BRAF inhibitor (BRAF-I) therapy for melanoma patients harboring the V600E mutation is initially highly effective, but almost all patients relapse within a few months.,Understanding the molecular mechanisms behind BRAF-I responsiveness and acquired resistance is therefore an important issue.,Here we assessed the role of urokinase type plasminogen activator receptor (uPAR) as a potentially valuable biomarker in the acquisition of BRAF-I resistance in V600E mutant melanoma cells.,We examined uPAR and EGFR levels by real time PCR and western blot analysis. uPAR loss of function was realized by knocking down uPAR by RNAi or using M25, a peptide that uncouples uPAR-integrin interaction.,We investigated uPAR-β1integrin-EGFR association by co-immunoprecipitation and confocal immuno-fluorescence analysis.,Acquired resistance to BRAF-I was generated by chronic exposure of cells to vemurafenib.,We proved that uPAR knockdown in combination with vemurafenib inhibits melanoma cell proliferation to greater extent than either treatment alone causing a decrease in AKT and ERK1/2 phosphorylation.,Conversely, we demonstrated that uPAR enforced over-expression results in reduced sensitivity to BRAF inhibition.,Moreover, by targeting uPAR and EGFR interaction with an integrin antagonist peptide we restored vemurafenib responsiveness in melanoma resistant cells.,Furthermore, we found significant detectable uPAR and EGFR levels in tumor biopsies of 4 relapsed patients.,We disclosed an unpredicted mechanism of reduced sensitiveness to BRAF inhibition, driven by elevated levels of uPAR and identified a potential therapeutic strategy to overcome acquired resistance.,Associazione Italiana Ricerca sul Cancro (AIRC); Ente Cassa di Risparmio di Firenze.
Once melanomas have progressed with acquired resistance to mitogen-activated protein kinase (MAPK)-targeted therapy, mutational heterogeneity presents a major challenge.,We therefore examined the therapy phase before acquired resistance had developed and discovered the melanoma survival oncogene MITF as a driver of an early non-mutational and reversible drug-tolerance state, which is induced by PAX3-mediated upregulation of MITF.,A drug-repositioning screen identified the HIV1-protease inhibitor nelfinavir as potent suppressor of PAX3 and MITF expression.,Nelfinavir profoundly sensitizes BRAF and NRAS mutant melanoma cells to MAPK-pathway inhibitors.,Moreover, nelfinavir is effective in BRAF and NRAS mutant melanoma cells isolated from patients progressed on MAPK inhibitor (MAPKi) therapy and in BRAF/NRAS/PTEN mutant tumors.,We demonstrate that inhibiting a driver of MAPKi-induced drug tolerance could improve current approaches of targeted melanoma therapy.,•MITF is a driver of a reversible non-mutational drug-tolerance phase in melanoma•Drug repositioning identifies nelfinavir mesylate as a suppressor of MITF expression•Nelfinavir sensitizes BRAF and NRAS mutant melanoma to MAPK inhibitor treatment•A nelfinavir combination therapy overcomes NRAS-driven acquired resistance,MITF is a driver of a reversible non-mutational drug-tolerance phase in melanoma,Drug repositioning identifies nelfinavir mesylate as a suppressor of MITF expression,Nelfinavir sensitizes BRAF and NRAS mutant melanoma to MAPK inhibitor treatment,A nelfinavir combination therapy overcomes NRAS-driven acquired resistance,Smith et al. discover PAX3-mediated overexpression of MITF as a reversible resistance mechanism to MAPK-pathway inhibition in BRAF mutant melanomas and identify nelfinavir, which inhibits this mechanism and sensitizes not only BRAF mutant but also BRAF and NRAS mutant melanoma cells to MAPK-pathway inhibitors.