Loss of nuclear progesterone receptors (PR) and low circulating progesterone levels are associated with increased ovarian cancer (OC) risk. However, PR are abundantly expressed in a significant percentage of serous and endometrioid ovarian tumors; patients with PR+ tumors typically experience longer progression-free survival relative to those with PR-null tumors. The molecular mechanisms of these protective effects are poorly understood. To study PR action in OC in the absence of added estrogen (i.e., needed to induce robust PR expression), we created ES-2 OC cells stably expressing vector control or GFP-tagged PR-B (GFP-PR). Progestin (R5020) stimulation of ES-2 cells stably expressing GFP-PR induced cellular senescence characterized by altered cellular morphology, prolonged survival, senescence-associated β-galactosidase activity, G1 cell cycle arrest and upregulation of the cell cycle inhibitor, p21, as well as the Forkhead-box transcription factor, FOXO1; these results repeated in unmodified ER+/PR+ PEO 4 OC cells. PR-B and FOXO1 were detected within the same PRE-containing regions of the p21 upstream promoter. Knockdown of p21 resulted in molecular compensation via FOXO1-dependent upregulation of numerous FOXO1 target genes (p15, p16, p27) and an increased rate of senescence. Inhibition of FOXO1 (with AS1842856) or stable FOXO1 knockdown inhibited progestin-induced p21 expression and blocked progestin-induced senescence. Overall, these findings support a role for PR as a tumor suppressor in OC cells, which exhibits inhibitory effects by inducing FOXO1-dependent cellular senescence. Clinical "priming" of the PRFOXO1-p21 signaling pathway using PR agonists may provide a useful strategy to induce irreversible cell cycle arrest and thereby sensitize OC cells to existing chemotherapies as part of combination "two-step" therapies.
Bibliographical noteFunding Information:
We would like to thank Dr Patricia Kruk (University of South Florida), Dr Amy Skubitz (University of Minnesota), Dr Sundaram Ramakrishnan (University of Minnesota) and Dr Scott Kaufmann (Mayo Clinic) for kindly providing cell lines utilized in this study. We would like to thank members of the University of Minnesota Masonic Cancer Center’s Flow Cytometry Core Facility and the University Imaging Centers for their assistance in data acquisition. This study was supported by NIH grant R01 CA159712 (to C.A.L.), the Minnesota Ovarian Cancer Alliance (to C.A.L.), Cancer Biology Training Grant NIH T32 CA009138 (to C.H.D.) and National Center for Advancing Translational Sciences of the National Institutes of Health Award UL1TR000114 (to C.H.D).
- Breast cancer
- Forkhead transcription factor
- Ovarian cancer
- Progesterone receptor