Mouse mammary tumor virus: transcriptional control and involvement in tumorigenesis

Mouse mammary tumor virus-like env sequences in human breast cancer

Mouse mammary tumor virus (MMTV) has long been known as a causal agent of breast cancer in mice. To date, varied MMTV-like envelope gene (env) sequences have been identified in up to 74% of human breast cancers. However, the role and origin of these MMTV-like sequences in human breast cancer remain uncertain. Our study was initiated to study the integration of MMTV-like env sequences in human breast cancer. PCR screening has identified 28 (56%) Australian breast cancer specimens and 7 (87.5%) human breast cancer cell lines to be positive for MMTV-like env sequence. In the MCF-7 genome, a fragment containing an MMTV-like env sequence of approximately 1.9 kb plus a downstream rodent-like sequence of approximately 200 bp was found to be integrated into a bacterial-like beta-lactamase sequence by insertional mutagenesis. The identified MMTV-rodent fragment is present in some MCF-7 sublines but absent in the screened specimens and other cell lines. Sporadic mutations found in this fragment indicate it has multiple copies in the MCF-7 genome. Sequence analysis has identified a novel ORF of approximately 1.6 kb which is 94-99% identical to MMTV env genes. RT-PCR was performed on the MCF-7 cDNA but no MMTV-like env transcript was detected. This is the first report to reveal the locus of MMTV-like env sequence in human cells. The MMTV-like env sequence was shown to be distinct from the human endogenous retroviral sequences and is closely related to rodents.

The Nuclear Factor I (NFI) gene family in mammary gland development and function

Mammary gland development and function require the coordinated spatial and temporal expression of a large fraction of the mammalian genome. A number of site-specific transcription factors are essential to achieve the appropriate growth, branching, expansion, and involution of the mammary gland throughout early postnatal development and the lactation cycle. One family of transcription factors proposed to play a major role in the mammary gland is encoded by the Nuclear Factor I (NFI) genes. The NFI gene family is found only in multicellular animals, with single genes being present in flies and worms and four genes in vertebrates. While the NFI family expanded and diversified prior to the evolution of the mammary gland, it is clear that several mammary-gland specific genes are regulated by NFI proteins. Here we address the structure and evolution of the NFI gene family and examine the role of the NFI transcription factors in the expression of mammary-gland specific proteins, including whey acidic protein and carboxyl ester lipase. We discuss current data showing that unique NFI proteins are expressed during lactation and involution and suggest that the NFI gene family likely has multiple important functions throughout mammary gland development.

Steroid hormone regulation of antiviral immunity

Recent observations in both humans and animals have demonstrated that stress is immunomodulatory and can alter the pathogenesis of microbial infections to the extent that it may be adverse to health. Stress disrupts homeostasis, and the body responds through endocrine and nervous system interactions in an effort to re-establish the health of the host. However, the resulting physiologic changes associated with stress, such as the rise in serum glucocorticoids (GCs), are implicated in suppression of antiviral immunity. Therefore, it would be of significance to counterregulate stress-mediated immunosuppression during viral infection to improve immune responses and limit virus-mediated damage. The data in this study focus upon the antiglucocorticoid influence of a native steroid hormone that has been shown to augment immune function and protect animals against lethal viral infections. Androstenediol (5-androstene-3 beta,17 beta-diol, AED), a metabolite of dehydroepiandrosterone (DHEA), confers protection against lethal infection with influenza A virus. The protective activity appears to counterbalance the function of the regulatory GCs because AED prevents GC-mediated suppression of IL-1, TNF-alpha, and IL-2 secretion. Furthermore, AED inhibits GC-induced transcription of a GC-sensitive reporter gene.

Reverse transcriptase of mouse mammary tumour virus: expression in bacteria, purification and biochemical characterization

We have constructed a plasmid that induces in bacteria the synthesis of an enzymically active reverse transcriptase (RT) of mouse mammary tumour virus (MMTV), a retrovirus with a typical B-type morphology. The highest catalytic activity was detected only when 27 residues from the C-terminus of the protease were included in the N-terminus of the recombinant RT, after an extra deoxyadenosine was added between the pro and pol genes to overcome the -1 frameshift event (which occurs naturally in virus-infected cells). The recombinant protein with a six-histidine tag was purified to homogeneity by a two-column purification procedure, Ni2+ nitriloacetic acid/agarose followed by carboxymethyl-Sepharose chromatography. Unlike most RTs, the purified MMTV RT is enzymically active as a monomer even after binding a DNA substrate. Like all RTs studied, the recombinant MMTV RT possesses RNA-dependent and DNA-dependent DNA polymerase activities as well as RNase H activity, all of which show a preference for Mg2+ over Mn2+ ions. Other features of these enzymic activities, such as extension of DNA primers, processivity of DNA synthesis, pH dependence, steady-state kinetic constants, effects of Na+ or K+ ions and sensitivity to a thiol-specific reagent and to a zinc chelator, have been evaluated. The catalytic properties of MMTV RT were compared with those of the well-studied RT of HIV-1, the causative agent of AIDS. Interestingly, MMTV RT exhibits a high sensitivity to nucleoside triphosphate analogues (which are known to be potent inhibitors of HIV RTs and are being used as the major anti-AIDS drugs), as high as that of HIV-1 and HIV-2 RTs. Furthermore the recombinant MMTV RT shows a processivity of DNA synthesis higher than that of HIV-1 RT.

Prolactin, epidermal growth factor or transforming growth factor-alpha activate a mammary cell-specific enhancer in mouse mammary tumor virus-long terminal repeat

Mammary specific expression of elevated levels of mouse mammary tumor virus (MMTV) contributes to mammary carcinogenesis. Mechanisms which regulate provirus expression have not been completely defined. Using a MMTV-long repeat terminal (MMTV-LRT) directed chloramphenicol-acetyltransferase (CAT) reporter gene system and a human breast cancer cell line T47D, we demonstrate that prolactin (PRL), epidermal growth factor (EGF), or transforming growth factor-alpha (TGF-alpha) act on a mammary cell-specific enhancer at the extreme 5' end of the MMTV-LTR involving sequences -1094 through -858. PRL and either EGF or TGF-alpha exert concerted roles in this activation of these sequences. In contrast, using a plasmid construct lacking this mammary cell-specific enhancer, EGF or TGF-alpha, but not PRL, act synergistically with progesterone to induce CAT activity, indicating that the action of PRL on regulatory elements of the MMTV-LTR is restricted to this mammary cell-specific enhancer involving sequences -1094 through -858. A mobility shift assay was used to demonstrate that PRL, EGF or TGF-alpha induce nuclear factors (MP4, MAF, and MGF) which bind directly to this mammary cell-specific enhancer element.

Transcription of human endogenous retroviral sequences related to mouse mammary tumor virus in human breast and placenta: similar pattern in most malignant and nonmalignant breast tissues

The human genome contains a large variety of sequences related to the mouse mammary tumor virus (MMTV). We have investigated the range of expression of human endogenous retroviral sequences (HERVs) related to MMTV (human MMTV-like; HML) as RNA in 60 breast cancers, 8 nonmalignant breast tissues, and 9 placentas. This was monitored using HML group-specific oligonucleotide probes in hybridizations toward PCR amplificates of HML pol sequences and internal control. The degree of expression of five HML groups varied between individuals and between tissues. On average, all HML groups were less expressed in breast tissues than in placenta. The hybridization signals of some HML RNAs were strongly correlated, indicating a nonstochastic mechanism and a concerted regulation of their expression. The PCR product from one breast cancer (BC 6), which gave an exceptionally high expression with probe hml-6, with a 20 times stronger signal than the rest of the cancers, was cloned and sequenced. The HML-6 transcript sequences were homogeneous in BC 6. The most predominant clone derived from the cancer was used as a probe in Southern hybridizations. The same restriction fragments were detected in human breast tissues, PBMCs (peripheral blood mononuclear cells), and breast cancer cell lines, except for one of the breast cancers and one of the nonmalignant breast tissues, which gave different banding patterns. A comparison of HML expression in normal and malignant breast tissue from the same individual would have been more precise than our comparison of samples from different persons. Bearing this limitation in mind, with a single exception, human MMTV-like sequences were not more actively expressed in malignant than in nonmalignant breast tissues. Nevertheless, an interesting diversity in their expression, especially between individuals, was found.

Nuclear factor binding to a DNA sequence element that represses MMTV transcription induces a structural transition and leads to the contact of single-stranded binding proteins with DNA

NRE1 is a DNA sequence element in the long terminal repeat of mouse mammary tumor virus through which viral transcription is repressed. In addition to double-stranded DNA binding, both upper- and lower-stranded NRE1 binding activities occur in nuclear extracts. All three binding activities appear to be important for transcriptional effects. We report that occupancy of NRE1 within linear double-stranded NRE1 induces a structural transition in upstream flanking DNA that is facilitated by Mg2+. This transition was reflected by the striking DNase I sensitivity of the DNA. As Mg2+ concentration was increased, discrete DNase I hypersensitivity on one face of the DNA progressed to complete degradation of template. On the DNA face opposite the DNase I hypersensitivity, Mg2+ promoted regularly spaced cleavage by the single-strand-specific cleavage agents KMnO4 and S1 nuclease. Induction of degradation by DNase I occurred independently of MMTV sequences flanking NRE1, because nuclear extract-dependent DNase I sensitivity was conferred to an unrelated DNA fragment by introduction of a 23-bp NRE1-containing oligonucleotide. UV protein-DNA cross-linking revealed that addition of Mg2+ to a double-stranded NRE1 DNA binding assay induced conversion from a double- to a single-stranded protein-DNA cross-linking pattern. Thus, nuclear factor binding to NRE1 induces changes in DNA topology that promote the direct contact of single-stranded NRE1 binding factors with DNA.

Prolactin acts on the extreme 5' portion of MMTV LTR involving a mammary cell-specific enhancer

We have previously shown that a human mammotropic polypeptide hormone, prolactin (PRL) can act synergistically with steroid hormones to regulate gene expression directed by the long terminal repeat of mouse mammary tumor virus (MMTV LTR) in a human ductal carcinoma cell line T47D cells using a chloramphenicol acetyltransferase reporter gene system and gene transfection methods. In the present study, using various recombinant plasmids we analyzed functional elements in the MMTV LTR that is essential for the PRL responses. We show that the PRL-responsive elements are located in the extreme 5' end of the MMTV LTR, a region previously described by others to be a mammary cell-specific enhancer.

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 mammary cell-specific enhancer in mouse mammary tumor virus DNA is composed of multiple regulatory elements including binding sites for CTF/NFI and a novel transcription factor, mammary cell-activating factor

Mouse mammary tumor virus (MMTV) is a milk-transmitted retrovirus involved in the neoplastic transformation of mouse mammary gland cells. The expression of this virus is regulated by mammary cell type-specific factors, steroid hormones, and polypeptide growth factors. Sequences for mammary cell-specific expression are located in an enhancer element in the extreme 5' end of the long terminal repeat region of this virus. This enhancer, when cloned in front of the herpes simplex thymidine kinase promoter, endows the promoter with mammary cell-specific response. Using functional and DNA-protein-binding studies with constructs mutated in the MMTV long terminal repeat enhancer, we have identified two main regulatory elements necessary for the mammary cell-specific response. These elements consist of binding sites for a transcription factor in the family of CTF/NFI proteins and the transcription factor mammary cell-activating factor (MAF) that recognizes the sequence G Pu Pu G C/G A A G G/T. Combinations of CTF/NFI- and MAF-binding sites or multiple copies of either one of these binding sites but not solitary binding sites mediate mammary cell-specific expression. The functional activities of these two regulatory elements are enhanced by another factor that binds to the core sequence ACAAAG. Interdigitated binding sites for CTF/NFI, MAF, and/or the ACAAAG factor are also found in the 5' upstream regions of genes encoding whey milk proteins from different species. These findings suggest that mammary cell-specific regulation is achieved by a concerted action of factors binding to multiple regulatory sites.

A mammary cell-specific enhancer in mouse mammary tumor virus DNA is composed of multiple regulatory elements including binding sites for CTF/NFI and a novel transcription factor, mammary cell-activating factor

Mouse mammary tumor virus (MMTV) is a milk-transmitted retrovirus involved in the neoplastic transformation of mouse mammary gland cells. The expression of this virus is regulated by mammary cell type-specific factors, steroid hormones, and polypeptide growth factors. Sequences for mammary cell-specific expression are located in an enhancer element in the extreme 5' end of the long terminal repeat region of this virus. This enhancer, when cloned in front of the herpes simplex thymidine kinase promoter, endows the promoter with mammary cell-specific response. Using functional and DNA-protein-binding studies with constructs mutated in the MMTV long terminal repeat enhancer, we have identified two main regulatory elements necessary for the mammary cell-specific response. These elements consist of binding sites for a transcription factor in the family of CTF/NFI proteins and the transcription factor mammary cell-activating factor (MAF) that recognizes the sequence G Pu Pu G C/G A A G G/T. Combinations of CTF/NFI- and MAF-binding sites or multiple copies of either one of these binding sites but not solitary binding sites mediate mammary cell-specific expression. The functional activities of these two regulatory elements are enhanced by another factor that binds to the core sequence ACAAAG. Interdigitated binding sites for CTF/NFI, MAF, and/or the ACAAAG factor are also found in the 5' upstream regions of genes encoding whey milk proteins from different species. These findings suggest that mammary cell-specific regulation is achieved by a concerted action of factors binding to multiple regulatory sites.

Chemosuppression of breast cancer with long-term tamoxifen therapy

The pharmacology of the antiestrogen tamoxifen is reviewed. The drug is currently used extensively in the treatment of all stages of breast cancer and is being considered as a preventive agent for women at high risk for breast cancer. Extensive laboratory studies demonstrate that tamoxifen is a tumoristatic agent in models of mammary carcinogenesis. Any clinical applications must therefore consider long-term (5-10 years) treatment strategies. Tamoxifen prevents rat mammary carcinogenesis. However, the timing of the carcinogenic insult is unknown among women. Tamoxifen must be considered to be a chemosuppressive agent to prevent the appearance of the primary tumor rather than to prevent the initial carcinogenic insult.

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.

Anti-oestrogenic and anti-tumour properties of prolonged tamoxifen therapy in C3H/OUJ mice

The anti-oestrogenic and anti-tumour properties of a sustained-release preparation of tamoxifen were evaluated in female C3H/OUJ mice. Tamoxifen decreased uterine weight compared with controls in intact mice but caused an initial uterotrophic response for 2 months in ovariectomized mice. Prolonged tamoxifen therapy in ovariectomized mice resulted in a uterine weight no different from controls, but these uteri were eventually (at 6 months) refractory to oestradiol. Spontaneous mammary tumours were detected in female mice between 6 months and 1 year of age during continuous cycles of pregnancy and weaning. A similar tumour frequency occurred after one cycle of pregnancy and weaning initiated at 31/2 months. Prolonged tamoxifen, started at 31/2 or 41/2 months of age (following pregnancy/weaning), reduced the appearance of tumours. Similarly ovariectomy at 31/2 months prevented mammary tumorigenesis and prolonged tamoxifen could not increase tumour incidence consistently in ovariectomized mice. Although tamoxifen is oestrogenic in short-term tests the compound has the properties of an anti-oestrogen during prolonged administration.

Isolation and characterization of a family of rat endogenous retroviral sequences

The complete nucleotide sequence of one representative rat genomic unit flanked on both sides with RAL elements, which have structural features specific to retroviral LTRs (1), was determined. The total unit was about 7.5 kbp long, and there was a partial homology to known retroviral sequences in gag, pol, and env regions. The sequence also contained minus- and plus-strand primer binding sites, thereby indicating a retroviral nature in replication. Transcription of the sequence was extensive in tumor cells and was strongly correlated with the state of methylation within 5' LTRs, which were highly methylated in the normal but not in the tumor state. In functional assays with bacterial chloramphenicol acetyltransferase constructs containing a series of deleted LTRs, there seemed to be both positive and negative cis-acting effector sequences.

Cycloheximide Increases Endogenous Retroviral RNA Levels in Murine Liver and Lung

The expression of moloney murine leukemia virus (Mo-MuLV), intracisternal A particle (IAP), virus-like 30S sequence (VL30), early transposon (ET) endogenous retroviral sequences was analysed in the liver of untreated C3Hf, C57BL/6J and AKR mice, and in the lungs of untreated A/J, BALB/c, C3Hf and C57BL/6J mice. C3Hf mice are genetically susceptible to hepatocarcinogenesis, whereas the other strains are resistant. A/J and BALB/c mice are genetically susceptible to lung carcinogenesis, whereas the other strains are resistant. Both in liver and lung tissues we found differences between murine strains in the pattern of basal retrovirus expression. The effect of inhibition of protein synthesis on the levels of retroviral mRNAs was studied in the same tissues and strains 3 h following in vivo cycloheximide treatment. Cycloheximide treatment increased the liver and lung levels of virus Mo-MuLV, IAP, ET related transcripts in a strain dependent way, whereas VL30 mRNA levels increased in both tissues of all strains examined. These results suggested the existence of labile proteins that regulate the abundance of specific retroviral mRNAs in murine liver and lung in a strain specific fashion. No clear relationships between pattern of retrovirus expression and genetical susceptibility to hepatocarcinogenesis was found. The strains genetically resistant to lung carcinogenesis (C3Hf, C57BL/6J) showed higher lung basal levels and higher cycloheximide inducibility of mRNAs homologous to Mo-MuLV than the susceptible strains (A/J, BALB/c).

The regulation of expression of mouse mammary tumor virus DNA by steroid hormones and growth factors

Mouse mammary tumor virus (MMTV) expression is associated with hyperplastic alveolar growth and subsequent development of mammary cancers in the mouse. The expression of this virus is also controlled by factors involved in the normal proliferation and differentiation of the mammary epithelium. During pregnancy when the mammary gland undergoes massive proliferation, MMTV expression is increased. Steroid hormones and growth factors that play an important role in the proliferation of mammary gland cells are responsible for the increased MMTV expression. In sarcomatous transformation of mouse mammary epithelial cells, MMTV expression is repressed. This repression is due to negative control of MMTV expression by transforming growth factor-beta (TGF beta). This growth factor is produced in high amounts when mammary epithelial cells progress into the transformed state. The expression of MMTV is therefore under multiple control by steroid hormones and growth factors.

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.

Genetic susceptibility to murine hepatocarcinogenesis is associated with high growth rate of NDEA-initiated hepatocytes

The murine hybrids (C57BL/6J X C3Hf)F1 (B6C3) and (C57BL/6J X BALB/c)F1 (B6C), which have a high and low spontaneous and induced incidence of hepatocellular tumors, respectively, were treated with a single dose of NDEA at 1 week of age followed by TCPOBOP, a phenobarbital-like promoter of liver carcinogenesis, or by vehicle, and sacrificed at 30 weeks of age. The frequency per liver of hepatocellular nodules was similar in the two hybrids. However, in male mice the mean volume of nodules was about 10-fold greater in B6C3 than in B6C mice receiving NDEA followed by vehicle, and the treatment with TCPOBOP after NDEA stimulated nodule growth, with a much greater response in B6C3 mice. In female mice no differences in the mean volume of nodules were seen between hybrids after NDEA and vehicle, whereas upon NDEA and TCPOBOP treatment the mean volume of nodules was 25-fold greater in B6C3 than in B6C females. In addition, a few hepatocellular adenomas and carcinomas were observed, mostly in animals treated with NDEA and TCPOBOP, and they were 3-fold more numerous among B6C3 than B6C mice. TCPOBOP alone induced the same biochemical and hyperplastic effects in the liver of both hybrids. Using DNA probes homologous to Moloney murine leukemia virus, intracisternal A particle and virus-like 30S sequences, no correlation was apparent between the expression of any of these endogenous retroviral families and the strain susceptibility to hepatocarcinogenesis. We hypothesize that the different susceptibility to hepatocarcinogenesis between B6C3 and B6C mice is related to a higher growth rate of B6C3 than B6C initiated liver cells.

Nonrandom distribution of MMTV proviral sequences in the mouse genome

Integrated sequences of mouse mammary tumor virus (MMTV) have been localized in the genomes of five inbred mouse strains (Balb/c, C3H, DBA/2, A.TH, 129-SV) and one mammary tumor cell line (GR). Two major classes of MMTV sequences have been detected in mouse DNA fractions as obtained by Cs2SO4/BAMD (3,6-bis-(acetatomercurimethyl)dioxane) density gradient centrifugation. The first one corresponds to previously described endogenous sequences (Mtv loci), whereas the second one corresponds to endogenous sequences not previously known, and/or recently acquired; in the case of GR cells exogenous sequences may also be present in this class. The genome distribution is somewhat different for the two classes of sequences, the first one being practically only present in the lightest DNA segments of the mouse genome (GC congruent to 38%); the second one being also represented in heavier segments (GC congruent to 43%). This integration pattern suggests that "ancient" endogenous sequences are practically only localized in genome segments of roughly matching composition, whereas exogenous and recently acquired endogenous MMTV sequences may also be present in heavier fractions.

A rare common integration site of proviruses of the mouse mammary tumor virus in P-type mammary tumors of mouse strain GR

The mouse mammary tumor virus (MMTV) can induce mammary tumors in mice by proviral activation of the cellular oncogenes int-1 or int-2. Activation of these genes, however, is observed in only a few hormone- and pregnancy-dependent mammary tumors of the mouse strain GR. To study the possible involvement of other oncogenes we cloned three MMTV proviral-host fragments (MT 40, 42, and 53) from different mammary tumors of GR with a single acquired MMTV provirus. From a genomic library of normal mouse DNA we isolated phages with insert DNAs that covered 20-30 kb of the uninterrupted regions. Suitable probes devoid of repetitive DNA sequences were isolated in order to screen other mammary tumors for MMTV proviral integrations in these regions. Only two mammary tumors, MT 40 and 42, showed integration of extra MMTV proviruses within the same region. The integrations occurred only 60 bp apart. The other mammary tumors, however, did not contain MMTV proviral integrations in this region, nor in the MT 53 region. Using mouse-hamster somatic cell hybrid DNA, the MT 40/42 integration region was assigned to mouse chromosome 7, and the second region, MT 53, to chromosome 16. The two regions bear no homology to known cellular oncogenes. We did not observe any mRNA being expressed from these cloned segments either in tumors or in normal mammary glands. These findings indicate that plaque(P)-type mammary tumors in mouse strain GR do not originate from MMTV provirus insertions in a particularly favored integration region, but that there may be a variety of integration sites in these tumors.

Androgen regulation by the long terminal repeat of mouse mammary tumor virus

Mouse mammary tumor virus (MMTV) has long been implicated in mouse mammary carcinogenesis, and it is now well established that the long terminal repeat (LTR) contains regulatory sequences responsible for glucocorticoid-mediated induction of viral RNA. However, we have demonstrated previously that androgens as well as glucocorticoids can regulate MMTV RNA in the S115 mouse mammary tumor cell line. To determine if androgens act directly on the LTR in these cells, plasmids were constructed with the MMTV LTR joined to the coding sequences of genes not normally expressed in the cells. Following transfection of these chimeric genes into S115 cells, we show that the expression of the genes is regulated by both androgens and glucocorticoids. Furthermore, hormonal regulation is also conferred by the LTR on the neighboring guanine phosphoribosyltransferase (gpt) gene. Thus, androgens can act on the LTR of MMTV when the appropriate receptors are present in the cells, and this interaction can influence the expression of additional adjacent genes.

Androgen regulation by the long terminal repeat of mouse mammary tumor virus

Mouse mammary tumor virus (MMTV) has long been implicated in mouse mammary carcinogenesis, and it is now well established that the long terminal repeat (LTR) contains regulatory sequences responsible for glucocorticoid-mediated induction of viral RNA. However, we have demonstrated previously that androgens as well as glucocorticoids can regulate MMTV RNA in the S115 mouse mammary tumor cell line. To determine if androgens act directly on the LTR in these cells, plasmids were constructed with the MMTV LTR joined to the coding sequences of genes not normally expressed in the cells. Following transfection of these chimeric genes into S115 cells, we show that the expression of the genes is regulated by both androgens and glucocorticoids. Furthermore, hormonal regulation is also conferred by the LTR on the neighboring guanine phosphoribosyltransferase (gpt) gene. Thus, androgens can act on the LTR of MMTV when the appropriate receptors are present in the cells, and this interaction can influence the expression of additional adjacent genes.

Decreased natural cytotoxicity in mice with high incidence of mammary adenocarcinoma

In an attempt to gather evidence relevant to the question of whether natural killer (NK) cells play a role in resisting the development of primary tumors, we compared natural cell-mediated cytotoxicity in two substrains of C3H mice. Animals of the C3H/OuJ substrain are at high risk for the formation of mammary adenocarcinomas, while C3Heb/FeJ mice have a low incidence of such tumors. Natural cytotoxicity of splenic mononuclear cells was lower in the high-risk substrain, suggesting that a lesion in NK cell activity may be involved in murine mammary tumorigenesis. This difference was observed in animals between 5 and 37 weeks of age. There was no significant difference in the number of splenic large granular lymphocytes between the substrains. A significant difference in the ability of splenic lymphocytes from the two substrains to bind to the target cells was noted. Since the binding capacity of lymphocytes was greater in mice with reduced NK cell activity, the lesion in cytotoxicity may exist at a postbinding step in the lytic sequence. It is felt that the C3H mouse may provide a useful model for studying the role of NK cells in controlling primary tumors.

Rearrangements in the long terminal repeat of extra mouse mammary tumor proviruses in T-cell leukemias of mouse strain GR result in a novel enhancer-like structure

Male GR mice develop T-cell leukemia at low frequency late in life. These leukemia cells invariably contain large amounts of mouse mammary tumor virus (MMTV) RNA and MMTV proteins and have extra MMTV proviruses integrated in their DNA. We show here that the extra MMTV proviruses are all derived from the endogenous MMTV provirus associated with the Mtv-2 locus and that the T-cell leukemias are clonal with respect to the acquired MMTV proviruses. The extra MMTV proviruses in six transplantable T-cell leukemia lines studied had rearranged, shortened long terminal repeats (LTRs); each T-cell leukemia, however, had a different LTR rearrangement within its extra MMTV provirus. The alteration within the extra LTRs of T-cell leukemia line 42 involved deletion of 453 nucleotides and generation of a tandem repeat region consisting of regions flanking the deletion. This alteration generated a sequence similar to the adenovirus enhancer core sequence. The viral RNAs in the T-cell leukemias contained corresponding alterations in their U3 regions. These results demonstrate that expression of MMTV in T-cell leukemias of GR mice may be the consequence of the generation of a novel enhancer, which could also stimulate expression of any adjacent cellular oncogene.

Rearrangements in the long terminal repeat of extra mouse mammary tumor proviruses in T-cell leukemias of mouse strain GR result in a novel enhancer-like structure

Male GR mice develop T-cell leukemia at low frequency late in life. These leukemia cells invariably contain large amounts of mouse mammary tumor virus (MMTV) RNA and MMTV proteins and have extra MMTV proviruses integrated in their DNA. We show here that the extra MMTV proviruses are all derived from the endogenous MMTV provirus associated with the Mtv-2 locus and that the T-cell leukemias are clonal with respect to the acquired MMTV proviruses. The extra MMTV proviruses in six transplantable T-cell leukemia lines studied had rearranged, shortened long terminal repeats (LTRs); each T-cell leukemia, however, had a different LTR rearrangement within its extra MMTV provirus. The alteration within the extra LTRs of T-cell leukemia line 42 involved deletion of 453 nucleotides and generation of a tandem repeat region consisting of regions flanking the deletion. This alteration generated a sequence similar to the adenovirus enhancer core sequence. The viral RNAs in the T-cell leukemias contained corresponding alterations in their U3 regions. These results demonstrate that expression of MMTV in T-cell leukemias of GR mice may be the consequence of the generation of a novel enhancer, which could also stimulate expression of any adjacent cellular oncogene.