Exercise training enhances muscle mitochondrial metabolism in diet-resistant obesity.

BACKGROUND: Current paradigms for predicting weight loss in response to energy restriction have general validity but a subset of individuals fail to respond adequately despite documented diet adherence. Patients in the bottom 20% for rate of weight loss following a hypocaloric diet (diet-resistant) have been found to have less type I muscle fibres and lower skeletal muscle mitochondrial function, leading to the hypothesis that physical exercise may be an effective treatment when diet alone is inadequate. In this study, we aimed to assess the efficacy of exercise training on mitochondrial function in women with obesity with a documented history of minimal diet-induced weight loss. METHODS: From over 5000 patient records, 228 files were reviewed to identify baseline characteristics of weight loss response from women with obesity who were previously classified in the top or bottom 20% quintiles based on rate of weight loss in the first 6 weeks during which a 900 kcal/day meal replacement was consumed. A subset of 20 women with obesity were identified based on diet-resistance (n=10) and diet sensitivity (n=10) to undergo a 6-week supervised, progressive, combined aerobic and resistance exercise intervention. FINDINGS: Diet-sensitive women had lower baseline adiposity, higher fasting insulin and triglycerides, and a greater number of ATP-III criteria for metabolic syndrome. Conversely in diet-resistant women, the exercise intervention improved body composition, skeletal muscle mitochondrial content and metabolism, with minimal effects in diet-sensitive women. In-depth analyses of muscle metabolomes revealed distinct group- and intervention- differences, including lower serine-associated sphingolipid synthesis in diet-resistant women following exercise training. INTERPRETATION: Exercise preferentially enhances skeletal muscle metabolism and improves body composition in women with a history of minimal diet-induced weight loss. These clinical and metabolic mechanism insights move the field towards better personalised approaches for the treatment of distinct obesity phenotypes. FUNDING: Canadian Institutes of Health Research (CIHR-INMD and FDN-143278; CAN-163902; CIHR PJT-148634).

Impaired mitochondrial oxidative phosphorylation and supercomplex assembly in rectus abdominis muscle of diabetic obese individuals.

AIMS/HYPOTHESIS: Skeletal muscle mitochondrial dysfunction has been documented in patients with type 2 diabetes mellitus; however, specific respiratory defects and their mechanisms are poorly understood. The aim of the current study was to examine oxidative phosphorylation and electron transport chain (ETC) supercomplex assembly in rectus abdominis muscles of 10 obese diabetic and 10 obese non-diabetic individuals. METHODS: Twenty obese women undergoing Roux-en-Y gastric bypass surgery were recruited for this study. Muscle samples were obtained intraoperatively and subdivided for multiple analyses, including high-resolution respirometry and assessment of supercomplex assembly. Clinical data obtained from referring physicians were correlated with laboratory findings. RESULTS: Participants in both groups were of a similar age, weight and BMI. Mitochondrial respiration rates were markedly reduced in diabetic vs non-diabetic patients. This defect was observed during maximal ADP-stimulated respiration in the presence of complex I-linked substrates and complex I- and II-linked substrates, and during maximal uncoupled respiration. There were no differences in fatty acid (octanoyl carnitine) supported respiration, leak respiration or isolated activity of cytochrome c oxidase. Intriguingly, significant correlations were found between glycated haemoglobin (HbA1c) levels and maximal respiration or respiration supported by complex I, complex I and II or fatty acid. In the muscle of diabetic patients, blue native gel electrophoresis revealed a striking decrease in complex I, III and IV containing ETC supercomplexes. CONCLUSIONS/INTERPRETATION: These findings support the hypothesis that ETC supercomplex assembly may be an important underlying mechanism of muscle mitochondrial dysfunction in type 2 diabetes mellitus.

Undernutrition during pregnancy in mice leads to dysfunctional cardiac muscle respiration in adult offspring.

Intrauterine growth restriction (IUGR) is associated with an increased risk of developing obesity, insulin resistance and cardiovascular disease. However, its effect on energetics in heart remains unknown. In the present study, we examined respiration in cardiac muscle and liver from adult mice that were undernourished in utero. We report that in utero undernutrition is associated with impaired cardiac muscle energetics, including decreased fatty acid oxidative capacity, decreased maximum oxidative phosphorylation rate and decreased proton leak respiration. No differences in oxidative characteristics were detected in liver. We also measured plasma acylcarnitine levels and found that short-chain acylcarnitines are increased with in utero undernutrition. Results reveal the negative impact of suboptimal maternal nutrition on adult offspring cardiac energy metabolism, which may have life-long implications for cardiovascular function and disease risk.

Macrophage Mitochondrial Energy Status Regulates Cholesterol Efflux and Is Enhanced by Anti-miR33 in Atherosclerosis.

RATIONALE: Therapeutically targeting macrophage reverse cholesterol transport is a promising approach to treat atherosclerosis. Macrophage energy metabolism can significantly influence macrophage phenotype, but how this is controlled in foam cells is not known. Bioinformatic pathway analysis predicts that miR-33 represses a cluster of genes controlling cellular energy metabolism that may be important in macrophage cholesterol efflux. OBJECTIVE: We hypothesized that cellular energy status can influence cholesterol efflux from macrophages, and that miR-33 reduces cholesterol efflux via repression of mitochondrial energy metabolism pathways. METHODS AND RESULTS: In this study, we demonstrated that macrophage cholesterol efflux is regulated by mitochondrial ATP production, and that miR-33 controls a network of genes that synchronize mitochondrial function. Inhibition of mitochondrial ATP synthase markedly reduces macrophage cholesterol efflux capacity, and anti-miR33 required fully functional mitochondria to enhance ABCA1-mediated cholesterol efflux. Specifically, anti-miR33 derepressed the novel target genes PGC-1α, PDK4, and SLC25A25 and boosted mitochondrial respiration and production of ATP. Treatment of atherosclerotic Apoe(-/-) mice with anti-miR33 oligonucleotides reduced aortic sinus lesion area compared with controls, despite no changes in high-density lipoprotein cholesterol or other circulating lipids. Expression of miR-33a/b was markedly increased in human carotid atherosclerotic plaques compared with normal arteries, and there was a concomitant decrease in mitochondrial regulatory genes PGC-1α, SLC25A25, NRF1, and TFAM, suggesting these genes are associated with advanced atherosclerosis in humans. CONCLUSIONS: This study demonstrates that anti-miR33 therapy derepresses genes that enhance mitochondrial respiration and ATP production, which in conjunction with increased ABCA1 expression, works to promote macrophage cholesterol efflux and reduce atherosclerosis.

Acylcarnitines: potential implications for skeletal muscle insulin resistance.

Insulin resistance may be linked to incomplete fatty acid ß-oxidation and the subsequent increase in acylcarnitine species in different tissues including skeletal muscle. It is not known if acylcarnitines participate in muscle insulin resistance or simply reflect dysregulated metabolism. The aims of this study were to determine whether acylcarnitines can elicit muscle insulin resistance and to better understand the link between incomplete muscle fatty acid ß-oxidation, oxidative stress, inflammation, and insulin-resistance development. Differentiated C2C12, primary mouse, and human myotubes were treated with acylcarnitines (C4:0, C14:0, C16:0) or with palmitate with or without carnitine acyltransferase inhibition by mildronate. Treatment with C4:0, C14:0, and C16:0 acylcarnitines resulted in 20-30% decrease in insulin response at the level of Akt phosphorylation and/or glucose uptake. Mildronate reversed palmitate-induced insulin resistance concomitant with an ∼25% decrease in short-chain acylcarnitine and acetylcarnitine secretion. Although proinflammatory cytokines were not affected under these conditions, oxidative stress was increased by 2-3 times by short- or long-chain acylcarnitines. Acylcarnitine-induced oxidative stress and insulin resistance were reversed by treatment with antioxidants. Results are consistent with the conclusion that incomplete muscle fatty acid ß-oxidation causes acylcarnitine accumulation and associated oxidative stress, raising the possibility that these metabolites play a role in muscle insulin resistance.

Lower mitochondrial proton leak and decreased glutathione redox in primary muscle cells of obese diet-resistant versus diet-sensitive humans.

CONTEXT: Weight loss success in response to energy restriction is highly variable. This may be due in part to differences in mitochondrial function and oxidative stress. OBJECTIVE: The objective of the study was to determine whether mitochondrial function, content, and oxidative stress differ in well-matched obese individuals in the upper [obese diet sensitive (ODS)] vs lower quintiles [obese diet resistant (ODR)] for rate of weight loss. DESIGN: Primary myotubes derived from muscle biopsies of individuals identified as ODS or ODR were studied. SETTING: Compliant ODS and ODR females who completed in the Ottawa Hospital Weight Management Program and identified as ODS and ODR participated in this study. PATIENTS OR OTHER PARTICIPANTS: Eleven ODS and nine ODR weight-stable females matched for age, body mass, and body mass index participated in this study. INTERVENTION: Vastus lateralis muscle biopsies were obtained and processed for muscle satellite cell isolation. MAIN OUTCOME MEASURES: Mitochondrial respiration, content, reactive oxygen species, and glutathione redox ratios were measured in the myotubes of ODS and ODR individuals. RESULTS: Mitochondrial proton leak was increased in myotubes of ODS compared with ODR (P < .05). Reduced and oxidized glutathione was decreased in the myotubes of ODR vs ODS (P < .05), indicating a more oxidized glutathione redox state. There were no differences in myotube mitochondrial content, uncoupling protein 3, or adenine nucleotide translocase levels. CONCLUSIONS: Lower rate of mitochondrial proton leak in muscle is a cell autonomous phenomenon in ODR vs ODS individuals, and this is associated with a more oxidized glutathione redox state in ODR vs ODS myotubes. The muscle of ODR subjects may thus have a lower capacity to adapt to oxidative stress as compared with ODS.

KCNMA1 encoded cardiac BK channels afford protection against ischemia-reperfusion injury.

Mitochondrial potassium channels have been implicated in myocardial protection mediated through pre-/postconditioning. Compounds that open the Ca2+- and voltage-activated potassium channel of big-conductance (BK) have a pre-conditioning-like effect on survival of cardiomyocytes after ischemia/reperfusion injury. Recently, mitochondrial BK channels (mitoBKs) in cardiomyocytes were implicated as infarct-limiting factors that derive directly from the KCNMA1 gene encoding for canonical BKs usually present at the plasma membrane of cells. However, some studies challenged these cardio-protective roles of mitoBKs. Herein, we present electrophysiological evidence for paxilline- and NS11021-sensitive BK-mediated currents of 190 pS conductance in mitoplasts from wild-type but not BK-/- cardiomyocytes. Transmission electron microscopy of BK-/- ventricular muscles fibres showed normal ultra-structures and matrix dimension, but oxidative phosphorylation capacities at normoxia and upon re-oxygenation after anoxia were significantly attenuated in BK-/- permeabilized cardiomyocytes. In the absence of BK, post-anoxic reactive oxygen species (ROS) production from cardiomyocyte mitochondria was elevated indicating that mitoBK fine-tune the oxidative state at hypoxia and re-oxygenation. Because ROS and the capacity of the myocardium for oxidative metabolism are important determinants of cellular survival, we tested BK-/- hearts for their response in an ex-vivo model of ischemia/reperfusion (I/R) injury. Infarct areas, coronary flow and heart rates were not different between wild-type and BK-/- hearts upon I/R injury in the absence of ischemic pre-conditioning (IP), but differed upon IP. While the area of infarction comprised 28±3% of the area at risk in wild-type, it was increased to 58±5% in BK-/- hearts suggesting that BK mediates the beneficial effects of IP. These findings suggest that cardiac BK channels are important for proper oxidative energy supply of cardiomyocytes at normoxia and upon re-oxygenation after prolonged anoxia and that IP might indeed favor survival of the myocardium upon I/R injury in a BK-dependent mode stemming from both mitochondrial post-anoxic ROS modulation and non-mitochondrial localizations.

Implications of mitochondrial uncoupling in skeletal muscle in the development and treatment of obesity.

Understanding the metabolic factors that contribute to obesity development and weight loss success are critical for combating obesity and obesity-related disorders. This review provides an overview of energy metabolism with a particular focus on mitochondrial function in health and in obesity. Mitochondrial proton leak contributes significantly to whole body energy expenditure and the potential role of energy uncoupling in weight loss success is discussed. We provide evidence to support the hypothesis that differences in energy efficiency are important regulators of body weight and weight loss success.

Increased proton leak and SOD2 expression in myotubes from obese non-diabetic subjects with a family history of type 2 diabetes.

Muscle insulin resistance is linked to oxidative stress and decreased mitochondrial function. However, the exact cause of muscle insulin resistance is still unknown. Since offspring of patients with type 2 diabetes mellitus (T2DM) are susceptible to developing insulin resistance, they are ideal for studying the early development of insulin resistance. By using primary muscle cells derived from obese non-diabetic subjects with (FH+) or without (FH-) a family history of T2DM, we aimed to better understand the link between mitochondrial function, oxidative stress, and muscle insulin resistance. Insulin-stimulated glucose uptake and glycogen synthesis were normal in FH+ myotubes. Resting oxygen consumption rate was not different between groups. However, proton leak was higher in FH+ myotubes. This was associated with lower ATP content and decreased mitochondrial membrane potential in FH+ myotubes. Surprisingly, mtDNA content was higher in FH+ myotubes. Oxidative stress level was not different between FH+ and FH- groups. Reactive oxygen species content was lower in FH+ myotubes when differentiated in high glucose/insulin (25mM/150pM), which could be due to higher oxidative stress defenses (SOD2 expression and uncoupled respiration). The increased antioxidant defenses and mtDNA content in FH+ myotubes suggest the existence of compensatory mechanisms, which may provisionally prevent the development of insulin resistance.

Enhanced glucose homeostasis in BHE/cdb rats with mutated ATP synthase.

The BHE/cdb rat has a mutation in adenosine triphosphate (ATP) synthase that impairs insulin secretion. However, male BHE/cdb rats have normal circulating glucose and enhanced glucose tolerance. The aim of the current study was to identify mechanisms of enhanced glucose tolerance. The respiratory exchange ratio was increased, indicating increased oxidation of carbohydrate in BHE/cdb rats, consistent with increases in liver pyruvate dehydrogenase activity and muscle citrate synthase activity. Liver also exhibited diminished phosphoenol pyruvate carboxykinase content, which correlated with a decreased counter-regulatory response in the insulin tolerance test. Signaling via Akt or AMP-dependent kinase pathways in the liver could not account for lower blood glucose. We conclude that chronically low insulin secretion leads to adaption in glucose metabolism primarily in liver to maintain euglycemia.

A single prior bout of exercise protects against palmitate-induced insulin resistance despite an increase in total ceramide content.

Ceramide accumulation has been implicated in the impairment of insulin-stimulated glucose transport in skeletal muscle following saturated fatty acid (FA) exposure. Importantly, a single bout of exercise can protect against acute lipid-induced insulin resistance. The mechanism by which exercise protects against lipid-induced insulin resistance is not completely known but may occur through a redirection of FA toward triacylglycerol (TAG) and away from ceramide and diacylglycerol (DAG). Therefore, in the current study, an in vitro preparation was used to examine whether a prior bout of exercise could confer protection against palmitate-induced insulin resistance and whether the pharmacological [50 µM fumonisin B(1) (FB1)] inhibition of ceramide synthesis in the presence of palmitate could mimic the protective effect of exercise. Soleus muscle of sedentary (SED), exercised (EX), and SED in the presence of FB1 (SED+FB1) were incubated with or without 2 mM palmitate for 4 h. This 2-mM palmitate exposure impaired insulin-stimulated glucose transport (-28%, P < 0.01) and significantly increased ceramide, DAG, and TAG accumulation in the SED group (P < 0.05). A single prior bout of exercise prevented the detrimental effects of palmitate on insulin signaling and caused a partial redistribution of FA toward TAG (P < 0.05). However, the net increase in ceramide content in response to palmitate exposure in the EX group was not different compared with SED, despite the maintenance of insulin sensitivity. The incubation of soleus from SED rats with FB1 (SED+FB1) prevented the detrimental effects of palmitate and caused a redirection of FA toward TAG accumulation (P < 0.05). Therefore, this research suggests that although inhibiting ceramide accumulation can prevent the detrimental effects of palmitate, a single prior bout of exercise appears to protect against palmitate-induced insulin resistance, which may be independent of changes in ceramide content.

Epinephrine-mediated regulation of PDK4 mRNA in rat adipose tissue.

Fatty acid reesterification in adipose tissue is dependent on the generation of glycerol 3-phosphate, and, at least in rodent adipose tissue, this appears to occur primarily through glyceroneogenesis. A key enzyme in this process is pyruvate dehydrogenase kinase 4 (PDK4). PDK4 is induced in white adipose tissue by thiazolidinediones (TZDs) and the inhibition or knockdown of PDK4 inhibits TZD-induced increases in glyceroneogenesis. Since TZDs have many unwanted side effects, we were interested in identifying alternative mechanisms that could regulate PDK4 mRNA expression in white adipose tissue. In this regard we hypothesized that exercise, fasting, and epinephrine would increase PDK4 mRNA levels in rat epididymal adipose tissue. We further postulated that the p38 mitogen-activated protein kinase (MAPK) and 5'-AMP-activated protein kinase (AMPK) signaling pathways would control PDK4 mRNA expression in cultured adipose tissue. Exercise, fasting, and in or ex vivo epinephrine treatment increased PDK4 mRNA levels. These perturbations did not increase the expression of PDK1, -2, or -3. Pyruvate dehydrogenase phosphorylation was increased after an overnight fast and 4 h after the cessation of exercise. In cultured adipose tissue, epinephrine increased p38 and AMPK signaling; however, the direct activation of AMPK by AICAR or metformin led to reductions in PDK4 mRNA levels. The p38 inhibitor SB202190 reduced epinephrine-mediated increases in p38 MAPK activation without altering hormone-sensitive lipase or AMPK phosphorylation or attenuating epinephrine-induced increases in lipolysis. Reductions in p38 MAPK signaling were associated with decreases in PDK4 mRNA expression. The inhibition of peroxisome proliferator-activated receptor-γ (PPARγ) also attenuated the induction of PDK4. Our results are the very first to demonstrate an epinephrine-mediated regulation of PDK4 mRNA levels in white adipose tissue and suggest that p38 MAPK and PPARγ could be involved in this pathway.

Skeletal muscle lipogenic protein expression is not different between lean and obese individuals: a potential factor in ceramide accumulation.

CONTEXT: Skeletal muscle lipid content is increased in obesity. Recent evidence suggests that fatty acid (FA) storage as triacylglycerol (TAG) represents a metabolically safe pool compared to the more bioactive diacylglycerol (DAG) and ceramide. OBJECTIVE/DESIGN: The purpose of this study was to compare the expression of lipogenic proteins and ceramide and DAG content in skeletal muscle of lean and obese humans. We hypothesized that lipogenic protein expression would be increased in obese to facilitate the storage of excess FA as TAG. PARTICIPANTS: Eighteen lean (BMI < or = 26 kg/m(2)) and 15 obese (BMI > 29 kg/m(2)) women participated in this study. RESULTS: There was no difference in the expression of any lipogenic (stearoyl-CoA desaturase-1, stearoyl retinol binding protein-1c, mitochondrial glycerol-3-phosphate acyltransferase, diacylglycerol acyltransferase-1) or sphingolipid proteins measured between lean and obese humans. Total ceramide was increased in muscle from obese humans (lean vs. obese, 529.4 +/- 54.8 vs. 672.4 +/- 57.4 nmol/g; P < 0.05), but there was no difference in total DAG content (lean vs. obese, 2244.1 +/- 278.2 vs. 1941.4 +/- 165.0 nmol/g). Content of protein phosphatase 2A, a ceramide target, was increased in muscle of obese humans (P < 0.05). CONCLUSIONS: We propose that in muscle of obese humans there is an insufficient lipogenic response to the lipid oversupply, allowing more FA to be stored as reactive lipid species, particularly ceramide, potentially contributing to subsequent metabolic complications.

Muscle-specific differences in the response of mitochondrial proteins to beta-GPA feeding: an evaluation of potential mechanisms.

Beta-Guanadinopropionic acid (beta-GPA) feeding leads to reductions in skeletal muscle phosphagen concentrations and has been used as a tool with which to study the effects of energy charge on skeletal muscle metabolism. Supplementing standard rodent diets with beta-GPA leads to increases in mitochondrial enzyme content in fast but not slow-twitch muscles from male rats. Given this apparent discrepancy between muscle types we used beta-GPA feeding as a model to study signaling pathways involved in mitochondrial biogenesis. We hypothesized that beta-GPA feeding would result in a preferential activation of p38 MAPK and AMPK signaling and reductions in RIP140 protein content in triceps but not soleus muscle. Despite similar reductions in high-energy phosphate concentrations, 6 wk of beta-GPA feeding led to increases in mitochondrial proteins in triceps but not soleus muscles. Differences in the response of mitochondrial proteins to beta-GPA feeding did not seem to be related to a differential activation of p38 MAPK and AMPK signaling pathways or discrepancies in the induction of PPARgamma coactivator (PGC)-1alpha and -1beta. The protein content and expression of the nuclear corepressor RIP140 was reduced in triceps but not soleus muscle. Collectively our results indicate that chronic reductions in high-energy phosphates lead to the activation of p38 MAPK and AMPK signaling and increases in the expression of PGC-1alpha and -1beta in both soleus and triceps muscles. The lack of an effect of beta-GPA feeding on mitochondrial proteins in the soleus muscles could be related to a fiber type-specific effect of beta-GPA on RIP140 protein content.

Resistin acutely impairs insulin-stimulated glucose transport in rodent muscle in the presence, but not absence, of palmitate.

Resistin is a cytokine implicated in the development of insulin resistance. However, there has been little investigation of the effects of resistin on fatty acid (FA) metabolism and insulin response in skeletal muscle, a key tissue for glucose disposal. The purpose of the present study was to examine the role of altered FA metabolism as a cause of resistin's inhibition of insulin-stimulated glucose transport in muscle. Isolated rat soleus muscles were incubated acutely (2 h) in the presence or absence of 600 ng/ml resistin, with or without 2 mM palmitate. Resistin acutely impaired insulin-stimulated glucose transport and Akt phosphorylation, but only in the presence of palmitate, implicating a role for altered FA metabolism. This impairment of glucose transport induced by resistin plus palmitate could be pharmacologically rescued by the inclusion of aimidazole carboxamide ribonucleotide, a stimulator of AMP-activated protein kinase and FA oxidation, as well as inhibitors of ceramide synthesis (myriocin, fumonisin). However, to our surprise, resistin actually blunted the palmitate-induced increase in muscle ceramide content; as expected, ceramide content was significantly lowered by fumonisin. In summary, the acute impairment of insulin response by resistin was manifested only in the presence of high palmitate and was alleviated when FA metabolism was manipulated (increased oxidation, inhibited ceramide synthesis). Resistin's acute impairment of insulin response does not appear to require an absolute increase in ceramide content; however, reducing ceramide content alleviated the impairment in glucose transport and insulin signaling.

PGC-1alpha's relationship with skeletal muscle palmitate oxidation is not present with obesity despite maintained PGC-1alpha and PGC-1beta protein.

Reduced skeletal muscle mitochondrial content and fatty acid oxidation are associated with obesity and insulin resistance. Although the exact mechanisms remain elusive, this may result from impaired mitochondrial biogenesis or reductions in the mitochondrial reticulum network. Therefore, the purpose of this study was to determine whether the protein contents of various transcription factors, including PGC-1alpha and PGC-1beta and proteins associated with mitochondrial fusion events, were reduced in skeletal muscle of nine obese (BMI = 37.6 +/- 2.2 kg/m(-2)) compared with nine age-matched lean (BMI = 23.3 +/- 0.7 kg/m(-2)) women. The protein contents of PGC-1alpha, PGC-1beta, PPARalpha, and tFAM were not reduced with obesity. In contrast, PPARgamma was increased (+22%, P < 0.05) with obesity, and there was a trend toward an increase (+31%, P = 0.13) in PPARdelta/beta. In lean individuals, PGC-1alpha protein correlated with citrate synthase (CS; r = 0.67) and rates of palmitate oxidation (r = 0.87), whereas PGC-1beta correlated with PPARgamma (r = 0.90), PPARdelta/beta (r = 0.63), and cytochrome c oxidase IV (COX-IV; r = 0.63). In obese individuals, the relationship between PGC-1alpha and CS was maintained (r = 0.65); however, the associations between PGC-1alpha and palmitate oxidation (r = -0.38) and PGC-1beta with PPARgamma (r = 0.14), PPARdelta/beta (r = 0.21), and COX-IV (r = 0.01) were lost. In addition, mitofusin-1 (MFN-1), MFN-2, and dynamin-related protein-1 (DRP-1) total protein contents were not altered with obesity (P > 0.05). These data suggest that altered regulation, and not reductions in the protein contents of transcription factors, is associated with insulin resistance. Also, it does not appear that alterations in the proteins associated with mitochondrial network formation and degradation can account for the observed decrease in mitochondrial content.

Palmitate acutely induces insulin resistance in isolated muscle from obese but not lean humans.

Exposure to high fatty acids (FAs) induces whole body and skeletal muscle insulin resistance. The globular form of the adipokine, adiponectin (gAd), stimulates FA oxidation and improves insulin sensitivity; however, its ability to prevent lipid-induced insulin resistance in humans has not been tested. The purpose of this study was to determine 1) whether acute (4 h) exposure to 2 mM palmitate would impair insulin signaling and glucose transport in isolated human skeletal muscle, 2) whether muscle from obese humans is more susceptible to the effects of palmitate, and 3) whether the presence of 2 mM palmitate + 2.5 mug/ml gAd (P+gAd) could prevent the effects of palmitate. Insulin-stimulated (10 mU/ml) glucose transport was not different, relative to control, following exposure to palmitate (-10%) or P+gAd (-3%) in lean muscle. In obese muscle, the absolute increase in glucose transport from basal to insulin-stimulated conditions was significantly decreased following palmitate (-55%) and P+gAd (-36%) exposure (control vs. palmitate; control vs. P+gAd, P < 0.05). There was no difference in the absolute increase in glucose transport between palmitate and P+gAd, indicating that in the presence of palmitate, gAd did not improve glucose transport. The palmitate-induced reduction in insulin-stimulated glucose transport in muscle from obese individuals may have been due to reduced Ser Akt (control vs. palmitate; P+gAd, P < 0.05) and Akt substrate 160 (AS160) phosphorylation (control vs. palmitate; P+gAd, P < 0.05). FA oxidation was significantly increased in muscle of lean and obese individuals in the presence of gAd (P < 0.05), suggesting that the stimulatory effects of gAd on FA oxidation may not be sufficient to entirely prevent palmitate-induced insulin resistance in obese muscle.

Conjugated linoleic acid increases skeletal muscle ceramide content and decreases insulin sensitivity in overweight, non-diabetic humans.

The effect of conjugated linoleic acid (CLA), a popular weight-loss supplement, on insulin sensitivity in humans is controversial and has not been extensively studied. To date no studies have examined the effects of CLA supplementation on human skeletal muscle metabolism or lipid content. It is also unknown whether CLA accumulates in human skeletal muscle with supplementation. In the present study, 9 overweight, non-diabetic individuals received 4 g/d of mixed CLA isomers in the form of 1 g supplements, for 12 weeks. CLA isomers significantly increased in both plasma and skeletal muscle following supplementation. Skeletal muscle ceramide content was also significantly increased, although there was no consistent change in muscle diacylglycerol or triacylglycerol content. Insulin sensitivity was significantly decreased (p

Skeletal muscle mitochondrial FAT/CD36 content and palmitate oxidation are not decreased in obese women.

A reduction in fatty acid oxidation has been associated with lipid accumulation and insulin resistance in the skeletal muscle of obese individuals. We examined whether this decrease in fatty acid oxidation was attributable to a reduction in muscle mitochondrial content and/or a dysfunction in fatty acid oxidation within mitochondria obtained from skeletal muscle of age-matched, lean [body mass index (BMI) = 23.3 +/- 0.7 kg/m2] and obese women (BMI = 37.6 +/- 2.2 kg/m2). The mitochondrial marker enzymes citrate synthase (-34%), beta-hydroxyacyl-CoA dehydrogenase (-17%), and cytochrome c oxidase (-32%) were reduced (P < 0.05) in obese participants, indicating that mitochondrial content was diminished. Obesity did not alter the ability of isolated mitochondria to oxidize palmitate; however, fatty acid oxidation was reduced at the whole muscle level by 28% (P < 0.05) in the obese. Mitochondrial fatty acid translocase (FAT/CD36) did not differ in lean and obese individuals, but mitochondrial FAT/CD36 was correlated with mitochondrial fatty acid oxidation (r = 0.67, P < 0.05). We conclude that the reduction in fatty acid oxidation in obese individuals is attributable to a decrease in mitochondrial content, not to an intrinsic defect in the mitochondria obtained from skeletal muscle of obese individuals. In addition, it appears that mitochondrial FAT/CD36 may be involved in regulating fatty acid oxidation in human skeletal muscle.