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

CDP is a repressor of mouse mammary tumor virus expression in the mammary gland

Mouse mammary tumor virus (MMTV) transcription is highest in the lactating mammary gland but is detectable in a variety of other tissues. Previous results have shown that MMTV expression is suppressed in lymphoid and other tissues through the binding of the homeodomain-containing repressor special AT-rich binding protein 1 to a negative regulatory element (NRE) in the MMTV long terminal repeat (LTR). Another homeoprotein repressor, CCAAT displacement protein (CDP), also binds to the MMTV NRE, but a role for CDP in MMTV transcriptional suppression has not yet been demonstrated. In this paper, we show that the level of CDP decreases during development of the mammary gland and that this decline in CDP level correlates with the known increase in MMTV expression observed during mammary gland differentiation. Moreover, CDP overexpression was able to suppress MMTV LTR-reporter gene activity up to 20-fold in transient-transfection assays of mouse mammary cells. To determine if this effect was due to direct binding of CDP to the promoter-proximal NRE, we performed DNase I protection assays to map two CDP-binding sites from +835 to +845 and +920 to +931 relative to the first base of the LTR. Mutations engineered into each of these sites decreased CDP binding to the proximal NRE, whereas a combination of these mutations further reduced binding. Subsequently, each of these mutations was introduced into the full-length MMTV LTR upstream of the luciferase reporter gene. Analysis of stable transfectants of LTR constructs showed that CDP binding site mutations in the proximal NRE elevated reporter gene expression two- to sixfold compared to wild-type LTR constructs. Thus, MMTV expression increases during mammary gland development, in part due to decreased CDP levels and CDP binding to the LTR. Together, these experiments provide the first evidence that CDP acts as a repressor of MMTV transcription in the mammary gland.

The mouse mammary tumor virus env gene is the source of a CD8+ T-cell-stimulating peptide presented by a major histocompatibility complex class I molecule in a murine thymoma

CD8+ cytotoxic T cells recognize their targets by the presence of unique peptide bound to a major histocompatibility complex (MHC) class I molecules on the cell surface. The MHC molecules normally display thousands of distinct peptides, making it difficult to identify individual antigenic peptides, their protein precursors, and their relative importance in the T-cell response. Here we used the EL-4 tumor-specific lacZ-inducible KZ30.6 T cell as a probe for detecting the peptide/MHC ligand that was generated in cells transfected with an EL-4 cDNA library. These expression screens allowed identification of a mouse mammary tumor virus (MMTV) transcript as the source of the antigenic peptide presented by the Kb MHC molecule. The antigenic activity was encoded within the MMTV env gene and was defined by the octapeptide ANYDFICV (AFV8). Synthetic AFV8 stimulated KZ30.6 T cells at picomolar concentrations and coeluted with one of two active peptides in HPLC-fractionated extracts of EL-4 cells. The AFV8/Kb complex was also recognized by two other EL-4-specific T cells. The results illustrate a novel strategy for identifying T-cell-stimulating antigens and suggest that the MMTV env gene and its naturally processed AFV8 peptide product can serve as a model for study of antigen processing and tumor immunotherapy.

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.

Influence of mammary cell differentiation on the expression of proteins encoded by endogenous BALB/c mouse mammary tumor virus genes

The interactions between differentiation-associated cellular events in the intact mammary gland or in cultured mammary cells and the post-transcriptional activity of the endogenous mouse mammary tumor virus (MMTV) loci were investigated. The transcriptional activities of the endogenous MMTV proviruses of the BALB/c mouse strain (Mtv-6, Mtv-8 and Mtv-9) appear to be regulated differentially during pregnancy-induced mammary gland development (J.E. Knepper, D. Medina and J.S. Butel, J. Virol. 59, 518-521, 1986). Analysis of MMTV-specific proteins at various stages of mammary gland development (virgin, midpregnant, lactating, regressing) established the presence of steady-state levels of a 67,000-Mr env precursor-type polypeptide at all physiological stages. However, processing to lower-molecular-weight env-specific proteins, including a predominant 50,000-Mr species, was detected only with the transition to the functional mammary gland phenotype. The contributions of cell proliferation, cell-matrix interactions, and modulation of functional activity to the pattern of endogenous MMTV protein expression were investigated using a 3-dimensional collagen type I culture system. Growth and cell-matrix interactions (cell polarization, lumen formation) leading to formation of 3-dimensional duct-like structures were permissive for the synthesis and processing of MMTV-specific proteins; accumulation of high levels of the 50,000-Mr env-specific polypeptide was associated with the onset of the fully functional mammary cell phenotype. Expression of MMTV-specific proteins was not due to amplification of a specific cell subpopulation. The potential of the full-length Mtv-8 and Mtv-9 proviruses to be transcribed, as indicated by their methylation status, was not dramatically different between differentiated and undifferentiated mammary cells in culture. This study indicates that MMTV transcriptional activity is reflected at the protein level in mammary tissue of BALB/c mice and that viral protein synthesis and processing may serve as important markers of different physiological stages of mammary epithelial cells. These observations also suggest a general approach to the examination of potential modulatory effects of cellular interactions (cell-cell, cell-matrix or both) known to be important in various differentiated epithelial cell systems for the expression of viral genes.

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.

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.