Developmental effects of endocrine-disrupting chemicals in wildlife and humans

Vitellogenin induction by xenobiotic estrogens in the red-eared turtle and African clawed frog

Many environmental pollutants have estrogenic activity in animals. Xenobiotic estrogens include many pesticides and industrial chemicals that biocumulate. The impact of these common pollutants on the reproductive success of wildlife may be considerable, particularly in threatened or endangered species. This research examined the use of plasma vitellogenin in males as a biomarker for estrogenic xenobiotics in reptiles and amphibians. Adult male turtles (Trachemys scripta) and frogs (Xenopus laevis) were given ip injections of estradiol-17 beta (E2), diethylstilbestrol (DES), or o,p'-DDT (1-chloro-2-[2,2,2-trichloro-1-(4-chlorophenyl)ethyl)benzene) daily for 7 days, and plasma was collected on day 14. The estrogenic activity of each compound was determined by measuring the induction of plasma vitellogenin. Vitellogenin was identified by precipitation, electrophoresis, Western blot, and enzyme-linked immunosorbant assay (ELISA). In both species, estradiol and DES treatments induced the most vitellogenin, whereas DDT treatments induced smaller amounts of vitellogenin in a dose-dependent fashion. These data indicate that induction of plasma vitellogenin in males may be a useful biomarker of xenobiotic estrogen activity in wild populations of reptiles and amphibians.

Defining the role of pollutants in the disruption of reproduction in wildlife

Although chemical exposure has been associated with reduced reproduction in certain North American fish, reptiles, and mammals, definitive cause-and-effect data are lacking in many instances. Because the increasing use and global transport of industrial chemicals pose significant risk to successful reproduction, methods should be developed that can define the geographic extent and magnitude of injury and risk to wildlife. Because industrial chemicals are articles of commerce, information about injury to wildlife has been contentious and too often ineffective in changing societal behavior. The following strategies are advocated for inferring causal relationships. First, a balanced and comprehensive assessment of the data is necessary to determine the geographic extent of exposure and reproductive effects associated with environmental pollution. Initial efforts to document reproductive injury should focus on specific ecosystems in which detrimental effects have been observed, but lack sufficient causal data. Model systems (including experimental mesocosms or field ecosystems) should be identified or designed that can adequately test multigenerational reproductive effects. Mechanistic data from supportive laboratory studies on reproductive toxicity, quantitative structure-activity relationships, and bioaccumulation can be used to predict effects of related pollutants and to determine risk. Such information is essential to prevent future injury to wildlife and to prioritize the numerous remediation decisions facing our society.

Green turtle fibropapillomatosis: challenges to assessing the role of environmental cofactors

Green turtle fibropapillomatosis (GTFP) is a growing threat to the survival of green turtle (Chelonia mydas) populations worldwide. Recent transmission studies point to an infectious etiology. Several field studies suggest that high GTFP prevalence is associated with marine habitats that have been impacted by agricultural, industrial, or urban development. Environmental contaminants could be involved in GTFP through several plausible mechanisms including cocarcinogenesis and contaminant-induced immune suppression. However, an association of contaminants with GTFP has not been established. A broader perspective is needed when studying infectious diseases such as GTFP in complex ecosystems. Alternative explanations for high GTFP prevalence in some near-shore habitats include the following: a) these habitats provide an optimum physical environment for survival and transmission of the infectious agent; b) these habitats attract a high density of susceptible turtles or harbor a higher density of potential vectors, facilitating transmission of the pathogen in a density-dependent fashion; and c) these habitats may contain other stressors that render turtles more susceptible to GTFP. Application of scientifically rigorous criteria in the epizootiology of GTFP in free-ranging populations remains a formidable challenge.

Tinkering with the tinkerer: pollution versus evolution

Pollutants can act as powerful selective forces by altering genetic variability, its intergenerational transfer, and the size, functional viability, adaptability, and survival of future generations. It is at the level of the cell and the individual that meiosis occurs, that genetic diversity is maintained, and behavior, reproduction, growth, and survival occur and are regulated. It is at this level that evolutionary processes occur and most pollutants exert their toxic effects. Chronic exposure to chemicals contributes to the cumulative stress on individuals and disrupts physiological processes and chemically mediated communication thereby threatening the diversity and long-term survival of sexually reproducing biota. Regional or global effects of pollution on the atmosphere, hydrosphere, and lithosphere have indirectly altered Earth's life-support systems, thereby modifying trace metal balance, reproduction, and incidence of UV-B-induced DNA damage in biota. By altering the competitive ability and survival of species, chemical pollutants potentially threaten evolutionary processes and the biodiversity and function of intercepting ecosystems.

Gonadal steroidogenesis in vitro from juvenile alligators obtained from contaminated or control lakes

The ubiquitous distribution of many contaminants and the nonlethal, multigenerational effects of such contaminants on reproductive, endocrine, and immune systems have led to concerns that wildlife worldwide are affected. Although the causal agents and effects are known for some species, the underlying physiological mechanisms associated with contaminant-induced reproductive modifications are still poorly understood and require extensive research. We describe a study examining the steroidogenic activity of gonads removed from juvenile alligators (Alligator mississippiensis) obtained from contaminated or control lakes in central Florida. Synthesis of estradiol-17 beta (E2) was significantly different when ovaries from the contaminated and control lakes were compared in vitro. Additionally, testes from males obtained from the contaminated lake. Lake Apopka, synthesized significantly higher concentrations of E2 when compared to testes obtained from control males. In contrast, testosterone (T) synthesis from all testes examined in this study displayed a normal pattern and produced concentrations greater than that observed from ovaries obtained from either lake. Interestingly, the pattern of gonadal steroidogenesis differs from previously reported plasma concentrations of these hormones obtained from the same individuals. We suggest that the differences between the in vivo and in vitro patterns are due to modifications in the hepatic degradation of plasma sex steroid hormones.

Possible mechanisms of action of environmental contaminants on St. Lawrence beluga whales (Delphinapterus leucas)

A small isolated population of beluga whales (Delphinapterus leucas) that are highly contaminated by pollutants, mostly of industrial origin, resides in the St. Lawrence estuary, Québec, Canada. Overhunting in the first half of the century was the probable cause for this population to dwindle from several thousand animals to the current estimate of 500. The failure of the population to recover might be due to contamination by organochlorine compounds, which are known to lead to reproductive failure and immunosuppression in domestic and laboratory animals and seals. Functional and morphological changes have been demonstrated in thyroid gland and adrenal cortex in many species exposed to organochlorinated compounds, including seals. Morphological lesions, although different, were also found in belugas. Functional evaluation of thyroid and adrenal glands of contaminated (St. Lawrence) versus much less contaminated (Arctic) belugas is currently under way. Necropsy of St. Lawrence belugas showed numerous severe and disseminated infections with rather mildly pathogenic bacteria, which suggests immunosuppression. Organochlorine compounds and other contaminants found in beluga whales cause immunosuppression in a variety of animal species including seals. Thirty-seven percent of all the tumors reported in cetaceans were observed in St. Lawrence beluga whales. This could be explained by two different mechanisms: high exposure to environmental carcinogens and suppression of immunosurveillance against tumors. Overall, St. Lawrence belugas might well represent the risk associated with long-term exposure to pollutants present in their environment and might be a good model to predict health problems that could emerge in highly exposed human populations over time.

Estrogenic pesticides: binding relative to estradiol in MCF-7 cells and effects of exposure during fetal life on subsequent territorial behaviour in male mice

Numerous chemicals released into the environment by man are able to disrupt the functioning of the endocrine system by binding to estrogen receptors in estrogen-responsive cells. The ability of o,p'-dichlorodiphenyl trichloroethane (DDT) and methoxychlor to compete with estradiol for binding to estrogen receptors in MCF-7 cells (relative binding affinity; RBA) was examined in both serum-free medium and 100% serum; this is referred to as a relative binding affinity-serum modified access (RBA-SMA) assay. RBA's ranged from 0.04% for o,p'-DDT (which showed enhanced access to cells in serum relative to serum-free medium) to 0.004% for methoxychlor (which did not show enhanced access in serum). Based on these findings, these pesticides, along with diethylstilbestrol (DES) as a positive control, were fed to pregnant mice from days 11-17 of pregnancy. When the male offspring were examined in adulthood for their rate of urine marking in a novel territory (territorial behaviour), the rate of urine marking increased dramatically with low doses of DES (relative to controls) and then decreased significantly at the highest dose administered prenatally. Relative binding in MCF-7 cells accurately predicted the doses of o,p'-DDT and methoxychlor that produced the same results, providing support for the hypothesis that effects on behaviour were mediated by binding to estrogen receptors in the developing brain.

Environmental and dietary estrogens and human health: is there a problem?

It has been hypothesized that organochlorine pesticides and other environmental and dietary estrogens may be associated with the increased incidence of breast cancer in women and decreased sperm concentrations and reproductive problems in men. However, elevation of organochlorine compounds such as dichlorodipehenyldichloroethylene (DDE) and polychlorinated biphenyls (PCBs) in breast cancer patients is not consistently observed. Reanalysis of the data showing that male sperm counts decreased by over 40% during 1940 to 1990 indicated that inadequate statistical methods were used and that the data did not support a significant decline in sperm count. Humans are exposed to both natural and industrial chemicals which exhibit estrogenic and antiestrogenic activities. For example, bioflavonoids, which are widely distributed in foods, and several industrial compounds, including organochlorine pesticides and various phenolic chemicals, exhibit estrogenic activity. Humans are also exposed to chemicals which inhibit estrogen-induced responses such as the aryl hydrocarbon receptor (AhR) agonist 2,3,7,8-tetrachlorodibenzo-p-dioxin and related chlorinated aromatics, polynuclear aromatic hydrocarbon combustion products, and indole-3-carbinol, which is found in cruciferous vegetables. Many of the weak estrogenic compounds, including bioflavonoids, are also antiestrogenic at some concentrations. A mass balance of dietary levels of industrial and natural estrogens, coupled with their estimated estrogenic potencies, indicates that the dietary contribution of estrogenic industrial compounds is 0.0000025% of the daily intake of estrogenic flavonoids in the diet.(ABSTRACT TRUNCATED AT 250 WORDS)

Formation of nuclear bodies in hepatocytes of estrogen-treated roosters

As a model for cellular growth and stimulation without accompanying proliferation, we have examined the induction and formation of nuclear bodies (NBs) in hepatocytes of estrogen-treated roosters. Four-week-old roosters were injected with a single intramuscular dose of estradiol and then killed at time points of 8 h, 48 h, and 4 wk post-injection. For immunofluorescence analyses, livers were excised and isolated hepatocyte nuclei were fixed and then labeled with antibody to the coiled body-specific protein, p80-coilin. In control animals (no estradiol) or in animals 8 h post-injection, each hepatocyte nucleus contained an average of 1.0 coiled body (CB), which appeared randomly distributed in the nucleoplasm. At 48 h post-injection, there were an average of 2.7 CBs/nucleus and many of these appeared to be in contact with the nucleolus. Pairs of CBs were also observed. By 4 wk post-injection an average of 1.5 CBs/nucleus were detected, with no apparent relationship to the nucleolus observed. By serial-section electron microscopy of intact livers, two different types of round NBs were observed, sometimes in close proximity to each other and to the expanded interchromatin granule region in maximally stimulated cells. One type of NB was a classical CB that averaged 0.35 microns in diameter and the other NB type was ring shaped, averaged 0.25 microns in diameter, was composed of a fibrous shell surrounding a hollow interior, and appeared as a simple NB when sectioned tangentially through its outer shell. Immunoelectron microscopy revealed that CBs were the only class of NBs that contained p80-coilin. From these data, we conclude that CBs proliferate in response to estrogen stimulation, possibly arising from the nucleolar surface and then increasing in number by replicative division.

Arctic air pollution and human health: what effects should be expected?

Persistent contaminants such as heavy metals and organochlorine compounds are transported from distant sources to the Arctic by oceanic and atmospheric currents. Natives inhabiting the Arctic can be exposed, because they exist at the highest trophic level of the arctic aquatic food chain, along which biomagnification of contaminants occurs. We reviewed the data available on heavy metal and organochlorine body burden in natives from different regions of Nunavik (northern Québec) and assessed the potential risk of health effects. In addition, we investigated the relationship between each contaminant plasma level and omega-3 fatty acid content of plasma phospholipid, a surrogate measure for aquatic food consumption. Cadmium exposure appears to be unrelated to the consumption of species from the aquatic food chain (r = 0.0004; P = 0.99), whereas PCBs and mercury were (r = 0.49 and 0.52, respectively; P < or = 0.0001). Mean blood mercury levels measured in northern Québec natives were below those associated with significant neurological disorders. Typical daily intakes of dioxin-like compounds, PCBs, DDE, and dieldrin were estimated from the mean concentration in milk fat and pharmacokinetic models. The calculated PCB intake (0.3 microgram/kg/day) exceeds the acceptable daily intake, with effects on reproduction and development being the most relevant to assess in future epidemiological studies.

Preliminary results of fasting on the kinetics of organochlorines in polar bears (Ursus maritimus)

We determined concentrations of polychlorinated biphenyls (PCBs), chlordanes (CHLORs), chlorobenzenes (CBzs), hexachlorocyclohexanes (HCHs), and dichlorodiphenyltrichloroethane (DDT) and its metabolites (DDD and DDE) in the tissues of individual polar bears (Ursus maritimus) before and after a lengthy period of fasting. Polar bears are an ideal model for such studies. They undergo one of the most extreme fasts known for any mammal and are located at the top of a long food chain, thus biomagnification of organochlorines (OC) is significant. Adipose tissue and milk were collected from different reproductive classes of adult females (solitary/pregnant, with cubs-of-the-year, with yearling cubs) and were analyzed for organochlorine content. As the fasting period progressed and adipose reserves decreased, concentrations of some organochlorines in the adipose tissue and milk increased on a lipid weight basis. The transfer of contaminants from mothers to offspring thus also increases with duration of the fasting period. This phenomenon could adversely influence the survival and growth of cubs during the critical early phase of their development.

Transport of organic environmental contaminants to animal products

A large number of chemical contaminants potentially may be present in agricultural environments, leading to exposure of animals and potential residues in animal products. The contamination may be either widespread, as a result of aerial transport of industrial emissions, or localized, as a result of accidental emissions and spills, improper waste disposal, contaminants in useful products, and areas of past use of products now banned. The halogenated hydrocarbons, including the polychlorinated dibenzo-p-dioxins (PCDDs), polychlorinated dibenzofurans (PCDFs), polychlorinated biphenyls (PCBs), and persistent organochlorine insecticides remaining from past use, are the contaminants of most concern. Depending on the degree and pattern of chlorine substitution, these compounds are resistant to degradation and tend to accumulate in the fat of animals and their products. Other classes of environmental contaminants as exemplified by the PAHs, phthalate esters, acid phenolics, and nitrosamines also may occur widely in the environment. These compounds are unlikely to be transported to animal products because the compounds are water-soluble or can be metabolized to water-soluble products, which are excreted in the urine and thus do not bioaccumulate in products such as milk and meat. The points of entry of environmental contaminants into agricultural environments usually are plants and soils. Lipophilic compounds such as the halogenated hydrocarbons are not taken up and translocated by plants. Contamination of plants is mainly a surface phenomenon resulting from aerial deposition of emissions or deposition of compounds volatilized from the surface of contaminated soil. Thus, fibrous roughages used primarily in feeding cattle and other ruminants will be the most important pathway of animal exposure and transport to human foods. The second pathway of animal exposure is by ingestion of contaminated soil while grazing or when confined to unpaved facilities. As in the case of feed sources, cattle is the species most vulnerable to exposure by the soil ingestion pathway under most commercial management systems, but poultry and swine are more vulnerable in those infrequent situations in which these species have access to contaminated soil.

In vivo DNA adduct formation by bisphenol A

We have previously shown that bisphenol A (BPA) is oxidized to bisphenol-o-quinone in the presence of activation system and that the chemical reaction of DNA or deoxyguanosine 3'-monophosphate (dGMP) with bisphenol-o-quinone produces adducts. In the present study, using the 32P-postlabeling technique, we have investigated the in vivo DNA adduct formation by BPA by examining covalent modification in DNA. Administration of a single or multiple dose of 200 mg/kg of BPA to CD1 male rats produced two major and several minor adducts in liver DNA. The two major in vivo adducts matched the adduct profile of DNA or dGMP-bisphenol-o-quinone. To determine how BPA may be converted to DNA-binding metabolites, adducts were examined after incubation of DNA with BPA in the presence of a microsomal activation system. The in vitro incubation of BPA with DNA in the presence of a microsomal activation system revealed one major adduct and several minor adducts. The formation of adducts in DNA by BPA in the presence of a microsomal activation system was drastically decreased by known inhibitors of cytochrome P450. Adduct formation in DNA when cumene hydroperoxide or NADPH was used as a cofactor showed adducts with similar chromatographic mobilities as those from the reaction of dGMP-bisphenol-o-quinone. These data demonstrate that BPA is capable of binding covalently to DNA. DNA binding can be inhibited by the inhibitors of cytochrome P450. One of the DNA-binding metabolite(s) both in vitro and in vivo may be bisphenol-o-quinone. Covalent modifications in DNA by in vivo exposure of BPA may be a factor in the induction of hepatotoxicity.

Screening and characterization of estrogenic activity from a hydroxystilbene library

BACKGROUND

Compounds that either inhibit or induce an estrogen response in vivo are important as potential drugs and biochemical tools. Non-steroidal stilbene analogs such as tamoxifen are known to function as both estrogen agonists and antagonists depending upon the analog structure. This family of compounds is amenable to parallel-manifold synthesis because stilbene analogs are easily synthesized using a single-step olefination reaction.

RESULTS

We have prepared a small 23-component hydroxystilbene library using a solid phase synthesis approach. The library was screened for estrogenic and antiestrogenic activity using a cell-based bioassay that measures estrogen receptor-mediated transcription of a reporter gene. Three of the analogs proved to have dose-dependent estrogenic activity with EC50 values between 5 microM and 15 microM. Further characterization of the hydroxystilbene-mediated estrogenic activity suggests that the agonist activity results from direct binding to the steroid site on the estrogen receptor with IC50 values of 1-10 microM.

CONCLUSIONS

The results of this study show that classic olefination chemistry can be adapted to a solid-phase format for parallel synthesis of analog libraries. Although yields varied for the individual analogs, sufficient quantity of pure material was obtained directly from the resin for structural characterization and biological evaluation. This study further validates solid-phase organic synthesis as a useful approach for rapid parallel-manifold library synthesis to augment both lead compound discovery and optimization.

The wildlife/human connection: modernizing risk decisions

This article proposes that genetic and molecular ecotoxicology can play an important role in making policy and risk assessment decisions concerning xenobiotics. It calls for a greater awareness by ecotoxicologists to the effects in wildlife and humans resulting from transgenerational exposure to synthetic chemicals that interfere with gene expression and differentiation. The difficulty of recognizing these effects on the endocrine, immune, and nervous systems in developing embryos is described and suggests why effects of this nature have traditionally not been addressed when determining risk to synthetic chemicals. Specific examples are cited of environmental effects on hormonally responsive tissue in wildlife populations which could be used as models for assessing human exposure to synthetic chemicals. Evidence is presented that the environmental load of synthetic chemicals has reached critical levels at which wildlife and human health are at risk.

Endocrine effects of prenatal exposure to PCBs, dioxins, and other xenobiotics: implications for policy and future research

Recent reports have suggested that environmental chemicals may be associated with endocrine alterations in people, wildlife, and experimental animals. Pharmacological investigations as well as natural poisoning episodes have led to the association between exogenous chemicals and alterations in multiple hormonal systems. Persistent environmental contaminants such as dioxins and PCBs have been shown to modulate the activities of several different hormones. The unborn child or the neonate may be at special risk from these chemicals because of rapid growth and development, in addition to enhanced exposure. Because most exposure to these persistent chemicals is via food, changes in dietary habits and/or reduced contamination of the food supply may be required.

Toxicokinetic and Toxicodynamic Influences on Endocrine Disruption by Polychlorinated Biphenyls

Polychlorinated biphenyl (PCB) mixtures and individual chlorobiphenyl (CB) congeners have various endocrine-disrupting effects, but ultimate responses may be altered by concurrent effects on enzyme levels and enzyme activities. The toxicodynamics of estrogenic PCBs and metabolites have been studied in vitro, but nonlinear dose-response relationships in vivo suggest that tests must integrate toxicokinetic parameters to explain whole-animal responses. To determine if any such interactions occurred, relatively large doses were subdivided into different treatment regimens for immature female Sprague-Dawley rats. Aroclor 1242 was uterotropic when 120 mg/kg (total) was administered (intraperitoneally) in two, three or five doses. CB 47 (2,2',4,4'-tetraCB) and CB 153 (2,2',4,4',5,5'-hexaCB) increased absolute uterine weights at 30 mg/kg on days 20 and 21. Results at 25 days in rats that received zero, two, three or five doses between days 20 and 24 were much more variable due to changes in tissue responsiveness and/or toxicokinetic interactions. In rats receiving treatment for 5 days, pentoxyresorufin O-dealkylase (PROD) activity was inversely related to CB serum residues; in rats receiving CB 153 for 2 days, PROD activity was directly related to serum residues. It was not clear whether PROD activity was the cause of or a reflection of lower serum residues; however, nonplanar CBs are better substrates for PROD than are planar CBs, and the longer-term dosing may enhance metabolism and excretion, changing the biological effects observed.

The pesticides endosulfan, toxaphene, and dieldrin have estrogenic effects on human estrogen-sensitive cells

Estrogenic pesticides such as DDT and chlordecone generate deleterious reproductive effects. An "in culture" bioassay was used to assess the estrogenicity of several pesticides. The E-screen test uses human breast estrogen-sensitive MCF7 cells and compares the cell yield achieved after 6 days of culture in medium supplemented with 5% charcoal-dextran stripped human serum in the presence (positive control) or absence (negative control) of estradiol and with diverse concentrations of xenobiotics suspected of being estrogenic. Among the organochlorine pesticides tested, toxaphene, dieldrin, and endosulfan had estrogenic properties comparable to those of DDT and chlordecone; the latter are known to be estrogenic in rodent models. The E-screen test also revealed that estrogenic chemicals may act cumulatively; when mixed together they induce estrogenic responses at concentrations lower than those required when each compound is administered alone.