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Hill KL, Mortensen ÅK, Teclechiel D, Willmore WG, Sylte I, Jenssen BM, Letcher RJ. In Vitro and in Silico Competitive Binding of Brominated Polyphenyl Ether Contaminants with Human and Gull Thyroid Hormone Transport Proteins. ENVIRONMENTAL SCIENCE & TECHNOLOGY 2018; 52:1533-1541. [PMID: 29283575 DOI: 10.1021/acs.est.7b04617] [Citation(s) in RCA: 14] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/07/2023]
Abstract
Tetradecabromo-1,4-diphenoxybenzene (TeDB-DiPhOBz) is a highly brominated additive flame retardant (FR). Debrominated photodegradates of TeDB-DiPhOBz are hydroxylated in vitro in liver microsomal assays based on herring gulls (Larus argentatus), including one metabolite identified as 4″-OH-2,2',2″,4-tetrabromo-DiPhOBz. Chemically related methoxylated tetra- to hexabromo-DiPhOBzs are known contaminants in herring gulls. Collectively, nothing is currently known about biological effects of these polybrominated (PB) DiPhOBz-based compounds. The present study investigated the potential thyroidogenicity of 2,2',2″,4-tetrabromo-(TB)-DiPhOBz along with its para-methoxy (MeO)- and hydroxy-(OH)-analogues, using an in vitro competitive protein binding assay with the human thyroid hormone (TH) transport proteins transthyretin (hTTR) and albumin (hALB). This model para-OH-TB-DiPhOBz was found to be capable of competing with thyroxine (T4) for the binding site on hTTR and hALB. In silico analyses were also conducted using a 3D homology model for gull TTR, to predict whether these TB-DiPhOBz-based compounds may also act as ligands for an avian TH transport protein despite evolutionary differences with hTTR. This analysis found all three TB-DiPhOBz analogues to be potential ligands for gull TTR and have similar binding efficacies to THs. Results indicate structure-related differences in binding affinities of these ligands and suggest there is potential for these contaminants to interact with both mammalian and avian thyroid function.
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Affiliation(s)
- Katie L Hill
- Ecotoxicology and Wildlife Health Division, Environment and Climate Change Canada, National Wildlife Research Centre, Carleton University, Ottawa, Ontario K1A 0H3, Canada
- Department of Biology, Carleton University , Ottawa, Ontario K1S 5B6, Canada
- Intrinsik Corp. , Ottawa, Ontario K1S 5R1, Canada
| | - Åse-Karen Mortensen
- Department of Biology, Norwegian University of Science and Technology , Trondheim, NO-7491, Norway
| | | | - William G Willmore
- Department of Biology, Carleton University , Ottawa, Ontario K1S 5B6, Canada
| | - Ingebrigt Sylte
- Department of Medical Biology, UiT - The Arctic University of Norway , Tromsø, NO-9037, Norway
| | - Bjørn M Jenssen
- Department of Biology, Norwegian University of Science and Technology , Trondheim, NO-7491, Norway
| | - Robert J Letcher
- Ecotoxicology and Wildlife Health Division, Environment and Climate Change Canada, National Wildlife Research Centre, Carleton University, Ottawa, Ontario K1A 0H3, Canada
- Department of Biology, Carleton University , Ottawa, Ontario K1S 5B6, Canada
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Hill KL, Hamers T, Kamstra JH, Willmore WG, Letcher RJ. Organophosphate triesters and selected metabolites enhance binding of thyroxine to human transthyretin in vitro. Toxicol Lett 2018; 285:87-93. [PMID: 29306024 DOI: 10.1016/j.toxlet.2017.12.030] [Citation(s) in RCA: 37] [Impact Index Per Article: 6.2] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/08/2017] [Revised: 11/10/2017] [Accepted: 12/31/2017] [Indexed: 01/01/2023]
Abstract
The toxicological properties of organophosphate (OP) triesters that are used as flame retardants and plasticizers are currently not well understood, though increasing evidence suggests they can affect the thyroid system. Perturbation of thyroid hormone (TH) transport is one mechanism of action that may affect thyroid function. The present study applied an in vitro competitive protein binding assay with thyroxine (T4) and human transthyretin (hTTR) transport protein to determine the potential for the OP triesters, TDCIPP (tris(1,3-dichloro-2-propyl) phosphate), TBOEP (tris(butoxyethyl) phosphate), TEP (triethyl phosphate), TPHP (triphenyl phosphate), p-OH-TPHP (para-hydroxy triphenyl phosphate), and the OP diester DPHP (diphenyl phosphate), to competitively displace T4 from hTTR. Enhancement of T4 binding to hTTR, rather than the hypothesized competition, was observed for the six OP esters and in a concentration-dependent manner. For example, T4-hTTR binding was significantly increased at concentrations of TBOEP as low as 64 nM, and up to 184% of controls at 5000 nM. A plausible explanation of these results, which to our knowledge has not been previously reported, may be allosteric interactions of the OP esters with hTTR allowing T4 to access the second site of the TH binding pocket. These in vitro results suggest a novel mechanism of OP ester toxicity via T4 binding enhancement, and possible dysregulation of T4-hTTR interactions.
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Affiliation(s)
- Katie L Hill
- Ecotoxicology and Wildlife Health Division, Environment and Climate Change Canada, National Wildlife Research Centre, Carleton University, Ottawa, Canada; Department of Biology, Carleton University, Ottawa, Canada; Intrinsik Corp., Ottawa, Canada
| | - Timo Hamers
- Department of Environment and Health, Vrije Universiteit Amsterdam, The Netherlands
| | - Jorke H Kamstra
- Faculty of Veterinary Medicine and Biosciences, Department of Basic Science and Aquatic Medicine, CoE CERAD, Norwegian University of Life Sciences, Oslo, 0033, Norway
| | | | - Robert J Letcher
- Ecotoxicology and Wildlife Health Division, Environment and Climate Change Canada, National Wildlife Research Centre, Carleton University, Ottawa, Canada; Department of Biology, Carleton University, Ottawa, Canada.
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