Regulation of CYP2A5 induction by porphyrinogenic agents in mouse primary hepatocytes

Abstract

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 cellular heme balance. To test the hypothesis that porphyrinogenic agents have the common feature of being able to increase 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 hepatic CYP2A5-mediated coumarin 7-hydroxylase (COH) activity (13.2-fold) and the amount of CYP2A5 steady-state mRNA (10.6-fold). Hepatocyte COH activity was increased also by the ferrochelatase inhibitor griseofulvin and the protoporphyrinogen oxidase inhibitor acifluorfen (about 9-fold induction). Of these inducers, only phenobarbital affected CYP1A12 and CYP2B10 expression. In contrast, many other porphyrinogenic agents such as cobalt, 2,2,4-trimethyl-1,2-dihydroquinoline (TMDQ), 1-[4-(3-acetyl-2,4,6-trimethylphenyl)-2,6-cyclohexanedionyl]-O-ethyl propionaldehyde oxime (ATMP), aminotriazole, and thioacetamide either decreased or had no effect on CYP2A5. The increases in COH activity and CYP2A5 mRNA were unaffected by combined treatment with the inducers and heme arginate, suggesting that heme is not a regulator of CYP2A5 induction. Treatment with actinomycin D totally abolished both constitutive CYP2A5 expression and its inducibility, suggesting that a transcriptional component is involved. These data suggest that, in mouse primary hepatocytes, CYP2A5 induction is not a universal response to disturbed cellular heme biosynthesis.