Induction of the hepatic mixed-function oxidase system by synthetic glucocorticoids. Transcriptional and post-transcriptional regulation.

Quantitative evaluation of transcriptional rates in isolated nuclei and intranuclear and cytoplasmic mRNA levels demonstrates that the catatoxic steroids pregnenolone-16 alpha-carbonitrile (PCN) and dexamethasone modulate microsomal drug metabolizing enzymes both positively and negatively at the transcriptional level. Additionally, a strong post-transcriptional influence, believed to be message stabilization, was found to be exerted by dexamethasone. Both NADPH-cytochrome P-450b-homologous mRNA are induced 7- and 12-fold, respectively, presumably through message stabilization. Under these conditions, however, the oxidoreductase content of the cell is only marginally increased while the cytochrome P-450b level is actually decreased by 40% within three days after a single injection of dexamethasone. In contrast, genes coding for cytochromes P-450PCN show a definite increase in transcription following administration of either pre gnenolone-16 alpha-carbonitrile or dexamethasone, whereas transcription of the epoxide hydrolase gene is markedly reduced by administration of these compounds.