An RNA-binding domain in the thyroid hormone receptor enhances transcriptional activation

Thyroid hormone plays important roles in development, differentiation, and metabolic homeostasis by binding to nuclear thyroid hormone receptors, which regulate target gene expression by interacting with DNA response elements and coregulatory proteins. We show that thyroid hormone receptors also are single-stranded RNA binding proteins and that this binding is functionally significant. By using a series of deletion mutants, a novel RNA-binding domain was localized to a 41-amino acid segment of thyroid hormone receptor alpha1 between the second zinc finger and the ligand-binding domain. This RNA-binding domain was necessary and sufficient for thyroid hormone receptor binding to the steroid receptor RNA activator (SRA). Although SRA does not bind directly to steroid receptors, it has been identified as a steroid receptor coactivator, and was thought not to be a coactivator for thyroid hormone receptors. However, transfection studies revealed that SRA enhances thyroid hormone induction of appropriate reporter genes and that the thyroid hormone receptor RNA-binding domain is important for this enhancement. We conclude that thyroid hormone receptors bind RNA through a novel domain and that the interaction of this domain with SRA, and perhaps other RNAs, enhances thyroid hormone receptor function.     

Making sense of cross-talk between steroid hormone receptors and intracellular signaling pathways: who will have the last word?

In classical models of nuclear steroid hormone receptor function, ligand binds receptor, heat shock proteins dissociate, and receptor dimers enter or are withheld in the nucleus and interact with coregulatory molecules to mediate changes in gene expression. The footnotes, "receptors become phosphorylated" and "dynamic nucleo-cytoplasmic shuttling occurs" describe well-accepted, but less well-understood aspects of receptor action. Recently, the idea that several protein kinases are activated in response to steroid hormone binding to cognate cytoplasmic or membrane-associated receptors has become fashionable. However, the precise role of steroid hormone receptor phosphorylation and our understanding of which cytoplasmic kinases are activated and their functional significance remain elusive. This review provides an overview of the primary ways in which steroid hormone receptor and growth factor cross-talk occurs, using the human progesterone receptor (PR) as a model. The functional consequences of PR phosphorylation by protein kinases classically activated in response to peptide growth factors and novel extranuclear or nongenomic functions of PR as potential independent initiators of signal transduction pathways are discussed. Intracellular protein kinases are emerging as key mediators of steroid hormone receptor action. Cross-talk between steroid receptor- and growth factor-initiated signaling events may explain how gene subsets are coordinately regulated by mitogenic stimuli in hormonally responsive normal tissues, and is suspected to play a role in their cancer biology.     

Crystal Structure of the Mineralocorticoid Receptor DNA Binding Domain in Complex with DNA

The steroid hormone receptors regulate important physiological functions such as reproduction, metabolism, immunity, and electrolyte balance. Mutations within steroid receptors result in endocrine disorders and can often drive cancer formation and progression. Despite the conserved three-dimensional structure shared among members of the steroid receptor family and their overlapping DNA binding preference, activation of individual steroid receptors drive unique effects on gene expression. Here, we present the first structure of the human mineralocorticoid receptor DNA binding domain, in complex with a canonical DNA response element. The overall structure is similar to the glucocorticoid receptor DNA binding domain, but small changes in the mode of DNA binding and lever arm conformation may begin to explain the differential effects on gene regulation by the mineralocorticoid and glucocorticoid receptors. In addition, we explore the structural effects of mineralocorticoid receptor DNA binding domain mutations found in type I pseudohypoaldosteronism and multiple types of cancer.     

Hormone-induced changes in the in vitro DNA-binding activity of the chicken progesterone receptor

Previous analyses have indicated that steroid hormone receptors undergo an allosteric change in structure upon binding by the steroid ligand. This structural change was envisioned as an intramolecular unmasking of the protein's DNA-binding domain, thus allowing the receptor to function in gene regulation. We report an analysis of the effect of hormone on the DNA-binding activity of the chicken progesterone receptor. Using an isocratic elution of DNA affinity columns we show that unliganded receptor (aporeceptor) can bind a 23-basepair progesterone response element with high affinity and a high degree of sequence preference. Hormone causes a 1.5-fold increase in affinity for the PRE sequence and a 2-fold decrease in affinity for non-specific DNA. Kinetic analysis of the off-rate of receptor-DNA complexes is consistent with this minor effect of hormone. In addition, gel retardation analysis of receptor-progesterone response element complexes further substantiates that hormone is not required for sequence-specific DNA binding. These results indicate that hormone is not necessary for the progesterone receptor to fold into a conformation that recognizes specific gene regulatory sequences.     

Identification of structurally distinct alpha 2-adrenergic receptors

Recent studies involving a variety of membrane receptors and ion channels indicate that diversity exists among these proteins as evidenced by tissue-specific and developmentally related expression of different isoforms. Alpha 2-Adrenergic receptors, plasma membrane proteins involved in sympathetic neurotransmission, may similarly represent a nonhomogeneous class of binding sites based on the following observations. First, their activation can elicit a wide variety of effector cell responses, which are apparently triggered by at least three different signal transduction mechanisms. Second, alpha 2-adrenergic receptors in various tissues and species exhibit marked differences in their ligand recognition properties. To determine if heterogeneity of the receptor protein itself is involved in generating this diversity, we structurally characterized the alpha 2-adrenergic receptor in two tissues that exhibit the greatest differences in ligand recognition properties, neonatal rat lung and human platelet. We report here that these differences in ligand recognition are maintained after partial receptor purification (50-100-fold) and are associated with distinct differences in the physical and structural properties of the receptor protein. The human platelet and neonatal rat lung receptor differ in the apparent molecular weight of their hormone-binding subunits (human platelet, Mr approximately 64,000 versus neonatal rat lung, Mr approximately 44,000) as well as in the number or type of their associated oligosaccharide moieties. The observed diversity is consistent with expression of isoforms of the alpha 2-adrenergic receptor and suggests the presence of more than one gene encoding similar but distinct receptor proteins.