Metabolic switching of human myotubes is improved by n-3 fatty acids.

The aim of the present study was to examine whether pretreatment with different fatty acids, as well as the liver X receptor (LXR) agonist T0901317, could modify metabolic switching of human myotubes. The n-3 FA eicosapentaenoic acid (EPA) increased suppressibility, the ability of glucose to suppress FA oxidation. Substrate-regulated flexibility, the ability to increase FA oxidation when changing from a high glucose, low fatty acid condition ("fed") to a high fatty acid, low glucose ("fasted") condition, was increased by EPA and other n-3 FAs. Adaptability, the capacity to increase FA oxidation with increasing FA availability, was enhanced after pretreatment with EPA, linoleic acid (LA), and palmitic acid (PA). T0901317 counteracted the effect of EPA on suppressibility and adaptability, but it did not affect these parameters alone. EPA per se accumulated less, however, EPA, LA, oleic acid, and T0901317 treatment increased the number of lipid droplets (LD) in myotubes. LD volume and intensity, as well as mitochondrial mass, were independent of FA pretreatment. Microarray analysis showed that EPA regulated more genes than the other FAs and that specific pathways involved in carbohydrate metabolism were induced only by EPA. The present study suggests a favorable effect of n-3 FAs on skeletal muscle metabolic switching and glucose utilization.

Liver X receptor antagonist reduces lipid formation and increases glucose metabolism in myotubes from lean, obese and type 2 diabetic individuals.

AIMS/HYPOTHESIS: Liver X receptors (LXRs) play important roles in lipid and carbohydrate metabolism. The purpose of the present study was to evaluate effects of the endogenous LXR agonist 22-R-hydroxycholesterol (22-R-HC) and its stereoisomer 22-S-hydroxycholesterol (22-S-HC), in comparison with the synthetic agonist T0901317 on lipid and glucose metabolism in human skeletal muscle cells (myotubes). METHODS: Myotubes established from lean and obese control volunteers and from obese type 2 diabetic volunteers were treated with LXR ligands for 4 days. Lipid and glucose metabolisms were studied with labelled precursors, and gene expression was analysed using real-time PCR. RESULTS: Treatment with T0901317 increased lipogenesis (de novo lipid synthesis) and lipid accumulation in myotubes, this increase being more pronounced in myotubes from type 2 diabetic volunteers than from lean volunteers. Furthermore, 22-S-HC efficiently counteracted the T0901317-induced enhancement of lipid formation. Moreover, synthesis of diacylglycerol, cholesteryl ester and free cholesterol from acetate was reduced below baseline by 22-S-HC, whereas glucose uptake and oxidation were increased. Both 22-S-HC and 22-R-HC, in contrast to T0901317, decreased the expression of genes involved in cholesterol synthesis, whereas only 22-R-HC, like T0901317, increased the expression of the gene encoding the reverse cholesterol transporter ATP-binding cassette subfamily A1 (ABCA1). CONCLUSIONS/INTERPRETATION: T0901317-induced lipogenesis and lipid formation was more pronounced in myotubes from type 2 diabetic patients than from lean individuals. 22-S-HC counteracted these effects and reduced de novo lipogenesis below baseline, while glucose uptake and oxidation were increased.

Lipid metabolism in human skeletal muscle cells: effects of palmitate and chronic hyperglycaemia.

This review focuses on the effect of exogenous factors known to be of importance for the development of insulin resistance in differentiated human myotubes. Recent data from our laboratory on the effects of fatty acid pre-treatment and chronic glucose oversupply on fatty acid and glucose metabolism, without and with acute insulin are presented, and discussed in the context of other recent publications in the field. Pre-treatment of myotubes with palmitate, chronic hyperglycaemia, and acute high concentrations of insulin changed fatty acid metabolism in favour of accumulation of intracellular lipids. Acute insulin exposure increased (14)C-oleate uptake and levels of free fatty acids (FFA) and triacylglycerol (TAG). Palmitate pre-treatment further increased oleate uptake, both under basal conditions and in the presence of insulin, with a marked increase in the phospholipid (PL) fraction, with a concomitant reduction in oleate oxidation. Chronic hyperglycaemia also promoted increased lipogenesis and elevated levels of cellular lipids. Changes in fatty acid metabolism in human muscle, in particular fatty acid oxidation, are probably crucial for the molecular mechanism behind skeletal muscle insulin resistance and impaired glucose metabolism. Differentiated human skeletal muscle cells may be an ideal system to further explore the mechanisms regulating lipid metabolism.

Chronic hyperglycaemia promotes lipogenesis and triacylglycerol accumulation in human skeletal muscle cells.

AIMS/HYPOTHESIS: The present study was conducted to evaluate the effect of hyperglycaemia in itself on glucose and lipid metabolism in human skeletal muscle cells. METHODS: Satellite cells were isolated from biopsy samples from the vastus lateralis muscle and differentiated into multinucleated myotubes in cultures. Metabolism studies were performed using isotopes ([3H]deoxyglucose, [14C]glucose, [14C]oleic acid and [14C]palmitic acid), and mRNA and protein levels were analysed by real-time RT-PCR and western blotting respectively. RESULTS: Exposure of myotubes to 20 mmol/l glucose for 4 days reduced insulin-stimulated glucose uptake and glycogen synthesis to 57+/-5% (p<0.0001) and 56+/-5% (p<0.0001) of normoglycaemic (5.5 mmol/l glucose) controls respectively. Basal glucose uptake and glycogen synthesis were both reduced, whereas glucose oxidation was unaltered. Total cell content of glycogen and expression of GLUT1 and GLUT4 mRNA were not affected. There was a significant increase in the incorporation of glucose into cellular NEFA (88+/-17% increase, p=0.006), triacylglycerol (44+/-21% increase, p=0.04) and cholesterol ester (89+/-36% increase, p=0.02) in hyperglycaemic myotubes compared with controls. Diacylglycerol tended to be increased though not significantly, and phospholipid formation were unchanged. Relative to controls, total cell content of triacylglycerol was increased by 25+/-7% (p=0.02) and acyl-CoA:1,2-diacylglycerol acyltransferase 1 activity was increased by 34+/-4% (p=0.004), whereas acyl-CoA:1,2-diacylglycerol acyltransferase 1 mRNA expression was unchanged. Total cellular uptake of palmitic acid was reduced by 18+/-3% (p=0.006) in hyperglycaemic cells compared with controls, while uptake of oleic acid was unchanged. Oxidation of palmitic acid or oleic acid was not affected by hyperglycaemia. CONCLUSIONS/INTERPRETATION: Chronic hyperglycaemia increased triacylglycerol accumulation and the incorporation of carbohydrate into triacylglycerol (i.e. de novo lipogenesis) concomitantly with a reduced insulin-stimulated glucose uptake and glycogen synthesis. Enhanced acyl-CoA:1,2-diacylglycerol acyltransferase 1 activity supported the increased triacylglycerol synthesis during hyperglycaemia.