Organization of a rainbow trout estrogen receptor gene

Intersex occurrence in rainbow trout (Oncorhynchus mykiss) male fry chronically exposed to ethynylestradiol

This study aimed to investigate the male-to-female morphological and physiological transdifferentiation process in rainbow trout (Oncorhynchus mykiss) exposed to exogenous estrogens. The first objective was to elucidate whether trout develop intersex gonads under exposure to low levels of estrogen. To this end, the gonads of an all-male population of fry exposed chronically (from 60 to 136 days post fertilization – dpf) to several doses (from environmentally relevant 0.01 µg/L to supra-environmental levels: 0.1, 1 and 10 µg/L) of the potent synthetic estrogen ethynylestradiol (EE2) were examined histologically. The morphological evaluations were underpinned by the analysis of gonad steroid (testosterone, estradiol and 11-ketotestosterone) levels and of brain and gonad gene expression, including estrogen-responsive genes and genes involved in sex differentiation in (gonads: cyp19a1a, ER isoforms, vtg, dmrt1, sox9a2; sdY; cyp11b; brain: cyp19a1b, ER isoforms). Intersex gonads were observed from the first concentration used (0.01 µg EE2/L) and sexual inversion could be detected from 0.1 µg EE2/L. This was accompanied by a linear decrease in 11-KT levels, whereas no effect on E2 and T levels was observed. Q-PCR results from the gonads showed downregulation of testicular markers (dmrt1, sox9a2; sdY; cyp11b) with increasing EE2 exposure concentrations, and upregulation of the female vtg gene. No evidence was found for a direct involvement of aromatase in the sex conversion process. The results from this study provide evidence that gonads of male trout respond to estrogen exposure by intersex formation and, with increasing concentration, by morphological and physiological conversion to phenotypic ovaries. However, supra-environmental estrogen concentrations are needed to induce these changes.

Structure, tissue distribution and estrogen regulation of splice variants of the sea bream estrogen receptor α gene

Estrogen actions are mainly mediated by specific nuclear estrogen receptors (ERs), for which different genes and a diversity of transcript variants have been identified, mainly in mammals. In this study, we investigated the presence of ER splice variants in the teleost fish gilthead sea bream (Sparus auratus), by comparison with the genomic organization of the related species Takifugu rubripes. Two exon2-deleted ERα transcript variants were isolated from liver cDNA of estradiol-treated fish. The ΔE2 variant lacks ERα exon 2, generating a premature termination codon and a putative C-terminal truncated receptor, while the ΔE2,3* variant contains an in-frame deletion of exon 2 and part of exon 3 and codes for a putative ERα protein variant lacking most of the DNA-binding domain. Both variants were expressed at very low levels in several female and male sea bream tissues, and their expression was highly inducible in liver by estradiol-17β treatment with a strong positive correlation with the typical wild-type (wt) ERα response in this tissue. These findings identify novel estrogen responsive splice variants of fish ERα, and provide the basis for future studies to investigate possible modulation of wt-ER actions by splice variants.

Gene expression profiles in rainbow trout, Onchorynchus mykiss, exposed to a simple chemical mixture

Among proposed uses for microarrays in environmental toxiciology is the identification of key contributors to toxicity within a mixture. However, it remains uncertain whether the transcriptomic profiles resulting from exposure to a mixture have patterns of altered gene expression that contain identifiable contributions from each toxicant component. We exposed isogenic rainbow trout Onchorynchus mykiss, to sublethal levels of ethynylestradiol, 2,2,4,4-tetrabromodiphenyl ether, and chromium VI or to a mixture of all three toxicants Fluorescently labeled complementary DNA (cDNA) were generated and hybridized against a commercially available Salmonid array spotted with 16,000 cDNAs. Data were analyzed using analysis of variance (p<0.05) with a Benjamani-Hochberg multiple test correction (Genespring [Agilent] software package) to identify up and downregulated genes. Gene clustering patterns that can be used as "expression signatures" were determined using hierarchical cluster analysis. The gene ontology terms associated with significantly altered genes were also used to identify functional groups that were associated with toxicant exposure. Cross-ontological analytics approach was used to assign functional annotations to genes with "unknown" function. Our analysis indicates that transcriptomic profiles resulting from the mixture exposure resemble those of the individual contaminant exposures, but are not a simple additive list. However, patterns of altered genes representative of each component of the mixture are clearly discernible, and the functional classes of genes altered represent the individual components of the mixture. These findings indicate that the use of microarrays to identify transcriptomic profiles may aid in the identification of key stressors within a chemical mixture, ultimately improving environmental assessment.

Molecular cloning, gene expression and characterization of the third estrogen receptor of the Nile tilapia, Oreochromis niloticus

Estrogens are essential for many reproductive and non-reproductive functions. In teleosts, it is well-known that several subtypes of estrogen receptors are required for the precise action of estrogens. Present study describes the cloning of the third estrogen receptor, ER- beta2, from the Nile tilapia by EST sequencing coupled microarray. The cloned ER-beta2 showed 77.7% amino acid identity with the reported Atlantic croaker ER-beta. Three ERs, ER-alpha, ER-beta1 and ER-beta2, from the fugu genome were also isolated to analyze their gene structures. Comparison of the intron/exon boundaries and exon numbers of fugu, tilapia, rainbow trout and zebrafish, and phylogenetic analysis of 63 ER sequences revealed that ER-beta probably underwent two successive lineage-specific duplications in teleost. The former took place only in zebrafish lineage, and the latter took place in advanced teleosts without the zebrafish lineage, whereas no duplication of the ER-alpha gene has been detected. Tissue distribution analysis by RT-PCR revealed that tilapia ER-alpha and ER-beta1 were expressed ubiquitously, whereas ER-beta2 is expressed only in the pituitary, liver, intestine, kidney and gonads, with the highest expression in the testis and the lowest level in the ovary. Northern blot analysis detected a single transcript of about 3.4 kb in the testis but not in the ovary mRNAs. In transient transfection assays using human embryonic kidney 293 (HEK293) cells, tilapia ER-beta2 showed estrodiol-17beta dependent transactivation.

Cloning, characterisation, and expression of three oestrogen receptors (ERalpha, ERbeta1 and ERbeta2) in the European sea bass, Dicentrarchus labrax

Three oestrogen receptor [ER] subtypes have been described in teleost fish, namely ERalpha, and two ERbeta subtypes, called ERbeta1 and ERbeta2 (or ERbeta and ERgamma in Atlantic croaker). Their expression during embryonic development and gonadal growth has evoked interest in their potential role in sexual differentiation and gonadal development in fish. We cloned three oestrogen receptors from adult liver (sb-ERalpha cDNA) and ovary (partial sb-ERbeta1 and sb-ERbeta2 cDNAs) of the European sea bass, and according to their phylogenetic relatedness to other ERs in teleosts, named them sea bass [sb-] ERalpha, ERbeta1 and ERbeta2. Deduced amino acid numbers for sb-ERalpha, sb-ERbeta1 and sb-ERbeta2 were 639, 517 and 608, respectively, representing in the case of sb-ERbeta1 and sb-ERbeta2 about 90% of the open reading frame. Highest amino acid identities were found for sb-ERalpha with eelpout ERalpha (88.7%), for sb-ERbeta1 with Atlantic croaker ERgamma (85.8%), and for sb-ERbeta2 with Atlantic croaker ERbeta (90.1%). Southern analysis confirmed that all three sea bass oestrogen receptors (sb-ERs) are the products of three distinct genes. In adult sea bass, ERalpha was predominantly expressed in liver and pituitary, while sb-ERbeta1 and sb-ERbeta2 were more ubiquitously expressed, with highest expression levels in pituitary. In a mixed-sex population of juvenile sea bass, sb-ERalpha expression was significantly elevated in gonads at 200 days posthatch (dph), while for sb-ERbeta1 and sb-ERbeta2 highest expression levels were observed in gonads at 250 dph. For sb-ERbeta2, expression was also significantly higher in the brain at 250 dph. The cloning of these three ER subtypes in the European sea bass together with the results obtained on expression levels in adult and juvenile animals has given us the foundation to investigate their possible role in sexual differentiation and development in this species in future studies.

Genomic structure and identification of a truncated variant message of the mouse estrogen receptor alpha gene

Estrogen receptor alpha (ERalpha) is a ligand-dependent transcription factor that directs the transcription of a wide number of estrogen-regulated genes. ERalpha mediates the effects of 17-beta-estradiol in both males and females, and was the first estrogen receptor identified. Despite the cloning of the mouse ERalpha cDNA over 15 years ago, the precise genomic organization of the mouse ERalpha gene has not yet been elucidated. In order to determine the structure of this gene, overlapping BAC and P1 clones containing partial genomic sequences of the mouse ERalpha cDNA were obtained from a mouse ES cell genomic library. Using standard restriction fragment analysis followed by Southern blotting, the mouse ERalpha gene was determined to be greater than 220 kb in length. The introns vary widely in size, from 1.8 to 60 kb in length. Sequencing of intron-exon boundaries shows that these boundaries are highly conserved between the human and mouse ERalpha genes. Additionally, we have identified a splice variant message of mouse ERalpha arising from a failure to properly splice at the 3' end of exon 4; the resulting message is predicted to produce a protein lacking the ligand-binding domain. Variant message was detected by RT-PCR in several tissues, including uterus, ovary, mammary gland, placenta and testis.

Molecular characterization of three estrogen receptor forms in zebrafish: binding characteristics, transactivation properties, and tissue distributions

There are two estrogen receptor (ER) subtypes in fish, ERalpha and ERbeta, and increasing evidence that the ERbeta subtype has more than one form. However, there is little information on the characteristics and functional significance of these ERs in adults and during development. Here, we report the cloning and characterization of three functional ER forms, zfERalpha, zfERbeta1, and zfERbeta2, in the zebrafish. The percentages of identity between these receptors suggest the existence of three distinct genes. Each cDNA encoded a protein that specifically bound estradiol with a dissociation constant ranging from 0.4 nM (zfERbeta2) to 0.75 nM (zfERalpha and zfERbeta1). In transiently transfected cells, all three forms were able to induce, in a dose-dependent manner, the expression of a reporter gene driven by a consensus estrogen responsive element; zfERbeta2 was slightly more sensitive than zfERalpha and zfERbeta1. Tissue distribution pattern, analyzed by reverse transcription polymerase chain reaction, showed that the three zfER mRNAs largely overlap and are predominantly expressed in brain, pituitary, liver, and gonads. In situ hybridization was performed to study in more detail the distribution of the three zfER mRNAs in the brain of adult females. The zfER mRNAs exhibit distinct but partially overlapping patterns of expression in two neuroendocrine regions, the preoptic area and the mediobasal hypothalamus. The characterization of these zfERs provides a new perspective for understanding the mechanisms underlying estradiol actions in a vertebrate species commonly used for developmental studies.

Mechanisms of estrogen action

Our appreciation of the physiological functions of estrogens and the mechanisms through which estrogens bring about these functions has changed during the past decade. Just as transgenic mice were produced in which estrogen receptors had been inactivated and we thought that we were about to understand the role of estrogen receptors in physiology and pathology, it was found that there was not one but two distinct and functional estrogen receptors, now called ER alpha and ER beta. Transgenic mice in which each of the receptors or both the receptors are inactive have revealed a much broader role for estrogens in the body than was previously thought. This decade also saw the description of a male patient who had no functional ER alpha and whose continued bone growth clearly revealed an important function of estrogen in men. The importance of estrogen in both males and females was also demonstrated in the laboratory in transgenic mice in which the aromatase gene was inactivated. Finally, crystal structures of the estrogen receptors with agonists and antagonists have revealed much about how ligand binding influences receptor conformation and how this conformation influences interaction of the receptor with coactivators or corepressors and hence determines cellular response to ligands.

Cloning of the Atlantic salmon (Salmo salar) estrogen receptor-alpha gene

A cDNA clone encoding most of an Atlantic salmon (Salmo salar) estrogen receptor (ER) was obtained from a liver cDNA library and the remainder of the coding sequence from the gene was isolated from a genomic library. Sequence comparisons showed that the cloned gene represents ER-alpha. Expression of the ER-alpha gene in male and female salmon parr was analysed by RT-PCR. Highest expression was found in brain and liver, with lower levels of ER-alpha mRNA present in all other tissues tested. There was little difference in expression of ER-alpha between male and female.

Two estrogen receptor (ER) isoforms with different estrogen dependencies are generated from the trout ER gene

A characteristic of all estrogen receptors (ER) cloned from fish to date is the lack of the first 37-42 N-terminal amino acids specific to the A domain. Here we report the isolation and characterization from trout ovary of a full-length complementary DNA (cDNA) clone encoding an N-terminal variant form of the rainbow trout ER (rtER). Sequence analysis of open reading frame of this cDNA predicts a 622-amino acid protein. The C-terminal region of this protein, from amino acid position 45 to the end, was very similar to the previously reported rtER (referred to as the short form, or rtER(S)). In contrast, this novel rtER cDNA (referred to as the long form, or rtER(L)) contains an additional in-frame ATG initiator codon that adds 45 residues to the N-terminal region of the protein. This new N-terminal region may represent the A domain of ER found in tetrapod species. The first 227 bp of this new cDNA were similar to the 3'-end intronic sequence of the rtER gene intron 1. These data together with S1 nuclease, primer extension, and RT-PCR experiments demonstrate that the rtER(L) represents a second isoform of rtER that arises from an alternative promoter within the first intron of the gene. Transcripts encoding both rtER forms were expressed in the liver. In vitro translation of the rtER(L) cDNA produced 2 proteins with molecular masses of 71 and 65 kDa, whereas rtER(S) cDNA produced 1 65-kDa protein. Interestingly, Western blot analysis with a specific antibody against the C-terminal region of rtER revealed 2 receptor forms of 65 and 71 kDa in trout liver nuclear extracts, in agreement with the presence of the 2 distinct classes of rtER messenger RNA in this tissue. Functional analysis of both rtER isoforms revealed that although rtER(S) consistently exhibited a basal (estrogen-independent) trans-activation activity that could be further increased in the presence of estrogens, the novel isoform rtER(L) is characterized by a strict estrogen-dependent transcriptional activity. These data suggest that the additional 45 residues at the N-terminal region of rtER(L) clearly modify the hormone-independent trans-activation function of the receptor.

Molecular cloning of an estrogen receptor beta subtype from the goldfish, Carassius auratus

The brain of many teleost fish species, including the goldfish Carassius auratus, expresses exceptionally high levels of cytochrome P450 aromatase (estrogen synthetase). To begin investigating the molecular and cellular targets of estrogen action in goldfish brain, a polymerase chain reaction (PCR) cloning strategy was used to isolate an estrogen receptor (ER) complementary DNA (cDNA). The 2283-bp cDNA isolated from goldfish liver encoded a protein of 568 amino acids (aa) with an estimated molecular weight of 63,539. The goldfish ER had high overall sequence identity when compared to other vertebrate ER sequences: eel (64%), human beta subtype (59%), human alpha subtype (46%), medaka (46%), and rainbow trout (47%). The highest degree of conservation was seen in the DNA-binding (94-100%) and ligand-binding (67-79%) domains. Phylogenetic analysis of the ER gene family indicated that the goldfish and eel ER are most closely related to mammalian ERbeta subtypes, whereas previously identified fish, amphibian, and avian ER forms cluster separately with mammalian ERalpha subtypes. Using the goldfish ER cDNA (here designated gfERbeta), multiple mRNA species (3.1- 8.6 kb) were detected by Northern blot analysis in goldfish liver and ovary but expression was below detection in brain. Using reverse transcription-PCR analysis, gfERbeta mRNA was detected in forebrain, mid/hindbrain, pituitary, retina, liver, ovary, and testis. Further studies are required to determine whether an additional ERalpha subtype is present in the goldfish and whether ERalpha or ERbeta forms have evolutionary precedence in vertebrates.

Synergism between a half-site and an imperfect estrogen-responsive element, and cooperation with COUP-TFI are required for estrogen receptor (ER) to achieve a maximal estrogen-stimulation of rainbow trout ER gene

In all oviparous, liver represents one of the main E2-target tissues where estrogen receptor (ER) constitutes the key mediator of estrogen action. The rainbow trout estrogen receptor (rtER) gene expression is markedly up-regulated by estrogens and the sequences responsible for this autoregulation have been located in a 0.2 kb upstream transcription start site within - 40/- 248 enhancer region. Absence of interference with steroid hormone receptors and tissue-specific factors and a conserved basal transcriptional machinery between yeast and higher eukaryotes, make yeast a simple assay system that will enable determination of important cis-acting regulatory sequences within rtER gene promoter and identification of transcription factors implicated in the regulation of this gene. Deletion analysis allowed to show a synergistic effect between an imperfect estrogen-responsive element (ERE) and a consensus half-ERE to achieve a high hormone-dependent transcriptional activation of the rtER gene promoter in the presence of stably expressed rtER. As in mammalian cells, here we observed a positive regulation of the rtER gene promoter by the chicken ovalbumin upstream promoter-transcription factor I (COUP-TFI) through enhancing autoregulation. Using a point mutation COUP-TFI mutant unable to bind DNA demonstrates that enhancement of rtER gene autoregulation requires the interaction of COUP-TFI to the DNA. Moreover, this enhancement of transcriptional activation by COUP-TFI requires specifically the AF-1 transactivation function of ER and can be observed in the presence of E2 or 4-hydroxytamoxifen but not ICI 164384. Thus, this paper describes the reconstitution of a hormone-responsive transcription unit in yeast in which the regulation of rtER gene promoter could be enhanced by the participation of cis-elements and/or trans-acting factors, such as ER itself or COUP-TF.

Identification of potential sites of cortisol actions on the reproductive axis in rainbow trout

The full length cDNA encoding a rainbow trout glucocorticoid receptor (rtGR) has been obtained from rainbow trout liver and intestine libraries. Northern blot analysis showed that the corresponding messengers are detected in the brain of trout with a size 7.5 kb similar to the size of rtGR mRNA in other target tissues. The distribution of the rtGR mRNA and protein was studied in the forebrain of the trout by means of both in situ hybridization and immunohistochemistry and compared with that of the oestrogen receptor (rtER). The GR and ER mRNAs and proteins were detected with a strong overlapping mainly in the: (a) preoptic region; (b) mediobasal hypothalamus; and (c) anterior pituitary, confirming their implication in the neuroendocrine control of pituitary functions. In both diencephalon and pituitary, the peptidergic phenotype of some neuron or cell categories expressing either type of receptors could be determined by double staining. Furthermore, double staining studies have demonstrated colocalization of the two receptors in the same neurons or pituitary cells. The rtER and rtGR were found to be co-expressed in the dopaminergic neurons inhibiting GTH2 secretion and in pituitary cells of the anterior lobe--notably the gonadotrophs. Given that the promoter of the ER gene contains several potential glucocorticoid-responsive elements (GRE) and that cortisol inhibits the oestradiol-stimulated ER expression in the liver, the possibility exists for modulation of ER gene expression by GR in the hypothalamo-pituitary complex. This could explain some of the well documented effects of stress on the reproductive performance in salmonids.

Antiestrogenic activity of anthropogenic and natural chemicals

A number of natural and man-made chemicals possess antiestrogenic activity, i.e. they antagonize a broad spectrum of estrogen-induced responses in vertebrates. Examples of antiestrogens include dioxin, furan and PCB congeners, certain PAHs, pesticides and indol-3-carbinol derivatives. Major mechanisms of anti-estrogenicity are antagonistic action of chemicals at the estrogen receptor, or binding of chemicals to the arylhydrocarbon (Ah) receptor and subsequent interaction with estrogen-responsive genes. Toxicological consequences resulting from antiestrogenic activity have not been conclusively demonstrated to date, although antiestrogenic compounds could critically affect sensitive reproductive and developmental processes.

Human estrogen receptor beta-gene structure, chromosomal localization, and expression pattern

The estrogen receptor (ER) is a ligand-activated transcription factor that mediates the effects of the steroid hormone 17 beta-estradiol, in both males and females. Since the isolation and cloning of ER, the consensus has been that only one such receptor exists. The finding of a second subtype of ER (ER beta) has caused considerable excitement amongst endocrinologists. In this article, we present data regarding the genomic structure and chromosomal localization of the human ER beta gene, demonstrating that two independent ER genes do exist in the human. Furthermore, we present data regarding the tissue distribution of human ER beta, showing that this receptor is expressed in multiple tissues. For instance, ER beta is found in developing spermatids of the testis, a finding of potential relevance for the ongoing debate on the effects of environmental estrogens on sperm counts. In addition, we find ER beta in ovarian granulosa cells, indicating that estrogens also participate in the regulation of follicular growth in the human.

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.

The nuclear orphan receptors COUP-TF and ARP-1 positively regulate the trout estrogen receptor gene through enhancing autoregulation

The rainbow trout estrogen receptor (rtER) is a positively autoregulated gene in liver cells. In a previous report, we showed that upregulation is mediated by an estrogen response element (ERE) located in the proximal promoter of the gene and that a half binding site for nuclear receptors (5'-TGACCT-3') located 15 bp upstream of the ERE is involved in the magnitude of the estrogen response. We now report that the human orphan receptor COUP-TF and a COUP-TF-like protein from trout liver are able to bind to the consensus half-site. When cotransfected with the rtER gene proximal promoter, COUP-TF had no regulatory functions on its own. Interestingly, COUP-TF enhanced rtER transactivation properties in the presence of estradiol in a dose-dependent manner when cotransfected with the rtER gene promoter. Unliganded retinoid receptor heterodimers had the same helper function as COUP-TF in the presence of estradiol but were switched to repressors when the ligand all-trans-retinoic acid was added. Mutation of the consensus half-site only slightly reduced COUP-TF helper function, suggesting that it actually results from a complex mechanism that probably involves both DNA binding of COUP-TF to the promoter and protein-protein interaction with another transcription factor bound to the promoter. Nevertheless, a DNA-binding-defective mutant of COUP-TF was also defective in ER helper function. Competition footprinting analysis suggested that COUP-TF actually establishes contacts with the consensus upstream half-site and the downstream ERE half-site that would form a DR-24-like response element. Interaction of COUP-TF with the DR-24 element was confirmed in footprinting assays by using nuclear extracts from Saccharomyces cerevisiae expressing COUP-TF. Finally, interaction of COUP-TF with mutants of the rtER gene promoter showed that COUP-TF recognizes the ERE when the upstream half-site is mutated. These data show that COUP-TF may activate transcription through interaction with other nuclear receptors. This cross-talk between liganded nuclear receptors and orphan receptors is likely to modulate the spectrum of action of a particular ligand-receptor complex and may participate in the cell-type specificity of the ligand effect.

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.

Transcription regulatory signals in the 5' and 3' regions of Oreochromis aureus ER gene

The Oreochromis aureus estrogen receptor (OaER) 5' region is 62.2% AT-rich and does not display well-positioned consensus TATA and CAAT boxes. A functional imperfect 13 bp ERE, TGTTAtggTGACC, deviating at 2 bases is located in the leader exon. Transient transfection assays indicate that this ERE confers a 5-fold increase in SEAP reporter gene activity at 20 h post E2-induction. The continued elevation of transcription after the initial peak could be responsible for the "memory' effect of E2-induced vitellogenesis. The 4.4 kb OaER 3' region is AT-rich and has a high representation of 1/2 EREs and GREs. There are 10 copies of the destabilizing pentamer ATTTA which, in transient transfection experiments strongly suppressed SEAP activity. The 3' EREs are functional, and on their own, induce a bimodal increase in SEAP activity of 9-fold at 6 h and 11-fold at 18 h post-E2 induction. A recombinant construct of SEAP gene flanked by the 5' upstream and 3' flanking regions of the OaER gene allowed these regulatory signals to "cross-talk' to achieve a 5- and 25-fold increase for the first and second peaks, respectively. This bimodal response to E2-induction is attributed to both transcriptional and translational controls over the SEAP gene. This work represents a novel illustration of a synergistic interaction between the 5' and 3' regulatory elements of a steroid receptor in autoregulation.

Molecular cloning and characterization of Japanese eel estrogen receptor cDNA

A cDNA encoding the Japanese eel, Anguilla japonica, estrogen receptor (ER) was isolated and sequenced. This cDNA contains a complete open reading frame encoding 573 amino acid residues, and the molecular weight of this protein is calculated to be 63417. The amino acid sequence of the eel ER shows high homology of the DNA binding (80%) and ligand binding (55%) domains with those of other species. However, the other domains show greatly reduced homology (10-20%). When the cDNA was ligated to the pSVL vector and transfected into COS7 cells, a protein was produced that had high affinity for estradiol-17 beta (E2) and specifically bound estrogens. Northern analysis showed that three ER mRNAs with lengths of 5.6, 3.8 and 1.2 kb were expressed in eel liver. Their expression was E2 inducible, with the 5.6 kb mRNA being strongly dependent on E2 stimulation.

Characterization of the transcription start point of the trout estrogen receptor-encoding gene: evidence for alternative splicing in the 5' untranslated region

The estrogen receptor (ER)-encoding gene (ER) regulates many genes implicated in the reproductive functions. Moreover, rainbow trout ER (rtER) is itself up-regulated by its own product. We have used Northern blot, RNase protection, primer extension and reverse transcription-polymerase chain reaction (RT-PCR) to study the position of the rtER mRNA transcription start point (tsp) in liver. This analysis has revealed the presence of a tsp positioned at the beginning of the cloned rtER cDNA. Functionality of this tsp was tested in transient transfections in CHO-K1 cells. The characterization of the rtER 5' untranslated region (UTR) showed that two transcripts exist in liver which differ in their 5'-UTR. The first one is 100% homologous to the cloned rtER cDNA sequence. The other one contains a 41-bp insertion. The isolation and sequencing of the first intron showed that this insertion arises from alternative splicing, due to the use of a splicing site internal to the first intron.

Molecular cloning and sequencing of the hormone-binding domain of Oreochromis aureus estrogen receptor gene

The nucleotide sequence of the estrogen receptor gene of Oreochromis aureus (OaER) indicates that the hormone-binding E domain is composed of 4 exons interspersed by short introns of only 0.18-1.3 kb each. All 4 E exons exhibit consensus sequences flanking the donor and acceptor splice sites. Analysis of introns revealed (i) numerous palindromic and half-palindromic steroid responsive elements including ERE, TRE and GRE, (ii) six alternative polyadenylation signals and (iii) putative control regions identified by the clustering of transcription factor binding sites. Of particular interest is the presence of a TATA and CAAT box in intron IV. The hydropathicity profile shows that the E exons are relatively hydrophobic. Two receptor dimerization regions have been observed: a conserved heptad repeat of hydrophobic residues (R168-M193) and a perfect leucine zipper (L36-L57). The presence of multiple sites for kinase activity in these regions suggests the importance of phosphorylation in the regulation of receptor functions and ligand-affinity.

The hormone-binding domain of Oreochromis aureus estrogen receptor gene: homology comparison with other steroid binding receptors

Although the estrogen receptor gene of Oreochromis aureus (OaER) shows 85% homology to rainbow trout ER (rtER), the molecular organization of its exons and introns in the hormone-binding E domain is more closely related to the human ER gene. Comparison with other vertebrates yielded reduced homologies of 64-67%, probably due to evolutionary speciation. The E1 and E2 exons of OaER are interspersed by a short intron of 1.3 kb which is flanked by consensus splice sites. This is in sharp contrast to the 11 kb intron separating E1 and E2 exons of rtER which also displayed a rare GC donor junction. Three conserved cys at 83, 112 and 195, which are important for formation of 3-D ligand-binding pocket were found in OaER. However, the 4th conserved cys is replaced by a ser. This substitution which is the result of a single base mutation probably suggests different affinity for estrogen or transactivation of the OaER gene. Two overlapping steroid binding and receptor dimerization domains have been observed. The E domain of OaER and rtER has diversified significantly from that of other non-piscine vertebrates, such that they form a separate subgroup in the UPGMA tree of steroid receptors.