Abstract 2911: Unravelling the role of AP-1 transcription factor in DNA damage signaling and response (DDR), platinum and PARP inhibitor resistance in ovarian cancers

Abstract Background: Hypoxia is a feature of high grade serous ovarian cancer (HGSOC) microenvironment and contributes to platinum and PARP inhibitor resistance. Hypoxia induces activator protein-1 (AP-1) transcription factor (TF) activity leading to sustained proliferation, invasion, metastasis and angiogenesis. The role of AP-1 in DNA damage signaling and repair (DDR) and platinum/PARP resistance is unclear in HGSOC. Methods: PARP sensitive and resistant HGSOC cells (PEO1, PEO1R) were monitored for the activity of 48 different TFs using a luciferase-based reporter assay. AP-1 subunits including c-JUN, JUND, JUNB, cFOS and FOSL2 were profiled for protein expression under normoxia and hypoxia (1% O2). PEO1 and PEO1R tumor xenografts were immunohistochemically evaluated for JUNB, FOSL2, MRE11, CA-9 and CD-31 expression. CRISPR knock outs (KO) of JUNB and FOSL2 were generated and investigated for DNA repair gene expression (by DNA repair profiler PCR arrays), whole genome RNA sequencing, proliferation, invasion, cisplatin sensitivity (clonogenic and 3D-spheroids). Functional assays included DNA double strand break (DSB) accumulation, cell cycle progression, apoptosis, immunofluorescence, protein stability assay, co-immunoprecipitation and chromatin immunoprecipitation. The clinicopathological significance of FOSL2, JUNB and MRE11 expression was investigated in 331 clinical epithelial ovarian cancers. Results: In platinum/PARP resistant PEO1R cells, upregulation of AP-1 transcription activity was evident compared to PEO1. Overexpression of JUNB and FOSL2 proteins was observed in normoxia and hypoxia (1% O2). PEO1 and PEO1R tumor xenografts showed high levels of JUNB and FOSL2 in tumor hypoxic areas. Compared to controls, JUNB and FOLS2_KO cells were less proliferative and have increased sensitivity to cisplatin/PARPi (olaparib), which was associated with increased DSBs, G2/M cell cycle arrest and increased apoptosis. DNA repair profiling revealed down regulation of several DNA repair genes in KO cells including MRE11, a key DDR factor. JUNB/FOSL2 proteins physically interacted with MRE11 and promoted its stability. RNA sequencing revealed enrichment of pathways such as platinum response, oxidative phosphorylation, translation and others in KO cells compared to control. In clinical cohorts, high FOSL2, high JUNB and high MRE11 expression was significantly associated with shorter progression-free survival (PFS) and worse overall survival (OS). Conclusion: Our data provides evidence that JUNB and FOSL2 may operate at the hypoxia-DDR interface in HGSOC. JUNB and FOSL2 not only have predictive and prognostic significance but could also be attractive anti-cancer targets including in platinum/PARP resistant HGSOC. Citation Format: Shatha Alqahtani, Rinad Mahmoud, Mashael Algethami, Asmaa Ibrahim, Ahmed Shoqafi, Jennie N. Jeyapalan, Sophie Kellaway, Nigel P. Mongan, Emad A. Rakha, Alan McIntyre, Srinivasan Madhusudan. Unravelling the role of AP-1 transcription factor in DNA damage signaling and response (DDR), platinum and PARP inhibitor resistance in ovarian cancers [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 2911.