The Role And Mechanism Of Metrnl In Lipid Metabolism

BackgroundLipid metabolism disorder is an important risk factor for chronic metabolic diseases.Metrnl is a novel secretory protein found in recent years,which can be secreted by different tissue,suggesting that Metrnl may have a extensive biological functions.In early study,we have proved that Metrnl is a new adipokine,and found that the expression of Metrnl in mouse adipose tissue can be induced by high fat diet.We also found that overexpression of Metrnl in adipose tissue can alleviate high triglyceridemia induced by high fat diet and the acute increase of blood triglyceride caused by intragastric administration of fat emulsion.Consistent with this,Knockout of Metrnl in adipose tissue can aggravate the high triglyceridemia induced by high fat diet.These results suggest that Metrnl may play a important role in the regulation of blood lipids.ObjectiveTo study the regulatory effect of Metrnl on blood lipid parameters and the corresponding mechanism.To find a new target for the treatment of lipid metabolism disorder.Methods1.According to the principle of Cre/Loxp recombination system,Ella-Cre mice and Metrnl^loxp/loxpmice were used to generate Metrnl global knockout mice(Metrnl^-/-),Alb-Cre mice and Metrnl^loxp/loxpmice were used to generate Metrnl liver tissue-specific knockout mice(Liver-Metrnl^-/-).Metrnl knockout were verified by detecting the expression of Metrnl m RNA in different tissues,including liver,intestine,subcutaneous white fat,brown fat with Real time PCR,and detecting the level of blood Metrnl with Elisa.2.To explore the effects of high fat diet on the expression of Metrnl,we detected the expression of Metrnl m RNA in liver and intestine from C57BL/J6 mice under 7 weeks high fat diet for as well as the level of blood Metrnl.3.To explore the effects of Metrnl on lipid parameters,we detected body weight,blood glucose for fasting 12 hours,HDL-C,LDL-C,TC,TG and NEFA with Metrnl^-/-mice under normal chow diet and 16 weeks high fat diet.4.To explore the effects of Metrnl tissue-specific knockout on blood lipid parameters,we detect body weight,blood glucose for fasting 12 hours,HDL-C,LDL-C,TC,TG and NEFA with Intestine-Metrnl^-/-mice,EC-Metrnl^-/-,Liver-Metrnl^-/-mice under normal chow diet and 16 weeks high fat diet.We have also detected blood Metrnl with liver-Metnl^-/-mice to explore the action pathway of liver Metrnl.5.To explore the mechanism of Metrnl on regulating blood cholesterol in liver,we detected the secretion of VLDL in liver with Liver-Metrnl^-/-mice under normal chow diet and high fat diet.We also detected the expression of gene related to lipid metabolism in liver from Liver-Metrnl^-/-mice under high fat diet or primary hepatocytes infected by Metrnl overexpression virus.Results1.Compared with control mice,the expression of tissues Metrnl m RNA and the level of blood Metrnl was undetected in Metrnl^-/-mice,tissues Metrnl m RNA expression of Liver-Metrnl^-/-mice was decreased approximately 75%in specific liver ranther other tissues,but blood Metrnl content was unchanged.Therefore,we have successfully established Metrnl global knockout animal model and Metrnl Liver tissue-specific knockout animal model.2.After high fat diet,the level of serum Metrnl was raised,but realtive expression of Metrnl m RNA was unchanged in liver and intestine.These results suggest that high fat diet can increase the expression of Metrnl in tissues,but it does not include liver and intestine.3.Deficiency of global Metrnl did not change mice body weight,blood sugar for fasting 12 hours and blood lipid under normal chow diet.But blood sugar and TG was increased,HDL-C（24%）and TC（16%）was decreased in Metrnl^-/-mice compared with wide type mice under high fat diet,suggesting that Metrnl could regulate blood HDL-C and TG.4.Deficiency of intestine Metrnl or endothelial cell Metrnl did not change the blood lipid,weight and blood sugar under normal chow diet or high fat diet.Blood glucose and body weight were same between Liver-Metrnl^-/-mice and control mice under normal chow diet.Deficiency of Liver Metrnl decreased blood HDL-C（24%）,LDL-C,TC（20%）,but did not change blood Metrnl under high fat diet.Therefore,liver participates in the regulation of blood HDL-C by Metrnl,intestine and endothelial cell does not participates in the regulation of blood lipid by Metrnl.5.The release of VLDL was same between Liver-Metrnl^-/-mice and control mice under normal chow diet or high fat diet,we have also found that deficiency of liver Metrnl did not change the expression of gene relating to lipid metabolism in liver under high fat diet.Consistent with it,overexpression of Metrnl in primary hepatocytes did not change the expression of gene relating to lipid metabolism.ConclusionsMetrnl could regulate blood HDL-C and TG,liver participates in the regulation of HDL-C by Metrnl,adipose participates in the regulation of TG by Metrnl,intestine and epithelial cells does not participate in the regulation of blood lipid by Metrnl.The regulatory effect of Metrnl on lipid metabolism is expected to be a new target for the treatment of hypertriglyceridemia and hypohdl.