NF-κB inducing kinase activates NF-κB transcriptional activity independently of IκB kinase γ through a p38 MAPK-dependent RelA phosphorylation pathway

Abstract

Molecular and biochemical analysis indicates that nuclear transcription factor κB (NF-κB)-inducing kinase (NIK) mediates IKK activation and NF-κB transcriptional activity. However, gene deletion studies suggest that NIK triggers gene expression without affecting IκBα degradation and NF-κB DNA binding activity. In order to investigate the role of NIK in NF-κB transcriptional activity, we used mouse embryonic fibroblasts (MEF) derived from wild-type (wt) and IκB kinase γ (IKKγ) gene deficient (IKKγ −/−) mice. We report that although TNF-induced NF-κB transcriptional activity is abolished in IKKγ −/− cells, adenoviral gene delivery of NIK (Ad5NIK) still enhanced transcriptional activity and IL-6 mRNA accumulation. Moreover, NIK targets the transactivation function of NF-κB through stimulation of the transactivation domain (TAD) of RelA (S536) in IKKγ −/− cells. Interestingly, Ad5NIK, but not TNF, induces RelA S536 and p38 mitogen-activated protein kinase (MAPK) phosphorylation in IKKγ −/− cells. Functional analysis demonstrated that Ad5NIK-induced NF-κB transcriptional activity, IL-6 mRNA expression and RelA phosphorylation are inhibited by the p38 inhibitor SB203580, suggesting a role for this MAPK in NIK signaling to NF-κB. These data demonstrate for the first time the presence of an IKKγ-independent NIK/p38 MAPK-dependent signaling pathway that activates NF-κB and induces pro-inflammatory gene expression through RelA phosphorylation.