Inflammatory Signaling And Diseases Mediated By TLR-TRAF6 In Macrophage

Interleukin-33(IL-33),an IL-1 family cytokine and nuclear alarmin,is constitutively expressed in epithelial barrier tissues and human blood vessels.However,little is known about the induced expression of IL-33 in monocytes and macrophages,which are major cytokine-producing cells of the innate immune system.Here,we report the induction of IL-33 expression in both human monocytes and mouse macrophages from C57BL/6 mice by the acute-phase protein serum amyloid A(SAA).SAA-induced transcriptional activation of the IL-33 gene,resulting in nuclear accumulation of the IL-33 protein.TLR2,one of the SAA receptors,was primarily responsible for the induction of IL-33.Progressive deletion of the human IL-33 promoter led to the identification of two potential binding sites for interferon regulatory factor 7(IRF7),one of which(-277/-257)was found to be important for SAA-stimulated IL-33 promoter activity.IRF7 was recruited to the IL-33 promoter upon SAA stimulation,and silencing IRF7 expression in THP-1 cells abrogated SAA-induced IL-33 expression.Taken together,these results identify IRF7 as a critical transcription factor for SAA-induced IL-33 expression in monocytes and macrophages.SAA also promoted an interaction between TNF receptor-associated factor 6 and IRF7.TRAF6 plays an important role,extensively exists in TLR,IL-1R and TNFR signaling pathway,in the activation of NF-κB.Dimerization and K63-linked polyubiquitination of TRAF6 is an critical way to mediated the downstream signaling pathway.The results of luciferase reporter assay indicated that an ubiquitin E3 ligase component,Cullin-5(Cul-5),acted as a positive regulator of TRAF6-dependent inflammation.Cullin-5 deficient mice displayed persistent and less expression of proinflammatory mediators in response to endotoxin.Accordingly,Cul-5 greatly suppressed LPS-induced NFmitogen-activated protein kinase activation and subsequent production of inflammatory mediators in macrophages.Mechanistically,we found that Cullin-5 can directly interact with the adaptor TRAF6 to enhance its dimerization and K63-linked polyubiquitination via C-terminal domain(CTD).Neddylation modification of Cul-5 further promoted dimerization of TRAF6 by facilitating the conformational change of Cul-5 and thus regulated polyubiquitination of TRAF6.Our findings identify Cul-5 as a critical regulator of TRAF6 activation and highlight a potential anti-inflammatory property of Cul-5 for treatment of inflammatory diseases.