Protein kinase Cdelta mediates insulin-induced glucose transport in primary cultures of rat skeletal muscle

Putative Role of Protein Palmitoylation in Cardiac Lipid-Induced Insulin Resistance

In the heart, inhibition of the insulin cascade following lipid overload is strongly associated with contractile dysfunction. The translocation of fatty acid transporter CD36 (SR-B2) from intracellular stores to the cell surface is a hallmark event in the lipid-overloaded heart, feeding forward to intracellular lipid accumulation. Yet, the molecular mechanisms by which intracellularly arrived lipids induce insulin resistance is ill-understood. Bioactive lipid metabolites (diacyl-glycerols, ceramides) are contributing factors but fail to correlate with the degree of cardiac insulin resistance in diabetic humans. This leaves room for other lipid-induced mechanisms involved in lipid-induced insulin resistance, including protein palmitoylation. Protein palmitoylation encompasses the reversible covalent attachment of palmitate moieties to cysteine residues and is governed by protein acyl-transferases and thioesterases. The function of palmitoylation is to provide proteins with proper spatiotemporal localization, thereby securing the correct unwinding of signaling pathways. In this review, we provide examples of palmitoylations of individual signaling proteins to discuss the emerging role of protein palmitoylation as a modulator of the insulin signaling cascade. Second, we speculate how protein hyper-palmitoylations (including that of CD36), as they occur during lipid oversupply, may lead to insulin resistance. Finally, we conclude that the protein palmitoylation machinery may offer novel targets to fight lipid-induced cardiomyopathy.

Stress Signals, Mediated by Membranous Glucocorticoid Receptor, Activate PLC/PKC/GSK-3β/β-catenin Pathway to Inhibit Wound Closure

Glucocorticoids (GCs), key mediators of stress signals, are also potent wound healing inhibitors. To understand how stress signals inhibit wound healing, we investigated the role of membranous glucocorticoid receptor (mbGR) by using cell-impermeable BSA-conjugated dexamethasone. We found that mbGR inhibits keratinocyte migration and wound closure by activating a Wnt-like phospholipase (PLC)/ protein kinase C (PKC) signaling cascade. Rapid activation of mbGR/PLC/PKC further leads to activation of known biomarkers of nonhealing found in patients, β-catenin and c-myc. Conversely, a selective inhibitor of PKC, calphostin C, blocks mbGR/PKC pathway, and rescues GC-mediated inhibition of keratinocyte migration in vitro and accelerates wound epithelialization of human wounds ex vivo. This novel signaling mechanism may have a major impact on understanding how stress response via GC signaling regulates homeostasis and its role in development and treatments of skin diseases, including wound healing. To test tissue specificity of this nongenomic signaling mechanism, we tested retinal and bronchial human epithelial cells and fibroblasts. We found that mbGR/PLC/PKC signaling cascade exists in all cell types tested, suggesting a more general role. The discovery of this nongenomic signaling pathway, in which glucocorticoids activate Wnt pathway via mbGR, provides new insights into how stress-mediated signals may activate growth signals in various epithelial and mesenchymal tissues.

The phosphatidylethanolamine derivative diDCP-LA-PE mimics intracellular insulin signaling

Insulin facilitates glucose uptake into cells by translocating the glucose transporter GLUT4 towards the cell surface through a pathway along an insulin receptor (IR)/IR substrate 1 (IRS-1)/phosphatidylinositol 3 kinase (PI3K)/3-phosphoinositide-dependent protein kinase-1 (PDK1)/Akt axis. The newly synthesized phosphatidylethanolamine derivative 1,2-O-bis-[8-{2-(2-pentyl-cyclopropylmethyl)-cyclopropyl}-octanoyl]-sn-glycero-3-phosphatidylethanolamine (diDCP-LA-PE) has the potential to inhibit protein tyrosine phosphatase 1B (PTP1B) and to directly activate PKCζ, an atypical isozyme, and PKCε, a novel isozyme. PTP1B inhibition enhanced insulin signaling cascades downstream IR/IRS-1 by preventing tyrosine dephosphorylation. PKCζ and PKCε directly activated Akt2 by phosphorylating at Thr309 and Ser474, respectively. diDCP-LA-PE increased cell surface localization of GLUT4 and stimulated glucose uptake into differentiated 3T3-L1 adipocytes, still with knocking-down IR or in the absence of insulin. Moreover, diDCP-LA-PE effectively reduced serum glucose levels in type 1 diabetes (DM) model mice. diDCP-LA-PE, thus, may enable type 1 DM therapy without insulin injection.

Retinol as electron carrier in redox signaling, a new frontier in vitamin A research

Nature uses carotenoids and retinoids as chromophores for diverse energy conversion processes. The key structural feature enabling the interaction with light and other manifestations of electro-magnetism is the conjugated double-bond system that all members of this superfamily share in common. Among retinoids, retinaldehyde alone was long known as the active chromophore of vision in vertebrates and invertebrates, as well of various light-driven proton and ion pumps in Archaea. Until now, vitamin A (retinol) was solely regarded as a biochemical precursor for bioactive retinoids such as retinaldehyde and retinoic acid (RA), but recent results indicate that this compound has its own physiology. It functions as an electron carrier in mitochondria. By electronically coupling protein kinase Cδ (PCKδ) with cytochrome c, vitamin A enables the redox activation of this enzyme. This review focuses on the biochemistry and biology of the PCKδ signaling system, comprising PKCδ, the adapter protein p66Shc, cytochrome c and retinol. This complex positively regulates the conversion of pyruvate to acetyl-coenzyme A (CoA) by the pyruvate dehydrogenase enzyme. Vitamin A therefore plays a key role in glycolytic energy generation. The emerging paradigm of retinol as electron-transfer agent is potentially transformative, opening new frontiers in retinoid research.

Retinol as a cofactor for PKCδ-mediated impairment of insulin sensitivity in a mouse model of diet-induced obesity

We previously defined that the mitochondria-localized PKCδ signaling complex stimulates the conversion of pyruvate to acetyl-coenzyme A by the pyruvate dehydrogenase complex. We demonstrated in vitro and ex vivo that retinol supplementation enhances ATP synthesis in the presence of the PKCδ signalosome. Here, we tested in vivo if a persistent oversupply of retinol would further impair glucose metabolism in a mouse model of diet-induced insulin resistance. We crossed mice overexpressing human retinol-binding protein (hRBP) under the muscle creatine kinase (MCK) promoter (MCKhRBP) with the PKCδ(-/-) strain to generate mice with a different status of the PKCδ signalosome and retinoid levels. Mice with a functional PKCδ signalosome and elevated retinoid levels (PKCδ(+/+)hRBP) developed the most advanced stage of insulin resistance. In contrast, elevation of retinoid levels in mice with inactive PKCδ did not affect remarkably their metabolism, resulting in phenotypic similarity between PKCδ(-/-)hRBP and PKCδ(-/-) mice. Therefore, in addition to the well-defined role of PKCδ in the etiology of metabolic syndrome, we present a novel PKCδ signaling pathway that requires retinol as a metabolic cofactor and is involved in the regulation of fuel utilization in mitochondria. The distinct role in whole-body energy homeostasis establishes the PKCδ signalosome as a promising target for therapeutic intervention in metabolic disorders.

Role of PKCδ in Insulin Sensitivity and Skeletal Muscle Metabolism

Protein kinase C (PKC)δ has been shown to be increased in liver in obesity and plays an important role in the development of hepatic insulin resistance in both mice and humans. In the current study, we explored the role of PKCδ in skeletal muscle in the control of insulin sensitivity and glucose metabolism by generating mice in which PKCδ was deleted specifically in muscle using Cre-lox recombination. Deletion of PKCδ in muscle improved insulin signaling in young mice, especially at low insulin doses; however, this did not change glucose tolerance or insulin tolerance tests done with pharmacological levels of insulin. Likewise, in young mice, muscle-specific deletion of PKCδ did not rescue high-fat diet-induced insulin resistance or glucose intolerance. However, with an increase in age, PKCδ levels in muscle increased, and by 6 to 7 months of age, muscle-specific deletion of PKCδ improved whole-body insulin sensitivity and muscle insulin resistance and by 15 months of age improved the age-related decline in whole-body glucose tolerance. At 15 months of age, M-PKCδKO mice also exhibited decreased metabolic rate and lower levels of some proteins of the OXPHOS complex suggesting a role for PKCδ in the regulation of mitochondrial mass at older age. These data indicate an important role of PKCδ in the regulation of insulin sensitivity and mitochondrial homeostasis in skeletal muscle with aging.

Protein kinase Cδ promotes proliferation and induces malignant transformation in skeletal muscle

In this paper, we investigated the isoform-specific roles of certain protein kinase C (PKC) isoforms in the regulation of skeletal muscle growth. Here, we provide the first intriguing functional evidence that nPKCδ (originally described as an inhibitor of proliferation in various cells types) is a key player in promoting both in vitro and in vivo skeletal muscle growth. Recombinant overexpression of a constitutively active nPKCδ in C2C12 myoblast increased proliferation and inhibited differentiation. Conversely, overexpression of kinase-negative mutant of nPKCδ (DN-nPKCδ) markedly inhibited cell growth. Moreover, overexpression of nPKCδ also stimulated in vivo tumour growth and induced malignant transformation in immunodeficient (SCID) mice whereas that of DN-nPKCδ suppressed tumour formation. The role of nPKCδ in the formation of rhabdomyosarcoma was also investigated where recombinant overexpression of nPKCδ in human rhabdomyosarcoma RD cells also increased cell proliferation and enhanced tumour formation in mouse xenografts. The other isoforms investigated (PKCα, β, ε) exerted only minor (mostly growth-inhibitory) effects in skeletal muscle cells. Collectively, our data introduce nPKCδ as a novel growth-promoting molecule in skeletal muscles and invite further trials to exploit its therapeutic potential in the treatment of skeletal muscle malignancies.

INTRACELLULAR SIGNALING BY BILE ACIDS

Bile acids, synthesized from cholesterol, are known to produce beneficial as well as toxic effects in the liver. The beneficial effects include choleresis, immunomodulation, cell survival, while the toxic effects include cholestasis, apoptosis and cellular toxicity. It is believed that bile acids produce many of these effects by activating intracellular signaling pathways. However, it has been a challenge to relate intracellular signaling to specific and at times opposing effects of bile acids. It is becoming evident that bile acids produce different effects by activating different isoforms of phosphoinositide 3-kinase (PI3K), Protein kinase Cs (PKCs), and mitogen activated protein kinases (MAPK). Thus, the apoptotic effect of bile acids may be mediated via PI3K-110γ, while cytoprotection induce by cAMP-GEF pathway involves activation of PI3K-p110α/β isoforms. Atypical PKCζ may mediate beneficial effects and nPKCε may mediate toxic effects, while cPKCα and nPKCδ may be involved in both beneficial and toxic effects of bile acids. The opposing effects of nPKCδ activation may depend on nPKCδ phosphorylation site(s). Activation of ERK1/2 and JNK1/2 pathway appears to mediate beneficial and toxic effects, respectively, of bile acids. Activation of p38α MAPK and p38β MAPK may mediate choleretic and cholestatic effects, respectively, of bile acids. Future studies clarifying the isoform specific effects on bile formation should allow us to define potential therapeutic targets in the treatment of cholestatic disorders.

PKCδ regulates hepatic insulin sensitivity and hepatosteatosis in mice and humans

C57BL/6J and 129S6/Sv (B6 and 129) mice differ dramatically in their susceptibility to developing diabetes in response to diet- or genetically induced insulin resistance. A major locus contributing to this difference has been mapped to a region on mouse chromosome 14 that contains the gene encoding PKCδ. Here, we found that PKCδ expression in liver was 2-fold higher in B6 versus 129 mice from birth and was further increased in B6 but not 129 mice in response to a high-fat diet. PRKCD gene expression was also elevated in obese humans and was positively correlated with fasting glucose and circulating triglycerides. Mice with global or liver-specific inactivation of the Prkcd gene displayed increased hepatic insulin signaling and reduced expression of gluconeogenic and lipogenic enzymes. This resulted in increased insulin-induced suppression of hepatic gluconeogenesis, improved glucose tolerance, and reduced hepatosteatosis with aging. Conversely, mice with liver-specific overexpression of PKCδ developed hepatic insulin resistance characterized by decreased insulin signaling, enhanced lipogenic gene expression, and hepatosteatosis. Therefore, changes in the expression and regulation of PKCδ between strains of mice and in obese humans play an important role in the genetic risk of hepatic insulin resistance, glucose intolerance, and hepatosteatosis; and thus PKCδ may be a potential target in the treatment of metabolic syndrome.

Imaging colon cancer response following treatment with AZD1152: a preclinical analysis of [18F]fluoro-2-deoxyglucose and 3'-deoxy-3'-[18F]fluorothymidine imaging

PURPOSE

To determine whether treatment response to the Aurora B kinase inhibitor, AZD1152, could be monitored early in the course of therapy by noninvasive [(18)F]-labeled fluoro-2-deoxyglucose, [(18)F]FDG, and/or 3'-deoxy-3'-[(18)F]fluorothymidine, [(18)F]FLT, PET imaging.

EXPERIMENTAL DESIGN

AZD1152-treated and control HCT116 and SW620 xenograft-bearing animals were monitored for tumor size and by [(18)F]FDG, and [(18)F]FLT PET imaging. Additional studies assessed the endogenous and exogenous contributions of thymidine synthesis in the two cell lines.

RESULTS

Both xenografts showed a significant volume-reduction to AZD1152. In contrast, [(18)F]FDG uptake did not demonstrate a treatment response. [(18)F]FLT uptake decreased to less than 20% of control values in AZD1152-treated HCT116 xenografts, whereas [(18)F]FLT uptake was near background levels in both treated and untreated SW620 xenografts. The EC(50) for AZD1152-HQPA was approximately 10 nmol/L in both SW620 and HCT116 cells; in contrast, SW620 cells were much more sensitive to methotrexate (MTX) and 5-Fluorouracil (5FU) than HCT116 cells. Immunoblot analysis demonstrated marginally lower expression of thymidine kinase in SW620 compared with HCT116 cells. The aforementioned results suggest that SW620 xenografts have a higher dependency on the de novo pathway of thymidine utilization than HCT116 xenografts.

CONCLUSIONS

AZD1152 treatment showed antitumor efficacy in both colon cancer xenografts. Although [(18)F]FDG PET was inadequate in monitoring treatment response, [(18)F]FLT PET was very effective in monitoring response in HCT116 xenografts, but not in SW620 xenografts. These observations suggest that de novo thymidine synthesis could be a limitation and confounding factor for [(18)F]FLT PET imaging and quantification of tumor proliferation, and this may apply to some clinical studies as well.

PKC{epsilon}-dependent and -independent effects of taurolithocholate on PI3K/PKB pathway and taurocholate uptake in HuH-NTCP cell line

The cholestatic bile acid taurolithocholate (TLC) inhibits biliary secretion of organic anions and hepatic uptake of taurocholate (TC). TLC has been suggested to induce retrieval of Mrp2 from the canalicular membrane via the phosphoinositide-3-kinase (PI3K)/PKB-dependent activation of novel protein kinase Cepsilon (nPKCepsilon) in rat hepatocytes. The aim of the present study was to determine whether TLC-induced inhibition of TC uptake may also involve PI3K-dependent activation of PKCepsilon in HuH7 cells stably transfected with human Na(+)-dependent TC-cotransporting polypeptide (NTCP) (HuH-NTCP cells). To avoid direct competition for uptake, cells were pretreated with TLC, washed, and then incubated with (3)H-TC to determine TC uptake. TLC produced time- and dose-dependent inhibition of TC uptake. TLC inhibited TC uptake competitively without affecting NTCP membrane translocation. A PI3K inhibitor failed to reverse TLC-induced TC uptake inhibition and TLC-inhibited PKB phosphorylation. TLC did activate nPKCepsilon as evidenced by increased membrane translocation and nPKCepsilon-Ser(729) phosphorylation. Overexpression of dominant negative-nPKCepsilon reversed TLC-induced inhibition of PKB phosphorylation but not of TC uptake. Finally, cAMP prevented TLC-induced inhibition of TC uptake via the PI3K pathway, and the prevention is due to the sum of cAMP-induced stimulation and TLC-induced inhibition of TC uptake. Taken together, these results suggest that TLC-induced inhibition of PKB, but not of TC uptake, is mediated via nPKCepsilon. Activation of nPKCepsilon and inhibition of TC uptake by TLC are not mediated via the PI3K/PKB pathway.

Lysophosphatidylcholine activates adipocyte glucose uptake and lowers blood glucose levels in murine models of diabetes

Glucose homeostasis is maintained by the orchestration of peripheral glucose utilization and hepatic glucose production, mainly by insulin. In this study, we found by utilizing a combined parallel chromatography mass profiling approach that lysophosphatidylcholine (LPC) regulates glucose levels. LPC was found to stimulate glucose uptake in 3T3-L1 adipocytes dose- and time-dependently, and this activity was found to be sensitive to variations in acyl chain lengths and to polar head group types in LPC. Treatment with LPC resulted in a significant increase in the level of GLUT4 at the plasma membranes of 3T3-L1 adipocytes. Moreover, LPC did not affect IRS-1 and AKT2 phosphorylations, and LPC-induced glucose uptake was not influenced by pretreatment with the PI 3-kinase inhibitor LY294002. However, glucose uptake stimulation by LPC was abrogated both by rottlerin (a protein kinase Cdelta inhibitor) and by the adenoviral expression of dominant negative protein kinase Cdelta. In line with its determined cellular functions, LPC was found to lower blood glucose levels in normal mice. Furthermore, LPC improved blood glucose levels in mouse models of type 1 and 2 diabetes. These results suggest that an understanding of the mode of action of LPC may provide a new perspective of glucose homeostasis.

Oxalate-induced activation of PKC-alpha and -delta regulates NADPH oxidase-mediated oxidative injury in renal tubular epithelial cells

Oxalate-induced oxidative stress contributes to cell injury and promotes renal deposition of calcium oxalate crystals. However, we do not know how oxalate stimulates reactive oxygen species (ROS) in renal tubular epithelial cells. We investigated the signaling mechanism of oxalate-induced ROS formation in these cells and found that oxalate significantly increased membrane-associated protein kinase C (PKC) activity while at the same time lowering cytosolic PKC activity. Oxalate markedly translocated PKC-alpha and -delta from the cytosol to the cell membrane. Pretreatment of LLC-PK1 cells with specific inhibitors of PKC-alpha or -delta significantly blocked oxalate-induced generation of superoxide and hydrogen peroxide along with NADPH oxidase activity, LDH release, lipid hydroperoxide formation, and apoptosis. The PKC activator PMA mimicked oxalate's effect on oxidative stress in LLC-PK1 cells as well as cytosol-to-membrane translocation of PKC-alpha and -delta. Silencing of PKC-alpha expression by PKC-alpha-specific small interfering RNA significantly attenuated oxalate-induced cell injury by decreasing hydrogen peroxide generation and LDH release. We believe this is the first demonstration that PKC-alpha- and -delta-dependent activation of NADPH oxidase is one of the mechanisms responsible for oxalate-induced oxidative injury in renal tubular epithelial cells. The study suggests that the therapeutic approach might be considered toward attenuating oxalate-induced PKC signaling-mediated oxidative injury in recurrent stone formers.

Oxidized LDL impair adipocyte response to insulin by activating serine/threonine kinases

Oxidized LDL (oxLDL) increase in patients affected by type-2 diabetes, obesity, and metabolic syndrome. Likewise, insulin resistance, an impaired responsiveness of target tissues to insulin, is associated with those pathological conditions. To investigate a possible causal relationship between oxLDL and the onset of insulin resistance, we evaluated the response to insulin of 3T3-L1 adipocytes treated with oxLDL. We observed that oxLDL inhibited glucose uptake (-40%) through reduced glucose transporter 4 (GLUT4) recruitment to the plasma membrane (-70%), without affecting GLUT4 gene expression. These findings were associated to the impairment of insulin signaling. Specifically, in oxLDL-treated cells insulin receptor (IR) substrate-1 (IRS-1) was highly degraded likely because of the enhanced Ser(307)phosphorylation. This process was largely mediated by the activation of the inhibitor of kappaB-kinase beta (IKKbeta) and the c-Jun NH(2)-terminal kinase (JNK). Moreover, the activation of IKKbeta positively regulated the nuclear content of nuclear factor kappaB (NF-kappaB), by inactivating the inhibitor of NF-kappaB (IkappaBalpha). The activated NF-kappaB further impaired per se GLUT4 functionality. Specific inhibitors of IKKbeta, JNK, and NF-kappaB restored insulin sensitivity in adipocytes treated with oxLDL. These data provide the first evidence that oxLDL, by activating serine/threonine kinases, impaired adipocyte response to insulin affecting pathways involved in the recruitment of GLUT4 to plasma membranes (PM). This suggests that oxLDL might participate in the development of insulin resistance.

Protein kinase C family: on the crossroads of cell signaling in skin and tumor epithelium

The protein kinase C (PKC) family represents a large group of phospholipid dependent enzymes catalyzing the covalent transfer of phosphate from ATP to serine and threonine residues of proteins. Phosphorylation of the substrate proteins induces a conformational change resulting in modification of their functional properties. The PKC family consists of at least ten members, divided into three subgroups: classical PKCs (alpha, betaI, betaII, gamma), novel PKCs (delta, epsilon, eta, theta), and atypical PKCs (zeta, iota/lambda). The specific cofactor requirements, tissue distribution, and cellular compartmentalization suggest differential functions and fine tuning of specific signaling cascades for each isoform. Thus, specific stimuli can lead to differential responses via isoform specific PKC signaling regulated by their expression, localization, and phosphorylation status in particular biological settings. PKC isoforms are activated by a variety of extracellular signals and, in turn, modify the activities of cellular proteins including receptors, enzymes, cytoskeletal proteins, and transcription factors. Accordingly, the PKC family plays a central role in cellular signal processing. Accumulating data suggest that various PKC isoforms participate in the regulation of cell proliferation, differentiation, survival and death. These findings have enabled identification of abnormalities in PKC isoform function, as they occur in several cancers. Specifically, the initiation of squamous cell carcinoma formation and progression to the malignant phenotype was found to be associated with distinct changes in PKC expression, activation, distribution, and phosphorylation. These studies were recently further extended to transgenic and knockout animals, which allowed a more direct analysis of individual PKC functions. Accordingly, this review is focused on the involvement of PKC in physiology and pathology of the skin.

Specific protein kinase C isoforms as transducers and modulators of insulin signaling

Recent studies implicate specific PKC isoforms in the insulin-signaling cascade. Insulin activates PKCs alpha, betaII, delta and zeta in several cell types. In addition, as will be documented in this review, certain members of the PKC family may also be activated and act upstream of PI3 and MAP kinases. Each of these isoforms has been shown one way or another either to mimic or to modify insulin-stimulated effects in one or all of the insulin-responsive tissues. Moreover, each of the isoforms has been shown to be activated by insulin stimulation or conditions important for effective insulin stimulation. Studies attempting to demonstrate a definitive role for any of the isoforms have been performed on different cells, ranging from appropriate model systems for skeletal muscle, liver and fat, such as primary cultures, and cell lines and even in vivo studies, including transgenic mice with selective deletion of specific PKC isoforms. In addition, studies have been done on certain expression systems such as CHO or HEK293 cells, which are far removed from the tissues themselves and serve mainly as vessels for potential protein-protein interactions. Thus, a clear picture for many of the isoforms remains elusive in spite of over two decades of intensive research. The recent intrusion of transgenic and precise molecular biology technologies into the research armamentarium has opened a wide range of additional possibilities for direct involvement of individual isoforms in the insulin signaling cascade. As we hope to discuss within the context of this review, whereas many of the long sought-after answers to specific questions are not yet clear, major advances have been made in our understanding of precise roles for individual PKC isoforms in mediation of insulin effects. In this review, in which we shall focus our attention on isoforms in the conventional and novel categories, a clear case will be made to show that these isoforms are not only expressed but are importantly involved in regulation of insulin metabolic effects.

Developing skeletal muscle cells express functional muscarinic acetylcholine receptors coupled to different intracellular signaling systems

This study analyzed the expression of muscarinic acetylcholine receptors (mAChRs) in the rat cultured skeletal muscle cells and their coupling to G protein, phospholipase C and adenylyl cyclase (AC). Our results showed the presence of a homogeneous population of [(3)H]methyl-quinuclidinyl benzilate-binding sites in the membrane fraction from the rat cultured muscle (K(D) = 0.4 nM, B(max) = 8.9 fmol mg protein(-1)). Specific muscarinic binding sites were also detected in denervated diaphragm muscles from adult rats and in myoblasts isolated from newborn rats. Activation of mAChRs with carbachol induced specific [(35)S]GTPgammaS binding to cultured muscle membranes and potentiated the forskolin-dependent stimulation of AC. These effects were totally inhibited by 0.1-1 microM atropine. In addition, mAChRs were able to stimulate generation of diacylglycerol (DAG) in response to acetylcholine, carbachol or selective mAChR agonist oxotremorine-M. The carbachol-dependent increase in DAG was inhibited in a concentration-dependent manner by mAChR antagonists atropine, pirenzepine and 4-DAMP mustard. Finally, activation of these receptors was correlated with increased synthesis of acetylcholinesterase, via a PKC-dependent pathway. Taken together, these results indicate that expression of mAChRs, coupled to G protein and distinct intracellular signaling systems, is a characteristic of noninnervated skeletal muscle cells and may be responsible for trophic influences of acetylcholine during formation of the neuromuscular synapse.

Human biliverdin reductase: a member of the insulin receptor substrate family with serine/threonine/tyrosine kinase activity

We describe here the tyrosine kinase activity of human biliverdin reductase (BVR) and its potential role in the insulin-signaling pathway. BVR is both a substrate for insulin receptor (IR) tyrosine kinase (IRK) activity and a kinase for serine phosphorylation of IR substrate 1 (IRS-1). Our previous studies have revealed serine/threonine kinase activity of BVR. Y198, in the YMKM motif found in the C-terminal domain of BVR, is shown to be a substrate for insulin-activated IRK. This motif in IRS proteins provides a docking site for proteins that contain a Src homology 2 domain. Additionally, Y228 in the YLSF sequence and Y291 are IRK substrates; the former sequence provides optimum recognition motif in the tyrosine phosphatase, SHP-1, and for SHC (Src homology 2 domain containing transfroming protein 1). BVR autophosphorylates N-terminal tyrosines Y72 and Y83. Serine residues in IRS-1 are targets for BVR phosphorylation, and point mutation of serine residues in the kinase domain of the reductase inhibits phosphotransferase activity. Because tyrosine phosphorylation of IRS-1 activates the insulin signaling pathway and serine phosphorylation of IRS-1 blocks insulin action, our findings that insulin increases BVR tyrosine phosphorylation and that there is an increase in glucose uptake in response to insulin when expression of BVR is "knocked down" by small interfering RNA suggest a potential role for BVR in the insulin signaling pathway.

PKC-delta-dependent activation of oxidative stress in adipocytes of obese and insulin-resistant mice: role for NADPH oxidase

Oxidative stress is thought to be one of the causative factors contributing to insulin resistance and type 2 diabetes. Previously, we showed that reactive oxygen species (ROS) production is significantly increased in adipocytes from high-fat diet-induced obese and insulin-resistant mice (HF). ROS production was also associated with the increased activity of PKC-delta. In the present studies, we hypothesized that PKC-delta contributes to ROS generation and determined their intracellular source. NADPH oxidase inhibitor diphenyleneiodonium chloride (DPI) reduced ROS levels by 50% in HF adipocytes, and inhibitors of NO synthase (L-NAME, 1 mM), xanthine oxidase (allopurinol, 100 microM), AGE formation (aminoguanidine, 10 microM), or the mitochondrial uncoupler (FCCP, 10 microM) had no effect. Rottlerin, a selective PKC-delta inhibitor, suppressed ROS levels by approximately 50%. However, neither GO-6976 nor LY-333531, effective inhibitors toward conventional PKC or PKC-beta, respectively, significantly altered ROS levels in HF adipocytes. Subsequently, adenoviral-mediated expression of wild-type PKC-delta or its dominant negative mutant (DN-PKC-delta) in HF adipocytes resulted in either a twofold increase in ROS levels or their suppression by 20%, respectively. In addition, both ROS levels and PKC-delta activity were sharply reduced by glucose depletion. Taken together, these results suggest that PKC-delta is responsible for elevated intracellular ROS production in HF adipocytes, and this is mediated by high glucose and NADPH oxidase.

Increased glucose uptake promotes oxidative stress and PKC-delta activation in adipocytes of obese, insulin-resistant mice

Increased oxidative stress is believed to be one of the mechanisms responsible for hyperglycemia-induced tissue damage and diabetic complications. In these studies, we undertook to characterize glucose uptake and oxidative stress in adipocytes of type 2 diabetic animals and to determine whether these promote the activation of PKC-delta. The adipocytes used were isolated either from C57Bl/6J mice that were raised on a high-fat diet (HF) and developed obesity and insulin resistance or from control animals. Basal glucose uptake significantly increased (8-fold) in HF adipocytes, and this was accompanied with upregulation of GLUT1 expression levels. Insulin-induced glucose uptake was inhibited in HF adipocytes and GLUT4 content reduced by 20% in these adipocytes. Reactive oxygen species (ROS) increased twofold in HF adipocytes compared with control adipocytes and were largely reduced with decreased glucose concentrations. At zero glucose, ROS levels were reduced to the normal levels seen in control adipocytes. The activity of PKC-delta increased twofold in HF adipocytes compared with control adipocytes and was further activated by H2O2. Moreover, PKC-delta activity was inhibited in HF adipocytes either by glucose deprivation or by treatment with the antioxidant N-acetyl-l-cysteine. In summary, we propose that increased glucose intake in HF adipocytes increases oxidative stress, which in turn promotes the activation of PKC-delta. These consequential events may be responsible, at least in part, for development of HF diet-induced insulin resistance in the fat tissue.

Differential effects of tumor necrosis factor-alpha on protein kinase C isoforms alpha and delta mediate inhibition of insulin receptor signaling

Tumor necrosis factor-alpha (TNF-alpha) is a multifunctional cytokine that interferes with insulin signaling, but the molecular mechanisms of this effect are unclear. Because certain protein kinase C (PKC) isoforms are activated by insulin, we examined the role of PKC in TNF-alpha inhibition of insulin signaling in primary cultures of mouse skeletal muscle. TNF-alpha, given 5 min before insulin, inhibited insulin-induced tyrosine phosphorylation of insulin receptor (IR), IR substrate (IRS)-1, insulin-induced association of IRS-1 with the p85 subunit of phosphatidylinositol 3-kinase (PI3-K), and insulin-induced glucose uptake. Insulin and TNF-alpha each caused tyrosine phosphorylation and activation of PKCs delta and alpha, but when TNF-alpha preceded insulin, the effects were less than that produced by each substance alone. Insulin induced PKCdelta specifically to coprecipitate with IR, an effect blocked by TNF-alpha. Both PKCalpha and -delta are constitutively associated with IRS-1. Whereas insulin decreased coprecipitation of IRS-1 with PKCalpha, it increased coprecipitation of IRS-1 with PKCdelta. TNF-alpha blocked the effects of insulin on association of both PKCs with IRS-1. To further investigate the involvement of PKCs in inhibitory actions of TNF-alpha on insulin signaling, we overexpressed specific PKC isoforms in mature myotubes. PKCalpha overexpression inhibited basal and insulin-induced IR autophosphorylation, whereas PKCdelta overexpression increased IR autophosphorylation and abrogated the inhibitory effect of TNF-alpha on IR autophosphorylation and signaling to PI3-K. Blockade of PKCalpha antagonized the inhibitory effects of TNF-alpha on both insulin-induced IR tyrosine phosphorylation and IR signaling to PI3-K. We suggest that the effects of TNF-alpha on IR tyrosine phosphorylation are mediated via alteration of insulin-induced activation and association of PKCdelta and -alpha with upstream signaling molecules.

Phosphatidylinositol 3-kinase redistribution is associated with skeletal muscle insulin resistance in gestational diabetes mellitus

Insulin resistance during pregnancy provokes gestational diabetes mellitus (GDM); however, the cellular mechanisms for this type of insulin resistance are not well understood. We evaluated the mechanisms(s) for insulin resistance in skeletal muscle from an animal model of spontaneous GDM, the heterozygous C57BL/KsJ-(db/+) mouse. Pregnancy triggered a novel functional redistribution of the insulin-signaling environment in skeletal muscle in vivo. This environment preferentially increases a pool of phosphatidylinositol (PI) 3-kinase activity associated with the insulin receptor, away from insulin receptor substrate (IRS)-1. In conjunction with the redistribution of PI 3-kinase to the insulin receptor, there is a selective increase in activation of downstream serine kinases Akt and p70S6. Furthermore, we show that redistribution of PI 3-kinase to the insulin receptor increases insulin-stimulated IRS-1 serine phosphorylation, impairs IRS-1 expression and its tyrosine phosphorylation, and decreases the ability of IRS-1 to bind and activate PI 3-kinase in response to insulin. Thus, the pool of IRS-1-associated PI 3-kinase activity is reduced, resulting in the inability of insulin to stimulate GLUT4 translocation to the plasma membrane. These defects are unique to pregnancy and suggest that redistribution of PI 3-kinase to the insulin receptor may be a primary defect underlying insulin resistance in skeletal muscle during gestational diabetes.

Activation of protein kinase C zeta induces serine phosphorylation of VAMP2 in the GLUT4 compartment and increases glucose transport in skeletal muscle

Insulin stimulates glucose uptake into skeletal muscle tissue mainly through the translocation of glucose transporter 4 (GLUT4) to the plasma membrane. The precise mechanism involved in this process is presently unknown. In the cascade of events leading to insulin-induced glucose transport, insulin activates specific protein kinase C (PKC) isoforms. In this study we investigated the roles of PKC zeta in insulin-stimulated glucose uptake and GLUT4 translocation in primary cultures of rat skeletal muscle. We found that insulin initially caused PKC zeta to associate specifically with the GLUT4 compartments and that PKC zeta together with the GLUT4 compartments were then translocated to the plasma membrane as a complex. PKC zeta and GLUT4 recycled independently of one another. To further establish the importance of PKC zeta in glucose transport, we used adenovirus constructs containing wild-type or kinase-inactive, dominant-negative PKC zeta (DNPKC zeta) cDNA to overexpress this isoform in skeletal muscle myotube cultures. We found that overexpression of PKC zeta was associated with a marked increase in the activity of this isoform. The overexpressed, active PKC zeta coprecipitated with the GLUT4 compartments. Moreover, overexpression of PKC zeta caused GLUT4 translocation to the plasma membrane and increased glucose uptake in the absence of insulin. Finally, either insulin or overexpression of PKC zeta induced serine phosphorylation of the GLUT4-compartment-associated vesicle-associated membrane protein 2. Furthermore, DNPKC zeta disrupted the GLUT4 compartment integrity and abrogated insulin-induced GLUT4 translocation and glucose uptake. These results demonstrate that PKC zeta regulates insulin-stimulated GLUT4 translocation and glucose transport through the unique colocalization of this isoform with the GLUT4 compartments.

Protein kinase C (PKC)-alpha activation inhibits PKC-zeta and mediates the action of PED/PEA-15 on glucose transport in the L6 skeletal muscle cells

Overexpression of the PED/PEA-15 protein in muscle and adipose cells increases glucose transport and impairs further insulin induction. Like glucose transport, protein kinase C (PKC)-alpha and -beta are also constitutively activated and are not further stimulatable by insulin in L6 skeletal muscle cells overexpressing PED (L6(PED)). PKC-zeta features no basal change but completely loses insulin sensitivity in L6(PED). In these cells, blockage of PKC-alpha and -beta additively returns 2-deoxy-D-glucose (2-DG) uptake to the levels of cells expressing only endogenous PED (L6(WT)). Blockage of PKC-alpha and -beta also restores insulin activation of PKC-zeta in L6(PED) cells, with that of PKC-alpha sixfold more effective than PKC-beta. Similar effects on 2-DG uptake and PKC-zeta were also achieved by 50-fold overexpression of PKC-zeta in L6(PED). In L6(WT), fivefold overexpression of PKC-alpha or -beta increases basal 2-DG uptake and impairs further insulin induction with no effect on insulin receptor or insulin receptor substrate phosphorylation. In these cells, overexpression of PKC-alpha blocks insulin induction of PKC-zeta activity. PKC-beta is 10-fold less effective than PKC-alpha in inhibiting PKC-zeta stimulation. Expression of the dominant-negative K(281)-->W PKC-zeta mutant simultaneously inhibits insulin activation of PKC-zeta and 2-DG uptake in the L6(WT) cells. We conclude that activation of classic PKCs, mainly PKC-alpha, inhibits PKC-zeta and may mediate the action of PED on glucose uptake in L6 skeletal muscle cells.

The role of protein kinase C isoforms in insulin action

Insulin action on target tissues is mediated by specific tyrosine kinase receptors. Upon ligand binding insulin receptors autophosphorylate and phosphorylate intracellular substrates on tyrosine residues. These early events of insulin action are followed by the activation of a number of enzymes, including protein kinase C (PKC). At least 14 PKC isoforms have been identified and cloned to date. PKCs appear to play dual roles in insulin signaling. For instance, they are involved in transduction of specific insulin signals but also contribute to the generation of insulin resistance. In this article, we will analyze the experimental evidence addressing the mechanism by which insulin might activate individual PKC isoforms as well as the role of single PKCs in insulin-induced bioeffects.

PKCdelta activation: a divergence point in the signaling of insulin and IGF-1-induced proliferation of skin keratinocytes

Insulin and insulin-like growth factor-1 (IGF-1) are members of the family of the insulin family of growth factors, which activate similar cellular downstream pathways. In this study, we analyzed the effects of insulin and IGF-1 on the proliferation of murine skin keratinocytes in an attempt to determine whether these hormones trigger the same signaling pathways. Increasing doses of insulin and IGF-1 promote keratinocyte proliferation in an additive manner. We identified downstream pathways specifically involved in insulin signaling that are known to play a role in skin physiology; these include activation of the Na+/K+ pump and protein kinase C (PKC). Insulin, but not IGF-1, stimulated Na+/K+ pump activity. Furthermore, ouabain, a specific Na+/K+ pump inhibitor, abolished the proliferative effect of insulin but not that of IGF-1. Insulin and IGF-1 also differentially regulated PKC activation. Insulin, but not IGF-1, specifically activated and translocated the PKCB isoform to the membrane fraction. There was no effect on PKC isoforms alpha, eta, epsilon, and zeta, which are expressed in skin. PKC8 overexpression increased keratinocyte proliferation and Na+/K+ pump activity to a degree similar to that induced by insulin but had no affect on IGF-1-induced proliferation. Furthermore, a dominant negative form of PKCdelta abolished the effects of insulin on both proliferation and Na+/K+ pump activity but did not abrogate induction of keratinocyte proliferation induced by other growth factors. These data indicate that though insulin or IGF-1 stimulation induce keratinocyte proliferation, only insulin action is specifically mediated via PKC8 and involves activation of the Na+/K+ pump.