Raf induces NF-kappaB by membrane shuttle kinase MEKK1, a signaling pathway critical for transformation.
NF-kappaB is regulated by inhibitor proteins (IkappaBs), which retain NF-kappaB in the cytoplasm. Signal-induced phosphorylation by the IkappaB-kinase complex containing the IkappaB-kinases 1 and 2 (IKK-1/2 or IKK-alpha/beta) and subsequent degradation of the IkappaB proteins are prerequisites for NF-kappaB activation. Many signals induce NF-kappaB, one of them being oncogenic Raf kinase. We investigated whether NF-kappaB induction is critical for Raf-mediated transformation. Here, we demonstrate that inhibition of NF-kappaB interferes with transformation by the Raf-oncogene, and we characterized the mechanism of NF-kappaB induction by activated Raf kinase and the tumor promoter phorbol 12-myristate 13-acetate (PMA). NF-kappaB activation by PMA and Raf critically depends on the IkappaB-kinase complex, most notably on IKK-2. A major signaling pathway induced by Raf is the mitogenic cytoplasmic kinase cascade. However, different inhibitors of this cascade do not affect PMA- and Raf-mediated NF-kappaB activation. Raf does not phosphorylate the IkappaB-kinase proteins directly. Raf rather synergizes with another membrane shuttle kinase MEKK1, and Raf-mediated activation of NF-kappaB is blocked by a dominant negative form of MEKK1. These results suggest that Raf induction of NF-kappaB is relayed by MEKK1, but not by the classical mitogenic cytoplasmic kinase cascade.
Baumann
B
B
Institut für Medizinische Strahlenkunde und Zellforschung, Universität Würzburg Versbacher Strasse 5, 97078 Würzburg, Germany.
Weber
C K
CK
Troppmair
J
J
Whiteside
S
S
Israel
A
A
Rapp
U R
UR
Wirth
T
T
D000818:Animals; D002460:Cell Line; D017930:Genes, Reporter; D051550:I-kappa B Kinase; D016328:NF-kappa B / Q000378:metabolism*; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism*; D019908:Proto-Oncogene Proteins c-raf / Q000378:metabolism*; D011993:Recombinant Fusion Proteins / Q000378:metabolism; D011994:Recombinant Proteins / Q000378:metabolism; D015398:Signal Transduction; D018411:Spodoptera; D013755:Tetradecanoylphorbol Acetate / Q000494:pharmacology; D014162:Transfection
D016328:NF-kappa B; D011993:Recombinant Fusion Proteins; D011994:Recombinant Proteins; D017346:Protein Serine-Threonine Kinases; D019908:Proto-Oncogene Proteins c-raf; D051550:I-kappa B Kinase; D013755:Tetradecanoylphorbol Acetate