The polyoma virus oncogenes. Coordinated functions of three distinct proteins in the transformation of rodent cells in culture

Role for PP2A in ARF signaling to p53

Activation of the ARF-p53 tumor suppressor pathway is one of the cell's major defense mechanisms against cancer induced by oncogenes. The ARF-p53 pathway is dysfunctional in a high proportion of human cancers. The regulation of the ARF-p53 signaling pathway has not yet been well characterized. In this study polyoma virus (Py) is used as a tool to better define the ARF-p53 signaling pathway. Py middle T-antigen (PyMT) induces ARF, which consequently up-regulates p53. We show that Py small T-antigen (PyST) blocks ARF-mediated activation of p53. This inhibition requires the small T-antigen PP2A-interacting domain. Our results reveal a previously unrecognized role of PP2A in the modulation of the ARF-p53 tumor suppressor pathway.

Polyoma virus disrupts ARF signaling to p53

Polyoma virus (Py) differs from other small DNA tumor viruses in not encoding a protein that inactivates p53. The complete Py early region encoding the large T-antigen (PyLT), middle T-antigen (PyMT) and small T-antigen (PyST) will transform primary rodent cells and REF52 cells, but PyMT, the main Py oncogene, by itself will only transform these cells when p53 or ARF is inactivated. We have related Py oncogene cooperation with the effects of the Py T-antigens on the ARF-p53 signaling pathway. PyMT activates an ARF-induced p53-mediated block to cell division explaining the inability of PyMT alone to generate dividing transformed cells. In contrast, in REF52 cells transformed by the whole Py early region (PyREF52), ARF is upregulated but p53 is not activated. Thus PyLT and/or PyST negates the PyMT-induced ARF-mediated block to cell division by disrupting the signaling pathway from ARF to p53. Although there is no detectable interaction or co-localization of endogenous ARF (nucleoli) and MDM2 (nucleoplasm) in PyREF52 cells, expression of transfected ectopic ARF results in an MDM2/ARF interaction and sequestration of MDM2 into the nucleoli. Sequestration of MDM2 by ARF in the nucleoli is not essential for a p53 response in REF52 cells as activation of Raf in REF52Raf-ER cells results in an ARF-induced p53-mediated cell cycle block in the absence of a detectable ARF-MDM2 interaction. Py may provide new insights into the cellular ARF-p53 signaling pathway.

Natural biology of polyomavirus middle T antigen

"It has been commented by someone that 'polyoma' is an adjective composed of a prefix and suffix, with no root between--a meatless linguistic sandwich" (C. J. Dawe). The very name "polyomavirus" is a vague mantel: a name given before our understanding of these viral agents was clear but implying a clear tumor life-style, as noted by the late C. J. Dawe. However, polyomavirus are not by nature tumor-inducing agents. Since it is the purpose of this review to consider the natural function of middle T antigen (MT), encoded by one of the seemingly crucial transforming genes of polyomavirus, we will reconsider and redefine the virus and its MT gene in the context of its natural biology and function. This review was motivated by our recent in vivo analysis of MT function. Using intranasal inoculation of adult SCID mice, we have shown that polyomavirus can replicate with an MT lacking all functions associated with transformation to similar levels to wild-type virus. These observations, along with an almost indistinguishable replication of all MT mutants with respect to wild-type viruses in adult competent mice, illustrate that MT can have a play subtle role in acute replication and persistence. The most notable effect of MT mutants was in infections of newborns, indicating that polyomavirus may be highly adapted to replication in newborn lungs. It is from this context that our current understanding of this well-studied virus and gene is presented.

Expression of the pRb-binding regions of E1A enables efficient transformation of primary epithelial cells by v-src

Primary cultures of rat embryo fibroblasts have been shown to be resistant to transformation by dominant oncogenes such as v-src. We sought to determine if similar resistance is displayed by primary epithelial cells, and, if so, whether an immortalizing oncogene such as E1A could enhance transformation of primary epithelial cells by v-src. Transformation of primary rat epithelial cells by v-src was synergistically enhanced when E1A expression plasmids were cotransfected with a v-src expression plasmid. Foci were more numerous and observed earlier (9 to 14 days) with E1A plus v-src than with v-src alone (18 to 28 days). This cotransformation ability was abrogated by deletions in CR1 or CR2 of E1A, which encode the binding regions for the pRb family and are responsible for E1A-mediated cell cycle activation. Mutations in the p300 binding site or the second exon, which abolish immortalization, did not affect v-src cooperation, in contrast to ras and adenovirus E1B. While kinase activation was required for growth in soft agar, differential activation of Src kinase did not correlate with transformation efficiency. Cell morphology and actin structures were not dramatically impacted by E1A expression; thus, hypertransformation, as previously described for ras cotransformation, was not observed with v-src and second-exon mutants of E1A. However, growth rates for cells expressing both E1A and v-Src were higher than those for cells expressing only v-Src. These results suggest that functions involved in cell cycle activation encoded by E1A first exon can enhance v-src transformation of primary epithelial cells.

Simian virus 40 as a vector: recombinant viruses expressing individual polyoma T antigens

We constructed simian virus 40 (SV40)/polyomavirus recombinants by replacing in SV40 the T antigen coding region with polyoma early region sequences, either cDNAs encoding small, middle or large T antigen or the wild-type sequence coding all three proteins. The recombinants maintained the SV40 late region and origin of replication and were propagated in COS cells yielding recombinant virus preparations with titers of 10(6)-10(7) infectious particles per milliliter. These viruses were characterized in productive infections of COS cells by analyzing early and late mRNA levels and by following synthesis of polyoma early proteins. In the absence of viral DNA replication, i.e. in infected monkey or mouse cells, expression of the polyoma T antigens was weak. Further experiments indicated that this was mostly due to high genomic instability during amplification, to lower levels of cDNA transcripts as compared to spliced mRNA, and possibly also to lower infectivity of the recombinant virions. It remains to be determined, whether these handicaps are unique to SV40/polyoma recombinants or whether SV40 is in general inadequate as a viral vector.

Functional interaction between the SH2 domain of Fyn and tyrosine 324 of hamster polyomavirus middle-T antigen

Middle-T antigen of mouse polyomavirus (MomT) associates with the cellular tyrosine kinases c-Src, c-Yes, and Fyn, while middle-T antigen of hamster polyomavirus (HamT) exclusively binds Fyn. This interaction is essential for polyomavirus-mediated transformation of cells in culture and tumor formation in animals. Here we show that the kinase domain of Fyn is sufficient for association with MomT but not for binding of HamT. We further demonstrate that a Fyn mutant lacking the SH2 domain is able to bind MomT but fails to associate with HamT, indicating that the SH2 domain of Fyn is essential for stable association with HamT. HamT, but not MomT, contains a tyrosine residue, Tyr-324, in the sequence context YEEI. Mutation of Tyr-324 to phenylalanine led to a drastic reduction of associated Fyn and abolished the oncogenicity of HamT. This suggests that Tyr-324 is the major phosphotyrosine residue mediating the binding of HamT to the SH2 domain of Fyn. These findings show that mouse and hamster polyomaviruses use different strategies to target Src-related tyrosine kinases.

Stepwise transformation of rat embryo fibroblasts: c-Jun, JunB, or JunD can cooperate with Ras for focus formation, but a c-Jun-containing heterodimer is required for immortalization

Among the Jun family of transcription factors, only c-Jun displays full transforming potential in cooperation with activated c-Ha-Ras in primary rat embryo fibroblasts. c-Jun in combination with Ras can both induce foci of transformed cells from rat embryo fibroblast monolayers and promote the establishment of these foci as tumoral cell lines. JunB can also cooperate with Ras to induce foci but is unable to promote immortalization. We report here that JunD, in cooperation with Ras, induces foci with an efficiency similar to that of JunB. Artificial Jun/eb1 derivatives from each of the three Jun proteins were also analyzed. These constructs carry a heterologous homodimerization domain from the viral EB1 transcription factor and are thought to form only homodimers in the cell. We show here that these Jun/eb1 chimeras are potent transactivators of AP1 sites and that they can cooperate with c-Ha-Ras to induce foci. However, among all the Ras-Jun and Ras-Jun/eb1 combinations tested, only foci from Ras-c-Jun can be efficiently expanded and maintained as long-term growing cultures. Therefore, we suggest that a heterodimer containing c-Jun might be required for in vitro establishment of these primary mammalian cells.

Polyomavirus middle-T antigen associates with the kinase domain of Src-related tyrosine kinases

Middle-T antigen of mouse polyomavirus, an oncogenic DNA virus, associates with and activates the cellular tyrosine kinases c-Src, c-Yes, and Fyn. This interaction is essential for polyomavirus-mediated transformation of cells in culture and tumor formation in animals. To determine the domain of c-Src directing association with middle-T, mutant c-Src proteins lacking the amino-terminal unique domain and the myristylation signal, the SH2 domain, the SH3 domain, or all three of these domains were coexpressed with middle-T in NIH 3T3 cells. All mutants were found to associate with middle-T, demonstrating that the kinase domain of c-Src, including the carboxy-terminal regulatory tail, is sufficient for association with middle-T. Moreover, we found that Hck, another member of the Src kinase family, does not bind middle-T, while chimeric kinases consisting of the amino-terminal domains of c-Src fused to the kinase domain of Hck or the amino-terminal domains of Hck fused to the kinase domain of c-Src associated with middle-T. Hck mutated at its carboxy-terminal regulatory residue, tyrosine 501, was also found to associate with middle-T. These results suggest that in Hck, the postulated intramolecular interaction between the carboxy-terminal regulatory tyrosine and the SH2 domain prevents association with middle-T. This intramolecular interaction apparently also limits the ability of c-Src to associate with middle-T, since removal of the SH2 or SH3 domain increases the efficiency with which middle-T binds c-Src.

Testicular adenoma and seminal vesicle engorgement in polyomavirus large-T antigen transgenic mice

Six lines of transgenic mice harboring the cDNA for polyomavirus large-T antigen (PVLT) linked to the mouse metallothionein-1 promoter were isolated. The transgene was expressed in testes in all lines isolated and in testes and seminal vesicles in two lines. Three lines developed enlarged testes and seminal vesicles. Development of the phenotype was divided into three stages separable by age and pathology. In stage 1, birth to 6 mo, PVLT was expressed in testes but no pathology was noted; in stage 2, 6-10 mo, PVLT was expressed solely in testes and not in seminal vesicles, yet the seminal vesicles were enlarged; and in stage 3, 10 mo and older, both testes and seminal vesicles expressed PVLT and both were enlarged. Testes were up to sevenfold heavier and increased up to fourfold to fivefold in each dimension. Seminal vesicles were enlarged up to 20-fold as the result of an accumulation of seminal vesicle fluid. In addition to the four major proteins of seminal vesicle fluid, extra proteins, initially found in stage 2, were increased in stage 3 seminal vesicle fluid. The Leydig cell was the dominant cell type in affected testes; there were few or no normal Sertoli cells or seminiferous tubules remaining by stage 3. The Leydig cells were physiologically active, as indicated by a 8.5-fold higher testosterone level in sera from stage 3 affected mice compared with sera from age-matched normal males. PVLT was present in the nuclei of the Leydig cells and was able to confer an immortal phenotype in vitro. Formation of the Leydig cell adenoma was dependent on PVLT expression, but since PVLT expression occurred much earlier than did pathology, additional secondary factors must determine the delay in phenotype development.

Immortalization of primary cells by DNA tumor viruses

Cellular senescence is characterized by a decline in sensitivity to growth factors resulting in cessation of cellular growth. The expression of cellular or viral oncogenes may result in the establishment of cell lines with unlimited proliferative potential ("immortalization"). A variety of viral and cellular oncogenes have been reported to immortalize cells, suggesting that multiple mechanisms may lead to an escape from senescence. Immortalization has been reported to occur as a result of an interaction of viral proteins with cellular suppressor gene products or may result from the elevated expression of "transforming" oncoproteins (such as the polyomavirus middle-t antigen). Here we speculate that a selection for cells with a further decreased probability of cell cycle withdrawal can occur during the growth of cells expressing viral early genes, resulting in a process of tumor progression. Explaining immortalization in terms of mitogenic stimulation due to the expression of viral oncogenes followed by genetic/epigenetic changes may help to explain why lytic DNA viruses have a biological activity which may not be necessary for their life cycle.

Myristylation of pp60c-src is not required for complex formation with polyomavirus middle-T antigen

Middle-T antigen (middle-T), the transforming gene product of polyomavirus, associates with several cellular tyrosine kinases, such as pp60c-src. Complex formation leads to kinase activation and is essential for cell transformation. Middle-T-associated as well as uncomplexed pp60c-src is predominantly found in the plasma membrane. We transfected mouse 3T3 fibroblasts with a mutated c-src gene (2Ac-src), allowing the expression of a protein containing alanine instead of glycine in position 2 of the primary translation product. Contrary to the wild-type c-src gene product, pp60c-src(2A) was not myristylated and accumulated in the cytoplasm instead of being transferred to cellular membranes. The mutant protein was able to associate with middle-T and was activated similarly to the wild-type c-src gene product. Both wild-type and 2A mutant protein were membrane associated upon complex formation with middle-T. This finding suggests that the putative carboxy-terminal membrane anchor sequence of middle-T is sufficient to hold middle-T-associated pp60c-src(2A) in the plasma membrane.

Karyotypic instability and viral integration in polyoma virus-induced mouse salivary gland tumors

Stable integration of polyoma viral DNA into the host-cell genome is a prerequisite for continuous expression of the transformed phenotype. In this study we have mapped the chromosomal location of integrated viral DNA sequences in a polyoma-induced mouse salivary gland adenocarcinoma. By in situ hybridization, a major integration site was assigned to chromosome 14, band B. The combined results from in situ hybridizations to metaphase chromosomes, primary tumors, and cultured tumor cells indicate the presence of both integrated and free polyoma viral DNA in the tumors. Cytogenetically, the tumors were characterized by a pronounced karyotypic instability. No abnormal cells with the same karyotype were observed in any of the tumors. Nevertheless, it was possible to recognize a preferential pattern of chromosomes variation with certain common recurrent and sporadic deviations. Clonal gains and/or losses of single chromosomes were seen in all tumors. It is concluded from these results that karyotypic instability may play an important role in the genesis and progression of polyoma virus-induced salivary gland tumors.

Development of new cell lines for animal cell biotechnology

Mammalian cell culture has been an important technique in laboratory-scale experimentation for many decades. Developments in large-scale culture have been due to the need to grow large numbers of cells to support the growth of viruses for vaccine production, and more recently, for growing hybridoma cells as a source of monoclonal antibody. Increasingly, however, pharmaceutical products such as hormones, enzymes, growth factors, and clotting factors are being produced from cell lines which have been manipulated by recombinant DNA techniques. It is clear, therefore, that the high cost of growing mammalian cells on a large scale does not necessarily prohibit their use for biotechnology, and indeed there is considerable evidence to suggest that animal cell biotechnology will continue to be a major growth area in the future.

Successive steps in the process of immortalization identified by transfer of separate bovine papillomavirus genes into rat fibroblasts

Transfer of neor and bovine papillomavirus type 1 (BPV1) DNA into rat embryo fibroblasts led to colony formation in G418-containing medium, with no detectable background in controls with neor DNA alone. More than 50% of the drug-resistant clones could be further propagated in culture. The genetic functions of BPV1 involved in colony formation and in long-term immortalization were investigated by both translation termination mutations in the full-length genome, which inactivate individual open reading frames, and constructs in which these open reading frames were separately expressed under control of long terminal repeat promoter enhancers. Expression of either open reading frame E2 or E5 was sufficient for formation of a drug-resistant colony, but long-term growth in culture required that of E6. No significant cooperative effect was observed upon cotransfection of BPV1 and ras oncogene DNAs. Expression of the early region of the human papillomavirus type 16 also led to immortalization of rat embryo fibroblast cells in the same assay, and, unlike what was previously reported in baby rat kidney cells, it required neither activation by a heterologous promoter, nor a cooperating ras oncogene.

Expression of biologically active middle T antigen of polyoma virus from recombinant baculoviruses

Two different recombinant baculoviruses have been generated for expressing the middle T antigen (MT) of polyoma virus in insect (Sf9) cells. One (pAcI-PyMT) produces moderate levels of MT and the other (pVL-PyMT) high levels. Indirect immunofluorescence and cellular fractionation studies with pAcI-PyMT infected Sf9 cells give results similar to those observed with wild type polyoma virus infected mouse cells, and show MT to be mainly associated with cytoplasmic membranes in the insect cell. In the latter, a sub-population of MT is phosphorylated in in vitro protein kinase assays. The yields of MT from pVL-PyMT infected cells are high enough to suggest that this protein can now be produced by this method in sufficient amounts for definitive biochemical and crystallographic analyses.

Tumorigenicity, invasiveness and metastatic capability of FR3T3 rat cells before and after transfection with bovine papilloma virus type 1 DNA

Fischer rat FR3T3 cells were tested for tumorigenicity, invasive and metastatic capabilities before and after transfection, either with the entire bovine papilloma virus type 1 (BPV-1) genome or with a plasmid (pV69) containing a 69 per cent Bam H1-Hind III fragment of the BPV-1 genome as well as bacterial sequences. Cell lines were grouped as parental, pV69-transfectants, BPV-1 transfectants, in vitro derivatives, and in vivo derivatives. The tumorigenic, invasive and metastatic capabilities of these cell lines were examined in vivo through s.c., and i.p. injections of cell suspensions and through s.c. implantations of cellular aggregates into syngeneic rats. Invasiveness was tested in vitro through confrontations with embryonic chick heart fragments in organ culture. All cell lines including parental lines, were found to be invasive in vitro and tumorigenic in vivo; all tumors were invasive. It is, therefore, not possible to draw conclusions about the role of BPV-1 gene sequences in the acquisition of the invasive phenotype. Transfection with BPV-1 genes conveyed the metastatic phenotype upon parental FR3T3 cells, which were themselves found to be non-metastatic. With regards to this, no differences were found between BPV-1 transfectants compared with pV69 transfectants. Untransfected cells became metastatic also through passage in vivo as an s.c. tumor. The expression of the metastatic phenotype was not noticeably correlated with alterations of growth characteristics of the cell lines. We concluded that the implication of BPV-1 gene sequences in conveying the metastatic phenotype upon FR3T3, if any, was indirect, presumably through alterations of the host cell genome. Our experiments illustrate the need for long-term observations with parental cell lines before drawing conclusions about the role of oncogenes in the acquisition of the malignant phenotype.

Thyroglobulin gene expression as a differentiation marker in primary cultures of calf thyroid cells

A system of calf thyroid follicular cells in primary cultures has been developed to investigate the control of thyroglobulin gene expression in normal cells in vitro. In low (0.1%) serum conditions, the cells remained quiescent and formed dense aggregates surrounded by slowly spreading cells. High expression of thyroid-specific differentiation markers such as thyroglobulin (Tg) mRNA accumulation and iodide transport required the continuous exposure of cells to thyrotropin (TSH) or other adenylate cyclase activators (cholera toxin and forskolin). In the absence of TSH, Tg mRNA decreased to low but still detectable levels. Addition of TSH, forskolin or cholera toxin restored high Tg gene expression. Hydrocortisone moderately stimulated basal Tg mRNA accumulation and strongly potentiated the effect of TSH. Growth promoters including serum (1-10%), epidermal growth factor (EGF), fibroblast growth factor (FGF) and 12-O-tetradecanoylphorbol 13-acetate (TPA) induced calf thyroid cells to develop as a monolayer and inhibited both basal and TSH-stimulated expression of specialized functions. Moreover, only a partial restoration of this expression was achieved after addition of TSH or forskolin to well spread-out cells that had proliferated in response to EGF or serum. The results show that in calf thyroid cells, iodide transport and Tg gene expression are regulated by TSH through cyclic AMP; hydrocortisone potentiates this effect on Tg gene expression, while all growth promoting factors inhibit the expression of these differentiated functions.

Characterization of polyoma virus early proteins expressed from vaccinia virus recombinants

We previously reported that live recombinant vaccinia viruses (VV) encoding either the large T (LT) or middle T (MT) antigens of polyoma virus (PyV) were able to induce rejection of tumors caused by PyV-transformed cells [Lathe et al., Nature 326 (1987) 878-880]. Here we present evidence that PyV early proteins expressed by the recombinants retain the biochemical characteristics of their authentic counterparts despite the cytopathic effect of VV infection. VV-encoded LT is a nuclear phosphoprotein, with specific DNA binding, ATPase and nucleotide-binding activities. VV-expressed MT associates with cellular kinases, particularly with pp60c-src, by which it is phosphorylated in vitro. Expression levels of LT and MT reached 10(6) molecules per infected cell. The use of VV as a vector is encouraged by the high expression level obtained and because VV infection does not seem to prevent appropriate post-translational processing of proteins encoded by VV recombinants.

Mutants of polyomavirus middle-T antigen

Polyomavirus middle-T antigen induces the transformation of established cell lines in culture and is known to interact with and/or modulate the activity of several enzymes (pp60c.src, protein kinase C and phosphatidylinositol kinase) in vitro. This review is a compilation of the reported mutants of middle-T antigen and their biochemical and biological properties as they relate to the transformation event. The mutants of polyomavirus middle-T antigen have been previously classified phenotypically. Given the now large number of mutants, the classification presented here is based upon the position within the molecule. A model of middle-T is presented in which the protein is considered as consisting of three domains: a hydrophobic domain (the putative membrane-binding domain), the amino-terminal half of the molecule (the putative pp60c.src-binding domain) and the intervening amino acids (the putative modulatory domain). A current model for the induction of transformation by polyomavirus middle-T is presented.

High affinity binding of the large T protein of polyoma virus to a genomic mouse DNA sequence

We purified a fragment of mouse DNA to which the large T protein of polyoma virus was bound in chromatin prepared from transformed mouse cells. This sequence, which is not repeated to a measurable extent within the mouse genome, does not show any significant homology to the viral ori region, except in a short region, which comprises a sequence related to the consensus for recognition by large T proteins ((A,T)GPuGGC). This region of pCG4 was confirmed by in vitro binding assays to be essential for T antigen binding.

A study of mechanisms of carcinogenesis by gene transfer of oncogenes into mammalian cells

Recent work has shown that individual oncogenes can be involved in several steps of the multistage process of carcinogenesis. Evidence comes from studies on the expression of cloned oncogenes transfected into early passage mammalian cells and into immortalized non-tumorigenic cell lines. Transformation of epithelial cells in vitro with cloned cellular and viral oncogenes is of special interest since most human tumors are of epithelial origin. An important aspect of cell transformation by oncogenes is the induction of transforming growth factors (TGFs). The role of oncogenes in differentiation has been examined by introducing the human myc and mutant T24 Ha-ras1 genes into mouse erythroleukemic cells which were then induced to differentiate. In several clones differentiation was inhibited by myc or ras genes. Studies are reported using oncogenes linked to transcriptional control elements that can be regulated in vitro, such as the human metallothionein (hMT-IIA) promoter region, by cadmium and dexamethasone. Phenotypic properties of transfectants including morphological transformation, anchorage dependence and TGF release are shown to be dependent on the regulators of the hMT-IIA control region.

Maintenance of expression of differentiated function of kidney cells following transformation by SV40 early region DNA

This study describes the isolation and characterization of epithelial cell lines that maintain their differentiated phenotype following the stable integration of SV40 genes. Epithelial cells were derived from a defined location of rabbit kidney, the thick ascending limb of Henle's loop, and were co-transfected with genes from the early region of SV40 together with pSV2-neo DNA (which confers resistance to the antibiotic G418). These cells were shown to be resistant to G418, express SV40 large T-antigen and continued to express differentiated characteristics typical of cells of their origin. Such characteristics include the expression of high levels of activity of both Na,K-ATPase and the functionally important Na,K,Cl-co-transport system, the synthesis of Tamm-Horsfall glycoprotein and the presence of a barium-sensitive K+ channel on the apical membrane surface.

Regulation of cellular phenotype and expression of polyomavirus middle T antigen in rat fibroblasts

Polyoma middle T antigen (mT) was expressed in rat F-111 cells under control of the dexamethasone-regulatable mouse mammary tumor virus promoter. Graded phenotypic responses to levels of mT induction by the hormone were seen, with morphological transformation, focus formation, and anchorage-independent growth requiring increasing levels of mT expression. The ability of different clones to form tumors reflected their maximum level of induction of mT-associated kinase and their ability to grow in soft agar. Expression of transformation parameters and tumorigenicity correlates with the level of mT phosphorylated by pp60c-src in immune complexes and not with the total amount of mT determined by metabolic labeling. We suggest that cellular factors regulate mT activity by forming a kinase-active fraction of mT molecules that controls the transformed state.

Germ line transmission of autonomous genetic elements in transgenic mouse strains

Upon microinjection into fertilized mouse eggs of circular molecules of plasmid pPyLT1 carrying the gene encoding the large T protein of polyoma virus within bacterial vector sequences, autonomous circular plasmids were stably maintained in low copy numbers in transgenic strains. These plasmids could be rescued in E. coli by transfection. Integrated forms could be detected neither in somatic tissues, nor in spermatozoa. Efficiency of paternal or maternal transmission was close to 100%. The plasmids had lost or had extensively rearranged the polyoma sequences. In addition, they had acquired defined segments of genomic mouse DNA, which might be responsible for correct segregation of daughter copies at both mitosis and meiosis (centromeric function).

Polyomavirus-transformed FR 3T3 rat cells are able to form metastases in syngeneic rats

A series of polyomavirus-transformed FR 3T3 rat cell lines were tested for their tumorigenic and metastatic properties after subcutaneous inoculation of syngeneic Fisher rats. All of them grew into tumors, which appeared with variable latency periods; the TD50 varied from cell line to cell line. Eight of the 18 transformants that were inoculated gave rise to metastases, always localized in the lung. The capacity to form metastases, though at a low frequency, was also conferred on FR 3T3 cells upon transformation with a recombinant plasmid encoding only the middle-T protein. Fibroblast-like cells were predominantly observed upon histological examination of the metastases. Culture cell lines were derived from independent tumors and metastases induced by two transformants with low and high metastatic potentials, respectively. Metastasis-derived cell lines exhibited metastatic potentials similar to those of the respective original transformants. All the tumor- and metastasis-derived cell lines synthesized the same early viral polypeptides as the respective original transformants; in contrast, the viral DNA integrations evolved during tumor and metastasis formation.

Malignant transformation of rat cells by the polyomavirus middle T gene

To gain an insight into the molecular mechanism of cooperation between the polyomavirus middle T gene and cellular genes in the tumorigenic process, we have examined various properties of rat cell lines transformed by middle T alone. Middle T transformants display a phenotype ranging from nontumorigenic (flat) to fully transformed (tumorigenic) and the phenotype of a given cell line correlates very well with its cellular level of middle T antigen. Highly transformed, tumorigenic variants arise spontaneously in the flat cells during their growth with a mutation rate of 2.2 X 10(-5) per cell per generation. These variants contain elevated levels of both middle T antigen and middle T transcripts, suggesting that fully transformed cells arise as a consequence of an efficient mode of viral gene expression.

Regulation of cellular phenotype and expression of polyomavirus middle T antigen in rat fibroblasts

Polyoma middle T antigen (mT) was expressed in rat F-111 cells under control of the dexamethasone-regulatable mouse mammary tumor virus promoter. Graded phenotypic responses to levels of mT induction by the hormone were seen, with morphological transformation, focus formation, and anchorage-independent growth requiring increasing levels of mT expression. The ability of different clones to form tumors reflected their maximum level of induction of mT-associated kinase and their ability to grow in soft agar. Expression of transformation parameters and tumorigenicity correlates with the level of mT phosphorylated by pp60c-src in immune complexes and not with the total amount of mT determined by metabolic labeling. We suggest that cellular factors regulate mT activity by forming a kinase-active fraction of mT molecules that controls the transformed state.

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.