Role of nuclear receptors and hepatocyte-enriched transcription factors for Ntcp repression in biliary obstruction in mouse liver

Abstract

Expression of the main hepatic bile acid uptake system, the Na+-taurocholate cotransporter (Ntcp), is downregulated during cholestasis. Bile acid-induced, farnesoid X receptor (FXR)-mediated induction of the nuclear repressor short heterodimer partner (SHP) has been proposed as a key mechanism reducing Ntcp expression. However, the role of FXR and SHP or other nuclear receptors and hepatocyte-enriched transcription factors in mediating Ntcp repression in obstructive cholestasis is unclear. FXR knockout (FXR−/−) and wild-type (FXR+/+) mice were subjected to common bile duct ligation (CBDL). Cholic acid (CA)-fed and LPS-treated FXR−/− and FXR+/+ mice were studied for comparison. mRNA levels of Ntcp and SHP and nuclear protein levels of hepatocyte nuclear factor (HNF)-1α, HNF-3β, HNF-4α, retinoid X receptor (RXR)-α, and retinoic acid receptor (RAR)-α and their DNA binding were assessed. Hepatic cytokine mRNA levels were also measured. CBDL and CA led to Ntcp repression in FXR+/+, but not FXR−/−, mice, whereas LPS reduced Ntcp expression in both genotypes. CBDL and LPS but not CA induced cytokine expression and reduced levels of HNF-1α, HNF-3β, HNF-4α, RXRα, and RARα to similar extents in FXR+/+ and FXR−/−. DNA binding of these transactivators was unaffected by CA in FXR+/+ mice but was markedly reduced in FXR−/− mice. In conclusion, Ntcp repression by CBDL and CA is mediated by accumulating bile acids via FXR and does not depend on cytokines, whereas Ntcp repression by LPS is independent of FXR. Reduced levels of HNF-1α, RXRα, and RARα in CBDL FXR−/− mice and reduced DNA binding in CA-fed FXR−/− mice, despite unchanged Ntcp levels, indicate that these factors may have a minor role in regulation of mouse Ntcp during cholestasis.

Mrp4−/− mice have an impaired cytoprotective response in obstructive cholestasis

“…22 These discrepancies may be related to strain differences and/or the severity of the cholestatic injury after 7 versus 14 days of CBDL. Ntcp, Oatp1, and Oct1 protein expression decreased in both wild-type and Mrp4Ϫ/Ϫ CBDL mice as previously reported in CBDL rats 21,23,26 and mice, 28,29 although Ntcp reached statistical significance only in the Mrp4Ϫ/Ϫ group (Fig. 4A-B).…”

Hepatobiliary Transporter Expression in Intercellular Adhesion Molecule 1 Knockout and Fas Receptor-Deficient Mice after Common Bile Duct Ligation Is Independent of the Degree of Inflammation and Oxidative Stress

“…In contrast to canalicular bile acid transporters, proinflammatory cytokines as well as bile acids are able to down-regulate Ntcp. In our experiments, Ntcp was down-regulated independent of the degree of inflammation and oxidative stress, suggesting that retention of bile acids may account for protective Ntcp repression in obstructive cholestasis (Geier et al, 2005b;Zollner et al, 2005). This finding is in line with our previous observation that Kupffer cell depletion and cytokine antagonists in CBDL rodents did not affect Ntcp down-regulation (Geier et al, 2005b).…”

The Human Organic Cation Transporter-1 Gene Is Transactivated by Hepatocyte Nuclear Factor-4α

“…HNF-4␣ plays an indispensable role in liver-specific gene expression (Hayhurst et al, 2001) and regulates several genes encoding hepatic membrane transporters (Jung and Kullak-Ublick, 2003;Popowski et al, 2005;Zollner et al, 2005). We hypothesized that the human OCT1 gene may also be a target for HNF-4␣-mediated regulation.…”