Effects Of Farnesoid X Receptor On The Expression And Activity Of Fatty Acid Synthetase And Hepatic Lipase In HepG2 And In Mice

Atherosclerosis(AS) is a common cardiac and cerebrovascular disease.Its formation has close relation with the increase of triglyceride.In the present study,it is shown that hypertriglyceridemia was an independent risk factor of coronary heart disease.Thus,it is meaningful to investigate the mechanism of triglyceride homeostasis for prevention and cure AS.The farnesoid X receptor(FXR) plays a central role in triglyceride metabolism. This experiment is important in the study of the central role of FXR in triglyceride homeostasis.A new approach has been put forward to sudy the mechanism and prevention of atherosclerosis.FXR is a member of the nuclear receptor superfamily that was also identified as bile acid receptor.The human FXR gene has been mapped to chromosome 12q23.1 and has four isoforms:FXRα1,FXRα2,FXRα3,FXRα4.FXR likes other members of this large superfamily of transcription factors,have a number of specific functional domains that usually include a poorly understood amino terminal transcriptional activation domain (AF-1),a DNA binding domain(DBD),a ligand binding domain(LBD),domains responsible for nuclear translocation and dimerization,and a transcriptional activation domain(AF-2) at the extreme carboxyl terminus.FXR modulates the expression of a wide variety of target genes by binding either as a monomer or as a heterodimer with the Retinoid X Receptor(RXR) to DNA sequence motifs called FXR response element(FXREs) or bile acid response element(BARE) in their promoter regions.Most of the FXR response elements correspond to an inverted repeat of the AGGTCA half site spaced by 1 nucleotide (IR-1).FXR is highly expressed in liver,intestine,kidney and adrenals and is lowly expressed in fatty tissue,heart,ovaries,thymus,eyes,spleen,testicle.FXR is activated by endogenic ligand bile acids(BA),such as the primary BA chenodeoxycholic acid(CDCA).In addition to BAs,synthetic FXR agonists have also been identified such as GW4064.BA activated FXR plays an important role in triglyceride,BA,cholesterol and lipid metabolism through regulating the expression of target genes.FXR plays an important role in triglyceride metabolism through regulating the expression of target genes.These target genes are key enzymes,significant lipoprotein and corresponding acceptors about triglyceride metabolism.Fatty acid synthase(FAS) and hepatic lipase(HL) are key enzymes of triglyceride metabolism.The expression and activity of FAS and HL are closely related to the content of triglyceride in vivo.Thus,we identify FAS and HL as FXR-regulated genes.This experiment is important in the study of the effection of FXR on the expression and activity of FAS and HL.A new approach has been put forward to sudy the mechanism of triglyceride and the prevention of atherosclerosis.Fatty acid synthase(FAS),the enzymes required for the de novo biosynthesis of fatty acids from acetyl and malonyl-CoA are integrated into a single polypeptide chain,which is highly expressed in liver,kidney,brain,lung and adipose tissue.Fatty acid synthase is the important enzyme responsible for the synthesis of fatty acid.The FAS gene is known to be regulated by a number of environmental,hormonal and nutritional signals.Significant regul -ation occurs at the transcriptional level where major metabolic signals such as insulin and carbohydrates,thyroid hormone,fatty acids,and sterols have all been shown to influence gene transcription.The other key enzyme in triglyceride metabolism is hepatic lipase.It is a glycoprotein synthesized by hepatocytes that exhibits phospholipase A1 and triglyceride hydrolase activities.The HL enzyme hydrolyzes triglycerides and phospholipids of HDL. Several lines of evidence demonstrate the important role of HL in HDL metabolism.HL may also play a role in facilitating the binding and uptake of apoB-containing lipoprotein -s.This two enzymes play major role in the metabolism of triglyceride.In this study,we will observe the effection of FXR on the expression and activity of FAS and HL in HepG2 cells and C57BL/6 mice.The major results were summarized as follows:1.HepG2 cells were treated with chenodeoxycholic acid(Oμmol/L,25μmol/L,50μmol/L,75μmol/L) for 48h and CDCA(75μmol/L) for 0h,6h,12h,24h,48h.The levels of FAS messenger RNA and protein were determined by using RT-PCR and Western Blot respectively,resulted in a dose-and-time-dependent decrease of FAS messenger RNA and protein levels.2.HepG2 cells were treated with chenodeoxycholic acid(Oμmol/L,25μmol/L,50μmol/L,75μmol/L) for 48h and CDCA(75μmol/L) for 0h,6h,12h,24h,48h.The levels of HL messenger RNA and protein were determined by using RT-PCR and Western Blot respectively,resulted in a dose-and-time-dependent decrease of HL messenger RNA and protein levels.3.C57BL/6 mice were treated with FXR agonists CDCA with different concentrations of0mg/kg,10mg/kg,20mg/kg,50mg/kg,90mg/kg,150mg/kg for seven days and the levels of FAS and HL messenger RNA were determined by using RT-PCR.The expression of FAS,HL and the activity of HL were decreased by CDCA.Meanwhile,the levels of triglyeride(TG) and cholesterol increased in low dose of CDCA but returned to control levels after treated with 90mg/kg of CDCA.It is demonstrated that FXR agonist CDCA can decreace FAS and HL expression at the mRNA level and protein level.So we identify that FAS and HL maybe novel FXR regulated genes in liver cells.But the metabolisms need further study in order to further elucidation the mechanism of FXR in regulation of triglyceride and to establish the experiment foundation for the prevention of atherosclerosis.