Campana Gabriele, Pasini Patrizia, Roda Aldo, Spampinato Santi
Department of Pharmacology, Alma Mater Studiorum-University of Bologna, 40126 Bologna, Italy.
Biochem Pharmacol. 2005 Jun 15;69(12):1755-63. doi: 10.1016/j.bcp.2005.03.019.
Ursodeoxycholic acid (UDCA) is beneficial in cholestatic diseases but its molecular mechanisms of action remain to be clearly elucidated. Other bile acids, such as chenodeoxycholic (CDCA), are agonists for the nuclear farnesoid X receptor (FXR) and regulate the expression of genes relevant for bile acid and cholesterol homeostasis. In ileal cells CDCA, through the FXR, up-regulates the expression of the ileal bile acid-binding protein (IBABP), implicated in the enterohepatic circulation of bile acids. We report that UDCA (100 and 200 microM) induced a moderate increase of IBABP mRNA (approximately 10% of the effect elicited by 50 microM CDCA) in enterocyte-like Caco-2 cells and approximately halved the potent effect of CDCA (50 microM). On the contrary, UDCA reduced by 80-90% CDCA-induced IBABP transcription in hepatocarcinoma derived HepG2 cells. We confirmed that these effects on IBABP transcription required the FXR by employing a cell-based transactivation assay. Finally, in a receptor binding assay, we found that UDCA binds to FXR expressed in CHO-K1 cells (K(d)=37.7 microM). Thus, UDCA may regulate IBABP in Caco-2 cells, which express it constitutively, by acting as a partial agonist through a FXR mediated mechanism. The observation that in HepG2 cells, which do not express constitutively IBABP, UDCA was able to almost completely prevent CDCA-induced activation of IBABP promoter, suggests that tissue-specific factors, other than FXR, may be required for bile acid regulation of FXR target genes.
熊去氧胆酸（UDCA）对胆汁淤积性疾病有益，但其分子作用机制仍有待明确阐释。其他胆汁酸，如鹅去氧胆酸（CDCA），是核法尼醇X受体（FXR）的激动剂，可调节与胆汁酸和胆固醇稳态相关基因的表达。在回肠细胞中，CDCA通过FXR上调回肠胆汁酸结合蛋白（IBABP）的表达，该蛋白与胆汁酸的肠肝循环有关。我们报道，在肠上皮样Caco-2细胞中，UDCA（100和200微摩尔）可使IBABP mRNA适度增加（约为50微摩尔CDCA所引发效应的10%），并使CDCA（50微摩尔）的强效作用减半。相反，在肝癌衍生的HepG2细胞中，UDCA可使CDCA诱导的IBABP转录降低80 - 90%。我们通过基于细胞的反式激活试验证实，这些对IBABP转录的影响需要FXR参与。最后，在受体结合试验中，我们发现UDCA可与CHO - K1细胞中表达的FXR结合（解离常数K(d)=37.7微摩尔）。因此，UDCA可能通过FXR介导的机制作为部分激动剂，调节组成型表达IBABP的Caco-2细胞中的IBABP。在未组成型表达IBABP的HepG2细胞中，UDCA几乎能完全阻止CDCA诱导的IBABP启动子激活，这一观察结果表明，除FXR外，可能还需要组织特异性因子来调节FXR靶基因的胆汁酸水平。
