The roles of individual polyoma virus early proteins in oncogenic transformation

Biological activities of v-myc and rearranged c-myc oncogenes in rat fibroblast cells in culture

Two distinct forms of the myc oncogene were assayed for their ability to induce, in cultured rat fibroblast cells, the alterations of cellular growth controls observed upon transfer of the gene of polyoma virus encoding only the large T protein (plt). Both of these rearranged myc genes and the plt gene had been previously shown to cooperate with ras oncogenes for transformation of rat embryo fibroblasts (REF) and were thought to induce the same early step ("immortalization") of the tumoral transformation pathway. We now report that these two different oncogenes elicite the same response in the following biological assays: (i) reduction of the requirements in serum factors for growth in culture of cells of the established FR3T3 line; (ii) expression of transformed properties in low serum medium after transfer into FR3T3 cells expressing only the middle T protein of polyoma virus (MTT lines); (iii) conferring on REF cells the ability to grow as clonal colonies after seeding at low cell density; (iv) conferring on REF cells the ability to grow continuously in cell culture. These congruent phenotypes suggest that the activities of the large T and myc proteins result in the induction of the same molecular events. These results also provide simple biological assays and selective systems for oncogenes of the myc class.

Immortalization of primary mouse embryo fibroblasts with SV40 virions, viral DNA, and a subgenomic DNA fragment in a quantitative assay

In order to investigate the capacity of the N-terminal half of large T antigen to convert primary cells to continuous cell lines conditions were standardized for transforming primary mouse embryo fibroblasts of C57Bl/6 origin (B6/MEF) with DNA. Using these conditions the transforming capability of SV40 virions, viral DNA, and a plasmid, pSV3T3-20-GV (C.E. Clayton, D. Murphey, M. Lovett, and P.W.J. Rigby, Nature (London), 299, 59-61, 1982) containing the 5' half of the SV40 early region, was determined in a quantitative immortalization assay. The plasmid pSV3T3-20-GV transformed B6/MEF at only 1/16 the frequency of a plasmid containing the entire early region. These results suggest that the 3' half of large T antigen which cannot be produced by this plasmid strongly influences the frequency of immortalization. It is not known whether this influence reflects the presence of a transformation domain in the carboxy terminus of large T antigen or, alternatively, results from an altered conformation or stability of the truncated polypeptide.

Transformation mediated by the SV40 T antigens: separation of the overlapping SV40 early genes with a retroviral vector

A murine retroviral vector has been used to separate physically the overlapping genes encoded by SV40. This minimal retroviral vector contains LTRs and other cis-acting signals required for infectious RNA virus propagation. We placed the SV40 early region within this DNA and after transfection of cells producing helper Moloney murine leukemia virus, SV40 retroviruses (MV40) could be rescued. Cytoplasmic spliced large T and small t transcripts, as well as unspliced transcripts, are packaged into virions with equal efficiency. Pure SV40 large T retroviruses can be cloned from these heterogeneous virus stocks by secondary transformation of rodent cells. The large T retrovirus stocks morphologically transform primary or established mouse and rat lines with high efficiency. There is little difference in transformation either by agar assay or focus formation between retroviruses carrying both SV40 genes or large T alone. We present quantitative data that demonstrate that abortive transformation of rodent cells by SV40, transient expression of the transformed phenotype after infection, is not manifested by MV40. Thus abortive transformation is not the result of a weakly dominant transforming gene, but rather of the normally inefficient mode of integration and early gene expression of SV40 upon infection of rodent cells.

ob17 cells transformed by the middle-T-only gene of polyoma virus differentiate in vitro and in vivo into adipose cells

Cell lines were derived from ob17 preadipocyte cells by focus formation after transfer of the complete early region of polyoma virus (ob17PY) or of a modified genome encoding only the middle T protein (ob17MT). Both ob17PY and ob17MT cell lines exhibited a high cloning efficiency in agarose medium containing 10% fetal bovine serum. Fully transformed ob17PY cells grew to high saturation densities and did not differentiate in vitro and in vivo. ob17MT cells and derived subclones did not grow in the absence of serum and were able to differentiate in vitro and to give rise in vivo to adipose tumors. Among these different clones an inverse relationship was observed in culture between their potentiality to overproliferate at low serum and their potentiality to convert into adipose cells. The expression of enzyme markers of adipose conversion was strictly dependent upon the presence of growth hormone. In addition, the hormonal requirements for differentiation were simpler than those of the original ob17 cells and the adipose conversion could take place in serum-free hormone-supplemented medium.

Malignant transformation of early passage rodent cells by a single mutated human oncogene

When linked to transcriptional enhancers, the mutant Ha-ras-1 gene from the T24 bladder carcinoma cell line induces the complete malignant transformation of early passage cells, while the normal Ha-ras-1 proto-oncogene only induces immortalization. Therefore, mutated Ha-ras-1 does not require a cooperating gene to trigger malignant conversion and ras genes may be involved in the process of tumorigenesis at an earlier stage than previously suspected.

Functional characteristics of cytotoxic T lymphocyte polyoma virus-transformed fibroblast hybrids

By two different fusions between a cytotoxic T-lymphocyte (CTL) clone and a polyoma virus (Py)-transformed fibroblast line, 40 hybrid clones have been generated. It has been demonstrated that they were all TCGF independent for multiplication. Moreover, some of these hybrids were functional for cytolytic expression, whether or not TCGF was present either at the time of fusion or in the selective media. Two clones generated from the same fusion were markedly cytolytic and were able to remove TCGF from their culture medium, suggesting that they possessed TCGF receptors. These clones also secreted discrete amounts of a TCGF-like factor. The effect of TCGF on hybrid cell proliferation is discussed.

Construction and functional characterization of polyomavirus genomes that separately encode the three early proteins

Modified polyomavirus genomes that individually encode the large and small T proteins were constructed by exchanging restriction endonuclease fragments between cDNA copies of the respective mRNAs and cloned genomic DNA. The efficacies of the new constructs, and that of the middle T protein gene described previously (R. Treisman , U. Novak, J. Favaloro , and R. Kamen , Nature [London] 292:595-600, 1981), were demonstrated with simian virus 40 (SV40)-polyomavirus recombinants in which part or all of the SV40 late region was replaced with the modified polyomavirus early genes. Each of the three recombinant viruses induced the synthesis of only the expected polyomavirus early protein in infected CV-1 cells. The rates of synthesis of large, middle, and small T proteins were ca. 1.5, 4.0, and 9.0 times the rate of synthesis of SV40 large T protein, respectively. The deletion of introns had no detrimental effect on mRNA biogenesis. Indeed, a further polyomavirus-SV40 recombinant, containing wild-type polyomavirus early region DNA, expressed an aberrant 58,000-dalton form of the middle T protein which we believe to result from utilization of a cryptic splice site. Immunofluorescence studied with monkey cells infected by the recombinant viruses allowed us to determine the cellular locations of the polyomavirus early proteins. Overproduction of the middle T protein did not result in a corresponding overproduction of the middle T protein-associated tyrosine phosphokinase activity.

mlt Mutation in the polyomavirus genome impairing a function of the middle T protein

The DNA from polyomavirus mlt mutant P155 transforms cells in culture as efficiently as wild-type DNA but has a much lower tumorigenic potential when injected into newborn rodents. The mutant has a 12-base-pair deletion between nucleotides 1347 and 1360, i.e., in a region which encodes parts of the middle and large T antigens (G elinas et al., J. Virol. 43:1072-1081, 1982). To determine which of the two viral gene functions was affected by the mutation, we transferred the latter into a modified polyomavirus genome encoding exclusively the middle T protein. Our results show that the P155 mutation alters a function of the polyomavirus middle T protein required for the induction of the tumorigenic process in vivo. Beside the 12-base-pair deletion at 96.3 map units, there is no other alteration in the coding sequence of P155 middle T with respect to that of P16, the wild-type parental strain. We conclude, therefore, that the deletion is the lesion affecting the tumorigenic potential of mutant P155 .

Polyoma middle T antigen requires cooperation from another gene to express the malignant phenotype in vivo

The oncogenic potential of polyomavirus in newborn hamsters can be expressed by a recombinant encoding only the middle T protein. However, polyoma middle T requires the cooperation from small T to induce tumors in newborn rats. Similar complementary functions such as cocarcinogens or tumor promotors can be exerted by the simian virus 40 T antigens as well as by one or several products of the early region 1A of adenovirus 2.

Transformation affects superoxide dismutase activity

A human, aging lung fibroblast cell strain manifested both copper-zinc superoxide dismutase activity (CuZn-SOD) and manganese-containing superoxide dismutase activity (MnSOD), and these activities were found to be modulated by physiological concentrations of thyroid hormone. The immortal cell lines examined, whether normal or malignant, had no MnSOD activity. The immortal normal cell lines examined had CuZnSOD activity which was modulated by thyroid hormone, whereas the immortal malignant cell lines had CuZnSOD activity which was not affected, or not as strongly affected, by the presence of thyroid hormone.

The transformed phenotype in culture and tumorigenicity of Fischer rat fibroblast cells (FR3T3) transformed with bovine papilloma virus type 1

Unlike cell lines transformed with Polyomaviruses, transformants derived by focus formation or colony formation in agarose medium after transfer into rat fibroblast cells (FR3T3 line) of Bovine Papilloma Type 1 (BPV1) DNA were consistently observed to grow poorly in suspension and to remain highly serum dependent for growth in culture. These cells did not produce detectable amounts of plasminogen activator, and kept the flat morphology and organized cytoskeleton characteristic of the normal fibroblast. However, they induced in syngeneic animals the development of tumors with a greater invasive potential than tumors induced by Polyomaviruses. By contrast with the original transformants, cells recovered from the tumors grew efficiently in suspension and produced high levels of plasminogen activator. They still had, however, extended cytoskeletal structures and remained completely dependent on high serum concentrations for growth in culture. The stepwise transformation process induced by BPV1 thus appears strikingly different from that previously observed with polyoma and SV40 viruses. The observed changes in transformation phenotype between transformed line and tumor cells do not correlate with any important modification of the number of autonomous copies of the viral genome, nor with any rearrangement of viral sequences detectable at the level of the blot analysis.

Molecular cloning and restriction mapping of a simian virus 40 deletion mutant derived from simian transformants expressing a non-karyophilic T antigen

A replication-defective Simian virus 40 genome, with a deletion of about 120 nucleotides in the region encoding the N-terminal fourth of the large T antigen, has been isolated from the DNA of Simian cells transformed by SV40. Both the original transformants, and the murine transformants obtained by transfection with this cloned mutant DNA, produced a large T antigen displaying in immunofluorescence an exclusively cytoplasmic localization. The protein apparent molecular mass (83 kDa) was about 6% smaller than that of normal karyophilic large T. Restriction analysis showed that the deletion eliminated two close HinfI sites, at nucleotides 4459 and 4376 (map unit 0.50).

Tumorigenic activity of polyoma virus and SV40 DNAs in newborn rodents

A procedure has been developed whereby the oncogenicity of the DNA from polyoma (Py) virus and Simian virus 40 (SV40) can be tested directly by injecting recombinant DNA into newborn rodents. Injection of 0.2-2.0 micrograms of linear DNA induced the development of subcutaneous liposarcomas and fibrosarcomas at the site of inoculation. Coinjection of high-molecular-weight rat DNA as carrier had little or no effect on tumor formation but plasmids pBR322, pAT153 , and pML2 behaved as strong inhibitors. Tumor induction by injecting DNA into newborn rodents provides an in vivo equivalent to a transformation assay but appears to be a more stringent and rigorous criterion of oncogenic transformation. The oncogenic potential of Py virus in newborn hamsters could be expressed by a recombinant encoding only the middle T protein, although with average tumor latencies 5-10 times longer than those observed with wild-type Py DNA. Py middle T required the cooperation from small T to induce tumors in newborn rats. SV40 DNA was tumorigenic only in newborn hamsters. delta 2005 DNA which is unable to produce the SV40 small T antigen was much less active and required a latent period about twice that of wild-type SV40 DNA. However, its tumorigenic potential was restored by addition of the Py small T antigen gene. This indicates that Py and SV40 small T antigens are interchangeable and that they probably play an identical role in malignant transformation. Finally, evidence was provided that intermolecular recombination or recombination between DNA fragments can occur in vivo.

A 61,000-dalton truncated large T-antigen is uniformly expressed in hamster cells transformed by polyomavirus

Various polyomavirus-transformed hamster cell lines derived from tumors or from infected hamster cell cultures synthesized polyoma middle and small tumor (T)-antigens but no full-size large T-antigen. Instead, all cell lines produced the same or similar polyoma T-antigen-related proteins of ca. 61 kilodaltons (kDal). Like large T-antigen synthesized in lytically infected mouse cells, the 61-kDal proteins were phosphoproteins showing electrophoretic and charge heterogeneities. Chromatographic analysis of the methionine-containing tryptic peptides indicated that the 61-kDal proteins were truncated forms of large T-antigen comprising amino acid residues 1 to 485 (+/- 25). Analysis of viral DNA present in hamster chromosomal DNA of three independently isolated cell lines confirmed that synthesis of the 61-kDal proteins was due to a discontinuity in the large T-antigen coding sequence, most likely located between 7 and 8.9 map units on the polyoma DNA map. The three cell lines yielded essentially the same patterns of viral DNA-containing restriction enzyme fragments, suggesting that insertion of viral DNA into the hamster chromosomes took place at closely similar sites.

Construction and expression of a recombinant DNA gene encoding a polyomavirus middle-size tumor antigen with the carboxyl terminus of the vesicular stomatitis virus glycoprotein G

We constructed a molecular clone encoding the N-terminal 379 amino acids of the polyomavirus middle-size tumor antigen, followed by the C-terminal 60 amino acids of the vesicular stomatitis virus glycoprotein G. This hybrid gene contained the coding region for the C-terminal hydrophobic membrane-spanning domain of the G protein in place of the C-terminal hydrophobic domain of the middle-size tumor antigen. The hybrid gene was expressed in COS-1 cells under the control of the simian virus 40 late promoter. The hybrid protein was located in cell membranes and was associated with a tyrosine-specific protein kinase activity, as was the middle-size tumor antigen. Plasmids encoding the hybrid protein failed to transform mouse NIH 3T3 or rat F2408 cells.

Transmission of the polyoma virus middle T gene as the oncogene of a murine retrovirus

Polyoma virus is a papovavirus that productively infects mouse cells. In cells of other species, such as rat cells, polyoma virus is virtually unable to replicate, and a small proportion of infected cells become stably transformed. The ability of polyoma virus to transform infected cells is determined by genes that encode the large, middle and small T antigens and which are found in the early region of the virus genome. We have inserted the transforming region of polyoma virus into a murine leukaemia virus (MLV) vector, to generate a replication-defective transforming retrovirus which for the first time allows efficient transformation of mouse cells by the polyoma virus middle T gene. During the life cycle of this recombinant virus the intervening sequence present in the original polyoma virus middle T gene was removed. The recombinant virus that we have constructed is analogous to other acutely transforming retroviruses, and demonstrates that the polyoma middle T gene is a dominant transforming oncogene.

Polyoma middle T antigen requires cooperation from another gene to express the malignant phenotype in vivo

The oncogenic potential of polyomavirus in newborn hamsters can be expressed by a recombinant encoding only the middle T protein. However, polyoma middle T requires the cooperation from small T to induce tumors in newborn rats. Similar complementary functions such as cocarcinogens or tumor promotors can be exerted by the simian virus 40 T antigens as well as by one or several products of the early region 1A of adenovirus 2.

DNA binding activity of polyoma virus large tumor antigen

Polyoma virus large tumor antigen from productively infected mouse cells has been purified to greater than 50% homogeneity by a simple immunoaffinity procedure using monoclonal antibodies. A radioimmunoreaction was devised for assaying purity. The purified large tumor antigen retained its antigenicity and its ability to bind DNA specifically. The regions on the polyoma virus genome recognized by the protein were characterized. Three binding regions were localized within the portion of the genome between the viral origin of DNA replication and the protein coding sequence, overlapping the early promoter and the sites of initiation of mRNAs that specify the viral tumor antigens. The binding regions each contain direct repeats of the pentanucleotide sequence G-R-G-G-C.

The role of superoxide dismutase and gene amplification in carcinogenesis

Chemical carcinogenesis is hypothesized to involve manganese superoxide dismutase and gene amplification. Initiation is hypothesized to be caused by destruction of the DNA that enables the cell to induce manganese superoxide dismutase. Tumor promotion then causes amplification of the manganese superoxide dismutase gene and the cell proliferation gene (oncogene) because of selective pressure exerted by the promoter. Because the promoter causes cell division and chromosomal rearrangements, unequal segregation of the amplified genes results. Because cells which have high amounts of the cell proliferation gene and low amounts of the manganese superoxide dismutase gene grow faster, these cells become dominant and a tumor forms.

Construction and expression of a recombinant DNA gene encoding a polyomavirus middle-size tumor antigen with the carboxyl terminus of the vesicular stomatitis virus glycoprotein G

We constructed a molecular clone encoding the N-terminal 379 amino acids of the polyomavirus middle-size tumor antigen, followed by the C-terminal 60 amino acids of the vesicular stomatitis virus glycoprotein G. This hybrid gene contained the coding region for the C-terminal hydrophobic membrane-spanning domain of the G protein in place of the C-terminal hydrophobic domain of the middle-size tumor antigen. The hybrid gene was expressed in COS-1 cells under the control of the simian virus 40 late promoter. The hybrid protein was located in cell membranes and was associated with a tyrosine-specific protein kinase activity, as was the middle-size tumor antigen. Plasmids encoding the hybrid protein failed to transform mouse NIH 3T3 or rat F2408 cells.

Transformation by polyoma virus is drastically reduced by substitution of phenylalanine for tyrosine at residue 315 of middle-sized tumor antigen

We used an oligonucleotide to introduce an A----T transversion at nucleotide position 1178 in polyoma virus DNA. The single effect of this mutation is to substitute phenylalanine for tyrosine at residue 315 of the middle-sized tumor (mT) protein (antigen). This site was previously identified as a major phosphate acceptor in the protein kinase reaction of immunocomplexes containing mT antigen. Reconstituted polyoma virus with the transversion, Py-1178-T, produces an altered mT protein that shows about 20% of the activity of wild-type mT antigen in the immunocomplex kinase assay. This residual activity appears to be directed primarily at another tyrosine at position 322 in the mT protein. The transforming ability of Py-1178-T is drastically reduced compared to wild-type virus. The efficiency of transformation by the mutant is less than 1% of that of wild type in focus assays and less than 0.1% in soft-agar growth assays. Cells identified in focus assays with Py-1178-T are generally less transformed in their phenotype than wild-type transformed cells.

Oncogenes: growth regulation and the papovaviruses polyoma and SV40

Cellular oncogenes and their activated and retrovirus-coded counterparts play an important role in cellular regulation. Here the relationship between such oncogenes and the genes coding for the transforming proteins of the papovaviruses, polyoma viruses, and simian virus 40 (SV40) is discussed. It is concluded that polyoma virus may transform established cells by a mechanism involving activation of a cellular oncogene product, whereas SV40 may transform by a mechanism involving a previously little studied cytoplasmic form of the transforming protein.

Polyomavirus-plasmid recombinants capable of replicating have an enhanced transforming potential

The frequency of transformation of rodent fibroblasts by polyomavirus is enhanced by a viral gene product, large T-antigen. However, this effect of large T-antigen cannot be demonstrated with pBR322-cloned viral DNA. Recently, it was discovered that pBR322 contains cis-acting sequences inhibitory to DNA replication in mammalian cells. Because polyomavirus large T-antigen is required for viral DNA replication, we examined the possibility that our inability to demonstrate a requirement for large T-antigen in transformation with pBR322-cloned viral DNA was due to the failure of the chimeric DNA to replicate in the transfected cells. To this end we constructed polyomavirus recombinant molecules with a plasmid (pML-2) that lacks these "poison" sequences and measured their capacity to transform cells. Here we report that recombinant plasmids capable of replicating in the transfected cells transform these cells at frequencies approximately sixfold greater than their replication-defective counterparts.

Antibodies against a nonapeptide of polyomavirus middle T antigen: cross-reaction with a cellular protein(s)

Antibodies were raised against the sequence Glu-Glu-Glu-Glu-Tyr-Met-Pro-Met -Glu, which represents a part of the middle T antigen of polyomavirus that is considered to be important in inducing the phenotype of transformed cells. The antibodies reacted with native as well as denatured middle T antigens. In addition, the antibodies immunoprecipitated a cellular protein with an apparent molecular weight of 130,000 (130K) from mouse and rat cells. In some cases, a 33K protein was also immunoprecipitated. Immunoprecipitation of middle T antigen as well as 130K and 33K proteins was blocked by the peptide. The antibodies labeled microfilaments of untransformed mouse, rat, human, and chicken cells by immunofluorescence. This labeling was also blocked by the peptide. The labeling pattern and distribution under a variety of conditions were indistinguishable from those of anti-actin antibodies, although no evidence has been obtained to indicate that the anti-peptide antibodies react with actin. The 130K protein migrated in sodium dodecyl sulfate-polyacrylamide gel electrophoresis slightly slower than chicken gizzard vinculin (130K) and slightly faster than myosin light-chain kinase of chicken smooth muscle (130K). Neither of these proteins absorbed the anti-peptide antibodies. The 33K protein does not seem to be tropomyosin (32K to 40K).

Role of the three polyoma virus early proteins in tumorigenesis

A modified polyoma virus genome which can encode the middle T protein but not the large or small T proteins transforms rat cells in culture with an efficiency about 20% that of the wild-type genome. Although middle T-transformed cells grow as tumors when transplanted into nude mice or syngeneic rats, the middle T gene alone is totally inactive when used in a more stringent and rigorous assay for tumorigenicity such as the injection of DNA into newborn rats. Thus, functions other than those expressed by middle T antigen are required for the elaboration of all the properties associated with tumorigenesis. To assess whether a complementary function could be exerted by the large or the small T antigen, we constructed plasmids containing two modified early regions which independently encoded middle T and one of the two other proteins. Both recombinants were tumorigenic in newborn rats. Cell lines derived by transfer of these plasmids under no special selective conditions did not acquire the property of growth in low-serum medium but exhibited the same tumorigenic properties as wild-type polyoma DNA-transformed cells. Furthermore, a recombinant which encoded the middle and small T antigens, but not the large T antigen, was tumorigenic in newborn rats. Although the small T antigen provides a complementary function for tumorigenicity, it cannot complement the middle T antigen for an efficient induction of transformation of cultured cells. This suggests that the complementary function exerted by the small T antigen is different from that of the N-terminal fragment of the large T protein.

Cellular oncogenes and multistep carcinogenesis

Two dozen cellular proto-oncogenes have been discovered to date through the study of retroviruses and the use of gene transfer. They form a structurally and functionally heterogeneous group. At least five distinct mechanisms are responsible for their conversion to active oncogenes. Recent work provides experimental strategies by which many of these oncogenes, as well as oncogenes of DNA tumor viruses, may be placed into functional categories. These procedures may lead to definition of a small number of common pathways through which the various oncogenes act to transform cells.

Activation of mouse genes in transformed cells

We have used molecular hybridization and cDNA cloning techniques to isolate mouse cellular genes activated in SV40-transformed cells and we show that many of the clones belong to one of four sets. We characterize the cytoplasmic transcripts and genomic sequences homologous to two of these sets. The Set 1 transcription unit(s) is activated in all SV40-transformed cell lines analyzed, and experiments with tsA-mutant-transformed lines show that activation appears to require functional large T-antigen. This transcription unit(s) is also activated in mouse fibroblasts transformed by other agents, including retroviruses and chemical carcinogens. Activation of the Set 2 transcription unit(s) is more restricted, being confined to cell lines transformed by SV40 and retroviruses with distinctive biological properties.

Polyomavirus-plasmid recombinants capable of replicating have an enhanced transforming potential

The frequency of transformation of rodent fibroblasts by polyomavirus is enhanced by a viral gene product, large T-antigen. However, this effect of large T-antigen cannot be demonstrated with pBR322-cloned viral DNA. Recently, it was discovered that pBR322 contains cis-acting sequences inhibitory to DNA replication in mammalian cells. Because polyomavirus large T-antigen is required for viral DNA replication, we examined the possibility that our inability to demonstrate a requirement for large T-antigen in transformation with pBR322-cloned viral DNA was due to the failure of the chimeric DNA to replicate in the transfected cells. To this end we constructed polyomavirus recombinant molecules with a plasmid (pML-2) that lacks these "poison" sequences and measured their capacity to transform cells. Here we report that recombinant plasmids capable of replicating in the transfected cells transform these cells at frequencies approximately sixfold greater than their replication-defective counterparts.

Enhancement of polyoma virus middle T antigen tyrosine phosphorylation by epidermal growth factor

Polyoma virus codes for three proteins involved in host cell transformation: the large, middle and small T antigens. Middle T antigen is a major transforming protein which is responsible for the induction of the phenotype of transformed cells and, without it, transformation does not occur (reviewed in refs 1-4). Middle T antigen alone can transform established cell lines, although large, and possibly small, T antigens are also required for the full expression of the phenotype of transformed cells in media with a low concentration of serum. A subfraction of middle T antigen is associated with a protein kinase activity which phosphorylates middle T antigen in vitro on tyrosine. There is a strong correlation between the level of this kinase activity and the degree of expression of the phenotype of transformed cells. We report here that epidermal growth factor (EGF) stimulates tyrosine phosphorylation of middle T antigen, suggesting the possibility that mitogenic growth factor(s) regulates this phosphorylation activity.

Adenovirus early region 1A enables viral and cellular transforming genes to transform primary cells in culture

The polyoma virus middle-T and the T24 Harvey ras1 genes are individually unable to transform primary baby rat kidney cells. Adenovirus early region 1A provides functions required by these genes to transform primary cells following DNA-mediated gene transfer. These results suggest that separate establishment and transforming functions are required for oncogenic transformation of primary cells in culture.

Tumorigenic conversion of primary embryo fibroblasts requires at least two cooperating oncogenes

Transfection of embryo fibroblasts by a human ras oncogene does not convert them into tumour cells unless the fibroblasts are established and immortalized before transfection. The embryo fibroblasts become tumorigenic if a second oncogene such as a viral or cellular myc gene or the gene for the polyoma large-T antigen is introduced together with the ras gene.

Role of the three polyoma virus early proteins in tumorigenesis

A modified polyoma virus genome which can encode the middle T protein but not the large or small T proteins transforms rat cells in culture with an efficiency about 20% that of the wild-type genome. Although middle T-transformed cells grow as tumors when transplanted into nude mice or syngeneic rats, the middle T gene alone is totally inactive when used in a more stringent and rigorous assay for tumorigenicity such as the injection of DNA into newborn rats. Thus, functions other than those expressed by middle T antigen are required for the elaboration of all the properties associated with tumorigenesis. To assess whether a complementary function could be exerted by the large or the small T antigen, we constructed plasmids containing two modified early regions which independently encoded middle T and one of the two other proteins. Both recombinants were tumorigenic in newborn rats. Cell lines derived by transfer of these plasmids under no special selective conditions did not acquire the property of growth in low-serum medium but exhibited the same tumorigenic properties as wild-type polyoma DNA-transformed cells. Furthermore, a recombinant which encoded the middle and small T antigens, but not the large T antigen, was tumorigenic in newborn rats. Although the small T antigen provides a complementary function for tumorigenicity, it cannot complement the middle T antigen for an efficient induction of transformation of cultured cells. This suggests that the complementary function exerted by the small T antigen is different from that of the N-terminal fragment of the large T protein.

Expression of the large T protein of polyoma virus promotes the establishment in culture of "normal" rodent fibroblast cell lines

Transfer into mouse and rat embryo fibroblasts in primary culture of cloned polyoma virus genes encoding only the large T protein led to the establishment of flat colonies in sparse subcultures at a frequency equal to that of transformation by wild-type virus. Cell lines could be derived from such colonies and maintained in culture for large numbers of generations without entering crisis. They exhibited a normal phenotype, by the criteria of growth on plastic to a low saturation density and of anchorage dependency. However, they required a lower serum concentration for growth than spontaneously established 3T3 cells. Similar results were obtained after transfer of recombinant DNA molecules encoding only the amino-terminal 40% of the large T protein, suggesting that this "immortalization" function corresponds to the activity of an amino-terminal domain of the protein. Immunoprecipitation analysis of T antigens in cell lines established after transfer of the full-size and of the truncated large T genes demonstrated the expression of the full-size large T protein and of a Mr 40,000 antigen expressed from the amino-terminal part of the gene, respectively. After transfer of a "large T only" plasmid that carries a tsa mutation, cell lines were established at 33 degrees C with the same efficiency as with the wild-type large T gene, but their growth was arrested after a shift to 40 degrees C, with a progressive loss in cell viability. This result indicates a continuous requirement for a large T function in the maintenance of "immortality."