Toxic bile acids induce hepatocyte apoptosis, for which p53 and cyclin D1 have been implicated as underlying mediators. Both p53 and cyclin D1 are targets of c-Myc, which is also up-regulated in cholestasis. Myc and Mnt use Max as a cofactor for DNA binding. Myc-Max typically activates transcription via E-box binding. Mnt-Max also binds the E-box sequence but serves as a repressor and inhibits the enhancer activity of Myc-Max. The current work tested the hypothesis that the switch from Mnt-Max to Myc-Max is responsible for p53 and cyclin D1 up-regulation and apoptosis during cholestasis. Following common bile duct ligation or left hepatic bile duct ligation, the expression of p53, c-Myc, and cyclin D1 increased markedly, whereas Mnt expression decreased. Nuclear binding activity of Myc to the E-box element of p53 and cyclin D1 increased, whereas that of Mnt decreased in a time-dependent fashion. Lithocholic acid (LCA) treatment of primary human hepatocytes and HuH-7 cells induced a similar switch from Mnt to Myc and increased p53 and cyclin D1 promoter activity and endogenous p53 and cyclin D1 expression and apoptosis. Blocking c-Myc induction in HuH-7 cells prevented the LCA-mediated increase in p53 and cyclin D1 expression and reduced apoptosis. Lowering Mnt expression further enhanced LCA's inductive effect on p53 and cyclin D1. Bile duct-ligated mice treated with a lentivirus harboring c-myc small interfering RNA were protected from hepatic induction of p53 and cyclin D1, a switch from Mnt to Myc nuclear binding to E-box, and hepatocyte apoptosis.
Conclusion: The switch from Mnt to Myc during bile duct ligation and in hepatocytes treated with LCA is responsible for the induction in p53 and cyclin D1 expression and contributes to apoptosis.