Expression of cytochrome P-450 and albumin genes in rat liver: effect of xenobiotics.

Thioacetamide, a hepatocarcinogen and an inhibitor of heme synthesis, blocks the phenobarbitone-mediated increase in the transcription of cytochrome P-450b+e messenger RNA in rat liver. This property is also shared by CoCl2 and 3-amino-1,2,4-triazole, two other inhibitors of heme synthesis. Thus, it appears feasible that heme may serve as a positive regulator of cytochrome P-450b+e gene transcription. Thioacetamide enhances albumin messenger RNA concentration, whereas phenobarbitone decreases the same. However, these changes in albumin messenger RNA concentration are not accompanied by corresponding changes in the transcription rates. Therefore, drug-mediated changes in albumin messenger RNA concentration are due to posttranscriptional regulation. The property of thioacetamide to enhance the albumin messenger RNA concentration is not shared by CoCl2 and 3-amino-1,2,4-triazole. Therefore, heme does not appear to be a regulatory molecule mediating the reciprocal changes brought about in the concentrations of cytochrome P-450b+e and albumin messenger RNAs.