University of Exeter, Biosciences, College of Life and Environmental Sciences, Geoffrey Pope Building, Stocker Rd., Exeter, EX4 4QD, UK.
University of Plymouth, School of Biological Sciences, Drake Circus, Plymouth, Devon, PL4 8AA, UK.
Aquat Toxicol. 2019 Apr;209:99-112. doi: 10.1016/j.aquatox.2019.01.022. Epub 2019 Jan 28.
Brominated flame retardants are known to disrupt thyroid hormone (TH) homeostasis in several vertebrate species, but the molecular mechanisms underlying this process and their effects on TH-sensitive tissues during the stages of early development are not well characterised. In this study, we exposed zebrafish (Danio rerio) embryo-larvae to 2,2',4,4'-tetrabromodiphenyl ether (BDE-47) and tetrabromobisphenol A (TBBPA) via the water for 96 h from fertilisation and assessed for lethality, effects on development and on the expression of a suite of genes in the hypothalamic-pituitary-thyroid (HPT) axis via both real time quantitative PCR (qRT-PCR) on whole body extracts and whole mount in situ hybridisation (WISH) to identify tissue targets. The 96-h lethal median concentration (96h-LC) for TBBPA was 0.9 μM and mortality was preceded by retardation of development (smaller animals) and morphological deformities including, oedemas in the pericardial region and tail, small heads, swollen yolk sac extension. Exposure to BDE-47 did not affect zebrafish embryo-larvae survival at any of the concentrations tested (1-100 μM) but caused yolk sac and craniofacial deformities, a curved spine and shorter tail at the highest exposure concentration. TBBPA exposure resulted in higher levels of mRNAs for genes encoding deiodinases (dio1), transport proteins (ttr), the thyroid follicle synthesis protein paired box 8 (pax8) and glucuronidation enzymes (ugt1ab) and lower levels of dio3b mRNAs in whole body extracts, with responses varying with developmental stage. BDE-47 exposure resulted in higher levels of thrb, dio1, dio2, pax8 and ugt1ab mRNAs and lower levels of ttr mRNAs in whole body extracts. TBBPA and BDE-47 therefore appear to disrupt the TH system at multiple levels, increasing TH conjugation and clearance, disrupting thyroid follicle development and altering TH transport. Compensatory responses in TH production/ metabolism by deiodinases were also evident. WISH analyses further revealed that both TBBPA and BDE-47 caused tissue-specific changes in thyroid receptor and deiodinase enzyme expression, with the brain, liver, pronephric ducts and craniofacial tissues appearing particularly responsive to altered TH signalling. Given the important role of TRs in mediating the actions of THs during key developmental processes and deiodinases in the control of peripheral TH levels, these transcriptional alterations may have implications for TH sensitive target genes involved in brain and skeletal development. These findings further highlight the potential vulnerability of the thyroid system to disruption by BFRs during early developmental windows.
溴化阻燃剂已被证实会破坏几种脊椎动物的甲状腺激素（TH）内环境稳定，但该过程的分子机制及其在早期发育阶段对 TH 敏感组织的影响尚未得到很好的描述。在这项研究中，我们通过水向斑马鱼（Danio rerio）胚胎-幼虫暴露于 2,2',4,4'-四溴二苯醚（BDE-47）和四溴双酚 A（TBBPA），从受精开始 96 小时，并通过实时定量 PCR（qRT-PCR）评估其致死率、对发育的影响以及下丘脑-垂体-甲状腺（HPT）轴中一系列基因的表达，通过全身提取物和整体原位杂交（WISH）来鉴定组织靶标。TBBPA 的 96 小时半致死浓度（96h-LC）为 0.9 μM，死亡率之前是发育迟缓（体型较小）和形态畸形，包括心包区和尾部水肿、小头部、卵黄囊肿胀延伸。暴露于 BDE-47 在任何测试浓度（1-100 μM）下均未影响斑马鱼胚胎-幼虫的存活率，但在最高暴露浓度下导致卵黄囊和颅面畸形、弯曲的脊柱和较短的尾巴。TBBPA 暴露导致编码脱碘酶（dio1）、转运蛋白（ttr）、甲状腺滤泡合成蛋白配对盒 8（pax8）和葡萄糖醛酸基转移酶（ugt1ab）的基因的 mRNA 水平升高，而全身提取物中的 dio3b mRNA 水平降低，反应随发育阶段而变化。BDE-47 暴露导致全身提取物中的 thrb、dio1、dio2、pax8 和 ugt1ab mRNA 水平升高，ttr mRNA 水平降低。因此，TBBPA 和 BDE-47 似乎在多个层面上破坏了 TH 系统，增加了 TH 的结合和清除，破坏了甲状腺滤泡的发育，并改变了 TH 的转运。脱碘酶介导的 TH 产生/代谢的代偿反应也很明显。WISH 分析进一步表明，TBBPA 和 BDE-47 均导致甲状腺受体和脱碘酶酶表达的组织特异性变化，大脑、肝脏、前肾管和颅面组织对改变的 TH 信号特别敏感。鉴于 TR 在介导 TH 对关键发育过程中的作用以及脱碘酶在控制外周 TH 水平中的作用，这些转录变化可能对涉及脑和骨骼发育的 TH 敏感靶基因产生影响。这些发现进一步强调了在早期发育窗口中，BFR 对甲状腺系统破坏的潜在脆弱性。
