Regulation of the human trh (htrh) gene by human thyroid hormone receptor β1 (hTRβ1) mutants

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Abstract
TRH is negatively regulated by T3 both in the hypothalamic paraventricular nucleus and transient transfection models. Mutations in hTRβ1 genes are associated with the syndrome of generalized resistance to thyroid hormone. To investigate potential effects of mutant TRs on T3 regulation of the hTRH gene, transient gene expression assays were performed in human neuroblastoma (HTB-11) cells with an hTRH promoter-luciferase construct, wild type (WT) hTRβ1, and three qualitatively distinct hTRβ1 mutant forms (ED, OK and PV). In the presence of T3 (10−9 M), liganded WT-hTRβ1 inhibited hTRH promoter activity significantly (40%). Co-transfection of each of the two mutants (ED and OK) achieved similar levels of inhibition only at 10 to 100 fold increased T3 concentrations. Of interest, a 10x excess of mutant ED or OK could also exert dominant negative effects upon WT hTRβ1-T3 mediated inhibitory actions on the hTRH promoter. In contrast, mutant TR-PV exerted neither inhibitory nor dominant negative effects at even higher concentrations of T3. Moreover, all three unliganded mutant forms stimulated TRH promoter activity significantly in the absence of T3, despite their different mutations in the ligand-binding domain (LBD). These data demonstrate that thyroid hormone resistance at the level of TRH gene regulation, due to reduced inhibitory actions of mutant TR-T3 complexes, as well as dominant negative effects upon WT hTRβ1 mediated inhibition, likely contribute to elevated TSH values observed in the syndrome of thyroid hormone resistance.

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