Functional Studies of Akt Isoform Specificity in Skeletal Muscle in Vivo; Maintained Insulin Sensitivity Despite Reduced Insulin Receptor Substrate-1 Expression

mTOR Ser1261 is an AMPK-dependent phosphosite in mouse and human skeletal muscle not required for mTORC2 activity

The kinases AMPK, and mTOR as part of either mTORC1 or mTORC2, are major orchestrators of cellular growth and metabolism. Phosphorylation of mTOR Ser1261 is reportedly stimulated by both insulin and AMPK activation and a regulator of both mTORC1 and mTORC2 activity. Intrigued by the possibilities that Ser1261 might be a convergence point between insulin and AMPK signaling in skeletal muscle, we investigated the regulation and function of this site using a combination of human exercise, transgenic mouse, and cell culture models. Ser1261 phosphorylation on mTOR did not respond to insulin in any of our tested models, but instead responded acutely to contractile activity in human and mouse muscle in an AMPK activity-dependent manner. Contraction-stimulated mTOR Ser1261 phosphorylation in mice was decreased by Raptor muscle knockout (mKO) and increased by Raptor muscle overexpression, yet was not affected by Rictor mKO, suggesting most of Ser1261 phosphorylation occurs within mTORC1 in skeletal muscle. In accordance, HEK293 cells mTOR Ser1261Ala mutation strongly impaired phosphorylation of mTORC1 substrates but not mTORC2 substrates. However, neither mTORC1 nor mTORC2-dependent phosphorylations were affected in muscle-specific kinase-dead AMPK mice with no detectable mTOR Ser1261 phosphorylation in skeletal muscle. Thus, mTOR Ser1261 is an exercise but not insulin-responsive AMPK-dependent phosphosite in human and murine skeletal muscle, playing an unclear role in mTORC1 regulation but clearly not required for mTORC2 activity.

KLF3 impacts insulin sensitivity and glucose uptake in skeletal muscle

Skeletal muscle is one of the predominant sites involved in glucose disposal, accounting for ∼80% of postprandial glucose uptake, and plays a critical role in maintaining glycemic homeostasis. Dysregulation of energy metabolism in skeletal muscle is involved in developing insulin resistance and type 2 diabetes (T2D). Transcriptomic responses of skeletal muscle to exercise found that the expression of Klf3 was increased in T2D Goto-Kakizaki (GK) rats and decreased after exercise with improved hyperglycemia and insulin resistance, implying that Klf3 might be associated with insulin sensitivity and glucose metabolism. We also found that knockdown of Klf3 promoted basal and insulin-stimulated glucose uptake in L6 myotubes, whereas overexpression of Klf3 resulted in the opposite. Through pairwise comparisons of L6 myotubes transcriptome, we identified 2,256 and 1,988 differentially expressed genes in Klf3 knockdown and overexpression groups, respectively. In insulin signaling, the expression of Slc2a4, Akt2, Insr, and Sorbs1 was significantly increased by Klf3 knockdown and decreased with Klf3 overexpression; Ptprf and Fasn were markedly downregulated in Klf3 reduced group and upregulated in Klf3 overexpressed group. Moreover, downregulation of Klf3 promoted the expression of glucose transporter 4 (GLUT4) and protein kinase B (AKT) proteins, as well as the translocation of GLUT4 to the cell membrane in the basal situation, and enhanced insulin sensitivity, characterized by increased insulin-stimulated GLUT4 translocation and AKT, TBC1 domain family member 1 (TBC1D1) and TBC1 domain family member 4 (TBC1D4) phosphorylation, whereas overexpression of Klf3 showed contrary results. These results suggest that Klf3 affects glucose uptake and insulin sensitivity via insulin signal transduction and intracellular metabolism, offering a novel potential treatment strategy for T2D.NEW & NOTEWORTHY The knockdown of Klf3 increased glucose uptake and improved insulin sensitivity in L6 myotubes, whereas its overexpression had the opposite effect. To explore the underlying mechanisms, we evaluated the transcriptional profiles of L6 myotubes after Klf3 knockdown and overexpression and revealed that metabolism and insulin-related pathways were significantly impacted. Klf3 also influenced the expression or modification of glucose transporter 4 (GLUT4), protein kinase B (AKT), TBC1 domain family member 1 (TBC1D1), and TBC1 domain family member 4 (TBC1D4) in the insulin signaling pathway, affecting insulin sensitivity and glucose uptake.

Methionine adenosyltransferase2A inhibition restores metabolism to improve regenerative capacity and strength of aged skeletal muscle

We investigate the age-related metabolic changes that occur in aged and rejuvenated myoblasts using in vitro and in vivo models of aging. Metabolic and signaling experiments reveal that human senescent myoblasts and myoblasts from a mouse model of premature aging suffer from impaired glycolysis, insulin resistance, and generate Adenosine triphosphate by catabolizing methionine via a methionine adenosyl-transferase 2A-dependant mechanism, producing significant levels of ammonium that may further contribute to cellular senescence. Expression of the pluripotency factor NANOG downregulates methionine adenosyltransferase 2 A, decreases ammonium, restores insulin sensitivity, increases glucose uptake, and enhances muscle regeneration post-injury. Similarly, selective inhibition of methionine adenosyltransferase 2 A activates Akt2 signaling, repairs pyruvate kinase, restores glycolysis, and enhances regeneration, which leads to significant enhancement of muscle strength in a mouse model of premature aging. Collectively, our investigation indicates that inhibiting methionine metabolism may restore age-associated impairments with significant gain in muscle function.

Effects of Acute Muscle Contraction on the Key Molecules in Insulin and Akt Signaling in Skeletal Muscle in Health and in Insulin Resistant States

Insulin signaling plays a key role in glucose uptake, glycogen synthesis, and protein and lipid synthesis. In insulin-resistant states like obesity and type 2 diabetes mellitus, these processes are dysregulated. Regular physical exercise is a potential therapeutic strategy against insulin resistance, as an acute bout of exercise increases glucose disposal during the activity and for hours into recovery. Chronic exercise increases the activation of proteins involved in insulin signaling and increases glucose transport, even in insulin resistant states. Here, we will focus on the effect of acute exercise on insulin signaling and protein kinase B (Akt) pathways. Activation of proximal proteins involved in insulin signaling (insulin receptor, insulin receptor substrate-1 (IRS-1), phosphoinoside-3 kinase (PI3K)) are unchanged in response to acute exercise/contraction, while activation of Akt and of its substrates, TBC1 domain family 1 (TBC1D1), and TBC domain family 4 (TBC1D4) increases in response to such exercise/contraction. A wide array of Akt substrates is also regulated by exercise. Additionally, AMP-activated protein kinase (AMPK) seems to be a main mediator of the benefits of exercise on skeletal muscle. Questions persist on how mTORC1 and AMPK, two opposing regulators, are both upregulated after an acute bout of exercise.

CORP: Gene delivery into murine skeletal muscle using in vivo electroporation

The strategy of gene delivery into skeletal muscles has provided exciting avenues in identifying new potential therapeutics towards muscular disorders and addressing basic research questions in muscle physiology through overexpression and knockdown studies. In vivo electroporation methodology offers a simple, rapidly effective technique for the delivery of plasmid DNA into post-mitotic skeletal muscle fibers and the ability to easily explore the molecular mechanisms of skeletal muscle plasticity. The purpose of this review is to describe how to robustly electroporate plasmid DNA into different hindlimb muscles of rodent models. Further, key parameters (e.g., voltage, hyaluronidase, plasmid concentration) which contribute to the successful introduction of plasmid DNA into skeletal muscle fibers will be discussed. In addition, details on processing tissue for immunohistochemistry and fiber cross-sectional area (CSA) analysis will be outlined. The overall goal of this review is to provide the basic and necessary information needed for successful implementation of in vivo electroporation of plasmid DNA and thus open new avenues of discovery research in skeletal muscle physiology.

The effects of exercise on kidney injury: the role of SIRT1

In patients with kidney injury, muscle mass and strength decrease with altered muscle protein synthesis and degradation along with complications such as inflammation and low physical activity. A treatment strategy to maintain muscle metabolism in kidney injury is important. One of the proposed strategies in this regard is exercise, which in addition to inducing muscle hypertrophy, reducing plasma creatinine and urea and decreasing the severity of tubal injuries, can boost immune function and has anti-inflammatory effects. One of the molecules that have been considered as a target in the treatment of many diseases is silent information regulator 1 (SIRT1). Exercise increases the expression of SIRT1 and improves its activity. Therefore, studies that examined the effect of exercise on kidney injury considering the role of SIRT1 in this effect were reviewed to determine the direction of kidney injury research in future regarding to its prevalence, especially following diabetes, and lack of definitive treatment. In this review, we found that SIRT1 can be one of renoprotective target pathways of exercise. However, further studies are needed to determine the role of SIRT1 in different kidney injuries following exercise according to the type and severity of exercise, and the type of kidney injury.

N1-Methylnicotinamide: Is It Time to Consider as a Dietary Supplement for Athletes?

Abstract:

Exercise is considered to be a “medicine” due to its modulatory roles in metabolic disorders such as diabetes and obesity. The intensity and duration of exercise determine the mechanism of energy production by various tissues of the body, especially by muscles, in which the requirement for adenosine triphosphate (ATP) increases by as much as 100-fold. Naturally, athletes try to improve their exercise performance by dietary supplementation with, e.g., vitamins, metabolites, and amino acids. MNAM, as a vitamin B3 metabolite, reduces serum levels and liver contents of triglycerides, and cholesterol and induces lipolysis. It stimulates gluconeogenesis and prohibits liver cholesterol and fatty acid synthesis through the expression of sirtuin1 (SIRT1). It seems that MNAM is not responsible for the actions of NNMT in the adipose tissues as MNAM inhibits the activity of NNMT in the adipose tissue and acts like inhibitors of its activity. NNMT-MNAM axis is more activated in the muscles of participants who were undergoing the high-volume-low-intensity exercise and caloric restriction. Therefore, MNAM could be an important myokine during exercise and fasting where it provides the required energy for muscles through the induction of lipolysis and gluconeogenesis in the liver and adipose tissues, respectively. Increased levels of MNAM in exercise and fasting led us to propose that the consumption of MNAM during training especially endurance training could boost exercise capacity and improves performance. Therefore, in this review, we shed light on the potential of MNAM as a dietary supplement in sports medicine.

Role of Akt isoforms in neuronal insulin signaling and resistance

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

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

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

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

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

Effect of exercise training on skeletal muscle protein expression in relation to insulin sensitivity: Per-protocol analysis of a randomized controlled trial (GO-ACTIWE)

Exercise training improves peripheral insulin sensitivity and leads to molecular adaptations in the skeletal muscle. We investigated changes in the expression of key muscle proteins in the glucose metabolic pathway following active commuting by bike or leisure-time exercise at two different intensities. In addition, potential associations between insulin sensitivity and muscle protein expression were examined. This per-protocol analysis included 72 out of 130 physically inactive, healthy women and men (20-45 years) with overweight/obesity (BMI: 25-35 kg/m2 ) who completed 6 months of no intervention (CON, n = 12), active commuting by bike (BIKE, n = 14), or leisure-time exercise of moderate (MOD, n = 28) or vigorous (VIG, n = 18) intensity. Exercise was prescribed 5 days/week with a weekly exercise energy expenditure of 1,600 kcal for women and 2,100 kcal for men. Insulin sensitivity was determined by a hyperinsulinemic euglycemic clamp and skeletal muscle biopsies were obtained from m. vastus lateralis and analyzed for protein expression at baseline and after 3 and 6 months of intervention. We found an increased expression of pyruvate dehydrogenase (PDH) in the exercise groups compared with the control group following 6 months of training. No differential effects were observed on the protein expression following moderate versus vigorous intensity exercise. In addition, we found a positive association between insulin sensitivity and the expression of glucose transporter type 4 as well as PDH. The positive association and the increase in expression of PDH after exercise training points toward a role for PDH in the training-induced enhancement of insulin sensitivity.

Insulin-Stimulated Muscle Glucose Uptake and Insulin Signaling in Lean and Obese Humans

PURPOSE

Skeletal muscle is the primary site for insulin-stimulated glucose disposal, and muscle insulin resistance is central to abnormal glucose metabolism in obesity. Whether muscle insulin signaling to the level of Akt/AS160 is intact in insulin-resistant obese humans is controversial.

METHODS

We defined a linear range of insulin-stimulated systemic and leg glucose uptake in 14 obese and 14 nonobese volunteers using a 2-step insulin clamp (Protocol 1) and then examined the obesity-related defects in muscle insulin action in 16 nonobese and 25 obese male and female volunteers matched for fitness using a 1-step, hyperinsulinemic, euglycemic clamp coupled with muscle biopsies (Protocol 2).

RESULTS

Insulin-stimulated glucose disposal (Si) was reduced by > 60% (P < 0.0001) in the obese group in Protocol 2; however, the phosphorylation of Akt and its downstream effector AS160 were not different between nonobese and obese groups. The increase in phosphorylation of Akt2 in response to insulin was positively correlated with Si for both the nonobese (r = 0.53, P = 0.03) and the obese (r = 0.55, P = 0.01) groups. Total muscle GLUT4 protein was 17% less (P < 0.05) in obese subjects.

CONCLUSIONS

We suggest that reduced muscle glucose uptake in obesity is not due to defects in the insulin signaling pathway at the level of Akt/AS160, which suggests there remain significant gaps in our knowledge of muscle insulin resistance in obesity. Our data imply that models of acute lipotoxicity do not replicate the pathophysiology of obesity.

The mechanisms of skeletal muscle atrophy in response to transient knockdown of the vitamin D receptor in vivo

KEY POINTS

Reduced vitamin D receptor (VDR) expression prompts skeletal muscle atrophy. Atrophy occurs through catabolic processes, namely the induction of autophagy, while anabolism remains unchanged. In response to VDR-knockdown mitochondrial function and related gene-set expression is impaired. In vitro VDR knockdown induces myogenic dysregulation occurring through impaired differentiation. These results highlight the autonomous role the VDR has within skeletal muscle mass regulation.

ABSTRACT

Vitamin D deficiency is estimated to affect ∼40% of the world's population and has been associated with impaired muscle maintenance. Vitamin D exerts its actions through the vitamin D receptor (VDR), the expression of which was recently confirmed in skeletal muscle, and its down-regulation is linked to reduced muscle mass and functional decline. To identify potential mechanisms underlying muscle atrophy, we studied the impact of VDR knockdown (KD) on mature skeletal muscle in vivo, and myogenic regulation in vitro in C2C12 cells. Male Wistar rats underwent in vivo electrotransfer (IVE) to knock down the VDR in hind-limb tibialis anterior (TA) muscle for 10 days. Comprehensive metabolic and physiological analysis was undertaken to define the influence loss of the VDR on muscle fibre composition, protein synthesis, anabolic and catabolic signalling, mitochondrial phenotype and gene expression. Finally, in vitro lentiviral transfection was used to induce sustained VDR-KD in C2C12 cells to analyse myogenic regulation. Muscle VDR-KD elicited atrophy through a reduction in total protein content, resulting in lower myofibre area. Activation of autophagic processes was observed, with no effect upon muscle protein synthesis or anabolic signalling. Furthermore, RNA-sequencing analysis identified systematic down-regulation of multiple mitochondrial respiration-related protein and genesets. Finally, in vitro VDR-knockdown impaired myogenesis (cell cycling, differentiation and myotube formation). Together, these data indicate a fundamental regulatory role of the VDR in the regulation of myogenesis and muscle mass, whereby it acts to maintain muscle mitochondrial function and limit autophagy.

Overexpression of the vitamin D receptor (VDR) induces skeletal muscle hypertrophy

OBJECTIVE

The Vitamin D receptor (VDR) has been positively associated with skeletal muscle mass, function and regeneration. Mechanistic studies have focused on the loss of the receptor, with in vivo whole-body knockout models demonstrating reduced myofibre size and function and impaired muscle development. To understand the mechanistic role upregulation of the VDR elicits in muscle mass/health, we studied the impact of VDR over-expression (OE) in vivo before exploring the importance of VDR expression upon muscle hypertrophy in humans.

METHODS

Wistar rats underwent in vivo electrotransfer (IVE) to overexpress the VDR in the Tibialis anterior (TA) muscle for 10 days, before comprehensive physiological and metabolic profiling to characterise the influence of VDR-OE on muscle protein synthesis (MPS), anabolic signalling and satellite cell activity. Stable isotope tracer (D2O) techniques were used to assess sub-fraction protein synthesis, alongside RNA-Seq analysis. Finally, human participants underwent 20 wks of resistance exercise training, with body composition and transcriptomic analysis.

RESULTS

Muscle VDR-OE yielded total protein and RNA accretion, manifesting in increased myofibre area, i.e., hypertrophy. The observed increases in MPS were associated with enhanced anabolic signalling, reflecting translational efficiency (e.g., mammalian target of rapamycin (mTOR-signalling), with no effects upon protein breakdown markers being observed. Additionally, RNA-Seq illustrated marked extracellular matrix (ECM) remodelling, while satellite cell content, markers of proliferation and associated cell-cycled related gene-sets were upregulated. Finally, induction of VDR mRNA correlated with muscle hypertrophy in humans following long-term resistance exercise type training.

CONCLUSION

VDR-OE stimulates muscle hypertrophy ostensibly via heightened protein synthesis, translational efficiency, ribosomal expansion and upregulation of ECM remodelling-related gene-sets. Furthermore, VDR expression is a robust marker of the hypertrophic response to resistance exercise in humans. The VDR is a viable target of muscle maintenance through testable Vitamin D molecules, as active molecules and analogues.

Endoplasmic reticulum stress causes insulin resistance by inhibiting delivery of newly synthesized insulin receptors to the cell surface

Accumulation of unfolded proteins in the endoplasmic reticulum (ER) causes ER stress and activates a signaling network known as the unfolded protein response (UPR). Here we characterize how ER stress and the UPR inhibit insulin signaling. We find that ER stress inhibits insulin signaling by depleting the cell surface population of the insulin receptor. ER stress inhibits proteolytic maturation of insulin proreceptors by interfering with transport of newly synthesized insulin proreceptors from the ER to the plasma membrane. Activation of AKT, a major target of the insulin signaling pathway, by a cytosolic, membrane-bound chimera between the AP20187-inducible FV2E dimerization domain and the cytosolic protein tyrosine kinase domain of the insulin receptor was not affected by ER stress. Hence, signaling events in the UPR, such as activation of the JNK mitogen-activated protein (MAP) kinases or the pseudokinase TRB3 by the ER stress sensors IRE1α and PERK, do not contribute to inhibition of signal transduction in the insulin signaling pathway. Indeed, pharmacologic inhibition and genetic ablation of JNKs, as well as silencing of expression of TRB3, did not restore insulin sensitivity or rescue processing of newly synthesized insulin receptors in ER-stressed cells. [Media: see text].

Local myostatin inhibition improves skeletal muscle glucose uptake in insulin-resistant high-fat diet-fed mice

Myostatin inhibition is thought to improve whole body insulin sensitivity and mitigate the development of insulin resistance in models of obesity. However, although myostatin is known to be a major regulator of skeletal muscle mass, the direct effects of myostatin inhibition in muscle on glucose uptake and the mechanisms that may underlie this are still unclear. We investigated the effect of local myostatin inhibition by adeno-associated virus-mediated overexpression of the myostatin propeptide on insulin-stimulated skeletal muscle glucose disposal in chow-fed or high fat diet-fed mice and evaluated the molecular pathways that might mediate this. We found that myostatin inhibition improved glucose disposal in obese high fat diet-fed mice alongside the induction of muscle hypertrophy but did not have an impact in chow-fed mice. This improvement was not associated with greater glucose transporter or peroxisome proliferator-activated receptor-γ coactivator-1α expression or 5' AMP-activated protein kinase activation as previously suggested. Instead, transcriptomic analysis suggested that the improvement in glucose disposal was associated with significant enrichment in genes involved in fatty acid metabolism and translation of mitochondrial genes. Thus, myostatin inhibition improves muscle insulin-stimulated glucose disposal in obese high fat diet-fed mice independent of muscle hypertrophy, potentially involving previously unidentified pathways.

Impacts of rat hindlimb Fndc5/irisin overexpression on muscle and adipose tissue metabolism

Myokines, such as irisin, have been purported to exert physiological effects on skeletal muscle in an autocrine/paracrine fashion. In this study, we aimed to investigate the mechanistic role of in vivo fibronectin type III domain-containing 5 (Fndc5)/irisin upregulation in muscle. Overexpression (OE) of Fndc5 in rat hindlimb muscle was achieved by in vivo electrotransfer, i.e., bilateral injections of Fndc5 harboring vectors for OE rats (n = 8) and empty vector for control rats (n = 8). Seven days later, a bolus of D₂O (7.2 mL/kg) was administered via oral gavage to quantify muscle protein synthesis. After an overnight fast, on day 9, 2-deoxy-d-glucose-6-phosphate (2-DG6P; 6 mg/kg) was provided during an intraperitoneal glucose tolerance test (2 g/kg) to assess glucose handling. Animals were euthanized, musculus tibialis cranialis muscles and subcutaneous fat (inguinal) were harvested, and metabolic and molecular effects were evaluated. Muscle Fndc5 mRNA increased with OE (~2-fold; P = 0.014), leading to increased circulating irisin (1.5 ± 0.9 to 3.5 ± 1.2 ng/mL; P = 0.049). OE had no effect on protein anabolism or mitochondrial biogenesis; however, muscle glycogen was increased, along with glycogen synthase 1 gene expression (P = 0.04 and 0.02, respectively). In addition to an increase in glycogen synthase activation in OE (P = 0.03), there was a tendency toward increased glucose transporter 4 protein (P = 0.09). However, glucose uptake (accumulation of 2-DG6P) was identical. Irisin elicited no endocrine effect on mitochondrial biogenesis or uncoupling proteins in white adipose tissue. Hindlimb overexpression led to physiological increases in Fndc5/irisin. However, our data indicate limited short-term impacts of irisin in relation to muscle anabolism, mitochondrial biogenesis, glucose uptake, or adipose remodeling.

Reduced insulin action in muscle of high fat diet rats over the diurnal cycle is not associated with defective insulin signaling

Objective

Energy metabolism and insulin action follow a diurnal rhythm. It is therefore important that investigations into dysregulation of these pathways are relevant to the physiology of this diurnal rhythm.

Methods

We examined glucose uptake, markers of insulin action, and the phosphorylation of insulin signaling intermediates in muscle of chow and high fat, high sucrose (HFHS) diet-fed rats over the normal diurnal cycle.

Results

HFHS animals displayed hyperinsulinemia but had reduced systemic glucose disposal and lower muscle glucose uptake during the feeding period. Analysis of gene expression, enzyme activity, protein abundance and phosphorylation revealed a clear diurnal regulation of substrate oxidation pathways with no difference in Akt signaling in muscle. Transfection of a constitutively active Akt2 into the muscle of HFHS rats did not rescue diet-induced reductions in insulin-stimulated glucose uptake.

Conclusions

These studies suggest that reduced glucose uptake in muscle during the diurnal cycle induced by short-term HFHS-feeding is not the result of reduced insulin signaling.

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Investigating metabolism in rodents over the diurnal cycle more accurately models normal animal physiology.

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Diurnal regulation of substrate oxidation is altered in muscle of HFHS-fed rats.

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There is a disconnect between glucose uptake and canonical insulin signaling in muscle.

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Activation of Akt2 does not rescue diet-induced reductions in insulin-stimulated glucose uptake in muscle.

Systematic tracking of altered modules identifies the key biomarkers involved in chronic lymphocytic leukemia

Key genes in chronic lymphocytic leukemia (CLL) were investigated through systematically tracking the dysregulated modules from protein-protein interaction (PPI) networks. Microarray data of normal subjects and CLL patients recruited from ArrayExpress database were applied to extract differentially expressed genes (DEGs). Additionally, we re-weighted the PPI network of normal and CLL conditions by means of Pearsons correlation coefficient (PCC). Furthermore, clique-merging method was applied to extract the modules and then the altered modules were screened out. The intersection genes were selected from miss and add genes in the altered modules. The common genes were screened from the intersection genes and DEGs in CLL. A total of 734 DEGs were screened by statistical analysis. In this investigation, there were 1,805 and 703 modules in normal as well as disease PPI network. In addition, 875 altered modules were obtained which included 145 miss genes, 353 add genes and 85 intersection genes. Finally, in-depth analysis revealed 9 mutual genes between the intersection genes and DEGs in CLL. Our analysis revealed several key genes associated with CLL by systematically tracking the dysregulated modules, which might be candidate targets for diagnosis and management of CLL.

Lack of muscle mTOR kinase activity causes early onset myopathy and compromises whole-body homeostasis

BACKGROUND

The protein kinase mechanistic target of rapamycin (mTOR) controls cellular growth and metabolism. Although balanced mTOR signalling is required for proper muscle homeostasis, partial mTOR inhibition by rapamycin has beneficial effects on various muscle disorders and age-related pathologies. Besides, more potent mTOR inhibitors targeting mTOR catalytic activity have been developed and are in clinical trials. However, the physiological impact of loss of mTOR catalytic activity in skeletal muscle is currently unknown.

METHODS

We have generated the mTORmKOKI mouse model in which conditional loss of mTOR is concomitant with expression of kinase inactive mTOR in skeletal muscle. We performed a comparative phenotypic and biochemical analysis of mTORmKOKI mutant animals with muscle-specific mTOR knockout (mTORmKO) littermates.

RESULTS

In striking contrast with mTORmKO littermates, mTORmKOKI mice developed an early onset rapidly progressive myopathy causing juvenile lethality. More than 50% mTORmKOKI mice died before 8 weeks of age, and none survived more than 12 weeks, while mTORmKO mice died around 7 months of age. The growth rate of mTORmKOKI mice declined beyond 1 week of age, and the animals showed profound alterations in body composition at 4 weeks of age. At this age, their body weight was 64% that of mTORmKO mice (P < 0.001) due to significant reduction in lean and fat mass. The mass of isolated muscles from mTORmKOKI mice was remarkably decreased by 38-56% (P < 0.001) as compared with that from mTORmKO mice. Histopathological analysis further revealed exacerbated dystrophic features and metabolic alterations in both slow/oxidative and fast/glycolytic muscles from mTORmKOKI mice. We show that the severity of the mTORmKOKI as compared with the mild mTORmKO phenotype is due to more robust suppression of muscle mTORC1 signalling leading to stronger alterations in protein synthesis, oxidative metabolism, and autophagy. This was accompanied with stronger feedback activation of PKB/Akt and dramatic down-regulation of glycogen phosphorylase expression (0.16-fold in tibialis anterior muscle, P < 0.01), thus causing features of glycogen storage disease type V.

CONCLUSIONS

Our study demonstrates a critical role for muscle mTOR catalytic activity in the regulation of whole-body growth and homeostasis. We suggest that skeletal muscle targeting with mTOR catalytic inhibitors may have detrimental effects. The mTORmKOKI mutant mouse provides an animal model for the pathophysiological understanding of muscle mTOR activity inhibition as well as for mechanistic investigation of the influence of skeletal muscle perturbations on whole-body homeostasis.

Microparticle Delivery of Protein Markers for Single-Cell Western Blotting from Microwells

Immunoblotting confers protein identification specificity beyond that of immunoassays by prepending protein electrophoresis (sizing) to immunoprobing. To accurately size protein targets, sample analysis includes concurrent analysis of protein markers with known molecular masses. To incorporate protein markers in single-cell western blotting, microwells are used to isolate individual cells and protein marker-coated microparticles. A magnetic field directs protein-coated microparticles to >75% of microwells, so as to 1) deliver a quantum of protein marker to each cell-laden microwell and 2) synchronize protein marker solubilization with cell lysis. Nickel-coated microparticles are designed, fabricated, and characterized, each conjugated with a mixture of histidine-tagged proteins (42.3-100 kDa). Imidazole in the cell lysis buffer solubilizes protein markers during a 30 s cell lysis step, with an observed protein marker release half-life of 4.46 s. Across hundreds of individual microwells and different microdevices, robust log-linear regression fits (R2 > 0.97) of protein molecular mass and electrophoretic mobility are observed. The protein marker and microparticle system is applied to determine the molecular masses of five endogenous proteins in breast cancer cells (GAPDH, β-TUB, CK8, STAT3, ER-α), with <20% mass error. Microparticle-delivered protein standards underpin robust, reproducible electrophoretic cytometry that complements single-cell genomics and transcriptomics.

Muscle and adipose tissue insulin resistance: malady without mechanism?

Insulin resistance is a major risk factor for numerous diseases, including type 2 diabetes and cardiovascular disease. These disorders have dramatically increased in incidence with modern life, suggesting that excess nutrients and obesity are major causes of "common" insulin resistance. Despite considerable effort, the mechanisms that contribute to common insulin resistance are not resolved. There is universal agreement that extracellular perturbations, such as nutrient excess, hyperinsulinemia, glucocorticoids, or inflammation, trigger intracellular stress in key metabolic target tissues, such as muscle and adipose tissue, and this impairs the ability of insulin to initiate its normal metabolic actions in these cells. Here, we present evidence that the impairment in insulin action is independent of proximal elements of the insulin signaling pathway and is likely specific to the glucoregulatory branch of insulin signaling. We propose that many intracellular stress pathways act in concert to increase mitochondrial reactive oxygen species to trigger insulin resistance. We speculate that this may be a physiological pathway to conserve glucose during specific states, such as fasting, and that, in the presence of chronic nutrient excess, this pathway ultimately leads to disease. This review highlights key points in this pathway that require further research effort.

Ablation of Grb10 Specifically in Muscle Impacts Muscle Size and Glucose Metabolism in Mice

Grb10 is an adaptor-type signaling protein most highly expressed in tissues involved in insulin action and glucose metabolism, such as muscle, pancreas, and adipose. Germline deletion of Grb10 in mice creates a phenotype with larger muscles and improved glucose homeostasis. However, it has not been determined whether Grb10 ablation specifically in muscle is sufficient to induce hypermuscularity or affect whole body glucose metabolism. In this study we generated muscle-specific Grb10-deficient mice (Grb10-mKO) by crossing Grb10flox/flox mice with mice expressing Cre recombinase under control of the human α-skeletal actin promoter. One-year-old Grb10-mKO mice had enlarged muscles, with greater cross-sectional area of fibers compared with wild-type (WT) mice. This degree of hypermuscularity did not affect whole body glucose homeostasis under basal conditions. However, hyperinsulinemic/euglycemic clamp studies revealed that Grb10-mKO mice had greater glucose uptake into muscles compared with WT mice. Insulin signaling was increased at the level of phospho-Akt in muscle of Grb10-mKO mice compared with WT mice, consistent with a role of Grb10 as a modulator of proximal insulin receptor signaling. We conclude that ablation of Grb10 in muscle is sufficient to affect muscle size and metabolism, supporting an important role for this protein in growth and metabolic pathways.

Biochemical and cellular properties of insulin receptor signalling

The mechanism of insulin action is a central theme in biology and medicine. In addition to the rather rare condition of insulin deficiency caused by autoimmune destruction of pancreatic β-cells, genetic and acquired abnormalities of insulin action underlie the far more common conditions of type 2 diabetes, obesity and insulin resistance. The latter predisposes to diseases ranging from hypertension to Alzheimer disease and cancer. Hence, understanding the biochemical and cellular properties of insulin receptor signalling is arguably a priority in biomedical research. In the past decade, major progress has led to the delineation of mechanisms of glucose transport, lipid synthesis, storage and mobilization. In addition to direct effects of insulin on signalling kinases and metabolic enzymes, the discovery of mechanisms of insulin-regulated gene transcription has led to a reassessment of the general principles of insulin action. These advances will accelerate the discovery of new treatment modalities for diabetes.

Promoting Glucose Transporter-4 Vesicle Trafficking along Cytoskeletal Tracks: PAK-Ing Them Out

Glucose is the principal cellular energy source in humans and maintenance of glucose homeostasis is critical for survival. Glucose uptake into peripheral skeletal muscle and adipose tissues requires the trafficking of vesicles containing glucose transporter-4 (GLUT4) from the intracellular storage compartments to the cell surface. Trafficking of GLUT4 storage vesicles is initiated via the canonical insulin signaling cascade in skeletal muscle and fat cells, as well as via exercise-induced contraction in muscle cells. Recent studies have elucidated steps in the signaling cascades that involve remodeling of the cytoskeleton, a process that underpins the mechanical movement of GLUT4 vesicles. This review is focused upon an alternate phosphoinositide-3 kinase-dependent pathway involving Ras-related C3 botulinum toxin substrate 1 signaling through the p21-activated kinase p21-activated kinase 1 and showcases related signaling events that co-regulate both the depolymerization and re-polymerization of filamentous actin. These new insights provide an enriched understanding into the process of glucose transport and yield potential new targets for interventions aimed to improve insulin sensitivity and remediate insulin resistance, pre-diabetes, and the progression to type 2 diabetes.

An overview of technical considerations for Western blotting applications to physiological research

The applications of Western/immunoblotting (WB) techniques have reached multiple layers of the scientific community and are now considered routine procedures in the field of physiology. This is none more so than in relation to skeletal muscle physiology (i.e., resolving the mechanisms underpinning adaptations to exercise). Indeed, the inclusion of WB data is now considered an essential aspect of many such physiological publications to provide mechanistic insight into regulatory processes. Despite this popularity, and due to the ubiquitous and relatively inexpensive availability of WB equipment, the quality of WB in publications and subsequent analysis and interpretation of the data can be variable, perhaps resulting in spurious conclusions. This may be due to poor laboratory technique and/or lack of comprehension of the critical steps involved in WB and what quality control procedures should be in place to ensure robust data generation. The present review aims to provide a detailed description and critique of WB procedures and technicalities, from sample collection through preparation, blotting and detection, to analysis of the data collected. We aim to provide the reader with improved expertise to critically conduct, evaluate, and troubleshoot the WB process, to produce reproducible and reliable blots.

A 2D-QSAR and Grid-Independent Molecular Descriptor (GRIND) Analysis of Quinoline-Type Inhibitors of Akt2: Exploration of the Binding Mode in the Pleckstrin Homology (PH) Domain

Protein kinase B-β (PKBβ/Akt2) is a serine/threonine-specific protein kinase that has emerged as one of the most important regulators of cell growth, differentiation, and division. Upregulation of Akt2 in various human carcinomas, including ovarian, breast, and pancreatic, is a well-known tumorigenesis phenomenon. Early on, the concept of the simultaneous administration of anticancer drugs with inhibitors of Akt2 was advocated to overcome cell proliferation in the chemotherapeutic treatment of cancer. However, clinical studies have not lived up to the high expectations, and several phase II and phase III clinical studies have been terminated prematurely because of severe side effects related to the non-selective isomeric inhibition of Akt2. The notion that the sequence identity of pleckstrin homology (PH) domains within Akt-isoforms is less than 30% might indicate the possibility of the development of selective antagonists against the Akt2 PH domain. Therefore, in this study, various in silico tools were utilized to explore the hypothesis that quinoline-type inhibitors bind in the Akt2 PH domain. A Grid-Independent Molecular Descriptor (GRIND) analysis indicated that two hydrogen bond acceptors, two hydrogen bond donors and one hydrophobic feature at a certain distance from each other were important for the selective inhibition of Akt2. Our docking results delineated the importance of Lys30 as an anchor point for mapping the distances of important amino acid residues in the binding pocket, including Lys14, Glu17, Arg25, Asn53, Asn54 and Arg86. The binding regions identified complement the GRIND-based pharmacophoric features.

Epigenomic and transcriptional control of insulin resistance

Insulin resistance is one of the defining features of type 2 diabetes and the metabolic syndrome and accompanies many other clinical conditions, ranging from obesity to lipodystrophy to glucocorticoid excess. Extraordinary efforts have gone into defining the mechanisms that underlie insulin resistance, with most attention focused on altered signalling as well as mitochondrial and endoplasmic reticulum stress. Here, nuclear mechanisms of insulin resistance, including transcriptional and epigenomic effects, will be discussed. Three levels of control involving transcription factors, transcriptional cofactors, and chromatin-modifying enzymes will be considered. Well-studied examples of the first include PPAR-γ in adipose tissue and the glucocorticoid receptor and FoxO1 in a variety of insulin-sensitive tissues. These proteins work in concert with cofactors such as PGC-1α and CRTC2, and chromatin-modifying enzymes including DNA methyltransferases and histone acetyltransferases, to regulate key genes that promote insulin-stimulated glucose uptake, gluconeogenesis or other pathways that affect systemic insulin action. Furthermore, genetic variation associated with increased risk of type 2 diabetes is often related to altered transcription factor binding, either by affecting the transcription factor itself, or more commonly by changing the binding affinity of a noncoding regulatory region. Finally, several avenues for therapeutic exploitation in the battle against metabolic disease will be discussed, including small-molecule inhibitors and activators of these factors and their related pathways.

Insulin resistance and sarcopenia: mechanistic links between common co-morbidities

Insulin resistance (IR) in skeletal muscle is a key defect mediating the link between obesity and type 2 diabetes, a disease that typically affects people in later life. Sarcopenia (age-related loss of muscle mass and quality) is a risk factor for a number of frailty-related conditions that occur in the elderly. In addition, a syndrome of 'sarcopenic obesity' (SO) is now increasingly recognised, which is common in older people and is applied to individuals that simultaneously show obesity, IR and sarcopenia. Such individuals are at an increased risk of adverse health events compared with those who are obese or sarcopenic alone. However, there are no licenced treatments for sarcopenia or SO, the syndrome is poorly defined clinically and the mechanisms that might explain a common aetiology are not yet well characterised. In this review, we detail the nature and extent of the clinical syndrome, highlight some of the key physiological processes that are dysregulated and discuss some candidate molecular pathways that could be implicated in both metabolic and anabolic defects in skeletal muscle, with an eye towards future therapeutic options. In particular, the potential roles of Akt/mammalian target of rapamycin signalling, AMP-activated protein kinase, myostatin, urocortins and vitamin D are discussed.

Effects of treadmill exercise on skeletal muscle mTOR signaling pathway in high-fat diet-induced obese mice

[Purpose] The aim of this study was to investigate the effects of regular treadmill exercise on skeletal muscle Rictor-Akt and mTOR-Raptor-S6K1 signaling pathway in high-fat diet-induced obese mice. [Subjects and Methods] Four- week-old C57BL/6 mice were adopted and classified into normal diet group (ND, n = 10), normal diet and training group (NDT, n = 10), high-fat diet group (HF, n = 10), and high-fat diet and training group (HFT, n = 10). The exercise program consisted of a treadmill exercise provided at low intensity for 1–4 weeks, and moderate intensity for 5–8 weeks. [Results] The Western blot method was used to measure the expression of mTOR, Raptor, S6K1, Rictor, and Akt proteins in the soleus muscle. mTOR levels were significantly higher in the HF group than in the ND and NDT groups. Raptor/mTORC1 and S6K1 levels were significantly higher in the HF group than in all the other groups. Akt levels were significantly lower in the HF group than in the NDT group. The risk of obesity may be associated with the overactivation of the mTOR-Raptor-S6K1 signaling pathway and a decrease in Akt levels. [Conclusion] This study also indicates that performing aerobic exercise may be associated with the downregulation of the mTOR-Raptor-S6K1 pathway.

Nuclear Mechanisms of Insulin Resistance

Insulin resistance is a sine qua non of type 2 diabetes and is associated with many other clinical conditions. Decades of research into mechanisms underlying insulin resistance have mostly focused on problems in insulin signal transduction and other mitochondrial and cytosolic pathways. By contrast, relatively little attention has been focused on transcriptional and epigenetic contributors to insulin resistance, despite strong evidence that such nuclear mechanisms play a major role in the etiopathogenesis of this condition. In this review, we summarize the evidence for nuclear mechanisms of insulin resistance, focusing on three transcription factors with a major impact on insulin action in liver, muscle, and fat.

Urocortin 3 activates AMPK and AKT pathways and enhances glucose disposal in rat skeletal muscle

Insulin resistance (IR) in skeletal muscle is an important component of both type 2 diabetes and the syndrome of sarcopaenic obesity, for which there are no effective therapies. Urocortins (UCNs) are not only well established as neuropeptides but also have their roles in metabolism in peripheral tissues. We have shown recently that global overexpression of UCN3 resulted in muscular hypertrophy and resistance to the adverse metabolic effects of a high-fat diet. Herein, we aimed to establish whether short-term local UCN3 expression could enhance glucose disposal and insulin signalling in skeletal muscle. UCN3 was found to be expressed in right tibialis cranialis and extensor digitorum longus muscles of rats by in vivo electrotransfer and the effects studied vs the contralateral muscles after 1 week. No increase in muscle mass was detected, but test muscles showed 19% larger muscle fibre diameter (P=0.030), associated with increased IGF1 and IGF1 receptor mRNA and increased SER256 phosphorylation of forkhead transcription factor. Glucose clearance into the test muscles after an intraperitoneal glucose load was increased by 23% (P=0.018) per unit mass, associated with increased GLUT1 (34% increase; P=0.026) and GLUT4 (48% increase; P=0.0009) proteins, and significantly increased phosphorylation of insulin receptor substrate-1, AKT, AKT substrate of 160 kDa, glycogen synthase kinase-3β, AMP-activated protein kinase and its substrate acetyl coA carboxylase. Thus, UCN3 expression enhances glucose disposal and signalling in muscle by an autocrine/paracrine mechanism that is separate from its pro-hypertrophic effects, implying that such a manipulation may have promised for the treatment of IR syndromes including sarcopaenic obesity.

Palmitoleic acid prevents palmitic acid-induced macrophage activation and consequent p38 MAPK-mediated skeletal muscle insulin resistance

Obesity and saturated fatty acid (SFA) treatment are both associated with skeletal muscle insulin resistance (IR) and increased macrophage infiltration. However, the relative effects of SFA and unsaturated fatty acid (UFA)-activated macrophages on muscle are unknown. Here, macrophages were treated with palmitic acid, palmitoleic acid or both and the effects of the conditioned medium (CM) on C2C12 myotubes investigated. CM from palmitic acid-treated J774s (palm-mac-CM) impaired insulin signalling and insulin-stimulated glycogen synthesis, reduced Inhibitor κBα and increased phosphorylation of p38 mitogen-activated protein kinase (MAPK) and c-Jun N-terminal kinase in myotubes. p38 MAPK inhibition or siRNA partially ameliorated these defects, as did addition of tumour necrosis factor-α blocking antibody to the CM. Macrophages incubated with both FAs generated CM that did not induce IR, while palmitoleic acid-mac-CM alone was insulin sensitising. Thus UFAs may improve muscle insulin sensitivity and counteract SFA-mediated IR through an effect on macrophage activation.

Over-expression of PRAS40 enhances insulin sensitivity in skeletal muscle

CONTEXT

Silencing proline-rich Akt substrate of 40-kDa (PRAS40) impairs insulin signalling in skeletal muscle.

OBJECTIVE

This study assessed the effects of over-expressing wild type or mutant AAA-PRAS40, in which the major phosphorylation sites and mTORC1-binding site were mutated, on insulin signalling in skeletal muscle.

RESULTS

Over-expression of WT-PRAS40, but not AAA-PRAS40, impaired the insulin-mediated activation of the mTORC1-pathway in human skeletal muscle cells (hSkMC). However, insulin-mediated Akt-phosphorylation was increased upon over-expression of WT-PRAS40 both in hSkMC and mouse skeletal muscle. Also over-expression of AAA-PRAS40 had an insulin-sensitizing effect, although to a lesser extent as WT-PRAS40. The insulin-sensitizing effect associated with increased IRS1 protein abundance and inhibition of proteasome activity. Finally, over-expression of WT-PRAS40 reversed hyperinsulinemia-induced insulin resistance.

CONCLUSION

This study identifies PRAS40 as a regulator of insulin sensitivity in hSkMC. In contrast to the mTORC1-pathway, the insulin-sensitizing action of PRAS40 occurs independent of binding of PRAS40 to the mTORC1-complex.

Local overexpression of the myostatin propeptide increases glucose transporter expression and enhances skeletal muscle glucose disposal

Insulin resistance (IR) in skeletal muscle is a prerequisite for type 2 diabetes and is often associated with obesity. IR also develops alongside muscle atrophy in older individuals in sarcopenic obesity. The molecular defects that underpin this syndrome are not well characterized, and there is no licensed treatment. Deletion of the transforming growth factor-β family member myostatin, or sequestration of the active peptide by overexpression of the myostatin propeptide/latency-associated peptide (ProMyo) results in both muscle hypertrophy and reduced obesity and IR. We aimed to establish whether local myostatin inhibition would have a paracrine/autocrine effect to enhance glucose disposal beyond that simply generated by increased muscle mass, and the mechanisms involved. We directly injected adeno-associated virus expressing ProMyo in right tibialis cranialis/extensor digitorum longus muscles of rats and saline in left muscles and compared the effects after 17 days. Both test muscles were increased in size (by 7 and 11%) and showed increased radiolabeled 2-deoxyglucose uptake (26 and 47%) and glycogen storage (28 and 41%) per unit mass during an intraperitoneal glucose tolerance test. This was likely mediated through increased membrane protein levels of GLUT1 (19% higher) and GLUT4 (63% higher). Interestingly, phosphorylation of phosphoinositol 3-kinase signaling intermediates and AMP-activated kinase was slightly decreased, possibly because of reduced expression of insulin-like growth factor-I in these muscles. Thus, myostatin inhibition has direct effects to enhance glucose disposal in muscle beyond that expected of hypertrophy alone, and this approach may offer potential for the therapy of IR syndromes.

Fatty acid metabolism, energy expenditure and insulin resistance in muscle

Fatty acids (FAs) are essential elements of all cells and have significant roles as energy substrates, components of cellular structure and signalling molecules. The storage of excess energy intake as fat in adipose tissue is an evolutionary advantage aimed at protecting against starvation, but in much of today's world, humans are faced with an unlimited availability of food, and the excessive accumulation of fat is now a major risk for human health, especially the development of type 2 diabetes (T2D). Since the first recognition of the association between fat accumulation, reduced insulin action and increased risk of T2D, several mechanisms have been proposed to link excess FA availability to reduced insulin action, with some of them being competing or contradictory. This review summarises the evidence for these mechanisms in the context of excess dietary FAs generating insulin resistance in muscle, the major tissue involved in insulin-stimulated disposal of blood glucose. It also outlines potential problems with models and measurements that may hinder as well as help improve our understanding of the links between FAs and insulin action.

Molecular mechanisms of muscle plasticity with exercise

The skeletal muscle phenotype is subject to considerable malleability depending on use. Low-intensity endurance type exercise leads to qualitative changes of muscle tissue characterized mainly by an increase in structures supporting oxygen delivery and consumption. High-load strength-type exercise leads to growth of muscle fibers dominated by an increase in contractile proteins. In low-intensity exercise, stress-induced signaling leads to transcriptional upregulation of a multitude of genes with Ca(2+) signaling and the energy status of the muscle cells sensed through AMPK being major input determinants. Several parallel signaling pathways converge on the transcriptional co-activator PGC-1α, perceived as being the coordinator of much of the transcriptional and posttranscriptional processes. High-load training is dominated by a translational upregulation controlled by mTOR mainly influenced by an insulin/growth factor-dependent signaling cascade as well as mechanical and nutritional cues. Exercise-induced muscle growth is further supported by DNA recruitment through activation and incorporation of satellite cells. Crucial nodes of strength and endurance exercise signaling networks are shared making these training modes interdependent. Robustness of exercise-related signaling is the consequence of signaling being multiple parallel with feed-back and feed-forward control over single and multiple signaling levels. We currently have a good descriptive understanding of the molecular mechanisms controlling muscle phenotypic plasticity. We lack understanding of the precise interactions among partners of signaling networks and accordingly models to predict signaling outcome of entire networks. A major current challenge is to verify and apply available knowledge gained in model systems to predict human phenotypic plasticity.

Characterization and regulation of mechanical loading-induced compensatory muscle hypertrophy

In mammalian systems, skeletal muscle exists in a dynamic state that monitors and regulates the physiological investment in muscle size to meet the current level of functional demand. This review attempts to consolidate current knowledge concerning development of the compensatory hypertrophy that occurs in response to a sustained increase in the mechanical loading of skeletal muscle. Topics covered include: defining and measuring compensatory hypertrophy, experimental models, loading stimulus parameters, acute responses to increased loading, hyperplasia, myofiber-type adaptations, the involvement of satellite cells, mRNA translational control, mechanotransduction, and endocrinology. The authors conclude with their impressions of current knowledge gaps in the field that are ripe for future study.

Skeletal muscle hypertrophy and regeneration: interplay between the myogenic regulatory factors (MRFs) and insulin-like growth factors (IGFs) pathways

Adult skeletal muscle can regenerate in response to muscle damage. This ability is conferred by the presence of myogenic stem cells called satellite cells. In response to stimuli such as injury or exercise, these cells become activated and express myogenic regulatory factors (MRFs), i.e., transcription factors of the myogenic lineage including Myf5, MyoD, myogenin, and Mrf4 to proliferate and differentiate into myofibers. The MRF family of proteins controls the transcription of important muscle-specific proteins such as myosin heavy chain and muscle creatine kinase. Different growth factors are secreted during muscle repair among which insulin-like growth factors (IGFs) are the only ones that promote both muscle cell proliferation and differentiation and that play a key role in muscle regeneration and hypertrophy. Different isoforms of IGFs are expressed during muscle repair: IGF-IEa, IGF-IEb, or IGF-IEc (also known as mechano growth factor, MGF) and IGF-II. MGF is expressed first and is observed in satellite cells and in proliferating myoblasts whereas IGF-Ia and IGF-II expression occurs at the state of muscle fiber formation. Interestingly, several studies report the induction of MRFs in response to IGFs stimulation. Inversely, IGFs expression may also be regulated by MRFs. Various mechanisms are proposed to support these interactions. In this review, we describe the general process of muscle hypertrophy and regeneration and decipher the interactions between the two groups of factors involved in the process.

Melatonin improves insulin sensitivity independently of weight loss in old obese rats

In aged rats, insulin signaling pathway (ISP) is impaired in tissues that play a pivotal role in glucose homeostasis, such as liver, skeletal muscle, and adipose tissue. Moreover, the aging process is also associated with obesity and reduction in melatonin synthesis from the pineal gland and other organs. The aim of the present work was to evaluate, in male old obese Wistar rats, the effect of melatonin supplementation in the ISP, analyzing the total protein amount and the phosphorylated status (immunoprecipitation and immunoblotting) of the insulin cascade components in the rat hypothalamus, liver, skeletal muscle, and periepididymal adipose tissue. Melatonin was administered in the drinking water for 8- and 12 wk during the night period. Food and water intake and fasting blood glucose remained unchanged. The insulin sensitivity presented a 2.1-fold increase both after 8- and 12 wk of melatonin supplementation. Animals supplemented with melatonin for 12 wk also presented a reduction in body mass. The acute insulin-induced phosphorylation of the analyzed ISP proteins increased 1.3- and 2.3-fold after 8- and 12 wk of melatonin supplementation. The total protein content of the insulin receptor (IR) and the IR substrates (IRS-1, 2) remained unchanged in all investigated tissues, except for the 2-fold increase in the total amount of IRS-1 in the periepididymal adipose tissue. Therefore, the known age-related melatonin synthesis reduction may also be involved in the development of insulin resistance and the adequate supplementation could be an important alternative for the prevention of insulin signaling impairment in aged organisms.

Inflammation as a potential link between nonalcoholic fatty liver disease and insulin resistance

Nonalcoholic fatty liver disease (NAFLD) has become a major health problem in developed countries. It has affected more than 30% of the general population and is commonly associated with insulin resistance, which is a major risk factor for the development of type 2 diabetes and a central feature of the metabolic syndrome. Furthermore, accumulating evidences reveal that NAFLD as well as insulin resistance is strongly related to inflammation. Cytokines and adipokines play a pivotal role in inflammatory processes. In addition, these inflammatory mediators regulate various functions including metabolic energy balance, inflammation, and immune response. However, their role in modulating ectopic lipids involved in the development of insulin resistance, such as diacylglycerols and ceramides, remains unknown. The aim of this review is first to describe the pathophysiology of insulin resistance in NAFLD. In particular, we discuss the role of ectopic lipid accumulation in the liver. Secondly, we also summarize recent findings emphasizing the role of main inflammatory markers in both NAFLD and insulin resistance and their potential role in modulating hepatic fat content in NAFLD and associated hepatic insulin resistance.

Insulin signalling mechanisms for triacylglycerol storage

Insulin signalling is uniquely required for storing energy as fat in humans. While de novo synthesis of fatty acids and triacylglycerol occurs mostly in liver, adipose tissue is the primary site for triacylglycerol storage. Insulin signalling mechanisms in adipose tissue that stimulate hydrolysis of circulating triacylglycerol, uptake of the released fatty acids and their conversion to triacylglycerol are poorly understood. New findings include (1) activation of DNA-dependent protein kinase to stimulate upstream stimulatory factor (USF)1/USF2 heterodimers, enhancing the lipogenic transcription factor sterol regulatory element binding protein 1c (SREBP1c); (2) stimulation of fatty acid synthase through AMP kinase modulation; (3) mobilisation of lipid droplet proteins to promote retention of triacylglycerol; and (4) upregulation of a novel carbohydrate response element binding protein β isoform that potently stimulates transcription of lipogenic enzymes. Additionally, insulin signalling through mammalian target of rapamycin to activate transcription and processing of SREBP1c described in liver may apply to adipose tissue. Paradoxically, insulin resistance in obesity and type 2 diabetes is associated with increased triacylglycerol synthesis in liver, while it is decreased in adipose tissue. This and other mysteries about insulin signalling and insulin resistance in adipose tissue make this topic especially fertile for future research.

AMPK and Exercise: Glucose Uptake and Insulin Sensitivity

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

Defining Akt actions in muscle differentiation

Muscle development in childhood and muscle regeneration in adults are highly regulated processes that are necessary for reaching and maintaining optimal muscle mass and strength throughout life. Muscle repair after injury relies on stem cells, termed satellite cells, whose activity is controlled by complex signals mediated by cell-cell contact, by growth factors, and by hormones, which interact with genetic programs controlled by myogenic transcription factors. Insulin-like growth factors (IGFs) play key roles in muscle development and help coordinate muscle repair after injury, primarily by stimulating the phosphatidylinositol 3-kinase-Akt signaling pathway, and both in vitro and in vivo studies have shown that Akt kinase activity is critical for optimal muscle growth and regeneration. Here we find that of the two Akts expressed in muscle, Akt1 is essential for initiation of differentiation in culture and is required for normal myoblast motility, while Akt2 is dispensable. Although Akt2 deficiency did lead to diminished myotube maturation, as assessed by a decline in myofiber area and in fusion index, either Akt1 or Akt2 could restore these processes toward normal. Thus levels of Akt expression rather than distinct actions of individual Akt species are critical for normal myofiber development during the later stages of muscle differentiation.

Overexpression of manganese superoxide dismutase ameliorates high-fat diet-induced insulin resistance in rat skeletal muscle

Elevated mitochondrial reactive oxygen species have been suggested to play a causative role in some forms of muscle insulin resistance. However, the extent of their involvement in the development of diet-induced insulin resistance remains unclear. To investigate, manganese superoxide dismutase (MnSOD), a key mitochondrial-specific enzyme with antioxidant modality, was overexpressed, and the effect on in vivo muscle insulin resistance induced by a high-fat (HF) diet in rats was evaluated. Male Wistar rats were maintained on chow or HF diet. After 3 wk, in vivo electroporation (IVE) of MnSOD expression and empty vectors was undertaken in right and left tibialis cranialis (TC) muscles, respectively. After one more week, insulin action was evaluated using hyperinsulinemic euglycemic clamp, and tissues were subsequently analyzed for antioxidant enzyme capacity and markers of oxidative stress. MnSOD mRNA was overexpressed 4.5-fold, and protein levels were increased by 70%, with protein detected primarily in the mitochondrial fraction of muscle fibers. This was associated with elevated MnSOD and glutathione peroxidase activity, indicating that the overexpressed MnSOD was functionally active. The HF diet significantly reduced whole body and TC muscle insulin action, whereas overexpression of MnSOD in HF diet animals ameliorated this reduction in TC muscle glucose uptake by 50% (P < 0.05). Decreased protein carbonylation was seen in MnSOD overexpressing TC muscle in HF-treated animals (20% vs. contralateral control leg, P < 0.05), suggesting that this effect was mediated through an altered redox state. Thus interventions causing elevation of mitochondrial antioxidant activity may offer protection against diet-induced insulin resistance in skeletal muscle.

Attenuation of unloading-induced rat soleus atrophy with the heat-shock protein inducer 17-(allylamino)-17-demethoxygeldanamycin

We hypothesized that pharmacological induction of HSP70 would attenuate soleus atrophy development under 3 d of rat hindlimb unloading. Male Wistar rats were divided into control (C; n=7), 3-d hindlimb unloading (HUL; n=7), HUL with HSP90 inducer administration, 17-allylamino-17-emethoxygeldanamycin (17-AAG; 60 mg/kg, HUL+17-AAG, n=8). The relative weight of soleus muscle to body weight [soleus wt (mg)/body wt (g)] in the HUL group was less than that of the C and HUL+17-AAG groups (P<0.05). We revealed HSP90, HSP70 mRNA decrease in the HUL group (but not the HUL+17-AAG group) vs. C (P<0.05). The unloading resulted in significant increases of μ-calpain and conjugated ubiquitin (Ub) levels (proteins as well as mRNAs) vs. the C group, whereas 17-AAG administration prevented these alterations (studied by SDS-PAGE and RT-PCR). pFOXO3 protein was decreased in the HUL group vs. C, but not in HUL+17-AAG. Content of E3-lygase (MuRF-1, MAFbx) mRNA was increased in both suspended groups. In summary, 17-AAG administration attenuates soleus muscle atrophy, μ-calpain, and Ub increases under hindlimb unloading as well as decrease of pFOXO3.

Phenotype selection reveals coevolution of muscle glycogen and protein and PTEN as a gate keeper for the accretion of muscle mass in adult female mice

We have investigated molecular mechanisms for muscle mass accretion in a non-inbred mouse model (DU6P mice) characterized by extreme muscle mass. This extreme muscle mass was developed during 138 generations of phenotype selection for high protein content. Due to the repeated trait selection a complex setting of different mechanisms was expected to be enriched during the selection experiment. In muscle from 29-week female DU6P mice we have identified robust increases of protein kinase B activation (AKT, Ser-473, up to 2-fold) if compared to 11- and 54-week DU6P mice or controls. While a number of accepted effectors of AKT activation, including IGF-I, IGF-II, insulin/IGF-receptor, myostatin or integrin-linked kinase (ILK), were not correlated with this increase, phosphatase and tensin homologue deleted on chromosome 10 (PTEN) was down-regulated in 29-week female DU6P mice. In addition, higher levels of PTEN phosphorylation were found identifying a second mechanism of PTEN inhibition. Inhibition of PTEN and activation of AKT correlated with specific activation of p70S6 kinase and ribosomal protein S6, reduced phosphorylation of eukaryotic initiation factor 2α (eIF2α) and higher rates of protein synthesis in 29-week female DU6P mice. On the other hand, AKT activation also translated into specific inactivation of glycogen synthase kinase 3ß (GSK3ß) and an increase of muscular glycogen. In muscles from 29-week female DU6P mice a significant increase of protein/DNA was identified, which was not due to a reduction of protein breakdown or to specific increases of translation initiation. Instead our data support the conclusion that a higher rate of protein translation is contributing to the higher muscle mass in mid-aged female DU6P mice. Our results further reveal coevolution of high protein and high glycogen content during the selection experiment and identify PTEN as gate keeper for muscle mass in mid-aged female DU6P mice.

The role of skeletal muscle glycogen breakdown for regulation of insulin sensitivity by exercise

Glycogen is the storage form of carbohydrates in mammals. In humans the majority of glycogen is stored in skeletal muscles (∼500 g) and the liver (∼100 g). Food is supplied in larger meals, but the blood glucose concentration has to be kept within narrow limits to survive and stay healthy. Therefore, the body has to cope with periods of excess carbohydrates and periods without supplementation. Healthy persons remove blood glucose rapidly when glucose is in excess, but insulin-stimulated glucose disposal is reduced in insulin resistant and type 2 diabetic subjects. During a hyperinsulinemic euglycemic clamp, 70-90% of glucose disposal will be stored as muscle glycogen in healthy subjects. The glycogen stores in skeletal muscles are limited because an efficient feedback-mediated inhibition of glycogen synthase prevents accumulation. De novo lipid synthesis can contribute to glucose disposal when glycogen stores are filled. Exercise physiologists normally consider glycogen's main function as energy substrate. Glycogen is the main energy substrate during exercise intensity above 70% of maximal oxygen uptake ([Formula: see text]) and fatigue develops when the glycogen stores are depleted in the active muscles. After exercise, the rate of glycogen synthesis is increased to replete glycogen stores, and blood glucose is the substrate. Indeed insulin-stimulated glucose uptake and glycogen synthesis is elevated after exercise, which, from an evolutional point of view, will favor glycogen repletion and preparation for new "fight or flight" events. In the modern society, the reduced glycogen stores in skeletal muscles after exercise allows carbohydrates to be stored as muscle glycogen and prevents that glucose is channeled to de novo lipid synthesis, which over time will causes ectopic fat accumulation and insulin resistance. The reduction of skeletal muscle glycogen after exercise allows a healthy storage of carbohydrates after meals and prevents development of type 2 diabetes.