Both cIAP1 and cIAP2 regulate TNFα-mediated NF-κB activation

The cellular inhibitor of apoptosis 1 and 2 (cIAP1 and cIAP2) proteins have been implicated in the activation of NF-kappa B by TNF alpha; however, genetic deletion of either cIAP1 or 2 did not support a physiologically relevant role, perhaps because of functional redundancy. To address this, we used combined genetic and siRNA knockdown approaches and report that cIAP1 and 2 are indeed critical, yet redundant, regulators of NF-kappa B activation upon TNFa treatment. Whereas NF-kappa B was properly activated by TNF alpha in cultured and primary cells deficient in either cIAP1 or 2, removal of both cIAPs severely blunted its activation. After treatment with TNFa, cIAP1 and 2 were rapidly recruited to the TNF receptor 1, along with the adapter protein TNF receptor associated factor 2. Importantly, either cIAP1 or 2 was required for proper TNF receptor 1 signalosome function. In their combined absence, polyubiquitination of receptor interacting protein 1, an upstream event necessary for NF-kappa B signaling, was attenuated. As a result, phosphorylation of the inhibitor of kappa B kinase beta was diminished, and signal transduction was severely blunted. Consequently, cells missing both cIAP1 and 2 were sensitized to TNF alpha-mediated apoptosis. Collectively, these data demonstrate that either cIAP1 or 2, is required for proper Rip1 polyubiquitination and NF-kappa B activation upon TNF alpha treatment.