hyperfilm™ MP (Amersham pharmacia biotech, Piscataway, NJ). Finally, the film was developed and scanned. To detect β-actin protein expression, the membrane was incubated in stripping buffer (100 mM 2- mercaptoethanol, 2% SDS, 62.5 mM Tris-HCl, pH 6.7) at 50 °C for 30 minutes with occasional agitation and rinsed in PBS-T twice for 10 minutes each. Then, the membrane was detected for β-actin protein expression by using ECL+Plus Western blotting detection reagents as described above. Unfortunately, since there is no commercial ERβ antibody available for western blotting, we couldn’t conduct the western blotting experiment to compare ERβ expression in these cell lines.
Immunohistochemical staining. Both normal human breast tissues and cancerous human breast tissues were fixed, dehydrated, embedded in paraffin according to protocols used for routine histopathological sample preparations and sectioned for immunohistochemical staining of ERα, ERβ, and PTPγ. As reference antibodies, we used an affinity-purified goat polyclonal antibody raised against a peptide corresponding to amino acids 1421-1438 mapping at the carboxy terminus of the PTPγ precursor of human origin (C-18; Cat. No. sc-1111, Santa Cruz Biotechnology, Santa Cruz, CA), an affinity- purified rabbit polyclonal antibody raised against a peptide mapping at the carboxy terminus of the ERα of human origin (HC-20; Cat. No. sc-543, Santa Cruz
Biotechnology, Santa Cruz, CA), and an affinity-purified rabbit polyclonal antibody raised against a peptide mapping at the amino acid terminus of the ERβ of human origin
(H-150; Cat. No. sc-8974, Santa Cruz Biotechnology, Santa Cruz, CA). The goat or rabbit ABC staining system (Cat. No. sc-2023 or sc-2018, Santa Cruz Biotechnology,
Santa Cruz, CA) was used to stain PTPγ, ERα, or ERβ in paraffin-embedded tissue 84 sections following the manufacturer’s instructions. An optimal primary antibody
concentration of 1.60 µg/ml was determined by titration in this system, and omission of primary antibody served as a negative control. Furthermore, the specificities of antibodies were confirmed by elimination of specific binding after preincubation of the antibodies with blocking peptides (sc-1111 P; sc-543 P; Santa Cruz Biotechnology, Santa Cruz,
CA).
RESULTS
Comparison of ERα and ERβ expression in MCF-7 cells and MDA-MB-231 cells.
As we know, MCF-7 cells have been claimed as an estrogen receptor (ER)-positive
human breast cancer cell line and MDA-MB-231 cells have been claimed as an ER-
negative human breast cancer cell line. About 10 years after the cloning of the ER (now it
is referred to as ERα), a novel member of ERs, termed ERβ, has been identified in cDNA
libraries from rat prostate [Kuiper et al., 1997; Katzenellenbogen et al., 1997]. To
compare mRNA expressions of ERα and ERβ in MCF-7 cells and MDA-MB-231 cells,
mRNA levels were determined by RT-PCR. The results showed that MCF-7 cells express
both ERα and ERβ; and MDA-MB-231 cells express only ERβ (Figure 4.1). Also, we
have determined ERα expression at protein level in MDA-MB-231, MDA-MB-231-
ERα-1000 and MCF-7 cells by western blotting assay, the results further support that
ERα protein is expressed in MDA-MB-231-ERα-1000 and MCF-7 cells not in MDA- 85 MB-231 cells (Figure 4.2). Unfortunately, there is no commercial ERβ antibody available for western blotting assay.
Regulation of PTPγ expression by E2, Z and ICI 182,780 (ICI) in MCF-7 cells,
MDA-MB-231 cells and MDA-MB-231-ERα-1000 cells. Estrogen diffuses through the plasma and nuclear membranes of the cells and binds to the ER in the nucleus. Once estrogen binds to the ER, heat shock proteins dissociate and a change in conformation and dimerization occurs [Macgregor et al., 1998]. ICI 182,780 is a pure antiestrogen without intrinsic estrogenic activities such as tamoxifen. Numerous reports demonstrate that pure antiestrogen-ER complexes can bind to estrogen response elements (EREs) but the transcriptional unit is inactive [Pink et al., 1996; Pham et al., 1991]. To investigate the effects of E2 and Z on PTPγ mRNA expression in these human breast cancer cell lines,
RT-PCR was used to determine PTPγ mRNA levels in cultured human breast cancer cell lines after the treatments with E2, or Z in the absence or presence of ICI. The expression levels of PTPγ mRNA in the E2- and Z-treated (30 nM, 24 h) MCF-7 cells were significantly suppressed compared to the PTPγ mRNA levels in the controls
(approximately a 60% reduction) and these effects were completely blocked by 1 µM of
ICI, however ICI alone had no effects on PTPγ mRNA expression (Figure 4.3). In contrast, E2, Z and ICI had no effects on PTPγ expression in MDA-MB-231 cells (Figure
4.4). Furthermore, PTPγ mRNA expression in the E2- and Z-treated (30 nM, 24 h) ERα- transfected MDA-MB-231 cells (MDA-MB-231-ERα-1000) were significantly suppressed in comparison to the PTPγ mRNA levels observed in the controls
(approximately an 50% reduction) and these effects were completely blocked by 100 nM
86 of ICI, however ICI alone had no blocking effects on PTPγ mRNA expression (Figure
4.5). These results suggested that downregulation of PTPγ expression by E2 and Z was intimately related with ERα status in human breast.
Comparison of ERα, ERβ and PTPγ expression levels in normal and cancerous human breast. Previous works showed that both ERα and ERβ expressed in many estrogen-targeted tissues [Kuiper et al., 1997]. To determine the relative distribution of
ERα and ERβ mRNA in normal and cancerous human breast tissues and cells, total RNA was isolated from both normal and cancerous human breast tissues and cells and RT-PCR was performed with specific primers for each ER subtype. Our results showed that
ERα mRNA expression was significantly higher in cancerous human breast tissues than in normal human breast tissues (Figure 4.6), while ERβ and PTPγ [Liu et al., 2002a] mRNA expression was higher in normal human breast tissues than in cancerous human breast tissues (Figure 4.7). In immunohistochemical staining study, the results showed that ERα protein level significantly higher in cancerous human breast tissues than in normal human breast tissues, while ERβ and PTPγ protein levels were higher in normal human breast tissues than in cancerous human breast tissues (Figure 4.8). These results indicated that lower PTPγ expression was associated with higher ERα expression in human breast cancerous tissues.
87 DISCUSSIONS
Previous reports indicated that ERα and ERβ share about 95% homology in the
DNA binding domain and 55% homology in the ligand binding domain, both bind to a consensus ERE [Tremblay et al., 1997] and exhibit similar ligand binding properties
[Kuiper et al., 1997]. Although the relative expression of ERα and ERβ varies in different cell types, their ligand binding, DNA binding, and transactivation properties are rather similar to one another. As we know, MDA-MB-231 cells have been traditionally classified as an ER-negative breast cancer cell lines. But, in our study, we found that
MCF-7 cells expressed both ERα and ERβ, and MDA-MB-231 cells expressed only
ERβ. Based on our results, the MDA-MB-231 cells we employed in the experiment should be recognized as an ERβ-positive breast cancer cell line.
In our present study, we investigated the effects of E2 and Z on PTPγ expression in both ERα (+) human breast cell lines and ERα (-) human breast cell lines, the results showed that both E2 and Z could downregulate PTPγ expression in only ERα (+) cell lines but not in ERα (-) cell lines and the estrogenic effects were blocked by ICI 182,780, which indicated that ERα played an important role in the regulation of PTPγ expression by estrogenically active agents in human breast and Z downregulated PTPγ expression in human breast through the similar pathway as E2 (ER pathway). Estrogens induce gene transcription both by binding to the classic estrogen response element(s) and by signaling through an AP-1 enhancer element requiring the products of c-fos and c-jun [Gaub et al.,
88 1990]. ERα and ERβ may have similar effects on gene transcription mediated via the
EREs but opposite effects on promoters containing AP-1 [Paech et al., 1997]. Estradiol
activates AP-1-mediated gene transcription when bound to ERα but inhibits promoter
activity when bound to ERβ [Paech et al., 1997]. We have briefly analyzed the promoter
region of human PTPγ gene, but we couldn’t find any consensus ERE in that region.
Therefore, we hypothesize that the regulation of PTPγ expression by E2 and Z might not
be through the EREs, which need to be determined by further experiments. Report
showed that ERα and ERβ expressed both in vitro and in vivo, formed heterodimers,
which bind to DNA with an affinity similar to that ERα and greater than that of ERβ
homodimers [Cowley et al., 1997]. Furthermore, the heterodimers, like the homodimers,
are capable of binding the steroid receptor co-activator-1 when bound to DNA and
stimulating transcription of a reporter gene in transfected cells [Cowley et al., 1997].
Based on our investigations, we are still not sure whether ERα alone or a combination of
ERα and ERβ are essential in the downregulation of PTPγ by estrogenically active agents
in human breast. For the future studies, we need to construct ERα- or ERβ- or both
transfected human breast cancer cell lines without any endogenous ER and use these
transfected human breast cancer cell lines to determine whether E2 and Z downregulate
PTPγ expression through ERα homodimers or ERα-ERβ heterodimers.
We have demonstrated that ERα expression level was significantly higher in
cancerous human breast than in normal human breast, while ERβ and PTPγ mRNA
expression was significantly higher in normal human breast than in cancerous human
breast. Our previous report indicated that PTPγ is a potential estrogen-regulated tumor
89 suppressor gene in human breast cancer that may play an important role in neoplastic
processes of human breast epithelium. Altogether, our results shed a light on the tight
relationship among estrogen, human breast cancer and PTPγ. But, the left question is
whether PTPγ can really inhibit human breast tumor formation. Some experiments about
the functional analysis of PTPγ in human breast need to be done in the future.
It is worth noting that the non-steroidal agent with estrogenic activities, Z, has
been employed in the current study. Z has been used as a growth promoter in the U.S.
beef industry to improve feed efficiency, weight gains and carcass quality. The U.S. Food
and Drug Administration (FDA) has a restricted legal limit at 150 parts per billion (ppb)
for the Z residues contained in edible beef for human consumption. At the present time,
there is no proved scientific evidence to suggest that the consumption of beef from the Z-
implanted cattle cause any risk of breast cancer. Our in vitro experimental data clearly
demonstrated that the natural estrogen (E2) and Z are capable of down regulating the
estrogen-regulated cancer suppressor gene, PTPγ in both human normal and cancerous
breast tissues as well as the cultured cells isolated from these tissues. Therefore, our
concern is whether the long-term exposure of extremely low levels of Z residues from the
consumption of beef from Z-implanted cattle has any health risk to the consumers. The
extrapolation of our in vitro findings to the linkage of potential risk in in vivo is not
appropriate. More studies in this important topic are warranted. In combination with the
results from other laboratory which showed that ERß could be a potent proliferation gatekeeper as well as an inhibitor of cell motility and invasion and the decreased
expression of ERß observed between normal and cancerous breast could be one of the
events leading to an uncontrolled proliferation of the cells, our results suggested that ERβ
90 might possess tumor suppressor activity in human breast, which might serve as a new concept in breast cancer research.
91
Figure 4.1. Comparison of estrogen receptors (α and β) mRNA expression in MCF-7 cells and MDA-MB-231 cells as determined by RT-PCR.
MCF-7 cells and MDA-MB-231 cells were cultured in 25-cm2 flasks (3×105 cells/flask)
without treatment. Total RNA was isolated from each cell line and RT-PCR was
performed.
A. Ethidium bromide-stained PCR products separated in a 1.5% agarose gel.
(M: Marker)
B. The mRNA ratios of ERα or ERβ to 36B4 as measured by densitometry.
The results showed that MCF-7 cells express both ERα and ERβ mRNA; and MDA-MB-
231 cells express only ERβ mRNA.
92
A. M
ERα (650 bp)
ERβ (528 bp)
36B4 (563 bp)
B. 0.6
0.5 ER α ER β on 0.4 6B4) pressi
x /3 β NA e ER 0.3
R or α ve m ti 0.2 (ER a Rel 0.1
0.0 7 F- -231 MC
MDA-MB
Figure 4.1
93
ERα (66 KDa)
β-actin (43 KDa)
1 2 3
Figure 4.2. Comparison of estrogen receptor α protein expression in MDA-MB-231 cells, MDA-MB-231-ERα-1000 cells and MCF-7 cells as determined by western blotting assay.
MDA-MB-231 cells, MDA-MB-231-ERα-1000 cells and MCF-7 cells were cultured in
75-cm2 flasks (1.5×106 cells/flask) without treatment. Total protein was isolated from
each cell line and western blotting assay was performed.
1: MDA-MB-231 cells; 2: MDA-MB-231-ERα-1000 cells; 3: MCF-7 cells
The result showed that ERα protein is expressed in MDA-MB-231-ERα-1000 and MCF-
7 cells not in MDA-MB-231 cells.
94
Figure 4.3. Effects of E2, Z and ICI 182,780 on PTPγ mRNA expression in MCF-7 cells as determined by RT-PCR.
MCF-7 cells were cultured in 25-cm2 flasks (3×105 cells/flask) and treated with 30 nM of
E2 or Z in the absence or presence of 10 nM, 100 nM or 1 µM of ICI 182,780, or ICI
182,780 alone for 24 hours. Total RNA was isolated from each treatment group and RT-
PCR was performed.
A. Ethidium bromide-stained PCR products separated in a 1.5% agarose gel.
(M: Marker)
B. The mRNA ratios of PTPγ to 36B4 as measured by densitometry.
These results showed that the expression levels of PTPγ mRNA in the E2- and Z-treated
MCF-7 cells were significantly suppressed compared to the PTPγ mRNA levels in the
controls (approximately a 60% reduction) and these effects were completely blocked by 1
µM of ICI, however ICI alone had no effects on PTPγ mRNA expression.
95
A. M PTPγ (492 bp)
36B4 (563 bp)
B.
0.40
0.35
0.30 on i s es r 0.25 4) exp 36B / NA
γ 0.20 P T
(P 0.15 ve mR i t a
Rel 0.10
0.05
0.00 l I I I I I Z tro CI ICI IC n M IC IC IC IC ICI n M I M M ICI Co nM E230 nM nM µ nM nM µΜ 30 0 0 1 0 µΜ 0 0 nM 1 1 10 10 n 0 + + 1 Z + 2 + 10 + 1 + 1 2 Z 2 E Z M M E M E nM 0 n n 0 nM 30 n 3 0 nM 3 0 30 3 3
Figure 4.3
96
Figure 4.4. Effects of E2, Z and ICI 182,780 on PTPγ mRNA expression in MDA-
MB-231 cells as determined by RT-PCR.
MDA-MB-231 cells were cultured in 25-cm2 flasks (3×105 cells/flask) and treated with
30 nM of E2 or Z in the absence or presence of 10 nM, 100 nM or 1 µM of ICI 182,780,
or ICI 182,780 alone for 24 hours. Total RNA was isolated from each treatment group
and RT-PCR was performed.
A. Ethidium bromide-stained PCR products separated in a 1.5% agarose gel.
(M: Marker)
B. The mRNA ratios of PTPγ to 36B4 as measured by densitometry.
The result showed that, in contrast to figure 4.3, E2, Z and ICI had no effects on PTPγ
expression in MDA-MB-231 cells.
97
A. M PTPγ (492 bp)
36B4 (563 bp)
B.
1.00 on 0.75 pressi 4) x e 6B /3 NA γ P R 0.50 (PT ve m i
Relat 0.25
0.00 l I I I 2 Z I ro ICI ICI C ICI ICI nt M ICI IC IC IC M E n M M M Co 0 n 30 nM µ nM nM I nM nM µΜ 3 0 1 µΜ 0 0 10 n 10 00 1 1 10 1 1 + + + 2 + Z + 10 Z + E 2 2 Z M M E n M M E nM nM n 0 30 n 0 3 30 n 30 3 30
Figure 4.4
98
Figure 4.5. Effects of E2, Z and ICI 182,780 on PTPγ mRNA expression in MDA-
MB-231-ERα-1000 cells as determined by RT-PCR.
MDA-MB-231-ERα-1000 cells were cultured in 25-cm2 flasks (3×105 cells/flask) and
treated with 30 nM of E2 or Z in the absence or presence of 10 nM, 100 nM or 1 µM of
ICI 182,780, or ICI 182,780 alone for 24 hours. Total RNA was isolated from each
treatment group and RT-PCR was performed.
A. Ethidium bromide-stained PCR products separated in a 1.5% agarose gel.
(M: Marker)
B. The mRNA ratios of PTPγ to 36B4 as measured by densitometry.
The result showed that PTPγ mRNA expression in the E2- and Z-treated (30 nM, 24 h)
ERα-transfected MDA-MB-231 cells (MDA-MB-231-ERα-1000) were significantly
suppressed in comparison to the PTPγ mRNA levels observed in the controls
(approximately an 50% reduction) and these effects were completely blocked by 100 nM
of ICI, however ICI alone had no blocking effects on PTPγ mRNA expression
99
A.
M PTPγ(492 bp)
36B4 (563 bp)
B.
1.00 on i s 0.75 es r 4) exp 36B / NA γ P 0.50 T (P ve mR i t a
Rel 0.25
0.00 l I I I I I Z tro CI ICI IC n M IC IC IC IC ICI n M I M M ICI Co nM E230 nM nM µ nM nM µΜ 30 0 0 1 0 µΜ 0 0 nM 1 1 10 10 n 0 + + 1 Z + 2 + 10 + 1 + 1 2 Z 2 E Z M M E M E nM 0 n n 0 nM 30 n 3 0 nM 3 0 30 3 3
Figure 4.5
100
0.6
Normal breast 0.5 Cancerous breast n
io 0.4 ress ) 4
exp
6B A
/3 0.3 N α R m (ER e 0.2
Relativ 0.1
0.0 s ll sues l Ce l Cells is a a T li he rom St Epit
Figure 4.6. Comparison of ERα mRNA expression in human breast as determined by RT-PCR.
Total RNA was isolated from both normal and cancerous human breast tissues, epithelial cells and stromal cells (Normal human breast tissues and cells are from the same patients;
Cancerous human breast tissues and cells are from the same patients) and RT-PCR was performed. The mRNA ratios of ERα to 36B4 was measured by densitometry.
Our results showed that ERα mRNA expression was significantly higher in cancerous human breast tissues and cells than in normal human breast tissues and cells.
101
0.4 Normal breast Cancerous breast n o i 0.3 ss e r p 4) x
e
/36B 0.2 NA β R (E ve mR i t a l 0.1 Re
0.0 s lls lls e C l Tissue lial Ce he roma St Epit
Figure 4.7. Comparison of ERβ mRNA expression in human breast as determined by RT-PCR.
Total RNA was isolated from both normal and cancerous human breast tissues, epithelial cells and stromal cells (Normal human breast tissues and cells are from the same patients;
Cancerous human breast tissues and cells are from the same patients) and RT-PCR was performed. The mRNA ratios of ERβ to 36B4 was measured by densitometry.
Our results showed that ERβ mRNA expression was significantly lower in cancerous human breast tissues and cells than in normal human breast tissues and cells.
102
Figure 4.8. Comparison of immunoreactivities of ERα, ERβ and PTPγ in normal and cancerous human breast tissues as determined by immunohistochemical staining.
Brown staining represents ERα, ERβ, or PTPγ immunoreactivities; Blue staining
represents nuclei. A. Normal human breast tissue stained for ERα; B. Cancerous human
breast tissue stained for ERα; C. Normal human breast tissue stained for ERβ; D.
Cancerous human breast tissue stained for ERβ; E. Normal human breast tissue stained
for PTPγ; F. Cancerous human breast tissue stained for PTPγ.
These results showed that ERα protein level significantly higher in cancerous human
breast tissues than in normal human breast tissues, while ERβ and PTPγ protein levels
were higher in normal human breast tissues than in cancerous human breast tissues.
103 AB
10 µm 10 µm
C D
10 µm 10 µm
EF
10 µm 10 µm
Figure 4.8
104 CHAPTER 5
FUNCTIONAL ANALYSIS OF ESTROGENICALLY REGULATED PROTEIN
TYROSINE PHOSPHATASE γ (PTPγ) IN HUMAN BREAST
CANCER CELL LINE MCF-7
ABSTRACT
Protein tyrosine phosphatase γ (PTPγ) is a member of the receptor-like family of
tyrosine-specific phosphatases and has been implicated as a tumor suppressor gene in
kidney and lung cancers. We have reported that PTPγ expression in normal human breast
was higher than that in cancerous human breast, and PTPγ expression was downregulated
through estrogen receptor α (ERα) homodimers or ERα and ERβ heterodimers by 17β-
estradiol (E2) and Zeranol (Z). Based on these findings, we hypothesize that PTPγ is a potential estrogen-regulated tumor suppressor gene in human breast cancer that may involve in neoplastic processes of human breast epithelium. The present study is to examine the action of PTPγ on in vitro growth of MCF-7 cells and compare the estrogenic responses of human breast cells with different expression levels of PTPγ. By
105 transfecting plasmid DNA with or without different cDNA inserts of PTPγ in pCR®3.1 vector into MCF-7 cells and selecting with G-418 (Geneticin), we established several stably transfected MCF-7 cell lines expressing different levels of PTPγ. By RT-PCR, we
found that full-length PTPγ-transfected MCF-7 cells [M7-PTPγ-800 (s)] had much higher
PTPγ mRNA expression than both wild type and mock-transfected MCF-7 cells [M7-
pCR®3.1-800 (s)], whereas, antisense PTPγ-transfected MCF-7 cells [M7-A24-800 (s)] had much lower PTPγ mRNA expression than wild type, M7-pCR®3.1-800 (s) and sense
PTPγ-transfected MCF-7 cells [M7-B19-800 (s)]. In doubling time assay, we demonstrated that the doubling time for M7-PTPγ-800 (s) was increased about 60%. In contrast, the doubling time for M7-A24-800 (s) was decreased about 34%. These results suggested that PTPγ is capable of inhibiting MCF-7 breast cancer cell growth and plays an important role in growth suppression of breast cancer. Our results further demonstrated that PTPγ overexpression led to a decrease in the anchorage-independent growth of MCF-7 cells in soft agar, and the reduction of PTPγ expression in MCF-7 cells resulted in an increase in the colony formation in soft agar. Furthermore, the results in the cell proliferation assay showed that PTPγ overexpression could reduce the estrogenic responses of MCF-7 cell proliferation to E2 and Z. Our data suggest that PTPγ is able to
inhibit proliferation and anchorage-independent growth of breast cancer cells in vitro and
has anti-estrogenic activities in human breast cancer cells, and PTPγ may function as an
important modulator in regulating the process of tumorigenesis in human breast. Studies
in progress are focused on examining the effect of PTPγ on the in vivo growth of human
breast cancer.
106
INTRODUCTION
Protein tyrosine phosphatases (PTPases) are a family of proteins which perform the enzymatic role to remove phosphate groups from phosphotyrosine residues of specific targets inside cells. PTPases regulate important cellular processes like gene expression, cell activation and proliferation, differentiation, development, transport, and locomotion, since they counterbalance the growth-promoting effects of protein tyrosine kinases
(PTKs), which catalyze the phosphorylation of tyrosine residues [Shock et al., 1995].
Therefore, alterations in PTPs activity might affect cell growth, neoplastic processes and transformation [Gaits et al., 1995]. Addition of the PTPase-inhibitor vanadate to cells in culture leads to increased amounts of phosphotyrosine-containing proteins and cellular transformation [Klarlund, 1985]. Therefore, a delicate balance between PTK and PTPase action is essential for normal functioning of cells.
PTPγ is a member of the receptor-like family of tyrosine-specific phosphatases as described in Chapter 3. The structure of the receptor-like PTPs (RPTPs) includes an extracellular, a transmembrane, and one or two tandemly repeated catalytic domains. This structure implies ligand-binding capability which may modulate enzymatic activity.
However, the putative ligands for most of the PTPs with receptor-like structures are not yet to be identified. Receptor-like PTPγ has a carbonic anhydrase-homologous amino terminus followed by a fibronectin type three domain, a cysteine-free domain, a 107 transmembrane domain and two intracellular PTPase catalytic domains [Barnea et al.,
1993; Levy et al., 1993; Krueger et al., 1992]. According to mRNA analysis [Barnea et al., 1993; LaForgia et al., 1991], PTPγ is a broadly expressed enzyme that exists in many tissues, including human lung, stomach, esophagus, colon, liver, spleen, and kidney
[Tsukamoto et al., 1992]. Based on the chromosomal location of the PTPγ gene (3p14.2)
[LaForgia et al., 1991; LaForgia et al., 1993] and studies showing loss of heterozygosity
(LOH) of the gene in kidney tumors [Lubinski et al., 1994], PTPγ has been implicated as a candidate tumor suppressor gene. More recently, PTPγ expression levels were shown to be reduced in ovarian and lung tumors [van Niekerk et al., 1999]. Our previous results showed lower PTPγ mRNA expression levels in diethylstilbestrol-induced kidney tumors in hamsters than in normal hamster kidney [Lin et al., 1994]. Also, we have shown that, in ACI rats, PTPγ is localized to the mammary epithelium and that zeranol (Z), a nonsteroidal agent with estrogenic activities that is used as a growth promoter in the U.S. beef, veal and lamb industries, can suppress PTPγ mRNA levels in mammary glands
[Kulp et al., 2000]. Furthermore, we have reported that PTPγ is expressed in normal and malignant human breast epithelium, and PTPγ mRNA levels can be suppressed by estrogens through an estrogen receptor-mediated mechanism [Zheng et al., 2000]. More recently, our results showed that PTPγ mRNA expression was lower in cancerous than in normal breast tissues; both 17β-estradiol (E2) and Z suppressed PTPγ mRNA levels in both cultured normal breast tissues and cultured breast cells isolated from normal breast tissues. In whole breast tissues, PTPγ was immunolocalized to the epithelium and the treatment with E2 or Z diminished PTPγ staining, which indicated reductions in PTPγ at
108 the protein level [Liu et al., 2002a]. These findings suggest that PTPγ is a potential estrogen-regulated tumor suppressor gene in human breast cancer that may play an important role in neoplastic processes of human breast epithelium, and indicate an intriguing relationship among cancer, estrogen and PTPγ.
The aim of the present study was to examine the effect of PTPγ on the growth of human breast cancer and compare the estrogenic responses of human breast cells with different expression levels of PTPγ. MCF-7 breast cancer cells were used as a model and we stably transfected a plasmid containing the whole PTPγ coding sequence or containing a small portion of PTPγ coding sequence in the antisense orientation into human MCF-7 breast carcinoma cells. PTPγ-overexpression MCF-7 cells normally show the characteristics with slow-growing and lower colony efficiency in soft agar, and show anti-estrogenic effects on human breast cell proliferation.
MATERIALS AND METHODS
Cell culture. The human breast cancer cell line MCF-7 cells were purchased from
American Type Culture Collection (ATCC, Manassas, VA). The cells were maintained in a mixture of Dulbecco's Modified Eagle's Medium and Ham's F12 Medium (1:1)
(DMEM/F12) without phenol red (Sigma Chemical Co., St. Louis, MO) supplemented with 5% Fetal Bovine Serum (FBS) (Atlanta Biologicals, Norcross, GA) and antibiotic- antimycotic (100 unit/ml penicillin G sodium, 100 mg/ml streptomycin sulfate and 0.25 109 mg/ml amphotericin B) (GibcoBRL, Bethesda, MD) and were plated separately in 75- cm2 culture flasks in a humidified incubator (5% CO2: 95% air, 37°C). The media were changed every two days. When the cells grew to about 85% confluence, cells were washed twice with calcium- and magnesium-free Phosphate Buffered Saline (PBS, pH7.3), and then trypsinized with 1 ml of 0.5% trypsin - 5.3 mM EDTA (GibcoBRL) in
PBS for 10 minutes at 37°C. The trypsinization was stopped by addition of 10 ml of culture medium with 5% FBS. After centrifugation, the viability of the dissociated cells were determined by using a hemacytometer, which showed that the viability was over
99%. The dissociated cells were resuspended in the same medium and subcultured into
75-cm2 culture flasks at a ratio of 1 flask to 5 flasks.
Transfection (Figure 5.1). The expression vector pCR®3.1 (Invitrogen, Carlsbad,
CA; Figure 5.2), containing a human full-length PTPγ cDNA (5.3 kb; Figure 5.3) in an
EcoRI site or a partial PTPγ cDNA fragment (756 bp amplified using the following primer set: 5’-CGTCACCAGTCTCCTATTGA-3’ and 5’-
GTCTGTCATGTCGTGGTTCC-3’; Figure 5.3) cloned in pCR®3.1 vector. Clones containing the construct in the sense or antisense orientation were selected and used for transfection [Sorio et al., 1995; Sorio et al., 1997]. As a control, we used an empty pCR®3.1 vector (mock). Plasmids (10 µg) were transfected into MCF-7 cells by using
Superfect (Qiagen, Valencia, CA) according to the manufacturer’s instructions. Forty- eight hours after transfection, the cells were transferred to selection medium containing different doses (0 µg/ml, 100 µg/ml, 200 µg/ml, 400 µg/ml, 600 µg/ml, 800 µg/ml, 1000
µg/ml) of G-418 (Geneticin) (Invitrogen, Carlsbad, CA) for 6 days to determine the optimal dose of G-418 for the selection. Finally, surviving single colonies from four to 110 five dishes with selection medium containing 800 µg/ml of G-418 were picked from each
stably transfected group for future culture and cells were treated with 400 µg/ml of G-418
in the culture medium every 2-3 passages to maintain the homogeneity of the cells. The
whole procedure for the establishment of cell lines is shown in Figure 5.1.
RNA isolation and Reverse transcription-polymerase chain reaction (RT-PCR).
1.5×105 viable cells from each single colony were plated in one well of 6-well plates with
5 ml of culture medium and cultured to ~ 85% confluence. Total RNA was isolated in 1 ml of TRIZOL Reagent (GibcoBRL) according to manufacturer’s instructions. RT-PCR
was performed in a gradient mastercycler (Eppendorf ®). PCR conditions were optimized
for MgCl2 concentration, annealing temperature and cycle number for the amplification
of each of the PCR products. Under optimal conditions, the amounts of PCR products
generated fell within the linear portion of the PCR amplification curve between twenty-
six and thirty-nine amplification cycles. Briefly, 1 µg of total RNA from cultured cells or
tissues was reverse transcribed with 200 U M-MLV Reverse Transcriptase (GibcoBRL)
at 42°C for 1 hour in the presence of 5 mM each of dATP, dCTP, dGTP and dTTP, 4 µl
5x 1st strand buffer (GibcoBRL), 0.01M DDT, 1 U RNA Guard RNase inhibitor
(Pharmacia Biotech, Uppsala, Sweden), and 2.5 mM random hexamers in a total volume
of 20 µl. The reaction was terminated by heating to 95°C for 3 minutes. The newly
synthesized cDNAs were used as templates for PCR after adjusting reagent
concentrations to 3.5 mM MgCl2, 2.5 µl 10× PCR Buffer (GibcoBRL), 1 U Platinum®
Taq DNA polymerase (GibcoBRL), and 0.24 µM primers. The reactant was incubated at
95 °C for 5 minutes. Then, thirty cycles of amplification were performed with each cycle
consisting of denaturation at 95°C for 1 minute, annealing at 63°C for 1 minute, and 111 extension at 72°C for 1 minute. The primer sequences generating 990 bp PCR products
for full-length PTPγ construct were 5′ _GTA TGG AGC AGT TTC AGC_ 3′ (sense @
PTPγ exon 29) and 5′ _TAG AAG GCA CAG TCG AGG_ 3′ (antisense @ pCR®3.1 reverse priming site). The primer sequences generating 583 bp PCR products for antisense PTPγ construct were 5′ _TAA TAC GAC TCA CTA TAG GG_ 3′ (sense @ T7
promoter priming site in pCR®3.1 vector) and 5′ _GAA CAC AGC ATC AAT GGC
AGG AGG_ 3′ (antisense @ PTPγ exon 4). The primer sequences generating 337 bp
PCR products for sense PTPγ construct were 5′ _ TAA TAC GAC TCA CTA TAG GG_
3′ (sense @ T7 promoter priming site in pCR®3.1 vector) and 5′ _CCT CCT GCC ATT
GAT GCT GTG TTC_ 3′ (antisense @ PTPγ exon 4). The primer sequences generating
492 bp PCR product for PTPγ were 5′ _GCG CAG CGA CTT TAG CCA GAC GA_ 3′
(sense @ PTPγ exon 8) and 5′ _GCT CCC GCT CCC CAT CCT CAC TC_ 3′ (antisense
@ PTPγ exon 11). The primer sequences generating 563 bp PCR products for 36B4 were
5′ _AAA CTG CTG CCT CAT ATC CG_ 3′ (sense @ 306 to 325) and 5′ _TTG ATG
ATA GAA TGG GGT ACT GAT G_ 3′ (antisense @ 868 to 848). The final PCR
products (10 µl) mixed with 1 µl of 10 × loading buffer were separated on a 1.5% agarose
gel containing ethidium bromide. The specific bands were quantified by ImageQuaNT
software (Molecular Dynamics, Sunnyvale, CA). The results are presented as the ratio of
each PCR product to 36B4.
Immunohistochemical staining. Wild type MCF-7 cells, mock-transfected MCF-7
cells (M7-pCR®3.1-800 (s)), PTPγ-overexpression MCF-7 cells (M7-PTPγ-800 (s)),
antisense PTPγ-transfected MCF-7 cells (M7-A24-800 (s)) and sense PTPγ-transfected
112 MCF-7 cells (M7-B19-800 (s)) were cultured on 24 × 30 mm cell culture cover slips
(Cat. No: 150067, Nalge Nunc Int., Naperville, IL) and washed in PBS for 3 times. After
–10 °C methanol fixation for 5 minutes and air-dry, cells were stained for PTPγ by using
VECTASTAIN Universal Quick Kit and DAB Substrate Kit (Cat. No: PK-8800, Cat.
No: SK-4100, Vector Laboratories, Inc., Burlingame, CA) according to the manufacturer’s instructions. The primary antibody was an affinity-purified rabbit polyclonal antibody raised against a self-designed in-house synthetic peptide (N–
SEDGEREHEEDGEKD –C) corresponding to amino acids 588-602 mapping in the extracellular domain of the human origin PTPγ and was produced by Sigma® Genosys
(The Woodlands, TX). An optimal primary antibody dilution of 1:100 was determined by titration in this system, and omission of primary antibody served as a negative control.
Doubling time assay. Wild type MCF-7 cells, M7-pCR®3.1-800 (s), M7-PTPγ-
800 (s), M7-A24-800 (s) and M7-B19-800 (s) were plated separately at a density of
0.5×104 cells / well in 24-well plates in a volume of 1 ml of fresh DMEM/F12 with 5%
FBS/well. After cells are attached to the wells, the medium was replaced with 2 ml of fresh DMEM/F12 with 5% FBS. At the same time (time 0 hour), a group of cells were counted. Cells were grown for 3 days and counted every 8 hours. Adherent cells were detached by rapid trypsinization. An adequate volume of medium containing trypan blue was added. Then cells were counted by using a hemacytometer. Experiments were performed in four replicate wells, and each experiment was repeated twice. Based on the counted cell numbers at different time points, a cell proliferation curve was generated.
Cell doubling (CD) was calculated by using the formula ln (Nj-Ni)/ln 2 where Nj or Ni is the cell numbers at different time point Tj or Ti (Tj > Ti) in the growth log phase of the 113 cells. Doubling time (DT) was consequently obtained by dividing the time interval (Tj >
Ti) by CD [Poliseno et al., 2002].
Soft agar assay for colony formation. Cells were cultured in 6-well plates first
covered with an agar layer (1 ml of phenol red-free DMEM/F12 with 0.5% agar and 10%
FBS. The middle layer contained different number of cells (2000, 4000, 8000 or 16000
cells) in 1 ml of phenol red-free DMEM/F12 with 0.35% agar and 10% FBS. The top
layer, consisting of 1 ml of culture medium, was added to prevent drying of the agar in
the plates. The plates were incubated for 21 days and another 1 ml of fresh culture
medium was added to the top layer at day 11. After 21 days’ incubation, the plates were
stained in 0.5 ml of 0.005% crystal violet for >1 hour and the cultures were inspected and
photographed. Colony efficiency (CE) was determined by a count of the number of
colonies greater than 150 µm in diameter, which was calculated as the average of
colonies counted at 40× magnification in five individual fields by using BioQuant NOVA
software.
Non-radioactive cell proliferation assay. 2×104 cells [Wild type MCF-7 cells,
M7-pCR®3.1-800 (s), M7-PTPγ-800 (s), M7-A24-800 (s) or M7-B19-800 (s)] were
seeded and cultured in each well of 24-well plates in phenol red-free high-calcium
DMEM/F12 supplemented with 5% FBS overnight. The medium was switch to phenol
red-free high-calcium DMEM/F12 supplemented with Dextran-coated charcoal (DCC)
(Dextran T-70, Pharmacia; activated charcoal, Sigma)-stripped FBS (5%). After 24
hours, cells were treated with the E2 or Z at 0, 7.5, 15, 30, 60 or 120 nM in the same fresh medium for 24 hours. Cell proliferation rate was quantified by using CellTiter 96TM
AQueous assay (Promega, Madison, WI). Briefly, at the end of treatment, the medium in
114 the wells were discarded and 250 µl of fresh medium with 50 µl of freshly combined
MTS (3-(4,5-dimethylthiazol-2-yl)-5-(3- carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-
tetrazolium, inner salt)/PMS (phenazine methosulfate) (the ratio of MTS:PMS is 20:1))
solution was added to each well. Then, the plates were incubated for 1.5 hours and the
color density was checked every 30 minutes. Finally, 100 µl of culture medium from
each well were transferred to one well of 96-well plates and optical density was read at
490 nm (O.D.490nm) by an ELISA plate reader.
Statistical analysis. The results for doubling time assay and non-radioactive cell
proliferation assay were presented as the mean ± standard deviation (SD) for 4 replicate
culture wells as one group. The results for colony efficiency assay were presented as the
mean ± standard deviation (SD) for 5 individual fields as one well. Analysis was
performed using Minitab statistical software for Windows (Minitab Inc., State College,
PA, USA). Statistical differences were determined by using one-way ANOVA for
independent groups. P-values of less than 0.05 were considered statistically significant.
RESULTS
Comparison of PTPγ expression in stably transfected MCF-7 cells. MCF-7 cells
were transfected with pCR®3.1 vector, full-length PTPγ, PTPγ antisense construct or
PTPγ sense construct. Based on the dose-response curve of each transfected cell line to
G-418, it was determined that 800 µg/ml was the optimal dose of G-418 for the single 115 colony selection in every transfected cell line. We have selected 5 single clones from
mock-transfected MCF-7 cells, 19 single clones from PTPγ-transfected MCF-7 cells, 17
single clones from antisense PTPγ-transfected MCF-7 cells and 11 single clones from
sense PTPγ-transfected MCF-7 cells. To identify the successfully and ideally transfected
single colonies, RT-PCR with a set of specific primers was used for each group of
transfection (Figure 5.4). The results showed that 13 out of 19 clones from M7-PTPγ-800
(s) expressed exogenous PTPγ mRNA, and all of the single clones from M7-pCR®3.1-
800 (s), M7-A24-800 (s) and M7-B19-800 (s) were successful. Furthermore, PTPγ
mRNA expression was compared among those successfully transfected cell lines. Our
results showed that M7-PTPγ-800 (s) had higher PTPγ mRNA expression than any of
wild type MCF-7, M7-pCR®3.1-800 (s), M7-A24-800 (s) and M7-B19-800 (s) cell lines.
M7-A24-800 (s) had lower PTPγ mRNA expression than any of wild type MCF-7, M7-
pCR®3.1-800 (s), M7-PTPγ-800 (s) and M7-B19-800 (s) cell lines (Figure 5.5). Also, the
results from immunohistochemical staining showed that the PTPγ staining were weak and
very similar among wild type MCF-7 cell (Figure 5.6A), M7-pCR®3.1-800 (s) (Figure
5.6B) and M7-B19-800 (s) (Figure 5.6C). Furthermore, the degree of staining was
observably increased in M7-PTPγ-800 (s) (Figure 5.6D) when compared to others;
however, the degree of staining was almost diminished in M7-A24-800 (s) (Figure 5.6E)
when compared to others. Thus, these results indicate that PTPγ is overexpressed in M7-
PTPγ-800 (s) and is greatly reduced in M7-A24-800 (s) compared to other cells. These
results are consistent with those presented in Figure 5.5 above.
Effects of PTPγ on the proliferation of MCF-7 cells. To check whether PTPγ level
116 has an anti-mitogenic effect on MCF-7 cells, a proliferation study was performed by
using doubling time assay. Wild type MCF-7 cells, M7-pCR®3.1-800 (s) and M7-B19-
800 (s) had the mean cell population doubling time of 24.0 ± 2 h, 24.3 ± 3 h and 23.8 ± 1
h, respectively, but it was increased to 40.0 ± 3 h for M7-PTPγ-800 (s) (~ 60% increase)
and it was decreased to 16.4 ± 1 h for M7-A24-800 (s) (~ 34% decrease) (Figure 5.7).
These results suggested that PTPγ can inhibit MCF-7 breast cancer cell growth and plays
an important role in tumor suppression of breast cancer.
Effects of PTPγ on anchorage-independent growth of MCF-7 cells. The
acquisition of anchorage-independent growth is generally considered to be one of the in
vitro properties associated with the malignancy of cells. The transfected cells were
examined for their ability to grow in soft agar. First, we checked the optimal number of
cells for the colony formation in 6-well plates seeded with different number of wild type
MCF-7 cells, M7-pCR®3.1-800 (s) or M7-PTPγ-800 (s). The results showed that the colony number in soft agar was increased while the seeded cell number was increased for each cell type, M7-PTPγ-800 (s) formed much less and smaller colonies in soft agar than wild type MCF-7 cells and M7- pCR®3.1-800 (s) in the same seeded cell number group
(Figure 5.8). Based on these results, we determined that 8000 cells/well was the optimal
MCF-7 cell number for the future soft agar assay in our experiment. Furthermore, we
found that M7-PTPγ-800 (s) formed less and smaller colonies in soft agar than wild type
MCF-7 cells and M7- pCR®3.1-800 (s), but M7-A24-800 (s) formed more and much
larger colonies in soft agar than wild type MCF-7 cells, M7- pCR®3.1-800 (s) and M7-
B19-800 (s) (Figure 5.9). These in vitro data indicated that PTPγ is able to inhibit proliferation and anchorage-independent growth of breast cancer cells. 117 Effects of PTPγ on estrogenic activities of E2 and Z on MCF-7 cell proliferation.
As we have known, E2 is a potent mitogen for human breast cancers. Z is a non-steroidal
anabolic growth promoter with estrogenic activities. Our previous results showed that
both Z and E2 could induce normal human breast epithelial cell transformation and
stimulate human breast cell proliferation in a dose-dependent manner, which suggested
that PTPγ is a potential estrogen-regulated tumor suppressor gene in human breast cancer
that may play an important role in neoplastic processes of human breast epithelium,
which indicates an intriguing relationship among cancer, estrogen and PTPγ. The
established stably transfected cell lines with different PTPγ expression levels were used
to examine whether PTPγ has any anti-estrogenic activities on MCF-7 cell proliferation.
Our results showed that both Z and E2 could stimulate the cell proliferation of wild type
MCF-7, M7-pCR®3.1-800 (s), M7-PTPγ-800 (s), M7-B19-800 (s) and M7-A24-800 (s) in a dose-dependent manner, and the potency of Z and E2 are very similar on the stimulation
of cell proliferation (Figure 5.10). Most interestingly, the degree of Z’s or E2’s
stimulation on M7-PTPγ-800 (s) cell proliferation (Figure 5.10D) is much less than that
on the cell proliferation of wild type MCF-7, M7-pCR®3.1-800 (s) or M7-B19-800 (s) cell proliferation (Figure 5.10A, 5.10B, 5.10C); however, the degree of Z’s or E2’s
stimulation on M7-A24-800 (s) cell proliferation (Figure 5.10E) is much greater than that
on the cell proliferation of wild type MCF-7, M7-pCR®3.1-800 (s) or M7-B19-800 (s) cell proliferation (Figure 5.10A, 5.10B, 5.10C). These results indicated that PTPγ overexpression could reduce the estrogenic responses of MCF-7 cell proliferation to E2 and Z and has anti-estrogenic activities on human breast cancer cells.
118 DISCUSSIONS
Transmembrane receptor tyrosine phosphatases represent a growing family of enzymes, the structural features of which suggest a role in the control of cellular phosphotyrosine balance. But the biological significance of PTPγ is only poorly understood. PTPγ has been considered as a candidate tumor suppressor gene due to the location of its gene on human chromosome 3p14.2, in a region found to be frequently deleted in certain types of renal and lung carcinomas [Barnea et al., 1993; LaForgia et al.,
1991; LaForgia et al., 1993]. Our previous results suggested that PTPγ is a potential estrogen-regulated tumor suppressor gene in human breast cancer that may play an important role in breast tumorigenesis [Lin et al., 1994; Kulp et al., 2000; Zheng et al.,
2000; Liu et al., 2002]. But, the function of PTPγ in human breast cancer is still not clear.
To examine the effect of PTPγ on the growth properties of human breast cancer cells in vitro and provide a biochemical background for understanding PTPγ function, we transfected PTPγ-expressing vector with a variant full-length PTPγ cDNA lacking the intracellular juxtamembrane exon of the reported PTPγ cDNA, which is expressed as the dominant isoform in most tissues [Sorio et al., 1995], or PTPγ antisense construct into
MCF-7 cells. It is clear from the data shown in Figure 5.7 that the relationship of link for the PTPγ and longer doubling time as well as less colony formation is intimately related.
When the breast cancer cells have higher levels of PTPγ mRNA or its protein, the proliferative ability of breast cancer cells will be greatly suppressed or blocked.
119 The results from doubling time assay and soft agar assay showed that MCF-7
cells overexpressing PTPγ has the characteristics with slow-growing and lower colony
efficiency in soft agar. Furthermore, the results from the cell proliferation assay showed
that MCF-7 cells overexpressing PTPγ can antagonize estrogenic effects of E2 and Z on
MCF-7 cell proliferation, which indicates PTPγ has anti-estrogenic effects on human breast cancer.
In combination with our previous results which showed that PTPγ mRNA
expression was significantly lower in cancerous than in normal breast tissues, and both E2 and Z suppressed PTPγ mRNA levels in cultured normal breast tissues and cultured normal breast cells isolated from the tissues, all these experimental data support our suggested concept that PTPγ is a potential estrogen-regulated breast cancer suppressor
gene. This molecular biomarker will be a powerful tool for investigating the estrogen
and/or nonsteroidal estrogenic agents in controlling the etiological process of
tumorigenesis in human breast and rodent mammary. It was claimed that the reduction of
PTPγ expression in human breast cancer is unlikely to be due to genetic events; therefore,
epigenetic mechanisms (such as methylation) might be responsible. Several genes have
been demonstrated to be hypermethylated in breast carcinomas, including HIN-1, p16, E-
cadherin, BRCA1, estrogen receptor, GSTP1 (glutathione S-transferase P1), MDGI
(mammary-derived growth inhibitor), HoxA5 and 14-3-3s (Krop et al., 2001). To identify
the mechanisms of PTPγ reduction in human breast cancer and PTPγ downregulation by
E2 and Z, human PTPγ promoter region analysis and investigation of hypermethylation in
PTPγ need to be conducted in the future.
120 Our data are the first evidence to demonstrate that the PTPγ does exert its suppressor capability to inhibit or slow down the proliferation of MCF-7 cells and does have the anti-estrogenic activities on human breast cancers exposed to E2 or Z. Although the in vivo function of PTPγ is unknown, its reduced expression in breast carcinomas and decreased colony formation following its overexpression suggest a tumor suppressor role.
If similar results can be reproduced in athymic mouse testing model, then the presence of this potential cancer suppressor gene and its protein will significantly prolong the survivability of human breast cancer patients by inhibiting or slowing down the proliferative ability of human breast cancer cells. The signaling pathway with PTPγ involvement may provide a new target for cancer prevention and treatment. In the future,
PTPγ might be able to be applied in clinical trials for cancer prevention and cancer therapy.
121
Figure 5.1. The procedures for the establishment of stably transfected cell lines.
The details were described in Materials and Methods. M7-pCR®3.1-800-a, b, … (s) are
single colonies selected with 800 µg/ml G418 from MCF-7 cells stably transfected pCR®3.1 vectors without inserts. M7-PTPγ-800-a, b, … (s) are single colonies selected
with 800 µg/ml G418 from MCF-7 cells stably transfected with full-length PTPγ cDNA
constructs (a pCR®3.1 vector carrying full-length PTPγ cDNA in the sense direction).
M7-A24-800-a, b … (s) are single colonies selected with 800 µg/ml G418 from MCF-7 cells stably transfected with PTPγ antisense constructs (a pCR®3.1 vector carrying a 756
bp PTPγ cDNA in the antisense direction). M7-B19-800-a, b … m (s) are single colonies
selected with 800 µg/ml G-418 from MCF-7 cells stably transfected with PTPγ sense
constructs (a pCR®3.1 vector carrying a 756 bp PTPγ cDNA in the sense direction).
122
EcoR I PTPγ cDNAs M7-pCR®3.1-800-a,b, … (s) pCR®3.1 M7-PTPγ-800-a,b, … (s) M7-A24-800-a,b, … (s) M7-B19-800-a,b, … (s) Cloning
Single clone selection
Transformation 100 200 400 600 800 1000 ….…...….... …...…...…. . ….…...….... …...…...…. . ….…..….... . ….…...….... …..…...…. . Preparation of plasmid DNA Geneticin (G 418) Transfection selection (ug/ml) ….…..….... . MCF-7 cells
Figure 5.1
123
PTPγ cDNA*
Figure 5.2. Diagram of pCR®3.1 vector from invitrogen and the inserting location of
PTPγ cDNA in the vector (Adapted from Invitrogen).
124
Figure 5.3. The sequences for the inserted PTPγ cDNAs.
The whole sequence represents human protein tyrosine phosphatase γ (PTPγ) mRNA
(www.ncbi.nih.gov; Accession number: L09247). The letters in blue color represent the partial PTPγ cDNA inserted to pCR®3.1 vector in sense or antisense direction; EcoRIsite:
EcoRI restriction enzyme site was ligated to the two ends of the inserted PTPγ cDNAs;
ATG: translation start codon; TGA: translation stop codon
125 EcoRIsite+AGGCTCGCACGGAGGCAAGAACTTATTCAACAAGTTTACCTCCCTGCTTTCCTCTTTTCGATGTGCGTTTTCG GACATGCGGAGGTTACTGGAACCGTGTTGGTGGATTTTGTTCCTGAAAATCACCAGTTCCGTGCTCCATTATGTCGTGTG CTTCCCCGCGTTGACAGAAGGCTACGTTGGGGCCCTGCACGAGAATAGACACGGCAGCGCAGTGCAGATCCGCAGGCG CAAGGCTTCAGGCGACCCGTACTGGGCCTACTCTGGTGCCTATGGTCCTGAGCACTGGGTCACGTCTAGTGTCAGCTGT GGGAGCCGTCACCAGTCTCCTATTGACATTTTAGACCAGTATGCGCGTGTTGGGGAAGAATACCAGGAACTGCAACTCG ATGGCTTCGACAATGAGTCTTCTAACAAAACCTGGATGAAAAACACAGGGAAAACAGTCGCCATCCTTCTGAAAGACG ACTATTTTGTCAGTGGAGCTGGTCTACCTGGCAGATTCAAAGCTGAGAAGGTGGAATTTCACTGGGGCCACAGCAATGG CTCAGCGGGCTCTGAACACAGCATCAATGGCAGGAGGTTTCCTGTTGAGATGCAGATTTTCTTTTACAATCCAGATGAC TTTGACAGCTTTCAAACCGCAATTTCTGAGAACAGAATAATCGGAGCCATGGCCATATTTTTTCAAGTCAGTCCGAGGG ACAATTCTGCACTGGATCCTATTATCCACGGGTTGAAGGGTGTCGTACATCATGAGAAGGAGACCTTTCTGGATCCTTT CGTCCTCCGGGACCTCCTGCCTGCATCCCTGGGCAGCTATTATCGGTACACAGGTTCCTTGACCACACCACCGTGTAGC GAAATAGTGGAGTGGATAGTCTTCCGGAGACCCGTCCCCATCTCTTACCATCAGCTTGAGGCTTTTTATTCCATCTTCAC CACGGAGCAGCAAGACCATGTCAAGTCGGTGGAGTATCTGAGAAATAACTTTCGACCACAGCAGCGTCTGCATGACAG GGTGGTGTCCAAGTCCGCCGTCCGTGACTCCTGGAACCACGACATGACAGACTTCTTAGAAAACCCACTGGGGACAGA AGCCTCTAAAGTTTGCAGCTCTCCACCCATCCACATGAAGGTGCAGCCTCTGAACCAGACGGCACTGCAGGTGTCCTGG AGCCAGCCGGAGACTATCTACCACCCACCCATCATGAACTACATGATCTCCTACAGCTGGACCAAGAATGAGGACGAG AAGGAGAAGACGTTTACAAAGGACAGCGACAAAGACTTGAAAGCCACCATTAGCCATGTCTCACCCGATAGCCTTTAC CTGTTCCGAGTCCAGGCCGTGTGTCGGAACGACATGCGCAGCGACTTTAGCCAGACGATGCTGTTTCAAGCTAATACCA CTCGAATATTCCAAGGGACCAGAATAGTGAAAACAGGAGTGCCCACAGCGTCTCCTGCCTCTTCAGCCGACATGGCCC CCATCAGCTCGGGGTCTTCTACCTGGACGTCCTCTGGCATCCCATTCTCATTTGTTTCCATGGCAACTGGGATGGGCCCC TCCTCCAGTGGCAGCCAGGCCACAGTGGCCTCGGTGGTCACCAGCACGCTGCTCGCCGGCCTGGGGTTCGGCGGTGGT GGCATCTCCTCTTTCCCCAGCACTGTGTGGCCCACGCGCCTCCCGACGGCCGCCTCAGCCAGCAAGCAGGCGGCTAGGC CAGTCCTAGCCACCACAGAGGCCTTGGCTTCTCCAGGGCCCGATGGTGATTCGTCACCAACCAAGGACGGCGAGGGCA CCGAGGAAGGAGAGAAGGATGAGAAAAGCGAGAGTGAGGATGGGGAGCGGGAGCACGAGGAGGATGGAGAGAAGG ACTCCGAAAAGAAGGAGAAGAGTGGGGTGACCCACGCTGCCGAGGAGCGGAATCAGACGGAGCCCAGCCCCACACCC TCGTCTCCTAACAGGACTGCCGAGGGAGGGCATCAGACTATACCTGGGCATGAGCAGGATCACACTGCCGTCCCCACA GACCAGACGGGCGGAAGGAGGGATGCCGGCCCAGGCCTGGACCCCGACATGGTCACCTCCACCCAAGTGCCCCCCACC GCCACAGAGGAGCAGTATGCAGGGAGTGATCCCAAGAGGCCCGAAATGCCATCTAAAAAGCCTATGTCCCGCGGGGA CCGATTTTCTGAAGACAGCAGATTTATCACTGTTAATCCAGCGGAAAAAAACACCTCTGGAATGATAAGCCGCCCTGCT CCAGGGAGGATGGAGTGGATCATCCCTCTGATTGTGGTATCAGCCTTGACCTTCGTGTGCCTCATCCTTCTCATTGCTGT GCTCGTTTACTGGAGAGGGTGTAACAAAATAAAGTCCAAGGGCTTTCCCAGACGTTTCCGTGAAGTGCCTTCTTCTGGG GAGAGAGGAGAGAAGGGGAGCAGAAAATGTTTTCAGACTGCTCATTTCTATGTGGAAGACAGCAGTTCACCTCGAGTG GTCCCTAATGAAAGTATTCCTATTATTCCTATTCCGGATGACATGGAAGCCATTCCTGTCAAACAGTTTGTCAAACACAT CGGTGAGCTCTATTCTAATAACCAGCATGGGTTCTCTGAGGATTTTGAGGAAGTCCAGCGCTGTACTGCTGATATGAAC ATCACTGCAGAGCATTCCAATCATCCAGAAAACAAGCACAAAAACAGATACATCAACATTTTAGCATATGATCACAGT AGGGTGAAGTTAAGACCTTTACCAGGAAAAGACTCTAAGCACAGCGACTACATTAATGCAAACTATGTTGATGGTTAC AACAAAGCAAAAGCCTACATTGCCACCCAAGGACCTTTGAAGTCTACATTTGAAGATTTCTGGAGGATGATTTGGGAA CAAAACACTGGAATCATTGTGATGATTACGAACCTTGTGGAAAAAGGAAGACGAAAATGTGATCAGTATTGGCCAACA GAGAACAGTGAGGAATATGGAAACATTATTGTCACGCTGAAGAGCACAAAAATACATGCCTGCTACACTGTTCGTCGT TTTTCAATCAGAAATACAAAAGTGAAAAAGGGTCAGAAGGGAAATCCCAAGGGTCGTCAGAATGAAAGGGTAGTGAT CCAGTATCACTATACACAGTGGCCTGACATGGGAGTTCCCGAGTATGCCCTTCCAGTACTGACTTTCGTGAGGAGATCC TCAGCAGCTCGGATGCCAGAAACGGGCCCTGTGTTGGTGCACTGCAGTGCTGGTGTGGGCAGAACAGGCACCTATATT GTAATAGACAGCATGCTGCAACAGATAAAAGACAAAAGCACAGTTAACGTCCTGGGATTCCTGAAGCATATCAGGACA CAGCGTAACTACCTCGTCCAGACTGAGGAGCAGTACATTTTCATCCATGATGCCTTGTTGGAAGCCATTCTTGGAAAGG AGACTGAAGTATCTTCAAATCAGCTGCACAGCTATGTTAACAGCATCCTTATACCAGGAGTAGGAGGAAAGACACGAC TGGAAAAGCAATTCAAGCTGGTCACACAGTGTAATGCAAAATATGTGGAATGTTTCAGTGCTCAGAAAGAGTGTAACA AAGAAAAGAACAGAAACTCTTCAGTTGTGCCATCTGAGCGTGCTCGAGTGGGTCTTGCACCATTGCCTGGAATGAAAG GAACAGATTACATTAATGCTTCTTATATCATGGGCTATTATAGGAGCAATGAATTTATTATAACTCAGCATCCTCTGCCA CATACTACGAAAGATTTCTGGCGAATGATTTGGGATCATAACGCACAGATCATTGTCATGCTGCCAGACAACCAGAGCT TGGCAGAAGATGAGTTTGTGTACTGGCCAAGTCGAGAAGAATCCATGAACTGTGAGGCCTTTACCGTCACCCTTATCAG CAAAGACAGACTGTGCCTCTCTAATGAAGAACAAATTATCATCCATGACTTTATCCTTGAAGCTACACAGGATGACTAT GTCTTAGAAGTTCGGCACTTTCAGTGTCCCAAATGGCCTAACCCAGATGCCCCCATAAGTAGTACCTTTGAACTTATCA ACGTCATCAAGGAAGAGGCCTTAACAAGGGATGGTCCCACCATTGTTCATGATGAGTATGGAGCAGTTTCAGCAGGAA TGTTATGTGCCCTTACCACCCTGTCCCAGCAACTGGAGAATGAAAATGCTGTGGATGTTTTCCAGGTTGCAAAAATGAT CAATCTTATGAGGCCTGGAGTATTCACAGACATTGAACAATACCAGTTCATCTATAAAGCAAGGCTTAGCTTGGTCAGC ACTAAAGAAAATGGAAATGGTCCCATGACAGTAGACAAAAATGGTGCTGTTCTTATTGCAGATGAATCAGACCCTGCT GAGAGCATGGAGTCCCTAGTGTGACTGGAATCCTGAAAGGGCACTTAATTTGTAAACTTCTGAAGACTGAGAACTTTTT TGAGGCCTTTTTTGCCAGACTCTAGGTTATACAATAACCCAGTTACTTTTTTACACTGATAAAAGTTTTGATATTTATTTT TTGCCATTTTATGTCTTAATGGTATCCTACTGAGCATTTGCACCTCTGTTCATTTCACACAGTGAAACGCAATTTTACCTA GTTTGCACTATATGATCAGTGTTACTGCCTATAATCTTATACAACAGCAAACCCTGATGTGACATTCCATGAC+EcoRISite Figure 5.3
126
Figure 5.4. Identification and confirmation of successfully transfected cell lines by
RT-PCR.
1.5×105 viable cells from wild type MCF-7 cells and each single colony were plated in one well of 6-well plates with 5 ml of culture medium separately and cultured to ~ 85% confluence, total RNA was isolated for each group, and RT-PCR was performed.
Ethidium bromide-stained PCR products separated in a 1.5% agarose gel. 36B4 was used as internal standard. Primer 305 is located in Bovine Growth Hormone (BGH) reverse priming site in pCR®3.1 vector; Primer 333 is located in the sense strand of full-length
PTPγ cDNA insert; Primer 83 is located in T7 promoter/priming site in pCR®3.1 vector;
Primer 360 is located in the antisense strand of the partial PTPγ cDNA insert; Primer 334
(complementary to primer 360) is located in the sense strand of the partial PTPγ cDNA insert. W.T.: wild type MCF-7 cells.
127
Primer pair 333 & 305 (990 bp)
36B4 (563 bp)
M W
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.
T r abcdeafbc de ghijklmnopqrs
k
.
e r M7-pCR3.1-800 (s) M7-PTPγ-800 (s)
Primer pair: 83&360 (583 bp) Primer pair: 83&334 (337 bp)
36B4 (563 bp)
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128
Figure 5.5. The expression of PTPγ mRNA in transfected MCF-7 cells.
1.5×105 viable cells from wild type MCF-7 cells, one of single colonies from mock transfected MCF-7 cells, and three of single colonies from M7-PTPγ-800 (s), M7-B19-
800 (s) or M7-A24-800 (s) were plated in one well of 6-well plates with 5 ml of culture
medium separately and cultured to ~ 85% confluence, total RNA was isolated for each
group, and RT-PCR was performed. Top: Ethidium bromide-stained PCR products
separated in a 1.5% agarose gel. Cells were not treated and total RNA was isolated from
each cell type separately. 36B4 was used as internal standard. Bottom: The mRNA ratios
of PTPγ to 36B4 as measured by densitometry.
Our results showed that M7-PTPγ-800 (s) had higher PTPγ mRNA expression than any
of wild type MCF-7, M7-pCR®3.1-800 (s), M7-A24-800 (s) and M7-B19-800 (s) cell
lines. M7-A24-800 (s) had lower PTPγ mRNA expression than any of wild type MCF-7,
M7-pCR®3.1-800 (s), M7-PTPγ-800 (s) and M7-B19-800 (s) cell lines.
129
M 650 bp 500 bp PTPγ (492 bp) 400 bp
650 bp 36B4 (563 bp) 500 bp
0.8
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Figure 5.5
130
Figure 5.6. Immunohistochemical staining for PTPγ in transfected MCF-7 cells.
Cells were cultured on 24 × 30 mm cell culture cover slips overnight and washed in PBS for 3 times. After –10 °C methanol fixation for 5 minutes and air-dry, cells were stained for PTPγ by using VECTASTAIN Universal Quick Kit and DAB Substrate Kit. Brown staining represents PTPγ immunoreactivity; Blue staining represents nuclei. A. Wild type
MCF-7 cells; B. M7-pCR®3.1-800 (s); C. M7-B19-800 (s); D. M7-PTPγ-800 (s); E. M7-
A24-800 (s); F. Negative control.
These results showed that the PTPγ staining were weak and very similar among wild type
MCF-7 cell, M7-pCR®3.1-800 (s) and M7-B19-800 (s). Furthermore, the degree of staining was observably increased in M7-PTPγ-800 (s) when compared to others; however, the degree of staining was almost diminished in M7-A24-800 (s) when compared to others.
131
A B C
D E F
Figure 5.6
132
Figure 5.7. Effects of PTPγ on the proliferation of MCF-7 cells.
5000 cells / well were seeded in 24-well plates and cells were grown for 3 days and counted every 8 hours. Experiments were performed in four replicate wells, and each experiment was repeated twice. Cell doubling (CD) was calculated by using the formula ln (Nj-Ni)/ln 2 where Nj or Ni is the cell numbers at different time point Tj or Ti (Tj > Ti) in the growth log phase of the cells. Doubling time (DT) was consequently obtained by dividing the time interval (Tj > Ti) by CD.
The results were presented as the mean ± standard deviation (SD) for 4 replicate culture wells as one group. Analysis was performed using Minitab statistical software for
Windows (Minitab Inc., State College, PA, USA). Statistical differences were determined by using one-way ANOVA for independent groups. P-values of less than 0.05 were considered statistically significant.
These results showed that M7-PTPγ-800 (s) has the longest doubling time, whereas M7-
A24-800 (s) has the shortest doubling time, which suggested that PTPγ can inhibit MCF-
7 breast cancer cell growth and plays an important role in tumor suppression of breast cancer.
133
Wild type MCF-7 cells: T1/2=24 hours
M7-pCR3.1-800 (s): T1/2=24.3 hours M7-PTP -800 (s) T =40 hours γ : 1/2 M7-B19-800 (s): T1/2=23.8 hours M7-A24-800 (s): T =16.4 hours 1/2 2.75 Wild type MCF-7 cells 2.50 M7-pCR3.1-800 (s) M7-PTPγ-800 (s) 2.25 M7-B19-800 (s) M7-A24-800 (s) )
4 2.00 0 1
x 1.75 ( r e b 1.50
1.25 ll num e C 1.00 0.75
0.50
0 8 16 24 32 40 48 56 64 72 Hours
Figure 5.7
134
Figure 5.8. Optimization of cell numbers for anchorage-independent growth of
MCF-7 cells.
A. Colony formation of different MCF-7 cell numbers in soft agar. Cells were cultured in
6-well plates first covered with an agar layer (phenol red-free high-calcium DMEM/F12
with 0.5% agar and 10% FBS). The middle layer contained different number of cells
(2000, 4000, 8000 or 16000 cells) in phenol red-free DMEM/F12 with 0.35% agar and
10% FBS. The top layer, consisting of medium, was added to prevent drying of the agar
in the plates. The plates were incubated for 21 days and another 1 ml of fresh culture
medium was added to the top layer at day 11. After 21 days’ incubation, the plates were
stained in 0.5 ml of 0.005% crystal violet for >1 hour and the cultures were inspected and
photographed. B. Comparison of colony efficiency (CE) among different MCF-7 cell
numbers. CE was determined by a count of the number of colonies greater than 15 µm in
diameter, which was calculated as the average of colonies counted at 50× magnification in five individual fields by using BioQuant NOVA software. Bar, 500 µm.
The results showed that the colony number in soft agar was increased while the seeded cell number was increased for each cell type, M7-PTPγ-800 (s) formed much less and smaller colonies in soft agar than wild type MCF-7 cells and M7- pCR®3.1-800 (s) in the
same seeded cell number group.
135
A.
Wild type MCF-7
M7-pCR®3.1-800 (s)
M7-PTPγ-800 (s)
B. 70 )
2 Wild type MCF-7 cells
m 60 M7-pCR3.1-800 (s) M7-PTPγ-800 (s) 50 * P<0.05 N=4
40 * (colonies/3.3 m 30
fficiency 20 * E * 10 * Colony 0 2000 4000 8000 16000 Cell number
Figure 5.8
136
Figure 5.9. Effects of PTPγ on anchorage-independent growth of MCF-7 cells.
A. Colony formation of different MCF-7 cell types in soft agar. The procedure is the
same as described in Figure 5.8 except for the seeded cell number (8000 cells/well). B.
Comparison of colony efficiency (CE) among different MCF-7 cell types. CE is
determined as described in Figure 5.8. Bar, 500 µm.
The results were presented as the mean ± standard deviation (SD) for 5 individual fields
as one well. Analysis was performed using Minitab statistical software for Windows
(Minitab Inc., State College, PA, USA). Statistical differences were determined by using
one-way ANOVA for independent groups. P-values of less than 0.05 were considered
statistically significant.
Our results showed that that M7-PTPγ-800 (s) formed less and smaller colonies in soft
agar than wild type MCF-7 cells and M7- pCR®3.1-800 (s), but M7-A24-800 (s) formed more and much larger colonies in soft agar than wild type MCF-7 cells, M7- pCR®3.1-
800 (s) and M7-B19-800 (s).
137
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Figure 5.9
138 Figure 5.10. Effects of PTPγ on E2’s and Z’s activities on MCF-7 cell proliferation.
4 2×10 cells were cultured in each well of 24-well plates and treated with the E2 or Z at 0,
7.5, 15, 30, 60 or 120 nM in phenol red-free high-calcium DMEM/F12 supplemented with DCC-stripped FBS (5%) for 24 hours. Cell proliferation rate was quantified by using
TM CellTiter 96 AQueous assay and optical density was read at 490nm (O.D.490nm) in 96-
well plates by an ELISA plate reader. Each point presented the mean ± standard deviation
(SD) of 4 replicate culture wells. * represents the significant difference from the control
group. A probability (P) of less than 0.05 was considered statistically significant
(P<0.05). A. Wild type MCF-7 cells; B. M7-pCR3.1-800 (s); C. M7-B19-800 (s); D.
M7-PTPγ-800 (s); E. M7-A24-800 (s)
The results were presented as the mean ± standard deviation (SD) for 4 replicate culture
wells as one group. Analysis was performed using Minitab statistical software for
Windows (Minitab Inc., State College, PA, USA). Statistical differences were determined
by using one-way ANOVA for independent groups. P-values of less than 0.05 were
considered statistically significant.
Our results showed that both Z and E2 could stimulate the cell proliferation of all tested cell types in a dose-dependent manner, and the potency of Z and E2 are very similar.
Most interestingly, the degree of Z’s or E2’s stimulation on M7-PTPγ-800 (s) cell
proliferation is much less than that on the cell proliferation of other cell types; however, the degree of Z’s or E2’s stimulation on M7-A24-800 (s) cell proliferation is much greater
than that on the cell proliferation of other cell types.
139
A.
3.00
2.75 E2 Z 2.50 N = 4 (well) 2.25 * P < 0.05 2.00 * ll 1.75
/ we 1.50 . * D. D . 1.25 * O 3.00 1.00 2.75 E2 * Z 0.75 2.50 N = 4 (well) 0.50 2.25 * P < 0.05 0.25 2.00 l 0.00 l 1.75 0.0 7.5 15.0 30.0 60.0 120.0 e
Concentrations (nM) / w 1.50 D. 1.25 O.
B. 1.00
3.00 0.75 * 2.75 E2 0.50 * * Z 2.50 0.25 N = 4 (well) 2.25 * P < 0.05 0.00 0.0 7.5 15.0 30.0 60.0 120.0 2.00 Concentrations (nM)
ll 1.75 * e *
/ w 1.50 .
D E. 1.25 O. * 3.00 1.00 * * 2.75 E2 0.75 Z * 2.50 0.50 N = 4 (well) 2.25 * P < 0.05 0.25 * 2.00 0.00
0.0 7.5 15.0 30.0 60.0 120.0 ll 1.75 e Concentrations (nM)
/ w 1.50 .
D * 1.25 O.
C. 1.00 3.00 0.75 2.75 E 2 0.50 Z 2.50 0.25 N = 4 (well) 2.25 * P < 0.05 0.00 2.00 0.0 7.5 15.0 30.0 60.0 120.0 Concentrations (nM) ll 1.75 e * *
/ w 1.50 . D 1.25 * O.
1.00 * 0.75
0.50
0.25
0.00 0.0 7.5 15.0 30.0 60.0 120.0 Concentrations (nM)
Figure 5.10
140 CHAPTER 6
CONCLUSIONS
PERSPECTIVES
Human health concerns: estrogenic activities of Zeranol (Z) in human breast
Epidemiological evidence of human breast cancer indicates that white American women have a 5-fold greater risk for human breast cancer than Asian women in China and Japan. Furthermore, the risk of acquiring breast cancer among Asian women immigrants in the U.S. approaches that of American women after 1 to 2 generations
(Kelsey and Berkowitz, 1988). These observations indicate a strong environmental, rather than genetic, component in the etiology of this disease (Buell, 1973). It has been speculated that dietary factors may contribute to this ethnic difference in human breast cancer incidence (Committee on Diet, Nutrition and Cancer, 1982). A recent study associated an increase in the risk for breast cancer with elevated red meat consumption, while fat, protein, dairy product, poultry or fish consumption was modestly or not at all associated with the incidence of the disease (Toniolo et al., 1994). In light of these
141 findings and the decreasing support for the association of dietary fat with breast cancer
(Toniolo et al., 1994; Lund, 1994), perhaps another component(s) within red meat may be involved in the postulated role of diet in breast cancer etiology. It is our concern that this component may be the estrogenic anabolic growth promotant used widely by the meat- producing industry.
As we have known, Zeranol (Z) (Ralgro), which is marketed as Ralgro
(Schering-Plough Corp, Kenilworth, NJ 07033), is synthesized from the mycotoxin
Zearalenone (ZL, a nonsteroidal, resorcyclic acid lactone compound produced by fungi of the genus Fusarium) and is a nonsteroidal agent with estrogenic activities that is used as a growth promoter in the U.S. beef, veal and lamb industries to accelerate weight gain, improve feed conversion efficiency and increase the lean meat-to-fat ratio. In recent years, the occurrence of compounds possessing estrogenic activities in the environment and in food products, either as natural constituents or as contaminants, has received increasing attention because of speculation concerning their ability to adversely affect human and animal endocrinology and their possible etiological role in estrogen-related carcinogenesis.
During the preparation of this dissertation, we have demonstrated that Z can induce human breast epithelial cell neoplastic transformation and has the same effects on some estrogen-targeted genes with the similar potency as E2 in our tested dose range. One possible explanation is that, since most exogenous hormone-like chemicals, including Z and the other synthetic growth promoting hormones, exhibit limited or no binding to the carrier proteins, the potential potency of Z is much larger than its actual concentrations suggested (up to 50 times) [Mastri et al., 1985; Shrimanker et al., 1985; Nagel et al.,
142 1998]. So, based on our current findings which provides a putative link between Z and
risk of human breast cancer, our concern is that the consumption of beef products,
derived from Z-implanted beef cattle may pass a potential health impact on human
consumers, particularly with respect to reproductive endocrinology and hormone-
sensitive organs, such as breasts.
PTPγ acts as an estrogen-targeted gene in human breast
Previous results, particularly our preliminary and published work as described in
this dissertation, indicate an intriguing relationship among cancer, estrogen and PTPγ.
We speculate that, in the human breast, PTPγ is an estrogen-regulated gene that possesses cancer suppressor activity, and estrogen-induced suppression of
PTPγ expression plays a role in breast tumorigenesis. Clearly, defining the mechanism(s)
of estrogenic regulation of PTPγ expression and elucidating its role in breast cancer
induction and progression is important. Such research could reveal novel mechanisms of
hormonal carcinogenesis, regulation of cancer cell growth and endocrine disruption by
environmental estrogens. As described in chapter 4 in this dissertation, we found that
downregulation of PTPγ expression by both E2 and Z is associated with ERα, which indicates this downregulation is through estrogen receptor-mediated pathway. However, there have been no reports to date of the presence of a functional estrogen response element (ERE) in the PTPγ gene, and this promotes us to hypothesize that there must be
some regulatory sequences responsible for estrogenic effects of E2 and Z on PTPγ
expression, which are not caused by the direct interaction of ligand-ER complex with an
ERE in the promoter region of the PTPγ gene in human breast. So, in the future, we need 143 to search for such regulatory sequences. Sequence analysis of the promoter of the human
PTPγ gene and evaluation of transcriptional activation through a series of truncated
PTPγ promoter-reporter constructs will allow us to identify such important regulatory domains within the 5’-flanking sequence of the PTPγ gene.
PTPγ inhibits cell proliferation and suppresses anchorage-independent growth of
human breast cancer cells
Previous results showed that alterations in PTPs activity might affect cell growth,
neoplastic processes and transformation [Gaits et al., 1995]. PTPγ has been implicated as
a tumor suppressor gene in kidney and lung cancers [Lubinski et al., 1994; van Niekerk et
al., 1999]. As described in chapter 5 in this dissertation, we examined the effect of PTPγ on the growth of human breast cancer and compare the estrogenic responses of human breast cells with different expression levels of PTPγ. We found that PTPγ overexpressing
MCF-7 cells normally show the characteristics with slow-growing and lower colony
efficiency on soft agar, and show anti-estrogenic effects on human breast cell
proliferation. In combination with our previous results, all these experimental data
support our suggested concept that PTPγ is a potential estrogen-regulated breast cancer
suppressor gene in vitro. In the future, in vivo experiments need to be done to further
support our suggested concept.
This molecular biomarker will be a powerful tool for investigating the estrogen
and/or nonsteroidal estrogenic agents in controlling the etiological process of
tumorigenesis in human breast and rodent mammary. It was claimed that the reduction of
144 PTPγ expression in human breast cancer is unlikely to be due to genetic events; therefore, epigenetic mechanisms (such as methylation) might be responsible.
FUTURE WORKS AND CLINICAL IMPLICATIONS
Mechanism(s) of estrogenic regulation of PTPγ expression in human breast –
Characterization and analysis of the human PTPγ promoter region
The work in this dissertation has shown that PTPγ can be downregulated by
estrogenically active agents E2 and Z, which is associated with ERα. These results suggested that this regulation is through estrogen receptor-mediated pathway. However, we couldn’t find any EREs in PTPγ promoter region, and it is not likely that E2 and Z
downregulated PTPγ expression by direct interaction of ligand-ER complex and EREs in
the human PTPγ gene. Future goals will focus on further characterization and analysis of
the human PTPγ promoter region by scanning of the promoter region with different
constructs of PTPγ:luciferase plasmid and look for the important elements in the
promoter region responsible for the estrogenic regulation of PTPγ expression. This work
will allow us to identify the possible molecules and/pathways involved in the estrogenic
regulation of PTPγ by E2 and Z in human breast and define the mechanisms for this
regulation.
145 Investigations of the anti-tumorigenicity of PTPγ in nude mice
During this dissertation, in combination with our previous results, we have
showed that there is an intriguing relationship among cancer, estrogen and PTPγ. All
these experimental data suggested that PTPγ is a potential estrogen-regulated breast
cancer suppressor gene, at least in vitro. If similar results can be reproduced in nude
mouse testing model and we can show that PTPγ overexpression can inhibit mammary
gland tumor growth in nude mice, then, taken together, these results indicate that PTPγ
overexpression can suppress the development of human breast cancer both in vitro and in
vivo, and we can further support our concept that PTPγ is a potential estrogen-regulated
breast cancer suppressor gene and the presence of this potential cancer suppressor gene.
Clinical relevance of PTPγ in human breast cancer patients
Based on these results, we conclude that there will be some clinical relevance for
PTPγ. PTPγ might be able to serve as a molecular biomarker for diagnosis or prognosis for breast cancer patient treatment. We expect that PTPγ protein will significantly prolong the survivability of human breast cancer patients by inhibiting or slow down the proliferative ability of human breast cancer cells. The signaling pathway with PTPγ involvement may provide a new target for cancer prevention and treatment. In the future,
PTPγ might be able to be applied in clinical trials for cancer prevention and cancer therapy.
146 CONCLUDING REMARKS
The studies in this dissertation were carried out with the intention to claim the estrogenic activities of Zeranol in human breast neoplastic transformation and identify a new potential tumor suppressor gene in human breast, which help us to understand some basic mechanisms regarding to the effects of environmental disruptors on human breast and contribute to our overall understanding of the tumor suppressing activities of PTPγ in human breast. But, there are still many questions to be answered with more investigations in the future. The underlying goal of this research was to provide rationale for the prevention and treatment of human breast cancer with some novel target sites. Thus, with this knowledge, we hope that it should be possible to design new therapies targeted to this novel tumor suppressor and lead to the improved treatment of human breast cancer, and hope that these studies can be used as a basis for the similar investigations in other human cancers.
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