Depletion of BRCA1 Potentiates Progestin-Induced Cytoskeletal Changes in an Ovarian Cancer Cell Model

Individuals with Hereditary Breast and Ovarian Cancer (HBOC) syndrome carry BRCA1/2 mutations that predispose them to various cancers. Notably, this condition increases the risk of developing an aggressive ovarian cancer (OC) subtype known as high-grade serous ovarian cancer (HGSOC). The impact of progesterone (P4) on the development of OC subtypes remains unclear. Research suggests a potential interplay between Progesterone Receptor (PR) signaling and BRCA1/2 actions. BRCA1 regulates PR expression and transcriptional activity, while P4 can drive estrogen receptor/PR complexes to BRCA1 DNA binding motifs. Normal fallopian tube tissues from healthy BRCA1/2 carriers (the cells of origin for HGSOC) exhibit gene signatures similar to those in HGSOC only during the luteal phase of the menstrual cycle, when circulating P4 levels are elevated. This implies a potential role for PR/BRCA complexes in modulating OC initiation and progression. To explore this interaction, BRCA1 was depleted in an ES-2 PR-B+ OC cell models using CRISPR-mediated gene editing. We hypothesized that BRCA1 depletion would alter PR-mediated signaling and reveal novel regulatory mechanisms. Depletion of BRCA1 led to increased total basal PR protein levels and site-specific hyperphosphorylation at S294 and S81 following R5020 (synthetic progestin) treatment. RNA sequencing of NE-1 and KO-BRCA1 pools treated with R5020 identified a high-confidence set of BRCA1-dependent, R5020-induced genes predominately involved with migration and cytoskeletal organization. Transwell assays revealed that R5020 treatment stimulates migration, an effect amplified upon BRCA1 depletion. Inhibition of PIP5K1C using UNC3230 blocked R5020-induced migration in one of the KO-BRCA1 cell pools with pan-Rho inhibitor CT04 showing similar results. The Rac1 inhibitor EHT-1864 suppressed all R5020-induced migration. In contrast, co-treatment with a pan-ROCK1/2 inhibitor Y-27632 resulted in an increase in R5020-induced migration, which was further enhanced with BRCA1 depletion. Overall, these findings establish a crosstalk between BRCA1 and PR signaling, where BRCA1 depletion sensitizes OC cells to progestin-induced migration by altering PR phosphorylation and remodeling cytoskeletal organization via PIP5K1C, Rho-GTPase, and Rac1-dependent mechanisms.