The Role of Estrogen Receptor in Xenopus Laevis Vitellogenin Gene Expression

Administration of estradiol 17 beta [estra-1,3,5(10)-triene-3,17-beta-diol] to male Xenopus laevis induces the massive synthesis by the liver of the egg yolk precursor phospholipoglycoprotein, vitellogenin, and its cognate mRNAs. Restimulation of male X. laevis that have been previously induced to synthesize vitellogenin mRNA but are inactive in vitellogenin mRNA synthesis at the time of restimulation with estrogen results in more rapid accumulation of vitellogenin mRNA and more efficient transcription of the vitellogenin genes than occurs following primary estrogen stimulation. The estrogen receptor system that mediates estrogen action in this organism exhibits several unusual properties. The cytoplasm of unstimulated liver cells contains high levels of a middle-affinity estrogen-specific binding protein and little if any estrogen receptor. The properties of the estrogen binding protein are consistent with a role in protecting estradiol 17 beta against metabolism, as a fraction of cytoplasmic estradiol 17 beta is not subject to rapid metabolism. In addition, similar binding activities are found in all Xenopus tissues surveyed that respond to steroid hormones. The induction of nuclear estrogen receptor is coincident with the onset of vitellogenin mRNA accumulation. However, an increased level of estrogen receptor is not responsible for the elevated rate of vitellogenin gene transcription observed following restimulation with estrogen.