Free thiol group of MD-2 as the target for inhibition of the lipopolysaccharide-induced cell activation

The emerging role of adaptor proteins in regulating innate immunity of sepsis

Sepsis is a life-threatening syndrome caused by a dysregulated immune response. A large number of adaptor proteins have been found to play a pivotal role in sepsis via protein-protein interactions, thus participating in inflammatory cascades, leading to the generation of numerous inflammatory cytokines, as well as oxidative stress and regulated cell death. Although available strategies for the diagnosis and management of sepsis have improved, effective and specific treatments are lacking. This review focuses on the emerging role of adaptor proteins in regulating the innate immunity of sepsis and evaluates the potential value of adaptor protein-associated therapeutic strategy for sepsis.

Inhibition of TLR4 signalling to dampen joint inflammation in osteoarthritis

Local and systemic low-grade inflammation, mainly involving the innate immune system, plays an important role in the development of OA. A receptor playing a key role in initiation of this inflammation is the pattern-recognition receptor Toll-like receptor 4 (TLR4). In the joint, various ligands for TLR4, many of which are damage-associated molecular patterns (DAMPs), are present that can activate TLR4 signalling. This leads to the production of pro-inflammatory and catabolic mediators that cause joint damage. In this narrative review, we will first discuss the involvement of TLR4 ligands and signalling in OA. Furthermore, we will provide an overview of methods for inhibit, TLR4 signalling by RNA interference, neutralizing anti-TLR4 antibodies, small molecules and inhibitors targeting the TLR4 co-receptor MD2. Finally, we will focus on possible applications and challenges of these strategies in the dampening of inflammation in OA.

Disulfiram blocks inflammatory TLR4 signaling by targeting MD-2

Toll-like receptor 4 (TLR4) sensing of lipopolysaccharide (LPS), the most potent pathogen-associated molecular pattern of gram-negative bacteria, activates NF-κB and Irf3, which induces inflammatory cytokines and interferons that trigger an intense inflammatory response, which is critical for host defense but can also cause serious inflammatory pathology, including sepsis. Although TLR4 inhibition is an attractive therapeutic approach for suppressing overexuberant inflammatory signaling, previously identified TLR4 antagonists have not shown any clinical benefit. Here, we identify disulfiram (DSF), an FDA-approved drug for alcoholism, as a specific inhibitor of TLR4-mediated inflammatory signaling. TLR4 cell surface expression, LPS sensing, dimerization and signaling depend on TLR4 binding to MD-2. DSF and other cysteine-reactive drugs, previously shown to block LPS-triggered inflammatory cell death (pyroptosis), inhibit TLR4 signaling by covalently modifying Cys133 of MD-2, a key conserved residue that mediates TLR4 sensing and signaling. DSF blocks LPS-triggered inflammatory cytokine, chemokine, and interferon production by macrophages in vitro. In the aggressive N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine mouse model of Parkinson's disease (PD) in which TLR4 plays an important role, DSF markedly suppresses neuroinflammation and dopaminergic neuron loss, and restores motor function. Our findings identify a role for DSF in curbing TLR4-mediated inflammation and suggest that DSF and other drugs that target MD-2 might be useful for treating PD and other diseases in which inflammation contributes importantly to pathogenesis.

The antimicrobial peptide LK2(6)A(L) exhibits anti-inflammatory activity by binding to the myeloid differentiation 2 domain and protects against LPS-induced acute lung injury in mice

Acute lung injury (ALI) is a life-threatening disease that is generally attributable to an uncontrolled inflammatory response in the lung, but there is a lack of effective treatments. At present, regulating the inflammatory response has become an important strategy for treating ALI. In the present study, LK2(6)A(L), a peptide derived from the natural antimicrobial peptide temporin-1CEa, inhibited lipopolysaccharide (LPS)-induced expression of tumor necrosis factor alpha (TNF-α), interleukin-6 (IL-6), and NO in RAW264.7 cells. Herein, the anti-inflammatory mechanism of LK2(6)A(L) was investigated. The RNA-sequencing (RNA-seq) results showed that LK2(6)A(L) significantly inhibited the TLR4-mediated NF-κB and MAPK signaling pathways in LPS-induced RAW264.7 cells. The results of co-immunoprecipitation (Co-IP), pull-down experiment, confocal laser scanning microscopy, and surface plasmon resonance (SPR) suggested that MD2 was the direct target of LK2(6)A(L). Chemical inhibition of MD2 and its knockdown abolished the anti-inflammatory effect of LK2(6)A(L). Molecular dynamic simulation indicated that LK2(6)A(L) could bind to the active domain of the MD2 hydrophobic pocket via six hydrogen bonds. The truncated peptides were designed based on analysis of the molecular docking of LK2(6)A(L) to MD2. The truncated peptide IS-7 showed strong affinity to MD2 and a remarkable inhibitory effect on pro-inflammatory factors that was comparable to the effect of LK2(6)A(L). Finally, LK2(6)A(L) and IS-7 relieved inflammatory symptoms and lung tissue destruction in the ALI mouse model. Overall, our study suggested that LK2(6)A(L) showed promising anti-inflammatory activity by targeting MD2, and the amino acid domain 7-13 was an important area that binds with MD2 and also an anti-inflammatory active region. LK2(6)A(L) and IS-7 may be potential new treatments for ALI and other acute inflammatory diseases.

6-shogaol treatment improves experimental knee OA exerting a pleiotropic effect over immune innate signaling response in chondrocytes

BACKGROUND AND PURPOSE

The pathogenesis of osteoarthritis (OA) implicates a low-grade inflammation associated to the activation of the innate immune system. Toll like receptor (TLR) stimulation triggers the release of inflammatory mediators, which aggravate OA severity. The aim was to study the preventive effect of 6-shogaol (6S), a potential TLR4 inhibitor, on the treatment of experimental knee OA.

EXPERIMENTAL APPROACH

OA was induced in C57BL6 mice by surgical section of the medial meniscotibial ligament, which received 6S for eight weeks. Cartilage damage, inflammatory mediator presence, and disease markers were assessed in the joint tissues by immunohistochemistry. Computational modelling was used to predict binding modes of 6S into the TLR4/MD2 receptor and its permeability across cellular membranes. Employing LPS-stimulated chondrocytes and MAPK assay, we clarified 6S action mechanisms.

KEY RESULTS

6S treatment was able to prevent articular cartilage lesions, synovitis, and the presence of pro-inflammatory mediators and disease markers in OA animals. Molecular modelling studies predicted 6S interaction with the TLR4/MD-2 heterodimer in an antagonist conformation through its binding into the MD-2 pocket. In cell culture, we confirmed that 6S reduced LPS-induced TLR4 inflammatory signaling pathways. Besides, MAPK assay demonstrated that 6S directly inhibits the ERK1/2 phosphorylation activity.

CONCLUSION AND IMPLICATIONS

6S evoked a preventive action on cartilage and synovial inflammation in OA mice. 6S effect may take place not only by hindering the interaction between TLR4 ligands and the TLR4/MD-2 complex in chondrocytes, but also through inhibition of ERK phosphorylation, implying a pleiotropic effect on different mediators activated during OA, which proposes it as an attractive drug for OA treatment.

Synthesis of small molecules targeting paclitaxel-induced MyD88 expression in triple-negative breast cancer cell lines

Acquired paclitaxel (PTX) chemoresistance in triple-negative breast cancer (TNBC) can be inferred from the overexpression of toll-like receptor 4 (TLR4) and myeloid differentiation primary response 88 (MyD88) proteins and the activation of the TLR4/MyD88 cascading signalling pathway. Finding a new inhibitor that can attenuate the activation of this pathway is a novel strategy for reducing PTX chemoresistance. In this study, a series of small molecule compounds were synthesised and tested in combination with PTX against TNBC cells. The trimethoxy-substituted compound significantly decreased MyD88 overexpression and improved PTX activity in MDA-MB-231^TLR4+ cells but not in HCC^TLR4- cells. On the contrary, the trifluoromethyl-substituted compound with PTX synergistically improved the growth inhibition in both TNBC subtypes. The fluorescence titrations indicated that both compounds could bind with MD2 with good and comparable binding affinities. This was further supported by docking analysis, in which both compounds fit perfectly well and form some critical binding interactions with MD2, an essential lipid-binding accessory to TLR4 involved in activating the TLR-4/MyD88-dependent pathway.

Inhibition of the 3CL Protease and SARS-CoV-2 Replication by Dalcetrapib

The severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) 3CL protease is a promising target for inhibition of viral replication by interaction with a cysteine residue (Cys145) at its catalytic site. Dalcetrapib exerts its lipid-modulating effect by binding covalently to cysteine 13 of a cholesteryl ester transfer protein. Because 12 free cysteine residues are present in the 3CL protease, we investigated the potential of dalcetrapib to inhibit 3CL protease activity and SARS-CoV-2 replication. Molecular docking investigations suggested that dalcetrapib-thiol binds to the catalytic site of the 3CL protease with a delta G value of -8.5 kcal/mol. Dalcetrapib inhibited both 3CL protease activity in vitro and viral replication in Vero E6 cells with IC₅₀ values of 14.4 ± 3.3 μM and an EC₅₀ of 17.5 ± 3.5 μM (mean ± SD). Near-complete inhibition of protease activity persisted despite 1000-fold dilution after ultrafiltration with a nominal dalcetrapib-thiol concentration of approximately 100 times below the IC₅₀ of 14.4 μM, suggesting stable protease-drug interaction. The inhibitory effect of dalcetrapib on the SARS-CoV-2 3CL protease and viral replication warrants its clinical evaluation for the treatment of COVID-19.

Disruption of disulfides within RBD of SARS-CoV-2 spike protein prevents fusion and represents a target for viral entry inhibition by registered drugs

The SARS-CoV-2 pandemic imposed a large burden on health and society. Therapeutics targeting different components and processes of the viral infection replication cycle are being investigated, particularly to repurpose already approved drugs. Spike protein is an important target for both vaccines and therapeutics. Insights into the mechanisms of spike-ACE2 binding and cell fusion could support the identification of compounds with inhibitory effects. Here, we demonstrate that the integrity of disulfide bonds within the receptor-binding domain (RBD) plays an important role in the membrane fusion process although their disruption does not prevent binding of spike protein to ACE2. Several reducing agents and thiol-reactive compounds are able to inhibit viral entry. N-acetyl cysteine amide, L-ascorbic acid, JTT-705, and auranofin prevented syncytia formation, viral entry into cells, and infection in a mouse model, supporting disulfides of the RBD as a therapeutically relevant target.

The oxidation state of cysteine thiols on the ectodomain of TLR2 and TLR4 influences intracellular signaling

Signal transduction by the Toll-like receptors (TLRs) is a key component of innate immunity against many pathogens and also underlies a large burden of human diseases. Therefore, the mechanisms and regulation of signaling from the TLRs are of considerable interest. Here we seek to determine the molecular mechanism by which TLR2 and TLR4, members of the Toll-like receptor family, are activated by bacterial LPS, hyperoxia, and zymosan respectively. Our central hypothesis is that the oxidation state of cysteine thiols on the ectodomain of TLR2 and TLR4 are critical for pathogen-initiated intracellular signaling as well in hyperoxia. Cysteine thiols of TLR4 and its co-receptor MD2 have been shown to aid binding between the two molecules and also bacterial LPS binding to the receptor complex. We extend these findings by demonstrating the oxidation of free thiols on the ectodomain of hTLR4, after exposure to LPS or hyperoxia suggesting that the cysteines on the ectodomain of TLR4 could form intra- or intermolecular disulfide bonds. We also demonstrated blockade of intracellular signaling from TLR4 and TLR2 by thiol-modifying compounds which suggest a novel therapeutic intervention for sepsis, hyperoxia-induced cell injury and yeast infection. In these experiments CHO-3E10, HEK293 cells expressing hTLR2 or hTLR4 and mouse peritoneal macrophages cells were pretreated with cell impermeable maleimides to alkylate thiols on the extracellular domain of TLRs, cells were then exposed to LPS, hyperoxia or zymosan. In all of these models, we detected decreased intracellular signaling from TLR2 or TLR4. Furthermore, incubation with phenyl arsine oxide - which forms stable complexes with vicinal cysteine residues - prevented LPS induced HEK293/hTLR4 intracellular signaling which was reversed by DMPS. Sequence analysis of different TLRs revealed Leucine-Rich Repeat C-terminal (LRRCT) domain that contains 4 conserved cysteines. Further work is required to pinpoint the role of each cysteine in receptor dimerization, pathogen binding, hyperoxia modulation, and intracellular signaling.

Inhibition of myeloid differentiation factor 2 by baicalein protects against acute lung injury

BACKGROUND

ALI/ARDS is characterized by severe hypoxemic respiratory failure attributed to inflammatory tissue injury. There are no treatment modalities able to prevent/reverse the dire pathological sequelae in these patients. Evidence links the inflammatory lung injury to uncontrolled activation of the immune signaling complex, TLR4-MD2 (Toll-like receptor-myeloid differentiation factor 2). Baicalein, a natural flavonoid, is reported to have robust anti-inflammatory properties, but its inhibition mechanism remains unclear.

HYPOTHESIS/PURPOSE

This study investigated the protective mechanisms of baicalein on ALI/ARDS.

METHODS

We used two experimental mouse models of LPS-induced ALI, pulmonary infection model (intratracheal LPS), and systemic infection model (intravenous LPS). Blood, BALF, lung and liver tissues were analyzed using routine methods. In vitro studies using peritoneal mouse macrophages or recombinant proteins were designed to elucidate inhibition mechanisms of baicalein.

RESULTS

Our critical new findings revealed that Baicalein was an MD2 inhibitor, directly bound to MD2, effectively suppressing TLR4-MD2 activation and the subsequent MAPK and NF-κB signaling. The inhibited MD2 prevented development of inflammatory tissue injury and improved survival. The importance of MD2 in the inflammatory injury in ALI was corroborated by data obtained from MD2^-/- mice, which did not develop the characteristic LPS-induced lung tissue damage. Thus, the findings indicated that MD2 was critical for development of ALI, functioning as an early upstream signal driving the progression of inflammatory injury.

CONCLUSION

Baicalein, as a direct and selective MD2 inhibitor, inhibited the early upstream TLR4-MD2 signaling and is a promising therapeutic agent for the treatment of ALI/ARDS.

S-Alk(en)ylmercaptocysteine suppresses LPS-induced pro-inflammatory responses in murine macrophages through inhibition of NF-κB pathway and modulation of thiol redox status

The Allium vegetable-derived metabolite, S-alk(en)ylmercaptocysteine (CySSR), has been reported to modulate oxidative stress and inflammatory responses. However, the underlying mechanisms of action and structure-activity relationships are not completely understood. We investigated the mechanistic basis of the protective effects of CySSR on pro-inflammatory responses involving redox/oxidative stress induced by E. coli lipopolysaccharide (LPS) using RAW 264.7 cells. CySSR (R = allyl, "A" or 1-propenyl, "Pe") pre-treatments conferred concentration-dependent reductions in cytokines (TNF-α, IL-1β and IL-6), NO production and iNOS (inducible nitric synthase) overexpression, and attenuated oxidant production in LPS-stimulated RAW 264.7 cells where viability remained > 90%. These protective effects were manifested through inhibited activation of the nuclear factor-kappa B (NF-κB) signaling pathway via suppression of the IκB kinases (IKK) phosphorylation possibly by transforming growth factor β-activated kinase 1 or a kinase further upstream the canonical NF-κB signaling pathway. The attenuation of LPS-induced inflammation by CySSRs was associated with enhanced levels of cellular cysteine (CySH) and glutathione (GSH) mediated by cellular import/reduction of CySSR and the induction of glutamate cysteine ligase (GCL), one of > 200 nuclear factor erythroid 2-related factor 2 (Nrf2) regulated proteins. The reduction of anti-inflammatory effect of CySSR following pretreatment of cells with L-buthionine-S,R-sulfoximine (BSO) implicates GSH having a major role in reducing inflammation, likely in the context of other Nrf2-regulated antioxidant enzymes that scavenge H₂O₂ and peroxides using GSH as co-substrate. The anti-inflammatory effect of CySSPe was significantly greater than CySSA for almost all indicators measured, and cell metabolites of CySSRs may have a role in attenuating NF-κB signaling.

New MD2 inhibitors derived from curcumin with improved anti-inflammatory activity

An overactive Toll-like receptor (TLR) signaling complex is a significant pathogenic factor of acute and chronic inflammatory diseases. The natural product curcumin is reported to inhibit the TLR4 co-receptor, MD2 (myeloid differentiation protein 2), but its low in vivo bioavailability limits its therapeutic potential. We developed new curcumin analogs (MACs) with removal of the β-diketone moiety and substituted residues in benzene rings, and identify these as potential MD2 inhibitors with improved inhibition potency and stability over that of curcumin. Specifically, MAC 17 and 28 showed the highest anti-inflammatory activity, with >90% inhibition of LPS-stimulated cytokine secretion from macrophages, and protected against LPS-induced acute lung injury and sepsis. The MACs inhibited the TLR4-MD2 signaling complex through competition with LPS for binding on MD2, likely at Arg⁹⁰. Our findings indicated that MAC 17 and 28 are promising candidates for future development as therapeutic drugs for inflammatory diseases with an endotoxin etiology.

Recent progress in the discovery of myeloid differentiation 2 (MD2) modulators for inflammatory diseases

Myeloid differentiation protein 2 (MD2), together with Toll-like receptor 4 (TLR4), binds lipopolysaccharide (LPS) with high affinity, inducing the formation of the activated homodimer LPS-MD2-TLR4. MD2 directly recognizes the Lipid A domain of LPS, leading to the activation of downstream signaling of cytokine and chemokine production, and initiation of inflammatory and immune responses. However, excessive activation and potent host responses generate severe inflammatory syndromes such as acute sepsis and septic shock. MD2 is increasingly being considered as an attractive pharmacological target for the development of potent anti-inflammatory agents. In this Keynote review, we provide a comprehensive overview of the recent advances in the structure and biology of MD2, and present MD2 modulators as promising agents for anti-inflammatory intervention.

An inhibitory epitope of human Toll-like receptor 4 resides on leucine-rich repeat 13 and is recognized by a monoclonal antibody

Lipopolysaccharide (LPS)-induced activation of Toll-like receptor 4 (TLR4) elicits the innate immune response and can trigger septic shock if excessive. Two antibodies (HT4 and HT52) inhibit LPS-induced human TLR4 activation via novel LPS binding-independent mechanisms. The HT52 epitope resides on leucine-rich repeat 2 (LRR2) and is a feature of many inhibitory antibodies; antigen specificity of HT4 does not reside in LRR2. Here, we identified an HT4 epitope on LRR13 located close to the TLR4 dimerization interface that plays a role in NFκB activation. HT4 and HT52 mutually enhanced TLR4 inhibition. LRR13 is a novel inhibitory epitope and may be useful for developing anti-TLR4 antibodies. Combination therapy with LRR2 and LRR13 may effectively inhibit TLR4 activation.

Extracellular vesicles in the pathogenesis of rheumatoid arthritis and osteoarthritis

Osteoarthritis (OA) and rheumatoid arthritis (RA) are both debilitating diseases that cause significant morbidity in the US population. Extracellular vesicles (EVs), including exosomes and microvesicles, are now recognized to play important roles in cell-to-cell communication by transporting various proteins, microRNAs (miRNAs), and mRNAs. EV-derived proteins and miRNAs impact cell viability and cell differentiation, and are likely to play a prominent role in the pathophysiology of both OA and RA. Some of the processes by which these membrane-bound vesicles can alter joint tissue include extracellular matrix degradation, cell-to-cell communication, modulation of inflammation, angiogenesis, and antigen presentation. For example, EVs from IL-1β-stimulated fibroblast-like synoviocytes have been shown to induce osteoarthritic changes in chondrocytes. RA models have shown that EVs stimulated with inflammatory cytokines are capable of inducing apoptosis resistance in T cells, presenting antigen to T cells, and causing extracellular damage with matrix-degrading enzymes. EVs derived from rheumatoid models have also been shown to induce secretion of COX-2 and stimulate angiogenesis. Additionally, there is evidence that synovium-derived EVs may be promising biomarkers of disease in both OA and RA. The characterization of EVs in the joint space has also opened up the possibility for delivery of small molecules. This article reviews current knowledge on the role of EVs in both RA and OA, and their potential role as therapeutic targets for modulation of these debilitating diseases.

Gram Negative Bacterial Inflammation Ameliorated by the Plasma Protein Beta 2-Glycoprotein I

Lipopolysaccharide (LPS) is a major component of the outer wall of gram negative bacteria. In high doses LPS contributes to the inflammation in gram negative sepsis, and in low doses contributes to the low grade inflammation characteristic of the metabolic syndrome. We wanted to assess the role of beta2-glycoprotein I (β2GPI) a highly conserved plasma protein and its different biochemical forms in a mouse model of LPS systemic inflammation. Normal and β2GPI deficient mice were administered LPS through their veins and assessed for a range of inflammation markers in their blood and liver. Different biochemical forms of β2GPI were measured in normal mice given either saline or LPS. We show that β2GPI has a significant role in inhibiting LPS induced inflammation. In this study we provide some evidence that β2GPI serves a protective role in a mouse model of LPS inflammation. This resolves the controversy of previous studies which used LPS and β2GPI in test tube based models of LPS induced activation of white cells. We also highlight the potential relevance of a newly discovered biochemical form of β2GPI in LPS mediated inflammation and we speculate that this form has a protective role against LPS induced pathology.

Discovery of new MD2 inhibitor from chalcone derivatives with anti-inflammatory effects in LPS-induced acute lung injury

Acute lung injury (ALI) is a life-threatening acute inflammatory disease with limited options available for therapy. Myeloid differentiation protein 2, a co-receptor of TLR4, is absolutely required for TLR4 sense LPS, and represents an attractive target for treating severe inflammatory diseases. In this study, we designed and synthesized 31 chalcone derivatives that contain the moiety of (E)-4-phenylbut-3-en-2-one, which we consider the core structure of current MD2 inhibitors. We first evaluated the anti-inflammatory activities of these compounds in MPMs. For the most active compound 20, we confirmed that it is a specific MD2 inhibitor through a series of biochemical experiments and elucidated that it binds to the hydrophobic pocket of MD2 via hydrogen bonds with Arg⁹⁰ and Tyr¹⁰² residues. Compound 20 also blocked the LPS-induced activation of TLR4/MD2 -downstream pro-inflammatory MAPKs/NF-κB signaling pathways. In a rat model with ALI induced by intracheal LPS instillation, administration with compound 20 exhibited significant protective effect against ALI, accompanied by the inhibition of TLR4/MD2 complex formation in lung tissues. Taken together, the results of this study suggest the specific MD2 inhibitor from chalcone derivatives we identified is a potential candidate for treating acute inflammatory diseases.

Determination of the binding mode for anti-inflammatory natural product xanthohumol with myeloid differentiation protein 2

It is recognized that myeloid differentiation protein 2 (MD-2), a coreceptor of toll-like receptor 4 (TLR4) for innate immunity, plays an essential role in activation of the lipopolysaccharide signaling pathway. MD-2 is known as a neoteric and suitable therapeutical target. Therefore, there is great interest in the development of a potent MD-2 inhibitor for anti-inflammatory therapeutics. Several studies have reported that xanthohumol (XN), an anti-inflammatory natural product from hops and beer, can block the TLR4 signaling by binding to MD-2 directly. However, the interaction between MD-2 and XN remains unknown. Herein, our work aims at characterizing interactions between MD-2 and XN. Using a combination of experimental and theoretical modeling analysis, we found that XN can embed into the hydrophobic pocket of MD-2 and form two stable hydrogen bonds with residues ARG-90 and TYR-102 of MD-2. Moreover, we confirmed that ARG-90 and TYR-102 were two necessary residues during the recognition process of XN binding to MD-2. Results from this study identified the atomic interactions between the MD-2 and XN, which will contribute to future structural design of novel MD-2-targeting molecules for the treatment of inflammatory diseases.

MD-2 as the target of a novel small molecule, L6H21, in the attenuation of LPS-induced inflammatory response and sepsis

BACKGROUND AND PURPOSE

Myeloid differentiation 2 (MD-2) recognizes LPS, which is required for TLR4 activation, and represents an attractive therapeutic target for severe inflammatory disorders. We previously found that a chalcone derivative, L6H21, could inhibit LPS-induced overexpression of TNF-α and IL-6 in macrophages. Here, we performed a series of biochemical experiments to investigate whether L6H21 specifically targets MD-2 and inhibits the interaction and signalling transduction of LPS-TLR4/MD-2.

EXPERIMENTAL APPROACH

The binding affinity of L6H21 to MD-2 protein was analysed using computer docking, surface plasmon resonance analysis, elisa, fluorescence measurements and flow cytometric analysis. The effects of L6H21 on MAPK and NF-κB signalling were determined using EMSA, fluorescence staining, Western blotting and immunoprecipitation. The anti-inflammatory effects of L6H21 were confirmed using elisa and RT-qPCR in vitro. The anti-inflammatory effects of L6H21 were also evaluated in septic C57BL/6 mice.

KEY RESULTS

Compound L6H21 inserted into the hydrophobic region of the MD-2 pocket, forming hydrogen bonds with Arg(90) and Tyr(102) in the MD-2 pocket. In vitro, L6H21 subsequently suppressed MAPK phosphorylation, NF-κB activation and cytokine expression in macrophages stimulated by LPS. In vivo, L6H21 pretreatment improved survival, prevented lung injury, decreased serum and hepatic cytokine levels in mice subjected to LPS. In addition, mice with MD-2 gene knockout were universally protected from the effects of LPS-induced septic shock.

CONCLUSIONS AND IMPLICATIONS

Overall, this work demonstrated that the new chalcone derivative, L6H21, is a potential candidate for the treatment of sepsis. More importantly, the data confirmed that MD-2 is an important therapeutic target for inflammatory disorders.

Toll-like receptor 4 senses oxidative stress mediated by the oxidation of phospholipids in extracellular vesicles

Oxidative stress produced in response to infection or sterile injury activates the innate immune response. We found that extracellular vesicles (EVs) isolated from the plasma of patients with rheumatoid arthritis or secreted from cells subjected to oxidative stress contained oxidized phospholipids that stimulated cells expressing Toll-like receptor 4 (TLR4) in a manner dependent on its co-receptor MD-2. EVs from healthy subjects or reconstituted synthetic EVs subjected to limited oxidation gained the ability to stimulate TLR4-expressing cells, whereas prolonged oxidation abrogated this property. Furthermore, we found that 15-lipoxygenase generated hydro(pero)xylated phospholipids that stimulated TLR4-expressing cells. Molecular modeling suggested that the mechanism of activation of TLR4 by oxidized phospholipids in EVs was structurally similar to that of the TLR4 ligand lipopolysaccharide (LPS). This was supported by experiments showing that EV-mediated stimulation of cells required MD-2, that mutations that block LPS binding to TLR4 abrogated the stimulatory effect of EVs, and that EVs induced TLR4 dimerization. On the other hand, analysis of gene expression profiles showed that genes encoding factors that resolve inflammation were more abundantly expressed in responses to EVs than in response to LPS. Together, these data suggest that EVs act as an oxidative stress-induced endogenous danger signal that underlies the pervasive role of TLR4 in inflammatory diseases.

Curcumin Analog L48H37 Prevents Lipopolysaccharide-Induced TLR4 Signaling Pathway Activation and Sepsis via Targeting MD2

Endotoxin-induced acute inflammatory diseases such as sepsis, mediated by excessive production of various proinflammatory cytokines, remain the leading cause of mortality in critically ill patients. Lipopolysaccharide (LPS), the characteristic endotoxin found in the outer membrane of Gram-negative bacteria, can induce the innate immunity system and through the myeloid differentiation protein 2 (MD2) and Toll-like receptor 4 (TLR4) complex, increase the production of inflammatory mediators. Our previous studies have found that a curcumin analog, L48H37 [1-ethyl-3,5-bis(3,4,5-trimethoxybenzylidene)piperidin-4-one], was able to inhibit LPS-induced inflammation, particularly tumor necrosis factor α and interleukin 6 production and gene expression in mouse macrophages. In this study, a series of biochemical experiments demonstrate L48H37 specifically targets MD2 and inhibits the interaction and signaling transduction of LPS-TLR4/MD2. L48H37 binds to the hydrophobic region of MD2 pocket and forms hydrogen bond interactions with Arg(90) and Tyr(102). Subsequently, L48H37 was shown to suppress LPS-induced mitogen-activated protein kinase phosphorylation and nuclear factor κB activation in macrophages; it also dose dependently inhibits the cytokine expression in macrophages and human peripheral blood mononuclear cells stimulated by LPS. In LPS-induced septic mice, both pretreatment and treatment with L48H37 significantly improved survival and protected lung injury. Taken together, this work identified a new MD2 specific inhibitor, L48H37, as a potential candidate in the treatment of sepsis.

Establishment of a mouse model to express bovine CD14 short hairpin RNA

Background

Cluster of differentiation 14 (CD14) functions as a co-receptor for Toll-like receptor (TLR)-4 and myeloid differentiation factor (MD)-2 in detecting bacterial lipopolysaccharide. Together, these complexes promote the phagocytosis and digestion of Gram-negative bacteria, and initiate immune responses. To date, much of our understanding of CD14 function during Gram-negative bacterial inflammation comes from studies on mouse knockout models and cell transfection. To identify the effect of CD14 knockdown in this process in large livestock animals, we established a mouse model expressing bovine CD14 short hairpin (sh) RNA. shRNA fragments targeting bovine CD14 were screened by co-transfection in HEK 293 cells, and the most effective CD14 shRNA fragment was cloned into the eukaryotic expression vector pSilencer4.1-CD14 shRNA-IRES (internal ribosome entry site) and transferred into mouse zygotes by pronuclear microinjection to obtain transgenic mice. Expression of the enhanced green fluorescent protein (EGFP) reporter and genes related to the TLR4 signaling pathway was detected by immunohistochemistry (IHC) and quantitative polymerase chain reaction (PCR), respectively.

Results

One effective shRNA fragment (shRNA-674) targeting bovine CD14 was obtained, the sequence of which was shown to be conserved between cows, buffalos, sheep, and humans. Thirty-seven founder pups were obtained by pronuclear microinjection, of which three were positive for the transgene. In the F₁ generation, 11 of 33 mice (33%) were positive for the transgene as detected by PCR. IHC analysis detected exogenous EGFP expression in the liver, kidney, and spleen of transgenic F₁ mice, indicating that they were chimeric. The expression of endogenous CD14 mRNA in the heart, liver, spleen, lung, and kidney of transgenic F₁ mice was decreased 8-, 3-, 19.5-, 6-, and 11-fold, respectively. The expression patterns of endogenous MD-2, TLR4, interleukin-6 and tumor necrosis factor-α genes in transgenic mice also varied.

Conclusions

This study confirms that transgenic mice expressing bovine CD14 shRNA can be generated by pronuclear microinjection, and demonstrates inhibited endogenous mouse CD14 expression that alters gene expression related to the TLR4 signaling pathway.

Kdo2 -lipid A: structural diversity and impact on immunopharmacology

3-deoxy-d-manno-octulosonic acid-lipid A (Kdo₂-lipid A) is the essential component of lipopolysaccharide in most Gram-negative bacteria and the minimal structural component to sustain bacterial viability. It serves as the active component of lipopolysaccharide to stimulate potent host immune responses through the complex of Toll-like-receptor 4 (TLR4) and myeloid differentiation protein 2. The entire biosynthetic pathway of Escherichia coli Kdo₂-lipid A has been elucidated and the nine enzymes of the pathway are shared by most Gram-negative bacteria, indicating conserved Kdo₂-lipid A structure across different species. Yet many bacteria can modify the structure of their Kdo₂-lipid A which serves as a strategy to modulate bacterial virulence and adapt to different growth environments as well as to avoid recognition by the mammalian innate immune systems. Key enzymes and receptors involved in Kdo₂-lipid A biosynthesis, structural modification and its interaction with the TLR4 pathway represent a clear opportunity for immunopharmacological exploitation. These include the development of novel antibiotics targeting key biosynthetic enzymes and utilization of structurally modified Kdo₂-lipid A or correspondingly engineered live bacteria as vaccines and adjuvants. Kdo₂-lipid A/TLR4 antagonists can also be applied in anti-inflammatory interventions. This review summarizes recent knowledge on both the fundamental processes of Kdo₂-lipid A biosynthesis, structural modification and immune stimulation, and applied research on pharmacological exploitations of these processes for therapeutic development.

Suppression of TLR signaling by targeting TIR domain-containing proteins

Toll-like receptors (TLRs) recognize molecules specific to pathogens and endogenous danger signals. Binding of agonists to the ectodomain of the receptor initiates TLR activation and is followed by the association of receptor cytosolic Toll/Interleukin-1 receptor (TIR) domains with TIR domains of adapter proteins leading to the assembly of signaling cascade of protein kinases that ultimately trigger the activation of transcription factors and expression of genes involved in the immune response. Excessive activation of TIR-domain mediated signaling has been implicated in inflammatory diseases (e.g. rheumatoid arthritis, systemic lupus erythematosus, colitis) as well as in the development of cancer. Targeting receptor-adapter interactions represents a potential strategy for the therapeutic TLR/IL-1R-specific inhibition due to the unique interacting domains involved. Peptide and protein-domain binding TLR inhibitors originating from the interacting surfaces of TIR-domain containing proteins can bind to the site on their target interacting protein thereby preventing the assembly of the functional signaling complex. Here we review protein-domain, peptide and peptidomimetic inhibitors targeting TIR-domain mediated interactions and their application demonstrated on in vitro and in vivo models. Recent structural data and elucidation of the molecular mechanisms of TIR-domain mediated signaling enabled the development of peptide inhibitors from TIR domains of TLRs and adapters, MyD88 intermediary domain as well as improved protein inhibitors based on TIR domain dimerization, mimicking bacterial TIR-domain containing immunosuppressors (TCPs) which we discuss with challenges concerning the delivery and specificity of inhibitors targeting TLR adapters.

Suppression of Toll-like receptor 4 activation by endogenous oxidized phosphatidylcholine, KOdiA-PC by inhibiting LPS binding to MD2

OBJECTIVE

Activation of Toll-like receptor 4 (TLR4) triggers immune and inflammatory events by sensing endogenous danger signals as well as invading pathogens and contributes to the development of chronic inflammatory diseases. In this study, we investigated effect of 1-palmitoyl-2-(5-keto-6-octenedioyl)-sn-glycero-3-phosphocholine (KOdiA-PC), an oxidized phosphatidylcholine, on TLR4 activation and the underlying regulatory mechanism.

METHODS

RAW264.7 macrophages were used for the study. The levels of TNF-α, IFN-β, and COX-2 mRNA and protein were determined by quantitative PCR and ELISA, respectively. Activation of TLR4-signaling was examined by immunoblot and luciferase reporter assays. In vitro binding assay was performed to determine LPS binding to MD2. Macrophage migration was analyzed using a transwell-culture system.

RESULTS

KOdiA-PC prevented the activation of TLR4-signaling components including ERK, JNK, p38, NF-κB, and IRF3 leading to decrease of TNF-α, IFN-β, and COX-2 expression. In vitro binding assay revealed that KOdiA-PC interrupted LPS binding to MD2, a TLR4 co-receptor. Consistently, KOdiA-PC suppressed LPS-induced macrophage migration.

CONCLUSION

The results demonstrate that KOdiA-PC can modulate TLR4 activation by regulating ligand-receptor interaction. Therefore, endogenously generated, oxidized phospholipids may play a role in resolving inflammation by terminating TLR activation and macrophage recruitment to the inflamed site.

Lipid A from lipopolysaccharide recognition: structure, dynamics and cooperativity by molecular dynamics simulations

Molecular dynamics simulations of Lipid A and its natural precursor Lipid IVA from E.coli have been carried out free in solution, bound to the myeliod differentiation protein 2 (MD2) and in the complex of MD2 with the toll like receptor 4 (TLR4). In addition, simulations of the ligand free MD2 and MD2-TLR4 complex were performed. A structural and energetic characterization of the bound and unbound states of Lipid A/IVA was generated. As the crystal structures depict, the main driving force for MD2-Lipid A/IVA are the hydrophobic interactions between the aliphatic tails and the MD2 cavity. The charged phosphate groups do strongly interact with positively charged residues, located at the surface of MD2. However, they are not essential for keeping the lipids in the cavity, indicating a more prominent role in binding recognition and ionic interactions with TLR4 at the MD2/TLR4 interface. Interestingly, in the absence of any ligand MD2 rapidly closes, blocking the binding cavity. The presence of TLR4, though changing the dynamics, was not able to impede the aforementioned closing event. We hypothesize that fluctuations of the H1 region are essential for this phenomenon, and it is plausible that an equilibrium between the open and closed states exists, although the lengths of our simulations are not sufficient to encompass the reversible process. The MD2/Lipid A-TLR4 complex simulations show that the presence of the ligand energetically stabilizes the complex relative to the ligand-free structures, indicating cooperativity in the binding process.

The biological activity of auranofin: implications for novel treatment of diseases

More than 30 years ago, auranofin was developed for the treatment of rheumatoid arthritis as a substitution for the injectable gold compounds aurothiomalate and aurothioglucose. Both the ease of oral administration over intramuscular injections and more potent anti-inflammatory effects in vitro made auranofin seem like an excellent substitute for the traditional injectable gold compounds. Despite efficacy in the treatment of both rheumatoid arthritis and psoriasis, currently, auranofin is seldom used as a treatment for patients with rheumatoid arthritis as more novel anti-rheumatic medications have become available. Despite the decline in its clinical applications, research on auranofin has continued as it shows promise in the treatment of several different diseases. In recent years, advances in technology have allowed researchers to use molecular techniques to identify novel mechanisms of action of auranofin. Additionally, researchers are discovering potential new applications of auranofin. Dual inhibition of inflammatory pathways and thiol redox enzymes by auranofin makes it a new candidate for cancer therapy and treating microbial infections. This review will summarize recently obtained data on the mechanisms of action of auranofin, and potential new applications of auranofin in the treatment of various diseases, including several types of leukaemia, carcinomas, and parasitic, bacterial, and viral infections.

Drosophila melanogaster NPC2 proteins bind bacterial cell wall components and may function in immune signal pathways

ML (MD-2 (myeloid differentiation factor-2)-related Lipid-recognition) is a conserved domain identified in MD-2, MD-1, NPC2 (Niemann-Pick disease type C2), and mite major allergen protein from animals, plants, and fungi. Vertebrate members of the ML family proteins, such as NPC2 and MD-2, play important roles in lipid metabolism and immune signaling pathway. MD-2 is an essential co-receptor in the lipopolysaccharide (LPS)/Toll-like receptor 4 (TLR4) signaling pathway. Insects contain multiple ML genes, arbitrarily named md-2- or npc2-like genes. However, whether insect ML genes have functions similar to vertebrate md-2 is unknown. In Drosophila melanogaster, there are eight npc2 genes (npc2a-h), and they can be further divided into three subgroups based on the numbers of cysteine residues (6, 7 and 8 Cys) in the mature proteins. The purpose of this study is to investigate whether any Drosophila npc2 genes may have functions in immune signaling pathways. We chose npc2a, npc2e and npc2h genes representing the three subgroups for this study. We showed that recombinant NPC2a, NPC2e and NPC2h not only bound to LPS and lipid A, but also bound to peptidoglycan (PG) and lipoteichoic acid (LTA), a property that has not been reported previously for vertebrate NPC2 or MD-2. More importantly, we showed that over-expression of NPC2a and NPC2e activated diptericin promoter reporter in S2 cells stimulated by PG, suggesting that NPC2e and NPC2a may play a role in the immune deficiency (Imd) pathway. This is the first in vitro study about NPC2 proteins in innate immunity of D. melanogaster.

Inhibition of LPS binding to MD-2 co-receptor for suppressing TLR4-mediated expression of inflammatory cytokine by 1-dehydro-10-gingerdione from dietary ginger

Myeloid differentiation protein 2 (MD-2) is a co-receptor of toll-like receptor 4 (TLR4) for innate immunity. Here, we delineated a new mechanism of 1-dehydro-10-gingerdione (1D10G), one of pungent isolates from ginger (Zingiber officinale), in the suppression of lipopolysaccharide (LPS)-induced gene expression of inflammatory cytokines. 1D10G inhibited LPS binding to MD-2 with higher affinity than gingerol and shogaol from dietary ginger. Moreover, 1D10G down-regulated TLR4-mediated expression of nuclear factor-κB (NF-κB) or activating protein 1 (AP1)-target genes such as tumor necrosis factor α (TNF-α) and interleukin-1β, as well as those of interferon (IFN) regulatory factor 3 (IRF3)-target IFN-β gene and IFN-γ inducible protein 10 (IP-10) in LPS-activated macrophages. Taken together, MD-2 is a molecular target in the anti-inflammatory action of 1D10G.

Myeloid differentiation 2 as a therapeutic target of inflammatory disorders

Lipopolysaccharide (LPS), an endotoxin of Gram-negative bacteria, activates the innate immunity system through a receptor complex of myeloid differentiation 2 (MD-2) and toll-like receptor 4 (TLR4). MD-2 directly recognizes the lipid A domain of LPS, which triggers MD-2/TLR4-mediated cellular response aimed at eliminating the invaded pathogen. However, excess production of inflammatory mediators is harmful to host tissue and this can cause septic death in extreme cases. MD-2 represents an attractive therapeutic target of inflammatory and immune diseases in human. In particular, eritoran is a synthetic tetraacylated lipid A that binds directly to MD-2 and antagonizes LPS binding to the same site, and it ameliorates various inflammatory conditions due to infection or sterile organ injury. In this review, we outline the recent advances in the structure biology of ligand interaction with MD-2/TLR4, and highlight the MD-2-directed LPS antagonists, which are natural and synthetic chemicals, under development to treat inflammatory diseases.

Intrinsic and extrinsic regulation of innate immune receptors

Pattern recognition receptors (PRRs) in innate immune cells play a pivotal role in the first line of host defense system. PRRs recognize pathogen-associated molecular patterns (PAMPs) or danger-associated molecular patterns (DAMPs) to initiate and regulate innate and adaptive immune responses. PRRs include Toll-like receptors (TLRs), RIG-I-like receptors (RLRs) and NOD-like receptors (NLRs), which have their own features in ligand recognition and cellular location. Activated PRRs deliver signals to adaptor molecules (MyD88, TRIF, MAL/TIRAP, TRAM, IPS-1) which act as important messengers to activate downstream kinases (IKK complex, MAPKs, TBK1, RIP-1) and transcription factors (NF-κB, AP-1, IRF3), which produce effecter molecules including cytokines, chemokines, inflammatory enzymes, and type I interferones. Since excessive PRR activation is closely linked to the development of chronic inflammatory diseases, the role of intrinsic and extrinsic regulators in the prevention of over- or unnecessary activation of PRRs has been widely studied. Intracellular regulators include MyD88s, SOCS1, TOLLIP, A20, and CYLD. Extrinsic regulators have also been identified with their molecular targets in PRR signaling pathways. TLR dimerization has been suggested as an inhibitory target for small molecules such as curcumin, cinnamaldehyde, and sulforaphane. TBK1 kinase can be a target for certain flavonoids such as EGCG, luteolin, quercetin, chrysin, and eriodictyol to regulate TRIF-dependent TLR pathways. This review focuses on the features of PRR signaling pathways and the therapeutic targets of intrinsic and extrinsic regulators in order to provide beneficial strategies for controlling the activity of PRRs and the related inflammatory diseases and immune disorders.

Biochemical characterization of cholesteryl ester transfer protein inhibitors

Cholesteryl ester transfer protein (CETP) has been identified as a novel target for increasing HDL cholesterol levels. In this report, we describe the biochemical characterization of anacetrapib, a potent inhibitor of CETP. To better understand the mechanism by which anacetrapib inhibits CETP activity, its biochemical properties were compared with CETP inhibitors from distinct structural classes, including torcetrapib and dalcetrapib. Anacetrapib and torcetrapib inhibited CETP-mediated cholesteryl ester and triglyceride transfer with similar potencies, whereas dalcetrapib was a significantly less potent inhibitor. Inhibition of CETP by both anacetrapib and torcetrapib was not time dependent, whereas the potency of dalcetrapib significantly increased with extended preincubation. Anacetrapib, torcetrapib, and dalcetrapib compete with one another for binding CETP; however anacetrapib binds reversibly and dalcetrapib covalently to CETP. In addition, dalcetrapib was found to covalently label both human and mouse plasma proteins. Each CETP inhibitor induced tight binding of CETP to HDL, indicating that these inhibitors promote the formation of a complex between CETP and HDL, resulting in inhibition of CETP activity.