The total dependence of amphibian metamorphosis on thyroid hormone (T3) provides a unique vertebrate model for studying the molecular mechanism of T3 receptor (TR) function in vivo. In vitro transcription and developmental expression studies have led to a dual function model for TR in amphibian development, ie, TRs act as transcriptional repressors in premetamorphic tadpoles and as activators during metamorphosis. We examined molecular mechanisms of TR action in T3-induced metamorphosis by using dominant-negative receptors (dnTR) ubiquitously expressed in transgenic Xenopus laevis. We showed that T3-induced activation of T3 target genes and morphological changes are blocked in dnTR transgenic animals. By using chromatin immunoprecipitation, we show that dnTR bound to target promoters, which led to retention of corepressors and continued histone deacetylation in the presence of T3. These results thus provide direct in vivo evidence for the first time for a molecular mechanism of altering gene expression by a dnTR. The correlation between dnTR-mediated gene repression and inhibition of metamorphosis also supports a key aspect of the dual function model for TR in development: during T3-induced metamorphosis, TR functions as an activator via release of corepressors and promotion of histone acetylation and gene activation.

Thyroid hormone (T3) affects a wide range of biological processes, from metabolism to development (72). The diverse effects of T3 are generally believed to be mediated through gene regulation by T3 receptors (TRs). In vitro and tissue culture studies have shown that T3 activates transcription by binding to TR, which most likely heterodimerizes with RXR (9-cis-retinoic acid receptor) and binds to thyroid hormone response elements (TREs) in T3 response genes. The binding of TREs by TR/RXR heterodimers is, however, independent of T3 (9, 35, 38, 43, 62, 69), implicating a role of unliganded TR in gene regulation. Indeed, various in vitro studies have revealed that unliganded TRs repress target transcription whereas, in the presence of T3, they enhance the transcription of these same genes (11, 23, 62, 69, 75). TRs exert these effects by recruiting TR-interacting cofactors. Many such cofactors have been isolated and characterized based on their ability to interact with TRs in the presence and/or absence of T3 (3, 4, 29, 39, 45, 70, 72, 75).