Soh Yamazaki

Regulation of Toll/IL-1-receptor-mediated gene expression by the inducible nuclear protein IκBζ

Toll-like receptors (TLRs) recognize microbial components and trigger the inflammatory and immune responses against pathogens. IκBζ (also known as MAIL and INAP) is an ankyrin-repeat-containing nuclear protein that is highly homologous to the IκB family member Bcl-3 (refs 1–6). Transcription of IκBζ is rapidly induced by stimulation with TLR ligands and interleukin-1 (IL-1). Here we show that IκBζ is indispensable for the expression of a subset of genes activated in TLR/IL-1R signalling pathways. IκBζ-deficient cells show severe impairment of IL-6 production in response to a variety of TLR ligands as well as IL-1, but not in response to tumour-necrosis factor-α. Endogenous IκBζ specifically associates with the p50 subunit of NF-κB, and is recruited to the NF-κB binding site of the IL-6 promoter on stimulation. Moreover, NF-κB1/p50-deficient mice show responses to TLR/IL-1R ligands similar to those of IκBζ-deficient mice. Endotoxin-induced expression of other genes such as Il12b and Csf2 is also abrogated in IκBζ-deficient macrophages. Given that the lipopolysaccharide-induced transcription of IκBζ occurs earlier than transcription of these genes, some TLR/IL-1R-mediated responses may be regulated in a gene expression process of at least two steps that requires inducible IκBζ.

Stimulus-specific Induction of a Novel Nuclear Factor-κB Regulator, IκB-ζ, via Toll/Interleukin-1 Receptor Is Mediated by mRNA Stabilization

We have recently identified an inducible nuclear factor-κB (NF-κB) regulator, IκB-ζ, which is induced by microbial ligands for Toll-like receptors such as lipopolysaccharide and the proinflammatory cytokine interleukin (IL)-1β but not by tumor necrosis factor (TNF)-α. In the present study, we examined mechanisms for stimulus-specific induction of IκB-ζ. The analysis of the IκB-ζ promoter revealed an essential role for an NF-κB binding sequence in transcriptional activation. The activation, however, did not account for the Toll-like receptor/IL-1 receptor-specific induction of IκB-ζ, because the promoter analysis and nuclear run-on analysis indicated that its transcription was similarly induced by TNF-α. To examine post-transcriptional regulation, we analyzed the decay of IκB-ζ mRNA, and we found that it was specifically stabilized by lipopolysaccharide or IL-1β but not by TNF-α. Furthermore, we found that costimulation with TNF-α and another proinflammatory cytokine, IL-17, elicited the IκB-ζ induction. Stimulation with IL-17 alone did not induce IκB-ζ but stabilized its mRNA. Therefore, IκB-ζ induction requires both NF-κB activation and stimulus-specific stabilization of its mRNA. Because IκB-ζ is essential for expression of a subset of NF-κB target genes, the stimulus-specific induction of IκB-ζ may be of great significance in regulation of inflammatory reactions.

Stimulus-specific induction of a novel NF-κB regulator, IκB- , via toll/interleukin-1 receptor is mediated by mRNA stabilization

We have recently identified an inducible nuclear factor-κB (NF-κB) regulator, IκB-ζ, which is induced by microbial ligands for Toll-like receptors such as lipopolysaccharide and the proinflammatory cytokine interleukin (IL)-1β but not by tumor necrosis factor (TNF)-α. In the present study, we examined mechanisms for stimulus-specific induction of IκB-ζ. The analysis of the IκB-ζ promoter revealed an essential role for an NF-κB binding sequence in transcriptional activation. The activation, however, did not account for the Toll-like receptor/IL-1 receptor-specific induction of IκB-ζ, because the promoter analysis and nuclear run-on analysis indicated that its transcription was similarly induced by TNF-α. To examine post-transcriptional regulation, we analyzed the decay of IκB-ζ mRNA, and we found that it was specifically stabilized by lipopolysaccharide or IL-1β but not by TNF-α. Furthermore, we found that costimulation with TNF-α and another proinflammatory cytokine, IL-17, elicited the IκB-ζ induction. Stimulation with IL-17 alone did not induce IκB-ζ but stabilized its mRNA. Therefore, IκB-ζ induction requires both NF-κB activation and stimulus-specific stabilization of its mRNA. Because IκB-ζ is essential for expression of a subset of NF-κB target genes, the stimulus-specific induction of IκB-ζ may be of great significance in regulation of inflammatory reactions.

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

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.

Analysis of the Functional Domain of the Rat Liver Mitochondrial Import Receptor Tom20

Tom20 is an outer mitochondrial membrane protein and functions as a component of the import receptor complex for the cytoplasmically synthesized mitochondrial precursor proteins. It consists of the N-terminal membrane-anchor segment, the tetratricopeptide repeat (TPR) motif, a charged amino acids-rich linker segment between the membrane anchor and the TPR motif, and the C-terminal acidic amino acid cluster. To assess the functional significance of these segments in mammalian Tom20, we cloned rat Tom20 and expressed mutant rat Tom20 proteins in Δtom20 yeast cells and examined their ability to complement the defects of respiration-driven growth and mitochondrial protein import. Tom20N69, a mutant consisting of the membrane anchor and the linker segments, was targeted to mitochondria and complemented the growth and import defects as efficiently as wild-type Tom20, whereas a mutant lacking the linker segment did not. In vitro protein import into mitochondria isolated from the complemented yeast cells revealed that the precursor targeted to yeast Tom70 was efficiently imported into the mitochondria via rat Tom20N69. Thus the linker segment is essential for the function of rat Tom20, whereas the TPR motif and the C-terminal acidic amino acids are not.

Essential roles for NF-κB and a Toll/IL-1 receptor domain-specific signal(s) in the induction of IκB-ζ

Abstract IκB-ζ, a new negative-regulator of nuclear factor-κB (NF-κB), is strongly induced by lipopolysaccharide or interleukin-1β stimulation, but not by tumor necrosis factor-α. Here, we analyzed the mechanisms for transcriptional induction of IκB-ζ. IκB-ζ mRNA was induced by overexpression of MyD88 or TRAF6, but not TRAF2. Stimulation of macrophages with peptidoglycan or CpG DNA, which activated Toll-like receptor 2 or 9, respectively, also resulted in IκB-ζ induction. Thus, activation of the MyD88-dependent signaling pathway, commonly found downstream of different Toll/interleukin-1 receptor (TIR) domains, is sufficient for IκB-ζ induction. The induction was inhibited by treatment with various inhibitors of NF-κB activation or by overexpressing IκB-α or β, indicating essential roles for NF-κB in IκB-ζ induction. However, overexpression of the NF-κB subunits induced IκB-α, but not IκB-ζ. These results indicate the existence of another signal essential for IκB-ζ induction, which is specifically mediated by the TIR domain-mediated signaling pathway.

Gene-specific requirement of a nuclear protein, IκB-ζ, for promoter association of inflammatory transcription regulators

Expression of many inflammatory genes is induced through activation of the transcription factor NF-κB. In contrast to the advanced understanding of cytoplasmic control of NF-κB activation, its regulation in the nucleus has not been fully understood despite its importance in selective gene expression. We previously identified an inducible nuclear protein, IκB-ζ, and demonstrated that this molecule is indispensable for the expression of a group of NF-κB-regulated genes. In this study, we established a unique gene induction system, in which IκB-ζ is expressed independently of inflammatory stimuli, to specifically investigate the molecular basis underlying IκB-ζ-mediated gene activation. We show that in the presence of IκB-ζ other primary response genes are dispensable for the expression of the target secondary response genes. ChIP analyses revealed that IκB-ζ is required for stimulus-induced recruitment of NF-κB onto the target promoter in a gene-specific manner. Surprisingly, IκB-ζ is also necessary for the gene-selective promoter recruitment of another inflammatory transcription factor, C/EBPβ, and the chromatin remodeling factor Brg1. We propose a new gene regulatory mechanism underlying the selective expression of inflammatory genes.

Attenuated Th1 induction by dendritic cells from mice deficient in the leukotriene B4 receptor 1

Dendritic cells (DCs) are important antigen-presenting cells that control Th1- and Th2-type immunological reactions by releasing cytokines and interacting directly with T cells. Leukotriene B4 (LTB4), a classical proinflammatory lipid mediator for phagocytes, was recently identified as an important attractant for effector CD4(+) and CD8(+) T cells. However, little information is available on the roles of LTB4 and its receptor BLT1 in DCs. Here we show that functional BLT1 expressed in mouse bone marrow-derived DCs (BMDCs) plays important role in initiating Th1-type immune response. Detailed analyses using BMDCs revealed that BLT1-deficient DCs produced less IL-12p70 than WT DCs, leading to attenuated IFN-gamma production in an allogeneic mixed lymphocyte reaction. Adoptive transfer of antigen-loaded BLT1-deficient DCs into naïve WT mice induced a weakened Th1- and enhanced Th2-response in vivo compared to WT DCs. BLT1-deficient mice consistently showed much attenuated delayed-type hypersensitivity (DTH), in which Th1-type cellular responses play a key role, and popliteal lymph node cells of BLT1-deficient mice showed reduced production of Th1 cytokines after DTH induction compared to cells from WT mice. Thus, in addition to its role in inflammation, the LTB4-BLT1 axis is important in initiating Th1-type immunological reactions mediated by DCs.

IκB-ζ, a new anti-inflammatory nuclear protein induced by lipopolysaccharide, is a negative regulator for nuclear factor-κB:

Activation of nuclear factor-κ B (NF-κ B), a prominent cellular response to bacterial endotoxin or other microbial products, must be strictly regulated because excessive activation leads to overproduction of cytotoxic cytokines that culminates in septic shock. During screening for genes up-regulated upon inflammation, we identified a new member of the IκB family proteins with the ankyrin-repeats. This protein, designated Iκ B-ζ , is hardly detectable in resting cells, but is strongly induced upon stimulation by lipopolysaccharide, which stimulates cells through the Toll-like receptor 4. Interleukin-1β stimulation also results in the strong induction of IκB-ζ, but tumor necrosis factor-α does not. In contrast to IκB-α or Iκ B-β , IκB-ζ localizes in the nucleus, where it inhibits NF-κB activity. NF-κ B activity is essential for the induction of IκB-ζ, but is not sufficient. Thus, this protein is a new anti-inflammatory protein, which is specifically induced upon inflammation to regulate NF-κB activity.

IkappaB-zeta, a new anti-inflammatory nuclear protein induced by lipopolysaccharide, is a negative regulator for nuclear factor-kappaB.

Activation of nuclear factor-kappaB (NF-kappaB), a prominent cellular response to bacterial endotoxin or other microbial products, must be strictly regulated because excessive activation leads to overproduction of cytotoxic cytokines that culminates in septic shock. During screening for genes up-regulated upon inflammation, we identified a new member of the IkappaB family proteins with the ankyrin-repeats. This protein, designated IkappaB-zeta, is hardly detectable in resting cells, but is strongly induced upon stimulation by lipopolysaccharide, which stimulates cells through the Toll-like receptor 4. Interleukin-1beta stimulation also results in the strong induction of IkappaB-zeta, but tumor necrosis factor-alpha does not. In contrast to IkappaB-alpha or IkappaB-beta, IkappaB-zeta localizes in the nucleus, where it inhibits NF-kappaB activity. NF-kappaB activity is essential for the induction of IkappaB-zeta, but is not sufficient. Thus, this protein is a new anti-inflammatory protein, which is specifically induced upon inflammation to regulate NF-kappaB activity.