IRF-8/Interferon (IFN) Consensus Sequence-binding Protein Is Involved in Toll-like Receptor (TLR) Signaling and Contributes to the Cross-talk between TLR and IFN-γ Signaling Pathways

Min Yang,Feng Zheng,Hee Jeong Kong,Milena Bogunovic,Lloyd Mayer,Hongxing Li,Chen Zhu,Jay Unkeless,Huabao Xiong,Bo Huang,Jie Zhao,Keiko Ozato

doi:10.1074/jbc.m507788200

Abstract

Toll-like receptor (TLR) and interferon-gamma (IFN-gamma) signaling pathways are important for both innate and adaptive immune responses. However, the cross-talk between these two signaling pathways is incompletely understood. Here we show that IFN-gamma and LPS synergistically induce the expression of proinflammatory factors, including interleukin-1 (IL-1), IL-6, IL-12, NO, and tumor necrosis factor-alpha (TNF-alpha). Comparable synergism was observed between IFN-gamma and peptidoglycan (PGN; a TLR2 ligand) and poly(I:C) (a TLR3 ligand) in the induction of IL-12 promoter activity. IFN-gamma enhanced lipopolysaccharide (LPS)-induced ERK and JNK phosphorylation but had no effect on LPS-induced NF-kappaB activation. Interestingly, we found that IRF-8-/- macrophages were impaired in the activation of LPS-induced ERK and JNK and the production of proinflammatory cytokines induced by LPS or IFN-gamma plus LPS. Retroviral transduction of IRF-8 into IRF-8-/- macrophages rescued ERK and JNK activation. Furthermore, co-immunoprecipitation experiments show that IRF-8 physically interacts with TRAF6 at a binding site between amino acid residues 356 and 305 of IRF-8. Transfection of IRF-8 enhanced TRAF6 ubiquitination, which is consistent with a physical interaction of IRF-8 with TRAF6. Taken together, the results suggest that the interaction of IRF-8 with TRAF6 modulates TLR signaling and may contribute to the cross-talk between IFN-gamma and TLR signal pathways.

Highlights

Toll-like receptors (TLRs),4 a family of evolutionarily conserved receptors, play a crucial role in early host defense against invading pathogens [1,2,3]
The results suggest that the interaction of IFN regulatory factor (IRF)-8 with tumor necrosis factor receptor-associated factor 6 (TRAF6) modulates TLR signaling and may contribute to the crosstalk between IFN-␥ and TLR signal pathways
In this study we investigate the role of IRF-8 in the cross-talk between IFN-␥ and TLR signaling

Summary

Introduction

Toll-like receptors (TLRs), a family of evolutionarily conserved receptors, play a crucial role in early host defense against invading pathogens [1,2,3]. IFN signaling involves a variety of trans- and cis-acting factors and is mediated through DNA motifs, designated the IFN-stimulated response element and IFN-␥-activated sequence, found in promoters of IFN-inducible genes [10] These genes play prominent roles in the control of growth, differentiation, and activation of the immune system. The loss of IRF-8 binding to an IFN-stimulated response element site in the IL-12 promoter region in IRF8-deficient macrophages results in a defect in IL-12 production This evidence does not necessarily rule out the possibility that IRF-8 may affect upstream signaling pathways leading to IL-12 production. IRF-8 binds to TRAF6 and regulates TRAF6 ubiquitination These results show that IRF-8 is an important molecule mediating cross-talk between the TLR and IFN-␥ signaling pathways and identify IRF-8 as a potential target of therapeutic intervention to control harmful immune responses

Methods

Results

Conclusion