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Excess Lipid Availability Increases Mitochondrial Fatty Acid Oxidative Capacity in Muscle

Evidence Against a Role for Reduced Fatty Acid Oxidation in Lipid-Induced Insulin Resistance in Rodents

¹ Diabetes and Obesity Program, Garvan Institute of Medical Research, Darlinghurst, Australia
² School of Health Sciences, University of Wollongong, Wollongong, Australia
³ Immunology and Inflammation Program, Garvan Institute of Medical Research, Darlinghurst, Australia
⁴ St. Vincent's Hospital Clinical School, University of New South Wales, Sydney, Australia

Address correspondence and reprint requests to Dr. Nigel Turner, Garvan Institute of Medical Research, 384 Victoria St., Darlinghurst, NSW 2010, Australia. E-mail: n.turner{at}garvan.org.au

Abbreviations: ßHAD, ß-hydroxyacyl CoA dehydrogenase; [³H]-2-DOG, [³H]-2-deoxyglucose; CPT, carnitine palmitoyl-transferase; MCAD, medium-chain acyl-CoA dehydrogenase; PGC, PPAR coactivator; PPAR, peroxisome proliferator–activated receptor; UCP, uncoupling protein

A reduced capacity for mitochondrial fatty acid oxidation in skeletal muscle has been proposed as a major factor leading to the accumulation of intramuscular lipids and their subsequent deleterious effects on insulin action. Here, we examine markers of mitochondrial fatty acid oxidative capacity in rodent models of insulin resistance associated with an oversupply of lipids. C57BL/6J mice were fed a high-fat diet for either 5 or 20 weeks. Several markers of muscle mitochondrial fatty acid oxidative capacity were measured, including ¹⁴C-palmitate oxidation, palmitoyl-CoA oxidation in isolated mitochondria, oxidative enzyme activity (citrate synthase, ß-hydroxyacyl CoA dehydrogenase, medium-chain acyl-CoA dehydrogenase, and carnitine palmitoyl-transferase 1), and expression of proteins involved in mitochondrial metabolism. Enzyme activity and mitochondrial protein expression were also examined in muscle from other rodent models of insulin resistance. Compared with standard diet–fed controls, muscle from fat-fed mice displayed elevated palmitate oxidation rate (5 weeks +23%, P < 0.05, and 20 weeks +29%, P < 0.05) and increased palmitoyl-CoA oxidation in isolated mitochondria (20 weeks +49%, P < 0.01). Furthermore, oxidative enzyme activity and protein expression of peroxisome proliferator–activated receptor coactivator (PGC)-1, uncoupling protein (UCP) 3, and mitochondrial respiratory chain subunits were significantly elevated in fat-fed animals. A similar pattern was present in muscle of fat-fed rats, obese Zucker rats, and db/db mice, with increases observed for oxidative enzyme activity and expression of PGC-1, UCP3, and subunits of the mitochondrial respiratory chain. These findings suggest that high lipid availability does not lead to intramuscular lipid accumulation and insulin resistance in rodents by decreasing muscle mitochondrial fatty acid oxidative capacity.

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