Disassociation of muscle insulin signaling and insulin-stimulated glucose uptake during endotoxemia

Lipopolysaccharide (LPS) elicits a strong immune response, which leads to the release of inflammatory cytokines. Increased cytokine production has been shown to impair insulin-mediated glucose disposal. LPS can alter other factors, such as muscle blood flow and insulin signaling in the myocyte, that can influence glucose disposal. We hypothesize that LPS induced impairments in cardiovascular function contribute to the associated impairments in insulin action in vivo. Male wild-type C57BL/6J mice had a catheter implanted in the jugular vein for infusions and the carotid artery for sampling 5 days prior to the hyperinsulinemic-euglycemic clamp. Mice were treated with vehicle, low- (1 ug/gBW) or high-dose (10 ug/gBW) LPS 4 hours prior to the clamp. Muscle glucose uptake (MGU) was assessed using [2-(14)C] deoxyglucose. While both low- and high-dose LPS inhibited insulin-stimulated MGU compared to vehicle-treated mice, the impairment was more significant with the high-dose treatment (～25% in soleus and ～70% in both gastrocnemius and vastus lateralis). Interestingly, insulin signaling through the PI3-kinase pathway in the muscle was not affected by this treatment suggesting that the decrease in MGU is not directly due to impairments in muscle insulin action. Echocardiography demonstrated that high-dose LPS treatment significantly decreased stroke volume (～30%), heart rate (～35%), and cardiac output (～50%). These observations were not seen with vehicle or low-dose LPS treatment. High-dose LPS treatment also significantly decreased muscle blood flow (～70%) and whole body oxygen consumption (～50%). Thus, in vivo acute endotoxemia does not impair insulin signaling through the PI3-kinase pathway in skeletal muscle and decreased tissue blood flow likely plays a central role in the impairment of glucose uptake in the muscle.     

Attenuation of insulin-stimulated insulin receptor substrate-1 serine 307 phosphorylation in insulin resistance of type 2 diabetes

Insulin resistance is a primary characteristic of type 2 diabetes and likely causally related to the pathogenesis of the disease. It is a result of defects in signal transduction from the cell surface receptor of insulin to target effects. We found that insulin-stimulated phosphorylation of serine 307 (corresponding to serine 302 in the murine sequence) in the immediate downstream mediator protein of the insulin receptor, insulin receptor substrate-1 (IRS1), is required for efficient insulin signaling and that this phosphorylation is attenuated in adipocytes from patients with type 2 diabetes. Inhibition of serine 307 phosphorylation by rapamycin mimicked type 2 diabetes and reduced the sensitivity of IRS1 tyrosine phosphorylation in response to insulin, while stimulation of the phosphorylation by okadaic acid, in cells from patients with type 2 diabetes, rescued cells from insulin resistance. EC(50) for insulin-stimulated phosphorylation of serine 307 was about 0.2 nM with a t(1/2) of about 2 min. The amount of IRS1 was similar in cells from non-diabetic and diabetic subjects. These findings identify a molecular mechanism for insulin resistance in non-selected patients with type 2 diabetes.     

Glucocorticoid-induced heat resistance in mammalian cells

Chinese hamster ovary cells were incubated for 24 h in a variety of steroid hormones (testosterone, progesterone, hydrocortisone, dexamethasone, and ecdysterone) to test their effect on the subsequent heat resistance of the cells. Only the glucocorticoids, hydrocortisone and dexamethasone, consistently induced heat resistance. Heat resistance induced by hydrocortisone at 10(-6)M developed after a lag of 2-3 h and was maximal by 20 h. Resistance was expressed in both asynchronous and plateau phase cells and was maintained for several days in medium without added hormone. Incubation of cells with hydrocortisone and a 100-fold excess of progesterone (a glucocorticoid antagonist) partially inhibited the development of resistance. Prior exposure to hydrocortisone did not inhibit the subsequent development of heat induced thermotolerance. However, cells made thermotolerant by prior heat shock did not display further heat resistance with hydrocortisone treatment. There was no evidence for the induction of heat shock proteins (HSP) by these steroid hormones although the 28 kDHSP was further enhanced by combined heat and hydrocortisone. Our results indicate that heat resistance in mammalian cells may be induced by physiological concentrations of glucocorticoids and that the characteristics of this resistance are consistent with a receptor mediated event.     

An amino acid mixture enhances insulin-stimulated glucose uptake in isolated rat epitrochlearis muscle

Protein and certain amino acids (AA) have been found to lower blood glucose. Although these glucose-lowering AA are important modulators of skeletal muscle metabolism, their impact on muscle glucose uptake remains unclear. We therefore examined how an AA mixture consisting of 2 mM isoleucine, 0.012 mM cysteine, 0.006 mM methionine, 0.0016 mM valine, and 0.014 mM leucine impacts skeletal muscle glucose uptake in the absence or presence of a submaximal (sINS) or maximal insulin (mINS) concentration. The AA mixture, sINS, and mINS significantly increased 2-[(3)H]deoxyglucose (2-DG) uptake by 63, 79, and 298% above basal, respectively. When the AA mixture was combined with sINS and mINS, 2-DG uptake was further increased significantly by 26% (P = 0.028) and 14% (P = 0.032), respectively. Western blotting analysis revealed that the AA mixture increased basal and sINS Akt substrate of 160 kDa (AS160) phosphorylation, while AA mixture did not change phosphorylation of Akt or mammalian target of rapamycin (mTOR) under these conditions. Interestingly, addition of the AA mixture to mINS increased phosphorylation of mTOR, Akt as well as AS160, compared with mINS alone. These data suggest that certain AA increase glucose uptake in the absence of insulin and augment insulin-stimulated glucose uptake in an additive manner. Furthermore, these effects appear to be mediated via a pathway that is independent from the canonical insulin cascade and therefore may prove effective as an alternative therapeutic treatment for insulin resistance.     

Ras inhibition induces insulin sensitivity and glucose uptake

Background

Reduced glucose uptake due to insulin resistance is a pivotal mechanism in the pathogenesis of type 2 diabetes. It is also associated with increased inflammation. Ras inhibition downregulates inflammation in various experimental models. The aim of this study was to examine the effect of Ras inhibition on insulin sensitivity and glucose uptake, as well as its influence on type 2 diabetes development.

Methods and Findings

The effect of Ras inhibition on glucose uptake was examined both in vitro and in vivo. Ras was inhibited in cells transfected with a dominant-negative form of Ras or by 5-fluoro-farnesylthiosalicylic acid (F-FTS), a small-molecule Ras inhibitor. The involvement of IκB and NF-κB in Ras-inhibited glucose uptake was investigated by immunoblotting. High fat (HF)-induced diabetic mice were treated with F-FTS to test the effect of Ras inhibition on induction of hyperglycemia. Each of the Ras-inhibitory modes resulted in increased glucose uptake, whether in insulin-resistant C2C12 myotubes in vitro or in HF-induced diabetic mice in vivo. Ras inhibition also caused increased IκB expression accompanied by decreased expression of NF-κB . In fat-induced diabetic mice treated daily with F-FTS, both the incidence of hyperglycemia and the levels of serum insulin were significantly decreased.

Conclusions

Inhibition of Ras apparently induces a state of heightened insulin sensitization both in vitro and in vivo. Ras inhibition should therefore be considered as an approach worth testing for the treatment of type 2 diabetes.     

Chemerin enhances insulin signaling and potentiates insulin-stimulated glucose uptake in 3T3-L1 adipocytes

To explore a novel adipokine, we screened adipocyte differentiation-related gene and found that TIG2/chemerin was strongly induced during the adipocyte differentiation. Chemerin was secreted by the mature 3T3-L1 adipocytes and expressed abundantly in adipose tissue in vivo as recently described. Intriguingly, the expression of chemerin was differently regulated in the liver and adipose tissue in db/db mice. In addition, serum chemerin concentration was decreased in db/db mice. Chemerin and its receptor/ChemR23 were expressed in mature adipocytes, suggesting its function in autocrine/paracrine fashion. Finally, chemerin potentiated insulin-stimulated glucose uptake concomitant with enhanced insulin signaling in the 3T3-L1 adipocytes. These data establish that chemerin is a novel adipokine that regulates adipocyte function.     

Uncarboxylated osteocalcin decreases insulin-stimulated glucose uptake without affecting insulin signaling and regulators of mitochondrial biogenesis in myotubes

Uncarboxylated osteocalcin (uOC) is a circulating bone matrix protein, which has previously been shown to regulate glucose uptake and systemic metabolism. However, the cellular mechanism by which uOC acts has yet to be elucidated. C2C12 mouse myotubes were treated for 72 h with uOC (1–100 ng/mL). Cellular metabolism was analyzed using oxygen consumption and extracellular acidification rate. Metabolic gene and protein expression were measured via quantitative real-time polymerase chain reaction and Western blot, respectively. Additionally, C2C12 myotubes were treated with 10 ng/mL uOC to examine glucose uptake and activation of insulin signaling with or without insulin resistance. Finally, cellular lipid content was measured via Oil Red O and Nile Red staining. uOC treatment resulted in dose-dependent alterations of oxygen consumption with little effect on regulators of mitochondrial metabolism. Basal expression of regulators of glucose uptake were unaffected by uOC treatment. However, insulin-stimulated glucose uptake was blunted by uOC treatment with no concurrent alterations in insulin signaling. While chronic insulin treatment resulted in suppressed activation of Akt, concurrent uOC treatment was unable to prevent these detrimental effects on insulin signaling. uOC treatment had no effect on markers of lipogenesis and cellular lipid content. These findings suggest that 72-h uOC treatment may alter oxygen consumption without effect on regulators of mitochondrial biogenesis. Additionally, uOC treatment suppressed insulin-stimulated glucose uptake in cultured myotubes but had little effect on insulin signaling or regulators of cellular metabolism and was unable to mitigate insulin resistance.     

Glucagon inhibition of insulin-stimulated 2-deoxyglucose uptake by rat adipocytes in the presence of adenosine deaminase.

Effects of adenosine deaminase and glucagon on insulin-stimulated 2-deoxyglucose uptake by rat adipocytes are reported. (1) Adenosine deaminase (10 micrograms/ml) caused a rightward shift in the dose-response curve for the stimulation by insulin of 2-deoxyglucose uptake, but the enzyme did not alter either the basal or the maximally insulin-stimulated uptake rate. (2) In adipocytes obtained from 24 h-starved rats, glucagon inhibited the effect of insulin on 2-deoxyglucose uptake in the presence (but not in the absence) of adenosine deaminase. Basal uptake rates were unaffected. (3) Glucagon inhibited insulin-stimulated 2-deoxyglucose uptake to a greater extent in cells isolated from starved rats than in cells from fed rats. (4) Adipocytes isolated from fed and from starved rats did not differ in their capacity for degradation of 125I-labelled glucagon. The results suggest that adenosine and glucagon are regulators of insulin action in adipose tissue.     

Role of insulin-stimulated protein phosphorylation in insulin action

Insulin promotes both the phosphorylation and dephosphorylation of proteins in its target cells. Insulin-induced dephosphorylation has long been thought to serve an important regulatory function; the role of insulin-stimulation phosphorylation is less certain. The proteins known to be substrates for this reaction are ATP citrate (pro-3S)-lyase, acetyl-CoA carboxylase, and the ribosomal subunit S6. The evidence as to the physiological role and mechanism underlying the insulin-stimulated phosphorylation of these proteins is summarized. Present information suggests that insulin-stimulated phosphorylation may serve an important regulatory role in certain actions of insulin.     

A novel insulin sensitizer (S15511) enhances insulin-stimulated glucose uptake in rat skeletal muscles.

Type 2 diabetes is preceded by the presence of skeletal muscle insulin resistance, and drugs that increase insulin sensitivity in skeletal muscle prevent the disease. S15511 is an original compound with demonstrated effects on insulin sensitivity in animal models of insulin resistance. However, the mechanisms behind the insulin-sensitizing effect of S15511 are unknown. The aim of our study was to explore whether S15511 improves insulin sensitivity in skeletal muscles. Insulin sensitivity was assessed in skeletal muscles from S15511-treated rats by measuring intracellular insulin-signaling activity and insulin-stimulated glucose transport in isolated muscles. In addition, GLUT4 expression and glycogen levels were assessed after treatment. S15511 treatment was associated with an increase in insulin-stimulated glucose transport in type IIb fibers, while type I fibers were unaffected. The enhanced glucose transport was mirrored by a fiber type-specific increase in GLUT4 expression, while no improvement in insulin-signaling activity was observed. S15511 is a novel insulin sensitizer that is capable of improving glucose homeostasis in nondiabetic rats. The compound enhances skeletal muscle insulin sensitivity and specifically targets type IIb muscle fibers by increasing GLUT4 expression. Together these data show S15511 to be a potentially promising new drug in the treatment and prevention of type 2 diabetes.     

Two systems in vitro that show insulin-stimulated serine kinase activity towards the insulin receptor.

Two systems in vitro are described that show insulin-stimulated phosphorylation of the insulin receptor on serine residues. In the first system, insulin receptor was purified partially from Fao rat hepatoma cells by direct solubilization of the cells in Triton X-100 and chromatography on wheat-germ-agglutinin-agarose. Phosphorylation of these preparations with [gamma-32P]ATP in the presence or absence of insulin resulted in 32P incorporation exclusively into phosphotyrosine residues. Serine kinase activity towards the insulin receptor was reconstituted by adding extracts of Fao cells. Prior exposure of the cells to insulin stimulated serine kinase activity towards the insulin receptor in extracts 7.2-fold. A receptor serine kinase activity enhanced by treatment of cells with cyclic AMP analogues was also retained in the reconstituted system. In the second system, insulin receptor and insulin-sensitive serine kinase activity towards the insulin receptor were co-purified from human placenta. The protocol involved preparation of membranes, before solubilization and chromatography on wheat-germ-agglutinin-agarose, by using gentle procedures designed not to disrupt a potentially labile association between the insulin receptor and the serine kinase. Serine kinase activity in these preparations towards the insulin receptor was stimulated up to 10-fold by insulin, and the stoicheiometry of serine phosphorylation was estimated to be approx 0.8 mol/mol of insulin receptor for phosphorylations performed in the presence of insulin. Thus a preparation of insulin receptor is described for the first time that is phosphorylated to high stoicheiometry on serine in an insulin-dependent manner. Conditions that facilitate recovery and assay of serine kinase activity are defined and discussed. These systems provide a basis for characterizing the nature of the insulin-sensitive serine kinase that phosphorylates the insulin receptor, and defining its role in insulin action and control of receptor function.     

Reduction of insulin-stimulated glucose uptake by peroxynitrite is concurrent with tyrosine nitration of insulin receptor substrate-1

Inducible nitric oxide synthetase plays an essential role in insulin resistance induced by a high-fat diet. The reaction of nitric oxide with superoxide leads to the formation of peroxynitrite (ONOO-), which can modify several proteins. In this study, we investigated whether peroxynitrite impairs insulin-signalling pathway. Our experiments showed that 3-(4-morpholinyl)sydnonimine hydrochloride (SIN-1), a constitutive producer of peroxynitrite, dose-dependently inhibited insulin-stimulated glucose uptake. While SIN-1 did not affect the insulin receptor protein level and tyrosine phosphorylation, it reduced the insulin receptor substrate-1 (IRS-1) protein level, and IRS-1 associated phosphatidylinositol-3 kinase (PI-3 kinase) activity. Although SIN-1 did not induce Ser307 phosphorylation of IRS-1, tyrosine nitration of IRS-1 was detected in SIN-1-treated-Rat1 fibroblasts expressing human insulin receptors. Mass spectrometry showed that peroxynitrite induced at least four nitrated tyrosine residues in rat IRS-1, including Tyr939, which is critical for association of IRS-1 with the p85 subunit of PI-3 kinase. Our results suggest that peroxynitrite reduces the IRS-1 protein level and decreases phosphorylation of IRS-1 concurrent with nitration of its tyrosine residues.     

Acute inhibition of insulin-stimulated glucose transport by the phosphatase inhibitor, okadaic acid.

Insulin is thought to exert its effects on cellular function through the phosphorylation or dephosphorylation of specific regulatory substrates. We have analyzed the effects of okadaic acid, a potent inhibitor of type 1 and 2A protein phosphatases, on the ability of insulin to stimulate glucose transport in rat adipocytes. Insulin and okadaic acid caused a 20-25- and a 3-6-fold increase, respectively, in the rate of 2-deoxyglucose accumulation by adipose cells. When added to cells previously treated with okadaic acid, insulin failed to stimulate 2-deoxyglucose accumulation beyond the levels observed with okadaic acid alone. Treatment of cells with okadaic acid did not inhibit the effect of insulin to stimulate tyrosine autophosphorylation of its receptor. These results indicate that okadaic acid potently inhibits the effects of insulin to stimulate glucose uptake and/or utilization at a step after receptor activation. To clarify the mechanism of inhibition by okadaic acid, the intrinsic activity of the plasma membrane glucose transporters was analyzed by measuring the rate of uptake of 3-O-methylglucose by adipose cells, and the concentration of adipocyte/skeletal muscle isoform of the glucose transporter (GLUT-4) in plasma membranes isolated from these cells. Insulin caused a 15-20-fold stimulation of 3-O-methylglucose uptake and a 2-3-fold increase in the levels of GLUT-4 detected by immunoblotting of isolated plasma membranes; okadaic acid caused a 2-fold increase in 3-O-methylglucose uptake, and a 1.5-fold increase in plasma membrane GLUT-4. Pretreatment of cells with okadaic acid blocked the effect of insulin to stimulate 3-O-methylglucose uptake and to increase the plasma membrane concentration of GLUT-4 beyond the levels observed with okadaic acid alone. These results indicate that the effect of okadaic acid to inhibit the effect of insulin on glucose uptake is exerted at a step prior to the recruitment of glucose transporters to the cell surface, and suggest that a phosphatase activity may be critical for this process.     

Glucose-induced insulin resistance of skeletal-muscle glucose transport and uptake.

The ability of glucose and insulin to modify insulin-stimulated glucose transport and uptake was investigated in perfused skeletal muscle. Here we report that perfusion of isolated rat hindlimbs for 5 h with 12 mM-glucose and 20,000 microunits of insulin/ml leads to marked, rapidly developing, impairment of insulin action on muscle glucose transport and uptake. Thus maximal insulin-stimulated glucose uptake at 12 mM-glucose decreased from 34.8 +/- 1.9 to 11.5 +/- 1.1 mumol/h per g (mean +/- S.E.M., n = 10) during 5 h perfusion. This decrease in glucose uptake was accompanied by a similar change in muscle glucose transport as measured by uptake of 3-O-[14C]-methylglucose. Simultaneously, muscle glycogen stores increased to 2-3.5 times initial values, depending on fibre type. Perfusion for 5 h in the presence of glucose but in the absence of insulin decreased subsequent insulin action on glucose uptake by 80% of the effect of glucose with insulin, but without an increase in muscle glycogen concentration. Perfusion for 5 h with insulin but without glucose, and with subsequent addition of glucose back to the perfusate, revealed glucose uptake and transport similar to initial values obtained in the presence of glucose and insulin. The data indicate that exposure to a moderately increased glucose concentration (12 mM) leads to rapidly developing resistance of skeletal-muscle glucose transport and uptake to maximal insulin stimulation. The effect of glucose is enhanced by simultaneous insulin exposure, whereas exposure for 5 h to insulin itself does not cause measurable resistance to maximal insulin stimulation.     

Prohibitin attenuates insulin-stimulated glucose and fatty acid oxidation in adipose tissue by inhibition of pyruvate carboxylase

Prohibitin (PHB-1) is a highly conserved protein involved in mitochondrial biogenesis and function. It is secreted in lipid droplets from adipocytes and is present in the circulation. In adipose tissue it functions as a membrane receptor and can target binding partners to the mitochondria. Here we report that PHB-1 has a hitherto undescribed role as an inhibitor of pyruvate carboxylase (PC). As a consequence, it can modulate insulin-stimulated glucose and fatty acid oxidation. It had no effect on insulin-stimulated 2-deoxglucose uptake by isolated adipocytes but inhibited insulin-stimulated oxidation of [14C]glucose with a half-maximal concentration of approximately 4 nM. It also inhibited oleic acid oxidation in glucose-depleted adipocytes via depletion of oxaloacetate. In vitro experiments using broken-cell assays confirmed that PHB-1 inhibited PC. MALDI-TOF analysis of proteins identified by cross-linking of PHB-1 to adipocyte membranes indicated that PHB-1 is closely associated with PC and EH domain 2 (EHD2). On the basis of these data, we propose that PHB-1 is recycled between the extracellular space and the mitochondria by a mechanism involving lipid rafts and EHD2 and can modulate mitochondrial fuel metabolism by inhibition of PC.     

Fatty acid transport proteins and insulin resistance

PURPOSE OF REVIEW: Disturbed fatty acid metabolism and homeostasis is associated with insulin resistance. The aim of this review, therefore, is to summarize recent developments relating to the relevance and importance of the fatty acid transport proteins (FATPs) in the aetiology of insulin resistance. In particular, the potential differences between the six members of the FATP family will be considered. RECENT FINDINGS: FATP1 knockout mice failed to develop insulin resistance associated with lipid infusion or a high-fat diet, as wild-type mice did. FATP1-mediated fatty acid uptake may cause intramuscular lipid accumulation leading to insulin resistance in muscle if the fatty acids are not oxidized. While mouse models demonstrated an absolute requirement for FATP4 for survival, they provided no direct evidence for a role of FATP4 in insulin resistance. However, expression of FATP4 in human adipose tissue was increased in obesity (independent of genetic factors). While other members of the FATP family have important roles in fatty acid metabolism, they have not been clearly linked to insulin resistance. FATP-mediated fatty acid uptake may be driven by intrinsic acyl-CoA synthase activity. SUMMARY: Any role in the development of insulin resistance is likely to be different for each member of the FATP family. So far, both FATP1 and FATP4 have been associated with parameters related to insulin resistance. Whether increased FATP-mediated fatty acid uptake is beneficial or detrimental may be dependent on the tissue in question and on the subsequent fate of the fatty acids. These issues remain to be resolved.     

Glucocorticoid-induced apoptosis and glucocorticoid resistance: molecular mechanisms and clinical relevance

The ability of glucocorticoids (GC) to efficiently kill lymphoid cells has led to their inclusion in essentially all chemotherapy protocols for lymphoid malignancies. This review summarizes recent findings related to the molecular basis of GC-induced apoptosis and GC resistance, and discusses their potential clinical implications. Accumulating evidence suggests that GC may induce cell death via different pathways resulting in apoptotic or necrotic morphologies, depending on the availability/responsiveness of the apoptotic machinery. The former might result from regulation of typical apoptosis genes such as members of the Bcl-2 family, the latter from detrimental GC effects on essential cellular functions possibly perpetuated by GC receptor (GR) autoinduction. Although other possibilities exist, GC resistance might frequently result from defective GR expression, perhaps the most efficient means to target multiple antileukemic GC effects. Numerous novel drug combinations are currently being tested to prevent resistance and improve GC efficacy in the therapy of lymphoid malignancies.     

Alpha-lipoic acid mitigates insulin resistance in Goto-Kakizaki rats.

Impaired glucose uptake and metabolism by peripheral tissues is a common feature in both type I and type II diabetes mellitus. This phenomenon was examined in the context of oxidative stress and the early events within the insulin signalling pathway using soleus muscles derived from non-obese, insulin-resistant type II diabetic Goto-Kakizaki (GK) rats, a well-known genetic rat model for human type II diabetes. Insulin-stimulated glucose transport was impaired in soleus muscle from GK rats. Oxidative and non-oxidative glucose disposal pathways represented by glucose oxidation and glycogen synthesis in soleus muscles of GK rats appear to be resistant to the action of insulin when compared to their corresponding control values. These diabetes-related abnormalities in glucose disposal were associated with a marked diminution in the insulin-mediated enhancement of protein kinase B (Akt/PKB) and insulin receptor substrate-1 (IRS-1)-associated phosphatidylinostol 3-kinase (PI 3-kinase) activities; these two kinases are key elements in the insulin signalling pathway. Moreover, heightened state of oxidative stress, as indicated by protein bound carbonyl content, was evident in soleus muscle of GK diabetic rats. Chronic administration of the hydrophobic/hydrophilic antioxidant alpha -lipoic-acid (ALA, 100 mg/kg, i.p.) partly ameliorated the diabetes-related deficit in glucose metabolism, protein oxidation as well as the activation by insulin of the various steps of the insulin signalling pathway, including the enzymes Akt/PKB and PI-3 kinase. Overall, the current investigation illuminates the concept that oxidative stress may indeed be involved in the pathogenesis of certain types of insulin resistance. It also harmonizes with the notion of including potent antioxidants such as ALA in the armamentarium of antidiabetic therapy.     

Chronic hypoxia increases insulin-stimulated glucose uptake in mouse soleus muscle

People living at high altitude appear to have lower blood glucose levels and decreased incidence of diabetes. Faster glucose uptake and increased insulin sensitivity are likely explanations for these findings: skeletal muscle is the largest glucose sink in the body, and its adaptation to the hypoxia of altitude may influence glucose uptake and insulin sensitivity. This study tested the hypothesis that chronic normobaric hypoxia increases insulin-stimulated glucose uptake in soleus muscles and decreases plasma glucose levels. Adult male C57BL/6J mice were kept in normoxia [fraction of inspired O? = 21% (Control)] or normobaric hypoxia [fraction of inspired O? = 10% (Hypoxia)] for 4 wk. Then blood glucose and insulin levels, in vitro muscle glucose uptake, and indexes of insulin signaling were measured. Chronic hypoxia lowered blood glucose and plasma insulin [glucose: 14.3 ± 0.65 mM in Control vs. 9.9 ± 0.83 mM in Hypoxia (P < 0.001); insulin: 1.2 ± 0.2 ng/ml in Control vs. 0.7 ± 0.1 ng/ml in Hypoxia (P < 0.05)] and increased insulin sensitivity determined by homeostatic model assessment 2 [21.5 ± 3.8 in Control vs. 39.3 ± 5.7 in Hypoxia (P < 0.03)]. There was no significant difference in basal glucose uptake in vitro in soleus muscle (1.59 ± 0.24 and 1.71 ± 0.15 μmol·g?1·h?1 in Control and Hypoxia, respectively). However, insulin-stimulated glucose uptake was 30% higher in the soleus after 4 wk of hypoxia than Control (6.24 ± 0.23 vs. 4.87 ± 0.37 μmol·g?1·h?1, P < 0.02). Muscle glycogen content was not significantly different between the two groups. Levels of glucose transporters 4 and 1, phosphoinositide 3-kinase, glycogen synthase kinase 3, protein kinase B/Akt, and AMP-activated protein kinase were not affected by chronic hypoxia. Akt phosphorylation following insulin stimulation in soleus muscle was significantly (25%) higher in Hypoxia than Control (P < 0.05). Neither glycogen synthase kinase 3 nor AMP-activated protein kinase phosphorylation changed after 4 wk of hypoxia. These results demonstrate that the adaptation of skeletal muscles to chronic hypoxia includes increased insulin-stimulated glucose uptake.     

High uric acid directly inhibits insulin signalling and induces insulin resistance

Background and aim

Accumulating clinical evidence suggests that hyperuricemia is strongly associated with abnormal glucose metabolism and insulin resistance. However, how high uric acid (HUA) level causes insulin resistance remains unclear. We aimed to determine the direct role of HUA in insulin resistance in vitro and in vivo in mice.

Methods

An acute hyperuricemia mouse model was created by potassium oxonate treatment, and the impact of HUA level on insulin resistance was investigated by glucose tolerance test, insulin tolerance test and insulin signalling, including phosphorylation of insulin receptor substrate 1 (IRS1) and Akt. HepG2 cells were exposed to HUA treatment and N-acetylcysteine (NAC), reactive oxygen species scavenger; IRS1 and Akt phosphorylation was detected by Western blot analysis after insulin treatment.

Results

Hyperuricemic mice showed impaired glucose tolerance with insulin resistance. Hyperuricemia inhibited phospho-Akt (Ser473) response to insulin and increased phosphor-IRS1 (Ser307) in liver, muscle and fat tissues. HUA induced oxidative stress, and the antioxidant NAC blocked HUA-induced IRS1 activation and Akt inhibition in HepG2 cells.

Conclusion

This study supplies the first evidence of HUA directly inducing insulin resistance in vivo and in vitro. Increased uric acid level may inhibit IRS1 and Akt insulin signalling and induce insulin resistance. The reactive oxygen species pathway plays a key role in HUA-induced insulin resistance.