Competitive Binding of Poly- and Perfluorinated Compounds to the Thyroid Hormone Transport Protein Transthyretin

TOXICOLOGICAL SCIENCES 109(2), 206–216 (2009) doi:10.1093/toxsci/kfp055 Advance Access publication March 17, 2009 Competitive Binding of Poly- and Perfluorinated Compounds to the Thyroid Hormone Transport Protein Transthyretin Jana M. Weiss,* Patrik L. Andersson,† Marja H. Lamoree,* Pim E. G. Leonards,* Stefan P. J. van Leeuwen,* and Timo Hamers*,1 *Institute for Environmental Studies, Department of Chemistry and Biology, VU University, 1081 HV Amsterdam, The Netherlands; and †Department of Chemistry, Umeå University, 90187 Umeå, Sweden Received December 10, 2008; accepted March 10, 2009 Due to their unique surfactant properties, poly- and perfluorinated compounds (PFCs) have been extensively used and can be found all over the environment. Concern about their environmental fate and toxicological properties has initiated several research projects. In the present study, we investigated if PFCs can compete with thyroxine (T4, i.e., the transport form of thyroid hormone) for binding to the human thyroid hormone transport protein transthyretin (TTR). Such competitive capacity may lead to decreased thyroid hormone levels as previously reported for animals exposed to PFCs. Twenty-four PFCs, together with 6 structurally similar natural fatty acids, were tested for binding capacity in a radioligand-binding assay. The binding potency decreased in the order: perfluorohexane sulfonate > perfluorooctane sulfonate/perfluorooctanoic acid > perfluoroheptanoic acid > sodium perfluoro-1-octanesulfinate > perfluorononanoic acid, with TTR binding potencies 12.5–50 times lower than the natural ligand T4. Some lower molecular weight compounds with structural similarity to these PFCs were > 100 times less potent than T4. Simple descriptors based on the two-dimensional molecular structures of the compounds were used to visualize the chemical variation and to model the structure-activity relationship for the competitive potencies of the TTR-binding compounds. The models indicated the dependence on molecular size and functional groups but demanded a more detailed description of the chemical properties and data for validation and further quantitative structure-activity relationship (QSAR) development. Competitive binding of PFCs to TTR, as observed for human TTR in the present study, may explain altered thyroid hormone levels described for PFC-exposed rats and monkeys. Median human blood levels of the most potent TTR-binding PFCs are one to two orders of magnitude lower than concentration at 50% inhibition (IC50) values determined in the present study. In addition, this study contributes to the understanding of the bioaccumulation of PFCs in man and possibly in other wildlife species. 1 To whom correspondence should be addressed at Institute for Environmental Studies, Department of Chemistry and Biology, VU University, De Boelelaan 1085, 1081 HV Amsterdam, The Netherlands. Fax: þ31 (0)20-5989553. E-mail: . Key Words: perfluorinated compounds; thyroid hormone; thyroxine; transthyretin binding; perfluorooctane sulfonate; endocrine disruption. Poly- and perfluorinated organic compounds (PFCs) are a class of substances characterized by a partially or fully fluorinated alkyl chain and a terminal functional group. The C–F bonds result in great stability under extreme heat and chemical stress and give the compound an oleophobic (oil repelling) property, whereas the polar head contributes to the excellent surfactant property of many PFC (e.g., perfluorinated sulfonates and carboxylates). These unique properties contribute to the widespread use of PFCs in a variety of commercial products, such as household surface finishes, food packaging, water- and stain-resistant materials, fire-fighting foams, etc. (Kissa, 2001). Environmental research initially focused on the compounds perfluorooctane sulfonate (PFOS) and perfluorooctanoic acid (PFOA), the perfluorinated contaminants predominantly analyzed and also found in the environment. However, a variety of other PFCs are now being found worldwide in the environment, animals, and humans from urban to remote areas in all trophic levels (Houde et al., 2006; Kallenborn et al., 2004; Kannan et al., 2004; Lau et al., 2007; van Leeuwen et al., 2006). The structure of many PFCs and their behavior within the body of organisms are comparable to free fatty acids (FAs), and as such they bind to liver FA–binding protein, and the protein albumin, which is mainly present in blood, liver, and eggs (Jones et al., 2003; Luebker et al., 2002; Martin et al., 2003). It is suggested that the polar hydrophobic nature of fluorinecontaining compounds can lead to increased affinity for proteins, despite the relatively weak dipolar interactions that characterize the hard C–F dipole. The polar hydrophobic concept can explain some of the protein-binding data characteristics of fluorinated compounds (Biffinger et al., 2004). Highest levels of PFCs in rodents, humans, and marine animals are accordingly found in the protein-rich blood and
The Author 2009. Published by Oxford University Press on behalf of the Society of Toxicology. All rights reserved. For permissions, please email: TRANSTHYRETIN BINDING POTENCY OF FLUORINATED COMPOUNDS liver (Kannan et al., 2004; Kudo and Kawashima, 2003; Lau et al., 2003; Luebker et al., 2002; Martin et al., 2003; Thibodeaux et al., 2003; Van den Heuvel et al., 1992). The toxicology of PFCs has recently been extensively reviewed (Kudo and Kawashima, 2003; Lau et al., 2007, 2008). Among other observations, decreased thyroid hormone levels after PFC exposure have been found in monkeys and rodents (Lau et al., 2003; Luebker et al., 2002; Seacat et al., 2002, 2003; Thibodeaux et al., 2003). It has recently been shown that PFOS does not affect the regulatory functions of the thyroid hormone system itself, but it is the competitive binding to transport proteins that alters the free thyroxine (T4) levels in blood (Chang et al., 2008; Lau et al., 2007). By altering thyroid hormone levels, PFCs may affect fetal and neonatal development, especially since PFOS can cross the placental barrier in both humans (Inoue et al., 2004) and rodents (Lau et al., 2003; Thibodeaux et al., 2003). Whether this is caused by diffusion over the membrane or mediated by transport proteins is not yet known. Thyroid hormones are associated (not covalently bound) to transport proteins such as transthyretin (TTR). This complex functions as a circulating reservoir to buffer changes in thyroid hormone levels. TTR is not only a highly conservative plasma protein and the main T4 carrier in cerebrospinal fluid (CSF) but also important in serum of most mammalian species and birds. TTR is composed of four identical 127 amino acid b sheet–rich subunits. X-ray studies of TTR show two funnel-shaped binding sites for T4 (Blake et al., 1978), each with an inner and an outer binding site. The hydroxyl group as well as the two adjacent iodine molecules on the outer ring of the T4 structu (...truncated)