Transcription of mouse mammary tumor virus: identification of a candidate mRNA for the long terminal repeat gene product

Convergent capture of retroviral superantigens by mammalian herpesviruses

Horizontal gene transfer from retroviruses to mammals is well documented and extensive, but is rare between unrelated viruses with distinct genome types. Three herpesviruses encode a gene with similarity to a retroviral superantigen gene (sag) of the unrelated mouse mammary tumour virus (MMTV). We uncover ancient retroviral sags in over 20 mammals to reconstruct their shared history with herpesviral sags, revealing that the acquisition is a convergent evolutionary event. A retrovirus circulating in South American primates over 10 million years ago was the source of sag in two monkey herpesviruses, and a different retrovirus was the source of sag in a Peruvian rodent herpesvirus. We further show through a timescaled phylogenetic analysis that a cross-species transmission of monkey herpesviruses occurred after the acquisition of sag. These results reveal that a diverse range of ancient sag-containing retroviruses independently donated sag twice from two separate lineages that are distinct from MMTV.

Horizontal gene transfer from retroviruses to mammals is rare between unrelated viruses. Here the authors show the convergent acquisition by herpesviruses of a virulence gene of ancient retroviruses, which occurred at least twice from different donor lineages, to distinct herpesviruses that infect mammals.

A novel, mouse mammary tumor virus encoded protein with Rev-like properties

We have identified a novel, multiple spliced, subgenomic mRNA species in MMTV producing cells of different origin containing an open reading frame encoding a 39-kDa Rev-like protein, Rem (regulator of expression of MMTV). An EGFP-Rem fusion protein is shown to be predominantly in the nucleolus. Further leptomycin B inhibits the nuclear export of nonspliced MMTV transcripts, implicating Rem in nuclear export by the Crm1 pathway in MMTV. Rem is thus reminiscent of the Rec protein from the related endogenous human retrovirus, HERV-K.

META-controlled env-initiated transcripts encoding superantigens of murine Mtv29 and Mtv7 and their possible role in B cell lymphomagenesis

Spontaneous germinal center (GC)-derived B cell lymphomas of SJL mice (RCS) transcribe a 1.8-kb Mtv-29 mRNA under control of the META-env promoter. The encoded vSAg29 stimulates syngeneic Vbeta16(+) CD4(+) T cells, thereby acquiring T cell help necessary for RCS growth. Other strains of B cell lymphoma-prone mice include Mtv29(+) C57L and MA/MyJ, and the Mtv29(-) Mtv7(+)-recombinant inbred strain, SW x J-1. The lymphomas of these mice produce similar mouse mtv-vSAg-encoding mRNA, as characterized by Northern blotting, PCR, and RNase protection. A 1.8-kb mRNA in C57L/J and MA/MyJ lymphomas hybridized with an Mtv29-specific oligonucleotide, whereas SW x J-1 lymphomas produced 1.8-kb transcripts hybridizing with an Mtv7-specific oligonucleotide. Similar META-env-initiated transcripts were absent from LPS-activated B cells from any strain examined but were detected in Peyer's patch RNA from SJL mice. Like typical SJL-derived RCS, all these lymphomas stimulated syngeneic CD4(+) T cells and Vbeta16(+) T hybridoma cells. Immunohistochemical staining of primary tumors showed the presence of peanut agglutinin binding (PNA(+)) highly mitotic lymphoblasts, suggesting their GC derivation. The findings indicate that this novel mRNA for Mtv29 is present in B cell lymphomas from several Mtv29(+) mouse strains. Additionally, this is the first description of the ability of Mtv7 to produce transcripts that are controlled and spliced identically to those of Mtv29 and that are expressed in SW x J-1, I-A(s+), lymphomas that also stimulate Vbeta16(+) T cells. Our results suggest an important role for mouse mtv-vSAgs and Vbeta16 T cell stimulation in the development of GC-derived murine B cell lymphomas.

C3H mouse mammary tumor virus superantigen function requires a splice donor site in the envelope gene

Mouse mammary tumor virus (MMTV) encodes a superantigen (Sag) that is required for efficient milk-borne transmission of virus from mothers to offspring. The mRNA used for Sag expression is controversial, and at least four different promoters (two in the long terminal repeat and two in the envelope gene) for sag mRNA have been reported. To determine which RNA is responsible for Sag function during milk-borne MMTV transmission, we mutated a splice donor site unique to a spliced sag RNA from the 5' envelope promoter. The splice donor mutation in an infectious provirus was transfected into XC cells and injected into BALB/c mice. Mice injected with wild-type provirus showed Sag activity by the deletion of Sag-specific T cells and induction of mammary tumors in 100% of injected animals. However, mice injected with the splice donor mutant gave sporadic and delayed T-cell deletion and a low percentage of mammary tumors with a long latency, suggesting that the resulting tumors were due to the generation of recombinants with endogenous MMTVs. Third-litter offspring of mice injected with wild-type provirus showed Sag-specific T-cell deletion and developed mammary tumors with kinetics similar to those for mice infected by nursing on MMTV-infected mothers, whereas the third-litter offspring of the splice donor mutant-injected mice did not. One of the fifth-litter progeny of splice donor mutant-injected mice showed C3H Sag activity and had recombinants that repaired the splice donor mutation, thus confirming the necessity for the splice donor site for Sag function. These experiments are the first to show that the spliced sag mRNA from the 5' envelope promoter is required for efficient milk-borne transmission of C3H MMTV.

Requirement for reverse immune surveillance for the growth of germinal center-derived murine lymphomas

The concept of reverse immune surveillance, first conceived over 12 years ago, described the relationship that existed between germinal center-derived B cell lymphoma cells and the host immune system in SjL/J mice. According to reverse immune surveillance, recognition of tumor cell antigens and a response by the host immune system is required for tumor growth. The phenomenon of reverse immune surveillance related to B cell lymphomas has recently also been characterized in another inbred mouse strain, C57L/J. Moreover, elements of reverse immune surveillance have been observed in several other mouse strains that develop B cell lymphomas, suggesting that this lymphomagenic mechanism may be more common than first envisioned. In SJL and C57L mice, the B lymphoma cells express an MMTV-encoded superantigen (vSAg29) that stimulates syngeneic CD4+ T cells bearing Vbeta16 in their TCR. In contrast to the mRNAs for other MMTVs in normal mouse B cells, vSAg29 mRNA initiates in the env (META) region, undergoes splicing in the 3' env region, and continues through the 3' LTR. Copious cytokine production, including IFN-gamma, IL-4 and IL-5 accompanies the response of the T cells to this vSAg. In addition to cytokines produced by vSAg-responsive T cells, more recent evidence indicates that another cytokine, LTalphabeta2, which is expressed on the lymphoma cell surface, also plays a role in the promotion of the B cell lymphoma growth. It is possible that interaction with LTbeta-R on follicular dendritic cells or other stromal elements facilitates tumor growth by preventing apoptosis of the malignant B cells. To what degree these findings in the mouse are relevant to the development and/or growth of human B lymphoma cells remains to be determined. However, endogenous retroviral sequences do exist in the human genome. Interestingly, some of these sequences are homologous to MMTV, and are transcribed in B lymphoblastoid cells. Moreover microorganisms that are infectious for human B cells, such as EBV and Herpes Virus 8, may also produce superantigens.

Mouse mammary tumor virus superantigen expression is reduced by glucocorticoid treatment

Expression of mouse mammary tumor virus (MMTV)-encoded superantigens in B lymphocytes are required for viral transmission and pathogenesis. Due to problems with detection and quantification of the superantigen protein, most reports about the mechanism of superantigen expression from the viral sag gene rely on the quantitative analysis of putative sag mRNAs. The description of multiple promoters as a source of putative sag mRNA has complicated the situation even further. All conclusions about the level of superantigen protein expression based on these data remain circumstantial. To test the effect of the glucocorticoid hormone dexamethasone on the total superantigen expression from an infectious MMTV provirus we used a quantitative assay that is based on a superantigen-luciferase fusion protein. MMTV gene expression from the major promoter in the 5' long terminal repeat (LTR) is strongly induced in the presence of glucocorticoid hormones. We now demonstrate that, in the presence of dexamethasone, sag gene expression is reduced despite increased transcription from the MMTV 5' LTR and increased amounts of putative sag mRNA initiated at the LTR promoter. These data show that the expression of the MMTV sag gene does not correlate with the activity of the major LTR promoter and thus differs from all other MMTV genes.

Cloning of an infectious milk-borne mouse mammary tumor virus (MMTV) DNA from a mammary tumor that developed in an endogenous MMTV-free wild mouse

Molecular characterization of infectious mouse mammary tumor viruses (MMTVs) has been hampered due to the problem of cloning a full-length exogenous virus into a plasmid. The present report describes our strategy for obtaining a full-length clone of an exogenous MMTV from a mouse mammary tumor that arose spontaneously in a wild Chinese mouse free of endogenous MMTV and shows that the cloned virus (JYG-MMTV) is expressed in rat RBA cells. Four-week-old C58/J x CBA/CaJ female mice, free of both endogenous and exogenous MMTVs, were injected with virus-secreting RBA cells. The progeny of these mice were bred, and their offspring were tested for the presence of MMTV. These third-generation mice were found to actively produce MMTV that was shed in their milk and transmitted to their offspring. The virus was detected not only in the mammary glands of these young mice, but also in their spleens and bone marrow. These results suggest that our plasmid-cloned exogenous JYG-MMTV is infectious. This virus can now be used effectively in manipulating the various genes of JYG-MMTV and other MMTV strains to understand their structure/function relationships.

Expression of mouse mammary tumor virus superantigen mRNA in the thymus correlates with kinetics of self-reactive T-cell loss

Mouse mammary tumor virus (MMTV) encodes a superantigen (Sag) that is expressed at the surface of antigen-presenting cells in conjunction with major histocompatibility complex (MHC) type II molecules. The Sag-MHC complex is recognized by entire subsets of T cells, leading to cytokine release and amplification of infected B and T cells that carry milk-borne MMTV to the mammary gland. Expression of Sag proteins from endogenous MMTV proviruses carried in the mouse germ line usually results in the deletion of self-reactive T cells during negative selection in the thymus and the elimination of T cells required for infection by specific milk-borne MMTVs. However, other endogenous MMTVs are unable to eliminate Sag-reactive T cells in newborn mice and cause partial loss of reactive T cells in adults. To investigate the kinetics of Sag-reactive T-cell deletion, backcross mice that contain single or multiple MMTVs were screened by a novel PCR assay designed to distinguish among highly related MMTV strains. Mice that contained Mtv-17 alone showed slow kinetics of reactive T-cell loss that involved the CD4(+), but not the CD8(+), subset. Deletion of CD4(+) or CD8(+) T cells reactive with Mtv-17 Sag was not detected in thymocytes. Slow kinetics of peripheral T-cell deletion by Mtv-17 Sag also was accompanied by failure to detect Mtv-17 sag-specific mRNA in the thymus, despite detectable expression in other tissues, such as spleen. Together, these data suggest that Mtv-17 Sag causes peripheral, rather than intrathymic, deletion of T cells. Interestingly, the Mtv-8 provirus caused partial deletion of CD4(+)Vbeta12(+) cells in the thymus, but other T-cell subsets appeared to be deleted only in the periphery. Our data have important implications for the level of antigen expression required for elimination of self-reactive T cells. Moreover, these experiments suggest that mice expressing endogenous MMTVs that lead to slow kinetics of T-cell deletion will be susceptible to infection by milk-borne MMTVs with the same Sag specificity.

Strain-specific expression of spliced MMTV RNAs containing the superantigen gene

The transmission of milk-borne or exogenous mouse mammary tumor virus (MMTV) requires infection of B cells in the gut-associated lymphoid tissue and expression of the superantigen (Sag) protein at the B-cell surface. Presentation of Sag at the B-cell surface is required for the transmission of MMTV to T cells and subsequent infection of the target mammary gland tissue. Because several different promoters have been reported for MMTV sag mRNA expression, we investigated whether the detection of spliced sag RNAs was dependent upon the cell type infected or the particular MMTV strain examined. In this study, we detected expression of spliced sag RNA from the standard promoter and from an internal U3 promoter in B-cell lines expressing endogenous Mtv-6 by RT-PCR, although expression from the standard promoter appeared to be at least 10-fold higher than that observed from the internal U3 promoter. Sag RNA originating from exogenous C3H MMTV was not observed from either of the U3 promoters in any cell type examined. However, spliced mRNAs containing the exogenous C3H MMTV, endogenous Mtv-8, or endogenous Mtv-17 sag genes could be detected from a previously described promoter in the envelope coding region regardless of the cell type infected. Because sag-specific RNAs can be initiated independently of the LTR promoters, there may be selection for independent control of MMTV sag and structural gene expression.

Quantitation of endogenous mouse mammary tumor virus superantigen expression by lymphocyte subsets

Superantigens (SAg) encoded by endogenous mouse mammary tumor viruses (Mtv) interact with the V beta domain of the T cell receptor (TcR-V beta). Presentation of Mtv SAg can lead to stimulation and/or deletion of the reactive T cells, but little is known about the quantitative aspects of SAg presentation. Although monoclonal antibodies have been raised against Mtv SAg, they have not been useful in quantitating SAg protein, which is present in very low amounts in normal cells. Alternative attempts to quantitate Mtv SAg mRNA expression are complicated by the fact that Mtv transcription occurs from multiple loci and in different overlapping reading frames. In this report we describe a novel competitive polymerase chain reaction assay which allows the locus-specific quantitation of SAg expression at the mRNA level in lymphocyte subsets from mouse strains with multiple endogenous Mtv loci. In B cells as well as T cells (CD4+ or CD8+), Mtv-6 SAg is expressed at the highest levels, followed by Mtv-7 SAg and (to a much lesser extent) Mtv-8,9. Consistent with functional Mtv-7 SAg presentation studies, we find that Mtv-7 SAg expression is higher in B cells than in CD8+ T cells and very low in the CD4+ subset. The overall hierarchy in Mtv SAg expression (i.e. Mtv-6 > Mtv-7 > Mtv 8,9) was also observed for mRNA isolated from neonatal thymus. Furthermore, the kinetics of intrathymic deletion of the corresponding TcR-V beta domains during ontogeny correlated with the levels of Mtv SAg expression. Collectively our data suggest that T cell responses to Mtv SAg are largely controlled by SAg expression levels on presenting cells.

Negative-acting factor and superantigen are separable activities of the mouse mammary tumor virus long terminal repeat

The open reading frame contained within the long terminal repeat (LTR) of mouse mammary tumor virus encodes Naf, a negative regulator of transcription, as well as a superantigen activity, Sag, which causes the deletion of specific classes of T cells. In the present study, the effect of Naf expression on different promoters and the coding requirements for Naf and Sag have been investigated. Sag activity was found to require only sequences in the LTR, whereas sequences located within the gag gene were additionally required for functional Naf activity. Surprisingly, both the classic promoter and a recently described promoter located in the LTR can give rise to both functional Naf and Sag. Further analysis of Naf revealed that the downregulatory effect was mediated by sequences located in the LTR and that heterologous promoters were also affected by Naf.

Structure and biological activity of the subgenomic Mtv-6 endogenous provirus

The Mtv-6 provirus has an incomplete genome, but retains a functional superantigen gene (sag) which directs the thymic deletion of CD4+ T cells expressing T cell receptors containing the V beta 3 or V beta 5 chains. To better understand the Mtv-6 superantigen, the structure and biological activity of the Mtv-6 provirus was analyzed. First, the complete nucleotide sequence was determined, and the mutation producing the subgenomic provirus was identified. Second, the nucleotide sequence of the 5' end of the sag gene transcript (including the splice junction) was determined by sequence analysis of a cDNA clone. Third, the superantigen activity of Mtv-6 was analyzed in mice carrying the Mtv-6 provirus isolated by selective breeding on a genetic background free of endogenous and exogenous mouse mammary tumor virus (MMTV). These studies demonstrate that (i) the Mtv-6 provirus contains a 6.2-kb deletion between two 12-bp direct repeats encompassing the central portion of the provirus but not affecting sag gene splicing or translation, (ii) the sag gene transcript has the structure predicted from previous S1 nuclease mapping studies, and (iii) the Mtv-6 superantigen can direct thymic deletion of target V beta 3+ and V beta 5+ T cells in the absence of gene products from full-length MMTV proviruses.

Endogenous superantigen expression controlled by a novel promoter in the MMTV long terminal repeat

Endogenous superantigens are encoded by the open reading frame contained within the mouse mammary tumour virus long terminal repeat (MMTV LTR). Superantigen expression results in T-cell proliferation and, during early ontogeny, T-cell deletion. Here we identify a novel promoter located upstream of the previously described MMTV promoter. Transcripts from this promoter initiate within the U3 region of the MMTV LTR and splice to the acceptor for endogenous superantigen coding region. The novel U3 promoter is active in B lymphocytes, which are cognate antigen-presenting cells for endogenous superantigen, and is able to direct expression of superantigen in the absence of the previously described MMTV promoter.

Factors controlling the expression of mouse mammary tumour virus

Images

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

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

Detection and characterization of a glycoprotein encoded by the mouse mammary tumor virus long terminal repeat gene

Mouse mammary tumor virus (MMTV) is a retrovirus that causes mammary tumors in susceptible mice. MMTV contains a unique open reading frame (ORF) in the unique 3' region of the proviral long terminal repeat (LTR) with the potential to encode a 36-kDa protein. However, the ORF gene product has not been detected in murine mammary tissues or cell lines. We utilized the baculovirus expression vector system to generate large amounts of the ORF protein. Putative ORF gene products of 36 and 45 kDa were detected as unique proteins in extracts of insect cells infected with recombinant baculovirus (LTR-ORF BV), and the identities of these proteins as viral gene products were confirmed immunologically. Antipeptide antisera were generated in rabbits against peptides chosen from computer-predicted hydrophilic regions of the ORF coding sequence. These antisera reacted specifically by immunoprecipitation and by immunoblot with the proteins expressed in LTR-ORF BV-infected insect cells, as well as with MMTV LTR ORF in vitro translation products. Polyclonal antisera were raised against two putative ORF protein species partially purified from insect cells. These sera specifically immunoprecipitated viral protein products translated in vitro. In vitro translation of MMTV LTR ORF transcripts in the presence of canine pancreatic microsomal membranes generated a higher-molecular-weight ORF gene product, indicating that the ORF protein is modified by N-linked glycosylation. This glycosylated ORF product comigrated with the larger ORF protein species produced in infected insect cells. The gp45 product was metabolically labeled with [3H] mannose, [3H] galactose, and [3H] N-acetyl-D-glucosamine in insect cells, whereas this incorporation was inhibited in the presence of tunicamycin. Digestion of gp45 with endoglycosidase H yielded the lower-molecular-weight ORF protein p36. These observations suggest that the ORF glycoprotein contains hybrid N-linked oligosaccharides. Demonstration of the modified nature of the ORF gene product will facilitate characterization of ORF protein expression in murine tissues.

naf, a trans-regulating negative-acting factor encoded within the mouse mammary tumor virus open reading frame region

The mouse mammary tumor virus (MMTV) long terminal repeat (LTR) open reading frame (ORF) encodes a negative acting factor (naf). In our test system, naf mediates its effect in trans on another MMTV provirus in which the 5' LTR has been replaced by that of Rous sarcoma virus. naf effects are evidenced at the level of transcriptional initiation rather than as reduced mRNA stability. The introduction of a premature termination codon into the MMTV LTR-encoded ORF abolishes the transcriptional down regulation localizing naf within the ORF. In addition, sequences in the gag/pol genes between +320 and +646 and between +3626 and +4590 relative to the site of transcription initiation are also involved in the MMTV-mediated transcriptional down regulation.

Altered mouse mammary tumor virus transcript synthesis in T-cell lymphoma cells

Proviral copies of mouse mammary tumor virus (MMTV) are known to be amplified in certain T-cell lymphomas. Transcription of the amplified MMTV proviruses was studied in detail in two T-cell lymphoma lines and showed the production of deletions and premature termination of env mRNAs and the premature termination of gag transcripts. EL-4 cells produce three env mRNAs, and sequence analysis of cDNAs of the two smaller transcripts revealed large deletions encompassing the 3' half of the env gene. The deletion in at least one of the altered transcripts appeared to be produced by a splicing mechanism. T-cell lymphoma line ML of DBA/2 mice also synthesizes two smaller env transcripts, both of which result from premature termination of transcription. Both lines transcribe high levels of gag mRNAs of about 0.8 kilobases in length, terminating at the end of the region encoding MMTV phosphoprotein pp21. Restriction enzyme BamHI analysis of the amplified proviruses of EL-4 and ML cells as well as of additional non-mammary tumor cell types containing amplified MMTV proviruses suggested that the amplified proviruses were derived from exogenous viruses, or activated endogenous provirus MTV-1 in the case of DBA/2 strain tumor cells.

Lipopolysaccharide and dexamethasone induce mouse mammary tumor proviral gene expression and differentiation in B lymphocytes through distinct regulatory pathways

Endogenous mouse mammary tumor virus (MMTV) proviral transcripts are up regulated during the normal course of B-lymphocyte differentiation. We report here that the regulatory mechanisms which lead to increased levels of MMTV transcripts in differentiating, lipopolysaccharide (LPS)-stimulated normal B cells and in the inducible B-cell lymphoma line CH12 are at least partially distinct from those controlling increases in immunoglobulin and J-chain gene expression. In studies designed to characterize the stimulatory pathways leading to MMTV expression in CH12 cells, we found that stimulation with either LPS or dexamethasone (Dex), a transcriptional activator of MMTV genes, induced not only MMTV expression but also differentiation to antibody secretion. Only Dex-induced and not LPS-induced MMTV expression and differentiation were inhibited by the glucocorticoid antagonist RU486, demonstrating that Dex and LPS stimulate B cells by distinct molecular pathways. Therefore, in B cells, MMTV expression can be regulated via either the conventional hormone receptor-dependent pathway or a hormone receptor-independent pathway. Furthermore, these results suggest that steroid stimulation of B cells can lead to alterations in the expression of other results suggest that steroid stimulation of B cells can lead to alterations in the expression of other steroid-responsive genes that can become involved in the process of B-cell differentiation.

Lipopolysaccharide and dexamethasone induce mouse mammary tumor proviral gene expression and differentiation in B lymphocytes through distinct regulatory pathways

Endogenous mouse mammary tumor virus (MMTV) proviral transcripts are up regulated during the normal course of B-lymphocyte differentiation. We report here that the regulatory mechanisms which lead to increased levels of MMTV transcripts in differentiating, lipopolysaccharide (LPS)-stimulated normal B cells and in the inducible B-cell lymphoma line CH12 are at least partially distinct from those controlling increases in immunoglobulin and J-chain gene expression. In studies designed to characterize the stimulatory pathways leading to MMTV expression in CH12 cells, we found that stimulation with either LPS or dexamethasone (Dex), a transcriptional activator of MMTV genes, induced not only MMTV expression but also differentiation to antibody secretion. Only Dex-induced and not LPS-induced MMTV expression and differentiation were inhibited by the glucocorticoid antagonist RU486, demonstrating that Dex and LPS stimulate B cells by distinct molecular pathways. Therefore, in B cells, MMTV expression can be regulated via either the conventional hormone receptor-dependent pathway or a hormone receptor-independent pathway. Furthermore, these results suggest that steroid stimulation of B cells can lead to alterations in the expression of other results suggest that steroid stimulation of B cells can lead to alterations in the expression of other steroid-responsive genes that can become involved in the process of B-cell differentiation.

Expression of the int-1 and int-2 loci in endogenous mouse mammary tumor virus-induced mammary tumorigenesis in the C3Hf mouse

The int-1 locus appears to be involved in over 80% of C3H exogenous mouse mammary tumor virus (MMTV)-induced mouse mammary tumors, and the int-2 locus appears to be involved in approximately 10% of these tumors. Analysis of 46 C3Hf mammary tumors resulting from endogenous, rather than exogenous, MMTV infection revealed that only 41% expressed int-1 RNA, while 2% expressed int-2 RNA. Our results suggest that in addition to the int-1 and int-2 loci, other loci may be involved in endogenous-MMTV-induced mammary tumors of the C3Hf mouse.

Spontaneous progression of hyperplastic outgrowths of the D1 lineage to mammary tumors: expression of mouse mammary tumor virus and cellular proto-oncogenes

Mammary cancer in mice is characterized by progression through defined stages of preneoplasia, with the most common preneoplastic stage being the hyperplastic alveolar nodule (HAN). We determined the relative levels of RNA expression of various cellular proto-oncogenes and endogenous mouse mammary tumor virus genes in outgrowths and tumors of three sublines of the transplantable D1 HAN preneoplastic outgrowth line. The three sublines differed in relative tumor-producing capabilities. Subline D1B produced a high incidence of tumors with short latency periods, whereas sublines D1C and D1D produced low incidences of tumors with long latency periods. No consistent alteration in proto-oncogene expression correlated with relative tumorigenicity, although tumors frequently contained higher levels of one or more proto-oncogene transcripts as compared with preneoplastic tissue. Slightly elevated (2- to 6-fold) levels of different oncogene transcripts were detected in 13 of 17 tumors as compared with outgrowth tissue, including abl (2 tumors), fps (5 tumors), Ha-ras (6 tumors), and Ki-ras (8 tumors). One tumor contained 45 times more Ki-ras-specific RNA than outgrowth tissue because of a comparable amplification of Ki-ras DNA sequences. Elevated levels of Ha-ras occurred more frequently in tumors of a high-incidence subline than in a less-aggressive subline (5/10 vs 1/7), but this difference was not statistically significant. However, consistent changes in MMTV expression accompanied progression from preneoplastic tissues to mammary tumors. All 17 tumors displayed reduced levels of the MMTV-specific long terminal repeat (LTR) transcript (1.6 kb) as compared with HAN tissue; tumors with moderate levels of LTR transcript expressed the 3.8-kb envelope message as well, one not detected in HANs. Expression of the LTR transcript is apparently influenced by factors in addition to the methylation status of endogenous mouse mammary tumor virus genes, which was similar in outgrowths and tumors. As the survey of representative proto-oncogenes failed to identify a uniform change between HAN and tumors, it is likely that other genes are involved in tumor progression in the mammary gland.

Phorbol diester-inducible, cyclosporine-suppressible transcription from a novel promoter within the mouse mammary tumor virus env gene

The mouse T-cell lymphoma cell line EL4.E1 constitutively synthesizes mouse mammary tumor virus (MMTV) transcripts encoding either the entire proviral genome or segments of it. In addition to these conventional mRNAs, however, an mRNA of about 1 kilobase accumulates after induction of these cells with phorbol myristate acetate (PMA). The accumulation of this transcript is strongly inhibited by the immunosuppressive agent cyclosporin A. Its pattern of induction by PMA and suppression by cyclosporin A is thus the same as seen for several lymphokine mRNAs in these cells, including interleukin-2 and granulocyte-macrophage colony-stimulating factor. The short MMTV transcript is the most abundant PMA-induced transcript in EL4.E1 cells, but was not found in a series of other leukocyte tumor cell lines. It is initiated from a novel promoter within the env gene, and a segment of 1,161 nucleotides is then spliced out. The major part of the transcript is a copy of the long terminal repeat (LTR) of MMTV. The MMTV proviral genomes in these cells, and the short transcript, contain a 491-nucleotide deletion in the LTR compared with the normal MMTV provirus. The resulting open reading frame could encode a protein of molecular weight 22,800, which is a likely candidate for an LTR-related protein with a similar molecular weight recently described in this system (J. Racevskis, J. Virol. 58:441-449, 1986).

Activation of endogenous MMTV proviruses in murine mammary cancer induced by chemical carcinogen

A study was undertaken to determine whether activation of expression of silent endogenous mouse mammary tumor virus (MMTV) proviruses may occur during tumor induction by a chemical carcinogen. A series of transplantable mammary tumors induced in BALB/c mice by treatment with dimethylbenz(alpha)anthracene (DMBA), pituitary isograft, or both was examined. The results obtained suggest that chemical carcinogens may induce mammary tumors through more than one pathway. Two of 9 tumor lines produced virus-specific products at levels above those observed during the course of normal mammary gland development. One tumor contained high levels of MMTV-specific envelope [3.8 kilobase (kb)] and genomic length (8.9 kb) RNAs. This tumor expressed core- and envelope-related proteins detectable by immunoblotting (including p28, gp52, and gp36), displayed an acquired provirus with a restriction map different from those of described exogenous MMTV strains, and contained abundant virus particles. The other tumor that expressed high levels of MMTV gene products contained envelope-specific (3.8 kb) and long-terminal-repeat-specific (1.6 kb) messages but no full-length RNA. It exhibited an aberrant 39 kDa, envelope-related protein, but no virus particles. Methylation data implicated the usually silent endogenous Mtv-8 provirus as the source of the abnormal envelope protein. None of the tumors expressed RNA from the putative mammary oncogenes, int-1 or int-2. We propose that chemical carcinogens may activate different cellular genes by mutation and that, in a subset of DMBA-induced mammary tumors, the target genes include endogenous MMTV proviruses that are normally not expressed. The effect on provirus expression varies from tumor to tumor, but is stable over passage of a given tumor. MMTV may be of etiological importance in the genesis of those DMBA-induced tumors which contain high levels of MMTV-specific products, but its action in the BALB/c system is not mediated through enhanced expression of the int-1 or int-2 preferred integration regions.

Complete nucleotide sequence of a milk-transmitted mouse mammary tumor virus: two frameshift suppression events are required for translation of gag and pol

We sequenced two recombinant DNA clones constituting a single provirus of the milk-transmitted mouse mammary tumor virus characteristic of BR6 mice. The complete provirus is 9,901 base pairs long, flanked by 6 base-pair duplications of cellular DNA at the site of integration. Five extensive blocks of open reading frame corresponding to the gag gene, the presumed protease, the pol and env genes, and the open reading frame orf within the long terminal repeat of the provirus were readily discernible. Translation of gag, protease, and pol involved three different translational reading frames to produce the three overlapping polyprotein precursors Pr77, Pr110, and Pr160 found in virus-infected cells. Synthesis of the reverse transcriptase and endonuclease therefore required two separate frameshifts to suppress the termination codons at the ends of the Pr77 and Pr110 domains. Direct evidence is presented for translational readthrough of both stop codons in an in vitro protein synthesis system.

Spleen specific expression of an MMTV related transcript associated with the Mtv-6 locus in BALB/c mice

We have detected an MMTV related transcript which is expressed in a spleen specific manner in BALB/cHeA mice. Using a recombinant inbred series between BALB/cHeA and STS/A mice (C X S RI series) we have identified RNA associated with the Mtv-3 locus of the STS/A strain. This transcript initiates at the same site in the MMTV LTR as already reported for Mtv-2 and Mtv-8. The novel spleen specific MMTV transcript in the BALB/cHeA strain has a different structure as compared to the transcripts associated with the Mtv-2, Mtv-3, or Mtv-8 loci. We have tentatively identified the Mtv-6 locus as the source of these unique transcripts.

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.

Differential expression of endogenous mouse mammary tumor virus genes during development of the BALB/c mammary gland

Expression of endogenous mouse mammary tumor virus sequences varied over the course of development of the mammary gland during primary pregnancy and lactation in virus-free BALB/c mice. Although RNA from all regions of the genome was detected, both the level and temporal regulation of expression were different for long terminal repeat-, env-, and gag-pol-specific RNAs. Analysis of the methylation status of proviral DNA indicated differential accessibility of the three endogenous units during development. The results demonstrated noncoordinate regulation of mouse mammary tumor virus expression with respect to provirus template utilized and specific transcripts accumulated.

Direct detection of exogenous mouse mammary tumor virus sequences in lymphoid cells of BALB/cfC3H female mice

The presence of exogenous mouse mammary tumor virus (MMTV) (C3H) DNA sequences in lymphoid tissue (spleen, bone marrow, and thymus) and nonlymphoid tissue (liver and kidney) of BALB/cfC3H female mice was directly assessed by DNA hybridization methods. Lymphoid tissues were found positive for integrated MMTV(C3H) sequences in females as young as 4 weeks. In most samples, the level of splenic MMTV(C3H) infection was low (2 to 5%). Infection remained throughout the life of the animal. The percentage of spleen samples found positive for exogenous viral infection was significantly higher in females bearing mammary tumors, whether virgin or multiparous. Liver and kidney DNAs were negative for exogenous MMTV sequences, suggesting tissue type selectivity in MMTV infection.

Expression of the protein product of the mouse mammary tumor virus long terminal repeat gene in phorbol ester-treated mouse T-cell-leukemia cells

Exposure of C57BL/6 mouse EL-4 T-cell leukemia cells to phorbol ester (12-O-tetradecanoylphorbol-13-acetate) (TPA) induced the synthesis of protein products encoded by the mouse mammary tumor virus (MMTV) long terminal repeat (LTR) region. Analysis of TPA-treated EL-4 cells with antiserum raised against a synthetic peptide predicted by the MMTV LTR open reading frame sequence detected a polypeptide migrating in gels with an apparent molecular weight of 37,000 Mr, as well as three less prominent proteins with apparent molecular weights of 31,000, 34,000, and 39,000. Tryptic peptide analysis established the identity of the immunoprecipitated cellular proteins with the LTR proteins obtained from in vitro translation of MMTV genomic RNA. All four proteins were glycosylated and were derived from one initial nonglycosylated translation product of 21,000 Mr. The 21,000-Mr apoprotein could be detected after digestion with endoglycosidase F or pretreatment of cells with tunicamycin. Untreated EL-4 cells synthesized three species of MMTV mRNA: 35S, 24S, and 20S. TPA treatment resulted in an increased level of transcription of the three mRNAs and the appearance of a new 1-kilobase mRNA. At least 10 acquired MMTV proviruses are present in the EL-4 genome, and examination of the degree of proviral methylation revealed extensive demethylation. However, no qualitative differences in the state of proviral methylation were apparent between TPA-treated and untreated cells.

An endogenous mouse mammary tumor virus genome common in inbred mouse strains is located on chromosome 6

We have examined EcoRI-restricted cellular DNA from mouse-hamster somatic cell hybrids. Results of this analysis show that the unit II mouse mammary tumor virus proviral genome is located on mouse chromosome 6. Restriction analysis of cellular DNA from (C3H/OuJ X Czech II) X Czech II backcross mice showed a strong linkage between unit II and Igk. The gene order of these markers on chromosome 6 relative to the Raf and Kirsten murine sarcoma virus ras-2 proto-oncogenes was established.

Production of mouse mammary tumor virus upon transfection of a recombinant proviral DNA into cultured cells

We have investigated the intracellular proteins synthesized in rat XC and feline kidney cells transfected with endogenous mouse mammary tumor virus (MMTV) proviral DNA. The endogenous provirus GR40, associated with the Mtv-8 locus, directs the synthesis of gag proteins indistinguishable from those found in MMTV-infected cells. The env precursor Pr73env and the mature gp52 proteins could not be detected in these cells. Instead an env-related protein of 68K is synthesized. In contrast to this endogenous provirus, a cloned exogenous proviral variant directs the synthesis of apparently normal env proteins upon transfection into the same cell lines. These results suggest that the env gene of the endogenous MMTV provirus GR40 is defective. The exogenous proviral variant is not expected to synthesize virus particles since it carries a rearrangement in the gag gene. In order to obtain an MMTV provirus capable of correctly expressing both gag and env functions, we have constructed a hybrid endogenous-exogenous provirus containing the 5' long terminal repeat (LTR)-gag of GR40 and the pol-env-3' LTR of the exogenous provirus. Upon transfection into feline kidney cells, this hybrid provirus directed the synthesis of apparently authentic gag and env proteins. Further, virus particles can be detected in the culture medium of the transfected cells by electron microscopy. Viral proteins obtained from viral particles banded in a sucrose gradient were detected by immunoprecipitation.

Identification and characterization of a mouse mammary tumor virus protein uniquely expressed on the surface of BALB/cV mammary tumor cells

A unique subline of BALB/c mice, designated BALB/cV, exhibits an intermediate mammary tumor incidence (47%) and harbors a distinct milk-transmitted mouse mammary tumor virus (MMTV). The BALB/cV subline was used to study the molecular basis of potential virus-host interactions involving cell surface-expressed MMTV proteins. Cell surface iodination identified virus-specific proteins expressed on BALB/cV primary mammary tumor cells grown in culture. In contrast to (C3H)MMTV-producing cell lines which expressed MMTV gp52, BALB/cV tumor cells lacked gp52 and expressed instead a 68K, env-related protein. The 68Kenv protein was also detected on the surface of metabolically labeled BALB/cV tumor cells by an external immunoprecipitation technique. The expression of 68Kenv was restricted to mammary tissues of BALB/cV mice that also expressed other MMTV proteins. Biochemical analysis established that 68Kenv was not modified by N-linked glycosylation. 125I-labeled 68Kenv was rapidly released into the media of tumor cell cultures and was recovered both in the form of a soluble protein and in a 100,000 g pellet. The biologic function of this cell surface-expressed viral protein remains unknown.

Molecular cloning, characterization, and genetic mapping of an endogenous murine mammary tumor virus proviral unit I of C3H/He mice

We have cloned and characterized a novel endogenous murine mammary tumor virus proviral unit of the C3H/He strain of mice. The cloned proviral unit is 16 kilobase pairs (kbp) in size and is composed of a 5.6-kbp 5' EcoRI segment of an endogenous provirus with 10.4-kbp flanking cellular sequences. A comparison of the restriction map of the cloned proviral DNA with that of an endogenous provirus of the GR strain of mice has revealed minor differences in restriction sites on the two proviruses. The restriction enzyme SstI, which does not cleave the 5' EcoRI fragment of GR DNA, cleaves the C3H/He proviral sequences once; MspI has an additional site in the C3H/He proviral sequences. By using a subcloned fragment containing unique cellular sequences as a hybridization probe, we (i) mapped the C3H/He proviral unit to chromosome 14 by using mouse-hamster somatic cell hybrids, and (ii) demonstrated that this proviral unit is also present in the genome of DBA/2 mice. From these results we conclude that the C3H/He strain of mice acquired this proviral unit from DBA stock by genetic transmission. Our data also indicate that the murine mammary tumor virus sequences present in the gag-specific proviral unit of C3H/He mice extend at least 2.45 kbp downstream of the EcoRI site in the genomic DNA. Since the structural organization and chromosomal location of this proviral unit are distinct from those of previously reported proviral units represented by similar-sized (16.7-kbp) EcoRI fragments, we tentatively propose to designate this proviral unit Mtv-7a.

Molecular basis of altered mouse mammary tumor virus expression in the D-2 hyperplastic alveolar nodule line of BALB/c mice

The preneoplastic D-2 hyperplastic outgrowth line, which was derived from a hormone-induced hyperplastic alveolar nodule (HAN) of a BALB/c mouse, was used for a detailed analysis of mouse mammary tumor virus (MMTV) expression. The D-2 HAN line has previously been shown to express viral RNA representative of the entire genome, although viral particles have been noted only rarely. The MMTV-specific mRNA, protein, and DNA content of the D-2 tissues was defined in an effort to better understand the molecular basis of the aberrant virus expression. Northern blotting techniques demonstrated the presence of properly processed 8.9 kb (genomic) and 3.6 kb (envelope) mRNA. Protein electroblotting procedures established the presence of properly processed viral core protein p28. In contrast, the envelope precursor polyprotein was not processed into detectable levels of gp52. Analysis of MMTV proviral content by Southern blot methodology revealed the presence of a newly acquired provirus which serves as a marker for the clonal nature of the D-2 line. The origin of the new provirus is unknown. Methylation studies established that the new proviral insert is hypomethylated and, therefore, is likely serving as the template for the MMTV expression observed in the D-2 HAN line. These characteristics of the D-2 line make it an excellent system in which to study the role, if any, of MMTV in the progression of D-2 preneoplastic tissues to the tumor phenotype.

Mammary tumorigenesis in C3Hf/Ki mice: examination of germinal mouse mammary tumor viruses and the int-1 and int-2 putative proto-oncogenes

The organization and expression of endogenous mouse mammary tumor virus (MMTV) proviruses in normal and neoplastic C3Hf/Ki tissues were examined. MMTV-containing EcoRI, HindIII, BamHI and PstI restriction fragments of C3Hf/Ki DNA were identical to those of C3H/StWi DNA. The full-length endogenous MMTV Units Ia (Mtv-7), II (Mtv-8), III (Mtv-9) and IV (Mtv-10), in addition to the subgenomic endogenous MMTV Units I (Mtv-6) and IX (Mtv-14), were germinally transmitted in C3Hf/Ki DNA. The previously uncharacterized Mtv-7 was contained in EcoRI fragments of 16.7 and 11.7 kbp. The endogenous MMTV Unit V (Mtv-1), which is responsible for virus production and mammary tumorigenesis in C3Hf/He mice, was absent from C3Hf/Ki DNA. The 9.0 kb gag-pol, the 3.8 kb env and the 1.7 kb LTR MMTV RNA transcripts were present in C3Hf/Ki mammary glands. MMTV proviruses, in addition to the endogenous C3Hf/Ki MMTV complement, were not detected in C3Hf/Ki mammary tumor DNA. The DNA organization and RNA expression of the putative mammary proto-oncogene regions int-1 and int-2 were also examined in C3Hf/Ki mammary tumors. The int-1 and int-2 regions did not appear rearranged, amplified, or expressed in C3Hf/Ki mammary tumors. These studies indicate that MMTV proviral activation of the int proto-oncogenes is not necessary for C3Hf/Ki mammary tumorigenesis.

Organization and expression of mouse mammary tumor virus sequences in normal and neoplastic C3Hf/HeSed mouse tissues

The organization and expression of germinally transmitted mouse mammary tumor virus (MMTV) proviruses in C3Hf/HeSed mouse tissues were examined. Digestion with the restriction enzymes EcoRI, BamHI, and HindIII and hybridization with cloned probes specific for the long terminal repeat and the 5' and 3' regions of the MMTV genome revealed three full-length (units Ib, II, and V) and two subgenomic (units I and IX) MMTV proviruses in C3Hf/HeSed mouse germ line DNA. The EcoRI fragments (15.0 and 5.7 kilobase pairs [kbp]) that contained unit Ib were previously described as separate, subgenomic MMTV proviruses. The methylated state of each full-length MMTV provirus was examined in DNA from C3Hf/HeSed mouse livers, spleens, mammary glands, and mammary tumors by digestion with EcoRI or BamHI in combination with the methyl-sensitive restriction enzymes HhaI or HpaII. Unit Ib contained HhaI- and HpaII-sensitive sites in spleen, mammary gland, and mammary tumor DNA but was completely methylated in liver DNA. Units II and V contained HhaI- and HpaII-sensitive sites in mammary gland and mammary tumor DNA, but the sites were extensively methylated in spleen and liver DNA. The HhaI-sensitive sites were mapped to the 5' end of the 5' and 3' long terminal repeats of each full-length MMTV provirus. C3Hf/HeSed mouse tissue RNA was examined for MMTV transcripts. Mammary glands contained MMTV RNA species of 9.0, 3.8, and 1.7 kb. Mammary tumors contained high levels of the 9.0- and 3.8-kb transcripts but lacked the 1.7-kb species. A very low level of the 3.8-kb MMTV transcript was present in spleens. Livers lacked detectable MMTV RNA. These results implicate mammary tissue as the site of unit V activation in the formation of MMTV virions.

Proteins encoded by the long terminal repeat region of mouse mammary tumor virus: identification by hybrid-selected translation

The long terminal repeat (LTR) region of mouse mammary tumor virus (MMTV) is known to contain an open reading frame of sufficient length to code for a protein of 36,000 Mr. The coding capacity of the 3' sequences of MMTV genomic RNA has been demonstrated by in vitro translation studies, which have reported the synthesis of four related proteins: p36, p24, p21, and p18. These proteins are overlapping translation products of the same open reading frame, with the smaller ones initiating at internal methionine codons. From the predicted amino acid sequence of the LTR protein, we have selected a region likely to be antigenic, obtained a synthetic peptide of that region, and raised antiserum to the peptide. The antipeptide serum specifically immunoprecipitated all four proteins from in vitro translated genomic 3' MMTV RNA, plus an additional one of 32,000 Mr. Published sequence data of MMRV LTRs show an internal AUG codon at a position which could initiate a protein of 32,000 Mr. The three smaller in vitro translation products (p24, p21, and p18) were consistently synthesized in much greater amounts than the p36 or p32 protein. The relative amount of each in vitro synthesized protein from genomic MMTV RNA could be predicted and was in good agreement with the postulated effect of flanking nucleotides on the efficiency of the respective AUG initiation codon. Polyadenylated RNAs, isolated from various mouse tissues, were selected by hybridization to plasmid DNA containing MMTV LTR sequences immobilized on nitrocellulose. In vitro translation of hybrid-selected mRNAs isolated from BALB/c mouse lactating mammary glands and carcinogen-induced mammary tumors, followed by immunoprecipitation with antipeptide serum, revealed that only one polypeptide was synthesized by the MMTV LTR-specific mRNA, the 36,000 Mr species.

Mouse mammary tumor virus DNA methylation: tissue-specific variation

Mouse mammary tumor virus-specific DNA sequences endogenous to the BALB/c mouse are shown to exhibit variable levels of methylation in a tissue-specific manner. In DNA from both lactating mammary gland and spleen, MMTV-specific sequences were hypomethylated at specific HpaII and HhaI sites. These variably methylated sites were found in the terminal repetitive sequences of the endogenous viral genomes. The specific hypomethylation of a HpaII site in Mtv-9 is associated with expression of a 1.6 kb transcript in the lactating mammary gland.

Two genetically transmitted BALB/c mouse mammary tumor virus genomes located on chromosomes 12 and 16

We have examined EcoRI-restricted cellular DNA from BALB/c mouse-hamster somatic cell hybrids by blot hybridization for the presence of mouse mammary tumor virus-related sequences. Results of this analysis show that mouse mammary tumor virus-related proviral copies are located on chromosomes 16 (16-kilobase-pair fragment) and 12 (10.5- and 7.7-kilobase-pair fragments).

Increased concentration of an indigenous proviral mouse mammary tumor virus long terminal repeat-containing transcript is associated with neoplastic transformation of mammary epithelium in C3H/Sm mice

Increased amounts of mouse mammary tumor virus (MMTV) proviral transcripts were found in RNA dot blots from MMTV-negative, C3H/Sm mouse mammary tumors which arose spontaneously or were induced by hormonal or chemical carcinogens or both. Other dot blots probed with a long terminal repeat (LTR) probe showed that LTR (MMTV)-containing transcripts were disproportionately represented in these tumor RNAs. Different segments of the MMTV genome were used in sequential hybridizations to Northern blots to determine relative sequence content and size of MMTV transcripts in transformed mammary tissues, as compared with those in lactating mammary glands. Increased amounts of 4.4-kilobase env and 8.1-kilobase genomic MMTV transcripts were detected with an env probe in many of the tumor RNAs examined. Hybridization of the same Northern blots containing tumor RNAs with an LTR probe revealed a 2.2-kilobase transcript which was prominent in RNAs from chemically-induced, hormonally-induced, and spontaneous mammary tumors relative to those from lactating mammary glands. The LTR-containing transcript did not possess significant homology to either env or gag-pol probes. This distinctive, transformation-enhanced, 2.2-kilobase transcript may contain mouse cellular sequences in addition to LTR sequences or it may represent the message for a nonstructural viral protein encoded within the LTR open reading frame of one or more of the four C3H/Sm MMTV proviral genes.

Transformation of mouse mammary epithelial cells by papovavirus SV40

Primary and established murine mammary epithelial cells and wild-type SV40 were employed to study the phenomenon of epithelial cell transformation. Thirteen independent transformed cell lines were derived. All contained SV40 intranuclear T antigen. Eight transformed mammary cell lines were examined ultrastructurally and all were found to exhibit pronounced epithelial cell characteristics, including desmosomes and tight junctions. Growth studies revealed that while normal mammary cells were unable to grow in low serum (2% FBS), established Cl S1 mammary cells and SV40-transformed mammary epithelial cells replicated well. Cell densities achieved by the transformants were only slightly elevated in high serum (13% FBS) over normal cell values. All the transformants formed colonies on plastic and exhibited anchorage-independent growth in methylcellulose. Five of the transformed lines were tumorigenic in syngeneic animals, in marked contrast to the lack of transplantability usually observed with SV40-transformed mouse fibroblasts. Anchorage-independent growth was not a predictor of tumorigenic potential in this system. The transformants exhibited a spectrum of responsiveness to exogenous growth factors. This study establishes that the SV40-murine mammary cell system is a valid model for analyses of the process and consequences of epithelial cell transformation, in general, and mammary cell transformation in particular.