Crucial roles of binding sites for NF-κB and C/EBPs in IκB-ζ-mediated transcriptional activation

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Abstract
IκB-ζ [inhibitor of NF-κB (nuclear factor κB) ζ] is a nuclear protein that is induced upon stimulation of TLRs (Toll-like receptors) and IL (interleukin)-1 receptor. IκB-ζ harbours C-terminal ankyrin repeats that interact with NF-κB. Our recent studies have shown that, upon stimulation, IκB-ζ is essential for the induction of a subset of inflammatory genes, represented by IL-6, whereas it inhibits the expression of TNF (tumour necrosis factor)-α. In the present study, we investigated mechanisms that determine the different functions of IκB-ζ. We found that co-expression of IκB-ζ and the NF-κB subunits synergistically activates transcription of the hBD-2 (human β-defensin 2) and NGAL (neutrophil gelatinase-associated lipocalin) genes, whereas it inhibits transcription of E-selectin. Reporter analyses indicated that, in addition to an NF-κB-binding site, a flanking C/EBP (CCAAT/enhancer-binding protein)-binding site in the promoters is essential for the IκB-ζ-mediated transcriptional activation. Using an artificial promoter consisting of the NF-κB- and C/EBP-binding sites, transcriptional activation was observed upon co-transfection with IκB-ζ and NF-κB, indicating that these sequences are minimal elements that confer the IκB-ζ-mediated transcriptional activation. Chromatin immunoprecipitation assays and knockdown experiments showed that both IκB-ζ and the NF-κB subunits were recruited to the NGAL promoter and were essential for the transcriptional activation of the hBD-2 and NGAL promoters on stimulation with IL-1β. The activation of the NGAL promoter by transfection of IκB-ζ and NF-κB was suppressed in C/EBPβ-depleted cells. Thus IκB-ζ acts as an essential transcriptional activator by forming a complex with NF-κB on promoters harbouring the NF-κB- and C/EBP-binding sites, upon stimulation of TLRs or IL-1 receptor.