Are environmental sentinels signaling?

There is an increasing perception that environmental contamination by chemicals no longer poses a significant health threat and that relaxation of environmental regulations is warranted. However, many wildlife populations are showing signs of developmental, behavioral, and reproductive dysfunction due to environmental contamination by endocrine-disrupting chemicals. Scientists, regulators, and legislators must mobilize to identify current health threats posed by environmental pollutants, develop testing protocols that will detect such properties of new chemicals, and strengthen legislation designed to protect environmental health.

The year 1995 marked the 25th anniversary of both the U.S. Environmental Protection Agency and Earth Day. The inception of both institutions signified the need to temper anthropogenic stresses on the environment or face unsettling consequences. Decades of environmental abuse culminated in the 1960s when public perception of the repercussion of unabated environmental pollution was heightened by Rachel Carson's graphic depictions (1). The pressing environmental problems of 25 years ago were blatant. Among the most significant of problems were chemical and sewage discharges making aquatic resources unsuitable for human use and habitation by aquatic organisms, and the use of pesticides, which posed a significant threat to nontarget species. In response, the Clean Water Act was instituted in 1972 to regulate waste discharge and to ensure that high water-quality standards were maintained. The Federal Insecticide, Fungicide and Rodenticide Act (FIFRA) was amended three times during the 1970s to provide safeguards against pesticide-mediated harm to human and environmental health. Such legislation provided the foundation upon which a sound and reasonable national environmental policy was established. This policy has resulted in significant improvement in environmental quality concurrent with population and economic growth. The success of the environmental protection policies of the United States is best exemplified when environmental quality of the United States is compared to that of industrialized countries of the former Soviet block and other countries where such policies were never significantly instituted (2)(3)(4).
With the current movement toward the reduction of government size and spending, the issue is often raised as to whether environmental legislation and supporting research programs could be relaxed without intolerable consequences. Major fish and wildlife kills due to chemical waste discharge and improper pesticide usage are ly relegated to distant memo] accepts the thesis that fish an4 species serve as sentinels for the of human health from environme. minants, then human health ml adequately protected from th health effects of pollution. Such ports the contention that envii legislation and research need not ed and could perhaps be relaxed. not factored into this argumer while the flagrant environmental of 25 years ago have been address subtle, though no less beguiling mental threats may persist. Cent issue is the question, are environn tinels currently signaling the ex such environmental hazards? Toxicant-mediated endocrir tion is one example of a toxicolog currently presenting itself in the ment. Endocrine-disrupting effect ronmental pollutants were first i while investigating mechanisms r for reproductive failure among species exposed to organochlorine (5). The observation that exposui chemicals can lead to reproduct led to the promulgation of re under FIFRA and subsequently e) nonpesticide chemicals under Substances Control Act requirin effects of chemical exposure on ti tion of viable offspring be determ tests, conducted in standard test birds, fish, mammals, and inv( involve chronically exposing t organisms to various concentra chemical, then assessing the n viable offspring produced (6). ' only subchronic testing, invt assessment of the effects of the cl survival and growth of larval fish, required. Assessments of reprodu( ity are mandated only if the no effect level established during the subchronic no loner toxicity test is greater than 1/10 the expected ewrrsnted. environmental concentration of the chemical inr1e" (6). Retrospective assessments have shown biurkals, that such approaches will adequately protect po1e4 by the environment against most chemicals (').
. However, unique toxicological properties of rp cronikc some chemicals can result in undetected toxtoxicy icity using these protocols. Endocrine-disrupting chemicals can be among these undetected toxicants because they may 1) elicit effects on the developing fetus that are not now large-manifested until the mature organism enters ry. If one its reproductive stage, 2) elicit specific biod wildlife chemical/physiological changes that affect an protection organism's reproductive capacity without ntal conta-affecting survival and growth as measured ast also be during subchronic testing, or 3) adversely e adverse affect endocrine processes characteristic of logic supsome species but absent in those surrogate ronmental species used in toxicity testing. Many pestibe expandcides, industrial chemicals, and wastes are However, among the toxicants that elicit such effects. nt is that, Shore birds such as gulls and terns typiproblems cally produce broods of two or three eggs. ssed, more Ornithologists began observing in the environ-1970s that broods of five or six eggs were tral to this not uncommon (8). This abnormal clutch mental sen-size was found to be due to multiple cistence of females sharing a nest (9). This femalefemale pairing appeared to be due to a define disrupciency in reproductively competent males ;ical hazard (10). Laboratory investigations demonstrate environ-ed that exposure to DDT feminized male cts of envi-gulls during embryonic development (11). recognized Further, incidence of female-female pairing responsible was higher in environments with significant some bird DDT contamination (JO). Thus, abnormal > pesticides breeding behavior in these birds appeared re to some to be due to reproductive deficiency in rive failure males caused by embryonic exposure to gulations environmental pollutants. This observation Kpanded to is not only of historical relevance, as the Toxic female-female pairing of terns has been Lg that the noted recently in areas contaminated with he produc-polychlorinated biphenyls (PCBs) (12). mined. Such Female Poeciliidae fish inhabiting areas species of receiving pulp mill effluent have been vertebrates, observed to undergo masculinization. Most ,he parent obvious is the modification of the anal fin in ttions of a lumber of With fish, living the chemical on is initially active toxic--observed-Commentary Environmental Sentinels affected females to a gonopodiumlike structure (used by males for sperm transmission) (13). Exposure to exogenous androgens has been shown to cause similar masculinization (14), and androgens generated by the action of bacteria on phytosterols present in the effluent are presumed to be responsible for this effect. Fish exposed to paper and pulp mill effluent can also experience altered steroid hormone titers (15), impaired gonad development (16), and reduced fecundity (17). Such effects, specific to reproduction, would not be detected in subchronic toxicity tests.
Propiconazole is a member of the imidazole-derivative class of fungicides. A common characteristic of these chemicals is their ability to inhibit enzymes responsible for steroid hormone biosynthesis and induce enzymes involved in steroid hormone metabolism (18,19). A consequence of this effect is severe reductions in some steroid hormone levels (18). This specific and potent effect has led to the consideration of some imidazole-derivatives for use as a male contraceptive (20). Propiconazole, which is used as an agricultural fungicide, shares these properties and thus has the potential to compromise reproductive success of chronically exposed organisms. These effects would not be detected in a subchronic toxicity test that did not evaluate reproduction. The Ecuadorian shrimp industry has called for a moratorium on the use of propiconazole for fear that it is responsible for the demise ofshrimp populations (21).
Tributyltin has been used extensively for more than 20 years as an antifoulant in marine paints. Tributyltin has been identified as the causative agent responsible for imposex in many marine mollusk populations. Imposex is the imposition of sex characteristics of one gender onto another (a form of pseudohermaphrodism). In the case of tributyltin-exposed mollusks, females develop a penis, vas deferens, and in severe cases, seminiferous tubules (22). Affected females can be rendered infertile because the vas deferens blocks the release of eggs from the oviduct. The mechanism responsible for this effect has not been conclusively established, but it seems to involve the neuroendocrine regulation of sexual differentiation (23). Tributyltin can cause imposex at low part per trillion concentrations and has caused the extinction of some affected populations (22). Certain mollusk species may be particularly sensitive to the effect of tributyltin owing to unique aspects of sexual differentiation in these organisms (24). Intersexuality also has been observed in some crustacean populations in the vicinity of sewage discharge, though causality has not been established (25). Peniand neona-tal exposure of rodent models to a variety of environmental chemicals including 2,3,7,8tetrachlorodibenzo-p-dioxin (26), PCBs (27), mirex (28), chlordecone (kepone) (29)(30)(31), dieldrin (28), aldrin (28), chlordane (32), and atrazine (33) have shown that these chemicals are capable of eliciting a variety of perturbations in the sexual differentiation of mammals.
Thus, it would appear that environmental sentinels are indeed signaling us that all is not well. Although the major environmental problems of the 1960s may have been successfully dealt with, we are faced in the 1990s with new problems to surmount. Speculation remains as to whether human heath issues such as increased incidence of breast cancer, prostate cancer, testicular cancer, endometriosis, birth defects in the male reproductive tract, and reductions in sperm count may be associated with the existence of endocrine-disrupting chemicals in the environment (12). Toxicity testing requirements for environmental chemicals must be expanded to consider effects that may go undetected using current guidelines. Existing toxicity testing requirements should be complemented with in vitro diagnostic tests designed to detect specific biological properties such as hormone agonistic or antagonistic activities. As discussed by McLachlan (34), the establishment of cell lines that have been transfected with specific receptor-reporter gene constructs would greatly facilitate the screening of chemicals for such properties. In addition, biomarkers must be identified that can be used as part of standard toxicity tests to identify chemicals that may pose risk of endocrine-disrupting effects. For example, a significant correlation has been shown between the percentage of males present in a litter of mice and the average anogenital distance in females in that litter (35). The intrauterine position of a female rodent with respect to the number of adjacent male siblings affects reproductive physiology and behavior (36) as well as anogenital distance (37). These observations suggest that the intrauterine hormonal environment affects the developmental and reproductive capacity of the offspring. Analyses of anogenital distance during conventional toxicity tests may thus serve as a biomarker of reproductive effects and as an indicator of the need for multigenerational toxicity tests. Production of estrogen-regulated proteins such as vitellogenin (38) and lactoferrin (395) in chemically exposed males would signal estrogenicity of the chemical, which would warrant more definitive testing protocols to explicitly characterize toxicity.
Changes in steroid hormone levels can also be indicative of endocrine-disrupting chemical exposure. Several studies have suggested that toxicant-induced alterations in steroid hormone levels or metabolism may contribute to reproductive impairment (40)(41)(42). Our laboratory has been conducting comparative studies of the effects of chemicals on reproductive capacity and steroid metabolism using the freshwater crustacean Daphnia magna in an attempt to validate this putative relationship. Experiments thus far indicate that concentrations of the toxicants that impair reproduction also perturb steroid metabolism (43). Furthermore, this effect on steroid metabolism can be detected after short-term exposure to the toxicant (44). These results suggest that for some reproductive toxicants, effects on steroid metabolism may be predictive of reproductive toxicity, and thus metabolic effects can serve as a biomarker of reproductive toxicity.
Clearly, many strategies exist that could improve our ability to detect endocrine-disrupting chemicals and identify exposure dosages at which effects are elicited. Further research is needed to better define such experimental approaches and validate their utility. Ultimately, testing requirements will need to be expanded to ensure the detection of endocrine-disrupting effects of environmental chemicals; environmental legislation must be strengthened to ensure protection against these and other chemicals that elicit subtle, yet devastating, effects. Legislators must be made aware that the absence of dead fish and wildlife is not justification for the relaxation of environmental legislation and supporting research. The deleterious consequences of chemicals in the environment continue. You just have to look a little harder to see them.