Endoplasmic reticulum stress inhibits expression of genes involved in thyroid hormone synthesis and their key transcriptional regulators in FRTL-5 thyrocytes

Abstract

Endoplasmic reticulum (ER) stress is characterized by the accumulation of misfolded proteins due to an impairment of ER quality control pathways leading to the activation of a defense system, called unfolded protein response (UPR). While thyrocytes are supposed to be highly susceptible to environmental conditions that cause ER stress due to the synthesis of large amounts of secretory proteins required for thyroid hormone synthesis, systematic investigations on the effect of ER stress on expression of key genes of thyroid hormone synthesis and their transcriptional regulators are lacking. Since the aim of the ER stress-induced UPR is to restore ER homeostasis and to facilitate cell survival through transient shutdown of ribosomal protein translation, we hypothesized that the expression of genes involved in thyroid hormone synthesis and their transcriptional regulators, all of which are not essential for cell survival, are down-regulated in thyrocytes during ER stress, while sterol regulatory element-binding proteins (SREBPs) are activated during ER stress in thyrocytes. Treatment of FRTL-5 thyrocytes with the ER stress inducer tunicamycin (TM) dose-dependently increased the mRNA and/or protein levels of known UPR target genes, stimulated phosphorylation of the ER stress sensor protein kinase RNA-like ER kinase (PERK) and of the PERK target protein eukaryotic initiation factor 2α (eIF2α) and caused splicing of the ER stress-sensitive transcription factor X-box binding protein (XBP-1) (P < 0.05). The mRNA levels and/or protein levels of genes involved in thyroid hormone synthesis, sodium/iodide symporter (NIS), thyroid peroxidase (TPO) and thyroglobulin (TG), their transcriptional regulators and thyrotropin (TSH) receptor and the uptake of Na125I were reduced at the highest concentration of TM tested (0.1 μg/mL; P < 0.05). Proteolytic activation of the SREBP-1c pathway was not observed in FRTL-5 cells treated with TM, whereas TM reduced proteolytic activation of the SREBP-2 pathway at 0.1 μg TM/mL (P < 0.05). In conclusion, the expression of key genes involved in thyroid hormone synthesis and their critical regulators and of the TSH receptor as well as the uptake of iodide is attenuated in thyrocytes during mild ER stress. Down-regulation of NIS, TPO and TG during ER stress is likely the consequence of impaired TSH/TSHR signaling in concert with reduced expression of critical transcriptional regulators of these genes.

IRE1α Implications in Endoplasmic Reticulum Stress-Mediated Development and Pathogenesis of Autoimmune Diseases

“…FRTL-5 thyrocytes treated with tunicamycin, an ER stress inducer, showed increased levels of XBP1 and other UPR molecules but also showed a reduction in thyroid hormone synthesis. This indicates the role of ER stress-activated molecules in thyroid hormone synthesis ( 319 ). However, IRE1α has not been studied extensively in this disease.…”

Thyrocyte cell survival and adaptation to chronic endoplasmic reticulum stress due to misfolded thyroglobulin

“…Thus perhaps these responses are to be expected, yet to us it is particularly notable that along with decreased levels of phospho-S6-kinase, thyrocytes adapted to chronic continuous ER stress maintain a lower overall protein synthesis level while exhibiting a growth rate that ultimately is comparable to that of unstressed cells. To us, this suggests the likelihood that protein synthesis machinery is less directed to proteins representing thyroid differentiated function (40) while preserving those proteins needed for continued cell growth.…”

ER stress contributes to high-fat diet-induced decrease of thyroglobulin and hypothyroidism

“…As Tg is the major secretory glycoprotein of thyrocytes, the disturbance of the thyrocytes ER homeostasis could interfere with the folding of Tg. Wen et al (39) reported that TM-treated FRTL-5 cells resulted in ER stress and the decrease of Tg protein, although the authors suggested the decreased mRNA level might be responsible. Other research using thapsigargintreated FRTL-5 cells revealed the block of Tg secretion, with the disturbance of Tg folding and retention in the ER (18).…”