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NKX3.1 Homeodomain Protein Binds to Topoisomerase I and Enhances Its Activity

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NKX3.1 Homeodomain Protein Binds to Topoisomerase I and Enhances Its Activity Research Article NKX3.1 Homeodomain Protein Binds to Topoisomerase I and Enhances Its Activity Cai Bowen,1 August Stuart,1 Jeong-Ho Ju,1 Jenny Tuan,1 Josip Blonder,2 Thomas P. Conrads,2 Timothy D. Veenstra,2 and Edward P. Gelmann1 1Departments of Oncology and Medicine, Lombardi Comprehensive Cancer Center, Georgetown University, Washington, District of Columbia and 2Laboratory of Proteomics and Analytical Technologies, Science Applications International Corporation-Frederick, Inc., National Cancer Institute at Frederick, Frederick, Maryland Abstract expression is reduced by an average of 30% from normal, indicating The prostate-specific homeodomain protein NKX3.1 is a that some compensatory expression occurs in response to allelic loss (10). Nearly complete loss of NKX3.1 expression occurs with tumor suppressor that is commonly down-regulated in human f prostate cancer. Using an NKX3.1 affinity column, we isolated tumor progression, such that 80% of metastatic lesions have topoisomerase I (Topo I)from a PC-3 prostate cancer cell no detectable expression of NKX3.1 (3). A missense mutation in extract. Topo I is a class 1B DNA-resolving enzyme that is the NKX3.1 homeodomain that reduced NKX3.1 DNA-binding ubiquitously expressed in higher organisms and many capacity caused predisposition to early prostate cancer in one prokaryotes. NKX3.1 interacts with Topo I to enhance for- family (11). mation of the Topo I-DNA complex and to increase Topo I Gene targeting studies in mice showed that Nkx3.1 haploinsuffi- cleavage of DNA. The two proteins interacted in affinity pull- ciency can predispose to prostate epithelial dysplasia and can down experiments in the presence of either DNase or RNase. cooperate with other oncogenic mutations to augment carcino- The NKX3.1 homeodomain was essential, but not sufficient, genesis (2, 12). Haploinsufficiency of Nkx3.1 is accompanied by for the interaction with Topo I. NKX3.1 binding to Topo I decreased expression of genes under the regulation of the Nkx3.l homeoprotein (13). Unlike human prostate cancer, Nkx3.1 occurred independently of the Topo I NH2-terminal domain. The binding of equimolar amounts of Topo I to NKX3.1 caused protein expression is lost in the earliest preinvasive lesions in displacement of NKX3.1 from its cognate DNA recognition murine prostate glands despite allelic retention and persistent +/À sequence. Topo I activity in prostates of Nkx3.1 and expression of mRNA (14). Unlike most human tumor suppressor À/À Nkx3.1 mice was reduced compared with wild-type mice, genes that undergo biallelic disruption to produce complete loss whereas Topo I activity in livers, where no NKX3.1 is expressed, of suppressor function, NKX3.1 is activated by partial down- was independent of Nkx3.1 genotype. Endogenous Topo I and regulation (10). NKX3.1 could be coimmunoprecipitated from LNCaP To understand the mechanism by which reduced NKX3.1 cells, where NKX3.1 and Topo I were found to colocalize in expression can contribute to prostate carcinogenesis and tumor the nucleus and comigrate within the nucleus in response progression, we have sought to understand the function of the to either ;-irradiation or mitomycin C exposure, two DNA- protein in the context of known mechanisms of molecular damaging agents. This is the first report that a homeodomain carcinogenesis. Homeodomain proteins bind both to DNA and to protein can modify the activity of Topo I and may have other proteins and determine gene expression in a time- and implications for organ-specific DNA replication, transcription, location-dependent manner. For example, NKX3.1 binds to its or DNA repair. [Cancer Res 2007;67(2):455–64] cognate DNA-binding sequence, whereby it down-regulates tran- scription (15), and also binds to serum response factor, whereby it Introduction activates transcription of genes downstream of serum response elements (16). To characterize the functional spectrum of NKX3.1 NKX3.1 is a prostate-specific protein that is a member of the NK in cells of prostate origin, we isolated proteins from a cellular family of homeodomain proteins that include several organ-specific extract that bound to NKX3.1. One of the proteins found to bind differentiation factors (1). NKX3.1 is expressed almost exclusively in NKX3.1 was topoisomerase (Topo) I. developing and mature prostate luminal epithelium (2–4). The Topo I is a DNA-resolving enzyme that participates in a wide NKX3.1 gene is located on chromosome 8p21.2 and is a target for range of functions, including transcription (17, 18), DNA replication loss of heterozygosity in the majority of human prostate cancers (19, 20), and DNA repair (21). Human Topo I is a type IB enzyme, (5–7). The contralateral NKX3.1 allele remains intact, and somatic indicating that after binding, it cleaves a single DNA strand where, mutations have not been found in human prostate cancer (8, 9). In in the course of catalysis, a covalent bond between a tyrosine and preinvasive and early-stage human prostate cancer, NKX3.1 the 3¶ phosphate in the cleaved DNA strand is formed (19). After single-strand cleavage, Topo I unwinds the DNA, religates the broken strand, and releases, all in an ATP-independent reaction. Topo I has been shown to play a role in repair of UV-induced DNA Note: Supplementary data for this article are available at Cancer Research Online damage and enhanced cell survival after UV irradiation (22). Topo I (http://cancerres.aacrjournals.org/). has also been shown to bind other tumor suppressor proteins, Requests for reprints: Edward P. Gelmann, Departments of Oncology and ARF Medicine, Lombardi Comprehensive Cancer Center, Georgetown University, 3800 such as p53 (23) and p14 (24). Here, we describe the first known Reservoir Road Northwest, Washington, DC 20007-2197. Phone: 202-444-2207; Fax: 202- interaction of a homeodomain protein with Topo I. This report 444-1229; E-mail: [email protected]. I2007 American Association for Cancer Research. focuses on the interaction of the proteins in vitro and in cells. doi:10.1158/0008-5472.CAN-06-1591 We found that NKX3.1 markedly enhanced the formation of the www.aacrjournals.org 455 Cancer Res 2007; 67: (2). January 15, 2007 Downloaded from cancerres.aacrjournals.org on September 25, 2021. © 2007 American Association for Cancer Research. Cancer Research Topo I-DNA complex and resultant DNA cleavage. We speculate polyacrylamide denaturing gels with 7 mol/L urea. After autoradiography, that the effect of NKX3.1 on Topo I may have an effect on the ability quantification of relative DNA cleavage was done with the Scion Image of the cell to repair DNA or to respond to stress. program. Religation reactions were initiated in the presence of a 1,000-fold excess (11 mer) acceptor oligonucleotides for an additional 60 min at 37jC. When cleavage preceded religation, cleavage was arrested with 500 mmol/L Materials and Methods NaCl and no SDS was used before the religation reaction. Cell culture. The human prostate cell lines PC-3 and LNCaP and human Electromobility shift assays. The Topo electromobility shift assay kidney cell line 293Twere cultured in modified IMEM (Invitrogen, Grand Island, (EMSA) probe was a 52-mer double-stranded oligonucleotide of the j ¶ NY) supplemented with 5% fetal bovine serum (FBS) at 5% CO2 and 37 C. sequence 5 -TCTAGAGGATTTCGAAGACTTAGAGAAATTTCGAAGATC- Affinity chromatography. Maltose-binding protein (MBP) and MBP- CCCGGGCGAGCTC-3¶. The NKX3.1 recognition sequence was a 15-mer NKX3.1 fusion proteins (3) expressed in Escherichia coli BL21 were bound to double-stranded oligonucleotide having the sequence 5¶-GTATATAAG- amylose resin beads (NEB, Beverly, MA). Protein-loaded beads were washed TAGTTG-3¶ (15). Oligonucleotides were 5¶ end labeled with T4 polynucle- with column buffer containing 20 mmol/L Tris-HCl, 200 mmol/L NaCl, and otide kinase in the presence of g-[32P]ATP and purified with the QIAquick 1 mmol/L EDTA. Crude lysates from PC-3 cells were obtained by treating Nucleotide Removal kit. Reactions were carried out with the indicated cells with 20 mmol/L Tris-HCl, 1% Triton X-100, 10% glycerol, 137 mmol/L amounts of Topo I and NKX3.1 at room temperature for 15 min in buffer NaCl, 1.5 mmol/L MgCl2, 1 mmol/L EDTA, 50 mmol/L NaF, 1 mmol/L containing 10 mmol/L Tris-HCl (pH 7.4), 40 mmol/L NaCl, 7.5 mmol/L Na3VO4, and 1 mmol/L phenylmethylsulfonyl fluoride (PMSF). Lysates were MgCl2, 1 mmol/L EDTA, 5% glycerol, 0.1% Triton X-100, 5% sucrose, 0.1% added to pre-equilibrated MBP or MBP-NKX3.1–loaded amylose beads. bovine serum albumin (BSA), 5 mmol/L DTT, 5 ng/AL deoxyinosine-deoxy- After incubation and exhaustive washing, retained proteins were then thymidine. Samples were incubated 15 min after addition of 1.5 nmol/L of released by boiling and analyzed by SDS-PAGE. Selected Coomassie-stained labeled probe. Samples were resolved in 8% native polyacrylamide gels and bands were subjected to in-gel digestion using trypsin, and the resultant visualized by autoradiography. A timed competition assay was done with peptides were analyzed using a capillary column liquid chromatography end-labeled oligonucleotide with the NKX3.1 binding motif incubated for (LC)-microelectrospray mass spectrometry (MS) system using a LCQ Deca 15 min with NKX3.1 at room temperature. Topo I or ovalbumin was then XP ion trap mass spectrometer (Thermo Finnigan, San Jose, CA) with added and incubated for different amounts of time. Samples were resolved Agilent 1100 microcapillary LC system (Palo Alto, CA). The peptides are in 8% native gel and visualized by autoradiography. analyzed using the data-dependent multitask capability of the instrument Coimmunoprecipitation of Topo I and NKX3.1. Nuclear extract was acquiring full scan mass spectra to determine peptide molecular weights isolated from 108 LNCaP cells cultured in modified IMEM with 10% FBS.
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