TNFα-induced noncanonical NF-κB activation is attenuated by RIP1 through stabilization of TRAF2

The current paradigm of noncanonical NF-kappa B signaling suggests that the loss of TRAF2, TRAF3 or cIAP1 and cIAP2 leads to stabilization of NF-kappa B-inducing kinase (NIK) to activate the noncanonical pathway. Although a crucial role of RIP1 in the TNF alpha-induced canonical NF-kappa B pathway has been well established, its involvement in noncanonical activation of NF-kappa B through the TNFR1 receptor, is unknown. Here we show that TNF alpha is capable of activating the noncanonical NF-kappa B pathway, but that activation of this pathway is negatively regulated by RIP1. In the absence of RIP1, TNFR1 stimulation leads to activation of the noncanonical NF-kappa B pathway through TRAF2 degradation, leading to NIK stabilization, IKK alpha phosphorylation and the processing of p100 to generate p52. Thus although RIP1(-/-) mouse embryonic fibroblasts are sensitive at early time points to cell death induced by TNF alpha, probably as a result of lack of canonical NF-kappa B activation, the late activation of the noncanonical NF-kappa B pathway protects the remaining cells from further cell death. The TNFR1-dependent noncanonical NF-kappa B activation in RIP1(-/-) cells suggests that there is functional interplay between the two NF-kappa B pathways during TNFR1 signaling, which might regulate the number and kinds of NF-kappa B transcription factors and thus finely control NF-kappa B-dependent gene transcription.