Abstract

Transactivation of the rat apical sodium-dependent bile acid transporter (ASBT; Slc10a2) by 1α,25-dihydroxyvitamin D₃ [1,25(OH)₂D₃] via the vitamin D receptor (VDR), was studied. Levels of ASBT protein and mRNA were low in the duodenum and high in the ileum, and both were induced by 1,25(OH)₂D₃. The nuclear receptor protein, VDR, was present uniformly in the duodenum, jejunum, and ileum of the rat small intestine. The physiological relevance of ASBT induction by 1,25(OH)₂D₃ was assessed by measuring absorption of cholylsarcosine, a non-metabolized synthetic bile acid analog, from duodenal or ileal closed loops of the perfused rat small intestine preparation. Absorption of cholylsarcosine was much greater from the ileal segment (28-fold that of the duodenum under control conditions) and was enhanced with 1,25(OH)₂D₃ treatment. Transient transfection analysis of the rat ASBT promoter in Caco-2 cells revealed concentration-dependent enhancement of luciferase reporter activity after treatment with 1,25(OH)₂D₃. The activation by 1,25(OH)₂D₃ was abrogated after site-directed mutagenesis or deletion of the vitamin D response element (VDRE) in the ASBT promoter. Gel-shift mobility assays of nuclear extracts from rat ileum showed that both rat retinoid X receptor and VDR were bound to the VDRE. The results indicate that rat ASBT gene expression is activated by 1,25(OH)₂D₃ by specific binding to the VDRE and that such activation enhances ileal bile acid transport. Human ABST mRNA and promoter activity were also increased in Caco-2 cells treated with 1,25(OH)₂D₃, suggesting a physiological role of VDR in human ileal bile acid homeostasis.