Examining thyroid hormone synthesis feedback loops as xenobiotic target for brominated flame retardant metabolites

项目摘要 多溴联苯醚(BDEs)在美国被用作阻燃剂，直到它们被逐步淘汰。 几年前，由于担心它们在环境中的持久性和毒性。然而，继续 人类对BDEs的暴露是公认的，并与神经发育缺陷和 循环中的甲状腺激素。虽然大多数研究都集中在这些污染物与 甲状腺激素受体参与生殖和代谢功能，但其作用机制尚未引起足够的重视 涉及甲状腺激素的生物合成途径。根据BDE之间的流行病学联系 暴露和循环中的甲状腺激素水平，以及缺乏研究探讨 这一现象，这一提议的总体科学前提是，暴露在OH-BDEs中会破坏 甲状腺激素生物合成的负反馈抑制。对这一重要生物合成的调节 通路可能是解释血液中BDEs和BDEs之间流行病学联系的缺失环节 甲状腺功能减退。我们假设存在两种未被研究的机制，即BDEs和它们的 代谢物可以调节循环中的甲状腺激素。第一个机制将检查OH-BDE代谢物 与甲状腺激素受体β2(TRβ2)的相互作用，这是一种甲状腺受体亚型，仅在 参与反馈抑制甲状腺激素合成的下丘脑和脑垂体。第二 机制将通过表观遗传机制研究OH-BDE-对甲状腺受体功能的间接调节。 特别是miRNAs。我们提出了两个高度独立但相关的具体目标来解决这些知识 差距。这些研究旨在检验与OH-BDE调节下丘脑上游有关的假说 控制甲状腺激素循环水平的反馈回路。特殊目标1将检验以下假设：OH- BDES将拮抗下丘脑和脑垂体特异性的甲状腺受体，trβ2，最终降低 促甲状腺激素释放激素(TRH)在下丘脑神经细胞中的表达。这些假设将通过以下方式进行测试 甲状腺激素受体报告分析和下丘脑细胞株。《特定目标2》将检验这一假设 这种甲状腺激素受体的表达在下丘脑中受到miRNA调节剂的抑制 OH-BDEs。这一假说将通过测量下丘脑细胞中TRs的miRNA调节器来检验 TRS表达与TRH的关系如果下丘脑中TRs的miRNA调节器与 合成TRH，我们将得出结论，miRNAs是OH-BDEs产生负面影响的关键调节因子 对甲状腺系统的影响。这些目标的完成将为观察到的甲状腺效应提供机械性的洞察 与接触溴化阻燃剂有关。这些检测方法也适用于其他甲状腺激素。 干扰物和这一机制可能在调节甲状腺激素循环水平方面具有重要意义 环境中的化学物质暴露。这些研究的结果将为支持未来 活体研究的建议集中在这些机制上。

Project Summary Polybrominated diphenyl ethers (BDEs) were used as flame retardants until they were phased out in the USA several years ago due to concerns about their environmental persistence and toxicity. However, continued human exposure to BDEs is well established and is linked to neurodevelopmental deficits and disruption of circulating thyroid hormones. While most studies have focused on interactions of these contaminants with thyroids receptors involved in reproductive and metabolic functions, little attention has been paid to mechanisms involving thyroid hormone biosynthesis pathways. In light of the epidemiological associations between BDE exposure and circulating thyroid hormone levels, and the lack of studies that examine the mechanistic drivers of this phenomenon, the overall scientific premise of this proposal is that exposure to OH-BDEs disrupts the negative feedback inhibition of thyroid hormone biosynthesis. Modulation of this important biosynthesis pathway may be the missing link to explain the epidemiological associations between BDEs in the blood and hypothyroidism. We posit that there are two understudied mechanisms through which BDEs and their metabolites may modulate circulating thyroid hormones. The first mechanism will examine OH-BDE metabolites interactions with the thyroid hormone receptor beta 2 (TRβ2), a thyroid receptor isoform found exclusively in the hypothalamus and pituitary where it is involved in feedback inhibition of thyroid hormone synthesis. The second mechanism will examine indirect modulation of thyroid receptor function by OH-BDE-via epigenetic mechanisms, specifically miRNAs. We propose two highly independent but related specific aims to address these knowledge gaps. These studies are designed to test hypotheses related to OH-BDE regulation of upstream hypothalamic feedback loops that control circulating levels of thyroid hormone. Specific aim 1 will test the hypothesis that OH- BDEs will antagonize the hypothalamic and pituitary specific thyroid receptor, TRβ2, ultimately decreasing thyrotropin-releasing hormone (TRH) in hypothalamic neuronal cells. These hypotheses will be tested using thyroid hormone receptor reporter assays and a hypothalamus cell line. Specific aim 2 will test the hypothesis that thyroid hormone receptor expression is repressed in the hypothalamus by miRNA regulators in the presence of OH-BDEs. This hypothesis will be tested by measuring miRNA regulators of TRs in hypothalamic cells in relation to TRs expression and TRH. If hypothalamic miRNA regulators of TRs correlate to changes in synthesized TRH, we will conclude that miRNAs are key regulators by which OH-BDEs impose negative effects on the thyroid system. Completion of these aims will lend mechanistic insight into the observed thyroid effects associated with exposure to brominated flame retardants. These assays are adaptable to other thyroid hormone disruptors and this mechanism may be significant in regulating circulating levels of thyroid hormones following environmental chemical exposures. Results from these studies will lend important evidence to support future in vivo research proposals focused on these mechanisms.