Coactivators and corepressors as mediators of nuclear receptor function: an update

Repression motif in HSF1 regulated by phosphorylation

The heat shock factor 1 (HSF1) is a transcription factor that itself is a sensor for stress and integrates various intrinsic or environmental stress sensing pathways. Thus HSF1 orchestrates the heat shock response (HSR) by translating these pathways into a distinct transcriptional program that aids the cells to cope with and adapt to proteotoxic stress. Although heavily researched the regulation of HSF1 activation is still not completely understood. A conserved reaction to stress is the hyperphosphorylation of the otherwise confined constitutive phosphorylated HSF1. Therefore, this stress specific phosphorylation is believed to be involved in the regulatory mechanism and hence, was and is focus of many studies, ascribing various effects to single phosphorylation sites. To gain additional insight into effects of phosphorylation, HSF1 carrying amino acid substitutions on up to 18 amino acids were tested for their transactivation potential on an HSR reporter plasmid. A pattern of eleven phosphor-mimicking and diminishing amino acid substitutions on well-known phosphorylation sites of HSF1 were introduced to produce transcriptional active [11 M(+)] or repressed [11 M(-)] phenotypes. It could be confirmed that heat activates HSF1 regardless of phosphorylation. Distinct cellular stress, obtained by chemical HSR inducers or mimicked by a constitutively active HSF1, showed clear differences in the activation potential of HSF1-11 M(+) and 11 M(-). Further refinement to the single amino acid level identified the S303/307 double-phosphorylation motif, wherein phosphorylation of S303 was sole responsible for the repressing effect. The effect could be reproduced in different cell lines and is not entirely based on degradation. A small repression motif could be dissociated from the HSF1 context, which is still capable of repressing the background transcription of a specifically designed reporter plasmid. Taken together these results indicate, that besides already described mechanisms of pS303/307 mediated repression of HSF1 activation, an additional mechanism repressing the transcriptional output of the entire HSE containing promoter is mediated by this small repressive motif.

Emerging Role of SMILE in Liver Metabolism

Small heterodimer partner-interacting leucine zipper (SMILE) is a member of the CREB/ATF family of basic leucine zipper (bZIP) transcription factors. SMILE has two isoforms, a small and long isoform, resulting from alternative usage of the initiation codon. Interestingly, although SMILE can homodimerize similar to other bZIP proteins, it cannot bind to DNA. As a result, SMILE acts as a co-repressor in nuclear receptor signaling and other transcription factors through its DNA binding inhibition, coactivator competition, and direct repression, thereby regulating the expression of target genes. Therefore, the knockdown of SMILE increases the transactivation of transcription factors. Recent findings suggest that SMILE is an important regulator of metabolic signals and pathways by causing changes in glucose, lipid, and iron metabolism in the liver. The regulation of SMILE plays an important role in pathological conditions such as hepatitis, diabetes, fatty liver disease, and controlling the energy metabolism in the liver. This review focuses on the role of SMILE and its repressive actions on the transcriptional activity of nuclear receptors and bZIP transcription factors and its effects on liver metabolism. Understanding the importance of SMILE in liver metabolism and signaling pathways paves the way to utilize SMILE as a target in treating liver diseases.

Emerging Role of SMILE in Liver Metabolism

Small heterodimer partner-interacting leucine zipper (SMILE) is a member of the CREB/ATF family of basic leucine zipper (bZIP) transcription factors. SMILE has two isoforms, a small and long isoform, resulting from alternative usage of the initiation codon. Interestingly, although SMILE can homodimerize similar to other bZIP proteins, it cannot bind to DNA. As a result, SMILE acts as a co-repressor in nuclear receptor signaling and other transcription factors through its DNA binding inhibition, coactivator competition, and direct repression, thereby regulating the expression of target genes. Therefore, the knockdown of SMILE increases the transactivation of transcription factors. Recent findings suggest that SMILE is an important regulator of metabolic signals and pathways by causing changes in glucose, lipid, and iron metabolism in the liver. The regulation of SMILE plays an important role in pathological conditions such as hepatitis, diabetes, fatty liver disease, and controlling the energy metabolism in the liver. This review focuses on the role of SMILE and its repressive actions on the transcriptional activity of nuclear receptors and bZIP transcription factors and its effects on liver metabolism. Understanding the importance of SMILE in liver metabolism and signaling pathways paves the way to utilize SMILE as a target in treating liver diseases.

Synthesis, biological evaluation and X-ray analysis of bicalutamide sulfoxide analogues for the potential treatment of prostate cancer

The androgen receptor (AR) is a pivotal target for the treatment of prostate cancer (PC) even when the disease progresses toward androgen-independent or castration-resistant forms. In this study, a series of sulfoxide derivatives were prepared and their antiproliferative activity evaluated in vitro against four different human prostate cancer cell lines (22Rv1, DU-145, LNCaP and VCap). Bicalutamide and enzalutamide were used as positive controls. Compound 28 displayed significant enhancement in anticancer activity across the four PC cell lines with IC₅₀ = 9.09 - 31.11 µM compared to the positive controls: bicalutamide (IC₅₀ = 45.20 -51.61 µM) and enzalutamide (IC₅₀ = 11.47 - 53.04 µM). Sulfoxide derivatives of bicalutamide were prepared efficiently from the corresponding sulfides using only one equivalent of mCPBA, limiting the reaction time to 15-30 min and maintaining the temperature at 0 °C. Interestingly, three pairs of sulfoxide diastereomers were separated and NMR comparison of their diastereotopic methylene (CH₂) group is presented. X-ray diffraction crystal structure analysis provided relative configuration assignment at the chiral sulfur and carbon centres. Molecular modelling study of the four diastereoisomers of compound 28 is described.

Synthesis and Biological Evaluation of Bicalutamide Analogues for the Potential Treatment of Prostate Cancer

The androgen receptor (AR) is a pivotal target for the treatment of prostate cancer (PC) even when the disease progresses toward androgen-independent or castration-resistant forms. In this study, a series of 15 bicalutamide analogues (sulfide, deshydroxy, sulfone, and O-acetylated) were prepared and their antiproliferative activity evaluated against four different human prostate cancer cell lines (22Rv1, DU-145, LNCaP, and VCap). Bicalutamide and enzalutamide were used as positive controls. Seven of these compounds displayed remarkable enhancement in anticancer activity across the four PC cell lines. The deshydroxy analogue (16) was the most active compound with IC₅₀ = 6.59-10.86 µM. Molecular modeling offers a plausible explanation of the higher activity of the sulfide analogues compared to their sulfone counterparts.

ICAT acts as a Coactivator in Regulating PPARγ Transcriptional Activity in Mesangial Cells

AIMS

Our study aims to explore the role of β-catenin interaction protein-1(ICAT) in regulating peroxisome proliferator-activated receptor γ (PPARγ) transcriptional activity in mesangial cells. The abnormal ICAT expression in mesangial cells under high glucose(HG) contributes to the development of diabetes and its complications such as diabetic nephropathy (DN).

METHODS

Human mesangial cells (HMCs) were cultured in either 5.5 (normal control) or 30 (high glucose) mmol/L glucose medium. Overexpression and knock-down of ICAT or β-catenin were carried out by transient transfection. PPARγ transcriptional activity was evaluated by luciferase assay. Protein-protein interactions were tested by Coimmunoprecipitation and GST-pull down assay. Cell phenotype transition of HMCs was detected by the expression level of α-SMA and fibronectin, as well as MTT assay.

RESULTS

High β-catenin protein expression but low ICAT was accompanied by low PPARγ transcriptional activity in HMCs cultured in HG. By using bioinformatics prediction, protein-protein and protein-DNA interaction experimental methods, ICAT and β-catenin were confirmed to act as coactivators in regulating PPARγ transcriptional activity. Overexpression of ICAT could mitigate the decrease of PPARγ transcriptional activity and partly relieve cell phenotype transition in HMCs.

CONCLUSIONS

β-catenin and ICAT interact as coactivator to modulate PPARγ transcriptional activation. In HMCs cultured in HG, the low expression of ICAT leads to low PPARγ transcriptional activation.

Construction of reporter gene assays using CWP and PDR mutant yeasts for enhanced detection of various sex steroids

Background

Sex steroid hormone receptors are classified into three classes of receptors: estrogen receptors (ER) α and β, androgen receptor (AR), and progesterone receptor (PR). They belong to the nuclear receptor superfamily and activate their downstream genes in a ligand-dependent manner. Since sex steroid hormones are involved in a wide variety of physiological processes and cancer development, synthetic chemical substances that exhibit sex steroid hormone activities have been applied as pharmaceuticals and consumed in large amounts worldwide. They are potentially hazardous contaminants as endocrine disruptors in the environment because they may induce inappropriate gene expression mediated by sex steroid hormone receptors in vivo.

Results

To develop simple reporter gene assays with enhanced sensitivity for the detection of sex steroid hormones, we newly established mutant yeast strains lacking the CWP and PDR genes encoding cell wall mannoproteins and plasma membrane drug efflux pumps, respectively, and expressing human ERα, ERβ, AR, and PR. Reporter gene assays with mutant yeast strains responded to endogenous and synthetic ligands more strongly than those with wild-type strains. Sex steroid hormone activities in some pharmaceutical oral tablets and human urine were also detectable in these yeast assays.

Conclusions

Yeast reporter gene assay systems for all six steroid hormone receptors, including previously established glucocorticoid receptor (GR) and mineralocorticoid receptor (MR) assay yeasts, are now available. Environmental endocrine disrupters with steroid hormone activity will be qualitatively detectable by simple and easy procedures. The yeast-based reporter gene assay will be valuable as a primary screening tool to detect and evaluate steroid hormone activities in various test samples. Our assay system will strongly support the detection of agonists, antagonists, and inverse agonists of steroid hormone receptors in the field of novel drug discovery and assessments of environmental pollutants.

Potential therapeutic roles of retinoids for prevention of neuroinflammation and neurodegeneration in Alzheimer's disease

All retinoids, which can be natural and synthetic, are chemically related to vitamin A. Both natural and synthetic retinoids use specific nuclear receptors such as retinoic acid receptors and retinoid X receptors to activate specific signaling pathways in the cells. Retinoic acid signaling is extremely important in the central nervous system. Impairment of retinoic acid signaling pathways causes severe pathological processes in the central nervous system, especially in the adult brain. Retinoids have major roles in neural patterning, differentiation, axon outgrowth in normal development, and function of the brain. Impaired retinoic acid signaling results in neuroinflammation, oxidative stress, mitochondrial malfunction, and neurodegeneration leading to progressive Alzheimer's disease, which is pathologically characterized by extra-neuronal accumulation of amyloid plaques (aggregated amyloid-beta) and intra-neurofibrillary tangles (hyperphosphorylated tau protein) in the temporal lobe of the brain. Alzheimer's disease is the most common cause of dementia and loss of memory in old adults. Inactive cholinergic neurotransmission is responsible for cognitive deficits in Alzheimer's disease patients. Deficiency or deprivation of retinoic acid in mice is associated with loss of spatial learning and memory. Retinoids inhibit expression of chemokines and neuroinflammatory cytokines in microglia and astrocytes, which are activated in Alzheimer's disease. Stimulation of retinoic acid receptors and retinoid X receptors slows down accumulation of amyloids, reduces neurodegeneration, and thereby prevents pathogenesis of Alzheimer's disease in mice. In this review, we described chemistry and biochemistry of some natural and synthetic retinoids and potentials of retinoids for prevention of neuroinflammation and neurodegeneration in Alzheimer's disease.

Regulation of Osteoblast Differentiation by Affinity Peptides of TGF-β1 Identified via Phage Display Technology

Transforming growth factor β1 (TGF-β1) plays a dual role in bone formation. In addition to promoting early differentiation of osteogenesis, it may also lead to uncontrolled extracellular matrix synthesis, inhibition of bone mineralization in the late stage, and aberrant bone remodeling. In this work, affinity peptides of TGF-β1 (Tβms) were identified from a phage display library to modify the TGF-β1 signal transduction. Tβms with more order and compact structures tended to have a higher affinity to TGF-β1 but maintained a greater immunoreactivity of TGF-β1. Tβms promoted the early osteoblast proliferation and had a negligible effect on the osteoblast differentiation. In synergy with exogenous TGF-β1, Tβms reduced the alkaline phosphatase (ALP) mRNA expression but significantly improved the expression of osteocalcin (OCN), along with impaired phosphorylation of Smad2/3. Moreover, osteoblasts showed an overall increase in ALP activity and Ca deposition than the blank control. These results demonstrated that Tβms could weaken the inhibition of TGF-β1 on osteogenic differentiation in the late stage. Depending on the impact features of Tβms on TGF-β1 response, these peptides may help to modify the implant surfaces to optimize the bone remodeling of interface, and be of interest in design of multidomain peptides.

Development of 5N-Bicalutamide, a High-Affinity Reversible Covalent Antiandrogen

Resistance to clinical antiandrogens has plagued the evolution of effective therapeutics for advanced prostate cancer. As with the first-line therapeutic bicalutamide (Casodex), resistance to newer antiandrogens (enzalutamide, ARN-509) develops quickly in patients, despite the fact that these drugs have ∼10-fold better affinity for the androgen receptor than bicalutamide. Improving affinity alone is often not sufficient to prevent resistance, and alternative strategies are needed to improve antiandrogen efficacy. Covalent and reversible covalent drugs are being used to thwart drug resistance in other contexts, and activated aryl nitriles are among the moieties being exploited for this purpose. We capitalized on the presence of an aryl nitrile in bicalutamide, and the existence of a native cysteine residue (Cys784) in the androgen receptor ligand binding pocket, to develop 5N-bicalutamide, a cysteine-reactive antiandrogen. 5N-bicalutamide exhibits a 150-fold improvement in K_i and 20-fold improvement in IC₅₀ over the parent compound. We attribute the marked improvement in affinity and activity to the formation of a covalent adduct with Cys784, a residue that is not among the more than 160 androgen receptor point mutations associated with prostate cancer. Increasing the residence time of bound antiandrogen via formation of a covalent adduct may forestall the drug resistance seen with current clinical antiandrogens.

The ability of thyroid hormone receptors to sense t4 as an agonist depends on receptor isoform and on cellular cofactors

T4 (3,5,3',5'-tetraiodo-l-thyronine) is classically viewed as a prohormone that must be converted to the T3 (3,5,3'-triiodo-l-thyronine) form for biological activity. We first determined that the ability of reporter genes to respond to T4 and to T3 differed for the different thyroid hormone receptor (TR) isoforms, with TRα1 generally more responsive to T4 than was TRβ1. The response to T4 vs T3 also differed dramatically in different cell types in a manner that could not be attributed to differences in deiodinase activity or in hormone affinity, leading us to examine the role of TR coregulators in this phenomenon. Unexpectedly, several coactivators, such as steroid receptor coactivator-1 (SRC1) and thyroid hormone receptor-associated protein 220 (TRAP220), were recruited to TRα1 nearly equally by T4 as by T3 in vitro, indicating that TRα1 possesses an innate potential to respond efficiently to T4 as an agonist. In contrast, release of corepressors, such as the nuclear receptor coreceptor NCoRω, from TRα1 by T4 was relatively inefficient, requiring considerably higher concentrations of this ligand than did coactivator recruitment. Our results suggest that cells, by altering the repertoire and abundance of corepressors and coactivators expressed, may regulate their ability to respond to T4, raising the possibility that T4 may function directly as a hormone in specific cellular or physiological contexts.

The two major isoforms of thyroid hormone receptor, TRα1 and TRβ1, preferentially partner with distinct panels of auxiliary proteins

Thyroid hormone receptors (TRs) are expressed primarily as two major isoforms, TRα1 and TRβ1, which are expressed at different times in development and at different tissue abundances in the adult. The transcription properties and biological properties of TRα1 and TRβ1 can differ. We report here that although overlapping, TRα1 and TRβ1 recruit distinct panels of partner proteins that may account for their divergent biological functions, and which appear to explain their distinct target gene regulatory properties.

Regulation of bcl-2 transcription by estrogen receptor-α and c-Jun in human endometrium

The estrogen-estrogen receptor (ER) signaling pathway plays crucial physiologic roles in not only the control of reproduction, but also in the generation of cancer in the breast and uterus. While some ER target genes have been identified containing the estrogen-responsive element (ERE), others are activated eventually by ER via protein-protein interaction without binding to ERE. In a previous study, we identified that the proliferative phase-specific expression of the bcl-2 gene in glandular cells could be regulated by the binding of c-Jun to its motifs in the promoter. Results from our present study indicate that the menstrual cyclic expression of bcl-2 could be controlled by either direct binding of ERα to ERE in the c-Jun promoter or the interaction of ERα with c-Jun that binds to its motifs in the bcl-2 gene. Intriguingly, the transcriptionally active form of c-Jun phosphorylated at Ser63 was identified binding to its motifs in the bcl-2 gene in a menstrual cyclic non-specific manner. Our study revealed a novel mechanism that transcriptionally regulates the expression of bcl-2 in the normal human endometrium.

PPARα in Obesity: Sex Difference and Estrogen Involvement

Peroxisome proliferator-activated receptor α (PPARα) is a member of the steroid hormone receptor superfamily and is well known to act as the molecular target for lipid-lowering drugs of the fibrate family. At the molecular level, PPARα regulates the transcription of a number of genes critical for lipid and lipoprotein metabolism. PPARα activators are further shown to reduce body weight gain and adiposity, at least in part, due to the increase of hepatic fatty acid oxidation and the decrease in levels of circulating triglycerides responsible for adipose cell hypertrophy and hyperplasia. However, these effects of the PPARα ligand fenofibrate on obesity are regulated with sexual dimorphism and seem to be influenced by the presence of functioning ovaries, suggesting the involvement of ovarian steroids in the control of obesity by PPARα. In female ovariectomized mice, 17β-estradiol inhibits the actions of fenofibrate on obesity through its suppressive effects on the expression of PPARα target genes, and these processes may be mediated by inhibiting the coactivator recruitment of PPARα. Thus, it is likely that PPARα functions on obesity may be enhanced in estrogen-deficient states.

CUE domain containing 2 regulates degradation of progesterone receptor by ubiquitin-proteasome

Accumulated evidence indicates that progesterone receptors (PR) are involved in proliferation of breast cancer cells and are implicated in the development of breast cancer. In this paper, a yeast two-hybrid screen for PR led to the identification of CUE domain containing 2 (CUEDC2), whose function is unknown. Our results demonstrate that CUEDC2 interacts with PR and promotes progesterone-induced PR degradation by the ubiquitin-proteasome pathway. The inhibition of endogenous CUEDC2 by siRNA nearly abrogated the progesterone-induced degradation of PR, suggesting that CUEDC2 is involved in progesterone-induced PR ubiquitination and degradation. Moreover, we identify the sumoylation site Lys-388 of PR as the target of CUEDC2-promoted ubiquitination. CUEDC2 decreases the sumoylation while promoting ubiquitination on Lys-388 of PRB. We also show that CUEDC2 represses PR transactivation, inhibits the ability of PR to stimulate rapid MAPK activity, and impairs the effect of progesterone on breast cancer cell growth. Therefore, our results identify a key post-translational mechanism that controls PR protein levels and for the first time provide an important insight into the function of CUEDC2 in breast cancer proliferation.

Comparison of crystal structures of human androgen receptor ligand-binding domain complexed with various agonists reveals molecular determinants responsible for binding affinity

Androgens exert their effects by binding to the highly specific androgen receptor (AR). In addition to natural potent androgens, AR binds a variety of synthetic agonist or antagonist molecules with different affinities. To identify molecular determinants responsible for this selectivity, we have determined the crystal structure of the human androgen receptor ligand-binding domain (hARLBD) in complex with two natural androgens, testosterone (Testo) and dihydrotestosterone (DHT), and with an androgenic steroid used in sport doping, tetrahydrogestrinone (THG), at 1.64, 1.90, and 1.75 A resolution, respectively. Comparison of these structures first highlights the flexibility of several residues buried in the ligand-binding pocket that can accommodate a variety of ligand structures. As expected, the ligand structure itself (dimension, presence, and position of unsaturated bonds that influence the geometry of the steroidal nucleus or the electronic properties of the neighboring atoms, etc.) determines the number of interactions it can make with the hARLBD. Indeed, THG--which possesses the highest affinity--establishes more van der Waals contacts with the receptor than the other steroids, whereas the geometry of the atoms forming electrostatic interactions at both extremities of the steroid nucleus seems mainly responsible for the higher affinity measured experimentally for DHT over Testo. Moreover, estimation of the ligand-receptor interaction energy through modeling confirms that even minor modifications in ligand structure have a great impact on the strength of these interactions. Our crystallographic data combined with those obtained by modeling will be helpful in the design of novel molecules with stronger affinity for the AR.

Heterodimers of retinoic acid receptors and thyroid hormone receptors display unique combinatorial regulatory properties

Nuclear receptors are ligand-regulated transcription factors that regulate key aspects of metazoan development, differentiation, and homeostasis. Nuclear receptors recognize target genes by binding to specific DNA recognition sequences, denoted hormone response elements (HREs). Many nuclear receptors can recognize HREs as either homodimers or heterodimers. Retinoid X receptors (RXRs), in particular, serve as important heterodimer partners for many other nuclear receptors, including thyroid hormone receptors (TRs), and RXR/TR heterodimers have been proposed to be the primary mediators of target gene regulation by T3 hormone. Here, we report that the retinoic acid receptors (RARs), a distinct class of nuclear receptors, are also efficient heterodimer partners for TRs. These RAR/TR heterodimers form with similar affinities as RXR/TR heterodimers on an assortment of consensus and natural HREs, and preferentially assemble with the RAR partner 5' of the TR moiety. The corepressor and coactivator recruitment properties of these RAR/TR heterodimers and their transcriptional activities in vivo are distinct from those observed with the corresponding RXR heterodimers. Our studies indicate that RXRs are not unique in their ability to partner with TRs, and that RARs can also serve as robust heterodimer partners and combinatorial regulators of T3-modulated gene expression.

Androgen deprivation therapy for prostate cancer: current status and future prospects

Androgens play a major role in promoting the development and progression of prostate cancer. As a result, androgen ablation or blockade of androgen action through the androgen receptor (AR) has been the cornerstone of treatment of advanced prostate cancer. Different strategies involving this hormonal therapy produce a significant clinical response in most of the patients, but most responders eventually lose dependency, resulting in mortality. Thus, whether hormonal therapy contributes to the improvement of overall survival rates, especially in patients with advanced prostate cancer, remains controversial. However, patients with advanced disease clearly have a benefit from androgen deprivation-based treatment for palliating their symptoms and for improving the quality of their lives. In order to improve overall survival, novel treatment strategies that prolong the androgen-dependent state and that are useful for androgen-independent disease based on specific molecular mechanisms need to be identified.

Cyclin D1 binding to the androgen receptor (AR) NH2-terminal domain inhibits activation function 2 association and reveals dual roles for AR corepression

The androgen receptor (AR) is a member of the nuclear receptor superfamily, the activity of which is critical for the development and progression of prostate cancer. We and others have previously demonstrated that cyclin D1 is a potent corepressor of the AR. Although cyclin D1 is suspected to recruit histone deacetylases to the AR complex, previous studies have demonstrated that this activity alone is insufficient for cyclin D1 function. Here, we uncover a novel, secondary means of cyclin D1-mediated repression, through modulation of AR amino-carboxy terminal interactions. We show that cyclin D1 predominantly binds the N-terminal domain of the AR, dependent on the AR 23FxxLF27 motif. Through this motif, cyclin D1 abrogates the ability of the AR N-terminal domain to interact with the C terminus. Secondary amino-terminal domain sites capable of fostering interaction with the C terminus were refractory to cyclin D1 action, indicating that the ability of cyclin D1 to modulate AR amino-carboxy terminal interactions is specific to 23FxxLF27. Deletion of the N-terminal cyclin D1 binding site severely compromised AR activity (due to loss of FxxLF) but unmasked a repressor action through interaction with the AR C terminus. In summary, these data reveal novel, unexpected mechanisms of cyclin D1 activity and demonstrate that this function of cyclin D1 is critical for AR modulation.

The role of corepressors in transcriptional regulation by nuclear hormone receptors

Nuclear receptors (also known as nuclear hormone receptors) are hormone-regulated transcription factors that control many important physiological and developmental processes in animals and humans. Defects in receptor function result in disease. The diverse biological roles of these receptors reflect their surprisingly versatile transcriptional properties, with many receptors possessing the ability to both repress and activate target gene expression. These bipolar transcriptional properties are mediated through the interactions of the receptors with two distinct classes of auxiliary proteins: corepressors and coactivators. This review focuses on how corepressors work together with nuclear receptors to repress gene transcription in the normal organism and on the aberrations in this process that lead to neoplasia and endocrine disorders. The actions of coactivators and the contributions of the same corepressors to the functions of nonreceptor transcription factors are also touched on.

RNA polymerase II association with the phosphoenolpyruvate carboxykinase (PEPCK) promoter is reduced in vitamin A-deficient mice

Phosphoenolpyruvate carboxykinase (PEPCK) gene expression is decreased in vitamin A-deficient (VAD) mice. However, the underlying molecular mechanism at the PEPCK promoter that contributes to this alteration in gene expression remains unexplained and thus serves as the basis for our investigation in this report. Using liver from vitamin A-sufficient (VAS) and VAD mice in the chromatin immunoprecipitation (ChIP) assay, we determined that histones H3 and H4 were in the acetylated or active state in VAS mice at each of the three retinoic acid response elements (RARE1, RARE2 and RARE3) of the PEPCK promoter. The same acetylation pattern was seen in VAD mice, but with relatively lower levels of acetylated H3 and H4 bound at the region encompassing PEPCK RARE1/RARE2. In ChIP assays conducted with an antibody to RNA polymerase II (RNA Pol II), the association of RNA Pol II with PEPCK RARE1/RARE2 was significantly decreased in vitamin A deficiency. The reduction in RNA Pol II association is indicative of an interruption in the direct interactions of RNA Pol II with the PEPCK promoter, with general transcription factors and/or with coregulator molecules that contribute to the activation of the PEPCK gene. These results increase our understanding of the molecular basis for decreased PEPCK gene expression in VAD mice in vivo and offer additional insight into the regulation of other retinoid-responsive genes.

Dexamethasone regulation of gastrin-releasing peptide receptor in human lung cells

We investigated the effects of the glucocorticoid, dexamethasone (Dex), on expression of the gastrin-releasing peptide (GRP) receptor by human small cell lung carcinoma (SCLC) SHP77 cells. After 12h of 10nM Dex exposure, a six-fold increase in the peak of GRP receptor mRNA compared with untreated controls (10.5+/-4 versus 1.65+/-0.15 attomols/microg total RNA, respectively, P<0.05) occurred. GRP receptor mRNA levels fell to less than 0.5 attomols/microg total RNA after 24h; in Dex-treated cells, these levels rose to 1.2 compared with 0.12 attomols/microg total RNA in the absence of Dex after 7 days. A significant increase (P<0.05) in the GRP receptor-specific binding was also found. Stimulation of SHP77 cell proliferation (25-35% in the presence of 10-100 nM Dex; P<0.0001) was observed after 4-8 days of exposure; this stimulation was inhibited by GRP receptor antagonists. SHP77 cell content and concentration of bombesin-like peptides (BLP) in conditioned medium (approximately 4 nM) was unchanged by Dex. Stimulation of human SCLC SHP77 cell proliferation by Dex may, in part, occur via effects on the GRP autocrine system in these cells.

The androgen receptor: structure, mutations, and antiandrogens

Androgens play a critical role not only in the physiological development of the prostate but also in the genesis of prostate cancer. The effects of androgen on the prostate gland and on the other tissues of the body are mediated by activation of the androgen receptor. The androgen receptor is a member of the superfamily of hormone receptors with a DNA-binding site, two zinc finger domains, and a hormone-binding site. Mutations in this receptor can be associated with loss of function or chronic endogeneous activation, depending upon the site of change. Androgens effect a conformal change in the structure of the androgen receptor associated with a change in protein phosphorylation. The androgen receptor can be activated by additional ligands affecting the hormone-binding site besides androgens. Activators and repressors of the androgen receptor modify this protein's function and are very delicately balanced such that disruptions of either function are associated with a disease state. Antiandrogens, which bind to the receptor and thus down-regulate the effects of endogeneous circulating androgens, remain the first line treatment for palliation of advanced prostate cancer. Mutations in the receptor are associated with a change in function of such compounds from antagonist to agonist in vitro. Newer evidence suggests there may be a role of intermittent androgen suppression rather than continuous suppression, perhaps by preventing overgrowth of hormone independent tumor cells. Future research focuses on the development of drugs directed at suppressing the androgen drive of the androgen sensitive clone of the tumor and making the nonsensitive subset more susceptible to cytotoxics.

CBP recruitment and histone acetylation in differential gene induction by glucocorticoids and progestins

We have analyzed histone acetylation at the steroid-responsive mouse mammary tumor virus (MMTV) promoter in five separate cell lines that express functional glucocorticoid and/or progesterone receptors. Chromatin immunoprecipitation assays reveal that glucocorticoid and progesterone receptors bind the MMTV promoter after hormone addition but that receptor binding is not associated with an increase in acetylation of histone H3 or H4. We have, however, found one exception to this rule. Previously we described a cell line [T47D(C&L)] that displayed a remarkable differential induction of MMTV by glucocorticoids and progestins. At one chromosomal locus (MMTV-luciferase), MMTV is preferentially induced by glucocorticoids, whereas at another locus within the same cell (MMTV-CAT), MMTV is activated by both glucocorticoids and progestins. Here we show that the glucocorticoid-mediated induction of MMTV-luciferase is accompanied by increased recruitment of CBP to the promoter and increased histone H3 and H4 acetylation, whereas the hormonal induction of MMTV-CAT in the same cell exhibits a more modest CBP recruitment without any increase in histone acetylation. These studies suggest that increased histone acetylation may serve a potentiating function for MMTV promoter activation at certain loci. However, increased histone acetylation is not requisite for steroid-mediated induction of transcription at all genes.

Thyroid hormone receptor-specific interactions with steroid receptor coactivator-1 in the pituitary

Steroid receptor coactivator-1 (SRC-1) is a transcription cofactor that enhances the hormone-dependent action mediated by the thyroid hormone (TH) receptor (TR) as well as other nuclear receptors. However, it is not known whether the SRC-1-mediated activation of TH-regulated gene transcription is TR isoform specific in the pituitary. We generated mice that were deficient in TRalpha and SRC-1 (TRalpha(0/0)SRC-1(-/-)), as well in TRbeta and SRC-1 (TRbeta(-/-)SRC-1(-/-)), and thyroid function tests and effects of TH deprivation and TH treatment were compared with wild-type mice or mice with deletion of either TRs or SRC-1 alone. We have shown that 1) TRbeta(-/-)SRC-1(-/-) mice demonstrate more severe TH resistance than either the SRC-1(-/-) or TRbeta(-/-) mice; the additive effect indicates that SRC-1 has an independent role in TH action over that of TRbeta; 2) SRC-1 facilitates TRbeta and TRalpha-mediated down-regulation of TSH, as TRalpha(0/0)SRC-1(-/-) mice demonstrate TH resistance rather than hypersensitivity as seen in TRalpha(0/0)mice; and 3) a compensatory increase in SRC-1 expression is associated with the TH hypersensitivity seen in TRalpha-deficient animals. We conclude that SRC-1 action in the pituitary mediates TH action via specific TR subtypes.

4-ethoxymethylphenol: a novel phytoestrogen that acts as an agonist for human estrogen receptors

Estrogen is the natural agonist of the estrogen receptor (ER). However, certain plant-derived compounds or phytoestrogens have been identified that mimic estrogens and act as agonists and/or antagonists of ERs, depending on subtype and target tissue. Using thin layer chromatography (TLC), gas chromatography-mass spectrometry (GC-MS), and proton nuclear magnetic resonance (1H-NMR), we identified a simple phenol, 4-ethoxymethylphenol (4EM), found in Maclura pomifera that acts as an agonist of ER-alpha and ER-beta in HeLa and MCF-7 cells. To study the effect of 4EM on ER-alpha and ER-beta activity, we performed transient transfection assays and showed that 4EM activated ER-dependent gene transcription in a dose-dependent manner on both ER subtypes and this activity was inhibited by trans-4-hydroxytamoxifen (4HT). Further, 4EM-mediated transcription in ER-alpha, like estrogen, was enhanced in the presence of coactivators, steroid receptor coactivator-1 (SRC-1), CREB binding proteins (CBP), and E6-associated protein (E6-AP). We found that 4EM was specific for ER and did not activate transcription of the progesterone receptor.

Mechanisms of decidualization

The uterus is composed of heterogeneous cell types that undergo cyclic synchronized waves of proliferation and differentiation in response to the rise and fall of ovarian oestrogen and progesterone. The spatial and temporal diversity in cellular responses to ovarian hormones within a given endometrial cell compartment is thought to be effected by locally released factors. These endometrial polypeptides bind to specific cell surface receptors on target cells, resulting in activation of signal transduction pathways by way of coupling to GTP-binding proteins (G proteins), or through autophosphorylation in response to conformational changes induced by the binding of ligand. Within this paradigm, the highly complex and coordinated expression of decidua-specific genes by differentiating endometrial stroma cells during the late secretory phase of the menstrual cycle could be regarded as the result of the convergence of liganded steroid hormone receptors and specific activated cytoplasmatic signalling pathways.

Update on glucocorticoid action and resistance

Glucocorticoids (GCs) are the most common group of medications used in the treatment of allergic and autoimmune disorders. They produce potent anti-inflammatory effects by inducing or repressing the expression of target genes. Although most patients with allergic diseases and autoimmune disorders respond to GC therapy, a small subset of patients demonstrate persistent tissue inflammation despite treatment with high doses of GCs. This condition results from an interaction between susceptibility genes, the host's environment, and immunologic factors. The treatment of these patients requires a systematic approach to rule out underlying conditions that lead to steroid resistance or treatment failure, as well as the use of alternative strategies to inhibit tissue inflammation.

Specificity of thyroid hormone receptor subtype and steroid receptor coactivator-1 on thyroid hormone action

Isoforms of the thyroid hormone receptor (TR)alpha and TRbeta genes mediate thyroid hormone action. How TR isoforms modulate tissue-specific thyroid hormone (TH) action remains largely unknown. The steroid receptor coactivator-1 (SRC-1) is among a group of transcriptional coactivator proteins that bind to TRs, along with other members of the nuclear receptor superfamily, and modulate the activity of genes regulated by TH. Mice deficient in SRC-1 possess decreased tissue responsiveness to TH and many steroid hormones; however, it is not known whether or not SRC-1-mediated activation of TH-regulated gene transcription in peripheral tissues, such as heart and liver, is TR isoform specific. We have generated mice deficient in TRalpha and SRC-1, as well as in TRbeta and SRC-1, and investigated thyroid function tests and effects of TH deprivation and TH treatment compared with wild-type (WT) mice or those deficient in either TR or SRC-1 alone. The data show that 1) in the absence of TRalpha or TRbeta, SRC-1 is important for normal growth; 2) SRC-1 modulates TRalpha and TRbeta effects on heart rate; 3) two new TRbeta-dependent markers of TH action in the liver have been identified, osteopontin (upregulated) and glutathione S-transferase (downregulated); and 4) SRC-1 may mediate the hypersensitivity to TH seen in liver of TRalpha-deficient mice.

Characterization of the two coactivator-interacting surfaces of the androgen receptor and their relative role in transcriptional control

The androgen receptor interacts with the p160 coactivators via two surfaces, one in the ligand binding domain and one in the amino-terminal domain. The ligand binding domain interacts with the nuclear receptor signature motifs, whereas the amino-terminal domain has a high affinity for a specific glutamine-rich region in the p160s. We here describe the implication of two conserved motifs in the latter interaction. The amino-terminal domain of the androgen receptor is a very strong activation domain constituent of Tau5, which is mainly active in the absence of the ligand binding domain, and Tau1, which is only active in the presence of the ligand binding domain. Both domains are, however, implicated in the recruitment of the p160s. Mutation analysis of the p160s has shown that the relative contribution of the two recruitment mechanisms via the signature motifs or via the glutamine-rich region depend on the nature of the enhancers tested. We propose, therefore, that the androgen receptor-coactivator complex has several alternative conformations, depending partially on the context of the enhancer.

Anabolic steroids, testosterone-precursors and virilizing androgens induce distinct activation profiles of androgen responsive promoter constructs

Different androgens, e.g. virilizing androgens such as testosterone and its precursors as well as synthetic anabolic steroids, respectively, induce diverse biological effects. The molecular basis for this variety in biological actions, however, is not well understood. We hypothesized that this variability of actions may be due to steroid-specific target gene expression profiles following androgen receptor (AR)-activation. Therefore, we investigated androgen receptor dependent transactivation of three structurally different androgen responsive promoter constructs ((ARE)(2)TATA-luc, MMTV-luc, GRE-OCT-luc) in co-transfected Chinese hamster ovary (CHO)-cells as an artificial model simulating different natural target genes. Three virilizing androgens (dihydrotestosterone, testosterone, methyltrienolone), three anabolic steroids (oxandrolone, stanozolol, nandrolone) and two testosterone-precursors of gonadal and adrenal origin (dehydroepiandrosterone, androstenedione) were used as ligands (0.001-100 nM). All steroids proved to be potent activators of the AR. Remarkably, anabolic steroids and testosterone-precursors showed characteristic promoter activation profiles distinct from virilizing androgens with significantly lower (ARE)(2)TATA-luc activation. Hierarchical clustering based on similarity of activation profiles lead to a dendrogram with two major branches: first virilizing androgens, and second anabolics/testosterone-precursors. We conclude that steroid-specific differences in gene transcription profiles due to androgen receptor activation could contribute to differences in biological actions of androgens.

Strain differences in tamoxifen sensitivity of Sprague-Dawley and Fischer 344 rats

Why some women are at increased risk for the development of endometrial carcinoma while taking the antiestrogen tamoxifen (Tam) for breast cancer treatment or prevention is unknown. Various strains of rodents display differences in sensitivity to compounds with estrogenic activity, but whether differences in Tam sensitivity exist in rodent strains has not been investigated. In the present study, we investigated whether rat strain differences in reproductive tract sensitivity to Tam and estrogen exist between Fischer 344 (F344) and Sprague Dawley (SD) rats. Immature (21-23 day; 6/group), ovariectomized F344 and SD rats were treated with vehicle (control), 17beta-estradiol (E2) [1 x 10 (-6) to 1.0 micro g/kg body weight (BW)] or 4-OH tamoxifen (4-OHT) (1 x 10 (-4) to 10 mg/kg BW) for 2 days and then sacrificed on day 3. Reproductive tracts were collected, weighed, and examined for changes in histomorphology and expression of ER and nuclear receptor co-regulators (SRC1, p300, CARM1, GRIP1, SPA, REA and Uba3). Treatment with E (1 x 10(-5) micro g/kg BW) increased ( <0.05) uterine epithelial cell height in F344 but not SD rats, demonstrating increased sensitivity of the F344 strain to E. Conversely, treatment with 1 x 10(-3) mg/kg BW 4-OHT increased ( <0.05) uterine weight and epithelial cell height in SD but not F344 rats, demonstrating that the SD strain is more sensitive to the antiestrogen. Northern and Western blot and immunohistochemical analysis revealed that ER expression levels in the SD and F344 uterus were not different. Expression of receptor co-regulators was higher in the uterus compared to the vagina regardless of strain and higher CARM1 expression was seen in SD uterus compared to F344 rats. Understanding differences in Tam sensitivity may help us to better understand why some women develop endometrial cancer while taking Tam and be beneficial in treatment decisions for breast cancer patients.

Identification and characterization of novel estrogen receptor-beta-sparing antiprogestins

The steroid hormones estrogen and progesterone together regulate the development and maintenance of the female reproductive system. The actions of these two hormones are mediated by their respective nuclear receptors located within overlapping cell populations in target organs. The molecular mechanism of action of these two hormones has been defined to a large extent using estrogen receptor (ER) and progesterone receptor (PR) antagonists. In the case of ER, the available antagonists are highly receptor selective. With respect to PR, however, the available antiprogestins also interact with the receptors for glucocorticoids, mineralocorticoids, and androgens. Whereas these cross-reactivities can usually be managed in studies of female reproductive function, it is the recent demonstration that RU486 is an effective antagonist of the beta-isoform of ER that suggested the need for more selective antiprogestins. In this study, we used cell-based transcriptional assays combined with screens using coactivator peptide analogs to identify two novel classes of antiprogestins that distinguish themselves from the antiprogestin RU486 in the manner they interact with PR. One class exhibits the characteristics of a pure antiprogestin in that its members bind to the receptor and induce a conformational change that prevents the presentation of two potential coactivator binding surfaces on the protein. The second class of compounds distinguish themselves from RU486 in that they are ERbeta sparing. When tested in vivo the ER-sparing antiprogestins were as effective as RU486 in suppressing superovulation. It is anticipated that the availability of these new antiprogestins will advance the studies of PR pharmacology in a manner similar to how the availability of selective ER modulators has helped the study of ER action.

Estrogen-related receptor alpha 1 actively antagonizes estrogen receptor-regulated transcription in MCF-7 mammary cells

The estrogen-related receptor alpha (ERRalpha) is an orphan member of the nuclear receptor superfamily. We show that the major isoform of the human ERRalpha gene, ERRalpha1, can sequence-specifically bind a consensus palindromic estrogen response element (ERE) and directly compete with estrogen receptor alpha (ERalpha) for binding. ERRalpha1 activates or represses ERE-regulated transcription in a cell type-dependent manner, repressing in ER-positive MCF-7 cells while activating in ER-negative HeLa cells. Thus, ERRalpha1 can function both as a modulator of estrogen responsiveness and as an estrogen-independent activator. Repression likely occurs in the absence of exogenous ligand since charcoal treatment of the serum had no effect on silencing activity. Mutational analysis revealed that repression is not simply the result of competition between ERalpha and ERRalpha1 for binding to the DNA. Rather, it also requires the presence of sequences within the carboxyl-terminal E/F domain of ERRalpha1. Thus, ERRalpha1 can function as either an active repressor or a constitutive activator of ERE-dependent transcription. We hypothesize that ERRalpha1 can play a critical role in the etiology of some breast cancers, thereby providing a novel therapeutic target in their treatment.

Signaling of monocytic differentiation by a non-hypercalcemic analog of vitamin D3, 1,25(OH)2-5,6 trans-16-ene-vitamin D3, involves nuclear vitamin D receptor (nVDR) and non-nVDR-mediated pathways

Exposure of leukemia cells to the physiologically active form of vitamin D3, 1,25-dihydroxyvitamin D3 (1,25D3) normalizes their phenotype to cells that resemble mature monocytes. One of the earliest detectable events in this process is an upregulation of the nuclear receptor for 1,25D3, the vitamin D receptor (nVDR). In contrast, the novel analog of 1,25D3, 1,25-dihydroxy-5,6 trans-16-ene-vitamin D3 (5,6-16D3), which has recently been reported to have low calcium-mobilizing activity in vivo, rapidly induced the expression of CD14, CD11b, and monocyte-specific esterase (MSE), classical markers of the mature monocyte, but upregulated nVDR expression less than 1,25D3. This upregulation was shown to be the result of altered degradation of the nVDR protein, while the levels of nVDR mRNA were constant. Knock-out of nVDR transcriptional activity by a decoy VDRE double-stranded deoxyoligonucleotide, markedly abrogated 1,25D3-induced differentiation, but incompletely inhibited 5,6-16D3-induced differentiation. These findings suggest that the unique ability of 5,6-16D3 to induce cell differentiation but not systemic hypercalcemia, may be due to the activation of pathways which initiate differentiation independently of nVDR.

Exposure of KS483 cells to estrogen enhances osteogenesis and inhibits adipogenesis

Osteoblasts and adipocytes arise from a common progenitor cell in bone marrow. Whether estrogen directly regulates the progenitor cells differentiating into osteoblasts or adipocytes remains unknown. Using a mouse clonal cell line KS483 cultured in charcoal-stripped fetal bovine serum (FBS), we showed that 17beta-estradiol (E2) stimulates the differentiation of progenitor cells into osteoblasts and concurrently inhibits adipocyte formation in an estrogen receptor (ER)-dependent way. E2 increased alkaline phosphate (ALP) activity and nodule formation and stimulated messenger RNA (mRNA) expression of core-binding factor alpha-1 (Cbfa1), parathyroid hormone/parathyroid hormone-related protein receptors (PTH/PTHrP-Rs), and osteocalcin. In contrast, E2 decreased adipocyte numbers and down-regulated mRNA expression of peroxisome proliferator-activated receptor-gamma (PPARgamma)2, adipocyte protein 2 (aP2), and lipoprotein lipase (LPL). Furthermore, the reciprocal control of osteoblast and adipocyte differentiation by E2 was observed also in the presence of the adipogenic mixture of isobutylmethylxanthine, dexamethasone, and insulin. Immunohistochemical staining showed that ERalpha and ERbeta were present in osteoblasts and adipocytes. A new mouse splice variant ERbeta2 was identified, which differed in two amino acid residues from the rat isoform. E2 down-regulated mRNA expression of ERalpha, ERbeta1, and ERbeta2. The effects of E2 are not restricted to the KS483 cell line because similar results were obtained in mouse bone marrow cell cultures. Our results indicate that estrogen, in addition to stimulation of osteogenesis, inhibits adipogenesis, which might explain the clinical observations that estrogen-deficiency leads to an increase in adipocytes.

Cyclin D1: mechanism and consequence of androgen receptor co-repressor activity

Androgen receptor regulation is pivotal for prostate growth and development. Activation of the receptor is dictated by association with androgen (ligand) and through interaction with co-activators and co-repressors. We have shown previously that cyclin D1 functions as a co-repressor to inhibit ligand-dependent androgen receptor activation. We demonstrate that cyclin D1 directly binds the N terminus of the androgen receptor and that this interaction is independent of ligand. Furthermore, we show that the interaction occurs in the nucleus and does not require the LXXLL motif of cyclin D1. Although two distinct transactivation domains exist in the N terminus (AF-1 and AF-5), the data shown support the hypothesis that cyclin D1 targets the AF-1 transactivation function. The constitutively active AF-5 domain was refractory to cyclin D1 inhibition. By contrast, cyclin D1 completely abolished androgen receptor activity, even in the presence of potent androgen receptor co-activators. This action of cyclin D1 at least partially required de-acetylase activity. Finally, we show that transient, ectopic expression of cyclin D1 results in reduced cell cycle progression in androgen-dependent LNCaP cells independent of CDK4 association. Collectively, our data support a model wherein cyclin D1 has a mitogenic (CDK4-dependent) function and an anti-mitogenic function (dependent on regulation of the AF-1 domain) that can collectively control the rate of androgen-dependent cellular proliferation. These findings provide insight into the non-cell cycle functions of cyclin D1 and provide the impetus to study its pleiotropic effects in androgen-dependent cells, especially prostatic adenocarcinomas.

Metabolic conversion as a pre-receptor control mechanism for lipophilic hormones

The majority of physiological effects mediated by steroids, retinoids and thyroids is accomplished by binding to members of the nuclear receptor superfamily of ligand activated transcription factors. The complex specific effects of lipid hormones depend not only on receptor expression, distribution and interactions, but also on the availability and metabolic conversion of the hormone itself. The cell-specific metabolic activation of inactive hormone precursors introduces a further level of hormonal regulation, and constitutes an important concept in endocrinology. The metabolic reactions carried out are achieved by dehydrogenases/reductases, hydroxylases and other enzymes, acting on ligands of the steroid/thyroid/retinoic hormone receptor superfamily. The concept implies that these tissue- and cell-specific metabolic conversions contribute to lipid hormone action, thus pointing to novel targets in drug development. All components of this signalling system, the hormone compounds, the receptor proteins, and modifying enzyme families originate from an early metazoan date, emphasizing the essential nature of all elements for development and diversification of vertebrate life.

T3-activation of the rat growth hormone gene is inhibited by a zinc finger/homeodomain protein

Since the transcription factor Zfhep is expressed in somatotropes and binds the rat growth hormone (rGH) gene T3-response element (TRE), we investigated whether Zfhep regulates the response of this gene to T3. In cotransfection experiments, Zfhep did not regulate the native rGH promoter in the absence of T3. However, Zfhep repressed T3-mediated activation significantly in either GH(3) or JEG-3 cells. Up to 70% repression was mediated through the rGH TRE in a heterologous promoter (thymidine kinase), but was not observed with the idealized DR4 or chicken lysozyme F2 TREs. Zfhep apparently does not repress T3-mediated activation simply by competition for binding to DNA since the C-terminal DNA-binding domain of Zfhep (which is sufficient for DNA-binding) is not sufficient for repression and since cotransfection of excess thyroid hormone receptor (TR) did not prevent repression by Zfhep. These data indicate that the rGH TRE is a composite element that can integrate Zfhep and T3 regulation.

Preventing estrogen receptor action with dimer-interface peptides

The human estrogen receptor-alpha (hER) is a ligand-activated transcription factor that functions as a homodimer. We sought to further understand the molecular processes involved in dimerization, and to develop a reagent that may function as an antiestrogen independent of the ligand binding site. To this end, we designed a 16-residue 'dimer-interface' oligopeptide derived from the helical region of the hER which is directly involved in dimerization. This peptide, termed the I-box peptide, has a high helical propensity in aqueous solution. The I-box peptide blocks hER action by causing aggregation and precipitation of both the ligand-bound and apo-hER. This effect is dependent on the helical nature of the peptide. A single Ile to Pro mutation in the helical region of the I-box peptide significantly reduces the helical content and abolishes the precipitation activity. Furthermore, the peptide activity appears to be specific for the hER. The I-box peptide does not significantly affect other proteins or steroid receptors tested. A homologous peptide derived from the nuclear receptor RXRalpha dimer interface, and a LXXLL-containing peptide from the coactivator TIF2 have no detectable in vitro effect on hER function or solubility. Our data suggest that rationally designed molecules capable of affecting steroid receptor quaternary structures may be potential avenues for the development of specific inhibitors of this class of proteins.

Modulation of gene expression by androgen and oestrogens in the testis and prostate of the adult rat following androgen withdrawal

Androgens are important for the structural and functional integrity of the testis and the prostate and this may in part be mediated by the aromatisation of testosterone to oestradiol. The aim of the present study was to establish an in vivo model that would allow the identification of genes, the expression of which was regulated acutely by androgen and/or oestrogen in the male reproductive system. In rats in which the Leydig cells were ablated by administration of ethane dimethane sulfonate (EDS) 6 days earlier, testosterone esters (T) were administered from day 0 (To), and additional animals were administered either T, 17beta-oestradiol benzoate (EB) or diethylstilbestrol (DES) for 1 or 4 h on day 6 after EDS-treatment. Nuclear immunoexpression of the androgen receptor (AR) was reduced or absent from the testis but unaffected in the ventral prostate following these treatments. ERbeta immunoexpression in these tissues was unchanged. Northern blot analysis showed that EB and DES as well as T administration 4 h earlier could modulate mRNA expression of two androgen-responsive genes, C3 and SGP-2, in the prostate. The co-administration of T or EB with the AR antagonist, flutamide, or with the ER antagonist, ICI 182,780 (ICI), did not block the suppression of SGP-2 mRNA expression by T or EB. In contrast, the upregulation of C3 mRNA expression by T was successfully antagonised by both flutamide and by ICI. A preliminary evaluation of the expression of three Sertoli cell and five germ cell mRNAs revealed that their expression was not steroid regulated. Our results support the hypothesis that the action of testosterone in the male reproductive system may in part be mediated by its conversion to oestradiol. This in vivo model should prove of value in future studies to identify androgen and oestrogen regulated genes in the male reproductive system.

Peroxisome proliferator activated receptor agonists

The peroxisome proliferator activated receptors (PPARs) represent a group of ligand-activated transcription factors that mediate the biological effects of various drugs, such as fibrates and thiazolidinediones. Three PPAR subtypes can be distinguished, alpha, beta and gamma, each of which has an unique pattern of expression among vertebrate tissues. The PPAR alpha receptor is activated by hypolipidemic drugs of the fibrate class, and regulates the expression of numerous genes involved in fatty acid catabolism. The PPAR gamma receptor is activated by hypoglycaemic drugs of the thiazolidinedione class, and is an important determinant of adipocyte differentiation. Little is currently known about PPAR beta. A heavy research effort is currently directed towards the identification of novel high-affinity, high-specificity agonists and antagonists that may be used in the treatment of hyperglycaemia, hyperlipidemia, and other diseases of metabolic origin.

Sex steroids and bone

Sex steroids are essential for skeletal development and the maintenance of bone health throughout adult life, and estrogen deficiency at menopause is a major pathogenetic factor in the development of osteoporosis in postmenopausal women. The mechanisms by which the skeletal effects of sex steroids are mediated remain incompletely understood, but in recent years there have been considerable advances in our knowledge of how estrogens and, to a lesser extent androgens, influence bone modeling and remodeling in health and disease. New insights into estrogen receptor structure and function, recent discoveries about the development and activity of osteoclasts, and lessons learned from human and animal genetic mutations have all contributed to increased understanding of the skeletal effects of estrogen, both in males and females. Studies of untreated and treated osteoporosis in postmenopausal women have also contributed to this knowledge and have provided unequivocal evidence for the potential of high-dose estrogen therapy to have anabolic skeletal effects. The development of selective estrogen receptor modulators has provided a new approach to the prevention of osteoporosis and other major diseases of menopause and has implications for the therapeutic use of other steroid hormones, including androgens. Further elucidation of the mechanisms by which sex steroids affect bone thus has the potential to improve the clinical management not only of osteoporosis, both in men and women, but also of a number of other diseases related to sex hormone status.

Phenotypic diversity and testosterone-induced normalization of mutant L712F androgen receptor function in a kindred with androgen insensitivity

Molecular causes of phenotypic diversity in androgen insensitivity syndrome, occurring even in the same family, have rarely been identified. We report on a family with four affected individuals, three brothers (B1-3) and their uncle, displaying strikingly different external genitalia: B1, ambiguous; B2, severe micropenis; B3, slight micropenis; and uncle, micropenis and penoscrotal hypospadias. All had been assigned a male gender. We detected the same L712F mutation of the androgen receptor (AR) gene in each subject. Methyltrienolone binding on cultured genital skin fibroblasts of B2 suggested moderate impairment of the ligand-binding domain [maximal binding capacity, 38.2 fmol/mg protein (normal); Kd, 0.21 nmol/L; normal range, 0.03-0.13 nmol/L]. In trans-activation assays, the mutant 712F-AR showed considerable deficiency at low concentrations of testosterone (0.01-0.1 nmol/L) or dihydrotestosterone (0.01 nmol/L). Remarkably, this could be fully neutralized by testosterone concentrations greater than 1.0 nmol/L. Hence, the 712F-AR could switch its function from subnormal to normal within the physiological concentration range of testosterone. This was reflected by an excellent response to testosterone therapy in B1, B2, and the uncle. Taking into account the well documented individual and time-dependent variation in testosterone concentration in early fetal development, our observations clearly illustrate the potential impact of varying ligand concentrations for distinct cases of phenotypic variability in androgen insensitivity syndrome.

Expression of estrogen receptor coactivators in the rat uterus

Nuclear receptor coactivators associate in a ligand-dependent manner with estrogen receptors (ER) and other nuclear receptors, and they enhance ligand-dependent transcriptional activation. This study examined basal coactivator expression in rat uterus to investigate if expression of these genes is regulated by estradiol-17 beta or tamoxifen. Ovariectomized mature and immature rats were injected with estradiol-17 beta, tamoxifen, or vehicle (i.e., sesame oil) alone. Uteri were collected and analyzed for changes in coactivator mRNA expression using Northern blot and in situ hybridization analyses. Constitutive uterine mRNA expression of switch protein for antagonist (SPA), SRC-1, GRIP1, RAC3, RIP140, and p300 mRNAs was observed in control uteri, and treatment with ER ligands did not alter coactivator mRNA levels. The data suggest that expression of these coactivator genes is not sensitive to estradiol or tamoxifen in the rat uterus. No cell type-specific pattern of expression was apparent in uterine sections from mature and immature rats; however, silver grains were more abundant in luminal and glandular epithelial cells compared with the stroma and myometrium, indicating that coactivator mRNA levels vary among the uterine compartments. Thus, to our knowledge, we show for the first time that there is constitutive expression of several uterine nuclear receptor coactivators in a physiological setting that remains insensitive to estrogenic regulation. Furthermore, we speculate that higher constitutive levels of coactivator expression in glandular and luminal epithelial cells may be associated with increased hormonal responsiveness by these uterine compartments.