Targeted therapies as BRAF and MEK inhibitor combination have been approved as first-line treatment for BRAF-mutant melanoma. However, disease progression occurs in most of the patients within few months of therapy. Metabolic adaptations have been described in the context of acquired resistance to BRAF inhibitors (BRAFi). BRAFi-resistant melanomas are characterized by an increase of mitochondrial oxidative phosphorylation and are more prone to cell death induced by mitochondrial-targeting drugs. BRAFi-resistant melanomas also exhibit an enhancement of oxidative stress due to mitochondrial oxygen consumption increase. To understand the mechanisms responsible for survival of BRAFi-resistant melanoma cells in the context of oxidative stress, we have established a preclinical murine model that accurately recapitulates in vivo the acquisition of resistance to MAPK inhibitors including several BRAF or MEK inhibitors alone and in combination. Using mice model and melanoma cell lines generated from mice tumors, we have confirmed that the acquisition of resistance is associated with an increase in mitochondrial oxidative phosphorylation as well as the importance of glutamine metabolism. Moreover, we have demonstrated that BRAFi-resistant melanoma can adapt mitochondrial metabolism to support glucose-derived glutamate synthesis leading to increase in glutathione content. Besides, BRAFi-resistant melanoma exhibits a strong activation of NRF-2 pathway leading to increase in the pentose phosphate pathway, which is involved in the regeneration of reduced glutathione, and to increase in xCT expression, a component of the xc—amino acid transporter essential for the uptake of cystine required for intracellular glutathione synthesis. All these metabolic modifications sustain glutathione level and contribute to the intracellular redox balance to allow survival of BRAFi-resistant melanoma cells.

Although MAPK pathway inhibitors are becoming a promising anticancer strategy, they are insufficient to fully eliminate cancer cells and their long-term efficacy is strikingly limited in patients with BRAF-mutant melanomas. It is well established that BRAF inhibitors (BRAFi) hamper glucose uptake before the apparition of cell death. Here, we show that BRAFi induce an extensive restructuring of mitochondria including an increase in mitochondrial activity and biogenesis associated with mitochondrial network remodeling. Furthermore, we report a close interaction between ER and mitochondria in melanoma exposed to BRAFi. This physical connection facilitates mitochondrial Ca2+ uptake after its release from the ER. Interestingly, Mfn2 silencing disrupts the ER–mitochondria interface, intensifies ER stress and exacerbates ER stress-induced apoptosis in cells exposed to BRAFi in vitro and in vivo. This mitochondrial control of ER stress-mediated cell death is similar in both BRAF- and NRAS-mutant melanoma cells exposed to MEK inhibitors. This evidence reinforces the relevance in combining MAPK pathway inhibitors with mitochondriotropic drugs to improve targeted therapies.