National Center for Computational Toxicology, Office of Research and Development, U.S. Environmental Protection Agency, Research Triangle Park, North Carolina, USA.
Center for Drug Evaluation and Research, U.S. Food and Drug Administration, Washington, DC, USA.
Environ Health Perspect. 2019 Sep;127(9):97009. doi: 10.1289/EHP5314. Epub 2019 Sep 30.
Thyroid hormone receptors (TRs) are critical endocrine receptors that regulate a multitude of processes in adult and developing organisms, and thyroid hormone disruption is of high concern for neurodevelopmental and reproductive toxicities in particular. To date, only a small number of chemical classes have been identified as possible TR modulators, and the receptors appear highly selective with respect to the ligand structural diversity. Thus, the question of whether TRs are an important screening target for protection of human and wildlife health remains.
Our goal was to evaluate the hypothesis that there is limited structural diversity among environmentally relevant chemicals capable of modulating TR activity via the collaborative interagency Tox21 project.
We screened the Tox21 chemical library (8,305 unique structures) in a quantitative high-throughput, cell-based reporter gene assay for TR agonist or antagonist activity. Active compounds were further characterized using additional orthogonal assays, including mammalian one-hybrid assays, coactivator recruitment assays, and a high-throughput, fluorescent imaging, nuclear receptor translocation assay.
Known agonist reference chemicals were readily identified in the TR transactivation assay, but only a single novel, direct agonist was found, the pharmaceutical betamipron. Indirect activation of TR through activation of its heterodimer partner, the retinoid-X-receptor (RXR), was also readily detected by confirmation in an RXR agonist assay. Identifying antagonists with high confidence was a challenge with the presence of significant confounding cytotoxicity and other, non-TR-specific mechanisms common to the transactivation assays. Only three pharmaceuticals-mefenamic acid, diclazuril, and risarestat-were confirmed as antagonists.
The results support limited structural diversity for direct ligand effects on TR and imply that other potential target sites in the thyroid hormone axis should be a greater priority for bioactivity screening for thyroid axis disruptors. https://doi.org/10.1289/EHP5314.
甲状腺激素受体（TRs）是至关重要的内分泌受体，可调节成年和发育中生物体的多种过程，特别是甲状腺激素的破坏对神经发育和生殖毒性的关注度很高。迄今为止，只有少数化学类别被确定为可能的 TR 调节剂，并且受体对配体结构多样性具有高度选择性。因此，TR 是否是保护人类和野生动物健康的重要筛选靶标仍然存在疑问。
我们的目标是评估以下假设，即在通过协作机构 Tox21 项目能够调节 TR 活性的环境相关化学物质中，存在有限的结构多样性。
我们在定量高通量细胞报告基因测定中筛选了 Tox21 化学库（8305 个独特结构），以检测 TR 激动剂或拮抗剂活性。使用其他正交测定法（包括哺乳动物单杂交测定法、共激活剂募集测定法和高通量荧光成像核受体易位测定法）进一步表征活性化合物。
在 TR 反式激活测定中，很容易识别出已知的激动剂参考化学物质，但仅发现了一种新型直接激动剂，即药物 betamipron。通过在 RXR 激动剂测定中确认，也很容易检测到通过激活其异二聚体伴侣视黄酸-X 受体（RXR）间接激活 TR。具有高置信度的拮抗剂的鉴定是一项挑战，因为存在明显的细胞毒性和其他非 TR 特异性的共激活剂募集测定中常见的机制。仅确认了三种药物（甲芬那酸、地克珠利和 risarestat）为拮抗剂。
结果支持直接配体对 TR 的有限结构多样性，并暗示甲状腺激素轴中的其他潜在靶位点应该成为甲状腺轴破坏物生物活性筛选的更高优先级。https://doi.org/10.1289/EHP5314.
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