The thyroid hormone inhibits the thyrotropin receptor promoter activity: evidence for a short loop regulation.

Thyrotropin, by binding its specific receptor on the plasma membrane of the thyrocyte, regulates thyroid function and differentiation. In FRTL5 cells, thyrotropin down-regulates the thyrotropin receptor (TSHr) promoter activity and induces the transcription of the alpha form of thyroid hormone receptor (TR-alpha 1). In this study we show that the thyrotropin receptor down-regulation, induced by thyrotropin, is mediated by TR-alpha 1. The thyroid hormone receptor binds, in vitro, the thyrotropin receptor minimal promoter and inhibits promoter activity in cotransfections experiments in CV 1 cells. The inhibition is achieved only in the presence of the thyroid hormone. The TSHr promoter mutated in the thyroid hormone receptor binding site does not bind TR, in vitro, and its activity is not inhibited, in cotransfection experiments, in CV 1 cells. The same mutation abolishes the TSH mediated down-regulation of the TSHr promoter activity in FRTL5 thyroid cells. These results support the hypothesis of a regulatory short loop of thyroid hormone in thyroid cells.