Colorectal cancer cells with the BRAFV600E mutation are addicted to the ERK1/2 pathway for growth factor-independent survival and repression of BIM

Abstract

The RAF–mitogen-activated protein kinase kinase 1/2–extracellular signal-regulated kinase 1/2 (RAF–MEK1/2–ERK1/2) pathway is activated in many human tumours and can protect cells against growth factor deprivation; however, most such studies have relied upon overexpression of RAF or MEK constructs that are not found in tumours. Here we show that expression of the endogenous BRAF^V600E allele in mouse embryonic fibroblasts from conditional knock-in transgenic mice activates ERK1/2, represses the BH3-only protein BIM and protects cells from growth factor withdrawal. Human colorectal cancer (CRC) cell lines harbouring BRAF^V600E are growth factor independent for the activation of ERK1/2 and survival. However, treatment with the MEK1/2 inhibitors U0126, PD184352 or the novel clinical candidate AZD6244 (ARRY-142886) overcomes growth factor independence, causing CRC cell death. BIM is de-phosphorylated and upregulated following MEK1/2 inhibition in all CRC cell lines studied and knockdown of BIM reduces cell death, indicating that repression of BIM is a major part of the ability of BRAF^V600E to confer growth factor-independent survival. We conclude that a single endogenous BRAF^V600E allele is sufficient to repress BIM and prevent death arising from growth factor withdrawal, and CRC cells with BRAF^V600E mutations are addicted to the ERK1/2 pathway for repression of BIM and growth factor-independent survival.