Dexamethasone-mediated induction of mouse mammary tumor virus RNA: a system for studying glucocorticoid action

Enhanced transgene expression from single-stranded AAV vectors in human cells in vitro and in murine hepatocytes in vivo

We identified that distal 10 nucleotides in the D-sequence in AAV2 inverted terminal repeat (ITR) share partial sequence homology to 1/2 binding site of glucocorticoid receptor-binding element (GRE). Here, we describe that (1) purified GR binds to AAV2 D-sequence, and the D-sequence competes with GR binding to its cognate binding site; (2) dexamethasone-mediated activation of GR pathway significantly increases the transduction efficiency of AAV2 vectors in human cells; (3) human osteosarcoma cells, U2OS, which lack expression of GR, are poorly transduced by AAV2 vectors, but stable transfection with a GR expression plasmid restores vector-mediated transgene expression; (4) replacement of the distal 10 nucleotides in the D-sequence of the AAV2 ITR with a full-length GRE consensus sequence significantly enhances transgene expression in human cells in vitro and in murine hepatocytes in vivo; and (5) none of the ITRs in AAV1, AAV3, AAV4, AAV5, and AAV6 genomes contains the GRE 1/2 binding site, and insertion of a full-length GRE consensus sequence in the AAV6-ITR also significantly enhances transgene expression from AAV6 vectors, both in vitro and in vivo. These novel vectors, termed generation Y AAV vectors, which are serotype, transgene, or promoter agnostic, should be useful in human gene therapy.

90 YEARS OF PROGESTERONE: Molecular mechanisms of progesterone receptor action on the breast cancer genome

Gene regulation by steroid hormones has been at the forefront in elucidating the intricacies of transcriptional regulation in eukaryotes ever since the discovery by Karlson and Clever that the insect steroid hormone ecdysone induces chromatin puffs in giant chromosomes. After the successful cloning of the hormone receptors toward the end of the past century, detailed mechanistic insight emerged in some model systems, in particular the MMTV provirus. With the arrival of next generation DNA sequencing and the omics techniques, we have gained even further insight into the global cellular response to steroid hormones that in the past decades also extended to the function of the 3D genome topology. More recently, advances in high resolution microcopy, single cell genomics and the new vision of liquid-liquid phase transitions in the context of nuclear space bring us closer than ever to unravelling the logic of gene regulation and its complex integration of global cellular signaling networks. Using the function of progesterone and its cellular receptor in breast cancer cells, we will briefly summarize the history and describe the present extent of our knowledge on how regulatory proteins deal with the chromatin structure to gain access to DNA sequences and interpret the genomic instructions that enable cells to respond selectively to external signals by reshaping their gene regulatory networks.

Transgenic mouse models--a seminal breakthrough in oncogene research

Transgenic mouse models are an integral part of modern cancer research, providing a versatile and powerful means of studying tumor initiation and progression, metastasis, and therapy. The present repertoire of these models is very diverse, with a wide range of strategies used to induce tumorigenesis by expressing dominant-acting oncogenes or disrupting the function of tumor-suppressor genes, often in a highly tissue-specific manner. Much of the current technology used in the creation and characterization of transgenic mouse models of cancer will be discussed in depth elsewhere. However, to gain a complete appreciation and understanding of these complex models, it is important to review the history of the field. Transgenic mouse models of cancer evolved as a new and, compared with the early cell-culture-based techniques, more physiologically relevant approach for studying the properties and transforming capacities of oncogenes. Here, we will describe early transgenic mouse models of cancer based on tissue-specific expression of oncogenes and discuss their impact on the development of this still rapidly growing field.

Fifty years ago: the quest for steroid hormone receptors

In 1963 Peter Karlson put forward the revolutionary "hormone-gene" hypothesis, which would change drastically the way in which steroid hormones were thought to act at the time. From a historical perspective, this review relates the acceptance of this initially controversial idea, the discovery of the steroid receptors and the key experiments that have led to the current understanding of the mechanism of steroid hormone action. It shows how, over 50years, the field has widened beyond all expectation and has contributed to major advances not only in endocrinology, but also in molecular biology, pharmacology and therapeutics.

Nuclear receptors and their selective pharmacologic modulators

Nuclear receptors are ligand-activated transcription factors and include the receptors for steroid hormones, lipophilic vitamins, sterols, and bile acids. These receptors serve as targets for development of myriad drugs that target a range of disorders. Classically defined ligands that bind to the ligand-binding domain of nuclear receptors, whether they are endogenous or synthetic, either activate receptor activity (agonists) or block activation (antagonists) and due to the ability to alter activity of the receptors are often termed receptor "modulators." The complex pharmacology of nuclear receptors has provided a class of ligands distinct from these simple modulators where ligands display agonist/partial agonist/antagonist function in a tissue or gene selective manner. This class of ligands is defined as selective modulators. Here, we review the development and pharmacology of a range of selective nuclear receptor modulators.

It takes two to tango: dimerisation of glucocorticoid receptor and its anti-inflammatory functions

For a number of years, there has been a widespread view that the adverse side-effects of prolonged glucocorticoid (GC) treatment are a result of glucocorticoid receptor (GR)-mediated gene activation, whilst the beneficial anti-inflammatory effects result from GR-mediated 'transrepression'. Since the introduction of the dimerisation-deficient GR mutant, GR(dim), was apparently unable to activate gene transcription, yet still able to repress pro-inflammatory gene transcription, the search for novel GR modulators has centred on the separation of gene activation from repression by prevention of GR dimerisation. However, recent work has questioned the conclusions drawn from these early GR(dim) studies, with evidence that GR(dim) mutants not only activate gene transcription, but that, in direct contradiction to the initial GR(dim) work, are also capable of forming dimers. This review of the current literature highlights the versatility of the GR in forming homodimer interactions, as well as the ability to bind to alternate nuclear receptors, and investigates the potential implications such varying GR dimer conformations may have for the design of GR ligands with a safer side effect profile.

Non-steroidal dissociated glucocorticoid agonists: indoles as A-ring mimetics and function-regulating pharmacophores

We report a SAR of non-steroidal glucocorticoid mimetics that utilize indoles as A-ring mimetics. Detailed SAR is discussed with a focus on improving PR and MR selectivity, GR agonism, and in vitro dissociation profile. SAR analysis led to compound (R)-33 which showed high PR and MR selectivity, potent agonist activity, and reduced transactivation activity in the MMTV and aromatase assays. The compound is equipotent to prednisolone in the LPS-TNF model of inflammation. In mouse CIA, at 30 mg/kg compound (R)-33 inhibited disease progression with an efficacy similar to the 3 mg/kg dose of prednisolone.

Identification of a lysosomal pathway that modulates glucocorticoid signaling and the inflammatory response

The antimalaria drug chloroquine has been used as an anti-inflammatory agent for treating systemic lupus erythematosus and rheumatoid arthritis. We report that chloroquine promoted the transrepression of proinflammatory cytokines by the glucocorticoid receptor (GR). In a mouse collagen-induced arthritis model, chloroquine enhanced the therapeutic effects of glucocorticoid treatment. By inhibiting lysosome function, chloroquine synergistically activated glucocorticoid signaling. Lysosomal inhibition by either bafilomycin A1 (an inhibitor of the vacuolar adenosine triphosphatase) or knockdown of transcription factor EB (TFEB, a master activator of lysosomal biogenesis) mimicked the effects of chloroquine. The abundance of the GR, as well as that of the androgen receptor and estrogen receptor, correlated with changes in lysosomal biogenesis. Thus, we showed that glucocorticoid signaling is regulated by lysosomes, which provides a mechanistic basis for treating inflammation and autoimmune diseases with a combination of glucocorticoids and lysosomal inhibitors.

Thyroid hormone receptor mutations in cancer and resistance to thyroid hormone: perspective and prognosis

Thyroid hormone, operating through its receptors, plays crucial roles in the control of normal human physiology and development; deviations from the norm can give rise to disease. Clinical endocrinologists often must confront and correct the consequences of inappropriately high or low thyroid hormone synthesis. Although more rare, disruptions in thyroid hormone endocrinology due to aberrations in the receptor also have severe medical consequences. This review will focus on the afflictions that are caused by, or are closely associated with, mutated thyroid hormone receptors. These include Resistance to Thyroid Hormone Syndrome, erythroleukemia, hepatocellular carcinoma, renal clear cell carcinoma, and thyroid cancer. We will describe current views on the molecular bases of these diseases, and what distinguishes the neoplastic from the non-neoplastic. We will also touch on studies that implicate alterations in receptor expression, and thyroid hormone levels, in certain oncogenic processes.

Retroviral Rem protein requires processing by signal peptidase and retrotranslocation for nuclear function

Mouse mammary tumor virus (MMTV) is a complex murine retrovirus that encodes an HIV Rev-like export protein, Rem, from a doubly spliced version of envelope (Env) mRNA. Previously, the N-terminal 98-amino acid sequence of Rem, which is identical to Env signal peptide (SP), and full-length Rem were shown to be functional in a reporter assay that measures a postexport function. Here we show that MMTV-infected cells or cells transfected with rem or env cDNAs express SP, which is the active component in the reporter assay. Uncleaved Rem was partially glycosylated, but mutations in both glycosylation sites within the C terminus prevented Rem function. Mutations that reduced Rem or Env cleavage by signal peptidase greatly reduced SP levels and functional activity in the reporter assay and allowed accumulation of the uncleaved protein. Fluorescence microscopy revealed that GFP-tagged cleavage-site mutants are unstable and lack fluorescence compared with wild-type Rem, suggesting improper folding. Proteasome inhibitors allowed accumulation of uncleaved Rem relative to SP and increased reporter activity, consistent with SP retrotranslocation and proteasome escape before nuclear entry. Expression of a dominant-negative p97 ATPase did not alter levels of unprocessed Rem and SP but decreased reporter activity, suggesting p97-facilitated retrotranslocation of SP. Our results provide an example of a SP that is processed by signal peptidase and retrotranslocated to allow nuclear localization and function.

Crosstalk in inflammation: the interplay of glucocorticoid receptor-based mechanisms and kinases and phosphatases

Glucocorticoids (GCs) are steroidal ligands for the GC receptor (GR), which can function as a ligand-activated transcription factor. These steroidal ligands and derivatives thereof are the first line of treatment in a vast array of inflammatory diseases. However, due to the general surge of side effects associated with long-term use of GCs and the potential problem of GC resistance in some patients, the scientific world continues to search for a better understanding of the GC-mediated antiinflammatory mechanisms. The reversible phosphomodification of various mediators in the inflammatory process plays a key role in modulating and fine-tuning the sensitivity, longevity, and intensity of the inflammatory response. As such, the antiinflammatory GCs can modulate the activity and/or expression of various kinases and phosphatases, thus affecting the signaling efficacy toward the propagation of proinflammatory gene expression and proinflammatory gene mRNA stability. Conversely, phosphorylation of GR can affect GR ligand- and DNA-binding affinity, mobility, and cofactor recruitment, culminating in altered transactivation and transrepression capabilities of GR, and consequently leading to a modified antiinflammatory potential. Recently, new roles for kinases and phosphatases have been described in GR-based antiinflammatory mechanisms. Moreover, kinase inhibitors have become increasingly important as antiinflammatory tools, not only for research but also for therapeutic purposes. In light of these developments, we aim to illuminate the integrated interplay between GR signaling and its correlating kinases and phosphatases in the context of the clinically important combat of inflammation, giving attention to implications on GC-mediated side effects and therapy resistance.

Histone acetylation characterizes chromatin presetting by NF1 and Oct1 and enhances glucocorticoid receptor binding to the MMTV promoter

Transcription from the mouse mammary tumor virus (MMTV) promoter is induced by the glucocorticoid receptor (GR). This switch was reconstituted in Xenopus oocytes. Previously, we showed that Nuclear Factor 1 (NF1) and Octamer Transcription Factor 1 (Oct1) bind constitutively to the MMTV promoter and thereby induce translational nucleosome positioning representing an intermediary, i.e. preset, state of nucleosome organization. Here we further characterize this NF1 and Oct1 induced preset chromatin in relation to the inactive and the hormone-activated state. The preset chromatin exhibits increased histone acetylation but does not cause dissociation of histone H1 as oppose to the hormone-activated state. Furthermore, upon hormone induction the preset MMTV chromatin displays an enhanced and prolonged GR binding capacity and transcription during an intrinsic and time-dependent silencing of the injected template. The silencing process correlates with a reduced histone acetylation. However, a histone deacetylase inhibitor, trichostatin A (TSA), does not counteract silencing in spite of its distinct stimulation of GR-DNA binding. The latter indicates the importance of histone acetylation to maintain DNA access for inducible factor binding. We discuss how constitutively bound factors such as NF1 and Oct1 may participate in the maintenance of tissue specificity of hormone responsive genes.

Control of pathogenicity and disease specificity of a T-lymphomagenic gammaretrovirus by E-box motifs but not by an overlapping glucocorticoid response element

Although transcription factors of the basic helix-loop-helix family have been shown to regulate enhancers of lymphomagenic gammaretroviruses through E-box motifs, the overlap of an E-box motif (Egre) with the glucocorticoid response element (GRE) has obscured their function in vivo. We report here that Egre, but not the GRE, affects disease induction by the murine T-lymphomagenic SL3-3 virus. Mutating all three copies of Egre prolonged the tumor latency period from 60 to 109 days. Further mutating an E-box motif (Ea/s) outside the enhancer prolonged the latency period to 180 days, suggesting that Ea/s works as a backup site for Egre. While wild-type SL3-3 and GRE and Ea/s mutants exclusively induced T-cell lymphomas with wild-type latencies mainly of the CD4(+) CD8(-) phenotype, Egre as well as the Egre and Ea/s mutants induced B-cell lymphomas and myeloid leukemia in addition to T-cell lymphomas. T-cell lymphomas induced by the two Egre mutants had the same phenotype as those induced by wild-type SL3-3, indicating the incomplete disruption of T-cell lymphomagenesis, which is in contrast to previous findings for a Runx site mutant of SL3-3. Mutating the Egre site or Egre and Ea/s triggered several tumor phenotype-associated secondary enhancer changes encompassing neighboring sites, none of which led to the regeneration of an E-box motif. Taken together, our results demonstrate a role for the E-box but not the GRE in T lymphomagenesis by SL3-3, unveil an inherent broader disease specificity of the virus, and strengthen the notion of selection for more potent enhancer variants of mutated viruses during tumor development.

Rapid spread of mouse mammary tumor virus in cultured human breast cells

BACKGROUND

The role of mouse mammary tumor virus (MMTV) as a causative agent in human breast carcinogenesis has recently been the subject of renewed interest. The proposed model is based on the detection of MMTV sequences in human breast cancer but not in healthy breast tissue. One of the main drawbacks to this model, however, was that until now human cells had not been demonstrated to sustain productive MMTV infection.

RESULTS

Here, we show for the first time the rapid spread of mouse mammary tumor virus, MMTV(GR), in cultured human mammary cells (Hs578T), ultimately leading to the infection of every cell in culture. The replication of the virus was monitored by quantitative PCR, quantitative RT-PCR and immunofluorescence imaging. The infected human cells expressed, upon cultivation with dexamethasone, MMTV structural proteins and released spiked B-type virions, the infectivity of which could be neutralized by anti-MMTV antibody. Replication of the virus was efficiently blocked by an inhibitor of reverse transcription, 3'-azido-3'-deoxythymidine. The human origin of the infected cells was confirmed by determining a number of integration sites hosting the provirus, which were unequivocally identified as human sequences.

CONCLUSION

Taken together, our results show that human cells can support replication of mouse mammary tumor virus.

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.

Mouse mammary tumor virus infects human cells

Mouse mammary tumor virus (MMTV) has long been speculated to be involved in human breast cancer and more recently in human primary biliary cirrhosis. Despite complete proviral sequences markedly homologous to MMTV being identified in human breast cancer tissue, no convincing evidence has been presented to date that MMTV can infect human cells. Using both wild-type and a genetically marked virus (MMTV-EGFP), we show here the successful infection of a number of different human cells by MMTV. Furthermore, infection of human cells is shown to be almost as efficient as the infection of murine mammary epithelial cells. Sequencing of PCR products from integrated proviruses reveals that reverse transcription and integration of the viral genome has occurred as expected. Furthermore, sequencing of two independent MMTV proviral integration sites reveal them to be present only in the human and not in the mouse genome. Infection requires an intact MMTV envelope protein and is blocked either by heat inactivation of the virus or by specific neutralizing anti-MMTV serum, ruling out a nonspecific mechanism of viral transfer. Thus, MMTV can infect human cells and this finding provides a possible explanation for the detection by others of MMTV sequences in human breast cancer patients.

Abundant authentic MMTV-Env production from a recombinant provirus lacking the major LTR promoter

As for all retroviruses, the env mRNA is thought to be a singly spliced product of the full-length transcript from the P1 promoter in the MMTV provirus. However, we show that envelope proteins can be produced in an inducible manner in the absence of the P1 promoter from an otherwise complete provirus. Furthermore, we demonstrate in both reporter assays and the proviral context that the R region is necessary for protein production in transiently transfected cells and in a number of independent, stably transfected cell clones. Using 5' RACE, we show that a sequence within the R region functions as a TATA less initiator. The most distal part of the 5' LTR (first 804 bases of the U3 region) is required for the activity of the R-initiator element only when the provirus is integrated. Transfection with a full-length proviral DNA carrying a deletion of P1 in the 5' LTR resulted in the establishment of stable cell clones able to produce Env in a dexamethasone-dependent manner but not infectious virions. We therefore conclude that in the absence of P1, R can drive transcription of the spliced env mRNA but not genomic viral RNA.

A Life-Long Search for the Molecular Pathways of Steroid Hormone Action

The O'Malley laboratory first showed that estrogen and progesterone act in the nucleus to stimulate synthesis of specific mRNAs (ovalbumin and avidin), coding for their respective inducible proteins. The overall molecular pathway of steroid-receptor-DNA-mRNA-protein-function was then established and provided a coherent foundation for future studies of the impact of estrogen and progesterone receptors on endocrine tissue development, adult function, and in pathologies such as cancer. The lab group went on to: biochemically demonstrate ligand-induced conformational activation of progesterone and estrogen receptors, discover the concept of ligand-independent activation of steroid receptors, discover key steroid receptor coactivator intermediary coactivators for receptor function, and define the role of coactivators/corepressors in selective receptor modulator drug action and in cell homeostasis. This body of work advanced our molecular understanding of the critical role of steroid hormones in normal and abnormal physiology and also generated a base of scientific knowledge that served to further modern hormonal therapy and disease management.

Chromatin-mediated restriction of nuclear factor 1/CTF binding in a repressed and hormone-activated promoter in vivo

Mouse mammary tumor virus (MMTV) promoter-driven transcription is induced by glucocorticoid hormone via binding of the glucocorticoid receptor (GR). The MMTV promoter also harbors a binding site for nuclear factor 1 (NF1). NF1 and GR were expressed in Xenopus oocytes; this revealed GR-NF1 cooperativity both in terms of DNA binding and chromatin remodeling but not transcription. A fraction of NF1 sites were occupied in a hormone-dependent fashion, but a significant and NF1 concentration-dependent fraction were constitutively bound. Activation of the MMTV promoter resulted in an approximately 50-fold increase in the NF1 accessibility for its DNA site. The hormone-dependent component of NF1 binding was dissociated by addition of a GR antagonist; however, the antagonist RU486, which supports partial GR-DNA binding, also maintained partial NF1 binding. Hence GR-NF1 cooperativity is independent of agonist-driven chromatin remodeling. NF1 induced the formation of a micrococcal-nuclease-resistant protein-DNA complex containing the DNA segment from -185 to -55, the MMTV enhanceosome. Coexpression of NF1 and Oct1 resulted in a significant stimulation of hormone-induced MMTV transcription and also in increased basal transcription. We propose that hormone-independent NF1 binding may be involved in maintaining transcriptional competence and establishment of tissue-specific gene networks.

Glucocorticoid hormone-induced receptor localization to the chromatin fibers formed on injected DNA in Xenopus oocytes

Oocytes from Xenopus laevis have provided a model system for studying the dynamic changes that occur in chromatin during gene activation. We have reconstituted glucocorticoid receptor (GR) induced transcription from the mouse mammary tumor virus (MMTV) promoter by intranuclear injection of an MMTV-driven reporter and cytoplasmic injection of synthetic mRNA(GR) into Xenopus oocytes. Here we investigate the intranuclear distribution of injected DNA, which is assembled into chromatin. We show that this chromatin is organized as an intranuclear fibrous network. Unliganded GR is located in the cytosol and hormone triggers its nuclear translocation and association with the chromatin fibers. Furthermore, we analyze the intranuclear distribution of other factors involved in transcription from the MMTV promoter. Indirect immunofluorescence microscopy on cryostat-sectioned oocytes revealed that BRG1, which is a subunit of the SWI/SNF chromatin remodeling complex, as well as RNA polymerase II and recombinantly expressed Xenopus nuclear factor 1-B, are all associated with the endogenous chromosomes and the chromatin fibers formed on injected DNA. This association does not depend on specific DNA binding sites and appears to be nonspecific.

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.

Bioassay of phytoestrogen in herbal medicine used for postmenopausal disorder using transformed MCF-7 cells

It is well known that plants contain substances with oestrogen activity comparable to that in animals. In animals, oestrogens are steroid hormones while plants contain oestrogen with various chemical characteristics, referred to as phytoestrogens. These especially, may be responsible for part of the effects of Chinese herbal prescriptions for postmenopausal disorders in which oestrogen deficiency is considered the primary cause. In addition, the presence or absence of oestrogen activity in herbal medicine is considered extremely important for patients in whom oestrogen administration is contraindicated. The most important action of oestrogen is the transcriptional regulation of specific genes via oestrogen receptors (ER). Therefore, we have established a highly sensitive bioassay system by placing oestrogen responsive elements (ERE) upstream to the reporter gene, and have used this assay to determine the oestrogen activity in herbal medicine. As a result, phytoestrogens were found in many Chinese herbal prescriptions for postmenopausal disorder, and some prescriptions were considered to effectively administer hormone replacement therapy (HRT). However, since oestrogen activities in most Chinese herbal prescriptions were thought to be weak, it was considered that these prescriptions improved postmenopausal disorder based on a mechanism differing from that of oestrogen replacement therapy.

Preferential binding of mouse mammary tumor virus to B lymphocytes

Mouse mammary tumor virus (MMTV) has been shown to preferentially infect B lymphocytes in vivo. We have used recombinant envelope-coated fluospheres and highly purified MMTV particles to study the distribution of the viral receptors on fresh mouse lymphocytes. A preferential dose-dependent binding to B lymphocytes was observed which could be competed with neutralizing antibodies. In contrast, T-lymphocyte binding remained at background levels. These results strongly suggest a higher density of viral receptor molecules on B lymphocytes than on T lymphocytes and correlate with the preferential initial infection of B lymphocytes observed in vivo.

Ku antigen-DNA conformation determines the activation of DNA-dependent protein kinase and DNA sequence-directed repression of mouse mammary tumor virus transcription

Mouse mammary tumor virus (MMTV) transcription is repressed by DNA-dependent protein kinase (DNA-PK) through a DNA sequence element, NRE1, in the viral long terminal repeat that is a sequence-specific DNA binding site for the Ku antigen subunit of the kinase. While Ku is an essential component of the active kinase, how the catalytic subunit of DNA-PK (DNA-PKcs) is regulated through its association with Ku is only beginning to be understood. We report that activation of DNA-PKcs and the repression of MMTV transcription from NRE1 are dependent upon Ku conformation, the manipulation of DNA structure by Ku, and the contact of Ku80 with DNA. Truncation of one copy of the overlapping direct repeat that comprises NRE1 abrogated the repression of MMTV transcription by Ku-DNA-PKcs. Remarkably, the truncated element was recognized by Ku-DNA-PKcs with affinity similar to that of the full-length element but was unable to promote the activation of DNA-PKcs. Analysis of Ku-DNA-PKcs interactions with DNA ends, double- and single-stranded forms of NRE1, and the truncated NRE1 element revealed striking differences in Ku conformation that differentially affected the recruitment of DNA-PKcs and the activation of kinase activity.

Ligand induction of retinoic acid receptors alters an acute infection by murine cytomegalovirus

Here we report that administration of retinoids can alter the outcome of an acute murine cytomegalovirus (MCMV) infection. We show that a crucial viral control element, the major immediate-early enhancer, can be activated by retinoic acid (RA) via multiple RA-responsive elements (DR2) that bind retinoid X receptor-retinoic acid receptor (RAR) heterodimers with apparent dissociation constants ranging from 15 to 33 nM. Viral growth is dramatically increased upon RA treatment of infected tissue culture cells. Using synthetic retinoid receptor-specific agonists and antagonists, we provide evidence that RAR activation in cells is required for mediating the response of MCMV to RA. Oral administration of RA to infected immunocompetent mice selectively exacerbates an infection by MCMV, while cotreatment with an RAR antagonist protects against the adverse effects of RA on MCMV infection. In conclusion, these chemical genetic experiments provide evidence that an RAR-mediated pathway can modulate in vitro and in vivo infections by MCMV.

Mapping of a mouse mammary tumor virus integration site by retroviral LTR--arbitrary polymerase chain reaction

The de novo integration of retroviral genomes within the mammalian genome is believed to contribute to the tumorigenic process. Integration may result in the disruption or inappropriate transcription of key regulatory genes. We describe the application of an arbitrarily primed PCR method for the mapping and cloning of genomic integration sites of the mouse mammary tumor virus (MMTV). We have amplified DNA sequences between a selected retroviral MMTV-LTR and random sites in the 3' flanking DNA. Using this technique we were able to visualize several proviral integration sites present in a MMTV-induced mammary tumor derived cell line that were absent from the germ line. Cloning and sequencing of the PCR product corresponding to one site established its identification as an unique 3' flanking sequence.

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.

Direct modulation of simian virus 40 late gene expression by thyroid hormone and its receptor

Transcription of the late genes of simian virus 40 (SV40) is repressed during the early phase of the lytic cycle of infection of primate cells by the binding of cellular factors, called IBP-s, to the SV40 late promoter; repression is relieved after the onset of viral DNA replication by titration of these repressors (S. R. Wiley, R. J. Kraus, F. R. Zuo, E. E. Murray, K. Loritz, and J. E. Mertz, Genes Dev. 7:2206-2219, 1993). Recently, we showed that IBP-s consists of several members of the steroid/thyroid hormone receptor superfamily (F. Zuo and J. E. Mertz, Proc. Natl. Acad. Sci. USA 92:8586-8590, 1995). Here, we show that the thyroid hormone receptor TRalpha1, in combination with retinoid X receptor alpha (RXRalpha), is specifically bound at the transcriptional initiation site of the major late promoter of SV40. This binding repressed transcription from the SV40 late promoter by preventing the formation of pre-initiation complexes. Addition of the thyroid hormone 3,5,3'-L-triiodothyronine (T3) resulted in reversal of this repression in cotransfected CV-1 cells. Interestingly, repression did not occur when this thyroid response element (TRE) was translocated to 50 bp upstream of the major late initiation site. Binding of TRalpha1/RXRalpha heterodimers to this TRE induced bending of the promoter DNA. We conclude that hormones and their receptors can directly affect the expression of SV40, probably by affecting protein-protein and protein-DNA interactions involved in the formation of functional preinitiation complexes.

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.

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.

Cisplatin inhibits chromatin remodeling, transcription factor binding, and transcription from the mouse mammary tumor virus promoter in vivo

The anticancer drug cis-diamminedichloro-platinum(II) (cisplatin) covalently modifies DNA, and these lesions are thought to lead to cell death by inhibiting DNA and RNA synthesis. By using in vivo analysis techniques, we have investigated the influence of cisplatin on hormone-induced transcription from the mouse mammary tumor virus (MMTV) promoter. Cisplatin substantially reduced glucocorticoid-induced expression from the MMTV promoter stably incorporated into mouse tumor cells. The glucocorticoid-receptor-dependent chromatin remodeling and loading of transcription factors that is a signature response of this promoter in the context of chromatin were significantly reduced by cisplatin but not by the clinically ineffective trans-isomer trans-diamminedichloroplatinum(II) (transplatin). Additional in vivo studies on transiently introduced nonchromatin MMTV templates demonstrated that cisplatin modification of DNA blocked binding of the transcription factor NF1. These results provide strong evidence that cisplatin influences transcription by interfering with the opening of repressive chromatin structures and by blocking transcription factor binding directly, each of which could contribute substantially to its toxicity.

Functional redundancy of octamer elements in the mouse mammary tumor virus promoter

The promoter of mouse mammary tumor virus contains three overlapping sequence elements related to the octamer consensus (ATGCAAAT) that are largely contained within two 10 bp direct repeats (CTTATGTAAA) separated by a 2 bp spacer between 60 and 39 relative to the start of transcription. Gel electrophoresis mobility shift competition assays demonstrate that the most distal of these octamer-related elements is recognized by a protein that also binds to the most proximal element, while the central octamer-related element is not efficiently recognized. Transient transfection assays with altered promoters reveal that the portion of the 10 bp repeat that is not related to the octamer consensus appears not to be important for transcription. The distal and proximal octamer-related elements are, at least to some extent, functionally redundant. Complete deletion of one element has little or no effect on promoter activity so long as certain spacing constraints among remaining promoter elements are maintained. Systematic variation of such spacing reveals a cyclic effect on promoter activity corresponding to the periodicity of Bform DNA, suggesting functional interactions between proteins bound to adjacent sites.

Endogenous superantigen expression controlled by a novel promoter in the MMTV long terminal repeat

Endogenous superantigens are encoded by the open reading frame contained within the mouse mammary tumour virus long terminal repeat (MMTV LTR). Superantigen expression results in T-cell proliferation and, during early ontogeny, T-cell deletion. Here we identify a novel promoter located upstream of the previously described MMTV promoter. Transcripts from this promoter initiate within the U3 region of the MMTV LTR and splice to the acceptor for endogenous superantigen coding region. The novel U3 promoter is active in B lymphocytes, which are cognate antigen-presenting cells for endogenous superantigen, and is able to direct expression of superantigen in the absence of the previously described MMTV promoter.

A mouse mammary tumor virus promoter element near the transcription initiation site

Transcription from the promoter of mouse mammary tumor virus is subject to both positive and negative control by cellular factors, and proviral promoter elements that mediate a basal level of transcription must in some way respond to these cellular regulatory signals. Several such elements, including a TATA box, a region containing three octamer-related sequences, and a binding site for nuclear factor 1, have been previously defined. Additional promoter mutations have allowed a fourth basal promoter element to be identified near the transcription initiation site between +2 and +10. Sequence alterations within this element affect transcription both in vivo and in vitro. Gel electrophoresis mobility shift and DNase I footprinting assays define a nuclear protein, termed initiation site-binding protein, that specifically recognizes this region of the promoter. Optimal levels of transcription from the mouse mammary tumor virus promoter require initiation site-binding protein, as demonstrated by a correlation between protein affinity and transcriptional activity and by specific inhibition of transcription in vitro by an oligonucleotide capable of titrating the protein from transcriptionally active fractions.

Interplay of steroid hormone receptors and transcription factors on the mouse mammary tumor virus promoter

The mouse mammary tumor virus (MMTV) promoter, that responds to glucocorticoids and progestins, contains a complex hormone response element (HRE) in the long terminal repeat (LTR) region covered by a phased nucleosome. Hormone treatment leads to alterations in chromatin structure that make the HRE region more accessible to digestion by DNase I and permit binding of transcription factors, including nuclear factor I (NFI), immediately downstream of the HRE. NFI acts as a basal transcription factor on the MMTV promoter in vitro but competes with the hormone receptors in terms of binding to free DNA. In uninduced chromatin, the precise positioning of the DNA double helix on the surface of the histone octamer precludes binding of NFI to its cognate sequence while still allowing recognition of the HRE by the hormone receptors. We postulate that receptor binding to the nucleosomally organized MMTV promoter disrupts the chromatin structure enabling NFI binding and subsequent formation of a stable transcription complex. Whether the receptor remains bound to DNA during induction or is displaced by NFI is not conclusively known, but our evidence supports a "hit and run" mechanism. NFI is not the only factor involved in hormonally induced transcription of the MMTV promoter. Two degenerated octamer motifs located immediately upstream of the TATA box are recognized by the ubiquitous transcription factor OTF-1 (Oct-1, NFIII), and are also important. In vitro, mutations in these motifs do not influence basal transcription, but completely abolish the stimulatory effect of purified progesterone receptor. Progesterone receptor bound to the HRE facilitates binding of OTF-1 to the two octamer motifs. Thus, OTF-1 is a natural mediator of progesterone induction of the MMTV promoter and acts through cooperation with the hormone receptor for binding to DNA.

Factors controlling the expression of mouse mammary tumour virus

Images

Protein-synthesis-dependent induction of annexin I by glucocorticoid

We demonstrate that annexin I/lipocortin I (lipo I) gene expression is regulated by dexamethasone (DEX) in mouse 3T3-L1 fibroblasts and LA-4 lung epithelial cells. We have characterized this induction further in 3T3-L1 fibroblasts. At 24 h after addition of DEX, the levels of lipo I mRNA and protein increased 5-fold and 1.5-fold respectively. Time-course experiments revealed that the induction was delayed by 2-4 h after DEX addition. Half-maximal induction of both lipo I mRNA and protein was achieved with 10 nM-DEX. Both actinomycin D and cycloheximide blocked the DEX effect on lipo I mRNA expression. These results indicate that the induction of lipo I by DEX has a transcriptional component and requires protein synthesis de novo.

Molecular cloning of a transcription factor, AGP/EBP, that belongs to members of the C/EBP family

A C/EBP-like transcription factor, AGP/EBP, that binds to three distinct motifs in the 5'-flanking region of alpha 1-acid glycoprotein gene (AGP) has been identified. Here we report the cloning and properties of cDNA corresponding to mouse AGP/EBP. AGP/EBP and C/EBP share 87% amino acid sequence homology in the "leucine zipper" and its associated DNA-binding domains, while their sequences outside these domains and the sizes of their mRNAs are different. Unlike the limited expression of C/EBP in tissues and cells, AGP/EBP appears to be ubiquitously expressed in tissues like lung, spleen, kidney, heart, testis, and liver and cell lines like p388D1, 129P (hepatoma cell line of C3H/HeJ), FO (mouse myeloma), and L929. Antibody against cloned and expressed AGP/EBP which was raised in rabbits could recognize AGP/EBP from nuclear extract of a number of cells and tissues. On the basis of our findings about the structural relationship and the similarity of motif recognition, we propose that a family of C/EBP-like transcription factors exists.

Molecular cloning of a transcription factor, AGP/EBP, that belongs to members of the C/EBP family

A C/EBP-like transcription factor, AGP/EBP, that binds to three distinct motifs in the 5'-flanking region of alpha 1-acid glycoprotein gene (AGP) has been identified. Here we report the cloning and properties of cDNA corresponding to mouse AGP/EBP. AGP/EBP and C/EBP share 87% amino acid sequence homology in the "leucine zipper" and its associated DNA-binding domains, while their sequences outside these domains and the sizes of their mRNAs are different. Unlike the limited expression of C/EBP in tissues and cells, AGP/EBP appears to be ubiquitously expressed in tissues like lung, spleen, kidney, heart, testis, and liver and cell lines like p388D1, 129P (hepatoma cell line of C3H/HeJ), FO (mouse myeloma), and L929. Antibody against cloned and expressed AGP/EBP which was raised in rabbits could recognize AGP/EBP from nuclear extract of a number of cells and tissues. On the basis of our findings about the structural relationship and the similarity of motif recognition, we propose that a family of C/EBP-like transcription factors exists.

Antitumor promotion and antiinflammation: down-modulation of AP-1 (Fos/Jun) activity by glucocorticoid hormone

Glucocorticoid hormones counteract inflammation and phorbol ester tumor promotion and drastically decrease the expression of several extracellular proteases, including collagenase I. Glucocorticoid hormone inhibits basal and induced transcription of collagenase by interfering with AP-1, the major enhancer factor of the collagenase promoter. The mechanism of interference is novel in that it does not require protein synthesis, it depends on the hormone receptor but not its binding to DNA, it occurs at hormone doses one order of magnitude below those required for gene activation, and it involves down-modulation of the trans-activating function of preexisting unbound and DNA-bound AP-1. Coprecipitation experiments suggest direct AP-1-hormone receptor interaction, which also possibly explains the reverse experiment: overexpression of Fos or Jun inhibits the expression of hormone-dependent genes.

Functional elements of the steroid hormone-responsive promoter of mouse mammary tumor virus

Transcription from the promoter of mouse mammary tumor virus is subject to induction by several classes of steroid hormones as well as to repression by a negative regulatory element present in the long terminal repeats of proviral DNA. In order to characterize the functional elements of the promoter that in some way must respond to these regulatory signals, a number of promoter mutations were constructed, including a set of linker-scanning mutations across the entire promoter region. Analysis of these mutated promoters with a transient-transfection assay defined at least three mutation-sensitive promoter elements that are required for both basal and hormone-induced transcription. One mutation-sensitive region contains a TATA element located at approximately position -30 with respect to the start of transcription. A second mutation-sensitive region contains two 10-base-pair direct repeats located between positions -60 and -38, within which are embedded three copies of octamer-related sequences; complete disruption of this region of the promoter leads to a more severe decrease in transcription than do any of the linker-scanning mutations, suggesting that the repeated sequences may be at least partially functionally redundant. Gel electrophoresis mobility shift assays were used to demonstrate specific binding of a nuclear protein to this region of the promoter. A third mutation-sensitive region contains a binding site for nuclear factor 1 (NF-1) located between positions -77 and -63. Site-directed mutations in the NF-1-binding site which increase the apparent affinity of NF-1 for the promoter in vitro do not decrease the hormone dependence of transcription, suggesting that transcriptional activation mediated by steroid hormone-receptor complexes cannot be explained by facilitation or stabilization of the interaction of promoter sequences with NF-1 and consistent with the idea that binding of NF-1 is not rate determining in transcription from the mouse mammary tumor virus promoter. None of the promoter mutations functionally separates basal from glucocorticoid-induced transcription, suggesting that hormone induction does not make the promoter independent of any of the DNA-binding factors required for its basal activity.