Skeletal Muscle Uptake Research Articles

Abscisic Acid Improves Insulin Action on Glycemia in Insulin-Deficient Mouse Models of Type 1 Diabetes.

Abscisic acid (ABA), a plant hormone, has recently been shown to play a role in glycemia regulation in mammals, by stimulating insulin-independent glucose uptake and metabolism in skeletal muscle. The aim of this study was to test whether ABA could improve glycemic control in a murine model of type 1 diabetes (T1D). Mice were rendered diabetic with streptozotocin and the effect of ABA administration, alone or with insulin, was tested on glycemia. Diabetic mice treated with a single oral dose of ABA and low-dose subcutaneous insulin showed a significantly reduced glycemia profile compared with controls treated with insulin alone. In diabetic mice treated for four weeks with ABA, the effect of low-dose insulin on the glycemia profile after glucose load was significantly improved, and transcription both of the insulin receptor, and of glycolytic enzymes in muscle, was increased. Moreover, a significantly increased transcription and protein expression of AMPK, PGC1-α, and GLUT4 was observed in the skeletal muscle from diabetic mice treated with ABA, compared with untreated controls. ABA supplementation in conjunction with insulin holds the promise of reducing the dose of insulin required in T1D, reducing the risk of hypoglycemia, and improving muscle insulin sensitivity and glucose consumption.

203-LB: The Role of the Exocyst Trafficking Complex in CD36 Fatty Acid Translocase Trafficking in Skeletal Muscle Cells

Skeletal muscle free fatty acid (FFA) uptake correlates with the amount of fatty acid translocase CD36 on the cell surface. In obesity, chronic membrane-localization of CD36, but not other FFA transporters, augments FFA uptake and intracellular lipid accumulation. This ectopic lipid accumulation then causes insulin resistance (IR) by inhibiting GLUT4 glucose transporter trafficking and glucose uptake in response to insulin. GLUT4 and CD36 cell-surface delivery is stimulated by insulin and contraction signaling, sharing conserved downstream effectors. The eight-protein exocyst complex is responsible for the targeted docking of membrane-bound vesicles, a process underlying cell-surface delivery of fuel transporters. The exocyst indeed regulates insulin-induced glucose uptake via GLUT4 trafficking to the membrane in adipocytes and skeletal muscle. In addition, it controls lipid uptake in adipocytes, but we do not know which of the FFA-transporting proteins the exocyst targets during this process. Based on the high degree of conservation of the fuel transporter trafficking mechanisms in adipose and skeletal muscle tissue, we hypothesized that the exocyst contributes to lipid uptake in skeletal muscle via the targeted trafficking of CD36. Our data shows that the exocyst plays a role in FFA uptake in skeletal muscle cells. Using an L6 GLUT4myc myoblast model we demonstrate that the exocyst is recruited to CD36 vesicles upon insulin and contraction signaling. We also show that disrupted exocyst activity leads to an impaired CD36 surface delivery and decreased FFA uptake in skeletal myoblasts. We are the first to demonstrate that the exocyst is necessary for insulin- and contraction-induced CD36 membrane trafficking and FFA metabolism in muscle cells. Further studies will continue to investigate the mechanism of exocyst-regulated FFA uptake and lipid metabolism in skeletal muscle to help our understanding of ectopic lipid accumulation leading to IR. Disclosure N. K. Nakamura: None. D. S. Tokunaga: None. H. Y. J. Ha: None. N. Polgar: None.

IRW (Isoleucine-Arginine-Tryptophan) Improves Glucose Tolerance in High Fat Diet Fed C57BL/6 Mice via Activation of Insulin Signaling and AMPK Pathways in Skeletal Muscle.

IRW (Isoleucine–Arginine–Tryptophan), has antihypertensive and anti-inflammatory properties in cells and animal models and prevents angiotensin-II- and tumor necrosis factor (TNF)-α-induced insulin resistance (IR) in vitro. We investigated the effects of IRW on body composition, glucose homeostasis and insulin sensitivity in a high-fat diet (HFD) induced insulin resistant (IR) model. C57BL/6 mice were fed HFD for 6 weeks, after which IRW was incorporated into the diet (45 or 15 mg/kg body weight (BW)) until week 14. IRW45 (at a dose of 45 mg/kg BW) reduced BW (p = 0.0327), fat mass gain (p = 0.0085), and preserved lean mass of HFD mice (p = 0.0065), concomitant with enhanced glucose tolerance and reduced fasting glucose (p < 0.001). In skeletal muscle, IRW45 increased insulin-stimulated protein kinase B (AKT) phosphorylation (p = 0.0132) and glucose transporter 4 (GLUT4) translocation (p < 0.001). Angiotensin 2 receptor (AT2R) (p = 0.0024), phosphorylated 5′-AMP-activated protein kinase (AMPKα) (p < 0.0124) and peroxisome proliferator-activated receptor gamma (PPARγ) (p < 0.001) were enhanced in skeletal muscle of IRW45-treated mice, as was the expression of genes involved in myogenesis. Plasma angiotensin converting enzyme-2 (ACE2) activity was increased (p = 0.0016). Uncoupling protein-1 in white adipose tissue (WAT) was partially restored after IRW supplementation. IRW improves glucose tolerance and body composition in HFD-fed mice and promotes glucose uptake in skeletal muscle via multiple signaling pathways, independent of angiotensin converting enzyme (ACE) inhibition.

Review of the Pathophysiologic and Clinical Aspects of Hypokalemia in Children and Young Adults: an Update.

This article examines the regulatory function of the skeletal muscle, renal, and adrenergic systems in potassium homeostasis. The pathophysiologic bases of hypokalemia, systematic approach for an early diagnosis, and therapeutic strategy to avert life-threatening complications are highlighted. By promoting skeletal muscle uptake, intense physical exercise (post), severe trauma, and several toxins produce profound hypokalemia. Hypovolemia due to renal and extra-renal fluid losses and ineffective circulation activate secondary aldosteronism causing urinary potassium wasting. In addition to hypokalemic alkalosis, primary aldosteronism causes low-renin hypertension. Non-aldosterone mineralocorticoid activation leading to low-renin and low-aldosterone hypertension occurs in Liddle’s syndrome and apparent mineralocorticoid excess. Although there is enzymatic inhibition of cortisol synthesis in congenital adrenal hyperplasia, precursors of aldosterone produce low-renin hypokalemic hypertension. In addition to the glucocorticoid effect, hypercortisolism activates mineralocorticoid receptors in Cushing’s syndrome. Genetic mutations involving furosemide-sensitive Na+-K+-2Cl− co-transporters and thiazide-sensitive Na+-Cl− transporters result in (non-hypertensive) salt-wasting nephropathy. Proximal and distal renal tubular acidosis is associated with hypokalemia. Eating disorders causing hypokalemia include bulimia, laxative abuse, and diuretic misuse. Low urinary potassium (<15 mmol/day) and/or low urinary chloride (<20 mol/L) suggest a gastrointestinal pathology. Co-morbidity of hypokalemia with chronic pulmonary and cardiovascular diseases may increase the fatality rate.

Atlas of non-pathological solitary or asymmetrical skeletal muscle uptake in [18F]FDG-PET

Positron emission tomography (PET) using 2-deoxy-2-[18F]fluoro-d-glucose ([18F]FDG) is widely used in oncology and other fields. In [18F]FDG PET images, increased muscle uptake is observed owing exercise load or muscle tension, in addition to malignant tumors and inflammation. Moreover, we occasionally observe non-pathological solitary or unilateral skeletal muscle uptake, which is difficult to explain the strict reason. In most cases, we can interpret them as not having pathological significance. However, it is important to recognize such muscle uptake patterns to avoid misdiagnoses with pathological ones. Therefore, the teaching point of this pictorial essay is to comprehend the patterns of solitary or asymmetrical skeletal muscle uptake seen in routine [18F]FDG-PET scans. As an educational goal, you will be able to mention muscles where intense physiological [18F]FDG uptake can be observed, differentiate between physiological muscle uptake and lesion, and discuss with any physicians or specialists about uncertain muscle uptake.

Cis-regulation of antisense non-coding RNA at the JAZF1 locus in type 2 diabetes.

Several genomic loci of type 2 diabetes (T2D) nominated in genome-wide association studies (GWAS) have been suggested to regulate metabolism in muscle. However, a large portion of the genetic risk and the underlying regulation remain unexplained. The present study aimed to localize the potentially functional regions or genes at juxtaposed with another zinc finger protein 1 (JAZF1) locus and interpret their possible biological mechanisms in the muscle of T2D. Seven GWAS datasets including 21,897 T2D patients and 32,710 healthy controls of 772 SNPs within JAZF1 locus were meta-analysed using unconditional logistic regression. The Sherlock and GTEx protal online algorithms were implemented to show the significant colocalizations. Multiple omics data were integrated to predict the potential biological functions of JAZF1-AS1 in muscle. The cis regulation of JAZF1-AS1 was analysed using in vitro cloning in Human skeletal muscle cells (HSkMC). With a cross-population meta-analysis of seven GWAS, we identified a linkage disequilibrium (LD) block within intron 1 of JAZF1 that was significantly associated with T2D (false discovery rate < 0.05). The colocalization analysis showed a significant association between genetically determined expression of JAZF1 in skeletal muscle and T2D with a strong probability of colocalization (PP4 = 75.09%). This region also encodes the upstream regulatory region of the antisense non-coding RNA JAZF1-AS1. Expression-quantitative trait loci analysis detected a regulatory SNP within this LD block, rs864745, which is associated with the expression of JAZF1-AS1 and JAZF1. With in vitro cloning, we further reported the role of JAZF1-AS1 in cis-regulating JAZF1 by directly forming RNA double strands. Downregulation of JAZF1, caused by JAZF1-AS1 depletion, inhibited the glucose uptake and lipid oxidation in skeletal muscle. The present study proposes a strategy for identifying a novel T2D gene at the reported locus and generating a model in which polymorphisms at JAZF1 influence T2D risk through antisense-mediated gene regulation.

Illumination of the Endogenous Insulin-Regulated TBC1D4 Interactome in Human Skeletal Muscle.

Insulin-stimulated muscle glucose uptake is a key process in glycemic control. This process depends on the redistribution of glucose transporters to the surface membrane, a process that involves regulatory proteins such as TBC1D1 and TBC1D4. Accordingly, a TBC1D4 loss-of-function mutation in human skeletal muscle is associated with an increased risk of type 2 diabetes, and observations from carriers of a TBC1D1 variant associate this protein to a severe obesity phenotype. Here, we identified interactors of the endogenous TBC1D4 protein in human skeletal muscle by an unbiased proteomics approach. We detected 76 proteins as candidate TBC1D4 interactors. The binding of 12 of these interactors was regulated by insulin, including proteins known to be involved in glucose metabolism (e.g., 14-3-3 proteins and α-actinin-4 [ACTN4]). TBC1D1 also coprecipitated with TBC1D4 and vice versa in both human and mouse skeletal muscle. This interaction was not regulated by insulin or exercise in young, healthy, lean individuals. Similarly, the exercise- and insulin-regulated phosphorylation of the TBC1D1-TBC1D4 complex was intact. In contrast, we observed an altered interaction as well as compromised insulin-stimulated phosphoregulation of the TBC1D1-TBC1D4 complex in muscle of obese individuals with type 2 diabetes. Altogether, we provide a repository of TBC1D4 interactors in human and mouse skeletal muscle that serve as potential regulators of TBC1D4 function and, thus, insulin-stimulated glucose uptake in human skeletal muscle.

Measurements of basal d-glucose transport through GLUT1 across the intact plasma membrane of isolated segments from single fast- and slow-twitch skeletal muscle fibres ofrat.

AimTo develop a method for direct measurement of the fluorescent d‐glucose analogue 2‐NBDG transport across the plasma membrane of single skeletal muscle fibres and derive the theoretical framework for determining the kinetic parameters for d‐glucose transport under basal conditions.MethodsA novel method is described for measuring free 2‐NBDG transport across plasma membrane of single rat muscle fibres at rest. The 2‐NBDG uptake was >90% suppressed by 100 µM cytochalasin B in both fast‐twitch and slow‐twitch fibres, indicating that the 2‐NBDG transport is GLUT‐mediated. Fibres were identified as fast‐twitch or slow‐twitch based on the differential sensitivity of their contractile apparatus to Sr2+.ResultsThe time course of 2‐NBDG uptake in the presence of 50 µM 2‐NBDG follows a one‐phase exponential plateau curve and is faster in fast‐twitch (rate constant 0.053 ± 0.0024 s‐1) than in slow‐twitch fibres (rate constant 0.031 ± 0.0021 s‐1). The rate constants were markedly reduced in the presence of 20 mM d‐glucose to 0.0082 ± 0.0004 s‐1 and 0.0056 ± 0.0002 s‐1 in fast‐twitch and slow‐twitch fibres respectively. 2‐NBDG transport was asymmetric, consistent with GLUT1 being the major functional GLUT isoform transporting 2‐NBDG in muscle fibres at rest. The parameters describing the transport kinetics for both 2‐NBDG and d‐glucose (dissociation constants, Michaelis–Menten constants, maximal rates of uptake and outflow) were calculated from the measurements made with 2‐NBDG.ConclusionFree 2‐NBDG and d‐glucose transport across the plasma membrane of single rat muscle fibres at rest is fast, conclusively showing that the rate‐limiting step in d‐glucose uptake in skeletal muscle is not necessarily the GLUT‐mediated transport of d‐glucose.

Gene deletion of γ-actin impairs insulin-stimulated skeletal muscle glucose uptake in growing mice but not in mature adult mice.

The cortical cytoskeleton, consisting of the cytoplasmic actin isoforms β and/or γ‐actin, has been implicated in insulin‐stimulated GLUT4 translocation and glucose uptake in muscle and adipose cell culture. Furthermore, transgenic inhibition of multiple actin‐regulating proteins in muscle inhibits insulin‐stimulated muscle glucose uptake. The current study tested if γ‐actin was required for insulin‐stimulated glucose uptake in mouse skeletal muscle. Based on our previously reported age‐dependent phenotype in muscle‐specific β‐actin gene deletion (−/−) mice, we included cohorts of growing 8–14 weeks old and mature 18–32 weeks old muscle‐specific γ‐actin−/− mice or wild‐type littermates. In growing mice, insulin significantly increased the glucose uptake in slow‐twitch oxidative soleus and fast‐twitch glycolytic EDL muscles from wild‐type mice, but not γ‐actin−/−. In relative values, the maximal insulin‐stimulated glucose uptake was reduced by ~50% in soleus and by ~70% in EDL muscles from growing γ‐actin−/− mice compared to growing wild‐type mice. In contrast, the insulin‐stimulated glucose uptake responses in mature adult γ‐actin−/− soleus and EDL muscles were indistinguishable from the responses in wild‐type muscles. Mature adult insulin‐stimulated phosphorylations on Akt, p70S6K, and ULK1 were not significantly affected by genotype. Hence, insulin‐stimulated muscle glucose uptake shows an age‐dependent impairment in young growing but not in fully grown γ‐actin−/− mice, bearing phenotypic resemblance to β‐actin−/− mice. Overall, γ‐actin does not appear required for insulin‐stimulated muscle glucose uptake in adulthood. Furthermore, our data emphasize the need to consider the rapid growth of young mice as a potential confounder in transgenic mouse phenotyping studies.

Gut microbiota-derived metabolites are novel targets for improving insulin resistance

The gut microbiota plays a key role in metabolic diseases. Gut-microbiota-derived metabolites are found in different dietary sources, including: Carbohydrate (acetate, propionate, butyrate, also known as short-chain fatty acids, as well as succinate); protein (hydrogen sulfide, indole, and phenylacetic acid); and lipids (resveratrol-, ferulic acid-, linoleic acid-, catechin- and berry-derived metabolites). Insulin resistance, which is a global pandemic metabolic disease that progresses to type 2 diabetes mellitus, can be directly targeted by these metabolites. Gut-microbiota-derived metabolites have broad effects locally and in distinct organs, in particular skeletal muscle, adipose tissue, and liver. These metabolites can modulate glucose metabolism, including the increase in glucose uptake and lipid oxidation in skeletal muscle, and decrease in lipogenesis and gluconeogenesis associated with lipid oxidation in the liver through activation of phosphatidylinositol 3-kinase - serine/threonine-protein kinase B and AMP-activated protein kinase. In adipose tissue, gut-microbiota-derived metabolites stimulate adipogenesis and thermogenesis, inhibit lipolysis, and attenuate inflammation. Importantly, an increase in energy expenditure and fat oxidation occurs in the whole body. Therefore, the therapeutic potential of current pharmacological and non-pharmacological approaches used to treat diabetes mellitus can be tested to target specific metabolites derived from intestinal bacteria, which may ultimately ameliorate the hyperglycemic burden.

P300 or CBP is required for insulin-stimulated glucose uptake in skeletal muscle and adipocytes.

While current thinking posits that insulin signaling to glucose transporter 4 (GLUT4) exocytic translocation and glucose uptake in skeletal muscle and adipocytes is controlled by phosphorylation-based signaling, many proteins in this pathway are acetylated on lysine residues. However, the importance of acetylation and lysine acetyltransferases to insulin-stimulated glucose uptake is incompletely defined. Here, we demonstrate that combined loss of the acetyltransferases E1A binding protein p300 (p300) and cAMP response element binding protein binding protein (CBP) in mouse skeletal muscle caused a complete loss of insulin-stimulated glucose uptake. Similarly, brief (i.e., 1 hour) pharmacological inhibition of p300/CBP acetyltransferase activity recapitulated this phenotype in human and rodent myotubes, 3T3-L1 adipocytes, and mouse muscle. Mechanistically, these effects were due to p300/CBP-mediated regulation of GLUT4 exocytic translocation and occurred downstream of Akt signaling. Taken together, we highlight a fundamental role for acetylation and p300/CBP in the direct regulation of insulin-stimulated glucose transport in skeletal muscle and adipocytes.

Effect of Taurine on the Regulation of Glucose Uptake in the Skeletal Muscle.

Diabetes is so common in Japan as to be called a national disease. Taurine, a free amino acid found abundantly in mammalian tissues that is also a key ingredient of many "energy drinks," has been shown to be effective in improving the hyperglycemic state caused by diabetes. Taurine administration is associated with increased insulin secretion from the pancreas, higher levels of insulin signaling-related factors, and higher expression of the glucose transporter, GLUT4. Skeletal muscle is the main target organ of insulin: Via cell surface GLUT4 molecules, myocytes take up blood glucose, enabling skeletal muscle contraction. The enhancing effect of taurine on blood glucose uptake in skeletal muscle has not been fully studied, and little is known about its mechanism. This review article summarizes what is known about the effects of taurine on insulin secretion from the pancreas and especially blood glucose uptake in skeletal muscle.

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.

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

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

A novel regulatory mechanism of geniposide for improving glucose homeostasis mediated by circulating RBP4

BackgroundSystemic insulin signal transduction is influenced by the inter-tissue crosstalk, which might be the potential therapeutic strategy for T2DM. Although anti-diabetic function of geniposide has been previously reported, the underlying mechanism was not completely clear in light of the complex pathogenesis of T2DM. PurposeThe present experiment is devoted to investigate the potential effects of geniposide on systemic insulin sensitivity mediated by hepatokine-RBP4 in high fat diet (HFD)-fed mice. MethodsThe HFD-fed wild type mice were administered with geniposide (25 or 50 mg/kg/d) by intraperitoneal injection, and the normal saline and Metformin were used as negative control group and positive control group, respectively. After administration for 4 weeks, the food intake, body weight, glucose tolerance tests, insulin tolerance tests and serum biochemical indices were examined, along with insulin signaling pathway-associated proteins and hepatic histomorphological analysis. The liver, gastrocnemius and mouse primary hepatocytes were also harvested for molecular mechanism study. ResultsAfter geniposide treatment for 4 weeks, the blood glucose level was reduced in HFD-fed mice. Furthermore, geniposide treatment improved insulin sensitivity both in the liver and gastrocnemius (GAS). In terms of mechanism, geniposide disturbed circulating RBP4 level including its synthesis, secretion and homeostasis. Moreover, geniposide modified fuel selection and promoted glucose uptake in skeletal muscle and reduced glycogen storage, which were closely related to impaired circulating RBP4 homeostasis, leading to ameliorative systemic insulin sensitivity. ConclusionOur current study proposes a novel regulatory mechanism of geniposide for improving glucose homeostasis through regulating circulating RBP4 level, which also provides new strategies for the prevention and treatment of T2DM.

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

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

Transforming Growth Factor Beta 1 Alters Glucose Uptake but Not Insulin Signalling in Human Primary Myotubes From Women With and Without Polycystic Ovary Syndrome.

Women with polycystic ovary syndrome (PCOS), commonly have profound skeletal muscle insulin resistance which can worsen other clinical features. The heterogeneity of the condition has made it challenging to identify the precise mechanisms that cause this insulin resistance. A possible explanation for the underlying insulin resistance may be the dysregulation of Transforming Growth Factor-beta (TGFβ) signalling. TGFβ signalling contributes to the remodelling of reproductive and hepatic tissues in women with PCOS. Given the systemic nature of TGFβ signalling and its role in skeletal muscle homeostasis, it may be possible that these adverse effects extend to other peripheral tissues. We aimed to determine if TGFβ1 could negatively regulate glucose uptake and insulin signalling in skeletal muscle of women with PCOS. We show that both myotubes from women with PCOS and healthy women displayed an increase in glucose uptake, independent of changes in insulin signalling, following short term (16 hr) TGFβ1 treatment. This increase occurred despite pro-fibrotic signalling increasing via SMAD3 and connective tissue growth factor in both groups following treatment with TGFβ1. Collectively, our findings show that short-term treatment with TGFβ1 does not appear to influence insulin signalling or promote insulin resistance in myotubes. These findings suggest that aberrant TGFβ signalling is unlikely to directly contribute to skeletal muscle insulin resistance in women with PCOS in the short term but does not rule out indirect or longer-term effects.

Atrophy of White Adipose Tissue Accompanied with Decreased Insulin-Stimulated Glucose Uptake in Mice Lacking the Small GTPase Rac1 Specifically in Adipocytes.

Insulin stimulates glucose uptake in adipose tissue and skeletal muscle by inducing plasma membrane translocation of the glucose transporter GLUT4. Although the small GTPase Rac1 is a key regulator downstream of phosphoinositide 3-kinase (PI3K) and the protein kinase Akt2 in skeletal muscle, it remains unclear whether Rac1 also regulates glucose uptake in white adipocytes. Herein, we investigated the physiological role of Rac1 in white adipocytes by employing adipocyte-specific rac1 knockout (adipo-rac1-KO) mice. Subcutaneous and epididymal white adipose tissues (WATs) in adipo-rac1-KO mice showed significant reductions in size and weight. Actually, white adipocytes lacking Rac1 were smaller than controls. Insulin-stimulated glucose uptake and GLUT4 translocation were abrogated in rac1-KO white adipocytes. On the other hand, GLUT4 translocation was augmented by constitutively activated PI3K or Akt2 in control, but not in rac1-KO, white adipocytes. Similarly, to skeletal muscle, the involvement of another small GTPase RalA downstream of Rac1 was demonstrated. In addition, mRNA levels of various lipogenic enzymes were down-regulated in rac1-KO white adipocytes. Collectively, these results suggest that Rac1 is implicated in insulin-dependent glucose uptake and lipogenesis in white adipocytes, and reduced insulin responsiveness due to the deficiency of Rac1 may be a likely explanation for atrophy of WATs.

The missing 'link'? β-Catenin's role in skeletal muscle glucose uptake during exercise and contraction.

The missing 'link'? β-Catenin's role in skeletal muscle glucose uptake during exercise and contraction.

Skeletal muscle energy metabolism in obesity.

Comparing energy metabolism in human skeletal muscle and primary skeletal muscle cells in obesity, while focusing on glucose and fatty acid metabolism, shows many common changes. Insulin-mediated glucose uptake in skeletal muscle and primary myotubes is decreased by obesity, whereas differences in basal glucose metabolism are inconsistent among studies. With respect to fatty acid metabolism, there is an increased uptake and storage of fatty acids and a reduced complete lipolysis, suggesting alterations in lipid turnover. In addition, fatty acid oxidation is decreased, probably at the level of complete oxidation, as -oxidation may be enhanced in obesity, which indicates mitochondrial dysfunction. Metabolic changes in skeletal muscle with obesity promote metabolic inflexibility, ectopic lipid accumulation, and formation of toxic lipid intermediates. Skeletal muscle also acts as an endocrine organ, secreting myokines that participate in interorgan cross talk. This review highlights interventions and some possible targets for treatment through action on skeletal muscle energy metabolism. Effects of exercisein vivoon obesity have been compared with simulation of endurance exercisein vitroon myotubes (electrical pulse stimulation). Possible pharmaceutical targets, including signaling pathways and drug candidates that could modify lipid storage and turnover or increase mitochondrial function or cellular energy expenditure through adaptive thermogenic mechanisms, are discussed.