SHOC2 and CRAF mediate ERK1/2 reactivation in mutant NRAS-mediated resistance to RAF inhibitor

Abstract

ERK1/2 signaling is frequently dysregulated in tumors through BRAF mutation. Targeting mutant BRAF with vemurafenib frequently elicits therapeutic responses; however, durable effects are often limited by ERK1/2 pathway reactivation via poorly defined mechanisms. We generated mutant BRAF V600E melanoma cells that exhibit resistance to PLX4720, the tool compound for vemurafenib, that co-expressed mutant (Q61K) NRAS. In these BRAFV600E/NRASQ61K co-expressing cells, reactivation of the ERK1/2 pathway during PLX4720 treatment was dependent on NRAS. Expression of mutant NRAS in parental BRAFV600 cells was sufficient to by-pass PLX4720 effects on ERK1/2 signaling, entry into S phase and susceptibility to apoptosis in a manner dependent on the RAF binding site in NRAS. ERK1/2 activation in BRAFV600E/NRASQ61K cells required CRAF only in the presence of PLX4720, indicating a switch in RAF isoform requirement. Both ERK1/2 activation and resistance to apoptosis of BRAFV600E/NRAS Q61K cells in the presence of PLX4720 was modulated by SHOC-2/Sur-8 expression, a RAS-RAF scaffold protein. These data show that NRAS mutations confer resistance to RAF inhibitors in mutant BRAF cells and alter RAF isoform and scaffold molecule requirements to re-activate the ERK1/2 pathway. © 2012 by The American Society for Biochemistry and Molecular Biology, Inc.