Defective insulin response of phosphorylase phosphatase in insulin-resistant humans

Deregulation of CREB signaling pathway induced by chronic hyperglycemia downregulates NeuroD transcription

CREB mediates the transcriptional effects of glucose and incretin hormones in insulin-target cells and insulin-producing β-cells. Although the inhibition of CREB activity is known to decrease the β-cell mass, it is still unknown what factors inversely alter the CREB signaling pathway in β-cells. Here, we show that β-cell dysfunctions occurring in chronic hyperglycemia are not caused by simple inhibition of CREB activity but rather by the persistent activation of CREB due to decreases in protein phophatase PP2A. When freshly isolated rat pancreatic islets were chronically exposed to 25 mM (high) glucose, the PP2A activity was reduced with a concomitant increase in active pCREB. Brief challenges with 15 mM glucose or 30 µM forskolin after 2 hour fasting further increased the level of pCREB and consequently induced the persistent expression of ICER. The excessively produced ICER was sufficient to repress the transcription of NeuroD, insulin, and SUR1 genes. In contrast, when islets were grown in 5 mM (low) glucose, CREB was transiently activated in response to glucose or forskolin stimuli. Thus, ICER expression was transient and insufficient to repress those target genes. Importantly, overexpression of PP2A reversed the adverse effects of chronic hyperglycemia and successfully restored the transient activation of CREB and ICER. Conversely, depletion of PP2A with siRNA was sufficient to disrupt the negative feedback regulation of CREB and induce hyperglycemic phenotypes even under low glucose conditions. Our findings suggest that the failure of the negative feedback regulation of CREB is the primary cause for β-cell dysfunctions under conditions of pathogenic hyperglycemia, and PP2A can be a novel target for future therapies aiming to protect β-cells mass in the late transitional phase of non-insulin dependent type 2 diabetes (NIDDM).

Palmitate action to inhibit glycogen synthase and stimulate protein phosphatase 2A increases with risk factors for type 2 diabetes

Recent studies have suggested that abnormal regulation of protein phosphatase 2A (PP2A) is associated with Type 2 diabetes in rodent and human tissues. Results with cultured mouse myotubes support a mechanism for palmitate activation of PP2A, leading to activation of glycogen synthase kinase 3. Phosphorylation and inactivation of glycogen synthase by glycogen synthase kinase 3 could be the mechanism for long-chain fatty acid inhibition of insulin-mediated carbohydrate storage in insulin-resistant subjects. Here, we test the effects of palmitic acid on cultured muscle glycogen synthase and PP2A activities. Palmitate inhibition of glycogen synthase fractional activity is increased in subjects with high body mass index compared with subjects with lower body mass index (r = -0.43, P = 0.03). Palmitate action on PP2A varies from inhibition in subjects with decreased 2-h plasma glucose concentration to activation in subjects with increased 2-h plasma glucose concentration (r = 0.45, P < 0.03) during oral glucose tolerance tests. The results do not show an association between palmitate effects on PP2A and glycogen synthase fractional activity. We conclude that subjects at risk for Type 2 diabetes have intrinsic differences in palmitate regulation of at least two enzymes (PP2A and glycogen synthase), contributing to abnormal insulin regulation of glucose metabolism.

Genetics of type 2 diabetes mellitus: status and perspectives

Throughout the last decade, molecular genetic studies of non-autoimmune diabetes mellitus have contributed significantly to our present understanding of this disease's complex aetiopathogenesis. Monogenic forms of diabetes (maturity-onset diabetes of the young, MODY) have been identified and classified into MODY1-6 according to the mutated genes that by being expressed in the pancreatic beta-cells confirm at the molecular level the clinical presentation of MODY as a predominantly insulin secretory deficient form of diabetes mellitus. Genomewide linkage studies of presumed polygenic type 2 diabetic populations indicate that loci on chromosomes 1q, 5q, 8p, 10q, 12q and 20q contain susceptibility genes. Yet, so far, the only susceptibility gene, calpain-10 (CAPN10), which has been identified using genomewide linkage studies, is located on chromosome 2q37. Mutation analyses of selected 'candidate' susceptibility genes in various populations have also identified the widespread Pro12Ala variant of the peroxisome proliferator-activated receptor-gamma and the common Glu23Lys variant of the ATP-sensitive potassium channel, Kir6.2 (KCNJ11). These variants may contribute significantly to the risk type 2 diabetes conferring insulin resistance of liver, muscle and fat (Pro12Ala) and a relative insulin secretory deficiency (Glu23Lys). It is likely that, in the near future, the recent more detailed knowledge of the human genome and insights into its haploblocks together with the developments of high-throughput and cheap genotyping will facilitate the discovery of many more type 2 diabetes gene variants in study materials, which are statistically powered and phenotypically well characterized. The results of these efforts are likely to be the platform for major progress in the development of personalized antidiabetic drugs with higher efficacy and few side effects.

Oral treatment with vanadium of Zucker fatty rats activates muscle glycogen synthesis and insulin-stimulated protein phosphatase-1 activity

Since the glucose-lowering effects of vanadium could be related to increased muscle glycogen synthesis, we examined the in vivo effects of vanadium and insulin treatment on glycogen synthase (GS) activation in Zucker fatty rats. The GS fractional activity (GSFA), protein phosphatase-1 (PP1), and glycogen synthase kinase-3 (GSK-3) activity were determined in fatty and lean rats following treatment with bis(maltolato)oxovanadium(IV) (BMOV) for 3 weeks (0.2 mmol/kg/day) administered in drinking water. Skeletal muscle was freeze-clamped before or following an insulin injection (5 U/kg i.v.). In both lean and fatty rats, muscle GSFA was significantly increased at 15 min following insulin stimulation. Vanadium treatment resulted in decreased insulin levels and improved insulin sensitivity in the fatty rats. Interestingly, this treatment stimulated muscle GSFA by 2-fold (p < 0.05) and increased insulin-stimulated PP1 activity by 77% (p < 0.05) in the fatty rats as compared to untreated rats. Insulin resistance, vanadium and insulin in vivo treatment did not affect muscle GSK-3beta activity in either fatty or lean rats. Therefore, an impaired insulin sensitivity in the Zucker fatty rats was improved following vanadium treatment, resulting in an enhanced muscle glucose metabolism through increased GS and insulin-stimulated PPI activity.

In vivo insulin regulation of skeletal muscle glycogen synthase in calorie-restricted and in ad libitum-fed rhesus monkeys

Chronic calorie restriction in primates has been shown to have profound and unexpected effects on basal and on in vivo insulin action on skeletal muscle glycogen synthase (GS) activity. The decreased ability of insulin to activate skeletal muscle GS is a hallmark of insulin resistance and type 2 diabetes. The mechanism and role of in vivo insulin regulation of skeletal muscle GS are not fully understood. Two pathways for the activation of GS by insulin have been described by Larner and others: 1) insulin activates glucose transport that results in an increase in glucose-6-phosphate (G6P), thereby activating protein phosphatase-1, which in turn dephosphorylates and activates GS, therefore, pushing substrate into glycogen; and 2) insulin activates GS (perhaps by forming low-molecular-weight mediators which may activate protein phosphatase-1 and 2C) and activated GS subsequently pulls intermediates (e.g., G6P and uridine 5'-diphosphoglucose) into glycogen. To determine whether in vivo insulin regulates glycogen synthesis primarily via a push or pull mechanism and how this mechanism might be affected by long-term calorie restriction, skeletal muscle samples were obtained before and during a euglycemic hyperinsulinemic clamp from 41 rhesus monkeys. The monkeys varied widely in their degree of insulin sensitivity and age and included chronically calorie-restricted (CR) monkeys and ad libitum-fed monkeys. The ad libitum-fed monkeys included spontaneously type 2 diabetic, prediabetic and clinically normal animals. The apparent affinity of GS for the allosteric activator G6P (G6P Ka of GS) was measured and compared with G6P content in the muscle samples. Basal G6P Ka of GS was lower in the CR monkeys compared with the 3 ad libitum-fed groups (P: < or = 0.05). Only the normal ad libitum-fed monkeys had a decrease in the G6P Ka of GS with insulin (P: < 0.005). The insulin effect (insulin-stimulated minus basal) on the G6P Ka of GS was strongly positively related to the insulin effect on G6P content (r = 0.80, P: < 0.0001) across the entire group of monkeys. This finding supports the hypothesis that activation/dephosphorylation of GS by insulin is related to a decrease in G6P content and that paradoxical inactivation/phosphorylation of GS by insulin is related to an increase in G6P content (as demonstrated in 4 of 6 CR monkeys). Therefore, during a euglycemic hyperinsulinemic clamp, insulin regulates skeletal muscle glycogen synthesis primarily via a pull mechanism in both CR and in ad libitum-fed rhesus monkeys.

Protein targeting to glycogen/PPP1R5: screening of coding and flanking genomic regions for polymorphisms and association analysis with insulin action in Pima Indians

Insulin resistance, a major predictor of type 2 diabetes mellitus, is genetically inherited in Pima Indians, a population with a high prevalence of the metabolically complex disease. Protein targeting to glycogen/PPP1R5 has recently been identified as a potential regulator of glycogen synthase, the rate-limiting enzyme of the insulin-induced glycogenesis. The gene is located on chromosome 10q23-24, where there is a suggestive linkage to insulin action in this population, establishing it as a functional and positional candidate gene. In this study, we discovered 2 novel polymorphisms upstream of the 5'UTR of the gene, with only one found in Pima Indians, but no polymorphism in the coding sequence. The genotype frequencies of the polymorphism and transcript levels of the gene in skeletal muscle do not correlate with insulin action in the subjects. These results exclude any significant role of protein targeting to glycogen/PPP1R5 in insulin resistance in Pima Indians.

Insulin decreases skeletal muscle cAMP-dependent protein kinase (PKA) activity in normal monkeys and increases PKA activity in insulin-resistant rhesus monkeys

Insulin activation of skeletal muscle glycogen synthase and glucose disposal is defective in both prediabetic and diabetic primates. Reduction in the activation of glycogen synthase by insulin could be the cause of lower glucose disposal rates, and could be the result, at least in part, of the failure of insulin to inhibit cAMP-dependent protein kinase activity (protein kinase A, PKA). To examine this proposed mechanism, PKA activity was measured in skeletal muscle (vastus lateralis) samples freeze-clamped in situ under basal fasting conditions before, and again during a euglycemic hyperinsulinemic clamp in 27 rhesus monkeys. Nine of the monkeys were normal (normal fasting glucose and insulin), eight were prediabetic (normal fasting glucose and hyperinsulinemia) and ten had spontaneous non-insulin-dependent diabetes (hyperglycemia). Insulin lowered PKA activity ratio in normal monkeys (basal vs insulin-stimulated, 14.4 +/- 3.2 vs 8.1 +/- 1.8%, p < 0.05), but raised PKA activity ratio in prediabetic monkeys (5.4 +/- 1.4 vs 10.5 +/- 2.6%, p < 0.05). PKA activity ratio was unaffected by insulin in the diabetic monkeys (6.7 +/- 1.8 vs 7.5 +/- 1.4%). Basal PKA activity ratio was higher in normal monkeys compared to prediabetic (p < 0.05) and diabetic monkeys (p < 0.05). Basal PKA activity ratio was inversely related to the insulin-stimulated change in PKA activity ratio (r = -0.72, p < 0.001). We conclude that in vivo insulin during euglycemic hyperinsulinemic clamp decreases skeletal muscle PKA activity ratio in normal monkeys but fails to decrease the activity ratio of PKA in insulin resistant (prediabetic and diabetic) monkeys. The insulin resistant state is characterized by low basal fasting skeletal muscle PKA activity ratio.

Postabsorptive respiratory quotient and insulin-stimulated glucose storage rate in nondiabetic pima indians are related To glycogen synthase fractional activity in cultured myoblasts

A decreased ratio of fat to carbohydrate oxidation rate (an elevated respiratory quotient) predicts the development of obesity. Skeletal muscle accounts for a major fraction of total body lipid oxidation and is the principle site for reduced glucose storage in insulin-resistant subjects. The potentially important role that muscle has in promoting obesity or insulin resistance may be based on metabolic control intrinsic to skeletal muscle. Cultured skeletal muscle provides a system to examine the importance of inherent metabolic traits in muscle biopsies from obese and insulin-resistant subjects. Glycogen synthase fractional activity (GSFA) was measured in cultured myoblasts from 21 Pima Indians characterized in vivo using indirect calorimetry and a euglycemic hyperinsulinemic clamp. Basal GSFA in cultured muscle cells is inversely correlated with postabsorptive respiratory quotient of the muscle donors (r = -0.66, P = 0.001) and with in vivo high dose insulin-stimulated glucose storage rates (r = 0.47, P = 0.04). These results indicate that the postabsorptive respiratory quotients and insulin-mediated glucose storage rates in vivo share a common regulatory mechanism with GSFA in cultured myoblasts. Abnormal regulation of glycogen synthase phosphorylation state may be an intrinsic defect in skeletal muscle associated with obesity and insulin resistance.

Relationship of glycogen synthase and glycogen phosphorylase to protein phosphatase 2C and cAMP-dependent protein kinase in liver of obese rhesus monkeys

The regulation of glycogen synthase (GS) and glycogen phosphorylase (GP) activity by phosphorylation/ dephosphorylation has been proposed to be via changes in activities of several different protein (serine/threonine) phosphatases and kinases, including protein phosphatase (PP) 1/2A, PP2C, and cAMP-dependent protein kinase (PKA). In order to determine whether PP1/2A, PP2C, and/or PKA activities are related to GS and/or GP activities, these enzymes were measured in freeze-clamped liver biopsies obtained under basal fasting conditions from 16 obese monkeys. Four monkeys were normoglycemic and normoinsulinemic, five were hyperinsulinemic, and seven had type 2 diabetes (NIDDM). Liver glycogen and glucose 6-phosphate (G6P) contents were also determine. Basal enzyme activities and basal substrate concentrations were not significantly different between the three group of obese monkeys; however, there were several significant linear relationships observed when the monkeys were treated as one group. Therefore, multiple regression was used to determine the correlation between key variables. GS fractional activity was correlated to GP fractional activity (p < 0.05) and to PP2C activity (p = 0.005) (adjusted R2, 53%). GP independent activity was correlated to GS independent activity (p < 0.07) and to PKA fractional activity (p = 0.005) (adjusted R2, 64%). PP2C activity was correlated to GS fractional activity (p < 0.0005) and to PP1/2A activity (p < 0.0001) (adjusted R2, 83%). PKA fractional activity was correlated to GP total activity (p < 0.0005) and to age (p = 0.001) (adjusted R2, 82%). G6P content was correlated to glycogen content (p < 0.05) and to PP2C activity (p = 0.0005) (adjusted R2, 73%). In conclusion, PP2C and PKA are involved in the regulation of GS and GP activity in the basal state in liver of obese monkeys with a wide range of glucose tolerance.

Asp905Tyr polymorphism of protein phosphatase 1 G subunit gene in hypertension

A possible pathogenic polymorphism in the gene for the G subunit of the glycogen-associated regulatory form of protein phosphatase 1 (PP1 G subunit), causing an Asp-to-Tyr substitution at codon 905 (Asp905Tyr), has been reported to be associated with insulin resistance and hypersecretion of insulin in the white population. Since marked heterogeneity has been reported in the association of mutations of candidate genes with essential hypertension between Japanese and other ethnic groups, we investigated the association of Asp905Tyr with essential hypertension in Japanese subjects. The frequency of the Tyr allele in Japanese control subjects (0.70) was much higher than that in the Danish population (0.10, P<1x10(-8)), indicating that the Tyr allele, previously reported as a rare variant in white subjects, is a common allele in our population. The genotype distribution in Japanese hypertensive patients (n=109; Asp/Asp=0.09, Asp/Tyr=0.39, Tyr/Tyr=0.52) was not significantly different (chi2=0.7, df=2, P>.6) from that in normotensive control subjects (n=148; Asp/Asp=0.12, Asp/Tyr=0.36, Tyr/Tyr=0.52). Among subjects with different PP1 G subunit genotypes, there was no difference in blood pressure, serum cholesterol, plasma glucose and insulin levels, and glucose disposal rate estimated by the euglycemic hyperinsulinemic clamp test. These data indicate that the Asp905Tyr polymorphism of the PP1 G subunit is not associated with essential hypertension, nor with insulin resistance and/or hyperinsulinemia in Japanese patients with essential hypertension, suggesting that the polymorphism plays little if any role in susceptibility to insulin resistance or hypertension.

The effect of modulating the glycogen-associated regulatory subunit of protein phosphatase-1 on insulin action in rat skeletal muscle cells

Recent studies from this laboratory have shown that insulin rapidly stimulates a membranous protein phosphatase-1 (PP-1) in cultured rat skeletal muscle cells and isolated rat adipocytes. Stimulation of PP-1 is accompanied by the phosphorylation of a 160-kDa regulatory subunit of PP-1 (PP-1G). To further evaluate the exact role of this subunit in insulin action, L6 rat skeletal muscle cells were stably transfected with a vector containing the gene for PP-1G in the sense and antisense orientations. Transfection with the vector containing the PP-1G gene in the sense orientation yielded three stable clones with a 4- to 6-fold increase in PP-1G protein expression compared to those of wild-type L6 cells and neo control cells harboring an empty expression vector. Compared to the neo control, overexpression of PP-1G resulted in a 3-fold increase in insulin-stimulated PP-1 catalytic activity bound to PP-1G immunoprecipitates. These cell lines were examined for insulin's effect on glucose uptake, glycogen synthase activity, and glycogen synthesis. Insulin treatment resulted in an approximately 2-fold increase in 2-deoxyglucose uptake in recombinant cells compared to control cells (P < 0.05). This increase in 2-deoxyglucose transport was accompanied by an approximately 2-fold increase in insulin-stimulated glycogen synthase fractional activity (P < 0.05) and a 2- to 4-fold increase in insulin-stimulated glycogen synthesis compared to control cells. In conjunction with these observations, we found that an 85% depletion of endogenous PP-1G, using antisense constructs, resulted in a complete lack of PP-1 activation and an inhibition of basal and insulin-stimulated glucose transport. We conclude that the PP-1G holoenzyme is the major phosphatase regulated by insulin in vivo and plays an important role in insulin-stimulated glycogen synthesis by regulating the catalytic activity of bound PP-1.

Associations between pituitary-adrenocortical function and abdominal obesity, hyperinsulinaemia and dyslipidaemia in normotensive males

OBJECTIVES

To examine the relationships between pituitary-adrenal cortical (PA) function, abdominal obesity, hyperinsulinaemia, and dyslipidaemia.

DESIGN

A prospective study.

SETTING

Helsinki University Central Hospital, Finland.

SUBJECTS

Seventy-one healthy males aged 30-55 years.

MAIN OUTCOME MEASURES

Insulin sensitivity was assessed by the oral glucose tolerance test (OGTT). Basal PA activity was examined by measuring urinary and serum concentrations of hormones, followed by dexamethasone suppression and corticotrophin (ACTH) stimulation tests to determine functional PA activity.

RESULTS

The means of waist-to-hip ratio (WHR), body-mass index (BMI), HDL-cholesterol and triglyceride levels, and insulin and C-peptide measurements during the OGTT were significantly different across the tertiles for insulin:glucose ratio. The ratio of 12-h urinary cortisol excretion to BMI, preceding the OGTT, and the mean basal cortisol level during the OGTT were decreased, while the net increments of cortisol and 17-hydroxyprogesterone (17-OHP) from 0 to 60 min, as well as the ratio of net 17-OHP to 11-deoxycortisol increments, after ACTH, were elevated in the upper compared with the lower tertile. The mean cortisol during the OGTT, and the ratio of urinary cortisol to BMI were negatively related, while absolute DHEA and cortisol responses to ACTH were positively related to fasting and mean insulin levels. Hormonal variables, WHR, insulin, and triglycerides were successfully integrated into a tentative mathematical model by the use of covariance structure (path) analyses.

CONCLUSIONS

Several alterations in the PA function, suggestive of decreased 21-hydroxylase activity, mild cortisol deficiency and slight adrenal hyperplasia, are associated with abdominal obesity which, in turn, appears to be an important prelude to insulin resistance and dyslipidaemia.

Rapid activation of glycogen synthase and protein phosphatase in human skeletal muscle after isometric contraction requires an intact circulation

The effects of isometric contraction (66% of maximal force) and recovery on glycogen synthase fractional activity (GSF) in human skeletal muscle have been studied. Biopsies were taken from the quadriceps femoris muscle at rest, at fatigue and 5 min postexercise on two occasions: after one of the contractions, the circulation to the thigh was occluded during the 5 min recovery (OCC), and after the other contraction, the circulation was intact (control, CON). During CON, GSF decreased from (mean +/- SE) 0.34 +/- 0.05 at rest to 0.24 +/- 0.02 at fatigue and then increased to 0.74 +/- 0.04 at 5 min postexercise; corresponding values for OCC were 0.37 +/- 0.04, 0.25 +/- 0.04 and 0.48 +/- 0.05 (P < 0.001 vs. CON for 5 min postexercise only). Compared with the value at fatigue, protein phosphatase activity (PP) increased by 79 +/- 16% during CON recovery (P < 0.01), whereas no change was observed during OCC recovery. Uridine diphosphate glucose increased by approximately 2.5-fold at fatigue, remained elevated during OCC recovery, but reverted to the preexercise level during CON recovery (P < 0.001 vs. OCC recovery). Glucose 6-P increased approximately 5-fold at fatigue and was higher at 5 min postexercise in OCC vs. CON recovery (8.6 +/- 1.5 vs. 4.1 +/- 0.9 mmol/kg dry wt; P < 0.01). It is concluded that the rapid increase in GSF after intense exercise with an intact circulation may be at least partly attributed to an increase in the specific activity of PP. The increase in GSF during recovery in OCC may be at least partly attributed to the high glucose 6-P content in vivo, which enhances the substrate suitability of GS for PP. Thus, separate mechanisms exist for the activation of PP and GS during recovery from intense short term exercise.

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.

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)

Insulin action, thermogenesis and obesity

The case for obesity per se being a major cause of insulin resistance has been made. There is evidence that each of the control points of insulin on glucose metabolism are negatively influenced by lipid oversupply, a characteristic of the obese state. The answer to the corollary, whether insulin resistance (a universal concomitant of obesity) can in turn lead to obesity via a decrease in thermogenesis, is more complex. Overall, the answer would appear to be no. On a population basis, obese individuals would not appear to have lower metabolic rates, whether expressed on a lean tissue or any other basis, than lean individuals. Even in the subpopulation of hypometabolic obese, there are no convincing data that the reduced metabolic rate is linked to particularly severe insulin resistance. Further, improving insulin action by weight loss would not appear to increase thermogenesis as would be predicted if insulin resistance impaired thermogenesis. A case can be made for reductions in a specific aspect of energy expenditure in obesity, that of meal-induced or glucose-induced thermogenesis, and this may be due to insulin resistance. However, meal-induced thermogenesis is a small component of total energy expenditure and total energy expenditure is not different between lean and obese. That leaves the intriguing possibility that a relative failure of prandial thermogenesis has an impact upon energy balance via impairment of satiety (related to reduced metabolic flux) and thus by increasing intake. While a potentially fruitful research avenue, too few data exist on this possibility for it to be anything more than speculative at this stage.

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.

Molecular cloning and chromosomal localization of a human skeletal muscle PP-1 gamma 1 cDNA

Type-1-protein phosphatase (PP-1) activity is reduced in skeletal muscle from human subjects with insulin resistance (Kida et al. 1990). This reduced phosphatase activity probably leads to the abnormal insulin action for glucose storage observed in insulin-resistant subjects. In the present study, a human homolog of rat liver PP-1 gamma 1 cDNA was isolated from human skeletal muscle. The nucleotide sequence contains a 957-nucleotide open reading frame encoding an amino acid sequence identical to that encoded by rat liver PP-1 gamma 1 cDNA. Northern blot analysis shows PP-1 gamma 1-specific mRNA is expressed in human heart, brain, placenta, lung, liver, skeletal muscle, kidney, and pancreas. PP-1 gamma 1 was localized to human Chromosome 12.

Pathogenesis of type 2 (non-insulin-dependent) diabetes mellitus: candidates for a signal transmitter defect causing insulin resistance of the skeletal muscle

Insulin resistance of skeletal muscle, liver and fat combined with an abnormality of insulin secretion characterizes Type 2 (non-insulin-dependent) diabetes mellitus. There is increasing evidence that the insulin resistance of the skeletal muscle plays a key role early in the development of Type 2 diabetes. As a consequence recent research efforts have focussed on the characterization of insulin signal transduction elements in the muscle which are candidates for a localization of a defect causing insulin resistance i.e. the insulin receptor, phosphatases related to insulin action, glycogen synthase and the glucose transporters. In this review we attempt to summarize present knowledge about abnormalities of these systems in skeletal muscle of Type 2 diabetic and pre-diabetic individuals. We try to classify abnormalities as secondary events or as candidates for putative primary molecular defects which might initiate the development of insulin resistance as early as in the "pre-diabetic" state.

Abnormal regulation of ribosomal protein S6 kinase by insulin in skeletal muscle of insulin-resistant humans

Insulin resistance in Pima Indians appears to result from a post-receptor impairment of insulin signal transduction that affects only some responses to insulin. To identify the primary lesion responsible for insulin resistance, we investigated the influence of insulin on ribosomal protein S6 kinase activities in skeletal muscle of insulin-sensitive and insulin-resistant nondiabetic Pima Indians during a 2-h hyperinsulinemic, euglycemic clamp. In sensitive subjects, S6 kinase activity was transiently activated fivefold over basal activity by 45 min of insulin infusion. Although basal activities in the two groups were similar, the response to insulin was delayed and restricted to about threefold over basal in subjects resistant to insulin. Two major S6 kinase activities in extracts of human muscle were resolved by chromatography on Mono Q. Peak 1, which accounted for basal activity owes to an enzyme antigenically related to the 90-kD S6 kinase II, a member of the rsk gene family. The major insulin-stimulated S6 kinase eluted as peak 2 and is antigenically related to a 70-kD S6 kinase. Our results show that insulin resistance impairs signaling to the 70-kD S6 kinase.