Ontogeny of the vitellogenic response to oestradiol and of the soluble nuclear oestrogen receptor in embryonic-chick liver

A comparative study of 3 alternative avian toxicity testing methods: Effects on hepatic gene expression in the chicken embryo

There is growing interest in developing alternative methods to screen and prioritize chemical hazards, although few studies have compared responses across different methods. The objective of the present study was to compare 3 alternative liver methods derived from white Leghorn chicken (Gallus gallus domesticus): primary hepatocyte culture, liver slices, and liver from in ovo injected embryos. We examined hepatic gene expression changes after exposure to 3 chemicals (17β-trenbolone [17βT], 17β-estradiol [E2], and 2,3,7,8-tetrachlorodibenzo-p-dioxin [TCDD]) using a custom quantitative polymerase chain reaction (qPCR) array with 7 genes (vitellogenin [VTG], apolipoprotein [Apo], cytochrome P450 1A4 [CYP1A4], liver basic fatty acid binding protein [LBFABP], 3β hydroxysteroid dehydrogenase [HSD3β1], stearoyl coenzyme A desaturase [SCD], and estrogen sulfotransferase [SULT1E1]). Gene expression across the 3 methods was examined using hierarchical clustering. Up-regulation of CYP1A4 in response to TCDD was consistent across all methods, and the magnitude was higher in hepatocytes (>150-fold) compared with slices (>31-fold) and in ovo liver (>27-fold). In hepatocytes, SCD and VTG up-regulation in response to 17βT and E2 was >4-fold and 16-fold, respectively. The rank order of cases with significant changes in gene expression among the 3 methods was: hepatocytes (22) > in ovo liver (11) > liver slices (6). Hierarchical clustering grouped liver slices and in ovo liver as more similar, whereas hepatocytes were grouped separately from in ovo liver. More introspective comparisons are needed to understand how and why alternative methods differ and to aid in their integration into toxicity testing. Environ Toxicol Chem 2019;38:2546-2555. © 2019 SETAC.

Establishment and Characterization of a Novel Tissue-specific DNA Construct and Culture System with Potential for Avian Bioreactor Generation

Transgenic chickens are of great interest for the production of recombinant proteins in their eggs. However, the use of constitutive strong promoters or the tissue-specific ovalbumin promoter for the generation of the transgenic chickens have different drawbacks that have to be overcome in order to make chicken bioreactor an efficient production system. This prompted us to investigate the use of an alternative tissue-specific promoter, the vitellogenin promoter, which could overcome the difficulties currently found in the generation of chicken bioreactors. In the present work we establish and characterize a DNA construct consisting of a fragment of the 5´-flanking region of the chicken vitellogenin II gene cloned in a reporter vector. This construct is capable of showing the ability of the promoter to drive expression of a reporting gene in a tissue-specific manner and in a way that closely resembles physiologic regulation of vitellogenin, making it an ideal candidate to be used in the future for generation of avian bioreactors. Besides, we validate an in vitro culture system to test the performance of the DNA construct under study that could be used as a practical tool before generating any transgenic chicken. These results are important since they provide the proof of concept for the use of the vitellogenin promoter for future genetic modification of chickens bioreactors with improved characteristics in terms of quality of the recombinant protein produced.

Endocrine disruption caused by oral administration of atrazine in European quail (Coturnix coturnix coturnix)

The widely used herbicide atrazine (ATZ) has been reported to exhibit reproductive toxicity in rats, fish and amphibians, with an avian LD(50) of 5000 mg/kg. In the present work, ATZ was administered as a single oral dose of 25 or 100 mg/kg to female European quail (Coturnix coturnix coturnix) at days 0, 5 and 10 of the experiment, being the animals sampled at days 15, 30 and 45. ATZ significantly increased the expression of hepatic estrogen receptor α (ERα) at both doses at day 30. An important increase was also observed in plasma 17β-estradiol (E2) concentrations. ATZ at 100 mg/kg increased the circulating concentration of vitellogenin (Vtg), but this effect was not related with an increase in hepatic Vtg mRNA levels. ATZ had no effect on the hepatic expression of both cytochrome P450 1A4 (CYP1A4) or the related biotransformation activity ethoxyresorufin-O-deethylase (EROD). These results led to the conclusion that ATZ provokes an estrogenic effect in sexually mature females of European quail. Further studies are necessary to establish the effect on sexual development or reproduction of female and male birds in the wild.

Activation of estrogen receptor alpha disrupts differentiation of the reproductive organs in chicken embryos

Gonadal estrogen plays an important role in the differentiation of a female phenotype in birds. Exogenous compounds that interfere with estrogen signaling, for instance by binding to the estrogen receptors alpha and beta (ERα and ERβ), are therefore potential disruptors of sexual differentiation in birds. The ERα agonist propyl-pyrazole-triol (PPT), the ERα antagonist methyl piperidino pyrazole (MPP) and the ERβ agonist diarylproprionitrile (DPN) were used in the present study to explore the roles of the ERs in normal and disrupted sex differentiation in the chicken embryo. Activation of ERα by PPT caused disturbed differentiation of the reproductive organs in both sexes. In male embryos, PPT caused left-side ovotestis formation and retention of the Müllerian ducts. In female embryos, PPT caused retention of the right Müllerian duct (which normally regresses) and malformation of both Müllerian ducts. PPT also induced hepatic expression of mRNA for the estrogen-regulated egg yolk protein apoVLDL II. Notably, none of these effects were observed following treatment with DPN. ERα-inactivation by MPP counteracted the action of PPT but had little effect by its own. Our results indicate that ERα plays an important role in sex differentiation of the reproductive tract in female chicken embryos and show that ERα can mediate xenoestrogen-induced disturbances of sex differentiation.

Expression of perivitelline membrane glycoprotein ZP1 in the liver of Japanese quail (Coturnix japonica) after in vivo treatment with diethylstilbestrol

Avian perivitelline membrane, an oocyte extracellular matrix homologous to the zona pellucida in mammals or chorion in fish, is composed of at least two glycoproteins. Previous studies have indicated that one of the components, a glycoprotein homologous to mammalian ZPC, is produced in the granulosa cells of the developing follicles of quail ovary on stimulation with testosterone. However, little is known about the molecular biology of the other component of the avian perivitelline membrane, ZP1, and information about gene expression is particularly lacking. We have cloned the ZP1 in Japanese quail and examined its gene expression. A cDNA encoding quail ZP1 was isolated from the livers of mature females using reverse transcription-polymerase chain reaction (RT-PCR) and rapid amplification of cDNA ends. It encoded a 934-amino acid protein that showed greatest homology (87.8% identity) with the chicken ZP1. RT-PCR amplification indicated that the ZP1 mRNA in the liver was restricted to mature laying females. The expression of ZP1 mRNA was stimulated by in vivo treatment with diethylstilbestrol in immature females as well as males. These results suggested that androgens and estrogens coordinately regulate the formation of quail perivitelline membrane proteins. In addition, the use of ZP1 transcriptional induction in males or immature females as a biological marker of environmental estrogens is discussed.

Reactivation of apolipoprotein II gene transcription by cycloheximide reveals two steps in the deactivation of estrogen receptor-mediated transcription

In this report, we describe apolipoprotein II (apoII) gene expression in cell lines derived by stable expression of the chicken estrogen receptor in LMH chicken hepatoma cells. In cell lines expressing high levels of receptor (LMH/2A), apoII gene expression is increased by estrogen 300-fold compared with levels in the receptor-deficient parent LMH line. LMH/2A cells show apoII mRNA induction and turnover kinetics similar to those in chicken liver. Inhibition of protein synthesis with cycloheximide (CHX) or puromycin following estrogen withdrawal superinduces apoII mRNA without affecting apoII mRNA stability. Superinduction is due to an estrogen-independent reactivation of apoII gene transcription. The apoII gene can be reactivated by CHX for up to 24 h following hormone withdrawal, suggesting that the gene is in a repressed yet transcriptionally competent state. These results reveal two distinct events necessary for termination of estrogen receptor-mediated transcription. The first event, removal of hormone, is sufficient to stop transcription when translation is ongoing. The second event is revealed by the CHX-induced superinduction of apoII mRNA following hormone withdrawal. This superinduction suggests that deactivation of estrogen receptor-mediated transcription requires a labile protein. Furthermore, reactivation of apoII gene expression by CHX and estrogen is additive, suggesting that estrogen is unable to overcome repression completely. Thus, a labile protein may act to repress estrogen receptor-mediated transcription of the apoII gene.

Reactivation of apolipoprotein II gene transcription by cycloheximide reveals two steps in the deactivation of estrogen receptor-mediated transcription

In this report, we describe apolipoprotein II (apoII) gene expression in cell lines derived by stable expression of the chicken estrogen receptor in LMH chicken hepatoma cells. In cell lines expressing high levels of receptor (LMH/2A), apoII gene expression is increased by estrogen 300-fold compared with levels in the receptor-deficient parent LMH line. LMH/2A cells show apoII mRNA induction and turnover kinetics similar to those in chicken liver. Inhibition of protein synthesis with cycloheximide (CHX) or puromycin following estrogen withdrawal superinduces apoII mRNA without affecting apoII mRNA stability. Superinduction is due to an estrogen-independent reactivation of apoII gene transcription. The apoII gene can be reactivated by CHX for up to 24 h following hormone withdrawal, suggesting that the gene is in a repressed yet transcriptionally competent state. These results reveal two distinct events necessary for termination of estrogen receptor-mediated transcription. The first event, removal of hormone, is sufficient to stop transcription when translation is ongoing. The second event is revealed by the CHX-induced superinduction of apoII mRNA following hormone withdrawal. This superinduction suggests that deactivation of estrogen receptor-mediated transcription requires a labile protein. Furthermore, reactivation of apoII gene expression by CHX and estrogen is additive, suggesting that estrogen is unable to overcome repression completely. Thus, a labile protein may act to repress estrogen receptor-mediated transcription of the apoII gene.

Tissue specific expression of avian vitellogenin gene is correlated with DNA hypomethylation and in vivo specific protein-DNA interactions

The avian vitellogenin gene is expressed only in the liver of egg-laying hens. It can, however, be activated in immature chicks or roosters by oestradiol. Parallel to the onset of transcription, there is a demethylation of specific mCpGs in the promoter region and in the oestrogen response element (ERE). The methylation pattern in the promoter region is hormone and expression specific, whereas in the ERE it is only hormone and not organ specific. The demethylation occurring in the promoter region is correlated with the appearance of DNase I hypersensitivity sites and changes in the specific protein-DNA interactions. In vivo genomic footprinting of the ERE with varying concentrations of dimethylsulphate revealed, upon gene activation, only minor changes in the protein-DNA interaction. We present evidence that there is another protein that binds with high affinity to the ERE, besides the oestrogen receptor.

Comparison of sex hormone dependent induction of delta-aminolevulinic acid dehydratase in chick liver and oviduct

1. Comparative study on induction of hepatic and oviduct delta-aminolevulinic acid dehydratase (EC 4.2.1.24, ALAD) was performed following estradiol-17 beta and/or testosterone administration in immature female chicken (Gallus domesticus). 2. The lowest amount of estradiol for maximal induction of hepatic and oviduct ALAD activity was 2 mg/day/bird. 3. When estradiol of 2 mg/day/bird was administered for 15 days successively, induction extent of oviduct ALAD molecule was markedly larger and became approximately 144-fold in comparison with that of liver. Testosterone (2 mg/day/bird) alone did not induce both hepatic and oviduct ALAD activity. 4. Synergistic and antagonistic effect of testosterone on estradiol-induced total ALAD activity in oviduct was dependent on testosterone amount administered, whereas testosterone antagonized the inductive effect of estradiol on liver ALAD activity, independently of its amount.

Isolation of chicken vitellogenin I and III cDNAs and the developmental regulation of five estrogen-responsive genes in the embryonic liver

The isolation of cDNA clones that code for portions of the two minor chicken vitellogenin (VTG) genes (VTGI and VTGIII) is reported. These clones represent unique sequences that are expressed exclusively in the livers of estrogenized birds. In the liver of the egg-laying hen, the levels of RNAs encoding VTGI, VTGII, and VTGIII are approximately 11,000, 30,000, and 3,000 molecules per cell, respectively. We have used the newly isolated clones, as well as the yolk protein cDNAs previously available [VTGII, apolipoprotein II (apoVLDLII), and apolipoprotein B], as probes to examine several aspects of the regulation of these genes by estradiol. First, we demonstrate that the capacity of each gene to respond to estradiol is acquired between 8 and 13 days in ovo. The response of four of these genes to estradiol is diminished during late fetal development, but the responsiveness is recovered within a week after hatching. Second, we demonstrate that these genes display distinct kinetic response profiles following the addition of estradiol. Third, as has been described previously for the VTGII and apoVLDLII genes, we demonstrate that a single injection of estradiol effects a long-term reprogramming event (hepatic memory) that allows a faster onset of the rapid accumulation of both VTGI and VTGIII RNAs following a subsequent rechallenge by estradiol. Collectively, these three sets of data suggest molecular parameters that may contribute to both the coordinate and noncoordinate regulation of this set of genes by estradiol.

Ah receptor for 2,3,7,8-tetrachlorodibenzo-p-dioxin: ontogeny in chick embryo liver

Aryl hydrocarbon hydroxylase (AHH, cytochrome P1-450) is induced in chick liver very early during embryonic development if embryos are treated with 3-methylcholanthrene-type compounds such as 3,4,3'4'-tetrachlorobiphenyl. In mammals, AHH induction is known to be mediated by the Ah receptor. Liver from embryonic and newly hatched chicks was found to contain a cytosolic receptor for 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) which has properties that are very similar to properties of the Ah receptor previously characterized in mammalian tissues. In chick embryo liver, cytosolic binding sites for TCDD were of high affinity (Kd for [3-H]-TCDD = 0.2 nM) and were specific for 3-methylcholanthrene-type inducers. The specific binding component sedimented at about 9S on sucrose density gradients prepared at low ionic strength. A high level of Ah receptor was detected in chick embryo liver by the fifth day of incubation (5 DI); this is at least 24 hours prior to the onset of AHH inducibility. The Ah receptor concentration increased from 5 DI to 8 DI, the period when chick liver is undergoing early morphological differentiation. After 8 DI, Ah receptor levels dropped substantially and remained low into the posthatching period. In contrast, AHH inducibility was high by 7 DI and remained high throughout embryonic development and into the posthatching period. The discrepancy between Ah receptor levels and the degree of AHH inducibility suggests that only a small fraction of the Ah receptor population is required for maximal AHH induction.

Renal cAMP and 1,25(OH)2D3 synthesis in estrogen-treated chick embryos and hens

Onset of sexual maturity in female chickens or administration of estrogen to mature males or to juveniles of either sex results in increased parathyroid hormone (PTH)-dependent adenylate cyclase activity and increased 25-hydroxyvitamin D3-1-hydroxylase activity in kidney. The relationship between estrogen-mediated alterations of these two enzyme systems was investigated in embryonic and mature, egg-laying chickens treated in vivo with 17 beta-estradiol (E2). Basal and PTH- and forskolin-stimulated adenylate cyclase activity in kidney plasma membrane preparations was not affected by E2 treatment of 19-day-old chick embryos or of 41-wk-old egg-laying females. High, possibly maximal, levels of catalytic activity in control embryos and hens may have precluded further stimulation by E2. In contrast, E2 significantly enhanced 25-hydroxyvitamin D3-1-hydroxylase activity of embryonic kidney up to 10-fold (P less than 0.005). In mature females, E2 caused cessation of egg laying accompanied by a significant reduction (P less than 0.005) of 25-hydroxyvitamin D3-1-hydroxylase activity. These results indicate that the PTH-dependent adenylate cyclase and the 25-hydroxyvitamin D3-1-hydroxylase systems of avian kidney can be regulated independently and suggest that factors in addition to estrogen are involved in their regulation.

Evidence for specific DNA sequences in the nuclear acceptor sites of the avian oviduct progesterone receptor

Recent studies have shown that saturable high-capacity nuclear binding sites (termed acceptor sites) for the avian oviduct progesterone receptor can be reconstituted by rehybridizing a specific oviduct chromatin protein fraction (CP-3) to pure hen DNA to generate a reconstituted nucleoacidic protein (NAP). Only a limited number of acceptor sites can be generated on hen DNA even at high protein/DNA ratios. This suggests the existence of a limited number of specific sequences in the avian genome that can participate in the acceptor sites. The studies presented in this paper show a specificity as to the source of DNA that can generate acceptor sites using hen oviduct CP-3 protein. The acceptor protein binds to all DNAs but generates acceptor sites only on DNAs from certain animals. The acceptor sites for the progesterone receptor, generated with heterologous mammalian DNAs and the avian oviduct CP-3 fraction, show saturation not only in number of acceptor sites generated on the DNAs but also in progesterone receptor binding. Binding to these sites is also receptor dependent. Using oviduct receptors from particular physiological states of the birds wherein the receptors do not bind to nuclear sites in vivo, it was found that the cell-free binding to these heterologous complexes of hen CP-3 protein and DNA from another species, termed heterologous NAP, is similarly absent. Thus, the cell-free binding to the native oviduct NAP and the heterologous NAP markedly resembles the nuclear binding in vivo. Interestingly, synthetic DNAs rich in adenine and thymine, but not those rich in guanine and cytosine, are capable of generating acceptor sites. Species-specific DNA sequences, as well as specific chromatin proteins, therefore, appear to be involved in the nuclear acceptor sites for the avian oviduct progesterone receptor. The DNA sequences appear to be conserved throughout most of the vertebrates but not among nonvertebrates as are the steroid hormones and their receptors. The exact numbers and distributions of these sequences in the avian genome are not known.

Specific modulation of the transcription of cloned avian vitellogenin II gene by estradiol-receptor complex in vitro

Avian vitellogenin-cauliflower mosaic virus hybrid gene is effectively transcribed in vitro in the homologous embryonic liver nuclei system. The transcription of the hybrid gene is modulated by the addition of an estradiol-receptor preparation that has been shown to bind selectively to an upstream region of cloned vitellogenin gene. Stimulation of the transcription of cloned vitellogenin hybrid gene by estradiol receptor is alpha-amanitin sensitive, hormone dependent, and promoter specific. Simian virus 40 and Escherichia coli promoters are not stimulated by the estradiol-receptor complex. The endogenous silent vitellogenin II gene (wild type) present in the nuclei is not turned on by the addition of estradiol-receptor complex. Deletion or inversion of the DNA sequence where the estradiol-receptor complex binds results in the complete suppression of the in vitro stimulation of transcription by estradiol receptor. Correct initiation of the transcription was demonstrated by primer extension studies of the newly synthesized RNA.

Hepatic estrogen receptors and plasma estrogen-binding activity in the Atlantic salmon

Livers of male and female immature Atlantic Salmon (Salmo salar) contain specific high-affinity [3H]estradiol binding sites in cytosol (Kd 2-4 nM, concentration about 0.6 pmol/g liver). Low levels of high-affinity binding are detectable in salt extracts of nuclei of untreated fish, but injections of estradiol result in transient depletion of the cytosol binder and in accumulation of high levels of binding sites in nuclear salt extracts (Kd 5-6 nM; concentration about 6 pmol/g liver). Both the cytosol and nuclear binding sites are temperature sensitive and are optimally assayed by incubation at 2 degrees. Both are specific for estradiol and diethylstilbestrol (DES) and no significant competition by dihydrotestosterone (DHT), progesterone, or hydrocortisone is seen. The triphenylethylene nonsteroidal antiestrogen, 4-hydroxytamoxifen, exhibits an affinity comparable to that of estradiol. The nuclear binding activity sediments with a coefficient of 3.6 S in salt-containing sucrose density gradients, and is stable on storage at -20 degrees for several months. The cytosol binder on the other hand is not stable on sucrose density gradients or on prolonged storage. Salmon plasma contains two [3H]estradiol binding components, one with a relatively high affinity for [3H]estradiol (kd 13 nM) and the other having a much lower affinity but present in high concentrations. The high-affinity plasma binder exhibits distinctive specificity with no affinity for DES or 4-hydroxytamoxifen but some affinity for DHT and progesterone. These properties serve to distinguish the plasma activity from the intrahepatic estrogen binders. The salmon liver estrogen receptor system has many features in common with typical estradiol receptors from other vertebrates. Immature salmon liver appears to be the richest source of hepatic estrogen receptor so far found for any vitellogenic species.

Unequal activation by estrogen of individual Xenopus vitellogenin genes during development

Using a technique of filter hybridization under very stringent conditions to HindIII fragments of complementary DNA cloned in plasmids, we have measured the accumulation in hepatocytes of mRNA specified by each of the four vitellogenin genes (A1, A2, B1, B2) at different stages of development of Xenopus laevis. The ontogenic competence of embryonic liver to respond to the first exposure to estradiol-17 beta, in terms of activation of transcription of this multigene family, is acquired late in metamorphosis at around Nieuwkoop-Faber stage 58. Upon hormonal induction, the four mRNAs accumulate under non-steady-state conditions at different rates and to different extents at all developmental stages in vivo and in cultured adult hepatocytes. A1 and B1 mRNAs appear more rapidly and accumulate to levels that are five- to eightfold those specified by genes A2 and B2, with higher amounts of B1 than A1 mRNA. A threefold higher absolute rate of synthesis of A1 and B1 mRNAs in hepatocyte cultures, relative to the A2-B2 pair, suggests that hormonal regulation of differential accumulation of vitellogenin mRNA occurs at the transcriptional level. At the early developmental stages (up to stage 61) of acquired competence, there appears to be no fixed pattern of expression, but a pattern of unequal activation of individual genes of the Xenopus vitellogenin multigene family is established thereafter and then retained at all developmental stages of tadpoles, froglets, and in both male and female adults.

Temporal order of chromatin structural changes associated with activation of the major chicken vitellogenin gene

The major chicken vitellogenin (VTG II) gene is marked at the chromatin level in hormone-responsive cells by a set of nuclease-hypersensitive sites that exist prior to hormone-mediated expression. When the gene is transcriptionally activated in the liver by treatment with 17 beta-estradiol, three additional hypersensitive sites are induced, which map near the 5' end of the gene. Two of these sites are stable and, moreover, appear to be propagated to daughter cells after hormone is withdrawn. In contrast, the third site (located 0.7 kb upstream from the gene) is observed only during periods of hormone treatment, perhaps due to a transient interaction with hormone receptor complex at this site. Whereas none of these nuclease-hypersensitive sites is observed in erythrocytes, brain, or fibroblasts, a subset is present in differentiated oviduct cells that have estrogen receptors, but do not express VTG II.

Juvenile hormone-dependent vitellogenin synthesis in Locusta migratoria fat body: inducibility related to sex and stage

Juvenile hormone (JH)-dependent vitellogenin (Vg) synthesis in the fat body of Locusta migratoria is normally limited to sexually mature adult females. As a step toward examining the basis of this limitation, we have tested female and male locusts in a series of stages after the third larval molt for inducibility of Vg synthesis by the synthetic JH analog, methoprene. We find that in the fourth and fifth larval instars fat body of both sexes can be induced to produce Vg, but in the adult stage females respond strongly while no more than trace amounts can be induced in males. Quantitative assays show relative responsiveness in the order: adult female greater than fifth instar female greater than fifth instar male much greater than adult male. During the fifth instar of both sexes, maximal vitellogenic response was obtained in midinstar. After the larval-adult ecdysis, female fat body was unresponsive during the first 4 days, then responsiveness increased and by Day 8 after ecdysis fat bodies were fully as competent to produce Vg as at Day 14, the usual maximum of the first vitellogenic cycle due to endogenous JH. Larval and adult female fat bodies implanted into male larvae are competent for Vg synthesis after metamorphosis, so that the differences between adult male and female cannot be imposed by the male milieu intérieur during the larval-adult molt. In male and female precocious adults, produced by treatment of fourth instars with precocene, fat body responded to methoprene as in normal adults. We conclude that factors intrinsic to the fat body cells, determined early in development, are responsible for differential gene programing in males and females, which is partially expressed by the fifth instar but fully manifest only after a molt in the absence of JH.

High affinity binding of anti-oestrogen to the chick liver nuclear oestrogen receptor

Tamoxifen is a potent inhibitor of specific oestrogen-induced yolk protein synthesis by chicken liver. The oestradiol receptor in salt extracts of liver nuclei from oestrogen-treated chicks has a K(d) for oestradiol of 0.7+/-0.2nm. Tamoxifen and its metabolite, monohydroxytamoxifen, compete for binding to the salt-soluble nuclear receptor with K(i) values of 2.6 and 0.1nm respectively. The anti-oestrogens show much less inhibition of [(3)H]oestradiol binding when assays are carried out using intact nuclei. The competition by unlabelled oestradiol for [(3)H]oestradiol binding to receptor is identical in both salt extracts and intact nuclei. This suggests that intact nuclei contain components which bind anti-oestrogens, but not oestradiol. While tamoxifen and desmethyltamoxifen will readily dissociate from the salt-soluble nuclear oestrogen receptor, monohydroxytamoxifen does not dissociate under the conditions generally used for exchange assays. A modified assay was developed in which 60-70% of monohydroxytamoxifen-bound sites were shown to be exchangeable for [(3)H]oestradiol. Soluble receptor preparations were first incubated in a 1.7% charcoal suspension at 37 degrees C for 15min before assay of specific oestradiol binding. This technique was used in examining the effects of tamoxifen and monohydroxytamoxifen given in vivo on the nuclear oestrogen receptor concentration. Despite their 30-fold difference in binding affinity for the receptor, both anti-oestrogens increase nuclear receptor levels to about the same degree. When given with oestradiol, both compounds have the same apparent partial inhibitory effect on the oestrogen-induced increase in nuclear receptor. These data are consistent with the metabolic hydroxylation of tamoxifen before binding to the hepatic oestrogen receptor.

Estrogen receptor and the development of estrogenic responses in embryonic chick liver

We have examined the development of responsiveness to estrogen by embryonic chick liver with a view to determining common and unique factors involved in the establishment of different genomic responses to th e hormone. The major apoproteins of chick VLDL, apo VLDL-B and apo VLDL-II, both appear to be estrogen inducible at an earlier stage of of embryonic development than is vitellogenin. Apo VLDL-B, but not vitellogenin, exhibits a significant level of hepatic synthesis in the absence of estrogen treatment. This basal synthesis in the absence of estrogen treatment. This basal synthesis is tamoxifen-resistant and is detectable at very early stages of hepatic development, well before estrogen responsiveness is seen. Immunological cross-reactivity, electrophoretic behavior and the results of limited proteolysis mapping suggest that the apo VLDL-B synthesized under basal and estrogen-stimulated conditions is the same (or a very similar) protein. Inducibility of the VLDL apoproteins appears to parallel the appearance of the hepatic estrogen receptor system at days 10-12 while vitellogenin induction is delayed by several days. Cytosol receptor concentration undergoes a gradual increase up to the 19th day of development and thereafter declines. The properties of the 19-day receptor are very similar to those of cytosol receptor in hatched chickens, but the fall in concentration does not appear to be proportionately related to inducibility of estrogenic responses, as measured by the relative rates of synthesis in vitro.

Synthesis of apoprotein B of very low density lipoprotein in embryonic-chick liver. Development of responsiveness of oestrogen

The influence of exogenous oestrogen on the rate of synthesis of the apoprotein B of very-low-density lipoprotein by embryonic chick liver has been examined at various stages of development. Significant synthesis of apoprotein B was found in the absence of hormone treatment as early as day 6 of egg incubation. This basal level of apoprotein B synthesis varied from 2.5--6% of total protein synthesis. Embryos at day 10 or earlier treated with oestradiol exhibited no increase in the basal level of apoprotein B synthesis. Embryos at day 11 responded to oestradiol slightly and, from days 12 to 20, oestrogen treatment raised the relative rate of apoprotein B synthesis significantly above basal values: the maximal rate was about 16% of total protein synthesis. The anti-oestrogen tamoxifen citrate abolished the hormone-induced increase in apoprotein B synthesis and was not itself oestrogenic. The basal level of apoprotein B production was not sensitive to tamoxifen citrate, either in relatively early or in later stages of development. The basal level of apoprotein B synthesis, therefore, is oestrogen-independent and under developmental control distinct from the hormone-sensitive synthesis. The ontogeny of oestrogen-responsiveness of apoprotein B production appears to parallel the acquisition of the hepatic oestrogen-receptor system [Lazier (1978) Biochem. J. 174, 143--152].

A high-affinity oestrogen-binding protein in cockerel liver cytosol

In contrast with several earlier reports, cytosol from cockerel liver contains a significant concentration of a protein that binds oestradiol with high affinity. To demonstrate the activity, certain alterations in the conventional method of preparation of cytosol must be made. Homogenization in sucrose-containing buffer at pH 8.4 in the presence of proteinase inhibitors and rapid fractionation of the cytosol with (NH4)2SO4 enables demonstration of a single class of oestradiol-binding sites with a Kd of about 1 nM and specificity only for oestrogens. The concentration is about 300 sites per cell in liver from 2-week-old cockerels. Oestradiol treatment in vivo decreases the number of exchangeable cytosol oestradiol-binding sites by about 80% for 1--4h, after which time it is gradually restored. Gel filtration of the cytosol preparation in the presence of high salt concentrations reveals that most of the oestradiol-binding activity is in high-molecular-weight aggregates, but a mild trypsin treatment generates a specific binding protein with an approximate mol.wt. of 40 000. This protein may be an oestrogen receptor.