The Death Domain Kinase RIP1 is Essential for Tumor Necrosis Factor Alpha Signaling to p38 Mitogen-Activated Protein Kinase

The cytokine tumor necrosis factor alpha (TNF-α) stimulates the NF-κB, SAPK/JNK, and p38 mitogen-activated protein (MAP) kinase pathways by recruiting RIP1 and TRAF2 proteins to the tumor necrosis factor receptor 1 (TNFR1). Genetic studies have revealed that RIP1 links the TNFR1 to the IκB kinase (IKK) complex, whereas TRAF2 couples the TNFR1 to the SAPK/JNK cascade. In transfection studies, RIP1 and TRAF2 stimulate p38 MAP kinase activation, and dominant-negative forms of RIP1 and TRAF2 inhibit TNF-α-induced p38 MAP kinase activation. We found TNF-α-induced p38 MAP kinase activation and interleukin-6 (IL-6) production impaired in rip1 ^-/- murine embryonic fibroblasts (MEF) but unaffected in traf2 ^-1- MEF. Yet, both rip1^-/- and traf2^-/- MEF exhibit a normal p38 MAP kinase response to inducers of osmotic shock or IL-1α. Thus, RIP1 is a specific mediator of the p38 MAP kinase response to TNF-α. These studies suggest that TNF-α-induced activation of p38 MAP kinase and SAPK/JNK pathways bifurcate at the level of RIP1 and TRAF2. Moreover, endogenous RIP1 associates with the MAP kinase kinase kinase (MAP3K) MEKK3 in TNF-α-treated cells, and decreased TNF-α-induced p38 MAP kinase activation is observed in Mekk3^-/- cells. Taken together, these studies suggest a mechanism whereby RIP1 may mediate the p38 MAP kinase response to TNF-α, by recruiting the MAP3K MEKK3.