Cholesterol-dependent down-regulation of mouse and human apical sodium-dependent bile acid transporter (ASBT) gene expression: molecular mechanism and physiological consequences

Charles Thomas ¹, Jean-François Landrier ², Dany Gaillard ¹, Jacques Grober ¹, Marie-Claude Monnot ¹, Anne Athias ³ and Philippe Besnard ^1*

¹ ENSBANA UMR 5170 CNRS/ 1214 INRA, France
² UMR 476 INSERM/1260 INRA, France
³ INSERM U498, France

^* To whom correspondence should be addressed. E-mail: pbesnard{at}u-bourgogne.fr.

Accepted 8 February 2006

Abstract

Fecal bile acid elimination greatly contributes to cholesterol homeostasis. Synthesized from cholesterol in the liver, bile acids are actively reclaimed in the ileum by the apical sodium-dependent bile acid transporter (ASBT). Although the expression level of ASBT affects body cholesterol balance, the impact of cholesterol on ASBT gene expression remains unclear. In this study, the effect of cholesterol on ASBT expression, and ileal bile acid uptake was explored in vivo and in vitro. ASBT gene expression was assessed by real-time quantitative PCR, Northern and/or Western blotting in mice subjected to a 2%-cholesterol diet for 2 weeks, in mouse ileal explants, or in human enterocyte-like Caco-2 cells cultured in sterol-enriched or -depleted media. Bile acid uptake was determined by measuring [3H]-taurocholic acid influx into in situ isolated ileal loops from mice or into differentiated Caco-2 cells. Molecular analysis of mouse, and human ASBT promoters was performed with reporter assays, site-directed mutagenesis and electrophoretic mobility shift assays. In mice, cholesterol-enriched diet triggered a down-regulation of ASBT expression (mRNA and protein), a drop in ileal bile acid uptake, and a rise the fecal excretion of bile acids. This effect was direct since it was reproduced ex vivo using mouse ileal explants, and in vitro in differentiated Caco-2 cells. This regulation, which involves an original partnership between SREBP-2 and HNF-1 transcription factors, affects ileal bile acid recycling, and thus might participate to the maintenance of body cholesterol homeostasis.

Keywords: ASBT, cholesterol homeostasis, HNF-1a, SREBP-2, bile acids

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