Regulation of IkB kinase family crosstalk by an N4BP1-caspase-8 axis [RNA-seq NGS3611]

The cell death protease caspase-8 plays an essential role in controlling inflammation, as severe immunodeficiency results from its loss. We previously found that caspase-8 promotes inflammatory responses by cleaving NEDD4-binding protein 1 (N4BP1), a suppressor of cytokine production, but the underlying mechanisms remained unclear. Here we find that N4BP1 curtails the duration, rather than initial induction, of proinflammatory signaling through a mechanism involving noncanonical IKK (ncIKK)-mediated inhibition of the canonical IkB kinase (IKK) complex, a crosstalk event among the IKK family facilitated by N4BP1. Accordingly, co-deletion of the ncIKKs or their adaptor protein TANK largely phenocopied deletion of N4BP1, augmenting cytokine responses by macrophages upon engagement of TRIF-independent toll-like receptors (TLR) 1/2, TLR7, or TLR9. Like N4BP1, TANK was largely prevented from inhibiting the TRIF-dependent TLR4 response due to caspase-8. Biochemically, N4BP1 binds both the canonical and noncanonical IKK complexes, in a manner promoted by linear and/or K63-linked polyubiquitin chain binding by N4BP1 and independent of its RNAse activity. Consistent with this, a knock-in mutant of N4BP1 with diminished ubiquitin chain-binding capacity led to increased proinflammatory cytokine responses. These findings thereby unveil a mechanism of late-phase inflammatory gene control, whereby N4BP1 prevents persistent IKK activity through ncIKK-mediated inhibition. This molecular crosstalk among caspase-8, N4BP1, and the IKKs and ncIKKs may have implications for our understanding of genetic immune diseases caused by mutations in caspase-8 or TBK1 and suggest a novel ‘guarding’ mechanism against pathogens that attempt to subvert the ncIKKs.