Enterohepatic bile salt transporters in normal physiology and liver disease (original) (raw)
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Hepatology, 2002
The bile salt export pump (BSEP or ABCB11) mediates the adenosine triphosphate–dependent transport of bile salts across the canalicular membrane of the hepatocyte. Mutations in the corresponding ABCB11 gene cause progressive familial intrahepatic cholestasis type 2. The aim of this study was to investigate the regulation of human ABCB11 gene transcription by bile salts. First, a 1.7-kilobase human ABCB11 promoter region was cloned. Sequence analysis for possible regulatory elements showed a farnesoid X receptor responsive element (FXRE) at position −180. The farnesoid X receptor (FXR) functions as a heterodimer with the retinoid X receptor α (RXRα) and can be activated by the bile salt chenodeoxycholic acid (CDCA). Luciferase reporter gene assays showed that the ABCB11 promoter is positively controlled by FXR, RXRα, and bile salts in a concentration-dependent manner. Mutation of the FXRE strongly represses the FXR-dependent induction. Second, endogenous ABCB11 transcription regulation was studied in HepG2 cells, stably expressing the rat sodium-dependent taurocholate transporter (rNtcp) cells. ABCB11 expression was induced by adding bile salts to the culture medium, and this effect was maximized by combining it with cotransfection of rFxr and hRXRα. Reducing endogenous FXR levels using RNA interference fully repressed the bile salt–induced ABCB11 expression. In conclusion, these results show that FXR is required for the bile salt–dependent transcriptional control of the human ABCB11 gene and that the cellular amount of FXR is critical for the level of activation of ABCB11 transcription. (HEPATOLOGY 2002;35:589-596.)
Journal of Biological Chemistry, 2001
and hierarchy of bile acids as ligands for the farnesyl/ bile acid receptor (FXR/BAR) paralleled their ability to induce BSEP in human hepatocyte cultures. FXR:RXR heterodimers bound to IR1 elements and enhanced bile acid transcriptional activation of the mouse and human BSEP/SPGP promoters. In FXR/BAR nullizygous mice, which have dramatically reduced BSEP/SPGP levels, hepatic CYP3A11 and CYP2B10 were strongly but unexpectedly induced. Notably, the rank order of bile acids as CYP3A4 inducers and activators of pregnane X receptor/steroid and xenobiotic receptor (PXR/SXR) closely paralleled each other but was markedly different from their hierarchy and potency as inducers of BSEP in human hepatocytes. Moreover, the hepatoprotective bile acid ursodeoxycholic acid, which reverses hydrophobic bile acid hepatotoxicity, activates PXR and efficaciously induces CYP3A4 (a bile-metabolizing enzyme) in primary human hepatocytes thus providing one mechanism for its hepatoprotection. Because serum and urinary bile acids increased in FXR/BAR ؊/؊ mice, we evaluated hepatic transporters for compensatory changes that might circumvent the profound decrease in BSEP/SPGP. We found weak MRP3 up-regulation. In contrast, MRP4 was substantially increased in the FXR/ BAR nullizygous mice and was further elevated by cholic acid. Thus, enhanced hepatocellular concentrations of bile acids, due to the down-regulation of BSEP/SPGPmediated efflux in FXR nullizygous mice, result in an alternate but apparent compensatory up-regulation of CYP3A, CYP2B, and some ABC transporters that is consistent with activation of PXR/SXR by bile acids.
Expression and regulation of hepatic drug and bile acid transporters
Toxicology, 2000
Transport across hepatocyte plasma membranes is a key parameter in hepatic clearance and usually occurs through different carrier-mediated systems. Sinusoidal uptake of compounds is thus mediated by distinct transporters, such as Na + -dependent or Na + -independent anionic transporters and by some cationic transporters. Similarly, several membrane proteins located at the apical pole of hepatocytes have been incriminated in the excretion of compounds into the bile. Indeed, biliary elimination of anionic compounds, including glutathione S-conjugates, is mediated by MRP2, whereas bile salts are excreted by a bile salt export pump (BSEP) and Class I-P-glycoprotein (P-gp) is involved in the secretion of amphiphilic cationic drugs, whereas class II-P-gp is a phospholipid transporter. The expression of hepatic transporters and their activity are regulated in various situations, such as ontogenesis, carcinogenesis, cholestasis, cellular stress and after treatment by hormones and xenobiotics. Moreover, a direct correlation between a defect and the absence of transporter with hepatic disease has been demonstrated for BSEP, MDR3-P-gp and MRP2.
Bile acid transporters and regulatory nuclear receptors in the liver and beyond
Journal of Hepatology, 2013
Bile acid (BA) transporters are critical for maintenance of the enterohepatic BA circulation where BAs exert their multiple physiological functions including stimulation of bile flow, intestinal absorption of lipophilic nutrients, solubilization and excretion of cholesterol, as well as antimicrobial and metabolic effects. Tight regulation of BA transporters via nuclear receptors is necessary to maintain proper BA homeostasis. Hereditary and acquired defects of BA transporters are involved in the pathogenesis of several hepatobiliary disorders including cholestasis, gallstones, fatty liver disease and liver cancer, but also play a role in intestinal and metabolic disorders beyond the liver. Thus, pharmacological modification of BA transporters and their regulatory nuclear receptors opens novel treatment strategies for a wide range of disorders.
Journal of Biological Chemistry, 2001
The bile salt excretory pump (BSEP, ABCb11) is critical for ATP-dependent transport of bile acids across the hepatocyte canalicular membrane and for generation of bile acid-dependent bile secretion. Recent studies have demonstrated that the expression of this transporter is sensitive to the flux of bile acids through the hepatocyte, possibly at the level of transcription of the BSEP gene. To determine the mechanisms underlying the regulation of BSEP by bile acids, the promoter of the BSEP gene was cloned. The sequence of the promoter contained an inverted repeat (IR)-1 element (5'-GGGACA T TGATCCT-3') at base pairs -63/-50 consisting of two nuclear receptor half-sites organized as an inverted repeat and separated by a single nucleotide. This IR-1 element has been shown in several recent studies to serve as a binding site for the farnesoid X receptor (FXR), a nuclear receptor for bile acids. FXR activity requires heterodimerization with RXR alpha, and when bound by bile acids, the complex effectively regulates the transcription of several genes involved in bile acid homeostasis. Gel mobility shift assays demonstrated specific binding of FXR/RXR alpha heterodimers to the IR-1 element in the BSEP promoter. In HepG2 cells, co-transfection of FXR and RXR alpha is required to attain full transactivation of the BSEP promoter by bile acids. Two FXR transactivation-deficient mutants (an AF-2 deletion and a W469A point mutant) failed to transactivate, indicating that the effect of bile acids is FXR-dependent. Further, mutational analysis confirms that the FXR/RXR alpha heterodimer activates transcription through the IR-1 site in the human BSEP promoter. These results demonstrate a mechanism by which bile acids transcriptionally regulate the activity of the bile salt excretory pump, a critical component involved in the enterohepatic circulation of bile acids.
Cholesterol modulates human intestinal sodium-dependent bile acid transporter
AJP: Gastrointestinal and Liver Physiology, 2005
Bile acids are efficiently absorbed from the intestinal lumen via the ileal apical sodium-dependent bile acid transporter (ASBT). ASBT function is essential for maintenance of cholesterol homeostasis in the body. The molecular mechanisms of the direct effect of cholesterol on human ASBT function and expression are not entirely understood. The present studies were undertaken to establish a suitable in vitro experimental model to study human ASBT function and its regulation by cholesterol. Luminal membrane bile acid transport was evaluated by the measurement of sodium-dependent 3H-labeled taurocholic acid (3H-TC) uptake in human intestinal Caco-2 cell monolayers. The relative abundance of human ASBT (hASBT) mRNA was determined by real-time PCR. Transient transfection and luciferase assay techniques were employed to assess hASBT promoter activity. Caco-2 cell line was found to represent a suitable model to study hASBT function and regulation. 25-Hydroxycholesterol (25-HCH; 2.5 μg/ml fo...