Isolation and characterization of a cDNA encoding a chicken beta thyroid hormone receptor.

We have isolated and characterized a cDNA encoding a chicken beta homolog of c-erbA, or thyroid hormone receptor (TR). Chicken liver cDNA libraries were screened with a rat TR beta-1 cDNA probe, and several cDNA inserts were isolated and characterized. The sequence of one cDNA predicts a 369-amino-acid open reading frame (ORF), with a protein sequence that possesses 96% identity with that of rat TR beta-1, but only 88% identity with chicken TR alpha. These data indicate that the cDNA likely encodes a beta form of TR that has the expected putative DNA and T3 binding domains. The chicken TR beta (chTR beta) in vitro translated protein binds T3 with high affinity, and binds both the thyroid hormone response element (TRE) from the rat growth hormone gene and the Xenopus vitellogenin A2 gene estrogen response element (ERE), similarly to that of the rat TR beta-1. Northern blot analysis revealed the expression of a 7.0-kb RNA in several tissues including cerebellum, pituitary, kidney, and liver. This chicken liver TR beta cDNA sequence varies in both the 5' and 3' untranslated regions from the chicken kidney TR beta cDNA sequence recently reported (Forrest et al., 1990). The 5' untranslated cDNA sequence divergence occurs near a potential splice site junction of the human TR beta gene, suggesting that this chicken liver cDNA may represent an alternatively spliced RNA product of the chicken TR beta gene.     

Ligand-induced recruitment of a histone deacetylase in the negative-feedback regulation of the thyrotropin beta gene.

We have investigated ligand-dependent negative regulation of the thyroid-stimulating hormone beta (TSHbeta) gene. Thyroid hormone (T3) markedly repressed activity of the TSHbeta promoter that had been stably integrated into GH(3 )pituitary cells, through the conserved negative regulatory element (NRE) in the promoter. By DNA affinity binding assay, we show that the NRE constitutively binds to the histone deacetylase 1 (HDAC1) present in GH(3 )cells. Significantly, upon addition of T3, the NRE further recruited the thyroid hormone receptor (TRbeta) and another deacetylase, HDAC2. This recruitment coincided with an alteration of in vivo chromatin structure, as revealed by changes in restriction site accessibility. Supporting the direct interaction between TR and HDAC, in vitro assays showed that TR, through its DNA binding domain, strongly bound to HDAC2. Consistent with the role for HDACs in negative regulation, an inhibitor of the enzymes, trichostatin A, attenuated T3-dependent promoter repression. We suggest that ligand-dependent histone deacetylase recruitment is a mechanism of the negative-feedback regulation, a critical function of the pituitary-thyroid axis.