Abstract

Phenobarbital induces cytochromes P450 (P450s) of not only the class IIB gene subfamily (i.e., P450b and P450e) but also the class IIIA gene subfamily (P450p and P450pcn2). To determine whether coinduction of these structurally dissimilar gene products involves the same mechanism, we examined the dose dependency of phenobarbital induction of the mRNAs for these four P450s in a new responsive system for primary monolayer cultures of adult rat hepatocytes on Matrigel, a reconstituted basement membrane. Two-day treatments of the cultures with phenobarbital produced marked dose-dependent increases in the levels of P450b, P450e, and P450p mRNAs, which reached maximal inductions ranging from 11- to greater than 193-fold. Although the dose-response relationships for the inductions of P450b and P450e mRNAs by phenobarbital were similar (ED50 = 1.5 x 10(-5) and 5.7 x 10(-6) M, respectively), the dose-response curve for the induction of P450p mRNA was positioned distinctly to the right (ED50 = 3.0 x 10(-4) M). This difference reflects a potency ratio of 20-fold for P450p/P450b mRNA induction. Phenobarbital also produced a weak dose-dependent induction of P450pcn2 mRNA, with a potency (ED50 = 3.4 x 10(-5) M) intermediate between those for P450b/e and P450p. In a similar experiment using two phenobarbital-like inducers, (trans)nonachlor and clotrimazole, the relative inductions of P450b, P450e, P450p, and P450pcn2 mRNAs proved to be similar to those produced by phenobarbital (P450p/P450b potency ratios = 14- and 16-fold, respectively). These findings provide strong further evidence in support of the newly emerging concept that "phenobarbital" induction of the responsive class IIB and class IIIA P450 isozymes likely reflects multiple mechanisms.