Two transcription activation functions in the amino terminus of the mouse estrogen receptor that are affected by the carboxy terminus

Determinants of the heightened activity of glucocorticoid receptor translational isoforms

Translational isoforms of the glucocorticoid receptor α (GR-A, -B, -C1, -C2, -C3, -D1, -D2, and -D3) have distinct tissue distribution patterns and unique gene targets. The GR-C3 isoform-expressing cells are more sensitive to glucocorticoid killing than cells expressing other GRα isoforms and the GR-D isoform-expressing cells are resistant to glucocorticoid killing. Whereas a lack of activation function 1 (AF1) may underlie the reduced activity of the GR-D isoforms, it is not clear how the GR-C3 isoform has heightened activity. Mutation analyses and N-terminal tagging demonstrated that steric hindrance is probably the mechanism for the GR-A, -B, -C1, and -C2 isoforms to have lower activity than the GR-C3 isoform. In addition, truncation scanning analyses revealed that residues 98 to 115 are critical in the hyperactivity of the human GR-C3 isoform. Chimera constructs linking this critical fragment with the GAL4 DNA-binding domain showed that GR residues 98 to 115 do not contain any independent transactivation activity. Mutations at residues Asp101 or Gln106 and Gln107 all reduced the activity of the GR-C3 isoform. In addition, functional studies indicated that Asp101 is crucial for the GR-C3 isoform to recruit coregulators and to mediate glucocorticoid-induced apoptosis. Thus, charged and polar residues are essential components of an N-terminal motif that enhances the activity of AF1 and the GR-C3 isoform. These studies, together with the observations that GR isoforms have cell-specific expression patterns, provide a molecular basis for the tissue-specific functions of GR translational isoforms.

In silico design of peptidic inhibitors targeting estrogen receptor alpha dimer interface

Human estrogen receptor alpha (ERα), which acts as a biomarker and as a therapeutic target for breast cancers, is activated by agonist ligands and co-activator proteins. Selective estrogen receptor modulators (SERM) act as antagonists in specific tissues and tamoxifen, a SERM, has served as a drug for decades for ERα-positive breast cancers. However, the ligand-selective and tissue-specific response of ERα biological activity and the resistance to tamoxifen treatment in advanced stages of ERα-positive breast cancers underscores the need to find a ligand-independent inhibitor for ERα. Here we present a ligand-independent approach of inhibiting ERα transactivation targeting its dimerization-a key process of ERα biological activity. Using in silico techniques, we first elucidated the hydrogen bond interactions involved in dimerization and identified three interfacial sequence motifs, where sequence I (DKITD) and sequence II (QQQHQRLAQ) of one monomer form hydrogen bonding with sequence II and sequence I of the second monomer, respectively, and sequence III (LSHIRHMSNK) hydrogen bonds with the same from the second monomer. Studying the structural stability and the binding affinity of the peptides derived from these sequence motifs, we found that an extended and ARG mutated version (LQQQHQQLAQ) of sequence II can act as a suitable template for designing peptidic inhibitors. It provides additional structural stability and interacts more strongly with ERα dimer interface groove formed by helices 9 and 10/11 and prevent ERα dimerization. Our result provides a novel therapeutic designing pipeline for ligand-independent inhibition of ERα.

Gene regulation by the glucocorticoid receptor: structure:function relationship

The glucocorticoid receptor (GR) belongs to the superfamily of ligand-activated transcription factors, the nuclear hormone receptors. Like other members of the family, the GR possesses a modular structure consisting of three major domains-the N-terminal (NTD), DNA binding (DBD), and ligand binding (LBD). Although the structures of independently expressed GR DBD and LBD are known, the structures of the NTD and of full-length GR are lacking. Both DBD and LBD possess overall globular structures. Not much is known about the structure of the NTD, which contains the powerful AF1/tau1/enh2 transactivation region. Several studies have shown that AF1 region is mostly unstructured and that it can acquire folded functional conformation under certain potentially physiological conditions, namely in the presence of osmolytes, when the GR DBD is bound to glucocorticoid response element (GRE), and when AF1 binds other transcription factor proteins. These conditions are discussed here. The functions of the GR will be fully understood only when its working three-dimensional structure is known. Based on the available data, we propose a model to explain data which are not adequately accounted for in the classical models of GR action. In this review, we summarize and discuss current information on the structure of the GR in the context of its functional aspects, such as protein:DNA and protein:protein interactions. Because of the close similarities in modular organization among the members of the nuclear hormone receptors, the principles discussed here for the GR should be applicable to many other receptors in the family as well.

Functional characterization of somatic point mutations of the human estrogen receptor alpha (hERalpha) in adenomyosis uteri

Endometriosis and adenomyosis uteri are chronic, benign diseases caused by the presence of endometrial tissue in ectopic locations, e.g. peritoneal or deep inside the myometrial wall of the uterus and/or in the rectovaginal septum. Although adenomyosis might be considered as a special form of endometriosis, both conditions differ with respect to clinical symptoms and treatment. Induction of a hypo-estrogenic state alone or in combination with surgical removal of the extra-uterine lesion is mostly sufficient for treatment of peritoneal endometriosis. By contrast, adenomyosis uteri rarely responds to hormonal therapy and usually requires a hysterectomy for cure. Consequently, the role of steroid hormone receptors with respect to the aetiology of either condition is still a matter of discussion. Using PCR/single strand conformation polymorphism analysis, we identified somatic estrogen receptor (ER) alpha gene mutations in three out of 55 samples from adenomyosis uteri. Functional characterization revealed that two of the mutant ERalpha proteins display severely impaired DNA-binding and transactivation properties secondary to an altered response to estrogens or changes in epidermal growth factor-mediated ligand-independent activation. Although the exact mechanism remains unknown, we suggest that mutation-related silencing of estrogen responsiveness might render endometriotic cells resistant to hypo-estrogenic conditions thereby accounting for failure of estrogen-ablative therapy in adenomyosis.

Estrogen receptor phosphorylation

Estrogen receptor alpha (ERalpha) is phosphorylated on multiple amino acid residues. For example, in response to estradiol binding, human ERalpha is predominately phosphorylated on Ser-118 and to a lesser extent on Ser-104 and Ser-106. In response to activation of the mitogen-activated protein kinase pathway, phosphorylation occurs on Ser-118 and Ser-167. These serine residues are all located within the activation function 1 region of the N-terminal domain of ERalpha. In contrast, activation of protein kinase A increases the phosphorylation of Ser-236, which is located in the DNA-binding domain. The in vivo phosphorylation status of Tyr-537, located in the ligand-binding domain, remains controversial. In this review, I present evidence that these phosphorylations occur, and identify the kinases thought to be responsible. Additionally, the functional importance of ERalpha phosphorylation is discussed.

Transactivation functions of the N-terminal domains of nuclear hormone receptors: protein folding and coactivator interactions

The N-terminal domains (NTDs) of many members of the nuclear hormone receptor (NHR) family contain potent transcription-activating functions (AFs). Knowledge of the mechanisms of action of the NTD AFs has lagged, compared with that concerning other important domains of the NHRs. In part, this is because the NTD AFs appear to be unfolded when expressed as recombinant proteins. Recent studies have begun to shed light on the structure and function of the NTD AFs. Recombinant NTD AFs can be made to fold by application of certain osmolytes or when expressed in conjunction with a DNA-binding domain by binding that DNA-binding domain to a DNA response element. The sequence of the DNA binding site may affect the functional state of the AFs domain. If properly folded, NTD AFs can bind certain cofactors and primary transcription factors. Through these, and/or by direct interactions, the NTD AFs may interact with the AF2 domain in the ligand binding, carboxy-terminal portion of the NHRs. We propose models for the folding of the NTD AFs and their protein-protein interactions.

Role of estrogen receptor beta in estrogen action

There was a time when the classification of sex hormones was simple. Androgens were male and estrogens female. What remains true today is that in young adults androgen levels are higher in males and estrogen levels higher in females. More recently we have learned that estrogens are necessary in males for regulation of male sexual behavior, maintenance of the skeleton and the cardiovascular system, and for normal function of the testis and prostate. The importance of androgen in females was never in doubt, it is after all the precursor of estrogen as the substrate for aromatase, the enzyme that produces estrogen. In addition, the tissue distribution of androgen receptors suggests that androgens themselves are important in the ovary, uterus, breast, and brain. New information promises to clarify some of the complex issues of the physiological roles of estrogen and the contribution of estrogen to the development of neoplastic diseases in humans. The discovery of the second estrogen receptor, the creation of mutant mice defective in both estrogen receptors and in the aromatase gene, the solution of the structures of the ligand-binding domains of estrogen receptor alpha (ERalpha) and estrogen receptor beta (ERbeta), the finding of novel routes through which estrogen receptors can modulate transcription, and the identification of a man with a bi-allelic disruptive mutation of the ERalpha gene are but some of the milestones. This review focuses on the mechanistic aspects of signal transduction mediated by ERs and on the physiological consequences of deficiency of estrogen or estrogen receptor in the available mouse models.

Interdomain signaling in a two-domain fragment of the human glucocorticoid receptor

Studies of individual domains or subdomains of the proteins making up the nuclear receptor family have stressed their modular nature. Nevertheless, these receptors function as complete proteins. Studies of specific mutations suggest that in the holoreceptors, intramolecular domain-domain interactions are important for complete function, but there is little knowledge concerning these interactions. The important transcriptional transactivation function in the N-terminal part of the glucocorticoid receptor (GR) appears to have little inherent structure. To study its interactions with the DNA binding domain (DBD) of the GR, we have expressed the complete sequence from the N-terminal through the DBD of the human GR. Circular dichroism analyses of this highly purified, multidomain protein show that it has a considerable helical content. We hypothesized that binding of its DBD to the cognate glucocorticoid response element would confer additional structure upon the N-terminal domain. Circular dichroism and fluorescence emission studies suggest that additional helicity as well as tertiary structure occur in the two-domain protein upon DNA binding. In sum, our data suggest that interdomain interactions consequent to DNA binding imparts structure to the portion of the GR that contains a major transactivation domain.

The structure of the nuclear hormone receptors

The functions of the group of proteins known as nuclear receptors will be understood fully only when their working three-dimensional structures are known. These ligand-activated transcription factors belong to the steroid-thyroid-retinoid receptor superfamily, which include the receptors for steroids, thyroid hormone, vitamins A- and D-derived hormones, and certain fatty acids. The majority of family members are homologous proteins for which no ligand has been identified (the orphan receptors). Molecular cloning and structure/function analyses have revealed that the members of the superfamily have a common functional domain structure. This includes a variable N-terminal domain, often important for transactivation of transcription; a well conserved DNA-binding domain, crucial for recognition of specific DNA sequences and protein:protein interactions; and at the C-terminal end, a ligand-binding domain, important for hormone binding, protein: protein interactions, and additional transactivation activity. Although the structure of some independently expressed single domains of a few of these receptors have been solved, no holoreceptor structure or structure of any two domains together is yet available. Thus, the three-dimensional structure of the DNA-binding domains of the glucocorticoid, estrogen, retinoic acid-beta, and retinoid X receptors, and of the ligand-binding domains of the thyroid, retinoic acid-gamma, retinoid X, estrogen, progesterone, and peroxisome proliferator activated-gamma receptors have been solved. The secondary structure of the glucocorticoid receptor N-terminal domain, in particular the taul transcription activation region, has also been studied. The structural studies available not only provide a beginning stereochemical knowledge of these receptors, but also a basis for understanding some of the topological details of the interaction of the receptor complexes with coactivators, corepressors, and other components of the transcriptional machinery. In this review, we summarize and discuss the current information on structures of the steroid-thyroid-retinoid receptors.

A novel human estrogen receptor beta: identification and functional analysis of additional N-terminal amino acids

A novel human estrogen receptor beta (hERbeta) was cloned from human testis mRNA, ovary and thymus cDNA utilizing PCR and 5' RACE methods. The 5' end of hERbeta contained an additional open reading frame, in-frame and upstream of the published clones. hERbeta encodes a protein of 530 amino acids with an approximate molecular weight of 63 kDa and is larger than the previously reported rat, mouse and human protein. To determine the functional role of additional N-terminal amino acids, we compared the transcription functions of receptor lacking (hERbetaT) and receptor containing (hERbetaL) this N-terminal extension. hERbetaL is more active than hERbetaT in transactivating ERE-based reporter genes. hERbetaL, but not hERbetaT, attenuated cytokine mediated NFkappaB activation. Taken together, the additional N-terminal amino acids appear to play a role in modulating estrogen responsive gene expression in vitro.

Changes in the structure of the ligand or substitutions to AF2 residues in the estrogen receptor make independent contributions to coactivator sensitivity by SRC-1

The estrogen receptor (ER) is a ligand-inducible transcription factor which depends, in part, upon the C-terminal activation function (AF2) in order to regulate the expression of target genes. AF2 residues fold into an amphipathic alpha-helix on helix 12 of the ER, with hydrophobic and acidic faces. It is believed that AF2 mediates the gene regulatory activities of ligand-activated ER by interacting with coactivator proteins. We have analyzed the contribution of acidic AF2 residues to the process of ER coactivation by the steroid receptor coactivator, SRC-1. In HeLa cells, SRC-1 coexpression was found to restore transcriptional potency to otherwise inert complexes of wild type ER and 4-hydroxyestratrien-17beta-ol. SRC-1 coexpression also enhanced transcriptional activity of reporter genes induced by an ER mutant with neutral replacements to acidic AF2 residues, in response to E2 or 4-hydroxyestratrien-17beta-ol. By contrast, ER complexes from ICI164,384-treated HeLa cells were both transcriptionally inactive and coactivator insensitive. It is concluded that changes to the structure of the ligand or substitutions to acidic residues in the AF2 region of the receptor contribute independently to the control of coactivator sensitivity in ER.