Excess iodide decreases transcription of NIS and VEGF genes in rat FRTL-5 thyroid cells

Abstract

Although it is well known that an excess of iodide suppresses thyroid function and blood flow in vivo, the underlying molecular mechanisms are not fully known. The functional effect of iodide occurs at multiple steps, which include inhibition of sodium/iodide symporter (NIS) expression, transient block of organification, and inhibition of hormonal release. The vascular effect likely involves suppression of the vascular endothelial growth factor (VEGF) gene. In this report, we show that excess iodide coordinately suppresses the expression of the NIS and VEGF genes in FRTL-5 thyroid cells. We also demonstrate that the mechanism of iodide suppression of NIS gene expression is transcriptional, which is synergized by the addition of thyroglobulin. Based on the findings of reporter gene assays and electrophoretic gel mobility shift analysis, we also report two novel DNA binding proteins that responded specifically to iodide and modulated NIS promoter activity. The results suggest that excess iodide affects thyroid vascular function in addition to iodide uptake. This study provides additional insights into the mechanism of action of excess iodide on thyroid function.

2010 Elsevier Inc. All rights reserved.

Figures

Fig. 1

Iodide suppression of NIS expression at the mRNA and protein levels in FRTL-5 thyroid cells. FRTL-5 cells were treated with either sodium chloride (NaCl) or sodium iodide (NaI) for 48 h. The final concentrations added to the cells were 0, 0.1, 1, 2.5, 5, 7.5, and 10 mM. NIS and β-actin were detected by Northern hybridization. (B) FRTL-5 cells were treated with 10 mM NaI for 6 or 48 h. NIS protein level was measured by Western blotting. Actin was used as a control.

Fig. 2

Iodide suppression of NIS promoter activity in FRTL-5 thyroid cells. Iodide suppressed the NIS promoter activity induced by thyrotropin (TSH). Results are expressed as mean ± SD from three different experiments performed in triplicate. Significant decreases are noted: * p

Fig. 3

Iodide suppression of DNA binding of nuclear protein to the NIS promoter in FRTL-5 thyroid cells. (A) The effect of iodide on DNA binding activity in the NIS 5’-flanking region was assessed by electrophoretic gel mobility shift analysis (EMSA). In (A), a solid arrow identifies a complex increased by thyrotropin (TSH), but decreased by sodium iodide (NaI). In (B), an open arrow identifies a complex decreased by NaI in the absence of TSH.

Fig. 4

Iodide and thyroglobulin (Tg) suppression of the expression of NIS and VEGF. Iodide suppressed the expression of NIS and VEGF at the mRNA level. A representative image of a Northern blot is shown in (A). The results of densitometric measurements are expressed relative to control in (B). (C) NIS and VEGF mRNA expression was restored by the removal of iodide from the medium. (D) Iodide and Tg synergistically suppressed the expression of NIS and VEGF. The results of densitometric measurements of different experiments are expressed as mean ± SD relative to control. Significant decreases are noted: ** p