Progesterone Enhances the Sensitivity of Ovarian Cancer Cells to Poly (ADP-Ribose) Polymerase (PARP) Inhibitors by Suggesting a Role for Transcription-Replication Conflict-Related Pathways: An In Vitro Study.

OBJECTIVE: Ovarian cancer is often diagnosed at an advanced stage with peritoneal dissemination and ascites. Despite initial chemosensitivity, most patients eventually relapse. Poly (ADP-ribose) polymerase (PARP) inhibitors have become important maintenance therapies, particularly for tumors with homologous recombination deficiencies. Transcription-replication conflicts (TRCs) are increasingly recognized as a key mechanism related to PARP inhibitor-induced cytotoxicity. Progesterone exerts rapid non-genomic effects via membrane progesterone receptors (mPRs), suppresses topoisomerase I (TOPO-I), and enhances irinotecan cytotoxicity in ovarian cancer cells. We hypothesized that combining progesterone with PARP inhibitors could enhance antitumor effects by modulating TRC-protective pathways.

METHODS: The BRCA1/2 wild-type ovarian cancer cell line SHIN-3 (PR-negative and mPR-positive), which is considered resistant to PARP inhibitors, was treated with progesterone (100-400 μM) and three PARP inhibitors (niraparib, olaparib, and AZD2461). Cell viability was assessed using a colorimetric assay to determine IC50 values. Transcriptional activity was transiently inhibited using 5,6-dichloro-1-β-D-ribofuranosyl benzimidazole (DRB, a transcription elongation inhibitor), which was used as a tool to probe TRC dependence, acknowledging its pleiotropic effects. Quantitative reverse transcription polymerase chain reaction (PCR) (RT-qPCR) was performed to analyze the expression of BRCA1/2 and TRC-protective factors, including PARP1/2/3, TOPO-I, TIMELESS, and TIPIN.

RESULTS: Progesterone significantly reduced the IC50 values of all three PARP inhibitors (1.3-1.6-fold increase in sensitivity; p < 0.01). This increase was abrogated by DRB treatment, consistent with a TRC-related mechanism; however, direct TRC assays were not performed. Progesterone did not alter BRCA1/2 expression but markedly suppressed the expression of PARP1/2/3, TOPO-I, TIMELESS, and TIPIN.

CONCLUSIONS: Progesterone enhances the sensitivity of ovarian cancer cells to PARP inhibitors by downregulating TRC-protective factors via mPR-mediated non-genomic actions. These in vitro findings suggest a potential preclinical rationale for combining progesterone with PARP inhibitors in BRCA-wild-type ovarian cancer; in vivo validation and dosing studies are needed before clinical consideration.