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

14-3-3 Family of Proteins: Biological Implications, Molecular Interactions, and Potential Intervention in Cancer, Virus and Neurodegeneration Disorders

The 14-3-3 family of proteins are highly conserved acidic eukaryotic proteins (25-32 kDa) abundantly present in the body. Through numerous binding partners, the 14-3-3 is responsible for many essential cellular pathways, such as cell cycle regulation and gene transcription control. Hence, its dysregulation has been linked to the onset of critical illnesses such as cancers, neurodegenerative diseases and viral infections. Interestingly, explorative studies have revealed an inverse correlation of 14-3-3 protein in cancer and neurodegenerative diseases, and the direct manipulation of 14-3-3 by virus to enhance infection capacity has dramatically extended its significance. Of these, COVID-19 has been linked to the 14-3-3 proteins by the interference of the SARS-CoV-2 nucleocapsid (N) protein during virion assembly. Given its predisposition towards multiple essential host signalling pathways, it is vital to understand the holistic interactions between the 14-3-3 protein to unravel its potential therapeutic unit in the future. As such, the general structure and properties of the 14-3-3 family of proteins, as well as their known biological functions and implications in cancer, neurodegeneration, and viruses, were covered in this review. Furthermore, the potential therapeutic target of 14-3-3 proteins in the associated diseases was discussed.

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.

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.

Co-ordinated control of the Aurora B abscission checkpoint by PKCε complex assembly, midbody recruitment and retention

A requirement for PKCε in exiting from the Aurora B dependent abscission checkpoint is associated with events at the midbody, however, the recruitment, retention and action of PKCε in this compartment are poorly understood. Here, the prerequisite for 14-3-3 complex assembly in this pathway is directly linked to the phosphorylation of Aurora B S227 at the midbody. However, while essential for PKCε control of Aurora B, 14-3-3 association is shown to be unnecessary for the activity-dependent enrichment of PKCε at the midbody. This localisation is demonstrated to be an autonomous property of the inactive PKCε D532N mutant, consistent with activity-dependent dissociation. The C1A and C1B domains are necessary for this localisation, while the C2 domain and inter-C1 domain (IC1D) are necessary for retention at the midbody. Furthermore, it is shown that while the IC1D mutant retains 14-3-3 complex proficiency, it does not support Aurora B phosphorylation, nor rescues division failure observed with knockdown of endogenous PKCε. It is concluded that the concerted action of multiple independent events facilitates PKCε phosphorylation of Aurora B at the midbody to control exit from the abscission checkpoint.

Modulatory Potential of Cannabidiol on the Opioid-Induced Inflammatory Response

Opioids are effective analgesics; however, there are many negative consequences of chronic use. One important side effect of chronic opioid use is the continuous engagement of the immune response that can exacerbate chronic pain. The opioid, morphine, initiates a Toll-like receptor 4 (TLR4) signaling cascade that drives the activation of NOD-, LRR-, and pyrin domain-containing protein 3 (NLRP3) inflammasome proteins, resulting in cytokine production and effectively creating a positive feedback loop for continuous TLR4 activation. In addition to driving cytokine production, morphine drives changes in proinflammatory lipid signaling. The alteration of both cytokine and lipid signaling systems by morphine suggests that its chronic use leads to a pathological immune response that would benefit from targeted therapy. Engaging the endogenous cannabinoid system has shown therapeutic benefit, particularly regarding its anti-inflammatory and immunosuppressive effects. Promising preclinical and clinical investigations suggest that cannabidiol (CBD) is an effective adjuvant for treatment of symptoms of opioid use disorders; however, the mechanism through which CBD drives this outcome is unclear. One potential source of insight into this mechanism is in how CBD regulates immune regulators such as cytokines and lipid signaling systems, including endocannabinoids and related immune-responsive lipids. In this review, we outline the immune response to chronic opioid use as well as CBD in the context of a lipopolysaccharide-induced immune response and speculate on the mechanism of CBD as a modulator of chronic opioid-induced immune system dysregulation.

TLR2 dimer-specific ligands selectively activate protein kinase C isoforms in Leishmania infection

Of the thirteen Toll-like receptors (TLRs) in mice, TLR2 has a unique ability of forming heterodimers with TLR1 and TLR6. Such associations lead to selective cellular signalling and cellular responses such as cytokine expression. One of the signalling intermediates is protein kinase C (PKC); of which, eight isoforms are expressed in macrophages. Leishmania-a protozoan parasite that resides and replicates in macrophages-selectively modulates PKC-α, PKC-β, PKC-δ and PKC-ζ isoforms in macrophages. As TLR2 plays significant roles in Leishmania infection, we examined whether these PKC isoforms play selective roles in TLR2 signalling and TLR2-induced anti-leishmanial functions. We observed that the TLR2 ligands-Pam₃ CSK₄ (TLR1/2), PGN (TLR2/2) and FSL (TLR2/6)-differentially phosphorylated and translocated PKC-α, PKC-β, PKC-δ and PKC-ζ isoforms to cell membrane in uninfected and L. major-infected macrophages. The PKC isoform-specific inhibitors differentially altered IL-10 and IL-12 expression, Th1 and Th2 responses and anti-leishmanial effects in macrophages and in BALB/c mice. While PKC isoforms' inhibitors had insignificant effects on the Pam3CSK4-induced anti-leishmanial functions, PGN-induced pro-leishmanial effects were enhanced by PKC-(α + β) inhibitors, whereas PKC-(δ + ζ) inhibitors enhanced the anti-leishmanial effects of FSL. These results indicated that the ligand-induced TLR2 dimerization triggered differential dose-dependent and kinetic profiles of PKC isoform activation and that selective targeting of PKC isoforms using their respective inhibitors in combination significantly modulated TLR2-induced anti-leishmanial functions. To the best of our knowledge, this is the first demonstration of TLR2 dimer signalling through PKC isoforms and TLR2-induced PKC isoform-targeted anti-leishmanial therapy.

The role of 14-3-3 proteins in cell signalling pathways and virus infection

14-3-3 proteins are highly conserved in species ranging from yeast to mammals and regulate numerous signalling pathways via direct interactions with proteins carrying phosphorylated 14-3-3-binding motifs. Recent studies have shown that 14-3-3 proteins can also play a role in viral infections. This review summarizes the biological functions of 14-3-3 proteins in protein trafficking, cell-cycle control, apoptosis, autophagy and other cell signal transduction pathways, as well as the associated mechanisms. Recent findings regarding the role of 14-3-3 proteins in viral infection and innate immunity are also reviewed.

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.

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.

Mechanical allodynia and enhanced responses to capsaicin are mediated by PI3K in a paclitaxel model of peripheral neuropathy

Paclitaxel chemotherapy treatment often leads to neuropathic pain resistant to available analgesic treatments. Recently spinal Toll-like receptor 4 (TLR4) and the transient receptor potential cation channel subfamily V member 1 (TRPV1) were identified to be involved in the pro-nociceptive effect of paclitaxel. The aim of this study was to investigate the role of phosphatidylinositol 3-kinase (PI3K) and serine/threonine kinases in this process, with the use of their antagonists (wortmannin, LY-294002, and staurosporine). The single paclitaxel administration (8 mg/kg i.p.) in mice induced robust mechanical allodynia measured as a reduced threshold to von Frey filament stimulation and generated reduced tachyphylaxis of capsaicin-evoked responses, recorded as changes in mEPSC frequency in patch-clamp recordings of dorsal horn neurons activity in vitro, for up to eight days. Paclitaxel application also induced increased Akt kinase phosphorylation in rat DRG neurons. All these paclitaxel-induced changes were prevented by the wortmannin in vivo pretreatment. Acute co-application of wortmannin or LY-294002 with paclitaxel in spinal cord slices also attenuated the paclitaxel effect on capsaicin-evoked responses. Staurosporine was effective in the acute in vitro experiments and on the first day after the paclitaxel treatment in vivo, but in contrast to wortmannin, it did not have a significant impact later. Our data suggest that the inhibition of PI3K signaling may help alleviate pathological pain syndromes in the paclitaxel-induced neuropathy.

An inflammatory bowel disease-risk variant in INAVA decreases pattern recognition receptor-induced outcomes

Inflammatory bowel disease (IBD) is characterized by dysregulation in both cytokines and responses to intestinal microbes, and proper regulation of pattern recognition receptor (PRR) signaling is critical for intestinal immune homeostasis. Altered functions for the IBD risk locus containing rs7554511, which encompasses the C1orf106 gene (recently named INAVA), and roles for the protein encoded by the INAVA gene are unknown. Here, we investigated the role of INAVA and INAVA genotype in regulating PRR-initiated outcomes in primary human cells. Both peripheral and intestinal myeloid cells expressed INAVA. Upon PRR stimulation, INAVA was required for optimal MAPK and NF-κB activation, cytokine secretion, and intracellular bacterial clearance. INAVA recruited 14-3-3τ, thereby contributing to recruitment of a signaling complex that amplified downstream signals and cytokines. Further, INAVA enhanced bacterial clearance by regulating reactive oxygen, reactive nitrogen, and autophagy pathways. Macrophages from rs7554511 C risk carriers expressed lower levels of INAVA RNA and protein. Lower expression was attributed in part to decreased transcription mediated directly by the intronic region containing the rs7554511 C variant. In rs7554511 C risk carrier macrophages, lower INAVA expression led to decreased PRR-induced activation of MAPK and NF-κB pathways, cytokines, and bacterial clearance pathways. Thus, IBD-associated polymorphisms in INAVA modulate PRR-initiated signaling, cytokines, and intracellular bacterial clearance, likely contributing to intestinal immune homeostasis.

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.

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.

Distinct contribution of protein kinase Cδ and protein kinase Cε in the lifespan and immune response of human blood monocyte subpopulations

Monocytes, key components of the immune system, are a heterogeneous population comprised of classical monocytes (CD16(-) ) and non-classical monocytes (CD16(+) ). Monocytes are short lived and undergo spontaneous apoptosis, unless stimulated. Dysregulation of monocyte numbers contribute to the pathophysiology of inflammatory diseases, yet the contribution of each subset remains poorly characterized. Protein kinase C (PKC) family members are central to monocyte biology; however, their role in regulating lifespan and immune function of CD16(-) and CD16(+) monocytes has not been studied. Here, we evaluated the contribution of PKCδ and PKCε in the lifespan and immune response of both monocyte subsets. We showed that CD16(+) monocytes are more susceptible to spontaneous apoptosis because of the increased caspase-3, -8 and -9 activities accompanied by higher kinase activity of PKCδ. Silencing of PKCδ reduced apoptosis in both CD16(+) and CD16(-) monocytes. CD16(+) monocytes express significantly higher levels of PKCε and produce more tumour necrosis factor-α in CD16(+) compared with CD16(-) monocytes. Silencing of PKCε affected the survival and tumour necrosis factor-α production. These findings demonstrate a complex network with similar topography, yet unique regulatory characteristics controlling lifespan and immune response in each monocyte subset, helping define subset-specific coordination programmes controlling monocyte function.

CXCR1/CXCR2 antagonist CXCL8(3-74)K11R/G31P blocks lung inflammation in swine barn dust-instilled mice

Inhalation of agricultural occupational dusts from swine confinement facilities can result in lung inflammation. The innate immune response to organic barn dusts results in production of a number of pro-inflammatory factors in the lungs of barn workers such as cytokines, chemokines, and an influx of neutrophils. Many of these inflammatory factors are influenced by the chemokine CXCL8/IL-8 (KC or MIP-2 in mice). Previously, we have demonstrated that an endotoxin-independent component of swine barn dust extract (SBE) elevates lung chemokines in a protein kinase C (PKC)-dependent manner resulting in the significant formation of lung inflammatory cell infiltrates in a mouse model of SBE injury. In this study we test the ability of a CXCR1/CXCR2 antagonist, CXCL8(3-74)K11R/G31P (G31P) to block many of the features of lung-inflammation in response to challenge with SBE in an established mouse exposure system. Injection of G31P concurrent with SBE nasal instillation over a course of 3 weeks significantly reduced neutrophil accumulation in the lungs of barn dust exposed animals compared to those given SBE alone. There was a similar reduction in pro-inflammatory cytokines and chemokines IL-6, KC, and MIP-2 in SBE plus G31P-treated mice. In addition to excreted products, the receptors ICAM-1, CXCR1, and CXCR2, which all were elevated with SBE exposure, were also decreased with G31P treatment. SBE activation of PKCα and PKCε was reduced as well with G31P treatment. Thus, G31P was found to be highly effective at reducing several features of lung inflammation in mice exposed to barn dust extracts.

Common mechanisms of Alzheimer's disease and ischemic stroke: the role of protein kinase C in the progression of age-related neurodegeneration

Ischemic stroke and Alzheimer's disease (AD), despite being distinct disease entities, share numerous pathophysiological mechanisms such as those mediated by inflammation, immune exhaustion, and neurovascular unit compromise. An important shared mechanistic link is acute and chronic changes in protein kinase C (PKC) activity. PKC isoforms have widespread functions important for memory, blood-brain barrier maintenance, and injury repair that change as the body ages. Disease states accelerate PKC functional modifications. Mutated forms of PKC can contribute to neurodegeneration and cognitive decline. In some cases the PKC isoforms are still functional but are not successfully translocated to appropriate locations within the cell. The deficits in proper PKC translocation worsen stroke outcome and amyloid-β toxicity. Cross talk between the innate immune system and PKC pathways contribute to the vascular status within the aging brain. Unfortunately, comorbidities such as diabetes, obesity, and hypertension disrupt normal communication between the two systems. The focus of this review is to highlight what is known about PKC function, how isoforms of PKC change with age, and what additional alterations are consequences of stroke and AD. The goal is to highlight future therapeutic targets that can be applied to both the treatment and prevention of neurologic disease. Although the pathology of ischemic stroke and AD are different, the similarity in PKC responses warrants further investigation, especially as PKC-dependent events may serve as an important connection linking age-related brain injury.

14-3-3ζ regulates immune response through Stat3 signaling in oral squamous cell carcinoma

Ectopic expression of 14-3-3ζ has been found in various malignancies, including lung cancer, liver cancer, head and neck squamous cell carcinoma (HNSCC), and so on. However, the effect of 14-3-3ζ in the regulation of interactions between tumor cells and the immune system has not been previously reported. In this study, we aimed to investigate whether and how 14-3-3ζ is implicated in tumor inflammation modulation and immune recognition evasion. In oral squamous cell carcinoma (OSCC) cell lines and cancer tissues, we found that 14-3-3ζ is overexpressed. In OSCC cells, 14-3-3ζ knockdown resulted in the up-regulated expression of inflammatory cytokines. In contrast, 14-3-3ζ introduction attenuated cytokine expression in human normal keratinocytes and fibroblasts stimulated with interferon-γ (IFN-γ) and lipopolysaccharide (LPS). Furthermore, supernatants from 14-3-3ζ knockdown OSCC cells dramatically altered the response of peritoneal macrophages, dendritic cells and tumor-specific T cells. Interestingly, Stat3 was found to directly interact with 14-3-3ζ and its disruption relieved the inhibition induced by 14-3-3ζ in tumor inflammation. Taken together, our studies provide evidence that 14-3-3ζ may regulate tumor inflammation and immune response through Stat3 signaling in OSCC.

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.

Phosphoproteomics combined with quantitative 14-3-3-affinity capture identifies SIRT1 and RAI as novel regulators of cytosolic double-stranded RNA recognition pathway

Viral double-stranded RNA (dsRNA) is the most important viral structure recognized by cytosolic pattern-recognition receptors of the innate immune system, and its recognition results in the activation of signaling cascades that stimulate the production of antiviral cytokines and apoptosis of infected cells. 14-3-3 proteins are ubiquitously expressed regulatory molecules that participate in a variety of cellular processes, and 14-3-3 protein-mediated signaling pathways are activated by cytoplasmic dsRNA in human keratinocytes. However, the functional role of 14-3-3 protein-mediated interactions during viral dsRNA stimulation has remained uncharacterized. Here, we used functional proteomics to identify proteins whose phosphorylation and interaction with 14-3-3 is modulated by dsRNA and to characterize the signaling pathways activated during cytosolic dsRNA-induced innate immune response in human HaCaT keratinocytes. Phosphoproteome analysis showed that several MAPK- and immune-response-related signaling pathways were activated after dsRNA stimulation. Interactome analysis identified RelA-associated inhibitor, high-mobility group proteins, and several proteins associated with host responses to viral infection as novel 14-3-3 target proteins. Functional studies showed that RelA-associated inhibitor regulated dsRNA-induced apoptosis and TNF production. Integrated network analyses of proteomic data revealed that sirtuin1 was a central molecule regulated by 14-3-3s during dsRNA stimulation. Further experiments showed that sirtuin 1 negatively regulated dsRNA-induced NFκB transcriptional activity, suppressed expression of antiviral cytokines, and protected cells from apoptosis in dsRNA-stimulated and encephalomyocarditis-virus-infected keratinocytes. In conclusion, our data highlight the importance of 14-3-3 proteins in antiviral responses and identify RelA-associated inhibitor and sirtuin 1 as novel regulators of antiviral innate immune responses.

Differential regulation of cyclin D1 expression by protein kinase C α and ϵ signaling in intestinal epithelial cells

Cellular accumulation of cyclin D1, a key regulator of cell proliferation and tumorigenesis, is subject to tight control. Our previous studies have identified PKCα as a negative regulator of cyclin D1 in the intestinal epithelium. However, treatment of non-transformed IEC-18 ileal crypt cells with PKC agonists has a biphasic effect on cyclin D1 expression. Initial PKCα-mediated down-regulation is followed by recovery and subsequent accumulation of the cyclin to levels markedly higher than those seen in untreated cells. Using protein overexpression strategies, siRNA, and pharmacological inhibitors, we now demonstrate that the recovery and hyperinduction of cyclin D1 reflect the combined effects of (a) loss of negative signals from PKCα due to agonist-induced PKCα down-regulation and (b) positive effects of PKCϵ. PKCϵ-mediated up-regulation of cyclin D1 requires sustained ERK stimulation and transcriptional activation of the proximal cyclin D1 (CCDN1) promoter, without apparent involvement of changes in protein stability or translation. PKCϵ also up-regulates cyclin D1 expression in colon cancer cells, through mechanisms that parallel those in IEC-18 cells. Although induction of cyclin D1 by PKCϵ is dependent on non-canonical NF-κB activation, the NF-κB site in the proximal promoter is not required. Instead, cyclin D1 promoter activity is regulated by a novel interaction between NF-κB and factors that associate with the cyclic AMP-response element adjacent to the NF-κB site. The differential effects of PKCα and PKCϵ on cyclin D1 accumulation are likely to contribute to the opposing tumor-suppressive and tumor-promoting activities of these PKC family members in the intestinal epithelium.

Bacteria induce prolonged PMN survival via a phosphatidylcholine-specific phospholipase C- and protein kinase C-dependent mechanism

Polymorphonuclear leukocytes (PMNs) are essential for the human innate immune defense, limiting expansion of invading microorganisms. PMN turnover is controlled by apoptosis, but the regulating signaling pathways remain elusive, largely due to inherent differences between mice and humans that undermine use of mouse models for understanding human PMN biology. Here, we aim to elucidate signal transduction mediating survival of human peripheral blood PMNs in response to bacteria, such as Yersinia pseudotuberculosis, an enteropathogen that causes the gastro-intestinal disease yersiniosis, as well as Escherichia coli and Staphylococcus aureus. Determinations of cell death reveal that uninfected control cells undergo apoptosis, while PMNs infected with either Gram-positive or -negative bacteria show profoundly increased survival. Infected cells exhibit decreased caspase 3 and 8 activities, increased mitochondrial integrity and are resistant to apoptosis induced by a death receptor ligand. This bacteria-induced response is accompanied by pro-inflammatory cytokine production including interleukin-8 and tumor necrosis factor-α competent to attract additional PMNs. Using agonists and pharmacological inhibitors, we show participation of Toll-like receptor 2 and 4, and interestingly, that protein kinase C (PKC) and phosphatidylcholine-specific phospholipase C (PC-PLC), but not tyrosine kinases or phosphatidylinositol-specific phospholipase C (PI-PLC) are key players in this dual PMN response. Our findings indicate the importance of prolonged PMN survival in response to bacteria, where general signaling pathways ensure complete exploitation of PMN anti-microbial capacity.

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 Cε-calcineurin cosignaling downstream of toll-like receptor 4 downregulates fibrosis and induces wound healing gene expression in cardiac myofibroblasts

The pathways which regulate resolution of inflammation and contribute to positive remodeling of the myocardium following injury are poorly understood. Here we show that protein kinase C epsilon (PKCε) cooperates with the phosphatase calcineurin (CN) to potentiate induction of cardioprotective gene expression while suppressing expression of fibrosis markers. This was achieved by detailed analysis of the regulation of cyclooxygenase 2 (COX-2) expression as a marker gene and by using gene expression profiling to identify genes regulated by coexpression of CN-Aα/PKCε in adult rat cardiac myofibroblasts (ARVFs) on a larger scale. GeneChip analysis of CN-Aα/PKCε-coexpressing ARVFs showed that COX-2 provides a signature for wound healing and is associated with downregulation of fibrosis markers, including connective tissue growth factor (CTGF), fibronectin, and collagens Col1a1, Col3a1, Col6a3, Col11a1, Col12a1, and Col14a1, with concomitant upregulation of cardioprotection markers, including COX-2 itself, lipocalin 2 (LCN2), tissue inhibitor of metalloproteinase 1 (TIMP-1), interleukin-6 (IL-6), and inducible nitric oxide synthase (iNOS). In primary rat cardiomyocyte cultures Toll-like receptor 4 (TLR4) agonist- or PKCε/CN-dependent COX-2 induction occurred in coresident fibroblasts and was blocked by selective inhibition of CN or PKC α/ε or elimination of fibroblasts. Furthermore, ectopic expression of PKCε and CN in ARVFs showed that the effects on COX-2 expression are mediated by specific NFAT sites within the COX-2 promoter as confirmed by site-directed mutagenesis and chromatin immunoprecipitation (ChIP). Therefore, PKCε may negatively regulate adverse myocardial remodeling by cooperating with CN to downregulate fibrosis and induce transcription of cardioprotective wound healing genes, including COX-2.

Vitamin D treatment modulates organic dust-induced cellular and airway inflammatory consequences

Exposure to organic dusts elicits airway inflammatory diseases. Vitamin D recently has been associated with various airway inflammatory diseases, but its role in agricultural organic dust exposures is unknown. This study investigated whether vitamin D reduces organic dust-induced inflammatory outcomes in cell culture and animal models. Organic dust extracts obtained from swine confinement facilities induced neutrophil chemokine production (human IL-8, murine CXCL1/CXCL2). Neutrophil chemokine induction was reduced in human blood monocytes, human bronchial epithelial cells, and murine lung slices pretreated with 1,25-(OH)(2) D(3) . Intranasal inhalation of organic dust extract induced neutrophil influx, and CXCL1/CXCL2 release was also decreased in mice fed a relatively high vitamin D diet as compared to mice fed a low vitamin D diet. These findings were associated with reduced tracheal epithelial cell PKCα and PKCε activity and whole lung TLR2 and TLR4 gene expression. Collectively, vitamin D plays a role in modulating organic dust-induced airway inflammatory outcomes.

14-3-3ε and 14-3-3σ inhibit Toll-like receptor (TLR)-mediated proinflammatory cytokine induction

Toll-like receptors (TLRs) are a group of pattern recognition receptors that play a crucial role in the induction of the innate immune response against bacterial and viral infections. TLR3 has emerged as a key sensor of viral double-stranded RNA. Thus, a clearer understanding of the biological processes that modulate TLR3 signaling is essential. Limited studies have applied proteomics toward understanding the dynamics of TLR signaling. Herein, a proteomics approach identified 14-3-3ε and 14-3-3σ proteins as new members of the TLR signaling complex. Toward the functional characterization of 14-3-3ε and 14-3-3σ in TLR signaling, we have shown that both of these proteins impair TLR2, TLR3, TLR4, TLR7/8, and TLR9 ligand-induced IL-6, TNFα, and IFN-β production. We also show that 14-3-3ε and 14-3-3σ impair TLR2-, TLR3-, TLR4-, TLR7/8-, and TLR9-mediated NF-κB and IFN-β reporter gene activity. Interestingly, although the 14-3-3 proteins inhibit poly(I:C)-mediated RANTES production, 14-3-3 proteins augment Pam(3)CSK(4), LPS, R848, and CpG-mediated production of RANTES (regulated on activation normal T cell expressed and secreted) in a Mal (MyD88 adaptor-like)/MyD88-dependent manner. 14-3-3ε and 14-3-3σ also bind to the TLR adaptors and to both TRAF3 and TRAF6. Our study conclusively shows that 14-3-3ε and 14-3-3σ play a major regulatory role in balancing the host inflammatory response to viral and bacterial infections through modulation of the TLR signaling pathway. Thus, manipulation of 14-3-3 proteins may represent novel therapeutic targets for inflammatory conditions and infections.

Protein kinase C epsilon is important in modulating organic-dust-induced airway inflammation

Organic dust samples from swine confinement facilities elicit pro-inflammatory cytokine/chemokine release from bronchial epithelial cells and monocytes, dependent, in part, upon dust-induced activation of the protein kinase C (PKC) isoform, PKCε. PKCε is also rapidly activated in murine tracheal epithelial cells following in vivo organic dust challenges, yet the functional role of PKCε in modulating dust-induced airway inflammatory outcomes is not defined. Utilizing an established intranasal inhalation animal model, experiments investigated the biologic and physiologic responses following organic dust extract (ODE) treatments in wild-type (WT) and PKCε knock-out (KO) mice. We found that neutrophil influx increased more than twofold in PKCε KO mice following both a one-time challenge and 3 weeks of daily challenges with ODE as compared with WT mice. Lung pathology revealed increased bronchiolar and alveolar inflammation, lymphoid aggregates, and T cell influx in ODE-treated PKCε KO mice. Airway hyperresponsiveness to methacholine increased in PKCε KO + ODE to a greater magnitude than WT + ODE animals. There were no significant differences in cytokine/chemokine release elicited by ODE treatment between groups. However, ODE-induced nitric oxide (NO) production differed in that ODE exposure increased nitrate levels in WT mice but not in PKCε KO mice. Moreover, ODE failed to upregulate NO from ex vivo stimulated PKCε KO lung macrophages. Collectively, these studies demonstrate that PKCε-deficient mice were hypersensitive to organic dust exposure and suggest that PKCε is important in the normative lung inflammatory response to ODE. Dampening of ODE-induced NO may contribute to these enhanced inflammatory findings.

Molecular mechanisms responsible for the selective and low-grade induction of proinflammatory mediators in murine macrophages by lipopolysaccharide

Low-dose endotoxemia is prevalent in humans with adverse health conditions, and it correlates with the pathogenesis of chronic inflammatory diseases such as atherosclerosis, diabetes, and neurologic inflammation. However, the underlying molecular mechanisms are poorly understood. In this study, we demonstrate that subclinical low-dose LPS skews macrophages into a mild proinflammatory state, through cell surface TLR4, IL-1R-associated kinase-1, and the Toll-interacting protein. Unlike high-dose LPS, low-dose LPS does not induce robust activation of NF-κB, MAPKs, PI3K, or anti-inflammatory mediators. Instead, low-dose LPS induces activating transcription factor 2 through Toll-interacting protein-mediated generation of mitochondrial reactive oxygen species, allowing mild induction of proinflammatory mediators. Low-dose LPS also suppresses PI3K and related negative regulators of inflammatory genes. Our data reveal novel mechanisms responsible for skewed and persistent low-grade inflammation, a cardinal feature of chronic inflammatory diseases.

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.

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.

Peptidoglycan derived from Staphylococcus epidermidis induces Connexin43 hemichannel activity with consequences on the innate immune response in endothelial cells

Gram-positive bacterial cell wall components including PGN (peptidoglycan) elicit a potent pro-inflammatory response in diverse cell types, including endothelial cells, by activating TLR2 (Toll-like receptor 2) signalling. The functional integrity of the endothelium is under the influence of a network of gap junction intercellular communication channels composed of Cxs (connexins) that also form hemichannels, signalling conduits that are implicated in ATP release and purinergic signalling. PGN modulates Cx expression in a variety of cell types, yet effects in endothelial cells remain unresolved. Using the endothelial cell line b.End5, a 6 h challenge with PGN induced IL-6 (interleukin 6), TLR2 and Cx43 mRNA expression that was associated with enhanced Cx43 protein expression and gap junction coupling. Cx43 hemichannel activity, measured by ATP release from the cells, was induced following 15 min of exposure to PGN. Inhibition of hemichannel activity with carbenoxolone or apyrase prevented induction of IL-6 and TLR2 mRNA expression by PGN, but had no effect on Cx43 mRNA expression levels. In contrast, knockdown of TLR2 expression had no effect on PGN-induced hemichannel activity, but reduced the level of TLR2 and Cx43 mRNA expression following 6 h of PGN challenge. PGN also acutely induced hemichannel activity in HeLa cells transfected to express Cx43, but had no effect in Cx43-deficient HeLa OHIO cells. All ATP responses were blocked with Cx-specific channel blockers. We conclude that acute Cx43 hemichannel signalling plays a role in the initiation of early innate immune responses in the endothelium.

Regulation of Protein Kinase C function by phosphorylation on conserved and non-conserved sites

Protein Kinase C (PKC) is a family of serine/threonine kinases whose function is influenced by phosphorylation. In particular, three conserved phosphorylation sites known as the activation-loop, the turn-motif and the hydrophobic-motif play important roles in controlling the catalytic activity, stability and intracellular localisation of the enzyme. Prevailing models of PKC phosphorylation suggest that phosphorylation of these sites occurs shortly following synthesis and that these modifications are required for the processing of newly-transcribed PKC to the mature (but still inactive) form; phosphorylation is therefore a priming event that enables catalytic activation in response to lipid second messengers. However, many studies have also demonstrated inducible phosphorylation of PKC isoforms at these sites following stimulation, highlighting that our understanding of PKC phosphorylation and its impact on enzymatic function is incomplete. Furthermore, inducible phosphorylation at these sites is often interpreted as catalytic activation, which could be misleading for some isoforms. Recent studies that include systems-wide phosphoproteomic profiling of cells has revealed a host of additional (and in many cases non-conserved) phosphorylation sites on PKC family members that influence their function. Many of these may in fact be more suitable than previously described sites as surrogate markers of catalytic activation. Here we discuss the role of phosphorylation in controlling PKC function and outline our current understanding of the mechanisms that regulate these phosphorylation sites.

Toll-like receptor 2 mediates inhibition of HCO(3)(-) absorption by bacterial lipoprotein in medullary thick ascending limb

Bacterial infection and sepsis are associated with renal tubule dysfunction and dysregulation of systemic electrolyte balance but the underlying mechanisms are incompletely understood. Recently, we demonstrated that HCO(3)(-) absorption by the medullary thick ascending limb (MTAL) is inhibited by gram-negative bacterial LPS through activation of Toll-like receptor 4 (TLR4). Here, we examined whether MTAL transport is altered by activation of TLR2, the receptor predominantly responsible for recognizing gram-positive bacteria. Confocal immunofluorescence showed expression of TLR2 in the basolateral membrane domain of rat and mouse MTALs. The functional role of TLR2 was examined in perfused MTALs using Pam(3)CSK(4), a bacterial lipoprotein analog that specifically activates TLR2. Adding Pam(3)CSK(4) to the bath decreased HCO(3)(-) absorption by 25%. The inhibition by Pam(3)CSK(4) was eliminated in MTALs from TLR2(-/-) mice. HCO(3)(-) absorption was also inhibited by the TLR2 agonists lipoteichoic acid and peptidoglycan, two cell wall components of gram-positive bacteria. The MEK/ERK inhibitor U0126 eliminated inhibition of HCO(3)(-) absorption by bath LPS but had no effect on inhibition by Pam(3)CSK(4). The inhibition by Pam(3)CSK(4) was eliminated by the protein kinase C inhibitors chelerythrine Cl and bisindolylmaleimide. Moreover, the inhibition by Pam(3)CSK(4), lipoteichoic acid, and peptidoglycan was additive to inhibition by LPS. Thus, agonists of basolateral TLR2 and TLR4 inhibit HCO(3)(-) absorption independently through distinct signaling pathways. We conclude that bacterial components act directly through TLRs to modify the transport function of renal tubules. During polymicrobial sepsis, gram-positive bacterial molecules acting through TLR2 and gram-negative LPS acting through TLR4 can function through parallel signaling pathways to impair MTAL transport. The inhibition of luminal acidification may impair the ability of the kidneys to correct systemic acidosis that contributes to sepsis pathogenesis.

The substrates and binding partners of protein kinase Cepsilon

The epsilon isoform of protein kinase C (PKCepsilon) has important roles in the function of the cardiac, immune and nervous systems. As a result of its diverse actions, PKCepsilon is the target of active drug-discovery programmes. A major research focus is to identify signalling cascades that include PKCepsilon and the substrates that PKCepsilon regulates. In the present review, we identify and discuss those proteins that have been conclusively shown to be direct substrates of PKCepsilon by the best currently available means. We will also describe binding partners that anchor PKCepsilon near its substrates. We review the consequences of substrate phosphorylation and discuss cellular mechanisms by which target specificity is achieved. We begin with a brief overview of the biology of PKCepsilon and methods for substrate identification, and proceed with a discussion of substrate categories to identify common themes that emerge and how these may be used to guide future studies.

Innate immunity mediates myocardial preconditioning through Toll-like receptor 2 and TIRAP-dependent signaling pathways

Recent studies have implicated Toll-like receptor 2 (TLR2) and TLR4 signaling in delimiting liver and brain injury following ischemia-reperfusion (I/R). To determine whether TLR2 and TLR4 conferred cytoprotection in the heart, we subjected hearts of wild-type (WT) mice and mice deficient in TLR2 (TLR2D), TLR4 (TLR4D), and TIR domain-containing adapter protein (TIRAP-D) to ischemic preconditioning (IPC). Langendorff-perfused hearts were subjected to 30 min ischemia and 60 min reperfusion with or without IPC. IPC resulted in a significant increase (P < 0.05) in the percent recovery of left ventricular developed pressure (%LVDP) in WT mouse hearts (54.4 +/- 2.7% of baseline), whereas there was no significant increase in %LVDP (P > 0.05) in TIRAP-D mouse hearts (43.8 +/- 1.9%) after I/R injury. IPC also resulted in a significant (P < 0.05) decrease in I/R-induced creatine kinase release and Evans blue dye uptake in WT but not TIRAP-D hearts. Interestingly, IPC resulted in a significant (P < 0.05) increase in %LVDP in TLR4-deficient hearts (52.7 +/- 3%) but not in TLR2D hearts (39.3 +/- 1.5%). Pretreatment with a specific TLR2 ligand (Pam3CSK) protected WT hearts against I/R-induced left ventricular dysfunction. The loss of IPC-induced cardioprotection in TIRAP-D mouse hearts was accompanied by a decreased translocation of protein kinase C-epsilon and decreased phosphorylation of GSK-3beta. Taken together, these data suggest that the cardioprotective effect of IPC is mediated, at least in part, through a TLR2-TIRAP-dependent pathway, suggesting that the modulation of this pathway represents a viable target for reducing I/R injury.

Structure and regulation of cytoplasmic adapter proteins involved in innate immune signaling

Initiation of the innate immune response requires agonist recognition by a pathogen recognition receptor. Following ligand binding, conformational rearrangement of the receptor creates a molecular scaffold from which signal transduction is propagated via complex cellular signaling pathways. This in turn leads to the induction of a pro-inflammatory immune response. A critical component of these signaling pathways is the homotypic interaction of receptor and adapter proteins via specific protein interaction domains. Within the innate immune signaling cascade, homotypic interactions between members of the death domain family and the Toll/interleukin-1 receptor domain are particularly important. Here we discuss the current understanding of the molecular basis of these homotypic receptor:adapter interactions and their role in innate immune signal transduction.

Signalling adaptors used by Toll-like receptors: an update

Research into the five Toll/IL1 receptor (TIR) adaptor proteins involved in innate immunity continues to advance. Here we outline some of the more recent findings. MyD88 has a key role in signalling by the IL1 receptor complex and TLRs. However, a MyD88-independent pathway of IL1beta signalling in neurons has been described which involves the protein kinase Akt, and which has an anti-apoptotic effect. This pathway may also be important for the mechanism whereby Alum exerts its adjuvant effect since this depends on IL1beta but is MyD88-independent. MyD88 is also involved in tumourigenesis in models of hepatocarcinoma and familial associated polyposis (FAP); negative regulation of TLR3 signalling and in PKCepsilon activation. The adaptor Mal is regulated by phosphorylation and caspase-1 cleavage. A variant form of Mal in humans termed S180L confers protection in multiple infectious diseases. TRAM is controlled by myristoylation and phosphorylation and the localisation of TRAM with TLR4 to endosomes is required for activation of IRF3 and induction of IFNbeta. Finally SARM has been shown to regulate TRIF and also appears to be involved in neuronal injury mediated by oxidative stress in mouse neurons. These advances confirm the importance for the TIR domain-containing adapters in host defence and inflammation.

The regulated assembly of a PKCepsilon complex controls the completion of cytokinesis

The cell cycle is exquisitely controlled by multiple sequential regulatory inputs to ensure fidelity. Here we demonstrate that the final step in division, the physical separation of daughter cells, is controlled by a member of the PKC gene superfamily. Specifically, we have identified three phosphorylation sites within PKCepsilon that control its association with 14-3-3. These phosphorylations are executed by p38 MAP kinase (Ser 350), GSK3 (Ser 346) and PKC itself (Ser 368). Integration of these signals is essential during mitosis because mutations that prevent phosphorylation of PKCepsilon and/or PKCepsilon binding to 14-3-3 also cause defects in the completion of cytokinesis. Using chemical genetic and dominant-negative approaches it is shown that selective inhibition of PKCepsilon halts cells at the final stages of separation. This arrest is associated with persistent RhoA activation at the midbody and a delay in actomyosin ring dissociation. This study therefore identifies a new regulatory mechanism that controls exit from cytokinesis, which has implications for carcinogenesis.