Bile acids are potentially toxic end products of
cholesterol metabolism and their concentrations must be tightly regulated. Homeostasis is maintained by both feed-forward regulation and feedback regulation. We used humanized transgenic mice incorporating 13 kb of the 5' regulatory flanking sequence of
CYP3A4 linked to a lacZ reporter gene to explore the in vivo relationship between
bile acids and physiological adaptive
CYP3A gene regulation in acute
cholestasis after bile duct
ligation (BDL). Male transgenic mice were subjected to BDL or
sham surgery prior to sacrifice on days 3, 6, and 10, and others were injected with intraperitoneal
lithocholic acid (LCA) or vehicle alone. BDL resulted in marked hepatic activation of the
CYP3A4/lacZ transgene in pericentral hepatocytes, with an 80-fold increase in transgene activation by day 10. Individual
bile acids were quantified by liquid chromatography/mass spectrometry. Serum 6beta-hydroxylated
bile acids were increased following BDL, confirming the physiological relevance of endogenous
Cyp3a induction to
bile acid detoxification. Although concentrations of conjugated primary
bile acids increased after BDL, there was no increase in LCA, a putative PXR
ligand, indicating that this cannot be the only endogenous
bile acid mediating this protective response. Moreover, in LCA-treated animals, 5-bromo-4-chloro-3-indolyl-beta-d-galactopyranoside staining showed hepatic activation of the
CYP3A4 transgene only on the liver capsular surface, and minimal parenchymal induction, despite significant liver injury. This study demonstrates that
CYP3A up-regulation is a significant in vivo adaptive response to
cholestasis. However, this up-regulation is not dependent on increases in circulating LCA and the role of other
bile acids as regulatory molecules requires further exploration.