Raf induces NF-κB by membrane shuttle kinase MEKK1, a signaling pathway critical for transformation

NF-kappa B is regulated by inhibitor proteins (I kappa Bs), which retain NF-kappa B in the cytoplasm. Signal-induced phosphorylation by the I kappa B-kinase complex containing the I kappa B-kinases 1 and 2 (IKK-1/2 or IKK-alpha/beta) and subsequent degradation of the I kappa B proteins are prerequisites for NF-kappa B activation. Many signals induce NF-kappa B, one of them being oncogenic Raf kinase. We investigated whether NF-kappa B induction is critical for Raf-mediated transformation. Here, we demonstrate that inhibition of NF-kappa B interferes with transformation by the Raf-oncogene, and we characterized the mechanism of NF-kappa B induction by activated Raf kinase and the tumor promoter phorbol 12-myristate 13-acetate (PMA). NF-kappa B activation by PMA and Raf critically depends on the I kappa B-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-kappa B activation. Raf does not phosphorylate the I kappa B-kinase proteins directly. Raf rather synergizes with another membrane shuttle kinase MEKK1. and Raf-mediated activation of NF-kappa B is blocked by a dominant negative form of MEKK1. These results suggest that Raf induction of NF-kappa B is relayed by MEKK1, but not by the classical mitogenic cytoplasmic kinase cascade.