title: Decreased BRCA1 confers tamoxifen resistance in breast cancer cells by altering estrogen receptor-coregulator interactions.
abstract: The breast cancer susceptibility gene 1 (BRCA1) is mutated in approximately 50% of hereditary breast cancers, and its expression is decreased in 30-40% of sporadic breast cancers, suggesting a general role in breast cancer development. BRCA1 physically and functionally interacts with estrogen receptor-alpha (ERalpha) and several transcriptional regulators. We investigated the relationship between cellular BRCA1 levels and tamoxifen sensitivity. Decreasing BRCA1 expression in breast cancer cells by small interfering RNA alleviated tamoxifen-mediated growth inhibition and abolished tamoxifen suppression of several endogenous ER-targeted genes. ER-stimulated transcription and cytoplasmic signaling was increased without detectable changes in ER or ER coregulator expression. Co-immunoprecipitation studies showed that with BRCA1 knockdown, tamoxifen-bound ERalpha was inappropriately associated with coactivators, and not effectively with corepressors. Chromatin immunoprecipitation studies demonstrated that with tamoxifen, BRCA1 knockdown did not change ERalpha promoter occupancy, but resulted in increased coactivator and decreased corepressor recruitment onto the endogenous cyclin D1 promoter. Our results suggest that decreased BRCA1 levels modify ERalpha-mediated transcription and regulation of cell proliferation in part by altering ERalpha-coregulator association. In the presence of tamoxifen, decreased BRCA1 expression results in increased coactivator and decreased corepressor recruitment on ER-regulated gene promoters.
authors.affiliation: Department of Molecular Physiology and Biological Physics, University of Virginia, Charlottesville, VA 22908, USA.
authors.forename: J
authors.initials: J
authors.lastname: Wen
authors.forename: R
authors.initials: R
authors.lastname: Li
authors.forename: Y
authors.initials: Y
authors.lastname: Lu
authors.forename: M A
authors.initials: MA
authors.lastname: Shupnik
mesh_terms: D018931:Antineoplastic Agents, Hormonal / Q000494:pharmacology* / Q000627:therapeutic use; D019313:BRCA1 Protein / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism*; D001943:Breast Neoplasms / Q000188:drug therapy / Q000235:genetics / Q000378:metabolism*; D045744:Cell Line, Tumor; D019938:Cyclin D1 / Q000235:genetics / Q000378:metabolism; D019008:Drug Resistance, Neoplasm / Q000187:drug effects* / Q000235:genetics; D047628:Estrogen Receptor alpha / Q000235:genetics / Q000378:metabolism*; D005260:Female; D015972:Gene Expression Regulation, Neoplastic / Q000187:drug effects / Q000235:genetics; D006801:Humans; D009154:Mutation; D011401:Promoter Regions, Genetic / Q000235:genetics; D034741:RNA, Small Interfering / Q000235:genetics; D015398:Signal Transduction / Q000187:drug effects / Q000235:genetics; D013629:Tamoxifen / Q000494:pharmacology* / Q000627:therapeutic use; D014158:Transcription, Genetic / Q000187:drug effects / Q000235:genetics
chemical_list: D018931:Antineoplastic Agents, Hormonal; D019313:BRCA1 Protein; C492913:BRCA1 protein, human; C530104:CCND1 protein, human; C506487:ESR1 protein, human; D047628:Estrogen Receptor alpha; D034741:RNA, Small Interfering; D013629:Tamoxifen; D019938:Cyclin D1