Identification of TTF-1/NKX2-1 as a key transcriptional regulator of SLC26A7 in the thyroid

Solute carrier family 26 member 7 (SLC26A7) is a recently identified iodide transporter localized to the apical membrane of thyroid follicular cells. Loss-of-function mutations in SLC26A7 cause congenital hypothyroidism, underscoring its essential role in iodide transport and thyroid hormone biosynthesis. We previously demonstrated that SLC26A7 mRNA and protein expression is markedly suppressed by thyroid‑stimulating hormone (TSH) or by intrafollicular concentrations of thyroglobulin (Tg). Reporter gene assays identified strong promoter activity within the −2,207 to −1,653 bp region upstream of SLC26A7 , where three TTF‑1 binding motifs were predicted to be located by in silico analysis. In the present study, we investigated whether TTF‑1 binds to these elements and whether such binding is required for SLC26A7 transcription. Electrophoretic mobility shift assay (EMSA) demonstrated that TTF‑1 binds specifically to each of the three predicted binding sequences. TTF‑1 binding was reduced in nuclear extracts derived from thyroid cells treated with TSH or Tg. Site-directed mutagenesis of the predicted TTF‑1 binding sites decreased both TTF‑1 binding activity, as assessed by EMSA, and promoter activity, as assessed by luciferase reporter assays. These findings suggest that TTF‑1 contributes as a key transcriptional regulator of SLC26A7 and indicate that TSH- or Tg-induced repression of SLC26A7 expression is mediated, at least in part, by attenuation of TTF‑1 binding.