Extra mouse mammary tumor proviruses in DBA/2 mouse lymphomas acquire a selective advantage in lymphocytes by alteration in the U3 region of the long terminal repeat

Exploring the link between viruses and cancer in companion animals: a comprehensive and comparative analysis

Currently, it is estimated that 15% of human neoplasms globally are caused by infectious agents, with new evidence emerging continuously. Multiple agents have been implicated in various forms of neoplasia, with viruses as the most frequent. In recent years, investigation on viral mechanisms underlying tumoral transformation in cancer development and progression are in the spotlight, both in human and veterinary oncology. Oncogenic viruses in veterinary medicine are of primary importance not only as original pathogens of pets, but also in the view of pets as models of human malignancies. Hence, this work will provide an overview of the main oncogenic viruses of companion animals, with brief notes of comparative medicine.

Presence of a mouse mammary tumour virus-like in feline lymphomas: a preliminary study

The mouse mammary tumour virus (MMTV) is implicated in the aetiology of murine mammary carcinomas and a variant of it, the type B leukemogenic virus, can cause murine thymic lymphomas. Interestingly, a MMTV-like virus is suspected to be involved in human breast cancer and feline mammary carcinomas. However, to date, no cases of MMTV-like sequence amplifications have been described in lymphoid neoplasms in veterinary literature. The aim of this study was to investigate the presence of env nucleotide sequences and protein 14 (p14) of a MMTV-like virus in fifty-three feline lymphoma samples. Our results show that MMTV-like sequences were detected in 5/53 tumours (9.4%): three gastrointestinal lymphomas (one B-type diffuse large, one B-type small non-cleaved, and one T-type diffuse mixed lymphoma); and two nasal lymphomas (one B-type diffuse small cleaved lymphoma and one B-type diffuse mixed lymphoma). P14 expression was detected in the cytoplasm, and rarely in nuclei, exclusively of neoplastic cells from PCR-positive tumours. The correlation between the presence of the MMTV-env like sequences (MMTVels) and p14 antigen was statistically significant in nasal lymphomas. All cats with MMTVels-positive lymphoma had a history of contact with the outdoor environment and/or catteries, and two deceased subjects shared their environment with cats that also died of lymphoma. In conclusion, this study succeeds in demonstrating the presence of MMTVels and p14 in feline lymphomas. The characterization of the immunophenotype of MMTVels-positive lymphomas could contribute to the understanding of a possible role of a MMTV-like virus in feline tumour aetiology. The significant association between the presence of the viral sequences in lymphoid tumours and their nasal localization, together with the data collected through supplementary anamnesis, should be further analysed in order to understand the epidemiology of the virus.

Mouse Mammary Tumor Virus (MMTV) and MMTV-like Viruses: An In-depth Look at a Controversial Issue

Since its discovery as a milk factor, mouse mammary tumor virus (MMTV) has been shown to cause mammary carcinoma and lymphoma in mice. MMTV infection depends upon a viral superantigen (sag)-induced immune response and exploits the immune system to establish infection in mammary epithelial cells when they actively divide. Simultaneously, it avoids immune responses, causing tumors through insertional mutagenesis and clonal expansion. Early studies identified antigens and sequences belonging to a virus homologous to MMTV in human samples. Several pieces of evidence fulfill a criterion for a possible causal role for the MMTV-like virus in human breast cancer (BC), though the controversy about whether this virus was linked to BC has raged for over 40 years in the literature. In this review, the most important issues related to MMTV, from its discovery to the present days, are retraced to fully explore such a controversial issue. Furthermore, the hypothesis of an MMTV-like virus raised the question of a potential zoonotic mouse–man transmission. Several studies investigate the role of an MMTV-like virus in companion animals, suggesting their possible role as mediators. Finally, the possibility of an MMTV-like virus as a cause of human BC opens a new era for prevention and therapy.

Lessons Learned from Mouse Mammary Tumor Virus in Animal Models

Mouse mammary tumor virus (MMTV), which was discovered as a milk-transmitted, infectious, cancer-inducing agent in the 1930s, has been used as an animal model for the study of retroviral infection and transmission, antiviral immune responses, and breast cancer and lymphoma biology. The main target cells for MMTV infection in vivo are cells of the immune system and mammary epithelial cells. Although the host mounts an immune response to the virus, MMTV has evolved multiple means of evading this response. MMTV causes mammary tumors when the provirus integrates into the mammary epithelial and lymphoid cell genome during viral replication and thereby activates cellular oncogene expression. Thus, tumor induction is a by-product of the infection cycle. A number of important oncogenes have been discovered by carrying out MMTV integration site analysis, some of which may play a role in human breast cancer.

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

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

Selection for c-myc integration sites in polyclonal T-cell lymphomas

Type B leukemogenic virus (TBLV) is highly related to mouse mammary tumor virus but induces rapidly appearing T-cell lymphomas in mice. Unlike other T-cell tumors induced by retroviruses, only 5 to 10% of TBLV-induced lymphomas have detectable viral integrations near c-myc by Southern blotting, whereas Northern blotting has shown that most tumors have two- to sixfold overexpression of c-myc RNA. In this report, PCR was used to demonstrate that at least 30% of these lymphomas have TBLV insertions near c-myc. Some tumors contained multiple TBLV proviruses in different locations and orientations, suggesting that the tumors are polyclonal. The integrated proviruses near c-myc had different numbers (two to four) of long terminal repeat (LTR) enhancer repeats, although LTRs with three-repeat enhancers dominated the proviral population. Passage of polyclonal tumors in immunocompetent mice and semiquantitative PCR revealed that only cells with particular integrations were selected for growth. In three of six tumors tested, proviruses containing four-repeat enhancers near c-myc were selected during tumor passage. Since tumor cell selection may be accomplished by overexpression of c-myc RNA due to proximity to the unique TBLV LTR enhancer, we inserted LTRs at various locations within a plasmid containing the entire c-myc locus and cellular flanking sequences. To quantitatively measure effects on transcription, the Renilla luciferase gene was substituted for most of c-myc exon 2, and transient transfections were performed with c-myc reporter constructs in two different T-cell lines. As expected, insertion of a TBLV LTR with three-repeat enhancers in either orientation, 5" and 3", of the myc gene elevated reporter activity from 2- to 160-fold, consistent with enhancer function, but four-repeat LTRs had lower levels of expression compared to three-repeat LTRs. Surprisingly, LTR insertions that gave maximal c-myc expression in transient-transfection assays declined in tumor cells selected for growth in vivo. Selection for clonal growth may occur in tumor cells that have modest c-myc overexpression after proviral insertion to prevent apoptosis.

Type B leukemogenic virus has a T-cell-specific enhancer that binds AML-1

Type B leukemogenic virus (TBLV) induces rapidly appearing T-cell tumors in mice. TBLV is highly related to mouse mammary tumor virus (MMTV) except that TBLV long terminal repeats (LTRs) have a deletion of negative regulatory elements and a triplication of sequences flanking the deletion. To determine if the LTR triplication represents a viral enhancer element, we inserted the triplication upstream and downstream in either orientation relative to the thymidine kinase promoter linked to the luciferase gene. These experiments showed that upregulation of reporter gene activity by the TBLV triplication was relatively orientation independent, consistent with the activity of eukaryotic enhancer elements. TBLV enhancer activity was observed in T-cell lines but not in fibroblasts, B cells, or mammary cells, suggesting that enhancer function is cell type dependent. To analyze the transcription factor binding sites that are important for TBLV enhancer function, we prepared substitution mutations in a reconstituted C3H MMTV LTR that recapitulates the deletion observed in the TBLV LTR. Transient transfections showed that a single mutation (556M) decreased TBLV enhancer activity at least 20-fold in two different T-cell lines. This mutation greatly diminished AML-1 (recently renamed RUNX1) binding in gel shift assays with a mutant oligonucleotide, whereas AML-1 binding to a wild-type TBLV oligomer was specific, as judged by competition and supershift experiments. The 556 mutation also reduced TBLV enhancer binding of two other protein complexes, called NF-A and NF-B, that did not appear to be related to c-Myb or Ets. AML-1 overexpression in a mammary cell line enhanced expression from the TBLV LTR approximately 30-fold. These data suggest that binding of AML-1 to the TBLV enhancer, likely in combination with other factors, is necessary for optimal enhancer function.

Identification of Notch1 as a frequent target for provirus insertional mutagenesis in T-cell lymphomas induced by leukemogenic mutants of mouse mammary tumor virus

In contrast to wild-type mouse mammary tumor virus (MMTV), the MMTV mutants with specific deletions in the U3 region of their long terminal repeats cause T-cell lymphomas. In 30% of T-cell lymphomas arising in BALB/c mice infected with MLA-MMTV, a leukemogenic MMTV mutant, we have found that MMTV proviruses were integrated into a short region of the Notch1 genome, so that truncated Notch1 transcripts encoding the transmembrane and the cytoplasmic domains of Notch1 protein could be expressed. Thus, Notch1 is a major target of provirus insertional mutagenesis in these T-cell lymphomas.

Synergistic action of GA-binding protein and glucocorticoid receptor in transcription from the mouse mammary tumor virus promoter

B lymphocytes are among the first cells to be infected by mouse mammary tumor virus (MMTV), and they play a crucial role in its life cycle. To study transcriptional regulation of MMTV in B cells, we have analyzed two areas of the long terminal repeat (LTR) next to the glucocorticoid receptor binding site, fp1 (at position -139 to -146 from the cap site) and fp2 (at -157 to -164). Both showed B-cell-specific protection in DNase I in vitro footprinting assays and contain binding sites for Ets transcription factors, a large family of proteins involved in cell proliferation and differentiation and oncogenic transformation. In gel retardation assays, fp1 and fp2 bound the heterodimeric Ets factor GA-binding protein (GABP) present in B-cell nuclear extracts, which was identified by various criteria: formation of dimers and tetramers, sensitivity to pro-oxidant conditions, inhibition of binding by specific antisera, and comigration of complexes with those formed by recombinant GABP. Mutations which prevented complex formation in vitro abolished glucocorticoid-stimulated transcription from an MMTV LTR linked to a reporter gene in transiently transfected B-cell lines, whereas they did not affect the basal level. Exogenously expressed GABP resulted in an increased level of hormone response of the LTR reporter plasmid and produced a synergistic effect with the coexpressed glucocorticoid receptor, indicating cooperation between the two. This is the first example of GABP cooperation with a steroid receptor, providing the opportunity for studying the integration of their intracellular signaling pathways.

The c-myc locus is a common integration site in type B retrovirus-induced T-cell lymphomas

Type B leukemogenic virus (TBLV) induces rapidly appearing T-cell leukemias. TBLV insertions near the c-myc gene were detectable in 2 of 30 tumors tested, whereas 80% of the tumors showed c-myc overexpression. TBLV insertions on chromosome 15 (including a newly identified locus, Pad7) may cause c-myc overexpression by cis-acting effects at a distance.

Murine leukemia provirus-mediated activation of the Notch1 gene leads to induction of HES-1 in a mouse T lymphoma cell line, DL-3

Constitutive activation of Notch signaling is known to be associated with tumorigenesis. In a mouse T lymphoma cell line, DL-3, we found that a murine leukemia provirus was inserted in the Notch1 locus, which led to marked expression of a virus-Notch1 fusion mRNA encoding an intracellular portion of the Notch1 protein. Furthermore, expression and nuclear localization of this constitutively active form of Notch1 protein were confirmed. Corresponding to this finding, the transcription of the hairy/enhancer of split (HES-1) gene, a known target of Notch1 signaling, was elevated in this cell line. A potential role for overexpressed HES-1 in the development of the lymphoma was discussed.

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

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

Sequence-specific binding of Ku autoantigen to single-stranded DNA

Glucocorticoid-induced transcription of mouse mammary tumor virus is repressed by Ku antigen/DNA-dependent protein kinase (DNA-PK) through a DNA sequence element (NRE1) in the viral long terminal repeat. Nuclear factors binding to the separated single strands of NRE1 have been identified that may also be important for transcriptional regulation through this element. We report the separation of the upper-stranded NRE1 binding activity in Jurkat T cell nuclear extracts into two components. One component was identified as Ku antigen. The DNA sequence preference for Ku binding to single-stranded DNA closely paralleled the sequence requirements of Ku for double-stranded DNA. Recombinant Ku bound the single, upper strand of NRE1 with an affinity that was 3-4-fold lower than its affinity for double-stranded NRE1. Sequence-specific single-stranded Ku binding occurred rapidly (t1/2 on = 2.0 min) and was exceptionally stable, with an off rate of t1/2= 68 min. While Ku70 cross-linked to the upper strand of NRE1 when Ku was bound to double-stranded and single-stranded DNAs, the Ku80 subunit only cross-linked to single-stranded NRE1. Intriguingly, addition of Mg2+ and ATP, the cofactors required for Ku helicase activity, induced the cross-linking of Ku80 to a double-stranded NRE1-containing oligonucleotide, without completely unwinding the two strands.

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.

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

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

Biology of mouse mammary tumor virus (MMTV)

Mouse mammary tumor viruses (MMTV) replicate in the mammary gland, appear as infectious particles in mother's milk and invade the sucking pups from the intestinal tract. The immune system is essential for MMTV in the gut to reach the mammary gland. These properties make the life cycle of MMTV unique. We review the oncologically and immunologically intriguing events caused by MMTV in relation to the life cycle of the virus.

Mouse Mammary Tumor Virus Proviral Integration in the DD/Tbr Mice

The patterns of mouse mammary tumor virus (MMTV) integration in the DNA of spontaneous-mammary tumors, salivary glands and livers of DD/Tbr mice were examined using MMTV env, int -1c and int-2c probes. The MMTV env probe revealed 1 to 7 new proviral insertions in all mammary tumors. MMTV integration into int-1 was observed in 10 of 18 mammary tumors, whereas that into int-2 was seen in only 2 of 18 tumors. Of the 13 salivary glands examined, only 3 showed new MMTV proviral integrations, but rearrangement in int-1 or int-2 loci by MMTV was not observed. Immuno-collidal gold electron microscopy revealed the presence of MMTV particles both in mammary tumors and in salivary glands, but no tumors were found to be developed in salivary glands. Taken together these results suggest that salivary glands support MMTV replication, but the virions thus produced may not lead to salivary gland tumorigenesis. It is suggested that the salivary gland is the source of horizontally transmitted MMTV in DD/Tbr mice.

A V beta 8.2-specific superantigen from exogenous mouse mammary tumor virus carried by FM mice

A number of endogenous mouse mammary tumor virus (MMTV) proviruses encode superantigen that have the ability to stimulate T cells with a certain T cell receptor (TCR) beta-chain variable region (V beta) and to mediate the V beta-specific clonal deletion. The tumorigenic milk-borne MMTV carried by C3H and GR mice also have superantigenic properties in vivo. In the present study we identified and characterized a novel V beta 8.2-specific superantigen of exogenous MMTV carried by FM mice. The open reading frame (ORF) in the 3' long terminal repeat of the MMTV was cloned by polymerase chain reaction with primers corresponding to conserved regions spanning the ORF coding region. Sequence analysis of the ORF revealed that there is no sequence identical to those in other known MMTV in the carboxy terminus implicated in TCR V beta recognition. Subcutaneous injection of the virus into adult BALB/c mice induced an approximately three- to fourfold enlargement of draining lymph nodes and a substantial increase of V beta 8.2+ CD4+ T cells in the lymph nodes within 6 days. The exposure of newborn BALB/c mice to the virus by foster nursing resulted in a marked deletion of V beta 8.2+ cells both in CD4+ and CD8+ T cells. Thus, a novel milk-borne MMTV in FM mice expresses strong superantigenic properties capable of stimulating V beta 8.2+ T cells. V beta 8.2+ T cells have been demonstrated to be frequently involved in recognition of conventional antigens and responsible for autoimmune diseases such as experimental allergic encephalomyelitis. Therefore, the MMTV (FM) may provide a new mouse model system for inducing immunodeficiency or autoimmune disease by retroviral infection.

Mouse mammary tumor virus with rearranged long terminal repeats causes murine lymphomas

Mouse mammary tumor virus (MMTV) is a slowly transforming retrovirus associated primarily with the induction of mammary tumors. It is widely accepted that T-cell lymphomas of various mouse strains are associated with extra proviruses of MMTV. These extra proviruses showed site-specific rearrangements in the U3 region of long terminal repeats (LTRs), consisting of about 400 nucleotide deletions and occasional substitution resulting in unique tandem repeats. However, the question of whether these mutant MMTVs cause lymphomas has not been experimentally resolved. Here we present distinct evidence that they do. We constructed chimeric MMTVs by replacing the LTR of the recently constructed pathogenic MMTV provirus clone with rearranged LTRs of MMTV proviruses obtained from two DBA/2 mouse lymphoma cell lines, MLA and DL-8, and inoculated them into BALB/c mice. These mice developed lymphomas, but no mammary tumors, 4 to 11 months postinoculation, whereas the original pathogenic MMTV clone alone induced mammary tumors. These results showed that the tissue specificity of MMTV tumorigenesis is determined by the LTR structures.

An activation-dependent, T-lymphocyte-specific transcriptional activator in the mouse mammary tumor virus env gene

Transcription of the complete mouse mammary tumor virus (MMTV) proviral genome in mouse cells is controlled by a strong promoter in its long terminal repeat. In the mouse T lymphoma EL4, there is a second, activation-dependent transcriptional initiation site within the envelope (env) gene, from which a short mRNA is generated, encoding the open reading frame of the long terminal repeat. We now report the isolation of a segment of the MMTV env gene (called META, for MMTV env transcriptional activator) which has the expected transcription-activating properties seen in EL4.E1 cells. Namely, it induces activation-dependent, T-lymphocyte-specific transcription of a chloramphenicol acetyltransferase reporter gene. It is active in mouse or human T-helper lymphocyte lines when they are stimulated to transcribe lymphokine genes but is inactive in unstimulated T-helper cells, fibroblasts, a cytotoxic T-lymphocyte line, and a mastocytoma cell line. Its activity is inhibited by cyclosporin A, a specific inhibitor of lymphokine transcription. Several forms of the META have been isolated from EL4.E1 cells, a mouse T-helper cell hybridoma, and from BALB/c spleen cells. Linked to the heterologous thymidine kinase promoter, a 400-bp portion of it is an inducible, orientation-independent, and cyclosporin A-sensitive transcriptional activator in T-helper cells.

An activation-dependent, T-lymphocyte-specific transcriptional activator in the mouse mammary tumor virus env gene

Transcription of the complete mouse mammary tumor virus (MMTV) proviral genome in mouse cells is controlled by a strong promoter in its long terminal repeat. In the mouse T lymphoma EL4, there is a second, activation-dependent transcriptional initiation site within the envelope (env) gene, from which a short mRNA is generated, encoding the open reading frame of the long terminal repeat. We now report the isolation of a segment of the MMTV env gene (called META, for MMTV env transcriptional activator) which has the expected transcription-activating properties seen in EL4.E1 cells. Namely, it induces activation-dependent, T-lymphocyte-specific transcription of a chloramphenicol acetyltransferase reporter gene. It is active in mouse or human T-helper lymphocyte lines when they are stimulated to transcribe lymphokine genes but is inactive in unstimulated T-helper cells, fibroblasts, a cytotoxic T-lymphocyte line, and a mastocytoma cell line. Its activity is inhibited by cyclosporin A, a specific inhibitor of lymphokine transcription. Several forms of the META have been isolated from EL4.E1 cells, a mouse T-helper cell hybridoma, and from BALB/c spleen cells. Linked to the heterologous thymidine kinase promoter, a 400-bp portion of it is an inducible, orientation-independent, and cyclosporin A-sensitive transcriptional activator in T-helper cells.

Expression pattern of mouse mammary tumor virus in transgenic mice carrying exogenous proviruses of different origins

To study the tissue specificity of mouse mammary tumor virus (MMTV) gene expression, we developed two series of transgenic mice, containing the MMTV proviral DNA of mammary (GR) and kidney (C3H-K) origin. The expression pattern in the MMTV(GR) transgenic mice is very similar to that observed in infected animals, e.g., a strong preference for viral expression in the lactating mammary glands and lower levels of expression in salivary glands, lymphoid tissues, and male reproductive organs. One line of transgenic mice carrying the C3H-K provirus has a similar expression pattern, indicating that MMTV(C3H-K), despite a striking alteration in the U3 region of its long terminal repeat, can be expressed in the same tissues as the wild-type MMTV.

Factors controlling the expression of mouse mammary tumour virus

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Two regions of the mouse mammary tumor virus long terminal repeat regulate the activity of its promoter in mammary cell lines

In vivo expression of the mouse mammary tumor virus (MMTV) is restricted to a few organs, with the highest rate of transcription found in the mammary gland. Using a series of mammary and nonmammary murine cell lines, we have identified two regulatory elements, located upstream of the hormone responsive element, that specifically regulate the MMTV promoter. The first element displays an enhancerlike activity and is coincident with the binding of a nuclear factor (designated MP4; position -1078 to -1052 in the long terminal repeat) whose presence is apparently restricted to mammary cell lines. The second regulatory region mediates a repressive activity and is mapped to the long terminal repeat segment from -415 to -483. This repression is specific for a particular subtype of mammary cells (RAC cells) able to grow under two differentiation states (A. Sonnenberg, H. Daams, J. Calafat, and J. Hilgers, Cancer Res. 46:5913-5922, 1986). The MMTV promoter in mammary cell lines thus appears to be modulated by two cis-acting elements that are likely to be involved in tissue-specific expression in vivo.

Two regions of the mouse mammary tumor virus long terminal repeat regulate the activity of its promoter in mammary cell lines

In vivo expression of the mouse mammary tumor virus (MMTV) is restricted to a few organs, with the highest rate of transcription found in the mammary gland. Using a series of mammary and nonmammary murine cell lines, we have identified two regulatory elements, located upstream of the hormone responsive element, that specifically regulate the MMTV promoter. The first element displays an enhancerlike activity and is coincident with the binding of a nuclear factor (designated MP4; position -1078 to -1052 in the long terminal repeat) whose presence is apparently restricted to mammary cell lines. The second regulatory region mediates a repressive activity and is mapped to the long terminal repeat segment from -415 to -483. This repression is specific for a particular subtype of mammary cells (RAC cells) able to grow under two differentiation states (A. Sonnenberg, H. Daams, J. Calafat, and J. Hilgers, Cancer Res. 46:5913-5922, 1986). The MMTV promoter in mammary cell lines thus appears to be modulated by two cis-acting elements that are likely to be involved in tissue-specific expression in vivo.

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.

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.