New approach methods to improve human health risk assessment of thyroid hormone system disruption-a PARC project

Abstract

Current test strategies to identify thyroid hormone (TH) system disruptors are inadequate for conducting robust chemical risk assessment required for regulation. The tests rely heavily on histopathological changes in rodent thyroid glands or measuring changes in systemic TH levels, but they lack specific new approach methodologies (NAMs) that can adequately detect TH-mediated effects. Such alternative test methods are needed to infer a causal relationship between molecular initiating events and adverse outcomes such as perturbed brain development. Although some NAMs that are relevant for TH system disruption are available-and are currently in the process of regulatory validation-there is still a need to develop more extensive alternative test batteries to cover the range of potential key events along the causal pathway between initial chemical disruption and adverse outcomes in humans. This project, funded under the Partnership for the Assessment of Risk from Chemicals (PARC) initiative, aims to facilitate the development of NAMs that are specific for TH system disruption by characterizing in vivo mechanisms of action that can be targeted by in embryo/in vitro/in silico/in chemico testing strategies. We will develop and improve human-relevant in vitro test systems to capture effects on important areas of the TH system. Furthermore, we will elaborate on important species differences in TH system disruption by incorporating non-mammalian vertebrate test species alongside classical laboratory rat species and human-derived in vitro assays.

Keywords: PARC; adverse outcome pathways; chemicals; endocrine disruption; non-animal test methods; regulatory toxicology; thyroid disruption.

FIGURE 1

The thyroid hormone (TH) system and tests to assess chemical effects that may lead to altered TH-signaling and adverse effects. The TH system has many components and molecules that can be targeted by environmental chemicals in one and/or all tissues. Targets can be grouped into classes according to the type of assay that can be used to test for potential effects of chemicals, e.g., by developing enzyme activity assays where each assay tests the activity of one enzyme, TPO, DIO1, DIO2, DEHAL1 etc. In vivo, each interaction may lead to altered TH-signaling with potential adverse effects on development and physiology. Abbreviations: DEHAL1: iodothyrosine dehalogenase, DIO: deiodinase, HPT: hypothalamic-pituitary-thyroid, MCT: monocarboxylate transporter, NIS: Sodium/Iodide Symporter, OATPs: organic-anion-transporting polypeptides, rT3: reverse T3, RXR: retinoid X receptor, SULTs: sulfotransferases, T2: 3,5-diiodo-L-thyronine, T3: triiodothyronine, T4: thyroxine, TBG: thyroxine-binding globulin, Tg: thyroglobulin, TH: thyroid hormone, TPO: thyroperoxidase, TR: thyroid receptor, TRE: thyroid response element, TRH: thyrotropin releasing hormone, TSH: thyroid stimulating hormone, TSHR: thyroid stimulating hormone receptor, TTR: transthyretin, UGTs: Uridine 5′-diphospho-glucuronosyltransferases.

FIGURE 2

The thyroid hormone system and fetal development. Maternal (A) and fetal (B) TH levels during human pregnancy. In this period the requirement for iodine in the mother’s diet increases considerably, as does the net demand for TH production by the maternal thyroid gland. Before the onset of fetal thyroid function the fetus is critically dependent on TH transferred from the maternal to the fetal circulation. Timing of ontogeny (C), function and autoregulation (D) of the fetal thyroid gland during pregnancy. (E) Maternal TH fully supply fetal requirements until fetal TH secretion begin at around 15 weeks of gestation after which fetal TH levels come from both the maternal and the fetal thyroid gland. (F) Timing of some TH mediated neurodevelopmental processes in the fetal brain. Early neuronal proliferation and migration is dependent on maternal TH in the first trimester of pregnancy. Also in the first trimester, inactivating type 3 deiodinase (DIO3) enzyme expression drops and development of the thyroid gland begins. In the second and third trimesters, the brain continues to develop, now increasingly relying on T4 produced by both the fetus and the mother. As the fetal hypothalamic-pituitary axis develops, a surge in thyroid-stimulating hormone (TSH) secretion occurs, initiating fetal TH production, expression of the activating enzyme deiodinase type 2 (DIO2), and increasing thyroid hormone nuclear receptors occupancy by T3. DIO: deiodinase, hCG: human chorionic gonadotropin, T3: triiodothyronine, T4: thyroxine, TBG: thyroxine-binding globulin, TH: thyroid hormone, TRH: thyrotropin releasing hormone, TSH: thyroid stimulating hormone, (Burrow et al., 1994; Howdeshell, 2002; López-Márquez et al., 2021).

FIGURE 3

The project: New approach methodologies (NAMs) for improved human health risk assessment. Building upon the adverse outcome pathway (AOP)-framework the project develops NAMs relating to specific molecular initiating events (MIE) and key events (KE) for thyroid hormone system disruption. Some of these relations are shown. The project also includes a number of activities to translate and/or validate assays, including leveraging data from zebrafish embryos to inform on human health and in vitro to in vivo translation.