Skeletal Muscle Mitochondria and Insulin Resistance: The Role of Exercise

ABSTRACT

Insulin resistance in skeletal muscle, liver, beta-cells, fat cells, the gastrointestinal track, alpha-cells, kidneys, and brain represents the core defect in obesity or type 2 diabetes (T2D). Among them, skeletal muscle insulin resistance due to obesity or T2D is manifested by decreased glucose uptake because skeletal muscle comprises 40-50% of the total human body mass. Many previous reports indicate that T2D patients or obese insulin-resistant individuals have less mitochondria in their skeletal muscles than lean control subjects. Whether or not mitochondria in skeletal muscle play a causal role in insulin resistance has been debated. A large number of studies demonstrated that skeletal muscle insulin resistance is associated with mitochondrial deficiency including 1) reduced fatty acid oxidation and increased accumulation of lipid intermediates (e.g., FA-CoA, DAG, ceramide), 2) increased mitochondrial overload and incomplete fatty acid oxidation, and 3) increased mitochondrial oxidative stress (e.g., H2O2) in skeletal muscle. In contrast, some studies demonstrated that mitochondrial dysfunction in skeletal muscle is not responsible for insulin resistance, suggesting that 1) the development of insulin resistance in high-fat diet animals occurs with increased muscle mitochondria, and 2) fatty acid oxidation is higher in T2D patients and obese insulin-resistant individuals compared with lean control subjects. However, various types of exercises (acute vs chronic, aerobic vs resistance) are critical in the treatment and prevention of insulin resistance in obesity and T2D.