MLK3 is required for mitogen activation of B-Raf, ERK and cell proliferation

Nat Cell Biol. 2004 Aug;6(8):770-6. doi: 10.1038/ncb1152. Epub 2004 Jul 18.


The ERK group of mitogen-activated protein kinases (MAPKs) is essential for cell proliferation stimulated by mitogens, oncogenic ras and raf (ref. 1). All MAPKs are activated by MAP3K/MEK/MAPK core pathways and the Raf proto-oncoproteins, especially B-Raf, are ERK-specific MAP3Ks (refs 1-3). Mixed lineage kinase-3 (MLK3) is a MAP3K that was thought to be a cytokine-activated, and comparatively selective, regulator of the JNK group of MAPKs (refs 1, 4-6). Here we report that silencing of mlk3 by RNAi suppressed mitogen and cytokine activation not only of JNK but of ERK and p38 as well. Silencing mlk3 also blocked mitogen-stimulated phosphorylation of B-Raf at Thr 598 and Ser 601, a step required for B-Raf activation. Furthermore, silencing mlk3 prevented serum-stimulated cell proliferation and the proliferation of tumour cells bearing either oncogenic Ki-Ras or loss-of-function neurofibromatosis-1 (NF1) or NF2 mutations. The proliferation of tumour cells containing activating B-raf or raf-1 mutations was unaffected by silencing mlk3. Our results define an unexpected role for MLK3 in mitogen regulation of B-Raf, ERK and cell proliferation.

Publication types

  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Cell Division*
  • Cell Line
  • Endothelium, Vascular / cytology
  • Endothelium, Vascular / enzymology
  • Enzyme Activation
  • Gene Silencing
  • HT29 Cells
  • Humans
  • Jurkat Cells
  • MAP Kinase Kinase Kinases / genetics
  • MAP Kinase Kinase Kinases / physiology*
  • Mitogen-Activated Protein Kinases / drug effects
  • Mitogen-Activated Protein Kinases / metabolism
  • Mitogen-Activated Protein Kinases / physiology*
  • Mitogens / pharmacology*
  • Phosphorylation
  • Proto-Oncogene Proteins c-raf / chemistry
  • Proto-Oncogene Proteins c-raf / drug effects
  • Proto-Oncogene Proteins c-raf / metabolism
  • Proto-Oncogene Proteins c-raf / physiology*
  • RNA Interference
  • Serine / chemistry
  • Threonine / chemistry
  • Umbilical Veins / cytology


  • Mitogens
  • Threonine
  • Serine
  • Proto-Oncogene Proteins c-raf
  • Mitogen-Activated Protein Kinases
  • MAP Kinase Kinase Kinases
  • mitogen-activated protein kinase kinase kinase 11