Amphibian Vitellogenin: Properties, Hormonal Regulation of Hepatic Synthesis and Ovarian Uptake, and Conversion to Yolk Proteins

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Abstract
Amphibian vitellogenin is a sex-limited protein found in the serum of normal females but not that of males. Its appearance can be induced in males, females, and hypophysectomized animals by estrogens, particularly estradiol-17rβ. The natural source of estradiol-17rβ is the ovary, and ovarian synthesis is markedly stimulated by gonadotropin(s). The pituitary is the natural source of gonadotropin(s), and hypophysectomy invariably leads to ovarian regression. Pituitary secretion in turn appears to depend primarily upon food supply rather than other environmental factors. The liver is the site of vitellogenin synthesis, and its induction by estrogen may involve both transcriptional and translational control mechanisms. Estrogen induction of unprimed liver (in which no vitellogenin synthesis occurs) may involve a cell differentiation or division phenomenon and has not yet been achieved in vitro. Estrogen induction of primed liver (in which a basal level of vitellogenin synthesis takes place) appears to be primarily a modulation phenomenon and can be duplicated to some extent in vitro. The secretion of vitellogenin by liver parenchymal cell is as yet poorly understood. Available evidence indicates that synthesis, phosphorylation, and lipid addition are closely linked events, and that approximately 2 hr are required for the newly synthesized peptide chain to be secreted from the liver. Serum vitellogenin is sequestered by the ovary, and specifically by the vitellogenic oocyte, by a micropinocytotic process. The uptake of vitellogenin is highly selective, is stimulated by gonadotropin(s), and may involve attachment to receptor sites on the oocyte membrane. Once incorporated into the oocyte, vitellogenin is converted within yolk platelets into the yolk proteins lipovitellin and phosvitin, for which it seems to be the sole source. The conversion process appears to involve a proteolytic cleavage of the vitellogenin peptides into those that make up lipovitellin and phosvitin.

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