Impaired glucose transport as a cause of decreased insulin-stimulated muscle glycogen synthesis in type 2 diabetes

Nitric oxide synthase inhibition reduces glucose uptake during exercise in individuals with type 2 diabetes more than in control subjects

Nitric oxide (NO) synthase inhibition reduces leg glucose uptake during cycling without reducing leg blood flow (LBF) in young, healthy individuals. This study sought to determine the role of NO in glucose uptake during exercise in individuals with type 2 diabetes. Nine men with type 2 diabetes and nine control subjects matched for age, sex, peak pulmonary oxygen uptake (VO(2) peak), and weight completed two 25-min bouts of cycling exercise at 60 +/- 2% VO(2) peak, separated by 90 min. N(G)-monomethyl-L-arginine (L-NMMA) (total dose 6 mg/kg) or placebo was administered into the femoral artery for the final 15 min of exercise in a counterbalanced, blinded, crossover design. LBF was measured by thermodilution in the femoral vein, and leg glucose uptake was calculated as the product of LBF and femoral arteriovenous glucose difference. During exercise with placebo, glucose uptake was not different between control subjects and individuals with diabetes; however, LBF was lower and arterial plasma glucose and insulin levels were higher in individuals with diabetes. L-NMMA had no effect on LBF or arterial plasma glucose and insulin concentrations during exercise in both groups. L-NMMA significantly reduced leg glucose uptake in both groups, with a significantly greater reduction (P = 0.04) in the diabetic group (75 +/- 13%, 5 min after L-NMMA) compared with the control group (34 +/- 14%, 5 min after L-NMMA). These data suggest a greater reliance on NO for glucose uptake during exercise in individuals with type 2 diabetes compared with control subjects.

Invited review: Effects of acute exercise and exercise training on insulin resistance

Insulin resistance of skeletal muscle glucose transport is a key defect in the development of impaired glucose tolerance and Type 2 diabetes. It is well established that both an acute bout of exercise and chronic endurance exercise training can have beneficial effects on insulin action in insulin-resistant states. This review summarizes the present state of knowledge regarding these effects in the obese Zucker rat, a widely used rodent model of obesity-associated insulin resistance, and in insulin-resistant humans with impaired glucose tolerance or Type 2 diabetes. A single bout of prolonged aerobic exercise (30-60 min at approximately 60-70% of maximal oxygen consumption) can significantly lower plasma glucose levels, owing to normal contraction-induced stimulation of GLUT-4 glucose transporter translocation and glucose transport activity in insulin-resistant skeletal muscle. However, little is currently known about the effects of acute exercise on muscle insulin signaling in the postexercise state in insulin-resistant individuals. A well-established adaptive response to exercise training in conditions of insulin resistance is improved glucose tolerance and enhanced skeletal muscle insulin sensitivity of glucose transport. This training-induced enhancement of insulin action is associated with upregulation of specific components of the glucose transport system in insulin-resistant muscle and includes increased protein expression of GLUT-4 and insulin receptor substrate-1. It is clear that further investigations are needed to further elucidate the specific molecular mechanisms underlying the beneficial effects of acute exercise and exercise training on the glucose transport system in insulin-resistant mammalian skeletal muscle.

Lactate induces insulin resistance in skeletal muscle by suppressing glycolysis and impairing insulin signaling

Elevation of plasma lactate levels induces peripheral insulin resistance, but the underlying mechanisms are unclear. We examined whether lactate infusion in rats suppresses glycolysis preceding insulin resistance and whether lactate-induced insulin resistance is accompanied by altered insulin signaling and/or insulin-stimulated glucose transport in skeletal muscle. Hyperinsulinemic euglycemic clamps were conducted for 6 h in conscious, overnight-fasted rats with or without lactate infusion (120 micromol x kg(-1) x min(-1)) during the final 3.5 h. Lactate infusion increased plasma lactate levels about fourfold. The elevation of plasma lactate had rapid effects to suppress insulin-stimulated glycolysis, which clearly preceded its effect to decrease insulin-stimulated glucose uptake. Both submaximal and maximal insulin-stimulated glucose transport decreased 25-30% (P < 0.05) in soleus but not in epitrochlearis muscles of lactate-infused rats. Lactate infusion did not alter insulin's ability to phosphorylate the insulin receptor, the insulin receptor substrate (IRS)-1, or IRS-2 but decreased insulin's ability to stimulate IRS-1- and IRS-2-associated phosphatidylinositol 3-kinase activities and Akt/protein kinase B activity by 47, 75, and 55%, respectively (P < 0.05 for all). In conclusion, elevation of plasma lactate suppressed glycolysis before its effect on insulin-stimulated glucose uptake, consistent with the hypothesis that suppression of glucose metabolism could precede and cause insulin resistance. In addition, lactate-induced insulin resistance was associated with impaired insulin signaling and decreased insulin-stimulated glucose transport in skeletal muscle.

Impaired ('diabetic') insulin signaling and action occur in fat cells long before glucose intolerance--is insulin resistance initiated in the adipose tissue?

This review postulates and presents recent evidence that insulin resistance is initiated in the adipose tissue and also suggests that the adipose tissue may play a pivotal role in the induction of insulin resistance in the muscles and the liver. Marked impairments in insulin's intracellular signaling cascade are present in fat cells from type 2 diabetic patients, including reduced IRS-1 gene and protein expression, impaired insulin-stimulated PI3-kinase and PKB/Akt activities. In contrast, upstream insulin signaling in skeletal muscle from diabetic subjects only shows modest impairments and PKB/Akt activation in vivo by insulin appears normal. However, insulin-stimulated glucose transport and glycogen synthesis are markedly reduced. Similar marked impairments in insulin signaling, including reduced IRS-1 expression, impaired insulin-stimulated PI3-kinase and PKB/Akt activities are also seen in some (approximately 30%) normoglycemic individuals with genetic predisposition for type 2 diabetes. In addition, GLUT4 expression is markedly reduced in these cells, similar to what is seen in diabetic cells. The individuals with reduced cellular expression of IRS-1 and GLUT4 are also markedly insulin resistant and exhibit several characteristics of the Insulin Resistance Syndrome.Thus, a 'diabetic' pattern is seen in the fat cells also in normoglycemic subjects and this is associated with a marked insulin resistance in vivo. It is proposed that insulin resistance and/or its effectors is initiated in fat cells and that this may secondarily encompass other target tissues for insulin, including the impaired glucose transport in the muscles.

Inhibition of glycogen synthesis by fatty acid in C(2)C(12) muscle cells is independent of PKC-alpha, -epsilon, and -theta

We have previously shown that glycogen synthesis is reduced in lipid-treated C(2)C(12) skeletal muscle myotubes and that this is independent of changes in glucose uptake. Here, we tested whether mitochondrial metabolism of these lipids is necessary for this inhibition and whether the activation of specific protein kinase C (PKC) isoforms is involved. C(2)C(12) myotubes were pretreated with fatty acids and subsequently stimulated with insulin for the determination of glycogen synthesis. The carnitine palmitoyltransferase-1 inhibitor etomoxir, an inhibitor of beta-oxidation of acyl-CoA, did not protect against the inhibition of glycogen synthesis caused by the unsaturated fatty acid oleate. In addition, although oleate caused translocation, indicating activation, of individual PKC isoforms, inhibition of PKC by pharmacological agents or adenovirus-mediated overexpression of dominant negative PKC-alpha, -epsilon, or -theta mutants was unable to prevent the inhibitory effects of oleate on glycogen synthesis. We conclude that neither mitochondrial lipid metabolism nor activation of PKC-alpha, -epsilon, or -theta plays a role in the direct inhibition of glycogen synthesis by unsaturated fatty acids.

Free fatty acids in obesity and type 2 diabetes: defining their role in the development of insulin resistance and beta-cell dysfunction

Plasma free fatty acids (FFA) play important physiological roles in skeletal muscle, heart, liver and pancreas. However, chronically elevated plasma FFA appear to have pathophysiological consequences. Elevated FFA concentrations are linked with the onset of peripheral and hepatic insulin resistance and, while the precise action in the liver remains unclear, a model to explain the role of raised FFA in the development of skeletal muscle insulin resistance has recently been put forward. Over 30 years ago, Randle proposed that FFA compete with glucose as the major energy substrate in cardiac muscle, leading to decreased glucose oxidation when FFA are elevated. Recent data indicate that high plasma FFA also have a significant role in contributing to insulin resistance. Elevated FFA and intracellular lipid appear to inhibit insulin signalling, leading to a reduction in insulin-stimulated muscle glucose transport that may be mediated by a decrease in GLUT-4 translocation. The resulting suppression of muscle glucose transport leads to reduced muscle glycogen synthesis and glycolysis. In the liver, elevated FFA may contribute to hyperglycaemia by antagonizing the effects of insulin on endogenous glucose production. FFA also affect insulin secretion, although the nature of this relationship remains a subject for debate. Finally, evidence is discussed that FFA represent a crucial link between insulin resistance and beta-cell dysfunction and, as such, a reduction in elevated plasma FFA should be an important therapeutic target in obesity and type 2 diabetes.

Mice deficient in the insulin-regulated membrane aminopeptidase show substantial decreases in glucose transporter GLUT4 levels but maintain normal glucose homeostasis

The insulin-regulated aminopeptidase (IRAP) is a zinc-dependent membrane aminopeptidase. It is the homologue of the human placental leucine aminopeptidase. In fat and muscle cells, IRAP colocalizes with the insulin-responsive glucose transporter GLUT4 in intracellular vesicles and redistributes to the cell surface in response to insulin, as GLUT4 does. To address the question of the physiological function of IRAP, we generated mice with a targeted disruption of the IRAP gene (IRAP-/-). Herein, we describe the characterization of these mice with regard to glucose homeostasis and regulation of GLUT4. Fed and fasted blood glucose and insulin levels in the IRAP-/- mice were normal. Whereas IRAP-/- mice responded to glucose administration like control mice, they exhibited an impaired response to insulin. Basal and insulin-stimulated glucose uptake in extensor digitorum longus muscle, and adipocytes isolated from IRAP-/- mice were decreased by 30-60% but were normal for soleus muscle from male IRAP-/- mice. Total GLUT4 levels were diminished by 40-85% in the IRAP-/- mice in the different muscles and in adipocytes. The relative distribution of GLUT4 in subcellular fractions of basal and insulin-stimulated IRAP-/- adipocytes was the same as in control cells. We conclude that IRAP-/- mice maintain normal glucose homeostasis despite decreased glucose uptake into muscle and fat cells. The absence of IRAP does not affect the subcellular distribution of GLUT4 in adipocytes. However, it leads to substantial decreases in GLUT4 expression.

Mouse models of insulin resistance

The hallmarks of type 2 diabetes are impaired insulin action in peripheral tissues and decreased pancreatic beta-cell function. Classically, the two defects have been viewed as separate entities, with insulin resistance arising primarily from impaired insulin-dependent glucose uptake in skeletal muscle, and beta-cell dysfunction arising from impaired coupling of glucose sensing to insulin secretion. Targeted mutagenesis and transgenesis involving components of the insulin action pathway have changed our understanding of these phenomena. It appears that the role of insulin signaling in the pathogenesis of type 2 diabetes has been overestimated in classic insulin target tissues, such as skeletal muscle, whereas it has been overlooked in liver, pancreatic beta-cells, and brain, which had been thought not to be primary insulin targets. We review recent progress and try to reconcile areas of apparent controversy surrounding insulin signaling in skeletal muscle and pancreatic beta-cells.

Obesity and gender differences in the risk of type 2 diabetes mellitus in Uganda

OBJECTIVE

We assessed the severity of type 2 diabetes in urban and rural communities of Uganda and characterized this disease according to sex, obesity, and hypertension.

METHODS

A total of 440 subjects was tested for high blood and urine glucose levels with the respective glucometers and sample strips. Body mass index and hypertension were determined by measuring height, weight, and blood pressure.

RESULTS

In a random study conducted in the communities of the districts of Kampala and Mokono, the prevalence of type 2 diabetes was found to be about 8.1% (n = 148). An association between obesity, hypertension, and risk of type 2 diabetes was found among the women, of whom nearly 80% were overweight. However, the men, who were primarily lean, did not exhibit this same correlation.

CONCLUSIONS

These epidemiologic data suggest a variance from the strong correlation of type 2 diabetes and obesity seen in Western countries and most of Europe. Specifically, the etiology of this disease in Uganda is different between men and women. The reasons for this phenomenon are not well elucidated, but it is likely that a long history of obesity in women may genetically or environmentally predispose them to this disease.

Effect of misoprostol (PGE1) on glucose metabolism in type-2-diabetic and control subjects

In vitro and in vivo studies have demonstrated that prostaglandins of the E series enhance muscle glucose uptake. We examined the effect of acute misoprostol (PGE1) administration on whole body insulin-mediated glucose disposal, as well as the major intracellular pathways of glucose metabolism in type 2 diabetic (n = 10) and non-diabetic (n = 4) subjects. Each subject received two 240-min euglycaemic insulin (40 mU/m2/min) clamp studies with tritiated glucose and indirect calorimetry. During one of the insulin clamp studies, 200 microg of misoprostol was ingested at 90 and 150 min after the start of the insulin infusion. Insulin-mediated total body glucose disposal, glycolysis, glycogenesis and glucose oxidation were similar during the insulin clamp studies performed without and with misoprostol in both the diabetic and non-diabetic groups. These results demonstrate that the acute administration of misoprostol does not enhance insulin-mediated glucose disposal in either type-2-diabetic or non-diabetic subjects.

Partitioning glucose distribution/transport, disposal, and endogenous production during IVGTT

We have separated the effect of insulin on glucose distribution/transport, glucose disposal, and endogenous production (EGP) during an intravenous glucose tolerance test (IVGTT) by use of a dual-tracer dilution methodology. Six healthy lean male subjects (age 33 +/- 3 yr, body mass index 22.7 +/- 0.6 kg/m(2)) underwent a 4-h IVGTT (0.3 g/kg glucose enriched with 3-6% D-[U-(13)C]glucose and 5-10% 3-O-methyl-D-glucose) preceded by a 2-h investigation under basal conditions (5 mg/kg of D-[U-(13)C]glucose and 8 mg/kg of 3-O-methyl-D-glucose). A new model described the kinetics of the two glucose tracers and native glucose with the use of a two-compartment structure for glucose and a one-compartment structure for insulin effects. Insulin sensitivities of distribution/transport, disposal, and EGP were similar (11.5 +/- 3.8 vs. 10.4 +/- 3.9 vs. 11.1 +/- 2.7 x 10(-2) ml small middle dot kg(-1) small middle dot min(-1) per mU/l; P = nonsignificant, ANOVA). When expressed in terms of ability to lower glucose concentration, stimulation of disposal and stimulation of distribution/transport accounted each independently for 25 and 30%, respectively, of the overall effect. Suppression of EGP was more effective (P < 0.01, ANOVA) and accounted for 50% of the overall effect. EGP was suppressed by 70% (52-82%) (95% confidence interval relative to basal) within 60 min of the IVGTT; glucose distribution/transport was least responsive to insulin and was maximally activated by 62% (34-96%) above basal at 80 min compared with maximum 279% (116-565%) activation of glucose disposal at 20 min. The deactivation of glucose distribution/transport was slower than that of glucose disposal and EGP (P < 0.02) with half-times of 207 (84-510), 12 (7-22), and 29 (16-54) min, respectively. The minimal-model insulin sensitivity was tightly correlated with and linearly related to sensitivity of EGP (r = 0.96, P < 0.005) and correlated positively but nonsignificantly with distribution/transport sensitivity (r = 0.73, P = 0.10) and disposal sensitivity (r = 0.55, P = 0.26). We conclude that, in healthy subjects during an IVGTT, the two peripheral insulin effects account jointly for approximately one-half of the overall insulin-stimulated glucose lowering, each effect contributing equally. Suppression of EGP matches the effect in the periphery.

Insulin resistance: concepts, controversies, and the role of nutrition

Insulin resistance is a prevalent condition, in which insulin loses its normal physiological action. Since people were first classified as insulin resistant over 60 years ago, one of the main discoveries has been that insulin resistance clusters with other risk factors such as obesity, elevated triglycerides, and low high-density lipoprotein cholesterol, increasing cardiovascular disease risk. Although insulin resistance appears to manifest first in the periphery and then in the liver, other sites, such as the brain and the pancreatic &b.beta-cell, may play pathogenic roles. Factors contributing to insulin resistance at these sites include perturbations in free fatty acids, glucose, and hormone-signalling, some of which may be linked to various genetic polymorphisms. Appropriate nutritional treatment for insulin resistance is controversial. Two main approaches are drawn from diabetes recommendations: i) a high-carbohydrate, low-fat, high-fibre diet emphasizing low glycemic-index foods and ii) sharing calories between monounsaturated fat and complex carbohydrate at the expense of saturated fat. Recent interest in insulin resistance has prompted the development of new guidelines. Promising data have also emerged, showing that a high-carbohydrate, high-fibre, low-fat diet plus exercise programs maintained through intensive counselling can decrease diabetes risk by over 40%. Additional research is required to confirm the sustainability of this approach and sort out the determinants of insulin resistance so that more effective nutritional interventions will result.

Indinavir inhibits the glucose transporter isoform Glut4 at physiologic concentrations

OBJECTIVES

To determine the relative sensitivities of glucose transporter isoforms to the protease inhibitor indinavir and to determine the kinetic mechanism of indinavir-mediated Glut4 isoform inhibition.

METHODS

The rate of 2-deoxyglucose uptake was measured in Xenopus laevis oocytes heterologously expressing mammalian Glut isoforms. 2-Deoxyglucose uptake was also measured in 3T3-L1 fibroblasts, 3T3-L1 adipocytes, and primary rat adipocytes.

RESULTS

The sensitivity to inhibition by indinavir among the Glut isoforms as assayed in the X. laevis oocyte system was as follows in decreasing order: Glut4 >> Glut2 > Glut3 > Glut1 approximately Glut8. 2-Deoxyglucose uptake measurements in insulin-stimulated primary rat adipocytes indicated a non-competitive mode of transport inhibition by indinavir under zero-trans conditions with a KI of 15 microM.

CONCLUSIONS

Indinavir appears to be a relatively selective inhibitor of the Glut4 isoform. As the concentration required to significantly inhibit insulin-stimulated glucose uptake in primary rat adipocytes is well within the physiologic range achieved in therapy, we conclude that direct inhibition of Glut4 contributes to the insulin resistance observed in patients receiving this drug.

Indinavir induces acute and reversible peripheral insulin resistance in rats

The use of HIV protease inhibitors (PIs) has been associated with several metabolic changes, including lipodystrophy, hyperlipidemia, and insulin resistance. The etiology of these adverse effects remains unknown. PIs have recently been found to cause acute and reversible inhibition of GLUT4 activity in vitro. To determine the acute in vivo effects of indinavir on whole-body glucose homeostasis, glucose tolerance tests were performed on PI-naïve Wistar rats immediately after a single intravenous dose of indinavir. Glucose and insulin levels were significantly elevated in indinavir-treated versus control rats (P < 0.05) during the initial 30 min of the glucose tolerance test. Under euglycemic- hyperinsulinemic clamp conditions, indinavir treatment acutely reduced the glucose infusion rate required to maintain euglycemia by 18 and 49% at indinavir concentrations of 14 and 27 micromol/l, respectively. Muscle 2-deoxyglucose uptake was similarly reduced under these conditions. Restoration of insulin sensitivity was observed within 4 h after stopping the indinavir infusion. Indinavir did not alter the suppression of hepatic glucose output under hyperinsulinemic conditions. These data demonstrate that indinavir causes acute and reversible changes in whole-body glucose homeostasis in rats and support the contribution of GLUT4 inhibition to the development of insulin resistance in patients treated with PIs.

N-3 polyunsaturated fatty acids prevent the defect of insulin receptor signaling in muscle

A high-fat diet containing polyunsaturated fatty acids (PUFA: n-3 or n-6) given for 4 wk to 5-wk-old male Wistar rats induced a clear hyperglycemia (10.4 +/- 0.001 mmol/l for n-6 rats and 10.1 +/- 0.001 for n-3 rats) and hyperinsulinemia (6.6 +/- 0.8 ng/ml for n-6 rats and 6.4 +/- 1.3 for n-3 rats), signs of insulin resistance. In liver, both diets (n-3 and n-6) significantly reduced insulin receptor (IR) number, IR and IR substrate (IRS)-1 tyrosine phosphorylation, and phosphatidylinositol (PI) 3'-kinase activity. In contrast, in leg muscle, IR density, as determined by Western blotting, was not affected, whereas IR and IRS-1 tyrosine phosphorylation in response to insulin treatment was restored in animals fed with n-3 PUFA to normal; in n-6 PUFA, the phosphorylation was depressed, as evidenced by Western blot analysis using specific antibodies. In addition, PI 3'-kinase activity and GLUT-4 content in muscle were maintained at normal levels in rats fed with n-3 PUFA compared with rats fed a normal diet. In rats fed with n-6 PUFA, both PI 3'-kinase activity and GLUT-4 content were reduced. Furthermore, in adipose tissue and using RT-PCR, we show that both n-3 and n-6 PUFA led to slight or strong reductions in p85 expression, respectively, whereas GLUT-4 and leptin expression was depressed in n-6 rats. The expression was not affected in n-3 rats compared with control rats. In conclusion, a high-fat diet enriched in n-3 fatty acids maintained IR, IRS-1 tyrosine phosphorylation, and PI 3'-kinase activity and total GLUT-44 content in muscle but not in liver. A high-fat diet (n-3) partially altered the expression of p85 but not that of GLUT-4 and leptin mRNAs in adipose tissue.

Mechanism of amino acid-induced skeletal muscle insulin resistance in humans

Plasma concentrations of amino acids are frequently elevated in insulin-resistant states, and a protein-enriched diet can impair glucose metabolism. This study examined effects of short-term plasma amino acid (AA) elevation on whole-body glucose disposal and cellular insulin action in skeletal muscle. Seven healthy men were studied for 5.5 h during euglycemic (5.5 mmol/l), hyperinsulinemic (430 pmol/l), fasting glucagon (65 ng/l), and growth hormone (0.4 microg/l) somatostatin clamp tests in the presence of low (approximately 1.6 mmol/l) and increased (approximately 4.6 mmol/l) plasma AA concentrations. Glucose turnover was measured with D-[6,6-(2)H(2)]glucose. Intramuscular concentrations of glycogen and glucose-6-phosphate (G6P) were monitored using (13)C and (31)P nuclear magnetic resonance spectroscopy, respectively. A approximately 2.1-fold elevation of plasma AAs reduced whole-body glucose disposal by 25% (P < 0.01). Rates of muscle glycogen synthesis decreased by 64% (180--315 min, 24 plus minus 3; control, 67 plus minus 10 micromol center dot l(-1) center dot min(-1); P < 0.01), which was accompanied by a reduction in G6P starting at 130 min (DeltaG6P(260--300 min), 18 plus minus 19; control, 103 plus minus 33 micromol/l; P < 0.05). In conclusion, plasma amino acid elevation induces skeletal muscle insulin resistance in humans by inhibition of glucose transport/phosphorylation, resulting in marked reduction of glycogen synthesis.

Regulation of glucose transport in human skeletal muscle

Glucose transport, the rate limiting step in glucose metabolism in skeletal muscle, is mediated by insulin-sensitive glucose transporter 4 (GLUT4) and can be activated in skeletal muscle by two separate and distinct signalling pathways: one stimulated by insulin and the second by muscle contractions. Skeletal muscle is the principal tissue responsible for insulin-stimulated glucose disposal and thus the major site of peripheral insulin resistance. Impaired glucose transport in skeletal muscle leads to impaired whole body glucose uptake, and contributes to the pathogenesis of Type 2 diabetes mellitus. A combination of genetic and environmental factors is likely to contribute to the pathogenesis of Type 2 diabetes mellitus; however, the primary defect is still unknown. Intense efforts are underway to define the molecular mechanisms that regulate glucose metabolism in insulin sensitive tissues. This review will present our current understanding of mechanisms regulating glucose transport in skeletal muscle in humans. Elucidation of the pathways involved in the regulation of glucose homeostasis will offer insight into the pathogenesis of insulin resistance and Type 2 diabetes mellitus and may lead to the identification of biochemical entry points for drug intervention to improve glucose homeostasis.

HIV inhibitors: problems and reality

The human immunodeficiency virus encodes three replication enzymes, which are required for a productive life-cycle. Currently, several anti-retroviral drugs are available for clinical use, and they are inhibitors of either the reverse transcriptase or the viral protease. The introduction of combination anti-retroviral therapy (HAART) changed the prognosis of HIV infection. However, current therapy is not able to eradicate the virus, only suppress it; therefore, long-term use of the drugs is required to keep the viral load under control. Most of the problems associated with the HIV therapy are the consequence of the necessarily long-term use of the drugs. The long-term effectiveness of current inhibitors as therapeutic agents is limited by the rapid development of drug-resistant variants. Furthermore, various side effects have been reported. These side effects include hypersensitivity, mitochondrial toxicity, lypodystrophy syndrome, insulin resistance and cardiovascular disorders. Further drug development is necessary to design new compounds that have efficacy similar to the currently used drugs in the management of HIV infection and that are potent against the resistant viruses but do not exhibit unwanted metabolic side effects.

Plasma fatty acids, adiposity, and variance of skeletal muscle insulin resistance in type 2 diabetes mellitus

Skeletal muscle insulin resistance (IR) is typically severe in type 2 diabetes mellitus (DM). However, the factors that account for interindividual differences in the severity of IR are not well understood. The current study was undertaken to examine the respective roles of plasma FFA, regional adiposity, and other metabolic factors as determinants of the severity of skeletal muscle IR in type 2 DM. Twenty-three subjects (12 women and 11 men) with type 2 DM underwent positron emission tomography imaging using [18F]2-fluoro-2-deoxyglucose during euglycemic insulin infusions (120 mU/min x m2) to measure skeletal muscle IR, using Patlak analysis of the tissue activity curves. Body composition analysis included body mass index, fat mass, and fat-free mass by dual energy x-ray tomography, and computed tomography determinations of visceral adiposity, thigh adipose tissue distribution, and muscle composition. Body mass index, fat mass, subfascial adiposity in the thigh, and visceral adipose tissue (VAT) were all significantly related to skeletal muscle IR (r = -0.48 to -0.63; P < 0.01). However, the strongest simple correlate of IR in skeletal muscle was insulin-suppressed plasma FFA (r = -0.81; P < 0.001). VAT was the sole component of adiposity that significantly correlated with insulin-suppressed plasma FFA concentration (r = 0.64; P < 0.001). These findings indicate that the severity of skeletal muscle IR in type 2 DM is closely related to the IR of suppressing lipolysis and that plasma fatty acids and VAT are key elements mediating the link between obesity and skeletal muscle IR in type 2 DM.

Idiopathic Type 1 diabetes

The purpose of this article is to review the available pathophysiological and clinical studies of patients with idiopathic Type 1 diabetes. Idiopathic Type 1 diabetes is a common form of diabetes most commonly seen in obese African American individuals living in large urban areas. This type of diabetes usually presents with the typical signs and symptoms of Type 1 diabetes such as diabetic ketoacidosis but its subsequent clinical course often resembles Type 2 diabetes. The natural history and pathogenesis of idiopathic Type 1 diabetes is unknown because most of these studies on these patients were done shortly after diagnosis. For the most part, these patients have been treated as if they had Type 2 diabetes with diet and/or oral agents but recent studies suggest that patients have a very variable response to diet and oral agents. They seem to have better long-term glycemic control with the use of insulin therapy. Although the pathogenesis of this disease is unknown, it may be related to lipotoxicity, glucose toxicity or transcription factors involved in fuel metabolism.

Management of the insulin resistance syndrome

The insulin resistance syndrome (IRS) is a common disorder, which has important clinical implications. It is a cluster of cardiovascular risk factors that include obesity, hypertension, dyslipidemia, glucose intolerance, and type 2 diabetes mellitus. Lifestyle modifications and insulin sensitizers are among the several therapeutic strategies available for the treatment of the IRS. Optimal treatment will not only improve glycemic control, but may also significantly lower cardiovascular disease.

Interactions of impaired glucose transport and phosphorylation in skeletal muscle insulin resistance: a dose-response assessment using positron emission tomography

It has been postulated that glucose transport is the principal site of skeletal muscle insulin resistance in obesity and type 2 diabetes, though a distribution of control between glucose transport and phosphorylation has also been proposed. The current study examined whether the respective contributions of transport and phosphorylation to insulin resistance are modulated across a dose range of insulin stimulation. Rate constants for transport and phosphorylation in skeletal muscle were estimated using dynamic positron emission tomography (PET) imaging of 2-deoxy-2[18F]fluoro-D-glucose ([18F]FDG) during insulin infusions at three rates (0, 40, and 120 mU/m2 per min) in lean glucose-tolerant, obese glucose-tolerant, and obese type 2 diabetic subjects. Parallel studies of arteriovenous fractional extraction across the leg of [18F]FDG and [2-3H] glucose were performed to measure the "lumped constant" (LC) (i.e., the analog effect) for [18F]FDG to determine whether this value is affected by insulin dose or insulin resistance. The value of the LC was similar across insulin doses and groups. Leg glucose uptake (LGU) also provided a measure of skeletal muscle glucose metabolism independent of PET. [18F]FDG uptake determined by PET imaging strongly correlated with LGU across groups and across insulin doses (r = 0.81, P < 0.001). Likewise, LGU correlated with PET parameters of glucose transport (r = 0.67, P < 0.001) and glucose phosphorylation (r = 0.86, P < 0.001). Glucose transport increased in response to insulin in the lean and obese groups (P < 0.05), but did not increase significantly in the type 2 diabetic group. A dose-responsive pattern of stimulation of glucose phosphorylation was observed in all groups of subjects (P < 0.05); however, glucose phosphorylation was lower in both the obese and type 2 diabetic groups compared with the lean group at the moderate insulin dose (P < 0.05). These findings indicate an important interaction between transport and phosphorylation in the insulin resistance of obesity and type 2 diabetes.

Sequence and functional analysis of GLUT10: a glucose transporter in the Type 2 diabetes-linked region of chromosome 20q12-13.1

We have carried out a detailed sequence and functional analysis of a novel human facilitative glucose transporter, designated GLUT10, located in the Type 2 diabetes-linked region of human chromosome 20q12-13.1. The GLUT10 gene is located between D20S888 and D20S891 and is encoded by 5 exons spanning 26.8 kb of genomic DNA. The human GLUT10 cDNA encodes a 541 amino acid protein that shares between 31 and 35% amino acid identity with human GLUT1-8. The predicted amino acid sequence of GLUT10 is nearly identical in length to the recently described GLUT9 homologue, but is longer than other known members of the GLUT family. In addition, we have cloned the mouse cDNA homolog of GLUT10 that encodes a 537 amino acid protein that shares 77.3% identity with human GLUT10. The amino acid sequence probably has 12 predicted transmembrane domains and shares characteristics of other mammalian glucose transporters. Human and mouse GLUT10 retain several sequence motifs characteristic of mammalian glucose transporters including VP497ETKG in the cytoplasmic C-terminus, G73R[K,R] between TMD2 and TMD3 (PROSITE PS00216), VD92RAGRR between TMD8 and TMD9 (PROSITE PS00216), Q242QLTG in TMD7, and tryptophan residues W430 (TMD10) and W454 (TMD11), that correspond to trytophan residues previously implicated in GLUT1 cytochalasin B binding and hexose transport. Neither human nor mouse GLUT10 retains the full P[E,D,N]SPR motif after Loop6 but instead is replaced with P186AG[T,A]. A PROSITE search also shows that GLUT10 has lost the SUGAR TRANSPORT 2 pattern (PS00217), a result of the substitution G113S in TMD4, while all other known human GLUTs retain the glycine and the pattern match. The significance of this substitution is unknown. Sites for N-linked glycosylation are predicted at N334ATG between TMD8 and TMD9 and N526STG in the cytoplasmic C-terminus. Northern hybridization analysis identified a single 4.4-kb transcript for GLUT10 in human heart, lung, brain, liver, skeletal muscle, pancreas, placenta, and kidney. By RT-PCR analysis, GLUT10 mRNA was also detected in fetal brain and liver. When expressed in Xenopus oocytes, human GLUT10 exhibited 2-deoxy-D-glucose transport with an apparent Km of approximately 0.3 mM. D-Glucose and D-galactose competed with 2-deoxy-D-glucose and transport was inhibited by phloretin. The gene localization and functional properties suggest a role for GLUT10 in glucose metabolism and Type 2 diabetes.

A hydroxychalcone derived from cinnamon functions as a mimetic for insulin in 3T3-L1 adipocytes

OBJECTIVES

These studies investigated the ability of a hydroxychalcone from cinnamon to function as an insulin mimetic in 3T3-LI adipocytes.

METHODS

Comparative experiments were performed with the cinnamon methylhydroxychalcone polymer and insulin with regard to glucose uptake, glycogen synthesis. phosphatidylinositol-3-kinase dependency, glycogen synthase activation and glycogen synthase kinase-3beta activity. The phosphorylation state of the insulin receptor was also investigated.

RESULTS

MHCP treatment stimulated glucose uptake and glycogen synthesis to a similar level as insulin. Glycogen synthesis was inhibited by both wortmannin and LY294002, inhibitors directed against the PI-3-kinase. In addition, MHCP treatment activated glycogen synthase and inhibited glycogen synthase kinase-3beta activities, known effects of insulin treatment. Analysis of the insulin receptor demonstrated that the receptor was phosphorylated upon exposure to the MHCP. This supports that the insulin cascade was triggered by MHCP. Along with comparing MHCP to insulin, experiments were done with MHCP and insulin combined. The responses observed using the dual treatment were greater than additive, indicating synergism between the two compounds.

CONCLUSION

Together, these results demonstrate that the MHCP is an effective mimetic of insulin. MHCP may be useful in the treatment of insulin resistance and in the study of the pathways leading to glucose utilization in cells.

Prevention of fat-induced insulin resistance by salicylate

Insulin resistance is a major factor in the pathogenesis of type 2 diabetes and may involve fat-induced activation of a serine kinase cascade involving IKK-beta. To test this hypothesis, we first examined insulin action and signaling in awake rats during hyperinsulinemic-euglycemic clamps after a lipid infusion with or without pretreatment with salicylate, a known inhibitor of IKK-beta. Whole-body glucose uptake and metabolism were estimated using [3-(3)H]glucose infusion, and glucose uptake in individual tissues was estimated using [1-(14)C]2-deoxyglucose injection during the clamp. Here we show that lipid infusion decreased insulin-stimulated glucose uptake and activation of IRS-1-associated PI 3-kinase in skeletal muscle but that salicylate pretreatment prevented these lipid-induced effects. To examine the mechanism of salicylate action, we studied the effects of lipid infusion on insulin action and signaling during the clamp in awake mice lacking IKK-beta. Unlike the response in wild-type mice, IKK-beta knockout mice did not exhibit altered skeletal muscle insulin signaling and action following lipid infusion. In summary, high-dose salicylate and inactivation of IKK-beta prevent fat-induced insulin resistance in skeletal muscle by blocking fat-induced defects in insulin signaling and action and represent a potentially novel class of therapeutic agents for type 2 diabetes.

A high-resolution 6.0-megabase transcript map of the type 2 diabetes susceptibility region on human chromosome 20

Recent linkage studies and association analyses indicate the presence of at least one type 2 diabetes susceptibility gene in human chromosome region 20q12-q13.1. We have constructed a high-resolution 6.0-megabase (Mb) transcript map of this interval using two parallel, complementary strategies to construct the map. We assembled a series of bacterial artificial chromosome (BAC) contigs from 56 overlapping BAC clones, using STS/marker screening of 42 genes, 43 ESTs, 38 STSs, 22 polymorphic, and 3 BAC end sequence markers. We performed map assembly with GraphMap, a software program that uses a greedy path searching algorithm, supplemented with local heuristics. We anchored the resulting BAC contigs and oriented them within a yeast artificial chromosome (YAC) scaffold by observing the retention patterns of shared markers in a panel of 21 YAC clones. Concurrently, we assembled a sequence-based map from genomic sequence data released by the Human Genome Project, using a seed-and-walk approach. The map currently provides near-continuous coverage between SGC32867 and WI-17676 ( approximately 6.0 Mb). EST database searches and genomic sequence alignments of ESTs, mRNAs, and UniGene clusters enabled the annotation of the sequence interval with experimentally confirmed and putative transcripts. We have begun to systematically evaluate candidate genes and novel ESTs within the transcript map framework. So far, however, we have found no statistically significant evidence of functional allelic variants associated with type 2 diabetes. The combination of the BAC transcript map, YAC-to-BAC scaffold, and reference Human Genome Project sequence provides a powerful integrated resource for future genomic analysis of this region.

Caveolae and the caveolins in human disease

There has been an exponential growth in caveolae research since the early 1990s. The caveolae membrane system comprises unique lipid and protein domains, and fulfills a role in a wide range of processes. At the plasma membrane caveolae serve to compartmentalise and integrate a wide range of signal transduction processes. A key structural and functional protein for caveolae is caveolin. Caveolin proteins possess a 'scaffolding' domain which for caveolins-1 and -3 appear central to many of the reported signal regulation functions for caveolae. Caveolae or caveolin protein are increasingly implicated in the molecular pathology of a number of diseases. Opportunities exist for basic and applied investigators working within the pharmaceutical sciences to exploit the caveolae membrane system to identify novel pharmacological targets and therapeutic strategies, including the delivery of pharmacologically active caveolin based peptides.

Glucose toxicity and the development of diabetes in mice with muscle-specific inactivation of GLUT4

Using cre/loxP gene targeting, transgenic mice with muscle-specific inactivation of the GLUT4 gene (muscle GLUT4 KO) were generated and shown to develop a diabetes phenotype. To determine the mechanism, we examined insulin-stimulated glucose uptake and metabolism during hyperinsulinemic-euglycemic clamp in control and muscle GLUT4 KO mice before and after development of diabetes. Insulin-stimulated whole body glucose uptake was decreased by 55% in muscle GLUT4 KO mice, an effect that could be attributed to a 92% decrease in insulin-stimulated muscle glucose uptake. Surprisingly, insulin's ability to stimulate adipose tissue glucose uptake and suppress hepatic glucose production was significantly impaired in muscle GLUT4 KO mice. To address whether these latter changes were caused by glucose toxicity, we treated muscle GLUT4 KO mice with phloridzin to prevent hyperglycemia and found that insulin-stimulated whole body and skeletal muscle glucose uptake were decreased substantially, whereas insulin-stimulated glucose uptake in adipose tissue and suppression of hepatic glucose production were normal after phloridzin treatment. In conclusion, these findings demonstrate that a primary defect in muscle glucose transport can lead to secondary defects in insulin action in adipose tissue and liver due to glucose toxicity. These secondary defects contribute to insulin resistance and to the development of diabetes.

Non-invasive studies of glycogen metabolism in human skeletal muscle using nuclear magnetic resonance spectroscopy

Nuclear magnetic resonance spectroscopy provides non-invasive and real-time assessment of the metabolic fluxes in skeletal muscle during exercise, recovery from exercise and stimulation by insulin. Carbon-13 nuclear magnetic resonance spectroscopy has proved that reduced glycogen synthesis is a consistent feature of insulin-resistant type 2 diabetic patients, their offspring, and obesity. Low intracellular glucose and glucose-6-phosphate concentrations indicate that decreased glucose transport is mainly responsible for common insulin resistance. An elevation of plasma free fatty acids causes similar alterations of muscle glucose metabolism, and could play a central role in the development of impaired muscle glucose transport associated with insulin resistance.

Glucose uptake and metabolism by cultured human skeletal muscle cells: rate-limiting steps

To use primary cultures of human skeletal muscle cells to establish defects in glucose metabolism that underlie clinical insulin resistance, it is necessary to define the rate-determining steps in glucose metabolism and to improve the insulin response attained in previous studies. We modified experimental conditions to achieve an insulin effect on 3-O-methylglucose transport that was more than twofold over basal. Glucose phosphorylation by hexokinase limits glucose metabolism in these cells, because the apparent Michaelis-Menten constant of coupled glucose transport and phosphorylation is intermediate between that of transport and that of the hexokinase and because rates of 2-deoxyglucose uptake and phosphorylation are less than those of glucose. The latter reflects a preference of hexokinase for glucose over 2-deoxyglucose. Cellular NAD(P)H autofluorescence, measured using two-photon excitation microscopy, is both sensitive to insulin and indicative of additional distal control steps in glucose metabolism. Whereas the predominant effect of insulin in human skeletal muscle cells is to enhance glucose transport, phosphorylation, and steps beyond, it also determines the overall rate of glucose metabolism.

13C NMR of intermediary metabolism: implications for systemic physiology

The study of intermediary metabolism in biomolecules has been given new directions by recent experiments in human muscle and brain by 13C NMR. Labeled substrates, generally glucose, have enabled the fluxes to be determined in vivo, whereas the naturally abundant 13C has enabled concentrations to be measured. In muscle the glycogen synthesis pathway has been measured and the flux control determined by metabolic control analysis of data, which shows that this pathway is mainly responsible for insulin-stimulated glucose disposal and that a deficiency in the glucose transporter in the pathway is responsible for hyperglycemia in non-insulin-dependent diabetics. From a physiological point of view the most surprising result was that the heavily regulated allosteric enzyme, glycogen synthase, does not control flux but is needed to maintain homeostasis during flux changes. This novel role for a phosphorylated allosteric enzyme is proposed to be a general phenomenon in metabolic and signaling pathways, which physiologically link different cellular activities. In human and rat brains 13C NMR measurements of the flow of labeled glucose into glutamate and glutamine simultaneously determine the rate of glucose oxidation and glutamate neurotransmitter cycling and reveal a 1:1 stoichiometry between the two fluxes. Implications for the interpretation of functional imaging studies and for psychology are discussed. These results demonstrate how intermediary metabolism serves to connect biochemistry with systemic physiology when measured and analyzed by in vivo NMR methods.

Tissue-specific overexpression of lipoprotein lipase causes tissue-specific insulin resistance

Insulin resistance in skeletal muscle and liver may play a primary role in the development of type 2 diabetes mellitus, and the mechanism by which insulin resistance occurs may be related to alterations in fat metabolism. Transgenic mice with muscle- and liver-specific overexpression of lipoprotein lipase were studied during a 2-h hyperinsulinemic-euglycemic clamp to determine the effect of tissue-specific increase in fat on insulin action and signaling. Muscle-lipoprotein lipase mice had a 3-fold increase in muscle triglyceride content and were insulin resistant because of decreases in insulin-stimulated glucose uptake in skeletal muscle and insulin activation of insulin receptor substrate-1-associated phosphatidylinositol 3-kinase activity. In contrast, liver-lipoprotein lipase mice had a 2-fold increase in liver triglyceride content and were insulin resistant because of impaired ability of insulin to suppress endogenous glucose production associated with defects in insulin activation of insulin receptor substrate-2-associated phosphatidylinositol 3-kinase activity. These defects in insulin action and signaling were associated with increases in intracellular fatty acid-derived metabolites (i.e., diacylglycerol, fatty acyl CoA, ceramides). Our findings suggest a direct and causative relationship between the accumulation of intracellular fatty acid-derived metabolites and insulin resistance mediated via alterations in the insulin signaling pathway, independent of circulating adipocyte-derived hormones.

[Diabetes mellitus, inflammation and coronary atherosclerosis: current and future perspectives]

Type 2 diabetes mellitus is a condition associated with an increased risk of coronary artery disease. This condition is currently reaching epidemic proportions in the Western world. Epidemiological studies have shown that insulin resistance and the constellation of metabolic alterations associated with type 2 diabetes mellitus such as dyslipidaemia, systemic hypertension, obesity and hypercoagulability, have an effect on the premature onset and severity of atherosclerosis. Albeit direct, the link between insulin resistance and atherogenesis is rather complex. It is likely that its complexity relates to the interaction between genes that predispose to insulin resistance and genes that independently regulate lipid metabolism, coagulation processes and biological responses of the arterial wall. The rapid development of molecular biology in recent years has resulted in a better understanding of the immune and inflammatory mechanisms that underlie insulin resistance and atherosclerosis. For example, it is known that nuclear transcription factors such as nuclear factor kappa beta and peroxisome proliferator-activated receptor are involved in atherosclerosis. The former modulates gene expression which encodes pro-inflammatory proteins vital for the development of the atheromatous plaque. In the presence of insulin resistance there are multiple activating factors that could explain the early onset and severity of atherosclerosis. Glitazones, the new oral antidiabetic drugs and agonists of peroxisome proliferator-activated receptor, have been shown to improve peripheral insulin sensitivity and to also delay atherosclerosis progression in experimental models. Their beneficial effects have been linked to their anti-inflammatory effect.

Insulin resistance: cellular and clinical concepts

Insulin resistance is defined as a clinical state in which a normal or elevated insulin level produces an attenuated biologic response. Specifically, the biologic response most studied is insulin-stimulated glucose disposal, yet the precise cellular mechanism responsible is not yet known. However, the presence of insulin resistance is observed many years before the onset of clinical hyperglycemia and the diagnosis of Type 2 diabetes. Insulin resistance at this stage appears to be significantly associated with a clustering of cardiovascular risk factors predisposing the individual to accelerated cardiovascular disease. An overview of insulin resistance and the associated clinical insulin resistant state will be discussed.

Effect of 5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside infusion on in vivo glucose and lipid metabolism in lean and obese Zucker rats

Activation of AMP-activated protein kinase (AMPK) with 5-aminoimidazole-4-carboxamide-1-beta-D-ribofurano-side (AICAR) increases glucose transport in skeletal muscle via an insulin-independent pathway. To examine the effects of AMPK activation on skeletal muscle glucose transport activity and whole-body carbohydrate and lipid metabolism in an insulin-resistant rat model, awake obese Zuckerfa/fa rats (n = 26) and their lean (n = 23) littermates were infused for 90 min with AICAR, insulin, or saline. The insulin infusion rate (4 mU.kg(-1).min(-1)) was selected to match the glucose requirements during AICAR (bolus, 100 mg/kg; constant, 10 mg.kg(-1).min(-1)) isoglycemic clamps in the lean rats. The effects of these identical AICAR and insulin infusion rates were then examined in the obese Zucker rats. AICAR infusion increased muscle AMPK activity more than fivefold (P < 0.01 vs. control and insulin) in both lean and obese rats. Plasma triglycerides, fatty acid concentrations, and glycerol turnover, as assessed by [2-13C]glycerol, were all decreased in both lean and obese rats infused with AICAR (P < 0.05 vs. basal), whereas insulin had no effect on these parameters in the obese rats. Endogenous glucose production rates, measured by [U-13C]glucose, were suppressed by >50% during AICAR and insulin infusions in both lean and obese rats (P < 0.05 vs. basal). In lean rats, rates of whole-body glucose disposal increased by more than two-fold (P < 0.05 vs. basal) during both AICAR and insulin infusion; [3H]2-deoxy-D-glucose transport activity increased to a similar extent, by >2.2-fold (both P < 0.05 vs. control), in both soleus and red gastrocnemius muscles of lean rats infused with either AICAR or insulin. In the obese Zucker rats, neither AICAR nor insulin stimulated whole-body glucose disposal or soleus muscle glucose transport activity. However, AICAR increased glucose transport activity by approximately 2.4-fold (P < 0.05 vs. control) in the red gastrocnemius from obese rats, whereas insulin had no effect. In summary, acute infusion of AICAR in an insulin-resistant rat model activates skeletal muscle AMPK and increases glucose transport activity in red gastrocnemius muscle while suppressing endogenous glucose production and lipolysis. Because type 2 diabetes is characterized by diminished rates of insulin-stimulated glucose uptake as well as increased basal rates of endogenous glucose production and lipolysis, these results suggest that AICAR-related compounds may represent a new class of antidiabetic agents.

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

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

Dipolar coupling and ordering effects observed in magnetic resonance spectra of skeletal muscle

Skeletal muscle is a biological structure with a high degree of organization at different spatial levels. This order influences magnetic resonance (MR) in vivo-in particular 1H-spectra-by a series of effects that have very distinct physical sources and biomedical applications: (a) bulk fat (extramyocellular lipids, EMCL) along fasciae forms macroscopic plates, changing the susceptibility within these structures compared to the spherical droplets that contain intra-myocellular lipids (IMCL); this effect leads to a separation of the signals from EMCL and IMCL; (b) dipolar coupling effects due to anisotropic motional averaging have been shown for 1H-resonances of creatine, taurine, and lactate; (c) aromatic protons of carnosine show orientation-dependent effects that can be explained by dipolar coupling, chemical shift anisotropy or by relaxation anisotropy; (d) limited rotational freedom and/or compartmentation may explain differences of 1H-MR-visibility of the creatine/phosphocreatine resonances; (e) lactate 1H-MR resonances are reported to reveal information on tissue compartmentation; (f) transverse relaxation of water and metabolites show multiple components, indicative of intra-, extracellular and/or macromolecular-bound pools, and in addition dipolar or J-coupling lead to a modulation of the signal decay, hindering straightforward interpretation; (g) diffusion weighted 31P-MRS has shown restricted diffusion of phosphocreatine; (h) magnetization transfer (MT) indicates that there is a motionally restricted proton pool in spin-exchange with free creatine; reduced availability or restricted motion of creatine is particularly important for an estimation of ADP from 31P-MR spectra, and in addition MT effects may alter the signal intensity of creatine 1H-resonances following water-suppression pulses; (i) transcytolemmal water-exchange can be studied in 1H-MRS by contrast-agents applied to the extracellular space; (k) transport of glucose across the cell membrane has been studied in diabetes patients using a combination of 13C- and 31P-MRS; and l residual quadrupolar interaction in 23Na MR spectra from human skeletal muscle suggest that sodium ions are bound to ordered muscular structures.

Studies of metabolic compartmentation and glucose transport using in vivo MRS

Organs consist of several types of cells with specialized functions. This cellular localization of function is often referred to as compartmentation. Due to the intrinsic low sensitivity of MR methods it is generally not possible in vivo to obtain images or spectra of single cells. Instead the MRS signal is the sum of the signal from millions of cells and multiple cell types. A major challenge in using MRS to study biological processes such as metabolism and transport is to devise measurements that provide cell-specific information from this mix. Fortunately nature has helped the MR scientist by in several cases nearly completely localizing metabolic pathways and their associated metabolites in specific cell types. The chemical specificity of MRS allows the concentrations and synthesis rates of these metabolites to be measured, providing information about the compartmentation of metabolism and function. In this review examples are presented from MRS studies of metabolic trafficking between neurons and astrocytes in the brain, brain glucose transport, and the role of muscle glucose transport in insulin resistance and diabetes. The concepts and approaches used in these studies are generally applicable for studying cellular metabolic compartmentation in a wide range of systems.

Skeletal muscle lipid content and oxidative enzyme activity in relation to muscle fiber type in type 2 diabetes and obesity

In obesity and type 2 diabetes, skeletal muscle has been observed to have a reduced oxidative enzyme activity, increased glycolytic activity, and increased lipid content. These metabolic characteristics are related to insulin resistance of skeletal muscle and are factors potentially related to muscle fiber type. The current study was undertaken to examine the interactions of muscle fiber type in relation to oxidative enzyme activity, glycolytic enzyme activity, and muscle lipid content in obese and type 2 diabetic subjects compared with lean healthy volunteers. The method of single-fiber analysis was used on vastus lateralis muscle obtained by percutaneous biopsy from 22 lean, 20 obese, and 20 type 2 diabetic subjects (ages 35+/-1, 42+/-2, and 52+/-2 years, respectively), with values for BMI that were similar in obese and diabetic subjects (23.7+/-0.7, 33.2+/-0.8, and 31.8+/-0.8 kg/m2, respectively). Oxidative enzyme activity followed the order of type I > type IIa > type IIb, but within each fiber type, skeletal muscle from obese and type 2 diabetic subjects had lower oxidative enzyme activity than muscle from lean subjects (P < 0.01). Muscle lipid content followed a similar pattern in relation to fiber type, and within each fiber type, muscle from obese and type 2 diabetic subjects had greater lipid content (P < 0.01). In summary, based on single-fiber analysis, skeletal muscle in obese and type 2 diabetic subjects mani-fests disturbances of oxidative enzyme activity and increased lipid content that are independent of the effect of fiber type.

Restoration of insulin-sensitive glucose transporter (GLUT4) gene expression in muscle cells by the transcriptional coactivator PGC-1

Muscle tissue is the major site for insulin-stimulated glucose uptake in vivo, due primarily to the recruitment of the insulin-sensitive glucose transporter (GLUT4) to the plasma membrane. Surprisingly, virtually all cultured muscle cells express little or no GLUT4. We show here that adenovirus-mediated expression of the transcriptional coactivator PGC-1, which is expressed in muscle in vivo but is also deficient in cultured muscle cells, causes the total restoration of GLUT4 mRNA levels to those observed in vivo. This increased GLUT4 expression correlates with a 3-fold increase in glucose transport, although much of this protein is transported to the plasma membrane even in the absence of insulin. PGC-1 mediates this increased GLUT4 expression, in large part, by binding to and coactivating the muscle-selective transcription factor MEF2C. These data indicate that PGC-1 is a coactivator of MEF2C and can control the level of endogenous GLUT4 gene expression in muscle.

Polycystic ovary syndrome, hyperandrogenism, and insulin resistance

Results from recent basic and clinical research investigations have greatly improved our understanding of insulin resistance in general and insulin resistance associated with PCOS in particular. With this understanding has come the possibility of using new methods to treat PCOS. This is particularly true when discussing the use of insulin-sensitizing drugs. Caution must be exercised in using these drugs because of unforeseen acute or remote adverse side effects. Postulated relationships among PCOS, hyperandrogenism, and insulin resistance do not completely solve the endocrinologic mystery of the patient with PCOS. For example, how does the partial destruction of the ovary (e.g., wedge biopsy or ovary drilling by laser or cautery), which does not affect insulin resistance, result in ovulatory cycles? Why does the administration of excessive exogenous insulin in the case of the insulin-dependent diabetic fail to cause hyperandrogenism? Certainly, much remains to be learned about the reproductive endocrine disturbance we now call PCOS.

Recent advances in our understanding of insulin action and insulin resistance

Insulin signaling at the target tissue results in a large array of biological outcomes. These events are essential for normal growth and development and for normal homeostasis of glucose, fat, and protein metabolism. Elucidating the intracellular events after activation of the IR has been the primary focus of a large number of investigators for decades, and for excellent reasons. Understanding the signaling pathways involved in insulin action could lead to a better understanding of the pathophysiology of insulin resistance associated with obesity and type 2 diabetes, and identifying key molecules and processes could lead to newer and more effective therapeutic agents for treating these common disorders.

This review summarizes our previous understanding of how insulin acts and outlines some recent developments in our understanding of insulin action and insulin resistance at the cellular level, beginning with a discussion on the discovery of evolutionarily conserved molecules of the insulin signaling pathways. This article will also provide a summary of a few in vitro and cellular models of insulin resistance and a description of some new paradigms in the cellular mechanisms of insulin action.

This review will not attempt to be all-inclusive; for a more comprehensive understanding, readers are referred to more complete reviews on insulin action (1–5).

Adipose-selective targeting of the GLUT4 gene impairs insulin action in muscle and liver

The earliest defect in developing type 2 diabetes is insulin resistance, characterized by decreased glucose transport and metabolism in muscle and adipocytes. The glucose transporter GLUT4 mediates insulin-stimulated glucose uptake in adipocytes and muscle by rapidly moving from intracellular storage sites to the plasma membrane. In insulin-resistant states such as obesity and type 2 diabetes, GLUT4 expression is decreased in adipose tissue but preserved in muscle. Because skeletal muscle is the main site of insulin-stimulated glucose uptake, the role of adipose tissue GLUT4 downregulation in the pathogenesis of insulin resistance and diabetes is unclear. To determine the role of adipose GLUT4 in glucose homeostasis, we used Cre/loxP DNA recombination to generate mice with adipose-selective reduction of GLUT4 (G4A-/-). Here we show that these mice have normal growth and adipose mass despite markedly impaired insulin-stimulated glucose uptake in adipocytes. Although GLUT4 expression is preserved in muscle, these mice develop insulin resistance in muscle and liver, manifested by decreased biological responses and impaired activation of phosphoinositide-3-OH kinase. G4A-/- mice develop glucose intolerance and hyperinsulinaemia. Thus, downregulation of GLUT4 and glucose transport selectively in adipose tissue can cause insulin resistance and thereby increase the risk of developing diabetes.