Role of the three polyoma virus early proteins in tumorigenesis

Hamster polyomavirus infection in a pet Syrian hamster (Mesocricetus auratus)

An approximately 8-week-old pet Syrian hamster (Mesocricetus auratus) with a 1-week history of dyspnea, hyporexia, and ataxia was submitted for necropsy. On gross examination, the hamster had multiple abdominal adhesions and enlargement of the mesenteric lymph node. Histologic evaluation revealed multicentric lymphoma of the liver, jejunum, mesenteric lymph node, testicular fat pad, and epididymis. Based on the hamster's age and the type and distribution of the lymphoma, a presumptive diagnosis of hamster polyomavirus-induced lymphoma was made. A specific polymerase chain reaction (PCR) was developed, which confirmed the diagnosis. An in situ PCR demonstrated hamster polyomavirus DNA within lymphocytes of the multicentric lymphoma and renal tubular epithelial cells and within clusters of enterocytes in the jejunum. These data are consistent with environmental dissemination of hamster polyomavirus virions through the renal tubular epithelium and into the urine and with fecal shedding of hamster polyomavirus virions; however, additional studies will be needed to confirm these observations.

Mechanisms of cell transformation induced by polyomavirus

Polyomavirus is a DNA tumor virus that induces a variety of tumors in mice. Its genome encodes three proteins, namely large T (LT), middle T (MT), and small T (ST) antigens, that have been implicated in cell transformation and tumorigenesis. LT is associated with cell immortalization, whereas MT plays an essential role in cell transformation by binding to and activating several cytoplasmic proteins that participate in growth factor-induced mitogenic signal transduction to the nucleus. The use of different MT mutants has led to the identification of MT-binding proteins as well as analysis of their importance during cell transformation. Studying the molecular mechanisms of cell transformation by MT has contributed to a better understanding of cell cycle regulation and growth control.

Introns enable the polyomavirus middle and small T antigens to stimulate the growth of primary rat embryo fibroblasts

We constructed spliceable vectors that separately encode polyomavirus MT and ST. The addition of an intron enables MT to transform and to immortalize more efficiently and ST to transiently stimulate the growth of primary rat embryo fibroblasts.

Primary rat cells expressing a hybrid polyomavirus-simian virus 40 large T antigen have altered growth properties

Expression of simian virus 40 (SV40) large T antigen efficiently immortalizes and transforms primary cells. We previously reported that a hybrid polyomavirus-SV40 large T antigen, PyT1-521-SVT336-708, binds to both p53 and pRb but does not transform an established rat cell line (J. J. Manfredi and C. Prives, J. Virol. 64:5250-5259, 1990). Here we show that this hybrid large T antigen is capable of immortalizing primary rat cells. Plasmids that express resistance to G418 sulfate and either SV40 large T antigen or PyT1-521-SVT336-708 were transfected into primary rat embryo fibroblasts, and cell lines were established. The cell lines that expressed PyT1-521-SVT336-708 were not fully transformed but did exhibit altered growth properties. Although these PyT1-521-SVT336-708-expressing lines did not form foci, they did grow in low serum and grew to a high saturation density; these cell lines also formed colonies in soft agar, but their colonies were much smaller than those seen with an SV40 large-T-antigen-expressing line. PyT1-521-SVT336-708 also demonstrated the ability to cooperate with activated Ha-ras to form foci on primary rat embryo fibroblasts. Surprisingly, two types of morphologies in such lines were observed: refractile and spindle shaped. Although there was no correlation between T-antigen level and morphology, all lines that displayed refractile morphology expressed high levels of p21ras. Since the p53 binding activity of PyT1-521-SVT336-708 appears to be intact, these results suggest that there are functions residing in the amino end of SV40 large T antigen which are necessary for full transformation that are missing from the amino end of polyomavirus large T antigen. Conversely, conferring the ability to bind to p53 on an amino-terminal fragment of polyomavirus large T antigen, although not enough to allow full transformation function, does increase its oncogenic activity in saturation density and soft agar growth assays.

Identification and characterization of the hamster polyomavirus middle T antigen

Hamster polyomavirus (HaPV) is associated with lymphoid and hair follicle tumors in Syrian hamsters. The early region of HaPV has the potential to encode three polypeptides (which are related to the mouse polyomavirus early proteins) and can transform fibroblasts in vitro. We identified the HaPV middle T antigen (HamT) as a 45-kDa protein. Like its murine counterpart, HamT was associated with serine/threonine phosphatase, phosphatidylinositol-3 kinase, and protein tyrosine kinase activities. However, whereas mouse middle T antigen associates predominantly with pp60c-src and pp62c-yes, HamT was associated with a different tyrosine kinase, p59fyn. The ability of HaPV to cause lymphoid tumors may therefore reside in its ability to associate with p59fyn, a potentially important tyrosine kinase in lymphocytes.

Polyomavirus large T mutants affected in retinoblastoma protein binding are defective in immortalization

To clarify the relationship between the various activities of the polyomavirus large T antigen and the contribution of this oncogene to neoplastic transformation, we constructed a series of mutants with small deletions or single-amino-acid substitutions in two separate regions of the protein. These sequences were targeted because they showed considerable similarity to conserved regions 1 and 2 of adenovirus E1A which are thought to be binding sites for the retinoblastoma gene product (pRB). The pRB-binding properties of the large T mutants were assessed with an in vitro coimmunoprecipitation assay. pRB binding was readily detected with wild-type large T, but coprecipitation was completely abolished by as little as a single amino acid substitution (Asp-141----Glu or Glu-146----Asp) in region 2 of the polyomavirus large T antigen. Mutants defective in pRB binding were unable to immortalize primary rat embryo fibroblasts, suggesting that association with pRB is an important component of immortalization mediated by polyomavirus large T. The mutations in region 1 affected pRB binding only marginally, yet some of them severely impaired immortalization, indicating that pRB binding may be essential but not sufficient for immortalization.

Mutations in polyomavirus large T affecting immortalization of primary rat embryo fibroblasts

To clarify the relationship between various functions of the polyomavirus large T antigen and the contribution of this oncogene toward neoplastic transformation, we have analyzed the properties of mutants with in-frame deletions in the second large T exon. dl45, dl96, and dl97 have retained the ability to immortalize primary rat embryo fibroblasts and to trans-activate viral promoters. dl8, dl23, and dl300, which are deficient immortalization, are also deficient in transactivation. However, a newly constructed mutant, designated dl141, which is deficient in immortalization, is still able to trans-activate both the polyoma and SV40 late promoters. This indicates that the ability to trans-activate promoters is not sufficient to confer on the large T antigen the ability to immortalize primary cells.

Chimerasome-mediated gene transfer in vitro and in vivo

Proteoliposome delivery vesicles can be prepared by the protein-cochleate method [Gould-Fogerite and Mannino, Anal. Biochem. 148 (1985) 15-25; Mannino and Gould-Fogerite, Biotechniques 6 (1988) 682-690]. Proteins which mediate the entry of enveloped viruses into cells are integrated in the lipid bilayer, and materials are encapsulated at high efficiency within the aqueous interior of these vesicles. We describe proteoliposome-mediated delivery of proteins and drugs into entire populations of cells in culture. Material can be delivered gradually by Sendai-virus-glycoprotein-containing proteoliposomes. Alternatively, synchronous delivery to a population can be achieved by exposing cell-bound influenza glycoprotein vesicles briefly to low pH buffer. When DNA is encapsulated, chimeric proteoliposome gene-transfer vesicles (chimerasomes), which mediate high-efficiency gene transfer in vitro and in vivo, are produced. Stable expression of a bovine papilloma virus-based plasmid in tissue-cultured cells, at 100,000 times greater efficiency than Ca.phosphate precipitation of DNA, with respect to the quantity of DNA used, has been achieved. Stable gene transfer and expression in mice has been obtained by subcutaneous injection of chimerasomes containing a plasmid expressing the early region of polyoma virus. In one experimental group, 50% of the mice developed tumors which were shown to express polyoma virus early proteins and contain the transferred DNA. This is the first report of stable gene transfer in animals mediated by a liposome- or proteoliposome-based system.

Mutations in polyomavirus middle T antigen affecting tumorigenesis

P155 is a polyomavirus mlt mutant with normal transforming ability but impaired tumorigenic potential. The mutation, a 12-bp deletion (nucleotides 1348-1359), removes amino acids 372 to 375 from middle T and affects its ability to function in tumorigenesis (C. Gelinas, S. Masse, and M. Bastin, 1984, J. Virol. 51, 242-246). We used deletion loop mutagenesis to introduce point mutations within the wild-type sequence spanned by the P155 deletion. A mutant phenotype resembling that of P155 could be produced by as little as one alanine to valine substitution at residue 373. The mutants were impaired in their ability to induce tumors in rats but they could still transform established cell lines or primary fibroblasts in culture. To define the biochemical defect, we examined the mutant middle T antigen both for association with pp60c-src, the cellular src gene product, as well as its pattern of phosphorylation. No obvious differences explaining the phenotype were observed. The mutant middle T associated with, and activated pp60c-src, but exhibited a slightly altered pattern of phosphorylation, presumably because of additional sites on the middle T protein.

Mechanisms of transformation by polyoma virus middle T antigen

This review addresses a fundamental question of polyoma virus biology: What is the molecular mechanism by which the polyoma virus middle T antigen (MTAg) transforms cells in culture? Since MTAg has no known intrinsic biochemical activity, it is believed to act by modulating the properties of the host cell's proteins (see review by Courtneidge [26]). Experiments to date have largely focused on the interaction of MTAg with the cellular tyrosine kinase, pp60c-src. However, recent data from a number of laboratories have demonstrated the importance of other MTAg-associating cellular proteins in MTAg-mediated transformation, including pp62c-yes and a phosphatidylinositol kinase. In this review, we will summarize what is presently known about the proteins interacting with MTAg. The extent to which the currently known details of the biochemistry of MTAg and its associated proteins can explain the transforming properties of the various mutant alleles of MTAg will be assessed.

Transfer of mlt mutations into polyomavirus intronless genomes by intramolecular recombination in bacteria

We describe a modification of the procedure of Weber and Weissmann [Nucl. Acids Res. 11 (1983) 5661-5669] for the formation of hybrid genes by in vivo recombination to introduce two separate mutations into the same gene. The mutants of interest are inserted as head-to-tail tandems in a bacterial plasmid in such a way that the 5'-proximal mutation is located upstream from the mutant with the more distal mutation. Propagation of the plasmid in a rec+ strain of Escherichia coli allows recombination between homologous sequences in the insert. DNA with the size expected for the recombinant plasmid is isolated by agarose gel electrophoresis, cloned in a recA strain, and characterized by restriction endonuclease mapping. Using this procedure, we have transferred the deletion from polyomavirus mutant dl-8 into other mutant genomes lacking the intervening sequences for either middle T or large T.

Site-directed mutagenesis of the polyomavirus genome: replication-defective large T mutants with increased immortalization potential

We used the deletion loop mutagenesis procedure to direct point mutations into a small region of the polyomavirus genome, at 0.97 map units, affecting the structure of both the middle and large T antigens. We describe the construction of six middle T mutants which have retained the ability to transform rat cells in culture and four large T mutants, three of which are deficient in viral DNA replication and capable of immortalizing primary rat embryo fibroblasts very efficiently.

Association of the polyomavirus middle-T antigen with c-yes protein

Expression of the middle-T antigen of polyomavirus is sufficient to induce transformation of fibroblasts in culture and tumour formation in whole animals. Middle-T can form a complex with the cellular src gene product (p60c-src) and can be phosphorylated by p60c-src in vitro. Studies using middle-T mutants have suggested that the association of middle-T with p60c-src may be necessary but not sufficient for transformation. Therefore, we addressed the possibility that middle-T could interact with other tyrosine protein kinases structurally related to p60c-src. Using antibody raised against a fusion protein between beta-galactosidase and amino-terminal sequences of p90gag-yes from Y73 virus (anti-yes antibody), we have found that middle-T can associate with and be phosphorylated by the c-yes proto-oncogene product, a protein of relative molecular mass (Mr) 62,000 (62K). This raises the possibility that the middle-T-p62c-yes complex contributes to transformation by polyomavirus.

Polyomavirus small t antigen: overproduction in bacteria, purification, and utilization for monoclonal and polyclonal antibody production

Polyomavirus small t antigen was purified from genetically engineered Escherichia coli and used as the immunogen for the production of polyclonal and monoclonal antibodies. A new series of plasmids for increased expression of polyomavirus T antigens or a T antigen-beta-galactosidase fusion protein was constructed by replacing sequences coding for the ribosome-binding site of previously published plasmids with a chemically synthesized sequence that has a higher degree of complementarity to the 3' end of the 16S rRNA. Cells expressing the fusion protein from the plasmid with the synthetic sequence contained 5- to 10-fold more fusion protein after a 3-h induction than did control cells. Pulse-labeling of cells bearing the new plasmids revealed that the T antigens were synthesized at high levels after induction: 10% of total synthesis for small t; 15% for Py-1387T middle T, a truncated mutant of middle T; and probably 1 to 5% for middle T. Small t and Py-1387T middle T, but not wild-type middle T, were seen as minor bands in total cell protein analyzed on sodium dodecyl sulfate-polyacrylamide gels stained with Coomassie blue. A simple, rapid procedure for purification of bacterial small t from the pellet of sonicated bacteria yielded 1 to 2 mg of small t per liter of bacterial culture at 80 to 90% homogeneity. High-titer polyclonal rabbit antisera raised against purified small t recognized all three T antigens and were suitable for immunoaffinity purification of middle T. Mouse monoclonal antibodies raised against bacterial small t were of four classes, immunoprecipitating either all three polyomavirus T antigens, small t and middle T only, primarily small t, or middle T and large T in preference to small t. One of the latter monoclonal antibodies also immunoprecipitated large T but not small t of simian virus 40, suggesting that the site recognized by this antibody may be functionally important. None of the monoclonal antibodies yielded an immunoprecipitate active in phosphorylating middle T in vitro.

Expression of the malignant phenotype in rat fibroblasts transfected with the polyomavirus transforming genes

As a step toward understanding the molecular mechanism of cooperation between viral and cellular genes in oncogenic transformation, we examined various properties of rat cells transfected with the polyomavirus transforming genes without selecting for a neoplastic phenotype. The cell lines displayed a phenotype ranging from nontumorigenic (flat) to fully transformed (tumorigenic). In the established FR3T3 cell line, acquisition of the fully transformed phenotype correlated with effective expression of the polyomavirus middle T (pmt) antigen. Flat cells carrying silent copies of pmt mutated spontaneously to the fully transformed state with a frequency of 2 to 6 X 10(-5) per cell per generation. In unestablished rat fibroblasts, simultaneous transfer of either pmt and small T or pmt and large T in the presence of the neo marker conferred only a partially transformed phenotype to most of the cell lines. The same results were obtained when wild-type genomic DNA was cotransfected with pSV2-neo. The flat transformants progressively acquired properties characteristic of fully transformed cells with passage in culture. However, in contrast to FR3T3 cells, the generation of fully transformed variants from the flat, unestablished fibroblasts was not caused by activation of pmt expression. This indicates that the functions conferred by the large and small T antigens, alone or in combination with each other, cannot substitute for all the functions expressed by the FR3T3 cell line as a result of in vitro establishment. Thus, polyomavirus-mediated transformation may require additional cellular alterations beyond the acquisition of the three viral oncogenes.

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.

Properties of cells transformed by the middle T-antigen-coding region of polyomavirus

A series of 10 Fischer rat transformed clonal cell lines were independently obtained in infections with a defective polyomavirus containing a scrambled genome except for an intact middle and small T-antigen-coding region. These cells synthesize middle and small T antigens; no fragment of large T antigen can be detected in any of them. The transformed phenotype of this set of cell lines (designated LT-) has been studied with respect to serum dependence, saturation density, and anchorage independence and compared with the phenotype of a set of six transformants (designated LT+) which synthesize detectable to high levels of shortened or normal-sized large T antigen. Both the LT+ and the LT- groups of polyomavirus transformants display a range of transformed phenotypes. These ranges overlap, and the variations within each group are larger than the variations between the two groups. Thus, the results suggest that, for established Fischer rat fibroblasts, the maintenance of any of the three phenotypes tested and, in particular, of serum independence is not necessarily correlated with the levels of large T antigen or fragments thereof.

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.

Transformation by polyoma ts-a mutants. I. Characterization of the transformed phenotype

Seven clonal lines of Fischer rat cells transformed with ts-a mutants of polyoma virus were studied. Four clones are characterized by a temperature-sensitive (ts) and three clones by a temperature-insensitive-transformed phenotype. Six clones have retained a functional though temperature-sensitive large T antigen, as judged by a 10- to 20-fold amplification of viral sequences in clones grown at low temperature compared to those grown at high temperature. No amplification is observed in one non-ts clone. As analyzed by Southern blotting, no obvious difference appears in the integration pattern of viral sequences in ts and non-ts clones concerning the number of sites of genome integration, the presence or absence of tandem repeats of the viral genome, or the absence of specific viral sequences. In autoradiograms of gel-electrophoresed immunoprecipitates, no correlation can be drawn between the amounts of either large T antigen or middle T antigen and the type of transformed state of the clones under the conditions tested. In assays of the middle T-antigen-associated kinase, no reproducible difference can be observed between the non-ts and ts clones. Finally, no correlation was observed between a temperature-insensitive phenotype and the production of an N-terminal fragment of large T antigen. Thus the molecular basis for the difference between ts-a transformants with ts or non-ts phenotypes remains elusive.

Recent progress in studies of polyomavirus tumour antigens

The polyomavirus tumour (T) antigens were originally identified by their reactivity with antisera from tumour-bearing animals. The primary structure of the three T-antigens has been established by combining the information from the nucleotide sequencing of DNA, RNA analysis, and peptide mapping. The functions of the T-antigens in productive infection and cellular transformation have largely been analysed by using virus mutants. The large T-antigen binds specifically to polyomavirus DNA. This binding is probably linked to the activity of the protein in the control of viral DNA and RNA synthesis. In addition, the large T-antigen has the ability to confer an unlimited growth potential to cells in culture. The middle T-antigen is a primary inducer of cellular transformation. The part of this protein that is located in the plasma membrane, is associated with a tyrosine kinase activity. The small T-antigen, finally, has not yet been studied extensively. However, small T-antigen has to be expressed to allow a complete productive infection cycle in mouse cells.

The major site of tyrosine phosphorylation in polyomavirus middle T antigen is not required for transformation

The induction of tumors and cellular transformation mediated by polyomavirus requires the action of middle T antigen. Accordingly, we have begun to define the domains of the viral protein important for these processes to learn more about its site and mechanism of action. One of the domains of middle T antigen which is thought to be important for its function includes a stretch of acidic amino acids and a vicinal tyrosine residue (tyrosine 315), the major site of tyrosine phosphorylation in vitro. To determine whether these acidic amino acids and tyrosine 315 are required to maintain the transforming activity of middle T antigen, we constructed deletions within the DNA sequences encoding these amino acids and measured the capacity of the resulting mutants to transform Rat-1 cells in culture. This was accomplished by using in vitro mutagenesis techniques with molecularly cloned polyomavirus DNA. Seven mutants were isolated. Five of these proved incapable of transforming Rat-1 cells and were found to contain deletions which altered the reading frame for middle T antigen. However, two mutants, pPdl1-4 and pPdl2-7, retained the capacity to transform Rat-1 cells at high frequencies. The middle T antigen encoded by one of these mutants, pPdl1-4, lacks part of the acidic string of amino acids but not tyrosine 315 (amino acids 304 through 310 are deleted), whereas the middle T antigen encoded by the other mutant, pPdl2-7, lacks the entire acidic amino acid stretch as well as tyrosine 315 (amino acids 285 through 323 are deleted). Rat-1 cells transformed by one or the other mutant DNA displayed a fully transformed phenotype, including the capacity to form tumors in animals. These results prove that the major site of tyrosine phosphorylation in middle T antigen and the acidic amino acids which precede it are not essential for its transforming activity.

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.