Regulation by estrogen receptor of vitellogenin gene transcription in Xenopus hepatocyte cultures

Natural estrogens in surface waters of a catchment with intensive livestock farming in Switzerland

Natural estrogens such as 17α-estradiol (E2α), 17β-estradiol (E2β), estrone (E1), and estriol (E3), released to surface waters from both urban and agricultural sources, are endocrine disrupting for fish. Here, we assess the prevalence of livestock farming derived natural estrogens in tributaries and ponds in the agriculturally dominated catchment of Lake Baldegg, Switzerland. Passive samplers were deployed in the main tributary and daily time-proportional water samples were collected in five tributaries for 30 days at the beginning of the vegetation period. Furthermore, we took grab samples of 12 ponds in the catchment. Aqueous samples were liquid-liquid extracted, derivatized, and analysed with LC-MS/MS and stream water samples additionally with ERα-CALUX, a bioassay for assessing total estrogenic activity. Natural estrogens were regularly detected, with mean concentrations ranging from below the limit of detection to 0.55 ng L^-1 for E2β and E1, respectively, and passive sampling and bioassay results largely confirmed these findings. Monte Carlo simulated mean natural estrogen concentrations underestimated measured ones by a factor of three to 11. An agricultural area's hydrological contribution and connectivity to surface waters seemed to be more important for the development of estrogen concentrations in streams than livestock densities in a catchment or the actual loads of slurry applied. Pond water occasionally contained natural estrogens in concentrations up to 8.6 ng L^-1 for E2α. The environmental quality standards of the European Union (0.4 ng L^-1 for E2β and 3.6 ng L^-1 for E1) were never exceeded for longer than a day in tributaries, but E1 reached critical concentrations for aquatic organisms in ponds.

Immunolocalization of hepatic estrogen and progesterone receptors in the female lizard Uromastyx acanthinura

The hormonal regulation of hepatic synthesis of vitellogenin during the annual reproductive cycle was performed for the first time in the deserticole, oviparous, diurnal and herbivorous Uromastyx acanthinura, a lizard belonging to the Agamidae family. In order to elucidate what kind of estrogen receptor is involved in this process, an immunohistochemical study was performed. Changes were obtained in the labeling and cellular distribution of the estrogen and progesterone receptors according to the period of the reproductive cycle and the experimental administration of 17β-estradiol. Only the ERβ subtype was present; it was found in all phases of the cycle with a variable localization: nuclear and cytosolic during vitellogenesis, mainly cytosolic in the female with egg retention (luteal phase) and strictly cytosolic in females at sexual rest. The progesterone receptors were present only at the luteal phase and during sexual rest and disappeared completely from females after 17β-estradiol treatment in sexual rest. Our data suggested that mediation of action of the 17β-estradiol in the vitellogenin synthesis in the lizard U. acanthinura occured via ERβ. PRA and PRB could both be necessary for the negative effect of progesterone on the hepatic synthesis of vitellogenin.

A dynamic model of transcriptional imprinting derived from the vitellogenesis memory effect

Transcriptional memory of transient signals can be imprinted on living systems and influence their reactivity to repeated stimulations. Although they are classically ascribed to structural chromatin rearrangements in eukaryotes, such behaviors can also rely on dynamic memory circuits with sustained self-amplification loops. However, these phenomena are either of finite duration, or conversely associated to sustained phenotypic changes. A mechanism is proposed, in which only the responsiveness of the target gene is durably reset at a higher level after primary stimulation, using the celebrated but still puzzling vitellogenesis memory effect. The basic ingredients of this system are: 1), a positive autoregulation of the estrogen receptor α gene; 2), a strongly cooperative action of the estradiol receptor on vitellogenin expression; and 3), a variant isoform of the estradiol receptor with two autonomous transcription-activating modules, one of which is signal-independent and the other, signal-dependent. Realistic quantification supports the possibility of a multistationary situation in which ligand-independent activity is unable by itself to prime the amplification loop, but can click the system over a memory threshold after a primary stimulation. This ratchet transcriptional mechanism can have developmental and ecotoxicological importance and explain lifelong imprinting of past exposures without apparent phenotypic changes before restimulation and without need for persistent chromatin modifications.

Mechanisms and significance of nuclear receptor auto- and cross-regulation

The number of functional hormone receptors expressed by a cell in large part determines its responsiveness to the hormonal signal. The regulation of hormone receptor gene expression is therefore a central component of hormone action. Vertebrate steroid and thyroid hormones act by binding to nuclear receptors (NR) that function as ligand-activated transcription factors. Nuclear receptor genes are regulated by diverse and interacting intracellular signaling pathways. Nuclear receptor ligands can regulate the expression of the gene for the NR that mediates the hormone's action (autoregulation), thus influencing how a cell responds to the hormone. Autoregulation can be either positive or negative, the hormone increasing or decreasing, respectively, the expression of its own NR. Positive autoregulation (autoinduction) is often observed during postembryonic development, and during the ovarian cycle, where it enhances cellular sensitivity to the hormonal signal to drive the developmental process. By contrast, negative autoregulation (autorepression) may become important in the juvenile and adult for homeostatic negative feedback responses. In addition to autoregulation, a NR can influence the expression other types of NRs (cross-regulation), thus modifying how a cell responds to a different hormone. Cross-regulation by NRs is an important means to temporally coordinate cell responses to a subsequent (different) hormonal signal, or to allow for crosstalk between hormone signaling pathways.

Regulation of estrogen receptors in primary cultured hepatocytes of the amphibian Xenopus laevis as estrogenic biomarker and its application in environmental monitoring

The present study aims to introduce the regulation of estrogen receptors (ER) in primary cultured hepatocytes of the amphibian Xenopus laevis as a further potential estrogenic biomarker. Time courses of free ER in cell cultures treated with 17beta-estradiol (E2), nonylphenol (NP), and bisphenol A (BPA) were determined by means of radioreceptorassay (RARA). All compounds led to an immediate drop of free ER followed by a significant increase. The estrogen specific induction of ER-mRNA in vitro during time course was verified by using semiquantitative RT-PCR demonstrating greatest differences after 36 h. Dose-response curves of ER-mRNA for E2, NP, and BPA revealed that E2 possessed highest estrogenicity starting at 10(-9) M, while NP and BPA induced significant increases at 10(-8) and 10(-7) M, respectively. Extracts of the river Alb were subjected to RARA for ER binding to cytosolic liver fraction as well as to primary cultured hepatocytes for assessment of ER-mRNA induction. The results by RARA demonstrated clearly that binding to ER was highest in sewage treatment plant effluents and increased during the course of the river. These findings could be correlated with induction of ER-mRNA levels in vitro indicating that both techniques are suitable for application in monitoring of estrogenic EDC.

Multihormonal control of vitellogenesis in lower vertebrates

The comparative approach on how and when vitellogenesis occurs in the diverse reproductive strategies displayed by aquatic and terrestrial lower vertebrates is presented in this chapter; moreover, attention has been paid to the multihormonal control of hepatic vitellogenin synthesis as it is related to seasonal changes and to vitellogenin use by growing oocytes. The hormonal mechanisms regulating vitellogenin synthesis are also considered, and the effects of environmental estrogens on the feminization process in wildlife and humans have been reported. It is then considered how fundamental nonmammalian models appear to be, for vitellogenesis research, addressed to clarifying the yolkless egg and the evolution of eutherian viviparity.

Endocrine effects of environmental pollution on Xenopus laevis and Rana temporaria

To determine the capacity of sewage treatment work effluents to disrupt the endocrine system under semifield conditions, two amphibian species, Xenopus laevis and Rana temporaria, were exposed to the effluent of a regional sewage treatment plant in South Bavaria during larval development until completion of metamorphosis. Exposure was carried out in river water (Würm) as a reference, and a 1:12-mixture sewage effluent representing the real situation on the spot, and in a higher concentration of sewage using a 1:2 mixture. An accidental impact of industrial wastewater into the reference and dilution medium, Würm, which was caused by a spate in the respective area during the sensitive period of sex differentiation of amphibian larvae, is assumed to be responsible for the relatively high percentage of females observed by histological analysis in all treatment groups. All of these values were higher than those determined in controls exposed to artificial tap water in laboratory experiments conducted in a comparable study design. Sex ratios between species, revealed by the semifield study with decreasing portions of females from control to 1:12 to 1:2, were strongly correlated. Determination of biomarker-mRNA-levels in Xenopus liver using semiquantitative RT-PCR at the end of the experimental phase, when exposure regime has turned into the initially expected situation with the highest load of potential estrogens in the effluent, followed by 1:2 and 1:12 mixture, resulted in a significant increase of Vitellogenin-mRNA in female juveniles exposed to the highest portion of sewage, whereas expression of both androgen and estrogen receptor-mRNA showed no clear differences. The results concerning the induction of estrogenic biomarkers are in accordance with our findings for estrogen receptor binding of sample extracts from the Würm and sewage taken in parallel at the end of the experiment, when sewage extracts possessed a much higher ability to displace [3H]estradiol from the estrogen receptor than the ones extracted from the mixtures.

Effect of gonadal steroids on progesterone receptor, estrogen receptor, and vitellogenin expression in male turtles (Chrysemys picta)

Hepatic vitellogenin (vtg) is a yolk precursor protein sequestered in follicular oocytes as nutrient supply for developing embryos in nonmammalian vertebrates. In prior research studies we have demonstrated that both progesterone (P) and testosterone (T) inhibit estrogen (E)-induced vitellogenesis in the male fresh water turtle (Chrysemys picta), and have suggested that these hormones may be involved in multihormonal regulation of vitellogenesis in the female turtle. However, the modes of action of progesterone and testosterone on estrogen-induced vitellogenesis are not known. We have proposed that progesterone inhibits vitellogenesis by modulation of progesterone receptor A (PRA) or B (PRB) isoforms and/or estrogen receptor (ER) gene transcription. In this study, we compare the vitellogenic responses of reproductively inactive male turtles to estradiol 17beta in the presence of exogenous testosterone or progesterone. Northern blot analysis was used to monitor the changes in vtg mRNA, ER mRNA, and PR mRNA expression; Western blotting to determine changes in PR isoform expression and a homologous ELISA for measurement of plasma vtg. Progesterone and testosterone reduced estrogen-induced vtg mRNA expression, but plasma vtg was not significantly reduced by these steroids. PRA and PRB were transcribed even though ER mRNA could not be detected, suggesting constitutive PR expression. However, in the presence of estradiol 17beta, both PR isoforms and mRNA transcripts were increased as a correlate of ER mRNA transcription, suggesting both transcriptional and translational effects; these effects were inhibited by testosterone and progesterone treatments. Since ER mRNA was sharply reduced by both testosterone and progesterone, and estradiol 17 beta increased PR mRNA transcription and translation, it is likely that the action of progesterone in reducing vtg mRNA is indirect via down regulation of ER mRNA, thus ER. This study provides further information on the role of progesterone and testosterone in the regulation of hepatic vitellogenesis, suggesting regulation of vitellogenesis mainly via modulation of hepatic ER mRNA.

Amphibians as a model for the study of endocrine disruptors

Evidence shows that environmental compounds can interfere with the endocrine systems of wildlife and humans. The main sink of such substances, called endocrine disruptors (EDs), which are mainly of anthropogenic origin, is surface water; thus, aquatic vertebrates such as fishes and amphibians are most endangered. Despite numerous reports on EDs in fishes, information about EDs in amphibians is scarce, and this paucity of information is of particular concern in view of the worldwide decline of amphibians. EDs could contribute to changes of amphibian populations via adverse effects on reproduction and the thyroid system. In amphibians, EDs can affect reproduction by (anti)estrogenic and (anti)androgenic modes of action that produce severe effects including abnormal sexual differentiation. ED actions on the thyroid system cause acceleration or retardation of metamorphosis, which may also affect population levels. Our broad knowledge of amphibian biology and endocrinology indicates that amphibians are very suitable models for the study of EDs. In particular, effects of EDs on the thyroid system triggering metamorphosis can be determined easily and most sensitively in amphibians compared to other vertebrates. A new classification of EDs according to their biological modes of action is proposed because EDs have quite heterogeneous chemical structures, which do not allow prediction of their biological effects. Methods and strategies are proposed for identification and risk assessment of EDs, whether as pure test substances or as mixtures from environmental samples. Effects of EDs on the thyroid system of amphibians can be assessed by a single animal model (Xenopus laevis), whereas the various types of reproduction need comparative studies to investigate whether general endocrine principles do exist among several species of anurans and urodeles. Thus, at least one anuran and one urodelean model are needed to determine ED interference with reproduction.

Seasonal changes in hepatic progesterone receptor mRNA, estrogen receptor mRNA, and vitellogenin mRNA in the painted turtle, Chrysemys picta

Previous studies using the fresh water turtle Chrysemys picta have demonstrated that progesterone (P) inhibits estradiol (E)-induced vitellogenin (vtg) secretion in this species. Further, there is evidence for the differential expression of the two P receptor isoforms (PRA and PRB) in the liver during the turtle seasonal cycle, correlating with hepatic vitellogenesis. In this study we report changes in the hepatic PR mPNA, ER mRNA, and vitellogenin (vtg) mRNA transcripts during the reproductive cycle of the turtle. Fragments of the turtle hepatic PR and ER cDNAs were cloned and sequenced and a previously cloned turtle vtg cDNA were used as probes in Northern blotting. No 3.7-kb PR mRNA, corresponding to the smaller PR transcript, PRA of other species was found, although, a smaller 1.8-kb transcript (putative PRC mRNA) was present. These observations suggest that the turtle as in the chicken and human, the 4.5-kb PR mRNA transcript encodes both PRA and PRB proteins. Only the larger PR mRNA transcript (4.5-kb), was found to vary significantly during the annual cycle, being highest when vitellogenesis was inhibited in winter and summer. Vtg mRNA could not be detected during the summer or winter, was highest during vitellogenesis in the spring, and reappeared during the fall period of vitellogenesis and ovarian recrudescence. ER mRNA followed a similar pattern, being highest during spring and early fall, when vtg synthesis is high. The data suggest that P/PR, as well as E/ER, may be involved in the seasonal regulation of hepatic vitellogenesis in this species.

Effects of dietary phytoestrogens in vivo and in vitro in rainbow trout and Siberian sturgeon: interests and limits of the in vitro studies of interspecies differences

A study of the effects of dietary genistein on trout and sturgeon in vivo showed that sturgeon was sensitive to 20 ppm of genistein, whereas trout was not. To analyze the origin of this interspecies difference in sensitivity, a cell culture technique was developed with hepatocytes from sturgeon and compared to results obtained with hepatocytes from trout in the same system. The hepatocyte culture proved to be useful as bioassay for estrogenicity. Vitellogenin (VTG), assayed by a specific enzyme-linked immunosorbent assay, was used as a biomarker of the estrogenic activity. 17 beta-Estradiol, its glucuronide and sulfate derivatives, and estradiol analogues (ethynylestradiol and diethylstilbestrol) were tested. Nonestrogenic compounds such as androgens, progesterone, and cortisol were tested as negative controls. VTG production was monitored at doses ranging from 1 nM to 10 microM estradiol. Phytoestrogens, from the isoflavone family, were tested individually at increasing doses exhibiting dose response curves for concentrations from 500 nM to 10 microM. With tamoxifen, an antagonist of estrogen receptors, the estrogenic effect was partially reduced. The effect was the same with ICI182,780 in sturgeon, whereas the effect was the opposite in trout. The estrogenic potency of the isoflavones ranged differently between the two species in the following order: biochanin A < daidzein = formononetin < genistein < equol in trout and biochanin A < genistein < daidzein < formononetin < equol in sturgeon. Further, in sturgeon, formononetin was the most potent phytoestrogen in vitro, whereas its activity was weakest in vivo. These data suggest that one must reconsider the relevance of heterologous estrogenic tests and of homologous in vitro tests for estrogenic potency of chemicals.

Characterization of a cytosolic estrogen receptor and its up-regulation by 17 beta-estradiol and the xenoestrogen 4-tert-octylphenol in the liver of eelpout (Zoarces viviparus)

Estrogen binding activity was revealed in the cytosolic fraction of hepatic extracts from adult male and female eelpout (Zoarces viviparus). The binding moiety was characterized by a single class of high affinity binding sites (K(d)=0.59+/-0.05 nM in males and 1.06+/-0.10 nM in females). The affinity was significantly higher in males. Binding sites were satiable and binding capacity was significantly elevated in vitellogenic females (2.92+/-0.28 pmol/g) compared to males (1.67+/-0.11 pmol/g). The binding was specific to known estrogens but not to other tested steroids. The binding moiety was able to bind to DNA-cellulose and was extractable by high salt concentrations. A time-course study of estrogen binding activity in liver cytosol and of vitellogenin (Vtg) in plasma, after intraperitoneal (i.p.) injections of 17 beta-estradiol (E(2)) in male eelpout, was carried out. It was shown that both are inducible by E(2). Estrogen binding activity was significantly elevated 48 h and Vtg 72 h after E(2) treatment. The binding moiety was hereafter designated as a cytosolic estrogen receptor (ER). The estrogenicity of 4-tert-octylphenol (OP) was evaluated by measuring ER and Vtg after i.p. treatment. OP-treatment increased both receptor levels and Vtg concentrations in male fish, indicating that OP acts as an estrogen in male eelpout.

Interactions of nitromusk parent compounds and their amino-metabolites with the estrogen receptors of rainbow trout (Oncorhynchus mykiss) and the South African clawed frog (Xenopus laevis)

Nitromusks, musk xylene (MX), musk ketone (MK) and musk moskene (MM) are synthetic fragrances. 4-Amino-MX (4X), 2-amino-MX (2X) and 2-amino-MK (2K) are nitromusk metabolites formed during the sewage treatment process and have been detected in effluent and surface water at concentrations four to 40 times higher than their parent compounds. To date, data to the aquatic toxicity of nitromusk compounds are limited to the parent compounds and the determination of acute and subacute effects in aquatic organisms. No data are available regarding the potential endocrine modulating effects of these compounds and/or their metabolites in aquatic organisms. Therefore, the competitive binding capability of nitromusks and their metabolites to the estrogen receptors (ER) in rainbow trout and xenopus was investigated. No binding of MX, MK and MM to the ER of either species was observed. In contrast, binding to the ER was observed for 4X, 2X and 2K in both species. The IC50 (competitive binding at the ER) of 2X in rainbow trout was 1.3 +/- 1.1 mM. In contrast, 4X, 2X and 2K bound to the xenopus ER with an IC50 of 30.8 +/- 28.5, 12.9 +/- 10.3 and 70.1 +/- 88.3 microM, respectively.

Amphibians as a model to study endocrine disruptors: II. Estrogenic activity of environmental chemicals in vitro and in vivo

Several environmental chemicals are known to have estrogenic activity by interacting with development and functions of endocrine systems in nearly all classes of vertebrates. In order to get a better insight of potential estrogenic effects on amphibians caused by environmental pollution this study aims to develop a model for investigating endocrine disruptors using the amphibian Xenopus laevis. In that model the potential estrogenic activity of endocrine disruptors is determined at several levels of investigation: (I) binding to liver estrogen receptor; (II) estrogenicity in vitro by inducing vitellogenin synthesis in primary cultured hepatocytes; and (III) in vivo effects on sexual development. Here we deal with establishing methods to assay estrogenic activity of environmental chemicals in vitro and in vivo. In vitro we used a semiquantitative reverse transcriptase-polymerase chain reaction (RT-PCR) technique to determine mRNA-induction of the estrogenic biomarker vitellogenin in primary cultured hepatocytes of male Xenopus laevis. Time courses of vitellogenin-mRNA in the presence and absence of 10(-6) M 17 beta-estradiol (E2) resulted in a marked loss of mRNA from controls after 2 days while E2 treatment kept vitellogenin-mRNA at a relatively stable level. After 36 h of incubation estrogenic activities of E2, 4-nonylphenol (NP), and 2,2-bis-(4-hydroxyphenyl)-propan (bisphenol A) at concentrations ranging from 10(-10) to 10(-5) M were assayed by RT-PCR of vitellogenin-mRNA and showed the following ranking of dose-dependent potency: E2 > NP > bisphenol A. These in vitro results were confirmed further by in vivo experiments determining sexual differentiation of Xenopus laevis after exposure to E2 and environmental chemicals during larval development. Concentrations of 10(-7) and 10(-8) M E2 as well as 10(-7) M of NP or bisphenol A caused a significant higher number of female phenotypes compared to controls indicating a similar ranking of estrogenic potencies in vivo as in vitro. In addition, butylhydroxyanisol and octylphenol, both showed feminization at 10(-7) M while octylphenol was also effective at 10(-8) M. In summary these results demonstrate for the first time the use of a semiquantitative RT-PCR technique for screening estrogenicity by assaying mRNA induction of the estrogenic biomarker vitellogenin in vitro. The combination of this newly developed method with classical exposure experiments is necessary for determination of the biological significance of estrogenic chemicals.

Amphibians as a model to study endocrine disruptors: I. Environmental pollution and estrogen receptor binding

Many chemicals released into the environment without toxicological risks have the capacities to disrupt the function of endocrine systems. These endocrine disruptors disturb normal endocrine mechanisms and have been observed in nearly all classes of vertebrates. The aim of this research is to develop a comprehensive model to study endocrine disruption using the amphibian Xenopus laevis. The assessment of estrogenic potencies of endocrine disruptors includes several levels of investigation: (I) binding to liver estrogen receptor, (II) estrogenic activity in vitro by inducing vitellogenin synthesis in primary cultured hepatocytes, and (III) in vivo effects on sexual development caused by exposure of larvae. The present paper is focused on the first part by establishing a radioreceptorassay for [3H]17 beta-estradiol ([3H]E2) binding using liver cytosol fraction. In order to get optimum binding conditions we performed kinetic, saturation, and competitive displacement experiments. Association of [3H]E2 to estrogen receptor revealed that maximum specific binding is achieved between 18 and 48 h of incubation. Scatchard analyses of saturation experiments resulted in a homogenous saturable population of estrogen receptors having no significant differences of binding parameters between both sexes. The values of Kd (dissociation constant) in males and females were 22.4 +/- 6.0 and 15.0 +/- 2.8 nM (mean +/- S.E.M.; n = 5), respectively, while corresponding Bmax (maximum binding capacity) revealed 89 +/- 46 and 136 +/- 46 fmol [3H]E2/mg protein. The specificity of estrogen receptors as shown by competitive displacement experiments demonstrated receptors being highly specific just for estrogens, but not for other endogenous steroids having the following ranking of binding affinities: E2 > estrone > dehydroepiandrosterone > aldosterone > or = testosterone > or = corticosterone > or = progesterone. The affinity ranking of environmental chemicals compared to E2 was: E2 > tetrachlorbiphenyl > diethylphthalate > 2,2-bis-(4-hydroxyphenyl)-propan (bisphenol A) > or = 4-nonylphenol > or = 3-t-butyl-4-hydroxyanisole > or = 4-octylphenol > dichlor-diphenyl-trichlor-ethan (4,4'-DDT). Analyses of five sewage effluents for displacement of [3H]E2 binding resulted in three samples displacing more than 50% of specific binding at their original concentration. Taken together the established radioreceptorassay for [3H]E2 binding in Xenopus laevis liver cytosol is useful to screen estrogen receptor binding of pure compounds or complex mixtures of them, which is the prerequisite for causing either estrogenic or antiestrogenic effects.

Prolactin inhibits auto- and cross-induction of thyroid hormone and estrogen receptor and vitellogenin genes in adult Xenopus (Amphibia) hepatocytes

It is well known that virtually every tissue of the amphibian larvae is highly sensitive to the mutually antagonistic actions of thyroid hormone (TH) and prolactin (PRL), but it is not known if adult amphibian tissues respond similarly to these two hormones. We have previously shown that very low doses of triiodothyronine (T3) rapidly and strongly potentiate the activation of silent vitellogenin (Vit) genes by estrogen (E2) and the autoinduction of estrogen receptor (ER) transcripts in primary cultures of adult Xenopus hepatocytes. This response to T3 is accompanied by the upregulation of thyroid hormone receptor <FONT FACE="Symbol">b</FONT> (TR<FONT FACE="Symbol">b</FONT>) mRNA. Using Northern blot and RNase protection assays, we now show that ovine PRL added for 12 h along with 2 x 10-9 M T3 will completely prevent potentiation of E2 induction of Vit mRNA in primary cultures of adult Xenopus hepatocytes. PRL also abolished the auto-upregulation of TR<FONT FACE="Symbol">b</FONT> mRNA and the cross-activation of autoinduction of ER mRNA. Thus, we show for the first time that the anti-TH action of PRL that is manifested in Xenopus tadpole tissues during metamorphosis is retained in adult liver, and suggest that the mutually antagonistic actions of the two hormones may be brought about by similar molecular mechanisms in larval and adult amphibian tissues

Differential regulation of two genes implicated in fish reproduction: vitellogenin and estrogen receptor genes

In rainbow trout as well as in other species, variability of estrogen receptor (ER) gene expression according to the cell type and the physiological state reflects a differential cell and gene sensitivity to estrogen. We previously demonstrated that expression of the rainbow trout estrogen receptor (rtER) and vitellogenin (Vg) genes were induced differently by estrogens in rainbow trout liver. Therefore, these two genes offered a suitable model to study the influence of ER concentration on gene transcriptional activities. In the present study we show that the transcription rate of rtER and Vg genes during an estrogenic treatment are affected differently by variation of cellular ER concentration. We demonstrate that rtER gene exhibits a low threshold response to loaded estrogen receptor, and increasing ER amounts do not affect the transcriptional response of this gene during an estrogenic stimulation. On the contrary, Vg gene expression requires the presence of a higher loaded estrogen receptor level to be induced, and its transcriptional response is directly proportional to the amount of synthesised ER.

Hormonal regulation of the estrogen receptor in primary cultures of hepatocytes from female rats

Estrogen treatment affects the hepatic synthesis and/or secretion of several proteins involved in clinically important pathological processes such as atherosclerosis, hypertension, and thrombosis. The endocrine regulation of the estrogen receptor (ER) concentration in primary cultures of rat hepatocytes was studied. Human growth hormone (hGH) and dexamethasone (DEX) in combination increased ER concentration 6-fold and ER mRNA levels 2.5-fold. These effects were not significantly different from those observed after treatment with the purely somatogenic bovine growth hormone (GH) in combination with DEX. Treatment with the lactogen ovine prolactin in the presence or absence of DEX did not significantly affect ER or ER mRNA concentrations. Triiodothyronine treatment at the most effective concentration (50 nM) increased ER and ER mRNA levels twofold. Medium supplementation with estradiol (0.1 nM) throughout the experiment did not affect the response to treatment with hGH and DEX. Treatment with high concentrations of ethinylestradiol in combination with hGH and DEX, however, increased the ER level twice as much as hGH and DEX without addition of estradiol or ethinylestradiol, whereas the ER mRNA concentration was the same in both the GH+DEX group and GH+ DEX+ (estradiol or ethinylestradiol) groups. These data indicate the importance of GH in combination with glucocorticoids for the maintenance of ER concentrations in the rat liver. Thyroid hormones may be of some, although minor importance, whereas the data suggest that prolactin is not directly involved in hepatic ER regulation.

Transcriptional and post-transcriptional regulation of rainbow trout estrogen receptor and vitellogenin gene expression

Estrogen receptor (ER) and vitellogenin (Vg) gene expression are strongly up-regulated by estrogens in rainbow trout liver. In this paper, we have used primary cultured hepatocytes to examine the mechanisms implicated in estrogen regulation of ER and Vg gene expression. Treatment of hepatocytes with 1 microM estradiol (E2) led to a rapid increase in ER and mRNA level (15 fold) followed by Vg and mRNA induction. Transcription rate and mRNA half-life determination carried out in the presence or absence of E2, demonstrated that E2 increases both the ER and Vg gene transcriptional activity and mRNA stability (ca. 3 fold). The effect of E2 was inhibited by an excess of antiestrogen, showing that E2-stimulation of ER and mRNA level is mediated by the estrogen receptor. Our data show that ER and Vg genes have different hormonal sensitivity. In fact, the Vg gene required a higher concentration of E2 to be stimulated compared to the ER gene. Examination of the mechanisms involved in post-transcriptional regulation of ER mRNA showed that the setting up and maintenance of this regulation process implies that estrogen receptor and the general translational activity within the cells, suggesting that ER mRNA depends on the synthesis of an estrogen-dependent protein. However, the cis and trans elements involved in E2-stabilization process remain to be identified.

The androgen receptor mRNA is up-regulated by testosterone in both the Harderian gland and thumb pad of the frog, Rana esculenta

Alpha 32P-labelled cDNA probe from plasmid containing rat androgen receptor (rAR) has been tested in hybridization experiments using RNAs from the Harderian gland and thumb pad of the edible frog, Rana esculenta. Northern blot analysis has shown a high degree of homology between the rAR cDNA and the frog androgen receptor mRNA (fAR mRNA); this has been supported by both the hybridization conditions (high stringency) and the molecular size of fAR mRNA which is quite similar to those described in mammals (9.4 kb). The role of androgens has been further investigated with respect to the kinetics of expression of fAR mRNA in in vivo experiments. In both the Harderian gland and thumb pad, testosterone has increased the levels of fAR mRNA as compared with the untreated groups. The use of cyproterone acetate (CPA) in combination with testosterone has resulted in a loss of the increase in fAR mRNA as compared to testosterone-treated groups, while CPA alone has resembled the control group. In primary cultures of frog Harderian gland and thumb pad cells, the steady-state levels of fAR mRNA have been increased in the cells exposed to testosterone as compared to those not exposed. These findings confirm that, in these androgen target tissues, testosterone exerts an up-regulation on its own receptors, increasing the accumulation of fAR mRNA in the same way as oestrogens up-regulate the expression of their own receptors in Xenopus liver and oviduct cells.

Thyroid hormone and glucocorticoid independently regulate the expression of estrogen receptor in male Xenopus liver cells

Earlier studies from our laboratory had shown that triiodothyronine (T3) strongly potentiates the activation by estradiol (E2) of silent vitellogenin (Vit) genes and the autoinduction of estrogen receptor (ER) mRNA in primary cultures of male Xenopus hepatocytes (Rabelo and Tata, 1993). It was, however, not known if T3, or other hormones, could up-regulate ER mRNA in the absence of exogenous E2. We now show that T3 and dexamethasone (Dex), but not progesterone and testosterone, directly induce ER mRNA within 4 h by separate pathways, at doses compatible with the Kd values of their receptors. This induction of ER mRNA is accompanied by a marked enhancement of the activation of the silent Vit B1 gene if E2 is added by 12 h after T3 and Dex, thus suggesting an elevated level of functional ER induced by the two hormones. This conclusion was supported by a higher rate of transcription from an estrogen response element (ERE)-tk-CAT construct transfected into cultured hepatocytes pre-treated with T3 and Dex before incubation with estrogen. Our findings emphasize the importance of hormonal interplay via auto- and cross-regulation of nuclear hormone receptors.

Autoregulation and crossregulation of nuclear receptor genes

Whereas crossregulation of nuclear receptors has been known for some time, recently several examples of autoregulation have been described, especially during development and specific gene expression. In this review, I discuss both these phenomena, based on some studies from our laboratory on amphibian metamorphosis and egg protein gene expression. These include autoinduction of estrogen receptor (ER) accompanying egg protein gene expression in adult and larval Xenopus; autoinduction of thyroid hormone receptor (TR) during metamorphosis and in adult Xenopus; crossregulation by triiodothyronine (T(3)) and dexamethasone of autoinduction of ER; and inhibition by PRL of autoinduction and crossinduction of TR and ER genes. A dual receptor threshold model to explain the interplay between T(3), estrogen and PRL is presented and its significance to the general question of nuclear receptor autoregulation and crossregulation during development is also discussed.

Interplay between thyroid hormone and estrogen in modulating expression of their receptor and vitellogenin genes during Xenopus metamorphosis

Many postembryonic developmental processes are regulated by an intricate interplay among hormones and growth factors. Thyroid hormone (TH) and estrogen are well known to be individually and obligatorily required for the initiation and progression of amphibian metamorphosis and vitellogenesis. However, whether or not a possible interplay between these two hormones would affect these two developmental processes is not known. Here we report on how triiodothyronine (T3) enhances the precocious activation of vitellogenin (Vit) genes by estradiol (E2) in Xenopus tadpoles during metamorphosis. Using a combination of filter hybridization, RNase protection assay and in situ hybridization, we first show that very low doses (10(-9) M) of exogenous T3 will autoinduce thyroid hormone receptor (TR) mRNA in several tissues of premetamorphic tadpoles. The same treatment enhances and accelerates the precocious activation of the silent vitellogenin genes by E2 at metamorphic climax (stages 60-64) but not before mid-metamorphosis (stages 56-58). This developmental stage dependency may be explained by our finding that, under the same experimental conditions, T3 fails to alter the autoinduction of ER mRNA at mid-metamorphosis but strongly potentiates it at metamorphic climax. Thus a developmental stage specific interplay between thyroid hormone and estrogen determines the kinetics and extent of activation of vitellogenin and estrogen receptor genes during Xenopus postembryonic development.

Thyroid hormone potentiates estrogen activation of vitellogenin genes and autoinduction of estrogen receptor in adult Xenopus hepatocytes

Although the important role of thyroid hormones in regulating metamorphosis of amphibian larvae is well known, it has not been clearly established if thyroid hormones have any function in the activities of adult amphibian tissues. We now describe a strong effect of 3,3',5-triiodothyronine (T3) on adult Xenopus liver cells. Low doses of T3 rapidly (within 6-12 h) potentiate the activation of vitellogenin (Vit) genes by estradiol-17 beta (E2) in primary cultures of adult male and female Xenopus hepatocytes. This effect is developmentally regulated and is first manifested during metamorphic climax. In an attempt to explain this potentiation, we find that T3 also upregulates thyroid hormone receptor beta, but not alpha, transcripts and rapidly enhances the autoinduction of estrogen receptor (ER) mRNA in adult Xenopus hepatocytes. In transient transfection of the Xenopus cell line XTC-2 with an estrogen response element--chloramphenicol transacetylase (ERE-CAT) construct T3 was found to potentiate the transcription by E2 from the transfected ERE, thus suggesting that it enhances the accumulation of functional ER. We conclude that T3 can function in adult amphibian tissues, and discuss the significance of thyroid hormone potentiation of responses to estrogen in reproductive processes.

Autoinduction of nuclear receptor genes and its significance

Although downregulation of receptors by their respective hormonal ligands is a well-studied phenomenon, relatively less is known about autoupregulation of receptors. However, an increasing number of observations of the latter process are now being reported. Here, we discuss the phenomenon of autoinduction of nuclear receptors of the steroid/thyroid hormone gene family, and its significance in the context of the developmental and gene regulatory function of the ligands. Much of this review is illustrated by recent work from our laboratory on the autoregulation of Xenopus estrogen (ER) and thyroid hormone (TR) receptors and their transcripts, accompanying or anticipating vitellogenesis and metamorphosis, respectively. The activation by estrogen (E2) of the silent vitellogenin genes and the induction of FOSP-1 genes in primary cultures of hepatocytes from male Xenopus and oviduct cells, respectively, are tightly coupled to a substantial upregulation of ER protein and its transcript. The developmental competence to activate vitellogenin in response to E2 was found to be acquired during late metamorphosis. Since the latter process is obligatorily controlled by thyroid hormones (TH), we extended our studies to the developmental and hormonal regulation of Xenopus TR genes. Although very low levels of TR alpha and beta mRNAs are detectable in embryos and early larvae, there is a large increase in the accumulation of both transcripts before the onset of metamorphosis (stage 54 tadpoles), by which time the larval thyroid gland has first begun to secrete TH. Filter and in situ hybridization revealed that the two transcripts were differentially regulated and were not equally distributed in all regions or tissues of the tadpole. Their concentration peaks at metamorphic climax and drops to low levels in froglets and adult Xenopus. Exogenous TH given to pre-metamorphic tadpoles is known to induce metamorphosis precociously. Administration of triiodothyronine (T3) to early tadpoles (stages 50/52) caused a rapid upregulation of TR alpha and beta mRNAs which was particularly marked for the beta transcript (20- to 50-fold increase in steady-state levels). This autoinduction, which is the earliest response to T3, is mimicked to variable degrees in some Xenopus cell lines. In XTC-2 cells, in which the in vivo process is fully reproduced, it was possible to show with cycloheximide that the increase in TR mRNA requires protein synthesis. It was also possible to show by transfection of XTC-2 cells with a reporter-promoter construct of Xenopus albumin gene, which is a target for T3, that the extra TR mRNA increases functional receptor in the cell. Although the role of TH is well-known in metamorphosis, we found that TR is also autoinduced in primary culture of adult male Xenopus hepatocytes. The significance of this finding lies in the fact that T3 potentiates the autoinduction of ER and the de novo activation of vitellogenin genes by E2. Prolactin (PRL) is known to exert a "juvenilizing" action by preventing the induction of amphibian metamorphosis by TH. It is therefore highly significant that PRL prevented both the autoinduction of TR alpha and beta mRNAs in whole tadpoles and organ cultures and the activation of TR target genes, such as those encoding albumin and 63 kDa adult-type keratin. Although how PRL exerts its antimetamorphic effect is not known, these findings lead us to propose a dual threshold model for the autoinduction of TR, whereby the autoinduction of TR genes requires a very low level of TR and TH to rapidly augment the amount of functional TR. This higher amount of receptor would be required to achieve a higher threshold to activate "downstream" or target genes which specify the adult phenotype at the end of metamorphosis. Finally, a survey of recent literature shows that the phenomenon of autoinduction is not restricted to Xenopus ER and TR but is more widespread among members of the nuclear receptor family.

Prolactin inhibits both thyroid hormone-induced morphogenesis and cell death in cultured amphibian larval tissues

We describe for the first time the successful organ culture, in a serum-free chemically defined medium, of hind limb buds from stage 54/55 Xenopus laevis tadpoles in which 2 x 10(-9) M triiodothyronine (T3) precociously induces morphogenesis to give rise to morphologically normal limbs within 7 days. It was important to retain the mesenchymal tissue joining the two limb buds in order to obtain limb development in culture. T3 added to tail explants from the same larvae, cultured in parallel with limb buds, induced regression and cell loss at rates comparable to those seen during T3-induced metamorphosis in intact tadpoles. We also demonstrate for the first time that 0.2 units of prolactin (PRL) added at the same time as 2 x 10(-9) M T3 totally blocked both limb development and tail regression over 8 days in culture. When added after T3 had initiated its metamorphic action. PRL arrested further morphogenesis and regression of these two tissues, respectively. Retinoic acid at 10(-7) M had only a marginal effect. Histological examination showed that T3 added to limb buds produced normal chondrogenesis and osteogenesis in vitro as well as skin, muscle, and digit formation, while it produced a rapid and marked histolysis of fin and connective tissue of the tail. The ease of hormonally manipulating both morphogenesis and cell death in culture in opposite directions offers a simple, effective model system for molecular analysis of mechanisms underlying hormone-regulated postembryonic developmental processes.

Changes in hepatic estrogen-receptor concentrations during the annual reproductive and ovarian cycles of a marine teleost, the spotted seatrout, Cynoscion nebulosus

The concentrations of estrogen and vitellogenin in plasma, and hepatic estrogen receptor in cytosolic (ERc) and nuclear (ERn) extracts were elevated throughout the reproductive season in females from a wild population of spotted seatrout, Cynoscion nebulosus. There were significant correlations between plasma estradiol and ERc and between ERn and plasma vitellogenin during the period of ovarian recrudescence (January-May), but not during the remainder of the reproductive season (June-September). During ovarian recrudescence ERc concentrations increased fourfold from 0.78 +/- 0.15 pmol/g liver (N = 26) in nonvitellogenic females to 3.23 +/- 0.26 pmol/g liver (N = 77) in late vitellogenic females and ERn concentrations increased eightfold from 0.16 +/- 0.07 pmol/g liver (N = 26) in nonvitellogenic females to 1.12 +/- 0.45 pmol/g (N = 10) in mid vitellogenic females. However, following this period of ovarian recrudescence, estrogen-receptor concentrations and plasma vitellogenin titers did not fluctuate during the remaining stages of the ovulatory cycle (hydration, ovulation, and spawning). In addition, both hepatic ERc and ERn were elevated for the rest of the seasonal reproductive cycle, during which several ovulatory cycles may have occurred in this multiple-spawning species. The affinity of ERc for estradiol did not vary (Kd = 1.26 +/- 0.06 nM, N = 68) regardless of the stage of ovarian development. Plasma estradiol titers declined in August, near the end of the reproductive season, prior to the decline in estrogen-receptor concentrations. The persistence of the estrogen receptor during the ovulatory cycle suggests that even if plasma estradiol titers declined between successive ovulatory periods, the hepatic responsiveness to estrogenic stimuli would not be diminished and thus vitellogenin synthesis, if interrupted at all, could be resumed soon after spawning.

Autoinduction of estrogen receptor is associated with FOSP-1 mRNA induction by estrogen in primary cultures of Xenopus oviduct cells

The number of nuclear and cytosolic estrogen receptors (ER) per cell and the steady-state levels of the mRNA encoding a tissue-specific, estrogen-inducible protein (FOSP-1) were measured as a function of time following the addition of estradiol-17 beta (E2) to primary cultures of Xenopus oviduct cells. After a lag period of about 12 h, 10(-9) to 10(-7) M E2 caused a 10 to 15-fold increase in FOSP-1 mRNA by 60 h, whereas it was only 2-fold with 10(-7) M progesterone. Under the same conditions, E2 doubled its own total receptor content within the first 12 h, reaching a 4-fold increase in nuclear ER by 48 h. Cycloheximide treatment in the presence of 10(-7) M estradiol reduced the functional ER content by 75.90%. Treatment with the anti-estrogen ICI 164,384 of oviduct cells in which FOSP-1 mRNA was pre-induced to high levels with the hormones caused a drastic reduction in nuclear ER and a total loss of FOSP-1 mRNA in 72 h. The close correlation between the kinetics of autoinduction of ER and the induction of FOSP-1 mRNA, as was shown earlier for vitellogenin mRNA in hepatocytes (Perlman et al. (1984) Mol. Cell. Endocrinol. 38, 151-161), strongly suggests that Xenopus egg protein gene activation by estrogen requires the up-regulation of its own receptor by the hormone.

Receptor interconversion model of hormone action. 2. Requirement of both kinase and phosphatase activities for conferring estrogen binding activity to the estrogen receptor

Three interconvertible forms of the estrogen receptor have been identified in the oviduct of estrogen-stimulated chicks. The non-estradiol binding form (Rnb) can be converted to the lower affinity binding form (Ry, Kd = 0.8 nM) by a process requiring the gamma-phosphoryl moiety of ATP. The enzymatic activity (Fy) essential for this "receptor potentiation" has been isolated from oviduct cytosol using ammonium sulfate fractionation, DEAE chromatography, and HPLC size-exclusion chromatography. The potentiation appears to require both kinase and phosphatase activities. The Fy kinase characteristically phosphorylates casein, histones, and glycogen synthase. Comparison of the kinase with casein kinase II, which also phosphorylates casein and glycogen synthase, indicates that Fy represents a distinct protein kinase since its activity is not stimulated by spermine or inhibited by heparin. Fy-mediated conversion of Rnb to Ry is blocked by the phosphatase inhibitors vanadate, fluoride, and pyrophosphate. The substrate specificity of the Fy phosphatase activity is distinct from that of the two well-characterized protein phosphatases 1 and 2A. Moreover, the requirement for Fy phosphatase activity in converting Rnb to Ry could not be mimicked by its substitution with purified protein phosphatases 1 or 2A. The unique substrate specificity of the oviduct protein phosphatase and protein kinase, which are apparently necessary to confer estradiol binding characteristics to the receptor, implies that these enzymes play a key role in the control of the estrogen receptor in its function as a transcription factor.

Binding characteristics of the hepatic estrogen receptor of the spotted seatrout, Cynoscion nebulosus

Estrogen receptors were identified in cytosolic and nuclear extracts of livers of adult female spotted seatrout, Cynoscion nebulosus. A single class of high affinity binding sites was found, Kd = 1.26 +/- 0.55 nM (N = 55) for the cytosolic extract and Kd = 1.96 +/- 0.42 nM (N = 8) for the nuclear extract. The Kd did not differ between males and females or between vitellogenic and nonvitellogenic females. The binding in both the cytosolic and nuclear extracts was specific for estrogens (DES greater than E2 much greater than E1 = E3). Receptor concentrations in cytosolic extracts from late vitellogenic females (14.61 +/- 1.07 pmol/g liver, N = 40) were significantly higher than those from nonvitellogenic females (3.91 +/- 0.73 pmol/g liver, N = 7). The nuclear binding capacity of livers from midvitellogenic females (1.12 +/- 0.45 pmol/g liver, N = 10) was significantly higher than the binding capacity in livers from nonvitellogenic females (0.16 +/- 0.07 pmol/g liver, N = 26), but not that of late vitellogenic females (0.80 +/- 0.09 pmol/g liver, N = 77). The concentration of estradiol-binding sites was greatest in the liver (liver much greater than ovary greater than heart greater than spleen greater than muscle greater than brain). No interference from other steroid-binding proteins was detected using a simple dextran-coated charcoal method to separate bound from free hormone. Approximately 14% of the binding in the cytosolic extract had DNA-binding affinity. Estrogen receptor binding activity was maximally extracted from nuclei with buffer containing 0.6 M KCl. Nuclear receptors eluted from gel filtration columns with an apparent molecular weight of 95 kDa.

Seasonal variation in hepatic binding of estrogen in the turtle, Chrysemys picta

The present study describes the seasonal changes of the estrogen receptor (ER) system in the liver of the turtle, Chrysemys picta. [3H]Estradiol ([3H]E2) binding capacities and affinities of liver cytosols and nuclear extracts were measured with established procedures and analyzed by Scatchard plots. Our data revealed significant seasonal variations in both receptor content and the ER's affinity for [3H]E2 in the liver of the turtle. Nuclear ER content remained at a fairly stable level of 70 fmol/g tissue throughout the year, but exhibited two sharp increases in the months of May and October, attaining values of approximately 150 fmol/g tissue. These rises in nuclear ER content coincided temporally with the two peaks of vitellogenin accumulation and estrogen surge in the annual cycle previously reported for the female turtle. Cytosolic ER level in the liver exhibited a much more complex pattern. It fluctuated with a much wider range of 80 to 250 fmol/g tissue. Right after oviposition in July, it declined to its lowest value of 80 +/- 9 fmol/g tissue but gradually increased to 160 +/- 15 fmol/g tissue in September and remained at this medium range for the remaining of the fall season but rose sharply again to the highest value of the year in December, reaching a value of 270 +/- 7 fmol/g tissue. In the following winter months, cytosolic ER level declined until March before it rebounded to a second peak value of 242 +/- 6 fmol/g tissue in May.(ABSTRACT TRUNCATED AT 250 WORDS)

Thyroid hormone directly induces hepatocyte competence for estrogen-dependent vitellogenin synthesis during the metamorphosis of Xenopus laevis

Hepatocytes competent for estrogen-dependent vitellogenin synthesis appeared and increased in number in the liver at the metamorphic climax of Xenopus laevis (A. Kawahara, S. Kohara, Y. Sugimoto, and M. Amano, 1987, Dev. Biol. 122, 139-145). The present study was conducted to determine whether cells competent for vitellogenin synthesis could be induced by thyroid hormone in a primary culture of larval hepatocytes. The thyroid hormone, triiodothyronine (T3), directly induced the competent cells in a primary culture of premetamorphic larval hepatocytes in a dose- and duration-dependent manner. The competency acquired in response to T3 persisted after removal of the hormone. Aphidicholin, an inhibitor of DNA synthesis, failed to block this induction, suggesting the presence of a "precursor cell fraction." This cell fraction in the hepatocyte population increased with the progress of metamorphosis. The thyroid hormone is thus considered the cause of competent cell formation at metamorphic climax.

FOSP-1 (frog oviduct-specific protein-1) gene: cloning of cDNA and induction by estrogen in primary cultures of Xenopus oviduct cells

We describe the isolation of a cloned cDNA from a cDNA library of oviduct of estrogen-treated adult Xenopus. Although the protein encoded by this cDNA is not known, it is designated as FOSP-1 (frog oviduct-specific protein-1). A partial restriction map of FOSP-1 cDNA, which is 1.5 kb in size, is presented. Northern hybridization analysis showed that FOSP-1 cDNA codes for a single species of mRNA of 2.6 kb which is exclusively expressed in Xenopus oviduct. Southern blot analysis showed that the gene was present in only one or two copies. Sequencing of partial FOSP-1 cDNA did not reveal homology with any protein in the sequence data bank. Measurement of steady-state levels of FOSP-1 mRNA in primary cultures of Xenopus oviduct cells by a technique of quantitative slot-blot analysis showed that both 17 beta and 17 alpha stereoisomers of estradiol caused a rapid 5-fold enhancement of accumulation of the mRNA with maximum values obtained at 5 X 10(-8) M estrogen. Progesterone caused only a small increase in FOSP-1 mRNA concentration. This hormone-specific induction of mRNA makes FOSP-1 a valuable candidate for exploring tissue specificity of regulation by estrogen of gene expression.

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.

Vitellogenin gene expression in primary culture of male rainbow trout hepatocytes

Using a primary culture of trout hepatocytes we compare the kinetics of accumulation of vitellogenin and its messenger RNA after estrogen administration. We found that the cells were more sensitive to estradiol than to other estrogens. The lowest effective concentration of estradiol was 10(-9) M. At 10(-6) M the androgens have no effect. Comparison of the primary and secondary stimulation with E2 shows that the initial rate of accumulation of vitellogenin is very much higher in the secondary stimulation. Over a time course of primary stimulation we show that after estradiol withdrawal the rate of accumulation of vitellogenin mRNA in the secondary is a function of time of the first stimulation.

Establishment and characterization of Xenopus oviduct cells in primary culture

Based on a previously established procedure for Xenopus hepatocytes, we describe tubular oviduct cells in primary culture which continue to secrete substantial quantities of egg jelly for several days, as can be visualized microscopically. Freshly isolated cells exhibited a culture shock response [A. P. Wolffe, J. F. Glover, and J. R. Tata (1984) Exp. Cell Res. 154, 581], from which they recovered by the third day in culture. This recovery was characterized by (a) the diminished synthesis of heat shock proteins hsp 70 and hsp 85, (b) the cessation of the drop in number of estrogen receptor, and (c) the enhanced rate of synthesis of cellular and secreted proteins. The oviduct estrogen receptor had the same characteristics as those in other estrogen target tissues and was present in the same amount as in adult female Xenopus hepatocytes [A. J. Perlman, A. P. Wolffe, J. Champion, and J. R. Tata (1984) Mol. Cell. Endocrinol. 38, 51]. Unlike the latter in primary culture [M. P. R. Tenniswood, P. F. Searle, A. P. Wolffe, and J. R. Tata (1983) Mol. Cell. Endocrinol. 30, 329], oviduct cell cultures did not actively metabolize either estradiol or progesterone (t1/2 approximately equal to 55 and 7 h, respectively). The successful establishment and characterization of primary cultures of both liver and oviduct cells now fulfil the conditions required for investigating the basis for tissue specificity of regulation by estrogen of Xenopus egg protein gene expression in primary cell culture.

Hormonal regulation of RNA synthesis and specific gene expression in Xenopus oviduct cells in primary culture

The studies described in this report were carried out as a first step towards the elucidation of mechanisms underlying the tissue specificity of regulation of gene expression by estrogen. Using a procedure established earlier in our laboratory for primary culture of Xenopus hepatocytes, we have characterized how estradiol-17 beta and progesterone affect the rate of synthesis of total RNA and that of accumulation of two oviduct-specific mRNAs in Xenopus oviduct cells in primary culture. In cells that had recovered from 'culture shock' 3 days after they were plated out, both hormones had only a slight or no effect on the overall rate of labelling of newly synthesized RNA over 24 h. Cloned cDNA probes for two mRNAs, termed 7F and 6G and specifying as yet unknown proteins expressed in the oviduct and not in the liver, were used to quantify the two mRNAs. The levels of both mRNAs declined for the first 2 days in culture after which they were stabilized. When added to the oviduct cell cultures 3 days after they were plated out, estradiol increased the steady-state concentration (relative to total RNA) of 7F and 6G mRNAs by 3- to 7-fold after 60-80 h, but with different time-course and dose-response kinetics for the two messages. The antiestrogen tamoxifen also exerted different degrees of antagonist effect on the estrogen-induced accumulation of 7F and 6G mRNAs. Although the protein products of these two oviduct-specific mRNAs have not yet been characterized, these studies set the stage for comparing the regulation by estrogen of their genes with that of vitellogenin genes in primary cultures of Xenopus oviduct cells and hepatocytes.

Estrogen and androgen receptors in the liver of cynomolgus monkeys (Macaca fascicularis)

Estrogen receptors (ER) and androgen receptors (AR) were evaluated in the hepatic cytosol from cynomolgus macaques to determine if there were differences associated with gender and endogenous hormone secretion. Saturable, high affinity binding (Kd = 0.2-0.8 nM) was demonstrated for both ER and AR from either male or female monkeys. Displacement of tritiated estradiol from the ER was estrogen specific (including ethinyl estradiol). Both androgens and the synthetic progestins (levonorgestrel and norethindrone) displaced tritiated mibolerone from the AR. Both 8S and 4S molecular forms of ER and AR were demonstrated on 5-20% sucrose density gradients. The ER levels were higher in females in the follicular phase of the menstrual cycle (40.5 +/- 1.9 fmol/mg protein) than levels in males (26.4 +/- 4.8 fmol/mg protein; P less than 0.01) or levels in luteal phase females (31.8 +/- 2.4 fmol/mg protein; P less than 0.05). AR levels were not different between females during different phases of the menstrual cycle (65.8 +/- 4.6 and 69.5 +/- 4.3 fmol/mg protein, follicular and luteal, respectively), but there was a tendency (P less than 0.10) for the levels in males (54.4 +/- 6.6 fmol/mg protein) to be lower than female levels. The demonstration of saturable, high affinity binding of androgens and estrogens in liver tissue of these primates, along with differences associated with gender and the stage of the menstrual cycle, suggests that hepatic receptors are functional and may play an important role in hepatic protein secretion.

Chromatin structural transitions and the phenomenon of vitellogenin gene memory in chickens

We have previously shown that the steroid hormone-mediated transcriptional activation of the chicken vitellogenin II gene (VTGII) in the liver is accompanied by a series of chromatin structural changes, including the formation of two sets of 5'-proximal nuclease-hypersensitive sites and the demethylation of a single 5'-flanking MspI site which lies within a region of DNA that recently has been shown by Jost and co-workers to specifically bind the estrogen receptor complex in vitro. To assay the stability and possible functional significance of these induced structural changes, we transiently activated the VTGII gene during embryonic development and then allowed the chickens to hatch and grow for various periods of time before analyzing their livers. By 7 weeks posthatching all of the induced 5'-flanking hypersensitive sites had decayed. Moreover, the loss of these sites occurred without consequence to the "memory effect," that is, these structural features did not need to be present in hormone withdrawn birds to allow this gene to be activated more rapidly in response to a secondary presentation of estradiol. Although the demethylation was more stable, it also appeared not to be the basis of the memory phenomenon. The birds that still exhibited memory after 25 weeks of hormone withdrawal were not more extensively demethylated within the receptor-binding site than were the birds which failed to show memory at this age. A similar uncoupling of these two parameters was also observed when embryos were first injected with submaximal doses of estradiol and then assayed 1 week after hatching; the chickens which acquired memory were not demethylated to any greater extent than those which did not acquire memory. Other parameters that may be relevant to memory are discussed.

Homologous up-regulation of the 1,25 (OH)2 vitamin D3 receptor in rats

This study investigates the ability of vitamin D-metabolites to regulate 1,25(OH)2D3 receptors in vivo. Rats made vitamin D-deficient were treated with 1,25(OH)2D3 or vehicle for 1-5 days. In treated animals, receptors for 1,25(OH)2D3 in kidney increased dramatically compared with control levels. An increase in specific binding to 220% of control was seen after 2 doses of hormone, which reached to 336% after 5 days of treatment. Intestinal receptors increased to only 130% of control levels after 5 days of treatment. In vitamin D-replete animals, the difference between control and treated groups was slightly greater when endogenously occupied sites were measured by exchange (TPCK). However, significant changes were observed only after 4 days of hormone treatment. The data indicate that homologous up-regulation of the 1,25(OH)2D3 receptor occurs in vivo. The difference in response in kidney and in intestine suggests differential importance of up-regulation in various organs.

Effects of antiestrogens on the induction of vitellogenin and its mRNA in Xenopus laevis

The egg yolk protein precursor vitellogenin is induced by estrogen in the liver of male Xenopus laevis. The large rise in serum vitellogenin is accompanied by a corresponding increase in intracellular levels of vitellogenin and its mRNA. In the present study this model system was used to examine the subcellular sites of action of triphenylethylene antiestrogens (e.g. tamoxifen). Tamoxifen was extensively metabolized to 4-hydroxytamoxifen in Xenopus and both of these antiestrogens were used in this study. Pre-injection with tamoxifen or 4-hydroxytamoxifen suppressed the estrogen-dependent induction of vitellogenin in serum. 4-Hydroxytamoxifen also inhibited the induction of intracellular vitellogenin and its mRNA by estrogen suggesting that this metabolite of tamoxifen is able to inhibit estrogen-induced transcription of the vitellogenin genes. Neither tamoxifen nor 4-hydroxytamoxifen stimulated the production of serum vitellogenin as assayed by a sensitive dot immunoblot assay. However either compound alone induced low amounts of vitellogenin mRNA and stimulated the production of intracellular vitellogenin to levels 10-40% of those produced by similar doses of estradiol. Since 10-40% of the serum levels of vitellogenin produced by estradiol would have been detected by the dot immunoblot assay, these data suggest that antiestrogens may have effects on post-translational processing or secretion of vitellogenin in addition to their effects on vitellogenin transcription.

Chromatin structural transitions and the phenomenon of vitellogenin gene memory in chickens

We have previously shown that the steroid hormone-mediated transcriptional activation of the chicken vitellogenin II gene (VTGII) in the liver is accompanied by a series of chromatin structural changes, including the formation of two sets of 5'-proximal nuclease-hypersensitive sites and the demethylation of a single 5'-flanking MspI site which lies within a region of DNA that recently has been shown by Jost and co-workers to specifically bind the estrogen receptor complex in vitro. To assay the stability and possible functional significance of these induced structural changes, we transiently activated the VTGII gene during embryonic development and then allowed the chickens to hatch and grow for various periods of time before analyzing their livers. By 7 weeks posthatching all of the induced 5'-flanking hypersensitive sites had decayed. Moreover, the loss of these sites occurred without consequence to the "memory effect," that is, these structural features did not need to be present in hormone withdrawn birds to allow this gene to be activated more rapidly in response to a secondary presentation of estradiol. Although the demethylation was more stable, it also appeared not to be the basis of the memory phenomenon. The birds that still exhibited memory after 25 weeks of hormone withdrawal were not more extensively demethylated within the receptor-binding site than were the birds which failed to show memory at this age. A similar uncoupling of these two parameters was also observed when embryos were first injected with submaximal doses of estradiol and then assayed 1 week after hatching; the chickens which acquired memory were not demethylated to any greater extent than those which did not acquire memory. Other parameters that may be relevant to memory are discussed.

Stimulation of estrogen receptor accumulation by estradiol in primary cultures of salmon hepatocytes

Hepatocytes of Salmo salar in primary culture form confluent monolayers and can be maintained at 11 degrees centigrade in serum-free medium for 8 days with minimal cell loss. Cultured hepatocytes from immature male salmon contain estrogen receptor both in nuclear and cytosol fractions (2000 and 2400 cites/cell, respectively). A single addition of estradiol results in an increase in the nuclear receptor to a level of 23 000 sites/cell after 24 h. This nuclear receptor concentration is similar to that in liver of estrogen-treated salmon in vivo, and is much higher than has been found for any other egg-laying vertebrate. The cultured salmon hepatocytes thus represent a highly sensitive system for the study of estrogen receptor dynamics and vitellogenesis in vitro.

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.