Sachs Laurent M, Jones Peter L, Havis Emmanuelle, Rouse Nicole, Demeneix Barbara A, Shi Yun-Bo
Unit on Molecular Morphogenesis, Laboratory of Gene Regulation and Development, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, Maryland 20892-5431, USA.
Mol Cell Biol. 2002 Dec;22(24):8527-38. doi: 10.1128/MCB.22.24.8527-8538.2002.
Thyroid hormone receptors (TR) act as activators of transcription in the presence of the thyroid hormone (T(3)) and as repressors in its absence. While many in vitro approaches have been used to study the molecular mechanisms of TR action, their physiological relevance has not been addressed. Here we investigate how TR regulates gene expression during vertebrate postembryonic development by using T(3)-dependent amphibian metamorphosis as a model. Earlier studies suggest that TR acts as a repressor during premetamorphosis when T(3) is absent. We hypothesize that corepressor complexes containing the nuclear receptor corepressor (N-CoR) are key factors in this TR-dependent gene repression, which is important for premetamorphic tadpole growth. To test this hypothesis, we isolated Xenopus laevis N-CoR (xN-CoR) and showed that it was present in pre- and metamorphic tadpoles. Using a chromatin immunoprecipitation assay, we demonstrated that xN-CoR was recruited to the promoters of T(3) response genes during premetamorphosis and released upon T(3) treatment, accompanied by a local increase in histone acetylation. Furthermore, overexpression of a dominant-negative N-CoR in tadpole tail muscle led to increased transcription from a T(3)-dependent promoter. Our data indicate that N-CoR is recruited by unliganded TR to repress target gene expression during premetamorphic animal growth, an important process that prepares the tadpole for metamorphosis.
甲状腺激素受体（TR）在甲状腺激素（T3）存在时作为转录激活因子起作用，而在其不存在时作为抑制因子起作用。虽然已使用多种体外方法来研究TR作用的分子机制，但其生理相关性尚未得到探讨。在此，我们以依赖T3的两栖类变态为模型，研究TR在脊椎动物胚胎后发育过程中如何调节基因表达。早期研究表明，在变态前T3不存在时，TR作为抑制因子起作用。我们假设，包含核受体共抑制因子（N-CoR）的共抑制复合物是这种依赖TR的基因抑制的关键因素，这对变态前蝌蚪的生长很重要。为了验证这一假设，我们分离出非洲爪蟾N-CoR（xN-CoR），并表明它存在于变态前和变态期的蝌蚪中。使用染色质免疫沉淀试验，我们证明在变态前xN-CoR被招募到T3反应基因的启动子上，并在T3处理后释放，同时伴随着组蛋白乙酰化的局部增加。此外，在蝌蚪尾肌中过表达显性负性N-CoR导致依赖T3的启动子转录增加。我们的数据表明，在变态前动物生长过程中，未结合配体的TR招募N-CoR来抑制靶基因表达，这是一个使蝌蚪为变态做准备的重要过程。
