Antiestrogens inhibit xenoestrogen-induced brain aromatase activity but do not prevent xenoestrogen-induced feminization in Japanese medaka (Oryzias latipes)

Reproductive toxicity of DDT in the Japanese medaka fish model: Revisiting the impacts of DDT+ on female reproductive health

The organochlorine pesticide dichlorodiphenyltrichloroethane (DDT) is an endocrine-disrupting compound (EDC) that has been banned by most countries for decades. However, it continues to be detected in nearly all humans and wildlife due to its biological and environmental persistence. The ovarian dysgenesis syndrome hypothesis speculates that exposure to EDCs during sensitive developmental windows such as early gonadal differentiation lead to reproductive disorders later in life. Yet, mechanisms by which DDT affects developing gonads remain unclear due to the inherent challenge of getting developmental exposure data from adults presenting with reproductive disease. The Japanese medaka (Oryzias latipes) is a valuable fish model for sex-specific toxicological studies due to its chromosomal sex determination, external embryonic development, short generation time, and extensively mapped genome. It is well documented that medaka exposed to DDT and its metabolites and byproducts (herein referred to as DDT+) at different developmental time points experience permanent alterations in gonadal morphology, reproductive success, and molecular and hormonal signaling. However, the overwhelming majority of studies focus primarily on functional and morphological outcomes in males and females and have rarely investigated long-term transcriptional or molecular effects. This review summarizes previous experimental findings and the state of our knowledge concerning toxic effects DDT + on reproductive development, fertility, and health in the valuable medaka model. It also identifies gaps in knowledge, emphasizing a need for more focus on molecular mechanisms of ovarian endocrine disruption using enhanced molecular tools that have become increasingly available over the past few decades. Furthermore, DDT forms a myriad of over 45 metabolites and transformation products in biota and the environment, very few of which have been evaluated for environmental abundance or health effects. This reinforces the demand for high throughput and economical in vivo models for predictive toxicology screening, and the Japanese medaka is uniquely positioned to meet this need.

A systematic review of the evaluation of endocrine-disrupting chemicals in the Japanese medaka (Oryzias latipes) fish

Japanese medaka (Oryzias latipes) is an acceptable small laboratory fish model for the evaluation and assessment of endocrine-disrupting chemicals (EDCs) found in the environment. In this research, we used this fish as a potential tool for the identification of EDCs that have a significant impact on human health. We conducted an electronic search in PubMed (http://www.ncbi.nlm.nih.gov/pubmed) and Google Scholar (https://scholar.google.com/) using the search terms, Japanese medaka, Oryzias latipes, and endocrine disruptions, and sorted 205 articles consisting of 128 chemicals that showed potential effects on estrogen-androgen-thyroid-steroidogenesis (EATS) pathways of Japanese medaka. From these chemicals, 14 compounds, namely, 17β-estradiol (E2), ethinylestradiol (EE2), tamoxifen (TAM), 11-ketotestosterone (11-KT), 17β-trenbolone (TRB), flutamide (FLU), vinclozolin (VIN), triiodothyronine (T3), perfluorooctanoic acid (PFOA), tetrabromobisphenol A (TBBPA), terephthalic acid (TPA), trifloxystrobin (TRF), ketoconazole (KTC), and prochloraz (PCZ), were selected as references and used for the identification of apical endpoints within the EATS modalities. Among these endpoints, during classification, priorities are given to sex reversal (masculinization of females and feminization of males), gonad histology (testis-ova or ovotestis), secondary sex characteristics (anal fin papillae of males), plasma and liver vitellogenin (VTG) contents in males, swim bladder inflation during larval development, hepatic vitellogenin (vtg) and choriogenin (chg) genes in the liver of males, and several genes, including estrogen-androgen-thyroid receptors in the hypothalamus-pituitary-gonad/thyroid axis (HPG/T). After reviewing 205 articles, we identified 108 (52.68%), 46 (22.43%), 19 (9.26%), 22 (17.18%), and 26 (12.68%) papers that represented studies on estrogen endocrine disruptors (EEDs), androgen endocrine disruptors (AEDs), thyroid endocrine disruptors (TEDs), and/or steroidogenesis modulators (MOS), respectively. Most importantly, among 128 EDCs, 32 (25%), 22 (17.18%), 15 (11.8%), and 14 (10.93%) chemicals were classified as EEDs, AEDs, TEDs, and MOS, respectively. We also identified 43 (33.59%) chemicals as high-priority candidates for tier 2 tests, and 13 chemicals (10.15%) show enough potential to be considered EDCs without any further tier-based studies. Although our literature search was unable to identify the EATS targets of 45 chemicals (35%) studied in 60 (29.26%) of the 205 articles, our approach has sufficient potential to further move the laboratory-based research data on Japanese medaka for applications in regulatory risk assessments in humans.

Aquatic ecotoxicology of anticancer drugs: A systematic review

Anticancer drugs in the aquatic environment have drawn a lot of attention in the last decade. Since wastewater treatment plants are inefficient at fully eliminating trace concentrations of anticancer drugs, these compounds are continuously discharged into the aquatic environment. Subsequently, non-target organisms such as the aquatic biota are directly exposed to a variety of anticancer drugs. To understand the potential impact on the aquatic organisms, a systematic review was conducted in compliance with the PRISMA guidelines. The results acquired from the 152 included studies were analysed and sorted into four categories: the impact of each included anticancer drug, the effect of metabolites, the effect of a mixture of drugs, and risk assessment. Findings showed that risk to the aquatic biota was unlikely to occur as the concentrations needed to induce effects were much higher than those detected in the environment. However, these data were based on acute toxicity and included only basic toxicity endpoints. The concentrations that produced significant effects were much lower when tested in the long-term or in multi-generational studies. Heterogeneity in results was also observed; this depended on the organism tested, the assessment adopted, and the endpoints selected. In this systematic review, an overall view of the research studies was generated by which all the variability factors to be considered were reported and recommendations to guide future studies were proposed.

Global expression response of genes in sex-undifferentiated Nile tilapia gonads after exposure to trace letrozole

The aromatase inhibitor letrozole can be found in rivers, effluents, and even drinking water. Studies have demonstrated that letrozole affects various metabolic pathways and may cause reproductive toxicity, especially in fish exposed during development. However, studies on the effect of a low concentration of letrozole at the whole-gonad transcriptomic level in the early stage of fish sexual development have not been investigated. The aim of our study was to explore the potential effects of a low concentration of letrozole on the gonad transcriptome of Nile tilapia at an early stage of sexual development. In this study, 9 dpf (days postfertilization) Nile tilapia were exposed to trace letrozole for 12 days. Letrozole exposure from 9 dpf to 21 dpf persistently altered phenotypic sex development and induced the male-biased sex ratio. The transcriptome results showed that 1173 differentially expressed genes (DEGs) were present in the female control vs 1.5 μg/L letrozole-treated female comparison group and that 1576 DEGs were present in the 1.5 μg/L letrozole-treated female vs male control comparison group. Differentially expressed gene enrichment analysis revealed several crucial pathways, including the drug metabolism-cytochrome P450 pathway, the ErbB-PI3K/Akt/mTOR pathway, and the calcium signalling pathway. Further analysis of these identified DEGs indicated that some key genes correlated with metabolism and epigenetic regulation were significantly affected by letrozole, such as UDP-glucuronosyltransferase (Ugt), glutathione S-transferase omega-1 (Gsto1), lysine-specific demethylase 6bb (Kdm6bb, original name is Kdm6a), jumonji and AT-rich interaction domain containing 2 (Jarid2b, original name is Jarid2), growth arrest and DNA damage inducible gamma (Gadd45g), and chromobox protein 7 (Cbx7). The qRT-PCR validation results for twelve DEGs showed that the Pearson's correlation of the log₁₀^{fold change} values between the qPCR and RNA-Seq results was 0.90, indicating the accuracy and reliability of the RNA-Seq results. Our study is the first to report the effect of letrozole on the transcriptome of gonads from fish during early-stage sexual development. These findings will be useful for understanding the toxic effects and molecular mechanisms of letrozole exposure at the early stage of gonad development on the sexual development of aquatic organisms.

Preparation and Characterization of Aptamers Against O,p'-DDT

The compound 1,1,1-trichloro-2-(p-chlorophenyl)-2-(o-chlorophenyl) ethane (o,p’-DDT) has been identified as one of the endocrine-disrupting chemicals causing adverse effects on wildlife and even humans through bioaccumulation. Its detection has become increasingly important. We have obtained candidate aptamers binding to o,p’-DDT by a systematic evolution of ligands by exponential enrichment (SELEX) protocol. Five out of seventeen candidate sequences were selected for preliminary characterization by SYBR Green I assay. One sequence with highest fluorescence response with o,p’-DDT, designated DDT_13, was chosen for further characterization. Its dissociation constant (K_d) was determined to be 412.3 ± 124.6 nM. DDT_13 exhibited low cross-binding activities on other tested small molecules. The good bioactivities of DDT_13 were demonstrated for the analysis of spiked lake water and tap water samples. This study provides a novel o,p’-DDT-specific probe for its future applications.

The Effects of Sertraline on Fathead Minnow (Pimephales promelas) Growth and Steroidogenesis

The purpose of this study was to evaluate the steroidogenic effects of sertraline, a popular selective serotonin reuptake inhibitor, on larval fathead minnow (FHM; Pimephales promelas) and adult FHM. Larvae were exposed to 0.1, 1, and 10 µg/L sertraline for 28 days and analyzed for differential mRNA expression of 11β-hydroxysteroid dehydrogenase (11β-HSD), 20β-hydroxysteroid dehydrogenase (20β-HSD), aromatase (CYP19a), nuclear thyroid receptor alpha (TRα), and normalized to RP-L8. Adult FHM were exposed to 3 or 10 µg/L sertraline for 7 days and analyzed for differential expression of the same genes with the addition of thyroid receptor beta (TRβ). Larval FHM exposed to 0.1 μg/L had a significant upregulation of both 20β-HSD and TRα while adult FHM exposed to 10 µg/L had a significant upregulation of 11β-HSD expression in brain tissue. The significance of these findings with respect to survival, growth and reproduction are currently unknown, but represent areas for future research.

An evaluation of the endocrine disruptive potential of crude oil water accommodated fractions and crude oil contaminated surface water to freshwater organisms using in vitro and in vivo approaches

Knowledge regarding the potential impacts of crude oil on endocrine signaling in freshwater aquatic vertebrates is limited. The expression of selected genes as biomarkers for altered endocrine signaling was studied in African clawed frog, Xenopus laevis, tadpoles and juvenile Mozambique tilapia, Oreochromis mossambicus, exposed to weathered bunker and unweathered refinery crude oil water accommodated fractions (WAFs). In addition, the expression of the aforementioned genes was quantified in X. laevis tadpoles exposed to surface water collected from the proximity of an underground oil bunker. The (anti)estrogenicity and (anti)androgenicity of crude oil, crude oil WAFs, and surface water were furthermore evaluated using recombinant yeast. Thyroid hormone receptor beta expression was significantly down-regulated in X. laevis in response to both oil WAF types, whereas a further thyroid linked gene, type 2 deiodinase, was up-regulated in O. mossambicus exposed to a high concentration of bunker oil WAF. In addition, both WAFs altered the expression of the adipogenesis-linked peroxisome proliferator-activated receptor gamma in X. laevis. The crude oil and WAFs exhibited antiestrogenic and antiandrogenic activity in vitro. However, O. mossambicus androgen receptor 2 was the only gene, representing the reproductive system, significantly affected by WAF exposure. Estrogenicity, antiestrogenicity, and antiandrogenicity were detected in surface water samples; however, no significant changes were observed in the expression of any of the genes evaluated in X. laevis exposed to surface water. The responses varied among the 2 model organisms used, as well as among the 2 types of crude oil. Nonetheless, the data provide evidence that crude oil pollution may lead to adverse health effects in freshwater fish and amphibians as a result of altered endocrine signaling. Environ Toxicol Chem 2017;36:1330-1342. © 2016 SETAC.

Effects of Tributyltin and Other Retinoid Receptor Agonists in Reproductive-Related Endpoints in the Zebrafish (Danio rerio)

Both field and experimental data examined the influence of exposure to environmental contaminant tributyltin (TBT) on marine organisms. Although most attention focused on the imposex phenomenon in gastropods, adverse effects were also observed in other taxonomic groups. It has been shown that imposex induction involves modulation of retinoid signaling in gastropods. Whether TBT influences similar pathways in fish is yet to be addressed. In this study, larvae of the model teleost Danio rerio were exposed to natural retinoids, all-trans-retinoic acid, 9-cis-retinoic acid, and all-trans-retinol, as well as to the RXR synthetic pan-agonist methoprene acid (MA) and to TBT. Larvae were exposed to TBT from 5 days post fertilization (dpf) to adulthood, and reproductive capacity was assessed and correlated with mode of action. TBT significantly decreased fecundity at environmentally relevant levels at 1 μg TBT Sn/g in diet. Interestingly, in contrast to previous reports, TBT altered zebrafish sex ratio toward females, whereas MA exposure biased sex toward males. Since fecundity was significantly altered in the TBT-exposed group with up to 62% decrease, the potentially affected pathways were investigated. Significant downregulation was observed in brain mRNA levels of aromatase b (CYP19a1b) in females and peroxisome proliferator activated receptor gamma (PPARg) in both males and females, suggesting an involvement of these pathways in reproductive impairment associated with TBT.

Complex effects of two presumably antagonistic endocrine disrupting compounds on the goldfish Carassius aumtus: a comprehensive study with multiple toxicological endpoints

We studied the effects of endocrine disrupting compounds nonylphenol (NP) and letrozole (LE) on the male goldfish Carassius aumtus. Exposure to NP (20 μg l(-1)) alone caused a significant up-regulation in the expression of aromatase, estrogen receptors and vitellogenin (VTG) genes, an increase in hepatic and plasma VTG concentration, but no obvious testicular impairment. Exposure to LE (1 mg kg(-1)) alone resulted in a significant decline in aromatase activity, reduced levels of plasma 17β-estradiol (E2), and enhanced sperm maturation. Co-exposure with LE (1 mg kg(-1)) could only partially affect some of the estrogenic effects caused by NP (20 μg l(-1)) (i.e. expression of hepatic and brain estrogen receptor genes, hepatic VTG concentration), but inhibit other estrogenic effects (i.e. brain and testicular aromatase activity, plasma E2). In addition, co-exposure resulted in impairment of liver mitochondria (i.e. detachment of ridges from the membrane, and uneven distribution of the cytoplasm with clusters of glycogen granules), but did not cause significant damage to the testes (i.e. the morphology, the spermatogonia and spermatozoa densities). Our results clearly showed that nonylphenol and letrozole co-exposure could induce profound effects on fish, and highlighted the importance of adopting multiple toxicological endpoints when evaluating the combined effects of endocrine disrupting compounds.

Developmental toxicity of endocrine disruptors in early life stages of zebrafish, a genetic and embryogenesis study

Endocrine disrupting compounds (EDCs) are capable of interfering with the endocrine system and are increasingly widespread in the aquatic environments. In the present study, zebrafish (Danio rerio) embryos and larvae were used to assess how EDCs may interfere with embryogenesis. Therefore, zebrafish embryos were exposed to 17α-ethinylestradiol (EE2: 0.4, 2, 4 and 20 ng/L), genistein (Gen: 2, 20, 200 and 2000 ng/L) and fadrozole (Fad: 2, 10, 50 and 250 μg/L), between 2 and 144 h post-fertilization (hpf). Somite development, heartbeat, malformations, mortality and hatching rates were evaluated. In parallel, the expression patterns of hormone receptors (esr1, esr2a, esr2b and ar) and apoptotic pathways related genes (p53 and c-jun) were determined using quantitative real-time PCR. Results showed that EE2, Gen and Fad caused a higher mortality and also malformations in larvae compared with control. A significant toxic effect was observed in the heartbeat rate, at 144 hpf, in larvae exposed to EE2 and Fad. QPCR revealed alterations in the expression levels of all the evaluated genes, at different time points. esr1 and c-jun genes were upregulated by EE2 and Gen exposure while the expression of esr2a, esr2b and ar genes was downregulated. Fad exposure decreased esr1, p53 and c-jun expression levels. This study shows a toxic effect of EE2, Gen and Fad to vertebrate embryogenesis and a relation between hormones action and apoptosis pathways.

Modulation of aromatase activity as a mode of action for endocrine disrupting chemicals in a marine fish

The steroidogenic enzyme aromatase catalyzes the conversion of androgens to estrogens and therefore plays a central role in reproduction. In contrast to most vertebrates, teleost fish have two distinct forms of aromatase. Because brain aromatase activity in fish is up to 1000 times that in mammals, fish may be especially susceptible to negative effects from environmental endocrine-disrupting chemicals (EDCs) that impact aromatase activity. In this study, the effects of estradiol (E2), ethynylestradiol (EE2), octylphenol (OP), and androstatrienedione (ATD) on reproduction and aromatase activity in brains and gonads from the marine fish cunner (Tautogolabrus adspersus) was investigated. The purpose of the study was to explore the relationship between changes in aromatase activity and reproductive output in a marine fish, as well as compare aromatase activity to two commonly used indicators of EDC exposure, plasma vitellogenin (VTG) and gonadosomatic index (GSI). Results with E2, EE2, and ATD indicate that aromatase activity in cunner brain and ovary are affected differently by exposure to these EDCs. In the case of E2 and EE2, male brain aromatase activity was signficantly increased by these treatments, female brain aromatase activity was unaffected, and ovarian aromatase activity was significantly decreased. Treatment with the aromatase inhibitor ATD resulted in significantly decreased aromatase activity in male and female brain, but had no significant impact on ovarian aromatase activity. Regardless of test chemical, a decrease or an increase in male brain aromatase activity relative to controls was associated with decreased egg production in cunner and was also correlated with significant changes in GSI in both sexes. E2 and EE2 significantly elevated plasma VTG in males and females, while ATD had no significant effect. Treatment of cunner with OP had no significant effect on any measured endpoint. Overall, results with these exposures indicate EDCs that impact aromatase activity also affect reproductive output in spawning cunner.

Sex-different effects of tributyltin on brain aromatase, estrogen receptor and retinoid X receptor gene expression in rockfish (Sebastiscus marmoratus)

Since the brain plays important roles in reproduction, the brain aromatase (Cyp19b), estrogen receptor (ER), retinoid X receptor (RXR) α and peroxisome proliferator-activated receptor γ were examined in rockfish after TBT exposure (1, 10, and 100 ng L(-1)). The results showed that the Cyp19b expression was elevated in the male rockfish, while no effect was produced in the females. Inconsistently, serum testosterone and 17β-estradiol showed no change in the males, while an increase of testosterone and a decrease of 17β-estradiol were observed in the females. TBT affected the ER expression in the males depending on the concentrations, however, no change was observed in the females. In addition, TBT elevated the RXRα expression in the males but produced an opposite effect in the females. In conclusion, TBT might have had sex-different effects on the brain Cyp19b, ER and RXR expression in rockfish, indicating a complex endocrine disrupting effect of TBT.

Endocrine-disrupting chemicals: associated disorders and mechanisms of action

The incidence and/or prevalence of health problems associated with endocrine-disruption have increased. Many chemicals have endocrine-disrupting properties, including bisphenol A, some organochlorines, polybrominated flame retardants, perfluorinated substances, alkylphenols, phthalates, pesticides, polycyclic aromatic hydrocarbons, alkylphenols, solvents, and some household products including some cleaning products, air fresheners, hair dyes, cosmetics, and sunscreens. Even some metals were shown to have endocrine-disrupting properties. Many observations suggesting that endocrine disruptors do contribute to cancer, diabetes, obesity, the metabolic syndrome, and infertility are listed in this paper. An overview is presented of mechanisms contributing to endocrine disruption. Endocrine disruptors can act through classical nuclear receptors, but also through estrogen-related receptors, membrane-bound estrogen-receptors, and interaction with targets in the cytosol resulting in activation of the Src/Ras/Erk pathway or modulation of nitric oxide. In addition, changes in metabolism of endogenous hormones, cross-talk between genomic and nongenomic pathways, cross talk with estrogen receptors after binding on other receptors, interference with feedback regulation and neuroendocrine cells, changes in DNA methylation or histone modifications, and genomic instability by interference with the spindle figure can play a role. Also it was found that effects of receptor activation can differ in function of the ligand.

Neuropeptides and enzymes are targets for the action of endocrine disrupting chemicals in the vertebrate brain

Endocrine-disrupting chemicals (EDC) are molecules that interfere with endocrine signaling pathways and produce adverse consequences on animal and human physiology, such as infertility or behavioral alterations. Some EDC act through binding to androgen or/and estrogen receptors primarily operating through a genomic mechanism regulating gene expression. This mechanism of action may induce profound developmental adverse effects, and the major targets of the EDC action are the gene products, i.e., mRNAs inducing the synthesis of various peptidic molecules, which include neuropeptides and enzymes related to neurotransmitters syntheses. Available immunohistochemical data on some of the systems that are affected by EDC in lower and higher vertebrates are detailed in this review.

Neuroendocrine effects of endocrine disruptors in teleost fish

Because a large proportion of potential endocrine disruptors (EDC) end up in surface waters, aquatic species are particularly vulnerable to their potential adverse effects. Recent studies identified a number of brain targets for EDC commonly present in environmentally relevant concentrations in surface waters. Among those neuronal systems disrupted by EDC are the gonadotropin-releasing hormone (GnRH) neurons, the dopaminergic and serotoninergic circuits, and more recently the Kiss/GPR54 system, which regulates gonadotropin release. However, one of the most striking effects of EDC, notably estrogen mimics, is their impact on the cyp19a1b gene that encodes the brain aromatase isoform in fish. Moreover, this is the only example in which the molecular basis of endocrine disruption is fully understood. The aims of this review were to (1) synthesize the most recent discoveries concerning the EDC effects upon neuroendocrine systems of fish and (2) provide, when possible, the underlying molecular basis of disruption for each system concerned. The potential adverse effects of EDC on neurogenesis, puberty, and brain sexualization are also described. It is important to point out the future environmental, social, and economical issues arising from endocrine disruption studies in the context of risk assessment.

Sensitivity of New Zealand mudsnail Potamopyrgus antipodarum (Gray) to a specific aromatase inhibitor

The freshwater prosobranch Potamopyrgus antipodarum (Molluska, Hydrobiidea, Smith 1889) has been proposed as a suitable species to assess the impact of endocrine disrupting compounds (EDC) in aquatic ecosystems. Steroid hormone biosynthesis pathway is potentially an important target for EDC, and vertebrate-like sex steroids seem to play a functional role in the control of mollusk reproduction. To assess the response and the sensitivity of P. antipodarum to disrupters of the steroid hormone biosynthesis pathway, we have experienced the action of a specific vertebrate aromatase inhibitor, fadrozole, acting on 17beta-estradiol synthesis in two separate 28 and 42d exposures. Fadrozole had effects consistent with the expected mechanism of action. A decrease of the reproduction parameters (such as on the number of neonates and number of embryos in the brood pouch) in a dose-dependant manner was observed. The steroids levels were also impaired with the ratio 17beta-estradiol/testosterone decreased by half in exposed snails. This shift of the steroids balance was accompanied by some alteration in the gonads histology and immunohistochemistry in fadrozole-exposed snails. This study highlights the value role of P. antipodarum as a test species for assessing EDC effects in aquatic wildlife.

Lack of a heritable reproductive defect in the offspring of male rainbow trout exposed to the environmental estrogen 17alpha-ethynylestradiol

Endocrine disruptors, including environmental estrogens, have been shown to induce heritable effects through both genetic and epigenetic mechanisms in mammals. Despite this information and the wealth of knowledge regarding the significant reproductive impacts endocrine disruptors impose on fishes, no studies have reported whether the observed effects are heritable. Without this information it is difficult to establish the long-term consequences for exposed populations. To determine potential consequences of long-term effects we must consider the possibility that induced reproductive defects in fishes may be heritable. Using rainbow trout (Oncorhynchus mykiss) as a model this study aims to determine whether a specific reproductive defect observed in 17alpha-ethynylestradiol exposed male parents, diminished progeny survival, is heritable in the unexposed surviving F1 males. Semen was collected from anesthetized males of the F1 generation upon sexual maturation at two time-points, one year old precocious males and two years old males. In vitro fertilization was used to produce an F2 generation. F2 embryos were then analyzed for survival at 19 days post-fertilization (eye pigmentation) and the different treatment groups statistically compared to the controls. Analysis indicated that F2 offspring survival from F1 males propagated from both exposed and unexposed parents survive normally and no heritable effect was observed in males from the F1 generation for this specific reproductive defect. These results provide scope for the recovery of fish populations exposed to environmental estrogens should the contaminant be removed.

Molecular biomarkers of endocrine disruption in small model fish

A wide range of environmental contaminants can interfere with hormonal regulation in vertebrates. These endocrine disrupting chemicals (EDCs) are of high relevance for human and wildlife health, since endocrine signalling controls many essential physiological processes which impact on the individual's health, such as growth and development, stress response, and ultimately reproduction and population development. Small fish represent a cost-effective model for testing potential EDCs allowing the possibility to integrate from molecular to phenotypic and functional effects. We have comprehensively reviewed exposure-effect data from four different small model fish: zebrafish, medaka, fathead minnow, and the three-spined stickleback. The majority of available data refer to EDCs interfering with reproductive hormones. However, we have also included interactions with other hormone systems, particularly the thyroid hormones. We demonstrate that the available data clearly indicates the predictive potential of molecular biomarkers, supporting the development and regulatory application of simple molecular-based screening assays using small model fish for EDC testing.

Interference of endocrine disrupting chemicals with aromatase CYP19 expression or activity, and consequences for reproduction of teleost fish

Many natural and synthetic compounds present in the environment exert a number of adverse effects on the exposed organisms, leading to endocrine disruption, for which they were termed endocrine disrupting chemicals (EDCs). A decrease in reproduction success is one of the most well-documented signs of endocrine disruption in fish. Estrogens are steroid hormones involved in the control of important reproduction-related processes, including sexual differentiation, maturation and a variety of others. Careful spatial and temporal balance of estrogens in the body is crucial for proper functioning. At the final step of estrogen biosynthesis, cytochrome P450 aromatase, encoded by the cyp19 gene, converts androgens into estrogens. Modulation of aromatase CYP19 expression and function can dramatically alter the rate of estrogen production, disturbing the local and systemic levels of estrogens. In the present review, the current progress in CYP19 characterization in teleost fish is summarized and the potential of several classes of EDCs to interfere with CYP19 expression and activity is discussed. Two cyp19 genes are present in most teleosts, cyp19a and cyp19b, primarily expressed in the ovary and brain, respectively. Both aromatase CYP19 isoforms are involved in the sexual differentiation and regulation of the reproductive cycle and male reproductive behavior in diverse teleost species. Alteration of aromatase CYP19 expression and/or activity, be it upregulation or downregulation, may lead to diverse disturbances of the above mentioned processes. Prediction of multiple transcriptional regulatory elements in the promoters of teleost cyp19 genes suggests the possibility for several EDC classes to affect cyp19 expression on the transcriptional level. These sites include cAMP responsive elements, a steroidogenic factor 1/adrenal 4 binding protein site, an estrogen-responsive element (ERE), half-EREs, dioxin-responsive elements, and elements related to diverse other nuclear receptors (peroxisome proliferator activated receptor, retinoid X receptor, retinoic acid receptor). Certain compounds including phytoestrogens, xenoestrogens, fungicides and organotins may modulate aromatase CYP19 activity on the post-transcriptional level. As is shown in this review, diverse EDCs may affect the expression and/or activity of aromatase cyp19 genes through a variety of mechanisms, many of which need further characterization in order to improve the prediction of risks posed by a contaminated environment to teleost fish population.

Toxicity of the aromatase inhibitor letrozole to Japanese medaka (Oryzias latipes) eggs, larvae and breeding adults

Letrozole is a synthetic aromatase inhibitor and interferes in the committed step in the synthesis of endogenous estrogens from androgens. To evaluate potential effects on the early life stages of Japanese medaka, larvae and fertilized eggs were exposed to letrozole for 96 h and 14 days, respectively. No larvae died and no adverse effects were found on embryonic development at concentrations up to 3125 microg/L. Reproductive effects were assessed by exposing adults to 1, 5, 25, 125 and 625 microg/L letrozole for 21 days. A dose-dependent decrease in fecundity (>25 microg/L) and fertility (>5 microg/L) accompanied by histological changes suggested the inhibition of oocyte growth and possibly maturation. At 625 microg/L, the fish ceased spawning during the last week of exposure. Letrozole (>5 microg/L) reduced plasma vitellogenin levels in females in a dose-dependent manner. Transgenerational effects were evaluated by removing freshly-laid F1 eggs from letrozole-contaminated water and raising them to 15 days post-hatching in control water. Hatchability and time to hatching were detrimentally affected (>5 microg/L), but no morphological deformities were observed. Furthermore, a dose-dependent increase in the proportion of genotypic F1 males was found (>5 microg/L).