Negative feedback regulation of NF-κB-inducing kinase is proteasome-dependent but does not require cellular inhibitors of apoptosis.

Non-canonical NF-κB signaling is controlled by the precise regulation of NF-κB inducing kinase (NIK) stability. NIK is constitutively ubiquitylated by cellular inhibitor of apoptosis (cIAP) proteins 1 and 2, leading to its complete proteasomal degradation in resting cells. Following stimulation, cIAP-mediated ubiquitylation of NIK ceases and NIK is stabilized, allowing for inhibitor of κB kinase (IKK)α activation and non-canonical NF-κB signaling. Non-canonical NF-κB signaling is terminated by feedback phosphorylation of NIK by IKKα that promotes NIK degradation; however, the mechanism of active NIK protein turnover remains unknown. To address this question, we established a strategy to precisely distinguish between basal degradation of newly synthesized endogenous NIK and induced active NIK in stimulated cells. Using this approach, we found that IKKα-mediated degradation of signal-induced activated NIK occurs through the proteasome. To determine whether cIAP1 or cIAP2 play a role in active NIK turnover, we utilized a Smac mimetic (GT13072), which promotes degradation of these E3 ubiquitin ligases. As expected, GT13072 stabilized NIK in resting cells. However, loss of the cIAPs did not inhibit proteasome-dependent turnover of signal-induced NIK showing that unlike the basal regulatory mechanism, active NIK turnover is independent of cIAP1 and cIAP2. Our results therefore establish that the negative feedback control of IKKα-mediated NIK turnover occurs via a novel proteasome-dependent and cIAP-independent mechanism.