Palmitate oxidation rate and action on glycogen synthase in myoblasts from insulin-resistant subjects

Endothelial autophagy-related gene 7 contributes to high fat diet-induced obesity

OBJECTIVE

Obesity-associated metabolic dysfunction is a major public health concern worldwide. Endothelial dysfunction is a hallmark of metabolic dysfunction, and endothelial cells affect metabolic functions. Because autophagy-related gene 7 (ATG7) is involved in various cellular physiology, we investigated the roles of endothelial cell-ATG7 (EC-ATG7) on high-fat diet-induced obesity and its related metabolic dysfunction.

METHODS

We generated an endothelial-specific Atg7 knock-out mouse by breeding Atg7flox/flox mouse with the Chd5-Cre mouse, and investigated the metabolic phenotypes associated with high-fat diet (HFD)-induced obesity. Body weight, food intake, glucose tolerance, insulin sensitivity, and liver fat accumulation were measured in endothelial Atg7 deficient (Atg7ΔEnd) and control mice (Atg7f/f). Adipose tissue inflammation was assessed by measuring the expression of pro-inflammatory genes. Furthermore, we performed indirect calorimetry and examined the insulin signaling pathway molecules.

RESULTS

We found that deletion of EC-Atg7 ameliorated HFD-induced weight gain, fatty liver, and adipocyte hypertrophy and inflammatory response in adipose tissue, and improved insulin sensitivity without changing glucose tolerance. These metabolic effects seem to be due to the reduced food intake because there were no differences in energy expenditure, energy excretion to feces, and physical activity. Interestingly, the deletion of EC-Atg7 protected from HFD-induced vascular rarefaction, and the knock-down of Atg7 in endothelial cells protected from fatty acid-induced cell death.

CONCLUSIONS

Our results suggest that EC-Atg7 deletion ameliorates HFD-induced obesity and its related metabolic dysfunction, such as insulin resistance and fatty liver by attenuating appetite and vascular rarefaction. The EC-Atg7 deletion may protect the endothelial cells from lipotoxicity and impaired angiogenesis, which preserves the endothelial function in metabolic tissues. These findings may have implications for developing new therapeutic strategies for preventing and treating obesity and its associated health risks.

MyoCount: a software tool for the automated quantification of myotube surface area and nuclear fusion index

It is often desirable to characterise the morphology of myogenic cultures. To achieve this, the surface area of myotubes is often quantified, along with the nuclear fusion index (NFI). Existing methods of such quantification are time-consuming and subject to error-prone human input. We have developed MyoCount, an open-source program that runs via the freely available MATLAB Runtime and quantifies myotube surface area and NFI. MyoCount allows the user to adjust its parameters to account for differences in image quality, magnification and the colour channels used in generating the image. MyoCount measures of myotube surface area and NFI were compared to the mean of measures performed by two blinded investigators using ImageJ software (surface area R ² = 0.89, NFI R ² =0.87). For NFI, the mean coefficient of variation (CV) between two investigators (17.6 ± 2.3%) was significantly higher than that between the investigator mean and MyoCount (13.5 ± 1.4%). For measurements of myotube area, the CV did not differ between both analysis methods. Given these results and the advantages of applying the same image analysis method uniformly across all images in an experiment, we suggest that MyoCount will be a useful research tool and we publish its source code and instructions for its use alongside this article.

Obese subcutaneous adipose tissue impairs human myogenesis, particularly in old skeletal muscle, via resistin-mediated activation of NFκB

Adiposity and adipokines are implicated in the loss of skeletal muscle mass with age and in several chronic disease states. The aim of this study was to determine the effects of human obese and lean subcutaneous adipose tissue secretome on myogenesis and metabolism in skeletal muscle cells derived from both young (18-30 yr) and elderly (>65 yr) individuals. Obese subcutaneous adipose tissue secretome impaired the myogenesis of old myoblasts but not young myoblasts. Resistin was prolifically secreted by obese subcutaneous adipose tissue and impaired myotube thickness and nuclear fusion by activation of the classical NFκB pathway. Depletion of resistin from obese adipose tissue secretome restored myogenesis. Inhibition of the classical NFκB pathway protected myoblasts from the detrimental effect of resistin on myogenesis. Resistin also promoted intramyocellular lipid accumulation in myotubes and altered myotube metabolism by enhancing fatty acid oxidation and increasing myotube respiration and ATP production. In conclusion, resistin derived from human obese subcutaneous adipose tissue impairs myogenesis of human skeletal muscle, particularly older muscle, and alters muscle metabolism in developing myotubes. These findings may have important implications for the maintenance of muscle mass in older people with chronic inflammatory conditions, or older people who are obese or overweight.

Effects of high-fat diet and AMP-activated protein kinase modulation on the regulation of whole-body lipid metabolism

Metabolic flexibility, the capacity to adapt to fuel availability for energy production, is crucial for maintaining whole-body energy homeostasis. An inability to adequately promote FA utilization is associated with lipid accumulation in peripheral tissues and contributes to the development of insulin resistance. In vivo assays to quantify whole-body lipid oxidation in mouse models of insulin resistance are lacking. We describe a method for assessing whole-body FA oxidation in vivo, as well as tissue-specific lipid uptake in conscious mice. The method relies on intravenous administration of [9,10-³H(N)]palmitic acid combined with a non-β-oxidizable palmitate analog, [1-¹⁴C]2-bromopalmitic acid. Pretreatment with etomoxir, a CPT1 inhibitor that prevents the shuttling of FAs into mitochondria, markedly reduced the appearance of the β-oxidation product ³H₂O in circulation and reduced lipid uptake by oxidative tissues including heart and soleus muscle. Whole-body fatty oxidation was unaltered between chow- or high-fat-fed WT and transgenic mice expressing a mutant form of the AMPK γ3 subunit (AMPKγ3^R225Q) in skeletal muscle. High-fat feeding increased lipid oxidation in WT and AMPKγ3^R225Q transgenic mice. In conclusion, this technique allows for the assessment of the effect of pharmaceutical agents, as well as gene mutations, on whole-body FA oxidation in mice.

Mitochondrial function in metabolic health: a genetic and environmental tug of war

BACKGROUND

The increased prevalence of obesity and its co-morbidities and their strong association with inactivity have produced an 'exercise-deficient phenotype' in which individuals with a particular combination of disease-susceptible genes collide with environmental influences to cross a biological 'threshold' that ultimately manifests as overt clinical conditions (i.e., risk-factors for disease states). These risk-factors have been linked to impairments in skeletal muscle mitochondrial function.

SCOPE OF REVIEW

The question of whether 'inborn' mitochondrial deficiencies and/or defective mitochondrial metabolism contribute to metabolic disease, or if environmental factors are the major determinant, will be examined.

MAJOR CONCLUSIONS

We contend that impaired whole-body insulin resistance along with impaired skeletal muscle handling of carbohydrate and lipid fuels (i.e., metabolic inflexibility) is associated with a reduced skeletal muscle mitochondrial content which, in large part, is a maladaptive response to an 'inactivity cycle' which predisposes to a reduced level of habitual physical activity. While genetic components play a role in the pathogenesis of metabolic disease, exercise is a powerful environmental stimulus capable of restoring the metabolic flexibility of fuel selection and reduces risk-factors for metabolic disease in genetically-susceptible individuals.

GENERAL SIGNIFICANCE

Given the apathy towards voluntary physical activity in most Western societies, it is clear that there is an urgent need for innovative, clinically-effective exercise strategies, coupled with changes in current attitudes and methods of delivering exercise prescription and dietary advice, in order to improve metabolic health and reduce metabolic disease risk at the population level. This article is part of a Special Issue entitled Frontiers of Mitochondrial Research.

Toll-like receptor 2 mediates high-fat diet-induced impairment of vasodilator actions of insulin

Obesity is characterized by a chronic proinflammatory state that leads to endothelial dysfunction. Saturated fatty acids (SFA) stimulate Toll-like receptors (TLR) that promote metabolic insulin resistance. However, it is not known whether TLR2 mediates impairment of vascular actions of insulin in response to high-fat diet (HFD) to cause endothelial dysfunction. siRNA knockdown of TLR2 in primary endothelial cells opposed palmitate-stimulated expression of proinflammatory cytokines and splicing of X box protein 1 (XBP-1). Inhibition of unfolding protein response (UPR) reduced SFA-stimulated expression of TNFα. Thus, SFA stimulates UPR and proinflammatory response through activation of TLR2 in endothelial cells. Knockdown of TLR2 also opposed impairment of insulin-stimulated phosphorylation of eNOS and subsequent production of NO. Importantly, insulin-stimulated vasorelaxation of mesenteric arteries from TLR2 knockout mice was preserved even on HFD (in contrast with results from arteries examined in wild-type mice on HFD). We conclude that TLR2 in vascular endothelium mediates HFD-stimulated proinflammatory responses and UPR that accompany impairment of vasodilator actions of insulin, leading to endothelial dysfunction. These results are relevant to understanding the pathophysiology of the cardiovascular complications of diabetes and obesity.

Palmitate action to inhibit glycogen synthase and stimulate protein phosphatase 2A increases with risk factors for type 2 diabetes

Recent studies have suggested that abnormal regulation of protein phosphatase 2A (PP2A) is associated with Type 2 diabetes in rodent and human tissues. Results with cultured mouse myotubes support a mechanism for palmitate activation of PP2A, leading to activation of glycogen synthase kinase 3. Phosphorylation and inactivation of glycogen synthase by glycogen synthase kinase 3 could be the mechanism for long-chain fatty acid inhibition of insulin-mediated carbohydrate storage in insulin-resistant subjects. Here, we test the effects of palmitic acid on cultured muscle glycogen synthase and PP2A activities. Palmitate inhibition of glycogen synthase fractional activity is increased in subjects with high body mass index compared with subjects with lower body mass index (r = -0.43, P = 0.03). Palmitate action on PP2A varies from inhibition in subjects with decreased 2-h plasma glucose concentration to activation in subjects with increased 2-h plasma glucose concentration (r = 0.45, P < 0.03) during oral glucose tolerance tests. The results do not show an association between palmitate effects on PP2A and glycogen synthase fractional activity. We conclude that subjects at risk for Type 2 diabetes have intrinsic differences in palmitate regulation of at least two enzymes (PP2A and glycogen synthase), contributing to abnormal insulin regulation of glucose metabolism.

Fuel use during glycogenesis in rainbow trout (Oncorhynchus mykissWalbaum) white muscle studiedin vitro

The purpose of this study was to examine fuel used during muscle glycogenesis in rainbow trout Oncorhynchus mykiss using an in vitro muscle slice preparation to test the hypothesis that intracellular lactate is the major glycogenic substrate and the muscle relies upon extracellular substrates for oxidation. Fish were exhaustively exercised to reduce muscle glycogen content, muscle slices were taken from exhausted fish and incubated for 1 h in medium containing various substrates at physiological concentrations. 14C-labeled lactate, glycerol or palmitate was added and 14C incorporation into muscle glycogen and/or CO2 was measured. Lactate clearance in the absence of net glycogenesis suggests that when suitable oxidizable extracellular substrates were lacking, intracellular lactate was oxidized. Only muscle incubated in lactate, glycerol or palmitate synthesized glycogen, with the greatest synthesis in muscle incubated in lactate plus glycerol. The major fate of these extracellular substrates was oxidative, with lactate oxidized at rates 10 times that of palmitate and 100 times that of glycerol. Neither extracellular lactate nor glycerol contributed significantly to glycogenesis,with lactate carbon contributing less than 0.1% of the total glycogen synthesized, and glycerol less than 0.01%. There was 100 times more extracellular lactate-carbon incorporated into CO2 than into glycogen. In the presence of extracellular lactate, palmitate or glycerol,intracellular lactate was spared an oxidative fate, allowing it to serve as the primary substrate for in situ glycogenesis, with oxidation of extracellular substrates driving ATP synthesis. The primary fate of extracellular lactate is clearly oxidative, while that of intracellular,glycolytically derived lactate is glycogenic, which suggests intracellular compartmentation of lactate metabolism.

Dynamic changes in fat oxidation in human primary myocytes mirror metabolic characteristics of the donor

Metabolic flexibility of skeletal muscle, that is, the preference for fat oxidation (FOx) during fasting and for carbohydrate oxidation in response to insulin, is decreased during insulin resistance. The aim of this study was to test the hypothesis that the capacity of myotubes to oxidize fat in vitro reflects the donor's metabolic characteristics. Insulin sensitivity (IS) and metabolic flexibility of 16 healthy, young male subjects was determined by euglycemic hyperinsulinemic clamp. Muscle samples were obtained from vastus lateralis, cultured, and differentiated into myotubes. In human myotubes in vitro, we measured suppressibility (glucose suppression of FOx) and adaptability (an increase in FOx in the presence of high palmitate concentration). We termed these dynamic changes in FOx metabolic switching. In vivo, metabolic flexibility was positively correlated with IS and maximal oxygen uptake and inversely correlated with percent body fat. In vitro suppressibility was inversely correlated with IS and metabolic flexibility and positively correlated with body fat and fasting FFA levels. Adaptability was negatively associated with percent body fat and fasting insulin and positively correlated with IS and metabolic flexibility. The interindividual variability in metabolic phenotypes was preserved in human myotubes separated from their neuroendocrine environment, which supports the hypothesis that metabolic switching is an intrinsic property of skeletal muscle.

Analysis of insulin-stimulated insulin receptor activation and glucose transport in cultured skeletal muscle cells from obese subjects

Obesity is associated with impaired insulin-stimulated glucose disposal in the skeletal muscle, but whether this is an intrinsic or acquired factor is unknown. In many patients with type 2 diabetes mellitus (T2D) and their nondiabetic relatives, who have a genetic predisposition for diabetes, insulin resistance is maintained in cultured muscle cells. To study the association of obesity with defects in insulin action, we investigated insulin stimulation of both insulin receptor (IR) autophosphorylation and subsequent glucose transport in primary skeletal muscle cell cultures obtained from both nonobese and obese nondiabetic subjects. In these 2 groups, there was no difference in the ability of insulin to induce autophosphorylation of the IR, phosphorylation of the downstream serine kinase Akt/PKB, or stimulation of glucose transport. Moreover, there were no major differences in cultured muscle cell content of either the IR, the IR antagonist PC-1, or GLUT 1 and GLUT 4. These data therefore indicate that the insulin resistance associated with obesity is not maintained in cultured muscle cells and suggest that this insulin resistance is an acquired feature of obesity.