Negative Regulation Of Tsh Alpha Target Gene By Thyroid Hormone Involves Histone Acetylation And Corepressor Complex Dissociation

Currently, little is known about histone modifications and molecular mechanisms of negatively regulated transcription. In pituitary cells, thyroid hormone (T-3) decreased transcription, and surprisingly increased histone acetylation, of TSH alpha promoter. This increase was mediated directly by thyroid hormone receptor. Histone acetylation of H3K9 and H3K18 sites, two modifications usually associated with transcriptional activation, occur in negative regulation of TSH alpha promoter. T-3 also caused release of a corepressor complex composed of histone deacetylase 3 (HDAC3), transducin beta-like protein 1, and nuclear receptor coprepressor (NCoR)/silencing mediator for retinoic and thyroid hormone receptor from TSH alpha promoter in chromatin immunoprecipitation assays. NCoR and HDAC3 overexpression selectively increased ligand-independent basal transcription. Two histone acetyltransferase inhibitors increased overall transcription but did not abrogate negative regulation or NCoR/HDAC3 complex release by T-3. Chromatin immunoprecipitation analyses of an endogenous positively regulated target gene showed increased histone acetylation and corepressor complex release with T-3 treatment. Finally, microarray analyses suggested there is a subset of negatively regulated genes with increased histone acetylation. These findings demonstrate the critical role of NCoR/HDAC3 complex in negative regulation of TSH alpha gene expression and show that similar complexes and overlapping epigenetic modifications can participate in both negative and positive transcriptional regulation. (Molecular Endocrinology 23: 600-609, 2009)