Multiple defects in muscle glycogen synthase activity contribute to reduced glycogen synthesis in non-insulin dependent diabetes mellitus

Insulin Resistance in Skeletal Muscle of Chronic Stroke

A stroke can lead to reduced mobility affecting skeletal muscle mass and fatty infiltration which could lead to systemic insulin resistance, but this has not been examined and the mechanisms are currently unknown. The objective was to compare the effects of in vivo insulin on skeletal muscle glycogen synthase (GS) activity in paretic (P) and nonparetic (NP) skeletal muscle in chronic stroke, and to compare to nonstroke controls. Participants were mild to moderately disabled adults with chronic stroke (n = 30, 60 ± 8 years) and sedentary controls (n = 35, 62 ± 8 years). Insulin sensitivity (M) and bilateral GS activity were determined after an overnight fast and during a hyperinsulinemic-euglycemic clamp. Stroke subjects had lower aerobic capacity than controls, but M was not significantly different. Insulin-stimulated activities of GS (independent, total, fractional), as well as absolute differences (insulin minus basal) and the percent change (insulin minus basal, relative to basal) in GS activities, were all significantly lower in P versus NP muscle. Basal GS fractional activity was 3-fold higher, and the increase in GS fractional activity during the clamp was 2-fold higher in control versus P and NP muscle. Visceral fat and intermuscular fat were associated with lower M. The effect of in vivo insulin to increase GS fractional activity was associated with M in control and P muscle. A reduction in insulin action on GS in paretic muscle likely contributes to skeletal muscle-specific insulin resistance in chronic stroke.

Morphological characteristics in diabetic cardiomyopathy associated with autophagy

Diabetic cardiomyopathy, clinically diagnosed as ventricular dysfunction in the absence of coronary atherosclerosis or hypertension in diabetic patients, is a cardiac muscle-specific disease that increases the risk of heart failure and mortality. Its clinical course is characterized initially by diastolic dysfunction, later by systolic dysfunction, and eventually by clinical heart failure from an uncertain mechanism. Light microscopic features such as interstitial fibrosis, inflammation, and cardiomyocyte hypertrophy are observed in diabetic cardiomyopathy, but are common to failing hearts generally and are not specific to diabetic cardiomyopathy. Electron microscopic studies of biopsy samples from diabetic patients with heart failure have revealed that the essential mechanism underlying diabetic cardiomyopathy involves thickening of the capillary basement membrane, accumulation of lipid droplets, and glycogen as well as increased numbers of autophagic vacuoles within cardiomyocytes. Autophagy is a conserved mechanism that contributes to maintaining intracellular homeostasis by degrading long-lived proteins and damaged organelles and is observed more often in cardiomyocytes within failing hearts. Diabetes mellitus (DM) impairs cardiac metabolism and leads to dysregulation of energy substrates that contribute to cardiac autophagy. However, a "snapshot" showing greater numbers of autophagic vacuoles within cardiomyocytes may indicate that autophagy is activated into phagophore formation or is suppressed due to impairment of the lysosomal degradation step. Recent in vivo studies have shed light on the underlying molecular mechanism governing autophagy and its essential meaning in the diabetic heart. Autophagic responses to diabetic cardiomyopathy differ between diabetic types: they are enhanced in type 1 DM, but are suppressed in type 2 DM. This difference provides important insight into the pathophysiology of diabetic cardiomyopathy. Here, we review recent advances in our understanding of the pathophysiology of diabetic cardiomyopathy, paying particular attention to autophagy in the heart, and discuss the therapeutic potential of interventions modulating autophagy in diabetic cardiomyopathy.

Pharmacological inhibition of diacylglycerol acyltransferase-1 and insights into postprandial gut peptide secretion

AIM

To examine the role that enzyme Acyl-CoA:diacylglycerol acyltransferase-1 (DGAT1) plays in postprandial gut peptide secretion and signaling.

METHODS

The standard experimental paradigm utilized to evaluate the incretin response was a lipid challenge. Following a lipid challenge, plasma was collected via cardiac puncture at each time point from a cohort of 5-8 mice per group from baseline at time zero to 10 h. Incretin hormones [glucagon like peptide-1 (GLP-1), peptide tyrosine-tyrosine (PYY) and glucose dependent insulinotropic polypeptide (GIP)] were then quantitated. The impact of pharmacological inhibition of DGAT1 on the incretin effect was evaluated in WT mice. Additionally, a comparison of loss of DGAT1 function either by genetic ablation or pharmacological inhibition. To further elucidate the pathways and mechanisms involved in the incretin response to DGAT1 inhibition, other interventions [inhibitors of dipeptidyl peptidase-IV (sitagliptin), pancreatic lipase (Orlistat), GPR119 knockout mice] were evaluated.

RESULTS

DGAT1 deficient mice and wildtype C57/BL6J mice were lipid challenged and levels of both active and total GLP-1 in the plasma were increased. This response was further augmented with DGAT1 inhibitor PF-04620110 treated wildtype mice. Furthermore, PF-04620110 was able to dose responsively increase GLP-1 and PYY, but blunt GIP at all doses of PF-04620110 during lipid challenge. Combination treatment of PF-04620110 and Sitagliptin in wildtype mice during a lipid challenge synergistically enhanced postprandial levels of active GLP-1. In contrast, in a combination study with Orlistat, the ability of PF-04620110 to elicit an enhanced incretin response was abrogated. To further explore this observation, GPR119 knockout mice were evaluated. In response to a lipid challenge, GPR119 knockout mice exhibited no increase in active or total GLP-1 and PYY. However, PF-04620110 was able to increase total GLP-1 and PYY in GPR119 knockout mice as compared to vehicle treated wildtype mice.

CONCLUSION

Collectively, these data provide some insight into the mechanism by which inhibition of DGAT1 enhances intestinal hormone release.

Molecular mechanisms of cardiac pathology in diabetes - Experimental insights

Diabetic cardiomyopathy is a distinct pathology independent of co-morbidities such as coronary artery disease and hypertension. Diminished glucose uptake due to impaired insulin signaling and decreased expression of glucose transporters is associated with a shift towards increased reliance on fatty acid oxidation and reduced cardiac efficiency in diabetic hearts. The cardiac metabolic profile in diabetes is influenced by disturbances in circulating glucose, insulin and fatty acids, and alterations in cardiomyocyte signaling. In this review, we focus on recent preclinical advances in understanding the molecular mechanisms of diabetic cardiomyopathy. Genetic manipulation of cardiomyocyte insulin signaling intermediates has demonstrated that partial cardiac functional rescue can be achieved by upregulation of the insulin signaling pathway in diabetic hearts. Inconsistent findings have been reported relating to the role of cardiac AMPK and β-adrenergic signaling in diabetes, and systemic administration of agents targeting these pathways appear to elicit some cardiac benefit, but whether these effects are related to direct cardiac actions is uncertain. Overload of cardiomyocyte fuel storage is evident in the diabetic heart, with accumulation of glycogen and lipid droplets. Cardiac metabolic dysregulation in diabetes has been linked with oxidative stress and autophagy disturbance, which may lead to cell death induction, fibrotic 'backfill' and cardiac dysfunction. This review examines the weight of evidence relating to the molecular mechanisms of diabetic cardiomyopathy, with a particular focus on metabolic and signaling pathways. Areas of uncertainty in the field are highlighted and important knowledge gaps for further investigation are identified. This article is part of a Special issue entitled Cardiac adaptations to obesity, diabetes and insulin resistance, edited by Professors Jan F.C. Glatz, Jason R.B. Dyck and Christine Des Rosiers.

Islet Transplantation under the Kidney Capsule Corrects the Defects in Glycogen Metabolism in Both Liver and Muscle of Streptozocin-Diabetic Rats

Insulin-deficient rats are characterized by multiple defects in the pathway of glycogen synthesis and breakdown in both liver and skeletal muscle. The aim of this study was to clarify whether islet transplantation under the kidney capsule, which is associated with delivery of insulin into the peripheral circulation, is able to normalize glycogen metabolism in liver and muscle of streptozotocin-diabetic rats. Three groups of male Lewis rats were studied under fasting condition: controls, untreated diabetics, and islet transplanted diabetics. Glycogen content, glucose-6-phosphate concentration, and glycogen synthase activity were measured in both liver and skeletal muscle. Untreated diabetic rats were characterized by an increase in glycogen content of 178% and a reduction of glucose-6-phosphate level of 50%. Both glycogen and glucose-6-phosphate contents were restored to normal in transplanted diabetic rats. Active glycogen synthase (0.35 ± 0.1 nmol/min/mg) and activity ratio (0.22 ± 0.04) were significantly impaired compared with controls (0.99 ± 0.2 nmol/min/mg and 0.43 ± 0.06, respectively) and were normalized by islet transplantation. In the skeletal muscle, glycogen content was similar in the three groups of animals, whereas muscle glucose-6-phosphate level was reduced by 28% and glycogen synthase was in a less active state in the untreated diabetic rats. Both the glucose-6-phosphate concentration and the kinetic profile of glycogen synthase were normalized by islet transplantation. In conclusion, islet transplantation under the kidney capsule corrects the diabetes-induced abnormalities in glycogen and glucose-6-phosphate content and glycogen synthase activity in both liver and skeletal muscle.

Type 2 diabetes and obesity induce similar transcriptional reprogramming in human myocytes

BACKGROUND

Skeletal muscle is one of the primary tissues involved in the development of type 2 diabetes (T2D). The close association between obesity and T2D makes it difficult to isolate specific effects attributed to the disease alone. Therefore, here we set out to identify and characterize intrinsic properties of myocytes, associated independently with T2D or obesity.

METHODS

We generated and analyzed RNA-seq data from primary differentiated myotubes from 24 human subjects, using a factorial design (healthy/T2D and non-obese/obese), to determine the influence of each specific factor on genome-wide transcription. This setup enabled us to identify intrinsic properties, originating from muscle precursor cells and retained in the corresponding myocytes. Bioinformatic and statistical methods, including differential expression analysis, gene-set analysis, and metabolic network analysis, were used to characterize the different myocytes.

RESULTS

We found that the transcriptional program associated with obesity alone was strikingly similar to that induced specifically by T2D. We identified a candidate epigenetic mechanism, H3K27me3 histone methylation, mediating these transcriptional signatures. T2D and obesity were independently associated with dysregulated myogenesis, down-regulated muscle function, and up-regulation of inflammation and extracellular matrix components. Metabolic network analysis identified that in T2D but not obesity a specific metabolite subnetwork involved in sphingolipid metabolism was transcriptionally regulated.

CONCLUSIONS

Our findings identify inherent characteristics in myocytes, as a memory of the in vivo phenotype, without the influence from a diabetic or obese extracellular environment, highlighting their importance in the development of T2D.

Impaired glucose metabolism and exercise capacity with muscle-specific glycogen synthase 1 (gys1) deletion in adult mice

Objective

Muscle glucose storage and muscle glycogen synthase (gys1) defects have been associated with insulin resistance. As there are multiple mechanisms for insulin resistance, the specific role of glucose storage defects is not clear. The aim of this study was to examine the effects of muscle-specific gys1 deletion on glucose metabolism and exercise capacity.

Methods

Tamoxifen inducible and muscle specific gys-1 KO mice were generated using the Cre/loxP system. Mice were subjected to glucose tolerance tests, euglycemic/hyperinsulinemic clamps and exercise tests.

Results

gys1-KO mice showed ≥85% reduction in muscle gys1 mRNA and protein concentrations, 70% reduction in muscle glycogen levels, postprandial hyperglycaemia and hyperinsulinaemia and impaired glucose tolerance. Under insulin-stimulated conditions, gys1-KO mice displayed reduced glucose turnover and muscle glucose uptake, indicative of peripheral insulin resistance, as well as increased plasma and muscle lactate levels and reductions in muscle hexokinase II levels. gys1-KO mice also exhibited markedly reduced exercise and endurance capacity.

Conclusions

Thus, muscle-specific gys1 deletion in adult mice results in glucose intolerance due to insulin resistance and reduced muscle glucose uptake as well as impaired exercise and endurance capacity.

In brief

This study demonstrates why the body prioritises muscle glycogen storage over liver glycogen storage despite the critical role of the liver in supplying glucose to the brain in the fasting state and shows that glycogen deficiency results in impaired glucose metabolism and reduced exercise capacity.

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Muscle-specific gys1 knockdown in adult mice results in 70% reduction in skeletal muscle glycogen levels.

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Muscle-specific gys1 knockdown leads to glucose intolerance and peripheral insulin resistance.

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Muscle glycogen depletion caused impaired performance, as well as fatigue development during exercise.

Breaking up of prolonged sitting over three days sustains, but does not enhance, lowering of postprandial plasma glucose and insulin in overweight and obese adults

To compare the cumulative (3-day) effect of prolonged sitting on metabolic responses during a mixed meal tolerance test (MTT), with sitting that is regularly interrupted with brief bouts of light-intensity walking. Overweight/obese adults (n=19) were recruited for a randomized, 3-day, outpatient, cross-over trial involving: (1) 7-h days of uninterrupted sitting (SIT); and (2) 7-h days of sitting with light-intensity activity breaks [BREAKS; 2-min of treadmill walking (3.2 km/h) every 20 min (total: 17 breaks/day)]. On days 1 and 3, participants underwent a MTT (75 g of carbohydrate, 50 g of fat) and the incremental area under the curve (iAUC) was calculated from hourly blood samples. Generalized estimating equation (GEE) models were adjusted for gender, body mass index (BMI), energy intake, treatment order and pre-prandial values to determine effects of time, condition and time × condition. The glucose iAUC was 1.3 ± 0.5 and 1.5 ± 0.5 mmol·h·l(-1) (mean differences ± S.E.M.) higher in SIT compared with BREAKS on days 1 and 3 respectively (condition effect: P=0.001), with no effect of time (P=0.48) or time × condition (P=0.8). The insulin iAUC was also higher on both days in SIT (day 1: ∆151 ± 73, day 3: ∆91 ± 73 pmol·h·l(-1), P=0.01), with no effect of time (P=0.52) or time × condition (P=0.71). There was no between-treatment difference in triglycerides (triacylglycerols) iAUC. There were significant between-condition effects but no temporal change in metabolic responses to MTT, indicating that breaking up of sitting over 3 days sustains, but does not enhance, the lowering of postprandial glucose and insulin.

Myocardial autophagic energy stress responses--macroautophagy, mitophagy, and glycophagy

An understanding of the role of autophagic processes in the management of cardiac metabolic stress responses is advancing rapidly and progressing beyond a conceptualization of the autophagosome as a simple cell recycling depot. The importance of autophagy dysregulation in diabetic cardiomyopathy and in ischemic heart disease - both conditions comprising the majority of cardiac disease burden - has now become apparent. New findings have revealed that specific autophagic processes may operate in the cardiomyocyte, specialized for selective recognition and management of mitochondria and glycogen particles in addition to protein macromolecular structures. Thus mitophagy, glycophagy, and macroautophagy regulatory pathways have become the focus of intensive experimental effort, and delineating the signaling pathways involved in these processes offers potential for targeted therapeutic intervention. Chronically elevated macroautophagic activity in the diabetic myocardium is generally observed in association with structural and functional cardiomyopathy; yet there are also numerous reports of detrimental effect of autophagy suppression in diabetes. Autophagy induction has been identified as a key component of protective mechanisms that can be recruited to support the ischemic heart, but in this setting benefit may be mitigated by adverse downstream autophagic consequences. Recent report of glycophagy upregulation in diabetic cardiomyopathy opens up a novel area of investigation. Similarly, a role for glycogen management in ischemia protection through glycophagy initiation is an exciting prospect under investigation.

Deficiency of a glycogen synthase-associated protein, Epm2aip1, causes decreased glycogen synthesis and hepatic insulin resistance

Glycogen synthesis is a major component of the insulin response, and defective glycogen synthesis is a major portion of insulin resistance. Insulin regulates glycogen synthase (GS) through incompletely defined pathways that activate the enzyme through dephosphorylation and, more potently, allosteric activation. We identify Epm2aip1 as a GS-associated protein. We show that the absence of Epm2aip1 in mice impairs allosteric activation of GS by glucose 6-phosphate, decreases hepatic glycogen synthesis, increases liver fat, causes hepatic insulin resistance, and protects against age-related obesity. Our work identifies a novel GS-associated GS activity-modulating component of insulin resistance.

Pharmacological inhibition to examine the role of DGAT1 in dietary lipid absorption in rodents and humans

Alterations in fat metabolism, in particular elevated plasma concentrations of free fatty acids and triglycerides (TG), have been implicated in the pathogenesis of Type 2 diabetes, obesity, and cardiovascular disease. Acyl-CoA:diacylglycerol acyltransferase 1 (DGAT1), a member of the large family of membrane-bound O-acyltransferases, catalyzes the final step in triacylglycerol formation. In the intestine, DGAT1 is one of the acyltransferases responsible for the reesterficiation of dietary TG. Following a single dose of a selective pharmacological inhibitor of DGAT1, PF-04620110, a dose-dependent inhibition of TG and vitamin A absorption postprandially was demonstrated in rodents and human subjects. In C57/BL6J mice, acute DGAT1 inhibition alters the temporal and spatial pattern of dietary lipid absorption. To understand the impact of DGAT1 inhibition on enterocyte lipid metabolism, lipomic profiling was performed in rat intestine and plasma as well as human plasma. DGAT1 inhibition causes an enrichment of polyunsaturated fatty acids within the TG class of lipids. This pharmacological intervention gives us insight as to the role of DGAT1 in human dietary lipid absorption.

Early diagnosis of pancreatic adenocarcinoma: role of stroma, surface proteases, and glucose-homeostatic agents

OBJECTIVES

New-onset diabetes in pancreatic adenocarcinoma is due to a combination of insulin resistance and decreased β-cell function. Its differentiation from the common type 2 diabetes is the prerequisite for early diagnosis of pancreatic adenocarcinoma. Little attention has been paid to pancreatic stroma and surface proteases.

METHODS

The activated fibroblasts selectively express fibroblast activation protein α, a structural homolog of the ubiquitously expressed dipeptidyl peptidase 4. Their role in pancreatic carcinogenesis is reviewed.

RESULTS

Homodimers and heterodimers of both enzymes display high specificity for peptides and proteins with penultimate proline or alanine. Most glucose-homeostatic agents are candidate substrates of these enzymes. The biological activity of truncated substrates is decreased or absent.

CONCLUSIONS

The interactions of surface proteases with glucose-homeostatic agents may adequately explain the evolution of diabetes associated with pancreatic adenocarcinoma and differentiate it from the common type 2 diabetes.

Effect of acute exercise on glycogen synthase in muscle from obese and diabetic subjects

Insulin stimulates glycogen synthase (GS) through dephosphorylation of serine residues, and this effect is impaired in skeletal muscle from insulin-resistant [obese and type 2 diabetic (T2DM)] subjects. Exercise also increases GS activity, yet it is not known whether the ability of exercise to affect GS is impaired in insulin-resistant subjects. The objective of this study was to examine the effect of acute exercise on GS phosphorylation and enzyme kinetic properties in muscle from insulin-resistant individuals. Lean normal glucose-tolerant (NGT), obese NGT, and obese T2DM subjects performed 40 min of moderate-intensity cycle exercise (70% of Vo(2max)). GS kinetic properties and phosphorylation were measured in vastus lateralis muscle before exercise, immediately after exercise, and 3.5 h postexercise. In lean subjects, GS fractional activity increased twofold after 40 min of exercise, and it remained elevated after the 3.5-h rest period. Importantly, exercise also decreased GS K(m) for UDP-glucose from ≈0.5 to ≈0.2 mM. In lean subjects, exercise caused significant dephosphorylation of GS by 50-70% (Ser(641), Ser(645), and Ser(645,649,653,657)), and phosphorylation of these sites remained decreased after 3.5 h; Ser⁷ phosphorylation was not regulated by exercise. In obese NGT and T2DM subjects, exercise increased GS fractional activity, decreased K(m) for UDP-glucose, and decreased GS phosphorylation as effectively as in lean NGT subjects. We conclude that the molecular regulatory process by which exercise promotes glycogen synthesis in muscle is preserved in insulin-resistant subjects.

Placental restriction reduces insulin sensitivity and expression of insulin signaling and glucose transporter genes in skeletal muscle, but not liver, in young sheep

Poor growth before birth is associated with impaired insulin sensitivity later in life, increasing the risk of type 2 diabetes. The tissue sites at which insulin resistance first develops after intrauterine growth restriction (IUGR), and its molecular basis, are unclear. We have therefore characterized the effects of placental restriction (PR), a major cause of IUGR, on whole-body insulin sensitivity and expression of molecular determinants of insulin signaling and glucose uptake in skeletal muscle and liver of young lambs. Whole-body insulin sensitivity was measured at 30 d by hyperinsulinaemic euglycaemic clamp and expression of insulin signaling genes (receptors, pathways, and targets) at 43 d in muscle and liver of control (n = 15) and PR (n = 13) lambs. PR reduced size at birth and increased postnatal growth, fasting plasma glucose (+15%, P = 0.004), and insulin (+115%, P = 0.009). PR reduced whole-body insulin sensitivity (-43%, P < 0.001) and skeletal muscle expression of INSR (-36%), IRS1 (-28%), AKT2 (-44%), GLUT4 (-88%), GSK3α (-35%), and GYS1 (-31%) overall (each P < 0.05) and decreased AMPKγ3 expression in females (P = 0.030). PR did not alter hepatic expression of insulin signaling and related genes but increased GLUT2 expression (P = 0.047) in males. Whole-body insulin sensitivity correlated positively with skeletal muscle expression of IRS1, AKT2, HK, AMPKγ2, and AMPKγ3 in PR lambs only (each P < 0.05) but not with hepatic gene expression in control or PR lambs. Onset of insulin resistance after PR and IUGR is accompanied by, and can be accounted for by, reduced expression of insulin signaling and metabolic genes in skeletal muscle but not liver.

Hyperglycaemia normalises insulin action on glucose metabolism but not the impaired activation of AKT and glycogen synthase in the skeletal muscle of patients with type 2 diabetes

AIMS/HYPOTHESIS

In type 2 diabetes, reduced insulin-stimulated glucose disposal, primarily glycogen synthesis, is associated with defective insulin activation of glycogen synthase (GS) in skeletal muscle. Hyperglycaemia may compensate for these defects, but to what extent it involves improved insulin signalling to glycogen synthesis remains to be clarified.

METHODS

Whole-body glucose metabolism was studied in 12 patients with type 2 diabetes, and 10 lean and 10 obese non-diabetic controls by means of indirect calorimetry and tracers during a euglycaemic-hyperinsulinaemic clamp. The diabetic patients underwent a second isoglycaemic-hyperinsulinaemic clamp maintaining fasting hyperglycaemia. Muscle biopsies from m. vastus lateralis were obtained before and after the clamp for examination of GS and relevant insulin signalling components.

RESULTS

During euglycaemia, insulin-stimulated glucose disposal, glucose oxidation and non-oxidative glucose metabolism were reduced in the diabetic group compared with both control groups (p < 0.05). This was associated with impaired insulin-stimulated GS and AKT2 activity, deficient dephosphorylation at GS sites 2 + 2a, and reduced Thr308 and Ser473 phosphorylation of AKT. When studied under hyperglycaemia, all variables of insulin-stimulated glucose metabolism were normalised compared with the weight-matched controls. However, insulin activation and dephosphorylation (site 2 + 2a) of GS as well as activation of AKT2 and phosphorylation at Thr308 and Ser473 remained impaired (p < 0.05).

CONCLUSIONS/INTERPRETATIONS

These data confirm that hyperglycaemia compensates for decreased whole-body glucose disposal in type 2 diabetes. In contrast to previous less well-controlled studies, we provide evidence that the compensatory effect of hyperglycaemia in patients with type 2 diabetes does not involve normalisation of insulin action on GS or upstream signalling in skeletal muscle.

Atorvastatin treatment reduces exercise capacities in rats: involvement of mitochondrial impairments and oxidative stress

Physical exercise exacerbates the cytotoxic effects of statins in skeletal muscle. Mitochondrial impairments may play an important role in the development of muscular symptoms following statin treatment. Our objective was to characterize mitochondrial function and reactive oxygen species (ROS) production in skeletal muscle after exhaustive exercise in atorvastatin-treated rats. The animals were divided into four groups: resting control (CONT; n = 8) and exercise rats (CONT+EXE; n = 8) as well as resting (ATO; n = 10) and exercise (ATO+EXE; n = 8) rats that were treated with atorvastatin (10 mg·kg(-1)·day(-1) for 2 wk). Exhaustive exercise showed that the distance that was covered by treated animals was reduced (P < 0.05). Using dihydroethidium staining, we showed that the ROS level was increased by 60% in the plantaris muscle of ATO compared with CONT rats and was highly increased in ATO+EXE (226%) compared with that in CONT+EXE rats. The maximal mitochondrial respiration (V(max)) was decreased in ATO rats compared with that in CONT rats (P < 0.01). In CONT+EXE rats, V(max) significantly increased compared with those in CONT rats (P < 0.05). V(max) was significantly lower in ATO+EXE rats (-39%) compared with that in CONT+EXE rats (P < 0.001). The distance that was covered by rats significantly correlated with V(max) (r = 0.62, P < 0.01). The glycogen content was decreased in ATO, CONT+EXE, and ATO+EXE rats compared with that in CONT rats (P < 0.05). GLUT-4 mRNA expression was higher after exhaustive exercise in CONT+EXE rats compared with the other groups (P < 0.05). Our results show that exhaustive exercise exacerbated metabolic perturbations and ROS production in skeletal muscle, which may reduce the exercise capacity and promote the muscular symptoms in sedentary atorvastatin-treated animals.

Not only students can express alcohol dehydrogenase: goldfish can too!

This article describes a novel approach to study the metabolic regulation of the respiratory system in vertebrates that suits physiology lessons for undergraduate students. It consists of an experimental demonstration of the goldfish's (Carassius auratus) adaptations to anoxia. The goldfish is one of the few vertebrates showing strong enzymatic plasticity for the expression of alcohol dehydrogenase (ADH), which allows it to survive long periods of severe anoxia. Therefore, we propose two simple laboratory exercises in which students are first asked to characterize the distribution of ADH isozymes in the goldfish by performing cellulose acetate electrophoresis. The second part of this laboratory lesson is the determination of liver glycogen. To further student comprehension, an interspecies comparative component is integrated, in which the same subjects are studied in an anoxia-sensitive species, the brook charr (Salvelinus fontinalis). ADH in goldfish is restricted to skeletal muscles, where it catalyzes alcoholic fermentation, permitting ethanol excretion through the gills and therefore preventing lactate acidosis caused by sustained glycolysis during anoxia. Electrophoresis also reveals the occurrence of a liver isozyme in the brook charr, which ADH catalyzes in the opposite pathway, allowing the usual ethanol degradation. As for the liver glycogen assay, it shows largely superior content in the goldfish liver compared with the brook charr, providing goldfish with a sustained energy supply during anoxia. The results of this laboratory exercise clearly demonstrate several physiological strategies developed by goldfish to cope with such a crucial environmental challenge as oxygen depletion.

GSK-3beta and control of glucose metabolism and insulin action in human skeletal muscle

The involvement of the beta-isoform of glycogen synthase kinase (GSK-3) in glucose metabolism and insulin action was investigated in cultured human skeletal muscle cells. A 60% reduction in GSK-3beta protein expression was attained by treatment with siRNA; GSK-3alpha expression was unaltered. GSK-3beta knockdown did not influence total glycogen synthase (GS) activity, but increased the phosphorylation-dependent activity (fractional velocity-FV) in the basal state. Insulin responsiveness of GSFV was doubled by GSK-3beta knockdown (p<0.05). Basal rates of glucose uptake (GU) were not significantly influenced by GSK-3beta knockdown, while insulin stimulation of GU was increased. Improvements in insulin action on GS and GU did not involve changes in protein expression of either IRS-1 or Akt 1/2. Maximal insulin stimulation of phosphorylation of Akt was unaltered by GSK-3beta knockdown. Unlike GSK-3alpha, GSK-3beta directly regulates both GS activity in the absence of added insulin and through control of insulin action.

Effect of a single bout of moderate exercise on glucose uptake in type 2 diabetes mellitus

BACKGROUND AND PURPOSE

Hypoglycemia during exercise is a serious problem in diabetic patients during cardiac rehabilitation, whereas normal subjects rarely experience hypoglycemia. Inappropriate glucose uptake by working muscles may be responsible. However, the precise characteristics of glucose uptake during exercise have not been fully studied. We have investigated the effect of acute exercise on glucose uptake in diabetic patients.

METHODS

Nine type 2 diabetic patients (age, 57 ± 6 years; HbA1c, 7.7 ± 1.3%) performed exercise at an intensity of anaerobic threshold for 15 min. Glucose utility was determined using euglycemic hyperinsulinemic clamp technique. Glucose infusion rate (GIR) was calculated throughout the exercise and recovery session.

RESULTS

Average GIR at rest was 3.4 ± 1.6 mg/(kg(BW)min). Fifteen minutes after starting exercise, it increased significantly (6.6 ± 2.4, p < 0.001). Thirty minutes after cessation of exercise, GIR decreased significantly (4.8 ± 1.9, p < 0.05) compared with peak value. Increase in GIR was greater as BMI or body fat ratio became greater (r=0.608 and 0.475). There was a weak correlation (r=0.344) between HbA1c and GIR improving ratio (GIR during exercise x 100/GIR at rest).

CONCLUSIONS

Glucose uptake was revealed to augment significantly within 15 min after the commencement of exercise. This improvement was more obvious in patients with greater body weight, fat accumulation, and poorer diabetic control.

Role of glycogen synthase kinase-3 alpha in insulin action in cultured human skeletal muscle cells

An association between glycogen synthase kinase-3 (GSK3) in skeletal muscle and insulin resistance has been demonstrated in type 2 diabetic patients. In addition, inhibition of GSK3 improves insulin action. The aim of the present study was to elucidate the role of the alpha-isoform of GSK3 in insulin resistance in human skeletal muscle cells from nondiabetic subjects maintained in culture. Transfection of muscle cells with specific antisense oligonucleotides resulted in a 30-50% decrease of GSK3alpha protein expression (P < 0.05). Whereas neither the basal fractional velocity of glycogen synthase (GS FV) (an indicator of the activation state of the enzyme) nor glucose uptake (GU) were altered, reducing GSK3alpha expression resulted in increases in insulin stimulation of both GS FV and GU. GSK3alpha overexpression (60-100% increase over control) did not alter basal GS FV or GU but impaired insulin stimulation of both responses. Knockdown of GSK alpha also led to an increase in insulin receptor substrate-1 protein expression but did not alter insulin stimulation of pS473-Akt phosphorylation. However, GSK3alpha overexpression impaired insulin action on pS473-Akt. In summary, we concluded the following: 1) modulation of GSK3alpha expression has no effect on basal GU and glycogen synthase activities; 2) reduction of GSK3alpha expression results in improvements in insulin action; and 3) elevation of GSK3alpha in human skeletal muscle cells can induce insulin resistance for several responses. We conclude that GSK3alpha is an important regulator of muscle insulin action.

Targeting glycogen synthase kinase-3 in insulin signalling

The renewed interest in an enzyme first discovered over 25 years ago stems from the potential of inhibitors of this enzyme to treat conditions as diverse as diabetes, Alzheimer's disease, stroke and bipolar disorder, and even to enhance the repopulating capacity of transplanted haematopoietic stem cells. The emergence of the first few potent and specific glycogen synthase kinase-3 (GSK-3) inhibitors will end years of speculation on their potential and finally allow the impact of GSK-3 inhibitors to be evaluated clinically. The next few years are likely to be particularly exciting ones for fans of this old enzyme. This review focuses on the role of GSK-3 in the insulin signalling pathway and highlights the evidence implicating the enzyme in insulin resistance. Pharmacological in vitro and in vivo proof-of-concept studies are also discussed, which establish the therapeutic potential of GSK-3 inhibitors as agents for the treatment of Type 2 diabetes.

Is non-insulin dependent glucose uptake a therapeutic alternative? Part 1: physiology, mechanisms and role of non insulin-dependent glucose uptake in type 2 diabetes

Several decades of research for treating type 2 diabetes have yielded new drugs but the actual experience with the available oral antidiabetic compounds clearly shows that therapeutic needs are not matched. This highlights the urgent need for exploring other pathways. All cell types have the capacity to take up glucose independently of insulin, whereby basal but also hyperglycaemia-promoted glucose supply is ensured. Although poorly explored, insulin-independent glucose uptake might nevertheless represent a therapeutic target, as an alternative to the clear limits of actual drug treatments. This review not only critically examines some major pathways not requiring insulin (although they may be influenced by the hormone) but importantly, this analysis extends to the clinical applicability of these potential therapeutic principles by also considering their predictable tolerability for long-term therapy. In particular vascular safety (the ultimate problem linked with diabetes) will be envisaged because of the ubiquitous distribution of glucose transporters and some linked mechanisms. Several mechanisms can be identified which do not require insulin for their functioning. The first part of this review deals with the description, the regulation and the limits of some mechanisms representing potential pharmacological targets capable of having a highly significant impact on glucose uptake. These selected topics are: a) unmasking and/or activation of glucose transporters in cell plasma membranes, b) insulin mimetics acting at postreceptor level, c) activation of AMPK, d) increasing nitric oxide and e) increasing glucose-6P and glycogen stores.

Characterization of the human skeletal muscle glycogen synthase gene (GYS1) promoter

BACKGROUND

Impaired activation of the human skeletal muscle glycogen synthase by insulin is typical for type 2 diabetic patients. Regulation of glycogen synthase occurs mainly by phosphorylation/dephoshorylation but little is known whether there also is transcriptional regulation. Therefore we studied transcriptional regulation of the human skeletal muscle glycogen synthase gene (GYS1) and evaluated the effects of insulin and forskolin on the promoter activity.

METHODS

Seven promoter fragments were expressed in C2C12 myoblasts and myotubes and in HEK293 cells, and the luciferase assay was used to determine transcriptional activity.

RESULTS

The highest luciferase activity, 350-fold of the promoterless vector, was obtained with nucleotides -692 to +59 in myotubes (P < 0.001), while the nucleotides -250 to +59 provided the highest, 45-fold, activity in the HEK293 cells (P < 0.001). Longer promoter constructs (nucleotides -971, -1707 and -2158 to +59, respectively) had low promoter activity in both cell types. Forskolin treatment for 24 h resulted in approximately 30% decreased promoter activity in myotubes (P < 0.05). Insulin treatment for 0.5-3 h did not increase GYS1 promoter activity; instead the activity was slightly but significantly decreased after 24 h in myotubes (P < 0.005).

CONCLUSIONS

From our results we conclude that basal GYS1 promoter activity is obtained from the first 250 nucleotides of the promoter, while the nucleotides -692 to -544 seem to be responsible for muscle-specific expression, and nucleotides -971 to -692 for negative regulation. In myotubes, the GYS1 promoter was sensitive to negative regulation by forskolin, whereas insulin did not increase GYS1 transcription.

Identification and characterization of a critical region in the glycogen synthase from Escherichia coli

The cysteine-specific reagent 5,5'-dithiobis(2-nitrobenzoic acid) inactivates the Escherichia coli glycogen synthase (Holmes, E., and Preiss, J. (1982) Arch. Biochem. Biophys. 216, 736-740). To find the responsible residue, all cysteines, Cys(7), Cys(379), and Cys(408), were substituted combinatorially by Ser. 5,5'-Dithiobis(2-nitrobenzoic acid) modified and inactivated the enzyme if and only if Cys(379) was present and it was prevented by the substrate ADP-glucose (ADP-Glc). Mutations C379S and C379A increased the S(0.5) for ADP-Glc 40- and 77-fold, whereas the specific activity was decreased 5.8- and 4.3-fold, respectively. Studies of inhibition by glucose 1-phosphate and AMP indicated that Cys(379) was involved in the interaction of the enzyme with the phosphoglucose moiety of ADP-Glc. Other mutations, C379T, C379D, and C379L, indicated that this site is intolerant for bulkier side chains. Because Cys(379) is in a conserved region, other residues were scanned by mutagenesis. Replacement of Glu(377) by Ala and Gln decreased V(max) more than 10,000-fold without affecting the apparent affinity for ADP-Glc and glycogen binding. Mutation of Glu(377) by Asp decreased V(max) only 57-fold indicating that the negative charge of Glu(377) is essential for catalysis. The activity of the mutation E377C, on an enzyme form without other Cys, was chemically restored by carboxymethylation. Other conserved residues in the region, Ser(374) and Gln(383), were analyzed by mutagenesis but found not essential. Comparison with the crystal structure of other glycosyltransferases suggests that this conserved region is a loop that is part of the active site. The results of this work indicate that this region is critical for catalysis and substrate binding.

A comprehensive search for HNF-3α-regulated genes in mouse hepatoma cells by 60K cDNA microarray and chromatin immunoprecipitation/PCR analysis

To characterize the regulatory pattern by a specific transcription regulatory factor, we used a combination of expression analysis with the mouse cDNA microarray composed of 60,000 cDNA clones and cross-linking/chromatin immunoprecipitation (X-ChIP) followed by comparative PCR. Overexpression of mouse hepatocyte nuclear factor-3alpha (HNF-3alpha) in a mouse hepatoma cell line resulted in accompanied perturbed expression of more than 1500 genes. Search for HNF-3alpha consensus recognition sequences in the upstream regions of their coding sequences, which were mapped on the mouse genome, enabled us to mine 300 genes as the potential HNF-3alpha-regulated genes and classify 135 annotated ones into several functional categories. Further X-ChIP/PCR analysis demonstrated in vivo binding of HNF-3alpha to the 5(')-flanking sequences of 25 members selected out of these genes. Besides known HNF-3alpha-regulated genes such as albumin and alpha-fetoprotein genes, the genes newly identified as the HNF-3alpha-regulated ones include three encoding CDP-diacylglycerol-inositol 3-phosphatidyltransferase, phosphatidylserine decarboxylase, and phospholipase A2, which are located en suite in the lipid metabolic pathway in liver. The potential usefulness of the present approach to extensive characterization of gene expression framework directed by a specific transcription regulatory factor is discussed.

Prevention of obesity and insulin resistance by glucokinase expression in skeletal muscle of transgenic mice

In type 2 diabetes, glucose phosphorylation, a regulatory step in glucose utilization by skeletal muscle, is impaired. Since glucokinase expression in skeletal muscle of transgenic mice increases glucose phosphorylation, we examined whether such mice counteract the obesity and insulin resistance induced by 12 wk of a high-fat diet. When fed this diet, control mice became obese, whereas transgenic mice remained lean. Furthermore, high-fat fed control mice developed hyperglycemia and hyperinsulinemia (a 3-fold increase), indicating that they were insulin resistant. In contrast, transgenic mice were normoglycemic and showed only a mild increase in insulinemia (1.5-fold). They also showed improved whole body glucose tolerance and insulin sensitivity and increased intramuscular concentrations of glucose 6-phosphate and glycogen. A parallel increase in uncoupling protein 3 mRNA levels in skeletal muscle of glucokinase-expressing transgenic mice was also observed. These results suggest that the rise in glucose phosphorylation by glucokinase expression in skeletal muscle leads to increased glucose utilization and energy expenditure that counteracts weight gain and maintains insulin sensitivity.

Association of muscle glycogen synthase polymorphism with insulin resistance in type 2 diabetic patients

The aim of the present study is to investigate whether Met416Val (M416V) polymorphism of glycogen synthase (GYS1) gene is associated with insulin resistance in type 2 diabetes. In 100 type 2 diabetic subjects (66 men and 34 women), the M416V polymorphism of GYS1 gene was analyzed by polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) as previously reported, and insulin resistance was assessed by euglycemic hyperinsulinemic clamp represented as M/I value, the mean of glucose infusion rate (M value) adjusted by steady state plasma insulin level. The means of age and body mass index (BMI) of the subjects were 53.1+/-11.6 (SD) years and 23.3+/-3.5 kg/m2. The allele frequencies of M416V polymorphism were 82.0% for MM, 16.0% for MV, and 2.0% for VV, and subjects were subsequently divided into V(+) group (n=18) and V(-) group (n=82) according to the presence or absence of V allele. There were no significant differences in age, BMI, blood pressure, fasting plasma glucose or insulin levels or glycosylated hemoglobin (HbA1c) levels between the V(+) and V(-) groups. No significant differences in either M or M/I value were found between the V(+) and V(-) groups (M value, 5.06+/-2.20 v 5.12+/-2.04 mg x kg(-1) x min(-1), P=.841; M/I value, 5.24+/-3.07 v 5.39+/-2.87 mg x kg(-1) x min(-1) x mU(-1) x L, P=.576). BMI showed the strongest independent contribution to M/I value, but the presence of V allele did not in multiple regression analysis. In conclusion, the M416V polymorphism of GYS1 gene is not associated with insulin resistance in type 2 diabetes.

Fat feeding impairs glycogen synthase activity in mice without effects on its gene expression

To examine whether the effects of high-fat feeding on glycogen synthase (GS) activity and mRNA levels differ between diabetes-prone (C57BL/6J) and diabetes-resistant mice (NMRI), we measured GS activity and mRNA levels in muscle from C57BL/6J and NMRI mice fed a high-fat or normal chow diet for 3, 6, and 15 months. As compared with chow feeding, fat feeding increased plasma insulin levels in C57BL/6J mice at 15 months (464 +/- 29 v 267 +/- 47 pmol/L, P =.005), which was associated with elevated plasma glucose levels at 15 months (5.3 +/- 0.3 v 3.8 +/- 0.2 mmol/L, P =.001). Fat feeding increased plasma insulin levels also in NMRI mice at 15 months (705 +/- 145 v 275 +/- 64 pmol/L, P =.01) without, however, a rise of plasma glucose levels. In parallel with increased insulin levels, decreased muscle GS fractional velocity (FV) was observed at 6 (49.0% +/- 2.6% v 69.1% +/- 7.3%, P =.04) and 15 (45.8% +/- 1.8% v 53.4% +/- 1.6 %, P <.01) months but not at 3 months in the fat-fed C57BL/6J mice. Similarly, there was a significant decrease in GS fractional activity at 3 (57.9% +/- 4.3% v 70.4% +/- 2.6 %, P <.03) and 15 (47.3% +/- 2.4% v 56.4% +/- 2.1%, P =.02) but not at 6 months in the fat-fed NMRI mice. The decrease in GS activity was not associated with changes in mRNA levels at any time points. We conclude that (1) fat feeding results in similar elevation of plasma insulin levels and impairs GS activity in C57BL/6J and NMRI mice, and (2) the changes in GS activity do not involve effects on gene expression.

Insulin resistance of glycogen synthase mediated by o-linked N-acetylglucosamine

We have investigated the mechanism by which high concentrations of glucose inhibit insulin stimulation of glycogen synthase. In NIH-3T3-L1 adipocytes cultured in low glucose (LG; 2.5 mm), the half-maximal activation concentration (A(0.5)) of glucose 6-phosphate was 162 +/- 15 microm. Exposure to either high glucose (HG; 20 mm) or glucosamine (GlcN; 10 mm) increased the A(0.5) to 558 +/- 61 or 612 +/- 34 microm. Insulin treatment with LG reduced the A(0.5) to 96 +/- 10 microm, but cells cultured with HG or GlcN were insulin-resistant (A(0.5) = 287 +/- 27 or 561 +/- 77 microm). Insulin resistance was not explained by increased phosphorylation of synthase. In fact, culture with GlcN decreased phosphorylation to 61% of the levels seen in cells cultured in LG. Hexosamine flux and subsequent enzymatic protein O-glycosylation have been postulated to mediate nutrient sensing and insulin resistance. Glycogen synthase is modified by O-linked N-acetylglucosamine, and the level of glycosylation increased in cells treated with HG or GlcN. Treatment of synthase in vitro with protein phosphatase 1 increased basal synthase activity from cells cultured in LG to 54% of total activity but was less effective with synthase from cells cultured in HG or GlcN, increasing basal activity to only 13 or 16%. After enzymatic removal of O-GlcNAc, however, subsequent digestion with phosphatase increased basal activity to over 73% for LG, HG, and GlcN. We conclude that O-GlcNAc modification of glycogen synthase results in the retention of the enzyme in a glucose 6-phosphate-dependent state and contributes to the reduced activation of the enzyme in insulin resistance.

Human obesity and type 2 diabetes are associated with alterations in SREBP1 isoform expression that are reproduced ex vivo by tumor necrosis factor-alpha

Sterol regulatory element binding protein (SREBP)-1 is a transcription factor with important roles in the control of fatty acid metabolism and adipogenesis. Little information is available regarding the expression of this molecule in human health or disease. Exposure of isolated human adipocytes to insulin enhanced SREBP1 gene expression and promoted its proteolytic cleavage to the active form. Furthermore, 3 h of in vivo hyperinsulinemia also significantly increased SREBP1 gene expression in human skeletal muscle. Transcript levels of SREBP1c, the most abundant isoform in adipose tissue, were significantly decreased in the subcutaneous adipose tissue of obese normoglycemic and type 2 diabetic subjects compared with that of nonobese normoglycemic control subjects. In skeletal muscle, SREBP1 expression was significantly reduced in type 2 diabetic subjects but not in obese subjects. Within the diabetic group, the extent of SREBP1 suppression was inversely related to metabolic control and was normalized by 3 h of in vivo hyperinsulinemia. Exposure of isolated human adipocytes to tumor necrosis factor-alpha (TNF-alpha) produced a marked and specific decrease in the mRNA encoding the SREBP1c isoform and completely blocked the insulin-induced cleavage of SREBP1 protein. Thus, both the expression and proteolytic maturation of human SREBP1 are positively modulated by insulin. The specific reduction in the SREBP1c isoform seen in the adipose tissue of obese and type 2 diabetic subjects can be recapitulated ex vivo by TNF-alpha, suggesting a possible mechanism for this association.

The diabetic phenotype is conserved in myotubes established from diabetic subjects: evidence for primary defects in glucose transport and glycogen synthase activity

The most well-described defect in the pathophysiology of type 2 diabetes is reduced insulin-mediated glycogen synthesis in skeletal muscles. It is unclear whether this defect is primary or acquired secondary to dyslipidemia, hyperinsulinemia, or hyperglycemia. We determined the glycogen synthase (GS) activity; the content of glucose-6-phosphate, glucose, and glycogen; and the glucose transport in satellite cell cultures established from diabetic and control subjects. Myotubes were precultured in increasing insulin concentrations for 4 days and subsequently stimulated acutely by insulin. The present study shows that the basal glucose uptake as well as insulin-stimulated GS activity is reduced in satellite cell cultures established from patients with type 2 diabetes. Moreover, increasing insulin concentrations could compensate for the reduced GS activity to a certain extent, whereas chronic supraphysiological insulin concentrations induced insulin resistance in GS and glucose transport activity. Our data suggest that insulin resistance in patients with type 2 diabetes comprises at least two important defects under physiological insulin concentrations: a reduced glucose transport under basal conditions and a reduced GS activity under acute insulin stimulation, implicating a reduced glucose uptake in the fasting state and a diminished insulin-mediated storage of glucose as glycogen after a meal.

Differential effects of metformin and troglitazone on cardiovascular risk factors in patients with type 2 diabetes

OBJECTIVE

Traditional cardiovascular risk factors (CVRF) only partly explain the excessive risk of cardiovascular disease in patients with type 2 diabetes. There is now an increasing appreciation for many novel CVRF that occur largely as a result of insulin resistance and hyperinsulinemia. Therefore, we investigated whether diabetes medications that vary in their mechanism of action and ability to reduce insulin resistance may differ in their effects on both traditional and novel CVRF.

RESEARCH DESIGN AND METHODS

We compared the addition of metformin or troglitazone therapy on CVRF in 22 subjects with type 2 diabetes who remained in poor glycemic control (with HbA1c >8.5%) while taking glyburide 10 mg twice daily. Subjects were initially randomized to either metformin 850 mg once daily or troglitazone 200 mg once daily. Both medications were then titrated upward as needed to achieve fasting plasma glucose <120 mg/dl. Measures of glucose control, insulin resistance, and CVRF (blood pressure, lipids, plasminogen activator inhibitor-1, C-reactive protein, fibrinogen, and small dense LDL) were assessed both before and after therapy.

RESULTS

After 4 months of treatment, both metformin and troglitazone led to similar decreases in fasting plasma glucose and HbA1c. The reduction in insulin resistance determined by hyperinsulinemic-euglycemic clamp was nearly twofold greater with troglitazone than metformin. Metformin did not induce significant changes in blood pressure, LDL cholesterol, LDL size, HDL cholesterol, triglycerides, or plasminogen activator inhibitor-1. However, C-reactive protein did decrease by 33% (6 +/- 1 to 4 +/- 1 mg/l; P < 0.01) [corrected]. Troglitazone therapy was associated with increases in LDL size (26.21 +/- 0.22 to 26.56 +/- 0.25 nm; P=0.04) and HDL cholesterol (33 +/- 3 to 36 +/- 3 mg/dl; P=0.05) and decreases in triglycerides (197 +/- 19 to 155 +/- 23 mg/dl; P=0.07) and C-reactive protein by 60% (8 +/- 3 to 3 +/- 1 mg/l, P < 0.01) [corrected].

CONCLUSIONS

For patients with type 2 diabetes in whom maximal sulfonylurea therapy failed, the addition of the insulin sensitizer troglitazone seemed to have greater benefits on several traditional and novel CVRF than metformin therapy. These differences were not related to glycemic improvement but reflected, in part, the greater reduction in insulin resistance obtained with addition of troglitazone. These data suggest that medications that more effectively address this underlying metabolic defect may be more beneficial in reducing cardiovascular risk in type 2 diabetes.

Effects of the long-acting insulin analog insulin glargine on cultured human skeletal muscle cells: comparisons to insulin and IGF-I

The aim of this study was to determine whether the long-acting insulin analog, insulin glargine, behaves like human insulin for metabolic and mitogenic responses in differentiated cultured human skeletal muscle cells from nondiabetic and diabetic subjects. Human insulin and insulin glargine were equipotent in their ability to compete for [(125)I]insulin binding. Insulin glargine displaced [(125)I]IGF-I from the IGF-I-binding site with approximately 0.5% the potency of IGF-I. In nondiabetic muscle cells, all three ligands stimulated glucose uptake similarly, whereas the sensitivity of glucose uptake was greatest in response to IGF-I and lower and equal for human insulin and insulin glargine. In diabetic muscle cells, the final responsiveness of glucose uptake was greatest for IGF-I and equivalent for human insulin and insulin glargine; sensitivities were the same as those for nondiabetic cells. Thymidine uptake into DNA was stimulated foremost by IGF-I, whereas human insulin and insulin glargine showed equivalent, but greatly reduced, sensitivities and potencies (<1% IGF-I). Stimulation of Akt phosphorylation was slightly more responsive to IGF-I compared with human insulin and insulin glargine, with sensitivities similar to glucose uptake stimulation. We conclude that in human skeletal muscle cells, insulin glargine is equivalent to human insulin for metabolic responses and does not display augmented mitogenic effects.

The basal kinetic parameters of glycogen synthase in human myotube cultures are not affected by chronic high insulin exposure

There is no consensus regarding the results from in vivo and in vitro studies on the impact of chronic high insulin and/or high glucose exposure on acute insulin stimulation of glycogen synthase (GS) kinetic parameters in human skeletal muscle. The aim of this study was to evaluate the kinetic parameters of glycogen synthase activity in human myotube cultures at conditions of chronic high insulin combined or not with high glucose exposure, before and after a subsequent acute insulin stimulation. Acute insulin stimulation significantly increased the fractional activity (FV(0.1)) of GS, increased the sensitivity of GS to the allosteric activator glucose 6-phosphate (A(0.5)) and increased the sensitivity of GS to its substrate UDPG (K(m(0.1))) when myotubes were precultured at low insulin with/without high glucose conditions. However, this effect of acute insulin stimulation was abolished in myotubes precultured at high insulin with or without high glucose. Furthermore, we found significant correlations between the fractional velocities FV(0.1) of GS and K(m(0.1)) (rho=-0.72, P<0.0001), between FV(0.1) and A(0.5) (rho=-0.82, P<0.0001) and between K(m(0.1)) and A(0.5) values (rho=0.71, P<0.0001). Our results show that chronic exposure of human myotubes to high insulin with or without high glucose did not affect the basal kinetic parameters but abolished the reactivity of GS to acute insulin stimulation. We suggest that insulin induced insulin resistance of GS is caused by a failure of acute insulin stimulation to decrease A(0.5) and K(m(0.1)) in human skeletal muscle.

Impaired muscle glycogen synthase in type 2 diabetes is associated with diminished phosphatidylinositol 3-kinase activation

Insulin signaling pathways potentially involved in regulation of skeletal muscle glycogen synthase were compared in differentiated human muscle cell cultures from nondiabetic and type 2 diabetic patients. Insulin stimulation of glycogen synthase activity as well as phosphorylation of MAPK, p70 S6 kinase, and protein kinase B (Akt) were blocked by the phosphatidylinositol 3-kinase inhibitors wortmannin (50 nM) and LY294002 (10 microM). In contrast to lean and obese nondiabetic subjects, where there were minimal effects (15-20% inhibition), insulin stimulation of glycogen synthase in muscle cultures from diabetic subjects was greatly diminished ( approximately 75%) by low concentrations of wortmannin (25 nM) or LY294002 (2 microM). This increased sensitivity of diabetic muscle to impairment of insulin-stimulated glycogen synthase activity occurs together with diminished insulin-stimulation (by 40%) of IRS-1-associated phosphatidylinositol 3-kinase activity in the same cells. Protein expression of IRS-1, p85, p110, Akt, p70 S6 kinase, and MAPK were normal in diabetic cells, as was insulin-stimulated phosphorylation of Akt, p70 S6 kinase, and MAPK. These studies indicate that, despite prolonged growth and differentiation of diabetic muscle under normal metabolic culture conditions, defects of insulin-stimulated phosphatidylinositol 3-kinase and glycogen synthase activity in diabetic muscle persist, consistent with intrinsic (rather than acquired) defects of insulin action.

A defect in glycogen synthesis characterizes insulin resistance in hypertensive patients with type 2 diabetes

A subgroup of patients with type 2 diabetes shows a clustering of abnormalities such as peripheral insulin resistance, hypertension, and microalbuminuria. To evaluate whether these traits reflect intrinsic disorders of cell function rather than in vivo environmental effects, we studied a group of 7 nondiabetic hypertensive subjects with an altered albumin excretion rate (AER) (HyMA+) and 3 groups of patients with type 2 diabetes: 7 with normal blood pressure and normal AER (DH-MA-), 7 with high blood pressure and normal AER (DH+MA-), and 7 with both high blood pressure and altered AER (DH+MA+). Glucose disposal was measured during an hyperinsulinemic clamp (40 mU. m(2)(-1). min(-1)) with primed deuterated [6.6 (2)H(2)] glucose infusion. In the same subjects, a skin biopsy was performed and the following parameters were investigated: glucose transport (as determined by [(3)H]2-deoxyglucose uptake); glycogen synthase activity (as determined by [(14)C] glucose incorporation from UDP-[U-(14)C] glucose into glycogen); glycogen phosphorylase activity (as measured by the incorporation of [U-(14)C]glucose 1-phosphate into glycogen); and total glycogen content. In vivo glucose disposal was significantly reduced in DH+MA- and DH+MA+, with respect to DH-MA-, HyMA+, and controls. Insulin-stimulated glucose transport was similar in the 3 groups of patients with diabetes. A significant reduction of intracellular glycogen content was observed in DH+MA- and DH+MA+ compared with DH-MA- in both basal and insulin-stimulated conditions, probably because of a major impairment of glycogen synthase activity. Glycogen phosphorylase activity did not show differences between the groups. These results suggest that (1) the combination of type 2 diabetes with hypertension and altered AER is associated with impaired insulin sensitivity, and (2) intrinsic, possibly genetic, factors may account for increased peripheral insulin resistance in hypertensive microalbuminuric patients with type 2 diabetes, pointing to the reduction of glycogen synthase activity as a shared common defect.

Effect of non-insulin-dependent diabetes mellitus on myocardial insulin responsiveness in patients with ischemic heart disease

BACKGROUND

Patients with non-insulin-dependent diabetes mellitus (NIDDM) exhibit poor clinical outcomes from myocardial ischemia. This may reflect an impairment in their cardiac insulin-response system.

METHODS AND RESULTS

We used AV balance and intracoronary infusion techniques to compare the intrinsic cardiac responsiveness to insulin in 26 coronary disease patients with (n=13) and without (n=13) NIDDM. During fasting, NIDDM hearts demonstrated lower fractional extraction of glucose from arterial plasma than controls (1.0+/-0.5% versus 2.1+/-0.5%, P<0.05) despite higher circulating insulin levels (26+/-5 versus 13+/-4 microU. mL, P<0.05). This was compensated for by higher circulating glucose levels, so that net cardiac glucose uptake in the 2 groups was equivalent (5.2+/-1.1 versus 5.3+/-1.1 micromol. min). Intracoronary insulin infusion produced an approximately 3-fold increase in fractional extraction and net uptake of glucose across the heart in both groups (to 3.7+/-0.4% and 18.3+/-3.5 micromol. min in NIDDM and to 5.4+/-0.7% and 17.7+/-4.3 micromol. min in controls) accompanied by an approximately 30% increase in net lactate uptake, suggesting preserved insulin action on both glucose uptake and glucose oxidation in the NIDDM heart. In nondiabetics, insulin consistently increased coronary blood flow, but this effect was absent in NIDDM.

CONCLUSIONS

In contrast to their peripheral tissues and coronary vasculature, the myocardium of patients with NIDDM expresses a competent insulin-response system with respect to glucose metabolism. This suggests that insulin resistance is mediated at the level of individual organs and that different mechanisms are involved in muscle and vascular tissue.

Flux control in the rat gastrocnemius glycogen synthesis pathway by in vivo 13C/31P NMR spectroscopy

To determine the relative contributions of glucose transport/hexokinase, glycogen synthase (GSase), and glycolysis to the control of insulin-stimulated muscle glycogen synthesis, we combined 13C and 31P NMR to quantitate the glycogen synthesis rate and glucose 6-phosphate (G-6-P) levels in rat (Sprague-Dawley) gastrocnemius muscle during hyperinsulinemia at euglycemic (E) and hyperglycemic (H) glucose concentrations under thiopental anesthesia. Flux control was calculated using metabolic control analysis. The combined control coefficient of glucose transport/hexokinase (GT/Hk) for glycogen synthesis was 1.1 +/- 0.03 (direct measure) and 1.14-1.16 (calculated for a range of glycolytic fluxes), whereas the control coefficient for GSase was much lower (0.011-0.448). We also observed that the increase in in vivo [G-6-P] from E to H (0.22 +/- 0.03 to 0.40 +/- 0.03 mM) effects a supralinear increase in the in vitro velocity of GSase, from 14.6 to 26.1 mU. kg(-1). min(-1) (1.8-fold). All measurements suggest that the majority of the flux control of muscle glycogen synthesis is at the GT/Hk step.

Insulin resistance and associated metabolic abnormalities in muscle: effects of exercise

Skeletal muscle is a major site of insulin resistance. In addition to glucose transport, oxidative disposal and storage defects, insulin resistant muscle exhibit many other metabolic abnormalities. After a brief review of insulin resistance determinants, we will focus on muscular abnormalities in obesity and type 2 diabetes. Glucose and lipid metabolism defects will be analysed and their interactions discussed. Exercise can improve many of these muscular abnormalities and the mechanisms underlying exercise-induced benefits have been clarified during the past decades. Therefore, exercise training has proved to be useful in the management of insulin resistant states, i.e. mainly obesity, especially in its truncal distribution, and type 2 diabetes. However, exercise prescription remains poorly codified, and results on glycaemic control are sometimes conflicting. In the last part of this review, we will emphasize the pathophysiological basis for an individualized exercise prescription in insulin resistant subjects.

The stimulation-induced increase in skeletal muscle glycogen synthase content is impaired in carriers of the glycogen synthase XbaI gene polymorphism

Associations between glycogen synthase gene (GYS1) polymorphism and states of insulin resistance and type 2 diabetes have been reported. The purpose of this study was to establish if the GYS1 genotype impacts on the content of glycogen synthase (GS) protein in muscle measured under basal and stimulated conditions. To examine this, GYS1 XbaI and Met416Val polymorphisms and thigh muscle GYS1 protein content were determined at rest, both before and after several weeks of neuromuscular electrical stimulation in carriers and noncarriers of the mutations. The allelic frequency was 0.086 for the XbaI mutation (A2) and 0.006 for the Met416Val in our cohort of French-Canadian subjects. When measured at rest, the GS protein content in muscle was similar among carriers and noncarriers of the XbaI variant. However, the stimulation-induced increase (23%) in the amount of GS muscle protein normally seen in wildtype individuals was impaired in those carrying the XbaI mutation. These data demonstrate that some individuals, because of their genetic background, are unable to stimulate the process of GS protein accumulation in skeletal muscle. These results could explain why some individuals appear to be genetically predisposed to developing skeletal muscle insulin resistance when exposed to unfavorable metabolic environments.

Identification of two essential glutamic acid residues in glycogen synthase

The detailed catalytic mechanism by which glycosyltransferases catalyze the transfer of a glycosyl residue from a donor sugar to an acceptor is not known. Through the multiple alignment of all known eukaryotic glycogen synthases we have found an invariant 17-amino acid stretch enclosed within the most conserved region of the members of this family. This peptide includes an E-X(7)-E motif, which is highly conserved in four families of retaining glycosyltransferases. Site-directed mutagenesis was performed in human muscle glycogen synthase to analyze the roles of the two conserved Glu residues (Glu-510 and Glu-518) of the motif. Proteins were transiently expressed in COS-1 cells as fusions to green fluorescence protein. The E510A and E518A mutant proteins retained the ability to translocate from the nucleus to the cytosol in response to glucose and to bind to intracellular glycogen. Although the E518A variant had approximately 6% of the catalytic activity shown by the green fluorescence protein-human muscle glycogen synthase fusion protein, the E510A mutation inactivated the enzyme. These results led us to conclude that the E-X(7)-E motif is part of the active site of eukaryotic glycogen synthases and that both conserved Glu residues are involved in catalysis. We propose that Glu-510 may function as the nucleophile and Glu-518 as the general acid/base catalyst.

Impact of the Xba1-polymorphism of the human muscle glycogen synthase gene on parameters of the insulin resistance syndrome in a Danish twin population

AIMS

To establish the impact on the insulin resistance syndrome of the intron 14 Xba1-polymorphism in human muscle glycogen synthase (GYS1).

METHODS

Parameters related to the insulin resistance syndrome were measured in 244 monozygotic twins and 322 dizygotic twins with or without impaired glucose tolerance. In addition a standard oral glucose tolerance test (OGTT) was performed. The twins were genotyped for Xba1-polymorphism in GYS1 intron 14.

RESULTS

The allele frequency of Xba1 non-cutters (A1) was 0.95 and of cutters (A2) was 0.05. Of the 566 twins examined, 90.0% had the genotype A1A1 and the remainder had the genotype A1A2. No A2A2-genotypes were detected. In 11 genotypic discordant dizygotic twin pairs the insulin resistance was significantly increased in the twins carrying the A1A2 genotype regardless of sex (HOMA index 1.81 (A1A1) vs. 2.57 (A1A2), P < 0.05). Diastolic blood pressure was increased in female carriers of the A2-allele with impaired glucose tolerance or Type 2 diabetes mellitus (79 +/- 1 vs. 94 +/- 4 mmHg, P < 0.01). Apart from a marginal increased waist-to-hip ratio, no other elements of the insulin resistance syndrome were associated with the polymorphism.

CONCLUSIONS

The Xba1-polymorphism of the human muscle glycogen synthase gene is correlated to insulin resistance and to diastolic blood pressure. The polymorphism does not involve any known transcription factor or any structural change in GYS1, and these correlations are therefore most probably caused by linkage to other functional polymorphisms in GYS1 or other gene polymorphisms on chromosome 19.

Regulation of MAP kinase pathway activity in vivo in human skeletal muscle

Insulin and exercise potently stimulate glucose metabolism and gene transcription in vivo in skeletal muscle. A single bout of exercise increases the rate of insulin-stimulated glucose uptake and metabolism in skeletal muscle in the postexercise period. The nature of the intracellular signaling mechanisms that control responses to exercise is not known. In mammalian tissues, numerous reports have established the existence of the mitogen-activated protein (MAP) kinase signaling pathway that is activated by a variety of growth factors and hormones. This study was undertaken to determine how a single bout of exercise and physiological hyperinsulinemia activate the MAP kinase pathway. The euglycemic-hyperinsulinemic clamp and cycle ergometer exercise techniques combined with percutaneous muscle biopsies were used to answer this question. In healthy subjects, within 30 min, insulin significantly increased MAP kinase [isoforms p42(MAPK) and p44(MAPK) (ERK1 and ERK2)] phosphorylation (141 +/- 2%, P < 0.05) and activity (177 +/- 5%, P < 0.05), and the activity of its upstream activator MEK1 (161 +/- 16%, P < 0.05). Insulin also increased the activity of the MAP kinase downstream substrate, the p90 ribosomal S6 kinase 2 (RSK2) almost twofold (198 +/- 45%, P < 0.05). In contrast, a single 30-min bout of moderate-intensity exercise had no effect on the MAP kinase pathway activation from MEK to RSK2 in muscle of healthy subjects. However, 60 min of exercise did increase extracellular signal-related kinase activity. Therefore, despite similar effects on glucose metabolism after 30 min, insulin and exercise regulate the MAP kinase pathway differently. Insulin more rapidly activates the MAP kinase pathway.

Impaired insulin-stimulated expression of the glycogen synthase gene in skeletal muscle of type 2 diabetic patients is acquired rather than inherited

To examine whether defective muscle glycogen synthase (GYS1) expression is associated with impaired glycogen synthesis in type 2 diabetes and whether the defect is inherited or acquired, we measured GYS1 gene expression and enzyme activity in muscle biopsies taken before and after an insulin clamp in 12 monozygotic twin pairs discordant for type 2 diabetes and in 12 matched control subjects. The effect of insulin on GYS1 fractional activity, when expressed as the increment over the basal values, was significantly impaired in diabetic (15.7 +/- 3.3%; P < 0.01), but not in nondiabetic (23.7 +/- 1.8%; P = NS) twins compared with that in control subjects (28.1 +/- 2.3%). Insulin increased GYS1 messenger ribonucleic acid (mRNA) expression in control subjects (from 0.14 +/- 0.02 to 1.74 +/- 0.10 relative units; P < 0.01) and in nondiabetic (from 0.24 +/- 0.05 to 1.81 +/- 0.16 relative units; P < 0.01) and diabetic (from 0.20 +/- 0.07 to 1.08 + 0.14 relative units; P < 0.01) twins. The effect of insulin on GYS1 expression was, however, significantly reduced in the diabetic (P < 0.003), but not in the nondiabetic, twins compared with that in control subjects. The postclamp GYS1 mRNA levels correlated strongly with the hemoglobin A1c levels (r = -0.61; P < 0.001). Despite the decrease in postclamp GYS1 mRNA levels, the GYS1 protein levels were not decreased in the diabetic twins compared with those in the control subjects (2.10 +/- 0.46 vs. 2.10 +/- 0.34 relative units; P = NS). We conclude that 1) insulin stimulates GYS1 mRNA expression; and 2) impaired stimulation of GYS1 gene expression by insulin in patients with type 2 diabetes is acquired and most likely is secondary to chronic hyperglycemia.