Distinct and histone-specific modifications mediate positive versus negative transcriptional regulation of TSHalpha promoter.

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. TSHalpha, a heterodimer partner for thyrotropin (TSH), is secreted by the pituitary gland. T(3) negatively regulates TSHalpha 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 TSHalpha gene by cAMP and T(3) using chromatin immunoprecipitation (ChIP) assays in stably-transfected rat pituitary cells containing the human TSHalpha promoter. Interestingly, cAMP selectively increased histone H4 acetylation whereas, as previously reported, T(3) induced histone H3 acetylation. In particular, cAMP increased H4K5 and H4K8 acetylation and decreased H4K20 trimethylation, modifications associated with transcriptional activation. T(3) 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 T(3) 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 T(3)-dependent negative regulation and partially blocked histone H3 modifications of the TSHalpha promoter. CONCLUSIONS/SIGNIFICANCE: Our findings show that non-overlapping and specific histone modifications determine positive versus negative transcriptional regulation, and integrate opposing hormonal and intracellular signals at the TSHalpha promoter. A mutant TR from a patient with RTH exerted dominant negative activity by blocking the histone modifications induced by T(3) on the TSHalpha promoter and likely contributes to the inappropriate TSH production observed in RTH.