Increased submaximal insulin-stimulated glucose uptake in mouse skeletal muscle after treadmill exercise

Acetaminophen influences musculoskeletal signaling but not adaptations to endurance exercise training

Acetaminophen (ACE) is a widely used analgesic and antipyretic drug with various applications, from pain relief to fever reduction. Recent studies have reported equivocal effects of habitual ACE intake on exercise performance, muscle growth, and risks to bone health. Thus, this study aimed to assess the impact of a 6-week, low-dose ACE regimen on muscle and bone adaptations in exercising and non-exercising rats. Nine-week-old Wistar rats (n = 40) were randomized to an exercise or control (no exercise) condition with ACE or without (placebo). For the exercise condition, rats ran 5 days per week for 6 weeks at a 5% incline for 2 min at 15 cm/s, 2 min at 20 cm/s, and 26 min at 25 cm/s. A human equivalent dose of ACE was administered (379 mg/kg body weight) in drinking water and adjusted each week based on body weight. Food, water intake, and body weight were measured daily. At the beginning of week 6, animals in the exercise group completed a maximal treadmill test. At the end of week 6, rats were euthanized, and muscle cross-sectional area (CSA), fiber type, and signaling pathways were measured. Additionally, three-point bending and microcomputer tomography were measured in the femur. Follow-up experiments in human primary muscle cells were used to explore supra-physiological effects of ACE. Data were analyzed using a two-way ANOVA for treatment (ACE or placebo) and condition (exercise or non-exercise) for all animal outcomes. Data for cell culture experiments were analyzed via ANOVA. If omnibus significance was found in either ANOVA, a post hoc analysis was completed, and a Tukey's adjustment was used. ACE did not alter body weight, water intake, food intake, or treadmill performance (p > .05). There was a treatment-by-condition effect for Young's Modulus where placebo exercise was significantly lower than placebo control (p < .05). There was no treatment by condition effects for microCT measures, muscle CSA, fiber type, or mRNA expression. Phosphorylated-AMPK was significantly increased with exercise (p < .05) and this was attenuated with ACE treatment. Furthermore, phospho-4EBP1 was depressed in the exercise group compared to the control (p < .05) and increased in the ACE control and ACE exercise group compared to placebo exercise (p < .05). A low dose of ACE did not influence chronic musculoskeletal adaptations in exercising rodents but acutely attenuated AMPK phosphorylation and 4EBP1 dephosphorylation post-exercise.

Ghrelin-responsive mediobasal hypothalamic neurons mediate exercise-associated food intake and exercise endurance

Previous studies have implicated the orexigenic hormone ghrelin as a mediator of exercise endurance and the feeding response postexercise. Specifically, plasma ghrelin levels nearly double in mice when they are subjected to an hour-long bout of high-intensity interval exercise (HIIE) using treadmills. Also, growth hormone secretagogue receptor-null (GHSR-null) mice exhibit decreased food intake following HIIE and diminished running distance (time until exhaustion) during a longer, stepwise exercise endurance protocol. To investigate whether ghrelin-responsive mediobasal hypothalamus (MBH) neurons mediate these effects, we stereotaxically delivered the inhibitory designer receptor exclusively activated by designer drugs virus AAV2-hSyn-DIO-hM4(Gi)-mCherry to the MBH of Ghsr-IRES-Cre mice, which express Cre recombinase directed by the Ghsr promoter. We found that chemogenetic inhibition of GHSR-expressing MBH neurons (upon delivery of clozapine-N-oxide) 1) suppressed food intake following HIIE, 2) reduced maximum running distance and raised blood glucose and blood lactate levels during an exercise endurance protocol, 3) reduced food intake following ghrelin administration, and 4) did not affect glucose tolerance. Further, HIIE increased MBH Ghsr expression. These results indicate that activation of ghrelin-responsive MBH neurons is required for the normal feeding response to HIIE and the usual amount of running exhibited during an exercise endurance protocol.

AS160 expression, but not AS160 Serine-588, Threonine-642, and Serine-704 phosphorylation, is essential for elevated insulin-stimulated glucose uptake by skeletal muscle from female rats after acute exercise

One exercise session can increase subsequent insulin-stimulated glucose uptake (ISGU) by skeletal muscle in both sexes. We recently found that muscle expression and phosphorylation of key sites of Akt substrate of 160 kDa (AS160; also called TBC1D4) are essential for the full-exercise effect on postexercise-ISGU (PEX-ISGU) in male rats. In striking contrast, AS160's role in increased PEX-ISGU has not been rigorously tested in females. Our rationale was to address this major knowledge gap. Wild-type (WT) and AS160-knockout (KO) rats were either sedentary or acutely exercised. Adeno-associated virus (AAV) vectors were engineered to express either WT-AS160 or AS160 mutated on key serine and threonine residues (Ser588, Thr642, and Ser704) to alanine to prevent their phosphorylation. AAV vectors were delivered to the muscle of AS160-KO rats to determine if WT-AS160 or phosphorylation-inactivated AS160 would influence PEX-ISGU. AS160-KO rats have lower skeletal muscle abundance of the GLUT4 glucose transporter protein. This GLUT4 deficit was rescued using AAV delivery of GLUT4 to determine if eliminating muscle GLUT4 deficiency would normalize PEX-ISGU. The novel results were as follows: (1) AS160 expression was required for greater PEX-ISGU; (2) rescuing muscle AS160 expression in AS160-KO rats restored elevated PEX-ISGU; (3) AS160's essential role for the postexercise increase in ISGU was not attributable to reduced muscle GLUT4 content; and (4) AS160 phosphorylation on Ser588, Thr642, and Ser704 was not essential for greater PEX-ISGU. In conclusion, these novel findings revealed that three phosphosites widely proposed to influence PEX-ISGU are not required for this important outcome in female rats.

Insulin and cancer: a tangled web

For a century, since the pioneering work of Otto Warburg, the interwoven relationship between metabolism and cancer has been appreciated. More recently, with obesity rates rising in the U.S. and worldwide, epidemiologic evidence has supported a link between obesity and cancer. A substantial body of work seeks to mechanistically unpack the association between obesity, altered metabolism, and cancer. Without question, these relationships are multifactorial and cannot be distilled to a single obesity- and metabolism-altering hormone, substrate, or factor. However, it is important to understand the hormone-specific associations between metabolism and cancer. Here, we review the links between obesity, metabolic dysregulation, insulin, and cancer, with an emphasis on current investigational metabolic adjuncts to standard-of-care cancer treatment.

Molecular Mechanisms of Exercise and Healthspan

Healthspan is the period of our life without major debilitating diseases. In the modern world where unhealthy lifestyle choices and chronic diseases taper the healthspan, which lead to an enormous economic burden, finding ways to promote healthspan becomes a pressing goal of the scientific community. Exercise, one of humanity's most ancient and effective lifestyle interventions, appears to be at the center of the solution since it can both treat and prevent the occurrence of many chronic diseases. Here, we will review the current evidence and opinions about regular exercise promoting healthspan through enhancing the functionality of our organ systems and preventing diseases.

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.

Exercise-Induced Improvement in Insulin-Stimulated Glucose Uptake by Rat Skeletal Muscle Is Absent in Male AS160-Knockout Rats, Partially Restored by Muscle Expression of Phosphomutated AS160, and Fully Restored by Muscle Expression of Wild-Type AS160

One exercise session can elevate insulin-stimulated glucose uptake (ISGU) in skeletal muscle, but the mechanisms remain elusive. Circumstantial evidence suggests a role for Akt substrate of 160 kDa (AS160 or TBC1D4). We used genetic approaches to rigorously test this idea. The initial experiment evaluated the role of AS160 in postexercise increase in ISGU using muscles from male wild-type (WT) and AS160-knockout (KO) rats. The next experiment used AS160-KO rats with an adeno-associated virus (AAV) approach to determine if rescuing muscle AS160 deficiency could restore the ability of exercise to improve ISGU. The third experiment tested if eliminating the muscle GLUT4 deficit in AS160-KO rats via AAV-delivered GLUT4 would enable postexercise enhancement of ISGU. The final experiment used AS160-KO rats and AAV delivery of AS160 mutated to prevent phosphorylation of Ser588, Thr642, and Ser704 to evaluate their role in postexercise ISGU. We discovered the following: 1) AS160 expression was essential for postexercise increase in ISGU; 2) rescuing muscle AS160 expression of AS160-KO rats restored postexercise enhancement of ISGU; 3) restoring GLUT4 expression in AS160-KO muscle did not rescue the postexercise increase in ISGU; and 4) although AS160 phosphorylation on three key sites was not required for postexercise elevation in ISGU, it was essential for the full exercise effect.

Exercise effects on γ3-AMPK activity, Akt substrate of 160 kDa phosphorylation, and glucose uptake in muscle of normal and insulin-resistant female rats

Previous studies demonstrated that acute exercise can enhance glucose uptake (GU), γ3-AMP-activated protein kinase (AMPK) activity, and Akt substrate of 160 kDa (AS160) phosphorylation in skeletal muscles from low-fat diet (LFD)- and high-fat diet (HFD)-fed male rats. Because little is known about exercise effects on these outcomes in females, we assessed postexercise GU by muscles incubated ± insulin, delta-insulin GU (GU of muscles incubated with insulin minus GU uptake of paired muscles incubated without insulin), and muscle signaling proteins from female rats fed a LFD or a brief HFD (2 wk). Rats were sedentary (LFD-SED, HFD-SED) or swim exercised. Immediately postexercise (IPEX) or 3 h postexercise (3hPEX), epitrochlearis muscles were incubated (no insulin IPEX; ±insulin 3hPEX) to determine GU. Muscle γ3-AMPK activity (IPEX, 3hPEX) and phosphorylated AS160 (pAS160; 3hPEX) were also assessed. γ3-AMPK activity and insulin-independent GU of IPEX rats exceeded sedentary rats without diet-related differences in either outcome. At 3hPEX, both GU by insulin-stimulated muscles and delta-insulin GU exceeded their respective diet-matched sedentary controls. GU by insulin-stimulated muscles, but not delta-insulin GU for LFD-3hPEX, exceeded HFD-3hPEX. LFD-3hPEX versus LFD-SED had greater γ3-AMPK activity and greater pAS160. HFD-3hPEX exceeded HFD-SED for pAS160 but not for γ3-AMPK activity. pAS160 and γ3-AMPK at 3hPEX did not differ between diet groups. These results revealed that increased γ3-AMPK activity at 3hPEX was not essential for greater GU in insulin-stimulated muscle or greater delta-insulin GU in HFD female rats. Similarly elevated γ3-AMPK activity in LFD-IPEX versus HFD-IPEX and pAS160 in LFD-3hPEX versus HFD-3hPEX may contribute to the comparable delta-insulin GU at 3hPEX in both diet groups.NEW & NOTEWORTHY Glucose uptake (GU) and phosphorylated AS160 (pAS160) by insulin-stimulated muscles at 3 h postexercise (3hPEX) exceeded diet-matched controls in female low-fat diet-fed (LFD) or high-fat diet-fed (HFD) rats. GU with insulin for LFD-3hPEX exceeded HFD-3hPEX, whereas pAS160 was similar between these groups. γ3-AMPK immediately postexercise (IPEX) was similarly elevated in LFD and HFD, but only LFD-3hPEX had increased γ3-AMPK. These results suggest that greater γ3-AMPK at IPEX and pAS160 at 3hPEX may contribute to elevated GU with insulin, but greater γ3-AMPK at 3hPEX was dispensable for female HFD rats.

Genes controlling skeletal muscle glucose uptake and their regulation by endurance and resistance exercise

Exercise improves the insulin sensitivity of glucose uptake in skeletal muscle. Due to that, exercise has become a cornerstone treatment for type 2 diabetes mellitus (T2DM). The mechanisms by which exercise improves skeletal muscle insulin sensitivity are, however, incompletely understood. We conducted a systematic review to identify all genes whose gain or loss of function alters skeletal muscle glucose uptake. We subsequently cross-referenced these genes with recently generated data sets on exercise-induced gene expression and signaling. Our search revealed 176 muscle glucose-uptake genes, meaning that their genetic manipulation altered glucose uptake in skeletal muscle. Notably, exercise regulates the expression or phosphorylation of more than 50% of the glucose-uptake genes or their protein products. This included many genes that previously have not been associated with exercise-induced insulin sensitivity. Interestingly, endurance and resistance exercise triggered some common but mostly unique changes in expression and phosphorylation of glucose-uptake genes or their protein products. Collectively, our work provides a resource of potentially new molecular effectors that play a role in the incompletely understood regulation of muscle insulin sensitivity by exercise.

Exercise as a drug for glucose management and prevention in type 2 diabetes mellitus

Physical inactivity and sedentary behavior are risk factors for type 2 diabetes mellitus (T2DM). Therefore, physical exercise (PE) together with medical treatment might be considered as a key strategy to counteract T2DM. Glycemic control is a central objective in the prevention and management of T2DM, and PE might be able to substantially affect the processes that determine it. Just like a drug, exercise can be dosed based on the characteristics of the individual to increase its benefits and reduce side effects. In this brief review, the mechanisms underlying the effects of PE on glucose metabolism in muscle are illustrated, and the effects of modulation of the parameters characterizing this atypical "drug" on glucose homeostasis are described.

AKT/AMPK-mediated phosphorylation of TBC1D4 disrupts the interaction with insulin-regulated aminopeptidase

TBC1D4 is a 160 kDa multidomain Rab GTPase-activating protein (RabGAP) and a downstream target of the insulin- and contraction-activated kinases AKT and AMPK. Phosphorylation of TBC1D4 has been linked to translocation of GLUT4 from storage vesicles (GSVs) to the cell surface. However, its impact on enzymatic activity is not well understood, as previous studies mostly investigated the truncated GAP domain lacking the known phosphorylation sites. In the present study, we expressed and purified recombinant full-length TBC1D4 using a baculovirus system. Size-exclusion chromatography and coimmunoprecipitation experiments revealed that full-length TBC1D4 forms oligomers of ∼600 kDa. Compared with the truncated GAP domain, full-length TBC1D4 displayed similar substrate specificity, but had a markedly higher specific GAP activity toward Rab10. Using high-resolution mass spectrometry, we mapped 19 Ser/Thr phosphorylation sites in TBC1D4. We determined Michaelis–Menten kinetics using in vitro phosphorylation assays with purified kinases and stable isotope-labeled γ-[¹⁸O₄]-ATP. These data revealed that Ser³²⁴ (K_M ∼6 μM) and Thr⁶⁴⁹ (K_M ∼25 μM) were preferential sites for phosphorylation by AKT, whereas Ser³⁴⁸, Ser⁵⁷⁷, Ser⁵⁹⁵ (K_M ∼10 μM), Ser⁷¹¹ (K_M ∼79 μM), and Ser⁷⁶⁴ were found to be preferred targets for AMPK. Phosphorylation of TBC1D4 by AKT or AMPK did not alter the intrinsic RabGAP activity, but did disrupt interaction with insulin-regulated aminopeptidase (IRAP), a resident protein of GSVs implicated in GLUT4 trafficking. These findings provide evidence that insulin and contraction may regulate TBC1D4 function primarily by disrupting the recruitment of the RabGAP to GLUT4 vesicles.

Nox4 mediates skeletal muscle metabolic responses to exercise

OBJECTIVE

The immediate signals that couple exercise to metabolic adaptations are incompletely understood. Nicotinamide adenine dinucleotide phosphate oxidase 4 (Nox4) produces reactive oxygen species (ROS) and plays a significant role in metabolic and vascular adaptation during stress conditions. Our objective was to determine the role of Nox4 in exercise-induced skeletal muscle metabolism.

METHODS

Mice were subjected to acute exercise to assess their immediate responses. mRNA and protein expression responses to Nox4 and hydrogen peroxide (H₂O₂) were measured by qPCR and immunoblotting. Functional metabolic flux was measured via ex vivo fatty acid and glucose oxidation assays using ¹⁴C-labeled palmitate and glucose, respectively. A chronic exercise regimen was also utilized and the time to exhaustion along with key markers of exercise adaptation (skeletal muscle citrate synthase and beta-hydroxyacyl-coA-dehydrogenase activity) were measured. Endothelial-specific Nox4-deficient mice were then subjected to the same acute exercise regimen and their subsequent substrate oxidation was measured.

RESULTS

We identified key exercise-responsive metabolic genes that depend on H₂O₂ and Nox4 using catalase and Nox4-deficient mice. Nox4 was required for the expression of uncoupling protein 3 (Ucp3), hexokinase 2 (Hk2), and pyruvate dehydrogenase kinase 4 (Pdk4), but not the expression of peroxisome proliferator-activated receptor gamma coactivator 1-alpha (Pgc-1α). Global Nox4 deletion resulted in decreased UCP3 protein expression and impaired glucose and fatty acid oxidization in response to acute exercise. Furthermore, Nox4-deficient mice demonstrated impaired adaptation to chronic exercise as measured by the time to exhaustion and activity of skeletal muscle citrate synthase and beta-hydroxyacyl-coA-dehydrogenase. Importantly, mice deficient in endothelial-Nox4 similarly demonstrated attenuated glucose and fatty acid oxidation following acute exercise.

CONCLUSIONS

We report that H₂O₂ and Nox4 promote immediate responses to exercise in skeletal muscle. Glucose and fatty acid oxidation were blunted in the Nox4-deficient mice post-exercise, potentially through regulation of UCP3 expression. Our data demonstrate that endothelial-Nox4 is required for glucose and fatty acid oxidation, suggesting inter-tissue cross-talk between the endothelium and skeletal muscle in response to exercise.

A milk casein hydrolysate-derived peptide enhances glucose uptake through the AMP-activated protein kinase signalling pathway in skeletal muscle cells

NEW FINDINGS

What is the central question of this study? How do common active ingredients contained in both Lactobacillus helveticus-fermented milk and milk casein hydrolysate (MCH) enhance glucose metabolism by skeletal muscle? What is the main finding and its importance? MCH enhanced glucose uptake in skeletal muscle cells by stimulating AMP-activated kinase, but not insulin, signalling. Moreover, the MCH-derived specific peptide Ile-Pro-Pro mimicked this effect, suggesting a mechanism for MCH-induced metabolic improvement.

ABSTRACT

Improvement of glucose metabolism in the skeletal muscle has a key role in exercise performance and prevention of metabolic diseases. In our previous study, we showed that intake of milk casein hydrolysate improves glucose metabolism in humans, but the mechanism of action was not elucidated. In this study, we aimed to investigate the mechanism of action of milk casein hydrolysate and its derived peptides on glucose uptake and glucose metabolic signalling in cultured skeletal muscle cells. Differentiated C2C12 myotubes were used for the experiments. The differentiated cells were incubated with milk casein hydrolysate, valine-proline-proline and isoleucine-proline-proline. Subsequently, the rate of 2-deoxy-glucose uptake and the phosphorylation levels of insulin-dependent and -independent signalling factors were examined. We found that the rate of 2-deoxy-glucose uptake in both milk casein hydrolysate and isoleucine-proline-proline-treated cells was higher than that in the control cells. Immunoblotting assays showed that the phosphorylation levels of AMP-activated protein kinase, a rate-limiting factor in insulin-independent signalling, and of liver kinase B1, an upstream factor of AMP-activated protein kinase, in both milk casein hydrolysate and isoleucine-proline-proline-treated cells were higher than those in the control cells. Such significant effects were not observed after treatment with valine-proline-proline. Moreover, the insulin-dependent signalling was not significantly affected under the different conditions. The findings of our study suggest that milk casein hydrolysate enhances glucose uptake by activating insulin-independent AMP-activated protein kinase signalling in skeletal muscle cells, which might be mediated by a milk casein hydrolysate-derived peptide, namely, isoleucine-proline-proline.

Modulation of Insulin Sensitivity by Exercise Training: Implications for Cardiovascular Prevention

The beneficial effects of physical activity on the cardiovascular system nowadays have achieved the relevance of clinical evidence. In fact, several studies have documented the benefits of exercise training in the prevention of the cardiovascular risk. Abnormalities of insulin signaling transduction account for the impairment of insulin sensitivity and development of insulin resistance, which, in turn, is responsible for the enhancement of cardiovascular risk. Insulin sensitivity is related to the degree of physical activity, and physical training has been shown to ameliorate insulin action in insulin-resistant subjects. This effect is mediated by the improvement of the molecular abnormalities that are responsible of the insulin resistance, contributing in this way to restore the physiological insulin sensitivity. However, it should be underlined that mechanisms that account for this phenomenon are extremely complex and still unclear. Further studies are required to better clarify the molecular basis of the exercise-evoked improvement of insulin signal.

Effects of metabolic state on the regulation of melanocortin circuits

Dysfunction in neurophysiological systems that regulate food intake and metabolism are at least partly responsible for obesity and related comorbidities. An important component of this process is the hypothalamic melanocortin system, where an imbalance can result in severe obesity and deficits in glucose metabolism. Exercise offers many health benefits related to cardiovascular improvements, hunger control, and blood glucose homeostasis. However, the molecular mechanism underlying the exercise-induced improvements to the melanocortin system remain undefined. Here, we review the role of the melanocortin system to sense hormonal, nutrient, and neuronal signals of energy status. This information is then relayed onto secondary neurons in order to regulate physiological parameters, which promote proper energy and glucose balance. We also provide an overview on the effects of physical exercise to induce biophysical changes in the melanocortin circuit which may regulate food intake, glucose metabolism and improve overall metabolic health.

Enhanced skeletal muscle insulin sensitivity after acute resistance-type exercise is upregulated by rapamycin-sensitive mTOR complex 1 inhibition

Acute aerobic exercise (AE) increases skeletal muscle insulin sensitivity for several hours, caused by acute activation of AMP-activated protein kinase (AMPK). Acute resistance exercise (RE) also activates AMPK, possibly improving insulin-stimulated glucose uptake. However, RE-induced rapamycin-sensitive mechanistic target of rapamycin complex 1 (mTORC1) activation is higher and has a longer duration than after AE. In molecular studies, mTORC1 was shown to be upstream of insulin receptor substrate 1 (IRS-1) Ser phosphorylation residue, inducing insulin resistance. Therefore, we hypothesised that although RE increases insulin sensitivity through AMPK activation, prolonged mTORC1 activation after RE reduces RE-induced insulin sensitising effect. In this study, we used an electrical stimulation-induced RE model in rats, with rapamycin as an inhibitor of mTORC1 activation. Our results showed that RE increased insulin-stimulated glucose uptake following AMPK signal activation. However, mTORC1 activation and IRS-1 Ser632/635 and Ser612 phosphorylation were elevated 6 h after RE, with concomitant impairment of insulin-stimulated Akt signal activation. By contrast, rapamycin inhibited these prior exercise responses. Furthermore, increases in insulin-stimulated skeletal muscle glucose uptake 6 h after RE were higher in rats with rapamycin treatment than with placebo treatment. Our data suggest that mTORC1/IRS-1 signaling inhibition enhances skeletal muscle insulin-sensitising effect of RE.

Effects of fasting on isolated murine skeletal muscle contractile function during acute hypoxia

Stored muscle carbohydrate supply and energetic efficiency constrain muscle functional capacity during exercise and are influenced by common physiological variables (e.g. age, diet, and physical activity level). Whether these constraints affect overall functional capacity or the timing of muscle energetic failure during acute hypoxia is not known. We interrogated skeletal muscle contractile properties in two anatomically distinct rodent hindlimb muscles that have well characterized differences in energetic efficiency (locomotory- extensor digitorum longus (EDL) and postural- soleus muscles) following a 24 hour fasting period that resulted in substantially reduced muscle carbohydrate supply. 180 mins of acute hypoxia resulted in complete energetic failure in all muscles tested, indicated by: loss of force production, substantial reductions in total adenosine nucleotide pool intermediates, and increased adenosine nucleotide degradation product-inosine monophosphate (IMP). These changes occurred in the absence of apparent myofiber structural damage assessed histologically by both transverse section and whole mount. Fasting and the associated reduction of the available intracellular carbohydrate pool (~50% decrease in skeletal muscle) did not significantly alter the timing to muscle functional impairment or affect the overall force/work capacities of either muscle type. Fasting resulted in greater passive tension development in both muscle types, which may have implications for the design of pre-clinical studies involving optimal timing of reperfusion or administration of precision therapeutics.

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.

Fiber type-specific effects of acute exercise on insulin-stimulated AS160 phosphorylation in insulin-resistant rat skeletal muscle

Muscle is a heterogeneous tissue composed of multiple fiber types. Earlier research revealed fiber type-selective postexercise effects on insulin-stimulated glucose uptake (ISGU) from insulin-resistant rats (increased for type IIA, IIB, IIBX, and IIX, but not type I). In whole muscle from insulin-resistant rats, the exercise increase in ISGU is accompanied by an exercise increase in insulin-stimulated AS160 phosphorylation (pAS160), an ISGU-regulating protein. We hypothesized that, in insulin-resistant muscle, the fiber type-selective exercise effects on ISGU would correspond to the fiber type-selective exercise effects on pAS160. Rats were fed a 2-wk high-fat diet (HFD) and remained sedentary (SED) or exercised before epitrochlearis muscles were dissected either immediately postexercise (IPEX) or at 3 h postexercise (3hPEX) using an exercise protocol that previously revealed fiber type-selective effects on ISGU. 3hPEX muscles and SED controls were incubated ± 100µU/mL insulin. Individual myofibers were isolated and pooled on the basis of myosin heavy chain (MHC) expression, and key phosphoproteins were measured. Myofiber glycogen and MHC expression were evaluated in muscles from other SED, IPEX, and 3hPEX rats. Insulin-stimulated pAkt^Ser473 and pAkt^Thr308 were unaltered by exercise in all fiber types. Insulin-stimulated pAS160 was greater for 3hPEX vs. SED on at least one phosphosite (Ser⁵⁸⁸, Thr⁶⁴², and/or Ser⁷⁰⁴) in type IIA, IIBX, and IIB fibers, but not in type I or IIX fibers. Both IPEX and 3hPEX glycogen were decreased versus SED in all fiber types. These results provided evidence that fiber type-specific pAS160 in insulin-resistant muscle may play a role in the previously reported fiber type-specific elevation in ISGU in some, but not all, fiber types.

The role of skeletal muscle Akt in the regulation of muscle mass and glucose homeostasis

OBJECTIVE

Skeletal muscle insulin signaling is a major determinant of muscle growth and glucose homeostasis. Protein kinase B/Akt plays a prominent role in mediating many of the metabolic effects of insulin. Mice and humans harboring systemic loss-of-function mutations in Akt2, the most abundant Akt isoform in metabolic tissues, are glucose intolerant and insulin resistant. Since the skeletal muscle accounts for a significant amount of postprandial glucose disposal, a popular hypothesis in the diabetes field suggests that a reduction in Akt, specifically in skeletal muscle, leads to systemic glucose intolerance and insulin resistance. Despite this common belief, the specific role of skeletal muscle Akt in muscle growth and insulin sensitivity remains undefined.

METHODS

We generated multiple mouse models of skeletal muscle Akt deficiency to evaluate the role of muscle Akt signaling in vivo. The effects of these genetic perturbations on muscle mass, glucose homeostasis and insulin sensitivity were assessed using both in vivo and ex vivo assays.

RESULTS

Surprisingly, mice lacking Akt2 alone in skeletal muscle displayed normal skeletal muscle insulin signaling, glucose tolerance, and insulin sensitivity despite a dramatic reduction in phosphorylated Akt. In contrast, deletion of both Akt isoforms (M-AktDKO) prevented downstream signaling and resulted in muscle atrophy. Despite the absence of Akt signaling, in vivo and ex vivo insulin-stimulated glucose uptake were normal in M-AktDKO mice. Similar effects on insulin sensitivity were observed in mice with prolonged deletion (4 weeks) of both skeletal muscle Akt isoforms selectively in adulthood. Conversely, short term deletion (2 weeks) of skeletal muscle specific Akt in adult muscles impaired insulin tolerance paralleling the effect observed by acute pharmacological inhibition of Akt in vitro. Mechanistically, chronic ablation of Akt induced mitochondrial dysfunction and activation of AMPK, which was required for insulin-stimulated glucose uptake in the absence of Akt.

CONCLUSIONS

Together, these data indicate that chronic reduction in Akt activity alone in skeletal muscle is not sufficient to induce insulin resistance or prevent glucose uptake in all conditions. Therefore, since insulin-stimulated glucose disposal in skeletal muscle is markedly impaired in insulin-resistant states, we hypothesize that alterations in signaling molecules in addition to skeletal muscle Akt are necessary to perturb glucose tolerance and insulin sensitivity in vivo.

Fiber type-selective exercise effects on AS160 phosphorylation

Earlier research using muscle tissue demonstrated that postexercise elevation in insulin-stimulated glucose uptake (ISGU) occurs concomitant with greater insulin-stimulated Akt substrate of 160 kDa (AS160) phosphorylation (pAS160) on sites that regulate ISGU. Because skeletal muscle is a heterogeneous tissue, we previously isolated myofibers from rat epitrochlearis to assess fiber type-selective ISGU. Exercise induced greater ISGU in type I, IIA, IIB, and IIBX but not IIX fibers. This study tested if exercise effects on pAS160 correspond with previously published fiber type-selective exercise effects on ISGU. Rats were studied immediately postexercise (IPEX) or 3.5 h postexercise (3.5hPEX) with time-matched sedentary controls. Myofibers dissected from the IPEX experiment were analyzed for fiber type (myosin heavy chain isoform expression) and key phosphoproteins. Isolated muscles from the 3.5hPEX experiment were incubated with or without insulin. Myofibers (3.5hPEX) were analyzed for fiber type, key phosphoproteins, and GLUT4 protein abundance. We hypothesized that insulin-stimulated pAS160 at 3.5hPEX would exceed sedentary controls only in fiber types characterized by greater ISGU postexercise. Values for phosphorylation of AMP-activated kinase substrates (acetyl CoA carboxylase^Ser79 and AS160^Ser704) from IPEX muscles exceeded sedentary values in each fiber type, suggesting exercise recruitment of all fiber types. Values for pAS160^Thr642 and pAS160^Ser704 from insulin-stimulated muscles 3.5hPEX exceeded sedentary values for type I, IIA, IIB, and IIBX but not IIX fibers. GLUT4 abundance was unaltered 3.5hPEX in any fiber type. These results advanced understanding of exercise-induced insulin sensitization by providing compelling support for the hypothesis that enhanced insulin-stimulated phosphorylation of AS160 is linked to elevated ISGU postexercise at a fiber type-specific level independent of altered GLUT4 expression.

Skeletal muscle fiber type-selective effects of acute exercise on insulin-stimulated glucose uptake in insulin-resistant, high-fat-fed rats

Insulin-stimulated glucose uptake (GU) by skeletal muscle is enhanced several hours after acute exercise in rats with normal or reduced insulin sensitivity. Skeletal muscle is composed of multiple fiber types, but exercise's effect on fiber type-specific insulin-stimulated GU in insulin-resistant muscle was previously unknown. Male rats were fed a high-fat diet (HFD; 2 wk) and were either sedentary (SED) or exercised (2-h exercise). Other, low-fat diet-fed (LFD) rats remained SED. Rats were studied immediately postexercise (IPEX) or 3 h postexercise (3hPEX). Epitrochlearis muscles from IPEX rats were incubated in 2-deoxy-[³H]glucose (2-[³H]DG) without insulin. Epitrochlearis muscles from 3hPEX rats were incubated with 2-[³H]DG ± 100 µU/ml insulin. After single fiber isolation, GU and fiber type were determined. Glycogen and lipid droplets (LDs) were assessed histochemically. GLUT4 abundance was determined by immunoblotting. In HFD-SED vs. LFD-SED rats, insulin-stimulated GU was decreased in type IIB, IIX, IIAX, and IIBX fibers. Insulin-independent GU IPEX was increased and glycogen content was decreased in all fiber types (types I, IIA, IIB, IIX, IIAX, and IIBX). Exercise by HFD-fed rats enhanced insulin-stimulated GU in all fiber types except type I. Single fiber analyses enabled discovery of striking fiber type-specific differences in HFD and exercise effects on insulin-stimulated GU. The fiber type-specific differences in insulin-stimulated GU postexercise in insulin-resistant muscle were not attributable to a lack of fiber recruitment, as indirectly evidenced by insulin-independent GU and glycogen IPEX, differences in multiple LD indexes, or altered GLUT4 abundance, implicating fiber type-selective differences in the cellular processes responsible for postexercise enhancement of insulin-mediated GLUT4 translocation.

Electrical pulse stimulation induces differential responses in insulin action in myotubes from severely obese individuals

KEY POINTS

Exercise/exercise training can enhance insulin sensitivity through adaptations in skeletal muscle, the primary site of insulin-mediated glucose disposal; however, in humans the range of improvement can vary substantially. The purpose of this study was to determine if obesity influences the magnitude of the exercise response in relation to improving insulin sensitivity in human skeletal muscle. Electrical pulse stimulation (EPS; 24 h) of primary human skeletal muscle myotubes improved insulin action in tissue from both lean and severely obese individuals, but responses to EPS were blunted with obesity. EPS improved insulin signal transduction in myotubes from lean but not severely obese subjects and increased AMP accumulation and AMPK Thr¹⁷² phosphorylation, but to a lesser degree in myotubes from the severely obese. These data reveal that myotubes of severely obese individuals enhance insulin action and stimulate exercise-responsive molecules with contraction, but in a manner and magnitude that differs from lean subjects.

ABSTRACT

Exercise/muscle contraction can enhance whole-body insulin sensitivity; however, in humans the range of improvements can vary substantially. In order, to determine if obesity influences the magnitude of the exercise response, this study compared the effects of electrical pulse stimulation (EPS)-induced contractile activity upon primary myotubes derived from lean and severely obese (BMI ≥ 40 kg/m² ) women. Prior to muscle contraction, insulin action was compromised in myotubes from the severely obese as was evident from reduced insulin-stimulated glycogen synthesis, glucose oxidation, glucose uptake, insulin signal transduction (IRS1, Akt, TBC1D4), and insulin-stimulated GLUT4 translocation. EPS (24 h) increased AMP, IMP, AMPK Thr¹⁷² phosphorylation, PGC1α content, and insulin action in myotubes of both the lean and severely obese subjects. However, despite normalizing indices of insulin action to levels seen in the lean control (non-EPS) condition, responses to EPS were blunted with obesity. EPS improved insulin signal transduction in myotubes from lean but not severely obese subjects and EPS increased AMP accumulation and AMPK Thr¹⁷² phosphorylation, but to a lesser degree in myotubes from the severely obese. These data reveal that myotubes of severely obese individuals enhance insulin action and stimulate exercise-responsive molecules with contraction, but in a manner and magnitude that differs from lean subjects.

Cellular and synaptic reorganization of arcuate NPY/AgRP and POMC neurons after exercise

OBJECTIVE

Hypothalamic Pro-opiomelanocortin (POMC) and Neuropeptide Y/Agouti-Related Peptide (NPY/AgRP) neurons are critical nodes of a circuit within the brain that sense key metabolic cues as well as regulate metabolism. Importantly, these neurons retain an innate ability to rapidly reorganize synaptic inputs and electrophysiological properties in response to metabolic state. While the cellular properties of these neurons have been investigated in the context of obesity, much less is known about the effects of exercise training.

METHODS

In order to further investigate this issue, we utilized neuron-specific transgenic mouse models to identify POMC and NPY/AgRP neurons for patch-clamp electrophysiology experiments.

RESULTS

Using whole-cell patch-clamp electrophysiology, we found exercise depolarized and increased firing rate of arcuate POMC neurons. The increased excitability of POMC neurons was concomitant with increased excitatory inputs to these neurons. In agreement with recent work suggesting leptin plays an important role in the synaptic (re)organization of POMC neurons, POMC neurons which express leptin receptors were more sensitive to exercise-induced changes in biophysical properties. Opposite to effects observed in POMC neurons, NPY neurons were shunted toward inhibition following exercise.

CONCLUSIONS

Together, these data support a rapid reorganization of synaptic inputs and biophysical properties in response to exercise, which may facilitate adaptations to altered energy balance and glucose metabolism.

Exercise training reduces the insulin-sensitizing effect of a single bout of exercise in human skeletal muscle

KEY POINTS

A single bout of exercise is capable of increasing insulin sensitivity in human skeletal muscle. Whether this ability is affected by training status is not clear. Studies in mice suggest that the AMPK-TBC1D4 signalling axis is important for the increased insulin-stimulated glucose uptake after a single bout of exercise. The present study is the first longitudinal intervention study to show that, although exercise training increases insulin-stimulated glucose uptake in skeletal muscle at rest, it diminishes the ability of a single bout of exercise to enhance muscle insulin-stimulated glucose uptake. The present study provides novel data indicating that AMPK in human skeletal muscle is important for the insulin-sensitizing effect of a single bout of exercise.

ABSTRACT

Not only chronic exercise training, but also a single bout of exercise, increases insulin-stimulated glucose uptake in skeletal muscle. However, it is not well described how adaptations to exercise training affect the ability of a single bout of exercise to increase insulin sensitivity. Rodent studies suggest that the insulin-sensitizing effect of a single bout of exercise is AMPK-dependent (presumably via the α₂ β₂ γ₃ AMPK complex). Whether this is also the case in humans is unknown. Previous studies have shown that exercise training decreases the expression of the α₂ β₂ γ₃ AMPK complex and diminishes the activation of this complex during exercise. Thus, we hypothesized that exercise training diminishes the ability of a single bout of exercise to enhance muscle insulin sensitivity. We investigated nine healthy male subjects who performed one-legged knee-extensor exercise at the same relative intensity before and after 12 weeks of exercise training. Training increased V ̇ O 2 peak and expression of mitochondrial proteins in muscle, whereas the expression of AMPKγ3 was decreased. Training also increased whole body and muscle insulin sensitivity. Interestingly, insulin-stimulated glucose uptake in the acutely exercised leg was not enhanced further by training. Thus, the increase in insulin-stimulated glucose uptake following a single bout of one-legged exercise was lower in the trained vs. untrained state. This was associated with reduced signalling via confirmed α₂ β₂ γ₃ AMPK downstream targets (ACC and TBC1D4). These results suggest that the insulin-sensitizing effect of a single bout of exercise is also AMPK-dependent in human skeletal muscle.

Postexercise improvement in glucose uptake occurs concomitant with greater γ3-AMPK activation and AS160 phosphorylation in rat skeletal muscle

A single exercise session can increase insulin-stimulated glucose uptake (GU) by skeletal muscle, concomitant with greater Akt substrate of 160 kDa (AS160) phosphorylation on Akt-phosphosites (Thr⁶⁴² and Ser⁵⁸⁸) that regulate insulin-stimulated GU. Recent research using mouse skeletal muscle suggested that ex vivo 5-aminoimidazole-4-carboxamide ribonucleotide (AICAR) or electrically stimulated contractile activity-inducing increased γ3-AMPK activity and AS160 phosphorylation on a consensus AMPK-motif (Ser⁷⁰⁴) resulted in greater AS160 Thr⁶⁴² phosphorylation and GU by insulin-stimulated muscle. Our primary goal was to determine whether in vivo exercise that increases insulin-stimulated GU in rat skeletal muscle would also increase γ3-AMPK activity and AS160 site-selective phosphorylation (Ser⁵⁸⁸, Thr⁶⁴², and Ser⁷⁰⁴) immediately postexercise (IPEX) and/or 3 h postexercise (3hPEX). Epitrochlearis muscles isolated from sedentary and exercised (2-h swim exercise; studied IPEX and 3hPEX) rats were incubated with 2-deoxyglucose to determine GU (without insulin at IPEX; without or with insulin at 3hPEX). Muscles were also assessed for γ1-AMPK activity, γ3-AMPK activity, phosphorylated AMPK (pAMPK), and phosphorylated AS160 (pAS160). IPEX versus sedentary had greater γ3-AMPK activity, pAS160 (Ser⁵⁸⁸, Thr⁶⁴², Ser⁷⁰⁴), and GU with unaltered γ1-AMPK activity. 3hPEX versus sedentary had greater γ3-AMPK activity, pAS160 Ser⁷⁰⁴, and GU with or without insulin; greater pAS160 Thr⁶⁴² only with insulin; and unaltered γ1-AMPK activity. These results using an in vivo exercise protocol that increased insulin-stimulated GU in rat skeletal muscle are consistent with the hypothesis that in vivo exercise-induced enhancement of γ3-AMPK activation and AS160 Ser⁷⁰⁴ IPEX and 3hPEX are important for greater pAS160 Thr⁶⁴² and enhanced insulin-stimulated GU by skeletal muscle.

Serum Is Not Necessary for Prior Pharmacological Activation of AMPK to Increase Insulin Sensitivity of Mouse Skeletal Muscle

Exercise, contraction, and pharmacological activation of AMP-activated protein kinase (AMPK) by 5-aminoimidazole-4-carboxamide ribonucleotide (AICAR) have all been shown to increase muscle insulin sensitivity for glucose uptake. Intriguingly, improvements in insulin sensitivity following contraction of isolated rat and mouse skeletal muscle and prior AICAR stimulation of isolated rat skeletal muscle seem to depend on an unknown factor present in serum. One study recently questioned this requirement of a serum factor by showing serum-independency with muscle from old rats. Whether a serum factor is necessary for prior AICAR stimulation to increase insulin sensitivity of mouse skeletal muscle is not known. Therefore, we investigated the necessity of serum for this effect of AICAR in mouse skeletal muscle. We found that the ability of prior AICAR stimulation to improve insulin sensitivity of mouse skeletal muscle did not depend on the presence of serum during AICAR stimulation. Although prior AICAR stimulation did not enhance proximal insulin signaling, insulin-stimulated phosphorylation of Tre-2/BUB2/CDC16- domain family member 4 (TBC1D4) Ser711 was greater in prior AICAR-stimulated muscle compared to all other groups. These results imply that the presence of a serum factor is not necessary for prior AMPK activation by AICAR to enhance insulin sensitivity of mouse skeletal muscle.

Ghrelin mediates exercise endurance and the feeding response post-exercise

Objective

Exercise training has several well-established health benefits, including many related to body weight, appetite control, and blood glucose homeostasis. However, the molecular mechanisms and, in particular, the hormonal systems that mediate and integrate these beneficial effects are poorly understood. In the current study, we aimed to investigate the role of the hormone ghrelin and its receptor, the growth hormone secretagogue receptor (GHSR; ghrelin receptor), in mediating the effects of exercise on food intake and blood glucose following exercise as well as in regulating exercise endurance capacity.

Methods

We used two mouse models of treadmill running to characterize the changes in plasma ghrelin with exercise. We also assessed the role of the ghrelin system to influence food intake and blood glucose after exercise, exercise endurance, and parameters potentially linked to responses to exercise. Mice lacking GHSRs (GHSR-null mice) and wild-type littermates were studied.

Results

An acute bout of exercise transiently elevated plasma acyl-ghrelin. Without the action of this increased ghrelin on GHSRs (as in GHSR-null mice), high intensity interval exercise markedly reduced food intake compared to control mice. The effect of exercise to acutely raise blood glucose remained unmodified in GHSR-null mice. Exercise-induced increases in plasma ghrelin positively correlated with endurance capacity, and time to exhaustion was reduced in GHSR-null mice as compared to wild-type littermates. In an effort to mechanistically explain their reduced exercise endurance, exercised GHSR-null mice exhibited an abrogated sympathoadrenal response, lower overall insulin-like growth factor-1 levels, and altered glycogen utilization.

Conclusions

Exercise transiently increases plasma ghrelin. GHSR-null mice exhibit decreased food intake following high intensity interval exercise and decreased endurance when submitted to an exercise endurance protocol. These data suggest that an intact ghrelin system limits the capacity of exercise to restrict food intake following exercise, although it enhances exercise endurance.

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High intensity exercise transiently increases plasma ghrelin.

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Without ghrelin action on its receptors (growth hormone secretagogue receptors), exercise markedly reduces food intake.

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An intact ghrelin system enhances exercise endurance.

AMPK in skeletal muscle function and metabolism

Skeletal muscle possesses a remarkable ability to adapt to various physiologic conditions. AMPK is a sensor of intracellular energy status that maintains energy stores by fine-tuning anabolic and catabolic pathways. AMPK’s role as an energy sensor is particularly critical in tissues displaying highly changeable energy turnover. Due to the drastic changes in energy demand that occur between the resting and exercising state, skeletal muscle is one such tissue. Here, we review the complex regulation of AMPK in skeletal muscle and its consequences on metabolism (e.g., substrate uptake, oxidation, and storage as well as mitochondrial function of skeletal muscle fibers). We focus on the role of AMPK in skeletal muscle during exercise and in exercise recovery. We also address adaptations to exercise training, including skeletal muscle plasticity, highlighting novel concepts and future perspectives that need to be investigated. Furthermore, we discuss the possible role of AMPK as a therapeutic target as well as different AMPK activators and their potential for future drug development.—Kjøbsted, R., Hingst, J. R., Fentz, J., Foretz, M., Sanz, M.-N., Pehmøller, C., Shum, M., Marette, A., Mounier, R., Treebak, J. T., Wojtaszewski, J. F. P., Viollet, B., Lantier, L. AMPK in skeletal muscle function and metabolism.

Nitric oxide is required for the insulin sensitizing effects of contraction in mouse skeletal muscle

KEY POINTS

People with insulin resistance or type 2 diabetes can substantially increase their skeletal muscle glucose uptake during exercise and insulin sensitivity after exercise. Skeletal muscle nitric oxide (NO) is important for glucose uptake during exercise, although how prior exercise increases insulin sensitivity is unclear. In the present study, we examined whether NO is necessary for normal increases in skeletal muscle insulin sensitivity after contraction ex vivo in mouse muscle. The present study uncovers, for the first time, a novel role for NO in the insulin sensitizing effects of ex vivo contraction, which is independent of blood flow.

ABSTRACT

The factors regulating the increase in skeletal muscle insulin sensitivity after exercise are unclear. We examined whether nitric oxide (NO) is required for the increase in insulin sensitivity after ex vivo contractions. Isolated C57BL/6J mouse EDL muscles were contracted for 10 min or remained at rest (basal) with or without the NO synthase (NOS) inhibition (N^G -monomethyl-l-arginine; l-NMMA; 100 μm). Then, 3.5 h post contraction/basal, muscles were exposed to saline or insulin (120 μU ml^-1 ) with or without l-NMMA during the last 30 min. l-NMMA had no effect on basal skeletal muscle glucose uptake. The increase in muscle glucose uptake with insulin (57%) was significantly (P < 0.05) greater after prior contraction (140% increase). NOS inhibition during the contractions had no effect on this insulin-sensitizing effect of contraction, whereas NOS inhibition during insulin prevented the increase in skeletal muscle insulin sensitivity post-contraction. Soluble guanylate cyclase inhibition, protein kinase G (PKG) inhibition or cyclic nucleotide phosphodiesterase inhibition each had no effect on the insulin-sensitizing effect of prior contraction. In conclusion, NO is required for increases in insulin sensitivity several hours after contraction of mouse skeletal muscle via a cGMP/PKG independent pathway.

Improvement of obesity-linked skeletal muscle insulin resistance by strength and endurance training

Obesity-linked insulin resistance is mainly due to fatty acid overload in non-adipose tissues, particularly skeletal muscle and liver, where it results in high production of reactive oxygen species and mitochondrial dysfunction. Accumulating evidence indicates that resistance and endurance training alone and in combination can counteract the harmful effects of obesity increasing insulin sensitivity, thus preventing diabetes. This review focuses the mechanisms underlying the exercise role in opposing skeletal muscle insulin resistance-linked metabolic dysfunction. It is apparent that exercise acts through two mechanisms: (1) it stimulates glucose transport by activating an insulin-independent pathway and (2) it protects against mitochondrial dysfunction-induced insulin resistance by increasing muscle antioxidant defenses and mitochondrial biogenesis. However, antioxidant supplementation combined with endurance training increases glucose transport in insulin-resistant skeletal muscle in an additive fashion only when antioxidants that are able to increase the expression of antioxidant enzymes and/or the activity of components of the insulin signaling pathway are used.

Role of AMP-Activated Protein Kinase for Regulating Post-exercise Insulin Sensitivity

Skeletal muscle insulin resistance precedes development of type 2 diabetes (T2D). As skeletal muscle is a major sink for glucose disposal, understanding the molecular mechanisms involved in maintaining insulin sensitivity of this tissue could potentially benefit millions of people that are diagnosed with insulin resistance. Regular physical activity in both healthy and insulin-resistant individuals is recognized as the single most effective intervention to increase whole-body insulin sensitivity and thereby positively affect glucose homeostasis. A single bout of exercise has long been known to increase glucose disposal in skeletal muscle in response to physiological insulin concentrations. While this effect is identified to be restricted to the previously exercised muscle, the molecular basis for an apparent convergence between exercise- and insulin-induced signaling pathways is incompletely known. In recent years, we and others have identified the Rab GTPase-activating protein, TBC1 domain family member 4 (TBC1D4) as a target of key protein kinases in the insulin- and exercise-activated signaling pathways. Our working hypothesis is that the AMP-activated protein kinase (AMPK) is important for the ability of exercise to insulin sensitize skeletal muscle through TBC1D4. Here, we aim to provide an overview of the current available evidence linking AMPK to post-exercise insulin sensitivity.

Exercise Increases Human Skeletal Muscle Insulin Sensitivity via Coordinated Increases in Microvascular Perfusion and Molecular Signaling

Insulin resistance is a major health risk, and although exercise clearly improves skeletal muscle insulin sensitivity, the mechanisms are unclear. Here we show that initiation of a euglycemic-hyperinsulinemic clamp 4 h after single-legged exercise in humans increased microvascular perfusion (determined by contrast-enhanced ultrasound) by 65% in the exercised leg and 25% in the rested leg (P < 0.05) and that leg glucose uptake increased 50% more (P < 0.05) in the exercised leg than in the rested leg. Importantly, infusion of the nitric oxide synthase inhibitor l-N^G-monomethyl-l-arginine acetate (l-NMMA) into both femoral arteries reversed the insulin-stimulated increase in microvascular perfusion in both legs and abrogated the greater glucose uptake in the exercised compared with the rested leg. Skeletal muscle phosphorylation of TBC1D4 Ser³¹⁸ and Ser⁷⁰⁴ and glycogen synthase activity were greater in the exercised leg before insulin and increased similarly in both legs during the clamp, and l-NMMA had no effect on these insulin-stimulated signaling pathways. Therefore, acute exercise increases insulin sensitivity of muscle by a coordinated increase in insulin-stimulated microvascular perfusion and molecular signaling at the level of TBC1D4 and glycogen synthase in muscle. This secures improved glucose delivery on the one hand and increased ability to take up and dispose of the delivered glucose on the other hand.

Enhanced Muscle Insulin Sensitivity After Contraction/Exercise Is Mediated by AMPK

Earlier studies have demonstrated that muscle insulin sensitivity to stimulate glucose uptake is enhanced several hours after an acute bout of exercise. Using AICAR, we recently demonstrated that prior activation of AMPK is sufficient to increase insulin sensitivity in mouse skeletal muscle. Here we aimed to determine whether activation of AMPK is also a prerequisite for the ability of muscle contraction to increase insulin sensitivity. We found that prior in situ contraction of m. extensor digitorum longus (EDL) and treadmill exercise increased muscle and whole-body insulin sensitivity in wild-type (WT) mice, respectively. These effects were not found in AMPKα1α2 muscle-specific knockout mice. Prior in situ contraction did not increase insulin sensitivity in m. soleus from either genotype. Improvement in muscle insulin sensitivity was not associated with enhanced glycogen synthase activity or proximal insulin signaling. However, in WT EDL muscle, prior in situ contraction enhanced insulin-stimulated phosphorylation of TBC1D4 Thr⁶⁴⁹ and Ser⁷¹¹ Such findings are also evident in prior exercised and insulin-sensitized human skeletal muscle. Collectively, our data suggest that the AMPK-TBC1D4 signaling axis is likely mediating the improved muscle insulin sensitivity after contraction/exercise and illuminates an important and physiologically relevant role of AMPK in skeletal muscle.

Novel single skeletal muscle fiber analysis reveals a fiber type-selective effect of acute exercise on glucose uptake

One exercise session can induce subsequently elevated insulin sensitivity that is largely attributable to greater insulin-stimulated glucose uptake by skeletal muscle. Because skeletal muscle is a heterogeneous tissue comprised of diverse fiber types, our primary aim was to determine exercise effects on insulin-independent and insulin-dependent glucose uptake by single fibers of different fiber types. We hypothesized that each fiber type featuring elevated insulin-independent glucose uptake immediately postexercise (IPEX) would be characterized by increased insulin-dependent glucose uptake at 3.5 h postexercise (3.5hPEX). Rat epitrochlearis muscles were isolated and incubated with 2-[³H]deoxyglucose. Muscles from IPEX and sedentary (SED) controls were incubated without insulin. Muscles from 3.5hPEX and SED controls were incubated ± insulin. Glucose uptake (2-[³H]deoxyglucose accumulation) and fiber type (myosin heavy chain isoform expression) were determined for single fibers dissected from the muscles. Major new findings included the following: 1) insulin-independent glucose uptake was increased IPEX in single fibers of each fiber type (types I, IIA, IIB, IIBX, and IIX), 2) glucose uptake values from insulin-stimulated type I and IIA fibers exceeded the values for the other fiber types, 3) insulin-stimulated glucose uptake for type IIX exceeded IIB fibers, and 4) the 3.5hPEX group vs. SED had greater insulin-stimulated glucose uptake in type I, IIA, IIB, and IIBX but not type IIX fibers. Insulin-dependent glucose uptake was increased at 3.5hPEX in each fiber type except for IIX fibers, although insulin-independent glucose uptake was increased IPEX in all fiber types (including type IIX). Single fiber analysis enabled the discovery of this fiber type-related difference for postexercise, insulin-stimulated glucose uptake.

Profiling Carbonylated Proteins in Heart and Skeletal Muscle Mitochondria from Trained and Untrained Mice

Understanding the relationship between physical exercise, reactive oxygen species, and skeletal muscle modification is important in order to better identify the benefits or the damages that appropriate or inappropriate exercise can induce. Heart and skeletal muscles have a high density of mitochondria with robust energetic demands, and mitochondria plasticity has an important role in both the cardiovascular system and skeletal muscle responses. The aim of this study was to investigate the influence of regular physical activity on the oxidation profiles of mitochondrial proteins from heart and tibialis anterior muscles. To this end, we used the mouse as animal model. Mice were divided into two groups: untrained and regularly trained. The carbonylated protein pattern was studied by two-dimensional gel electrophoresis followed by Western blot with anti-dinitrophenyl hydrazone antibodies. Mass spectrometry analysis allowed the identification of several different protein oxidation sites, including methionine, cysteine, proline, and leucine residues. A large number of oxidized proteins were found in both untrained and trained animals. Moreover, mitochondria from skeletal muscles and heart showed almost the same carbonylation pattern. Interestingly, exercise training seems to increase the carbonylation level mainly of mitochondrial proteins from skeletal muscle.

Rac1 in Muscle Is Dispensable for Improved Insulin Action After Exercise in Mice

Exercise has a potent insulin-sensitivity enhancing effect on skeletal muscle, but the intracellular mechanisms that mediate this effect are not well understood. In muscle, Ras-related C3 botulinum toxin substrate 1 (Rac1) regulates both insulin- and contraction-stimulated glucose transport and is dysregulated in insulin resistant muscle. However, whether Rac1 is involved in mediating enhanced insulin sensitivity after an acute bout of exercise is unresolved. To address this question, we investigated after exercise whole-body (insulin tolerance test) as well as muscle (insulin-stimulated 2-deoxyglucose transport in isolated soleus muscle) insulin sensitivity in inducible muscle-specific Rac1 knockout (mKO) and wild-type (WT) littermate mice. Previous exercise enhanced whole-body insulin sensitivity by 40% in WT mice and rescued the insulin intolerance in Rac1 mKO mice by improving whole-body insulin sensitivity by 230%. In agreement, previous exercise significantly improved insulin sensitivity by 20% in WT and by 40% in Rac1 mKO soleus muscles. These findings suggest that muscle Rac1 is dispensable for the insulin sensitizing effect of exercise. Moreover, insulin resistance in Rac1 mKO mice can be completely normalized by previous exercise explaining why insulin resistant patients can increase insulin action with exercise despite dysfunctional Rac1 activity in muscle.

Mechanisms for greater insulin-stimulated glucose uptake in normal and insulin-resistant skeletal muscle after acute exercise

Enhanced skeletal muscle and whole body insulin sensitivity can persist for up to 24-48 h after one exercise session. This review focuses on potential mechanisms for greater postexercise and insulin-stimulated glucose uptake (ISGU) by muscle in individuals with normal or reduced insulin sensitivity. A model is proposed for the processes underlying this improvement; i.e., triggers initiate events that activate subsequent memory elements, which store information that is relayed to mediators, which translate memory into action by controlling an end effector that directly executes increased insulin-stimulated glucose transport. Several candidates are potential triggers or memory elements, but none have been conclusively verified. Regarding potential mediators in both normal and insulin-resistant individuals, elevated postexercise ISGU with a physiological insulin dose coincides with greater Akt substrate of 160 kDa (AS160) phosphorylation without improved proximal insulin signaling at steps from insulin receptor binding to Akt activity. Causality remains to be established between greater AS160 phosphorylation and improved ISGU. The end effector for normal individuals is increased GLUT4 translocation, but this remains untested for insulin-resistant individuals postexercise. Following exercise, insulin-resistant individuals can attain ISGU values similar to nonexercising healthy controls, but after a comparable exercise protocol performed by both groups, ISGU for the insulin-resistant group has been consistently reported to be below postexercise values for the healthy group. Further research is required to fully understand the mechanisms underlying the improved postexercise ISGU in individuals with normal or subnormal insulin sensitivity and to explain the disparity between these groups after similar exercise.

Prior AICAR stimulation increases insulin sensitivity in mouse skeletal muscle in an AMPK-dependent manner

An acute bout of exercise increases glucose uptake in skeletal muscle by an insulin-independent mechanism. In the period after exercise, insulin sensitivity to increased glucose uptake is enhanced. The molecular mechanisms underpinning this phenomenon are poorly understood but appear to involve an increased cell surface abundance of GLUT4. While increased proximal insulin signaling does not seem to mediate this effect, elevated phosphorylation of TBC1D4, a downstream target of both insulin (Akt) and exercise (AMPK) signaling, appears to play a role. The main purpose of this study was to determine whether AMPK activation increases skeletal muscle insulin sensitivity. We found that prior AICAR stimulation of wild-type mouse muscle increases insulin sensitivity to stimulate glucose uptake. However, this was not observed in mice with reduced or ablated AMPK activity in skeletal muscle. Furthermore, prior AICAR stimulation enhanced insulin-stimulated phosphorylation of TBC1D4 at Thr(649) and Ser(711) in wild-type muscle only. These phosphorylation events were positively correlated with glucose uptake. Our results provide evidence to support that AMPK activation is sufficient to increase skeletal muscle insulin sensitivity. Moreover, TBC1D4 phosphorylation may facilitate the effect of prior AMPK activation to enhance glucose uptake in response to insulin.

Acute exercise increases insulin sensitivity in adult sheep: a new preclinical model

In healthy humans and rodents, chronic and acute exercise improves subsequent insulin sensitivity of skeletal muscle. A large animal species with similar metabolic responses to exercise would permit longitudinal studies, including repeated biopsies of muscle and other tissues not possible in rodents, and enable study of interactions with insulin-resistant physiological states not feasible in humans. Therefore, we examined whether acute exercise increases insulin sensitivity in adult sheep. Insulin sensitivity was measured by hyperinsulinemic euglycemic clamp (HEC) in mature female sheep ( n = 7). Sheep were familiarized to treadmill walking and then performed an acute exercise bout (30 min, 8% slope, up to 4.4 km/h). A second HEC was conducted ∼18 h after the acute exercise. Musculus semimembranosus biopsies were obtained before and after each HEC. Glucose infusion rate during the HEC increased 40% ( P = 0.003) and insulin sensitivity (glucose infusion rate/plasma insulin concentration) increased 32% ( P = 0.028) after acute exercise. Activation of proximal insulin signaling in skeletal muscle after the HEC, measured as Ser473 phosphorylation of Akt, increased approximately five-fold in response to insulin ( P < 0.001) and was unaltered by acute exercise performed 18 h earlier. PGC1α and GLUT4 protein, glycogen content and citrate synthase activity in skeletal muscle did not change in response to insulin or exercise. In conclusion, improved insulin sensitivity and unchanged proximal insulin signaling on the day after acute exercise in sheep are consistent with responses in humans and rodents, suggesting that the sheep is an appropriate large-animal model in which to study responses to 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.

Postexercise improvement in insulin-stimulated glucose uptake occurs concomitant with greater AS160 phosphorylation in muscle from normal and insulin-resistant rats

Earlier research on rats with normal insulin sensitivity demonstrated that acute exercise increased insulin-stimulated glucose uptake (GU) concomitant with greater phosphorylation of Akt substrate of 160 kDa (pAS160). Because mechanisms for exercise effects on GU in insulin-resistant muscle are unknown, our primary objective was to assess insulin-stimulated GU, proximal insulin signaling (insulin receptor [IR] tyrosine phosphorylation, IR substrate 1-phosphatidylinositol-3-kinase, and Akt phosphorylation and activity), and pAS160 in muscles from acutely exercised (one session) and sedentary rats fed either chow (low-fat diet [LFD]; normal insulin sensitivity) or a high-fat diet (HFD; for 2 weeks, insulin-resistant). At 3 h postexercise (3hPEX), isolated epitrochlearis muscles were used for insulin-stimulated GU and insulin signaling measurements. Although exercise did not enhance proximal signaling in either group, insulin-stimulated GU at 3hPEX exceeded respective sedentary control subjects (Sedentary) in both diet groups. Furthermore, insulin-stimulated GU for LFD-3hPEX was greater than HFD-3hPEX values. For HFD-3hPEX muscles, pAS160 exceeded HFD-Sedentary, but in muscle from LFD-3hPEX rats, pAS160 was greater still than HFD-3hPEX values. These results implicated pAS160 as a potential determinant of the exercise-induced elevation in insulin-stimulated GU for each diet group and also revealed pAS160 as a possible mediator of greater postexercise GU of insulin-stimulated muscles from the insulin-sensitive versus insulin-resistant group.

Insulin- and contraction-induced glucose transporter 4 traffic in muscle: insights from a novel imaging approach

Insulin- and contraction-mediated glucose transporter 4 (GLUT4) trafficking have different kinetics in mature skeletal muscle. Intravital imaging indicates that insulin-stimulated GLUT4 trafficking differs between t-tubules and sarcolemma. In contrast, contraction-induced GLUT4 trafficking does not differ between membrane surfaces. This distinction likely is caused by differences in the underlying signaling pathways regulating GLUT4 vesicle depletion, GLUT4 membrane fusion, and GLUT4 reinternalization.

Role of oxidative stress in impaired insulin signaling associated with exercise-induced muscle damage

Skeletal muscle is a major tissue that utilizes blood glucose. A single bout of exercise improves glucose uptake in skeletal muscle through insulin-dependent and insulin-independent signal transduction mechanisms. However, glucose utilization is decreased in muscle damage induced by acute, unaccustomed, or eccentric exercise. The decrease in glucose utilization is caused by decreased insulin-stimulated glucose uptake in damaged muscles with inhibition of the membrane translocation of glucose transporter 4 through phosphatidyl 3-kinase/Akt signaling. In addition to inflammatory cytokines, reactive oxygen species including 4-hydroxy-2-nonenal and peroxynitrate can induce degradation or inactivation of signaling proteins through posttranslational modification, thereby resulting in a disturbance in insulin signal transduction. In contrast, treatment with factors that attenuate oxidative stress in damaged muscle suppresses the impairment of insulin sensitivity. Muscle-damaging exercise may thus lead to decreased endurance capacity and muscle fatigue in exercise, and it may decrease the efficiency of exercise therapy for metabolic improvement.

Muscle-specific Pikfyve gene disruption causes glucose intolerance, insulin resistance, adiposity, and hyperinsulinemia but not muscle fiber-type switching

The evolutionarily conserved kinase PIKfyve that synthesizes PtdIns5P and PtdIns(3,5)P₂ has been implicated in insulin-regulated GLUT4 translocation/glucose entry in 3T3-L1 adipocytes. To decipher PIKfyve's role in muscle and systemic glucose metabolism, here we have developed a novel mouse model with Pikfyve gene disruption in striated muscle (MPIfKO). These mice exhibited systemic glucose intolerance and insulin resistance at an early age but had unaltered muscle mass or proportion of slow/fast-twitch muscle fibers. Insulin stimulation of in vivo or ex vivo glucose uptake and GLUT4 surface translocation was severely blunted in skeletal muscle. These changes were associated with premature attenuation of Akt phosphorylation in response to in vivo insulin, as tested in young mice. Starting at 10-11 wk of age, MPIfKO mice progressively accumulated greater body weight and fat mass. Despite increased adiposity, serum free fatty acid and triglyceride levels were normal until adulthood. Together with the undetectable lipid accumulation in liver, these data suggest that lipotoxicity and muscle fiber switching do not contribute to muscle insulin resistance in MPIfKO mice. Furthermore, the 80% increase in total fat mass resulted from increased fat cell size rather than altered fat cell number. The observed profound hyperinsulinemia combined with the documented increases in constitutive Akt activation, in vivo glucose uptake, and gene expression of key enzymes for fatty acid biosynthesis in MPIfKO fat tissue suggest that the latter is being sensitized for de novo lipid anabolism. Our data provide the first in vivo evidence that PIKfyve is essential for systemic glucose homeostasis and insulin-regulated glucose uptake/GLUT4 translocation in skeletal muscle.

A persistent increase in insulin-stimulated glucose uptake by both fast-twitch and slow-twitch skeletal muscles after a single exercise session by old rats

Exercise has been demonstrated to enhance subsequent insulin-stimulated glucose uptake (GU) by predominantly type II (fast-twitch) muscle of old rats, but previous research has not evaluated exercise effects on GU by type I (slow-twitch) muscle from old rats. Accordingly, we studied male Fischer 344/Brown Norway rats (24 months old) and determined GU (0, 100, 200, and 5,000 μU/ml insulin) of isolated soleus (predominantly type I) and epitrochlearis (predominantly type II) muscles after one exercise session. Epitrochlearis (100, 200, and 5,000 μU/ml insulin) and soleus (100 and 200 μU/ml insulin) GU were greater at 3-h postexercise vs. age-matched sedentary controls. Insulin receptor tyrosine phosphorylation (Tyr1162/1163) was unaltered by exercise in either muscle. Akt phosphorylation (pAkt) was greater for exercised vs. sedentary rats in the epitrochlearis (Ser473 and Thr308 with 100 and 200 μU/ml, respectively) and soleus (Ser473 with 200 μU/ml). AS160 phosphorylation (pAS160) was greater for exercised vs. sedentary rats in the epitrochlearis (Thr642 with 100 μU/ml), but not the soleus. Exercised vs. sedentary rats did not differ for total protein abundance of insulin receptor, Akt, AS160, or GLUT4 in either muscle. These results demonstrate that both predominantly type I and type II muscles from old rats are susceptible to exercise-induced improvement in insulin-mediated GU by mechanisms that are independent of enhanced insulin receptor tyrosine phosphorylation or altered abundance of important signaling proteins or GLUT4. Exercise-induced elevation in pAkt, and possibly pAS160, may contribute to this effect in the epitrochlearis of old rats, but other mechanisms are likely important for the soleus.