Lack of association between CYP2A5 induction and apoptosis in mouse primary hepatocytes

Upregulation of mouse hepatic cytochrome P450 2A5 (CYP2A5) is a process closely associated with hepatocellular damage and formation of liver tumours. 2-Aminopurine, a protein kinase inhibitor modulating cell cycle control, was recently shown to strongly induce CYP2A5 in mouse hepatocytes. The objective of this study was to determine the association between CYP2A5 induction and apoptosis in mouse primary hepatocytes. Five well-characterised CYP2A5 inducers were tested for their ability to affect apoptosis rate, determined by immunohistochemical in situ 3'-end-labelling technique, in a primary mouse hepatocyte model. Transforming growth factor beta (TGFbeta) was used as a positive (proapoptotic) control. Phenobarbital, pyrazole and the mitogen-activated protein kinase inhibitor PD98059 did not significantly affect apoptosis rate in hepatocytes. Norcocaine induced apoptosis at 6 hr (1.8-fold) and 2-aminopurine 12 hr (1.4-fold) after treatment, which is considerably earlier than peaks in the amount of CYP2A5 mRNA. TGFbeta reduced CYP2A5 marker activity, coumarin 7-hydroxylase by 74%. These results indicate that in a primary hepatocyte model (a) there is no systematic correlation between apoptosis and CYP2A5 induction; (b) phenobarbital does not significantly affect the rate of apoptosis; and (c) the induction of apoptosis caused by the chemicals tested occurs considerable earlier than elevation of CYP2A5 expression. Thus, no causal link appears to exist between induction of CYP2A5 and apoptotic rate.