The Effect Of Ursodeoxycholic Acid On The Bile Acid Enterohepatic Circulation

Background and purposeRecently,bile acids(BAs)have been increasingly recognized as signaling molecules that help regulate lipid,glucose and energy metabolism and thus,maintain the BA homeostasis.One of its key receptors,farnesoid x receptor(FXR),plays an important role in maintaining bile acids homeostasis in enterohepatic circulation.However,the influence of individual bile acid on FXR signaling pathway was not consistent with each other and their effects on enterohepatic circulation has been under-investigated.Ursodeoxycholic acid(UDCA)is a BA with choleretic,anti-inflammatory,and cytoprotective properties and is a firstline treatment approved by the Food and Drug Administration(FDA)for primary biliary cholangitis(PBC).UDCA has been shown to enhance the expression of hepatobiliary secretion transporters,which are positively regulated by the FXR.However,UDCA has also been reported to be a FXR antagonist in human,leaving some questions as to the mechanism by which UDCA facilitates hepatic BA secretion and exerts liver protective effects.UDCA and its derivatives have been tested in other liver diseases in addition to PBC.Nonalcoholic steatohepatitis(NASH)with the increasing prevalence rate is becoming a major cause of liver disease.UDCA and its derivatives also have been tested in NASH.One study found a reduction in steatosis,while randomized placebo-controlled trials reported no improvement in ballooning and inflammation in patients with NASH.On the other hand,taurine-conjugated ursodeoxycholic acid(TUDCA)reduced hepatic steatosis and enhanced the activity of insulin in mouse liver,muscle,and adipose tissues.Therefore,their therapeutic potential should be further determined because of the inconsistent results.The aim of this study was to study the influence of individual bile acids on mice BA profile,and study the effect of UDCA on the kinetics of BA enterohepatic circulation.Additionally,we will investigate the effect of UDCA on BA and fatty acid(FA)profile of a high fat diet-induced obese mouse model and investigate the potential mechanism.MethodsWe performed the targeted BA metabolomics to delineat the dynamic changes in BA enterohepatic circulation in response to individual bile acid interventions in mice.In vitro molecular biology measurements were conducted to detect the changes of expression and function of BA transporters in liver and ileum as a result of exposure to UDCA.In addition,we applied isotope dilution techniques and biochemical analyses in vivo and in vitro to study the kinetics of BA enterohepatic circulation after oral administration of UDCA and the mechanistic pathways involved.In the high fat diet(HFD)-induced obese mice model,we profiled BAs and FAs in serum,liver,white adipose tissue and brown adipose tissue,and studied the correlations between BAs and FAs.Furthermore,gene expressions involved in FA metabolism were determined by techniques of molecular biology to explore potential mechanism.Results1.Based on the change of BAs profile after feeding different bile acids,we found administration of UDCA lead to decreased BA pool size,in which non 12-OH BAs proportion increased sharply,and TUDCA was dominated.BAs distributed more in distal intestine and less in gallbladder.The amount of BAs excretion in feces increased after UDCA administration.In contrast,administration of deoxycholic acid(DCA)led to a very different BA profile,in which 12-OH BAs proportion increased significantly and taurineconjugated cholic acid(TCA)was the dominating component.Lower levels of BAs were found in distal intestine and the size of BA pool remained unchanged.2.Oral administration of UDCA upregulated the expression of apical sodium dependent bile acid transporter(ASBT),which was main transporter involved in BA active reabsorption in the terminal ileum.Besides,transporters Na+-dependent taurocholic cotransporting polypeptide(NTCP)and organic anion transport polypeptide 1(OATP1)responsible for BAs taken up back to liver,and transporters multidrug resistance related protein 2(MRP2)and bile-salt export pump(BSEP)responsible for the excretion of BAs into the bile through the canalicular membrane were induced in UDCA group.Meanwhile,we found UDCA administration accelerated BAs enterohepatic circulation.After further study,we found intestinal FXR signaling pathway was inhibited after feeding UDCA,but liver FXR signaling pathway was not.3.We mimicked BA profile that was identified in UDCA-treated mice in cell culture studies,and found that intestinal FXR signaling pathway was inhibited,which was consistent with the results in vivo.However,liver FXR signaling pathway was also inhibited in vitro,which was inconsistent with that in vivo.The changes in transporters we observed in liver cell culture were consistent with what have been reported,but was not in accordance with what we saw for BSEP in vivo.From the above,it may be inferred that some other factors participate in controlling some of the BA transporters in liver.4.We next investigated the fibroblast growth factor 15(FGF15),which is a gut-derived hormone connecting cross-talk between intestine and liver.Level of FGF15 was downregulated in the UDCA group from the in vivo study,so we treated cultured liver cells with FGF19(homologous analog of FGF15 in human)at different concentrations to observe its influence on BA transporters and found liver BA transporters MRP2 and BSEP were inhibited by FGF19 in a dose dependent manner.After intraperitoneal injection of FGF19 in mice,we found that the expressions of NTCP,OATP1,MRP2,and BSEP were significantly decreased.Besides,UDCA intervention led to a much higher fold change in cholecystokinin(CCK)levels between the postprandial and fasted status.Meanwhile,we observed that the size of the gallbladder in the UDCA group was smaller especially when compared with the DCA group.After intraperitoneal injection of FGF19 in vivo,the weight of gallbladder increased.5.UDCA improved metabolic dysfunction in HFD-induced obese mice,including improvement of body weight,liver index,fat index,oral glucose tolerance test(OGTT)and insulin tolerance test(ITT).6.UDCA resulted in a unique BA profile in which TUDCA and UDCA dominated the composition.The levels of saturated fatty acids(SFA)decreased in most tissues,while polyunsaturated fatty acids(PUFA),in particular,n3 PUFA,significantly increased in the serum of mice.7.The levels of UDCA and its derivatives were positively correlated with those of n-3 PUFA,TUDCA particularly.On the other hand,lithocholic acid(LCA)species were negatively correlated with SFA in liver.8.Finally,we found that genes involved in de novo synthesis and uptake of FFA and triglyceride formation were downregulated by UDCA administration,whereas genes related to FFA oxidation were upregulated.ConclusionOur study demonstrates that oral administration of UDCA accelerated BA enterohepatic circulation by inhibiting intestinal FXR signaling pathway,thus increased the output of BAs in feces and decreased BA pool size.Meanwhile,UDCA improved FAs profile and had beneficial effect on the HFD-induced obesity in mice.This study provides a new therapeutic strategy for metabolic diseases such as obesity using UDCA or UDCA derivatives.