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

Mouse Mammary Tumor Virus (MMTV) and MMTV-like Viruses: An In-depth Look at a Controversial Issue

Since its discovery as a milk factor, mouse mammary tumor virus (MMTV) has been shown to cause mammary carcinoma and lymphoma in mice. MMTV infection depends upon a viral superantigen (sag)-induced immune response and exploits the immune system to establish infection in mammary epithelial cells when they actively divide. Simultaneously, it avoids immune responses, causing tumors through insertional mutagenesis and clonal expansion. Early studies identified antigens and sequences belonging to a virus homologous to MMTV in human samples. Several pieces of evidence fulfill a criterion for a possible causal role for the MMTV-like virus in human breast cancer (BC), though the controversy about whether this virus was linked to BC has raged for over 40 years in the literature. In this review, the most important issues related to MMTV, from its discovery to the present days, are retraced to fully explore such a controversial issue. Furthermore, the hypothesis of an MMTV-like virus raised the question of a potential zoonotic mouse–man transmission. Several studies investigate the role of an MMTV-like virus in companion animals, suggesting their possible role as mediators. Finally, the possibility of an MMTV-like virus as a cause of human BC opens a new era for prevention and therapy.

The Glucocorticoid Receptor Regulates the ANGPTL4 Gene in a CTCF-Mediated Chromatin Context in Human Hepatic Cells

Glucocorticoid signaling through the glucocorticoid receptor (GR) plays essential roles in the response to stress and in energy metabolism. This hormonal action is integrated to the transcriptional control of GR-target genes in a cell type-specific and condition-dependent manner. In the present study, we found that the GR regulates the angiopoietin-like 4 gene (ANGPTL4) in a CCCTC-binding factor (CTCF)-mediated chromatin context in the human hepatic HepG2 cells. There are at least four CTCF-enriched sites and two GR-binding sites within the ANGPTL4 locus. Among them, the major CTCF-enriched site is positioned near the ANGPTL4 enhancer that binds GR. We showed that CTCF is required for induction and subsequent silencing of ANGPTL4 expression in response to dexamethasone (Dex) and that transcription is diminished after long-term treatment with Dex. Although the ANGPTL4 locus maintains a stable higher-order chromatin conformation in the presence and absence of Dex, the Dex-bound GR activated transcription of ANGPTL4 but not that of the neighboring three genes through interactions among the ANGPTL4 enhancer, promoter, and CTCF sites. These results reveal that liganded GR spatiotemporally controls ANGPTL4 transcription in a chromosomal context.

The HSP90 molecular chaperone cycle regulates cyclical transcriptional dynamics of the glucocorticoid receptor and its coregulatory molecules CBP/p300 during ultradian ligand treatment

HSP90 regulates cyclical glucocorticoid receptor activity, cofactor recruitment, histone acetylation and transcriptional pulsing at the Period 1 promoter in response to ultradian glucocorticoid exposure.

Glucocorticoid (GC) hormones are secreted from the adrenal gland in a characteristic pulsatile pattern. This ultradian secretory activity exhibits remarkable plasticity, with distinct changes in response to both physiological and stressful stimuli in humans and experimental animals. It is therefore important to understand how the pattern of GC exposure regulates intracellular signaling through the GC receptor (GR). We have previously shown that each pulse of ligand initiates rapid, transient GR activation in several physiologically relevant and functionally diverse target cell types. Using chromatin immunoprecipitation assays, we detect cyclical shifts in the net equilibrium position of GR association with regulatory elements of GC-target genes and have investigated in detail the mechanism of pulsatile transcriptional regulation of the GC-induced Period 1 gene. Transient recruitment of the histone acetyl transferase complex cAMP response element-binding protein (CREB) binding protein (CBP)/p300 is found to precisely track the ultradian hormone rhythm, resulting in transient localized net changes in lysine acetylation at GC-regulatory regions after each pulse. Pulsatile changes in histone H4 acetylation and concomitant recruitment of RNA polymerase 2 precede ultradian bursts of Period 1 gene transcription. Finally, we report the crucial underlying role of the intranuclear heat shock protein 90 molecular chaperone complex in pulsatile GR regulation. Pharmacological interference of heat shock protein 90 (HSP90) with geldanamycin during the intranuclear chaperone cycle completely ablated GR's cyclical activity, cyclical cAMP response element-binding protein (CREB) binding protein (CBP)/p300 recruitment, and the associated cyclical acetylation at the promoter region. These data imply a key role for an intact nuclear chaperone cycle in cyclical transcriptional responses, regulated in time by the pattern of pulsatile hormone.

DNA of mouse mammary tumor virus-like virus is present in human tumors influenced by hormones

Mouse mammary tumor virus (MMTV) is a hormonally regulated, oncogenic virus of mice. MMTV-like virus DNA has previously been detected in human breast cancers, liver disease, and liver cancers. It is hypothesized that local hormonal effects might be of primary importance in determining MMTV-like virus detection in human tumors. MMTV-like virus envelope (env) DNA was determined using nested PCR in 89 ovarian, 147 prostate, 50 endometrial, 141 skin, and 51 lung cancers. Viral-positive sequences were compared with published MMTV-like viral sequences from human breast cancer, liver cancer and MMTV. Immunohistochemistry for estrogen receptor (ER-alpha) and progesterone receptor (PgR) was performed on a subset of tumors. MMTV-like virus env DNA was detected in ovarian cancers (14/89; 16%), prostate cancers (53/147; 36%), endometrial cancers (5/50; 10%), skin cancers (13/141; 9%) but not in lung cancers (0/51). Phylogenetic analysis of the viral-positive sequences showed no clustering of the isolates according to tissue type. A significant association was observed between the presence of hormone receptors and detection of MMTV-like virus in the human cancers screened (P = 0.01). A significant association between MMTV-like virus and PgR was noted in skin cancers (P = 0.003). Therefore, unlike the mouse model, the detection of MMTV-like env sequences in human cancers in addition to breast indicates that MMTV-like viral expression is not breast cancer-specific and may relate to hormone-dependent viral expression.

Structural and functional relationships of the steroid hormone receptors' N-terminal transactivation domain

Steroid hormone receptors are members of a family of ligand inducible transcription factors, and regulate the transcriptional activation of target genes by recruiting coregulatory proteins to the pre-initiation machinery. The binding of these coregulatory proteins to the steroid hormone receptors is often mediated through their two activation functional domains, AF1, which resides in the N-terminal domain, and the ligand-dependent AF2, which is localized in the C-terminal ligand-binding domain. Compared to other important functional domains of the steroid hormone receptors, our understanding of the mechanisms of action of the AF1 are incomplete, in part, due to the fact that, in solution, AF1 is intrinsically disordered (ID). However, recent studies have shown that AF1 must adopt a functionally active and folded conformation for its optimal activity under physiological conditions. In this review, we summarize and discuss current knowledge regarding the molecular mechanisms of AF1-mediated gene activation, focusing on AF1 conformation and coactivator binding. We further propose models for the binding/folding of the AF1 domains of the steroid hormone receptors and their protein:protein interactions. The population of ID AF1 can be visualized as a collection of many different conformations, some of which may be assuming the proper functional folding for other critical target binding partners that result in the ultimate assembly of AF1:coactivator complexes and subsequent gene regulation. Knowledge of the mechanisms involved therein will significantly help in understanding how signals from a steroid to a specific target gene are conveyed.

Intrinsic promoter nucleosome stability/dynamics variations support a novel targeting mechanism

Genomic processes like transcription initiation typically involve the alteration of nucleosome structure, to expose DNA elements for regulatory factor binding. Nucleosome altering/modifying complexes have been identified, but precisely how these complexes find their specific targets remains unclear. We have shown that nucleosomes can exhibit significant DNA sequence-dependent stability and dynamics variations and have suggested that these inherent variations could facilitate nucleosome recognition and thus aid in specific targeting. Here, we confirm an important prediction of the model, namely, that stability and DNA dynamics features can correlate with the transcriptional involvement of specific promoter nucleosomes. A transcriptionally inert Mouse Mammary Tumor Virus promoter-region nucleosome (MMTV-D) has greater inherent stability than and reduced dynamics compared to a nearby nucleosome (MMTV-B) that is the initial target of transcription activation-associated processes on this promoter. MMTV-D stability could help direct activation-associated events to the less stable and more dynamic target, MMTV-B.

Glucocorticoid receptor dynamics and gene regulation

The glucocorticoid receptor regulates the expression of a large number of genes in mammalian cells. The interaction of this receptor with regulatory elements has been discovered to be highly dynamic, with occupancy states measured in seconds, rather than minutes or hours. This finding has led to a paradigm shift in our understanding of receptor function throughout the genome. The mechanisms involved in these rapid exchange events, as well as the implications for receptor function, are discussed.

Single-cell analysis of glucocorticoid receptor action reveals that stochastic post-chromatin association mechanisms regulate ligand-specific transcription

The glucocorticoid receptor (GR) dynamically interacts with response elements in the mouse mammary tumor virus (MMTV) promoter to regulate steroid-dependent transcription. In a clonal mammary carcinoma cell line containing a tandem array of MMTV promoter-reporter gene cassettes integrated at a single genomic locus, direct binding of a green fluorescent protein (GFP)-GR fusion protein to the MMTV regulatory elements can be observed in living cells. After ligand treatment, MMTV-dependent transcription in individual cells was detected by RNA fluorescence in situ hybridization (FISH). High-resolution fluorescence images were acquired from large numbers of randomly selected cells. Images were analyzed with a novel automated computer algorithm, measuring the RNA FISH signal and the relative GFP-GR fluorescence intensity at the MMTV array for each cell. Although dexamethasone increased the mean RNA FISH signal approximately 10-fold, RU486 produced only about a 2-fold induction, as expected for this mixed antagonist. For all treatment conditions, the relative GFP-GR fluorescence at the array for the averaged cells paralleled the RNA FISH measurements, suggesting that image analysis accurately detected an increase in steady-state GR association with the MMTV array that was responsible for the increase in transcriptional activity. The antagonist-dependent decreases in GR association with the MMTV promoter were confirmed by chromatin immunoprecipitation experiments, supporting the image analysis results. A pronounced cell-to-cell variability was observed in RNA FISH signal and GR-MMTV association within treatment groups. We observed a nonlinear relationship between GR-MMTV association and RNA FISH in individual cells, indicating that differences in GR-MMTV interaction account for some, but not all, of the transcriptional heterogeneity between individual cells. In selected cell subpopulations with equal levels of GR-MMTV association, there was a decrease in RNA FISH signal with RU486 treatment compared with dexamethasone treatment. These results indicate that stochastic events occurring after GR-promoter association, such as the actions of chromatin remodeling complexes or other cofactors, change in a ligand-dependent manner and regulate heterogeneous transcription in individual cells.

Retrotransposons and regulatory suites

Cellular differentiation and multicellular development require the programmed expression of coregulated suites of genetic loci dispersed throughout the genome. How do functionally diverse loci come to share common regulatory motifs? A new paper finds that retrotransposons (RTEs) may play a role in providing common regulation to a group of functions expressed during the development of oocytes and preimplantation embryos. Examining cDNA libraries, Peaston et al. find that 13% of all processed transcripts in full-grown mouse oocytes contain RTE sequences, mostly from the MT family of retroviral-like elements. Smaller but still significant percentages of RTE sequences are found in cDNA libraries from 2-cell embryos and blastocysts. A quarter of these RTE sequences are at the 5' ends of chimeric transcripts that also contain exons from endogenous mouse loci. These chimeric transcripts display restricted expression in oocytes and preimplantation embryos and presumably originate from developmentally regulated LTR promoters. Some, but not all, chimeric transcripts encode novel protein products.

The distinct agonistic properties of the phenylpyrazolosteroid cortivazol reveal interdomain communication within the glucocorticoid receptor

Recent structural analyses of the nuclear receptors establish a paradigm of receptor activation, in which agonist binding induces the ligand binding domain (LBD)/activation function-2 helix to form a charge clamp for coactivator recruitment. However, these analyses have not sufficiently addressed the mechanisms for differential actions of various synthetic steroids in terms of fine tuning of multiple functions of whole receptor molecules. In the present study, we used the glucocorticoid receptor (GR)-specific agonist cortivazol (CVZ) to probe the plasticity and functional modularity of the GR. Structural docking analysis revealed that although CVZ is more bulky than other agonists, it can be accommodated in the ligand binding pocket of the GR by reorientation of several amino acid side chains but without major alterations in the active conformation of the LBD. In this induced fit model, the phenylpyrazole A-ring of CVZ establishes additional contacts with helices 3 and 5 of the LBD that may contribute to a more stable LBD configuration. Structural and functional analysis revealed that CVZ is able to compensate for the deleterious effects of a C-terminal deletion of the LBD in a manner that mimics the stabilizing influence of the F602S point mutation. CVZ-mediated productive recruitment of transcriptional intermediary factor 2 to the C-terminally deleted LBD requires the receptor's own DNA binding domain and is positively influenced by the N-terminal regions of GR or progesterone receptor. These results support a model where ligand-dependent conformational changes in the LBD play a role in GR-mediated gene regulation via modular interaction with the DBD and activation function-1.

Chromatin-dependent E1A activity modulates NF-kappaB RelA-mediated repression of glucocorticoid receptor-dependent transcription

The role of chromatin-dependent regulatory mechanisms in the repression of glucocorticoid-dependent transcription from the murine mammary tumor virus (MMTV) promoter by p65 and E1A was investigated by using chromatin and transiently transfected reporters. The p65 RelA subunit of NF-kappaB represses MMTV expression on either transient or integrated reporters. In contrast, the viral oncoprotein E1A represses a transient but not an integrated MMTV. E1A repression is attenuated by chromatin, suggesting p65 but not E1A manipulates chromatin appropriately to inhibit the GR. Coexpression of p65 and E1A additively represses the transient MMTV but restores the transcriptional activation of the chromatin MMTV in response to glucocorticoids. This indicates that E1A has a dominant chromatin-dependent activity that attenuates repression by p65. E1A, p65, and GR bind the MMTV promoter, and chromatin remodeling enhances binding on both repressed and activated promoters. In addition, p65 requires Brg for repression of the integrated MMTV. This suggests that neither p65 repression nor E1A attenuation of repression results from an inhibition of remodeling that prevents transcription factor binding. Furthermore, p300/CBP is also required for both repression and attenuation by p65 and E1A. E1A and p65 mutants that do not bind p300/CBP are inactive, indicative of a requirement for p300/CBP-dependent complex formation for both repression and attenuation with chromatin. These data suggest that both the p65-dependent repression and the E1A-mediated attenuation of repression require the Brg1-dependent chromatin remodeling function and p300/CBP-dependent complex formation at a promoter assembled within chromatin.

Liver receptor homolog-1 stimulates the progesterone biosynthetic pathway during follicle-stimulating hormone-induced granulosa cell differentiation

FSH-stimulated granulosa cell differentiation is associated with the induction of the LH receptor (LHr) as well as induction of the estrogen and progesterone biosynthetic pathways. Although activation of the cAMP-protein kinase A pathway is sufficient to stimulate progesterone production, additional pathways are required for the induction of the LHr and p450 aromatase. The orphan nuclear receptor, liver receptor homolog-1 (LRH-1), is expressed in granulosa cells and has been shown to synergize with the cAMP signaling system to regulate the gonadal type II aromatase promoter in transient transfection assays. To determine whether LRH-1 can interact with the cAMP pathway in the induction of aromatase and the LHr, we examined the effects of an adenoviral vector that directs the expression of human LRH-1 (Ad-LRH-1) on FSH-stimulated granulosa cell differentiation. Infection of undifferentiated granulosa cells with LRH-1 alone had no effect on estrogen production, progesterone production, or the expression of the LHr. However, combination of FSH stimulation and Ad-LRH-1 infection led to significantly greater progesterone production and increases in mRNA for p450 side-chain cleavage and 3beta-hydroxysteroid dehydrogenase than granulosa cells stimulated by FSH alone. However, infection with Ad-LRH-1 did not stimulate estradiol production or increases in mRNA for p450 aromatase or the LHr above that seen with FSH treatment alone. Moreover, infection with Ad-LRH-1 was able to overcome H-89 inhibition of FSH-stimulated progesterone but not estrogen production. Collectively, these observations support a direct role for LRH-1 in the induction of the progesterone but not the estrogen biosynthetic pathway during granulosa cell differentiation.

Suv39h histone methyltransferases interact with Smads and cooperate in BMP-induced repression

Smad proteins transduce signals from transforming growth factor-beta (TGF-beta) superfamily ligands to regulate the expression of target genes. In order to identify novel partners of Smad proteins in transcriptional regulation, we performed a two-hybrid screen using Smad5, a protein that is activated predominantly by bone morphogenetic protein (BMP) signaling. We identified an interaction between Smad5 and suppressor of variegation 3-9 homolog 2 (Suv39h2), a chromatin modifier enzyme. Suv39h proteins are histone methyltransferases that methylate histone H3 on lysine 9, resulting in transcriptional repression or silencing of target genes. Biochemical studies in mammalian cells demonstrated that Smad5 binds to both known mammalian isoforms of Suv39h proteins, and that Smad proteins activated by the TGF-beta signaling pathway, Smad2 and Smad3, do not bind with significant affinity. Functional studies using the muscle creatine kinase (MCK) promoter, which is suppressed by BMP signaling, demonstrate that Suv39h proteins and Smads cooperate to repress promoter activity. These data suggest a model where association of Smad proteins with Suv39h methyltransferases can repress or silence genes involved in developmental processes, and argues that inefficient gene repression may result in the alteration of the differentiated phenotype. Thus, examination of the Smad-Suv interaction may provide insight into the mechanism of phenotypic determination mediated by BMP signaling.

Dynamics of nuclear receptor movement and transcription

Following a hormone signal, steroid/nuclear receptors bind regulatory elements in chromatin and initiate the recruitment of a variety of multi-protein complexes to promoter sequences. These complexes ultimately lead to the recruitment of general transcription factors and the initiation of transcription. Traditional models suggest that these factors remain statically bound to each other and to chromatin until other signals are received to reduce transcription. Recent findings demonstrate that the processes and actions involved are much more complex than traditional models convey, and that the movement of receptors and coactivators is remarkably dynamic. Transcription factors are highly mobile in the nuclear environment, and interact only briefly with target sites in the nucleus. As a result of these transient interactions, promoters move through many states during activation and repression. Two general concepts emerge from current data: (1) Various transcription factors appear to follow "ordered recruitment" to promoters on a time scale of minutes to hours in response to a stimulus. During this response, the proteins that interact with chromatin may cycle on and off the promoter multiple times. (2) During these ordered recruitment cycles, the individual molecules that form functional complexes often exchange rapidly on a time scale of seconds. This rapid exchange of molecules within a formed complex occurs independently of long-term cycling on chromatin. Several processes are implicated in rapid nuclear dynamics, including potential roles for molecular chaperones, the proteasome degradation machinery and chromatin remodeling complexes.

The interplay between the glucocorticoid receptor and nuclear factor-kappaB or activator protein-1: molecular mechanisms for gene repression

The inflammatory response is a highly regulated physiological process that is critically important for homeostasis. A precise physiological control of inflammation allows a timely reaction to invading pathogens or to other insults without causing overreaction liable to damage the host. The cellular signaling pathways identified as important regulators of inflammation are the signal transduction cascades mediated by the nuclear factor-kappaB and the activator protein-1, which can both be modulated by glucocorticoids. Their use in the clinic includes treatment of rheumatoid arthritis, asthma, allograft rejection, and allergic skin diseases. Although glucocorticoids have been widely used since the late 1940s, the molecular mechanisms responsible for their antiinflammatory activity are still under investigation. The various molecular pathways proposed so far are discussed in more detail.

Transcription activation by the ecdysone receptor (EcR/USP): identification of activation functions

The ecdysone receptor is a heterodimer of the two nuclear receptors EcR and ultraspiracle (USP). We have identified the regions of Drosophila EcR and USP responsible for transcriptional activation of a semisynthetic Eip71CD promoter in Kc cells. The isoform-specific A/B domains of EcR-B1 and B2, but not those of EcR-A or USP, exhibit strong activation activity [activation function 1 (AF1)], both in isolation and in the context of the intact receptor. AF1 activity in isoform B1 derives from dispersed elements; the B2-specific AF1 consists of a 17-residue amphipathic helix. AF2 function was studied using a two-hybrid assay in Kc cells, based on the observation that potent hormone-dependent activation by the EcR/USP ligand-binding domain heterodimer requires the participation of both partners. Mutagenesis reveals that AF2 function depends on EcR helix 12, but not on the cognate USP region. EcR helix 12 mutants (F645A and W650A) exhibit a dominant negative phenotype. Thus, in the setting tested, the ecdysone receptor can activate transcription using the AF1 regions of EcR-B1 or -B2 and the AF2 region of EcR. USP acts as an allosteric effector for EcR, but does not contribute any intrinsic function.

EcR isoforms in Drosophila: testing tissue-specific requirements by targeted blockade and rescue

The three Drosophila EcR isoforms differ only at their N termini; thus, they share the conserved ligand-binding domain transcriptional activation function (AF2) and only differ in the unconserved A/B region, which contains a second, isoform-specific, activation function (AF1). We have developed a dominant-negative mutant EcR (EcR-DN), expressed it in flies with the GAL4/UAS system, and used it to block ecdysone signaling in eight tissues or groups of tissues. Localized EcR-DN arrests ecdysone-dependent development in the target cells and often--because of a molting checkpoint--arrests development globally. Simultaneously expressing individual wild-type EcR isoforms in the same target tissues suppresses the EcR-DN phenotype and identifies the rescuing isoform as sufficient to support the development of the target. Every isoform, and even an N-terminal truncated EcR that lacks any AF1, supports development in the fat body, eye discs, salivary glands, EH-secreting neurosecretory cells and in the dpp expression domain, implying that AF1 is dispensable in these tissues. By contrast, only EcR-A is able to support development in the margins of the wing discs, and only EcR-B2 can do so in the larval epidermis and the border cells of the developing egg chamber. In light of our results, the simplest explanations for the widespread spatial and temporal variations in EcR isoform titers appear untenable.