Glycine Ameliorates Nonalcoholic Fatty Liver Disease Through Activating AMPK Signaling Pathway In Mice

Nonalcoholic fatty liver disease(NAFLD)is characterized by that hepatic lipid accumulation of more than 5%of liver weight and has an estimated prevalence of 27.1%in China.The incidence of NAFLD is still rapidly rising,which is attributed to the increased rate of obesity.NAFLD is believed as the result from fat accumulation in the liver.However,no effective pharmacological intervention is available this disease.Glycine is the smallest non-essential,neutral and metabolically inert amino acid in the human body.It is an essential substrate for the synthesis of several important biomolecules.It has a broad spectrum of anti-inflammatory,cytoprotective and immunomodulatory properties.Circulating glycine level is consistently low in patients with type 2 diabietes and correlates negatively with obesity and insulin resistance.Male mice were fed on high fat diet(HFD)for 16 weeks to generate an obese model.Combination therapy with glycine and metformin extended glucose-reducing effects and improved glucose metabolism.Compared with metformin,the compound MGG synthesized from glycine and metformin provided better random glycemic control and apparently extended effects on reducing plasma glucose level,but increased incidence of hypoglycaemia.The MGG analogue MG had equivalently hypoglycemic effect with metformin at a dose of 300 mg/kg.MG at dosage of 400 mg/kg showed strick glycemic control effect but increased incidence of hypoglycaemia compared with metformin.Mice received MG at dosage of 600 mg/kg by intragastrical gavage died within 2 days(n=4),which challenges its therapeutic benefit as diabetic drugs.Plasma level of glycine is also positively associated with glucose disposal which is increased by managements improving glucose homeostasis such as exercise and bariatric surgery.However,role for glycine in the regulation of glucose,beyond being a potential biomarker,is still unknown.In the present study we investigated role of glycine in glucolipid metabolism and NAFLD.We found that 5 mM glycine apparently stimulated AMPK phosphorylation in mouse hepatocytes.Hepatic AMPK was suppressed in the conditions of high glucose level(30 mM),which could be reversed by treating with 1 mM or 5 mM of glycine.Ex vivo experiment in murine hepatocyte,AMPK was magnificantly activated by 500 mg/kg glycine from mice fed with standard chow diet.Next,obese mice fed on HFD for 24 weeks were used to evaluate the plasma glucose level.Briefly,the DIO mice were randomly divided into two groups:saline-treated and glycine-treated(500 mg/kg/d)groups.Glycine was orally administered into mice for additional 4 weeks plus HFD treatment.We found that glycine significantly ameliorated high-fat diet induced insulin resistance,glucose metabolic imbalance,hepatic lipid accumulation,and increased insulin secretion.Glycine activated LKB1-AMPK-ACC pathway both in mice liver and hepatic cells at specific concentration.Mechanistically,significant decreases in mRNA levels of PEPCK,G6PC,HMGCR,SREBP1C,FAS,ACC and PPARy,and increases in mRNA levels ABCG1,PPARα,CPT1α and MCAD were detected in livers of mice treated with glycine.Additionally,by electron microscope,we found glycine promoted mitophagy through inhibition of the mammalian target of rapamycin(mTOR)complex via AMPK activation.Taken together,our results reveal that glycine may conferre protective effects against HFD-induced NAFLD through activating AMPK signaling pathway in mouse liver.