Transforming potential of deletion mutants of the SV40 T antigen coding gene in Syrian hamster cells

Quantitation of simian virus 40 T-antigen correlated with the cell cycle of permissive and non-permissive cells

These studies examined cell cycle progression and quantitative changes in T-antigen following infection by SV40. Single cells were assayed by multiparameter flow cytometric analysis (FCM) for DNA content and T-antigen expression. Conditions were used which permitted permissive, semi-permissive, and non-permissive cells to be monitored through two rounds of DNA synthesis induced by SV40. The permissive cells included the monkey kidney cell lines; CV-1, Vero and BSC-1 and the COS-1 and COS-7 which are CV-1 cells transformed with an origin defective SV40. The non-permissive cell strains included mouse embryo fibroblasts, Chinese hamster fibroblasts, and IMR-90, a human diploid fibroblast. Cell types differed in the maximal amount of T-antigen expressed per cell. Additionally, all cell types expressed a limited quantity of T-antigen for each cell cycle phase and the quantity increased in each successive phase. The level in each phase was increased only two-fold when 100 times more virus was used. Thus, for an infected population the quantity of T-antigen was dependent on cell cycle distribution. High levels of T-antigen were not required for permissive infection; however, permissive cells were distinguished from non-permissive cells by the G2 levels. Permissive G2 cells had more than double the T-antigen content expressed in G1, while nonpermissive G2 cells had less than a two-fold increase over G1 levels. The appearance of cells with tetraploid DNA content and the failure to undergo mitosis correlated to the higher T-antigen levels in the G2 of the permissive cells. Two other strains of SV40, 776, and VA45 exhibit similar values for T-antigen expression and movement into tetraploid DNA content. This study establishes the levels of T-antigen correlated to the cell cycle and cell type.

Expression of growth-regulated genes in normal and SV40 transformed hamster fibroblasts

Transformation by the oncogenic virus SV40 has been shown to alter the expression of cellular genes at the level of RNA abundance. Many of these genes have yet to be identified. We have determined, by Northern blot analysis, the abundance levels of several growth-regulated genes in SV40-transformed cell lines to determine if their expression is altered and correlates with the ability of SV40 transformed cells to grow in low serum containing media. The mRNA abundance levels of the G1-specific genes 2A9/calcyclin, 2F1/translocase, and 4F1/vimentin were determined in the parental hamster fibroblast cell line, tk-ts13, and in two SV40 transformants, HR5 and HR8 cells, grown in medium containing 10% calf serum (normal medium) and in HR5 and HR8 cells adapted to passage in medium containing low serum. A spontaneous transformant of the parental line capable of growth in low serum in the absence of SV40 transformation (tk-ts13/1%), was also included in these studies. The low serum adapted SV40-transformed cells and the spontaneous tk-ts13 transformed cells grew more vigorously than their nonadapted counterparts in medium containing low serum. The low serum adapted cells also grew to higher saturation densities in low serum and to densities comparable to those in high serum, whereas the nonadapted cells grew to low saturation densities in low serum, but not as low as the untransformed parental.(ABSTRACT TRUNCATED AT 250 WORDS)

Growth suppression induced by wild-type p53 protein is accompanied by selective down-regulation of proliferating-cell nuclear antigen expression

The p53 gene is a frequent target of mutation in a wide variety of human cancers. Previously, it was reported that conditional expression of wild-type p53 protein in a cell line (GM47.23) derived from a human glioblastoma multiform tumor had a negative effect on cell proliferation. We have now investigated the effect that induction of wild-type p53 protein in this cell line has on the expression of the proliferating-cell nuclear antigen gene. The proliferating-cell nuclear antigen gene encodes a nuclear protein that is an auxiliary factor of DNA polymerase delta and part of the DNA replication machinery of the cell. We show that inhibition of cell cycle progression into S-phase after induction of wild-type p53 protein is accompanied by selective down-regulation of proliferating-cell nuclear antigen mRNA and protein expression.

Human papovavirus BK early gene regulation in nonpermissive cells

The human papovavirus BK latently infects a majority of the population worldwide, and its DNA has been found in human tumor tissue. BKV is known to be highly oncogenic in rodents, and is capable of transforming cells in vitro. Rearrangements in the transcriptional regulatory sequences controlling expression of the transforming early gene, T antigen, are known to affect both the tumorigenic and transforming properties of this virus. Little is known about the mechanism by which this occurs. We have examined several aspects of BKV early promoter/enhancer regulation in cell types which the virus transforms, baby hamster kidney (BHK) and newborn rat kidney (NRK) cells, and compare them to the same processes in monkey kidney CV1 cells. We find that BKV early transcriptional efficiency requires the same enhancer repeat elements in all three cell types, but that requirements for sequences to the early and late side of these repeats vary between these cells. While the BKV T antigen was found to repress early gene expression from the BKV early promoter in CV1 cells, this effect was lower in BHK cells and essentially absent in NRK cells. The impaired autoregulation observed in rodent cells may be the result of inefficient T antigen production in these cells. DNA replication from the BKV origin was not detected in either BHK or NRK cells. Finally, we find no correlation between the efficiency of the BKV early regulatory region in directing gene expression and the ability to transform NRK cells.

Growth-related expression of a 72,000 molecular weight poly(A)+ mRNA binding protein

In this communication, we have studied a 72,000 mol w (p72) host protein which reacts with a mouse monoclonal antibody (PAb6) directed against antigenic determinants on the Simian virus 40 (SV40) large T antigen protein that map 5' of 0.42 map units on the viral genome. The p72 protein is an abundant basic (pI greater than 7) cytoplasmic protein found in both SV40-transformed and untransformed parental cells and in cell lines derived from normal human and tumor tissue. By two-dimensional gel analysis and Western blot analysis the p72 protein identified by PAb6 is indistinguishable from the 72,000 mol w protein PABP associated with the poly(A)+ tract of cytoplasmic messenger RNA molecules. In normal human peripheral blood mononuclear cells stimulated to proliferate with the T-cell-specific mitogenic lectin phytohemagglutinin the synthesis and cytoplasmic accumulation of p72 occurs very early during the G0----G1-phase transition. The p72 protein is also expressed in proliferating and differentiated human promyelocytic HL60 cells indicating that the expression of this protein is not strictly limited to cycling cells.

Characterization of the in vitro interaction between SV40 T antigen and p53: mapping the p53 binding site

An efficient in vitro system for generating soluble complexes between simian virus 40 T antigen and the cellular protein p53 was developed. A p53 cDNA was inserted 3' to the SP6 promoter in pGEM-1 (Promega-Biotec) and transcribed by SP6 polymerase. In vitro translation of the cRNA generated p53 which was immunoprecipitable with all five monoclonal antibodies tested (PAb122, PAb421, PAb242, PAb246, and PAb248). The p53 sedimented at about 8-10 S in sucrose gradients, possibly corresponding to a tetramer. T-antigen-p53 complexes were produced by the addition of immunoaffinity-purified T antigen to p53-containing translation lysates. Equivalent amounts of p53 were immunoprecipitated with the anti-T-antigen antibodies PAb416, PAb419, and PAb101, suggesting that in vitro made p53 complexed mostly to a population of T-antigen molecules that had matured at least 15 min in the cell. The complexes sedimented at 18-20 S in sucrose gradients. In order to map the p53 binding site on T antigen, p53 was complexed in vitro to labeled proteolytic fragments of T antigen. A 46K fragment, spanning residues 131-517, was immunoprecipitated with the anti-p53 monoclonal PAb122 and therefore is likely to contain the p53 binding site. This region contains T-antigen sequences necessary for the efficient transformation of nonpermissive cells and for the induction of cellular rRNA synthesis. It also contains the binding sites for DNA polymerase alpha and ATP. We suggest a possible role for T-p53 complexes in T-antigen-associated functions.

Regulation of the expression of the SV40 T-antigen coding gene under the control of an rDNA promoter

We have constructed a hybrid gene in which the SV40 T-antigen coding gene is driven by a mouse rDNA promoter and we have compared its expression to that of an SV40 T-antigen coding gene under the control of its own promoter. The comparison has been carried out in microinjected cells, in transfected cells, and in stable cell lines carrying the respective T-antigen coding genes in an integrated form. These cell lines were derived from ts AF8 cells, a mutant which is temperature sensitive for RNA polymerase II activity. The hybrid gene clearly expresses T-antigen, albeit less efficiently than when the T antigen coding gene is under the control of the SV40-promoter. We also show that the expression of T-antigen by the hybrid gene is 50% inhibited by an antibody against RNA polymerase I. In tsAF8 cells carrying the hybrid gene, T-antigen is still expressed at the restrictive temperature (where RNA polymerase II is inactive) at a level again about 50% of controls. However, our findings also confirm those of Smale and Tjian (Mol. Cell. Biol. 5:352, 1985) that such hybrid genes are in part transcribed by RNA polymerase II and generate abnormal transcripts.

Differential requirement for SV40 early genes in immortalization and transformation of primary rat and human embryonic cells

A series of recombinant plasmids carrying various DNA fragments of SV40 early region were used to test for their ability to immortalize primary cultures of rat embryo (RE) and human embryonic kidney (HEK) cells. When primary RE cells were transfected with plasmids containing an entire early region of wild-type SV40- or a deletion mutant in the small tumor (t) antigen, dl 1410-DNA, they were all immortalized. The immortalized cells could grow in soft-agar medium and produced large tumor (T)-antigen. Cultured RE cells transfected with pW2-t, which contains a deletion in the large-T-specific coding region, also gave rise to continuous cell lines. Interestingly, two of nine RE lines immortalized by pW2-t could also grow in soft-agar medium. The plasmid pW-t8 carrying a similar fragment of SV40 DNA as pW2-t, but lacking the processing and polyadenylation signal sequences, also immortalized RE cells. Surprisingly, the plasmid pD-t1 which contains neither the intact large-T nor the small-t function also immortalized RE cells. However, the RE lines immortalized by pW-t8 or pD-t1 were unable to grow in soft-agar medium and displayed a wide range of growth phenotypes. On the contrary, when primary HEK cells were used for immortalization experiments, only those SV40 plasmids carrying the intact large-T function were able to generate immortalized lines. The growth properties of these immortalized HEK lines can be categorized into two groups. Those HEK lines immortalized by the large T alone grew slightly denser and rounder than their parental normal HEK cells, while those immortalized by both the large-T and small-t antigens grew extremely fast, reached higher density, piled up on each other, and were anchorage independent. In addition, when these SV40 plasmids were used to directly transform primary HEK cells by the focus assay, the large-T clone, pD3-05, only transformed HEK cells to form light foci. Transfection by the large-T plus the small-t sequences either in cis or in trans, did increase the frequency of focus formation, and gave rise to dense foci which could grow in soft-agar medium.

Deletion of 43 amino acids in the NH2-terminal half of the large tumor antigen of simian virus 40 results in a non-karyophilic protein capable of transforming established cells

We have characterized a simian virus 40 (SV40) mutant, derived from the viral DNA insertion present in simian cell transformants, which carries a deletion affecting the NH2-terminal region of the SV40 large tumor antigen. This mutant protein is 6% smaller than normal, has lost the typical nuclear localization of the SV40 large tumor antigen, and accumulates in the cytoplasm. The deletion begins at nucleotide position 4490 of the SV40 DNA and ends in-frame at nucleotide position 4362. The missing 43 amino acids begin with proline-110 and end with serine-152 of the predicted sequence; they include a cluster of basic residues, presumably important for the viral origin-DNA binding, and most of the phosphorylation sites present in the NH2-terminal half of the molecule. The protein can still be phosphorylated considerably in vivo. This mutant viral genome is replication-defective but has conserved the competence to transform established cells, such as NIH/3T3 cells. Transfection of cloned mutant DNA into such cells resulted in the production of full transformants. Full transformants were not produced in similar transfections carried out in primary rat embryo fibroblasts, although some primary transfectants expressing the non-karyophilic large tumor antigen might be considered minimally transformed.

Demonstration of T antigens on the surface of cells transformed with primate polyoma viruses

Primate polyoma virus-transformed hamster, mouse, and rat cell lines were examined by indirect immunofluorescence staining for cell surface-associated T antigens, by using a rabbit antiserum prepared against sodium dodecyl sulfate-denatured large T antigen of simian virus 40 (anti-SV40-SDS-T serum). Positive surface staining was shown not only on SV40-transformed cells, but also on BK and JC virus-transformed cells. In contrast, normal cells and cells transformed with mouse polyoma-, human adeno-, and murine sarcoma viruses were negative. The data on SV40-transformed cells confirmed the reports of others demonstrating the cell surface location of SV40 large T antigen, and the data on BK and JC virus-transformed cells proved that these cells have cell-surface T antigens that cross-react with anti-SV40-SDS-T serum.