Liver receptor homologue-1 mediates species- and cell line-specific bile acid-dependent negative feedback regulation of the apical sodium-dependent bile acid transporter

Intestinal reclamation of bile salts is mediated in large part by the apical sodium-dependent bile acid transporter (ASBT). The bile acid responsiveness of ASBT is controversial. Bile acid feeding in mice results in decreased expression of ASBT protein and mRNA. Mouse but not rat ASBT promoter activity was repressed in Caco-2, but not IEC-6, cells by chenodeoxycholic acid. A potential liver receptor homologue-1 (LRH-1) cis-acting element was identified in the bile acid-responsive region of the mouse but not rat promoter. The mouse, but not rat, promoter was activated by LRH-1, and this correlated with nuclear protein binding to the mouse but not rat LRH-1 element. The short heterodimer partner diminished the activity of the mouse promoter and could partially offset its activation by LRH-1. Interconversion of the potential LRH-1 cis-elements between the mouse and rat ASBT promoters was associated with an interconversion of LRH-1 and bile acid responsiveness. LRH-1 protein was found in Caco-2 cells and mouse ileum, but not IEC-6 cells or rat ileum. Bile acid response was mediated by the farnesoid X receptor, as shown by the fact that overexpression of a dominant-negative farnesoid X-receptor eliminated the bile acid mediated down-regulation of ASBT. In addition, ASBT expression in farnesoid X receptor null mice was unresponsive to bile acid feeding. In summary cell line- and species-specific negative feedback regulation of ASBT by bile acids is mediated by farnesoid X receptor via small heterodimer partner-dependent repression of LRH-1 activation of the ASBT promoter.     

Cholesterol modulates human intestinal sodium-dependent bile acid transporter

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 microg/ml for 24 h) significantly inhibited Na(+)-dependent 3H-TC uptake in Caco-2 cells. This inhibition was associated with a 50% decrease in the V(max) of the transporter with no significant changes in the apparent K(m). The inhibition in hASBT activity was associated with reduction in both the level of hASBT mRNA and its promoter activity. Our data show the inhibition of hASBT function and expression by 25-HCH in Caco-2 cells. These data provide novel evidence for the direct regulation of human ASBT function by cholesterol and suggest that this phenomenon may play a central role in cholesterol homeostasis.     

The small heterodimer partner interacts with the pregnane X receptor and represses its transcriptional activity

SHP (small heterodimer partner, NR1I0) is an atypical orphan member of the nuclear receptor subfamily in that it lacks a DNA-binding domain. It is mostly expressed in the liver, where it binds to and inhibits the function of nuclear receptors. SHP is up-regulated by primary bile acids, through the activation of their receptor farnesoid X receptor, leading to the repression of cholesterol 7alpha-hydroxylase (CYP7alpha) expression, the rate-limiting enzyme in bile acid production from cholesterol. PXR (pregnane X receptor, NR1I2) is a broad-specificity sensor that recognizes a wide variety of synthetic drugs as well as endogenous compounds such as bile acid precursors. Upon activation, PXR induces CYP3A and inhibits CYP7alpha, suggesting that PXR can act on both bile acid synthesis and elimination. Indeed, CYP7alpha and CYP3A are involved in biochemical pathways leading to cholesterol conversion into primary bile acids, whereas CYP3A is also involved in the detoxification of toxic secondary bile acid derivatives. Here, we show that PXR is a target for SHP. Using pull-down assays, we show that SHP interacts with both murine and human PXR in a ligand-dependent manner. From transient transfection assays, SHP is shown to be a potent repressor of PXR transactivation. Furthermore, we report that chenodeoxycholic acid and cholic acid, two farnesoid X receptor ligands, induce up-regulation of SHP and provoke a repression of PXR-mediated CYP3A induction in human hepatocytes as well as in vivo in mice. These results reveal an elaborate regulatory cascade, tightly controlled by SHP, for both the maintenance of bile acid production and detoxification in the liver.     

beta-Klotho and FGF-15/19 inhibit the apical sodium-dependent bile acid transporter in enterocytes and cholangiocytes

beta-Klotho, a newly described membrane protein, regulates bile acid synthesis. Fibroblast growth factor-15 (FGF-15) and FGF receptor-4 (FGFR4) knockout mice share a similar phenotype with beta-Klotho-deficient mice. FGF-15 secretion by the intestine regulates hepatic bile acid biosynthesis. The effects of beta-Klotho and FGF-15 on the ileal apical sodium bile transporter (ASBT) are unknown. beta-Klotho siRNA treatment of the mouse colon cancer cell line, CT-26, and the human intrahepatic biliary epithelial cells (HIBEC) resulted in upregulation of endogenous ASBT expression that was associated with reduced expression of the farnesoid X receptor (FXR) and the short heterodimer partner (SHP). Silencing beta-Klotho activated the ASBT promoter in CT-26, Mz-ChA-1 (human cholangiocarcinoma), and HIBEC cells. Site-directed mutagenesis of liver receptor homolog-1 (mouse) or retinoic acid receptor/retinoid X receptor (RAR/RXR) (human) cis-elements attenuated the basal activity of the ASBT promoter and abrogated its response to beta-Klotho silencing. siSHP, siFXR, or dominant-negative FXR treatment also eliminated the beta-Klotho response. FGF-15 secretion into cell culture media by CT-26 cells was diminished after siFGF-15 or sibeta-Klotho treatment and enhanced by chenodeoxycholic acid. Exogenous FGF-19 repressed ASBT protein expression in mouse ileum, gallbladder, and in HIBEC and repressed ASBT promoter activity in Caco-2, HIBEC, and Mz-ChA-1 cells. Promoter repression was dependent on the expression of FGFR4. These results indicate that both beta-Klotho and FGF-15/19 repress ASBT in enterocytes and cholangiocytes. These novel signaling pathways need to be considered in analyzing bile acid homeostasis.     

Human bile acid transporter ASBT (SLC10A2) forms functional non-covalent homodimers and higher order oligomers

The human apical sodium-dependent bile acid transporter, hASBT/SLC10A2, plays a central role in cholesterol homeostasis via the efficient reabsorption of bile acids from the distal ileum. hASBT has been shown to self-associate in higher order complexes, but while the functional role of endogenous cysteines has been reported, their implication in the oligomerization of hASBT remains unresolved. Here, we determined the self-association architecture of hASBT by site-directed mutagenesis combined with biochemical, immunological and functional approaches. We generated a cysteine-less form of hASBT by creating point mutations at all 13 endogenous cysteines in a stepwise manner. Although Cysless hASBT had significantly reduced function correlated with lowered surface expression, it featured an extra glycosylation site that facilitated its differentiation from wt-hASBT on immunoblots. Decreased protein expression was associated with instability and subsequent proteasome-dependent degradation of Cysless hASBT protein. Chemical cross-linking of wild-type and Cysless species revealed that hASBT exists as an active dimer and/or higher order oligomer with apparently no requirement for endogenous cysteine residues. This was further corroborated by co-immunoprecipitation of differentially tagged (HA-, Flag-) wild-type and Cysless hASBT. Finally, Cysless hASBT exhibited a dominant-negative effect when co-expressed with wild-type hASBT which validated heterodimerization/oligomerization at the functional level. Combined, our data conclusively demonstrate the functional existence of hASBT dimers and higher order oligomers irrespective of cysteine-mediated covalent bonds, thereby providing greater understanding of its topological assembly at the membrane surface.     

The nuclear receptor for bile acids, FXR, transactivates human organic solute transporter-alpha and -beta genes

Bile acids are synthesized from cholesterol in the liver and are excreted into bile via the hepatocyte canalicular bile salt export pump. After their passage into the intestine, bile acids are reabsorbed in the ileum by sodium-dependent uptake across the apical membrane of enterocytes. At the basolateral domain of ileal enterocytes, bile acids are extruded into portal blood by the heterodimeric organic solute transporter OSTalpha/OSTbeta. Although the transport function of OSTalpha/OSTbeta has been characterized, little is known about the regulation of its expression. We show here that human OSTalpha/OSTbeta expression is induced by bile acids through ligand-dependent transactivation of both OST genes by the nuclear bile acid receptor/farnesoid X receptor (FXR). FXR agonists induced endogenous mRNA levels of OSTalpha and OSTbeta in cultured cells, an effect that was not discernible upon inhibition of FXR expression by small interfering RNAs. Furthermore, OST mRNAs were induced in human ileal biopsies exposed to the bile acid chenodeoxycholic acid. Reporter constructs containing OSTalpha or OSTbeta promoters were transactivated by FXR in the presence of its ligand. Two functional FXR binding motifs were identified in the OSTalpha gene and one in the OSTbeta gene. Targeted mutation of these elements led to reduced inducibility of both OST promoters by FXR. In conclusion, the genes encoding the human OSTalpha/OSTbeta complex are induced by bile acids and FXR. By coordinated control of OSTalpha/OSTbeta expression, bile acids may adjust the rate of their own efflux from enterocytes in response to changes in intracellular bile acid levels.     

Characterization of cloned mouse Na+/taurocholate cotransporting polypeptide by transient expression in COS-7 cells

The mouse Na+/taurocholate cotransporting polypeptide transiently expressed in COS-7 cells caused sodium-dependent uptake of [3H]taurocholic acid with Km and Vmax values of 18 microM and 102 pmol/mg protein/min, respectively. This Km value is comparable to that for rat NTCP and higher than that for human NTCP. Substrate specificity was evaluated by measuring inhibitory effects of unlabeled bile acids on [3H]taurocholic acid transport.     

FXR agonists and FGF15 reduce fecal bile acid excretion in a mouse model of bile acid malabsorption

Bile acid malabsorption, which in patients leads to excessive fecal bile acid excretion and diarrhea, is characterized by a vicious cycle in which the feedback regulation of bile acid synthesis is interrupted, resulting in additional bile acid production. Feedback regulation of bile acid synthesis is under the control of an endocrine pathway wherein activation of the nuclear bile acid receptor, farnesoid X receptor (FXR), induces enteric expression of the hormone, fibroblast growth factor 15 (FGF15). In liver, FGF15 acts together with FXR-mediated expression of small heterodimer partner to repress bile acid synthesis. Here, we show that the FXR-FGF15 pathway is disrupted in mice lacking apical ileal bile acid transporter, a model of bile acid malabsorption. Treatment of Asbt-/- mice with either a synthetic FXR agonist or FGF15 downregulates hepatic cholesterol 7alpha-hydroxylase mRNA levels, decreases bile acid pool size, and reduces fecal bile acid excretion. These findings suggest that FXR agonists or FGF15 could be used therapeutically to interrupt the cycle of excessive bile acid production in patients with bile acid malabsorption.     

Disrupted coordinate regulation of farnesoid X receptor target genes in a patient with cerebrotendinous xanthomatosis

Cerebrotendinous xanthomatosis (CTX), sterol 27-hydroxylase (CYP27A1) deficiency, is associated with markedly reduced chenodeoxycholic acid (CDCA), the most powerful activating ligand for farnesoid X receptor (FXR). We investigated the effects of reduced CDCA on FXR target genes in humans. Liver specimens from an untreated CTX patient and 10 control subjects were studied. In the patient, hepatic CDCA concentration was markedly reduced but the bile alcohol level exceeded CDCA levels in control subjects (73.5 vs. 37.8 +/- 6.2 nmol/g liver). Cholesterol 7alpha-hydroxylase (CYP7A1) and Na+/taurocholate-cotransporting polypeptide (NTCP) were upregulated 84- and 8-fold, respectively. However, small heterodimer partner (SHP) and bile salt export pump were normally expressed. Marked CYP7A1 induction with normal SHP expression was not explained by the regulation of liver X receptor alpha (LXRalpha) or pregnane X receptor. However, another nuclear receptor, hepatocyte nuclear factor 4alpha (HNF4alpha), was induced 2.9-fold in CTX, which was associated with enhanced mRNA levels of HNF4alpha target genes, CYP7A1, 7alpha-hydroxy-4-cholesten-3-one 12alpha-hydroxylase, CYP27A1, and NTCP. In conclusion, the coordinate regulation of FXR target genes was lost in CTX. The mechanism of the disruption may be explained by a normally stimulated FXR pathway attributable to markedly increased bile alcohols with activation of HNF4alpha caused by reduced bile acids in CTX liver.