mTORC2 regulates BRAFV600E melanoma cells metabolism and sensitivity to BRAF inhibition

Melanoma is the most aggressive type of skin cancer. Gene sequencing analyses have shown that activated BRAF mutations are present in more than 50% of malignant melanomas and contribute to constitutive activation of the MAPK pathway. Among them, the most common is a single amino acid substitution of valine with glutamic acid at residue 600 in the BRAF oncogene (BRAFV600E). For this reason, several clinical studies have suggested the antitumor effects of BRAF inhibitor (BRAFi) alone or in combination with MEK inhibitors (MEKi) against BRAF-mutant melanoma. Although BRAF inhibitors rapidly suppress melanoma growth, median progression-free survival remains unsatisfactory. In this work we investigated the molecular mechanisms at the base of BRAF inhibitors resistance. In particular, we demonstrated that sustained downregulation of Rictor or mSin1- which are indispensable component of mTORC2- in BRAFV600E melanoma cells causes metabolic alterations that lead to tumor resistance to BRAFi and/or BRAFi/MEKi combined therapy. Rictor and mSin1 knockdown and knockout cells, in fact, display the characteristic metabolic changes found in BRAF inhibitor resistant (BiR) cells, such as reduced glycolysis, glutamine dependence and increased mitochondrial oxidative phosphorylation and NAD+ production. These metabolic alterations represent potential targets that need to be exploited in order to establish a more effective therapy for BRAFi resistant melanomas. Indeed, we investigated the potential role of metabolic inhibitors - such as the NAMPT inhibitor FK866, alone or in combination with the BRAFi Vemurafenib, in reverting BRAFV600E melanoma cells acquired resistance to canonical treatments.