Progesterone Receptor Isoform Signaling In Ovarian Cancer Cellular Senescence

University of Minnesota Ph.D. dissertation. August 2015. Major: Microbiology, Immunology and Cancer Biology. Advisor: Carol Lange. 1 computer file (PDF); x, 120 pages.As the deadliest of all gynecologic malignancies, ovarian cancer has a death rate of more than 50% due to late detection and diagnosis of the disease and intrinsic or acquired resistance to current therapeutic regimens. The identification of robust biomarkers for early detection will have a substantial impact on survival rates, while prognostic molecular markers may allow for efficacious targeted therapeutic strategies. Progesterone plays a pivotal role in the development and progression of hormone-regulated tumors. In the breast, progesterone promotes a proliferative, pro-survival response, but inhibits growth in the uterus and ovary. The opposing biology in ovarian versus breast cancer cells may be largely dependent on cell context. The paradoxical effects of progesterone observed in ovarian relative to breast cancer cells may be attributed to actions of the nuclear progesterone receptor (PR) and its isoforms, PR-B and PR-A and their relative regulation (i.e. by post-translational modifications and co-factor binding partners), differential cross-talk between PR and growth factor-mediated signaling pathways (i.e. protein kinases), and/or altered expression levels in the target cells. In contrast to breast cancer, the detailed mechanisms of progesterone action in ovarian cancer are poorly understood. The goals of our studies were to determine the level of PR isoform expression in primary and cell models of ovarian cancer, to define the biological consequences of PR expression and activity on ovarian cancer cell biology, and to identify the key cofactor(s) required for mediating PR-dependent signaling actions. We demonstrated that ligand-activated PR-B induced a non-proliferative cell fate, known as cellular senescence, through a FOXO1-dependent mechanism in ovarian cancer cells. PR-B and FOXO1 were co-recruited to the same PRE-containing region of the upstream promoter of p21 upon progestin (R5020) treatment. Both proteins are required to cooperatively activate p21 expression based on data from PR-null control cells and lentiviral-delivered shRNA FOXO1-knockdown studies. Stable knock-down of FOXO1 inhibited progestin-induced p21 expression in ES-2 cells stably expressing GFP-tagged PR-B and blocked PR-dependent cellular senescence. Next, we investigated the biological consequences of PR isoform-specific gene regulation in ovarian cancer models, as well as directly compared PR isoform-selective transcriptomes between ovarian and breast cancer models. In ovarian cancer cells, PR-A is relatively insensitive to hormone, and PR-B (but not PR-A) is capable of inducing FOXO1 and p21 expression required for progestin-mediated cellular senescence in ovarian cancer cells. Furthermore, our studies revealed FOXO1 as a critical cofactor and determinant for the regulation of PR hormone sensitivity. Namely, activated FOXO1 confers PR-B-like behavior to PR-A, and in the presence of FOXO1, PR-A trans-activates classical PR-B target genes and induces robust cellular senescence. Finally, we utilized a novel ex vivo explant culture system of human primary ovarian tumors that recapitulated our in vitro findings. Significantly, identifying the mechanisms governing PR-A versus PR-B specific gene regulation driven by FOXO1 may provide a means to promote PR-B driven cellular senescence in ovarian cancer and provide clues to inducing the protective actions of PR-A in other hormone-driven cancers, such as breast and uterine. PR-targeted strategies could provide a safe and useful means to improve treatment outcomes and increase overall ovarian cancer patient survival