Induction of Aryl Hydrocarbon (Benzo[a]pyrene) Hydroxylase in Liver Cell Culture by Temporary Inhibition of Protein Synthesis

The microsomal enzyme complex, aryl hydrocarbon (benzo[a]pyrene) hydroxylase, is inducible by benz[a]anthracene in cells cloned from Buffalo rat liver. The enzyme complex can also be induced to high activity, without adding a polycyclic hydrocarbon inducer, by temporarily inhibiting protein synthesis with cycloheximide or puromycin. This increase in enzyme activity is a function of both the degree and the duration of protein synthesis inhibition. It requires RNA synthesis during the period of inhibition of protein synthesis but not subsequent to the removal of the block; and it is prolonged in the presence of actinomycin D, when administered after the release of the block in protein synthesis. Combined exposure to benz[a]anthracene and cycloheximide is followed by an increase in enzyme activity which is greater than the sum of the increases which follow exposure to each compound alone. Qualitatively similar results were observed in four other mammalian cell culture systems. The results suggest two events in the regulation of aryl hydrocarbon hydroxylase induction: the inducer, benz[a]anthracene, acts primarily at the level of transcription, and a labile protein(s) regulates the translation of induction-specific RNA.