title: Cholesterol modulates human intestinal sodium-dependent bile acid transporter.
abstract: 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.
authors.affiliation: Section of Digestive Diseases and Nutrition, Department of Medicine, University of Illinois at Chicago and Jesse Brown Veteran Affairs Medical Center, Medical Research Service (600/151 820 South Damen Ave., Chicago, Illinois 60612, USA. walrefai@uic.edu
authors.forename: Waddah A
authors.initials: WA
authors.lastname: Alrefai
authors.forename: Zaheer
authors.initials: Z
authors.lastname: Sarwar
authors.forename: Sangeeta
authors.initials: S
authors.lastname: Tyagi
authors.forename: Seema
authors.initials: S
authors.lastname: Saksena
authors.forename: Pradeep K
authors.initials: PK
authors.lastname: Dudeja
authors.forename: Ravinder K
authors.initials: RK
authors.lastname: Gill
mesh_terms: D018938:Caco-2 Cells; D005786:Gene Expression Regulation / Q000502:physiology*; D006801:Humans; D006888:Hydroxycholesterols / Q000494:pharmacology*; D029363:Organic Anion Transporters, Sodium-Dependent / Q000378:metabolism*; D011401:Promoter Regions, Genetic / Q000502:physiology; D012333:RNA, Messenger / Q000378:metabolism; D027981:Symporters / Q000378:metabolism*; D013656:Taurocholic Acid / Q000378:metabolism
chemical_list: D006888:Hydroxycholesterols; D029363:Organic Anion Transporters, Sodium-Dependent; D012333:RNA, Messenger; D027981:Symporters; C071908:sodium-bile acid cotransporter; D013656:Taurocholic Acid; C007997:25-hydroxycholesterol