Environmental Toxicology and Pharmacology 23 (2007) 302–307 Thyroid hormone receptor isoform selectivity of thyroid hormone disrupting compounds quantified with an in vitro reporter gene assay Merijn Schriks a,∗ , Julie M. Roessig b , Albertinka J. Murk a , J. David Furlow b a Toxicology Section, Wageningen University, Tuinlaan 5, 6703 HE Wageningen, The Netherlands b Section of Neurobiology, Physiology and Behavior, University of California, Davis, CA 95616-8519, USA Received 3 July 2006; accepted 14 November 2006 Available online 18 November 2006 Abstract Some compounds, including brominated diphenyl ethers (BDEs), can interfere with thyroid hormone (TH) receptor (TR)-mediated TH-signalling. In this study, the TR isoform selectivity of some TH disrupting compounds was investigated with TR/ specific reporter gene assays. For this purpose, the effects of compounds on 3,3 ′ ,5-triiodothyronine (T 3 )-induced TR- or TR-activation were tested in green monkey kidney fibroblast (CV-1) cells transiently transfected with Xenopus TRs and a luciferase reporter gene. The T 3 -like BDE-OH and diiodobiphenyl (DIB) increased T 3 - induced TR-activation, but not T 3 -induced TR-activation. BDE28 (100 nM) did not act via TR, but almost tripled T 3 -induced TR-activation relative to T 3 at its EC 50 . BDE206 (100 nM) was antagonistic on both TRs with a maximum repression -54% relative to T 3 at its EC 50 . Contrary to previous results obtained with the T-screen, HBCD was inactive. The present study illustrates the importance of testing potential TH disrupting compounds in model systems that enable independent characterization of effects on both T 3 -induced TRs. © 2006 Elsevier B.V. All rights reserved. Keywords: PHAHs; Brominated flame retardants; BDE206; HBCD; Transient transfections; Differential expression 1. Introduction Thyroid hormones (THs) regulate several essential physio- logical processes such as energy metabolism, growth, formation of the central nervous system (CNS), tissue differentiation and reproduction (Tata, 1993; Kaltenbach, 1996; Wu and Koenig, 2000). The molecular action of THs is mediated via the thyroid hormone receptors (TRs) (Wu and Koenig, 2000) which, after ligand binding, activate genes by binding to the thyroid hormone response elements (TREs). Vertebrates have two separate genes that encode the TRs, designated TRα and TRβ (Sap et al., 1986; Weinberger et al., 1986). Considering the importance of THs on various physiologi- cal and morphogenetical processes, it is not surprising that the interference of environmental contaminants on multiple levels of thyroid hormone signalling is of concern to both scientists as well as to legislators demanding that newly marketed com- pounds are tested for TH disrupting potential (OECD, 2006). ∗ Corresponding author. Tel.: +31 317 485271; fax: +31 317 484931. E-mail address: Merijn.schriks@wur.nl (M. Schriks). A group of persistent organic pollutants (POPs) known to interfere with the TH-axis at several levels, are the polyhalo- genated aromatic hydrocarbons (PHAHs). These compounds and their hydroxylated metabolites can compete with 3,3 ′ ,5,5 ′ - l-thyroxine (T 4 ) for binding to the plasma transport protein transthyretin (TTR) (Lans et al., 1993, 1994; Cheek et al., 1999; Meerts et al., 2000; Ishihara et al., 2003; Yamauchi et al., 2000, 2003) and they also inhibit deiodinase activity (Adams et al., 1990), T 4 -glucuronidation and T 4 -sulfation (Schuur et al., 1998a,b; Brouwer et al., 1998). Only recently, more attention is paid to direct interaction with the TRs and related complex effects on TH signalling (Zoeller, 2005). In recent TR-binding studies it has been shown that bisphenol-A binds to TR/ with relative high affinity (Kitamura et al., 2002; Moriyama et al., 2002), but that polychlorinated biphenyls (PCBs) do not (Gauger et al., 2004). In the presence of the most bioac- tive form of TH, 3,3 ′ ,5-triiodo-l-thyronine (T 3 ), it is to be expected that specific compounds can bind to and/or modu- late the activity of TR or TR in a highly selective way (McKinney and Waller, 1994, 1998). Beeren van et al. (1995) and Bakker et al. (1994) showed, for example, that the phar- maceutical compound desethylamiodarone is a noncompetitive 1382-6689/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.etap.2006.11.007