Thyroid hormone induces competence for oestrogen-dependent vitelogenin synthesis in developing Xenopus laevis liver

Abstract

OESTROGEN-DEPENDENT synthesis of vitellogenin, the precursor of the major yolk proteins, in the liver of egg-laying vertebrates is an attractive model for investigating regulatory mechanisms of gene expression. Induction of vitellogenin synthesis by oestrogen occurs not only in the liver of females but also in males which normally do not produce this protein, thus showing that responsiveness to oestrogen is not linked to sex (for review see ref. 1). However, Follet and Redshaw² reported that liver of Xenopus laevis larvae is refractory to oestrogen, becoming responsive shortly after completion of metamorphosis. As amphibian metamorphosis is known to be obligatorily dependent on thyroid hormones (for review see ref. 3), the question arises as to whether acquisition of competence to respond to oestrogen by the Xenopus liver cells is part of the hormonally regulated metamorphic changes or merely reflects age-dependent maturation. Here we present evidence that during normal development of Xenopus laevis the liver cells become responsive to oestrogen during metamorphosis and before vitellogenin synthesis has been initiated by endogeneous oestrogens. Moreover, we demonstrate that responsiveness to oestrogen fails to appear when metamorphosis is suppressed, but readily appears after induction of metamorphosis by thyroxine, suggesting that acquisition of competence to respond to oestrogen by the Xenopus liver cells is controlled by thyroid hormones.