Getting the mOST from OST: Role of organic solute transporter, OSTalpha-OSTbeta, in bile acid and steroid metabolism.
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ABSTRACT:
SUBMITTER: Dawson PA
PROVIDER: S-EPMC2911127 | biostudies-literature | 2010 Sep
REPOSITORIES: biostudies-literature
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Getting the mOST from OST: Role of organic solute transporter, OSTalpha-OSTbeta, in bile acid and steroid metabolism.
Dawson Paul A PA Hubbert Melissa L ML Rao Anuradha A
Biochimica et biophysica acta 20100609 9
The organic solute transporter (OST)(alpha)-OST(beta) is an unusual heteromeric carrier expressed in a variety of tissues including the small intestine, colon, liver, biliary tract, kidney, and adrenal gland. In polarized epithelial cells, OSTalpha-OSTbeta protein is localized on the basolateral membrane and functions in the export or uptake of bile acids and steroids. This article reviews recent results including studies of knockout mouse models that provide new insights to the role of OSTalpha ...[more]
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Project description:Blocking intestinal bile acid (BA) absorption by inhibiting or inactivating the apical sodium-dependent BA transporter (Asbt) classically induces hepatic BA synthesis. In contrast, blocking intestinal BA absorption by inactivating the basolateral BA transporter, organic solute transporter alpha-beta (Ost?-Ost?) is associated with an altered homeostatic response and decreased hepatic BA synthesis. The aim of this study was to determine the mechanisms underlying this phenotype, including the role of the farnesoid X receptor (FXR) and fibroblast growth factor 15 (FGF15).BA and cholesterol metabolism, intestinal phenotype, expression of genes important for BA metabolism, and intestinal FGF15 expression were examined in wild type, Ost?(-/-), Fxr(-/-), and Ost?(-/-)Fxr(-/-) mice.Inactivation of Ost? was associated with decreases in hepatic cholesterol 7?-hydroxylase (Cyp7a1) expression, BA pool size, and intestinal cholesterol absorption. Ost?(-/-) mice exhibited significant small intestinal changes, including altered ileal villus morphology, and increases in intestinal length and mass. Total ileal FGF15 expression was elevated almost 20-fold in Ost?(-/-) mice as a result of increased villus epithelial cell number and ileocyte FGF15 protein expression. Ost?(-/-)Fxr(-/-) mice exhibited decreased ileal FGF15 expression, restoration of intestinal cholesterol absorption, and increases in hepatic Cyp7a1 expression, fecal BA excretion, and BA pool size. FXR deficiency did not reverse the intestinal morphological changes or compensatory decrease for ileal Asbt expression in Ost?(-/-) mice.These results indicate that signaling via FXR is required for the paradoxical repression of hepatic BA synthesis but not the complex intestinal adaptive changes in Ost?(-/-) mice.
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Project description:The organic solute and steroid transporter (OST/Ost) is a unique membrane transport protein heterodimer composed of subunits designated alpha and beta, that transports conjugated steroids and prostaglandin E(2) across the plasma membrane. Ost was first identified in the liver of the cartilaginous fish Leucoraja erinacea, the little skate, and subsequently was found in many other species, including humans and rodents. The present study describes the isolation of a new cell line, LEE-1, derived from an early embryo of L. erinacea, and characterizes the expression of Ost in these cells. The mRNA size and amino acid sequence of Ost-beta in LEE-1 were identical to that previously reported for Ost-beta from skate liver, and the primary structure was identical to that of the spiny dogfish shark (Squalus acanthias) with the exception of a single amino acid. Ost-beta was found both on the plasma membrane and intracellularly in LEE-1 cells, consistent with its localization in other cell types. Interestingly, arachidonic acid, the precursor to eicosanoids, strongly induced Ost-beta expression in LEE-1 cells and a lipid mixture containing arachidonic acid also induced Ost-alpha. Overall, the present study describes the isolation of a novel marine cell line, and shows that this cell line expresses relatively high levels of Ost when cultured in the presence of arachidonic acid. Although the function of this transport protein in embryo-derived cells is unknown, it may play a role in the disposition of eicosanoids or steroid-derived molecules.
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Project description:Ileal bile acid absorption is mediated by uptake via the apical sodium-dependent bile acid transporter (ASBT), and export via the basolateral heteromeric organic solute transporter ?-? (OST?-OST?). In this study, we investigated the cytotoxic effects of enterocyte bile acid stasis in Ost?-/- mice, including the temporal relationship between intestinal injury and initiation of the enterohepatic circulation of bile acids.Ileal tissue morphometry, histology, markers of cell proliferation, gene, and protein expression were analyzed in male and female wild-type and Ost?-/- mice at postnatal days 5, 10, 15, 20, and 30. Ost?-/-Asbt-/- mice were generated and analyzed. Bile acid activation of intestinal Nrf2-activated pathways was investigated in Drosophila.As early as day 5, Ost?-/- mice showed significantly increased ileal weight per length, decreased villus height, and increased epithelial cell proliferation. This correlated with premature expression of the Asbt and induction of bile acid-activated farnesoid X receptor target genes in neonatal Ost?-/- mice. Expression of reduced nicotinamide adenine dinucleotide phosphate oxidase-1 and Nrf2-anti-oxidant responsive genes were increased significantly in neonatal Ost?-/- mice at these postnatal time points. Bile acids also activated Nrf2 in Drosophila enterocytes and enterocyte-specific knockdown of Nrf2 increased sensitivity of flies to bile acid-induced toxicity. Inactivation of the Asbt prevented the changes in ileal morphology and induction of anti-oxidant response genes in Ost?-/- mice.Early in postnatal development, loss of Ost? leads to bile acid accumulation, oxidative stress, and a restitution response in ileum. In addition to its essential role in maintaining bile acid homeostasis, Ost?-Ost? functions to protect the ileal epithelium against bile acid-induced injury. NCBI Gene Expression Omnibus: GSE99579.
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Project description:The heteromeric steroid transporter organic solute transporter ?/? (OST?/?, SLC51A/B) was discovered over a decade ago, but its physiological significance in the liver remains uncertain. A major challenge has been the lack of suitable models expressing OST?/?. Based on observations first reported here that hepatic OST?/? is upregulated in nonalcoholic steatohepatitis, the aim of this research was to develop an in vitro model to evaluate OST?/? function and interaction with drugs and bile acids. OST?/? expression in human liver tissue was analyzed by quantitative RT-PCR, Western blotting, and immunofluorescence. Radiolabeled compounds were used to determine OST?/?-mediated transport in the established in vitro model. The effect of bile acids and drugs, including those associated with cholestatic drug-induced liver injury, on OST?/?-mediated transport was evaluated. Expression of OST?/? was elevated in the liver of patients with nonalcoholic steatohepatitis and primary biliary cholangitis, whereas hepatocyte expression of OST?/? was low in control liver tissue. Studies in the novel cell-based system showed rapid and linear OST?/?-mediated transport for all tested compounds: dehydroepiandrosterone sulfate, digoxin, estrone sulfate, and taurocholate. The interaction study with 26 compounds revealed novel OST?/? inhibitors: a biomarker for cholestasis, glycochenodeoxycholic acid; the major metabolite of troglitazone, troglitazone sulfate; and a macrocyclic antibiotic, fidaxomicin. Additionally, some drugs (e.g., digoxin) consistently stimulated taurocholate uptake in OST?/?-overexpressing cells. Our findings demonstrate that OST?/? is an important transporter in liver disease and imply a role for this transporter in bile acid-bile acid and drug-bile acid interactions, as well as cholestatic drug-induced liver injury. NEW & NOTEWORTHY The organic solute transporter OST?/? is highly expressed in hepatocytes of liver tissue obtained from patients with nonalcoholic steatohepatitis and primary biliary cholangitis. OST?/? substrates exhibit rapid, linear, and concentration-driven transport in an OST?/?-overexpressing cell line. Drugs associated with hepatotoxicity modulate OST?/?-mediated taurocholate transport. These data suggest that hepatic OST?/? plays an essential role in patients with cholestasis and may have important clinical implications for bile acid and drug disposition.
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Project description:UnlabelledOrganic solute transporter alpha-beta (Ostalpha-Ostbeta) is a heteromeric bile acid and sterol transporter that facilitates the enterohepatic and renal-hepatic circulation of bile acids. Hepatic expression of this basolateral membrane protein is increased in cholestasis, presumably to facilitate removal of toxic bile acids from the liver. In this study, we show that the cholestatic phenotype induced by common bile duct ligation (BDL) is reduced in mice genetically deficient in Ostalpha. Although Ostalpha(-/-) mice have a smaller bile acid pool size, which could explain lower serum and hepatic levels of bile acids after BDL, gallbladder bilirubin and urinary bile acid concentrations were significantly greater in Ostalpha(-/-) BDL mice, suggesting additional alternative adaptive responses. Livers of Ostalpha(-/-) mice had higher messenger RNA levels of constitutive androstane receptor (Car) than wild-type BDL mice and increased expression of Phase I enzymes (Cyp7a1, Cyp2b10, Cyp3a11), Phase II enzymes (Sult2a1, Ugt1a1), and Phase III transporters (Mrp2, Mrp3). Following BDL, the bile acid pool size increased in Ostalpha(-/-) mice and protein levels for the hepatic basolateral membrane export transporters, multidrug resistance-associated protein 3 (Mrp3) and Mrp4, and for the apical bilirubin transporter, Mrp2, were all increased. In the kidney of Ostalpha(-/-) mice after BDL, the apical bile acid uptake transporter Asbt is further reduced, whereas the apical export transporters Mrp2 and Mrp4 are increased, resulting in a significant increase in urinary bile acid excretion.ConclusionThese findings indicate that loss of Ostalpha provides protection from liver injury in obstructive cholestasis through adaptive responses in both the kidney and liver that enhance clearance of bile acids into urine and through detoxification pathways most likely mediated by the nuclear receptor Car.
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