Background: Hypoxia, common in high-grade serous ovarian cancer (HGSOC) microenvironments, contributes to resistance to platinum-based treatments and PARP inhibitors by activating AP-1 transcription factors. AP-1 promotes proliferation, invasion, metastasis, and angiogenesis, but its role in DNA damage signaling, repair, and resistance mechanisms remains unclear.

Methods: TF activity was assessed in PARP-sensitive and -resistant HGSOC cells (PEO1, PEO1R, OVCAR4) using a luciferase reporter assay. AP-1 subunit (c-JUN, JUND, JUNB, cFOS, FOSL2) expression was analyzed under normoxic and hypoxic (1% O₂) conditions. Immunohistochemistry for JUNB, FOSL2, MRE11, CA-9, and CD-31 was conducted on PEO1 and PEO1R tumor xenografts. CRISPR knockouts of JUNB and FOSL2 were studied for effects on DNA repair gene expression, proliferation, invasion, and cisplatin sensitivity using DNA repair profiling, RNA sequencing, and functional assays. Protein stability and co-immunoprecipitation were analyzed. The clinical significance of FOSL2, JUNB, and MRE11 expression was evaluated in a cohort of 331 ovarian cancer patients.

Results: In platinum/PARP-resistant PEO1R cells, AP-1 transcription activity was elevated compared to PEO1 cells, with JUNB and FOSL2 overexpressed under normoxic and hypoxic (1% O₂) conditions. Tumor xenografts showed high JUNB and FOSL2 in hypoxic regions. Knockout (KO) of JUNB and FOSL2 reduced proliferation and increased cisplatin and olaparib sensitivity, linked to higher double-strand breaks (DSBs), G2/M arrest, and apoptosis. KO cells showed a downregulation of DNA repair genes, including MRE11, which JUNB and FOSL2 interact with to enhance stability. RNA sequencing revealed enriched pathways in platinum response, oxidative phosphorylation, and translation. Clinically, higher FOSL2, JUNB, and MRE11 expression correlated with shorter progression-free survival (PFS) and poorer overall survival (OS).

Conclusion: JUNB and FOSL2 are key players at the intersection of hypoxia and DNA damage response (DDR) in high-grade serous ovarian cancer (HGSOC). They have predictive and prognostic value and may serve as therapeutic targets in platinum/PARP-resistant HGSOC.