Quantitative Analysis of Protein Synthesis Altered by Estrogen in Cultured Xenopus Liver Parenchymal Cell

Sandwich ELISAs for quantification of Xenopus laevis vitellogenin and albumin and their application to measurement of estradiol-17 beta effects on whole animals and primary-cultured hepatocytes

Sandwich enzyme-linked immunosorbent assay (ELISA) was developed for quantification of vitellogenin (VTG) and albumin (ALB) in Xenopus laevis. Working ranges of the ELISAs were 2-1000 ng/ml for VTG and 1-300 ng/ml for ALB. Recoveries of plasma VTG by ELISA were over 90% in dilutions of more than 200 times. The VTG-inducing activity of estradiol-17beta (E2) was measured in whole animals and primary cultured hepatocytes. Immersion of mature male animals in more than 1 nM E2 induced a detectable amount of plasma VTG. VTG induction in younger animals was less potent than in the mature animals but the youngest animals (1.5-3 g body mass) was applicable to the exposure test, irrespective of sex. In vitro exposure of hepatocytes to more than 0.1 nM E2 dose-dependently induced secretion of VTG into the culture medium, while ALB secretion was not significantly affected by E2 treatment. When the VTG-induction levels were normalized by use of a concentration ratio of VTG to ALB, the values obtained from three independent experiments were mutually comparable irrespective of differences in cell density and hepatocyte preparation. Thus, this ratio is thought to be useful for large-scale in vitro screening of estrogenic activities of chemical substances.

Presence of ornithine decarboxylase antizyme in primary cultured hepatocytes of the frog Xenopus laevis

Ornithine decarboxylase (ODC; EC 4.1.1.17) could be induced in primary cultured hepatocytes of the frog, Xenopus laevis, by a hypotonic treatment. Addition of 10 mM putrescine caused a rapid decay of preinduced ODC after a lag period of 30 min. The putrescine-induced ODC decay was faster than the ODC decay in the presence of cycloheximide. Simultaneous addition of cycloheximide blocked the putrescine-induced acceleration of ODC decay, indicating an involvement of protein synthesis. Addition of putrescine to normal medium caused complete loss of ODC activity in 2 h and then ODC-inhibitory activity appeared and progressively increased. The inhibitory factor was non-dialysable and temperature-sensitive and showed a time-independent and stoichiometric pattern of ODC inhibition. On the basis of these observations the inhibitory factor was identified as ODC antizyme. These results indicated that in frog hepatocytes, like in mammalian cells and tissues, ODC is under negative feedback regulation mediated by antizyme.

A change of the hepatocyte population is responsible for the progressive increase of vitellogenin synthetic capacity at and after metamorphosis of Xenopus laevis

Synthesis of the egg yolk precursor protein, vitellogenin, can be induced in adult, but not in larval, amphibian hepatocytes by estrogen treatment. The transition process for this inducibility of hepatocytes during development of Xenopus laevis was examined, using primary cultures of hepatocytes. This was found to occur at about the metamorphic climax of stage 62, although the level of vitellogenin production was very limited at this stage. This low level seemed due neither to insufficient estradiol-17 beta nor to high estrogen-degrading activity. The level of synthesis gradually increased following metamorphosis. Immunohistochemical analysis showed that fewer than 5% of the hepatocytes at stage 62 could be stained with antivitellogenin antibody and that the stained cell fraction subsequently increased gradually for several months after metamorphosis. These findings indicate that adult-type cells capable of synthesizing vitellogenin appear at metamorphosis and then expand their population in the liver during postmetamorphic maturation.

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.

In vitro growth of adult amphibian (Xenopus laevis) hepatocytes and characterization of hepatocyte-proliferating activity in homologous serum

Adult frog (Xenopus laevis) hepatocytes were found to proliferate in a culture medium containing adult homologous serum. Insulin and dexamethasone were required for a net proliferation of hepatocytes. Dose-response analysis showed that a low concentration of serum (greater than or equal to 0.5%) was enough to induce DNA synthesis and mitosis, but a higher concentration (5%) caused certain necrotic changes. Under optimal conditions, there was a two- to threefold increase in nuclei per culture 10 days after serum treatment. Heterologous sera (fetal bovine, calf and chick) showed less proliferative activity. Based on our results, hepatocyte-proliferating activity in adult frog serum is considered to be heat-unstable and acidic protein(s). Thus, adult frog serum may contain hepatopoietin possibly different from well-known growth factors.

Primary culture, cellular stress and differentiated function

Isolation of specialized cell types for the analysis of tissue-specific gene function often results in loss of the differentiated phenotype. Examples of this type of phenotypic change following tissue disaggregation are reviewed together with possible explanations. Close similarities between the effects of cell isolation with those of other cellular stresses such as heat or anoxia point to common biochemical mechanisms being involved. This suggests that the study of freshly isolated cells will contribute significantly to out understanding of the nature of cellular stress and its consequences for the maintenance of phenotype and induction of tissue specific gene expression.

The effect of insulin on amino acid metabolism and glycogen content in isolated liver cells of juvenile coho salmon, Oncorhynchus kisutch

Amino acid transport and [14C]leucine incorporation into liver proteins as well as the secretion of proteins into incubation medium were studied in liver cells isolated from coho salmon (Oncorhynchus kisutch) parr. Pink salmon (Oncorhynchus gorbuscha) or mammalian (bovine) insulin caused a significant increase in TCA-precipitable radioactivity from both cells and incubation medium. The effects appeared at insulin concentration of 10(-8) M with a maximal response at 5 X 10(-8) M. The radioactivity of the TCA-soluble fraction was not changed by insulin. Insulin increased the amount of the non-metabolized amino acid [14C]cycloleucine, in the TCA-soluble fraction of hepatocytes. The glycogen content of hepatocytes was increased in the presence of insulin at 10(-9) M but was not changed from the control value in the presence of insulin at 10(-8) M.

Regulation of protein synthesis by estradiol 17 beta, dexamethasone and insulin in primary cultured Xenopus hepatocytes

Changes in the protein synthesis of Xenopus hepatocytes caused by insulin, estradiol-17 beta (estradiol) and dexamethasone were studied by using a primary culture in serum-free medium. All of these hormones stimulated the synthesis of secretory and intracellular proteins. Dexamethasone induced or stimulated the synthesis of many proteins (though limited in number), whereas estradiol induced or stimulated relatively few proteins, including the yolk precursor protein vitellogenin. The majority of these proteins differed in molecular weight and/or isoelectric point. When hepatocytes were treated with both steroids, most of the proteins were synthesized at the rates expected from the single treatment of the respective steroids. Thus, each steroid selectively stimulated the synthesis of its specific proteins. However, exceptional proteins were observed, whose syntheses were stimulated only by double treatment. In contrast, insulin seemed to cause an overall increase in individual secretory protein synthesis.