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

Abstract

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.