Gastroenterology

Gastroenterology

Volume 123, Issue 6, December 2002, Pages 2005-2016
Gastroenterology

Basic–Alimentary Tract
Inflammatory-mediated repression of the rat ileal sodium-dependent bile acid transporter by c-fos nuclear translocation,☆☆

https://doi.org/10.1053/gast.2002.37055Get rights and content

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 down-regulation 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.

GASTROENTEROLOGY 2002;123:2005-2016

Section snippets

Indomethacin-induced ileitis

All animal studies were approved by the Animal Care and Use Committees at the Mount Sinai School of Medicine and at the University of North Carolina. Acute ileitis was induced in female Lewis rats (180–200 g) by subcutaneous injection of 10 mg/kg of indomethacin daily for 2 days.22, 23 Control animals were injected with sodium bicarbonate/10% ethanol vehicle only. Forty-eight hours after the first indomethacin injection, the terminal ileum was removed. The number of ulcers was quantified; ileal

Ileal inflammation significantly upregulates c-fos and c-jun and reduces steady-state ASBT expression

Ileitis was induced in Lewis rats by subcutaneous injection of indomethacin. Indomethacin-injected rats (n = 6) had an average of 36 ulcers in the ileum (range, 19–55), whereas control injected rats (n = 6) had none. Phosphorimager quantification of Northern blot analysis showed a marked reduction in steady-state ASBT mRNA levels and equal RNA loading as assessed by quantification of 28S ribosomal RNA (ASBT: indomethacin, 38,900 ± 15,800 vs. control, 97,800 ± 40,300, P < 0.001; 28S RNA:

Discussion

Intestinal inflammation is associated with a variety of abnormalities in solute transport.16, 40, 41, 42 Clinical studies in humans indicate that bile acid malabsorption is observed in various forms of ileitis.11, 12, 13, 14, 15 Molecular investigations have not yet been performed in humans to determine if this is mediated by alterations in the expression or function of ASBT. Previously, 2 distinct animal models had shown that ileitis induces marked reductions in the expression of ASBT.16, 17

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      As our results show, indomethacin also repressed the expression of ASBT, the ileal transporter responsible for the reuptake of bile acids. This is in agreement with previous studies [56], and may contribute to both impaired activation of the nuclear FXR, and increased luminal proportion of conjugated bile acids, for which it has higher affinity than for the unconjugated forms [57]. Another important finding of the present study is that indomethacin did not increase the hydrophobicity index of bile in the ileum, in contrast to the biliary tract [10,11].

    • ASBT(SLC10A2): A promising target for treatment of diseases and drug discovery

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      Inflammatory process is the main mechanism leading to inflammatory bowel disease. The downregulation mechanism of ASBT mediated by inflammation is related to the increase of levels of IL-1β, IL-6 and TNF-α, which can inhibit ASBT through the combination of c-jun/c-fos heterodimer and dAP-1 [17,86,87]. It can cause malabsorption of BAs to increase the level of BAs in the colon, activate chloride ion channels and cause watery diarrhea.

    • An important intestinal transporter that regulates the enterohepatic circulation of bile acids and cholesterol homeostasis: The apical sodium-dependent bile acid transporter (SLC10A2/ASBT)

      2017, Clinics and Research in Hepatology and Gastroenterology
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      Inflammatory bowel disease (IBD; e.g., Crohn's disease) is characterized by chronic intestinal inflammation along with morbidities, such as malnutrition, weight loss, and diarrhoea [64–66]. BA malabsorption by inflammatory cytokines that inhibit ASBT increases the BA levels in the colon, thus activating chloride channels and causing watery diarrhoea [55,67,68]. In addition, the ASBT levels in the ileum of IBD patients and 23 control subjects were quantitatively measured and were significantly lower in the former than in the latter [66,69].

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    Address requests for reprints to: Benjamin Shneider, M.D., Mount Sinai School of Medicine Box 1656, One Gustave L. Levy Place, New York, New York 10029. e-mail: [email protected]; fax: (212) 427-1951.

    ☆☆

    Supported by National Institutes of Health grants DK 02076, DK 54165, DK 34987, and DK 40249, and a research grant from Lehman Brothers.

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