The Role Of SIRT1 In Improving The High-fat Diet Induced Insulin Resistance And Its Mechanism

Objective:High-fat diet (HFD), skeletal muscle lipid metabolism disorders and insulin resistance (IR) are closely related. Although resveratrol (RES) is implicated in the regulation of insulin sensitivity in rodents, the exact mechanism underlying this effect remains unclear. Therefore, we intended to interfered the HFD rats with SIRT1 agonist RES in order to increase SIRT1 expression and activity, and observe how RES impacts the skeletal muscle lipid transportation and lipid oxidation of mitochondria in HFD-induced IR rats, thus exploring the internal mechanism.Materials/Methods:Male SD rats, aged 8 weeks, were randomly divided into 3 groups, i.e. normal chow for 8 weeks (NC8 group), HFD for 8 weeks (HF8 group), HFD administered with RES for 8 weeks (HF8E group).Systemic and skeletal muscle insulin sensitivity together with expressions of several genes related to mitochondrial biogenesis and skeletal muscle lipid transportation were studied in rats fed a normal diet, a HFD and a HFD with intervention of RES for 8 weeks. Besides, citrate synthase (CS), electron transport chain (ETC) activities and several enzymes for mitochondrialβ-oxidation were assessed in subsarcolemmal (SS) and intermyofibrillar (IMF) mitochondria from skeletal muscle.Results:HFD fed rats exhibited obvious systemic and skeletal muscle IR as well as intramuscular lipid accumulation (IMCL). SIRT1 protein content, activity and expression of genes related to mitochondrial biogenesis were greatly declined whereas gene for lipid transportation, FAT/CD36 was up-regulated (p<0.05). SS but not IMF mitochondria displayed lower CS, ETC andβ-oxidation activities. By contrast, RES treatment protected rats against diet induced IMCL and IR, increased SIRT1 protein content, activity and mitochondrial biogenesis, reverted the decline in SS mitochondrial CS and ETC activities. Importantly, though expression of FAT/CD36 was increased (11%;P<0.05), activities of SS mitochondrialβ-oxidation enzymes were largely enhanced (41～67%;P<0.05).Conclusions:This study suggests that SIRTl agonist RES could increase SIRTl protein content and activity, and ameliorates insulin sensitivity by improving the balance between skeletal muscle lipid transportation and SS mitochondrial P-oxidation in HFD rats. Thus, this study, to a certain extent, reflects the role of SIRTl in improving HFD induced IR and the internal mechanisms. Objective:Skeletal muscle mitochondrial biogenesis and dysfunction are closely related to 1R. SIRT1 is known to improve IR but whether this effect is direct or not is still unclear and this question has not been addressed in mammals in vivo at the organ level, such as in the skeletal muscle. Therefore, we intended to specifically transfect skeletal muscle with SIRT1 in vivo and test if acute overexpression of SIRT1 in skeletal muscle of HFD rats in vivo would affect SS and IMF mitochondrial different functions and explore its internal mechanism to improve insulin sensitivity.Materials/Methods:Rats were fed normal chow diet or HFD for 8 weeks. In vivo electrotransfer was used to overexpress SIRT1 in the skeletal muscle of rats fed HFD. Systemic, skeletal muscle insulin sensitivity and downstream effects of SIRT1 on AMPK and mitochondrial biogenesis were studied. CS and complexes I-IV activities as well as oxidative stress levels and antioxidant enzymes activities were assessed in SS and IMF mitochondria.Results:HFD fed rats showed systemic and skeletal muscle IR as well as decreased SIRT1 level, expression of AMPK, mtDNA and mitochondrial biogenesis(P<0.05). SS but not IMF mitochondria displayed lower CS, complexes I-IV and antioxidant enzyme activities (p<0.05). By contrast, moderate (1.5～3 folds) SIRT1 overexpression attenuated HFD induced skeletal muscle IR. This improvement was associated with increased expressions of AMPK (-50%; p<0.05), PGC-la (70～80%; p<0.05) and mtDNA as well as SS mitochondrial CS and complexes I-IV activities in skeletal muscle. Importantly, SIRT1 overexpression largely restored antioxidant enzyme activities, enhanced complex I but not complexes II-IV functions in individual SS and IMF mitochondria.Conclusions:This study suggests that SIRT1 overexpression improved insulin sensitivity at least partly by targeting complex I functions of SS and IMF mitochondria thereby notably decreasing the skeletal muscle oxidative stress through the activation of SIRT1, AMPK and PGC-la. Therefore, on the one hand, this study confirms the role of SIRT1 in improving skeletal muscle IR. On the other hand, it further reveals the internal mechanism of SIRT1 in improving the insulin sensitivity of HFD from another angle.