Raf induces NF-kappa B 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-alp ha/beta) and subsequent degradation of the I kappa B proteins are prerequis ites 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 i s critical for Raf-mediated transformation. Here, we demonstrate that inhib ition of NF-kappa B interferes with transformation by the Raf-oncogene, and we characterized the mechanism of NF-kappa B induction by activated Raf ki nase 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 com plex, most notably on IKK-2. A major signaling pathway induced by Raf is th e mitogenic cytoplasmic kinase cascade. However. different inhibitors of th is cascade do not affect PMA- and Raf-mediated NF-kappa B activation. Raf d oes not phosphorylate the I kappa B-kinase proteins directly. Raf rather sy nergizes with another membrane shuttle kinase MEKK1. and Raf-mediated activ ation 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 n ot by the classical mitogenic cytoplasmic kinase cascade.