Endocrine Abstracts

Resistance to endocrine treatment develops with time in virtually all patients with metastatic breast cancer (BC), and eventually most of them are treated with chemotheray. Our group and others discovered a group of mutations in the ligand binding domain (LBD) of the estrogen receptor (ER), that are the direct cause of resistance in ~40% of the patients. The most common mutations are Y537S and D538G ER. Clinical data indicate an association between those ER mutations worse prognosis. The aim of this study was to study the role of ER mutations in promoting (i) resistance to chemotherapy; (ii) identify mechanisms mediating chemo-resistance. We studied chemo-resistance, using viability, colony formation and apoptosis assays, in MCF7 BC cells stabely expressing the WT-ER or Y537S and D538G ER mutations (mut-ER). The results showed that mut-ER cells exhibit elevated resistance to paclitaxel and doxorubicin. Nest, we studied the expression of the MDR1 gene, that encodes an ATP pump that is associated with chemoresistance. We observed increased expression of MDR1 mRNA and protein in mut-ER cells. Furthermore, we observed activation of the JNK pathway in mut-ER cells as evidanced by increased JNK and cjun phosphorylation, as well as cjun transcriptional activity. Our next goal was to find out whether cjun regulates MDR1 expression in mut-ER cells. Indeed, chromatin immunoprecipitation assay showed that cjun bound MDR1 promoter. Inhibition of JNK using SP600125 decreased MDR1 expression and also restored sensitivity to chemotherapy. Moreover, immunohistochemical studies of mut-ER tumors in mice, showed higher expression of MDR1 and cjun compared to WT-ER tumors. Finally, we analyzed duration of response to chemotherapy among 51 breast cancer patients with endocrine resistance, of which mut-ER was detected in 20 patients, whereas 31 had WT-ER. The analysis showed a significant shorter duration of chemotherapy treatment among patients with mut-ER, indicative of refractory response. In conclusion, ER activating mutations are associated with resistance to chemotherapy, mediated by the cjun-MDR1 axis. Our studies suggest novel, non-endocrine, therapeutic approaches aimed at targeting chemoresistance.