MEDIATION OF THE HORMONE-DEPENDENT AND SERUM-DEPENDENT REGULATION OF THYROGLOBULIN GENE-EXPRESSION BY THYROID-TRANSCRIPTION FACTORS IN RAT-THYROID FRTL-5 CELLS

The molecular mechanism for hormone- and serum-dependent regulation of thyroglobulin (TG) gene expression was studied. A construct of rat TG promoter (- 178 to - 3) Linked to a luciferase gene was transfected i nto TSH-, insulin- and serum-deprived FRTL-5 cells. Addition of TSH, i nsulin or serum augmented the luciferase activity. The endogenous TG m RNA level was also increased, indicating that the promoter used confer s responsiveness of TG gene to these additives. The possible involveme nt of thyroid-transcription factors, TTF-1, TTF-2 and Pax-8, in the in duction of TG gene transcription was studied using an electrophoretic mobility shift assay. Since the protein/DNA ratio in FRTL-5 cell extra cts was significantly increased by these additives, binding activities of these factors per unit of DNA were examined. It was demonstrated t hat TSH, insulin or serum increased not only TTF-2 binding activity bu t also the binding activities of TTF-1 and Pax-8. However, the magnitu de of the increase in TTF-1 and Pax-8 mRNA levels per unit of DNA was less than that of the binding activity. Taken together, our results su ggest that TSH, insulin and serum increase the binding activities of T TF-1 and Pax-8 to the TG promoter presumably through the posttranslati onal modification of the factors, thereby enhancing TG gene transcript ion.