Induction of estrogen receptor and reversal of the nuclear/cytoplasmic receptor ratio during vitellogenin synthesis and withdrawal in Xenopus laevis

Estrogen signaling encourages blastocyst development and implantation potential

PURPOSE

Estrogen is well-known for preparing uterine receptivity. However, its roles in regulating embryo development and implantation are unclear. Our objective was to characterize estrogen receptor 1 (ESR1) in human and mouse embryos and determine the effect of estradiol (E2) supplementation on pre- and peri-implantation blastocyst development.

METHODS

Mouse embryos, 8-cell through hatched blastocyst stages, and human embryonic days 5-7 blastocysts were stained for ESR1 and imaged using confocal microscopy. We then treated 8-cell mouse embryos with 8 nM E2 during in vitro culture (IVC) and examined embryo morphokinetics, blastocyst development, and cell allocation into the inner cell mass (ICM) and trophectoderm (TE). Finally, we disrupted ESR1, using ICI 182,780, and evaluated peri-implantation development.

RESULTS

ESR1 exhibits nuclear localization in early blastocysts followed by aggregation, predominantly in the TE of hatching and hatched blastocysts, in human and mouse embryos. During IVC, most E2 was absorbed by the mineral oil, and no effect on embryo development was found. When IVC was performed without an oil overlay, embryos treated with E2 exhibited increased blastocyst development and ICM:TE ratio. Additionally, embryos treated with ICI 182,780 had significantly decreased trophoblast outgrowth during extended embryo culture.

CONCLUSION

Similar ESR1 localization in mouse and human blastocysts suggests a conserved role in blastocyst development. These mechanisms may be underappreciated due to the use of mineral oil during conventional IVC. This work provides important context for how estrogenic toxicants may impact reproductive health and offers an avenue to further optimize human-assisted reproductive technology (ART) to treat infertility.

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.

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.

Analysis of ligand dependence and hormone response element synergy in transcription by estrogen receptor

In this work we examined two questions: (1) Is the low, but readily detectable, ability of estrogen receptor (ER) to activate transcription in the absence of added 17beta-estradiol caused by traces of estrogen in the growth medium, or by a weak ligand-independent ability of ER to activate transcription? (2) Does the ER exhibit synergistic activation of transcription on reporter genes containing multiple estrogen response elements (EREs)? To study these questions we developed a powerful new reporter gene, containing four EREs, which achieves inductions of up to 330-fold in the presence of liganded ER. We provided several types of evidence indicating that under standard cell culture conditions unliganded ER is unable to activate transcription. We demonstrated that when cells are grown in serum-free medium, estrogenic compounds may be in the base tissue culture medium. We demonstrated a strong cell and ER-dependence in transcriptional synergy, and suggest that cooperative binding of ER to multiple EREs can be responsible for transcriptional synergy in vivo.

Up-regulation of estrogen receptor mRNA and estrogen receptor activity by estradiol in liver of rainbow trout and other teleostean fish

Injection of estradiol (E2) into immature rainbow trout resulted in the induction of the hepatic vitellogenin gene mediated by the nuclear estrogen receptor (ER). Liver ER mRNA rose markedly on E2 treatment in three groups of trout kept at different temperatures. Only in the group kept at 4 degrees C did the total cellular ER, as measured by [3H]estradiol-binding activity in nuclear and cytosol fractions, parallel the ER mRNA level. In fish kept at 9 degrees C and 15 degrees C, the ratio of total ER activity to ER mRNA fell during chronic E2 treatment, probably reflecting translational of post-translational control mechanisms. Upregulation of ER mRNA also occurred in sea raven, sculpin, winter flounder, and Atlantic salmon after E2 treatment. Intrahepatic ER activity rose proportionately in Atlantic salmon kept at 6-9 degrees C but not in sea raven, sculpin, or flounder. We conclude that the regulation of ER expression in teleosts is complex and includes transcriptional, translational, and post-translational elements and is influenced by environmental temperature.

Regulation of Xenopus laevis estrogen receptor gene expression is mediated by an estrogen response element in the protein coding region

To investigate the 17 beta-estradiol induction of the mRNA coding for the Xenopus laevis estrogen receptor (XER), we cloned the promoter and the 5'-flanking region of the ER gene. Transcription initiation sites were identified by primer extension, and confirmed by the polymerase chain reaction. The promoter and 5'-flanking region contain an imperfect TATA box and a potential CAAT box at -51. Sequence analysis and transfections indicated that no functional estrogen response element (ERE) was present in approximately 3 kb of 5'-flanking region. An imperfect ERE, GGTCAGTTTGACG, which differs from the consensus ERE sequence by 1 nucleotide, was detected in the protein coding region of the gene, approximately 480 nucleotides downstream of the transcription initiation site. In transient transfections using a simple promoter containing two copies of this Xenopus estrogen receptor ERE (XERE), we observed an estrogen-dependent increase in CAT activity of four- to five-fold, to a level approximately 20-fold greater than the activity of the control plasmid lacking the XEREs. In competition gel mobility-shift assays, the XERE exhibited a weak, but clearly detectable, ability to compete for binding of human ER to a labeled consensus ERE. Because it exhibits sequence-specific binding to the ER in competition gel mobility-shift assays, and is able to confer estrogen-dependent transcription on a simple synthetic promoter, the novel XERE, located in the protein coding region of the XER gene appears to represent a weak, but functional, ERE.

Comparison of sex steroid hormone-dependent induction of chick oviduct delta-aminolaevulinic acid dehydratase during primary and secondary stimulation

1. A comparative study on primary and secondary stimulation of oviduct delta-aminolaevulinic acid dehydratase (ALAD) (EC 4.2.1.24) was carried out with oestradiol-17 beta and/or testosterone administration in immature female chickens during 15-day-primary stimulation, 20-day-withdrawal and 15-day-secondary stimulation periods. 2. Compared with primary stimulation in oestrogenized birds, synthesis and degradation rates of oviduct ALAD molecule during secondary stimulation increased 3.4- and 1.8-fold respectively, resulting in a rapid induction of the enzyme. 3. Specific activity of oviduct ALAD in oestradiol-plus-testosterone treated birds became significantly higher than that of oestradiol alone during secondary stimulation, whereas no significant changes were observed during primary stimulation.

Human estrogen receptor introduced into the Xenopus oocyte represses expression from an artificial frog estrogen response element

Although the estrogen responsiveness and estrogen receptors of Xenopus hepatocytes have been well described, oocytes of this species have not previously been shown to contain estrogen receptors (ER). Recombinant human ER (HER) was expressed in oocytes in a dose dependent fashion as measured by [35S]methionine incorporation into newly synthesized proteins. Chloramphenicol acetyl transferase (CAT) reporter plasmids, driven by a herpes simplex thymidine kinase promotor with or without a 17 base pair estrogen response element (ERE) from the vitellogenin A2 gene, were also injected into oocytes. When injected without the accompanying HER sequences, the construct containing the ERE expressed 10-fold more CAT activity, and this response was saturable as demonstrated by injecting increasing amounts of reporter plasmid. These results suggest either the activity of small amounts of a Xenopus ER (measured here by LH-20 assay), or the presence of some endogenous oocyte protein other than the ER that can interact with this ERE. When HER was co-expressed with ERECAT, CAT expression was suppressed over a wide range of HER concentrations. This unexpected repression may be due to displacement of an estrogen receptor or other endogenous oocyte regulatory protein on the ERE. HER's positive regulatory activity may require transcription factors that are lacking or insufficient in the oocyte. Alternatively the simple 17 base pair ERE may not provide DNA binding sites for such transcription factors.

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)

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.

High-affinity binding of [3H]estradiol-17 beta by an estrogen receptor in the liver of the turtle

Specific [3H]estradiol-17 beta ([3H]E2) binding activity (EBA) with characteristics of an estrogen receptor (ER) was demonstrated in cytosols and nuclear extracts of the female turtle, Chrysemys picta. Three different receptor assays (dextran-coated charcoal assay, hydroxylapatite batch procedure, and DNA-cellulose chromatography) were evaluated in terms of their applicability in analyzing large numbers of samples. For the measurement of cytosolic EBA, the hydroxylapatite batch procedure was found to be the most reliable assay. On the other hand, the dextran-coated charcoal assay was found to be the most appropriate method for the measurement of nuclear EBA. Turtle hepatic EBA binds [3H]E2 with high affinity (cytosolic, 17.4 +/- 2.8 X 10(9) M-1; nuclear, 17.7 +/- 1.9 X 10(9) M-1), limited capacity (cytosolic, 133.7 +/- 4.6 fmol/g tissue; nuclear, 81.1 +/- 9.0 fmol/g tissue), and strict steroid specificity. The EBA bound natural estrogens (E2, estrone, estriol) as well as the nonsteroidal estrogen, diethylstilbestrol, but exhibited little affinity for androgens, progesterone, or corticosterone. The turtle hepatic EBA resembled mammalian and avian ERs in terms of binding characteristics; however, unlike mammalian and avian ERs it was shown to be heat-labile. Incubation at 30 degrees caused rapid loss of [3H]E2 binding activity in both cytosolic and nuclear fractions. The exchange between [3H]E2 and the endogenously bound estrogen was slow at 4 and 15 degrees, but the exchange process was facilitated in the presence of the chaotropic salt, NaSCN. Establishment of quantitation methods for both cytosolic and nuclear forms of EBA will enable future investigation of the mechanism and regulation of estrogen action in the liver of this turtle species.

Transcriptional and post-transcriptional inhibition of albumin gene expression by estrogen in Xenopus liver

The purpose of this study was to investigate the suppression of albumin mRNA by estrogen in Xenopus liver. A single dose of estradiol rapidly suppressed albumin mRNA to 30% of the control level. Albumin mRNA remained at this new steady-state level for 9 days, after which it returned to the control level. Transcription 'run-on' experiments in isolated liver nuclei demonstrated a transient decrease of 60-90% in albumin transcription after 2-6 h and approached constitutive transcription by 12 h. Albumin gene transcription then remained constant for the following 12 days. Prolonged and enhanced suppression of albumin mRNA was observed in animals treated repeatedly with estrogen for 12 days. In these animals, albumin gene transcription was decreased 80-90% from the constitutive control level. These data indicate that albumin mRNA is suppressed by both transcriptional and post-transcriptional mechanisms.

Estrogen-dependent DNA synthesis and parenchymal cell proliferation in the liver of adult male Xenopus frogs

Estradiol-17 beta treatment of adult male Xenopus laevis induces liver parenchymal cells to synthesize DNA and proliferate. DNA synthesis begins 3 to 4 days after estrogen treatment and continues for approximately 10 days. Over this 2-week period, the total number of liver parenchymal cells increases fourfold, the wet weight of the liver remains constant, and there is a 50% reduction in cell volume. The elevated number of cells persists for several months and then returns to the control value. The extent of proliferation is hormone dose dependent. Pulse-chase experiments demonstrate that as a result of hormone treatment a minority of the parenchymal cells in the initial population enter the cell cycle, and via repeated divisions become the majority (79%) of the population by Day 14. The implications of this phenomenon for estrogen-induced liver cell differentiation and vitellogenin gene function are discussed.

Estrogen induction of biotin-binding protein in immature chicks: kinetics, hormonal specificity and modulation

Employing a specific radioimmunoassay for quantification, the kinetics of estrogen-induced elevation in the plasma concentration of biotin-binding protein (BBP) in immature male chicks was investigated. A single injection of the steroid hormone enhanced the plasma BBP content several-fold at 6 h, reaching peak levels around 48 h and declining thereafter. A 2-fold amplification of the response was evident during secondary stimulation with the hormone. The magnitude of the response was hormonal dose-dependent while the initial lag phase and the time of peak protein accumulation were unaltered within the hormonal doses tested. The circulatory half-life of the specific protein in normal and estrogenized birds was 10 h. Hyperthyroidism markedly decreased the hormonal response while the opposite effect was seen during hypothyroidism. The antiestrogens E- and Z-clomiphene citrate effectively blocked the protein induction whereas progesterone, either alone or in combination with estrogen, was ineffective in modulating the induction. Cycloheximide administration drastically inhibited the inductive response. The above observations clearly suggest that the genes corresponding to the two isofunctional proteins of chicken egg, viz. BBP and avidin, are differentially regulated.

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.

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

We have used primary cell cultures of hepatocytes from male or female Xenopus laevis to study the mechanisms by which estrogen induces vitellogenin gene transcription and how primary exposure to estrogen renders cells more responsive to secondary stimulation. We have characterized the estrogen receptor in hormonally naïve cells and in hepatocytes treated with estrogen under a variety of conditions. Under all conditions the receptor has a Kd congruent to 4 X 10(-10) M. Hormonally naïve male cells contain 300 binding sites whereas female cells or male cells previously exposed to estradiol exhibit 6-7-fold higher levels. In parallel cultures, the absolute rate of vitellogenin gene transcription was determined by hybridization of newly synthesized RNA pulse-labelled with [3H]uridine to cloned Xenopus vitellogenin cDNA. Naïve male cells on primary stimulation with estradiol synthesized vitellogenin mRNA at an average rate of approximately 150 moles/cell/h compared to 1200 moles/cell/h for cells previously exposed to estrogen, thus bearing a close correlation with receptor number. Furthermore, we show that the kinetics of the induced up-regulation of receptor exactly parallel those of the increase in the rate of vitellogenin gene transcription upon secondary hormonal stimulation following various periods of primary exposure to estrogen. Addition of cycloheximide to cell cultures during primary estrogen treatment abolishes both receptor up-regulation and increased rate of vitellogenin gene transcription on secondary stimulation. In addition, primary treatment with the antiestrogen tamoxifen prevents both receptor up-regulation and an enhanced rate of transcription or accumulation of vitellogenin mRNA on secondary hormonal exposure. These results demonstrate that estrogen treatment of male Xenopus hepatocytes results in the rapid up-regulation of its own receptor to female levels via new receptor synthesis, and that receptor number is rate-limiting in vitellogenin gene transcription.

Enhanced hormonal responsiveness in mammary glands from parous mice: molecular mechanisms

Mammary glands from parous mice required lower concentrations of hormones in vitro than those from virgins to effect differentiation, as measured by lactose synthetase activity. This phenomenon could not be explained by changes in receptor levels, since both mammary gland insulin and prolactin binding, although elevated at midpregnancy, returned to baseline in tissue from parous mice. Ethidium bromide, an intercalating dye, was a potent inhibitor of lactose synthetase induction in explants from virgins but much less potent in tissue from pregnant mice; explants from parous animals displayed intermediate sensitivity, suggesting that DNA structure was permanently altered. However, casein synthesis in glands from parous mice required hormone concentrations as high as in virgins and are just as susceptible to ethidium bromide as in virgins. Similarly, the vulnerability of the casein gene to DNAase I digestion is low in mammary epithelial cells from virgins, high in cells from pregnant mice, and low again in cells from parous animals. These data suggest that during the first pregnancy of mice, there are changes in the chromatin configuration that may facilitate the transcription of milk-related mRNA. Furthermore, after mammary gland involution these changes in the casein gene undergo reversion, while those involved with lactose synthetase activity persist; this may explain the disparate hormonal responsiveness seen in these animals with respect to casein and lactose synthesis.

Loss of differentiative potential of the mammary gland in ovariectomized mice: altered estrogen responsiveness in parous mice

Previously, estrogen had been shown to be essential for the maintenance of differentiative potential in the mammary gland of virgin mice; the purpose of this study was to evaluate the role of these steroids in nonpregnant, nonlactating parous mice. To this end, parous mice were either ovariectomized or adrenalectomized and ovariectomized for 4-6 weeks before hormonally induced differentiation was attempted in culture. Although mammary glands from virgin mice lose their differentiative potential in vitro following ovariectomy, tissue from castrated parous mice exhibit only a partial loss of activity. Since combined adrenalectomy and ovariectomy in parous animals virtually abolished differentiation and since exogenous estradiol preserved it, it would appear that mammary gland differentiation in parous mice, as in virgins, requires estrogens, but that tissue from the former is more sensitive. This difference between virgins and parous mice is not related to differences in either the capacity or affinity of the cytosol estrogen receptor, as these parameters are identical in all experimental groups.

Estrogen stabilizes vitellogenin mRNA against cytoplasmic degradation

We have demonstrated, by DNA excess filter hybridizations to pulse-labeled cell RNA, that estrogen selectively stabilizes Xenopus liver vitellogenin mRNA against cytoplasmic degradation. The half-life of vitellogenin mRNA is approximately 3 weeks in the presence of estrogen and 16 hr after estrogen is withdrawn from the culture medium. Total poly(A) mRNA exhibits the same half-life (16 hr) in the presence or absence of estrogen. The rapid cytoplasmic degradation of vitellogenin mRNA in the absence of estrogen is fully reversible upon restimulation with estrogen, indicating that nuclear modification of vitellogenin RNA transcripts is not responsible for their stability. Intermediate levels of vitellogenin mRNA stability and changes in the relative rate of vitellogenin gene transcription are not observed late in estrogen induction, when vitellogenin mRNA levels plateau. Instead, Xenopus liver cells achieve fine control over the level of vitellogenin mRNA through down-regulation of the overall rate of total nuclear RNA synthesis.

Possible significance of new target tissues for 1,25-dihydroxyvitamin D3

A sensitive sucrose gradient procedure provided evidence for specific 3.6S 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] receptor-like binding components with low affinity for 25(OH)D3 in several reproductive target tissues in the rat, including testis, uterus and probably epididymis. Thus, the presence of 1,25(OH)2D3 receptors in non-vitamin D endocrine tissues is not restricted to the hormone secreting glands. Estrogen-stimulated uterine growth paralleled increased levels of the putative 1,25(OH)2D3 receptors, providing in vivo evidence for the concept of a relationship between these phenomena. However, a similar correlation was not observed in the growing testis (prepubertal vs mature rats). Whether this result stems from receptor localization in a cell type prevalent in the adult or a true dissociation between testis growth and receptor levels is unclear. Finally, significant levels (10.3 +/- 1.0% vs intestinal mucosa) of the putative 1,25(OH)2D3 receptors were found in the heart, an organ in which calcium mediates many specialized functions. Taken collectively, these observations on new target tissues of quite different overall function suggest that 1,25(OH)2D3 and its receptors may play a role in intracellular calcium homeostasis and possibly in regulating specialized intracellular functions of calcium.

Rapid estrogen metabolism and vitellogenin gene expression in Xenopus hepatocyte cultures

Male hepatocytes metabolized estradiol-17 beta, 17 alpha-ethinylestradiol and mestranol extremely rapidly (t 1/2 = 40, 60 and 300 min, respectively), whereas these were more stable in cultures of female hepatocytes (t 1/2 = 120, 150 and 640 min, respectively). Vitellogenin mRNA accumulated for only 12 h after a single addition of 10(-6) M estradiol to male hepatocyte cultures; mestranol, but not 17 alpha-ethinylestradiol or diethylstilbestrol, was more potent than the natural hormone. The level and rate of accumulation of vitellogenin mRNA were 5-15 times higher in female than in male hepatocytes, mestranol and estradiol being more potent than 17 alpha-ethinylestradiol and diethylstilbestrol. Ovariectomy, 60 days prior to cell culture, did not alter the metabolism of estradiol or the vitellogenic response of female hepatocytes. On the other hand, a single administration of estradiol in vivo to male Xenopus caused a long-lasting shift (at least 16 weeks) to the female pattern of its metabolism, although the enhanced inducibility of vitellogenin genes was partially reversed between 4 and 16 weeks after hormonal treatment. The addition of fresh estradiol every 4 h to male hepatocyte cultures to compensate for its rapid metabolism resulted in a continuous and sustained accumulation of vitellogenin mRNA at rates comparable to those attained in vivo. Our findings explain the requirement for high levels of estrogen to activate vitellogenin genes and establish Xenopus hepatocyte cultures as a reproducible system for analysing the expression of this multigene family.

Coordinate and non-coordinate estrogen-induced expression of A and B groups of vitellogenin genes in male and female Xenopus Hepatocytes in culture

The concentration of mRNA transcribed from A and B groups of vitellogenin genes, induced by estrogen added to primary cultures of hepatocytes from male and female adult Xenopus, was measured by a technique of filter disc hybridization with cloned Xenopus vitellogenin cDNA probes. In cells from naive male Xenopus (i.e. not previously exposed to hormone in vivo or in vitro), the two groups of the multigene family were non-coordinately expressed during the early stages of response to estradiol in vitro. Only B group transcripts could be detected for the first 2-3 h. At later times, or upon successive additions of estradiol to the male cell cultures, both A and B group mRNAs accumulated at the same rate and to the same extent. In female hepatocytes both groups of mRNAs accumulated in parallel at all stages of their response to estrogen, reaching levels 10-fold higher than in naive male cells similarly exposed to the hormone. However, hepatocytes from male Xenopus that had received a single injection of estradiol 5 weeks before the cells were prepared, now exhibited identical rates and extent of accumulation of A and B groups of vitellogenin mRNAs to those observed in female cells. Pulse-labeling of RNA in cultured male Xenopus hepatocytes confirmed that the coordinate and non-coordinate accumulation of mRNAs were largely a function of differential or equal transcription of the A and B groups of vitellogenin genes. A phenomenon analogous to the hormone-induced shift from non-coordinate to coordinate expression of two groups of genes of the same multigene family has not been described previously, and we discuss possible mechanisms underlying the transition.

The role of estrogen receptor in Xenopus laevis vitellogenin gene expression

Administration of estradiol 17 beta [estra-1,3,5(10)-triene-3,17-beta-diol] to male Xenopus laevis induces the massive synthesis by the liver of the egg yolk precursor phospholipoglycoprotein, vitellogenin, and its cognate mRNAs. Restimulation of male X. laevis that have been previously induced to synthesize vitellogenin mRNA but are inactive in vitellogenin mRNA synthesis at the time of restimulation with estrogen results in more rapid accumulation of vitellogenin mRNA and more efficient transcription of the vitellogenin genes than occurs following primary estrogen stimulation. The estrogen receptor system that mediates estrogen action in this organism exhibits several unusual properties. The cytoplasm of unstimulated liver cells contains high levels of a middle-affinity estrogen-specific binding protein and little if any estrogen receptor. The properties of the estrogen binding protein are consistent with a role in protecting estradiol 17 beta against metabolism, as a fraction of cytoplasmic estradiol 17 beta is not subject to rapid metabolism. In addition, similar binding activities are found in all Xenopus tissues surveyed that respond to steroid hormones. The induction of nuclear estrogen receptor is coincident with the onset of vitellogenin mRNA accumulation. However, an increased level of estrogen receptor is not responsible for the elevated rate of vitellogenin gene transcription observed following restimulation with estrogen.