Newly expressed progesterone receptor cannot activate stable, replicated mouse mammary tumor virus templates but acquires transactivation potential upon continuous expression

Chromatin remodeling during glucocorticoid receptor regulated transactivation

Steroid hormone receptor (SR) signaling leads to widespread changes in gene expression, and aberrant SR signaling can lead to malignancies including breast, prostate, and lung cancers. Chromatin remodeling is an essential component of SR signaling, and defining the process of chromatin and nucleosome remodeling during signaling is critical to the continued development of related therapies. The glucocorticoid receptor (GR) is a key SR that activates numerous promoters including the well defined MMTV promoter. The activation of MMTV by GR provides an excellent model for teasing apart the sequence of events between hormone treatment and changes in gene expression. Comparing hormone-induced transcription from stably integrated promoters with defined nucleosomal structure to that from transiently expressed, unstructured promoters permits key distinctions between interactions that require remodeling and those that do not. The importance of co-activators and histone modifications prior to remodeling and the formation of the preinitiation complex that follows can also be clarified by defining key transition points in the propagation of hormonal signals. Combined with detailed mapping of proteins along the promoter, a temporal and spatial understanding of the signaling and remodeling processes begins to emerge. In this review, we examine SR signaling with a focus on GR activation of the MMTV promoter. We also discuss the ATP-dependent remodeling complex SWI/SNF, which provides the necessary remodeling activity during GR signaling and interacts with several SRs. BRG1, the central ATPase of SWI/SNF, also interacts with a set of BAF proteins that help determine the specialized function and fine-tuned regulation of BRG1 remodeling activity. BRG1 regulation comes from its own subdomains as well as its interactive partners. In particular, the HSA domain region of BRG1 and unique features of its ATPase homology appear to play key roles in regulating remodeling function. Details of the inter-workings of this chromatin remodeling protein continue to be revealed and promise to improve our understanding of the mechanism of chromatin remodeling during steroid hormone signaling. This article is part of a Special Issue entitled: Chromatin in time and space.

A composite intronic element directs dynamic binding of the progesterone receptor and GATA-2

The progesterone receptor (PR) plays a pivotal role in proper development and function of the mammary gland and has also been implicated in mammary tumorigenesis. PR is a ligand-activated transcription factor; however, relatively, little is known about its mechanisms of action at endogenous target promoters. The aim of our study was to identify a natural PR-responsive gene and investigate its transcriptional regulation in the mammary microenvironment. Our experiments revealed FKBP5 as a direct target of the PR, because it exhibited a rapid activation by progestin that was cycloheximide independent and correlated with recruitment of RNA polymerase II to the promoter. Site-directed mutagenesis and chromatin immunoprecipitation assays showed that progestin responsiveness is mediated through a composite element in the first intron, to which the PR binds concomitantly with GATA-2. Mutational analysis of the element revealed that the GATA-2 site is essential for progestin activation. Direct binding of PR to DNA contributes to the efficiency of activation but is not sufficient, suggesting that the receptor makes important protein-protein interactions as part of its mechanism of action at the FKBP5 promoter. Using chromatin immunoprecipitation assays we also determined that the intronic region is in communication with the promoter, probably via DNA looping. Time course analysis revealed a cyclical pattern of PR recruitment to the FKBP5 gene but a persistent recruitment to the mouse mammary tumor virus promoter, indicating that receptor cycling is a gene-specific phenomenon rather than a characteristic of the receptor itself. Our study offers new insight in the nature of PR-regulated transcription in mammary cancer cells.

Development of Assays for Nuclear Receptor Modulators Using Fluorescently Tagged Proteins

This chapter describes a method for designing cell-based assays to screen for nuclear receptor modulators. The basic strategy consists in following the movement of the receptors from the cytoplasm into the nucleus in response to ligand binding or analogous activating events. The receptors are tagged with green fluorescent protein for automated, fluorescent detection. In the case of constitutively nuclear receptors, they are engineered for cytoplasmic retention in the absence of an activating signal by fusing them to specific regions of the glucocorticoid receptor, which is found predominantly in the cytoplasm of cultured cells. The resulting chimeras respond to ligands or receptor modulators by translocating into the nucleus. This movement is monitored easily by automated fluorescent microscopy and serves as the basis for screening libraries. Finally, secondary assays built into the cell system can differentiate between modulators that stimulate, inhibit, or do not affect the transcriptional activity of the receptor under study. This approach has been validated for both the estrogen receptor and the retinoic acid receptor and should be applicable to any member of the superfamily, facilitating the identification of new ligands and selective receptor modulators.

Progesterone-independent effects of human progesterone receptors (PRs) in estrogen receptor-positive breast cancer: PR isoform-specific gene regulation and tumor biology

Progesterone receptors (PRs) are prognostic markers in breast cancers irrespective of the patient's progestational status. However, there are two PR isoforms, PR-A and PR-B, that are equimolar in the normal breast but dysregulated in advanced disease. Postmenopausal, tamoxifen-treated patients with estrogen receptor (ER)-positive, PR-A-rich tumors have much faster disease recurrence than patients with PR-B-rich tumors. To study the mechanisms we engineered ER+ breast cancer cells that express each PR isoform under control of an inducible promoter. We identified 79 genes regulated by progesterone (P), mainly by PR-B, and 51 genes regulated without progesterone, mainly by PR-A. Only nine genes were regulated with and without ligand, leading to definition of three classes: I) genes regulated only by liganded PR; II) genes regulated only by unliganded PR; III) genes regulated by both. Unliganded PR-A and PR-B differentially regulate genes that coordinate extracellular signaling pathways and influence tumor cell biology. Indeed, in the absence of P, compared with ER+/PR-B+ or PR- cells, ER+, PR-A+ cells exhibit an aggressive phenotype, are more adhesive to an extracellular matrix, and are more migratory. Additionally, unliganded PR-A and PR-B both inhibit cell growth and provoke resistance to Taxol-induced apoptosis. We propose that PR-A:PR-B ratios, even in the absence of P, influence the biology and treatment response of ER+ tumors, that PR-A isoforms are functionally dominant in P-deficient states, and that PR-A rich tumors are especially aggressive.

Progesterone receptor deficient in chromatin binding has an altered cellular state

Our previous work has shown that the progesterone receptor (PR) can exist in two distinct functional states in mammary adenocarcinoma cells. The differences in function included the ability to activate a promoter in organized chromatin, sensitivity to ligand, and ligand-independent activation. To determine whether these functional differences were because of altered cellular processing, we carried out biochemical analyses of the functionally distinct PRs. Although the majority of PR is localized to the nucleus, biochemical partitioning resulted in a loosely bound (cytosolic) fraction, and a tightly bound (nuclear) fraction. In the absence of progestins, the functionally distinct PRs differed significantly in partitioning between the two fractions. To characterize these fractions further, we analyzed interactions of unliganded PR with chaperones by coimmunoprecipitation. We determined that PR in the cytosolic fraction associated with hsp90 and p23. In contrast, PR in the nuclear fraction consisted of complexes containing hsp90, p23, and FKBP51 as well as PR that was dimerized and highly phosphorylated. Hormone treatment significantly reduced the formation of all PR-chaperone complexes. The hsp90 inhibitor, geldanamycin, similarly blocked transcriptional activity of both functionally distinct receptors. However, the two forms of the PR differed in their ability to associate with the mouse mammary tumor virus promoter in organized chromatin. These findings provide new information about the composition and distribution of mature progesterone receptor complexes in mammary adenocarcinoma cells, and suggest that differences in receptor subcellular distribution have a significant impact on their function. These findings also reveal that transiently expressed steroid receptors may not always be processed like their endogenous counterparts.

New human breast cancer cells to study progesterone receptor isoform ratio effects and ligand-independent gene regulation

All known progesterone target cells coexpress two functionally different progesterone receptor (PR) isoforms: 120-kDa B-receptors (PR-B) and N-terminally truncated, 94-kDa A-receptors (PR-A). Their ratio varies in normal and malignant tissues. In human breast cancer cells, homodimers of progesterone-occupied PR-A or PR-B regulate different gene subsets. To study PR homo- and heterodimers, we constructed breast cancer cell lines in which isoform expression is controlled by an inducible system. PR-negative cells or cells that stably express one or the other isoform were used to construct five sets of cells: (i) PR-negative control cells (Y iNull), (ii) inducible PR-A cells (Y iA), (iii) inducible PR-B cells (Y iB), (iv) stable PR-B plus inducible PR-A cells (B iA), and (v) stable PR-A plus inducible PR-B cells (A iB). Expression levels of each isoform and/or the PR-A/PR-B ratios could be tightly controlled by the dose of inducer as demonstrated by immunoblotting and transcription studies. Induced PRs underwent normal progestin-dependent phosphorylation and down-regulation and regulated exogenous promoters as well as endogenous gene expression. Transcription of exogenous promoters was dependent on the PR-A/PR-B ratio, whereas transcription of endogenous genes was more complex. Finally, we have described several genes that are regulated by induced PR-A even in the absence of ligand.

Glucocorticoid receptor domain requirements for chromatin remodeling and transcriptional activation of the mouse mammary tumor virus promoter in different nucleoprotein contexts

The glucocorticoid receptor (GR) contains several activation domains, tau1 (AF-1), tau2, and AF-2, which were initially defined using transiently transfected reporter constructs. Using domain mutations in the context of full-length GR, this study defines those domains required for activation of the mouse mammary tumor virus (MMTV) promoter in two distinct nucleoprotein configurations. A transiently transfected MMTV template with a disorganized, accessible chromatin structure was largely dependent on the AF-2 domain for activation. In contrast, activation of an MMTV template in organized, replicated chromatin requires both domains but has a relatively larger dependence on the tau1 domain. Domain requirements for GR-induced chromatin remodeling of the latter template were also investigated. Mutation of the AF-2 helix 12 domain partially inhibits the induction of nuclease hypersensitivity, but the inhibition was relieved in the absence of tau1, suggesting the occurrence of an important interaction between the two domains. Further mutational analysis indicates that GR-induced chromatin remodeling requires the ligand-binding domain in the region of helix 3. Our study shows that the GR activation surfaces required for transcriptional modulation of a target promoter were determined in part by its chromatin structure. Within a particular cellular environment the GR appears to possess a significant degree of versatility in the mechanism by which it activates a target promoter.

The viral transactivator E1A regulates the mouse mammary tumor virus promoter in an isoform- and chromatin-specific manner

Proteins encoded by the adenovirus E1A gene regulate both cellular and viral genes to mediate effects on cell cycle, differentiation, and cell growth control. We have identified the mouse mammary tumor virus (MMTV) promoter as a target of E1A action and investigated the role nucleoprotein structure plays in its response to E1A. Both 12 and 13 S forms target the MMTV promoter when it has a disorganized and accessible chromatin configuration. However, whereas the 13 S form is stimulatory, the 12 S form is repressive. When the MMTV promoter adopts an organized and repressed chromatin structure, it is targeted only by the 13 S form, which stimulates it. Although evidence indicates that E1A interacts with the SWI/SNF remodeling complex, E1A had no effect on chromatin remodeling at the MMTV promoter in organized chromatin. Analysis of E1A mutants showed that stimulation of the MMTV promoter is mediated solely through conserved region 3 and does not require interaction with Rb, p300/CBP-associated factor, or CBP/p300. Imaging analysis showed that E1A colocalizes with MMTV sequences in vivo, suggesting that it functions directly at the promoter. These results indicate that E1A stimulates the MMTV promoter in a fashion independent of chromatin conformation and through a direct mechanism involving interaction with the basal transcription machinery.

Nuclear receptor peroxisome proliferator-activated receptor alpha (PPARalpha) is expressed in resting murine lymphocytes. The PPARalpha in T and B lymphocytes is both transactivation and transrepression competent

Peroxisome proliferator-activated receptors (PPARs) are transcription factors that belong to the nuclear hormone receptor superfamily. PPARalpha and PPARgamma ligands have been demonstrated to exert anti-inflammatory activities in macrophages by repressing the activities of several transcription factors. PPARgamma is expressed in T lymphocytes and may play a role in cytokine production, cellular proliferation, and susceptibility to apoptosis. Herein, we demonstrate that T and B lymphocytes constitutively express PPARalpha. PPARalpha represents the predominant isoform expressed in lymphocytes, whereas PPARgamma dominates in all cell types of the myeloid lineage. PPARalpha expression was down-regulated following T-cell activation while PPARgamma expression increased under the same activating conditions. PPARalpha expression in T cells may be regulated by microenvironmental factors, because Peyer's patch T cells expressed far greater levels of PPARalpha than T cells isolated from peripheral lymphoid organs. Exposure to specific ligand determined that PPARalpha in lymphocytes can effectively transactivate a peroxisome proliferator response element reporter construct. PPARalpha's ability to regulate endogenous genes, however, required treatment with histone deacetylase inhibitors. Finally, ligand activation of lymphocyte PPARalpha antagonized NF-kappaB. Our observation that a functional PPARalpha exists within T cells and B lymphocytes suggests an expanding role for this nuclear receptor in cells of the immune system.

LCR-mediated, long-term tissue-specific gene expression within replicating episomal plasmid and cosmid vectors

Locus control regions (LCRs) are transcriptional regulatory elements, which possess a dominant chromatin remodelling and transcriptional activating capability conferring full physiological levels of expression on a gene linked in cis, when integrated into the host cell genome. Using the human beta-globin LCR (betaLCR) as a model, we show that this class of control element can drive high levels of tissue-specific gene expression in stably transfected cultured cells from within an Epstein-Barr virus-based plasmid REV. Furthermore, a 38-kb betaLCR minilocus-REV cosmid vector was efficiently retained and maintained therapeutic levels of beta-globin transgene expression in the absence of drug selective pressure over a 2-month period of continuous culture equivalent to at least 60 generations. This demonstrates for the first time the feasibility of using REVs for gene therapy of the haemoglobinopathies. Importantly, our results demonstrate that as in the case of integrated transgenes, expression from within REVs is prone to silencing but that the inclusion of the betaLCR prevented this repression of gene function. Therefore, appropriate control elements to provide and maintain tissue-specific gene expression, as well as the episomal status of REVs is a crucial feature in vector design. Our data suggest that LCRs can contribute to this vital function.

A glucocorticoid/retinoic acid receptor chimera that displays cytoplasmic/nuclear translocation in response to retinoic acid. A real time sensing assay for nuclear receptor ligands

Members of the nuclear receptor superfamily play key roles in a host of physiologic and pathologic processes from embryogenesis to cancer. Some members, including the retinoic acid receptor (RAR), are activated by ligand binding but are unaffected in their subcellular distribution, which is predominantly nuclear. In contrast, several members of the steroid receptor family, including the glucocorticoid receptor, are cytoplasmic and only translocate to the nucleus after ligand binding. We have constructed chimeras between RAR and glucocorticoid receptor that selectively respond to RAR agonists but display cytoplasmic localization in the absence of ligand. These chimeric receptors manifest both nuclear translocation and gene activation functions in response to physiological concentrations of RAR ligands. The ability to achieve regulated subcellular trafficking with a heterologous ligand binding domain has implications both for current models of receptor translocation and for structural-functional conservation of ligand binding domains broadly across the receptor superfamily. When coupled to the green fluorescent protein, chimeric receptors offer a powerful new tool to 1) study mechanisms of steroid receptor translocation, 2) detect dynamic and graded distributions of ligands in complex microenvironments such as embryos, and 3) screen for novel ligands of "orphan" receptors in vivo.

Transactivation specificity of glucocorticoid versus progesterone receptors. Role of functionally different interactions of transcription factors with amino- and carboxyl-terminal receptor domains

A major unanswered question of glucocorticoid and progesterone action is how different whole cell responses arise when both of the cognate receptors can bind to, and activate, the same hormone response elements. We have documented previously that the EC(50) of agonist complexes, and the partial agonist activity of antagonist complexes, of both glucocorticoid receptors (GRs) and progesterone receptors (PRs) are modulated by increased amounts of homologous receptor and of coregulators. We now ask whether these components can differentially alter GR and PR transcriptional properties. To remove possible cell-specific differences, we have examined both receptors in the common environment of a line of mouse mammary adenocarcinoma (1470.2) cells. In order to segregate the responses that might be due to unequal nucleosome reorganization by the two receptors from those reflecting interactions with other components, we chose a transiently transfected reporter containing a simple glucocorticoid response element (i.e. GREtkLUC). No significant differences are found with elevated levels of either receptor. However, major, qualitative differences are seen with the corepressors SMRT and NCoR, which afford opposite responses with GR and PR. Studies with chimeric GR/PR receptors indicate that no one segment of PR or GR is responsible for these properties and that the composite response likely involves interactions with both the amino and carboxyl termini of receptors. Collectively, the data suggest that GR and PR induction of responsive genes in a given cell can be differentially controlled, in part, by unequal interactions of multiple receptor domains with assorted nuclear cofactors.

Characterization of transiently and constitutively expressed progesterone receptors: evidence for two functional states

Activated steroid receptors induce chromatin remodeling events in the promoters of some target genes. We previously reported that transiently expressed progesterone receptor (PR) cannot activate mouse mammary tumor virus (MMTV) promoter when it adopts the form of ordered chromatin. However, when expressed continuously, the PR acquires this ability. In this study we explored whether this gain of function occurs through alterations in nucleoprotein structure at the MMTV promoter or through changes in receptor status. We observed no major structural differences at the MMTV promoter in the presence of constitutively expressed PR and found its mechanism of activation to be very similar to that of the glucocorticoid receptor (GR). However, a systematic comparison of the functional behavior of the transiently and constitutively expressed PR elucidated significant differences. The transiently expressed PR is activated in the absence of ligand by cAMP and by components in FBS and has significantly increased sensitivity to progestins. In contrast, the constitutively expressed PR is refractory to activation by cAMP and serum and has normal sensitivity to its ligand. In addition, while the PR is localized to the nucleus in both cases, a significant fraction of the transiently expressed PR is tightly bound to the nucleus even in the absence of ligand, while the majority of constitutively expressed PR is not. These results strongly suggest that the PR undergoes processing in the cell subsequent to its initial expression and that this processing is important for various aspects of its function, including its ability to productively interact with target genes that require chromatin remodeling for activation.

A histone deacetylase inhibitor potentiates estrogen receptor activation of a stably integrated vitellogenin promoter in HepG2 cells

To compare the role of histone deactylation in estrogen activation of a transiently transfected vitellogenin (VIT) promoter and an integrated VIT promoter in the same cells, we produced three HepG2, human hepatoma, cell lines (HepG2ERV cells) stably expressing human estrogen receptor alpha (hERalpha) and containing an integrated VIT promoter-chloramphenicol acetyltransferase (VIT-CAT) reporter gene. The three ER-positive HepG2ERV cell lines and wild-type, ER-negative, HepG2 cells cotransfected with cytomegalovirus-hERalpha exhibited similar MOX-dependent inductions of 20- to 50-fold with a transiently transfected VIT-luciferase reporter and 15- to 50-fold with a transfected 4-estrogen response element-TATA-luciferase reporter gene. The histone deacetylase inhibitor, trichostatin A, did not enhance MOX induction of the transiently transfected VIT promoter in the HepG2ERV cells. In contrast, trichostatin A dramatically potentiated MOX induction of the stably integrated VIT-CAT reporter gene, resulting in MOX-ER-dependent increases in CAT activity of up to 600-fold. These data demonstrate that although liganded ER exhibits the capacity to fully activate a transiently transfected VIT promoter, under some circumstances the ability to reorganize a repressive chromatin structure may be limiting for steroid receptor action.

Opposing effects of corepressor and coactivators in determining the dose-response curve of agonists, and residual agonist activity of antagonists, for glucocorticoid receptor-regulated gene expression

A distinguishing, but unexplained, characteristic of steroid hormone action is the dose-response curve for the regulation of gene expression. We have previously reported that the dose-response curve for glucocorticoid induction of a transfected reporter gene in CV-1 and HeLa cells is repositioned in the presence of increasing concentrations of glucocorticoid receptors (GRs). This behavior is now shown to be independent of the reporter, promoter, or enhancer, consistent with our proposal that a transacting factor(s) was being titrated by added receptors. Candidate factors have been identified by the observation that changes in glucocorticoid induction parameters in CV-1 cells could be reproduced by varying the cellular levels of coactivators [transcriptional intermediary factor 2 (TIF2), steroid receptor coactivator 1 (SRC-1), and amplified in breast cancer 1 (AIB1)], comodulator [CREB-binding protein (CBP)], or corepressor [silencing mediator for retinoid and thyroid-hormone receptors (SMRT)] without concomitant increases in GR. Significantly, the effects of TIF2 and SMRT were mutually antagonistic. Similarly, additional SMRT could reverse the action of increased levels of GRs in HeLa cells, thus indicating that the effects of cofactors on transcription may be general for GR in a variety of cells. These data further indicate that GRs are yet an additional target of the corepressor SMRT. At the same time, these cofactors were found to be capable of regulating the level of residual agonist activity displayed by antiglucocorticoids. Finally, these observations suggest that a novel role for cofactors is to participate in processes that determine the dose-response curve, and partial agonist activity, of GR-steroid complexes. This new activity of cofactors is disconnected from their ability to increase or decrease GR transactivation. An equilibrium model is proposed in which the ratio of coactivator-corepressor bound to either receptor-agonist or -antagonist complexes regulates the final transcriptional properties.

Using inducible vectors to study intracellular trafficking of GFP-tagged steroid/nuclear receptors in living cells

Intracellular trafficking and localization of proteins can now be efficiently visualized by fusion of a polypeptide to the green fluorescent protein (GFP). Many spectral variants of this reagent are now available, providing powerful tools for studies in living cells. This approach is particularly useful for members of the steroid/nuclear receptor superfamily, since these molecules frequently undergo rapid subcellular redistribution on ligand activation. A major roadblock in the application of this technology concerns problems associated with transient transfections. This technique produces cell populations that are highly heterogeneous with respect to the newly introduced protein and usually contain the protein in a highly overexpressed state. In addition, long-term studies related to cell cycle and cellular differentiation are essentially impossible with this approach. These problems can be overcome by introduction of the GFP fusion into cells under appropriate induction control. We describe application of the tetracycline regulatory system to inducible control of a glucocorticoid receptor (GR)/GFP chimera. Intracellular concentrations of GFP-GR can be very effectively controlled in this system, providing an ideal environment in which to study subcellular trafficking of the receptor and interactions with a variety of intracellular targets.

Comparison of chromatin remodeling and transcriptional activation of the mouse mammary tumor virus promoter by the androgen and glucocorticoid receptor

We examined the interaction between the androgen (AR) and glucocorticoid receptor (GR) at the transcriptional level using mouse fibroblast cell lines harboring an integrated mouse mammary tumor virus (MMTV) promoter. We found that the AR, after induction with dihydrotestosterone (DHT), caused a progressive increase in MMTV-CAT reporter activity over 72 h which was correlated to an increase in chromatin remodeling of the MMTV promoter in the vicinity of the hormone response element (HRE). In contrast, stimulation of the GR by the synthetic glucocorticoid dexamethasone (Dex) caused a transient increase in MMTV transcriptional activity which returned to basal levels after 72 h. These changes were correlated to a transient increase in chromatin remodeling in the region of the HRE. Neither cotreatment nor pretreatment with Dex affected the DHT response. In fact, there was a more than additive effect of the two hormones on transcription at early time points. This suggests that the inability of GR to remodel chromatin, after 24 h of hormone treatment, is most likely related to changes in the GR itself and not the chromatin remodeling process. Consistent with this, nuclear GR levels dropped by greater than 50% after Dex treatment whereas the AR was induced fourfold after 24 h of DHT treatment. We conclude that a promoter with an ordered chromatin structure can still respond to androgens even after its glucocorticoid responsiveness is lost. This may be one mechanism cells utilize to establish target gene specificity for nuclear receptors that recognize identical DNA sequences.

Nuclear factor I-mediated repression of the mouse mammary tumor virus promoter is abrogated by the coactivators p300/CBP and SRC-1

To better understand the function of nuclear factor I (NFI) proteins in transcription, we have used transient transfection assays to assess transcriptional modulation by NFI proteins on the NFI-dependent mouse mammary tumor virus (MMTV) promoter. Expression of NFI-C or NFI-X, but not NFI-A or NFI-B proteins, represses glucocorticoid induction of the MMTV promoter in HeLa cells. Repression is DNA binding-independent as a deletion construct expressing the NH2-terminal 160 residues of NFI-C represses but does not bind DNA. Repression by NFI-C is cell type-dependent and occurs in HeLa and COS-1 cells but not 293 or JEG-3 cells. NFI-C does not repress progesterone induction of the MMTV promoter in HeLa cells, suggesting that progesterone induction of the promoter differs mechanistically from glucocorticoid induction. NFI-C-mediated repression is alleviated by overexpression of glucocorticoid receptor (GR), suggesting that NFI-C represses the MMTV promoter by preventing GR function. However, repression by NFI-C occurs with only a subset of glucocorticoid-responsive promoters, as the chimeric NFIGREbeta-gal promoter that is activated by GR is not repressed by NFI-C. Since the coactivator proteins p300/CBP, SRC-1A, and RAC3 had previously been shown to function at steroid hormone-responsive promoters, we asked whether they could influence NFI-C-mediated repression of MMTV expression. Expression of p300/CBP or SRC-1A alleviates repression by NFI-C, whereas RAC3 has no effect. This abrogation of NFI-C-mediated repression by p300/CBP and SRC-1A suggests that repression by NFI-C may occur by interference with coactivator function at the MMTV promoter.

Gene silencing by methyl-CpG-binding proteins

An important consequence of CpG methylation is the local silencing of gene expression. In part this can be mediated by direct interference of methylation with the binding of transcription factors. The major component of silencing, however, appears to be the binding of repressors that have an affinity for methyl-CpG. We have studied two proteins that bind to methylated DNA, methyl-CpG-binding protein 1 (MeCP1) and MeCP2. MeCP2 is a relatively abundant chromosomal protein whose localization in the nucleus is primarily dependent on CpG methylation. We find that MeCP2 is a potent transcriptional repressor with a genome-wide distribution. MeCP1 requires multiple methylated CpGs for binding and has previously been implicated as a methyl-CpG-dependent transcriptional repressor. Recent cloning of a candidate gene for a component of MeCP1 may provide clues to its mechanism of action.

Identification of glucocorticoid receptor domains necessary for transcriptional activation of the mouse mammary tumor virus promoter integrated in the genome

It has previously been determined that the mouse mammary tumor virus (MMTV) promoter when integrated in the genome assumes a defined chromatin structure which is disrupted upon addition of glucocorticoids. In contrast, a transiently introduced MMTV promoter has a random nucleoprotein structure. To reveal glucocorticoid receptor (GR) domains necessary for transcriptional activation of the MMTV promoter we compared the effects of mutations of the GR on transcriptional activation of the stably integrated versus transiently introduced MMTV promoter. For this purpose we generated a GR-negative cell line which has an MMTV promoter/reporter construct integrated in the genome and studied the transcriptional activation of this construct by different GR mutants introduced into the cells. Transcriptional activation of the integrated and transiently introduced promoter was achieved by the wild-type GR or a chimeric receptor in which the MR hormone-binding domain (HBD) replaced the GR HBD. In contrast, we found that deletion of the HBD of the GR or replacement of this region with the equivalent domain of the estrogen receptor produced receptors that were unable to activate the MMTV promoter integrated in the genome although these receptors efficiently activated the transiently introduced MMTV promoter. The HBD was not the sole determinant of MMTV transcriptional activation when integrated in the genome. Chimeric receptors which harbored the MR amino terminal domain or the wild-type MR were also unable to activate the integrated MMTV promoter. Taken together, these data indicate a rigid requirement for sequences in both the GR amino and the carboxy terminal domains for transcriptional activation of a hormone response element in the defined chromatin context of the MMTV promoter.

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.

Differential activity of progesterone and glucocorticoid receptors on mouse mammary tumor virus templates differing in chromatin structure

In vivo, transcription factors interact with promoters having complex nucleoprotein structures. The transiently expressed progesterone receptor (PR) efficiently activates a transfected mouse mammary tumor virus (MMTV) promoter but is a poor activator of the MMTV promoter when it acquires an ordered chromatin structure as an endogenous, replicating gene. We show that the deficiency in PR activity is not due to insufficient expression of either B or A isoforms or competition between the two types of MMTV templates. Rather, this deficiency reflects an inability to induce the chromatin remodeling event that is required for activation of the replicated MMTV template. To determine whether this characteristic is common to transiently expressed steroid receptors or specific to the PR, we examined the activity of transiently expressed glucocorticoid (GR) receptor. Unlike the PR, the transiently expressed GR is an effective activator of both MMTV templates and efficiently induces the necessary chromatin remodeling event at the replicated template. These results indicate that the GR and PR have unique requirements for activation of promoters with ordered chromatin structure. These differences may provide a mechanism for establishing target gene specificity in vivo for steroid receptors that recognize and bind to identical DNA sequences.

Induction properties of a transiently transfected glucocorticoid-responsive gene vary with glucocorticoid receptor concentration

Transient transfections of steroid receptors have yielded much of the data used to construct the current models of steroid hormone action. These experiments invariably examine the ability of receptors to regulate transcription when occupied by saturating concentrations of steroid. We now report that other induction properties of a transiently transfected gene are not constant but vary with the concentration of transiently transfected glucocorticoid receptors. Thus, the percentage of maximal induction seen with subsaturating concentrations of glucocorticoid could be dramatically increased, and an antiglucocorticoid could be converted into a partial glucocorticoid, simply by increasing the concentration of glucocorticoid receptors. This behavior was observed in HeLa cells, containing endogenous receptors, or in CV-1 cells, containing almost no endogenous receptor, with either homologous or heterologous receptors. These increases were relatively insensitive to the concentration of reporter gene, suggesting the titration of some transcription factor(s) involved in regulating the position of the glucocorticoid dose-response curve and the agonist activity of an antiglucocorticoid. This property of transfected glucocorticoid receptors required a full-length, functionally active receptor but was retained, albeit reduced in magnitude, in the absence of binding to a glucocorticoid response element. Furthermore, this phenomenon was specific in that the A form of the human progesterone receptor had no effect under the same conditions. These variations in induction properties of antiglucocorticoids and of subsaturating concentrations of glucocorticoid, in a manner that was proportional to the amount of transfected receptor, reveal processes that are not operative with saturating concentrations of glucocorticoid. These variations also demonstrate that caution should be exercised in making mechanistic conclusions based solely on experiments conducted with saturating concentrations of glucocorticoid.

Immunolocalization of estrogen receptor protein in the mouse blastocyst during normal and delayed implantation

We previously showed that estrogen receptor (ER) mRNA is present in preimplantation mouse embryos. The apparent synthesis of ER mRNA by the blastocyst at the time of implantation when estrogen is required was of special interest. A demonstration of the presence of ER protein would support the idea that estrogen can act directly on the embryo. The mouse embryo at the blastocyst stage is differentiated into two cell types, the trophectoderm and the inner cell mass. To determine whether ER mRNA is translated into ER protein and its cell-specific distribution, immunocytochemical analyses were performed in mouse blastocysts. ER protein was detected in all cell types of the normal, dormant, or activated blastocyst. To trace the fate of ER in these cell types, immunocytochemistry was performed in implanting blastocysts and early egg cylinder stage embryos developed in culture. Again, ER was detected in all cells of the implanting blastocyst. At the early egg cylinder stage, continued expression of ER was observed in cells derived from the inner cell mass or the trophoblast. In trophoblast giant cells, ER was concentrated in small regions of the nucleus, possibly the nucleoli, which was similar to that observed in dormant and activated blastocysts. The embryonic expression of ER at such early stages in a broad array of cells suggests that ER may have a general role during early development.

Glucocorticoids and protein kinase A coordinately modulate transcription factor recruitment at a glucocorticoid-responsive unit

The rat tyrosine aminotransferase gene is a model system to study transcriptional regulation by glucocorticoid hormones. We analyzed transcription factor binding to the tyrosine aminotransferase gene glucocorticoid-responsive unit (GRU) at kb -2.5, using in vivo footprinting studies with both dimethyl sulfate and DNase I. At this GRU, glucocorticoid activation triggers a disruption of the nucleosomal structure. We show here that various regulatory pathways affect transcription factor binding to this GRU. The binding differs in two closely related glucocorticoid-responsive hepatoma cell lines. In line H4II, glucocorticoid induction promotes the recruitment of hepatocyte nuclear factor 3 (HNF3), presumably through the nucleosomal disruption. However, the footprint of the glucocorticoid receptor (GR) is not visible, even though a regular but transient interaction of the GR is necessary to maintain HNF3 binding. In contrast, in line FTO2B, HNF3 binds to the GRU in the absence of glucocorticoids and nucleosomal disruption, showing that a "closed" chromatin conformation does not repress the binding of certain transcription factors in a uniform manner. In FTO2B cells, the footprint of the GR is detectable, but this requires the activation of protein kinase A. In addition, protein kinase A stimulation also improves the recruitment of HNF3 independently of glucocorticoids and enhances the glucocorticoid response mediated by this GRU in an HNF3-dependent manner. In conclusion, the differences in the behavior of this regulatory sequence in the two cell lines show that various regulatory pathways are integrated at this GRU through modulation of interrelated events: transcription factor binding to DNA and nucleosomal disruption.

Nucleosome positioning on the MMTV LTR results from the frequency-biased occupancy of multiple frames

The translational positions of nucleosomes in the promoter region of the mouse mammary tumor virus (MMTV) were defined at high resolution. Nucleosome boundaries were determined in primer extension assays using full-length single-stranded mononucleosomal DNA prepared from cells treated with formaldehyde, a reversible protein-DNA cross-linking agent. Multiple boundaries were observed in both the nucleosome A (Nuc-A) and Nuc-B region of the promoter, indicating multiple nucleosome translational frames. The different nucleosome frames in both the Nuc-A and Nuc-B regions were occupied unequally. The most frequently occupied frames were found clustered within 50-60 bases of each other, resulting in a distribution centered in the positions defined previously at low resolution for Nuc-A and Nuc-B. The most abundant 5' ends of the frames in the B region were found between -235 and -187, and the 3' ends between -86 and -36, whereas in the A region the most abundant 5' ends were between -22 and +42, and the 3' ends between +121 and +186. Although frames in the Nuc-B region of the LTR extend at a low frequency in the 5' direction toward the Nuc-C region, there is a sharp discontinuity in the 3' direction toward Nuc-A, suggesting the presence of a boundary constraint in the A-B linker. The positions and relative occupancies of nucleosome frames, in either the B or the A region, did not change when the promoter was activated with dexamethasone.

A functional cyclic AMP response element plays a crucial role in neuroendocrine cell type-specific expression of the secretory granule protein chromogranin A

Chromogranin A, a soluble acidic protein, is a ubiquitous component of secretory vesicles throughout the neuroendocrine system. We reported previously the cloning and initial characterization of the mouse chromogranin A gene promoter, which showed that the promoter contains both positive and negative domains and that a proximal promoter spanning nucleotides -147 to +42 bp relative to the transcriptional start site is sufficient for neuroendocrine cell type-specific expression. The current study was undertaken to identify the particular elements within this proximal promoter that control tissue-specific expression. We found that deletion or point mutations in the potential cAMP response element (CRE) site at -68 bp virtually abolished promoter activity specifically in neuroendocrine (PC12 chromaffin or AtT20 corticotrope) cells, with little effect on activity in control (NIH3T3 fibroblast) cells; thus, the CRE box is necessary for neuroendocrine cell type-specific activity of the chromogranin A promoter. Furthermore, the effect of the CRE site is enhanced in the context of intact (wild-type) promoter sequences between -147 and -100 bp. DNase I footprint analysis showed that these regions (including the CRE box) bind nuclear proteins present in both neuroendocrine (AtT20) and control (NIH3T3) cells. In AtT20 cells, electrophoretic mobility shift assays and factor-specific antibody supershifts showed that an oligonucleotide containing the chromogranin A CRE site formed a single, homogeneous protein-DNA complex containing the CRE-binding protein CREB. However, in control NIH3T3 cells we found evidence for an additional immunologically unrelated protein in this complex. A single copy of this oligonucleotide was able to confer neuroendocrine-specific expression to a heterologous (thymidine kinase) promoter, albeit with less fold selectivity than the full proximal chromogranin A promoter. Hence, the CRE site was partially sufficient to explain the neuroendocrine cell type specificity of the promoter. The functional activity of the CRE site was confirmed through studies of the endogenous chromogranin A gene. Northern mRNA analysis showed that expression of the endogenous chromogranin A gene was stimulated seven- to eightfold by cAMP in PC12 cells, whereas no induction occurred in the NIH3T3 cells. Similar cAMP induction was obtained with the transfected chromogranin A promoter in PC12 cells, and abolition of the CRE site (by deletion or point mutation) eliminated the induction. Thus, the CRE site in the chromogranin A proximal promoter is functional and plays a crucial, indeed indispensable, role in neuroendocrine-specific expression of the gene. These results also provide insight into transcriptional mechanisms governing acquisition of the neuroendocrine secretory phenotype.

Nucleoprotein structure influences the response of the mouse mammary tumor virus promoter to activation of the cyclic AMP signalling pathway

Recent studies have provided evidence of crosstalk between steroid receptors and cyclic AMP (cAMP) signalling pathways in the regulation of gene expression. A synergism between intracellular phosphorylation inducers and either glucocorticoids or progestins has been shown to occur during activation of the mouse mammary tumor virus (MMTV) promoter. We have investigated the effect of 8-Br-cAMP and okadaic acid, modulators of cellular kinases and phosphatases, on the hormone-induced activation of the MMTV promoter in two forms: a transiently transfected template with a disorganized, accessible nucleoprotein structure and a stably replicating template with an ordered, inaccessible nucleoprotein structure. Both okadaic acid and 8-Br-cAMP synergize significantly with either glucocorticoids or progestins in activating the transiently transfected MMTV template. In contrast, 8-Br-cAMP, but not okadaic acid, is antagonistic to hormone-induced activation of the stably replicating MMTV template. Nuclear run-on experiments demonstrate that this inhibition is a transcriptional effect on both hormone-induced transcription and basal transcription. Surprisingly, 8-Br-cAMP does not inhibit glucocorticoid-induced changes in restriction enzyme access and nuclear factor 1 binding. However, association of a complex with the TATA box region is inhibited in the presence of 8-Br-cAMP. Thus, cAMP treatment interferes with the initiation process but does not inhibit interaction of the receptor with the template. Since the replicated, ordered MMTV templates and the transfected, disorganized templates show opposite responses to 8-Br-cAMP treatment, we conclude that chromatin structure can influence the response of a promoter to activation of the cAMP signalling pathway.

Mouse mammary tumor virus chromatin in human breast cancer cells is constitutively hypersensitive and exhibits steroid hormone-independent loading of transcription factors in vivo

We have stably introduced a reporter gene under the control of the mouse mammary tumor virus (MMTV) long terminal repeat (LTR) into human T47D breast cancer cells to study the action of the progesterone receptor (PR) on transcription from a chromatin template. Unexpectedly, the chromatin organization of the MMTV LTR in these human breast cancer cells differed markedly from what we have observed previously. The region adjacent to the transcription start site (-221 to -75) was found to be constitutively hypersensitive to restriction enzyme cleavage in the absence of hormone. This region is normally encompassed within the second nucleosome of a phased array of six nucleosomes that is assembled when the MMTV LTR is stably maintained in mouse cells. Characteristically, in these rodent cells, the identical DNA sequences show increased restriction enzyme cleavage only in the presence of glucocorticoid. The increased access of restriction enzymes observed in the human PR+ cells was not observed in adjacent nucleosomes and was unaffected by treatment with the progesterone antagonist RU486. In addition, exonuclease III-dependent stops corresponding to the binding sites for nuclear factor 1 and the PR were observed before and after hormone treatment. These results indicate that MMTV chromatin replicated in these cells is organized into a constitutively open architecture and that this open chromatin state is accompanied by hormone-independent loading of a transcription factor complex that is normally excluded from uninduced chromatin.

Antiprogestins prevent progesterone receptor binding to hormone responsive elements in vivo

Antiprogestins inhibit progesterone action by competing for binding to the progesterone receptor and are potentially important pharmaceuticals in fertility control and cancer therapy. Why the complex of antiprogestins and progesterone receptor is functionally inactive is unclear. Present models are based on indirect evidence, such as transfection competition assays and in vitro DNA binding studies, partly because of difficulties in visualizing the receptor bound to DNA in vivo. Here we used genomic footprinting analysis to show ligand-dependent binding of endogenous progesterone receptor to the hormone responsive elements (HREs) of a chromosomally integrated mouse mammary tumor virus long terminal repeat in a human mammary carcinoma cell line. The antiprogestins RU 486 and ZK 98299 do not promote binding of the progesterone receptor to this natural HRE in vivo, even at concentrations that completely inhibit the agonistic effects of potent synthetic progestins. Moreover, antiprogestins cause a rapid disappearance of the agonist-induced progesterone receptor footprint. We conclude that antiprogestins interfere with receptor function by preventing its specific DNA binding.