Pax-8 is essential for regulation of the thyroglobulin gene by transforming growth factor-beta1.

Transforming growth factor-beta1 (TGF-beta1) is a multifunctional cytokine that is thought to play a major role in the regulation of growth and differentiation of thyroid cells. However, little is known of its detailed mechanisms of action in thyrocytes. We have therefore studied the molecular mechanisms of TGF-beta1 action on thyroglobulin (TG) gene expression by focusing our attention on TGF-beta1 regulation of thyroid-specific transcription factors. TGF-beta1 decreased TG messenger RNA (mRNA) expression both in the presence and in the absence of TSH in rat thyroid FRTL-5 cells. Transfected into FRTL-5 cells, the activity of reporter plasmids containing the rat TG promoter ligated to a luciferase gene was significantly suppressed by the addition of TGF-beta1. When the nuclear extracts prepared from TGF-beta1-treated FRTL-5 cells were used in gel mobility shift assays, the amount of protein-DNA complex formed by Pax-8 was reduced, both in the presence and in the absence of TSH, but protein-DNA complexes formed by thyroid transcription factor-1 (TTF-1) and TTF-2 were not. The suppressive effect of TGF-beta1 on Pax-8/DNA complex formation is in part due to the suppression of Pax-8 mRNA and protein levels by TGF-beta1. Expressions of Pax-8 mRNA and protein, which were assessed by Northern blot and Western blot analyses, respectively, were decreased by TGF-beta1 treatment of FRTL-5 cells in a concentration-dependent manner. In a transfection experiment, mutation of the Pax-8-binding site caused a loss of both TGF-beta1- and TSH-responsiveness in TG promoter activity. Overexpression of Pax-8 abolished the TGF-beta1 suppression of TG promoter activity. These results indicate that TGF-beta1 decreases Pax-8 mRNA levels as well as Pax-8 DNA-binding activity, which, at least in part, seems to be involved in the TGF-beta1-induced suppression of TG gene expression.