Translocation In Skeletal Muscle Research Articles

Dietary stimulators of GLUT4 expression and translocation in skeletal muscle: a mini-review.

Chronic insulin resistance can lead to type II diabetes mellitus, which is also directly influenced by an individual's genetics as well as their lifestyle. Under normal circumstances, insulin facilitates glucose uptake in skeletal muscle and adipose tissue by stimulating glucose transporter 4 (GLUT4) translocation and activity. GLUT4 activity is directly correlated with the ability to clear elevated blood glucose and insulin sensitivity. In diabetes, energy excess and prolonged hyperinsulinemia suppress muscle and adipose response to insulin, in part through reduced GLUT4 membrane levels. This work uniquely describes much of the experimental data demonstrating the effects of various dietary components on GLUT4 expression and translocation in skeletal muscle. These observations implicate several individual dietary chemicals as potential adjuvant therapies in the maintenance of diabetes and insulin resistance.

Glycogen Content Regulates Peroxisome Proliferator Activated Receptor-∂ (PPAR-∂) Activity in Rat Skeletal Muscle

Performing exercise in a glycogen depleted state increases skeletal muscle lipid utilization and the transcription of genes regulating mitochondrial β-oxidation. Potential candidates for glycogen-mediated metabolic adaptation are the peroxisome proliferator activated receptor (PPAR) coactivator-1α (PGC-1α) and the transcription factor/nuclear receptor PPAR-∂. It was therefore the aim of the present study to examine whether acute exercise with or without glycogen manipulation affects PGC-1α and PPAR-∂ function in rodent skeletal muscle. Twenty female Wistar rats were randomly assigned to 5 experimental groups (n = 4): control [CON]; normal glycogen control [NG-C]; normal glycogen exercise [NG-E]; low glycogen control [LG-C]; and low glycogen exercise [LG-E]). Gastrocnemius (GTN) muscles were collected immediately following exercise and analyzed for glycogen content, PPAR-∂ activity via chromatin immunoprecipitation (ChIP) assays, AMPK α1/α2 kinase activity, and the localization of AMPK and PGC-1α. Exercise reduced muscle glycogen by 47 and 75% relative to CON in the NG-E and LG-E groups, respectively. Exercise that started with low glycogen (LG-E) finished with higher AMPK-α2 activity (147%, p<0.05), nuclear AMPK-α2 and PGC-1α, but no difference in AMPK-α1 activity compared to CON. In addition, PPAR-∂ binding to the CPT1 promoter was significantly increased only in the LG-E group. Finally, cell reporter studies in contracting C2C12 myotubes indicated that PPAR-∂ activity following contraction is sensitive to glucose availability, providing mechanistic insight into the association between PPAR-∂ and glycogen content/substrate availability. The present study is the first to examine PPAR-∂ activity in skeletal muscle in response to an acute bout of endurance exercise. Our data would suggest that a factor associated with muscle contraction and/or glycogen depletion activates PPAR-∂ and initiates AMPK translocation in skeletal muscle in response to exercise.

Peripheral Effects of Nesfatin-1 on Glucose Homeostasis

Aims/hypothesisThe actions of peripherally administered nesfatin-1 on glucose homeostasis remain controversial. The aim of this study was to characterize the mechanisms by which peripheral nesfatin-1 regulates glucose metabolism.MethodsThe effects of nesfatin-1 on glucose metabolism were examined in mice by continuous infusion of the peptide via osmotic pumps. Changes in AKT phosphorylation and Glut4 were investigated by Western blotting and immnuofluorescent staining. Primary myocytes, adipocytes and hepatocytes were isolated from male mice.ResultsContinuous peripheral infusion of nesfatin-1 altered glucose tolerance and insulin sensitivity in mice fed either normal or high fat diet, while central administration of nesfatin-1 demonstrated no effect. Nesfatin-1 increases insulin secretion in vivo, and in vitro in cultured min6 cells. In addition, nesfatin-1 up-regulates the phosphorylation of AKT in pancreas and min6 islet cells. In mice fed normal diet, peripheral nesfatin-1 significantly increased insulin-stimulated phosphorylation of AKT in skeletal muscle, adipose tissue and liver; similar effects were observed in skeletal muscle and adipose tissue in mice fed high fat diet. At basal conditions and after insulin stimulation, peripheral nesfatin-1 markedly increased GLUT4 membrane translocation in skeletal muscle and adipose tissue in mice fed either diet. In vitro studies showed that nesfatin-1 increased both basal and insulin-stimulated levels of AKT phosphorylation in cells derived from skeletal muscle, adipose tissue and liver.ConclusionsOur studies demonstrate that nesfatin-1 alters glucose metabolism by mechanisms which increase insulin secretion and insulin sensitivity via altering AKT phosphorylation and GLUT 4 membrane translocation in the skeletal muscle, adipose tissue and liver.

Muscle-specificPikfyvegene disruption causes glucose intolerance, insulin resistance, adiposity, and hyperinsulinemia but not muscle fiber-type switching

The evolutionarily conserved kinase PIKfyve that synthesizes PtdIns5P and PtdIns(3,5)P₂ has been implicated in insulin-regulated GLUT4 translocation/glucose entry in 3T3-L1 adipocytes. To decipher PIKfyve's role in muscle and systemic glucose metabolism, here we have developed a novel mouse model with Pikfyve gene disruption in striated muscle (MPIfKO). These mice exhibited systemic glucose intolerance and insulin resistance at an early age but had unaltered muscle mass or proportion of slow/fast-twitch muscle fibers. Insulin stimulation of in vivo or ex vivo glucose uptake and GLUT4 surface translocation was severely blunted in skeletal muscle. These changes were associated with premature attenuation of Akt phosphorylation in response to in vivo insulin, as tested in young mice. Starting at 10-11 wk of age, MPIfKO mice progressively accumulated greater body weight and fat mass. Despite increased adiposity, serum free fatty acid and triglyceride levels were normal until adulthood. Together with the undetectable lipid accumulation in liver, these data suggest that lipotoxicity and muscle fiber switching do not contribute to muscle insulin resistance in MPIfKO mice. Furthermore, the 80% increase in total fat mass resulted from increased fat cell size rather than altered fat cell number. The observed profound hyperinsulinemia combined with the documented increases in constitutive Akt activation, in vivo glucose uptake, and gene expression of key enzymes for fatty acid biosynthesis in MPIfKO fat tissue suggest that the latter is being sensitized for de novo lipid anabolism. Our data provide the first in vivo evidence that PIKfyve is essential for systemic glucose homeostasis and insulin-regulated glucose uptake/GLUT4 translocation in skeletal muscle.

Propolis extract promotes translocation of glucose transporter 4 and glucose uptake through both PI3K‐ and AMPK‐dependent pathways in skeletal muscle

It is well known that propolis has the ability to prevent hyperglycemia. However, the underlying mechanism is not yet fully understood. We therefore investigated whether a Brazilian propolis ethanol extract affects glucose uptake and translocation of insulin-sensitive glucose transporter (GLUT) 4 in skeletal muscle cells. In L6 myotubes, the extract at 1 μg/mL significantly promoted GLUT4 translocation and glucose uptake activity. Regarding the mechanism of GLUT4 translocation, propolis extract induced both PI3K and AMPK phosphorylation in a dose-dependent manner in L6 myotubes. However, we could not define which pathway was preferentially associated with GLUT4 translocation, because both PI3K and AMPK inhibitors revealed off-target effects to each other. The main polyphenols found in the propolis extract, artepillin C, coumaric acid, and kaempferide, promoted GLUT4 translocation in L6 myotubes. Additionally, these compounds activated both PI3K- and AMPK-dependent dual-signaling pathways. However, only kaempferide increased glucose uptake activity under our experimental conditions. Single oral administrations of propolis extract, at 250 mg/kg body weight, lowered postprandial blood glucose levels in ICR mice. The extract promoted GLUT4 translocation in skeletal muscle of rats and mice, but did not inhibit α-glucosidase activity in the small intestine under our experimental conditions. It was confirmed that propolis extract promoted phosphorylation of both PI3K and AMPK in rat skeletal muscle. In conclusion, we show that Brazilian propolis has the potential to prevent hyperglycemia through the promotion of GLUT4 translocation in skeletal muscle and that kaempferide is one of the candidates for active compound in propolis.

Resistance Training for Diabetes Prevention and Therapy: Experimental Findings and Molecular Mechanisms

Type 2 diabetes mellitus (T2D) is characterized by insulin resistance, impaired glycogen synthesis, lipid accumulation, and impaired mitochondrial function. Exercise training has received increasing recognition as a cornerstone in the prevention and treatment of T2D. Emerging research suggests that resistance training (RT) has the power to combat metabolic dysfunction in patients with T2D and seems to be an effective measure to improve overall metabolic health and reduce metabolic risk factors in diabetic patients. However, there is limited mechanistic insight into how these adaptations occur. This review provides an overview of the intervention data on the impact of RT on glucose metabolism. In addition, the molecular mechanisms that lead to adaptation in skeletal muscle in response to RT and that are associated with possible beneficial metabolic responses are discussed. Some of the beneficial adaptations exerted by RT include increased GLUT4 translocation in skeletal muscle, increased insulin sensitivity and hence restored metabolic flexibility. Increased energy expenditure and excess postexercise oxygen consumption in response to RT may be other beneficial effects. RT is increasingly establishing itself as an effective measure to improve overall metabolic health and reduce metabolic risk factors in diabetic patients.

Doc2b is a key effector of insulin secretion and skeletal muscle insulin sensitivity.

Exocytosis of intracellular vesicles, such as insulin granules, is carried out by soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) and Sec1/Munc18 (SM) proteins. An additional regulatory protein, Doc2b (double C2 domain), has recently been implicated in exocytosis from clonal β-cells and 3T3-L1 adipocytes. Here, we investigated the role of Doc2b in insulin secretion, insulin sensitivity, and the maintenance of whole-body glucose homeostasis. Doc2b heterozygous (Doc2b+/−) and homozygous (Doc2b−/−) knockout mice exhibited significant whole-body glucose intolerance and peripheral insulin resistance, compared with wild-type littermates. Correspondingly, Doc2b+/− and Doc2b−/− mice exhibited decreased responsiveness of pancreatic islets to glucose in vivo, with significant attenuation of both phases of insulin secretion ex vivo. Peripheral insulin resistance correlated with ablated insulin-stimulated glucose uptake and GLUT4 vesicle translocation in skeletal muscle from Doc2b-deficient mice, which was coupled to impairments in Munc18c-syntaxin 4 dissociation and in SNARE complex assembly. Hence, Doc2b is a key positive regulator of Munc18c-syntaxin 4–mediated insulin secretion as well as of insulin responsiveness in skeletal muscle, and thus a key effector for glucose homeostasis in vivo. Doc2b’s actions in glucose homeostasis may be related to its ability to bind Munc18c and/or directly promote fusion of insulin granules and GLUT4 vesicles in a stimulus-dependent manner.

Syntaxin4 Interacting Protein (Synip) Binds Phosphatidylinositol (3,4,5) Triphosphate

The insulin responsive Glut4 transport vesicles contain the v-SNARE protein Vamp2 that associate with the plasma membrane t-SNARE protein Syntaxin 4 to drive insulin-stimulated Glut4 translocation in skeletal muscle and adipocytes. The syntaxin 4 interacting protein (Synip) binds to syntaxin 4 in the basal state and dissociates in the insulin-stimulated state allowing for the subsequent binding of Vamp2 containing Glut4 vesicles and fusion with the plasma membrane. In this study, we have found that Synip binds phosphatidylinositol 3,4,5-triphosphate (PIP3), but not phosphatidylinositol 3 phosphate (PIP) or phosphatidylinositol 3,4-biphosphate (PIP2) through the Synip WW domain as deletion of this domain (Synip ΔWW) failed to bind PIP3. Over-expressed Synip ΔWW in 3T3L1 adipocytes reduced the basal levels of Glut4 at the plasma membrane with no effect on the binding to syntaxin 4 in vitro. Subcellular fractionation demonstrated that the amount of Synip ΔWW at the PM was decreased in response to insulin in 3T3L1 adipocytes whereas the amount of Synip WT increased. These data suggest that in the presence of insulin, the dissociated Synip remains anchored to the plasma membrane by binding to PIP3.

An intervention study in obese mice with astaxanthin, a marine carotenoid – effects on insulin signaling and pro-inflammatory cytokines

Astaxanthin (ASX), a xanthophyll carotenoid from the marine algae Hematococcus pluvialis, has anti-obesity and insulin-sensitivity effects. The specific molecular mechanisms of its actions are not yet established. The present study was designed to investigate the mechanisms underlying the insulin sensitivity effects of ASX in a non-genetic insulin resistant animal model. A group of male Swiss albino mice was divided into two and fed either a starch-based control diet or a high fat-high fructose diet (HFFD). Fifteen days later, mice in each dietary group were divided into two and were treated with either ASX (6 mg kg(-1) per day) in olive oil or olive oil alone. At the end of 60 days, glucose, insulin and pro-inflammatory cytokines in plasma, lipids and oxidative stress markers in skeletal muscle and adipose tissue were assessed. Further, post-receptor insulin signaling events in skeletal muscle were analyzed. Increased body weight, hyperglycemia, hyperinsulinemia and increased plasma levels of tumor necrosis factor-α and interleukin-6 observed in HFFD-fed mice were significantly improved by ASX addition. ASX treatment also reduced lipid levels and oxidative stress in skeletal muscle and adipose tissue. ASX improved insulin signaling by enhancing the autophosphorylation of insulin receptor-β (IR-β), IRS-1 associated PI3-kinase step, phospho-Akt/Akt ratio and GLUT-4 translocation in skeletal muscle. This study demonstrates for the first time that chronic ASX administration improves insulin sensitivity by activating the post-receptor insulin signaling and by reducing oxidative stress, lipid accumulation and proinflammatory cytokines in obese mice.

The plasma 5′-AMP acts as a potential upstream regulator of hyperglycemia in type 2 diabetic mice

Increased plasma free fatty acid (FFA) level is a hallmark of type 2 diabetes. However, the underlying molecular basis for FFA-caused hyperglycemia remains unclear. Here we identified plasma 5'-adenosine monophosphate (pAMP) markedly elevated in the plasma of type 2 diabetic mice. High levels of FFAs induced damage in vein endothelial cells and contributed to an increase in pAMP. Administration of synthetic 5'-AMP caused hyperglycemia and impaired insulin action in lean wild-type mice. 5'-AMP elevated blood glucose in mice deficient in adenosine receptors with equal efficiency as wild-type mice. The function of pAMP was initiated by the elevation of cellular adenosine levels, directly stimulating G-6-Pase enzyme activity, attenuating insulin-dependent GLUT4 translocation in skeletal muscle, and displaying a rapid and steep increase in blood glucose and a decrease in hepatic glycogen level. It was followed by an increase in the gene expression of hepatic Foxo1 and its targeting gene Pepck and G6Pase, which was similar to diabetic phenotype in db/db mice. Our results suggest that pAMP is a potential upstream regulator of hyperglycemia in type 2 diabetes.

Inhibition or Ablation of p21-activated Kinase (PAK1) Disrupts Glucose Homeostatic Mechanisms in Vivo

The p21-activated kinase PAK1 is implicated in tumorigenesis, and efforts to inhibit PAK1 signaling as a means to induce tumor cell apoptosis are underway. However, PAK1 has also been implicated as a positive effector of mechanisms in clonal pancreatic beta cells and skeletal myotubes that would be crucial to maintaining glucose homeostasis in vivo. Of relevance, human islets of Type 2 diabetic donors contained ~80% less PAK1 protein compared with non-diabetics, implicating PAK1 in islet signaling/scaffolding functions. Mimicking this, islets from PAK1(-/-) knock-out mice exhibited profound defects in the second/sustained-phase of insulin secretion. Reiteration of this specific defect by human islets treated with the PAK1 signaling inhibitor IPA3 revealed PAK1 signaling to be of primary functional importance. Analyses of human and mouse islet beta cell signaling revealed PAK1 activation to be 1) dependent upon Cdc42 abundance, 2) crucial for signaling downstream to activate ERK1/2, but 3) dispensable for cofilin phosphorylation. Importantly, the PAK1(-/-) knock-out mice were found to exhibit whole body glucose intolerance in vivo. Exacerbating this, the PAK1(-/-) knock-out mice also exhibited peripheral insulin resistance. Insulin resistance was coupled to ablation of insulin-stimulated GLUT4 translocation in skeletal muscle from PAK1(-/-) knock-out mice, and in sharp contrast to islet beta cells, skeletal muscle PAK1 loss was underscored by defective cofilin phosphorylation but normal ERK1/2 activation. Taken together, these data provide the first human islet and mammalian in vivo data unveiling the key and crucial roles for differential PAK1 signaling in the multi-tissue regulation of whole body glucose homeostasis.

Intensive insulin treatment induces insulin resistance in diabetic rats by impairing glucose metabolism-related mechanisms in muscle and liver

Insulin replacement is the only effective therapy to manage hyperglycemia in type 1 diabetes mellitus (T1DM). Nevertheless, intensive insulin therapy has inadvertently led to insulin resistance. This study investigates mechanisms involved in the insulin resistance induced by hyperinsulinization. Wistar rats were rendered diabetic by alloxan injection, and 2 weeks later received saline or different doses of neutral protamine Hagedorn insulin (1.5, 3, 6, and 9 U/day) over 7 days. Insulinopenic-untreated rats and 6U- and 9U-treated rats developed insulin resistance, whereas 3U-treated rats revealed the highest grade of insulin sensitivity, but did not achieve good glycemic control as 6U- and 9U-treated rats did. This insulin sensitivity profile was in agreement with glucose transporter 4 expression and translocation in skeletal muscle, and insulin signaling, phosphoenolpyruvate carboxykinase/glucose-6-phosphatase expression and glycogen storage in the liver. Under the expectation that insulin resistance develops in hyperinsulinized diabetic patients, we believe insulin sensitizer approaches should be considered in treating T1DM.

Epigallocatechin-3-O-gallate (EGCG) attenuates FFAs-induced peripheral insulin resistance through AMPK pathway and insulin signaling pathway in vivo

We aimed to investigate the effects and possible mechanisms of Epigallocatechin-3-O-gallate (EGCG) on free fatty acids (FFAs)-induced peripheral insulin resistance in vivo. Overnight-fasted Wistar rats were subjected to 48-h intravenous infusion of either saline or Intralipid plus heparin (IH) with or without different doses of EGCG co-injection. Hyperinsulinemic–euglycemic clamp was performed in awake rats to assess peripheral insulin sensitivity. Co-injection with EGCG significantly prevented FFAs-induced peripheral insulin resistance, decreased plasma markers of oxidative stress: malondialdehyde (MDA) and 8-isoprostaglandin, and increased antioxidant enzymes: superoxide dismutases (SOD) and Glutathione peroxidase (GPx). Furthermore, EGCG treatment reversed IH-induced: (1) decrease in Thr172 phosphorylation of AMP activated protein kinase (AMPK); (2) increase in protein kinase Cθ(PKCθ) membrane translocation and Ser307 phosphorylation of insulin receptor substrate-1 (IRS-1); (3) decrease in Ser473 phosphorylation of Akt and Glucose transporter 4 (GLUT4) translocation in skeletal muscle and adipose tissue. Our data suggest that EGCG treatment ameliorated FFAs-induced peripheral insulin resistance in vivo, and this might be through decreasing oxidative stress and PKCθ membrane translocation, activating the AMPK pathway and improving insulin signaling pathway in vivo. This study suggests the therapeutic value of EGCG in protecting from insulin resistance caused by elevated FFAs.

Chinese herbal extracts (SK0506) as a potential candidate for the therapy of the metabolic syndrome

The metabolic syndrome has reached epidemic proportions worldwide, but currently there is a lack of effective therapies for this multifactorial endocrine disease. TCM (traditional Chinese medicine) has been utilized to treat a wide variety of diseases for centuries in the People's Republic of China, subsequently becoming a promising source for the development of new therapeutic agents. Chinese medicinal herbs Gynostemma pentaphyllum, Coptis chinensis and Salvia miltiorrhiza have been shown to have anti-atherosclerotic and antidiabetic properties. In this study, we have investigated the metabolic effects of a mixture of these three herbal extracts (SK0506) in a rodent model of the metabolic syndrome induced by an HFD (high-fat diet). SD (Sprague-Dawley) rats that were fed on an HFD for 4 weeks gained 33% more weight compared with chow-fed rats (P<0.05). Four weeks treatment with SK0506 prevented weight gain with decreased visceral fat (P<0.01 compared with vehicle treatment). SK0506 also significantly reduced plasma triacylglycerols (triglycerides), NEFAs (non-esterified fatty acids) and cholesterol. SK0506 exerted similar effects to RSG (rosiglitazone) on impaired glucose intolerance. SK0506 also significantly enhanced glucose uptake and glycogen synthesis in adipose tissue during hyperinsulinaemic-euglycaemic clamp. Western blotting analysis revealed that SK0506 enhanced GLUT4 (glucose transporter 4) expression in adipose tissue, and RSG markedly up-regulated GLUT4 translocation in skeletal muscle. Overall, the present study has discovered that SK0506 can reverse several components of the metabolic syndrome primarily through acting on hyperlipidaemia and visceral obesity. The results from the present study suggest that it is worthwhile to conduct a randomized clinical trial to confirm the potential that SK0506 may be a new oral agent for treating the metabolic syndrome and preventing Type 2 diabetes.

Exercise‐induced TBC1D1 Ser237 phosphorylation and 14‐3‐3 protein binding capacity in human skeletal muscle

TBC1D1 is a Rab-GTPase activating protein involved in regulation of GLUT4 translocation in skeletal muscle. We here evaluated exercise-induced regulation of TBC1D1 Ser237 phosphorylation and 14-3-3 protein binding capacity in human skeletal muscle. In separate experiments healthy men performed all-out cycle exercise lasting either 30 s, 2 min or 20 min. After all exercise protocols, TBC1D1 Ser237 phosphorylation increased (∼70-230%, P < 0.005), with the greatest response observed after 20 min of cycling. Interestingly, capacity of TBC1D1 to bind 14-3-3 protein showed a similar pattern of regulation, increasing 60-250% (P < 0.001). Furthermore, recombinant 5AMP-activated protein kinase (AMPK) induced both Ser237 phosphorylation and 14-3-3 binding properties on human TBC1D1 when evaluated in vitro. To further characterize the role of AMPK as an upstream kinase regulating TBC1D1, extensor digitorum longus muscle (EDL) from whole body α1 or α2 AMPK knock-out and wild-type mice were stimulated to contract in vitro. In wild-type and α1 knock-out mice, contractions resulted in a similar ∼100% increase (P < 0.001) in Ser237 phosphorylation. Interestingly, muscle of α2 knock-out mice were characterized by reduced protein content of TBC1D1 (∼50%, P < 0.001) as well as in basal and contraction-stimulated (∼60%, P < 0.001) Ser237 phosphorylation, even after correction for the reduced TBC1D1 protein content. This study shows that TBC1D1 is Ser237 phosphorylated and 14-3-3 protein binding capacity is increased in response to exercise in human skeletal muscle. Furthermore, we show that the catalytic α2 AMPK subunit is the main (but probably not the only) donor of AMPK activity regulating TBC1D1 Ser237 phosphorylation in mouse EDL muscle.

Kinetics of Contraction-Induced GLUT4 Translocation in Skeletal Muscle Fibers From Living Mice

OBJECTIVEExercise is an important strategy for the treatment of type 2 diabetes. This is due in part to an increase in glucose transport that occurs in the working skeletal muscles. Glucose transport is regulated by GLUT4 translocation in muscle, but the molecular machinery mediating this process is poorly understood. The purpose of this study was to 1) use a novel imaging system to elucidate the kinetics of contraction-induced GLUT4 translocation in skeletal muscle and 2) determine the function of AMP-activated protein kinase α2 (AMPKα2) in this process.RESEARCH DESIGN AND METHODSConfocal imaging was used to visualize GLUT4-enhanced green fluorescent protein (EGFP) in transfected quadriceps muscle fibers in living mice subjected to contractions or the AMPK-activator AICAR.RESULTSContraction increased GLUT4-EGFP translocation from intracellular vesicle depots to both the sarcolemma and t-tubules with similar kinetics, although translocation was greater with contractions elicited by higher voltage. Re-internalization of GLUT4 did not begin until 10 min after contractions ceased and was not complete until 130 min after contractions. AICAR increased GLUT4-EGFP translocation to both sarcolemma and t-tubules with similar kinetics. Ablation of AMPKα2 activity in AMPKα2 inactive transgenic mice did not change GLUT4-EGFP′s basal localization, contraction-stimulated intracellular GLUT4-EGFP vesicle depletion, translocation, or re-internalization, but diminished AICAR-induced translocation.CONCLUSIONSWe have developed a novel imaging system to study contraction-stimulated GLUT4 translocation in living mice. Contractions increase GLUT4 translocation to the sarcolemma and t-tubules with similar kinetics and do not require AMPKα2 activity.

Measuring GLUT4 translocation in mature muscle fibers

Skeletal muscle is the major tissue for postprandial glucose disposal. Facilitated glucose uptake into muscle fibers is mediated by increases in surface membrane levels of the glucose transporter GLUT4 via insulin- and/or muscle contraction-mediated GLUT4 translocation. However, the regulatory mechanisms controlling GLUT4 translocation in skeletal muscle have been difficult to characterize at the cell biology level due to muscle tissue complexity. Muscle cell culture models have improved our understanding of GLUT4 translocation and glucose transport regulation, but in vitro muscle models lack many of the characteristics of mature muscle fibers. Thus, the molecular and cellular details of GLUT4 translocation in mature skeletal muscle are deficient. The objective of this review is to highlight how advances in recent experimental approaches translate into an enhanced understanding of the regulation of GLUT4 translocation and glucose transport in mature skeletal muscle.

Muscles as molecular and metabolic machines

skeletal muscle is the largest organ in humans, comprising ∼50% of the total body mass. While this tissue functions to sustain normal daily physical activity, skeletal muscle is also involved in regulating metabolic homeostasis. For example, early studies in skeletal muscle revealed that glucose

Astragalus polysaccharide improves insulin sensitivity in KKAy mice: Regulation of PKB/GLUT4 signaling in skeletal muscle

Ethnopharmacological relevance Astragalus polysaccharide (APS) is an important bioactive component of Astragalus membranaceus Bunge (Leguminosae) that has been used in traditional Chinese medicine for treating diabetes. Aim of the study To study the mechanisms by which APS ameliorates diabetes, we examined whether treatment with APS improves insulin sensitivity in insulin-resistant mice and whether this is associated with an improvement of dysregulated protein kinase B and glucose transporter 4 expressions in skeletal muscle. Methods APS (700 mg kg −1 day −1) or vehicle was administered to 12-week-old diabetic KKAy and nondiabetic C57BL/6J mice for 8 weeks. Changes in body weight, blood glucose level, insulin resistance index, and oral glucose tolerance were routinely evaluated. The expressions of protein kinase B and glucose transporter 4 in skeletal muscle tissues were determined with Western blot. Results KKAy mice developed persistent hyperglycemia, impaired glucose tolerance and insulin resistance. Insulin-stimulated protein kinase B phosphorylation and glucose transporter 4 translocation were significantly decreased in KKAy compared to age-matched C57BL/6J mice. APS treatment ameliorated hyperglycemia and insulin resistance. Although the content of protein kinase B and glucose transporter 4 in KKAy skeletal muscle were not affected by APS, insulin-induced protein kinase B Ser-473 phosphorylation and glucose transporter 4 translocation in skeletal muscle were partially restored by APS treatment. In contrast, APS did not have any effect on C57BL/6J mice. Conclusions These results indicate that APS can regulate part of the insulin signaling in insulin-resistant skeletal muscle, and that APS could be a potential insulin sensitizer for the treatment of type 2 diabetes.

Effects of pyridoxamine (K-163) on glucose intolerance and obesity in high-fat diet C57BL/6J mice

Advanced glycation end products (AGEs) contribute to the pathogenesis of diabetes-associated complications. Previously, we reported the possible effect of pyridoxamine (K-163), an AGE inhibitor, on improvement of glucose intolerance in type 2 diabetes mellitus KK-A y/Ta mice. Recently, AGEs and oxidative stress have been shown to induce insulin resistance. The objective of the present study is to examine the effect of pyridoxamine on glucose intolerance and oxidative stress. C57BL/6J mice were divided into 3 groups as follows: low-fat diet, high-fat diet, and high-fat diet with pyridoxamine treatment. Body and adipose tissue weight, serum insulin, hydrogen peroxide, malondialdehyde and AGE, and urinary 8-hydroxy-2′-deoxyguanosine levels were measured. Nicotinamide adenine dinucleotide phosphate subunits, antioxidant enzymes, and adipocytokine messenger RNA expressions in the adipose tissues were evaluated. Akt/protein kinase B activity and glucose transporter 4 translocation in skeletal muscle were also evaluated. Body and adipose tissue weights of the pyridoxamine treatment group were significantly decreased compared with those of the high-fat diet group. Pyridoxamine attenuated serum hydrogen peroxide, malondialdehyde and AGE, and urinary 8-hydroxy-2′-deoxyguanosine and nicotinamide adenine dinucleotide phosphate oxidase expression; increased antioxidant enzyme expression; and improved dysregulation of adipocytokines in adipose tissues. Pyridoxamine improved blood glucose levels after glucose injection and fasting hyperinsulinemia. Suppressed Akt/protein kinase B activity and glucose transporter 4 translocation in skeletal muscle in high-fat diet mice were improved by pyridoxamine treatment. It appears that the antioxidative effect of pyridoxamine is associated with improvement of glucose intolerance and obesity in C57BL/6J mice fed a high-fat diet. We assume that pyridoxamine may be useful in the treatment of the obesity-associated metabolic syndrome.