Exposure, postexposure, and density-mediated effects of atrazine on amphibians: breaking down net effects into their parts

Most toxicology studies focus on effects of contaminants during exposure. This is disconcerting because subsequent survival may be affected. For instance, contaminant-induced mortality can be later ameliorated by reduced competition among the survivors, a concept we refer to as "density-mediated compensation." Alternatively, it can be exacerbated by toxicant effects that persist or appear after exposure, a phenomenon we term "carryover effects." We developed a laboratory framework for testing the contribution of exposure, density-mediated, and carryover effects to net survival, by exposing embryos and larvae of the streamside salamander (Ambystoma barbouri) to atrazine (0, 4, 40, 400 ppb; 3 ppb is the U.S. drinking water maximum) and quantifying survival during and 14 months after exposure. Atrazine is the most commonly used herbicide in the United States and a documented endocrine disruptor. We show that atrazine-induced mortality during exposure was ameliorated by density-dependent survival after exposure, but complete density-mediated compensation was precluded by significant carryover effects of atrazine. Consequently, salamanders exposed to >or=4 ppb of atrazine had significantly lower survival than did control animals 14 months postexposure. The greatest change in survival occurred at low exposure concentrations. These nonlinear, long-term, postexposure effects of atrazine have similarities to effects of early development exposure to other endocrine disruptors. Together with evidence of low levels of atrazine impairing amphibian gonadal development, the results here raise concerns about the role of atrazine in amphibian declines and highlight the importance of considering persistent, postexposure effects when evaluating the impact of xenobiotics on environmental health.

A rapid, physiologic protocol for testing transcriptional effects of thyroid-disrupting agents in premetamorphic Xenopus tadpoles

Increasing numbers of substances present in the environment are postulated to have endocrine-disrupting effects on vertebrate populations. However, data on disruption of thyroid signaling are fragmentary, particularly at the molecular level. Thyroid hormone (TH; triiodothyronine, T3) acts principally by modulating transcription from target genes; thus, thyroid signaling is particularly amenable to analysis with a transcriptional assay. Also, T3 orchestrates amphibian metamorphosis, thereby providing an exceptional model for identifying thyroid-disrupting chemicals. We combined these two advantages to develop a method for following and quantifying the transcriptional action of T3 in Xenopus laevis tadpoles. This technology provides a means of assessing thyroid activity at the molecular level in a physiologically relevant situation. Moreover, translucent tadpoles are amenable to "on-line" imaging with fluorescent reporter constructs that facilitate in vivo measurement of transcriptional activity. We adapted transgenesis with TH-responsive elements coupled to either luciferase or green fluorescent protein to follow T3-dependent transcription in vivo. To reduce time of exposure and to synchronize responses, we optimized a physiologic pretreatment protocol that induced competence to respond to T3 and thus to assess T3 effects and T3 disruption within 48 hr. This pretreatment protocol was based on a short (24 hr), weak (10(-12) M) pulse of T3 that induced TH receptors, facilitating and synchronizing the transcriptional responses. This protocol was successfully applied to somatic and germinal transgenesis with both reporter systems. Finally, we show that the transcriptional assay allows detection of the thyroid-disrupting activity of environmentally relevant concentrations (10(-8) M) of acetochlor, a persistent herbicide.

Cigarette smoking and effects on hormone function in premenopausal women

Cigarette smoke contains compounds that are suspected to cause reproductive damage and possibly affect hormone activity; therefore, we examined hormone metabolite patterns in relation to validated smoking status. We previously conducted a prospective study of women of reproductive age (n = 403) recruited from a large health maintenance organization, who collected urine daily during an average of three to four menstrual cycles. Data on covariates and daily smoking habits were obtained from a baseline interview and daily diary, and smoking status was validated by cotinine assay. Urinary metabolite levels of estrogen and progesterone were measured daily throughout the cycles. For the present study, we measured urinary levels of the pituitary hormone follicle-stimulating hormone (FSH) in a subset of about 300 menstrual cycles, selected by smoking status, with the time of transition between two cycles being of primary interest. Compared with nonsmokers, moderate to heavy smokers (>/= 10 cigarettes/day) had baseline levels (e.g., early follicular phase) of both steroid metabolites that were 25-35% higher, and heavy smokers (>/= 20 cigarettes/day) had lower luteal-phase progesterone metabolite levels. The mean daily urinary FSH levels around the cycle transition were increased at least 30-35% with moderate smoking, even after adjustment. These patterns suggest that chemicals in tobacco smoke alter endocrine function, perhaps at the level of the ovary, which in turn effects release of the pituitary hormones. This endocrine disruption likely contributes to the reported associations of smoking with adverse reproductive outcomes, including menstrual dysfunction, infertility, and earlier menopause.

Declining sex ratio in a first nation community

Members of the Aamjiwnaang First Nation community near Sarnia, Ontario, Canada, voiced concerns that there appeared to be fewer male children in their community in recent years. In response to these concerns, we assessed the sex ratio (proportion of male births) of the Aamjiwnaang First Nation over the period 1984-2003 as part of a community-based participatory research project. The trend in the proportion of male live births of the Aamjiwnaang First Nation has been declining continuously from the early 1990s to 2003, from an apparently stable sex ratio prior to this time. The proportion of male births (m) showed a statistically significant decline over the most recent 10-year period (1994-2003) (m = 0.412, p = 0.008) with the most pronounced decrease observed during the most recent 5 years (1999-2003) (m = 0.348, p = 0.006). Numerous factors have been associated with a decrease in the proportion of male births in a population, including a number of environmental and occupational chemical exposures. This community is located within the Great Lakes St. Clair River Area of Concern and is situated immediately adjacent to several large petrochemical, polymer, and chemical industrial plants. Although there are several potential factors that could be contributing to the observed decrease in sex ratio of the Aamjiwnaang First Nation, the close proximity of this community to a large aggregation of industries and potential exposures to compounds that may influence sex ratios warrants further assessment into the types of chemical exposures for this population. A community health survey is currently under way to gather more information about the health of the Aamjiwnaang community and to provide additional information about the factors that could be contributing to the observed decrease in the proportion of male births in recent years.

Differential effects of glyphosate and roundup on human placental cells and aromatase

Roundup is a glyphosate-based herbicide used worldwide, including on most genetically modified plants that have been designed to tolerate it. Its residues may thus enter the food chain, and glyphosate is found as a contaminant in rivers. Some agricultural workers using glyphosate have pregnancy problems, but its mechanism of action in mammals is questioned. Here we show that glyphosate is toxic to human placental JEG3 cells within 18 hr with concentrations lower than those found with agricultural use, and this effect increases with concentration and time or in the presence of Roundup adjuvants. Surprisingly, Roundup is always more toxic than its active ingredient. We tested the effects of glyphosate and Roundup at lower nontoxic concentrations on aromatase, the enzyme responsible for estrogen synthesis. The glyphosate-based herbicide disrupts aromatase activity and mRNA levels and interacts with the active site of the purified enzyme, but the effects of glyphosate are facilitated by the Roundup formulation in microsomes or in cell culture. We conclude that endocrine and toxic effects of Roundup, not just glyphosate, can be observed in mammals. We suggest that the presence of Roundup adjuvants enhances glyphosate bioavailability and/or bioaccumulation.

Effects of ammonium perchlorate on thyroid function in developing fathead minnows, Pimephales promelas

Perchlorate is a known environmental contaminant, largely due to widespread military use as a propellant. Perchlorate acts pharmacologically as a competitive inhibitor of thyroidal iodide uptake in mammals, but the impacts of perchlorate contamination in aquatic ecosystems and, in particular, the effects on fish are unclear. Our studies aimed to investigate the effects of concentrations of ammonium perchlorate that can occur in the environment (1, 10, and 100 mg/L) on the development of fathead minnows, Pimephales promelas. For these studies, exposures started with embryos of < 24-hr postfertilization and were terminated after 28 days. Serial sectioning of thyroid follicles showed thyroid hyperplasia with increased follicular epithelial cell height and reduced colloid in all groups of fish that had been exposed to perchlorate for 28 days, compared with control fish. Whole-body thyroxine (T4) content (a measure of total circulating T4 in fish exposed to 100 mg/L perchlorate was elevated compared with the T4 content of control fish, but 3,5,3-triiodothyronine (T3) content was not significantly affected in any exposure group. Despite the apparent regulation of T3, after 28 days of exposure to ammonium perchlorate, fish exposed to the two higher levels (10 and 100 mg/L) were developmentally retarded, with a lack of scales and poor pigmentation, and significantly lower wet weight and standard length than were control fish. Our study indicates that environmental levels of ammonium perchlorate affect thyroid function in fish and that in the early life stages these effects may be associated with developmental retardation.

Intersexuality and the cricket frog decline: historic and geographic trends

Exposure to anthropogenic endocrine disruptors has been listed as one of several potential causes of amphibian declines in recent years. We examined gonads of 814 cricket frogs (Acris crepitans) collected in Illinois and deposited in museum collections to elucidate relationships between the decline of this species in Illinois and the spatial and temporal distribution of individuals with intersex gonads. Compared with the preorganochlorine era studied (1852-1929), the percentage of intersex cricket frogs increased during the period of industrial growth and initial uses of polychlorinated biphenyls (PCBs) (1930-1945), was highest during the greatest manufacture and use of p,p-dichlorodiphenyltrichloroethane (DDT) and PCBs (1946-1959), began declining with the increase in public concern and environmental regulations that reduced and then prevented sales of DDT in the United States (1960-1979), and continued to decline through the period of gradual reductions in environmental residues of organochlorine pesticides and PCBs in the midwestern United States (1980-2001). The proportion of intersex individuals among those frogs was highest in the heavily industrialized and urbanized northeastern portion of Illinois, intermediate in the intensively farmed central and northwestern areas, and lowest in the less intensively managed and ecologically more diverse southern part of the state. Records of deposits of cricket frog specimens into museum collections indicate a marked reduction in numbers from northeastern Illinois in recent decades. These findings are consistent with the hypothesis that endocrine disruption contributed to the decline of cricket frogs in Illinois.

Long-term exposure to environmental concentrations of the pharmaceutical ethynylestradiol causes reproductive failure in fish

Heightened concern over endocrine-disrupting chemicals is driven by the hypothesis that they could reduce reproductive success and affect wildlife populations, but there is little evidence for this expectation. The pharmaceutical ethynylestradiol (EE2) is a potent endocrine modulator and is present in the aquatic environment at biologically active concentrations. To investigate impacts on reproductive success and mechanisms of disruption, we exposed breeding populations (n = 12) of zebrafish (Danio rerio) over multiple generations to environmentally relevant concentrations of EE2. Life-long exposure to 5 ng/L EE2 in the F1 generation caused a 56% reduction in fecundity and complete population failure with no fertilization. Conversely, the same level of exposure for up to 40 days in mature adults in the parental F0 generation had no impact on reproductive success. Infertility in the F1 generation after life-long exposure to 5 ng/L EE2 was due to disturbed sexual differentiation, with males having no functional testes and either undifferentiated or intersex gonads. These F1 males also showed a reduced vitellogenic response when compared with F0 males, indicating an acclimation to EE2 exposure. Depuration studies found only a partial recovery in reproductive capacity after 5 months. Significantly, even though the F1 males lacked functional testes, they showed male-pattern reproductive behavior, inducing the spawning act and competing with healthy males to disrupt fertilization. Endocrine disruption is therefore likely to affect breeding dynamics and reproductive success in group-spawning fish. Our findings raise major concerns about the population-level impacts for wildlife of long-term exposure to low concentrations of estrogenic endocrine disruptors.

Is bone mineral composition disrupted by organochlorines in east Greenland polar bears (Ursus maritimus)?

We analyzed bone mineral density (BMD) in skulls of polar bears (Ursus maritimus) (n = 139) from East Greenland sampled during 1892-2002. Our primary goal was to detect possible changes in bone mineral content (osteopenia) due to elevated exposure to organochlorine [polychlorinated biphenyls (PCBs), dichlorodiphenyl trichloroethane (DDT) and its metabolites, chlordanes (CHLs), dieldrin, hexacyclohexanes, hexachlorobenzene] and polybrominated diphenyl ether (PBDE) compounds. To ensure that the BMD value in skull represented the mineral status of the skeletal system in general, we compared BMD values in femur and three lumbar vertebrae with skull in a subsample. We detected highly significant correlations between BMD in skull and femur (r = 0.99; p < 0.001; n = 13) and skull and vertebrae (r = 0.97; p < 0.001; n = 8). BMD in skulls sampled in the supposed pre-organochlorine/PBDE period (1892-1932) was significantly higher than that in skulls sampled in the supposed pollution period (1966-2002) for subadult females, subadult males, and adult males (all, p < 0.05) but not adult females (p = 0.94). We found a negative correlation between organochlorines and skull BMD for the sum of PCBs (SigmaPCB; p < 0.04) and SigmaCHL (p < 0.03) in subadults and for dieldrin (p < 0.002) and SigmaDDT (p < 0.02) in adult males; indications for SigmaPBDE in subadults were also found (p = 0.06). In conclusion, the strong correlative relationships suggest that disruption of the bone mineral composition in East Greenland polar bears may have been caused by organochlorine exposure.

Androgenic and estrogenic response of green mussel extracts from Singapore's coastal environment using a human cell-based bioassay

In the last decade, evidence of endocrine disruption in biota exposed to environmental pollutants has raised serious concern. Human cell-based bioassays have been developed to evaluate induced androgenic and estrogenic activities of chemical compounds. However, bioassays have been sparsely applied to environmental samples. In this study we present data on sex hormone activities in the green mussel, Perna viridis, in Singapore's coastal waters. P.viridis is a common bioindicator of marine contamination, and this study is a follow-up to an earlier investigation that reported the presence of sex hormone activities in seawater samples from Singapore's coastal environment. Specimens were collected from eight locations around the Singapore coastline and analyzed for persistent organic pollutants (POPs) and heavy metals. Tissue extracts were then screened for activities on androgen receptors (ARs) and estrogen receptors (ER-alpha and ER-beta) using a reporter gene bioassay based on a HeLa human cell line. Mussel extracts alone did not exhibit AR activity, but in the presence of the reference androgenic hormone dihydrotestosterone (DHT), activities were up to 340% higher than those observed for DHT alone. Peak activities were observed in locations adjacent to industrial and shipping activities. Estrogenic activities of the mussel extract both alone and in the presence of reference hormone were positive. Correlations were statistically investigated between sex hormone activities, levels of pollutants in the mussel tissues, and various biological parameters (specimen size, sex ratio, lipid and moisture content). Significant correlations exist between AR activities, in the presence of DHT, and total concentration of POPs (r= 0.725, p < 0.05).

Survivorship patterns of larval amphibians exposed to low concentrations of atrazine

Amphibians can be exposed to contaminants in nature by many routes, but perhaps the most likely route is agricultural runoff in amphibian breeding sites. This runoff results in high-level pulses of pesticides. For example, atrazine, the most widely used pesticide in the United States, can be present at several parts per million in agricultural runoff. However, pesticide levels are likely to remain in the environment at low levels for longer periods. Nevertheless, most studies designed to examine the impacts of contaminants are limited to short-term (approximately 4 days) tests conducted at relatively high concentrations. To investigate longer-term (approximately 30 days) exposure of amphibians to low pesticide levels, we exposed tadpoles of four species of frogs--spring peepers (Pseudacris crucifer), American toads (Bufo americanus, green frogs (Rana clamitans), and wood frogs (Rana sylvatica)--at early and late developmental stages to low concentrations of a commercial preparation of atrazine (3, 30, or 100 ppb; the U.S. Environmental Protection Agency drinking water standard is 3 ppb). We found counterintuitive patterns in rate of survivorship. Survival was significantly lower for all animals exposed to 3 ppb compared with either 30 or 100 ppb, except the late stages of B. americanus and R. sylvatica. These survival patterns highlight the importance of investigating the impacts of contaminants with realistic exposures and at various developmental stages. This may be particularly important for compounds that produce greater mortality at lower doses than higher doses, a pattern characteristic of many endocrine disruptors.

Problems associated with the recognition and confirmation of low-dose endocrine toxicities

Our attempts to confirm reports of low-dose/hormetic effects in rodent endocrine toxicity studies are reviewed. It is concluded that our present failure to confirm any such effects is due, in large part, to a general lack of understanding of confounding influences and the failure of most investigators to confirm their findings before publication. The major potential confounding factor is suggested to be variability of the parameters under study within control groups, a factor that assumes increased importance when attempting to demonstrate weak low-dose effects. This is illustrated by our studies with bisphenol A in the mouse uterotrophic assay and of finasteride in the Hershberger antiandrogenicity assay. In both of these cases our ability to demonstrate a low-dose effect is dependent on whether concurrent or recent control values are used.

Sex-change chemicals and their influence on the brain

The potential for man-made chemicals to mimic or antagonise natural hormones is a controversial issue, but one for which increasing amounts of evidence are being gathered worldwide. The controversy surrounds not so much the matter of whether these chemicals can mimic hormones in vitro--this phenomenon has been widely accepted in the scientific world - but more whether, as a result, they can disrupt reproduction in a wildlife situation. It has, nevertheless, been acknowledged that many wildlife populations are exhibiting reproductive and/or developmental abnormalities such as intersex gonads in wild roach populations in the U.K. and various reproductive disorders in alligators in Lake Apopka, Florida. However, the causative agents for many of these effects are difficult to specify, due to the extensive mixtures of chemicals--each of which may act via different pathways--to which wild populations are exposed, together with the wide variability observed even in natural (uncontaminated) habitats. As a result, any information detailing fundamental mechanism of action of the so-called endocrine disrupting chemicals (EDCs) is of use in determining whether or not these chemicals, as they are present in the environment, may in fact be capable of causing some of the effects observed in wildlife over recent years.

Symbiotic gene activation is interrupted by endocrine disrupting chemicals

Endocrine disrupting chemicals (EDCs) include organochlorine pesticides, plastics manufacturing by-products, and certain herbicides. These chemicals have been shown to disrupt hormonal signaling in exposed wildlife, lab animals, and mammalian cell culture by binding to estrogen receptors (ER-+/- and ER-) and affecting the expression of estrogen responsive genes. Additionally, certain plant chemicals, termed phytoestrogens, are also able to bind to estrogen receptors and modulate gene expression, and as such also may be considered EDCs. One example of phytoestrogen action is genistein, a phytochemical produced by soybeans, binding estrogen receptors, and changing expression of estrogen responsive genes which certain studies have linked to a lower incidence of hormonally related cancers in Japanese populations. Why would plants make compounds that are able to act as estrogens in the human body? Obviously, soybeans do not intentionally produce phytoestrogens to prevent breast cancer in Japanese women.

Urinary 17-KS and 17-OHCS as markers of endocrine disruption by air pollution in primary school children

The present study was undertaken to determine the effects of air pollution on urinary excretion of 17-ketosteroids (17-KS) and 17-hydroxycorticosteroids (17-OHCS), which are markers of adrenal cortex functions, in primary school children from five schools with different levels of air pollution. One hundred and twentynine fourth-grade children were included in the study. Urinary excretion of 17-KS and 17-OHCS were significantly lower in male children living in polluted areas than in children living in clean areas. We conclude that adrenal cortex functions before puberty can serve as practical markers of endocrine disruption by environmental factors in epidemiological studies.