Defective activation of atypical protein kinase C zeta and lambda by insulin and phosphatidylinositol-3,4,5-(PO4)(3) in skeletal muscle of rats following high-fat feeding and streptozotocin-induced diabetes

Effects of Acute Muscle Contraction on the Key Molecules in Insulin and Akt Signaling in Skeletal Muscle in Health and in Insulin Resistant States

Insulin signaling plays a key role in glucose uptake, glycogen synthesis, and protein and lipid synthesis. In insulin-resistant states like obesity and type 2 diabetes mellitus, these processes are dysregulated. Regular physical exercise is a potential therapeutic strategy against insulin resistance, as an acute bout of exercise increases glucose disposal during the activity and for hours into recovery. Chronic exercise increases the activation of proteins involved in insulin signaling and increases glucose transport, even in insulin resistant states. Here, we will focus on the effect of acute exercise on insulin signaling and protein kinase B (Akt) pathways. Activation of proximal proteins involved in insulin signaling (insulin receptor, insulin receptor substrate-1 (IRS-1), phosphoinoside-3 kinase (PI3K)) are unchanged in response to acute exercise/contraction, while activation of Akt and of its substrates, TBC1 domain family 1 (TBC1D1), and TBC domain family 4 (TBC1D4) increases in response to such exercise/contraction. A wide array of Akt substrates is also regulated by exercise. Additionally, AMP-activated protein kinase (AMPK) seems to be a main mediator of the benefits of exercise on skeletal muscle. Questions persist on how mTORC1 and AMPK, two opposing regulators, are both upregulated after an acute bout of exercise.

Culprits or consequences: Understanding the metabolic dysregulation of muscle in diabetes

The prevalence of type 2 diabetes (T2D) continues to rise despite the amount of research dedicated to finding the culprits of this debilitating disease. Skeletal muscle is arguably the most important contributor to glucose disposal making it a clear target in insulin resistance and T2D research. Within skeletal muscle there is a clear link to metabolic dysregulation during the progression of T2D but the determination of culprits vs consequences of the disease has been elusive. Emerging evidence in skeletal muscle implicates influential cross talk between a key anabolic regulatory protein, the mammalian target of rapamycin (mTOR) and its associated complexes (mTORC1 and mTORC2), and the well-described canonical signaling for insulin-stimulated glucose uptake. This new understanding of cellular signaling crosstalk has blurred the lines of what is a culprit and what is a consequence with regard to insulin resistance. Here, we briefly review the most recent understanding of insulin signaling in skeletal muscle, and how anabolic responses favoring anabolism directly impact cellular glucose disposal. This review highlights key cross-over interactions between protein and glucose regulatory pathways and the implications this may have for the design of new therapeutic targets for the control of glucoregulatory function in skeletal muscle.

Insulin-Sensitive Protein Kinases (Atypical Protein Kinase C and Protein Kinase B/Akt): Actions and Defects in Obesity and Type II Diabetes

Glucose transport into muscle is the initial process in glucose clearance and is uniformly defective in insulin-resistant conditions of obesity, metabolic syndrome, and Type II diabetes mellitus. Insulin regulates glucose transport by activating insulin receptor substrate-1 (IRS-1)-dependent phosphatidylinositol 3-kinase (PI3K) which, via increases in PI-3, 4, 5-triphosphate (PIP3), activates atypical protein kinase C (aPKC) and protein kinase B (PKB/Akt). Here, we review (i) the evidence that both aPKC and PKB are required for insulin-stimulated glucose transport, (ii) abnormalities in muscle aPKC/PKB activation seen in obesity and diabetes, and (iii) mechanisms for impaired aPKC activation in insulin-resistant conditions. In most cases, defective muscle aPKC/PKB activation reflects both impaired activation of IRS-1/PI3K, the upstream activator of aPKC and PKB in muscle and, in the case of aPKC, poor responsiveness to PIP3, the lipid product of PI3K. Interestingly, insulin-sensitizing agents (e.g., thiazolidinediones, metformin) improve aPKC activation by insulin in vivo and PIP3 in vitro, most likely by activating 5′-adenosine monophosphate-activated protein kinase, which favorably alters intracellular lipid metabolism. Differently from muscle, aPKC activation in the liver is dependent on IRS-2/PI3K rather than IRS-1/PI3K and, surprisingly, the activation of IRS-2/PI3K and aPKC is conserved in high-fat feeding, obesity, and diabetes. This conservation has important implications, as continued activation of hepatic aPKC in hyperinsulinemic states may increase the expression of sterol regulatory element binding protein-1c, which controls genes that increase hepatic lipid synthesis. On the other hand, the defective activation of IRS-1/PI3K and PKB, as seen in diabetic liver, undoubtedly and importantly contributes to increases in hepatic glucose output. Thus, the divergent activation of aPKC and PKB in the liver may explain why some hepatic actions of insulin (e.g., aPKC-dependent lipid synthesis) are increased while other actions (e.g., PKB-dependent glucose metabolism) are diminished. This may explain the paradox that the liver secretes excessive amounts of both very low density lipoprotein triglycerides and glucose in Type II diabetes. Previous reviews from our laboratory that have appeared in the Proceedings have provided essentials on phospholipid-signaling mechanisms used by insulin to activate several protein kinases that seem to be important in mediating the metabolic effects of insulin. During recent years, there have been many new advances in our understanding of how these lipid-dependent protein kinases function during insulin action and why they fail to function in states of insulin resistance. The present review will attempt to summarize what we believe are some of the more important advances.

PKCiota promotes ovarian tumor progression through deregulation of cyclin E

The high frequency of relapse of epithelial ovarian tumors treated with standard chemotherapy has highlighted the necessity to identify targeted therapies that can improve patient outcomes. The dynamic relationship between Cyclin E and PKCiota frequent overexpression in high-grade ovarian tumors poses a novel pathway for therapeutic investigation. We hypothesized that a PI3K dependent signaling pathway activating PKCiota perpetuates cyclin E deregulation during ovarian tumorigenesis. We observed a positive correlation between PKCiota and cyclin E in a panel of 19 ovarian cancer cell lines. Modulation of cyclin E had no effect on PKCiota knockdown/overexpression however PKCiota differentially regulated cyclin E expression. In the serous ovarian cancer cells (IGROV, OVCAR-3), shPKCiota decreased proliferation, caused a G1 arrest, and significantly prolonged overall survival in xenograft mouse models. In vitro shPKCiota decreased the ability of IGROV cells to grow under anchorage independent conditions and form aberrant acini, which was dependent upon Ad-cyclin E or Ad-LMW-E expression. RPPA analysis of PKCiota wild-type, catalytic active, dominant negative protein isoforms strengthened the association between phospho-PKCiota levels and PI3K pathway activation. Inhibitors of PI3K coordinately decreased phospho-PKCiota and Cyclin E protein levels. In conclusion, we have identified a PI3K/PKCiota/Cyclin E signaling pathway as a therapeutic target during ovarian tumorigenesis.

Effects of exercise on AMPK signaling and downstream components to PI3K in rat with type 2 diabetes

Exercise can increase skeletal muscle sensitivity to insulin, improve insulin resistance and regulate glucose homeostasis in rat models of type 2 diabetes. However, the potential mechanism remains poorly understood. In this study, we established a male Sprague–Dawley rat model of type 2 diabetes, with insulin resistance and β cell dysfunction, which was induced by a high-fat diet and low-dose streptozotocin to replicate the pathogenesis and metabolic characteristics of type 2 diabetes in humans. We also investigated the possible mechanism by which chronic and acute exercise improves metabolism, and the phosphorylation and expression of components of AMP-activated protein kinase (AMPK) and downstream components of phosphatidylinositol 3-kinase (PI3K) signaling pathways in the soleus. As a result, blood glucose, triglyceride, total cholesterol, and free fatty acid were significantly increased, whereas insulin level progressively declined in diabetic rats. Interestingly, chronic and acute exercise reduced blood glucose, increased phosphorylation and expression of AMPKα1/2 and the isoforms AMPKα1 and AMPKα2, and decreased phosphorylation and expression of AMPK substrate, acetyl CoA carboxylase (ACC). Chronic exercise upregulated phosphorylation and expression of AMPK upstream kinase, LKB1. But acute exercise only increased LKB1 expression. In particular, exercise reversed the changes in protein kinase C (PKC)ζ/λ phosphorylation, and PKCζ phosphorylation and expression. Additionally, exercise also increased protein kinase B (PKB)/Akt1, Akt2 and GLUT4 expression, but AS160 protein expression was unchanged. Chronic exercise elevated Akt (Thr³⁰⁸) and (Ser⁴⁷³) and AS160 phosphorylation. Finally, we found that exercise increased peroxisome proliferator-activated receptor-γ coactivator 1 (PGC1) mRNA expression in the soleus of diabetic rats. These results indicate that both chronic and acute exercise influence the phosphorylation and expression of components of the AMPK and downstream to PIK3 (aPKC, Akt), and improve GLUT4 trafficking in skeletal muscle. These data help explain the mechanism how exercise regulates glucose homeostasis in diabetic rats.

Defective Activation of Protein Kinase C-z in Muscle by Insulin and Phosphatidylinositol-3,4,5,-(PO(4))(3) in Obesity and Polycystic Ovary Syndrome

Insulin resistance occurs frequently in metabolic syndrome components, obesity, and the polycystic ovary syndrome, and is partly due to impaired glucose transport into skeletal muscle, but underlying mechanisms are uncertain. Atypical protein kinase C and protein kinase B, operating downstream of phosphatidylinositol 3-kinase, mediate insulin effects on glucose transport, but their importance in these syndromes is poorly understood. Presently, we examined these signaling factors in muscle biopsies obtained during euglycemic/hyperinsulinemic clamp studies. In lean subjects, insulin provoked approximately twofold increases in muscle atypical protein kinase C activity. In obese subjects and obese subjects who had evidence of the polycystic ovary syndrome, insulin-stimulated glucose disposal and atypical protein kinase C activation were diminished, whereas activation of insulin receptor substrate-1-dependent phosphatidylinositol 3-kinase and protein kinase B trended lower, but not significantly. Interestingly, direct activation of atypical protein kinase C by phosphatidylinositol-3,4,5-(PO(4))(3), the lipid product of phosphatidylinositol 3-kinase, was readily apparent in immunoprecipitates prepared from muscles of lean subjects, but to a lesser degree or poorly if at all in subjects who were obese or had the obesity/polycystic ovary syndrome. Our findings suggest that activation of muscle atypical protein kinase C by insulin and phosphatidylinositol-3,4,5-(PO(4))(3) is defective and may contribute to skeletal muscle insulin resistance in women who are obese, or have obesity associated with the polycystic ovary syndrome.

Insulin signaling and insulin sensitizing in muscle and liver of obese monkeys: peroxisome proliferator-activated receptor gamma agonist improves defective activation of atypical protein kinase C

Obesity, the metabolic syndrome, and aging share several pathogenic features in both humans and non-human primates, including insulin resistance and inflammation. Since muscle and liver are considered key integrators of metabolism, we sought to determine in biopsies from lean and obese aging rhesus monkeys the nature of defects in insulin activation and, further, the potential for mitigation of such defects by an in vivo insulin sensitizer, rosiglitazone, and a thiazolidinedione activator of the peroxisome proliferator-activated receptor gamma. The peroxisome proliferator-activated receptor gamma agonist reduced hyperinsulinemia, improved insulin sensitivity, lowered plasma triglycerides and free fatty acids, and increased plasma adiponectin. In muscle of obese monkeys, previously shown to exhibit defective insulin signaling, the insulin sensitizer improved insulin activation of atypical protein kinase C (aPKC), the defective direct activation of aPKC by phosphatidylinositol (PI)-3,4,5-(PO₄)₃, and 5'-AMP-activated protein kinase and increased carnitine palmitoyltransferase-1 mRNA expression, but it did not improve insulin activation of insulin receptor substrate (IRS)-1-dependent PI 3-kinase (IRS-1/PI3K), protein kinase B, or glycogen synthase. We found that, although insulin signaling was impaired in muscle, insulin activation of IRS-1/PI3K, IRS-2/PI3K, protein kinase B, and aPKC was largely intact in liver and that rosiglitazone improved insulin signaling to aPKC in muscle by improving responsiveness to PI-3,4,5-(PO₄)₃.

Improved insulin sensitivity after exercise: focus on insulin signaling

After a single bout of exercise, the ability of insulin to stimulate glucose uptake is markedly improved locally in the previously active muscles. This makes exercise a potent stimulus counteracting insulin resistance characterizing type 2 diabetes (T2D). It is believed that at least part of the mechanism relates to an improved ability of insulin to stimulate translocation of glucose transporters (GLUT4) to the muscle membrane after exercise. How this is accomplished is still unclear; however, an obvious possibility is that exercise interacts with the insulin signaling pathway to GLUT4 translocation allowing for a more potent insulin response. Parallel to unraveling of the insulin signaling cascade, this has been investigated within the past 25 years. Reviewing existing studies clearly indicates that improved insulin action can occur independent of interactions with proximal insulin signaling. In contrast, more recent observations indicate that interactions exist at the distal signaling level of AS160 and atypical protein kinase C (aPKC). Although the functional interpretation is lacking, these novel observations may present a breakthrough in understanding the beneficial interplay between exercise and insulin action.

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

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

The critical role of atypical protein kinase C in activating hepatic SREBP-1c and NFkappaB in obesity

Obesity is frequently associated with systemic insulin resistance, glucose intolerance, and hyperlipidemia. Impaired insulin action in muscle and paradoxical diet/insulin-dependent overproduction of hepatic lipids are important components of obesity, but their pathogenesis and inter-relationships between muscle and liver are uncertain. We studied two murine obesity models, moderate high-fat-feeding and heterozygous muscle-specific PKC-lambda knockout, in both of which insulin activation of atypical protein kinase C (aPKC) is impaired in muscle, but conserved in liver. In both models, activation of hepatic sterol receptor element binding protein-1c (SREBP-1c) and NFkappaB (nuclear factor-kappa B), major regulators of hepatic lipid synthesis and systemic insulin resistance, was chronically increased in the fed state. In support of a critical mediatory role of aPKC, in both models, inhibition of hepatic aPKC by adenovirally mediated expression of kinase-inactive aPKC markedly diminished diet/insulin-dependent activation of hepatic SREBP-1c and NFkappaB, and concomitantly improved hepatosteatosis, hypertriglyceridemia, hyperinsulinemia, and hyperglycemia. Moreover, in high-fat-fed mice, impaired insulin signaling to IRS-1-dependent phosphatidylinositol 3-kinase, PKB/Akt and aPKC in muscle and hyperinsulinemia were largely reversed. In obesity, conserved hepatic aPKC-dependent activation of SREBP-1c and NFkappaB contributes importantly to the development of hepatic lipogenesis, hyperlipidemia, and systemic insulin resistance. Accordingly, hepatic aPKC is a potential target for treating obesity-associated abnormalities.

Roles of protein kinase Calpha isozyme in the regulation of oxidative stress and neuropeptide Y gene expression in phenylpropanolamine-mediated appetite suppression

Hypothalamic neuropeptide Y (NPY) is an appetite stimulant in the brain. Although regulation of NPY expression has been reported to contribute to the appetite-suppressing effect of phenylpropanolamine (PPA), it is still unknown if protein kinase C (PKC) is involved in this effect. Rats were daily treated with PPA for 4 days. Changes in food intake, hypothalamic NPY, PKC, and proopiomelanocortin (POMC) mRNA levels were assessed and compared. Results showed that the NPY gene was down-regulated following PPA treatment, which was parallel with the decrease of feeding. Moreover, several isotypes of PKC mRNA level (alpha, betaI, betaII, gamma, delta, eta, lambda, epsilon, and zeta) were changed. Among these, alpha, delta, and lambda PKC were up-regulated along with POMC gene expression which coincided with down-regulation of the NPY gene. To further determine if PKCalpha was involved, infusions of antisense oligonucleotide into the cerebroventricle were performed at 1 h before daily PPA treatment in free-moving rats. Results showed that PKCalpha knock-down could modify both anorexia induced by PPA and the NPY mRNA levels. Moreover, PKCalpha knock-down could also modify superoxide dismutase (SOD) gene expression. It is suggested that PKCalpha participates in the regulation of PPA-mediated appetite suppression via the modulation of NPY and SOD gene expression.

Increased lipid accumulation and insulin resistance in transgenic mice expressing DGAT2 in glycolytic (type II) muscle

Insulin resistance and type 2 diabetes are frequently accompanied by lipid accumulation in skeletal muscle. However, it is unknown whether primary lipid deposition in skeletal muscle is sufficient to cause insulin resistance or whether the type of muscle fiber, oxidative or glycolytic fiber, is an important determinant of lipid-mediated insulin resistance. Here we utilized transgenic mice to test the hypothesis that lipid accumulation specifically in glycolytic muscle promotes insulin resistance. Overexpression of DGAT2, which encodes an acyl-CoA:diacylglycerol acyltransferase that catalyzes triacylglycerol (TG) synthesis, in glycolytic muscle of mice increased the content of TG, ceramides, and unsaturated long-chain fatty acyl-CoAs in young adult mice. This lipid accumulation was accompanied by impaired insulin signaling and insulin-mediated glucose uptake in glycolytic muscle and impaired whole body glucose and insulin tolerance. We conclude that DGAT2-mediated lipid deposition specifically in glycolytic muscle promotes insulin resistance in this tissue and may contribute to the development of diabetes.

Muscle-specific knockout of PKC-lambda impairs glucose transport and induces metabolic and diabetic syndromes

Obesity, the metabolic syndrome, and type 2 diabetes mellitus (T2DM) are major global health problems. Insulin resistance is frequently present in these disorders, but the causes and effects of such resistance are unknown. Here, we generated mice with muscle-specific knockout of the major murine atypical PKC (aPKC), PKC-lambda, a postulated mediator for insulin-stimulated glucose transport. Glucose transport and translocation of glucose transporter 4 (GLUT4) to the plasma membrane were diminished in muscles of both homozygous and heterozygous PKC-lambda knockout mice and were accompanied by systemic insulin resistance; impaired glucose tolerance or diabetes; islet beta cell hyperplasia; abdominal adiposity; hepatosteatosis; elevated serum triglycerides, FFAs, and LDL-cholesterol; and diminished HDL-cholesterol. In contrast to the defective activation of muscle aPKC, insulin signaling and actions were intact in muscle, liver, and adipocytes. These findings demonstrate the importance of aPKC in insulin-stimulated glucose transport in muscles of intact mice and show that insulin resistance and resultant hyperinsulinemia owing to a specific defect in muscle aPKC is sufficient to induce abdominal obesity and other lipid abnormalities of the metabolic syndrome and T2DM. These findings are particularly relevant because humans who have obesity, impaired glucose tolerance, and T2DM reportedly have defective activation and/or diminished levels of muscle aPKC.

Muscle-specific knockout of PKC-lambda impairs glucose transport and induces metabolic and diabetic syndromes

Obesity, the metabolic syndrome, and type 2 diabetes mellitus (T2DM) are major global health problems. Insulin resistance is frequently present in these disorders, but the causes and effects of such resistance are unknown. Here, we generated mice with muscle-specific knockout of the major murine atypical PKC (aPKC), PKC-lambda, a postulated mediator for insulin-stimulated glucose transport. Glucose transport and translocation of glucose transporter 4 (GLUT4) to the plasma membrane were diminished in muscles of both homozygous and heterozygous PKC-lambda knockout mice and were accompanied by systemic insulin resistance; impaired glucose tolerance or diabetes; islet beta cell hyperplasia; abdominal adiposity; hepatosteatosis; elevated serum triglycerides, FFAs, and LDL-cholesterol; and diminished HDL-cholesterol. In contrast to the defective activation of muscle aPKC, insulin signaling and actions were intact in muscle, liver, and adipocytes. These findings demonstrate the importance of aPKC in insulin-stimulated glucose transport in muscles of intact mice and show that insulin resistance and resultant hyperinsulinemia owing to a specific defect in muscle aPKC is sufficient to induce abdominal obesity and other lipid abnormalities of the metabolic syndrome and T2DM. These findings are particularly relevant because humans who have obesity, impaired glucose tolerance, and T2DM reportedly have defective activation and/or diminished levels of muscle aPKC.

Exercise reverses high-fat diet-induced impairments on compartmentalization and activation of components of the insulin-signaling cascade in skeletal muscle

The aims of this investigation were 1) to determine whether endurance exercise training could reverse impairments in insulin-stimulated compartmentalization and/or activation of aPKCzeta/lambda and Akt2 in skeletal muscle from high-fat-fed rodents and 2) to assess whether the PPARgamma agonist rosiglitazone could reverse impairments in skeletal muscle insulin signaling typically observed after high-fat feeding. Sprague-Dawley rats were placed on chow (NORCON, n = 16) or high-fat (n = 64) diets for 4 wk. During a subsequent 4-wk experimental period, high-fat-fed rats were allocated (n = 16/group) to either sedentary control (HFC), exercise training (HFX), rosiglitazone treatment (HFRSG), or a combination of both exercise training and rosiglitazone (HFRX). Following the 4-wk experimental period, animals underwent hindlimb perfusions. Insulin-stimulated plasma membrane-associated aPKCzeta and -lambda protein concentration, aPKCzeta/lambda activity, GLUT4 protein concentration, cytosolic Akt2, and aPKCzeta/lambda activities were reduced (P < 0.05) in HFC compared with NORCON. Cytosolic Akt2, aPKCzeta, and aPKClambda protein concentrations were not affected in HFC compared with NORCON. Exercise training reversed the deleterious effects of the high-fat diet such that insulin-stimulated compartmentalization and activation of components of the insulin-signaling cascade in HFX were normalized to NORCON. High-fat diet-induced impairments to skeletal muscle glucose metabolism were not reversed by rosiglitazone administration, nor did rosiglitazone augment the effect of exercise. Our findings indicate that chronic exercise training, but not rosiglitazone, reverses high-fat diet induced impairments in compartmentalization and activation of components of the insulin-signaling cascade in skeletal muscle.

Effects of endurance exercise training on insulin signaling in human skeletal muscle: interactions at the level of phosphatidylinositol 3-kinase, Akt, and AS160

The purpose of this study was to investigate the mechanisms explaining improved insulin-stimulated glucose uptake after exercise training in human skeletal muscle. Eight healthy men performed 3 weeks of one-legged knee extensor endurance exercise training. Fifteen hours after the last exercise bout, insulin-stimulated glucose uptake was approximately 60% higher (P < 0.01) in the trained compared with the untrained leg during a hyperinsulinemic-euglycemic clamp. Muscle biopsies were obtained before and after training as well as after 10 and 120 min of insulin stimulation in both legs. Protein content of Akt1/2 (55 +/- 17%, P < 0.05), AS160 (25 +/- 8%, P = 0.08), GLUT4 (52 +/- 19%, P < 0.001), hexokinase 2 (HK2) (197 +/- 40%, P < 0.001), and insulin-responsive aminopeptidase (65 +/- 15%, P < 0.001) increased in muscle in response to training. During hyperinsulinemia, activities of insulin receptor substrate-1 (IRS-1)-associated phosphatidylinositol 3-kinase (PI3-K) (P < 0.005), Akt1 (P < 0.05), Akt2 (P < 0.005), and glycogen synthase (GS) (percent I-form, P < 0.05) increased similarly in both trained and untrained muscle, consistent with increased phosphorylation of Akt Thr(308), Akt Ser(473), AS160, glycogen synthase kinase (GSK)-3alpha Ser(21), and GSK-3beta Ser(9) and decreased phosphorylation of GS site 3a+b (all P < 0.005). Interestingly, training improved insulin action on thigh blood flow, and, furthermore, in both basal and insulin-stimulated muscle tissue, activities of Akt1 and GS and phosphorylation of AS160 increased with training (all P < 0.05). In contrast, training reduced IRS-1-associated PI3-K activity (P < 0.05) in both basal and insulin-stimulated muscle tissue. Our findings do not support generally improved insulin signaling after endurance training; rather it seems that improved insulin-stimulated glucose uptake may result from hemodynamic adaptations as well as increased cellular protein content of individual insulin signaling components and molecules involved in glucose transport and metabolism.

Dissociation of hepatic steatosis and insulin resistance in mice overexpressing DGAT in the liver

Hepatic steatosis, the accumulation of lipids in the liver, is widely believed to result in insulin resistance. To test the causal relationship between hepatic steatosis and insulin resistance, we generated mice that overexpress acyl-CoA:diacylglycerol acyltransferase 2 (DGAT2), which catalyzes the final step of triacylglycerol (TG) biosynthesis, in the liver (Liv-DGAT2 mice). Liv-DGAT2 mice developed hepatic steatosis, with increased amounts of TG, diacylglycerol, ceramides, and unsaturated long-chain fatty acyl-CoAs in the liver. However, they had no abnormalities in plasma glucose and insulin levels, glucose and insulin tolerance, rates of glucose infusion and hepatic glucose production during hyperinsulinemic-euglycemic clamp studies, or activities of insulin-stimulated signaling proteins in the liver. DGAT1 overexpression in the liver also failed to induce glucose or insulin intolerance. Our results indicate that DGAT-mediated lipid accumulation in the liver is insufficient to cause insulin resistance and show that hepatic steatosis can occur independently of insulin resistance.

Exercise improves phosphatidylinositol-3,4,5-trisphosphate responsiveness of atypical protein kinase C and interacts with insulin signalling to peptide elongation in human skeletal muscle

We investigated if acute endurance-type exercise interacts with insulin-stimulated activation of atypical protein kinase C (aPKC) and insulin signalling to peptide chain elongation in human skeletal muscle. Four hours after acute one-legged exercise, insulin-induced glucose uptake was approximately 80% higher (N = 12, P < 0.05) in previously exercised muscle, measured during a euglycaemic-hyperinsulinaemic clamp (100 microU ml(-1)). Insulin increased (P < 0.05) both insulin receptor substrate (IRS)-1 and IRS-2 associated phosphatidylinositol (PI)-3 kinase activity and led to increased (P < 0.001) phosphorylation of Akt on Ser(473) and Thr(308) in skeletal muscle. Interestingly, in response to prior exercise IRS-2-associated PI-3 kinase activity was higher (P < 0.05) both at basal and during insulin stimulation. This coincided with correspondingly altered phosphorylation of the extracellular-regulated protein kinase 1/2 (ERK 1/2), p70S6 kinase (P70S6K), eukaryotic elongation factor 2 (eEF2) kinase and eEF2. aPKC was similarly activated by insulin in rested and exercised muscle, without detectable changes in aPKC Thr(410) phosphorylation. However, when adding phosphatidylinositol-3,4,5-triphosphate (PIP3), the signalling product of PI-3 kinase, to basal muscle homogenates, aPKC was more potently activated (P = 0.01) in previously exercised muscle. Collectively, this study shows that endurance-type exercise interacts with insulin signalling to peptide chain elongation. Although protein turnover was not evaluated, this suggests that capacity for protein synthesis after acute endurance-type exercise may be improved. Furthermore, endurance exercise increased the responsiveness of aPKC to PIP3 providing a possible link to improved insulin-stimulated glucose uptake after exercise.

Contrasting insulin dose-dependent defects in activation of atypical protein kinase C and protein kinase B/Akt in muscles of obese diabetic humans

AIMS/HYPOTHESIS

Insulin-stimulated glucose transport in muscle is impaired in obesity and type 2 diabetes, but alterations in levels of relevant signalling factors, i.e. atypical protein kinase C (aPKC) and protein kinase B (PKB/Akt), are still uncertain. Clamp studies using maximal insulin concentrations have revealed defects in activation of aPKC, but not PKB, in both obese non-diabetic and obese diabetic subjects. In contrast, clamp studies using submaximal insulin concentrations revealed defects in PKB activation/phosphorylation in obese non-diabetic and diabetic subjects, but changes in aPKC were not reported. The aim of this study was to test the hypothesis that dose-related effects of insulin may account for the reported differences in insulin signalling to PKB in diabetic muscle.

SUBJECTS AND METHODS

We compared enzymatic activation of aPKC and PKB, and PKB phosphorylation (threonine-308 and serine-473) during hyperinsulinaemic-euglycaemic clamp studies using both submaximal (400-500 pmol/l) and maximal (1400 pmol/l) insulin levels in non-diabetic control and obese diabetic subjects.

RESULTS

In lean control subjects, the submaximal insulin concentration increased aPKC activity and glucose disposal to approximately 50% of the maximal level and PKBbeta activity to 25% of the maximal level, but PKBalpha activity was not increased. In these subjects, phosphorylation of PKBalpha and PKBbeta was increased to near-maximal levels at submaximal insulin concentrations. In obese diabetic subjects, whereas aPKC activation was defective at submaximal and maximal insulin concentrations, PKBbeta activation and the phosphorylation of PKBbeta and PKBalpha were defective at submaximal, but not maximal, insulin concentrations.

CONCLUSIONS/INTERPRETATIONS

Defective PKBbeta activation/phosphorylation, seen on submaximal insulin stimulation in diabetic muscle, may largely reflect impaired activation of insulin signalling factors present in concentrations greater than those needed for full PKB activation/phosphorylation. Defective aPKC activation, seen at all insulin levels, appears to reflect, at least partly, an impaired action of distal factors needed for aPKC activation, or poor aPKC responsiveness.

Metformin improves atypical protein kinase C activation by insulin and phosphatidylinositol-3,4,5-(PO4)3 in muscle of diabetic subjects

AIMS/HYPOTHESIS

Metformin is widely used for treating type 2 diabetes mellitus, but its actions are poorly understood. In addition to diminishing hepatic glucose output, metformin, in muscle, activates 5'-AMP-activated protein kinase (AMPK), which alone increases glucose uptake and glycolysis, diminishes lipid synthesis, and increases oxidation of fatty acids. Moreover, such lipid effects may improve insulin sensitivity and insulin-stimulated glucose uptake. Nevertheless, the effects of metformin on insulin-sensitive signalling factors in human muscle have only been partly characterised to date. Interestingly, other substances that activate AMPK, e.g., aminoimidazole-4-carboxamide-1-beta-D: -riboside (AICAR), simultaneously activate atypical protein kinase C (aPKC), which appears to be required for the glucose transport effects of AICAR and insulin.

METHODS

Since aPKC activation is defective in type 2 diabetes, we evaluated effects of metformin therapy on aPKC activity in muscles of diabetic subjects during hyperinsulinaemic-euglycaemic clamp studies.

RESULTS

After metformin therapy for 1 month, basal aPKC activity increased in muscle, with little or no change in insulin-stimulated aPKC activity. Metformin therapy for 8 to 12 months improved insulin-stimulated, as well as basal aPKC activity in muscle. In contrast, IRS-1-dependent phosphatidylinositol (PI) 3-kinase activity and Ser473 phosphorylation of protein kinase B were not altered by metformin therapy, whereas the responsiveness of muscle aPKC to PI-3,4,5-(PO(4))(3), the lipid product of PI 3-kinase, was improved.

CONCLUSIONS/INTERPRETATION

These findings suggest that the activation of AMPK by metformin is accompanied by increases in aPKC activity and responsiveness in skeletal muscle, which may contribute to the therapeutic effects of metformin.

Both saturated and n-6 polyunsaturated fat diets reduce phosphorylation of insulin receptor substrate-1 and protein kinase B in muscle during the initial stages of in vivo insulin stimulation

Our aim was to determine the importance of changes in phosphorylation of key insulin signaling intermediates in the insulin resistance observed in skeletal muscle of rats fed diets high in saturated or n-6 polyunsaturated fat. We used phospho-specific antibodies to measure the time course of phosphorylation of key components of the insulin signaling pathway by immunoblotting during the initial stages of a physiological elevation in the circulating insulin concentration. The phosphorylation of insulin receptor at Tyr1162/1163 (IR Tyr1162/1163) increased over 20 min of insulin infusion, whereas the downstream phosphorylation of insulin receptor substrate-1 Tyr612 (IRS-1 Tyr612) peaked at 5 min and declined thereafter. Interestingly, phosphorylation of IRS-1 at Tyr895 continued to increase over the 20-min period, and protein kinase B (PKB) phosphorylation at Ser473 reached a plateau by 5 min, demonstrating that different profiles of phosphorylation are involved in transmission of the insulin signal despite a constant level of insulin stimulation. In muscle from rats fed high n-6 polyunsaturated or saturated fat diets, however, there was no insulin-stimulated increase in IRS-1 Tyr612 phosphorylation and a temporal difference in PKB Ser473 phosphorylation despite no difference in IR Tyr1162/1163 phosphorylation, IRS-1 Tyr895 phosphorylation, and ERK phosphorylation. These results demonstrate that under conditions of increased insulin, similar to those used to assess insulin action in vivo, chronic high-fat feeding impairs insulin signal transduction related to glucose metabolism at the level of IRS-1 Tyr612 and PKB Ser473 and that these effects are independent of the type of fat used in the high-fat diet.

Cellular location of insulin-triggered signals and implications for glucose uptake

Insulin stimulation of glucose uptake into muscle and fat cells requires movement of GLUT4-containing vesicles from intracellular compartments to the plasma membrane. Accordingly, insulin-derived signals must arrive at and be recognized by the appropriate intracellular GLUT4 pools. We describe the insulin signals participating in GLUT4 translocation, and review evidence that they are recruited to intracellular membranes in conjunction with cytoskeletal elements. Such segregation may facilitate the encounter between signals and target vesicles. In most animal and cellular models of insulin resistance, insulin-stimulated GLUT4 translocation to the plasma membrane is reduced. Insulin resistance caused by oxidative stress does not affect early insulin signals, rather their intracellular localization is altered. In this and several other insulin-resistant states, insulin-induced actin remodelling is concomitantly diminished. We summarize evidence suggesting that spatial localization of signals is critical for efficient insulin action, and that the cytoskeleton may act as a scaffold to promote efficient translocation of GLUT4 to the cell surface.

Transcriptional involvement of protein kinase C-alpha isozyme in amphetamine-mediated appetite suppression

Amphetamine (AMPH) is known as an anorectic agent. The anorectic action of AMPH has been attributed to its inhibitory action on hypothalamic neuropeptide Y (NPY), an appetite stimulant in the brain. The molecular mechanisms behind this anorectic action of AMPH are still unclear. This study investigated the possible role of protein kinase C (PKC) isotypes in this anorectic action. Results revealed that most PKC isotypes (alpha, betaII, gamma, delta, eta, lambda and zeta), except betaI and epsilon isotypes, were stimulated during a repeated treatment of AMPH. Among these stimulated isotypes, three isotypes (alpha, delta, lambda) were activated and expressed in a similar manner, while the other isotypes were expressed differently and specifically. To determine if PKCalpha was involved in the anorectic response of AMPH, the infusions of antisense oligonucleotide into the brain were performed 1 h before daily AMPH treatment in freely moving rats, and the results showed that PKCalpha knock down could block the anorectic response and restore NPY mRNA levels in AMPH-treated rats. These results suggest that PKC isotypes- (at least the alpha isotype), related modification of NPY gene expression in hypothalamus might play an essential role in the central regulation of AMPH-mediated feeding suppression.

Chronic aerobic exercise enhances components of the classical and novel insulin signalling cascades in Sprague-Dawley rat skeletal muscle

AIM

The aim of this study was to provide a more extensive evaluation of the effects of chronic aerobic exercise on various components of the insulin signalling cascade in normal rodent skeletal muscle because of the limited body of literature that exists in this area of investigation.

METHODS

Male Sprague-Dawley rats were assigned to either control (n = 7) or chronic aerobic exercise (n = 7) groups. Aerobic exercise animals were run 3 day week(1) for 45 min on a motor-driven treadmill (32 m min(1), 15% grade) for a 12 week period. Following the training period, all animals were subjected to hind limb perfusion in the presence of 500 microU mL(1) insulin to determine what effect chronic aerobic training had on various components of the insulin signalling cascade, c-Cbl protein concentration and c-Cbl phosphorylation.

RESULTS

Twelve weeks of aerobic training did not alter skeletal muscle Akt 1/2 protein concentration, Akt Ser 473 phosphorylation, Akt Thr 308 phosphorylation, Akt 1 activity, aPKC-zeta protein concentration, aPKC-lambda protein concentration or c-Cbl protein concentration. In contrast, chronic aerobic exercise increased insulin-stimulated phosphatidylinositol 3-kinase, Akt 2 kinase and aPKC-zeta/lambda kinase activities, as well as c-Cbl tyrosine phosphorylation, in a fibre type specific response to aerobic training. In addition, chronic aerobic exercise enhanced insulin-stimulated plasma membrane glucose transporter 4 (GLUT4) protein concentration.

CONCLUSION

Collectively, these findings suggest that chronic aerobic exercise enhances components of both the classical and novel insulin signalling cascades in normal rodent skeletal muscle, which may contribute to an increased insulin-stimulated plasma membrane GLUT4 protein concentration.

Atypical protein kinase C in insulin action and insulin resistance

It now seems clear that aPKC (atypical protein kinase C) isoforms are required for insulin-stimulated glucose transport in muscle and adipocytes. Moreover, there are marked defects in the activation of aPKCs under a variety of insulin-resistant conditions in humans, monkeys and rodents. In humans, defects in aPKC in muscle are seen in Type II diabetes and its precursors, obesity, the obesity-associated polycystic ovary syndrome and impaired glucose tolerance. These defects in muscle aPKC activation are due to both impaired activation of insulin receptor substrate-1-dependent PI3K (phosphoinositide 3-kinase) and the direct activation of aPKCs by the lipid product of PI3K, PI-3,4,5-(PO4)3. Although it is still uncertain which underlying defect comes first, the resultant defect in aPKC activation in muscle most certainly contributes significantly to the development of skeletal muscle insulin resistance. Of further note, unlike the seemingly ubiquitous presence of defective aPKC activation in skeletal muscle in insulin-resistant states, the activation of aPKC is normal or increased in livers of Type II diabetic and obese rodents. The maintenance of aPKC activation in the liver may explain how insulin-dependent lipid synthesis is maintained in these states, as aPKCs function mainly in the activation of enzymes important for lipid synthesis. Thus increased activation of liver aPKC in hyperinsulinaemic states may contribute significantly to the development of hyperlipidaemia in insulin-resistant states.

Insulin-stimulated plasma membrane association and activation of Akt2, aPKC zeta and aPKC lambda in high fat fed rodent skeletal muscle

Several recent reports using cell lines have suggested that both Akt and atypical protein kinase C (aPKC) zeta/lambda are translocated to the plasma membrane (PM) in response to insulin. However, it has yet to be determined in skeletal muscle whether: (1) insulin increases PM-associated Akt2, aPKC zeta and/or lambda protein concentration, (2) the activity of these kinases is altered by insulin at the PM, and (3) high fat feeding alters the insulin-stimulated PM concentration and/or activity of Akt2 and aPKC zeta/lambda. Sprague-Dawley rats were randomly assigned to either normal (n=16) or high fat (n=16) dietary groups. Following a 12 week dietary period, animals were subjected to hind limb perfusions in the presence (n=8 per group) or absence (n=8 per group) of insulin. In normal skeletal muscle, total PI3-kinase, Akt2 and aPKC zeta/lambda activities were increased by insulin. PM-associated aPKC zeta and lambda, and aPKC zeta/lambda activity, but not Akt2 or Akt2 activity, were increased by insulin in normal muscle. High fat feeding did not alter total skeletal muscle Akt2, aPKC zeta or aPKC lambda protein concentration. Insulin-stimulated total PI3-kinase, Akt2 and aPKC zeta/lambda activities were reduced in the high fat fed animals. Insulin-stimulated PM aPKC zeta, aPKC lambda, aPKC zeta/lambda activity and GLUT4 protein concentration were also reduced in high fat fed animals. These findings suggest that in skeletal muscle, insulin stimulates translocation of aPKC zeta and lambda, but not Akt2, to the PM. In addition, high fat feeding impairs insulin-stimulated activation of total aPKC zeta/lambda and Akt2, as well as PM association and activation of aPKC zeta and lambda.

Insulin-nonspecific reduction in skeletal muscle glucose transport in high-fat-fed rats

High-fat feeding diminishes insulin-stimulated glucose transport in skeletal muscle. However, conflicting results are reported regarding whether phosphatidylinositol (PI)-3 kinase-independent glucose transport is also impaired in insulin-resistant high-fat-fed rodents. The aim of the present study was to study whether non-insulin-dependent mechanisms for stimulation of glucose transport are defective in skeletal muscle from high-fat-fed rats. Rats were fed normal chow diet or high-fat diet for 4 weeks and isolated epitrochlearis muscles were used for measuring glucose transport. Insulin-stimulated glucose transport was significantly lower in rats fed the high-fat diet compared with chow-fed rats (P < .05). Hypoxia-stimulated glucose transport was also reduced in high-fat-fed rats (P < .05). Nevertheless, hypoxia-stimulated adenosine monophosphate-activated protein kinase (AMPK) phosphorylation (Thr172) level was not affected by high-fat feeding. Glucose transport by sodium nitroprusside stimulation was reduced in high-fat-fed rats (P < .05). Protein content of glucose transporter (GLUT)-4 and AMPK-alpha, and glycogen content were comparable between both groups. Our findings provide evidence that high-fat feeding can affect not only insulin but also non-insulin-stimulated glucose transport. A putative defect in common steps in glucose transport may play a role to account for impaired insulin-stimulated glucose transport in rats fed a high-fat diet.

Active involvement of PKC for insulin-mediated rates of muscle protein synthesis in Zucker rats

A recent report from our group demonstrated that insulin facilitates muscle protein synthesis in obese Zucker rats. The purpose of this study was to determine whether PKC, a probable modulator of insulin signal transduction and/or mRNA translation, has a role in this insulin-mediated anabolic response. In the first portion of the study, gastrocnemius muscles of lean and obese Zucker rats (n = 5-7 for each phenotype) were bilaterally perfused with or without insulin to assess cytosolic and membrane PKC activity. Limbs perfused with insulin demonstrated greater PKC activity in both lean and obese Zucker rats (P < 0.05) compared with no insulin, but overall activity was greater in obese animals (by approximately 27% compared with lean, P < 0.05). To determine whether PKC plays a role in muscle protein synthesis, hindlimbs (n = 6-8 for each phenotype) were bilaterally perfused with or without insulin and/or GF-109203X (GF; a PKC inhibitor). The presence of GF did not influence the rates of insulin-mediated protein synthesis in gastrocnemius muscle of lean Zucker rats. However, when obese rats were perfused with GF (P < 0.05), the effect of insulin on elevating rates of protein synthesis was not observed. We also used phorbol 12-myristate 13-acetate (TPA, a PKC activator; n = 5-7 for each phenotype) with and without insulin to determine the effect of PKC activation on muscle protein synthesis. TPA alone did not elevate muscle protein synthesis in lean or obese rats. However, TPA plus insulin resulted in elevated rates of protein synthesis in both phenotypes that were similar to rates of insulin alone of obese rats. These results suggest that PKC is a modulator and is necessary, but not sufficient, for insulin-mediated protein anabolic responses in skeletal muscle.

Transgenic overexpression of protein-tyrosine phosphatase 1B in muscle causes insulin resistance, but overexpression with leukocyte antigen-related phosphatase does not additively impair insulin action

Previous studies implicate protein-tyrosine phosphatase 1B (PTP1B) and leukocyte antigen-related phosphatase (LAR) as negative regulators of insulin signaling. The expression and/or activity of PTP1B and LAR are increased in muscle of insulin-resistant rodents and humans. Overexpression of LAR selectively in muscle of transgenic mice causes whole body insulin resistance. To determine whether overexpression of PTP1B also causes insulin resistance, we generated transgenic mice overexpressing human PTP1B selectively in muscle at levels similar to those observed in insulin-resistant humans. Insulin-stimulated insulin receptor (IR) tyrosyl phosphorylation and phosphatidylinositol 3'-kinase activity were impaired by 35% and 40-60% in muscle of PTP1B-overexpressing mice compared with controls. Insulin stimulation of protein kinase C (PKC)lambda/zeta activity, which is required for glucose transport, was impaired in muscle of PTP1B-overexpressing mice compared with controls, showing that PTP1B overexpression impairs activation of these PKC isoforms. Furthermore, hyperinsulinemic-euglycemic clamp studies revealed that whole body glucose disposal and muscle glucose uptake were decreased by 40-50% in PTP1B-overexpressing mice. Overexpression of PTP1B or LAR alone in muscle caused similar impairments in insulin action; however, compound overexpression achieved by crossing PTP1B- and LAR-overexpressing mice was not additive. Antibodies against specific IR phosphotyrosines indicated overlapping sites of action of PTP1B and LAR. Thus, overexpression of PTP1B in vivo impairs insulin sensitivity, suggesting that overexpression of PTP1B in muscle of obese humans and rodents may contribute to their insulin resistance. Lack of additive impairment of insulin signaling by PTP1B and LAR suggests that these PTPs have overlapping actions in causing insulin resistance in vivo.

Resistance training enhances components of the insulin signaling cascade in normal and high-fat-fed rodent skeletal muscle

Our laboratory recently reported that chronic resistance training (RT) improved insulin-stimulated glucose transport in normal rodent skeletal muscle, owing, in part, to increased GLUT-4 protein concentration (Yaspelkis BB III, Singh MK, Trevino B, Krisan AD, and Collins DE. Acta Physiol Scand 175: 315-323, 2002). However, it remained to be determined whether these improvements resulted from alterations in the insulin signaling cascade as well. In addition, the possibility existed that RT might improve skeletal muscle insulin resistance. Thirty-two male Sprague-Dawley rats were assigned to four groups: control diet (Con)-sedentary (Sed); Con-RT; high-fat diet (HF)-Sed; and HF-RT. Animals consumed their respective diets for 9 wk; then RT animals performed 12 wk of training (3 sets, 10 repetitions at 75% one-repetition maximum, 3x/wk). Animals remained on their dietary treatments over the 12-wk period. After the training period, animals were subjected to hindlimb perfusions. Insulin-stimulated insulin receptor substrate-1-associated phosphatidylinositol-3 kinase activity was enhanced in the red gastrocnemius and quadriceps of Con-RT and HF-RT animals. Atypical PKC-zeta/lambda and Akt activities were reduced in HF-Sed and normalized in HF-RT animals. Resistance training increased GLUT-4 protein concentration in red gastrocnemius and quadriceps of Con-RT and HF-RT animals. No differences were observed in total protein concentrations of insulin receptor substrate-1, Akt, atypical PKC-zeta/lambda, or phosphorylation of Akt. Collectively, these findings suggest that resistance training increases insulin-stimulated carbohydrate metabolism in normal skeletal muscle and reverses high-fat diet-induced skeletal muscle insulin resistance by altering components of both the insulin signaling cascade and glucose transporter effector system.

Increased atypical PKC activity in endurance-trained human skeletal muscle

Exercise training may modulate protein content and enzyme activities in skeletal muscle. However, it is not known whether atypical protein kinase C (aPKC) is affected by training. Thus, we investigated aPKC, extracellular-regulated protein kinase 1/2 (ERK 1/2), and P38 mitogen-activated protein kinase (P38 MAPK) activities and expression in skeletal muscle from untrained and endurance-trained subjects at rest and after 20min of cycle exercise (80% of VO(2peak)). Activities of aPKC (P<0.05) and ERK 1/2 (P=0.06), but not phosphorylation of P38 MAPK, were higher in trained than in sedentary subjects at rest. Exercise increased the activities of ERK 1/2 (P<0.01) and aPKC (P<0.05) and the phosphorylation (Thr180/Tyr182) of P38 MAPK (P<0.01) similarly in muscle from trained and sedentary subjects. Protein expression of the kinases was similar in trained and sedentary muscle. The increased aPKC activity in exercise-trained subjects could be important in explaining the enhanced insulin action in these individuals.

Activation of protein kinase C-zeta by insulin and phosphatidylinositol-3,4,5-(PO4)3 is defective in muscle in type 2 diabetes and impaired glucose tolerance: amelioration by rosiglitazone and exercise

Insulin resistance in type 2 diabetes is partly due to impaired glucose transport in skeletal muscle. Atypical protein kinase C (aPKC) and protein kinase B (PKB), operating downstream of phosphatidylinositol (PI) 3-kinase and its lipid product, PI-3,4,5-(PO(4))(3) (PIP(3)), apparently mediate insulin effects on glucose transport. We examined these signaling factors during hyperinsulinemic-euglycemic clamp studies in nondiabetic subjects, subjects with impaired glucose tolerance (IGT), and type 2 diabetic subjects. In nondiabetic control subjects, insulin provoked twofold increases in muscle aPKC activity. In both IGT and diabetes, aPKC activation was markedly (70-80%) diminished, most likely reflecting impaired activation of insulin receptor substrate (IRS)-1-dependent PI 3-kinase and decreased ability of PIP(3) to directly activate aPKCs; additionally, muscle PKC-zeta levels were diminished by 40%. PKB activation was diminished in patients with IGT but not significantly in diabetic patients. The insulin sensitizer rosiglitazone improved insulin-stimulated IRS-1-dependent PI 3-kinase and aPKC activation, as well as glucose disposal rates. Bicycle exercise, which activates aPKCs and stimulates glucose transport independently of PI 3-kinase, activated aPKCs comparably to insulin in nondiabetic subjects and better than insulin in diabetic patients. Defective aPKC activation contributes to skeletal muscle insulin resistance in IGT and type 2 diabetes, rosiglitazone improves insulin-stimulated aPKC activation, and exercise directly activates aPKCs in diabetic muscle.