Catherine C. Thompson

Direct repeats as selective response elements for the thyroid hormone, retinoic acid, and vitamin D3 receptors

We report here the identification of thyroid hormone response elements (TREs) that consist of a direct repeat, not a palindrome, of the half-sites. Unlike palindromic TREs, direct repeat TREs do not confer a retinoic acid response. The tandem TRE can be converted into a retinoic acid response element by increasing the spacing between the half-sites by 1 nucleotide, and the resulting retinoic acid response element is no longer a TRE. Decreasing the half-site spacing by 1 nucleotide converts the TRE to a vitamin D3 response element, while eliminating response to T3. These results correlate well with DNA-binding affinities of the thyroid hormone, retinoic acid, and vitamin D3 receptors. This study points to the general importance of tandem repeat hormone response elements and suggests a simple physiologic code exists in which half-site spacing plays a critical role in achieving selective hormonal response.

Novel Mechanism of Nuclear Receptor Corepressor Interaction Dictated by Activation Function 2 Helix Determinants

ABSTRACT Transcriptional regulation by nuclear receptors is controlled by the concerted action of coactivator and corepressor proteins. The product of the thyroid hormone-regulated mammalian gene hairless (Hr) was recently shown to function as a thyroid hormone receptor corepressor. Here we report that Hr acts as a potent repressor of transcriptional activation by RORα, an orphan nuclear receptor essential for cerebellar development. In contrast to other corepressor-nuclear receptor interactions, Hr binding to RORα is mediated by two LXXLL-containing motifs, a mechanism associated with coactivator interaction. Mutagenesis of conserved amino acids in the ligand binding domain indicates that RORα activity is ligand-dependent, suggesting that corepressor activity is maintained in the presence of ligand. Despite similar recognition helices shared with coactivators, Hr does not compete for the same molecular determinants at the surface of the RORα ligand binding domain, indicating that Hr-mediated repression is not simply through displacement of coactivators. Remarkably, the specificity of Hr corepressor action can be transferred to a retinoic acid receptor by exchanging the activation function 2 (AF-2) helix. Repression of the chimeric receptor is observed in the presence of retinoic acid, demonstrating that in this context, Hr is indeed a ligand-oblivious nuclear receptor corepressor. These results suggest a novel molecular mechanism for corepressor action and demonstrate that the AF-2 helix can play a dynamic role in controlling corepressor as well as coactivator interactions. The interaction of Hr with RORα provides direct evidence for the convergence of thyroid hormone and RORα-mediated pathways in cerebellar development.

A Neural Thyroid Hormone Receptor Gene

One of the prominent questions surrounding tissue-specific gene activation is how a single hormone type such as thyroxine can have such diverse physiological effects. Generally, two mechanisms contribute to the particular variety of proteins synthesized either during development or in response to required physiological changes. On the one hand, specific DNA tertiary structure induced by associated nuclear proteins probably presets the transcriptional activity of target cell gene networks (1). An additional constraint is likely provided by hormones or growth factors mediating changing gene expression patterns (2). Each cell produces distinct receptor proteins which determine the effective response to hormonal stimulation. In this manner, both the ontogenetic history of a particular cell type and the hormone receptor field, or its distribution in specific cell types, limit the scope of induced proteins during animal development and homeostasis.