Wenji Piao

TLR4/MyD88/PI3K interactions regulate TLR4 signaling

TLRs activate immune responses by sensing microbial structures such as bacterial LPS, viral RNA, and endogenous “danger” molecules released by damaged host cells. MyD88 is an adapter protein that mediates signal transduction for most TLRs and leads to activation of NF‐κB and MAPKs and production of proinflammatory cytokines. TLR4‐mediated signaling also leads to rapid activation of PI3K, one of a family of kinases involved in regulation of cell growth, apoptosis, and motility. LPS stimulates phosphorylation of Akt, a downstream target of PI3K, in wild‐type (WT) mouse macrophages. LPS‐induced phosphorylation of Akt serine 473 was blunted in MyD88−/− macrophages and was completely TLR4‐dependent. MyD88 and p85 were shown previously to co‐immunoprecipitate, and a YXXM motif within the Toll‐IL‐1 resistance (TIR) domain of MyD88 was suggested to be important for this interaction. To test this hypothesis, we compared expressed MyD88 variants with mutations within the YXXM motif or lacking the TIR domain or death domain and measured their capacities to bind PI3K p85, MyD88, and TLR4 by co‐immunoprecipitation analyses. The YXXM → YXXA mutant MyD88 bound more strongly to p85, TLR4, and WT MyD88 than the other variants, yet was significantly less active than WT MyD88, suggesting that sustained interaction of MyD88/PI3K with the TLR4 intracellular “signaling platform” negatively regulates signaling. We propose a hypothetical model in which sustained PI3K activity at the membrane limits the availability of the PI3K substrate, thereby negatively regulating signaling.

Endotoxin tolerance dysregulates MyD88- and Toll/IL-1R domain-containing adapter inducing IFN-β-dependent pathways and increases expression of negative regulators of TLR signaling

Endotoxin tolerance reprograms cell responses to LPS by repressing expression of proinflammatory cytokines, while not inhibiting production of anti‐inflammatory cytokines and antimicrobial effectors. Molecular mechanisms of induction and maintenance of endotoxin tolerance are incompletely understood, particularly with regard to the impact of endotoxin tolerization on signalosome assembly, activation of adaptor‐kinase modules, and expression of negative regulators of TLR signaling in human cells. In this study, we examined LPS‐mediated activation of MyD88‐dependent and Toll‐IL‐1R‐containing adaptor inducing IFN‐β (TRIF)‐dependent pathways emanating from TLR4 and expression of negative regulators of TLR signaling in control and endotoxin‐tolerant human monocytes. Endotoxin tolerization suppressed LPS‐inducible TLR4‐TRIF and TRIF‐TANK binding kinase (TBK)1 associations, induction of TBK1 kinase activity, activation of IFN regulatory factor (IRF)‐3, and expression of RANTES and IFN‐β. Tolerance‐mediated dysregulation of the TLR4‐TRIF‐TBK1 signaling module was accompanied by increased levels of suppressor of IκB kinase‐ε (SIKE) and sterile α and Armadillo motif‐containing molecule (SARM). LPS‐tolerant cells showed increased expression of negative regulators Toll‐interacting protein (Tollip), suppressor of cytokine signaling (SOCS)‐1, IL‐1R‐associated kinase‐M, and SHIP‐1, which correlated with reduced p38 phosphorylation, IκB‐α degradation, and inhibited expression of TNF‐α, IL‐6, and IL‐8. To examine functional consequences of increased expression of Tollip in LPS‐tolerized cells, we overexpressed Tollip in 293/TLR4/MD‐2 transfectants and observed blunted LPS‐inducible activation of NF‐κB and RANTES, while TNF‐α responses were not affected. These data demonstrate dysregulation of TLR4‐triggered MyD88‐ and TRIF‐dependent signaling pathways and increased expression of negative regulators of TLR signaling in endotoxin‐tolerant human monocytes.

Analysis of proteinase-activated receptor 2 and TLR4 signal transduction: a novel paradigm for receptor cooperativity.

Proteinase-activated receptor 2 (PAR2), a seven-transmembrane G protein-coupled receptor, is activated at inflammatory sites by proteolytic cleavage of its extracellular N terminus by trypsin-like enzymes, exposing a tethered, receptor-activating ligand. Synthetic agonist peptides (AP) that share the tethered ligand sequence also activate PAR2, often measured by Ca2+ release. PAR2 contributes to inflammation through activation of NF-κB-regulated genes; however, the mechanism by which this occurs is unknown. Overexpression of human PAR2 in HEK293T cells resulted in concentration-dependent, PAR2 AP-inducible NF-κB reporter activation that was protein synthesis-independent, yet blocked by inhibitors that uncouple Gi proteins or sequester intracellular Ca2+. Because previous studies described synergistic PAR2- and TLR4-mediated cytokine production, we hypothesized that PAR2 and TLR4 might interact at the level of signaling. In the absence of TLR4, PAR2-induced NF-κB activity was inhibited by dominant negative (DN)-TRIF or DN-TRAM constructs, but not by DN-MyD88, findings confirmed using cell-permeable, adapter-specific BB loop blocking peptides. Co-expression of TLR4/MD-2/CD14 with PAR2 in HEK293T cells led to a synergistic increase in AP-induced NF-κB signaling that was MyD88-dependent and required a functional TLR4, despite the fact that AP exhibited no TLR4 agonist activity. Co-immunoprecipitation of PAR2 and TLR4 revealed a physical association that was AP-dependent. The response to AP or lipopolysaccharide was significantly diminished in TLR4–/– and PAR –/–2 macrophages, respectively, and SW620 colonic epithelial cells exhibited synergistic responses to co-stimulation with AP and lipopolysaccharide. Our data suggest a unique interaction between two distinct innate immune response receptors and support a novel paradigm of receptor cooperativity in inflammatory responses.

The Asp299Gly Polymorphism Alters TLR4 Signaling by Interfering with Recruitment of MyD88 and TRIF

Asp299Gly (D299G) and, to a lesser extent, Thr399Ile (T399I) TLR4 polymorphisms have been associated with Gram-negative sepsis and other infectious diseases, but the mechanisms by which they affect TLR4 signaling are unclear. In this study, we determined the impact of the D299G and T399I polymorphisms on TLR4 expression, interactions with myeloid differentiation factor 2 (MD2), LPS binding, and LPS-mediated activation of the MyD88- and Toll/IL-1R resistance domain-containing adapter inducing IFN-β (TRIF) signaling pathways. Complementation of human embryonic kidney 293/CD14/MD2 transfectants with wild-type (WT) or mutant yellow fluorescent protein-tagged TLR4 variants revealed comparable total TLR4 expression, TLR4–MD2 interactions, and LPS binding. FACS analyses with anti-TLR4 Ab showed only minimal changes in the cell-surface levels of the D299G TLR4. Cells transfected with D299G TLR4 exhibited impaired LPS-induced phosphorylation of p38 and TANK-binding kinase 1, activation of NF-κB and IFN regulatory factor 3, and induction of IL-8 and IFN-β mRNA, whereas T399I TLR4 did not cause statistically significant inhibition. In contrast to WT TLR4, expression of the D299G mutants in TLR4−/− mouse macrophages failed to elicit LPS-mediated induction of TNF-α and IFN-β mRNA. Coimmunoprecipitation revealed diminished LPS-driven interaction of MyD88 and TRIF with the D299G TLR4 species, in contrast to robust adapter recruitment exhibited by WT TLR4. Thus, the D299G polymorphism compromises recruitment of MyD88 and TRIF to TLR4 without affecting TLR4 expression, TLR4–MD2 interaction, or LPS binding, suggesting that it interferes with TLR4 dimerization and assembly of intracellular docking platforms for adapter recruitment.

Endotoxin Tolerance Impairs IL-1 Receptor-Associated Kinase (IRAK) 4 and TGF-β-activated Kinase 1 Activation, K63-linked Polyubiquitination and Assembly of IRAK1, TNF Receptor-associated Factor 6, and IκB Kinase γ and Increases A20 Expression

Endotoxin tolerance reprograms Toll-like receptor 4 responses by impairing LPS-elicited production of pro-inflammatory cytokines without inhibiting expression of anti-inflammatory or anti-microbial mediators. In septic patients, Toll-like receptor tolerance is thought to underlie decreased pro-inflammatory cytokine expression in response to LPS and increased incidence of microbial infections. The impact of endotoxin tolerance on recruitment, post-translational modifications and signalosome assembly of IL-1 receptor-associated kinase (IRAK) 4, IRAK1, TNF receptor-associated factor (TRAF) 6, TGF-β-activated kinase (TAK) 1, and IκB kinase (IKK) γ is largely unknown. We report that endotoxin tolerization of THP1 cells and human monocytes impairs LPS-mediated receptor recruitment and activation of IRAK4, ablates K63-linked polyubiquitination of IRAK1 and TRAF6, compromises assembly of IRAK1-TRAF6 and IRAK1-IKKγ platforms, and inhibits TAK1 activation. Deficiencies in these signaling events in LPS-tolerant cells coincided with increased expression of A20, an essential deubiquitination enzyme, and sustained A20-IRAK1 associations. Overexpression of A20 inhibited LPS-induced activation of NF-κB and ablated NF-κB reporter activation driven by ectopic expression of MyD88, IRAK1, IRAK2, TRAF6, and TAK1/TAB1, while not affecting the responses induced by IKKβ and p65. A20 shRNA knockdown abolished LPS tolerization of THP1 cells, mechanistically linking A20 and endotoxin tolerance. Thus, deficient LPS-induced activation of IRAK4 and TAK1, K63-linked polyubiquitination of IRAK1 and TRAF6, and disrupted IRAK1-TRAF6 and IRAK1-IKKγ assembly associated with increased A20 expression and A20-IRAK1 interactions are new determinants of endotoxin tolerance.

Tyrosine phosphorylation of MyD88 adapter-like (Mal) is critical for signal transduction and blocked in endotoxin tolerance

Toll-like receptor 4 (TLR4) recognition of lipopolysaccharide triggers signalosome assembly among TLR4, sorting (e.g. MyD88 adapter-like (Mal)) and signaling (e.g. MyD88) adapters, initiating recruitment and activation of kinases, activation of transcription factors, and production of inflammatory mediators. In this study we examined whether tyrosine phosphorylation of Mal regulates its interactions with TLR4, MyD88, interleukin-1 (IL-1) receptor-associated kinase (IRAK)-2, and tumor necrosis factor receptor-associated factor (TRAF)-6 and is important for signaling. Overexpression of wild-type Mal in human embryonic kidney 293T cells induced its constitutive tyrosine phosphorylation and led to activation of p38, NF-κB, and IL-8 gene expression. Mutagenesis of Tyr-86, Tyr-106, and Tyr-159 residues within the Toll-IL-1 receptor domain impaired Mal tyrosine phosphorylation, interactions with Bruton tyrosine kinase, phosphorylation of p38, and NF-κB activation. Lipopolysaccharide triggered tyrosine phosphorylation of endogenous Mal and initiated Mal-Bruton-tyrosine kinase interactions in 293/TLR4/MD-2 cells and human monocytes that were suppressed in endotoxin-tolerant cells. Compared with wild-type Mal, Y86A-, Y06A-, and Y159A-Mal variants exhibited higher interactions with TLR4 and MyD88, whereas associations with IRAK-2 and TRAF-6 were not affected. Overexpression of Y86A- and Y106A-Mal in 293/TLR4/MD-2 cells exerted dominant-negative effects on TLR4-inducible p38 phosphorylation and NF-κB reporter activation to the extent comparable with P125H-Mal-mediated suppression. In contrast, tyrosine-deficient Mal species did not affect NF-κB activation when signaling was initiated at the post-receptor level by overexpression of MyD88, IRAK-2, or TRAF-6. Thus, tyrosine phosphorylation of Mal is required for adapter signaling, regulates Mal interactions with TLR4 and receptor signaling, and is inhibited in endotoxin tolerance.

Targeting Toll-like Receptor (TLR) Signaling by Toll/Interleukin-1 Receptor (TIR) Domain-containing Adapter Protein/MyD88 Adapter-like (TIRAP/Mal)-derived Decoy Peptides

Background: Assembly of TLR signaling complexes is mediated by a cooperative interaction of TIR domains present in TLRs and TLR adapters. Results: This work identifies several TIRAP/Mal-derived peptides that inhibit TLR4 and TLR2 signaling in vitro and in vivo. Conclusion: TIRAP/Mal-derived inhibitory peptides block TLR signaling by interfering with signaling complex assembly. Significance: Inhibitory peptides indicate TIRAP TIR interfaces and provide leads for development of TLR-targeting drugs. Toll/interleukin-1 receptor (TIR) domain-containing adapter protein/MyD88 adapter-like (TIRAP/Mal) is an adapter protein that facilitates recruitment of MyD88 to TLR4 and TLR2 signaling complexes. We previously generated a library of cell-permeating TLR4 TIR-derived decoy peptides fused to the translocating segment of the Drosophila Antennapedia homeodomain and examined each peptide for the ability to inhibit TLR4 signaling (Toshchakov, V. Y., Szmacinski, H., Couture, L. A., Lakowicz, J. R., and Vogel, S. N. (2011) J. Immunol. 186, 4819–4827). We have now expanded this study to test TIRAP decoy peptides. Five TIRAP peptides, TR3 (for TIRAP region 3), TR5, TR6, TR9, and TR11, inhibited LPS-induced cytokine mRNA expression and MAPK activation. Inhibition was confirmed at the protein level; select peptides abolished the LPS-induced cytokine production measured in cell culture 24 h after a single treatment. Two of the TLR4 inhibitory peptides, TR3 and TR6, also inhibited cytokine production induced by a TLR2/TLR1 agonist, S-(2,3-bis(palmitoyloxy)-(2R,2S)-propyl)-N-palmitoyl-(R)-Cys-Ser-Lys4-OH; however, a higher peptide concentration was required to achieve comparable inhibition of TLR2 versus TLR4 signaling. Two TLR4 inhibitory peptides, TR5 and TR6, were examined for the ability to inhibit TLR4-driven cytokine induction in mice. Pretreatment with either peptide significantly reduced circulating TNF-α and IL-6 in mice following LPS injection. This study has identified novel TLR inhibitory peptides that block cellular signaling at low micromolar concentrations in vitro and in vivo. Comparison of TLR4 inhibition by TLR4 and TIRAP TIR-derived peptides supports the view that structurally diverse regions mediate functional interactions of TIR domains.

Inhibition of TLR4 Signaling by TRAM-Derived Decoy Peptides In Vitro and In Vivo

Toll/IL-1R (TIR) domain-containing adapter-inducing IFN-β (TRIF)–related adapter molecule (TRAM) serves as a bridging adapter that enables recruitment of TRIF to activated TLR4 and thereby mediates the induction of TRIF-dependent cytokines. A library of cell-permeating decoy peptides derived from TRAM TIR domain has been screened for the ability of individual peptides to inhibit TLR4 signaling in primary murine macrophages. Peptides derived from TRAM TIR BB loop (TM4) and C helix (TM6) inhibited the LPS-induced activation of MyD88-dependent and TRIF-dependent cytokines, as well as MAPK activation. TM4 and TM6 did not block macrophage activation induced by TLR2, TLR9, or retinoic acid–inducible gene 1–like receptor agonists. Both TM4 and TM6 blocked coimmunoprecipitation of TRAM and TLR4 ectopically expressed in HEK293T cells. Both peptides also blocked the LPS-induced recruitment of MyD88 to TLR4 in primary murine macrophages. In vivo examination of TRAM-derived peptides demonstrated that all peptides that were inhibitory in vitro profoundly suppressed systemic inflammatory response elicited in mice by a sublethal LPS dose, and protected mice against a lethal LPS challenge. This research identifies novel TLR inhibitors effective in vitro and in vivo and validates the approach taken in this study as a rational way for development of signaling inhibitors and lead therapeutics.

Recruitment of TLR adapter TRIF to TLR4 signaling complex is mediated by the second helical region of TRIF TIR domain

Significance This study sheds light on the mechanisms of Toll-like receptor 4 (TLR4) signaling complex assembly. Particularly, the interaction site that mediates Toll/IL-1R resistance (TIR) domain–containing adapter-inducing IFN-β (TRIF) recruitment to the TLR4 signaling complex is identified within the second helical region of the TRIF TIR domain. This finding is significant because “a mimic” of this site potently inhibits TLR4 signaling in cells by a competitive mechanism and decreases LPS-induced mortality in mice by more than 80%. Toll/IL-1R resistance (TIR) domain–containing adapter-inducing IFN-β (TRIF) is a Toll-like receptor (TLR) adapter that mediates MyD88-independent induction of type I interferons through activation of IFN regulatory factor 3 and NFκB. We have examined peptides derived from the TRIF TIR domain for ability to inhibit TLR4. In addition to a previously identified BB loop peptide (TF4), a peptide derived from putative helix B of TRIF TIR (TF5) strongly inhibits LPS-induced cytokine and MAPK activation in wild-type cells. TF5 failed to inhibit LPS-induced cytokine and kinase activation in TRIF-deficient immortalized bone-marrow–derived macrophage, but was fully inhibitory in MyD88 knockout cells. TF5 does not block macrophage activation induced by TLR2, TLR3, TLR9, or retinoic acid-inducible gene 1/melanoma differentiation-associated protein 5 agonists. Immunoprecipitation assays demonstrated that TF4 binds to TLR4 but not TRIF-related adaptor molecule (TRAM), whereas TF5 binds to TRAM strongly and TLR4 to a lesser extent. Although TF5 prevented coimmunoprecipitation of TRIF with both TRAM and TLR4, site-directed mutagenesis of the TRIF B helix residues affected TRIF–TRAM coimmunoprecipitation selectively, as these mutations did not block TRIF–TLR4 association. These results suggest that the folded TRIF TIR domain associates with TRAM through the TRIF B helix region, but uses a different region for TRIF–TLR4 association. The B helix peptide TF5, however, can associate with either TRAM or TLR4. In a mouse model of TLR4-driven inflammation, TF5 decreased plasma cytokine levels and protected mice from a lethal LPS challenge. Our data identify TRIF sites that are important for interaction with TLR4 and TRAM, and demonstrate that TF5 is a potent TLR4 inhibitor with significant potential as a candidate therapeutic for human sepsis.

A Decoy Peptide that Disrupts TIRAP Recruitment to TLRs Is Protective in a Murine Model of Influenza

Toll-like receptors (TLRs) activate distinct, yet overlapping sets of signaling molecules, leading to inflammatory responses to pathogens. Toll/interleukin-1 receptor (TIR) domains, present in all TLRs and TLR adapters, mediate protein interactions downstream of activated TLRs. A peptide library derived from TLR2 TIR was screened for inhibition of TLR2 signaling. Cell-permeable peptides derived from the D helix and the segment immediately N-terminal to the TLR2 TIR domain potently inhibited TLR2-mediated cytokine production. The D-helix peptide, 2R9, also potently inhibited TLR4, TLR7, and TLR9, but not TLR3 or TNF-α signaling. Cell imaging, co-immunoprecipitation, and in vitro studies demonstrated that 2R9 preferentially targets TIRAP. 2R9 diminished systemic cytokine responses elicited in vivo by synthetic TLR2 and TLR7 agonists; it inhibited the activation of macrophages infected with influenza strain A/PR/8/34 (PR8) and significantly improved the survival of PR8-infected mice. Thus, 2R9 represents a TLR-targeting agent that blocks protein interactions downstream of activated TLRs.