Abstract 1713: Disrupting ER-AR crosstalk using AR degraders: a novel approach to treating endocrine resistant breast cancer

Background: Estrogen receptor positive (ER+) breast cancer accounts for approximately 70% of all breast cancer cases and is the most diagnosed subtype amongst women. Current treatments include aromatase inhibitors (AIs), which block the biosynthesis of estrogens, tamoxifen (ER antagonist), and fulvestrant (ER degrader) (FUL). Despite therapeutic effectiveness, 30% of patients develop resistance, necessitating a deeper understanding of the underling mechanisms. Recent evidence points to the upregulation of androgen receptor (AR) signaling pathway as a potential mechanism of acquired resistance in breast cancer. To address the role of ER/AR crosstalk in acquired endocrine resistance, we generated an AI-resistant breast cancer cell line, MCF-7:5C (5C), mimicking the resistance seen in patients undergoing AI treatment. Methods: The inhibitory effect of FUL and ARCC4 (AR PROTAC degrader) on AI-resistant 5C and parental MCF-7 cells was examined using CellTiter-Blue® Cell Viability assay and trypan blue cell counting. Western blot, immunofluorescence, and nuclear fractionation assays were performed to determine ER and AR protein expression and cellular localization in response to FUL and ARCC4 treatments. siRNAs were used to knockdown ER and AR expression in resistant 5C cells to assess functional significance. Additionally, flow cytometry analysis using the RhodB-PTP (10mg/mL) was used to probe for surface expression of plectin in resistant cells. Results: AI-resistant 5C cells expressed markedly elevated levels of ER and AR and were more sensitive to the inhibitory effects of FUL and ARCC4 compared to parental MCF-7 cells. Immunofluorescence and nuclear fractionation assays revealed predominant nuclear localization of ER and AR in 5C cells compared to MCF-7 cells, and both proteins were markedly reduced (>95%) by FUL and ARCC4 treatments and siRNAs targeting both ER and AR. Notably, flow cytometry data revealed a strong correlation between ER, AR, and surface Plectin in 5C cells, thus identifying an additional target in these cells. Conclusions: Plectin is a cytoskeleton linker protein found in non-transformed cells, however, evidence suggests that its cancer-specific mislocalization to the cell surface contributes to its oncogenic function. As such, therapeutic targeting of surface plectin, along with ER/AR using FUL and ARCC4, represents an attractive opportunity to impede tumorigenesis and possible reverse endocrine resistance in breast cancer. Citation Format: Matt Konda, Obdulia Covarrubias-Zambrano, Stefan H. Bossmann, Joan Lewis-Wambi. Disrupting ER-AR crosstalk using AR degraders: a novel approach to treating endocrine resistant 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 1713.