Dose response for induction of two cytochrome P-450 isozymes and their mRNAs by 3,4,5,3'4'5'-hexachlorobiphenyl indicating coordinate regulation in rat liver.

The present study compares the time course and dose-response curves for induction of the two major 3-methylcholanthrene (3-MC)-inducible isozymes of cytochrome P-450 and their mRNAs in livers of male rats after administration of 3,4,5,3'4'5'-hexachlorobiphenyl (HCB). Isozyme concentrations were measured by radioimmunoassay. The corresponding translatable mRNAs were measured by translation of polysomes in a cell-free translational system followed by immunoprecipitation and electrophoretic analysis of the translational products. The time course for induction of the two isozymes by HCB indicated that cytochrome P-448MC (P-450c) peaked sooner than P-448HCB (P450d). However, the time course for induction of the two mRNAs was identical. The dose-response curves for induction of the two isozymes and their mRNAs demonstrated that the ED50 for induction of P-448MC was identical to that of P-448HCB, suggesting that the two proteins are induced coordinately by this compound in liver. HCB did not induce P-450PB (the major phenobarbital-inducible isozyme) or affect mRNA levels for this isozyme. Although cytochrome P-448HCB is the predominant cytochrome in liver microsomes from HCB-induced rats, the magnitude of the induction of this isozyme (40-fold) is lower than that of P-448MC (600-fold), because cytochrome P-448HCB is present in higher concentrations in livers of untreated rats than P-448MC (90 versus 3 pmol/mg). Polysomes from control rats also contain more translationally active P-448HCB mRNA than P-448MC mRNA (0.009 versus 0.003% of the total translational products). The increase in the translatable mRNAs (12-fold for P-448HCB mRNA and 40-fold for P-448MC mRNA) was less than the increase in the isozymes. The discrepancy between the magnitude of the induction of the isozymes and their respective mRNAs suggests that factors other than an increase in mRNA influence the magnitude of the increase of the isozymes by HCB. However, HCB did not affect translational efficiency of total mRNA as measured in vitro in the present study. Differences in half-lives of the proteins or effects of HCB on the stability of the proteins might account for the magnitude of the increase in the isozymes after HCB treatment.