PKCzeta at the crossroad of NF-kappaB and Jak1/Stat6 signaling pathways

Suppressive and pro-inflammatory roles for IL-4 in the pathogenesis of experimental drug-induced liver injury: a review

IMPORTANCE OF THE FIELD

Idiosyncratic drug reactions resulting in drug-induced liver injury (DILI) account for approximately 13% of acute liver failure cases in the US. Idiosyncratic drug reactions are the third most common cause of liver transplantation, exceeded only by acetaminophen and indeterminate causes. Clinical evidence suggests that idiosyncratic DILI is triggered by drug hapten-altered self proteins resulting in hepatocellular injury. An example of this type of DILI is hepatitis that develops in susceptible individuals following administration of halogenated volatile anesthetics, dihydralazine, carbamazepine or diclofenac.

AREAS COVERED IN THIS REVIEW

In this review, we describe research in animal models that supports a critical role for suppressive and pro-inflammatory roles for IL-4 in the pathogenesis of immune-mediated DILI.

WHAT THE READER WILL GAIN

The reader will gain insights into the roles of IL-4 in the development of experimental DILI. The reader will gain tools to assist in the translation of these findings to those in patients with immune-mediated DILI, as well as other inflammatory diseases of the liver. The reader will then be made aware of gaps in knowledge in the pathogenesis of DILI where research could result in significant advances in the care of these complicated patients.

TAKE HOME MESSAGE

In experimental immune-mediated DILI, IL-4 suppresses regulatory responses to CYP2E1 autoantigens but induces pro-inflammatory responses to drug haptens.

PKCzeta-regulated inflammation in the nonhematopoietic compartment is critical for obesity-induced glucose intolerance

Obesity-induced inflammation is critical for the development of insulin resistance. Here, we show that genetic inactivation of PKCzeta in vivo leads to a hyperinflammatory state in obese mice that correlates with a higher glucose intolerance and insulin resistance. Previous studies implicated PKCzeta in the regulation of type 2 inflammatory responses in T cells. By using ex vivo and in vivo experiments, we demonstrate that although PKCzeta is involved in the alternative (M2) activation of macrophages, surprisingly, PKCzeta ablation in the nonhematopoietic compartment but not in the hematopoietic system is sufficient to drive inflammation and IL-6 synthesis in the adipose tissue, as well as insulin resistance. Experiments using PKCzeta/IL-6 double-knockout mice demonstrated that IL-6 production accounts for obesity-associated glucose intolerance induced by PKCzeta deficiency. These results establish PKCzeta as a critical negative regulator of IL-6 in the control of obesity-induced inflammation in adipocytes.

7-ketocholesterol-induced inflammation: involvement of multiple kinase signaling pathways via NFκB but independently of reactive oxygen species formation

PURPOSE

7-Ketocholesterol (7KCh) accumulates in oxidized lipoprotein deposits and is known to be involved in macrophage foam cell formation and atherosclerosis. 7-KCh is present in the primate retina and is associated with oxidized lipoprotein deposits located in the choriocapillaris, Bruch's membrane, and retinal pigment epithelium (RPE). 7-KCh can also be formed in the retina as a consequence of light-induced iron release. The purpose of this study was to examine the signaling pathways involved in the 7KCh-mediated inflammatory response focusing on three cytokines, VEGF, IL-6, and IL-8.

METHODS

ARPE-19 cells were treated with 7KCh solubilized in hydroxypropyl-β-cyclodextrin. Cytokines were quantified by qRT-PCR (mRNA) and ELISA (protein) using commercially available products. NFκB activation was determined by IκBα mRNA induction.

RESULTS

Treatment of ARPE-19 cells with 15 μM 7KCh markedly induced the expression of VEGF, IL-6, and IL-8. No increase in NOX-4 expression or ROS formation was detected. 7KCh induced the phosphorylation of ERK1/2 and p38MAPK, and inhibitors to these kinases markedly reduced the cytokine expression but did not affect the IκBα mRNA expression. By contrast, inhibition of PI3K and PKCζ significantly decreased the cytokine and IκBα mRNA expression. Inhibition of the IκB kinase complex essentially ablated all cytokine induction.

CONCLUSIONS

7KCh induces cytokines via three kinase signaling pathways, AKT-PKCζ-NFκB, p38 MAPK, and ERK. The MAPK/ERK pathways seem to preferentially enhance cytokine induction downstream from NFκB activation. The results of this study suggest that 7KCh activates these pathways through interactions in the plasma membrane, but the mechanism(s) remains unknown.

Atypical protein kinase C zeta exhibits a proapoptotic function in ovarian cancer

Intracellular signaling governed by serine/threonine kinases comprises the molecular interface between cell surface receptors and the nuclear transcriptional machinery. The protein kinase C (PKC) family members are involved in the control of many signaling processes directing cell proliferation, motility, and survival. Here, we examined a role of different PKC isoenzymes in protein phosphatase 2A (PP2A) and HRSL3 tumor suppressor-dependent cell death induction in the ovarian carcinoma cell line OVCAR-3. Phosphorylation and activity of PKC isoenzymes were measured in response to PP2A or phosphoinositide 3-kinase inhibition or HRSL3 overexpression. These experiments indicated a regulation of PKC, epsilon, zeta, and iota through PP2A and/or HRSL3, but not of PKCalpha and beta. Using isoform-specific peptide inhibitors and overexpression approaches, we verified a contribution to PP2A- and HRLS3-dependent apoptosis only for PKCzeta, suggesting a proapoptotic function of this kinase. We observed a significant proportion of human ovarian carcinomas expressing high levels of PKCzeta, which correlated with poor prognosis. Primary ovarian carcinoma cells isolated from patients also responded to okadaic acid treatment with increased phosphorylation of PKCzeta and apoptosis induction. Thus, our data indicate a contribution of PKCzeta in survival control in ovarian carcinoma cells and suggest that upregulation or activation of tyrosine kinase receptors in this tumor might impinge onto apoptosis control through the negative regulation of the atypical PKCzeta.

Molecular basis of lysophosphatidic acid-induced NF-κB activation

PKC, β-arrestin 2, CARMA3, BCL10, MALT1, TRAF6 and MEKK3 are signaling proteins that have a key role in G protein-coupled receptor (GPCR)-mediated activation of nuclear factor-κB (NF-κB) pathway in nonhematopoietic cells in response to lysophosphatidic acid (LPA) stimulation. The PKC, β-arrestin 2, CARMA3-BCL10-MALT1-TRAF6 signalosome, and MEKK3 functions as a link between GPCR signaling and IKK-NF-κB activation. Here we briefly summarize recent progress in the understanding of the molecular and biological functions of these proteins in GPCR-mediated NF-κB activation in nonhematopoietic cells.

G alpha(q) acts as an adaptor protein in protein kinase C zeta (PKCzeta)-mediated ERK5 activation by G protein-coupled receptors (GPCR)

G(q)-coupled G protein-coupled receptors (GPCR) mediate the actions of a variety of messengers that are key regulators of different cellular functions. These receptors can regulate a highly interconnected network of biochemical routes that control the activity of several members of the mitogen-activated protein kinase (MAPK) family. The ERK5 MAPK has been shown to be activated by G(q)-coupled GPCR via unknown mechanisms. We find that the atypical protein kinase C (PKCzeta), previously reported to interact with the ERK5 activator MEK5 and to be involved in epidermal growth factor-mediated ERK5 stimulation, plays a crucial role in the activation of the ERK5 pathway by G(q)-coupled GPCR. Stimulation of ERK5 by G(q)-coupled GPCR is abolished upon pharmacological inhibition of PKCzeta as well as in embryonic fibroblasts obtained from PKCzeta-deficient mice. Both PKCzeta and MEK5 associate to G alpha(q) upon activation of GPCR, thus forming a ternary complex that seems essential for the activation of ERK5. These data put forward a novel function of G alpha(q) as a scaffold protein involved in the modulation of the ERK5 cascade by GPCR that could be relevant in G(q)-mediated physiological functions.

Involvement of protein kinase C in IL-1beta-induced expression of cyclooxygenase-2 in human gingival fibroblasts

Interleukin-1beta (IL-1beta) stimulates expression of the highly inducible enzyme cyclooxygenase-2 (COX-2) via activation of nuclear factor kappaB (NFkappaB), and consequently provokes prostaglandin E(2) (PGE(2)) synthesis, which induces inflammatory responses. In this study, the contribution of protein kinase C (PKC) to IL-1beta-induced PGE(2) synthesis in human gingival fibroblasts was investigated. The PKC activator phorbol 12-myristate 13-acetate (PMA) stimulated PGE(2) release and COX-2 mRNA expression, as shown in human gingival fibroblasts stimulated by IL-1beta. However, PMA showed only a weak effect on the formation of COX-2-NFkappaB DNA-protein complex, whereas IL-1beta had a clearly stimulatory effect. In cells in which PMA-dependent PKC was down-regulated, PMA failed to induce the formation of NFkappaB DNA-protein complex and reduced the release of PMA-induced PGE(2), whereas IL-1beta stimulated the formation of COX-2-NFkappaB DNA-protein complex and PGE(2) release. The atypical PKC (aPKC) inhibitor Gö6983 clearly suppressed the formation of COX-2-NFkappaB DNA-protein complex and PGE(2) release stimulated by IL-1beta but not the inhibitor of conventional PKC (cPKC) and the novel PKC (nPKC) inhibitor Gö6976. These observations suggest that aPKC is involved in IL-1beta-induced PGE(2) synthesis, which is controlled by transcription of the COX-2 gene via the NFkappaB-dependent pathway in human gingival fibroblasts.

The C2 domains of classical and novel PKCs as versatile decoders of membrane signals

The C2 domains of classical and novel protein kinases C play a very important role in decoding signals, which trigger the translocation of these enzymes to the plasma membrane and/or other membrane subcellular compartments. The C2 domain of classical PKCs has a long reputation as a paradigm of protein responding to intracytosolic Ca2+ elevations through a calcium-binding region, where this cation acts as a bridge with the phosphatidylserine located in the inner leaflet of the plasma membrane. However, more recently, it has been discovered that a second site on the C2 domain interacts specifically with the phosphoinositide, PtdIns(4,5)P(2). Furthermore, several in vivo studies have shown that both calcium and PtdIns(4,5)P(2)-interacting regions are essential for the translocation of classical PKCs to the membrane. Other molecules like arachidonic and retinoic acid have also been observed to bind to these domains, modulating the activity of classical PKCs. The C2 domains of novel PKCs, on the other hand, were supposed to play only a secondary role with respect to the C1 domain in the activation process of these enzymes. New insights reveal that these C2 domains may also receive regulatory inputs and play an important role in the localization and activation of these enzymes. In this way, the C2 domain of PKCepsilon has been observed to respond to phosphatidic acid and to act together with the C1 domain in the membrane anchorage and activation of the protein. These domains are also regulated by lipid-independent events like protein-protein interactions and phosphorylation. In this review we will focus in describing the recent findings on structural and functional properties of the C2 domains of PKCs, mainly as lipid-interacting modules able to integrate a wide variety of signals in the cell.

Protein kinase Czeta: not-so-innocent bystander or unusual suspect in kidney transplant rejection?

Antibodies against non-HLA targets are increasingly recognized in the context of transplant rejection. However, their specific role remains largely elusive, as evidence exists supporting both their occurrence as an epiphenomenon and their actual pathogenicity in the rejection process. Sutherland et al. describe protein kinase Czeta as a novel, non-HLA antigenic target in the setting of graft rejection.

Atypical protein kinase C activity in the hypothalamus is required for lipopolysaccharide-mediated sickness responses

By activating the Toll-like receptor 4-nuclear factor-kappaB signal transduction pathway, the bacterial endotoxin lipopolysaccharide (LPS) induces anorexia, weight loss, fever, and other components of the sickness response. By comparison, the hormones leptin and insulin cause anorexia without sickness via a central mechanism involving the phosphatidylinositol-3 kinase signaling pathway. In the current study, we investigated whether a common Toll-like receptor 4 and phosphatidylinositol-3 kinase signaling intermediate, atypical protein kinase Czeta/lambda (aPKC), contributes to changes of energy balance induced by these stimuli. Immunohistochemistry analysis revealed that aPKC is expressed in the arcuate and paraventricular nuclei of the hypothalamus, key sites of leptin, insulin, and LPS action. Although administration of LPS, insulin, and leptin each acutely increased hypothalamic aPKC activity at doses that also reduce food intake, LPS treatment caused over 10-fold greater activation of hypothalamic a PKC signaling than that induced by leptin or insulin. Intracerebroventricular pretreatment with an aPKC inhibitor blocked anorexia induced by LPS but not insulin or leptin. Similarly, LPS-induced hypothalamic inflammation (as judged by induction of proinflammatory cytokine gene expression) and neuronal activation in the paraventricular nucleus (as judged by c-fos induction) were reduced by central aPKC inhibition. Although intracerebroventricular aPKC inhibitor administration also abolished LPS-induced fever, it had no effect on sickness-related hypoactivity or weight loss. We conclude that although hypothalamic aPKC signaling is not required for food intake inhibition by insulin or leptin, it plays a key role in inflammatory anorexia and fever induced by LPS.

Aurora A moonlights in neurite extension

Aurora A, an integral mitotic kinase, is essential for microtubule dynamics of post-mitotic neurons. PKCzeta activates Aurora A, which in turn phosphorylates NDEL1 to promote neurite extension. This raises the possibility that Aurora A may also be involved in establishing cell polarity and axon/dendrite elaboration in young neurons.

Of the atypical PKCs, Par-4 and p62: recent understandings of the biology and pathology of a PB1-dominated complex

The recent identification of a novel protein-protein interaction module, termed PB1, in critical signaling molecules such as p62 (also known as sequestosome1), the atypical PKCs, and Par-6, has unveiled the existence of a new set of signaling complexes, which can be central to several biological processes from development to cancer. In this review, we will discuss the most recent advances on the role that the different components of these complexes have in vivo and that are relevant to human disease. In particular, we will review what we are learning from new data from knockout mice, and the indications from human mutations on the real role of these proteins in the physiology and biology of human diseases. The role that PKCzeta, PKClambda/iota, and Par-4 have in lung and prostate cancer in vivo and in humans will be extensively covered in this article, as will the multifunctional role of p62 as a novel hub in cell signaling during cancer and inflammation, and the mechanistic details and controversial data published on its potential role in aggregate formation and signaling. All this published information is shedding new light on the proposed pathological implications of these PB1-regulators in disease and shows their important role in cell physiology.

Suppressive and pro-inflammatory roles for IL-4 in the pathogenesis of experimental drug-induced liver injury

The pathogenesis of immune-mediated drug-induced liver injury (DILI) following halogenated anesthetics, carbamazepine or alcohol has not been fully elucidated. Detecting cytochrome P450 2E1 (CYP2E1) IgG4 auto-antibodies in anesthetic DILI patients suggests a role for IL-4 in this hapten-mediated process. We investigated IL-4-mediated mechanisms using our model of experimental DILI induced by immunizing BALB/c (WT) and IL-4(-/-) (KO) mice with S100 liver proteins covalently modified by a trifluoroacetyl chloride (TFA) hapten formed following halogenated anesthetic metabolism by CYP2E1. WT mice developed more hepatitis, TFA and S100 antibodies (p<0.01), as well as T-cell proliferation to CYP2E1 and TFA (p<0.01) than KO mice. Additionally, WT CD4(+) T cells adoptively transferred hepatitis to naïve Rag(-/-) mice (p<0.01). Pro-inflammatory cytokines were expectedly decreased in TFA hapten-stimulated KO splenocyte supernatants (p<0.001); however, IL-2 and IFN-gamma (p<0.05), as well as IL-6 and IL-10 (p<0.001) levels were elevated in CYP2E1-stimulated KO splenocyte supernatants, suggesting dual IL-4-mediated pro-inflammatory and regulatory responses. Anti-IL-10 administered to KO mice increased hepatitis, TFA and CYP2E1 antibodies in KO mice confirming a critical role for IL-4. This is the first demonstration of dual roles for IL-4 in the pathogenesis of immune-mediated DILI by suppressing auto-antigen-induced regulatory responses while promoting hapten-induced pro-inflammatory responses.

Identification of the serine 307 of LKB1 as a novel phosphorylation site essential for its nucleocytoplasmic transport and endothelial cell angiogenesis

LKB1, a master kinase that controls at least 13 downstream protein kinases including the AMP-activated protein kinase (AMPK), resides mainly in the nucleus. A key step in LKB1 activation is its export from the nucleus to the cytoplasm. Here, we identified S307 of LKB1 as a putative novel phosphorylation site which is essential for its nucleocytoplasmic transport. In a cell-free system, recombinant PKC-zeta phosphorylates LKB1 at S307. AMPK-activating agents stimulate PKC-zeta activity and LKB1 phosphorylation at S307 in endothelial cells, hepatocytes, skeletal muscle cells, and vascular smooth muscle cells. Like the kinase-dead LKB1 D194A mutant (mutation of Asp194 to Ala), the constitutively nucleus-localized LKB1 SL26 mutant and the LKB1 S307A mutant (Ser307 to Ala) exhibit a decreased association with STRAD alpha. Interestingly, the PKC-zeta consensus sequence surrounding LKB1 S307 is disrupted in the LKB1 SL26 mutant, thus providing a likely molecular explanation for this mutation causing LKB1 dysfunction. In addition, LKB1 nucleocytoplasmic transport and AMPK activation in response to peroxynitrite are markedly reduced by pharmacological inhibition of CRM1, which normally facilitates nuclear export of LKB1-STRAD complexes. In comparison to the LKB1 wild type, the S307A mutant complexes show reduced association with CRM1. Finally, adenoviral overexpression of wild-type LKB1 suppresses, while the LKB1 S307A mutant increases, tube formation and hydrogen peroxide-enhanced apoptosis in cultured endothelial cells. Taken together, our results suggest that, in multiple cell types the signaling pathways engaged by several physiological stimuli converge upon PKC-zeta-dependent LKB1 phosphorylation at S307, which directs the nucleocytoplasmic transport of LKB1 and consequent AMPK activation.

Nephrin deficiency activates NF-kappaB and promotes glomerular injury

Increasing evidence implicates activation of NF-kappaB in a variety of glomerular diseases, but the mechanisms involved are unknown. Here, upregulation of NF-kappaB in the podocytes of transgenic mice resulted in glomerulosclerosis and proteinuria. Absence of the podocyte protein nephrin resulted in NF-kappaB activation, suggesting that nephrin negatively regulates the NF-kappaB pathway. Signal transduction assays supported a functional relationship between nephrin and NF-kappaB and suggested the involvement of atypical protein kinase C (aPKCzeta/lambda/iota) as an intermediary. We propose that disruption of the slit diaphragm leads to activation of NF-kappaB; subsequent upregulation of NF-kappaB-driven genes results in glomerular damage mediated by NF-kappaB-dependent pathways. In summary, nephrin may normally limit NF-kappaB activity in the podocyte, suggesting a mechanism by which it might discourage the evolution of glomerular disease.

Phosphorylation of TRAF2 within its RING domain inhibits stress-induced cell death by promoting IKK and suppressing JNK activation

Tumor necrosis factor (TNF) receptor-associated factor 2 (TRAF2) is an adaptor protein that modulates the activation of the c-Jun NH(2) terminal kinase (JNK)/c-Jun and IkappaB kinase (IKK)/nuclear factor-kappaB (NF-kappaB) signaling cascades in response to TNFalpha stimulation. Although many serine/threonine kinases have been implicated in TNFalpha-induced IKK activation and NF-kappaB-dependent gene expression, most of them do not directly activate IKK. Here, we report that protein kinase Czeta phosphorylates TRAF2 at Ser(55), within the RING domain of the protein, after TNFalpha stimulation. Although this phosphorylation event has a minimal effect on induction of the immediate/transient phase of IKK and JNK activation by TNFalpha, it promotes the secondary/prolonged phase of IKK activation and inhibits that of JNK. Importantly, constitutive TRAF2 phosphorylation increased both basal and inducible NF-kappaB activation and rendered Ha-Ras-V12-transformed cells resistant to stress-induced apoptosis. Moreover, TRAF2 was found to be constitutively phosphorylated in some malignant cancer cell lines and Hodgkin's lymphoma. These results reveal a new level of complexity in TNFalpha-induced IKK activation modulated by TRAF2 phosphorylation and suggest that TRAF2 phosphorylation is one of the events that are responsible for elevated basal NF-kappaB activity in certain human cancers.

Par complex in cancer: a regulator of normal cell polarity joins the dark side

In the past 20 years, the discovery and characterization of the molecular machinery that controls cellular polarization have enabled us to achieve a better understanding of many biological processes. Spatial asymmetry or establishment of cell polarity during embryogenesis, epithelial morphogenesis, neuronal differentiation, and migration of fibroblasts and T cells are thought to rely on a small number of evolutionarily conserved proteins and pathways. Correct polarization is crucial for normal cell physiology and tissue homeostasis, and is lost in cancer. Thus, cell polarity signaling is likely to have an important function in tumor progression. Recent findings have identified a regulator of cell polarity, the Par complex, as an important signaling node in tumorigenesis. In normal cell types, the Par complex is part of the molecular machinery that regulates cell polarity and maintains normal cell homeostasis. As such, the polarity regulators are proposed to have a tumor suppressor function, consistent with the loss of polarity genes associated with hyperproliferation in Drosophila melanogaster. However, recent studies showing that some members of this complex also display pro-oncogenic activities suggest a more complex regulation of the polarity machinery during cellular transformation. Here, we examine the existing data about the different functions of the Par complex. We discuss how spatial restriction, binding partners and substrate specificity determine the signaling properties of Par complex proteins. A better understanding of these processes will very likely shed some light on how the Par complex can switch from a normal polarity regulation function to promotion of transformation downstream of oncogenes.

Protein kinase C isoforms: Multi-functional regulators of cell life and death

The protein kinase C (PKC) family consists of 10 related serine/threonine protein kinases some of which are critical regulators of cell proliferation, survival and cell death. While early studies relied on broad spectrum chemical activators or inhibitors of this family, the generation of isoform specific tools has greatly facilitated our understanding of the contribution of specific PKC isoforms to cell proliferation and apoptosis. These studies suggest that PKC-alpha, PKC-epsilon, and the atypical PKC's, PKC-lambda/iota and PKC-zeta, preferentially function to promote cell proliferation and survival, while the novel isoform, PKC-delta is an important regulator of apoptosis. The essential role of this kinase family in both cell survival and apoptosis suggests that specific isoforms may function as molecular sensors, promoting cell survival or cell death depending on environmental cues. Given their central role in cell and tissue homeostasis, it is not surprising that the expression or activity of some of these kinases is altered in human diseases, particularly cancer.

2-(6-Phenyl-1H-indazol-3-yl)-1H-benzo[d]imidazoles: design and synthesis of a potent and isoform selective PKC-zeta inhibitor

The inhibition of PKC-zeta has been proposed to be a potential drug target for immune and inflammatory diseases. A series of 2-(6-phenyl-1H indazol-3-yl)-1H-benzo[d]imidazoles with initial high crossover to CDK-2 has been optimized to afford potent and selective inhibitors of protein kinase c-zeta (PKC-zeta). The determination of the crystal structures of key inhibitor:CDK-2 complexes informed the design and analysis of the series. The most selective and potent analog was identified by variation of the aryl substituent at the 6-position of the indazole template to give a 4-NH(2) derivative. The analog displays good selectivity over other PKC isoforms (alpha, betaII, gamma, delta, epsilon, mu, theta, eta and iota/lambda) and CDK-2, however it displays marginal selectivity against a panel of other kinases (37 profiled).

Structural basis of protein kinase C isoform function

Protein kinase C (PKC) isoforms comprise a family of lipid-activated enzymes that have been implicated in a wide range of cellular functions. PKCs are modular enzymes comprised of a regulatory domain (that contains the membrane-targeting motifs that respond to lipid cofactors, and in the case of some PKCs calcium) and a relatively conserved catalytic domain that binds ATP and substrates. These enzymes are coexpressed and respond to similar stimulatory agonists in many cell types. However, there is growing evidence that individual PKC isoforms subserve unique (and in some cases opposing) functions in cells, at least in part as a result of isoform-specific subcellular compartmentalization patterns, protein-protein interactions, and posttranslational modifications that influence catalytic function. This review focuses on the structural basis for differences in lipid cofactor responsiveness for individual PKC isoforms, the regulatory phosphorylations that control the normal maturation, activation, signaling function, and downregulation of these enzymes, and the intra-/intermolecular interactions that control PKC isoform activation and subcellular targeting in cells. A detailed understanding of the unique molecular features that underlie isoform-specific posttranslational modification patterns, protein-protein interactions, and subcellular targeting (i.e., that impart functional specificity) should provide the basis for the design of novel PKC isoform-specific activator or inhibitor compounds that can achieve therapeutically useful changes in PKC signaling in cells.

Kinome-wide RNAi screen implicates at least 5 host hepatocyte kinases in Plasmodium sporozoite infection

Plasmodium sporozoites, the causative agent of malaria, are injected into their vertebrate host through the bite of an infected Anopheles mosquito, homing to the liver where they invade hepatocytes to proliferate and develop into merozoites that, upon reaching the bloodstream, give rise to the clinical phase of infection. To investigate how host cell signal transduction pathways affect hepatocyte infection, we used RNAi to systematically test the entire kinome and associated genes in human Huh7 hepatoma cells for their potential roles during infection by P. berghei sporozoites. The three-phase screen covered 727 genes, which were tested with a total of 2,307 individual siRNAs using an automated microscopy assay to quantify infection rates and qRT-PCR to assess silencing levels. Five protein kinases thereby emerged as top hits, all of which caused significant reductions in infection when silenced by RNAi. Follow-up validation experiments on one of these hits, PKCς (PKCzeta), confirmed the physiological relevance of our findings by reproducing the inhibitory effect on P. berghei infection in adult mice treated systemically with liposome-formulated PKCς-targeting siRNAs. Additional cell-based analyses using a pseudo-substrate inhibitor of PKCς added further RNAi-independent support, indicating a role for host PKCς on the invasion of hepatocytes by sporozoites. This study represents the first comprehensive, functional genomics-driven identification of novel host factors involved in Plasmodium sporozoite infection.

During a mammalian malaria infection, Plasmodium sporozoites injected by an infected mosquito travel to the liver where they invade hepatocytes and multiply into thousands of new parasites. These newly formed merozoites are then released into the bloodstream where they infect red blood cells and cause the symptoms of the disease. Although asymptomatic, the liver stage of malaria is an obligatory step in the parasite's lifecycle and constitutes an appealing target for prophylatic intervention. The marked tropism of sporozoites for hepatocytes suggests the latter may provide the parasite with a molecular environment that it can exploit to its own benefit. The identification of host factors that influence hepatic infection can thus provide clues for potential anti-malarial strategies. To this end, we carried out an RNA interference screen of the entire human kinome and associated signaling molecules and assessed the effect of knockdown of their expression in the infection of a human hepatoma cell line by Plasmodium. This strategy identified at least 5 kinases whose down-regulation leads to a marked decrease in infection. Further characterisation of one of these proteins, PKCζ, confirmed that it plays a role in infection by influencing the parasite's invasion of the host liver cells.

Protein kinase Czeta represses the interleukin-6 promoter and impairs tumorigenesis in vivo

Gene alterations in tumor cells that confer the ability to grow under nutrient- and mitogen-deficient conditions constitute a competitive advantage that leads to more-aggressive forms of cancer. The atypical protein kinase C (PKC) isoform, PKCzeta, has been shown to interact with the signaling adapter p62, which is important for Ras-induced lung carcinogenesis. Here we show that PKCzeta-deficient mice display increased Ras-induced lung carcinogenesis, suggesting a new role for this kinase as a tumor suppressor in vivo. We also show that Ras-transformed PKCzeta-deficient lungs and embryo fibroblasts produced more interleukin-6 (IL-6), which we demonstrate here plays an essential role in the ability of Ras-transformed cells to grow under nutrient-deprived conditions in vitro and in a mouse xenograft system in vivo. We also show that PKCzeta represses histone acetylation at the C/EBPbeta element in the IL-6 promoter. Therefore, PKCzeta, by controlling the production of IL-6, is a critical signaling molecule in tumorigenesis.

Loss of protein kinase Cϵ results in impaired cutaneous wound closure and myofibroblast function

Cutaneous wound repair requires the de novo induction of a specialized form of fibroblast, the α-smooth muscle actin (α-SMA)-expressing myofibroblast, which migrates into the wound where it adheres to and contracts extracellular matrix (ECM), resulting in wound closure. Persistence of the myofibroblast results in scarring and fibrotic disease. In this report, we show that, compared with wild-type littermates, PKCϵ-/- mice display delayed impaired cutaneous wound closure and a reduction in myofibroblasts. Moreover, both in the presence and absence of TGFβ, dermal fibroblasts from PKCϵ-/- mice cultured on fibronectin show impaired abilities to form `supermature' focal adhesions and α-SMA stress fibers, and reduced pro-fibrotic gene expression. Smad3 phosphorylation in response to TGFβ1 was impaired in PKCϵ-/- fibroblasts. PKCϵ-/- fibroblasts show reduced FAK and Rac activation, and adhesive, contractile and migratory abilities. Overexpressing constitutively active Rac1 rescues the defective FAK phosphorylation, cell migration, adhesion and stress fiber formation of these PKCϵ-/- fibroblasts, indicating that Rac1 operates downstream of PKCϵ, yet upstream of FAK. These results suggest that loss of PKCϵ severely impairs myofibroblast formation and function, and that targeting PKCϵ may be beneficial in selectively modulating wound healing and fibrotic responses in vivo.

HIV-1-Tat potentiates CXCL12/stromal cell-derived factor 1-induced downregulation of membrane CXCR4 in T lymphocytes through protein kinase C zeta

We have investigated the role of intracellular HIV-1 Tat on CXCR4 expression on T cells. We found that stable or doxycycline-regulated expression of HIV-1 Tat on Jurkat T cells results in lower cell surface expression of CXCR4, but not of other chemokine receptors. This effect was not due to an alteration in CXCR4 transcription, and total CXCR4 levels remained unaltered. Rather, when cells were treated with CXCL12/Stromal Cell-Derived Factor 1, a faster downmodulation of CXCR4 was observed although resurfacing was unaffected. Similar effect was seen in peripheral human T cells transiently transfected with Tat. At the molecular level Tat did not alter cellular levels of G-coupled receptor kinases 2 and 6 and beta-arrestin, proteins involved in CXCR4 downregulation. Neither Tat significantly affected phosphatidylinositol 3-kinase activation in response to CXCL12. Interestingly, in Jurkat cell clones stably expressing both Protein kinase (PK)-Czeta and HIV-1 Tat, CXCL12 induced a faster CXCR4 internalization than in cells only expressing HIV-1 Tat. In contrast in Jurkat cell stably expressing a dominant negative PKCzeta, Tat enhancement of CXCR4 internalization was abrogated. Thus, our results show a new function of HIV-1 Tat, its ability to regulate CXCR4 expression via PKCzeta. The significance of those results is discussed.

Par-4 inhibits Akt and suppresses Ras-induced lung tumorigenesis

The atypical PKC-interacting protein, Par-4, inhibits cell survival and tumorigenesis in vitro, and its genetic inactivation in mice leads to reduced lifespan, enhanced benign tumour development and low-frequency carcinogenesis. Here, we demonstrate that Par-4 is highly expressed in normal lung but reduced in human lung cancer samples. We show, in a mouse model of lung tumours, that the lack of Par-4 dramatically enhances Ras-induced lung carcinoma formation in vivo, acting as a negative regulator of Akt activation. We also demonstrate in cell culture, in vivo, and in biochemical experiments that Akt regulation by Par-4 is mediated by PKCzeta, establishing a new paradigm for Akt regulation and, likely, for Ras-induced lung carcinogenesis, wherein Par-4 is a novel tumour suppressor.

Mycobacterium tuberculosis lipoprotein-induced association of TLR2 with protein kinase C zeta in lipid rafts contributes to reactive oxygen species-dependent inflammatory signalling in macrophages

Membrane lipid rafts are enriched in cholesterol and play an important role as signalling platforms. However, the roles of lipid rafts and associated signalling molecules in the innate immune responses to mycobacteria remain unknown. Here we show that stimulation with Mycobacterium tuberculosis 19 kDa lipoprotein, a TLR2/1 agonist, results in translocation of TLR2 to lipid rafts, coalescence of lipid rafts and production of reactive oxygen species (ROS) that drive pro-inflammatory responses. Disruption of lipid raft organization markedly reduced lipoprotein-induced ROS and inflammatory responses. Remarkably, the atypical protein kinase C (PKC) zeta was specifically recruited into detergent-resistant membrane fractions and associated with TLR2. PKCzeta activity was critical for lipoprotein-dependent ROS generation, raft coalescence and the pro-inflammatory responses by macrophages. Moreover, lipid raft organization was required for 19 kDa mediated PKCzeta activation. These results demonstrate that TLR2 trafficking and raft coalescence play an essential role for the initiation of lipoprotein-induced innate immune responses via TLR2 and ROS signalling. In addition, PKCzeta targets to lipid rafts and may act as a critical adaptor molecule to regulate lipid raft dynamics during TLR2 signalling.

The actin cytoskeleton in T cell activation

T cell cytoarchitecture differs dramatically depending on whether the cell is circulating within the bloodstream, migrating through tissues, or interacting with antigen-presenting cells. The transition between these states requires important signaling-dependent changes in actin cytoskeletal dynamics. Recently, analysis of actin-regulatory proteins associated with T cell activation has provided new insights into how T cells control actin dynamics in response to external stimuli and how actin facilitates downstream signaling events and effector functions. Among the actin-regulatory proteins that have been identified are nucleation-promoting factors such as WASp, WAVE2, and HS1; severing proteins such as cofilin; motor proteins such as myosin II; and linker proteins such as ezrin and moesin. We review the current literature on how signaling pathways leading from diverse cell surface receptors regulate the coordinated activity of these and other actin-regulatory proteins and how these proteins control T cell function.

Ceramide signaling in cancer and stem cells

Most of the previous work on the sphingolipid ceramide has been devoted to its function as an apoptosis inducer. Recent studies, however, have shown that in stem cells, ceramide has additional nonapoptotic functions. In this article, ceramide signaling will be reviewed in light of 'systems interface biology': as an interconnection of sphingolipid metabolism, membrane biophysics and cell signaling. The focus will be on the metabolic interconversion of ceramide and sphingomyelin or sphingosine-1-phosphate. Lipid rafts and sphingolipid-induced protein scaffolds will be discussed as a membrane interface for lipid-controlled cell signaling. Ceramide/sphingomyelin and ceramide/sphingosine-1-phosphate-interdependent cell-signaling pathways are significant for the regulation of cell polarity, apoptosis and/or proliferation, and as novel pharmacologic targets in cancer and stem cells.

Cyclosporin A up-regulates and activates protein kinase C-zeta in EBV-infected and EBV-transformed human B-cells

BACKGROUND

Protein Kinase C (PKC) is a family of enzymes that plays a key role in cell signaling pathways leading to cellular activation and proliferation. Conventional PKC (cPKC) is dependent on calcium for activation. We have proposed that cyclosporin A (CsA), despite being a calcineurin inhibitor, will activate PKC in B cells, thus promoting Epstein-Barr virus (EBV)-induced transformation. Here we show that CsA promoted atypical PKC isoform PKC-zeta in B cells.

MATERIALS AND METHODS

Western-blot was used to assay PKC-zeta protein level in EBV-B cells. Confocal microscopy was used to assay PKC-zeta translocation from cytosol to cell membrane, a known process of PKC activation.

RESULTS

CsA (500 ng/mL) time dependently increased PKC-zeta from control of 7055 units to 7145, 10,805, 10,914, and 12,705 units, respectively, after 15 min, 1 h, 12 h, and 24 h of incubation in EBV-transformed human B-cell line (LCL). CsA increased PKC-zeta expression was inhibited 50% by Vit.E (40 microM) indicating that this effect may be due to oxidative stress induced by CsA. Indeed, after oxidant H(2)O(2) (0.1 mM) treatment, PKC-zeta protein level in LCL cells increased 124%, 257%, 349%, and 359% after 15 min, 1 h, 12 h, and 24 h of culture compared with control. Addition of Vit.E (40 microM) in H(2)O(2) (0.1 mM) treatment and then with Vit.E in the culture decreased PKC-zeta level in LCL cells 26%, 20%, 41%, and 60% after 15 min, 1 h, 12 h, and 24 h of culture. In confocal microscopy in Jurkat T cell line, phorbol 12-myristate 13-acetate (PMA) activated cPKC isoform PKCalpha after 30 min treatment and activated PKC-zeta after 60 min treatment. CsA inhibited PMA activation of PKC-alpha, but not PKC-zeta. CsA alone did not activate PKC-alpha or PKC-zeta in Jurkat T cells. In LCL and in EBV-infected human B-cells, PMA stimulated PKC-alpha activation after 30 min treatment and stimulated PKC-zeta activation after 60 min treatment. CsA inhibited PMA activation of PKC-alpha, but not PKC-zeta. In addition, CsA activated PKC-zeta in the EBV-transformed and EBV-infected human B cells.

CONCLUSION

These experiments show that CsA-induced oxidative stress caused PKC-zeta up-regulation in LCL cells, and show the differential effect of CsA in the PKC signaling pathways in T cells versus B cells. CsA-induced PKC-zeta activation may be an important signaling step in EBV-induced post-transplant lymphoproliferative disorders.

Activation of keratinocyte protein kinase C zeta in psoriasis plaques

PKCzeta (protein kinase C-zeta), a member of protein kinase C family, plays an important role in cell proliferation, differentiation, and apoptosis. It acts as a downstream molecule for TNF-alpha (tumor necrosis factor) signal transduction and also regulates the expression of CD1d, an HLA-class I-like molecule. The interaction of CD1d with natural killer T (NKT) cells has been shown to be important in their Th1 cytokine production in psoriasis. In this study, we examined PKCzeta in psoriasis in order to define its role in the pathogenesis of the disease. We found that T-cell receptor (TCR) V alpha24+ V beta11+ NKT cells and CD1d molecules within psoriatic skin were increased. Moreover, there was an associated increase in PKCzeta mRNA and protein expression with membrane translocation in psoriasis lesions compared to uninvolved skin. Furthermore, cultured keratinocytes exhibited increased PKCzeta activity and membrane translocation upon stimulation by TNF-alpha, a cytokine known to play an important role in the pathogenesis of psoriasis. These results implied that PKCzeta is an important transduction molecule downstream of TNF-alpha signaling and is associated with increased expression of CD1d that may enhance CD1d-NKT cell interactions in psoriasis lesions. This makes PKCzeta a tempting target for possible pharmacological intervention in modifying the downstream effects of TNF-alpha in psoriasis.

PKCtheta promotes c-Rel-driven mammary tumorigenesis in mice and humans by repressing estrogen receptor alpha synthesis

The vast majority of primary human breast cancer tissues display aberrant nuclear NF-kappaB c-Rel expression. A causal role for c-Rel in mammary tumorigenesis has been demonstrated using a c-Rel transgenic mouse model; however, tumors developed with a long latency, suggesting a second event is needed to trigger tumorigenesis. Here we show that c-Rel activity in the mammary gland is repressed by estrogen receptor alpha (ERalpha) signaling, and we identify an epigenetic mechanism in breast cancer mediated by activation of what we believe is a novel PKCtheta-Akt pathway that leads to downregulation of ERalpha synthesis and derepression of c-Rel. ERalpha levels were lower in c-Rel-induced mammary tumors compared with normal mammary gland tissue. PKCtheta induced c-Rel activity and target gene expression and promoted growth of c-Rel- and c-RelxCK2alpha-driven mouse mammary tumor-derived cell lines. RNA expression levels of PKCtheta and c-Rel target genes were inversely correlated with ERalpha levels in human breast cancer specimens. PKCtheta activated Akt, thereby inactivating forkhead box O protein 3a (FOXO3a) and leading to decreased synthesis of its target genes, ERalpha and p27(Kip1). Thus we have shown that activation of PKCtheta inhibits the FOXO3a/ERalpha/p27(Kip1) axis that normally maintains an epithelial cell phenotype and induces c-Rel target genes, thereby promoting proliferation, survival, and more invasive breast cancer.

PKC zeta controls DNA topoisomerase-dependent human caspase-2 pre-mRNA splicing

Caspase-2 exists as two main isoforms: the caspase-2L long isoform, which is pro-apoptotic, and the caspase-2S short isoform, which may be anti-apoptotic. Topoisomerase inhibitors drive inclusion of exon 9, specific for Casp-2S mRNA, and lower Casp-2L [corrected] mRNA and protein. With cell lines engineered to express various PKC isoforms, we demonstrate that PKC zeta, but not PKCalpha, positively regulates Casp-2S mRNA assembly triggered by topoisomerase inhibitors. In addition, exon 9 inclusion is lowered in mitosis but increased in the G1/S phase. Hence, the control of caspase-2 exon 9 inclusion by topoisomerase inhibitors depends on phosphorylation and/or dephosphorylation events, and on the cell cycle phase.

TNFR1 and TNFR2 Signaling Interplay in Cardiac Myocytes

Tumor necrosis factor alpha (TNFalpha) plays a major role in chronic heart failure, signaling through two different receptor subtypes, TNFR1 and TNFR2. Our aim was to further delineate the functional role and signaling pathways related to TNFR1 and TNFR2 in cardiac myocytes. In cardiac myocytes isolated from control rats, TNFalpha induced ROS production, exerted a dual positive and negative action on [Ca(2+)] transient and cell fractional shortening, and altered cell survival. Neutralizing anti-TNFR2 antibodies exacerbated TNFalpha responses on ROS production and cell death, arguing for a major protective role of the TNFR2 pathway. Treatment with either neutralizing anti-TNFR1 antibodies or the glutathione precursor, N-acetylcysteine (NAC), favored the emergence of TNFR2 signaling that mediated a positive effect of TNFalpha on [Ca(2+)] transient and cell fractional shortening. The positive effect of TNFalpha relied on TNFR2-dependent activation of the cPLA(2) activity, independently of serine 505 phosphorylation of the enzyme. Together with cPLA(2) redistribution and AA release, TNFalpha induced a time-dependent phosphorylation of ERK, MSK1, PKCzeta, CaMKII, and phospholamban on the threonine 17 residue. Taken together, our results characterized a TNFR2-dependent signaling and illustrated the close interplay between TNFR1 and TNFR2 pathways in cardiac myocytes. Although apparently predominant, TNFR1-dependent responses were under the yoke of TNFR2, acting as a critical limiting factor. In vivo NAC treatment proved to be a unique tool to selectively neutralize TNFR1-mediated effects of TNFalpha while releasing TNFR2 pathways.

Biliverdin reductase: PKC interaction at the cross-talk of MAPK and PI3K signaling pathways

Biliverdin reductase (BVR) was characterized some 25 years ago as a unique dual-cofactor/pH-dependent enzyme that catalyzes the reduction of biliverdin-IXa. Our knowledge of functions of BVR has increased enormously in recent years. hBVR functions in the IR/IGF-1-controlled regulation of the MAPK and PI3K cascades that are linked by the PKC enzymes. The first of the two culminates in the activation of transcription factors for oxidative stress-responsive genes, including ho-1, where BVR functions as both a bZip (basic leucine zipper) transcription factor and a kinase. The second pathway amplifies the insulin/growth-factor signal for protein/DNA synthesis and glucose transport downstream of PI3K. hBVR is a transactivator of PKC-betaII, and thus an integral component of the "activation loop" linking MAPK, PKC-betaII, and PI3K to insulin/growth-factor signaling. The emergence of biliverdin and bilirubin as a newly defined category of modulators of cell signaling and kinase activity further underscores the critical input of hBVR in the response of intracellular pathways into the external environment. Structural features of BVR and recent findings relevant to its function in cell-signaling pathways are reviewed here and are intended to complement a recent commentary on the role of BVR in linking heme metabolism and cell signaling.

Regulation of TNF-alpha-activated PKC-zeta signaling by the human biliverdin reductase: identification of activating and inhibitory domains of the reductase

Human biliverdin reductase (hBVR) is a dual function enzyme: a catalyst for bilirubin formation and a S/T/Y kinase that shares activators with protein kinase C (PKC) -zeta, including cytokines, insulin, and reactive oxygen species (ROS). Presently, we show that hBVR increases PKC-zeta autophosphorylation, stimulation by TNF-alpha, as well as cytokine stimulation of NF-kappaB DNA binding and promoter activity. S149 in hBVR S/T kinase domain and S230 in YLS230F in hBVR's docking site for the SH2 domain of signaling proteins are phosphorylation targets of PKC-zeta. Two hBVR-based peptides, KRNRYLS230F (#1) and KKRILHC281 (#2), but not their S-->A or C-->A derivatives, respectively, blocked PKC-zeta stimulation by TNF-alpha and its membrane translocation. The C-terminal-based peptide KYCCSRK296 (#3), enhanced PKC-zeta stimulation by TNF-alpha; for this, Lys296 was essential. In metabolically 32P-labeled HEK293 cells transfected with hBVR or PKC-zeta, TNF-alpha increased hBVR phosphorylation. TNF-alpha did not stimulate PKC-zeta in cells infected with small interfering RNA for hBVR or transfected with hBVR with a point mutation in the nucleotide-binding loop (G17), S149, or S230; this was similar to the response of "kinase-dead" PKC-zeta(K281R). We suggest peptide #1 blocks PKC-zeta-docking site interaction, peptide #2 disrupts function of the PKC-zeta C1 domain, and peptide #3 alters ATP presentation to the kinase. The findings are of potential significance for development of modulators of PKC-zeta activity and cellular response to cytokines.

Flow Antagonizes TNF-α Signaling in Endothelial Cells by Inhibiting Caspase-Dependent PKCζ Processing

Unidirectional laminar flow is atheroprotective, in part by inhibiting cytokine-mediated endothelial cell (EC) inflammation and apoptosis. Previously, we showed that flow inhibited TNF-α signaling by preventing activation of JNK. Recently, PKCζ was identified as the PKC isoform most strongly regulated by flow pattern, with increased PKCζ activity in regions of disturbed flow versus unidirectional flow. Interestingly, PKCζ is cleaved by caspases after TNF-α stimulation to generate a 50-kDa truncated form (CATζ, catalytic domain of PKCζ) with a higher kinase activity than the full-length protein. We hypothesized that flow would inhibit TNF-α–mediated PKCζ cleavage and thereby CATζ formation. We found that PKCζ activity was required for TNF-α–mediated JNK and caspase-3 activation in ECs. PKCζ was rapidly cleaved to generate CATζ in cultured bovine and human aortic ECs and in intact rabbit vessels stimulated with TNF-α. This truncated form of PKCζ enhanced JNK and caspase-3 activation. Interestingly, PKCζ cleavage was prevented by inhibitors of PKCζ, JNK, and caspase activities, suggesting that these enzymes, via regulating CATζ formation, modulate caspase-3 activity in ECs. Finally, we found that flow reduced caspase-dependent processing of PKCζ and caspase-3 activation. These results define a novel role for PKCζ as a shared signaling mediator for flow and TNF-α, and important for flow-mediated inhibition of proinflammatory and apoptotic events in ECs.

Induction of nitric oxide synthase and activation of signaling proteins in Anopheles mosquitoes by the malaria pigment, hemozoin

Anopheles stephensi, a major vector for malaria parasite transmission, responds to Plasmodium infection by synthesis of inflammatory levels of nitric oxide (NO), which can limit parasite development in the midgut. We have previously shown that Plasmodium falciparum glycosylphosphatidylinositols (PfGPIs) can induce A. stephensi NO synthase (AsNOS) expression in the midgut epithelium in vivo in a manner similar to the manner in which cytokines and NO are induced by PfGPIs in mammalian cells. In mosquito cells, signaling by PfGPIs and P. falciparum merozoites is mediated through Akt/protein kinase B (Akt/PKB), the mitogen-activated protein kinase kinase DSOR1, and extracellular signal-regulated kinase (ERK). In mammalian cells, a second parasite factor, malaria pigment or hemozoin (Hz), signals NOS induction through ERK- and nuclear factor kappa B-dependent pathways and has been demonstrated to be a novel proinflammatory ligand for Toll-like receptor 9. In this study, we demonstrate that Hz can also induce AsNOS gene expression in immortalized A. stephensi and Anopheles gambiae cell lines in vitro and in A. stephensi midgut tissue in vivo. In mosquito cells, Hz signaling is mediated through transforming growth factor beta-associated kinase 1, Akt/PKB, ERK, and atypical protein kinase C zeta/lambda. Our results show that Hz is a prominent parasite-derived signal for Anopheles and that signaling pathways activated by PfGPIs and Hz have both unique and shared components. Together with our previous findings, our data indicate that parasite signaling of innate immunity is conserved in mosquito and mammalian cells.

Ras oncogenes and their downstream targets

RAS proteins are small GTPases, which serve as master regulators of a myriad of signaling cascades involved in highly diverse cellular processes. RAS oncogenes have been originally discovered as retroviral oncogenes, and ever since constitutively activating RAS mutations have been identified in human tumors, they are in the focus of intense research. In this review, we summarize the biochemical properties of RAS proteins, trace down the evolution of RAS signaling and present an overview of the spatio-temporal activation of major RAS isoforms. We further discuss RAS effector pathways, their role in normal and transformed cell physiology and summarize ongoing attempts to interfere with aberrant RAS signaling. Finally, we comment on the role of micro RNAs in modulating RAS expression, contribution of RAS to stem cell function and on high-throughput analyses of RAS signaling networks.

Role of aPKC isoforms and their binding partners Par3 and Par6 in epidermal barrier formation

The skin water barrier, essential for terrestrial life, is formed by a multilayered stratifying epithelium, which shows a polarized distribution of both differentiation and intercellular junction markers. Recently, several reports showed the crucial importance of tight junctions for the in vivo water barrier function of the skin. In simple epithelial cells, intercellular junction formation is closely coupled to the establishment of polarity. However, if and how polarity proteins contribute to epidermal differentiation and junction formation is not yet known. Here, we have characterized the localization and isoform expression of the polarity protein atypical PKC (aPKC) and its binding partners Par3 and Par6 in epidermis and primary keratinocytes of mice. Their distribution is only partially overlapping in the granular layer, the site of functional tight junctions, suggesting that next to a common Par3/Par6/aPKC function they also may have functions independent of each other. Both aPKCzeta and aPKCiota/lambda, are expressed in the epidermis but only aPKCiota/lambda showed a strong enrichment in the junctions, suggesting that this aPKC isoform is important for epidermal tight junction function. Indeed, inhibition of aPKC function showed that endogenous aPKC is crucial for in vitro barrier function and this required the presence of both the Par3 and Par6 binding sites.

Keeping up NF-κB appearances: Epigenetic control of immunity or inflammation-triggered epigenetics

Controlled expression of cytokine genes is an essential component of an immune response and is crucial for homeostasis. In order to generate an appropriate response to an infectious condition, the type of cytokine, as well as the cell type, dose range and the kinetics of its expression are of critical importance. The nuclear factor-kappaB (NF-kappaB) family of transcription factors has a crucial role in rapid responses to stress and pathogens (innate immunity), as well as in development and differentiation of immune cells (acquired immunity). Although quite a number of genes contain NF-kappaB-responsive elements in their regulatory regions, their expression pattern can significantly vary from both a kinetic and quantitative point of view, reflecting the impact of environmental and differentiative cues. At the transcription level, selectivity is conferred by the expression of specific NF-kappaB subunits and their respective posttranslational modifications, and by combinatorial interactions between NF-kappaB and other transcription factors and coactivators, that form specific enhanceosome complexes in association with particular promoters. These enhanceosome complexes represent another level of signaling integration, whereby the activities of multiple upstream pathways converge to impress a distinct pattern of gene expression upon the NF-kappaB-dependent transcriptional network. Today, several pieces of evidence suggest that the chromatin structure and epigenetic settings are the ultimate integration sites of both environmental and differentiative inputs, determining proper expression of each NF-kappaB-dependent gene. We will therefore discuss in this review the multilayered interplay of NF-kappaB signaling and epigenome dynamics, in achieving appropriate gene expression responses and transcriptional activity.

Cell Signaling and Function Organized by PB1 Domain Interactions

The PB1-domain-containing proteins p62, aPKC, MEKK2/MEKK3, MEK5, and Par-6 play roles in critical cell processes like osteoclastogenesis, angiogenesis, and early cardiovascular development or cell polarity. PB1 domains are scaffold modules that adopt the topology of ubiquitin-like beta-grasp folds that interact with each other in a front-to-back mode to arrange heterodimers or homo-oligomers. The different PB1 domain adaptors provide specificity for PB1 kinases to ensure the effective transmission of cellular signals. Also, recent data suggest that PB1 domains may serve to orchestrate signaling cascades not involving other PB1 domains, such as the MEK5-ERK5 and p62-ERK1 interactions.

Protein kinase C regulatory domains: the art of decoding many different signals in membranes

Protein kinase C (PKC) is a member of a family of Ser/Thr phosphotransferases that are involved in many cellular signaling pathways. These enzymes possess two regulatory domains, C1 and C2, that are the targets of different second messengers. The purpose of this review is to describe in molecular terms the diverse mechanisms of activation of PKCs in the light of very significant advances made in this field over recent years. The role of some critical amino acid residues concerning activation of the enzymes and their location within known structures of isolated domains will be presented. For example, the recently deduced 3D structures of the C2 domains show that these domains can additionally act as PtdIns(4,5)P(2)-binding or phosphotyrosine-binding modules depending on the isoenzyme. All these capacities to play different roles in the cell wide web of signals underline the notion that we are dealing with a multifunctional family of enzymes which, after 30 years of investigation, we are just beginning to understand.

Nuclear factor-kappaB dimer exchange promotes a p21(waf1/cip1) superinduction response in human T leukemic cells

The nuclear factor-kappaB (NF-kappaB)/Rel transcription factors are recognized as critical apoptosis regulators. We reported previously that NF-kappaB contributes to chemoresistance of CEM human T leukemic cells in part through its ability to induce p21(waf1/cip1). Here, we provide evidence that sequential NF-kappaB-activating signals induce heightened NF-kappaB DNA binding and p21(waf1/cip1) induction in CEM and additional T leukemic cell lines. This response arises from exceedingly low basal expression of the p105/p50 NF-kappaB subunit encoded by the NFKB1 gene in these cell lines. An initial NF-kappaB activation event enhances the recruitment of p65 and ELF1 to the NFKB1 promoter, leading to p65- and ELF1-dependent synthesis of p105/p50, which promotes an exchange of NF-kappaB complexes to p50-containing complexes with an increased DNA-binding activity to certain NF-kappaB target elements. Subsequent stimulation of these cells with an anticancer agent, etoposide, results in augmented NF-kappaB-dependent p21(waf1/cip1) induction and increased chemoresistance of the leukemia cells. Thus, we propose that low basal NFKB1 expression coupled with sequential NF-kappaB activation events can promote increased chemoresistance in certain T leukemic cells.