Vemurafenib Drives Epithelial-to-Mesenchymal Transition Gene Expression in BRAF Inhibitor‒Resistant BRAF/NRAS Melanoma Enhancing Tumor Growth and Metastasis in a Bioluminescent Murine Model.

BRAF inhibitor (BRAFi) resistance compromises long-term survivorship of patients with malignant melanoma, and mutant NRAS is a major mediator of BRAFi resistance. In this study, employing phenotypic and transcriptomic analysis of isogenic melanoma cells that differ only by NRAS mutational status (BRAFi-sensitive A375-BRAF/NRAS vs. BRAFi-resistant A375-BRAF/NRAS), we show that BRAFi (vemurafenib) treatment selectively targets BRAF/NRAS cells upregulating epithelial-to-mesenchymal transition (EMT) gene expression, paradoxically promoting invasiveness and metastasis in vitro and in vivo. First, NanoString nCounter transcriptomic analysis identified the upregulation of specific gene expression networks (EMT and EMT to metastasis) as a function of NRAS status. Strikingly, BRAFi treatment further exacerbated the upregulation of genes promoting EMT in BRAF/NRAS cells (with opposing downregulation of EMT-driver genes in the BRAF/NRAS genotype) as detected by EMT-focused RT Profiler qPCR array analysis. In BRAF/NRAS cells, BRAFi treatment enhanced proliferation and invasiveness, together with activation of phosphorylated protein kinase B (Ser473), with opposing phenotypic effects observable in BRAF/NRAS cells displaying downregulation of phosphorylated protein kinase B and phosphorylated extracellular signal-regulated kinase 1/2. In a SCID mouse bioluminescent melanoma metastasis model, BRAFi treatment enhanced lung tumor burden imposed by BRAF/NRAS cells while blocking BRAF/NRAS metastasis. These preclinical data document the BRAFi-driven enhancement of tumorigenesis and metastasis in BRAFi-resistant human BRAF/NRAS melanoma, a finding with potential clinical implications for patients with NRAS-driven BRAFi-resistant tumors receiving BRAFi treatment.