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

Co-administering Melatonin With an Estradiol-Progesterone Menopausal Hormone Therapy Represses Mammary Cancer Development in a Mouse Model of HER2-Positive Breast Cancer

Melatonin has numerous anti-cancer properties reported to influence cancer initiation, promotion, and metastasis. With the need for effective hormone therapies (HT) to treat menopausal symptoms without increasing breast cancer risk, co-administration of nocturnal melatonin with a natural, low-dose HT was evaluated in mice that develop primary and metastatic mammary cancer. Individually, melatonin (MEL) and estradiol-progesterone therapy (EPT) did not significantly affect mammary cancer development through age 14 months, but, when combined, the melatonin-estradiol-progesterone therapy (MEPT) significantly repressed tumor formation. This repression was due to effects on tumor incidence, but not latency. These results demonstrate that melatonin and the HT cooperate to decrease the mammary cancer risk. Melatonin and EPT also cooperate to alter the balance of the progesterone receptor (PR) isoforms by significantly increasing PRA protein expression only in MEPT mammary glands. Melatonin significantly suppressed amphiregulin transcripts in MEL and MEPT mammary glands, suggesting that amphiregulin together with the higher PRA:PRB balance and other factors may contribute to reducing cancer development in MEPT mice. Melatonin supplementation influenced mammary morphology by increasing tertiary branching in the mouse mammary glands and differentiation in human mammary epithelial cell cultures. Uterine weight in the luteal phase was elevated after long-term exposure to EPT, but not to MEPT, indicating that melatonin supplementation may reduce estrogen-induced uterine stimulation. Melatonin supplementation significantly decreased the incidence of grossly-detected lung metastases in MEL mice, suggesting that melatonin delays the formation of metastatic lesions and/or decreases aggressiveness in this model of HER2⁺ breast cancer. Mammary tumor development was similar in EPT and MEPT mice until age 8.6 months, but after 8.6 months, only MEPT continued to suppress cancer development. These data suggest that melatonin supplementation has a negligible effect in young MEPT mice, but is required in older mice to inhibit tumor formation. Since melatonin binding was significantly decreased in older mammary glands, irrespective of treatment, melatonin supplementation may overcome reduced melatonin responsiveness in the aged MEPT mice. Since melatonin levels are known to decline near menopause, nocturnal melatonin supplementation may also be needed in aging women to cooperate with HT to decrease breast cancer risk.

DC-SCRIPT regulates glucocorticoid receptor function and expression of its target GILZ in dendritic cells

Dendritic cells (DCs) play a central role in the immune system; they can induce immunity or tolerance depending on diverse factors in the DC environment. Pathogens, but also tissue damage, hormones, and vitamins, affect DC activation and maturation. In particular, glucocorticoids (GCs) are known for their immunosuppressive effect on DCs, creating tolerogenic DCs. GCs activate the type I nuclear receptor (NR) glucocorticoid receptor (GR), followed by induced expression of the transcription factor glucocorticoid-inducible leucine zipper (GILZ). GILZ has been shown to be necessary and sufficient for GC-induced tolerogenic DC generation. Recently, we have identified the DC-specific transcript (DC-SCRIPT) as an NR coregulator, suppressing type I steroid NRs estrogen receptor and progesterone receptor. In this study, we analyzed the effect of DC-SCRIPT on GR activity. We demonstrate that DC-SCRIPT coexists with GR in protein complexes and functions as a corepressor of GR-mediated transcription. Coexpression of DC-SCRIPT and GR is shown in human monocyte-derived DCs, and DC-SCRIPT knockdown enhances GR-dependent upregulation of GILZ mRNA expression in DCs. This demonstrates that DC-SCRIPT serves an important role in regulating GR function in DCs, corepressing GR-dependent upregulation of the tolerance-inducing transcription factor GILZ. These data imply that by controlling GR function and GILZ expression DC-SCRIPT is potentially involved in the balance between tolerance and immunity.

Progesterone receptors, their isoforms and progesterone regulated transcription

This review discusses mechanisms by which progesterone receptors (PR) regulate transcription. We examine available data in different species and tissues regarding: (1) regulation of PR levels; and (2) expression profiling of progestin-regulated genes by total PRs, or their PRA and PRB isoforms. (3) We address current views about the composition of progesterone response elements, and postulate that PR monomers acting through "half-site" elements are common, entailing cooperativity with neighboring DNA-bound transcription factors. (4) We summarize transcription data for multiple progestin-regulated promoters as directed by total PR, or PRA vs. PRB. We conclude that current models and methods used to study PR function are problematical, and recommend that future work employ cells and receptors appropriate to the species, focusing on analyses of the effects of endogenous receptors targeting endogenous genes in native chromatin.

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.

A 5' distal palindrome within the mouse mammary tumor virus-long terminal repeat recruits a mammary gland-specific complex and is required for a synergistic response to progesterone plus prolactin

Progesterone (P) and prolactin (PRL) fulfill crucial roles during growth and differentiation of the mammary epithelium, and each has been implicated in the pathogenesis of mammary cancer. We previously identified that these hormones synergistically stimulate the proliferation of mouse mammary epithelial cells in vivo, although the mechanism(s) underlying their cooperative effect are unknown. We now report a novel pathway by which P and PRL synergize to activate transcription from the long terminal repeat (LTR) of the mouse mammary tumor virus-LTR (MMTV-LTR) in T47D breast cancer cells. Using serial 5' and 3' deletions of the MMTV-LTR, in addition to selective mutations, we identified that a previously uncharacterized inverted palindrome on the distal enhancer (-941/-930), in addition to a signal transducer and activator of transcription 5 site, was essential for the synergistic activation of transcription by P and PRL. Notably, hormone synergy occurred via a mechanism that was independent of the P receptor DNA-binding elements found in the proximal MMTV-LTR hormone-response element. The palindrome specifically recruited a protein complex (herein termed mammary gland-specific complex) that was almost exclusive to normal and cancerous mammary cells. The synergy between P and PRL occurred via a Janus kinase 2 and c-Src/Fyn-dependent signaling cascade downstream of P and PRL receptors. Combined, our data outline a novel pathway in T47D cells that may facilitate the action(s) of P and PRL during mammary development and breast cancer.

Cofactor of BRCA1 modulates androgen-dependent transcription and alternative splicing

Transcriptional activity of nuclear receptors (NRs) is influenced by a large number of coregulators that exert their actions predominantly at the transcription initiation step. Unlike most well-characterized NR coregulators, cofactor of BRCA1 (COBRA1), a subunit of the negative elongation factor (NELF), binds to estrogen receptor alpha (ERalpha) and modulates estrogen-dependent transcription by impeding the movement of RNA polymerase II (RNAPII) during the transcription elongation stage. Here we show that, in addition to ERalpha, COBRA1 also displays various degrees of affinity for several other NRs. In particular, COBRA1 binds strongly to androgen receptor (AR) via its ligand-binding domain (LBD). Small hairpin RNA (shRNA)-mediated reduction of endogenous COBRA1 enhances androgen-mediated transcription. The effect of COBRA1 knockdown can be rescued by a silent mutant COBRA1 that is refractory to the shRNA action. Using a reporter assay for alternative splicing, we also provide evidence for a role of COBRA1 in influencing the exon skipping/inclusion of nascent transcripts produced from an androgen-dependent promoter. These findings suggest that COBRA1 may coordinate multiple steps in ligand-dependent gene expression, which in turn ensures both the quantity and quality of hormone-stimulated gene products.

Androgen regulation of the human hair follicle: the type I hair keratin hHa7 is a direct target gene in trichocytes

Previous work had shown that most members of the complex human hair keratin family were expressed in terminal scalp hairs. An exception to this rule was the type I hair keratin hHa7, which was only detected in some but not all vellus hairs of the human scalp (Langbein et al, 1999). Here we show that hHa7 exhibits constitutive expression in medullary cells of all types of male and female sexual hairs. Medullated beard, axillary, and pubic hairs arise during puberty from small, unmedullated vellus hairs under the influence of circulating androgens. This suggested an androgen-controlled expression of the hHa7 gene. Further evidence for this assumption was provided by the demonstration of androgen receptor (AR) expression in the nuclei of medullary cells of beard hairs. Moreover, homology search for the semipalindromic androgen receptor-binding element (ARE) consensus sequence GG(A)/(T)ACAnnnTGTTCT in the proximal hHa7 promoter revealed three putative ARE motifs. Electrophoretic mobility shift assays demonstrated the specific binding of AR to all three hHa7 AREs. Their function as AR-responsive elements, either individually or in concert within the hHa7 promoter, could be further confirmed by transfection studies with or without an AR expression vector in PtK2 and prostate PC3-Arwt cells, respectively in the presence or absence of a synthetic androgen. Our study detected hHa7 as the first gene in hair follicle trichocytes whose expression appears to be directly regulated by androgens. As such, hHa7 represents a marker for androgen action on hair follicles and might be a suitable tool for investigations of androgen-dependent hair disorders.

A G577R mutation in the human AR P box results in selective decreases in DNA binding and in partial androgen insensitivity syndrome

We have characterized a novel mutation of the human AR, G577R, associated with partial androgen insensitivity syndrome. G577 is the first amino acid of the P box, a region crucial for the selectivity of receptor/DNA interaction. Although the equivalent amino acid in the GR (also Gly) is not involved in DNA interaction, the residue at the same position in the ER (Glu) interacts with the two central base pairs in the PuGGTCA motif. Using a panel of 16 palindromic probes that differ in these base pairs (PuGNNCA) in gel shift experiments with either the AR DNA-binding domain or the full length receptor, we observed that the G577R mutation does not induce binding to probes that are not recognized by the wild-type AR. However, binding to the four PuGNACA elements recognized by the wild-type AR was affected to different degrees, resulting in an altered selectivity of DNA response element recognition. In particular, AR-G577R did not interact with PuGGACA palindromes. Modeling of the complex between mutant AR and PuGNACA motifs indicates that the destabilizing effect of the mutation is attributable to a steric clash between the C beta of Arg at position 1 of the P box and the methyl group of the second thymine residue in the TGTTCPy arm of the palindrome. In addition, the Arg side chain can interact with G or T at the next position (PuGCACA and PuGAACA elements, respectively). The presence of C is not favorable, however, because of incompatible charges, abrogating binding to the PuGGACA element. Transactivation of several natural or synthetic promoters containing PuGGACA motifs was drastically reduced by the G577R mutation. These data suggest that androgen target genes may be differentially affected by the G577R mutation, the first natural mutation characterized that alters the selectivity of the AR/DNA interaction. This type of mutation may thus contribute to the diversity of phenotypes associated with partial androgen insensitivity syndrome.

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

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

Beneficial role of human endogenous retroviruses: facts and hypotheses

Human endogenous retroviruses (HERVs) have recently been suggested as mediators of normal biological processes such as cellular differentiation and regulation of gene expression. Moreover, a direct role for HERVs in pathogenesis and the development of disease is now better appreciated. Elucidation of the mechanisms regulating HERV biology should provide information about fundamental cellular activities and the pathogenesis of multifactorial diseases such as cancer and autoimmune disease. The importance of understanding the roles of HERVs is underscored by the recently obtained insight that activation of endogenous retroviruses poses potential risks following xenotransplantation and in gene therapy using retroviral vectors. Furthermore, HERV-encoded superantigens have recently been implicated as causes of autoimmune disease. This review discusses the established and possible biological roles of HERVs, and proposes hypotheses concerning their involvement as mediators of fundamental cellular responses. We propose that the evolutionary persistence of endogenous retroviruses in the genomes of eukaryotic cells reflects their indispensability in important normal functions in specialized cellular environments. HERVs can also be potentially hazardous through their involvement in the development of disease. In addition, the creation of new retroviruses can occur through recombination, between different HERVs and between HERVs and exogenous retroviruses.

Differential gene induction by glucocorticoid and progesterone receptors

The question of how glucocorticoid, progesterone, androgen, and mineralocorticoid receptors can generate distinct patterns of gene expression despite similar, if not identical, DNA sequence recognition properties is a central question in steroid biology. This study addresses the hypothesis that glucocorticoid and progesterone receptors can differentially utilize the promoter context created by a series of receptor recognition sites. This hypothesis predicts that for different receptors an individual (often suboptimal) binding site contributes to the overall response element activity to a different degree. Therefore, mutations to an individual binding site of a multipartite hormone response element may differentially affect the response to different steroids. This study tests this hypothesis by introducing mutations into a receptor recognition site that is part of a hormone responsive promoter derived from the mouse mammary tumor virus. We find that weakening one site of a multipartite element differentially affects glucocorticoid and progestin signaling both in fibroblasts and in mammary carcinoma cells. A similar test was done in a simplified promoter context comprised only of a TATA box and a pair of receptor recognition sites. Mutations introduced into one of these sites impaired glucocorticoid response more than the progestin response. However, high receptor expression can partially overcome the effect of some target site mutations. Thus both receptor expression levels and the inherent ability to utilize the context created by a multipartite response element contribute to quantitative differences in the response to receptors that share DNA sequence recognition properties.

Antiprogestins mediate differential effects on glucocorticoid receptor remodeling of chromatin structure

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

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

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

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.

Steroid hormone receptor status defines the MMTV promoter chromatin structure in vivo

The ability to respond to small signalling molecules such as steroid hormones is important for many physiological processes. Steroid hormones act through a group of high affinity receptors that regulate transcription by binding to hormone response elements (HREs) located within the promoters of target genes, which themselves are organized with nuclear proteins to form chromatin. To dissect the mechanisms(s) of steroid hormone action we have used the steroid inducible mouse mammary tumor virus (MMTV) promoter as a model system. The MMTV promoter is assembled into a phased array of nucleosomes that are specifically positioned in rodent cells. Induction of transcription by glucocorticoids is accompanied by the appearance of a hypersensitive region in the proximal promoter which allows the hormone dependent assembly of a preinitiation complex including transcription factors such as nuclear factor 1 (NF1) and the octamer transcription factor (OTF). Surprisingly, when introduced by transient transfection, the progesterone receptor (PR) is unable to activate this promoter in vivo, a finding that may result from its inability to alter MMTV promoter chromatin. In an attempt to investigate the failure of the PR to activate the promoter, we have stably introduced the MMTV promoter into human T47D breast cancer cells that express high levels of the PR. In contrast to what has been observed previously in rodent cells, the MMTV templates resident in human breast cancer cells adopt a novel and constitutively open chromatin structure. The constitutively open chromatin structure is accompanied by the hormone independent loading of transcription factors including the PR and NF1. In T47D cells that stably express the glucocorticoid receptor, the MMTV promoter responds to glucocorticoids, but not progestins, and displays glucocorticoid induced restriction enzyme hypersensitivity and transcription factor loading. These findings suggest that the organization of the MMTV chromatin structure is dependent upon the cell type and receptor status of the recipient cell into which the MMTV promoter is stably introduced.

Chromatin structure modulates transcription factor binding to the mouse mammary tumor virus (MMTV) promoter

The MMTV promoter contains a complex hormone responsive region (HRR) upstream of a binding site for the transcription factor nuclear factor I (NFI). Hormonal induction of MMTV expression requires the integrity of both the HRR and the NFI binding site. However, in vitro NFI acts as a basal transcription factor on the MMTV promoter that does not cooperate but rather competes with the hormone receptors in terms of binding to MMTV-DNA. Fragments that contain the HRR and the NFI binding site have been reconstituted into mononucleosomes. Steroid hormone receptors bind efficiently to these nucleosomes, NFI does not. Therefore it has been postulated that the chromatin structure may be responsible for the inability of NFI to bind to the chromosomally organized inactive MMTV promoter. In vivo DNaseI and methidium-propyl-EDTA-Fe(II) (MPE) digestion pattern indicate the presence of a nucleosome covering the HRR and the NFI binding site. Genomic footprinting shows that in vivo the rotational setting of the MMTV promoter DNA in this nucleosome is identical to that previously reported for reconstituted nucleosomes in which the major grooves of the NFI half palindromes are facing towards the histone octamer and appear not to be accessible to NFI. These results indicate that MMTV promoter sequences are determining nucleosome positioning in vivo and supports the concept that rotational positioning of DNA in this nucleosome constitutively represses the MMTV promoter.

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.

Stable transfection of androgen receptor and MMTV-CAT into mammalian cells: inhibition of cat expression by anti-androgens

A hormone-inducible transcriptional system has been established, based on the stable transfection of the rat androgen receptor (rAR) and a reporter plasmid containing the mouse mammary tumour virus promoter linked to the chloramphenicol acetyltransferase gene (pMMTV-CAT) into steroid receptor-negative CV-1 cells. First, the rAR was stably introduced into CV-1 cells. Single clones were tested for stable expression of functionally active AR by analysing the effect of dihydrotestosterone on induction of transiently transfected pMMTV-CAT. Stable transfection and the expression of AR was confirmed by steroid-binding assays. In a second step, a clone expressing physiological amounts of AR protein (30 fmol/mg protein) was stably transfected with pMMTV-CAT to yield a permanent cell line that stably expresses functional AR and MMTV-CAT sequences. This cell line provides a powerful tool for the efficient and accurate determination and quantification of the effects of androgens and anti-androgens on reporter gene transcription. This was demonstrated by investigating the action of the three anti-androgens hydroxyflutamide, casodex and cyproterone acetate. The three compounds were shown to reverse the effects of the androgen R1881 on gene expression but were themselves devoid of agonistic activity.

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.

In two genes, synergism of steroid hormone action is not mediated by cooperative binding of receptors to adjacent sites

Synergistic action of multiple steroid hormone response elements (HREs) has been proposed to be due to cooperative binding of receptors. We have studied the cooperativity of steroid hormone receptor binding to synergistic HREs in two natural genes. In the mouse mammary tumor virus long terminal repeat that contains four progesterone receptor binding sites, no cooperativity in receptor binding was observed between the single distal and the three proximal sites whereas a low level of cooperativity in receptor binding (about 2-fold) was found between the three proximal sites. This contrasted with the very strong synergism of these four HREs in stimulation of transcription. In the chicken vitellogenin II gene upstream sequences, an estrogen and a progestin response elements act synergistically. In this case again, no cooperativity of binding of the estrogen and progesterone receptors to their respective binding sites was observed. We therefore conclude that cooperative receptor binding may not always be required for synergistic action of multiple HREs.

Identification of a novel mammary cell line-specific enhancer element in the long terminal repeat of mouse mammary tumor virus, which interacts with its hormone-responsive element

We analyzed the long terminal repeat (LTR) of mouse mammary tumor virus for sequences that influence its promoter activity by using the chloramphenicol acetyltransferase assay. A series of LTR deletion mutants and recombinants between LTR and simian virus 40 regulatory sequences were used for these studies. Through transfection experiments in three different human cell lines (T47D, MCF-7, and HeLa), we identified a novel mammary cell line-specific enhancer element on a 98-bp BanII fragment (from position -1075 to -978 upstream of the start site of transcription) which interacts with the hormone-responsive element of LTR. We also identified nuclear factors that specifically interacted with this BanII fragment in the nuclear extract from the mammary tumor cell line, T47D, but not from the HeLa cell line.

Oestrogen enhances the responsiveness of the MMTV-LTR to glucocorticoid in ZR-75-1 human breast cancer cells

Treatment of ZR-75-1 human breast cancer cells with oestrogen has no direct effect on the expression of a transfected MMTV-LTR but enhances its inducibility in response to glucocorticoid treatment. This effect which can be produced with both oestradiol and diethylstilbestrol is specific to induction of the MMTV-LTR, no effect of the treatment on expression driven by the RSV-LTR being observed. The effect can be observed in cells pre-treated with oestrogen prior to removal of DNA and glucocorticoid addition but not in cells where oestrogen is added after removal of the DNA. The possible mechanisms of these effects and their relationship to the induction of oestrogen-responsive genes by this hormone are discussed.