Exploiting Paradoxical Activation of Oncogenic MAPK Signaling by Targeting Mitochondria to Sensitize NRAS Mutant-Melanoma to Vemurafenib

Vemurafenib is a BRAF (rapidly accelerated fibrosarcoma B-type)-targeted therapy used to treat patients with advanced, unresectable melanoma. It inhibits the MAPK (mitogen-activated protein kinase)/ERK (extracellular signal-regulated kinase) pathway and tumor proliferation in BRAF^V600E-mutated melanoma cells. Resistance to vemurafenib has been reported in melanoma patients due to secondary <i>NRAS</i> (neuroblastoma RAS viral oncogene homolog) mutations, which lead to paradoxical MAPK pathway activation and tumor proliferation. However, the impact of this paradoxical activation on mitochondrial dynamics and function in <i>NRAS</i>-mutated melanoma is unclear. Here, we investigated the effects of vemurafenib on NRAS^Q61R-mutated melanoma cells, focusing on mitochondrial dynamics and function. As expected, vemurafenib did not exhibit cytotoxicity in SK-MEL-147 NRAS^Q61R-mutated melanoma cells, even after 72 h of incubation. However, it significantly enhanced the MAPK/ERK signaling through paradoxical activation, accompanied by decreased expression of mitochondrial fusion proteins and activation of the fission protein DRP1 (dynamin-related protein 1), leading to small, rounded mitochondrial morphology. These observations were corroborated by transcriptome data obtained from <i>NRAS</i>-mutated melanoma patients, showing <i>MFN1</i> (mitofusin 1) and <i>OPA1</i> (optic atrophy 1) downregulation and <i>DNM1L</i> (DRP1 gene) upregulation. Interestingly, inhibition of mitochondrial fission with mdivi-1 or modulation of oxidative phosphorylation via respiratory chain inhibition or uncoupling significantly sensitized NRAS^Q61R-mutated melanoma cells to vemurafenib. Despite vemurafenib's low cytotoxicity in <i>NRAS</i>-mutated melanoma, targeting mitochondrial dynamics and/or oxidative phosphorylation may offer a promising strategy for combined therapy.