Inhibition of mTORC1/2 Overcomes Resistance to MAPK Pathway Inhibitors Mediated by PGC1α and Oxidative Phosphorylation in Melanoma

Metabolic heterogeneity is a key factor in cancer pathogenesis. We found that a subset of BRAF- and NRAS-mutant human melanomas resistant to the MEK inhibitor selumetinib displayed increased oxidative phosphorylation (OxPhos) mediated by the transcriptional coactivator PGC1 alpha. Notably, all selumetinib-resistant cells with elevated OxPhos could be resensitized by cotreatment with the mTORC1/2 inhibitor AZD8055, whereas this combination was ineffective in resistant cell lines with low OxPhos. In both BRAF- and NRAS-mutant melanoma cells, MEK inhibition increased MITF expression, which in turn elevated levels of PGC1 alpha. In contrast, mTORC1/2 inhibition triggered cytoplasmic localization of MITF, decreasing PGC1 alpha expression and inhibiting OxPhos. Analysis of tumor biopsies from patients with BRAF-mutant melanoma progressing on BRAF inhibitor +/- MEK inhibitor revealed that PGC1 alpha levels were elevated in approximately half of the resistant tumors. Overall, our findings highlight the significance of OxPhos in melanoma and suggest that combined targeting of the MAPK and mTORC pathways may offer an effective therapeutic strategy to treat melanomas with this metabolic phenotype. (C) 2014 AACR.