Oestrogen receptor levels and vitellogenin synthesis during development of Xenopus laevis.

Abstract

The synthesis of vitellogenin, the precursor of the major eggyolk proteins, becomes inducible by oestrogen in Xenopus laevis larval liver during metamorphosis1–4, well before vitellogenin is required for oocyte growth. The synthesis in vivo of some liver proteins is responsive to oestradiol at earlier stages, but vitellogenin inducibility does not appear until a precise point during metamorphic climax, stage 62, when many ‘adult’ liver proteins first start to be synthesized5. This suggests that vitellogenin inducibility is not determined simply by the presence of an oestrogen receptor. The receptor identified and characterized in fully inducible adult liver6,7 (also detected by others8) is unusual because it is present at very low concentrations and 50% is found in the nucleus in unstimulated animals6,7, and is itself synthesized in response to oestrogen, raising the level in the nucleus9. Here we have measured nuclear receptor levels and the effect of oestrogen on these during Xenopus metamorphosis. Larvae which cannot be induced to synthesize vitellogenin contain an oestrogen receptor with the same affinity and specificity as the adult receptor. Treatment with oestrogen raises the level of nuclear oestrogen receptor in inducible but not in uninducible larvae. These and additional results from- thyrostatic larvae suggest that an increase in the level of nuclear receptor is necessary for the induction of vitellogenin synthesis, and that the onset of vitellogenin inducibility might be determined by the ability of oestrogen to increase nuclear receptor levels.