Conditions for optimal MTV synthesis in vitro and the effect of steroid hormones on virus production

Characterization of fatty acid synthase in cell lines derived from experimental mammary tumors

The lipogenic enzyme fatty acid synthase (FAS) is elevated in various human primary cancers and certain human cancer cell lines. FAS overexpression in human neoplasia has clinical relevance because of its association with tumor aggression and potential chemotherapeutic intervention. Here, we surveyed FAS in cell lines established from normal murine mammary epithelium (NMuMG) and from mammary tumors induced by either rodent polyoma (Py) virus or murine mammary tumor virus (MMTV). Western blotting revealed greater content of FAS in Py-transformed A1-1 and T1 than NMuMG or MMTV-transformed Mm5MT, RIIIMT and MMT060562. These data suggest that signaling events mediated by Py transformation may increase cellular amounts of FAS. Although FAS content was elevated to similar levels in A1-1 and T1, specific activities were significantly different as enzyme activity in T1 was 3-fold higher than A1-1. Likewise, FAS activity in NMuMG was about 0.5-fold higher than the MMTV-transformed lines, even though enzyme content was similar. Immunoprecipitation studies employing anti-phosphoamino acid antibodies followed by immunoblot analysis with anti-FAS antisera (and vice versa) were used to characterize the constitutive phosphorylation state of the enzyme. Phosphoserine and phosphothreonine residues were detected in the more active FAS from T1 and NMuMG, but not in the less active FAS from Mm5MT or A1-1. Discovery of phosphorylated FAS suggests that the enzyme may have more immediate control over lipogenesis than previously thought. High-dose (10-4 M) dexamethasone induced FAS content and activity in NMuMG and MMTV-transformed lines but not Py-transformed cells. Lower concentrations (10-8, 10-6 M) of dexamethasone also activated FAS but without concomitant elevation of its protein content, which was consistent with a phosphorylated form of FAS. Finally, cell lines were treated with the FAS inhibitor cerulenin: almost all breast cancer lines were growth inhibited at significantly lower amounts of drug than normal cell lineages, suggesting that FAS plays a greater role in viability of tumor cells than normal cells. Pretreatment with palmitate (a primary end-product of FAS) prior to cerulenin rescued A1-1 cells only slightly from growth inhibition, whereas pretreatment with oleate (a monounsaturated fatty acid synthesized from palmitate) synergized cerulenin's cytotoxic effects.

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.

Stimulation of polydnavirus replication by 20-hydroxyecdysone

During oviposition the endoparasitic wasp Campoletis sonorensis, introduces a polydnavirus into parasitized insects where viral gene expression is required for endoparasite survival. The polydnavirus is integrated into wasp chromosomal DNA and replicates only in the ovary. Ecdysteroids regulate the developmental expression of many insect genes and may regulate polydnavirus replication. Direct verification of viral replication was performed by dot blot hybridization and by amplifying DNA sequences containing the viral integration site; this 'junction' fragment cannot be amplified from integrated virus. Thoracic ligations and in vitro ecdysteroid treatments of wasp ovaries support the hypothesis that polydnavirus DNA replication is regulated by ecdysteroid during parasite development.

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

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

Factors controlling the expression of mouse mammary tumour virus

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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.

Estrogen influence on maturational pathway of murine mammary tumor virus: an immunoelectron microscopy study

Modifications induced by estrogens on hormone-independent murine mammary tumor (MMT) and its main etiological agent, the MMT virus (MMTV), are reported. High doses of estrogens released continuously from silastic capsules delay significantly the development of transplanted tumors into syngeneic hosts. Neoplastic cells present a striking cytoplasmic vacuolization and changes in the MMTV differentiation pattern. Mature virions are detected budding into cytoplasmic vacuoles instead of the extracellular space as in spontaneous and untreated transplanted tumors. This phenomenon is reversed after estrogen withdrawal at the first sign of tumor development. Application of electron microscope immunocytochemistry with colloidal gold-protein A complex and multiple monospecific antibodies reveals several interesting features. In spontaneous and untreated tumor grafts, structural viral proteins p14 and p25 appear in both intracytoplasmic capsids and mature extracellular viruses. By contrast glycoprotein gp55 labels only the envelope of mature virus. In estrogen-treated tumors this antigenic pattern is modified and the gp55 is detected in those atypical virions maturing into the intracytoplasmic vacuoles. These observations led to the conclusions that the delay in the development of hormone-independent mammary tumors caused by estrogen is due to an abnormal maturational viral process and that estrogens induce alterations of polarity in the translocation process of viral envelope glycoproteins.

Effects of cytoskeletal disrupting agents on mouse mammary tumor virus replication

Alterations in mouse mammary tumor virus (MMTV) production and composition were induced by exposure of mammary tumor cells to cytodisruptive agents. Treatment with 2.1 microM cytochalasin D (CD) for 24 h reduced MMTV yield by 80% and electron microscopic examination of these cells did not reveal budding virions. Immune precipitation and quantitative immunofluorescence studies demonstrated that CD had no significant effect on MMTV polypeptide synthesis or surface expression suggesting that CD inhibited late steps in MMTV maturation. Decreases in MMTV production were also observed as a result of 24 h exposure of the cells to 2.1 microM cytochalasin B (CB). However, an initial 70% increase in the levels of extracellular virions within the first 18 h of treatment preceded diminution of virus production. In addition, CB was unable to abrogate maturation and release of MMTV particles as revealed by electron microscopic evaluation of thin sections of treated cells. Colcemid at 0.28 microM had no effect on virus production during the first 24 h of exposure although MMTV yield was reduced by 60-70% after 36 h of treatment. Polypeptide profiles of MMTV purified from cell cultures treated with any of the three cytodisruptive agents were altered and included 5-7 polypeptides not typically present in MMTV from untreated cells. These cytodisruptive agents did not significantly affect viability and protein metabolism of MJY-alpha cells; the data suggest that alterations in MMTV replication were due to disruption of the cellular cytoskeleton.

Exogenous mouse mammary tumor virus proviral DNA isolated from a kidney adenocarcinoma cell line contains alterations in the U3 region of the long terminal repeat

Mouse mammary tumor virus (MMTV) is a B-type retrovirus which induces predominantly mammary carcinomas after a relatively long latency period. To date, very little is known about the reasons for the strict tissue specificity of MMTV. The BALB/cf/Cd strain of mice, which was infected with milk-borne MMTV (C3H), shows a high incidence of kidney adenocarcinomas, and our data suggest that MMTV might be involved in the formation of these tumors. Newly integrated exogenous MMTV proviruses were found in the genome of transplanted tumor cells as well as in the DNA of a cell line derived from one tumor, but not in normal cells of BALB/cf/Cd mice. The MMTV DNA in these tumor cells was transcribed and viral RNA synthesis was strongly stimulated by glucocorticoid hormones. Viral structural polypeptides, comparable in size and antigenicity to MMTV polypeptides of infected mammary tumor cells were synthesized and processed normally in the cell line and were organized correctly into intracytoplasmic particles. Heteroduplex analysis of the molecularly cloned MMTV proviral DNAs of kidney and mammary tumor origin revealed a high degree of homology in the gag, pol, and env genes. A striking difference, however, was observed in the U3 region of the two LTRs that might relate to the different tissue specificity of the two viruses.

Prostaglandin regulation of murine mammary tumor virus production: a basis for some of the glucocorticoid and prolactin actions on mammary tumor cell cultures

Hormonal regulation of mouse mammary tumor virus (MuMTV) production has been studied in cell cultures derived from mammary carcinomas of GR mice. The purpose of this study was to define the role of prolactin in dexamethasone-induced MuMTV production and to evaluate the mechanism(s) of action of these hormones. Results of our investigations in vitro establish a central role for prostaglandins, in particular PGF2 alpha. Prolactin treatment up to 1000 ng/ml increased the MuMTV production only slightly when added alone but a maximal stimulatory response was expressed when prolactin was added simultaneously with dexamethasone 2 micrograms/ml. This prolactin-related enhancement of MuMTV production was probably mediated by PGs since it was inhibited by indomethacin, an inhibitor of prostaglandin synthesis. When higher concentrations of indomethacin were used in dexamethasone-treated cultures, MuMTV production, although inhibited, was restored in part by addition of exogeneous PGF2 alpha. PGF2 alpha added alone stimulated the MuMTV production and, in combination with dexamethasone, showed a synergistic effect on stimulation of MuMTV production. Two compounds, PGF2 alpha and TXA2 (measured as TXB2), were produced in significant concentrations by these cultures. These findings suggest that PGF2 alpha exerts a regulating role on production of MuMTV in GR cells and indicate that the actions of both dexamethasone and prolactin are mediated at least in part by prostaglandins.

Role of natural prostaglandins in the control of murine mammary tumor virus expression

The regulation of murine mammary tumor virus (MuMTV) production by mammotropic hormones, hormonomimetic substances, and cyclic nucleotides was investigated. The virus produced in control and treated mammary tumor cell cultures was quantitated by measuring the supernatant reverse transcriptase activity in exogenous reaction using poly(rC).oligo(dG) as template-primer. Two days after exposure, the synthetic glucocorticoid, dexamethasone (DXMT), increased spontaneous MuMTV production at optimal concentration (0.1 mumol) up to ten times. Dibutyryl derivative of cyclic AMP had no effect on spontaneous MuMTV production, whereas the drug potentiated suboptimal concentrations of the glucocorticoid. Natural prostaglandins, potent agonists of adenylate cyclase catalyzing intracellular synthesis of cyclic AMP, enhanced both basal (up to five times) and DXMT-stimulated (up to 1.6 times) MuMTV replication. The MuMTV-stimulating activity of prostaglandins decreased in the order of PGA1 greater than PGE1 greater than PGB1 greater than PGF2 alpha. Prostaglandins can be replaced partially by norepinephrine and isoproterenol by enhancing the DXMT-mediated MuMTV stimulation, whereas these drugs remained without effect on spontaneous MuMTV production. Theophylline, an antagonist of cAMP-phosphodiesterase converting cAMP to AMP, enhanced the virus-stimulating activity of DXMT as well as of prostaglandins. The enhancement of MuMTV production by adenylate cyclase agonists do not correlate absolutely with the estimates of intracellular cAMP levels, since the highest amounts of cAMP has been repeatedly observed in cells treated with PGE1 and norepinephrine. The results indicate that besides hormones, other hormone-like substances and cyclic nucleotides may be involved in the complex mechanism of hormone-regulated MuMTV genome expression.

Replication of human syncytium-forming virus in human cells: effect of certain biological factors and selective chemicals

The growth characteristics of the human syncytium-forming virus (HSFV) were examined in several human cell lines of normal and malignant origins and composing of either fibroblastic or epithelial-like cells. Virus production occurred only in the fibroblastic diploid cell lines: HEF (human embryonic cells, Flow #5,000) and HFDL #645 (human fetal diploid lung), but not in the epithelial-like heteroploid cell lines: RA (a continuous line of human amnion), #999 (human bone marrow), and KB (carcinoma of the nasopharynx). While the single-cycle growth pattern of the virus in HEF and HFDL #645 cell lines were essentially similar, the virus yield per cell was greater in the HFDL #645 cells. Furthermore, the physiological state of the cell had a marked effect on virus production. Subconfluent actively growing HFDL #645 cells produced higher yields of virus than density-inhibited confluent HFDL #645 cell cultures. The replication of HSFV was inhibited by actinomycin D at concentrations that did not interfere with poliovirus replication (0.001 to 0.01 microgram/ml). Pretreatment and posttreatments of infected cell cultures with either the polycation polybrene (hexadimethrine bromide) or the synthetic glucocorticosteroid dexamethasone did not enhance HSFV production.

Glucocorticoid regulation of mouse mammary tumor virus gene expression

Glucocorticoid hormones act rapidly and specifically to stimulate the synthesis of mouse mammary tumor virus RNA in a variety of mouse mammary tumor cells and infected heterologous cells. The increase in viral RNA production appears to be mediated by receptor proteins and requires the presence of basal levels of viral RNA. Infection of heterologous cells with MMTV may alter host cell responses to glucocorticoids; in addition, production of unintegrated viral DNA in these cells has provided reagents required for studying the structure and function of the viral DNA itself. The advent of new techniques for genetic manipulation of eukaryotic cells and for isolation of large amounts of specific DNA sequences should now permit detailed analyses of steroid hormone action in this system.

Simultaneous production of casein and mammary tumor virus in mouse mammary epithelial cells grown on floating collagen gels

Simultaneous production of casein and mammary tumor virus (MTV) was analyzed in monolayer cultures of mammary epithelial cells from pregnant BALB/cfC3H mice. A comparison of the 2 cell culture substrata, plastic culture dish and floating collagen gel, showed that the latter supported a much higher degree of simultaneous casein and MTV production in the presence of insulin, cortisol and prolactin in serum-free culture medium. Importance of floating collagen gel was further shown by delaying the flotation of gels. When the release of gels was delayed, there were concomitant delays in the increase of casein and MTV production. These results indicate that hormones, nature of substratum and flotation regulate the degree of differentiation of mammary epithelial cells in vitro.

Regulation of mouse mammary tumor virus gene expression by glucocorticoid hormones

Several laboratories have documented that glucocorticoid hormones markedly stimulate the expression of mouse mammary tumor virus genes in a variety of mouse mammary tumor cells and in infected heterologous cells. The effect of the hormone appears to be a rapid and specific augmentation of the synthesis of viral RNA, mediated by interaction with glucocorticoid receptor proteins. The availability of virus-specific reagents and recent developments in the molecular biology of RNA tumor viruses now permit a highly refined analysis of hormonal regulation in this experimental system.

Effects of testosterone on messenger ribonucleic acid and protein synthesis in rat seminal vesicle

In a previous report [Higgins et al. (1976) Biochem. J.158, 271-282] we described the effects of alterations in androgen status on the synthesis of two basic secretory proteins of the rat seminal vesicle. In the present paper we examine the effects of testosterone on the activity of mRNA in the seminal vesicle. Total cellular poly(A)-rich RNA was isolated and translated in a cell-free system prepared from wheat germ. Translation products were separated on denaturing polyacrylamide gels and the protein bands corresponding to the two basic secretory proteins were identified immunologically. Incorporation of radioactive methionine into these bands was taken as a measure of the individual mRNA activities. Total mRNA activity was estimated by radioactivity in total acid-precipitable material. The results show that 1 to 2 weeks after castration the activities of mRNA molecules for the basic secretory proteins were decreased 10-20-fold on a tissue basis. Testosterone given in vivo rapidly and substantially restores mRNA activity to normal. Since these changes correlate closely with variations in the rates of synthesis of the secretory proteins in whole cells it suggests that androgenic steroids control protein synthesis chiefly via mRNA availability. In this respect their action resembles those of other steroid hormones acting in other systems. However, these effects of testosterone on the mRNA molecules for the major secretory proteins could not be distinguished from those on total mRNA. Thus the proportion of the total mRNA population accounted for by the two specific mRNA molecules showed less than a 2-fold variation with androgen status. Similarly the two secretory proteins always accounted for 25-33% of general protein synthesis. This is in sharp contrast with the markedly differential effects of other steroid hormones controlling synthesis of major proteins in other well-studied systems. We interpret our results as indicating that testosterone regulates the mRNA population of the seminal vesicle as a whole.

Demonstration of components of serum-free culture medium effecting maximum in vitro expression of mouse mammary tumor virus

By using a chemically defined serum-free (SF) medium for propagation of Mm5mtc1 mouse adenocarcinoma cell cultures and clonal derivatives, medium components including hormones, glucose and individual amino acids were evaluated as to modulation of mouse mammary tumor virus (MMTV) porduction. Insulin, hydrocortisone and dexamethasone each increased MMTV production on a per cell basis over constitutive expression that occurs in SF medium devoid of hormones. Maximum production occurred when all three hormones were present. Hormone-stimulated virus expression also was influenced by glucose concentration. Cell growth and maximum MMTV expression increased when thyroxine, asparagine, proline and serine were omitted from the medium formulation. The resulting modified SF medium provides and ideal system for the propagation of high MMTV-producer clones and for the study of the biochemical regulation of MMTV expression.

Mouse mammary tumor virus production stimulated by hormones and polyamines in cells grown in semi-synthetic in vitro conditions

Hormonal regulation of mouse mammary tumor virus (MMTV) synthesis was studied in the CCL-51-SF cell subline derived from the Sykes' mammary tumor cell line CCL-51 and adapted to grow in semi-synthetic in vitro conditions. The virus was quantitated by measuring the supernatant reverse transcriptase activity in exogenous reaction using poly (rA)-oligo (dT) and poly (rC)-oligo (dG) as template/primers. The cells produced a low but significant amount of virus in the absence of any hormones and serum proteins. The synthetic glucocorticoid dexamethasone increased production considerably, up to 100-fold. Pretreatment of CCL-51-SF cells with serum or 5-bromodeoxyuridine, BrdUrd, partly reduced the stimulation by dexamethasone of MMTV production. Insulin and prolactin alone or in combination had no stimulating effect on spontaneous MMTV synthesis and cell growth. Prolactin, and more efficiently the prolactin-insulin combination, enhanced the MMTV production stumulated with dexamethasone. Insulin alone remained without effect. The polyamine spermidine, but not spermine, increased the MMTV production over the control by a factor of 2. Polyamines did not influence cell replication at the concentrations used.

Infection of cultured rat hepatoma cells by mouse mammary tumor virus

A continuous line of buffalo rat hepatoma (HTC) cells has been successfully infected with mouse mammary tumor virus (MMTV) produced by the GR mammary tumor cell line. Uniform infection required initial exposure of the HTC cells to greater than 10(5) MMTV particles per cell. The resultant chronically infected cell population was found to have stably acquired 20-30 copies of MMTV DNA. The infected cells contain viral RNA and express viral antigens; however, very few MMTV particles are released into the culture medium. In spite of the biochemical evidence for infection, we have not detected any alterations in the morphology or growth properties of the infected HTC cells. As is the case in mammary tumor cells, the intracellular concentration of viral RNA is strongly stimulated (50-150 fold) by the synthetic glucorcorticoid, dexamethasone. Thus it appears that the mechanisms by which glucorticoids regulate MMTV gene expression in mouse cells are maintained when this virus infects nonmurine cells.

Cell culture factors influencing in vitro expression of mouse mammary tumor virus

Several cell culture factors were found to influence in vitro expression of mouse mammary tumor virus (MMTV) in the mouse adenocarcinoma cell line Mm5mt/c1. Cells were propagated in a variety of commercially available cell culture media to which dexamethasone (DXM) was added as a stimulator of MMTV production. Culture seeding density, culture medium type, and glucose concentration each influenced MMTV production when expressed on a per cell basis. Maximum cell growth occurred in cultures grown in RPMI-1640 medium containing insulin. Those media which provided good cell growth were not necessarily optimal for virus expression. Addition of insulin did not stimulate MMTV synthesis although dexamethasone alone was stimulatory in all media used; however, maximum MMTV expression was obtained when dexamethasone and insulin were used in concert. Equivalent levels of MMTV-specific cell membrane antigen, MMTV-specific protein, and virus particles were produced at incubation temperatures of 32 degrees, 34 degrees or 37 degrees C; however, higher levels of virus-related RNA-dependent DNA polymerase (RDDP) activity were recovered from cultures incubated at 32 degrees and 34 degrees C than at 37 degrees C. Decreased levels of RDDP were attributed to enzyme thermolability at 37 degrees C incubation.

Relationship between receptor and mammary tumour virus production after stimulation by glucocorticoid

A short exposure of primary cultures of mouse mammary tumour cells to glucocorticoids results in at least a three-fold stimulation of mammary tumour virus (MTV) production. Specific interaction of glucocorticoids with the cytoplasmic and nuclear receptors can also be demonstrated. The biological potency of various steroids to stimulate MTV is related directly to the retention of the steroid-receptor complex in the nuclei. Progesterone has a high affinity for the cytoplasmic receptor, is not retained by the nuclei and does not stimulate or block the basal level of MTV production. It is, however, quite effective in abolishing the glucocorticoid-mediated stimulation of MTV and thus behaves as an antagonist of glucocorticoid.

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

We have investigated the mechanisms by which dexamethasone (a synthetic glucocorticoid) stimulates the production of mouse mammary tumor virus (MMTV) by cell cultures derived from mammary carcinomas of GR mice. Treatment of these cells with dexamethasone stimulates a rapid accumulation of intracellular virus-specific RNA which is dependent upon RNA synthesis but not upon DNA or protein synthesis. The effect of dexamethasone is probably mediated by a specific and saturable glucocorticoid receptor. We conclude that the accumulation of MMTV RNA is a primary response to dexamethasone and that the rate of synthesis of MMTV RNA is probably accelerated by treatment with dexamethasone.