Insulin resistance and muscle insulin receptor substrate-1 serine hyperphosphorylation.

Insulin resistance in metabolic syndrome subjects is profound in spite of muscle insulin receptor and insulin-responsive glucose transporter (GLUT4) expression being nearly normal. Insulin receptor tyrosine kinase phosphorylation of insulin receptor substrate-1 (IRS-1) at Tyr896 is a necessary step in insulin stimulation of translocation of GLUT4 to the cell surface. Serine phosphorylation of IRS-1 by some kinases diminishes insulin action in mice. We evaluated the phosphorylation status of muscle IRS-1 in 33 subjects with the metabolic syndrome and seventeen lean controls. Each underwent euglycemic insulin clamps and a thigh muscle biopsy before and after 8 weeks of either strength or endurance training. Muscle IRS-1 phosphorylation at six sites was quantified by immunoblots. Metabolic syndrome muscle IRS-1 had excess phosphorylation at Ser337 and Ser636 but not at Ser307, Ser789, or Ser1101. Ser337 is a target for phosphorylation by glycogen synthase kinase 3 (GSK3) and Ser636 is phosphorylated by c-Jun N-terminal kinase 1 (JNK1). Exercise training without weight loss did not change the IRS-1 serine phosphorylation. These data suggest that baseline hyperphosphorylation of at least two key serines within muscle IRS-1 diminishes the transmission of the insulin signal and thereby decreases the insulin-stimulated translocation of GLUT4. Excess fasting phosphorylation of muscle IRS-1 at Ser636 may be a major cause of the insulin resistance seen in obesity and might prevent improvement in insulin responsiveness when exercise training is not accompanied by weight loss.

Insulin responsiveness in metabolic syndrome after eight weeks of cycle training.

INTRODUCTION: Insulin resistance in obesity is decreased after successful diet and exercise. Aerobic exercise training alone was evaluated as an intervention in subjects with the metabolic syndrome. METHODS: Eighteen nondiabetic, sedentary subjects, 11 with the metabolic syndrome, participated in 8 wk of increasing intensity stationary cycle training. RESULTS: Cycle training without weight loss did not change insulin resistance in metabolic syndrome subjects or sedentary control subjects. Maximal oxygen consumption (V·O 2max), activated muscle AMP-dependent kinase, and muscle mitochondrial marker ATP synthase all increased. Strength, lean body mass, and fat mass did not change. The activated mammalian target of rapamycin was not different after training. Training induced a shift in muscle fiber composition in both groups but in opposite directions. The proportion of type 2× fibers decreased with a concomitant increase in type 2a mixed fibers in the control subjects, but in metabolic syndrome, type 2× fiber proportion increased and type 1 fibers decreased. Muscle fiber diameters increased in all three fiber types in metabolic syndrome subjects. Muscle insulin receptor expression increased in both groups, and GLUT4 expression increased in the metabolic syndrome subjects. The excess phosphorylation of insulin receptor substrate 1 (IRS-1) at Ser337 in metabolic syndrome muscle tended to increase further after training in spite of a decrease in total IRS-1. CONCLUSIONS: In the absence of weight loss, the cycle training of metabolic syndrome subjects resulted in enhanced mitochondrial biogenesis and increased the expression of insulin receptors and GLUT4 in muscle but did not decrease the insulin resistance. The failure for the insulin signal to proceed past IRS-1 tyrosine phosphorylation may be related to excess serine phosphorylation at IRS-1 Ser337, and this is not ameliorated by 8 wk of endurance exercise training.

Slow-twitch fiber proportion in skeletal muscle correlates with insulin responsiveness.

CONTEXT: The metabolic syndrome, characterized by central obesity with dyslipidemia, hypertension, and hyperglycemia, identifies people at high risk for type 2 diabetes. OBJECTIVE: Our objective was to determine how the insulin resistance of the metabolic syndrome is related to muscle fiber composition. DESIGN: Thirty-nine sedentary men and women (including 22 with the metabolic syndrome) had insulin responsiveness quantified using euglycemic clamps and underwent biopsies of the vastus lateralis muscle. Expression of insulin receptors, insulin receptor substrate-1, glucose transporter 4, and ATP synthase were quantified with immunoblots and immunohistochemistry. PARTICIPANTS AND SETTING: Participants were nondiabetic, metabolic syndrome volunteers and sedentary control subjects studied at an outpatient clinic. MAIN OUTCOME MEASURES: Insulin responsiveness during an insulin clamp and the fiber composition of a muscle biopsy specimen were evaluated. RESULTS: There were fewer type I fibers and more mixed (type IIa) fibers in metabolic syndrome subjects. Insulin responsiveness and maximal oxygen uptake correlated with the proportion of type I fibers. Insulin receptor, insulin receptor substrate-1, and glucose transporter 4 expression were not different in whole muscle but all were significantly less in the type I fibers of metabolic syndrome subjects when adjusted for fiber proportion and fiber size. Fat oxidation and muscle mitochondrial expression were not different in the metabolic syndrome subjects. CONCLUSION: Lower proportion of type I fibers in metabolic syndrome muscle correlated with the severity of insulin resistance. Even though whole muscle content was normal, key elements of insulin action were consistently less in type I muscle fibers, suggesting their distribution was important in mediating insulin effects.

Impaired muscle AMPK activation in the metabolic syndrome may attenuate improved insulin action after exercise training.

CONTEXT: Strength training induces muscle remodeling and may improve insulin responsiveness. OBJECTIVE: This study will quantify the impact of resistance training on insulin sensitivity in subjects with the metabolic syndrome and correlate this with activation of intramuscular pathways mediating mitochondrial biogenesis and muscle fiber hypertrophy. DESIGN: Ten subjects with the metabolic syndrome (MS) and nine sedentary controls underwent 8 wk of supervised resistance exercise training with pre- and posttraining anthropometric and muscle biochemical assessments. SETTING: Resistance exercise training took place in a sports laboratory on a college campus. MAIN OUTCOME MEASURES: Pre- and posttraining insulin responsiveness was quantified using a euglycemic clamp. Changes in expression of muscle 5-AMP-activated protein kinase (AMPK) and mammalian target of rapamycin (mTOR) pathways were quantified using immunoblots. RESULTS: Strength and stamina increased in both groups. Insulin sensitivity increased in controls (steady-state glucose infusion rate = 7.0 ± 2.0 mg/kg · min pretraining training vs. 8.7 ± 3.1 mg/kg · min posttraining; P < 0.01) but did not improve in MS subjects (3.3 ± 1.3 pre vs. 3.1 ± 1.0 post). Muscle glucose transporter 4 increased 67% in controls and 36% in the MS subjects. Control subjects increased muscle phospho-AMPK (43%), peroxisome proliferator-activated receptor γ coactivator 1α (57%), and ATP synthase (60%), more than MS subjects (8, 28, and 21%, respectively). In contrast, muscle phospho-mTOR increased most in the MS group (57 vs. 32%). CONCLUSION: Failure of resistance training to improve insulin responsiveness in MS subjects was coincident with diminished phosphorylation of muscle AMPK, but increased phosphorylation of mTOR, suggesting activation of the mTOR pathway could be involved in inhibition of exercise training-related increases in AMPK and its activation and downstream events.

Brain glucose transporter (Glut3) haploinsufficiency does not impair mouse brain glucose uptake.

Mouse brain expresses three principal glucose transporters. Glut1 is an endothelial marker and is the principal glucose transporter of the blood-brain barrier. Glut3 and Glut6 are expressed in glial cells and neural cells. A mouse line with a null allele for Glut3 has been developed. The Glut3(-/-) genotype is intrauterine lethal by 7days post-coitis, but the heterozygous (Glut3(+/-)) littermate survives, exhibiting rapid post-natal weight gain, but no seizures or other behavioral aberrations. At 12weeks of age, brain uptake of tail vein-injected ((3))H-2-deoxy glucose in Glut3(+/-) mice was not different from Glut3(+/+) littermates, despite 50% less Glut3 protein expression in the brain. The brain uptake of injected ((18))F-2-fluoro-2-deoxy glucose was similarly not different from Glut3(+/-) littermates in the total amount, time course, or brain imaging in the Glut3(+/-) mice. Glut1 and Glut6 protein expressions evaluated by immunoblots were not affected by the diminished Glut3 expression in the Glut3(+/-) mice. We conclude that a 50% decrease in Glut3 is not limiting for the uptake of glucose into the mouse brain, since Glut3 haploinsufficiency does not impair brain glucose uptake or utilization.