The Effect of Estrogen on Regulatory Enzyme Levels and Albumin Gene Expression in Xenopus Laevis Hepatocytes

Abstract

The estrogen induced synthesis of vitellogenin in male Xenopus laevis is a unique system in which to study the molecular mechanism of hormone action on gene expression. Following administration of estradiol-17 to male Xenopus laevis a specific set of egg yolk proteins together with all components required for the membrane dependent synthesis of a secretory protein are coordinately induced with vitellogenin synthesis.

In an effort to identify genes that are coordinately controlled by estrogen, I quantitated the activity of 3-hydroxy-3-methylglutaryl CoA reductase (HMG-CoA reductase), the key regulatory enzyme in cholesterol synthesis, and acetyl-CoA carboxylase and fatty acid synthetase, regulatory enzymes in the fatty acid synthetic pathway, during estrogen induced vitellogenin synthesis. HMG-CoA reductase and acetyl-CoA carboxylase activity show an increase in parallel with the synthesis of their respective endproducts upon administration of estrogen, while fatty acid synthetase activity is unaffected by the hormone.

The effect of estrogen on the constitutive synthesis of Xenopus serum albumin was examined by hybridization analysis, using cloned albumin cDNA as a probe. This clone was constructed by insertion of an RNA:DNA hybrid into pBR322 and identified by hybridization-translation assay and hybridization to RNA immobilized on diazo-benzyloxymethyl paper. A restriction map was constructed to further characterize the cloned sequence. Hybridization studies using this cloned albumin sequence determined that albumin synthesis was unaffected by estrogen and by the massive increase in vitellogenin synthesis and secretion.