The thyroid hormone receptor (TR) is a member of the nuclear receptor superfamily which plays important roles in a wide variety of biological processes, including growth, development, differentiation, and tumorigenesis, through its capacity to regulate target gene expression in response to thyroid hormone. In eukaryotic cells, genomic DNA is packaged with histones and non-histone proteins into chromatin structure. Both genetic and biochemical evidence accumulated in the last two decades indicate that chromatin organization is an integral component of transcription regulation in eukaryotic cells. Recent evidence indicates that transcriptional regulation by TR and its heterodimeric partner retinoid X receptor (RXR) requires coactivators and corepressors. The observations that corepresses are associated with histone deacetylase (HD) and that coactivators either possess intrinsic or are associated with histone acetyltransferase activity (HAT) suggest that TR may regulate gene expression by targeting dynamic acetylation of chromatin structure. In accord with this model, we have demonstrated, using Xenopus oocytes as a model system, that nucleosome assembly has a role in both the silencing and activation of transcription by TR/RXR and that HD activity is required for the repression by unliganded TR/RXR on chromatin templates. In addition, we have shown that liganded TR/RXR induces extensive chromatin disruption during transcription activation, which could not be accounted by hyperacetylation of histones, suggesting a mechanism distinct from histone acetylation is also involved. We have recently established an in vitro hormone-dependent transcription system using purified TR/RXR, HeLa nuclear extracts, and in vitro assembled chromatin templates. We hypothesize that coactivators and corepressors play important roles in transcriptional regulation by TR/RXR in the context of chromatin and that chromatin remodeling activity(s) also plays an important role in transactivation by TR/RXR. We therefore propose the following three specific aims to test these hypotheses. A). Test the hypothesis that dynamic acetylation is involved in transcriptional regulation by TR/RXR; B) Investigate the role of coactivators and corepressors in transcriptional regulation of TR/RXR in the context of chromatin; C) Investigate the mechanism of chromatin remodeling induced by liganded TR/RXR and characterize the chromatin remodeling factor(s) involved.
