Iodotyrosine Deiodinase, a Novel Target of Environmental Halogenated Chemicals for Disruption of the Thyroid Hormone System in Mammals

Abstract

Many synthetic chemicals have been identified as environmental contaminants with activity to disrupt normal function of the thyroid hormone system. Thyroid hormones play important roles in growth, development, differentiation, and basal metabolic homeostasis, as well as in brain development in human fetus and children, and thyroid dysfunction can have very serious consequences, including mental retardation. Environmental chemicals may affect thyroid hormone action in multiple ways, including reduced thyroid hormone synthesis owing to direct toxicity at the thyroid gland, interaction with thyroid hormone receptors and transporters such as transthyretin, and disturbance of thyroid hormone metabolism (e.g., glucuronidation, sulfation and deiodination). In addition, iodotyrosine deiodinase, which is involved in iodide salvage by catalyzing deiodination of iodinated by-products of thyroid hormone production, was recently identified as a possible new target for disruption of thyroid hormone homeostasis by environmental halogenated chemicals. This topic, after briefly summarizing findings on the thyroid hormone-disrupting action of environmental chemicals in mammals, focuses on the effects of environmental halogenated chemicals on iodotyrosine deiodinase activity.

1. INTRODUCTION

Thyroid hormones, thyroxine (T4) and 3,3′,5-triiodotyronine (T3) (Fig. 1), play an important role in a wide range of physiological processes in mammals, including growth, development, differentiation, and basal metabolic homeostasis,¹⁾ and are essential for brain development in the human fetus and children.²⁾ Thyroid hormone deficiency in pregnant women is associated with severe mental disorders in their offspring.³⁾ Thyroid hormone T4 is synthesized via thyroglobulin in the thyroid gland, and partially converted into the more active T3.¹⁾ Both T4 and T3 are released into serum in a ratio of approximately 17 : 1,¹⁾ and then transported to target organs by the serum transport protein transthyretin (TTR).⁴⁾ In peripheral tissues, conversion of T4 to T3 by outer ring deiodination is catalyzed by type 1 and type 2 iodothyronine deiodinases (D1 and D2, respectively), which serve to regulate the balance of these hormones in the circulation.⁵⁾ Type 3 iodothyronine deiodinase (D3) also regulates intracellular T3 levels by catalyzing inner ring deiodination of T4, converting T4 into 3,3′,5′-triiodothyronine (reverse T3 [rT3]).⁶⁾ Both T3 and rT3 undergo additional deiodination to 3,3′-diiodothyronine (T2).⁷⁾ T2 and rT3 are both biologically inactive. Following deiodination, thyroid hormones are conjugated with sulfate or uridine 5′-diphosphate (UDP)-glucuronic acid by sulfotransferases (SULTs) or UDP-glucuronosyltransferases (UGTs).⁸⁾ Iodotyrosine deiodinase (IYD) functions in iodide salvage by catalyzing deiodination of iodinated by-products of thyroid hormone production.⁹⁾

Many environmental chemicals, classified as endocrine-disrupting chemicals (EDCs), disrupt the normal function of the endocrine system, influencing hormone biosynthesis, metabolism, or activity in wildlife and humans, and thereby modulating hormone-mediated responses.¹⁰⁾ Some of them specifically affect the thyroid hormone system.^11,12) This topic first briefly summarizes findings on the thyroid hormone-disrupting action of environmental chemicals in mammals, and then focuses on the effects of environmental halogenated chemicals on IYD activity.

Fig. 1. Structural Similarity of Hydroxylated PCBs and PBDEs to Thyroid Hormones

Cl ‘m/n’ and Br ‘m/n’ are number of chlorine and bromine atom substitution on a phenyl ring, respectively.

2. THYROID HORMONE DISRUPTION BY ENVIRONMENTAL HALOGENATED CHEMICALS IN MAMMALS

Many thyroid hormone-disrupting agents are halogenated aromatic hydrocarbons, which are structurally similar to the endogenous thyroid hormones. Polychlorinated biphenyls (PCBs) and polybrominated diphenyl ethers (PBDEs) are well-known environmental contaminants that disrupt thyroid hormone homeostasis (Fig. 1). They have been detected in wildlife and human serum, breast milk, and adipose tissue at levels of the order of pg-ng/g lipid, and are considered to be accumulated due to their lipophilicity.^13–16) Laboratory animals exposed to PCBs and PBDEs develop lesions in thyroid follicular cells, with decreased levels of circulating serum thyroid hormones.^17,18) PCBs and PBDEs exert neurotoxic effects on the developing brain by inducing hypothyroidism.^19,20) These compounds, including their hydroxylated metabolites, bind competitively at thyroid hormone receptors in mammals.^21–23) Hydroxylated PCBs and PBDEs also show high binding affinity for serum TTR.^24,25) The activity and gene expression of thyroid hormone metabolic enzymes, SULTs, UGTs and Ds, are affected by hydroxylated metabolites of PCBs and PBDEs.^26–29)

The chemical structures of several agrichemicals, antiparasitics, pharmaceuticals and food colorants resemble those of thyroid hormones. Among them, triclosan, a chlorinated phenolic antibacterial agent, decreases serum T4 concentration in male juvenile rats.³⁰⁾ Bithionol, closantel and rafoxanide, which are used as antiparasitics in animals, exhibit thyroid hormone-like activity in reporter gene assays using a rat pituitary cell line, MtT/E2.³¹⁾ Erythrosine B and rose bengal, which are used as food colorants, disrupt thyroid function in rodents: erythrosine B affects the pituitary-thyroid axis in rats³²⁾ and rose bengal inhibits thyroid peroxidase in vitro.³³⁾ These colorants were also reported to induce thyroid follicular cell hyperplastic lesions and adenomas in mice and rats after administration at high doses for a long period.^34,35) In addition, we have recently shown that IYD is a target of environmental halogenated chemicals, and this interaction may also have potential for disruption of thyroid hormone homeostasis.³⁶⁾

3. IODOTYROSINE DEIODINASE

IYD is critical for iodide recycling in the thyroid gland. It contains a flavin mononucleotide (FMN) moiety that is involved in reduced nicotinamide adenine dinucleotide phosphate (NADPH)-dependent reductive deiodination of 3-iodo-L-tyrosine (MIT) and 3,5-diiodo-L-tyrosine (DIT), which are released along with the thyroid hormones T4 and T3 during thyroglobulin proteolysis³⁷⁾ (Fig. 2). Deiodinases D1–3 also contribute to iodide recycling by catalyzing reduction of iodothyronines with release of iodide.³⁸⁾ Released iodide enters the circulatory system, and is transported back into the thyroid by the Na⁺/I⁻ symporter.³⁹⁾ However, proteolysis of thyroglobulin within thyroid follicular cells releases 6–7 times more iodide via MIT and DIT than is released from T4 and T3. Thus, IYD action on MIT and DIT is a major source of of iodide, which is reused for thyroid hormone biosynthesis.⁴⁰⁾

Fig. 2. IYD-Catalyzed Reductive Dehalogenation of Halogenated Tyrosines⁴⁴⁾

DEHAL1 has been identified as the gene encoding IYD in thyroid.⁴¹⁾ Two missense mutations and a deletion of three base pairs in DEHAL1 were identified in patients with hypothyroidism.⁴²⁾ Interestingly, carriers of DEHAL1 mutation have increased serum TSH levels in birth, and so cannot identify by neonatal mass screening.⁴²⁾ Ultimately, patients with IYD deficiency show inadequate intrathyroidal iodide retention and develop goitrous hypothyroidism.⁴³⁾

More recently, IYD has been discovered to act as a general dehalogenase, promoting reductive dehalogenation of 3-bromo- and 3-chloro-L-tyrosine, though not 3-fluoro-L-tyrosine.⁴⁴⁾ Thus, halogenated phenolic chemicals, such as hydroxylated PCBs and PBDEs, may inhibit IYD activity due to their structural similarity to halogenated tyrosines. This represents a new potential route of thyroid hormone disruption, and its possible significance needs to be addressed.

4. INHIBITION OF IODOTYROSINE DEIODINASE ACTIVITY BY ENVIRONMENTAL HALOGENATED CHEMICALS

To investigate the possible influence of environmental chemicals on thyroid hormone metabolism through inhibition of IYD activity, we examined the inhibitory effects of various halogenated chemicals on IYD activity in vitro using microsomal fraction of HEK-293 T cells expressing recombinant human IYD. We found that various halogenated phenolic chemicals, including hydroxylated PCBs and PBDEs, agrichemicals, antiparasitics, pharmaceuticals and food colorants, showed IYD-inhibitory activity³⁶⁾ (Fig. 3). These inhibitors may disrupt thyroid hormone homeostasis by blocking iodide recycling through inhibition of IYD activity in the thyroid. However, hydroxylated chemicals without a halogen atom, such as bisphenol A and uranine (fluorescein disodium salt), and halogenated chemicals without a hydroxyl group, such as non-hydroxylated PCBs (e.g., 2,2′,4,4′-tetrachlorobiphenyl) and PBDEs (e.g., 2,2′,4,4′-tetrabromodiphenyl ether [BDE-47]), nitrofen, trichlabendazole, miconazole and amiodarone, lacked IYD-inhibitory activity. Thus, the combination of halogen atom and hydroxyl group substitution on a phenyl ring is required for inhibition of IYD activity. Interestingly, the structural requirements of PCBs and PBDEs for thyroid hormone-disrupting action (e.g., affinity for thyroid hormone receptor and TTR) and for inhibition of SULTs and Ds also include hydroxylation and halogenation on a phenyl ring.^{21,22,24,26,28,29)} Therefore, a halogenated phenolic ring seems to be a critical structural requirement for thyroid hormone disruption, including IYD inhibition, by environmental chemicals.

Fig. 3. Chemical Structures of IYD-Inhibitory Agents³⁶⁾

Rose bengal, erythrosine B and phloxine B, which are xanthene food colorants that contain hydroxylated and halogenated phenyl ring structure, are also potent IYD inhibitors. Although rose bengal and erythrosine B induce thyroid adenomas in rodents,^34,35) the mechanism is not established. It is possible that IYD inhibition may be involved. Further research is necessary to uncover the mechanisms of induction of thyroid adenomas by environmental halogenated chemicals.

5. CONCLUSION AND PERSPECTIVES

There is extensive evidence that environmental halogenated chemicals have thyroid hormone-disrupting action in mammals. Many halogenated chemicals, such as PCBs and PBDEs, are highly lipophilic, and are accumulated in mammals through biological concentration in the food chain. These chemicals and their metabolites have been detected in wildlife and human serum, breast milk, and adipose tissue. Various environmental halogenated chemicals that contain hydroxylated and halogenated phenyl ring structure have been shown to inhibit IYD activity. Further work is necessary to evaluate the contribution of the IYD-inhibitory activity of such chemicals to disruption of in vivo thyroid hormone homeostasis and thyroid carcinogenesis.

Acknowledgment

The author is grateful to Prof. Kazumi Sugihara at Hiroshima International University, Prof. Shigeyuki Kitamura at Nihon Pharmaceutical University, and Prof. Shigeru Ohta at Hiroshima University for their advice and encouragement. The author is also grateful to Dr. Masafumi Yamaguchi at Hiroshima International University for providing HEK-293 T cells expressing recombinant human IYD, Prof. Hiroaki Kuroki at Daiichi University of Pharmacy for PCBs synthesis, and Dr. Naoto Uramaru at Nihon Pharmaceutical University for PBDEs synthesis. This study was supported by a Grant-in-Aid for Young Scientists (B) from the Japan Society for the Promotion of Science (JSPS KAKENHI Grant Number 24710071).

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