Regulation of Mixed Lineage Kinase-3 (MAP3K11) and Its Role in Breast Cancer Pathogenesis

Mixed Lineage Kinase-3 (MLK-3) is a MAP Kinase Kinase Kinase and belongs to a novel family of MLKs. The physiological functions of MLKs are not known; however, it is reported that activation of MLKs promote neurodegeneration. The functional role of MLKs in cancer is still elusive. Earlier, we reported that MLK-3 kinase activity was down regulated by both IGF-1 and estrogen via activation of PI3K/AKT pathway. Specifically, the AKT activation, either by IGF-1 or estrogen directly phosphorylated MLK-3 on Ser⁶⁷⁴ site, and this phosphorylation suppressed the MLK-3 kinase activity and prevented cancer cell death. Since Human Epidermal Growth Factor Receptor 2 (HER2) is significantly overexpressed in 20-30 percent of breast cancer, we asked whether HER2 also plays any role in regulating MLK-3 kinase activity and could play a functional role in survival/death of HER2+ breast cancer cells. We noticed a significant reduction in the kinase activity of MLK-3 in HER2 positive (HER2+) but not in HER2 negative (HER2-) breast cancer tissues. Furthermore, the kinase activity of MLK-3 was inversely correlated with HER2+ tumor grades. Moreover, HER2-directed therapies, such as trastuzumab and lapatinib, as well as depletion of HER2 or HER3, stimulated MLK-3 kinase activity in HER2+ breast cancer cell lines. In addition, the noted inhibitory effect of HER2 on MLK-3 kinase activity was mediated via its phosphorylation on Ser⁶⁷⁴ by AKT and pharmacological inhibitors of PI3K/AKT prevented trastuzumab- and lapatinib-induced stimulation of MLK-3 activity. Consistent with the pro-apoptotic function of MLK-3, stable knockdown of MLK-3 in HER2+ cell line blunted the pro-apoptotic effects of trastuzumab and lapatinib. These findings suggest that HER2 activation inhibits the pro-apoptotic function of MLK-3, which plays a mechanistic role in mediating anti-tumor activities of HER2-directed therapies. Based on our published results and current findings, we propose that MLK-3 is a nodal point by which breast cancer cells survive via activation of PI3K/AKT pathway by IGF-1R, ER and HER2. Therefore, targeting MLK-3 by using its activator could block the escape pathways that promote breast cancer cell survival, therapy resistance and metastasis.