Protein kinase C is increased in the liver of humans and rats with non-insulin-dependent diabetes mellitus: an alteration not due to hyperglycemia

Inhibition of protein kinase Cepsilon prevents hepatic insulin resistance in nonalcoholic fatty liver disease

Nonalcoholic fatty liver disease is strongly associated with hepatic insulin resistance and type 2 diabetes mellitus, but the molecular signals linking hepatic fat accumulation to hepatic insulin resistance are unknown. Three days of high-fat feeding in rats results specifically in hepatic steatosis and hepatic insulin resistance. In this setting, PKCepsilon, but not other isoforms of PKC, is activated. To determine whether PKCepsilon plays a causal role in the pathogenesis of hepatic insulin resistance, we treated rats with an antisense oligonucleotide against PKCepsilon and subjected them to 3 days of high-fat feeding. Knocking down PKCepsilon expression protects rats from fat-induced hepatic insulin resistance and reverses fat-induced defects in hepatic insulin signaling. Furthermore, we show that PKCepsilon associates with the insulin receptor in vivo and impairs insulin receptor kinase activity both in vivo and in vitro. These data support the hypothesis that PKCepsilon plays a critical role in mediating fat-induced hepatic insulin resistance and represents a novel therapeutic target for type 2 diabetes.

Inhibition of protein kinase Cepsilon prevents hepatic insulin resistance in nonalcoholic fatty liver disease

Nonalcoholic fatty liver disease is strongly associated with hepatic insulin resistance and type 2 diabetes mellitus, but the molecular signals linking hepatic fat accumulation to hepatic insulin resistance are unknown. Three days of high-fat feeding in rats results specifically in hepatic steatosis and hepatic insulin resistance. In this setting, PKCepsilon, but not other isoforms of PKC, is activated. To determine whether PKCepsilon plays a causal role in the pathogenesis of hepatic insulin resistance, we treated rats with an antisense oligonucleotide against PKCepsilon and subjected them to 3 days of high-fat feeding. Knocking down PKCepsilon expression protects rats from fat-induced hepatic insulin resistance and reverses fat-induced defects in hepatic insulin signaling. Furthermore, we show that PKCepsilon associates with the insulin receptor in vivo and impairs insulin receptor kinase activity both in vivo and in vitro. These data support the hypothesis that PKCepsilon plays a critical role in mediating fat-induced hepatic insulin resistance and represents a novel therapeutic target for type 2 diabetes.

Inhibition of protein kinase Cepsilon prevents hepatic insulin resistance in nonalcoholic fatty liver disease

Nonalcoholic fatty liver disease is strongly associated with hepatic insulin resistance and type 2 diabetes mellitus, but the molecular signals linking hepatic fat accumulation to hepatic insulin resistance are unknown. Three days of high-fat feeding in rats results specifically in hepatic steatosis and hepatic insulin resistance. In this setting, PKCepsilon, but not other isoforms of PKC, is activated. To determine whether PKCepsilon plays a causal role in the pathogenesis of hepatic insulin resistance, we treated rats with an antisense oligonucleotide against PKCepsilon and subjected them to 3 days of high-fat feeding. Knocking down PKCepsilon expression protects rats from fat-induced hepatic insulin resistance and reverses fat-induced defects in hepatic insulin signaling. Furthermore, we show that PKCepsilon associates with the insulin receptor in vivo and impairs insulin receptor kinase activity both in vivo and in vitro. These data support the hypothesis that PKCepsilon plays a critical role in mediating fat-induced hepatic insulin resistance and represents a novel therapeutic target for type 2 diabetes.

Regulation of insulin receptor substrate 1 pleckstrin homology domain by protein kinase C: role of serine 24 phosphorylation

Phosphorylation of insulin receptor substrate (IRS) proteins on serine residues is an important posttranslational modification that is linked to insulin resistance. Several phosphoserine sites on IRS1 have been identified; the majority are located proximal to the phosphotryosine-binding domain or near key receptor tyrosine kinase substrate- and/or Src-homology 2 domain-binding sites. Here we report on the characterization of a serine phosphorylation site in the N-terminal pleckstrin homology (PH) domain of IRS1. Bioinformatic tools identify serine 24 (Ser24) as a putative substrate site for the protein kinase C (PKC) family of serine kinases. We demonstrate that this site is indeed a bona fide substrate for conventional PKC. In vivo, IRS-1 is also phosphorylated on Ser24 after phorbol 12-myristate 13-acetate treatment of cells, and isoform-selective inhibitor studies suggest the involvement of PKCalpha. By comparing the pharmacological characteristics of phorbol 12-myristate 13-acetate-stimulated Ser24 phosphorylation with phosphorylation at two other sites previously linked to PKC activity (Ser307 and Ser612), we show that PKCalpha is likely to be directly involved in Ser24 phosphorylation, but indirectly involved in Ser307 and Ser612 phosphorylation. Using Ser24Asp IRS-1 mutants to mimic the phosphorylated residue, we demonstrate that the phosphorylation status of Ser24 does play an important role in regulating phosphoinositide binding to, and the intracellular localization of, the IRS1-PH domain, which can ultimately impinge on insulin-stimulated glucose uptake. Hence we provide evidence that IRS1-PH domain function is important for normal insulin signaling and is regulated by serine phosphorylation in a manner that could contribute to insulin resistance.

Involvement of novel PKC isoforms in FFA induced defects in insulin signaling

Involvement of novel PKCs (nPKCs) in the negative regulation of insulin-signaling pathway is a current interest of many workers investigating the cause for insulin resistance and type 2 diabetes. Free fatty acids (FFAs) are recently shown to be the major players in inducing insulin resistance in insulin target cells. They are also found to be involved in activating nPKCs associated with the impairment of insulin sensitivity. In this overview, we describe PKC delta, theta and epsilon linked to the FFA induced damage of insulin-signaling molecules.

Reversal of diet-induced hepatic steatosis and hepatic insulin resistance by antisense oligonucleotide inhibitors of acetyl-CoA carboxylases 1 and 2

Hepatic steatosis is a core feature of the metabolic syndrome and type 2 diabetes and leads to hepatic insulin resistance. Malonyl-CoA, generated by acetyl-CoA carboxylases 1 and 2 (Acc1 and Acc2), is a key regulator of both mitochondrial fatty acid oxidation and fat synthesis. We used a diet-induced rat model of nonalcoholic fatty liver disease (NAFLD) and hepatic insulin resistance to explore the impact of suppressing Acc1, Acc2, or both Acc1 and Acc2 on hepatic lipid levels and insulin sensitivity. While suppression of Acc1 or Acc2 expression with antisense oligonucleotides (ASOs) increased fat oxidation in rat hepatocytes, suppression of both enzymes with a single ASO was significantly more effective in promoting fat oxidation. Suppression of Acc1 also inhibited lipogenesis whereas Acc2 reduction had no effect on lipogenesis. In rats with NAFLD, suppression of both enzymes with a single ASO was required to significantly reduce hepatic malonyl-CoA levels in vivo, lower hepatic lipids (long-chain acyl-CoAs, diacylglycerol, and triglycerides), and improve hepatic insulin sensitivity. Plasma ketones were significantly elevated compared with controls in the fed state but not in the fasting state, indicating that lowering Acc1 and -2 expression increases hepatic fat oxidation specifically in the fed state. These studies suggest that pharmacological inhibition of Acc1 and -2 may be a novel approach in the treatment of NAFLD and hepatic insulin resistance.

Inhibition of insulin receptor gene expression and insulin signaling by fatty acid: interplay of PKC isoforms therein

Fatty acids are known to play a key role in promoting the loss of insulin sensitivity causing insulin resistance and type 2 diabetes. However, underlying mechanism involved here is still unclear. Incubation of rat skeletal muscle cells with palmitate followed by I(125)- insulin binding to the plasma membrane receptor preparation demonstrated a two-fold decrease in receptor occupation. In searching the cause for this reduction, we found that palmitate inhibition of insulin receptor (IR) gene expression effecting reduced amount of IR protein in skeletal muscle cells. This was followed by the inhibition of insulin-stimulated IRbeta tyrosine phosphorylation that consequently resulted inhibition of insulin receptor substrate 1 (IRS 1) and IRS 1 associated phosphatidylinositol-3 kinase (PI3 Kinase), phosphoinositide dependent kinase-1 (PDK 1) phosphorylation. PDK 1 dependent phosphorylation of PKCzeta and Akt/PKB were also inhibited by palmitate. Surprisingly, although PKCepsilon phosphorylation is PDK1 dependent, palmitate effected its constitutive phosphorylation independent of PDK1. Time kinetics study showed translocation of palmitate induced phosphorylated PKCepsilon from cell membrane to nuclear region and its possible association with the inhibition of IR gene transcription. Our study suggests one of the pathways through which fatty acid can induce insulin resistance in skeletal muscle cell.

New twist on neuronal insulin receptor signaling in health, disease, and therapeutics

Long after the pioneering studies documenting the existence of insulin (year 1967) and insulin receptor (year 1978) in brain, the last decade has witnessed extraordinary progress in the understanding of brain region-specific multiple roles of insulin receptor signalings in health and disease. In the hypothalamus, insulin regulates food intake, body weight, peripheral fat deposition, hepatic gluconeogenesis, reproductive endocrine axis, and compensatory secretion of counter-regulatory hormones to hypoglycemia. In the hippocampus, insulin promotes learning and memory, independent of the glucoregulatory effect of insulin. Defective insulin receptor signalings are associated with the dementia in normal aging and patients with age-related neurodegenerative diseases (e.g., Alzheimer's disease); the cognitive impairment can be reversed with systemic administration of insulin in the euglycemic condition. Intranasal administration of insulin enhances memory and mood and decreases body weight in healthy humans, without causing hypoglycemia. In the hypothalamus, insulin-induced activation of the phosphoinositide 3-kinase pathway followed by opening of ATP-sensitive K+ channel has been shown to be related to multiple effects of insulin. However, the precise molecular mechanisms of insulin's pleiotropic effects still remain obscure. More importantly, much remains unknown about the quality control mechanisms ensuring correct conformational maturation of the insulin receptor, and the cellular mechanisms regulating density of cell surface functional insulin receptors.

Shp2 is required for protein kinase C-dependent phosphorylation of serine 307 in insulin receptor substrate-1

The function of insulin receptor substrate-1 (IRS-1), a key molecule of insulin signaling, is modulated by phosphorylation at multiple serine/threonine residues. Phorbol ester stimulation of cells induces phosphorylation of two inhibitory serine residues in IRS-1, i.e. Ser-307 and Ser-318, suggesting that both sites may be targets of protein kinase C (PKC) isoforms. However, in an in vitro system using a broad spectrum of PKC isoforms (alpha, beta1, beta2, delta, epsilon, eta, mu), we detected only Ser-318, but not Ser-307 phosphorylation, suggesting that phorbol ester-induced phosphorylation of this site in intact cells requires additional signaling elements and serine kinases that link PKC activation to Ser-307 phosphorylation. As we have observed recently that the tyrosine phosphatase Shp2, a negative regulator of insulin signaling, is a substrate of PKC, we studied the role of Shp2 in this context. We found that phorbol ester-induced Ser-307 phosphorylation is reduced markedly in Shp2-deficient mouse embryonic fibroblasts (Shp2-/-) whereas Ser-318 phosphorylation is unaltered. The Ser-307 phosphorylation was rescued by transfection of mouse embryonic fibroblasts with wild-type Shp2 or with a phosphatase-inactive Shp2 mutant, respectively. In this cell model, tumor necrosis factor-alpha-induced Ser-307 phosphorylation as well depended on the presence of Shp2. Furthermore, Shp2-dependent phorbol ester effects on Ser-307 were blocked by wortmannin, rapamycin, and the c-Jun NH2-terminal kinase (JNK) inhibitor SP600125. This suggests an involvement of the phosphatidylinositol 3-kinase/mammalian target of rapamycin cascade and of JNK in this signaling pathway resulting in IRS-1 Ser-307 phosphorylation. Because the activation of these kinases does not depend on Shp2, it is concluded that the function of Shp2 is to direct these activated kinases to IRS-1.

Prevention of hepatic steatosis and hepatic insulin resistance in mitochondrial acyl-CoA:glycerol-sn-3-phosphate acyltransferase 1 knockout mice

In order to investigate the role of mitochondrial acyl-CoA:glycerol-sn-3-phosphate acyltransferase 1 (mtGPAT1) in the pathogenesis of hepatic steatosis and hepatic insulin resistance, we examined whole-body insulin action in awake mtGPAT1 knockout (mtGPAT1(-/-)) and wild-type (wt) mice after regular control diet or three weeks of high-fat feeding. In contrast to high-fat-fed wt mice, mtGPAT1(-/-) mice displayed markedly lower hepatic triacylglycerol and diacylglycerol concentrations and were protected from hepatic insulin resistance possibly due to a lower diacylglycerol-mediated PKC activation. Hepatic acyl-CoA has previously been implicated in the pathogenesis of insulin resistance. Surprisingly, compared to wt mice, mtGPAT1(-/-) mice exhibited increased hepatic insulin sensitivity despite an almost 2-fold elevation in hepatic acyl-CoA content. These data suggest that mtGPAT1 might serve as a novel target for treatment of hepatic steatosis and hepatic insulin resistance and that long chain acyl-CoA's do not mediate fat-induced hepatic insulin resistance in this model.

PKCepsilon induces interleukin-6 expression through the MAPK pathway in 3T3-L1 adipocytes

Recent reports have suggested that PKCepsilon contributes to systemic insulin resistance, and is involved in the pathogenesis of type 2 diabetes, however, the exact mechanism is still unknown. To elucidate the possible involvement of PKCepsilon in the pathogenesis of type 2 diabetes, we examined the role of PKCepsilon in differentiated adipocytes using mouse 3T3-L1 adipocytes. We found that the over-expression of PKCepsilon resulted in the increase of IL-6 expression in differentiated adipocytes. This PKCepsilon-induced IL-6 expression could be completely inhibited by U0126, an inhibitor of mitogen-activated protein/extracellular signal-regulated kinase (ERK) kinase. We also demonstrated that PKCepsilon increased the transcriptional activity of Est-like transcription factor (Elk-1) as well as the DNA-binding activity of activator protein-1 (AP-1) in differentiated 3T3-L1 adipocytes. These results suggest that PKCepsilon is able to increase IL-6 expression via the ERK-AP-1 pathway in differentiated adipocytes, and that PKCepsilon is involved in systemic insulin resistance by regulating plasma IL-6 concentrations.

Altered body composition and metabolism in the male offspring of high fat-fed rats

An intrauterine environment may play a role in predisposing a developing fetus to metabolic diseases during adulthood. We investigated the hypothesis that a maternal diet high in omega-6 polyunsaturated fat can modify the programming of an offspring's glucose tolerance, insulin sensitivity, body composition, lipid metabolism, and insulin signaling. High omega-6 polyunsaturated fat diets were fed to female rats 4 weeks before mating and throughout the gestation period. The offspring were maintained on chow diet. At 3 months of age, indirect calorimetry, oral glucose tolerance tests, and dual x-ray absorptiometry measurements were performed. Triglyceride content and beta-hydroxyacyl coenzyme A dehydrogenase activity were determined in the liver and quadriceps muscle. Expression levels of key insulin signaling pathway proteins were measured in the liver and quadriceps muscle of the 3-month-old offspring. Offspring from the fat-fed dams had significantly increased proportions of both total body fat and abdominal fat. All offspring displayed normal insulin sensitivity and glucose tolerance, although the offspring from the fat-fed dams were significantly more hyperinsulinemic 15 minutes after an oral glucose challenge. Whole body fuel oxidation was not altered. The offspring of fat-fed dams had significantly elevated liver triglyceride content. Insulin signaling protein expression levels in the offspring of fat-fed dams were consistent with reduced hepatic insulin sensitivity but increased quadriceps insulin sensitivity. A maternal diet high in omega-6 polyunsaturated fat evokes programming within the metabolic processes of the offspring that may predispose the offspring to the development of metabolic diseases.

Inhibition of cellular responses to insulin in a rat liver cell line. A role for PKC in insulin resistance

The initial response of liver cells to insulin is mediated through exocytosis of Cl- channel-containing vesicles and a subsequent opening of plasma membrane Cl- channels. Intracellular accumulation of fatty acids leads to profound defects in metabolism, and is closely associated with insulin resistance. It is not known whether the activity of Cl- channels is altered in insulin resistance and by which mechanisms. We studied the effects of fatty acid accumulation on Cl- channel opening in a model liver cell line. Overnight treatment with amiodarone increased the fat content by approximately 2-fold, and the rates of gluconeogenesis by approximately 5-fold. The ability of insulin to suppress gluconeogenesis was markedly reduced indicating that amiodarone treatment induces insulin resistance. Western blot analysis showed that these cells express the same number of insulin receptors as control cells. However, insulin failed to activate exocytosis and Cl- channel opening. These inhibitory effects were mimicked in control cells by exposures to arachidonic acid (15 microm). Further studies demonstrated that fatty acids stimulate the PKC activity, and inhibition of PKC partially restored exocytosis and Cl- channel opening in insulin-resistant cells. Accordingly, activation of PKC with PMA in control cells potently inhibited the insulin responses. These results suggest that stimulation of PKC activity in insulin resistance contributes to the inhibition of cellular responses to insulin in liver cells.

Mechanisms of the free fatty acid-induced increase in hepatic glucose production

The associations between obesity, insulin resistance, and type 2 diabetes mellitus are well documented. Free fatty acids (FFA), which are often elevated in obesity, have been implicated as an important link in these associations. Contrary to muscle glucose metabolism, the effects of FFA on hepatic glucose metabolism and the associated mechanisms have not been extensively investigated. It is still controversial whether FFA have substantial effects on hepatic glucose production, and the mechanisms responsible for these putative effects remain unknown. We review recent progress in this area and try to clarify controversial issues regarding the mechanisms responsible for the FFA-induced increase in hepatic glucose production in the postabsorptive state and during hyperinsulinemia.

Exploring levels of hexosamine biosynthesis pathway intermediates and protein kinase C isoforms in muscle and fat tissue of Zucker Diabetic Fatty rats

Many studies suggest that insulin resistance develops and/or is maintained by an increased flux of glucose through the hexosamine biosynthesis pathway. This pathway may attenuate insulin-stimulated glucose uptake by activating protein kinase C (PKC). Therefore, we investigated whether the concentrations of the major hexosamine metabolites, uridine diphosphate- N-acetyl-glucosamine (UDP-GlcNAc) and uridine diphosphate- N-acetyl-galactosamine (UDP-GalNAc), and the expression levels of PKC isoforms were affected in Zucker Diabetic Fatty (ZDF) rats, an animal model widely used to study type 2 diabetes mellitus. At the age of 6 wk, control and ZDF rats were normoglycemic. Whereas control rats remained normoglycemic, the ZDF rats became hyperglycemic. The amount of UDP-GlcNAc and UDP-GalNAc in muscle tissue of ZDF rats was similar at 6, 12, 18, and 24 wk of age. Moreover, the concentration of both hexosamines did not differ among ZDF, phlorizin-treated ZDF, and control rats. Western blot analysis revealed that PKCalpha, delta, epsilon, andzeta, but not PKCbeta and gamma, were expressed in muscle and fat tissues from 6- and 24-wk-old control and ZDF rats. In addition, we did not observe changes in the expression levels of the PKC isoforms following prolonged hyperglycemia. Taken together, these findings indicate that the amounts of several metabolites from the hexosamine biosynthesis pathway and PKC isoforms, both hypothesized to be important in the development and/or maintenance of the insulin-resistant state of muscle and fat tissue, are not different in ZDF compared with nondiabetic rats.

High levels of palmitic acid lead to insulin resistance due to changes in the level of phosphorylation of the insulin receptor and insulin receptor substrate-1

Insulin resistance is defined as the decrease in the glucose disposal in response to insulin by the target tissues. High concentrations of nonesterified fatty acids (NEFA) in plasma have been implicated with many insulin resistance states. We evaluated several aspects of the insulin resistance induced by palmitic acid in rats and found that after treatment with 0.09 g/kg of palmitic acid there is a delay in the curve of tolerance to glucose. We measured the changes in protein phosphorylation in samples from abdominus rectus muscle and there was a decrease of 64 and 75% in the levels of phosphorylation in tyrosine of the insulin receptor and insulin receptor substrate-1, respectively. This diminution in the tyrosine phosphorylation is consistent with a decrease in the main pathway known to be activated after insulin treatment, the mitogen activated protein kinases (MAPKs). If the animals were treated with inhibitors of PKC, like sphingosine, there was a prevention of the effect of palmitic acid determined at the level of tyrosine phosphorylation. According with this result, we found an increase in the phosphorylations in serine of the insulin receptor after the treatment with palmitate. These results suggest that PKC has a role as negative regulator (by phosphorylation in serine) of the insulin receptors activation in the insulin resistance induced by palmitic acid.

Free fatty acid-induced hepatic insulin resistance: a potential role for protein kinase C-delta

The mechanisms of the impairment in hepatic glucose metabolism induced by free fatty acids (FFAs) and the importance of FFA oxidation in these mechanisms remain unclear. FFA-induced peripheral insulin resistance has been linked to membrane translocation of novel protein kinase C (PKC) isoforms, but the role of PKC in hepatic insulin resistance has not been assessed. To investigate the biochemical pathways that are induced by FFA in the liver and their relation to glucose metabolism in vivo, we determined endogenous glucose production (EGP), the hepatic content of citrate (product of acetyl-CoA derived from FFA oxidation and oxaloacetate), and hepatic PKC isoform translocation after 2 and 7 h Intralipid + heparin (IH) or SAL in rats. Experiments were performed in the basal state and during hyperinsulinemic clamps (insulin infusion rate, 5 mU. kg(-1). min(-1)). IH increased EGP in the basal state (P < 0.001) and during hyperinsulinemia (P < 0.001) at 2 and 7 h. Also, 7-h infusion of IH induced resistance to the suppressive effect of insulin on EGP (P < 0.05). Glycerol infusion (resulting in plasma glycerol levels similar to IH infusion) did not have any effect on EGP. IH increased hepatic citrate content by twofold, independent of the insulin levels and the duration of IH infusion. IH induced hepatic PKC-delta translocation from the cytosolic to membrane fraction in all groups. PKC-delta translocation was greater at 7 compared with 2 h (P < 0.05). In conclusion, 1) increased FFA oxidation may contribute to the FFA-induced increase in EGP in the basal state and during hyperinsulinemia but is not associated with FFA-induced hepatic insulin resistance, and 2) the progressive insulin resistance induced by FFA in the liver is associated with a progressive increase in hepatic PKC-delta translocation.

Protein kinase C and lipid-induced insulin resistance in skeletal muscle

Insulin resistance of skeletal muscle in humans, animals, and cells is often strongly correlated with increased lipid availability. The elevation of certain intracellular lipid species can lead to the activation of signal transduction pathways that inhibit normal insulin action. Thus, increased diacylglycerol levels in muscle are associated with the activation of one or more isoforms of the protein kinase C family, which is known to attenuate insulin signaling, especially at the level of IRS-1. In addition, de novo synthesis of ceramide can inhibit more distal sites by the activation of protein phosphatase 2A and hence promote the dephosphorylation and inactivation of protein kinase B. Such mechanisms may account at least in part for the reduced insulin sensitivity occurring in obesity and type 2 diabetes where lipid oversupply is a major factor.

From receptor to effector: insulin signal transduction in skeletal muscle from type II diabetic patients

Insulin resistance is a characteristic feature of type II diabetes mellitus and obesity. Although defects in glucose homeostasis have been recognized for decades, the molecular mechanisms accounting for impaired whole body glucose uptake are still not fully understood. Skeletal muscle constitutes the largest insulin-sensitive organ in humans; thus, insulin resistance in this tissue will have a major impact on whole body glucose homeostasis. Intense efforts are under way to define the molecular mechanisms that regulate glucose metabolism and gene expression in insulin-sensitive tissues. Knowledge of the human genome sequence, used in concert with gene and/or protein array technology, will provide a powerful means to facilitate efforts in revealing molecular targets that regulate glucose homeostasis in type II diabetes mellitus. This will offer quicker ways forward to identifying gene expression profiles in insulin-sensitive and insulin-resistant human tissue. This review will present our current understanding of potential defects in insulin signal transduction pathways, with an emphasis on mechanisms regulating glucose transport in skeletal muscle from people with type II diabetes mellitus. Elucidation of the pathways involved in the regulation of glucose homeostasis will offer insight into the causation of insulin resistance and type II diabetes mellitus. Furthermore, this will identify biochemical entry points for drug intervention to improve glucose homeostasis.

Protein kinase C and the development of diabetic vascular complications

Hyperglycemic control in diabetes is key to preventing the development and progression of vascular complications such as retinopathy, nephropathy and neuropathy. Increased activation of the diacylglycerol (DAG)-protein kinase C (PKC) signal transduction pathway has been identified in vascular tissues from diabetic animals, and in vascular cells exposed to elevated glucose. Vascular abnormalities associated with glucose-induced PKC activation leading to increased synthesis of DAG include altered vascular blood flow, extracellular matrix deposition, basement membrane thickening, increased permeability and neovascularization. Preferential activation of the PKCbeta isoform by elevated glucose is reported to occur in a variety of vascular tissues. This has lead to the development of LY333531, a PKCbeta isoform specific inhibitor, which has shown potential in animal models to be an orally effective and nontoxic therapy able to produce significant improvements in diabetic retinopathy, nephropathy, neuropathy and cardiac dysfunction. Additionally, the antioxidant vitamin E has been identified as an inhibitor of the DAG-PKC pathway, and shows promise in reducing vascular complications in animal models of diabetes. Given the overwhelming evidence indicating a role for PKC activation in contributing to the development of diabetic vascular complications, pharmacological therapies that can modulate this pathway, particularly with PKC isoform selectivity, show great promise for treatment of vascular complications, even in the presence of hyperglycemia.

Obesity is associated with impaired ventricular protein kinase C-MAP kinase signaling and altered ANP mRNA expression in the heart of adult Zucker rats

BACKGROUND

In the obesity model of the Zucker rat, myocardial protein kinase C (PKC) activation by phorbol ester is impaired. The influence of obesity on myocardial cell signaling was investigated by studying the activation of PKC isozymes and MAP kinases (MAPK) p38 and p42/44 as well as the induction of ANP mRNA.

METHODS

Isolated hearts obtained from 17-week-old lean and obese Zucker rats were perfused with 200 nM phorbol 12-myristate 13-acetate (PMA) at different time periods. Immunodetectable PKC isozymes, phosphorylated-MAPK, and ANP mRNA were determined by Western and Northern blots, respectively.

RESULTS

PMA promoted a marked transient translocation of ventricular PKCalpha from the cytosol to the membranes within 10 minutes in lean rats, whereas it had a much weaker effect in obese rats. Moreover, PMA induced a significant activation of PKCdelta in lean but not in obese rat hearts. After PKC activation, increases in phosphorylation levels of myocardial p38 and p42 MAPK were approximately 3-fold higher in lean rats than in obese animals. Concerning the induction of ANP, PMA transiently tripled ANP mRNA within 60 minutes in lean but not in obese rats.

CONCLUSIONS

In the genetically obese Zucker rat, the myocardial signal transduction cascade PKC-MAPK-ANP mRNA seems to be markedly impaired. It can be speculated that this abnormal cardiac cell signaling in obese rats reflects an early phase in the cardiac pathogenesis accompanying obesity.

Nonesterified fatty acids in blood pressure control and cardiovascular complications

The fact that cardiovascular risk factors cluster among individuals with the insulin resistance syndrome strongly suggests a common pathogenetic denominator. For many years, abnormalities of nonesterified fatty acid metabolism have been implicated in the disturbances of carbohydrate and lipid metabolism that characterize the cluster. However, until more recently, evidence implicating fatty acids in the hemodynamic and vascular abnormalities that affect patients with this syndrome was lacking. Observations from epidemiological, clinical, and basic science suggest that fatty acids can raise blood pressure and contribute to the development of hypertension. The effects of fatty acids on blood pressure may be mediated in part by inhibition of endothelial nitric oxide synthase activity and endothelium-dependent vasodilation. Fatty acids can also increase alpha1-adrenoceptor-mediated vascular reactivity and induce vascular smooth muscle migration and proliferation. The adverse effects of fatty acids appear to be mediated in part through induction of oxidative stress. Fatty acids interact with other components of the risk factor cluster, including increased angiotensin II, to synergistically augment oxidative stress in cultured vascular smooth muscle cells. Oxidative stress is implicated in the pathogenesis of insulin resistance, hypertension, vascular remodeling, and vascular complications. A clearer definition of the specific reactive oxygen signaling pathways involved and interventions aimed at altering these pathways could lead to more rationale antioxidant therapy and improved outcomes.

Protein kinase C activation and its pharmacological inhibition in vascular disease

Vascular complications in diabetes mellitus are known to be associated with the activation of the protein kinase C (PKC) pathway through the de novo synthesis of diacylglycerol (DAG) from glycolytic intermediates. Specific PKC isoforms, mainly the beta- and delta-isoforms, have been shown to be persistently activated in diabetic mellitus. Multiple studies have reported that the activation of PKC leads to increased production of extracellular matrix and cytokines, enhances contractility, permeability and vascular cell proliferation, induces the activation of cytosolic phospholipase A2 and inhibits the activity of Na+-K+-ATPase. These events are not only frequently observed in diabetes mellitus but are also involved in the actions of vasoactive agents or oxidative stress. Inhibition of PKC by two different kinds of PKC inhibitors - LY333531, a selective PKC-beta-isoform inhibitor, and vitamin E, d-alpha-tocopheron - were able to prevent or reverse the various vascular dysfunctions in vitro and in vivo. Clinical studies using these compounds are now ongoing to evaluate the significance of DAG-PKC pathway activation in the development of vascular complications in diabetic patients.

Cellular mechanism of nutritionally induced insulin resistance in Psammomys obesus: overexpression of protein kinase Cepsilon in skeletal muscle precedes the onset of hyperinsulinemia and hyperglycemia

The sand rat (Psammomys obesus) is an animal model of nutritionally induced diabetes. We report here that several protein kinase C (PKC) isoforms (alpha, epsilon, and zeta, representing all three subclasses of PKC) are overexpressed in the skeletal muscle of diabetic animals of this species. This is most prominent for the epsilon isotype of PKC. Interestingly, increased expression of PKCepsilon could already be detected in normoinsulinemic, normoglycemic (prediabetic) animals of the diabetes-prone (DP) line when compared with a diabetes-resistant (DR) line. In addition, plasma membrane (PM)-associated fractions of PKCalpha and PKCepsilon were significantly increased in skeletal muscle of diabetic animals, suggesting chronic activation of these PKC isotypes in the diabetic state. The increased PM association of these PKC isotypes revealed a significant correlation with the diacylglycerol content in the muscle samples. Altered expression/activity of PKCepsilon, in particular, may thus contribute to the development of diabetes in these animals; along with other PKC isotypes, it may be involved in the progression of the disease. This may possibly occur through inhibition of insulin receptor (IR) tyrosine kinase activity mediated by serine/threonine phosphorylation of the IR or insulin receptor substrate 1 (IRS-1). However, overexpression of PKCepsilon also mediated down-regulation of IR numbers in a cell culture model (HEK293), resulting in attenuation of insulin downstream signaling (reduced protein kinase B [PKB]/Akt activity). In accordance with this, we detected decreased 125I-labeled insulin binding, probably reflecting a downregulation of IR numbers, in skeletal muscle of Psammomys animals from the DP line. The number of IRs was inversely correlated to both the expression and PM-associated levels of PKCepsilon. These data suggest that overexpression of PKCepsilon may be causally related to the development of insulin resistance in these animals, possibly by increasing the degradation of IRs.

Diacylglycerol kinase zeta in hypothalamus interacts with long form leptin receptor. Relation to dietary fat and body weight regulation

Leptin and its long form receptor, Ob-Rb, in hypothalamic nuclei play a key role in regulating energy balance. The mutation of Ob-Rb into one of its natural variants, Ob-Ra, results in severe obesity in rodents. We demonstrate here that diacylglycerol kinase zeta (DGKzeta) interacts, via its ankyrin repeats, with the cytoplasmic portion of Ob-Rb in yeast two-hybrid systems, in protein precipitation experiments in vitro and in vivo. It does not interact, however, with the short form, Ob-Ra, which mediates the entry of leptin into the brain. Furthermore, we show by in situ hybridization that DGKzeta is expressed in neurons of hypothalamic nuclei known to synthesize Ob-Rb and to participate in energy homeostasis. The mutant ob-/ob- and db-/db- mice exhibit increased hypothalamic DGKzeta mRNA level compared with their wild-type controls, suggesting a role for the leptin/OB-Rb system in regulating DGKzeta expression. Further experiments show that hypothalamic DGKzeta mRNA level is stimulated by the consumption of a high-fat diet. In addition, DGKzeta mRNA is statistically significantly lower in rats and inbred mice that become obese on a high-fat diet compared with their lean counterparts. In fact, it is strongly, negatively correlated with both body fat and circulating levels of leptin. Taken together, our evidence suggests that DGKzeta constitutes a downstream component of the leptin signaling pathway and that reduced hypothalamic DGKzeta mRNA, and possibly activity, is associated with obesity.

Signaling events mediating the additive effects of oleic acid and angiotensin II on vascular smooth muscle cell migration

Obese hypertensive patients with cardiovascular risk factor clustering and increased risk for atherosclerotic disease have increased plasma nonesterified fatty acid levels, including oleic acid (OA), and a more active renin-angiotensin-aldosterone system. Vascular smooth muscle cell (VSMC) migration and proliferation participate in the development of atherosclerotic plaque. OA and angiotensin (Ang) II induce synergistic mitogenic responses in VSMCs through sequential signaling pathways dependent on the activation of protein kinase C (PKC), oxidants (reactive oxygen species, ROS), and extracellular signal-regulated kinase (ERK) activation. We tested the hypotheses that (1) OA and Ang II have additive or synergistic effects on VSMC migration and (2) PKC, ROS, and mitogen-activated protein kinase are critical signaling molecules. OA at 100 micromol/L increases VSMC migration 60+/-10% over control (P:<0.001). Ang II (10(-)(9) mol/L) increases VSMC migration by 62+/-13% and 73% over control, respectively (P:<0.01). Coincubation of cells with OA and Ang II produces a nearly additive increase in VSMC cell migration at 107+/-20% (P:<0.01). Increases in VSMC migration induced by OA alone and combined with Ang II were reduced by PKC inhibition and downregulation. VSMC migration in response to OA alone and with Ang II was also inhibited by N:-acetyl-cysteine, MEK inhibition, and ERK antisense. VSMC migration in response to OA alone or combined with Ang II is dependent on activation of PKC, ROS, and ERK activation, further raising the possibility that increased plasma nonesterified fatty acids and an activated renin-angiotensin-aldosterone system in subjects with the risk factor cluster contribute to accelerated atherosclerosis through a PKC, ROS, and ERK-dependent signaling pathway.

Acute reversal of lipid-induced muscle insulin resistance is associated with rapid alteration in PKC-theta localization

Muscle insulin resistance in the chronic high-fat-fed rat is associated with increased membrane translocation and activation of the novel, lipid-responsive, protein kinase C (nPKC) isozymes PKC-theta and -epsilon. Surprisingly, fat-induced insulin resistance can be readily reversed by one high-glucose low-fat meal, but the underlying mechanism is unclear. Here, we have used this model to determine whether changes in the translocation of PKC-theta and -epsilon are associated with the acute reversal of insulin resistance. We measured cytosol and particulate PKC-alpha and nPKC-theta and -epsilon in muscle in control chow-fed Wistar rats (C) and 3-wk high-fat-fed rats with (HF-G) or without (HF-F) a single high-glucose meal. PKC-theta and -epsilon were translocated to the membrane in muscle of insulin-resistant HF-F rats. However, only membrane PKC-theta was reduced to the level of chow-fed controls when insulin resistance was reversed in HF-G rats [% PKC-theta at membrane, 23.0 +/- 4.4% (C); 39.7 +/- 3.4% (HF-F, P < 0.01 vs. C); 22.5 +/- 2.7% (HF-G, P < 0.01 vs. HF-F), by ANOVA]. We conclude that, although muscle localization of both PKC-epsilon and PKC-theta are influenced by chronic dietary lipid oversupply, PKC-epsilon and PKC-theta localization are differentially influenced by acute withdrawal of dietary lipid. These results provide further support for an association between PKC-theta muscle cellular localization and lipid-induced muscle insulin resistance and stress the labile nature of high-fat diet-induced insulin resistance in the rat.

Insulin-treated Zucker diabetic fatty rats retain the hypertriglyceridemia associated with obesity

Lipoprotein and apolipoprotein changes were evaluated in 10-week-old Zucker diabetic fatty (ZDF) male rats following 12 weeks of insulin treatment, which normalized blood glucose and maintained weight gaining characteristic of nondiabetic Zucker fatty rats. Compared with untreated ZDF rats (saline-injected), insulin treatment resulted in increased very-low-density lipoprotein (VLDL; d < 1.006 g/mL) and decreased alpha lipoprotein on agarose gel electrophoresis. These findings were consistent with an observed increase in VLDL triglyceride and cholesterol, and decreased high-density lipoprotein (HDL) cholesterol with insulin treatment in isolated lipoproteins. B100 levels were unchanged by insulin treatment, but B48 levels were significantly increased in the VLDL fraction. Insulin treatment depressed apolipoprotein (apo) A-I levels in HDL, but had little effect on total apo E, apo A-IV, or apo C, although apo C was redistributed to the VLDL fraction. These results suggest that insulin treatment of ZDF rats normalizes hyperglycemia and prevents age-related changes in lipoprotein parameters associated with development of insulinopenic diabetes. Insulin therapy in ZDF rats thereby sustains the hyperlipidemic lipoprotein pattern associated with hyperinsulinemia and obesity.

Signalling aspects of insulin resistance in skeletal muscle: mechanisms induced by lipid oversupply

A reduced capacity for insulin to elicit increases in glucose uptake and metabolism in target tissues such as skeletal muscle is a common feature of obesity and diabetes. The association between lipid oversupply and such insulin resistance is well established, and evidence for mechanisms through which lipids could play a causative role in the generation of muscle insulin resistance is reviewed. While the effects of lipids may in part be mediated by substrate competition through the glucose-fatty acid cycle, interference with insulin signal transduction by lipid-activated signalling pathways is also likely to play an important role. Thus, studies of insulin resistance in Type 2 diabetes, obesity, fat-fed animals and lipid-treated cells have identified defects both at the level of insulin receptor-mediated tyrosine phosphorylation and at downstream sites such as protein kinase B (PKB) activation. Lipid signalling molecules can be derived from free fatty acids, and include diacylglycerol, which activates isozymes of the protein kinase C (PKC) family, and ceramide, which has several effectors including PKCs and a protein phosphatase. In addition, elevated lipid availability can increase flux through the hexosamine biosynthesis pathway which can also lead to activation of PKC as well as protein glycosylation and modulation of gene expression. The mechanisms giving rise to decreased insulin signalling include serine/threonine phosphorylation of insulin receptor substrate-1, but also direct inhibition of components such as PKB. Thus lipids can inhibit glucose disposal by causing interference with insulin signal transduction, and most likely by more than one pathway depending on the prevalent species of fatty acids.

Protein kinase C-alpha and -epsilon down-regulate cell surface sodium channels via differential mechanisms in adrenal chromaffin cells

In cultured bovine adrenal chromaffin cells, our [3H]saxitoxin ([3H]STX) binding, immunoblot, and northern blot analyses specified protein kinase C (PKC) isoform-specific posttranscriptional and posttranslational mechanisms that direct down-regulation of cell surface Na channels. Immunoblot analysis showed that among 11 PKC isoforms, adrenal chromaffin cells contained only conventional (c)PKC-alpha, novel (n)PKC-epsilon, and atypical (a)PKC-zeta. Treatment of adrenal chromaffin cells with 100 nM 12-O-tetradecanoylphorbol 13-acetate (TPA) or 100 nM phorbol 12,13-dibutyrate (PDBu) caused a rapid (<15 min) and sustained (>15 h) translocation of PKC-alpha and -epsilon (but not -zeta) from cytosol to membranes, whereas a biologically inactive 4alpha-TPA had no effect. Thymeleatoxin (TMX), an activator of cPKC, produced similar membrane association of only PKC-alpha at 100 nM, with the potency of TMX being comparable with those of TPA and PDBu. Treatment with either 100 nM TPA or 100 nM TMX reduced cell surface [3H]STX binding to a comparable extent at 3, 6, and 12 h, whereas TPA lowered the binding to a greater extent than TMX at 15, 18, and 24 h; at 15 h, Gö6976, a specific inhibitor of cPKC, completely blocked TMX-induced decrease of [3H]STX binding while preventing by merely 57% TPA-induced decrease of [3H]STX binding. Treatment with 100 nM TPA lowered the Na channel alpha-subunit mRNA level between 3 and 12 h, with its maximum 52% fall at 6 h, and it was accompanied by a subsequent 61 % rise of the beta1-subunit mRNA level at 24 h. Gö6976 failed to prevent TPA-induced reduction of the alpha-subunit mRNA level; TMX did not change the alpha- and beta1-subunit mRNA levels throughout the 24-h treatment. Brefeldin A, an inhibitor of vesicular exit from the trans-Golgi network, augmented TPA- and TMX-induced decrease of [3H]STX binding at 1 and 3 h. Our previous and present studies suggest that PKC down-regulates cell surface Na channels without altering the allosteric gating of Na channels via PKC isoform-specific mechanisms; cPKC-alpha promotes Na channel internalization, whereas nPKC-epsilon decreases the alpha-subunit mRNA level by shortening the half-life of alpha-subunit mRNA without changing its gene transcription.

Protein kinase C modulates insulin action in human skeletal muscle

There is good evidence from cell lines and rodents that elevated protein kinase C (PKC) overexpression/activity causes insulin resistance. Therefore, the present study determined the effects of PKC activation/inhibition on insulin-mediated glucose transport in incubated human skeletal muscle and primary adipocytes to discern a potential role for PKC in insulin action. Rectus abdominus muscle strips or adipocytes from obese, insulin-resistant, and insulin-sensitive patients were incubated in vitro under basal and insulin (100 nM)-stimulated conditions in the presence of GF 109203X (GF), a PKC inhibitor, or 12-deoxyphorbol 13-phenylacetate 20-acetate (dPPA), a PKC activator. PKC inhibition had no effect on basal glucose transport. GF increased (P < 0.05) insulin-stimulated 2-deoxyglucose (2-DOG) transport approximately twofold above basal. GF plus insulin also increased (P < 0.05) insulin receptor tyrosine phosphorylation 48% and phosphatidylinositol 3-kinase (PI 3-kinase) activity approximately 50% (P < 0.05) vs. insulin treatment alone. Similar results for GF on glucose uptake were observed in human primary adipocytes. Further support for the hypothesis that elevated PKC activity is related to insulin resistance comes from the finding that PKC activation by dPPA was associated with a 40% decrease (P < 0.05) in insulin-stimulated 2-DOG transport. Incubation of insulin-sensitive muscles with GF also resulted in enhanced insulin action ( approximately 3-fold above basal). These data demonstrate that certain PKC inhibitors augment insulin-mediated glucose uptake and suggest that PKC may modulate insulin action in human skeletal muscle.

High insulin levels do not influence PC-1 gene expression and protein content in human muscle tissue and hepatoma cells

BACKGROUND

To verify whether insulin levels influence PC-1 tissue content, we studied PC-1 gene expression and protein content in skeletal muscle of patients with insulinoma, a model of primary hyperinsulinemia. Data were compared with those obtained in matched insulin sensitive or resistant healthy subjects. In addition, the effect of high insulin concentration on PC-1 protein content was studied in HepG2 cells.

METHODS

The following measurements were performed: insulin sensitivity by euglycemic clamp; PC-1 protein content and insulin receptor autophosphorylation by specific ELISAs; PC-1 gene expression by competitive polymerase chain reaction (PCR); phosphatidyl-inositol-3 kinase by immunoprecipitation and thin layer chromatography; glycogen synthesis by (14)C-glucose incorporation.

RESULTS

Muscle PC-1 content was similar in the insulinoma patients and in insulin sensitive controls but higher (p<0.01) in insulin resistant controls (21.9+/-4.6 ng/mg protein, 23.8+/-3.9, 48.0+/-8.7, respectively). PC-1 protein content was inversely correlated with insulin sensitivity (r=-0.5, p<0.015) but with neither plasma insulin nor glucose levels. PC-1 protein content was correlated with PC-1 gene expression (r=0.53, p<0.05, n=14). Exposure to high insulin (100 nmol/l for 16 h) caused a significant (p<0.05-0.01) impairment of insulin receptor autophosphorylation, phosphatidyl-inositol-3 kinase activity and glycogen synthesis, but not of PC-1 protein content (114+/-3 vs 102+/-14 ng/mg protein) in HepG2 cells.

CONCLUSION

These findings suggest that chronic high insulin levels do not influence PC-1 expression.

Changes in cardiac protein kinase C activities and isozymes in streptozotocin-induced diabetes

To understand cardiac dysfunction in diabetes, the activity of protein kinase C (PKC) and protein contents of its isozymes (PKC-alpha, -beta, -epsilon, and -zeta) were examined in diabetic rats upon injection of streptozotocin (65 mg/kg iv). The hearts were removed at 1, 2, 4, and 8 wk, and some of the 6-wk diabetic animals had been injected with insulin (3 U/day) for 2 wk. The Ca(2+)-dependent PKC activity was increased by 43 and 51% in the homogenate fraction and 31 and 70% in the cytosolic fraction from the 4- and 8-wk diabetic hearts, respectively, in comparison with control values. The Ca(2+)-independent PKC activity was increased by 24 and 32% in the homogenate fraction and 52 and 89% in the cytosolic fraction from the 4- and 8-wk diabetic hearts, respectively, in comparison with control values. The relative protein contents of PKC-alpha, -beta, -epsilon, and -zeta isozymes were increased by 43, 31, 48, and 38%, respectively, in the homogenate fraction and by 126, 119, 148, and 129%, respectively, in the cytosolic fraction of the 8-wk diabetic heart. The observed changes in heart homogenate and cytosolic fractions were partially reversible upon treatment of the diabetic rats with insulin. The results suggest that the increased myocardial PKC activity and increased protein contents of the cytosolic PKC isozymes are associated with subcellular alterations and cardiac dysfunction in the diabetic heart.

A soluble PC-1 circulates in human plasma: relationship with insulin resistance and associated abnormalities

An increased tissue content of PC-1, an inhibitor of insulin receptor signaling, may play a role in insulin resistance. Large scale prospective studies to test this hypothesis are difficult to carry out because of the need for tissue biopsies. The aim of this study was to investigate whether PC-1 is measurable in human plasma and whether its concentration is related to insulin sensitivity. A soluble PC-1, with mol wt and enzymatic activity similar to those of tissue PC-1, was measurable in human plasma by a specific enzyme-linked immunosorbent assay and was inversely correlated to skeletal muscle PC-1 content (r = -0.5; P < 0.01). The plasma PC-1 concentration was decreased (P < 0.05) in insulin-resistant (22.7 +/- 3.0 ng/mL; n = 25) compared to insulin-sensitive (36.7 +/- 4.5; n = 25) nondiabetic subjects and was correlated negatively with the waist/hip ratio (r = -0.48; P < 0.001) and mean blood pressure (r = -0.3; P < 0.05) and positively with high density lipoprotein/total cholesterol (r = 0.38; P < 0.01) and both the M value and the plasma free fatty acid level decrement at clamp studies (r = 0.28; n = 50; P = 0.05 and r = 0.43; n = 22; P < 0.05, respectively). A plasma PC-1 concentration of 19 ng/mL or less identified a cluster of insulin resistance-related alterations with 75% accuracy. In conclusion, PC-1 circulates in human plasma, and its concentration is related to insulin sensitivity. This may help to plan studies aimed at understanding the role of PC-1 in insulin resistance.

Can protein kinase C inhibition and vitamin E prevent the development of diabetic vascular complications?

Hyperglycemia causes vascular complications of diabetes possible by the activation of protein kinase C (PKC). We have provided substantial evidence that activation of PKC can lead to a whole host of vascular dysfunction in diabetes. The activation of PKC induced by hyperglycemia appears to be due to an increase in diacylglycerol (DAG) levels, a physiological activator of PKC. Studies involving cultural cells, animal models of diabetes and patients have shown that inhibition of PKC by specific PKC inhibitor was able to reverse many of the vascular dysfunctions in the retina, kidney and cardiovascular systems induced by either hyperglycemia or diabetes. In addition high doses of vitamin E were shown to decrease the level of DAG and PKC induced by diabetes or hyperglycemia. Thus animal and clinical studies have shown that high doses of vitamin E treatment can apparently reverse some of the changes in the retinal and renal vessels.

Vascular effects of non-esterified fatty acids: implications for the cardiovascular risk factor cluster

Insulin resistance emerges as a central component of the risk factor cluster and is a likely contributor to vascular disease independently of traditional risk factors such as hypertension and diabetes mellitus. However, the intermediary mechanisms by which atherosclerosis is accelerated among patients with the insulin resistance syndrome remain inadequately defined. Most of the attention has centered on hyperinsulinemia and defects of insulin-mediated glucose disposal. However, we observed that obese hypertensive patients have elevated plasma concentrations of non-esterified fatty acids (NEFAs), including oleic acid, which are highly resistant to suppression by insulin. Resistance to insulin's fatty acid lowering action correlate with blood pressure in obese subjects independently of defects in glucose disposal. This observation raises the possibility that NEFAs have biologically significant effects on the cardiovascular system. In fact, oleic acid impairs nitric oxide synthase activity and endothelium-dependent vasorelaxation in vitro. Moreover, raising NEFAs in normal human volunteers to levels observed in obese hypertensive patients impairs lower extremity endothelium-dependent vasodilation and augments local and systemic vascular alpha1-adrenoceptor reactivity in normal volunteers. Thus, raising NEFAs replicates in healthy subjects important functional vascular changes implicated in the hypertension and atherosclerosis observed in patients with the risk factor cluster. At a molecular level, experiments in cultured vascular smooth muscle cells demonstrate that oleic acid activates a mitogenic signaling cascade which includes protein kinase C, reactive oxygen species and extracellular signal-regulated kinases. Each of these signaling events has been implicated in the structural and functional vascular changes which accompany the risk factor cluster. Collectively, these observations raise the possibility that fatty acids contribute to functional and structural vascular changes among insulin-resistant individuals. A better understanding of the signaling mechanisms by which NEFAs exert their vascular effects may facilitate novel and more effective therapeutic approaches to managing the cardiovascular risk factor cluster.

Increased expression of liver PKC alpha in hypoinsulinemic diabetic rats: a post-translational effect

Ca2+-dependent protein kinase C (cPKC) activity and expression have been studied in livers from hypoinsulinemic streptozotocin (STZ)-induced diabetic and untreated control rats. In diabetic rats, cPKC activity was slightly decreased in liver total particulate and nuclear fractions but was unchanged in mitochondrial-lysosomal, microsomal and cytosolic fractions. On Western immunoblot analysis, PKC alpha was identified as two distinct proteins of 90 and 81 kDa. In diabetic rats, the abundance of the 90 kDa protein was increased in most subcellular fractions with a maximum in the cytosolic and microsomal fractions (180%) but that of the 81 kDa protein was unchanged. PKC beta2 was detected as a single 81 kDa protein in cytosolic and microsomal fractions with unchanged levels in diabetic rats. Liver PKC alpha mRNA levels as measured by reverse transcription and competitive PCR amplification were similar in diabetic and control rats. The increased expression of PKC alpha protein in diabetic rats was reversed by insulin but not by phlorizin, suggesting that it did not result from hyperglycemia. We conclude that STZ-induced diabetes induces the expression of a biologically inactive form of PKC alpha which differs from active PKC alpha by an undefined post-translational modification, possibly an increase in phosphorylation state.

Insulin induction of protein kinase C alpha expression is independent of insulin receptor Tyr1162/1163 residues and involves mitogen-activated protein kinase kinase 1 and sustained activation of nuclear p44MAPK

We examined the effect of insulin on protein kinase C alpha (PKCalpha) expression and the implication of the mitogen-activated protein kinase kinase 1 mitogen-activated protein kinase (MAPK) pathway in this effect. PKCalpha expression was measured by quantitative RT-PCR and Western blotting using Chinese hamster ovary (CHO) cells overexpressing human insulin receptors of the wild type (CHO-R) or insulin receptors mutated at Tyr1162/1163 autophosphorylation sites (CHO-Y2). In CHO-R cells, insulin caused a time- and concentration-dependent increase in PKCalpha messenger RNA, with a maximum at 6 h and 10-(8)M insulin. This increase involved a transcriptional mechanism, as it was not due to stabilization of PKCalpha messenger RNA and was associated with a similar increase in the immunoreactive PKCalpha level. Insulin induction of PKCalpha expression involved the MEK1MAPK pathway, as it was 1) almost completely suppressed by the potent MEK1 inhibitor PD98059, 2) mimicked by the dominant-active MEK1 (S218D/S222D) mutant, and 3) associated with sustained MAPK activation. In CHO-Y2 cells in which the early phase of MAPK activation by insulin was lost and only the late and sustained phase of activation was observed, insulin signaling of PKCalpha expression was preserved and again involved the MEK1-MAPK pathway. Moreover, we show that in both CHO-R and CHO-Y2 cells, insulin stimulation of PKCalpha gene expression was associated with prolonged activation of nuclear p44MAPK. These results indicate that induction of PKCalpha gene expression by insulin is independent of Tyr1162/1163 autophosphorylation sites and correlates with sustained activation of p44MAPK at the nuclear level.

Translocation of PKC delta by insulin in a rat hepatoma cell line

The aim of this study was to examine the effects of insulin and phorbol 12-myristate 13-acetate (PMA), an activator of classic and novel PKCs, on the translocation of PKC from cytosol to membrane in H4IIE (H4) rat hepatoma cells. Six PKC isoforms were expressed, including PKC-mu and PKC-lambda, identified for the first time in this hepatoma-cell line. Insulin induced translocation of PKC-delta from the cytosol to the membrane fraction as early as 15 min and maximally at 60 min with levels returning to that of controls by 180 min. Insulin also decreased levels of PKC-zeta in membranes at 5, 10, 15, and 30 min, but had no effect on cytosol levels. Ten minutes of PMA treatment translocated PKC-delta completely, and 24 h of PMA treatment downregulated PKC-delta. Neither acute nor chronic PMA had any effect on PKC-zeta. These studies establish the ability of both insulin and PMA to activate PKC-delta in H4 cells, and coupled with our previous work demonstrating a diminution of the effect of insulin on gene transcription in PKC downregulated cells, suggest that insulin may exert specific effects, in part, through a PKC-dependent pathway.

Experimental diabetes is associated with functional activation of protein kinase C epsilon and phosphorylation of troponin I in the heart, which are prevented by angiotensin II receptor blockade

A cardiomyopathy that is characterized by an impairment in diastolic relaxation and a loss of calcium sensitivity of the isolated myofibril has been described in chronic diabetic animals and humans. To explore a possible role for protein kinase C (PKC)-mediated phosphorylation of myofibrillar proteins in this process, we characterized the subcellular distribution of the major PKC isoforms seen in the adult heart in cardiocytes isolated from diabetic rats and determined patterns of phosphorylation of the major regulatory proteins, including troponin I (TnI). Rats were made diabetic with a single injection of streptozotocin, and myocardiocytes were isolated and studied 3 to 4 weeks later. In nondiabetic animals, 76% of the PKC epsilon isoform was located in the cytosol and 24% was particulate, whereas in diabetic animals, 55% was cytosolic and 45% was particulate (P < .05). PKC delta, the other major PKC isoform seen in adult cardiocytes, did not show a change in subcellular localization. In parallel, TnI phosphorylation was increased 5-fold in cardiocytes isolated from the hearts of diabetic animals relative to control animals (P < .01). The change in PKC epsilon distribution and in TnI phosphorylation in diabetic animals was completely prevented by rendering the animals euglycemic with insulin or by concomitant treatment with a specific angiotensin II type-1 receptor (AT1) antagonist. Since PKC phosphorylation of TnI has been associated with a loss of calcium sensitivity of intact myofibrils, these data suggest that angiotensin II receptor-mediated activation of PKC may play a role in the contractile dysfunction seen in chronic diabetes.

Reversal of chronic alterations of skeletal muscle protein kinase C from fat-fed rats by BRL-49653

We have recently shown that the reduction in insulin sensitivity of rats fed a high-fat diet is associated with the translocation of the novel protein kinase C epsilon (nPKC epsilon) from cytosolic to particulate fractions in red skeletal muscle and also the downregulation of cytosolic nPKC theta. Here we have further investigated the link between insulin resistance and PKC by assessing the effects of the thiazolidinedione insulin-sensitizer BRL-49653 on PKC isoenzymes in muscle. BRL-49653 increased the recovery of nPKC isoenzymes in cytosolic fractions of red muscle from fat-fed rats, reducing their apparent activation and/or downregulation, whereas PKC in control rats was unaffected. Because BRL-49653 also improves insulin-stimulated glucose uptake in fat-fed rats and reduces muscle lipid storage, especially diglyceride content, these results strengthen the association between lipid availability, nPKC activation, and skeletal muscle insulin resistance and support the hypothesis that chronic activation of nPKC isoenzymes is involved in the generation of muscle insulin resistance in fat-fed rats.

Intrarenal distribution of rabbit PKC zeta

To elucidate roles of protein kinase C (PKC) zeta in rabbit kidney, PKC zeta was cloned from a rabbit kidney cortex cDNA library. Sequencing revealed a 2113 m insert with an open reading frame encoding a protein of 591 amino acids. The predicted amino acid sequence is 93.7% identical with rat PKC zeta. In situ hybridization in rabbit kidney with a riboprobe generated from the cloned cDNA, showed PKC zeta mRNA is highly expressed in proximal tubule, thick limb, and collecting duct. No message was detected over glomerular cells. Immunohistochemical studies using a monoclonal antibody against PKC zeta confirmed this distribution with low expression in vascular elements and high expression in tubule epithelium. Confocal microscopy showed diffuse cytosolic immunoreactivity in confluent cultured cortical collecting ducts (CCDs). However, in subconfluent cells, immunoreactivity was restricted to the peri-nuclear area. This differential distribution of PKC zeta in the CCD suggests that PKC zeta action be involved in growth and differentiation of the collecting duct. In conclusion, PKC zeta is differentially expressed in the rabbit kidney with high expression in the tubule epithelium and little expression in vascular elements. These studies suggest an important role for PKC zeta along the nephron.

Transcriptional regulation of insulin receptor substrate 1 by protein kinase C

Insulin receptor substrate-1 (IRS-1) is involved in insulin signal transduction distal to receptor occupation. Targeted disruption of IRS-1 leads to insulin resistance and hyperglycemia in mice, which suggests that altered IRS-1 expression could contribute to the insulin resistance seen in non-insulin-dependent diabetes mellitus. In vitro studies using phorbol esters have implicated the protein kinase C (PKC) pathway as being involved in the pathogenesis of insulin resistance. Using the MCF-7 breast cancer cell, a role for PKC in regulating IRS-1 expression was examined. In an MCF-7 cell line (MCF-7-PKC-alpha) that exhibits multiple alterations in PKC isoform expression, IRS-1 content was reduced to negligible levels relative to parental MCF-7 cells. This decrease in IRS-1 content was associated with a 30-fold reduction in IRS-1 transcription. In parental MCF-7 cells, PKC inhibitors (GF109203X (bisindolylmaleimide I) and staurosporine) reduced IRS-1 content. Chronic exposure to 12-O-tetradecanoylphorbol-13-acetate (TPA; >8 h) reduced IRS-1 content and down-regulated the novel PKC-delta isoform. Bryostatin 1 inhibited TPA-induced depletion of both IRS-1 and PKC-delta expression in MCF-7 cells. Associated with TPA-induced reduction in IRS-1 content was a reduction in IRS-1 transcription. These data demonstrate that PKC can modulate IRS-1 content and suggest a potential role for PKC-delta in positively regulating IRS-1 expression.