bluuebunny/crossref_metadata_2025 huggingface dataset:
Toxicity vs. beneficial effects of phytoestrogens
Authors: Wanibuchi H. , Kang J. S. , Salim E. I. , Morimura K. , Fukushima S.
Published: October 2007

Phytoestrogens are nonsteroidal plant-derived compounds possessing estrogenic activity. These include two major classes: isoflavonoids and lignans. Phytoestrogens have received recently great attention because of their beneficial effects, which include the prevention of cancer, atherosclerosis, and bone density loss. However, they have estrogenic activity and may affect as endocrine disruptors. In this review, we pay attention to both the toxic and beneficial effects of phytoestrogens.

Epidemiological data support that isoflavonoids help prevent cancer of breast, prostate, stomach, and lung. However, there have been some reports about a positive association between some phytoestrogens (e.g., campesterol and stigmasterol) and prostate cancer risk. Animal experiments and in vitro experiments have shown that the biological effects of phytoestrogens may be organ-specific, inhibiting cancer development in some sites, yet showing no effect or an enhancing effect on tumorigenesis at other sites. Also, their effects may be dependent on the timing and duration of exposure. For example, several studies in rodents have established that the favorable effect of an isoflavone-rich diet on breast cancer risk may be significant only if consumption occurs before puberty or during adolescence. On the other hand, prenatal exposure of genistein was reported to have a carcinogenic effect on the uterus of rodents. Phytoestrogen, especially soy products, has been used as hormone-replacement therapy, reducing such symptoms as hot flashes, vaginal dryness, and mood changes while protecting women from osteoporosis and heart disease. However, the animal data suggest that the timing of exposure to such compounds is crucial, with neonatal exposure having the most pronounced effects. Given the exposure of neonates to phytoestrogens, this should be a cause for concern.
Evaluation of thyroid function in neonatal and adult rats: The neglected endocrine mode of action
Authors: Christian M. S. , Trenton N. A.
Published: October 2007

Although known to regulate growth and development, cellular metabolism, the use of oxygen, and basal metabolic rate, thyroid hormones have been only minimally evaluated in neonatal rodents at critical times of development. Despite some modulation of metabolic rate by other hormones, such as testosterone, growth hormone, and norepinephrine, 3,5,3'-triiodothyronine (T3) and 3,5,3',5'-tetraiodothyronine (T4) are the most important metabolic rate modulators. Endpoints used for thyroid function assessment in neonatal and adult rats include thyroid-stimulating hormone (TSH), T3, and T4 levels and histopathology. In rodents, decreased serum levels of T3 and T4 and increased serum TSH levels, with sustained release of TSH and resultant follicular cell hypertrophy/hyperplasia, are typical hormonal and histopathological findings attributable to compounds altering thyroid function. Hypothyroidism early in the neonatal period can affect reproductive endpoints in both male and female rats, with the critical period of exposure being the first two weeks postnatal. Hypothyroidism has been shown to reduce gonadotrophin levels and delay pubertal spermatogenesis in male rats and to block gonadotropin-induced first ovulation in immature female rats by decreasing FSH and luteinizing hormone (LH) serum concentrations. Inclusion of evaluations of TSH, T3, and T4 assays in multigeneration and developmental neurotoxicity protocols may assist in risk assessments.
Modification of endocrine active potential by mixtures
Authors: Gaido Kevin , You Li , Safe S.
Published: October 2007

Wildlife and humans are exposed to a complex mixture of endocrine active chemicals. The activity of a specific chemical in any mixture can be modified through interactions with other components of the mixture. The toxic equivalency factor (TEF) approach for risk assessment was developed for chemicals such as halogenated aromatics that induce their effects through ligand-activated receptors. For persistent halogenated aromatic AhR agonists, this approach has some utility. However, the use of the TEF approach for endocrine active compounds is confounded by the unique tissue- and response-specific activities of these structurally diverse compounds. The term selective receptor modulator describes the ability of a natural or synthetic receptor ligand to manifest agonist activity in one tissue or for one response and antagonist activity in other tissues or for another response in the same tissue. Thus, it is possible for chemicals in a mixture to behave in an additive manner for one response and an antagonist manner for another response. A mechanisms-based hazard risk assessment of endocrine active chemical mixtures must account for these multiple variables.
Experience with new testing guidelines with endocrine-sensitive end-points
Authors: Tyl R. W.
Published: October 2007

Concerns about the effects of endocrine disruptors on humans and wildlife have resulted in revised governmental testing guidelines (e.g., U.S. Environmental Protection Agency, Organization for Economic Cooperation and Development, U.S. Food and Drug Administration), adding endpoints to enhance their capability to detect endocrine active compounds. Based on experience with these testing guidelines, I present my opinions and data on study design, performance, results, endpoints, interpretation, and recommendations for improvement. New regulatory endpoints must be reproducible, robust, sensitive, relevant, and consistent. These new endpoints are appropriate: anogenital distance, examination of culled pups on postnatal day 4, examination for retained nipples/areolae in preweanling males, weanling necropsy, acquisition of puberty in offspring, prebreed estrous cyclicity, reproductive organ weights and histopathology, and andrology. Endpoints considered not appropriate are ovarian primordial follicle counts, stage of estrous for parental females at demise, and single blood sample at necropsy to measure circulating levels of relevant hormones. In conclusion: (1) regulatory guidelines represent only the minimum requirements and should/will change as science improves and scientific and societal concerns arise; (2) scientists must interact to identify and refine new methods, endpoints, and mechanisms; (3) the objective of these studies is to provide good science and useful information for risk assessment. Regulatory agencies should convene workshops with stakeholders to discuss issues such as new fundamental/applied information, mechanisms, and current and new endpoints. Interactions will lead to enhanced science, periodic revisions of testing guidelines, and improved risk assessment.
Critical evaluation of observed adverse effects of endocrine active substances on reproduction and development, the immune system, and the nervous system
Authors: O'Connor J. C. , Chapin R. E.
Published: October 2007

The last 40 years have seen many reports that man-made chemicals and environmental pollutants cause adverse effects in humans and wildlife; however, actually linking an exposure with a mechanism and an effect has yet to be done for endocrine disruption. Certainly, studies in experimental animals have shown that sufficient doses of select compounds can disrupt the endocrine system and produce the attendant adverse outcomes. The purpose of this contribution is to evaluate some of the recent reports of the adverse effects on reproduction and development, the immune system, and the nervous system that have been observed in experimental animals after treatment with man-made chemicals and environmental pollutants. Space limitations prevent us from presenting a comprehensive review of all reported endocrine active chemicals and their effects. Instead, we have focused on drawing conclusions as to the scope and etiology of the adverse effects in experimental animals using examples from the scientific literature, and on suggesting a path forward for further work.
Significance of experimental studies for assessing adverse effects of endocrine-disrupting chemicals
Authors: Gray L. E. , Foster P. M. D.
Published: October 2007

The U.S. Environmental Protection Agency (USEPA) is developing an endocrine disruptor screening and testing program to detect chemicals that alter hypothalamic-pituitary-gonadal (HPG) function, estrogen, androgen, and thyroid (EAT) hormone synthesis or metabolism and induce androgen (AR) and estrogen (ER) receptor-mediated effects in mammals and other animals. The utility of this approach is based upon the knowledge that mechanisms of endocrine-disrupting chemical (EDC) action are highly conserved at the cellular and molecular levels among vertebrates. Some EDC mechanisms also are shared with invertebrates. High-priority chemicals would be evaluated in a Tier 1 screening (T1S) battery, and chemicals that are positive in T1S would then be tested in Tier 2 (T2). T1S includes in vitro ER and AR receptor binding and/or gene expression, an assessment of steroidogenesis and mammalian (rat) and nonmammalian (fish) in vivo assays. In vivo, the uterotropic assay detects estrogens and antiestrogens, while steroidogenesis, antithyroid activity, antiestrogenicity, and HPG function are assessed in a pubertal female assay. Antiandrogens are detected in the Hershberger assay (weight of androgen-dependent tissues in castrate-immature-male rats). Fish and amphibian assays are also being developed to identify EDCs. Several alternative mammalian in vivo assays have been proposed. Of these, a short-term pubertal male rat assay appears most promising. T1S is designed to be sensitive to EAT activities, but many of the effects detected at the screening level would not be considered adverse, the dosage levels may be high, and the route of administration used may not be the most relevant. However, issues of adversity, dose response, and route(s) of exposure would be resolved in the testing phase. In addition to using an enhanced multigenerational test for Tier 2, an in utero-lactational screening protocol is also being evaluated by USEPA for use in T2 or T1S. For T2, the numbers of endocrine-sensitive end-points and offspring (F1) examined in multigenerational tests need to be expanded for EDCs in a thoughtful manner, based in part upon the results of T1S. In addition, for some chemicals histological examination of 10 adult F1 per sex in only the control and high-dose groups provides inadequate statistical power to detect low-dose lesions induced during development. In these cases, we propose that all the offspring be examined after puberty for gross and histological reproductive abnormalities. Since EDCs, like the phthalates and AR-antagonists, produce characteristic profiles, or syndromes, of adverse effects, data need to be reported in a manner that clearly identifies the proportion of animals displaying one or more of the abnormalities in a syndrome. Consideration should be given to tailoring T2, based on the results of T1S to assure that all of the effects in such chemically induced developmental syndromes are included in the study.
Determination of acceptable exposure levels for humans for endocrine active substances: Use of animal models
Authors: Fenner-Crisp P. A.
Published: October 2007

Regulatory authorities and other scientific organizations around the world have developed hazard/risk assessment practices that involve the derivation of numerical values said to represent "acceptable" or "safe" levels of human exposure to individual chemical substances. Human data would be preferable for this purpose since interspecies extrapolation would not be necessary. In most cases, however, these data are not available or are inadequate for this purpose. Therefore, results from studies conducted in non-human mammalian species are used as the principal or sole basis for hazard/risk assessment. Existing risk assessment frameworks developed for the evaluation of endpoints of toxicity for chemical substances are sufficiently flexible to incorporate knowledge concerning the mode(s)/mechanism(s) of action by which these endpoints occur, including those involving disturbance of normal endocrine status ("endocrine disruption"). Background on nomenclature, traditional practices using non-human animal models, and the nature and adequacy of data sets for deriving acceptable human exposures for chemicals, including endocrine active substances, are described, as is how/why the existing frameworks are adequate for the purpose of deriving numerical values for endocrine active substances. Projections on how assessment practices for these substances may evolve further in the future also are offered.
Use of NOAEL, benchmark dose, and other models for human risk assessment of hormonally active substances
Authors: Setzer R. W. , Kimmel C. A.
Published: October 2007

The benchmark dose (BMD) is the dose of a substance that is expected to result in a prespecified level of effect, the benchmark response level or BMR. It is a general approach to characterizing dose response, applicable to any toxicant and endpoint. A BMD is conceptually superior to a "no observed adverse effect level" (NOAEL) for this purpose because of being less determined by experimental design, because it is a precisely defined entity, and because its precision can be estimated. Since a BMD is a single number, just as an NOAEL, it is tempting to use the BMD as a straightforward replacement for the NOAEL in the assessment process for calculating allowable daily intakes. However, the level of toxic response at a NOAEL is unknown, while that at a BMD is well defined. Use of the BMD approach potentially adds consistency and objectivity to the process of deriving reference values (RfDs, RfCs, or ADIs) for setting regulatory levels. To take advantage of this, BMRs need to be selected in a consistent way across studies and endpoints. This paper discusses some issues affecting the selection of BMRs, and presents an example of a BMD calculated for the effects of peripubertal exposure to the fungicide vinclozolin.
Endocrine active substances and dose response for individuals and populations
Authors: Barton H. A.
Published: October 2007

Dose-response characteristics for endocrine disruption have been major focuses in efforts to understand potential impacts on human and ecological health. Issues include assumptions of thresholds for developmental effects, effects at low doses with nonmonotonic (e.g., "U-shaped") behaviors, population vs. individual responses, and background exposures (e.g., dietary phytoestrogens). Dose-response analysis presents a challenge because it is multidisciplinary, involving biologists and mathematicians. Statistical analyses can be valuable for evaluating issues such as the reproducibility of data as illustrated for contradictory findings on low-dose effects. Mechanistically based modeling provides insights into how perturbations of biological systems by endocrine active substances can create different dose-response behaviors. These analyses have demonstrated that higher order behaviors resulting from the interaction of component parts may appear highly nonlinear, thresholded, low-dose linear, or nonmonotonic, or exhibit hysteresis. Some effects need to be evaluated as population impacts. For example, alterations in male:female ratio may be important at the population level even though not adverse for the individual. Descriptions of the contributions of background exposures to dose-response behaviors are essential. The challenge for improving dose-response analyses is to better understand how system characteristics create different dose-response behaviors. Such generalizations could then provide useful guidance for developing risk assessment approaches.
Endocrine disruption occurring at doses lower than those predicted by classical chemical toxicity evaluations: The case bisphenol A
Authors: Ashby J.
Published: October 2007

The meaning of the term "low dose" is discussed in relation to endocrine toxicity data for chemicals. Consideration is also given to experimental conditions likely to impinge on the interpretation and extrapolation of such low-dose effects, and the importance of gathering appropriate control data is emphasized. In the specific case of bisphenol A (BPA), it is concluded that despite the extensive endocrine disruptor (ED) database available for this chemical, it is still not possible to locate a single study that passes the most rudimentary scientific requirements-that the observations are capable of independent confirmation. Two possible explanations for this are considered. First, that BPA possesses subtle low-dose ED toxicities that only become evident under certain undefined experimental conditions. Until these conditions are defined and understood, it will be a matter of chance what individual investigators observe experimentally for BPA or any other chemical. Second, that the general failure of investigators to define and understand natural variability among control parameters monitored in ED studies allows artefactual positive results to be encountered for chemicals, especially in limited and nonreproduced studies. Whichever of these conclusions is correct, the positive low-dose data currently available for BPA cannot be extrapolated to humans with any confidence.
Environmental estrogens and sperm counts
Authors: Fisch Harry , Golden Robert
Published: October 2007

The term "environmental estrogen" refers to chemical substances that exhibit some degree of estrogen-like activity. The primary emphasis for potential adverse effects resulting from exposure to environmental estrogens is on in utero exposure because such exposure can occur during critical periods of organogenesis. Assessment of biological plausibility can be based, in part, on the extensive data on the effects of diethylstilbestrol (DES). The available evidence is too limited to judge with any confidence whether sperm counts have declined during the past 50 years. Based on both animal and human data with DES, it is biologically plausible that in utero exposures to exogenous estrogenic compounds are capable of reducing sperm production in adult men. However, the apparent existence of a maternal dose threshold for DES-induced effects on sperm counts undermines the likelihood that environmental estrogens, which are substantially less potent, are capable of causing similar effects.
Historical perspective on endocrine disruption in wildlife
Authors: Matthiessen P.
Published: October 2007

Endocrine disruption (ED) as a named field of research has been very active for over 10 years, but effects in wildlife that would now be labeled as ED have been studied since the 1940s. This paper briefly surveys the progress in wildlife studies that has been made to date and draws out the major themes and issues that have been identified. In particular, it discusses information concerning causative substances, modes of action, ubiquity of effects across taxa, individual- and population-level impacts, and the importance of low-dose and mixture effects. The main conclusion is that while most wildlife taxa are showing some ED effects at some locations, good evidence for population-level impacts is still limited to a few groups. In order to improve both the interpretation of field observations and the way in which environmental risk assessments are conducted, we need to develop an enhanced ability to predict effects on populations and communities from a knowledge of effects on individuals.
Endocrine disruption in invertebrates
Authors: Oehlmann J. , Schulte-Oehlmann U.
Published: October 2007

Recent reports have shown that a number of xenobiotics in the environment are capable of interfering with the normal endocrine function in a variety of animals. The overwhelming majority of the studies on the effects of hormone-mimetic industrial chemicals were focused on findings in vertebrates. More detailed information about the effects on and mechanisms of action in invertebrates has only been obtained from a few cases, although invertebrates represent more than 95 % of the known species in the animal kingdom and are extremely important with regard to ecosystem structure and function. The limited number of examples for endocrine disruption (ED) in invertebrates is partially due to the fact that their hormonal systems are rather poorly understood in comparison with vertebrates. Deleterious endocrine changes following an exposure to certain compounds may easily be missed or simply be unmeasurable at present, even though a number of studies show that endocrine disruption has probably occurred. The well-documented case studies of tributyltin effects in mollusks and of insect growth regulators, the latter as purposely synthesized endocrine disruptors, are explained to support this view. According to our present knowledge, there is no reason to suppose that such far-reaching changes are in any sense unique. The additional existing evidence for ED in invertebrates from laboratory and field studies are summarized as an update and amendment of the EDIETA report from 1998. Finally, conclusions about the scale and implications of the observed effects are drawn and further research needs are defined.
Effects of endocrine disruptors in aquatic mammals
Authors: Fossi M. C. , Marsili Letizia
Published: October 2007

In the last few decades, various studies have shown that aquatic mammals are sensitive to the toxicological effects of certain xenobiotic compounds, including the large class of endocrine-disrupting chemicals (EDCs). Since some EDCs, particularly organochlorines, tend to bioaccumulate and biomagnify in the aquatic food chain, various aquatic mammals, particularly those high in the food chain, such as pinnipeds, odontocete cetaceans, and polar bears, are potentially "at risk". The main aim of this chapter is to define the state of the art on effects of endocrine disruptors in aquatic mammals, both freshwater and marine. Another aim is to formulate recommendations for future research in this field and finally to define what can be done internationally for hazard/risk assessment and communication of the findings.
Endocrine disruption in marine fish
Authors: Matthiessen P.
Published: October 2007

This topic reviews the whole field of endocrine disruption (ED) in marine fish and compares this with our knowledge of the situation in freshwater species. In broad terms, similar types of ED have been observed in the two groups, although effects in the marine environment tend to be less marked, presumably due to dispersion and dilution. There are, however, some data which suggest that marine fish that are top-predators can experience ED due to biomagnification of organochlorines. Processes such as smoltification, metamorphosis, and hermaphroditism, which are common in some marine species, may be particularly susceptible to ED, but have as yet been scarcely studied. As with freshwater fish, firm links to population-level effects have not yet been demonstrated, although it is not unreasonable to suppose that they are occurring in some locations. The topic concludes with some recommendations for future research.
Deformed frogs and environmental retinoids
Authors: Gardiner David , Ndayibagira A. , Grün Felix , Blumberg Bruce
Published: October 2007

Since the early 1990s, a substantial number of deformed frogs have been observed in North America, particularly in the upper Midwest and Canada. Attempts to understand the etiology of the deformed frog problem have met with limited success to date with nearly as many proposed explanations as research groups working on the problem. Models for the mechanism underlying the development of deformed frogs include parasite/predation, ultraviolet radiation, and chemical exposure. Each model has its strengths and weaknesses. Despite contentious debate among researchers, there is an overall consensus that the increasing prevalence of deformed frogs is the result of a water-borne contaminant that has recently appeared, or reached a critical concentration. Our detailed analysis of malformed frogs collected in Minnesota ponds and lakes suggested that limb patterning was being modified by the disruption of a retinoid-sensitive developmental signaling pathway. Accordingly, we focused in the identification and characterization of bioactive retinoids from lake water and showed that retinoid treatment of frog embryos at sensitive times of development could recapitulate the full spectrum of limb abnormalities observed in field specimens in the laboratory. These data have led to the conclusion that inappropriate modulation of retinoid signaling by environmental contaminants is the mechanism underlying the increased incidence of frog malformations.
Contaminant-induced endocrine and reproductive alterations in reptiles
Authors: Guillette L. J. , Iguchi Taisen
Published: October 2007

Many chemicals introduced into the environment by humans adversely affect embryonic development and the functioning of the vertebrate reproductive system. It has been hypothesized that many developmental alterations are due to the endocrine-disruptive effects of various environmental contaminants. The endocrine system exhibits an organizational effect on the developing embryo, altering gene expression and dosing. Thus, a disruption of the normal hormonal signals can permanently modify the organization and future functioning of the reproductive and endocrine system. We have worked extensively with contaminant-exposed and reference populations of the American alligator (Alligator mississippiensis) as well as performed a number of experimental studies exposing developing embryos to various persistent and nonpersistent pesticides. Using this species, we have described altered steroidogenesis, circulating hormone levels, and hepatic transformation of androgen and endocrine organ (gonad, thyroid) morphology in juvenile alligators living in polluted environments. Given the adverse observations reported to date, we recommend several important future needs:

Further development of "receptor zoos" and other molecular tools that include key reptiles from various major ecosystems, in addition to freshwater ecosystems.

Global studies extending the current knowledge base on crocodilians and freshwater turtles to comparable ecosystems on other continents, such as linked studies examining and extending current molecular to population level studies in Florida (USA) to tropical and temporate regions of Africa, Australia, and South America.

Further studies of actual exposure, assimilation and excretion of contaminants by ectothermic vertebrates, especially reptiles that occupy high levels of the food chain.
Review of the effects of endocrine-disrupting chemicals in birds
Authors: Giesy J. P. , Feyk L. A. , Jones P. D. , Kannan Kurunthachalam , Sanderson T.
Published: October 2007

There have been several case studies of the impact of chemical contaminants on birds at the level of individuals or populations. While many of the chemicals involved in these incidents have been classified as endocrine-disrupting chemicals or endocrine active substances (EASs) the mechanisms by which these chemicals affect birds are not clearly or fully understood.
Wildlife as models for the study of how mixtures, low doses, and the embryonic environment modulate the action of endocrine-disrupting chemicals
Authors: Crews David , Putz O. , Thomas P. , Hayes Tyrone , Howdeshell Kembra
Published: October 2007

This paper will review briefly the use of wildlife as models in the study of how mixtures, low doses, and the embryonic environment modulate the action of endocrine active substances (EASs). In so doing it will show how the issue of low dosages must be considered within the context of mixtures present in the environment and the endocrine background of the exposed individual. That is, in nature, EASs usually are found in mixtures in which the constituent parts are in concentrations well below their no observed adverse effect level (NOAEL) as determined in single compound studies in the laboratory. In addition, exposure always occurs on organisms in various endocrine states. Thus, the issue of mixtures and dosages must always be considered within the context of the endocrine background. Finally, the effects of exposure are passed down through the generations. The question of exposure then at the level of the individual becomes very complicated, as it must take into account that at every life stage, the naturally occurring endocrine milieu of the organism (or tissue), any EAS burden inherited from the mother or built up over the individual's life, and the social environment in which the individual develops and interacts as an adult, will influence the response to acute exposure.
Interactions of endocrine-disrupting chemicals with stress responses in wildlife
Authors: Pottinger T. G.
Published: October 2007

The extent to which nonreproductive aspects of the endocrine system are affected by environmental contaminants is to a large extent unknown. However, an emerging body of data demonstrates that the neuroendocrine stress response is a sensitive target for disruption by a range of environmental contaminants, at a number of discrete loci. Several mechanisms are responsible for generating and sustaining the corticosteroid response to a stressor, including synthesis of the steroid, negative feedback at the pituitary and hypothalamus, and clearance via metabolism and conjugation in peripheral tissues and the liver. Laboratory and field studies provide evidence that these elements of the stress response are susceptible to interference by EAS. The functional significance to the individual of interference with this important adaptive mechanism remains to be established.
Effects of endocrine active substances in wildlife species: Genetic, biochemical, and physiological factors in variable susceptibility to endocrine disruptors
Authors: Kawai Shin'ichiro , Kobayashi M. , Kaneko Hideo
Published: October 2007

Responses to endocrine active substances (EASs) in animals are various, and differences between the responses among individuals, populations and species are well known. These differences are observed not only in EASs but in most environmental chemicals including synthetic and naturally occurring ones. The basic differences in sensitivity to EASs are attributed to that of affinity or specificity of the receptors to EASs at the cellular level. Although the nucleotide sequences encoding for estrogen receptor proteins have been documented in several species and the functions of the receptors are the same, the ability to bind the natural hormones and the estrogenic xenobiotics is not necessarily identical. The reproductive endocrine system is basically common among vertebrates, but chemical types of hormones, physiological roles of hormones and the basal blood levels of hormones differ among each species, especially in sex steroids. These differences cause various types of responses and sensitivity to EASs among animal species. Xenobiotic metabolism is important for the genetical, biochemical and physiological factors concerning the influence of EASs. Some EASs directly inhibit cytochrome P450 (CYP) activity as was reported in tributyltin that inhibits CYP19 (aromatase) activity causing imposex in neogastropods. Some organochlorines including dioxins stimulate aryl hydrocarbon (Ah) receptor-mediated xenobiotic metabolism, and result in the metabolic disruption of steroid hormones such as estrogen as were reported in eggshell thinning in birds of prey and uterus occlusion in seals. CYP activity greatly differs among wildlife species in both terrestrial and aquatic organisms, and these differences are significantly responsible for the multiple effects or toxicity of EASs. Sex and age differences also cause different responses to EASs and are largely due to the differences in xenobiotic metabolizing activities.
Development of fish tests for endocrine disruptors
Authors: Hutchinson T. H. , Yokota H. , Hagino S. , Ozato K.
Published: October 2007

International concern over endocrine active substances (EASs) has led to intensive research programmes to establish fish reproductive and developmental toxicity tests for use in environmental (ecological) risk assessment. This chapter gives an overview of key themes of in vivo ecotoxicology research, including fish screening assays, partial life-cycle tests (the draft OECD fish reproduction test and the new fish development test) and fish full life-cycle tests. In the context of the OECD test guidelines program, fish species of primary interest include fathead minnow, medaka and zebrafish, while guppy, rainbow trout, sheepshead minnow, and three-spined stickleback are also of scientific importance. Critical factors for evaluation include baseline reproductive biology and definition of EAS sensitive life-stages. For regulatory applications, a critical review of existing fish EAS data suggests that apical adverse effect endpoints, namely development, growth and reproduction (e.g., fecundity, fertilization rates, and hatching success) should be used to derive predicted no-effect concentrations (PNECs) for the environmental risk assessment of EASs. In support of these apical adverse effect endpoints, biomarker responses (e.g., vitellogenin, gonadal-somatic index, and gonad histopathology) should be used to provide mechanistic data, compare species (e.g., cyprinids vs. salmonids), and allow extrapolation between laboratory and field studies.
Endocrine disruption in wildlife: The future?
Authors: Sumpter J. P.
Published: October 2007

Probably the only thing that can be said with certainty about the future of this field of ecotoxicology is that predicting it is foolish; the chances of being right are very slim. Instead, it seems to me likely that unexpected discoveries will probably have more influence on the field of endocrine disruption than the outcomes of all the planned experiments. It is certainly true that chance discoveries, such as masculinized fish in rivers receiving paper-mill effluent, imposex in molluscs due to exposure to tributyltin and feminized fish in rivers receiving effluent from sewage-treatment works, have been pivotal in the development of the field of endocrine disruption in wildlife. I consider that further such discoveries are likely, but I do not know which species will be affected, what effects will be found, what chemical(s) will be the cause, or what endocrine mechanism(s) will underlie the effects. The recent realization that many pharmaceuticals are present in the aquatic environment only underscores the range of effects that could, in theory at least, occur in exposed wildlife. What is somewhat easier to predict is the research that will be conducted in the immediate future, which will build upon what is known already. For example, it is clear that wildlife is rarely, if ever, exposed to single chemicals, but instead is exposed to highly complex, ill-defined mixtures of chemicals, including many that are endocrine active in various ways. We need to understand much better how chemicals interact, and what overall effects will occur upon exposure to such mixtures. We also need to move from assessing effects at the individual organism level, to understanding the consequences of these effects at the population level. Then, we need to determine the significance of any population-level effects due to endocrine disruption in comparison with the impact of many other significant stressors (e.g., over-exploitation, habitat loss, climate change) that also negatively impact wildlife. Such research will be difficult, and time-consuming, and will probably produce many surprises. All I can be fairly certain about is that the next few years are likely to be as interesting and exciting as the last few have been.
Mechanism-based QSAR approach to the study of the toxicity of endocrine active substances
Authors: Selassie C. D. , Garg R. , Mekapati S.
Published: October 2007

Mechanism-based QSAR models for interactions between various ligands and the estrogenic receptor are developed by using well-developed physicochemical parameters. Common features of these QSARs are identified, and deficiencies in some datasets are highlighted. The relative binding affinities of various substituted hexestrols to estrogen receptors are examined in terms of their steric, electronic, and hydrophobic attributes. Different QSARs for hexestrols and tamoxifens reveal that steric effects are of overriding importance in variations of binding affinity. In the few cases where a large number of diverse substituents are located on aromatic rings, electronic effects emerge and suggest that electron-donating groups enhance binding to the receptor while hydrophobicity plays a marginal role in decreasing binding affinity. With substituted phenols bearing alkyl-type substituents and substituted hydroxy-biphenyls, the binding is strictly dependent on hydrophobicity and size. These QSAR models are described in detail and examined together to illustrate the utility of lateral validation in mechanistic interpretation.
Regulatory application of SAR/QSAR for priority setting of endocrine disruptors: A perspective
Authors: Tong W. , Fang Hong , Hong H. , Xie Q. , Perkins R. , Anson Jeanne , Sheehan D. M.
Published: October 2007

Some seven years have passed since the U.S. legislature mandated the EPA to develop and implement a screening and testing program for chemicals that may disrupt the delicate endocrine system. The envisioned EPA program has evolved to incorporate a tiered scheme of in vitro and in vivo assays, and considered QSAR as a viable method to set testing priorities. At the U.S. FDA's National Center for Toxicological Research (NCTR), the Endocrine Disruptor Knowledge Base Project has developed models to predict estrogen and androgen receptor binding. Our approach rationally integrates various QSAR models into a sequential "Four-Phase" scheme according to the strength of each type of model. In four hierarchical phases, models predict the inactive chemicals that are then eliminated from the pool of chemicals to which increasingly precise but more time-consuming models are subsequently applied. Each phase employs different models selected to work complementarily in representing key activity-determining structure features in order to absolutely minimize the rate of false negatives, an outcome we view as paramount for regulatory use. In this paper, the QSAR models developed at NCTR, and particularly how we integrated these models into the "Four-Phase" system will be discussed for a number of datasets, including 58 000 chemicals identified by the U.S. EPA.
QSAR prioritization of chemical inventories for endocrine disruptor testing
Authors: Schmieder P. , Mekenyan Ovanes , Bradbury Steven , Veith G.
Published: October 2007

Binding affinity between chemicals and the estrogen receptor (ER) serves as an indicator of the potential to cause endocrine disruption through this receptor-mediated endocrine pathway. Estimating ER-binding affinity is, therefore, one strategic approach to reducing the costs of screening chemicals for potential risks of endocrine disruption. While measuring ER binding with in vitro assays may be the first choice in prioritizing chemicals for additional in vitro or in vivo estrogenicity testing, the time and costs associated with screening thousands of chemicals is prohibitive. Recent advances in 3D modeling of the reactivity of flexible structures make in silico methods for estimating ER binding possible. One technique, the common reactivity pattern (COREPA) approach, was applied to development of reactivity patterns for ER relative binding affinity based on global nucleophilicity, interatomic distances between nucleophilic sites, and local electron donor capability of the nucleophilic sites. The reactivity patterns provided descriptor profiles for order-of-magnitude RBA ranges of training set chemicals. An exploratory expert system was subsequently developed to predict RBA and rank chemicals with respect to potential estrogenicity. A strategy is presented for extending initial exploratory 3D QSAR models beyond current training sets to increase applicability to more diverse structures in large chemical inventories.
The nature of the ligand-binding pocket of estrogen receptor alpha and beta: The search for subtype-selective ligands and implications for the prediction of estrogenic activity
Authors: Katzenellenbogen J. A. , Muthyala R. , Katzenellenbogen B. S.
Published: October 2007

The ligand-binding pockets of estrogen receptor alpha and beta (ERα and ERβ) appear to have subpockets of different size and flexibility. To find ligands that will discriminate between the two ER subtypes on the basis of affinity or efficacy, we have prepared compounds of varying size, shape and structure. We have evaluated the binding affinity of these compounds and their potency and efficacy as transcriptional activators through ERα and ERβ. In this manner, we have identified a number of ligands that show pronounced ER subtype selectivity. These studies also highlight the eclectic structure–activity relationships of estrogens and the challenges inherent in developing computational methods for the prediction of estrogenic activity.
Fragment molecular orbital study of the binding energy of ligands to the estrogen receptor
Authors: Fukuzawa Kaori , Kitaura Kazuo , Nakata K. , Kaminuma Tsuguchika , Nakano T.
Published: October 2007

We examined the published data for the binding affinity of typical ligands to the α-subtype of the human estrogen receptor with use of an approximate molecular orbital method applicable to interacting molecular clusters. An ab initio procedure for "molecular fragments" proposed recently to deal with such macromolecules as proteins was applied to the molecular orbital calculations. The receptor protein was primarily modeled using 50 amino acid residues surrounding the ligand. For a few ligand-receptor complexes, the binding energy was also calculated with use of 241 amino acid residues contained in the entire binding domain. No significant difference was found in the calculated binding energy between the complex modeled with ligand-surrounding 50 amino acids and that with residues of the entire domain. The calculated binding energy was correlated very well with the published relative binding affinity for typical ligands.
Toxicogenomics: Impact on human health
Authors: Selkirk J. K.
Published: October 2007

Toxicology is the science of adverse effects of chemicals, drugs, environmental agents, and stressors. Genomics defines the structure, sequence (code), and function of the entire DNA complement of organisms. The interface of these diverse disciplines is called toxicogenomics and is based upon the application of genomic technologies to define globally the changes in gene expression (both mRNA and proteins) as a consequence of exposures. DNA microarray technology enables the simultaneous measurement of transcription of thousands of genes using microchips containing thousands of probes of complementary DNA (cDNA) immobilized in a predetermined array. The ultimate application of this technology to toxicology holds great promise but faces several formidable problems. With the solution to these problems, it will be possible to develop a substantial database of Chemical Effects in Biological Systems (CEBS). Such a database will provide the capacity to relate specific changes in gene expression to specific adverse effects and to look for similar pathways in different organisms. Such data will provide an objective way of assessing surrogate systems for reporting or predicting potential adverse effects in humans. While the potential for toxicogenomics is thus very high for studying active substances, the task must be approached in a deliberate, incremental manner to insure that only high-quality data are compiled and analyzed. This workshop will present the latest results of research conducted in leading international laboratories studying endocrine-mediated toxicity.
Temporal responses to estrogen in the uterus
Authors: Fertuck K. C. , Zacharewski T. R.
Published: October 2007

The estrogen receptor acts primarily as an estrogen-inducible transcription factor in target organs, however, until recently the identification of individual responsive transcripts has been cumbersome. In the present study, oligonucleotide GeneChip microarrays were used to describe and analyze the levels of approximately 6500 transcripts in the uteri of immature, ovariectomized mice at various times following oral exposure to 100 μg/kg of ethynyl estradiol. The most reproducible responses were identified and subjected to K-means clustering, and functional annotations were obtained for the transcripts within each cluster. Connections were made between the observed transcriptional responses and the known physiological responses to estrogen exposure, focusing here on effects on uterine cell cycle progression leading to estrogen-stimulated uterine growth.
Application of toxicogenomics to the endocrine disruption issue
Authors: Shirai T. , Asamoto Makoto
Published: October 2007

Toxicogenomics can be expected to be a useful method for detecting the carcinogenic potential of endocrine active substances (EASs) in the short term with the generation of understanding of mode-of-action and mechanisms when a reliable database with information about proteomics and informatics is established. At present, there are no concrete epidemiological data supporting any exogenous EAS contribution to hormone-related organ carcinogenesis in humans. However, with the establishment of appropriate animal models and analysis of genomic-scale gene expression, risk identification and evaluation should be facilitated within a relatively short period, and this approach eventually promises to contribute a great deal of risk management regarding EASs.
Transcript profiles elicited by developmental exposure to endocrine-mediated toxicants
Authors: Daston George P.
Published: October 2007

Genomics can be applied in toxicology as a means of identifying modes of action, for generating hypotheses on the relationships of gene activity and toxicity, and for better characterizing the nature of dose-response relationships at low doses. This paper provides examples from our research on endocrine active compounds that illustrate each of these applications. We have determined that agents that bind estrogen receptors produce a characteristic transcript profile in estrogen-responsive tissues of the fetal and juvenile rat. The transcript profile is diagnostic of the mechanism of action. The transcripts that are up- or down-regulated by estrogens belong to a number of functional groups, such as growth factors, pro-apoptotic factors, transcription factors, and steroid metabolizing enzymes. There are a number of testable hypotheses that can be generated from these data regarding the relationships between changes in these genes and developmental or physiological responses to estrogens. We have determined that the sensitivity of gene expression changes is high, making it possible to define the shape of the dose-response curve at dose levels of estrogen several orders of magnitude below those that cause a physiological response (in this case, a uterotrophic response). The shape of the dose response is monotonic: both the number of genes changed and the intensity of the up- or down-regulation decreases with decreasing dose.
Use of gene expression profiling to understand the transcriptional program associated with estrogen-induced uterine growth
Authors: Moggs J. G. , Deavall D. G. , Orphanides G.
Published: October 2007

The use of gene expression data in predictive and mechanistic toxicology is hindered by a lack of information on the relationships between transcriptional events and physiological and pathological changes. We discuss the analysis of these relationships using the rodent uterotrophic response as a model experimental system for estrogen-induced uterine growth.