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

Deborah N Chadee1, John M Kyriakis

  • 1The Molecular Cardiology Research Institute, Department of Medicine, Tufts-New England Medical Center and The Department of Medicine, Tufts University School of Medicine, 750 Washington Street, Box 8486, Boston, MA 02111, USA.

Nature Cell Biology
|July 20, 2004
PubMed

Insights

Mixed lineage kinase-3 (MLK3) unexpectedly regulates ERK and cell proliferation. Silencing MLK3 blocks B-Raf phosphorylation, impacting mitogen and cytokine signaling pathways essential for cell growth.

Area of Science:

  • Cellular signaling pathways
  • Molecular biology
  • Cancer research

Background:

  • Mitogen-activated protein kinases (MAPKs) like ERK are crucial for cell proliferation.
  • Raf proteins, particularly B-Raf, act as specific MAP3Ks activating the ERK pathway.
  • Mixed lineage kinase-3 (MLK3) was previously considered a selective regulator of JNK signaling.

Purpose of the Study:

  • To investigate the role of MLK3 in regulating MAPK pathways beyond JNK.
  • To determine MLK3's involvement in B-Raf activation and cell proliferation.
  • To elucidate MLK3's function in cancer cell growth driven by specific mutations.

Main Methods:

  • RNA interference (RNAi) to silence MLK3 expression.
  • Assessing the activation of JNK, ERK, and p38 MAPKs.
  • Analyzing B-Raf phosphorylation at specific sites (Thr 598 and Ser 601).
  • Evaluating cell proliferation in response to mitogens, cytokines, and specific genetic mutations (Ras, Raf, NF1, NF2).

Main Results:

  • MLK3 silencing suppressed the activation of JNK, ERK, and p38 pathways by mitogens and cytokines.
  • MLK3 depletion blocked mitogen-stimulated B-Raf phosphorylation, a key activation step.
  • Silencing MLK3 inhibited serum-stimulated cell proliferation and the growth of tumor cells with oncogenic Ki-Ras or NF1/NF2 mutations.
  • Tumor cell proliferation driven by activating B-raf or raf-1 mutations was not affected by MLK3 silencing.

Conclusions:

  • MLK3 plays an unexpected and critical role in the regulation of B-Raf and ERK signaling.
  • MLK3 is essential for mitogen-driven cell proliferation, particularly in contexts involving Ras or NF1/NF2 mutations.
  • These findings reveal a novel mechanism linking MLK3 to cell growth control and offer potential therapeutic targets in specific cancers.

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