Distinct and Histone-Specific Modifications Mediate Positive versus Negative Transcriptional Regulation of TSHα Promoter

Dongqing Wang,Xianmin Xia,Roy E Weiss,Samuel Refetoff,Paul M Yen,Mary Bryk

doi:10.1371/journal.pone.0009853

Abstract

BackgroundHormonally-regulated histone modifications that govern positive versus negative transcription of target genes are poorly characterized despite their importance for normal and pathological endocrine function. There have been only a few studies examining chromatin modifications on target gene promoters by nuclear hormone receptors. Moreover, these studies have focused on positively-regulated target genes.TSHα, a heterodimer partner for thyrotropin (TSH), is secreted by the pituitary gland. T3 negatively regulates TSHα gene expression via thyroid hormone receptors (TRs) which belong to the nuclear hormone receptor superfamily, whereas thyrotropin releasing hormone (TRH) positively regulates via the TRH receptor, a G protein-coupled receptor.Methodology/Principal FindingsWe studied regulation of the TSHα gene by cAMP and T3 using chromatin immunoprecipitation (ChIP) assays in stably-transfected rat pituitary cells containing the human TSHα promoter. Interestingly, cAMP selectively increased histone H4 acetylation whereas, as previously reported, T3 induced histone H3 acetylation. In particular, cAMP increased H4K5 and H4K8 acetylation and decreased H4K20 trimethylation, modifications associated with transcriptional activation. T3 increased H3K9 and H3K18 acetylation and H3K4 trimethylation; however, it also decreased H3K27 acetylation and increased H3K27 trimethylation which are associated with transcriptional repression. Of note, cAMP recruited pCREB, CBP/p300, and PCAF to the promoter whereas T3 caused dissociation of NCoR/SMRT and HDAC3. Overexpression of a dominant negative mutant thyroid hormone receptor (TR) from a patient with resistance to thyroid hormone (RTH) led to less T3-dependent negative regulation and partially blocked histone H3 modifications of the TSHα promoter.Conclusions/SignificanceOur findings show that non-overlapping and specific histone modifications determine positive versus negative transcriptional regulation, and integrate opposing hormonal and intracellular signals at the TSHα promoter. A mutant TR from a patient with RTH exerted dominant negative activity by blocking the histone modifications induced by T3 on the TSHα promoter and likely contributes to the inappropriate TSH production observed in RTH.

Highlights

Thyroid hormone receptors (TRs) are members of the nuclear hormone receptor superfamily that are responsible for many of the physiological and cellular effects by thyroid hormones
Negative regulation was observed in cells treated with both T3 and cAMP; the level of transcription was higher than T3 alone
In the presence of T3, this complex dissociates from the promoter resulting in increased H3 acetylation whereas in the presence of cAMP, it remains bound to the TSHa promoter. pCREB recruits coactivators with histone acetyltransferase (HAT) activity, leading to both increased H4 acetylation and transcriptional activity

Summary

Introduction

Thyroid hormone receptors (TRs) are members of the nuclear hormone receptor superfamily that are responsible for many of the physiological and cellular effects by thyroid hormones. In positively-regulated target genes, unliganded TRs bind to thyroid hormone response elements (TREs) in the promoters of target genes They associate with corepressors such as nuclear receptor corepressor (NCoR) or silencing mediator for retinoic and thyroid hormone receptors (SMRT) in corepressor complexes containing transducin b-like protein 1 (TBL1) and histone deacetylase 3 (HDAC3). ATP-dependent chromatin remodeling complexes similar to yeast SWI/SNF which contain the ATPase subunit, Brahma-related gene (BRG1) are recruited to the promoter [13,14] and likely facilitate chromatin renodeling necessary for HAT activity by SRC complexes and demethylases Another major complex, Mediator, interacts with liganded TRs and recruits RNA polymerase II (RNA pol II) to the transcriptional start site [6,15,16]. T3 negatively regulates TSHa gene expression via thyroid hormone receptors (TRs) which belong to the nuclear hormone receptor superfamily, whereas thyrotropin releasing hormone (TRH) positively regulates via the TRH receptor, a G protein-coupled receptor

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Conclusion