Final Report: Field and Laboratory Studies of the Effects of Polychlorinated Biphenyls and Other Persistent Organic Pollutants on Thyroid Function During Avian DevelopmentEPA Grant Number: R827400
Title: Field and Laboratory Studies of the Effects of Polychlorinated Biphenyls and Other Persistent Organic Pollutants on Thyroid Function During Avian Development
Investigators: McNabb, F.M. Anne
Institution: Virginia Polytechnic Institute and State University
EPA Project Officer: Carleton, James N
Project Period: August 1, 1999 through July 31, 2001 (Extended to December 31, 2003)
Project Amount: $256,587
RFA: Endocrine Disruptors (1999) RFA Text | Recipients Lists
Research Category: Economics and Decision Sciences , Health , Safer Chemicals , Endocrine Disruptors
The objectives of this research project were to: (1) evaluate the current status of thyroid function in developing Herring Gulls (Larus argentatus) from Great Lakes sites with different chemical pollutant exposures; and (2) determine, using laboratory studies on developing chickens (Gallus domesticus), if the disruption of avian thyroid function by polychlorinated biphenyls (PCBs) is mechanistically similar to that in laboratory mammals. PCBs are key contaminants at many Great Lakes sites.
We have studied Herring Gulls, a fish-eating, top-predator, sentinel species that has been used to monitor the concentrations and biological effects of environmental contaminants in the Great Lakes for more than 25 years. Past research has focused on mortality, developmental abnormalities, and reproductive effects of chemical pollutants in the Great Lakes. Studies of thyroid histology and developmental effects in the 1980s suggested that thyroid function was disrupted in these gulls and might be a causal factor in some of the developmental problems observed. There have been, however, no measurements of thyroid function that would confirm this conclusion. Our studies have examined the thyroid status of developing gulls environmentally exposed to PCB mixtures, and developing chickens exposed to specific PCB congeners. The purpose of these studies was to examine the mechanisms whereby PCBs may disrupt thyroid function in birds in comparison to the known mechanisms of disruption in mammals.
Our studies have been in collaboration with Glen Fox (Contaminant Effects Specialist, Canadian Wildlife Service) and Keith Grasman (Associate Professor of Biological Sciences, Wright State University), who have collected the gulls at the Great Lakes sites as part of their grant-funded projects. PCB analyses were done by the Canadian National Wildlife Research Centre. For the studies of specific PCB congeners, the egg dosing and incubation, as well as the sampling of embryos, was done in Dr. Grasman's laboratory.
Accomplishments in Relation to Objective 1
Thyroid Function in Embryonic and Prefledgling Herring Gulls From Great Lakes Sites in 1998, 1999, and 2000, in Relation to Yolk Sac (for Embryos) and Liver (for Prefledglings) PCB Concentrations. Herring Gull pipped embryos and prefledgling chicks from some "high-PCB sites" showed evidence of decreases in thyroid function in a number of the collections in all 3 years compared with those at the reference sites at Kent Island in the Bay of Fundy and Chantry Island in Lake Huron. Sum PCB concentrations (total of 60 PCB congeners) in yolk sacs at the high-PCB sites ranged from approximately 7-28 times those at the Kent Island reference site.
In Herring Gull pipped embryos, thyroid gland T4 content (the most sensitive index of altered thyroid function), was significantly decreased at three of five high-PCB sites in 1998, at one of two in 1999, and at all five high-PCB sites in 2000 (in 2000, the thyroidal T4 at four of five high-PCB sites was <10 percent than that at the reference site). Organismal hypothyroidism, as indicated by significantly depressed plasma T4 concentrations, was less frequent than significant decreases in thyroid gland T4. These findings indicate that although embryos at some high-PCB sites are able to maintain circulating hormone concentrations, they are doing so at the expense of depleted hormone stores. The depleted hormone stores indicate that these birds have little scope for making adaptive responses to environmental challenges that require increases in circulating thyroid hormones.
In Herring Gull prefledglings, the picture was more complex. Although the thyroidal T4 content per unit weight of gland was depleted significantly at all high-PCB sites in all 3 years, at some sites the hypothalamic-pituitary-thyroid (HPT) axis response (increased thyroid weights) of prefledglings compensated and the total hormone content per thyroid gland was not significantly different from that of birds at the reference sites. Thus, prefledglings, which have a more mature HPT axis than pipped embryos, appear sometimes to be able to compensate for PCB effects.
To date, the thyroid data have only been evaluated in relation to the sum PCBs (i.e., the sum of 60 congeners). At present, we are evaluating the data in relation to a number of the other contaminants measured (e.g., PCBs expressed as Aroclor 1254 and 1260 equivalents and certain specific PCB congeners thought to be significantly affecting liver enzyme systems that can alter thyroid hormone excretion).
Thyroid Function in Adult Herring Gulls at a Reference Site and Three "Areas of Concern" in 2001. Adult Herring Gulls at two high-PCB Areas of Concern sites in the Great Lakes had significantly depleted thyroidal T4 content compared to gulls at the two reference sites (one in the Great Lakes and one in the Bay of Fundy). Other thyroid variables did not differ between the high-PCB and reference sites.
A Basic Study of the Pattern of Thyroid Development in Gull Embryos. Herring Gulls are classed as semiprecocial, with respect to their patterns of physiological and behavioral development. The pattern of thyroid development, however, which is distinctly different in precocial and altricial birds, has not previously been investigated in any semiprecocial bird. Thus, to provide background information for our studies, we conducted a descriptive study of thyroid development in Herring Gull embryos incubated in the laboratory (in collaboration with Angela Lorenzen, Canadian Wildlife Service).
The Effects of PCBs on T4 Displacement From the Hormone Binding Protein (Transthyretin) and the Effect of Such Displacement on T4 Deposition in Avian Eggs. Some PCB congeners can displace T4 from hormone binding proteins in Herring Gull plasma in vitro, the percent of free T4 in gull plasma tended to show an increase in prefledglings (1998) and adults (2001) at Great Lakes sites with high-PCB exposures compared to reference sites and gull egg T4 deposition, which tended to show a decrease when exposed to high PCBs. Note, the 1998 prefledglings and the 2001 adults were the only collections for which sufficient plasma was available for these free hormone measurements.
Thyroid Effects of PCBs on Other Species in Contexts That Add to the Understanding of the Main Parts of This Project: Black Guillemots Exposed to an Aroclor 1260 Spill, Turtles Exposed to Mixed Pollutants in Areas of Concern and the Links Between Thyroid Effects and Molting in PCB Exposed Kestrels. Black guillemots from a location in Labrador, where the only contaminant present was Aroclor 1260, were compared at a reference site, a slightly contaminated island site, and a contaminated beach site (birds at the beach site had liver PCB concentrations approximately 11-130 times those at the reference site). There were no significant differences in thyroid function between the sites, presumably because of the wide differences in liver PCBs in individuals at the high-PCB sites. Analyses of data for individuals have not yet been done. If there are no effects of PCB exposure alone, these data may suggest that in the Great Lakes sites, PCBs as well as other contaminants are playing a role in the altered thyroid function observed in Herring Gulls.
Turtles hatched from eggs collected at Areas of Concern sites in the Great Lakes, but that were reared in the laboratory, had significantly decreased thyroidal T4 content and significant thyroid hypertrophy compared to those from reference sites. These alterations in thyroid function were correlated with the in ovo exposure to 1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane) and 1,1-dichloro-2,2-bis(p-chlorophenyl)ethylene), indicating that these contaminants may be playing a contributing role in thyroid changes in animals in the Great Lakes (in collaboration with Kim Fernie, Canadian Wildlife Service).
Thyroid hormones are involved with molting in birds. Kestrels fed Aroclor 1242 for 6 months, however, did not have altered plumage characteristics, but they did have significantly depressed plasma T4 concentrations compared to controls (in collaboration with Michael Quinn, University of Maryland).
The Sensitivity of Different Thyroid Variables Measured for Detecting Thyroid Disruption Based on Data From This Project in Gulls, Several of the Collaborations Listed Above, and Another Project in Our Laboratory on Perchlorate Exposure in Bobwhite Quail and Mallard Ducks. The projects described above (funded by the U.S. Department of Defense, the U.S. Department of Energy, and the U.S. Environmental Protection Agency) have allowed us to compare the sensitivity of a number of thyroid variables as well as other growth-related variables that are influenced by thyroid function for detecting thyroid disruption. In all cases, when we have measured several thyroid variables, thyroidal T4 is by far the most sensitive measure of thyroid alterations, with plasma hormones and thyroid gland hypertrophy being much less sensitive indicators. Growth-related variables, such as body weight, limb growth, and plumage changes, are very insensitive compared to direct measurements of thyroid function. Thus, only thyroid alterations severe enough to result in sustained organismal hypothyroidism are sufficient to cause these types of target organ effects.
Accomplishments in Relation to Objective 2
The Same Thyroid Variables Used for the Gull Studies Have Been Employed To Assess the Effects of PCB-126 (A Coplanar, Dioxin-Like PCB Congener; 3,3',4, 4',5-Pentachlorobiphenyl) on Chicken Embryos Exposed Throughout Embryonic Life (some studies also addressed the posthatch effects of embryonic exposure to PCB 126, and some studies have used another coplanar congener [PCB 77]). Although the range of PCB 126 doses we used was chosen based on immunological effects at all doses, we found essentially no effect on any of the thyroid variables measured in chicken embryos. Currently, we are analyzing some additional samples to increase the number of data points. Overall, our work to date suggests that PCB 126, the most toxic and most dioxin-like of the PCBs, has toxic effects (40 percent mortality at our highest dose) before any possible thyroid effects are apparent. These results from the PCB 126 dosed eggs were the same for both embryos and for chicks that hatched and were sampled at several posthatch times. We also did not find altered thyroid function in embryos or chicks from eggs dosed with PCB 77. In addition to the thyroid variables described above, we measured the activity of 5'-deiodinase (5'-D) in brain tissue in the embryos. This enzyme, which converts T4 to the metabolically active enzyme T3, responds to hypothyroid conditions by increasing activity and "protecting" central nervous system development from thyroid hormone deficiency. We hypothesized that this enzyme might be a very sensitive indicator of minor alterations in thyroid function. We did not find, however, consistent changes in 5'-D with embryonic PCB 126 exposure in chicken embryos, although chickens generally are considered to be more sensitive to PCBs than many other avian species.
Development of Methods for Measuring Uridine Diphosphate Glucuronosyl Transferase (UDP-GT) Activity in Avian Liver, Measurement of the Pattern of UDP-GT Development and Measurement of Hepatic UDP-GT Activity in Some of the Studies of Embryonic Chickens Exposed to PCB 126. Because there have been essentially no measurements of UDP-GT activity in birds, we modified and validated an enzymatic assay using para-nitrophenol as a substrate for use with avian tissues. In addition, we described the pattern of UDP-GT activity in a galliform bird to provide basic information about the developmental patterns of this enzyme. The assay then was used to measure UDP-GT activity in chicken embryos in the PCB 126 studies described above. There was a trend towards increased UDP-GT activity with increasing PCB exposure, but there were no statistically significant effects on thyroid function up to PCB 126 doses that caused 40 percent mortality. We also did preliminary work on a UDP-GT assay using labeled T4 as a substrate. A new graduate student in the laboratory is continuing this work in hopes that it will provide a more sensitive method for measuring enzyme activity in stored liver samples from Herring Gulls.
Our studies of Herring Gulls in the Great Lakes show that gulls from areas with high-PCB exposures have altered thyroid function that compromises their capability to mount responses to altered environmental conditions requiring increases in thyroid hormone secretion. We have found evidence of decreased thyroid function in relation to increasing PCB exposure in embryos, prefledglings, and adult Herring Gulls. Both our gull studies and our collaborative studies on turtles indicate these animals also have decreases in thyroid function at sites with the highest PCB exposures. A number of those sites have been designated as Areas of Concern in the Great Lakes. Comparison of our measurements of thyroid gland weights to published data (the only type of thyroid measurement previously published) in this study suggests that the current degree of thyroid alteration is less than that present in the late 1980s, when PCB exposures were much higher than at present. Our studies of PCB 126 and 77 effects in chicken embryos suggest the mechanisms of toxicity of these coplanar PCB congeners are not through endocrine disruption. These findings suggest that some of the less toxic PCBs that are present in much higher concentrations are the ones likely to be responsible for most of the effects of PCBs on thyroid function.
Finally, studies in this project and others in our laboratory show that thyroidal hormone content is a much more sensitive indicator of alterations in thyroid function than measurements of either plasma thyroid hormones or thyroid weight. Thus, the measurement of thyroid gland hormone content should be developed as an assessment tool for studies of thyroid disruption.
Journal Articles on this Report : 5 Displayed | Download in RIS Format
|Other project views:||All 39 publications||8 publications in selected types||All 5 journal articles|
||Catena ML, Porter TE, McNabb FM, Ottinger AA. Cloning of a partial cDNA for Japanese quail thyroid-stimulating hormone beta and effects of methimazole on the thyroid and reproductive axes. Poultry Science 2003;82(3):381-387||
||McNabb FM. Biomarkers for the assessment of avian thyroid disruption by chemical contaminants. Avian and Poultry Biology Reviews 2005;16(1):3-10||
||McNabb FMA, Fox GA. Avian thyroid development in chemically contaminated environments: is there evidence of alterations in thyroid function and development? Evolution & Development 2003;5(1):76-82.||
||Quinn MJ, French JB, McNabb FMA, Ottinger MA. The effects of polychlorinated biphenyls (Aroclor 1242) on thyroxine, estradiol, molt, and plumage characteristics in the American kestrel (Falco sparverius). Environmental Toxicology and Chemistry 2002;21(7):1417-1422.||
||Scanes CG, McNabb FM. Avian models for research in toxicology and endocrine disruption. Avian and Poultry Biology Reviews 2003;14(1):21-52||