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

Association between remote resistance exercises programs delivered by a smartphone application and skeletal muscle mass among elderly patients with type 2 diabetes- a retrospective real-world study

Objective

We aimed to explore the relationship between remote resistance exercise programs delivered via a smartphone application and skeletal muscle mass among elderly patients with type 2 diabetes, utilizing real-world data.

Methods

The resistance exercises were provided through Joymotion®, a web-based telerehabilitation smartphone application (Shanghai Medmotion Medical Management Co., Ltd). The primary outcome was the changes in skeletal muscle index (SMI) before and after the remote resistance exercises programs. The secondary outcomes were changes in skeletal muscle cross-sectional area (SMA), skeletal muscle radiodensity (SMD) and intermuscular adipose tissue (IMAT).

Results

A total of 101 elderly patients with type 2 diabetes were analyzed. The participants had an average age of 72.9 ± 6.11 years for males and 74.4 ± 4.39 years for females. The pre- and post-intervention SMI mean (± SE) was 31.64 ± 4.14 vs. 33.25 ± 4.22 cm2/m2 in male, and 22.72 ± 3.24 vs. 24.28 ± 3.60 cm2/m2 in female respectively (all P < 0.001). Similarly, a statistically significant improvement in SMA, IMAT, and SMD for both male and female groups were also observed respectively (P < 0.001). Multiple linear regression models showed potential confounding factors of baseline hemoglobin A1c and duration of diabetes with changes in SMI in male, while hemoglobin A1c and high density lipoprotein cholesterol with changes in SMI in female.

Conclusion

Remote resistance exercises programs delivered by a smartphone application were feasible and effective in helping elderly patients with type 2 diabetes to improve their skeletal muscle mass.

Independent and combined effects of calorie restriction and AICAR on glucose uptake and insulin signaling in skeletal muscles from 24-month-old female and male rats

We assessed the effects of two levels of calorie restriction (CR; eating either 15% or 35% less than ad libitum, AL, food intake for 8 weeks) by 24-month-old female and male rats on glucose uptake (GU) and phosphorylation of key signaling proteins (Akt; AMP-activated protein kinase, AMPK; Akt substrate of 160 kDa, AS160) measured in isolated skeletal muscles that underwent four incubation conditions (without either insulin or AICAR, an AMPK activator; with AICAR alone; with insulin alone; or with insulin and AICAR). Regardless of sex: (1) neither CR group versus the AL group had greater GU by insulin-stimulated muscles; (2) phosphorylation of Akt in insulin-stimulated muscles was increased in 35% CR versus AL rats; (3) prior AICAR treatment of muscle resulted in greater GU by insulin-stimulated muscles, regardless of diet; and (4) AICAR caused elevated phosphorylation of acetyl CoA carboxylase, an indicator of AMPK activation, in all diet groups. There was a sexually dimorphic diet effect on AS160 phosphorylation, with 35% CR exceeding AL for insulin-stimulated muscles in male rats, but not in female rats. Our working hypothesis is that the lack of a CR-effect on GU by insulin-stimulated muscles was related to the extended duration of the ex vivo incubation period (290 min compared to 40-50 min that was previously reported to be effective). The observed efficacy of prior treatment of muscles with AICAR to improve glucose uptake in insulin-stimulated muscles supports the strategy of targeting AMPK with the goal of improving insulin sensitivity in older females and males.

Local low-frequency vibration accelerates healing of full-thickness wounds in a hyperglycemic rat model

AIMS/INTRODUCTION

Local low-frequency vibration (LLFV) promotes vasodilation and blood flow, enhancing wound healing in diabetic foot ulcers with angiopathy. However, vibration-induced vasodilation does not occur, owing to chronic hyperglycemia and inflammation. We hypothesized that LLFV improves glycometabolism and inflammation, leading to vasodilation and angiogenesis in diabetic wounds. Therefore, this study investigated the effect of LLFV on wound healing in hyperglycemic rats, primarily focusing on glycometabolism, inflammation, vasodilation, and angiogenesis.

MATERIALS AND METHODS

Streptozotocin-induced hyperglycemic Sprague-Dawley rats were used in this study. We applied LLFV to experimentally-induced wounds at 50 Hz and 0, 600, 1,000 or 1,500 mVpp for 40 min/day from post-wounding days (PWD) 1-14.

RESULTS

The relative wound areas in the 600 and 1,000 mVpp groups on PWD 5-7 were significantly smaller than those at 0 mVpp. The expression of Glo-1 (1,500 mVpp) and Slc2A4 (1,000 and 1,500 mVpp) was upregulated on PWD 4 and 14, respectively. However, there was no difference in methylglyoxal expression levels in any group until PWD 14. At 1,000 mVpp, the expression of Tnfa on PWD 4, and that of Ptx3 and Ccl2 on PWD 14 was downregulated. Furthermore, the M1/M2 macrophage ratio was considerably decreased on both days. The expression of Nos3, Vegfa and vascular endothelial growth factor A was upregulated on PWD 4. In addition, vasodilation and angiogenesis were more obvious on PWD 14 with 1,000 mVpp.

CONCLUSIONS

The results suggest that LLFV promotes wound healing, improves glycometabolism and inflammation, and enhances vasodilation and angiogenesis in hyperglycemic wounds.

Fasting potentiates insulin-mediated glucose uptake in rested and prior-contracted rat skeletal muscle

A single bout of exercise can potentiate the effect of insulin on skeletal muscle glucose uptake via activation of the AMPK-TBC1 domain family member 4 (TBC1D4) pathway, which suggests a positive correlation between AMPK activation and insulin sensitization. In addition, prolonged fasting in rodents is known to upregulate and thereby synergistically enhance the effect of exercise on muscle AMPK activation. Therefore, fasting may potentiate the insulin-sensitizing effect of exercise. In the present study, we mimicked exercise by in situ muscle contraction and evaluated the effect of a 36-h fast on muscle contraction-induced insulin sensitization. Male Wistar rats weighing 150-170 g were allocated to either a 36-h fasting or feeding group. The extensor digitorum longus (EDL) muscles were electrically contracted via the common peroneal nerve for 10 min followed by a 3-h recovery period. EDL muscles were dissected and incubated in the presence or absence of submaximal insulin. Our results demonstrated that acute muscle contraction and 36 h of fasting additively upregulated AMPK pathway activation. Insulin-stimulated muscle glucose uptake and site-specific TBC1D4 phosphorylation were enhanced by prior muscle contraction in 36-h-fasted rats, but not in fed rats. Moreover, enhanced insulin-induced muscle glucose uptake and Akt phosphorylation due to 36 h of fasting were associated with a decrease in tribbles homolog 3 (TRB3), a negative regulator of Akt activation. In conclusion, fasting and prior muscle contraction synergistically enhance insulin-stimulated TBC1D4 phosphorylation and glucose uptake, which is associated with augmented AMPK pathway activation in rodents.NEW & NOTEWORTHY In this study, we revealed that 36 h of fasting additively upregulated acute muscle contraction-induced AMPK pathway activation in rats. Besides, fasting and muscle contraction synergistically enhanced insulin-stimulated site-specific TBC1D4 phosphorylation and glucose uptake, which was associated with augmented AMPK pathway activation. These results contribute to understanding the regulation of muscle insulin sensitivity.

Prior AICAR induces elevated glucose uptake concomitant with greater γ3-AMPK activation and reduced membrane cholesterol in skeletal muscle from 26-month-old rats

Attenuated skeletal muscle glucose uptake (GU) has been observed with advancing age. It is important to elucidate the mechanisms linked to interventions that oppose this detrimental outcome. Earlier research using young rodents and (or) cultured myocytes reported that treatment with 5-aminoimidazole-4-carboxamide-1-β-d-ribofuranoside (AICAR; an AMP-activated protein kinase (AMPK) activator) can increase γ3-AMPK activity and reduce membrane cholesterol content, each of which has been proposed to elevate GU. However, the effect of AICAR treatment on γ3-AMPK activity and membrane cholesterol in skeletal muscle of aged animals has not been reported. Our purpose was to evaluate the effects of AICAR treatment on these potential mechanisms for enhanced glucose uptake in the skeletal muscle of aged animals. Epitrochlearis muscles from 26–27-month-old male rats were isolated and incubated ± AICAR, followed by 3 h incubation without AICAR, and then incubation with 3- O-methyl-[3 H] glucose (to assess GU ± insulin). Muscles were also analyzed for γ3-AMPK activity and membrane cholesterol content. Prior AICAR treatment led to increased γ3-AMPK activity, reduced membrane cholesterol content, and enhanced glucose uptake in skeletal muscle from aged rats. These observations revealed that two potential mechanisms for greater GU previously observed in younger animals and (or) cell models are also potentially relevant for enhanced GU by muscles from older animals.

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.

Effect of Exercise Training on Fat Loss-Energetic Perspectives and the Role of Improved Adipose Tissue Function and Body Fat Distribution

In obesity, excessive abdominal fat, especially the accumulation of visceral adipose tissue (VAT), increases the risk of metabolic disorders, such as type 2 diabetes mellitus (T2DM), cardiovascular disease, and non-alcoholic fatty liver disease. Excessive abdominal fat is associated with adipose tissue dysfunction, leading to systemic low-grade inflammation, fat overflow, ectopic lipid deposition, and reduced insulin sensitivity. Physical activity is recommended for primary prevention and treatment of obesity, T2DM, and related disorders. Achieving a stable reduction in body weight with exercise training alone has not shown promising effects on a population level. Because fat has a high energy content, a large amount of exercise training is required to achieve weight loss. However, even when there is no weight loss, exercise training is an effective method of improving body composition (increased muscle mass and reduced fat) as well as increasing insulin sensitivity and cardiorespiratory fitness. Compared with traditional low-to-moderate-intensity continuous endurance training, high-intensity interval training (HIIT) and sprint interval training (SIT) are more time-efficient as exercise regimens and produce comparable results in reducing total fat mass, as well as improving cardiorespiratory fitness and insulin sensitivity. During high-intensity exercise, carbohydrates are the main source of energy, whereas, with low-intensity exercise, fat becomes the predominant energy source. These observations imply that HIIT and SIT can reduce fat mass during bouts of exercise despite being associated with lower levels of fat oxidation. In this review, we explore the effects of different types of exercise training on energy expenditure and substrate oxidation during physical activity, and discuss the potential effects of exercise training on adipose tissue function and body fat distribution.

Reduced membrane cholesterol content in skeletal muscle is not essential for greater insulin-stimulated glucose uptake after acute exercise by rats

One exercise session can elevate insulin-stimulated glucose uptake (GU) by skeletal muscle, but it is uncertain if this effect is accompanied by altered membrane cholesterol content, which is reportedly inversely related to insulin-stimulated GU. Muscles from sedentary (SED) or exercised 3 h post-exercise (3hPEX) rats were evaluated for GU, membrane cholesterol, and phosphorylation of cholesterol regulatory proteins (pHMCGR^Ser872 and pABCA1^Ser2054). Insulin-stimulated GU for 3hPEX exceeded SED. Membrane cholesterol, pHMCGR^Ser872 and pABCA1^Ser2054 did not differ between groups. Novelty: Alterations in membrane cholesterol and phosphorylation of proteins that regulate muscle cholesterol are not essential for elevated insulin-stimulated GU in skeletal muscle after acute exercise.

Effects of short-term fasting and pharmacological activation of AMPK on metabolism of rat adipocytes

AMP-activated protein kinase (AMPK) is a key intracellular energy sensor and regulates processes associated with energy metabolism. In the present study, effects of AICAR, a pharmacological activator of AMPK, on metabolism of adipocytes of non-fasted and 12-h fasted rats were compared. It was shown that in fat cells of control rats, epinephrine- and dibutyryl-cAMP-induced lipolysis was markedly reduced in the presence of AICAR. However, in adipocytes of fasted animals, the lipolytic response was not significantly affected by AICAR. Moreover, in cells of control rats, the inhibitory effect of insulin on epinephrine-induced lipolysis was markedly deepened in the presence of AICAR. However, this effect was not shown in fat cells of fasted rats. This indicates that pharmacological activation of AMPK by AICAR influences metabolism of adipocytes of non-fasted rats, however, AICAR fails to affect metabolism of these cells under conditions of fasting.

Biochemical and Molecular Mechanisms of Glucose Uptake Stimulated by Physical Exercise in Insulin Resistance State: Role of Inflammation

Obesity associated with systemic inflammation induces insulin resistance (IR), with consequent chronic hyperglycemia. A series of reactions are involved in this process, including increased release of proinflammatory cytokines, and activation of c-Jun N-terminal kinase (JNK), nuclear factor-kappa B (NF-κB) and toll-like receptor 4 (TLR4) receptors. Among the therapeutic tools available nowadays, physical exercise (PE) has a known hypoglycemic effect explained by complex molecular mechanisms, including an increase in insulin receptor phosphorylation, in AMP-activated protein kinase (AMPK) activity, in the Ca2+/calmodulin-dependent protein kinase kinase (CaMKK) pathway, with subsequent activation of peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α), Rac1, TBC1 domain family member 1 and 4 (TBC1D1 and TBC1D4), in addition to a variety of signaling molecules, such as GTPases, Rab and soluble N-ethylmaleimide-sensitive factor attached protein receptor (SNARE) proteins. These pathways promote greater translocation of GLUT4 and consequent glucose uptake by the skeletal muscle. Phosphoinositide-dependent kinase (PDK), atypical protein kinase C (aPKC) and some of its isoforms, such as PKC-iota/lambda also seem to play a fundamental role in the transport of glucose. In this sense, the association between autophagy and exercise has also demonstrated a relevant role in the uptake of muscle glucose. Insulin, in turn, uses a phosphoinositide 3-kinase (PI3K)-dependent mechanism, while exercise signal may be triggered by the release of calcium from the sarcoplasmic reticulum. The objective of this review is to describe the main molecular mechanisms of IR and the relationship between PE and glucose uptake.

Potential role of angiotensin-(1-7) in the improvement of vascular insulin sensitivity after a bout of exercise

NEW FINDINGS

What is the central question of this study? What is the mechanism by which a bout of exercise increases subsequent insulin-stimulated vasodilatation? What is the main finding and its importance? Angiotensin-(1-7) through the Mas receptor participates in enhanced insulin-induced vasorelaxation after a bout of exercise in healthy rats. This new potential role of angiotensin-(1-7) could help in understanding how physical activity improves vascular insulin sensitivity in normal and insulin-resistant states.

ABSTRACT

Exercise increases insulin-stimulated vasodilatation, but the mechanisms involved are unclear. This study was performed to investigate the possible involvement of angiotensin-(1-7) (Ang-(1-7)), a vasoactive peptide of the renin-angiotensin system (RAS), in enhanced vascular insulin sensitivity after a bout of exercise. Male Wistar rats were subjected to swimming for 2.5 h. After exercise, carbachol- or insulin-induced relaxation in aorta was assessed. Prior exercise improved insulin-stimulated vasorelaxation; however, this insulin-sensitizing effect was prevented by the selective Mas receptor (MasR; an Ang-(1-7) receptor) antagonist A779. Carbachol-mediated vascular relaxation was not modified by exercise. These results suggest that Ang-(1-7) acting through MasR participates in the enhancement of vascular insulin sensitivity after an exercise session. This new potential role of Ang-(1-7) could help in understanding how exercise improves vascular insulin sensitivity in normal and insulin-resistant states.

Angiotensin-(1-7) Participates in Enhanced Skeletal Muscle Insulin Sensitivity After a Bout of Exercise

A single bout of exercise increases subsequent insulin-stimulated glucose uptake in skeletal muscle; however, it is unknown whether angiotensin-(1-7) (Ang-(1-7)), a vasoactive peptide of the renin-angiotensin system, participates in this process. The aim of this study was to investigate the possible involvement of Ang-(1-7) in enhanced skeletal muscle insulin sensitivity after an exercise session. Male Wistar rats were forced to swim for 2.5 hours. Two hours after exercise, insulin tolerance tests and 2-deoxyglucose uptake in isolated soleus muscle were assessed in the absence or presence of the selective Mas receptor (MasR, Ang-(1-7) receptor) antagonist A779. Ang II and Ang-(1-7) levels were quantified in plasma and soleus muscle by HPLC. The protein abundance of angiotensin-converting enzyme (ACE), ACE2, Ang II type 1 receptor (AT₁R), and MasR was measured in soleus muscle by Western blot. Prior exercise enhanced insulin tolerance and insulin-mediated 2-deoxyglucose disposal in soleus muscle. Interestingly, these insulin-sensitizing effects were abolished by A779. After exercise, the Ang-(1-7)/Ang II ratio decreased in plasma, whereas it increased in muscle. In addition, exercise reduced ACE expression, but it did not change the protein abundance of AT₁R, ACE2, and MasR. These results suggest that Ang-(1-7) acting through MasR participates in enhanced insulin sensitivity of skeletal muscle after a bout of exercise.

Effect of a Low-Carbohydrate High-Fat Diet and a Single Bout of Exercise on Glucose Tolerance, Lipid Profile and Endothelial Function in Normal Weight Young Healthy Females

Low-carbohydrate-high-fat (LCHF) diets are efficient for weight loss, and are also used by healthy people to maintain bodyweight. The main aim of this study was to investigate the effect of 3-week energy-balanced LCHF-diet, with >75 percentage energy (E%) from fat, on glucose tolerance and lipid profile in normal weight, young, healthy women. The second aim of the study was to investigate if a bout of exercise would prevent any negative effect of LCHF-diet on glucose tolerance. Seventeen females participated, age 23.5 ± 0.5 years; body mass index 21.0 ± 0.4 kg/m², with a mean dietary intake of 78 ± 1 E% fat, 19 ± 1 E% protein and 3 ± 0 E% carbohydrates. Measurements were performed at baseline and post-intervention. Fasting glucose decreased from 4.7 ± 0.1 to 4.4 mmol/L (p < 0.001) during the dietary intervention whereas fasting insulin was unaffected. Glucose area under the curve (AUC) and insulin AUC did not change during an OGTT after the intervention. Before the intervention, a bout of aerobic exercise reduced fasting glucose (4.4 ± 0.1 mmol/L, p < 0.001) and glucose AUC (739 ± 41 to 661 ± 25, p = 0.008) during OGTT the following morning. After the intervention, exercise did not reduce fasting glucose the following morning, and glucose AUC during an OGTT increased compared to the day before (789 ± 43 to 889 ± 40 mmol/L∙120min^–1, p = 0.001). AUC for insulin was unaffected. The dietary intervention increased total cholesterol (p < 0.001), low-density lipoprotein (p ≤ 0.001), high-density lipoprotein (p = 0.011), triglycerides (p = 0.035), and free fatty acids (p = 0.021). In conclusion, 3-week LCHF-diet reduced fasting glucose, while glucose tolerance was unaffected. A bout of exercise post-intervention did not decrease AUC glucose as it did at baseline. Total cholesterol increased, mainly due to increments in low-density lipoprotein. LCHF-diets should be further evaluated and carefully considered for healthy individuals.

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.

Resistance training restores metabolic alterations induced by monosodium glutamate in a sex-dependent manner in male and female rats

Despite resistance exercises being associated with health outcomes, numerous issues are still unresolved and further research is required before the exercise can faithfully be prescribed as medicine. The goal of this study was to investigate whether there are sex differences in resistance training effects on metabolic alterations induced by monosodium glutamate (MSG), a model of obesity, in male and female rats. Male and female Wistar rats received MSG (4 g/kg body weight/day, s.c.) from postnatal day 1 to 10. After 10 days from MSG administration, the rats were separated into two groups: MSG-sedentary and MSG-exercised. At postnatal day 60, the animals started a resistance training protocol in an 80 degrees inclined vertical ladder apparatus and performed it for 7 weeks. Control rats received saline solution and were divided in saline-sedentary and saline-exercised. Resistance training restored all plasma biochemical parameters (glucose, cholesterol, triglycerides, aspartate aminotransferase, and alanine aminotransferase) increased in male and female rats treated with MSG. The MSG administration induced hyperglycemia associated with a decrease in the skeletal muscle glucose transporter 4 (GLUT4) levels and accompanied by deregulation in proteins, G-6Pase, and tyrosine aminotransferase, involved in hepatic glucose metabolism of male and female rats. MSG induced dyslipidemia and lipotoxicity in the liver and skeletal muscle of male rats. Regarding female rats, lipotoxicity was found only in the skeletal muscle. The resistance training had beneficial effects against metabolic alterations induced by MSG in male and female rats, through regulation of proteins (GLUT2, protein kinase B, and GLUT4) involved in glucose and lipid pathways in the liver and 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.

Dicarbonyl Stress and Glyoxalase-1 in Skeletal Muscle: Implications for Insulin Resistance and Type 2 Diabetes

Glyoxalase-1 (GLO1) is a ubiquitously expressed cytosolic protein which plays a role in the natural maintenance of cellular health and is abundantly expressed in human skeletal muscle. A consequence of reduced GLO1 protein expression is cellular dicarbonyl stress, which is elevated in obesity, insulin resistance and type 2 diabetes (T2DM). Both in vitro and pre-clinical models suggest dicarbonyl stress per se induces insulin resistance and is prevented by GLO1 overexpression, implicating a potential role for GLO1 therapy in insulin resistance and type 2 diabetes (T2DM). Recent work has identified the therapeutic potential of novel natural agents as a GLO1 inducer, which resulted in improved whole-body metabolism in obese adults. Given skeletal muscle is a major contributor to whole-body glucose, lipid, and protein metabolism, such GLO1 inducers may act, in part, through mechanisms in skeletal muscle. Currently, investigations examining the specificity of dicarbonyl stress and GLO1 biology in human skeletal muscle are lacking. Recent work from our lab indicates that dysregulation of GLO1 in skeletal muscle may underlie human insulin resistance and that exercise training may impart therapeutic benefits. This minireview will summarize the existing human literature examining skeletal muscle GLO1 and highlight the emerging therapeutic concepts for GLO1 gain-of-function in conditions such as insulin resistance and cardiometabolic disease.