title: Understanding the molecular mechanism of dominant negative action of mutant thyroid hormone beta 1-receptors: the important role of the wild-type/mutant receptor heterodimer.
abstract: The clinical manifestations of patients with resistance to thyroid hormone result from inhibition of the functions of wild-type thyroid hormone receptors (wTRs) by the dominant negative effect of mutant TR beta 1 receptors (mTR beta 1). One of the proposed mechanisms by which mTR beta 1 exerts its dominant negative action is via formation of the putative inactive wTR beta 1/mTR beta 1 heterodimer. However, the nature of the wTR beta 1/mTR beta 1 heterodimer is poorly understood. The present study characterizes the wTR beta 1/mTR beta 1 heterodimer by electrophoretic mobility shift assay. The mutant TR beta 1 used was PV, which contains a frame shift mutation in the C-terminal part of TR beta 1 and has less than 1% of the T3 binding affinity of the wTR beta 1. Because of the difficulty in resolving wTR beta 1 and mutant PV dimers, we used a truncated wTR beta 1 in which the A/B domain was deleted (delta TR beta 1) to demonstrate the formation of the heterodimer on thyroid hormone response elements (TREs) in which the half-site binding motifs are oriented in an inverted repeat (F2), a direct repeat separated by four nucleotides (DR4), or an inverted repeat (Pal). Deletion of the A/B domain had no effect on the binding of T3 and TREs to wTR beta 1. In the presence of equal amounts of delta TR beta 1 and PV, three types of molecular complexes. delta TR beta 1 homodimer, delta TR beta 1/PV heterodimer, and PV homodimer bound to each TRE in a ratio of approximately 1:2:1. The identities of these complexes were confirmed by their ability to be supershifted by anti-TR beta 1 and/or anti-PV antibodies. delta TR beta 1/PV heterodimer formation varied with different TREs. The ratio of apparent affinity constant (Ka) in the binding of delta TR beta 1/PV to TREs was F2:DR4:Pal = approximately 6:2:1. The effect of T3 on delta TR beta 1/PV heterodimer formation was TRE dependent. No T3-induced dissociation was observed for the delta TR beta 1/PV heterodimer when bound to F2 and Pal. In contrast, the delta TR beta 1/PV heterodimer bound to DR4 was dissociated by T3 with an ED50 of 3.9 +/- 0.9 nM. The T3-induced dissociation of delta TR beta 1 homodimer bound to F2, DR4, and Pal had ED50 values of 4.1 +/- 1.2, 1.3 +/- 0.3, and more than 100 nM, respectively. By transfection assays, the dominant negative action of PV was found to be TRE dependent with the rank order of F2 >> Pal > ME (a DR4-like TRE in the rat malic enzyme gene). Taken together, these results indicate a strong correlation between wTR beta 1/mTR beta 1 heterodimer formation and the dominant negative potency of PV. These results suggest that the wTR beta 1/mTR beta 1 heterodimer could play an important role in the dominant negative action of mTR beta 1.
authors.affiliation: Division of Cancer Biology, National Cancer Institute, National Institutes of Health, Bethesda, Maryland 20892, USA.
authors.forename: X G
authors.initials: XG
authors.lastname: Zhu
authors.forename: C L
authors.initials: CL
authors.lastname: Yu
authors.forename: P
authors.initials: P
authors.lastname: McPhie
authors.forename: R
authors.initials: R
authors.lastname: Wong
authors.forename: S Y
authors.initials: SY
authors.lastname: Cheng
mesh_terms: D000818:Animals; D001483:Base Sequence; D001665:Binding Sites; D005799:Genes, Dominant; D008969:Molecular Sequence Data; D009154:Mutation; D015345:Oligonucleotide Probes / Q000235:genetics; D011401:Promoter Regions, Genetic; D051381:Rats; D011988:Receptors, Thyroid Hormone / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism; D013963:Thyroid Hormones / Q000235:genetics; D014284:Triiodothyronine / Q000494:pharmacology
chemical_list: D015345:Oligonucleotide Probes; D011988:Receptors, Thyroid Hormone; D013963:Thyroid Hormones; D014284:Triiodothyronine