Role of receptor-interacting protein in tumor necrosis factor-alpha -dependent MEKK1 activation.

Receptor-interacting protein (RIP), a death domain serine/threonine kinase, has been shown to play a critical role in tumor necrosis factor-alpha (TNF-alpha)-induced activation of the nuclear factor-kappaB signaling pathway. We demonstrate here that ectopically expressed RIP induces I-kappaB kinase-beta (IKKbeta) activation in intact cells and that RIP-induced IKKbeta activation can be blocked by a kinase-inactive form of MEKK1, MEKK1(K1253M). Interestingly, RIP physically associated with MEKK1 both in vitro and in vivo. RIP phosphorylated MEKK1 at Ser-957 and Ser-994. Our data also indicate that RIP induced the stimulation of MEKK1 but not MEKK1(S957A/S994A) in transfected cells. Furthermore, overexpressed MEKK1(S957A/S994A) inhibited the RIP-induced activation of both IKKbeta and nuclear factor-kappaB. We also demonstrated that the TNF-alpha-induced MEKK1 activation was defective in RIP-deficient Jurkat cells. Taken together, our results suggest that RIP phosphorylates and activates MEKK1 and that RIP is involved in TNF-alpha-induced MEKK1 activation.