Ligand T3 Dependent and Independent Effects of Thyroid Hormone Receptors upon Human TRH Gene Transcription in Neuroblastoma Cells

Abstract

Thyrotropin releasing hormone (TRH) gene is regulated negatively at the transcriptional level by thyroid hormone (T3) in rat anterior hypothalamus. The actions of T3 upon other target genes are known to be mediated through the thyroid hormone receptors (TR), TRα and TRβ. To explore whether the inhibitory regulation of human (h) TRH gene transcription by T3 is TR isoform specific and whether TRH gene transcription can be modulated as well by unliganded TR isoforms, transient gene expression studies have been carried out using hTRH-luciferase (TRH-Luc) chimeric constructs and TR expression constructs, co-transfected into a human neuroblastoma cell line (HTB-11). Data herein demonstrate T3-dependent inhibitory regulation of the hTRH gene promoter by TR-T3 complexes. Moreover, significant inhibition (39%-60%) could be achieved by T3 bound to either hTRα1, hTRβ1, or rTRβ1, β2 and was comparable quantitatively, indicating an absence of TR isoform specificity for T3 inhibition. Conversely, basal promoter activity of the hTRH gene could be activated significantly by unliganded hTRα1, β1, rTRβ1, and β2 (150% to 334%), but not by hTRα2. Thus, TRs appear to exert opposite effects on hTRH gene transcription, depending on the presence or absence of ligand T3. These dual effects of TR suggest that the addition of the T3 ligand effects conformational changes that can abrogate the initiation of transcription.