title: MLK3 is required for mitogen activation of B-Raf, ERK and cell proliferation.
abstract: 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.
authors.affiliation: The 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.
authors.forename: Deborah N
authors.initials: DN
authors.lastname: Chadee
authors.forename: John M
authors.initials: JM
authors.lastname: Kyriakis
mesh_terms: D002455:Cell Division; D002460:Cell Line; D004730:Endothelium, Vascular / Q000166:cytology / Q000201:enzymology; D004789:Enzyme Activation; D020868:Gene Silencing; D019073:HT29 Cells; D006801:Humans; D019169:Jurkat Cells; D020930:MAP Kinase Kinase Kinases / Q000235:genetics / Q000502:physiology*; D020928:Mitogen-Activated Protein Kinases / Q000187:drug effects / Q000378:metabolism / Q000502:physiology*; D008934:Mitogens / Q000494:pharmacology*; D010766:Phosphorylation; D019908:Proto-Oncogene Proteins c-raf / Q000737:chemistry / Q000187:drug effects / Q000378:metabolism / Q000502:physiology*; D034622:RNA Interference; D012694:Serine / Q000737:chemistry; D013912:Threonine / Q000737:chemistry; D014471:Umbilical Veins / Q000166:cytology; D000097767:Mitogen-Activated Protein Kinase Kinase Kinase 11
chemical_list: D020930:MAP Kinase Kinase Kinases; D020928:Mitogen-Activated Protein Kinases; D008934:Mitogens; D019908:Proto-Oncogene Proteins c-raf; D012694:Serine; D013912:Threonine; D000097767:Mitogen-Activated Protein Kinase Kinase Kinase 11