PAK Signaling Drives Acquired Drug Resistance to MAPK Inhibitors in BRAF-mutant Melanomas

Targeted BRAF inhibition (BRAFi) and combined BRAF and MEK inhibition (BRAFi+MEKi) therapies have significantly improved clinical outcomes in patients with metastatic melanoma. Unfortunately, the efficacy is beset by the acquisition of drug resistance(1–6). Here we investigated molecular mechanisms underlying acquired resistance to BRAFi (BRAFi resistance, “BR”) and BRAFi+MEKi (combination therapy resistance, “CR”). Consistent with previous studies, BR is mediated by ERK pathway re-activation. CR is, however, mediated by mechanisms independent of re-activation of ERK in many therapy-resistant cell lines and clinical samples. p21-activated kinases (PAKs) become activated in acquired drug resistant cells and play a pivotal role in mediating both BR and CR. Our screening using reverse phase protein array (RPPA) revealed distinct mechanisms by which PAKs mediate BR and CR. In BR, PAKs phosphorylate CRAF and MEK to reactivate ERK. In CR, PAKs regulate JNK and β-catenin phosphorylation, mTOR pathway activation, and inhibit apoptosis, thereby bypassing ERK. Together, our results provide new insights into molecular mechanisms underlying acquired drug resistance to current targeted therapies, and may help to direct novel drug development efforts to overcome acquired drug resistance.