Abstract GMM-038: DECIPHERING THE ROLE OF ESTROGEN RECEPTOR ALPHA IN OVARIAN CANCER

Ovarian cancer is the leading cause of death among gynecologic malignancies. Despite years of research, the 5-year survival rate for ovarian cancer patients has remained poor and the standard of care is largely the same as it was decades ago. Epidemiological evidence and previous studies have suggested a role for steroid hormones in the pathogenesis of ovarian cancer, however, the role of steroid hormones in ovarian cancer remains understudied. We hypothesized that Estrogen Receptor α (ERα) is transcriptionally active and drives a transcriptional program that is sufficient to promote ovarian cancer cell proliferation and survival in a subset of ER+ ovarian cancers. Preliminary data generated in our laboratory has suggested that estradiol (E2) treatment increases cell proliferation in PEO1 cells, an ERα+ ovarian cancer cell line. This increase in cell proliferation can be inhibited by co-administration with the selective estrogen receptor modulator, Tamoxifen, and the selective estrogen receptor degrader, Fulvestrant. To further interrogate the mechanism of action of ERα in ovarian cancer, we performed RNA-seq on PEO1 cells treated with Vehicle, 10 nM E2, 100 nM Fulvestrant, and E2+Fulvestrant for 24 hours. We identified 659 significantly differentially expressed genes following E2 treatment. Fulvestrant inhibited the majority of E2-induced differentially expressed genes, confirming that these genes are dependent upon ERα. Gene Set Enrichment Analysis (GSEA) indicated that the Hallmark early and late estrogen responses are enriched in the E2 dataset, confirming that classical ER activity is intact in these cells. Furthermore, we identified G2/M checkpoint as positively enriched in our dataset, indicating that proliferation genes are upregulated by ERα in these cells. Additionally, we found that apoptosis was negatively enriched in our dataset, indicating that apoptosis pathway genes are suppressed by E2 treatment. To further characterize the transcriptional role of ERα in ovarian cancer cells, ChIP-seq was performed on PEO1 cells treated with Vehicle, E2, Tamoxifen, or E2+Tamoxifen for 45 minutes. E2 treatment robustly increased ER recruitment to its regulatory regions (25,717 binding sites) when compared to Vehicle (8,370 binding sites) or Tamoxifen (18,935 binding sites) alone. Motif analysis of these ERα binding sites demonstrated a significant enrichment in members of the AP-1 transcription factor family, but not in known ERα cofactors FOXA1 and GATA3. FOXA1 and GATA3 are important regulators of ERα-dependent transcription in breast cancer cells; our results may suggest that mechanisms governing ERα transcriptional activity are lineage dependent. Finally, Binding and expression target analysis (BETA) was performed in order to predict the activating and repressing function of ERα. This analysis demonstrated that ERα has significant repressive function in PEO1 cells, indicating that ER may be acting as a strong transcriptional repressor in ovarian cancer cells. Future investigation in this project will center on the role of AP-1 in mediating ERα-dependent transcription. Additionally, we will use CRISPR/Cas9 screening to determine genetic sensitizers to ER antagonists. We believe that these studies will provide additional drug targets that may suggest a combinatorial therapeutic approach in conjunction with endocrine therapies for the treatment of ovarian cancer. Citation Format: Irene I Lee, Myles Brown. DECIPHERING THE ROLE OF ESTROGEN RECEPTOR ALPHA IN OVARIAN CANCER [abstract]. In: Proceedings of the 12th Biennial Ovarian Cancer Research Symposium; Sep 13-15, 2018; Seattle, WA. Philadelphia (PA): AACR; Clin Cancer Res 2019;25(22 Suppl):Abstract nr GMM-038.