Abstract MIP-054: ESTROGEN RECEPTOR BETA – P53 – FOXM1 SIGNALING AXIS AS A NOVEL MOLECULAR TARGET TO OVERCOME THERAPEUTIC RESISTANCE IN HIGH–GRADE SEROUS OVARIAN CANCER

The high–grade serous ovarian carcinoma (HGSOC), the most aggressive ovarian cancer subtype, accounts for two–thirds of ovarian cancer deaths. It is a poorly understood disease with high therapeutic resistance and extremely poor prognosis. The current study is an attempt to analyze molecular mechanisms underlying HGSOC and to identify new therapeutic targets. The Cancer Genome Atlas (TCGA) analysis of ovarian carcinoma demonstrated that p53 mutations are near universal (>90%) in HGSOC and forkhead box M1 protein (FOXM1) network is significantly altered in about 87% of HGSOC leading to transcriptional upregulation of proliferation–related target genes. Moreover, mutant p53 and FOXM1 have important roles in resistance to chemotherapy. In the present study, we have used various experimental procedures such as RNAi technology, immunoprecipitation, quantitative real–time PCR, immunoblot analysis, proximity ligation assay (PLA) (to determine protein–protein interaction in situ) and assays for cell cycle, apoptosis and proliferation to analyze estrogen receptor beta (ER–beta) – mutant p53 –FOXM1 signaling crosstalk in OVCAR3 (HGSOC) cells. Knocking down ER–beta resulted in increased mut–p53 transcript levels suggesting transcription of mut–p53 is repressed by ER–beta in OVCAR cells. Similarly, mut–p53 represses transcription of ER–beta gene. We show that not only mut–p53 and ER–beta physically interact, but also they mutually regulate each other9s expression at the gene transcriptional level leading to changes in expression of FOXM1 and its downstream signaling targets such as . We show that when ER–beta was knocked down, FOXM1 levels were decreased and resistance to cisplatin and carboplatin, major therapeutic agents currently in use for ovarian cancer, was decreased as evidenced by increased apoptosis and decreased colony formation. Surprisingly, ER–beta binds to both p53 and FOXM1. Our data provides new insight into the mechanisms underlying HGSOC by illustrating how signaling crosstalk between ER–beta and mut–p53 leads to increased expression of pro–proliferative and pro–tumorigenic downstream targets such as FOXM1, overexpression of which has been associated with resistance to therapeutic agents.in HGSOC patients. Our study will be the first, to the best of our knowledge, to investigate whether the crosstalk between ER–beta and mut–p53 along with FOXM1 plays an important role in the onset and progression of HGSOC. We have identified ER–beta–mutant p53–FOXM1 signaling axis as a novel therapeutic target in HGSOC. Correlation between ER–beta, mut–p53, and FOXM1 expression, their interaction, and disease characteristics are being analyzed in HGSOC patient tissue microarrays (TMAs). This study was supported by a Pilot Study Award from the Rivkin Center for Ovarian Cancer. Citation Format: Chetan Oturkar and Gokul M. Das. ESTROGEN RECEPTOR BETA – P53 – FOXM1 SIGNALING AXIS AS A NOVEL MOLECULAR TARGET TO OVERCOME THERAPEUTIC RESISTANCE IN HIGH–GRADE SEROUS OVARIAN CANCER [abstract]. In: Proceedings of the 11th Biennial Ovarian Cancer Research Symposium; Sep 12-13, 2016; Seattle, WA. Philadelphia (PA): AACR; Clin Cancer Res 2017;23(11 Suppl):Abstract nr MIP-054.