Dietary conjugated linoleic acid lowered tumor necrosis factor-α content and altered expression of genes related to lipid metabolism and insulin sensitivity in the skeletal muscle of Zucker rats

Type-2 diabetes is characterized by obesity-related insulin resistance. Insulin resistance and accompanying hyperinsulinemia have been reported to play an important role in pathogenesis of the metabolic syndrome. Conjugated linoleic acid (CLA), a mixture of positional and geometric isomers of linoleic acid, has attracted considerable attention because of its potentially beneficial biological effects. Previous studies showed that dietary CLA alleviates diabetes through improvement of glucose tolerance and insulin-stimulated glucose transport activity in skeletal muscle of diabetic rats. Skeletal muscle plays an important role both in insulin-mediated glucose metabolism and in lipid metabolism. In the present study, we evaluated comprehensively the effect of dietary CLA on the expression of genes related to lipid metabolism and insulin sensitivity in the skeletal muscle of obese, diabetic Zucker rats. After 8 weeks of feeding, expression of lipogenic genes was decreased in tendency, while expression of lipolytic genes was markedly increased by dietary CLA. Additionally, expression of genes-related insulin sensitivity, such as adiponectin receptor 1, was significantly enhanced, and mRNA level of peroxisome proliferator activated receptor-α, known as a transcriptional factor related lipid metabolism and insulin signaling in skeletal muscle, was markedly increased in CLA-fed rats. We also showed that dietary CLA significantly decreased the level of tumor necrosis factor-α (TNF-α), associated with the development of insulin resistance, in the skeletal muscle of Zucker rats. We suppose that the attenuated TNF-α accumulation in skeletal muscle may contribute to the alteration of expression of several genes and the alleviation of insulin resistance in CLA-fed Zucker rats.