Cloning of the human glucocorticoid receptor cDNA

Retinoic acid receptors at 35 years

For almost a century, vitamin A has been known as a nutrient critical for normal development, differentiation, and homeostasis; accordingly, there has been much interest in understanding its mechanism of action. This review is about the discovery of specific receptors for the vitamin A derivative, retinoic acid (RA), which launched extensive molecular, genetic, and structural investigations into these new members of the nuclear receptor superfamily of transcriptional regulators. These included two families of receptors, the RAR isotypes (α, β, and γ) along with three RXR isotypes (α, β, and γ), which bind as RXR/RAR heterodimers to cis-acting response elements of RA target genes to generate a high degree of complexity. Such studies have provided deep molecular insight into how the widespread pleiotropic effects of RA can be generated.

Glucocorticoid Receptor: A Multifaceted Actor in Breast Cancer

Breast cancer (BC) is one of the most common cancers in women worldwide. Even though the role of estrogen receptor alpha (ERα) is extensively documented in the development of breast tumors, other members of the nuclear receptor family have emerged as important players. Synthetic glucocorticoids (GCs) such as dexamethasone (dex) are commonly used in BC for their antiemetic, anti-inflammatory, as well as energy and appetite stimulating properties, and to manage the side effects of chemotherapy. However, dex triggers different effects depending on the BC subtype. The glucocorticoid receptor (GR) is also an important marker in BC, as high GR expression is correlated with a poor and good prognosis in ERα-negative and ERα-positive BCs, respectively. Indeed, though it drives the expression of pro-tumorigenic genes in ERα-negative BCs and is involved in resistance to chemotherapy and metastasis formation, dex inhibits estrogen-mediated cell proliferation in ERα-positive BCs. Recently, a new natural ligand for GR called OCDO was identified. OCDO is a cholesterol metabolite with oncogenic properties, triggering mammary cell proliferation in vitro and in vivo. In this review, we summarize recent data on GR signaling and its involvement in tumoral breast tissue, via its different ligands.

The nuclear receptor superfamily: A structural perspective

Nuclear receptors (NRs) are a family of transcription factors that regulate numerous physiological processes such as metabolism, reproduction, inflammation, as well as the circadian rhythm. NRs sense changes in lipid metabolite levels to drive differential gene expression, producing distinct physiologic effects. This is an allosteric process whereby binding a cognate ligand and specific DNA sequences drives the recruitment of diverse transcriptional co-regulators at chromatin and ultimately transactivation or transrepression of target genes. Dysregulation of NR signaling leads to various malignances, metabolic disorders, and inflammatory disease. Given their important role in physiology and ability to respond to small lipophilic ligands, NRs have emerged as valuable therapeutic targets. Here, we summarize and discuss the recent progress on understanding the complex mechanism of action of NRs, primarily from a structural perspective. Finally, we suggest future studies to improve our understanding of NR signaling and better design drugs by integrating multiple structural and biophysical approaches.

Antiproliferative effect of dexamethasone in the MCF-7 breast cancer cell line

Glucocorticoids (GCs) are used in the treatment of cancer to induce programmed cell death in the transformed cells of the hematopoietic system and to reduce side effects. Additionally, GCs are described as an inhibitor of certain chemotherapy or radiation-induced apoptosis and also an inhibitor of cancer progression by downregulating or upregulating the expression of several genes. The present study used immunofluorescence to investigate the presence of the glucocorticoid receptor (GR) in MCF-7 cells, and the cell culture growth was determined by cell counting the number of cells following exposure to GC and/or dexamethasone (Dex). The presence and immunoreactivity of the GR were confirmed, and treatment with Dex (10⁻⁸–10⁻⁷ M) caused an inhibitory effect (30–35%) on the proliferative activity of the MCF-7 cells. This growth inhibitory effect was possibly produced by the pro-apopotic effect of Dex. Since Dex is administered systematically prior to breast cancer chemotherapy, the possible interactions between these drugs require further investigation.

Clinical and biological significance of glucocorticoid receptor (GR) expression in breast cancer

The glucocorticoid receptor (GR) is a member of the nuclear receptor superfamily of transcription factors, which exerts anti-proliferative and anti-apoptotic activities. The GR is expressed in a large proportion of breast cancer (BC) although levels generally decrease during cancer progression. This study aimed to determine the clinical and biological significance of GR expression using a large series of early-stage BC with long-term follow-up and BC cell lines. Immunohistochemistry was used to assess the expression of GR in 999 cases of primary invasive BC prepared as tissue microarrays. Reverse phase protein microarray was used to assess the expression of GR in MCF7 and MDA-MB-231 cell lines. Nuclear expression of GR was observed in 61.6 % of breast tumours and was associated with features of good prognosis including smaller tumour size and lower grade with less pleomorphism and low mitotic count. GR expression was positively correlated with expression of oestrogen (ER) and progesterone receptors. In ER-positive tumours, GR was associated with other features of favourable outcome including FOXA1, GATA3 and BEX1 expression, while low GR expression was associated with high Ki67, p53 and CD71 expression. GR expression is associated with features of good outcome but does not provide prognostic information independent of size, stage and grade. Understanding the receptor and its effects on BC behaviour is essential for avoiding any unwanted effects from the use of glucocorticoids in routine oncology practice.

RXRs: collegial partners

Retinoid X Receptors (RXR) were initially identified as nuclear receptors binding with stereo-selectivity the vitamin A derivative 9-cis retinoic acid, although the relevance of this molecule as endogenous activator of RXRs is still elusive. Importantly, within the nuclear receptor superfamily, RXRs occupy a peculiar place, as they are obligatory partners for a number of other nuclear receptors, thus integrating the corresponding signaling pathways. In this chapter, we describe the structural features allowing RXR to form homo- and heterodimers, and the functional consequences of this unique ability. Furthermore, we discuss the importance of studying RXR activity at a genome-wide level in order to comprehensively address the biological implications of their action that is fundamental to understand to what extent RXRs could be exploited as new therapeutic targets.

History of retinoic acid receptors

The discovery of retinoic acid receptors arose from research into how vitamins are essential for life. Early studies indicated that Vitamin A was metabolized into an active factor, retinoic acid (RA), which regulates RNA and protein expression in cells. Each step forward in our understanding of retinoic acid in human health was accomplished by the development and application of new technologies. Development cDNA cloning techniques and discovery of nuclear receptors for steroid hormones provided the basis for identification of two classes of retinoic acid receptors, RARs and RXRs, each of which has three isoforms, α, β and ɣ. DNA manipulation and crystallographic studies revealed that the receptors contain discrete functional domains responsible for binding to DNA, ligands and cofactors. Ligand binding was shown to induce conformational changes in the receptors that cause release of corepressors and recruitment of coactivators to create functional complexes that are bound to consensus promoter DNA sequences called retinoic acid response elements (RAREs) and that cause opening of chromatin and transcription of adjacent genes. Homologous recombination technology allowed the development of mice lacking expression of retinoic acid receptors, individually or in various combinations, which demonstrated that the receptors exhibit vital, but redundant, functions in fetal development and in vision, reproduction, and other functions required for maintenance of adult life. More recent advancements in sequencing and proteomic technologies reveal the complexity of retinoic acid receptor involvement in cellular function through regulation of gene expression and kinase activity. Future directions will require systems biology approaches to decipher how these integrated networks affect human stem cells, health, and disease.

Glucocorticoid receptor and breast cancer

Stress enhances glucocorticoid (GC) synthesis, which alters inflammation and immune responses, as well as cellular proliferation and apoptosis in a number of tissues. Increasingly, stress has been associated with cancer progression, and in particular in breast cancer. Consequently, an operational glucocorticoid receptor system in breast tissue influences breast cancer development. In this review, we summarize the data on the GC/GR system in normal and tumoral breast tissue. We also review the molecular mechanisms by which GCs control apoptosis and proliferation in breast cancer models and how GCs alter the chemotherapy of breast cancer treatment when used in combination. Finally, we discuss the participation of GR in breast tumorigenesis under hormone replacement therapy.

Nuclear hormone receptors in parasitic helminths

Nuclear receptors (NRs) belong to a large protein superfamily that are important transcriptional modulators in metazoans. Parasitic helminths include parasitic worms from the Lophotrochozoa (Platyhelminths) and Ecdysozoa (Nematoda). NRs in parasitic helminths diverged into two different evolutionary lineages. NRs in parasitic Platyhelminths have orthologues in Deuterostomes, in arthropods or both with a feature of extensive gene loss and gene duplication within different gene groups. NRs in parasitic Nematoda follow the nematode evolutionary lineage with a feature of multiple duplication of SupNRs and gene loss.

Liver X receptors as regulators of macrophage inflammatory and metabolic pathways

The liver X receptors (LXRα and LXRβ) are members of the nuclear receptor family of transcription factors that play essential roles in the transcriptional control of lipid metabolism. LXRs are endogenously activated by modified forms of cholesterol known as oxysterols and control the expression of genes important for cholesterol uptake, efflux, transport, and excretion in multiple tissues. In addition to their role as cholesterol sensors, a number of studies have implicated LXRs in the modulation of innate and adaptive immune responses. Both through activation and repression mechanisms, LXRs regulate diverse aspects of inflammatory gene expression in macrophages. The ability of LXRs to coordinate metabolic and immune responses constitutes an attractive therapeutic target for the treatment of chronic inflammatory disorders. This article is part of a Special Issue entitled: Translating nuclear receptors from health to disease.

Mechanisms regulating the susceptibility of hematopoietic malignancies to glucocorticoid-induced apoptosis

Glucocorticoids (GCs) are commonly used in the treatment of hematopoietic malignancies owing to their ability to induce apoptosis of these cancerous cells. Whereas some types of lymphoma and leukemia respond well to this drug, others are resistant. Also, GC-resistance gradually develops upon repeated treatments ultimately leading to refractory relapsed disease. Understanding the mechanisms regulating GC-induced apoptosis is therefore uttermost important for designing novel treatment strategies that overcome GC-resistance. This review discusses updated data describing the complex regulation of the cell's susceptibility to apoptosis triggered by GCs. We address both the genomic and nongenomic effects involved in promoting the apoptotic signals as well as the resistance mechanisms opposing these signals. Eventually we address potential strategies of clinical relevance that sensitize GC-resistant lymphoma and leukemia cells to this drug. The major target is the nongenomic signal transduction machinery where the interplay between protein kinases determines the cell fate. Shifting the balance of the kinome towards a state where Glycogen synthase kinase 3alpha (GSK3alpha) is kept active, favors an apoptotic response. Accumulating data show that it is possible to therapeutically modulate GC-resistance in patients, thereby improving the response to GC therapy.

Dienogest is a selective progesterone receptor agonist in transactivation analysis with potent oral endometrial activity due to its efficient pharmacokinetic profile

Dienogest was introduced as an oral progestin. Yet its strong oral potency on endometrial activity is not clearly explained. To circumvent this situation, steroid hormone receptor profiling using transactivation assay and endometrial activity test in rabbits were carried out with determination of plasma drug concentration. Agonistic/antagonistic activity on human progesterone receptor (PR), androgen receptor (AR), glucocorticoid receptor (GR), mineralocorticoid receptor (MR), estrogen receptor alpha (ERalpha), or estrogen receptor beta (ERbeta) were determined. Dienogest activate PR (EC50=3.4 or 10.5 nmol/l) with antagonistic activity on AR (EC50=420.6 or 775.0 nmol/l) but not agonistic nor antagonistic action on GR, MR (3000 nmol/l). Dienogest activate neither ERalpha nor ERbeta (3000 nmol/l). Progesterone activated PR with antagonistic activity on AR and on MR. Dydrogesterone showed a similar profile to progesterone. Norethisterone activated PR, AR, and ERalpha. Medroxyprogesterone acetate activated PR, AR, and GR. Danazol activated PR and AR. Collectively, dienogest has a good specificity to PR compared with the other drugs. By oral treatment, dienogest showed the strongest endometrial activity (ED50=0.0042 mg/kg) in McPhail test among other progestins (ED50 values for MPA, DYG, NES were 0.074, 1.9, >0.05 mg/kg, respectively). Dienogest showed higher plasma concentrations than those of the other progestins with higher doses. The estimated plasma concentration of dienogest at ED50 (3.66 nmol/l) was close to its EC50 value to activate PR. Thus, the stronger oral activity of dienogest could be explained simply by its in vitro potency on PR and its oral pharmacokinetic profile.

Retinoid X receptors: X-ploring their (patho)physiological functions

Retinoid X receptor (RXR) belongs to a family of ligand-activated transcription factors that regulate many aspects of metazoan life. A class of nuclear receptors requires RXR as heterodimerization partner for their function. This places RXR in the crossroad of multiple distinct biological pathways. This and the fact that the debate on the endogenous ligand requirement for RXR is not yet settled make RXR still an enigmatic transcription factor. Here, we review some of the biology of RXR. We place RXR into the evolution of nuclear receptors, review structural details and ligands of the receptor. Then processes regulated by RXR are discussed focusing on the developmental roles deduced from studies on knockout animals and metabolic roles in diseases such as diabetes and atherosclerosis deduced from pharmacological studies. Finally, aspects of RXR's involvement in myeloid differentiation and apoptosis are summarized along with issues on RXR's suitability as a therapeutic target.

Regulation of RNA Polymerase II Transcription by Sequence-Specific DNA Binding Factors

In eukaryotes, transcription of the diverse array of tens of thousands of protein-coding genes is carried out by RNA polymerase II. The control of this process is predominantly mediated by a network of thousands of sequence-specific DNA binding transcription factors that interpret the genetic regulatory information, such as in transcriptional enhancers and promoters, and transmit the appropriate response to the RNA polymerase II transcriptional machinery. This review will describe some early advances in the discovery and characterization of the sequence-specific DNA binding transcription factors as well as some of the properties of these regulatory proteins.

Glucocorticoid and thyroid hormone receptors in mitochondria of animal cells

This article concerns the localization of glucocorticoid and thyroid hormone receptors in mitochondria of animal cells. The receptors are discussed in terms of their potential role in the regulation of mitochondrial transcription and energy production by the oxidative phosphorylation pathway, realized both by nuclear-encoded and mitochondrially encoded enzymes. A brief survey of the role of glucocorticoid and thyroid hormones on energy metabolism is presented, followed by a description of the molecular mode of action of these hormones and of the central role of the receptors in regulation of transcription. Subsequently, the structure and characteristics of glucocorticoid and thyroid hormone receptors are described, followed by a section on the effects of glucocorticoid and thyroid hormones on the transcription of mitochondrial and nuclear genes encoding subunits of OXPHOS and by an introduction to the mitochondrial genome and its transcription. A comprehensive description of the data demonstrates the localization of glucocorticoid and thyroid hormone receptors in mitochondria as well as the detection of potential hormone response elements that bind to these receptors. This leads to the conclusion that the receptors potentially play a role in the regulation of transcription of mitochondrial genes. The in organello mitochondrial system, which is capable of sustaining transcription in the absence of nuclear participation, is presented, responding to T3 with increased transcription rates, and the central role of a thyroid receptor isoform in the transcription effect is emphasized. Lastly, possible ways of coordinating nuclear and mitochondrial gene transcription in response to glucocorticoid and thyroid hormones are discussed, the hormones acting directly on the genes of the two compartments by way of common hormone response elements and indirectly on mitochondrial genes by stimulation of nuclear-encoded transcription factors.

Genomics versus orphan nuclear receptors--a half-time report

Following the successful cloning of the orphan nuclear receptors during the 1990s we entered the 21st century with knowledge of the full complement of human nuclear receptors. Many of these proteins are ligand-activated transcription factors that act as the cognate receptors for steroid, retinoid, and thyroid hormones. In addition to these well characterized endocrine hormone receptors, there are a large number of orphan receptors of which less is known about the nature and function of their ligands. The task of deciphering the physiological function of these orphan receptors has been aided by a new generation of genomic technologies. Through application of chemical, structural, and functional genomics, several orphan nuclear receptors have emerged as pharmaceutical drug targets for the treatment of important human diseases. The significant progress that has been made in the functional analysis of more than half of the nuclear receptor gene family provides an opportunity to review the impact of genomics in this endeavor.

Expression and regulation of nuclear receptor coactivators in glucocorticoid action

Nuclear receptor coactivators are involved in receptor-mediated transcriptional activation of target genes in a hormone-sensitive manner, and the mechanism of their transactivation has been studied in recent years. The glucocorticoid receptor (GR) interacts with several coactivators, including steroid receptor coactivator-1 (SRC-1) family and CREB-binding protein (CBP). Since coactivators function as transcription amplifiers, subtle changes in expression levels of coactivators in certain cells would markedly intensify receptor-mediated transcriptional activity. The regulation of coactivators by glucocorticoid action, however, has not yet been clarified. In this study, we have shown that one of the coactivators interacting with GR, SRC-1, is downregulated by dexamethasone (DEX) both in vivo and in vitro. In experiments on Sprague-Dawley rats in vivo, the downregulation of SRC-1 was observed in heart, stomach, kidney, liver, and cerebrum, and in experiments on two types of kidney-derived cells in vitro, similar downregulation of SRC-1 was demonstrated in both types of cells. DEX-mediated downregulation of SRC-1 mRNA recovered in 4-8 h, while the downregulation of SRC-1 protein lasted for 12 h and its levels returned to the basal level, 24 h after DEX treatment. Other coactivators examined in this study showed no remarkable changes in expression by DEX treatment, implying that ligand-mediated downregulation of SRC-1 has a pivotal role in the physiology of glucocorticoid action.

Nuclear cytoplasmic shuttling by thyroid hormone receptors. multiple protein interactions are required for nuclear retention

In this report, we have studied the intracellular dynamics and distribution of the thyroid hormone receptor-beta (TRbeta) in living cells, utilizing fusions to the green fluorescent protein. Wild-type TRbeta was mostly nuclear in both the absence and presence of triiodothyronine; however, triiodothyronine induced a nuclear reorganization of TRbeta. By mutating defined regions of TRbeta, we found that both nuclear corepressor and retinoid X receptor are involved in maintaining the unliganded receptor within the nucleus. A TRbeta mutant defective in DNA binding had only a slightly altered nuclear/cytoplasmic distribution compared with wild-type TRbeta; thus, site-specific DNA binding is not essential for maintaining TRbeta within the nucleus. Both ATP depletion studies and heterokaryon analysis demonstrated that TRbeta rapidly shuttles between the nuclear and the cytoplasmic compartments. Cotransfection of nuclear corepressor and retinoid X receptor markedly decreased the shuttling by maintaining unliganded TRbeta within the nucleus. In summary, our findings demonstrate that TRbeta rapidly shuttles between the nucleus and the cytoplasm and that protein-protein interactions of TRbeta with various cofactors, rather than specific DNA interactions, play the predominant role in determining the intracellular distribution of the receptor.

Novel mechanism of steroid action in skin through glucocorticoid receptor monomers

Glucocorticoids (GCs), important regulators of epidermal growth, differentiation, and homeostasis, are used extensively in the treatment of skin diseases. Using keratin gene expression as a paradigm of epidermal physiology and pathology, we have developed a model system to study the molecular mechanism of GCs action in skin. Here we describe a novel mechanism of suppression of transcription by the glucocorticoid receptor (GR) that represents an example of customizing a device for transcriptional regulation to target a specific group of genes within the target tissue, in our case, epidermis. We have shown that GCs repress the expression of the basal-cell-specific keratins K5 and K14 and disease-associated keratins K6, K16, and K17 but not the differentiation-specific keratins K3 and K10 or the simple epithelium-specific keratins K8, K18, and K19. We have identified the negative recognition elements (nGREs) in all five regulated keratin gene promoters. Detailed footprinting revealed that the function of nGREs is to instruct the GR to bind as four monomers. Furthermore, using cotransfection and antisense technology we have found that, unlike SRC-1 and GRIP-1, which are not involved in the GR complex that suppresses keratin genes, histone acetyltransferase and CBP are. In addition, we have found that GR, independently from GREs, blocks the induction of keratin gene expression by AP1. We conclude that GR suppresses keratin gene expression through two independent mechanisms: directly, through interactions of keratin nGREs with four GR monomers, as well as indirectly, by blocking the AP1 induction of keratin gene expression.

Imidazoline receptor antisera-selected (IRAS) cDNA: cloning and characterization

The imidazoline-1 receptor (IR1) is considered a novel target for drug discovery. Toward cloning an IR1, a truncated cDNA clone was isolated from a human hippocampal lambda gt11 cDNA expression library by relying on the selectivity of two antisera directed against candidate IR proteins. Amplification reactions were performed to extend the 5' and 3' ends of this cDNA, followed by end-to-end PCR and conventional cloning. The resultant 5131-basepair molecule, designated imidazoline receptor-antisera-selected (IRAS) cDNA, was shown to encode a 1504-amino acid protein (IRAS-1). No relation exists between the amino acid sequence of IRAS-1 and proteins known to bind imidazolines (e.g., it is not an alpha2-adrenoceptor or monoamine oxidase subtype). However, certain sequences within IRAS-1 are consistent with signaling motifs found in cytokine receptors, as previously suggested for an IR1. An acidic region in IRAS-1 having an amino acid sequence nearly identical to that of ryanodine receptors led to the demonstration that ruthenium red, a dye that binds the acidic region in ryanodine receptors, also stained IRAS-1 as a 167-kD band on SDS gels and inhibited radioligand binding of native I1 sites in untransfected PC-12 cells (a source of authentic I1 binding sites). Two epitope-selective antisera were also generated against IRAS-1, and both reacted with the same 167-kD band on Western blots. In a host-cell-specific manner, transfection of IRAS cDNA into Chinese hamster ovary cells led to high-affinity I1 binding sites by criteria of nanomolar affinity for moxonidine and rilmenidine. Thus, IRAS-1 is the first protein discovered with characteristics of an IR1.

Hormone receptors in vestibular schwannomas

The possibility that steroid hormones play a role in vestibular schwannoma proliferation has been suggested by a number of investigators. There is conflicting information about the presence of steroid hormone receptors in these tumors. The aim of this study was to examine the expression of androgen, progesterone, glucocorticoid and estrogen receptor messenger ribonucleic acid levels (mRNA) in twenty-one vestibular schwannomas by either Northern blot analysis or the polymerase chain reaction (PCR). Glucocorticoid receptor mRNA was expressed in all twenty-one tumors examined. Only two male specimens were positive for androgen receptor mRNA expression by PCR-Southern blot analysis. Thirty-three percent of the schwannomas (7/21) showed a strong band for progesterone receptor mRNA by PCR-Southern blot analysis; there were an equal number of males and females in this group. Estrogen receptor mRNA levels were undetectable in all tumors examined by PCR-Southern blot analysis. These studies suggest that the pattern of steroid receptor expression is different in schwannomas than in meningiomas. Individual vestibular schwannomas need to be examined for their steroid receptor mRNA expression mRNA expression to know whether they will be responsive.

Effects of intermittent androgen suppression on the stem cell composition and the expression of the TRPM-2 (clusterin) gene in the Shionogi carcinoma

The proportion of tumorigenic stem cells and the expression of the apoptosis-related gene, TRPM-2 (clusterin), were studied in populations of Shionogi carcinoma cells subjected to multiple cycles of androgen withdrawal and replacement (intermittent androgen suppression). The parent androgen-dependent cell line was initially transplanted into a male mouse which was castrated when the estimated weight of the resultant tumour became approximately 3 g. After the tumour had regressed to 40% or less of the original weight, it was transplanted into the next non-castrated male. This was repeated for four cycles of transplantation and castration-induced apoptosis before the tumour progressed to an androgen-independent state. The proportion of total stem cells in the tumour, as determined by in vivo limiting dilution assays in male mice, was constant during the first three cycles but increased 15-fold between the third and fourth cycles. In the parent androgen-dependent tumour before androgen ablation, the androgen-independent stem cell population formed 0.8% of the total stem cell compartment. After the fourth cycle this population increased to 47%; a population of similar size (33%, P = 0.8) was found in the androgen-independent recurrent form of the tumour induced by one-time castration. Whether androgen withdrawal therapy was intermittent or continuous, conversion to androgen independence thus occurred when one-third to one-half of the total stem cell compartment was populated by androgen-independent stem cells. The androgen-repressed TRPM-2 (clusterin) gene was actively expressed in regressing tumours after androgen ablation, and also became constitutively expressed in non-regressing tumours after the first and subsequent cycles of androgen withdrawal. Staining of cytoplasm and nuclei with anti-clusterin antibody was observed in androgen-dependent tumour cells after each cycle of intermittent androgen suppression; the nuclear staining was more intense in recurrent androgen-independent cells. The anomalous nuclear localization of clusterin, an anti-cytolytic TRPM-2 encoded protein, may serve to inhibit early events in the apoptotic process and thereby foster the generation and outgrowth of androgen-independent stem cells in an androgen-depleted environment.

Steroid hormone receptors in astrocytic neoplasms

The presence of specific steroid hormone-binding receptors has been correlated with the clinical response to hormonal therapy in a number of different neoplasias, including breast and prostate cancer. In this article, we investigated the expression of the androgen, estrogen, glucocorticoid, and progesterone receptor messenger ribonucleic acid (mRNA) and protein in a number of astrocytic neoplasms of various histological grades. Androgen and glucocorticoid receptor mRNA were detected in all astrocytic neoplasms examined, regardless of histological subtype. In contrast, progesterone receptor mRNA was observed more frequently in high-grade tumors than in low-grade tumors. Estrogen receptor mRNA was undetectable in all astrocytic tumors examined. These studies suggest a possible adjunct clinical use of hormonal therapy for the treatment of astrocytomas. Specific antagonists and agonists may allow the modulation of the growth of these tumors. Development of this body of knowledge may lead to the development of better treatment for these aggressive tumors.

Transcription activation of mouse mammary tumor virus-chloramphenicol acetyltransferase: a model to study the metabolism of cortisol

The human 11 beta-hydroxysteroid dehydrogenase (h11 beta-HSD) inactivates the active corticosteroid cortisol to its inactive metabolite cortisone. We have developed transactivation analyses of the reporter chimeric gene mouse mammary tumor virus-chloramphenicol acetyltransferase (MMTV-CAT) to study the catalytic activity of h11 beta-HSD introduced by cotransfection into receptor and 11 beta-HSD deficient CV-1 cells. Assay of 11 beta-HSD expressed in CV-1 cells by cotransfection showed that the catalyzed dehydrogenation of cortisol to cortisone was 2-fold higher in the presence of NADP. The reductase activity was dependent on the coenzyme NADPH. The addition of increasing concentrations of the inhibitor carbenoxolone (CBX) in the incubates blocked the enzyme activity in a dose dependent fashion. In CV-1 cells cotransfected with expression vectors of either human glucocorticoid (hGR1-777) or mineralocorticoid (hMR1-984) and the reporter plasmid MMTV-CAT, dexamethasone (DEX), aldosterone (ALDO), cortisol, and corticosterone induction of CAT activity was dose dependent. Cotransfection of CV-1 cells transfected with 10 micrograms of 11 beta-HSD expression vector reduced the transactivation of MMTV-CAT by hGR or hMR in the presence of either cortisol or corticosterone to basal values. The concomitant addition of 100 nM cortisone and 1 microM NADPH to these transfectants elevated CAT activity. These data show that transactivation analyses can be used to study the 11 beta-HSD-catalyzed regulation of corticosteroid levels, which triggers physiological processes and in certain cases provides an alternative to animal experimentation.

Regulation of glucocorticoid receptor (GR) mRNA and protein levels by phorbol ester in MCF-7 cells. Mechanism of GR mRNA induction and decay

Treatment of MCF-7 cells with the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) (10(-7) M) was associated with a time-dependent increase in specific binding of [3H]dexamethasone (34.8 +/- 4.6 fmol/mg protein after 9 h of TPA treatment compared with 16.0 +/- 2.3 fmol/mg protein in control cells) as well as a transient induction in the level of glucocorticoid receptor (GR) mRNA (4- to 8-fold stimulation after 2-3 h, followed by a decline towards the control value after 6 h). In the presence of the transcription inhibitor actinomycin D (AMD) (5.0 micrograms/ml) the TPA-dependent induction of GR mRNA was completely abolished, and GR mRNA showed a gradual decline with a half-life of 2-3 h. In contrast, treatment with TPA and the protein synthesis inhibitor cycloheximide (50 microM) resulted in a superinduction of GR mRNA (> 50-fold after 6 h). Inhibition of a half-life of 2-3 h, which is identical to that observed in non-treated cells. We conclude that the increase in GR mRNA in the presence of TPA is dependent on ongoing transcription, whereas the rate by which GR transcripts are degraded, is not altered by TPA.

Induction of peroxisomal acyl-CoA oxidase by 3-thia fatty acid, in hepatoma cells and hepatocytes in culture is modified by dexamethasone and insulin

The effects of tetradecylthioacetic acid (TTA) (50 microM), dexamethasone (0.25 microM) and insulin (0.4 microM) on induction of peroxisomal acyl-CoA oxidase activity and mRNA levels were studied in short term cultures of Morris 7800C1 and MH1C1 hepatoma cells and of rat hepatocytes. Dexamethasone and TTA resulted in parallel increases in the enzyme activity and the steady state mRNA content in the hepatoma cells. Combination of dexamethasone and TTA resulted in a synergistic and parallel stimulation of both the enzyme activity and the mRNA levels up to 11-12-fold and maximal changes were observed after 14 days of treatment. Semiquantitative immunoblot analyses of acyl-CoA oxidase were in concordance with enzyme and mRNA results. Insulin counteracted the inductive effects of dexamethasone and TTA on all parameters. The half-life of the acyl-CoA oxidase mRNA increased after treatment with the 3-thia fatty acid (t1/2 = 10.0 h +/- 0.4) compared to control (t1/2 = 5.9 h +/- 0.3). However, in combination with dexamethasone there was no further increase in the mRNA stability (t1/2 = 8.0 h +/- 0.3). Southern blot analysis did not reveal any changes on the oxidase gene level in any treatment group. TTA alone or in combination with dexamethasone did not affect the expression of either the glucocorticoid receptor or the peroxisomal proliferator acting receptor (PPAR) steady state mRNA levels. In cultured hepatocytes the acyl-CoA oxidase was modified in similar manner by these treatments, but the changes were less marked. We suggest that the changes in peroxisomal acyl-CoA oxidase activity in hepatoma cells are due to a major effect on the level of mRNA, involving both transcriptional effects and message stabilization.

Differential regulation of mouse mammary tumor virus-bacterial chloramphenicol acetyltransferase chimeric gene by human mineralocorticoid hormone-receptor complexes

The brain tissues of the rat and mouse express two types of corticosteroid binding proteins, the glucocorticoid (GR) and aldosterone (MR) receptors. Unlike the type II (GR) receptor, type I receptor has a high affinity for aldosterone (ALDO) and corticosterone and is structurally similar to the kidney mineralocorticoid receptor (MR). The results reported in this study provide direct evidence for the interaction of dexamethasone (DEX), triamcinolone acetonide (TA), dexamethasone-21-mesylate (DXM) and 11-deoxycorticosterone (DOC) with human MR expressed in cells by transient co-transfection of a hMR expression vector. The interactions of hMR with DEX, TA, DXM, DOC, promegestone (R5020) and methyltrienelone (R1881) were measured by trans-activation of mouse mammary tumor virus long terminal repeat fused to bacterial chloramphenicol acetyltransferase (MMTV-tk-CAT) in gene co-transfection experiments and by cell free hormone binding assay. The incubation of various steroid hormones in the presence of [3H]ALDO in a competition assay with extracts prepared from HeLa cells co-transfected with hMR expression vector, showed that hMR expressed under these conditions has a high relative affinity for DEX which is similar to ALDO, TA and DOC. Incubation with DXM under these conditions showed very little competition, as was observed with R1881 and R5020. Incubation of the co-transfected cells with DEX, ALDO, DOC, R5020, TA, R1881 and DXM demonstrated that the level of trans-activation did not reflect the previously observed order of binding affinity for the hMR. The level of transactivation was always higher with DEX and TA compared to ALDO and DOC. Analysis of the binding of labeled glucocorticoid regulatory element (GRE) and hMR incubated with DEX, ALDO and DXM by gel shift analysis demonstrated that the trans-activation of MMTV-tk-CAT by hMR is a result of the interaction of hMR with GRE in the MMTV-LTR.

Human glucocorticoid receptor gene promotor-homologous down regulation

To study the regulation of the human glucocorticoid receptor (hGR), we characterized the promoter region by primer extension, S1 nuclease mapping and by DNA sequencing. We found that the promoter is extremely G + C rich (72% GC content) and contains a "TAATA" and a "CAT" box, eight "GGGCGG", three "CCGCCC" and two "CACCC" motifs and a motif similar to the glucocorticoid responsive element (GRE) which included two interchanged nucleotides "TCTTGT". In contrast to other steroid receptor genes, exon I or GHGR contains the major part of the 5' non-coding sequences of hGR mRNA while exon II contains coding sequences for the first 394 amino acid residues of the A/B region of hGR. The major transcriptional start site was found to be 134 bp upstream of the ATG initiation codon. Transfection of HeLa cells with plasmids containing various deletions of GHGR promoter fused to a promoterless CAT vector suggested the region between -470 and -1030, at the 5' end of the mRNA start site, to contain sequences required for down regulation by hormone.

Steroid hormone receptors

In the three decades since the original discovery of receptors for steroid hormones, much has been learned about the biochemical processes by which these regulatory agents exert their effects in target tissues. The intracellular receptor proteins are potential transcription factors, needed for optimal gene expression in hormone-dependent cells. They are present in an inactive form until association with the hormone converts them to a functional state that can react with target genes. Transformation of the receptor protein to the nuclear binding form appears to involve the removal of both macromolecular and micromolecular factors that act to keep the receptor form reacting with DNA. Much of the native receptor is present in the nucleus, loosely bound and readily extractable, but for some and possibly all steroid hormones, some receptor is in the cytoplasm, perhaps in equilibrium with a nuclear pool. Methods have been developed for the stabilization, purification, and characterization of receptor proteins, and through cloning and sequencing of their cDNAs, primary structures for these receptors are now known. This has led to the recognition of structural similarities among the family of receptors for the different steroid hormones and to the identification of regions in the protein molecule responsible for the various aspects of their function. Monoclonal antibodies recognizing specific molecular domains are available for most receptors. Despite the knowledge that has been acquired, many important questions remain unsolved. How does association with the steroid remove factors keeping the receptor protein in its native state, and how does binding of the transformed receptor to the response element in the promoter region enhance gene transcription? Once it has converted the receptor to the nuclear binding state, is there a further role for the steroid in modulating transcription? Still not entirely clear is the involvement of phosphorylation and/or dephosphorylation in hormone binding, receptor transformation, and transcriptional activation. Less vital to basic understanding but important in the overall picture is whether the native receptors for gonadal hormones are entirely confined to the nucleus or whether there is an intracellular distribution equilibrium. With the effort now being devoted to this field, and with the application of new experimental techniques, especially those of molecular biology, our understanding of receptor function is progressing rapidly. The precise mechanism of steroid hormone action should soon be completely established.

An auto-inducible vector conferring high glucocorticoid inducibility upon stable transformant cells

A new gene expression system in mammalian cells was developed by using the glucocorticoid receptor (GR) as an inducible positive feedback factor. Mouse Ltk- cells were transfected with plasmids carrying the GR-encoding gene and the lacZ reporter gene, both of which were fused with the glucocorticoid-inducible enhancer/promotor of the mouse mammary tumor virus (MTV). The GR gene was first induced to supply the receptor protein, which further induced the expression of both GR and reporter genes. Stable transformants induced with dexamethasone, a synthetic glucocorticoid hormone, demonstrated beta-galactosidase activity 60-140-fold higher than uninduced controls. Similarly, the human alpha-interferon-encoding gene fused with the MTV enhancer/promoter was induced more than 12,000-fold. This system allowed us to increase the expression of the reporter or target genes without augmenting basal levels of expression significantly, and may be useful to investigate the unknown function of a cloned gene, particularly when the gene product of interest is cytotoxic or growth-inhibiting.

A highly inducible system of gene expression by positive feedback production of glucocorticoid receptors

To investigate the function of a cloned gene, inducible enhancer/promoters are preferred for controlling the level of expression experimentally. Highly inducible gene expression systems with both low basal levels and enhanced maximal levels of expression are especially sought. We have developed a new expression system using a positive feedback mechanism. Glucocorticoid receptor cDNA was fused to the enhancer/promoter of mouse mammary tumor virus (MTV) to induce receptor production. Simultaneous introduction of this receptor plasmid into mouse Ltk- cells augmented the expression of the reporter gene linked with the MTV enhancer/promoter by 10- to 100-fold in an induced state, but did not significantly change the expression of that in an uninduced state. This novel system of gene expression is a good candidate for an efficient genetic switch.

Androgen receptor in sexual differentiation

Androgens play an essential role in sexual differentiation and their action is mediated by the androgen receptor (AR). The normal AR is a soluble protein, highly thermolabile, with a mol. wt of 90 kDa and a pI of 5.2 as determined by 2 dimensional (2D) gel electrophoresis. It is regulated by androgens in culture conditions but the physiological relevance of this regulation remains controversial. The presence of a functional AR is an absolute requirement for male sexual differentiation and its absence results in complete insensitivity. However, androgen insensitivity (complete or partial) can develop in the presence of a normal androgen binding capacity and there is no correlation between the clinical and the biochemical findings. A number of qualitative abnormalities have been described to explain the failure of androgen action in these cases: they all emphasize the extreme instability of the abnormal AR. It is difficult at the present time to determine whether these abnormalities result from structural mutations of the AR gene, transcriptional or post-transcriptional abnormalities. Further elucidation of these defects awaits for an antibody and/or a cDNA probe for the AR.

The structure and function of steroid receptor proteins

This review has highlighted several topics in the study of steroid hormone action. The unanswered questions regarding the mechanism of ligand-controlled LRF activity, the extent of evolutionary conservation and specificity of DNA binding, and the validity of various models of transcriptional regulation mediated through gene networks point to the future direction of research in this field. Steroid hormones are used extensively in clinical treatments, especially glucocorticoids. Our laboratory is attempting to determine which gene networks are responsible for some of these clinical phenotypes. Figure 5 points out that the study of glucocorticoid action holds a unique position because it spans both the basic sciences and the field of applied molecular biology. Now that we have a fundamental knowledge of the necessary elements required for steroid-dependent regulation of gene expression, we can better investigate the clinical responses to steroid therapy (which include devastating side effects) by isolating and characterizing the important target gene(s). In this author's opinion, future directions in the study of steroid responsiveness will have to include a systematic approach toward deciphering a variety of these LRF-regulated gene networks in experimentally feasible systems. Hopefully, work in this area may be revealing and perhaps beneficial to ongoing clinical studies. In addition, the study of mechanisms of transcriptional induction and repression, using the model system of LRFs, could be applicable to many gene regulatory systems which are controlled by such processes as development and differentiation.

Abdominal segmentation of the Drosophila embryo requires a hormone receptor-like protein encoded by the gap gene knirps

The body pattern along the anterior-posterior axis of the insect embryo is thought to be established by two organizing centres localized at the ends of the egg. Genetic analysis of the polarity-organizing centres in Drosophila has identified three distinct classes of maternal effect genes that organize the anterior, posterior and terminal pattern elements of the embryo. The factors provided by these gene classes specify the patterns of expression of the segmentation genes at defined positions along the longitudinal axis of the embryo. The system responsible for organizing the posterior segment pattern is a group of at least seven maternal genes and the zygotic gap gene knirps (kni). Their mutant phenotype has adjacent segments in the abdominal region of the embryo deleted. Genetic analysis and cytoplasmic transplantation experiments suggested that these maternal genes are required to generate a 'posterior activity' that is thought to activate the expression of kni (reviewed in ref. 2). The molecular nature of the members of the posterior group is still unknown. Here we report the molecular characterization of the kni gene that codes for a member of the steroid/thyroid receptor superfamily of proteins which in vertebrates act as ligand-dependent DNA-binding transcription regulators.

Rat antibodies as probes for the characterization of progesterone receptor A and B proteins from laying hen oviduct cytosol

The chicken oviduct contains two different hormone binding forms of the progesterone receptor, A and B. We have prepared rat antisera against both forms of the receptor partially purified from laying hen oviduct. The anti-progesterone receptor A antiserum reacts with both receptor forms on Western blots, while the anti-progesterone receptor B antiserum reacts mainly with the B form. Both antisera also react with the native progesterone receptor proteins as shown by sedimentation analysis of the antibody-receptor complexes. Receptors A and B are recognized on Western blots of total protein from dissolved tissue, indicating that both forms are likely to be physiological components. Epitope mapping experiments show that immunogenicity of both receptor molecules is restricted to structurally related protein domains of 28 kDa in receptor A and of 52 kDa in receptor B.

Functional domains of the human estrogen receptor

Two domains of the human estrogen receptor, responsible for hormone binding (region E) and tight nuclear binding (region C), are essential for the receptor to activate efficiently the transcription of estrogen-responsive genes. Region D, which joins the DNA- and hormone-binding domains, can be altered without affecting activation. Deletion of the N-terminal domain (region A/B) has no effect on activation of a reporter gene containing a vitellogenin estrogen-responsive element (ERE) and the HSV-tk promoter, whereas it severely impairs activation of the human pS2 gene promoter. Deletion of most or all of the hormone-binding domain leads to only about 5% constitutive transcriptional activity, yet these mutants appear to bind efficiently to an ERE in vivo. Apparently, region C recognizes the ERE of target genes, and the hormone-binding domain plays an essential role for efficient activation of transcription.

Oestradiol induction of a glucocorticoid-responsive gene by a chimaeric receptor

Steroid hormone receptors are a class of cell-specific trans-acting transcription regulatory factors whose activity is controlled by specific binding of the hormone. The hormone-receptor complex appears to associate with promoter/enhancer elements of specific target genes, resulting in activation of transcription (see refs 1 and 2 for reviews). Sequence comparison between the oestrogen, glucocorticoid and progesterone receptors (refs 7, 8 and unpublished results) and site-directed mutation analysis, has identified in each at least two functional domains important for steroid receptor function. Region E (Fig. 1a), is the hormone-binding domain; region C is a 66-amino-acid region (Figs 1a,b) that is more highly conserved than the hormone-binding domain and has the potential to form at least two zinc-stabilized 'DNA-binding fingers' analogous to those proposed for the Xenopus transcription factor TFIIIA. We and others have suggested that this region may be the receptor's DNA-binding domain. We show here that point mutations replacing two cysteines by two histidines in the first potential DNA-binding finger of the human oestrogen receptor (hER) prevent it from activating gene transcription. We further show that a chimaeric receptor formed by replacing this 66-amino-acid region of the hER with that of the human glucocorticoid receptor (hGR) activates expression of a glucocorticoid-inducible gene, but not of an oestrogen-inducible gene, in the presence of oestradiol. Thus, region C determines the receptor's specificity for target genes.

Genetics of glucocorticoid receptors

The lymphocytolytic effect of glucocorticoids has been used for isolating receptor mutants. They fall into several groups with defects either in the hormone binding domain or the DNA binding domain or with part of the receptor polypeptide missing. These truncated receptors have increased binding affinity for general DNA and are synthesized from 5'-truncated messages. In addition, a phenotype has been identified in which a receptor allele, although apparently normal, is shut-off with no gene product detectable. The wild-type receptor polypeptide of about 95,000 molecular weight is synthesized from two mRNAs of 7 kb and 5 kb which differ in the lengths of their 3'-untranslated regions. A receptor model with three linearly arranged and functionally distinct domains is discussed. The DNA binding domain is rich in basic amino acids and cysteines and is located in the middle of the polypeptide. This region has the highest degree of homology with the estrogen receptor and with the v-erb-A oncogene product.

Glucocorticoid mechanism of action: monoclonal antibodies as experimental tools

In order to provide a further insight into glucocorticoid receptor (GR)-mediated action of glucocorticoid hormones, we produced ten monoclonal antibodies against rat GR. In studies combining physicochemical separation methods with antibody methodology, we established that the molybdate-stabilised GR contains one steroid-binding monomer. Using a monoclonal anti-GR antibody-based immunoaffinity chromatographic procedure, we purified two non-ligand-binding proteins, with molecular weights of 80,000 and 90,000, present in the molybdate-stabilised GR complex. These proteins are not recognised by monoclonal antibodies directed against GR. The possible relation of these two proteins to heat shock proteins remains to be established. Immunohistochemical studies of GR in the central nervous system of the rat provided new information on the distribution of GR, particularly in the hypothalamus. Studies of intracellular receptor localisation in rat brain after endocrine manipulations gave results in support of the classical concept of translocation of GR from cytoplasm to cell nucleus. Studies with a cell culture system also supported the existence of GR in the cytoplasm as well as in the cell nucleus.

Down-regulation of glucocorticoid receptor mRNA by glucocorticoid hormones and recognition by the receptor of a specific binding sequence within a receptor cDNA clone

A cDNA clone for the rat glucocorticoid receptor (GR) was used to study mechanisms of GR mRNA regulation. Treatment of rat hepatoma culture cells with 0.5 microM dexamethasone caused a small, initial increase in the GR mRNA level after 6 hr as well as a 50% to 95% reduction of the GR mRNA level after 24 hr of incubation when studied by RNA blot hybridization. After 72 hr, the initial GR mRNA level was restored. The down-regulation of GR mRNA levels appears to be independent of protein synthesis, since it also was observed in the presence of cycloheximide. However, cycloheximide caused a 4-fold increase in intracellular levels of GR mRNA. Using an immunoprecipitation assay, we could demonstrate that the GR specifically interacts with a GR cDNA clone, which represents a 2.6-kilobase fragment of the 3' nontranslated region of the GR mRNA. Nuclease protection experiments indicate the presence of several internal GR-binding regions in the above fragment.

Cloning and expression of cDNA for anti-müllerian hormone

Messenger RNA, prepared from fetal bovine testicular tissue, was used to construct a cDNA library in lambda gt11 phage. The library was screened with an antibody probe directed against bovine anti-Müllerian hormone and three positive clones were isolated. Cross-hybridizing cDNA inserts carried by clones 4 and 5 (1.2 and 0.08 kilobases long, respectively) code for a fragment of authentic anti-Müllerian hormone, as shown by the ability of the anti-epitope antibodies eluted from fusion protein 4 to bind strongly to anti-Müllerian hormone on immunoblots and by the capacity of anti-epitope antibodies 4 and 5 to precipitate radioiodinated bovine anti-Müllerian hormone. A probe prepared from insert 4 hybridizes with an mRNA present only in tissues that are known producers of anti-Müllerian hormone, such as the fetal testis and adult ovarian follicles. The amount of specific mRNA in tissues of males and females is related to the rate of their anti-Müllerian hormone production. The 2.1-kilobase size of this mRNA species is large enough to code for the Mr 62,000 anti-Müllerian hormone polypeptide chain. Insert 4 also hybridizes with an mRNA of similar size in human and rat fetal testicular tissue. The third isolated clone, clone 8, which does not cross-hybridize with the others, carries a cDNA insert coding for a ubiquitous protein, smaller than anti-Müllerian hormone, with which it apparently shares an epitope.

Cloning of the chicken progesterone receptor

Monospecific antibodies directed against the chicken progesterone receptor (PR) form B were used to screen a randomly primed phage lambda gt11 cDNA expression library prepared from size-fractionated chicken oviduct mRNA. Two independent immunoreactive clones, lambda cPR1 and lambda cPR2, were isolated. Antibodies selected from anti-PR form B antiserum on matrices of lambda cPR1 and lambda cPR2 fusion proteins detected two proteins on electrophoretic immunoblots of crude and purified PR preparations. These proteins had the same apparent molecular weights as did PR forms A and B crosslinked with the tritiated progestin R 5020. Thus, lambda cPR1 and lambda cPR2 fusion proteins contain epitopes present in both PR forms A and B. A cDNA clone, lambda cPR3, containing the inserts of both lambda cPR1 and lambda cPR2, was isolated from a randomly primed lambda gt10 oviduct cDNA library, indicating that both cDNA inserts were derived from the same oviduct mRNA. Additional evidence that these cDNAs correspond to PR mRNA was provided by sequencing the lambda cPR3 cDNA insert, since it was found to encode the sequence of three tryptic peptides prepared from purified PR form B. A fourth and a fifth cDNA clone, lambda cPR4 and lambda cPR5, were sequentially isolated from the same lambda gt10 cDNA library beginning with a probe derived from the 3' end of the lambda cPR3 insert. Partial DNA sequencing of lambda cPR4 and lambda cPR5 revealed the presence of a sequence coding for a cysteine-rich domain that is strikingly homologous to the amino acid sequences present in the putative DNA-binding domain of the human and chicken estrogen receptors, human glucocorticoid receptor, and v-erbA gene product of the avian erythroblastosis virus.