Obesity,Liver Insulin Resistance Regulation And Type 2 Diabetes

ObjectiveObesity is closely related to the occurrence of non-alcoholic fatty liver disease(NAFLD)and type 2 diabetes(T2D),and can be divided into general obesity and abdominal obesity according to the distribution of body fat.Body fat percentage(BF %)and body mass index(BMI)are two main indexes to evaluate general obesity.Previous studies have shown that the increase of BMI is significantly associated with the risk of incident T2 D in prediabetic subjects.However,it is unclear whether the changes of BMI or BF% in Chinese population with normal glucose tolerance(NGT)are associated with the incidence of T2 D.Abdominal obesity plays an important role in the pathogenesis of hepatic insulin resistance leading the development of T2 D and NAFLD,of which the exact pathogenesis is not clear.Neuronal nitric oxide synthase(nNOS)is one of the three forms of nitric oxide synthase.We found previously that nNOS activity was increased in the liver of obese mice.Therefore,the aim of the study was to 1)characterize BF% and BMI changes in NGT subjects and explore the effect of their changes on the risk of future T2D;2)evaluate the effects of nNOS on hepatic glucose and lipid metabolism and insulin sensitivity,and its underlying mechanisms.MethodsThe subjects were from a community-based prospective cohort study in Shanghai Diabetes Study.A total of 1857 NGT subjects aged 20-75 were included and followed up for an average period of 44.57±11.43 months.Subjects were grouped based on the BF%and/or BMI changes between baseline and follow-up.Liver nNOS overexpression was achieved by tail-vein-injection of adenovirus-nNOS in C57BL/6 mice fed with a high-fat diet(HFD).nNOS specific inhibitor(L-NPA)was used to change the activity of nNOS in ob/ob mice.Hematoxylin and eosin(HE),oil red O and Periodic Acid-Schiff stain were used to evaluate lipid accumulation and glycogen storage in livers.Western blot was used to evaluate the protein expression.Real-time PCR was used to detect the gene expression.Results11.5% or 9.2% of the community population with normal BF% or BMI at baseline became overweight or obese at follow-up,23.1% or 16.5% of individuals with abnormal BF% or abnormal BMI at baseline became normal BF% or BMI at follow-up.After44.57±11.43 months of follow-up,28(1.5%)NGT participants developed T2 D.Compared with subjects with stable normal BF% or BMI,subjects who became overweight/obesity at follow-up had a higher risk to develop T2D(BF% gain: RR=7.903,95% CI 1.952-31.999;BMI gain: RR=8.916,95% CI 2.943-27.009,P<0.05),while no difference in diabetic risk could be viewed between BF% or BMI loss group and stable normal BF% or BMI group,the results kept the same after adjustment of age,family history of diabetes and fasting glucose.Moreover,when we combined the changes of BF% and BMI to compare,subjects with normal BMI at both baseline and follow-up,but abnormal BF% at baseline and/or follow-up still had a higher risk of developing T2D(RR= 4.790,95%CI 1.061-21.621,P<0.05)compared with those who kept normal BMI and BF% both at baseline and follow-up,while those who had normal BF% at baseline and follow-up,but abnormal BMI at baseline and/or follow-up did not had an increased risk of T2 D.nNOS expressed in livers from humans and mice,and its phosphorylation in Ser1417 increased in the livers of high-fat diet(HFD)and ob/ob mice.Overexpression of nNOS in the livers of HFD mice further impaired the glucose tolerance,increased the intrahepatic lipid accumulation,decreased the glycogen storage,and blunted insulin-induced phosphorylation of IR? and AKT in livers(all P <0.05).Similarly,overexpression of nNOS increased the triglyceride(TG)production,decreased the glucose utilization,and insulin-induced phosphorylation of IR?,AKT and GSK3? in Hep G2 cells(all P <0.05).In contrast,inhibition of nNOS with L-NPA in ob/ob mice improved the glucose tolerance and increased the insulin-induced phosphorylation of IR? and AKT in livers(all P <0.05).Moreover,phosphorylation of P38 MAPK increased in nNOS-overexpressed Hep G2 cells.Inhibition of P38 MAPK with SB203580 significantly reversed the decrease of insulin-induced phosphorylation of IR?,AKT and GSK3? by nNOS overexpression(all P<0.05).ConclusionsIn NGT populations,subjects from normal BF% or BMI at baseline to overweight or obese at follow-up are associated with an increased risk of T2 D.Maintaining normal BF%is more relevant than BMI in preventing diabetes.Hepatic nNOS could inhibit insulin signaling pathway through the activation of P38 MAPK and thereafter affects hepatic glucose and lipid homeostasis.nNOS may serve as a potential therapeutic target for the prevention and treatment of NAFLD and T2 D.