The Transcriptomic Response of the Murine Thyroid Gland to Iodide Overload and the Role of the Nrf2 Antioxidant System.

Ilias Georgakopoulos-Soares,Ioannis Habeos,Ioannis P Trougakos,Gerasimos P Sykiotis,Adam A T Smith,Ana Claudia Marques,Mark Ibberson,Dionysios V Chartoumpekis,Panos G Ziros,Nicholas K H Khoo,Peter A Kopp

doi:10.3390/antiox9090884

Abstract

Background: Thyroid follicular cells have physiologically high levels of reactive oxygen species because oxidation of iodide is essential for the iodination of thyroglobulin (Tg) during thyroid hormone synthesis. Thyroid follicles (the functional units of the thyroid) also utilize incompletely understood autoregulatory mechanisms to defend against exposure to excess iodide. To date, no transcriptomic studies have investigated these phenomena in vivo. Nuclear erythroid factor 2 like 2 (Nrf2 or Nfe2l2) is a transcription factor that regulates the expression of numerous antioxidant and other cytoprotective genes. We showed previously that the Nrf2 pathway regulates the antioxidant defense of follicular cells, as well as Tg transcription and Tg iodination. We, thus, hypothesized that Nrf2 might be involved in the transcriptional response to iodide overload. Methods: C57BL6/J wild-type (WT) or Nrf2 knockout (KO) male mice were administered regular water or water supplemented with 0.05% sodium iodide for seven days. RNA from their thyroids was prepared for next-generation RNA sequencing (RNA-Seq). Gene expression changes were assessed and pathway analyses were performed on the sets of differentially expressed genes. Results: Analysis of differentially expressed messenger RNAs (mRNAs) indicated that iodide overload upregulates inflammatory-, immune-, fibrosis- and oxidative stress-related pathways, including the Nrf2 pathway. Nrf2 KO mice showed a more pronounced inflammatory–autoimmune transcriptional response to iodide than WT mice. Compared to previously published datasets, the response patterns observed in WT mice had strong similarities with the patterns typical of Graves’ disease and papillary thyroid carcinoma (PTC). Long non-coding RNAs (lncRNAs) and microRNAs (miRNAs) also responded to iodide overload, with the latter targeting mRNAs that participate mainly in inflammation pathways. Conclusions: Iodide overload induces the Nrf2 cytoprotective response and upregulates inflammatory, immune, and fibrosis pathways similar to autoimmune hyperthyroidism (Graves’ disease) and PTC.

Highlights

The thyroid gland normally produces high amounts of hydrogen peroxide (H2O2), which is necessary for the oxidation of iodine to iodide, the iodination of thyroglobulin (Tg), and the production of thyroid hormones, which are essentially iodinated derivatives of selected tyrosines of Tg
We recently showed that the transcription factor nuclear erythroid factor 2 like 2 (Nfe2l2 or Nrf2) plays an important role in the positive regulation of these enzymes and is a direct positive regulator of Tg transcription and a negative regulator of Tg iodination [2]
This means that the effects of genotype (WT or KO), iodide treatment, or their combination each led to a distinct gene expression signature. This is evident from the principal component analysis (PCA) of gene expression values (Figure 1B), where the samples clustered clearly depending on treatment and genotype

Summary

Introduction

The thyroid gland normally produces high amounts of hydrogen peroxide (H2O2), which is necessary for the oxidation of iodine to iodide, the iodination of thyroglobulin (Tg), and the production of thyroid hormones, which are essentially iodinated derivatives of selected tyrosines of Tg. The Nrf pathway is of high medical interest due to its druggable nature, and Nrf inducers have been tested in clinical trials for cancer chemoprevention, chronic kidney disease, neurodegenerative diseases, and diabetes [13] Natural compounds such as sulforaphane from broccoli sprout extracts can enhance Nrf signaling in thyroid follicular cells [2,14] and provide a promising method to enhance cytoprotection in the thyroid. Nuclear erythroid factor 2 like 2 (Nrf or Nfe2l2) is a transcription factor that regulates the expression of numerous antioxidant and other cytoprotective genes. We showed previously that the Nrf pathway regulates the antioxidant defense of follicular cells, as well as Tg transcription and Tg iodination. Conclusions: Iodide overload induces the Nrf cytoprotective response and upregulates inflammatory, immune, and fibrosis pathways similar to autoimmune hyperthyroidism (Graves’ disease) and PTC

Methods

Results

Conclusion