SUN-004 Unique Cell Fates and Metastatic Phenotypes Are Activated by Progesterone Receptor (PR) Isoform-specific Gene Expression in Fallopian Tube Models of HGSC

High grade serous ovarian carcinoma (HGSC), the most prevalent and aggressive form of ovarian cancer, contains abundant receptors for the ovarian steroid hormones, estrogen (ER; 76%) and progesterone (PR; 35%). These receptors are known to contribute to breast and reproductive cancer development. Our understanding, however, of the mechanisms of PR isoform action in the initiation and progression of ovarian cancer is limited. HGSC is thought to originate from the fallopian tube epithelium (FTE), and accumulating evidence suggests that serous tubal intraepithelial carcinomas (STICs) are precursor lesions to most HGSCs. Our IHC analysis of normal FTE and STICs showed expression of both total PR and activated phopho-Ser294 PR (p-PR). Interestingly, STIC lesions had greater intensity of focal nuclear p-PR, indicative of highly active transcriptional complexes. To investigate PR isoform signaling in early stage HGSC, we utilized p53-dominant negative mutant FTE cells to generate stable cell lines expressing either the PR-A or PR-B isoform. Progestin (progesterone; R5020) treatment of 2D adherent cultures revealed functional PR signaling through MAPK-dependent p-PR as well as isoform specific expression of PR target genes encoding adhesion molecules (e.g. HEF1), cell cycle regulators (e.g. FOXO1), and glucocorticoid signaling proteins (e.g. CRISPLD2, NDRG1). Progestin treatment also dramatically inhibited proliferation which was recovered following growth factor stimulus, suggesting that PR signaling may promote cell quiescence (G0). To mimic dissemination of FTE cells from STICs, a 3D spheroid model was established. Interestingly, we observed that PR expression and activation greatly increased the formation, size and number of total spheroids compared to PR negative controls. Cells within these spheroids are predominantly Ki67-negative indicating that they are likely non-proliferative and potentially G0-arrested cells. When the spheroids were embedded into collagen, to mimic tumor cell invasion into the peritoneum, only PR+ spheroids exhibited invasive behavior. Additionally, PR-B+ spheroids were more invasive than PR-A+ spheroids; progestin treatment during 3D formation dramatically increased spheroid invasion. Notably, this is the opposite of our results in 2D cultures, where PR-A+ FTE cells display greater proliferative and migratory phenotypes relative to PR-B+ FTE cells. Taken together, our data suggest that activation of PR signaling promotes and enhances the formation and survival of non-adherent 3D FTE structures. Such effects of progesterone are predicted to influence the shedding, aggregation and dissemination of early STIC lesions that form ovarian and peritoneal metastases. Our findings demonstrate the importance of understanding the impact of steroid hormone receptors, including PR isoforms, on early ovarian cancer development.