Morphological and biochemical changes in the hepatic endoplasmic reticulum and golgi apparatus of male Xenopus laevis after induction of egg-yolk protein synthesis by oestradiol-17 beta

Microscopic Analysis of Nuclear Speckles in a Viviparous Reptile

Nuclear speckles are compartments enriched in splicing factors present in the nucleoplasm of eucaryote cells. Speckles have been studied in mammalian culture and tissue cells, as well as in some non-mammalian vertebrate cells and invertebrate oocytes. In mammals, their morphology is linked to the transcriptional and splicing activities of the cell through a recruitment mechanism. In rats, speckle morphology depends on the hormonal cycle. In the present work, we explore whether a similar situation is also present in non-mammalian cells during the reproductive cycle. We studied the speckled pattern in several tissues of a viviparous reptile, the lizard Sceloporus torquatus, during two different stages of reproduction. We used immunofluorescence staining against splicing factors in hepatocytes and oviduct epithelium cells and fluorescence and confocal microscopy, as well as ultrastructural immunolocalization and EDTA contrast in Transmission Electron Microscopy. The distribution of splicing factors in the nucleoplasm of oviductal cells and hepatocytes coincides with the nuclear-speckled pattern described in mammals. Ultrastructurally, those cell types display Interchromatin Granule Clusters and Perichromatin Fibers. In addition, the morphology of speckles varies in oviduct cells at the two stages of the reproductive cycle analyzed, paralleling the phenomenon observed in the rat. The results show that the morphology of speckles in reptile cells depends upon the reproductive stage as it occurs in mammals.

Vitellogenin-inducing activities of natural, synthetic, and environmental estrogens in primary cultured Xenopus laevis hepatocytes

Vitellogenin (VTG)-inducing activities of natural estrogens (E1: estrone, E2:17beta-estradiol, E3: estriol, alpha-E2: 17alpha-estradiol), synthetic estrogens (EE2: 17alpha-ethynyl estradiol, DES: diethylstilbestrol,), phytoestrogen (GEN: genistein), and xeno-estrogens (BPA: bisphenol A, NP: nonylphenol, OP: octylphenol) were investigated by an assay system using primary-cultured hepatocytes of Xenopus laevis. An enzyme-linked immunoabsorbent assay (ELISA) was able to detect VTG at a minimum detection limit of 0.06 ng/mL. Relative estrogenic activities of the compounds were determined from their dose-response curves. The activities relative to E2 activity were 138% for DES, 121% for EE2, 6.1% for E3, 0.33% for E1, 0.29% for alpha-E2, 0.037% for GEN, 0.008% for BPA, 0.005% for NP, and 0.002% for OP. Comparison with data reported for other bioassay systems revealed that there were significant interspecies-and cell-type-differences in the activities of DES, E3, E1 and alpha-E2. BPA was found to have a substantial antagonistic activity (approximately 0.8% of tamoxifen activity) under the influence of physiological concentrations of E2. Complex-effects of endocrine disrupters on aquatic animals will be discussed.

Eggshell and egg yolk proteins in fish: hepatic proteins for the next generation: oogenetic, population, and evolutionary implications of endocrine disruption

The oocyte is the starting point for a new generation. Most of the machinery for DNA and protein synthesis needed for the developing embryo is made autonomously by the fertilized oocyte. However, in fish and in many other oviparous vertebrates, the major constituents of the egg, i.e. yolk and eggshell proteins, are synthesized in the liver and transported to the oocyte for uptake. Vitellogenesis, the process of yolk protein (vitellogenin) synthesis, transport, and uptake into the oocyte, and zonagenesis, the synthesis of eggshell zona radiata proteins, their transport and deposition by the maturing oocyte, are important aspects of oogenesis. The many molecular events involved in these processes require tight, coordinated regulation that is under strict endocrine control, with the female sex steroid hormone estradiol-17beta in a central role. The ability of many synthetic chemical compounds to mimic this estrogen can lead to unscheduled hepatic synthesis of vitellogenin and zona radiata proteins, with potentially detrimental effects to the adult, the egg, the developing embryo and, hence, to the recruitment to the fish population. This has led to the development of specific and sensitive assays for these proteins in fish, and the application of vitellogenin and zona radiata proteins as informative biomarkers for endocrine disrupting effects of chemicals and effluents using fish as test organisms. The genes encoding these important reproductive proteins are conserved in the animal kingdom and are products of several hundred million years of evolution.

Determination of vitellogenin kinetics in male fathead minnows (Pimephales promelas)

A lack of knowledge persists concerning the combination of kinetics on protein and mRNA levels of the most commonly used biomarker for estrogenic influences-vitellogenin (VTG). Consequently, male fathead minnows were exposed to 17alpha-ethinylestradiol (EE(2)) for 35 days, followed by an equally long depuration period in a flow-through system. VTG mRNA levels reached a plateau after 3 days of exposure, which remained stable until 3 days after EE(2) removal. Control levels were re-attained within 7 days of the depuration phase. VTG protein accumulated in the plasma following a two-phased model. The first phase depicting an exponential increase lasted 15 days and was followed by a saturation phase approaching a plateau of approximately 47 mg VTG/ml plasma. Clearance kinetics could be described by a two-compartment open model, with half-lives of 2.17 and 21.32 days for the alpha- and beta-phases, respectively. In addition, a high VTG protein synthesis rate seemed to adversely affect fitness and mortality of the fish.

Estradiol-induced vitellogenin gene expression in a teleost fish, Oreochromis aureus

Vitellogenesis presents a versatile model for the study of hormone-induced gene expression. We report here the effects of estradiol-17 beta-propionate on vitellogenin gene expression in male Oreochromis aureus, a teleost fish. Vitellogenin mRNA of 6500 nucleotides has been elucidated from the livers of female and estradiol-treated male O. aureus. By hybridization with a specific O. aureus cDNA probe, the vitellogenin mRNA transcript was detected as early as 1 hr following primary and secondary estradiol-stimulations, although for the latter, the rate of accumulation of vitellogenin-specific mRNA was 20-fold higher. The vitellogenin mRNA peaked at 72 and 48 hr, respectively, for primary and secondary stimulations. At the translational level, the increase in plasma vitellogenin was further enhanced during the secondary stimulation. There was a distinct shift in the peak of plasma vitellogenin from Day 14 in the primary induction to Day 3 in the secondary stimulation. The plasma vitellogenin presented in two forms, 300 and 500 kDa, both of which were immunologically confirmed by Western blot analysis to be vitellogenin proteins.

Estrogen treatment and its influence on protein synthesis and amino acid metabolism in Zoarces viviparus (L) males

Overall protein synthesis was studied in estradiol-treated and control Zoarces viviparus males. Significant increases in total RNA, the RNA:DNA and the RNA:protein ratios in the liver were correlated with a 6-fold increase in total calcium levels in the plasma, indicating increased protein synthetic activity and the presence of circulating vitellogenin respectively in the induced males. The time-course incorporation of (14)C-phenylalanine into protein in a cell free liver system showed a linear increase during the first 30 min after poly (U) was added to the incubation medium in control as well as in the estradiol-treated males. The incorporation rate in the induced liver, however, was increased significantly above the level of the control livers. Estradiol-treatment altered the concentration of most amino acids in the plasma and a few in the liver with particular marked effects on alanine and tyrosine in the plasma and glutamate in the liver.

Vitellogenin gene expression in male rainbow trout (Salmo gairdneri)

Estrogen has a marked effect on the expression of vitellogenin, the egg yolk precursor protein in the liver of egg-laying vertebrates. cDNA clones specific for trout vitellogenin mRNA have been used to study the expression of the vitellogenin genes in rainbow trout (Salmo gairdneri). The steady-state levels of vitellogenin mRNA in the liver of male rainbow trout were measured during primary and secondary stimulation with estradiol. The kinetics of induction in trout appear to be very similar to those seen in Xenopus and chicken in that a lag of approximately 2 days is observed in the accumulation of serum vitellogenin during primary induction. This lag is not observed during the secondary stimulation. The primary induction of vitellogenin mRNA in trout liver, using a single injection of estradiol (3 mg/kg body wt) results in a short-lived rise, reaching a maximum level of 260 ppm total RNA on Day 2. Using silastic implants of estradiol to induce a primary response produces a large increase in the steady-state level of vitellogenin mRNA which reaches a maximum of 2750 ppm total RNA on Day 10. During secondary stimulation, using silastic tubing the maximum level reached was 1200 ppm of the total RNA on Day 7, approximately half the level seen in the primary induction using the silastic implants. The difference in these two levels is due to an increase in the steady-state levels of rRNA, which appear to increase between Days 10 and 21 after the primary stimulation. These results demonstrate that the induction of vitellogenesis in the trout by estradiol involves changes in the steady-state levels of a number of different mRNA and rRNA sequences and resembles that seen in Xenopus and chicken.

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.

Deinduction of transcription of Xenopus 74-kDa albumin genes and destabilization of mRNA by estrogen in vivo and in hepatocyte cultures

The goal of this study is to explain the molecular basis of the marked deinduction of Xenopus albumin synthesis and secretion accompanying the activation of vitellogenin genes by estrogen. We have characterized by restriction analysis, DNA sequencing and hybrid-selected translation of mRNA, a cloned cDNA specifying the two 74-kDa albumins which constitute the predominant circulating form of albumin in Xenopus laevis. Using this recombinant DNA plasmid as a hybridization probe, we have determined the steady-state levels of albumin mRNA, the rate of transcription of the two 74-kDa albumin genes and the stability of the mRNA in male and female Xenopus hepatocytes in vivo and in primary cell cultures following estrogen treatment. In both whole liver and cultured hepatocytes estradiol caused a rapid drop in the steady-state levels of 74-kDa albumin mRNAs, which was reversed spontaneously in the continued presence of the hormone. The concentration of albumin mRNA was substantially higher in male than in female hepatocytes, the hormonal effect being more marked in male than in female hepatocytes. The decrease in steady-state levels of mRNA was anticipated in male hepatocytes by a 70% inhibition of rate of transcription of albumin genes within 2 h of exposure to estradiol, as measured by run-off transcription in liver nuclei isolated from animals treated in vivo or by determining the absolute transcription rate in cell cultures. In the latter the diminished transcription rate returned to normal within 12 h in the continued presence of the hormone. Estradiol caused a threefold destabilization of albumin mRNA in both male and female hepatocyte cultures to t 1/2 = 3 h and 2 h respectively. The combined effects on rate of or transcription and mRNA stability largely explain the changes in the steady-state levels of mRNA caused by hormone administration. Comparison of the kinetics of transcription rates of vitellogenin and albumin genes in vivo and in vitro reveals a striking reciprocity in the selective activation of the inducible genes and deinduction of the constitutively expressed genes at the early stages of response of Xenopus hepatocytes to estrogen.

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.

A correlated morphometric and biochemical study of estrogen-induced vitellogenesis in male Rana pipiens

Adult male Rana pipiens were administered estradiol-17 beta to induce vitellogenesis. Liver and blood were taken from control animals and experimental animals on Days 1, 2, 4, 8, 12, 16, and 120 following hormone treatment. Stereological analysis of livers showed that mitochondrial structural parameters remained constant while rough endoplasmic reticulum (RER) parameters increased significantly by 4 days and to more than four times control, or 120-day levels, by 8-16 days. Liver RNA concentration increased 2.5-fold and in parallel with RER, while liver protein and DNA concentrations did not change. Increases in total plasma protein and plasma vitellogenin (Vg) lagged behind increases in liver RER and RNA. Of the total plasma protein, Vg constituted 6% by 4 days, 40% by 12-16 days and less than 2% by 120 days. The half-life of plasma Vg was estimated to be no greater than 22 days. These studies provide the first quantitative correlations between ultrastructural and biochemical changes occurring in frog tissues following estrogen administration.

Estradiol-induced hepatic protein synthesis and transaminase activity in the male flounder, Platichthys flesus (L.)

A continued accumulation of vitellogenin was observed in the serum through 28 days after administration of spaced large doses of estradiol. The level of estradiol in the serum reached a maximum on Day 4, followed by a continued decrease until a basal concentration was reached at Day 15. The estradiol treatment increased the in vitro hepatic protein synthesis activity, measured as polyphenylalanine synthesis per unit cytoplasmic RNA, within 2-4 days reaching a maximum after 7 days. The effect was transitory and reduced to less than 50% within 15 days. After restimulation a second rise in protein synthesis activity was observed. The cellular content of bulk RNA i the liver showed a slow but steady accumulation through the 28 days. The specific activity of glutamate-oxaloacetate transaminase and glutamate-pyruvate transaminase in the liver decreased 35 and 20% relative to control, respectively, within 7 days after the first stimulation and again 14 days after restimulation. The concentration of serum protein showed a steady increase throughout the experimental period. The concentration of serum ninhydrine positive substances was lowered to 50% relative to the controls within the first 7 days after start of treatment with hormone.

Sex differences in liver non-specific esterases of the green frog Rana esculenta

1. Liver esterases of the green frog Rana esculenta have been fractionated on disc electrophoresis, thin-layer electrofocusing and column electrofocusing. 2. The enzyme resolves in several molecular forms some of which result sex dependent, since they disappear in castrated animals and can be induced by sex hormone administration. 3. The enzyme molecular forms which depend on female hormones might be involved in cellular modifications of hepatocytes related to yolk protein synthesis. induced by sex hormone administration. 3. The enzyme molecular forms which depend on female hormones might be involved in cellular modifications of hepatocytes related to yolk protein synthesis.

Vitellogenesis and the vitellogenin gene family

Vitellogenin is synthesized under estrogen control in the liver, extensively modified, transported to the ovary, and there processed to the yolk proteins lipovitellin and phosvitin. In the frog Xenopus laevis there are at least four distinct but related vitellogenin genes. The two genes A1 and A2 have a 95 percent sequence homology in their messenger RNA coding regions, and contain 33 introns that interrupt the coding region (exons) at homologous positions. Sequences and lengths of analogous introns differ, and many introns contain repetitive DNA elements. The introns in these two genes that have apparently arisen by duplication have diverged extensively by events that include deletions, insertions, and probably duplications. Rapid evolutionary change involving rearrangements and the presence of repeated DNA suggests that the bulk of the sequences within introns may not have any specific function.

Xenopus liver: ontogeny of estrogen responsiveness

Estradiol-17 beta stimulates the synthesis of numerous proteins exported into the culture medium by Xenopus tadpole liver tissue obtained after stage 50 and throughout metamorphosis to stage 66. Although estrogen-induced vitellogenin can be detected as early as stage 54, it is a minor percentage of the exported proteins until after the completion of metamorphosis. In hepatic tissue obtained after metamorphosis, the hormone evokes the synthesis of vitellogenin specifically without affecting the labeling of other secreted proteins.

Dose response kinetics of serum vitellogenin, liver DNA, RNA, protein and lipid after induction by estradiol-17 beta in male flounders (Platichthys flesus L.)

1. Male flounders receiving 100 micrograms estradiol each second day were fully induced to vitellogenin synthesis within 11 days, while fishes given 5 micrograms doses continued to accumulate vitellogenin in the serum at a progressive rate through 17 days. 2. Liver DNA per unit fish remained constant, while RNA per unit fish in flounders given 100 and 5 micrograms doses attained values 80 and 25% respectively, above the values found in control animals. 3. Liver RNA per unit DNA increased at maximal rate within 6 days in fishes receiving 100 micrograms doses. RNA synthesis continued at a progressive rate through 17 days in fishes given 5 micrograms doses of estradiol. 4. Liver protein per unit DNA elevated at a plateau 60% above control within 6 days with 100 micrograms doses. Doses of 5 micrograms had only little effect on liver protein. 5. Estradiol had a lipogenic effect on the liver. Cellular lipid rose 120 and 60% above control after treatment with 100 and 5 micrograms respectively. 6. Liver dry weight per unit DNA increased 60 and 55% above control with 100 and 5 micrograms doses respectively. Cellular hypertrophy in fishes receiving the smaller dose was primarily associated with an increase in lipid concentration, while protein and lipid contributed almost equally to cellular growth in fishes receiving the high dose.

Synthesis of vitellogenin, an attractive model for investigating hormone-induced gene activation

The estrogen-induced synthesis of vitellogenin in the frog Xenopus and the chicken is an attractive system for investigating the molecular events leading to the activation of a specific gene. In this review article the events occurring at the level of the protein and mRNA in the cytoplasm are discussed. The available data show that the induction of vitellogenin synthesis is due to the accumulation of the corresponding vitellogenin mRNA in the cytoplasm. This suggests that transcriptional or posttranscriptional events in the nucleus are activated by the hormone. The few experiments investigating the processes in the nuclear compartment are reviewed.

Oestrogen-induced cholesterol and fatty acid biosynthesis in Xenopus laevis liver during vitellogenic response

1. Oestradiol-17beta induces livers of Xenopus laevis (South African clawed toad) to synthesize and secrete into the serum large quantities of the egg-yolk-protein precursor, vitellogenin. The peak of this response occurs 9-16 days after hormone treatment [Dolphin, Ansari, Lazier, Munday & Akhtar (1971) Biochem. J.124, 751-758]. It is now shown that 6 days after hormone treatment a 120-160-fold stimulation of the synthesis of cholesterol and fatty acid compared with control values occurred. 2. A cell-free system, derived from Xenopus liver, which synthesizes squalene and fatty acid is described. By using this system, several hundredfold stimulation of incorporation of [(14)C]acetate into squalene was recorded 6 days after the administration of oestradiol-17beta, compared with a 3-4-fold stimulation of incorporation of [(3)H]mevalonate compared with control values. It is argued that oestradiol-17beta must affect enzyme(s) catalysing step(s) between acetate and mevalonate in the biosynthetic pathway to cholesterol. 3. In incubation of liver slices in vitro, most of the lipid and cholesterol synthesized in response to the steroid hormone was associated with those subcellular fractions that contained membranes. Moreover, pulse-labelling experiments in vivo showed that 70% of this lipid and cholesterol was retained in the liver. The remainder appeared in the serum, where it was equally distributed between vitellogenin and vitellogenin-free serum. 4. G.l.c. analyses of the cholesterol content of liver microsomal fractions of Xenopus laevis indicated that the cholesterol content was at least 50% higher in microsomal fractions obtained from livers that had been exposed to oestradiol-17beta. Meanwhile, g.l.c. analysis of the lipid moiety of secreted vitellogenin showed that up to 35% of its lipid was cholesterol.

Fenestrae in the rough endoplasmic reticulum of Xenopus laevis hepatocytes

The rough endoplasmic reticulum (RER) of Xenopus laevis hepatocytes was examined by freeze-fracture and by conventional thin section electron microscopy. Much of the RER was present as stacks of cisternae at the cell periphery but, in addition, large whorls of cisternae were seen in the cytoplasm in most sections. Freeze-fracture replicas revealed fenestrae in both stacked and whorled cisternae, although the fenestrae were more numerous in the whorls. The role of these fenestrae is unknown, but such structures would facilitate access of precursors to the protein synthetic machinery in this highly metabolically active cell type. This would be particularly important in RER whorls, where the innermost cisternae would otherwise be isolated from the rest of the cytoplasm.