Molecular mechanisms of action of steroid/thyroid receptor superfamily members

The interleukin-1 type 2 receptor gene displays immediate early gene responsiveness in glucocorticoid-stimulated human epidermal keratinocytes

Human epidermal keratinocytes (HEKs) in primary culture (P2-P4) were used to study glucocorticoid (GC)-mediated transcription of the genes encoding the constitutively expressed interleukin-1 type 1 receptor (IL-1R1) and the inducible interleukin-1 type 2 receptor (IL-1R2). Utilizing Northern dot blot analysis and a quantitative reverse transcription-polymerase chain reaction protocol for IL-1R1 and IL-1R2, dexamethasone and, in particular, the budesonide epimer R were shown to effectively and rapidly induce transcription from the IL-IR2 gene when compared with IL-1R1 or beta-actin RNA message levels in the same sample. Southern blot analysis of newly generated IL-1R2 reverse transcription-polymerase chain reaction products using end-labeled IL-1R2 intron probes suggested that GC enhancement of IL-1R2 expression was regulated primarily at the level of de novo transcription. GC-induced IL-1R2 gene transcription displayed features characteristic of a classical immediate early gene response, including a signal transduction function, a relatively low basal abundance, a rapid, transient induction, cycloheximide superinduction, actinomycin D suppression, and a rapid decay of IL-1R2 RNA message. Parallel time course kinetic analysis of IL-1R2 RNA message levels with Western immunoblotting revealed tight coupling of de novo IL-IR2 gene transcription with translation of the IL-1R2 RNA message; a newly synthesized ( approximately 46-kDa) IL-1R2 protein was detected in the HEK growth medium as early as 1 h after budesonide epimer R treatment. These data indicate that different GC compounds can variably up-regulate the IL-1R2 response in HEKs through transcription-mediated mechanisms and, for the first time, suggest that a gene encoding a soluble cytokine receptor can respond like an immediate early gene.

Dominant negative activity of thyroid hormone receptor variant alpha2 and interaction with nuclear corepressors

The splicing variant of the thyroid hormone receptor alpha (TRalpha) gene, TR variant alpha2 (TRv alpha2), lacks the second half of the ninth heptad, a domain thought to be important for heterodimerization with retinoid X receptors (RXRs). In transient transfection studies, TRv alpha2 exhibits weak dominant negative inhibition of TRalpha1-mediated transcription. In contrast, a TRv alpha2 mutant in which the ninth heptad was restored (alpha2 + 9H), exhibits very strong dominant negative activity. We have examined the role of nuclear corepressors (CoRs) in the dominant negative activity of TRv alpha2 and alpha2 + 9H. Glutathione S-transferase pull down experiments revealed that TRv alpha2 barely interacts with CoRs, whereas alpha2 + 9H interaction with CoRs is as strong as that of TRalpha1. A P160R CoR box mutation was introduced in the context of TRv alpha2 and alpha2 + 9H, which nearly abolishes the ability of these receptors to interact with CoRs. In transient transfection the dominant negative activity of TRv alpha2 was only marginally impaired by the P160R mutation. In contrast, alpha2 + 9H-P160R had approximately 66% less dominant negative activity than alpha2 + 9H. These results suggest that the weak dominant negative activity of TRv alpha2 is due in part to its lack of interaction with CoRs, and that restoration of the ninth heptad restores CoR interaction and strong dominant negative activity. Further, the data reveal aspects of the dominant negative action that are dependent on the orientation of the TRE.

Cell cycle and hormonal control of nuclear-cytoplasmic localization of the serum- and glucocorticoid-inducible protein kinase, Sgk, in mammary tumor cells. A novel convergence point of anti-proliferative and proliferative cell signaling pathways

The serum- and glucocorticoid-inducible kinase (sgk) is a novel serine/threonine protein kinase that is transcriptionally regulated in rat mammary tumor cells by serum under proliferative conditions or by glucocorticoids that induce a G1 cell cycle arrest. Our results establish that the subcellular distribution of Sgk is under stringent cell cycle and hormonal control. Sgk is localized to the perinuclear or cytoplasmic compartment as a 50-kDa hypophosphorylated protein in cells arrested in G1 by treatment with the synthetic glucocorticoid dexamethasone. In serum-stimulated cells, Sgk was transiently hyperphosphorylated and resided in the nucleus. Laser scanning cytometry, which monitors Sgk localization and DNA content in individual mammary tumor cells of an asynchronously growing population, revealed that Sgk actively shuttles between the nucleus (in S and G2/M) and the cytoplasm (in G1) in synchrony with the cell cycle. In cells synchronously released from the G1/S boundary, Sgk localized to the nucleus during progression through S phase. The forced retention of exogenous Sgk in either the cytoplasmic compartment, using a wild type sgk gene, or the nucleus, using a nuclear localization signal-containing sgk gene (NLS-Sgk), suppressed the growth and DNA synthesis of serum-stimulated cells. Thus, our study implicates the nuclear-cytoplasmic shuttling of sgk as a requirement for cell cycle progression and represents a novel convergence point of anti-proliferative and proliferative signaling in mammary tumor cells.

Growth and phenotypic characterization of porcine coronary artery smooth muscle cells

Vascular smooth muscle cell (VSMC) proliferation significantly contributes to atherosclerotic plaque formation and limits the success rate of percutaneous transluminal coronary angioplasty. We derived a population of porcine coronary artery SMCs to characterize VSMC proliferation and phenotype in preparation to study the molecular actions of VSMC mitogens and antiproliferative agents. Growth assays were designed to minimize the estrogen content in the culture medium, since this steroid hormone significantly influences VSMC growth and the expression of VSMC mitogens and their receptors. Culture conditions were identified such that this criterion was achieved while maintaining a significant VSMC growth rate. Cells cultured in serum-free medium, regardless of growth factor supplements, did not remain adherent to a plastic culture substrate, nor did they proliferate. Dextran-coated charcoal (DCC)-treated sera, including fetal bovine, calf, and porcine, supported VSMC adhesion, but not growth. Whole fetal bovine serum (FBS) produced the best proliferative response. A type-I collagen-coated culture surface significantly enhanced VSMC growth, but only in culture medium containing non-DCC-treated FBS. Flow cytometry analyses confirmed the mitogenic effects of this substrate. The VSMCs exhibited a morphological change on type-I collagen, but this was not accompanied by a change in VSMC phenotype. Our data indicate that culture of these porcine coronary artery SMCs in 2.5% FBS plus 10 ng platelet-derived growth factor-BB per ml in phenol red-free medium on type-I collagen may be the optimal conditions for studying the molecular aspects of VSMC mitogens and antiproliferative agents.

Chromatin recycling of glucocorticoid receptors: implications for multiple roles of heat shock protein 90

Unliganded glucocorticoid receptors (GRs) released from chromatin after hormone withdrawal remain associated with the nucleus within a novel subnuclear compartment that serves as a nuclear export staging area. We set out to examine whether unliganded nuclear receptors cycle between distinct subnuclear compartments or require cytoplasmic transit to regain hormone and chromatin-binding capacity. Hormone-withdrawn rat GrH2 hepatoma cells were permeabilized with digitonin to deplete cytoplasmic factors, and then hormone-binding and chromatin-binding properties of the recycled nuclear GRs were measured. We found that recycled nuclear GRs do not require cytosolic factors or ATP to rebind hormone. Nuclear GRs that rebind hormone in permeabilized cells target to high-affinity chromatin-binding sites at 30 C, but not 0 C, in the presence of ATP. Since geldanamycin, a heat shock protein-90 (hsp90)-binding drug, inhibits hormone binding to recycled nuclear GRs, hsp90 may be required to reassemble the receptor into a form capable of productive interactions with hormone. Geldanamycin also inhibits GR release from chromatin during hormone withdrawal, suggesting that hsp90 chaperone function may play multiple roles to facilitate chromatin recycling of GR.

Recent advances in understanding thyroid hormone receptor coregulators

Thyroid hormones (L-triiodothyronine, T3; L-tetraiodothyronine, T4) regulate normal cellular growth and development, and general metabolism as well. Their various actions are mediated by the thyroid hormone receptor, a ligand-dependent transcriptional factor belonging to the nuclear hormone receptor superfamily. The recent discovery of coregulators (coactivators, corepressors, and cointegrators) has greatly enhanced our understanding of thyroid hormone receptor functions. Hence we review and discuss, in brief, the potential role of thyroid hormone receptor coregulators involved in diverse cellular activities.

Synergistic activation of yeast-expressed rat androgen receptor by modulators of protein kinase-A

We have employed a yeast (Saccharomyces cerevisiae) based rat androgen receptor expression system to examine the cross-talk between different signalling pathways. We report here the synergistic modulation of androgen regulated transcriptional activation of beta-galactosidase reporter activity by the activators of protein kinase-A, like forskolin and 8-bromo-cyclic AMP. A similar ligand-dependent enhancement of reporter activity compared to a DHT treated control has been noticed with okadaic acid, which is a potent inhibitor of protein phosphatase. The activation could be blocked by protein kinase-A/C inhibitor, H7. Forskolin treatment neither altered levels of receptor mRNA nor [3H]R1881 binding to the receptor. Although it promotes binding of receptor to an androgen response element, forskolin was unable to activate subsequent interaction with the transcription machinery in the absence of androgen. Additionally, the synergistic actions of these activators were independent of the degree of androgen response element occupancy. Anti-androgens, cyproterone acetate and flutamide, which failed to exhibit antagonistic behaviour with yeast expressed receptor, were able to antagonize only the forskolin mediated augmentation of reporter activity. Finally, analyses of mutants established the role of DNA and steroid binding domains of receptor for this synergism.

UNC-55, an orphan nuclear hormone receptor, orchestrates synaptic specificity among two classes of motor neurons in Caenorhabditis elegans

Loss of UNC-55 function in the nematode Caenorhabditis elegans causes one motor neuron class, the ventral D (VD) motor neurons, to adopt the synaptic pattern of another motor neuron class, the dorsal D (DD) motor neurons. Here we show that unc-55 encodes a member of the nuclear hormone receptor gene family that is similar to the vertebrate chicken ovalbumin upstream promoter transcription factors. Although the VD and DD motor neuron classes arise from different lineages at different developmental stages, they share a number of structural and functional features that appear to be the product of identical genetic programs. UNC-55 is expressed in the VD but not the DD motor neurons to modify this genetic program and to create the synaptic pattern that distinguishes the two motor neuron classes from one another.

Molecular mechanism of the regulation of glutathione synthesis by tumor necrosis factor-alpha and dexamethasone in human alveolar epithelial cells

Glutathione (GSH) is an important physiological antioxidant in lung epithelial cells and lung lining fluid. We studied the regulation of GSH synthesis in response to the pro-inflammatory cytokine tumor necrosis factor-alpha (TNF-alpha) and the anti-inflammatory agent dexamethasone in human alveolar epithelial cells (A549). TNF-alpha (10 ng/ml) exposure increased GSH levels, concomitant with a significant increase in gamma-glutamylcysteine synthetase (gamma-GCS) activity and the expression of gamma-GCS heavy subunit (gamma-GCS-HS) mRNA at 24 h. Treatment with TNF-alpha also increased chloramphenicol acetyltransferase (CAT) activity of a gamma-GCS-HS 5'-flanking region reporter construct, transfected into alveolar epithelial cells. Mutation of the putative proximal AP-1-binding site (-269 to -263 base pairs), abolished TNF-alpha-mediated activation of the promoter. Gel shift and supershift analysis showed that TNF-alpha increased AP-1 DNA binding which was predominantly formed by dimers of c-Jun. Dexamethasone (3 microM) produced a significant decrease in the levels of GSH, decreased gamma-GCS activity and gamma-GCS-HS mRNA expression at 24 h. The increase in GSH levels, gamma-GCS-HS mRNA, gamma-GCS-HS promoter activity, and AP-1 DNA binding produced by TNF-alpha were abrogated by co-treating the cells with dexamethasone. Thus these data demonstrate that TNF-alpha and dexamethasone modulate GSH levels and gamma-GCS-HS mRNA expression by their effects on AP-1 (c-Jun homodimer). These data have implications for the oxidant/antioxidant balance in inflammatory lung diseases.

The molecular chaperone Hsp90 can negatively regulate the activity of a glucocorticosteroid-dependent promoter

Hsp90, a molecular chaperone required for the functioning of glucocorticosteroid receptor (GR), ensures, by direct interaction, the conformational competence of the steroid-binding pocket. In addition to having this positive function, Hsp90 maintains steroid receptors in an inactive form in the absence of hormone. However, neither the participation of Hsp90 once the pathway has been activated by the ligand nor the importance of increased Hsp90 levels in determining the amplitude of the response has ever been assessed directly. Here, by increasing the Hsp90/GR ratio in the nuclear compartment, we found an attenuation of the response to glucocorticosteroids that was not due to a nonspecific or toxic effect of the Hsp90 modified by nuclear targeting. Since this negative effect was more pronounced at high levels of hormone, when receptor and Hsp90 are maximally dissociated, the possibility of an interaction between Hsp90 and GR, already activated to a DNA-binding form, was directly investigated. Indeed GR, after in vivo activation by ligand, was still able to reassociate with Hsp90, suggesting that this interaction plays a role in vivo, possibly in receptor recycling. Moreover, the GR binding to its DNA response element was inhibited by an excess of Hsp90, pointing to a function of Hsp90 in the nuclear compartment. It is thus proposed that an increased Hsp90/GR ratio influences the responsiveness to ligand at a step that is after receptor activation. This increased ratio may be of pathophysiological relevance in the different circumstances that lead to an elevated level of nuclear Hsp90.

The 70-kDa heat shock cognate protein (Hsc73) gene is enhanced by ovarian hormones in the ventromedial hypothalamus

Estrogen (E) and progesterone (P) orchestrate many cellular responses involved in female reproductive physiology, including reproductive behaviors. E- and P-binding neurons important for lordosis behavior have been located within the ventromedial hypothalamus (VMH), and several hormone-responsive genes have been observed there as well. In attempts to identify additional E- and P-responsive genes in the VMH that may contribute to sexual behaviors, we used the differential display mRNA screening technique. One of the genes identified encodes the 73-kDa heat shock cognate protein (Hsc73). Quantitative in situ hybridization analysis of brains from naturally cycling female rats revealed a significant increase in Hsc73 mRNA in the VMH and arcuate nucleus of animals during proestrus compared with those at diestrus-1. To confirm that these increases were steroid hormone dependent, we compared vehicle-treated ovariectomized females with ovariectomized females treated with estradiol benzoate and P. Northern analysis and in situ hybridizations showed that the Hsc73 gene is enhanced by E and P in the pituitary and subregions of the VMH. Incidentally, by examining the primary amino acid sequence of rat, human, and chicken progesterone receptors, we noticed that putative Hsc73 binding sites are conserved across species with similar sites existing in the androgen and glucocorticoid receptors. Together these findings suggest a possible mechanism through which E could influence the activities of progesterone, androgen, and glucocorticoid receptors, by enhancing the expression of Hsc73 in cells where these proteins colocalize.

An additional region of coactivator GRIP1 required for interaction with the hormone-binding domains of a subset of nuclear receptors

Transcriptional coactivators of the p160 family (SRC-1, GRIP1, and p/CIP) associate with DNA-bound nuclear receptors (NRs) and help the NRs to recruit an active transcription initiation complex to the promoters of target genes. Previous studies have demonstrated the importance of the NR interaction domain (NID) of p160 proteins containing three NR box motifs (LXXLL) for the interaction with the hormone-binding domains of NRs. Here we report that, in addition to NID, another region of coactivator GRIP1 (amino acids 1011-1121), called the auxiliary NID (NIDaux), is required in vitro and in vivo for efficient interaction with a subset of NRs, including the glucocorticoid receptor (GR), androgen receptor, and retinoic acid receptor alpha. A second group of NRs, which includes the progesterone receptor, retinoid X receptor alpha, thyroid hormone receptor beta1, and vitamin D receptor, required only NID for efficient interaction. For binding to GR, the NID and NIDaux of GRIP1 must act in cis, but deletion of up to 144 amino acids between the two regions did not reduce binding efficiency. Amino acids 1011-1121 of GRIP1 also contain a p300 interaction domain, but mutational analysis indicated that the p300 interaction function within this region is separable from the ability to contribute to GR hormone-binding domain binding. SRC-1 lacks an NIDaux activity equivalent to that in GRIP1.

Signal perception and transduction: the role of protein kinases

Cells can react to environmental changes by transduction of extracellular signals, to produce intracellular responses. Membrane-impermeable signal molecules are recognized by receptors, which are localized on the plasma membrane of the cell. Binding of a ligand can result in the stimulation of an intrinsic enzymatic activity of its receptor or the modulation of a transducing protein. The modulation of one or more intracellular transducing proteins can finally lead to the activation or inhibition of a so-called 'effector protein'. In many instances, this also results in altered gene expression. Phosphorylation by protein kinases is one of the most common and important regulatory mechanisms in signal transmission. This review discusses the non-channel transmembrane receptors and their downstream signaling, with special focus on the role of protein kinases.

Regulation of amyloid precursor protein cleavage

Multiple lines of evidence suggest that increased production and/or deposition of the beta-amyloid peptide, derived from the amyloid precursor protein, contributes to Alzheimer's disease. A growing list of neurotransmitters, growth factors, cytokines, and hormones have been shown to regulate amyloid precursor protein processing. Although traditionally thought to be mediated by activation of protein kinase C, recent data have implicated other signaling mechanisms in the regulation of this process. Moreover, novel mechanisms of regulation involving cholesterol-, apolipoprotein E-, and stress-activated pathways have been identified. As the phenotypic changes associated with Alzheimer's disease encompass many of these signaling systems, it is relevant to determine how altered cell signaling may be contributing to increasing brain amyloid burden. We review the myriad ways in which first messengers regulate amyloid precursor protein catabolism as well as the signal transduction cascades that give rise to these effects.

Phosphorylation of human estrogen receptor alpha by protein kinase A regulates dimerization

Phosphorylation provides an important mechanism by which transcription factor activity is regulated. Estrogen receptor alpha (ERalpha) is phosphorylated on multiple sites, and stimulation of a number of growth factor receptors and/or protein kinases leads to ligand-independent and/or synergistic increase in transcriptional activation by ERalpha in the presence of estrogen. Here we show that ERalpha is phosphorylated by protein kinase A (PKA) on serine-236 within the DNA binding domain. Mutation of serine-236 to glutamic acid prevents DNA binding by inhibiting dimerization by ERalpha, whereas mutation to alanine has little effect on DNA binding or dimerization. Furthermore, PKA overexpression or activation of endogenous PKA inhibits dimerization in the absence of ligand. This inhibition is overcome by the addition of 17beta-estradiol or the partial agonist 4-hydroxy tamoxifen. Interestingly, treatment with the complete antagonist ICI 182,780 does not overcome the inhibitory effect of PKA activation. Our results indicate that in the absence of ligand ERalpha forms dimers through interaction between DNA binding domains and that dimerization mediated by the ligand binding domain only occurs upon ligand binding but that the complete antagonist ICI 182,780 prevents dimerization through the ligand-binding domain. Heterodimer formation between ERalpha and ERbeta is similarly affected by PKA phosphorylation of serine 236 of ERalpha. However, 4-hydroxytamoxifen is unable to overcome inhibition of dimerization by PKA. Thus, phosphorylation of ERalpha in the DNA binding domain provides a mechanism by which dimerization and thereby DNA binding by the estrogen receptor is regulated.

Differential expression of estrogen receptor-beta and estrogen receptor-alpha in the rat ovary

Immunohistochemical localization of two estrogen receptor (ER) subtypes, ER beta and ER alpha, was performed in neonatal, early postnatal, immature, and adult rats to determine whether ER alpha and ER beta are differentially expressed in the ovary. ER beta and ER alpha were visualized using a polyclonal anti-ER beta antibody and a monoclonal ER alpha (ID5) antibody, respectively. Postfixed frozen sections and antigen-retrieved paraffin sections of the ovary revealed nuclear ER beta immunoreactivity (IR) in granulosa cells, which was prevented when peptide-adsorbed antibody was used instead. In immature and adult rat ovaries, ER beta was expressed exclusively in nuclei of granulosa cells of primary, secondary, and mature follicles. Atretic follicle granulosa cells showed only weak or no staining. No specific nuclear ER beta IR was detected in thecal cells, luteal cells, interstitial cells, germinal epithelium, or oocytes. In neonatal rat ovary, no ER beta expression was found. In ovaries of 5- and 10-day-old rats, weak ER beta IR was observed in granulosa cells of primary and secondary follicles, but no staining was detected in the primordial follicles. ER alpha protein exhibited a differential distribution in the ovary with no detectable expression in the granulosa cells but evidence of ER alpha IR in germinal epithelium, interstitial cells, and thecal cells. In the oviduct and uterus, IR for ER alpha, but not ER beta, was found in luminal epithelium, stromal cells, muscle cells, and gland cells. Our present study demonstrates that ER beta and ER alpha proteins are expressed in distinctly different cell types in the ovary. The exclusive presence of ER beta in granulosa cells implies that this specific new subtype of ER beta mediates some effects of estrogen action in the regulation of growth and maturation of ovarian follicles.

Potential tissue selectivity of dietary phytoestrogens and estrogens

The recent discovery of a second estrogen receptor subtype, estrogen receptor-beta, may significantly advance our understanding of tissue specific effects of estrogenic compounds, both natural and synthetic. Although specific effects mediated by estrogen receptor beta in vivo remain to be elucidated, hypothetically the existence of two estrogen receptor subtypes (differing in both tissue distribution and biological activity) may help to explain the curious pharmacological behaviour of many estrogenic and antiestrogenic compounds, including the naturally occurring dietary phytoestrogens.

The Angelman syndrome-associated protein, E6-AP, is a coactivator for the nuclear hormone receptor superfamily

In this study, we found that the E6-associated protein (E6-AP/UBE3A) directly interacts with and coactivates the transcriptional activity of the human progesterone receptor (PR) in a hormone-dependent manner. E6-AP also coactivates the hormone-dependent transcriptional activities of the other members of the nuclear hormone receptor superfamily. Previously, it was shown that E6-AP serves the role of a ubiquitin-protein ligase (E3) in the presence of the E6 protein from human papillomavirus types 16 and 18. Our data show that the ubiquitin-protein ligase function of E6-AP is dispensable for its ability to coactivate nuclear hormone receptors, showing that E6-AP possesses two separable independent functions, as both a coactivator and a ubiquitin-protein ligase. Disruption of the maternal copy of E6-AP is correlated with Angelman syndrome (AS), a genetic neurological disorder characterized by severe mental retardation, seizures, speech impairment, and other symptoms. However, the exact mechanism by which the defective E6-AP gene causes AS remains unknown. To correlate the E6-AP coactivator function and ubiquitin-protein ligase functions with the AS phenotype, we expressed mutant forms of E6-AP isolated from AS patients and assessed the ability of each of these mutant proteins to coactivate PR or provide ubiquitin-protein ligase activity. This analysis revealed that in the majority of the AS patients examined, the ubiquitin-protein ligase function of E6-AP was defective whereas the coactivator function was intact. This finding suggests that the AS phenotype results from a defect in the ubiquitin-proteosome protein degradation pathway.

Electrogenic Na+ transport in rat late distal colon by natural and synthetic glucocorticosteroids

The potency of in vitro-added corticosteroids to stimulate electrogenic Na+ absorption (JNa, the Na+ absorptive short-circuit current blockable by 10(-4) M amiloride) was determined in rat late distal colon. JNa was determined 8 h after steroid addition from the drop in short-circuit current caused by 10(-4) M amiloride. The concentration dependency of JNa was obtained for seven corticosteroids and compared with that established for aldosterone. Apparent mineralocorticoid potencies as determined from apparent Michaelis-Menten constant (Km) values were as follows: aldosterone 1. 2 nM >> RU-28362 20 nM = deoxycorticosterone 20 nM > deoxycortisol 36 nM >/= dexamethasone 37 nM >> corticosterone 170 nM > cortisol 210 nM. These steroids exhibited Vmax values of 9-13 micromol. h-1. cm-2 and similar concentration dependencies. Hill coefficients were between 1.6 and 2.1, suggesting cooperative effects between activated receptors. We conclude that corticosteroids exhibit graded mineralocorticoid potency instead of a sharp partition into exclusive groups of mineralocorticoid and nonmineralocorticoid hormones. The low apparent Km value of RU-28362 for mineralocorticoid action and the need for high concentrations of the mineralocorticoid antagonist mespirenone to block this response indicated that JNa in a native mammalian epithelium can be mediated by the glucocorticoid receptor. Glucocorticoid receptor-specific amounts of RU-28362 in combination with mineralocorticoid receptor-specific amounts of aldosterone or of the mineralocorticoid antagonist spironolactone showed cooperative action, suggesting a heterodimeric activation of JNa by the glucocorticoid receptor and mineralocorticoid receptor.

Direct visualization of the human estrogen receptor alpha reveals a role for ligand in the nuclear distribution of the receptor

The human estrogen receptor alpha (ER alpha) has been tagged at its amino terminus with the S65T variant of the green fluorescent protein (GFP), allowing subcellular trafficking and localization to be observed in living cells by fluorescence microscopy. The tagged receptor, GFP-ER, is functional as a ligand-dependent transcription factor, responds to both agonist and antagonist ligands, and can associate with the nuclear matrix. Its cellular localization was analyzed in four human breast cancer epithelial cell lines, two ER+ (MCF7 and T47D) and two ER- (MDA-MB-231 and MDA-MB-435A), under a variety of ligand conditions. In all cell lines, GFP-ER is observed only in the nucleus in the absence of ligand. Upon the addition of agonist or antagonist ligand, a dramatic redistribution of GFP-ER from a reticular to punctate pattern occurs within the nucleus. In addition, the full antagonist ICI 182780 alters the nucleocytoplasmic compartmentalization of the receptor and causes partial accumulation in the cytoplasm in a process requiring continued protein synthesis. GFP-ER localization varies between cells, despite being cultured and treated in a similar manner. Analysis of the nuclear fluorescence intensity for variation in its frequency distribution helped establish localization patterns characteristic of cell line and ligand. During the course of this study, localization of GFP-ER to the nucleolar region is observed for ER- but not ER+ human breast cancer epithelial cell lines. Finally, our work provides a visual description of the "unoccupied" and ligand-bound receptor and is discussed in the context of the role of ligand in modulating receptor activity.

Retinoid isomers differ in the ability to induce release of SMRT corepressor from retinoic acid receptor-alpha

Nuclear hormone receptors are ligand-regulated transcription factors that modulate the expression of specific target genes in response to the binding of small, hydrophobic hormone ligands. Many nuclear hormone receptors, such as the retinoic acid receptors, can both repress and activate target gene expression; these bimodal transcription properties are mediated by the ability of these receptors to tether auxiliary factors, denoted corepressors and coactivators. Corepressors are typically bound by receptors in the absence of cognate hormone, whereas binding of an appropriate hormone agonist induces an allosteric alteration in the receptor resulting in release of the corepressor and recruitment of coactivator. Structural analysis indicates that there is a close induced fit between the hormone ligand and the receptor polypeptide chain. This observation suggests that different ligands, once bound, may confer distinct conformations on the receptor that may invoke, in turn, distinct functional consequences. We report here that different retinoids do differ in the ability to release corepressor once bound to retinoic acid receptor and suggest that these differences in corepressor release may manifest as differences in transcriptional regulation.

Progesterone regulates proliferation of endothelial cells

The use of steroid hormones in postmenopausal replacement therapy has been associated with prevention of cardiovascular disease. Although the contribution of estradiol to endothelial cell function has been addressed, little information is available on the effect of progestins on this cell type. Here, we provide direct evidence for the presence of functional nuclear progesterone receptor in endothelial cells and demonstrate that physiological levels of progesterone inhibit proliferation through a nuclear receptor-mediated mechanism. The effects of progesterone were blocked by pretreatment with a progesterone receptor antagonist, and progesterone receptor-deficient endothelial cells failed to respond to the hormone. We evaluated the effect of progesterone by analysis of aorta re-endothelialization experiments in wild-type and progesterone receptor knockout mice. The rate of re-endothelialization was significantly decreased in wild-type mice when in the presence of progesterone, whereas there was no difference between control and progesterone-treated progesterone receptor knockout mice. FACS analysis showed that progestins arrest endothelial cell cycle in G1. The lag in cell cycle progression involved reduction in cyclin-dependent kinase activity, as shown by down-regulation in retinoblastoma protein phosphorylation. In addition, treatment of endothelial cells with progestins altered the expression of cyclin E and A in accordance with G1 arrest. These results have important implications to our current knowledge of the effect of steroids on endothelial cell function and to the overall contribution of progesterone to vascular repair.

Activation of human O6-methylguanine-DNA methyltransferase gene by glucocorticoid hormone

O6-methylguanine-DNA methyltransferase (MGMT), a ubiquitous DNA repair protein, removes the mutagenic DNA adduct O6-alkylguanine, which is synthesized both endogenously and after exposure to alkylnitrosamines and alkylating antitumor drugs such as 2-chloroethyl-N-nitrosourea (CNU). The MGMT gene is highly regulated in mammalian cells and its overexpression, observed in many types of tumor cells, is often associated with cellular resistance to CNU. Dexamethasone, a synthetic glucocorticoid hormone, was found to increase MGMT expression in HeLa S3 cells, concomitant with their increased resistance to CNU. Two putative glucocorticoid responsive elements (GREs) were identified in the human MGMT (hMGMT) promoter. Transient expression of the luciferase reporter gene driven by an hMGMT promoter fragment containing these GREs was activated by dexamethasone. DNase I footprinting assays demonstrated the binding of glucocorticoid receptor to these sequences. In vitro transcription experiment showed that these DNA sequences are functional in glucocorticoid receptor signal-mediated activation of transcription. These results suggest glucocorticoid-mediated induction of the MGMT gene contributes to high level expression of MGMT.

Investigation of the binding interactions of progesterone using muteins of the human progesterone receptor ligand binding domain designed on the basis of a three-dimensional protein model

The aim of this study was to investigate the binding interactions of the human progesterone receptor (hPR) with its natural ligand. Therefore, a homology-derived model of the hPR ligand binding domain has been constructed and used to predict residues potentially involved in interactions with progesterone. These residues and the free cysteines have been mutated (in total 13 residues with 15 mutations). All exchanges have been designed to preserve the three-dimensional structure of the protein. With respect to the binding characteristics towards progesterone, the muteins fall into three groups displaying no, reduced, or wildtype-like binding activity.

Ligand-independent activation of the glucocorticoid receptor by beta2-adrenergic receptor agonists in primary human lung fibroblasts and vascular smooth muscle cells

The glucocorticoid receptor (GR) is a ubiquitously expressed transcription factor present in most cell types. Upon ligand binding, the GR is activated and translocates into the nucleus where it transmits the anti-inflammatory actions of glucocorticoids. Here, we describe the ligand-independent activation of GR by the beta2-adrenergic receptor (beta2-AR) agonists, salbutamol and salmeterol, in primary human lung fibroblasts and vascular smooth muscle cells. Immunohistochemistry demonstrated expression of GR and the beta2-AR by fibroblasts and vascular smooth muscle cells. Treatment of the cells with the beta2-AR agonists, salbutamol or salmeterol, resulted in translocation of GR into the nucleus beginning at 30 min, as shown by immunohistochemistry and Western blotting of cytosolic and nuclear cell extracts. In comparison, activation of GR induced by the corticosteroids dexamethasone and fluticasone occurred at the same time after treatment (30 min) but resulted in a more complete depletion of GR from the cytosolic compartment. Electrophoretic mobility shift assays confirmed that nuclear GR, activated by both beta2-AR agonists and glucocorticoids, actively bound to the GR consensus sequence (GR element). Functional activation of the GR was confirmed by a Luciferase reporter gene assay, using a GR driven promoter fragment from the p21((WAF1/CIP1)) gene. The effects of the beta2-AR agonists, salbutamol and salmeterol, were dependent upon binding to the beta2-AR, because blocking of beta2-AR with propranolol abrogated GR activation. GR activation appeared to involve cAMP. In summary, these data show that beta2-AR agonists are potent activators of GR. Ligand-independent activation of GR by beta2-AR agonists may substantially mediate the anti-inflammatory actions of these drugs observed in vitro and in vivo.

3,3'4,4'-Tetrachlorobiphenyl exhibits antiestrogenic and antitumorigenic activity in the rodent uterus and mammary cells and in human breast cancer cells

3,3',4,4'-Tetrachlorobiphenyl (tetraCB) binds to the aryl hydrocarbon receptor (AhR), and several reports have demonstrated that AhR agonists exhibit antiestrogenic and antitumorigenic activities in human breast cancer cells, the rodent uterus and breast. In contrast, a recent study showed that 3,3',4,4'-tetraCB bound the estrogen receptor (ER) and exhibited ER agonist activities, and we therefore have reinvestigated the estrogenic and antiestrogenic activities of 3,3',4,4'-tetraCB. Our results showed that 3,3',4,4'tetraCB and a structurally related analog, 3,3',4,4',5-pentaCB, did not bind the mouse uterine or human ER, did not induce proliferation of MCF-7 or T47D human breast cancer cells or induce reporter gene activity in cells transfected with E2-responsive constructs derived from the creatine kinase B (pCKB) or cathepsin D (pCD) gene promoters. Moreover, 3,3',4,4'-tetraCB and 3,3',4,4',5-pentaCB did not induce an increase in uterine wet weight, peroxidase activity or progesterone receptor binding in the 21-25-day-old female B6C3F1 mouse uterus. In contrast, both compounds inhibited 17beta-estradiol (E2)-induced cell proliferation and transactivation in MCF-7/T47D cells and uterine responses in B6C3F1 mice; surprisingly inhibition of E2-induced reporter gene activity was not observed in T47D cells transfected with pCKB, and this was observed as a cell-specific response with other AhR agonists. Additionally, 3,3',4,4'-tetraCB significantly inhibited mammary tumor growth in female Sprague-Dawley rats initiated with 7,12-dimethylbenzanthracene. Our results indicate that 3,3',4,4'-tetraCB does not exhibit ER agonist activity but exhibits a broad spectrum of antiestrogenic responses consistent with ligand-mediated AhR-ER crosstalk.

New screening methods for chemicals with hormonal activities using interaction of nuclear hormone receptor with coactivator

The endocrine system exerts important functions in a multitude of physiological processes including embryogenesis, differentiation, and homeostasis. Xenobiotics may modify natural endocrine function and so affect human health and wildlife. It is necessary, therefore, to understand the degree to which xenobiotics can disrupt endocrine systems. The key targets of endocrine disruptors are nuclear hormone receptors, which bind to steroid hormones and regulate their gene transcription. We have developed relevant assay systems based on the ligand-dependent interaction between nuclear hormone receptor and coactivator. The coactivators used in this study contained CBP, p300, RIP140, SRC1, TIF1, and TIF2. By two hybrid assay in yeast, the interactions of estrogen receptor with RIP140, SRC1, TIF1, and TIF2 were detected and they were completely dependent on the presence of estrogen. Specificity of this assay was assessed by determining the effect of steroids, known estrogen receptor agonists, and phytoestrogens. The pattern of response to chemicals were consistent with estrogenic activity measured by other assay systems, indicating that this assay system is reliable for measuring estrogenic activity. In addition, we carried out in vitro binding studies: GST pull-down assay and surface plasmon resonance analysis. The estrogen receptor also bound to coactivator in response to chemicals depending on their estrogenic activity in vitro. These data demonstrate that the measurement of interaction between steroid hormone receptor and coactivator serves as a useful tool for identifying chemicals that interact with steroid receptors.

Response of primary rabbit kidney proximal tubule cells to estrogens

The effects of estrogens on the growth and function of primary rabbit kidney proximal tubule (RPT) cells have been examined in hormonally defined phenol red-free medium. 17beta-estradiol was observed to stimulate growth at dosages as low as 10(-10) M. The growth stimulatory effects of 17beta-estradiol were mitigated in the presence of hydrocortisone, suggesting that these two steroid hormones acted at least in part by common mechanisms. The effects of other steroids known to interact with the estrogen receptor were examined. Alpha estradiol was found to be growth stimulatory over a concentration range of 10(-9) to 10(-8) M, albeit to a lower extent than beta estradiol. In addition, the anti-estrogen tamoxifen was also growth stimulatory (unlike the case with the human mammary tumor cell line MCF-7). The effects of several metabolic precursors of 17beta-estradiol were examined, including testosterone, which was growth stimulatory, and progesterone, which was growth inhibitory. The growth stimulatory effects of 17beta-estradiol, alpha estradiol, and tamoxifen could possibly be explained by their interaction with an estrogen receptor. Indeed, metabolic labelling and immunoprecipitation studies indicated the presence of such an estrogen receptor in the primary cultures. The rate of biosynthesis of the estrogen receptor was found to be affected by the presence of exogenously added 17beta-estradiol. 17beta-estradiol was also observed to increase the activity of two brush border enzymes, alkaline phosphatase and gamma glutamyl transpeptidase, during the growth phase of the primary cultures.

Comparison of estrogen receptor alpha and beta subtypes based on comparative molecular field analysis (CoMFA)

A substantial body of evidence indicates that both humans and wildlife suffer adverse health effects from exposure to environmental chemicals that are capable of interacting with the endocrine system. The recent cloning of the estrogen receptor beta subtype (ER-beta) suggests that the selective effects of estrogenic compounds may arise in part by the control of different subsets of estrogen-responsive promoters by the two ER subtypes, ER-alpha and ER-beta. In order to identify the structural prerequisites for ligand-ER binding and to discriminate ER-alpha and ER-beta in terms of their ligand-binding specificities, Comparative Molecular Field Analysis (CoMFA) was employed to construct a three-dimensional Quantitative Structure-Activity Relationship (3D-QSAR) model on a data set of 31 structurally-diverse compounds for which competitive binding affinities have been measured against both ER-alpha and ER-beta. Structural alignment of the molecules in CoMFA was achieved by maximizing overlap of their steric and electrostatic fields using the Steric and Electrostatic ALignment (SEAL) algorithm. The final CoMFA models, generated by correlating the calculated 3D steric and electrostatic fields with the experimentally observed binding affinities using partial least-squares (PLS) regression, exhibited excellent self-consistency (r2 > 0.99) as well as high internal predictive ability (q2 > 0.65) based on cross-validation. CoMFA-predicted values of RBA for a test set of compounds outside of the training set were consistent with experimental observations. These CoMFA models can serve as guides for the rational design of ER ligands that possess preferential binding affinities for either ER-alpha or ER-beta. These models can also prove useful in risk assessment programs to identify real or suspected EDCs.

Estrogen induces adenosine deaminase gene expression in MCF-7 human breast cancer cells: role of estrogen receptor-Sp1 interactions

Adenosine deaminase (ADA) gene expression is induced by 17beta-estradiol (E2) in MCF-7 human breast cancer cells, whereas the antiestrogens 4'-hydroxytamoxifen and ICI 182,780 exhibit partial estrogen receptor (ER) agonist/antagonist and antagonist activities, respectively. Previous studies have shown that the -211 to +11 region of the ADA gene promoter contains six GC-rich sites (I-VI) that bind Sp1 protein, and these elements are required for high basal expression. In transient transfection studies with pADA211, which contains the -211 to +11 ADA gene promoter linked to a bacterial chloramphenicol acetyl transferase (CAT) reporter gene, E2 and tamoxifen (but not ICI 182,780) induced CAT activity. Ligand-induced transactivation was observed only in cells cotransfected with expression plasmids for wild-type ER or HE11, which does not contain the DNA-binding domain of the ER. Cotransfection with HE15 and HE19, which contain the DNA-binding domain and activation function-1 (AF-1) and AF-2 of the ER, respectively, did not result in E2-induced activity. Subsequent deletion analysis of the ADA gene promoter showed that Sp1 binding site IV (-79 to -73) was primarily responsible for hormone responsiveness. ER activation of ADA gene expression is another example of an E2-induced gene that is dependent on ER/Sp1 interactions with a site-specific GC-rich motif.

Plasticity of hippocampal corticosteroid receptors during aging in the rat

Aging is commonly associated with dysregulation of the hypothalamo-pituitary-adrenal axis and cognitive impairment. On the basis of suggestions that these disruptions ensue from changes in the hippocampal complement of corticosteroid (mineralocorticoid and glucocorticoid) receptors (MR and GR), we examined the availability of hippocampal MR and GR by measuring the in vivo uptake of 3H-aldosterone and 3H-dexamethasone (selective MR and GR agonists, respectively); MR and GR mRNA levels were also measured. We observed age-related declines in both the synthesis of MR and GR and the uptake of their respective ligands. Whereas MR mRNA levels and ligand uptake declined in parallel, GR binding declined more steeply than GR mRNA. This latter result, together with our finding that aged rats show impaired corticosteroid receptor mRNA and protein up-regulation after corticosteroid withdrawal, indicates decreased transcription of MR and GR genes and posttranslational modification of GR mRNA during aging. Given that corticosteroids can influence MR and GR synthesis and binding, and based on the finding that aged subjects show reduced basal secretion of corticosterone, we propose that this relative hypocorticalism may be responsible for the changes observed in MR and GR activity, which then leads to disturbances in neuroendocrine regulation and cognitive function in aged subjects.

Progesterone signaling and mammary gland morphogenesis

Progesterone was identified as a mammogenic hormone several years ago but until now its precise role in mammary development has remained obscure. Recently with the generation of several transgenic mouse models and development of reagents for analysis of progesterone receptor expression, the role of progesterone signaling in mammary development is becoming more clear. The most significant observations to emerge from these studies are (1) progesterone receptors (PR) are present in a heterogeneous manner in the epithelial cells and undetectable in the surrounding fat pad; (2) they are essential for lobuloalveolar and not for ductal morphogenesis; (3) progesterone signaling through progesterone receptors, leading to lobuloalveolar development, is initiated in the epithelium and may occur through paracrine mechanisms; and (4) a regulated expression of the two isoforms of progesterone receptor is critical for maintaining appropriate responsiveness to progesterone and hence, epithelial cell replicative homeostasis. These studies also reveal that the consequences of progesterone signaling through progesterone receptor may depend on the cell context, cell-cell and cell-extracellular matrix interactions, the dynamics of PR turnover and the fate of PR positive cells.

Structural analysis of the thyroid hormone receptor ligand binding domain: studies using a quadrupole time-of-flight tandem mass spectrometer

The overall architecture of the ligand binding domain (LBD) of members of the nuclear receptor superfamily are similar. There are now standard procedures to express and purify these proteins. A rapid and sensitive method for the structural analysis of these proteins is nano-electrospray tandem mass spectrometry. In the present study we have analysed the LBD of the human thyroid hormone receptor-beta-1 (TR-beta) by quadrupole time-of-flight tandem mass spectrometry. The intact protein was analysed in a carboxymethylated form in an attempt to identify which cysteine residues are located on the surface. The protein molecular weight (31 652.5 Da) was determined with an accuracy of +/-1 Da, while masses of tryptic fragments were determined with an accuracy of at least 75 ppm. The sequence coverage of the tryptic peptide mass map was 93.2 %. Tryptic peptides were subjected to collision-induced dissociation (CID) and the resulting product ions were mass measured with an accuracy of about 100 ppm. When accurate mass measurements were made with internal calibration, mass accuracies were improved to +/-2 ppm in mass spectra, and +/-20 ppm in CID spectra. From these data it was possible to determine the presence of post-translational modifications, locate the sites of carboxymethylation and, in addition, confirm the amino acid sequence of the expressed protein. To the best of our knowledge, this is the first characterisation of the TR-LBD-beta at the protein level.

Methods for xenoestrogen testing

Research in our laboratories has focused on development of a battery of in vivo and in vitro bioassays for determining estrogenic activity and potency of different classes of natural and synthetic industrial-derived estrogenic compounds (xenoestrogens) including food/beverage extracts, phytoestrogens, phenolic compounds, organochlorine pesticides and pollutants. For many of the weak estrogenic compounds, their activity as estrogen receptor (ER) agonists or antagonists is dependent on the gene/gene promoter, cell context and expression of ER(alpha) or ER(beta) isoform. For example, extracts of red wine, bound to the ER, exhibited estrogenic activity in T47D, MCF-7 (breast) and Hep G2 (liver) human cancer cell lines, whereas reconstituted organochlorine pesticide residues found in food were active only in Hep G2 cells that transiently expressed ER(alpha) or ER(beta). The relative potencies of red wine extracts versus reconstituted organochlorine pesticides were assay-dependent; however, estrogen equivalent daily intakes from a glass of red wine (approximately 0.5-2 microg estrogen equivalents/day) were at least 10(3) higher than observed for the reconstituted organochlorine pesticide mixtures. Risk assessment of xenoestrogens and other synthetic chemicals which modulate endocrine responses must take into account high dietary levels of natural products in food, drugs and health food store extracts which also modulate endocrine responses.

The structural basis of estrogen receptor/coactivator recognition and the antagonism of this interaction by tamoxifen

Ligand-dependent activation of transcription by nuclear receptors (NRs) is mediated by interactions with coactivators. Receptor agonists promote coactivator binding, and antagonists block coactivator binding. Here we report the crystal structure of the human estrogen receptor alpha (hER alpha) ligand-binding domain (LBD) bound to both the agonist diethylstilbestrol (DES) and a peptide derived from the NR box II region of the coactivator GRIP1 and the crystal structure of the hER alpha LBD bound to the selective antagonist 4-hydroxytamoxifen (OHT). In the DES-LBD-peptide complex, the peptide binds as a short alpha helix to a hydrophobic groove on the surface of the LBD. In the OHT-LBD complex, helix 12 occludes the coactivator recognition groove by mimicking the interactions of the NR box peptide with the LBD. These structures reveal the two distinct mechanisms by which structural features of OHT promote this "autoinhibitory" helix 12 conformation.

Retinoic acid and thyroid hormone may function through similar and competitive pathways in regenerating axolotls

The objective of this study was to determine whether thyroid hormone (TH) would interfere with retinoic acid (RA), which proximalizes axolotl larvae regenerate limb pattern. RA and TH are ligands for members of the steroid hormone thyroid hormone nuclear binding protein superfamily which form functional homodimers, but may also form stable heterodimers with the RXR protein and may recognize identical DNA sequences. TH alone does not affect limb pattern but induces metamorphosis in regenerating animals. Coinjected animals do not metamorphose, and when compared to RA controls regenerate more proximal and in some cases anteroposterior (AP) and dorsoventral (DV) duplicate limb structures. In addition, the tissues that are normally lost or changed during metamorphosis appear to be sensitized resulting in the formation of (1) new dorsal gill lamellae accompanied by bifurcation and broadening of the original gill lamellae, (2) partial resorption of the tail fin, and (3) changes in eye position and snout morphology. Bifurcation of gill lamellae tips, but not the formation of supernumerary gills, is also observed in animals treated with RA alone. These results indicate that the molecular mechanism of RA and TH function through similar and perhaps competitive pathways.

Agonist and antagonists induce homodimerization and mixed ligand heterodimerization of human progesterone receptors in vivo by a mammalian two-hybrid assay

This study utilizes the mammalian two-hybrid system to examine the role of ligand in the dimerization of human progesterone receptor (hPR). The GAL4 DNA-binding domain and the herpes simplex virus VP16 transactivation domain were fused to the amino terminus of full-length hPR (both the A and B isoforms) to produce chimeric proteins. PR dimerization was detected by the ability of cotransfected GAL4/PR and VP16/PR chimeras in COS cells to induce expression of a reporter gene under the control of GAL4-binding sites (pG5CAT). Hormone agonist-dependent interactions were observed between the two like isoforms of PR (A-A and B-B) and between PR-A and PR-B (A-B), indicating that hormone can stimulate the formation of the three possible dimeric forms of PR within cells. In contrast, neither type I (ZK98299) nor type II (RU486, ZK112993) progestin antagonists stimulated interaction between these same hybrid PR proteins. However, activation of the VP16/PR chimera by antagonists on a progesterone response element-controlled reporter gene (DHRE-E1b-CAT) was only a fraction (4-13%) of that stimulated by agonist R5020. One possibility for the failure to detect an induction in the two-hybrid assay is antagonist-induced repression of the activity of the VP16/PR fusion protein rather than a failure of antagonists to stimulate interaction between the hybrid proteins. To test this idea, an UP-1 carboxyl-terminal truncation mutant of PR was used to construct the two-hybrid proteins. PR-UP-1 selectively binds antagonists, but not agonists, and is fully activated in response to antagonists. Both types of progestin antagonists stimulated interactions between GAL4/PR(UP-1) and VP16/PR(UP-1) hybrid proteins, indicating that antagonists are capable of stimulating PR dimerization in cells and do not function by disrupting or preventing dimerization. To determine whether PR bound to an antagonist can dimerize in whole cells with PR bound to agonist, GAL4/PR(UP-1) was paired in the two-hybrid assay with a VP16/PR fusion protein harboring a point mutation in PR at amino acid 722 (Gly-Cys) that specifically binds progestin agonist but not antagonist. Neither R5020 nor RU486 alone stimulated interaction between these ligand-specific PR hybrid proteins. However, strong interaction was detected by addition of both agonist and antagonists, indicating the formation of mixed ligand heterodimers and that both PR partners require ligand for dimerization to occur. Based on electrophoretic gel mobility shift assays (EMSAs), these heterodimers appear to have substantially reduced DNA binding activity. Progestin antagonists inhibit agonist activation of PR at concentrations that are too low to be accounted for by a simple competition mechanism for binding to PR. We propose that antiprogestin inactivation of PR in trans by heterodimerization contributes to the biological potency of these compounds.

CREB binding protein is a coactivator for the androgen receptor and mediates cross-talk with AP-1

Androgens are critical in the development and maintenance of the male reproductive system and important in the progression of prostate cancer. The effects of androgens are mediated through the androgen receptor (AR), which is a ligand-modulated transcription factor that belongs to the nuclear receptor superfamily. In addition to its ability to activate transcription from androgen response elements, AR can inhibit activator protein-1 (AP-1) activity, composed of Jun and Fos oncoproteins, in a ligand-dependent manner. Conversely, when activated, AP-1 can block AR activity. We found that CREB (cAMP response element-binding protein) binding protein (CBP) had a direct role in both of these activities of AR. CBP significantly increased the ability of endogenous AR in LNCaP cells to activate transcription from an AR-dependent reporter construct. On the other hand, repression of AR activity by treatment of LNCaP cells with an activator of AP-1 was largely relieved when CBP was ectopically expressed. AR and CBP can physically interact in vitro as was shown in glutathione S-transferase pulldown assays. Whereas both the N terminus and ligand-binding domain of AR can interact with CBP, a short region in the N terminus of CBP is required for these interactions. As opposed to the interaction of CBP with other nuclear receptors studied so far, CBP-AR interactions were not affected by ligand binding to AR in vitro. These data suggest that CBP is a coactivator for AR in vivo and that the transcriptional interference between AR and AP-1 is the result of competition for limiting amounts of CBP in the cell.

Putative estrogen receptor beta and alpha mRNA expression in male and female rhesus macaques

The profound effects of estrogen on different tissues may involve at least two estrogen receptor (ER) subtypes, ERalpha and the recently discovered ERbeta. Where and how the two ER subtypes differentially or cooperatively mediate estrogen actions, however, are still unknown. In this study, we report the cloning of a specific ERbeta cDNA fragment and the expression of ERalpha and ERbeta mRNAs in various endocrine and non-endocrine tissues of male and female rhesus macaques. Total RNA from monkey tissues was isolated and subjected to reverse transcription-polymerase chain reaction (RT-PCR) using human-specific ERbeta primers. A 126 bp RT-PCR product was identified in ovarian tissue and subsequently transfected into pGEM-T vectors for DNA sequencing. The cloned rhesus monkey ERbeta fragment contained a sequence nearly identical to the corresponding sequence in the human (four mismatched nucleotides out of 126). Because complete monkey ERbeta and ERalpha DNA sequences have not been established, the expression of the ERbeta and ERalpha fragments in monkey tissues by RT-PCR reflects 'putative' ERbeta and ERalpha mRNA expression, respectively. Both ERbeta and ERalpha mRNAs were present in male and female reproductive organs, in several endocrine and non-endocrine tissues, and in various regions of the brain, whereas several tissues, including liver, frontal cortex, caudate nucleus, locus coeruleus and cerebellum, expressed only ERalpha message. In some brain regions, i.e. the putamen, internal capsule, hippocampus and paraventricular hypothalamus, the ERbeta fragment was expressed in the female but not in the male. These data suggest that ERalpha mRNA is widely distributed in both female and male tissues, while ERbeta mRNA is more widely distributed in female than in male brain.

Cross-talk between orphan nuclear hormone receptor RZRalpha and peroxisome proliferator-activated receptor alpha in regulation of the peroxisomal hydratase-dehydrogenase gene

The genes encoding the peroxisomal beta-oxidation enzymes enoyl-CoA hydratase/3-hydroxyacyl-CoA dehydrogenase (HD) and fatty acyl-CoA oxidase (AOx) are coordinately regulated by peroxisome proliferator-activated receptor alpha (PPARalpha)/9-cis-retinoic acid receptor (RXRalpha) heterodimers that transactivate these genes in a ligand-dependent manner via upstream peroxisome proliferator response elements (PPRE). Here we demonstrate that the monomeric orphan nuclear hormone receptor, RZRalpha, modulates PPARalpha/RXRalpha-dependent transactivation in a response-element dependent manner. Electrophoretic mobility shift analysis showed that RZRalpha bound specifically as a monomer to the HD-PPRE but not the AOx-PPRE. Determinants in the HD-PPRE for binding of RZRalpha were distinct from those required for interaction with PPARalpha/RXRalpha heterodimers. In transient transfections, RZRalpha stimulated ligand-mediated transactivation by PPARalpha from an HD-PPRE luciferase reporter in the absence of exogenously added RXRalpha, but did not affect PPARalpha-dependent transactivation of an AOx-PPRE reporter gene. These data illustrate cross-talk between the RZRalpha and PPARalpha signaling pathways at the level of the HD-PPRE in the regulation of the HD gene and characterize additional factors governing the regulation of peroxisomal beta-oxidation.

Differential regulation of androgen and glucocorticoid receptors by retinoblastoma protein

The androgen receptor (AR) plays a major role in the development and maintenance of male primary and secondary sexual characteristics. The growth promoting effects of androgens are clearly seen in prostate cancer where treatment by androgen ablation usually leads to tumor regression, followed sometime later, by growth of tumor cells that are resistant to endocrine therapy. We have found that the level of pRB in cells controls AR activity. Overexpression of pRB leads to increased transcriptional activity of the AR. This is similar to the previously reported potentiation of glucocorticoid receptor activity by pRB. In contrast, loss of pRB activity inhibits AR but not glucocorticoid receptor activity. This inhibition correlates with the unique ability of the AR to form a protein-protein complex with pRB in vitro. The site of interaction with pRB lies within the N-terminal domain of the AR and co-localizes with the region of the AR that specifies a requirement for pRB. Thus, the AR has a novel requirement for pRB raising the possibility that the growth promoting activity of AR is due to its direct interaction with pRB. Furthermore, loss of pRB activity during progression of prostate cancer may directly result in a decreased response to androgens.

Defective release of corepressor by hinge mutants of the thyroid hormone receptor found in patients with resistance to thyroid hormone

On positive thyroid hormone response elements (pTREs), thyroid hormone receptor (TR) binding to DNA in the absence of ligand (thyroid hormone, T3) decreases transcription (silencing). Silencing is due to a family of recently described nuclear corepressor proteins (NCoR and SMRT) which bind to the CoR box in the hinge region of TR. Ligand-dependent activation of TR is associated with displacement of corepressors and recruitment of coactivating proteins. Resistance to thyroid hormone (RTH) is due to mutations in the beta isoform of the thyroid hormone receptor (TR-beta). To date, three RTH mutations reportedly with near-normal T3 binding (A234T, R243Q, and R243W) have been described in or near the CoR box. To determine the mechanism of RTH caused by these mutants, the interaction of wild type (wt) and mutant TRs with the corepressor, NCoR, and the coactivator, SRC-1, was tested in gel-shift assays. As expected, NCoR bound wt TR in the absence of T3 and dissociated from TR with increasing T3 concentration. SRC-1 failed to bind wt TR in the absence of T3, but bound to TR with increasing avidity as T3 concentrations rose. At no T3 concentration did both NCoR and SRC-1 bind to wt TR, indicating that their binding to TR was mutually exclusive. Hinge mutants bound NCoR normally in the absence of T3; however, dissociation of NCoR and recruitment of SRC-1 was markedly impaired except at very high T3 concentrations. Importantly, hinge mutant TRs when complexed to DNA bound T3 poorly despite their near-normal T3 binding in solution. These binding studies correlated with functional assays showing defective transactivation of pTREs by hinge mutants except at high T3 concentrations. Thus, we describe a novel mechanism of RTH whereby TR hinge mutants selectively affect T3 binding when complexed to DNA, and prevent NCoR dissociation from TR. Our data also suggest that solution T3 binding by RTH mutants may not accurately reflect physiologically relevant T3 binding by TR when bound to DNA.