The Effects Of Hepatic Lipid Metabolism In Insulin-resistant Mice Induced By Long-term And Short-term High Fat Diet

ObjectiveInsulin resistance(IR)is the common pathophysiological mechanism of many metabolic diseases,which is closely related to the occurrence and development of type 2 diabetes mellitus.Those related metabolic diseases are mainly associated with liver,fat and muscle.IR in liver plays an important role in the abnormal metabolism of glucose and lipid and fatty liver.The mechanism of IR is not yet clear.It is essential to exert further research on IR to figure out the understanding of metabolic diseases.Glycerol-3-phosphate Acyltransferase 4(GPAT4)is a key rate-limiting enzyme isoform in the first step of glycerol-phosphate pathway for triacylglycerols(TAG)and glycerophosphatide synthesis.Several relevant studies have shown that GPAT4 plays a crucial role of TAG synthesis in adipose tissue and liver in mice.Research shows that the accumulation of unfolded protein in endoplasmic reticulum and unfolded protein response(UPR)triggered by high-fat diet can up-regulate the expression of some lipases,further leading to hepatic and systematic IR.Mice with chronic high-fat diet leads to fat deposition in liver and obesity.At present,it is not clear whether liver GPAT4 plays an important role in the development of obesity-related insulin resistance,what its specific role is and whether it is related to the occurrence of endoplasmic reticulum stress in liver induced by long-term high-fat diet.Therefore,this study will focus on the effect of liver GPAT4 and the role of other lipid synthases on IR induced by long-term and short-term high-fat diet,further exploring its mechanism.Methods1.The obesity model was established by long-term and short-term high-fat diet to replicate human obesity model.(Four-week-old C57BL/6 mice were fed with high fatdiet(HFD)and chow diet(CD)after 7 days of adaptive feeding.)2.Long-term high fat diet group(LTHFD)and long-term chow diet group(LTCD)were observed the metabolic index such as body weight and food intake weekly.Glucose and insulin tolerance test were measured at the 14 th week of this experiment.At the 18 th week of the experiment,the mice were sacrificed.Serum glucose,TAG,cholesterol and insulin,hepatic GPAT4,TAG,lipid synthase and endoplasmic reticulum stress index were measured.The changes of liver tissue structure and hepatic lipid content in mice were detected by pathological technology.3.Short-term high fat diet group(STHFD)and short-term chow diet group(STCD)were sacrificed at the third day.Serum glucose,TAG,cholesterol,insulin,hepatic GPAT4 and lipid synthase expression and hepatic TAG were measured.Results1.After 10 weeks of feeding with different diets,there were differences in body weight and food intake between LTHFD and LTCD group(P < 0.05);the weight of liver,white and brown adipose tissue in LTHFD group was significantly higher than that in LTCD group(P < 0.05);fasting blood glucose,TAG,insulin and cholesterol in LTHFD group were significantly higher than those in LTCD group,and there was no significant difference in liver enzymes between the two groups(P > 0.05).2.Compared with LTCD group,the blood glucose of LTHFD group increased significantly at 15,30,60 and 120 minutes(P < 0.05 or P < 0.01)in IPGTT and IPITT.Compared with control group,hepatic TAG content of LTHFD and STFD groups were significantly increased(P < 0.05).Compared with LTCD group,lipid deposition and structural changes of adipocytes were observed in LTHFD group.3.Hepatic lipid synthase expression: Compared with STCD group,the expression of GPAT4,SREBP1-c,CHREBP and MGAT2 in STHFD group increased significantly(P < 0.05),while the expression of ACC2 and SCD-1 decreased significantly(P <0.01),the expression of GPAT1 and GPAT3 have no change(P > 0.05).Compared with LTCD group,the expressions of GPAT4,ACC2,GPAT1,GPAT3,SCD-1 andMGAT in LTHFD group decreased significantly(P < 0.05),while the expressions of SREBP1-c and CHREBP increased(P < 0.05).Compared with STCD group,the expression of GPAT4 protein in liver increased in STHFD group(P < 0.05),and decreased in LTHFD group(P < 0.05).4.Endoplasmic reticulum stress(ERS)related gene expression: Compared with LTHFD group,liver GRP78,CHOP mRNA and protein levels in LTHFD group had no difference(P > 0.05).Conclusion1.Both long-term and short-term high-fat diet group could increase hepatic TAG content and decrease systemic insulin sensitivity in mice.2.Long-term high-fat diet inhibits the synthesis of endogenous and exogenous lipids in mice liver.Short-term high-fat diet activates the expression of GPAT4 in liver,and inhibited the expression of other endogenous lipid synthase in the liver,suggesting that GPAT4 may be a key factor in the occurrence of early hepatic steatosis and IR.3.ERS may not be the initial mechanism of hepatic steatosis and decreased insulin sensitivity in mice fed with high-fat diet for a long time.