Ileal reclamation of bile salts, a critical determinant of their enterohepatic circulation, is mediated primarily by the apical sodium-dependent bile acid transporter (ASBT=SLC10A2). We have defined mechanisms involved in the transcriptional regulation of ASBT. The ASBT gene extends over 17 kilobases and contains five introns. Primer extension analysis localized two transcription initiation sites 323 and 255 base pairs upstream of the initiator methionine. Strong promoter activity is imparted by both a 2.7- and 0.2-kilobase 5′-flanking region of ASBT. The promoter activity is cell line specific (Caco-2, not Hep-G2, HeLa-S3, or Madin-Darby canine kidney cells). Four distinct specific binding proteins were identified by gel shift and cross-linking studies using Caco-2 or rat ileal nuclear extracts. Two AP-1 consensus sites were identified in the proximal promoter. DNA binding and promoter activity could be abrogated by mutation of the proximal AP-1 site. Supershift analysis revealed binding of c-Jun and c-Fos to this AP-1 element. Co-expression of c-Jun enhanced promoter activity in Caco-2 cells and activated the promoter in Madin-Darby canine kidney cells. Region and developmental stage-specific expression of ASBT in the rat intestine correlated with the presence of one of these DNA-protein complexes and both c-Fos and c-Jun proteins. A specific AP-1 element regulates transcription of the rat ASBT gene.
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Published, JBC Papers in Press, August 16, 2001, DOI 10.1074/jbc.M104511200
This work was supported by National Institutes of Health Grants DK 02076 and DK 54165.The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
