Discovery of TBC1D1 as an insulin-, AICAR-, and contraction-stimulated signaling nexus in mouse skeletal muscle

Mice with AS160/TBC1D4-Thr649Ala knockin mutation are glucose intolerant with reduced insulin sensitivity and altered GLUT4 trafficking

AS160 has emerged as a key player in insulin-mediated glucose transport through controlling GLUT4 trafficking, which is thought to be regulated by insulin-stimulated phosphorylation of sites including the 14-3-3 binding phospho-Thr649 (equivalent to Thr642 in human AS160). To define physiological roles of AS160-Thr649 phosphorylation and 14-3-3 binding in glucose homeostasis, we substituted this residue by a nonphosphorylatable alanine by knockin mutation in mice. The mutant protein was expressed at normal levels, while insulin-stimulated AS160 binding to 14-3-3s was abolished in homozygous knockin mice. These animals displayed impaired glucose disposal and insulin sensitivity, which were associated with decreased glucose uptake in vivo. Insulin-stimulated glucose transport and cell surface GLUT4 content were reduced in isolated muscles, but not in adipocytes. These results provide genetic evidence that insulin-induced AS160-Thr649 phosphorylation and/or its binding to 14-3-3 play an important role in regulating whole-body glucose homeostasis, at least in part through regulating GLUT4 trafficking in muscle.

Impaired insulin-induced site-specific phosphorylation of TBC1 domain family, member 4 (TBC1D4) in skeletal muscle of type 2 diabetes patients is restored by endurance exercise-training

AIMS/HYPOTHESIS

Insulin-mediated glucose disposal rates (R(d)) are reduced in type 2 diabetic patients, a process in which intrinsic signalling defects are thought to be involved. Phosphorylation of TBC1 domain family, member 4 (TBC1D4) is at present the most distal insulin receptor signalling event linked to glucose transport. In this study, we examined insulin action on site-specific phosphorylation of TBC1D4 and the effect of exercise training on insulin action and signalling to TBC1D4 in skeletal muscle from type 2 diabetic patients.

METHODS

During a 3 h euglycaemic-hyperinsulinaemic (80 mU min⁻¹ m⁻²) clamp, we obtained M. vastus lateralis biopsies from 13 obese type 2 diabetic and 13 obese, non-diabetic control individuals before and after 10 weeks of endurance exercise-training.

RESULTS

Before training, reductions in insulin-stimulated R (d), together with impaired insulin-stimulated glycogen synthase fractional velocity, Akt Thr³⁰⁸ phosphorylation and phosphorylation of TBC1D4 at Ser³¹⁸, Ser⁵⁸⁸ and Ser⁷⁵¹ were observed in skeletal muscle from diabetic patients. Interestingly, exercise-training normalised insulin-induced TBC1D4 phosphorylation in diabetic patients. This happened independently of increased TBC1D4 protein content, but exercise-training did not normalise Akt phosphorylation in diabetic patients. In both groups, training-induced improvements in insulin-stimulated R(d) (~20%) were associated with increased muscle protein content of Akt, TBC1D4, α2-AMP-activated kinase (AMPK), glycogen synthase, hexokinase II and GLUT4 (20-75%).

CONCLUSIONS/INTERPRETATION

Impaired insulin-induced site-specific TBC1D4 phosphorylation may contribute to skeletal muscle insulin resistance in type 2 diabetes. The mechanisms by which exercise-training improves insulin sensitivity in type 2 diabetes may involve augmented signalling of TBC1D4 and increased skeletal muscle content of key insulin signalling and effector proteins, e.g., Akt, TBC1D4, AMPK, glycogen synthase, GLUT4 and hexokinase II.

A Potent and Selective AMPK Activator That Inhibits de Novo Lipogenesis

AMP-activated protein kinase (AMPK) is a heterotrimeric kinase that regulates cellular energy metabolism by affecting energy-consuming pathways such as de novo lipid biosynthesis and glucose production as well as energy-producing pathways such as lipid oxidation and glucose uptake. Accordingly, compounds that activate AMPK represent potential drug candidates for the treatment of hyperlipidemia and type 2 diabetes. Screening of a proprietary library of AMP mimetics identified the phosphonic acid 2 that bears little structural resemblance to AMP but is capable of activating AMPK with high potency (EC50 = 6 nM vs AMP EC50 = 6 μM) and specificity. Phosphonate prodrugs of 2 inhibited de novo lipogenesis in cellular and animal models of hyperlipidemia.

Myo1c regulates glucose uptake in mouse skeletal muscle

Contraction and insulin promote glucose uptake in skeletal muscle through GLUT4 translocation to cell surface membranes. Although the signaling mechanisms leading to GLUT4 translocation have been extensively studied in muscle, the cellular transport machinery is poorly understood. Myo1c is an actin-based motor protein implicated in GLUT4 translocation in adipocytes; however, the expression profile and role of Myo1c in skeletal muscle have not been investigated. Myo1c protein abundance was higher in more oxidative skeletal muscles and heart. Voluntary wheel exercise (4 weeks, 8.2 ± 0.8 km/day), which increased the oxidative profile of the triceps muscle, significantly increased Myo1c protein levels by ∼2-fold versus sedentary controls. In contrast, high fat feeding (9 weeks, 60% fat) significantly reduced Myo1c by 17% in tibialis anterior muscle. To study Myo1c regulation of glucose uptake, we expressed wild-type Myo1c or Myo1c mutated at the ATPase catalytic site (K111A-Myo1c) in mouse tibialis anterior muscles in vivo and assessed glucose uptake in vivo in the basal state, in response to 15 min of in situ contraction, and 15 min following maximal insulin injection (16.6 units/kg of body weight). Expression of wild-type Myo1c or K111A-Myo1c had no effect on basal glucose uptake. However, expression of wild-type Myo1c significantly increased contraction- and insulin-stimulated glucose uptake, whereas expression of K111A-Myo1c decreased both contraction-stimulated and insulin-stimulated glucose uptake. Neither wild-type nor K111A-Myo1c expression altered GLUT4 expression, and neither affected contraction- or insulin-stimulated signaling proteins. Myo1c is a novel mediator of both insulin-stimulated and contraction-stimulated glucose uptake in skeletal muscle.

Contraction regulates site-specific phosphorylation of TBC1D1 in skeletal muscle

TBC1D1 (tre-2/USP6, BUB2, cdc16 domain family member 1) is a Rab-GAP (GTPase-activating protein) that is highly expressed in skeletal muscle, but little is known about TBC1D1 regulation and function. We studied TBC1D1 phosphorylation on three predicted AMPK (AMP-activated protein kinase) phosphorylation sites (Ser231, Ser660 and Ser700) and one predicted Akt phosphorylation site (Thr590) in control mice, AMPKα2 inactive transgenic mice (AMPKα2i TG) and Akt2-knockout mice (Akt2 KO). Muscle contraction significantly increased TBC1D1 phosphorylation on Ser231 and Ser660, tended to increase Ser700 phosphorylation, but had no effect on Thr590. AICAR (5-aminoimidazole-4-carboxyamide ribonucleoside) also increased phosphorylation on Ser231, Ser660 and Ser700, but not Thr590, whereas insulin only increased Thr590 phosphorylation. Basal and contraction-stimulated TBC1D1 Ser231, Ser660 and Ser700 phosphorylation were greatly reduced in AMPKα2i TG mice, although contraction still elicited a small increase in phosphorylation. Akt2 KO mice had blunted insulin-stimulated TBC1D1 Thr590 phosphorylation. Contraction-stimulated TBC1D1 Ser231 and Ser660 phosphorylation were normal in high-fat-fed mice. Glucose uptake in vivo was significantly decreased in tibialis anterior muscles overexpressing TBC1D1 mutated on four predicted AMPK phosphorylation sites. In conclusion, contraction causes site-specific phosphorylation of TBC1D1 in skeletal muscle, and TBC1D1 phosphorylation on AMPK sites regulates contraction-stimulated glucose uptake. AMPK and Akt regulate TBC1D1 phosphorylation, but there must be additional upstream kinases that mediate TBC1D1 phosphorylation in skeletal muscle.

Exercise-induced TBC1D1 Ser237 phosphorylation and 14-3-3 protein binding capacity in human skeletal muscle

TBC1D1 is a Rab-GTPase activating protein involved in regulation of GLUT4 translocation in skeletal muscle. We here evaluated exercise-induced regulation of TBC1D1 Ser237 phosphorylation and 14-3-3 protein binding capacity in human skeletal muscle. In separate experiments healthy men performed all-out cycle exercise lasting either 30 s, 2  min or 20  min. After all exercise protocols, TBC1D1 Ser237 phosphorylation increased (∼70-230%, P < 0.005), with the greatest response observed after 20  min of cycling. Interestingly, capacity of TBC1D1 to bind 14-3-3 protein showed a similar pattern of regulation, increasing 60-250% (P < 0.001). Furthermore, recombinant 5AMP-activated protein kinase (AMPK) induced both Ser237 phosphorylation and 14-3-3 binding properties on human TBC1D1 when evaluated in vitro. To further characterize the role of AMPK as an upstream kinase regulating TBC1D1, extensor digitorum longus muscle (EDL) from whole body α1 or α2 AMPK knock-out and wild-type mice were stimulated to contract in vitro. In wild-type and α1 knock-out mice, contractions resulted in a similar ∼100% increase (P < 0.001) in Ser237 phosphorylation. Interestingly, muscle of α2 knock-out mice were characterized by reduced protein content of TBC1D1 (∼50%, P < 0.001) as well as in basal and contraction-stimulated (∼60%, P < 0.001) Ser237 phosphorylation, even after correction for the reduced TBC1D1 protein content. This study shows that TBC1D1 is Ser237 phosphorylated and 14-3-3 protein binding capacity is increased in response to exercise in human skeletal muscle. Furthermore, we show that the catalytic α2 AMPK subunit is the main (but probably not the only) donor of AMPK activity regulating TBC1D1 Ser237 phosphorylation in mouse EDL muscle.

AMP-activated protein kinase and its downstream transcriptional pathways

The AMP-activated protein kinase (AMPK) is a key regulator of catabolic versus anabolic processes. Its properties as an energy sensor allow it to couple the energy status of the cell to the metabolic environment. These adaptations not only take place through the acute modulation of key metabolic enzymes via direct phosphorylation, but also through a slower transcriptional adaptative response. The question of how AMPK regulates the expression of a number of gene sets, such as those related to mitochondrial biogenesis, energy production and oxidative protection, is only beginning to be elucidated, and still many questions remain to be answered. In this review we will try to integrate our current knowledge on how AMPK regulates transcription in muscle and liver, which will serve as examples to illustrate the major advances in the field and the key challenges ahead.

TBC1D1 regulates insulin- and contraction-induced glucose transport in mouse skeletal muscle

OBJECTIVE

TBC1D1 is a member of the TBC1 Rab-GTPase family of proteins and is highly expressed in skeletal muscle. Insulin and contraction increase TBC1D1 phosphorylation on phospho-Akt substrate motifs (PASs), but the function of TBC1D1 in muscle is not known. Genetic linkage analyses show a TBC1D1 R125W missense variant confers risk for severe obesity in humans. The objective of this study was to determine whether TBC1D1 regulates glucose transport in skeletal muscle.

RESEARCH DESIGN AND METHODS

In vivo gene injection and electroporation were used to overexpress wild-type and several mutant TBC1D1 proteins in mouse tibialis anterior muscles, and glucose transport was measured in vivo.

RESULTS

Expression of the obesity-associated R125W mutant significantly decreased insulin-stimulated glucose transport in the absence of changes in TBC1D1 PAS phosphorylation. Simultaneous expression of an inactive Rab-GTPase (GAP) domain of TBC1D1 in the R125W mutant reversed this decrease in glucose transport caused by the R125W mutant. Surprisingly, expression of TBC1D1 mutated to Ala on four conserved Akt and/or AMP-activated protein kinase predicted phosphorylation sites (4P) had no effect on insulin-stimulated glucose transport. In contrast, expression of the TBC1D1 4P mutant decreased contraction-stimulated glucose transport, an effect prevented by concomitant disruption of TBC1D1 Rab-GAP activity. There was no effect of the R125W mutation on contraction-stimulated glucose transport.

CONCLUSIONS

TBC1D1 regulates both insulin- and contraction-stimulated glucose transport, and this occurs via distinct mechanisms. The R125W mutation of TBC1D1 impairs skeletal muscle glucose transport, which could be a mechanism for the obesity associated with this mutation.

Emerging molecular targets for the treatment of nonalcoholic fatty liver disease

Nonalcoholic fatty liver disease (NAFLD) is characterized by hepatic fat accumulation in the absence of significant ethanol consumption, viral infection, or other specific causes of liver disease. Currently the most common chronic liver disease, affecting 30% of the Western world, NAFLD may progress to cirrhosis and end-stage liver disease and may increase the risk of developing diabetes and cardiovascular disease. Although its pathogenesis is unclear, NAFLD is tightly associated with insulin resistance and the metabolic syndrome. No established treatment exists, and current research is targeting new molecular mechanisms that underlie NAFLD and associated cardiometabolic disorders. This review discusses some of these emerging molecular mechanisms and their therapeutic implications for the treatment of NAFLD: microRNAs, incretin analogs/antagonists, liver-specific thyromimetics, AMP-activated protein kinase activators, and nuclear receptors farnesoid X receptor and pregane X receptor.

Contraction-related stimuli regulate GLUT4 traffic in C2C12-GLUT4myc skeletal muscle cells

Muscle contraction stimulates glucose uptake acutely to increase energy supply, but suitable cellular models that faithfully reproduce this complex phenomenon are lacking. To this end, we have developed a cellular model of contracting C(2)C(12) myotubes overexpressing GLUT4 with an exofacial myc-epitope tag (GLUT4myc) and explored stimulation of GLUT4 traffic by physiologically relevant agents. Carbachol (an acetylcholine receptor agonist) induced a gain in cell surface GLUT4myc that was mediated by nicotinic acetylcholine receptors. Carbachol also activated AMPK, and this response was sensitive to the contractile myosin ATPase inhibitor N-benzyl-p-toluenesulfonamide. The gain in surface GLUT4myc elicited by carbachol or by the AMPK activator 5-amino-4-carboxamide-1 beta-ribose was sensitive to chemical inhibition of AMPK activity by compound C and partially reduced by siRNA-mediated knockdown of AMPK catalytic subunits or LKB1. In addition, the carbachol-induced gain in cell surface GLUT4myc was partially sensitive to chelation of intracellular calcium with BAPTA-AM. However, the carbachol-induced gain in cell surface GLUT4myc was not sensitive to the CaMKK inhibitor STO-609 despite expression of both isoforms of this enzyme and a rise in cytosolic calcium by carbachol. Therefore, separate AMPK- and calcium-dependent signals contribute to mobilizing GLUT4 in response to carbachol, providing an in vitro cell model that recapitulates the two major signals whereby acute contraction regulates glucose uptake in skeletal muscle. This system will be ideal to further analyze the underlying molecular events of contraction-regulated GLUT4 traffic.

In vivo exercise followed by in vitro contraction additively elevates subsequent insulin-stimulated glucose transport by rat skeletal muscle

The cellular mechanisms whereby prior exercise enhances insulin-stimulated glucose transport (GT) are not well understood. Previous studies suggested that a prolonged increase in phosphorylation of Akt substrate of 160 kDa (AS160) may be important for the postexercise increase in insulin sensitivity. In the current study, the effects of in vivo exercise and in vitro contraction on subsequent insulin-stimulated GT were studied separately and together. Consistent with results from previous studies, prior exercise resulted in an increase in AS160 (642)Thr phosphorylation immediately after exercise in rat epitrochlearis muscles, and this increase remained 3 h postexercise concomitant with enhanced insulin-stimulated GT. For experiments with in vitro contraction, isolated rat epitrochlearis muscles were electrically stimulated to contract in the presence or absence of rat serum. As expected, insulin-stimulated GT measured 3 h after electrical stimulation in serum, but not after electrical stimulation without serum, exceeded resting controls. Immediately after electrical stimulation with or without serum, phosphorylation of both AS160 (detected by phospho-Akt substrate, PAS, antibody, or phospho-(642)Thr antibody) and its paralog TBC1D1 (detected by phospho-(237)Ser antibody) was increased. However, both AS160 and TBC1D1 phosphorylation had reversed to resting values at 3 h poststimulation with or without serum. Increasing the amount of exercise (from 1 to 2 h) or electrical stimulation (from 5 to 10 tetani) did not further elevate insulin-stimulated GT. In contrast, the combination of prior exercise and electrical stimulation had an additive effect on the subsequent increase in insulin-stimulated GT, suggesting that these exercise and electrical stimulation protocols may amplify insulin-stimulated GT through distinct mechanisms, with a persistent increase in AS160 phosphorylation potentially important for increased insulin sensitivity after exercise, but not after in vitro contraction.

Role of ataxia telangiectasia mutated in insulin signalling of muscle-derived cell lines and mouse soleus

AIM

Ataxia telangiectasia mutated (ATM) reportedly plays a role in insulin-stimulated activation of Akt in some cell types but not in others. The role of ATM in insulin signalling has not been firmly resolved for skeletal muscle cells, for which Akt phosphorylation is a pivotal step in stimulation of glucose transport. Accordingly, our aim was to determine the role of ATM in insulin effects for cell lines derived from skeletal muscle and for skeletal muscle.

METHODS

We examined insulin effects in L6 myotubes, mouse soleus, C2C12 myotubes and differentiated rhabdomyosarcoma (RD) cells in the presence and absence of a low concentration (1 microm) of the ATM inhibitor KU55933. We also compared insulin signalling in C2C12 cells expressing shRNA against ATM and control cell lines (empty vector; cells expressing non-targeting shRNA).

RESULTS

In L6 myotubes and mouse soleus muscle, KU55933 inhibited insulin-stimulated phosphorylation of the 160 kDa substrate of Akt (AS160) despite no effect on Akt. In contrast, KU55933 prevented insulin-stimulated Akt phosphorylation in C2C12 myotubes. Furthermore, C2C12 myotubes expressing shRNA against ATM displayed reduced insulin-stimulated Akt phosphorylation compared to controls. KU55933 also decreased insulin-stimulated Akt phosphorylation in differentiated RD cells.

CONCLUSION

These model-dependent differences in the role of ATM in insulin action demonstrate a role of ATM in insulin-stimulated phosphorylation of Akt (in C2C12 and RD cells) but also allow the elucidation of a novel, Akt-independent role of ATM (in L6 myotubes and mouse soleus, at the level of AS160) in insulin signalling.

Exercise and insulin: Convergence or divergence at AS160 and TBC1D1?

Akt substrate of 160 kDa (called AS160 or TBC1D4) and TBC1D1, Rab GTPase-activating proteins that regulate glucose transport, become phosphorylated with exercise or insulin stimulation. Evidence suggests that this convergence may prove to be imperfect, and each stimulus will produce a unique phosphosignature, providing a plausible mechanism for their apparently unique and overlapping roles in exercise- and insulin-stimulated glucose transport.

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.

Effects of excess corticosterone on LKB1 and AMPK signaling in rat skeletal muscle

Cushing's syndrome is characterized by marked central obesity and insulin insensitivity, effects opposite those seen with chronic AMP-activated protein kinase (AMPK) activation. This study was designed to determine whether chronic exposure to excess glucocorticoids influences LKB1/AMPK signaling in skeletal muscle. Corticosterone pellets were implanted subcutaneously in rats (hypercorticosteronemia, Hypercort) for 2 wk. Controls were sham operated and fed ad libitum or were sham operated and food restricted (pair-weighted group, Pair) to produce body weights similar to Hypercort rats. At the end of the 2-wk treatment period, rats were anesthetized, and the right gastrocnemius-plantaris (gastroc) and soleus muscles were removed. Left muscles were removed after electrical stimulation for 5 min. No significant differences were noted between treatment groups in ATP, creatine phosphate, or LKB1 activity. The alpha- and beta-subunit isoforms were not significantly influenced in gastroc by corticosterone treatment. Expression of the gamma3-subunit decreased, and gamma1- and gamma2-subunit expression increased. Both alpha2-AMPK and alpha1-AMPK activities were increased in the gastroc in response to electrical stimulation, but the magnitude of the increase was less for alpha2 in the Hypercort rats. Despite elevated plasma insulin and elevated plasma leptin in the Hypercort rats, phosphorylation of TBC1D1 was lower in both resting and stimulated muscle compared with controls. Malonyl-CoA content was elevated in gastroc muscles of resting Hypercort rats. These changes in response to excess glucocorticoids could be responsible, in part, for the decrease in insulin sensitivity and adiposity seen in Cushing's syndrome.

Role of the AMPKgamma3 isoform in hypoxia-stimulated glucose transport in glycolytic skeletal muscle

Skeletal muscle glucose transport is regulated via the canonical insulin-signaling cascade as well as by energy-sensing signals. 5'-AMP-activated protein kinase (AMPK) has been implicated in the energy status regulation of glucose transport. We determined the role of the AMPKgamma3 isoform in hypoxia-mediated energy status signaling and glucose transport in fast-twitch glycolytic extensor digitorum longus (EDL) muscle from AMPKgamma3-knockout (KO) mice and wild-type mice. Although hypoxia increased glucose transport (P < 0.001) in wild-type mice, this effect was attenuated in AMPKgamma3-KO mice (45% reduction, P < 0.01). The role of Ca(2+)-mediated signaling was tested using the Ca(2+)/calmodulin competitive inhibitor KN-93. KN-93 exposure reduced hypoxia-mediated glucose transport in AMPKgamma3-KO and wild-type mice (P < 0.05). To further explore the underlying signaling mechanisms, phosphorylation of CaMKII, AMPK, ACC, and TBC1D1/D4 as well as isoform-specific AMPK activity was determined. Basal and hypoxia-mediated phosphorylation of CaMKII, AMPK, and ACC as well as alpha1- and alpha2-associated AMPK activity was comparable between AMPKgamma3-KO and wild-type mice. KN-93 reduced hypoxia-mediated CaMKII phosphorylation in AMPKgamma3-KO and wild-type mice (P < 0.05), whereas phosphorylation of AMPK and ACC as well as alpha1- and alpha2-associated AMPK activity was unaltered. Hypoxia increased TBC1D1/D4 phosphorylation in AMPKgamma3-KO and wild-type mice (P < 0.001). KN-93 exposure prevented this effect in AMPKgamma3-KO, but not in wild-type mice. Taken together, we provide direct evidence for a role of the AMPKgamma3 isoform in hypoxia-mediated glucose transport in glycolytic muscle. Moreover, hypoxia-mediated TBC1D1/D4 phosphorylation was uncoupled from glucose transport in AMPKgamma3-KO mice, indicating that TBC1D1/D4-independent mechanisms contribute to glucose transport in skeletal muscle.

Identification of a novel phosphorylation site on TBC1D4 regulated by AMP-activated protein kinase in skeletal muscle

TBC1D4 (also known as AS160) regulates glucose transporter 4 (GLUT4) translocation and glucose uptake in adipocytes and skeletal muscle. Its mode of action involves phosphorylation of serine (S)/threonine (T) residues by upstream kinases resulting in inactivation of Rab-GTPase-activating protein (Rab-GAP) activity leading to GLUT4 mobilization. The majority of known phosphorylation sites on TBC1D4 lie within the Akt consensus motif and are phosphorylated by insulin stimulation. However, the 5'-AMP-activated protein kinase (AMPK) and other kinases may also phosphorylate TBC1D4, and therefore we hypothesized the presence of additional phosphorylation sites. Mouse skeletal muscles were contracted or stimulated with 5-aminoimidazole-4-carboxamide-1-beta-d-ribofuranoside (AICAR), and muscle lysates were subjected to mass spectrometry analyses resulting in identification of novel putative phosphorylation sites on TBC1D4. The surrounding amino acid sequence predicted that S711 would be recognized by AMPK. Using a phosphospecific antibody against S711, we found that AICAR and contraction increased S711 phosphorylation in mouse skeletal muscle, and this increase was abolished in muscle-specific AMPKalpha2 kinase-dead transgenic mice. Exercise in human vastus lateralis muscle also increased TBC1D4 S711 phosphorylation. Recombinant AMPK, but not Akt1, Akt2, or PKCzeta, phosphorylated purified muscle TBC1D4 on S711 in vitro. Interestingly, S711 was also phosphorylated in response to insulin in an Akt2- and rapamycin-independent, but a wortmannin-sensitive, manner, suggesting this site is regulated by one or more additional upstream kinases. Despite increased S711 phosphorylation with AICAR, contraction, and insulin, mutation of S711 to alanine did not alter glucose uptake in response to these stimuli. S711 is a novel TBC1D4 phosphorylation site regulated by AMPK in skeletal muscle.

Restoring AS160 phosphorylation rescues skeletal muscle insulin resistance and fatty acid oxidation while not reducing intramuscular lipids

We examined whether AICAR or leptin rapidly rescued skeletal muscle insulin resistance via increased palmitate oxidation, reductions in intramuscular lipids, and/or restoration of insulin-stimulated AS60 phosphorylation. Incubation with palmitate (2 mM, 0-18 h) induced insulin resistance in soleus muscle. From 12-18 h, palmitate was removed or AICAR or leptin was provided while 2 mM palmitate was maintained. Palmitate oxidation, intramuscular triacylglycerol, diacylglycerol, ceramide, AMPK phosphorylation, basal and insulin-stimulated glucose transport, plasmalemmal GLUT4, and Akt and AS160 phosphorylation were examined at 0, 6, 12, and 18 h. Palmitate treatment (12 h) increased intramuscular lipids (triacylglycerol +54%, diacylglycerol +11%, total ceramide +18%, C16:0 ceramide +60%) and AMPK phosphorylation (+118%), whereas it reduced fatty acid oxidation (-60%) and insulin-stimulated glucose transport (-70%), GLUT4 translocation (-50%), and AS160 phosphorylation (-40%). Palmitate removal did not rescue insulin resistance or associated parameters. The AICAR and leptin treatments did not consistently reduce intramuscular lipids, but they did rescue palmitate oxidation and insulin-stimulated glucose transport, GLUT4 translocation, and AS160 phosphorylation. Increased AMPK phosphorylation was associated with these improvements only when AICAR and leptin were present. Hence, across all experiments, AMPK phosphorylation did not correlate with any parameters. In contrast, palmitate oxidation and insulin-stimulated AS160 phosphorylation were highly correlated (r = 0.83). We speculate that AICAR and leptin activate both of these processes concomitantly, involving activation of unknown kinases in addition to AMPK. In conclusion, despite the maintenance of high concentrations of palmitate (2 mM), as well as increased concentrations of intramuscular lipids (triacylglycerol, diacylglycerol, and ceramide), the rapid AICAR- and leptin-mediated rescue of palmitate-induced insulin resistance is attributable to the restoration of insulin-stimulated AS160 phosphorylation and GLUT4 translocation.

Development of protein kinase activators: AMPK as a target in metabolic disorders and cancer

AMP-activated protein kinase (AMPK) is a cellular energy sensor activated by metabolic stresses that either inhibit ATP synthesis or accelerate ATP consumption. Activation of AMPK in response to an increase in the cellular AMP:ATP ratio results in inhibition of ATP-consuming processes such as gluconeogenesis and fatty acid synthesis, while stimulating ATP-generating processes, including fatty acid oxidation. These alterations in lipid and glucose metabolism would be expected to ameliorate the pathogenesis of obesity, type 2 diabetes and other metabolic disorders. Recently, AMPK has also been identified as a potential target for cancer prevention and/or treatment. Cell growth and proliferation are energetically demanding, and AMPK may act as an "energy checkpoint" that permits growth and proliferation only when energy reserves are sufficient. Thus, activators of AMPK could have potential as novel therapeutics both for metabolic disorders and for cancer, which together constitute two of the most prevalent groups of diseases worldwide.

Insulin-stimulated phosphorylation of the Rab GTPase-activating protein TBC1D1 regulates GLUT4 translocation

Insulin stimulates the translocation of the glucose transporter GLUT4 from intracellular locations to the plasma membrane in adipose and muscle cells. Prior studies have shown that Akt phosphorylation of the Rab GTPase-activating protein, AS160 (160-kDa Akt substrate; also known as TBC1D4), triggers GLUT4 translocation, most likely by suppressing its Rab GTPase-activating protein activity. However, the regulation of a very similar protein, TBC1D1 (TBC domain family, member 1), which is mainly found in muscle, in insulin-stimulated GLUT4 translocation has been unclear. In the present study, we have identified likely Akt sites of insulin-stimulated phosphorylation of TBC1D1 in C2C12 myotubes. We show that a mutant of TBC1D1, in which several Akt sites have been converted to alanine, is considerably more inhibitory to insulin-stimulated GLUT4 translocation than wild-type TBC1D1. This result thus indicates that similar to AS160, Akt phosphorylation of TBC1D1 enables GLUT4 translocation. We also show that in addition to Akt activation, activation of the AMP-dependent protein kinase partially relieves the inhibition of GLUT4 translocation by TBC1D1. Finally, we show that the R125W variant of TBC1D1, which has been genetically associated with obesity, is equally inhibitory to insulin-stimulated GLUT4 translocation, as is wild-type TBC1D1, and that healthy and type 2 diabetic individuals express approximately the same level of TBC1D1 in biopsies of vastus lateralis muscle. In conclusion, phosphorylation of TBC1D1 is required for GLUT4 translocation. Thus, the regulation of TBC1D1 resembles that of its paralog, AS160.

Genetic disruption of AMPK signaling abolishes both contraction- and insulin-stimulated TBC1D1 phosphorylation and 14-3-3 binding in mouse skeletal muscle

TBC1D1 is a Rab-GTPase-activating protein (GAP) known to be phosphorylated in response to insulin, growth factors, pharmacological agonists that activate 5'-AMP-activated protein kinase (AMPK), and muscle contraction. Silencing TBC1D1 in L6 muscle cells by siRNA increases insulin-stimulated GLUT4 translocation, and overexpression of TBC1D1 in 3T3-L1 adipocytes with low endogenous TBC1D1 expression inhibits insulin-stimulated GLUT4 translocation, suggesting a role of TBC1D1 in regulating GLUT4 translocation. Aiming to unravel the regulation of TBC1D1 during contraction and the potential role of AMPK in intact skeletal muscle, we used EDL muscles from wild-type (WT) and AMPK kinase dead (KD) mice. We explored the site-specific phosphorylation of TBC1D1 Ser(237) and Thr(596) and their relation to 14-3-3 binding, a proposed mechanism for regulation of GAP function of TBC1D1. We show that muscle contraction increases 14-3-3 binding to TBC1D1 as well as phosphorylation of Ser(237) and Thr(596) in an AMPK-dependent manner. AMPK activation by AICAR induced similar Ser(237) and Thr(596) phosphorylation of, and 14-3-3 binding to, TBC1D1 as muscle contraction. Insulin did not increase Ser(237) phosphorylation or 14-3-3 binding to TBC1D1. However, insulin increased Thr(596) phosphorylation, and intriguingly this response was fully abolished in the AMPK KD mice. Thus, TBC1D1 is differentially regulated in response to insulin and contraction. This study provides genetic evidence to support an important role for AMPK in regulating TBC1D1 in response to both of these physiological stimuli.

AMPK in Health and Disease

The function and survival of all organisms is dependent on the dynamic control of energy metabolism, when energy demand is matched to energy supply. The AMP-activated protein kinase (AMPK) alphabetagamma heterotrimer has emerged as an important integrator of signals that control energy balance through the regulation of multiple biochemical pathways in all eukaryotes. In this review, we begin with the discovery of the AMPK family and discuss the recent structural studies that have revealed the molecular basis for AMP binding to the enzyme's gamma subunit. AMPK's regulation involves autoinhibitory features and phosphorylation of both the catalytic alpha subunit and the beta-targeting subunit. We review the role of AMPK at the cellular level through examination of its many substrates and discuss how it controls cellular energy balance. We look at how AMPK integrates stress responses such as exercise as well as nutrient and hormonal signals to control food intake, energy expenditure, and substrate utilization at the whole body level. Lastly, we review the possible role of AMPK in multiple common diseases and the role of the new age of drugs targeting AMPK signaling.

Knockout of the predominant conventional PKC isoform, PKCalpha, in mouse skeletal muscle does not affect contraction-stimulated glucose uptake

Conventional (c) protein kinase C (PKC) activity has been shown to increase with skeletal muscle contraction, and numerous studies using primarily pharmacological inhibitors have implicated cPKCs in contraction-stimulated glucose uptake. Here, to confirm that cPKC activity is required for contraction-stimulated glucose uptake in mouse muscles, contraction-stimulated glucose uptake ex vivo was first evaluated in the presence of three commonly used cPKC inhibitors (calphostin C, Gö-6976, and Gö-6983) in incubated mouse soleus and extensor digitorum longus (EDL) muscles. All potently inhibited contraction-stimulated glucose uptake by 50-100%, whereas both Gö compounds, but not calphostin C, inhibited insulin-stimulated glucose uptake modestly. AMP-activated protein kinase (AMPK) and eukaryotic elongation factor 2 phosphorylation was unaffected by the blockers. PKCalpha was estimated to account for approximately 97% of total cPKC protein expression in skeletal muscle. However, in muscles from PKCalpha knockout (KO) mice, neither contraction- nor phorbol ester-stimulated glucose uptake ex vivo differed compared with the wild type. Furthermore, the effects of calphostin C and Gö-6983 on contraction-induced glucose uptake were similar in muscles lacking PKCalpha and in the wild type. It can be concluded that PKCalpha, representing approximately 97% of cPKC in skeletal muscle, is not required for contraction-stimulated glucose uptake. Thus the effect of the PKC blockers on glucose uptake is either nonspecific working on other parts of contraction-induced signaling or the remaining cPKC isoforms are sufficient for stimulating glucose uptake during contractions.

Increased AS160 phosphorylation, but not TBC1D1 phosphorylation, with increased postexercise insulin sensitivity in rat skeletal muscle

A single exercise bout can increase insulin-independent glucose transport immediately postexercise and insulin-dependent glucose transport (GT) for several hours postexercise. Akt substrate of 160 kDa (AS160) and TBC1D1 are paralog Rab GTPase-activating proteins that have been proposed to contribute to these exercise effects. Previous research demonstrated greater AS160 and Akt threonine phosphorylation in rat skeletal muscle at 3-4 h postexercise concomitant with enhanced insulin-stimulated GT. To further probe whether these signaling events or TBC1D1 phosphorylation were important for the enhanced postexercise insulin-stimulated GT, male Wistar rats were studied using four experimental protocols (2-h swim exercise, differing with regard to timing of muscle sampling and whether food was provided postexercise) that were known to vary in their influence of insulin-independent and insulin-dependent GT postexercise. The results indicated that, in isolated rat epitrochlearis muscle, 1) elevated phosphorylation of AS160 (measured using anti-phospho-Akt substrate, PAS-AS160, and phosphospecific anti-Thr(642)-AS160, pThr(642)-AS160) consistently tracked with elevated insulin-stimulated GT; 2) PAS-TBC1D1 was not different from sedentary values at 3 or 27 h postexercise, when insulin sensitivity was increased; 3) insulin-stimulated Akt activity was not increased postexercise in muscles with increased insulin sensitivity; 4) PAS-TBC1D1 was increased immediately postexercise, when insulin-independent GT was elevated, and reversed at 3 and 27 h postexercise, when insulin-independent GT was also reversed; and 5) there was no significant effect of exercise or insulin on total abundance of AS160, TBC1D1, Akt, or GLUT4 protein with any of the protocols. The results are consistent with increased AS160 phosphorylation (PAS-AS160 or pThr(642)-AS160) but not increased PAS-TBC1D1 or Akt activity, which is important for increased postexercise insulin-stimulated GT in rat skeletal muscle. They also support the idea that increased TBC1D1 phosphorylation may play a role in the insulin-independent increase in GT postexercise.

AMP-activated protein kinase--a sensor of glycogen as well as AMP and ATP?

The classical role of the AMP-activated protein kinase (AMPK) is to act as a sensor of the immediate availability of cellular energy, by monitoring the concentrations of AMP and ATP. However, the beta subunits of AMPK contain a glycogen-binding domain, and in this review we develop the hypothesis that this is a regulatory domain that allows AMPK to act as a sensor of the status of cellular reserves of energy in the form of glycogen. We argue that the pool of AMPK that is bound to the glycogen particle is in an active state when glycogen particles are fully synthesized, causing phosphorylation of glycogen synthase at site 2 and providing a feedback inhibition of further extension of the outer chains of glycogen. However, when glycogen becomes depleted, the glycogen-bound pool of AMPK becomes inhibited due to binding to alpha1-->6-linked branch points exposed by the action of phosphorylase and/or debranching enzyme. This allows dephosphorylation of site 2 on glycogen synthase by the glycogen-bound form of protein phosphatase-1, promoting rapid resynthesis of glycogen and replenishment of glycogen stores. This is an extension of the classical role of AMPK as a 'guardian of cellular energy', in which it ensures that cellular energy reserves are adequate for medium-term requirements. The literature concerning AMPK, glycogen structure and glycogen-binding proteins that led us to this concept is reviewed.

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.

Potential role of TBC1D4 in enhanced post-exercise insulin action in human skeletal muscle

AIMS/HYPOTHESIS

TBC1 domain family, member 4 (TBC1D4; also known as AS160) is a cellular signalling intermediate to glucose transport regulated by insulin-dependent and -independent mechanisms. Skeletal muscle insulin sensitivity is increased after acute exercise by an unknown mechanism that does not involve modulation at proximal insulin signalling intermediates. We hypothesised that signalling through TBC1D4 is involved in this effect of exercise as it is a common signalling element for insulin and exercise.

METHODS

Insulin-regulated glucose metabolism was evaluated in 12 healthy moderately trained young men 4 h after one-legged exercise at basal and during a euglycaemic-hyperinsulinaemic clamp. Vastus lateralis biopsies were taken before and immediately after the clamp.

RESULTS

Insulin stimulation increased glucose uptake in both legs, with greater effects (approximately 80%, p < 0.01) in the previously exercised leg. TBC1D4 phosphorylation, assessed using the phospho-AKT (protein kinase B)substrate antibody and phospho- and site-specific antibodies targeting six phosphorylation sites on TBC1D4, increased at similar degrees to insulin stimulation in the previously exercised and rested legs (p < 0.01). However, TBC1D4 phosphorylation on Ser-318, Ser-341, Ser-588 and Ser-751 was higher in the previously exercised leg, both in the absence and in the presence of insulin (p < 0.01; Ser-588, p = 0.09; observed power = 0.39). 14-3-3 binding capacity for TBC1D4 increased equally (p < 0.01) in both legs during insulin stimulation.

CONCLUSION/INTERPRETATION

We provide evidence for site-specific phosphorylation of TBC1D4 in human skeletal muscle in response to physiological hyperinsulinaemia. The data support the idea that TBC1D4 is a nexus for insulin- and exercise-responsive signals that may mediate increased insulin action after exercise.

Inhibition of contraction-stimulated AMP-activated protein kinase inhibits contraction-stimulated increases in PAS-TBC1D1 and glucose transport without altering PAS-AS160 in rat skeletal muscle

OBJECTIVE

Phosphorylation of two members of the TBC1 domain family of proteins, Akt substrate of 160 kDa (AS160, also known as TBC1D4) and TBC1D1, has been implicated in the regulation of glucose transport in skeletal muscle. Insulin-stimulated phosphorylation (measured using the phospho-Akt substrate [PAS] antibody) of AS160 and TBC1D1 appears to occur in an Akt-dependent manner, but the kinases responsible for contraction-stimulated PAS-AS160 and PAS-TBC1D1 remain unclear. AMP-activated protein kinase (AMPK) and Akt, both activated by contraction, can each phosphorylate AS160 and TBC1D1 in cell-free assays.

RESEARCH DESIGN AND METHODS

To evaluate the roles of AMPK and Akt on insulin- or contraction-stimulated PAS-AS160, PAS-TBC1D1, and glucose transport, rat epitrochlearis was incubated with and without compound C (inhibitor of AMPK) or Wortmannin (inhibitor of phosphatidylinositol [PI] 3-kinase, which is upstream of Akt) before and during insulin stimulation or contraction.

RESULTS

Insulin-stimulated glucose transport and phosphorylation of both AS160 and TBC1D1 were completely inhibited by Wortmannin. Wortmannin eliminated contraction stimulation of phospho-Ser(21/9)glycogen synthase kinase 3alpha/beta (pGSK3; Akt substrate) and PAS-AS160 but did not significantly alter pAMPK, phospho-Ser79acetyl CoA carboxylase (pACC; AMPK substrate), PAS-TBC1D1, or glucose transport in contraction-stimulated muscle. Compound C completely inhibited contraction-stimulated pACC and PAS-TBC1D1 and partially blocked glucose transport, but it did not significantly alter pAkt, pGSK3, or PAS-AS160.

CONCLUSIONS

These data suggest that 1) insulin stimulates glucose transport and phosphorylation of AS160 and TBC1D1 in a PI 3-kinase/Akt-dependent manner, 2) contraction stimulates PAS-AS160 (but not PAS-TBC1D1 or glucose transport) in a PI 3-kinase/Akt-dependent manner, and 3) contraction stimulates PAS-TBC1D1 and glucose transport (but not PAS-AS160) in an AMPK-dependent manner.

A myosin II ATPase inhibitor reduces force production, glucose transport, and phosphorylation of AMPK and TBC1D1 in electrically stimulated rat skeletal muscle

Contraction-stimulated glucose transport by skeletal muscle appears to be caused by the cumulative effects of multiple inputs [potentially including AMP-activated protein kinase (AMPK), Ca(2+) flux, and force production], making it challenging to isolate the roles of these putative regulatory factors. To distinguish the effects of force production from the direct consequences of Ca(2+) flux, the predominantly type II rat epitrochlearis muscle was incubated without (vehicle) or with N-benzyl-p-toluenesulfonamide (BTS), a highly specific myosin II ATPase inhibitor that prevents force production by electrically stimulated (ES) type II fibers without altering cytosolic Ca(2+). In ES muscles, BTS vs. vehicle had an 84% reduction in force production and a 57% decrement in contraction-stimulated 3-O-methylglucose transport (3MGT). BTS did not alter the ES increase in phosphorylation of CaMKII (indicative of cytosolic Ca(2+)) or the amount of glycogen depletion. ES caused significant reductions in ATP (48%) and phosphocreatine (67%) concentrations for vehicle-treated muscles. For BTS-treated muscles, ES did not reduce ATP and caused only a 42% decrease in phosphocreatine. There was an ES increase in phosphorylation of AMPK, acetyl-CoA carboxylase (an AMPK substrate), and TBC1D1 for vehicle-treated muscles but not for BTS-treated muscles. These results point toward an essential role for tension-related events, including AMPK activation, in the 57% contraction-stimulated increase in 3MGT that was inhibited by BTS and further suggest a possible role for TBC1D1 phosphorylation. Non-tension-related events (e.g., increased cytosolic Ca(2+) rather than increased AMPK and TBC1D1 phosphorylation) are implicated in the contraction-stimulated increase in 3MGT that persisted in the presence of BTS.

Regulation of glucose transporter 4 traffic by energy deprivation from mitochondrial compromise

Skeletal muscle is the major store and consumer of fatty acids and glucose. Glucose enters muscle through glucose transporter 4 (GLUT4). Upon insufficient oxygen availability or energy compromise, aerobic metabolism of glucose and fatty aids cannot proceed, and muscle cells rely on anaerobic metabolism of glucose to restore cellular energy status. An increase in glucose uptake into muscle is a key response to stimuli requiring rapid energy supply. This chapter analyses the mechanisms of the adaptive regulation of glucose transport that rescue muscle cells from mitochondrial uncoupling. Under these conditions, the initial drop in ATP recovers rapidly, through a compensatory increase in glucose uptake. This adaptive response involves AMPK activation by the initial ATP drop, which elevates cell surface GLUT4 and glucose uptake. The gain in surface GLUT4 involves different signals and routes of intracellular traffic compared with those engaged by insulin. The hormone increases GLUT4 exocytosis through phosphatidylinositol 3-kinase and Akt, whereas energy stress retards GLUT4 endocytosis through AMPK and calcium inputs. Given that energy stress is a component of muscle contraction, and that contraction activates AMPK and raises cytosolic calcium, we hypothesize that the increase in glucose uptake during contraction may also involve a reduction in GLUT4 endocytosis.

Ciliary neurotrophic factor stimulates muscle glucose uptake by a PI3-kinase-dependent pathway that is impaired with obesity

OBJECTIVE

Ciliary neurotrophic factor (CNTF) reverses muscle insulin resistance by increasing fatty acid oxidation through gp130-LIF receptor signaling to the AMP-activated protein kinase (AMPK). CNTF also increases Akt signaling in neurons and adipocytes. Because both Akt and AMPK regulate glucose uptake, we investigated muscle glucose uptake in response to CNTF signaling in lean and obese mice.

RESEARCH DESIGN AND METHODS

Mice were injected intraperitoneally with saline or CNTF, and blood glucose was monitored. The effects of CNTF on skeletal muscle glucose uptake and AMPK/Akt signaling were investigated in incubated soleus and extensor digitorum longus (EDL) muscles from muscle-specific AMPKalpha2 kinase-dead, gp130(DeltaSTAT), and lean and obese ob/ob and high-fat-fed mice. The effect of C2-ceramide on glucose uptake and gp130 signaling was also examined.

RESULTS

CNTF reduced blood glucose and increased glucose uptake in isolated muscles in a time- and dose-dependent manner with maximal effects after 30 min with 100 ng/ml. CNTF increased Akt-S473 phosphorylation in soleus and EDL; however, AMPK-T172 phosphorylation was only increased in soleus. Incubation of muscles from AMPK kinase dead (KD) and wild-type littermates with the PI3-kinase inhibitor LY-294002 demonstrated that PI3-kinase, but not AMPK, was essential for CNTF-stimulated glucose uptake. CNTF-stimulated glucose uptake and Akt phosphorylation were substantially reduced in obesity (high-fat diet and ob/ob) despite normal induction of gp130/AMPK signaling--effects also observed when treating myotubes with C2-ceramide.

CONCLUSIONS

CNTF acutely increases muscle glucose uptake by a mechanism involving the PI3-kinase/Akt pathway that does not require AMPK. CNTF-stimulated glucose uptake is impaired in obesity-induced insulin resistance and by ceramide.

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.

Estradiol stimulates Akt, AMP-activated protein kinase (AMPK) and TBC1D1/4, but not glucose uptake in rat soleus

Post-menopausal women exhibit decreases in circulating estrogen levels and whole body insulin sensitivity, suggesting that estrogen regulates skeletal muscle glucose disposal. Thus, we assessed whether estrogen stimulates glucose uptake or enhances insulin sensitivity in skeletal muscle. Ex vivo muscle stimulation with 17beta-estradiol (10 nM) resulted in a rapid (10 min) increase in the phosphorylation of Akt, AMP-activated protein kinase (AMPK), and TBC1D1/4, key signaling proteins that regulate glucose uptake in muscle. Treatment with the estrogen receptor antagonist, ICI 182,780, only partly inhibited signaling, suggesting both an estrogen receptor-dependent and independent mechanism of estradiol action. 17beta-Estradiol did not stimulate ex vivo muscle [(3)H]-2-deoxyglucose uptake or enhance insulin-induced glucose uptake, demonstrating discordance between the estradiol-induced stimulation of signaling proteins and muscle glucose uptake. This study is the first to demonstrate that estradiol stimulates Akt, AMPK, and TBC1D1/4 in intact skeletal muscle, but surprisingly, estradiol does not stimulate muscle glucose uptake.

Variation in the ovarian reserve is linked to alterations in intrafollicular estradiol production and ovarian biomarkers of follicular differentiation and oocyte quality in cattle

The mechanisms whereby the high variation in numbers of morphologically healthy oocytes and follicles in ovaries (ovarian reserve) may have an impact onovarian function, oocyte quality, and fertility are poorly understood. The objective was to determine whether previously validated biomarkers for follicular differentiation and function, as well as oocyte quality differed between cattle with low versus a high antral follicle count (AFC). Ovaries were removed (n = 5 per group) near the beginning of the nonovulatory follicular wave, before follicles could be identified via ultrasonography as being dominant, from heifers with high versus a low AFC. The F1, F2, and F3 follicles were dissected and diameters determined. Follicular fluid and thecal, granulosal, and cumulus cells and the oocyte were isolated and subjected to biomarker analyses. Although the size and numerous biomarkers of differentiation, such as mRNAs for the gonadotropin receptors, were similar, intrafollicular concentrations of estradiol and the abundance of mRNAs for CYP19A1 in granulosal cells and ESR1, ESR2, and CTSB in cumulus cells were greater, whereas mRNAs for AMH in granulosal cells and TBC1D1 in thecal cells were lower for animals with low versus a high AFC during follicle waves. Hence, variation in the ovarian reserve may have an impact on follicular function and oocyte quality via alterations in intrafollicular estradiol production and expression of key genes involved in follicle-stimulating hormone action (AMH) and estradiol (CYP19A1) production by granulosal cells, function and survival of thecal cells (TBC1D1), responsiveness of cumulus cells to estradiol (ESR1, ESR2), and cumulus cell determinants of oocyte quality (CTSB).

Intracellular retention and insulin-stimulated mobilization of GLUT4 glucose transporters

GLUT4 glucose transporters are expressed nearly exclusively in adipose and muscle cells, where they cycle to and from the plasma membrane. In cells not stimulated with insulin, GLUT4 is targeted to specialized GLUT4 storage vesicles (GSVs), which sequester it away from the cell surface. Insulin acts within minutes to mobilize these vesicles, translocating GLUT4 to the plasma membrane to enhance glucose uptake. The mechanisms controlling GSV sequestration and mobilization are poorly understood. An insulin-regulated aminopeptidase that cotraffics with GLUT4, IRAP, is required for basal GSV retention and insulin-stimulated mobilization. TUG and Ubc9 bind GLUT4, and likely retain GSVs within unstimulated cells. These proteins may be components of a retention receptor, which sequesters GLUT4 and IRAP away from recycling vesicles. Insulin may then act on this protein complex to liberate GLUT4 and IRAP, discharging GSVs into a recycling pathway for fusion at the cell surface. How GSVs are anchored intracellularly, and how insulin mobilizes these vesicles, are the important topics for ongoing research. Regulation of GLUT4 trafficking is tissue-specific, perhaps in part because the formation of GSVs requires cell type-specific expression of sortilin. Proteins controlling GSV retention and mobilization can then be more widely expressed. Indeed, GLUT4 likely participates in a general mechanism by which the cell surface delivery of various membrane proteins can be controlled by extracellular stimuli. Finally, it is not known if defects in the formation or intracellular retention of GSVs contribute to human insulin resistance, or play a role in the pathogenesis of type 2 diabetes.

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.

Contractile C2C12 myotube model for studying exercise-inducible responses in skeletal muscle

Adequate exercise leads to a vast variety of physiological changes in skeletal muscle as well as other tissues/organs and is also responsible for maintaining healthy muscle displaying enhanced insulin-responsive glucose uptake via GLUT4 translocation. We generated highly developed contractile C(2)C(12) myotubes by manipulating intracellular Ca(2+) transients with electric pulse stimulation (EPS) that is endowed with properties similar to those of in vivo skeletal muscle in terms of 1) excitation-induced contractile activity as a result of de novo sarcomere formation, 2) activation of both the AMP kinase and stress-activated MAP kinase cascades, and 3) improved insulin responsiveness as assessed by GLUT4 recycling. Tbc1d1, a Rab-GAP implicated in exercise-induced GLUT4 translocation in skeletal muscle, also appeared to be phosphorylated on Ser(231) after EPS-induced contraction. In addition, a switch in myosin heavy-chain (MHC) expression from "fast type" to "slow type" was observed in the C(2)C(12) myotubes endowed with EPS-induced repetitive contractility. Taking advantage of these highly developed contractile C(2)C(12) myotubes, we identified myotube-derived factors responsive to EPS-evoked contraction, including the CXC chemokines CXCL1/KC and CXCL5/LIX, as well as IL-6, previously reported to be upregulated in contracting muscles in vivo. Importantly, animal treadmill experiments revealed that exercise significantly increased systemic levels of CXCL1/KC, perhaps derived from contracting muscle. Taken together, these results confirm that we have established a specialized muscle cell culture model allowing contraction-inducible cellular responses to be explored. Utilizing this model, we identified contraction-inducible myokines potentially linked to the metabolic alterations, immune responses, and angiogenesis induced by exercise.

Resistance exercise increases human skeletal muscle AS160/TBC1D4 phosphorylation in association with enhanced leg glucose uptake during postexercise recovery

Akt substrate of 160 kDa (AS160/TBC1D4) is associated with insulin and contraction-mediated glucose uptake. Human skeletal muscle AS160 phosphorylation is increased during aerobic exercise but not immediately following resistance exercise. It is not known whether AS160 phosphorylation is altered during recovery from resistance exercise. Therefore, we hypothesized that muscle AS160/TBC1D4 phosphorylation and glucose uptake across the leg would be increased during recovery following resistance exercise. We studied 9 male subjects before, during, and for 2 h of postexercise recovery. We utilized femoral catheterizations and muscle biopsies in combination with indirect calorimetry and immunoblotting to determine whole body glucose and fat oxidation, leg glucose uptake, muscle AMPKalpha2 activity, and the phosphorylation of muscle Akt and AS160/TBC1D4. Glucose oxidation was reduced while fat oxidation increased ( approximately 35%) during postexercise recovery (P <or= 0.05). Glucose uptake increased during exercise and postexercise recovery (P <or= 0.05). Akt phosphorylation was increased at 1 h and AMPKalpha2 activity increased at 2 h postexercise (P <or= 0.05). Phospho(Ser/Thr)-Akt substrate (PAS) phosphorylation (often used as a marker for AS160) was unchanged immediately postexercise and increased at 1 h (P <or= 0.05) and 2 h postexercise (P = 0.07). The PAS antibody is not always specific for AS160/TBC1D4 and can detect proteins at a similar molecular weight. Therefore, we immunoprecipitated AS160/TBC1D4 and then blotted with the PAS antibody, which confirmed that PAS phosphorylation is occurring on AS160/TBC1D4. There was also a positive correlation between PAS phosphorylation and leg glucose uptake during recovery (P < 0.05). We conclude that resistance exercise increases AS160/TBC1D4 phosphorylation in association with an increase in leg glucose uptake during postexercise recovery.

Contraction-stimulated glucose transport in rat skeletal muscle is sustained despite reversal of increased PAS-phosphorylation of AS160 and TBC1D1

Akt substrate of 160 kDa (AS160), the most distal insulin signaling protein known to be important for insulin-stimulated glucose transport, becomes phosphorylated with skeletal muscle contraction. Akt, AMP-activated protein kinase (AMPK), and Ca(2+)/calmodulin-dependent kinase II (CaMKII) have been implicated in regulating AS160 and/or glucose transport. Our primary aim was to assess time courses for contraction's effects on glucose transport and phosphorylation of Akt, AMPK, CaMKII, and AS160. Isolated rat epitrochlearis muscles were studied without or with contraction (5, 10, 20, 40, 60 min). Phospho-Akt substrate (PAS) antibody was used to measure AS160 PAS phosphorylation by quantifying the approximately 160-kDa band on PAS immunoblots (PAS-160); a separate band at 150 kDa (PAS-150) that responded similarly to contraction was also identified. Using specific antibodies for AS160 or TBC1D1 on immunoblots, the molecular mass of PAS-160 was found to correspond with that of AS160 and not TBC1D1, whereas PAS-150 corresponded with TBC1D1 and not AS160. Furthermore, supernatant of sample immunodepleted with anti-AS160 had greatly reduced PAS-160, whereas supernatant of sample immunodepleted with anti-TBC1D1 had greatly reduced PAS-150, providing further evidence that PAS-160 and PAS-150 correspond with PAS-AS160 and PAS-TBC1D1, respectively. Contraction induced transient increases in PAS-160, PAS-150, phospho-glycogen synthase kinase 3 (an Akt substrate) and phospho-CaMKII; glucose transport and phospho-AMPK increases were maintained for 60 min of contraction. These data suggest the following: 1) PAS-160 (AS160) and PAS-150 (TBC1D1) respond to contraction transiently, despite sustained stimulation; 2) continual AMPK activation was insufficient for sustained increase in PAS-160 or PAS-150; and 3) sustained elevation of PAS-160 or PAS-150 was unnecessary to maintain contraction-stimulated glucose transport for up to 60 min.

Muscle cells engage Rab8A and myosin Vb in insulin-dependent GLUT4 translocation

Insulin causes translocation of glucose transporter 4 (GLUT4) to the membrane of muscle and fat cells, a process requiring Akt activation. Two Rab-GTPase-activating proteins (Rab-GAP), AS160 and TBC1D1, were identified as Akt substrates. AS160 phosphorylation is required for insulin-stimulated GLUT4 translocation, but the participation of TBC1D1 on muscle cell GLUT4 is unknown. Moreover, there is controversy as to the AS160/TBC1D1 target Rabs in fat and muscle cells, and Rab effectors are unknown. Here we examined the effect of knockdown of AS160, TBC1D1, and Rabs 8A, 8B, 10, and 14 (in vitro substrates of AS160 and TBC1D1 Rab-GAP activities) on insulin-induced GLUT4 translocation in L6 muscle cells. Silencing AS160 or TBC1D1 increased surface GLUT4 in unstimulated cells but did not prevent insulin-induced GLUT4 translocation. Knockdown of Rab8A and Rab14, but not of Rab8B or Rab10, inhibited insulin-induced GLUT4 translocation. Furthermore, silencing Rab8A or Rab14 but not Rab8B or Rab10 restored the basal-state intracellular retention of GLUT4 impaired by AS160 or TBC1D1 knockdown. Lastly, overexpression of a fragment of myosin Vb, a recently identified Rab8A-interacting protein, inhibited insulin-induced GLUT4 translocation and altered the subcellular distribution of GTP-loaded Rab8A. These results support a model whereby AS160, Rab8A, and myosin Vb are required for insulin-induced GLUT4 translocation in muscle cells, potentially as part of a linear signaling cascade.

Emerging role for AS160/TBC1D4 and TBC1D1 in the regulation of GLUT4 traffic

Vesicular traffic of the glucose transporter GLUT4 occurs in response to insulin, muscle contraction, and metabolic stimuli that lead to changes in the energy status of the cell. These stimuli are associated with linked kinase cascades that lead to changes in glucose uptake that meet the energy challenges imposed on the highly regulated cell types in insulin-responsive tissues. The need to mechanistically link these kinase-associated stimuli to identifiable intermediates in vesicular traffic has long been known but has been difficult to fulfill. The Rab-GTPase-activating proteins AS160 and TBC1D1 have now emerged as strong candidates to fill this void. Here we review the initial discovery of these proteins as phosphorylated substrates for Akt and the more recent emerging data that indicate that these proteins are substrates for additional kinases that are downstream of contraction and energy status signaling. The mechanism of coupling these phosphorylated proteins to vesicle traffic appears to be dependent on linking to small GTPase of the Rab family. We examine the current state of a hypothesis that suggests that phosphorylation of the Rab-GTPase-activating proteins leads to increased GTP loading of Rab proteins on GLUT4 vesicles and subsequently to increased interaction with Rab effectors that control GLUT4 vesicle translocation.

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.