Abstract PO2-05-09: Beyond endocrine resistance: estrogen receptor (ESR1) activating mutations mediate chemotherapy resistance through the JNK/c-Jun MDR1 pathway in breast cancer

Abstract Purpose: All patients with metastatic breast cancer (MBC) expressing estrogen receptor-α (ESR1) will eventually develop resistance to endocrine therapies. In up to 40% of patients, this resistance is caused by activating mutations in the ligand-binding domain (LBD) of ESR1. Accumulating clinical evidence indicate adverse outcomes for these patients, beyond that expected by resistance to endocrine therapy. We hypothesized that ESR1 mutations may also confer resistance to chemotherapy. Experimental Design: MCF-7 cells harboring Y537S and D538G ESR1 mutations (mut-ER) were employed to study response to chemotherapy using viability and apoptotic assay in vitro, and tumor growth in vivo. JNK/c-Jun/MDR1 pathway was studied using qRT-PCR, western-blot, gene-reporter and ChIP assays. MDR1 expression was analyzed in clinical samples using IHC. Results: Cell harboring ESR1 mutations displayed relative chemoresistance, evidenced by higher viability and reduced apoptosis as well as resistance to paclitaxel in vivo. To elucidate the underlying mechanism, MDR1 expression was examined and elevated levels were observed in mut-ER cells, and in clinical BC samples. MDR1 is regulated by the JNK/cJun pathway, and indeed, we detected higher JNK/cJun expression and activity in mut-ER cells, as well as increased occupancy of c-Jun in MDR1 promoter. Importantly, JNK inhibition decreased MDR1 expression, particularly of D538G-cells, and reduced viability in response to chemotherapy. Conclusions: Taken together, these data indicate that ESR1 mutations confer chemoresistance in BC through activation of the JNK/MDR1 axis. Targeting this pathway may restore sensitivity to chemotherapy and serve as a novel treatment strategy for MBC patients carrying ESR1 mutations. Citation Format: Marwa Taya, Keren Merenbakh-Lamin, Asia Zubkov, Zohar Honig, Ori Mayer, Noam Shomron, Wolf Ido, Tami Rubinek. Beyond endocrine resistance: estrogen receptor (ESR1) activating mutations mediate chemotherapy resistance through the JNK/c-Jun MDR1 pathway in breast cancer [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO2-05-09.