ABSTRACT
In this Letter, two relevant references were omitted; see the accompanying Amendment for details. The original Letter has not been corrected.
ABSTRACT
Targeted BRAF inhibition (BRAFi) and combined BRAF and MEK inhibition (BRAFi and MEKi) therapies have markedly improved the clinical outcomes of patients with metastatic melanoma. Unfortunately, the efficacy of these treatments is often countered by the acquisition of drug resistance. Here we investigated the molecular mechanisms that underlie acquired resistance to BRAFi and to the combined therapy. Consistent with previous studies, we show that resistance to BRAFi is mediated by ERK pathway reactivation. Resistance to the combined therapy, however, is mediated by mechanisms independent of reactivation of ERK in many resistant cell lines and clinical samples. p21-activated kinases (PAKs) become activated in cells with acquired drug resistance and have a pivotal role in mediating resistance. Our screening, using a reverse-phase protein array, revealed distinct mechanisms by which PAKs mediate resistance to BRAFi and the combined therapy. In BRAFi-resistant cells, PAKs phosphorylate CRAF and MEK to reactivate ERK. In cells that are resistant to the combined therapy, PAKs regulate JNK and ß-catenin phosphorylation and mTOR pathway activation, and inhibit apoptosis, thereby bypassing ERK. Together, our results provide insights into the molecular mechanisms underlying acquired drug resistance to current targeted therapies, and may help to direct novel drug development efforts to overcome acquired drug resistance.
Subject(s)
Drug Resistance, Neoplasm/drug effects , Melanoma/drug therapy , Melanoma/genetics , Mitogen-Activated Protein Kinases/antagonists & inhibitors , Mutation , Protein Kinase Inhibitors/pharmacology , Proto-Oncogene Proteins B-raf/genetics , Signal Transduction/drug effects , p21-Activated Kinases/metabolism , Animals , Apoptosis/drug effects , Cell Line, Tumor , Drug Resistance, Neoplasm/genetics , Enzyme Activation/drug effects , Female , Humans , JNK Mitogen-Activated Protein Kinases/chemistry , JNK Mitogen-Activated Protein Kinases/metabolism , MAP Kinase Signaling System/drug effects , Melanoma/enzymology , Mice , Mitogen-Activated Protein Kinase Kinases/chemistry , Mitogen-Activated Protein Kinase Kinases/metabolism , Phosphorylation/drug effects , Proto-Oncogene Proteins c-raf/chemistry , Proto-Oncogene Proteins c-raf/metabolism , TOR Serine-Threonine Kinases/metabolism , beta Catenin/chemistry , beta Catenin/metabolism , p21-Activated Kinases/antagonists & inhibitors , p21-Activated Kinases/geneticsABSTRACT
Targeting multiple components of the MAPK pathway can prolong the survival of patients with BRAFV600E melanoma. This approach is not curative, as some BRAF-mutated melanoma cells are intrinsically resistant to MAPK inhibitors (MAPKi). At the systemic level, our knowledge of how signaling pathways underlie drug resistance needs to be further expanded. Here, we have shown that intrinsically resistant BRAF-mutated melanoma cells with a low basal level of mitochondrial biogenesis depend on this process to survive MAPKi. Intrinsically resistant cells exploited an integrated stress response, exhibited an increase in mitochondrial DNA content, and required oxidative phosphorylation to meet their bioenergetic needs. We determined that intrinsically resistant cells rely on the genes encoding TFAM, which controls mitochondrial genome replication and transcription, and TRAP1, which regulates mitochondrial protein folding. Therefore, we targeted mitochondrial biogenesis with a mitochondrium-targeted, small-molecule HSP90 inhibitor (Gamitrinib), which eradicated intrinsically resistant cells and augmented the efficacy of MAPKi by inducing mitochondrial dysfunction and inhibiting tumor bioenergetics. A subset of tumor biopsies from patients with disease progression despite MAPKi treatment showed increased mitochondrial biogenesis and tumor bioenergetics. A subset of acquired drug-resistant melanoma cell lines was sensitive to Gamitrinib. Our study establishes mitochondrial biogenesis, coupled with aberrant tumor bioenergetics, as a potential therapy escape mechanism and paves the way for a rationale-based combinatorial strategy to improve the efficacy of MAPKi.