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 endogenousBRAFV600Eallele 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 harbouringBRAFV600Eare 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 BRAFV600E to confer growth factor-independent survival. We conclude that a single endogenousBRAFV600Eallele is sufficient to repress BIM and prevent death arising from growth factor withdrawal, and CRC cells with BRAFV600E mutations are addicted to the ERK1/2 pathway for repression of BIM and growth factor-independent survival.
Oncogene (2008) 27, 7150-7161; doi:10.1038/onc.2008.335; published online 22 September 2008