Summary of reference chemicals evaluated by the fish short-term reproduction assay, OECD TG229, using Japanese Medaka, Oryzias latipes

Effect of diisobutyl adipate on the expression of biomarker genes that respond to endocrine disruption and on gonadal sexual differentiation in Japanese medaka (Oryzias latipes)

Phthalate and non-phthalate plasticizers are used in polymer materials, such as plastic and rubber. It has recently been found that diisobutyl adipate (DIBA), which is considered an environmentally safe non-phthalate plasticizer, potentially acts as a thyroid disruptor in fish. Here, we investigated the sexual hormone effects of DIBA based on the expression levels of genes that respond to endocrine disruption and sexual hormone activity in the livers and gonads, and on gonadal sexual differentiation in Japanese medaka. Compared with the control group, the mRNA expression of chgH, vtg1, vtg2, and esr1 was significantly suppressed in the livers of DIBA exposed XX individuals. Furthermore, the mRNA expression of gsdf was significantly upregulated and downregulated in the gonads of XX and XY individuals, respectively. The mRNA expressions of esr1 and esr2b were significantly suppressed by DIBA exposure in the gonads of both XX and XY individuals. These observations suggest that DIBA has potential androgenic activity in Japanese medaka. However, normal testes and ovaries were observed in respective XY and XX medaka after DIBA exposure; therefore, these results suggest that DIBA may have weak androgenic activity.

An international cross-laboratory survey on fish vitellogenin analysis: Methodological challenges and opportunities for best practice

Vitellogenin (VTG) is a biomarker for possible endocrine activity of chemicals acting via the estrogen, androgen, or steroidogenesis pathways. VTG is assessed in standardised fish guideline studies conducted for regulatory safety assessment of chemicals. VTG data can be highly variable leading to concerns for potential equivocal, false positive and/or negative outcomes. Consequently, additional fish testing may be required to address uncertainties in the VTG response, and possibly erroneous/missed identification of endocrine activity. To better understand the technical challenges of VTG assessment and reporting for regulatory purposes, a survey was sent to 27 testing laboratories performing these analyses. The survey results from 16 respondents (6 from the UK, 3 from the USA, and 7 from the EU) were analysed and discussed in a follow-up webinar. High variability in background VTG concentrations was widely acknowledged and thought to be associated with fish batch, husbandry, laboratory practices, and several methodological aspects. These include sample collection and storage, VTG quantification, data handling, and the benchmarks used for data acceptability. Information gathered in the survey provides a basis for improving and harmonizing the measurement of VTG in fish, and an opportunity to reassess the suitability of current acceptability criteria in test guidelines.

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.

Influence of phthalate and non-phthalate plasticizers on reproductive endocrine system-related gene expression profiles in Japanese medaka (Oryzias latipes)

Plasticizers containing phthalates have the potential to alter endocrine function in vertebrates. While non-phthalate plasticizers were previously considered to be environmentally friendly and safe, our research team discovered that bis-(2-ethylhexyl) adipate (DEHA) and acetyl tributyl citrate (ATBC) disrupt thyroid hormones in Japanese medaka (Oryzias latipes). We assessed reproductive- and estrogen-responsive gene expression patterns in Japanese medaka to determine whether the phthalate plasticizers bis-(2-ethylhexyl) phthalate (DEHP, positive control) and the non-phthalate plasticizers DEHA and ATBC disrupt endocrine signaling. The results showed that the levels of choriogenin H (chgH) and vitellogenin (vtg) genes increased after exposure to DEHP and ATBC, suggesting that these plasticizers may have estrogenic activity. Exposure to DEHP and DEHA resulted in the upregulation of kisspeptin (kiss), gonadotropin-releasing hormone (gnrh), and follicle-stimulating hormone beta (fshβ) genes, suggesting that these plasticizers may interfere with reproductive function. To the best of our knowledge, this is the first study to demonstrate that the non-phthalate plasticizers DEHA and ATBC can disrupt reproduction-related hormonal activity in fish.

Are changes in vitellogenin concentrations in fish reliable indicators of chemical-induced endocrine activity?

Vitellogenin (VTG), a biomarker for endocrine activity, is a mechanistic component of the regulatory assessment of potential endocrine-disrupting properties of chemicals. This review of VTG data is based on changes reported for 106 substances in standard fish species. High intra-study and inter-laboratory variability in VTG concentrations was confirmed, as well as discrepancies in interpretation of results based on large differences between fish in the dilution water versus solvent control, or due to the presence of outlier measurements. VTG responses in fish were ranked against predictions for estrogen receptor agonist activity and aromatase inhibition from bioactivity model output and ToxCast in vitro assay results, respectively. These endocrine mechanisms explained most of the VTG responses in the absence of systemic toxicity, the magnitude of the VTG response being proportional to the in vitro potency. Interpretation of the VTG data was sometimes confounded by an alternative endocrine mechanism of action. There was evidence for both false positive and negative responses for VTG synthesis, but overall, it was rare for substances without endocrine activity in vitro to cause a concentration-dependent VTG response in fish in the absence of systemic toxicity. To increase confidence in the VTG results, we recommend improvements in the VTG measurement methodologies and greater transparency in reporting of VTG data (including quality control criteria for assay performance). This review supports the application of New Approach Methodologies (NAMs) by demonstrating that endocrine activity in vitro from mammalian cell lines is predictive for in vivo VTG response in fish, suggesting that in vitro mechanistic data could be used more broadly in decision-making to help reduce animal testing.

Impact of acetyl tributyl citrate on gonadal sex differentiation and expression of biomarker genes for endocrine disruption in Japanese medaka

Plasticizers are broadly classified as phthalate or nonphthalate. Recently, acetyl tributyl citrate (ATBC), an environmentally friendly nonphthalate plasticizer, was revealed to have the ability to disrupt thyroid hormone activity in fish species. Therefore, we aimed to assess whether ATBC exhibits any sex hormone (i.e., androgenic or estrogenic) activities. First, we examined the effects of ATBC on gonadal sex differentiation. Subsequently, we analyzed the different expression of biomarker genes that respond to endocrine-disrupting chemicals (EDCs) with sexual hormone activity in the liver. We observed normal testes and ovaries after both XX and XY medakas were exposed to ATBC, indicating that ATBC is not an EDCs with strong sex hormone activity and that it does not induce intersex (testis-to-ova or ovo-to-testis) or sex changes in Japanese medaka. The vitellogenin 1 (vtg1) and vitellogenin 2 (vtg2) mRNA expression levels in the liver of XX medakas were significantly reduced compared with those in the control group, whereas the expression levels of these genes in the liver of XY medakas remained unchanged. Finally, we examined the changes in the expression of biomarker genes that respond to EDCs with sex hormone activity in the gonads. The expression levels of biomarker genes did not differ significantly from that of the control group, although the expression levels of gsdf mRNA tended to increase while that of aromatase mRNA tended to decrease in the ovary of XX medakas following ATBC exposure. Conversely, the expression levels of gsdf and aromatase mRNAs in the testis of XY medakas remained unchanged. These results suggest that ATBC does not exhibit estrogenic activity, although it may have weak androgenic activity or no sexual hormone activity.

Detection of anti-androgenic activity of chemicals in fish studies: a data review

A systematic review was conducted on the sensitivity of fish testing guidelines to detect the anti-androgenic activity of substances. Sequence Alignment to Predict Across Species Susceptibility (SeqAPASS) was used to investigate the conservation of the androgen receptor (AR) between humans and fish, and among fish species recommended in test guidelines. The AR is conserved between fish species and humans (i.e. ligand binding domain [LBD] homology ≥70%) and among the recommended fish species (LBD homology >85%). For model anti-androgens, we evaluated literature data on in vitro anti-androgenic activity in fish-specific receptor-based assays and changes in endpoints indicative of endocrine modulation from in vivo studies. Anti-androgenic activity was most consistently and reliably detected in in vitro and in vivo mechanistic studies with co-exposure to an androgen (spiggin in vitro assay, Rapid Androgen Disruption Activity Reporter [RADAR] Assay, and Androgenised Female Stickleback Screen). Regardless of study design (Fish Short-Term Reproduction Assay [FSTRA], Fish Sexual Development Test [FSDT], partial or full life-cycle tests), or endpoint (vitellogenin, secondary sexual characteristics, gonadal histopathology, sex ratio), there was no consistent evidence for detecting anti-androgenic activity in studies without androgen co-exposure, even for the most potent substances (while less potent substances may induce no (clear) response). Therefore, based on studies without androgen co-exposure (35 FSTRAs and 22 other studies), the other studies (including the FSDT) do not outperform the FSTRA for detecting potent anti-androgenic activity, which if suspected, would be best addressed with a RADAR assay. Overall, fish do not appear particularly sensitive to mammalian anti-androgens.

Effects of non-phthalate plasticizer bis(2-ethylhexyl) sebacate (DEHS) on the endocrine system in Japanese medaka (Oryzias latipes)

Water pollution due to plasticizers is one of the most severe environmental problems worldwide. Phthalate plasticizers can act as endocrine disruptors in vertebrates. In this study, we investigated whether the non-phthalate bis(2-ethylhexyl) sebacate (DEHS) plasticizer can act as an endocrine disruptor by evaluating changes in the expression levels of thyroid hormone-related, reproduction-related, and estrogen-responsive genes of Japanese medaka (Oryzias latipes) exposed to the plasticizer. Following the exposure, the gene expression levels of thyroid-stimulating hormone subunit beta (tshβ), deiodinase 1 (dio1), and thyroid hormone receptor alpha (trα) did not change. Meanwhile, DEHS suppressed dio2 expression, did not induce swim bladder inflation, and eventually reduced the swimming performance of Japanese medaka. These findings indicate that DEHS can potentially disrupt the thyroid hormone-related gene expression and metabolism of these fish. However, exposure to DEHS did not induce changes in the gene expression levels of kisspeptin 1 (kiss1), gonadotropin-releasing hormone (gnrh), follicle-stimulating hormone beta (fshβ), luteinizing hormone beta (lhβ), choriogenin H (chgH), and vitellogenin (vtg) in a dose-dependent manner. This is the first report providing evidence that DEHS can disrupt thyroid hormone-related metabolism in fish.

Bisphenol A alters sexual dimorphism and gene expression in marine medaka Oryzias melastigma

Bisphenol A (BPA) is an endocrine disruptor that is present in freshwater and marine environments. However, conclusive evidence for the toxicity of chronic BPA exposure to marine fishes remains lacking. Therefore, we investigated the influence of BPA on male marine medaka (Oryzias melastigma). BPA exposure induced formation of testis-ova at 2610 µg/L, and male-type anal fins became more female type in a concentration-dependent manner. Some males with female-type anal fins had normal testes, indicating that anal fin shape is more sensitive to BPA. Gonadal soma-derived factor (gsdf) expression decreased after BPA exposure in the 746 and 2610 µg/L exposure groups, although the changes were not statistically significant. Additionally, liver vitellogenin (vtg) expression increased in a dose-dependent manner and was significantly higher in all exposure groups. vtg and gsdf are likely to be useful biomarkers for the impact of estrogenic endocrine disrupters in O. melastigma.

Effects of plasticizer diisobutyl adipate on the Japanese medaka (Oryzias latipes) endocrine system

Plasticizer pollution of the water environment is one of the world's most serious environmental issues. Phthalate plasticizers can disrupt endocrine function in vertebrates. Therefore, this study analyzed thyroid-related, reproduction-related, and estrogen-responsive genes in Japanese medaka (Oryzias latipes) to determine whether non-phthalate diisobutyl adipate (DIBA) plasticizer could affect endocrine hormone activity or not. Developmental toxicity during fish embryogenesis was also evaluated. At a concentration of 11.57 mg/l, embryonic exposure to DIBA increased the mortality rate. Although abnormal development, including body curvature, edema, and lack of swim bladder inflation, was observed at 3.54 and 11.57 mg/l DIBA, growth inhibition and reduced swimming performance were also observed. In addition, DIBA exposure increased the levels of thyroid-stimulating hormone beta-subunit (tshβ) and deiodinase 1 (dio1) but decreased the levels of thyroid hormone receptor alpha (trα) and beta (trβ). These results suggest that DIBA has thyroid hormone-disrupting activities in fish. However, kisspeptin (kiss1 and kiss2), gonadotropin-releasing hormone (gnrh1), follicle-stimulating hormone beta (fshβ), luteinizing hormone beta (lhβ), choriogenin H (chgH), and vitellogenin (vtg1) expression did not change dose-dependently in response to DIBA exposure, whereas gnrh2 and vtg2 expression was elevated. These results indicate that DIBA has low estrogenic activity and does not disrupt the endocrine reproduction system in fish. Overall, this is the first report indicating that non-phthalate DIBA plasticizer is embryotoxic and disrupt thyroid hormone activity in fish.

Adverse effects of thyroid-hormone-disrupting chemicals 6-propyl-2-thiouracil and tetrabromobisphenol A on Japanese medaka (Oryzias latipes)

Thyroid-hormone-disrupting chemicals are increasingly attracting attention because of their potential harmful effects on animal health, including on fishes. Here, we investigated the effects of exposure to the thyroid-hormone-disrupting chemicals 6-propyl-2-thiouracil (PTU) and tetrabromobisphenol A (TBBPA) on swim bladder inflation, eye development, growth, swimming performance, and the expression of thyroid-related genes in Japanese medaka (Oryzias latipes). PTU exposure resulted in reductions in eye size, growth, and swim bladder inflation, and these effects led to poorer swimming performance. These phenotypic effects were accompanied by increased expression of the thyroid-stimulating hormone subunit beta (tshβ) paralog tshβ-like, but there were no significant changes in expression for tshβ, deiodinase 1 (dio1), deiodinase 2 (dio2), and thyroid hormone receptor alpha (trα) and beta (trβ). For PTU exposure, we identified the key event (swim bladder inflation reduction) and an adverse outcome (swimming performance reduction). No significant effects from TBBPA exposure were seen on swim bladder inflation, eye development, growth, or swimming performance. However, expression of tshβ-like and tshβ (significantly enhanced) and trα and trβ (significantly reduced) were affected by TBBPA exposure albeit not in dose-dependent manners. There were no effects of TBBPA on the expression of dio1 and dio2. We thus show that the two thyroid-hormone-disrupting chemicals PTU and TBBPA differ in their effect profiles with comparable effects on the studied phenotypes and thyroid-related gene expression to those reported in zebrafish.

Summary of 17 chemicals evaluated by OECD TG229 using Japanese Medaka, Oryzias latipes in EXTEND 2016

In June 2016, the Ministry of the Environment of Japan announced a program "EXTEND2016" on the implementation of testing and assessment for endocrine active chemicals, consisting of a two-tiered strategy. The aim of the Tier 1 screening and the Tier 2 testing is to identify the impacts on the endocrine system and to characterize the adverse effects to aquatic animals by endocrine disrupting chemicals detected in the aquatic environment in Japan. For the consistent assessment of the effects on reproduction associated with estrogenic, anti-estrogenic, androgenic, and/or anti-androgenic activities of chemicals throughout Tier 1 screening to Tier 2 testing, a unified test species, Japanese medaka (Oryzias latipes), has been used. For Tier 1 screening, the in vivo Fish Short-Term Reproduction Assay (OECD test guideline No. 229) was conducted for 17 chemicals that were nominated based on the results of environmental monitoring, existing knowledge obtained from a literature survey, and positive results in reporter gene assays using the estrogen receptor of Japanese medaka. In the 17 assays using Japanese medaka, adverse effects on reproduction (i.e., reduction in fecundity and/or fertility) were suggested for 10 chemicals, and a significant increase of hepatic vitellogenin in males, indicating estrogenic (estrogen receptor agonistic) potency, was found for eight chemicals at the concentrations in which no overt toxicity was observed. Based on these results, and the frequency and the concentrations detected in the Japanese environment, estrone, 4-nonylphenol (branched isomers), 4-tert-octylphenol, triphenyl phosphate, and bisphenol A were considered as high priority candidate substances for the Tier 2 testing.

Summary of reference chemicals evaluated by the fish short-term reproduction assay, OECD TG229, using Japanese Medaka, Oryzias latipes

Under the Organisation for Economic Co-operation and Development (OECD), the Ministry of the Environment of Japan (MOE) added Japanese medaka (Oryzias latipes) to the test guideline fish short-term reproduction assay (FSTRA) developed by the United States Environmental Protection Agency (US EPA) using fathead minnow (Pimephales promelas). The FSTRA was designed to detect endocrine disrupting effects of chemicals interacting with the hypothalamic-pituitary-gonadal axis (HPG axis) such as agonists or antagonists on the estrogen receptor (Esr) and/or the androgen receptor (AR) and steroidogenesis inhibitors. We conducted the FSTRA with Japanese medaka, in accordance with OECD test guideline number 229 (TG229), for 16 chemicals including four Esr agonists, two Esr antagonists, three AR agonists, two AR antagonists, two steroidogenesis inhibitors, two progesterone receptor agonists, and a negative substance, and evaluated the usability and the validity of the FSTRA (TG229) protocol. In addition, in vitro reporter gene assays (RGAs) using Esr1 and ARβ of Japanese medaka were performed for the 16 chemicals, to support the interpretation of the in vivo effects observed in the FSTRA. In the present study, all the test chemicals, except an antiandrogenic chemical and a weak Esr agonist, significantly reduced the reproductive status of the test fish, that is, fecundity or fertility, at concentrations where no overt toxicity was observed. Moreover, vitellogenin (VTG) induction in males and formation of secondary sex characteristics (SSC), papillary processes on the anal fin, in females was sensitive endpoints to Esr and AR agonistic effects, respectively, and might be indicators of the effect concentrations in long-term exposure. Overall, it is suggested that the in vivo FSTRA supported by in vitro RGA data can adequately detect effects on the test fish, O. latipes, and probably identify the mode of action (MOA) of the chemicals tested.