INDUCTION OF CYP2A5 BY PYRAZOLE AND ITS DERIVATIVES IN MOUSE PRIMARY HEPATOCYTES

Mouse liver CYP2A5 is induced by several structurally unrelated compou nds. In intact mouse liver, pyrazole (PYR) and 4-hydroxypyrazole (4-OH ) induce selectively the expression of CYP2A5 while expression of othe r CYPs is decreased. In this study we exposed mouse primary hepatocyte s to PYR, 4-OH, 4-methylpyrazole (4Me; 0.1-20mM) and 4-iodopyrazole (4 -I; 0.1-5.0 mM). PYR and its derivatives increased coumarin 7-hydroxyl ase activity, with 4-1 and 4-OH being the strongest inducers, by 114-f old and 41-fold, respectively. However, only 4-1 treatment increased m arkedly the CYP2A5 protein content. CYP2B9/10-mediated pentoxyresorufi n O-deethylase activity (PROD) was decreased by 80% by 4-Me and 4-1, a nd by 50% by 4-OH while PYR had no marked effect. PYR and 4-Me increas ed 2- to 3-fold the CYPA1/2-mediated ethoxyresorufin O-deethylase acti vity (EROD) while 4-OH and 4-1 had no marked effect on this enzyme. Th e time of exposure markedly affected the inducibility of 4-OH such tha t induction was 7-fold stronger when it was added to the incubation me dium 24 h after the isolation of hepatocytes compared to exposure 3 h after their isolation. Cimetidine prevented the induction of coumarin 7-hydroxylase activity by PYR and 4-OH by 46 and 74%, respectively ind icating that their effects on the expression of CYP2A5 are, at least p artly, mediated via their metabolites. The data demonstrate that the r egulation of CYP2A5 is different from other monooxygenases and that th e effects of pyrazole and its derivatives are different in vivo and in vitro. Also, the timing of exposure markedly affects the inducibility of 4-OH in hepatocytes.