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

Normal insulin-dependent activation of Akt/protein kinase B, with diminished activation of phosphoinositide 3-kinase, in muscle in type 2 diabetes

To determine whether the serine/threonine kinase Akt (also known as protein kinase B) is activated in vivo by insulin administration in humans, and whether impaired activation of Akt could play a role in insulin resistance, we measured the activity and phosphorylation of Akt isoforms in skeletal muscle from 3 groups of subjects: lean, obese nondiabetic, and obese type 2 diabetic. Vastus lateralis biopsies were taken in the basal (overnight fast) and insulin-stimulated (euglycemic clamp) states. Insulin-stimulated glucose disposal was reduced 31% in obese subjects and 63% in diabetic subjects, compared with lean subjects. Glycogen synthase (GS) activity in the basal state was reduced 28% in obese subjects and 49% in diabetic subjects, compared with lean subjects. Insulin-stimulated GS activity was reduced 30% in diabetic subjects. Insulin treatment activated the insulin receptor substrate-1-associated (IRS-1-associated) phosphoinositide 3-kinase (PI 3-kinase) 6.1-fold in lean, 3.7-fold in obese, and 2.4-fold in diabetic subjects. Insulin also stimulated IRS-2-associated PI 3-kinase activity 2.2-fold in lean subjects, but only 1.4-fold in diabetic subjects. Basal activity of Akt1/Akt2 (Akt1/2) and Akt3 was similar in all groups. Insulin increased Akt1/2 activity 1.7- to 2. 0-fold, and tended to activate Akt3, in all groups. Insulin-stimulated phosphorylation of Akt1/2 was normal in obese and diabetic subjects. In lean subjects only, insulin-stimulated Akt1/2 activity correlated with glucose disposal rate. Thus, insulin activation of Akt isoforms is normal in muscle of obese nondiabetic and obese diabetic subjects, despite decreases of approximately 50% and 39% in IRS-1- and IRS-2-associated PI 3-kinase activity, respectively, in obese diabetic subjects. It is therefore unlikely that Akt plays a major role in the resistance to insulin action on glucose disposal or GS activation that is observed in muscle of obese type 2 diabetic subjects.

Effects of troglitazone on substrate storage and utilization in insulin-resistant rats

Elevated serum and tissue lipid stores are associated with skeletal muscle insulin resistance and diminished glucose-stimulated insulin secretion, the hallmarks of type 2 diabetes. We studied the effects of 6-wk treatment with the insulin sensitizer troglitazone on substrate storage and utilization in lean control and Zucker diabetic fatty (ZDF) rats. Troglitazone prevented development of diabetes and lowered serum triglycerides (TG) in ZDF rats. Soleus muscle glycogen and TG content were elevated twofold in untreated ZDF rats, and both were normalized by troglitazone to lean control levels (P < 0.05). Troglitazone also normalized insulin-stimulated glucose uptake as well as basal and insulin-stimulated glycogen synthesis, implying increased skeletal muscle glycogen turnover. The proportion of active pyruvate dehydrogenase (PDH) in soleus muscle was reduced in ZDF relative to lean control rat muscle (16 +/- 2 vs. 21 +/- 2%) but was restored by troglitazone treatment (30 +/- 3%). Increased PDH activation was associated with a 70% increase in glucose oxidation. Muscle lipoprotein lipase activity was decreased by 35% in ZDF compared with lean control rats and was increased twofold by troglitazone. Palmitate oxidation and incorporation into TG were higher in ZDF relative to lean control rats but were unaffected by troglitazone treatment. Troglitazone decreased the incorporation of glucose into the acyl group of TG by 60% in ZDF rats. In summary, ZDF rats demonstrate increased skeletal muscle glycogen and TG stores, both of which were reduced by troglitazone treatment. Troglitazone appears to increase both glycogen and TG turnover in skeletal muscle. Normalization of PDH activity and decreased glucose incorporation into acyl TG may underlie the improvements in intracellular substrate utilization and energy stores, which lead to decreased serum TG and glucose.

Insulin resistance, impaired glucose tolerance and non-insulin-dependent diabetes, pathologic mechanisms and treatment: current status and therapeutic possibilities

Impaired glucose tolerance and non-insulin-dependent diabetes (NIDDM) are the pathologic consequence of two co-incident and interacting conditions, namely insulin resistance and relative insulin deficiency. Recognised by the World Health Authority as a global health problem there are at 1995 estimates at least 110 million diagnosed diabetics world wide with at least the same number undiagnosed. Diabetes is the 4th leading cause of death in developed countries and its management exerts a vast economic and social burden. Insulin resistance is established as the characteristic pathologic feature of patients with glucose intolerance and NIDDM describing a state in which insulin stimulated glucose uptake and utilisation in liver, skeletal muscle and adipose tissue is impaired and coupled to impaired suppression of hepatic glucose output. Although the biochemical mechanisms underpinning both defects are becoming better understood, the genetic and molecular causes remain elusive; and whether insulin resistance or relative insulin deficiency represents the primary defect in patients with NIDDM is the matter of some debate. In this article we review the biochemical and molecular nature of the defects in insulin sensitivity and glucose uptake, and discuss some of the potential causative mechanisms. The genetic and environmental basis of insulin resistance is reviewed and presented, and potential therapeutic targets including thiazolidinediones are discussed.

Pathogenesis of non-insulin-dependent (type II) diabetes mellitus (NIDDM) - genetic predisposition and metabolic abnormalities

Non-insulin-dependent diabetes mellitus (NIDDM), also known as type II diabetes, is characterized by abnormal glucose homeostasis, resulting in hyperglycemia, and is associated with microvascular, macrovascular, and neuropathic complications. NIDDM is a complex disease with many causes. Both genetic and environmental factors play important roles in the pathogenesis of NIDDM. Cumulative evidence on the high prevalence of NIDDM in certain ethnic groups, the high concordance rate for the disease in monozygotic twins, familial aggregation, and familial transmission patterns suggests that the genetic component plays an important etiological role in the development of NIDDM. In genetically predisposed individuals, there is a slow progression from a normal state to hyperglycemia, largely due to a combination of insulin resistance and defects in insulin secretion. Although numerous candidate genes responsible for insulin resistance and for the defects in insulin secretion have been reported, no specific gene(s) accounting for the majority of cases of the common type of NIDDM has been identified. Considerable evidence indicates that environmental and other factors, including diet, stress, physical activity, obesity and aging, also play an important role in the development of the disease. In conclusion, the pathogenic process of NIDDM depends on a complex interaction between genetic and environmental factors.

The effect of low intensity bicycle exercise on the insulin-induced glucose uptake in obese patients with type 2 diabetes

OBJECTIVE

The present study was undertaken to reveal the effect of low intensity bicycle exercise on the insulin-induced glucose uptake in obese patients.

SUBJECTS AND METHODS

Seven obese men with Type 2 diabetes (OBDM) and seven healthy young men (HY) participated in this study. The glucose infusion rate (GIR) was determined by glucose clamp procedure at an insulin infusion rate of 40 mU m-2 min-1 (plasma insulin concentrations: 700-800 pmol l-1). Confirming stabilized GIR, a 30-min bicycle exercise was performed during the glucose clamp which was continued for 120 min after exercise.

RESULTS

Average GIR in OBDM for last 30 min prior to exercise were significantly lower than HY (28.3 +/- 1.7, 47.4 +/- 1.8 mumol kg-1 min-1 respectively, P < 0.05). GIR abruptly increased during exercise and gradually decreased after exercise to the nadir almost at the time from 30 to 60 min in recovery period in both groups. GIR in OBDM, however, gradually increased significantly over pre-exercise levels (P < 0.05), following exercise and reached the same levels compared to HY after 80 min of recovery period.

CONCLUSION

These results indicated that in obese Type 2 diabetes, 30 min of low intensity bicycle exercise significantly enhances the lower level of insulin-induced glucose uptake shortly after exercise and might be useful for the treatment of post-prandial hyperglycemia.

Insulin resistance in cirrhosis: prolonged reduction of hyperinsulinemia normalizes insulin sensitivity

Insulin resistance is present in nearly all patients with cirrhosis, but its etiology remains unknown. Chronic hyperinsulinemia has been suspected as a potential candidate, and we therefore tested the hypothesis that, in cirrhosis, prolonged reduction of the hyperinsulinemia restores insulin sensitivity. Whole-body insulin sensitivity (euglycemic insulin-clamp technique), glucose turnover (6,6-2H2-glucose isotope dilution), glucose oxidation (indirect calorimetry), non-oxidative glucose disposal, and fractional glycogen synthase activity in muscle (biopsies) were measured in eight clinically stable patients with cirrhosis before and at the end of a 4-day continuous subcutaneous infusion of the somatostatin-analogue octreotide (200 microg/24 h) designed to continuously reduce plasma insulin levels. Baseline data were compared with results obtained in healthy individuals matched for sex, age, and weight (n = 8). During the baseline (pre-octreotide) study, patients demonstrated a significant decrease in insulin-mediated glucose uptake compared with controls (5.75 +/- 0.21 vs. 7.98 +/- 0.84 mg/kg/min; P < .03), which was entirely accounted for by an impairment in non-oxidative glucose disposal (P < .04). Four-day infusion of octreotide to cirrhotic patients: 1) reduced postabsorptive and meal-stimulated plasma insulin levels by approximately 35% to 45% without significantly affecting glucose tolerance; 2) did not significantly alter plasma free fatty acids (FFA), growth hormone, and glucagon levels in the postabsorptive state and during the meal test; 3) normalized insulin-mediated whole-body glucose disposal (7.63 +/- 0.72 mg/kg/min post-octreotide; P = not significant vs. control). Restoration of insulin-mediated glucose utilization was entirely caused by normalization of non-oxidative glucose disposal; 4) was associated with a considerably more pronounced stimulation by insulin of the fractional glycogen synthase in muscle compared with pre-octreotide results (increment above baseline pre: 0.035 +/- 0.010 vs. post: 0.060 +/- 0.023 nmol/min/mg protein; P < .04). Fractional glycogen activity significantly correlated with non-oxidative glucose disposal during insulin infusion (r = .69; P < .03). Prolonged reduction of hyperinsulinemia for 96 hours in cirrhotic patients normalizes insulin-mediated glucose uptake and glycogen synthesis in muscle. We conclude that chronic hyperinsulinemia causes insulin resistance in cirrhosis.

Troglitazone regulation of glucose metabolism in human skeletal muscle cultures from obese type II diabetic subjects

To determine the effects of troglitazone on abnormal skeletal muscle glucose metabolism, muscle cultures from type II diabetic patients were grown for 4-6 weeks and then fused for 4 days either without or with troglitazone (1-5 micrograms/mL; chronic studies) or had troglitazone added for 90 min (1-5 micrograms/mL) at completion of fusion (acute studies). Acute troglitazone treatment stimulated glucose uptake, but not glycogen synthase (GS) activity 2-fold (P < 0.05) in a dose-dependent fashion and to the same extent as the addition of maximal (33 nmol/L) insulin. Maximal chronic troglitazone (5 micrograms/mL for 4 days) increased both glucose uptake (from 9.0 +/- 1.5 to 40.9 +/- 8.1 pmol/mg protein.min; P < 0.05) and GS fractional velocity (from 5.4 +/- 0.7% to 20.6 +/- 6.3%; P < 0.05) by approximately 4-fold. At each concentration of chronic troglitazone, glucose uptake rates were similar in the absence and presence of maximal (33 nmol/L) insulin concentrations. In contrast, insulin-stimulated GS activity was greater (P < 0.05) when maximal chronic troglitazone and acute insulin were combined than when chronic troglitazone alone was used. After 4 days of troglitazone, GLUT1 messenger ribonucleic acid and protein increased about 2-fold (P < 0.05) without a change in GLUT4 or GS messenger ribonucleic acid and protein. We conclude that troglitazone has both acute and chronic effects to improve skeletal muscle glucose metabolism of obese type II diabetic subjects. These effects involve direct insulin mimetic stimulatory actions as well as indirect insulin-sensitizing properties.

New variants in the glycogen synthase gene (Gln71His, Met416Val) in patients with NIDDM from eastern Finland

Impaired glycogen synthesis after insulin stimulation accounts for most of the insulin resistance in patients with non-insulin-dependent diabetes mellitus (NIDDM). The glycogen synthase gene (GYS1), which encodes the rate-limiting enzyme for glycogen synthesis, is a promising candidate gene for NIDDM. Therefore, we screened all 16 exons of this gene by single-strand conformation polymorphism analysis in 40 patients with NIDDM (age 67 +/- 2 years, body mass index 28.2 +/- 0.6 kg/m2) from Taipalsaari, eastern Finland. The Gly464Ser variant (exon 11) and a silent polymorphism TTC342TTT (exon 7) have been reported previously. In addition, we found a new variant Gln71His (exon 2) and a new amino acid polymorphism Met416Val (exon 10). An additional sample of 65 patients with NIDDM and 82 normoglycaemic men (age 54 +/- 1 years, body mass index 26.3 +/- 1.4 kg/m2) were screened. The allele frequency of the TTC342TTT silent substitution was 0.29 in both NIDDM and normoglycaemic subjects. The Gln71His and Gly464Ser variants were found in 1 (1%) and 3 (3%) subjects, respectively, of the 105 NIDDM patients and in none of the 82 normoglycaemic men. The Met416Val polymorphism was found in 16 (15%) of the 105 NIDDM patients and in 14 (17%) of the 82 control subjects (all heterozygous). The Met416Val polymorphism was not associated with insulin resistance in two groups of normoglycaemic subjects. In conclusion, the new Gln71His and Met416Val substitutions and other variants of the glycogen synthase gene are unlikely to make a major contribution to insulin resistance and NIDDM in diabetic patients from eastern Finland.

Peripheral but not hepatic insulin resistance in mice with one disrupted allele of the glucose transporter type 4 (GLUT4) gene

Glucose transporter type 4 (GLUT4) is insulin responsive and is expressed in striated muscle and adipose tissue. To investigate the impact of a partial deficiency in the level of GLUT4 on in vivo insulin action, we examined glucose disposal and hepatic glucose production (HGP) during hyperinsulinemic clamp studies in 4-5-mo-old conscious mice with one disrupted GLUT4 allele [GLUT4 (+/-)], compared with wild-type control mice [WT (+/+)]. GLUT4 (+/-) mice were studied before the onset of hyperglycemia and had normal plasma glucose levels and a 50% increase in the fasting (6 h) plasma insulin concentrations. GLUT4 protein in muscle was approximately 45% less in GLUT4 (+/-) than in WT (+/+). Euglycemic hyperinsulinemic clamp studies were performed in combination with [3-3H]glucose to measure the rate of appearance of glucose and HGP, with [U-14C]-2-deoxyglucose to estimate muscle glucose transport in vivo, and with [U-14C]lactate to assess hepatic glucose fluxes. During the clamp studies, the rates of glucose infusion, glucose disappearance, glycolysis, glycogen synthesis, and muscle glucose uptake were approximately 55% decreased in GLUT4 (+/-), compared with WT (+/+) mice. The decreased rate of in vivo glycogen synthesis was due to decreased stimulation of glucose transport since insulin's activation of muscle glycogen synthase was similar in GLUT4 (+/-) and in WT (+/+) mice. By contrast, the ability of hyperinsulinemia to inhibit HGP was unaffected in GLUT4 (+/-). The normal regulation of hepatic glucose metabolism in GLUT4 (+/-) mice was further supported by the similar intrahepatic distribution of liver glucose fluxes through glucose cycling, gluconeogenesis, and glycogenolysis. We conclude that the disruption of one allele of the GLUT4 gene leads to severe peripheral but not hepatic insulin resistance. Thus, varying levels of GLUT4 protein in striated muscle and adipose tissue can markedly alter whole body glucose disposal. These differences most likely account for the interindividual variations in peripheral insulin action.

More marked stimulation by lithium than insulin of the glycogenic pathway in rat skeletal muscle

Lithium's impact on glucose metabolism was compared with that of insulin in isolated rat soleus muscle. Lithium chloride (20 mmol/l) induced a 4.8-fold more pronounced increment over basal glycogen synthase activity than insulin (10 nmol/l) (nmol UDP-glucose into glycogen in synthase activity assay.g-1.min-1: lithium, +22.1 +/- 1.8 vs. insulin, +4.6 +/- 3.9; P < 0.01). In parallel, lithium was less efficient than insulin in stimulating glucose transport (counts per minute 2-deoxy-D-[3H]glucose.mg-1.h-1: lithium, +211 +/- 19 vs. insulin, +311 +/- 57; P < 0.05) and lactate release (mumol.g-1.h-1: lithium, +1.0 +/- 0.5 vs. insulin, +3.9 +/- 0.5; P < 0.01), and similar increments were induced in glycogen synthesis (mumol glucose into glycogen.g-1.h-1: lithium, +3.32 +/- 0.43 vs. insulin, +3.46 +/- 0.47; not significant). Full additivity of glycogenic effects and divergent dependency on phosphatidylinositol 3-kinase activation provided further evidence for different mechanisms of action. In muscle from insulin-resistant obese Zucker rats (fa/fa), failure of lithium to reverse deficits in glucose metabolism suggested a primary deficit in muscle glucose uptake rather than glycogen synthesis. Hence lithium distinctly stimulates glycogen synthase activity in skeletal muscle and may therefore be regarded as a candidate for the treatment of disorders associated with primary deficits in the glycogenic pathway.

Polymorphism of the glycogen synthase gene and non-insulin-dependent diabetes mellitus in the Russian population

Recently, a polymorphism in the glycogen synthase gene was shown to be associated with the development of non-insulin-dependent diabetes mellitus (NIDDM) and identified patients with a strong family history of diabetes and hypertension in the Finnish population. However, no association was found in French and Japanese populations. We investigated the possible association between the XbaI polymorphism of the glycogen synthase gene and NIDDM in the Russian population. One hundred fifty NIDDM patients and 109 healthy controls were studied. In 16 of 150 Russian NIDDM patients (10.7%), the XbaI polymorphism was found, and 17 of 109 controls (15.6%) showed the XbaI polymorphism (P > .05). These results suggest that the XbaI polymorphism of the glycogen synthase gene cannot be used as a marker for NIDDM in the Russian population.

Glycogen synthase activity is reduced in cultured skeletal muscle cells of non-insulin-dependent diabetes mellitus subjects. Biochemical and molecular mechanisms

To determine whether glycogen synthase (GS) activity remains impaired in skeletal muscle of non-insulin-dependent diabetes mellitus (NIDDM) patients or can be normalized after prolonged culture, needle biopsies of vastus lateralis were obtained from 8 healthy nondiabetic control (ND) and 11 NIDDM subjects. After 4-6 wk growth and 4 d fusion in media containing normal physiologic concentrations of insulin (22 pM) and glucose (5.5 mM), both basal (5.21 +/- 0.79 vs 9.01 +/- 1.25%, P < 0.05) and acute insulin-stimulated (9.35 +/- 1.81 vs 16.31 +/- 2.39, P < 0.05) GS fractional velocity were reduced in NIDDM compared to ND cells. Determination of GS kinetic constants from muscle cells of NIDDM revealed an increased basal and insulin-stimulated Km(0.1) for UDP-glucose, a decreased insulin-stimulated Vmax(0.1) and an increased insulin-stimulated activation constant (A(0.5)) for glucose-6-phosphate. GS protein expression, determined by Western blotting, was decreased in NIDDM compared to ND cells (1.57 +/- 0.29 vs 3.30 +/- 0.41 arbitrary U/mg protein, P < 0.05). GS mRNA abundance also tended to be lower, but not significantly so (0.168 +/- 0.017 vs 0.243 +/- 0.035 arbitrary U, P = 0.08), in myotubes of NIDDM subjects. These results indicate that skeletal muscle cells of NIDDM subjects grown and fused in normal culture conditions retain defects of basal and insulin-stimulated GS activity that involve altered kinetic behavior and possibly reduced GS protein expression. We conclude that impaired regulation of skeletal muscle GS in NIDDM patients is not completely reversible in normal culture conditions and involves mechanisms that may be genetic in origin.

Glycogen synthase polymorphism, insulin resistance and hypertension

The A(2) allele of the human glycogen synthase gene may be associated with hypertension in diabetic and non-diabetic Finnish subjects. The prevalence of the A(2) allele was investigated in 64 non-diabetic hypertensive subjects with borderline hypertension or established hypertension. Ambulatory blood pressure was performed on all subjects. Insulin sensitivity index (S(I)) was determined in subjects with borderline hypertension. The DNA fragment containing the XBaI restriction site was amplified by the polymerase chain reaction, digested by the XBaI enzyme and compared by gel electrophoresis with a positive control from Finland. Mean age +/- SD for age, S(I) and ambulatory blood pressure were respectively: 39 +/- 10 yrs, 60 +/- 30 min(-1)(nmol/mL) and 132 +/- 7/ 83 +/- 6 mmHg. Sixteen of the subjects were insulin resistant as determined by S(I) <70.0 and they had significantly higher BP and BMI than insulin sensitive subjects. The A(2) allele of the glycogen synthase was not detected in any subject. This suggests that the relation between the XBaI polymorphism of the glycogen synthase gene, insulin resistance and elevated blood pressure may be restricted to a limited and genetically uniform Finnish population.

Expression of the gene encoding glycogen phosphorylase is elevated in diabetic rat skeletal muscle and is regulated by insulin and cyclic AMP

Glycogen phosphorylase regulates the breakdown of glycogen into glucose, but as previous studies have demonstrated, the control of glycogen metabolism becomes deregulated in diabetes mellitus. Messenger RNA levels encoding several different proteins are altered in skeletal muscle biopsies of patients with insulin-dependent and non-insulin-dependent diabetes. The possible alteration of expression of the gene encoding the skeletal muscle isoform of glycogen phosphorylase during diabetes has not previously been investigated. We examined the effect of streptozotocin-induced diabetes and insulin treatment on glycogen phosphorylase mRNA in rat skeletal muscle; glycogen phosphorylase mRNA levels were elevated in diabetic rat muscle tissue, but were partially suppressed in diabetic rat muscle following insulin treatment. To distinguish between the effects of insulin and counter-regulatory hormones on glycogen phosphorylase mRNA levels, we employed differentiating rat L6 myoblasts in culture. Insulin stimulated the accumulation of glycogen phosphorylase mRNA as determined by Northern blot analysis. Moreover, insulin and dibutyryl cAMP stimulated expression of a transiently transfected chloramphenicol acetyl transferase reporter gene under the control of the muscle glycogen phosphorylase promoter in differentiating myotubes in culture, suggesting that the effects of insulin and counter-regulatory hormones on glycogen phosphorylase mRNA are at the level of transcription. These results suggest that insulin and epinephrine may participate in the induction of the glycogen phosphorylase gene during myogenesis; moreover, activation of this gene in muscle tissue may be a contributing factor in impaired glycogen storage during uncontrolled diabetes.

Insulin stimulation of mitogen-activated protein kinase, p90rsk, and p70 S6 kinase in skeletal muscle of normal and insulin-resistant mice. Implications for the regulation of glycogen synthase

Skeletal muscles from mice stimulated with insulin in vivo were used to evaluate relationships between the insulin receptor tyrosine kinase, mitogen-activated protein (MAP) kinase, p90rsk, p70 S6 kinase (p70S6k), and glycogen synthase. Two models of insulin resistance were also evaluated: (a) transgenic mice with a severe insulin receptor defect and (b) gold thioglucose (GTG) mice (obesity with minimal insulin receptor dysfunction). In normal mice, insulin stimulated MAP kinase (6-fold), p90rsk (RSK2, 5-fold), p70S6k (10-fold), and glycogen synthase (30-50% increase in fractional velocity). In transgenic mice, stimulation of MAP kinase and RSK2 were not detectable, whereas activation of p70S6k and glycogen synthase were preserved. In GTG mice, activation of MAP kinase, RSK2, p70S6k, and glycogen synthase were impaired. Since p70S6k and glycogen synthase were correlated, rapamycin was used to block p70S6k, and glycogen synthase activation was unaffected in normal mice; however, it was partially impaired in transgenic mice.

CONCLUSIONS

(a) stimulation of p70S6k and glycogen synthase are selectively preserved in muscles with a severe insulin receptor kinase defect, indicating signal amplification in pathways leading to these effects; (b) MAP kinase-RSK2 and p70S6k activation are impaired in obese mice, suggesting multiple loci for postreceptor insulin resistance; (c) glycogen synthase was dissociated from MAP kinase and RSK2, indicating that they are not required for this effect of insulin; and (d) p70S6k is not essential for glycogen synthase activation, but it may participate in redundant signaling pathways leading to this effect of insulin.

The effect of acute (60 minute) insulin stimulation upon human skeletal muscle glycogen synthase and protein phosphatase-1 in non-insulin-dependent diabetic patients and control subjects

Previous studies have established that activation of muscle glycogen synthase (GS) is abnormal in non-insulin-dependent diabetes mellitus (NIDDM). Insulin-mediated activation of GS depends upon protein phosphatase-1 (PP1), which dephosphorylates the relevant sites of GS. In order to determine whether defects in PP1 activation cause subnormal activation of GS or whether PP1 activation itself is normal, we administered a short insulin infusion to 8 NIDDM subjects and 8 healthy controls matched for gender, age, and body mass index (BMI). GS fractional activity and PP1 activity were determined in biopsies taken from the gastrocnemius muscle before and after 60 min insulin infusion (0.1 U kg h-1). In the NIDDM group, muscle GS fractional activity was 6.8 +/- 1.6 and 10.0 +/- 1.5% (mean +/- SEM) (p = 0.11) before and after insulin infusion. In the control group, muscle GS fractional activity increased from 7.3 +/- 2.0 to 13.3 +/- 2.7% (p < 0.02). PP1 activity had returned towards basal levels after insulin infusion; NIDDM group 156 +/- 24.7 to 184.1 +/- 28.1 U mg-1; control group 220.8 +/- 30.1 to 233.8 +/- 29.8 U mg-1. In the NIDDM group there was a positive correlation between the increases in GS fractional activity and PP1 activity following insulin stimulation r = 0.77; p < 0.025). These data indicate that in vivo insulin-dependent activation of muscle PP1 is transient in normal subjects but is delayed in NIDDM. The defect in GS activation in NIDDM is likely to be proximal to PP1 in the pathway of transmission of the insulin signal.

Effect of troglitazone (CS-045) and bezafibrate on glucose tolerance, liver glycogen synthase activity, and beta-oxidation in fructose-fed rats

To clarify the relationship between lipid and glucose metabolism abnormalities in fructose-fed rats, we examined whether an improvement of insulin sensitivity by troglitazone (CS-045) or a decrease in plasma lipids by bezafibrate affects the relationship between serum levels of lipid and glucose. In addition, we also examined changes in liver glycogen metabolism and beta-oxidation in fructose-fed rats. Troglitazone ameliorated fasting hyperlipidemia, hyperglycemia, and hyperinsulinemia. In addition, it augmented glycogen synthase activity by 53%, and decreased the mitochondrial palmitic acid beta-oxidation rate and ketone body production rate by 27% and 55%, respectively. However, hyperglycemia and liver glycogen synthase activity were not improved by bezafibrate treatment despite a marked reduction of serum triglyceride (TG) levels resulting from a 1.76-fold increase in mitochondrial oxidation and a 2.04-fold increase in hepatic ketone body production. These results suggest that abnormalities in glucose and lipid metabolism in fructose-fed rats, which are ameliorated by troglitazone, may be closely linked to reduced glycogen synthase activity in the liver.

Defects in liver and muscle glycogen metabolism in neonatal and adult New Zealand obese mice

Impaired glycogen synthesis is present in subjects at risk for developing non-insulin-dependent diabetes mellitus (NIDDM), suggesting that it is a primary defect in NIDDM. To examine whether defects in glycogen metabolism are present at birth in an animal model of NIDDM, glycogen synthase (GS), glycogen phosphorylase (GP), and total glycogen content were measured in liver and quadriceps muscle of 1-day- and 20-week-old insulin-resistant New Zealand Obese (NZO) mice and control (NZC) mice. In livers of both neonatal and adult NZO mice, active GS was reduced by 54% and 36%, respectively, as compared with that in NZC mice (P < .03). Total liver GS activity was the same in neonates, but was 65% higher in adult NZO as compared with NZC mice (P < .02). Liver glycogen was 28% lower at birth in NZO mice (P < .03), but was 49% higher at 20 weeks of age. Active and total GP were the same in NZO and NZC animals, despite hyperinsulinemia in 20-week-old NZO mice. In muscle, active GS was reduced by 41% in both 1-day- and 20-week-old NZO mice (P < .02). Total GS was also lower in NZC mice at 1 day of age (P < .01), but not at 20 weeks. No differences were detected in GP activity or in total glycogen content in muscle. Therefore, reduced GS activity is an early defect present at birth in the insulin-resistant NZO mouse in both liver and muscle. However, it is not the sole determinant of the amount of glycogen deposited in tissues.(ABSTRACT TRUNCATED AT 250 WORDS)

Unchanged gene expression of glycogen synthase in muscle from patients with NIDDM following sulphonylurea-induced improvement of glycaemic control

We have previously shown that the mRNA expression of muscle glycogen synthase is decreased in non-insulin-dependent diabetic (NIDDM) patients; the objective of the present protocol was to examine whether the gene expression of muscle glycogen synthase in NIDDM is affected by chronic sulphonylurea treatment. Ten obese patients with NIDDM were studied before and after 8 weeks of treatment with a weight-maintaining diet in combination with the sulphonylurea gliclazide. Gliclazide treatment was associated with significant reductions in HbA1C (p=0.001) and fasting plasma glucose (p=0.005) as well as enhanced beta-cell responses to an oral glucose load. During euglycaemic, hyperinsulinaemic clamp (2 mU x kg-1 x min-1) in combination with indirect calorimetry, a 35% (p=0.005) increase in whole-body insulin-stimulated glucose disposal rate, predominantly due to an increased non-oxidative glucose metabolism (p=0.02) was demonstrated in teh gliclazide-treated patients when compared to pre-treatment values. In biopsies obtained from vastus lateralis muscle during insulin infusion, the half-maximal activation of glycogen synthase was achieved at a significantly lower concentration of the allosteric activator glucose 6-phosphate (p=0.01). However, despite significant increases in both insulin-stimulated non-oxidative glucose metabolism and muscle glycogen synthase activation in gliclazide-treated patients no changes were found in levels of glycogen synthase mRNA or immunoreactive protein in muscle. In conclusion, improved blood glucose control in gliclazide-treated obese NIDDM patients has no impact on the gene expression of muscle glycogen synthase.

[Etiopathogenesis of non-insulin-dependent diabetes]

Two major mechanisms, peripheral insulin resistance and impaired insulin secretion participate concomitantly but to a variable extent to the pathogenesis of non-insulin-dependent diabetes, whose heterogeneity, suspected for a long time, is now confirmed by the recent discoveries of the molecular biology. Mutations of several genes governing key-steps of the recognition of the glucose signal, insulin secretion or its peripheral effect have been found in some particular cases, but presently not at a large scale among non insulin-dependent diabetic patients. The tendency to worsening of the metabolic disturbances with the time, even under adequate therapy, can be explained by the vicious circle of glucose toxicity, but other mechanisms like amylin, responsible of the deposition of amyloid in the islets, may play a role. So, despite the acquisition of many new knowledges, the pathogenesis of non-insulin-dependent diabetes keeps nowadays a part of its mystery.

Insulin stimulation of glycogen synthesis and glycogen synthase activity is blocked by wortmannin and rapamycin in 3T3-L1 adipocytes: evidence for the involvement of phosphoinositide 3-kinase and p70 ribosomal protein-S6 kinase

We have investigated the involvement of phosphoinositide (PI) 3-kinase and p70 ribosomal protein-S6 kinase (p70s6k) in mediating insulin stimulation of glycogen synthesis in 3T3-L1 adipocytes using specific inhibitors. Wortmannin inhibited PI 3-kinase activity (IC50 approximately 10 nM), inhibition being complete at 100 nm. Wortmannin (100 nM) completely blocked the ability of insulin to activate glycogen synthase in 3T3-L1 adipocytes and the ability of insulin to stimulate glucose incorporation into glycogen in 3T3-L1 fibroblasts. Rapamycin, which blocks insulin-stimulated activation of p70s6k, decreased insulin activation of glycogen synthase in a dose-dependent manner (IC50 approximately 0.8 ng/ml), with a maximum approx. 75% inhibition of insulin's stimulatory effect. Rapamycin inhibited insulin-stimulated glucose incorporation into glycogen to a similar extent and with similar dose-dependency, while having no effect on insulin-stimulated glucose transport. We conclude that PI 3-kinase and p70s6k are involved in the signalling pathways by which insulin stimulates glycogen synthase in 3T3-L1 adipocytes.

Glycogen synthase: a putative locus for diet-induced hyperglycemia

Inbred mouse strains fed a diabetogenic diet have different propensities to develop features analogous to type 2 diabetes mellitus. To define chromosomal locations that control these characteristics, recombinant inbred strains from diabetes-prone C57BL/6J (B/6J) and diabetes-resistant A/J strains were studied. Insulin levels and hyperglycemia correlated with two different regions of mouse chromosome 7 (two point LOD scores > 3.0). For insulin levels, 15 of 16 recombinant inbred strains were concordant with a region that contains the tubby mutation that results in hyperinsulinemia. For hyperglycemia, 19 of 23 strains were concordant with the D7Mit25 marker and 20 of 23 strains with the Gpi-1 locus on proximal mouse chromosome 7. Using more stringent criteria for hyperglycemia, 10 of 11 strains characterized as A/J or B/6J like were concordant with D7Mit25. This putative susceptibility locus is consistent with that of the glycogen synthase gene (Gys) recently suggested as a candidate locus by analyses of type 2 diabetes patients. Fractional glycogen synthase activity in isolated muscle was significantly lower in normal B/6J diabetic-prone mice compared with normal diabetic-resistant A/J mice, a finding similar to that reported in relatives of human patients with type 2 diabetes. These data, taken together, raise the possibility that defects in the Gys gene may in part be responsible for the propensity to develop type 2 diabetes.

Mechanisms of fatty acid-induced inhibition of glucose uptake

Increased plasma FFA reduce insulin-stimulated glucose uptake. The mechanisms responsible for this inhibition, however, remain uncertain. It was the aim of this study to determine whether the FFA effect was dose dependent and to investigate its mechanism. We have examined in healthy volunteers (13 male/1 female) the effects of three steady state plasma FFA levels (approximately 50, approximately 550, approximately 750 microM) on rates of glucose uptake, glycolysis (both with 3-3H-glucose), glycogen synthesis (determined with two independent methods), carbohydrate (CHO) oxidation (by indirect calorimetry), hepatic glucose output, and nonoxidative glycolysis (glycolysis minus CHO oxidation) during euglycemic-hyperinsulinemic clamping. Increasing FFA concentration (from approximately 50 to approximately 750 microM) decreased glucose uptake in a dose-dependent fashion (from approximately 9 to approximately 4 mg/kg per min). The decrease was caused mainly (approximately 2/3) by a reduction in glycogen synthesis and to a lesser extent (approximately 1/3) by a reduction in CHO oxidation. We have identified two independent defects in glycogen synthesis. The first consisted of an impairment of muscle glycogen synthase activity. It required high FFA concentration (approximately 750 microM), was associated with an increase in glucose-6-phosphate, and developed after 4-6 h of fat infusion. The second defect, which preceded the glycogen synthase defect, was seen at medium (approximately 550 microM) FFA concentration, was associated with a decrease in muscle glucose-6-phosphate concentration, and was probably due to a reduction in glucose transport/phosphorylation. In addition, FFA and/or glycerol increased insulin-suppressed hepatic glucose output by approximately 50%. We concluded that fatty acids caused a dose-dependent inhibition of insulin-stimulated glucose uptake (by decreasing glycogen synthesis and CHO oxidation) and that FFA and/or glycerol increased insulin-suppressed hepatic glucose output and thus caused insulin resistance at the peripheral and the hepatic level.

Multiple lipolysis defects in the insulin resistance (metabolic) syndrome

Bearing in mind the importance of upper-body obesity for the insulin resistance (or metabolic) syndrome and the abnormalities in free fatty acid metabolism associated with this disorder, the regulation of lipolysis in isolated subcutaneous adipocytes was investigated in 13 72-yr old upper-body obese men with insulin resistance and glucose intolerance and in 10 healthy 72-yr-old men. There was a marked resistance to the lipolytic effect of noradrenaline in the metabolic syndrome due to defects at two different levels in the lipolytic cascade. First, an 80-fold decrease in sensitivity to the beta 2-selective agonist terbutaline (P < 0.001) which could be ascribed to a 50% reduced number of beta 2-receptors (P < 0.005) as determined with radioligand binding. The groups did not differ as regards dobutamine (beta 1) or clonidine (alpha-2) sensitivity, nor beta 1-receptor number. The mRNA levels for beta 1- and beta 2-receptors were similar in the two groups. Second, the maximum stimulated lipolytic rate was markedly reduced in the metabolic syndrome. This was true for isoprenaline (nonselective beta-agonist), forskolin (activating adenylyl cyclase), and dibutyryl cAMP (activating protein kinase). In regression analysis, the observed abnormalities in lipolysis regulation correlated in an independent way with the degree of glucose intolerance (r = -0.67) and beta 2-receptor number with insulin resistance (r = 0.67). In conclusion, the results of this study indicate the existence of lipolytic resistance to catecholamines in the adipose tissue of elderly men with the metabolic syndrome, which may be of importance for impaired insulin action and glucose intolerance. The resistance is located at a posttranscriptional level of beta 2-receptor expression and at the protein kinase-hormone sensitive lipase level.

Simple tandem repeat DNA polymorphism in the human glycogen synthase gene is associated with NIDDM in Japanese subjects

We investigated the possible association between alleles of a simple tandem repeat DNA polymorphism in the human glycogen synthase gene and non-obese non-insulin-dependent diabetes (NIDDM) in Japanese subjects. Nine alleles (-4G, -3G, -2G, -1G, 0G, 1G, 2G, 3G, and 4G) were identified in the study group of 164 patients with NIDDM and 115 non-diabetic subjects. The overall frequency distribution of the glycogen synthase gene alleles was significantly different between the two groups (p = 0.0316). The 2G allele was found more frequently in patients with NIDDM than in non-diabetic subjects (17.7% vs 8.7%, p = 0.0016). These results suggest that the 2G allele could be a genetic marker of NIDDM in Japanese subjects.

A liver factor increasing glucose uptake in rat hindquarters

The effect of the liver on glucose uptake in muscle was studied in the isolated rat hindquarter, perfused with and without an isolated rat liver included in a recirculating system. Glucose uptake in the hindquarter was 0.75 +/- 0.10 mumol.min-1 x 100 g bw-1 (mean and SEM), and increased to 1.30 +/- 0.12 during the period when the liver was included in the perfusion (N = 17; p = 0.0001). In experiments where tissue extracts were added to the perfusate during the second period, raw liver extracts increased the glucose uptake from 0.87 +/- 0.11 to 1.25 +/- 0.10 (N = 6; p = 0.003). Deproteinized liver and kidney extracts increased glucose uptake similarly. When the liver and hindquarter were perfused together during the first period, the glucose uptake in the hindquarter was 1.55 +/- 0.16 mumol.min-1 x 100 g bw-1 and decreased gradually during the period of isolated perfusion to 1.27 +/- 0.16 (N = 4; p = 0.15). In control experiments where tissue free extract fluid was added to the perfusate during the second period, or where the hindquarter was perfused either alone or with the liver during both periods, glucose uptake decreased slightly from 0.87 +/- 0.18 to 0.80 +/- 0.13 (N = 9; p = 0.2). In conclusion, a factor from the liver may increase the glucose uptake in muscle tissues. This factor is present in extracts from the liver and kidney, it is probably not a protein, but its nature is not known. It is speculated that glucose intolerance in patients with liver disease may be related to a lack of this factor.

Diabetes mellitus: a disease of abnormal cellular calcium metabolism?

Although the pathogenesis of the diabetes mellitus syndrome remains poorly understood, both insulin-dependent diabetes mellitus and non-insulin-dependent diabetes mellitus predispose the individual to a similar spectrum of complications, including hypertension, macrovascular and microvascular disease, cataracts cardiomyopathy, neuropathy, and premature aging, suggesting that these complications develop along a pathway common to both diabetic conditions. Yet not all diabetic persons are affected by all of these complications or to the same degree. What causes this marked variability in the clinical manifestations of the diabetes syndrome remains an enigma. Accumulating data from animal models of diabetes and from studying patients with diabetes reveal that intracellular calcium levels are increased in most tissues. The activities of the membrane, adenosine triphosphatase (ATPase) associated cation pumps, which determine intracellular calcium level (i.e., calcium-ATPase and [sodium + potassium]-ATPase), are also altered. The nature of the alteration is often tissue specific and may depend on the level of blood glucose or insulin, or both. In this review we discuss the potential contribution of these changes in intracellular calcium regulation, whether acquired or genetically determined, to the pathogenesis of the diabetes syndrome, to the abnormalities in insulin secretion and action (mainly in non-insulin-dependent diabetes), and to the complications of both diabetes syndromes. Altered intracellular calcium metabolism may represent a common, underlying abnormality linking the metabolic, cardiovascular, ocular, and neural manifestations of the diabetic disease process.

Non-esterified fatty acids regulate lipid and glucose oxidation and glycogen synthesis in healthy man

We examined the interrelationship of lipid and glucose metabolism in the basal state and during insulin stimulus in 19 healthy men (27 +/- 2 years, body mass index 23.6 +/- 0.6 kg/m2). In each subject, we performed a 4-h euglycaemic (5.3 +/- 0.1 mmol/l) hyperinsulinaemic (647 +/- 21 pmol/l) insulin clamp with indirect calorimetry in the basal state and during insulin infusion, and muscle biopsies before and at the end of the clamp. In the basal state, serum non-esterified fatty acid levels correlated directly with lipid oxidation (r = 0.56, p < 0.05) and indirectly with glucose oxidation (r = -0.80, p < 0.001). Lipid and glucose oxidation rates were inversely related in the basal state (r = -0.47, p < 0.05) and during insulin infusion (r = -0.65, p < 0.01). Basal lipid oxidation and glycogen synthase total activity correlated inversely (r = -0.54, p < 0.05). Lipid oxidation both in the basal state (r = -0.61, p < 0.01) and during insulin infusion (r = -0.62, p < 0.05) was inversely related to muscle glycogen content after the insulin clamp. Fasting plasma triglyceride concentration correlated directly to fasting insulin (r = 0.55, p < 0.05) and C-peptide (r = 0.50, p < 0.03) concentrations and inversely to non-oxidative glucose disposal rate at the end of clamp (r = -0.54, p < 0.05).

IN CONCLUSION

1) Serum non-esterified fatty acid concentration enhances lipid and reduces glucose oxidation. 2) Lipid oxidation is inversely related to total glycogen synthase activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Skeletal muscle glycogenolysis is more sensitive to insulin than is glucose transport/phosphorylation. Relation to the insulin-mediated inhibition of hepatic glucose production

The effects of minimal increments in plasma insulin concentrations on hepatic glucose production and glucose uptake, skeletal muscle net glycogen synthesis and glycogenolysis, glycogen synthase and phosphorylase activity, glucose-6-phosphate and uridinediphosphoglucose (UDPG) concentrations were examined in 24-h and in 6-h fasted conscious rats. Insulin was infused for 120 min at rates of 1.5, 3, 6, 12, 24, and 108 pmol/kg per min in 24-h fasted rats and at rates of 3, 6, 9, 12, 36, and 108 pmol/kg per min in 6-h fasted rats while endogenous insulin release was inhibited by SRIF infusion and plasma glucose was maintained at the basal level. All rats received an infusion of [3-3H]glucose. The portion of the muscle glucose-6-phosphate (G6P) pool derived from net glycogenolysis was estimated from the ratio of specific activities of muscle UDPG and plasma glucose. Minimal increments in the circulating insulin levels, which did not stimulate glucose uptake, caused: (a) the increase in skeletal muscle glycogen synthase activity and the decrease in the rate of muscle glycogenolysis and in the G6P concentration; (b) the inhibition of hepatic glucose production. Net muscle glycogen synthesis was not stimulated despite submaximal activation of glycogen synthase, and its onset correlated with the rise in muscle G6P levels. Thus, insulin's inhibition of muscle glycogenolysis is the most sensitive insulin action on skeletal muscle and its dose-response characteristics resemble those for the inhibition of hepatic glucose production. These findings indicate that skeletal muscle glycogen synthase may play a major role in carbohydrate homeostasis even under postabsorptive (basal insulin) conditions and support the notion that insulin may exert some of its effects on the liver through an indirect or peripheral mechanism.

Liver and peripheral tissue glycogen metabolism in obese mice: effect of a mixed meal

Glycogen metabolism in the liver, skeletal muscle, cardiac muscle, and white adipose tissue was studied in gold thioglucose (GTG) obese mice after fasting and during refeeding. Prolonged (48 h) fasted control and GTG mice were refed with standard laboratory diet for 24 h. During fasting and refeeding, the changes in glycogen content and the activity of glycogen synthase I and R and phosphorylase alpha in the liver were similar in lean and GTG mice. However, the glycogen storage in the livers from GTG mice was always greater than that in lean animals. In GTG mice the activity of liver glycogen synthase I and R was significantly higher than that in lean animals 3 and 6 h after refeeding. The activity of liver phosphorylase alpha in GTG mice was higher than that in lean mice after refeeding. There were no significant differences in the glycogen content of white adipose tissue, cardiac muscle, and skeletal muscle from lean and GTG mice during the entire study. The results of this study suggest that increased glycogen storage in the liver is a major alteration in nonoxidative glucose metabolism and contributes to the development of insulin resistance and glucose intolerance in GTG obese mice.

Transmembrane glucose transport in skeletal muscle of patients with non-insulin-dependent diabetes

Insulin resistance for glucose metabolism in skeletal muscle is a key feature in non-insulin-dependent diabetes mellitus (NIDDM). Which cellular effectors of glucose metabolism are involved is still unknown. We investigated whether transmembrane glucose transport in vivo is impaired in skeletal muscle in nonobese NIDDM patients. We performed euglycemic insulin clamp studies in combination with the forearm balance technique (brachial artery and deep forearm vein catheterization) in six nonobese NIDDM patients and five age- and weight-matched controls. Unlabeled D-mannitol (a nontransportable molecule) and radioactive 3-O-methyl-D-glucose (the reference molecular probe to assess glucose transport activity) were simultaneously injected into the brachial artery, and the washout curves were measured in the deep venous effluent blood. In vivo transmembrane transport of 3-O-methyl-D-glucose in forearm muscle was determined by computerized analysis of the washout curves. At similar steady-state plasma concentrations of insulin (approximately 500 pmol/liter) and glucose (approximately 5.15 mmol/liter), transmembrane inward transport of 3-O-methyl-D-glucose in skeletal muscle was markedly reduced in the NIDDM patients (6.5 x 10(-2) +/- 0.56 x 10(-2).min-1) compared with controls (12.5 x 10(-2) +/- 1.5 x 10(-2).min-1, P < 0.005). Mean glucose uptake was also reduced in the diabetics both at the whole body level (9.25 +/- 1.84 vs. 28.3 +/- 2.44 mumol/min per kg, P < 0.02) and in the forearm tissues (5.84 +/- 1.51 vs. 37.5 +/- 7.95 mumol/min per kg, P < 0.02). When the latter rates were extrapolated to the whole body level, skeletal muscle accounted for approximately 80% of the defect in insulin action seen in NIDDM patients. We conclude that transmembrane glucose transport, when assessed in vivo in skeletal muscle, is insensitive to insulin in nonobese NIDDM patients, and plays a major role in determining whole body insulin resistance.

Glycogen synthase and phosphofructokinase protein and mRNA levels in skeletal muscle from insulin-resistant patients with non-insulin-dependent diabetes mellitus

In patients with non-insulin-dependent diabetes mellitus (NIDDM) and matched control subjects we examined the interrelationships between in vivo nonoxidative glucose metabolism and glucose oxidation and the muscle activities, as well as the immunoreactive protein and mRNA levels of the rate-limiting enzymes in glycogen synthesis and glycolysis, glycogen synthase (GS) and phosphofructokinase (PFK), respectively. Analysis of biopsies of quadriceps muscle from 19 NIDDM patients and 19 control subjects showed in the basal state a 30% decrease (P < 0.005) in total GS activity and a 38% decrease (P < 0.001) in GS mRNA/microgram DNA in NIDDM patients, whereas the GS protein level was normal. The enzymatic activity and protein and mRNA levels of PFK were all normal in diabetic patients. In subgroups of NIDDM patients and control subjects an insulin-glucose clamp in combination with indirect calorimetry was performed. The rate of insulin-stimulated nonoxidative glucose metabolism was decreased by 47% (P < 0.005) in NIDDM patients, whereas the glucose oxidation rate was normal. The PFK activity, protein level, and mRNA/microgram DNA remained unchanged. The relative activation of GS by glucose-6-phosphate was 33% lower (P < 0.02), whereas GS mRNA/micrograms DNA was 37% lower (P < 0.05) in the diabetic patients after 4 h of hyperinsulinemia. Total GS immunoreactive mass remained normal. In conclusion, qualitative but not quantitative posttranslational abnormalities of the GS protein in muscle determine the reduced insulin-stimulated nonoxidative glucose metabolism in NIDDM.

Association between polymorphism of the glycogen synthase gene and non-insulin-dependent diabetes mellitus

BACKGROUND

The storage of glucose as glycogen in skeletal muscle is frequently impaired in patients with non-insulin-dependent diabetes mellitus (NIDDM) and their nondiabetic relatives. Despite an intensive search for candidate genes associated with NIDDM, no data have been available on the gene coding for the key enzyme of this pathway, glycogen synthase.

METHODS AND RESULTS

Using a human complementary DNA probe, the restriction enzyme Xbal, and Southern blot analysis, we identified two polymorphic alleles, A1 and A2, in the glycogen synthase gene. The gene was localized to chromosome 19. The A1A2 or A2A2 genotype was found in 30 percent of 107 patients with NIDDM but in only 8 percent of 164 nondiabetic subjects without a family history of NIDDM (P < 0.001). The diabetic patients with the A2 allele had a stronger family history of NIDDM (P = 0.019), a higher prevalence of hypertension (P = 0.008), and a more severe defect in insulin-stimulated glucose storage (P = 0.001) than the diabetic patients with the A1 allele. The concentration of the glycogen synthase protein in biopsy specimens of skeletal muscle from the patients with the A2 allele was normal, however, suggesting that expression of the gene was unaltered. The Xbal polymorphism was due to a change of a single base in an intron.

CONCLUSIONS

The Xbal polymorphism of the glycogen synthase gene identifies a subgroup of patients with NIDDM characterized by a strong family history of NIDDM, a high prevalence of hypertension, and marked insulin resistance.

Multiple metabolic effects of CGRP in conscious rats: role of glycogen synthase and phosphorylase

Calcitonin gene-related peptide (CGRP) is a neuropeptide that is released at the neuromuscular junction in response to nerve excitation. To examine the relationship between plasma CGRP concentration and intracellular glucose metabolism in conscious rats, we performed insulin (22 pmol.kg-1.min-1) clamp studies combined with the infusion of 0, 20, 50, 100, 200, and 500 pmol.kg-1.min-1 CGRP (plasma concentrations ranging from 2 x 10(-11) to 5 x 10(-9) M). CGRP antagonized insulin's suppression of hepatic glucose production at plasma concentrations (approximately 10(-10) M) that are only two- to fivefold its basal portal concentration. Insulin-mediated glucose disposal was decreased by 20-32% when CGRP was infused at 50 pmol.kg-1.min-1 (plasma concentration 3 x 10(-10) M) or more. The impairment in insulin-stimulated glycogen synthesis in skeletal muscle accounted for all of the CGRP-induced decrease in glucose disposal, while whole body glycolysis was increased despite the reduction in total glucose uptake. The muscle glucose 6-phosphate concentration progressively increased during the CGRP infusions. CGRP inhibited insulin-stimulated glycogen synthase in skeletal muscle with a 50% effective dose of 1.9 +/- 0.36 x 10(-10) M. This effect on glycogen synthase was due to a reduction in enzyme affinity for UDP-glucose, with no changes in the maximal velocity. In vitro CGRP stimulated both hepatic and skeletal muscle adenylate cyclase in a dose-dependent manner. These data suggest that 1) CGRP is a potent antagonist of insulin at the level of muscle glycogen synthesis and hepatic glucose production; 2) inhibition of glycogen synthase is its major biochemical action in skeletal muscle; and 3) these effects are present at concentrations of the peptide that may be in the physiological range for portal vein and skeletal muscle. These data underscore the potential role of CGRP in the physiological modulation of intracellular glucose metabolism.

Role of impaired intracellular glucose metabolism in the insulin resistance of aging

The insulin resistance of aging is characterized by both reduced glucose uptake and impaired intracellular glucose metabolism. The aim of this study was to determine whether impaired intracellular glucose metabolism contributes to insulin resistance in the elderly independent of reduced glucose uptake. To address this question, glucose uptake in non-obese elderly males was matched to controls using the glucose clamp technique, and intracellular glucose metabolism was assessed in vivo by indirect calorimetry and in vitro by skeletal muscle biopsy for glycogen synthase activity. When elderly subjects were compared with controls at an equivalent basal glucose uptake of approximately 2.5 mg/kg fat-free mass (FFM)/min, muscle glycogen synthase activity was similar (fractional velocity of glycogen synthase at 0.1 mmol/L glucose-6-phosphate [FV0.1], 0.06 +/- 0.1 and 0.07 +/- 0.1), but whole-body rates of glucose oxidation were reduced (1.36 +/- 0.12 v 1.90 +/- 0.11 mg/kg FFM/min, P less than .05). During 40-mU/m2/min hyperinsulinemic clamps at matched rates of glucose uptake (approximately 10.7 mg/kg FFM/min in both groups), glycogen synthase activity was again similar (FV0.1, 0.15 +/- 0.02 and 0.14 +/- 0.02), and glucose oxidation remained reduced in the elderly (4.18 +/- 0.25 v 4.77 +/- 0.17 mg/kg FFM/min, P less than .05). Only during clamps in the maximal range of glucose uptake (approximately 29.5 mg/kg FFM/min) was glucose oxidation between the groups comparable (5.97 +/- 0.50 and 5.75 +/- 0.31 mg/kg FFM/min). Plasma free fatty acid (FFA) concentrations, fat oxidation, and protein oxidation were similar under all study conditions.(ABSTRACT TRUNCATED AT 250 WORDS)

Glucose transport in human skeletal muscle cells in culture. Stimulation by insulin and metformin

Primary human muscle cell cultures were established and the regulation of glucose transport was investigated. Primary cultures were allowed to proceed to the stage of myotubes through fusion of myoblasts or were used for clonal selection based on fusion potential. In clonally selected cultures, hexose (2-deoxy-glucose) uptake into myotubes was linear within the time of study and inhibitable by cytochalasin B (IC50 = 400 nM). Cytochalasin B photolabeled a protein(s) of 45,000-50,000 D in a D-glucose-protectable manner, suggesting identity with the glucose transporters. In the myotube stage, the cells expressed both the GLUT1 and GLUT4 glucose transporter protein isoforms at an average molar ratio of 7:1. Preincubation in media of increasing glucose concentrations (range 5-25 mM) progressively decreased the rate of 2-deoxyglucose uptake. Insulin elevated 2-deoxyglucose uptake in a dose-dependent manner, with half maximal stimulation achieved at 3.5 nM. Insulin also stimulated the transport of the nonmetabolizable hexose 3-O-methylglucose, as well as the activity of glycogen synthase, responsible for nonoxidative glucose metabolism. The oral antihyperglycemic drug metformin stimulated the cytochalasin B-sensitive component of both 2-deoxyglucose and 3-O-methylglucose uptake. Maximal stimulation was observed at 8 h of exposure to 50 microM metformin, and this effect was not prevented by incubation with the protein-synthesis inhibitor cycloheximide. The relative effect of metformin was higher in cells incubated in 25 mM glucose than in 5 mM glucose, consistent with its selective action in hyperglycemic conditions in vivo. Metformin (50 microM for 24 h) was more effective than insulin (1 microM for 1 h) in stimulating hexose uptake and the hormone was effective on top of the stimulation caused by the biguanide, suggesting independent mechanisms of action.

Decreased insulin activation of glycogen synthase in skeletal muscles in young nonobese Caucasian first-degree relatives of patients with non-insulin-dependent diabetes mellitus

Insulin resistance in non-insulin-dependent diabetes is associated with a defective insulin activation of the enzyme glycogen synthase in skeletal muscles. To investigate whether this may be a primary defect, we studied 20 young (25 +/- 1 yr) Caucasian first-degree relatives (children) of patients with non-insulin-dependent diabetes, and 20 matched controls without a family history of diabetes. Relatives and controls had a normal oral glucose tolerance, and were studied by means of the euglycemic hyperinsulinemic clamp technique, which included performance of indirect calorimetry and muscle biopsies. Insulin-stimulated glucose disposal was decreased in the relatives (9.2 +/- 0.6 vs 11.5 +/- 0.5 mg/kg fat-free mass per (FFM) min, P less than 0.02), and was due to a decreased rate of insulin-stimulated nonoxidative glucose metabolism (5.0 +/- 0.5 vs 7.5 +/- 0.4 mg/kg fat-free mass per min, P less than 0.001). The insulin-stimulated, fractional glycogen synthase activity (0.1/10 mmol liter glucose-6-phosphate) was decreased in the relatives (46.9 +/- 2.3 vs 56.4 +/- 3.2%, P less than 0.01), and there was a significant correlation between insulin-stimulated, fractional glycogen synthase activity and nonoxidative glucose metabolism in relatives (r = 0.76, P less than 0.001) and controls (r = 0.63, P less than 0.01). Furthermore, the insulin-stimulated increase in muscle glycogen content over basal values was lower in the relatives (13 +/- 25 vs 46 +/- 9 mmol/kg dry wt, P = 0.05). We conclude that the defect in insulin activation of muscle glycogen synthase may be a primary, possibly genetically determined, defect that contributes to the development of non-insulin-dependent diabetes.

Hyperglycaemia compensates for the defects in insulin-mediated glucose metabolism and in the activation of glycogen synthase in the skeletal muscle of patients with type 2 (non-insulin-dependent) diabetes mellitus

Insulin resistance and a defective insulin activation of the enzyme glycogen synthase in skeletal muscle during euglycaemia may have important pathophysiological implications in Type 2 (non-insulin-dependent) diabetes mellitus. Hyperglycaemia may serve to compensate for these defects in Type 2 diabetes by increasing glucose disposal through a mass action effect. In the present study, rates of whole-body glucose oxidation and glucose storage were measured during fasting hyperglycaemia and isoglycaemic insulin infusion (40 mU.m-2.min-1, 3 h) in 12 patients with Type 2 diabetes. Eleven control subjects were studied during euglycaemia. Biopsies were taken from the vastus lateralis muscle. Fasting and insulin-stimulated glucose oxidation, glucose storage and muscle glycogen synthase activation were all fully compensated (normalized) during hyperglycaemia in the diabetic patients. The insulin-stimulated increase in muscle glycogen content was the same in the diabetic patients and in the control subjects. Besides hyperglycaemia, the diabetic patients had elevated muscle free glucose and glucose 6-phosphate concentrations. A positive correlation was demonstrated between intracellular free glucose concentration and muscle glycogen synthase fractional velocity insulin activation (0.1 mmol/l glucose 6-phosphate: r = 0.65, p less than 0.02 and 0.0 mmol/l glucose 6-phosphate: r = 0.91, p less than 0.0001). In conclusion, this study indicates an important role for hyperglycaemia and elevated muscle free glucose and glucose 6-phosphate concentrations in compensating (normalizing) intracellular glucose metabolism and skeletal muscle glycogen synthase activation in Type 2 diabetes.