Differential hepatocellular zonation pattern of cholesterol 7α-hydroxylase (Cyp7a1) and sterol 12α-hydroxylase (Cyp8b1) in the mouse

The synthesis of primary bile acids is confined to the hepatocytes. This study aimed to evaluate the expression pattern within the liver architecture of the rate-limiting enzyme of the neutral pathway, cholesterol 7 alpha-hydroxylase (Cyp7a1), and sterol 12 alpha-hydroxylase (Cyp8b1), the enzyme necessary for the synthesis of cholic acid. Specific Cyp8b1 and Cyp7a1 peptide antiserums were used for immunohistochemical staining of livers from wild type and Cyp8b1 null mice, the latter instead expressing beta-galactosidase (beta-Gal) as a replacement reporter gene. Cyp8b1 was mainly expressed in the hepatocytes in a zonal pattern surrounding the central vein while the areas surrounding the portal zones showed much lower levels. The zonation was maintained in cholic acid-depleted mice using beta-Gal as a reporter protein. Cyp7a1 expression in wild type mice also showed a zonal distribution pattern, although less distinct, with a maximal expression within a 1-2 cell thick layer of hepatocytes surrounding the central vein. In Cyp8b1 null mice, a more intense staining was obtained, in accordance with the higher expression level of Cyp7a1, although the overall expression pattern was maintained. Our results in mice indicate possible differences in the regulation of the cellular zonation of Cyp7a1 and Cyp8b1. Also, cholic acid affects the setpoint of Cyp7a1 expression but not its zonal distribution.