Inflammatory-mediated repression of the rat ileal sodium-dependent bile acid transporter by c-Fos nuclear translocation

Abstract

Background & Aims: Ileal malabsorption of bile salts is observed in Crohn's ileitis. We define the transcriptional mechanisms involved in cytokine-mediated repression of the rat apical sodium-dependent bile acid transporter (ASBT). Methods: ASBT regulation was studied in IL-1β-treated IEC-6 and Caco-2 cells and in indomethacin-treated rats. Results: Indomethacin-induced ileitis in Lewis rats leads to specific reductions in ileal ASBT messenger RNA and protein levels, whereas c-jun and c-fos are induced. The proinflammatory cytokines interleukin-1β and tumor necrosis factor repress the activity of the ASBT promoter in Caco-2 and intestinal epithelial cell-6 cells. This effect is blocked by the proteasome inhibitor, MG-132, or by the phosphatidyl inositol 3-kinase inhibitor, wortmannin. Indomethacin (in vivo) or proinflammatory cytokine (in vitro) treatment leads to serine phosphorylation and nuclear translocation of c-fos. Mutation of a 5′ activated protein (AP)-1 site inactivates the ASBT promoter, whereas mutation of the 3′ site abrogates the proinflammatory cytokine-mediated repression. The 5′ site binds a c-jun homodimer, whereas the 3′ site binds a c-jun/c-fos heterodimer. c-Jun overexpression enhances ASBT promoter activity, whereas a dominant negative c-jun construct inactivates the promoter. c-Fos overexpression represses promoter activity. A 27 base pair cis-element from the 3′ site in the ASBT promoter imparts cytokine-mediated downregulation to a heterologous SV40 promoter construct. Conclusions: The ASBT promoter contains 2 distinct cis AP-1 elements; the 5′ element binds homodimeric c-jun and mediates basal transcription. Inflammation is associated with up-regulation, phosphorylation, and nuclear translocation of c-fos, which then represses ASBT promoter activity via binding of the 3′ AP-1 element by a c-fos/c-jun heterodimer.