Abstract

Several of the hepatic microsomal cytochromes P450 can be induced by various drugs and xenobiotics, among them the barbiturate phenobarbital. Rat hepatoma cells (Fao and its derivatives) respond to phenobarbital or dexamethasone treatment with an increased accumulation of CYP2C6 mRNA and thus provide a culture system to investigate the mechanisms involved. Examination of the kinetics of CYP2C6 mRNA induction revealed that the response to dexamethasone is rapid, whereas induction by phenobarbital occurs only slowly after an 8-10-hr lag. Run-on transcription measurements demonstrated that phenobarbital treatment led to a 3-4-fold increase in CYP2C6 gene transcription. Surprisingly, induction by phenobarbital of both accumulation of CYP2C6 mRNA and transcription of the gene was blocked by the antiprogestin-antiglucocorticoid RU486, suggesting the involvement of a steroid receptor in the induction process. Transfection of promoter constructs containing a reporter gene whose expression is driven by a 1.4-kilobase 5' flanking segment of the CYP2B1 or CYP2B2 genes, which are highly inducible by phenobarbital in rat liver, led to > 3-fold increases in reporter gene activity in the presence of the drug. Again, phenobarbital induction was prevented by RU486. The RU486 inhibition of the phenobarbital induction of both the endogenous CPY2C6 gene and the transfected CYP2B1 and CYP2B2 promoter constructs leads us to propose a model whereby the drug acts indirectly to cause the accumulation of an endogenous steroid, and this molecule, acting via its receptor, would be the direct inducer of cytochromes P450. Whether or not this model proves to be correct, the results presented here provide the first evidence of the involvement of a steroid receptor in phenobarbital induction.