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

Human muscle fiber type-specific insulin signaling: impact of obesity and type 2 diabetes

Skeletal muscle is a heterogeneous tissue composed of different fiber types. Studies suggest that insulin-mediated glucose metabolism is different between muscle fiber types. We hypothesized that differences are due to fiber type-specific expression/regulation of insulin signaling elements and/or metabolic enzymes. Pools of type I and II fibers were prepared from biopsies of the vastus lateralis muscles from lean, obese, and type 2 diabetic subjects before and after a hyperinsulinemic-euglycemic clamp. Type I fibers compared with type II fibers have higher protein levels of the insulin receptor, GLUT4, hexokinase II, glycogen synthase (GS), and pyruvate dehydrogenase-E1α (PDH-E1α) and a lower protein content of Akt2, TBC1 domain family member 4 (TBC1D4), and TBC1D1. In type I fibers compared with type II fibers, the phosphorylation response to insulin was similar (TBC1D4, TBC1D1, and GS) or decreased (Akt and PDH-E1α). Phosphorylation responses to insulin adjusted for protein level were not different between fiber types. Independently of fiber type, insulin signaling was similar (TBC1D1, GS, and PDH-E1α) or decreased (Akt and TBC1D4) in muscle from patients with type 2 diabetes compared with lean and obese subjects. We conclude that human type I muscle fibers compared with type II fibers have a higher glucose-handling capacity but a similar sensitivity for phosphoregulation by insulin.

Roles of TBC1D1 and TBC1D4 in insulin- and exercise-stimulated glucose transport of skeletal muscle

This review focuses on two paralogue Rab GTPase activating proteins known as TBC1D1 Tre-2/BUB2/cdc 1 domain family (TBC1D) 1 and TBC1D4 (also called Akt Substrate of 160 kDa, AS160) and their roles in controlling skeletal muscle glucose transport in response to the independent and combined effects of insulin and exercise. Convincing evidence implicates Akt2-dependent TBC1D4 phosphorylation on T642 as a key part of the mechanism for insulin-stimulated glucose uptake by skeletal muscle. TBC1D1 phosphorylation on several insulin-responsive sites (including T596, a site corresponding to T642 in TBC1D4) does not appear to be essential for in vivo insulin-stimulated glucose uptake by skeletal muscle. In vivo exercise or ex vivo contraction of muscle result in greater TBC1D1 phosphorylation on S237 that is likely to be secondary to increased AMP-activated protein kinase activity and potentially important for contraction-stimulated glucose uptake. Several studies that evaluated both normal and insulin-resistant skeletal muscle stimulated with a physiological insulin concentration after a single exercise session found that greater post-exercise insulin-stimulated glucose uptake was accompanied by greater TBC1D4 phosphorylation on several sites. In contrast, enhanced post-exercise insulin sensitivity was not accompanied by greater insulin-stimulated TBC1D1 phosphorylation. The mechanism for greater TBC1D4 phosphorylation in insulin-stimulated muscles after acute exercise is uncertain, and a causal link between enhanced TBC1D4 phosphorylation and increased post-exercise insulin sensitivity has yet to be established. In summary, TBC1D1 and TBC1D4 have important, but distinct roles in regulating muscle glucose transport in response to insulin and exercise.

Increased postexercise insulin sensitivity is accompanied by increased AS160 phosphorylation in slow-twitch soleus muscle

A single bout of exercise can enhance insulin‐stimulated glucose uptake in both fast‐twitch (type II) and slow‐twitch (type I) skeletal muscle for several hours postexercise. Akt substrate of 160 kDa (AS160) is most distal insulin signaling proteins that have been proposed to contribute to the postexercise enhancement of insulin action in fast‐twitch muscle. In this study, we examined whether the postexercise increase in insulin action of glucose uptake in slow‐twitch muscle is accompanied by increased phosphorylation of AS160 and its paralog TBC1D1. Male Wistar rats (~1‐month‐old) were exercised on a treadmill for 180 min (9 m/min). Insulin (50 μU/mL)‐stimulated glucose uptake was increased at 2 h after cessation of exercise in soleus muscle composed of predominantly slow‐twitch fibers. This postexercise increase in insulin action of glucose uptake was accompanied by increased phosphorylation of AS160 (detected by phospho‐Thr642 and phospho‐Ser588 antibody). On the other hand, prior exercise did not increase phosphorylation of TBC1D1 (detected by phospho‐Thr590) at 2 h postexercise. These results suggest the possibility that an enhancement in AS160 phosphorylation but not TBC1D1 phosphorylation is involved with increased postexercise insulin action of glucose uptake in slow‐twitch muscle.

In slow‐twitch soleus muscle, phosphorylation of AS160 Thr642 and Ser588 was increased together with the enhanced insulin action of the glucose uptake at 2 h postexercise. The phosphosite of TBC1D1 (Thr590), which is possibly important for insulin‐stimulated glucose uptake, did not increase phosphorylation at 2 h postexercise. These results suggest that the increased phosphorylation of AS160, but not TBC1D1, can account for the postexercise enhancement in the insulin action of the glucose uptake in slow‐twitch muscle.

AMP-activated protein kinase: a key regulator of energy balance with many roles in human disease

The AMP-activated protein kinase (AMPK) is a sensor of cellular energy status that regulates cellular and whole-body energy balance. A recently reported crystal structure has illuminated the complex regulatory mechanisms by which AMP and ADP cause activation of AMPK, involving phosphorylation by the upstream kinase LKB1. Once activated by falling cellular energy status, AMPK activates catabolic pathways that generate ATP whilst inhibiting anabolic pathways and other cellular processes that consume ATP. A role of AMPK is implicated in many human diseases. Mutations in the γ2 subunit cause heart disease due to excessive glycogen storage in cardiac myocytes, leading to ventricular pre-excitation. AMPK-activating drugs reverse many of the metabolic defects associated with insulin resistance, and recent findings suggest that the insulin-sensitizing effects of the widely used antidiabetic drug metformin are mediated by AMPK. The upstream kinase LKB1 is a tumour suppressor, and AMPK may exert many of its antitumour effects. AMPK activation promotes the oxidative metabolism typical of quiescent cells, rather than the aerobic glycolysis observed in tumour cells and cells involved in inflammation, explaining in part why AMPK activators have both antitumour and anti-inflammatory effects. Salicylate (the major in vivo metabolite of aspirin) activates AMPK, and this could be responsible for at least some of the anticancer and anti-inflammatory effects of aspirin. In addition to metformin and salicylates, novel drugs that modulate AMPK are likely to enter clinical trials soon. Finally, AMPK may be involved in viral infection: downregulation of AMPK during hepatitis C virus infection appears to be essential for efficient viral replication.

Exercise and type 2 diabetes: molecular mechanisms regulating glucose uptake in skeletal muscle

Exercise is a well-established tool to prevent and combat type 2 diabetes. Exercise improves whole body metabolic health in people with type 2 diabetes, and adaptations to skeletal muscle are essential for this improvement. An acute bout of exercise increases skeletal muscle glucose uptake, while chronic exercise training improves mitochondrial function, increases mitochondrial biogenesis, and increases the expression of glucose transporter proteins and numerous metabolic genes. This review focuses on the molecular mechanisms that mediate the effects of exercise to increase glucose uptake in skeletal muscle.

The adaptor protein APPL2 inhibits insulin-stimulated glucose uptake by interacting with TBC1D1 in skeletal muscle

Insulin stimulates glucose uptake by promoting the trafficking of GLUT4 to the plasma membrane in muscle cells, and impairment of this insulin action contributes to hyperglycemia in type 2 diabetes. The adaptor protein APPL1 potentiates insulin-stimulated Akt activation and downstream actions. However, the physiological functions of APPL2, a close homolog of APPL1, in regulating glucose metabolism remain elusive. We show that insulin-evoked plasma membrane recruitment of GLUT4 and glucose uptake are impaired by APPL2 overexpression but enhanced by APPL2 knockdown. Likewise, conditional deletion of APPL2 in skeletal muscles enhances insulin sensitivity, leading to an improvement in glucose tolerance. We identified the Rab-GTPase-activating protein TBC1D1 as an interacting partner of APPL2. Insulin stimulates TBC1D1 phosphorylation on serine 235, leading to enhanced interaction with the BAR domain of APPL2, which in turn suppresses insulin-evoked TBC1D1 phosphorylation on threonine 596 in cultured myotubes and skeletal muscle. Substitution of serine 235 with alanine diminishes APPL2-mediated inhibition on insulin-dependent TBC1D1 phosphorylation on threonine 596 and the suppressive effects of TBC1D1 on insulin-induced glucose uptake and GLUT4 translocation to the plasma membrane in cultured myotubes. Therefore, the APPL2-TBC1D1 interaction is a key step to fine tune insulin-stimulated glucose uptake by regulating the membrane recruitment of GLUT4 in skeletal muscle.

TBC1D1 reduces palmitate oxidation by inhibiting β-HAD activity in skeletal muscle

In skeletal muscle the Rab-GTPase-activating protein TBC1D1 has been implicated in the regulation of fatty acid oxidation by an unknown mechanism. We determined whether TBC1D1 altered fatty acid utilization via changes in protein-mediated fatty acid transport and/or selected enzymes regulating mitochondrial fatty acid oxidation. We also determined the effects of TBC1D1 on glucose transport and oxidation. Electrotransfection of mouse soleus muscles with TBC1D1 cDNA increased TBC1D1 protein after 2 wk (P<0.05), without altering its paralog AS160. TBC1D1 overexpression decreased basal palmitate oxidation (-22%) while blunting 5-aminoimidazole-4-carboxamide ribonucleotide (AICAR)-stimulated palmitate oxidation (-18%). There was a tendency to increase fatty acid esterification (+10 nmol·g(-1)·60 min(-1), P=0.07), which reflected the reduction in fatty acid oxidation (-12 nmol·g(-1)·60 min(-1)). Concomitantly, basal (+21%) and AICAR-stimulated glucose oxidation (+8%) were increased in TBC1D1-transfected muscles relative to their respective controls (P<0.05), independent of changes in GLUT4 and glucose transport. The reductions in TBC1D1-mediated fatty acid oxidation could not be attributed to changes in the transporter FAT/CD36, muscle mitochondrial content, CPT1 expression or the expression and phosphorylation of AS160, acetyl-CoA carboxylase, or AMPK. However, TBC1D1 overexpression reduced β-HAD enzyme activity (-18%, P<0.05). In conclusion, TBC1D1-mediated reduction of muscle fatty acid oxidation appears to occur via inhibition of β-HAD activity.

A small-molecule benzimidazole derivative that potently activates AMPK to increase glucose transport in skeletal muscle: comparison with effects of contraction and other AMPK activators

AMPK (AMP-activated protein kinase) is an attractive therapeutic drug target for treating metabolic disorders. We studied the effects of an AMPK activator developed by Merck (ex229 from patent application WO2010036613), comparing chemical activation with contraction in intact incubated skeletal muscles. We also compared effects of ex229 with those of the Abbott A769662 compound and AICAR (5-amino-4-imidazolecarboxamide riboside). In rat epitrochlearis muscle, ex229 dose-dependently increased AMPK activity of α1-, α2-, β1- and β2-containing complexes with significant increases in AMPK activity seen at a concentration of 50 μM. At a concentration of 100 μM, AMPK activation was similar to that observed after contraction and importantly led to an ~2-fold increase in glucose uptake. In AMPK α1-/α2-catalytic subunit double-knockout myotubes incubated with ex229, the increases in glucose uptake and ACC (acetyl-CoA carboxylase) phosphorylation seen in control cells were completely abolished, suggesting that the effects of the compound were AMPK-dependent. When muscle glycogen levels were reduced by ~50% after starvation, ex229-induced AMPK activation and glucose uptake were amplified in a wortmannin-independent manner. In L6 myotubes incubated with ex229, fatty acid oxidation was increased. Furthermore, in mouse EDL (extensor digitorum longus) and soleus muscles, ex229 increased both AMPK activity and glucose uptake at least 2-fold. In summary, ex229 efficiently activated skeletal muscle AMPK and elicited metabolic effects in muscle appropriate for treating Type 2 diabetes by stimulating glucose uptake and increasing fatty acid oxidation.

Sustained AS160 and TBC1D1 phosphorylations in human skeletal muscle 30 min after a single bout of exercise

BACKGROUND

phosphorylation of AS160 and TBC1D1 plays an important role for GLUT4 mobilization to the cell surface. The phosphorylation of AS160 and TBC1D1 in humans in response to acute exercise is not fully characterized.

OBJECTIVE

to study AS160 and TBC1D1 phosphorylation in human skeletal muscle after aerobic exercise followed by a hyperinsulinemic euglycemic clamp.

DESIGN

eight healthy men were studied on two occasions: 1) in the resting state and 2) in the hours after a 1-h bout of ergometer cycling. A hyperinsulinemic euglycemic clamp was initiated 240 min after exercise and in a time-matched nonexercised control condition. We obtained muscle biopsies 30 min after exercise and in a time-matched nonexercised control condition (t = 30) and after 30 min of insulin stimulation (t = 270) and investigated site-specific phosphorylation of AS160 and TBC1D1.

RESULTS

phosphorylation on AS160 and TBC1D1 was increased 30 min after the exercise bout, whereas phosphorylation of the putative upstream kinases, Akt and AMPK, was unchanged compared with resting control condition. Exercise augmented insulin-stimulated phosphorylation on AS160 at Ser(341) and Ser(704) 270 min after exercise. No additional exercise effects were observed on insulin-stimulated phosphorylation of Thr(642) and Ser(588) on AS160 or Ser(237) and Thr(596) on TBC1D1.

CONCLUSIONS

AS160 and TBC1D1 phosphorylations were evident 30 min after exercise without simultaneously increased Akt and AMPK phosphorylation. Unlike TBC1D1, insulin-stimulated site-specific AS160 phosphorylation is modified by prior exercise, but these sites do not include Thr(642) and Ser(588). Together, these data provide new insights into phosphorylation of key regulators of glucose transport in human skeletal muscle.

A link between hepatic glucose production and peripheral energy metabolism via hepatokines

Type 2 diabetes is characterized by a deterioration of glucose tolerance, which associates insulin resistance of glucose uptake by peripheral tissues and increased endogenous glucose production. Here we report that the specific suppression of hepatic glucose production positively modulates whole-body glucose and energy metabolism. We used mice deficient in liver glucose-6 phosphatase that is mandatory for endogenous glucose production. When they were fed a high fat/high sucrose diet, they resisted the development of diabetes and obesity due to the activation of peripheral glucose metabolism and thermogenesis. This was linked to the secretion of hepatic hormones like fibroblast growth factor 21 and angiopoietin-like factor 6. Interestingly, the deletion of hepatic glucose-6 phosphatase in previously obese and insulin-resistant mice resulted in the rapid restoration of glucose and body weight controls. Therefore, hepatic glucose production is an essential lever for the control of whole-body energy metabolism during the development of obesity and diabetes.

Overexpression of TRB3 in muscle alters muscle fiber type and improves exercise capacity in mice

Increasing evidence suggests that TRB3, a mammalian homolog of Drosophila tribbles, plays an important role in cell growth, differentiation, and metabolism. In the liver, TRB3 binds and inhibits Akt activity, whereas in adipocytes, TRB3 upregulates fatty acid oxidation. In cultured muscle cells, TRB3 has been identified as a potential regulator of insulin signaling. However, little is known about the function and regulation of TRB3 in skeletal muscle in vivo. In the current study, we found that 4 wk of voluntary wheel running (6.6 ± 0.4 km/day) increased TRB3 mRNA by 1.6-fold and protein by 2.5-fold in the triceps muscle. Consistent with this finding, muscle-specific transgenic mice that overexpress TRB3 (TG) had a pronounced increase in exercise capacity compared with wild-type (WT) littermates (TG: 1,535 ± 283; WT: 644 ± 67 joules). The increase in exercise capacity in TRB3 TG mice was not associated with changes in glucose uptake or glycogen levels; however, these mice displayed a dramatic shift toward a more oxidative/fatigue-resistant (type I/IIA) muscle fiber type, including threefold more type I fibers in soleus muscles. Skeletal muscle from TRB3 TG mice had significantly decreased PPARα expression, twofold higher levels of miR208b and miR499, and corresponding increases in the myosin heavy chain isoforms Myh7 and Myb7b, which encode these microRNAs. These findings suggest that TRB3 regulates muscle fiber type via a peroxisome proliferator-activated receptor-α (PPAR-α)-regulated miR499/miR208b pathway, revealing a novel function for TRB3 in the regulation of skeletal muscle fiber type and exercise capacity.

Insulin signaling, resistance, and the metabolic syndrome: insights from mouse models into disease mechanisms

Insulin resistance is a major underlying mechanism responsible for the 'metabolic syndrome', which is also known as insulin resistance syndrome. The incidence of the metabolic syndrome is increasing at an alarming rate, becoming a major public and clinical problem worldwide. The metabolic syndrome is represented by a group of interrelated disorders, including obesity, hyperglycemia, hyperlipidemia, and hypertension. It is also a significant risk factor for cardiovascular disease and increased morbidity and mortality. Animal studies have demonstrated that insulin and its signaling cascade normally control cell growth, metabolism, and survival through the activation of MAPKs and activation of phosphatidylinositide-3-kinase (PI3K), in which the activation of PI3K associated with insulin receptor substrate 1 (IRS1) and IRS2 and subsequent Akt→Foxo1 phosphorylation cascade has a central role in the control of nutrient homeostasis and organ survival. The inactivation of Akt and activation of Foxo1, through the suppression IRS1 and IRS2 in different organs following hyperinsulinemia, metabolic inflammation, and overnutrition, may act as the underlying mechanisms for the metabolic syndrome in humans. Targeting the IRS→Akt→Foxo1 signaling cascade will probably provide a strategy for therapeutic intervention in the treatment of type 2 diabetes and its complications. This review discusses the basis of insulin signaling, insulin resistance in different mouse models, and how a deficiency of insulin signaling components in different organs contributes to the features of the metabolic syndrome. Emphasis is placed on the role of IRS1, IRS2, and associated signaling pathways that are coupled to Akt and the forkhead/winged helix transcription factor Foxo1.

Insulin receptor signaling in normal and insulin-resistant states

In the wake of the worldwide increase in type-2 diabetes, a major focus of research is understanding the signaling pathways impacting this disease. Insulin signaling regulates glucose, lipid, and energy homeostasis, predominantly via action on liver, skeletal muscle, and adipose tissue. Precise modulation of this pathway is vital for adaption as the individual moves from the fed to the fasted state. The positive and negative modulators acting on different steps of the signaling pathway, as well as the diversity of protein isoform interaction, ensure a proper and coordinated biological response to insulin in different tissues. Whereas genetic mutations are causes of rare and severe insulin resistance, obesity can lead to insulin resistance through a variety of mechanisms. Understanding these pathways is essential for development of new drugs to treat diabetes, metabolic syndrome, and their complications.

Constitutively active CaMKKα stimulates skeletal muscle glucose uptake in insulin-resistant mice in vivo

In insulin-sensitive skeletal muscle, the expression of constitutively active Ca(2+)/calmodulin-dependent protein kinase kinase α (caCaMKKα) stimulates glucose uptake independent of insulin signaling (i.e., Akt and Akt-dependent TBC1D1/TBC1D4 phosphorylation). Our objectives were to determine whether caCaMKKα could stimulate glucose uptake additively with insulin in insulin-sensitive muscle, in the basal state in insulin-resistant muscle, and if so, to determine whether the effects were associated with altered TBC1D1/TBC1D4 phosphorylation. Mice were fed a control or high-fat diet (60% kcal) for 12 weeks to induce insulin resistance. Muscles were transfected with empty vector or caCaMKKα plasmids using in vivo electroporation. After 2 weeks, caCaMKKα protein was robustly expressed. In insulin-sensitive muscle, caCaMKKα increased basal in vivo [(3)H]-2-deoxyglucose uptake approximately twofold, insulin increased glucose uptake approximately twofold, and caCaMKKα plus insulin increased glucose uptake approximately fourfold. caCaMKKα did not increase basal TBC1D1 (Ser(237), Thr(590), Ser(660), pan-Thr/Ser) or TBC1D4 (Ser(588), Thr(642), pan-Thr/Ser) phosphorylation. In insulin-resistant muscle, caCaMKKα increased basal glucose uptake approximately twofold, and attenuated high-fat diet-induced basal TBC1D1 (Thr(590), pan-Thr/Ser) and TBC1D4 (Ser(588), Thr(642), pan-Thr/Ser) phosphorylation. In cell-free assays, CaMKKα increased TBC1D1 (Thr(590), pan-Thr/Ser) and TBC1D4 (Ser(588), pan-Thr/Ser) phosphorylation. Collectively, these results demonstrate that caCaMKKα stimulates glucose uptake additively with insulin, and in insulin-resistant muscle, and alters the phosphorylation of TBC1D1/TBC1D4.

Cellular regulation of glucose uptake by glucose transporter GLUT4

GLUT4 is regulated by its intracellular localization. In the absence of insulin, GLUT4 is efficiently retained intracellularly within storage compartments in muscle and fat cells. Upon insulin stimulation (and contraction in muscle), GLUT4 translocates from these compartments to the cell surface where it transports glucose from the extracellular milieu into the cell. Its implication in insulin-regulated glucose uptake makes GLUT4 not only a key player in normal glucose homeostasis but also an important element in insulin resistance and type 2 diabetes. Nevertheless, how GLUT4 is retained intracellularly and how insulin acts on this retention mechanism is largely unclear. In this review, the current knowledge regarding the various molecular processes that govern GLUT4 physiology is discussed as well as the questions that remain.

Metabolic syndrome and insulin resistance: underlying causes and modification by exercise training

Metabolic syndrome (MS) is a collection of cardiometabolic risk factors that includes obesity, insulin resistance, hypertension, and dyslipidemia. Although there has been significant debate regarding the criteria and concept of the syndrome, this clustering of risk factors is unequivocally linked to an increased risk of developing type 2 diabetes and cardiovascular disease. Regardless of the true definition, based on current population estimates, nearly 100 million have MS. It is often characterized by insulin resistance, which some have suggested is a major underpinning link between physical inactivity and MS. The purpose of this review is to: (i) provide an overview of the history, causes and clinical aspects of MS, (ii) review the molecular mechanisms of insulin action and the causes of insulin resistance, and (iii) discuss the epidemiological and intervention data on the effects of exercise on MS and insulin sensitivity.

Acute exercise and physiological insulin induce distinct phosphorylation signatures on TBC1D1 and TBC1D4 proteins in human skeletal muscle

We investigated the phosphorylation signatures of two Rab-GTPase activating proteins TBC1D1 and TBC1D4 in human skeletal muscle in response to physical exercise and physiological insulin levels induced by a carbohydrate rich meal using a paired experimental design. Eight healthy male volunteers exercised in the fasted or fed state and muscle biopsies were taken before and immediately after exercise. We identified TBC1D1/4 phospho-sites that (1) did not respond to exercise or postprandial increase in insulin (TBC1D4: S666), (2) responded to insulin only (TBC1D4: S318), (3) responded to exercise only (TBC1D1: S237, S660, S700; TBC1D4: S588, S751), and (4) responded to both insulin and exercise (TBC1D1: T596; TBC1D4: S341, T642, S704). In the insulin-stimulated leg, Akt phosphorylation of both T308 and S473 correlated significantly with multiple sites on both TBC1D1 (T596) and TBC1D4 (S318, S341, S704). Interestingly, in the exercised leg in the fasted state TBC1D1 phosphorylation (S237, T596) correlated significantly with the activity of the α2/β2/γ3 AMPK trimer, whereas TBC1D4 phosphorylation (S341, S704) correlated with the activity of the α2/β2/γ1 AMPK trimer. Our data show differential phosphorylation of TBC1D1 and TBC1D4 in response to physiological stimuli in human skeletal muscle and support the idea that Akt and AMPK are upstream kinases. TBC1D1 phosphorylation signatures were comparable between in vitro contracted mouse skeletal muscle and exercised human muscle, and we show that AMPK regulated phosphorylation of these sites in mouse muscle. Contraction and exercise elicited a different phosphorylation pattern of TBC1D4 in mouse compared with human muscle, and although different circumstances in our experimental setup may contribute to this difference, the observation exemplifies that transferring findings between species is problematic.

Nonalcoholic fatty liver disease: molecular pathways and therapeutic strategies

Along with rising numbers of patients with metabolic syndrome, the prevalence of nonalcoholic fatty liver disease (NAFLD) has increased in proportion with the obesity epidemic. While there are no established treatments for NAFLD, current research is targeting new molecular mechanisms that underlie NAFLD and associated metabolic disorders. This review discusses some of these emerging molecular mechanisms and their therapeutic implications for the treatment of NAFLD. The basic research that has identified potential molecular targets for pharmacotherapy will be outlined.

Identification and Association of SNPs in TBC1D1 Gene with Growth Traits in Two Rabbit Breeds

The TBC1D1 plays a key role in body energy homeostasis by regulating the insulin-stimulated glucose uptake in skeletal muscle. The present study aimed to identify the association between genetic polymorphisms of TBC1D1 and body weight (BW) in rabbits. Among the total of 12 SNPs detected in all 20 exons, only one SNP was non-synonymous (c.214G>A. p.G72R) located in exon 1. c.214G>A was subsequently genotyped among 491 individuals from two rabbit breeds by the high-resolution melting method. Allele A was the predominant allele with frequencies of 0.7780 and 0.6678 in European white rabbit (EWR, n = 205) and New Zealand White rabbit (NZW, n = 286), respectively. The moderate polymorphism information content (0.250.05). Our results implied that the c.214G>A of TBC1D1 gene might be one of the candidate loci affecting the trait of 35 d BW in the rabbit.

Exercise, GLUT4, and skeletal muscle glucose uptake

Glucose is an important fuel for contracting muscle, and normal glucose metabolism is vital for health. Glucose enters the muscle cell via facilitated diffusion through the GLUT4 glucose transporter which translocates from intracellular storage depots to the plasma membrane and T-tubules upon muscle contraction. Here we discuss the current understanding of how exercise-induced muscle glucose uptake is regulated. We briefly discuss the role of glucose supply and metabolism and concentrate on GLUT4 translocation and the molecular signaling that sets this in motion during muscle contractions. Contraction-induced molecular signaling is complex and involves a variety of signaling molecules including AMPK, Ca(2+), and NOS in the proximal part of the signaling cascade as well as GTPases, Rab, and SNARE proteins and cytoskeletal components in the distal part. While acute regulation of muscle glucose uptake relies on GLUT4 translocation, glucose uptake also depends on muscle GLUT4 expression which is increased following exercise. AMPK and CaMKII are key signaling kinases that appear to regulate GLUT4 expression via the HDAC4/5-MEF2 axis and MEF2-GEF interactions resulting in nuclear export of HDAC4/5 in turn leading to histone hyperacetylation on the GLUT4 promoter and increased GLUT4 transcription. Exercise training is the most potent stimulus to increase skeletal muscle GLUT4 expression, an effect that may partly contribute to improved insulin action and glucose disposal and enhanced muscle glycogen storage following exercise training in health and disease.

Insulin stimulation regulates AS160 and TBC1D1 phosphorylation sites in human skeletal muscle

INTRODUCTION:

Individuals with obesity and type 2 diabetes (T2D) are typically insulin resistant, exhibiting impaired skeletal muscle glucose uptake. Animal and cell culture experiments have shown that site-specific phosphorylation of the Rab-GTPase-activating proteins AS160 and TBC1D1 is critical for GLUT4 translocation facilitating glucose uptake, but their regulation in human skeletal muscle is not well understood.

METHODS:

Here, lean, obese and T2D subjects underwent a euglycemic-hyperinsulinemic clamp, and vastus lateralis muscle biopsies were obtained before, and at 30 and 180 min post insulin infusion.

RESULTS:

Obese and T2D subjects had higher body mass indexes and fasting insulin concentrations, and T2D subjects showed insulin resistance. Consistent with the clamp findings, T2D subjects had impaired insulin-stimulated phosphorylation of AS160 Thr⁶⁴², a site previously shown to be important in glucose uptake in rodents. Interestingly, insulin-stimulated phosphorylation of TBC1D1 Thr⁵⁹⁰, a site shown to be regulated by insulin in rodents, was only increased in T2D subjects, although the functional significance of this difference is unknown.

CONCLUSION:

These data show that insulin differentially regulates AS160 and TBC1D1 phosphorylation in human skeletal muscle. Impaired insulin-stimulated glucose uptake in T2D subjects is accompanied by dysregulation of AS160 and TBC1D1 phosphorylation in skeletal muscle, suggesting that these proteins may regulate glucose uptake in humans.

Role of energy- and nutrient-sensing kinases AMP-activated protein kinase (AMPK) and mammalian target of rapamycin (mTOR) in adipocyte differentiation

Recent advances have demonstrated that the adipose tissue plays a central role in regulating overall energy balance. Obesity results from a chronic deregulation of energy balance, with energy intake exceeding energy expenditure. Recently, new mechanisms that control the obesity phenotype such as the equilibrium between white and brown adipose tissue function has been identified. In this context, it is becoming increasingly clear that in addition to cellular growth, AMP-activated protein kinase (AMPK) and mammalian target of rapamycin (mTOR) also regulate lipid metabolism and adipogenesis. Here, we review recent advances in the understanding of the molecular mechanisms involved in white and brown differentiation programs focusing on AMPK and mTOR signaling pathways, which may play differential roles in white adipose tissue and brown adipose tissue development. In view of the worldwide epidemic of obesity and its associated metabolic disorders such as insulin resistance and type 2 diabetes, targeting these kinases may represent a potential approach for reducing adiposity and improving obesity-related diseases. © 2013 IUBMB Life, 65(7):572-583, 2013.

Structural and population-based evaluations of TBC1D1 p.Arg125Trp

Obesity is now a leading cause of preventable death in the industrialised world. Understanding its genetic influences can enhance insight into molecular pathogenesis and potential therapeutic targets. A non-synonymous polymorphism (rs35859249, p.Arg125Trp) in the N-terminal TBC1D1 phosphotyrosine-binding (PTB) domain has shown a replicated association with familial obesity in women. We investigated these findings in the Avon Longitudinal Study of Parents and Children (ALSPAC), a large European birth cohort of mothers and offspring, and by generating a predicted model of the structure of this domain. Structural prediction involved the use of three separate algorithms; Robetta, HHpred/MODELLER and I-TASSER. We used the transmission disequilibrium test (TDT) to investigate familial association in the ALSPAC study cohort (N = 2,292 mother-offspring pairs). Linear regression models were used to examine the association of genotype with mean measurements of adiposity (Body Mass Index (BMI), waist circumference and Dual-energy X-ray absorptiometry (DXA) assessed fat mass), and logistic regression was used to examine the association with odds of obesity. Modelling showed that the R125W mutation occurs in a location of the TBC1D1 PTB domain that is predicted to have a function in a putative protein:protein interaction. We did not detect an association between R125W and BMI (mean per allele difference 0.27 kg/m² (95% Confidence Interval: 0.00, 0.53) P = 0.05) or obesity (odds ratio 1.01 (95% Confidence Interval: 0.77, 1.31, P = 0.96) in offspring after adjusting for multiple comparisons. Furthermore, there was no evidence to suggest that there was familial association between R125W and obesity (χ² = 0.06, P = 0.80). Our analysis suggests that R125W in TBC1D1 plays a role in the binding of an effector protein, but we find no evidence that the R125W variant is related to mean BMI or odds of obesity in a general population sample.

Electrical stimuli release ATP to increase GLUT4 translocation and glucose uptake via PI3Kγ-Akt-AS160 in skeletal muscle cells

Skeletal muscle glucose uptake in response to exercise is preserved in insulin-resistant conditions, but the signals involved are debated. ATP is released from skeletal muscle by contractile activity and can autocrinely signal through purinergic receptors, and we hypothesized it may influence glucose uptake. Electrical stimulation, ATP, and insulin each increased fluorescent 2-NBD-Glucose (2-NBDG) uptake in primary myotubes, but only electrical stimulation and ATP-dependent 2-NBDG uptake were inhibited by adenosine-phosphate phosphatase and by purinergic receptor blockade (suramin). Electrical stimulation transiently elevated extracellular ATP and caused Akt phosphorylation that was additive to insulin and inhibited by suramin. Exogenous ATP transiently activated Akt and, inhibiting phosphatidylinositol 3-kinase (PI3K) or Akt as well as dominant-negative Akt mutant, reduced ATP-dependent 2-NBDG uptake and Akt phosphorylation. ATP-dependent 2-NBDG uptake was also inhibited by the G protein βγ subunit-interacting peptide βark-ct and by the phosphatidylinositol 3-kinase-γ (PI3Kγ) inhibitor AS605240. ATP caused translocation of GLUT4myc-eGFP to the cell surface, mechanistically mediated by increased exocytosis involving AS160/Rab8A reduced by dominant-negative Akt or PI3Kγ kinase-dead mutants, and potentiated by myristoylated PI3Kγ. ATP stimulated 2-NBDG uptake in normal and insulin-resistant adult muscle fibers, resembling the reported effect of exercise. Hence, the ATP-induced pathway may be tapped to bypass insulin resistance.

Exploratory study on association of genetic variation in TBC1D1 with antipsychotic-induced weight gain

BACKGROUND

Previous studies have shown that antipsychotics with high propensity for antipsychotic-induced weight gain (AIWG) influence glucose transporter type 4 (GLUT4) mediated glucose intake. Variation in the gene encoding TBC1 domain family member 1 (TBC1D1), a Rab-GTPase activating protein regulating GLUT4 trafficking, has been associated with obesity. Therefore, we investigated the impact of TBC1D1 polymorphisms on AIWG.

METHODS

We analyzed rs9852 and rs35859249 in TBC1D1 in 195 schizophrenia subjects treated mostly with clozapine or olanzapine for up to 14 weeks. Association was tested using analysis of variance and analysis of covariance with change (%) from baseline weight as the dependent variable.

RESULTS

Analysis of covariance showed a non-significant trend for lower weight gain in carriers of the T-allele of rs9852 than in C-allele homozygotes (p = 0.063). This effect was more pronounced in the subgroup of patients treated with clozapine or olanzapine (p = 0.024). For rs35859249, no significant association with AIWG could be detected.

CONCLUSIONS

This is the first study examining the association between TBC1D1 and AIWG. The moderate association of rs9852, located in the 3'UTR near a miRNA binding site, indicates an influence of TBC1D1 on AIWG. Further investigations remain necessary to elucidate the role of this gene in AIWG.

Exercise-induced AMPK activity in skeletal muscle: role in glucose uptake and insulin sensitivity

The energy/fuel sensor 5'-AMP-activated protein kinase (AMPK) is viewed as a master regulator of cellular energy balance due to its many roles in glucose, lipid, and protein metabolism. In this review we focus on the regulation of AMPK activity in skeletal muscle and its involvement in glucose metabolism, including glucose transport and glycogen synthesis. In addition, we discuss the plausible interplay between AMPK and insulin signaling regulating these processes.

AMPK and Exercise: Glucose Uptake and Insulin Sensitivity

AMPK is an evolutionary conserved sensor of cellular energy status that is activated during exercise. Pharmacological activation of AMPK promotes glucose uptake, fatty acid oxidation, mitochondrial biogenesis, and insulin sensitivity; processes that are reduced in obesity and contribute to the development of insulin resistance. AMPK deficient mouse models have been used to provide direct genetic evidence either supporting or refuting a role for AMPK in regulating these processes. Exercise promotes glucose uptake by an insulin dependent mechanism involving AMPK. Exercise is important for improving insulin sensitivity; however, it is not known if AMPK is required for these improvements. Understanding how these metabolic processes are regulated is important for the development of new strategies that target obesity-induced insulin resistance. This review will discuss the involvement of AMPK in regulating skeletal muscle metabolism (glucose uptake, glycogen synthesis, and insulin sensitivity).

Electrical vs manual acupuncture stimulation in a rat model of polycystic ovary syndrome: different effects on muscle and fat tissue insulin signaling

In rats with dihydrotestosterone (DHT)-induced polycystic ovary syndrome (PCOS), repeated low-frequency electrical stimulation of acupuncture needles restores whole-body insulin sensitivity measured by euglycemic hyperinsulinemic clamp. We hypothesized that electrical stimulation causing muscle contractions and manual stimulation causing needle sensation have different effects on insulin sensitivity and related signaling pathways in skeletal muscle and adipose tissue, with electrical stimulation being more effective in DHT-induced PCOS rats. From age 70 days, rats received manual or low-frequency electrical stimulation of needles in abdominal and hind limb muscle five times/wk for 4–5 wks; controls were handled but untreated rats. Low-frequency electrical stimulation modified gene expression (decreased Tbc1d1 in soleus, increased Nr4a3 in mesenteric fat) and protein expression (increased pAS160/AS160, Nr4a3 and decreased GLUT4) by western blot and increased GLUT4 expression by immunohistochemistry in soleus muscle; glucose clearance during oral glucose tolerance tests was unaffected. Manual stimulation led to faster glucose clearance and modified mainly gene expression in mesenteric adipose tissue (increased Nr4a3, Mapk3/Erk, Adcy3, Gsk3b), but not protein expression to the same extent; however, Nr4a3 was reduced in soleus muscle. The novel finding is that electrical and manual muscle stimulation affect glucose homeostasis in DHT-induced PCOS rats through different mechanisms. Repeated electrical stimulation regulated key functional molecular pathways important for insulin sensitivity in soleus muscle and mesenteric adipose tissue to a larger extent than manual stimulation. Manual stimulation improved whole-body glucose tolerance, an effect not observed after electrical stimulation, but did not affect molecular signaling pathways to the same extent as electrical stimulation. Although more functional signaling pathways related to insulin sensitivity were affected by electrical stimulation, our findings suggest that manual stimulation of acupuncture needles has a greater effect on glucose tolerance. The underlying mechanism of the differential effects of the intermittent manual and the continuous electrical stimulation remains to be elucidated.

Regulatory mode shift of Tbc1d1 is required for acquisition of insulin-responsive GLUT4-trafficking activity

Tbc1d1 is involved in AICAR-dependent GLUT4 liberation. Tbc1d1 acquires temporal insulin responsiveness with AICAR pretreatment. This shift in regulatory mode requires Ser- 237 phosphorylation and the PTB1 domain. PTB1 mutants exhibit no shift in regulatory mode and thus no insulin responsiveness.

Tbc1d1 is key to skeletal muscle GLUT4 regulation. By using GLUT4 nanometry combined with a cell-based reconstitution model, we uncover a shift in the regulatory mode of Tbc1d1 by showing that Tbc1d1 temporally acquires insulin responsiveness, which triggers GLUT4 trafficking only after an exercise-mimetic stimulus such as aminoimidazole carboxamide ribonucleotide (AICAR) pretreatment. The functional acquisition of insulin responsiveness requires Ser-237 phosphorylation and an intact phosphotyrosine-binding (PTB) 1 domain. Mutations in PTB1, including R125W (a natural mutant), thus result in complete loss of insulin-responsiveness acquisition, whereas AICAR-responsive GLUT4-liberation activity remains intact. Thus our data provide novel insights into temporal acquisition/memorization of Tbc1d1 insulin responsiveness, relying on the PTB1 domain, possibly a key factor in the beneficial effects of exercise on muscle insulin potency.

Metabolic syndrome and insulin resistance: underlying causes and modification by exercise training

Metabolic syndrome (MS) is a collection of cardiometabolic risk factors that includes obesity, insulin resistance, hypertension, and dyslipidemia. Although there has been significant debate regarding the criteria and concept of the syndrome, this clustering of risk factors is unequivocally linked to an increased risk of developing type 2 diabetes and cardiovascular disease. Regardless of the true definition, based on current population estimates, nearly 100 million have MS. It is often characterized by insulin resistance, which some have suggested is a major underpinning link between physical inactivity and MS. The purpose of this review is to: (i) provide an overview of the history, causes and clinical aspects of MS, (ii) review the molecular mechanisms of insulin action and the causes of insulin resistance, and (iii) discuss the epidemiological and intervention data on the effects of exercise on MS and insulin sensitivity.

Greater filamin C, GSK3α, and GSK3β serine phosphorylation in insulin-stimulated isolated skeletal muscles of calorie restricted 24 month-old rats

Moderate calorie restriction (CR) can improve insulin-stimulated Akt phosphorylation and glucose uptake in muscles from 24 month-old rats, but the specific Akt substrates linking CR-effects on Akt to glucose uptake and other cellular processes are uncertain. We probed CR's influence on site-specific phosphorylation of five Akt substrates (AS160(Ser588), TBC1D1(Thr596), FLNc(Ser2213), GSK3α(Ser21), and GSK3β(Ser9)) in predominantly fast-twitch (epitrochlearis) and predominantly slow-twitch (soleus) muscles. We observed no CR-effect on phosphorylation of AS160(Ser588) or TBC1D1(Thr596), but there was a CR-induced increase in insulin-stimulated FLNc(Ser2213), GSK3α(Ser21), and GSK3β(Ser9) phosphorylation for both muscles. These results indicate that CR does not uniformly affect insulin-mediated phosphorylation of Akt substrates in fast- or slow-twitch muscles from 24 month-old rats.

Deletion of Rab GAP AS160 modifies glucose uptake and GLUT4 translocation in primary skeletal muscles and adipocytes and impairs glucose homeostasis

Tight control of glucose uptake in skeletal muscles and adipocytes is crucial to glucose homeostasis and is mediated by regulating glucose transporter GLUT4 subcellular distribution. In cultured cells, Rab GAP AS160 controls GLUT4 intracellular retention and release to the cell surface and consequently regulates glucose uptake into cells. To determine AS160 function in GLUT4 trafficking in primary skeletal muscles and adipocytes and investigate its role in glucose homeostasis, we characterized AS160 knockout (AS160(-/-)) mice. We observed increased and normal basal glucose uptake in isolated AS160(-/-) adipocytes and soleus, respectively, while insulin-stimulated glucose uptake was impaired and GLUT4 expression decreased in both. No such abnormalities were found in isolated AS160(-/-) extensor digitorum longus muscles. In plasma membranes isolated from AS160(-/-) adipose tissue and gastrocnemius/quadriceps, relative GLUT4 levels were increased under basal conditions and remained the same after insulin treatment. Concomitantly, relative levels of cell surface-exposed GLUT4, determined with a glucose transporter photoaffinity label, were increased in AS160(-/-) adipocytes and normal in AS160(-/-) soleus under basal conditions. Insulin augmented cell surface-exposed GLUT4 in both. These observations suggest that AS160 is essential for GLUT4 intracellular retention and regulation of glucose uptake in adipocytes and skeletal muscles in which it is normally expressed. In vivo studies revealed impaired insulin tolerance in the presence of normal (male) and impaired (female) glucose tolerance. Concurrently, insulin-elicited increases in glucose disposal were abolished in all AS160(-/-) skeletal muscles and liver but not in AS160(-/-) adipose tissues. This suggests AS160 as a target for differential manipulation of glucose homeostasis.

Regulation and dysregulation of glucose transport in cardiomyocytes

The ability of the heart muscle to derive energy from a wide variety of substrates provides the myocardium with remarkable capacity to adapt to the ever-changing metabolic environment depending on factors including nutritional state and physical activity. There is increasing evidence that loss of metabolic flexibility of the myocardium contributes to cardiac dysfunction in disease conditions such as diabetes, ischemic heart disease and heart failure. At the level of glucose metabolism reduced metabolic adaptation in most cases is characterized by impaired stimulation of transarcolemmal glucose transport in the cardiomyocytes in response to insulin, referred to as insulin resistance, or to other stimuli such as energy deficiency. This review discusses cellular mechanisms involved in the regulation of glucose uptake in cardiomyocytes and their potential implication in impairment of stimulation of glucose transport under disease conditions. This article is part of a Special Issue entitled: Cardiomyocyte Biology: Cardiac Pathways of Differentiation, Metabolism and Contraction.

The Rab-GTPase-activating protein TBC1D1 regulates skeletal muscle glucose metabolism

The Rab-GTPase-activating protein TBC1D1 has emerged as a novel candidate involved in metabolic regulation. Our aim was to determine whether TBC1D1 is involved in insulin as well as energy-sensing signals controlling skeletal muscle metabolism. TBC1D1-deficient congenic B6.SJL-Nob1.10 (Nob1.10(SJL)) and wild-type littermates were studied. Glucose and insulin tolerance, glucose utilization, hepatic glucose production, and tissue-specific insulin-mediated glucose uptake were determined. The effect of insulin, AICAR, or contraction on glucose transport was studied in isolated skeletal muscle. Glucose and insulin tolerance tests were normal in TBC1D1-deficient Nob1.10(SJL) mice, yet the 4-h-fasted insulin concentration was increased. Insulin-stimulated peripheral glucose utilization during a euglycemic hyperinsulinemic clamp was similar between genotypes, whereas the suppression of hepatic glucose production was increased in TBC1D1-deficient mice. In isolated extensor digitorum longus (EDL) but not soleus muscle, glucose transport in response to insulin, AICAR, or contraction was impaired by TBC1D1 deficiency. The reduction in glucose transport in EDL muscle from TBC1D1-deficient Nob1.10(SJL) mice may be explained partly by a 50% reduction in GLUT4 protein, since proximal signaling at the level of Akt, AMPK, and acetyl-CoA carboxylase (ACC) was unaltered. Paradoxically, in vivo insulin-stimulated 2-deoxyglucose uptake was increased in EDL and tibialis anterior muscle from TBC1D1-deficient mice. In conclusion, TBC1D1 plays a role in regulation of glucose metabolism in skeletal muscle. Moreover, functional TBC1D1 is required for AICAR- or contraction-induced metabolic responses, implicating a role in energy-sensing signals.

Thr649Ala-AS160 knock-in mutation does not impair contraction/AICAR-induced glucose transport in mouse muscle

AS160 and its closely related protein TBC1D1 have emerged as key mediators for both insulin- and contraction-stimulated muscle glucose uptake through regulating GLUT4 trafficking. Insulin increases AS160 phosphorylation at multiple Akt/PKB consensus sites, including Thr(649), and promotes its binding to 14-3-3 proteins through phospho-Thr(649). We recently provided genetic evidence that AS160-Thr(649) phosphorylation/14-3-3 binding plays a key role in mediating insulin-stimulated glucose uptake in muscle. Contraction has also been proposed to increase phosphorylation of AS160 and TBC1D1 via AMPK, which could be detected by a generic phospho-Akt substrate (PAS) antibody. Here, analysis of AS160 immunoprecipitates from muscle extracts with site-specific phospho-antibodies revealed that contraction and AICAR caused no increase but rather a slight decrease in phosphorylation of the major PAS recognition site AS160-Thr(649). In line with this, contraction failed to enhance 14-3-3 binding to AS160. Consistent with previous reports, we also observed that in situ contraction stimulated the signal intensity of PAS antibody immunoreactive protein of ∼150-160 kDa in muscle extracts. Using a TBC1D1 deletion mutant mouse, we showed that TBC1D1 protein accounted for the majority of the PAS antibody immunoreactive signals of ∼150-160 kDa in extracts of contracted muscles. Consistent with the proposed role of AS160-Thr(649) phosphorylation/14-3-3 binding in mediating glucose uptake, AS160-Thr(649)Ala knock-in mice displayed normal glucose uptake upon contraction and AICAR in isolated muscles. We conclude that the previously reported PAS antibody immunoreactive band ∼150-160 kDa, which were increased upon contraction, does not represent AS160 but TBC1D1, and that AS160-Thr(649)Ala substitution impairs insulin- but neither contraction- nor AICAR-stimulated glucose uptake in mouse skeletal muscle.

AMPK enhances insulin-stimulated GLUT4 regulation via lowering membrane cholesterol

AMP-activated protein kinase (AMPK) enhances glucose transporter GLUT4 regulation. AMPK also suppresses energy-consuming pathways such as cholesterol synthesis. Interestingly, recent in vitro and in vivo data suggest that excess membrane cholesterol impairs GLUT4 regulation. Therefore, this study tested whether a beneficial, GLUT4-regulatory aspect of AMPK stimulation involved cholesterol lowering. Using L6 myotubes stably expressing an exofacial myc-epitope-tagged-GLUT4, AMPK stimulation by 5-aminoimidazole-4-carboxamide-1-β-d-ribonucleoside (AICAR; 45 min, 1 mm) or 2,4-dinitrophenol (DNP; 30 min, 200 μm) increased cell surface GLUT4myc labeling by approximately ≈ 25% (P < 0.05). Insulin (20 min, 100 nm) also increased GLUT4myc labeling by about 50% (P < 0.05), which was further enhanced (≈ 25%, P < 0.05) by AICAR or DNP. Consistent with AMPK-mediated suppression of cholesterol synthesis, AICAR and DNP decreased membrane cholesterol by 20-25% (P < 0.05). Whereas AMPK knockdown prevented the enhanced basal and insulin-stimulated GLUT4myc labeling by AICAR and DNP, cholesterol replenishment only blocked the AMPK-associated enhancement in insulin action. Cells cultured in a hyperinsulinemic milieu, resembling conditions in vivo that promote the progression/worsening of insulin resistance, displayed an increase in membrane cholesterol. This occurred concomitantly with a loss of cortical filamentous actin (F-actin) and defects in GLUT4 regulation by insulin. These derangements were prevented by AMPK stimulation. Examination of skeletal muscle from insulin-resistant Zucker rats revealed a similar elevation in membrane cholesterol and loss of F-actin. Lowering cholesterol to control levels restored F-actin structure and insulin sensitivity. In conclusion, these data suggest a novel aspect of GLUT4 regulation by AMPK involves membrane cholesterol lowering. Moreover, this AMPK-mediated process protected against hyperinsulinemia-induced insulin resistance.

Regulation of glucose transporter translocation in health and diabetes

To enhance glucose uptake into muscle and fat cells, insulin stimulates the translocation of GLUT4 glucose transporters from intracellular membranes to the cell surface. This response requires the intersection of insulin signaling and vesicle trafficking pathways, and it is compromised in the setting of overnutrition to cause insulin resistance. Insulin signals through AS160/Tbc1D4 and Tbc1D1 to modulate Rab GTPases and through the Rho GTPase TC10α to act on other targets. In unstimulated cells, GLUT4 is incorporated into specialized storage vesicles containing IRAP, LRP1, sortilin, and VAMP2, which are sequestered by TUG, Ubc9, and other proteins. Insulin mobilizes these vesicles directly to the plasma membrane, and it modulates the trafficking itinerary so that cargo recycles from endosomes during ongoing insulin exposure. Knowledge of how signaling and trafficking pathways are coordinated will be essential to understanding the pathogenesis of diabetes and the metabolic syndrome and may also inform a wide range of other physiologies.

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.

Leptin reduces the expression and increases the phosphorylation of the negative regulators of GLUT4 traffic TBC1D1 and TBC1D4 in muscle of ob/ob mice

Leptin improves insulin sensitivity in skeletal muscle. Our goal was to determine whether proteins controlling GLUT4 traffic are altered by leptin deficiency and in vivo leptin administration in skeletal muscle of wild type and ob/ob mice. Leptin-deficient ob/ob mice were divided in three groups: control, leptin-treated (1 mg/kg/d) and leptin pair-fed ob/ob mice. Microarray analysis revealed that 1,546 and 1,127 genes were regulated by leptin deficiency and leptin treatment, respectively. Among these, we identified 24 genes involved in intracellular vesicle-mediated transport in ob/ob mice. TBC1 domain family, member 1 (Tbc1d1), a negative regulator of GLUT4 translocation, was up-regulated (P = 0.001) in ob/ob mice as compared to wild types. Importantly, leptin treatment reduced the transcript levels of Tbc1d1 (P<0.001) and Tbc1d4 (P = 0.004) in the leptin-treated ob/ob as compared to pair-fed ob/ob animals. In addition, phosphorylation levels of TBC1D1 and TBC1D4 were enhanced in leptin-treated ob/ob as compared to control ob/ob (P = 0.015 and P = 0.023, respectively) and pair-fed ob/ob (P = 0.036 and P = 0.034, respectively) mice. Despite similar GLUT4 protein expression in wild type and ob/ob groups a different immunolocalization of this protein was evidenced in muscle sections. Leptin treatment increased GLUT4 immunoreactivity in gastrocnemius and extensor digitorum longus sections of leptin-treated ob/ob mice. Moreover, GLUT4 protein detected in immunoprecipitates from TBC1D4 was reduced by leptin replacement compared to control ob/ob (P = 0.013) and pair-fed ob/ob (P = 0.037) mice. Our findings suggest that leptin enhances the intracellular GLUT4 transport in skeletal muscle of ob/ob animals by reducing the expression and activity of the negative regulators of GLUT4 traffic TBC1D1 and TBC1D4.

Protein kinase D1 is essential for contraction-induced glucose uptake but is not involved in fatty acid uptake into cardiomyocytes

Increased contraction enhances substrate uptake into cardiomyocytes via translocation of the glucose transporter GLUT4 and the long chain fatty acid (LCFA) transporter CD36 from intracellular stores to the sarcolemma. Additionally, contraction activates the signaling enzymes AMP-activated protein kinase (AMPK) and protein kinase D1 (PKD1). Although AMPK has been implicated in contraction-induced GLUT4 and CD36 translocation in cardiomyocytes, the precise role of PKD1 in these processes is not known. To study this, we triggered contractions in cardiomyocytes by electric field stimulation (EFS). First, the role of PKD1 in GLUT4 and CD36 translocation was defined. In PKD1 siRNA-treated cardiomyocytes as well as cardiomyocytes from PKD1 knock-out mice, EFS-induced translocation of GLUT4, but not CD36, was abolished. In AMPK siRNA-treated cardiomyocytes and cardiomyocytes from AMPKα2 knock-out mice, both GLUT4 and CD36 translocation were abrogated. Hence, unlike AMPK, PKD1 is selectively involved in glucose uptake. Second, we analyzed upstream factors in PKD1 activation. Cardiomyocyte contractions enhanced reactive oxygen species (ROS) production. Using ROS scavengers, we found that PKD1 signaling and glucose uptake are more sensitive to changes in intracellular ROS than AMPK signaling or LCFA uptake. Furthermore, silencing of death-activated protein kinase (DAPK) abrogated EFS-induced GLUT4 but not CD36 translocation. Finally, possible links between PKD1 and AMPK signaling were investigated. PKD1 silencing did not affect AMPK activation. Reciprocally, AMPK silencing did not alter PKD1 activation. In conclusion, we present a novel contraction-induced ROS-DAPK-PKD1 pathway in cardiomyocytes. This pathway is activated separately from AMPK and mediates GLUT4 translocation/glucose uptake, but not CD36 translocation/LCFA uptake.

Clustering of GLUT4, TUG, and RUVBL2 protein levels correlate with myosin heavy chain isoform pattern in skeletal muscles, but AS160 and TBC1D1 levels do not

Skeletal muscle is a heterogeneous tissue. To further elucidate this heterogeneity, we probed relationships between myosin heavy chain (MHC) isoform composition and abundance of GLUT4 and four other proteins that are established or putative GLUT4 regulators [Akt substrate of 160 kDa (AS160), Tre-2/Bub2/Cdc 16-domain member 1 (TBC1D1), Tethering protein containing an UBX-domain for GLUT4 (TUG), and RuvB-like protein two (RUVBL2)] in 12 skeletal muscles or muscle regions from Wistar rats [adductor longus, extensor digitorum longus, epitrochlearis, gastrocnemius (mixed, red, and white), plantaris, soleus, tibialis anterior (red and white), tensor fasciae latae, and white vastus lateralis]. Key results were 1) significant differences found among the muscles (range of muscle expression values) for GLUT4 (2.5-fold), TUG (1.7-fold), RUVBL2 (2.0-fold), and TBC1D1 (2.7-fold), but not AS160; 2) significant positive correlations for pairs of proteins: GLUT4 vs. TUG (R = 0.699), GLUT4 vs. RUVBL2 (R = 0.613), TUG vs. RUVBL2 (R = 0.564), AS160 vs. TBC1D1 (R = 0.293), and AS160 vs. TUG (R = 0.246); 3) significant positive correlations for %MHC-I: GLUT4 (R = 0.460), TUG (R = 0.538), and RUVBL2 (R = 0.511); 4) significant positive correlations for %MHC-IIa: GLUT4 (R = 0.293) and RUVBL2 (R = 0.204); 5) significant negative correlations for %MHC-IIb vs. GLUT4 (R = -0.642), TUG (R = -0.626), and RUVBL2 (R = -0.692); and 6) neither AS160 nor TBC1D1 significantly correlated with MHC isoforms. In 12 rat muscles, GLUT4 abundance tracked with TUG and RUVBL2 and correlated with MHC isoform expression, but was unrelated to AS160 or TBC1D1. Our working hypothesis is that some of the mechanisms that regulate GLUT4 abundance in rat skeletal muscle also influence TUG and RUVBL2 abundance.

Exercise increases TBC1D1 phosphorylation in human skeletal muscle

Exercise and weight loss are cornerstones in the treatment and prevention of type 2 diabetes, and both interventions function to increase insulin sensitivity and glucose uptake into skeletal muscle. Studies in rodents demonstrate that the underlying mechanism for glucose uptake in muscle involves site-specific phosphorylation of the Rab-GTPase-activating proteins AS160 (TBC1D4) and TBC1D1. Multiple kinases, including Akt and AMPK, phosphorylate TBC1D1 and AS160 on distinct residues, regulating their activity and allowing for GLUT4 translocation. In contrast to extensive rodent-based studies, the regulation of AS160 and TBC1D1 in human skeletal muscle is not well understood. In this study, we determined the effects of dietary intervention and a single bout of exercise on TBC1D1 and AS160 site-specific phosphorylation in human skeletal muscle. Ten obese (BMI 33.4 ± 2.4, M-value 4.3 ± 0.5) subjects were studied at baseline and after a 2-wk dietary intervention. Muscle biopsies were obtained from the subjects in the resting (basal) state and immediately following a 30-min exercise bout (70% Vo(2 max)). Muscle lysates were analyzed for AMPK activity and Akt phosphorylation and for TBC1D1 and AS160 phosphorylation on known or putative AMPK and Akt sites as follows: AS160 Ser(711) (AMPK), TBC1D1 Ser(231) (AMPK), TBC1D1 Ser(660) (AMPK), TBC1D1 Ser(700) (AMPK), and TBC1D1 Thr(590) (Akt). The diet intervention that consisted of a major shift in the macronutrient composition resulted in a 4.2 ± 0.4 kg weight loss (P < 0.001) and a significant increase in insulin sensitivity (M value 5.6 ± 0.6), but surprisingly, there was no effect on expression or phosphorylation of any of the muscle-signaling proteins. Exercise increased muscle AMPKα2 activity but did not increase Akt phosphorylation. Exercise increased phosphorylation on AS160 Ser(711), TBC1D1 Ser(231), and TBC1D1 Ser(660) but had no effect on TBC1D1 Ser(700). Exercise did not increase TBC1D1 Thr(590) phosphorylation or TBC1D1/AS160 PAS phosphorylation, consistent with the lack of Akt activation. These data demonstrate that a single bout of exercise regulates TBC1D1 and AS160 phosphorylation on multiple sites in human skeletal muscle.

Current understanding of increased insulin sensitivity after exercise - emerging candidates

Exercise counteracts insulin resistance and improves glucose homeostasis in many ways. Apart from increasing muscle glucose uptake quickly, exercise also clearly increases muscle insulin sensitivity in the post-exercise period. This review will focus on the mechanisms responsible for this increased insulin sensitivity. It is believed that increased sarcolemmal content of the glucose transporter GLUT4 can explain the phenomenon to some extent. Surprisingly no improvement in the proximal insulin signalling pathway is observed at the level of the insulin receptor, IRS1, PI3K or Akt. Recently more distal signalling component in the insulin signalling pathway such as aPKC, Rac1, TBC1D4 and TBC1D1 have been described. These are all affected by both insulin and exercise which means that they are likely converging points in promoting GLUT4 translocation and therefore possible candidates for regulating insulin sensitivity after exercise. Whereas TBC1D1 does not appear to regulate insulin sensitivity after exercise, correlative evidence in contrast suggests TBC1D4 to be a relevant candidate. Little is known about aPKC and Rac1 in relation to insulin sensitivity after exercise. Besides mechanisms involved in signalling to GLUT4 translocation, factors influencing the trans-sarcolemmal glucose concentration gradient might also be important. With regard to the interstitial glucose concentration microvascular perfusion is particular relevant as correlative evidence supports a connection between insulin sensitivity and microvascular perfusion. Thus, there are new candidates at several levels which collectively might explain the phenomenon.

Beneficial effects of resistance exercise on glycemic control are not further improved by protein ingestion

Purpose

To investigate the mechanisms underpinning modifications in glucose homeostasis and insulin sensitivity 24 h after a bout of resistance exercise (RE) with or without protein ingestion.

Methods

Twenty-four healthy males were assigned to a control (CON; n = 8), exercise (EX; n = 8) or exercise plus protein condition (EX+PRO; n = 8). Muscle biopsy and blood samples were obtained at rest for all groups and immediately post-RE (75% 1RM, 8×10 repetitions of leg-press and extension exercise) for EX and EX+PRO only. At 24 h post-RE (or post-resting biopsy for CON), a further muscle biopsy was obtained. Participants then consumed an oral glucose load (OGTT) containing 2 g of [U-¹³C] glucose during an infusion of 6, 6-[²H₂] glucose. Blood samples were obtained every 10 min for 2 h to determine glucose kinetics. EX+PRO ingested an additional 25 g of intact whey protein with the OGTT. A final biopsy sample was obtained at the end of the OGTT.

Results

Fasted plasma glucose and insulin were similar for all groups and were not different immediately post- and 24 h post-RE. Following RE, muscle glycogen was 26±8 and 19±6% lower in EX and EX+PRO, respectively. During OGTT, plasma glucose AUC was lower for EX and EX+PRO (75.1±2.7 and 75.3±2.8 mmol·L⁻¹∶120 min, respectively) compared with CON (90.6±4.1 mmol·L⁻¹∶120 min). Plasma insulin response was 13±2 and 21±4% lower for EX and CON, respectively, compared with EX+PRO. Glucose disappearance from the circulation was ∼12% greater in EX and EX+PRO compared with CON. Basal 24 h post-RE and insulin-stimulated PAS-AS160/TBC1D4 phosphorylation was greater for EX and EX+PRO.

Conclusions

Prior RE improves glycemic control and insulin sensitivity through an increase in the rate at which glucose is disposed from the circulation. However, co-ingesting protein during a high-glucose load does not augment this response at 24 h post-exercise in healthy, insulin-sensitive individuals.

Metabolic functions of AMPK: aspects of structure and of natural mutations in the regulatory gamma subunits

AMP-activated protein kinase, AMPK, is widely accepted as the master regulator of energy levels within the cell. Responding quickly to changing energy demands, AMPK works to restore levels of ATP during times of cellular stress by promoting ATP producing catabolic pathways and inhibiting ATP consuming anabolic ones. As a heterotrimeric protein complex, AMPK's subunits each act in unique and crucial ways to control AMPK function and its localization within the cell. Research in the last decade has identified and begun to characterize the impact of naturally occurring mutations in the gamma regulatory subunits. Mutations in the γ2 subunit have implications for cardiac function and disease, while the R225W mutation in the γ3 subunit have implications for skeletal muscle fuel metabolism and resistance to fatigue. Research focused on structure-function aspects of AMPK regulatory subunits will lead to a better understanding of the roles of AMPK in health and disease.

Mapping insulin/GLUT4 circuitry

One of the most important metabolic actions of insulin is catalysing glucose uptake into skeletal muscle and adipose tissue. This is accomplished via activation of the phosphatidylinositol-3-kinase/Akt signalling pathway and subsequent translocation of GLUT4 from intracellular storage vesicles to the plasma membrane. As such, this represents an ideal system for studying the convergence of signal transduction and protein trafficking. The GLUT4 translocation process is complex, but can be dissected into at least four discrete trafficking steps. This raises the question as to which of these is the major regulated step in insulin-stimulated GLUT4 translocation. Numerous molecules have been reported to regulate GLUT4 trafficking. However, with the exception of TBC1D4, the molecular details of these distal signalling arms of the insulin signalling network and how they modify distinct steps of GLUT4 trafficking have not been established. We discuss the need to adopt a more global approach to expand and deepen our understanding of the molecular processes underpinning this system. Strategies that facilitate the generation of detailed models of the entire insulin signalling network will enable us to identify the critical nodes that control GLUT4 traffic and decipher emergent properties of the system that are not currently apparent.

Metabolism, migration and memory in cytotoxic T cells

The transcriptional and metabolic programmes that control CD8(+) T cells are regulated by a diverse network of serine/threonine kinases. The view has been that the kinases AKT and mammalian target of rapamycin (mTOR) control T cell metabolism. Here, we challenge this paradigm and discuss an alternative role for these kinases in CD8(+) T cells, namely to control cell migration. Another emerging concept is that AMP-activated protein kinase (AMPK) family members control T cell metabolism and determine the effector versus memory fate of CD8(+) T cells. We speculate that one link between metabolism and immunological memory is provided by kinases that originally evolved to control T cell metabolism and have subsequently acquired the ability to control the expression of key transcription factors that regulate CD8(+) T cell effector function and migratory capacity.

Effect of exercise intensity and AICAR on isoform-specific expressions of murine skeletal muscle PGC-1α mRNA: a role of β₂-adrenergic receptor activation

There are three isoforms of peroxisome proliferator-activated receptor-γ coactivator 1α (PGC-1α) mRNA, which promotes mitochondrial biogenesis in skeletal muscles. Compared with PGC-1α-a mRNA, PGC-1α-b or PGC-1α-c mRNA is transcribed by a different exon 1 of the PGC-1α gene. In this study, effects of exercise intensity and 5-aminoimidazole-4-carboxamide-1β-d-ribofuranoside (AICAR) on isoform-specific expressions of PGC-1α were investigated. All isoforms were increased in proportion to exercise intensity of treadmill running (10-30 m/min for 30 min). Preinjection of β₂-adrenergic receptor (AR) antagonist (ICI 118551) inhibited the increase in PGC-1α-b and PGC-1α-c mRNAs, but not the increase in PGC-1α-a mRNA, in response to high-intensity exercise. Although high-intensity exercise activated α2-AMP-activated protein kinase (α2-AMPK) in skeletal muscles, inactivation of α2-AMPK activity did not affect high-intensity exercise-induced mRNA expression of all PGC-1α isoforms, suggesting that activation of α2-AMPK is not mandatory for an increase in PGC-1α mRNA by high-intensity exercise. A single injection in mice of AICAR, an AMPK activator, increased mRNAs of all PGC-1α isoforms. AICAR increased blood catecholamine concentrations, and preinjection of β₂-AR antagonist inhibited the increase in PGC-1α-b and PGC-1α-c mRNAs but not the increase in PGC-1α-a mRNA. Direct exposure of epitrochlearis muscle to AICAR increased PGC-1α-a but not the -b isoform. These data indicate that exercise-induced PGC-1α expression was dependent on the intensity of exercise. Exercise or AICAR injection increased PGC-1α-b and PGC-1α-c mRNAs via β₂-AR activation, whereas high-intensity exercise increased PGC-1α-a expression by a multiple mechanism in which α2-AMPK is one of the signaling pathways.