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

Combined effect of AMPK/PPAR agonists and exercise training in mdx mice functional performance

The present investigation was undertaken to test whether exercise training (ET) associated with AMPK/PPAR agonists (EM) would improve skeletal muscle function in mdx mice. These drugs have the potential to improve oxidative metabolism. This is of particular interest because oxidative muscle fibers are less affected in the course of the disease than glycolitic counterparts. Therefore, a cohort of 34 male congenic C57Bl/10J mdx mice included in this study was randomly assigned into four groups: vehicle solution (V), EM [AICAR (AMPK agonist, 50 mg/Kg-1.day-1, ip) and GW 1516 (PPARδ agonist, 2.5 mg/Kg-1.day-1, gavage)], ET (voluntary running on activity wheel) and EM+ET. Functional performance (grip meter and rotarod), aerobic capacity (running test), muscle histopathology, serum creatine kinase (CK), levels of ubiquitined proteins, oxidative metabolism protein expression (AMPK, PPAR, myoglobin and SCD) and intracellular calcium handling (DHPR, SERCA and NCX) protein expression were analyzed. Treatments started when the animals were two months old and were maintained for one month. A significant functional improvement (p<0.05) was observed in animals submitted to the combination of ET and EM. CK levels were decreased and the expression of proteins related to oxidative metabolism was increased in this group. There were no differences among the groups in the intracellular calcium handling protein expression. To our knowledge, this is the first study that tested the association of ET with EM in an experimental model of muscular dystrophy. Our results suggest that the association of ET and EM should be further tested as a potential therapeutic approach in muscular dystrophies.

Reduced AMPK-ACC and mTOR signaling in muscle from older men, and effect of resistance exercise

AMP-activated protein kinase (AMPK) is a key energy-sensitive enzyme that controls numerous metabolic and cellular processes. Mammalian target of rapamycin (mTOR) is another energy/nutrient-sensitive kinase that controls protein synthesis and cell growth. In this study we determined whether older versus younger men have alterations in the AMPK and mTOR pathways in skeletal muscle, and examined the effect of a long term resistance type exercise training program on these signaling intermediaries. Older men had decreased AMPKα2 activity and lower phosphorylation of AMPK and its downstream signaling substrate acetyl-CoA carboxylase (ACC). mTOR phosphylation also was reduced in muscle from older men. Exercise training increased AMPKα1 activity in older men, however, AMPKα2 activity, and the phosphorylation of AMPK, ACC and mTOR, were not affected. In conclusion, older men have alterations in the AMPK-ACC and mTOR pathways in muscle. In addition, prolonged resistance type exercise training induces an isoform-selective up regulation of AMPK activity.

The effects of altitude training on the AMPK-related glucose transport pathway in the red skeletal muscle of both lean and obese Zucker rats

INTRODUCTION

The skeletal muscle AMP-activated protein kinase (AMPK)-related glucose transport pathway is involved in glucose homeostasis.

AIM

In this study, we examined whether obese control Zucker rats had abnormal expression of proteins in the LKB1-AMPK-AS160-GLUT4 pathway in red gastrocnemius muscle compared to that in lean (normal) control Zucker rats. We also compared the chronic training effects of exercise, hypoxia, and altitude training on this pathway in lean and obese rats.

METHODS

At sea level, lean and obese rats were divided into 4 groups for 6 weeks training as follows: 1) control; 2) exercise (progressive daily swimming-exercise training with comparable exercise signals between the two groups); 3) hypoxia (8 hours of daily 14% O2 exposure); and 4) exercise plus hypoxia (also called altitude training). Seven animals were used for each group.

RESULTS

The obese rats in the control group had higher body weights, elevated fasting insulin and glucose levels, and higher baseline levels of muscle AMPK and AS160 phosphorylation compared with those of lean control rats. For obese Zucker rats in the exercise or hypoxia groups, the muscle AMPK phosphorylation level was significantly decreased compared with that of the control group. For obese Zucker rats in the altitude training group, the levels of AMPK, AS160 phosphorylation, fasting insulin, and fasting glucose were decreased concomitant with an approximate 50% increase in the muscle GLUT4 protein level compared with those of the control group. In lean rats, the altitude training efficiently lowered fasting glucose and insulin levels and increased muscle AMPK and AS160 phosphorylation as well as GLUT4 protein levels.

CONCLUSION

Our results provide evidence that long-term altitude training may be a potentially effective nonpharmacological strategy for treating and preventing insulin resistance based on its effects on the skeletal muscle AMPK-AS160-GLUT4 pathway.

EMG-normalised kinase activation during exercise is higher in human gastrocnemius compared to soleus muscle

In mice, certain proteins show a highly confined expression in specific muscle groups. Also, resting and exercise/contraction-induced phosphorylation responses are higher in rat skeletal muscle with low mitochondrial content compared to muscles with high mitochondrial content, possibly related to differential reactive oxygen species (ROS)-scavenging ability or resting glycogen content. To evaluate these parameters in humans, biopsies from soleus, gastrocnemius and vastus lateralis muscles were taken before and after a 45 min inclined (15%) walking exercise bout at 69% VO2_max aimed at simultaneously activating soleus and gastrocnemius in a comparable dynamic work-pattern. Hexokinase II and GLUT4 were 46–59% and 26–38% higher (p<0.05) in soleus compared to the two other muscles. The type I muscle fiber percentage was highest in soleus and lowest in vastus lateralis. No differences were found in protein expression of signalling proteins (AMPK subunits, eEF2, ERK1/2, TBC1D1 and 4), mitochondrial markers (F1 ATPase and COX1) or ROS-handling enzymes (SOD2 and catalase). Gastrocnemius was less active than soleus measured as EMG signal and glycogen use yet gastrocnemius displayed larger increases than soleus in phosphorylation of AMPK Thr172, eEF2 Thr56 and ERK 1/2 Thr202/Tyr204 when normalised to the mean relative EMG-signal. In conclusion, proteins with muscle-group restricted expression in mice do not show this pattern in human lower extremity muscle groups. Nonetheless the phosphorylation-response is greater for a number of kinase signalling pathways in human gastrocnemius than soleus at a given activation-intensity. This may be due to the combined subtle effects of a higher type I muscle fiber content and higher training status in soleus compared to gastrocnemius muscle.

PPARbeta activation induces rapid changes of both AMPK subunit expression and AMPK activation in mouse skeletal muscle

AMP-activated protein kinases (AMPK) are heterotrimeric, αβγ, serine/threonine kinases. The γ3-AMPK subunit is particularly interesting in muscle physiology because 1) it is specifically expressed in skeletal muscle, 2) α2β2γ3 is the AMPK heterotrimer activated during exercise in humans, and 3) it is down-regulated in humans after a training period. However, mechanisms underlying this decrease of γ3-AMPK expression remained unknown. We investigated whether the expression of AMPK subunits and particularly that of γ3-AMPK are regulated by the PPARβ pathway. We report that PPARβ activation with GW0742 induces a rapid (2 h) and sustained down-regulation of γ3-AMPK expression both in mouse skeletal muscles and in culture myotubes. Concomitantly, phosphorylation levels of both AMPK and acetyl-coenzyme A carboxylase are rapidly modified. The γ3-AMPK down-regulation is also observed in muscles from young and adult transgenic mice with muscle-specific overexpression of peroxisome proliferator-activated receptor β (PPARβ). We showed that γ3-AMPK down-regulation is a rapid physiological muscle response observed in mouse after running exercise or fasting, two situations leading to PPARβ activation. Finally, using C2C12, we demonstrated that dose and time-dependent down-regulation of γ3-AMPK expression upon GW0742 treatment, is due to decrease γ3-AMPK promoter activity.

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

CONTEXT

Strength training induces muscle remodeling and may improve insulin responsiveness.

OBJECTIVE

This study will quantify the impact of resistance training on insulin sensitivity in subjects with the metabolic syndrome and correlate this with activation of intramuscular pathways mediating mitochondrial biogenesis and muscle fiber hypertrophy.

DESIGN

Ten subjects with the metabolic syndrome (MS) and nine sedentary controls underwent 8 wk of supervised resistance exercise training with pre- and posttraining anthropometric and muscle biochemical assessments.

SETTING

Resistance exercise training took place in a sports laboratory on a college campus.

MAIN OUTCOME MEASURES

Pre- and posttraining insulin responsiveness was quantified using a euglycemic clamp. Changes in expression of muscle 5-AMP-activated protein kinase (AMPK) and mammalian target of rapamycin (mTOR) pathways were quantified using immunoblots.

RESULTS

Strength and stamina increased in both groups. Insulin sensitivity increased in controls (steady-state glucose infusion rate = 7.0 ± 2.0 mg/kg · min pretraining training vs. 8.7 ± 3.1 mg/kg · min posttraining; P < 0.01) but did not improve in MS subjects (3.3 ± 1.3 pre vs. 3.1 ± 1.0 post). Muscle glucose transporter 4 increased 67% in controls and 36% in the MS subjects. Control subjects increased muscle phospho-AMPK (43%), peroxisome proliferator-activated receptor γ coactivator 1α (57%), and ATP synthase (60%), more than MS subjects (8, 28, and 21%, respectively). In contrast, muscle phospho-mTOR increased most in the MS group (57 vs. 32%).

CONCLUSION

Failure of resistance training to improve insulin responsiveness in MS subjects was coincident with diminished phosphorylation of muscle AMPK, but increased phosphorylation of mTOR, suggesting activation of the mTOR pathway could be involved in inhibition of exercise training-related increases in AMPK and its activation and downstream events.

Cafeteria diet-induced insulin resistance is not associated with decreased insulin signaling or AMPK activity and is alleviated by physical training in rats

Excess energy intake via a palatable low-fat diet (cafeteria diet) is known to induce obesity and glucose intolerance in rats. However, the molecular mechanisms behind this adaptation are not known, and it is also not known whether exercise training can reverse it. Male Wistar rats were assigned to 12-wk intervention groups: chow-fed controls (CON), cafeteria diet (CAF), and cafeteria diet plus swimming exercise during the last 4 wk (CAF(TR)). CAF feeding led to increased body weight (16%, P < 0.01) and increased plasma glucose (P < 0.05) and insulin levels (P < 0.01) during an IVGTT, which was counteracted by training. In the perfused hindlimb, insulin-stimulated glucose transport in red gastrocnemius muscle was completely abolished in CAF and rescued by exercise training. Apart from a tendency toward an approximately 20% reduction in both basal and insulin-stimulated Akt Ser(473) phosphorylation (P = 0.051) in the CAF group, there were no differences in insulin signaling (IR Tyr(1150/1151), PI 3-kinase activity, Akt Thr(308), TBC1D4 Thr(642), GSK3-alpha/beta Ser(21/9)) or changes in AMPKalpha1 or -alpha2, GLUT4, Munc18c, or syntaxin 4 protein expression or in phosphorylation of AMPK Thr(172) among the groups. In conclusion, surplus energy intake of a palatable but low-fat cafeteria diet resulted in obesity and insulin resistance that was rescued by exercise training. Interestingly, insulin resistance was not accompanied by major defects in the insulin-signaling cascade or in altered AMPK expression or phosphorylation. Thus, compared with previous studies of high-fat feeding, where insulin signaling is significantly impaired, the mechanism by which CAF diet induces insulin resistance seems different.

Cycle training increased GLUT4 and activation of mammalian target of rapamycin in fast twitch muscle fibers

PURPOSE

To determine whether cycle training of sedentary subjects would increase the expression of the principle muscle glucose transporters, six volunteers completed 6 wk of progressively increasing intensity stationary cycle cycling.

METHODS

In vastus lateralis muscle biopsies, changes in expression of GLUT1, GLUT4, GLUT5, and GLUT12 were compared using quantitative immunoblots with specific protein standards. Regulatory pathway components were evaluated by immunoblots of muscle homogenates and immunohistochemistry of microscopic sections.

RESULTS

GLUT1 was unchanged, GLUT4 increased 66%, GLUT12 increased 104%, and GLUT5 decreased 72%. A mitochondrial marker (cytochrome c) and regulators of mitochondrial biogenesis (peroxisome proliferator-activated receptor gamma coactivator 1 alpha and phospho-5'-adenosine monophosphate-activated protein kinase) were unchanged, but the muscle hypertrophy pathway component, phospho-mammalian target of rapamycin (mTOR), increased 83% after the exercise program. In baseline biopsies, GLUT4 by immunohistochemical techniques was 37% greater in Type I (slow twitch, red) muscle fibers, but the exercise training increased GLUT4 expression in Type II (fast twitch, white) fibers by 50%, achieving parity with the Type I fibers. Baseline phospho-mTOR expression was 50% higher in Type II fibers and increased more in Type II fibers (62%) with training but also increased in Type I fibers (34%).

CONCLUSION

Progressive intensity stationary cycle training of previously sedentary subjects increased muscle insulin-responsive glucose transporters (GLUT4 and GLUT12) and decreased the fructose transporter (GLUT5). The increase in GLUT4 occurred primarily in Type II muscle fibers, and this coincided with activation of the mTOR muscle hypertrophy pathway. There was little impact on Type I fiber GLUT4 expression and no evidence of change in mitochondrial biogenesis.

Impaired skeletal muscle beta-adrenergic activation and lipolysis are associated with whole-body insulin resistance in rats bred for low intrinsic exercise capacity

Rats selectively bred for high endurance running capacity (HCR) have higher insulin sensitivity and improved metabolic health compared with those bred for low endurance capacity (LCR). We investigated several skeletal muscle characteristics, in vitro and in vivo, that could contribute to the metabolic phenotypes observed in sedentary LCR and HCR rats. After 16 generations of selective breeding, HCR had approximately 400% higher running capacity (P < 0.001), improved insulin sensitivity (P < 0.001), and lower fasting plasma glucose and triglycerides (P < 0.05) compared with LCR. Skeletal muscle ceramide and diacylglycerol content, basal AMP-activated protein kinase (AMPK) activity, and basal lipolysis were similar between LCR and HCR. However, the stimulation of lipolysis in response to 10 mum isoproterenol was 70% higher in HCR (P = 0.004). Impaired isoproterenol sensitivity in LCR was associated with lower basal triacylglycerol lipase activity, Ser660 phosphorylation of HSL, and beta2-adrenergic receptor protein content in skeletal muscle. Expression of the orphan nuclear receptor Nur77, which is induced by beta-adrenergic signaling and is associated with insulin sensitivity, was lower in LCR (P < 0.05). Muscle protein content of Nur77 target genes, including uncoupling protein 3, fatty acid translocase/CD36, and the AMPK gamma3 subunit were also lower in LCR (P < 0.05). Our investigation associates whole-body insulin resistance with impaired beta-adrenergic response and reduced expression of genes that are critical regulators of glucose and lipid metabolism in skeletal muscle. We identify impaired beta-adrenergic signal transduction as a potential mechanism for impaired metabolic health after artificial selection for low intrinsic exercise capacity.

Genetic and metabolic effects on skeletal muscle AMPK in young and older twins

The protein complex AMP-activated protein kinase (AMPK) is believed to play an important role in the regulation of skeletal muscle glucose and lipid metabolism. Defects in the AMPK system might therefore be an important factor in the pathogenesis of type 2 diabetes. We aimed to identify genetic and environmental mechanisms involved in the regulation of AMPK expression and activity and to examine the association between AMPK protein levels and activity on the one hand, and glucose and fat metabolism on the other. We investigated skeletal muscle biopsies from 100 young and 82 older mono- and dizygotic nondiabetic twins excised during the basal and insulin-stimulated states of a physiological hyperinsulinemic-euglycemic clamp. AMPKalpha1, -alpha2, and -gamma3 mRNA expression was investigated using real-time PCR, and Western blotting was employed to measure protein levels. Multiple regression analyses indicated that skeletal muscle AMPK mRNA and protein expression as well as activity were regulated by sex, age, obesity, and aerobic capacity. Comparison of intraclass correlations on AMPK measurements from mono- and dizygotic twins suggested that skeletal muscle AMPK expression was under minor genetic influence. AMPKgamma3 protein expression and activity were negatively related to whole body glucose uptake through the nonoxidative metabolic pathway and positively related to phosphorylation of glycogen synthase. Our results suggest that skeletal muscle AMPK expression is under minor genetic control but regulated by age and sex and associated with obesity and aerobic capacity. Furthermore, our results indicate a role for gamma3-containing AMPK complexes in downregulation of insulin-stimulated nonoxidative glucose metabolism possibly through inhibition of glycogen synthase activity.

Metformin promotes induction of lipoprotein lipase in skeletal muscle through activation of adenosine monophosphate-activated protein kinase

Metformin is known to increase lipoprotein lipase (LPL) mass level in serum. Lipoprotein lipase is produced by adipose tissue and skeletal muscles. This study aimed to examine the effect of metformin on LPL production in adipocytes and skeletal muscle cells and to investigate the mechanism by which metformin enhances LPL production. 3T3-L1 preadipocytes and L6 skeletal muscle cells were incubated with metformin or 5-aminoimidazole-4-carboxamide ribonucleoside (AICAR). Lipoprotein lipase activity, LPL protein expression, and LPL messenger RNA (mRNA) expression were measured. Metformin increased LPL activity only in skeletal muscle cells. To clarify the mechanism of this phenomenon, AICAR, which is well known as an activator of adenosine monophosphate-activated protein kinase (AMPK), was used. Metformin and AICAR enhanced phosphorylated AMPK in skeletal muscle cells by Western blot analysis. Like metformin, AICAR increased LPL activity only in skeletal muscle cells. Both metformin and AICAR also enhanced LPL protein and LPL mRNA expressions in skeletal muscle cells but not in adipocytes. Phosphorylated AMPK protein expression was decreased when AMPK signaling was interfered by AMPKalpha small interfering RNA. Lipoprotein lipase activity and LPL expression, which were enhanced by 1 mumol/L metformin, were reduced by AMPKalpha small interfering RNA. These results suggest that metformin increases LPL activity, LPL protein expression, and LPL mRNA expression through activation of AMPK in skeletal muscle cells but not in adipocytes.

Glucose ingestion during endurance training does not alter adaptation

Glucose ingestion during exercise attenuates activation of metabolic enzymes and expression of important transport proteins. In light of this, we hypothesized that glucose ingestion during training would result in 1) an attenuation of the increase in fatty acid uptake and oxidation during exercise, 2) lower citrate synthase (CS) and β-hydroxyacyl-CoA dehydrogenase (β-HAD) activity and glycogen content in skeletal muscle, and 3) attenuated endurance performance enhancement in the trained state. To investigate this we studied nine male subjects who performed 10 wk of one-legged knee extensor training. They trained one leg while ingesting a 6% glucose solution (Glc) and ingested a sweetened placebo while training the other leg (Plc). The subjects trained their respective legs 2 h at a time on alternate days 5 days a week. Endurance training increased peak power (Pmax) and time to fatigue at 70% of Pmax ∼14% and ∼30%, respectively. CS and β-HAD activity increased and glycogen content was greater after training, but there were no differences between Glc and Plc. After training the rate of oxidation of palmitate (Rox) and the % of rate of disappearance that was oxidized (%Rdox) changed. %Rdox was on average 16.4% greater during exercise after training whereas, after exercise %Rdox was 30.4% lower. Rox followed the same pattern. However, none of these parameters were different between Glc and Plc. We conclude that glucose ingestion during training does not alter training adaptation related to substrate metabolism, mitochondrial enzyme activity, glycogen content, or performance.

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.

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.

Divergent cell signaling after short-term intensified endurance training in human skeletal muscle

Endurance training represents one extreme in the continuum of skeletal muscle plasticity. The molecular signals elicited in response to acute and chronic exercise and the integration of multiple intracellular pathways are incompletely understood. We determined the effect of 10 days of intensified cycle training on signal transduction in nine inactive males in response to a 1-h acute bout of cycling at the same absolute workload (164 +/- 9 W). Muscle biopsies were taken at rest and immediately and 3 h after the acute exercise. The metabolic signaling pathways, including AMP-activated protein kinase (AMPK) and mammalian target of rapamycin (mTOR), demonstrated divergent regulation by exercise after training. AMPK phosphorylation increased in response to exercise ( approximately 16-fold; P < 0.05), which was abrogated posttraining (P < 0.01). In contrast, mTOR phosphorylation increased in response to exercise ( approximately 2-fold; P < 0.01), which was augmented posttraining (P < 0.01) in the presence of increased mTOR expression (P < 0.05). Exercise elicited divergent effects on mitogen-activated protein kinase (MAPK) pathways after training, with exercise-induced extracellular signal-regulated kinase (ERK) 1/2 phosphorylation being abolished (P < 0.01) and p38 MAPK maintained. Finally, calmodulin kinase II (CaMKII) exercise-induced phosphorylation and activity were maintained (P < 0.01), despite increased expression ( approximately 2-fold; P < 0.05). In conclusion, 10 days of intensified endurance training attenuated AMPK, ERK1/2, and mTOR, but not CaMKII and p38 MAPK signaling, highlighting molecular pathways important for rapid functional adaptations and maintenance in response to intensified endurance exercise and training.

Ablation of AMP-activated protein kinase alpha2 activity exacerbates insulin resistance induced by high-fat feeding of mice

OBJECTIVE

We determined whether muscle AMP-activated protein kinase (AMPK) has a role in the development of insulin resistance.

RESEARCH DESIGN AND METHODS

Muscle-specific transgenic mice expressing an inactive form of the AMPK alpha2 catalytic subunit (alpha2i TG) and their wild-type littermates were fed either a high-fat (60% kcal fat) or a control (10% kcal fat) diet for 30 weeks.

RESULTS

Compared with wild-type mice, glucose tolerance in alpha2i TG mice was slightly impaired on the control diet and significantly impaired on the high-fat diet. To determine whether the whole-body glucose intolerance was associated with impaired insulin sensitivity in skeletal muscle, glucose transport in response to submaximal insulin (450 microU/ml) was measured in isolated soleus muscles. On the control diet, insulin-stimulated glucose transport was reduced by approximately 50% in alpha2i TG mice compared with wild-type mice. High-fat feeding partially decreased insulin-stimulated glucose transport in wild-type mice, while high-fat feeding resulted in a full blunting of insulin-stimulated glucose transport in the alpha2i TG mice. High-fat feeding in alpha2i TG mice was accompanied by decreased expression of insulin signaling proteins in gastrocnemius muscle.

CONCLUSIONS

The lack of skeletal muscle AMPK alpha2 activity exacerbates the development of glucose intolerance and insulin resistance caused by high-fat feeding and supports the thesis that AMPK alpha2 is an important target for the prevention/amelioration of skeletal muscle insulin resistance through lifestyle (exercise) and pharmacologic (e.g., metformin) treatments.

AMPK and PPARdelta agonists are exercise mimetics

The benefits of endurance exercise on general health make it desirable to identify orally active agents that would mimic or potentiate the effects of exercise to treat metabolic diseases. Although certain natural compounds, such as reseveratrol, have endurance-enhancing activities, their exact metabolic targets remain elusive. We therefore tested the effect of pathway-specific drugs on endurance capacities of mice in a treadmill running test. We found that PPARbeta/delta agonist and exercise training synergistically increase oxidative myofibers and running endurance in adult mice. Because training activates AMPK and PGC1alpha, we then tested whether the orally active AMPK agonist AICAR might be sufficient to overcome the exercise requirement. Unexpectedly, even in sedentary mice, 4 weeks of AICAR treatment alone induced metabolic genes and enhanced running endurance by 44%. These results demonstrate that AMPK-PPARdelta pathway can be targeted by orally active drugs to enhance training adaptation or even to increase endurance without exercise.

Differential effects of resistance and endurance exercise in the fed state on signalling molecule phosphorylation and protein synthesis in human muscle

Resistance (RE) and endurance (EE) exercise stimulate mixed skeletal muscle protein synthesis. The phenotypes induced by RE (myofibrillar protein accretion) and EE (mitochondrial expansion) training must result from differential stimulation of myofibrillar and mitochondrial protein synthesis. We measured the synthetic rates of myofibrillar and mitochondrial proteins and the activation of signalling proteins (Akt-mTOR-p70S6K) at rest and after an acute bout of RE or EE in the untrained state and after 10 weeks of RE or EE training in young healthy men. While untrained, RE stimulated both myofibrillar and mitochondrial protein synthesis, 67% and 69% (P < 0.02), respectively. After training, only myofibrillar protein synthesis increased with RE (36%, P = 0.05). EE stimulated mitochondrial protein synthesis in both the untrained, 154%, and trained, 105% (both P < 0.05), but not myofibrillar protein synthesis. Acute RE and EE increased the phosphorylation of proteins in the Akt-mTOR-p70S6K pathway with comparatively minor differences between two exercise stimuli. Phosphorylation of Akt-mTOR-p70S6K proteins was increased after 10 weeks of RE training but not by EE training. Chronic RE or EE training modifies the protein synthetic response of functional protein fractions, with a shift toward exercise phenotype-specific responses, without an obvious explanatory change in the phosphorylation of regulatory signalling pathway proteins.

AMPK alpha1 activation is required for stimulation of glucose uptake by twitch contraction, but not by H2O2, in mouse skeletal muscle

Background

AMPK is a promising pharmacological target in relation to metabolic disorders partly due to its non-insulin dependent glucose uptake promoting role in skeletal muscle. Of the 2 catalytic α-AMPK isoforms, α₂ AMPK is clearly required for stimulation of glucose transport into muscle by certain stimuli. In contrast, no clear function has yet been determined for α₁ AMPK in skeletal muscle, possibly due to α-AMPK isoform signaling redundancy. By applying low-intensity twitch-contraction and H₂O₂ stimulation to activate α₁ AMPK, but not α₂ AMPK, in wildtype and α-AMPK transgenic mouse muscles, this study aimed to define conditions where α₁ AMPK is required to increase muscle glucose uptake.

Methodology/Principal Findings

Following stimulation with H₂O₂ (3 mM, 20 min) or twitch-contraction (0.1 ms pulse, 2 Hz, 2 min), signaling and 2-deoxyglucose uptake were measured in incubated soleus muscles from wildtype and muscle-specific kinase-dead AMPK (KD), α₁ AMPK knockout or α₂ AMPK knockout mice. H₂O₂ increased the activity of both α₁ and α₂ AMPK in addition to Akt phosphorylation, and H₂O₂-stimulated glucose uptake was not reduced in any of the AMPK transgenic mouse models compared with wild type. In contrast, twitch-contraction increased the activity of α₁ AMPK, but not α₂ AMPK activity nor Akt or AS160 phosphorylation. Glucose uptake was markedly lower in α₁ AMPK knockout and KD AMPK muscles, but not in α₂ AMPK knockout muscles, following twitch stimulation.

Conclusions/Significance

These results provide strong genetic evidence that α₁ AMPK, but not α₂ AMPK, Akt or AS160, is necessary for regulation of twitch-contraction stimulated glucose uptake. To our knowledge, this is the first report to show a major and essential role of α₁ AMPK in regulating a physiological endpoint in skeletal muscle. In contrast, AMPK is not essential for H₂O₂-stimulated muscle glucose uptake, as proposed by recent studies.

Effect of training in the fasted state on metabolic responses during exercise with carbohydrate intake

Skeletal muscle gene response to exercise depends on nutritional status during and after exercise, but it is unknown whether muscle adaptations to endurance training are affected by nutritional status during training sessions. Therefore, this study investigated the effect of an endurance training program (6 wk, 3 day/wk, 1-2 h, 75% of peak Vo(2)) in moderately active males. They trained in the fasted (F; n = 10) or carbohydrate-fed state (CHO; n = 10) while receiving a standardized diet [65 percent of total energy intake (En) from carbohydrates, 20%En fat, 15%En protein]. Before and after the training period, substrate use during a 2-h exercise bout was determined. During these experimental sessions, all subjects were in a fed condition and received extra carbohydrates (1 g.kg body wt(-1) .h(-1)). Peak Vo(2) (+7%), succinate dehydrogenase activity, GLUT4, and hexokinase II content were similarly increased between F and CHO. Fatty acid binding protein (FABPm) content increased significantly in F (P = 0.007). Intramyocellular triglyceride content (IMCL) remained unchanged in both groups. After training, pre-exercise glycogen content was higher in CHO (545 +/- 19 mmol/kg dry wt; P = 0.02), but not in F (434 +/- 32 mmol/kg dry wt; P = 0.23). For a given initial glycogen content, F blunted exercise-induced glycogen breakdown when compared with CHO (P = 0.04). Neither IMCL breakdown (P = 0.23) nor fat oxidation rates during exercise were altered by training. Thus short-term training elicits similar adaptations in peak Vo(2) whether carried out in the fasted or carbohydrate-fed state. Although there was a decrease in exercise-induced glycogen breakdown and an increase in proteins involved in fat handling after fasting training, fat oxidation during exercise with carbohydrate intake was not changed.

Effect of sex differences on human MEF2 regulation during endurance exercise

Women exhibit an enhanced capability for lipid metabolism during endurance exercise compared with men. The underlying regulatory mechanisms behind this sex-related difference are not well understood but may comprise signaling through a myocyte enhancer factor 2 (MEF2) regulatory pathway. The primary purpose of this study, therefore, was to investigate the protein signaling of MEF2 regulatory pathway components at rest and during 90 min of bicycling exercise at 60% Vo(2peak) in healthy, moderately trained men (n = 8) and women (n = 9) to elucidate the potential role of these proteins in substrate utilization during exercise. A secondary purpose was to screen for mRNA expression of MEF2 isoforms and myogenic regulatory factor (MRF) family members of transcription factors at rest and during exercise. Muscle biopsies were obtained before and immediately after exercise. Nuclear AMP-activated protein kinase-alpha (alphaAMPK) Thr(172) (P < 0.001), histone deacetylase 5 (HDAC5) Ser(498) (P < 0.001), and MEF2 Thr (P < 0.01) phosphorylation increased with exercise. No significant sex differences were observed at rest or during exercise. At rest, no significant sex differences were observed in mRNA expression of the measured transcription factors. mRNA for transcription factors MyoD, myogenin, MRF4, MEF2A, MEF2C, MEF2D, and peroxisome proliferator-activated receptor-gamma coactivator 1alpha (PGC1alpha) were significantly upregulated by exercise. Of these, MEF2A mRNA increased 25% specifically in women (P < 0.05), whereas MEF2D mRNA tended to increase in men (P = 0.11). Although minor sex differences in mRNA expression were observed, the main finding of the present study was the implication of a joint signaling action of AMPK, HDAC5, and PGC1alpha on MEF2 in the immediate regulatory response to endurance exercise. This signaling response was independent of sex.

Exercise training-induced improvements in insulin action

Individuals with insulin resistance are characterized by impaired insulin action on whole-body glucose uptake, in part due to impaired insulin-stimulated glucose uptake into skeletal muscle. A single bout of exercise increases skeletal muscle glucose uptake via an insulin-independent mechanism that bypasses the typical insulin signalling defects associated with these conditions. However, this 'insulin sensitizing' effect is short-lived and disappears after approximately 48 h. In contrast, repeated physical activity (i.e. exercise training) results in a persistent increase in insulin action in skeletal muscle from obese and insulin-resistant individuals. The molecular mechanism(s) for the enhanced glucose uptake with exercise training have been attributed to the increased expression and/or activity of key signalling proteins involved in the regulation of glucose uptake and metabolism in skeletal muscle. Evidence now suggests that the improvements in insulin sensitivity associated with exercise training are also related to changes in the expression and/or activity of proteins involved in insulin signal transduction in skeletal muscle such as the AMP-activated protein kinase (AMPK) and the protein kinase B (Akt) substrate AS160. In addition, increased lipid oxidation and/or turnover is likely to be another mechanism by which exercise improves insulin sensitivity: exercise training results in an increase in the oxidative capacity of skeletal muscle by up-regulating lipid oxidation and the expression of proteins involved in mitochondrial biogenesis. Determination of the underlying biological mechanisms that result from exercise training is essential in order to define the precise variations in physical activity that result in the most desired effects on targeted risk factors, and to aid in the development of such interventions.

The molecular bases of training adaptation

Skeletal muscle is a malleable tissue capable of altering the type and amount of protein in response to disruptions to cellular homeostasis. The process of exercise-induced adaptation in skeletal muscle involves a multitude of signalling mechanisms initiating replication of specific DNA genetic sequences, enabling subsequent translation of the genetic message and ultimately generating a series of amino acids that form new proteins. The functional consequences of these adaptations are determined by training volume, intensity and frequency, and the half-life of the protein. Moreover, many features of the training adaptation are specific to the type of stimulus, such as the mode of exercise. Prolonged endurance training elicits a variety of metabolic and morphological changes, including mitochondrial biogenesis, fast-to-slow fibre-type transformation and substrate metabolism. In contrast, heavy resistance exercise stimulates synthesis of contractile proteins responsible for muscle hypertrophy and increases in maximal contractile force output. Concomitant with the vastly different functional outcomes induced by these diverse exercise modes, the genetic and molecular mechanisms of adaptation are distinct. With recent advances in technology, it is now possible to study the effects of various training interventions on a variety of signalling proteins and early-response genes in skeletal muscle. Although it cannot presently be claimed that such scientific endeavours have influenced the training practices of elite athletes, these new and exciting technologies have provided insight into how current training techniques result in specific muscular adaptations, and may ultimately provide clues for future and novel training methodologies. Greater knowledge of the mechanisms and interaction of exercise-induced adaptive pathways in skeletal muscle is important for our understanding of the aetiology of disease, maintenance of metabolic and functional capacity with aging, and training for athletic performance. This article highlights the effects of exercise on molecular and genetic mechanisms of training adaptation in skeletal muscle.

Recovery of skeletal muscle mass after extensive injury: positive effects of increased contractile activity

The present study was designed to test the hypothesis that increasing physical activity by running exercise could favor the recovery of muscle mass after extensive injury and to determine the main molecular mechanisms involved. Left soleus muscles of female Wistar rats were degenerated by notexin injection before animals were assigned to either a sedentary group or an exercised group. Both regenerating and contralateral intact muscles from active and sedentary rats were removed 5, 7, 14, 21, 28 and 42 days after injury (n = 8 rats/group). Increasing contractile activity through running exercise during muscle regeneration ensured the full recovery of muscle mass and muscle cross-sectional area as soon as 21 days after injury, whereas muscle weight remained lower even 42 days postinjury in sedentary rats. Proliferator cell nuclear antigen and MyoD protein expression went on longer in active rats than in sedentary rats. Myogenin protein expression was higher in active animals than in sedentary animals 21 days postinjury. The Akt-mammalian target of rapamycin (mTOR) pathway was activated early during the regeneration process, with further increases of mTOR phosphorylation and its downstream effectors, eukaryotic initiation factor-4E-binding protein-1 and p70(s6k), in active rats compared with sedentary rats (days 7-14). The exercise-induced increase in mTOR phosphorylation, independently of Akt, was associated with decreased levels of phosphorylated AMP-activated protein kinase. Taken together, these results provided evidence that increasing contractile activity during muscle regeneration ensured early and full recovery of muscle mass and suggested that these beneficial effects may be due to a longer proliferative step of myogenic cells and activation of mTOR signaling, independently of Akt, during the maturation step of muscle regeneration.

Gene expression profiling in lung fibroblasts reveals new players in alveolarization

Little is known about the molecular basis of lung alveolarization. We used a microarray profiling strategy to identify novel genes that may regulate the secondary septation process. Rat lung fibroblasts were extemporaneously isolated on postnatal days 2, 7, and 21, i.e., before, during, and after septation, respectively. Total RNA was extracted, and cRNAs were hybridized to Affymetrix rat genome 230 2.0 microarrays. Expression levels of a selection of genes were confirmed by real-time PCR. In addition to genes already known to be upregulated during alveolarization including drebrin, midkine, Fgfr3, and Fgfr4, the study allowed us to identify two remarkable groups of genes with opposite profiles, i.e., gathering genes either transiently up- or downregulated on day 7. The former group includes the transcription factors retinoic acid receptor (RXR)-gamma and homeobox (Hox) a2, a4, and a5 and genes involved in Wnt signaling (Wnt5a, Fzd1, and Ndp); the latter group includes the extracellular matrix components Comp and Opn and the signal molecule Slfn4. Profiling in whole lung from fetal life to adulthood confirmed that changes were specific for alveolarization. Two treatments that arrest septation, hyperoxia and dexamethasone, inhibited the expression of genes that are upregulated during alveolarization and conversely enhanced that of genes weakly expressed during alveolarization and upregulated thereafter. The possible roles of these genes in secondary septation are discussed. Gene expression profiling analysis on freshly isolated cells represents a powerful approach to provide new information about differential regulation of genes during alveolarization and pathways potentially involved in the pathogenesis of bronchopulmonary dysplasia.

Gain-of-function R225W mutation in human AMPKgamma(3) causing increased glycogen and decreased triglyceride in skeletal muscle

Background

AMP-activated protein kinase (AMPK) is a heterotrimeric enzyme that is evolutionarily conserved from yeast to mammals and functions to maintain cellular and whole body energy homeostasis. Studies in experimental animals demonstrate that activation of AMPK in skeletal muscle protects against insulin resistance, type 2 diabetes and obesity. The regulatory γ₃ subunit of AMPK is expressed exclusively in skeletal muscle; however, its importance in controlling overall AMPK activity is unknown. While evidence is emerging that gamma subunit mutations interfere specifically with AMP activation, there remains some controversy regarding the impact of gamma subunit mutations [1]–[3]. Here we report the first gain-of-function mutation in the muscle-specific regulatory γ₃ subunit in humans.

Methods and Findings

We sequenced the exons and splice junctions of the AMPK γ₃ gene (PRKAG3) in 761 obese and 759 lean individuals, identifying 87 sequence variants including a novel R225W mutation in subjects from two unrelated families. The γ₃ R225W mutation is homologous in location to the γ₂R302Q mutation in patients with Wolf-Parkinson-White syndrome and to the γ₃R225Q mutation originally linked to an increase in muscle glycogen content in purebred Hampshire Rendement Napole (RN^-) pigs. We demonstrate in differentiated muscle satellite cells obtained from the vastus lateralis of R225W carriers that the mutation is associated with an approximate doubling of both basal and AMP-activated AMPK activities. Moreover, subjects bearing the R225W mutation exhibit a ∼90% increase of skeletal muscle glycogen content and a ∼30% decrease in intramuscular triglyceride (IMTG).

Conclusions

We have identified for the first time a mutation in the skeletal muscle-specific regulatory γ₃ subunit of AMPK in humans. The γ₃R225W mutation has significant functional effects as demonstrated by increases in basal and AMP-activated AMPK activities, increased muscle glycogen and decreased IMTG. Overall, these findings are consistent with an important regulatory role for AMPK γ₃ in human muscle energy metabolism.

HIF-1alpha in endurance training: suppression of oxidative metabolism

During endurance training, exercising skeletal muscle experiences severe and repetitive oxygen stress. The primary transcriptional response factor for acclimation to hypoxic stress is hypoxia-inducible factor-1alpha (HIF-1alpha), which upregulates glycolysis and angiogenesis in response to low levels of tissue oxygenation. To examine the role of HIF-1alpha in endurance training, we have created mice specifically lacking skeletal muscle HIF-1alpha and subjected them to an endurance training protocol. We found that only wild-type mice improve their oxidative capacity, as measured by the respiratory exchange ratio; surprisingly, we found that HIF-1alpha null mice have already upregulated this parameter without training. Furthermore, untrained HIF-1alpha null mice have an increased capillary to fiber ratio and elevated oxidative enzyme activities. These changes correlate with constitutively activated AMP-activated protein kinase in the HIF-1alpha null muscles. Additionally, HIF-1alpha null muscles have decreased expression of pyruvate dehydrogenase kinase I, a HIF-1alpha target that inhibits oxidative metabolism. These data demonstrate that removal of HIF-1alpha causes an adaptive response in skeletal muscle akin to endurance training and provides evidence for the suppression of mitochondrial biogenesis by HIF-1alpha in normal tissue.

The molecular bases of training adaptation

Skeletal muscle is a malleable tissue capable of altering the type and amount of protein in response to disruptions to cellular homeostasis. The process of exercise-induced adaptation in skeletal muscle involves a multitude of signalling mechanisms initiating replication of specific DNA genetic sequences, enabling subsequent translation of the genetic message and ultimately generating a series of amino acids that form new proteins. The functional consequences of these adaptations are determined by training volume, intensity and frequency, and the half-life of the protein. Moreover, many features of the training adaptation are specific to the type of stimulus, such as the mode of exercise. Prolonged endurance training elicits a variety of metabolic and morphological changes, including mitochondrial biogenesis, fast-to-slow fibre-type transformation and substrate metabolism. In contrast, heavy resistance exercise stimulates synthesis of contractile proteins responsible for muscle hypertrophy and increases in maximal contractile force output. Concomitant with the vastly different functional outcomes induced by these diverse exercise modes, the genetic and molecular mechanisms of adaptation are distinct. With recent advances in technology, it is now possible to study the effects of various training interventions on a variety of signalling proteins and early-response genes in skeletal muscle. Although it cannot presently be claimed that such scientific endeavours have influenced the training practices of elite athletes, these new and exciting technologies have provided insight into how current training techniques result in specific muscular adaptations, and may ultimately provide clues for future and novel training methodologies. Greater knowledge of the mechanisms and interaction of exercise-induced adaptive pathways in skeletal muscle is important for our understanding of the aetiology of disease, maintenance of metabolic and functional capacity with aging, and training for athletic performance. This article highlights the effects of exercise on molecular and genetic mechanisms of training adaptation in skeletal muscle.

Mechanical ventilation with 40% oxygen reduces pulmonary expression of genes that regulate lung development and impairs alveolar septation in newborn mice

Mechanical ventilation with 40% oxygen reduces pulmonary expression of genes that regulate lung development and impairs alveolar septation in newborn mice. Am J Physiol Lung Cell Mol Physiol 293: , 2007. First published August 17, 2007; - Mechanical ventilation (MV) with O(2)-rich gas offers life-saving treatment for extremely premature infants with respiratory failure but often leads to neonatal chronic lung disease (CLD), characterized by defective formation of alveoli and blood vessels in the developing lung. We discovered that MV of 2- to 4-day-old mice with 40% O(2) for 8 h, compared with unventilated control pups, reduced lung expression of genes that regulate lung septation and angiogenesis (VEGF-A and its receptor, VEGF-R2; PDGF-A; and tenascin-C). MV with air for 8 h yielded similar results for PDGF-A and tenascin-C but did not alter lung mRNA expression of VEGF or VEGF-R2. MV of 4- to 6-day-old mice with 40% O(2) for 24 h reduced lung protein abundance of VEGF-A, VEGF-R2, PDGF-A, and tenascin-C and resulted in lung structural abnormalities consistent with evolving CLD. After MV with 40% O(2) for 24 h, lung volume was similar to unventilated controls, whereas distal air space size, assessed morphometrically, was greater in lungs of ventilated pups, indicative of impaired septation. Immunostaining for vimentin, which is expressed in myofibroblasts, was reduced in distal lung after 24 h of MV with 40% O(2). These molecular, cellular, and structural changes occurred without detectable lung inflammation as evaluated by histology and assays for proinflammatory cytokines, myeloperoxidase activity, and water content in lung. Thus lengthy MV of newborn mice with O(2)-rich gas reduces lung expression of genes and proteins that are critical for normal lung growth and development. These changes yielded lung structural defects similar to those observed in evolving CLD.

5-Aminoimidazole-4-carboxamide-1-beta-4-ribofuranoside stimulates tyrosine hydroxylase activity and catecholamine secretion by activation of AMP-activated protein kinase in PC12 cells

The activity of AMP-activated protein kinase (AMPK) is regulated by the metabolic and nutritional state of the cell. 5-Aminoimidazole-4-carboxamide-1-beta-4-ribofuranoside (AICAR) is transformed into riboside monophosphate (ZMP) via phosphorylation by adenosine kinase inside the cell and exerts it effect by stimulating AMPK. AICAR significantly induces an increase in AMPK activity in a dose- and time-dependent manner in the rat pheochromocytoma cell line PC12. In addition, compound C, an AMPK inhibitor, as well as 5'-amino-5'-dAdo, an adenosine kinase inhibitor, inhibits the AICAR-induced AMPK activity. AICAR significantly stimulates tyrosine hydroxylase (TH) (the rate-limiting enzyme in the biosynthesis of catecholamine) activity and the corresponding mRNA level, which closely matches with the TH protein level. In addition, AICAR provokes a rapid and long-lasting increase in the phosphorylation of TH at Ser19, Ser31 and Ser40. AICAR also markedly activates ERKs, JNK and p38. The MEK-1-inhibitor (PD-098059) causes a partial, but significant, inhibition of AICAR-induced TH enzyme activity by phosphorylation of Ser31 without affecting phosphorylation at the two other sites. By contrast, neither the JNK-inhibitor nor the p38-inhibitor affects TH enzyme activity and phosphorylation. Similarly, PD-098059 partially, but significantly, inhibits the AICAR-induced increase in the TH mRNA level. Furthermore, AICAR increases the level of cAMP in PC12 cells. The present study also shows that H89, a protein kinase A inhibitor, abolishes the AICAR-induced increase in the level of TH mRNA, as well as the corresponding enzyme activity and Ser40 phosphorylation. Finally, AICAR significantly increases dopamine secretion from PC12 cells. These findings indicate that AICAR activates catecholamine synthesis and secretion through AMPK activation in chromaffin cells.

Effects of endurance exercise training on insulin signaling in human skeletal muscle: interactions at the level of phosphatidylinositol 3-kinase, Akt, and AS160

The purpose of this study was to investigate the mechanisms explaining improved insulin-stimulated glucose uptake after exercise training in human skeletal muscle. Eight healthy men performed 3 weeks of one-legged knee extensor endurance exercise training. Fifteen hours after the last exercise bout, insulin-stimulated glucose uptake was approximately 60% higher (P < 0.01) in the trained compared with the untrained leg during a hyperinsulinemic-euglycemic clamp. Muscle biopsies were obtained before and after training as well as after 10 and 120 min of insulin stimulation in both legs. Protein content of Akt1/2 (55 +/- 17%, P < 0.05), AS160 (25 +/- 8%, P = 0.08), GLUT4 (52 +/- 19%, P < 0.001), hexokinase 2 (HK2) (197 +/- 40%, P < 0.001), and insulin-responsive aminopeptidase (65 +/- 15%, P < 0.001) increased in muscle in response to training. During hyperinsulinemia, activities of insulin receptor substrate-1 (IRS-1)-associated phosphatidylinositol 3-kinase (PI3-K) (P < 0.005), Akt1 (P < 0.05), Akt2 (P < 0.005), and glycogen synthase (GS) (percent I-form, P < 0.05) increased similarly in both trained and untrained muscle, consistent with increased phosphorylation of Akt Thr(308), Akt Ser(473), AS160, glycogen synthase kinase (GSK)-3alpha Ser(21), and GSK-3beta Ser(9) and decreased phosphorylation of GS site 3a+b (all P < 0.005). Interestingly, training improved insulin action on thigh blood flow, and, furthermore, in both basal and insulin-stimulated muscle tissue, activities of Akt1 and GS and phosphorylation of AS160 increased with training (all P < 0.05). In contrast, training reduced IRS-1-associated PI3-K activity (P < 0.05) in both basal and insulin-stimulated muscle tissue. Our findings do not support generally improved insulin signaling after endurance training; rather it seems that improved insulin-stimulated glucose uptake may result from hemodynamic adaptations as well as increased cellular protein content of individual insulin signaling components and molecules involved in glucose transport and metabolism.

Effect of endurance exercise training on Ca2+ calmodulin-dependent protein kinase II expression and signalling in skeletal muscle of humans

Here the hypothesis that skeletal muscle Ca(2+)-calmodulin-dependent kinase II (CaMKII) expression and signalling would be modified by endurance training was tested. Eight healthy, young men completed 3 weeks of one-legged endurance exercise training with muscle samples taken from both legs before training and 15 h after the last exercise bout. Along with an approximately 40% increase in mitochondrial F(1)-ATP synthase expression, there was an approximately 1-fold increase in maximal CaMKII activity and CaMKII kinase isoform expression after training in the active leg only. Autonomous CaMKII activity and CaMKII autophosphorylation were increased to a similar extent. However, there was no change in alpha-CaMKII anchoring protein expression with training. Nor was there any change in expression or Thr(17) phosphorylation of the CaMKII substrate phospholamban with training. However, another CaMKII substrate, serum response factor (SRF), had an approximately 60% higher phosphorylation at Ser(103) after training, with no change in SRF expression. There were positive correlations between the increases in CaMKII expression and SRF phosphorylation as well as F(1)ATPase expression with training. After training, there was an increase in cyclic-AMP response element binding protein phosphorylation at Ser(133), but not expression, in muscle of both legs. Taken together, skeletal muscle CaMKII kinase isoform expression and SRF phosphorylation is higher with endurance-type exercise training, adaptations that are restricted to active muscle. This may contribute to greater Ca(2+) mediated regulation during exercise and the altered muscle phenotype with training.

Alpha-catalytic subunits of 5'AMP-activated protein kinase display fiber-specific expression and are upregulated by chronic low-frequency stimulation in rat muscle

5'-AMP-activated protein kinase (AMPK) signaling initiates adaptive changes in skeletal muscle fibers that restore homeostatic energy balance. The purpose of this investigation was to examine, in rats, the fiber-type protein expression patterns of the alpha-catalytic subunit isoforms in various skeletal muscles, and changes in their respective contents within the tibialis anterior (TA) after chronic low-frequency electrical stimulation (CLFS; 10 Hz, 10 h daily), applied for 4 +/- 1.2 or 25 +/- 4.8 days. Immunocytochemical staining of soleus (SOL) and medial gastrocnemius (MG) showed that 86 +/- 4.1 to 97 +/- 1.4% of type IIA fibers stained for both the alpha1- and alpha2-isoforms progressively decreased to 63 +/- 12.2% of type IID/X and 9 +/- 2.4% of IIB fibers. 39 +/- 11.4% of IID/X and 83 +/- 7.9% of IIB fibers expressed only the alpha2 isoform in the MG, much of which was localized within nuclei. alpha1 and alpha2 contents, assessed by immunoblot, were lowest in the white gastrocnemius [WG; 80% myosin heavy chain (MHC) IIb; 20% MHCIId/x]. Compared with the WG, alpha1 content was 1.6 +/- 0.08 (P < 0.001) and 1.8 +/- 0.04 (P < 0.0001)-fold greater in the red gastrocnemius (RG: 13%, MHCIIa) and SOL (21%, MHCIIa), respectively, and increased in proportion to MHCIIa content. Similarly, alpha2 content was 1.4 +/- 0.10 (P < 0.02) and 1.5 +/- 0.07 (P < 0.001)-fold greater in RG and SOL compared with WG. CLFS induced 1.43 +/- 0.13 (P < 0.007) and 1.33 +/- 0.08 (P < 0.009)-fold increases in the alpha1 and alpha2 contents of the TA and coincided with the transition of faster type IIB and IID/X fibers toward IIA fibers. These findings indicate that fiber types differ with regard to their capacity for AMPK signaling and that this potential is increased by CLFS.

Role of mitochondria in non-alcoholic fatty liver disease

Mitochondrial dysfunction is involved in the three stages of the transition from lack of exercise and excessive food intake to insulin resistance, diabetes and non-alcoholic steatohepatitis (NASH). In muscle, lack of exercise, a fat-rich diet, a polymorphism in peroxisome proliferator-activated receptor gamma coactivator-1 (PGC-1), and possibly age-related mitochondrial DNA (mtDNA) mutations may variously combine their effects to decrease PGC-1 expression, mitochondrial biogenesis and fat oxidation. Together with excessive food intake, insufficient fat oxidation causes fat accumulation and cellular stress in myocytes. The activation of Jun N-terminal kinase and protein kinase C-theta triggers the serine phosphorylation and inactivation of the insulin receptor substrate, and hampers the insulin-mediated translocation of glucose transporter-4 to the plasma membrane. Initially, the trend for increased blood glucose increases insulin secretion by pancreatic beta-cells. High plasma insulin levels compensate for insulin resistance in muscle and maintain normal blood glucose levels. Eventually, however, increased uncoupling protein-2 expression and possibly acquired mtDNA mutations in pancreatic beta-cells can blunt glucose-mediated adenosine triphosphate (ATP) formation and insulin secretion, to cause diabetes in some patients. High plasma glucose and/or insulin levels induce hepatic lipogenesis and cause hepatic steatosis. In fat-engorged hepatocytes, several vicious cycles involving tumor necrosis factor-alpha, reactive oxygen species (ROS), peroxynitrite, and lipid peroxidation products alter respiratory chain polypeptides and mtDNA, thus partially blocking the flow of electrons in the respiratory chain. The overreduction of upstream respiratory chain complexes increases mitochondrial ROS and peroxynitrite formation. Oxidative stress increases the release of lipid peroxidation products and cytokines, which together trigger the liver lesions of NASH.

Energy sensing and regulation of gene expression in skeletal muscle

Major modifications in energy homeostasis occur in skeletal muscle during exercise. Emerging evidence suggests that changes in energy homeostasis take part in the regulation of gene expression and contribute to muscle plasticity. A number of energy-sensing molecules have been shown to sense variations in energy homeostasis and trigger regulation of gene expression. The AMP-activated protein kinase, hypoxia-inducible factor 1, peroxisome proliferator-activated receptors, and Sirt1 proteins all contribute to altering skeletal muscle gene expression by sensing changes in the concentrations of AMP, molecular oxygen, intracellular free fatty acids, and NAD+, respectively. These molecules may therefore sense information relating to the intensity, duration, and frequency of muscle exercise. Mitochondria also contribute to the overall response, both by modulating the response of energy-sensing molecules and by generating their own signals. This review seeks to examine our current understanding of the roles that energy-sensing molecules and mitochondria can play in the regulation of gene expression in skeletal muscle.

Role of AMPKalpha2 in basal, training-, and AICAR-induced GLUT4, hexokinase II, and mitochondrial protein expression in mouse muscle

We investigated the role of AMPKalpha2in basal, exercise training-, and AICAR-induced protein expression of GLUT4, hexokinase II (HKII), mitochondrial markers, and AMPK subunits. This was conducted in red (RG) and white gastrocnemius (WG) muscle from wild-type (WT) and alpha2-knockout (KO) mice after 28 days of activity wheel running or daily AICAR injection. Additional experiments were conducted to measure acute activation of AMPK by exercise and AICAR. At basal, mitochondrial markers were reduced by approximately 20% in alpha2-KO muscles compared with WT. In both muscle types, AMPKalpha2 activity was increased in response to both stimuli, whereas AMPKalpha1 activity was increased only in response to exercise. Furthermore, AMPK signaling was estimated to be 60-70% lower in alpha2-KO compared with WT muscles. In WG, AICAR treatment increased HKII, GLUT4, cytochrome c, COX-1, and CS, and the alpha2-KO abolished the AICAR-induced increases, whereas no AICAR responses were observed in RG. Exercise training increased GLUT4, HKII, COX-1, CS, and HAD protein in WG, but the alpha2-KO did not affect training-induced increases. Furthermore, AMPKalpha1, -alpha2, -beta1, -beta2, and -gamma3 subunits were reduced in RG, but not in WG, by 30-60% in response to exercise training. In conclusion, the alpha2-KO was associated with an approximately 20% reduction in mitochondrial markers in both muscle types and abolished AICAR-induced increases in protein expression in WG. However, the alpha2-KO did not reduce training-induced increases in HKII, GLUT4, COX-1, HAD, or CS protein in WG, suggesting that AMPKalpha2 may not be essential for metabolic adaptations of skeletal muscles to exercise training.

Exercise-induced improvement in vasodilatory function accompanies increased insulin sensitivity in obesity and type 2 diabetes mellitus

OBJECTIVE

The present study was undertaken to determine whether improved vasodilatory function accompanies increased insulin sensitivity in overweight, insulin-resistant subjects (OW) and type 2 diabetic patients (T2DM) who participated in an 8-wk exercise training regimen.

DESIGN

Before and after training, subjects had euglycemic clamps to determine insulin sensitivity. Brachial artery catheterization was done on another occasion for measurement of vasodilatory function. A lean, healthy, untrained group was studied as nonexercised controls.

RESULTS

Training increased oxygen consumption (VO2) peak [OW, 29 +/- 1 to 37 +/- 4 ml/kg fat-free mass (FFM).min; T2DM, 33 +/- 2 to 43 +/- 3 ml/kg FFM.min; P < 0.05] and improved insulin-stimulated glucose disposal (OW, 6.5 +/- 0.5 to 7.2 +/- 0.4 mg/kg FFM.min; T2DM, 3.8 +/- 0.3 to 4.2 +/- 0.3 mg/kg FFM.min; P < 0.05) in insulin resistance. OW and T2DM, before training, had decreased acetylcholine chloride (ACh)- and sodium nitroprusside-mediated vasodilation and decreased reactive hyperemia compared with lean controls. Training increased the vasodilatory response to ACh [OW (30 microg ACh/min), 12.2 +/- 3.4 to 19 +/- 4.2 ml/100 g.min; T2DM (30 microg ACh/min), 10.1 +/- 1.5 to 14.2 +/- 2.1 ml/100 g.min; P < 0.05] in both groups without affecting nitroprusside response.

CONCLUSION

Because vasodilatory dysfunction has been postulated to contribute to insulin resistance, the exercise-induced improvement in vasodilatory function may signify changes in the endothelium that could contribute to the improvement in insulin sensitivity observed after aerobic exercise training.

Diverse patterns of cell-specific gene expression in response to glucocorticoid in the developing small intestine

Although glucocorticoids are known to elicit functional maturation of the gastrointestinal tract, the molecular mechanisms of glucocorticoid action on the developing intestine have not been fully elucidated. Our previous microarray studies identified 66 transcripts as being rapidly induced in the jejunum following dexamethasone (Dex) administration to suckling mice. Now we report the specific cellular location of a subset of these transcripts. Mouse pups at P8 received Dex or vehicle and intestinal segments were collected 3-4 h later. Robotic-based in situ hybridization (ISH) was performed with digoxygenin-labeled riboprobes. Transcripts studied included Ndrg1, Sgk1, Fos, and two unknown genes (Gene 9 and Gene 36). As predicted, ISH revealed marked diversity of cellular expression. In small intestinal segments, Sgk1 mRNA was in all epithelial cells; Fos mRNA was confined to epithelial cells at the villus tip; and Ndrg1 and Gene 36 mRNAs were localized to epithelial cells of the upper crypt and villus base. The remaining transcript (Gene 9) was induced modestly in villus stroma and strongly in the muscle layers. In the colon, Ndrg1, Sgk1, and Gene 36 were induced in all epithelial cells; Gene 9 was in muscle layers only; and Fos was not detectable. For jejunal segments, quantitation of ISH signals in tissue from Dex-treated and vehicle-treated mice demonstrated mRNA increases very similar to those measured by Northern blotting. We conclude that glucocorticoid action in the intestine reflects diverse molecular mechanisms operating in different cell types and that quantitative ISH is a valuable tool for studying hormone action in this tissue.

Impact of endurance training on murine spontaneous activity, muscle mitochondrial DNA abundance, gene transcripts, and function

We hypothesized that enhanced skeletal muscle mitochondrial function following aerobic exercise training is related to an increase in mitochondrial transcription factors, DNA abundance [mitochondrial DNA (mtDNA)], and mitochondria-related gene transcript levels, as well as spontaneous physical activity (SPA) levels. We report the effects of daily treadmill training on 12-wk-old FVB mice for 5 days/wk over 8 wk at 80% peak O(2) consumption and studied the training effect on changes in body composition, glucose tolerance, muscle mtDNA muscle, mitochondria-related gene transcripts, in vitro muscle mitochondrial ATP production capacity (MATPC), and SPA levels. Compared with the untrained mice, the trained mice had higher peak O(2) consumption (+18%; P < 0.001), lower percentage of abdominal (-25.4%; P < 0.02) and body fat (-19.5%; P < 0.01), improved glucose tolerance (P < 0.04), and higher muscle mitochondrial enzyme activity (+19.5-43.8%; P < 0.04) and MATPC (+28.9 to +32.4%; P < 0.01). Gene array analysis showed significant differences in mRNAs of mitochondria-related ontology groups between the trained and untrained mice. Training also increased muscle mtDNA (+88.4 to +110%; P < 0.05), peroxisome proliferative-activated receptor-gamma coactivator-1alpha protein (+99.5%; P < 0.04), and mitochondrial transcription factor A mRNA levels (+21.7%; P < 0.004) levels. SPA levels were higher in trained mice (P = 0.056, two-sided t-test) and significantly correlated with two separate substrate-based measurements of MATPC (P < 0.02). In conclusion, aerobic exercise training enhances muscle mitochondrial transcription factors, mtDNA abundance, mitochondria-related gene transcript levels, and mitochondrial function, and this enhancement in mitochondrial function occurs in association with increased SPA.

Promoting training adaptations through nutritional interventions

Training and nutrition are highly interrelated in that optimal adaptation to the demands of repeated training sessions typically requires a diet that can sustain muscle energy reserves. As nutrient stores (i.e. muscle and liver glycogen) play a predominant role in the performance of prolonged, intense, intermittent exercise typical of the patterns of soccer match-play, and in the replenishment of energy reserves for subsequent training sessions, the extent to which acutely altering substrate availability might modify the training impulse has been a key research area among exercise physiologists and sport nutritionists for several decades. Although the major perturbations to cellular homeostasis and muscle substrate stores occur during exercise, the activation of several major signalling pathways important for chronic training adaptations take place during the first few hours of recovery, returning to baseline values within 24 h after exercise. This has led to the paradigm that many chronic training adaptations are generated by the cumulative effects of the transient events that occur during recovery from each (acute) exercise bout. Evidence is accumulating that nutrient supplementation can serve as a potent modulator of many of the acute responses to both endurance and resistance training. In this article, we review the molecular and cellular events that occur in skeletal muscle during exercise and subsequent recovery, and the potential for nutrient supplementation (e.g. carbohydrate, fat, protein) to affect many of the adaptive responses to training.

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.

Mining frequent patterns for AMP-activated protein kinase regulation on skeletal muscle

Background

AMP-activated protein kinase (AMPK) has emerged as a significant signaling intermediary that regulates metabolisms in response to energy demand and supply. An investigation into the degree of activation and deactivation of AMPK subunits under exercise can provide valuable data for understanding AMPK. In particular, the effect of AMPK on muscle cellular energy status makes this protein a promising pharmacological target for disease treatment. As more AMPK regulation data are accumulated, data mining techniques can play an important role in identifying frequent patterns in the data. Association rule mining, which is commonly used in market basket analysis, can be applied to AMPK regulation.

Results

This paper proposes a framework that can identify the potential correlation, either between the state of isoforms of α, β and γ subunits of AMPK, or between stimulus factors and the state of isoforms. Our approach is to apply item constraints in the closed interpretation to the itemset generation so that a threshold is specified in terms of the amount of results, rather than a fixed threshold value for all itemsets of all sizes. The derived rules from experiments are roughly analyzed. It is found that most of the extracted association rules have biological meaning and some of them were previously unknown. They indicate direction for further research.

Conclusion

Our findings indicate that AMPK has a great impact on most metabolic actions that are related to energy demand and supply. Those actions are adjusted via its subunit isoforms under specific physical training. Thus, there are strong co-relationships between AMPK subunit isoforms and exercises. Furthermore, the subunit isoforms are correlated with each other in some cases. The methods developed here could be used when predicting these essential relationships and enable an understanding of the functions and metabolic pathways regarding AMPK.

Long-term exercise stimulates adenosine monophosphate-activated protein kinase activity and subunit expression in rat visceral adipose tissue and liver

Adenosine monophosphate-activated protein kinase (AMPK) is activated in response to adenosine triphosphate depletion caused by the metabolic and nutritional state. Mammalian AMPK is a heterotrimeric enzyme composed of a catalytic alpha subunit and 2 regulatory subunits (beta and gamma). Although much attention has been focused on exercise-induced AMPK activation in skeletal muscle, little information is available on the role of AMPK in adipose tissue and liver. Acetyl-coenzyme A carboxylase (ACC) is a well-known downstream target of AMPK. The ACC contains serine residues that are phosphorylated by AMPK. The present study was undertaken to determine whether long-term exercise of medium intensity (60% of Vo2max for 12 weeks) may influence AMPK enzyme activity, gene/protein expression, and subsequent ACC phosphorylation in rat adipose tissue (visceral and subcutaneous) and liver. We initially demonstrated that long-term exercise induced a significant increase in phosphorylation of Thr172 in the AMPK alpha1 subunit and of Ser79 in ACC in visceral adipose tissue rather than subcutaneous tissue. We also demonstrated that the AMPK alpha1-,alpha2-subunit messenger RNA (mRNA) level as well as the corresponding protein levels were increased in response to long-term exercise, whereas the other subunits were not altered significantly. In contrast to that of visceral adipose tissue, long-term exercise did not induce any significant effect on any of the AMPK subunit mRNA levels or alpha1-,alpha2-subunit protein levels in subcutaneous adipose tissue. In addition to adipose tissue, we demonstrated that long-term exercise induced an increase in both AMPK/ACC phosphorylation and alpha1-,alpha2-subunit mRNA/protein expression in the liver. Although the precise physiologic relevance of AMPK activation in these tissues remains unknown, it is possible that it might play an important role in long-term exercise-induced adaptation mechanisms and may lead to an improvement in certain metabolic abnormalities in metabolic diseases.

Activation of AMP-activated protein kinase within the ventromedial hypothalamus amplifies counterregulatory hormone responses in rats with defective counterregulation

Defective counterregulatory responses (CRRs) to hypoglycemia are associated with a marked increase in the risk of severe hypoglycemia. The mechanisms leading to the development of defective CRRs remain largely unknown, although they are associated with antecedent hypoglycemia. Activation of AMP-activated protein kinase (AMPK) in the ventromedial hypothalamus (VMH) amplifies the counterregulatory increase in glucose production during acute hypoglycemia. To examine whether activation of AMPK in the VMH restores defective CRR, controlled hypoglycemia ( approximately 2.8 mmol/l) was induced in a group of 24 Sprague-Dawley rats, all of which had undergone a 3-day model of recurrent hypoglycemia before the clamp study. Before the acute study, rats were microinjected to the VMH with either 5-aminoimidazole-4-carboxamide (AICAR; n=12), to activate AMPK, or saline (n=12). In a subset of rats, an infusion of H(3)-glucose was additionally started to calculate glucose turnover. Stimulation of AMPK within the VMH was found to amplify hormonal CRR and increase endogenous glucose production. In addition, analysis of tissue from both whole hypothalamus and VMH showed that recurrent hypoglycemia induces an increase in the gene expression of AMPK alpha(1) and alpha(2). These findings suggest that the development of novel drugs designed to selectively activate AMPK in the VMH offer a future therapeutic potential for individuals with type 1 diabetes who have defective CRRs to hypoglycemia.

Exercise training decreases the concentration of malonyl-CoA and increases the expression and activity of malonyl-CoA decarboxylase in human muscle

The study was designed to evaluate whether changes in malonyl-CoA and the enzymes that govern its concentration occur in human muscle as a result of physical training. Healthy, middle-aged subjects were studied before and after a 12-wk training program that significantly increased VO2 max by 13% and decreased intra-abdominal fat by 17%. Significant decreases (25-30%) in the concentration of malonyl-CoA were observed after training, 24-36 h after the last bout of exercise. They were accompanied by increases in both the activity (88%) and mRNA (51%) of malonyl-CoA decarboxylase (MCD) in muscle but no changes in the phosphorylation of AMP kinase (AMPK, Thr172) or of acetyl-CoA carboxylase. The abundance of peroxisome proliferator-activated receptor (PPAR)gamma coactivator-1alpha (PGC-1alpha), a regulator of transcription that has been linked to the mediation of MCD expression by PPARalpha, was also increased (3-fold). In studies also conducted 24-36 h after the last bout of exercise, no evidence of increased whole body insulin sensitivity or fatty acid oxidation was observed during an euglycemic hyperinsulinemic clamp. In conclusion, the concentration of malonyl-CoA is diminished in muscle after physical training, most likely because of PGC-1alpha-mediated increases in MCD expression and activity. These changes persist after the increases in AMPK activity and whole body insulin sensitivity and fatty acid oxidation, typically caused by an acute bout of exercise in healthy individuals, have dissipated.

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.

LKB1-AMPK signaling in muscle from obese insulin-resistant Zucker rats and effects of training

AMPK is a key regulator of fat and carbohydrate metabolism. It has been postulated that defects in AMPK signaling could be responsible for some of the metabolic abnormalities of type 2 diabetes. In this study, we examined whether insulin-resistant obese Zucker rats have abnormalities in the AMPK pathway. We compared AMPK and ACC phosphorylation and the protein content of the upstream AMPK kinase LKB1 and the AMPK-regulated transcriptional coactivator PPARgamma coactivator-1 (PGC-1) in gastrocnemius of sedentary obese Zucker rats and sedentary lean Zucker rats. We also examined whether 7 wk of exercise training on a treadmill reversed abnormalities in the AMPK pathway in obese Zucker rats. In the obese rats, AMPK phosphorylation was reduced by 45% compared with lean rats. Protein expression of the AMPK kinase LKB1 was also reduced in the muscle from obese rats by 43%. In obese rats, phosphorylation of ACC and protein expression of PGC-1alpha, two AMPK-regulated proteins, tended to be reduced by 50 (P = 0.07) and 35% (P = 0.1), respectively. There were no differences in AMPKalpha1, -alpha2, -beta1, -beta2, and -gamma3 protein content between lean and obese rats. Training caused a 1.5-fold increase in AMPKalpha1 protein content in the obese rats, although there was no effect of training on AMPK phosphorylation and the other AMPK isoforms. Furthermore, training also significantly increased LKB1 and PGC-1alpha protein content 2.8- and 2.5-fold, respectively, in the obese rats. LKB1 protein strongly correlated with hexokinase II activity (r = 0.75, P = 0.001), citrate synthase activity (r = 0.54, P = 0.02), and PGC-1alpha protein content (r = 0.81, P < 0.001). In summary, obese insulin-resistant rodents have abnormalities in the LKB1-AMPK-PGC-1 pathway in muscle, and these abnormalities can be restored by training.