Perchlorate and Nitrate Treatments Disrupt the Endoderm and Thyroid Development Through Epigenetic Mechanisms

Abstract Nitrate and perchlorate competitively inhibit iodide uptake in thyrocytes and disrupt thyroid function in rodents and humans. Our previous data indicated that the intrauterine exposure to perchlorate or nitrate induced thyroid dysfunction in the offspring rats during adult life. Therefore, this study aimed to investigate the effects of these endocrine disruptors during the embryonic period on the endoderm and thyroid development. Additionally, it was also investigated the role of epigenetic modifications in the programming of gene expression of the evaluated tissues. For this purpose, CD1 pregnant female mice received filtered water (control) or filtered water supplemented with sodium perchlorate (0.3 or 1 ppm) or sodium nitrate (20 or 50 ppm). At gestational day 16.5 (GD16.5), the embryonic thyroid lobes were collected and processed for molecular analysis. Besides the in vivo model, the effect of these thyroid disruptors was also evaluated during the differentiation of mouse embryonic stem cells (mESc) into endoderm and thyroid cells. Endoderm cells differentiation was achieved through the treatment of mESc with several growth factors. During the entire protocol the cells were exposed or not to sodium perchlorate or sodium nitrate (10-5 or 10-7 M). The effects of perchlorate and nitrate were also evaluated during the differentiation of mESC into thyroid cells. For this purpose, mESC-derived endoderm cells were transiently transfected with Pax8/Nkx2.1 expressing vectors. During the endoderm-to-thyrocytes differentiation protocol, the cells were also exposed or not to perchlorate or nitrate (10-5 or 10-7 M). The results demonstrated that both thyroid disruptors reduced the mRNA and protein expression of several endoderm markers (Foxa1, Gata4, Sox17) in the mESc-derived endoderm cells. Moreover, perchlorate or nitrate treatment also reduced the expression of thyroid transcription factors (Pax8, Nkx2.1, Foxe1) and thyroid differentiation markers (Slc5a5, Tpo, Tshr, Tg) both in the embryonic thyroid lobes and in the mESc-derived thyrocytes. Epigenetic mechanisms related to transcription repression seem to be involved in the gene expression downregulation both in vivo and in vitro, since perchlorate and nitrate increased the mRNA expression of Dnmt1, Dnmt3, Hdac and reduced the expression of Hat. Additionally, the methylation of histone H3 was increased, and the acetylation status of this histone was decreased in perchlorate- or nitrate-exposed thyroid lobes and mESc-derived endoderm/thyroid cells. In conclusion, our data strongly suggest that the programming of thyroid dysfunction induced by intrauterine exposure to perchlorate or nitrate involve the disruption of the endoderm and thyroid development during embryonic life through epigenetic mechanisms.