Intramyocellular Triacylglycerol Research Articles

Intramyocellular triacylglycerol accumulation across weight loss strategies; Sub-study of the CENTRAL trial.

BackgroundIntramyocellular triacylglycerol (IMTG) is utilized as metabolic fuel during exercise and is linked to insulin resistance, but the long-term effect of weight loss strategies on IMTG among participants with abdominal fat, remain unclear.MethodsIn an 18-month trial, sedentary participants with abdominal fat/dyslipidemia were randomized to either a low-fat (LF) or Mediterranean/low-carbohydrate (MED/LC) diet (including 28g·day-1 of walnuts). After 6-months, the participants were re-randomized to moderate intense physical activity (PA+) or non-physical activity (PA-). Magnetic resonance imaging (MRI) was used to quantify changes of IMTG, abdominal sub-depots, hepatic and intermuscular fats.ResultsAcross the 277 participants [86% men, age = 48 years, body-mass-index (BMI) = 31kg/m2, visceral fat = 33%] 86% completed the 18-m trial. At baseline, women had higher IMTG than men (3.4% vs. 2.3%, p<0.001) and increased IMTG was associated with aging and higher BMI, visceral and intermuscular fats, HbA1c%, HDL-c and leptin(p<0.05), but not with intra-hepatic fat. After 18 month of intervention and a -3 kg mean weight loss, participants significantly increased IMTG by 25%, with a distinct effect in the MED/LCPA+ group as compared to the other intervention groups (57% vs. 9.5–18.5%, p<0.05). Changes in IMTG were associated with visceral and intermuscular fat, metabolic syndrome, insulin and leptin (p<0.05 for all), however, these associations did not remain after adjustment for visceral fat changes.ConclusionsLifestyle strategies differentially affect IMTG accumulation; combination of exercise with decreased carbohydrate/increased unsaturated fat proportion intake greatly increase IMTG. Our findings suggest that increased IMTG during diet-induced moderate weight loss may not be directly related to cardiometabolic risk.Trial registrationClinicalTrials.gov NCT01530724

Calcitriol concomitantly enhances insulin sensitivity and alters myocellular lipid partitioning in high fat-treated skeletal muscle cells.

Vitamin D reduces myocellular insulin resistance, but the effects of vitamin D on intramyocellular lipid (IMCL) partitioning are unknown. The purpose of this study was to understand how calcitriol, the active vitamin D metabolite, affects insulin sensitivity and lipid partitioning in skeletal muscle cells. C2C12 myotubes were treated with calcitriol (100nM) or vehicle control for 96h. Insulin-stimulated Akt phosphorylation (Thr 308) was determined by western blot. Intramyocellular triacylglycerol (IMTG), diacylglycerol (DAG), and ceramide content were measured by LC/MS. IMTG partitioning and lipid droplet accumulation were assessed by oil red O. Expression of genes involved in lipid droplet packaging and lipolysis were measured by RT-PCR. Compared to vehicle-treated myotubes, calcitriol augmented insulin-stimulated pAkt. Calcitriol increased total ceramides and DAG in a subspecies-specific manner. Specifically, calcitriol preferentially increased ceramide 24:1 (1.78 fold) and di-18:0 DAG (46.89 fold). Calcitriol increased total IMTG area as assessed by oil red O, but decreased the proportion of lipid within myotubes. Calcitriol increased mRNA content of genes involved in lipid droplet packaging (perilipin 2; PLIN 2, 2.07 fold) and lipolysis (comparative gene identification-58; CGI-58 and adipose triglyceride lipase; ATGL, ~1.80 fold). Calcitriol alters myocellular lipid partitioning and lipid droplet packaging which may favor lipid turnover and partially explain improvements in insulin sensitivity.

Opposite Regulation of Insulin Sensitivity by Dietary Lipid Versus Carbohydrate Excess.

To understand the mechanisms in lipid-induced insulin resistance, a more physiological approach is to enhance fatty acid (FA) availability through the diet. Nine healthy men ingested two hypercaloric diets (in 75% excess of habitual caloric intake) for 3 days, enriched in unsaturated FA (78 energy % [E%] fat) (UNSAT) or carbohydrates (80 E% carbohydrate) (CHO) as well as a eucaloric control diet (CON). Compared with CON, the UNSAT diet reduced whole-body and leg glucose disposal during a hyperinsulinemic-euglycemic clamp, while decreasing hepatic glucose production. In muscle, diacylglycerol (DAG) and intramyocellular triacylglycerol were increased. The accumulated DAG was sn-1,3 DAG, which is known not to activate PKC, and insulin signaling was intact. UNSAT decreased PDH-E1α protein content and increased inhibitory PDH-E1α Ser300 phosphorylation and FA oxidation. CHO increased whole-body and leg insulin sensitivity, while increasing hepatic glucose production. After CHO, muscle PDH-E1α Ser300 phosphorylation was decreased, and glucose oxidation increased. After UNSAT, but not CHO, muscle glucose-6-phosphate content was 103% higher compared with CON during the clamp. Thus, PDH-E1α expression and covalent regulation, and hence the tricarboxylic acid cycle influx of pyruvate-derived acetyl-CoA relative to β-oxidation-derived acetyl-CoA, are suggested to impact on insulin-stimulated glucose uptake. Taken together, the oxidative metabolic fluxes of glucose and FA are powerful and opposite regulators of insulin action in muscle.

Impact of Endurance Exercise Training in the Fasted State on Muscle Biochemistry and Metabolism in Healthy Subjects: Can These Effects be of Particular Clinical Benefit to Type2 Diabetes Mellitus and Insulin-Resistant Patients?

Exercise trainingintervention is a cornerstone in the care of type2 diabetes mellitus (T2DM) and insulin resistance (IR), and it is pursued in order to optimize exercise interventions for these patients. In this regard, the nutritional state of patients during exercise (being in the fed or fasted state) can be of particular interest. The aim of the present review is to describe the impact of endurance exercise (training) in the fasted versus fed state on parameters of muscle biochemistry and metabolism linked to glycemic control or insulin sensitivity in healthy subjects. From these data it can then be deduced whether exercise training in the fasted state may be relevant to patients with T2DM or IR. In healthy subjects, acute endurance exercise in the fasted state is accompanied by lower blood insulin and elevated blood free fatty acid concentrations, stable blood glucose concentrations (in the first 60-90min), superior intramyocellular triacylglycerol oxidation and whole-body lipolysis, and muscle glycogen preservation. Long-term exercise training in the fasted state in healthy subjects is associated with greater improvements in insulin sensitivity, basal muscle fat uptake capacity, and oxidation. Therefore, promising results of exercise (training) in the fasted state have been found in healthy subjects on parameters of muscle biochemistry and metabolism linked to insulin sensitivity and glycemic control. Whether exercise training intervention in which exercise sessions are organized in the fasted state may be more effective in improving insulin sensitivity or glycemic control in T2DM patients and insulin-resistant individuals warrants investigation.

Extremely rapid increase in fatty acid transport and intramyocellular lipid accumulation but markedly delayed insulin resistance after high fat feeding in rats

The mechanisms for diet-induced intramyocellular lipid accumulation and its association with insulin resistance remain contentious. In a detailed time-course study in rats, we examined whether a high-fat diet increased intramyocellular lipid accumulation via alterations in fatty acid translocase (FAT/CD36)-mediated fatty acid transport, selected enzymes and/or fatty acid oxidation, and whether intramyocellular lipid accretion coincided with the onset of insulin resistance. We measured, daily (on days 1-7) and/or weekly (for 6 weeks), the diet-induced changes in circulating substrates, insulin, sarcolemmal substrate transporters and transport, selected enzymes, intramyocellular lipids, mitochondrial fatty acid oxidation and basal and insulin-stimulated sarcolemmal GLUT4 and glucose transport. We also examined whether upregulating fatty acid oxidation improved glucose transport in insulin-resistant muscles. Finally, in Cd36-knockout mice, we examined the role of FAT/CD36 in intramyocellular lipid accumulation, insulin sensitivity and diet-induced glucose intolerance. Within 2-3 days, diet-induced increases occurred in insulin, sarcolemmal FAT/CD36 (but not fatty acid binding protein [FABPpm] or fatty acid transporter [FATP]1 or 4), fatty acid transport and intramyocellular triacylglycerol, diacylglycerol and ceramide, independent of enzymatic changes or muscle fatty acid oxidation. Diet-induced increases in mitochondria and mitochondrial fatty acid oxidation and impairments in insulin-stimulated glucose transport and GLUT4 translocation occurred much later (≥21 days). FAT/CD36 ablation impaired insulin-stimulated fatty acid transport and lipid accumulation, improved insulin sensitivity and prevented diet-induced glucose intolerance. Increasing fatty acid oxidation in insulin-resistant muscles improved glucose transport. High-fat feeding rapidly increases intramyocellular lipids (in 2-3 days) via insulin-mediated upregulation of sarcolemmal FAT/CD36 and fatty acid transport. The 16-19 day delay in the onset of insulin resistance suggests that additional mechanisms besides intramyocellular lipids contribute to this pathology.

Mechanisms Of Insulin Resistance After 24 Hours Physical Inactivity In Murine Soleus Muscle.

BACKGROUND AND AIMS: Long term physical inactivity induces obesity and lead to insulin resistance. We have recently demonstrated that only 24 hours physical inactivity by leg immobilization induces insulin resistance in murine skeletal muscle. To elucidate its mechanism, we performed hind-limb cast immobilization (HCI) for 24 hours in C57BL6J mice and investigated ex-vivo insulin-stimulated 2-deoxy glucose uptake, insulin signal transduction and lipid composition in soleus muscle. RESULTS: Twenty-four hours HCI significantly decreased insulin-stimulated 2-deoxy glucose uptake by 41%. In parallel with this change, we observed decreased insulin stimulated serine phosphorylation of Akt and tyrosine phosphorylation of insulin receptor substrate (IRS)-1 after 24h HCI. It has been hypothesized that intramyocellular diacylglycerol (IMDG) accumulation and PKC activation impairs insulin signal transduction. Consistent with this hypothesis, we found that 24h HCI dramatically increased IMDG by 192%, while the amount of intramyocellular triacylglycerol (IMTG) was not changed. In addition, we also observed increase of PKCe translocation to the plasma membrane after 24 HCI. Associated with the IMDG accumulation, expression level of diacylglycerol acyltransferase (DGAT)-1, which converts diacylglycerol to triacylglycerol, was decreased by 53%, and activity of Lipin1, which converts phosphatidic acid to diacylglycerol, was increased by 141% after 24h HCI. Finally, overexpression of dominant negative Lipin1 in soleus muscle partially inhibited diacylglycerol accumulation after 24h HCI. CONCLUSION: These data suggested that 24h HCI induce insulin resistance through IMDG accumulation and trans-location of PKCe to the plasma membrane in skeletal muscle. Given that increased Lipin1 activity and decreased DGAT-1 expression might be involved in the mechanism of IMDG accumulation, suppression of lipin1 can be a drug target for insulin resistance induced by physical inactivity.

Intramyocellular Triacylglycerol is Associated with Peroxisomal Biogenesis in Skeletal Muscle from Lean and Obese Humans

Obesity is associated with elevated levels of lipids (intramyocellular triacylglycerols; IMTG) and reductions in mitochondrial fatty acid oxidation in skeletal muscle, both which are associated with insulin resistance. Known predominately for their actions in liver, peroxisomes are subcellular compartments essential for lipid disposal by chain‐shortening very long‐ and long‐chain fatty acids to acyl‐carnitines, permitting CPT‐1 independent entry into the mitochondria for oxidation. We hypothesized that peroxisome proliferation in human skeletal muscle occurs as an adaptive response to elevated lipid supply and accumulation (IMTG) which could facilitate mitochondrial fatty acid oxidation. Assays for the peroxisomal biogenesis marker PEX19 protein content and IMTG were assessed using vastus lateralis from obese and lean humans. Peroxisomal membrane protein 70 (PMP70) was measured after lipid incubation of human skeletal muscle primary myotubes. Results: 1. A significant relationship between tissue IMTG and PEX19 was identified in human skeletal muscle tissue (P&lt;0.05) but not % body fat or BMI 2) PEX19 protein content was elevated (P&lt;0.05) in tissues from obese individuals with high IMTG and 3) An increase in peroxisomal PMP70 was observed (P&lt;0.05) with high fat incubation of human primary myotubes.ConclusionThis is the first report associating lipid oversupply, IMTG accumulation, and peroxisomal biogenesis in human skeletal muscle suggesting a functional‐adaptive role for peroxisomes in mitochondrial lipid oxidation and obesity.

Adaptations in lipid metabolism in adipose tissue induced by high intensity training

This study investigated the effects of high intensity training on metabolic regulation of lipid metabolism. Male Wistar were randomized to the controlor training group. Progressive tests of running speed were performed in order to determine the maximal velocity (Vmax) and total time exercise (TTF). On five consecutive days, during ten weeks, the rats performed 60 min of exercise at 75%Vmax. PPARα and CPT1 mRNA expression were determined by qRT-PCR. AMPKp, ACC, ATGL and CD36 protein quantities were determined by western blotting. Intramyocellular triacylglycerol (IMTG) was extracted. Citrate synthase activity (CSA) was analyzed. Data were analyzed: unpaired Student's t-test (P<0.05). The training protocol increased TTF on the trained group and this response was followed by reduction on adiposity index, without changes in body weight. The trained group showed reduction in IMTG and increase in CSA in muscle tissue. It was observed increase in ATGL and CD36 in muscle and adipose tissues and no changes in PPARα and AMPKp, however CPT1 was reduced in adipose tissue, while, it was increased in muscle tissue in trained rats. ACC in adipose tissue was increased in trained rats, but in muscle tissue it was not different. The high intensity training promotes changes in lipid metabolism in adipose tissue and these responses could improve the energy supply to active muscle. Supported: CNPq

Perilipin 5-Driven Lipid Droplet Accumulation in Skeletal Muscle Stimulates the Expression of Fibroblast Growth Factor 21.

Perilipin 5 (PLIN5) is a lipid droplet protein and is highly expressed in oxidative tissue. Expression of the PLIN5 gene is regulated by peroxisome proliferator–activated receptor-α, fasting, and exercise. However, the effect of increased muscle PLIN5 expression on whole-body energy homeostasis remains unclear. To examine this, we developed a mouse line with skeletal muscle PLIN5 overexpression (MCK-Plin5). We show that MCK-Plin5 mice have increased energy metabolism and accumulate more intramyocellular triacylglycerol but have normal glucose and insulin tolerance. MCK-Plin5 mice fed high-fat chow manifest lower expression of inflammatory markers in their liver and increased expression of “browning” factors in adipose tissue. This muscle-driven phenotype is, at least in part, mediated by myokines; the MCK-Plin5 mice have 80-fold higher FGF21 gene expression in muscle and increased serum FGF21 concentration. The increase in FGF21 occurs mainly in muscles with a predominance of fast-twitch fibers, suggesting that fiber type–specific lipid storage may be part of the mechanism conferring metabolic protection in MCK-Plin5 mice. In conclusion, upregulating the PLIN5 level in skeletal muscle drives expression of the FGF21 gene in fast-twitch fibers and is metabolically protective. These findings provide insight into the physiology of PLIN5 and the potential contribution of its upregulation during exercise.

Adipose triglyceride lipase deletion from adipocytes, but not skeletal myocytes, impairs acute exercise performance in mice.

Adipose triglyceride lipase (ATGL) is the rate-limiting enzyme mediating triacylglycerol hydrolysis in virtually all cells, including adipocytes and skeletal myocytes, and hence, plays a critical role in mobilizing fatty acids. Global ATGL deficiency promotes skeletal myopathy and exercise intolerance in mice and humans, and yet the tissue-specific contributions to these phenotypes remain unknown. The goal of this study was to determine the relative contribution of ATGL-mediated triacylglycerol hydrolysis in adipocytes vs. skeletal myocytes to acute exercise performance. To achieve this goal, we generated murine models with adipocyte- and skeletal myocyte-specific targeted deletion of ATGL. We then subjected untrained mice to acute peak and submaximal exercise interventions and assessed exercise performance and energy substrate metabolism. Impaired ATGL-mediated lipolysis within adipocytes reduced peak and submaximal exercise performance, reduced peripheral energy substrate availability, shifted energy substrate preference toward carbohydrate oxidation, and decreased HSL Ser(660) phosphorylation and mitochondrial respiration within skeletal muscle. In contrast, impaired ATGL-mediated lipolysis within skeletal myocytes was not sufficient to reduce peak and submaximal exercise performance or peripheral energy substrate availability and instead tended to enhance metabolic flexibility during peak exercise. Furthermore, the expanded intramyocellular triacylglycerol pool in these mice was reduced following exercise in association with preserved HSL phosphorylation, suggesting that HSL may compensate for impaired ATGL action in skeletal muscle during exercise. These data suggest that adipocyte rather than skeletal myocyte ATGL-mediated lipolysis plays a greater role during acute exercise in part because of compensatory mechanisms that maintain lipolysis in muscle, but not adipose tissue, when ATGL is absent.

Novel regulatory mechanisms in muscle metabolism during exercise.

Novel regulatory mechanisms in muscle metabolism during exercise.

Short-term muscle disuse atrophy is not associated with increased intramuscular lipid deposition or a decline in the maximal activity of key mitochondrial enzymes in young and older males

Aging is generally accompanied by a progressive loss of skeletal muscle mass and impairments in metabolic function. Even a few days of muscle disuse (such as that occurring during injury or illness) leads to considerable loss of muscle mass and strength. It has been speculated that short, successive periods of muscle disuse throughout the lifespan may be largely responsible for the age-related loss of muscle mass. However, it remains unknown whether such short periods of disuse also induce impairments in metabolic function within skeletal muscle. Here, we investigated the effects of a five day period of muscle disuse on intramyocellular triacylglycerol (IMTG) content, muscle oxidative capacity, and the expression of key genes that regulate oxidative metabolism in healthy young and elderly men. Muscle biopsies were collected from healthy, young (n=12; 23±1y) and elderly (n=12; 70±1y) men prior to and immediately after a five day period of one-legged knee immobilization by way of a full leg cast. At baseline, elderly men had a greater IMTG content when compared with the young (56.2±5.1 and 34.8±7.3μmol·g−1, respectively; P<0.05) with no changes in either group following immobilization (53.4±5.0 and 35.7±5.0μmol·g−1, respectively; P>0.05). In line, five days of disuse did not lower citrate synthase, β-HAD or cytochrome C oxidase activity in skeletal muscle tissue. Pyruvate dehydrogenase activity increased following immobilization in the older subjects only, from 0.39±0.06 to 0.55 0.05μmol·g−1·min−1 (71±33%; P<0.01). The skeletal muscle mRNA expression of PGC1α and citrate synthase both declined following immobilization in both the young and elderly subjects. We conclude that five days of muscle disuse does not increase intramuscular lipid deposition or reduce the maximal activity of key mitochondrial enzymes within the skeletal muscle of young or older men.

Effect of inhaled terbutaline on substrate utilization and 300-kcal time trial performance.

In a randomized, double-blind crossover design, we investigated the effect of the beta2-agonist terbutaline (TER) on endurance performance and substrate utilization in nine moderately trained men [maximum oxygen uptake (V̇O(2 max)) 58.9 ± 3.1 ml·min(-1)·kg(-1)]. Subjects performed 60 min of submaximal exercise (65-70% of V̇O(2 max)) immediately followed by a 300-kcal time trial with inhalation of either 15 mg of TER or placebo (PLA). Pulmonary gas exchange was measured during the submaximal exercise, and muscle biopsies were collected before and after the exercise bouts. Time trial performance was not different between TER and PLA (1,072 ± 145 vs. 1,054 ± 125 s). During the submaximal exercise, respiratory exchange ratio, glycogen breakdown (TER 266 ± 32, PLA 195 ± 28 mmol/kg dw), and muscle lactate accumulation (TER 20.3 ± 1.6, PLA 13.2 ± 1.2 mmol/kg dw) were higher (P < 0.05) with TER than PLA. There was no difference between TER and PLA in net muscle glycogen utilization or lactate accumulation during the time trial. Intramyocellular triacylglycerol content did not change with treatment or exercise. Pyruvate dehydrogenase-E1α phosphorylation at Ser(293) and Ser(300) was lower (P < 0.05) before submaximal exercise with TER than PLA, with no difference after the submaximal exercise and the time trial. Before submaximal exercise, acetyl-CoA carboxylase 2 (ACC2) phosphorylation at Ser(221) was higher (P < 0.05) with TER than PLA. There was no difference in phosphorylation of alpha 5'-AMP-activated protein kinase (αAMPK) at Thr(172) between treatments. The present study suggests that beta2-agonists do not enhance 300-kcal time trial performance, but they increase carbohydrate metabolism in skeletal muscles during submaximal exercise independent of AMPK and ACC phosphorylation, and that this effect diminishes as drug exposure time, exercise duration, and intensity are increased.

PLIN5 deletion remodels intracellular lipid composition and causes insulin resistance in muscle.

Defective control of lipid metabolism leading to lipotoxicity causes insulin resistance in skeletal muscle, a major factor leading to diabetes. Here, we demonstrate that perilipin (PLIN) 5 is required to couple intramyocellular triacylglycerol lipolysis with the metabolic demand for fatty acids. PLIN5 ablation depleted triacylglycerol stores but increased sphingolipids including ceramide, hydroxylceramides and sphingomyelin. We generated perilipin 5 (Plin5)−/− mice to determine the functional significance of PLIN5 in metabolic control and insulin action. Loss of PLIN5 had no effect on body weight, feeding or adiposity but increased whole-body carbohydrate oxidation. Plin5−/− mice developed skeletal muscle insulin resistance, which was associated with ceramide accumulation. Liver insulin sensitivity was improved in Plin5−/− mice, indicating tissue-specific effects of PLIN5 on insulin action. We conclude that PLIN5 plays a critical role in coordinating skeletal muscle triacylglycerol metabolism, which impacts sphingolipid metabolism, and is requisite for the maintenance of skeletal muscle insulin action.

Relatively low endogenous fatty acid mobilization and uptake helps preserve insulin sensitivity in obese women.

BackgroundAlthough obesity is commonly linked with metabolic disease risk, some obese adults do not develop metabolic abnormalities, such as insulin resistance.ObjectivesThe primary aim of this study was to determine whether alterations in fatty acid mobilization and uptake underlie differences in insulin sensitivity (Si) among a seemingly homogeneous cohort of obese women.MethodsInsulin sensitivity (FSIVGTT), basal fatty acid rate of disappearance from plasma (Rd), resting whole-body fat oxidation, intramyocellular triacylglycerol (IMTG) concentration, and markers of skeletal muscle inflammation were measured in 21 obese women. Participants were divided into tertiles based on their Si. The subset of participants with the lowest Si (LOW-Si; Si ≤2.1 (mU/L)−1·min−1; n=7) was compared with the subset of participants with the highest Si, who exhibited relatively normal insulin sensitivity (NORM-Si; Si ≥3.4 (mU/L)−1·min−1; n=8).ResultsDespite nearly identical physical characteristics in LOW-Si vs. NORM-Si (BMI: 34±2 vs. 34±1 kg/m2; %body fat: 48±1% vs. 47±1%; waist circumference: 104±2 vs. 104±2 cm; VO2max: 2.2±0.2 vs. 2.3±0.1 L/min), fatty acid Rd was nearly 30% lower in NORM (P=0.02). Importantly, the greater rate of fatty acid uptake in LOW-Si vs. NORMSi did not translate to higher rate of fat oxidation (3.5±0.2 vs. 3.7±0.2 μmol/kg/min) or to a measureable difference in IMTG content, (68.3±12.7 vs. 63.7±6.7 μmol/g dry weight). In conjunction with the lower fatty acid Rd in NORM-Si vs. LOW-Si, activation of inflammatory pathways known to impair insulin action in skeletal muscle was also lower (i.e. lower phosphorylated JNK, higher IκBα abundance). In contrast, LOW-Si and NORM-Si exhibited no differences in plasma markers of inflammation (i.e. TNFα, IL-6, MCP-1).ConclusionThese findings suggest that obese women who maintain a relatively low rate of endogenous fatty acid uptake may be somewhat “protected” against the development of insulin resistance potentially by less activation of inflammatory pathways within skeletal muscle.

Short‐term muscle disuse atrophy is not associated with increased skeletal muscle lipid accumulation or impaired oxidative enzyme activity in young or old men (863.1)

Aging is accompanied by a loss of skeletal muscle mass (sarcopenia) and metabolic function. A period of disuse (e.g. injury/illness) also causes muscle atrophy. The accumulation of periods of disuse throughout the lifespan has been proposed as a key factor in the development of sarcopenia. However, it is unknown whether brief periods of disuse also impair muscle metabolic function. We investigated the effects of 5 days of limb immobilization in healthy young (n=12; 23±1 y) and old (n=70±1 y) men on intramyocellular triacylglycerol (IMTG) content, the maximal activity and activation status of selected mitochondrial enzymes and the mRNA expression of genes associated with mitochondrial biogenesis, in muscle biopsies taken before and after disuse. At baseline, IMTG content was greater in old compared with young men (56.3±6.8 and 34.8±7.3 µmol.g‐1, respectively; P&lt;0.05) but no changes were observed following immobilization. Disuse did not reduce muscle citrate synthase, β‐HAD or cytochrome C oxidase activity. However, muscle pyruvate dehydrogenase complex activation status increased following immobilization in the elderly men. The muscle mRNA expression of PGC1α and citrate synthase declined following immobilization in both groups. We conclude that 5 days of muscle disuse is not accompanied by a net increase in muscle lipid deposition or a decline in muscle oxidative capacity in young or elderly men.

Comment on: Sitnick et al. Skeletal Muscle Triacylglycerol Hydrolysis Does Not Influence Metabolic Complications of Obesity. Diabetes 2013;62:3350–3361

Sitnick et al. (1) provide compelling evidence that skeletal muscle–specific modulations of adipose triglyceride lipase (ATGL) expression in vivo in mice impacts intramyocellular triacylglycerol (IMTG) content with no measurable effects on other lipid species (diacylglycerols [DAG], ceramides, and long-chain acyl-coenzyme A) and insulin sensitivity. The authors speculate that the fatty acid (FA) cycle observed in cardiac muscle, with FA largely trafficking through IMTG before …

Skeletal Muscle Triacylglycerol Hydrolysis Does Not Influence Metabolic Complications of Obesity

Intramyocellular triacylglycerol (IMTG) accumulation is highly associated with insulin resistance and metabolic complications of obesity (lipotoxicity), whereas comparable IMTG accumulation in endurance-trained athletes is associated with insulin sensitivity (the athlete’s paradox). Despite these findings, it remains unclear whether changes in IMTG accumulation and metabolism per se influence muscle-specific and systemic metabolic homeostasis and insulin responsiveness. By mediating the rate-limiting step in triacylglycerol hydrolysis, adipose triglyceride lipase (ATGL) has been proposed to influence the storage/production of deleterious as well as essential lipid metabolites. However, the physiological relevance of ATGL-mediated triacylglycerol hydrolysis in skeletal muscle remains unknown. To determine the contribution of IMTG hydrolysis to tissue-specific and systemic metabolic phenotypes in the context of obesity, we generated mice with targeted deletion or transgenic overexpression of ATGL exclusively in skeletal muscle. Despite dramatic changes in IMTG content on both chow and high-fat diets, modulation of ATGL-mediated IMTG hydrolysis did not significantly influence systemic energy, lipid, or glucose homeostasis, nor did it influence insulin responsiveness or mitochondrial function. These data argue against a role for altered IMTG accumulation and lipolysis in muscle insulin resistance and metabolic complications of obesity.

Contraction‐induced lipolysis is not impaired by inhibition of hormone‐sensitive lipase in skeletal muscle

In skeletal muscle hormone-sensitive lipase (HSL) has long been accepted to be the principal enzyme responsible for lipolysis of intramyocellular triacylglycerol (IMTG) during contractions. However, this notion is based on in vitro lipase activity data, which may not reflect the in vivo lipolytic activity. We investigated lipolysis of IMTG in soleus muscles electrically stimulated to contract ex vivo during acute pharmacological inhibition of HSL in rat muscles and in muscles from HSL knockout (HSL-KO) mice. Measurements of IMTG are complicated by the presence of adipocytes located between the muscle fibres. To circumvent the problem with this contamination we analysed intramyocellular lipid droplet content histochemically. At maximal inhibition of HSL in rat muscles, contraction-induced breakdown of IMTG was identical to that seen in control muscles (P < 0.001). In response to contractions IMTG staining decreased significantly in both HSL-KO and WT muscles (P < 0.05). In vitro TG hydrolase activity data revealed that adipose triglyceride lipase (ATGL) and HSL collectively account for ∼98% of the TG hydrolase activity in mouse skeletal muscle, other TG lipases accordingly being of negligible importance for lipolysis of IMTG. The present study is the first to demonstrate that contraction-induced lipolysis of IMTG occurs in the absence of HSL activity in rat and mouse skeletal muscle. Furthermore, the results suggest that ATGL is activated and plays a major role in lipolysis of IMTG during muscle contractions.

Hormone Sensitive Lipase knockout mice have higher Post Exercise Insulin Sensitivity despite accumulation of diacylglycerol

AimTo evaluate an accumulation of diacylglycerol (DAG) on post exercise insulin sensitivity.MethodInsulin‐stimulated 2‐deoxyglucose uptake, using a submaximal insulin concentration, was measured in incubated mouse soleus (SOL) and extensor digitorum longus (EDL) muscle from wildtype (WT) and HSL Knock Out (KO) mice 90 min after completion of 40 min treadmill running.ResultsPost exercise insulin stimulated glucose uptake was 14% and 25% higher in SOL and EDL respectively in HSL KO compared to WT. Intramyocellular triacylglycerol was reduced to the same extent in both WT and HSL KO (p&lt;0.01). Muscle DAG was 40% higher after exercise in HSL KO mice compared to WT (p&lt;0.05), there were no differences in muscle DAG levels at rest. Phosphorylation of AMPK thr172 and ACC ser212 were not increased post exercise in both genotypes. Insulin induced signaling (phosphorylation of AKT ser473 and AKT thr308) was similar in both genotypes in both SOL and EDL. Total GLUT 4 protein expression was 20% and 25% higher in SOL and EDL respectively (p&lt;0.05) in HSL KO compared to WT.ConclusionPost exercise insulin sensitivity was higher in HSL KO despite accumulation of DAG. Moreover, the data suggests that reduced fatty acid availability for mitochondrial oxidation plays a role in the observed increased post exercise insulin sensitivity.