Molecular basis for increased synthesis of albumin in rat liver after thioacetamide administration.

Administration of thioacetamide to rats for 4 days has been shown previously to increase the activity of liver messenger RNA (mRNA) and to disproportionately increase the activity of albumin mRNA in the wheat germ translation system. We have explored the possibility that administration of thioacetamide quantitatively alters the transcription of rat liver unique DNA. Nucleic acid hybridization between the radioactive rat liver unique DNA and RNA from untreated as well as 4-day thioacetamide-treated rat liver indicates that the normal extent of transcription is not altered by the drug treatment. We also investigated the possibility that thioacetamide treatment increases the level of mRNA in general or of albumin mRNA in particular. Albumin mRNA, which is th most abundantly represented population in rat liver cytoplasmic mRNA, was further enriched by sucrose gradient fractionation of the liver mRNA. Synthesis of complementary DNA (cDNA) and then use of the strategy of limited hybridization yielded the cDNA corresponding to albumin mRNA. Hybridization of the fractionated cDNA with mRNA from untreated livers as well as from livers with increasing days of treatment shows no change in the level of albumin mRNA. The results indicate that drug treatment induces the augmented synthesis of albumin by disproportionately increasing the translational activity of albumin mRNA. Quantitation of gene dosage by comparison of hybridization kinetics of fractionated cDNA and unique [3H]DNA with total rat cellular DNA indicates that there are not more than two copies of the albumin gene present in the rat genome.