The affinity for thyroid hormone receptor (TR) and estrogen receptor α (ER) of polybromodiphenyl ethers (PBDEs) and hydroxylated PBDEs was examined. 4-Hydroxy-2,2',3,4',5-pentabromodiphenyl ether and 3-hydroxy-2,2',4,4'-tetrabromodiphenyl ether markedly inhibited the binding of triiodothyronine (1×10-10 M) to TR in the concentration range of 1×10-6 - 1×10-4 M. 2,3,4,5,6-Pentabromophenol also showed an inhibitory effect at 1×10-5 - 1×10-4 M. However, 2,2',3,4,4',5'-hexabromodiphenyl ether, decabromodiphenyl ether, 4-methoxy-2,2',3,4',5-pentabromodiphenyl ether, 4-hydroxy-2,2',4',5 -tetrabromodiphenyl ether, 4-hydroxy-2,2',3,4'-tetrabromodiphenyl ether, 4'-hydroxy-2,2',4-tribromodiphenyl ether, 3'-hydroxy-2,4-dibromodiphenyl ether, 2,4,6-tribromophenol and tetrabromohydroquinone did not show affinity for TR. In contrast, 4'-hydroxy-2,2',4-tribromodiphenyl ether and 3'-hydroxy-2,4-dibromodiphenyl ether exhibited estrogenic activity in estrogen-responsive reporter assay using MCF-7 cells at the concentration of 1×10-5 M. However, adjacent bromo substitution of 3- or 4-hydroxylated PBDEs markedly decreased the estrogenic activity. These results suggest that hydroxylated PBDEs act as thyroid hormone-like agents, as well as estrogens.
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