RESUMO
The effect of dexamethasone on rat alpha-fetoprotein (AFP) expression has been further examined. Quantitation of serum AFP levels from newborns treated with dexamethasone showed a dose-response relationship between the quantity of dexamethasone administered and the reduction in AFP serum level. RNA blots, utilizing cloned AFP cDNA as probe, showed a marked reduction in AFP mRNA in dexamethasone treated livers. The extent of AFP mRNA depletion was correlated with dexamethasone dosage. The effect of dexamethasone on AFP mRNA concentration was relatively rapid; a substantial reduction occurred 12 hours after a single injection. The effect of dexamethasone appeared to be irreversible as hormone withdrawal did not cause AFP mRNA levels to rise. One putative AFP nuclear RNA precursor was identified which rapidly disappeared following dexamethasone treatment. AFP mRNA synthesis was also diminished in nuclei transcribed in vitro. The direct inhibitory effect of glucocorticoid hormone on AFP gene transcription was demonstrated in a reconstituted cell-free nuclear system.
Assuntos
Núcleo Celular/metabolismo , Dexametasona/farmacologia , Genes/efeitos dos fármacos , Fígado/metabolismo , Transcrição Gênica/efeitos dos fármacos , alfa-Fetoproteínas/genética , Animais , Animais Recém-Nascidos , Clonagem Molecular , Citosol/metabolismo , DNA/metabolismo , Cinética , Neoplasias Hepáticas Experimentais/metabolismo , Hibridização de Ácido Nucleico , RNA/isolamento & purificação , RNA Mensageiro/genética , Ratos , Ratos Endogâmicos , Receptores de Glucocorticoides/metabolismo , alfa-Fetoproteínas/análiseRESUMO
Suppression of serum alpha-fetoprotein (AFP) levels in glucocorticoid treated newborn rats was investigated. Daily intraperitoneal injection of 2 micrograms/g body weight of dexamethasone into newborn rats greatly reduced the concentration of AFP in the serum and liver cytosol. In contrast, this treatment stimulated liver ornithine decarboxylase activity. The reduction in AFP levels is not due to a change of distribution of AFP molecular variants, inhibition of secretion of synthesized AFP by the liver or disruption of liver polysomes. Glucocorticoids decrease the AFP levels in hormone-treated rats by supressing the synthesis of AFP. The size of AFP polysomes isolated from the livers of dexamethasone-treated rats were as large as those from normal rats. However, the amount of AFP-producing polysomes in hormone-treated rat liver is only 14% of the controls. By hybridization assays, it was found that dexamethasone treated livers contained decreased amounts of AFP mRNA sequences in liver cytoplasmic and nuclear RNAs. The decreased amounts of AFP mRNA sequences in hormone-treated liver are caused by both a decrease in the rate of AFP mRNA transcription and in AFP mRNA stability.