Disruption of thyroid hormone binding to sea bream recombinant transthyretin by ioxinyl and polybrominated diphenyl ethers

Abstract

A number of chemicals released into the environment share structural similarity to the thyroid hormones (THs), thyroxine (T₄) and triiodothyronine (T₃) and it is thought that they may interfere with the thyroid axis and behave as endocrine disruptors (EDs). One of the ways by which such environmental contaminants may disrupt the TH axis is by binding to TH transporter proteins. Transthyretin (TTR) is one of the thyroid hormone binding proteins responsible for TH transport in the blood. TTR forms a stable tetramer that binds both T₄ and T₃ and in fish it is principally synthesized in the liver but is also produced by the brain and intestine. In the present study, we investigate the ability of some chemicals arising from pharmaceutical, industrial or agricultural production and classified as EDs, to compete with [I¹²⁵]-T₃ for sea bream recombinant TTR (sbrTTR). Ioxinyl, a common herbicide and several polybrominated diphenyl ethers were strong inhibitors of [I¹²⁵]-T₃ binding to TTR and some showed even greater affinity than the natural ligand T₃. The TTR competitive binding assay developed offers a quick and effective tool for preliminary risk assessment of chemicals which may disrupt the thyroid axis in teleost fish inhabiting vulnerable aquatic environments.

Introduction

Environmental contamination by endocrine disruptors is presently a major issue of concern. Such synthetic chemicals can mimic or block hormones interfering with the endocrine system and eventually compromising crucial biological processes. The increasing contamination of the biosphere with chemicals with potential endocrine disrupting effects can become a serious threat to human and wildlife populations.

Thyroid hormones (THs) are known to play a crucial role in many metabolic processes and are essential for normal growth, differentiation and development of vertebrates (Hadley, 1996). Their importance in early life stages is well established in mammals and in amphibians THs are crucial for metamorphosis (Yun-Bo Shi, 1996). In fish, THs are implicated in reproduction and appear to be important in the regulation of development. High concentrations are present in fish eggs and increased levels are reported during metamorphosis or during the larval-juvenile transition (for review see Leatherland, 1994, Power et al., 2001, Yamano, 2005). In light of this evidence, disruption of the thyroid axis may seriously compromise normal development, differentiation, growth or reproduction in many vertebrates (Brown et al., 2004, Boas et al., 2006).

The thyroid hormones have a small hydrophobic thyronine nucleus that mediates their action by binding to specific nuclear receptors, which act directly on target genes bringing about a cellular response (Yen and Chin, 1994). 3,5,3′-l-Triiodothyronine (T₃) is the most active hormone and binds with high affinity to nuclear receptors (TRs), while l-thyroxine (T₄), which is the precursor of T₃ binds with low affinity and has few direct actions (Hadley, 1996, Darras et al., 1998). Almost all THs circulating in the plasma are bound to transporter proteins (Larsson et al., 1985) and only free hormones enter cells to elicit a response (Ekins et al., 1982, Mendel, 1989). The balance of free to bound THs in the plasma depends on plasma proteins, although the importance of this interaction is not entirely understood.

In vertebrates, thyroid hormone-binding proteins (THBP) include transthyretin (TTR), thyroxine-binding globulin (TBG) and albumin (ALB). Transthyretin in its tetrameric form transports THs and is the major plasma TH carrier in rodents and the main THBP in cerebrospinal fluid of both rodents and humans (Schreiber and Richardson, 1997). In small eutherians and lower vertebrates both albumin and transthyretin seem to be important for TH transport (Power et al., 2000). However, in tadpoles and fish, TTR is proposed to be the major TH carrier protein (Yamauchi et al., 1993, Yamauchi et al., 1999) and is reported to have higher affinity for T₃ than T₄ in birds, reptiles and amphibians (Yamauchi et al., 1993, Yamauchi et al., 2000, Chang et al., 1999). The TH affinity of TTR in teleost fish is less clear cut and in some species it is reported to have higher affinity for T₃ (Yamauchi et al., 1999), although sbrTTR binds T₃ and T₄ with similar affinities (Morgado et al., 2006).

A number of chemicals released into the environment share structural similarity with THs (Fig. 1) and may behave as endocrine disrupting chemicals (EDCs) if they substitute THs and disrupt the thyroid axis. Many of these chemicals, usually benzenic halogenated compounds, arising from either the industrial, medical or agricultural sector have been reported to interact strongly with plasma THBPs, especially TTR in vertebrates (Rickenbacher et al., 1986, Lans et al., 1993, McKinney and Waller, 1994, Brouwer et al., 1999, Cheek et al., 1999, Yamauchi et al., 2000, Ishihara et al., 2003a, Ishihara et al., 2003b). Other compounds structurally even closer to THs are the brominated flame retardants, extensively used in electronic and plastic materials. These organohalogenic chemicals are produced as commercial mixtures that include a wide variety of bromine-containing derivatives, like the polybrominated diphenyl ethers (PBDE) or tetrabromobisphenol A (TBBPA), and many of them are found as persistent pollutants in the environment with bioaccumulating potencies. In the past few years, brominated flame retardants have been associated with endocrine disruption (Hallgren and Darnerud, 2002, Legler and Brouwer, 2003) and many of them were found to mimic THs (Kitamura et al., 2002, Zhou et al., 2002) and bind human TTR in vitro (Meerts et al., 2000, Hamers et al., 2006).

The binding and transport of exogenous compounds by THBPs might be expected to affect TH balance as they may be preferentially taken up by cells leading to thyroid axis disruption. The aquatic environment and organisms such as fish are constantly threatened by pollution resulting from human activity and a previous study has shown that EDCs can bind to TTR in masu salmon (Ishihara et al., 2003b). The large number of chemicals released into the aquatic environment (from industrial residues, land run-offs and acid rains) and the desire of society to minimize the use of animals for testing highlights the need for quick robust and cheap predictive in vitro assays.

In the present study, a competitive binding assay was developed and the effect of putative EDCs of agricultural, medical and industrial origin on [I¹²⁵]-T₃ binding to sbrTTR was assessed. Teleost fish are frequently used as sentinel organisms for aquatic ecosystem contamination and the present in vitro assay with teleost TTR should be a more adequate risk assessment tool than equivalent assays with mammalian TTR.