Skeletal muscle insulin resistance: the interplay of local lipid excess and mitochondrial dysfunction

This review explores the complex relationship between excess lipid exposure, mitochondrial dysfunction, and insulin resistance at the level of human skeletal muscle. Lipotoxicity, that is, the elevation of lipids and/or associated lipid metabolites within blood and tissues with subsequent metabolic derangement, has been proposed as a possible mechanism of skeletal muscle insulin resistance. Intravenous lipid infusion is a well-documented method of inducing insulin resistance. Although IMCL content has been correlated with insulin resistance, there is increasing evidence that lipid metabolites such as 4-HNE, DAG, ceramide, and LC-CoA may play a more significant role than TGs in producing skeletal muscle insulin resistance. The association between mitochondrial dysfunction and insulin resistance is unclear, particularly because of the varied options for measuring mitochondrial function. The effect of acute lipid exposure producing skeletal muscle insulin resistance in humans is well documented. The effects of sustained lipid exposure from dietary ingestion on skeletal muscle insulin resistance and skeletal muscle mitochondrial function remain disputed. The effects of skeletal muscle mitochondrial dysfunction on accumulation of lipotoxic species and skeletal muscle insulin resistance also remain uncertain. Certainly, pursuit of the role of lipid metabolites and of their roles in the generation of skeletal muscle insulin resistance remains an exciting area for future research. Moreover, alteration in skeletal muscle insulin resistance does not occur in isolation, as functional perturbations of any component of the glucose homeostatic system may initiate development of insulin resistance in skeletal muscle through cross talk between tissues. Nevertheless, understanding pathophysiology of skeletal muscle insulin resistance remains critically important because of its role in preceding and facilitating the development of DM2.