Oncogenic B-RafV600E inhibits apoptosis and promotes ERK-dependent inactivation of Bad and Bim

J Biol Chem. 2008 Aug 8;283(32):22128-35. doi: 10.1074/jbc.M800271200. Epub 2008 May 27.

Abstract

Recent studies have revealed that B-Raf mutations are very common in malignant melanoma and are required for tumor growth and maintenance. The majority of melanoma-associated B-Raf mutations involve a single point mutation, V600E, which results in greatly elevated B-Raf kinase activity and constitutive activation of MAPK/ERK downstream. Here we show that B-Raf(V600E) increases resistance to apoptosis induced by chemotherapeutic drugs and promotes ERK-dependent phosphorylation of the BH3-only proteins Bim and Bad that are involved in setting thresholds for apoptosis. ERK-dependent phosphorylation of Bim resulted in degradation of this BH3-only protein, whereas phosphorylation of Bad has previously been shown to result in its sequestration by 14-3-3 proteins. Consistent with this, inhibition of ERK activity in a panel of melanoma cell lines resulted in stabilization of Bim and dephosphorylation of Bad. Furthermore, apoptosis induced through overexpression of Bad or Bim was efficiently blocked by coexpression of mutant B-Raf(V600E). However, small interfering RNA-mediated silencing of Bim and Bad expression conferred only modest protection against cytotoxic drugs, whereas oncogenic B-Raf strongly protected against the same stimuli. These observations suggest that B-Raf-initiated inactivation of Bad and Bim only partly contributes to the anti-apoptotic activities of this oncogene and that other points within the cell death machinery are also targeted by deregulated ERK signaling.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Apoptosis Regulatory Proteins / metabolism*
  • Apoptosis*
  • Bcl-2-Like Protein 11
  • Cell Line, Tumor
  • Extracellular Signal-Regulated MAP Kinases / metabolism*
  • HeLa Cells
  • Humans
  • Melanoma / physiopathology
  • Membrane Proteins / metabolism*
  • Mitochondria / metabolism
  • Oncogenes / genetics*
  • Phosphorylation
  • Point Mutation
  • Protein Transport
  • Proto-Oncogene Proteins / metabolism*
  • Proto-Oncogene Proteins B-raf / genetics*
  • Proto-Oncogene Proteins B-raf / metabolism*
  • Serine / metabolism
  • bcl-2-Associated X Protein / metabolism
  • bcl-Associated Death Protein / metabolism*

Substances

  • Apoptosis Regulatory Proteins
  • BAD protein, human
  • BCL2L11 protein, human
  • Bcl-2-Like Protein 11
  • Membrane Proteins
  • Proto-Oncogene Proteins
  • bcl-2-Associated X Protein
  • bcl-Associated Death Protein
  • Serine
  • Proto-Oncogene Proteins B-raf
  • Extracellular Signal-Regulated MAP Kinases