Impact of exercise and modulation of triancylglycerol dynamics on lipid metabolism in human skeletal muscle cells

The prevalence of metabolic disorders such as obesity and type 2 diabetes (T2D) has increased dramatically worldwide the last decades. Lifestyle factors such as diet and physical activity have preventive as well as therapeutic benefits for these metabolic disturbances. Skeletal muscle, which constitutes up to 40-50% of total body mass, is one of the main regulators of glucose and fatty acid (FA) metabolism in the body. Furthermore, skeletal muscle displays a remarkable adaptability in response to exercise. Increasing evidence suggests that dysregulation of lipid metabolism in skeletal muscle have significant impact on whole-body energy metabolism, and lipid turnover is a potential key to understand the impairments attributed to skeletal muscle in conditions such as obesity and T2D. Approximately 50-60% of free FAs taken up by skeletal muscle are stored as triacylglycerol (TAG) in lipid droplets (LDs). LDs are dynamic organelles that continuously are regulated between storage and breakdown of TAG by lipases to generate FAs available for energy production and re-synthesis of lipids. Thus, TAG dynamics is a critical factor that balancing FA utilization between storage, accumulation of lipid intermediates and FA oxidation. The studies presented in this thesis, were aimed to examine the regulation of lipid metabolism in cultured skeletal muscle cells related to exercise, obesity and T2D as well as by modulation of TAG dynamics. Biopsy-derived cultured human myotubes from overweight subjects after exercise showed increased FA metabolism as well as increased fractional glucose oxidation compared to myotubes retrieved prior to exercise. By studying myotubes established from lean, obese and obese T2D donors, we observed that myotubes from obese donors had an increased FA uptake and lower complete-to-incomplete FA oxidation ratio, associated with a reduced mitochondrial function, compared to myotubes from lean donors. Myotubes from the same group of donors were challenged with electrical pulse stimulation (EPS), mimicking exercise in vivo. We observed that myotubes from non-diabetic (lean and obese) versus diabetic donors (obese T2D) responded differently to EPS. Thus, myotubes from T2D individuals were partially refractory to contraction-mediated effects on lipid handling. TAG dynamics were studied in myotubes established from lean, healthy donors by using selective inhibitors of diacylglycerol acyltransferases 1 and 2 (DGAT), which catalyse the final step of TAG synthesis. We explored that the two distinct enzymes, DGAT1 and DGAT2, played different roles in lipid metabolism in skeletal muscle cells, where DGAT1 was the main enzyme for re-synthesis of TAG while DGAT2 mainly contributed to de novo synthesis of TAG. Increases in all processes of FA turnover, i.e. accumulation, lipolysis and oxidation, combined with enhanced mitochondrial function, were observed after acute and chronic exposure to eicosapentaenoic acid (EPA). EPA also increased accumulation of FAs into TAG, indicating an enhanced lipid turnover in myotubes. In conclusion, the results presented in this thesis have contributed to new knowledge about impact of exercise and modulation of TAG dynamics on lipid metabolism in human skeletal muscle cells. Cultured skeletal muscle cells established from satellite cells, activated, proliferated and differentiated into myotubes has been proven to be a valuable model system for studying mechanisms related to exercise and metabolic diseases. We presented data which confirm that myotubes preserve, to some extent, the phenotype of their donors in vitro, and that interventions like exercise, both in vivo and in vitro, as well as potential pharmacological agents, can positively influence lipid metabolism and mitochondrial function in skeletal muscle cells