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<p>Polybrominated diphenyl ethers (PBDEs) are a class of brominated flame retardant
chemicals that are added to plastics, electronic components, furniture foam, and textiles
to reduce their combustibility. Of the three commercial mixtures historically marketed,
only DecaBDE, which is constituted almost entirely (~97%) of the fully brominated
congener decabromodiphenyl ether (BDE-209), continues to be used in the U.S. today.
While decaBDE is scheduled for phase-out in the U.S. at the end of 2013, exposures
to BDE-209 and other PBDEs will continue into the foreseeable future as products that
contain them continue to be used, recycled, and discarded. In addition, decaBDE use
continues to be largely unrestricted across Asia, although restricted from use in
electronic equipment in Europe. </p><p>Despite limits placed on PBDE uses, they are
ubiquitous contaminants detected worldwide in humans and wildlife. Major health effect
concerns for PBDEs come largely from evidence in laboratory rodents demonstrating
neurotoxicity, reproductive and developmental impairments, and thyroid disruption.
The potential for PBDEs, particularly BDE-209, to disrupt thyroid regulation and elicit
other toxic outcomes in fish is less clear. Thus, the overall objective of this thesis
research was to answer questions concerning how fish, as important indicators of overall
environmental health, are metabolizing PBDEs and whether and how PBDEs are disrupting
thyroid hormone regulation. The central hypothesis was that PBDE metabolism in fish
is mediated by iodothyronine deiodinase (dio) enzymes, which are responsible for activating
and inactivating thyroid hormones, and that PBDE exposures are causing thyroid system
dysfunction across fish life stages. </p><p>Under the first research aim, in vitro
experiments conducted in liver tissues isolated from common carp (Cyprinus carpio)
suggested a role for dio enzymes in catalyzing the reductive debromination of PBDEs.
Carp liver microsomes efficiently debrominated BDE-99 to BDE-47, and enzymes catalyzing
this reaction were associated predominantly with the endoplasmic reticulum (i.e.,
microsomal fraction) where dio enzymes are located. Competitive substrate experiments
in carp liver microsomes also demonstrated that rates of BDE-99 debromination to BDE-47
were significantly inhibited upon challenges with 3,3',5'-triiodothyronine (rT3) and
thyroxine (T4). This finding supported the hypothesis that enzymes involved in the
metabolism of PBDEs may have high affinities for thyroid hormones. Indeed, experiments
to determine apparent enzymatic kinetics (apparent Vmax and Km values) of BDE-99 hepatic
metabolism suggested that enzymes responsible for the catalytic activity appeared
to have a higher affinity for native thyroid hormone than BDE-99. </p><p>The second
and third research aims were focused on evaluating BDE-209 accumulation, metabolism,
and thyroid toxicity in juvenile and adult life stages of fish using the fathead minnow
(Pimephales promelas) as a model. BDE-209 bioaccumulated and was debrominated to several
reductive metabolites ranging from penta- to octaBDEs in both juvenile and adult fish
exposed to BDE-209. In addition, thyroid hormone regulation in juvenile and adult
male fathead minnows was severely disrupted by BDE-209 at low, environmentally relevant
exposures. In juvenile minnows, the activity of dio enzymes (T4-outer ring deiodination;
T4-ORD and T4-inner ring deiodination; T4-IRD) declined by ~74% upon oral doses of
9.8 ± 0.2 µg/g wet weight (ww) food at 3% body weight (bw)/day for 28 days, compared
to controls. Declines in dio activity were accompanied by thyroid follicle hypertrophy
indicative of over-stimulation and injury. In addition to thyroid disruption, a distinctive
liver phenotype characterized by vacuolated hepatocyte nuclei was measured in ~48%
of hepatocytes from treated fish that was not observed in controls. </p><p>Under the
third research aim, adult male fathead minnows received dietary treatments of BDE-209
at a low dose (95.3 ± 0.41 ng/g-food at 3% bw/day) and a high dose (10.1 ± 0.10 µg/g-food
at 3% bw/day) for 28 days followed by a 14-day depuration period to evaluate recovery.
Compared to negative controls, adult male fish exposed orally to BDE-209 at the low
dose tested for 28 days experienced a 53% and 46% decline in circulating total T4
and T3, respectively, while fish at the high BDE-209 dose tested had total T4 and
T3 deficits of 59% and 62%, respectively. Depressed levels of plasma thyroid hormones
were accompanied by a 45-50% decline in the rate of T4-ORD in brains of all treatments
by day 14 of the exposure. The decreased T4-ORD continued in the brain at day 28 with
a ~65% decline measured at both BDE-209 doses. BDE-209 exposures also caused transient,
tissue-specific upregulations of relative mRNA transcripts encoding dio enzymes (dio1,
dio2), thyroid hormone receptors (TR&alpha, TR&beta), and thyroid hormone transporters
(MCT8, OATP1c1) in the brain and liver in patterns that varied with time and dose,
possibly as a compensatory response to hypothyroidism. In addition, thyroid perturbations
at the low dose tested generally were equal to those measured at the high dose tested,
suggesting non-linear relationships between PBDE exposures and thyroid dysfunction
in adult fish. Thus, mechanisms for BDE-209 induced disruption of thyroid regulation
can be proposed in adult male minnows that involve altered patterns of thyroid hormone
signaling at several important steps in their transport and activation. </p><p>A
growing body of evidence describing PBDE toxicity in biota, including data generated
here, along with studies showing continued and rising PBDE body burdens, raises concern
for human and wildlife health. Long delays in removing PBDEs from the market, their
ongoing presence in many products still in use, and their active use outside the U.S.
and European Union will leave a lasting legacy of rising contamination unless more
concerted regulatory and policy actions are taken to reduce future exposures and harm.</p>
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