IRAK-4 as the central TIR signaling mediator in innate immunity

Toll-like receptors (TLRs), which recognize pathogen-associated molecular patterns and members of the proinflammatory interleukin-1 receptor (IL-1R) family, share homologies in their cytoplasmic domains. Engagement of members of both of these families initiates a common intracellular signaling cascade, in which MyD88 and tumor necrosis factor (TNF) receptor-associated factor 6 (TRAF6) are key adaptor proteins. Signaling between MyD88 and TRAF6 is mediated by members of the IL-1R-associated kinase (IRAK) family; however, the exact function of each IRAK protein remains controversial. IRAK-1 is required for the optimal transduction of IL-1R- and TLR-mediated signals, but IRAK-1 can be replaced by other IRAKs. Surprisingly, gene targeting studies show that the newest IRAK protein, IRAK-4, has an essential role in mediating signals initiated by IL-1R and TLR engagement. The kinase activity of IRAK-4 might be necessary to functionally modify IRAK-1 and perhaps other signal transducing substrates. Understanding the role of IRAK-4 in innate immunity will enable us to design novel strategies for therapeutic intervention in human infectious disease.     

IL-1beta-induced phosphorylation of PKB/Akt depends on the presence of IRAK-1

IL-1, IL-18 and LPS are recognized by specific receptor complexes of the Toll/IL-1R family, characterized by a common intracellular domain indispensable for downstream signaling. Upon ligand binding, these receptors activate the central MyD88-IRAK-TRAF6 signaling module, resulting in the activation of NF-kappaB. Ligated receptors also induce activation of other signaling cascades, suchas the PI3-kinase (PI3-K) and the p38 mitogen-activated protein kinase (MAPK) pathways. Unlike the p38MAPK pathway, which couples to the central signaling module, the PI3-K pathway seems to directly interact with the receptor molecules. Thus, activation of the PI3-K pathway is thought to be independent of the IRAK-containing signaling module. Employing two cell lines, we show that the PI3-K pathways can be activated by IL-1, IL-18 or LPS with comparable, but cell type specific kinetics, which can be correlated to biological consequences. This indicates that activation of the PI3-K pathways may be regulated by an element common for all three receptor types, the MyD88-IRAK-TRAF6 module being a candidate for this function. Using an IRAK-1-deficient cell line, we demonstrate that the IRAK-1-containing signaling module is essential for the IL-1-induced activation of the PI3-K pathway. Possible models of the interaction between IRAK-1 and the PI3-K pathway are discussed.     

Differential regulation of interleukin 1 receptor and Toll-like receptor signaling by MEKK3

Interleukin 1 receptor (IL-1R) and Toll-like receptors (TLRs) induce inflammatory genes through the complex of MyD88, IL-1R-associated protein kinase (IRAK) and tumor necrosis factor receptor-associated factor 6 (TRAF6), which is believed to function 'upstream' of the cascades of IkappaB kinase (IKK) and nuclear factor-kappaB (NF-kappaB); extracellular signal-regulated protein kinase (ERK); c-Jun N-terminal kinase (JNK); and p38 mitogen-activated protein kinase (MAPK). Here we show that MAPK-ERK kinase kinase (MEKK3) is an essential signal transducer of the MyD88-IRAK-TRAF6 complex in IL-1R-TLR4 signaling. MEKK3 forms a complex with TRAF6 in response to IL-1 and lipopolysaccharide (LPS) but not CpG, and is required for IL-1R- and TLR4-induced IL-6 production. Furthermore, MEKK3 is crucial for IL-1- and LPS-induced activation of NF-kappaB and JNK-p38 but not ERK, indicating that MAPKs are differentially activated during IL-1R-TLR4 signaling. These data demonstrate that MEKK3 is crucial for IL-1R and TLR4 signaling through the IKK-NF-kappaB and JNK-p38 MAPK pathways.*Note: In the version of this article originally published online, the third author's name was incorrect. The correct author name should be Yong Lin. This error has been corrected for the HTML and print versions of this article.     

Minimal structure of IRAK-1 to induce degradation of TRAF6

Excessive activation of Toll-like receptor (TLR) leads to sepsis. Inflammatory responses to various microbiological components are initiated via different TLR proteins, but all TLR signals are transmitted by TRAF6. We reported that TRAF6 associated with ubiquitinated IRAK-1 undergoes proteasome-mediated degradation, suggesting that IRAK-1 has a negative regulatory role in TLR signaling. Here, we investigated the minimal structural region of IRAK-1 needed for degradation of TRAF6. The IRAK-1 protein contains an N-terminal death domain (DD; amino acids 1–102), a serine/proline/threonine-rich ProST domain (amino acids 103–197), a central kinase domain with an activation loop (amino acids 198–522), and the C-terminal C1 and C2 domains (amino acids 523–712), which contain two and one putative TRAF6-binding (TB) sites, respectively. TRAF6 degradation was severely impaired by deletion of the DD or C1 domain, and a mutant (DC1) containing only the DD and C1 domains could induce TRAF6 degradation. IRAK-1 mutants lacking the N- or C-terminal amino acids of DD induced little degradation. Deletion or mutation of TB2 (amino acids 585–591) in the C1 domain also inhibited TRAF6 degradation. An IRAK-1 mutant possessing only DD and TB2 did not induce TRAF6 degradation, although a mutant in which a short spacer was inserted between DD and TB2 induced TRAF6 degradation, which and DC1-induced degradation were inhibited by proteasome inhibitors. All IRAK-1 mutants that induced TRAF6 degradation could be immunoprecipitated with TRAF6. Meanwhile, NF-κB activation was observed for all IRAK-1 mutants–including those that failed to induce degradation and was severely impaired only for a mutant carrying mutations in both TBs of C1. These results demonstrate that only DD and TB2 separated by an appropriate distance can induce TRAF6 degradation. Conformational analysis of this minimal structural unit may aid development of low molecular compounds that negatively regulate TLR signaling.     

Role of IRAK4 and IRF3 in the control of intracellular infection with Chlamydia pneumoniae

TLR signal transduction involves a MyD88-mediated pathway, which leads to recruitment of the IL-1 receptor (IL-1R)-associated kinase 4 (IRAK4) and Toll/IL-1R translation initiation region domain-containing adaptor-inducing IFN-beta-mediated pathway, resulting in the activation of IFN regulatory factor (IRF)3. Both pathways can lead to expression of IFN-beta. TLR-dependent and -independent signals converge in the TNF receptor-associated factor 6 (TRAF6) adaptor, which mediates the activation of NF-kappaBeta. Infection of murine bone marrow-derived macrophages (BMM) with Chlamydia pneumoniae induces IFN-alpha/beta- and NF-kappaBeta-dependent expression of IFN-gamma, which in turn, will control bacterial growth. The role of IRAK4 and IRF3 in the regulation of IFN-alpha/beta expression and NF-kappaBeta activation was studied in C. pneumoniae-infected BMM. We found that levels of IFN-alpha, IFN-beta, and IFN-gamma mRNA were reduced in infected IRAK4(-/-) BMM compared with wild-type (WT) controls. BMM also showed an IRAK4-dependent growth control of C. pneumoniae. No increased IRF3 activation was detected in C. pneumoniae-infected BMM. Similar numbers of intracellular bacteria, IFN-alpha, and IFN-gamma mRNA titers were observed in C. pneumoniae-infected IRF3(-/-) BMM. On the contrary, IFN-beta(-/-) BMM showed lower IFN-alpha and IFN-gamma mRNA levels and higher bacterial titers compared with WT controls. C. pneumoniae infection-induced activation of NF-kappaBeta and expression of proinflammatory cytokines were shown to be TRAF6-dependent but did not require IRAK4 or IRF3. Thus, our data indicate that IRAK4, but not IRF3, controls C. pneumoniae-induced IFN-alpha and IFN-gamma secretion and bacterial growth. IRAK4 and IRF3 are redundant for infection-induced NF-kappaB activation, which is regulated by TRAF6.     

Common and distinct signalling cascades in the production of tumour necrosis factor-α and interleukin-13 induced by lipopolysaccharide in RBL-2H3 cells

BACKGROUND: Activation of mast cells by lipopolysaccharide (LPS) results in the production of TNF-alpha and IL-13. TNF-alpha and IL-13 are key mediators in the development of neutrophilic and allergic inflammation, respectively. LPS-induced TNF-alpha and IL-13 production in mast cells has been reported to be mediated by Toll-like receptor 4 (TLR4) signalling, but differences in signal transduction mechanisms leading to the production of these cytokines are not clearly defined. OBJECTIVE: We investigated the molecular mechanisms responsible for LPS-induced TNF-alpha and IL-13 production in mast cells. METHODS: TNF-alpha and IL-13 production by LPS was assessed by transfecting RBL-2H3 cells with dominant-negative (DN) expression vectors. RESULTS: Transfection of RBL-2H3 cells with plasmids encoding DN mutants of myeloid differentiation protein (MyD88) and TNFR-associated factor (TRAF6) inhibited both LPS-induced TNF-alpha and IL-13 production. IkappaBalpha-DN inhibited LPS-induced production of TNF-alpha, but not IL-13. We also found that inhibition of p38 kinase suppressed both TNF-alpha and IL-13 induction by LPS, and inhibition of JNK reduced IL-13 production, but not TNF-alpha. Furthermore, we found that protein kinase R (PKR) was activated by LPS in these cells. Treatment with 2-aminopurine, a PKR inhibitor, attenuated LPS-induced nuclear factor-kappaB activation and TNF-alpha production, whereas inhibition of PKR had little effect on IL-13 production. CONCLUSION: These findings indicate that the production of TNF-alpha and IL-13 by LPS required TLR4/MyD88/TRAF6 signalling as a common pathway of mast cell-mediated inflammation. We furthermore found that TNF-alpha and IL-13 production were differentially regulated by signalling cascades through PKR and mitogen-activated protein kinases downstream of TRAF6 in mast cells.     

Rapid CD40-mediated rescue from CD95-induced apoptosis requires TNFR-associated factor-6 and PI3K

The activation molecule CD40 and the death receptor CD95/Fas play important roles in regulating B cells so that effective antimicrobial immunity occurs without autoimmunity. CD40 signaling increases CD95 expression, sensitizing cells to apoptosis, but sustained CD40 signals rescue B cells from CD95 killing. Here we describe a mechanism of early CD40-mediated rescue from CD95-induced apoptosis in B cells. Maximal rescue was achieved when CD40 signals were given within 1-2 h of initiating CD95 apoptosis. CD40 signaling did not block association of Fas-associated death domain-containing protein with CD95, but decreased CD95-induced activation of caspases 3 and 8. Rapid CD40 rescue did not require NF-kappaB activation and was independent of de novo protein synthesis, but was dependent upon active PI3 K. Signaling via a CD40 mutant that does not bind TNFR-associated factor (TRAF)1, TRAF2, and TRAF3 rescued B cells from CD95-induced apoptosis. TRAF1/2/3-independent rescue was confirmed in B cell lines made deficient in these TRAF molecules by gene targeting. In contrast, CD40 rescue was completely abrogated in TRAF6-deficient B cells, which showed reduced activation of Akt in response to CD40 engagement. These results reveal a new rapid mechanism to balance B cell activation and apoptosis.     

天抗(TK)对脂多糖诱导小鼠炎症模型的抗炎作用及其机制

目的研究天抗(TK)对脂多糖(LPS)诱导的小鼠炎症模型的抗炎作用及机制研究。方法将42只昆明小鼠随机分为正常对照(NC)组、模型对照(LPS)组、地塞米松(DXM)组、天抗低(TK-L)、中(TK-M)和高(TK-H)剂量组(0.2,0.8和3.2 g/kg)。各组分别灌胃给药7 d后,腹腔注射30 mg/kg的LPS诱导小鼠急性炎性模型,6 h后处死小鼠,检测小鼠脾脏指数,ELISA测定小鼠血清中IL-1β、IL-6和TNF-α的表达水平;生化法检测小鼠血清中SOD和MDA的表达;qRT-PCR检测小鼠脾脏TLR4、MyD88、TRAF6、p65、IL-1β、IL-6和TNF-αmRNA的表达水平;Western blot检测小鼠脾脏TLR4、MyD88、TRAF6、p-p65和p65蛋白表达水平。结果与LPS组相比,TK组小鼠的脾脏指数明显降低,血清和脾脏组织中IL-1β、IL-6、TNF-α和MDA水平显著下降,SOD水平明显升高,小鼠脾脏组织的TLR4、MyD88、TRAF6和p-p65等蛋白及mRNA表达水平均明显降低。结论天抗对LPS诱导的小鼠急性炎症模型具有抗炎作用,其作用机制可能是通过TLR4/MyD88/NF-κB(p-65)信号通路抑制炎症因子的释放。     

Unresponsiveness of MyD88-deficient mice to endotoxin.

MyD88 is a general adaptor protein that plays an important role in the Toll/IL-1 receptor family signalings. Recently, Toll-like receptors 2 and 4 (TLR2 and TLR4) have been suggested to be the signaling receptors for lipopolysaccharide (LPS). In this study, we demonstrate that MyD88 knockout mice lack the ability to respond to LPS as measured by shock response, B cell proliferative response, and secretion of cytokines by macrophages and embryonic fibroblasts. However, activation of neither NF-kappaB nor the mitogen-activated protein (MAP) kinase family is abolished in MyD88 knockout mice. These findings demonstrate that signaling via MyD88 is essential for LPS response, but the inability of MyD88 knockout mice to induce LPS-dependent gene expression cannot simply be attributed to lack of the activation of MAP kinases and NF-kappaB.     

Porin of Shigella dysenteriae activates mouse peritoneal macrophage through Toll-like receptors 2 and 6 to induce polarized type I response

Porin of Shigella dysenteriae type 1 coexpressed Toll-like receptor (TLR) 2 and TLR6 on peritoneal cavity (PerC) macrophages (MPhi) of C57BL/6 mice implicating that both the TLRs are essential as a combinatorial repertoire to recognize the protein. Besides TLRs, mRNA for MyD88 and TRAF6, and nuclear translocation of NF-kappaB were enhanced that indicate their involvement in tandem in the activity of porin. The protein selectively up-regulated CD80 on the activated MPhi together with MHC class II molecule and CD40, and had no effect on CD86 expression. The porin-induced profile of MIP-1alpha, MIP-1beta and RANTES showed strong bias for chemokines correlated with M1 polarization. Intracellular expression and release of TNF-alpha and IL-12 in presence of porin was found to be TLR2 and NF-kappaB dependent. Induction of TNF-alpha and IL-12 along with the chemokine profile suggests type I polarization of the MPhi that would influence Th1-type response.     

Molecular cloning and functional characterization of a novel isoform of chicken myeloid differentiation factor 88 (MyD88)

Myeloid differentiation factor 88 (MyD88) is an adaptor protein involved in the interleukin-1 receptor- and Toll-like receptor-induced activation of nuclear factor-kappaB (NF-kappaB). A novel isoform of chicken MyD88, designated chicken MyD88-2, has been cloned and functionally characterized. Its open reading frame is of length 900bp, and it encodes a predicted 299 residue protein, similar in length to its mammalian orthologues, but, respectively, 77 and 69 amino acids shorter than the previously described chicken MyD88-1 and -3. The amino acid sequence of chicken MyD88-2 displays 96.9%, 96.9%, 70.4% and 70.2% identity with, respectively, chicken MyD88-1, -3, human and mouse MyD88. Chicken MyD88-2 expression was detected in a range of tissues tested, but no expression of either chicken MyD88-1 or -3 was observed. The over-expression of chicken MyD88-2 significantly induced the activation of NF-kappaB in vitro, suggesting that chicken MyD88-2 plays an important role in the innate immune responses of chicken.     

TAK1-binding protein 2 facilitates ubiquitination of TRAF6 and assembly of TRAF6 with IKK in the IL-1 signaling pathway

TAK1 mitogen-activated protein kinase kinase kinase participates in the Interleukin-1 (IL-1) signaling pathway by mediating activation of JNK, p38, and NF-kappaB. TAK1-binding protein 2 (TAB2) was previously identified as an adaptor that links TAK1 to an upstream signaling intermediate, tumor necrosis factor receptor-associated factor 6 (TRAF6). Recently, ubiquitination of TRAF6 was shown to play an essential role in the activation of TAK1. However, the mechanism by which IL-1 induces TRAF6 ubiquitination remains to be elucidated. Here we report that TAB2 functions to facilitate TRAF6 ubiquitination and thereby mediates IL-1-induced cellular events. A conserved ubiquitin binding domain in TAB2, the CUE domain, is important for this function. We also found that TAB2 promotes the assembly of TRAF6 with a downstream kinase, IkappaB kinase (IKK). These results show that TAB2 acts as a multifunctional signaling molecule, facilitating both IL-1-dependent TRAF6 ubiquitination and assembly of the IL-1 signaling complex.     

TLR4-MyD88-TRAF6-TAK1 Complex-Mediated NF-κB Activation Contribute to the Anti-Inflammatory Effect of V8 in LPS-Induced Human Cervical Cancer SiHa Cells

The synthetic compound 7-4-[Bis-(2-hydroxyethyl)-amino]-butoxy-5-hydroxy-8-methoxy-2-phenylchromen-4-one (V8) is a novel flavonoid-derived compound. In this study, we investigated the effects of V8 on Toll-like receptor 4 (TLR4)-mediated inflammatory reaction in human cervical cancer SiHa cells and lipopolysaccharide (LPS)-induced TLR4 activity in cervical cancer SiHa (HPV16+) cells, but not in HeLa (HPV18+) and C33A (HPV?) cells. In addition, V8 inhibited LPS-induced expression of TLR4, MyD88, TRAF6 and phosphorylation of TAK1, and their interaction with TLR4 in SiHa cells, resulting in an inhibition of TLR4-MyD88-TRAF6-TAK1 complex. Moreover, V8 blocked LPS-induced phosphorylation of IκB and IKK, resulting in inhibition of the nuclear translocation of P65-NF-κB in SiHa cells. We also found that V8 reduced the expression of NF-κB target genes, such as those for COX-2, iNOS, IL-6, IL-8, CCL-2, and TNF-α in LPS-stimulated SiHa cells. These results suggested that V8 exerted an anti-inflammatory effect on SiHa cells by inhibiting the TLR4-MyD88-TRAF6-TAK1 complex-mediated NF-κB activation.