Distinct signals regulate AS160 phosphorylation in response to insulin, AICAR, and contraction in mouse skeletal muscle

Sex-specific increases in myostatin and SMAD3 contribute to obesity-related insulin resistance in human skeletal muscle and primary human myotubes

The purpose of the present study was to determine the effects of obesity and biological sex on myostatin expression in humans and to examine the direct effects of myostatin, SMAD2, and SMAD3 on insulin signaling in primary human skeletal muscle cells (HSkMCs). For cohort 1, 15 lean [body mass index (BMI): 22.1 ± 0.5 kg/m2; n = 8 males; n = 7 females] and 14 obese (BMI: 40.6 ± 1.4 kg/m2; n = 7 males; n = 7 females) individuals underwent skeletal muscle biopsies and an oral glucose tolerance test. For cohort 2, 14 young lean (BMI: 22.4 ± 1.9 kg/m2; n = 6 males; n = 8 females) and 14 obese (BMI: 39.3 ± 7.9 kg/m2; n = 6 males; n = 8 females) individuals underwent muscle biopsies for primary HSkMC experiments. Plasma mature myostatin (P = 0.041), skeletal muscle precursor myostatin (P = 0.048), and skeletal muscle SMAD3 (P = 0.029) were elevated in obese females compared to lean females, and plasma mature myostatin (r = 0.58, P = 0.029) and skeletal muscle SMAD3 (r = 0.56, P = 0.037) were associated with insulin resistance in females but not males. Twenty-four hours of myostatin treatment impaired insulin signaling in primary HSkMCs derived from females (P < 0.024) but not males. Overexpression of SMAD3, but not SMAD2, impaired insulin-stimulated AS160 phosphorylation in HSkMCs derived from lean females (-27%, P = 0.040), whereas silencing SMAD3 improved insulin-stimulated AS160 phosphorylation and insulin-stimulated glucose uptake (25%, P < 0.014) in HSkMCs derived from obese females. These results suggest for the first time that myostatin-induced impairments in skeletal muscle insulin signaling are sex specific and that increased body fat in females is associated with detrimental elevations in myostatin and SMAD3, which contribute to obesity-related insulin resistance.NEW & NOTEWORTHY Obesity is considered a main risk factor for the development of insulin resistance and type 2 diabetes. The present study utilizes in vivo and in vitro experiments in human skeletal muscle to demonstrate for the first time that females are inherently more susceptible to myostatin-induced insulin resistance, which is further enhanced with obesity due to increased myostatin and SMAD3 expression.

AMPK and Beyond: The Signaling Network Controlling RabGAPs and Contraction-Mediated Glucose Uptake in Skeletal Muscle

Impaired skeletal muscle glucose uptake is a key feature in the development of insulin resistance and type 2 diabetes. Skeletal muscle glucose uptake can be enhanced by a variety of different stimuli, including insulin and contraction as the most prominent. In contrast to the clearance of glucose from the bloodstream in response to insulin stimulation, exercise-induced glucose uptake into skeletal muscle is unaffected during the progression of insulin resistance, placing physical activity at the center of prevention and treatment of metabolic diseases. The two Rab GTPase-activating proteins (RabGAPs), TBC1D1 and TBC1D4, represent critical nodes at the convergence of insulin- and exercise-stimulated signaling pathways, as phosphorylation of the two closely related signaling factors leads to enhanced translocation of glucose transporter 4 (GLUT4) to the plasma membrane, resulting in increased cellular glucose uptake. However, the full network of intracellular signaling pathways that control exercise-induced glucose uptake and that overlap with the insulin-stimulated pathway upstream of the RabGAPs is not fully understood. In this review, we discuss the current state of knowledge on exercise- and insulin-regulated kinases as well as hypoxia as stimulus that may be involved in the regulation of skeletal muscle glucose uptake.

ATROPHIC REMODELING OF THE HEART DURING VITAMIN D DEFICIENCY AND INSUFFICIENCY IN A RAT MODEL

Vitamin D deficiency (VDD) is associated with skeletal muscle wasting and impaired cardiac function in humans and animals. However, the molecular events that cause cardiac dysfunction in VDD are poorly understood, and therefore, therapeutic approaches are limited. In the present study, we investigated the effects of VDD on heart function with an emphasis on signaling pathways that regulate anabolism/catabolism in cardiac muscle. A Vitamin D deficient or insufficient rat model was employed. Heart electrical activity was measured by electrocardiography. Gene expression was monitored by qPCR, while protein expression was assessed by western blotting. Catalytic activities of the proteasome, lysosomal cathepsin activity, and apoptotic caspases were measured by fluorimetry. Vitamin D insufficiency and deficiency led to cardiac arrhythmia, a decrease in heart weight, and an increase in apoptosis and interstitial fibrosis.  Ex-vivo cultures of atria revealed an increase in total protein degradation and a decrease in de-novo protein synthesis. The catalytic activities of the major proteolytic systems: ubiquitin-proteasome system, autophagy-lysosome, and calpains were upregulated in the heart of VDD and insufficient rats. In contrast, the mTOR pathway that regulates protein synthesis was suppressed. These catabolic events were exacerbated by a decrease in the expression of myosin heavy chain and troponin genes, as well as decreased expression and activities of metabolic enzymes. These latter changes occurred despite the activation of the energy sensor, AMPK. Our results provide, compelling evidence for cardiac atrophy in Vitamin D deficient rats. Unlike the skeletal muscle, the heart responded to VDD by activating all three proteolytic systems.

Serpentine Enhances Insulin Regulation of Blood Glucose through Insulin Receptor Signaling Pathway

Insulin sensitizers targeting insulin receptors (IR) are a potential drug for the treatment of diabetes. Serpentine is an alkaloid component in the root of Catharanthus roseus (L.) G. Don. Serpentine screened by surface plasmon resonance (SPR) technology has the ability to target IR. The objective of this study was to investigate whether serpentine could modulate the role of insulin in regulating blood glucose through insulin receptors in cells and in animal models of diabetes. SPR technology was used to detect the affinity of different concentrations of serpentine with insulin receptors. The Western blotting method was used to detect the expression levels of key proteins of the insulin signaling pathway in C2C12 cells and 3T3-L1 cells as well as in muscle and subcutaneous adipose tissue of diabetic mice after serpentine and insulin treatment. Diabetic mice were divided into four groups and simultaneously injected with insulin or serpentine, and the blood glucose concentration and serum levels of insulin, glucagon, and C-peptide were measured 150 min later. mRNA levels of genes related to lipid metabolism and glucose metabolism in liver, muscle, and subcutaneous adipose tissue were detected by RT-PCR. Serpentine was able to bind to the extracellular domain of IR with an affinity of 2.883 × 10^-6 M. Serpentine combined with insulin significantly enhanced the ability of insulin to activate the insulin signaling pathway and significantly enhanced the glucose uptake capacity of C2C12 cells. Serpentine enhanced the ability of low-dose insulin (1 nM) and normal-dose insulin (100 nM) to activate the insulin signaling pathway. Serpentine also independently activated AMPK phosphorylation, thus stimulating glucose uptake by C2C12 cells. In high-fat-diet/streptozotocin (HFD/STZ)-induced diabetic mice, serpentine significantly prolonged the hypoglycemic time of insulin, significantly reduced the use of exogenous insulin, and inhibited endogenous insulin secretion. In addition, serpentine alone significantly increased the expression of GSK-3β mRNA in muscle tissue, thus enhancing glucose uptake, and at the same time, serpentine significantly increased glucagon secretion and liver gluconeogenesis. Serpentine enhances the ability of insulin to regulate blood glucose through the insulin receptor, and can also regulate blood glucose alone, but it has a negative regulation mechanism and cannot produce a hypoglycemic effect. Therefore, serpentine may be useful as an insulin sensitizer to assist insulin to lower blood glucose.

Post-translational Modifications: The Signals at the Intersection of Exercise, Glucose Uptake, and Insulin Sensitivity

Diabetes is a global epidemic, of which type 2 diabetes makes up the majority of cases. Nonetheless, for some individuals, type 2 diabetes is eminently preventable and treatable via lifestyle interventions. Glucose uptake into skeletal muscle increases during and in recovery from exercise, with exercise effective at controlling glucose homeostasis in individuals with type 2 diabetes. Furthermore, acute and chronic exercise sensitizes skeletal muscle to insulin. A complex network of signals converge and interact to regulate glucose metabolism and insulin sensitivity in response to exercise. Numerous forms of post-translational modifications (eg, phosphorylation, ubiquitination, acetylation, ribosylation, and more) are regulated by exercise. Here we review the current state of the art of the role of post-translational modifications in transducing exercise-induced signals to modulate glucose uptake and insulin sensitivity within skeletal muscle. Furthermore, we consider emerging evidence for noncanonical signaling in the control of glucose homeostasis and the potential for regulation by exercise. While exercise is clearly an effective intervention to reduce glycemia and improve insulin sensitivity, the insulin- and exercise-sensitive signaling networks orchestrating this biology are not fully clarified. Elucidation of the complex proteome-wide interactions between post-translational modifications and the associated functional implications will identify mechanisms by which exercise regulates glucose homeostasis and insulin sensitivity. In doing so, this knowledge should illuminate novel therapeutic targets to enhance insulin sensitivity for the clinical management of type 2 diabetes.

Exocytosis Proteins: Typical and Atypical Mechanisms of Action in Skeletal Muscle

Insulin-stimulated glucose uptake in skeletal muscle is of fundamental importance to prevent postprandial hyperglycemia, and long-term deficits in insulin-stimulated glucose uptake underlie insulin resistance and type 2 diabetes. Skeletal muscle is responsible for ~80% of the peripheral glucose uptake from circulation via the insulin-responsive glucose transporter GLUT4. GLUT4 is mainly sequestered in intracellular GLUT4 storage vesicles in the basal state. In response to insulin, the GLUT4 storage vesicles rapidly translocate to the plasma membrane, where they undergo vesicle docking, priming, and fusion via the high-affinity interactions among the soluble N-ethylmaleimide sensitive factor attachment protein receptor (SNARE) exocytosis proteins and their regulators. Numerous studies have elucidated that GLUT4 translocation is defective in insulin resistance and type 2 diabetes. Emerging evidence also links defects in several SNAREs and SNARE regulatory proteins to insulin resistance and type 2 diabetes in rodents and humans. Therefore, we highlight the latest research on the role of SNAREs and their regulatory proteins in insulin-stimulated GLUT4 translocation in skeletal muscle. Subsequently, we discuss the novel emerging role of SNARE proteins as interaction partners in pathways not typically thought to involve SNAREs and how these atypical functions reveal novel therapeutic targets for combating peripheral insulin resistance and diabetes.

Physiological responses to acute fasting: implications for intermittent fasting programs

Intermittent fasting (IF) is a dietary strategy that involves alternating periods of abstention from calorie consumption with periods of ad libitum food intake. There is significant interest in the body of literature describing longitudinal adaptations to IF. Less attention has been given to the acute physiological responses that occur during the fasting durations that are commonly employed by IF practitioners. Thus, the purpose of this review was to examine the physiological responses - including alterations in substrate metabolism, systemic hormones, and autophagy - that occur throughout an acute fast. Literature searches were performed to locate relevant research describing physiological responses to acute fasting and short-term starvation. A single fast demonstrated the ability to alter glucose and lipid metabolism within the initial 24 hours, but variations in protein metabolism appeared to be minimal within this time frame. The ability of an acute fast to elicit significant increases in autophagy is still unknown. The information summarized in this review can be used to help contextualize existing research and better inform development of future IF interventions.

AMPK and the Adaptation to Exercise

Noncommunicable diseases are chronic diseases that contribute to death worldwide, but these diseases can be prevented and mitigated with regular exercise. Exercise activates signaling molecules and the transcriptional network to promote physiological adaptations, such as fiber type transformation, angiogenesis, and mitochondrial biogenesis. AMP-activated protein kinase (AMPK) is a master regulator that senses the energy state, promotes metabolism for glucose and fatty acid utilization, and mediates beneficial cellular adaptations in many vital tissues and organs. This review focuses on the current, integrative understanding of the role of exercise-induced activation of AMPK in the regulation of system metabolism and promotion of health benefits.

Co-overexpression of CD36 and FABPpm increases fatty acid transport additively, not synergistically, within muscle

We aimed to determine the combined effects of over-expressing FABPpm and CD36 on skeletal muscle fatty acid transport to establish if these transport proteins function collaboratively. Electrotransfection with either FABPpm or CD36 increased their protein content at the plasma membrane (+75% and +64%), increased fatty acid transport rates +24% for FABPpm and +62% for CD36, resulting in a calculated transport efficiency of ~0.019 and ~0.053 per unit protein change for FABPpm and CD36, respectively. We subsequently used these data to determine if increasing both proteins additively or synergistically increased fatty acid transport. Co-transfection of FABPpm and CD36 simultaneously increased protein content in whole muscle (FABPpm, +46%; CD36, +45%) and at the sarcolemma (FABPpm, +41% and CD36, +42%), as well as fatty acid transport rates (+50%). Since the relative effects of changing FABPpm and CD36 content had been independently determined, we were able to a predict a change in fatty acid transport based on the overexpression of plasmalemmal transporters in the co-transfection experiments. This prediction yielded an increase in fatty acid transport of +0.984 and +1.722 pmol/mg prot/15sec for FABPpm and CD36, respectively, for a total increase of +2.96 pmol/mg prot/15sec. This calculated determination was remarkably consistent with the measured change in transport, namely +2.89 pmol/mg prot/15sec. Altogether, these data indicate that increasing CD36 and FABPpm alters fatty acid transport rates additively, but not synergistically, suggesting an independent mechanism-of-action within muscle for each transporter. This conclusion was further supported by the observation that plasmalemmal CD36 and FABPpm did not co-immunoprecipitate.

Enhanced Oxygen Utilization Efficiency With Concomitant Activation of AMPK-TBC1D1 Signaling Nexus in Cyclophilin-D Conditional Knockout Mice

We have previously reported in HEK 293 T cells and in constitutive cyclophilin-D (Cyp-D) knockout (KO) mice that Cyp-D ablation downregulates oxygen consumption (VO₂) and triggers an adaptive response that manifest in higher exercise endurance with less VO₂. This adaptive response involves a metabolic switch toward preferential utilization of glucose via AMPK-TBC1D1 signaling nexus. We now investigated whether a similar response could be triggered in mice after acute ablation of Cyp-D using tamoxifen-induced ROSA26-Cre-mediated (i.e., conditional KO, CKO) by subjecting them to treadmill exercise involving five running sessions. At their first treadmill running session, CKO mice and controls had comparable VO₂ (208.4 ± 17.9 vs. 209.1 ± 16.8 ml/kg min⁻¹), VCO₂ (183.6 ± 17.2 vs. 184.8 ± 16.9 ml/kg min⁻¹), and RER (0.88 ± 0.043 vs. 0.88 ± 0.042). With subsequent sessions, CKO mice displayed more prominent reduction in VO₂ (genotype & session interaction p = 0.000) with less prominent reduction in VCO₂ resulting in significantly increased RER (genotype and session interaction p = 0.013). The increase in RER was consistent with preferential utilization of glucose as respiratory substrate (4.6 ± 0.8 vs. 4.0 ± 0.9 mg/min, p = 0.003). CKO mice also performed a significantly higher treadmill work for given VO₂ expressed as a power/VO₂ ratio (7.4 ± 0.2 × 10⁻³ vs. 6.7 ± 0.2 10⁻³ ratio, p = 0.025). Analysis of CKO skeletal muscle tissue after completion of five treadmill running sessions showed enhanced AMPK activation (0.669 ± 0.06 vs. 0.409 ± 0.11 pAMPK/β-tubulin ratio, p = 0.005) and TBC1D1 inactivation (0.877 ± 0.16 vs. 0.565 ± 0.09 pTBC1D1/β-tubulin ratio, p < 0.05) accompanied by increased glucose transporter-4 levels consistent with activation of the AMPK-TBC1D1 signaling nexus enabling increased glucose utilization. Taken together, our study demonstrates that like constitutive Cyp-D ablation, acute Cyp-D ablation also induces a state of increased O₂ utilization efficiency, paving the way for exploring the use of pharmacological approach to elicit the same response, which could be beneficial under O₂ limiting conditions.

Role of Akt isoforms in neuronal insulin signaling and resistance

The aim of the present study was to determine the role of Akt isoforms in insulin signaling and resistance in neuronal cells. By silencing Akt isoforms individually and in pairs, in Neuro-2a and HT22 cells we observed that, in insulin-sensitive condition, Akt isoforms differentially reduced activation of AS160 and glucose uptake with Akt2 playing the major role. Under insulin-resistant condition, phosphorylation of all isoforms and glucose uptake were severely affected. Over-expression of individual isoforms in insulin-sensitive and resistant cells differentially reversed AS160 phosphorylation with concomitant reversal in glucose uptake indicating a compensatory role of Akt isoforms in controlling neuronal insulin signaling. Post-insulin stimulation Akt2 translocated to the membrane the most followed by Akt3 and Akt1, decreasing glucose uptake in the similar order in insulin-sensitive cells. None of the Akt isoforms translocated in insulin-resistant cells or high-fat-diet mediated diabetic mice brain cells. Based on our data, insulin-dependent differential translocation of Akt isoforms to the plasma membrane turns out to be the key factor in determining Akt isoform specificity. Thus, isoforms play parallel with predominant role by Akt2, and compensatory yet novel role by Akt1 and Akt3 to regulate neuronal insulin signaling, glucose uptake, and insulin-resistance.

AKT ISOFORMS-AS160-GLUT4: The defining axis of insulin resistance

The Akt isoforms-AS160-GLUT4 axis is the primary axis that governs glucose homeostasis in the body. The first step on the path to insulin resistance is deregulated Akt isoforms. This could be Akt isoform expression, its phosphorylation, or improper isoform-specific redistribution to the plasma membrane in a specific tissue system. The second step is deregulated AS160 expression, its phosphorylation, improper dissociation from glucose transporter storage vesicles (GSVs), or its inability to bind to 14-3-3 proteins, thus not allowing it to execute its function. The final step is improper GLUT4 translocation and aberrant glucose uptake. These processes lead to insulin resistance in a tissue-specific way affecting the whole-body glucose homeostasis, eventually progressing to an overt diabetic phenotype. Thus, the relationship between these three key proteins and their proper regulation comes out as the defining axis of insulin signaling and -resistance. This review summarizes the role of this central axis in insulin resistance and disease in a new light.

Short-term physical inactivity induces diacylglycerol accumulation and insulin resistance in muscle via lipin1 activation

Physical inactivity impairs muscle insulin sensitivity. However, its mechanism is unclear. To model physical inactivity, we applied 24-h hind-limb cast immobilization (HCI) to mice with normal or high-fat diet (HFD) and evaluated intramyocellular lipids and the insulin signaling pathway in the soleus muscle. Although 2-wk HFD alone did not alter intramyocellular diacylglycerol (IMDG) accumulation, HCI alone increased it by 1.9-fold and HCI after HFD further increased it by 3.3-fold. Parallel to this, we found increased protein kinase C ε (PKCε) activity, reduced insulin-induced 2-deoxyglucose (2-DOG) uptake, and reduced phosphorylation of insulin receptor β (IRβ) and Akt, key molecules for insulin signaling pathway. Lipin1, which converts phosphatidic acid to diacylglycerol, showed increase of its activity by HCI, and dominant-negative lipin1 expression in muscle prevented HCI-induced IMDG accumulation and impaired insulin-induced 2-DOG uptake. Furthermore, 24-h leg cast immobilization in human increased lipin1 expression. Thus, even short-term immobilization increases IMDG and impairs insulin sensitivity in muscle via enhanced lipin1 activity.NEW & NOTEWORTHY Physical inactivity impairs muscle insulin sensitivity. However, its mechanism is unclear. To model physical inactivity, we applied 24-h hind-limb cast immobilization to mice with normal or high-fat diet and evaluated intramyocellular lipids and the insulin signaling pathway in the soleus muscle. We found that even short-term immobilization increases intramyocellular diacylglycerol and impairs insulin sensitivity in muscle via enhanced lipin1 activity.

Impact of Dietary Carbohydrate Restriction versus Energy Restriction on Exogenous Carbohydrate Oxidation during Aerobic Exercise

Individuals with high physical activity levels, such as athletes and military personnel, are likely to experience periods of low muscle glycogen content. Reductions in glycogen stores are associated with impaired physical performance. Lower glycogen stores in these populations are likely due to sustained aerobic exercise coupled with sub-optimal carbohydrate or energy intake. Consuming exogenous carbohydrate during aerobic exercise may be an effective intervention to sustain physical performance during periods of low glycogen. However, research is limited in the area of carbohydrate recommendations to fuel performance during periods of sub-optimal carbohydrate and energy intake. Additionally, the studies that have investigated the effects of low glycogen stores on exogenous carbohydrate oxidation have yielded conflicting results. Discrepancies between studies may be the result of glycogen stores being lowered by restricting carbohydrate or restricting energy intake. This narrative review discusses the influence of low glycogen status resulting from carbohydrate restriction versus energy restriction on exogenous carbohydrate oxidation and examines the potential mechanism resulting in divergent responses in exogenous carbohydrate oxidation. Results from this review indicate that rates of exogenous carbohydrate oxidation can be maintained when glycogen content is lower following carbohydrate restrictions, but may be reduced following energy restriction. Reductions in exogenous carbohydrate oxidation following energy restriction appear to result from lower insulin sensitivity and glucose uptake. Exogenous carbohydrate may thus be an effective intervention to sustain performance following short-term energy adequate carbohydrate restriction, but may not be an effective ergogenic aid when glycogen stores are low due to energy restriction.

Effect of Caloric Restriction on Aging: Fixing the Problems of Nutrient Sensing in Postmitotic Cells?

The review discusses the role of metabolic disorders (in particular, insulin resistance) in the development of age-related diseases and normal aging with special emphasis on the changes in postmitotic cells of higher organisms. Caloric restriction helps to prevent such metabolic disorders, which could probably explain its ability to prolong the lifespan of laboratory animals. Maintaining metabolic homeostasis is especially important for the highly differentiated long-lived body cells, whose lifespan is comparable to the lifespan of the organism itself. Normal functioning of these cells can be ensured only upon correct functioning of the cytoplasm clean-up system and availability of all required nutrients and energy sources. One of the central problems in gerontology is the age-related disruption of glucose metabolism leading to obesity, diabetes, metabolic syndrome, and other related pathologies. Along with the adipose tissue, skeletal muscles are the main consumers of insulin; hence the physical activity of muscles, which supports their energy metabolism, delays the onset of insulin resistance. Insulin resistance disrupts the metabolism of cardiomyocytes, so that they fail to utilize the nutrients to perform their functions even being surrounded by a nutrient-rich environment, which contributes to the development of age-related cardiovascular diseases. Metabolic pathologies also alter the nutrient sensitivity of neurons, thus disrupting the action of insulin in the central nervous system. In addition, there is evidence that neurons can develop insulin resistance as well. It has been suggested that affecting nutritional sensors (e.g., AMPK) in postmitotic cells might improve the state of the entire multicellular organism, slow down its aging, and increase the lifespan.

Culprits or consequences: Understanding the metabolic dysregulation of muscle in diabetes

The prevalence of type 2 diabetes (T2D) continues to rise despite the amount of research dedicated to finding the culprits of this debilitating disease. Skeletal muscle is arguably the most important contributor to glucose disposal making it a clear target in insulin resistance and T2D research. Within skeletal muscle there is a clear link to metabolic dysregulation during the progression of T2D but the determination of culprits vs consequences of the disease has been elusive. Emerging evidence in skeletal muscle implicates influential cross talk between a key anabolic regulatory protein, the mammalian target of rapamycin (mTOR) and its associated complexes (mTORC1 and mTORC2), and the well-described canonical signaling for insulin-stimulated glucose uptake. This new understanding of cellular signaling crosstalk has blurred the lines of what is a culprit and what is a consequence with regard to insulin resistance. Here, we briefly review the most recent understanding of insulin signaling in skeletal muscle, and how anabolic responses favoring anabolism directly impact cellular glucose disposal. This review highlights key cross-over interactions between protein and glucose regulatory pathways and the implications this may have for the design of new therapeutic targets for the control of glucoregulatory function in skeletal muscle.

A xanthene derivative, DS20060511, attenuates glucose intolerance by inducing skeletal muscle-specific GLUT4 translocation in mice

Reduced glucose uptake into the skeletal muscle is an important pathophysiological abnormality in type 2 diabetes, and is caused by impaired translocation of glucose transporter 4 (GLUT4) to the skeletal muscle cell surface. Here, we show a xanthene derivative, DS20060511, induces GLUT4 translocation to the skeletal muscle cell surface, thereby stimulating glucose uptake into the tissue. DS20060511 induced GLUT4 translocation and stimulated glucose uptake into differentiated L6-myotubes and into the skeletal muscles in mice. These effects were completely abolished in GLUT4 knockout mice. Induction of GLUT4 translocation by DS20060511 was independent of the insulin signaling pathways including IRS1-Akt-AS160 phosphorylation and IRS1-Rac1-actin polymerization, eNOS pathway, and AMPK pathway. Acute and chronic DS20060511 treatment attenuated the glucose intolerance in obese diabetic mice. Taken together, DS20060511 acts as a skeletal muscle-specific GLUT4 translocation enhancer to facilitate glucose uptake. Further studies of DS20060511 may pave the way for the development of novel antidiabetic medicines.

Biglycan reduces body weight by regulating food intake in mice and improves glucose metabolism through AMPK/AKT dual pathways in skeletal muscle

While biglycan (BGN) is suggested to direct diverse signaling cascades, the effects of soluble BGN as a ligand on metabolic traits have not been studied. Herein, we tested the effects of BGN on obesity in high-fat diet (HFD)-induced obese animals and glucose metabolism, with the underlying mechanism responsible for observed effects in vitro. Our results showed that BGN administration (1 mg/kg body weight, intraperitoneally) significantly prevented HFD-induced obesity, and this was mainly attributed to reduced food intake. Also, intracerebroventricular injection of BGN reduced food intake and body weight. The underlying mechanism includes modulation of neuropeptides gene expression involved in appetite in the hypothalamus in vitro and in vivo. In addition, BGN regulates glucose metabolism as shown by improved glucose tolerance in mice as well as AMPK/AKT dual pathway-driven enhanced glucose uptake and GLUT4 translocation in L6 myoblast cells. In conclusion, our results suggest BGN as a potential therapeutic target to treat risk factors for metabolic diseases.

14-Deoxy-11,12-Didehydroandrographolide Ameliorates Glucose Intolerance Enhancing the LKB1/AMPK[Formula: see text]/TBC1D1/GLUT4 Signaling Pathway and Inducing GLUT4 Expression in Myotubes and Skeletal Muscle of Obese Mice

14-Deoxy-11,12-didehydroandrographolide (deAND), a bioactive component of Andrographis paniculata, has antidiabetic activity. AMP-activated protein kinase (AMPK) regulates glucose transport and ameliorates insulin resistance. The aim of the present study was to investigate whether activation of AMPK is involved in the mechanism by which deAND ameliorates insulin resistance in muscles. deAND amounts up to 40 [Formula: see text]M dose-dependently activated phosphorylation of AMPK[Formula: see text] and TBC1D1 in C2C12 myotubes. In addition, deAND significantly activated phosphorylation of LKB1 at 6 h after treatment, and this activation was maintained up to 48 h. deAND increased glucose uptake at 18 h after treatment, and this increase was time dependent up to 72 h. Compound C, an inhibitor of AMPK, suppressed deAND-induced phosphorylation of AMPK[Formula: see text] and TBC1D1 and reversed the effect on glucose uptake. In addition, the expression of GLUT4 mRNA and protein in C2C12 myotubes was up-regulated by deAND in a time-dependent manner. Promotion of GLUT4 gene transcription was verified by a pGL3-GLUT4 (837 bp) reporter assay. deAND also increased the nuclear translocation of MEF-2A and PPAR[Formula: see text]. After 16 weeks of feeding, the high-fat diet (HFD) inhibited phosphorylation of AMPK[Formula: see text] and TBC1D1 in skeletal muscle of obese C57BL/6JNarl mice, and deactivation of AMPK[Formula: see text] and TBC1D1 by the HFD was abolished by deAND supplementation. Supplementation with deAND significantly promoted membrane translocation of GLUT4 compared with the HFD group. Supplementation also significantly increased GLUT4 mRNA and protein expression in skeletal muscle compared with the HFD group. The hypoglycemic effects of deAND are likely associated with activation of the LKB1/AMPK[Formula: see text]/TBC1D1/GLUT4 signaling pathway and stimulation of MEF-2A- and PPAR[Formula: see text]-dependent GLUT4 gene expression, which account for the glucose uptake into skeletal muscle and lower blood glucose levels.

Induction of Glutathione Synthesis Provides Cardioprotection Regulating NO, AMPK and PPARa Signaling in Ischemic Rat Hearts

Glutathione (GSH) is essential for antioxidant defence, and its depletion is associated with tissue damage during cardiac ischemia-reperfusion (I/R). GSH is synthesized by the glutamate-cysteine ligase enzyme (GCL) from L-cysteine, which alternatively might be used for hydrogen sulfide production by cystathionine-gamma-lyase (CSE). Here, we have investigated whether in vivo treatment with L-cysteine and an inhibitor of CSE,D,L-propargylglycine (PAG), can modulate cardiac glutathione and whether this treatment can influence heart resistance to I/R in a Langendorff isolated rat hearts model. Pretreatment with PAG + L-cysteine manifested in pronounced cardioprotection, as there was complete recovery of contractile function; preserved constitutive NOS activity; and limited the production of reactive oxygen and nitrogen species in the ischemized myocardium. Cardiac GSH and GSSG levels were increased by 3.5- and 2.1-fold in PAG + L-cysteine hearts and were 3.3- and 3.6-fold higher in PAG + L-cysteine + I/R compared to I/R heart. The cardioprotective effect of PAG + L-cysteine was completely abolished by an inhibitor of GCL, DL-buthionine-(S,R)-sulfoximine. Further analysis indicated diminished fatty acid β-oxidation, increased glucose consumption and anaerobic glycolysis, and promoted OXPHOS proteins and SERCA2 in PAG + L-cysteine + I/R compared to the I/R group. PAG + L-cysteine inhibited PPARα and up-regulated AMPK signalling in the heart. Thus, induction of glutathione synthesis provided cardioprotection regulating NO, AMPK and PPARa signaling in ischemic rat hearts.

Rosemary extract activates AMPK, inhibits mTOR and attenuates the high glucose and high insulin-induced muscle cell insulin resistance

Impaired action of insulin in skeletal muscle, termed insulin resistance, leads to increased blood glucose levels resulting in compensatory increase in insulin levels. The elevated blood glucose and insulin levels exacerbate insulin resistance and contribute to the pathogenesis of type 2 diabetes mellitus. In previous studies we found attenuation of free fatty acid-induced muscle cell insulin resistance by rosemary extract (RE). In the present study we investigated the effects of RE on high glucose (HG) and high insulin (HI)-induced muscle cell insulin resistance. Exposure of L6 myotubes to 25 mmol/L glucose and 100 nmol/L insulin for 24 h, to mimic hyperglycemia and hyperinsulinemia, abolished the acute insulin-stimulated glucose uptake, increased the serine phosphorylation of IRS-1 and the phosphorylation/activation of mTOR and p70S6K. Treatment with RE significantly improved the insulin-stimulated glucose uptake and increased the acute insulin-stimulated tyrosine phosphorylation while reducing the HG+HI-induced serine phosphorylation of IRS-1 and phosphorylation of mTOR and p70S6K. Additionally, treatment with RE significantly increased the phosphorylation of AMPK, its downstream effector ACC and the plasma membrane GLUT4 levels. Our data indicate a potential of RE to counteract muscle cell insulin resistance and more studies are required to investigate its effectiveness in vivo. Novelty: RE phosphorylated muscle cell AMPK and ACC under both normal and HG+HI conditions. The HG+HI-induced serine phosphorylation of IRS-1 and activation of mTOR and p70S6K were attenuated by RE. RE restored the insulin-stimulated glucose uptake by enhancing GLUT4 glucose transporter translocation to plasma membrane.

Medium-Intensity Treadmill Exercise Exerts Beneficial Effects on Bone Modeling Through Bone Marrow Mesenchymal Stromal Cells

As a type of multipotential cells, bone marrow mesenchymal stromal cells (BMMSCs) can differentiate into chondrocytes, osteoblasts, and adipocytes under different loading condition or specific microenvironment. Previous studies have shown that BMMSCs and their lineage-differentiated progeny (for example, osteoblasts), and osteocytes are mechanosensitive in bone. The appropriate physical activity and exercise could help attenuate bone loss, effectively stimulate bone formation, increase bone mineral density (BMD), prevent the progression of osteoporosis, and reduce the risk of bone fractures. Bone morphogenetic protein (BMP) is originally discovered as a protein with heterotopic bone-inducing activity in the bone matrix that exerts a critical role in multiple stages of bone metabolism. In the present study, the medium-intensity treadmill exercise enhanced bone formation and increased osteocalcin (OCN) and osteopontin (OPN) mRNA expression as well as activation of the BMP-Smad signaling pathway in vivo. In order to investigate the effect of a BMP-Smad signaling pathway, we injected mice with activated enzyme inhibitors (LDN-193189HCL) and subjected the mice to treadmill exercise intervention. LDN-193189HCL attenuated the BMD and bone mass mediated by medium-intensity exercise and BMP-Smad signaling pathway.

Integrin-associated ILK and PINCH1 protein content are reduced in skeletal muscle of maintenance haemodialysis patients

KEY POINTS

Patients with renal failure undergoing maintenance haemodialysis are associated with insulin resistance and protein metabolism dysfunction. Novel research suggests that disruption to the transmembrane protein linkage between the cytoskeleton and the extracellular matrix in skeletal muscle may contribute to reduced amino acid metabolism and insulin resistance in haemodialysis. ILK, PINCH1 and pFAK^Tyr397 were significantly decreased in haemodialysis compared to controls, whereas Rac1 and Akt2 showed no different between groups. Rac1 deletion in the Rac1 knockout model did not alter the expression of integrin-associated proteins. Phenylalanine kinetics were reduced in the haemodialysis group at 30 and 60 min post meal ingestion compared to controls; both groups showed similar levels of insulin sensitivity and β-cell function. Key proteins in the integrin-cytoskeleton linkage are reduced in haemodialysis patients, suggesting for the first time that integrin-associated proteins dysfunction may contribute to reduced phenylalanine flux without affecting insulin resistance in haemodialysis patients.

ABSTRACT

Muscle atrophy, insulin resistance and reduced muscle phosphoinositide 3-kinase-Akt signalling are common characteristics of patients undergoing maintenance haemodialysis (MHD). Disruption to the transmembrane protein linkage between the cytoskeleton and the extracellular matrix in skeletal muscle may contribute to reduced amino acid metabolism and insulin resistance in MHD patients. Eight MHD patients (age: 56 ± 5 years: body mass index: 32 ± 2 kg m^-2 ) and non-diseased controls (age: 50 ± 2 years: body mass index: 31 ± 1 kg m^-2 ) received primed continuous l-[ring-² H₅ ]phenylalanine before consuming a mixed meal. Phenylalanine metabolism was determined using two-compartment modelling. Muscle biopsies were collected prior to the meal and at 300 min postprandially. In a separate experiment, skeletal muscle tissue from muscle-specific Rac1 knockout (Rac1 mKO) was harvested to investigate whether Rac1 depletion disrupted the cytoskeleton-integrin linkage, allowing for cross-model examination of proteins of interest. ILK, PINCH1 and pFAK^Tyr397 were significantly lower in MHD (P < 0.01). Rac1 and Akt showed no difference between groups for the human trial. Rac1 deletion in the Rac1 mKO model did not alter the expression of integrin-associated proteins. Phenylalanine rates of appearance and disappearance, as well as metabolic clearance rates, were lower in the MHD group at 30 and 60 min post meal ingestion compared to controls (P < 0.05). Both groups showed similar levels of insulin sensitivity and β-cell function. Key proteins in the integrin-cytoskeleton linkage are reduced in MHD patients, suggesting for the first time that integrin-associated proteins dysfunction may contribute to reduced phenylalanine flux without affecting insulin resistance in haemodialysis patients.

Structure, function and regulation of mammalian glucose transporters of the SLC2 family

The SLC2 genes code for a family of GLUT proteins that are part of the major facilitator superfamily (MFS) of membrane transporters. Crystal structures have recently revealed how the unique protein fold of these proteins enables the catalysis of transport. The proteins have 12 transmembrane spans built from a replicated trimer substructure. This enables 4 trimer substructures to move relative to each other, and thereby alternately opening and closing a cleft to either the internal or the external side of the membrane. The physiological substrate for the GLUTs is usually a hexose but substrates for GLUTs can include urate, dehydro-ascorbate and myo-inositol. The GLUT proteins have varied physiological functions that are related to their principal substrates, the cell type in which the GLUTs are expressed and the extent to which the proteins are associated with subcellular compartments. Some of the GLUT proteins translocate between subcellular compartments and this facilitates the control of their function over long- and short-time scales. The control of GLUT function is necessary for a regulated supply of metabolites (mainly glucose) to tissues. Pathophysiological abnormalities in GLUT proteins are responsible for, or associated with, clinical problems including type 2 diabetes and cancer and a range of tissue disorders, related to tissue-specific GLUT protein profiles. The availability of GLUT crystal structures has facilitated the search for inhibitors and substrates and that are specific for each GLUT and that can be used therapeutically. Recent studies are starting to unravel the drug targetable properties of each of the GLUT proteins.

Effect of chitinase-3-like protein 1 on glucose metabolism: In vitro skeletal muscle and human genetic association study

We investigated the effect of chitinase-3-like protein 1 (CHI3L1) on glucose metabolism and its underlying mechanisms in skeletal muscle cells, and evaluated whether the observed effects are relevant in humans. CHI3L1 was associated with increased glucose uptake in skeletal muscles in an AMP-activated protein kinase (AMPK)-dependent manner, and with increased intracellular calcium levels via PAR2. The improvement in glucose metabolism observed in an intraperitoneal glucose tolerance test on male C57BL/6J mice supported this association. Inhibition of the CaMKK was associated with suppression of CHI3L1-mediated glucose uptake. Additionally, CHI3L1 was found to influence glucose uptake through the PI3K/AKT pathway. Results suggested that CHI3L1 stimulated the phosphorylation of AS160 and p38 MAPK downstream of AMPK and AKT, and the resultant GLUT4 translocation. In primary myoblast cells, stimulation of AMPK and AKT was observed in response to CHI3L1, underscoring the biological relevance of CHI3L1. CHI3L1 levels were elevated in cells under conditions that mimic exercise in vitro and in exercised mice in vivo, indicating that CHI3L1 is secreted during muscle contraction. Finally, similar associations between CHI3L1 and metabolic parameters were observed in humans alongside genotype associations between CHI3L1 and diabetes at the population level. CHI3L1 may be a potential therapeutic target for the treatment of diabetes.

Glucose transporters in cardiovascular system in health and disease

Glucose transporters are essential for the heart to sustain its function. Due to its nature as a high energy-consuming organ, the heart needs to catabolize a huge quantity of metabolic substrates. For optimized energy production, the healthy heart constantly switches between various metabolites in accordance with substrate availability and hormonal status. This metabolic flexibility is essential for the maintenance of cardiac function. Glucose is part of the main substrates catabolized by the heart and its use is fine-tuned via complex molecular mechanisms that include the regulation of the glucose transporters GLUTs, mainly GLUT4 and GLUT1. Besides GLUTs, glucose can also be transported by cotransporters of the sodium-glucose cotransporter (SGLT) (SLC5 gene) family, in which SGLT1 and SMIT1 were shown to be expressed in the heart. This SGLT-mediated uptake does not seem to be directly linked to energy production but is rather associated with intracellular signalling triggering important processes such as the production of reactive oxygen species. Glucose transport is markedly affected in cardiac diseases such as cardiac hypertrophy, diabetic cardiomyopathy and heart failure. These alterations are not only fingerprints of these diseases but are involved in their onset and progression. The present review will depict the importance of glucose transport in healthy and diseased heart, as well as proposed therapies targeting glucose transporters.

Role of Skeletal Muscle in Insulin Resistance and Glucose Uptake

The skeletal muscle is the largest organ in the body, by mass. It is also the regulator of glucose homeostasis, responsible for 80% of postprandial glucose uptake from the circulation. Skeletal muscle is essential for metabolism, both for its role in glucose uptake and its importance in exercise and metabolic disease. In this article, we give an overview of the importance of skeletal muscle in metabolism, describing its role in glucose uptake and the diseases that are associated with skeletal muscle metabolic dysregulation. We focus on the role of skeletal muscle in peripheral insulin resistance and the potential for skeletal muscle-targeted therapeutics to combat insulin resistance and diabetes, as well as other metabolic diseases like aging and obesity. In particular, we outline the possibilities and pitfalls of the quest for exercise mimetics, which are intended to target the molecular mechanisms underlying the beneficial effects of exercise on metabolic disease. We also provide a description of the molecular mechanisms that regulate skeletal muscle glucose uptake, including a focus on the SNARE proteins, which are essential regulators of glucose transport into the skeletal muscle. © 2020 American Physiological Society. Compr Physiol 10:785-809, 2020.

An AMPK/Axin1-Rac1 signaling pathway mediates contraction-regulated glucose uptake in skeletal muscle cells

Contraction stimulates skeletal muscle glucose uptake predominantly through activation of AMP-activated protein kinase (AMPK) and Rac1. However, the molecular details of how contraction activates these signaling proteins are not clear. Recently, Axin1 has been shown to form a complex with AMPK and liver kinase B1 during glucose starvation-dependent activation of AMPK. Here, we demonstrate that electrical pulse-stimulated (EPS) contraction of C2C12 myotubes or treadmill exercise of C57BL/6 mice enhanced reciprocal coimmunoprecipitation of Axin1 and AMPK from myotube lysates or gastrocnemius muscle tissue. Interestingly, EPS or exercise upregulated total cellular Axin1 levels in an AMPK-dependent manner in C2C12 myotubes and gastrocnemius mouse muscle, respectively. Also, direct activation of AMPK with 5-aminoimidazole-4-carboxamide ribonucleotide treatment of C2C12 myotubes or gastrocnemius muscle elevated Axin1 protein levels. On the other hand, siRNA-mediated Axin1 knockdown lessened activation of AMPK in contracted myotubes. Further, AMPK inhibition with compound C or siRNA-mediated knockdown of AMPK or Axin1 blocked contraction-induced GTP loading of Rac1, p21-activated kinase phosphorylation, and contraction-stimulated glucose uptake. In summary, our results suggest that an AMPK/Axin1-Rac1 signaling pathway mediates contraction-stimulated skeletal muscle glucose uptake.

Exercise effects on γ3-AMPK activity, phosphorylation of Akt2 and AS160, and insulin-stimulated glucose uptake in insulin-resistant rat skeletal muscle

One exercise session can increase subsequent insulin-stimulated glucose uptake (ISGU) by skeletal muscle. Prior research on healthy muscle suggests that enhanced postexercise ISGU depends on elevated γ3-AMPK activity leading to greater phosphorylation of Akt substrate of 160 kDa (pAS160) on an AMPK-phosphomotif (Ser⁷⁰⁴). Phosphorylation of AS160^Ser704, in turn, may favor greater insulin-stimulated pAS160 on an Akt-phosphomotif (Thr⁶⁴²) that regulates ISGU. Accordingly, we tested if exercise-induced increases in γ3-AMPK activity and pAS160 on key regulatory sites accompany improved ISGU at 3 h postexercise (3hPEX) in insulin-resistant muscle. Rats fed a high-fat diet (HFD; 2-wk) that induces insulin resistance either performed acute swim-exercise (2 h) or were sedentary (SED). SED rats fed a low-fat diet (LFD; 2 wk) served as healthy controls. Isolated epitrochlearis muscles from 3hPEX and SED rats were analyzed for ISGU, pAS160, pAkt2 (Akt-isoform that phosphorylates pAS160^Thr642), and γ1-AMPK and γ3-AMPK activity. ISGU was lower in HFD-SED muscles versus LFD-SED, but this decrement was eliminated in the HFD-3hPEX group. γ3-AMPK activity, but not γ1-AMPK activity, was elevated in HFD-3hPEX muscles versus both SED controls. Furthermore, insulin-stimulated pAS160^Thr642, pAS160^Ser704, and pAkt2^Ser474 in HFD-3hPEX muscles were elevated above HFD-SED and equal to values in LFD-SED muscles, but insulin-independent pAS160^Ser704 was unaltered at 3hPEX. These results demonstrated, for the first time in an insulin-resistant model, that the postexercise increase in ISGU was accompanied by sustained enhancement of γ3-AMPK activation and greater pAkt2^Ser474. Our working hypothesis is that these changes along with enhanced insulin-stimulated pAS160 increase ISGU of insulin-resistant muscles to values equaling insulin-sensitive sedentary controls.NEW & NOTEWORTHY Earlier research focusing on signaling events linked to increased insulin sensitivity in muscle has rarely evaluated insulin resistant muscle after exercise. We assessed insulin resistant muscle after an exercise protocol that improved insulin-stimulated glucose uptake. Prior exercise also amplified several signaling steps expected to favor enhanced insulin-stimulated glucose uptake: increased γ3-AMP-activated protein kinase activity, greater insulin-stimulated Akt2 phosphorylation on Ser474, and elevated insulin-stimulated Akt substrate of 160 kDa phosphorylation on Ser588, Thr642, and Ser704.

Sex and fiber type independently influence AMPK, TBC1D1, and TBC1D4 at rest and during recovery from high-intensity exercise in humans

Women and men present different metabolic responses to exercise, yet whether this phenomenon results from differences in fiber type (FT) composition or other sex-specific factors remains unclear. Therefore, our aim was to examine the effects of sex and FT independently on AMP-activated protein kinase (AMPK), acetyl-CoA carboxylase (ACC), Tre-2/BUB2/CDC1 domain family (TBC1D)1, and TBC1D4 in response to acute exercise. Segregated pools of myosin heavy chain (MHC) I and MHC IIa fibers were prepared from vastus lateralis biopsies of young trained men and women at rest and during recovery (0 min, 45 min, 90 min, or 180 min) from high-intensity interval exercise (6 × 1.5 min at 95% maximum oxygen uptake). In resting MHC I vs. IIa fibers, AMPKα2, AMPKγ3, and TBC1D1 were higher and TBC1D4 expression was lower in both sexes, along with higher phospho (p)-TBC1D1^Ser660 and lower p-TBC1D4^Thr642. Women expressed higher ACC than men in MHC IIa fibers and higher AMPKβ1, AMPKβ2, TBC1D1, and TBC1D4 in both FTs. Immediately after exercise, p-AMPKα^Thr172 increased only in MHC IIa fibers, whereas p-ACC^Ser221 increased in both FTs, with no change in p-TBC1D1^Ser660 or p-TBC1D4^Thr642. During recovery, delayed responses were observed for p-AMPKα^Thr172 in MHC I (45 min), p-TBC1D4^Thr642 in both FTs (45 min), and p-TBC1D1^Ser660 (180 min). FT-specific phosphorylation responses to exercise were similar between men and women. Data indicate that sex and FT independently influence expression of AMPK and its substrates. Thus failing to account for sex or FT may reduce accuracy and precision of metabolic protein measurements and conceal key findings.NEW & NOTEWORTHY This investigation is the first to compare muscle fiber type (FT)-specific analysis of proteins between the sexes, providing comprehensive data on AMP-activated protein kinase (AMPK), acetyl-CoA carboxylase (ACC), Tre-2/BUB2/CDC1 domain family (TBC1D)1, and TBC1D4 before and in the hours following high-intensity interval exercise (HIIT). Expression and phosphorylation of specific AMPK isoforms, ACC, TBC1D1, and TBC1D4 were shown to be FT dependent, sex dependent, or both, and TBC1D1 showed an unexpected delay in FT-dependent phosphorylation in the time period following HIIT.

Loss of FoxOs in muscle reveals sex-based differences in insulin sensitivity but mitigates diet-induced obesity

OBJECTIVE

Gender influences obesity-related complications, including diabetes. Females are more protected from insulin resistance after diet-induced obesity, which may be related to fat accumulation and muscle insulin sensitivity. FoxOs regulate muscle atrophy and are targets of insulin action, but their role in muscle insulin sensitivity and mitochondrial metabolism is unknown.

METHODS

We measured muscle insulin signaling, mitochondrial energetics, and metabolic responses to a high-fat diet (HFD) in male and female muscle-specific FoxO1/3/4 triple knock-out (TKO) mice.

RESULTS

In male TKO muscle, insulin-stimulated AKT activation was decreased. AKT2 protein and mRNA levels were reduced and insulin receptor protein and IRS-2 mRNA decreased. These changes contributed to decreased insulin-stimulated glucose uptake in glycolytic muscle in males. In contrast, female TKOs maintain normal insulin-mediated AKT phosphorylation, normal AKT2 levels, and normal glucose uptake in glycolytic muscle. When challenged with a HFD, fat gain was attenuated in both male and female TKO mice, and associated with decreased glucose levels, improved glucose homeostasis, and reduced muscle triglyceride accumulation. Furthermore, female TKO mice showed increased energy expenditure, relative to controls, due to increased lean mass and maintenance of mitochondrial function in muscle.

CONCLUSIONS

FoxO deletion in muscle uncovers sexually dimorphic regulation of AKT2, which impairs insulin signaling in male mice, but not females. However, loss of FoxOs in muscle from both males and females also leads to muscle hypertrophy and increases in metabolic rate. These factors mitigate fat gain and attenuate metabolic abnormalities in response to a HFD.

Impact of Endurance and Resistance Training on Skeletal Muscle Glucose Metabolism in Older Adults

Aging is associated with insulin resistance and the development of type 2 diabetes. While this process is multifaceted, age-related changes to skeletal muscle are expected to contribute to impaired glucose metabolism. Some of these changes include sarcopenia, impaired insulin signaling, and imbalances in glucose utilization. Endurance and resistance exercise training have been endorsed as interventions to improve glucose tolerance and whole-body insulin sensitivity in the elderly. While both types of exercise generally increase insulin sensitivity in older adults, the metabolic pathways through which this occurs can differ and can be dependent on preexisting conditions including obesity and type 2 diabetes. In this review, we will first highlight age-related changes to skeletal muscle which can contribute to insulin resistance, followed by a comparison of endurance and resistance training adaptations to insulin-stimulated glucose metabolism in older adults.

Endurance running exercise is an effective alternative to estradiol replacement for restoring hyperglycemia through TBC1D1/GLUT4 pathway in skeletal muscle of ovariectomized rats

Menopause is a risk factor for impaired glucose metabolism. Alternative treatment of estrogen for postmenopausal women is required. The present study was designed to investigate the effects of 5-week endurance running exercise (Ex) by treadmill on hyperglycemia and signal pathway components mediating glucose transport in ovariectomized (OVX) placebo-treated rats, compared with 4-week 17β-estradiol (E2) replacement or pair-feeding (PF) to the E2 group. Ex improved the hyperglycemia and insulin resistance index in OVX rats as much as E2 or PF did. However, Ex had no effect on body weight gain in the OVX rats. Moreover, Ex enhanced the levels of GLUT4 and phospho-TBC1D1 proteins in the gastrocnemius of the OVX rats, but E2 or PF did not. Instead, the E2 increased the Akt2/AS160 expression and activation in the OVX rats. This study suggests that endurance Ex training restored hyperglycemia through the TBC1D1/GLUT4 pathway in muscle by an alternative mechanism to E2 replacement.

Effects of Nutrient Intake Timing on Post-Exercise Glycogen Accumulation and its Related Signaling Pathways in Mouse Skeletal Muscle

We investigated the effects of nutrient intake timing on glycogen accumulation and its related signals in skeletal muscle after an exercise that did not induce large glycogen depletion. Male ICR mice ran on a treadmill at 25 m/min for 60 min under a fed condition. Mice were orally administered a solution containing 1.2 mg/g carbohydrate and 0.4 mg/g protein or water either immediately (early nutrient, EN) or 180 min (late nutrient, LN) after the exercise. Tissues were harvested at 30 min after the oral administration. No significant difference in blood glucose or plasma insulin concentrations was found between the EN and LN groups. The plantaris muscle glycogen concentration was significantly (p < 0.05) higher in the EN group—but not in the LN group—compared to the respective time-matched control group. Akt Ser⁴⁷³ phosphorylation was significantly higher in the EN group than in the time-matched control group (p < 0.01), while LN had no effect. Positive main effects of time were found for the phosphorylations in Akt substrate of 160 kDa (AS160) Thr⁶⁴² (p < 0.05), 5′-AMP-activated protein kinase (AMPK) Thr¹⁷² (p < 0.01), and acetyl-CoA carboxylase Ser⁷⁹ (p < 0.01); however, no effect of nutrient intake was found for these. We showed that delayed nutrient intake could not increase muscle glycogen after endurance exercise which did not induce large glycogen depletion. The results also suggest that post-exercise muscle glycogen accumulation after nutrient intake might be partly influenced by Akt activation. Meanwhile, increased AS160 and AMPK activation by post-exercise fasting might not lead to glycogen accumulation.

Wogonin Alleviates Hyperglycemia Through Increased Glucose Entry into Cells Via AKT/GLUT4 Pathway

:

Insulin resistance and type 2 Diabetes mellitus resulting in chronic hyperglycemia is a major health problem in the modern world. Many drugs have been tested to control hyperglycemia which is believed to be the main factor behind many of the diabetes-related late-term complications. Wogonin is a famous herbal medicine which has been shown to be effective in controlling diabetes and its complications. In our previous work, we showed that wogonin is beneficial in many ways in controlling diabetic cardiomyopathy. In this review, we mainly explained wogonin anti-hyperglycemic property through AKT/GLUT4 pathway. Here we briefly discussed that wogonin increases Glut4 trafficking to plasma membrane which allows increased entry of glucose and thus alleviates hyperglycemia.

Conclusion:

Wogonin can be used as an anti-diabetic and anti-hyperglycemic drug and works via AKT/GLUT4 pathway.

Chemical Inducers of Obesity-Associated Metabolic Stress Activate Inflammation and Reduce Insulin Sensitivity in 3T3-L1 Adipocytes

Obesity is accompanied by dyslipidemia, hypoxia, endoplasmic reticulum (ER) stress, and inflammation, representing the major risk factor for the development of insulin resistance (IR) and type 2 diabetes. We modeled these conditions in cultured 3T3-L1 adipocytes and studied their effect on insulin signaling, glucose uptake, and inflammatory response via activation of stress-dependent JNK1/2 kinases. Decreased insulin-induced phosphorylation of the insulin cascade components IRS, Akt, and AS160 was observed under all tested conditions (lipid overloading of cells by palmitate, acute inflammation induced by bacterial lipopolysaccharide, hypoxia induced by Co2+, and ER stress induced by brefeldin A). In all the cases, except the acute inflammation, glucose uptake by adipocytes was reduced, and the kinetics of JNK1/2 activation was bi-phasic exhibiting sustained activation for 24 h. By contrast, in acute inflammation, JNK1/2 phosphorylation increased transiently and returned to the basal level within 2-3 h of stimulation. These results suggest a critical role of sustained (latent) vs. transient (acute) inflammation in the induction of IR and impairment of glucose utilization by adipose tissue. The components of the inflammatory signaling can be promising targets in the development of new therapeutic approaches for preventing IR and type 2 diabetes.

Effects of β-hydroxybutyrate treatment on glycogen repletion and its related signaling cascades in epitrochlearis muscle during 120 min of postexercise recovery

We investigated the effects of β-hydroxybutyrate (β-HB), the most abundant type of ketone body in mammals, on postexercise glycogen recovery in skeletal muscle by using an in vitro experimental model. Male ICR mice swam for 60 min and then their epitrochlearis muscles were removed and incubated with either physiological levels of glucose (8 mmol/L) and insulin (60 μU/mL) or glucose and insulin plus 1, 2, or 4 mmol/L of sodium β-HB. Four millimoles per liter β-HB had a significant positive effect on glycogen repletion in epitrochlearis muscle at 120 min after exercise (p < 0.01), while 2 mmol/L of β-HB showed a tendency to increase the glycogen level (p < 0.09), and 1 mmol/L of β-HB had no significant effect. We further investigated the effect of 4 mmol/L β-HB treatment on the signaling cascade related to glycogen repletion in the epitrochlearis muscles throughout a 120-min recovery period. After incubating the muscles in 4 mmol/L of β-HB for 15 min postexercise, the Akt substrate of 160 kDa Thr⁶⁴² (p < 0.05) and Akt Thr³⁰⁸ (p < 0.05) phosphorylations were significantly increased compared with the control treatment. At the same time point, 5'-AMP-activated protein kinase and acetyl-coenzyme A carboxylase phosphorylations were significantly lower (p < 0.05) in the epitrochlearis muscle incubated with 4 mmol/L of β-HB than in the control muscle. Our results demonstrate that postexercise 4 mmol/L β-HB administration enhanced glycogen repletion in epitrochlearis muscle. Four millimoles per liter β-HB treatment was associated with alternation of the phosphorylated status of several proteins involved in glucose uptake and metabolic/energy homeostasis at the early stage of postexercise.

Whole body glucoregulation and tissue-specific glucose uptake in a novel Akt substrate of 160 kDa knockout rat model

Akt substrate of 160 kDa (also called AS160 or TBC1D4) is a Rab GTPase activating protein and key regulator of insulin-stimulated glucose uptake which is expressed by multiple tissues, including skeletal muscle, white adipose tissue (WAT) and the heart. This study introduces a novel rat AS160-knockout (AS160-KO) model that was created using CRISPR/Cas9 technology. We compared male AS160-KO versus wildtype (WT) rats for numerous metabolism-related endpoints. Body mass, body composition, energy expenditure and physical activity did not differ between genotypes. Oral glucose intolerance was detected in AS160-KO versus WT rats (P<0.005). A hyperinsulinemic-euglycemic clamp (HEC) revealed insulin resistance for glucose infusion rate (P<0.05) with unaltered hepatic glucose production in AS160-KO versus WT rats. Genotype-effects on glucose uptake during the HEC: 1) was significantly lower in epitrochlearis (P<0.01) and extensor digitorum longus (P<0.05) of AS160-KO versus WT rats, and tended to be lower for AS160-KO versus WT rats in the soleus (P<0.06) and gastrocnemius (P<0.08); 2) tended to be greater for AS160-KO versus WT rats in white adipose tissue (P = 0.09); and 3) was significantly greater in the heart (P<0.005) of AS160-KO versus WT rats. GLUT4 protein abundance was significantly lower for AS160-KO versus WT rats in each tissue analyzed (P<0.01–0.001) except the gastrocnemius. Ex vivo insulin-stimulated glucose uptake was significantly lower (P<0.001) for AS160-KO versus WT rats in isolated epitrochlearis or soleus. Insulin-stimulated Akt phosphorylation (in vivo or ex vivo) did not differ between genotypes for any tissue tested. Ex vivo AICAR-stimulated glucose uptake by isolated epitrochlearis was significantly lower for AS160-KO versus WT rats (P<0.01) without genotype-induced alteration in AMP-activated protein phosphorylation. This unique AS160-KO rat model, which elucidated striking genotype-related modifications in glucoregulation, will enable future research aimed at understanding AS160’s roles in numerous physiological processes in response to various interventions (e.g., diet and/or exercise).

Whole Egg Consumption Impairs Insulin Sensitivity in a Rat Model of Obesity and Type 2 Diabetes

BACKGROUND

The literature regarding the relation between egg consumption and type 2 diabetes (T2D) is inconsistent and there is limited evidence pertaining to the impact of egg consumption on measures of insulin sensitivity.

OBJECTIVES

The aim of this study was to investigate the effect of dietary whole egg on metabolic biomarkers of insulin resistance in T2D rats.

METHODS

Male Zucker diabetic fatty (ZDF) rats (n = 12; 6 wk of age) and age-matched lean controls (n = 12) were randomly assigned to be fed a casein- or whole egg-based diet. At week 5 of dietary treatment, an insulin tolerance test (ITT) was performed on all rats and blood glucose was measured by glucometer. After 7 wk of dietary treatment, rats were anesthetized and whole blood was collected via a tail vein bleed. Following sedation, the extensor digitorum longus muscle was removed before and after an intraperitoneal insulin injection, and insulin signaling in skeletal muscle was analyzed by Western blot. Serum glucose and insulin were analyzed by ELISA for calculation of the homeostatic model assessment of insulin resistance (HOMA-IR).

RESULTS

Mean ITT blood glucose over the course of 60 min was 32% higher in ZDF rats fed the whole egg-based diet than in ZDF rats fed the casein-based diet. Furthermore, whole egg consumption increased fasting blood glucose by 35% in ZDF rats. Insulin-stimulated phosphorylation of key proteins in the insulin signaling pathway did not differ in skeletal muscle of ZDF rats fed casein- and whole egg-based diets. In lean rats, no differences were observed in insulin tolerance, HOMA-IR and skeletal muscle insulin signaling, regardless of experimental dietary treatment.

CONCLUSIONS

These data suggest that whole body insulin sensitivity may be impaired by whole egg consumption in T2D rats, although no changes were observed in skeletal muscle insulin signaling that could explain this finding.

Aberrant insulin receptor expression is associated with insulin resistance and skeletal muscle atrophy in myotonic dystrophies

Myotonic dystrophy type 1 (DM1) and type 2 (DM2) are autosomal dominant multisystemic disorders linked to two different genetic loci and characterized by several features including myotonia, muscle atrophy and insulin resistance. The aberrant alternative splicing of insulin receptor (IR) gene and post-receptor signalling abnormalities have been associated with insulin resistance, however the precise molecular defects that cause metabolic dysfunctions are still unknown. Thus, the aims of this study were to investigate in DM skeletal muscle biopsies if beyond INSR missplicing, altered IR protein expression could play a role in insulin resistance and to verify if the lack of insulin pathway activation could contribute to skeletal muscle wasting. Our analysis showed that DM skeletal muscle exhibits a lower expression of the insulin receptor in type 1 fibers which can contribute to the defective activation of the insulin pathway. Moreover, the aberrant insulin signalling activation leads to a lower activation of mTOR and to an increase in MuRF1 and Atrogin-1/MAFbx expression, possible explaining DM skeletal muscle fiber atrophy. Taken together our data indicate that the defective insulin signalling activation can contribute to skeletal muscle features in DM patients and are probably linked to an aberrant specific-fiber type expression of the insulin receptor.

Apolipoprotein A-I enhances insulin-dependent and insulin-independent glucose uptake by skeletal muscle

Therapeutic interventions that increase plasma high density lipoprotein (HDL) and apolipoprotein (apo) A-I levels have been reported to reduce plasma glucose levels and attenuate insulin resistance. The present study asks if this is a direct effect of increased glucose uptake by skeletal muscle. Incubation of primary human skeletal muscle cells (HSKMCs) with apoA-I increased insulin-dependent and insulin–independent glucose uptake in a time- and concentration-dependent manner. The increased glucose uptake was accompanied by enhanced phosphorylation of the insulin receptor (IR), insulin receptor substrate-1 (IRS-1), the serine/threonine kinase Akt and Akt substrate of 160 kDa (AS160). Cell surface levels of the glucose transporter type 4, GLUT4, were also increased. The apoA-I-mediated increase in glucose uptake by HSKMCs was dependent on phosphatidylinositol-4,5-bisphosphate 3-kinase (PI3K)/Akt, the ATP binding cassette transporter A1 (ABCA1) and scavenger receptor class B type I (SR-B1). Taken together, these results establish that apoA-I increases glucose disposal in skeletal muscle by activating the IR/IRS-1/PI3K/Akt/AS160 signal transduction pathway. The findings suggest that therapeutic agents that increase apoA-I levels may improve glycemic control in people with type 2 diabetes.

AMPK: a therapeutic target of heart failure-not only metabolism regulation

Heart failure (HF) is a serious disease with high mortality. The incidence of this disease has continued to increase over the past decade. All cardiovascular diseases causing dysfunction of various physiological processes can result in HF. AMP-activated protein kinase (AMPK), an energy sensor, has pleiotropic cardioprotective effects and plays a critical role in the progression of HF. In this review, we highlight that AMPK can not only improve the energy supply in the failing heart by promoting ATP production, but can also regulate several important physiological processes to restore heart function. In addition, we discuss some aspects of some potential clinical drugs which have effects on AMPK activation and may have value in treating HF. More studies, especially clinical trials, should be done to evaluate manipulation of AMPK activation as a potential means of treating HF.

Cross-talk of healthy and impaired human tissues for dissection of disease pathogenesis

Systemic diseases affect multiple tissues that interact with each other within a network difficult to explore at the body level. However, understanding the interdependences between tissues could be of high relevance for drug target identification, especially at the first stages of disease development. In vitro systems have the advantages of accessibility to measurements and precise controllability of culture conditions, but currently have limitations in mimicking human in vivo systemic tissue response. In this work, we present an in vitro model of cross-talk between an ex vivo culture of adipose tissue from an obese donor and a skeletal muscle in vitro model from a healthy donor. This is relevant to understand type 2 diabetes mellitus pathogenesis, as obesity is one of its main risk factors. The human adipose tissue biopsy was maintained as a three-dimensional culture for 48 h. Its conditioned culture medium was used to stimulate a human skeletal muscle-on-chip, developed by differentiating primary cells of a patient's biopsy under topological cues and molecular self-regulation. This system has been characterized to demonstrate its ability to mimic important features of the normal skeletal muscle response in vivo. We then found that the conditioned medium from a diseased adipose tissue is able to perturb the normal insulin sensitivity of a healthy skeletal muscle, as reported in the early stages of diabetes onset. In perspective, this work represents an important step toward the development of technological platforms that allow to study and dissect the systemic interaction between unhealthy and healthy tissues in vitro. © 2018 American Institute of Chemical Engineers Biotechnol. Prog., 35: e2766, 2019.

Chemical biology probes of mammalian GLUT structure and function

The structure and function of glucose transporters of the mammalian GLUT family of proteins has been studied over many decades, and the proteins have fascinated numerous research groups over this time. This interest is related to the importance of the GLUTs as archetypical membrane transport facilitators, as key limiters of the supply of glucose to cell metabolism, as targets of cell insulin and exercise signalling and of regulated membrane traffic, and as potential drug targets to combat cancer and metabolic diseases such as type 2 diabetes and obesity. This review focusses on the use of chemical biology approaches and sugar analogue probes to study these important proteins.

Body temperature elevation during exercise is essential for activating the Akt signaling pathway in the skeletal muscle of type 2 diabetic rats

This study examined the effect of changes in body temperature during exercise on signal transduction-related glucose uptake in the skeletal muscle of type 2 diabetic rats. Otsuka Long-Evans Tokushima Fatty rats (25 weeks of age), which have type 2 diabetes, were divided into the following four weight-matched groups; control (CON, n = 6), exercised under warm temperature (WEx, n = 8), exercised under cold temperature (CEx, n = 8), and heat treatment (HT, n = 6). WEx and CEx animals were subjected to running on a treadmill at 20 m/min for 30 min under warm (25°C) or cold (4°C) temperature. HT animals were exposed to single heat treatment (40–41°C for 30 min) in a heat chamber. Rectal and muscle temperatures were measured immediately after exercise and heat treatment, and the gastrocnemius muscle was sampled under anesthesia. Rectal and muscle temperatures increased significantly in rats in the WEx and HT, but not the CEx, groups. The phosphorylation levels of Akt, AS160, and TBC1D1 (Thr590) were significantly higher in the WEx and HT groups than the CON group (p < 0.05). In contrast, the phosphorylation levels of AMP-activated protein kinase, ACC, and TBC1D1 (Ser660) were significantly higher in rats in the WEx and CEx groups than the CON group (p < 0.05) but did not differ significantly between rats in the WEx and CEx groups. Body temperature elevation by heat treatment did not activate the AMPK signaling. Our data suggest that body temperature elevation during exercise is essential for activating the Akt signaling pathway in the skeletal muscle of rats with type 2 diabetic rats.

Exercise and Mitochondrial Remodeling in Skeletal Muscle in Type 2 Diabetes

Exercise is regarded as a potent stimulus in modulation of glucose utility and mitochondrial adaptations in skeletal muscle, leading to enhanced metabolic health. As mitochondria play a crucial role in sustaining metabolic homeostasis, and disturbances in mitochondrial function are highly linked with development of metabolic diseases, a comprehensive understanding of exercise-mediated mitochondrial remodeling under the pathophysiological condition of type 2 diabetes is warranted to develop an efficient therapeutic strategy. Although it is evident that the primary etiology of type 2 diabetes is insulin resistance, there is accumulating evidence linking abnormal mitochondrial functional and morphological properties to development of type 2 diabetes. Despite this, the precise molecular and cellular events that underline these phenomena remain uncertain. Mitochondria are highly dynamic subcellular organelles that can change mass and shape as necessary via coordinated processes such as mitochondrial fusion, fission, and biogenesis. Mitochondrial fusion is controlled by proteins, including mitofusin-1, mitofusin-2, and optic atrophy protein 1, while the fission process is mainly modulated by control of fission protein 1 and dynamin-related protein 1. Peroxisome proliferator-activated receptor gamma coactivator-1α acts as a master controller of mitochondrial biogenesis. The present review’s primary aims were to briefly discuss the cellular mechanisms of muscle fiber type-dependent glucose uptake and to highlight emerging evidence linking disturbances in mitochondrial dynamics to development of insulin resistance and type 2 diabetes. The potential for exercise to normalize type 2 diabetes-induced aberrant mitochondrial integrity is also addressed.

Long-Term Chronic Intermittent Hypobaric Hypoxia Induces Glucose Transporter (GLUT4) Translocation Through AMP-Activated Protein Kinase (AMPK) in the Soleus Muscle in Lean Rats

Background: In chronic hypoxia (CH) and short-term chronic intermittent hypoxia (CIH) exposure, glycemia and insulin levels decrease and insulin sensitivity increases, which can be explained by changes in glucose transport at skeletal muscles involving GLUT1, GLUT4, Akt, and AMPK, as well as GLUT4 translocation to cell membranes. However, during long-term CIH, there is no information regarding whether these changes occur similarly or differently than in other types of hypoxia exposure. This study evaluated the levels of AMPK and Akt and the location of GLUT4 in the soleus muscles of lean rats exposed to long-term CIH, CH, and normoxia (NX) and compared the findings.

Methods: Thirty male adult rats were randomly assigned to three groups: a NX (760 Torr) group (n = 10), a CIH group (2 days hypoxia/2 days NX; n = 10) and a CH group (n = 10). Rats were exposed to hypoxia for 30 days in a hypobaric chamber set at 428 Torr (4,600 m). Feeding (10 g daily) and fasting times were accurately controlled. Measurements included food intake (every 4 days), weight, hematocrit, hemoglobin, glycemia, serum insulin (by ELISA), and insulin sensitivity at days 0 and 30. GLUT1, GLUT4, AMPK levels and Akt activation in rat soleus muscles were determined by western blot. GLUT4 translocation was measured with confocal microscopy at day 30.

Results: (1) Weight loss and increases in hematocrit and hemoglobin were found in both hypoxic groups (p < 0.05). (2) A moderate decrease in glycemia and plasma insulin was found. (3) Insulin sensitivity was greater in the CIH group (p < 0.05). (4) There were no changes in GLUT1, GLUT4 levels or in Akt activation. (5) The level of activated AMPK was increased only in the CIH group (p < 0.05). (6) Increased GLUT4 translocation to the plasma membrane of soleus muscle cells was observed in the CIH group (p < 0.05).

Conclusion: In lean rats experiencing long-term CIH, glycemia and insulin levels decrease and insulin sensitivity increases. Interestingly, there is no increase of GLUT1 or GLUT4 levels or in Akt activation. Therefore, cellular regulation of glucose seems to primarily involve GLUT4 translocation to the cell membrane in response to hypoxia-mediated AMPK activation.

Voluntary exercise opposes insulin resistance of skeletal muscle glucose transport during liquid fructose ingestion in rats

We have recently reported that male rats given liquid fructose ingestion exhibit features of cardiometabolic abnormalities including non-obese insulin resistance with impaired insulin signaling transduction in skeletal muscle (Rattanavichit Y et al. Am J Physiol Regul Integr Comp Physiol 311: R1200-R1212, 2016). While exercise can attenuate obesity-related risks of cardiometabolic syndrome, the effectiveness and potential mechanism by which exercise modulates non-obese insulin resistance have not been fully studied. The present investigation evaluated whether regular exercise by voluntary wheel running (VWR) can reduce cardiometabolic risks induced by fructose ingestion. Moreover, the potential cellular adaptations following VWR on key signaling proteins known to influence insulin-induced glucose transport in skeletal muscle of fructose-ingested rats were investigated. Male Sprague-Dawley rats were given either water or liquid fructose (10% wt/vol) without or with access to running wheel for 6 weeks. We demonstrated that VWR restored insulin-stimulated glucose transport in the soleus muscle by improving the functionality of several signaling proteins, including insulin-stimulated IRβ Tyr¹¹⁵⁸/Tyr¹¹⁶²/Tyr¹¹⁶³ (82%), IRS-1 Tyr⁹⁸⁹ (112%), Akt Ser⁴⁷³ (56%), AS160 Thr⁶⁴² (76%), and AS160 Ser⁵⁸⁸ (82%). These effects were accompanied by lower insulin-stimulated phosphorylation of IRS-1 Ser³⁰⁷ (37%) and JNK Thr¹⁸³/Tyr¹⁸⁵ (49%), without significant changes in expression of proteins in the renin-angiotensin system. Intriguingly, multiple cardiometabolic abnormalities were not observed in fructose-ingested rats with access to VWR. Collectively, this study demonstrates that the development of cardiometabolic abnormalities as well as insulin resistance of skeletal muscle and defective signaling molecules in rats induced by fructose ingestion could be opposed by VWR.

Electrical pulse stimulation induces GLUT4 translocation in C2C12 myotubes that depends on Rab8A, Rab13, and Rab14

The signals mobilizing GLUT4 to the plasma membrane in response to muscle contraction are less known than those elicited by insulin. This disparity is undoubtedly due to lack of suitable in vitro models to study skeletal muscle contraction. We generated C₂C₁₂ myotubes stably expressing HA-tagged GLUT4 (C2C12-GLUT4 HA) that contract in response to electrical pulse stimulation (EPS) and investigated molecular mechanisms regulating GLUT4 HA. EPS (60 min, 20 V, 1 Hz, 24-ms pulses at 976-ms intervals) elicited a gain in surface GLUT4 HA (GLUT4 translocation) comparably to insulin or 5-amino imidazole-4-carboxamide ribonucleotide (AICAR). A myosin II inhibitor prevented EPS-stimulated myotube contraction and reduced surface GLUT4 by 56%. EPS stimulated AMPK and CaMKII phosphorylation, and EPS-stimulated GLUT4 translocation was reduced in part by small interfering (si)RNA-mediated AMPKα1/α2 knockdown, compound C, siRNA-mediated Ca²⁺/calmodulin-dependent protein kinase (CaMKII)δ knockdown, or CaMKII inhibitor KN93. Key regulatory residues on the Rab-GAPs AS160 and TBC1D1 were phosphorylated in response to EPS. Stable expression of an activated form of the Rab-GAP AS160 (AS160-4A) diminished EPS- and insulin-stimulated GLUT4 translocation, suggesting regulation of GLUT4 vesicle traffic by Rab GTPases. Knockdown of each Rab8a, Rab13, or Rab14 reduced, in part, GLUT4 translocation induced by EPS, whereas only Rab8a, or Rab14 knockdown reduced the AICAR response. In conclusion, EPS involves Rab8a, Rab13, and Rab14 to elicit GLUT4 translocation but not Rab10; moreover, Rab10 and Rab13 are not engaged by AMPK activation alone. C2C12-GLUT4 HA cultures constitute a valuable in vitro model to investigate molecular mechanisms of contraction-stimulated GLUT4 translocation.