Inhibition of AR and DNA-PK for Radio-Sensitization of AR-driven Cancers.

e12593 Background: Breast cancer (BC) is the most diagnosed cancer globally with over 2.2 million new cases and 680,000 deaths annually. Radiotherapy (RT) is essential for curative BC treatment; however, 20% of BC cases are radio-resistant, with the underlying mechanisms remaining poorly understood. Previous studies in prostate cancer (PCa) have shown that androgen receptor (AR) promotes radio-resistance (RT-resistance) through transcriptional regulation and increased DNA repair capacity, including through upregulation of DNA repair proteins DNA-PK and PARP1. Although AR is expressed in 60% of BCs, the implications of AR in BC RT-resistance are not yet fully characterized. We hypothesized that AR may drive RT-resistance in BC through a similar AR-DNA repair loop. Methods: To characterize the repertoire of cofactors that cooperate with AR on the TF complex, we exposed a BC cell line to RT and performed rapid immunoprecipitation mass spectrometry of endogenous proteins (RIME) proteomics. We identified several candidate proteins that interact with AR to further investigate as targets for RT-sensitization. Survival analyses in BC and PCa cell lines were performed to identify RT-sensitizing agents. Molecular mechanisms of RT-sensitization were assessed using ATAC-seq and RNA-seq analyses. Results: Using RIME, we identified several DNA repair proteins including DNA-PK, PARP-1, and XRCC5 as differentially recruited to chromatin-bound AR. This suggests a cooperative role and provides potential druggable targets to improve RT-sensitivity. Based on these results, we also performed a survival analysis in AR-driven BC and PCa cell lines and showed that inhibition of either AR or DNA-PK led to RT-sensitization, and dual inhibition provided an additive effect. We sought to characterize the molecular mechanisms driving the RT-sensitizing effects of AR and DNA-PK inhibition and performed ATAC-seq and RNA-seq on treated and untreated cells. Combining enzalutamide with a DNA-PK inhibitor altered expression of several pathways implicated in RT-sensitization, although these pathways were largely distinct in BC and PCa. Further, we identified distinct genes that may be responsible for this phenotype in AR-driven BC. Additionally, we found a larger effect in chromatin rearrangement after treatment in PCa than BC. Conclusions: Identifying mechanisms of RT-resistance in hormone-driven cancers is essential to develop therapeutic strategies and improve patient outcomes. This study provides key evidence for AR-driven RT-resistance in BC and PCa, and identifies DNA-PK inhibition as a potential drug target. Distinct mechanisms appear to be driving this phenotype across BC and PCa, in our models. Understanding the cooperation of DNA repair and hormone signaling pathways will help support the use of DNA-damaging agents for hormone-driven cancers, and potentially inform rational design of novel drug-RT combinations.