The human thyrotropin-releasing hormone gene is regulated by thyroid hormone through two distinct classes of negative thyroid hormone response elements

TRH is the principal positive regulator of TSH synthesis and secretion in man. T3 is able to control TRH synthesis through feedback inhibition at the transcriptional level, presumably by binding to its receptor which interacts with one or more negative thyroid hormone response elements (TREs) present within the human TRH promoter. In the present study we have identified the specific negative TREs within the TRH promoter and characterized their ability to interact with thyroid hormone receptors (TRs), and the retinoid X receptor (RXR). Our analysis demonstrates that ligand-independent and dependent regulation of the human TRH promoter is restricted to the TRβ 1 isoform. Deletional analysis of the TRH promoter identified two discrete regions that are responsible for mediating ligand-dependent negative regulation of the TRH promoter. Mutagenesis of potential TR binding half-sites within these regions identified three separate half-sites (site 4 from -55 to -60 base pairs (bp); site 5, +14 to +19 bp; and site 6, +37 to +42 bp) which act in combination to allow for negative regulation. Mutation and/or deletion of each of these sites leads to a loss of negative regulation of the TRH promoter by T3. Gel-mobility shift assays of site 4 and its surrounding nucleotides revealed that this region of the promoter is capable of binding TR monomers, homodimers, and TR-RXR heterodimers. Mutagenesis of site 4 leads to a loss of all binding to this region. The region encompassing sites 5 and 6 binds only TR monomer, and the addition of RXR to the binding reaction leads to a loss of specific monomeric binding. To assess the functional importance of site 4 and its surrounding nucleotides we cotransfected RXR isoforms along with TRβ with TRH promoter constructs containing either site 4 or its mutant. In the presence of wild type site 4 sequence, cotransfected RXR enhanced negative regulation of the TRH promoter. Mutation and or deletion of site 4 leads to a loss of this enhancement. These data demonstrate that two structurally different negative TREs cooperate to allow for negative regulation of the human TRH promoter and that negative regulation is TR isoform-specific and modulated by the RXR-signaling pathway through a novel negative TRE.