DNA sequences bound specifically by glucocorticoid receptor in vitro render a heterologous promoter hormone responsive in vivo

Sequence polymorphisms in the long terminal repeat of bovine leukemia virus: evidence for selection pressures in regulatory sequences

Bovine leukemia virus (BLV) is an oncogenic virus widespread in cattle. It belongs to the genus Deltaretrovirus of the family Retroviridae along with human and simian T-lymphotropic viruses. The BLV transcriptional promoter is located in the proviral 5' long terminal repeat (LTR), composed of U3, R, and U5 regions. BLV LTR contains multiple cis-acting elements important for promoter activity, a short coding sequence (encoding the NH(2) terminus of the G4 regulatory protein), and non-regulatory/non-coding regions. Variation in coding sequences of BLV structural proteins has been studied extensively, but little work has been done on sequence variability of non-coding regions, mostly located in LTR. Here, we report the first study on the natural diversity of the BLV LTR, using viral isolates from 52 cattle in several different areas worldwide. Nucleotide variations from the consensus sequence were observed in most isolates and clustered phylogenetically, corresponding to the geographic distribution of donor cattle. Overall, regulatory regions were significantly more conserved than non-regulatory regions in the BLV LTR, as well as in LTR sub-regions (U3, R, and U5). Evidence of selection pressures in BLV LTR suggests that selection occurs not only in coding sequences, but may also involve regulatory sequences.

Chromatin Remodeling and Control of Cell Proliferation by Progestins via Cross Talk of Progesterone Receptor with the Estrogen Receptors and Kinase Signaling Pathways

Transcription from the mouse mammary tumor virus (MMTV) promoter can be induced by glucocorticoids or progestins. Progesterone treatment of cultured cells carrying an integrated single copy of an MMTV transgene leads to recruitment of progesterone receptor (PR), SWI/SNF, and SNF2h-related complexes to MMTV promoter. Recruitment is accompanied by selective displacement of histones H2A and H2B from the nucleosome B. In nucleosomes assembled on promoter sequences, SWI/SNF displaces histones H2A and H2B from MMTV nucleosome B, but not from other MMTV nucleosomes or from an rDNA promoter nucleosome. Thus, the outcome of nucleosome remodeling by purified SWI/SNF depends on the DNA sequence. On the other hand, 5 min after hormone treatment, the cytoplasmic signaling cascade Src/Ras/Erk is activated via an interaction of PR with the estrogen receptor, which activates Src. As a consequence of Erk activation PR is phosphorylated, Msk1 is activated, and a ternary complex PR-Erk-Msk1 is recruited to MMTV nucleosome B. Msk1 phosphorylates H3 at serine 10, which is followed by acetylation at lysine 14, displacement of HP1gamma, and recruitment of Brg1, PCAF, and RNA polymerase II. Blocking Erk activation or Msk1 activity prevents induction of the MMTV transgene. Thus, the rapid nongenomic effects of progestins are essential for their transcriptional effects on certain progestin target genes. In rat endometrial stromal cells, picomolar concentrations of progestins trigger the cross talk of PR with ERbeta that activates the Erk and Akt kinase pathways leading to cell proliferation in the absence of direct transcriptional effects of the ligand-activated PR. Thus, depending on the cellular context rapid kinase activation and transcriptional effect play different roles in the physiological response to progestins.

Effect of 17beta-estradiol and progesterone on the expression of FeLV in chronically infected cells

In a previous study, it was found that even though more male cats were infected by feline leukaemia virus (FeLV), females seemed to progress easier to overt disease. To study the effect of female hormones, 17beta-estradiol and progesterone were added in different concentrations (10(-3) M to 10(-12) M) to a culture of persistently FeLV-infected cells. The effect of both hormones was very similar. After 24 h the cell viability was very low at 10(-3) M and 10(-4) M but similar to controls at the remaining concentrations. Liberation of viral particles was estimated by the reverse transcriptase activity (RT), which was the lowest also at 10(-3) M and 10(-4) M. However, low viability could not account for this low RT, as when cells were lysed with lysis buffer RT was high. Thus, cells were dying without freeing viral particles, suggestive of apoptosis. This possibility was confirmed by staining hormone-treated cells with annexin V and propidium iodide. The FeLV antigen p27 measured in the cultures had a maximum at 10(-3) M and 10(-4) M, higher than controls and lysed cells, so the presence of p27 in the supernatant was not only due to cell lysis but a consequence of hormone effect. In conclusion, 17beta-estradiol and progesterone induce death of FeLV-infected cells at high concentrations, probably through a process of apoptosis, which might limit the spread of the infection, as infective viral particles would be hampered from budding.

A robust characterization of retinoic acid response elements based on a comparison of sites in three species

The availability of high-throughput genomic sequencing has allowed us to construct a more robust characterization of retinoic acid response elements than was possible in the past. We located human, mouse, and rat homologs for each of 51 well-documented, conserved retinoic acid response elements. Mathematical and statistical analyses of these 153 sites, 78 of which are new, shows that 92% of response elements have direct-repeat symmetry, but that only 76% exhibit canonical spacing attributes. While the familiar '(a/g)g(g/t)tca' hexamer motif is upheld, the more relaxed sequence, '(a/g)g(g/t)(g/t)(g/c)a', represents a 10% consensus. Sites are as likely to be on the coding strand as on the non-coding strand, and 86% of them are in upstream locations. From a statistical point of view, DR1 elements are fundamentally different from DR2 and DR5 elements, but this is only evident in the 5' hexamer. While there is considerable variation in core positions, and while no nucleotide can be considered forbidden at any position, variation among species at a fixed locus appears surprisingly constrained once a functional site has been attained.

DNA damage stabilizes interaction of CSB with the transcription elongation machinery

The Cockayne syndrome B (CSB) protein is essential for transcription-coupled DNA repair (TCR), which is dependent on RNA polymerase II elongation. TCR is required to quickly remove the cytotoxic transcription-blocking DNA lesions. Functional GFP-tagged CSB, expressed at physiological levels, was homogeneously dispersed throughout the nucleoplasm in addition to bright nuclear foci and nucleolar accumulation. Photobleaching studies showed that GFP-CSB, as part of a high molecular weight complex, transiently interacts with the transcription machinery. Upon (DNA damage-induced) transcription arrest CSB binding these interactions are prolonged, most likely reflecting actual engagement of CSB in TCR. These findings are consistent with a model in which CSB monitors progression of transcription by regularly probing elongation complexes and becomes more tightly associated to these complexes when TCR is active.

A single amino acid change in the first zinc finger of the DNA binding domain of the glucocorticoid receptor regulates differential promoter selectivity

Mammalian species are well known to differ in their sensitivity to glucocorticoids, but the molecular basis for this difference remains largely uncharacterized. To address this issue, the transcriptional activity of the mouse and human glucocorticoid receptor (GR) was analyzed on two model glucocorticoid-responsive promoters. Mouse GR (mGR) displayed unique promoter discrimination in response to a range of glucocorticoids, with enhanced activity on a simple glucocorticoid response element (GRE)-based promoter and diminished activity on the complex mouse mammary tumor virus promoter compared with human GR (hGR). Promoter discrimination between mGR and hGR was mapped to a single amino acid change at residue 437 (glycine to valine) of mGR and to sequence differences within individual GREs of the different promoters. Mouse GR displayed higher activation on GREs with a guanine rather than a thymine at the -6 position. Binding studies indicated mGR (mGR437V) displayed a weaker affinity for GREs containing a thymine at the -6 position than a mGR mutant containing a glycine at residue 437 (mGR437G). Despite distinct transcriptional activities, both receptors had similar affinities for response elements that contain a guanine at the -6 position. Our findings support a model by which the presence of a valine residue at position 437 of mGR induces a conformational change that leads to alterations in affinity and/or transcriptional activation in a promoter-dependent context.

Mechanisms of mineralocorticoid action: determinants of receptor specificity and actions of regulated gene products

The mineralocorticoid receptor (MR) and its close cousin, the glucocorticoid receptor (GR), share considerable structural and functional similarity, including indistinguishable DNA binding properties, yet they mediate distinct physiological responses in some tissues. Specificity is determined by their distinct interactions with other protein factors and modification by peptides, including the small ubiquitin modifier SUMO1. Serum and glucocorticoid-regulated kinase 1 (sgk1) is one key target gene of both MR and GR, and encodes a serine-threonine kinase that stimulates the apical membrane localization of the epithelial sodium channel ENaC. Sgk1 exerts its effects, at least in part, by inhibiting an isoform of the ENaC inhibitory ubiquitin ligase Nedd4-2. This review briefly summarizes two areas of mineralocorticoid research: molecular determinants of MR specificity, and the role of Sgk1 in mediating the effects of aldosterone on epithelial Na(+) transport.

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.

Identification of glucocorticoid receptor domains involved in transrepression of transforming growth factor-beta action

The transforming growth factor-beta (TGF-beta) and glucocorticoid signaling pathways interact both positively and negatively in regulating a variety of physiological and pathologic processes. We previously reported that liganded glucocorticoid receptor (GR) repressed TGF-beta induction of human plasminogen activator inhibitor-1 gene transcription by directly targeting the transcriptional activation function of Smad3. To identify the domain(s) in the glucocorticoid receptor involved in this repression, we have examined the ability of various GR truncation, deletion, and substitution mutants to repress TGF-beta transactivation in Hep3B human hepatoma cells that lack functional endogenous GR. Partial deletions in the ligand-binding domain (LBD), including the tau2 and tauc regions, greatly reduced or eliminated GR repression, whereas deletion of the N-terminal AF1 (tau1) domain and substitution mutations in the DNA-binding domain had little or no effect. Liganded androgen receptor repressed TGF-beta transactivation, whereas mineralocorticoid receptor did not, and studies with rat GR-mineralocorticoid receptor chimeras confirmed that the GR C-terminal domains were required for repression. RU486, a strong antagonist of transactivation by GR, partially reversed repression by wild type GR. Co-immunoprecipitation experiments in Hep3B cells indicated that physical interaction between GR and Smad3 is necessary but not sufficient for repression. Physical interaction required activation of Smad3 by TGF-beta but not dexamethasone binding to GR. Glutathione S-transferase pull-down assays demonstrated that several regions of the LBD could mediate GR-Smad3 physical interaction. We conclude that the LBD of GR, but not the DNA-binding domain or the N-terminal activation domain, is required for GR-mediated transrepression of TGF-beta transactivation.

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.

The upstream regulatory region of human papillomavirus type 31 is insensitive to glucocorticoid induction

The upstream regulatory region (URR) of various types of human papillomaviruses (HPVs) has been shown to contain functional glucocorticoid response elements (GREs), including HPV type 11 (HPV11), HPV16, and HPV18. Glucocorticoids have been demonstrated to induce the transcriptional activity of the early promoters of these HPV types. Although it has been assumed that the URR of HPV31 contains at least one GRE, no functionality has been demonstrated. We attempt to show here inducibility of the URR of HPV31 by the synthetic glucocorticoid dexamethasone (dex). By sequence analysis we identified three potential GREs in the URR of HPV31. Gel shift analysis indicated that each of these three sites has the potential to be a functional GRE. However, constructs containing the full-length URR, 5' deletions of the URR, and an internal fragment of the URR containing all three putative GREs were only weakly inducible by dex. Linker scanning mutants, whereby each potential GRE was replaced individually, in double combination, or in triple combination by a unique polylinker, had no effect on dex inducibility. Replacement of each of the three HPV31 GREs with the GRE of HPV18 failed to induce a response to dex. Placement of the HPV18 GRE into the URR of HPV31 in a region similar to its location in the HPV18 URR was also unable to result in a strong dex induction of the HPV31 URR. These data suggest that the lack of dex inducibility is due to the overall context of the HPV31 URR and may be dependent on the requirements of the major early promoter for transcriptional activation. Finally, replacement of the HPV18 GRE with each of the HPV31 GREs in HPV18 only showed weak inducibility, indicating that the three GREs of HPV31 are in fact only weak inducers of dex. Overall, these data suggest that dex responsiveness, along with oncogenic potential, may provide a possible explanation for the classification of HPV31 as an intermediate-risk virus and demonstrate the complexity of transcriptional regulation of the URR of HPV.

TEF-1 transcription factors regulate activity of the mouse mammary tumor virus LTR

The mouse mammary tumor virus long terminal repeat (LTR) is a potent transcriptional enhancer. We identified several putative binding sites for the TEF-1 family of transcription factors (TEF-1, RTEF-1, DTEF-1, and ETF) in the proximal negative regulatory element of the LTR. Gel mobility shift assays revealed strong TEF-1 factor binding to one site using nuclear extracts from CV-1 cells and from the human breast cancer cell line MCF-7. Mutation of this site increased basal activity of the LTR. In transient transfection assays, TEF-1 squelched the basal LTR activity and completely abrogated the response to the glucocorticoid dexamethasone. RTEF-1 and DTEF-1 had little effect on the basal activity, whereas ETF activated the LTR. These TEF-1 factors also interfered with the response to dexamethasone. Taken together, our results reveal an important new role for TEF-1 factors in regulating MMTV LTR activity and suggest that TEF-1 factors might participate in mammary tumorigenesis.

Glucocorticoid Hormones Negatively Regulate Nerve Growth Factor Expression In Vivo and in Cultured Rat Fibroblasts

Sciatic nerve transection leads to an up-regulation of nerve growth factor (NGF) production in non-neuronal cells surrounding the axons. The lesion-mediated increase in NGF-mRNA levels in the nerve can be blocked by pretreating the animals with the synthetic glucocorticoid dexamethasone. Dexamethasone also reduces NGF-mRNA levels in cultured sciatic fibroblasts stimulated with fetal calf serum or interleukin-1. In order to study at which level glucocorticoids down-regulate NGF expression, sciatic fibroblasts where transfected with a construct in which a reporter gene (chloramphenicol acetyltransferase) is expressed under the control of the NGF promotor. The results demonstrated that dexamethasone effectively represses NGF gene transcription. Deletion experiments showed that a 162 nucleotide promotor region mediates the glucocorticoid hormone suppression of NGF expression. The negative regulation of NGF synthesis by glucocorticoids is a factor to be considered in the treatment of patients with peripheral nerve lesions.

Species-specific differences in the glucocorticoid receptor transactivation function upon binding with betamethasone-esters

Glucocorticoids (GCs) are the most effective drugs for anti-inflammatory diseases. A number of adverse side effects, however, limit chronic treatment with GCs. To improve their therapeutic usefulness, attempts have been made to dissociate the two main actions of the glucocorticoid receptor (GR), transactivation and transrepression, which are believed to be responsible for the side effects and anti-inflammatory effects, respectively. We report here species-specific differences in the transactivation response mediated by GR. Dexamethasone (DEX), betamethasone (BM), and their esterified-derivatives had full transrepression agonistic activity in a reporter assay using CV-1 cells transfected with either human or rat GR. These GCs also had full transactivation agonistic activity in CV-1 cells transfected with human GR. The esterified-BM, however, had only partial transactivation agonistic activity in cells transfected with rat GR, whereas BM and esterified-DEX had full transactivation agonistic activity. Moreover, in rat hepatoma H4-II-E cells, the esterified-BM failed to induce tyrosine aminotransferase, which is regulated by GR-mediated transactivation activity. There were no significant differences between the binding affinity of these GCs to human and rat GR. Consistent with the weak transactivation activity of esterified-BM mediated by rat GR, there were few side effects, evaluated by thymus involution and body weight loss, in an antigen-induced asthmatic model in rats. These results suggest that the potency of esterified-BM to induce transactivation activity is different between species and that this difference is not due to differences in receptor binding.

Determinants of subnuclear organization of mineralocorticoid receptor characterized through analysis of wild type and mutant receptors

The mineralocorticoid receptor (MR) is a hormone-dependent regulator of gene transcription that in the absence of ligand resides both in the cytoplasm and the nucleus. Agonists but not antagonists increase the number of MRs residing in the nucleus and cause aggregation of MRs into distinct clusters. To identify the functional determinants of MR nuclear organization, we examined the localization pattern of wild type MR and a series of mutants in the presence and absence of ligands using fluorescent protein chimeras in living cells. Our data show that although MR DNA binding is not necessary to mediate nuclear localization, it is absolutely required for wild type cluster formation as is an intact N-terminal or C-terminal activation function. In contrast, destabilization of a dimerization motif within the DNA-binding domain has no effect on subnuclear receptor architecture. These data suggest that normal MR cluster formation is dependent on both DNA binding and intact transcriptional activation functions but not on DNA-dependent receptor dimerization. Because dimer mutants bind with high affinity to hormone response element DNA multimers but not to single palindromic DNA sites, we suggest that clusters represent MR aggregates bound to DNA response element multimers in the vicinity of regulated genes.

Factor recruitment and TIF2/GRIP1 corepressor activity at a collagenase-3 response element that mediates regulation by phorbol esters and hormones

To investigate determinants of specific transcriptional regulation, we measured factor occupancy and function at a response element, col3A, associated with the collagenase-3 gene in human U2OS osteosarcoma cells; col3A confers activation by phorbol esters, and repression by glucocorticoid and thyroid hormones. The subunit composition and activity of AP-1, which binds col3A, paralleled the intracellular level of cFos, which is modulated by phorbol esters and glucocorticoids. In contrast, a similar AP-1 site at the collagenase-1 gene, not inducible in U2OS cells, was not bound by AP-1. The glucocorticoid receptor (GR) associated with col3A through protein-protein interactions with AP-1, regardless of AP-1 subunit composition, and repressed transcription. TIF2/GRIP1, reportedly a coactivator for GR and the thyroid hormone receptor (TR), was recruited to col3A and potentiated GR-mediated repression in the presence of a GR agonist but not antagonist. GRIP1 mutants deficient in GR binding and coactivator functions were also defective for corepression, and a GRIP1 fragment containing the GR-interacting region functioned as a dominant-negative for repression. In contrast, repression by TR was unaffected by GRIP1. Thus, the composition of regulatory complexes, and the biological activities of the bound factors, are dynamic and dependent on cell and response element contexts. Cofactors such as GRIP1 probably contain distinct surfaces for activation and repression that function in a context-dependent manner.

Inhibition of AP-1 by the glucocorticoid-inducible protein GILZ

The immunosuppressive effects of glucocorticoids arise largely by inhibition of cytokine gene expression, which has been ascribed to interference between the glucocorticoid receptor and transcription factors such as AP-1 and NF-kappa B as well as by competition for common coactivators. Here we show that glucocorticoid-induced inhibition of interleukin-2 mRNA expression in activated normal T cells required new protein synthesis, suggesting that this phenomenon is secondary to expression of glucocorticoid-regulated genes. One of the most prominent glucocorticoid-induced genes is glucocorticoid-induced leucine zipper (GILZ), which has been reported to inhibit activation-induced up-regulation of Fas ligand (FasL) mRNA. Indeed, transient expression of GILZ in Jurkat T cells blocked induction of a reporter construct driven by the FasL promoter. This could be accounted for by GILZ-mediated inhibition of Egr-2 and Egr-3, NFAT/AP-1-inducible transcription factors that bind a regulatory element in the FasL promoter and up-regulate FasL expression. GILZ also potently inhibited AP-1-driven and IL-2 promoter-driven reporter constructs, and recombinant GILZ specifically interacted with c-Fos and c-Jun in vitro and inhibited the binding of active AP-1 to its target DNA. Whereas homodimerization of GILZ required the presence of its leucine zipper, the interaction with c-Fos and c-Jun occurred through the N-terminal 60-amino acid region of GILZ. Thus, GILZ represents a glucocorticoid-induced gene product that can inhibit a variety of activation-induced events, at least in part by direct interference with AP-1, and is therefore a candidate for a mediator of glucocorticoid-induced immunosuppression.

Interleukin-2 inhibits glucocorticoid receptor transcriptional activity through a mechanism involving STAT5 (signal transducer and activator of transcription 5) but not AP-1

Cytokines and glucocorticoids (GCs) signaling pathways interfere with each other in the regulation of apoptosis and gene expression in the immune system. Interleukin-2 (IL-2), through the Janus kinase/signal transducers and activators of transcription (Jak/STAT) and mitogen-activated protein kinase (MAPK) pathways, activates STAT5 and activated protein-1 (AP-1) transcription factors, respectively, which are known to repress glucocorticoid receptor (GR) activity, at least in part, through protein-protein interactions. In this work, we have analyzed the mechanisms whereby IL-2 down-regulates the GC-induced transactivation of the mouse mammary tumor virus long terminal repeat (MMTV-LTR) in murine CTLL-2 T lymphocytes. Mutagenesis studies revealed that the MMTV-LTR STAT5 binding site (-923/-914) was not required for IL-2-mediated inhibition but identified both glucocorticoid response elements (GREs) and the -104/+1 region as critical elements for this negative response. The DNA binding activities of transcription factors required for GC-mediated activation of the MMTV-LTR promoter and that bind to the -104/+1 region (nuclear factor-1, Oct-1) were not affected by IL-2 treatment. Overexpression of wild-type STAT5B enhanced the effect of IL-2 on MMTV-LTR activity, and a dominant negative form of STAT5B (Y699F) abolished the IL-2-mediated MMTV-LTR inhibition, whereas AP-1 activation had no effect in this system. Direct interaction between liganded GR and STAT5 was observed in CTLL-2 cells in a STAT5 phosphorylation-independent manner. Overexpression of nuclear coactivators CBP (CREB-binding protein) or SRC-1a (steroid receptor coactivator 1a) did not blunt IL-2 inhibitory effects. We suggest that the STAT5-repressive activity on the GC-dependent transcription may involve direct interaction of STAT5 with GR, is dependent on the promoter context and STAT5 activation level, and occurs independently of coactivators levels in T cells.

Estrogen receptors in the human forebrain and the relation to neuropsychiatric disorders

The steroid hormone estrogen influences brain function and neuropsychiatric disorders, but neuroanatomical information about the estrogen receptors (ERs) are rather limited. The main focus of this article is to provide an overview of the current status of the ER distribution and possible function in the human brain. The ERs are ligand activated transcription factors that belong to the steroid hormone receptors, included in the nuclear receptor superfamily. To date, there are two known ER subtypes, alpha and beta. In the human forebrain, both estrogen receptor subtypes are predominantly expressed in limbic-related areas, although they show distinct distribution patterns. The ERalpha mRNA expression appears to dominate in the hypothalamus and amygdala, indicating that the alpha-subtype might modulate neuronal cell populations involved in autonomic and reproductive neuroendocrine functions as well as emotional interpretation and processing. In contrast, the hippocampal formation, entorhinal cortex, and thalamus appear to be ERbeta dominant areas, suggesting a putative role for ERbeta in cognition, non-emotional memory and motor functions. Clinical observations of estrogenic effects together with the information available today regarding ER expression in the primate brain provide important clues as to the functional aspects of the two ER subtypes. However, further characterization of the different phenotypes of the ER expressing cells in the human brain is needed as well as the delineation of the genes which are regulated by the ERs and how this transcriptional control correlates with human behavior and mental status.

Understanding nuclear receptor function: from DNA to chromatin to the interphase nucleus

The regulation of gene expression by steroid receptors is the fundamental mechanism by which these important bioregulatory molecules exert their action. As such, mechanisms utilized by receptors in the modulation of genetic expression have been intensively studied since the first identification of hormone-binding proteins. Although these mechanisms include both posttranscriptional (1) and posttranslational (2) components, the primary level of control involves direct modulation of the rate of transcription, and it is this process that has been the major focus of research in the field.

Role of accessory factors and steroid receptor coactivator 1 in the regulation of phosphoenolpyruvate carboxykinase gene transcription by glucocorticoids

In the liver, glucocorticoids induce a 10-15-fold increase in the rate of transcription of the phosphoenolpyruvate carboxykinase (PEPCK) gene, which encodes a key gluconeogenic enzyme. This induction requires a multicomponent glucocorticoid response unit (GRU) comprised of four glucocorticoid accessory factor (AF) elements and two glucocorticoid receptor binding sites. We show that the AFs that bind the gAF1, gAF2, and gAF3 elements (hepatocyte nuclear factor [HNF]4/chicken ovalbumin upstream promoter transcription factor 1 and HNF3beta) all interact with steroid receptor coactivator 1 (SRC1). This suggests that the AFs function in part by recruiting coactivators to the GRU. The binding of a GAL4-SRC1 chimeric protein completely restores the glucocorticoid induction that is lost when any one of these elements is replaced with a GAL4 binding site. Thus, when SRC1 is recruited directly to gAF1, gAF2, or gAF3, the requirement for the corresponding AF is bypassed. Surprisingly, glucocorticoid receptor is still required when SRC1 is recruited directly to the GAL4 site, suggesting a role for the receptor in activating SRC1 in the context of the GRU. Structural variants of GAL4-SRC1 were used to identify requirements for the basic-helix-loop-helix and histone acetyltransferase domains of SRC1, and these are specific to the region of the promoter to which the coactivator is recruited.

Identification of a glucocorticoid-responsive element in the promoter region of the mouse tyrosine hydroxylase gene

It has been known for nearly 30 years that glucocorticoid receptor stimulation induces increased tyrosine hydroxylase (TH) gene expression. However, the mechanism mediating this effect has remained elusive. Sequences with homology to known glucocorticoid-responsive elements (GRE) have been identified in the 5' flanking region of the TH gene of several vertebrate species, but none has been shown to be functional. To identify the GRE element(s) in the TH promoter, we generated chimeric constructs in which different lengths of the 5' flanking sequences of the mouse TH gene (3.6, 1.1 and 0.8 kb) were ligated to a luciferase reporter gene. Dexamethasone treatment increased luciferase expression only in cells transiently transfected with the construct containing 3.6 kb of the TH 5' flanking DNA. Co-administration of mifepristone (RU486), a glucocorticoid receptor antagonist, blocked this effect. We identified a TH-GRE sequence (5'-GGCACAGTGTGGTCT) in the mouse 5' flanking DNA between -2435 and -2421 from the transcription start. Responsiveness to dexamethasone was lost following deletion of this sequence. To determine the ability of this element to function in a heterologous promoter, we prepared a chimeric construct in which the TH-GRE sequence was cloned just upstream of a minimal thymidine kinase (TK) promoter. Promoter activity was increased 2-fold in dexamethasone-treated PC12 cells transfected with the TH-GRE-TK construct. These results provide strong evidence that the 15 base-pair sequence in the 5' flanking DNA of the mouse TH gene functions as a glucocorticoid response element. This is the first report identifying a functional glucocorticoid response element in the promoter region of the TH gene of any species.

Differential gene expression of dopamine D-2 receptor subtypes in rat chromaffin cells and sympathetic neurons in culture

Chromaffin cells and sympathetic neurons arise from a common bipotential progenitor which, if exposed to nerve growth factor (NGF), matures into a sympathetic neuron, but if exposed to glucocorticoids (GCs), differentiates into a mature chromaffin cell. Pharmacological evidence indicates that, in adrenal medulla and sympathetic neurons, dopamine (DA) receptors belonging to the D-2 family inhibit catecholamine secretion. The molecular characterization of these receptors, however, is not been yet described. Our data suggest that bipotential cells obtained from newborn rat adrenal medulla express both isoforms of the D-2 receptor, while D-3 receptor and D-4 receptor messenger RNAs (mRNAs) are not present. GC-mediated maturation induces the expression of D-4 receptors, without modification of D-2 isoforms. Sympathetic neurons differentiated in vitro selectively express the D-2short mRNA. Taken together, present results suggest that NGF and GCs play a role in regulating D-2 family receptor expression in neural crest-derived cells.

Mechanisms of Glucocorticoid-receptor-mediated Repression of Gene Expression

It is hoped that this review will give the reader a taste of some of the mechanisms used by the glucocorticoid receptor to repress gene function. These mechanisms include direct binding to DNA, antagonism of other transcription factor families and sequestration of necessary cofactors. Each of these mechanisms, and others, are discussed.

Identification of a protein that recognizes a distal negative regulatory element within the mouse mammary tumor virus long terminal repeat

The mouse mammary tumor virus (MMTV) long terminal repeat contains a distal negative regulatory element (dNRE) that selectively represses activity of the proviral promoter in the absence of steroid hormone receptor-mediated activation. A protein, termed MMTV NRE-binding protein 1 (MNBP-1), that recognizes long terminal repeat sequences between -433 and -418 was identified by gel electrophoresis mobility shift assays and methylation interference footprinting in nuclear extracts of HeLa and Ltk(-) cells. Mutations within the defined binding site affect dNRE-mediated promoter repression in vivo. MNBP-1 has an apparent molecular mass of approximately 100 kDa as determined by gel filtration chromatography.

Distinct glucocorticoid receptor transcriptional regulatory surfaces mediate the cytotoxic and cytostatic effects of glucocorticoids

Glucocorticoids act through the glucocorticoid receptor (GR), which can function as a transcriptional activator or repressor, to elicit cytostatic and cytotoxic effects in a variety of cells. The molecular mechanisms regulating these events and the target genes affected by the activated receptor remain largely undefined. Using cultured human osteosarcoma cells as a model for the GR antiproliferative effect, we demonstrate that in U20S cells, GR activation leads to irreversible growth inhibition, apoptosis, and repression of Bcl2. This cytotoxic effect is mediated by GR's transcriptional repression function, since transactivation-deficient mutants and ligands still bring about apoptosis and Bcl2 down-regulation. In contrast, the antiproliferative effect of GR in SAOS2 cells is reversible, does not result in apoptosis or repression of Bcl2, and is a function of the receptor's ability to stimulate transcription. Thus, the cytotoxic versus cytostatic outcome of glucocorticoid treatment is cell context dependent. Interestingly, the cytostatic effect of glucocorticoids in SAOS2 cells involves multiple GR activation surfaces. GR mutants and ligands that disrupt individual transcriptional activation functions (activation function 1 [AF-1] and AF-2) or receptor dimerization fail to fully inhibit cellular proliferation and, remarkably, discriminate between the targets of GR's cytostatic action, the cyclin-dependent kinase inhibitors p21(Cip1) and p27(Kip1). Induction of p21(Cip1) is agonist dependent and requires AF-2 but not AF-1 or GR dimerization. In contrast, induction of p27(Kip1) is agonist independent, does not require AF-2 or AF-1, but depends on GR dimerization. Our findings indicate that multiple GR transcriptional regulatory mechanisms that employ distinct receptor surfaces are used to evoke either the cytostatic or cytotoxic response to glucocorticoids.

The structure of the nuclear hormone receptors

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

Cyclic AMP response element mediates dexamethasone induced suppression of prostaglandin H synthase-2 gene expression in human amnion derived WISH cells

A human PGHS-2 promoter fragment (300 BP) linked to the luciferase reporter was used to study the regulation of PGHS-2 gene expression in human amnion-derived WISH cells. A cyclic AMP (cAMP) response element (CRE) was found to be important in the induction of PGHS-2 gene expression. This was demonstrated by showing that coexpression of CREB stimulated native but not CRE mutant promoter and that IL-1beta and PMA induced less activity with the mutant promoter as compared to the native promoter. The effect of dexamethasone on IL-1beta and PMA induced promoter activities was further examined. IL-1beta or PMA induced activity was blocked by dexamethasone, whereas IL-1beta or PMA induced mutant activity was not responsive to dexamethasone. Direct activation of CRE by a cAMP elevating agent, isoproterenol, was found to be inhibited significantly dexamethasone. These results suggest that CRE may mediate the induction of PGHS-2 by IL-1beta and PMA as well as the suppression of expression by dexamethasone in amnion-derived cells.

Glucocorticoid-mediated suppression of the promoter activity of the cyclooxygenase-2 gene is modulated by expression of its receptor in vascular endothelial cells

Cyclooxygenase-2 (COX-2), an inducible isozyme of cyclooxygenase, is expressed selectively in response to various inflammatory stimuli such as lipopolysaccharide (LPS) and its expression is suppressed by the glucocorticoid dexamethasone (DEX) in numerous types of cells. However, LPS-enhanced production of prostacyclin in bovine arterial endothelial cells (BAEC) was not significantly decreased by treatment with DEX but was suppressed by selective COX-2 inhibitors. This is consistent with the finding that DEX was not effective at preventing the expression of LPS-induced COX-2 mRNA. Transient transfection analysis showed that DEX did not suppress the LPS-induced promoter activity of the 5'-flanking region of the human COX-2 gene (nucleotides -327 to +59). Since RNA blot analysis indicated low-level expression of glucocorticoid receptor (GR) mRNA in BAEC, a GR-expression vector was transfected to evaluate the role of the GR in the COX-2 promoter activity. It was found that DEX mediated the suppression of the LPS-induced COX-2 promoter activity in a dose-dependent manner. These results suggest that the DEX-mediated suppression of LPS-induced promoter activity of the COX-2 gene is modulated by expression of the GR, which will be possible to account for a unique expression pattern of the COX-2 gene in BAEC.

Steroid-selective initiation of chromatin remodeling and transcriptional activation of the mouse mammary tumor virus promoter is controlled by the site of promoter integration

The mouse mammary tumor virus (MMTV) promoter has target sequences recognized by several steroid receptors. We present evidence for a novel mechanism that confers hormone specificity to this promoter. We show that remodeling of MMTV chromatin and the concomitant activation of the MMTV promoter are induced equally by glucocorticoids and progestins in one chromosomal context but are selective for glucocorticoids in another. Furthermore, increased histone acetylation modulates MMTV promoter regulation disparately at the two chromosomal locations. Together, these data indicate that chromosomal architecture commands a crucial role in gene regulation, imposing locus-specific selectivity between regulators with similar sequence recognition.

Characterization of the gene and promoter for RTI40, a differentiation marker of type I alveolar epithelial cells

In an effort to understand the processes that establish and maintain the differentiated state of the alveolar epithelium, we have analyzed the gene for rat type I cell 40 kD protein (RTI40), an apical integral plasma membrane protein expressed in type I but not type II alveolar epithelial cells. The RTI40 gene spans 35 kilobase pairs; it contains 6 exons and at least 6 rat Identifier repetitive elements. Three exons encode the predicted RTI40 extracellular domain and one encodes the single transmembrane spanning domain. The final exon encodes one amino acid followed by a stop codon. RTI40 gene transcription starts downstream from a TATA homology, which is immediately adjacent to putative binding sites for thyroid transcription factor 1 and Sp1. In H441 cell transfections, mutagenesis of a 5'-flanking fragment (-2496 to +104) revealed two regions that contribute to promoter activity: -1247 through -795 and -163 through -81. Heterologous promoter fusion experiments suggest that a cooperative interaction between these regions activates transcription. In transfected type II cells, deletion across the proximal region produced a 6-fold drop in promoter activity, whereas deletion across the distal region was without apparent effect. These results provide a foundation to analyze further the factors that govern alveolar epithelial cell phenotype.

Gene structure and properties of TIGR, an olfactomedin-related glycoprotein cloned from glucocorticoid-induced trabecular meshwork cells

Expression of the trabecular meshwork inducible glucocorticoid response (TIGR) gene progressively increases from barely detectable levels to greater than 2% of total cellular mRNA over 10 days exposure of trabecular meshwork (TM) cells to dexamethasone. Cycloheximide blocked most of the TIGR mRNA induction, suggesting a requirement for ongoing protein synthesis. The genomic structure of TIGR (approximately 20 kilobases) consists of 3 exons, and a 5-kilobase promoter region that contains 13 predicted hormone response elements, including several glucocorticoid regulatory elements, and other potentially important regulatory motifs. TIGR cDNA encodes an olfactomedin-related glycoprotein of 504 amino acids with motifs for N- and O-linked glycosylation, glycosaminoglycan initiation, hyaluronic acid binding, and leucine zippers. Recombinant TIGR (rTIGR) showed oligomerization and specific binding to TM cells. Anti-rTIGR antibody detected multiple translational/post-translational forms of TIGR produced by the cells (including secreted 66 kDa/55 kDa glycoproteins/proteins in the media and 55 kDa cellular proteins), whereas Northern blot showed a single mRNA species. The findings suggest potential mechanisms by which TIGR could obstruct the aqueous humor fluid flow and participate in the pathogenesis of glaucoma.

Antiprogestins mediate differential effects on glucocorticoid receptor remodeling of chromatin structure

We examined the mechanism(s) by which the progesterone receptor (PR) is able to inhibit glucocorticoid receptor (GR) activation from the mouse mammary tumor virus (MMTV) promoter in vivo. Using specific hormone antagonists, we demonstrate that the PR complexed with an type II antiprogestin blocks glucocorticoid-induced activation of the MMTV promoter. However, when complexed with a type I antiprogestin the PR is unable to block glucocorticoid-induced activation. PR repression of GR activity results from the inhibition of the ability of the GR to remodel chromatin such that the antiprogestin-occupied/PR prevents the glucocorticoid induced assembly of a preinitiation complex at MMTV promoter. These experiments suggest that the specific chromatin organization of the MMTV promoter provides a mechanism for regulating cross-talk between the GR and PR in vivo.

Hormone regulation of bovine leukemia virus via the long terminal repeat

The hormone regulation of viruses has been of great interest since the discovery of glucocorticoid stimulation of mouse mammary tumor virus via a hormone response element in the viral long terminal repeat (LTR) promoter region. This report describes the investigation of the hormone responsiveness of bovine leukemia virus (BLV), an oncogenic retrovirus that infects dairy and beef cattle worldwide. It is a member of the human T cell leukemia (HTLV)/BLV group of retroviruses, which encode a protein, Tax, that is essential for regulating transcription of their own proviruses and for transforming host cells. We investigated the responsiveness of BLV to the hormones 17 beta-estradiol, progesterone, prolactin, insulin, and dexamethasone, a potent glucocorticoid. Only dexamethasone, in combination with insulin or insulin/prolactin, consistently stimulated BLV expression, as measured by reverse transcriptase activity, RNA blot hybridization (Northern blots), and CAT (chloramphenicol acetyltransferase) reporter assays of cell lines transiently or stably transfected with the BLV LTR. This effect required the presence of glucocorticoid receptors and Tax. This is the first report of hormone responsiveness in a virus of the HTLV/BLV group.

Glucocorticoid receptor-mediated cell cycle arrest is achieved through distinct cell-specific transcriptional regulatory mechanisms

Glucocorticoids inhibit proliferation of many cell types, but the events leading from the activated glucocorticoid receptor (GR) to growth arrest are not understood. Ectopic expression and activation of GR in human osteosarcoma cell lines U2OS and SAOS2, which lack endogenous receptors, result in a G1 cell cycle arrest. GR activation in U2OS cells represses expression of the cyclin-dependent kinases (CDKs) CDK4 and CDK6 as well as their regulatory partner, cyclin D3, leading to hypophosphorylation of the retinoblastoma protein (Rb). We also demonstrate a ligand-dependent reduction in the expression of E2F-1 and c-Myc, transcription factors involved in the G1-to-S-phase transition. Mitogen-activated protein kinase, CDK2, cyclin E, and the CDK inhibitors (CDIs) p27 and p21 are unaffected by receptor activation in U2OS cells. The receptor's N-terminal transcriptional activation domain is not required for growth arrest in U2OS cells. In Rb-deficient SAOS2 cells, however, the expression of p27 and p21 is induced upon receptor activation. Remarkably, in SAOS2 cells that express a GR deletion derivative lacking the N-terminal transcriptional activation domain, induction of CDI expression is abolished and the cells fail to undergo ligand-dependent cell cycle arrest. Similarly, murine S49 lymphoma cells, which, like SAOS2 cells, lack Rb, require the N-terminal activation domain for growth arrest and induce CDI expression upon GR activation. These cell-type-specific differences in receptor domains and cellular targets linking GR activation to cell cycle machinery suggest two distinct regulatory mechanisms of GR-mediated cell cycle arrest: one involving transcriptional repression of G1 cyclins and CDKs and the other involving enhanced transcription of CDIs by the activated receptor.

Disruption of the glucocorticoid receptor assembly with heat shock protein 90 by a peptidic antiglucocorticoid

Association of glucocorticoid (GR) and progesterone (PR) receptors with a set of molecular chaperones, including the 90-kDa heat shock protein (hsp90), is a dynamic process required for proper folding and maintaining these nuclear receptors under a transcriptionally inactive, ligand-responsive state. Mutational studies of the chicken hsp90 complementary DNA suggested that three regions of this protein (A, B, and Z) interact with the hormone-binding domain of GR, whereas region A is dispensable for hsp90 binding to PR. We found that this 69-amino acid region can be narrowed down to a 35-mer alpha-helical, acidic peptide, which is by itself able to inhibit hsp90 association to GR translated in vitro. The hsp90-free GR did not bind ligand, but was devoid of any specific DNA-binding activity, and higher peptide concentrations specifically inhibited the binding of activated GR to DNA. When overexpressed in cultured cells, this peptide acted as an antiglucocorticoid and inhibited the antiactivating protein-1 activity and the ligand-dependent nuclear transfer of GR. None of these effects, either in vivo and in vitro, was observed for PR. The region from residue 232 to residue 265 of hsp90 is, therefore, a domain critical for its association to GR, an association that is a prerequisite for receptor transcriptional activity. More importantly, these results demonstrate that targeting specific protein/protein interaction interfaces is a powerful means to specifically modulate nuclear receptor signaling pathways in a ligand-independent manner.

Regulation of heme oxygenase-2 by glucocorticoids in neonatal rat brain: characterization of a functional glucocorticoid response element

Heme oxygenase-2 (HO-2) is constitutively expressed in mammalian tissues; together with HO-1 (HSP32) it catalyzes the cleavage of heme to produce biliverdin IX alpha, CO and Fe. Detection of a consensus sequence of the glucocorticoid response element (GRE) in the promoter region of the HO-2 gene prompted the present study which has investigated the role of glucocorticoids (Gcs) in the regulation of HO-2 protein and transcript development in the newborn rat brain and has examined the promoter activity of the GRE in HeLa cells. Using in situ hybridization histochemistry, we noted a pronounced increase in signal for HO-2 mRNA in the brain of 14-day-old rats postnatally treated with corticosterone (5 microg/g, 4 x, starting 24-36 h after birth). And, using immunohistochemistry, a striking increase in neuronal HO-2 immunostaining in treated brains was detected. The HO-2 GRE was tested for responsiveness to dexamethasone (DX) using both a promoterless CAT expression vector, and a heterologous promoter containing luciferase expression vector in HeLa cells. The HO-2 promoter containing the GRE and transcription start site induced CAT reporter gene activity in response to DX, whereas mutation or deletion in the GRE abolished hormone responsiveness. Similarly, constructs containing the GRE conferred responsiveness to DX in an orientation-independent manner and increased relative luciferase activity. Further, specific binding of glucocorticoid receptor protein to the GRE was observed; binding could be competed out only by excess cold GRE and not by mutated HO-2 GRE, or AP1. HO-2 mRNAs (approximately 1.3 and approximately 1.9 kb) increased in HeLa cells treated with DX (5 microM), the level reached a maximum at 24 h. DX did not effect HO-1 mRNA level. The increase in the HO-2 transcript was accompanied by an increase in HO-2 protein, as assessed by Western blot analysis, and an increase in HO activity, as measured by bilirubin formation. Also, an increase in intensity of immunostaining was noted in DX-treated HeLa cells. We conclude that the GRE present in the HO-2 gene promoter region is functional, and propose the direct involvement of the adrenal glucocorticoids in modulation of HO-2 gene expression. In the context of biological functions of heme degradation products, we suggest that this regulation may be of significance, particularly to the neurons.

Determining selectivity of drugs by quantitative two-dimensional gel analysis. A study of tenidap, piroxicam, and dexamethasone

In vitro pharmacologic measures of drug specificity are well established, i.e. drug interaction with a specific target such as an enzyme, receptor, or ion channel. However, in vitro measures of drug selectivity, defined as effects on secondary targets, are lacking. Two-dimensional gel electrophoresis (2-D gel) was examined as a measure of drug selectivity by comparing the effects of three drugs, tenidap, piroxicam, and dexamethasone, on the synthesis of intracellular proteins in lipopolysaccharide (LPS)-stimulated murine macrophages. A set of 902 35S-methionine-labeled proteins were separated consistently, identified by their coordinates of apparent isoelectric point and molecular weight, and quantified. LPS altered the concentrations of 45 proteins. Tenidap, at 10 microM, affected a total of five proteins (suppressed three; stimulated two), whereas piroxicam, at 10 microM, suppressed two proteins. Dexamethasone at 0.01 microM suppressed eight proteins and stimulated one. Thus, none of the drugs reversed the LPS-induced changes. Two of the eight proteins suppressed by dexamethasone were also suppressed by tenidap and were identified as proIL-1 alpha and proIL-1 beta. Since the subset of affected proteins provided a unique protein "fingerprint" for each drug, the three drugs were mechanistically differentiated by 2-D gel analysis. Compared to LPS (5% affected proteins), all three drugs were selective (< or = 1% affected) with piroxicam > tenidap > dexamethasone. With identification of affected proteins, this technique can provide a useful in vitro assessment of drug selectivity.

Glucocorticoid-mediated repression of NFkappaB activity in endothelial cells does not involve induction of IkappaBalpha synthesis

Repression of NFkappaB-dependent gene expression is one of the major elements of immunosuppression by glucocorticoids. Protein-protein interactions between the glucocorticoid receptor and NFkappaB have been characterized and shown to be a possible mechanism of mutual inhibition of transactivation properties. More recently, glucocorticoid-mediated induction of IkappaBalpha, an inhibitor of NFkappaB, has been described in monocytes and lymphocytes; an increase in IkappaBalpha mRNA and protein resulted in inactivation and cytosolic retention of NFkappaB. Thus, rather than the physical interaction between the glucocorticoid receptor and NFkappaB, the up-regulation of IkappaBalpha was presented as the key element in immunosuppression by glucocorticoids. In contrast, we show that the IkappaBalpha pathway is not involved in glucocorticoid-mediated inhibition of NFkappaB activity in endothelial cells. Although transcriptional activation by NFkappaB was significantly reduced in the presence of glucocorticoids, we did not detect induction of IkappaBalpha protein that could prevent nuclear translocation of NFkappaB upon stimulation with lipopolysaccharide or tumor necrosis factor alpha. Furthermore, treatment with glucocorticoids did not seem to affect the transcription rate or mRNA stability of IkappaBalpha. We therefore conclude that, although induction of IkappaBalpha expression by glucocorticoids seems to be of importance in monocytes and lymphocytes, it cannot explain inhibition of NFkappaB-dependent gene expression in endothelial cells. Our results emphasize the relevance of physical interaction between the glucocorticoid receptor and NFkappaB in endothelial cells and thus in suppression of inflammation by glucocorticoids.

Glucocorticoid and progestin receptors are differently involved in the cooperation with a structural element of the mouse mammary tumor virus promoter

We have previously characterized a regulatory element located between -294 and -200 within the mouse mammary tumor virus (MMTV) long terminal repeat (LTR). This element termed AA element cooperates with the glucocorticoid response elements (GREs) for glucocorticoid activation. Here we show that in a MMTV LTR wild type context, the deletion of this element significantly reduces both glucocorticoid and progestin activation of the promoter. Deletion of the two most distal GREs forces the glucocorticoid receptor (GR) and the progestin receptor (PR) to bind the same response elements and results in a dramatic decrease in the inducibility of the MMTV promoter by the two hormones. The simultaneous deletion of the two distal GREs and of the AA element abolishes completely the glucocorticoid-induced activation of the promoter. In contrast it restores a significant level of progestin-induced activation. This different effect of the double deletion on glucocorticoid- and progestin-induced MMTV promoter activation is not cell specific because it is also observed, and is even stronger, when either GR or PR is expressed in the same cell line (NIH 3T3). This is the first description of a mutated MMTV promoter that, although retaining GREs, is activated by progestins and not by glucocorticoids. This suggests a different functional cooperation between protein(s) interacting with the AA element and GR or PR. Cotransfections with constructs containing wild-type or mutated MMTV LTR with either PR lacking its C-terminal domain or GR/PR chimeras in which the N-terminal domains have been exchanged demonstrate that the N-terminal domains of the receptors specify the different behavior of GR and PR regarding the AA element.

Interactions of a transcriptional activator in the env gene of the mouse mammary tumor virus with activation-dependent, T cell-specific transacting factors

The mouse mammary tumor virus env gene contains a transcriptional activator (META) that can control transcription of the adjacent long terminal repeat region. Transcriptional control by META parallels that of several lymphokine genes, being specific to T cells, dependent on their activation, and inhibited by the immunosuppressive drug cyclosporine (CsA). DNase I footprinting indicated that nuclear factors from activated T lymphocytes bound a promoter-proximal site, META(P), and a promoter-distal site, META(D+), within the 400-base pair META region. Nuclear factors from unstimulated, but not from activated cells, bound a site, META(D-), adjacent to META(D+). META(D+) directed transcription of a linked luciferase gene, and gel shift analysis revealed binding of inducible, CsA-sensitive T cell factors, in parallel with transfection results. Authentic NFAT and NF-kappaB targets did not compete for the META(D+) binding factor(s). The SV40 core sequence competed for META(D+) binding factors, but META(D+) failed to compete for the complexes obtained with the SV40 probe. Our results, taken together, indicate that META(D+) is a novel transcriptional enhancer element that is similar in its cell-type specificity, activation dependence, and CsA sensitivity to the NFAT element. It may be relevant to the role of MMTV in expression of Mls antigens or the induction of T cell lymphomas.

Chromatin structure of the MMTV promoter and its changes during hormonal induction

1. The packaging of nuclear DNA in chromatin determines the conversion of the genetic information into a defined phenotype by influencing the availability of DNA sequences for interactions with regulatory proteins and transcription factors. 2. We have studied the influence of the first level of chromatin organization, the nucleosome, on the activity of the mouse mammary tumor virus (MMTV) promoter. The MMTV promoter is strongly transcribed in response to steroid hormones but is virtually silent in the absence of hormonal stimuli. Full hormonal induction requires binding of the hormone receptors to four hormone-responsive elements (HREs), as well as binding of nuclear factor I (NFI) and the octamer transcription factor 1 (OTF-1 or Oct-1) to sites located between the HREs and the TATA box. A full loading with transcription factors cannot be achieved on free DNA due to steric hindrance between hormone receptor and NFI and between NFI and OTF-1. 3. The low basal activity of the MMTV promoter is most likely due to its organization in a positioned nucleosome. In the intact cell, as well in reconstituted chromatin, the regulatory region of the MMTV promoter is wrapped around a histone octamer in a precise rotational orientation, which permits access of the hormone receptors to only two of the four HREs, while precluding binding of NFI and OTF-1 to their respective sites. Upon hormone induction, the nucleosome is remodeled and the path of its DNA altered in a way which makes the nucleosomal dyad axis more accessible to DNase I and enables occupancy of all relevant sites: the four HREs, as well as the binding sites for NFI and OTF-1. 4. These results suggest that the nucleosomal organization of the MMTV promoter not only is responsible for the low activity prior to hormone treatment, but also may be a prerequisite for full loading with transcription factors after hormone induction. We conclude that the DNA contains topological information which modulates the expression of the genetic program.

Glucocorticoid receptor localization in human epidermal cells

Glucocorticoids, which are widely used in therapy, exert their immunosuppressive actions through specific receptors. These receptors have been characterized in cultured human skin fibroblasts and keratinocytes, but their localization in vitro and in vivo has not been established. To determine the tissue and cellular distribution of glucocorticoid receptors (GR), two specific polyclonal rabbit anti-human GR antibodies were used to detect these receptors in skin biopsy specimens, in freshly isolated and cultured human epidermal cells and in keratinocyte cell lines. Immunoreactive GR were only faintly detected in normal and abnormal differentiated cells and as well as those in the stratum granulosum and corneocytes. These immunolocalization studies were confirmed by fluorescence cell sorter analysis of isolated basal and suprabasal keratinocytes. Immunoreactive GR were highly expressed in normal cultured human keratinocytes, Langerhans cells and several cell lines whereas they were less expressed in melanocytes. Based upon these results the main targets of glucocorticoids in the epidermis appear to be basal and Langerhans cells.

A redundant nuclear protein binding site contributes to negative regulation of the mouse mammary tumor virus long terminal repeat

The tissue specificity of mouse mammary tumor virus (MMTV) expression is controlled by regulatory elements in the MMTV long terminal repeat (LTR). These regulatory elements include the hormone response element, located approximately between -200 and -75, as well as binding sites for NF-1, Oct-1 (OTF-1), and mammary gland enhancer factors. Naturally occurring MMTV deletion variants isolated from T-cell and kidney tumors, transgenic-mouse experiments with MMTV LTR deletions, and transient transfection assays with LTR constructs indicate that there are additional transcription regulatory elements, including a negative regulatory element (NRE), located upstream of the hormone response element. To further define this regulatory region, we have constructed a series of BAL 31 deletion mutants in the MMTV LTR for use in transient transfection assays. These assays indicated that deletion of two regions (referred to as promoter-distal and -proximal NREs) between -637 and -201 elevated basal MMTV promoter activity in the absence of glucocorticoids. The region between -637 and -264 was surveyed for the presence of nuclear protein binding sites by gel retardation assays. Only one type of protein complex (referred to as NRE-binding protein or NBP) bound exclusively to sites that mapped to the promoter-distal and -proximal NREs identified by BAL 31 mutations. The promoter-proximal binding site was mapped further by linker substitution mutations and transfection assays. Mutations that mapped to a region containing an inverted repeat beginning at -287 relative to the start of transcription elevated basal expression of a reporter gene driven by the MMTV LTR. A 59-bp DNA fragment from the distal NRE also bound the NBP complex. Gel retardation assays showed that mutations within both inverted repeats of the proximal NRE eliminated NBP binding and mutations within single repeats altered NBP binding. Intriguingly, the NBP complex was detected in extracts from T cells and lung cells but was absent from mammary gland cells. These results suggest that a factor contributing to high-level expression of MMTV in the mammary gland is the lack of negative regulation by NBP.