The Effect Of The Exenatide On Liver Mitochondrial Dysfunction In Obese-Mice

Background:Obesity is a global health problem and the obese population reached 700 million in 2015.There is a clear association between obesity and the metabolic syndrome that includes nonalcoholic fatty liver disease(NAFLD),type 2 diabetes,hyperlipidemia and hypertension.Insulin resistance is a central feature of the metabolic syndrome.However,the development of insulin resistance in the liver precedes the development of systemic insulin resistance,and the impaired insulin signal transduction in the liver may be the key problem.However,liver lipid deposition and insulin resistance can cause mitochondrial oxidative stress,and mitochondrial disfunction(such as respiratory enzyme activity and generation for ATP)of the liver caused by mitochondrial oxidative stress can also in turn aggravate the liver lipid deposition and insulin resistance,thus forming a vicious cycle.Hence,it is important to improve the sensitivity of insulin and reduce the accumulation of liver lipids,which is essential to prevent the secondary attack on the liver from oxidative stress.Glucagon-like peptide(GLP)-1 is an incretin hormone,which is released by the L-cells of the small intestine in response to ingesting food and primarily regulates blood glucose through the glucose-dependent insulin release.The reportedly beneficial effects of GLP-1 include inhibiting gastric emptying,increasing lower gastrointestinal motility,acting on the brain to induce satiety,promoting ?-cell proliferation and reducing the plasma glucagon concentration.Exenatide is the agonist of GLP-1 receptor.It was also reported to have beneficial effects including reducing body weight in animal models of obesity,decreasing excess body weight in obese subjects through decreased food intake and increased energy expenditure.These advantages break the vicious cycle of diabetes,obesity and insulin resistance.Hence,exenatide is more suitable for patients with type 2 diabetes and obesity,and differs from other traditional hypoglycemic agents.Meanwhile,with the discovery of GLP-1 receptor expression in the liver,numerous studies have shown that GLP-1 plays an important role in improving liver lipid toxicity,especially in diabetics.Although some studies have reported that exenatide has a beneficial effect on hepatic insulin resistance and lipid toxicity,which may be related to its effects on oxidative stress and mitochondria,the underlying mechanism involved in exenatide action is still unclear and controversial.Objective:1 To determine whether liver oxidative stress could be attenuated by in vivo treatment with exenatide in high fat diet-induced obese mice2 To explore the effect of exenatide on mitochondrial dysfunction in high fat diet-induced obese mice3 To determine whether insulin resistance could be reduced in obese mice induced by high-fat diet and its possible mechanism.Methods:1 Animals and Treatment:Forty-eight four-week-old male C57BL/6 mice were randomly assigned to standard diet group(STD)and high-fat diet obese group(DIO)for 12 weeks.STD mice and DIO mice were treated with subcutaneous injection ofexenatide(10 ug/kg body weight,twice a day)or saline during the final four weeks of the study.Mice were divided into the following groups:STD(n=12),STD+Exenatide(n=12),DIO(n=12)and DIO+Exenatide(n=12).2 Oral glucose tolerance tests(OGTT)and intraperitoneal insulin tolerance tests(IPITT)were performed before and after the treatment with exenatide,and the weight were measured and recorded weekly.3.A venous blood sample was collected from all the fasting mice at the end of the study,and the liver function,blood lipid,glycated albumin(GA%),glucose and insulin were also measured.Homeostatic model assessment was calculated by the following equation:HOMA-IR =[FBG(mmol/L)*FINS(mlU/L)/22.5].4 HE staining,oil-red O staining and transmission electron microscopy were used to observe the morphology,lipid accumulation and ultrastructure of the liver.5 We detected the insulin signaling pathway related proteins(IRS1,phospho-IRS1/2,Akt,phospho-Akt(Ser473))in liver tissue by Western blot.6 The MDA content of the liver tissue was determined according to the Lipid Peroxidation MDA assay kit.The fluorescent,lipophilic and cationic probe,JC-1(Beyotime,China),was used to measure the mitochondrial membrane potential(??m)of liver tissue.7 The mitochondrial respiratory chain(RC)enzymatic activities on liver were measured by enzyme-substrate reaction.8 Adenine nucleotide levels(AMP,ADP,ATP)were determined by the method of high performance liquid chromatography(HPLC).The energy charge was calculated by the following equation:[ATP + ADP/2]/[ATP + ADP + AMP],where AMP,ADP,and ATP are the respective tissue concentrations.9 The body metabolism of mice were measured by metabolic chambers.,which including food and water intake,physical activity,carbon dioxide production(VC02),oxygen consumption(V02),respiratory quotient(RQ),heat production and basal metabolic rate(BMR).Results:1 Exenatide reduced the net weight gain and fat content of liver tissue in DIO mice.2 Exenatide improves OGTT,IP ITT and effects on IR in DIO mice.3 Exenatide reduces oxidative stress and improves mitochondrial structure in DIO mice.4 Exenatide improves mitochondrial respiratory chain(RC)enzymatic activities in the liver of DIO mice.5 Exenatide effects on adenine nucleotide levels.6 Exenatide effects on the whole body metabolic status.Conclusion:Data from this investigation domenstrate that exenatide reduced hepatic steatosis,decreased oxidative stress,and improved insulin resistance in DIO mice,in concert with improvements in the insulin metabolic signaling,mitochondrial respiratory chain enzymatic activation,adenine nucleotide production,organism metabolism and weight gain.