Transformation of mouse mammary epithelial cells by papovavirus SV40

SV40 lymphomagenesis in Syrian golden hamsters

Simian virus 40 (SV40) isolates differ in oncogenic potential in Syrian golden hamsters following intraperitoneal inoculation. Here we describe the effect of intravenous exposure on tumor induction by SV40. Strains SVCPC (simple regulatory region) and VA45-54(2E) (complex regulatory region) were highly oncogenic following intravenous inoculation, producing a spectrum of tumor types. Three lymphoma cell lines were established; all expressed SV40 T-antigen, were immortalized for growth in culture, and were tumorigenic following transplantation in vivo. New monoclonal antibodies directed against hamster lymphocyte surface antigens are described. The cell lines expressed MHC class II and macrophage markers and were highly phagocytic, indicating a histiocytic origin. Many hamsters that remained tumor-free developed SV40 T-antigen antibodies, suggesting that viral replication occurred. This study shows that route of exposure influences the pathogenesis of SV40-mediated carcinogenesis, that SV40 strain VA45-54(2E) is lymphomagenic in hamsters, that hamster lymphoid cells of histiocytic origin can be transformed in vivo and established in culture, and that reagents to hamster leukocyte differentiation molecules are now available.

Tumorigenic effect of transcription factor hAP-2alpha and the intricate link between hAP-2alpha activation and squelching

Overexpression of human activator protein-2alpha (hAP-2alpha) is carcinogenic. Its aberrant regulation is the underlying tumorigenic event in the human teratocarcinoma cell line PA-1. In this cell line excess hAP-2alpha protein binds and sequesters coactivators, which interferes with the activity of other activators and with its own activity. The N-terminus of hAP-2alpha, which contains an activation domain, is critical in squelching and tumorigenicity. Mutation analyses of the N-terminus region showed that activation and squelching were intricately linked; nevertheless, squelching could occur in the absence of activity. Cells overexpressing squelching-proficient mutants grew efficiently on soft agar irrespective of their ability to activate transcription, which indicates that these cells are tumorigenic. Mutants that lacked both properties were nontumorigenic. These results suggest that squelching, but not activation, causes transformation and that the factors that are sequestered at this region are critical in tumorigenesis.

Inhibitors of COP-mediated transport and cholera toxin action inhibit simian virus 40 infection

Simian virus 40 (SV40) is a nonenveloped virus that has been shown to pass from surface caveolae to the endoplasmic reticulum in an apparently novel infectious entry pathway. We now show that the initial entry step is blocked by brefeldin A and by incubation at 20 degrees C. Subsequent to the entry step, the virus reaches a domain of the rough endoplasmic reticulum by an unknown pathway. This intracellular trafficking pathway is also brefeldin A sensitive. Infection is strongly inhibited by expression of GTP-restricted ADP-ribosylation factor 1 (Arf1) and Sar1 mutants and by microinjection of antibodies to betaCOP. In addition, we demonstrate a potent inhibition of SV40 infection by the dipeptide N-benzoyl-oxycarbonyl-Gly-Phe-amide, which also inhibits late events in cholera toxin action. Our results identify novel inhibitors of SV40 infection and show that SV40 requires COPI- and COPII-dependent transport steps for successful infection.

Natural isolates of simian virus 40 from immunocompromised monkeys display extensive genetic heterogeneity: new implications for polyomavirus disease

Simian virus 40 (SV40) DNAs in brain tissue and peripheral blood mononuclear cells (PBMCs) of eight simian immunodeficiency virus-infected rhesus monkeys with SV40 brain disease were analyzed. We report the detection, cloning, and identification of five new SV40 strains following a quadruple testing-verification strategy. SV40 genomes with archetypal regulatory regions (containing a duplication within the G/C-rich regulatory region segment and a single 72-bp enhancer element) were recovered from seven animal brains, two tissues of which also contained viral genomes with nonarchetypal regulatory regions (containing a duplication within the G/C-rich regulatory region segment as well as a variable duplication within the enhancer region). In contrast, PBMC DNAs from five of six animals had viral genomes with both regulatory region types. It appeared, based on T-antigen variable-region sequences, that nonarchetypal virus variants arose de novo within each animal. The eighth animal exclusively yielded a new type of SV40 strain (SV40-K661), containing a protoarchetypal regulatory region (lacking a duplication within the G/C-rich segment of the regulatory region and containing one 72-bp element in the enhancer region), from both brain tissue and PBMCs. The presence of SV40 in PBMCs suggests that hematogenous spread of viral infection may occur. An archetypal version of a virus similar to SV40 reference strain 776 (a kidney isolate) was recovered from one brain, substantiating the idea that SV40 is neurotropic as well as kidney-tropic. Indirect evidence suggests that maternal-infant transmission of SV40 may have occurred in one animal. These findings provide new insights for human polyomavirus disease.

Serological evidence of SV40 infections in HIV-infected and HIV-negative adults

SV40 is a simian polyomavirus that was a contaminant of some viral vaccines administered to people between 1955 and 1962. SV40 DNA has recently been found associated with several types of human tumors, suggesting that the virus is present in humans. We examined sera from patients infected with human immunodeficiency virus type 1 (HIV-1) as well as from HIV-1-negative controls to determine the prevalence of SV40 neutralizing antibodies using a specific plaque reduction assay. We found that 16.1% of HIV-infected patients (n = 236) were seropositive for SV40, as compared to 12.0% of HIV-negative control volunteers (n = 108) and 11.1% of HIV-negative patients (n = 72). These differences were not statistically significant. As individuals born between 1941 and 1962 had the highest chance of having received SV40-contaminated poliovaccines, we analyzed SV40 seropositivity rates based on year of birth. SV40 antibody rates for HIV-infected patients born before 1941, between 1941 and 1962, and after 1962 were 17.1%, 16.3%, and 11.8%, respectively. For the HIV-negative subjects, the rates were 12.5%, 12.0%, and 9.7%, respectively. There was no correlation between SV40 seropositivity and either the stage of disease in HIV-infected patients or the race/ethnicity. Also, there was no correlation between the presence of SV40 neutralizing antibody and the titer of neutralizing antibody to human polyomavirus BKV. The SV40 seropositivity rates in the patients born between 1941 and 1962 may be explained by the likelihood of those individuals having received SV40-contaminated vaccines, but the detection of SV40 neutralizing antibody in individuals born after 1962 (with no risk of having received contaminated vaccines) is significant. Although cross-reactive antibodies might theoretically contribute to the observed reactivities, these results suggest that SV40 neutralizing antibodies are present in certain individuals and raise the possibility that SV40 continues to infect humans long after vaccines were freed from contamination.

Establishment and characterization of a bovine mammary myoepithelial cell line

The thermolabile large T-antigen, encoded by the simian virus 40 early region mutant tsA58, was used to establish clonal cell lines (BMM-UV) from primary bovine myoepithelial cells. The BMM-UV cells have undergone more than 300 population doublings without any signs of senescence, and they contain the intranuclear large T antigen. At low confluency, they grow in a spindlelike manner and develop very long projections that most likely allow for communication of cells at a distance from each other. Establishment results in a decrease in the number of cells that contract in response to oxytocin compared with the parental nontransfected cells (20% versus 45%). Oxytocin responsiveness of BMM-UV cells increases when the cells are cultured in a medium supplemented with staphylococcal proteases. Proliferation of BMM-UV cells increases when they are cultured in the presence of epidermal growth factor (10 ng/ml) or insulinlike growth factor I (50 ng/ml). The BMM-UV cells may become a useful model to study growth properties, cell-to-cell communication, and the function of bovine mammary myoepithelial cells.

Signal-mediated nuclear transport in simian virus 40-transformed cells is regulated by large tumor antigen

Transformation of cultured cells with simian virus 40 (SV40), or transfection with the early region of the SV40 genome, causes a significant increase in both the rate of signal-mediated nuclear transport and the functional size of the transport channels (located in the pore complexes). By microinjecting purified large tumor (T) antigen into the cytoplasm of murine BALB/c 3T3 cells, we have demonstrated that this protein alone can account for the increase in transport capacity. The T antigen-dependent changes can be partially inhibited by cycloheximide and require a functional nuclear localization sequence. Although necessary, the nuclear localization sequence by itself cannot produce the observed variations in nuclear permeability and presumably function in a "helper" capacity, in association with another, as yet unidentified domain.

Differential effects of the simian virus 40 early genes on mammary epithelial cell growth, morphology, and gene expression

To study the effect of SV40 T-antigen in mammary epithelial cells, a rat beta-casein promoter-driven SV40 early-region construct was stably introduced into the clonal mouse mammary epithelial cell line HC11. With the expression of the viral T-antigens under the control of a hormone-inducible promoter, it was possible to dissociate the effects of different levels of T-antigen expression on cell growth, morphology, and gene expression. Following hormonal induction, a rapid but transient induction of T-antigen was observed, followed by a delayed induction of H4 histone mRNA. In T-antigen-positive HC11 cells cultured in the absence of EGF, the expression of basal levels of T-antigen (in the absence of hormonal induction) led to a decreased doubling time and an increased cell density. In the presence of EGF, T-antigen expression resulted additionally in an altered cell morphology. Despite the effects of T-antigen on cell growth and gene expression, the cells were unable to form colonies in soft agar and were nontumorigenic when transplanted into cleared mammary fat pads. They were, however, weakly tumorigenic in nude mice. Relatively high levels of p53 protein synthesis were observed in both the transfected HC11 cells and the parental COMMA-D cells, as compared to 3T3E fibroblasts and another mammary epithelial cell line. The HC11 and COMMA-D cells synthesized approximately equal levels of wild-type and mutated p53 proteins as defined by their reactivities with monoclonal antibodies PAb246 and PAb240, respectively. Interactions between excess p53 and T-antigen may, in part, explain the failure of these cells to display a completely transformed phenotype.

Spontaneous acquisition of tumorigenicity and invasiveness by mouse lens explant cells during culture in vitro

The lens of the eye is one of the rare organs in which tumors do not occur spontaneously. It therefore appeared to us that lens cells would not present the background of spontaneous transformation toward malignancy found with many other cell cultures. We have cultured C3H/HeA mouse lens explant (MLE) cells for 70 wk and analyzed changes in malignancy-related phenotypes in function of the number of passages. In vitro, we studied morphology, colony forming efficiency on tissue culture plastic substrate (CFEtc) and in soft agar, population doubling time, saturation density, and invasiveness into precultured chick heart fragments. In vivo, tumorigenicity, invasion, and metastasis were analyzed after injection of cell suspensions subcutaneously and intraperitoneally, after implantation of cells aggregated to collagen sponges under the renal capsule and after implantation of cell aggregates subcutaneously into the tail and into the pinna. The CFEtc, population doubling time, and saturation density increased as the number of passages of culture in vitro increased, but colony formation in soft agar was never observed. MLE cells till passage 16 were not invasive in vitro, but hereafter consistently were found to be invasive. After about 17 passages, corresponding to 25 wk of culture, MLE cells acquired the capacity to form tumors in syngeneic mice. These tumors were invasive but metastases were not observed. We concluded that MLE cells acquired in an apparently spontaneous way a number of malignancy-related phenotypes, without, however, reaching the stage of metastasis.

The immortalization of thymic nurse cells by SV40 virus

Thymic nurse cells (TNCs) are stromal elements that contain between 20 and 200 T cells within their cytoplasm. Because of this unique feature they are believed to play a role in thymocyte development. Unfortunately, it has been difficult to obtain pure TNCs in quantities sufficient for extensive evaluation of their thymic function. As a result, only a limited amount of information is available that characterizes TNCs or the T cell population(s) found within their cytoplasm. We have now used SV40 to infect and immortalize TNCs from C57BL/6 mice. SV40-transformed TNCs were found to specifically bind and internalize cells from an immature thymocyte line isolated in our laboratory. These results describe a method of obtaining pure populations of TNCs for future studies of their thymic function, and suggest that binding to specific subpopulations of lymphoblasts may be necessary for internalization.

Tumorigenicity of simian virus 40-hepatocyte cell lines: effect of in vitro and in vivo passage on expression of liver-specific genes and oncogenes

Five simian virus 40 (SV40)-hepatocyte cell lines were examined for tumorigenicity and the effect of in vitro passage on the expression of four liver-specific genes (albumin, transferrin, alpha 1-antitrypsin, and phosphoenolpyruvate carboxykinase), two oncogenes (c-Ha-ras and c-raf), and two genes associated with hepatocarcinogenesis (alpha-fetoprotein and placental-type glutathione-S-transferase). At low passage (12 to 22), all five cell lines expressed the four liver-specific genes at levels similar to those in the liver and were not tumorigenic or were weakly tumorigenic. At high passage (33 to 61), the cell lines formed carcinomas, and four out of five cell lines produced primary tumors that metastasized. At least two cell lines produced well-differentiated hepatocellular carcinomas that expressed liver-specific RNAs. Levels of expression of liver-specific genes changed with time in culture. Some of the changes in liver-specific gene expression in the tumor tissue (such as for the phosphoenolpyruvate carboxykinase gene) paralleled those that occurred with in vitro passage, while other changes (such as for the albumin gene) did not parallel those that occurred with in vitro passage. Correlations between enhanced expression of c-Ha-ras and tumorigenic potential and between the process of SV40 immortalization and induced expression of c-raf and glutathione-S-transferase-P were observed. Induction of alpha-fetoprotein was detected with in vitro and in vivo passage only in the CWSV14 cell line and was paralleled by diminished albumin expression. In conclusion, we developed a model system with five SV40-hepatocyte cell lines, tumors induced by them, and tumor cell lines to examine changes in gene expression that accompany the progression from a normal cell to a hepatocellular carcinoma. Because the SV40-hepatocyte cell lines and tumor cell lines remain highly differentiated and vary in the magnitude of expression of specific genes, they can be used to study the molecular mechanisms regulating gene expression, in particular those regulating specific genes associated with differentiation.

Changes in in vitro growth behaviour of the mammary epithelial cell line NMuMG caused by the v-fos oncogene

A defective retrovirus was constructed to investigate the effect of the expression of the v-fos oncogene from FBJ-MSV on the in vitro growth properties of the mammary epithelial cell line NMuMG. Clearly visible areas of overgrowth in monolayer cultures of NMuMG were seen in cells infected with the v-fos-containing retrovirus but not in cells infected with control virus which did not contain an oncogene. Two cell lines, representing two morphological types of infected cell, were isolated from a morphologically altered region and further characterized. Fos.3.1.NMuMG grew as very spindly cells, achieving a higher density than control cells in 5% foetal calf serum (FCS) but growing very poorly in 1% FCS or in soft agar. Fos.3.3.NMuMG grew to a high density in 5% FCS and to a limited extent in low serum. This cell line also grew in soft agar. Fos.3.3.NMuMG seemed to be more transformed than fos.3.1.NMuMG using the criteria of growth in soft agar and low serum. All the cells used in this study were shown to retain epithelial characteristics by staining for cytokeratins and to contain at least one viral genome by Southern blotting. fos mRNA expression was raised over control levels in the two transformed cell lines.

Tumorigenicity of simian virus 40-hepatocyte cell lines: effect of in vitro and in vivo passage on expression of liver-specific genes and oncogenes

Five simian virus 40 (SV40)-hepatocyte cell lines were examined for tumorigenicity and the effect of in vitro passage on the expression of four liver-specific genes (albumin, transferrin, alpha 1-antitrypsin, and phosphoenolpyruvate carboxykinase), two oncogenes (c-Ha-ras and c-raf), and two genes associated with hepatocarcinogenesis (alpha-fetoprotein and placental-type glutathione-S-transferase). At low passage (12 to 22), all five cell lines expressed the four liver-specific genes at levels similar to those in the liver and were not tumorigenic or were weakly tumorigenic. At high passage (33 to 61), the cell lines formed carcinomas, and four out of five cell lines produced primary tumors that metastasized. At least two cell lines produced well-differentiated hepatocellular carcinomas that expressed liver-specific RNAs. Levels of expression of liver-specific genes changed with time in culture. Some of the changes in liver-specific gene expression in the tumor tissue (such as for the phosphoenolpyruvate carboxykinase gene) paralleled those that occurred with in vitro passage, while other changes (such as for the albumin gene) did not parallel those that occurred with in vitro passage. Correlations between enhanced expression of c-Ha-ras and tumorigenic potential and between the process of SV40 immortalization and induced expression of c-raf and glutathione-S-transferase-P were observed. Induction of alpha-fetoprotein was detected with in vitro and in vivo passage only in the CWSV14 cell line and was paralleled by diminished albumin expression. In conclusion, we developed a model system with five SV40-hepatocyte cell lines, tumors induced by them, and tumor cell lines to examine changes in gene expression that accompany the progression from a normal cell to a hepatocellular carcinoma. Because the SV40-hepatocyte cell lines and tumor cell lines remain highly differentiated and vary in the magnitude of expression of specific genes, they can be used to study the molecular mechanisms regulating gene expression, in particular those regulating specific genes associated with differentiation.

Tumorigenesis in transgenic mice by a nuclear transport-defective SV40 large T-antigen gene

The SV40(cT) mutant encodes a large tumor antigen (cT-ag) that is defective for transport from the cell cytoplasm into the nucleus. This mutant is able to transform established cell lines at near wild-type virus efficiencies, but has a markedly decreased ability to transform primary cells and to induce tumors in newborn hamsters (R. E. Lanford, C. Wong, and J. S. Butel, 1985, Mol. Cell. Biol. 5, 1043-1050). To explore the biology of transport-defective T-ag in vivo, transgenic mice carrying the cT-ag gene were produced. Five of eight founder animals died early in life of choroid plexus tumors (mean age +/- SE, 52 +/- 11.0 days); renal and thymic lesions were also observed. Mice of an SV40(cT) transgenic line regularly succumb to brain tumors (mean age, 81 +/- 1.2 days). SV40 T-ag is expressed in the tumor cells and is retained in the cytoplasm. The observation that SV40(cT) is equivalent to wild-type virus at tumor induction in transgenic mice emphasizes the probable importance of extranuclear forms of SV40 T-ag in brain tumor formation. This study also indicates that in vitro cell transformation assays may not always be accurate reflections of the oncogenic potential of a transforming gene in vivo, because of the different cell types involved.

Establishment of two rabbit mammary epithelial cell lines with distinct oncogenic potential and differentiated phenotype after microinjection of transforming genes

The goal of this work was to establish an assay for transformation of epithelial cells. Two epithelial cell lines were obtained after microinjecting transforming genes into primary rabbit mammary secretory cells. The cell lines were analyzed for their oncogenic potential and for the maintenance of a differentiated phenotype. A fully transformed cell line, which retained epithelial cell organization, was obtained by coinjecting simian virus 40 DNA and the activated human c-Ha-ras gene. The proliferation rate of these cells was high, with a doubling time of 16 h. Their growth was anchorage independent, and they had lost contact inhibition. The cells were tumorigenic in nude mice, but had no metastatic potential. Both microinjected DNAs were efficiently transcribed and translated, in contrast to the casein genes, which were expressed in primary cells but not in the transformed cell line. An immortalized cell line established after injection with simian virus 40 DNA alone was characterized by a moderate rate of proliferation with a doubling time of approximately 30 h. The growth of these cells was contact inhibited and anchorage dependent. The cells were not tumorigenic in nude mice. The viral DNA was expressed during early passages, as shown by the presence of the large T antigen in cell nuclei, but not at later passages. A high number of lactogenic hormone receptors were found associated with the cell surface. Despite the presence of these receptors, no induction of genes coding for milk proteins was observed after addition of prolactin. These data demonstrate that this assay system can be used to assess the immortalizing and transforming potential of candidate oncogenes in epithelial cells.

Fluctuation of simian virus 40 (SV40) super T-antigen expression in tumors induced by SV40-transformed mouse mammary epithelial cells

Higher-molecular-weight forms of the simian virus 40 (SV40) large tumor antigen (T-Ag), designated super T-Ag, are commonly found in SV40-transformed rodent cells. We examined the potential role of super T-Ag in neoplastic progression by using a series of clonal SV40-transformed mouse mammary epithelial cell lines. We confirmed an association between the presence of super T-Ag and cellular anchorage-independent growth in methylcellulose. However, tumorigenicity in nude mice did not correlate with the expression of super T-Ag. In the tumors that developed in nude mice, super T-Ag expression fluctuated almost randomly. Cell surface iodination showed that super T-Ag molecules were transported to the epithelial cell surface. The biological functions of super T-Ag remain obscure, but it is clear that it is not important for tumorigenicity by SV40-transformed mouse mammary epithelial cells. Super T-Ag may be most important as a marker of genomic rearrangements by the resident viral genes in transformed cells.

Establishment of two rabbit mammary epithelial cell lines with distinct oncogenic potential and differentiated phenotype after microinjection of transforming genes

The goal of this work was to establish an assay for transformation of epithelial cells. Two epithelial cell lines were obtained after microinjecting transforming genes into primary rabbit mammary secretory cells. The cell lines were analyzed for their oncogenic potential and for the maintenance of a differentiated phenotype. A fully transformed cell line, which retained epithelial cell organization, was obtained by coinjecting simian virus 40 DNA and the activated human c-Ha-ras gene. The proliferation rate of these cells was high, with a doubling time of 16 h. Their growth was anchorage independent, and they had lost contact inhibition. The cells were tumorigenic in nude mice, but had no metastatic potential. Both microinjected DNAs were efficiently transcribed and translated, in contrast to the casein genes, which were expressed in primary cells but not in the transformed cell line. An immortalized cell line established after injection with simian virus 40 DNA alone was characterized by a moderate rate of proliferation with a doubling time of approximately 30 h. The growth of these cells was contact inhibited and anchorage dependent. The cells were not tumorigenic in nude mice. The viral DNA was expressed during early passages, as shown by the presence of the large T antigen in cell nuclei, but not at later passages. A high number of lactogenic hormone receptors were found associated with the cell surface. Despite the presence of these receptors, no induction of genes coding for milk proteins was observed after addition of prolactin. These data demonstrate that this assay system can be used to assess the immortalizing and transforming potential of candidate oncogenes in epithelial cells.

Comparative analysis of casein synthesis during mammary cell differentiation in collagen and mammary gland development in vivo

Substrata upon which epithelial cells are cultured modulate their morphology,growth, and ability to differentiate. Mouse mammary epithelial cells cannot be induced to synthesize caseins, a marker of cell differentiation, when grown on a plastic surface. An analysis was made of the effect of time within a collagen matrix on the ability of normal mammary epithelial cells to be induced to synthesize caseins and that response was compared to mammary gland development in vivo. Primary cultures of mammary cells from unprimed virgin BALB/c mice were embedded in rat-tail collagen gel mixtures and maintained in growth medium. Induction medium containing lactogenic hormones was added at various times. The cells were monitored every 3-7 days over a period of 8 weeks for cell growth, casein synthesis, and ability to grow in vivo in cleared mammary fat pads. Casein accumulation was assayed quantitatively by an ELISA competition assay and qualitatively by the immunoblot procedure using specific antisera prepared against purified mouse caseins. No marked differences in cell numbers and transplantability potential were observed among cells cultured for various times in collagen. Mammary cells grown in collagen for up to 8 weeks retained the capacity to grow in vivo as normal ductal outgrowths. The duration of culture within collagen prior to hormonal stimulation did influence the kinetics of casein synthesis. Cells cultured for 1 week in growth medium did not accumulate detectable levels of casein until after 3 weeks of induction, whereas cells cultured for 2 or 4 weeks responded by accumulating caseins after 2 weeks and 3 days of induction, respectively. While the levels of total caseins that accumulated under optimal conditions of induction in culture approached levels found during lactation in vivo, the relative proportion of specific casein polypeptides synthesized in culture was altered from alpha casein (43K) in favor of the beta casein (30K) species. These results suggest that a period of culture within collagen is required to permit mammary epithelial cells to become responsive for hormone-induced differentiation. It is possible that during growth within the collagen the cells synthesize and deposit extracellular matrix components important in modulating gene expression.

New acceptor cell for transfected genomic DNA: oncogene transfer into a mouse mammary epithelial cell line

A line of mouse mammary epithelial cells (NMuMG) has been characterized for its ability to be stably transfected with exogenous DNA. A transfection frequency of at least 1 cell per 1,000 was obtained with the pSV2neo plasmid. Several thousand G418-resistant NMuMG cell clones can easily be generated in cotransfection of genomic DNA and pSV2neo. The NMuMG cells were isolated from normal mammary glands and do not form malignant lesions when injected into nude mice. We have cotransfected NMuMG cells with pSV2neo and genomic DNA from the human EJ bladder carcinoma line, a cell line which contains an activated c-rasH oncogene. When a pool of 4,700 G418-resistant colonies was injected into nude mice, tumors were obtained. These tumors contain a transfected human rasH gene. Genomic DNA transfection into a line of mouse epithelial cells, in combination with the selection of stable transfectants and tumor induction in nude mice, can be used to screen human tumor DNA for the presence of activated oncogenes.

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.

New acceptor cell for transfected genomic DNA: oncogene transfer into a mouse mammary epithelial cell line

A line of mouse mammary epithelial cells (NMuMG) has been characterized for its ability to be stably transfected with exogenous DNA. A transfection frequency of at least 1 cell per 1,000 was obtained with the pSV2neo plasmid. Several thousand G418-resistant NMuMG cell clones can easily be generated in cotransfection of genomic DNA and pSV2neo. The NMuMG cells were isolated from normal mammary glands and do not form malignant lesions when injected into nude mice. We have cotransfected NMuMG cells with pSV2neo and genomic DNA from the human EJ bladder carcinoma line, a cell line which contains an activated c-rasH oncogene. When a pool of 4,700 G418-resistant colonies was injected into nude mice, tumors were obtained. These tumors contain a transfected human rasH gene. Genomic DNA transfection into a line of mouse epithelial cells, in combination with the selection of stable transfectants and tumor induction in nude mice, can be used to screen human tumor DNA for the presence of activated oncogenes.

Construction and characterization of an SV40 mutant defective in nuclear transport of T antigen

An SV40-adenovirus 7 hybrid virus, PARA(cT), has been described that is defective for the nuclear transport of SV40 large tumor antigen. An SV40(cT) mutant was constructed using SV40 early and late region DNA fragments derived from PARA(cT) and wild-type SV40 respectively. The SV40(cT)-3 construct is defective for viral replication, but can be propagated in COS-1 cells. T antigen induced by SV40(cT)-3 is localized in the cytoplasm of infected cells. The cT mutation also inhibits the transport of wild-type T antigen; COS-1 cells lose their constitutive expression of nuclear T antigen after infection with SV40(cT)-3. Sequence analysis revealed that the cT mutation results in the replacement of a positively charged lysine in wild-type T antigen with a neutral asparagine at amino acid number 128, demonstrating that the alteration of a single amino acid is sufficient to abolish nuclear transport. Implications of the cT mutation on possible mechanisms for the transport of proteins to the nucleus are discussed.