Previous studies have suggested that insulin resistance develops secondary to diminished fat oxidation and resultant accumulation of cytosolic lipid molecules that impair insulin signaling. Contrary to this model, the present study used targeted metabolomics to find that obesity-related insulin resistance in skeletal muscle is characterized by excessive beta-oxidation, impaired switching to carbohydrate substrate during the fasted-to-fed transition, and coincident depletion of organic acid intermediates of the tricarboxylic acid cycle. In cultured myotubes, lipid-induced insulin resistance was prevented by manipulations that restrict fatty acid uptake into mitochondria. These results were recapitulated in mice lacking malonyl-CoA decarboxylase (MCD), an enzyme that promotes mitochondrial beta-oxidation by relieving malonyl-CoA-mediated inhibition of carnitine palmitoyltransferase 1. Thus, mcd(-/-) mice exhibit reduced rates of fat catabolism and resist diet-induced glucose intolerance despite high intramuscular levels of long-chain acyl-CoAs. These findings reveal a strong connection between skeletal muscle insulin resistance and lipid-induced mitochondrial stress.

Nearly unlimited supplies of energy-dense foods and technologies that encourage sedentary behaviour have introduced a new threat to the survival of our species: obesity and its co-morbidities. Foremost among the co-morbidities is type 2 diabetes, which is projected to afflict 300 million people worldwide by 2020. Compliance with lifestyle modifications such as reduced caloric intake and increased physical activity has proved to be difficult for the general population, meaning that pharmacological intervention may be the only recourse for some. This epidemiological reality heightens the urgency for gaining a deeper understanding of the processes that cause metabolic failure of key tissues and organ systems in type 2 diabetes, as reviewed here.

Uncoupling protein 3 (UCP3) is a member of the mitochondrial anion carrier superfamily. Based upon its high homology with UCP1 and its restricted tissue distribution to skeletal muscle and brown adipose tissue, UCP3 has been suggested to play important roles in regulating energy expenditure, body weight, and thermoregulation. Other postulated roles for UCP3 include regulation of fatty acid metabolism, adaptive responses to acute exercise and starvation, and prevention of reactive oxygen species (ROS) formation. To address these questions, we have generated mice lacking UCP3 (UCP3 knockout (KO) mice). Here, we provide evidence that skeletal muscle mitochondria lacking UCP3 are more coupled (i.e. increased state 3/state 4 ratio), indicating that UCP3 has uncoupling activity. In addition, production of ROS is increased in mitochondria lacking UCP3. This study demonstrates that UCP3 has uncoupling activity and that its absence may lead to increased production of ROS. Despite these effects on mitochondrial function, UCP3 does not seem to be required for body weight regulation, exercise tolerance, fatty acid oxidation, or cold-induced thermogenesis. The absence of such phenotypes in UCP3 KO mice could not be attributed to up-regulation of other UCP mRNAs. However, alternative compensatory mechanisms cannot be excluded. The consequence of increased mitochondrial coupling in UCP3 KO mice on metabolism and the possible role of yet unidentified compensatory mechanisms, remains to be determined. Uncoupling protein 3 (UCP3)1 (1-3) is a member of the mitochondrial anion carrier superfamily with high homology (57%) to UCP1, a well characterized uncoupling protein (4, 5). UCP3 together with UCP1, UCP2 (6, 7), and possibly BMCP1 (brain mitochondrial carrier protein) (8) and UCP4 (9), form a family of uncoupling proteins located in the inner mitochondrial membrane. The evidence supporting the uncoupling activity of these proteins comes from studies where UCPs have been heterologously expressed in yeast or reconstituted into proteoliposomes. The expression of UCP2 and -3 decreases the mitochondrial membrane potential, as assessed by uptake of fluorescent membrane potential-sensitive dyes in whole yeast. They also increase state 4 respiration in isolated mitochondria, which serves as an indicator of inner membrane proton leak (3, 6, 10). More recently, reconstitution of UCPs into liposomes has shown that UCP2 and UCP3, like UCP1, mediate proton transport across bilipid layers (11). It is well established that UCP1 is exclusively expressed in brown fat, where it plays a key role in facultative thermogenesis in rodents. Although there is controversy about the molecular mechanisms involved (12-16), it is clear that activated UCP1 catalyzes a proton leak across the mitochondrial inner membrane leading to thermogenesis. The activity of UCP1 is highly regulated, facilitated by fatty acids and inhibited by purine ribose di-and trinucleotides (ATP, ADP, GTP, GDP) (17). UCP1 is also highly regulated at the transcriptional level (18) by cat...