Abstract 3435: Characterizing the dependence of AP-1 transcription factors on Estrogen Receptor Alpha transcriptional activity and Fulvestrant sensitivity in ovarian cancer cells

Ovarian cancer is the leading cause of death among gynecologic malignancies. Epidemiological evidence has suggested a role for steroid hormones in the pathogenesis of ovarian cancer, however, their precise role in ovarian cancer remains unknown. We hypothesized that Estrogen Receptor α (ERα) drives a transcriptional program that is sufficient to promote ovarian cancer cell proliferation in ER+ ovarian cancers. Preliminary data generated in our laboratory has suggested that estradiol (E2) treatment increases cell proliferation in PEO1 and OVKATE cells, ERα+ ovarian cancer cell lines. 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 4 and 24 hours. We identified 86 significantly differentially expressed genes following 4 hr E2 treatment and 659 significantly differentially expressed genes following 24 hr 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 and G2/M checkpoint as positively enriched in our dataset, indicating that classical ER pathways are intact in these cells. 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 when compared to Vehicle or Tamoxifen 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. Furthermore, using GIGGLE analysis to identify similar ChIP-seq datasets to the PEO1 dataset, we identified FOSL1, FOSL2, JUND, JUN, and FOS as among the datasets with the highest similarity scores to the E2 PEO1 dataset. In order to identify potential combinatorial endocrine therapies for ovarian cancer, we performed a genome-wide CRISPR screen in the presence of Fulvestrant in PEO1 cells. Among the top positively selected genes in the presence of Fulvestrant was the AP-1 family member, FOSL2, indicating that loss of FOSL2 contributes to Fulvestrant resistance and further confirmed the role of AP-1 factors in estrogen function. We also identified genes that confer sensitivity to Fulvestrant treatment; the E2F, PI3K/mTOR, and steroid hormone biosynthesis pathways are enriched in the Fulvestrant sensitizer genes. Further analysis is currently underway to determine synergistic combinations with Fulvestrant for the treatment of ovarian cancer. Citation Format: Irene I. Lee, Myles Brown. Characterizing the dependence of AP-1 transcription factors on Estrogen Receptor Alpha transcriptional activity and Fulvestrant sensitivity in ovarian cancer cells [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 3435.