The CaMKK2/CaMKIV Relay Is an Essential Regulator of Hepatic Cancer

Hepatic cancer is one of the most lethal cancers worldwide. Here, we report that the expression of Ca2+/calmodulin-dependent protein kinase kinase 2 (CaMKK2) is significantly up-regulated in hepatocellular carcinoma (HCC) and negatively correlated with HCC patient survival. The CaMKK2 protein is highly expressed in all eight hepatic cancer cell lines evaluated and is markedly up-regulated relative to normal primary hepatocytes. Loss of CaMKK2 function is sufficient to inhibit liver cancer cell growth, and the growth defect resulting from loss of CaMKK2 can be rescued by ectopic expression of wild-type CaMKK2 but not by kinase-inactive mutants. Cellular ablation of CaMKK2 using RNA interference yields a gene signature that correlates with improvement in HCC patient survival, and ablation or pharmacological inhibition of CaMKK2 with STO-609 impairs tumorigenicity of liver cancer cells in vivo. Moreover, CaMKK2 expression is up-regulated in a time-dependent manner in a carcinogen-induced HCC mouse model, and STO-609 treatment regresses hepatic tumor burden in this model. Mechanistically, CaMKK2 signals through Ca2+/calmodulin-dependent protein kinase 4 (CaMKIV) to control liver cancer cell growth. Further analysis revealed that CaMKK2 serves as a scaffold to assemble CaMKIV with key components of the mammalian target of rapamycin/ribosomal protein S6 kinase, 70 kDa, pathway and thereby stimulate protein synthesis through protein phosphorylation. Conclusion: The CaMKK2/CaMKIV relay is an upstream regulator of the oncogenic mammalian target of rapamycin/ribosomal protein S6 kinase, 70 kDa, pathway, and the importance of this CaMKK2/CaMKIV axis in HCC growth is confirmed by the potent growth inhibitory effects of genetically or pharmacologically decreasing CaMKK2 activity; collectively, these findings suggest that CaMKK2 and CaMKIV may represent potential targets for hepatic cancer. (Hepatology 2015;62:505–520