SHORT COMMUNICATION Differential recruitment of nuclear receptor coregulators in ligand-dependent transcriptional repression by estrogen receptor-a KW Merrell, JD Crofts, RL Smith, JH Sin, KE Kmetzsch, A Merrell, RO Miguel, NR Candelaria and C-Y Lin 1 Department of Microbiology and Molecular Biology, Brigham Young University, Provo, UT, USA Estrogen receptors (ERs) are normally expressed in breast tissues and mediate hormonal functions during develop- ment and in female reproductive physiology. In the majority of breast cancers, ERs are involved in regulating tumor cell proliferation and serve as prognostic markers and therapeutic targets in the management of hormone- dependent tumors. At the molecular level, ERs function as ligand-dependent transcription factors and activate target- gene expression following hormone stimulation. Recent transcriptomic and whole-genome-binding studies suggest, however, that ligand-activated ERs can also repress the expression of a significant subset of target genes. To characterize the molecular mechanisms of transcriptional repression by ERs, we examined recruitment of nuclear receptor coregulators, histone modifications and RNA polymerase II docking at ER-binding sites and cis- regulatory regions adjacent to repressed target genes. Moreover, we utilized gene expression data from patient samples to determine potential roles of repressed target genes in breast cancer biology. Results from these studies indicate that nuclear receptor corepressor recruitment is a key feature of ligand-dependent transcriptional repression by ERs, and some repressed target genes are associated with disease progression and response to endocrine therapy. These findings provide preliminary insights into a novel aspect of the molecular mechanisms of ER functions and their potential roles in hormonal carcino- genesis and breast cancer biology. Oncogene (2011) 30, 1608–1614; doi:10.1038/onc.2010.528; published online 22 November 2010 Keywords: estrogen receptor; transcriptional regula- tion; hormonal carcinogenesis; coregulators Estrogens and their cellular receptors play key roles during development and in reproductive physiology and endocrine-related diseases, including breast cancers (Gruber et al., 2002; Yager and Davidson, 2006). Two estrogen receptor (ER) subtypes, ERa and ERb, have been identified and exhibit differential target tissue distribution and functions. In the majority of breast cancers, ERa is expressed in tumor cells and is involved in promoting tumor cell proliferation and invasiveness (Ali and Coombes, 2000). The role of ERb in breast cancer biology remains to be determined, although some findings suggest that it may attenuate ERa functions (Lindberg et al., 2003; Cappelletti et al., 2004; Lin et al., 2007a). ER status is an important prognostic marker in breast cancer management and a determinant for treatment with selective estrogen receptor modulators. Treatment with selective estrogen receptor modulators has also been shown to be effective in reducing the risk of breast cancer (Fisher et al., 1998, 2005; Vogel et al., 2006). However, inhibiting estrogen or ER functions elicits menopausal symptoms in premenopausal women and negates other beneficial effects, such as bone maintenance and cardiovascular protection. Long-term application of endocrine therapy may result in drug resistance and hormone-independence in tumor cells (Bookman, 2005). Overcoming these challenges will necessitate a more complete understanding of the molecular mechanisms of ER function. ER is a nuclear receptor and affects target-gene expression directly as a ligand-dependent transcription factor (Nilsson and Gustafsson, 2002). Upon activation, ER upregulates target-gene expression by binding cis-regulatory estrogen response elements (EREs) and nucleating coregulator complexes that modify histones and render the DNA accessible to the transcriptional machinery (Shang et al., 2000; Metivier et al., 2003). ER coregulators include those which enhance transcrip- tional activity by affecting nucleosome spatial orienta- tioin or by modifying histones through acetylation (SRC1, CBP/p300, p/CAF and p/CIP/AIB1) and methylation (CARM1 and PRMT1) (Halachmi et al., 1994; Onate et al., 1995; Lin et al., 1996; Ogryzko et al., 1996; Wang et al., 1996, 2001; Anzick et al., 1997; Chen et al., 1999; Phelan et al., 1999; Sterner and Berger, 2000). Coregulators, such as NCoR, SMRT, NRIP1, LCoR and REA, function as nuclear receptor corepres- sors, and have been shown to bind ER following selective estrogen receptor modulator treatments, but their roles in normal ER transcriptional activity are not Received 7 January 2010; revised 5 October 2010; accepted 6 October 2010; published online 22 November 2010 Correspondence: Dr C-Y Lin, Department of Microbiology and Molecular Biology, Brigham Young University, 753 WIDB, Provo, UT 84602, USA. E-mail: clin23@central.uh.edu 1 Current address: Center for Nuclear Receptors and Cell Signaling, Department of Biology and Biochemistry, University of Houston, Houston, TX 77204, USA. Oncogene (2011) 30, 1608–1614 & 2011 Macmillan Publishers Limited All rights reserved 0950-9232/11 www.nature.com/onc