Induction of cytochrome P450IA1 in mouse hepatoma cells by several chemicals: Phenobarbital and TCDD induce the same form of cytochrome P450

Abstract

The mouse hepatoma cell line Hepa-1 was studied for aryl hydrocarbon hydroxylase (AHH) inducibility by sixteen compounds known to be inducers of cytochrome P450 of different “classes”. Both 2,3,7,8-tetrachlorodibenzo- p-dioxin (TCDD) and sodium phenobarbital induced AHH activity. A cytochrome P450IA1-specific (P 1-450) mouse cDNA probe was used to quantitate mRNA induction. There was a good correlation between the amount of cytochrome P450IA1 mRNA induced and AHH activity. Immunoblots with monoclonal antibody 1-7-1, which recognizes rat liver P450IA1 and P450IA2 (P450c and P450d, respectively), showed that both phenobarbital and TCDD increase the amount of a P450 isozyme immunorelated to P450IA1 in this cell line. Hepa-1 mutants with no AHH inducibility (no functional P450IA1 structural gene; no Ah receptor; no nuclear translocation of the inducerreceptor complex; and presence of dominant repressor) did not respond to phenobarbital. The cytosolic receptor for TCDD ( Ah receptor) was characterized to see if phenobarbital induced cytochrome P450IA1 mRNA and the hydroxylase enzyme through the same mechanism as TCDD. 20 mM Phenobarbital almost completely abolished the binding of 3H-TCDD to the cytosolic receptor. These data indicate that phenobarbital can be a weak ligand for the Ah receptor and thus induce cytochrome P450IA1 and AHH activity. The observation increases the list of different P450 forms inducible by phenobarbital.