Metabolism of PBDEs in Fathead Minnows (Pimephales Promelas) and Effects on Thyroid RegulationEPA Grant Number: FP917145
Title: Metabolism of PBDEs in Fathead Minnows (Pimephales Promelas) and Effects on Thyroid Regulation
Investigators: Noyes, Pamela Diane
Institution: Duke University
EPA Project Officer: Cobbs-Green, Gladys M.
Project Period: August 1, 2010 through July 31, 2013
Project Amount: $111,000
RFA: STAR Graduate Fellowships (2010) RFA Text | Recipients Lists
Research Category: Academic Fellowships , Fellowship - Pesticides and Toxic Substances
Polybrominated diphenyl ethers (PBDEs) are flame-retardant chemicals added to consumer products such as furniture foam, carpets, car seats, and electronics to reduce their flammability. PBDEs are released into the environment through numerous pathways, and they are now environmentally ubiquitous. This contamination is highest in people and wildlife in North America. Evidence suggests that PBDEs may be neurodevelopmental toxins. They also perturb the endocrine system of vertebrates by impairing thyroid function, which plays a key role in growth, development, and metabolism of all vertebrates. Core objectives of this research are to determine how young and adult fish, as important indicators of overall environmental health, are metabolizing PBDEs to more persistent, bioavailable, and potentially toxic metabolites, and how these compounds may disrupt fish thyroid regulation.
Polybrominated diphenyl ethers (PBDEs) are flame-retardant chemicals added to consumer products such as furniture, car seats, and electronics to reduce their flammability. They are now widespread contaminants in both living and non-living parts of the environment. The core questions I am seeking to answer center on determining: 1) how young and adult fish are breaking down PBDEs to more persistent and toxic products; and 2) how these compounds are potentially disrupting the fish thyroid system.
This research will focus primarily on decabromodiphenyl ether (BDE-209). BDE 209 is the primary congener in the high production volume mixture known as DecaBDE, which is the only commercial PBDE mixture still used today. The first phase of this research project will measure the accumulation of BDE-209 and its metabolites in fathead minnow (Pimephales promelas) adults, larvae, and juveniles receiving dietary exposures to BDE-209. This initial phase will also involve using in vitro assays of sub-cellular fractions to help identify enzyme systems catalyzing PBDE metabolism by examining the activity of potential biotransforming enzymes across different fathead minnow life stages. PBDE metabolism in rodents and humans appears to occur primarily through oxidative, cytochrome P450 (CYP)-mediated pathways, which generate hydroxylated metabolites (e.g., OH-BDEs). In fish, however, studies suggest that PBDE metabolism occurs via a reductive dehalogenation pathway. No hydroxylated metabolites have been observed to form metabolically in PBDE-exposed fish. An important hypothesis of this research is that deiodinase (DI) enzymes, which aid in vertebrate thyroid hormone homeostasis, may be involved in this reductive debromination pathway in fish. This will be the first study to apply liquid chromatography tandem mass spectrometry (LC/MS-MS) methods, recently developed in our laboratory, to measure DI activity and contaminant effects on DI activity. The second phase of this research will explore thyroidal effects and mechanisms of thyroid dysfunction at different levels of the thyroid axis of adult fathead minnows exposed to BDE-209 via the diet. This phase of the research will examine BDE-209 effects on circulating thyroid hormone levels (i.e., thyroxine (T4) and triiodothyronine (T3)) and thyroid follicle morphology. It will also examine whether these BDE-209 exposures alter the expression of genes encoding important thyroidal and hepatic proteins involved in thyroid hormone homeostasis. Genes that will be examined include those encoding DI enzymes, thyroid hormone receptors and transporters, thyroid stimulating hormone (TSH), and hepatic metabolizing enzymes. This research will be the first evaluation of the mRNA expression of DI isoforms in fish exposed to PBDEs.
PBDE effects on fish and other wildlife continue to be poorly understood, and this research will contribute to filling this data gap. It will increase our understanding of PBDE metabolic pathways and mechanisms of thyroid dysfunction in fish exposed to this important class of contaminants. Substantial differences in the biotransformation of PBDEs have been observed between mammals and fish. While the reductive debromination of PBDEs to potentially more persistent and bioactive congeners appears to be a major metabolic pathway in fish, the involvement and role of specific enzyme systems are largely unknown. This will be the first research to more clearly address why there appear to be substantial differences between how mammals and fish metabolize PBDEs. Moreover, DI-catalyzed metabolism of an environmental contaminant would be a novel pathway not observed previously in vertebrates. By examining PBDE metabolism across different life-stages, this research will help to elucidate whether early life-stages of fish may be especially sensitive to these contaminants. Finally, work under this project will further our understanding of PBDE effects across different levels of the fish thyroid system, including their potential to alter peripheral thyroid hormone levels, thyroid hormone-regulated gene transcription and metabolic activity, and thyroid follicle morphology.
Potential to Further Environmental/Human Health Protection:
Data collected under this research project will help to inform decision-making to balance the benefits gained from the use of PBDEs with their potential to cause adverse effects in sensitive wildlife populations and humans. Because the thyroid system is highly conserved across vertebrates, results of this work could have broader applications to elucidate PBDE effects on human health. Finally, this PBDE effects research will combine the use of chemical, biochemical, sub-cellular, histological, and molecular assays and techniques. This type of integrative approach may serve as a useful model for detecting contaminant impacts among wild fish populations and for evaluating the potential for other contaminants to cause thyroid disruption.