Chronic elevated calcium blocks AMPK-induced GLUT-4 expression in skeletal muscle

Muscle contraction stimulates glucose transport independent of insulin. Glucose uptake into muscle cells is positively related to skeletal muscle-specific glucose transporter (GLUT-4) expression. Therefore, our objective was to determine the effects of the contraction-mediated signals, calcium and AMP-activated protein kinase (AMPK), on glucose uptake and GLUT-4 expression under acute and chronic conditions. To accomplish this, we used pharmacological agents, cell culture, and pigs possessing genetic mutations for increased cytosolic calcium and constitutively active AMPK. In C2C12 myotubes, caffeine, a sarcoplasmic reticulum calcium-releasing agent, had a biphasic effect on GLUT-4 expression and glucose uptake. Low-concentration (1.25 to 2 mM) or short-term (4 h) caffeine treatment together with the AMPK activator, 5-aminoimidazole-4-carboxamide-1-beta-D-ribonucleoside (AICAR), had an additive effect on GLUT-4 expression. However, high-concentration (2.5 to 5 mM) or long-term (4 to 30 h) caffeine treatment decreased AMPK-induced GLUT-4 expression without affecting cell viability. The negative effect of caffeine on AICAR-induced GLUT-4 expression was reduced by dantrolene, which desensitizes the ryanodine receptor. Consistent with cell culture data, increases in GLUT-4 mRNA and protein expression induced by AMPK were blunted in pigs possessing genetic mutations for both increased cytosolic calcium and constitutively active AMPK. Altogether, these data suggest that chronic exposure to elevated cytosolic calcium concentration blocks AMPK-induced GLUT-4 expression in skeletal muscle.     

AMP-activated protein kinase regulates beta-catenin transcription via histone deacetylase 5

AMP-activated protein kinase (AMPK) is a key regulator of energy metabolism; it is inhibited under obese conditions and is activated by exercise and by many anti-diabetic drugs. Emerging evidence also suggests that AMPK regulates cell differentiation, but the underlying mechanisms are unclear. We hypothesized that AMPK regulates cell differentiation via altering β-catenin expression, which involves phosphorylation of class IIa histone deacetylase 5 (HDAC5). In both C3H10T1/2 cells and mouse embryonic fibroblasts (MEFs), AMPK activity was positively correlated with β-catenin content. Chemical inhibition of HDAC5 increased β-catenin mRNA expression. HDAC5 overexpression reduced and HDAC5 knockdown increased H3K9 acetylation and cellular β-catenin content. HDAC5 formed a complex with myocyte enhancer factor-2 to down-regulate β-catenin mRNA expression. AMPK phosphorylated HDAC5, which promoted HDAC5 exportation from the nucleus; mutation of two phosphorylation sites in HDAC5, Ser-259 and -498, abolished the regulatory role of AMPK on β-catenin expression. In conclusion, AMPK promotes β-catenin expression through phosphorylation of HDAC5, which reduces HDAC5 interaction with the β-catenin promoter via myocyte enhancer factor-2. Thus, the data indicate that AMPK regulates cell differentiation and development via cross-talk with the wingless and Int (Wnt)/β-catenin signaling pathway.     

Effects of AICAR and exercise on insulin-stimulated glucose uptake, signaling, and GLUT-4 content in rat muscles.

Physical activity is known to increase insulin action in skeletal muscle, and data have indicated that 5'-AMP-activated protein kinase (AMPK) is involved in the molecular mechanisms behind this beneficial effect. 5-Aminoimidazole-4-carboxamide-1-beta-d-ribofuranoside (AICAR) can be used as a pharmacological tool to repetitively activate AMPK, and the objective of this study was to explore whether the increase in insulin-stimulated glucose uptake after either long-term exercise or chronic AICAR administration was followed by fiber-type-specific changes in insulin signaling and/or changes in GLUT-4 expression. Wistar rats were allocated into three groups: an exercise group trained on treadmill for 5 days, an AICAR group exposed to daily subcutaneous injections of AICAR, and a sedentary control group. AMPK activity, insulin-stimulated glucose transport, insulin signaling, and GLUT-4 expression were determined in muscles characterized by different fiber type compositions. Both exercised and AICAR-injected animals displayed a fiber-type-specific increase in glucose transport with the most marked increase in muscles with a high content of type IIb fibers. This increase was accompanied by a concomitant increase in GLUT-4 expression. Insulin signaling as assessed by phosphatidylinositol 3-kinase and PKB/Akt activity was enhanced only after AICAR administration and in a non-fiber-type-specific manner. In conclusion, chronic AICAR administration and long-term exercise both improve insulin-stimulated glucose transport in skeletal muscle in a fiber-type-specific way, and this is associated with an increase in GLUT-4 content.     

Chronic activation of 5'-AMP-activated protein kinase increases GLUT-4, hexokinase, and glycogen in muscle.

This study was designed to determine whether chronic chemical activation of AMP-activated protein kinase (AMPK) would increase glucose transporter GLUT-4 and hexokinase in muscles similarly to periodic elevation of AMPK that accompanies endurance exercise training. The adenosine analog, 5-aminoimidazole-4-carboxamide ribonucleoside (AICAR), has previously been shown to be taken up by cells and phosphorylated to form a compound (5-aminoimidazole-4-carboxamide ribonucleotide) that mimics the effect of AMP on AMPK. A single injection of AICAR resulted in a marked increase in AMPK in epitrochlearis and gastrocnemius/plantaris muscles 60 min later. When rats were injected with AICAR (1 mg/g body wt) for 5 days in succession and were killed 1 day after the last injection, GLUT-4 was increased by 100% in epitrochlearis muscle and by 60% in gastrocnemius muscle in response to AICAR. Hexokinase was also increased approximately 2. 5-fold in the gastrocnemius/plantaris. Gastrocnemius glycogen content was twofold higher in AICAR-treated rats than in controls. Chronic chemical activation of AMPK, therefore, results in increases in GLUT-4 protein, hexokinase activity, and glycogen, similarly to those induced by endurance training.     

Effect of carbohydrate supplementation on postexercise GLUT-4 protein expression in skeletal muscle.

The effect of carbohydrate supplementation on skeletal muscle glucose transporter GLUT-4 protein expression was studied in fast-twitch red and white gastrocnemius muscle of Sprague-Dawley rats before and after glycogen depletion by swimming. Exercise significantly reduced fast-twitch red muscle glycogen by 50%. During a 16-h exercise recovery period, muscle glycogen returned to control levels (25.0 +/- 1.4 micromol/g) in exercise-fasted rats (24.2 +/- 0. 3 micro). However, when carbohydrate supplementation was provided during and immediately postexercise by intubation, muscle glycogen increased 77% above control (44.4 +/- 2.1 micromol/g). Exercise-fasting resulted in an 80% increase in fast-twitch red muscle GLUT-4 mRNA but only a 43% increase in GLUT-4 protein concentration. Conversely, exercise plus carbohydrate supplementation elevated fast-twitch red muscle GLUT-4 protein concentration by 88% above control, whereas GLUT-4 mRNA was increased by only 40%. Neither a 16-h fast nor carbohydrate supplementation had an effect on fast-twitch red muscle GLUT-4 protein concentration or on GLUT-4 mRNA in sedentary rats, although carbohydrate supplementation increased muscle glycogen concentration by 40% (35.0 +/- 0.9 micromol/g). GLUT-4 protein in fast-twitch white muscle followed a pattern similar to fast-twitch red muscle. These results indicate that carbohydrate supplementation, provided with exercise, will enhance GLUT-4 protein expression by increasing translational efficiency. Conversely, postexercise fasting appears to upregulate GLUT-4 mRNA, possibly to amplify GLUT-4 protein expression on an increase in glucose availability. These regulatory mechanisms may help control muscle glucose uptake in accordance with glucose availability and protect against postexercise hypoglycemia.     

AMPK signaling in contracting human skeletal muscle: acetyl-CoA carboxylase and NO synthase phosphorylation

AMP-activated protein kinase (AMPK) is a metabolic stress-sensing protein kinase responsible for coordinating metabolism and energy demand. In rodents, exercise accelerates fatty acid metabolism, enhances glucose uptake, and stimulates nitric oxide (NO) production in skeletal muscle. AMPK phosphorylates and inhibits acetyl-coenzyme A (CoA) carboxylase (ACC) and enhances GLUT-4 translocation. It has been reported that human skeletal muscle malonyl-CoA levels do not change in response to exercise, suggesting that other mechanisms besides inhibition of ACC may be operating to accelerate fatty acid oxidation. Here, we show that a 30-s bicycle sprint exercise increases the activity of the human skeletal muscle AMPK-alpha1 and -alpha2 isoforms approximately two- to threefold and the phosphorylation of ACC at Ser(79) (AMPK phosphorylation site) approximately 8.5-fold. Under these conditions, there is also an approximately 5.5-fold increase in phosphorylation of neuronal NO synthase-mu (nNOSmu;) at Ser(1451). These observations support the concept that inhibition of ACC is an important component in stimulating fatty acid oxidation in response to exercise and that there is coordinated regulation of nNOSmu to protect the muscle from ischemia/metabolic stress.     

Exercise and insulin signaling: a historical perspective.

Over the past 30 years, a considerable body of evidence has revealed that a prior bout of exercise can increase the ability of insulin to stimulate glucose transport and glycogen synthesis in skeletal muscle. Apart from its clinical implications, this work has led to a considerable effort to determine at a molecular level how exercise causes this effect and, in particular, whether it does so by enhancing specific events in the insulin-signaling cascade. The objective of this review is to discuss from a historical perspective how our current thinking in this area has evolved and the people responsible for it. Areas to be discussed include the effect or lack of effect of prior exercise on the insulin-signaling pathway, effects of exercise on the regulation by insulin of the GLUT-4 glucose transporter in muscle, and the emerging role of AMP-activated protein kinase as a mediator of exercise-induced signaling events. In addition, we will discuss briefly some of the avenues that research in this area is likely to follow.     

Mechanisms underlying impaired GLUT-4 translocation in glycogen-supercompensated muscles of exercised rats

Exercise training induces an increase in GLUT-4 in muscle. We previously found that feeding rats a high-carbohydrate diet after exercise, with muscle glycogen supercompensation, results in a decrease in insulin responsiveness so severe that it masks the effect of a training-induced twofold increase in GLUT-4 on insulin-stimulated muscle glucose transport. One purpose of this study was to determine whether insulin signaling is impaired. Maximally insulin-stimulated phosphatidylinositol (PI) 3-kinase activity was not significantly reduced, whereas protein kinase B (PKB) phosphorylation was approximately 50% lower (P < 0.01) in muscles of chow-fed, than in those of fasted, exercise-trained rats. Our second purpose was to determine whether contraction-stimulated glucose transport is also impaired. The stimulation of glucose transport and the increase in cell surface GLUT-4 induced by contractions were both decreased by approximately 65% in glycogen-supercompensated muscles of trained rats. The contraction-stimulated increase in AMP kinase activity, which has been implicated in the activation of glucose transport by contractions, was approximately 80% lower in the muscles of the fed compared with the fasted rats 18 h after exercise. These results show that both the insulin- and contraction-stimulated pathways for muscle glucose transport activation are impaired in glycogen-supercompensated muscles and provide insight regarding possible mechanisms.     

Modulation of glucose transport in skeletal muscle by reactive oxygen species.

Glucose transport is an essential physiological process that is characteristic of all eukaryotic cells, including skeletal muscle. In skeletal muscle, glucose transport is mediated by the GLUT-4 protein under conditions of increased carbohydrate utilization. The three major physiological stimuli of glucose transport in muscle are insulin, exercise/contraction, and hypoxia. Here, the role of reactive oxygen species (ROS) in modulating glucose transport in skeletal muscle is reviewed. Convincing evidence for ROS involvement in insulin- and hypoxia-mediated transport in muscle is lacking. Recent experiments, based on pharmacological and genetic approaches, support a role for ROS in contraction-mediated glucose transport. During contraction, endogenously produced ROS appear to mediate their effects on glucose transport via AMP-activated protein kinase.     

Regulation of glucose transport in skeletal muscle.

The entry of glucose into muscle cells is achieved primarily via a carrier-mediated system consisting of protein transport molecules. GLUT-1 transporter isoform is normally found in the sarcolemmal (SL) membrane and is thought to be involved in glucose transport under basal conditions. With insulin stimulation, glucose transport is accelerated by translocating GLUT-4 transporters from an intracellular pool out to the T-tubule and SL membranes. Activation of transporters to increase the turnover number may also be involved, but the evidence is far from conclusive. When insulin binds to its receptor, it autophosphorylates tyrosine and serine residues on the beta-subunit of the receptor. The tyrosine residues are thought to activate tyrosine kinases, which in turn phosphorylate/activate as yet unknown second messengers. Insulin receptor antibodies, however, have been reported to increase glucose transport without increasing kinase activity. Insulin resistance in skeletal muscle is a major characteristic of obesity and diabetes mellitus, especially NIDDM. A decrease in the number of insulin receptors and the ability of insulin to activate receptor tyrosine kinase has been documented in muscle from NIDDM patients. Most studies report no change in the intracellular pool of GLUT-4 transporters available for translocation to the SL. Both the quality and quantity of food consumed can regulate insulin sensitivity. A high-fat, refined sugar diet, similar to the typical U.S. diet, causes insulin resistance when compared with a low-fat, complex-carbohydrate diet. On the other hand, exercise increases insulin sensitivity. After an acute bout of exercise, glucose transport in muscle increases to the same level as with maximum insulin stimulation. Although the number of GLUT-4 transporters in the sarcolemma increases with exercise, neither insulin or its receptor is involved. After an initial acute phase, which may involve calcium as the activator, a secondary phase of increased insulin sensitivity can last for up to a day after exercise. The mechanism responsible for the increased insulin sensitivity with exercise is unknown. Regular exercise training also increases insulin sensitivity, which can be documented several days after the final bout of exercise, and again the mechanism is unknown. An increase in the muscle content of GLUT-4 transporters with training has recently been reported. Even though significant progress has been made in the past few years in understanding glucose transport in skeletal muscle, the mechanisms involved in regulating transport are far from being understood.     

Increased expression of GLUT-4 and hexokinase in rat epitrochlearis muscles exposed to AICAR in vitro.

Exercise acutely stimulates muscle glucose transport and also brings about an adaptive increase in the capacity of muscle for glucose uptake by inducing increases in GLUT-4 and hexokinase.(1) Recent studies have provided evidence that activation of AMP protein kinase (AMPK) is involved in the stimulation of glucose transport by exercise. The purpose of this study was to determine whether activation of AMPK is also involved in mediating the adaptive increases in GLUT-4 and hexokinase. To this end, we examined the effect of incubating rat epitrochlearis muscles in culture medium for 18 h in the presence or absence of 5-aminoimidazole-4-carboxamide ribonucleoside (AICAR), which enters cells and is converted to the AMP analog ZMP, thus activating AMPK. Exposure of muscles to 0.5 mM AICAR in vitro for 18 h resulted in an approximately 50% increase in GLUT-4 protein and an approximately 80% increase in hexokinase. This finding provides strong evidence in support of the hypothesis that the activation of AMPK that occurs in muscle during exercise is involved in mediating the adaptive increases in GLUT-4 and hexokinase.