Expression and translocation of cloned human estrogen receptor in the Xenopus oocyte does not induce expression of the endogenous oocyte vitellogenin genes.

Estrogen-receptor complex activates the genes coding for the egg yolk protein precursor vitellogenin in hepatocytes of oviparous vertebrates, while oocyte vitellogenin genes are unresponsive to the hormone. Localization of [3H]steroid hormones (estradiol, progesterone, testosterone, and dexamethasone) was assayed in 10% trichloroacetic acid-precipitated dissected Xenopus oocytes (germinal vesicle vs. the rest of the oocyte). Whether hormones were introduced by incubation in the medium surrounding the oocytes or by injection of an equivalent amount into the oocyte cytoplasm, all hormones partitioned into the nucleus at equivalent levels (approximately 5%), reflecting that portion of the oocyte volume occupied by its nucleus. Therefore, intracellular receptors for these hormones were not detectable. Subsequently, we introduced a species-heterologous estrogen receptor into the Xenopus oocyte via a recombinant plasmid containing the coding sequence for the human estrogen receptor (HER) housed in a vector that ensures highly efficient transcription and translation of inserted sequences. HER synthesis was directed from injected plasmid (2 ng/oocyte germinal vesicle) as shown by [35S]methionine incorporation into newly synthesized proteins; however, vitellogenin was not synthesized under these conditions. When HER plasmid-injected oocytes were incubated in [3H]estradiol, they translocated to the nucleus 38% of the radiolabeled estradiol taken up by the cells, compared to 5% nuclear localization for vector-injected controls. Therefore, although the oocyte can readily transcribe and translate HER sequences as well as appropriately partition the completed protein in the nuclear compartment, the endogenous, potentially estrogen-responsive vitellogenin genes of the oocyte are not expressed.