Abstract

Cytochromes P450b and P450e (IIB1 and IIB2, respectively) are two remarkably similar microsomal hemoproteins whose inductions in rat liver are generally believed to be coordinately controlled by such xenobiotics as phenobarbital. To critically examine this assumption, we used a new system of primary cultures of adult rat hepatocytes on Matrigel to evaluate whether organochlorine pesticides, as "phenobarbital-like" agents, directly induce these cytochromes in parallel in the liver parenchymal cell. For 14 of the pesticides we tested, as well as for phenobarbital, P450b and P450e mRNAs, measured on Northern blots, rose in concert as much as 58- and 6-fold, respectively, over the amounts in incubated control cultures. Kepone (chlordecone) treatment of the cultures increased P450e mRNA in a dose-dependent manner that disclosed a 10-fold greater potency, compared with cultures exposed to phenobarbital. Kepone also resembled phenobarbital in these experiments, in that there were dose-dependent increases in the amounts of hepatocellular P450p, P450pcn2, P450PB-1, P450f, and NADPH-cytochrome P450 oxidoreductase mRNAs. However, in the same kepone-treated cells, P450b mRNA or P450b immunoreactive protein was induced only slightly, if at all. In contrast, additions to the medium of mirex, a structural analog of kepone, effectively induced both P450b and P450e mRNAs and their proteins. Selective induction of P450e by kepone in the hepatocyte cultures, the first pharmacologic dissociation of the induction of P450b and P450e mRNAs and proteins, was not apparent in kepone-treated rats, where both P450b and P450e mRNAs were increased to equivalent extents. We conclude that the P450b and P450e genes may be expressed independently by process(es), possibly involving extrahepatic factors, that can be defined with the present hepatocyte culture system.