Subfragments of the large terminal repeat cause glucocorticoid-responsive expression of mouse mammary tumor virus and of an adjacent gene

Revisiting progesterone receptor (PR) actions in breast cancer: Insights into PR repressive functions

Progesterone receptor (PR) is a master regulator in female reproductive tissues that controls developmental processes and proliferation and differentiation during the reproductive cycle and pregnancy. PR also plays a role in progression of endocrine-dependent breast cancer. As a member of the nuclear receptor family of ligand-dependent transcription factors, the main action of PR is to regulate networks of target gene expression in response to binding its cognate steroid hormone, progesterone. Liganded-PR transcriptional activation has been thoroughly studied and associated mechanisms have been described while progesterone-mediated repression has remained less explored. The present work summarizes recent advances in the understanding of how PR-mediated repression is accomplished in breast cancer cells and highlights the significance of fully understanding the determinants of context-dependent PR action.

Endocrine-Disrupting Activity of Hydraulic Fracturing Chemicals and Adverse Health Outcomes After Prenatal Exposure in Male Mice

Oil and natural gas operations have been shown to contaminate surface and ground water with endocrine-disrupting chemicals. In the current study, we fill several gaps in our understanding of the potential environmental impacts related to this process. We measured the endocrine-disrupting activities of 24 chemicals used and/or produced by oil and gas operations for five nuclear receptors using a reporter gene assay in human endometrial cancer cells. We also quantified the concentration of 16 of these chemicals in oil and gas wastewater samples. Finally, we assessed reproductive and developmental outcomes in male C57BL/6J mice after the prenatal exposure to a mixture of these chemicals. We found that 23 commonly used oil and natural gas operation chemicals can activate or inhibit the estrogen, androgen, glucocorticoid, progesterone, and/or thyroid receptors, and mixtures of these chemicals can behave synergistically, additively, or antagonistically in vitro. Prenatal exposure to a mixture of 23 oil and gas operation chemicals at 3, 30, and 300 μg/kg · d caused decreased sperm counts and increased testes, body, heart, and thymus weights and increased serum testosterone in male mice, suggesting multiple organ system impacts. Our results suggest possible adverse developmental and reproductive health outcomes in humans and animals exposed to potential environmentally relevant levels of oil and gas operation chemicals.

Unliganded progesterone receptor-mediated targeting of an RNA-containing repressive complex silences a subset of hormone-inducible genes

A close chromatin conformation precludes gene expression in eukaryotic cells. Genes activated by external cues have to overcome this repressive state by locally changing chromatin structure to a more open state. Although much is known about hormonal gene activation, how basal repression of regulated genes is targeted to the correct sites throughout the genome is not well understood. Here we report that in breast cancer cells, the unliganded progesterone receptor (PR) binds genomic sites and targets a repressive complex containing HP1γ (heterochromatin protein 1γ), LSD1 (lysine-specific demethylase 1), HDAC1/2, CoREST (corepressor for REST [RE1 {neuronal repressor element 1} silencing transcription factor]), KDM5B, and the RNA SRA (steroid receptor RNA activator) to 20% of hormone-inducible genes, keeping these genes silenced prior to hormone treatment. The complex is anchored via binding of HP1γ to H3K9me3 (histone H3 tails trimethylated on Lys 9). SRA interacts with PR, HP1γ, and LSD1, and its depletion compromises the loading of the repressive complex to target chromatin-promoting aberrant gene derepression. Upon hormonal treatment, the HP1γ-LSD1 complex is displaced from these constitutively poorly expressed genes as a result of rapid phosphorylation of histone H3 at Ser 10 mediated by MSK1, which is recruited to the target sites by the activated PR. Displacement of the repressive complex enables the loading of coactivators needed for chromatin remodeling and activation of this set of genes, including genes involved in apoptosis and cell proliferation. These results highlight the importance of the unliganded PR in hormonal regulation of breast cancer cells.

A new role for an old player: steroid receptor RNA Activator (SRA) represses hormone inducible genes

In breast cancer cells the Steroid Receptor ¬RNA Activator (SRA) acts as scaffold of a complex containing HP1γ, LSD1, HDAC1/2 and CoREST, which contributes to repression of key hormone-inducible genes that must be kept silent in the absence of hormone.

Impact of chromatin structure and dynamics on PR signaling. The initial steps in hormonal gene regulation

Gene regulation requires access of transcription factors to DNA sequences of target genes, which is limited by the compaction of DNA in chromatin. Based on our studies on the Progesterone receptor (PR)-dependent hormonal induction of mouse mammary tumor virus (MMTV) promoter we found that remodeling of the various levels of chromatin organization is a complex and necessary prerequisite for regulation. Two consecutive cycles are essential for transcriptional activation, both involving the collaboration between activated protein kinases, histone modifying enzymes and ATP-dependent chromatin remodelers. The first cycle ends with the displacement of histone H1 and decompaction of higher order chromatin structure. The second cycle leads to the displacement of dimers of histones H2A and H2B resulting in opening of nucleosomes. In both cases the hormone receptor recruits an ATP-dependent chromatin remodeler, whose binding to chromatin is stabilized by distinct histone modifications. The final result is to facilitate full occupancy of the cis regulatory sites and access for the basal transcription machinery. Thus, activation of PR-target genes involves a very rapid coordination of enzymatic activities via crosstalk with various kinase-signaling pathways.

Promoter complex in the central part of the mouse mammary tumor virus long terminal repeat

Unique among the retroviruses, mouse mammary tumor virus (MMTV) carries, in addition to the usual long terminal repeat (LTR) promoter, another promoter, P2, which is located in the central part of the proviral U3 sequence, within the LTR open reading frame (ORF). Using an in vitro reporter system based on a sensitive luciferase expression assay, we investigated the regulation of the P2 promoter in the context of the Mtv-2 and Mtv-8 genomes. Irrespective of the genomic source, the activity of the P2 promoter is regulated by a downstream-located enhancer and an upstream-located negative regulatory element (NRE), the activity of which overrides the activator. During this study, we unexpectedly detected another independent neighboring promoter that we called P3. The novel P3 promoter does not seem to be controlled by any NRE but is influenced by the same enhancer that modulates the P2 promoter. The respective transcription starts of the two promoters located in this tight cluster are only 61 bases apart. The transcripts originating from this promoter complex carry the same first intron, which is bound by canonical splice donor and splice acceptor sites located in the LTR. One novel doubly spliced transcript carrying a 459-nucleotide-long ORF was detected in several MMTV-carrying murine cells and could be successfully expressed in murine cells as a His-tagged fusion product. The novel viral protein, the function of which remains to be elucidated, has an apparent molecular mass of 20 kDa.

MMTV accessory factor Naf affects cellular gene expression

Mouse mammary tumour virus (MMTV) encodes a viral superantigen (Sag) and a negative acting factor (Naf) which share parts of their coding sequence. Using 2-dimensional gel electrophoresis (2D-DIGE), we could show that at least 10 different cellular proteins were differentially expressed in Naf positive cells. Also, luciferase reporter expression was down-regulated in Naf expressing cells independent of the promoter used and further experiments suggested that this effect was due in part to a decrease in cellular growth rates. Although in Naf positive cells expression of the major sag containing transcript was strongly induced by the synthetic glucocorticoid dexamethasone, the hormone analogue neither influenced luciferase expression nor mRNA expression of selected cellular proteins identified by 2D-DIGE. Taken together, these data support the previous finding that Naf and Sag have separable activities and suggest that Naf may play a role in modulating host cell gene expression during MMTV infection.

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.

Cross-talk between glucocorticoid receptor and AP-1

Cross-talk between different transcription factors, notably between the glucocorticoid receptor and AP-1, has been discovered more than 10 years ago: a bona fide transcription factor, without apparent need for its own direct DNA contact, influences the activity of another transcription factor. Recent experiments have added interesting aspects: in addition to major insights into the mechanism of cross-talk, it is now clear that the cross-talk ability of glucocorticoid receptor is essential for mouse development, while the activation of target promoters carrying a glucocorticoid response element (GRE), is surprisingly, dispensable for survival under animal house conditions. Interestingly, the cross-talk function is responsible for almost all regulatory actions of cortisol in the immune system. It is possible that the two functions of the glucocorticoid receptor can be activated separately by specific ligands. Future goals will be to define whether adverse effects of long-term corticosteroid treatment, e.g. osteoporosis, joint necroses, metabolic effects, can be ascribed to GRE-target gene activation and thus be dissociated from the desirable actions in the treatment e.g. of autoimmune disease.

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

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

The stressful condition as a nutritionally dependent adaptive dichotomy

The injured body manifests a cascade of cytokine-induced metabolic events aimed at developing defense mechanisms and tissue repair. Rising concentrations of counterregulatory hormones work in concert with cytokines to generate overall insulin and insulin-like growth factor 1 (IGF-1), postreceptor resistance and energy requirements grounded on lipid dependency. Salient features are self-sustained hypercortisolemia persisting as long as cytokines are oversecreted and down-regulation of the hypothalamo-pituitary-thyroid axis stabilized at low basal levels. Inhibition of thyroxine 5'-deiodinating activity (5'-DA) accounts for the depressed T3 values associated with the sparing of both N and energy-consuming processes. Both the liver and damaged territories adapt to stressful signals along up-regulated pathways disconnected from the central and peripheral control systems. Cytokines stimulate liver 5'-DA and suppress the synthesis of transthyretin (TTR), causing the drop of retinol-binding protein (RBP) and the leakage of increased amounts of T4 and retinol in free form. TTR and RBP thus work as prohormonal reservoirs of precursor molecules which need to be converted into bioactive derivatives (T3 and retinoic acids) to reach transcriptional efficiency. The converting steps (5'-DA and cellular retinol-binding protein-I) are activated by T4 and retinol, themselves operating as limiting factors of positive feedback loops. Healthy adults with normal macrophage functioning and liver parenchymal integrity, who submitted to a stress of medium severity, are characterized by TTR-RBP plasma levels reduced by half and an estimated ten-fold increase in free ligand disposal to target cells during the days ensuing injury. This transient hyperthyroid and hyperretinoid climate creates a second defense line strengthening and fine-tuning the effects primarily initiated by cytokines. The suicidal behavior of thyroxine-binding globulin (TBG), corticosteroid-binding globulin (CBG), and IGFBP-3 allows the occurrence of peak endocrine and mitogenic influences at the site of inflammation. The production rate of TTR by the liver is the main determinant of both the hepatic release and blood transport of holoRBP, which explains why poor nutritional status concomitantly impairs thyroid- and retinoid-dependent acute-phase responses, hindering the stressed body to appropriately face the survival crisis. The prognostic significance of low TT4 blood levels may be assigned to the exhaustion of extrathyroidal hormonal pools normally stored in liver and plasma but markedly shrunken in protein-depleted states. These data offer new insights into the mechanisms whereby preexisting malnutrition and stressful complications are interrelated, emphasizing the pivotal role played by TTR in that context.

The position and length of the steroid-dependent hypersensitive region in the mouse mammary tumor virus long terminal repeat are invariant despite multiple nucleosome B frames

Stimulation of the mouse mammary tumor virus with steroids results in the generation of a DNase I-hypersensitive region (HSR) spanning the hormone responsive element (HRE) in the long terminal repeat. Restriction enzymes were used to characterize the accessibility of various sites within the HSR of mouse mammary tumor virus long terminal repeat-reporter constructions in four different cell lines. The glucocorticoid-dependent HSR was found to span minimally 187 bases, a stretch of DNA longer than that associated with histones in the core particle. Although the 5'-most receptor binding site within the HRE is downstream of -190, hypersensitive sites were found further upstream to at least -295. The relationship in the accessibility between pairs of sites in the vicinity of the HSR was further examined in one cell line by a two-enzyme restriction access assay. In the uninduced state, the accessibilities at these sites were found to be independent of each other. In contrast, when stimulated with hormone, the accessibilities at these sites were observed to become linked. That is, once a distinct promoter was activated, all of the sites within the HSR of that molecule became accessible. The HSR formed along an invariant stretch of DNA sequence despite the multiplicity of nucleosome frames in the nucleosome B region, where the HRE is located. The results indicate that the macroscopic length of the HSR does not arise from core length-remodeling events in molecules containing Nuc-B in alternative positions.

Functional interactions between Stat5 and the glucocorticoid receptor

Signal transduction pathways enable extracellular signals to activate latent transcription factors in the cytoplasm of cells. Dimerization, nuclear localization and binding to specific DNA sequences result in the induction of gene transcription by these proteins. These events are necessary for the functioning of the JAK/STAT pathway and of the glucocorticoid-receptor pathway. In the former, the protein Stat5, which is a member of a family of signal transducers and activators of transcription, is activated by cytokines, hormones and growth factors. These polypeptide ligands bind at the outside of the cell to specific transmembrane receptors and activate intracellular Janus protein tyrosine kinases (JAKs) to tyrosine-phosphorylate STAT proteins; interaction with the SH2 domain of the dimerization partner then confers the ability to bind to DNA at the STAT-response element and induce transcription. In the glucocorticoid-receptor pathway, the receptor interacts with its steroid hormone ligand in the cytoplasm, undergoes an allosteric change that enables the hormone receptor complex to bind to specific DNA-response elements (glucocorticoid response elements, or GRE) and modulate transcription. Although these pathways appear to be unrelated, we show here that the glucocorticoid receptor can act as a transcriptional co-activator for Stat5 and enhance Stat5-dependent transcription. Stat5 forms a complex with the glucocorticoid receptor which binds to DNA independently of the GRE. This complex formation between Stat5 and the glucocorticoid receptor diminishes the glucocorticoid response of a GRE-containing promoter.

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

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

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.

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

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

Stimulation of basal transcription from the mouse mammary tumor virus promoter by Oct proteins

The steroid hormone-inducible promoter of mouse mammary tumor virus (MMTV) contains three overlapping sequences related to the consensus octamer motif ATGCAAAT. Basal promoter activity in the absence of hormone induction from a template in which all three octamer elements were mutated was decreased by two-to threefold in in vitro transcription assays. Oct-1 protein purified from HeLa cell nuclear extracts, as well as recombinant Oct-1 expressed in bacteria, recognized MMTV octamer-related sequences, as shown by DNase I footprinting. Furthermore, rabbit polyclonal antiserum directed against recombinant Oct-1 completely inhibited the formation of specific complexes between MMTV octamer-related sequences and proteins present in nuclear extracts of HeLa cells, indicating that Oct-1 is the major protein in HeLa nuclear extracts that recognizes octamer-related sequences in the MMTV promoter. In addition, depletion of Oct-1 from the nuclear extract by using Oct-1-specific antiserum or a sequence-specific DNA affinity resin decreased in vitro transcription from the wild-type MMTV promoter to a level identical to that obtained from a promoter in which all three octamer-related sequences were mutated. Addition of purified HeLa Oct-1 or recombinant Oct-1 to the depleted extract selectively increased transcription from the wild-type relative to the mutated promoter, demonstrating that Oct-1 transcription factor stimulates basal transcription from the MMTV promoter. A similar effect was observed when purified recombinant Oct-2 was added to the Oct-1-depleted extract, suggesting that Oct-2 may play an important role in MMTV transcription in B cells.

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.

Tissue-specific and ubiquitous factors binding next to the glucocorticoid receptor modulate transcription from the mouse mammary tumor virus promoter

Steroid hormones complexed with their receptors play an essential role in the regulation of mouse mammary tumor virus (MMTV) transcription. However, the need for additional tissue-specific regulatory factors is suggested by the lack of virus expression in liver, in which glucocorticoid receptors are highly abundant, and by the tissue-specific transcription of reporter genes linked to an MMTV long terminal repeat in transgenic mice. In this study, we characterized two distal-region regulatory elements, DRa and DRc, which, together with the distal glucocorticoid receptor binding site (DRb), increased transcription from the MMTV promoter in permissive cells. This was demonstrated by transfection of these sequences (DRa, DRb, and DRc) in different combinations with the natural MMTV promoter in mouse fibroblasts and mammary epithelial cells, followed by quantitative S1 nuclease mapping of the transcripts. We further showed by DNase I footprinting, methylation interference, and gel retardation assays with various nuclear extracts from permissive or nonpermissive tissues and cell lines that the factors binding to the DRa site are distinct and tissue-specific whereas those binding to DRc are ubiquitous.

Stably integrated mouse mammary tumor virus long terminal repeat DNA requires the octamer motifs for basal promoter activity

In the mouse mammary tumor virus promoter, a tandem of octamer motifs, recognized by ubiquitous and tissue-restricted Oct transcription factors, is located upstream of the TATA box and next to a binding site for the transcription factor nuclear factor I (NF-I). Their function was investigated with mutant long terminal repeats under different transfection conditions in mouse Ltk- cells and quantitative S1 nuclease mapping of the transcripts. In stable transfectants, which are most representative of the state of proviral DNA with respect to both number of integrated DNA templates and chromatin organization, a long terminal repeat mutant of both octamer sites showed an average 50-fold reduction of the basal transcription level, while the dexamethasone-stimulated level was unaffected. DNase I in vitro footprinting assays with L-cell nuclear protein extracts showed that the mutant DNA was unable to bind octamer factors but had a normal footprint in the NF-I site. I conclude that mouse mammary tumor virus employs the tandem octamer motifs of the viral promoter, recognized by the ubiquitous transcription factor Oct-1, for its basal transcriptional activity and the NF-I binding site, as previously shown, for glucocorticoid-stimulated transcription. A deletion mutant with only one octamer site showed a marked base-level reduction at high copy number but little reduction at low copies of integrated plasmids. The observed transcription levels may depend both on the relative ratio of transcription factors to DNA templates and on the relative affinity of binding sites, as determined by oligonucleotide competition footprinting.

Stably integrated mouse mammary tumor virus long terminal repeat DNA requires the octamer motifs for basal promoter activity

In the mouse mammary tumor virus promoter, a tandem of octamer motifs, recognized by ubiquitous and tissue-restricted Oct transcription factors, is located upstream of the TATA box and next to a binding site for the transcription factor nuclear factor I (NF-I). Their function was investigated with mutant long terminal repeats under different transfection conditions in mouse Ltk- cells and quantitative S1 nuclease mapping of the transcripts. In stable transfectants, which are most representative of the state of proviral DNA with respect to both number of integrated DNA templates and chromatin organization, a long terminal repeat mutant of both octamer sites showed an average 50-fold reduction of the basal transcription level, while the dexamethasone-stimulated level was unaffected. DNase I in vitro footprinting assays with L-cell nuclear protein extracts showed that the mutant DNA was unable to bind octamer factors but had a normal footprint in the NF-I site. I conclude that mouse mammary tumor virus employs the tandem octamer motifs of the viral promoter, recognized by the ubiquitous transcription factor Oct-1, for its basal transcriptional activity and the NF-I binding site, as previously shown, for glucocorticoid-stimulated transcription. A deletion mutant with only one octamer site showed a marked base-level reduction at high copy number but little reduction at low copies of integrated plasmids. The observed transcription levels may depend both on the relative ratio of transcription factors to DNA templates and on the relative affinity of binding sites, as determined by oligonucleotide competition footprinting.

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.

Characterization of the interaction of the human mineralocorticosteroid receptor with hormone response elements

Although the mineralocorticosteroid receptor (MR) belongs to the superfamily of hormone-dependent transcription factors, the molecular mechanism by which it regulates gene expression is poorly understood. Binding of the MR to target gene promoters has never been characterized, and specific mineralocorticosteroid response elements (MREs) remain to be identified. The human MR (hMR) was overexpressed in Sf21 insect cells using the baculovirus system. The high degree of similarity between the glucocorticosteroid receptor (GR) and the MR prompted us to examine the DNA-binding properties of the recombinant MR with glucocorticosteroid-regulated genes. Gel shift mobility assays demonstrated that the recombinant receptor interacted with oligonucleotides containing perfect and imperfect palindromic sequences of GRE. A monoclonal anti-hMR antibody (FD4) induced a supershift of protein-DNA complexes and identified the MR in Western blot analysis. In vitro DNAse I protection assays with the hormone-regulated murine mammary tumour virus promoter showed that recombinant hMR generated four footprints whose limits encompassed the GRE motifs. By means of these two complementary approaches, no difference between the interaction of free, agonist- or antagonist-bound MR and DNA was detected. We provide evidence that hMR functions as a sequence-specific DNA-binding protein.

Regulation of expression of mouse mammary tumor virus through sequences located in the hormone response element: involvement of cell-cell contact and a negative regulatory factor

Mouse mammary tumor virus (MMTV) is a latently oncogenic retrovirus responsible for the neoplastic transformation of mammary epithelial cells. Its expression is regulated by steroids, polypeptide growth factors, and cell-type-specific factors. Using GR mouse mammary cells and NIH 3T3 fibroblasts stably transfected with chimeric constructs of the long terminal repeat region of MMTV, we have demonstrated a novel mechanism of cell-type-specific expression of this virus. In confluent monolayer cultures that permit cell-cell interaction, MMTV long terminal repeat expression is positively regulated by sequences within the hormone response element (HRE) that bind the transcription factors CTF/NFI and OTFI. Although these factors are present in NIH 3T3 cells, MMTV expression in these cells is not regulated by cell density. This is partially due to a negative regulatory factor that binds sequences between -164 and -151 in the HRE. Mutations that destroy the binding site for this factor restored in part the cell density-regulated expression of MMTV to NIH 3T3 fibroblasts. The HRE is thus a central coordinator of regulatory pathways that positively or negatively influence the expression of MMTV.

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.

Factors controlling the expression of mouse mammary tumour virus

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Down-regulation of beta-adrenergic receptors by pindolol in Gs alpha-transfected S49 cyc- murine lymphoma cells

The role of the alpha subunit of the guanine nucleotide-binding regulatory protein that stimulates adenylyl cyclase (GS alpha) in the down-regulation of beta-adrenergic receptors by pindolol was studied in S49 cyc- cells (normally GS alpha-deficient) transfected to express functional recombinant rat GS alpha. An inducible cell line (S49 GS alpha IND) was derived from S49 cyc- cells transfected with a vector containing the full-length coding sequence of GS alpha under the inducible control of the mouse mammary tumor virus long-terminal repeat promoter. GS alpha was not detectable in S49 GS alpha IND cells by immunoblot or by ADP-ribosylation in the presence of cholera toxin and [alpha-32P]NAD. When cells were grown in 100 nM dexamethasone, isoproterenol-stimulated cyclic AMP accumulation increased within 3 h. After 15 h, GS alpha was present at a level 40-50% of that found in S49 wild-type (WT) cells as measured either by immunoblot analysis or by [alpha-32P]ADP-ribosylation. Membranes prepared from GS alpha IND cells grown in the presence of dexamethasone bound agonist with high affinity, and this binding was sensitive to guanine nucleotides. A second vector, DzbGS alpha +, contained the coding sequence of GS alpha under the constitutive regulatory control of the SV40 early promoter. This vector was introduced into cyc- cells, and the resulting cells, S49 GS alpha CST cells, expressed GS alpha at a level comparable to that found in S49 WT cells as measured by immunoblot analysis. Isoproterenol-stimulated cyclic AMP accumulation in S49 GS alpha CST cells was at least as great as in S49 WT cells. When cells were grown in the presence of dexamethasone, exposure to 50 nM pindolol for 12 h down-regulated the density of beta-adrenergic receptors in S49 WT cells to 60% of that in cells grown in the absence of pindolol, but pindolol had no effect on the density of receptors on cyc- or GS alpha IND cells. When GS alpha CST cells were exposed to 50 nM pindolol for 12 h, the density of beta-adrenergic receptors was down-regulated by the same amount as in S49 WT cells. These results suggest that GS alpha is necessary to restore the ability of pindolol to down-regulate beta-adrenergic receptors in S49 cyc- cells and that the protein must be expressed at a level comparable to that found in S49 WT cells.

Expression of the mouse mammary tumor virus ORF gene in cultured cells

We have recently shown that expression vectors harboring the open reading frame of the long terminal repeat region of mouse mammary tumor virus direct the synthesis of a product which acts as a superantigen in transgenic mice. The detection of the ORF protein has been hampered by the extremely low levels of expression observed in these mice, as estimated from the low levels of specific mRNA. To study the properties of the ORF protein, we attempted its expression in different cell types in culture. The experiments performed in yeast show that the ORF gene product is a glycoprotein of approximately 45 kDA. As expected from the derived primary sequence, the unglycosylated product made in the presence of tunicamycin has a molecular weight of 36 kDA. No secretion of the glycosylated protein was observed. Curiously, the full-length molecule was made in lower amounts than a truncated version which contains only the C-terminal half of the protein. Transfection experiments in different mammalian cells suggest that high expression of the ORF protein might have an adverse effect on survival of cells in culture.

Cooperation between structural elements in hormono-regulated transcription from the mouse mammary tumor virus promoter

The mouse mammary tumor virus (MMTV) promoter is under the control of several types of regulatory agents. The proximal promoter within the long terminal repeat (LTR), from -200 to the CAP site and its regulation by steroid hormones have been extensively studied. However the precise role of sequences located upstream of this region remain unclear. We have constructed MMTV LTR deletion mutants coupled to the luciferase reporter gene and assayed their activities after transient transfection into transformed mammary epithelial cells (34i) and immortalized fibroblasts (NIH-3T3). In the absence of hormone, the MMTV promoter is almost silent, and deletions in the LTR have no significant effect on basal activity. In the presence of hormone, deletions spanning from the 5'-end to -455 have only slight effects on luciferase levels. In contrast, deletion of the region spanning from -450 to -201 leads to a dramatic decrease in transcription. A substantial decrease, more marked in 34i cells, is also clear when 90bp between -290 and -201 are deleted. At least one element cooperating positively with the glucocorticoid response element (GRE) is present between -223 and -201, as supported by the results of substitution mutation experiments. In 34i cell line, dexamethasone stimulates the MMTV LTR transcriptional activity to a level comparable to that of SV40. In contrast, in NIH-3T3 cells, MMTV promoter inducibility is weak. This results from a glucocorticoid receptor content 10-fold lower in NIH-3T3 cells than in 34i cells. Transfection of a glucocorticoid receptor expression plasmid allows recovery of a high inducibility of the MMTV promoter. This was true with all the MMTV LTR mutants studied here and suggests that NIH-3T3 cells possess all the factors necessary to cooperate with the steroid hormone in order to achieve a high transcriptional activity.

A superantigen encoded in the open reading frame of the 3' long terminal repeat of mouse mammary tumour virus

Mice express a collection of superantigens, which bind to class II major histocompatibility proteins and interact with T cells bearing particular V beta chains as part of their alpha beta receptors. These superantigens have been suggested to be encoded by exogenous or endogenous mouse mammary tumour viruses. One such superantigen is now shown to be encoded in the open reading frame of the long terminal repeat of a mammary tumour virus, a gene of previously unknown function.

Binding of the glucocorticoid receptor induces a topological change in plasmids containing the hormone-responsive element of mouse mammary tumor virus

Glucocorticoid hormones stimulate transcription of the mouse mammary tumor virus (MMTV) promoter by means of an interaction of the hormone receptor with a set of upstream DNA-binding sites called the hormone-responsive element (HRE). In an attempt to understand the underlying molecular mechanism, we have analyzed the influence of receptor binding on the topological state of plasmids carrying different parts of the HRE. Incubation of negatively supercoiled plasmids containing the intact HRE with the 94-kD purified glucocorticoid receptor, followed by extensive digestion with topoisomerase I, leads to a distribution of topoisomers shifted by two turns toward positively supercoiled circles. If relaxed plasmids are incubated with receptor and treated with topoisomerase I, the average linking number of the resulting distribution of topoisomers is also increased by two with respect to the controls. This effect depends on the presence of an intact HRE and is almost undetectable with the 40-kD form of the receptor. Thus, binding of the hormone receptor to functional DNA regulatory sites located in closed circular plasmids results in a topological change that could be related to the hormonal activation of transcription.

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.

Herpes simplex virus type 1 encodes two Fc receptors which have different binding characteristics for monomeric immunoglobulin G (IgG) and IgG complexes

Two herpes simplex virus type 1 glycoproteins, gE and gI, have been shown to form a complex that binds the Fc domain of immunoglobulin G (IgG). We demonstrate that this complex is required for the binding of monomeric nonimmune IgG but that gE alone is sufficient for binding polymeric IgG in the form of IgG complexes. Evidence that gE but not gI is required for binding IgG complexes is as follows. IgG complexes bound equally well to cells infected with gI-negative mutants or with wild-type virus, whereas cells infected with gE-negative mutants did not bind IgG complexes. Furthermore, L cells transiently transfected to express gE bound IgG complexes. Additional evidence that gI fails to augment binding of IgG complexes comes from experiments in which the gI gene was inducibly expressed in cells after infection. Inducible gI expression failed to increase binding of IgG complexes to infected cells in comparison with cells not capable of inducible gI expression. In contrast, expression of both gE and gI was necessary for binding of monomeric IgG, as demonstrated by flow cytometry using cells infected with gE-negative and gI-negative mutants. These observations demonstrate that herpes simplex virus type 1 Fc receptors (FcRs) have different binding characteristics for monomeric IgG and IgG complexes. Furthermore, it appears that gE is the FcR for IgG complexes and that gE and gI form the FcR for monomeric IgG.

The long terminal repeat region of the mouse mammary tumour virus contains multiple regulatory elements

Mouse mammary tumour virus (MMTV) is the major aetiologic agent of mouse mammary tumour formation. The expression of this virus is regulated by steroid hormones and cell type specific factors. The nucleotide sequence that controls the steroid hormone response has already been localized between -202 and -59 upstream of the start of transcription in the long terminal repeat (LTR) region of the proviral DNA. Through transfection experiments in three different cultured mouse cell lines (NIH3T3, NMuMG and GR), we have investigated which sequences in the MMTV LTR play a role in the cell type specific expression at the proviral promoter. We have identified two elements on the MMTV LTR from -631 to -560 and from -428 to -364 that have the potential to influence expression at the MMTV LTR promoter. The -631 to -560 element mediated a negative response in all the cell types we studied whereas the -428 to -364 element had negative effects in the mouse fibroblast NIH3T3 and the normal mouse mammary gland cell NMuMG but not in the mouse mammary tumour epithelial GR cells. The -428 to -364 element therefore contributes to the cell type specific expression at MMTV LTR promoter. We have also identified another regulatory element between -1094 and -739 that had a slight positive regulatory effect at the MMTV LTR promoter but greatly enhanced expression at a foreign promoter when present at this promoter in an orientation that is the reverse of its own orientation in the MMTV LTR. This orientation-dependent effect was only observed in the mouse mammary epithelial cells NMuMG and GR but not in mouse fibroblastic cell line NIH3T3. This element may be important in regulating the expression of neighbouring genes in a cell type specific manner. These results show that the MMTV LTR contains multiple regulatory elements necessary for the control of expression at its own promoter and the expression of neighbouring genes.

Nuclear factor-I and activator protein-2 bind in a mutually exclusive way to overlapping promoter sequences and trans-activate the human growth hormone gene

Transcription of the human growth hormone (hGH) gene and its regulation are controlled by trans-acting factors that bind to hGH gene promoter sequences. Several DNase I footprints have been described within 500 bp of this promoter, one of which (-289 to -267) has not yet been ascribed to a defined factor. By DNase I footprinting, gel mobility shift, and methylation interference assays with extracts from HeLa cells and GH-producing pituitary tumor (GC) cells, we show that this factor belongs to the NF-I family. When NF-I was competed out of the cell extracts, the trans-acting factor AP-2 bound to the same site as NF-I. AP-2 was present not only in HeLa cells, but also in GC cells albeit at a much lower concentration. Consistent with the mutually exclusive binding of NF-I and AP-2, their methylation interference patterns included four guanine residues that were crucial for binding of both NF-I and AP-2. Cell-free transcription from the hGH gene promoter showed that these two factors can transactivate this gene.

Mutations in the hormone regulatory element of mouse mammary tumor virus differentially affect the response to progestins, androgens, and glucocorticoids

Transcription of the mouse mammary tumor virus DNA is known to be induced by several steroid hormones. Using chimeric MMTV plasmids containing mutations within the hormone regulatory element, we have previously studied the regions required for the glucocorticoid response in mouse fibroblasts. Here we report the characterization of elements essential for the stimulation by progestins and androgens as compared with glucocorticoids. The same set of mutant plasmids was transfected into the human mammary tumor cell line T47D, and the specific transcripts were analyzed by an S1 nuclease protection assay. Androgen-mediated stimulation, although weak, showed an extended sensitivity to mutations, with a slight preference for the proximal region. The results with progestin suggest that sequences within all the described sites protected by the receptor in vitro are required and that the promoter-proximal region (-128 to -78 from the RNA start site) is more important than the distal one (-190 to -160). Moreover, a binding site for nuclear factor I was not required for the progestin response, whereas it was required for glucocorticoids. Thus, the various steroid receptors play a role in the differential regulation of mouse mammary tumor virus transcription by recognizing distinct sequence differences in the hormone regulatory element and interacting with different factors bound to the promoter.

Independent glucocorticoid induction and repression of two contiguous responsive genes

Specific DNA sequence elements which contain binding sites for the glucocorticoid receptor mediate the action of glucocorticoid hormones on gene transcription. In glucocorticoid-inducible genes, these glucocorticoid-responsive elements behave as hormone-inducible enhancers of transcription. We have taken advantage of the bovine papillomavirus (BPV) system to test the stringency of glucocorticoid regulation of transcription. BPV episomes were constructed to contain two hormone-regulated transcription units in close proximity; one transcription unit is under control of a glucocorticoid-inducible promoter (mouse mammary tumor virus) while the other is under control of a glucocorticoid-inhibited promoter (pro-opiomelanocortin). Glucocorticoids independently regulated transcription of the two physically linked transcription units, irrespective of their relative orientation and of their proximity on the BPV episomes. This result contrasts with the so-called position-independent activity of enhancers and suggests that the multicomponent organization of eucaryotic promoters restricts the action of hormone-responsive regulatory elements to a specific transcription unit, thus accounting for the stringency of hormonal regulation observed in vivo.

Use of a glucocorticoid-inducible promoter for expression of herpes simplex virus type 1 glycoprotein gC1, a cytotoxic protein in mammalian cells

Abundant expression of herpes simplex virus type 1 glycoprotein gC (gC1) in transfected mammalian cells has not previously been achieved, possibly because gC1 protein is toxic to cells. To approach this problem, the gC1 coding sequence was placed under the control of the weak but inducible glucocorticoid-responsive promoter from the mouse mammary tumor virus (MMTV) long terminal repeat (LTR). As controls to evaluate for gC1 cytotoxicity, the MMTV LTR promoter was used to express glycoprotein gD1, and a strong, constitutive promoter from the Moloney murine sarcoma virus LTR was used to express gC1. L cells were transfected with these constructs, and a clone expressing gC1 from the inducible MMTV LTR promoter was analyzed. In the absence of glucocorticoid (dexamethasone) stimulation, only a low level of gC1 mRNA expression was detected; after overnight stimulation with dexamethasone, transcription increased approximately 200-fold. Abundant gC1 protein that was functionally active in that it bound complement component C3b, was produced. From passages 5 through 26 (70 cell population doublings), the gC1-producing clone became less responsive to overnight dexamethasone stimulation. The block to gC1 expression occurred at the level of transcription and was associated with hypermethylation of the MMTV LTR DNA. Treatment of the clone with 5-aza-2'-deoxycytidine partially reversed the block in gC1 protein production. Late-passage cells assumed a gC1-negative phenotype that appeared to offer a selective growth advantage, which suggested that gC1 was cytotoxic. Several findings support this view: (i) some cells expressing gC1 after overnight stimulation with dexamethasone assumed bizarre, syncytial shapes; (ii) continuous stimulation with dexamethasone for 5 weeks resulted in death of most cells; (iii) cells transfected with gC1 under the control of the strong Moloney murine sarcoma virus promoter assumed bizarre shapes, and stable gC1-expressing clones could not be established; and (iv) cells induced to express gD1 retained a normal appearance after overnight stimulation or 15 weeks of continuous stimulation with dexamethasone. The inducible MMTV LTR promoter is useful for expressing gC1 and may have applications for expressing other cytotoxic proteins.

Mutations in the hormone regulatory element of mouse mammary tumor virus differentially affect the response to progestins, androgens, and glucocorticoids

Transcription of the mouse mammary tumor virus DNA is known to be induced by several steroid hormones. Using chimeric MMTV plasmids containing mutations within the hormone regulatory element, we have previously studied the regions required for the glucocorticoid response in mouse fibroblasts. Here we report the characterization of elements essential for the stimulation by progestins and androgens as compared with glucocorticoids. The same set of mutant plasmids was transfected into the human mammary tumor cell line T47D, and the specific transcripts were analyzed by an S1 nuclease protection assay. Androgen-mediated stimulation, although weak, showed an extended sensitivity to mutations, with a slight preference for the proximal region. The results with progestin suggest that sequences within all the described sites protected by the receptor in vitro are required and that the promoter-proximal region (-128 to -78 from the RNA start site) is more important than the distal one (-190 to -160). Moreover, a binding site for nuclear factor I was not required for the progestin response, whereas it was required for glucocorticoids. Thus, the various steroid receptors play a role in the differential regulation of mouse mammary tumor virus transcription by recognizing distinct sequence differences in the hormone regulatory element and interacting with different factors bound to the promoter.

Progesterone and glucocorticoid response elements occur in the long control regions of several human papillomaviruses involved in anogenital neoplasia

We have previously identified in the long control region of the genome of human papillomavirus type 16 (HPV-16) a DNA segment which functions as a cell-type-specific enhancer as well as mediating glucocorticoid response. It contains multiple transcription-factor-binding sites, including several for nuclear factor I and one for the glucocorticoid receptor, which binds to the partially palindromic sequence TGTACANNNTGTCAT. We report here that this sequence element, when separated from the surrounding transcription-factor-binding sites and placed as an oligonucleotide into a test vector, retains its function as a glucocorticoid response element (GRE) in HeLa cells. In T47D cells, which express the progesterone receptor, the HPV-16 enhancer fragment mediates progesterone responsiveness. A point mutant in this fragment and the response of the oligonucleotide clone to both steroids prove the identity of the progesterone response element (PRE) with the GRE. The antiprogesterone and antiglucocorticoid RU486 interferes with both hormonal responses. In SiHa cells, the HPV-16 GRE mediates an increase in transcripts encoding E6 and E7 proteins, which are involved in transformation by HPV-16. Hormonal regulation is not restricted to HPV-16: DNA segments containing the cell-type-specific enhancers of HPV-11 and HPV-18 also mediate glucocorticoid and progesterone response. We identified sequence elements in the long control regions of HPV-11 and HPV-18 which function as GRE/PREs when tested as oligonucleotides. These findings suggest that GRE/PREs are an integral part of gene expression regulation in genital HPVs.

Independent glucocorticoid induction and repression of two contiguous responsive genes

Specific DNA sequence elements which contain binding sites for the glucocorticoid receptor mediate the action of glucocorticoid hormones on gene transcription. In glucocorticoid-inducible genes, these glucocorticoid-responsive elements behave as hormone-inducible enhancers of transcription. We have taken advantage of the bovine papillomavirus (BPV) system to test the stringency of glucocorticoid regulation of transcription. BPV episomes were constructed to contain two hormone-regulated transcription units in close proximity; one transcription unit is under control of a glucocorticoid-inducible promoter (mouse mammary tumor virus) while the other is under control of a glucocorticoid-inhibited promoter (pro-opiomelanocortin). Glucocorticoids independently regulated transcription of the two physically linked transcription units, irrespective of their relative orientation and of their proximity on the BPV episomes. This result contrasts with the so-called position-independent activity of enhancers and suggests that the multicomponent organization of eucaryotic promoters restricts the action of hormone-responsive regulatory elements to a specific transcription unit, thus accounting for the stringency of hormonal regulation observed in vivo.

Use of a glucocorticoid-inducible promoter for expression of herpes simplex virus type 1 glycoprotein gC1, a cytotoxic protein in mammalian cells

Abundant expression of herpes simplex virus type 1 glycoprotein gC (gC1) in transfected mammalian cells has not previously been achieved, possibly because gC1 protein is toxic to cells. To approach this problem, the gC1 coding sequence was placed under the control of the weak but inducible glucocorticoid-responsive promoter from the mouse mammary tumor virus (MMTV) long terminal repeat (LTR). As controls to evaluate for gC1 cytotoxicity, the MMTV LTR promoter was used to express glycoprotein gD1, and a strong, constitutive promoter from the Moloney murine sarcoma virus LTR was used to express gC1. L cells were transfected with these constructs, and a clone expressing gC1 from the inducible MMTV LTR promoter was analyzed. In the absence of glucocorticoid (dexamethasone) stimulation, only a low level of gC1 mRNA expression was detected; after overnight stimulation with dexamethasone, transcription increased approximately 200-fold. Abundant gC1 protein that was functionally active in that it bound complement component C3b, was produced. From passages 5 through 26 (70 cell population doublings), the gC1-producing clone became less responsive to overnight dexamethasone stimulation. The block to gC1 expression occurred at the level of transcription and was associated with hypermethylation of the MMTV LTR DNA. Treatment of the clone with 5-aza-2'-deoxycytidine partially reversed the block in gC1 protein production. Late-passage cells assumed a gC1-negative phenotype that appeared to offer a selective growth advantage, which suggested that gC1 was cytotoxic. Several findings support this view: (i) some cells expressing gC1 after overnight stimulation with dexamethasone assumed bizarre, syncytial shapes; (ii) continuous stimulation with dexamethasone for 5 weeks resulted in death of most cells; (iii) cells transfected with gC1 under the control of the strong Moloney murine sarcoma virus promoter assumed bizarre shapes, and stable gC1-expressing clones could not be established; and (iv) cells induced to express gD1 retained a normal appearance after overnight stimulation or 15 weeks of continuous stimulation with dexamethasone. The inducible MMTV LTR promoter is useful for expressing gC1 and may have applications for expressing other cytotoxic proteins.