Dissociation of AMPK activity and ACCbeta phosphorylation in human muscle during prolonged exercise

Interleukin-6 modifies mRNA expression in mouse skeletal muscle

AIM

The aim of this study was to test the hypothesis that interleukin (IL)-6 plays a role in exercise-induced peroxisome proliferator-activated receptor γ co-activator (PGC)-1α and tumor necrosis factor (TNF)-α mRNA responses in skeletal muscle and to examine the potential IL-6-mediated AMP-activated protein kinase (AMPK) regulation in these responses.

METHODS

Whole body IL-6 knockout (KO) and wildtype (WT) male mice (4 months of age) performed 1 h treadmill exercise. White gastrocnemius (WG) and quadriceps (Quad) muscles were removed immediately (0') or 4 h after exercise and from mice not run acutely.

RESULTS

Acute exercise reduced only in WT muscle glycogen concentration to 55 and 35% (P < 0.05) of resting level in Quad and WG respectively. While AMPK and Acetyl CoA carboxylase (ACC) phosphorylation increased 1.3-fold (P < 0.05) in WG and twofold in Quad immediately after exercise in WT mice, no change was detected in WG in IL-6 KO mice. The PGC-1α mRNA content was in resting WG 1.8-fold higher (P < 0.05) in WT mice than in IL-6 KO mice. Exercise induced a delayed PGC-1α mRNA increase in Quad in IL-6 KO mice (12-fold at 4 h) relative to WT mice (fivefold at 0'). The TNF-α mRNA content was in resting Quad twofold higher (P < 0.05) in IL-6 KO than in WT, and WG TNF-α mRNA increased twofold (P < 0.05) immediately after exercise only in IL-6 KO.

CONCLUSION

In conclusion, IL-6 affects exercise-induced glycogen use, AMPK signalling and TNF-α mRNA responses in mouse skeletal muscle.

Caffeine activates preferentially α1-isoform of 5'AMP-activated protein kinase in rat skeletal muscle

AIM

Caffeine activates 5'AMP-activated protein kinase (AMPK), a signalling intermediary implicated in the regulation of glucose, lipid and energy metabolism in skeletal muscle. Skeletal muscle expresses two catalytic α subunits of AMPK, α1 and α2, but the isoform specificity of caffeine-induced AMPK activation is unclear. The aim of this study was to determine which α isoform is preferentially activated by caffeine in vitro and in vivo using rat skeletal muscle.

METHODS

Rat epitrochlearis muscle was isolated and incubated in vitro in the absence or presence of caffeine. In another experiment, the muscle was dissected after intravenous injection of caffeine. Isoform-specific AMPK activity, the phosphorylation status of AMPKα Thr(172) and acetyl-CoA carboxylase (ACC) Ser(79) , the concentrations of ATP, phosphocreatine (PCr) and glycogen, and 3-O-methyl-d-glucose (3MG) transport activity were estimated.

RESULTS

Incubation of isolated epitrochlearis muscle with 1 mm of caffeine for 15 min increased AMPKα1 activity, but not AMPKα2 activity; concentrations of ATP, PCr and glycogen were not affected. Incubation with 3 mm of caffeine activated AMPKα2 and reduced PCr and glycogen concentrations. Incubation with 1 mm of caffeine increased the phosphorylation of AMPK and ACC and enhanced 3MG transport. Intravenous injection of caffeine (5 mg kg(-1) ) predominantly activated AMPKα1 and increased 3MG transport without affecting energy status.

CONCLUSION

Our results suggest that of the two α isoforms of AMPK, AMPKα1 is predominantly activated by caffeine via an energy-independent mechanism and that the activation of AMPKα1 increases glucose transport and ACC phosphorylation in skeletal muscle.

Reconstituted high-density lipoprotein infusion modulates fatty acid metabolism in patients with type 2 diabetes mellitus

We recently demonstrated that reconstituted high-density lipoprotein (rHDL) modulates glucose metabolism in humans via both AMP-activated protein kinase (AMPK) in muscle and by increasing plasma insulin. Given the key roles of both AMPK and insulin in fatty acid metabolism, the current study investigated the effect of rHDL infusion on fatty acid oxidation and lipolysis. Thirteen patients with type 2 diabetes received separate infusions of rHDL and placebo in a randomized, cross-over study. Fatty acid metabolism was assessed using steady-state tracer methodology, and plasma lipids were measured by mass spectrometry (lipidomics). In vitro studies were undertaken in 3T3-L1 adipocytes. rHDL infusion inhibited fasting-induced lipolysis (P = 0.03), fatty acid oxidation (P < 0.01), and circulating glycerol (P = 0.04). In vitro, HDL inhibited adipocyte lipolysis in part via activation of AMPK, providing a possible mechanistic link for the apparent reductions in lipolysis observed in vivo. In contrast, circulating NEFA increased after rHDL infusion (P < 0.01). Lipidomic analyses implicated phospholipase hydrolysis of rHDL-associated phosphatidylcholine as the cause, rather than lipolysis of endogenous fat stores. rHDL infusion inhibits fasting-induced lipolysis and oxidation in patients with type 2 diabetes, potentially through both AMPK activation in adipose tissue and elevation of plasma insulin. The phospholipid component of rHDL also has the potentially undesirable effect of increasing circulating NEFA.

Glucagon and lipid interactions in the regulation of hepatic AMPK signaling and expression of PPARalpha and FGF21 transcripts in vivo

Hepatic glucagon action increases in response to accelerated metabolic demands and is associated with increased whole body substrate availability, including circulating lipids. The hypothesis that increases in hepatic glucagon action stimulate AMP-activated protein kinase (AMPK) signaling and peroxisome proliferator-activated receptor-α (PPARα) and fibroblast growth factor 21 (FGF21) expression in a manner modulated by fatty acids was tested in vivo. Wild-type (gcgr(+/+)) and glucagon receptor-null (gcgr(-/-)) littermate mice were studied using an 18-h fast, exercise, and hyperglucagonemic-euglycemic clamps plus or minus increased circulating lipids. Fasting and exercise in gcgr(+/+), but not gcgr(-/-) mice, increased hepatic phosphorylated AMPKα at threonine 172 (p-AMPK(Thr(172))) and PPARα and FGF21 mRNA. Clamp results in gcgr(+/+) mice demonstrate that hyperlipidemia does not independently impact or modify glucagon-stimulated increases in hepatic AMP/ATP, p-AMPK(Thr(172)), or PPARα and FGF21 mRNA. It blunted glucagon-stimulated acetyl-CoA carboxylase phosphorylation, a downstream target of AMPK, and accentuated PPARα and FGF21 expression. All effects were absent in gcgr(-/-) mice. These findings demonstrate that glucagon exerts a critical regulatory role in liver to stimulate pathways linked to lipid metabolism in vivo and shows for the first time that effects of glucagon on PPARα and FGF21 expression are amplified by a physiological increase in circulating lipids.

Genetic downregulation of AMPK-alpha isoforms uncovers the mechanism by which metformin decreases FA uptake and oxidation in skeletal muscle cells

Metformin is known to improve insulin sensitivity in part via a rise in AMP-activated protein kinase (AMPK) activity and alterations in muscle metabolism. However, a full understanding of how metformin alters AMPK-α(1) vs. AMPK-α(2) activation remains unknown. To study this question, L6 skeletal muscle cells were treated with or without RNAi oligonucleotide sequences to downregulate AMPK-α(1) or AMPK-α(2) protein expression and incubated with or without 5-aminoimidazole-4-carboxamide-1-β-d-ribofuranoside (AICAR) or metformin and/or insulin. In contrast to AICAR, which preferentially activated AMPK-α(2), metformin preferentially activated AMPK-α(1) in a dose- and time-dependent manner. Metformin increased (P < 0.05) glucose uptake and plasma membrane (PM) Glut4 in a dose- and time-dependent manner. Metformin significantly reduced palmitate uptake (P < 0.05) and oxidation (P < 0.05), and this was accompanied by a similar decrease (P < 0.05) in PM CD36 content but with no change in acetyl-CoA carboxylase (ACC) phosphorylation (P > 0.05). AICAR and metformin similarly increased (P < 0.05) nuclear silent mating-type information regulator 2 homolog 1 (SIRT1) activity. Downregulation of AMPK-α(1) completely prevented the metformin-induced reduction in palmitate uptake and oxidation but only partially reduced the metformin-induced increase in glucose uptake. Downregulation of AMPK-α(2) had no effect on metformin-induced glucose uptake, palmitate uptake, and oxidation. The increase in SIRT1 activity induced by metformin was not affected by downregulation of either AMPK-α(1) or AMPK-α(2). Our data indicate that, in muscle cells, the inhibitory effects of metformin on fatty acid metabolism occur via preferential phosphorylation of AMPK-α(1), and the data indicate that cross talk between AMPK and SIRT1 does not favor either AMPK isozyme.

CFTR depletion results in changes in fatty acid composition and promotes lipogenesis in intestinal Caco 2/15 cells

Background

Abnormal fatty acid composition (FA) in plasma and tissue lipids frequently occurs in homozygous and even in heterozygous carriers of cystic fibrosis transmembrane conductance regulator (CFTR) mutations. The mechanism(s) underlying these abnormalities remained, however, poorly understood despite the potentially CFTR contributing role.

Methodology/Principal Findings

The aim of the present study was to investigate the impact of CFTR depletion on FA uptake, composition and metabolism using the intestinal Caco-2/15 cell line. shRNA-mediated cftr gene silencing induced qualitative and quantitative modifications in FA composition in differentiated enterocytes as determined by gas-liquid chromatography. With the cftr gene disruption, there was a 1,5 fold increase in the total FA amount, largely attributable to monounsaturated and saturated FA compared to controls. The activity of delta-7 desaturase, estimated by the 16:1(n-7)/16:0, was significantly higher in knockdown cells and consistent with the striking elevation of the n-7 FA family. When incubated with [¹⁴C]-oleic acid, CFTR-depleted cells were capable of quick incorporation and export to the medium concomitantly with the high protein expression of L-FABP known to promote intracellular FA trafficking. Accordingly, lipoprotein vehicles (CM, VLDL, LDL and HDL), isolated from CFTR knockdown cells, exhibited higher levels of radiolabeled FA. Moreover, in the presence of [¹⁴C]-acetate, knockdown cells exhibited enhanced secretion of newly synthesized phospholipids, triglycerides, cholesteryl esters and free FA, thereby suggesting a stimulation of the lipogenic pathway. Conformably, gene expression of SREBP-1c, a key lipogenic transcription factor, was increased while protein expression of the phosphorylated and inactive form of acetylCoA carboxylase was reduced, confirming lipogenesis induction. Finally, CFTR-depleted cells exhibited lower gene expression of transcription factors (PPARα, LXRα, LXRβ and RXRα).

Conclusions/Significance

Collectively, our results indicate that CFTR depletion may disrupt FA homeostasis in intestinal cells through alterations in FA uptake and transport combined with stimulation of lipogenesis that occurs by an LXR/RXR-independent mechanism. These findings exclude a contributing role of CFTR in CF-associated fat malabsorption.

SIRT1, AMP-activated protein kinase phosphorylation and downstream kinases in response to a single bout of sprint exercise: influence of glucose ingestion

This study was designed to examine potential in vivo mechanisms of AMP-activated protein kinase (AMPK) phosphorylation inhibition and its downstream signaling consequences during the recovery period after a single bout of sprint exercise. Sprint exercise induces Thr(172)-AMPK phosphorylation and increased PGC-1alpha mRNA, by an unknown mechanism. Muscle biopsies were obtained in 15 young healthy men in response to a 30-s sprint exercise (Wingate test) randomly distributed into two groups: the fasting (n = 7, C) and the glucose group (n = 8, G), who ingested 75 g of glucose 1 h before exercising to inhibit AMPKalpha phosphorylation. Exercise elicited different patterns of Ser(221)-ACCbeta, Ser(473)-Akt and Thr(642)-AS160 phosphorylation, during the recovery period after glucose ingestion. Thirty minutes after the control sprint, Ser(485)-AMPKalpha1/Ser(491)-AMPKalpha2 phosphorylation was reduced by 33% coinciding with increased Thr(172)-AMPKalpha phosphorylation (both, P < 0.05). Glucose abolished the 30-min Thr(172)-AMPKalpha phosphorylation. Ser(221)-ACCbeta phosphorylation was elevated immediately following and 30 min after exercise in C and G, implying a dissociation between Thr(172)-AMPKalpha and Ser(221)-ACCbeta phosphorylation. Two hours after the sprint, PGC-1alpha protein expression remained unchanged while SIRT1 (its upstream deacetylase) was increased. Glucose ingestion abolished the SIRT1 response without any significant effect on PGC-1alpha protein expression. In conclusion, glucose ingestion prior to a sprint exercise profoundly affects Thr(172)-AMPKalpha phosphorylation and its downstream signaling during the recovery period.

Additive effect of contraction and insulin on glucose uptake and glycogen synthase in muscle with different glycogen contents

Insulin and contraction regulate glucose uptake and glycogen synthase (GS) via distinct mechanisms in skeletal muscles, and an additive effect has been reported. Glycogen content is known to influence both contraction- and insulin-stimulated glucose uptake and GS activity. Our study reports that contraction and insulin additively stimulate glucose uptake in rat epitrochlearis muscles with normal (NG) and high (HG) glycogen contents, but the additive effect was only partial. In muscles with low glycogen (LG) content no additive effect was seen, but glucose uptake was higher in LG than in NG and HG during contraction, insulin stimulation, and when the two stimuli were combined. In LG, contraction-stimulated AMP-activated protein kinase (AMPK) activity and insulin-stimulated PKB phosphorylation were higher than in NG and HG, but phosphorylation of Akt substrate of 160 kDa was not elevated correspondingly. GLUT4 content was 50% increased in LG (rats fasted 24 h), which may explain the increased glucose uptake. Contraction and insulin also additively increased GS fractional activity in NG and HG but not in LG. GS fractional activity correlated most strongly with GS Ser641 phosphorylation (R -0.94, P<0.001). GS fractional activity also correlated with GS Ser7,10 phosphorylation, but insulin did not reduce GS Ser7,10 phosphorylation. In conclusion, an additive effect of contraction and insulin on glucose uptake and GS activity occurs in muscles with normal and high glycogen content but not in muscles with low glycogen content. Furthermore, contraction, insulin, and glycogen content all regulate GS Ser641 phosphorylation and GS fractional activity in concert.

The 5' adenosine monophosphate-activated protein kinase: regulating the ebb and flow of cellular energetics

The 5' adenosine monophosphate-activated protein kinase (AMPK) is a heterotrimeric, evolutionary conserved enzyme which has emerged as a critical regulator of skeletal muscle cellular bioenergetics. AMPK is activated by both chemical (adipokines) and mechanical (stretch, contraction) stimuli leading to metabolic changes within muscle cells that include increased fatty acid oxidation, glucose uptake and glycolysis, as well as the stimulation and regulation of mitochondrial biogenesis. Collectively these acute responses and chronic adaptations act to reduce cellular disturbances, resulting in tighter metabolic control and maintenance of energy homeostasis. This brief review will describe the structure, function and activation of AMPK in skeletal muscle and how this ubiquitous molecule may be a plausible target for the treatment of several lifestyle-related metabolic disorders.

AMP-activated protein kinase in contraction regulation of skeletal muscle metabolism: necessary and/or sufficient?

In skeletal muscle, the contraction-activated heterotrimeric 5'-AMP-activated protein kinase (AMPK) protein is proposed to regulate the balance between anabolic and catabolic processes by increasing substrate uptake and turnover in addition to regulating the transcription of proteins involved in mitochondrial biogenesis and other aspects of promoting an oxidative muscle phenotype. Here, the current knowledge on the expression of AMPK subunits in human quadriceps muscle and evidence from rodent studies suggesting distinct AMPK subunit expression pattern in different muscle types is reviewed. Then, the intensity and time dependence of AMPK activation in human quadriceps and rodent muscle are evaluated. Subsequently, a major part of this review critically examines the evidence supporting a necessary and/or sufficient role of AMPK in a broad spectrum of skeletal muscle contraction-relevant processes. These include glucose uptake, glycogen synthesis, post-exercise insulin sensitivity, fatty acid (FA) uptake, intramuscular triacylglyceride hydrolysis, FA oxidation, suppression of protein synthesis, proteolysis, autophagy and transcriptional regulation of genes relevant to promoting an oxidative phenotype.

AMPK activation is fiber type specific in human skeletal muscle: effects of exercise and short-term exercise training

AMP-activated protein kinase (AMPK) has been extensively studied in whole muscle biopsy samples of humans, yet the fiber type-specific expression and/or activation of AMPK is unknown. We examined basal and exercise AMPK-alpha Thr(172) phosphorylation and AMPK subunit expression (alpha(1), alpha(2), and gamma(3)) in type I, IIa, and IIx fibers of human skeletal muscle before and after 10 days of exercise training. Before training basal AMPK phosphorylation was greatest in type IIa fibers (P < 0.05 vs. type I and IIx), while an acute bout of exercise increased AMPK phosphorylation in all fibers (P < 0.05), with the greatest increase occurring in type IIx fibers. Exercise training significantly increased basal AMPK phosphorylation in all fibers, and the exercise-induced increases were uniformly suppressed compared with pretraining exercise. Expression of AMPK-alpha(1) and -alpha(2) was similar between fibers and was not altered by exercise training. However, AMPK-gamma(3) was differentially expressed in skeletal muscle fibers (type IIx > type IIa > type I), irrespective of training status. Thus skeletal muscle AMPK phosphorylation and AMPK expression are fiber type specific in humans in the basal state, as well as during exercise. Our findings reveal fiber type-specific differences that have been masked in previous studies examining mixed muscle samples.

Role of 5'AMP-activated protein kinase in skeletal muscle

5'AMP-activated protein kinase (AMPK) is recognized as an important intracellular energy sensor, shutting down energy-consuming processes and turning on energy-generating processes. Discovery of target proteins of AMPK has dramatically increased in the past 10 years. Historically, AMPK was first shown to regulate fatty acid and cholesterol synthesis, but is now hypothesized to take part in the regulation of energy/fuel balance not only at the cellular level but also at the level of the whole organism. In this brief review we will discuss some of the roles of AMPK in skeletal muscle.

AMPK and the biochemistry of exercise: implications for human health and disease

AMPK (AMP-activated protein kinase) is a phylogenetically conserved fuel-sensing enzyme that is present in all mammalian cells. During exercise, it is activated in skeletal muscle in humans, and at least in rodents, also in adipose tissue, liver and perhaps other organs by events that increase the AMP/ATP ratio. When activated, AMPK stimulates energy-generating processes such as glucose uptake and fatty acid oxidation and decreases energy-consuming processes such as protein and lipid synthesis. Exercise is perhaps the most powerful physiological activator of AMPK and a unique model for studying its many physiological roles. In addition, it improves the metabolic status of rodents with a metabolic syndrome phenotype, as does treatment with AMPK-activating agents; it is therefore tempting to attribute the therapeutic benefits of regular physical activity to activation of AMPK. Here we review the acute and chronic effects of exercise on AMPK activity in skeletal muscle and other tissues. We also discuss the potential role of AMPK activation in mediating the prevention and treatment by exercise of specific disorders associated with the metabolic syndrome, including Type 2 diabetes and Alzheimer's disease.

The glucagon-like peptide-1 receptor regulates endogenous glucose production and muscle glucose uptake independent of its incretin action

Glucagon-like peptide-1 (GLP-1) diminishes postmeal glucose excursions by enhancing insulin secretion via activation of the beta-cell GLP-1 receptor (Glp1r). GLP-1 may also control glucose levels through mechanisms that are independent of this incretin effect. The hyperinsulinemic-euglycemic clamp (insulin clamp) and exercise were used to examine the incretin-independent glucoregulatory properties of the Glp1r because both perturbations stimulate glucose flux independent of insulin secretion. Chow-fed mice with a functional disruption of the Glp1r (Glp1r(-/-)) were compared with wild-type littermates (Glp1r(+/+)). Studies were performed on 5-h-fasted mice implanted with arterial and venous catheters for sampling and infusions, respectively. During insulin clamps, [3-(3)H]glucose and 2[(14)C]deoxyglucose were used to determine whole-body glucose turnover and glucose metabolic index (R(g)), an indicator of glucose uptake. R(g) in sedentary and treadmill exercised mice was determined using 2[(3)H]deoxyglucose. Glp1r(-/-) mice exhibited increased glucose disappearance, muscle R(g), and muscle glycogen levels during insulin clamps. This was not associated with enhanced muscle insulin signaling. Glp1r(-/-) mice exhibited impaired suppression of endogenous glucose production and hepatic glycogen accumulation during insulin clamps. This was associated with impaired liver insulin signaling. Glp1r(-/-) mice became significantly hyperglycemic during exercise. Muscle R(g) was normal in exercised Glp1r(-/-) mice, suggesting that hyperglycemia resulted from an added drive to stimulate glucose production. Muscle AMP-activated protein kinase phosphorylation was higher in exercised Glp1r(-/-) mice. This was associated with increased relative exercise intensity and decreased exercise endurance. In conclusion, these results show that the endogenous Glp1r regulates hepatic and muscle glucose flux independent of its ability to enhance insulin secretion.

Contractions but not AICAR increase FABPpm content in rat muscle sarcolemma

In the present study, it was investigated whether acute muscle contractions in rat skeletal muscle increased the protein content of FABPpm in the plasma membrane. Furthermore, the effect of AICAR stimulation on FAT/CD36 and FABPpm protein content in sarcolemma of rat skeletal muscle was evaluated.

METHODS

Male wistar rats (150 g) were anesthetized and either subjected to in situ electrically induced contractions (hindlimb muscles: 20 min, 10-20 V, 200 ms trains, 100 Hz) or stimulated with the pharmacological activator of AMPK, AICAR. To investigate changes in the content of FABPpm and FAT/CD36 in the plasma membrane by these stimuli, the giant sarcolemma vesicle (GSV) technique was applied. The hindlimb muscles were removed and used for the production of GSV and lysates. All samples were analyzed using the western blotting technique.

RESULTS

Electrical stimulation of rat hindlimb muscle resulted in an increase in FABPpm protein content in the GSV of 61% (P < 0.05) and in FAT/CD36 protein content in the GSV of 33% (P < 0.05). AICAR stimulation increased FAT/CD36 protein content in GSV by 22% (P < 0.05), whereas FABPpm protein content in GSV was unaffected by AICAR treatment. There was no change in total FAT/CD36 and FABPpm protein expression, measured in lysates with western blotting, by either stimulus. AMPK thr172 and ERK1/2 thr202/204 phosphorylation were significantly increased with muscle contractions (P < 0.05), whereas only AMPK thr172 phosphorylation was increased with AICAR stimulation (P < 0.05).

CONCLUSION

These data show that contractions increase both FAT/CD36 and FABPpm protein content in skeletal muscle plasma membrane, whereas only FAT/CD36 protein content is increased when muscle are stimulated with AICAR. This suggests that AMPK is involved in regulation of FAT/CD36, but not FABPpm in skeletal muscle. However, since both ERK1/2 thr202/204 and AMPK thr172 phosphorylation are increased during muscle contractions, the present study cannot rule out that both could play a significant role in regulation of FAT/CD36 and FABPpm during muscle contractions.

Alpha2-AMPK activity is not essential for an increase in fatty acid oxidation during low-intensity exercise

A single bout of exercise increases glucose uptake and fatty acid oxidation in skeletal muscle, with a corresponding activation of AMP-activated protein kinase (AMPK). While the exercise-induced increase in glucose uptake is partly due to activation of AMPK, it is unclear whether the increase of fatty acid oxidation is dependent on activation of AMPK. To examine this, transgenic mice were produced expressing a dominant-negative (DN) mutant of alpha(1)-AMPK (alpha(1)-AMPK-DN) in skeletal muscle and subjected to treadmill running. alpha(1)-AMPK-DN mice exhibited a 50% reduction in alpha(1)-AMPK activity and almost complete loss of alpha(2)-AMPK activity in skeletal muscle compared with wild-type littermates (WT). The fasting-induced decrease in respiratory quotient (RQ) ratio and reduced body weight were similar in both groups. In contrast with WT mice, alpha(1)-AMPK-DN mice could not perform high-intensity (30 m/min) treadmill exercise, although their response to low-intensity (10 m/min) treadmill exercise was not compromised. Changes in oxygen consumption and the RQ ratio during sedentary and low-intensity exercise were not different between alpha(1)-AMPK-DN and WT. Importantly, at low-intensity exercise, increased fatty acid oxidation in response to exercise in soleus (type I, slow twitch muscle) or extensor digitorum longus muscle (type II, fast twitch muscle) was not impaired in alpha(1)-AMPK-DN mice, indicating that alpha(1)-AMPK-DN mice utilize fatty acid in the same manner as WT mice during low-intensity exercise. These findings suggest that an increased alpha(2)-AMPK activity is not essential for increased skeletal muscle fatty acid oxidation during endurance exercise.

Acute exercise does not cause sustained elevations in AMPK signaling or expression

PURPOSE

No study has examined the response of skeletal muscle AMP-activated protein kinase (AMPK) signaling beyond the first 3 h after an acute exercise bout in humans. The purpose of this study was to assess AMPK signaling in human skeletal muscle immediately after a single bout of moderate-intensity endurance exercise and 3 and 24 h after the exercise bout.

METHODS

We examined AMPK signaling, and protein expression of AMPK alpha, ACC-beta, and nNOS mu in untrained individuals (four females and four males) during the 24-h period after a 60-min bout of moderate-intensity (63 +/- 1% VO2peak) cycling endurance exercise.

RESULTS

AMPK alpha2 activity, AMPK alpha2 Thr172 phosphorylation, and ACC-beta Ser222 phosphorylation were increased immediately after exercise. These increases had all returned to basal levels at 3 and 24 h after exercise. Furthermore, an acute bout of exercise did not alter AMPK alpha1, AMPK alpha2, ACC-beta, or nNOS mu protein expression during the 24-h period after exercise.

CONCLUSION

Although an acute bout of exercise elicits increases in AMPK signaling, this alone is not sufficient to induce sustained increases in either AMPK signaling or protein expression during the postexercise period.

AMP activated protein kinase: a next generation target for total metabolic control

Metabolic syndrome is characterized by a cluster of metabolic disorders, such as reduced glucose tolerance, hyperinsulinemia, hypertension, visceral obesity and lipid disorders. The benefit of exercise in maintaining total metabolic control is well known and recent research indicates that AMP-activated protein kinase (AMPK) may play an important role in exercise-related effects. AMPK is considered as a master switch in regulating glucose and lipid metabolism. AMPK is an enzyme that works as a fuel gauge, being activated in conditions of high phosphate depletion. In the liver, activation of AMPK results in decreased production of plasma glucose, cholesterol, triglyceride and enhanced fatty acid oxidation. AMPK is also robustly activated by skeletal muscle contraction and myocardial ischemia, and is involved in the stimulation of glucose transport and fatty acid oxidation by these stimuli. In adipose tissue, activated AMPK inhibits deposition of fat, but enhances breakdown and burning of stored fat, resulting in reduction of body weight. The two leading diabetic drugs, namely metformin and rosiglitazone, and adipokines, such as adiponectin and leptin, show their metabolic effects partially through AMPK. These data suggest that AMPK may be a key player in the development of new treatments for obesity, Type 2 diabetes and the metabolic syndrome. In this review, the author provide insight into the role of AMPK as a probable target for treatment of metabolic syndrome.

Lack of AMPKalpha2 enhances pyruvate dehydrogenase activity during exercise

5'-AMP-activated protein kinase (AMPK) was recently suggested to regulate pyruvate dehydrogenase (PDH) activity and thus pyruvate entry into the mitochondrion. We aimed to provide evidence for a direct link between AMPK and PDH in resting and metabolically challenged (exercised) skeletal muscle. Compared with rest, treadmill running increased AMPKalpha1 activity in alpha(2)KO mice (90%, P < 0.01) and increased AMPKalpha2 activity in wild-type (WT) mice (110%, P < 0.05), leading to increased AMPKalpha Thr(172) (WT: 40%, alpha(2)KO: 100%, P < 0.01) and ACCbeta Ser(227) phosphorylation (WT: 70%, alpha(2)KO: 210%, P < 0.01). Compared with rest, exercise significantly induced PDH-E(1)alpha site 1 (WT: 20%, alpha(2)KO: 62%, P < 0.01) and site 2 (only alpha(2)KO: 83%, P < 0.01) dephosphorylation and PDH(a) [ approximately 200% in both genotypes (P < 0.01)]. Compared with WT, PDH dephosphorylation and activation was markedly enhanced in the alpha(2)KO mice both at rest and during exercise. The increased PDH(a) activity during exercise was associated with elevated glycolytic flux, and muscles from the alpha(2)KO mice displayed marked lactate accumulation and deranged energy homeostasis. Whereas mitochondrial DNA content was normal, the expression of several mitochondrial proteins was significantly decreased in muscle of alpha(2)KO mice. In isolated resting EDL muscles, activation of AMPK signaling by AICAR did not change PDH-E(1)alpha phosphorylation in either genotype. PDH is activated in mouse skeletal muscle in response to exercise and is independent of AMPKalpha2 expression. During exercise, alpha(2)KO muscles display deranged energy homeostasis despite enhanced glycolytic flux and PDH(a) activity. This may be linked to decreased mitochondrial oxidative capacity.

Role of AMPK in skeletal muscle gene adaptation in relation to exercise

The 5′-AMP-activated protein kinase (AMPK) functions as an intracellular fuel sensor that affects metabolism and gene expression. AMPK is activated in skeletal muscle in response to exercise and is therefore believed to be an important signalling molecule in regulating adaptation of skeletal muscle to exercise training. This review first focuses on mechanisms regulating AMPK activity during muscle contraction. We then discuss the role of AMPK in regulating expression of genes encoding various enzymes in muscle in the basal state and in relation to exercise training. Although decreased AMPK activity in muscle causes reduced protein expression of mitochondrial enzymes in the basal state, AMPK does not appear to be indispensable for exercise-training induced increase in mitochondrial enzyme expression.

Acute exercise increases triglyceride synthesis in skeletal muscle and prevents fatty acid-induced insulin resistance

Fatty acid oversupply is a key mediator of skeletal muscle insulin resistance in obesity, primarily via accumulation of fatty acid metabolites and activation of proinflammatory pathways. Herein, we demonstrate that fatty acid-induced insulin resistance in humans is completely prevented the day after 1 session of endurance exercise. Because skeletal muscle is the primary site for systemic glucose disposal and is highly susceptible to impaired insulin action by elevated fatty acid availability, we obtained skeletal muscle samples to investigate possible mechanisms mediating this protective effect of exercise. Prevention of fatty acid-induced insulin resistance after exercise accompanied enhanced skeletal muscle protein expression of key lipogenic enzymes and an increase in muscle triglyceride synthesis. Partitioning more fatty acids toward triglyceride synthesis within muscle reduced the accumulation of fatty acid metabolites and suppressed the proinflammatory response in skeletal muscle, as evidenced by decreased phosphorylation and activation of JNK and increased abundance of inhibitor of NF-kappaB alpha (I kappa B-alpha) and I kappa B-beta. We believe this is the first study to demonstrate that 1 session of exercise completely reverses fatty acid-induced insulin resistance in humans. Reversal of insulin resistance accompanied enhanced lipogenic capacity within skeletal muscle, reduced accumulation of highly bioactive fatty acid metabolites, and suppressed activation of proinflammatory pathways known to impair insulin action.

AS160 phosphorylation is associated with activation of alpha2beta2gamma1- but not alpha2beta2gamma3-AMPK trimeric complex in skeletal muscle during exercise in humans

We investigated time- and intensity-dependent effects of exercise on phosphorylation of Akt substrate of 160 kDa (AS160) in human skeletal muscle. Subjects performed cycle exercise for 90 min (67% VO2 peak, n=8), 20 min (80% VO2 peak, n=11), 2 min (110% of peak work rate, n=9), or 30 s (maximal sprint, n=10). Muscle biopsies were obtained before, during, and after exercise. In trial 1, AS160 phosphorylation increased at 60 min (60%, P=0.06) and further at 90 min of exercise (120%, P<0.05). alpha2beta2gamma3-AMP-activated protein kinase (AMPK) activity increased significantly to a steady-state level after 30 min, whereas alpha2beta2gamma1-AMPK activity increased after 60 min of exercise with a further significant increase after 90 min. alpha2beta2gamma1-AMPK activity and AS160 phosphorylation correlated positively (r2=0.55). In exercise trials 2, 3, and 4, alpha2beta2gamma3-AMPK activity but neither AS160 phosphorylation nor alpha2beta2gamma1-AMPK activity increased. Akt Ser473 phosphorylation was unchanged in all trials, whereas Akt Thr308 phosphorylation increased significantly in trial 3 and 4 only. These results show that AS160 is phosphorylated in a time-dependent manner during moderate-intensity exercise and suggest that alpha2beta2gamma1- but not alpha2beta2gamma3-AMPK may act in a pathway responsible for exercise-induced AS160 phosphorylation. Furthermore, we show that AMPK complexes in skeletal muscle are activated differently depending on exercise intensity and duration.

Reduced glycogen availability is associated with increased AMPKalpha2 activity, nuclear AMPKalpha2 protein abundance, and GLUT4 mRNA expression in contracting human skeletal muscle

Glycogen availability can influence glucose transporter 4 (GLUT4) expression in skeletal muscle through unknown mechanisms. The multisubstrate enzyme AMP-activated protein kinase (AMPK) has also been shown to play an important role in the regulation of GLUT4 expression in skeletal muscle. During contraction, AMPK alpha2 translocates to the nucleus and the activity of this AMPK isoform is enhanced when skeletal muscle glycogen is low. In this study, we investigated if decreased pre-exercise muscle glycogen levels and increased AMPK alpha2 activity reduced the association of AMPK with glycogen and increased AMPK alpha2 translocation to the nucleus and GLUT4 mRNA expression following exercise. Seven males performed 60 min of exercise at approximately 70% VO(2) (peak) on 2 occasions: either with normal (control) or low (LG) carbohydrate pre-exercise muscle glycogen content. Muscle samples were obtained by needle biopsy before and after exercise. Low muscle glycogen was associated with elevated AMPK alpha2 activity and acetyl-CoA carboxylase beta phosphorylation, increased translocation of AMPK alpha2 to the nucleus, and increased GLUT4 mRNA. Transfection of primary human myotubes with a constitutively active AMPK adenovirus also stimulated GLUT4 mRNA, providing direct evidence of a role of AMPK in regulating GLUT4 expression. We suggest that increased activation of AMPK alpha2 under conditions of low muscle glycogen enhances AMPK alpha2 nuclear translocation and increases GLUT4 mRNA expression in response to exercise in human skeletal muscle.

Predominant alpha2/beta2/gamma3 AMPK activation during exercise in human skeletal muscle

5'AMP-activated protein kinase (AMPK) is a key regulator of cellular metabolism and is regulated in muscle during exercise. We have previously established that only three of 12 possible AMPK alpha/beta/gamma-heterotrimers are present in human skeletal muscle. Previous studies describe discrepancies between total AMPK activity and regulation of its target acetyl-CoA-carboxylase (ACC)beta. Also, exercise training decreases expression of the regulatory gamma3 AMPK subunit and attenuates alpha2 AMPK activity during exercise. We hypothesize that these observations reflect a differential regulation of the AMPK heterotrimers. We provide evidence here that only the alpha2/beta2/gamma3 subunit is phosphorylated and activated during high-intensity exercise in vivo. The activity associated with the remaining two AMPK heterotrimers, alpha1/beta2/gamma1 and alpha2/beta2/gamma1, is either unchanged (20 min, 80% maximal oxygen uptake ) or decreased (30 or 120 s sprint-exercise). The differential activity of the heterotrimers leads to a total alpha-AMPK activity, that is decreased (30 s trial), unchanged (120 s trial) and increased (20 min trial). AMPK activity associated with the alpha2/beta2/gamma3 heterotrimer was strongly correlated to gamma3-associated alpha-Thr-172 AMPK phosphorylation (r(2) = 0.84, P < 0.001) and to ACCbeta Ser-221 phosphorylation (r(2) = 0.65, P < 0.001). These data single out the alpha2/beta2/gamma3 heterotrimer as an important actor in exercise-regulated AMPK signalling in human skeletal muscle, probably mediating phosphorylation of ACCbeta.

Acute exercise and GLUT4 expression in human skeletal muscle: influence of exercise intensity

To examine the influence of exercise intensity on the increases in vastus lateralis GLUT4 mRNA and protein after exercise, six untrained men exercised for 60 min at 39 ± 3% peak oxygen consumption (V̇o2 peak) (Lo) or 27 ± 2 min at 83 ± 2% V̇o2 peak(Hi) in counterbalanced order. Preexercise muscle glycogen levels were not different between trials (Lo: 408 ± 35 mmol/kg dry mass; Hi: 420 ± 43 mmol/kg dry mass); however, postexercise levels were lower ( P < 0.05) in Hi (169 ± 18 mmol/kg dry mass) compared with Lo (262 ± 35 mmol/kg dry mass). Thus calculated muscle glycogen utilization was greater ( P < 0.05) in Hi (251 ± 24 mmol/kg) than in Lo (146 ± 34). Exercise resulted in similar increases in GLUT4 gene expression in both trials. GLUT4 mRNA was increased immediately at the end of exercise (∼2-fold; P < 0.05) and remained elevated after 3 h of postexercise recovery. When measured 3 h after exercise, total crude membrane GLUT4 protein levels were 106% higher in Lo (3.3 ± 0.7 vs. 1.6 ± 0.3 arbitrary units) and 61% higher in Hi (2.9 ± 0.5 vs. 1.8 ± 0.5 arbitrary units) relative to preexercise levels. A main effect for exercise was observed, with no significant differences between trials. In conclusion, exercise at ∼40 and ∼80% V̇o2 peak, with total work equal, increased GLUT4 mRNA and GLUT4 protein in human skeletal muscle to a similar extent, despite differences in exercise intensity and duration.

Role of AMPK in skeletal muscle metabolic regulation and adaptation in relation to exercise

The 5'-AMP-activated protein kinase (AMPK) is a potent regulator of skeletal muscle metabolism and gene expression. AMPK is activated both in response to in vivo exercise and ex vivo contraction. AMPK is therefore believed to be an important signalling molecule in regulating muscle metabolism during exercise as well as in adaptation of skeletal muscle to exercise training. The first part of this review is focused on different mechanisms regulating AMPK activity during muscle work such as alterations in nucleotide concentrations, availability of energy substrates and upstream AMPK kinases. We furthermore discuss the possible role of AMPK as a master switch in skeletal muscle metabolism with the main focus on AMPK in metabolic regulation during muscle work. Finally, AMPK has a well established role in regulating expression of genes encoding various enzymes in muscle, and this issue is discussed in relation to adaptation of skeletal muscle to exercise training.

Oral glucose ingestion attenuates exercise-induced activation of 5'-AMP-activated protein kinase in human skeletal muscle

5'-AMP-activated protein kinase (AMPK) has been suggested to be a 'metabolic master switch' regulating various aspects of muscle glucose and fat metabolism. In isolated rat skeletal muscle, glucose suppresses the activity of AMPK and in human muscle glycogen loading decreases exercise-induced AMPK activation. We hypothesized that oral glucose ingestion during exercise would attenuate muscle AMPK activation. Nine male subjects performed two bouts of one-legged knee-extensor exercise at 60% of maximal workload. The subjects were randomly assigned to either consume a glucose containing drink or a placebo drink during the two trials. Muscle biopsies were taken from the vastus lateralis before and after 2 h of exercise. Plasma glucose was higher (6.0 +/- 0.2 vs. 4.9 +/- 0.1 mmol L-1, P < 0.001), whereas glycerol (44.8 +/- 7.8 vs. 165.7 +/- 22.3 micromol L-1), and free fatty acid (169.3 +/- 9.5 vs. 1161 +/- 144.9 micromol L-1) concentrations were lower during the glucose compared to the placebo trial (both P < 0.001). Calculated fat oxidation was lower during the glucose trial (0.17 +/- 0.02 vs. 0.25 +/- 0.03 g min-1, P < 0.001). Activation of alpha2-AMPK was attenuated in the glucose trial compared to the placebo trial (0.24 +/- 0.07 vs. 0.46 +/- 0.14 pmol mg-1 min-1, P = 0.03), whereas the alpha1-AMPK activity was not different between trials or affected by exercise. AMPK and the downstream target of AMPK, acetyl-CoA carboxylase-beta, were phosphorylated as a response to exercise, but neither was significantly different between the two trials. We conclude that oral glucose ingestion attenuates the exercise-induced activation of alpha2-AMPK, bringing further support for a fuel-sensing role of AMPK in skeletal muscle.

Higher skeletal muscle alpha2AMPK activation and lower energy charge and fat oxidation in men than in women during submaximal exercise

5'AMP-activated protein kinase (AMPK) is an energy sensor activated by perturbed cellular energy status such as during muscle contraction. Activated AMPK is thought to regulate several key metabolic pathways. We used sex comparison to investigate whether AMPK signalling in skeletal muscle regulates fat oxidation during exercise. Moderately trained women and men completed 90 min bicycle exercise at 60% VO2peak. Both AMPK Thr172 phosphorylation and alpha2AMPK activity were increased by exercise in men (approximately 200%, P < 0.001) but not significantly in women. The sex difference in muscle AMPK activation with exercise was accompanied by an increase in muscle free AMP (approximately 164%, P < 0.01), free AMP/ATP ratio (159%, P < 0.05), and creatine (approximately 42%, P < 0.001) in men but not in women (NS), suggesting that lack of AMPK activation in women was due to better maintenance of muscle cellular energy balance compared with men. During exercise, fat oxidation per kg lean body mass was higher in women than in men (P < 0.05). Regression analysis revealed that a higher proportion of type 1 muscle fibres (approximately 23%, P < 0.01) and a higher capillarization (approximately 23%, P < 0.05) in women than in men could partly explain the sex difference in alpha2AMPK activity (r = -0.54, P < 0.05) and fat oxidation (r = 0.64, P < 0.05) during exercise. On the other hand, fat oxidation appeared not to be regulated via AMPK. In conclusion, during prolonged submaximal exercise at 60% VO2peak, higher fat oxidation in women cannot be explained by higher AMPK signalling but is accompanied by improved muscle cellular energy balance in women probably due to sex specific muscle morphology.

Effect of exercise intensity and hypoxia on skeletal muscle AMPK signaling and substrate metabolism in humans

We compared in human skeletal muscle the effect of absolute vs. relative exercise intensity on AMP-activated protein kinase (AMPK) signaling and substrate metabolism under normoxic and hypoxic conditions. Eight untrained males cycled for 30 min under hypoxic conditions (11.5% O(2), 111 +/- 12 W, 72 +/- 3% hypoxia Vo(2 peak); 72% Hypoxia) or under normoxic conditions (20.9% O(2)) matched to the same absolute (111 +/- 12 W, 51 +/- 1% normoxia Vo(2 peak); 51% Normoxia) or relative (to Vo(2 peak)) intensity (171 +/- 18 W, 73 +/- 1% normoxia Vo(2 peak); 73% Normoxia). Increases (P < 0.05) in AMPK activity, AMPKalpha Thr(172) phosphorylation, ACCbeta Ser(221) phosphorylation, free AMP content, and glucose clearance were more influenced by the absolute than by the relative exercise intensity, being greatest in 73% Normoxia with no difference between 51% Normoxia and 72% Hypoxia. In contrast to this, increases in muscle glycogen use, muscle lactate content, and plasma catecholamine concentration were more influenced by the relative than by the absolute exercise intensity, being similar in 72% Hypoxia and 73% Normoxia, with both trials higher than in 51% Normoxia. In conclusion, increases in muscle AMPK signaling, free AMP content, and glucose disposal during exercise are largely determined by the absolute exercise intensity, whereas increases in plasma catecholamine levels, muscle glycogen use, and muscle lactate levels are more closely associated with the relative exercise intensity.

Carbohydrate metabolism during prolonged exercise and recovery: interactions between pyruvate dehydrogenase, fatty acids, and amino acids

During prolonged exercise, carbohydrate oxidation may result from decreased pyruvate production and increased fatty acid supply and ultimately lead to reduced pyruvate dehydrogenase (PDH) activity. Pyruvate also interacts with the amino acids alanine, glutamine, and glutamate, whereby the decline in pyruvate production could affect tricarboxycylic acid cycle flux as well as gluconeogenesis. To enhance our understanding of these interactions, we studied the time course of changes in substrate utilization in six men who cycled at 44+/-1% peak oxygen consumption (mean+/-SE) until exhaustion (exhaustion at 3 h 23 min+/-11 min). Femoral arterial and venous blood, blood flow measurements, and muscle samples were obtained hourly during exercise and recovery (3 h). Carbohydrate oxidation peaked at 30 min of exercise and subsequently decreased for the remainder of the exercise bout (P<0.05). PDH activity peaked at 2 h of exercise, whereas pyruvate production peaked at 1 h of exercise and was reduced (approximately 30%) thereafter, suggesting that pyruvate availability primarily accounted for reduced carbohydrate oxidation. Increased free fatty acid uptake (P<0.05) was also associated with decreasing PDH activity (P<0.05) and increased PDH kinase 4 mRNA (P<0.05) during exercise and recovery. At 1 h of exercise, pyruvate production was greatest and was closely linked to glutamate, which was the predominant amino acid taken up during exercise and recovery. Alanine and glutamine were also associated with pyruvate metabolism, and they comprised approximately 68% of total amino-acid release during exercise and recovery. Thus reduced pyruvate production was primarily associated with reduced carbohydrate oxidation, whereas the greatest production of pyruvate was related to glutamate, glutamine, and alanine metabolism in early exercise.

Skeletal Muscle Lipid Metabolism in Exercise and Insulin Resistance

Lipids as fuel for energy provision originate from different sources: albumin-bound long-chain fatty acids (LCFA) in the blood plasma, circulating very-low-density lipoproteins-triacylglycerols (VLDL-TG), fatty acids from triacylglycerol located in the muscle cell (IMTG), and possibly fatty acids liberated from adipose tissue adhering to the muscle cells. The regulation of utilization of the different lipid sources in skeletal muscle during exercise is reviewed, and the influence of diet, training, and gender is discussed. Major points deliberated are the methods utilized to measure uptake and oxidation of LCFA during exercise in humans. The role of the various lipid-binding proteins in transmembrane and cytosolic transport of lipids is considered as well as regulation of lipid entry into the mitochondria, focusing on the putative role of AMP-activated protein kinase (AMPK), acetyl CoA carboxylase (ACC), and carnitine during exercise. The possible contribution to fuel provision during exercise of circulating VLDL-TG as well as the role of IMTG is discussed from a methodological point of view. The contribution of IMTG for energy provision may not be large, covering ∼10% of total energy provision during fasting exercise in male subjects, whereas in females, IMTG may cover a larger proportion of energy delivery. Molecular mechanisms involved in breakdown of IMTG during exercise are also considered focusing on hormone-sensitive lipase (HSL). Finally, the role of lipids in development of insulin resistance in skeletal muscle, including possible molecular mechanisms involved, is discussed.

AMPK activation increases fatty acid oxidation in skeletal muscle by activating PPARalpha and PGC-1

AMP-activated protein kinase (AMPK) activation increases fatty acid oxidation in skeletal muscle by decreasing malonyl CoA concentrations. However, this may not explain the long-term effects of AMPK activation. Here we show that AMPK activation by 5-aminoimidazole-4-carboxamide ribonucleoside (AICAR) increases mRNA expression of PPARalpha target genes and PGC-1 in cultured muscle cells and mouse skeletal muscle, and that inhibition of PPARalpha and PGC-1 by siRNAs prevents AICAR-stimulated increase in fatty acid oxidation. These data suggest that a novel transcriptional regulatory mechanism involving PPARalpha and PGC-1 exists that is responsible for long-term stimulation of fatty acid oxidation in skeletal muscle by AICAR.

Contraction-mediated phosphorylation of AMPK is lower in skeletal muscle of adenylate kinase-deficient mice

The activity of AMP-activated protein kinase (AMPK) increases during muscle contractions as a result of elevated AMP concentration. We tested whether activation of AMPK would be altered during contractions in adenylate kinase (AK) 1-deficient (AK1-/-) mice, because they have a reduced capacity to form AMP. The right gastrocnemius-soleus-plantaris muscle group was stimulated via the sciatic nerve at 2 Hz for 30 min in both wild-type (WT) and AK1-/- animals. Initial force production was not different between the two groups (129.2 +/- 3.3 g vs. 140.9 +/- 8.5 g for WT and AK1-/-, respectively); however, force production by AK1-/- mice was significantly greater over the 30-min stimulation period, and final tension was 85 +/- 4.5% of initial in WT and 102 +/- 3.2% of initial in AK1-/- mice. Western blot analysis showed that AMPK phosphorylation with contractions was clearly increased in WT muscles (4.0 +/- 1.1 above resting values), but did not change noticeably with AK deficiency (1.6 +/- 0.4 above WT resting values). However, increases in phosphorylation of acetyl CoA carboxylase were robust in both WT and AK1-/- muscles and not different between the two groups. These results suggest that reduced formation of AMP during contractions in skeletal muscle of AK1-/- mice results in reduced phosphorylation of AMPK. However, altered AMPK signaling was not apparent in the phosphorylation status of acetyl CoA carboxylase, a typical marker of AMPK activity.

Short-term exercise training in humans reduces AMPK signalling during prolonged exercise independent of muscle glycogen

We examined the effect of short-term exercise training on skeletal muscle AMP-activated protein kinase (AMPK) signalling and muscle metabolism during prolonged exercise in humans. Eight sedentary males completed 120 min of cycling at 66 +/- 1% , then exercise trained for 10 days, before repeating the exercise bout at the same absolute workload. Participants rested for 72 h before each trial while ingesting a high carbohydrate diet (HCHO). Exercise training significantly (P < 0.05) attenuated exercise-induced increases in skeletal muscle free AMP: ATP ratio and glucose disposal and increased fat oxidation. Exercise training abolished the 9-fold increase in AMPK alpha2 activity observed during pretraining exercise. Since training increased muscle glycogen content by 93 +/- 12% (P < 0.01), we conducted a second experiment in seven sedentary male participants where muscle glycogen content was essentially matched pre- and post-training by exercise and a low CHO diet (LCHO; post-training muscle glycogen 52 +/- 7% less than in HCHO, P < 0.001). Despite the difference in muscle glycogen levels in the two studies we obtained very similar results. In both studies the increase in ACCbeta Ser(221) phosphorylation was reduced during exercise after training. In conclusion, there is little activation of AMPK signalling during prolonged exercise following short-term exercise training suggesting that other factors are important in the regulation of glucose disposal and fat oxidation under these circumstances. It appears that muscle glycogen is not an important regulator of AMPK activation during exercise in humans when exercise is begun with normal or high muscle glycogen levels.

Long-chain acyl-CoA esters inhibit phosphorylation of AMP-activated protein kinase at threonine-172 by LKB1/STRAD/MO25

Activation of the AMP-activated protein kinase (AMPK) results in acute changes in cellular metabolism and transcriptional events that make the cell more robust when encountering an energy challenge. AMPK is thought to be inhibited by glycogen, the major storage form of intracellular carbohydrate. We hypothesized that long-chain acyl-CoA esters (LCACEs) might also inhibit AMPK signaling. Cytosolic LCACEs are available for immediate transport and oxidation within the mitochondria and accordingly may be representative of the lipid energy charge of the cell. We found that LCACEs inhibited phosphorylation of AMPK by the recombinant AMPK kinase (AMPKK) LKB1/STRAD/MO25 in a concentration-dependent manner. Palmitoyl-CoA (PCoA) did not affect the activity of phosphothreonine-172 AMPK. PCoA potently inhibited AMPKK purified from liver. Conversely, PCoA stimulated the kinase activity of LKB1/STRAD/MO25 toward the peptide substrate LKB1tide. Octanoyl-CoA, palmitate, and palmitoylcarnitine did not inhibit AMPKK activity. Removal of AMP from the reaction mixture resulted in reduced AMPKK activity in the presence of PCoA. In conclusion, these results demonstrate that the AMPKK activity of LKB1/STRAD/MO25 is substrate specific and distinct from the kinase activity of LKB1/STRAD/MO25 toward the peptide substrate LKB1tide. They also demonstrate that LCACEs inhibit the AMPKK activity of LKB1/STRAD/MO25 in a specific manner with a dependence on both a long fatty chain and a CoA moiety. These results suggest that the AMPK signaling cascade may directly sense and respond to the lipid energy charge of the cell.

Acute renal ischemia rapidly activates the energy sensor AMPK but does not increase phosphorylation of eNOS-Ser1177

A fundamental aspect of acute renal ischemia is energy depletion, manifest as a falling level of ATP that is associated with a simultaneous rise in AMP. The energy sensor AMP-activated protein kinase (AMPK) is activated by a rising AMP-to-ATP ratio, but its role in acute renal ischemia is unknown. AMPK is activated in the ischemic heart and is reported to phosphorylate both endothelial nitric oxide synthase (eNOS) and acetyl-CoA carboxylase. To study activation of AMPK in acute renal ischemia, the renal pedicle of anesthetized Sprague-Dawley rats was cross-clamped for increasing time intervals. AMPK was strongly activated within 1 min and remained so after 30 min. However, despite the robust activation of AMPK, acute renal ischemia did not increase phosphorylation of the AMPK phosphorylation sites eNOS-Ser(1177) or acetyl-CoA carboxylase-Ser(79). Activation of AMPK in bovine aortic endothelial cells by the ATP-depleting agent antimycin A and the antidiabetic drug phenformin also did not increase phosphorylation of eNOS-Ser(1177), confirming that AMPK activation and phosphorylation of eNOS are dissociated in some situations. Immunoprecipitation studies demonstrated that the dissociation between AMPK activation and phosphorylation of eNOS-Ser(1177) was not due to changes in the physical associations between AMPK, eNOS, or heat shock protein 90. In conclusion, acute renal ischemia rapidly activates the energy sensor AMPK, which is known to maintain ATP reserves during energy stress. The substrates it phosphorylates, however, are different from those in other organs such as the heart.

Transcriptional regulation of pyruvate dehydrogenase kinase 4 in skeletal muscle during and after exercise

The pyruvate dehydrogenase complex (PDC) has a key position in skeletal muscle metabolism as it represents the entry of carbohydrate-derived fuel into the mitochondria for oxidation. PDC is regulated by a phosphorylation-dephosphorylation cycle, in which the pyruvate dehydrogenase kinase (PDK) phosphorylates and inactivates the complex. PDK exists in four isoforms, of which the PDK4 isoform is predominantly expressed in skeletal and heart muscle. PDK4 transcription and PDK4 mRNA are markedly increased in human skeletal muscle during prolonged exercise and after both short-term high-intensity and prolonged low-intensity exercise. The exercise-induced transcriptional response of PDK4 is enhanced when muscle glycogen is lowered before the exercise, and intake of a low-carbohydrate high-fat diet during recovery from exercise results in increased transcription and mRNA content of PDK4 when compared with intake of a high-carbohydrate diet. The activity of pyruvate dehydrogenase (PDH) is increased during the first 2 h of low-intensity exercise, followed by a decrease towards resting levels, which is in line with the possibility that the increased PDK4 expressed influences the PDH activity already during prolonged exercise. PDK4 expression is also increased in response to fasting and a high-fat diet. Thus, increased PDK4 expression when carbohydrate availability is low seems to contribute to the sparing of carbohydrates by preventing carbohydrate oxidation. The impact of substrate availability on PDK4 expression during recovery from exercise also underlines the high metabolic priority given to replenishing muscle glycogen stores and re-establishing intracellular homeostasis after exercise.

Regulation and role of hormone-sensitive lipase activity in human skeletal muscle

Hormone-sensitive lipase (HSL) is believed to play a regulatory role in initiating the degradation of intramuscular triacylglycerol (IMTG) in skeletal muscle. A series of studies designed to characterise the response of HSL to three stimuli: exercise of varying intensities and durations; adrenaline infusions; altered fuel supply have recently been conducted in human skeletal muscle. In an attempt to understand the regulation of HSL activity the changes in the putative intramuscular and hormonal regulators of the enzyme have also been measured. In human skeletal muscle at rest there is a high constitutive level of HSL activity, which is not a function of biopsy freezing. The combination of low adrenaline and Ca(2+) levels and resting levels of insulin appear to dictate the level of HSL activity at rest. During the initial minute of low and moderate aerobic exercise HSL is activated by contractions in the apparent absence of increases in circulating adrenaline. During intense aerobic exercise, adrenaline may contribute to the early activation of HSL. The contraction-induced activation may be related to increased Ca(2+) and/or other unknown intramuscular activators. As low- and moderate-intensity exercise continues beyond a few minutes, activation by adrenaline through the cAMP cascade may also occur. With prolonged moderate-intensity exercise beyond 1-2 h and sustained high-intensity exercise, HSL activity decreases despite continuing increases in adrenaline, possibly as a result of increasing accumulations of free AMP, activation of AMP kinase and phosphorylation of inhibitory sites on HSL. The existing work in human skeletal muscle also suggests that there are numerous levels of control involved in the regulation of IMTG degradation, with control points downstream from HSL also being important. For example, it must be remembered that the actual flux (IMTG degradation) through HSL may be allosterically inhibited during prolonged exercise as a result of the accumulation of long-chain fatty acyl-CoA.

β‐adrenergic stimulation of skeletal muscle HSL can be overridden by AMPK signaling

Hormone-sensitive lipase (HSL), an important regulatory enzyme for triacylglycerol hydrolysis within skeletal muscle, is controlled by beta-adrenergic signaling as well as intrinsic factors related to contraction and energy turnover. In the current study, we tested the capacity of 5'AMP-activated protein kinase (AMPK) to suppress beta-adrenergic stimulation of HSL activity. Eight male subjects completed 60 min of cycle exercise at 70% VO2 peak on two occasions: either with normal (CON) or low (LG) pre-exercise muscle glycogen content, which is known to enhance exercise-induced AMPK activity. Muscle samples were obtained before and immediately after exercise. Pre-exercise glycogen averaged 375 +/- 35 and 163 +/- 27 mmol x kg(-1) dm for CON and LG, respectively. AMPK alpha-2 was not different between trials at rest and was increased (3.7-fold, P<0.05) by exercise during LG only. HSL activity did not differ between trials at rest and increased (0 min: 1.67 +/- 0.13; 60 min: 2.60 +/- 0.26 mmol x min(-1) x kg(-1) dm) in CON. The exercise-induced increase in HSL activity was attenuated by AMPK alpha-2 activation in LG. The attenuated HSL activity during LG occurred despite higher plasma epinephrine levels (60 min: CON, 1.96 +/- 0.29 vs LG, 4.25 +/- 0.60 nM, P<0.05) compared with CON. Despite the attenuated HSL activity in LG, IMTG was decreased by exercise (0 min: 27.1 +/- 2.0; 60 min: 22.5 +/- 2.0 mmol x kg(-1) dm, P<0.05), whereas no net reduction occurred in CON. To confirm the apparent effect of AMPK on HSL activity, we performed experiments in muscle cell culture. The epineprine-induced increase in HSL activity was totally attenuated (P<0.05) by AICAR administration in L6 myotubes. These data provide new evidence indicating that AMPK is a major regulator of skeletal muscle HSL activity that can override beta-adrenergic stimulation. However, the increased IMTG degradation in LG suggests factors other than HSL activity are important for IMTG degradation.

Intensified exercise training does not alter AMPK signaling in human skeletal muscle

The AMP-activated protein kinase (AMPK) cascade has been linked to many of the acute effects of exercise on skeletal muscle substrate metabolism, as well as to some of the chronic training-induced adaptations. We determined the effect of 3 wk of intensified training (HIT; 7 sessions of 8 x 5 min at 85% Vo2 peak) in skeletal muscle from well-trained athletes on AMPK responsiveness to exercise. Rates of whole body substrate oxidation were determined during a 90-min steady-state ride (SS) pre- and post-HIT. Muscle metabolites and AMPK signaling were determined from biopsies taken at rest and immediately after exercise during the first and seventh HIT sessions, performed at the same (absolute) pre-HIT work rate. HIT decreased rates of whole body carbohydrate oxidation (P < 0.05) and increased rates of fat oxidation (P < 0.05) during SS. Resting muscle glycogen and its utilization during intense exercise were unaffected by HIT. However, HIT induced a twofold decrease in muscle [lactate] (P < 0.05) and resulted in tighter metabolic regulation, i.e., attenuation of the decrease in the PCr/(PCr + Cr) ratio and of the increase in [AMPfree]/ATP. Resting activities of AMPKalpha1 and -alpha2 were similar post-HIT, with the magnitude of the rise in response to exercise similar pre- and post-HIT. AMPK phosphorylation at Thr172 on both the alpha1 and alpha2 subunits increased in response to exercise, with the magnitude of this rise being similar post-HIT. Acetyl-coenzyme A carboxylase-beta phosphorylation was similar at rest and, despite HIT-induced increases in whole body rates of fat oxidation, did not increase post-HIT. Our results indicate that, in well-trained individuals, short-term HIT improves metabolic control but does not blunt AMPK signaling in response to intense exercise.

Integration of metabolic and mitogenic signal transduction in skeletal muscle

Exercise has diverse effects on metabolic and mitogenic signaling pathways in human skeletal muscle, implying specificity of intracellular signaling cascades. The role of several parallel signaling cascades are discussed in an effort to assign a physiological role for these targets in the regulation of exercise-mediated responses on metabolism and gene expression.

5'-AMP-activated protein kinase activity and protein expression are regulated by endurance training in human skeletal muscle

The 5'-AMP-activated protein kinase (AMPK) is proposed to be involved in signaling pathways leading to adaptations in skeletal muscle in response to both a single exercise bout and exercise training. This study investigated the effect of endurance training on protein content of catalytic (alpha1, alpha2) and regulatory (beta1, beta2 and gamma1, gamma2, gamma3) subunit isoforms of AMPK as well as on basal AMPK activity in human skeletal muscle. Eight healthy young men performed supervised one-legged knee extensor endurance training for 3 wk. Muscle biopsies were obtained before and 15 h after training in both legs. In response to training the protein content of alpha1, beta2 and gamma1 increased in the trained leg by 41, 34, and 26%, respectively (alpha1 and beta2 P < 0.005, gamma1 P < 0.05). In contrast, the protein content of the regulatory gamma3-isoform decreased by 62% in the trained leg (P = 0.01), whereas no effect of training was seen for alpha2, beta1, and gamma2. AMPK activity associated with the alpha1- and the alpha2-isoforms increased in the trained leg by 94 and 49%, respectively (both P < 0.005). In agreement with these observations, phosphorylation of alpha-AMPK-(Thr172) and of the AMPK target acetyl-CoA carboxylase-beta(Ser221) increased by 74 and 180%, respectively (both P < 0.001). Essentially similar results were obtained in four additional subjects studied 55 h after training. This study demonstrates that protein content and basal AMPK activity in human skeletal muscle are highly susceptible to endurance exercise training. Except for the increase in gamma1 protein, all observed adaptations to training could be ascribed to local contraction-induced mechanisms, since they did not occur in the contralateral untrained muscle.

5??? Adenosine Monophosphate-Activated Protein Kinase, Metabolism and Exercise

The 5' adenosine monophosphate-activated protein kinase (AMPK) is a member of a metabolite-sensing protein kinase family that functions as a metabolic 'fuel gauge' in skeletal muscle. AMPK is a ubiquitous heterotrimeric protein, consisting of an alpha catalytic, and beta and gamma regulatory subunits that exist in multiple isoforms and are all required for full enzymatic activity. During exercise, AMPK becomes activated in skeletal muscle in response to changes in cellular energy status (e.g. increased adenosine monophosphate [AMP]/adenosine triphosphate [ATP] and creatine/phosphocreatine ratios) in an intensity-dependent manner, and serves to inhibit ATP-consuming pathways, and activate pathways involved in carbohydrate and fatty-acid metabolism to restore ATP levels. Recent evidence shows that although AMPK plays this key metabolic role during acute bouts of exercise, it is also an important component of the adaptive response of skeletal muscles to endurance exercise training because of its ability to alter muscle fuel reserves and expression of several exercise-responsive genes. This review discusses the putative roles of AMPK in acute and chronic exercise responses, and suggests avenues for future AMPK research in exercise physiology and biochemistry.

Knockout of the alpha2 but not alpha1 5'-AMP-activated protein kinase isoform abolishes 5-aminoimidazole-4-carboxamide-1-beta-4-ribofuranosidebut not contraction-induced glucose uptake in skeletal muscle

We investigated the importance of the two catalytic alpha-isoforms of the 5'-AMP-activated protein kinase (AMPK) in 5-aminoimidazole-4-carboxamide-1-beta-4-ribofuranoside (AICAR) and contraction-induced glucose uptake in skeletal muscle. Incubated soleus and EDL muscle from whole-body alpha2- or alpha1-AMPK knockout (KO) and wild type (WT) mice were incubated with 2.0 mm AICAR or electrically stimulated to contraction. Both AICAR and contraction increased 2DG uptake in WT muscles. KO of alpha2, but not alpha1, abolished AICAR-induced glucose uptake, whereas neither KO affected contraction-induced glucose uptake. AICAR and contraction increased alpha2- and alpha1-AMPK activity in wild type (WT) muscles. During AICAR stimulation, the remaining AMPK activity in KO muscles increased to the same level as in WT. During contraction, the remaining AMPK activity in alpha2-KO muscles was elevated by 100% probably explained by a 2-3-fold increase in alpha1-protein. In alpha1-KO muscles, alpha2-AMPK activity increased to similar levels as in WT. Both interventions increased total AMPK activity, as expressed by AMPK-P and ACCbeta-P, in WT muscles. During AICAR stimulation, this was dramatically reduced in alpha2-KO but not in alpha1-KO, whereas during contraction, both measurements were essentially similar to WT in both KO-muscles. The results show that alpha2-AMPK is the main donor of basal and AICAR-stimulated AMPK activity and is responsible for AICAR-induced glucose uptake. In contrast, during contraction, the two alpha-isoforms seem to substitute for each other in terms of activity, which may explain the normal glucose uptake despite the lack of either alpha2- or alpha1-AMPK. Alternatively, neither alpha-isoform of AMPK is involved in contraction-induced muscle glucose uptake.

Physiological role of AMP-activated protein kinase in the heart: graded activation during exercise

AMP-activated protein kinase (AMPK) is emerging as a key signaling pathway that modulates cellular metabolic processes. In skeletal muscle, AMPK is activated during exercise. Increased myocardial substrate metabolism during exercise could be explained by AMPK activation. Although AMPK is known to be activated during myocardial ischemia, it remains uncertain whether AMPK is activated in response to the physiological increases in cardiac work associated with exercise. Therefore, we evaluated cardiac AMPK activity in rats at rest and after 10 min of treadmill running at moderate (15% grade, 16 m/min) or high (15% grade, 32 m/min) intensity. Total AMPK activity in the heart increased in proportion to exercise intensity (P < 0.05). AMPK activity associated with the alpha2-catalytic subunit increased 2.8 +/- 0.4-fold (P < 0.02 vs. rest) and 4.5 +/- 0.6-fold (P < 0.001 vs. rest) with moderate- and high-intensity exercise, respectively. AMPK activity associated with the alpha1-subunit increased to a lesser extent. Phosphorylation of the Thr172-regulatory site on AMPK alpha-catalytic subunits increased during exercise (P < 0.001). There was no increase in Akt phosphorylation during exercise. The changes in AMPK activity during exercise were associated with physiological AMPK effects (GLUT4 translocation to the sarcolemma and ACC phosphorylation). Thus cardiac AMPK activity increases progressively with exercise intensity, supporting the hypothesis that AMPK has a physiological role in the heart.

Effect of exercise intensity on skeletal muscle AMPK signaling in humans

The effect of exercise intensity on skeletal muscle AMP-activated protein kinase (AMPK) signaling and substrate metabolism was examined in eight men cycling for 20 min at each of three sequential intensities: low (40 +/- 2% VO(2) peak), medium (59 +/- 1% VO(2) peak), and high (79 +/- 1% VO(2) peak). Muscle free AMP/ATP ratio only increased at the two higher exercise intensities (P < 0.05). AMPK alpha 1 (1.5-fold) and AMPK alpha 2 (5-fold) activities increased from low to medium intensity, with AMPK alpha 2 activity increasing further from medium to high intensity. The upstream AMPK kinase activity was substantial at rest and only increased 50% with exercise, indicating that, initially, signaling through AMPK did not require AMPK kinase posttranslational modification. Acetyl-CoA carboxylase (ACC)-beta phosphorylation was sensitive to exercise, increasing threefold from rest to low intensity, whereas neuronal NO synthase (nNOS) micro phosphorylation was only observed at the higher exercise intensities. Glucose disappearance (tracer) did not increase from rest to low intensity, but increased sequentially from low to medium to high intensity. Calculated fat oxidation increased from rest to low intensity in parallel with ACC beta phosphorylation, then declined during high intensity. These results indicate that ACC beta phosphorylation is especially sensitive to exercise and tightly coupled to AMPK signaling and that AMPK activation does not depend on AMPK kinase activation during exercise.

Insulin signalling: effects of prior exercise

After the discovery and clinical use of insulin for treatment of diabetes it became clear that some of the biological effect of insulin was dependent on the circumstances under which it was given. Relevant for this review is the notion that physical activity, in addition to its own direct metabolic effects also markedly affects the ability of insulin to stimulate a range of metabolic processes. More specifically, during and for a prolonged period after, exercise elicits effects on processes such as insulin-induced muscle glucose uptake and glucose metabolism which influence systemic glucose homeostasis. These phenomena are probably responsible for the improvement in glucose homeostasis and metabolic control that typically occurs with exercise in people with insulin resistance and probably contributes to the reduced risk for development of type 2 diabetes in individuals who engage in regular exercise. Here we focus on the influence of a single bout of exercise on the action of insulin on processes such as glucose uptake and glucose storage in skeletal muscle.

Utilization of long-chain fatty acids in human skeletal muscle during exercise

Long-chain fatty acids (LCFA) are important sources of energy in contracting skeletal muscle: during the course of endurance exercise the contribution of LCFA in energy metabolism increases whereas when the intensity of exercise increases, the energy need is covered more and more by carbohydrates. Although this has been known for nearly 100 years, the mechanisms controlling fatty acid uptake and oxidation during various exercise modes are still not completely elucidated. Besides passive diffusion, data suggest that both membrane-associated and cytosolic fatty acid binding proteins are involved in the uptake of LCFA into skeletal muscle. However, data from human studies suggest that the regulation of fatty acid utilization in skeletal muscle during exercise lies mainly within the entrance into the mitochondria or metabolism within the mitochondria. Although possible compartmentalization within the cell makes definitive conclusions difficult, available evidence suggests that changes in malonyl CoA concentration in muscle do not play a major regulatory role in controlling LCFA oxidation during exercise in man. In contrast, it is suggested that the availability of free carnitine may play a major regulatory role in oxidation of LCFA during exercise.

Hormone-sensitive lipase activity and fatty acyl-CoA content in human skeletal muscle during prolonged exercise

Hormone-sensitive lipase (HSL) catalyzes the hydrolysis of intramuscular triacylglycerols (IMTGs), but HSL regulation is poorly understood in skeletal muscle. The present study measured human skeletal muscle HSL activity at rest and during 120 min of cycling at 60% of peak O2 uptake. Several putative HSL regulators were also measured, including muscle long-chain fatty acyl-CoA (LCFA CoA) and free AMP contents and plasma epinephrine and insulin concentrations. HSL activity increased from resting levels by 10 min of exercise (from 2.09 +/- 0.19 to 2.56 +/- 0.22 mmol. min-1x kg dry mass-1, P < 0.05), increased further by 60 min (to 3.12 +/- 0.27 mmol x min-1x kg dry mass-1, P < 0.05), and decreased to near-resting rates after 120 min of cycling. Skeletal muscle LCFA CoA increased (P < 0.05) above rest by 60 min (from 15.9 +/- 3.0 to 50.4 +/- 7.9 micromol/kg dry mass) and increased further by 120 min. Estimated free AMP increased (P < 0.05) from rest to 60 min and was approximately 20-fold greater than that at rest by 120 min. Epinephrine was increased above rest (P < 0.05) at 60 (1.47 +/- 0.15 nM) and 120 min (4.87 +/- 0.76 nM) of exercise. Insulin concentrations decreased rapidly and were lower than resting levels by 10 min and continued to decrease throughout exercise. In summary, HSL activity was increased from resting levels by 10 min, increased further by 60 min, and decreased to near-resting values by 120 min. The increased HSL activity at 60 min was associated with the stimulating effect of increased epinephrine and decreased insulin levels. After 120 min, the decreased HSL activity was associated with the proposed inhibitory effects of increased free AMP. The accumulation of LCFA CoA in the 2nd h of exercise may also have reduced the flux through HSL and accounted for the reduction in IMTG utilization previously observed late in prolonged exercise.