Vanadate and rapamycin synergistically enhance insulin-stimulated glucose uptake

Tyrosine dephosphorylation, serine phosphorylation, and proteasomal degradation of insulin receptor substrates (IRSs) are implicated in the negative regulation of insulin action. Here we show that simultaneous inhibition of IRS-1 tyrosine dephosphorylation and proteasomal degradation synergistically augments insulin-responsive glucose uptake. L6 skeletal muscle cells (L6 cells) were treated with inhibitors of protein-tyrosine phosphatases, proteasomal degradation, and mammalian target of rapamycin (mTOR), and the effects of insulin on glucose uptake, IRS-1 tyrosine phosphorylation, phosphatidylinositol (PI) 3-kinase activity, and IRS-1 mass were examined. Pretreatment of L6 cells with sodium orthovanadate (Na(3)VO(4)) plus the mTOR inhibitor rapamycin caused a 5-fold increase in insulin-responsive glucose uptake at 2 hours when compared to insulin alone. Evaluation of IRS-1 associated PI 3-kinase activity, IRS-1-associated p85 mass, and IRS-1 tyrosine phosphorylation showed that 2 hours after insulin addition they were reduced by 70% from maximal activity. Likewise, IRS-1 mass was reduced by 50%. When L6 cells were pretreated with Na(3)VO(4) plus the proteasome inhibitor MG-132 or the mTOR inhibitor rapamycin prior to insulin addition, IRS-1 mass loss as well as IRS-1/PI-3 kinase complex decay was blocked at 2 hours and PI 3-kinase activity was increased 2.5-fold and 4-fold, respectively, over insulin alone. Finally, treatment of L6 cells with subtherapeutic amounts of vanadyl sulfate and rapamycin induced a synergistic 3-fold increase in insulin-induced glucose uptake at 2 hours. These findings indicate that vanadium and rapamycin synergize to enhance glucose uptake by preventing IRS-1 mass loss and IRS-1/PI 3-kinase complex decay and may offer a new approach to enhance glucose transport in diabetes.     

Molecular mechanisms of insulin-stimulated glucose uptake in adipocytes

The stimulation of muscle and adipose tissue glucose metabolism, which is ultimately responsible for bringing about post-absorptive blood glucose clearance, is the primary clinically relevant action of insulin. Insulin acts on many steps of glucose metabolism, but one of the most important effects is its ability to increase the rate of cellular glucose transport. This results from the translocation of the insulin-responsive transporter isoform, GLUT4, from intra-cellular vesicular storage sites to the plasma membrane. In adipocytes, a substantial amount of cellular GLUT4 is located in a specific highly insulin-responsive storage pool, termed GLUT4 Storage Vesicles (GSVs). GLUT4 can also translocate to the plasma membrane from the recycling endosomal pool which also additionally contains the GLUT1 isoform of glucose transporter and the transferrin receptor. In this article we review the molecular mechanism by which insulin stimulates GLUT4 translocation in adipose cells, including the nature of the signaling pathways involved and the role of the cytoskeleton.     

Intraportal hyperinsulinemia decreases insulin-stimulated glucose uptake in the dog

Hyperinsulinemia and insulin resistance are commonly seen in obese and non-insulin-dependent diabetes mellitis (NIDDM) patients. While it is known that chronic exposure to severe hyperinsulinemia can lead to an insulin-resistant state and mild hyperinsulinemia for rather short durations (20 to 40 hours) and can also lead to insulin resistance, it is less clear whether mild hyperinsulinemia for a more prolonged duration can lead to insulin resistance. In the present study we determined the effects of chronic (28 days) exposure to mild hyperinsulinemia on insulin-stimulated glucose use. Chronic hyperinsulinemia was produced by an intraportal infusion of porcine insulin (425 microU/kg/min), which raised the basal peripheral insulin levels by approximately 50%. Insulin responsiveness was assessed using the euglycemic hyperinsulinemic clamp (2 mU/kg/min) in dogs before the induction of chronic hyperinsulinemia (day 0), after 28 days of hyperinsulinemia (day 28), and 28 days after discontinuation of the chronic insulin infusion (day 56). The amount of glucose (M) required to maintain euglycemia during the euglycemic hyperinsulinemic clamp was decreased (relative to day 0) 39% +/- 3% on day 28 and 18% +/- 3% on day 56 (P less than .05). In control animals that received a chronic infusion of saline for the 28-day period the glucose infusion rate (M) was not changed significantly (decreasing 2% +/- 5% and 5% +/- 10% on days 28 and 56, respectively). In conclusion insulin resistance can be produced by a mild hypersecretion of insulin and discontinuation of the chronic insulin infusion tends to reverse the resistance.     

Variations in insulin-stimulated glucose uptake in healthy individuals with normal glucose tolerance

Measurements were made of both glucose disposal (M) during hyperinsulinemic clamp studies and plasma glucose and insulin responses to an oral glucose challenge in 100 individuals with normal glucose tolerance. The subjects were divided into 4 quartiles on the basis of M values, ranging from a low mean (+/- SEM) value of 140 +/- 3 mg/m2 X min (quartile 1) to a high of 349 mg/m2 X min (quartile 4). The plasma insulin response to oral glucose inversely correlated with the M value (r = -0.60; P less than 0.001), being highest in those with the lowest M (quartile 1) and lowest in those with the highest M (quartile 4). On the other hand, the plasma glucose responses of the 4 quartiles were virtually identical. These results document that insulin-stimulated glucose uptake varies widely in subjects with normal glucose tolerance, and that these differences are independent of any change in the plasma glucose response to oral glucose. Furthermore, the results indicate that insulin resistance in normal individuals is associated with hyperinsulinemia.     

The physiology of amylin and insulin: maintaining the balance between glucose secretion and glucose uptake

Glucose homeostasis is accomplished through intricate, and arguably, elegant interactions among several organs and hormones. Historically, glucose homeostasis has been viewed somewhat narrowly--insulin from pancreatic beta cells regulated glucose disposal, while glucagon from pancreatic alpha [corrected] cells regulated glucose appearance during fasting states. But more recent characterization and understanding of the role of incretin hormones from the gut--notably, glucagon-like peptide 1 and gastric inhibitory polypeptide--and amylin from pancreatic beta cells has led to a more complete model of glucose homeostasis. Furthermore, availability of pharmacologic agents to replace, mimic, or enhance the actions of these hormones allows application of this more complete model of glucose homeostasis to the treatment of type 1 and type 2 diabetes. This article provides an overview of the role of the pancreatic hormone amylin in glucose homeostasis and of Pramlintide, a analogue of native amylin, recently approved as adjunct therapy to insulin in people with type 1 and type 2 diabetes.     

胰淀素对胰岛素刺激的人脂肪细胞糖摄取的影响

观察胰淀素对2型糖尿病及非糖尿病者脂肪细胞糖摄取的影响以及胰岛素对此影响的干预作用。使用2-脱氧-D-[1-3H]-葡萄糖直接检测法观察脂肪细胞葡萄糖摄取量。结果发现胰淀素抑制人脂肪细胞糖摄取，大剂量胰岛素可以减低胰淀素的抑制作用。在糖尿病组胰淀素的抑制较明显，同时胰岛素的改善作用也较为明显。     

Normal insulin-stimulated endothelial function and impaired insulin-stimulated muscle glucose uptake in young adults with low birth weight

Low birth weight has been linked to insulin resistance and cardiovascular disease. We hypothesized that insulin sensitivity of both muscle and vascular tissues were impaired in young men with low birth weight. Blood flow was measured by venous occlusion plethysmography during dose-response studies of acetylcholine and sodium nitroprusside in the forearm of fourteen 21-yr-old men with low birth weight and 16 controls of normal birth weight. Glucose uptake was measured during intraarterial insulin infusion. Dose-response studies were repeated during insulin infusion. The maximal blood flow during acetylcholine infusion was 14.1 +/- 2.7 and 14.4 +/- 2.1 [ml x (100 ml forearm)(-1) x min(-1)] in low and normal birth weight subjects, respectively. Insulin coinfusion increased acetylcholine-stimulated flow in both groups: 18.0 +/- 3.1 vs. 17.9 +/- 3.1 [ml x (100 ml forearm)(-1) x min(-1)], NS. Insulin infusion increased glucose uptake significantly in the normal birth weight group, compared with the low birth weight group: 0.40 +/- 0.09 to 1.00 +/- 0.16 vs. 0.44 +/- 0.09 to 0.59 +/- 0.1 [ micro mol glucose x (100 ml forearm)(-1) x min(-1)], P = 0.04. Young men with low birth weight have normal insulin-stimulated endothelial function and impaired insulin-stimulated forearm glucose uptake. Thus, endothelial dysfunction does not necessarily coexist with metabolic alterations in subjects with low birth weight.     

TC10alpha is required for insulin-stimulated glucose uptake in adipocytes

Previous studies have suggested that activation of the Rho family member GTPase TC10 is necessary but not sufficient for the stimulation of glucose transport by insulin. We show here that endogenous TC10alpha is rapidly activated in response to insulin in 3T3L1 adipocytes in a phosphatidylinositol 3-kinase-independent manner, whereas platelet-derived growth factor was without effect. Knockdown of TC10alpha but not TC10beta by RNA interference inhibited insulin-stimulated glucose uptake as well as the translocation of the insulin-sensitive glucose transporter GLUT4 from intracellular sites to the plasma membrane. In contrast, loss of TC10alpha had no effect on the stimulation of Akt by insulin. Additionally, knockdown of TC10alpha inhibited insulin-stimulated translocation of its effector CIP4. These data indicate that TC10alpha is specifically required for insulin-stimulated glucose uptake in adipocytes.     

GLUT8 is a glucose transporter responsible for insulin-stimulated glucose uptake in the blastocyst

Mammalian preimplantation blastocysts exhibit insulin-stimulated glucose uptake despite the absence of the only known insulin-regulated transporter, GLUT4. We describe a previously unidentified member of the mammalian facilitative GLUT superfamily that exhibits approximately 20-25% identity with other murine facilitative GLUTs. Insulin induces a change in the intracellular localization of this protein, which translates into increased glucose uptake into the blastocyst, a process that is inhibited by antisense oligoprobes. Presence of this transporter may be necessary for successful blastocyst development, fuel metabolism, and subsequent implantation. Moreover, the existence of an alternative transporter may explain examples in other tissues of insulin-regulated glucose transport in the absence of GLUT4.     

Magnesium reduces insulin-stimulated glucose uptake and serum lipid concentrations in type 1 diabetes

A magnesium (Mg) deficit has been described in patients with type 1 diabetes, and it has been related to the development of cardiovascular disease. We tested the hypothesis that type 1 diabetic patients have deficits in dietary Mg intake and that proper long-term (24 weeks) oral Mg supplementation would reduce cardiovascular risk factors. Therefore, the Mg status, dietary Mg intake, and the effect of Mg supplementation were evaluated in 10 type 1 diabetic patients and 5 control subjects. Muscle Mg content was decreased by 7% in the type 1 diabetic patients, and it increased by 5% after 24 weeks of oral MgO supplementation. Acute and chronic Mg supplementation decreased serum total cholesterol, serum low-density lipoprotein (LDL)-cholesterol, and apolipoprotein B. Insulin-stimulated glucose uptake decreased by 35% after 24 weeks of oral MgO supplementation. Eight of 10 patients with type 1 diabetes had a daily intake of Mg below 90% of the recommended daily allowance. In conclusion, a Mg deficit was found in type 1 diabetic patients. The deficit might be due partly to a relatively Mg-deficient diet. Mg repletion was associated with a decrease in atherogenic lipid fractions and a reduced insulin-stimulated glucose uptake.     

Enhancement of insulin-stimulated myocardial glucose uptake in patients with Type 2 diabetes treated with rosiglitazone.

AIMS: Peroxisome proliferator-activated receptor gamma (PPARgamma) activators have recently been identified as regulators of cellular proliferation, inflammatory responses and lipid and glucose metabolism. These agents prevent coronary arteriosclerosis and improve left ventricular remodelling and function in heart failure after myocardial infarction. Improvement in myocardial metabolic state may be one of the mechanisms behind these findings. The aim of this study was to investigate the effects of rosiglitazone on myocardial glucose uptake in patients with Type 2 diabetes. Placebo and metformin were used as control treatments. METHODS: Forty-four patients were randomized to treatment with rosiglitazone (4 mg b.i.d.), metformin (1 g b.i.d.) or placebo in a 26-week double-blinded trial. Myocardial glucose uptake was measured using [(18)F]-2-fluoro-2-deoxy-D-glucose ([(18)F]FDG) and positron emission tomography (PET) during euglycaemic hyperinsulinaemia before and after the treatment. RESULTS: Rosiglitazone increased insulin-stimulated myocardial glucose uptake by 38% (from 38.7 +/- 3.4 to 53.3 +/- 3.6 micromol 100 g(-1) min(-1), P = 0.004) and whole body glucose uptake by 36% (P = 0.01), while metformin treatment had no significant effect on myocardial (40.5 +/- 3.5 vs. 36.6 +/- 5.2, NS) or whole body glucose uptake. Myocardial work as determined by the rate-pressure-product was similar between the groups. Neither treatment had any significant effect on fasting serum free fatty acids (FFA) but the FFA levels during hyperinsulinaemia were more suppressed in the rosiglitazone group (-47%, P = 0.02). Myocardial glucose uptake correlated inversely to FFA concentrations both before (r =-0.54, P = 0.002) and after (r = -0.43, P = 0.01) the treatment period in the pooled data. Furthermore, the increase in myocardial glucose uptake correlated inversely with interleukin-6 (IL-6) concentrations (r = -0.58, P = 0.03). CONCLUSIONS: In addition to the improvement in whole body insulin sensitivity, rosiglitazone treatment enhances insulin stimulated myocardial glucose uptake in patients with Type 2 diabetes, most probably due to its suppression of the serum FFAs.     

Chronic leptin administration increases insulin-stimulated skeletal muscle glucose uptake and transport.

Leptin, the product of the ob gene, has been shown to reduce fat mass, food intake, hyperglycemia, and hyperinsulinemia and to increase whole-body glucose disposal. However, it is unknown if leptin improves insulin action in skeletal muscle. Therefore, the purpose of this investigation was to determine if chronic leptin administration increases insulin-stimulated skeletal muscle glucose uptake and transport. Sixty-nine female Sprague-Dawley rats (240 to 250 g) were randomly assigned to one of three groups: (1) control, (2) pair-fed, and (3) leptin. All animals were subcutaneously implanted with miniosmotic pumps that delivered 0.5 mg leptin/kg/d to the leptin animals and vehicle to the control and pair-fed animals for 14 days. Following this 14-day period, all animals were subjected to hindlimb perfusion to determine the rates of skeletal muscle glucose uptake and 3-O-methyl-D-glucose (3-MG) transport under basal, submaximal (500 microU/mL), and maximal (10,000 microU/mL) insulin concentrations. Chronic leptin treatment significantly increased (P < .05) the rate of glucose uptake across the hindlimb by 27%, 32%, and 47% under basal, submaximal, and maximal insulin, respectively, compared with the control and pair-fed condition. However, when the submaximal rate of glucose uptake was expressed as a percentage of maximal insulin-stimulated glucose uptake, no differences existed among the groups, indicating that leptin treatment does not increase insulin sensitivity. Rates of 3-MG transport in the soleus, plantaris, and white and red portions of the gastrocnemius (WG and RG) were significantly increased (P < .05) in leptin animals under all perfusion conditions. 3-MG transport was not different between control and pair-fed animals. Collectively, these findings suggest that improvements in insulin-stimulated skeletal muscle glucose uptake and transport following chronic leptin treatment result from increased insulin responsiveness.     

Disruption of Sur2-containing K(ATP) channels enhances insulin-stimulated glucose uptake in skeletal muscle

ATP-sensitive potassium channels (K(ATP)) are involved in a diverse array of physiologic functions including protection of tissue against ischemic insult, regulation of vascular tone, and modulation of insulin secretion. To improve our understanding of the role of K(ATP) in these processes, we used a gene-targeting strategy to generate mice with a disruption in the muscle-specific K(ATP) regulatory subunit, SUR2. Insertional mutagenesis of the Sur2 locus generated homozygous null (Sur2(-/-)) mice and heterozygote (Sur2(+/-)) mice that are viable and phenotypically similar to their wild-type littermates to 6 weeks of age despite, respectively, half or no SUR2 mRNA expression or channel activity in skeletal muscle or heart. Sur2(-/-) animals had lower fasting and fed serum glucose, exhibited improved glucose tolerance during a glucose tolerance test, and demonstrated a more rapid and severe hypoglycemia after administration of insulin. Enhanced glucose use was also observed during in vivo hyperinsulinemic euglycemic clamp studies during which Sur2(-/-) mice required a greater glucose infusion rate to maintain a target blood glucose level. Enhanced insulin action was intrinsic to the skeletal muscle, as in vitro insulin-stimulated glucose transport was 1.5-fold greater in Sur2(-/-) muscle than in wild type. Thus, membrane excitability and K(ATP) activity, to our knowledge, seem to be new components of the insulin-stimulated glucose uptake mechanism, suggesting possible future therapeutic approaches for individuals suffering from diabetes mellitus.     

Relationships between insulin secretion, insulin metabolism and insulin resistance in mild glucose intolerance

The aim of this study was to evaluate whether the correlation between insulin resistance and peripheral hyperinsulinaemia existing in mild glucose intolerance corresponds to a relationship between insulin resistance and insulin overproduction by the pancreas. In addition, the possibility that insulin resistance is related to insulin metabolism was examined. Twenty five subjects with fasting normoglycaemia and an abnormal glucose response to the oral glucose tolerance test (OGTT) were studied. Insulin secretion by the pancreas was estimated by means of fasting C-peptide levels in peripheral blood. Insulin resistance was estimated by the rate of glucose disappearance from plasma after i.v. insulin injection. Insulin metabolism was estimated indirectly by the C-peptide: insulin molar ratio. A negative correlation was found between the glucose disappearance rate from plasma after i.v. insulin injection and fasting insulin levels (r = -0.677, p less than 0.001), but not fasting C-peptide concentrations (r = -0.164, p = NS). Glucose disappearance rate from plasma correlated positively with the C-peptide: insulin molar ratio (r = 0.626, p less than 0.001). These results suggest that in mild glucose intolerance insulin resistance and insulin secretion by the pancreas are not related phenomena, and that the defect responsible for insulin resistance might also be implicated in the impaired insulin metabolism.     

Relationship between insulin sensitivity, insulin secretion and glucose tolerance in cirrhosis

Hepatic insulin extraction is difficult to measure in humans; as a result, the interrelationship between defective insulin secretion and insulin insensitivity in the pathogenesis of glucose intolerance in cirrhosis remains unclear. To reassess this we used recombinant human C-peptide to measure C-peptide clearance in cirrhotic patients and controls and thus derive C-peptide and insulin secretion rates after a 75-gm oral glucose load and during a 10 mmol/L hyperglycemic clamp. Cirrhotic patients were confirmed as insulin-insensitive during a euglycemic clamp (glucose requirement: 4.1 +/- 0.1 mg/kg/min vs. 8.1 +/- 0.5 mg/kg/min; p less than 0.001), which also demonstrated a low insulin metabolic clearance rate (p less than 0.001). Although intolerant after oral glucose, the cirrhotic patients had glucose requirements identical to those of controls during the hyperglycemic clamp (cirrhotic patients: 6.1 +/- 1.0 mg/kg/min; controls: 6.3 +/- 0.7 mg/kg/min), suggesting normal intravenous glucose tolerance. C-peptide MCR was identical in cirrhotic patients (2.93 +/- 0.16 ml/min/kg) and controls (2.96 +/- 0.24 ml/min/kg). Insulin secretion was higher in cirrhotic patients, both fasting (2.13 +/- 0.26 U/hr vs. 1.09 +/- 0.10 U/hr; p less than 0.001) and from min 30 to 90 of the hyperglycemic clamp (5.22 +/- 0.70 U/hr vs. 2.85 +/- 0.22 U/hr; p less than 0.001). However, with oral glucose the rise in serum C-peptide concentration was relatively delayed, and the insulin secretion index (secretion/area under 3-hr glucose curve) was not elevated. Hepatic insulin extraction was reduced both in fasting and during the hyperglycemic clamp (p less than 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)