Abstract 4675: GR activation modifies ER transcriptional activity and results in decreased estrogen-mediated breast cancer cell proliferation

Proceedings: AACR 106th Annual Meeting 2015; April 18-22, 2015; Philadelphia, PA Approximately 75% of breast cancers (BC) express estrogen receptor-alpha (ER). The proliferative and anti-apoptotic actions of estrogen signaling in ER+ BC are well-established. Our laboratory recently reported that approximately 40% of ER+ breast cancers express significant glucocorticoid receptor (GR) mRNA; a meta-analysis of early-stage ER+ BC gene expression unexpectedly found that patients with tumors expressing high GR had significantly improved relapse-free survival (p-value 7.8E-8). Consistent with the improved outcome observed in ER+/GR+ BC patients, our laboratory and others have reported that GR activation can alter ER-mediated transcription and also antagonize estradiol (E2)-mediated cell proliferation. We therefore hypothesized that GR activation directly antagonizes the expression of ER-mediated pro-proliferative genes. Recapitulating observations reported by others, we first performed cell cycle analysis and found that the synthetic glucocorticoid dexamethasone (Dex) decreased E2-mediated proliferation of MCF-7 BC cells. To understand how GR activation inhibits ER-mediated pro-proliferative activity in ER+ BC cells, we performed chromatin immunoprecipitation followed by high-throughput sequencing (ChIP-seq) to examine global GR chromatin occupancy in MCF-7 cells in the presence or absence of ER activation (+/- E2 at 10 nM). Surprisingly, in GR ChIP experiments we observed that in the presence of Dex, E2 treatment resulted in significantly increased GR association with ER response elements (EREs). Likewise, ER ChIP experiments showed that in the presence of E2, Dex treatment significantly increased the association of ER with GR response elements (GREs). Using genome-wide gene expression array analyses of MCF-7 cells, the gene expression signatures resulting from Dex and E2 co-treatment compared to E2 treatment alone are being examined. In analyses done to date, we found that in the presence of E2, Dex treatment inhibits the induction of ER-mediated pro-proliferative genes related to the cell cycle such as CCND1 (Cyclin D1) and CDC6. Furthermore, microarray analysis suggested that E2-mediated ER activation represses pro-apoptotic CASP7 and BIK gene expression, while co-treatment with Dex inhibits this repression. Additionally, anti-GR ChIP-seq analysis showed GR enrichment at CASP7 regulatory regions upon co-treatment with Dex and E2. Together, these data suggest that GR expression and subsequent GR activation may inhibit ER-mediated pro-proliferative gene expression as well as inhibit ER-mediated repression of anti-apoptotic genes through direct GR-chromatin interaction. These experiments suggest that GR activation can be an important component modulating ER activity in ER+/GR+ BC. Citation Format: Diana C. West, Charles F. Pierce, Deng Pan, Kyle Hernandez, Masha Kocherginsky, Suzanne D. Conzen. GR activation modifies ER transcriptional activity and results in decreased estrogen-mediated breast cancer cell proliferation. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 4675. doi:10.1158/1538-7445.AM2015-4675