The roles of individual polyoma virus early proteins in oncogenic transformation

A paradigm for oncogene complementation in human breast cancer

Multiple cellular oncogenes are amplified in malignant tumours, and it is possible to invoke gene dosage as a possible base for product activity. In vitro data have shown that two different oncogenes can cooperate in converting a normal cell into one that is tumorigenic. This suggested that multiple cooperative alterations might be involved in cancer progression. Breast cancers have a broad spectrum of clinical behaviours ranging from highly aggressive neoplasms to almost chronic diseases. Fifty months of clinical follow-up were studied in 143 patients with primary breast cancers. Uni- and multivariate analyses were performed in order to determine any synergistic effect of amplified c-myc, erbB-2 and int-2 genes on the disease-free state and overall survival. We showed that c-myc amplification was associated with early recurrence and shorter survival; in contrast, erbB-2 and int-2 extra copies resulted in later relapse events, especially in patients whose tumours showed a normal copy number of c-myc genes. This pointed out sequential activation of complex regulatory cascades within the cell. Such particular behaviour enabled us to categorize erbB-2 and int-2 oncogenes into a group showing delayed action, in contrast to c-myc involvement in rapid spread of the tumour. As expected, co-amplified c-myc and erbB-2 genes showed positive cooperation with respect to recurrence and shortening of overall survival. Finally, the harmful effects of amplified c-myc and erbB-2 oncogenes were dramatically increased in patient subgroups showing a normal copy number of the int-2 gene. Multivariate analysis was used to evaluate disease-free survival and to test for potential interactions of oncogene covariates. It pointed out multiple independent combinations which enabled us to define complementation groups with respect to clinical patient behaviour.

A small nuclear RNA, U5, can transform cells in vitro

Low-molecular-weight RNA exhibiting transforming potential was identified in chemically induced lymphoma cells by the transformation of mink lung cells after transfection. The RNA was sequenced by the direct chemical method and was shown to be a small nuclear RNA, U5. The transforming potential of the RNA was further studied in quantitative transformation assays using 3Y1, a rat fibroblastic cell line. Transformed foci appeared with a latency of 3 to 4 weeks after transfection. U5-transformed 3Y1 cells frequently carried an amplified c-myc oncogene. In addition, U5 induced chromosome aberrations in transfected cells, indicating that the RNA acts as a clastogen. Transforming and clastogenic potentials were specifically inactivated when U5 was incubated with RNase H in the presence of a complementary oligonucleotide. We discuss a possible mechanism of U5-induced cell transformation.

Resistance to oncogenic transformation in revertant R1 of human ras-transformed NIH 3T3 cells

A flat revertant, R1, was isolated from human activated c-Ha-ras-1 (hu-ac-Ha-ras) gene-transformed NIH 3T3 cells (EJ-NIH 3T3) treated with mutagens. R1 contained unchanged transfected hu-ac-Ha-ras DNA and expressed high levels of hu-ac-Ha-ras-specific mRNA and p21 protein. Transfection experiments revealed that NIH 3T3 cells could be transformed by DNA from R1 cells but R1 cells could not be retransformed by Kirsten sarcoma virus, DNA from EJ-NIH 3T3 cells, hu-ac-Ha-ras, v-src, v-mos, simian virus 40 large T antigen, or polyomavirus middle T antigen. Somatic cell hybridization studies showed that R1 was not retransformed by fusion with NIH 3T3 cells and suppressed anchorage independence of EJ-NIH 3T3 and hu-ac-Ha-ras gene-transformed rat W31 cells in soft agar. These results suggest that the reversion and resistance to several oncogenes in R1 is due not to cellular defects in the production of the transformed phenotype but rather to enhancement of cellular mechanisms that suppress oncogenic transformation.

Cell lines established by a temperature-sensitive simian virus 40 large-T-antigen gene are growth restricted at the nonpermissive temperature

The thermolabile large T antigen, encoded by the simian virus 40 early-region mutant tsA58, was used to establish clonal cell lines derived from rat embryo fibroblasts. These cell lines grew continuously at the permissive temperature but upon shift-up to the nonpermissive temperature showed rapidly arrested growth. The growth arrest occurred in either the G1 or G2 phase of the cell cycle. After growth arrest, the cells remained metabolically active as assayed by general protein synthesis and the ability to exclude trypan blue. The inability of these cell lines to divide at the nonpermissive temperature was not readily complemented by the exogenous introduction of other nuclear oncogenes. This finding suggests that either these genes establish cells via different pathways or that immortalization by one oncogene results in a finely balanced cellular state which cannot be adequately complemented by another establishment gene.

Perturbed development of the mouse lens by polyomavirus large T antigen does not lead to tumor formation

To study how the oncogenic process may involve effects on differentiation, we overexpressed an immortalizing oncogene in a developing tissue in transgenic mice. By use of a gene fusion of the alpha A-crystallin promoter to the viral immortalizing oncogene, polyoma large T antigen (PyLT), we created transgenic mice that express PyLT specifically in ocular lens. Expression of large T antigen during embryonic development led to a perturbation in lens development, specifically, an interference with the normal program of fiber cell differentiation. This resulted in microphthalmia, which persisted throughout the life of the animal. Histological analysis revealed impairment of cell elongation, denucleation, and mitotic senescence in both primary and secondary fiber cell differentiation. Strikingly, there was no evidence for hyperplasia or for tumor development in vivo, unlike the consequences of many immortalizing oncogenes on tissues in other transgenic mice. In vitro, however, the developmentally perturbed cells derived from the transgenic lens showed high proliferative capacity. Our results suggest that a primary effect of aberrant expression of an immortalizing gene is an interference with normal tissue development; however, this interference may not necessarily induce proliferation or lead to tumor formation.

Endothelioma cells expressing the polyoma middle T oncogene induce hemangiomas by host cell recruitment

Mouse endothelioma cells expressing the polyoma middle T oncogene induced hemangiomas in a variety of species such as mice, rats, chicks, and quails. In embryos and newborn mice the hemangiomas expanded within 10-18 hr of injection, disrupting the vasculature and causing the death of the animal. In contrast, the hemangiomas formed a stable structure reminiscent of benign human hemangiomas in adult mice within 5 days. Analysis of the cells comprising the hemangioma revealed that over 95% of the endothelial cells were host derived. No induction of host cell proliferation was detected, and no endothelial mitogens were secreted by the endothelioma cells in vitro. The maintenance of the hemangioma appeared to require the continuous presence of endothelioma cells. The results indicate that these endothelioma cells act as a potent stimulating agent in the rapid formation of hemangiomas by recruiting nonproliferating host endothelial cells.

The murine p53 protein blocks replication of SV40 DNA in vitro by inhibiting the initiation functions of SV40 large T antigen

We have characterized the effect of murine p53 on SV40 DNA replication in vitro. Purified wild-type murine p53 dramatically inhibited the ability of SV40 T antigen to mediate the replication of a plasmid bearing the viral origin (ori-DNA) in vitro. In contrast, polyoma ori-DNA replication in vitro was unaffected by p53. Surprisingly, both unbound p53 and SV40 T antigen-bound p53 were equally detrimental to SV40 ori-DNA replication. Thus, p53 interferes with interactions between T antigen molecules that are required for DNA synthesis. p53 inhibited the binding to and subsequent unwinding of the SV40 origin by T antigen and thus selectively blocked the initial stages of ori-DNA replication. In contrast to the nononcogenic wild-type murine p53, high concentrations of a mutant transforming p53 failed to block SV40 ori-DNA replication in vitro. These observations may provide insight into a possible role for p53 in the cell.

Mutational analysis of polyomavirus small-T-antigen functions in productive infection and in transformation

The function of polyomavirus small T antigen in productive infection and in transformation was studied. Transfection of permissive mouse cells with mixtures of mutants that express only one type of T antigen showed that small T antigen increased large-T-antigen-dependent viral DNA synthesis approximately 10-fold. Under the same conditions, small T antigen was also essential for the formation of infectious virus particles. To analyze these activities of small T antigen, mutants producing protein with single amino acid replacements were constructed. Two mutants, bc1073 and bc1075, were characterized. Although both mutations led to the substitution of amino acid residues of more than one T antigen, the phenotype of both mutants was associated with alterations of the small T antigen. Both mutant proteins had lost their activity in the maturation of infectious virus particles. The bc1075 but not the bc1073 small T antigen had also lost its ability to stimulate viral DNA synthesis in mouse 3T6 cells. Finally, both mutants retained a third activity of small T antigen: to confer on rat cells also expressing middle T antigen the ability to grow efficiently in semisolid medium. The phenotypes of the mutants in these three assays suggest that small T antigen has at least three separate functions.

Resistance to oncogenic transformation in revertant R1 of human ras-transformed NIH 3T3 cells

A flat revertant, R1, was isolated from human activated c-Ha-ras-1 (hu-ac-Ha-ras) gene-transformed NIH 3T3 cells (EJ-NIH 3T3) treated with mutagens. R1 contained unchanged transfected hu-ac-Ha-ras DNA and expressed high levels of hu-ac-Ha-ras-specific mRNA and p21 protein. Transfection experiments revealed that NIH 3T3 cells could be transformed by DNA from R1 cells but R1 cells could not be retransformed by Kirsten sarcoma virus, DNA from EJ-NIH 3T3 cells, hu-ac-Ha-ras, v-src, v-mos, simian virus 40 large T antigen, or polyomavirus middle T antigen. Somatic cell hybridization studies showed that R1 was not retransformed by fusion with NIH 3T3 cells and suppressed anchorage independence of EJ-NIH 3T3 and hu-ac-Ha-ras gene-transformed rat W31 cells in soft agar. These results suggest that the reversion and resistance to several oncogenes in R1 is due not to cellular defects in the production of the transformed phenotype but rather to enhancement of cellular mechanisms that suppress oncogenic transformation.

Cell lines established by a temperature-sensitive simian virus 40 large-T-antigen gene are growth restricted at the nonpermissive temperature

The thermolabile large T antigen, encoded by the simian virus 40 early-region mutant tsA58, was used to establish clonal cell lines derived from rat embryo fibroblasts. These cell lines grew continuously at the permissive temperature but upon shift-up to the nonpermissive temperature showed rapidly arrested growth. The growth arrest occurred in either the G1 or G2 phase of the cell cycle. After growth arrest, the cells remained metabolically active as assayed by general protein synthesis and the ability to exclude trypan blue. The inability of these cell lines to divide at the nonpermissive temperature was not readily complemented by the exogenous introduction of other nuclear oncogenes. This finding suggests that either these genes establish cells via different pathways or that immortalization by one oncogene results in a finely balanced cellular state which cannot be adequately complemented by another establishment gene.

Multistage carcinogenesis induced by ras and myc oncogenes in a reconstituted organ

ras and myc oncogenes were able to induce distinct phenotypic alterations, resembling different types of premalignant lesions, when introduced into approximately 0.1% of the cells used to reconstitute the mouse prostate gland. While ras induced dysplasia in combination with angiogenesis, myc induced a hyperplasia of the otherwise normally developed organ. ras and myc together induced primarily carcinomas. However, tumor progression was also associated with additional genetic alterations involving gene amplification. Our data indicate that specific types of benign premalignant lesions may reflect the activation of different single oncogenes, and that the consecutive activation of multiple oncogenes could be a causal event in the step-like progression of tumorigenesis.

Separation of host range from transformation functions of the hr-t gene of polyomavirus

hr-t mutants of polyomavirus are defective in virus growth as well as in cell transformation, and have genetic alterations that invariably affect both the middle and small T proteins. We have examined the growth properties of three site-directed mutants that either eliminate or alter the middle T without affecting the small T protein. Mutant 808A encodes large and small T proteins but no middle T; it grew poorly in NIH 3T3 cells. In contrast, mutants 1387T and 1178T which express altered middle T along with normal large and small T proteins grew nearly as well as wild-type virus. Thus, although the altered middle T proteins encoded by 1387T and 1178T are defective for cell transformation, they retained the ability to induce expression of a cellular permissivity factor(s) required for virus production. At the biochemical level, the induction of permissivity by middle T was manifested primarily in terms of phosphorylation of VP1 on threonine and in efficient encapsidation of viral DNA to form infectious virus. The natural role of middle T involves regulation of phosphorylation events, and can be enacted, at least in part, independently of interactions with pp60c-src.

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.

Conditional immortalization and/or transformation of rat cells carrying v-abl or EJras oncogene in the presence or absence of glucocorticoid hormone

Early-passaged rat chondroblasts (RX cells) and embryonal fibroblasts (RE cells) are hardly transformed by transfection of activated human H-ras (EJras) or by Abelson murine leukemia virus v-abl oncogene. However, these cells were transformed by v-abl or EJras gene when dexamethasone (DX) was added in the culture medium as well as when co-transfected with retrovirus LTR-linked mouse c-myc gene. RX cell lines carrying v-abl (RXabl), RE cell lines carrying v-abl (REabl) and RX cell lines carrying EJras (RXEJ) were established from transformed colonies in the DX-added soft agar. In the absence and in the presence of DX, RXabl cells showed mortal and immortalized, REabl cells showed mortal and transformed, and RXEJ cells showed immortalized and transformed phenotypes, respectively. Especially, immortalization and transformation of REabl1 and REabl3 lines were switched on and off by addition and depletion of DX. v-abl or EJras mRNA levels in tested REabl, RXabl and RXEJ lines cultured without DX was not decreased compared to those cultured with the hormone. The above suggests that, like myc gene, glucocorticoid collaborates with v-abl or activated ras oncogene to transform unestablished rat cells and that the transformation phenotypes were determined not only by the introduced oncogene but by the cellular condition including their tissue origin. Transformation of senescent REabl cells in the absence of DX was tested by transfecting different oncogenes. Among nuclear oncogenes tested, only adenovirus 12 E1A gene could induce transformation of G0-arrested REabl cells in a cooperative fashion with the integrated v-abl gene.

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.

Cellular proteins that associate with the middle and small T antigens of polyomavirus

We have used two-dimensional gel electrophoresis to analyze in more detail the cellular proteins which associate with the middle and small tumor antigens (MT and ST, respectively) of polyomavirus. Proteins with molecular masses of 27, 29, 36, 51, 61, 63, and 85 kilodaltons (kDa) that specifically coimmunoprecipitated with MT were identified on these gels. The 36-, 51-, 61-, 63-, and 85-kDa proteins are probably the same as the proteins of similar sizes previously reported by a number of groups, whereas the 27- and 29-kDa proteins represent proteins that are heretofore undescribed. The 27- and 29-kDa proteins were abundant cellular proteins, whereas the others were minor cellular constituents. The association of each of these proteins with MT was sensitive to one or more mutations in MT that rendered it transformation defective. The association of the 85-kDa protein was the most sensitive indicator of the transformation competence of MT mutants. In addition, the 85-kDa protein was the only associated protein whose association with MT changed consistently in parallel with MT-associated phosphatidylinositol kinase activity. Furthermore, the fraction of the 85-kDa protein which was found associated with the MT complex contained 15 to 20% of its phosphate content on tyrosine. The 36- and 63-kDa proteins complexed with both polyomavirus MT and ST and comigrated on two-dimensional gels with two simian virus 40 ST-associated proteins originally described by Rundell and coworkers (K. Rundell, E. O. Major, and M. Lampert, J. Virol. 37:1090-1093, 1981). None of the other MT-associated proteins associated significantly with ST.

The E1a gene of adenovirus type 2 reduces the metastatic potential of ras-transformed rat embryo cells

We have previously demonstrated that second-passage rat embryo cells transformed by the ras oncogene alone are both tumorigenic and highly metastatic when injected into nude mice. In contrast, rat embryo cells cotransformed with the ras oncogene and the adenovirus type 2 (Ad2) E1a gene are tumorigenic but either fail to metastasize or exhibit a very low metastatic potential. In this report, we demonstrate that transfection of the Ad2 E1a gene into four independent ras-transformed rat embryo cell lines results in a dramatic reduction in metastatic potential relative to that of the parental cell line. Transfection of cDNAs for the 12S and 13S E1a transcripts showed that the 12S cDNA was highly effective in reducing the metastatic potential of ras-transformed cell lines, while the 13S cDNA showed an effect in only one of the two cell lines tested. This effect is specific to the Ad2 E1a gene, since ras-transformed cell lines expressing the Ad12 E1a gene maintained their high metastatic potential. We hypothesize that the Ad2 E1a gene may regulate the expression of one or more cellular genes that contribute to the metastatic phenotype.

Recombinant retroviruses that transduce middle T antigen cDNAs derived from polyomavirus mutants: separation of focus formation and soft-agar growth in transformation assays and correlations with kinase activities in vitro

To study correlations between cellular transformation and the biochemical properties of polyomavirus middle T antigen, middle T cDNAs have been derived from the polyomavirus mutants dl1015, dl23, and NG59b and have been introduced into rodent fibroblast cell lines by using a retrovirus vector. It was found that all three mutants are completely defective in inducing growth in soft agar but possess a range of activities in assays of focus formation on cell monolayers. Furthermore, when assays of middle T antigen-associated kinase activities were performed in vitro, a correlation between the level of associated phosphatidylinositol kinase activity and the ability of mutant middle T antigens to induce focus formation was observed. However, the association of this activity with middle T antigen does not appear to be sufficient to bring about full transformation, since the middle T antigen derived from dl1015 is completely defective for soft-agar growth but is associated with a level of phosphatidylinositol kinase activity which is comparable to that of the wild type. Therefore, some other unidentified middle T antigen function may also be required for full transformation.

Transformation of primary human fibroblast cells with human papillomavirus type 16 DNA and EJ-ras

Human papillomavirus type 16 (HPV type 16) has been implicated as an etiological agent in human cervical cancer. This virus contains sequences which, under the proper transcriptional control, can increase the tumorigenicity of established mouse cells and cooperate with EJ-ras in transforming primary baby rat kidney (BRK) cells. These data argue that this virus contains oncogenic sequences. Because this is a human virus, it was important to study the effect on primary human cells of HPV type-16 DNA in both the presence and absence of EJ-ras. We now present data which demonstrate that HPV type-16 DNA, under the transcriptional control of Moloney murine leukemia viral long terminal repeats (MoMuLV-LTR), can extend the life span of primary human fibroblast cells in culture. Co-transfection of the HPV type-16 DNA containing plasmid together with an activated EJ-ras oncogene gives rise to transformed cells which grow faster, are morphologically different from and more aneuploid than cells established with only HPV type-16 DNA. Molecular analysis of these established and partially transformed human cells reveals that they contain and express the transfected DNA. These data argue that primary human cells can be transformed with HPV type-16 and EJ-ras sequences, whereas cells harboring only HPV type 16 DNA are largely normal but have an extended life span.

Transforming but not immortalizing oncogenes activate the transcription factor PEA1

The transcription factor PEA1 (a homologue of AP1 and c-jun) is highly active in several fibroblast cell lines, compared to its low activity in a myeloma and an embryo-carcinoma (EC) cell line. Serum components are essential to attain these high levels of PEA1 activity in fibroblasts. This serum requirement is abrogated by transformation with the oncogenes c-Ha-ras, v-src and polyoma middle T (Py-MT) but not by immortalization with polyoma large T (Py-LT), v-myc, c-myc or SV40 large T (SV40T). Expression in myeloma cells of the same transforming oncogenes, as well as v-mos and c-fos, activates PEA1, whereas expression of the same immortalizing oncogenes and EIA does not. These results suggest that a common target for transforming oncogenes is PEA1. Serum components have no effect on PEA1 activity in the myeloma and EC cell lines. In contrast, retinoic acid treatment of F9 EC cells augments PEA1 activity. These results suggest that transforming oncogene expression compensates for the absence of cell type-specific factors which are required to activate PEA1. Activation of PEA1 may lead to altered transcription of a set of transformation-related genes.

Recombinant polyoma--vaccinia viruses: T antigen expression vectors and anti-tumor immunization agents

Live vaccinia virus recombinants expressing viral antigens have recently been developed as effective anti-viral vaccines. We have examined the possibility of extending this approach to specific anti-tumor immunity, using tumors induced by the polyoma virus (PyV) as a model system. Three recombinant vaccinia viruses, separately encoding the three early proteins of the polyoma virus (large, middle and small tumor (T) antigens) were constructed. Each recombinant efficiently expresses the appropriate T antigen, which exhibits biochemical properties and subcellular localization of the authentic PyV protein. The potential of the recombinants to elicit immunity towards PyV-induced tumors was assessed in rats by a challenge injection of syngeneic PyV-transformed cells. After prior immunization with the large-T or the middle-T viruses, small tumors developed, which later regressed and were eliminated in more than 50% of the animals. In contrast, the small-T virus failed to elicit tumor rejection. Established tumors could also be eliminated by curative vaccinations. No circulating antibodies directed against PyV large-T or middle-T antigens were detected in animals vaccinated with the large-T or middle-T viruses, suggesting that rejection may be due to a cell-mediated immune response.

The E1a gene of adenovirus type 2 reduces the metastatic potential of ras-transformed rat embryo cells

We have previously demonstrated that second-passage rat embryo cells transformed by the ras oncogene alone are both tumorigenic and highly metastatic when injected into nude mice. In contrast, rat embryo cells cotransformed with the ras oncogene and the adenovirus type 2 (Ad2) E1a gene are tumorigenic but either fail to metastasize or exhibit a very low metastatic potential. In this report, we demonstrate that transfection of the Ad2 E1a gene into four independent ras-transformed rat embryo cell lines results in a dramatic reduction in metastatic potential relative to that of the parental cell line. Transfection of cDNAs for the 12S and 13S E1a transcripts showed that the 12S cDNA was highly effective in reducing the metastatic potential of ras-transformed cell lines, while the 13S cDNA showed an effect in only one of the two cell lines tested. This effect is specific to the Ad2 E1a gene, since ras-transformed cell lines expressing the Ad12 E1a gene maintained their high metastatic potential. We hypothesize that the Ad2 E1a gene may regulate the expression of one or more cellular genes that contribute to the metastatic phenotype.

Transformation of NIH 3T3 cells by cotransfection with c-src and nuclear oncogenes

pp60c-src, the cellular homolog of the Rous sarcoma virus transforming protein, does not completely transform cells even when present at high levels, but has been shown to be involved in polyomavirus-induced transformation when activated by polyomavirus middle T (pmt)-antigen binding. Here we show that cotransfection, but not solo transfection, of expression plasmids for c-src and either adenovirus E1A, v-myc, c-myc, or the 5' half of polyomavirus large T (pltN) antigen into NIH 3T3 cells induces anchorage-independent growth, enhanced focus formation, and, for pltN cotransfection, tumorigenicity in adult NFS mice. Enhancement of transformation was not observed with polyomavirus small t (pst) antigen. Cotransfection of c-src with pltN induced modification of pp60c-src that altered its electrophoretic mobility and in vivo phosphorylation state and stimulated its in vitro kinase activity. Similar alterations were not seen after c-src-E1A cotransfection, suggesting that at least two different mechanisms of enhancement are involved.

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.

Regulation of the metastatic phenotype by the E1A gene of adenovirus-2

We have previously demonstrated that rat embryo cells transformed by the ras oncogene alone are both tumorigenic and highly metastatic when injected into nude mice. In contrast, rat embryo cells transformed with the ras oncogene and the adenovirus 2 (Ad2) Ela gene are tumorigenic but either fail to metastasize, or exhibit a very low metastatic potential. Here we demonstrate that transfection of the Ad2 Ela gene into several of the ras transformed rat embryo cell lines results in a dramatic reduction in metastatic potential relative to the parental cell line. Transfection of cDNAs for the 12S and 13S Ela transcripts showed that both gene products are capable of reducing the metastatic potential of the ras transformed cell lines, however the 12S cDNA was more effective. This effect is specific to the Ad2 Ela gene as ras transformed cell lines expressing the Ad12 Ela gene or the human N-myc gene maintained their high metastatic potential. We hypothesize that the Ad2 Ela gene may regulate the expression of one or more cellular genes that contribute to the metastatic phenotype.

Requirements for immortalization of primary mouse embryo fibroblasts probed with mutants bearing deletions in the 3' end of SV40 gene A

The influence of specific contiguous stretches of amino acids predominantly in the carboxy terminal third of the SV40 large T antigen on the immortalization of cells in culture was investigated. Mutants that bear either small in-phase or frameshift deletions in the large T antigen coding sequence were transfected into primary mouse embryo fibroblasts of C57Bl/6 origin (B6/MEF). The frequency of immortalization was determined as the number of colonies that developed from cells escaping senescence. The results indicated that the terminal 81 amino acids of large T antigen are not needed for efficient immortalization or tumorigenicity. In contrast removal of as few as three amino acids encoded in the vicinity of the Dde-1 site at 0.234 map units (m.u.) severely restricted immortalization, suggesting that this region of the coding sequence either structurally or functionally is essential to at least one parameter of the transformed cell phenotype. The T antigen produced by dlA2433 which bears a deletion of nine nucleotides at 0.234 m.u. fails to associate stably with the cellular protein p53. The results showed that the addition of long stretches of amino acids (96 or 97 residues) from the open reading frame at the 3' end of the early region inactivated immortalizing functions, although the addition of as many as 18 amino acids from other reading frames was not detrimental. The evidence presented also confirmed that wild-type levels of ATPase activity are not necessary for immortalization or tumorigenicity of B6/MEF. Finally, we show that one of the mutants that immortalized primary cells did not produce dense foci on a cell monolayer. This last result indicated that independent functions are required for these two parameters of the transformed cell phenotype.

Two regions of the adenovirus early region 1A proteins are required for transformation

Regions of the adenovirus type 5 early region 1A (E1A) proteins that are required for transformation were defined by using a series of deletion mutants. Deletion mutations collectively spanning the entire protein-coding region of E1A were constructed and assayed for their ability to cooperate with an activated ras oncogene to induce transformation in primary baby rat kidney cells. Two regions of E1A (amino acids 1 to 85 and 121 to 127) were found to be essential for transformation. Deletion of all or part of the region from amino acids 121 to 127 resulted in a total loss of transforming ability. An adjacent stretch of amino acids (residues 128 to 139), largely consisting of acidic residues, was found to be dispensable for transformation but appeared to influence the efficiency of transformation. Amino acids 1 to 85 made up a second region of the E1A protein that was essential for transformation. Deletion of all or part of this region resulted in a loss of the transforming activity. Even a mutation resulting in a single amino acid change at position 2 of the polypeptide chain was sufficient to eliminate transformation. Deletion of amino acids 86 to 120 or 128 to 289 did not eliminate transformation, although some mutations in these regions had lowered efficiencies of transformation. Foci induced by transformation-competent mutants could be expanded into cell lines that retained their transformed morphology and constitutively expressed the mutant E1A proteins.

Interactions between polyomavirus medium T antigen and three cellular proteins of 88, 61, and 37 kilodaltons

Affinity-purified medium T antigen of wild-type polyomavirus and dl8, a transforming mutant with a deletion in the medium T gene, is associated with three cellular proteins with apparent molecular weights of 88,000 (88K protein), 61,000 (61K protein), and 37,000 (37K protein). Medium T antigen encoded by the nontransforming hrt mutants fails to associate with these proteins, whereas medium T antigen of the nontransforming mutant dl1015 is able to do so. Medium T antigen of the nontransforming mutant dl23 binds to the 61K and 37K proteins; however, binding to the 88K protein is uncertain. The pattern of complex formation between these proteins and medium T antigen resembles that of pp60c-src and medium T antigen. The binding of medium T antigen to the 88K, 61K, and 37K proteins, as well as to pp60c-src, might represent a necessary but insufficient step in transformation. By mixing extracts from infected and uninfected cells, complex formation between medium T antigen and the 88K, 61K, and 37K proteins can be demonstrated in vitro. Pulse-chase experiments indicated that in vivo the association between medium T antigen and the 61K and 37K proteins is a slow process. The latter two proteins are probably bound to each other in uninfected cells. On two-dimensional gels of whole-cell extract, the 61K protein comigrated with a minor protein with an isoelectric point of 5.2. The 61K protein was neither phosphorylated nor glycosylated. Polyomavirus tumor serum precipitated the 61K and 37K proteins independently of medium T antigen. Therefore, the 61K protein or the 37K protein or both have the properties of a cellular tumor antigen.

Polyomavirus early region alternative poly(A) site: 3'-end heterogeneity and altered splicing pattern

The position of an alternative polyadenylation [poly(A)] site at the 3' end of the polyomavirus middle T antigen (T-Ag) coding sequences suggests the possibility of a functional role for this site in early gene regulation. The fine structure of this alternative poly(A) site was determined by cDNA sequence and 3' S1 analyses. Cleavage-poly(A) was found to be heterogeneous, occurring at multiple CA dinucleotides downstream from the AATAAA signal sequence. About 50% of the alternative poly(A) takes place upstream from the middle T-Ag stop codon. In addition, the pattern of splicing of transcripts with the alternative poly(A) site differed from that with the major poly(A) site at the end of the early region. The ratio of the small and middle T-Ag splices to the large T-Ag splice for the alternative poly(A)+ mRNAs is about 2.5 times that found for mRNAs with the major poly(A) site. The altered splicing pattern and 3'-end heterogeneity of the alternative poly(A)+ mRNAs would result in preferential translation of small T-Ag (to a greater degree) and middle T-Ag over large T-Ag at later times in the polyomavirus lytic cycle.

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.

Endothelial cell tumors develop in transgenic mice carrying polyoma virus middle T oncogene

Inoculation of newborn mice with the murine polyoma (Py) virus leads to tumor formation in a wide range of tissues. In order to investigate viral oncogenesis, we generated transgenic mice carrying either the Py large T antigen (LT) gene or the Py middle T antigen (MT) gene linked to Py early region regulatory sequences. While Py LT mice exhibit no phenotype, Py MT mice develop multifocal tumors of the vascular endothelium. These hemangiomas are lethal to the animals and can be passaged in vivo. Transgene RNAs and protein are present in both hemangiomas and the testes of these mice, and the Py middle T protein in both tissues is complexed to a cellular tyrosine kinase. The expression of complexed middle T protein in both tumorigenic endothelial cells and unperturbed testes implies that endothelial cells may be particularly susceptible to the action of the middle T oncogene. These observations indicate that Py middle T disrupts the normal strict controls on vascular growth, and suggest that Py MT transgenic mice will provide a model for studying the control of angiogenesis.

Transformation of differentiated rat hepatocytes with adenovirus and adenovirus DNA

Primary cultures of hepatocytes isolated by collagenase perfusion of adult rats were transformed by infection with adenovirus type 5 or transfection with adenovirus DNA. Total virion DNA or recombinant plasmid DNA containing the adenovirus E1A and E1B genes transformed hepatocytes at comparable frequencies. No foci of replicating hepatocytes were detected after transfection with a plasmid containing the E1A gene alone. The frequency of transformation by the adenovirus E1A and E1B genes was dependent on the composition of the culture medium. Transformation occurred at a low frequency when the transfected hepatocytes were maintained in a chemically defined medium (CDM), but the frequency was enhanced 8- to 10-fold when the cells were maintained in (i) serum-supplemented medium or (ii) CDM supplemented with epidermal growth factor. Cell lines derived from the adenovirus-transformed colonies of hepatocytes expressed adenovirus E1A and E1B RNAs. When hepatocytes were maintained in CDM supplemented with dimethyl sulfoxide and transfected with plasmids containing the E1A and E1B genes, it was possible to derive cell lines that retained the ability to express several liver-specific genes, including albumin, transferrin, hemopexin, and the third component of complement. The amount of albumin secreted per cell varied from 1 to 5 pg per cell per 24 h, and in one cell line it was below detectable levels by passage 9. Adenovirus-transformed hepatocytes were not tumorigenic when inoculated subcutaneously into neonatal syngeneic rats. We conclude that the adenovirus E1A and E1B genes are capable of transforming adult rat hepatocytes, a differentiated epithelial cell type.

Transformation of NIH 3T3 cells by cotransfection with c-src and nuclear oncogenes

pp60c-src, the cellular homolog of the Rous sarcoma virus transforming protein, does not completely transform cells even when present at high levels, but has been shown to be involved in polyomavirus-induced transformation when activated by polyomavirus middle T (pmt)-antigen binding. Here we show that cotransfection, but not solo transfection, of expression plasmids for c-src and either adenovirus E1A, v-myc, c-myc, or the 5' half of polyomavirus large T (pltN) antigen into NIH 3T3 cells induces anchorage-independent growth, enhanced focus formation, and, for pltN cotransfection, tumorigenicity in adult NFS mice. Enhancement of transformation was not observed with polyomavirus small t (pst) antigen. Cotransfection of c-src with pltN induced modification of pp60c-src that altered its electrophoretic mobility and in vivo phosphorylation state and stimulated its in vitro kinase activity. Similar alterations were not seen after c-src-E1A cotransfection, suggesting that at least two different mechanisms of enhancement are involved.

Transforming and mitogenic effects of avian leukemia virus E26 on chicken hematopoietic cells and fibroblasts, respectively, correlate with level of expression of the provirus

We have investigated the effect of E26, an avian leukemia retrovirus, on the growth properties of chicken embryo fibroblasts (CEFs). E26-infected CEFs were not transformed, according to several transformation parameters, but exhibited an activated growth in vitro. They started to grow without latency in serum-supplemented medium, maintained long-term growth in regular or low-serum medium, and could grow when seeded at low cell density in low-serum medium. We compared the integration and the level of expression of the proviral DNA in E26-infected CEFs and E26-transformed hematopoietic cells. An average of two provirus copies were found in each kind of cells. However, whereas high contents of both viral mRNA and E26-specific protein products were found in transformed hematopoietic cells, we detected only low amounts of viral mRNA and no E26 protein in infected CEFs. These data show that the level of expression of the E26 provirus is lower in CEFs than in hematopoietic cells. They suggest that transformation efficiency of the virus depends on its level of expression.

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

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

Cooperation of middle and small T antigens of polyomavirus in transformation of established fibroblast and epithelial-like cell lines

We have reported recently that small T antigen of polyomavirus stimulates the growth of NIH 3T3 cells beyond their saturation density and induces weak anchorage-independent growth (T. Noda, M. Satake, T. Robins, and Y. Ito, J. Virol. 60:105-113, 1986). We examined whether small T antigen would cooperate with middle T antigen in the in vitro transformation of NIH 3T3 (fibroblasts) and NRK-52E (epitheliallike) cells. The small-T-antigen gene, when cotransfected with the middle-T-antigen gene, had no additional effect on the efficiency or size of dense foci formation induced by the middle-T-antigen gene on a monolayer of NIH 3T3 cells. However, the small-T-antigen gene dramatically increased the rate of growth of NIH 3T3 cells transformed by middle T antigen in semisolid medium. Introduction of the small-T-antigen gene into middle-T-antigen-transformed cells did not disturb the integrated middle-T gene, alter expression of the middle-T gene, or enhance middle-T-antigen-associated tyrosine protein kinase activity. For NRK-52E cells, the expression of middle T antigen alone resulted in small, slow-growing foci on a monolayer. These cells did not show anchorage-independent growth, despite the fact that middle-T-antigen-associated tyrosine protein kinase activity was clearly detected in these cells. NRK-52E cells expressing both middle and small T antigens formed faster growing foci on a monolayer than middle-T-antigen-expressing cells did and grew in semisolid medium, even when the amounts of middle T antigen and its associated kinase activities were lower than those of middle-T-antigen-expressing cells. We conclude that small T antigen cooperates with middle T antigen in the in vitro transformation of established cell lines of fibroblast and epitheliallike cells, that it does not share the middle-T-antigen function even though they are structurally related, and that it has a significantly more important role in the transformation of NRK-52E cells than that of NIH 3T3 cells.

Expression of the v-erbA oncogene in chicken embryo fibroblasts stimulates their proliferation in vitro and enhances tumor growth in vivo

In contrast to uninfected chicken embryo fibroblasts (CEFs), CEFs infected with a retroviral vector that carries the v-erbA gene of avian erythroblastosis virus displayed new properties. These included limited anchorage-independent growth in soft agar, growth without latency in serum-supplemented medium, ability to overcome quiescence induced by serum deprivation, growth at low cell density, and an extended life span in vitro. Furthermore, when explanted in vivo onto the chorioallantoic membrane of chicken embryo, the transformed CEFs expressing v-erbA in addition to v-erbB exhibited a high proliferative rate, giving rise to fibrosarcoma tumors that were ten times larger than those developed from transformed CEFs expressing v-erbB alone. All these data show that CEFs expressing the v-erbA oncogene display activated growth and suggest that the v-erbA product interferes with the mechanisms regulating the growth and/or differentiation of primary CEFs.

Lifespan extension of basal cell nevus syndrome fibroblasts by transfection with mouse pro or v-myc genes

Dermal fibroblasts from patients with the autosomal dominant cancer-prone disease Basal Cell Nevus Syndrome (BCNS) exhibit a serum dependence, anchorage dependence and in vitro lifespan (about 20 population doublings or less) similar to those of fibroblasts from normal age-, race- and sex-matched controls. Transfection with v-myc or with an activated mouse pro-I gene (which specifies sensitivity to promotion of neoplastic transformation in JB6 mouse epidermal cells) specifically conferred partial immortality on the BCNS fibroblasts by substantially extending their population doubling levels by more than 19 population doublings. This suggests that either v-myc or pro-I gene can cooperate with BCNS gene(s) to produce an extension of lifespan or partial immortality. However, the transfected BCNS fibroblasts that escaped senescence were anchorage-dependent even after exposure to the tumor promoters 12-O-tetradecanoyl-phorbol-13-acetate (TPA), epidermal growth factor (EGF) or platelet-derived growth factor (PDGF). These observations indicate that BCNS fibroblasts differ from their normal counterparts in susceptibility to extended growth and may therefore be pre-neoplastic. It is clear that they require more than an activated pro or myc gene for progression to the tumor cell phenotype.

Biological effects of a murine retrovirus carrying an activated N-ras gene of human origin

We have introduced a genomic DNA clone of a mutated human N-ras gene from a T-cell leukemia cell line into a retroviral vector equipped with a neo resistance gene and with SV40 and pBR322 origins of replication. The helper free N-ras virus, which was recovered after transfection of the construction in the psi 2 packaging cell line, contained a correctly spliced N-ras gene. Proviral DNA was amplified in cos cells and subsequently cloned in bacteria. Nucleic acid sequence analysis of the activated N-ras gene revealed a point mutation at codon 12 resulting in a glycine to aspartic acid substitution. The N-ras virus was able to transform mouse fibroblastic cell lines, but failed to fully transform mouse primary embryo fibroblasts. MoMuLV or amphotropic 4070A pseudotypes of the virus were injected intraperitoneally into newborn mice. The MoMuLV pseudotype produced only helper-virus-induced leukemias. The amphotropic pseudotype caused fibrosarcomas after a long latent period. The results of these and other in vivo experiments are discussed in relation to known pathogenic effects of other retroviruses carrying H-ras or K-ras genes.

Tumour prevention and rejection with recombinant vaccinia

Tumour-specific antigens (TSA; ref. 1) have been exploited in the diagnosis and imaging of human cancer and anti-TSA antibodies have therapeutic potential. Vaccination with TSA or anti-idiotypic (TSA) antibodies has also been used to control tumour growth in model systems. An effective immune response nevertheless demands copresentation of antigen with host histocompatibility determinants. We therefore examined whether live vaccinia virus recombinants expressing TSA in cells of the vaccinated host might better elicit tumour immunity. Polyoma virus (PY) is tumorigenic in rodents; because killed PY-transformed cells can elicit tumour immunity, a PY-specific TSA has been postulated. Tumorigenesis involves expression of three early PY proteins, large-T (LT), middle-T (MT) and small-T (ST), but their role as TSAs is unclear. We therefore expressed the three T proteins in separate vaccinia recombinants. Rejection of PY tumours was observed in rats immunized with recombinants expressing either LT or MT. Further, tumour-bearing animals could be induced to reject their tumours by inoculation of recombinants.

Biochemical properties associated with the immortalizing domain of the large T protein of polyoma virus

A fragment of polyoma virus DNA lacking the carboxy-terminal part of the large T antigen coding region is sufficient to express the functions of the entire gene in cell transformation. Two cell lines expressing this truncated DNA were studied, one of them producing a large T-related protein of the expected size (37 kDa) and the other one, a shorter product (34 kDa). Both proteins were phosphorylated and localized in the nucleus, but devoid of ATPase and nucleotide binding activities. As the complete protein, the larger product, but not the shorter variant, exhibited sequence-specific DNA binding properties. These results indicate that ATPase and nucleotide-binding activities are not required for immortalization, and suggest that recognition of specific DNA sequences may be dispensable.

Detection of phosphotyrosine-containing proteins in polyomavirus middle tumor antigen-transformed cells after treatment with a phosphotyrosine phosphatase inhibitor

Cells transformed with the middle tumor antigen (mT) of polyomavirus were treated with sodium orthovanadate (Na3VO4), an inhibitor of phosphotyrosine phosphatases, to enhance for the detection of cellular proteins which are phosphorylated on tyrosine. Na3VO4 treatment of mT-transformed rat F1-11 cells resulted in a 16-fold elevation in the level of phosphotyrosine associated with total cellular proteins. Parental F1-11 cells displayed only a twofold increase in phosphotyrosine following Na3VO4 treatment. The abundance of phosphotyrosine in Na3VO4-treated mT-transformed F1-11 cells was twofold higher than in untreated Rous sarcoma virus (RSV)-transformed F1-11 cells and 3.5-fold lower than in Na3VO4-treated RSV-transformed F1-11 cells. Tyrosine phosphorylation of many cellular proteins, including p36, the major substrate of the RSV pp60v-src protein, was detected in Na3VO4-treated mT-transformed F1-11 cells at levels comparable to those observed in RSV-transformed cells. Some of the major protein species recognized by antiphosphotyrosine antibodies in Na3VO4-treated mT-transformed cells displayed electrophoretic mobilities similar to those detected in RSV-transformed F1-11 cells. Tyrosine phosphorylation of p36 was also detected in fibroblasts infected with polyomavirus. There was no detectable difference in the kinase activity of pp60c-src:mT extracted from untreated and Na3VO4-treated mT-transformed cells; however, Na3VO4 treatment of F1-11 and mT-transformed F1-11 cells was shown to inhibit the activity of phosphotyrosine phosphatases in a crude assay of total cellular activity with pp60v-src as the substrate. Thus, Na3VO4 treatment may allow the detection of phosphotyrosine-containing proteins in mT-transformed cells by preventing the turnover of phosphate on substrates phosphorylated by activated cellular protein-tyrosine kinases associated with mT. These results suggest that tyrosine phosphorylation of cellular proteins may be involved in the events that are responsible for mT-induced cellular transformation.

Overproduction of polyomavirus middle T antigen in mammalian cells through the use of an adenovirus vector

To overproduce biologically active polyomavirus middle T antigen, we used an adenovirus vector and human 293 cells as hosts. Two helper-independent recombinant adenoviruses were isolated that contain a hybrid transcription unit, in differing orientations, at a site in the adenovirus genome from which the E1a and most of the E1b transcription units have been deleted. The hybrid transcription unit consists of the adenovirus type 2 major late promoter and tripartite leader and a cDNA segment capable of encoding polyomavirus middle T antigen and accompanying 3' RNA-processing signals. Both recombinant viruses were stable and replicated to high titers in human 293 cells. The polyomavirus sequences were expressed, predominantly at late times after infection of 293 cells, to yield mRNAs that encoded middle T antigen. One of the recombinant viruses also expressed a middle T antigen-related protein in 293 cells. The latter was translated from one of several novel mRNA species that resulted from aberrant splicing and incomplete RNA processing of precursor RNA transcripts. Comparison of the amount of middle T antigen produced in 3T6 cells infected with polyomavirus with that in 293 cells infected with either of the recombinant adenoviruses, under optimal conditions for each system, revealed at least a 10-fold greater yield of the protein on a per-cell basis in the latter system than in the former. The recombinant-virus-encoded middle T antigen was biologically active, as evidenced by its ability to associate with and serve as a substrate for human pp60c-src. The functionality of the middle T antigen was further confirmed by demonstrating that both recombinant viruses efficiently transformed Rat-1 cells. These recombinant viruses will be useful to overproduce middle T antigen and to introduce the polyomavirus oncogene into a wide variety of mammalian cells.

Analysis of polyomavirus middle-T-antigen-transformed rat cell variants expressing different levels of pp60c-src

We characterize two independent variant cellular clones which arose following in vitro passage of polyomavirus middle-T-antigen (MTAg)-transformed FR3T3 cells expressing RNA complementary to c-src mRNA. These clones were initially flat and underwent morphologic transformation at a high frequency to a phenotype indistinguishable from that of parental MTAg-transformed FR3T3 cells. Biochemical analysis of the flat clones prior to phenotypic conversion revealed that these cells synthesized little detectable pp60c-src and had correspondingly low levels of pp60c-src protein kinase activity and MTAg-associated protein kinase activity. The flat cell clones did not possess detectable focus-forming activity, were not capable of detectable anchorage-independent growth, and had saturation densities and doubling times below those normally observed for FR3T3 cells. Following conversion of the flat clones to a shape resembling that of typical MTAg-transformed cells, the abundance of pp60c-src, pp60c-src kinase activity, and MTAg-associated in vitro protein kinase activity were all restored to the levels found in the parental MTAg transformants. These cells had growth rates, focus-forming activities, anchorage-independent growth rates, and saturation densities similar to those of the parental MTAg-transformed rat cells. These data provide additional evidence that maintenance of a transformed phenotype by polyomavirus MTAg in established rat cell lines depends, at least in part, on a minimal threshold level of pp60c-src.

Abundant expression of polyomavirus middle T antigen and dihydrofolate reductase in an adenovirus recombinant

A modular gene with a cDNA encoding the polyomavirus middle T antigen positioned behind the adenovirus type 2 major late promoter and tripartite leader was substituted for the E1a region in an adenovirus vector. Permissive human cells infected with this recombinant produce middle T protein at levels as high as those of the most abundant late adenoviral proteins, e.g., hexon or fiber. This level represents at least a 40-fold increase over that observed in a polyomavirus lytic infection of murine cells. Partial proteolytic mapping showed that this protein has the same primary structure as middle T protein produced in polyomavirus-infected murine cells. The adenovirus recombinant-generated middle T protein exhibited in vitro kinase activity, although at an approximately 10-fold-lower specific activity than that of middle T protein from polyomavirus-infected murine cells. Comparison of the expression levels of this middle T antigen-containing adenovirus vector with a similar construction encoding dihydrofolate reductase suggested that the translation efficiency of the inserted gene was dependent upon the proximity of its initiation codon to the tripartite leader. We tested this possibility by comparing three dihydrofolate reductase recombinants among which the spacing between the initiation codon and tripartite leader varied from 188 to 36 nucleotides. The efficiency of expression of dihydrofolate reductase protein dramatically increased as this spacing was reduced.

Immortalization of rat embryo fibroblasts by mutant polyomavirus large T antigens deficient in DNA binding

We have identified a putative DNA-binding domain in polyomavirus large T antigen. Mutations introduced into the gene between amino acids 290 and 310 resulted in proteins that no longer bound to the high-affinity binding sites on the polyomavirus genome, showed no detectable nonspecific DNA binding, and were not able to initiate DNA replication from the viral origin. These mutant T antigen genes were introduced into rat embryo fibroblasts together with the neomycin resistance gene to allow selection for growth in the presence of G418. All the mutations tested facilitated the establishment of these cells in long-term culture at an efficiency indistinguishable from that of the wild-type protein.