Transcriptional Axis of EZH2-ERα-GREB1 Regulates Tamoxifen Resistance in Breast Cancer

WU, YANMING; PROIETTI, CECILIA JAZMÍN; ELIZALDE, PATRICIA VIRGINIA; CENCIARINI, MAURO EZEQUIEL; LIAO, YIJI; ZHANG, ZHAO; YANG, MEI; XU, KEXIN

Austin

Conferencia; Innovations in Cancer Prevention and Research Conference; 2017

Cancer Prevention and Research Institute of Texas

Introduction: The ef cacy of endocrine therapy in estrogen receptor (ER)-positive breast cancer is often limited by either intrinsic or acquired resistance. Emerging evidence suggests that epigenetic alterations contribute to the development of endocrine resistance. Enhancer of zeste homolog 2 (EZH2), a histone methyltransferase, is frequently overexpressed in breast cancer and strongly associated with aggressive phenotypes of cancer cells. However, the involvement of EZH2 in endocrine resistance remains poorly explored. Here we discovered a critical transcriptional axis consisting of EZH2, ERα and its cofactor GREB1 in driving tamoxifen resistance (TamR). Methods: Association between EZH2 and tamoxifen resistance was investigated bioinformatically, qRT-PCR, Western blot, cell proliferation assays and immunohistochemistry (IHC) staining. RNA- seq and ChIP-qPCR were applied to con rm epigenetic reprogramming of speci c transcriptomes to favor the endocrine resistant phenotypes. Xenograft models were established to further validate the ef cacy of EZH2 inhibitors in vivo. Results: Our studies demonstrated that higher EZH2 levels are associated with poorer response to tamoxifen in breast cancer patients. EZH2 represses the expression of the ERα cofactor GREB1 by maintaining DNA hypermethylation of a particular CpG-enriched region at the GREB1 promoter, which is negatively correlated with GREB1 levels in clinical specimens and highly associated with cell sensitivity to endocrine treatment. We also revealed a novel function of GREB1 in ensuring proper cellular responses to different ligands by recruiting distinct sets of ERα cofactors to cis-regulatory elements. This explains the opposing biological effects of GREB1 on breast cancer cell growth in response to estrogen or anti-estrogen. EZH2-dependent repression of GREB1 in hormone refractory cells results in chromatin reallocation of ERα coregulators, converting the anti-estrogen into an agonist. Levels of EZH2 and GREB1 are negatively correlated in clinical samples from patients receiving adjuvant tamoxifen treatment, and together predict response to endocrine therapy. Conclusions: Our work provides insights into an epigenetic mechanism of endocrine therapy resistance and a potential novel therapeutic strategy to overcome tamoxifen resistance in aggressive breast cancer.