| ObjectivesIn recent years,studies have found that diabetes and bile acid metabolism are closely related.Bile acids are mainly derived from the conversion of cholesterol.Sterol regulatory element binding protein-2(SREBP-2)mainly regulates cholesterol synthesis.Farnesoid X receptor(FXR)regulates bile acid synthesis as a bile acid receptor.Some studies have shown that metformin not only reduces blood glucose but can also improve blood lipids.The aim of this study is to investigate the effects of metformin on cholesterol metabolism in streptozotocin(STZ)induced type 2 diabetic rats.We investigated the protein expression of liver SREBP-cleavage activating protein(SCAP),SREBP-2,low density lipoprotein receptor(LDLR)and FXR,as well as the mRNA expression of cholesterol 7?-hydroxylase(CYP7a1),to analyze the mechanism of metformin in improving cholesterol metabolism in a further way.MethodsThirty 8-week-old Wistar male rats were randomly divided into three groups: normal control group(CON),diabetic group(DM)and metformin treatment group(DM+MET).The normal control group had a standard diet and the other two groups had a high-fat diet.After 8 weeks,DM group and DM+MET group were given a small dose of STZ intraperitoneally to induce type 2 diabetes.The DM+MET group was treated with metformin for 12 weeks(500 mg·kg-1·d-1)after successful modeling.At the 22 th week,the fasting blood biochemical indexes of each group were detected.Furthermore,the Lipid accumulation was examined by hematoxylin and eosin(H&E),Oil Red O staining and hepatocyte cholesterol quantitative assay.The protein expression of molecules involved in SCAP,SREBP-2,LDLR and FXR was examined by Western blot.The mRNA expression of FXR and CYP7a1 was detected by PCR.Results1.The changes of diet,drinking and body weight in three groups of rats.Compared with the CON group,the DM group's drinking and diet were increased,while the weight was lost.In the DM + MET group,the drinking and diet were increased than those in the CON group,but the weight was lost.Compared with the DM group,the DM + MET group's drinking and diet were decreased and the weight was lost,the difference was statistically significant.2.The changes of serum biochemical indexes in three groups of rats.The levels of FPG,TG,TC,LDL-C and TABs in DM group were significantly higher than those in CON group,and the levels of FPG,TG,TC,LDL-C and TABs in DM + MET group were significantly lower than those in DM group,the difference was statistically significant.3.The changes of liver morphology and intrahepatic cholesterol in three groups of rats.Compared with CON,the results of H&E and Oil Red O staining showed that the hepatic fat vacuoles in DM group were large and dense.The morphological structure of liver cells was disorganized.The contents of TC and LDL-C in liver cells were increased by blood chemistry,and the difference was statistically significant.Compared with DM group,In the DM + MET group,the morphology of hepatic fat vacuoles became smaller and fewer,the morphology of hepatocytes changed slightly,and the levels of TC and LDL-C in liver tissue decreased,with statistical significance.4.The expression of SCAP,SREBP-2,LDLR,FXR and CYP7a1 in three groups of rats.Compared with CON group,the expression of SCAP,SREBP-2 protein and CYP7a1 mRNA in DM group increased,while the expression of LDLR,FXR protein and FXR mRNA decreased in DM group.Compared with DM group,the expression of SCAP,SREBP-2,LDLR protein and CYP7a1 mRNA in DM + MET group were significantly increased,while the expression of FXR mRNA and protein were decreased,the difference was statistically significant.Conclusion1.Metformin reduced the diet,drinking and body weight in diabetic rats and improved their glucose,insulin,cholesterol and bile acid metabolism.2.Metformin improved liver morphology and reduced hepatic cholesterol levels in DM rats.However,metformin up-regulated the expression of SCAP and SREBP-2 protein,which had the effect of promoting cholesterol synthesis.At the same time,metformin up-regulated the expression of LDLR protein to promote peripheral cholesterol uptake and clearance,thereby lowering serum cholesterol levels.3.Metformin promoted the expression of CY7a1 by down-regulating FXR in DM rats,promoted the conversion of hepatic cholesterol into bile acid,and reduced the level of hepatic cholesterol.Simultaneously,metformin reduced serum bile acid levels in DM rats,which indicated that metformin promotes bile acid metabolism.4.Metformin improved cholesterol and bile acid metabolism by coordinating the SREBP-2 related cholesterol synthesis pathway and the FXR-related transformed pathway. |
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