Protein kinase Cdelta binds TIRAP/Mal to participate in TLR signaling

Dorzolamide intermediates with potential anti-inflammatory activity

Dorzolamide (DZD), a Carbonic anhydrase (CA) inhibitor clinically used to lower intraocular pressure, exhibits anti-inflammatory effects owing to the drug's ability to inhibit the TIR domain-containing adaptor protein (TIRAP)-mediated signalling in macrophages. Here, we investigated whether DZD intermediates also demonstrate any anti-inflammatory property like DZD but with a reduced inhibition of CA. We found that several intermediates of DZD show increased binding to TIRAP at the common interface of kinases, such as Protein kinase C-delta (PKCδ) and Bruton's tyrosine kinase (BTK). Such binding results in a decreased activity of TIRAP, p38 Mitogen-activating protein kinases (MAPK), and p65, which are essential for major inflammatory signaling pathways. Remarkably, the DZD intermediates were more effective than DZD in decreasing the mRNA expression levels of pro-inflammatory cytokines in Lipopolysaccharide (LPS)-stimulated RAW 264.7 cells. The DZD intermediates also exhibit a reduced binding energy to CA II and CA IV, highlighting their improved specificity as anti-inflammatory compounds with decreased unwanted biological effects. Furthermore, we validated the anti-inflammatory effect of the most efficient DZD intermediate, DRZ V, in a model of mouse sepsis. DRZ V-treated septic mice exhibited improved survival compared to DZD-treated septic mice. Our data indicate that the tested DZD intermediates are more effectual in dampening TIRAP-mediated inflammatory signaling as compared to DZD. Thus, DZD intermediates may be a promising option for developing novel anti-inflammatory therapeutics.

Protein kinase C signaling "in" and "to" the nucleus: Master kinases in transcriptional regulation

PKC is a multifunctional family of Ser-Thr kinases widely implicated in the regulation of fundamental cellular functions, including proliferation, polarity, motility, and differentiation. Notwithstanding their primary cytoplasmic localization and stringent activation by cell surface receptors, PKC isozymes impel prominent nuclear signaling ultimately impacting gene expression. While transcriptional regulation may be wielded by nuclear PKCs, it most often relies on cytoplasmic phosphorylation events that result in nuclear shuttling of PKC downstream effectors, including transcription factors. As expected from the unique coupling of PKC isozymes to signaling effector pathways, glaring disparities in gene activation/repression are observed upon targeting individual PKC family members. Notably, specific PKCs control the expression and activation of transcription factors implicated in cell cycle/mitogenesis, epithelial-to-mesenchymal transition and immune function. Additionally, PKCs isozymes tightly regulate transcription factors involved in stepwise differentiation of pluripotent stem cells toward specific epithelial, mesenchymal, and hematopoietic cell lineages. Aberrant PKC expression and/or activation in pathological conditions, such as in cancer, leads to profound alterations in gene expression, leading to an extensive rewiring of transcriptional networks associated with mitogenesis, invasiveness, stemness, and tumor microenvironment dysregulation. In this review, we outline the current understanding of PKC signaling "in" and "to" the nucleus, with significant focus on established paradigms of PKC-mediated transcriptional control. Dissecting these complexities would allow the identification of relevant molecular targets implicated in a wide spectrum of diseases.

Dorzolamide suppresses PKCδ -TIRAP-p38 MAPK signaling axis to dampen the inflammatory response

Background: Sepsis is a syndrome due to microbial infection causing impaired multiorgan function. Its underlying cause is immune dysfunction and macrophages play an essential role. Methods: TIRAP interaction with PKCδ in macrophage was studied, revealing downstream signaling by Western blot and quantitative reverse transcriptase PCR. Dorzolamide (DZD) disrupting TIRAP-PKCδ interaction was identified by virtual screening and validated in vitro and in septic mice. Results: The study highlights the indispensable role of TIRAP-PKCδ in p38 MAPK-activation, NF-κB- and AP-1-mediated proinflammatory cytokines expression, whereas DZD significantly attenuated the signaling. Conclusion: Targeting TIRAP-PKCδ interaction by DZD is a novel therapeutic approach for treating sepsis.

Identification of novel inhibitors targeting TIRAP interactions with BTK and PKCδ in inflammation through an in silico approach

Advanced computational tools focusing on protein-protein interaction (PPI) based drug development is a powerful platform to accelerate the therapeutic development of small lead molecules and repurposed drugs. Toll/interleukin-1 receptor (TIR) domain-containing adapter protein (TIRAP) and its interactions with other proteins in macrophages signalling are crucial components of severe or persistent inflammation. TIRAP activation through Bruton's tyrosine kinase (BTK) and Protein Kinase C delta (PKCδ) is essential for downstream inflammatory signalling. We created homology-based structural models of BTK and PKCδ in MODELLER 9.24. TIRAP interactions with BTK and PKCδ in its non-phosphorylated and phosphorylated states were determined by multiple docking tools including HADDOCK 2.4, pyDockWEB and ClusPro 2.0. Food and Drug Administration (FDA)-approved drugs were virtually screened through Discovery Studio LibDock and Autodock Vina tools to target the common TIR domain residues of TIRAP, which interact with both BTK and PKC at the identified interfacial sites of the complexes. Four FDA-approved drugs were identified and found to have stable interactions over a range of 100 ns MD simulation timescales. These drugs block the interactions of both kinases with TIRAP in silico. Hence, these drugs have the potential to dampen downstream inflammatory signalling and inflammation-mediated disease.

Calcium Dobesilate Modulates PKCδ-NADPH Oxidase- MAPK-NF-κB Signaling Pathway to Reduce CD14, TLR4, and MMP9 Expression during Monocyte-to-Macrophage Differentiation: Potential Therapeutic Implications for Atherosclerosis

Monocyte-to-macrophage differentiation results in the secretion of various inflammatory mediators and oxidative stress molecules necessary for atherosclerosis pathogenesis. Consequently, this differentiation represents a potential clinical target in atherosclerosis. Calcium dobesilate (CaD), an established vasoactive and angioprotective drug in experimental models of diabetic microvascular complications reduces oxidative stress and inhibits inflammation via diverse molecular targets; however, its effect on monocytes/macrophages is poorly understood. In this study, we investigated the anti-inflammatory mechanism of CaD during phorbol 12-myristate 13-acetate (PMA)-induced monocyte-to-macrophage differentiation in in vitro models of sepsis (LPS) and hyperglycemia, using THP-1 monocytic cell line. CaD significantly suppressed CD14, TLR4, and MMP9 expression and activity, lowering pro-inflammatory mediators, such as IL1β, TNFα, and MCP-1. The effects of CaD translated through to studies on primary human macrophages. CaD inhibited reactive oxygen species (ROS) generation, PKCδ, MAPK (ERK1/2 and p38) phosphorylation, NOX2/p47phox expression, and membrane translocation. We used hydrogen peroxide (H2O2) to mimic oxidative stress, demonstrating that CaD suppressed PKCδ activation via its ROS-scavenging properties. Taken together, we demonstrate for the first time that CaD suppresses CD14, TLR4, MMP9, and signature pro-inflammatory cytokines, in human macrophages, via the downregulation of PKCδ/NADPH oxidase/ROS/MAPK/NF-κB-dependent signaling pathways. Our data present novel mechanisms of how CaD alleviates metabolic and infectious inflammation.

TIRAP in the Mechanism of Inflammation

The Toll-interleukin-1 Receptor (TIR) domain-containing adaptor protein (TIRAP) represents a key intracellular signalling molecule regulating diverse immune responses. Its capacity to function as an adaptor molecule has been widely investigated in relation to Toll-like Receptor (TLR)-mediated innate immune signalling. Since the discovery of TIRAP in 2001, initial studies were mainly focused on its role as an adaptor protein that couples Myeloid differentiation factor 88 (MyD88) with TLRs, to activate MyD88-dependent TLRs signalling. Subsequent studies delineated TIRAP’s role as a transducer of signalling events through its interaction with non-TLR signalling mediators. Indeed, the ability of TIRAP to interact with an array of intracellular signalling mediators suggests its central role in various immune responses. Therefore, continued studies that elucidate the molecular basis of various TIRAP-protein interactions and how they affect the signalling magnitude, should provide key information on the inflammatory disease mechanisms. This review summarizes the TIRAP recruitment to activated receptors and discusses the mechanism of interactions in relation to the signalling that precede acute and chronic inflammatory diseases. Furthermore, we highlighted the significance of TIRAP-TIR domain containing binding sites for several intracellular inflammatory signalling molecules. Collectively, we discuss the importance of the TIR domain in TIRAP as a key interface involved in protein interactions which could hence serve as a therapeutic target to dampen the extent of acute and chronic inflammatory conditions.

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.

Longifolioside A inhibits TLR4-mediated inflammatory responses by blocking PKCδ activation in LPS-stimulated THP-1 macrophages

Longifolioside A is an iridoid glucoside compound isolated from Pseudolysimachion rotundum var. subintegrum, which has been used in traditional herbal medicines to treat respiratory inflammatory diseases. Logifolioside A is a potent antioxidant; however, its underlying pharmacological mechanisms of action in inflammatory diseases are unknown. Here, we investigated the inhibitory effects of longifolioside A in lipopolysaccharide (LPS)-stimulated toll-like receptor 4 (TLR4) signal transduction systems using human THP-1 macrophages and HEK293 cells stably expressing human TLR4 protein (293/HA-hTLR4). Longifolioside A significantly reduced the release of inflammatory cytokines such as interleukin (IL)-6, -8, and tumor necrosis factor (TNF)-α in LPS-stimulated THP-1 macrophages. Furthermore, longifolioside A inhibited the expression of inflammatory mediator genes such as inducible nitric oxide synthase (iNOS) and cyclooxygenase (COX)-2 that produce nitric oxide (NO) and prostaglandin E2 (PGE2), respectively. Longifolioside A suppressed the phosphorylation of PKCδ, IRAK4, IKKα/β, IκBα, and mitogen-activated protein (MAP) kinases (ERK 1/2 and JNK, but not p38), thereby inactivating the nuclear localization of NF-κB and AP-1, and thus decreasing the expression of inflammatory response genes. Notably, longifolioside A disrupted the interaction between human TLR4 and the TIR domain-containing adaptor protein (TIRAP), an early step during TLR4 activation, thereby reducing IL-8 secretion in 293/HA-hTLR4 cells. This inhibitory effect was comparable to that of TAK-242 (a TLR4 inhibitor, or resatorvid). Our results indicate that longifolioside A prevents inflammatory response by suppressing TLR4 activation required for NF-κB and AP-1 activation.

Protein kinase C-delta inhibition is organ-protective, enhances pathogen clearance, and improves survival in sepsis

Sepsis is a leading cause of morbidity and mortality in intensive care units. Previously, we identified Protein Kinase C-delta (PKCδ) as an important regulator of the inflammatory response in sepsis. An important issue in development of anti-inflammatory therapeutics is the risk of immunosuppression and inability to effectively clear pathogens. In this study, we investigated whether PKCδ inhibition prevented organ dysfunction and improved survival without compromising pathogen clearance. Sprague Dawley rats underwent sham surgery or cecal ligation and puncture (CLP) to induce sepsis. Post-surgery, PBS or a PKCδ inhibitor (200µg/kg) was administered intra-tracheally (IT). At 24 hours post-CLP, there was evidence of lung and kidney dysfunction. PKCδ inhibition decreased leukocyte influx in these organs, decreased endothelial permeability, improved gas exchange, and reduced blood urea nitrogen/creatinine ratios indicating organ protection. PKCδ inhibition significantly decreased bacterial levels in the peritoneal cavity, spleen and blood but did not exhibit direct bactericidal properties. Peritoneal chemokine levels, neutrophil numbers, or macrophage phenotypes were not altered by PKCδ inhibition. Peritoneal macrophages isolated from PKCδ inhibitor-treated septic rats demonstrated increased bacterial phagocytosis. Importantly, PKCδ inhibition increased survival. Thus, PKCδ inhibition improved survival and improved survival was associated with increased phagocytic activity, enhanced pathogen clearance, and decreased organ injury.

The effect of palmitic acid on inflammatory response in macrophages: an overview of molecular mechanisms

Palmitic acid is a saturated fatty acid whose blood concentration is elevated in obese patients. This causes inflammatory responses, where toll-like receptors (TLR), TLR2 and TLR4, play an important role. Nevertheless, palmitic acid is not only a TLR agonist. In the cell, this fatty acid is converted into phospholipids, diacylglycerol and ceramides. They trigger the activation of various signaling pathways that are common for LPS-mediated TLR4 activation. In particular, metabolic products of palmitic acid affect the activation of various PKCs, ER stress and cause an increase in ROS generation. Thanks to this, palmitic acid also strengthens the TLR4-induced signaling. In this review, we discuss the mechanisms of inflammatory response induced by palmitic acid. In particular, we focus on describing its effect on ER stress and IRE1α, and the mechanisms of NF-κB activation. We also present the mechanisms of inflammasome NLRP3 activation and the effect of palmitic acid on enhanced inflammatory response by increasing the expression of FABP4/aP2. Finally, we focus on the consequences of inflammatory responses, in particular, the effect of TNF-α, IL-1β and IL-6 on insulin resistance. Due to the high importance of macrophages and the production of proinflammatory cytokines by them, this work mainly focuses on these cells.

PKC-δ isoform plays a crucial role in Tat-TLR4 signalling pathway to activate NF-κB and CXCL8 production

HIV-1 Tat protein induces the production of CXCL8 chemokine in a TLR4/MD2 and PKC dependent manner. The objective of this study was to understand whether these two pathways were distinct or constituted a single common pathway, and to determine the nature of the PKC isoforms involved and their interrelation with the activation of NF-κB and CXCL8 gene product expression. Here, we show that Tat-induced CXCL8 production is essentially dependent on the activation of PKC delta isoform, as shown a) by the capacity of PKC delta dominant negative (DN), and Rottlerin, a selective PKC delta pharmacological inhibitor, to inhibit Tat-induced CXCL8 production and b) by the ability of the constitutively active (CAT) isoform of PKC delta to induce CXCL8 production in a HEK cell line in the absence of Tat stimulation. The finding that comparable amounts of CXCL8 were produced following stimulation with either Tat protein, PKC-delta CAT transfection, or both, argue for the implication of one common pathway where PKC delta is activated downstream of TLR4 recruitment and leads to the activation of NF-κB. Altogether, our results underline the crucial role of PKC delta isoform in activating gene expression of CXCL8, a cytokine largely implicated in the physiopathology of HIV-1 infection.

Heterotrimeric complex of p38 MAPK, PKCδ, and TIRAP is required for AP1 mediated inflammatory response

Inflammation could be described as a physiological response of the body to tissue injury, pathogen invasion, and irritants. During the inflammatory phase, cells of both the innate as well as adaptive immune system are activated and recruited to the site of inflammation. These mediators are downstream targets for the transcription factors; activator protein-1 (AP1), nuclear factor kappa-light-chain-enhancer (NF-κB), signal transducers and activators of transcription factors (STAT1), as well as interferon regulatory factors (IRFs), which control the expression of most immunomodulatory genes. There is a significant increase in active p38 mitogen-activated protein kinase (p38MAK) immediately after lipopolysaccharide (LPS) stimulation, which results in the activation of AP-1 transcription factor and expression of proinflammatory cytokines, IL-12 and IL-23. We studied the novel mechanism of p38 MAPK activation through the formation of a heterotrimeric complex of Protein kinase C delta type (PKCδ), Toll-Interleukin 1 Receptor (TIR) Domain Containing Adaptor Protein (TIRAP), and p38 proteins. TIRAP serves as an adaptor molecule which brings PKCδ and p38 in close proximity. The complex facilitates the activation of p38MAPK by PKCδ. Therefore, we propose that disruption of the heterotrimeric complex may be a good strategy to dampen the inflammatory response. Structure-based design of small molecules or peptides targetting PKCδ-TIRAP or TIRAP-p38 interfaces would be beneficial for therapy in AP1 mediated inflammatory diseases.

RAGE-TLR Crosstalk Sustains Chronic Inflammation in Neurodegeneration

Chronic inflammatory reactions are consistenly present in neurodegeneration of Alzheimer type and are considered important factors that accelerate progression of the disease. Receptors of innate immunity participate in triggering and driving inflammatory reactions. For example, Toll-like receptors (TLRs) and receptor for advanced glycation end product (RAGE), major receptors of innate immunity, play a central role in perpetuation of inflammation. RAGE activation should be perceived as a primary mechanism which determines self-perpetuated chronic inflammation, and RAGE cooperation with TLRs amplifies inflammatory signaling. In this review, we highlight and discuss that RAGE-TLR crosstalk emerges as an important driving force of chronic inflammation in Alzheimer's disease.

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.

Molecular mechanisms of regulation of Toll-like receptor signaling

TLRs play a critical role in the detection of microbes and endogenous "alarmins" to initiate host defense, yet they can also contribute to the development and progression of inflammatory and autoimmune diseases. To avoid pathogenic inflammation, TLR signaling is subject to multilayer regulatory control mechanisms, including cooperation with coreceptors, post-translational modifications, cleavage, cellular trafficking, and interactions with negative regulators. Nucleic acid-sensing TLRs are particularly interesting in this regard, as they can both recognize host-derived structures and require internalization of their ligand as a result of intracellular sequestration of the nucleic acid-sensing TLRs. This review summarizes the regulatory mechanisms of TLRs, including regulation of their access to ligands, receptor folding, intracellular trafficking, and post-translational modifications, as well as how altered control mechanism could contribute to inflammatory and autoimmune disorders.

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.

Role of protein kinase C in cytokine secretion by lung epithelial cells during infection with Paracoccidioides brasiliensis

In this study, we investigated the role of protein kinases C (PKCs) in interleukin (IL)-6 and IL-8 secretion by human lung epithelial A549 cells during infection with the fungal pathogen Paracoccidioides brasiliensis. Rottlerin and the broad spectrum PKC inhibitor Go 6983 reduced cytokine levels in A549 cell-P. brasiliensis cultures. Next, by western blot, we verified that infection with this fungus led to phosphorylation of PKC δ (Thr(505)). By using a peptide inhibitor for PKC δ or PKC δ short interfering RNA technique, IL-6 and IL-8 levels in A549-P. brasiliensis cultures were also reduced. Together, these results indicate that P. brasiliensis promotes IL-6 and IL-8 secretion by A549 cells in a PKC δ-dependent manner.

Lipid-mediated muscle insulin resistance: different fat, different pathways?

Increased dietary fat intake and lipolysis result in excessive lipid availability, which relates to impaired insulin sensitivity. Over the last years, several mechanisms possibly underlying lipid-mediated insulin resistance evolved. Lipid intermediates such as diacylglycerols (DAG) associate with changes in insulin sensitivity in many models. DAG activate novel protein kinase C (PKC) isoforms followed by inhibitory serine phosphorylation of insulin receptor substrate 1 (IRS1). Activation of Toll-like receptor 4 (TLR4) raises another lipid class, ceramides (CER), which induce pro-inflammatory pathways and lead to inhibition of Akt phosphorylation. Inhibition of glucosylceramide and ganglioside synthesis results in improved insulin sensitivity and increased activatory tyrosine phosphorylation of IRS1 in the muscle. Incomplete fat oxidation can increase acylcarnitines (ACC), which in turn stimulate pro-inflammatory pathways. This review analyzed the effects of lipid metabolites on insulin action in skeletal muscle of humans and rodents. Despite the evidence for the association of both DAG and CER with insulin resistance, its causal relevance may differ depending on the subcellular localization and the tested cohorts, e.g., athletes. Nevertheless, recent data indicate that individual lipid species and their degree of fatty acid saturation, particularly membrane and cytosolic C18:2 DAG, specifically activate PKCθ and induce both acute lipid-induced and chronic insulin resistance in humans.

Role of amplification in phospholipase Cγ2 activation in modulation of gastric mucosal inflammatory responses to Helicobacter pylori: effect of ghrelin

Phosphoinositide-specific phospholipase C (PLC) enzymes are crucial elements of signal transduction pathways that provide a common link of communication integrating specific receptor responses to a variety of hormones, growth factors, and bacterial endotoxins with the intended intracellular targets. Here, we examined the involvement of PLC in modulation of gastric mucosal inflammatory responses to Helicobacter pylori LPS by peptide hormone, ghrelin. We show that stimulation of gastric mucosal cells with the LPS leads to the activation and membrane translocation of the γ2 isoform of PLC, phosphorylated on Tyr as well as Ser, while the effect of ghrelin is reflected in the translocation and phosphorylation of membrane-associated PLCγ2 on Tyr mainly. Moreover, we demonstrate that PLCγ2 phosphorylation on Tyr remains under the control of the Src family protein tyrosine kinases (SFK-PTKs), and is intimately linked to PLCγ2 membrane localization, while the LPS-induced phosphorylation of membrane-recruited PLCγ2 on Ser displays dependence on protein kinase Cδ (PKCδ) and leads to the amplification in PLCγ2 activation. Thus, our findings link the extent of H. pylori-elicited gastric mucosal inflammatory involvement to the PKCδ-mediated amplification in PLCγ2 activation through phosphorylation on Ser.

Lipin-1 integrates lipid synthesis with proinflammatory responses during TLR activation in macrophages

Lipin-1 is a Mg(2+)-dependent phosphatidic acid phosphatase involved in the de novo synthesis of phospholipids and triglycerides. Using macrophages from lipin-1-deficient animals and human macrophages deficient in the enzyme, we show in this work that this phosphatase acts as a proinflammatory mediator during TLR signaling and during the development of in vivo inflammatory processes. After TLR4 stimulation lipin-1-deficient macrophages showed a decreased production of diacylglycerol and activation of MAPKs and AP-1. Consequently, the generation of proinflammatory cytokines like IL-6, IL-12, IL-23, or enzymes like inducible NO synthase and cyclooxygenase 2, was reduced. In addition, animals lacking lipin-1 had a faster recovery from endotoxin administration concomitant with a reduced production of harmful molecules in spleen and liver. These findings demonstrate an unanticipated role for lipin-1 as a mediator of macrophage proinflammatory activation and support a critical link between lipid biosynthesis and systemic inflammatory responses.

Modulation of gastric mucosal inflammatory responses to Helicobacter pylori via ghrelin-induced protein kinase Cδ tyrosine phosphorylation

A peptide hormone, ghrelin, plays a key role in modulation of gastric mucosal inflammatory responses to Helicobacter pylori by controlling the activation of constitutive nitric oxide synthase via Src/Akt-dependent phosphorylation that requires phosphatidylinositol 3-kinase (PI3K) participation. Here, we examined the relationship among PI3K; its upstream effector, protein kinase C (PKC); and cSrc. We show that stimulation of gastric mucosal cells with H. pylori LPS leads to the activation and membrane translocation of Ser-phosphorylated PKCδ, while the effect of ghrelin is reflected in the phosphorylation of membrane-associated PKCδ on Tyr. Further, we demonstrate that in response to the LPS-induced PKCδ activation both PI3K and Src show a marked increase in their Ser phosphorylation, while the effect of ghrelin is manifested in the phosphorylation of PI3K and cSrc at Tyr. Moreover, whereas Tyr phosphorylation of PKCδ exhibited susceptibility to cSrc inhibitor (PP2), the inhibitor of PKC (GF109203X) but not that of cSrc (PP2) blocked the Tyr phosphorylation of PI3K, while ghrelin-induced cSrc phosphorylation at Tyr was subject to inhibition by the inhibitors of PKC and PI3K. Thus, our findings stipulate the prerequisite of PKCδ in the activation of PI3K as well as cSrc, and imply that PI3K activation provides an essential platform for ghrelin-induced cSrc activation through autophosphorylation at Tyr(416). We also reveal that ghrelin-elicited up-regulation in PKCδ activation by Tyr phosphorylation shows dependence on cSrc activity.

Structural evaluation of BTK and PKCδ mediated phosphorylation of MAL at positions Tyr86 and Tyr106

A number of diseases including sepsis, rheumatoid arthritis, diabetes, cardiovascular diseases and hyperinflammatory immune disorders have been associated with Toll like receptor (TLR) 2 and TLR4. Endogenous adaptor protein known as MyD88 adapter-like protein (MAL) bind exclusively to the cytosolic portions of TLR2 and TLR4 to initiate downstream signalling. Brutons tyrosine kinase (BTK) and protein kinase C delta (PKCδ) have been implicated to phosphorylate MAL and activate it to initiate downstream signalling. BTK has been associated with phosphorylation at positions Tyr86 and Tyr106, necessary for the activation of MAL but definite residual target of PKCδ in MAL is still to be explored. To produce a better understanding of the functional domains involved in the formation of MAL-kinase complexes, computer-aided studies were used to characterize the protein-protein interactions (PPIs) of phosphorylated BTK and PKCδ with MAL. Docking and physicochemical studies indicated that BTK was involved in close contact with Tyr86 and Tyr106 of MAL whereas PKCδ may phosphorylate Tyr106 only. Moreover, the electrostatics charge distribution of binding interfaces of BTK and PKCδ were distinct but compatible with respective regions of MAL. Our results implicate that position of Tyr86 is specifically phosphorylated by BTK whereas Tyr106 can be phosphorylated by competitive action of both BTK and PKCδ. Additionally, the residues of MAL which are necessary for interaction with TLR2, TLR4, MyD88 and SOCS-1 also play their roles in maintaining interaction with kinases and can be targeted in future to reduce TLR2 and TLR4 induced pathological responses.

MyD88 adaptor-like (Mal) functions in the epithelial barrier and contributes to intestinal integrity via protein kinase C

MyD88 adapter-like (Mal)-deficient mice displayed increased susceptibility to oral but not intraperitoneal infection with Salmonella Typhimurium. Bone marrow chimeras demonstrated that mice with Mal-deficient non-hematopoietic cells were more susceptible to infection, indicating a role for Mal in non-myeloid cells. We observed perturbed barrier function in Mal(-/-) mice, as indicated by reduced electrical resistance and increased mucosa blood permeability following infection. Altered expression of occludin, Zonula occludens-1, and claudin-3 in intestinal epithelia from Mal(-/-) mice suggest that Mal regulates tight junction formation, which may in part contribute to intestinal integrity. Mal interacted with several protein kinase C (PKC) isoforms in a Caco-2 model of intestinal epithelia and inhibition of Mal or PKC increased permeability and bacterial invasion via a paracellular route, while a pan-PKC inhibitor increased susceptibility to oral infection in mice. Mal signaling is therefore beneficial to the integrity of the intestinal barrier during infection.

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.

Allergens as immunomodulatory proteins: the cat dander protein Fel d 1 enhances TLR activation by lipid ligands

Allergic responses can be triggered by structurally diverse allergens. Most allergens are proteins, yet extensive research has not revealed how they initiate the allergic response and why the myriad of other inhaled proteins do not. Among these allergens, the cat secretoglobulin protein Fel d 1 is a major allergen and is responsible for severe allergic responses. In this study, we show that similar to the mite dust allergen Der p 2, Fel d 1 substantially enhances signaling through the innate receptors TLR4 and TLR2. In contrast to Der p 2, however, Fel d 1 does not act by mimicking the TLR4 coreceptor MD2 and is not able to bind stably to the TLR4/MD2 complex in vitro. Fel d 1 does, however, bind to the TLR4 agonist LPS, suggesting that a lipid transfer mechanism may be involved in the Fel d 1 enhancement of TLR signaling. We also show that the dog allergen Can f 6, a member of a distinct class of lipocalin allergens, has very similar properties to Fel d 1. We propose that Fel d 1 and Can f 6 belong to a group of allergen immunomodulatory proteins that enhance innate immune signaling and promote airway hypersensitivity reactions in diseases such as asthma.

PKCδ as a regulator for TGFβ1-induced α-SMA production in a murine nonalcoholic steatohepatitis model

The precise mechanism of TGFβ1 signaling in the progression of non-alcoholic steatohepatitis (NASH) has remained unclear. In particular, a potential regulatory mechanism by which PKCδ affects profibrogenic gene expression had never been explored. In this study, therefore, the role of PKCδ in TGFβ1 mediated α-SMA expression was investigated using NASH model mice. In preparation of the NASH model, male C57BL6/J mice were fed a methionine-choline-deficient (MCD) diet for 3 weeks, after which time they were intraperitoneally injected with lipopolysaccharide (LPS). In addition, Tlr4(Lps-d) (CH3/HeJ) mice were used to demonstrate the TGFβ1 signaling's dependency on TLR4 induction. Liver histology and hepatic hepatitis markers were investigated, and hepatic gene expression levels were determined by real-time PCR. Acute liver injury by LPS injection specifically elevated not only α-SMA expression but also phospho-PKCδ in this model. In contrast, Tlr4(Lps-d) (CH3/HeJ) and blockade of TGFβ1 receptor by SB431542 resulted in a significant reduction of PKCδ activation and α-SMA expression. Moreover, the TGFβ1-induced α-SMA production was significantly reduced by a specific PKCδ inhibitor. These findings suggested that PKCδ plays a critical role in TGFβ1-induced α-SMA production in a NASH model. Thus, this was the first demonstration of the involvement of PKCδ in the regulation of α-SMA expression in NASH liver tissues, and the impaired induction of PKCδ phosphorylation by LPS in a steatohepatitis condition. Interestingly, treatment by PKCδ inhibitor caused dramatic reduction of myofibroblast activation, indicating that PKCδ represents a promising target for treating NASH.

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.

Helminth-derived immunomodulators: can understanding the worm produce the pill?

Helminths may protect humans against allergic and autoimmune diseases and, indeed, defined helminth-derived products have recently been shown to prevent the development of such inflammatory diseases in mouse models. Here, we propose that helminth-derived products not only have therapeutic potential but can also be used as unique tools for defining key molecular events in the induction of an anti-inflammatory response and, therefore, for defining new therapeutic targets.

The tangled web of signaling in innate immunity

Similarities in innate immune signaling exist between mammals and the nematode Caenorhabditis elegans. Now, Ziegler et al. (2009) and Ren et al. (2009) demonstrate that a protein kinase C delta homolog in C. elegans is involved in innate immunity, providing evidence that the conservation of immune signaling networks extends further than previously thought.

Analysis of ubiquitin degradation and phosphorylation of proteins

Both ubiquitination and phosphorylation are crucial mediators involved in controlling the functions of numerous proteins belonging to the Toll-like receptor (TLR) signaling pathways. Altering the aforementioned post-translational events can be detrimental to the host survival. Therefore, the importance of these modifications cannot be overestimated. This chapter describes techniques used to examine if a protein is ubiquitinated and/or phosphorylated. In addition, a method is provided to identify the modified amino acids. We have previously shown using these techniques that the protein MyD88 adapter-like (Mal) is phosphorylated and ubiquitinated following activation of the TLR2 and TLR4 signaling pathways. Both post-translational modifications are essential for the activation and degradation of Mal, and thus are crucial steps, in regulating these TLR signaling cascades and consequently the innate immune response.

Phosphatase PTP1B negatively regulates MyD88- and TRIF-dependent proinflammatory cytokine and type I interferon production in TLR-triggered macrophages

Toll-like receptors (TLRs) are primary sensors to detect conserved patterns on microorganisms, thus acting as the important components of innate immunity against invading pathogens. Protein tyrosine phosphatase-1B (PTP1B) has been shown to be a critical negative regulator of insulin pathway and other cellular signaling, however, whether and how PTP1B regulates TLR-triggered innate response remain to be investigated. We report here that PTP1B can markedly decrease TNF-alpha, IL-6 and IFN-beta production by macrophages stimulated with LPS, CpG ODN, or Poly I:C. Accordingly, knockdown of endogenous PTP1B expression increases production of TNF-alpha, IL-6 and IFN-beta in macrophages stimulated with TLR ligands. Phosphatase activity-disrupted mutant PTP1B cannot inhibit TLR-triggered production of proinflammatory cytokines and IFN-beta, indicating PTP1B exerts its suppressive activity in phosphatase-dependent manner. PTP1B inhibits TLR ligands-induced activation of MAPKs, NF-kappaB, and IRF3, furthermore, co-transfection of PTP1B inhibits both MyD88- and TRIF-induced transcription of TNF-alpha and IFN-beta reporter genes in a dose-dependent manner. In addition, PTP1B inhibits LPS-induced Tyk2 and STAT1 activation. Therefore, we demonstrate that phosphatase PTP1B is a physiological negative regulator of TLR signaling via suppression of both MyD88- and TRIF-dependent production of proinflammatory cytokine and IFN-beta in macrophages. Our results provide new mechanistic explanation for negative regulation TLR response and suggest PTP1B as a potential target for the intervention of the inflammatory diseases.

Toll-like receptor-4 modulates survival by induction of left ventricular remodeling after myocardial infarction in mice

Left ventricular (LV) remodeling is known to contribute to morbidity and mortality after myocardial infarction (MI). Because LV remodeling is strongly associated with an inflammatory response, we investigated whether or not TLR-4 influences LV remodeling and survival in a mice model of MI. Six days after MI induction, TLR4 knockout (KO)-MI mice showed improved LV function 32 and reduced LV remodeling as indexed by reduced levels of atrial natriuretic factor and total collagen as well as by a reduced heart weight to body weight ratio when compared with WT-MI mice. This was associated with a reduction of protein levels of the intracellular TLR4 adapter protein MyD88 and enhanced protein expression of the anti-hypertrophic JNK in KO-MI mice when compared with wild-type (WT)-MI mice. In contrast, protein activation of the pro-hypertrophic kinases protein kinase Cdelta and p42/44 were not regulated in KO-MI mice when compared with WT-MI mice. Improved LV function, reduced cardiac remodeling, and suppressed intracellular TLR4 signaling in KO-MI mice were associated with significantly improved survival compared with WT-MI mice (62 vs 23%; p < 0.0001). TLR4 deficiency led to improved survival after MI mediated by attenuated left ventricular remodeling.

The scaffold MyD88 acts to couple protein kinase Cepsilon to Toll-like receptors

Mice lacking protein kinase Cepsilon (PKCepsilon) are hypersensitive to both Gram-positive and Gram-negative bacterial infections; however, the mechanism of PKCepsilon coupling to the Toll-like receptors (TLRs), responsible for pathogen detection, is poorly understood. Here we sought to investigate the mechanism of PKCepsilon involvement in TLR signaling and found that PKCepsilon is recruited to TLR4 and phosphorylated on two recently identified sites in response to lipopolysaccharide (LPS) stimulation. Phosphorylation at both of these sites (Ser-346 and Ser-368) resulted in PKCepsilon binding to 14-3-3beta. LPS-induced PKCepsilon phosphorylation, 14-3-3beta binding, and recruitment to TLR4 were all dependent on expression of the scaffold protein MyD88. In mouse embryo fibroblasts and activated macrophages from MyD88 knock-out mice, LPS-stimulated PKCepsilon phosphorylation was reduced compared with wild type cells. Acute knockdown of MyD88 in LPS-responsive 293 cells also resulted in complete loss of Ser-346 phosphorylation and TLR4/PKCepsilon association. By contrast, MyD88 overexpression in 293 cells resulted in constitutive phosphorylation of PKCepsilon. A general role for MyD88 was evidenced by the finding that phosphorylation of PKCepsilon was induced by the activation of all TLRs tested that signal through MyD88 (i.e. all except TLR3) both in RAW cells and in primary human macrophages. Functionally, it is established that phosphorylation of PKCepsilon at these two sites is required for TLR4- and TLR2-induced NFkappaB reporter activation and IkappaB degradation in reconstituted PKCepsilon(-/-) cells. This study therefore identifies the scaffold protein MyD88 as the link coupling TLRs to PKCepsilon recruitment, phosphorylation, and downstream signaling.

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

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

Structure, function and regulation of the Toll/IL-1 receptor adaptor proteins

The Toll/IL-1 receptor (TIR) domain plays a central role in Toll-like receptor (TLR) signalling. All TLRs contain a cytoplasmic TIR domain, which, upon activation, acts as a scaffold to recruit adaptor proteins. The adaptor proteins MyD88, Mal, TRIF, TRAM and SARM are also characterized by the presence of a TIR domain. MyD88, Mal, TRIF and TRAM associate with the TLRs via homophilic TIR domain interactions whereas SARM utilizes its TIR domain to negatively regulate TRIF. It is well established that the differential recruitment of adaptors to TLRs provides a significant amount of specificity to the TLR-signalling pathways. Despite this, the TIR-TIR interface has not been well defined. However, structural studies have indicated the importance of TIR domain surfaces in mediating specific TIR-TIR interactions. Furthermore, recent findings regarding the regulation of adaptors provide further insight into the crucial role of the TIR domain in TLR signalling.