Immunological selection of tumour cells which have lost SV40 antigen expression

Transgenic oncogenes induce oncogene-independent cancers with individual karyotypes and phenotypes

Cancers are clones of autonomous cells defined by individual karyotypes, much like species. Despite such karyotypic evidence for causality, three to six synergistic mutations, termed oncogenes, are generally thought to cause cancer. To test single oncogenes, they are artificially activated with heterologous promoters and spliced into the germ line of mice to initiate cancers with collaborating spontaneous oncogenes. Because such cancers are studied as models for the treatment of natural cancers with related oncogenes, the following must be answered: 1) which oncogenes collaborate with the transgenes in cancers; 2) how do single transgenic oncogenes induce diverse cancers and hyperplasias; 3) what maintains cancers that lose initiating transgenes; 4) why are cancers aneuploid, over- and underexpressing thousands of normal genes? Here we try to answer these questions with the theory that carcinogenesis is a form of speciation. We postulate that transgenic oncogenes initiate carcinogenesis by inducing aneuploidy. Aneuploidy destabilizes the karyotype by unbalancing teams of mitosis genes. This instability thus catalyzes the evolution of new cancer species with individual karyotypes. Depending on their degree of aneuploidy, these cancers then evolve new subspecies. To test this theory, we have analyzed the karyotypes and phenotypes of mammary carcinomas of mice with transgenic SV40 tumor virus- and hepatitis B virus-derived oncogenes. We found that (1) a given transgene induced diverse carcinomas with individual karyotypes and phenotypes; (2) these karyotypes coevolved with newly acquired phenotypes such as drug resistance; (3) 8 of 12 carcinomas were transgene negative. Having found one-to-one correlations between individual karyotypes and phenotypes and consistent coevolutions of karyotypes and phenotypes, we conclude that carcinogenesis is a form of speciation and that individual karyotypes maintain cancers as they maintain species. Because activated oncogenes destabilize karyotypes and are dispensable in cancers, we conclude that they function indirectly, like carcinogens. Such oncogenes would thus not be valid models for the treatment of cancers.

Criteria for establishing that a virus is oncogenic

To prove that a particular infectious agent causes a disease is much more difficult in human subjects than in other animals for both ethical and practical reasons. Where the disease is a malignant tumour with a long latent period the situation is even more difficult. For these reasons, it is often necessary to concentrate in the first instance on association of the virus with the disease, and this is discussed in the context of papillomaviruses. Association of a virus with a tumour may occur for a variety of reasons other than the virus being the cause of the tumour. This is illustrated by several examples of parvoviruses and DNA tumour viruses. Conversely, the absence of any sign of virus or viral nucleic acid in a tumour does not prove that the tumour was not induced by a virus. Apart from association of a virus with a tumour it is also necessary to show that the virus in question is oncogenic. Again this cannot normally be done directly, so that indirect evidence from animal experiments or from in vitro transformation is likely to be the best available alternative. In the final analysis the best proof of oncogenicity may be the effectiveness of intervention directed at the virus.

In vivo evolution of adenovirus 2-transformed cell virulence associated with altered E1A gene function

Neoplastic cell populations may evolve to a state of higher virulence in immunocompetent hosts. Transforming gene involvement in this process of tumor progression was evaluated using adenovirus type 2 (Ad2)-transformed hamster cells that are highly susceptible to destruction by natural killer cells and activated macrophages, due to Ad E1A gene function, and are nontumorigenic in immunocompetent animals. Cells selected for increased tumorigenicity retained parental cell patterns of viral gene integration and methylation and expressed Ad2 E1A proteins but exhibited altered E1A function evidenced by decreased susceptibility to killer cell-mediated lysis and inability to support E1A(-) mutant virus replication. The data suggest that an interruption in cellular pathways of E1A expression may result in increased transformed cell virulence.

Quantitation of a 55K cellular protein: similar amount and instability in normal and malignant mouse cells

Quantitative expression of a specific 55,000 (55K)-molecular-weight cellular protein was studied in two groups of mouse embryo fibroblast (clonal) cells originating from two parent clones, one of which possessed high tumorigenicity and the other of which possessed very low tumorigenicity. From the clone with low tumorigenicity, tumor lines and clones were obtained by selecting rare spontaneously transformed highly tumorigenic (mutant) cells. Cells were labeled during exponential growth for 3 h at 37 degrees C, with [35S]methionine, and the cellular 55K protein was immunoprecipitated with a monoclonal antibody and quantitated. There were low and approximately equal amounts of 55K protein in cells (clones) with both low and high tumorigenicity from both groups of cells, and there was no correlation at all between quantitative expression of 55K protein and of cellular tumorigenicity. There was approximately 10- to 20-fold more 55K protein in all simian virus 40-transformed T antigen-positive derivative clones, as shown previously. The T antigen-negative revertant tumor lines and clones obtained by an immunological in vivo selection method had low amounts of 55K protein, similar to the parent cell before simian virus 40 transformation. In all of the T antigen-negative cells, including the highly tumorigenic cells, degradation (turnover?) of the 55K protein was rapid, and a half-life of 15 to 60 min was estimated from pulse-chase experiments. In all of the T antigen-positive cells the 55K protein was stable (half-life greater than 10 h). In primary cells established from the tumors induced by highly tumorigenic cells there was a very low or no detectable amount of the 55K protein. This is in contrast to the primary cells obtained from early murine embryos in which we have reported high amounts of (stable) 55K proteins.

The transformation-related protein p53 is not bound to the SV40 T antigen in BALB 3T12 cells expressing T antigen

In most murine cells transformed by the SV40 virus, virtually all of the cellular phosphoprotein p53 is in a complex with the SV40 T antigen. Here, we report that, in SV40-infected T-antigen-positive Balb 3T12 mouse cells, most (approximately 80%) of the p53 is not in complex. Complex formation was determined by measuring the amounts of [35S]methionine-labeled p53 which coprecipitated with T antigen when using monoclonal antibody to T antigen. The amount of complex formation was expressed as a percentage of total p53 present, measured by the amount of p53 precipitated with the monoclonal antibody to the p53. The values were confirmed by Western blotting procedure, in which the steady-state levels of the proteins were measured. In these measurements after complete precipitation with antibody to T antigen, the residual p53 in the supernatant was precipitated by antibody to p53, and this amount was denoted as free p53. There was no significant difference seen between the [35S]methionine-labeled tryptic peptides of complexed and the free p53 (or between complexed and free T antigens) as determined by two-dimensional gel electrophoresis and chromatography. Virus rescue experiments and retransformation by the rescued virus showed that there was no mutation in the SV40 DNA coding for the T antigen which could account for the lack of complex formation. Both p53 and T antigen were underphosphorylated in cells which exhibited reduced complex formation. Tumorigenicity in syngeneic mice and anchorage-independent cell growth in culture of various cloned mouse cells with or without T antigen expression was compared. The changes in the biologic properties were explainable solely on the basis of known or expected effects of expression of the T antigen and were independent of complex formation or of absence of complex formation between p53 and T antigen.

Immunologic selection of simian virus 40 (SV40) T-antigen-negative tumor cells which arise by excision of early SV40 DNA

A clonal line of highly oncogenic spontaneously transformed mouse cells (104C) was transformed in tissue culture by simian virus 40 (SV40) and subsequently recloned (106CSC). This 106CSC cell line expressed T antigen and transplantation antigen but was about 100 times less tumorigenic than the 104C parent. When 10(5) 106CSC cells were injected into immunocompetent syngeneic mice, tumors were produced. From such tumors, cell lines were established in culture, all of which were consistently negative for T antigen. We found previously by solution DNA hybridization methods that the tumor cells were depleted in the early region of SV40 DNA which codes for the T antigen. We postulated that this loss occurs through a DNA rearrangement of unknown mechanism in one or a few 106CSC cells and that the tumors are then produced from such a cell or cells, whereas all the T-antigen-positive 106CSC cells are rejected by immunologic means. In this investigation we showed by the DNA transfer method using appropriately selected SV40 DNA probes that indeed the tumor cell clone (130CSCT) we selected to investigate came from one 106CSC cell in which the T-antigen-coding SV40 DNA sequences (but not all the early SV40 DNA sequences) were lost by an excision and recombination mechanism. We also showed that the 130CSCT cells, which are highly tumorigenic, could again be transformed by SV40 and that the resulting T-antigen-positive cloned derivative cells became much less tumorigenic (approximately 10(5)-fold), apparently again because of immunologic recognition and rejection. Indeed, when 10(7) T-antigen-positive cloned cells were injected, all the T-antigen-positive cells were rejected and the tumor was produced again from one or more T-antigen-negative cells. Thus, a one-step in vivo transplantation experiment allowed a selection (for tumorigenicity and against the SV40 T antigen) of a mutant mammalian cell with a DNA deletion at a definable site.

Characterization of tumor progression from threshold tumor inocula: evidence for natural resistance

An examination of the variant generation and selection hypothesis of tumor progression was undertaken using the NK-sensitive, NAb-sensitive SL2-5 lymphoma in the threshold inoculum model of tumor progression. Tumor cells obtained from the i.p. injection site expressed increased heterogeneity for sensitivity to syngeneic NAb and to NR measured in the 131IUdR-labelled tumor elimination assay. Cells retrieved from the s.c. injection site exhibited reductions in sensitivity to NR which correlated with decreases in sensitivity to syngeneic NAb and NK cells in vitro. These data confirm and extend our previous observations with the NK-resistant L5178Y-F9 lymphoma and further substantiate the evidence for the participation of NAb and NK cells in host-mediated anti-tumor resistance. Characterization of the model revealed that the selection for reduced sensitivity to NR in thymus-depleted AT x BM mice and normal animals could not be distinguished, suggesting that thymus-independent mechanisms may be major contributors to the surveillance of nascent tumors. The decreases in susceptibility to NR occurred in a stepwise and time-dependent manner in accord with the sequential multistage concept of progression. Furthermore, the selection for tumor cells which exhibited reductions in sensitivity to NR correlated with selection for increased tumorigenicity, in keeping with the idea that progression is associated with development towards an increasingly autonomous tumor.

Cell lines derived from tumors induced in syngeneic rats by FR 3T3 SV40 transformants no longer synthesize the early viral proteins

Several SV40-transformed FR 3T3 rat cell lines formed tumors upon inoculation to syngeneic immunocompetent Fisher rats. These tumors, which appeared only after a long latency period, showed a fast rate of growth. Tumor-derived (TD) cell lines were established in culture from several tumors induced by independent transformants, and their properties were analyzed. Though TD cells were highly tumorigenic, their level of transformation in culture was similar to that of the original transformants. They did not synthesize detectable amounts of the two early viral gene products, the large-T and small-T polypeptides. However, the transformation-associated cellular p53 protein was detected in all of them by [35S]methionine labeling and immune precipitation with monoclonal antibodies directed against the mouse p53. Growth in the animal apparently counterselected the cells expressing the early viral proteins, and hence, possibly, the tumor-specific transplantation antigen. This selection was mediated at least in part by the T-cell immune response, as the tumors induced by the same transformants in nu/nu mice still expressed the nuclear T-antigen. Absence of expression of the early viral region was frequently correlated with the loss of the integrated SV40 DNA. Some tumors, however, still contained early viral DNA sequences, which were, in even fewer cases, transcribed into RNA. These results altogether suggest that tumor formation by the FR 3T3-SV40 transformed cells in immunocompetent rats requires two events, the selection for the acquisition of a high tumorigenic potential, and against the expression of the early viral genes. Only the first of these two events was observed upon tumor formation in nude mice.

Biology of simian virus 40 (SV40) transplantation antigen (TrAg). X. Tumorigenic potential of mouse cells transformed by SV40 in high responder C57BL/6 mice and correlation with the persistence of SV40 TrAg, early proteins and sequences

Primary mouse embryo fibroblasts of C57Bl/6 origin and cells derived from a tumor induced by polyoma virus in a C57Bl/6 mouse were transformed with SV40. The tumorigenic potential of these cells in normal adult and SV40-immunized mice was correlated with the synthesis of SV40 tumor antigens including the transplantation rejection antigen (TrAg) and with the presence of SV40 early region DNA sequences. Primary cells transformed by SV40 (B6/WT-3) induced tumors in immunocompetent adult syngeneic mice after adaptation in the immunosuppressed host. Passage of these tumor cells (B6/WT-3-T) through SV40-immunized mice resulted in the retention of both T and t antigens and TrAg. However, passage of SV40-transformed polyoma tumor cells through SV40-immunized immunocompetent adult mice but not in nonimmunized mice resulted in the loss of expression of SV40 tumor antigens including TrAg. This loss correlated with the loss of SV40 early region sequences from these double transformed cells. These results demonstrate that the establishment of in vitro SV40-transformed primary mouse cells into a tumor capable of progressive growth in high responder mice does not lead to the selection of variants which have lost the expression of early region DNA sequences.

Rat cells transformed by simian virus 40 give rise to tumor cells which contain no viral proteins and often no viral DNA

Rat 3T3 cells transformed by simian virus 40 were injected into rats to examine their capacity to develop into tumors. Both large T-dependent (N) transformants and large T-independent (A) transformants were used. All the transformed cell lines contained large T and small t and could multiply efficiently in agar. Only some transformants could develop into tumors. All tumor cells examined had lost both large T and small t. Tumor cells in which the viral genome could still be detected were found together with tumor cells in which the simian virus 40 DNA could no longer be detected. N transformants which displayed the transformed phenotype in a temperature-sensitive manner became temperature insensitive during tumor formation.

Rat cells transformed by simian virus 40 give rise to tumor cells which contain no viral proteins and often no viral DNA

Rat 3T3 cells transformed by simian virus 40 were injected into rats to examine their capacity to develop into tumors. Both large T-dependent (N) transformants and large T-independent (A) transformants were used. All the transformed cell lines contained large T and small t and could multiply efficiently in agar. Only some transformants could develop into tumors. All tumor cells examined had lost both large T and small t. Tumor cells in which the viral genome could still be detected were found together with tumor cells in which the simian virus 40 DNA could no longer be detected. N transformants which displayed the transformed phenotype in a temperature-sensitive manner became temperature insensitive during tumor formation.

Quantitation of a 55K cellular protein: similar amount and instability in normal and malignant mouse cells

Quantitative expression of a specific 55,000 (55K)-molecular-weight cellular protein was studied in two groups of mouse embryo fibroblast (clonal) cells originating from two parent clones, one of which possessed high tumorigenicity and the other of which possessed very low tumorigenicity. From the clone with low tumorigenicity, tumor lines and clones were obtained by selecting rare spontaneously transformed highly tumorigenic (mutant) cells. Cells were labeled during exponential growth for 3 h at 37 degrees C, with [35S]methionine, and the cellular 55K protein was immunoprecipitated with a monoclonal antibody and quantitated. There were low and approximately equal amounts of 55K protein in cells (clones) with both low and high tumorigenicity from both groups of cells, and there was no correlation at all between quantitative expression of 55K protein and of cellular tumorigenicity. There was approximately 10- to 20-fold more 55K protein in all simian virus 40-transformed T antigen-positive derivative clones, as shown previously. The T antigen-negative revertant tumor lines and clones obtained by an immunological in vivo selection method had low amounts of 55K protein, similar to the parent cell before simian virus 40 transformation. In all of the T antigen-negative cells, including the highly tumorigenic cells, degradation (turnover?) of the 55K protein was rapid, and a half-life of 15 to 60 min was estimated from pulse-chase experiments. In all of the T antigen-positive cells the 55K protein was stable (half-life greater than 10 h). In primary cells established from the tumors induced by highly tumorigenic cells there was a very low or no detectable amount of the 55K protein. This is in contrast to the primary cells obtained from early murine embryos in which we have reported high amounts of (stable) 55K proteins.

Transforming genes and gene products of polyoma and SV40

The small DNA-containing viruses, SV40 and polyoma, transform cells in vitro and induce tumors in vivo. For both viruses two genes required for transformation have been found. The genes required for transformation are also involved in productive infection. Although the two viruses are similar in their effects on cells, the organization of the transforming genes and gene products is different. The purpose of this review is to compare what is known about the biology and the biochemistry of the early regions of the two viruses. The genetic and biochemical studies defining the sequences important for transformation will be reviewed. Then, the products of the transforming genes, called T antigens, will be discussed in detail. There is a substantial body of descriptive information on those products, and studies on the function of the T antigens have also begun.

An embryo protein induced by SV40 virus transformation of mouse cells

A specific protein of molecular weight (MW) approximately 55,000 (55K) was found recently by immunoprecipitation in all SV40 virus-transformed mammalian cells, in addition to the SV40 large T antigen (appoximately 94K) and small antigen (approximately 17K), which are the only proteins coded by the 'early half' of the SV40 genome. The 55K protein is encoded by cellular DNA; its peptide pattern is different from that of the SV40 antigens and it is species specific in mouse, rat, hamster, monkey and human SV40-transformed (or infected) cells. A 55K protein with a similar peptide pattern was found in mouse embryonal carcinoma cells not exposed to SV40. Similar proteins were reported in mouse sarcomas and leukaemias induced by a great variety of aetiological agents and also in a spontaneously transformed mouse fibroblast cell line, and it has been suggested that the protein may be a general correlated of cellular tumorigenicity. We now report that the approximately 55K protein is present in primary cell cultures from 12-14 day old mouse embryos, but not in 16-day old mouse embryos. The embryo protein has a peptide pattern virtually indistinguishable from that of the SV40-induced protein. We also show by comparing closely related cell families that spontaneously transformed highly tumorigenic mouse cells do not possess the 55K protein.

Subcellular distribution of simian virus 40 T antigen species in various cell lines: the 56K protein

The subcellular distribution of SV40 anti-T serum-specific species was examined in SV40-transformed, T-antigen-positive tissue culture cell lines of rat and of AL/N and BALB/c mouse origin. Cells were labelled with [35S]methionine. The cytoplasm, nuclear and membrane fractions were obtained, and their radioimmunoprecipitates analyzed by gel electrophoresis. Tests were performed to determine the purity of these subcellular fractions, and negligible cross-contamination was found. The cytoplasm fractions lacked detectable anti-T serum reactivity. Large amounts of both large T antigen and a 56K protein were always present together both in the nuclear fractions and, in a somewhat lesser amount, in the plasma membrane fractions of all cell lines examined. Analysis of density gradient sedimentation profiles of the immunoprecipitates of whole-cell extracts indicated these species were associated in some fashion, probably with each other. The activity of the 56K protein may be associated with its presence on the cell surface where, either alone or acting together with the large T antigen, it might provide the surface activity responsible for tumor-specific surface and/or transplantation antigen activities.