p97/VCP promotes Cullin-RING-ubiquitin-ligase/proteasome-dependent degradation of IκBα and the preceding liberation of RelA from ubiquitinated IκBα

Cullin-RING-ubiquitin-ligase (CRL)-dependent ubiquitination of the nuclear factor kappa B (NF-kappa B) inhibitor I kappa B alpha and its subsequent degradation by the proteasome usually precede NF-kappa B/RelA nuclear activity. Through removal of the CRL-activating modification of their cullin subunit with the ubiquitin (Ub)-like modifier NEDD8, the COP9 signalosome (CSN) opposes CRL Ub-ligase activity. While RelA phosphorylation was observed to mediate NF-kappa B activation independent of Ub-proteasome-pathway (UPP)-dependent turnover of I kappa B alpha in some studies, a strict requirement of the p97/VCP ATPase for both, I kappa B alpha degradation and NF-kappa B activation, was reported in others. In this study, we thus aimed to reconcile the mechanism for tumour necrosis factor (TNF)-induced NF-kappa B activation. We found that inducible phosphorylation of RelA is accomplished in an IKK-complex-dependent manner within the NF-kappa B/RelA-I kappa B alpha-complex contemporaneous with the phosphorylation of I kappa B alpha, and that RelA phosphorylation is not sufficient to dissociate NF-kappa B/RelA from I kappa B alpha. Subsequent to CRL-dependent I kappa B alpha ubiquitination functional p97/VCP is essentially required for efficient liberation of (phosphorylated) RelA from I kappa B alpha, preceding p97/VCP-promoted timely and efficient degradation of I kappa B alpha well as simultaneous NF-kappa B/RelA nuclear translocation. Collectively, our data add new facets to the knowledge about maintenance of I kappa B alpha RelA expression, likely depending on p97/VCP-supported scheduled basal NF-kappa B activity, and the mechanism of TNF-induced NF-kappa B activation.