Abstract 4681: Multiomic insights into mechanisms of androgen receptor-driven radioresistance in triple-negative breast cancer

Background: Due to the lack of molecular-targeted therapies for triple-negative breast cancer (TNBC), there have been efforts to use multimodal strategies to enhance the efficacy of existing therapies such as radiation. The androgen receptor (AR) has been identified as a radiosensitizing target and is expressed in 25-50% of TNBC tumors. The combination of AR inhibitors, such as enzalutamide (enza), and radiation therapy (RT), leads to radiosensitization in AR+ TNBC models but the mechanism by which AR mediates radioresistance is not known, and targeting this radioresistance could provide a novel strategy to improve outcomes in this aggressive breast cancer subtype. Following RT, AR is translocated to the nucleus suggesting that AR may drive transcriptional changes in response to RT. Methods: Bulk RNA-seq with and without R1881 stimulation and RT was used to assess transcriptional changes at 24 hours. RPPA analysis evaluated protein and phophoprotein changes. AR ChIP-seq was used to identify changes in AR binding and Bru-seq to profile changes in nascent RNA. These experiments were performed in two AR+ TNBC models, MDA-MB-453 and MFM-223 cells, with and without enza and RT. Clonogenic survival assays were performed to assess for radiosensitivity. Results: RNA-seq and RPPA analysis demonstrated numerous changes in gene and protein expression after enza-, RT-, and combination treatment. Gene set enrichment analysis (GSEA) of these changes nominated alterations of MAPK pathway genes with R1881 and R1881+RT compared to controls. To assess the MAPK/ERK pathway, constitutively active ERK was overexpressed in AR+ TNBC models which induced radioresistance, confirming this relationship. As expected, AR ChIP-seq data showed enza treatment induced a significant reduction in AR peaks compared to DMSO-treated controls. Radiation induced fewer changes in AR binding than enza treatment. Bru-seq GSEA showed significant upregulation of MYC target gene sets in response to RT in control conditions. Conversely, with enza treatment, there was no significant MYC target gene set upregulation. While there were no changes in MYC expression or AR binding to known MYC enhancers, there was increased binding of AR to known MYC binding sites in control samples in response to RT. This RT-induced binding to MYC sites was not seen in the enza-treated samples suggesting that AR inhibition may dampen the RT induction of MYC signaling leading to radiosensitization. Conclusions: These findings provide new insights into the molecular underpinnings of AR-mediated radioresistance and identify MAPK and MYC pathways as potential therapeutic targets to enhance radiosensitivity in AR+ TNBC. Ongoing studies aim to translate these findings into clinically actionable strategies. This work expands our understanding of AR-mediated radioresistance which may uncover resistance mechanisms to combination AR inhibition and RT. Citation Format: Benjamin Hauk, Breanna McBean, Anna Michmerhuizen, Douglas Gurdak, Savannah Tidmore, Michelle Paulsen, Lynn Lerner, Connor Ward, Kassidy Jungles, Daniel Spratt, Lori Pierce, Mats Ljungman, Corey Speers. Multiomic insights into mechanisms of androgen receptor-driven radioresistance in triple-negative breast cancer [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 4681.