Efficacy of pharmacological estrogen receptor antagonists in blocking activation of zebrafish estrogen receptors

The Dicentrarchus labrax estrogen screen test: A relevant tool to screen estrogen-like endocrine disrupting chemicals in the aquatic environment

In response to the need for the diversification of regulatory bioassays to screen estrogen-like endocrine disrupting chemical (EEDC) in the environment, we propose the use of a reporter gene assay involving all nuclear estrogen receptors from Dicentrarchus labrax (i.e., sbEsr1, sbEsr2a, or sbEsr2b). Named DLES test (D. labrax estrogen screen), it aims at complementing existing standardized in vitro tests by implementing more estrogen receptors notably those that do not originate from humans. Positive responses were obtained with all three estrogen receptors, and-consistently with observations from other species-variations in sensitivity to E2 were measured. Sensitivity and EC50 values could be classified as follows: sbEsr2b < sbEsr2a < sbEsr1. The pharmacological characterization with a human estrogen receptor antagonist (fulvestrant) successfully validated the specific involvement of each sbEsr and evidenced the capacity of the DLES test to highlight antagonist interactions. The DLES test was applied to WWTP contaminant extracts. A positive response was detected in the inflow sample in accordance with the YES test, but not in the outflow sample. Notwithstanding, the DLES test (sbEsr2b) exhibited greater sensitivity for the screening of those samples. This study demonstrates the need for more comprehensive testing including representatives of marine species for a better detection of EEDCs. The DLES test appears as a pertinent tool to predict adverse effects and to widen the scope of screening and hazard assessment of EEDCs in the environment.

Modeling Sex-Bias in Anxiety: Pros and Cons of a Larval Zebrafish Model

Anxiety disorders are the most prevalent psychiatric disorders, exhibiting strong female bias. Clinical studies implicate declining estradiol levels in the exacerbation of anxiety symptoms in the premenstrual phase of the menstrual cycle. This study aimed to simulate estradiol fluctuation-linked anxiety behavior in larval zebrafish, using an estradiol treatment withdrawal model. Contrary to model aims, estradiol treatment withdrawal decreased both basal activity and anxiety-like hyperlocomotion (ANOVA main effect of dose, P < 0.0001 and P < 0.01, respectively) in the light/dark transition test. The accuracy of the estradiol washout model was not improved by longer durations of treatment or withdrawal. Basal activity was slightly altered by supraphysiological concentrations of WAY-200070 in the absence of added estradiol. Estrogen receptor (ER) β expression was not upregulated in larvae exposed to physiologically relevant, low concentrations of estradiol. Longer exposure to low concentrations of estradiol increased antioxidant capacity (P < 0.01). In addition, acute exposure to low concentrations of estradiol increased basal activity. Data suggest that in the current models, estradiol-associated altered activity levels were linked to more favorable redox status, rather than reflecting altered anxiety levels. As such, it is recommended that zebrafish larval behavioral analysis be conducted in parallel with mechanistic studies such as redox indicators, for investigations focused on ER signaling.

Neurodegenerative Disease: Roles for Sex, Hormones, and Oxidative Stress

Neurodegenerative diseases cause severe impairments in cognitive and motor function. With an increasing aging population and the onset of these diseases between 50 and 70 years, the consequences are bound to be devastating. While age and longevity are the main risk factors for neurodegenerative diseases, sex is also an important risk factor. The characteristic of sex is multifaceted, encompassing sex chromosome complement, sex hormones (estrogens and androgens), and sex hormone receptors. Sex hormone receptors can induce various signaling cascades, ranging from genomic transcription to intracellular signaling pathways that are dependent on the health of the cell. Oxidative stress, associated with aging, can impact the health of the cell. Sex hormones can be neuroprotective under low oxidative stress conditions but not in high oxidative stress conditions. An understudied sex hormone receptor that can induce activation of oxidative stress signaling is the membrane androgen receptor (mAR). mAR can mediate nicotinamide adenine dinucleotide-phosphate (NADPH) oxidase (NOX)-generated oxidative stress that is associated with several neurodegenerative diseases, such as Alzheimer disease. Further complicating this is that aging can alter sex hormone signaling. Prior to menopause, women experience more estrogens than androgens. During menopause, this sex hormone profile switches in women due to the dramatic ovarian loss of 17β-estradiol with maintained ovarian androgen (testosterone, androstenedione) production. Indeed, aging men have higher estrogens than aging women due to aromatization of androgens to estrogens. Therefore, higher activation of mAR-NOX signaling could occur in menopausal women compared with aged men, mediating the observed sex differences. Understanding of these signaling cascades could provide therapeutic targets for neurodegenerative diseases.

The ERβ-CXCL19/CXCR4-NFκB pathway is critical in mediating the E2-induced inflammation response in the orange-spotted grouper (Epinephelus coioides)

The main physiological function of 17β-estradiol (E2) in vertebrates is to regulate sexual development and reproduction. In fish, especially hermaphroditic fish, estrogen is often used to aid reproduction, but it also can trigger an inflammatory response. However, the molecular mechanism for this E2-induced inflammatory reaction is not clear. In this study, we found that the ERβ-CXCL19/CXCR4-NFκB cascade regulated the E2-induced inflammatory response in the orange-spotted grouper (Epinephelus coioides). Strikingly, E2 treatment resulted in significantly high expression of inflammatory cytokines and induced phosphorylation and degradation of IκBα and translocation of NFκB subunit p65 to the nucleus in grouper spleen cells. However, the E2-induced inflammatory response could be prevented by the broad estrogen receptor (ER) ligand ICI 182,780. Moreover, the luciferase assay showed that E2 induced the inflammatory response by activating the promotor of chemokine CXCL19 through ERβ1 and ERβ2. Knockdown of CXCL19 blocked the E2-induced inflammatory response and NFκB nucleus translocation. Additionally, knockdown of chemokines CXCR4a and CXCR4b together, but not alone, blocked the E2-induced inflammatory response. The immunofluorescence assay and co-immunoprecipitation analysis showed that CXCL19 mediated the E2-induced inflammatory response by activating CXCR4a or CXCR4b. Taken together, these results showed that the ERβ-CXCL19/CXCR4-NFκB pathway mediated the E2-induced inflammatory response in grouper. These findings are valuable for future comparative immunological studies and provide a theoretical basis for mitigating the adverse reactions that occur when using E2 to help fish reproduce.

Effect of bisphenol A on craniofacial cartilage development in zebrafish (Danio rerio) embryos: A morphological study

Bisphenol A (BPA), an endocrine-disrupting chemical, is present in everyday-used consumables and common household products. Although the side effects of BPA have been sufficiently explored, little is known the effects of environmentally relevant low levels of BPA on chondrogenesis in skeletal development. Here we used a morphological approach to investigate whether exposure to BPA (0, 0.0038, 0.05, 0.1, 1.0 μM) could affect craniofacial cartilage development of zebrafish embryo. Furthermore, we sought to determine receptor-mediated BPA induced chondrogenesis toxicity by co-exposing developing embryos to BPA and various inhibitors. Low-dose BPA affected heart rate and induced body and head elongation of larvae. Quantitative morphometric and histopathological analysis revealed that BPA exposure changed the angle and length of craniofacial cartilage elements and disrupted chondrocytes. BPA induced pharyngeal cartilage defects via multiple cellular pathways, including estrogen receptor, androgen receptor, and estrogen-related receptors. Our findings demonstrate that BPA alters the normal development of cartilage and craniofacial structures in zebrafish embryos. Furthermore, in this study we find multiple cellular pathways mediating the effects of BPA-induced craniofacial chondrogenesis toxicity. Further experiments will allow for establishing a connection between BPA and increased risk of congenital malformation of the facial cranium in BPA-exposed populations.

HPLC-electrospray ionization-mass spectrometry optimization by high-performance design of experiments for astrocyte glutamine measurement

The amino acid glutamine (Gln) is a likely source of energy in the brain during neuroglucopenia. Effects of glucose deficiency on astrocyte Gln homeostasis remain unclear, as analytical tools of requisite sensitivity for quantification of intracellular levels of this molecule are not currently available. Here, a primary hypothalamic astrocyte culture model was used in conjunction with design of experiments (DOE)-refined high-performance liquid chromatography-electrospray ionization-mass spectrometry (LC-ESI-MS) methodology to investigate the hypothesis that glucoprivation alters astrocyte Gln content in a sex-specific manner. Critical mass spectrometric parameters for Gln derivative chromatographic response were identified by comparing the performance of central composite design, Box-Behnken design, and Optimal Design (OD)-A, -D, -I, -Distance, and -Modified Distance DOE models. The outcomes showed that the OD-A-generated response was superior relative to other design outcomes. Forecasted surface plot critical mass spectrometric parameters were maximized by OD-A, OD-Distance, and OD-Modified Distance designs. OD-A produced a high-performance method that yielded experimental run and forecasted surface plot maximal responses. Optimized mass spectrometric analysis of male versus female astrocyte Gln content provides novel evidence that glucoprivation significantly depletes this amino acid in female, but not in male, and that this sex-specific response may involve differential sensitivity to estrogen receptor signaling. This technological advance will facilitate efforts to ascertain how distinctive physiological and pathophysiological stimuli impact astrocyte Gln metabolism in each sex.

Sex differences in glucoprivic regulation of glycogen metabolism in hypothalamic primary astrocyte cultures: Role of estrogen receptor signaling

Hypoglycemia causes sex-reliant changes in hypothalamic astrocyte glycogen metabolism in vivo. The role of nuclear versus membrane astrocyte estrogen receptors (ER) in glucoprivic regulation of glycogen is unclear. Here, primary hypothalamic astrocyte cultures were treated with selective ER antagonists during glucoprivation to investigate the hypothesis that ER mediate sex-specific glycogen responses to glucoprivation. Results show that glucoprivic down-regulation of glycogen synthase expression is mediated by transmembrane G protein-coupled ER-1 (GPER) signaling in each sex and estrogen receptor (ER)-beta (ERβ) activity in females. Glucoprivic inhibition of glycogen phosphorylase involves GPER and ERβ in females, but ER-independent mechanisms in males. GPER, ERβ, and ER-alpha (ERα) inhibit or stimulate AMPK protein expression in male versus female astrocytes, respectively. Glucoprivic augmentation of phospho-AMPK profiles in male glia was opposed by GPER activation, whereas GPER and ERβ suppress this protein in females. Astrocyte ERα and GPER content was down-regulated in each sex during glucose deficiency, whereas ERβ levels was unaltered (males) or increased (females). Glucoprivation correspondingly elevated or diminished male versus female astrocyte glycogen content; ER antagonism reversed this response in males, but not females. Results identify distinctive ER variants involved in sex-similar versus sex-specific astrocyte protein responses to withdrawal of this substrate fuel. Notably, glucoprivation elicits a directional switch or gain-of-effect of GPER and ERβ on specific glial protein profiles. Outcomes infer that ERs are crucial for glucoprivic regulation of astrocyte glycogen accumulation in males. Alternatively, estradiol may act independently of ER signaling to disassemble this reserve in females.

Species differences in ligand interaction and activation of estrogen receptors in fish and human

Estrogen receptor (ER) sequences vary between species and this suggests that there are differences in the ligand-specificity, leading to species-specific effects. This would indicate that it is not possible to generalize effects across species. In this study, we investigated the differences in activation potencies and binding affinities of ER´s alpha (α) and beta (β) in human, zebrafish and sea bream to elucidate species differences in response to estradiol, estrone, estriol and methyltestosterone. In vitro analysis showed that estradiol had the highest activity for all the ER´s except for human ERβ and seabream ERβ2. Alignment of the ligand binding domain and ligand binding pocket (LBP) residues of the three species showed that different residues were involved in the LBPs which led to differences in pocket volume, affected binding affinity and orientation of the ligands. By combining in silico and in vitro results, it was possible to identify the ligand specificities of ER´s. The results demonstrated that the human ER´s show lower resolution in ligand-dependent activation, suggesting higher promiscuity, than the zebrafish and seabream ER´s. These results show species-specificity of ER´s and suggest that species-specific differences must be taken into consideration when studying different exposure scenarios.

Interaction of nuclear ERs and GPER in vitellogenesis in zebrafish

Estrogens exert their biological functions through the estrogen receptors (ERs). In zebrafish, three nuclear estrogen receptors (nERs) named ERα, ERβ1 and ERβ2 and one membrane-bound G protein-coupled estrogen receptor (GPER) are identified. Vitellogenin (Vtg) is predominantly expressed in liver and strongly response to the stimulation of estrogen. It has been proposed that all three nERs are functionally involved in vitellogenesis and ERα may act as the major mediator in teleost. However, the role of GPER and its interaction with nERs in this process are not yet defined in teleost species. In the present study, we provide genetic evidence for the functional significance of ERα that the expression of Vtg genes (vtg1, vtg2, vtg3) and their response to estradiol stimulation were significantly decreased in esr1 mutant zebrafish. Activation of ERβ1 and ERβ2 induced Vtg expression through ERα. Moreover, the involvement of GPER in vitellogenesis and its interaction with nERs in zebrafish were firstly proposed in this work. Activation of GPER induced Vtg genes expression while inhibition of GPER significantly attenuated the estrogenic effect on Vtg. Both treatments altered the expression levels of nERs, suggesting GPER acts interactively with nERs. Collectively, the involvement of both nERs and GPER in regulation of vitellogenesis is demonstrated. ERα is the central factor, acting interactively with ERβ1, ERβ2 and GPER, and GPER regulates vitellogenesis directly and interactively with nERs.

Evaluation of the estrogen receptor alpha as a possible target of bifenthrin effects in the estrogenic and dopaminergic signaling pathways in zebrafish embryos

Bifenthrin (BF) is a pyrethroid insecticide widely used in urban and agricultural applications. Previous studies in embryos of zebrafish have shown that BF can affect estradiol biosynthesis and the dopaminergic system. To examine the role of the estrogen receptor (ER) in the endocrine effects of BF, embryos were exposed for 96 h to a mixture of 0.15 and 1.5 μg/L BF and an ER agonist (17α-ethynylestradiol - EE2) at 0.09 μg/L. Transcripts related to estrogenic (vitellogenin VTG) and dopaminergic (tyrosine hydroxylase (TH), dopamine receptor 1 (DR1), monoamine oxidase (MAO), and catechol-O-methyltransferase b (COMTb)) signaling pathways were investigated by qRT-PCR. Dopamine (DA) and its metabolites (homovanillic acid (HVA) and 3,4-dihydroxyphenylacetic acid (DOPAC)) were also measured. There was a significant increase in VTG, DR1, MAO and COMTb mRNA levels and HVA-DA ratios within all zebrafish embryos exposed to EE2, including EE2 alone, 0.15 μg/L BF + EE2 and 1.5 μg/L BF + EE2. A significant decrease in homogenate concentrations of DA was observed within all zebrafish embryos exposed to EE2, which included EE2 alone, 0.15 μg/L BF + EE2 and 1.5 μg/L BF + EE2. Co-exposure of BF with EE2 failed to diminish estrogenic or dopaminergic signaling in embryos. Additionally, embryos with diminished ERα expression by morpholino injection were exposed to 0.15 μg/L BF, 1.5 μg/L BF and 0.09 μg/L EE2, with subsequent gene expression measurements. ERα knockdown did not prevent the effects of BF, indicating ERα may have a limited role in the estrogenic and dopaminergic effects caused by BF in zebrafish embryos.

Molecular docking of genistein on estrogen receptors, promoter region of BCLX, caspase-3, Ki-67, cyclin D1, and telomere activity

Objectives

This study aims to investigate the modulation of estrogen receptors by estrogen and the role of genistein in the transcriptional process that regulates genes involved in the proliferation, apoptosis, and telomere activity.

Methods

The research was conducted in silico, wherein docking, the most important method, was carried out using Hex 8.0 software and HADDOCK web server. Interaction analysis was subsequently done to observe the interactions between genistein and several related proteins and BCLX, Casp3, Ki-67, CyclinD1, hTERT, and POT1 genes using Discovery Studio, LigPlus, and NUCPLOT.

Results

The interaction between ERα with genistein was not found to form a single bond. Thus, the interaction that may occur will not be effective because it is not stable. Conversely, when interacting with ERβ, two hydrogen bonds and four hydrophobic bonds, MPP dihydrochloride interacted with ERα via two hydrogen bonds and three hydrophobic bonds. The ERβ/eNOS complex will be comparatively easier to induced by the transcriptional activation of BCLX, Casp3, Ki-67, CyclinD1, hTERT and POT1 genes.

Conclusions

Administration of genistein can increase the genomic activities of the estrogen-eNOS receptor complexes related to apoptosis, proliferation, and telomere activity.

Estrogen directly stimulates LHb expression at the pituitary level during puberty in female zebrafish

The LHb expression is up-regulated during puberty in female zebrafish. However, the molecular mechanism underlying how LHb expression is regulated during puberty remains largely unknown. In this study, we found that the mRNA expression levels of lhb, fshb and cyp19a1b were up-regulated along with the puberty onset in zebrafish. Among the three nuclear estrogen receptors (nERs), the esr2b is the only type whose expression is significantly up-regulated during puberty onset in the pituitary. However, in situ hybridization results revealed that lhb mRNA was colocalized with esr1 and esr2a but not esr2b. Exposure to estradiol (E₂) significantly stimulates LHb expression in both wild-type and kiss1^-/-;kiss2^-/-;gnrh3^-/- triple knockout pubertal zebrafish. Moreover, exposure of cultured pituitary cells to E₂ increased the LHb expression, indicating that the estrogenic effect on LHb expression could be acted at the pituitary level. Finally, we cloned and analyzed the promoter of lhb by luciferase assay. Our results indicated that the E₂ responsive regions of lhb promoter for ERα and ERβ2 are identical, suggesting that ERα and ERβ2 could bind to the same half ERE region of the promoter of lhb, exhibiting a classical ERE-dependent pathway. In summary, we demonstrate that E₂ could directly act on the pituitary level to stimulate LHb transcription during puberty in zebrafish.

How does oestradiol influence the AVT/IT system in female round gobies during different reproductive phases?

In this in vitro gradient perfusion study, we determined whether there is a functional relationship between oestradiol and the arginine vasotocin/isotocin (AVT/IT) system in the female round goby (Neogobius melanostomus). Brain explants were perfused in medium supplemented with 17β-oestradiol (E₂) at doses mimicking the plasma levels of this hormone in nature during the spawning-capable phase and regressing phase. We aimed to establish which pathway, genomic or non-genomic, is involved in this mechanism in different reproductive phases. For this purpose, brain explants were perfused in medium supplemented with Fulvestrant (ICI 182.780) or Actinomycin D (Act D) separately or in combination with E₂ The contents of AVT and IT in the perfusion media were determined using high-performance liquid chromatography (HPLC) with fluorescence and UV detection. During the spawning-capable phase, the effect of E₂ on AVT release is mediated through oestrogen receptors (ERs) via both genomic and non-genomic pathways, while IT release is mediated through ERs via a genomic pathway only. In the regressing phase, release of both nonapeptides is mediated through ERs via a genomic pathway. This is the first study to present a feasible mechanism of oestradiol action on the AVT/IT system in female fish during different phases of the reproductive cycle.

Cloning, expression and functional characterization on vitellogenesis of estrogen receptors in Scatophagus argus

Estrogen receptors (Er) play a critical role in vitellogenesis. Three ers (erα, erβ1 and erβ2) and vitellogenins (vtg-A, vtg-B and vtg-C) subtypes were isolated in various fish species, while the contribution of each Er to the regulation of vtgs expression was not analyzed in detail. Here, erα, erβ1 and erβ2 were cloned and all were found to be expressed in female liver in Scatophagus argus. During proteic vitellogenesis stage, erα was simultaneously up-regulated, while erβ1 and erβ2 were not, with three vtgs in female liver. The effects of 17β-estradiol (E₂) alone or combined with Er antagonists on ers, vtgs mRNA expressions and Vtg protein content in incubated male liver were examined by real-time PCR and enzyme-linked immunosorbent assay (ELISA), respectively. The expressions of erα, erβ1, vtgs mRNA and Vtg protein increased significantly after 24h incubation with E₂ (0.1, 1 and 10μM), while Er nonselective antagonist ICI 182 780 (0.01, 0.1 and 1μM) significantly attenuated the up-regulation effects of E₂ on ers, vtgs mRNA and Vtg protein in a dose-dependent manner. Erα selective antagonist Methyl-piperidinopyrazole (MPP) (0.01, 0.1 and 1μM) significantly attenuated the up-regulation effects of E₂ on erα, vtg-B, vtg-C mRNA and Vtg protein, while promoted the expression of erβ1 and vtg-A. Erβ selective antagonist Cyclofenil (0.01, 0.1 and 1μM) attenuated the up-regulation effects of E₂ on erβ1, erβ2, vtg-A, vtg-C mRNA and Vtg protein while promoted the expression of erα and vtg-B. Our results suggest that the regulation of Ers on different vtgs was divergent in S. argus.

Zebrafish: A Versatile Animal Model for Fertility Research

The utilization of zebrafish in biomedical research is very common in the research world nowadays. Today, it has emerged as a favored vertebrate organism for the research in science of reproduction. There is a significant growth in amount numbers of scientific literature pertaining to research discoveries in reproductive sciences in zebrafish. It has implied the importance of zebrafish in this particular field of research. In essence, the current available literature has covered from the very specific brain region or neurons of zebrafish, which are responsible for reproductive regulation, until the gonadal level of the animal. The discoveries and findings have proven that this small animal is sharing a very close/similar reproductive system with mammals. More interestingly, the behavioral characteristics and along with the establishment of animal courtship behavior categorization in zebrafish have laid an even stronger foundation and firmer reason on the suitability of zebrafish utilization in research of reproductive sciences. In view of the immense importance of this small animal for the development of reproductive sciences, this review aimed at compiling and describing the proximate close similarity of reproductive regulation on zebrafish and human along with factors contributing to the infertility, showing its versatility and its potential usage for fertility research.

Actions of Bisphenol A and Bisphenol S on the Reproductive Neuroendocrine System During Early Development in Zebrafish

Bisphenol A (BPA) is a well-known environmental, endocrine-disrupting chemical, and bisphenol S (BPS) has been considered a safer alternative for BPA-free products. The present study aims to evaluate the impact of BPA and BPS on the reproductive neuroendocrine system during zebrafish embryonic and larval development and to explore potential mechanisms of action associated with estrogen receptor (ER), thyroid hormone receptor (THR), and enzyme aromatase (AROM) pathways. Environmentally relevant, low levels of BPA exposure during development led to advanced hatching time, increased numbers of GnRH3 neurons in both terminal nerve and hypothalamus, increased expression of reproduction-related genes (kiss1, kiss1r, gnrh3, lhβ, fshβ, and erα), and a marker for synaptic transmission (sv2). Low levels of BPS exposure led to similar effects: increased numbers of hypothalamic GnRH3 neurons and increased expression of kiss1, gnrh3, and erα. Antagonists of ER, THRs, and AROM blocked many of the effects of BPA and BPS on reproduction-related gene expression, providing evidence that those three pathways mediate the actions of BPA and BPS on the reproductive neuroendocrine system. This study demonstrates that alternatives to BPA used in the manufacture of BPA-free products are not necessarily safer. Furthermore, this is the first study to describe the impact of low-level BPA and BPS exposure on the Kiss/Kiss receptor system during development. It is also the first report of multiple cellular pathways (ERα, THRs, and AROM) mediating the effects of BPA and BPS during embryonic development in any species.

Estrogenic and anti-estrogenic influences in cultured brown trout hepatocytes: Focus on the expression of some estrogen and peroxisomal related genes and linked phenotypic anchors

Estrogens, estrogenic mimics and anti-estrogenic compounds are known to target estrogen receptors (ER) that can modulate other nuclear receptor signaling pathways, such as those controlled by the peroxisome proliferator-activated receptor (PPAR), and alter organelle (inc. peroxisome) morphodynamics. By using primary isolated brown trout (Salmo trutta f. fario) hepatocytes after 72 and 96h of exposure we evaluated some effects in selected molecular targets and in peroxisomal morphological features caused by: (1) an ER agonist (ethinylestradiol-EE2) at 1, 10 and 50μM; (2) an ER antagonist (ICI 182,780) at 10 and 50μM; and (3) mixtures of both (Mix I-10μM EE2 and 50μM ICI; Mix II-1μM EE2 and 10μM ICI and Mix III-1μM EE2 and 50μM ICI). The mRNA levels of the estrogenic targets (ERα, ERβ-1 and vitellogenin A-VtgA) and the peroxisome structure/function related genes (catalase, urate oxidase-Uox, 17β-hydroxysteroid dehydrogenase 4-17β-HSD4, peroxin 11α-Pex11α and PPARα) were analyzed by real-time polymerase chain reaction (RT-PCR). Stereology combined with catalase immunofluorescence revealed a significant reduction in peroxisome volume densities at 50μM of EE2 exposure. Concomitantly, at the same concentration, electron microscopy showed smaller peroxisome profiles, exacerbated proliferation of rough endoplasmic reticulum, and a generalized cytoplasmic vacuolization of hepatocytes. Catalase and Uox mRNA levels decreased in all estrogenic stimuli conditions. VtgA and ERα mRNA increased after all EE2 treatments, while ERβ-1 had an inverse pattern. The EE2 action was reversed by ICI 182,780 in a concentration-dependent manner, for VtgA, ERα and Uox. Overall, our data show the great value of primary brown trout hepatocytes to study the effects of estrogenic/anti-estrogenic inputs in peroxisome kinetics and in ER and PPARα signaling, backing the still open hypothesis of crosstalk interactions between these pathways and calling for more mechanistic experiments.

Effects of pharmaceuticals present in aquatic environment on Phase I metabolism in fish

The fate of pharmaceuticals in aquatic environments is an issue of concern. Current evidence indicates that the risks to fish greatly depend on the nature and concentrations of the pharmaceuticals and might be species-specific. Assessment of risks associated with the presence of pharmaceuticals in water is hindered by an incomplete understanding of the metabolism of these pharmaceuticals in aquatic species. In mammals and fish, pharmaceuticals are primarily metabolized by cytochrome P450 enzymes (CYP450). Thus, CYP450 activity is a crucial factor determining the detoxification abilities of organisms. Massive numbers of toxicological studies have investigated the interactions of human pharmaceuticals with detoxification systems in various fish species. In this paper, we review the effects of pharmaceuticals found in aquatic environments on fish hepatic CYP450. Moreover, we discuss the roles of nuclear receptors in cellular regulation and the effects of various groups of chemicals on fish, presented in the recent literature.

Selectivity of natural, synthetic and environmental estrogens for zebrafish estrogen receptors

Zebrafish, Danio rerio, is increasingly used as an animal model to study the effects of pharmaceuticals and environmental estrogens. As most of these estrogens have only been tested on human estrogen receptors (ERs), it is necessary to measure their effects on zebrafish ERs. In humans there are two distinct nuclear ERs (hERα and hERβ), whereas the zebrafish genome encodes three ERs, zfERα and two zfERβs (zfERβ1 and zfERβ2). In this study, we established HeLa-based reporter cell lines stably expressing each of the three zfERs. We first reported that estrogens more efficiently activate the zfERs at 28°C as compared to 37°C, thus reflecting the physiological temperature of zebrafish in wildlife. We then showed significant differences in the ability of agonist and antagonist estrogens to modulate activation of the three zfER isotypes in comparison to hERs. Environmental compounds (bisphenol A, alkylphenols, mycoestrogens) which are hER panagonists and hERβ selective agonists displayed greater potency for zfERα as compared to zfERβs. Among hERα selective synthetic agonists, PPT did not activate zfERα while 16α-LE2 was the most zfERα selective compound. Altogether, these results confirm that all hER ligands control in a similar manner the transcriptional activity of zfERs although significant differences in selectivity were observed among subtypes. The zfER subtype selective ligands that we identified thus represent new valuable tools to dissect the physiological roles of the different zfERs. Finally, our work also points out that care has to be taken in transposing the results obtained using the zebrafish as a model for human physiopathology.

Transgenic zebrafish reveal tissue-specific differences in estrogen signaling in response to environmental water samples

BACKGROUND

Environmental endocrine disruptors (EEDs) are exogenous chemicals that mimic endogenous hormones such as estrogens. Previous studies using a zebrafish transgenic reporter demonstrated that the EEDs bisphenol A and genistein preferentially activate estrogen receptors (ERs) in the larval heart compared with the liver. However, it was not known whether the transgenic zebrafish reporter was sensitive enough to detect estrogens from environmental samples, whether environmental estrogens would exhibit tissue-specific effects similar to those of BPA and genistein, or why some compounds preferentially target receptors in the heart.

METHODS

We tested surface water samples using a transgenic zebrafish reporter with tandem estrogen response elements driving green fluorescent protein expression (5xERE:GFP). Reporter activation was colocalized with tissue-specific expression of ER genes by RNA in situ hybridization.

RESULTS

We observed selective patterns of ER activation in transgenic fish exposed to river water samples from the Mid-Atlantic United States, with several samples preferentially activating receptors in embryonic and larval heart valves. We discovered that tissue specificity in ER activation was due to differences in the expression of ER subtypes. ERα was expressed in developing heart valves but not in the liver, whereas ERβ2 had the opposite profile. Accordingly, subtype-specific ER agonists activated the reporter in either the heart valves or the liver.

CONCLUSION

The use of 5xERE:GFP transgenic zebrafish revealed an unexpected tissue-specific difference in the response to environmentally relevant estrogenic compounds. Exposure to estrogenic EEDs in utero was associated with adverse health effects, with the potentially unanticipated consequence of targeting developing heart valves.

17β-Estradiol induces supernumerary primordial germ cells in embryos of the polychaete Platynereis dumerilii

In the polychaete Platynereis dumerilii exactly four primordial germ cells (PGCs) arise in early development and are subject to a transient mitotic arrest until the animals enter gametogenesis. In order to unravel the mechanisms controlling the number of PGCs in Platynereis, we tested whether the steroid 17β-estradiol (E2) is able to induce PGC proliferation, as it had been described in other species. Our data provide strong support for such a mechanism, showing that E2 significantly increases the occurrence of larvae with supernumerary PGCs in Platynereis in a dose dependent manner. E2 responsiveness is restricted to early developmental stages, when the PGCs are specified. During these stages, embryos exhibit high expression levels of the estradiol receptor (ER). The ER transcript localizes to the yolk-free cytoplasm of unfertilized eggs and segregates into the micromeres during cleavage stages. Nuclear ER protein is found asymmetrically distributed between daughter cells. Neither transcript nor protein is detectable in PGCs at larval stages. Addition of the specific estradiol receptor inhibitor ICI-182,780 (ICI) abolishes the proliferative effect of E2, suggesting that it is mediated by ER signaling. Our study reports for the first time an ER mediated proliferative effect of E2 on PGCs in an invertebrate organism.

Morpholino-mediated knockdown of ERα, ERβa, and ERβb mRNAs in zebrafish (Danio rerio) embryos reveals differential regulation of estrogen-inducible genes

Genetically distinct estrogen receptor (ER) subtypes (ERα and ERβ) play a major role in mediating estrogen actions in vertebrates, but their unique and overlapping functions are not entirely clear. Although mammals have 1 gene of each subtype (ESR1 and ESR2), teleost fish have a single esr1 (ERα) and 2 esr2 (ERβa and ERβb) genes. To determine the in vivo role of different ER isoforms in regulating estrogen-inducible transcription targets, zebrafish (Danio rerio) embryos were microinjected with esr-specific morpholino (MO) oligonucleotides to disrupt splicing of the exon III/intron III junction in the DNA-binding domain. Each MO knocked down its respective normal transcript and increased production of variants with a retained intron III (esr1 MO) or a deleted or mis-spliced exon III (esr2a and esr2b MOs). Both esr1 and esr2b MOs blocked estradiol induction of vitellogenin and ERα mRNAs, predominant hepatic genes, but esr2b was the only MO that blocked induction of cytochrome P450 aromatase B mRNA, a predominant brain gene. Knockdown of ERβa with the esr2a MO had no effect on estrogen induction of the 3 mRNAs but, when coinjected with esr1 MO, attenuated the effect of ERα knockdown. Results indicate that ERα and ERβb, acting separately or cooperatively on specific gene targets, are positive transcriptional regulators of estrogen action, but the role of ERβa, if any, is unclear. We conclude that MO technology in zebrafish embryos is an advantageous approach for investigating the interplay of ER subtypes in a true physiological context.

Metal and pharmaceutical mixtures: is ion loss the mechanism underlying acute toxicity and widespread additive toxicity in zebrafish?

The acute toxicities and mechanisms of action of a variety of environmental contaminants were examined using zebrafish larvae (Danio rerio; 4-8 days post fertilization). Toxic interactions were observed between metals. For example, the addition of a sublethal level of nickel (15% of the LC50, one third of the LC01) to all copper treatments decreased the copper 96 h LC50 by 58%, while sublethal copper exposure (6% of the copper LC50, 13% of the LC01) decreased the cadmium 96 h LC50 by 47%. Two predictive models were assessed, the concentration addition (CA) model, which assumes similar mechanisms of action, and the independent action (IA) model, which assumes different mechanisms of action. Quantitative comparisons indicated the CA model performed better than the IA model; the latter tended to underestimate combined toxicity to a greater extent. The effects of mixtures with nickel or ammonia were typically additive, while mixtures with copper or cadmium were typically greater than additive. Larvae exposed to cadmium, copper or nickel experienced whole body ion loss. Decreases were greatest for Na(+) followed by K(+) (as high as 19% and 9%, respectively, in 24h). Additive toxicity between copper and other pharmaceutical compounds such as fluoxetine (Prozac™), β-naphthoflavone, estrogen and 17α-ethinylestradiol were also observed. Similar to metals, acutely toxic concentrations of fluoxetine, β-naphthoflavone and ammonia all decreased whole body Na(+) and K(+). Overall, whole body Na(+) loss showed the greatest correlation with mortality across a variety of toxicants. We theorize that a disruption of ion homeostasis may be a common mechanism underlying the acute additive toxicity of many contaminants in fish.

Differential regulation of gonadotropin receptors (fshr and lhcgr) by estradiol in the zebrafish ovary involves nuclear estrogen receptors that are likely located on the plasma membrane

FSH and LH are gonadotropins (GTH) that control all major events of gonadal function. FSH and LH signal through their cognate receptors, FSH receptor and LH/choriogonadotropin receptor, respectively, across vertebrates. Compared with the information in mammals, very little is known about these receptors in fish, especially the regulation of their expression. In female zebrafish, fshr and lhcgr exhibit significant temporal difference in expression, with fshr increasing first when the follicles are activated to enter the vitellogenic growth phase and lhcgr lagging behind. This raises an interesting question on the differential regulation of these two GTH receptors (GTHR) during folliculogenesis. Using a primary follicle cell culture, the present study demonstrated that 17β-estradiol (E2), but not testosterone, was a potent endocrine hormone that differentially regulated the expression of fshr and lhcgr. Although E2 stimulated both receptors, its effect on the steady-state level of lhcgr mRNA was much higher (>8-fold up-regulation) than that of fshr (∼0.5-fold increase). E2 likely acted at the transcription level via its nuclear estrogen receptors (ERα and ERβ), because ICI 182,780 could abolish its effects. However, our evidence suggested that these receptors might be localized on the plasma membrane, because β-estradiol 6-(O-carboxy methyl)oxime:BSA could fully mimic the effects of E2. Demonstrating that E2 is likely one of the differentiating factors for the distinct expression of the two GTHR in the zebrafish ovary, this study sheds important light on the functions of the two GTH and their receptors in fish as well as the conservation and diverse aspects of GTHR regulation across vertebrates.

Selective activation of zebrafish estrogen receptor subtypes by chemicals by using stable reporter gene assay developed in a zebrafish liver cell line

The number of environmental chemical contaminants suspected to act as endocrine disruptor compounds by interacting with estrogen receptor (ER) signaling pathway has been continuously increasing. To study such interaction, the use of stable reporter gene assays is relevant, but species-specific in vitro screening assays are still lacking to address hazard assessment of estrogenic chemicals in aquatic vertebrates. Here, we describe the development of stable reporter gene assays based on stable expression of subtypes of zebrafish ER (zfERα, zfERβ1, and zfERβ2) coupled to estrogen response element-driven luciferase in a zebrafish liver (ZFL) cell line. The three established cell models, named ZELH-zfERα, ZELH-zfERβ1, and ZELH-zfERβ2, expressed stable and significant basal luciferase signal, which was induced by 17β-estradiol (E2) in a sensitive and dose-response manner at EC(50)s of 0.2, 0.03, and 0.05 nM, respectively. In addition, E2 significantly altered cell proliferation in ZELH-zfERα and ZELH-zfERβ2 cells, but not in parental ZFL and ZELH-zfERβ1 cells, suggesting a functionality of these two receptors to modulate endogenous gene expression in the transfected clones. The screening of various xenoestrogens from different classes in the three models resulted in different luciferase response patterns. Natural and synthetic estrogens and 1,1,1-trichloro-2-(2 chlorophenyl)-2-(4-chlorophenyl)ethane were active at lower concentrations in ZELH-zfERβ1 and ZELH-zfERβ2 than in ZELH-zfERα cells, whereas genistein and zearalenone metabolites as well as three benzophenone derivatives preferentially activated zfERα. Altogether, the newly established models provide specific and convenient in vitro tool for comparative assessment of zfERs selective activation by chemicals within ZFL cell context.