Taken together, these data indicate that ESR1 mutations confer chemoresistance through activation of the JNK/MDR1 axis. These finding suggest a novel treatment option for BC tumors expressing ESR1 mutations.

MCF-7 cells harboring Y537S and D538G ESR1 mutations (mut-ER) were employed to study the response to chemotherapy drugs, paclitaxel and doxorubicin, 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.

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. Here we aimed to study the role of ESR1 mutations in conferring chemoresistance in BC cells.

Cell harboring ESR1 mutations displayed relative chemoresistance compared to WT-ER, 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 c-Jun pathway, and we showed high correlation between these two genes in BC using TCGA databases. Accordingly, we detected higher JNK/c-Jun expression and activity in ESR1-mutated cells, as well as increased occupancy of c-Jun in MDR1 promoter. Importantly, JNK inhibition decreased MDR1 expression and restored sensitivity to chemotherapy. Conclusions: Taken together, these data indicate that ESR1 mutations confer chemoresistance through activation of the JNK/MDR1 axis. These finding suggest a novel treatment option for BC tumors expressing ESR1 mutations.