Role of Notch Signaling in Cell-Fate Determination of Human Mammary

Available online http://breast-cancer-research.com/content/6/6/R605

ResearchVol 6 No 6 article Open Access Role of Notch signaling in cell-fate determination of human mammary stem/progenitor cells Gabriela Dontu, Kyle W Jackson, Erin McNicholas, Mari J Kawamura, Wissam M Abdallah and Max S Wicha

Comprehensive Cancer Center, Department of Internal Medicine, University of Michigan, Ann Arbor, Michigan, USA

Corresponding author: Gabriela Dontu, [email protected]

Received: 6 Apr 2004 Revisions requested: 10 May 2004 Revisions received: 28 Jun 2004 Accepted: 15 Jul 2004 Published: 16 Aug 2004

Breast Cancer Res 2004, 6:R605-R615 (DOI 10.1186/bcr920)http://breast-cancer-research.com/content/6/6/R605 © 2004 Dontu et al.; licensee BioMed Central Ltd. This is an Open Access article: verbatim copying and redistribution of this article are permitted in all media for any purpose, provided this notice is preserved along with the article's original URL.

Abstract

Introduction Notch signaling has been implicated in the renewal and on early progenitor cells to promote their regulation of cell-fate decisions such as self-renewal of adult proliferation, as demonstrated by a 10-fold increase in stem cells and differentiation of progenitor cells along a secondary mammosphere formation upon addition of a Notch- particular lineage. Moreover, depending on the cellular and activating DSL peptide. In addition to acting on stem cells, developmental context, the Notch pathway acts as a regulator of Notch signaling is also able to act on multipotent progenitor cell survival and cell proliferation. Abnormal expression of Notch cells, facilitating myoepithelial lineage-specific commitment and receptors has been found in different types of epithelial proliferation. Stimulation of this pathway also promotes metaplastic lesions and neoplastic lesions, suggesting that branching morphogenesis in three-dimensional Matrigel Notch may act as a proto-oncogene. The vertebrate Notch1 and cultures. These effects are completely inhibited by a Notch4 Notch4 homologs are involved in normal development of the blocking antibody or a gamma secretase inhibitor that blocks mammary gland, and mutated forms of these genes are Notch processing. In contrast to the effects of Notch signaling associated with development of mouse mammary tumors. on mammary stem/progenitor cells, modulation of this pathway has no discernable effect on fully committed, differentiated, Methods In order to determine the role of Notch signaling in mammary epithelial cells. mammary cell-fate determination, we have utilized a newly described in vitro system in which mammary stem/progenitor cells can be cultured in suspension as nonadherent Conclusion These studies suggest that Notch signaling plays a 'mammospheres'. Notch signaling was activated using critical role in normal human mammary development by acting exogenous ligands, or was inhibited using previously on both stem cells and progenitor cells, affecting self-renewal characterized Notch signaling antagonists. and lineage-specific differentiation. Based on these findings we propose that abnormal Notch signaling may contribute to Results Utilizing this system, we demonstrate that Notch mammary carcinogenesis by deregulating the self-renewal of signaling can act on mammary stem cells to promote self- normal mammary stem cells.

Keywords: mammary gland development, mammary progenitor cells, mammary stem cells, Notch

Introduction of identified specific cell surface markers. Furthermore, Stem cells in adult tissues are characterized by their ability investigation of the mechanisms underlying cell-fate deci- to undergo self-renewal and multilineage differentiation [1]. sions in mammary stem/progenitor cells has been limited The elucidation of pathways that govern stem cell functions by the lack of suitable in vitro culture systems, which main- is essential for understanding normal development and org- tain these cells in an undifferentiated state [3]. anogenesis. Moreover, there is increasing evidence that defects in these pathways play an important role in carcino- We have recently described an in vitro culture system that genesis [2]. The isolation of stem cells from the mammary allows for the propagation of primary human mammary epi- gland in humans and rodents has been hindered by the lack thelial stem cells and progenitor cells in an undifferentiated

ADAM = a disintegrin and metalloproteases; FACS = fluoresence-activated cell sorting; FBS = fetal bovine serum; FITC = fluorescein isothiocyanate; GSI = gamma secretase inhibitor; HBSS = Hanks Balanced Salt Solution; RT-PCR = reverse transcriptase-polymerase chain reaction. R605 Breast Cancer Research Vol 6 No 6 Dontu et al.

state, based on their ability to proliferate in suspension as Mammospheres were collected by gentle centrifugation spherical structures, which we have termed 'nonadherent (800 rpm) after 7–10 days, and were dissociated enzymat- mammospheres' [4]. As has previously been described for ically (10 min in 0.05% trypsin, 0.53 mM EDTA-4Na; Invit- neuronal stem cells and progenitor cells cultured as neuro- rogen) and mechanically, using a fire-polished Pasteur spheres [5], we have demonstrated that mammospheres pipette. The cells obtained from dissociation were sieved are composed of stem cells and progenitor cells capable of through a 40 µm sieve and analyzed microscopically for sin- self-renewal and multilineage differentiation [4]. We have gle cellularity. Cells plated at low densities (1000 cells/ml) utilized this system to investigate the role of Notch signal- were grown in conditioned medium from high-density pri- ing in mammary cell-fate determination. mary cultures in suspension.

In other systems, Notch signaling has been shown to play Differentiating culture conditions an important role in cell-fate determination, as well as in cell Single cell suspensions were plated on collagen-coated survival and proliferation [6,7]. The Notch proteins, repre- plates at a density of 2000 viable cells/10 cm diameter sented by four homologs in mammals (Notch1–Notch4), dish. Cells were grown in Ham's F-12 medium (BioWhit- interact with a number of surface-bound or secreted lig- taker) with 5% fetal bovine serum (FBS), 5 µg/ml insulin, 1 ands (Delta-like 1, Delta-like 3, Delta-like 4, Jagged 1 and µg/ml hydrocortisone, 10 µg/ml cholera toxin (Sigma, St Jagged 2) [8-10]. These interactions are modulated by Louis, MO, USA), 10 ng/ml epidermal growth factor (BD modifier proteins from the Fringe family (Lunatic, Manic, Biosciences) and 1 × Pen/Strep/Fungizone Mix (BioWhit- and Radical Fringe) [11]. Upon ligand binding, Notch taker). Three-dimensional cultures in Matrigel were estab- receptors are activated by serial cleavage events involving lished as previously described [18]. members of the ADAM protease family, as well as an intramembranous cleavage regulated by gamma secretase Immunostaining (presinilin). This intramembranous cleavage is followed by To assess the lineage composition of the colonies, cells translocation of the intracellular domain on Notch to the were fixed on plates for 20 min in methanol, at -20°C, and nucleus, where it acts on downstream targets [11]. The ver- were then stained using Peroxidase Histostain-Plus and tebrate Notch4 gene has been shown to be involved in nor- Alkaline-phosphatase Histostain-Plus kits (Zymed, South mal mammary development [12]. In vitro, overexpression of San Francisco, CA, USA), according to the manufacturer's a constitutively active form of Notch4 inhibits differentiation protocol. The ductal–alveolar structures were fixed in of normal breast epithelial cells [13]. In vivo, transgenic Matrigel in 4% neutral formalin, and treated with 1 mg/ml mice expressing a constitutively active form of Notch4 fail proteinase K for 10–15 min at room temperature, prior to to develop normal mammary glands and subsequently staining. For immunostaining in Matrigel, all antibody incu- develop mammary tumors [14]. These studies suggest the bations were for 60–90 min at 37°C, and washes were for involvement of Notch signaling in normal breast develop- 20 min at 37°C, with a final overnight wash at 4°C. ment, and that alterations in Notch signaling may play a role in breast cancer development [15,16]. The primary antibodies – epithelial-specific antigen and cytokeratin 18 for epithelial cells, CD10 and cytokeratin 14 Materials and methods (Novocastra, Newcastle-upon-Tyne, UK) for myoepithelial Dissociation of mammary tissue cells – were used at the dilutions indicated by the manufac- Normal breast tissue from reduction mammoplasties was turer. The Notch4 antibody was used at a dilution of 1:50 dissociated mechanically and enzymatically, as previously for immunohistochemistry and of 1:20 for immunostaining described [17]. Cells were sieved sequentially through a structures within Matrigel. AEC and DAB (Zymed) were 100 µm cell strainer and a 40 µm cell strainer (BD Falcon, used as substrates for peroxidase, and NBT/BCIP (Gibco/ Palo Alto, CA, USA) to obtain a single cell suspension. BRL, Gaithersburg, MD, USA) was used for alkaline phosphatase. FITC-labeled and Texas-red labeled secondary Mammosphere culture antibodies (Jackson Labs, West Grove, PA, USA) were Single cells were plated on ultralow attachment plates used for fluorescent microscopy. (Corning, Acton, MA, USA) at a density of 20,000 viable cells/ml in primary culture and a density of 1000 cells/ml in Luciferase gene reporter assay passages. Cells were grown in serum-free mammary epi- MCF-7 cells were grown in RPMI 10% FBS on tissue cul- thelial growth medium (BioWhittaker, Rockland, ME, USA), ture-treated plastic dishes. Cells were collected by trypsini- supplemented with B27 (Invitrogen, Carlsbad, CA, USA) zation and RNA was isolated as previously described and and 10 ng/ml epidermal growth factor (BD Biosciences, used to test expression of Notch1 and Notch4 by RT-PCR. Palo Alto, CA, USA). The bovine pituitary extract was MCF-7 cells expressed both Notch1 and Notch4 so they excluded. were further used for the gene reporter assay. Expression of Notch4 in MCF-7 cells was also confirmed by R606 Available online http://breast-cancer-research.com/content/6/6/R605

immunohistochemistry. The staining was mostly nuclear which treatment was applied for longer than 48 hours, it and cytoplasmic, although membrane staining was also was added every 2 or 3 days when the medium was present, confirming detectable levels of active Notch4 in changed. The Notch4 antibody and antigenic peptide used MCF-7 cells. for treating cells in culture did not contain sodium azide.

The specificity of the antibody was tested by the manufac- FACS analysis turer via western blotting using cell lysates from a variety of Cells were fixed in methanol and immunostained with anti- human cancer cell lines (Jurkat cells, HeLa, H4, etc.) that epithelial-specific antigen, CD10 and Muc1 antibodies express all four homologs of Notch. The antibody was (Novocastra), at the concentration recommended by the raised against a synthetic peptide that does not show manufacturer. Incubation was performed for 30 min at room sequence similarity with any of the other Notch members, temperature in Hanks Balanced Salt Solution (HBSS) with as shown by Blast search analysis. Using the Lipo- 2% FBS, followed by washing in HBSS with 2% FBS. The fectamine–Plus kit (Promega, Madison, WI, USA), cells same procedure was followed for staining with secondary were co-transfected with pcDNA3 (Invitrogen) and a antibody, anti-goat IgG or anti-rabbit IgG, labeled with FITC pGL3-promoter vector (Promega) containing a Hes-1 pro- (1:200 dilution; Jackson Labs). After incubation, cells were moter-derived sequence upstream of luciferase, according washed once with HBSS (BioWhittaker) and were resus- to the manufacturer's protocol. Stable transfection was pended in HBSS supplemented with 5% FBS. Cells were achieved by selecting with 800 µg/ml Geneticin (Gibco/ stained with 1 µg/ml propidium iodide (Sigma) for 30 min BRL). for DNA cell cycle analysis or for 5 min for viability. Analysis was performed using FACStarPLUS (Becton Dickinson, Palo Cells were incubated for 4, 8 and 16 hours in the presence Alto, CA, USA) and the Elite SP (Becton Dickinson) flow of agonists or antagonists of Notch signaling as follows: cytometer. blocking Notch4 antibody (Santa-Cruz, Santa Cruz, CA, USA) was used in dilutions of 1:100, 1:50, 1:25 and 1:10, Results gamma secretase inhibitor (GSI) type I (Z-Leu-Leu-Nle- Experimental strategy CHO; Calbiochem, San Diego, CA, USA) was used at 20 In order to investigate the role of the Notch pathway in the µM, 10 µM, 5 µM, 1 µM, 100 nM and 10 nM concentration, development of the mammary gland, we examined the and the DSL peptide (CDDYYYGFGCNKFCRPR; effect of agonists and antagonists of Notch signaling on Research Genetics, Huntsville, AL, USA) was used at 10 cell-fate determination utilizing mammary stem/progenitor µM, 1 µM, 100 nM, 10 nM and 1 nM. The control scram- cells cultured in suspension, as mammospheres, and differ- bled peptide (DYGKRCYGCFPDYFNCR) was used at 10 entiated mammary cells, obtained by passaging these cells µM concentration. Dimethylsulfoxide 1:50 was used as a on a collagen substratum. The experimental strategy is out- control for GSI. Direct evidence that GSI blocks Notch4 lined in Fig. 1. The activators and inhibitors of Notch sign- and Notch1 cleavage in vitro has been provided previously aling were applied at three different stages of in vitro [19]. development, which we have previously characterized [4]. First, the treatment was applied in suspension culture, The treatment with antagonist involved incubating for 1 which is enriched in stem/progenitor cells (self-renewal of hour at 37°C, prior to treatment with the DSL peptide. Incu- stem cells and the proliferation of progenitor cells occur in bation of the antibody with the peptide against which it was these conditions) (Fig. 1a). Second, the treatment was raised (Santa Cruz) was carried out at antibody to peptide applied in cultures on collagen substratum, under condi- ratios of 1:1, 1:2. 1:3 and 1:4 (vol:vol). Luciferase activity tions that promote differentiation of mammosphere-derived was assessed using the Luciferase assay system cells (multipotent progenitors commit to a specific lineage) (Promega) and a Turner Designs luminometer TD-20/20 (Fig. 1b). Finally, the treatment was applied in cultures on a (Turner Designs, Sunnyvale, CA, USA). collagen substratum, using differentiated cells passaged on a collagen substratum (lineage-restricted progenitors Treatment with Notch agonists and antagonists undergo terminal differentiation) (Fig. 1c). Treatment with Notch agonists and antagonists was carried out as described for the reporter assay. The DSL pep- A synthetic peptide derived from the DSL (Delta-Serrate- tide was used at a concentration of 100 nM. The LAG 2) domain conserved in all Notch ligands [9] and a recombinant Delta 1 Fc ligand was prepared and used as previously described recombinant Delta 1 ligand fused to previously described [20]. Anti-human Fc antibody (Jack- the immunoglobulin Fc fragment (used in combination with son Labs) was added to the medium at 1:100 dilution, to an anti-human Fc antibody, for ligand clustering) [20] were cluster the recombinant ligand–receptor complexes. The utilized as agonists of Notch signaling. A Notch4-specific blocking Notch4 antibody was used at a dilution of 1:50, antibody or a GSI that blocks the intramembranous cleav- and the GSI at a concentration of 5 µM. In experiments in age of Notch, required for signaling [21], was used as the R607 Breast Cancer Research Vol 6 No 6 Dontu et al.

Figure 1

Suspensionculture Dissociated Dissociation Collagenculture tissue

Stem cell Multipotent progenitors Differentiated,cells Differentiated,cells

Massive anoikis Self-renewal Early differentiation Late differentiation Survival of small population

Dissociation Read-out in Read-out in colony morphology, Last of Read-out in and passage secondary distribution of colony types and/or two distribution of sphere distribution of cell lineages passages cell lineages formation Treatment application:

a bc

Effecttal strategy of modulators of Notch signaling on self-renewal of mammary stem cells and on lineage commitment of mammary progenitor cells: experimental strategy. Treatment was applied (a) to mammospheres in suspension culture, (b) to mammosphere-derived cells cultivated on a collagen substratum, and (c) to differentiated cells cultivated on a collagen substratum. The assay for self-renewal was mammosphere formation. The assay for multilineage potential or lineage commitment was lineage-specific immunostaining of clonogenic cultures on the collagen substratum.

antagonist. The synthetic DSL peptide, the recombinant Utilizing retroviral marking studies, we also have demon- Delta fused to Fc and GSI are all well characterized with strated that mammospheres are clonally derived and do not respect to their effect on Notch signaling [9,20]. We uti- result from cellular aggregation [4]. Primary, secondary, lized a reporter assay in MCF-7 cells, in which luciferase and tertiary generation mammospheres were formed in the expression was driven by the Hes-1 promotor, a gene reg- presence or absence of the Notch-activating DSL peptide. ulated by Notch signaling [22], to confirm the ability of acti- Although DSL peptide had only modest effects on primary vating or inhibiting ligands to affect Notch signaling. mammosphere formation, it increased secondary and tertiary mammosphere formation 10-fold, compared with con- As shown in Fig. 2a, DSL peptide upregulated Hes-1- trol cultures (Fig. 3a,3b,3d,3e,3g). If this effect was due to driven luciferase expression. These results are consistent stem cell self-renewal, we would predict that cells derived with previous characterization of these Notch signaling from these spheres would retain their multipotent differen- agonists in reporter assays and in experiments using other tiation ability. We therefore examined the ability of second- cell types [9]. Notch signaling was blocked using a ary and tertiary mammosphere-derived cells to clonally Notch4-specific antibody, or a GSI, which abrogates the differentiate along the multiple lineages present in the adult intramembranous cleavage of Notch, required for signaling mammary gland. [19,21]. Both of these inhibitors blocked endogenous Hes- 1-driven luciferase expression, as well as the increase in The adult mammary gland has a lobuloalveolar structure luciferase expression induced by the DSL ligand (Fig. 2a). composed of three cell lineages: myoepithelial cells that The specificity of the blocking antibody was confirmed by form the basal layer of ducts and alveoli, ductal epithelial successful competition with the antigenic peptide used to cells that line the lumen of ducts, and alveolar epithelial produce the antibody (data not shown). Furthermore, these cells that synthesize milk proteins [23]. Mammospheres effects were dose dependent (Fig. 2b). grown in the presence or absence of Notch agonists or antagonists were dissociated into single cells and plated at Effects of Notch signaling on self-renewal of mammary clonogenic densities on collagen substrata in a medium stem cells that promotes differentiation [4]. After 7 days of cultivation, We examined the effects of these agonists and antagonists the clonally derived colonies were immunostained using lin- of Notch signaling on primary, secondary and tertiary mam- eage-specific markers (epithelial-specific antigen and mosphere formation (Fig. 1a). This assay has previously cytokeratin 18 for ductal epithelial cells, and cytokeratin 14 been used for the in vitro study of neural stem cell self- and smooth muscle actin for myoepithelial cells). The renewal [19] and relies on the observation that neuro- number of multipotent cells after one passage was seven sphere formation is initiated clonally by neural stem cells times higher, and that after two passages 100 times higher, [5]. in mammospheres cultured in the presence of DSL, R608 Available online http://breast-cancer-research.com/content/6/6/R605

Figure 2

N4Ab (a)(b) 350 10 µM DSL+N4Ab GSI 300 10 µM DSL+GSI RLU R2 =0.8264 Linear (10 µM DSL+GSI) Linear (10 µM DSL+N4Ab) 60000 250 Linear (GSI) Linear (N4Ab) 50000 200 R2 =0.8811 40000 150 30000 R2 =0.989 Luciferaseactivity(%) 20000 100

10000 50 R2 =0.893 0 No DSL N4Ab N4Ab+DSL GSI GSI+DSL 0 treatment 0 124 6µg/ml N4Ab 00.11 5 25µM GSI Concentrationinhibitor

Effect of Notch agonist and antagonist treatment, assessed by Hes-1-driven luciferase expression in MCF-7 cells.cells (a) DSL treatment (100 nM) increased luciferase expression twofold compared with control, whereas treatment with the N4 blocking antibody (N4Ab) or gamma secretase inhibitor (GSI) (5 µM) reduced both endogenous and DSL-induced luciferase expression. (b) Dose–effect of Notch signaling antagonists. The difference in luciferase activity in cells treated with the maximum dose of inhibitory agent and in nontreated cells cells was statistically significant for each of the four different treatments (P < 0.005). The R2 values and trendlines for the four sets of experiments are shown. compared with mammospheres cultured without added tures. The viability of cells within a mammosphere was sim- DSL. These calculations are based on the number of ilar in the presence of and the absence of DSL. These spheres (Fig. 3g) and the corresponding percentage of results indicate that the Notch activation increases prolifer- multilineage colonies, in each experiment (Fig. 3h). ation of progenitor cells.

The ability of Notch activating ligands to increase mammos- In order to further investigate the role of Notch signaling, phere formation, as well as the production of multilineage and in particular Notch4, in mammary stem cell self- progenitors, suggests that Notch activation promotes renewal, we assayed the effect of Notch inhibitors on mam- mammary stem cell self-renewal. Similar effects to the DSL mosphere formation. The addition of a Notch4 blocking peptide on mammosphere formation were obtained with antibody to primary cultures had no effect on primary recombinant Delta Fc fragment (data not shown). In addi- sphere formation, but completely abolished secondary tion to increasing the number of mammosphere initiating mammosphere formation (Fig. 3a,3c,3d,3f,3h). A similar cells and bipotent progenitor cells, treatment with Notch effect was obtained by blocking Notch activation with a agonists increased their proliferation potential, as shown by GSI at 1–5 µM, a concentration found to effectively block the increased size of mixed lineage colonies, generated Notch activation in the luciferase reporter assay (Fig. 2a). In clonally on collagen culture by these cells (Fig. 3i). Treat- contrast to the effect of adding Notch4 blocking antibody ment with Notch agonists was applied in suspension cul- at the time of mammosphere formation, this treatment had ture only, and was not applied to sphere-derived cells no effect when added 24–48 hours after mammosphere subsequently cultured on collagen substrata, suggesting formation. This suggests that the effects of Notch activation that the effect on proliferation is irreversible. This positive on mammary stem cell self-renewal may occur during the effect of Notch activation on proliferation potential of stem/ initial stages of mammosphere formation. The cells progenitor cells also occurs in the mammospheres, as responding to modulation of Notch signaling are the sphere demonstrated by an increased size of the primary and sec- initiating cells, capable of survival and proliferation in ondary mammosphere diameter in the DSL-treated suspen- suspension. sion cultures (mean ± standard error of the mean, 245 ± 11.5 µM versus 130 ± 3 µM; n = 69 and n = 89). This is Effect of Notch signaling on lineage specification of due to an increased number of cells per sphere, as demon- mammary progenitor cells strated by cell counts that are approximately double in In other developing systems, Notch signaling has been DSL-treated mammospheres compared with control cul- shown to effect lineage commitment as well as stem cell R609 Breast Cancer Research Vol 6 No 6 Dontu et al.

Figure 3

Primary spheres Secondary spheres 600 Primary spheres Secondary sphe 500 Tertiary spheres DSL + peptide + + 400 N4 Ab + + 300

200

#spheres/10,000cells 100

0 Control DSL peptide N4 Ab (a)(b)(c)(d)(e)(f) (g)

2500 Mixed Mixed Epithelilal 100 Epithelial 2000 Myoepithelial 90 Myoepithelial 80 70 1500 60 50 1000 40 Colonytype(%) 30 500

20 Colonysize(avg#ofcells/col) 10 0 0 Control DSL 1 DSL 2 Control DSL 1 DSL 2 (h) (i)

100 70

90 60 Control 80 DSL peptide Ductal epithelial-green 50 70 Myoepithelial-red 60 40

50 30 40 %positivecells Celltype(%) 30 20

20 10 10 0 0 ESA Muc 1 CD10 Control DSL (j)(k)

Effect of modulators of Notch signaling on self-renewal of mammary stem cells and on lineage commitment of mammary progenitor cellsells. (a)–(g) Effect of Notch agonist and antagonist treatment on primary and secondary mammosphere formation. Primary and secondary mammospheres were grown in standard conditions (a, d), in the presence of DSL peptide (b, e) and in the presence of N4 blocking antibody (N4Ab) (c, f). Primary mammospheres grown in the presence of N4 antibody (c) are smaller than the control (a). Secondary mammosphere formation is completely blocked by treatment with Notch4 blocking antibody (d, f). The addition of DSL peptide increases primary, secondary and tertiary mammosphere formation (a, b, d, e, g). Data are presented as the mean ± standard error of the mean. The calculated number of multilineage progenitor cells (number of spheres × % multilineage progenitors) shows a sevenfold increase after one passage, and a 100-fold increase after two passages, in mammospheres cultured in the presence of DSL compared with mammospheres cultured without added DSL. (h)–(k) Effect of Notch activation on lineage specification of human mammary progenitor cells. DSL treatment of mammospheres in suspension culture (DSL1) and on mammosphere-derived cells on the collagen substratum (DSL2) increases the number of myoepithelial progenitors, as shown by the clonogenic assay (h), and increases the rate of proliferation of bipotent and myoepithelial progenitors, as reflected by the colony size (i). DSL treatment in suspension culture increases the percentage of myoepithelial cells, as shown by flow-cytometry analysis (j). Cells were stained red (phycoerythrin [PE]) with myoepithelial marker CD10, and green (FITC) with the ductal epithelial marker epithelial-specific antigen (ESA). Treatment with DSL peptide does not have a lineage selective effect on differentiated cells, as shown by flow-cytometry analysis of human mammary epithelial cells passaged twice on collagen (k). Data are presented as the mean ± standard error of the mean.

self-renewal [20,24-26]. The inability of previously ble of multilineage differentiation, we utilized this system to described culture systems of mammary cells to maintain determine whether Notch signaling could also act on mam- progenitor cells in an uncommitted state has limited the in mary progenitor cells to affect lineage-specific commit- vitro study of lineage commitment. Since we have previ- ment. In order to test this, we added the DSL peptide to ously demonstrated that, in addition to mammary stem mammosphere cultures (Fig. 1a), to cultures of mammos- cells, mammospheres contain mammary progenitors capa- phere-derived cells on collagen substrata in conditions that R610 Available online http://breast-cancer-research.com/content/6/6/R605

promote their differentiation (Fig. 1b) or to cultures of differentiated mammary cells, we have utilized three-dimensional entiated cells (Fig. 1c). Matrigel cultures to explore the morphogenic differentiation potential of these cells. Following 3–4 weeks of cultivation Cells were assayed for lineage-specific commitment, using in Matrigel, mammospheres develop extensive ductal lobu- the clonogenic assay described earlier. As shown in Fig. 3h loalveolar structures similar in morphology to those found in (DSL1), the percentage of myoepithelial progenitor cells vivo (Fig. 4a,4b). We utilized this system to examine the generated by DSL-treated mammospheres was increased role of Notch signaling in morphogenesis. Secondary mam- approximately 13-fold compared with cells from mammos- mospheres were imbedded in Matrigel and treated with pheres cultured in the absence of DSL ligand. The percent- DSL peptide, Notch4 antibody or GSI. Treatment with DSL age of myoepithelial progenitor cells generated by peptide promoted earlier development, as well as mammosphere-derived cells treated with DSL starting from increased length and number of branching structures, com- the moment of their plating on collagen culture (DSL2) gen- pared with control cultures (Fig. 4c,4d). In contrast, the erated sevenfold more myoepithelial progenitor cells com- Notch4 antibody completely inhibited branching morpho- pared with controls. The significant effect of DSL peptide, genesis (Fig. 4c,4d). Similar effects were produced by when added only to suspension-cultured mammospheres, blocking Notch signaling with GSI (data not shown). suggests that Notch activation increased the number of myoepithelial progenitors and/or their rate of proliferation in Expression of Notch4 in human mammary epithelial cells an irreversible manner. The increased size of myoepithelial in vitro and bilineage colonies (Fig. 3i) formed in the presence of We and other workers have found that expression of Notch DSL peptide suggests that Notch stimulation may promote receptors in the adult human mammary gland was below proliferation of myoepithelial and bilineage progenitors. the level of detection using immunostaining of tissue sec- tions, but was detectable by RT-PCR [28]. Moreover, In order to determine whether the increase in colony size expression of Notch4 in dissociated mammary tissue was results from an effect of Notch on cell survival, we used pro- below the level of detection using flow cytometry. The pidium iodide staining in the same clonogenic assay. Treat- experiments already described, however, suggested that ment with DSL peptide did not have a discernable effect on Notch receptors are present in mammary stem cells and the cell-death rate of either ductal epithelial cells or myoep- progenitor cells, and are downregulated in differentiated ithelial cells. To assess more accurately the increase in total cells. Previous gene expression profile analysis by micro- number of myoepithelial cells upon treatment with DSL in array and real-time RT-PCR showed that mammosphere- suspension culture, we repeated the same experiment derived cells passaged in suspension culture express all described earlier, utilizing flow cytometry analysis, to quan- Notch receptors at a higher level (twofold to fourfold) than titate the cell lineage (Fig. 3j). The results showed a 15-fold the same cells cultured under conditions that induce differ- increase in myoepithelial cells upon addition of DSL pep- entiation [4]. tide to the culture. These results suggest that Notch receptors are expressed Effect of Notch signaling on differentiated mammary at higher levels in more primitive mammary cells, such as epithelial cells those found in the developing mammary gland, compared In contrast to the significant effects of Notch signaling on with the mature gland. To test this hypothesis, we immu- uncommitted mammary cells, modulation of the Notch nostained structures generated by mammospheres in pathway had no significant effect on differentiated mam- Matrigel culture, at different stages of growth (between 4 mary epithelial cells. The addition of DSL peptide to differ- days and 2 weeks). We found that cells expressing Notch4 entiated cells cultured on collagen substrata (Fig. 1c) had were uniformly distributed in spherical structures at the no effect on lineage specification, as shown by flow-cytom- beginning of cultivation (Fig. 5a,5b). As branching morpho- etry analysis using lineage-specific markers (Fig. 3k). Fur- genesis occurred, Notch4 expression became limited to thermore, the addition of Notch antibody or the GSI at 1– the subterminal end of branches (Fig. 5c,5d). The corre- 10 µM had no significant effect on cell number or lineage sponding areas in vivo have previously been postulated to commitment (data not shown). This also demonstrates that contain mammary stem cells and progenitor cells [23]. Notch activation is not necessary for the survival of fully differentiated mammary cells. Discussion In order to study the role of Notch signaling in mammary Effect of Notch signaling on branching morphogenesis cell-fate determination, we have utilized an experimental It has previously been demonstrated that culture of human system that allows the analysis of cell self-renewal and dif- or rodent mammary cells in reconstituted basement mem- ferentiation in vitro. This enabled us to study the effect of brane (Matrigel) promotes morphogenic differentiation Notch activation and inhibition at a single cell level and [18,27]. Since mammospheres are composed of undiffer- under conditions where the timing and the duration of R611 Breast Cancer Research Vol 6 No 6 Dontu et al.

Figure 4

(a)(b)

Control

Spheres Branched structures 100 90 80 DSL peptide 70 60 50 40 30 Morphotype(%) N4 Ab 20 10 0 Control D SL peptide N4 Ab 2 8121624 days (c)(d)

Effect of Notch signaling on branching morphogenesis.morphogenesis (a) Fragment of the human mammary gland, after 8 hours of collagenase digestion of the reduction mammoplasty sample. (b) Complex ductal–alveolar structure developed in 18 days, from a mammosphere embedded in Matrigel. (c) Sec- ondary mammospheres were embedded in Matrigel and treated with DSL peptide or Notch4 blocking antibody (N4Ab). Longer branches developed earlier in the presence of DSL peptide. (d) In addition, more spheres developed branching structures. The Notch4 blocking antibody completely inhibited proliferation and branching in Matrigel (c, d). Images shown in (c) are representative of 100–150 structures per experiment, scored in three different experiments with cells derived from three different patients. Data are presented as the mean ± standard error of the mean.

Notch signaling could be controlled. Our findings suggest effect of Notch activation on mammosphere formation that Notch signaling regulates cell-fate decisions in the underscores the value of the mammosphere cultivation sys- mammary gland at several distinct developmental stages. tem in studying in vitro self-renewal regulation. It has previously been shown in suspension culture that neural stem We propose that in the mammary gland, as has been cells can generate mixed colonies, termed neurospheres, reported in other developmental systems, Notch activation composed of stem cells and progenitor cells. We have acts as a regulator of asymmetric cell-fate decisions. Notch demonstrated that mammospheres are composed of stem activation promotes self-renewal in stem cells, whereas in cells and progenitor cells capable of self-renewal and mul- later stages of development it biases cell-fate decisions in tilineage differentiation [4]. Our clonal analysis of individual mammary progenitor cells towards adoption of a myoepi- cells from single spheres suggested that mammospheres thelial cell fate versus an epithelial cell fate. are composed of approximately 150–200 progenitor cells, which cannot generate new spheres but have bilineage or The increase in the number of mammosphere-initiating cells trilineage differentiation potential, and of one or two puta- reflects an increase in the number of cells capable of self- tive stem cells able to generate a new sphere with the same renewal, in the experimental system used in this study. The cellular composition as the parental sphere. R612 Available online http://breast-cancer-research.com/content/6/6/R605

Figure 5 gests that Notch activation increases the proliferation potential of both bipotent and myoepithelial progenitors. These effects are irreversible and may involve an instructive mechanism or the selective proliferation of myoepithelial cells. In contrast to the effects of Notch signaling on mammary stem cells and progenitor cells, activation or inhibition of these pathways in fully differentiated mammary cells had 100 µM no apparent effect. (a)(b) The increased branching we observed in the three-dimensional cultivation system upon Notch activation is consistent with previous studies demonstrating that proliferation of myoepithelial cells can facilitate branching morphogenesis. Moreover, Notch signaling appears to be absolutely required for branching and growth in three-dimensional cultures as development of branching structures is completely 100 µM blocked by Notch4 antibody or GSI. The expression of (c)(d) Notch4 protein in the growing mammary tree in vitro is also consistent with an important role of Notch signaling in mor- Notch4derived fromprotein mammospheres detected by immunostaining embedded in Matrigel (FITC-green) in structures phogenesis of the mammary gland. The apparent discrep- derived from mammospheres embedded in Matrigel. (a),(b) Spherical structures at 5 days of cultivation (Nomarski contrast-phase image and ancy between our results and the previous findings by immunofluorescence). (c),(d) Branching structures at 16 days of Uyttendaele and colleagues [13] (who showed that overex- cultivation. pressing Notch4 in the mouse mammary epithelial TAC-2 cell line prevented branching morphogenesis in response The constant frequency of sphere formation during multiple to hepatocyte growth factor or transforming growth factor passages, observed in passages of both individual and beta) underscores the importance of the experimental sys- pooled mammospheres, suggests that only asymmetric tems utilized. This discrepancy may be accounted for by self-divisions or a small number of symmetric self-renewal differences in the experimental systems, such as the use of divisions contribute to sphere formation (Fig. 6a). If bipo- an immortalized mouse epithelial cell line in the Uyttendaele tent or monopotent progenitor cells were able to survive in study compared with primary normal human cells in our suspension, they would generate mammospheres of study. Furthermore, Uyttendaele and colleagues utilized increasingly smaller size and, eventually, they would stop overexpression of activated Notch, whereas we used exog- growing in suspension after several passages (Fig. 6b), enous modulation of the Notch pathway. which did not occur in the experimental setting used here. An increase in the number of symmetric self-renewal divi- Smith and colleagues [12] demonstrated that Notch4 has sions would result in an amplification of the cellular popula- an important role in both normal mammary development tion, by progressively increasing the number of and carcinogenesis. Transgenic mice harboring a constitu- mammospheres at each passage (Fig. 6c). This effect was tively active Notch4 under mouse mammary tumor virus obtained upon exogenous activation of Notch signaling. In promoter regulation exhibited arrested mammary gland this discussion we adopted the operational definition of development, and eventually developed poorly differenti- stem cells as the only normal cells capable of self-renewal ated adenocarcinomas. Questions remain in interpreting [29,30]. the report that the Notch4 knockout mouse has no apparent defect in mammary gland development [31]. This may Treatment with Notch agonists increased the commitment indicate that in vivo there are compensatory mechanisms of bilineage progenitors to myoepithelial lineage, when absent in our in vitro experimental system. Alternatively, the applied early in suspension culture (Fig. 1, DSL1) or later expression or activity of different homologs of Notch can be on collagen culture (Fig. 1, DSL2). The effect on lineage interdependent, as suggested by previous studies [32]. commitment appears to be irreversible, since early activation of Notch, in suspension culture, resulted in an increase Conclusions in the number of myoepithelial progenitors – although the Our findings regarding the role of Notch in promoting self- treatment with Notch agonists was not maintained on renewal of mammary stem cells, in addition to previous collagen cultures. In addition to an increase in the number observations that it can function as a proto-oncogene of bipotent and myoepithelial progenitor cells, treatment [15,16,28,33-38], suggest that abnormal Notch signaling with Notch agonists resulted in a significant increase in the may be involved in carcinogenesis, through deregulation of size of both mixed and myoepithelial colonies. This sug- normal mammary stem cell self-renewal. Indeed, it has R613 Breast Cancer Research Vol 6 No 6 Dontu et al.

Figure 6

Asymmetricself- Restrictive Symmetricself- renewaldivisions divisions renewaldivisions

Stem cell Multipotent progenitor Lineage restricted progenitor Differentiated, lineage committed cell Differentiated, lineage committed cell

Efficiencyofsphereformation Same Decreases Increases

(a) (b) (c)

Effect of stem cell self-renewal and restrictive didivisionsvisions onon thethe efficiencyefficiency ofof mammospheremammosphere formation.formation (a) Asymmetric stem cell self-renewal divisions result in constant mammosphere numbers in serial passages. (b) Proliferation of stem cells or progenitor cells through restrictive divisions results in a decreasing number of spheres in serial passages. (c) Symmetric self-renewal divisions of stem cells result in an increasing number of mammospheres in serial passages.

recently been proposed that stem cells or early progenitor Competing interests cells constitute the primary targets of transformation [2]. MW has financial holdings in Oncomed, which has applied for a patent on cancer stem cell technologies. We have recently described the existence of a tumorigenic subset of human breast carcinoma cells, with a definable Acknowledgements phenotype, which is responsible for tumor formation as well Thanks are due to Dr Thomas Giordano for tissue procurement, the Uni- as for generating the phenotypic heterogeneity found in versity of Michigan Cancer Center Flow Cytometry core, Dr Michael breast cancers [39]. Tumorigenic cells, like their normal Clarke for technical advice and Dr Sean Morrison for providing the stem cell counterparts, can therefore undergo self-renewal recombinant Delta 1-Fc recombinant ligand and for critical review of the paper. This work was supported by NIH grants CA66233 and as well as differentiation, albeit aberrant. We have postu- CA101860, and in part by the University of Michigan Cancer Center lated that the tumor stem cell population drives tumorigen- NIH Support Grant 5 P 30 CA46592. esis, recurrence and metastasis [40]. References In addition to our findings regarding the role of Notch in 1. Weissman IL: Stem cells: units of development, units of regen- self-renewal of mammary stem cells, these findings suggest eration, and units in evolution. Cell 2000, 100:157-168. 2. Reya T, Morrison SJ, Clarke MF, Weissman IL: Stem cells, cancer, that carcinogenesis may involve the deregulation of proc- and cancer stem cells. Nature 2001, 414:105-111. esses involved in normal stem cell self-renewal. Elucidation 3. Smith GH, Chepko G: Mammary epithelial stem cells. Microsc Res Technique 2001, 52:190-203. of key elements of these pathways may identify new targets 4. Dontu G, Abdallah W, Foley J, Jackson K, Clarke M, Kawamura M, for cancer therapy [32]. Wicha M: In vitro propagation and transcriptional profiling of R614 Available online http://breast-cancer-research.com/content/6/6/R605

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