Trif-related adapter molecule is phosphorylated by PKC{epsilon} during Toll-like receptor 4 signaling

New Promising Routes in Peptic Ulcers: Toll-like Receptors and Semaphorins

Peptic ulcers (PU) are one of the commonest yet problematic diseases found to be existing in the majority of the population. Today, drugs from a wide range of therapeutic classes are available for the management of the disease. Still, the complications of the condition are difficult to tackle and the side effect profile is quite a concern. The literature indicates that Toll-like receptors (TLRs) and Semaphorins (SEMAs) have been under study for their various pharmacological actions over the past few decades. Both these signalling pathways are found to regulate immunological and inflammatory responses. Moreover, receptors and signalling molecules from the family of TLRs and SEMAs are found to have bacterial recognition and antibacterial properties which are essential in eradicating Helicobacter pylori (H. pylori), one of the major causative agents of PU. Our understanding of SEMAs, a class of proteins involved in cell signalling, is relatively less developed compared to TLRs, another class of proteins involved in the immune response. SEMAs and TLRs play different roles in biological processes, with SEMAs primarily involved in guiding cell migration and axon guidance during development, while TLRs are responsible for recognizing pathogens and initiating an immune response. Here, in this review, we will discuss in detail the signalling cascade of TLRs and SEMAs and thereby understand its association with PU for future therapeutic targeting. The review also aims at providing an overview of the study that has been into exploring the role of these signalling pathways in the management of PU.

Thromboxane A2-TP axis promotes adipose tissue macrophages M1 polarization leading to insulin resistance in obesity

Aberrant arachidonic acid metabolism has been implicated in multiple pathophysiological conditions, and the downstream prostanoids levels are associated with adipocyte dysfunction in obesity. However, the role of thromboxane A₂ (TXA₂) in obesity remains unclear. We observed that TXA₂, through its receptor TP, is a candidate mediator in obesity and metabolic disorders. Obese mice with upregulated TXA₂ biosynthesis (TBXAS1) and TXA₂ receptor (TP) expression in caused insulin resistance and macrophage M1 polarization in white adipose tissue (WAT), which can be prevented by treatment with aspirin. Mechanistically, the activation of TXA₂-TP signaling axis leads to accumulation of protein kinase Cɛ (PKCɛ), thereby enhancing free fat acid (FFA) induced Toll-like receptor4 (TLR4) proinflammatory macrophage activation and the tumor necrosis factor-a (TNF-a) production in adipose tissues. Importantly, TP knockout mice reduced the accumulation of proinflammatory macrophages and adipocyte hypertrophy in WAT. Thus, our findings demonstrate that TXA₂-TP axis plays a crucial role in obesity-induced adipose macrophage dysfunction, and rational targeting TXA₂ pathway may improve obesity and its associated metabolic disorders in future. In this work, we establish previously unknown role of TXA₂-TP axis in WAT. These findings might provide new insight into the molecular pathogenesis of insulin resistance, and indicate rational targeting TXA₂ pathway to improve obesity and its associated metabolic disorders in future.

Regulatory role of phosphoproteins in the development of bovine small intestine during early life

The small intestine is the primary site of nutrient digestion and absorption, which plays a key role in the survival of neonatal calves. A comprehensive assessment of the phosphoproteomic changes in the small intestine of neonatal calves is unavailable; therefore, we used phosphopeptide enrichment coupled with liquid chromatography-tandem mass spectrometry to investigate the changes in the phosphoproteome profile in the bovine small intestine during the first 36 h of life. Twelve neonatal male calves were assigned to one of the following groups: (1) calves not fed colostrum and slaughtered approximately 2 h postpartum (n = 3), (2) calves fed colostrum at 1 to 2 h and slaughtered 8 h postpartum (n = 3), (3) calves fed 2 colostrum meals (at 1-2 and 10-12 h) and slaughtered 24 h postpartum (n = 3), (4) calves fed 3 colostrum meals (at 1-2, 10-12, and 22-24 h) and slaughtered 36 h postpartum (n = 3). Mid-duodenal, jejunal, and ileal samples of the calves were collected after slaughter. We identified 1,678 phosphoproteins with approximately 3,080 phosphosites, which were mainly Ser (89.9%), Thr (9.8%), and Tyr (0.3%) residues; they belonged to the prodirected (52.9%), basic (20.4%), acidic (16.6%), and Tyr-directed (1.7%) motif categories. The regional differentially expressed phosphoproteins included zonula occludens 2, sorting nexin 12, and protein kinase C, which are mainly associated with developmental processes, intracellular transport, vesicle-mediated transport, and immune system process. They are enriched in the endocytosis, tight junction, insulin signaling, and focal adhesion pathways. The temporal differentially expressed phosphoproteins included occludin, epsin 1, and bridging integrator 1, which were mainly associated with macromolecule metabolic process, cell adhesion, and growth. They were enriched in the spliceosomes, adherens junctions, and tight junctions. The observed changes in the phosphoproteins in the tissues of small intestine suggest the protein phosphorylation plays an important role in nutrient transport and immune response of calves during early life, which needs to be confirmed in a larger study.

Molecular Signature of Neuroinflammation Induced in Cytokine-Stimulated Human Cortical Spheroids

Crucial in the pathogenesis of neurodegenerative diseases is the process of neuroinflammation that is often linked to the pro-inflammatory cytokines Tumor necrosis factor alpha (TNFα) and Interleukin-1beta (IL-1β). Human cortical spheroids (hCSs) constitute a valuable tool to study the molecular mechanisms underlying neurological diseases in a complex three-dimensional context. We recently designed a protocol to generate hCSs comprising all major brain cell types. Here we stimulate these hCSs for three time periods with TNFα and with IL-1β. Transcriptomic analysis reveals that the main process induced in the TNFα- as well as in the IL-1β-stimulated hCSs is neuroinflammation. Central in the neuroinflammatory response are endothelial cells, microglia and astrocytes, and dysregulated genes encoding cytokines, chemokines and their receptors, and downstream NFκB- and STAT-pathway components. Furthermore, we observe sets of neuroinflammation-related genes that are specifically modulated in the TNFα-stimulated and in the IL-1β-stimulated hCSs. Together, our results help to molecularly understand human neuroinflammation and thus a key mechanism of neurodegeneration.

The Anti-Inflammatory Effect of Nanoarchaeosomes on Human Endothelial Cells

Archaebacterias are considered a unique source of novel biomaterials of interest for nanomedicine. In this perspective, the effects of nanoarchaeosomes (ARC), which are nanovesicles prepared from polar lipids extracted from the extreme halophilic Halorubrum tebenquinchense, on human umbilical vein endothelial cells (HUVEC) were investigated in physiological and under inflammatory static conditions. Upon incubation, ARC (170 nm mean size, −41 mV ζ) did not affect viability, cell proliferation, and expression of intercellular adhesion molecule-1 (ICAM-1) and E-selectin under basal conditions, but reduced expression of both molecules and secretion of IL-6 induced by lypopolysaccharide (LPS), Pam3CSK4 or Escherichia coli. Such effects were not observed with TNF-α or IL-1β stimulation. Interestingly, ARC significantly decreased basal levels of von Willebrand factor (vWF) and levels induced by all stimuli. None of these parameters was altered by liposomes of hydrogenated phosphatidylcholine and cholesterol of comparable size and concentration. Only ARC were endocytosed by HUVEC and reduced mRNA expression of ICAM-1 and vWF via NF-ĸB and ERK1/2 in LPS-stimulated cells. This is the first report of the anti-inflammatory effect of ARC on endothelial cells and our data suggest that its future use in vascular disease may hopefully be of particular interest.

Molecular Mechanisms of Lipopolysaccharide (LPS) Induced Inflammation in an Immortalized Ovine Luteal Endothelial Cell Line (OLENDO)

Escherichia coli (E. coli) is the most common Gram-negative bacterium causing infection of the uterus or mammary gland and is one of the major causes of infertility in livestock. In those animals affected by E. coli driven LPS-mediated infections, fertility problems occur in part due to disrupted follicular and luteal functionality. However, the molecular mechanisms by which LPS induces inflammation, and specifically, the role of LPS in the disruption of capillary morphogenesis and endothelial barrier function remain unclear. Here, we hypothesized that LPS may lead to alterations in luteal angiogenesis and vascular function by inducing inflammatory reactions in endothelial cells. Accordingly, OLENDO cells were treated with LPS followed by evaluation of the expression of selected representative proinflammatory cytokines: NF-kB, IL6, IL8, TNFα, and ICAM 1. While TNFα was not affected by treatment with LPS, transcripts of NF-kB, IL6, and IL8 were affected in a dosage-dependent manner. Additionally, the activity of TLR2 and TLR4 was blocked, resulting in suppression of the LPS-induced expression of ICAM 1, NF-kB, IL6, and IL8. Inhibition of the PKA or MAPK/ERK pathways suppressed the LPS-stimulated expression of NF-kB, IL6, and IL8, whereas blocking the PKC pathway had the opposite effect. Furthermore, LPS-induced phosphorylation of Erk1 and Erk2 was inhibited when the TLR4 or MAPK/ERK pathways were blocked. Finally, LPS seems to induce inflammatory processes in OLENDO cells via TLR2 and TLR4, utilizing different signaling pathways.

Achyranthes bidentata Polysaccharide Activates Nuclear Factor-Kappa B and Promotes Cytokine Production in J774A.1 Cells Through TLR4/MyD88 Signaling Pathway

Achyranthes bidentata Blume, a traditional Chinese medicine, is widely acknowledged for its function of invigorating the liver and kidneys and as a stranguria-relieving diuretic and used in the treatment of edema, gonorrhea, and other diseases. Polysaccharide (ABPS), isolated from Achyranthes bidentata Blume, has been demonstrated to have multiple biological activities including immunomodulatory effects. However, the mechanisms underlying the effects of ABPS have not been fully investigated. The present study is conducted to explore the underlying mechanism of immunomodulatory activities of ABPS. Results showed that ABPS significantly increased the secretion of IL-1β and TNF-α in J744 A.1 cells. Nitric oxide (NO) also significantly increased after ABPS treatment. The special antibodies (Toll-like receptor 4 (TLR4) antibody and CD14/TLR4 antibody) significantly decreased the activation, while the Toll-like receptor 2 (TLR2) antibody could not abolish this activation. Meanwhile, pyrrolidine dithiocarbamate (PDTC), a specific inhibitor of NF-κB, remarkably inhibited the secretion of IL-1β and TNF-α induced by ABPS in J744 A.1 cells. Western blotting (WB) and confocal laser scanning microscopy (CLSM) showed that ABPS promoted NF-κB translocation into the nucleus. Furthermore, the mRNA and protein expression of TLR4 and MyD88 were significantly increased after ABPS treatment. Taken together, these findings suggested that the immunomodulatory mechanism of ABPS was associated with the secretion of cytokines by stimulating the NF-κB pathway through TLR4/MyD88 signaling.

Designed PKC-targeting bryostatin analogs modulate innate immunity and neuroinflammation

Neuroinflammation characterizes multiple neurologic diseases, including primary inflammatory conditions such as multiple sclerosis and classical neurodegenerative diseases. Aberrant activation of the innate immune system contributes to disease progression, but drugs modulating innate immunity, particularly within the central nervous system (CNS), are lacking. The CNS-penetrant natural product bryostatin-1 attenuates neuroinflammation by targeting innate myeloid cells. Supplies of natural bryostatin-1 are limited, but a recent scalable good manufacturing practice (GMP) synthesis has enabled access to it and its analogs (bryologs), the latter providing a path to more efficacious, better tolerated, and more accessible agents. Here, we show that multiple synthetically accessible bryologs replicate the anti-inflammatory effects of bryostatin-1 on innate immune cells in vitro, and a lead bryolog attenuates neuroinflammation in vivo, actions mechanistically dependent on protein kinase C (PKC) binding. Our findings identify bryologs as promising drug candidates for targeting innate immunity in neuroinflammation and create a platform for evaluation of synthetic PKC modulators in neuroinflammatory diseases.

Protein N-myristoylation: functions and mechanisms in control of innate immunity

Protein N-myristoylation is an important fatty acylation catalyzed by N-myristoyltransferases (NMTs), which are ubiquitous enzymes in eukaryotes. Specifically, attachment of a myristoyl group is vital for proteins participating in various biological functions, including signal transduction, cellular localization, and oncogenesis. Recent studies have revealed unexpected mechanisms indicating that protein N-myristoylation is involved in host defense against microbial and viral infections. In this review, we describe the current understanding of protein N-myristoylation (mainly focusing on myristoyl switches) and summarize its crucial roles in regulating innate immune responses, including TLR4-dependent inflammatory responses and demyristoylation-induced innate immunosuppression during Shigella flexneri infection. Furthermore, we examine the role of myristoylation in viral assembly, intracellular host interactions, and viral spread during human immunodeficiency virus-1 (HIV-1) infection. Deeper insight into the relationship between protein N-myristoylation and innate immunity might enable us to clarify the pathogenesis of certain infectious diseases and better harness protein N-myristoylation for new therapeutics.

Signal Transduction in Immune Cells and Protein Kinases

Immune response relies upon several intracellular signaling events. Among the protein kinases involved in these pathways, members of the protein kinase C (PKC) family are prominent molecules because they have the capacity to acutely and reversibly modulate effector protein functions, controlling both spatial distribution and dynamic properties of the signals. Different PKC isoforms are involved in distinct signaling pathways, with selective functions in a cell-specific manner.In innate system, Toll-like receptor signaling is the main molecular event triggering effector functions. Various isoforms of PKC can be common to different TLRs, while some of them are specific for a certain type of TLR. Protein kinases involvement in innate immune cells are presented within the chapter emphasizing their coordination in many aspects of immune cell function and, as important players in immune regulation.In adaptive immunity T-cell receptor and B-cell receptor signaling are the main intracellular pathways involved in seminal immune specific cellular events. Activation through TCR and BCR can have common intracellular pathways while others can be specific for the type of receptor involved or for the specific function triggered. Various PKC isoforms involvement in TCR and BCR Intracellular signaling will be presented as positive and negative regulators of the immune response events triggered in adaptive immunity.

Toll-Like Receptors, Associated Biochemical Signaling Networks, and S100 Ligands

ABSTRACT

Host cells recognize molecules that signal danger using pattern recognition receptors (PRRs). Toll-like receptors (TLRs) are the most studied class of PRRs and detect pathogen-associated molecular patterns and danger-associated molecular patterns. Cellular TLR activation and signal transduction can therefore contain, combat, and clear danger by enabling appropriate gene transcription. Here, we review the expression, regulation, and function of different TLRs, with an emphasis on TLR-4, and how TLR adaptor protein binding directs intracellular signaling resulting in activation or termination of an innate immune response. Finally, we highlight the recent progress of research on the involvement of S100 proteins as ligands for TLR-4 in inflammatory disease.

Immunological regulatory effect of flavonoid baicalin on innate immune toll-like receptors

As an essential component of the innate immune system, Toll-like receptors (TLRs) are a family of well-recognized ligand-binding receptors found in various organisms and initiate host immune responses. Activation of TLRs signaling pathways lead to the induction of numerous genes that function in host defense. Baicalin is a natural compound from the dry raw root of Scutellaria baicalensis (S. baicalensis) and it has been found to exhibit several pharmaceutical actions, such as anti-inflammation, anti-tumor and antivirus. These biological activities are mainly related to the regulatory effect of baicalin on the host immune response. In this review, we provide an overview of the regulation of baicalin on TLRs signaling pathways in various pathological conditions, and highlight potential targets for the development of the regulatory effect of natural compound from traditional Chinese medicine on innate immune system.

Signalling, sorting and scaffolding adaptors for Toll-like receptors

Toll-like receptors (TLRs) are danger-sensing receptors that typically propagate self-limiting inflammatory responses, but can unleash uncontrolled inflammation in non-homeostatic or disease settings. Activation of TLRs by pathogen- and/or host-derived stimuli triggers a range of signalling and transcriptional pathways to programme inflammatory and anti-microbial responses, including the production of a suite of inflammatory cytokines and other mediators. Multiple sorting and signalling adaptors are recruited to receptor complexes on the plasma membrane or endosomes where they act as scaffolds for downstream signalling kinases and effectors at these sites. So far, seven proximal TLR adaptors have been identified: MyD88, MAL, TRIF (also known as TICAM1), TRAM (TICAM2), SARM (SARM1), BCAP (PIK3AP1) and SCIMP. Most adaptors tether directly to TLRs through homotypic Toll/interleukin-1 receptor domain (TIR)-TIR interactions, whereas SCIMP binds to TLRs through an atypical TIR-non-TIR interaction. In this Review, we highlight the key roles for these adaptors in TLR signalling, scaffolding and receptor sorting and discuss how the adaptors thereby direct the differential outcomes of TLR-mediated responses. We further summarise TLR adaptor regulation and function, and make note of human diseases that might be associated with mutations in these adaptors.

A unique protein kinase C-dependent pathway for tissue factor downregulation in pericytes

Essentials Many mediators increase tissue factor (TF) expression in a wide variety of cell types. The only known example of TF downregulation is by pericytes during wound healing angiogenesis. Downregulation of TF mRNA and protein in cultured pericytes is Protein Kinase C (PKC) dependent. Pericyte TF regulation is unique, since PKC mediates increased TF in all other cell types tested. SUMMARY: Background Embryonic and tumor-associated angiogenesis are linked to elevated expression of the procoagulant transmembrane receptor tissue factor (TF). In contrast, we have reported that high baseline TF expression by perivascular cells (pericytes) is dramatically reduced during angiogenesis at sites of wound healing. This is the only setting in which active TF downregulation has been reported, thus revealing a novel mechanism of TF regulation. Objectives To define the mechanisms underlying the unique pattern of TF expression in pericytes. Methods TF expression in primary cultures of human pericytes is not altered by angiogenic cytokines or growth factors, but is actively downregulated by phorbol 12-myristate 13-acetate (PMA). We characterized TF transcription, protein stability and trafficking in response to PMA. Results Exposure to PMA reduced TF mRNA synthesis and shortened the half-life of TF protein from 11 h to 4.5 h. Addition of PMA rapidly triggered endocytosis of cell surface TF, followed by degradation in lysosomes. Cell surface TF coagulant activity was maintained until internal stores were depleted. Reduction of TF transcription, TF endocytosis and enhanced degradation of TF protein were all blocked by broad-spectrum inhibitors of protein kinase C (PKC). This was a surprising finding, because PKC activation increases TF expression in other cell types that have been tested. Conclusions The unique PKC-dependent pathway of TF downregulation in pericytes suggests that TF downregulation may play a functional role in angiogenesis. Distinct pathways regulating pathological and physiological TF expression could be utilized to modulate TF expression for therapeutic purposes.

Early immune anergy towards recall antigens and mitogens in patients at onset of septic shock

The pathology of sepsis is typically characterized by an infection and excessive initial inflammation including a cytokine storm, followed by a state of immune suppression or paralysis. This classical view of a two peak kinetic immune response is currently controversially discussed. This study was a sub-study of the randomized clinical Trial SISPCT registered with www.clinicaltrials.gov (NCT00832039, Registration date: 29/01/2009). Blood samples from 76 patients with severe sepsis and septic shock were incubated for 48 h at 37 °C in vitro with bacterial or fungal recall-antigens or specific mitogen antigens within 24 hours of sepsis onset. Recall-antigen stimulation led to a severe dampening of normal cytokine release. This immunologic anergy was similarly observed after mitogen stimulation. Moreover, patients under hydrocortisone therapy or with lowered arterial oxygen tension had further reductions in cytokine levels upon B- and T-cell mitogen stimulation. This investigation reveals an early onset of immunoparalysis during sepsis. This immune incompetence in mounting an adequate response to further infections includes previously sensitized pathogens, as seen with recall-antigens. Also, the immune-suppressive role of hydrocortisone and low PaO₂ is highlighted. Aside from early broad-spectrum antimicrobial therapy, our findings reinforce the need for maximal immunological support and protection against further infections at the onset of sepsis.

Berkeleyacetal C, a meroterpenoid isolated from the fungus Penicillium purpurogenum MHZ 111, exerts anti-inflammatory effects via inhibiting NF-κB, ERK1/2 and IRF3 signaling pathways

Berkeleyacetal C (BAC), a meroterpenoid compound, was isolated from the fungus Penicillium purpurogenum MHZ 111 and showed favorable activity of inhibiting nitrogen oxide (NO) production of macrophages stimulated by lipopolysaccharide (LPS) in our preliminary screening. In order to develop novel therapeutic drug for acute and chronic inflammatory diseases, the anti-inflammatory activity and underlying mechanisms of BAC were investigated in macrophages and neutrophils. The results showed that BAC significantly inhibited the expression of inducible nitric oxide synthase (iNOS) and the following NO production by macrophages. The expression and secretion of key pro-inflammatory factors and chemokines, including tumor necrosis factor-α (TNF-α)，interleukin-6 (IL-6), interleukin-1β (IL-1β), macrophage inflammatory protein-1α (MIP-1α), and monocyte chemotactic protein-1 (MCP-1) were also intensively suppressed by BAC. Furthermore, BAC also markedly inhibited activation of neutrophils and reactive oxygen species production. In mechanism study, BAC selectively suppressed phosphorylation of nuclear factor-κB (NF-κB), extracellular signal-regulated protein kinases 1 and 2 (ERK1/2), and interferon regulatory transcription factor 3 (IRF3) during the activation of NF-κB, mitogen-activated protein kinase (MAPK), signal transducer and activator of transcription 1 and 3 (STAT1/3), and IRF3 signaling pathways induced by LPS. In summary, BAC exerts strong anti-inflammatory effects by inhibiting NF-κB, ERK1/2 and IRF3 signaling pathways and thereby shows great potential to be developed into therapeutic agent for inflammatory disorders.

Susceptibility to Urinary Tract Infection: Benefits and Hazards of the Antibacterial Host Response

A paradigm shift is needed to improve and personalize the diagnosis of infectious disease and to select appropriate therapies. For many years, only the most severe and complicated bacterial infections received more detailed diagnostic and therapeutic attention as the efficiency of antibiotic therapy has guaranteed efficient treatment of patients suffering from the most common infections. Indeed, treatability almost became a rationale not to analyze bacterial and host parameters in these larger patient groups. Due to the rapid spread of antibiotic resistance, common infections like respiratory tract- or urinary-tract infections (UTIs) now pose new and significant therapeutic challenges. It is fortunate and timely that infectious disease research can offer such a wealth of new molecular information that is ready to use for the identification of susceptible patients and design of new suitable therapies. Paradoxically, the threat of antibiotic resistance may become a window of opportunity, by encouraging the implementation of new diagnostic and therapeutic approaches. The frequency of antibiotic resistance is rising rapidly in uropathogenic organisms and the molecular and genetic understanding of UTI susceptibility is quite advanced. More bold translation of the new molecular diagnostic and therapeutic tools would not just be possible but of great potential benefit in this patient group. This chapter reviews the molecular basis for susceptibility to UTI, including recent advances in genetics, and discusses the consequences for diagnosis and therapy. By dissecting the increasingly well-defined molecular interactions between bacteria and host and the molecular features of excessive bacterial virulence or host-response malfunction, it is becoming possible to isolate the defensive from the damaging aspects of the host response. Distinguishing "good" from "bad" inflammation has been a long-term quest of biomedical science and in UTI, patients need the "good" aspects of the inflammatory response to resist infection while avoiding the "bad" aspects, causing chronicity and tissue damage.

PKC-epsilon and TLR4 synergistically regulate resistin-mediated inflammation in human macrophages

BACKGROUND AND AIMS

Resistin has been associated with atherosclerotic inflammation and cardiovascular complications. We and others have previously shown that PKC-epsilon (PKCε) is involved in resistin-induced smooth muscle cell (VSMC) dysfunction at a high pathological concentration. This study aimed to evaluate the role and potential pathways of resistin at a physiological concentration, in atherosclerosis-related inflammation.

METHODS

Plasma from patients with atherosclerosis was analyzed for resistin concentration. Patients were divided into tertiles based on resistin levels and cytokines were compared between tertiles. Macrophages were then treated with resistin in the presence or absence of PKCε inhibitor and/or TLR4 blocking-antibody, and their inflammatory state was evaluated with ELISA, RT-PCR, immunocytochemistry, and Western blot.

RESULTS

We observed significant associations between plasma resistin levels and TNF-α, IL-6, IL-12, MIP-1α, MIP-1β, and CD40L. Our in vitro analyses revealed that resistin activated PKCε via TLR4. This was followed by NF-kB activation and induction of a pro-inflammatory phenotype in macrophages, significantly upregulating CD40, downregulating CD206 and stimulating gene expression and secretion of the inflammatory cytokines, for which we found association in our plasma analysis. Resistin also induced persistent TRAM and CD40L upregulation up to 36 h after resistin treatment. PKCε and TLR4 inhibitors suppressed gene expression to levels similar to control, especially when used in combination.

CONCLUSIONS

Resistin, at a physiological concentration, exacerbates the inflammatory response of macrophages. PKCε is a key upstream mediator in resistin-induced inflammation that may interact synergistically with TLR4 to promote NF-kB activation, while TRAM is an important signal. PKCε and TRAM may represent novel molecular targets for resistin-associated chronic atherosclerotic inflammation.

The helminth product, ES-62 modulates dendritic cell responses by inducing the selective autophagolysosomal degradation of TLR-transducers, as exemplified by PKCδ

We have previously shown that ES-62, a phosphorylcholine (PC)-containing glycoprotein secreted by the parasitic filarial nematode Acanthocheilonema viteae targets dendritic cell (DC) responses, specifically by suppressing TLR4 signalling to inhibit Th1/Th17-driven inflammation. We have now investigated the molecular mechanisms underpinning such immunomodulation and show here that ES-62-mediated downregulation of protein kinase C-δ (PKC-δ), a TLR4-associated signalling mediator required for full activation of LPS-driven pro-inflammatory responses, is associated with induction of a low level of autophagic flux, as evidenced by upregulation and trafficking of p62 and LC3 and their consequent autophagolysosomal degradation. By contrast, the classical TLR4 ligand LPS, strongly upregulates p62 and LC3 expression but under such canonical TLR4 signalling this upregulation appears to reflect a block in autophagic flux, with these elements predominantly degraded in a proteasomal manner. These data are consistent with autophagic flux acting to homeostatically suppress proinflammatory DC responses and indeed, blocking of PKC-δ degradation by the autophagolysosomal inhibitors, E64d plus pepstatin A, results in abrogation of the ES-62-mediated suppression of LPS-driven release of IL-6, IL-12p70 and TNF-α by DCs. Thus, by harnessing this homeostatic regulatory mechanism, ES-62 can protect against aberrant inflammation, either to promote parasite survival or serendipitously, exhibit therapeutic potential in inflammatory disease.

HIV-1 Tat Protein Activates both the MyD88 and TRIF Pathways To Induce Tumor Necrosis Factor Alpha and Interleukin-10 in Human Monocytes

In this study, we show that the HIV-1 Tat protein interacts with rapid kinetics to engage the Toll-like receptor 4 (TLR4) pathway, leading to the production of proinflammatory and anti-inflammatory cytokines. The pretreatment of human monocytes with Tat protein for 10 to 30 min suffices to irreversibly engage the activation of the TLR4 pathway, leading to the production of tumor necrosis factor alpha (TNF-α) and interleukin-10 (IL-10), two cytokines strongly implicated in the chronic activation and dysregulation of the immune system during HIV-1 infection. Therefore, this study analyzed whether the HIV-1 Tat protein is able to activate these two pathways separately or simultaneously. Using three complementary approaches, including mice deficient in the MyD88, TIRAP/MAL, or TRIF adaptor, biochemical analysis, and the use of specific small interfering RNAs (siRNAs), we demonstrated (i) that Tat was able to activate both the MyD88 and TRIF pathways, (ii) the capacity of Tat to induce TIRAP/MAL degradation, (iii) the crucial role of the MyD88 pathway in the production of Tat-induced TNF-α and IL-10, (iv) a reduction but not abrogation of IL-10 and TNF-α by Tat-stimulated macrophages from mice deficient in TIRAP/MAL, and (v) the crucial role of the TRIF pathway in Tat-induced IL-10 production. Further, we showed that downstream of the MyD88 and TRIF pathways, the Tat protein activated the protein kinase C (PKC) βII isoform, the mitogen-activated protein (MAP) kinases p38 and extracellular signal-regulated kinase 1/2 (ERK1/2), and NF-κB in a TLR4-dependent manner. Collectively, our data show that by recruiting the TLR4 pathway with rapid kinetics, the HIV-1 Tat protein leads to the engagement of both the MyD88 and TRIF pathways and to the activation of PKC, MAP kinase, and NF-κB signaling to induce the production of TNF-α and IL-10.

IMPORTANCE

In this study, we demonstrate that by recruiting the TLR4 pathway with rapid kinetics, the HIV-1 Tat protein leads to the engagement of both the MyD88 and TRIF pathways and to the activation of PKC-βII, MAP kinase, and NF-κB signaling to induce the production of TNF-α and IL-10, two cytokines strongly implicated in the chronic activation and dysregulation of the immune system during HIV-1 infection. Thus, it may be interesting to target Tat as a pathogenic factor early after HIV-1 infection. This could be achieved either by vaccination approaches including Tat as an immunogen in potential candidate vaccines or by developing molecules capable of neutralizing the effect of the Tat protein.

TRIF-dependent TLR signaling, its functions in host defense and inflammation, and its potential as a therapeutic target

Toll/IL-1R domain-containing adaptor-inducing IFN-β (TRIF)-dependent signaling is required for TLR-mediated production of type-I IFN and several other proinflammatory mediators. Various pathogens target the signaling molecules and transcriptional regulators acting in the TRIF pathway, thus demonstrating the importance of this pathway in host defense. Indeed, the TRIF pathway contributes to control of both viral and bacterial pathogens through promotion of inflammatory mediators and activation of antimicrobial responses. TRIF signaling also has both protective and pathologic roles in several chronic inflammatory disease conditions, as well as an essential function in wound-repair processes. Here, we review our current understanding of the regulatory mechanisms that control TRIF-dependent TLR signaling, the role of the TRIF pathway in different infectious and noninfectious pathologic states, and the potential for manipulating TRIF-dependent TLR signaling for therapeutic benefit.

Src kinases central to T-cell receptor signaling regulate TLR-activated innate immune responses from human T cells

TLRs have a fundamental role in immunity. We have recently reported that stimulation of TLR2 and TLR5 in freshly isolated and activated human T cells with microbial ligands without concomitant activation through the TCR brings about secretion of neutrophil chemoattractant, CXCL8, and effector cytokine, IFN-γ, respectively. However, the mechanism of TLR signaling in T cells has not been worked out. Here, we show that the Src family kinases, p56(lck)(Lck) and p59(fyn)(Fyn), which are essential for activation of T cells through the TCR, are also critical for signal transduction through TLRs in human T cells. The secretion of CXCL8 following stimulation of the model human T cell line, Jurkat, with the TLR5 ligand, flagellin, was reduced in presence of the Src-kinase inhibitor, PP2 and specific inhibitors of Lck and Fyn. These inhibitors suppressed generation of activated JNK and p38, which were both required for TLR-induced CXCL8 production. The Lck-deficient derivative of Jurkat, JCam1.6, responded poorly to TLR2, TLR5 and TLR7 agonists, and did not generate active signaling intermediates. Lck and Fyn inhibitors also reduced TLR5-induced IFN-γ secretion from the activated T cell phenotype-representing T cell line, HuT78, without modulating JNK and p38 activation. These results reveal that TCR and TLRs share key proximal signaling regulators in T cells.

Protein kinase C in the immune system: from signalling to chromatin regulation

Protein kinase C (PKC) form a key family of enzymes involved in signalling pathways that specifically phosphorylates substrates at serine/threonine residues. Phosphorylation by PKC is important in regulating a variety of cellular events such as cell proliferation and the regulation of gene expression. In the immune system, PKCs are involved in regulating signal transduction pathways important for both innate and adaptive immunity, ultimately resulting in the expression of key immune genes. PKCs act as mediators during immune cell signalling through the immunological synapse. PKCs are traditionally known to be cytoplasmic signal transducers and are well embedded in the signalling pathways of cells to mediate the cells' response to a stimulus from the plasma membrane to the nucleus. PKCs are also found to transduce signals within the nucleus, a process that is distinct from the cytoplasmic signalling pathway. There is now growing evidence suggesting that PKC can directly regulate gene expression programmes through a non-traditional role as nuclear kinases. In this review, we will focus on the role of PKCs as key cytoplasmic signal transducers in immune cell signalling, as well as its role in nuclear signal transduction. We will also highlight recent evidence for its newly discovered regulatory role in the nucleus as a chromatin-associated kinase.

The transcription factor TFEB acts as a molecular switch that regulates exogenous antigen-presentation pathways

Dendritic cells (DCs) can initiate immune responses by presenting exogenous antigens to T cells via both major histocompatibility complex (MHC) class I pathways and MHC class II pathways. Lysosomal activity has an important role in modulating the balance between these two pathways. The transcription factor TFEB regulates lysosomal function by inducing lysosomal activation. Here we report that TFEB expression inhibited the presentation of exogenous antigen by MHC class I while enhancing presentation via MHC class II. TFEB promoted phagosomal acidification and protein degradation. Furthermore, we found that the activation of TFEB was regulated during DC maturation and that phagosomal acidification was impaired in DCs in which the gene encoding TFEB was silenced. Our data indicate that TFEB is a key participant in the differential regulation of the presentation of exogenous antigens by DCs.

Phosphatase PTPN4 preferentially inhibits TRIF-dependent TLR4 pathway by dephosphorylating TRAM

TLR4 recruits TRIF-related adaptor molecule (TRAM, also known as TICAM2) as a sorting adaptor to facilitate the interaction between TLR4 and TRIF and then initiate TRIF-dependent IRF3 activation. However, the mechanisms by which TRAM links downstream molecules are not fully elucidated. In this study, we show that TRAM undergoes tyrosine phosphorylation upon TLR4 activation and that is required for TLR4-induced IRF3 activation. Protein tyrosine phosphatase nonreceptor type 4 (PTPN4), a protein tyrosine phosphatase, inhibits tyrosine phosphorylation and subsequent cytoplasm translocation of TRAM, resulting in the disturbance of TRAM-TRIF interaction. Consequently, PTPN4 specifically inhibits TRIF-dependent IRF3 activation and IFN-β production in TLR4 pathway. Therefore, our results provide new insight into the TLR4 pathway and identify PTPN4 as a specific inhibitor of TRIF-dependent TLR4 pathway. Targeting PTPN4 would be beneficial for the development of new strategy to control TLR4-associated diseases without unwanted side effects.

Role of lipid microdomains in TLR-mediated signalling

Over the last twenty years, evidence has been provided that the plasma membrane is partitioned with microdomains, laterally mobile in the bilayer, providing the necessary microenvironment to specific membrane proteins for signalling pathways to be initiated. We discuss here the importance of such microdomains for Toll-like receptors (TLR) localization and function. First, lipid microdomains favour recruitment and clustering of the TLR machinery partners, i.e. receptors and co-receptors previously identified to be required for ligand recognition and signal transmission. Further, the presence of the so-called Cholesterol Recognition Amino-Acid Consensus (CRAC) sequences in the intracellular juxtamembrane domain of several Toll-like receptors suggests a direct role of cholesterol in the activation process. This article is part of a Special Issue entitled: Lipid-protein interactions.

Assembly and localization of Toll-like receptor signalling complexes

Signal transduction by the Toll-like receptors (TLRs) is central to host defence against many pathogenic microorganisms and also underlies a large burden of human disease. Thus, the mechanisms and regulation of signalling by TLRs are of considerable interest. In this Review, we discuss the molecular basis for the recognition of pathogen-associated molecular patterns, the nature of the protein complexes that mediate signalling, and the way in which signals are regulated and integrated at the level of allosteric assembly, post-translational modification and subcellular trafficking of the components of the signalling complexes. These fundamental molecular mechanisms determine whether the signalling output leads to a protective immune response or to serious pathologies such as sepsis. A detailed understanding of these processes at the molecular level provides a rational framework for the development of new drugs that can specifically target pathological rather than protective signalling in inflammatory and autoimmune disease.

Bruton's tyrosine kinase and protein kinase C µ are required for TLR7/9-induced IKKα and IRF-1 activation and interferon-β production in conventional dendritic cells

Stimulation of TLR7/9 by their respective ligands leads to the activation of IκB kinase α (IKKα) and Interferon Regulatory Factor 1 (IRF-1) and results in interferon (IFN)-β production in conventional dendritic cells (cDC). However, which other signaling molecules are involved in IKKα and IRF-1 activation during TLR7/9 signaling pathway are not known. We and others have shown that Bruton's Tyrosine Kinase (BTK) played a part in TLR9-mediated cytokine production in B cells and macrophages. However, it is unclear if BTK participates in TLR7/9-induced IFN-β production in cDC. In this study, we show that BTK is required for IFN-β synthesis in cDC upon TLR7/9 stimulation and that stimulated BTK-deficient cDC are defective in the induction of IKKα/β phosphorylation and IRF-1 activation. In addition, we demonstrate that Protein Kinase C µ (PKCµ) is also required for TLR7/9-induced IRF-1 activation and IFN-β upregulation in cDC and acts downstream of BTK. Taken together, we have uncovered two new molecules, BTK and PKCµ, that are involved in TLR7/9-triggered IFN-β production in cDC.

The TLR signaling adaptor TRAM interacts with TRAF6 to mediate activation of the inflammatory response by TLR4

TLRs act as sentinels in professional immune cells to detect and initiate the innate immune response to pathogen challenge. TLR4 is a widely expressed TLR, responsible for initiating potent immune responses to LPS. TRAM acts to bridge TLR4 with TRIF, orchestrating the inflammatory response to pathogen challenge. We have identified a putative TRAF6-binding motif in TRAM that could mediate a novel signaling function for TRAM in TLR4 signaling. TRAM and TRAF6 association was confirmed by immunoprecipitation of endogenous, ectopically expressed and recombinant proteins, which was ablated upon mutation of a key Glu residue in TRAM (TRAM E183A). TRAF6 and TRAM were observed colocalizing using confocal microscopy following ectopic expression in cells and the ability of TRAM and TRAM E183A to activate luciferase-linked reporter assays was determined in HEK293 and TRAF6-deficient cells. Importantly, TRAM-deficient macrophages reconstituted with TRAM E183A display significantly reduced inflammatory TNF-α, IL-6, and RANTES protein production compared with WT TRAM. These results demonstrate a novel role for TRAM in TLR4-mediated signaling in regulating inflammatory responses via its interaction with TRAF6, distinct from its role as a bridging adaptor between TLR4 and TRIF.

Protein kinase C and acute respiratory distress syndrome

The acute respiratory distress syndrome (ARDS) is a major public health problem and a leading source of morbidity in intensive care units. Lung tissue in patients with ARDS is characterized by inflammation, with exuberant neutrophil infiltration, activation, and degranulation that is thought to initiate tissue injury through the release of proteases and oxygen radicals. Treatment of ARDS is supportive primarily because the underlying pathophysiology is poorly understood. This gap in knowledge must be addressed to identify urgently needed therapies. Recent research efforts in anti-inflammatory drug development have focused on identifying common control points in multiple signaling pathways. The protein kinase C (PKC) serine-threonine kinases are master regulators of proinflammatory signaling hubs, making them attractive therapeutic targets. Pharmacological inhibition of broad-spectrum PKC activity and, more importantly, of specific PKC isoforms (as well as deletion of PKCs in mice) exerts protective effects in various experimental models of lung injury. Furthermore, PKC isoforms have been implicated in inflammatory processes that may be involved in the pathophysiologic changes that result in ARDS, including activation of innate immune and endothelial cells, neutrophil trafficking to the lung, regulation of alveolar epithelial barrier functions, and control of neutrophil proinflammatory and prosurvival signaling. This review focuses on the mechanistic involvement of PKC isoforms in the pathogenesis of ARDS and highlights the potential of developing new therapeutic paradigms based on the selective inhibition (or activation) of specific PKC isoforms.

Protein kinase C and its inhibitors in the regulation of inflammation: inducible nitric oxide synthase as an example

Protein kinase C (PKC) is a family of ten isoenzymes that play a crucial role in cellular signal transduction. Studies with PKC knockout animals have revealed that many of the isoenzymes are involved in cell growth, proliferation and differentiation. Several PKC isoenzymes have also been shown to be important mediators in inflammation and immunity, particularly in lymphocyte responses. However, less is known about the role of PKC in the regulation of the expression of inflammatory genes. In inflammatory processes, nitric oxide is primarily produced by inducible nitric oxide synthase (iNOS) in inflammatory cells, such as macrophages. In innate immunity, nitric oxide functions as an effector molecule towards the infectious organisms. Increased levels of nitric oxide are also produced by inflammatory and tissue cells in inflammatory diseases, such as asthma and arthritis. In this MiniReview, the role of PKC isoenzymes in the pathogenesis and as a potential drug target in inflammation will be discussed presenting iNOS as an example of an inflammatory gene regulated by the pleiotropic PKC signalling pathway.

IL-32-PAR2 axis is an innate immunity sensor providing alternative signaling for LPS-TRIF axis

Interleukin (IL)-32 is known to exert adujvant effects on innate immune response, however, receptors and downstream signaling pathways remain to be clarified. Here we found that IL-32γ upregulated serine protease activity of proteinase-3 (PR3), in turn triggering protease-activated receptor 2 (PAR2) signaling. Interestingly, silencing of PR3 or PAR2 using siRNA markedly diminished IL-32γ-induced TNFα and IFN-β mRNA expression. IL-32γ-PAR2 axis utilized TRIF and Ras-Raf-1 pathways. On stimulation with lipopolysaccharide (LPS), differential activation of protein kinase C isoforms modulated the balance between LPS-TLR4-TRIF and IL-32-PAR2-TRIF axes, because LPS was a strong inducer of IL-32γ. IL-32-PAR2-TRIF axis might serve not only as an extracellular sensor of bacterial and autologous proteases, but also as a modulator of innate and adaptive immunity during infection.

Expression pattern of protein kinase C ϵ during mouse embryogenesis

BACKGROUND

Protein kinase C epsilon (PKCϵ) belongs to the novel PKC subfamily, which consists of diacylglycerol dependent- and calcium independent-PKCs. Previous studies have shown that PKCϵ is important in different contexts, such as wound healing or cancer. In this study, we contribute to expand the knowledge on PKCϵ by reporting its expression pattern during murine midgestation using the LacZ reporter gene and immunostaining procedures.

RESULTS

Sites showing highest PKCϵ expression were heart at ealier stages, and ganglia in older embryos. Other stained domains included somites, bone, stomach, kidney, and blood vessels.

CONCLUSIONS

The seemingly strong expression of PKCϵ in heart and ganglia shown in this study suggests a important role of this isoform in the vascular and nervous systems during mouse development. However, functional redundancy with other PKCs during midgestation within these domains and others reported here possibly exists since PKCϵ deficient mice do not display obvious embryonic developmental defects.

Wheat amylase trypsin inhibitors drive intestinal inflammation via activation of toll-like receptor 4

Pest resistance molecules, α-amylase/trypsin inhibitors from wheat, activate innate immune cells through engagement of TLR4 to elicit inflammatory responses in the intestine.

Ingestion of wheat, barley, or rye triggers small intestinal inflammation in patients with celiac disease. Specifically, the storage proteins of these cereals (gluten) elicit an adaptive Th1-mediated immune response in individuals carrying HLA-DQ2 or HLA-DQ8 as major genetic predisposition. This well-defined role of adaptive immunity contrasts with an ill-defined component of innate immunity in celiac disease. We identify the α-amylase/trypsin inhibitors (ATIs) CM3 and 0.19, pest resistance molecules in wheat, as strong activators of innate immune responses in monocytes, macrophages, and dendritic cells. ATIs engage the TLR4–MD2–CD14 complex and lead to up-regulation of maturation markers and elicit release of proinflammatory cytokines in cells from celiac and nonceliac patients and in celiac patients’ biopsies. Mice deficient in TLR4 or TLR4 signaling are protected from intestinal and systemic immune responses upon oral challenge with ATIs. These findings define cereal ATIs as novel contributors to celiac disease. Moreover, ATIs may fuel inflammation and immune reactions in other intestinal and nonintestinal immune disorders.

IFN-β expression is directly activated in human neutrophils transfected with plasmid DNA and is further increased via TLR-4-mediated signaling

Upon LPS binding, TLR4 activates a MyD88-dependent pathway leading to the transcriptional activation of proinflammatory genes, as well as a MyD88-independent/TRIF-dependent pathway, responsible for the transcriptional induction of IFN-β. Previous findings delineated that human neutrophils are unable to induce the transcription of IFN-β in response to TLR4 stimulation. Because neutrophils do not express protein kinase C ε, a molecule recently reported as essential for initiating the MyD88-independent/TRIF-dependent pathway, we optimized an electroporation method to transfect PKCε into neutrophils with very high efficiency. By doing so, a significant IFN-β mRNA expression was induced, in the absence of LPS stimulation, not only in PKCε-overexpressing neutrophils but also in cells transfected with a series of empty DNA plasmids; however, LPS further upregulated the IFN-β transcript levels in plasmid-transfected neutrophils, regardless of PKCε overexpression. Phosphoimmunoblotting studies, as well as chromatin immunoprecipitation assays targeting the IFN-β promoter, revealed that IFN-β mRNA induction occurred through the cooperative action of IRF3, activated by transfected DNA, and NF-κB, activated by LPS. Additional immunoblotting and coimmunoprecipitation studies revealed that neutrophils constitutively express various cytosolic DNA sensors, including IFN-inducible protein 16, leucine-rich repeat (in Flightless I) interacting protein-1, and DDX41, as well as that IFN-inducible protein 16 is the intracellular receptor recognizing transfected DNA. Consistently, infection of neutrophils with intracellular pathogens, such as Bartonella henselae, Listeria monocytogenes, Legionella pneumophila, or adenovirus type 5, promoted a marked induction of IFN-β mRNA expression. Taken together, these data raise questions about the role of PKCε in driving the MyD88-independent/TRIF-dependent response and indicate that human neutrophils are able to recognize and respond to microbial cytosolic DNA.

TRAM is involved in IL-18 signaling and functions as a sorting adaptor for MyD88

MyD88, a Toll/interleukin-1 receptor homology (TIR) domain-containing adaptor protein, mediates signals from the Toll-like receptors (TLR) or IL-1/IL-18 receptors to downstream kinases. In MyD88-dependent TLR4 signaling, the function of MyD88 is enhanced by another TIR domain-containing adaptor, Mal/TIRAP, which brings MyD88 to the plasma membrane and promotes its interaction with the cytosolic region of TLR4. Hence, Mal is recognized as the “sorting adaptor” for MyD88. In this study, a direct interaction between MyD88-TIR and another membrane-sorting adaptor, TRAM/TICAM-2, was demonstrated in vitro. Cell-based assays including RNA interference experiments and TRAM deficient mice revealed that the interplay between MyD88 and TRAM in cells is important in mediating IL-18 signal transduction. Live cell imaging further demonstrated the co-localized accumulation of MyD88 and TRAM in the membrane regions in HEK293 cells. These findings suggest that TRAM serves as the sorting adaptor for MyD88 in IL-18 signaling, which then facilitates the signal transduction. The binding sites for TRAM are located in the TIR domain of MyD88 and actually overlap with the binding sites for Mal. MyD88, the multifunctional signaling adaptor that works together with most of the TLR members and with the IL-1/IL-18 receptors, can interact with two distinct sorting adaptors, TRAM and Mal, in a conserved manner in a distinct context.

Inhibition of clathrin/dynamin-dependent internalization interferes with LPS-mediated TRAM-TRIF-dependent signaling pathway

Recognition of lipopolysaccharide (LPS) by Toll-like receptor 4 (TLR4) activates two district proinflammatory signaling pathway and initiates LPS internalization. To investigate roles of LPS internalization, a traditionally regarded metabolic pathway for LPS, in regulation of these two pathways, three internalization inhibitors, monodansylcadaverine (MDC, a clathrin inhibitor), dynasore (DS, a dynamin inhibitor) and chloroquine (CQ, an endosome acidifying maturation inhibitor) were applied to induce internalization dysfunction in macrophages. Results showed MDC and DS affected LPS internalization but did not interfere with their colocalization. Additionally, they decreased cytokines and chemokines release and inhibited signaling molecules activation mediated by TRAM-TRIF-dependent pathway as determined by protein array. In contrast, CQ did not inhibit LPS internalization but affected the colocalization. It also suppressed macrophage activation mediated by both MyD88-dependent and TRAM-TRIF-dependent pathways. The above data indicated that LPS internalization was clathrin/dynamin dependent and it was essential for activation of TRAM-TRIF-dependent signaling pathway.

Myeloid differentiation factor-2 interacts with Lyn kinase and is tyrosine phosphorylated following lipopolysaccharide-induced activation of the TLR4 signaling pathway

Stimulation with LPS induces tyrosine phosphorylation of numerous proteins involved in the TLR signaling pathway. In this study, we demonstrated that myeloid differentiation factor-2 (MD-2) is also tyrosine phosphorylated following LPS stimulation. LPS-induced tyrosine phosphorylation of MD-2 is specific; it is blocked by the tyrosine kinase inhibitor, herbimycin A, as well as by an inhibitor of endocytosis, cytochalasin D, suggesting that MD-2 phosphorylation occurs during trafficking of MD-2 and not on the cell surface. Furthermore, we identified two possible phospho-accepting tyrosine residues at positions 22 and 131. Mutant proteins in which these tyrosines were changed to phenylalanine had reduced phosphorylation and significantly diminished ability to activate NF-κB in response to LPS. In addition, MD-2 coprecipitated and colocalized with Lyn kinase, most likely in the endoplasmic reticulum. A Lyn-binding peptide inhibitor abolished MD-2 tyrosine phosphorylation, suggesting that Lyn is a likely candidate to be the kinase required for MD-2 tyrosine phosphorylation. Our study demonstrated that tyrosine phosphorylation of MD-2 is important for signaling following exposure to LPS and underscores the importance of this event in mediating an efficient and prompt immune response.

Signaling through Toll-like receptor 4 and mast cell-dependent innate immunity responses

Signal transduction through Toll-like receptors (TLRs) has been one of the main topics in immunology research in recent years. Because of their signaling particularities based on the homotypic recognition of protein domains in multiple adaptors and selective activation of protein kinases, TLRs have become a paradigm to study ligand recognition coupled to dynamic and highly specific transcriptional and secretory responses in immune cells. Particularly, deleterious effects of Gram-negative bacteria-associated immune reactions has promoted intense research in the field, leading to the description of a number of canonical molecules connecting lipopolysaccharide-induced TLR4 activation with NFκB-dependent transcription. However, the diversity of immune cell phenotypes and the activity of distinct immune receptors in the same cell, strongly suggest that a number of elements in TLR4 signaling cascade, such as novel coreceptors, tyrosine kinases, and molecules regulating the secretion of preformed mediators remain to be described. Recent investigations have placed the mast cells, widely known by their role on allergic responses, as important effectors of innate immunity reactions against Gram-negative bacteria. Their remarkable capacity of cytokine storage, synthesis and release, and the large number of inflammatory reactions controlled by their activation, suggest the existence of new modulators of TLR4 signaling in this particular cell type.

Identification and functional characterization of 14-3-3 in TLR2 signaling

The Interleukin-1/Toll-like receptor signaling pathway is a crucial signaling pathway within the innate immune system and the use of mass spectrometric techniques became valuable to investigate signal transduction pathways. To date only a few reports exist that focus on the mass spectrometric identification of novel signaling intermediates within the TLR signal transduction pathway. Here we used this approach systematically to identify new interaction partners of the TLR signaling pathway and subsequently characterized them functionally. We identified 14-3-3 theta as a new member of the TLR signaling complex. With genetic complementation assays, we demonstrate that 14-3-3 negatively regulates TLR2-dependent NF-κB activity and amplifies the TLR4-dependent activation of the transcription factor. While 14-3-3 has no effect on TLR-induced apoptosis in innate immune cells, it controls the release of the inflammatory, IRF3-dependent cytokines like RANTES and IP-10 after stimulation with LPS. Most strikingly, 14-3-3 controls the production of proinflammatory cytokines like IL-6, IL-8, and TNFα in a different manner. Our results identify 14-3-3 theta as a new and important regulatory protein in the TLR signaling suppressing the MyD88-dependent pathway.

Protein kinase C and toll-like receptor signaling

Protein kinase C (PKC) is a family of kinases that are implicated in a plethora of diseases, including cancer and cardiovascular disease. PKC isoforms can have different, and sometimes opposing, effects in these disease states. Toll-like receptors (TLRs) are a family of pattern recognition receptors that bind pathogens and stimulate the secretion of cytokines. It has long been known that PKC inhibitors reduce LPS-stimulated cytokine secretion by macrophages, linking PKC activation to TLR signaling. Recent studies have shown that PKC-α, -δ, -ε, and -ζ are directly involved in multiple steps in TLR pathways. They associate with the TLR or proximal components of the receptor complex. These isoforms are also involved in the downstream activation of MAPK, RhoA, TAK1, and NF-κB. Thus, PKC activation is intimately involved in TLR signaling and the innate immune response.

Shedding of the Mer tyrosine kinase receptor is mediated by ADAM17 protein through a pathway involving reactive oxygen species, protein kinase Cδ, and p38 mitogen-activated protein kinase (MAPK)

Mer tyrosine kinase (MerTK) is an integral membrane protein that is preferentially expressed by phagocytic cells, where it promotes efferocytosis and inhibits inflammatory signaling. Proteolytic cleavage of MerTK at an unidentified site leads to shedding of its soluble ectodomain (soluble MER; sMER), which can inhibit thrombosis in mice and efferocytosis in vitro. Herein, we show that MerTK is cleaved at proline 485 in murine macrophages. Site-directed deletion of 6 amino acids spanning proline 485 rendered MerTK resistant to proteolysis and suppression of efferocytosis by cleavage-inducing stimuli. LPS is a known inducer of MerTK cleavage, and the intracellular signaling pathways required for this action are unknown. LPS/TLR4-mediated generation of sMER required disintegrin and metalloproteinase ADAM17 and was independent of Myd88, instead requiring TRIF adaptor signaling. LPS-induced cleavage was suppressed by deficiency of NADPH oxidase 2 (Nox2) and PKCδ. The addition of the antioxidant N-acetyl cysteine inhibited PKCδ, and silencing of PKCδ inhibited MAPK p38, which was also required. In a mouse model of endotoxemia, we discovered that LPS induced plasma sMER, and this was suppressed by Adam17 deficiency. Thus, a TRIF-mediated pattern recognition receptor signaling cascade requires NADPH oxidase to activate PKCδ and then p38, culminating in ADAM17-mediated proteolysis of MerTK. These findings link innate pattern recognition receptor signaling to proteolytic inactivation of MerTK and generation of sMER and uncover targets to test how MerTK cleavage affects efferocytosis efficiency and inflammation resolution in vivo.

Comparative and phylogenetic analyses of three TIR domain-containing adaptors in metazoans: implications for evolution of TLR signaling pathways

Toll-like receptor adaptor molecule 1/2 (TICAM-1/2) and Toll-interleukin 1 receptor (TIR) domain-containing adaptor protein (TIRAP) play key roles in the Toll-like receptor (TLR) signaling pathways, which respond to viral and bacterial infections. These genes have been identified and studied in several vertebrates. However, our understanding of their evolutionary history and their roles in immune responses is far from complete. In this study, comparative and evolutionary analyses were performed for TICAM-1, TICAM-2 and TIRAP within the range of 25 representative species. Our data show that the origin of the TICAM-like and TIRAP-like genes may coincide with the origin of chordates (amphioxus). Several putative TICAMs and TIRAPs were identified for different chordate species. Shark is the only non-mammalian species whose genome contains a TICAM-2 gene. Structural modeling and comparison of TIR domains of these adaptors support their potential functional motifs and residues. Together with analyses of other genes involved in the TLR signaling pathways, we speculate that TICAM-1, TICAM-2 and TIRAP might have co-evolved with the TLR3/22 antivirus signaling, the LPS-specific TLR4 signaling and the Gram-positive bacteria-induced TLR2 signaling pathways, respectively. Our results are valuable contributions to the understanding of TICAM/TIRAP evolutional functions and may provide targets for therapeutic intervention in TLR-mediated vertebrate diseases.

Novel approach for interleukin-23 up-regulation in human dendritic cells and the impact on T helper type 17 generation

Interleukin-23 (IL-23) is important for T helper type 17 (Th17) responses and strategies to regulate IL-23 in human dendritic cells (DC) are limited. This study describes a novel means to control IL-23 secretion by conditioning DC with a phosphatidyl inositol 3-kinase inhibitor Wortmannin (WM). Treatment of monocyte-derived DC with WM increased Toll-like receptor (TLR) -dependent IL-23 secretion 10-fold and IL-12p70 twofold, but IL-27 was unaffected. The effect of WM was restricted to TLR3/4 pathways, did not occur through TLR2, TLR7/8 or Dectin-1, and was characterized by increased p19, p35 and p40 transcription. These responses were not solely dependent on phosphatidyl inositol 3-kinase as the alternative inhibitor LY294002 did not modulate IL-23 production. The normal patterns of activation of mitogen-activated protein kinase pathways were unaffected by WM-conditioning but IL-23 secretion required p38, ERK and JNK pathways. Importantly, this effect was manifest in populations of blood DC. Conditioning freshly isolated myeloid DC with WM before TLR3 or TLR4 triggering resulted in high levels of IL-23 secretion and an absence of IL-12p70. These WM-conditioned myeloid DC were highly effective at priming Th17 responses from naive CD4(+) T cells. Our findings provide a novel means to generate IL-23-rich environments and Th17 responses and suggest as yet unidentified regulatory factors, identification of which will provide new approaches to control IL-23-dependent immunity in infectious disease, autoimmunity and malignancy.

A conventional protein kinase C inhibitor targeting IRF-3-dependent genes differentially regulates IL-12 family members

Protein kinase C (PKC) isoforms play a critical role in the regulation of innate immune responses. We have previously demonstrated that conventional PKC (cPKC) α is involved in interferon regulatory factor 3 (IRF-3) activation and IFN-β synthesis. Herein, we investigated the role of cPKCs in the regulation of IL-12 family members expression mediated by the Toll-like receptor 3 (TLR3) and TLR4. First, inhibition of cPKCs activity in human DCs by a cPKC-specific inhibitor, Gö6976 downregulated the expression of IL-12p70 and IL-27p28 but not IL-12/IL-23p40, IL-23, IL-27EBI3 induced by LPS or poly(I:C). Furthermore, reporter gene assays in RAW 264.7 macrophages showed that cPKCs regulate IL-12p35 and IL-27p28 promoter activities since Gö6976 repressed LPS and poly(I:C)-mediated transcriptional activities of IL-12p35 and IL-27p28. In contrast, no effect was observed with IL-12/IL-23p40 and IL-23p19 reporter constructs. These results prompted us to study the role of IRF-3 on IL-23 expression. Bone marrow-derived DC (BMDCs) from IRF-3(-/-) mice produced comparable levels of IL-23 induced by both LPS and poly(I:C) as compared to wild type BMDCs, indicating that IRF-3 is not involved in IL-23 production. Finally, BMDCs from PKCα(-/-) mice displayed a reduced synthesis of IL-27 induced by poly(I:C). Collectively, these data identify cPKCs as critical components that control IRF-3-dependent IL-12p35 and IL-27p28 gene expression downstream of TLR3 and TLR4.

Macroscopic law of conservation revealed in the population dynamics of Toll-like receptor signaling

Stimulating the receptors of a single cell generates stochastic intracellular signaling. The fluctuating response has been attributed to the low abundance of signaling molecules and the spatio-temporal effects of diffusion and crowding. At population level, however, cells are able to execute well-defined deterministic biological processes such as growth, division, differentiation and immune response. These data reflect biology as a system possessing microscopic and macroscopic dynamics. This commentary discusses the average population response of the Toll-like receptor (TLR) 3 and 4 signaling. Without requiring detailed experimental data, linear response equations together with the fundamental law of information conservation have been used to decipher novel network features such as unknown intermediates, processes and cross-talk mechanisms. For single cell response, however, such simplicity seems far from reality. Thus, as observed in any other complex systems, biology can be considered to possess order and disorder, inheriting a mixture of predictable population level and unpredictable single cell outcomes.

Regulation of lipopolysaccharide-induced translation of tumor necrosis factor-alpha by the toll-like receptor 4 adaptor protein TRAM

Lipopolysaccharide (LPS)-induced production of tumor necrosis factor (TNF)-α requires the recruitment of two pairs of adaptors to the Toll-like receptor 4 cytoplasmic domain. The contribution of one pair - Toll-interleukin-1 receptor domain-containing adaptor inducing interferon-β (TRIF) and TRIF-related adaptor molecule (TRAM) - to TNF-α expression is not well understood. To clarify this issue, we studied TRAM knockout bone marrow-derived macrophages (BMDM). LPS-stimulated TRAM-deficient BMDM had decreased TNF-α protein expression even at times when TNF-α mRNA levels were normal, suggesting impaired translation. Consistent with this idea, knockdown of TRAM in RAW264.7 macrophages decreased translation of a reporter controlled by the TNF-α 3' untranslated region, while transfection of TRAM in HEK293T cells increased translation of this reporter. Also consistent with a role for TRAM in TNF-α translation, LPS-induced activation of MK2, a kinase involved in this process, was impaired in TRAM-deficient BMDM. TRIF did not increase translation of the TNF-α 3' untranslated region reporter when expressed in HEK293T cells. However, BMDM that lacked functional TRIF produced reduced levels of TNF-α protein in response to LPS despite normal amounts of the mRNA. Unlike BMDM, LPS-stimulated TRAM-deficient peritoneal macrophages displayed equivalent reductions in TNF-α protein and mRNA. Our results indicate that TRAM- and TRIF-dependent signals have a previously unappreciated, cell type-specific role in regulating TNF-α translation.

Absence of TRIF signaling in lipopolysaccharide-stimulated murine mast cells

In macrophages, two signaling pathways, dependent on MyD88 or TIR domain-containing adaptor-inducing IFN-β (TRIF) signaling, emanate from the LPS receptor TLR4/MD-2. In this study, we show that in murine bone marrow-derived mast cells (BMMCs), only the MyD88-dependent pathway is activated by LPS. The TRIF signaling branch leading both to NF-κB activation and enhanced proinflammatory cytokine production, as well as to IRF3 activation and subsequent IFN-β production, is absent in LPS-stimulated BMMCs. IRF3 activation is also absent in peritoneal mast cells from LPS-injected mice. We observed strongly diminished TRAM expression in BMMCs, but overexpression of TRAM only moderately enhanced IL-6 and did not boost IFN-β responses to LPS in these cells. A combination of very low levels of TRAM and TLR4/MD-2 with the known absence of membrane-bound CD14 are expected to contribute to the defective TRIF signaling in mast cells. We also show that, unlike in macrophages, in BMMCs the TRIF-dependent and -independent IFN-αβ responses to other recognized IFN inducers (dsRNA, adenovirus, and B-DNA) are absent. These results show how the response to the same microbial ligand using the same receptor can be regulated in different cell types of the innate immune system.

What the Myddosome structure tells us about the initiation of innate immunity

Signaling by the toll-like receptor (TLR) and interleukin-1 receptor superfamily requires the adapter protein myeloid differentiation primary response protein 88 (MyD88). The recent determination of the structure of the so-called Myddosome provides us with new insights into the structural basis for innate immune signaling. Other information on the biochemistry and genetics of MyD88 and other adapters, such as MyDD adapter-like and TRIF-related adapter molecule, allows us to describe in some detail the signaling process activated by TLRs and provides new insights into the role these important proteins play in innate immunity.