Coordinate estrogen induction of vitellogenin and a small serum protein mRNA in Xenopus laevis liver

Identification and preliminary characterization of white sturgeon (Acipenser transmontanus) vitellogenin mRNA

Vitellogenesis was induced in white sturgeon by administration of estrogen through silastic implants. Vitellogenin mRNA was identified by agarose gel electrophoresis and cell-free translation. A highly abundant 5.7-kb mRNA was induced in the liver of estrogen-treated sturgeon. Cell-free translation of poly(A)+ mRNA showed the induction of two high-molecular-weight proteins of 180 and 120 kDa. These two proteins, encoded by the 5.7-kb mRNA(s), were immunoprecipitated by antiserum to serum from vitellogenic sturgeon. Immunoprecipitations also showed the presence of four other serum proteins synthesized by the liver of estrogen-treated sturgeon. The induction of vitellogenesis by estrogen in sturgeon, which are a primitive teleost, was found to be similar to induction of vitellogenesis in amphibians, avians, and other teleosts. Estrogen treatment induced a highly abundant vitellogenin mRNA as well as several mRNAs for other serum proteins. However, the presence of two distinct vitellogenin monomers in the cell-free translation assay was significantly different from the results in other species.

Estrogen induction of a 45 kDa secreted protein coordinately with vitellogenin in Xenopus liver

In the frog Xenopus laevis, vitellogenin is the major estrogen-induced protein in the liver. We have characterized an additional secreted protein, of 45,000 Da and designated Ep45, which is also responsive to estrogen treatment. Like vitellogenin, Ep45 is not normally found in the plasma nor synthesized by the liver of the male frog. Its synthesis increases 6-fold between days 2 and 8 following a single 2 mg injection of estradiol-17 beta. For comparison, we have also studied a third estrogen-regulated protein, Ep20, with a molecular weight of approximately 20,000. This protein exhibits a different set of characteristics with regard to hormone responsiveness. Ep20 is synthesized in the liver of normal males and therefore is not absolutely hormone-dependent. Its level increases only about 4-fold following estrogen stimulation. The messenger RNAs for both Ep45 and Ep20 have been identified and purified, using a high-resolution RNA fractionation technique. By this procedure, it was possible to demonstrate that following high doses of estrogen the predominant mRNAs in the liver are those coding for vitellogenin, Ep45 and Ep20. Thus estrogen suppression of virtually all other liver proteins appears to act at the messenger RNA level for intracellular as well as secreted proteins.

Estrogen-dependent DNA synthesis in cultures of Xenopus liver parenchymal cells

We have recently shown that extensive proliferation of liver parenchymal cells takes place in adult male Xenopus frogs in response to estradiol-17 beta, which also induces synthesis and secretion of vitellogenin, the precursor of yolk proteins. We demonstrate here that liver parenchymal cells from adult male animals can be maintained for several weeks in a defined tissue culture medium containing added insulin, dexamethasone, and triiodothyronine. Under these conditions the cells do not divide, but can synthesize DNA. Maximum DNA synthesis occurs in cells that have achieved monolayer morphology under low plating densities. Estradiol-17 beta causes a dose-dependent increase in the number of cells synthesizing DNA, as well as inducing synthesis of vitellogenin. Estrogen-dependent, but not background, DNA synthesis is inhibited by the antiestrogen tamoxifen. These results imply that estradiol-17 beta acts directly on liver cells to initiate DNA replication, probably by interaction with a receptor protein and induction of specific gene transcription.

Dexamethasone and estrogen regulate Xenopus laevis albumin mRNA levels

The regulation of albumin mRNA levels by steroid hormones was examined in a primary Xenopus liver culture system. In the absence of exogenous steroid hormone, albumin mRNA levels declined rapidly in culture. Dexamethasone is required for preservation of albumin mRNA levels in culture and effects a greater than 10 fold induction of albumin mRNA in cultures maintained in steroid hormone-free medium. Estrogen can override the effect of dexamethasone and elicits a decline of greater than 80% in albumin mRNA levels. Our cDNA clones of the mRNAs encoding the two 74,000 dalton and one 72,000 dalton cellular albumins allowed us to show that all three albumin mRNAs were down regulated by estrogen.