It is now generally accepted that activation of AMPactivated protein kinase (AMPK) is involved in the stimulation of glucose transport by muscle contractions. However, earlier studies provided evidence that increases in cytosolic Ca 2؉ mediate the effect of muscle contractions on glucose transport. The purpose of this study was to test the hypothesis that both the increase in cytosolic Ca 2؉ E xercise and insulin stimulate glucose transport by separate pathways, and their maximal effects on muscle glucose uptake are additive (1). Both muscle contractions and insulin increase glucose transport by inducing translocation of the GLUT4 isoform of the glucose transporter from intracellular sites to the cell surface (1). Results of early studies suggested that the increase in cytosolic Ca 2ϩ during contractile activity initiates the process that leads to increased muscle glucose transport (2-5). However, because increases in Ca 2ϩ caused the muscles to contract, it was not possible to clearly dissociate the effects of Ca 2ϩ from the metabolic consequences of the contraction-induced decrease in highenergy phosphates. This problem was surmounted in experiments in which caffeine or W-7, agents that release Ca 2ϩ from the sarcoplasmic reticulum, were used to raise cytosolic Ca 2ϩ to levels too low to cause muscle contraction or a decrease in high-energy phosphates (6). In these experiments, glucose transport increased in response to raising cytosolic Ca 2ϩ to subcontraction levels (6). We interpreted this finding as evidence supporting our hypothesis that Ca 2ϩ mediates the effect of exercise on muscle glucose transport.More recent studies have shown that 5-aminoimidazole-4-carboxamide ribonucleoside (AICAR) stimulates glucose transport in skeletal muscle (7)(8)(9)(10)(11). AICAR is taken up by cells and converted to the AMP analog ZMP, which mimics the stimulatory effect of AMP on AMP kinase (AMPK) (12). Like the effect of exercise, the stimulation of glucose transport by AICAR is not inhibited by wortmannin and is additive to that of a maximal insulin stimulus (8). AMPK is activated by increases in the AMP-to-ATP ratio and decreases in phosphocreatine, and thus is activated during muscle contractions (12).Although it seems firmly established that activation of AMPK is involved in mediating the stimulation of glucose uptake by muscle contractions, it does not appear to account for all of the increase in glucose transport activity. This is evidenced by the finding of Mu et al. (13) that expression of a dominant inhibitory mutant of AMPK in mouse muscle results in only an ϳ30 -40% decrease in contraction-stimulated glucose transport. In this context, the purpose of the present study was to test the hypothesis that both the increase in cytosolic Ca 2ϩ and the activation of AMPK during muscle contractions are involved in mediating the stimulation of glucose transport by contractile activity. AICAR, 5-aminoimidazole-4-carboxamide ribonucleoside; AMPK, AMP kinase; CAMK, calmodulin-dependent protein kinase; 2-DG, 2-[1,2-3 H]d...

The transcriptional coactivator peroxisome proliferator-activated receptor ␥ coactivator-1␣ (PGC-1␣) has been identified as an inducible regulator of mitochondrial function. Skeletal muscle PGC-1␣ expression is induced post-exercise. Therefore, we sought to determine its role in the regulation of muscle fuel metabolism. Studies were performed using conditional, musclespecific, PGC-1␣ gain-of-function and constitutive, generalized, loss-of-function mice. Forced expression of PGC-1␣ increased muscle glucose uptake concomitant with augmentation of glycogen stores, a metabolic response similar to postexercise recovery. Induction of muscle PGC-1␣ expression prevented muscle glycogen depletion during exercise. Conversely, PGC-1␣-deficient animals exhibited reduced rates of muscle glycogen repletion post-exercise. PGC-1␣ was shown to increase muscle glycogen stores via several mechanisms including stimulation of glucose import, suppression of glycolytic flux, and by down-regulation of the expression of glycogen phosphorylase and its activating kinase, phosphorylase kinase ␣. These findings identify PGC-1␣ as a critical regulator of skeletal muscle fuel stores.Glucose and fatty acids are the chief fuel sources for skeletal muscle. During prolonged bouts of low intensity exercise, muscle energy needs are met through utilization of both substrates with mitochondrial fatty acid oxidation serving a "glucose sparing" function (1, 2). During acute high intensity exercise, glucose derived from hepatic and muscle glycogen stores serves as the chief energy source (reviewed in Refs. 3-5). Rapid glycogen repletion following a bout of exhausting intense exercise is an important adaptive response, preparing the muscle for subsequent bouts of activity. With endurance exercise training, the capacity for mitochondrial oxidation of fatty acids is augmented and muscle glycogen reserves increase (2). In disease states such as diabetes and heart failure, the capacity for muscle energy substrate utilization is reduced due to alterations in glucose metabolism and derangements in mitochondrial function (6, 7) (reviewed in Ref. 8).The molecular regulatory mechanisms involved in the control of muscle fuel metabolism are incompletely understood. Recent evidence implicates the transcriptional coactivator, peroxisome proliferator-activated receptor (PPAR) 5 -␥ coactivator 1␣ (PGC-1␣), in the regulation of striated muscle energy metabolism and function (9 -13). PGC-1␣ levels are rapidly induced in skeletal muscle following bouts of activity in rodents and humans (14 -22). PGC-1␣ coactivates multiple transcription factors involved in mitochondrial biogenesis, oxidative phosphorylation, and fatty acid oxidation, including the estrogen-related receptor ␣, PPAR␣, and nuclear respiratory factors 1 and 2 (6, 23-26). PGC-1␣ gain-and loss-of-function studies conducted in cells and in mice have demonstrated that PGC-1␣ stimulates gene regulatory programs that augment mitochondrial oxidative capacity in tissues with high energy demands, such as heart and ske...

To determine whether uncoupling respiration from oxidative phosphorylation in skeletal muscle is a suitable treatment for obesity and type 2 diabetes, we generated transgenic mice expressing the mitochondrial uncoupling protein (Ucp) in skeletal muscle. Skeletal muscle oxygen consumption was 98% higher in Ucp-L mice (with low expression) and 246% higher in Ucp-H mice (with high expression) than in wild-type mice. Ucp mice fed a chow diet had the same food intake as wild-type mice, but weighed less and had lower levels of glucose and triglycerides and better glucose tolerance than did control mice. Ucp-L mice were resistant to obesity induced by two different high-fat diets. Ucp-L mice fed a high-fat diet had less adiposity, lower levels of glucose, insulin and cholesterol, and an increased metabolic rate at rest and with exercise. They were also more responsive to insulin, and had enhanced glucose transport in skeletal muscle in the setting of increased muscle triglyceride content. These data suggest that manipulating respiratory uncoupling in muscle is a viable treatment for obesity and its metabolic sequelae.

The role of the peroxisome proliferator-activated receptor-alpha (PPARalpha) in the development of insulin-resistant diabetes was evaluated using gain- and loss-of-function approaches. Transgenic mice overexpressing PPARalpha in muscle (MCK-PPARalpha mice) developed glucose intolerance despite being protected from diet-induced obesity. Conversely, PPARalpha null mice were protected from diet-induced insulin resistance in the context of obesity. In skeletal muscle, MCK-PPARalpha mice exhibited increased fatty acid oxidation rates, diminished AMP-activated protein kinase activity, and reduced insulin-stimulated glucose uptake without alterations in the phosphorylation status of key insulin-signaling proteins. These effects on muscle glucose uptake involved transcriptional repression of the GLUT4 gene. Pharmacologic inhibition of fatty acid oxidation or mitochondrial respiratory coupling prevented the effects of PPARalpha on GLUT4 expression and glucose homeostasis. These results identify PPARalpha-driven alterations in muscle fatty acid oxidation and energetics as a potential link between obesity and the development of glucose intolerance and insulin resistance.

BACKGROUND & AIMS An increased number of macrophages in adipose tissue is associated with insulin resistance and metabolic dysfunction in obese people. However, little is known about other immune cells in adipose tissue from obese people, and whether they contribute to insulin resistance. We investigated the characteristics of T cells in adipose tissue from metabolically abnormal insulin-resistant obese (MAO) subjects, metabolically normal insulin-sensitive obese (MNO) subjects, and lean subjects. Insulin sensitivity was determined by using the hyperinsulinemic euglycemic clamp procedure. METHODS We assessed plasma cytokine concentrations and subcutaneous adipose tissue CD4+ T-cell populations in 9 lean, 12 MNO, and 13 MAO subjects. Skeletal muscle and liver samples were collected from 19 additional obese patients undergoing bariatric surgery to determine the presence of selected cytokine receptors. RESULTS Adipose tissue from MAO subjects had 3- to 10-fold increases in numbers of CD4+ T cells that produce interleukin (IL)-22 and IL-17 (a T-helper [Th] 17 and Th22 phenotype) compared with MNO and lean subjects. MAO subjects also had increased plasma concentrations of IL-22 and IL-6. Receptors for IL-17 and IL-22 were expressed in human liver and skeletal muscle samples. IL-17 and IL-22 inhibited uptake of glucose in skeletal muscle isolated from rats and reduced insulin sensitivity in cultured human hepatocytes. CONCLUSIONS Adipose tissue from MAO individuals contains increased numbers of Th17 and Th22 cells, which produce cytokines that cause metabolic dysfunction in liver and muscle in vitro. Additional studies are needed to determine whether these alterations in adipose tissue T cells contribute to the pathogenesis of insulin resistance in obese people.