Acid Metabolism In Skeletal Muscle Research Articles

MiR-122/PPARβ axis is involved in hypoxic exercise and modulates fatty acid metabolism in skeletal muscle of obese rats

Hypoxic exercise is an effective intervention for obesity, because it promotes weight loss by regulating fatty acid (FA) metabolism. The regulation of peroxisome proliferator-activated receptor β (PPARβ) by miR-122 may be involved in this process, but the detailed mechanisms are unknown. In order to address this issue, we probed how miR-122 affected the expression of factors associated with FA metabolism in skeletal muscle of obese rats undergoing hypoxic training. By injecting adeno-associated virus 9 containing miR-122 overexpression vector or miR-122 inhibitor into skeletal muscles of rats with a 4-week hypoxic exercise regimen, the miR-122 expression level can be regulated. Body composition and blood lipid levels were analyzed, and PPARβ, carnitine palmitoyltransferase 1b (CPT1b), acetylCoA carboxylase 2 (ACC2), and FA synthase (FAS) mRNA and protein levels were evaluated using quantitative reverse transcription quantitative PCR(RT-qPCR) and Western blot analysis. We found that miR-122 overexpression increased low-density lipoprotein cholesterol (LDL-C) and triglyceride (TG) levels and decreased PPARβ, ACC2, and FAS expression. Conversely, miR-122 inhibition decreased TG level, increased high-density lipoprotein cholesterol (HDL-C) level, and upregulated PPARβ, ACC2, FAS, and CPT1b. These data indicated that the negative regulation of PPARβ by miR-122 promotes FA metabolism by altering the levels of the factors related to FA metabolism in skeletal muscle of obese rat under hypoxic training, thus providing molecular-level insight into the beneficial effects of this intervention.

Ketogenic propensity is differentially related to lipid-induced hepatic and peripheral insulin resistance.

Determine the ketogenic response (β-hydroxybutyrate, a surrogate of hepatic ketogenesis) to a controlled lipid overload in humans. In total, nineteen young, healthy adults (age: 28.4 ± 1.7 years; BMI: 22.7 ± 0.3 kg/m2 ) received either a 12 h overnight lipid infusion or saline in a randomized, crossover design. Plasma ketones and inflammatory markers were quantified by colorimetric and multiplex assays. Hepatic and peripheral insulin sensitivity was assessed by the hyperinsulinemic-euglycemic clamp. Skeletal muscle biopsies were obtained to quantify gene expression related to ketone body metabolism and inflammation. By design, the lipid overload-induced hepatic (50%, p < 0.001) and peripheral insulin resistance (73%, p < 0.01) in healthy adults. Ketones increased with hyperlipidemia and were subsequently reduced with hyperinsulinemia during the clamp procedure (Saline: Basal = 0.22 mM, Insulin = 0.07 mM; Lipid: Basal = 0.78 mM, Insulin = 0.51 mM; 2-way ANOVA: Lipid p < 0.001, Insulin p < 0.001, Interaction p = 0.07). In the saline control condition, ketones did not correlate with hepatic or peripheral insulin sensitivity. Conversely, in the lipid condition, ketones were positively correlated with hepatic insulin sensitivity (r = 0.59, p < 0.01), but inversely related to peripheral insulin sensitivity (r = -0.64, p < 0.01). Hyperlipidemia increased plasma inflammatory markers, but did not impact skeletal muscle inflammatory gene expression. Gene expression related to ketone and fatty acid metabolism in skeletal muscle increased in response to hyperlipidemia. This work provides important insight into the role of ketones in human health and suggests that ketone body metabolism is altered at the onset of lipid-induced insulin resistance.

1644-P: Exercise and Caloric Restriction Exert Different Benefits on Metabolism and Mechanical Function of Skeletal Muscle in Aging Condition

Exercise and caloric restriction improve skeletal muscle metabolism. However, the benefits of exercise and caloric restriction on metabolism and mechanical function of skeletal muscle in aging condition have never been compared. Seven-week-old male Wistar rats (n = 24) were divided into 4 groups (n = 6 per group) to receive subcutaneous injection with either 1) normal saline solution as a vehicle for 28 weeks, 2) 150 mg/kg/day of D-galactose [D-gal] for 28 weeks to induce premature aging, 3) D-gal for 28 weeks and exercise for 16 weeks (week 13-28), and 4) D-gal for 28 weeks and 30% caloric restriction for 16 weeks (week 13-28). The 17-month-old male Wistar rats (n = 6) were subcutaneously injected with the vehicle for 28 weeks as the naturally aged control. At the end of week 28, whole-body substrate oxidation during vigorous physical activity, total walking distance, and fitness level were determined. Then, all rats were sacrificed to collect the tibialis anterior muscle for protein expression analyses and mass spectrometry-based metabolomics study. We found that skeletal muscle of D-gal successfully mimicked that of natural aging. Exercise and caloric restriction equally improved whole-body carbohydrate oxidation during vigorous physical activity, myogenesis, and amino acid metabolism in skeletal muscle. However, exercise was superior to caloric restriction in terms of improving fatty acid oxidation, oxidative phosphorylation, and mechanical function of skeletal muscle. Interestingly, caloric restriction decreased oxidative stress, whereas exercise increased oxidative stress in skeletal muscle. These findings suggested that the benefits of exercise and caloric restriction on metabolism and mechanical function of skeletal muscle in aging were different. Hence, either combined exercise and caloric restriction, exercise with antioxidant supplement, or caloric restriction with fatty acid oxidation enhancement is highly recommended. Disclosure C.Thonusin: None. P.Pantiya: None. W.Nawara: None. B.Arunsak: None. S.Sriwichaiin: None. N.Chattipakorn: None. S.C.Chattipakorn: None.

Cold Exposure Affects Lipid Metabolism, Fatty Acids Composition and Transcription in Pig Skeletal Muscle.

Cold exposure promotes glucose oxidation and modulates the lipid metabolism in adipose tissue, but it is still not fully clear whether cold exposure could affect meat quality and fatty acid metabolism in skeletal muscle of pig in vivo. Here, we kept finishing pigs under cold or room temperature overnight and determined the effects of cold exposure on meat quality, fatty acids composition and transcriptional changes in skeletal muscle of pigs. We found that cold exposure significantly reduced the meat colour24 h and pH24 h, without affecting carcass characteristics and other meat quality traits. Considerable changes were found in the proportions of individual fatty acids and the total content of saturated fatty acid, polyunsaturated fatty acids, monounsaturated fatty acid and n3-fatty acids. RNA-seq results showed upregulated fatty acid biosynthesis genes and downregulated mitochondrial beta-oxidation genes. The lipid metabolism in cold-treated longissimus dorsi muscle might be regulated by functions of the lipoprotein particle, the extracellular matrix, and the PPAR signaling pathways. Our study revealed the potential of cold exposure to regulate the lipid metabolism and fatty acid composition in skeletal muscle of farmed animals.

498-P: Resistance Exercise Rapidly Alters Citric Acid Cycle and Amino Acid Metabolism in Skeletal Muscle

Muscle glucose metabolism is improved by either aerobic or resistance exercise (RE) training, but the mechanisms leading to improved metabolism for each of these exercise modes are not identical. Much work has been done onimproved oxidative metabolism by aerobic exercise, but far less is known about how RE impacts fuel metabolism. Here, TCA and amino acid metabolites are measured in skeletal muscle after a single bout of RE. Twenty one healthy subjects performed a session of one-legged knee extension RE that consisted of 3 sets of 10 reps at 70% of their 1 rep max. Two muscle biopsies were obtained from the vastus lateralis of each leg. From the RE leg, biopsies were obtained ≤10min post-RE (0hr PEX) and approximately 1hr post-RE (1hr PEX), respectively. From the non-RE leg, biopsies were obtained before and 1hr PEX, respectively. Muscle samples were frozen and stored until mass spectrometry analysis of metabolites was performed. A 2-fold increase in plasma lactate (P&amp;lt;0.05) and a 30-40% increase in muscle glycolytic enzyme activity (P&amp;lt;0.05) at 0hr PEX suggest increased activation of the glycolytic pathway. Also at 0hr PEX, TCA metabolites citrate and isocitrate were increased (P&amp;lt;0.05) compared to the pre-RE biopsy (0.35 and 0.01 mmol/μg, respectively). Further, multiple amino acid metabolites Arg, Asn, Lys, Thr, and Val were increased (P&amp;lt;0.05) in the REmuscle at 0hr PEX. These amino acids can enter the TCA cycle at various steps, indicating increased flux through the TCA cycle immediately after RE. By 1hr PEX, TCA intermediates malate and fumarate were reduced (P&amp;lt;0.05)compared to the non-RE leg (-0.65 and -0.35 mmol/μg, respectively), and multiple amino acid metabolites were also reduced (P&amp;lt;0.05). The decrease in these metabolites at 1hr PEX suggests increased use of metabolites for mitochondrial respiration. These results suggest activation of glycolytic and amino metabolism pathways, contributing to citric acid flux immediately following RE in order to meet energy needs in skeletal muscle. Disclosure M. W. Pataky: None. A. Prabha kumar: None. K. Klaus: None. K. Nair: None. Funding National Institutes of Health (R01AG062859)

Novel regulation of cardiac branched-chain amino acid metabolism through AMP deaminase: a possible therapeutic target for diabetic cardiomyopathy

Abstract Background Dysregulation of branched-chain amino acid (BCAA) metabolism has been shown to be associated with type 2 diabetes mellitus (T2DM) and heart failure. BCAA reportedly protects cells from fatty acid-induced mitochondrial injury via sequestration of fatty acids in intracellular lipid droplets from mitochondria. We previously reported that up-regulation of AMP deaminase 3 (AMPD3) impairs cardiac energetics in T2DM hearts, and AMPD3 was recently shown to participate in regulation of amino acid metabolism in skeletal muscle. Purpose We hypothesized that AMPD3 regulates cardiac amino acid metabolism by interaction with branched-chain α-ketoacid dehydrogenase (BCKDH) complex and that cardioprotective effect of sodium glucose cotransporter 2 inhibitors is mediated by modification of BCAA metabolism in diabetic cardiomyocytes. Methods and results Proteomic analyses of immunoprecipitates with an anti-AMPD3 antibody in rat hearts revealed that AMPD3 interacted with the E1α component of BCKDH complex. Whereas BCKDH has been reported to localize in mitochondria matrix as a rate-limiting enzyme for BCAA catabolism, immunoblotting using subcellular fractions revealed that BCKDH E1α is present in cytosol and endoplasmic reticulum as well. AMPD3-BCKDH E1α interaction was decreased by 68% in T2DM rats (OLETF) compared to that in control rats (LETO), and significant accumulation of BCAAs was observed in OLETF hearts (317±30 vs. 213±16 nmol/g). Survival rate at 48 hours after myocardial infarction (MI) was significantly lower in OLETF than in LETO (40% vs 84%). Empagliflozin treatment (10 mg/kg/day, 14 days) before MI improved the survival rate in OLETF to 70%, increased BCAAs as the top of 92 detected metabolites (791±187 nmol/g) and significantly preserved tissue ATP in the non-MI remote region. Electron microscopy showed a significantly higher prevalence of myocardium lipid droplets in OLETF, which was further increased by empagliflozin. Conclusions Results of the present analyses support the hypotheses that conversion of BCAA-derived branched-chain α-ketoacid to branched-chain acyl-CoA is suppressed by reduced AMPD3-BCKDH interaction in the myocardium of T2DM and that empagliflozin induces compensation of the dysregulated cardiac BCAA metabolism by augmentation of BCAA influx and promotion of fatty acid sequestration in intracellular lipid droplets. Funding Acknowledgement Type of funding source: Foundation. Main funding source(s): Boehringer Ingelheim

Editorial introductions

Editorial introductions

165-OR: High- and Moderate-Intensity Exercise Training Increased Skeletal Muscle Acylcarnitine and Phospholipid Abundance in Obese Adults

The primary aim of this study was to determine the impact of high-intensity interval training [HIIT] and moderate-intensity continuous training [MICT] on skeletal muscle lipidomic profile in obese adults. 17 obese adults (BMI = 34±3 kg·m-2; age = 31±6 years) completed 12 weeks of either HIIT (10 x 1 min @ 90%HRmax + 1 min recovery between intervals; n=8) or MICT (45 min steady-state exercise at 70% HRmax; n=9). Subjects exercised 4d/week and body weight was strictly maintained to assess the direct effects of exercise, independent of changes in body weight or fat mass. Skeletal muscle samples were collected from the vastus lateralis after an overnight fast both before training and 4 days after the last exercise session (to wash out the acute effects of exercise). Using shotgun lipidomics (LC MS/MS) we detected 734 lipid species from 26 different classes. Neither MICT nor HIIT significantly altered abundance of the primary muscle acylglycerides (tri- and di-acylglyceride) or ceramide abundance. Muscle acylcarnitine abundance increased after both MICT and HIIT (main effect, P = 0.04), specifically polyunsaturated acylcarnitines (P = 0.03). We also found phosphatidylcholine (PC) increased after training in both groups (main effect, P = 0.03), while the increase in phosphatidylethanolamine (PE) was greater after HIIT compared with MICT (P = 0.01). When normalized to cardiolipin (which is found almost exclusively in the inner mitochondrial membrane), the increased abundances of PC and PE were no longer apparent. Therefore, the training-induced increase in these phospholipids may largely reflect increased mitochondrial biosynthesis with training. In contrast, the observed increase in polyunsaturated acylcarnitines may represent an important adaptive response in the coordination of fatty acid metabolism in skeletal muscle after training. Overall, changes in muscle lipid profile were surprisingly similar in HIIT and MICT. Disclosure M.W. Schleh: None. B.J. Ryan: None. A. Ludzki: None. J.B. Gillen: None. J.F. Horowitz: None. Funding National Institutes of Health (R01DK077966, P30DK089503, F32DK117522, U24DK097153, UL1TR002240); Canadian Institutes of Health Research (DFS146190)

Exchange protein directly activated by cAMP (Epac) 1 plays an essential role in stress-induced exercise capacity by regulating PGC-1α and fatty acid metabolism in skeletal muscle.

Exchange protein directly activated by cAMP (Epac) mediates cAMP-mediated cell signal independent of protein kinase A (PKA). Mice lacking Epac1 displayed metabolic defect suggesting possible functional involvement of skeletal muscle and exercise capacity. Epac1 was highly expressed, but not Epac 2, in the extensor digitorum longus (EDL) and soleus muscles. The exercise significantly increased protein expression of Epac 1 in EDL and soleus muscle of wild-type (WT) mice. A global proteomics and pathway analyses revealed that Epac 1 deficiency mainly affected "the energy production and utilization" process in the skeletal muscle. We have tested their forced treadmill exercise tolerance. Epac1-/- mice exhibited significantly reduced exercise capacity in the forced treadmill exercise and lower number of type 1 fibers than WT mice. The basal protein level of proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α) was reduced in the Epac1-/- mice. Furthermore, increasing expression of PGC-1α by exercise was also significantly attenuated in the skeletal muscle of Epac1-/- mice. The expressions of downstream target genes of PGC-1α, which involved in uptake and oxidation of fatty acids, ERRα and PPARδ, and fatty acid content were lower in muscles of Epac1-/-, suggesting a role of Epac1 in forced treadmill exercise capacity by regulating PGC-1α pathway and lipid metabolism in skeletal muscle. Taken together, Epac1 plays an important role in exercise capacity by regulating PGC-1α and fatty acid metabolism in the skeletal muscle.

291-LB: Long Noncoding RNA H19 Improves Insulin Resistance in Skeletal Muscle by Regulating Heterogeneous Nuclear Ribonucleoprotein A1

Skeletal muscles play a pivotal role in glucose and lipid metabolism, and it has become a consensus that impaired lipid metabolism leads to ectopic lipid accumulation in skeletal muscles, which ultimately causes systematic insulin resistant. Previous reports have shown that down-regulation of long non-coding RNA(lncRNA) H19 in skeletal muscle impairs glucose metabolism. To better understand the impact of H19 on insulin resistance, here we focused on the relationships between H19 and fatty acid metabolism in skeletal muscle. In our research, decreased H19 expression was found in skeletal muscle of db/db mice compared with db/m mice. In vivo overexpression of H19 via injection of recombinant H19-adenovirus by tail-vein in db/db mice significantly improved glucose tolerance, lowered serum insulin, TG and skeletal muscle TG levels, which were consistent with improved skeletal muscle H&amp;E and oil red staining contrast to vehicle-treated db/db mice, indicating that overexpression of H19 in db/db mice ameliorated ectopic lipid accumulation in skeletal muscle. As expected, similar effects were observed in vitro. Overexpression of H19 in muscle cells treated with palmitate acid reduced lipid contents,promoted mitochondrial synthesis and ATP productions,and increased the expression of fatty acid oxidation related genes and mitochondrial respiratory chain complexes.Finally, it was drawn from RNA pull-down and RIP that heterogeneous nuclear ribonucleoprotein A1(hnRNPA1)is the target gene of H19, whose function in mitochondria has not been reported yet. Furthermore, we identified PGC1a and CPT1b were among the target genes of hnRNPA1 by searching widely used bioinformatics databases and preliminary RIP validation. Based on these results, we put forth our hypothesis that lncRNA H19 could improve insulin resistance by promoting mitochondrial biosynthesis and fatty acid oxidation in skeletal muscle, through regulating hnRNPA1. Disclosure X. Lin: None. W. Gui: None. H. Li: None. Funding National Natural Science Foundation of China (81873653)

Enhanced GLUT4-Dependent Glucose Transport Relieves Nutrient Stress in Obese Mice Through Changes in Lipid and Amino Acid Metabolism

Impaired GLUT4-dependent glucose uptake is a contributing factor in the development of whole-body insulin resistance in obese patients and obese animal models. Previously, we demonstrated that transgenic mice engineered to express the human GLUT4 gene under the control of the human GLUT4 promoter (i.e., transgenic [TG] mice) are resistant to obesity-induced insulin resistance. A likely mechanism underlying increased insulin sensitivity is increased glucose uptake in skeletal muscle. The purpose of this study was to investigate the broader metabolic consequences of enhanced glucose uptake into muscle. We observed that the expression of several nuclear and mitochondrially encoded mitochondrial enzymes was decreased in TG mice but that mitochondrial number, size, and fatty acid respiration rates were unchanged. Interestingly, both pyruvate and glutamate respiration rates were decreased in TG mice. Metabolomics analyses of skeletal muscle samples revealed that increased GLUT4 transgene expression was associated with decreased levels of some tricarboxylic acid intermediates and amino acids, whereas the levels of several glucogenic amino acids were elevated. Furthermore, fasting acyl carnitines in obese TG mice were decreased, indicating that increased GLUT4-dependent glucose flux decreases nutrient stress by altering lipid and amino acid metabolism in skeletal muscle.

Short-duration intermittent hypoxia enhances endurance capacity by improving muscle fatty acid metabolism in mice.

This study was designed to (1) investigate the effects of acute short‐duration intermittent hypoxia on muscle mRNA and microRNA expression levels; and (2) clarify the mechanisms by which short‐duration intermittent hypoxia improves endurance capacity. Experiment‐1: Male mice were subjected to either acute 1‐h hypoxia (12% O2), acute short‐duration intermittent hypoxia (12% O2 for 15 min, room air for 10 min, 4 times, Int‐Hypo), or acute endurance exercise (Ex). The expression of vascular endothelial growth factor‐A mRNA was significantly greater than the control at 0 h post Ex and 6 h post Int‐Hypo in the deep red region of the gastrocnemius muscle. miR‐16 expression levels were significantly lower at 6 and 10 h post Int‐Hypo. Peroxisome proliferator‐activated receptor gamma coactivator 1‐alpha (PGC‐1α) mRNA levels were significantly greater than the control at 3 h post Ex and 6 h post Int‐Hypo. miR‐23a expression levels were lower than the control at 6–24 h post Int‐Hypo. Experiment‐2: Mice were subjected to normoxic exercise training with or without intermittent hypoxia for 3 weeks. Increases in maximal exercise capacity were significantly greater by training with short‐duration intermittent hypoxia (IntTr) than without hypoxia. Both 3‐Hydroxyacyl‐CoA‐dehydrogenase and total carnitine palmitoyl transferase activities were significantly enhanced in IntTr. Peroxisome proliferator‐activated receptor delta and PGC‐1α mRNA levels were both significantly greater in IntTr than in the sedentary controls. These results suggest that exercise training under normoxic conditions with exposure to short‐duration intermittent hypoxia represents a beneficial strategy for increasing endurance performance by enhancing fatty acid metabolism in skeletal muscle.

Effect of different exercise protocols on metabolic profiles and fatty acid metabolism in skeletal muscle in high-fat diet-fed rats.

To evaluate the efficacy of mild-intensity endurance, high-intensity interval, and concurrent exercise on preventing high-fat diet-induced obesity. Male rats were divided into five groups, control diet/sedentary group, high-fat diet/sedentary, high-fat diet/endurance exercise, high-fat diet/interval exercise (HI), and high-fat diet/concurrent exercise. All exercise groups were made to exercise for 10 weeks, with matched running distances. Body weight, fat content, blood metabolites, quantitative insulin sensitivity check index (QUICKI), and adipocyte and liver lipid droplet size were assessed, and the expression of fatty acid metabolism-related genes was quantified. All exercise protocols reduced body weight, adiposity, serum triglycerides, and fasting glucose and also improved QUICKI to some extent. However, only HI prevented obesity and its associated pathologies completely. The expression of stearoyl-coenzyme A desaturase-1 was elevated in all rats fed a high-fat diet whereas carnitine palmitoyltransferase 1 (CPT1) expression was increased with exercise. Rev-erbα expression was elevated only in the HI group, which also had the highest level of CPT1 expression. The HI-induced increase in Rev-erbα and CPT1 expression was associated with the complete prevention of diet-induced obesity. Moreover, the increased caloric expenditure achieved with this protocol was preferential over other exercise regimens, and might be used to improve lipid metabolism.

SUMO-Specific Protease 2 (SENP2) Is an Important Regulator of Fatty Acid Metabolism in Skeletal Muscle.

Small ubiquitin-like modifier (SUMO)-specific proteases (SENPs) that reverse protein modification by SUMO are involved in the control of numerous cellular processes, including transcription, cell division, and cancer development. However, the physiological function of SENPs in energy metabolism remains unclear. Here, we investigated the role of SENP2 in fatty acid metabolism in C2C12 myotubes and in vivo. In C2C12 myotubes, treatment with saturated fatty acids, like palmitate, led to nuclear factor-κB–mediated increase in the expression of SENP2. This increase promoted the recruitment of peroxisome proliferator–activated receptor (PPAR)δ and PPARγ, through desumoylation of PPARs, to the promoters of the genes involved in fatty acid oxidation (FAO), such as carnitine-palmitoyl transferase-1 (CPT1b) and long-chain acyl-CoA synthetase 1 (ACSL1). In addition, SENP2 overexpression substantially increased FAO in C2C12 myotubes. Consistent with the cell culture system, muscle-specific SENP2 overexpression led to a marked increase in the mRNA levels of CPT1b and ACSL1 and thereby in FAO in the skeletal muscle, which ultimately alleviated high-fat diet–induced obesity and insulin resistance. Collectively, these data identify SENP2 as an important regulator of fatty acid metabolism in skeletal muscle and further implicate that muscle SENP2 could be a novel therapeutic target for the treatment of obesity-linked metabolic disorders.

Enhanced Glutamine Availability Exerts Different Effects on Protein and Amino Acid Metabolism in Skeletal Muscle From Healthy and Septic Rats.

Enhanced glutamine (GLN) intake may affect the catabolism of branched-chain amino acids (BCAAs; valine, leucine, and isoleucine), which play a regulatory role in protein turnover. We examined the effects of enhanced GLN availability on leucine oxidation, amino acid concentrations, and protein metabolism in muscles from healthy and septic rats. Cecal ligation and puncture were used as a model of sepsis. Twenty-four hours after surgery, the soleus (SOL, red muscle) and the extensor digitorum longus (EDL, white muscle) were incubated in medium containing 0.5 or 2.0 mM GLN. Protein breakdown, protein synthesis, and leucine oxidation were determined via 3-methylhistidine release, muscle L-[1-(14)C]leucine radioactivity, and the radioactivity of released (14)CO2, respectively. In muscles from septic animals, increased proteolysis and leucine oxidation and decreased protein synthesis were detected. These effects were more pronounced in the EDL. In septic muscles, the addition of GLN decreased leucine oxidation in both muscles and increased protein synthesis in the EDL. In muscles from untreated animals, decreased leucine oxidation after the addition of GLN to the medium was associated with decreased protein synthesis in the SOL and decreased concentrations of serine, glycine, histidine, alanine, arginine, proline, and lysine in both muscles. White muscle fibers are more sensitive to septic stimuli than red fibers are. In sepsis, enhanced GLN intake may ameliorate GLN deficiency, inhibit BCAA catabolism, and stimulate protein synthesis. In the healthy state, surplus of GLN may lead to severe alterations in the intramuscular concentration of several amino acids and impair protein synthesis.

Glutamine deficiency in extracellular fluid exerts adverse effects on protein and amino acid metabolism in skeletal muscle of healthy, laparotomized, and septic rats

Characteristic feature of critical illness, such as trauma and sepsis, is muscle wasting associated with activated oxidation of branched-chain amino acids (valine, leucine, isoleucine) and enhanced release of glutamine (GLN) to the blood. GLN consumption in visceral tissues frequently exceeds its release from muscle resulting in GLN deficiency that may exert adverse effects on the course of the disease. In the present study, we investigated the effects of GLN depletion in extracellular fluid on GLN production and protein and amino acid metabolism in skeletal muscle of healthy, laparotomized, and septic rats. Cecal ligation and puncture (CLP) was used as a model of sepsis. After 24 h, soleus muscle (SOL, slow-twitch, red muscle) and extensor digitorum longus (EDL, fast-twitch, white muscle) were isolated and incubated in a medium containing 0.5 mM GLN or without GLN. L-[1-(14)C]leucine was used to estimate protein synthesis and leucine oxidation, 3-methylhistidine release was used to evaluate myofibrillar protein breakdown. CLP increased GLN release from muscle, protein breakdown and leucine oxidation, and decreased protein synthesis. The effects were more pronounced in EDL. Alterations induced by laparotomy were similar to those observed in sepsis, but of a lower extent. GLN deficiency in medium enhanced GLN release and decreased intramuscular GLN concentration, decreased protein synthesis in muscles of intact and laparotomized rats, and enhanced leucine oxidation in SOL of intact and protein breakdown in SOL of laparotomized rats. It is concluded that (1) fast-twitch fibers are more sensitive to septic stimuli than slow-twitch fibers, (2) extracellular GLN deficiency may exert adverse effects on protein and amino acid metabolism in skeletal muscle, and (3) muscles of healthy and laparotomized animals are more sensitive to GLN deficiency than muscles of septic animals.

First-Degree Relatives of Type 2 Diabetic Patients Have Reduced Expression of Genes Involved in Fatty Acid Metabolism in Skeletal Muscle

First-degree relatives of patients with type 2 diabetes (FH+) have been shown to have decreased energy expenditure and decreased expression of mitochondrial genes in skeletal muscle. In previous studies, it has been difficult to distinguish whether mitochondrial dysfunction and differential regulation of genes are primary (genetic) or due to reduced physical activity, obesity, or other correlated factors. The aim of this study was to investigate whether mitochondrial dysfunction is a primary defect or results from an altered metabolic state. We compared gene expression in skeletal muscle from 24 male subjects with FH and 26 without FH matched for age, glucose tolerance, VO(2peak) (peak oxygen uptake), and body mass index using microarrays. Additionally, type fiber composition, mitochondrial DNA content, and citrate synthase activity were measured. The results were followed up in an additional cohort with measurements of in vivo metabolism. FH+ vs. FH- subjects showed reduced expression of mitochondrial genes (P = 2.75 × 10(-6)), particularly genes involved in fatty acid metabolism (P = 4.08 × 10(-7)), despite similar mitochondrial DNA content. Strikingly, a 70% reduced expression of the monoamine oxidase A (MAOA) gene was found in FH+ vs. FH- individuals (P = 0.0009). Down-regulation of the genes involved in fat metabolism was associated with decreased in vivo fat oxidation and increased glucose oxidation examined in an additional cohort of elderly men. These results suggest that genetically altered fatty acid metabolism predisposes to type 2 diabetes and propose a role for catecholamine-metabolizing enzymes like MAOA in the regulation of energy metabolism.

Regulatory role of leptin in glucose and lipid metabolism in skeletal muscle

Leptin is a hormone secreted by adipocytes that plays a pivotal role in regulation of food intake, energy expenditure, and neuroendocrine function. Several lines of evidences indicate that independent of the anorexic effect, leptin regulates glucose and lipid metabolism in peripheral tissues in rodents and humans. It has been shown that leptin improves the diabetes phenotype in lipodystrophic patients and rodents. Moreover, leptin suppresses the development of severe, progressive impairment of glucose metabolism in insulin-deficient diabetes in rodents. We found that leptin increases glucose uptake and fatty acid oxidation in skeletal muscle in rats and mice in vivo. Leptin increases glucose uptake in skeletal muscle via the hypothalamic–sympathetic nervous system axis and β-adrenergic mechanism, while leptin stimulates fatty acid oxidation in muscle via AMP-activated protein kinase (AMPK). Leptin-induced fatty acid oxidation results in the decrease of lipid accumulation in muscle, which can lead to functional impairments called as “lipotoxicity.” Activation of AMPK occurs by direct action of leptin on muscle and through the medial hypothalamus–sympathetic nervous system and α-adrenergic mechanism. Thus, leptin plays an important role in the regulation of glucose and fatty acid metabolism in skeletal muscle.

Enzyme Catalytic Activities in Chronic Obstructive Pulmonary Disease

Altered muscle amino acid metabolism resulting in skeletal muscle dysfunction is one of the systemic effects of chronic obstructive pulmonary disease (COPD) associated with systemic oxidative stress and inflammation. The aim of the study was to investigate the existence and extent of changes in the activities of the enzymes catalyzing transamination reactions (aminotransferases), the enzyme involved in bone rearrangement (alkaline phosphatase), and the enzyme reflecting hypoxia that is characteristic of these patients (lactate dehydrogenase). In addition, the effect of cigarette smoking on these enzyme activities was also assessed. Enzyme activities such as alanine aminotransferase, aspartate aminotransferase, alkaline phosphatase, gamma-glutamyltransferase and lactate dehydrogenase were determined by standard analysis in sera of 29 COPD patients (FEV(1) = 46.6 +/- 12.1%) and 58 healthy subjects (21 nonsmokers, 17 ex-smokers and 20 smokers). The activity of aspartate aminotransferase and alanine aminotransferase was significantly decreased, and the activity of lactate dehydrogenase increased in sera of COPD patients as compared with the group of healthy nonsmokers. According to centile values, the activity of alkaline phosphatase, gamma-glutamyltransferase and lactate dehydrogenase was increased in 50, 5, and 50% of COPD patients, respectively. Study results revealed significant changes in the activities of transamination enzymes in patient sera, thus supporting the reports on altered amino acid metabolism in skeletal muscle in COPD. The elevated activity of alkaline phosphatase provides additional evidence for altered bone rearrangement in these patients. Smoking was not found to have any major effect on these enzyme activities in the present study.

Intracellular Fatty Acid Metabolism in Skeletal Muscle and Insulin Resistance

Triglyceride accumulation in skeletal muscle is increased in subjects with insulin resistance. Increased intracellular lipolysis from stored triglyceride may induce insulin resistance in skeletal muscle by activating the glucose-fatty acid cycle. However, inconsistent with this hypothesis, intracellular lipolysis from skeletal muscle is decreased in high fat-fed, insulin resistant rats. Therefore, it is suggested that an increase in triglyceride accumulation is the result of decreased mitochondrial fatty acid oxidation in the cells. As evidence, fenofibrate (a PPARalpha activator), rosiglitazone (a PPARgamma activator) and alpha-lipoic acid completely prevented the development of diabetes in obese diabetes-prone rats. All three drugs increased fatty acid oxidation and decreased triglyceride accumulation in skeletal muscle. Administration of ALA activated AMPK and increased fatty acid oxidation. It is suggested that decreased fatty acid oxidation in skeletal muscle is one of the major factors leading to an accumulation of lipid metabolites and insulin resistance.