Effects of endocrine-disrupting contaminants on amphibian oogenesis: methoxychlor inhibits progesterone-induced maturation of Xenopus laevis oocytes in vitro

Human and fish differences in steroid receptors activation: A review

Steroid receptors (SRs) are transcription factors activated by steroid hormones (SHs) that belong to the nuclear receptors (NRs) superfamily. Several studies have shown that SRs are targets of endocrine disrupting chemicals (EDCs), widespread substances in the environment capable of interfering with the endogenous hormonal pathways and causing adverse health effects in living organisms and/or their progeny. Cell lines with SRs reporter gene are currently used for in vitro screening of large quantities of chemicals with suspected endocrine-disrupting activities. However, most of these cell lines express human SRs and therefore the toxicological data obtained are also extrapolated to non-mammalian species. In parallel, in vivo tests have recently been developed on fish species whose data are also extrapolated to mammalian species. As some species-specific differences in SRs activation by natural and synthetic chemicals have been recently reported, the aim of this review is to summarize those between human and fish SRs, as representatives of mammalian and non-mammalian toxicology, respectively. Overall, this literature study aims to improve inter-species extrapolation of toxicological data on EDCs and to understand which reporter gene cell lines expressing human SRs are relevant for the assessment of effects in fish and whether in vivo tests on fish can be properly used in the assessment of adverse effects on human health.

Sex Determination and Ovarian Development in Reptiles and Amphibians: From Genetic Pathways to Environmental Influences

BACKGROUND

Reptiles and amphibians provide untapped potential for discovering how a diversity of genetic pathways and environmental conditions are incorporated into developmental processes that can lead to similar functional outcomes. These groups display a multitude of reproductive strategies, and whereas many attributes are conserved within groups and even across vertebrates, several aspects of sexual development show considerable variation.

SUMMARY

In this review, we focus our attention on the development of the reptilian and amphibian ovary. First, we review and describe the events leading to ovarian development, including sex determination and ovarian maturation, through a comparative lens. We then describe how these events are influenced by environmental factors, focusing on temperature and exposure to anthropogenic chemicals. Lastly, we identify critical knowledge gaps and future research directions that will be crucial to moving forward in our understanding of ovarian development and the influences of the environment in reptiles and amphibians.

KEY MESSAGES

Reptiles and amphibians provide excellent models for understanding the diversity of sex determination strategies and reproductive development. However, a greater understanding of the basic biology of these systems is necessary for deciphering the adaptive and potentially disruptive implications of embryo-by-environment interactions in a rapidly changing world.

Ethylene oxide derived glycol ethers: A review of the alkyl glycol ethers potential to cause endocrine disruption

The 'ethylene glycol ethers' (EGE) are a broad family of solvents and hydraulic fluids produced through the reaction of ethylene oxide and a monoalcohol. Certain EGE derived from methanol and ethanol are well known to cause toxicity to the testes and fetotoxicity and that this is caused by the common metabolites methoxy and ethoxyacetic acid, respectively. There have been numerous published claims that EGE fall into the category of 'endocrine disruptors' often without substantiated evidence. This review systematically evaluates all of the available and relevant in vitro and in vivo data across this family of substances using an approach based around the EFSA/ECHA 2018 guidance for the identification of endocrine disruptors. The conclusion reached is that there is no significant evidence to show that EGE target any endocrine organs or perturb endocrine pathways and that any toxicity that is seen occurs by non-endocrine modes of action.

Prediction of chemical reproductive toxicity to aquatic species using a machine learning model: An application in an ecological risk assessment of the Yangtze River, China

The endocrine disrupting chemicals (EDCs) have been at the forefront of environmental issues for over 20 years and are a principle factor considered in every ecological risk assessment, but this kind of risk assessment faces difficulties. The expense, time cost of in vivo tests, and lack of toxicity data are key limiting factors for the ability to conduct ecological risk assessments of EDCs to aquatic species. In this study, a machine learning model named the support vector machine (SVM) was used to predict the reproductive toxicity of EDCs, and the performance of the models was evaluated. The results showed that the SVM model provided more accurate toxicity prediction data compared with the interspecies correlation estimation (ICE) model developed by previous study to predict the reproductive toxicity. The application of the predicted toxicity data was an important supplement to the observed data for the ecological risk assessment of EDCs in the Yangtze River, where estrogens and phenolic compounds have been found at some sampling sites in the middle and lower reaches. The results showed that the ecological risk of estrone, 17β-estradiol, and ethinyl estradiol were significant. This study revealed the application potential of machine learning models for the prediction of reproductive toxicity effects of EDCs. This can provide reliable alternative toxicity data for the ecological risk assessments of EDCs.

Effects of endocrine disrupting chemicals in pigs

Endocrine-disrupting chemicals (EDCs) are compounds that interfere with the expression, synthesis, and activity of hormones in organisms. They are released into the environment from flame retardants and products containing plasticizers. Persistent pesticides, such as dichlorodiphenyltrichloroethane (DDT) and hexachlorobenzene, also disrupt the endocrine system through interaction with hormone receptors. Endogenous hormones, such as 17β-estradiol (E2), are released in the urine and feces of farm animals and seep into terrestrial and aquatic ecosystems through sewage. Pigs are widely used as animal models to determine the effects of EDCs because they are physiologically, biochemically, and histologically similar to humans. EDCs primarily disrupt the reproductive and nervous systems of pigs. Moreover, embryonic development during the prenatal and early postnatal periods is particularly sensitive to EDCs. Mycotoxins, such as zearalenone, are food contaminants that alter hormonal activities in pigs. Mycotoxins also alter the innate immune system in pigs, making them vulnerable to diseases. It has been reported that farm animals are exposed to various types of EDCs, which accumulate in tissues, such as those of gonads, livers, and intestines. There is a lack of an integrated understanding of the impact of EDCs on porcine reproduction and development. Thus, this article aims to provide a comprehensive review of literature regarding the effects of EDCs in pigs.

Exposures to chemical contaminants: What can we learn from reproduction and development endpoints in the amphibian toxicology literature?

Environmental contamination is one of the major factors or cofactors affecting amphibian populations. Since 2000, the number of studies conducted in laboratory conditions to understand impacts of chemical exposures increased. They aimed to characterize biological effects on amphibians. This review proposes an overview of biological responses reported after exposures to metals, phytopharmaceuticals or emerging organic contaminants and focuses on endpoints relating to reproduction and development. Due to amphibian peculiar features, these periods of their life cycle are especially critical to pollutant exposures. Despite the large range of tested compounds, the same model species are often used as biological models and morphological alterations are the most studied observations. From the results, the laboratory-to-field extrapolation remained uneasy and exposure designs have to be more elaborated to be closer to environmental conditions. Few studies proposed such experimental approaches. Lastly, gametes, embryos and larvae constitute key stages of amphibian life cycle that can be harmed by exposures to freshwater pollutants. Specific efforts have to be intensified on the earliest stages and notably germ cells.

Oocyte maturation in the toad Rhinella arenarum (Amphibia, Anura): Evidence of cAMP involvement in steroid production and action

In this work, we describe the participation of the adenylate cyclase/3'-5'-cyclic adenonsine monophosphate (cAMP) pathway in the seasonal follicular secretion of progesterone (P₄ ) and testosterone (T), and its relationship with the maturation of Rhinella arenarum oocytes. Under gonadotropin stimulation, P₄ secretion was the dominant steroid produced during the reproductive period, resulting in 100% germinal vesicle breakdown (GVBD) in oocytes in vitro; in contrast, T and estradiol (E₂ ) secretion increased (∼16 nM/20 follicles and ∼80 pM/20 follicles, respectively) during the non-reproductive period, but only yielded 50% GVBD. Treatment of the follicles with dibutyryl-cAMP or forskolin induced a significant increase in T secretion during both periods, but P₄ secretion did not significantly change and GVBD did not occur. These results suggest that high cAMP levels in the oocyte maintain meiotic arrest and prevent the induction effect of follicular steroids. An increase in cAMP levels in denuded oocytes, however, negatively regulated T-induced maturation since treatment with increasing db-cAMP or forskolin inhibited their maturation. Therefore, we hypothesize that an elevation in T during the non-reproductive period favors its aromatization to E₂ , leading to follicle growth. During the reproductive period, P₄ production might promote oocyte maturation when environmental conditions are favorable for reproduction. Together, the results indicate that steroidogenesis is seasonal and depends on gonadotropic activity in R. arenarum.

Inhibition of germinal vesicle breakdown in Xenopus oocytes in vitro by a series of substituted glycol ethers

A 24 hour in vitro Xenopus oocyte maturation (germinal vesicle breakdown [GVBD]) assay developed by Pickford and Morris (Environmental Health Perspectives, 1999, 107, 285-292) was used to screen a series of substituted glycol ethers (GEs). Substituted GEs included: ethylene glycol monomethyl ether (EGME); EG monoethyl ether (EGEE); EG monopropyl ether (EGPE); EG monobutyl ether (EGBE); EG monohexyl ether (EGHE); diethylene glycol monomethyl ether (DGME); triethylene glycol monomethyl ether (TGME); ethylene glycol monophenyl ether (EGPhE); EG monobenzyl ether (EGBeE); EG diphenyl ether (EGDPhE); and propylene glycol monophenyl ether (PGPhE). The GEs inhibited progesterone- or androstenedione-induced GVBD with the following relative potency: EGPhE > PGPhE > EGME >> EGEE ≥ EGBeE > EGPE >> EGBE >EGHE > EGDPhE >> DGME ≥ TGME, or EGPhE >> PGPhE >> EGBeE > EGDPhE > EGEE > EGME > EGPE > EGBE, EGHE, DGME and TGME, respectively. Further, [³ H]progesterone or [³ H]androstenedione binding affinities to the oocyte plasma membrane progesterone receptor (OMPR) or classical androgen receptor (AR) were: EGME > EGPhE ≥ PGPhE ≥ EGEE > EGBeE >> EGPE >> EGBE ≥ EGHE > EGDPhE, TGME, and DGME, or EGPhE > PGPhE >> EGBeE > EGDPhE >> EGEE ≥ EGME >> EGPE, EGBE, and EGHE > DGME and TGME, respectively. Binary joint mixture studies with the GVBD model using flutamide (AR antagonist) and EGPhE indicated that flutamide/EGPhE mixture acted in a concentration additive manner. The effects of substituted GE series, however, may be mediated through the OMPR; the potency of EGPhE may be the result of bimodal inhibition of both the OMPR and AR pathways.

Endocrine disruption by environmental gestagens in amphibians - A short review supported by new in vitro data using gonads of Xenopus laevis

Endocrine disruption caused by various anthropogenic compounds is of persisting concern, especially for aquatic wildlife, because surface waters are the main sink of these so-called endocrine disruptors (ED). In the past, research focused on (anti)estrogenic, (anti)androgenic, and (anti)thyroidal substances, affecting primarily reproduction and development in vertebrates; however, other endocrine systems might be also targeted by ED. Environmental gestagens, including natural progestogens (e.g. progesterone (P4)) and synthetic progestins used for contraception, are supposed to affect vertebrate reproduction via progesterone receptors. In the present paper, we review the current knowledge about gestagenic effects in amphibians, focussing on reproduction and the thyroid system. In addition, we support the literature data with results of recent in vitro experiments, demonstrating direct impacts of the gestagens levonorgestrel (LNG) and P4 on sexually differentiated gonads of larval Xenopus laevis. The results showed a higher susceptibility of female over male gonads to gestagenic ED. Only in female gonads LNG, but not P4, had direct inhibitory effects on gene expression of steroidogenic acute regulatory protein and P₄₅₀ side chain cleavage enzyme, whereas aromatase expression decreased in reaction to both gestagens. Surprisingly, beyond the expected ED effects of gestagens on reproductive physiology in amphibians, LNG drastically disrupted the thyroid system, which resembles direct effects on thyroid glands and pituitary along the pituitary-thyroid axis disturbing metamorphic development. In amphibians, environmental gestagens not only affect the reproductive system but at least LNG can impact also development by disruption of the thyroid system.

Hormonally active agents in the environment: a state-of-the-art review

After the Second World War, infatuation with modern products has exponentially widened the spectrum of chemicals used. Some of them are capable of hijacking the endocrine system by blocking or imitating a hormone and are referred to as hormonally active chemicals or endocrine disruptors. These are chemicals that the body was not designed for evolutionarily and they are present in every matrix of the environment. We are living in a chemical world where the exposures are ubiquitous and take place in combinations that can interact with the endocrine system and some other metabolic activities in unexpected ways. The complexity of interaction of these compounds can be understood by the fact that they interfere with gene expression at extremely low levels, consequently harming an individual life form, its offspring or population. As the endocrine system plays a critical role in many biological or physiological functions, by interfering body's endocrine system, endocrine disrupting compounds (EDCs) have various adverse effects on human health, starting from birth defects to developmental disorders, deadly deseases like cancer and even immunological disorders. Most of these compounds have not been tested yet for safety and their effects cannot be assessed by the available techniques. The establishment of proper exposure measurement techniques and integrating correlation is yet to be achieved to completely understand the impacts at various levels of the endocrine axis.

Effects of Endocrine-disrupting Chemicals on Female Reproductive Health

Endocrine-disrupting chemicals (EDCs) are increasingly prevalent in the environment and the evidence demonstrates that they affect reproductive health, has been accumulating for the last few decades. In this review of recent literature, we present evidence of the effects of estrogen-mimicking EDCs on female reproductive health especially the ovaries and uteri. As representative EDCs, data from studies with a pharmaceutical estrogen, diethylstilbestrol (DES), an organochlorine pesticide methoxychlor (MXC), a phytoestrogen (genistein), and a chemical used in plastics, bisphenol a (BPA) have been presented. We also discuss the effects of a commonly found plasticizer in the environment, a phthalate (DEHP), even though it is not a typical estrogenic EDC. Collectively, these studies show that exposures during fetal and neonatal periods cause developmental reprogramming leading to adult reproductive disease. Puberty, estrous cyclicity, ovarian follicular development, and uterine functions are all affected by exposure to these EDCs. Evidence that epigenetic modifications are involved in the progression to adult disease is also presented.

In vitro steroid-induced meiosis in Rhinella arenarum oocytes: role of pre-MPF activation

In this work we showed the relationship between seasonal periods and the response of R. arenarum follicles and oocytes to different steroids. Using in vitro germinal vesicle breakdown (GVBD) assays, we demonstrated that P4 is the main steroid capable of inducing maturation in R. arenarum oocytes and follicles. In the second part of this work we showed that androgens can activate pre-maturation promoting factors (pre-MPFs) such as P4, by cytoplasm microinjection experiments. The results indicated that the steroids assayed induced oocyte and follicle maturation in a dose- and time-dependent manner. In oocytes, P4 was the most efficient steroid as a maturation inducer (EC50 of the reproductive period, 6 nM, EC50 of the non-reproductive period ≅ 30 nM). Androgens (DHEA, dehydroepiandrosterone; T, testosterone; and AD, androstenedione) were less efficient maturation inducers than P4 (EC50 reproductive period ≅ 50, 120 and 600 nM respectively). Similar results were obtained with intact follicles in both seasonal periods. Although the response of follicles to the different androgens was variable, in no case was it above the above the response induced by P4. Independently of the season, oocytes and follicles incubated in P4, P5 and T underwent GVBD after 6–10 h while oocytes and follicles incubated in DHEA and AD matured more slowly. Furthermore, we demonstrated that microinjection of mature cytoplasm from androgen-treated oocytes is sufficient to promote GVBD in immature recipient oocytes (DHEA, 57 ± 12%; AD, 60 ± 8%; T, 56 ± 13%). Thus, androgens such as DHEA, T and AD are as competent as P4 to activate pre-MPF.

Amphibian (Euphlyctis cyanophlyctis) in vitro ovarian culture system to assess impact of aquatic agrochemical contaminants on female reproduction

The present study is an attempt to screen impacts of aquatic agrochemical contaminants (acephate, atrazine and cypermethrin) on development and growth of follicles, in in vitro-cultured ovarian fragments of frog (Euphlyctis cyanophlyctis). Ovarian lobes removed surgically from gravid females were cut into small pieces and cultured in vitro in presence of graded (0.01 or 0.1 μg/ml of culture medium) concentrations of test chemicals or estradiol-17β (positive controls) or culture medium alone (controls) in quadruplicate sets at 23 ± 1 °C temperature for 20 days in a humidified sterile chamber. On 21st day, they were fixed in Bouin's fluid and used for differential follicle counting (n = 3 sets) and histology (n = 1 set). In vitro exposure of ovarian fragments to test chemicals caused a decline in previtellogenic follicles, maintenance of large yolky follicles, incorporation of brown granules into early vitellogenic follicles and decrease in follicular atresia compared to corresponding controls. These results suggest that ovarian follicles are greatly sensitive to chemical exposure during their transition from previtellogenic to vitellogenic growth phase and in vitro ovarian culture system may be used as a tool to assess the effects of aquatic agrochemical contaminants on ovarian function.

Development of a New Decision Tree to Rapidly Screen Chemical Estrogenic Activities of Xenopus laevis

During the last past decades, there is an increasing number of studies about estrogenic activities of the environmental pollutants on amphibians and many determination methods have been proposed. However, these determination methods are time-consuming and expensive, and a rapid and simple method to screen and test the chemicals for estrogenic activities to amphibians is therefore imperative. Herein is proposed a new decision tree formulated not only with physicochemical parameters but also a biological parameter that was successfully used to screen estrogenic activities of the chemicals on amphibians. The biological parameter, CDOCKER interaction energy (Ebinding ) between chemicals and the target proteins was calculated based on the method of molecular docking, and it was used to revise the decision tree formulated by Hong only with physicochemical parameters for screening estrogenic activity of chemicals in rat. According to the correlation between Ebinding of rat and Xenopus laevis, a new decision tree for estrogenic activities in Xenopus laevis is finally proposed. Then it was validated by using the randomly 8 chemicals which can be frequently exposed to Xenopus laevis, and the agreement between the results from the new decision tree and the ones from experiments is generally satisfactory. Consequently, the new decision tree can be used to screen the estrogenic activities of the chemicals, and combinational use of the Ebinding and classical physicochemical parameters can greatly improves Hong's decision tree.

Transcriptional signature of progesterone in the fathead minnow ovary (Pimephales promelas)

A growing number of studies have examined transcriptional responses to sex steroids along the hypothalamic-pituitary-gonadal axis in teleost fishes. However, data are lacking on the molecular cascades that underlie progesterone signaling. The objective of this study was to characterize the transcriptional response in the ovary of fathead minnows (Pimephales promelas) in response to progesterone (P4). Fathead minnow ovaries were exposed in vitro to 500 ng P4/L. Germinal vesicle migration and breakdown (GVBD) was observed and microarrays were used to identify gene cascades affected by P4. Microarray analysis identified 1702 differentially expressed transcripts after P4 treatment. Functional enrichment analysis revealed that transcripts involved in the molecular functions of protein serine/threonine kinase activity, ATP binding, and activity of calcium channels were increased after P4 treatment. There was an overwhelming decrease in levels of transcripts of genes that are structural constituents of ribosomes with P4 treatment. There was also evidence for gene expression changes in steroid and maturation-related transcripts. Pathway analyses identified cell cycle regulation, insulin action, hedgehog, and B cell activation as pathways containing an over-representation of highly regulated transcripts. Significant regulatory sub-networks of P4-mediated transcripts included genes regulated by tumor protein p53 and E2F transcription factor 1. These data provide novel insight into the molecular signaling cascades that underlie P4-signaling in the ovary and identify genes and processes that may indicate premature GVBD due to environmental pollutants that mimic progestins.

In vitro induction/inhibition of germinal vesicle breakdown (GVBD) in frog (Euphlyctis cyanophlyctis) oocytes by endocrine active compounds

Oocyte maturation is transformation of oocytes into a fertilizable egg. This study examined the effects of four classes of chemicals: 1) acephate (organophosphate); 2) atrazine (herbicide); 3) cypermethrin and fenvalerate (synthetic pyrethroids); and 4) carbaryl (carbamate) on in vitro germinal vesicle breakdown (GVBD) of Euphlyctis cyanophlyctis oocytes. Follicles were isolated and defolliculated from surgically removed ovaries of E. cyanophlyctis and exposed to either progesterone (1 μM/mL) or graded concentrations (1, 5, 10, 15, and 20 μg/mL) of test chemicals. GVBD was evident by the presence of a white spot in the animal pole as well as the absence of germinal vesicles in sectioned heat-fixed oocytes. Percent GVBD was scored every 4 hours until 24 hours. Progesterone induced 77-84% GVBD, compared to 29-33% in controls, at 24 hours. Acephate induced 46-67% GVBD, whereas atrazine elicited 58-77% of GVBD. In cypermethrin or carbaryl- or fenvalerate-exposed oocytes, GVBD was limited to 22-28, 17-29 and 18-24%, respectively. The study infers that some chemical contaminants in the aquatic system may interfere with GVBD in amphibians. Because oocyte maturation is a prerequisite for the production of fertilizable eggs, any alteration in this process potentially impairs the fecundity of females.

Endocrine-disrupting chemicals in ovarian function: effects on steroidogenesis, metabolism and nuclear receptor signaling

Endocrine-disrupting chemicals (EDCs) are exogenous agents with the ability to interfere with processes regulated by endogenous hormones. One such process is female reproductive function. The major reproductive organ in the female is the ovary. Disruptions in ovarian processes by EDCs can lead to adverse outcomes such as anovulation, infertility, estrogen deficiency, and premature ovarian failure among others. This review summarizes the effects of EDCs on ovarian function by describing how they interfere with hormone signaling via two mechanisms: altering the availability of ovarian hormones, and altering binding and activity of the hormone at the receptor level. Among the chemicals covered are pesticides (e.g. dichlorodiphenyltrichloroethane and methoxychlor), plasticizers (e.g. bisphenol A and phthalates), dioxins, polychlorinated biphenyls, and polycyclic aromatic hydrocarbons (e.g. benzo[a]pyrene).

Aromatase expression in Xenopus oocytes: a three cell-type model for the ovarian estradiol synthesis

In contrast to the classical model describing the synthesis of androgens and estrogens as restricted to somatic cells, a previous study demonstrated that Xenopus laevis oocytes participate in androgen synthesis. The objective of our study was to determine whether Xenopus oocytes are also involved in estrogen synthesis. More precisely, we analyzed aromatase expression by in situ hybridization and RT-QPCR and measured aromatase activity. Aromatase, the enzyme responsible for estrogen synthesis, appears to be expressed and active not only in the follicular cells but also in the vitellogenic oocytes. During late oogenesis, aromatase oocyte expression and activity decreased concomitantly with the trend observed in surrounding follicular layers. In order to investigate the role of estradiol-17β (E(2)), we studied its effect on oocyte meiotic resumption. It appears that, as in Rana pipiens, E(2) inhibited the follicle-enclosed maturation of Xenopus oocytes, likely through inhibition of LH-induced maturation-inducing steroid synthesis. In addition, E(2) exerted a slight enhancing action on denuded oocyte maturation whose biological significance remains unclear. Together, our results demonstrate that Xenopus oocyte significantly participates in ovarian E(2) synthesis and this may be a common feature of vitellogenic vertebrates.

In vivo induction of oocyte maturation and ovulation in zebrafish

The maturation of fish oocytes is a well-characterized system induced by progestins via non-genomic actions. In a previous study, we demonstrated that diethylstilbestrol (DES), a non-steroidal estrogen, induces fish oocyte maturation via the membrane progestin receptor (mPR). Here, we attempted to evaluate the effect of DES as an environmental endocrine disrupting chemical (EDC) upon fish oocyte maturation using live zebrafish. DES triggered oocyte maturation within several hours in vivo when administrated directly into the surrounding water. The natural teleost maturation-inducing hormone, 17alpha, 20beta-dihydroxy-4-pregnen-3-one (17,20beta-DHP) also induced oocyte maturation in vivo. Steroids such as testosterone, progesterone or 17alpha-hydroxyprogesterone were also effective in vivo. Further studies indicated that externally applied 17,20beta-DHP even induced ovulation. In contrast to 17,20beta -DHP, DES induced maturation but not ovulation. Theoretically this assay system provides a means to distinguish pathways involved in the induction of ovulation, which are known to be induced by genomic actions from the pathway normally involved in the induction of oocyte maturation, a typical non-genomic action-dependent pathway. In summary, we have demonstrated the effect of EDCs on fish oocyte maturation in vivo. To address the effects, we have explored a conceptually new approach to distinguish between the genomic and non-genomic actions induced by steroids. The assay can be applied to screens of progestin-like effects upon oocyte maturation and ovulation for small molecules of pharmacological agents or EDCs.

Embryos as targets of endocrine disrupting contaminants in wildlife

Environmental contaminants are now a ubiquitous part of the ecological landscape, and a growing literature describes the ability of many of these chemicals to alter the developmental trajectory of the embryo. Because many environmental pollutants readily bioaccumulate in lipid rich tissues, wildlife can attain considerable body burdens. Embryos are often exposed to these pollutants through maternal transfer, and a growing number of studies report long-term or permanent developmental consequences. Many biological mechanisms are reportedly affected by environmental contaminants in the developing embryo and fetus, including neurodevelopment, steroidogenesis, gonadal differentiation, and liver function. Embryos are not exposed to one chemical at a time, but are chronically exposed to many chemicals simultaneously. Mixture studies show that for some developmental disorders, mixtures of chemicals cause a more deleterious response than would be predicted from their individual toxicities. Synergistic responses to low dose mixtures make it difficult to estimate developmental outcomes, and as such, traditional toxicity testing often results in an underestimate of exposure risks. In addition, the knowledge that biological systems do not necessarily respond in a dose-dependent fashion, and that very low doses of a chemical can prove more harmful than higher doses, has created a paradigm shift in studies of environmental contaminant-induced dysfunction. Although laboratory studies are critical for providing dose-response relationships and determining specific mechanisms involved in disease etiology, wildlife sentinels more accurately reflect the genetic diversity of real world exposure conditions, and continue to alert scientists and health professionals alike of the consequences of developmental exposures to environmental pollutants.

Epigenetic effects of endocrine-disrupting chemicals on female reproduction: an ovarian perspective

The link between in utero and neonatal exposure to environmental toxicants, such as endocrine-disrupting chemicals (EDCs) and adult female reproductive disorders is well established in both epidemiological and animal studies. Recent studies examining the epigenetic mechanisms involved in mediating the effects of EDCs on female reproduction are gathering momentum. In this review, we describe the developmental processes that are susceptible to EDC exposures in female reproductive system, with a special emphasis on the ovary. We discuss studies with select EDCs that have been shown to have physiological and correlated epigenetic effects in the ovary, neuroendocrine system, and uterus. Importantly, EDCs that can directly target the ovary can alter epigenetic mechanisms in the oocyte, leading to transgenerational epigenetic effects. The potential mechanisms involved in such effects are also discussed.

Developmental disruptions induced by insect growth regulator (Novaluron) in Bufo melanostictus tadpoles

Novaluron is an insect growth regulator (IGR) used against fruit-borers and domestic pests. In this study, effects of different concentrations (0.5, 0.75, 1.0, 1.5 microg l(-1)) of novaluron on the tail regeneration, limb development and metamorphosis were examined in Bufo melanostictus. Thyroxine (1.0, 2.0, 3.5, 5.0 microg l(-1)), which promotes amphibian development/metamorphosis, and vitamin A (5, 20, 40, 60 IU l(-1)), which disrupts development and induce polymelia, were used for comparison. Bufo melanostictus tadpoles were raised in the laboratory from eggs collected around Dharwad in August 2007. The tail of tadpoles was amputated at limb-bud stage under ether anesthesia. Ten tadpoles were exposed to each concentration of chemicals in triplicate for 5 days and then reared in conditioned water. Tadpoles were fed on boiled spinach. In controls (tadpoles exposed to ringer solution), the tail regenerated on the 5th day, hind-limbs and fore-limbs appeared on days 15 and 24, respectively; metamorphosis was complete on the 30th day. In 1 and 2 microg l(-1) thyroxine exposed tadpoles, regeneration of tail, development of limbs and metamorphosis were preponed to the 4th, 7th, 11th and 23rd day respectively. Exposure to vitamin A resulted in the regeneration of laterally drooped tails, delay in limb development and arrest of metamorphosis in all the concentrations used. Novaluron at lower dose (0.5 microg l(-1)) was not effective, but at 0.75 microg and higher concentrations it elicited results comparable to those of vitamin A. The study indicates that novaluron interferes with amphibian development if found as contaminant in the water bodies where amphibians live and reproduce.

Detection of potential (anti)progestagenic endocrine disruptors using a recombinant human progesterone receptor binding and transactivation assay

The present work describes the identification of (anti)progestin endocrine disrupting chemicals (EDC) using a two step screening system. In the first step a competitive binding assay was developed using recombinant human progesterone receptor (hPR). The tested chemicals were of various classes like insecticides, their metabolites, industrial chemicals and waste water treatment plant (WWTP) effluents. All the tested chemicals demonstrated a high affinity binding for hPR. The average IC50 values of the test chemicals were within the range of 1-25microM. In the second step of screening, a mammalian cell-based hPR transactivation assay was developed where HEK 293 cells were co-transfected with hPR and luciferase reporter gene under the control of progesterone-response element. Stimulation of the cells with progesterone resulted in about 25-fold up regulation of luciferase activity, with EC50 value of 4nM. Potent anti-progesterone, RU486, significantly inhibited progesterone-induced transactivation and non-progestagenic steroids failed to transactivate hPR till 1microM concentrations. The chemicals showing high binding affinities in competitive binding assays were then tested in transactivation assay and all of them were found to be anti-progestative except WWTP effluents. Transactivation assays using extracted water samples from five different WWTP effluents showed that it was rich in progestative compounds. The levels of induction caused by these effluents were in the range of 15-25% of induction by progesterone and they represented about 6ng/l equivalent progesterone activities. In conclusion, we demonstrated that this two step assay provides an efficient screening tool for the detection of (anti)progestative EDC in various samples.

Effects of BPA and DES on longchin goby (Chasmichthys dolichognathus) in vitro during the oocyte maturation

This study was performed in order to assess whether bisphenol (BPA) and diethylstilbestrol (DES) had agonistic or antagonistic effects on oocyte maturation using marine fish. We tested the effects of these chemicals on in vitro maturation, germinal vesicle breakdown (GVBD), assay using oocytes from the longchin goby, Chasmichthys dolichognathus. During the maturation process, low concentrations of BPA and DES triggered GVBD depending on the stage of oocyte development; BPA at 0.044 nM and DES at 0.037, 0.37, and 3.73 nM induced GVBD in 0.82-0.88 mm diameter oocytes (germinal vesicle located near the center of oocytes). In 0.76-0.80 mm diameter oocytes (fully vitellogenic oocytes), BPA induced GVBD at relatively higher concentrations (4.38, 43.8, and 438 nM). In 0.86-0.90 mm diameter oocytes, BPA and DES had no observable effect on GVBD at the concentrations tested. Oocytes with diameters between 0.82 and 0.88 mm appeared to be more sensitive to these chemicals. Moreover, our results showed that BPA and DES did not inhibit GVBD.

Interactions of diethylstilbestrol (DES) and DES analogs with membrane progestin receptor-alpha and the correlation with their nongenomic progestin activities

Progestin induction of oocyte maturation (OM) in fish is a useful model for investigating endocrine disruption of nongenomic steroid actions. Although diethylstilbestrol (DES) analogs have been shown to mimic the actions of progestins to induce meiotic maturation of goldfish and zebrafish oocytes, their molecular mechanisms of action remain unclear. The ability of these endocrine-disrupting chemicals (EDCs) to interact with the progestin receptor mediating OM was investigated in receptor binding assays using plasma membranes from goldfish ovaries and breast cancer cells transfected with goldfish membrane progestin receptor (mPR)-alpha. Membranes prepared from both ovaries and mPRalpha-transfected cells showed high-affinity, saturable, displaceable, single binding sites specific for the goldfish maturation-inducing steroid, 17alpha,20beta-dihydroxy-4-pregnen-3-one (17,20beta-DHP). DES and DES analogs (dipropionate-DES and hexestrol), which induce OM in goldfish, bound to the receptor and caused concentration-dependent displacement of [3H]-17,20beta-DHP, whereas dimethyl ether-DES had no affinity for the receptor. Scatchard plot analysis of specific 17,20beta-DHP binding in the presence of different amounts of DES showed that DES binding is of the noncompetitive type. The activities of DES and DES analogs to induce meiotic maturation of goldfish oocytes were examined in an in vitro bioassay. Whereas a concentration-dependent induction of OM was observed in response to DES, dipropionate-DES, and hexestrol, dimethyl ether-DES did not show any OM-inducing activity. The close correspondence between binding of DES and its analogs to the mPRalpha protein and their OM-inducing activities suggests a mechanism of endocrine disruption mediated by binding to mPRalpha resulting in its activation, thereby mimicking the nongenomic action of the progestin 17,20beta-DHP.

Sexual dimorphic responses in wildlife exposed to endocrine disrupting chemicals

Understanding the gender similarities and differences in how organisms respond following exposure to environmental chemicals is important if we are to determine the relative risk of these agents to wildlife and human populations. In this paper, we have chosen to focus on the sex determination and differentiation of fishes, amphibians, and reptiles, because of their close association with the environment and the number of environmental factors (e.g., temperature and endocrine disrupting chemicals) that are known to affect these phenomena in these taxa. We have discussed examples of gender differences in response to exposure to endocrine disrupting chemicals and found gender similarities about as often as we found differences. We found that most studies examined either one sex exclusively, or the experimental design did not include examining the effect of sex as a variable. Given the central role of sex steroid hormones in the sex determination and sexual differentiation of fishes, amphibians, and reptiles, we recommend that future research purposefully include sex as a factor, so that risk assessment by government agencies can address the probable gender differences in effects from exposure to chemicals in the environment.

The hormonal herbicide, 2,4-dichlorophenoxyacetic acid, inhibits Xenopus oocyte maturation by targeting translational and post-translational mechanisms

The widely used hormonal herbicide, 2,4-dichlorophenoxyacetic acid, blocks meiotic maturation in vitro and is thus a potential environmental endocrine disruptor with early reproductive effects. To test whether maturation inhibition was dependent on protein kinase A, an endogenous maturation inhibitor, oocytes were microinjected with PKI, a specific PKA inhibitor, and exposed to 2,4-D. Oocytes failed to mature, suggesting that 2,4-D is not dependent on PKA activity and likely acts on a downstream target, such as Mos. De novo synthesis of Mos, which is triggered by mRNA poly(A) elongation, was examined. Oocytes were microinjected with radiolabelled in vitro transcripts of Mos RNA and exposed to progesterone and 2,4-D. RNA analysis showed progesterone-induced polyadenylation as expected but none with 2,4-D. 2,4-D-activated MAPK was determined to be cytoplasmic in localization studies but poorly induced Rsk2 phosphorylation and activation. In addition to inhibition of the G2/M transition, 2,4-D caused abrupt reduction of H1 kinase activity in MII phase oocytes. Attempts to rescue maturation in oocytes transiently exposed to 2,4-D failed, suggesting that 2,4-D induces irreversible dysfunction of the meiotic signaling mechanism.

Pesticide mixtures, endocrine disruption, and amphibian declines: are we underestimating the impact?

Amphibian populations are declining globally at an alarming rate. Pesticides are among a number of proposed causes for these declines. Although a sizable database examining effects of pesticides on amphibians exists, the vast majority of these studies focus on toxicological effects (lethality, external malformations, etc.) at relatively high doses (parts per million). Very few studies focus on effects such as endocrine disruption at low concentrations. Further, most studies examine exposures to single chemicals only. The present study examined nine pesticides (four herbicides, two fungicides, and three insecticides) used on cornfields in the midwestern United States. Effects of each pesticide alone (0.1 ppb) or in combination were examined. In addition, we also examined atrazine and S-metolachlor combined (0.1 or 10 ppb each) and the commercial formulation Bicep II Magnum, which contains both of these herbicides. These two pesticides were examined in combination because they are persistent throughout the year in the wild. We examined larval growth and development, sex differentiation, and immune function in leopard frogs (Rana pipiens). In a follow-up study, we also examined the effects of the nine-compound mixture on plasma corticosterone levels in male African clawed frogs (Xenopus laevis). Although some of the pesticides individually inhibited larval growth and development, the pesticide mixtures had much greater effects. Larval growth and development were retarded, but most significantly, pesticide mixtures negated or reversed the typically positive correlation between time to metamorphosis and size at metamorphosis observed in controls: exposed larvae that took longer to metamorphose were smaller than their counterparts that metamorphosed earlier. The nine-pesticide mixture also induced damage to the thymus, resulting in immunosuppression and contraction of flavobacterial meningitis. The study in X. laevis revealed that these adverse effects may be due to an increase in plasma levels of the stress hormone corticosterone. Although it cannot be determined whether all the pesticides in the mixture contribute to these adverse effects or whether some pesticides are effectors, some are enhancers, and some are neutral, the present study revealed that estimating ecological risk and the impact of pesticides on amphibians using studies that examine only single pesticides at high concentrations may lead to gross underestimations of the role of pesticides in amphibian declines.

Endocrine disrupting contaminants--beyond the dogma

Descriptions of endocrine disruption have largely been associated with wildlife and driven by observations documenting estrogenic, androgenic, antiandrogenic, and antithyroid actions. These actions, in response to exposure to ecologically relevant concentrations of various environmental contaminants, have now been established in numerous vertebrate species. However, many potential mechanisms and endocrine actions have not been studied. For example, the DDT [1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane] metabolite, p,p -DDE [1,1-dichloro-2,2-bis(p-chlorophenyl)ethylene] is known to disrupt prostaglandin synthesis in the uterus of birds, providing part of the explanation for DDT-induced egg shell thinning. Few studies have examined prostaglandin synthesis as a target for endocrine disruption, yet these hormones are active in reproduction, immune responses, and cardiovascular physiology. Future studies must broaden the basic science approach to endocrine disruption, thereby expanding the mechanisms and endocrine end points examined. This goal should be accomplished even if the primary influence and funding continue to emphasize a narrower approach based on regulatory needs. Without this broader approach, research into endocrine disruption will become dominated by a narrow dogma, focusing on a few end points and mechanisms.

Induction and inhibition of oocyte maturation by EDCs in zebrafish

BACKGROUND

Oocyte maturation in lower vertebrates is triggered by maturation-inducing hormone (MIH), which acts on unidentified receptors on the oocyte surface and induces the activation of maturation-promoting factor (MPF) in the oocyte cytoplasm. We previously described the induction of oocyte maturation in fish by an endocrine-disrupting chemical (EDC), diethylstilbestrol (DES), a nonsteroidal estrogen.

METHODS

In this study, stimulatory and inhibitory effects of EDCs and natural steroids on oocyte maturation were examined in zebrafish. For effective agents, some details about the mechanism in induction or inhibition of maturation were examined. Possible groups of DES interacting with the MIH receptor are discussed based on relative potency of steroids to induce maturation.

RESULTS

Among agents tested, tamoxifen (TAM) and its metabolite 4-hydroxytamoxifen (4-OHT) showed stimulatory activity similar to DES. The time courses of the change in germinal vesicle breakdown and an intracellular molecular event (the synthesis of cyclin B) induced by TAM were indistinguishable from those induced by MIH. In contrast, pentachlorophenol (PCP) had a potent inhibitory effect on MIH-induced oocyte maturation. PCP inhibited not only MIH-induced maturation but also DES- and TAM-induced maturation. Methoxychlor also inhibited maturation when oocytes were pre-treated with this agent.

CONCLUSION

These results suggest that EDCs act as agonists or antagonists in the induction of oocyte maturation in fish.

Inhibition of progesterone receptor activity in recombinant yeast by soot from fossil fuel combustion emissions and air particulate materials

Numerous environmental pollutants have been detected for estrogenic activity by interacting with the estrogen receptor, but little information is available about their interactions with the progesterone receptor. In this study, emission samples generated by fossil fuel combustion (FFC) and air particulate material (APM) collected from an urban location near a traffic line in a big city of China were evaluated to interact with the human progesterone receptor (hPR) signaling pathway by examining their ability to interact with the activity of hPR expressed in yeast. The results showed that the soot of a petroleum-fired vehicle possessed the most potent anti-progesteronic activity, that of coal-fired stove and diesel fired agrimotor emissions took the second place, and soot samples of coal-fired heating work and electric power station had lesser progesterone inhibition activity. The anti-progesteronic activity of APM was between that of soot from petroleum-fired vehicle and soot from coal-fired establishments and diesel fired agrimotor. Since there was no other large pollution source near the APM sampling sites, the endocrine disrupters were most likely from vehicle emissions, tire attrition and house heating sources. The correlation analysis showed that a strong relationship existed between estrogenic activity and anti-progesteronic activity in emissions of fossil fuel combustion. The discoveries that some environmental pollutants with estrogenic activity can also inhibit hPR activity indicate that further studies are required to investigate potential mechanisms for the reported estrogenic activities of these pollutants.

Disruption of rapid, nongenomic steroid actions by environmental chemicals: interference with progestin stimulation of sperm motility in Atlantic croaker

Several nongenomic steroid actions, like genomic ones, can be disrupted by estrogenic xenobiotics (xenoestrogens), but the extent and sensitivity of this alternative mechanism of steroid action to chemical interference remain unclear. The effects of environmentally realistic concentrations of a broad range of organic contaminants on the nongenomic action of a progestin (17,20beta,21-trihydroxy-4-pregnen-3-one or 20beta-S) to upregulate Atlantic croaker sperm motility were examined in an in vitro bioassay. Pretreatment of sperm for 10 min in vitro with estrogenic compounds (estradiol-17beta, o,p'-DDT derivatives, zearalenone, bisphenol A, 2',3',4',5'-PCB-4-OH, kepone, chlordane, methoxyclor) and nonestrogenic organic compounds (p,p'-DDT derivatives, atrazine, Aroclor 1254, naphthalene, benzene) at concentrations ranging from 0.01 to 10 microM did not decrease the percent of motile sperm, but all of them partially or completely blocked the response to 20beta-S. Most of the compounds impaired this endocrine mechanism at a concentration of 0.1 microM (approximately 30-40ppb), whereas o,p'-DDT and atrazine were effective at lower concentrations. The antagonistic actions of o,p'-DDT were partially reversed with 10-fold higher concentrations of 20beta-S, which is consistent with a hormone receptor-mediated mechanism of DDT action. The finding that low concentrations of a wide range of organic environmental contaminants can interfere with a rapid, nongenomic steroid action suggests that this mechanism of endocrine disturbance is of toxicological importance.

Agonistic effect of imidazole and triazole fungicides on in vitro oocyte maturation in rainbow trout (Oncorhynchus mykiss)

The purpose of the present study was to assess the in vitro effect of some imidazole (prochloraz, imazalil) and triazole (epoxiconazole) agricultural fungicides on gonadotropin-induced oocyte maturation in rainbow trout. Results show that prochloraz, epoxiconazole and imazalil strongly potentiated the induction of oocyte maturation by gonadotropin in a dose-dependent manner. Furthermore, 10(-5) M prochloraz and epoxiconazole alone induced oocyte maturation. The mRNA biosynthesis inhibitor, actinomycin d, completely inhibited oocyte maturation induced by fungicides, suggesting that the gonadotropin-like effect of these chemicals is at least dependent on de novo gene expression.

Effects of methoxychlor, dieldrin and lindane on sea urchin fertilization and early development

We have studied the effects of methoxychlor (MXC), dieldrin, and lindane on fertilization and early development of sea urchin egg. These organochlorine pesticides have often been found in polluted ground and water near agricultural sites, and have therefore been detected from time to time in the food chain and in drinking water. They have been reported to alter various reproduction functions in various animals including marine populations. We observed that the rate of fertilization decreased when the sperm was incubated with dieldrin or lindane. Treatment of eggs with each pesticide did not prevent fertilization, but increased the rate in polyspermy, delayed or blocked the first mitotic divisions, and altered early embryonic development. Moreover, all pesticides could alter several intracellular biochemical pathways that control first mitotic divisions and early development, including intracellular calcium homeostasis, MPF (mitosis promoting factor) activity and formation of the bipolar mitotic spindle. We found that lindane was the most potent of the three pesticides to alter all biochemical events. All these effects were observed at relatively high concentrations. However, bio-accumulation in sediments and aquatic organisms have been reported. Sea urchin eggs may then be in contact with very high concentrations of these pesticides in areas where these pesticides are not handled or stocked properly, and then develop into abnormal embryos.

The development of an ELISA for chicken apolipoprotein II quantitation

Polyclonal antibodies raised against chicken apoll was characterised for its use in Western blotting and ELISA detection systems of apoll in chicken plasma. The antibody has a high avidity and specificity for apolipoprotein II (apoII). Western blots show that the antibody reacts with a single band at 15 kDa. The antibody was used for setting up both direct and indirect ELISA assays for apoll. The indirect ELISA has a broader detection range (10-1,600U/mL) than the direct ELISA (10-100U/mL). It was found that both ELISA systems discriminate very well between vitellogenic (laying hen) and non-vitellogenic (rooster) plasma. The in- direct ELISA, due to its broad detection range, can potentially be used for monitoring female reproductive cycles, accidental and environmental exposure of males to estrogen, and for apoII secretion by cultured hepatocytes and hepatomas.

Inhibition of gonadotropin-induced oviposition and ovarian steroidogenesis in the African clawed frog (Xenopus laevis) by the pesticide methoxychlor

Concern over the role of environmental toxicants in amphibian population declines has highlighted the need to develop more comprehensive ecotoxicological test methods for this at-risk group. With continued interest in environmental endocrine disrupters (EDs), and the paucity of data pertaining to endocrine disrupting effects in amphibia, such tests should incorporate reproductive and endocrine endpoints. We investigated the effects of in vivo exposure to the pesticide methoxychlor (MXC) on reproductive and endocrine function in adult female African clawed frogs, (Xenopus laevis). Frogs were exposed to MXC (0.5-500 microg/l) in tank water throughout a cycle of oogenesis stimulated by exogenous gonadotropins. Gonadotropin-induced oviposition was delayed, and reduced numbers of unfertilizable eggs of increased size were oviposited by frogs exposed to 500 microg/l MXC. Reduced egg output was mirrored by increased gonado-somatic index in MXC-treated frogs. Post-oviposition, plasma sex steroid profiles were altered in MXC-exposed frogs as estradiol/progesterone and estradiol/testosterone ratios were elevated. Ex vivo synthesis of progesterone by ovarian explants was significantly reduced for frogs exposed to MXC> or = 0.5 microg/l. Additionally, plasma vitellogenin concentrations were significantly depressed in frogs exposed to 500 microg/l MXC. These data indicate that reproductive and endocrine dysfunction can occur in adult amphibia exposed to high concentrations of an environmental toxin with endocrine disrupting activity. Such effects may be indicative of the potential for adverse effects on amphibian wildlife exposed to environmental EDs.

Amphibians as a model for the study of endocrine disruptors

Evidence shows that environmental compounds can interfere with the endocrine systems of wildlife and humans. The main sink of such substances, called endocrine disruptors (EDs), which are mainly of anthropogenic origin, is surface water; thus, aquatic vertebrates such as fishes and amphibians are most endangered. Despite numerous reports on EDs in fishes, information about EDs in amphibians is scarce, and this paucity of information is of particular concern in view of the worldwide decline of amphibians. EDs could contribute to changes of amphibian populations via adverse effects on reproduction and the thyroid system. In amphibians, EDs can affect reproduction by (anti)estrogenic and (anti)androgenic modes of action that produce severe effects including abnormal sexual differentiation. ED actions on the thyroid system cause acceleration or retardation of metamorphosis, which may also affect population levels. Our broad knowledge of amphibian biology and endocrinology indicates that amphibians are very suitable models for the study of EDs. In particular, effects of EDs on the thyroid system triggering metamorphosis can be determined easily and most sensitively in amphibians compared to other vertebrates. A new classification of EDs according to their biological modes of action is proposed because EDs have quite heterogeneous chemical structures, which do not allow prediction of their biological effects. Methods and strategies are proposed for identification and risk assessment of EDs, whether as pure test substances or as mixtures from environmental samples. Effects of EDs on the thyroid system of amphibians can be assessed by a single animal model (Xenopus laevis), whereas the various types of reproduction need comparative studies to investigate whether general endocrine principles do exist among several species of anurans and urodeles. Thus, at least one anuran and one urodelean model are needed to determine ED interference with reproduction.

Thymic atrophy induced by methoxychlor in rat pups

The effect of Methoxychlor (MXC) on the thymus was examined in rat pups that were delivered from dams receiving MXC at a dietary concentration of 0 or 1500 ppm for a period from pregnancy to lactation. The pups of both sexes were euthanized on postnatal days (PNDs) 7, 14, and 21. Histologically, the thymus showed marked depletion of cortical lymphocytes on PND 7 and also had an increase in lymphophagocytosis in the cortical area on PNDs 14 and 21. Morphometrical analysis disclosed that both cortex and medulla of the thymus from treated pups were reduced in size, but the reduction was more evident in the cortex. A significant increase in transferase-mediated dUTP nick end labeling-positive cells was detected in the cortex area, corresponding to the presence of lymphophagocytosis. Flow cytometric analysis revealed a significant decrease in the double positive (CD3(int)CD4(+)CD8(+)) immature cells on PND 21. These results have suggested that MXC may impair maturation of thymic lymphocytes in rat pups, which results in enhancement of apoptosis leading to thymic atrophy during the postnatal period.

Effect of endocrine disrupting chemicals on germinal vesicle breakdown in Xenopus in vitro

Currently, no standardized and well-validated alternative models exist for screening for progesterone-responsive endocrine disrupting chemicals (EDCs). Because of this, a rapid assay for evaluating progestin/antiprogestin activity using Xenopus oocyte germinal vesicle breakdown (GVBD) as a model was evaluated. Five compounds, including progesterone (P), ethinyl estradiol (EE), ethylene glycol monomethyl ether (EGME), cadmium (Cd), and boric acid (B) were used to validate the model on a preliminary basis. Each test material was tested for progestin/anti-progestin activity. The binding affinity of each test material to the oocyte plasma membrane receptor (OPMR) relative to progesterone was then determined. Results from the present studies suggested that both EE and EGME were capable of inhibiting GVBD in a concentration-dependent manner. Cd had a subtle inhibitory effect at high concentrations. B had no effect on GVBD even at concentrations of 100 mg B/L, and thus appears to have no EDC activity in this model. The binding capacity of the test substances to the OMPR relative to progesterone was low. Thus, the relative inhibitory potential of the test materials study was EE > EGME >> Cd > B. However, the relative binding affinity of the toxicants to the OMPR can be expressed as P >> EGME > EE > Cd > B. Although EE was more effective in inhibiting GVBD, EGME appeared to bind more tightly to the OMPR.

Aromatase activity in the ovary and brain of the eastern mosquitofish (Gambusia holbrooki) exposed to paper mill effluent

Studies have shown that female mosquitofish living downstream of a paper mill located on the Fenholloway River, Florida, have masculinized secondary sex characteristics, including altered anal fin development and reproductive behavior. Masculinization can be caused by exposure to androgens in the water or from an alteration in aromatase activity in the fish. We hypothesized that aromatase activity would be inhibited by a component(s) of the paper mill effluent. Aromatase inhibition could masculinize the hormonal profile and, subsequently, secondary sex characteristics of the exposed females. Therefore, we predicted that ovarian and brain aromatase activity would be lower in the female mosquitofish from the Fenholloway River compared with the reference site, the Econfina River. Adult females were collected and standard length, body mass, anal fin length, and segment number were measured. Ovarian and brain aromatase activity were determined using a tritiated water assay. Fenholloway females had masculinized anal fin development as indicated by an increase in the number of segments in the longest anal fin ray (p < 0.0001), yet the length of the ray did not differ between sites (p = 0.95). Fenholloway females exhibited higher ovarian (p = 0.0039) and brain (p = 0.0003) aromatase activity compared with reference site fish. These data do not support aromatase inhibition as the mechanism for masculinization, suggesting that the masculinization of the Fenholloway female mosquitofish is due to androgenic contaminants. Future studies should examine the relationship between aromatase enzyme activity and exposure to environmental androgens.

Octylphenol and UV-B radiation alter larval development and hypothalamic gene expression in the leopard frog (Rana pipiens)

We assessed octylphenol (OP), an estrogenic endocrine-disrupting chemical, and UV-B radiation, a known stressor in amphibian development, for their effects on hypothalamic gene expression and premetamorphic development in the leopard frog Rana pipiens. Newly hatched tadpoles were exposed for 10 days to OP alone at two different dose levels; to subambient UV-B radiation alone; and to two combinations of OP and UV-B. Control animals were exposed to ethanol vehicle (0.01%) exposure, a subset of tadpoles from each treatment group was raised to metamorphosis to assess differences in body weight and time required for hindlimb emergence. Tadpoles from one of the OP/UV-B combination groups had greater body weight and earlier hindlimb emergence (p < 0.05), but neither OP nor UV-B alone produced significant changes in body weight or hindlimb emergence, indicating a potential mechanism of interaction between OP and UV-B. We hypothesized that the developing hypothalamus might be a potential environmental sensor for neurotoxicologic studies because of its role in the endocrine control of metamorphosis. We used a differential display strategy to identify candidate genes differentially expressed in the hypothalamic region of the exposed tadpoles. Homology cloning was performed to obtain R. pipiens glutamate decarboxylases--GAD65 and GAD67, enzymes involved in the synthesis of the neurotransmitter gamma-aminobutyric acid (GABA). cDNA expression profiles revealed that OP and UV-B affected the levels of several candidate transcripts in tadpole (i.e., Nck, Ash, and phospholipase C gamma-binding protein 4 and brain angiogenesis inhibitor-3) and metamorph (i.e., GAD67, cytochrome C oxidase, and brain angiogenesis inhibitor-2 and -3) brains. This study represents a novel approach in toxicology that combines physiologic and molecular end points and indicates that levels of OP commonly found in the environment and subambient levels of UV-B alter the expression of important hypothalamic genes and disrupt tadpole growth patterns.

The diverse mechanism of action of dichlorodiphenyldichloroethylene (DDE) and methoxychlor in ovarian cells in vitro

Dichlorodiphenyldichloroethylene (DDE), the most stable metabolite of the organochlorine insecticide dichlorodiphenyltrichloroethane (DDT), and the DDT analog methoxychlor can have adverse effects on reproduction. These chemicals have been identified as having estrogenic activity. The aim of the current study was to examine the effects of dichlorodiphenyldichloroethylene (DDE), methoxychlor, and estradiol-17 beta on steroidogenesis and FSH responsiveness in ovarian cells in vitro. Experiments were performed on a primary culture of porcine granulosa cells and a culture of Chinese hamster ovary (CHO) cells, the latter stably transfected with the FSH receptor (CHO-FSH-R). DDE (10 microM) and estradiol-17 beta (0.1 microM) but not methoxychlor (10 microM), increased proliferation of the granulosa cells. DDE (100 and 10 microM, respectively) decreased FSH-stimulated cAMP synthesis in the granulosa and CHO-FSH-R cells. DDE also decreased progesterone synthesis in the granulosa cells. Methoxychlor (10 microM) inhibited progesterone synthesis in the granulosa cells, but it did not affect the generation of cAMP in either type of cells studied. However, methoxychlor inhibited estradiol-17 beta-stimulated progesterone synthesis in the granulosa cells. We conclude that DDE primarily inhibited the generation of cAMP, while methoxychlor suppressed progesterone synthesis through a mechanism distal to cAMP generation. The present results indicate that DDE and methoxychlor are not limited to a mimicking of the endocrine effects of estradiol-17 beta in cultured ovarian cells. Therefore, a non-estrogenic component of the endocrine disrupting activities of DDE and methoxychlor must be considered in evaluating their reproductive toxicity.