REGULATION OF CYP 2A5 INDUCTION BY PORPHYRINOGENIC AGENTS IN MOUSE PRIMARY HEPATOCYTES

All cytochrome P450 (CYP) enzymes contain heme as a prosthetic group. In contrast to other CYP enzymes, murine CYP2A5 is upregulated in vivo by several agents that disturb heme balance. To test the hypothesis t hat porphyrinogenic agents have the common feature of being able to in crease CYP2A5 expression, mouse liver primary hepatocytes were exposed to various porphyrinogenic chemicals and changes in CYP2A5 catalytic activity and levels of mRNA were monitored. Phenobarbital increased he patic CYP2A5-mediated coumarin 7-hydroxylase (COH) activity (13.2-fold ) and the amount of CYP2A5 steady-state mRNA (10.6-fold). Hepatocyte C OH activity was increased also by the ferrochelatase inhibitor griseof ulvin and the protoporphyrinogen oxidase inhibitor acifluorfen (about 9-fold induction). Of these inducers, only phenobarbital affected CYP1 A12 and CYP2B10 expression. In contrast, many other porphyrinogenic ag ents such as cobalt, 2,2,4-trimethyl-1,2-dihydroquinoline (TMDQ), l-2, 4,6-trimethylphenyl)-2,6-cyclohexanedionyl]-O- ethyl propionaldehyde o xime (ATMP), aminotriazole, and thioacetamide either decreased or had no effect on CYP2A5. The increases in COH activity and CYP2A5 mRNA wer e unaffected by combined treatment with the inducers and heme arginate , suggesting that heme is not a regulator of CYP2A5 induction. Treatme nt with actinomycin D totally abolished both constitutive CYP2A5 expre ssion and its inducibility, suggesting that a transcriptional componen t is involved. These data suggest that, in mouse primary hepatocytes, CYP2A5 induction is not a universal response to disturbed cellular hem e biosynthesis.