Estrogen Negatively Regulates the Pro-apoptotic Function of Mixed Lineage Kinase 3 in Estrogen Receptor Positive Breast Cancer

Estrogen stimulates growth and inhibits apoptosis of breast cancer cells via genomic and non-genomic actions. However, the detailed mechanism by which estrogen inhibits the pro-apoptotic pathways that might impede the normal homeostasis and action of chemotherapeutic drugs in breast cancer cells is not well understood. Here, we report a negative regulation of a pro-apoptotic kinase, M ixed L ineage K inase 3 (MLK3) by 17β-estradiol (E 2 ) that hinders cytotoxic drug-induced cell death in estrogen receptor positive (ER + ) breast cancer cells. MLK3 kinase activities were significantly higher in estrogen receptor negative (ER − ), progesterone receptor negative (PR − ) primary human breast tumors, suggesting that E 2 might have a negative role in regulating MLK3 kinase activity. The kinase activities of MLK3 and its downstream target, JNK were rapidly inhibited by E 2 in ER + but not in ER − breast cancer cells. The inhibition of MLK3 kinase activity by E 2 was mediated via activation of protein kinase B (PKB/AKT) because specific knockdown of AKT1/2 prevented the E 2 -induced inhibition of MLK3. Furthermore, E 2 -induced inhibition of MLK3 kinase activity involved a direct phosphorylation of MLK3 at Ser 674 site by AKT, which resulted in an attenuation of the pro-apoptotic function of MLK3. In addition, a pan-MLK inhibitor (CEP-11004) significantly attenuated Taxol-induced cell death, which was further synergized by E 2 . Thus, our data suggest that E 2 negatively regulates the pro-apoptotic function of MLK3 during breast cancer pathogenesis and therefore MLK3 and other MLK family members might play an important role in cytotoxic drug-induced cell death in ER + breast cancer cells.