Mechanisms and time course of impaired skeletal muscle glucose transport activity in streptozocin diabetic rats

Research advances in the therapy of metabolic syndrome

Metabolic syndrome refers to the pathological state of metabolic disorder of protein, fat, carbohydrate, and other substances in the human body. It is a syndrome composed of a group of complex metabolic disorders, whose pathogenesis includes multiple genetic and acquired entities falling under the category of insulin resistance and chronic low-grade inflammationand. It is a risk factor for increased prevalence and mortality from diabetes and cardiovascular disease. Cardiovascular diseases are the predominant cause of morbidity and mortality globally, thus it is imperative to investigate the impact of metabolic syndrome on alleviating this substantial disease burden. Despite the increasing number of scientists dedicating themselves to researching metabolic syndrome in recent decades, numerous aspects of this condition remain incompletely understood, leaving many questions unanswered. In this review, we present an epidemiological analysis of MetS, explore both traditional and novel pathogenesis, examine the pathophysiological repercussions of metabolic syndrome, summarize research advances, and elucidate the mechanisms underlying corresponding treatment approaches.

The Emerging Importance of Cirsimaritin in Type 2 Diabetes Treatment

Cirsimaritin is a dimethoxy flavon that has different biological activities such as antiproliferative, antimicrobial, and antioxidant activities. This study aims to investigate the anti-diabetic effects of cirsimaritin in a high-fat diet and streptozotocin-(HFD/STZ)-induced rat model of type 2 diabetes mellitus (T2D). Rats were fed HFD, followed by a single low dose of STZ (40 mg/kg). HFD/STZ diabetic rats were treated orally with cirsimaritin (50 mg/kg) or metformin (200 mg/kg) for 10 days before terminating the experiment and collecting plasma, soleus muscle, adipose tissue, and liver for further downstream analysis. Cirsimaritin reduced the elevated levels of serum glucose in diabetic rats compared to the vehicle control group (p < 0.001). Cirsimaritin abrogated the increase in serum insulin in the treated diabetic group compared to the vehicle control rats (p < 0.01). The homeostasis model assessment of insulin resistance (HOMA-IR) was decreased in the diabetic rats treated with cirsimaritin compared to the vehicle controls. The skeletal muscle and adipose tissue protein contents of GLUT4 (p < 0.01 and p < 0.05, respectively) and pAMPK-α1 (p < 0.05) were upregulated following treatment with cirsimaritin. Cirsimaritin was able to upregulate GLUT2 and AMPK protein expression in the liver (p < 0.01, <0.05, respectively). LDL, triglyceride, and cholesterol were reduced in diabetic rats treated with cirsimaritin compared to the vehicle controls (p < 0.001). Cirsimaritin reduced MDA, and IL-6 levels (p < 0.001), increased GSH levels (p < 0.001), and reduced GSSG levels (p < 0.001) in diabetic rats compared to the vehicle control. Cirsimaritin could represent a promising therapeutic agent to treat T2D.

Metformin and Insulin Resistance: A Review of the Underlying Mechanisms behind Changes in GLUT4-Mediated Glucose Transport

Metformin is the most commonly used treatment to increase insulin sensitivity in insulin-resistant (IR) conditions such as diabetes, prediabetes, polycystic ovary syndrome, and obesity. There is a well-documented correlation between glucose transporter 4 (GLUT4) expression and the level of IR. Therefore, the observed increase in peripheral glucose utilization after metformin treatment most likely comes from the induction of GLUT4 expression and its increased translocation to the plasma membrane. However, the mechanisms behind this effect and the critical metformin targets are still largely undefined. The present review explores the evidence for the crucial role of changes in the expression and activation of insulin signaling pathway mediators, AMPK, several GLUT4 translocation mediators, and the effect of posttranscriptional modifications based on previously published preclinical and clinical models of metformin’s mode of action in animal and human studies. Our aim is to provide a comprehensive review of the studies in this field in order to shed some light on the complex interactions between metformin action, GLUT4 expression, GLUT4 translocation, and the observed increase in peripheral insulin sensitivity.

The many actions of insulin in skeletal muscle, the paramount tissue determining glycemia

As the principal tissue for insulin-stimulated glucose disposal, skeletal muscle is a primary driver of whole-body glycemic control. Skeletal muscle also uniquely responds to muscle contraction or exercise with increased sensitivity to subsequent insulin stimulation. Insulin's dominating control of glucose metabolism is orchestrated by complex and highly regulated signaling cascades that elicit diverse and unique effects on skeletal muscle. We discuss the discoveries that have led to our current understanding of how insulin promotes glucose uptake in muscle. We also touch upon insulin access to muscle, and insulin signaling toward glycogen, lipid, and protein metabolism. We draw from human and rodent studies in vivo, isolated muscle preparations, and muscle cell cultures to home in on the molecular, biophysical, and structural elements mediating these responses. Finally, we offer some perspective on molecular defects that potentially underlie the failure of muscle to take up glucose efficiently during obesity and type 2 diabetes.

Effects of Acute Dietary Polyphenols and Post-Meal Physical Activity on Postprandial Metabolism in Adults with Features of the Metabolic Syndrome

Approximately 22% of U.S. adults and 25% of adults globally have metabolic syndrome (MetS). Key features, such as dysglycemia and dyslipidemia, predict type 2 diabetes, cardiovascular disease, premature disability, and death. Acute supplementation of dietary polyphenols and post-meal physical activity hold promise in improving postprandial dysmetabolism. To our knowledge, no published review has described the effects of either intervention on postprandial glucose, insulin, lipids, and markers of oxidative damage and inflammation in adults with features of MetS. Thus, we conducted this review of controlled clinical trials that provided dietary polyphenols from oils, fruits, teas, and legumes during a dietary challenge, or implemented walking, cycling, and stair climbing and descending after a dietary challenge. Clinical trials were identified using ClinicalTrials.gov, PubMed, and Google Scholar and were published between 2000 and 2019. Dietary polyphenols from extra virgin olive oil, grapes, blackcurrants, strawberries, black tea, and black beans improved postprandial glucose, insulin, and markers of oxidative damage and inflammation, but results were not consistent among clinical trials. Freeze-dried strawberry powder distinctly improved postprandial insulin and markers of oxidative damage and inflammation. Post-meal physical activity attenuated postprandial glucose, but effects on postprandial lipids and markers of oxidative damage and inflammation were inconclusive. Consuming dietary polyphenols with a meal and completing physical activity after a meal may mitigate postprandial dysmetabolism in adults with features of MetS.

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

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

Recovery of insulin sensitivity and Slc2a4 mRNA expression depend on T3 hormone during refeeding

OBJECTIVE

GLUT4 protein, encoded by the Slc2a4 gene, plays a key role in muscle glucose uptake, and its expression decreases in muscles under insulin resistance. Slc2a4/GLUT4 decreases with fasting and rapidly increases with refeeding and the same occurs to plasma glucose, amino acids, insulin and T3. Thus, they might be potential regulators of the Slc2a4 gene, which makes them promising targets for strategies to improve GLUT4 expression. Herein, we investigate the role of metabolic-hormonal parameters triggered by refeeding upon the Slc2a4 expression.

MATERIALS/METHODS

Plasma glucose/insulin/T3, and gastrocnemius Slc2a4 mRNA contents were measured in rats studied at the end of 48-h fasting, and subsequently at: i) 2-4h after spontaneous refeeding; ii) 2-4h after T3 injection, without refeeding; and iii) 0.5-2h after intravenous infusion of insulin, insulin+glucose and insulin+amino acids, without refeeding.

RESULTS

Refeeding increased plasma glucose/insulin/T3 and muscle Slc2a4 mRNA, reverting insulin resistance. Post-fasting infusions surprisingly induced a further Slc2a4 mRNA decrease (~20%, P<0.05 vs. fasting), but T3 injection induced a ~2-fold increase in Slc2a4 mRNA, 2-4h later (P<0.001). Moreover, T3 increased glycemia and insulinemia to the 2h-refed rats levels, suggesting that T3 elevation is a key factor to the mechanisms of metabolic balance during refeeding.

CONCLUSIONS

Refeeding induces a rapid increase in muscle Slc2a4 expression, not associated with increased plasma glucose, insulin or amino acids, but highly correlated to increased plasma T3 concentration. This result points out T3 hormone as a powerful Slc2a4 enhancer, an effect that may be acutely explored in situations of insulin resistance.

Biochemical adaptations of mammalian hibernation: exploring squirrels as a perspective model for naturally induced reversible insulin resistance

An important disease among human metabolic disorders is type 2 diabetes mellitus. This disorder involves multiple physiological defects that result from high blood glucose content and eventually lead to the onset of insulin resistance. The combination of insulin resistance, increased glucose production, and decreased insulin secretion creates a diabetic metabolic environment that leads to a lifetime of management. Appropriate models are critical for the success of research. As such, a unique model providing insight into the mechanisms of reversible insulin resistance is mammalian hibernation. Hibernators, such as ground squirrels and bats, are excellent examples of animals exhibiting reversible insulin resistance, for which a rapid increase in body weight is required prior to entry into dormancy. Hibernator studies have shown differential regulation of specific molecular pathways involved in reversible resistance to insulin. The present review focuses on this growing area of research and the molecular mechanisms that regulate glucose homeostasis, and explores the roles of the Akt signaling pathway during hibernation. Here, we propose a link between hibernation, a well-documented response to periods of environmental stress, and reversible insulin resistance, potentially facilitated by key alterations in the Akt signaling network, PPAR-γ/PGC-1α regulation, and non-coding RNA expression. Coincidentally, many of the same pathways are frequently found to be dysregulated during insulin resistance in human type 2 diabetes. Hence, the molecular networks that may regulate reversible insulin resistance in hibernating mammals represent a novel approach by providing insight into medical treatment of insulin resistance in humans.

Infusion of mesenchymal stem cells ameliorates hyperglycemia in type 2 diabetic rats: identification of a novel role in improving insulin sensitivity

Infusion of mesenchymal stem cells (MSCs) has been shown to effectively lower blood glucose in diabetic individuals, but the mechanism involved could not be adequately explained by their potential role in promoting islet regeneration. We therefore hypothesized that infused MSCs might also contribute to amelioration of the insulin resistance of peripheral insulin target tissues. To test the hypothesis, we induced a diabetic rat model by high-fat diet/streptozotocin (STZ) administration, performed MSC infusion during the early phase (7 days) or late phase (21 days) after STZ injection, and then evaluated the therapeutic effects of MSC infusion and explored the possible mechanisms involved. MSC infusion ameliorated hyperglycemia in rats with type 2 diabetes (T2D). Infusion of MSCs during the early phase not only promoted β-cell function but also ameliorated insulin resistance, whereas infusion in the late phase merely ameliorated insulin resistance. Infusion of MSCs resulted in an increase of GLUT4 expression and an elevation of phosphorylated insulin receptor substrate 1 (IRS-1) and Akt (protein kinase B) in insulin target tissues. This is the first report of MSC treatment improving insulin sensitivity in T2D. These data indicate that multiple roles and mechanisms are involved in the efficacy of MSCs in ameliorating hyperglycemia in T2D.

Correction of glycaemia and GLUT1 level by mildronate in rat streptozotocin diabetes mellitus model

Anti-ischaemic drug mildronate suppresses fatty acid metabolism and increases glucose utilization in myocardium. It was proposed that it could produce a favourable effect on metabolic parameters and glucose transport in diabetic animals. Rats with streptozotocin diabetes mellitus were treated with mildronate (100 mg/kg daily, per os, 6 weeks). Therapeutic effect of mildronate was monitored by measuring animal weight, concentrations of blood glucose, insulin, blood triglycerides, free fatty acids, blood ketone bodies and cholesterol, glycated haemoglobin per cent (HbA1c%) and glucose tolerance. GLUT1 mRNA and protein expression in kidneys, heart, liver and muscles were studied by means of real time RT-PCR and immunohistochemistry correspondingly. In the streptozotocin + mildronate group, mildronate treatment caused a significant decrease in mean blood glucose, cholesterol, free fatty acid and HbA1c concentrations and improved glucose tolerance. Induction of streptozotocin diabetes mellitus provoked increase of both GLUT1 gene and protein expression in kidneys, heart and muscle, mildronate treatment produced normalization of the GLUT1 expression levels. In the liver a similar effect was observed for GLUT1 protein expression, while GLUT1 gene expression was increased by mildronate. Mildronate produces therapeutic effect in streptozotocin diabetes model. Mildronate normalizes the GLUT1 expression up-regulated by streptozotocin diabetes mellitus in kidneys, heart, muscle and liver. Copyright © 2011 John Wiley & Sons, Ltd.

17beta-Estradiol and/or progesterone protect from insulin resistance in STZ-induced diabetic rats

Recent clinical and experimental evidences suggest that sex steroids protect from insulin resistance associated with diabetes. Therefore, we have assessed the influence of E2 and/or P4 on insulin sensitivity by euglicaemic-hyperinsulinaemic clamp in ovariectomized streptozotocin-induced diabetic rats, focusing on key proteins of insulin signaling in skeletal muscle. Although low plasma levels of E2 (days 6 and 11) increased Glut-4 plasma membrane content and subsequent improved insulin sensitivity, they could not fully reverse hyperglycaemia negative effects on p85alpha-IRS-1 association and IRS-1 content during 11 days. However, high plasma levels of E2 (day 16) could reverse hyperglycaemia effects not only on Glut-4 plasma membrane content but also on p85alpha-IRS-1 association and IRS-1 protein content level. In contrast, P4 treatment only improved insulin sensitivity when its plasma concentration was low (days 6 and 11) and its effects were not associated with any proteins study in this paper. The combined therapy had a synergic effect on insulin sensitivity when their plasma levels were low (day 6) or high (day 16), that could be associated with Glut-4 plasma membrane content modulation, p85alpha-IRS-1 association and IRS-1 amount. These new findings improve our understanding of biochemical basis of insulin resistance due to hyperglycaemia and could open up new possibilities of treatment in uncontrolled type 1 DM.

Voltage-gated potassium channel Kv1.3 regulates GLUT4 trafficking to the plasma membrane via a Ca2+-dependent mechanism

Kv1.3 is a voltage-gated K(+) channel expressed in insulin-sensitive tissues. We previously showed that gene inactivation or pharmacological inhibition of Kv1.3 channel activity increased peripheral insulin sensitivity independently of body weight by augmenting the amount of GLUT4 at the plasma membrane. In the present study, we further examined the effect Kv1.3 on GLUT4 trafficking and tested whether it occurred via an insulin-dependent pathway. We found that Kv1.3 inhibition by margatoxin (MgTX) stimulated glucose uptake in adipose tissue and skeletal muscle and that the effect of MgTX on glucose transport was additive to that of insulin. Furthermore, whereas the increase in uptake was wortmannin insensitive, it was completely inhibited by dantrolene, a blocker of Ca(2+) release from intracellular Ca(2+) stores. In white adipocytes in primary culture, channel inhibition by Psora-4 increased GLUT4 translocation to the plasma membrane. In these cells, GLUT4 protein translocation was unaffected by the addition of wortmannin but was significantly inhibited by dantrolene. Channel inhibition depolarized the membrane voltage and led to sustained, dantrolene-sensitive oscillations in intracellular Ca(2+) concentration. These results indicate that the apparent increase in insulin sensitivity observed in association with inhibition of Kv1.3 channel activity is mediated by an increase in GLUT4 protein at the plasma membrane, which occurs largely through a Ca(2+)-dependent process.

High-Yield Functional Expression of Human Sodium/d-Glucose Cotransporter1 inPichia pastorisand Characterization of Ligand-Induced Conformational Changes as Studied by Tryptophan Fluorescence†

Studies on the structure-function relationship of transporters require the availability of sufficient amounts of the protein in a functional state. In this paper, we report the functional expression, purification, and reconstitution of the human sodium/d-glucose cotransporter1 (hSGLT1) in Pichia pastoris and ligand-induced conformational changes of hSGLT1 in solution as studied by intrinsic tryptophan fluorescence. hSGLT1 gene containing FLAG tag at position 574 was cloned into pPICZB plasmid, and the resulting expression vector pPICZB-hSGLT1 was introduced into P. pastoris strain GS115 by electroporation. Purification of recombinant hSGLT1 by nickel-affinity chromatography yields about 3 mg of purified recombinant hSGLT1 per 1-liter of cultured Pichia cells. Purified hSGLT1 migrates on SDS-PAGE with an apparent mass of 55 kDa. Kinetic analysis of hSGLT1 in proteoliposomes revealed sodium-dependent, secondary active, phlorizin-sensitive, and stereospecific alpha-methyl-d-glucopyranoside transport, demonstrating its full catalytic activity. The position of the maximum intrinsic tryptophan fluorescence and titration with hydrophilic collisional quenchers KI, acrylamide, and trichloroethanol suggested that most of Trps in hSGLT1 in solution are in a hydrophobic environment. In the presence of sodium, sugars that have been identified earlier as substrate for the transporter increase intrinsic fluorescence in a saturable manner by a maximum of 15%. alpha-Methyl-d-glucopyranoside had the highest affinity (K(d) = 0.71 mM), followed by d-glucose, d-galactose, d-mannose, and d-allose which showed a much lower affinity. l-Glucose was without effect. d-Glucose also increased the accessibility of the Trps to hydrophilic collisional quenchers. On the contrary phlorizin, the well-established inhibitor of SGLT1, decreased intrinsic fluorescence by a maximum of 50%, and induced a blue shift of maximum (5 nm). Again, the effects were sodium-dependent and saturable and a high affinity K(d) of 5 muM was observed. In addition the surface of hSGLT1 was labeled with 1-anilinonaphthalene-8-sulfonic acid, a reporter molecule for the surface hydrophobicity. In the presence of sodium, addition of d-glucose decreased ANS fluorescence whereas phlorizin increased ANS fluorescence. Thus three conformational states of SGLT1 could be defined which differ in their packing density and hydrophobicity of their surface. They reflect properties of the empty carrier, the d-glucose loaded carrier facing the outside of membrane and the complex of the outside-orientated carrier with phlorizin.

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

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

Effects of keishi-ka-jutsubu-to (traditional herbal medicine: Gui-zhi-jia-shu-fu-tang) on in vivo insulin action in streptozotocin-induced diabetic rats

This study investigated the effects of the traditional herbal medicine, Keishi-ka-jutsubu-to (KJT) on insulin action in vivo and insulin signaling in skeletal muscle in STZ-induced diabetes. Rats were divided into single and 7-days oral administration groups. Euglycemic clamp (insulin infusion rates: 3 and 30 mU/kg/min) was used in awaked rats and the insulin signaling in skeletal muscle was evaluated. At low-dose insulin infusion, the decreased metabolic clearance rates of glucose (MCR) in diabetic rats were improved by a single and 7-days administration of KJT (800 mg/kg BW, p.o.; acute effect: 6.7 +/- 0.6 vs. 12.3 +/- 1.2, and 7-days effect: 6.3 +/- 0.5 vs. 13.9 +/- 1.0 ml/kg/min, P<0.001, respectively). During high-dose insulin infusion, the MCR was increased in 7-days KJT treated diabetes compared with saline diabetes, but, these changes were not observed after a single KJT treatment. About 90% of the increasing effect in MCR induced by the 7-days KJT treatment was blocked by L-NMMA. However, no further additive effects were seen in KJT + SNP treatment. IRbeta protein increase and decreased IRS-1 protein expression in diabetes were significantly improved by KJT treatment. KJT had no effect on the GLUT4 protein content. The increased tyrosine phosphorylation level of IRbeta, IRS-1, and IRS-1 associated with PI 3-kinase were significantly inhibited in KJT treated diabetes. The present study suggests that the improvement of impaired insulin action in STZ-diabetes by administration of KJT may be due, at least in part, to enhanced insulin signaling, which may be involved with production of nitric oxide (NO).

Modulation of glucose transporters in rat diaphragm by sodium tungstate

Oral administration of sodium tungstate is an effective treatment for diabetes in animal models. We examined the effects of 6 weeks of oral administration of tungstate on glucose transporters (GLUT) in streptozotocin-induced diabetic rat diaphragm. Diabetes decreased GLUT4 expression while tungstate treatment normalized not only GLUT4 protein but also GLUT4 mRNA in the diabetic rats. Furthermore, treatment increased GLUT4 protein in plasma and internal membranes, suggesting a stimulation of its translocation to the plasma membrane. Tungstate had no effect on healthy animals. There were no differences in the total amount of GLUT1 transporter in any group. We conclude that the normoglycemic effect of tungstate may be partly due to a normalization of the levels and subcellular localization of GLUT4, which should result in an increase in muscle glucose uptake.

Activation of PI 3-kinase by the hexosamine biosynthesis pathway

It has been shown that hyperglycaemia-induced defects in glucose transport and insulin action are mediated by increased flux of excess glucose through the hexosamine biosynthesis pathway (HBP). We have previously demonstrated that in rat adipocytes, increased flux through the HBP activates protein kinase C (PKC). The aim of the present study was to explore the mechanism for HBP-mediated activation of PKC. We show that activation of the HBP by either high glucose or glucosamine causes the translocation of PKC-zeta/lambda and PKC-epsilon but not other PKC isoforms tested (alpha, beta, delta). This translocation was inhibited by wortmannin, a PI 3-kinase inhibitor. Both high glucose and glucosamine caused widespread cellular activation of PI 3-kinase. We demonstrate that HBP-mediated activation of PI 3-kinase has an insulin-like effect to translocate GLUT4. We conclude that an acute increase of glucose flux through the HBP activates PI 3-kinase.

Transdermal delivery of insulin from a novel biphasic lipid system in diabetic rats

Noninvasive transdermal insulin delivery could provide diabetic patients with sustained physiological levels of basal insulin in a pain-free manner. We have developed a novel transdermal lipid-based system (Biphasix) suitable for macromolecule delivery across the skin. The objective of this study was to evaluate the pharmacological effects of the Biphasix-insulin delivery system in a diabetic rat model. Transdermal patches (one per animal) containing Biphasix-insulin formulation (10 mg of recombinant human insulin dose) were applied to the shaved abdominal skin of streptozotocin-induced diabetic rats for 48 h. Blood glucose was monitored every 2-4 h using a Lifescan glucose meter. Serum insulin levels were analysed by enzyme-linked immunosorbent assay. A decrease in blood glucose of 43.7 +/- 3.8% (mean +/- SEM, n = 25) was observed compared with initial blood glucose levels. The duration of the response was 51.5 +/- 3.7 h (mean +/- SEM, n = 25). Serum insulin after application of the transdermal Biphasix-insulin patch was 20.08 +/- 5.44 micro IU/mL (mean +/- SEM, n = 13) during the steady state, which was not statistically different from the insulin levels obtained 2 h after subcutaneous injection of 1 mg of recombinant human insulin solution. Insulin bioavailability from the transdermal Biphasix-insulin patches was 21.5 +/- 6.9% (mean +/- SEM, n = 13) based on serum insulin and 39.5 +/- 8.5% (mean +/- SEM, n = 25) based on the pharmacodynamic blood glucose-lowering effects. The Biphasix system successfully delivered insulin transdermally, as evidenced by a significant sustained decrease in blood glucose in diabetic rats, with a corresponding increase in serum insulin. These results support the feasibility of developing a transdermal insulin patch for human applications.

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.

Expression of glucokinase in skeletal muscle: a new approach to counteract diabetic hyperglycemia

Chronic hyperglycemia is responsible for diabetes-specific microvascular and macrovascular complications. To reduce hyperglycemia, key tissues may be engineered to take up glucose. To determine whether an increase in skeletal muscle glucose phosphorylation leads to increased glucose uptake and to normalization of diabetic alterations, the liver enzyme glucokinase (GK) was expressed in muscle of transgenic mice. GK has a high Km for glucose and its activity is not inhibited by glucose 6-phosphate. The presence of GK activity in skeletal muscle resulted in increased concentrations of glucose 6-phosphate and glycogen. These mice showed lower glycemia and insulinemia, increased serum lactate levels, and higher blood glucose disposal after an intraperitoneal glucose tolerance test. Furthermore, transgenic mice were more sensitive to injection of low doses of insulin, which led to increased blood glucose disposal. In addition, streptozotocin (STZ)-treated transgenic mice showed lower levels of blood glucose than STZ-treated controls and maintained body weight. Moreover, injection of insulin to STZ-treated transgenic mice led to normoglycemia, while STZ-treated control mice remained highly hyperglycemic. Thus, these results are consistent with a key role of glucose phosphorylation in regulating glucose metabolism in skeletal muscle. Furthermore, this study suggests that engineering skeletal muscle to express GK may be a new approach to the therapy of diabetes mellitus.

Hyperglycemia contributes insulin resistance in hepatic and adipose tissue but not skeletal muscle of ZDF rats

To determine the contribution of hyperglycemia to the insulin resistance in various insulin-sensitive tissues of Zucker diabetic fatty (ZDF) rats, T-1095, an oral sodium-dependent glucose transporter (SGLT) inhibitor, was administered by being mixed into food. Long-term treatment with T-1095 lowered both fed and fasting blood glucose levels to near normal ranges. A hyperinsulinemic euglycemic clamp study that was performed after 4 wk of T-1095 treatment demonstrated partial recovery of the reduced glucose infusion rate (GIR) in the T-1095-treated group. In the livers of T-1095-treated ZDF rats, hepatic glucose production rate (HGP) and glucose utilization rate (GUR) showed marked recovery, with almost complete normalization of reduced glucokinase/glucose-6-phosphatase (G-6-Pase) activities ratio. In adipose tissues, decreased GUR was also shown to be significantly improved with a normalization of insulin-induced GLUT-4 translocation. In contrast, in skeletal muscles, the reduced GUR was not significantly improved in response to amelioration of hyperglycemia by T-1095 treatment. These results suggest that the contribution of hyperglycemia to insulin resistance in ZDF rats is very high in the liver and considerably elevated in adipose tissues, although it is very low in skeletal muscle.

Soluble dietary fibre improves insulin sensitivity by increasing muscle GLUT-4 content in stroke-prone spontaneously hypertensive rats

1. The effects of soluble dietary fibre (psyllium) on peripheral insulin sensitivity and skeletal muscle GLUT-4 protein expression were studied in 12 male stroke-prone spontaneously hypertensive rats (SHRSP) fed a high-caloric diet from 5 to 9 weeks of age. 2. In the psyllium-supplemented group, fasting plasma glucose was significantly reduced and glucose levels following an oral glucose tolerance test were significantly lower than in the cellulose-supplemented group at 30 (P < 0.05) and 60 min (P < 0.01). However, there was no difference in insulin secretion. 3. In the psyllium-supplemented group, skeletal muscle GLUT-4 content was significantly increased in the plasma membrane (P < 0.001), but not in the intracellular membrane. 4. No significant difference was found in phosphatidylinositol 3 (PI3)-kinase activity between cellulose and psyllium diet not only in the basal state but also when stimulated by insulin. 5. These results demonstrate that psyllium increases blood glucose disposal by increasing skeletal muscle plasma membrane GLUT-4 content without PI3-kinase activation.

Mobilization of GLUT-4 from intracellular vesicles by insulin and K(+) depolarization in cultured H9c2 myotubes

The insulin-responsive glucose transporter, GLUT-4, moves from an intracellular compartment to the cell surface in response to insulin and/or muscle contraction. Treatment of H9c2 myotubes with insulin significantly increased uptake of 2-deoxyglucose. Depolarization of the myotubes by increasing extracellular [K(+)], which mimics the initial phases of excitation-contraction coupling, also increased 2-deoxyglucose uptake. The K(+)- but not insulin-evoked increase was blocked by dantrolene, an inhibitor of Ca(2+) release from the sarcoplasmic reticulum. In contrast, wortmannin, an inhibitor of phosphatidylinositol 3-kinase, blocked insulin- but not K(+)-stimulated 2-deoxyglucose uptake. Increased glucose uptake in response to insulin or K(+) depolarization was associated with increased GLUT-4 in plasma membranes and depletion of a population of small intracellular GLUT-4-containing vesicles. Similarly, in H9c2 cells transfected with c-myc-tagged GLUT-4, translocation of c-myc GLUT-4 to the cell surface was increased after stimulation with insulin or K(+) depolarization. Taken together, these data demonstrate that insulin and K(+) depolarization increase glucose uptake by recruiting GLUT-4 from intracellular vesicles to the plasma membrane of H9c2 myotubes via distinct signaling mechanisms.

Increased diaphragm expression of GLUT4 in control and streptozotocin-diabetic rats by fish oil-supplemented diets

Dietary fat intake influences plasma glucose concentration through modifying glucose uptake and utilization by adipose and skeletal muscle tissues. In this paper, we studied the effects of a low-fat diet on diaphragm GLUT4 expression and fatty acid composition in control and streptozotocin-induced diabetic rats. Control as well as diabetic rats were divided into three different dietary groups each. Either 5% olive oil, 5% sunflower oil, or 5% fish oil was the only fat supplied by the diet. Feeding these low-fat diets for 5 wk induced major changes in fatty acid composition, both in control and in diabetic rats. Arachidonic acid was higher in diabetic olive and sunflower oil-fed rats with respect to fish oil-fed, opposite to docosahexaenoic acid which was higher in diabetic fish oil-fed rats with respect to the other two groups. Animals receiving a fish oil diet had the lowest plasma glucose concentration. GLUT4 expression in diaphragm, as indicated by GLUT4 protein and mRNA, is modulated both by diabetes and by diet fatty acid composition. Diabetes induced a decrease in expression in all dietary groups. Plasma glucose levels correlated well with the increased amount of GLUT4 protein and mRNA found in fish oil-fed groups. Results are discussed in terms of the influence that arachidonic and n-3 polyunsaturated fatty acids may exert on the transcriptional and translational control of the GLUT4 gene.

Nutrient-induced insulin resistance

Impaired function of the hormone insulin (insulin resistance) is a major feature of type 2 diabetes, a condition that is expected to afflict over 200 million people by early next century. Intensive investigation has failed to find a genetic basis for insulin resistance in the majority of cases. In this brief review the evidence that insulin resistance may be caused by excess nutrient supply will be presented. Both excess glucose and excess fat can cause insulin resistance in muscle and fat tissue, while excess fat can cause impaired suppression of endogenous glucose production. Each nutrient may impair insulin action by several mechanisms, at least one of which may be common to both.

Effects of streptozocin-induced diabetes and islet cell transplantation on insulin signaling in rat skeletal muscle

Streptozocin-induced diabetes is associated with alterations in insulin signaling in rat skeletal muscle, including increased insulin receptor substrate-1 phosphorylation and phosphotidylinositol 3-kinase activity. In the current study, we determined the effects of streptozocin-induced diabetes and treatment of diabetes by islet cell transplantation on several proximal insulin-activated signaling proteins. Three groups of male Lewis rats (untreated streptozocin-diabetic animals, islet cell-transplanted diabetic rats, and nondiabetic control rats) were studied in the basal state or 30 min after i.p. insulin injection (20 U/rat). Mixed hindlimb skeletal muscle lysates were used to determine the expression and enzymatic activities of the extracellular regulated kinase 2 (ERK2), p90 ribosomal S6 kinase (RSK2), Akt, and p70 S6 kinase (p70S6k). In all three groups of rats, insulin significantly increased ERK2, RSK2, Akt, and p70S6k activities. There was no effect of diabetes on insulin-stimulated ERK2 activity or ERK2 protein levels. RSK2 expression and insulin-stimulated RSK2 activity were significantly elevated in diabetic rats compared with those in the control animals. Insulin-stimulated Akt activity was also significantly greater in the diabetic animals, but there was no change in protein expression. In contrast, there was a decrease in insulin-stimulated p70S6k activity with no change in protein expression in the diabetic rats. Islet transplantation partially (RSK2) or fully (Akt, p70S6k) normalized these diabetes-induced changes in insulin signaling proteins. We conclude that streptozocin diabetes results in the dysregulation of several critical insulin-activated proteins in rat skeletal muscle, but islet cell transplantation is an effective therapy to partially correct these alterations in insulin signaling.

D-glucose combined chronic toxicity and carcinogenicity studies in Sprague-Dawley rats and Syrian golden hamsters

After an initial period of 16 weeks with increasing concentrations, D-glucose was administered at 30% in the diet to 50 male and 50 female Sprague-Dawley rats from the 17th to the 112th study week. Additional 10 male and 10 female animals were treated for 14 months and then sacrificed for interim examination. Groups of 60 male and 60 female Syrian golden hamsters received D-glucose in the form of 20% solution in tap water for a period of 80 weeks. In each case, groups consisting of an equal number of untreated animals served as controls. General behavior and mortality were not affected by the treatment. The rats and hamsters treated with glucose showed significantly higher body weights of up to a maximum of 16% in male and 26% in female rats, or 15% in male and female hamsters. In rats, the increase was evident by week 14, and in the hamsters by week 10. Glucose-dosed rats displayed a slightly increased feed intake and a reduced water intake. Both parameters, however, were not influenced in hamsters. Hematological and histopathological examination showed no pertinent changes in hematopoetic tissue. Sharply increased blood glucose and renal glucose excretion values were present in rats beginning with 18 months and were indicative of the development of non-insulin-dependent diabetes mellitus (NIDDM). The insulin concentrations in peripheral blood were not appreciably affected, although there was a trend to higher values in males at all evaluation times and in females only at 3 months. Pathological evaluation did not show any compound related non-neoplastic lesions. The incidences of islet cell adenomas in the pancreas of male rats were significantly increased and the cortical adenomas in the adrenals of females were decreased. In addition, the mammary gland adenomas (in females) and the Leydig cell tumors of the testes were decreased. In hamsters, the incidence of adrenocortical adenomas were increased in the females. No other pertinent neoplastic changes were observed. In conclusion, the increases and decreases in benign neoplasms of hormone-sensitive tissues, appear to be the result of nutritionally/metabolism-induced modulation of the homeostasis in these 4 tissues in both species, and not the result of chronic glucose administration.

Possible role for gp160 in constitutive but not insulin-stimulated GLUT4 trafficking: dissociation of gp160 and GLUT4 localization

GLUT4-containing vesicles are constantly cycling in both basal and insulin-stimulated states. Our previous studies have shown that basal cycling of GLUT4 is impaired under conditions of high glucose or glucosamine and, as a consequence, GLUT4 is retained intracellularly in low-density microsomes [Filippis A., Clark, S., and Proietto, J. (1997) Biochem. J. 324, 981-985]. In addition to GLUT4 itself, a major protein component of GLUT4-containing vesicles is a glycoprotein of Mr 160000 (gp160). In all studies so far published gp160 has been co-localized with GLUT4 under all conditions. In this study, we show that retention of GLUT4 in low-density microsomes (enriched in Golgi apparatus) is associated with a decrease in gp160 levels in this compartment. A concomitant increase of gp160 in high-density microsomes (enriched in endoplasmic reticulum), demonstrates for the first time a dissociation in the localization of gp160 and GLUT4. Despite the marked decrease in gp160 levels in the GLUT4-containing compartment, insulin-stimulated translocation was normal, while little gp160 appeared in the plasma membrane in response to insulin. The retention of gp160 in the high-density microsomes is apparently not due to a change in the glycosylation state of gp160 as measured by [3H]mannose incorporation. It is concluded that, in rat adipocytes, gp160 is not required for insulin-stimulated translocation, but may be necessary for constitutive trafficking of the GLUT4-containing vesicle.

Regulatory elements in the insulin-responsive glucose transporter (GLUT4) gene

GLUT4, the insulin responsive-glucose transporter, mediates the rate limiting step of glucose metabolism in skeletal muscle and adipose tissue. GLUT4 expression is up-regulated by exercise training and thyroid hormone treatment and is down-regulated by fasting, streptozotocin-induced diabetes, obesity, high-fat diet, and denervation. Since overexpression of GLUT4 in insulin resistant db/db mice and high-fat diet-fed mice has been observed to dramatically improve glycemic control, increasing GLUT4 expression may be an effective strategy with which to alleviate insulin resistance. This review discusses recent findings on the regulation of the GLUT4 gene and on progress in the identification of regulatory elements in the promoter of the gene.

Regulated expression of 5'-deleted mouse GLUT4 minigenes in transgenic mice: effects of exercise training and high-fat diet

Fourteen kb murine GLUT4 minigene (= -7395 GLUT4) contains DNA sequence that confers tissue specific, exercise-induced up-regulation of the GLUT4 gene in skeletal muscle and high-fat diet induced-down-regulation in white adipose tissue. To identify the DNA sequences required for regulated expression, we generated GLUT4 minigene transgenic mice harboring 3237, 2000, 1000, and 442 bp of 5'-flanking region, all exons and introns, and 1 kb of 3'-flanking sequence of the mouse GLUT4 gene. The -3237-, -2000-, and -1000-GLUT4 constructs were expressed in a tissue-specific manner identical to the endogenous GLUT4. Exercise-induced up-regulation and high-fat diet-induced down-regulation of these constructs also paralleled those of the endogenous GLUT4 gene. In contrast, the -442 GLUT4 construct was expressed substantially in skeletal muscle (gastrocnemius and quadriceps) and heart, but was only expressed very weakly in white adipose tissue and was not expressed in brown adipose tissue. Furthermore, this -442 GLUT4 construct failed to respond to exercise or a high-fat diet in either muscle or adipose tissue. These results indicate that brown and white adipocyte-specific enhancer(s) and exercise- and high-fat diet-responsive elements are located between bases -1000 and -442 of the murine GLUT4 5'-flanking region.

Increased flux through the hexosamine biosynthesis pathway inhibits glucose transport acutely by activation of protein kinase C

The hexosamine biosynthesis pathway and protein kinase C (PKC) activation mediate hyperglycaemia-induced impaired glucose transport, but the relative role of each pathway is unknown. Following a 2 h preincubation of rat adipocytes in the presence of either high glucose (30 mM) plus insulin (0.7 nM) or glucosamine (3 mM), both high glucose and glucosamine inhibited subsequent basal and insulin-stimulated glucose transport, measured at 5.0 mM glucose. Azaserine, an inhibitor of the enzyme glutamine:fructose-6-phosphate aminotransferase, abolished the effect of high glucose, but not that of glucosamine. Ro-31-8220, an inhibitor of PKC, reversed the effects of both high glucose and glucosamine, suggesting that flux through the hexosamine biosynthesis pathway impaired glucose transport acutely by activating PKC. Both high glucose and glucosamine caused a 3-fold increase in PKC activity; this effect of high glucose, but not that of glucosamine, was partially decreased by azaserine. Neither high glucose nor glucosamine altered basal or insulin-stimulated plasma membrane GLUT1 levels, whereas both treatments decreased basal, but not insulin-stimulated, GLUT4 levels. Azaserine abolished the effect of high glucose, but not that of glucosamine, on basal plasma membrane GLUT4 levels. Ro-31-8220, which returned glucose transport to control values, caused a further decrease in plasma membrane GLUT4 levels. It is concluded that, in rat adipocytes, an acute increase in flux through the hexosamine biosynthesis pathway inhibits glucose transport by activation of PKC.

Chronic growth hormone treatment in normal rats reduces post-prandial skeletal muscle plasma membrane GLUT1 content, but not glucose transport or GLUT4 expression and localization

Whether skeletal muscle glucose transport system is impaired in the basal, post-prandial state during chronic growth hormone treatment is unknown. The current study was designed to determine whether 4 weeks of human growth hormone (hGH) treatment (3.5 mg/kg per day) would impair glucose transport and/or the number of glucose transporters in plasma membrane vesicles isolated from hindlimb skeletal muscle of Sprague-Dawley rats under basal, post-prandial conditions. hGH treatment was shown to have no effect on glucose influx (Vmax or K(m)) determined under equilibrium exchange conditions in isolated plasma membrane vesicles. Plasma membrane glucose transporter number (Ro) measured by cytochalasin B binding was also unchanged by hGH treatment. Consequently, glucose transporter turnover number (Vmax/Ro), a measure of average glucose transporter intrinsic activity, was similar in hGH-treated and control rats. hGH did not change GLUT4 protein content in whole muscle or in the plasma membrane, and muscle content of GLUT4 mRNA also was unchanged. In contrast, GLUT1 protein content in the plasma membrane fraction was significantly reduced by hGH treatment. This was associated with a modest, although not significant, decrease in muscle content of GLUT1 mRNA. In conclusion, high-dose hGH treatment for 4 weeks did not alter post-prandial skeletal muscle glucose transport activity. Neither the muscle level nor the intracellular localization of GLUT4 was changed by the hormone treatment. On the contrary, the basal post-prandial level of GLUT1 in the plasma membrane was reduced by hGH. The mRNA data suggest that this reduction might result from a decrease in the synthesis of GLUT1.

Islet transplantation under the kidney capsule fully corrects the impaired skeletal muscle glucose transport system of streptozocin diabetic rats

Chronic insulin therapy improves but does not restore impaired insulin-mediated muscle glucose uptake in human diabetes or muscle glucose uptake, transport, and transporter translocation in streptozocin diabetic rats. To determine whether this inability is due to inadequate insulin replacement, we studied fasted streptozocin-induced diabetic Lewis rats either untreated or after islet transplantation under the kidney capsule. Plasma glucose was increased in untreated diabetics and normalized by the islet transplantation (110 +/- 5, 452 +/- 9, and 102 +/- 3 mg/dl in controls, untreated diabetics, and transplanted diabetics, respectively). Plasma membrane and intracellular microsomal membrane vesicles were prepared from hindlimb skeletal muscle of basal and maximally insulin-stimulated rats. Islet transplantation normalized plasma membrane carrier-mediated glucose transport Vmax, plasma membrane glucose transporter content, and insulin-induced transporter translocation. There were no differences in transporter intrinsic activity (Vmax/Ro) among the three groups. Microsomal membrane GLUT4 content was reduced by 30% in untreated diabetic rats and normal in transplanted diabetics, whereas the insulin-induced changes in microsomal membrane GLUT4 content were quantitatively similar in the three groups. There were no differences in plasma membrane GLUT1 among the groups and between basal and insulin stimulated states. Microsomal membrane GLUT1 content was increased 60% in untreated diabetics and normalized by the transplantation. In conclusion, an adequate insulin delivery in the peripheral circulation, obtained by islet transplantation, fully restores the muscle glucose transport system to normal in streptozocin diabetic rats.