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Reduced expression of the NAD+-dependent deacetylase, SIRT3, has been associated with insulin resistance and metabolic dysfunction in humans and rodents. In this study, we investigated whether specific overexpression of SIRT3 in vivo in skeletal muscle could prevent high-fat diet (HFD)-induced muscle insulin resistance. To address this, we used a muscle-specific adeno-associated virus (AAV) to overexpress SIRT3 in rat tibialis and extensor digitorum longus (EDL) muscles. Mitochondrial substrate oxidation, substrate switching and oxidative enzyme activity were assessed in skeletal muscles with and without SIRT3 overexpression. Muscle-specific insulin action was also assessed by hyperinsulinaemic–euglycaemic clamps in rats that underwent a 4-week HFD-feeding protocol. Ex vivo functional assays revealed elevated activity of selected SIRT3-target enzymes including hexokinase, isocitrate dehydrogenase and pyruvate dehydrogenase that was associated with an increase in the ability to switch between fatty acid- and glucose-derived substrates in muscles with SIRT3 overexpression. However, during the clamp, muscles from rats fed an HFD with increased SIRT3 expression displayed equally impaired glucose uptake and insulin-stimulated glycogen synthesis as the contralateral control muscle. Intramuscular triglyceride content was similarly increased in the muscle of high-fat-fed rats, regardless of SIRT3 status. Thus, despite SIRT3 knockout (KO) mouse models indicating many beneficial metabolic roles for SIRT3, our findings show that muscle-specific overexpression of SIRT3 has only minor effects on the acute development of skeletal muscle insulin resistance in high-fat-fed rats. [ABSTRACT FROM AUTHOR]

Intracellular calcium influences an array of pathways and affects cellular processes. With the rapidly progressing research investigating the molecular identity and the physiological roles of the mitochondrial calcium uniporter (MCU) complex, we now have the tools to understand the functions of mitochondrial Ca2 + in the regulation of pathophysiological processes. Herein, we describe the role of key MCU complex components in insulin resistance in mouse and human adipose tissue. Adipose tissue gene expression was analyzed from several models of obese and diabetic rodents and in 72 patients with obesity as well as in vitro insulin-resistant adipocytes. Genetic manipulation of MCU activity in 3T3-L1 adipocytes allowed the investigation of the role of mitochondrial calcium uptake. In insulin-resistant adipocytes, mitochondrial calcium uptake increased and several MCU components were upregulated. Similar results were observed in mouse and human visceral adipose tissue (VAT) during the progression of obesity and diabetes. Intriguingly, subcutaneous adipose tissue (SAT) was spared from overt MCU fluctuations. Furthermore, MCU expression returned to physiological levels in VAT of patients after weight loss by bariatric surgery. Genetic manipulation of mitochondrial calcium uptake in 3T3-L1 adipocytes demonstrated that changes in mitochondrial calcium concentration ([Ca2+]mt) can affect mitochondrial metabolism, including oxidative enzyme activity, mitochondrial respiration, membrane potential, and reactive oxygen species formation. Finally, our data suggest a strong relationship between [Ca2+]mt and the release of IL-6 and TNFa in adipocytes. Altered mitochondrial calcium flux in fat cells may play a role in obesity and diabetes and may be associated with the differential metabolic profiles of VAT and SAT. [ABSTRACT FROM AUTHOR]

SIRT1 is a NAD+-dependent deacetylase thought to regulate cellular metabolic pathways in response to alterations in nutrient flux. In the current study we investigated whether acute changes in SIRT1 expression affect markers of muscle mitochondrial content and also determined whether SIRT1 influenced muscle insulin resistance induced by acute glucose oversupply. In male Wistar rats either SIRT1 or a deacetylase inactive mutant form (H363Y) was electroprated into the tibialis cranialis (TC) muscle. The other leg was electroporated with an empty control vector. One week later, glucose was infused and hyperglycaemia was maintained at ~11mM. After 5 hours, 11mM glucose induced significant insulin resistance in skeletal muscle. Interestingly, overexpression of either SIRT1 or SIRT1 (H363Y) for 1 week did not change markers of mitochondrial content or function. SIRT1 or SIRT1 (H363Y) overexpression had no effect on the reduction in glucose uptake and glycogen synthesis in muscle in response to hyperglycemia. Therefore we conclude that acute increases in SIRT1 protein have little impact on mitochondrial content and that overexpressing SIRT1 does not prevent the development of insulin resistance during hyperglycaemia. [ABSTRACT FROM AUTHOR]

Abstract: Glucose infusion into rats causes skeletal muscle insulin resistance that initially occurs without changes in insulin signaling. The aim of the current study was to prolong glucose infusion and evaluate other events associated with the transition to muscle insulin resistance. Hyperglycemia was produced in rats by glucose infusion for 3, 5 and 8h. The rate of infusion required to maintain hyperglycemia was reduced at 5 and 8h. Glucose uptake into red quadriceps (RQ) and its incorporation into glycogen decreased between 3 and 5h, further decreasing at 8h. The earliest observed change in RQ was decreased AMPKα2 activity associated with large increases in muscle glycogen content at 3h. Activation of the mTOR pathway occurred at 5h. Akt phosphorylation (Ser473) was decreased at 8h compared to 3 and 5, although no decrease in phosphorylation of downstream GSK-3β (Ser9) and AS160 (Thr642) was observed. White quadriceps showed a similar but delayed pattern, with insulin resistance developing by 8h and decreased AMPKα2 activity at 5h. These results indicate that, in the presence of a nutrient overload, alterations in muscle insulin signaling occur, but after insulin resistance develops and appropriate changes in energy/nutrient sensing pathways occur. [Copyright &y& Elsevier]

OBJECTIVE--Medium-chain fatty acids (MCFAs) have been reported to be less obesogenic than long-chain fatty acids (LCFAs); however, relatively little is known regarding their effect on insulin action. Here, we examined the tissue-specific effects of MCFAs on lipid metabolism and insulin action. RESEARCH DESIGN AND METHODS--C57BL6/J mice and Wistar rats were fed either a low-fat control diet or high-fat diets rich in MCFAs or LCFAs for 4-5 weeks, and markers of mitochondrial oxidative capacity, lipid levels, and insulin action were measured. RESULTS--Mice fed the MCFA diet displayed reduced adiposity and better glucose tolerance than LCFA-fed animals. In skeletal muscle, triglyceride levels were increased by the LCFA diet (77%, P < 0.01) but remained at low-fat diet control levels in the MCFA-fed animals. The LCFA diet increased (20-50%, P < 0.05) markers of mitochondrial metabolism in muscle compared with low-fat diet-fed controls; however; the increase in oxidative capacity was substantially greater in MCFA-fed animals (50-140% versus low-fat-fed controls, P < 0.01). The MCFA diet induced a greater accumulation of liver triglycerides than the LCFA diet, likely due to an upregulation of several lipogenic enzymes. In rats, isocaloric feeding of MCFA or LCFA high-fat diets induced hepatic insulin resistance to a similar degree; however, insulin action was preserved at the level of low-fat diet-fed controls in muscle and adipose from MCFA-fed animals. CONCLUSIONS--MCFAs reduce adiposity and preserve insulin action in muscle and adipose, despite inducing steatosis and insulin resistance in the liver. Dietary supplementation with MCFAs may therefore be beneficial for preventing obesity and peripheral insulin resistance. Diabetes 58:2547-2554, 2009 [ABSTRACT FROM AUTHOR]