Impaired Muscle Glucose Uptake Research Articles

1632-P: Effects of MTOR Signaling in Muscle-Specific Irs1/2 Knockout Mice

Disruption of Irs1 and Irs2 expression in skeletal and cardiac muscle (MDKO mice) caused death from dilated cardiomyopathy within the first 30 days of life. MDKO showed muscle atrophy but were not glucose intolerant, despite impaired muscle glucose uptake. Beiging of white adipose fat increased glucose uptake into white adipose tissue of MDKO mice, and glucose uptake into brown adipose was also increased. Knockout of atrophy-driving genes FoxO1 and FoxO3a in MDKO mice (MQKO mice) extended life span to 70 days, when MQKO mice died with cardiomyopathy. High fat diet feeding increased life span in both MDKO and MQKO mice; however, genetic activation of the MTOR pathway in muscle of MDKO mice by Tsc1 knockout (MTKO mice) increased life span to 220 days. Regardless, MTKO mice developed cardiac hypertrophy with reduced ejection fraction by 24 weeks. Cardiac glucose uptake in MTKO mice was impaired during non-insulin as well as during insulin-stimulated conditions. Moreover, fat mass of MTKO mice decreased over time, implying a shift in consumption by MTKO hearts from glucose to fatty acids as source for cardiac energy. Tracing of radiolabeled palmitic acid did not support increased cardiac fatty acid uptake in MTKO mice; however, expression of lactate transporters was significantly elevated in MTKO hearts and circulating lactate generated by skeletal muscle was significantly reduced, indicating a potential use of lactate in MTKO hearts. These results show more robust effects of activating the MTOR pathway in insulin resistant hearts of MDKO mice rather than manipulating atrophy-related genes FoxO1/3a to counter early death from cardiomyopathy. Furthermore, activated MTOR signaling potentiated a metabolic switch in MDKO hearts. Disclosure O.Stoehr: None. K.D.Copps: None. R.Tao: None. M.F.White: Board Member; Housey Pharma.

Loss of Hematopoietic Cell-Derived Oncostatin M Worsens Diet-Induced Dysmetabolism in Mice.

Innate immune cells infiltrate growing adipose and propagate inflammatory clues to metabolically distant tissues, thereby promoting glucose intolerance and insulin resistance. Cytokines of the IL-6 family and gp130 ligands are among such signals. The role played by Oncostatin M (OSM) in the metabolic consequences of overfeeding is debated at least in part because prior studies did not distinguish OSM sources and dynamics. Here, we explored the role of OSM in metabolic responses and used bone marrow transplantation to test the hypothesis that hematopoietic cells are major contributors to the metabolic effects of OSM. We show that OSM is required to adapt during the development of obesity as OSM concentrations are dynamically modulated during high-fat diet (HFD) and Osm-/- mice displayed early-onset glucose intolerance, impaired muscle glucose uptake, worsened liver inflammation and damage. We found that OSM is mostly produced by blood cells, and that deletion of OSM in hematopoietic cells phenocopied glucose intolerance of whole-body Osm-/- mice on HFD, and recapitulated liver damage with increased aminotransferase levels. We thus uncover that modulation of OSM is involved in the metabolic response to overfeeding and that hematopoietic cell-derived OSM can regulate metabolism, likely via multiple effects in different tissues.

Effects of Simvastatin on Lipid Metabolism in Wild-Type Mice and Mice with Muscle PGC-1α Overexpression.

Previous studies suggest that statins may disturb skeletal muscle lipid metabolism potentially causing lipotoxicity with insulin resistance. We investigated this possibility in wild-type mice (WT) and mice with skeletal muscle PGC-1α overexpression (PGC-1α OE mice). In WT mice, simvastatin had only minor effects on skeletal muscle lipid metabolism but reduced glucose uptake, indicating impaired insulin sensitivity. Muscle PGC-1α overexpression caused lipid droplet accumulation in skeletal muscle with increased expression of the fatty acid transporter CD36, fatty acid binding protein 4, perilipin 5 and CPT1b but without significant impairment of muscle glucose uptake. Simvastatin further increased the lipid droplet accumulation in PGC-1α OE mice and stimulated muscle glucose uptake. In conclusion, the impaired muscle glucose uptake in WT mice treated with simvastatin cannot be explained by lipotoxicity. PGC-1α OE mice are protected from lipotoxicity of fatty acids and triglycerides by increased the expression of FABP4, formation of lipid droplets and increased expression of CPT1b.

The functional significance of the skeletal muscle clock: lessons from Bmal1 knockout models

The circadian oscillations of muscle genes are controlled either directly by the intrinsic muscle clock or by extrinsic factors, such as feeding, hormonal signals, or neural influences, which are in turn regulated by the central pacemaker, the suprachiasmatic nucleus of the hypothalamus. A unique feature of circadian rhythms in skeletal muscle is motor neuron-dependent contractile activity, which can affect the oscillation of a number of muscle genes independently of the muscle clock. The role of the intrinsic muscle clock has been investigated using different Bmal1 knockout (KO) models. A comparative analysis of these models reveals that the dramatic muscle wasting and premature aging caused by global conventional KO are not present in muscle-specific Bmal1 KO or in global Bmal1 KO induced in the adult, therefore must reflect the loss of Bmal1 function during development in non-muscle tissues. On the other hand, muscle-specific Bmal1 knockout causes impaired muscle glucose uptake and metabolism, supporting a major role of the muscle clock in anticipating the sleep-to-wake transition, when glucose becomes the predominant fuel for the skeletal muscle.

Activation of CaMKKα Stimulates Skeletal Muscle Glucose Uptake in Insulin Resistant Mice

Activation of Ca2+/calmodulin‐dependent protein kinase kinase α (CaMKKα) signaling stimulates skeletal muscle glucose uptake in healthy, insulin‐sensitive mice. The goal of this study was to determine if CaMKKα could stimulate glucose uptake in muscle from insulin resistant mice. To induce insulin resistance, male C57BL6/J mice were fed a high fat (60% kcal fat) diet. After 12 weeks, insulin (600 μU/ml)‐stimulated ex vivo muscle [3H]‐2‐deoxyglucose uptake was significantly decreased (30%) in high fat fed mice, demonstrating skeletal muscle insulin resistance. To stimulate CaMKKα signaling, muscles from control and insulin‐resistant mice were transfected with plasmid DNA containing constitutively active CaMKKα or empty vector by in vivo electroporation. After 2 weeks, active CaMKKα protein levels were robustly increased and were not different between control and insulin‐resistant mice. In vivo muscle [3H]‐2‐deoxyglucose uptake was examined in the absence of insulin stimulation. In both control and insulin‐resistant mice, active CaMKKα increased glucose uptake ~2‐fold. Collectively, these results show that activation of CaMKKα signaling can stimulate glucose uptake in both insulin‐sensitive and insulin‐resistant skeletal muscle, and suggest that CaMKKα may be a novel therapeutic target for impaired muscle glucose uptake in type 2 diabetes. SUPPORT: NIH R00AR056298, ECU start‐up funds.

Effect of age and moderate food restriction on insulin sensitivity in Wistar rats: role of adiposity

Insulin resistance develops with ageing in humans and rodents. Here, we have studied the evolution of insulin sensitivity with ageing trying to discriminate the role of adiposity from that of ageing in this process. We performed oral glucose tolerance tests and determined overall and tissue-specific glucose utilization under euglycemic-hyper-insulinemic conditions in 3-, 8-, and 24-month-old rats fed ad libitum, and in 8- and 24-month-old rats after 3 months of calorie restriction. Body composition and adipocyte-derived cytokines such as leptin, resistin, and adiponectin were analyzed. Overall insulin sensitivity decreases with ageing. Calorie restriction improves global insulin sensitivity in 8- but not in 24-month-old rats. Insulin-stimulated glucose utilization in adipose tissues decreases in 8 months, while in oxidative muscles it reaches significance only in older rats. Calorie restriction restores adipose tissue insulin sensitivity only in 8-month-old rats and no changes are observed in muscles of 24-month-old rats. Resistin and leptin increase with ageing. Food restriction lowers resistin and increases adiponectin in 8-month-old rats and decreases leptin in both ages. Visceral and total fat increase with ageing and decrease after calorie restriction. We conclude that accretion of visceral fat plays a key role in the development of insulin resistance after sexual maturity, which is reversible by calorie restriction. With aging, accumulation of retroperitoneal and total body fat leads to impaired muscle glucose uptake and to a state of insulin resistance that is difficult to reverse.

Severe diabetes, age-dependent loss of adipose tissue, and mild growth deficiency in mice lacking Akt2/PKB beta.

The serine/threonine kinase Akt/PKB plays key roles in the regulation of cell growth, survival, and metabolism. It remains unclear, however, whether the functions of individual Akt/PKB isoforms are distinct. To investigate the function of Akt2/PKBbeta, mice lacking this isoform were generated. Both male and female Akt2/PKBbeta-null mice exhibit mild growth deficiency and an age-dependent loss of adipose tissue or lipoatrophy, with all observed adipose depots dramatically reduced by 22 weeks of age. Akt2/PKBbeta-deficient mice are insulin resistant with elevated plasma triglycerides. In addition, Akt2/PKBbeta-deficient mice exhibit fed and fasting hyperglycemia, hyperinsulinemia, glucose intolerance, and impaired muscle glucose uptake. In males, insulin resistance progresses to a severe form of diabetes accompanied by pancreatic beta cell failure. In contrast, female Akt2/PKBbeta-deficient mice remain mildly hyperglycemic and hyperinsulinemic until at least one year of age. Thus, Akt2/PKBbeta-deficient mice exhibit growth deficiency similar to that reported previously for mice lacking Akt1/PKBalpha, indicating that both Akt2/PKBbeta and Akt1/PKBalpha participate in the regulation of growth. The marked hyperglycemia and loss of pancreatic beta cells and adipose tissue in Akt2/PKBbeta-deficient mice suggest that Akt2/PKBbeta plays critical roles in glucose metabolism and the development or maintenance of proper adipose tissue and islet mass for which other Akt/PKB isoforms are unable to fully compensate.

Peripheral glucose metabolism in patients with essential hypertension.

The present study was designed to determine the effect of essential hypertension on peripheral glucose metabolism during the postabsorptive state and after an oral glucose challenge. Ten normal subjects and nine patients with essential hypertension were studied after an overnight fast (12-14 h) and for 3 h after the ingestion of 75 g of glucose. Peripheral glucose metabolism was analyzed by the forearm technique to estimate muscle exchange of substrate combined with indirect calorimetry. Decreased forearm glucose uptake was observed in hypertensive patients compared to normal subjects (4.9+/-0.6 vs. 8.6+/-0.5 mmol x 100 ml forearm(-1) x 3 h(-1)) with diminished nonoxidative glucose metabolism (2.7+/-0.5 vs. 6.9+/-0.6 mmol x 100 ml forearm(-1) x 3 h(-1)). Muscle glucose oxidation did not differ significantly between groups. Both serum free fatty acid levels and lipid oxidation rates were similar in the normal subjects and the hypertensive patients, and declined in a similar fashion after glucose ingestion. Basal serum insulin levels did not differ significantly between normal and hypertensive patients, whereas the insulinemic response to glucose load was greater among the patients with essential hypertension. These data show that insulin resistance occurring in patients with essential hypertension is accompanied by impaired muscle glucose uptake and nonoxidative metabolism.

Acute vasoconstriction-induced insulin resistance in rat muscle in vivo.

Insulin-mediated changes in blood flow are associated with altered blood flow distribution and increased capillary recruitment in skeletal muscle. Studies in perfused rat hindlimb have shown that muscle metabolism can be regulated by vasoactive agents that control blood flow distribution within the hindlimb. In the present study, the effects of a vasoconstrictive agent that has no direct effect on skeletal muscle metabolism but that alters perfusion distribution in rat hindlimb was investigated in vivo to determine its effects on insulin-mediated vascular action and glucose uptake. We measured the effects of alpha-methylserotonin (alpha-met5HT) on mean arterial blood pressure, heart rate, femoral blood flow, hindlimb vascular resistance, and glucose uptake in control and euglycemic insulin-clamped (10 mU x min(-1) x kg(-1)) anesthetized rats. Blood flow distribution within the hindlimb muscles was assessed by measuring the metabolism of 1-methylxanthine (1-MX), an exogenously added substrate for capillary xanthine oxidase. Alpha-met5HT (20 microg x min(-1) x kg(-1)) infusion alone increased mean arterial blood pressure by 25% and increased hindlimb vascular resistance but caused no change in femoral blood flow. These changes were associated with decreased hindlimb 1-MX metabolism indicating less capillary flow. Insulin infusion caused decreased hindlimb vascular resistance that was associated with increased femoral blood flow and 1-MX metabolism. Treatment with alpha-met5HT infusion commenced before insulin infusion prevented the increase in femoral blood flow and inhibited the stimulation of 1-MX metabolism. Alpha-met5HT infusion had no effect on hindlimb glucose uptake but markedly inhibited the insulin stimulation of glucose uptake (P < 0.05) and was associated with decreased glucose infusion rates to maintain euglycemia (P < 0.05). A significant correlation (P < 0.05) was observed between 1-MX metabolism and hindlimb glucose uptake but not between femoral blood flow and glucose uptake. The results indicate that in vivo, certain types of vasoconstriction in muscle such as elicited by 5HT2 agonists, which prevent normal insulin recruitment of capillary flow, cause impaired muscle glucose uptake. Moreover, if vasoconstriction of this kind results from stress-induced increase in sympathetic outflow, then this may provide a clue as to the link between hypertension and insulin resistance that is often observed in humans.

Muscle glucose uptake during and after exercise is normal in insulin-resistant rats.

It is not generally known whether impaired stimulation of muscle glucose metabolism in insulin-resistant states is specific to insulin stimulation. Our aim was to examine whether glucose uptake responded normally to exercise and postexercise recovery in insulin-resistant high-fat-fed (HFF) rats. Three-week HFF or Chow-fed [control (Con)] adult rats were studied 5 days after cannulation. Before, during, or immediately after (recovery) 50 min of treadmill exercise, bolus 2-deoxy-[3H]glucose and [14C]glucose were administered to estimate muscle glucose uptake (R'g) and glycogen incorporation rates. Mean exercise and recovery plasma glucose levels were similar in HFF and Con rats. In hindlimb muscles sampled, exercise and recovery R'g were similar in HFF and Con (e.g., red quadriceps exercise 104 +/- 13 vs. 113 +/- 8, recovery 45.3 +/- 3.9 vs. 47.7 +/- 4.5 mumol.100 g-1.min-1, respectively). Moreover, muscle glucose transporter (GLUT-4) content was not reduced in HFF rats. Glycogen resynthesis accounted almost entirely for R'g during recovery and was equivalent between groups. We conclude that impaired muscle glucose uptake and glycogen synthesis in HFF rats are characteristic of insulin but not of exercise or postexercise stimulation.

Peripheral Glucose Metabolism in Acromegaly

The present study was designed to determine the effect of chronic GH excess on forearm muscle glucose uptake and oxidation during the postabsorptive state and after an oral glucose challenge. Nine normal subjects and 10 nondiabetic acromegalic patients (5 of them with normal glucose tolerance) were studied after an overnight fast (12-14 h) and for 3 h after the ingestion of 75 g glucose. Peripheral glucose metabolism was analyzed by the forearm technique to estimate muscle exchange of substrate combined with indirect calorimetry. Decreased forearm glucose uptake was observed in the acromegalic patients compared to that in the normal subjects (380 +/- 84 vs. 709 +/- 56 mumol/100 mL forearm.3 h) with diminished nonoxidative glucose metabolism (262 +/- 81 vs. 572 +/- 53 mumol/100 mL forearm.3 h). The acromegalics with normal glucose tolerance also showed decreased forearm glucose uptake and nonoxidative glucose metabolism compared to normal subjects (271 +/- 124 vs. 709 +/- 56 and 133 +/- 110 vs. 572 +/- 53 mumol/100 mL forearm.3 h, respectively). Muscle glucose oxidation did not differ significantly in normal subjects, the entire group of acromegalic patients, and the acromegalics with normal glucose tolerance (137 +/- 18 vs. 118 +/- 22 vs. 138 +/- 34 mumol/100 mL forearm.3 h, respectively). Serum FFA levels and lipid oxidation rates were similar in the normal subjects and the acromegalic patients, and declined in a similar fashion after glucose ingestion. Insulin levels were significantly higher in acromegalic patients than in normal subjects before and after glucose loading. In conclusion, this study showed that the insulin resistance occurring in the presence of chronic GH excess is accompanied by impaired muscle glucose uptake and nonoxidative glucose metabolism, which are early derangements because they are also observed in acromegalic patients with normal glucose tolerance.