In Vitro and In Vivo Analysis of the Thyroid System–Disrupting Activities of Brominated Phenolic and Phenol Compounds in Xenopus laevis

Abstract

We investigated the effects of the brominated phenolic and phenol compounds, some of which are brominated flame retardants, on the binding of (125)I-3,3',5-L-triiodothyronine ((125)I-T(3)) to purified Xenopus laevis transthyretin (xTTR) and to the ligand-binding domain of X. laevis thyroid hormone receptor beta (xTR LBD), on the induction of a T(3)-responsive reporter gene in a recombinant X. laevis cell line (XL58-TRE-Luc) and on T(3)-induced or spontaneous metamorphosis in X. laevis tadpoles. Of the brominated phenolic and phenol compounds tested, 3,3',5-tribromobisphenol A and 3,3'-dibromobisphenol A were the most potent competitors of (125)I-T(3) binding to xTTR and the xTR LBD, respectively. Structures with a bromine in either ortho positions with respect to the hydroxy group competed more efficiently with T(3) binding to xTTR and the xTR LBD. 3,3',5-Tribromobisphenol A and 3,3',5,5'-tetrabromobisphenol A, at 0.1-1.0 microM, exerted both T(3) agonist and antagonist activities in the T(3)-responsive reporter gene assay. Sera obtained from fetal bovine and bullfrog tadpoles weakened the T(3) agonist and antagonist activities of 3,3',5-tribromobisphenol A, but not the T(3) antagonist activity of o-t-butylphenol, for which xTTR has no significant affinity. The T(3) agonist and antagonist activities of 0.5 microM 3,3',5-tribromobisphenol A were confirmed in the in vivo, short-term gene expression assay in premetamorphic X. laevis tadpoles using endogenous, T(3)-responsive genes as molecular markers. Our results suggest that 3,3',5-tribromobisphenol A affects T(3) binding to xTTR and xTR and that it interferes with the intracellular T(3) signaling pathway.

Characterization of little skate (Leucoraja erinacea) recombinant transthyretin: Zinc-dependent 3,3′,5-triiodo-l-thyronine binding

“…Tetrabromobisphenol A and pentabromophenol were the most potent chemicals in halogenated phenolic compounds with two phenolic rings and with a single ring, respectively. The structural preference of ligand binding was well conserved in vertebrate evolution (Kudo et al, 2006;Meerts et al, 2000;Morgado et al, 2007;Yamauchi et al, 2003). Furthermore, in the structureactivity relationships, halogen species-dependency (Br À > Cl À in pentahalogenated phenols; I À > Br À % Cl À in trihalogented phenols; Br À > Cl À > no halogen in bisphenol A) and halogen numberdependency (penta-> tri-in brominated and chlorinated phenols) are similar with previous studies using human (Meerts et al, 2000), chicken and amphibian TTRs (Kudo et al, 2006;Yamauchi et al, 2003).…”

A screening assay for thyroid hormone signaling disruption based on thyroid hormone-response gene expression analysis in the frog Pelophylax nigromaculatus

“…In addition, one in vitro study using GH3.TRE-Luc cell lines also revealed a non-monotonic dose-dependent TH signaling disrupting action of TBBPA ranging from 100 nmol/L-10 μmol/L in the presence of T3 (Freitas et al, 2011). However, other in vitro reporter gene assays could not detect non-monotonic dose-dependent effects of TBBPA on TH signaling, even when a wide concentration range of TBBPA was tested (Hofmann et al, 2009;Kudo et al, 2006;Oka et al, 2012). It is known that non-monotonic dose-response for endocrine disrupting chemicals is observed frequently (Beausoleil et al, 2013;Vandenberg, 2014).…”

Brominated flame retardants as possible endocrine disrupters

“…The effects of BFR on TH have chiefly been studied in mammals but have also been observed in, for example, frogs and birds (Fernie et al. , 2005; Kudo et al. , 2006).…”

“…The effects of BFR on TH have chiefly been studied in mammals but have also been observed in, for example, frogs and birds (Fernie et al, 2005;Kudo et al, 2006). In the frog, a TBBPA derivative affected triiodothyronine (T3) binding to TH-transporting protein and to hormone receptor, and in the American kestrel penta-BDE lowered the plasma thyroxine (T4) levels.…”