Thyroid hormone receptors (TR) belong to a family of nuclear hormone receptor proteins. These receptors function as hormone-mediated transcriptional switches that, when activated by hormone, bind to specific DNA elements to regulate gene expression. TR binding elements that positively mediate gene expression have been extensively studied, with the identification of many naturally occurring positive elements and a well-developed understanding of the structural organization of TR as it interacts with DNA. In contrast to thyroid hormone receptor-mediated induction, little is known about the mechanism of its physiologically important role in T3-dependent gene repression. Consensus DNA sequences for negative TR binding elements and the receptor structure that differentially leads to inhibition of gene expression is unknown. We have used the human thyrotropin beta gene as a model system to study triiodothyronine (T3) inhibition of gene expression. We present preliminary data within this proposal that identify a thyroid hormone receptor binding site in the proximal promoter of the human TSH-beta gene within a region that confers inhibition. Direct comparison of this novel site and other published negative TR binding elements to positive TR response elements identify characteristics which appear unique to TR repression: they are composed of a single TR binding site, bind TR only as a homodimer, do not bind TR as monomer after T3 treatment, and are resistant to heterodimer formation with TR accessory proteins. Thus, the specific aims of this proposal will: 1) define DNA sequence characteristics necessary to mediate T3 inhibition of gene expression, and 2) characterize the differential organization of TR and other accessory proteins on negative and positive TR binding sites and identify their relative roles in T3-mediated gene regulation. Preliminary data shown are the first description of molecular differences between TR-induced repression and induction. Results of proposed experiments will provide insight into at present speculative mechanisms of the bifunctional nature of the thyroid hormone receptor.
