NAD+-Precursor Supplementation With L-Tryptophan, Nicotinic Acid, and Nicotinamide Does Not Affect Mitochondrial Function or Skeletal Muscle Function in Physically Compromised Older Adults.

BACKGROUND: Boosting NAD+ via supplementation with niacin equivalents has been proposed as a potential modality capable of promoting healthy aging and negating age-dependent declines of skeletal muscle mass and function. OBJECTIVES: We investigated the efficacy of NAD+-precursor supplementation (tryptophan, nicotinic acid, and nicotinamide) on skeletal muscle mitochondrial function in physically compromised older adults. METHODS: A randomized, double-blind, controlled trial was conducted in 14 (female/male: 4/10) community-dwelling, older adults with impaired physical function [age, 72.9 ± 4.0 years; BMI, 25.2 ± 2.3 kg/m2]. Participants were supplemented with 207.5 mg niacin equivalents/day [intervention (INT)] and a control product (CON) that did not contain niacin equivalents, each for 32 days. The primary outcomes tested were mitochondrial oxidative capacity and exercise efficiency, analyzed by means of paired Student's t-tests. Secondary outcomes, such as NAD+ concentrations, were analyzed accordingly. RESULTS: Following supplementation, skeletal muscle NAD+ concentrations [7.5 ± 1.9 compared with 7.9 ± 1.6 AU, respectively] in INT compared with CON conditions were not significantly different compared to the control condition, whereas skeletal muscle methyl-nicotinamide levels were significantly higher under NAD+-precursor supplementation [INT, 0.098 ± 0.063 compared with CON, 0.025 ± 0.014; P = 0.001], suggesting an increased NAD+ metabolism. Conversely, neither ADP-stimulated [INT, 82.1 ± 19.0 compared with CON, 84.0 ± 19.2; P = 0.716] nor maximally uncoupled mitochondrial respiration [INT, 103.4 ± 30.7 compared with CON, 108.7 ± 33.4; P = 0.495] improved under NAD+-precursor supplementation, nor did net exercise efficiency during the submaximal cycling test [INT, 20.2 ± 2.77 compared with CON, 20.8 ± 2.88; P = 0.342]. CONCLUSIONS: Our findings are consistent with previous findings on NAD+ efficacy in humans, and we show in community-dwelling, older adults with impaired physical function that NAD+-precursor supplementation through L-tryptophan, nicotinic acid, and nicotinamide does not improve mitochondrial or skeletal muscle function. This study was registered at clinicaltrials.gov as NCT03310034.

Exercise counteracts lipotoxicity by improving lipid turnover and lipid droplet quality.

The incidence of obesity and metabolic disease, such as type 2 diabetes mellitus (T2D), is rising globally. Dietary lipid over supply leads to lipid accumulation at ectopic sites, such as skeletal muscle. Ectopic lipid storage is highly correlated with insulin resistance and T2D, likely due to a loss of metabolic flexibility - the capacity to switch between fat and glucose oxidation upon insulin stimulation - and cellular dysfunction because of lipotoxicity. However, muscle lipid levels are also elevated in endurance-trained athletes, presenting a paradoxical phenotype of increased intramuscular lipids along with high insulin sensitivity - the 'athletes' paradox'. This review focuses on recent human data to characterize intramuscular lipid species in order to elucidate some of the underlying mechanisms driving skeletal muscle lipotoxicity. There is evidence that lipotoxicity is characterized by an increase in bioactive lipid species, such as ceramide. The athletes' paradox supports the notion that regular physical exercise has health benefits that might originate from the alleviation of lipotoxicity. Indeed, exercise training alleviates intramuscular ceramide content in obese individuals without a necessary decrease in ectopic lipid storage. Furthermore, evidence shows that exercise training elevates markers of lipid droplet dynamics such as the PLIN proteins, and triglyceride lipases ATGL and HSL, as well as mitochondrial efficiency, potentially explaining the improved lipid turnover and a reduction in the accumulation of lipotoxic intermediates observed with the athelets' paradox.

Low brown adipose tissue activity in endurance-trained compared with lean sedentary men.

BACKGROUND/OBJECTIVES: It has now been unequivocally demonstrated that humans possess functional brown adipose tissue (BAT) and that human BAT can be recruited upon chronic cold stimulation. Recruitment of BAT has been postulated as a potential strategy to counteract the current global obesity epidemic. Recently, it was shown in rodents that endurance exercise training could stimulate the recruitment of brown-like adipocytes within white adipose tissue (WAT) via exercise-induced myokines such as irisin (the cleaved circulating product of the type 1 membrane protein FNDC5) and interleukin-6 (IL-6). Our objective was to test whether endurance-trained athletes had increased cold-stimulated BAT activity and browning of subcutaneous WAT compared with lean sedentary males. SUBJECTS/METHODS: Twelve endurance-trained athletes and 12 lean sedentary males were measured during 2 h of mild cold exposure to determine cold-induced BAT activity via [(18)F]fluorodeoxyglucose-positron emission tomography-computed tomography ([(18)F]FDG-PET-CT) scanning. Skeletal muscle FNDC5 expression, as well as plasma irisin and IL-6 levels were determined. In addition, a subcutaneous abdominal WAT biopsy was taken to measure gene expression of several markers for browning of WAT. RESULTS: Cold-induced BAT activity was significantly lower in athletes, and no differences in gene expression of classical brown and beige adipocyte markers were detected in subcutaneous WAT between the groups. As expected, mRNA expression of FNDC5 in skeletal muscle was significantly higher in endurance athletes but plasma irisin and Il-6 levels were similar in both groups. CONCLUSIONS: These results indicate that chronic endurance exercise is not associated with brown and beige adipocyte recruitment; in fact endurance training appears to be linked to lower the metabolic activity of BAT in humans.

Mitochondrial coupling and capacity of oxidative phosphorylation in skeletal muscle of Inuit and Caucasians in the arctic winter.

During evolution, mitochondrial DNA haplogroups of arctic populations may have been selected for lower coupling of mitochondrial respiration to ATP production in favor of higher heat production. We show that mitochondrial coupling in skeletal muscle of traditional and westernized Inuit habituating northern Greenland is identical to Danes of western Europe haplogroups. Biochemical coupling efficiency was preserved across variations in diet, muscle fiber type, and uncoupling protein-3 content. Mitochondrial phenotype displayed plasticity in relation to lifestyle and environment. Untrained Inuit and Danes had identical capacities to oxidize fat substrate in arm muscle, which increased in Danes during the 42 days of acclimation to exercise, approaching the higher level of the Inuit hunters. A common pattern emerges of mitochondrial acclimatization and evolutionary adaptation in humans at high latitude and high altitude where economy of locomotion may be optimized by preservation of biochemical coupling efficiency at modest mitochondrial density, when submaximum performance is uncoupled from VO2max and maximum capacities of oxidative phosphorylation.

Overexpression of PLIN5 in skeletal muscle promotes oxidative gene expression and intramyocellular lipid content without compromising insulin sensitivity.

AIMS/HYPOTHESIS: While lipid deposition in the skeletal muscle is considered to be involved in obesity-associated insulin resistance, neutral intramyocellular lipid (IMCL) accumulation per se does not necessarily induce insulin resistance. We previously demonstrated that overexpression of the lipid droplet coat protein perilipin 2 augments intramyocellular lipid content while improving insulin sensitivity. Another member of the perilipin family, perilipin 5 (PLIN5), is predominantly expressed in oxidative tissues like the skeletal muscle. Here we investigated the effects of PLIN5 overexpression - in comparison with the effects of PLIN2 - on skeletal muscle lipid levels, gene expression profiles and insulin sensitivity. METHODS: Gene electroporation was used to overexpress PLIN5 in tibialis anterior muscle of rats fed a high fat diet. Eight days after electroporation, insulin-mediated glucose uptake in the skeletal muscle was measured by means of a hyperinsulinemic euglycemic clamp. Electron microscopy, fluorescence microscopy and lipid extractions were performed to investigate IMCL accumulation. Gene expression profiles were obtained using microarrays. RESULTS: TAG storage and lipid droplet size increased upon PLIN5 overexpression. Despite the higher IMCL content, insulin sensitivity was not impaired and DAG and acylcarnitine levels were unaffected. In contrast to the effects of PLIN2 overexpression, microarray data analysis revealed a gene expression profile favoring FA oxidation and improved mitochondrial function. CONCLUSIONS/INTERPRETATION: Both PLIN2 and PLIN5 increase neutral IMCL content without impeding insulin-mediated glucose uptake. As opposed to the effects of PLIN2 overexpression, overexpression of PLIN5 in the skeletal muscle promoted expression of a cluster of genes under control of PPARα and PGC1α involved in FA catabolism and mitochondrial oxidation.

The effects of 30 days resveratrol supplementation on adipose tissue morphology and gene expression patterns in obese men.

Polyphenolic compounds, such as resveratrol, have recently received widespread interest because of their ability to mimic effects of calorie restriction. The objective of the present study was to gain more insight into the effects of 30 days resveratrol supplementation on adipose tissue morphology and underlying processes. Eleven healthy obese men were supplemented with placebo and resveratrol for 30 days (150 mg per day), separated by a 4-week washout period in a double-blind randomized crossover design. A postprandial abdominal subcutaneous adipose tissue biopsy was collected to assess adipose tissue morphology and gene expression using microarray analysis. Resveratrol significantly decreased adipocyte size, with a shift toward a reduction in the proportion of large and very-large adipocytes and an increase in small adipocytes. Microarray analysis revealed downregulation of Wnt and Notch signaling pathways and upregulation of pathways involved in cell cycle regulation after resveratrol supplementation, suggesting enhanced adipogenesis. Furthermore, lysosomal/phagosomal pathway and transcription factor EB were upregulated reflecting an alternative pathway of lipid breakdown by autophagy. Further research is necessary to investigate whether resveratrol improves adipose tissue function.

Thirty days of resveratrol supplementation does not affect postprandial incretin hormone responses, but suppresses postprandial glucagon in obese subjects.

AIMS: Resveratrol, a natural polyphenolic compound produced by various plants (e.g. red grapes) and found in red wine, has glucose-lowering effects in humans and rodent models of obesity and/or diabetes. The mechanisms behind these effects have been suggested to include resveratrol-induced secretion of the gut incretin hormone glucagon-like peptide-1. We investigated postprandial incretin hormone and glucagon responses in obese human subjects before and after 30 days of resveratrol supplementation. METHODS: Postprandial plasma responses of the incretin hormones glucagon-like peptide-1 and glucose-dependent insulinotropic polypeptide and glucagon were evaluated in 10 obese men [subjects characteristics (mean ± standard error of the mean): age 52 ± 2 years; BMI 32 ± 1 kg/m(2), fasting plasma glucose 5.5 ± 0.1 mmol/l] who had been given a dietary supplement of resveratrol (Resvida(®) 150 mg/day) or placebo for 30 days in a randomized, double-blind, crossover design with a 4-week washout period. At the end of each intervention period a standardized meal test (without co-administration of resveratrol) was performed. RESULTS: Resveratrol supplementation had no impact on fasting plasma concentrations or postprandial plasma responses (area under curve values) of glucose-dependent insulinotropic polypeptide (11.2 ± 2.1 vs. 11.8 ± 2.2 pmol/l, P = 0.87; 17.0 ± 2.2 vs. 14.8 ± 1.6 min × nmol/l, P = 0.20) or glucagon-like peptide-1 (15.4 ± 1.0 vs. 15.2 ± 0.9 pmol/l, P = 0.84; 5.6 ± 0.4 vs. 5.7 ± 0.3 min × nmol/l, P = 0.73). Resveratrol supplementation significantly suppressed postprandial glucagon responses (4.4 ± 0.4 vs. 3.9 ± 0.4 min × nmol/l, P = 0.01) without affecting fasting glucagon levels (15.2 ± 2.2 vs. 14.5 ± 1.5 pmol/l, P = 0.56). CONCLUSIONS: Our data suggest that 30 days of resveratrol supplementation does not affect fasting or postprandial incretin hormone plasma levels in obese humans, but suppresses postprandial glucagon responses.

Prolonged fasting and the effects on biomarkers of inflammation and on adipokines in healthy lean men.

Obesity and insulin resistance are associated with low-grade systemic inflammation, which is related to increased concentrations of plasma FFAs, glucose, or insulin. Prolonged fasting induces insulin resistance due to elevated plasma FFAs, but is not accompanied by hyperinsulinemia or hyperglycemia. This makes it possible to study effects of physiologically increased FFA concentrations on inflammatory markers, when insulin and glucose concentrations are not increased. In random order, 10 healthy young lean men (mean BMI: 22.8 kg/m2) were fasted or fed in energy balance for 60 h with a 2-week wash-out period. Subjects stayed in a respiration chamber during the 60-h periods. Blood samples were taken after 12, 36, and 60 h. Then, a hyperinsulinemic-euglycemic clamp was performed.Fasting decreased insulin sensitivity by 45% and increased FFA concentrations 5-fold. Fasting did not change concentrations of the inflammatory cytokines TNF-α, IL-1ß, IL-6 and IL-8, or of hs-CRP. Effects on vascular endothelial growth factor (VEGF)--which may positively relate to insulin resistance, and on chemerin and leptin--adipokines related to obesity, and obesity-related pathologies, were also studied. At t=60 h, VEGF concentrations were significantly increased during the fasted period (p<0.05). At the same time point, chemerin (p<0.01) and leptin (p<0.01) were significantly decreased after fasting. For leptin, this decrease was also significant after 36 h (p<0.01). Adiponectin levels remained unchanged. In healthy young lean men, fasting-induced increases in FFAs leading to insulin resistance do not cause changes in concentrations of the inflammatory cytokines. VEGF concentrations increased and those of chemerin decreased.

Targeting of mitochondrial reactive oxygen species production does not avert lipid-induced insulin resistance in muscle tissue from mice.

AIMS/HYPOTHESIS: High-fat, high-sucrose diet (HF)-induced reactive oxygen species (ROS) levels are implicated in skeletal muscle insulin resistance and mitochondrial dysfunction. Here we investigated whether mitochondrial ROS sequestering can circumvent HF-induced oxidative stress; we also determined the impact of any reduced oxidative stress on muscle insulin sensitivity and mitochondrial function. METHODS: The Skulachev ion (plastoquinonyl decyltriphenylphosphonium) (SkQ), a mitochondria-specific antioxidant, was used to target ROS production in C2C12 muscle cells as well as in HF-fed (16 weeks old) male C57Bl/6 mice, compared with mice on low-fat chow diet (LF) or HF alone. Oxidative stress was measured as protein carbonylation levels. Glucose tolerance tests, glucose uptake assays and insulin-stimulated signalling were determined to assess muscle insulin sensitivity. Mitochondrial function was determined by high-resolution respirometry. RESULTS: SkQ treatment reduced oxidative stress in muscle cells (-23% p < 0.05), but did not improve insulin sensitivity and glucose uptake under insulin-resistant conditions. In HF mice, oxidative stress was elevated (56% vs LF p < 0.05), an effect completely blunted by SkQ. However, HF and HF+SkQ mice displayed impaired glucose tolerance (AUC HF up 33%, p < 0.001; HF+SkQ up 22%; p < 0.01 vs LF) and disrupted skeletal muscle insulin signalling. ROS sequestering did not improve mitochondrial function. CONCLUSIONS/INTERPRETATION: SkQ treatment reduced muscle mitochondrial ROS production and prevented HF-induced oxidative stress. Nonetheless, whole-body glucose tolerance, insulin-stimulated glucose uptake, muscle insulin signalling and mitochondrial function were not improved. These results suggest that HF-induced oxidative stress is not a prerequisite for the development of muscle insulin resistance.

Increased intramyocellular lipids but unaltered in vivo mitochondrial oxidative phosphorylation in skeletal muscle of adipose triglyceride lipase-deficient mice.

Adipose triglyceride lipase (ATGL) is a lipolytic enzyme that is highly specific for triglyceride hydrolysis. The ATGL-knockout mouse (ATGL(-/-)) accumulates lipid droplets in various tissues, including skeletal muscle, and has poor maximal running velocity and endurance capacity. In this study, we tested whether abnormal lipid accumulation in skeletal muscle impairs mitochondrial oxidative phosphorylation, and hence, explains the poor muscle performance of ATGL(-/-) mice. In vivo ¹H magnetic resonance spectroscopy of the tibialis anterior of ATGL(-/-) mice revealed that its intramyocellular lipid pool is approximately sixfold higher than in WT controls (P = 0.0007). In skeletal muscle of ATGL(-/-) mice, glycogen content was decreased by 30% (P < 0.05). In vivo ³¹P magnetic resonance spectra of resting muscles showed that WT and ATGL(-/-) mice have a similar energy status: [PCr], [P(i)], PCr/ATP ratio, PCr/P(i) ratio, and intracellular pH. Electrostimulated muscles from WT and ATGL(-/-) mice showed the same PCr depletion and pH reduction. Moreover, the monoexponential fitting of the PCr recovery curve yielded similar PCr recovery times (τPCr; 54.1 ± 6.1 s for the ATGL(-/-) and 58.1 ± 5.8 s for the WT), which means that overall muscular mitochondrial oxidative capacity was comparable between the genotypes. Despite similar in vivo mitochondrial oxidative capacities, the electrostimulated muscles from ATGL(-/-) mice displayed significantly lower force production and increased muscle relaxation time than the WT. These findings suggest that mechanisms other than mitochondrial dysfunction cause the impaired muscle performance of ATGL(-/-) mice.

Exercise-induced modulation of cardiac lipid content in healthy lean young men.

Cardiac lipid accumulation is associated with decreased cardiac function and energy status (PCr/ATP). It has been suggested that elevated plasma fatty acid (FA) concentrations are responsible for the cardiac lipid accumulation. Therefore, the aim of the present study was to investigate if elevating plasma FA concentrations by exercise results in an increased cardiac lipid content, and if this influences cardiac function and energy status. Eleven male subjects (age 25.4 ± 1.1 years, BMI 23.6 ± 0.8 kg/m²) performed a 2-h cycling protocol, once while staying fasted and once while ingesting glucose, to create a state of high versus low plasma FA concentrations, respectively. Cardiac lipid content was measured by proton magnetic resonance spectroscopy (¹H-MRS) at baseline, directly after exercise and again 4 h post-exercise, together with systolic function (by multi-slice cine-MRI) and cardiac energy status (by ³¹P-MRS). Plasma FA concentrations were increased threefold during exercise and ninefold during recovery in the fasted state compared with the glucose-fed state (p < 0.01). Cardiac lipid content was elevated at the end of the fasted test day (from 0.26 ± 0.04 to 0.44 ± 0.04%, p = 0.003), while it did not change with glucose supplementation (from 0.32 ± 0.03 to 0.26 ± 0.05%, p = 0.272). Furthermore, PCr/ATP was decreased by 32% in the high plasma FA state compared with the low FA state (n = 6, p = 0.014). However, in the high FA state, the ejection fraction 4 h post-exercise was higher compared with the low FA state (63 ± 2 vs. 59 ± 2%, p = 0.018). Elevated plasma FA concentrations, induced by exercise in the fasted state, lead to increased cardiac lipid content, but do not acutely hamper systolic function. Although the lower cardiac energy status is in line with a lipotoxic action of cardiac lipid content, a causal relationship cannot be proven.

TNF-alpha impairs regulation of muscle oxidative phenotype: implications for cachexia?

Chronic obstructive pulmonary disease (COPD) is characterized by weight loss, muscle wasting (in advanced disease ultimately resulting in cachexia), and loss of muscle oxidative phenotype (oxphen). This study investigates the effect of inflammation (as a determinant of muscle wasting) on muscle oxphen by using cell studies combined with analyses of muscle biopsies of patients with COPD and control participants. We analyzed markers (citrate synthase, ß-hydroxyacyl-CoA dehydrogenase, and cytochrome c oxidase IV) and regulators (PGC-1α, PPAR-α, and Tfam) of oxphen in vastus lateralis muscle biopsies of patients with advanced COPD and healthy smoking control participants. Here 17 of 73 patients exhibited elevated muscle TNF-α mRNA levels. In these patients, significantly lower mRNA levels of all oxidative markers/regulators were found. Interestingly, these patients also had a significantly lower body mass index and tended to have less muscle mass. In cultured muscle cells, mitochondrial protein content and myosin heavy chain isoform I (but not II) protein and mRNA levels were reduced on chronic TNF-α stimulation. TNF-α also reduced mitochondrial respiration in a nuclear factor kappaB (NF-κB) -dependent manner. Importantly, TNF-α-induced NF-κB activation decreased promoter transactivation and transcriptional activity of regulators of mitochondrial biogenesis and muscle oxphen. In conclusion, these results demonstrate that TNF-α impairs muscle oxphen in a NF-κB-dependent manner.

Impact of aging and exercise on skeletal muscle mitochondrial capacity, energy metabolism, and physical function.

The relationship between the age-associated decline in mitochondrial function and its effect on skeletal muscle physiology and function remain unclear. In the current study, we examined to what extent physical activity contributes to the decline in mitochondrial function and muscle health during aging and compared mitochondrial function in young and older adults, with similar habitual physical activity levels. We also studied exercise-trained older adults and physically impaired older adults. Aging was associated with a decline in mitochondrial capacity, exercise capacity and efficiency, gait stability, muscle function, and insulin sensitivity, even when maintaining an adequate daily physical activity level. Our data also suggest that a further increase in physical activity level, achieved through regular exercise training, can largely negate the effects of aging. Finally, mitochondrial capacity correlated with exercise efficiency and insulin sensitivity. Together, our data support a link between mitochondrial function and age-associated deterioration of skeletal muscle.

Exercise training increases mitochondrial content and ex vivo mitochondrial function similarly in patients with type 2 diabetes and in control individuals.

AIMS/HYPOTHESIS: We previously showed that type 2 diabetic patients are characterised by compromised intrinsic mitochondrial function. Here, we examined if exercise training could increase intrinsic mitochondrial function in diabetic patients compared with control individuals. METHODS: Fifteen male type 2 diabetic patients and 14 male control individuals matched for age, BMI and VO(2max) enrolled in a 12 week exercise intervention programme. Ex vivo mitochondrial function was assessed by high-resolution respirometry in permeabilised muscle fibres from vastus lateralis muscle. Before and after training, insulin-stimulated glucose disposal was examined during a hyperinsulinaemic-euglycaemic clamp. RESULTS: Diabetic patients had intrinsically lower ADP-stimulated state 3 respiration and lower carbonyl cyanide 4-(trifluoro-methoxy)phenylhydrazone (FCCP)-induced maximal oxidative respiration, both on glutamate and on glutamate and succinate, and in the presence of palmitoyl-carnitine (p < 0.05). After training, diabetic patients and control individuals showed increased state 3 respiration on the previously mentioned substrates (p < 0.05); however, an increase in FCCP-induced maximal oxidative respiration was observed only in diabetic patients (p < 0.05). The increase in mitochondrial respiration was accompanied by a 30% increase in mitochondrial content upon training (p < 0.01). After adjustment for mitochondrial density, state 3 and FCCP-induced maximal oxidative respiration were similar between groups after training. Improvements in mitochondrial respiration were paralleled by improvements in insulin-stimulated glucose disposal in diabetic patients, with a tendency for this in control individuals. CONCLUSIONS/INTERPRETATION: We confirmed lower intrinsic mitochondrial function in diabetic patients compared with control individuals. Diabetic patients increased their mitochondrial content to the same extent as control individuals and had similar intrinsic mitochondrial function, which occurred parallel with improved insulin sensitivity.

PPARgamma inhibits NF-kappaB-dependent transcriptional activation in skeletal muscle.

Skeletal muscle pathology associated with a chronic inflammatory disease state (e.g., skeletal muscle atrophy and insulin resistance) is a potential consequence of chronic activation of NF-kappaB. It has been demonstrated that peroxisome proliferator-activated receptors (PPARs) can exert anti-inflammatory effects by interfering with transcriptional regulation of inflammatory responses. The goal of the present study, therefore, was to evaluate whether PPAR activation affects cytokine-induced NF-kappaB activity in skeletal muscle. Using C(2)C(12) myotubes as an in vitro model of myofibers, we demonstrate that PPAR, and specifically PPARgamma, activation potently inhibits inflammatory mediator-induced NF-kappaB transcriptional activity in a time- and dose-dependent manner. Furthermore, PPARgamma activation by rosiglitazone strongly suppresses cytokine-induced transcript levels of the NF-kappaB-dependent genes intracellular adhesion molecule 1 (ICAM-1) and CXCL1 (KC), the murine homolog of IL-8, in myotubes. To verify whether muscular NF-kappaB activity in human subjects is suppressed by PPARgamma activation, we examined the effect of 8 wk of rosiglitazone treatment on muscular gene expression of ICAM-1 and IL-8 in type 2 diabetes mellitus patients. In these subjects, we observed a trend toward decreased basal expression of ICAM-1 mRNA levels. Subsequent analyses in cultured myotubes revealed that the anti-inflammatory effect of PPARgamma activation is not due to decreased RelA translocation to the nucleus or reduced RelA DNA binding. These findings demonstrate that muscle-specific inhibition of NF-kappaB activation may be an interesting therapeutic avenue for treatment of several inflammation-associated skeletal muscle abnormalities.

Peroxisome proliferator-activated receptors: a therapeutic target in COPD?

Extrapulmonary pathology significantly impairs clinical outcome in chronic obstructive pulmonary disease (COPD). The peroxisome proliferator-activated receptors (PPARs) are implicated in the regulation of several hallmarks of systemic COPD pathology, including cachexia, decreased oxidative muscle metabolism, oxidative stress and systemic inflammation. Recently, expression of PPARs and related cofactors was shown to be reduced in peripheral skeletal muscle of patients with moderate-to-severe COPD and muscle weakness. The current authors hypothesise that impaired peroxisome proliferator-activated receptor signalling may underlie some of the muscular disturbances in chronic obstructive pulmonary disease. Proposed mechanisms will be outlined in the present article, as well as the therapeutic potential of peroxisome proliferator-activated receptor modulation in the treatment of skeletal muscle dysfunction.

Effect of training in the fasted state on metabolic responses during exercise with carbohydrate intake.

Skeletal muscle gene response to exercise depends on nutritional status during and after exercise, but it is unknown whether muscle adaptations to endurance training are affected by nutritional status during training sessions. Therefore, this study investigated the effect of an endurance training program (6 wk, 3 day/wk, 1-2 h, 75% of peak Vo(2)) in moderately active males. They trained in the fasted (F; n = 10) or carbohydrate-fed state (CHO; n = 10) while receiving a standardized diet [65 percent of total energy intake (En) from carbohydrates, 20%En fat, 15%En protein]. Before and after the training period, substrate use during a 2-h exercise bout was determined. During these experimental sessions, all subjects were in a fed condition and received extra carbohydrates (1 g.kg body wt(-1) .h(-1)). Peak Vo(2) (+7%), succinate dehydrogenase activity, GLUT4, and hexokinase II content were similarly increased between F and CHO. Fatty acid binding protein (FABPm) content increased significantly in F (P = 0.007). Intramyocellular triglyceride content (IMCL) remained unchanged in both groups. After training, pre-exercise glycogen content was higher in CHO (545 +/- 19 mmol/kg dry wt; P = 0.02), but not in F (434 +/- 32 mmol/kg dry wt; P = 0.23). For a given initial glycogen content, F blunted exercise-induced glycogen breakdown when compared with CHO (P = 0.04). Neither IMCL breakdown (P = 0.23) nor fat oxidation rates during exercise were altered by training. Thus short-term training elicits similar adaptations in peak Vo(2) whether carried out in the fasted or carbohydrate-fed state. Although there was a decrease in exercise-induced glycogen breakdown and an increase in proteins involved in fat handling after fasting training, fat oxidation during exercise with carbohydrate intake was not changed.

Skeletal muscle uncoupling protein 3 expression is a determinant of energy expenditure in Pima Indians.

The recent discovery of uncoupling protein (UCP)-2 and UCP-3, and their high expression in skeletal muscle, has renewed interest in a possible role for these proteins in underlying the variability in energy expenditure and therefore metabolic efficiency. Using reverse transcription-polymerase chain reaction, levels of expression of UCP-2 and long and short forms of UCP-3 were measured in skeletal muscle of 19 nondiabetic, male Pima Indians covering a wide range of body weight. Twenty-four-hour energy expenditure was measured in a respiratory chamber in 16 of these individuals. BMI was negatively correlated with the expression levels of the long (r = -0.53, P = 0.025) and short (r = -0.46, P = 0.047) forms of UCP-3. BMI was not correlated with UCP-2 expression. Metabolic rate during sleep, adjusted for fat-free mass and fat mass, was positively correlated with the long form of UCP-3 (r = 0.69, P = 0.006). These results indicate that UCP-3 may be a determinant of energy expenditure and metabolic efficiency in Pima Indians.

Increase in fat oxidation on a high-fat diet is accompanied by an increase in triglyceride-derived fatty acid oxidation.

The aim of this study is to investigate the mechanism behind the slow increase in fat oxidation on a high-fat diet. Therefore, we determined 24-h substrate oxidation using respiration chambers and the rate of appearance and oxidation of plasma-derived fatty acids in seven healthy nonobese men (age 23 +/- 2 years, height 1.85 +/- 0.03 m, weight 70.4 +/- 2.3 kg, % body fat 13 +/- 1). Before testing, they consumed a low-fat diet (30% fat, 55% carbohydrate) at home for 3 days. Measurements were performed after 1 day consumption of either a low-fat diet (LF), a high-fat diet (HF1, 60% fat, 25% carbohydrate), or a high-fat diet preceded by a glycogen-lowering exercise test (HF1+EX), and after 7 days on a high-fat diet (HF7). After an overnight fast, an infusion of [U-13C]palmitate (0.00806 micromol x min(-1) x kg(-1)) was started and continued for 2 h at rest followed by 1 h of exercise at 50% of maximal power output (Wmax). Whole-body fat oxidation was measured using indirect calorimetry, and plasma-derived fatty acid oxidation was evaluated by measuring breath 13CO2 enrichment and corrected with the acetate recovery factor. Twenty-four-hour fat oxidation gradually increased on the high-fat diet. Both at rest and during exercise, there was no change in rate of appearance of fatty acids and plasma-derived fatty acid oxidation. Triglyceride-derived fatty acid oxidation tended to be higher after 7 days of high-fat diet at rest (P < 0.07). This difference was significant during exercise (P < 0.05). In conclusion, the results from this study suggest that triglyceride-derived fatty acid oxidation (VLDL or intramuscular triglycerides) plays a role in the increase in fat oxidation on a high-fat diet, but plasma-derived fatty acids remain the major source for fat oxidation.

Uncoupling protein 3 content is decreased in skeletal muscle of patients with type 2 diabetes.

Recently, a role for uncoupling protein-3 (UCP3) in carbohydrate metabolism and in type 2 diabetes has been suggested. Mice overexpressing UCP3 in skeletal muscle showed reduced fasting plasma glucose levels, improved glucose tolerance after an oral glucose load, and reduced fasting plasma insulin levels. However, data regarding the expression of UCP3 in patients with type 2 diabetes is inconsistent, and so far, there have been no reports of UCP3 protein content. Here we compared, for the first time, the protein levels of UCP3 in vastus lateralis muscle in 14 male type 2 diabetic patients (age 49.8 +/- 2.1 years; BMI 27.2 +/- 1.2 kg/m(2); mean +/- SE) with 16 male control subjects (age 48.0 +/- 1.9 years; BMI 23.4 +/- 0.6 kg/m(2)). We found that UCP3 protein levels were twice as low in patients with type 2 diabetes compared with control subjects (117 +/- 16 vs. 58 +/- 12 AU; P = 0.007). There was no correlation between UCP3 content and BMI. In conclusion, UCP3 content is lower in type 2 diabetic patients compared with healthy control subjects. These results are consistent with a role for UCP3 in glucose homeostasis and suggest a role for UCP3 in type 2 diabetes.