ASSIGNMENT OF THE BETA-THYROID HORMONE RECEPTOR TO 3,5,3'-TRIIODOTHYRONINE-DEPENDENT INHIBITION OF TRANSCRIPTION FROM THE THYROTROPIN-RELEASING-HORMONE PROMOTER IN CHICK HYPOTHALAMIC NEURONS

Thyroid hormone, T3, is essential to the normal development and metabolism of vertebrates. Fine tuning of circulating levels of T3 is critical and involves feedback inhibition of the TRH and TSH genes by T3 at the hypothalamic and hypophyseal levels. However, the molecular basis of T3 inhibition of TRH gene expression in the hypothalamus is not known. The actions of T3 on target gene expression are mediated through nuclear receptor proteins, TRalpha and TRbeta. To examine their effects on T3-dependent transcription from the rat TRH promoter, we used a gene transfer technique to express TRalpha and TRbeta in cultured embryonic chick hypothalamic cells. Transcription from the TRH promoter construct transfected into these cultures was depressed in the presence of 10(-9) M T3. Cotransfecting TRalpha or TRbeta activated transcription from the TRH promoter. However, only TRbeta-dependent TRH transcription was differentially modulated by T3. Physiological concentrations of T3 decreased TRbeta-dependent TRH transcription 4-fold. Thus, when T3 levels increase, TRbeta mediates inhibition of TRH expression, a key step in down-regulating the hypophyseal-thyroid axis. This study demonstrates for the first time a T3-dependent differential regulation of the TRH promoter by TRbeta and not TRalpha. Thus, the negative regulation of the TRH promoter in transiently transfected primary embryonic chick hypothalamic neurons provides a useful system for studying the molecular actions of thyroid hormone receptors.