In vitro models in endocrine disruptor screening

Differential susceptibility to endocrine disruptor-induced epimutagenesis

There is now considerable evidence indicating the potential for endocrine disrupting chemicals to alter the epigenome and for subsets of these epigenomic changes or "epimutations" to be heritably transmitted to offspring in subsequent generations. While there have been many studies indicating how exposure to endocrine disrupting chemicals can disrupt various organs associated with the body's endocrine systems, there is relatively limited information regarding the relative susceptibility of different specific organs, tissues, or cell types to endocrine disrupting chemical-induced epimutagenesis. Here we review available information about different organs, tissues, cell types, and/or cell lines which have been shown to be susceptible to specific endocrine disrupting chemical-induced epimutations. In addition, we discuss possible mechanisms that may be involved, or impacted by this tissue- or cell type-specific, differential susceptibility to different endocrine disrupting chemicals. Finally, we summarize available information indicating that certain periods of development display elevated susceptibility to endocrine disrupting chemical exposure and we describe how this may affect the extent to which germline epimutations can be transmitted inter- or transgenerationally. We conclude that cell type-specific differential susceptibility to endocrine disrupting chemical-induced epimutagenesis is likely to directly impact the extent to, or manner in, which endocrine disrupting chemical exposure initially induces epigenetic changes to DNA methylation and/or histone modifications, and how these endocrine disrupting chemical-induced epimutations can then subsequently impact gene expression, potentially leading to the development of heritable disease states.

In vitro profiling of the potential endocrine disrupting activities affecting steroid and aryl hydrocarbon receptors of compounds and mixtures prevalent in human drinking water resources

Prioritizing chemicals posing threats to drinking water resources is crucial for legislation considering the cost of water treatment, remediation, and monitoring. We profiled in vitro potential endocrine disrupting activities (both agonistic and antagonistic) of 18 contaminants most prevalent in Walloon raw water resources intended for drinking water production, including several compound groups: pesticides, perfluorinated compounds, polycyclic aromatic hydrocarbons, a corrosion inhibitor, and bisphenol A. Mixtures thereof relevant for human realistic exposure were also investigated. Seven luciferase reporter gene cell lines were used i.e. three (human and rat) responsive to dioxins through the aryl hydrocarbon receptor (AhR) and four (human) responsive to steroids through the estrogen (ER), androgen (AR), progesterone (PR), and glucocorticoid (GR) receptors. Among the 18 compounds, ten caused at least one response in at least one receptor. Specifically, chlorpyrifos, bisphenol A, fluoranthene, phenanthrene, and benzo [a]pyrene displayed significant activities on several receptors. Bisphenol A agonized ER, but abolished the cells' response to androgen and progesterone. While fluoranthene and phenanthrene strongly reduced human AhR and AR transactivation, benzo [a]pyrene strongly activated AhR and ER, but inhibited GR and AR. In human breast cancer cells, benzo [a]pyrene dramatically activated AhR, inducing a 10-fold higher response than 2,3,7,8-tetrachlorodibenzodioxin (TCDD) at concentrations possibly found realistically in human blood. The mixture of the 18 compounds exerted both ER and rat AhR agonism, with the main contribution being from benzo [a]pyrene or its combination with bisphenol A. Moreover, the mixture significantly inhibited TCDD-induced CYP1A activity (detected only by EROD assays) in human hepatoma cells.

LC-MS/MS based profiling and dynamic modelling of the steroidogenesis pathway in adrenocarcinoma H295R cells

Endocrine disrupting chemicals have been reported to exert effects directly on enzymes involved in steroid biosynthesis. Here, we present a new liquid chromatography-tandem mass spectrometry (LC-MS/MS) method for profiling the steroid metabolome of H295R human adrenocarcinoma cells. Our method can simultaneously analyse 19 precursors, intermediates and end-products, representing the adrenal steroid biosynthesis pathway. In order to obtain better insights into the processes of steroidogenesis, we investigated the dose-response relationship of forskolin, an activator of adenylate cyclase, on steroid production in H295R cells. We observed that 1.5 μM forskolin stimulated steroid production at approximately 50% of the maximum rate for most steroids. Hence, we studied the time course for steroid synthesis over 72 h in H295R cells that were stimulated with forskolin. At 24 h, we observed a peak in steroid levels for the intermediate metabolites, such as progesterone and pregnenolone, while end-products such as testosterone and cortisol continued to increase until 72 h. Finally, we show how global data provide a unique basis to develop a comprehensive, dynamic model for steroidogenesis using first order kinetics. The timeline data made it possible to estimate all reaction rate constants of the network. We propose this method as a unique and sensitive screening tool to identify effects on adrenal steroidogenesis by endocrine disrupting compounds.

In vitro observations and in silico predictions of xenoestrogen mixture effects in T47D-based receptor transactivation and proliferation assays

Within endocrine disruptor research, evaluation and interpretation of mixture effects and the predictive value for downstream responses still warrant more in-depth investigations. We used an estrogen receptor (ER)-mediated reporter gene assay (ER-CALUX®) and a cell proliferation assay (WST-1 assay), both based on the T47D breast cancer cell line, to test mixtures of heterogeneous xenoestrogens. Observed concentration-response curves were compared to those predicted by the concepts of concentration addition (CA), generalized concentration addition (GCA), and a novel full logistic model (FLM). CA performed better regarding mixture potency (EC50 values), whereas GCA was superior in predicting mixture efficacy (maximal response). In comparison, FLM proved to be highly suitable for in silico mixture effect prediction, combining advantages of both CA and GCA. The inter-assay comparison revealed that ER activation is not necessarily predictive for induction of cell proliferation. The results support the use of models like CA, GCA, or FLM in mixture effect evaluation. However, we conclude that reliable estimations regarding the disruptive potential of mixtures of endocrine active substances require an integrative approach considering more than one assay/endpoint to avoid misinterpretations. The formazan-based WST-1 proliferation assay might be a possible alternative to commonly used proliferation assays in endocrine disrupter research.

In vitro and in vivo studies of the endocrine disrupting potency of cadmium in roach (Rutilus rutilus) liver

Cadmium has been reported to exert estrogenic, antiestrogenic or both effects in vertebrate species. To elucidate the endocrine disrupting action of CdCl2, ex vivo and in vivo experiments were performed in roach (Rutilus rutilus). Roach liver explants were exposed to a range of CdCl2 concentrations alone (0.1-50μM) or with an effective concentration (100nM) of 17β-estradiol (E2). In addition, juvenile roach were intraperitoneally injected with CdCl2 (0.1-2.5mg/kg) with or without 1mg E2/kg. Subsequent analysis evaluated the effect of CdCl2 on vitellogenin (VTG) synthesis both at the mRNA and protein level, on estrogen receptors (erα and erβ1) and on androgen receptor (ar) mRNA expression. Ex vivo and in vivo experiments indicated that CdCl2 is strongly anti-estrogenic as, when co-exposed to E2, CdCl2 significantly inhibited VTG production as well as vtg and erα mRNA expressions. Moreover, CdCl2 compromised the E2-mediated induction of the ar mRNA expression in vivo.

Bacterial biosensors for evaluating potential impacts of estrogenic endocrine disrupting compounds in multiple species

To study the effects and possible mechanisms of suspected endocrine disrupting compounds (EDCs), a wide variety of assays have been developed. In this work, we generated engineered Escherichia coli biosensor strains that incorporate the ligand-binding domains (LBDs) of the β-subtype estrogen receptors (ERβ) from Solea solea (sole), and Sus scrofa (pig). These strains indicate the presence of ligands for these receptors by changes in growth phenotype, and can differentiate agonist from antagonist and give a rough indication of binding affinity via dose-response curves. The resulting strains were compared with our previously reported Homo sapiens ERβ biosensor strain. In initial tests, all three of the strains correctly identified estrogenic test compounds with a high degree of certainly (Z' typically greater than 0.5), including the weakly binding test compound bisphenol A (BPA) (Z' ≈ 0.1-0.3). The modular design of the sensing element in this strain allows quick development of new species-based biosensors by simple LBD swapping, suggesting its use in initial comparative analysis of EDC impacts across multiple species. Interestingly, the growth phenotypes of the biosensor strains indicate similar binding for highly estrogenic control compounds, but suggest differences in ligand binding for more weakly binding EDCs.

A computational model to predict rat ovarian steroid secretion from in vitro experiments with endocrine disruptors

A finely tuned balance between estrogens and androgens controls reproductive functions, and the last step of steroidogenesis plays a key role in maintaining that balance. Environmental toxicants are a serious health concern, and numerous studies have been devoted to studying the effects of endocrine disrupting chemicals (EDCs). The effects of EDCs on steroidogenic enzymes may influence steroid secretion and thus lead to reproductive toxicity. To predict hormonal balance disruption on the basis of data on aromatase activity and mRNA level modulation obtained in vitro on granulosa cells, we developed a mathematical model for the last gonadal steps of the sex steroid synthesis pathway. The model can simulate the ovarian synthesis and secretion of estrone, estradiol, androstenedione, and testosterone, and their response to endocrine disruption. The model is able to predict ovarian sex steroid concentrations under normal estrous cycle in female rat, and ovarian estradiol concentrations in adult female rats exposed to atrazine, bisphenol A, metabolites of methoxychlor or vinclozolin, and letrozole.

A review of the tissue residue approach for organic and organometallic compounds in aquatic organisms

This paper reviews the tissue residue approach (TRA) for toxicity assessment as it applies to organic chemicals and some organometallic compounds (Sn, Hg, and Pb) in aquatic organisms. Specific emphasis was placed on evaluating key factors that influence interpretation of critical body residue (CBR) toxicity metrics including data quality issues, lipid dynamics, choice of endpoints, processes that alter toxicokinetics and toxicodynamics, phototoxicity, species- and life stage-specific sensitivities, and biotransformation. The vast majority of data available on TRA is derived from laboratory studies of acute lethal responses to organic toxicants exhibiting baseline toxicity. Application of the TRA to various baseline toxicants as well as substances with specific modes of action via receptor-mediated processes, such as chlorinated aromatic hydrocarbons, pesticides, and organometallics is discussed, as is application of TRA concepts in field assessments of tissue residues. In contrast to media-based toxicity relationships, CBR values tend to be less variable and less influenced by factors that control bioavailability and bioaccumulation, and TRA can be used to infer mechanisms of toxic action, evaluate the toxicity of mixtures, and interpret field data on bioaccumulated toxicants. If residue-effects data are not available, body residues can be estimated, as has been done using the target lipid model for baseline toxicants, to derive critical values for risk assessment. One of the primary unresolved issues complicating TRA for organic chemicals is biotransformation. Further work on the influence of biotransformation, a better understanding of contaminant lipid interactions, and an explicit understanding of the time dependency of CBRs and receptor-mediated toxicity are all required to advance this field. Additional residue-effects data on sublethal endpoints, early life stages, and a wider range of legacy and emergent contaminants will be needed to improve the ability to use TRA for organic and organometallic compounds.

Toxicity testing in the 21st century: a vision and a strategy

With the release of the landmark report Toxicity Testing in the 21st Century: A Vision and a Strategy, the U.S. National Academy of Sciences, in 2007, precipitated a major change in the way toxicity testing is conducted. It envisions increased efficiency in toxicity testing and decreased animal usage by transitioning from current expensive and lengthy in vivo testing with qualitative endpoints to in vitro toxicity pathway assays on human cells or cell lines using robotic high-throughput screening with mechanistic quantitative parameters. Risk assessment in the exposed human population would focus on avoiding significant perturbations in these toxicity pathways. Computational systems biology models would be implemented to determine the dose-response models of perturbations of pathway function. Extrapolation of in vitro results to in vivo human blood and tissue concentrations would be based on pharmacokinetic models for the given exposure condition. This practice would enhance human relevance of test results, and would cover several test agents, compared to traditional toxicological testing strategies. As all the tools that are necessary to implement the vision are currently available or in an advanced stage of development, the key prerequisites to achieving this paradigm shift are a commitment to change in the scientific community, which could be facilitated by a broad discussion of the vision, and obtaining necessary resources to enhance current knowledge of pathway perturbations and pathway assays in humans and to implement computational systems biology models. Implementation of these strategies would result in a new toxicity testing paradigm firmly based on human biology.

Alternatives to animal experimentation for hormonal compounds research

Alternatives to animal testing and the identification of reliable methods that may decrease the need for animals are currently the subject of intense investigation worldwide. Alternative testing procedures are particularly important for synthetic and natural chemicals that exert their biological actions through binding nuclear receptors, called nuclear receptors-interacting compounds (NR-ICs), for which research is increasingly emphasizing the limits of several models in the accurate estimation of the physiological consequences of exposure to these compounds. In particular, estrogen receptor interacting compounds (ER-ICs) have a great impact on human health from the therapeutic, nutritional, and toxicological point of view due to the highly permissive nature of the estrogen receptors towards a large number of natural and synthetic compounds. Similar to in vitro systems, recently generated animal models (e.g., animal models generated for the study of estrogen receptor ligands) may fulfill the 3R principles: refine, reduce, and replace. If used correctly, NR-regulated models, such as reporter mice, xenopus, or zebrafish, and models obtained by somatic gene transfer in reporter systems, combined with imaging technologies, may contribute to strongly decreasing the overall number of animals required for NR-IC testing and research. With these models, flexible and highly standardized parameters and reporter marker quantification can be obtained. Here, we highlight the need for the substitution of currently used testing models with more appropriate ones that can reproduce the features and reactivity of specific mammalian target tissue/organs. We consider the promotion of this advancement a research priority bearing scientific, economic, social, and ethical relevance.

In-vitro screening of the antiestrogenic activity of chemicals

BACKGROUND

Many chemicals have the potential to interfere with the endocrine systems of humans and wildlife, leading to adverse health effects. In the tiered testing strategies developed for regulatory hazard assessment, in-vitro screens could serve for prioritisation of compounds and for guiding subsequent testing.

OBJECTIVE

To describe in-vitro assays to detect antiestrogenic activity of chemicals.

METHODS

Antiestrogenicity was considered in this review as any inhibition or reduction of estrogen-induced processes due to interference with the normal functioning of the estrogen receptor pathway. Accordingly, in-vitro screening assays for antiestrogenicity have to consider all the possible mechanisms by which this inhibition may occur. Such assays include binding assays, cell proliferation assays, reporter gene assays, and gene activation/protein production assays.

RESULTS/CONCLUSIONS

While binding assays appear to be of limited value in assessing antiestrogenicity, assays using differentiated cells with metabolic competence and a varied receptor/regulatory factor equipment have the capability to detect various modes of antiestrogenic action.

Assessing and managing risks arising from exposure to endocrine-active chemicals

Managing risks to human health and the environment produced by endocrine-active chemicals (EAC) is dependent on sound principles of risk assessment and risk management, which need to be adapted to address the uncertainties in the state of the science of EAC. Quantifying EAC hazard identification, mechanisms of action, and dose-response curves is complicated by a range of chemical structure/toxicology classes, receptors and receptor subtypes, and nonlinear dose-response curves with low-dose effects. Advances in risk science including toxicogenomics and quantitative structure-activity relationships (QSAR) along with a return to the biological process of hormesis are proposed to complement existing risk assessment strategies, including that of the Endocrine Disruptor Screening and Testing Advisory Committee (EDSTAC 1998). EAC represents a policy issue that has captured the public's fears and concerns about environmental health. This overview describes the process of EAC risk assessment and risk management in the context of traditional risk management frameworks, with emphasis on the National Research Council Framework (1983), taking into consideration the strategies for EAC management in Canada, the United States, and the European Union.

Systematic Reviews of Animal Experiments Demonstrate Poor Human Clinical and Toxicological Utility

The assumption that animal models are reasonably predictive of human outcomes provides the basis for their widespread use in toxicity testing and in biomedical research aimed at developing cures for human diseases. To investigate the validity of this assumption, the comprehensive Scopus biomedical bibliographic databases were searched for published systematic reviews of the human clinical or toxicological utility of animal experiments. In 20 reviews in which clinical utility was examined, the authors concluded that animal models were either significantly useful in contributing to the development of clinical interventions, or were substantially consistent with clinical outcomes, in only two cases, one of which was contentious. These included reviews of the clinical utility of experiments expected by ethics committees to lead to medical advances, of highly-cited experiments published in major journals, and of chimpanzee experiments — those involving the species considered most likely to be predictive of human outcomes. Seven additional reviews failed to clearly demonstrate utility in predicting human toxicological outcomes, such as carcinogenicity and teratogenicity. Consequently, animal data may not generally be assumed to be substantially useful for these purposes. Possible causes include interspecies differences, the distortion of outcomes arising from experimental environments and protocols, and the poor methodological quality of many animal experiments, which was evident in at least 11 reviews. No reviews existed in which the majority of animal experiments were of good methodological quality. Whilst the effects of some of these problems might be minimised with concerted effort (given their widespread prevalence), the limitations resulting from interspecies differences are likely to be technically and theoretically impossible to overcome. Non-animal models are generally required to pass formal scientific validation prior to their regulatory acceptance. In contrast, animal models are simply assumed to be predictive of human outcomes. These results demonstrate the invalidity of such assumptions. The consistent application of formal validation studies to all test models is clearly warranted, regardless of their animal, non-animal, historical, contemporary or possible future status. Likely benefits would include, the greater selection of models truly predictive of human outcomes, increased safety of people exposed to chemicals that have passed toxicity tests, increased efficiency during the development of human pharmaceuticals and other therapeutic interventions, and decreased wastage of animal, personnel and financial resources. The poor human clinical and toxicological utility of most animal models for which data exists, in conjunction with their generally substantial animal welfare and economic costs, justify a ban on animal models lacking scientific data clearly establishing their human predictivity or utility.

The use of human cell line reporter gene-based assays in chemical toxicity testing

Genetically modified rodents allow greater sensitivity in monitoring DNA damage or gene expression than traditional rodent bioassays and have become increasingly used for toxicity testing, particularly with the greater availability of protein and DNA-based toxicity biomarkers. Here, the advantages and limitations of several in vitro reporter assays already used to study the mechanisms of toxicity are discussed in relation to the in vivo traditional and reporter-based bioassays for carcinogenicity, mutagenicity, endocrine changes and inflammation endpoints to examine the scope for refining and replacing transgenic in vivo models.

Vitellogenin synthesis in primary cultures of fish liver cells as endpoint for in vitro screening of the (anti)estrogenic activity of chemical substances

Concern over possible adverse effects of endocrine-disrupting compounds on fish has caused the development of appropriate testing methods. In vitro screening assays may provide initial information on endocrine activities of a test compound and thereby may direct and optimize subsequent testing. Induction of vitellogenin (VTG) is used as a biomarker of exposure of fish to estrogen-active substances. Since VTG induction can be measured not only in vivo but also in fish hepatocytes in vitro, the use of VTG induction response in isolated fish liver cells has been suggested as in vitro screen for identifying estrogenic-active substances. The main advantages of the hepatocyte VTG assay are considered its ability to detect effects of estrogenic metabolites, since hepatocytes in vitro remain metabolically competent, and its ability to detect both estrogenic and anti-estrogenic effects. In this article, we critically review the current knowledge on the VTG response of cultured fish hepatocytes to (anti)estrogenic substances. In particular, we discuss the sensitivity, specificity, and variability of the VTG hepatocyte assay. In addition, we review the available data on culture factors influencing basal and induced VTG production, the response to natural and synthetic estrogens as well as to xenoestrogens, the detection of indirect estrogens, and the sources of assay variability. The VTG induction in cultured fish hepatocytes is clearly influenced by culture conditions (medium composition, temperature, etc.) and culture system (hepatocyte monolayers, aggregates, liver slices, etc.). The currently available database on estrogen-mediated VTG induction in cultured teleost hepatocytes is too small to support conclusive statements on whether there exist systematic differences of the VTG response between in vitro culture systems, VTG analytical methods or fish species. The VTG hepatocyte assay detects sensitively natural and synthetic estrogens, whereas the response to xenoestrogens appears to be more variable. The detection of weak estrogens can be critical due to the overshadow with cytotoxic concentrations. Moreover, the VTG hepatocyte assay is able to detect antiestrogens as well as indirect estrogens, i.e substances which require metabolic activation to induce an estrogenic response. Nevertheless, more chemicals need to be analysed to corroborate this statement. It will be necessary to establish standardized protocols to minimize assay variability, and to develop a set of pass-fail criteria as well as cut-offs for designating positive and negative responses.

Selecting Appropriate Animal Models and Experimental Designs for Endocrine Disruptor Research and Testing Studies

Evidence that chemicals in the environment may cause developmental and reproductive abnormalities in fish and wildlife by disrupting normal endocrine functions has increased concern about potential adverse human health effects from such chemicals. US laws have now been enacted that require the US Environmental Protection Agency (EPA) to develop and validate a screening program to identify chemicals in food and water with potential endocrine-disrupting activity. EPA subsequently proposed an Endocrine Disruptor Screening Program that uses in vitro and in vivo test systems to identify chemicals that may adversely affect humans and ecologically important animal species. However, the endocrine system can be readily modulated by many experimental factors, including diet and the genetic background of the selected animal strain or stock. It is therefore desirable to minimize or avoid factors that cause or contribute to experimental variation in endocrine disruptor research and testing studies. Standard laboratory animal diets contain high and variable levels of phytoestrogens, which can modulate physiologic and behavioral responses similar to both endogenous estrogen as well as exogenous estrogenic chemicals. Other studies have determined that some commonly used outbred mice and rats are less responsive to estrogenic substances than certain inbred mouse and rat strains for various estrogen-sensitive endpoints. It is therefore critical to select appropriate biological models and diets for endocrine disruptor studies that provide optimal sensitivity and specificity to accomplish the research or testing objectives. An introduction is provided to 11 other papers in this issue that review these and other important laboratory animal experimental design considerations in greater detail, and that review laboratory animal and in vitro models currently being used or evaluated for endocrine disruptor research and testing. Selection of appropriate animal models and experimental design parameters for endocrine disruptor research and testing will minimize confounding experimental variables, increase the likelihood of replicable experimental results, and contribute to more reliable and relevant test systems.