Monoclonal antibody to simian virus 40 small t

No Implication of Simian virus 40 in pathogenesis of Malignant Pleural Mesothelioma in Slovenia

Background and aim

Malignant mesothelioma is predominantly caused by asbestos exposure, although the association of Simian virus 40 in its pathogenesis is currently still under debate. Simian virus 40, a DNA rhesus monkey virus with oncogenic properties, accidentally contaminated early batches of polio vaccine in the 1960s. In the 1990s, viral sequences and proteins were discovered in several human tumors, which triggered research to find a link between Simian virus 40 and human cancers, especially malignant mesothelioma. The aim of our study was to establish an effective laboratory procedure for Simian virus 40 detection and to investigate the presence of Simian virus 40 DNA and small t antigen in mesothelioma samples from Slovenian patients.

Methods and study design

Paraffin-embedded malignant pleural mesothelioma specimens from 103 Slovenian patients were collected and used for total DNA isolation and real-time polymerase chain reaction for Simian virus 40 small t and large T DNA analysis. Special attention was devoted to primer design, good laboratory practice and polymerase chain reaction contamination prevention. Polymerase chain reaction products were sequenced and BLAST aligned. One 5 μm thick paraffin section from each patient's tissue block was stained with hematoxylin and eosin for histological typing and one for immunohistochemical detection of Simian virus 40 small t antigen using a monoclonal antibody against Simian virus 40 (Pab280). SV40-expressing Wi-38 cells were used as positive control in both PCR and immunohistochemistry.

Results

In real-time polymerase chain reaction analyses, only 4 samples gave products with primer pairs amplifying small t antigen and were inconsistent and poorly reproducible. BLAST alignment showed no homology with any deposited SV40 sequences. No immunopositive staining for SV40 small t antigen was found in any of the samples.

Conclusions

We found no evidence of SV40 presence in tissue samples from 103 Slovenian patients with malignant pleural mesothelioma. Asbestos exposure remains the main risk factor for malignant pleural mesothelioma in Slovenia. Free full text available at www.tumorionline.it

SV40: Cell transformation and tumorigenesis

The story of SV40-induced tumorigenesis and cellular transformation is intimately entwined with the development of modern molecular biology. Because SV40 and other viruses have small genomes and are relatively easy to manipulate in the laboratory, they offered tractable systems for molecular analysis. Thus, many of the early efforts to understand how eukaryotes replicate their DNA, regulate expression of their genes, and translate mRNA were focused on viral systems. The discovery that SV40 induces tumors in certain laboratory animals and transforms many types of cultured cells offered the first opportunity to explore the molecular basis for cancer. The goal of this article is to highlight some of the experiments that have led to our current view of SV40-induced transformation and to provide some context as to how they contributed to basic research in molecular biology and to our understanding of cancer.

Phosphorylcholine is favorable for antibody production from hybridoma cells

Growth of antibody-secreting hybridomas requires special conditions such as serum-free defined media containing growth factors and vitamins. However, the surface on which these cells can proliferate has been shown to play an important role. Phosphorylcholine (PC)-based polymers are zwitterionic compounds with nonbiofouling properties. These polymers are characterized by having reduced protein absorption properties. Our aim was to determine whether well-established hybridoma cell lines were able to proliferate and produce measurable amounts of monoclonal antibodies when grown on PC-polymer-coated surfaces. Comparative experiments using four well-known hybridoma cell lines (PAb421, PAb246, PAb1801 which recognize p53, and PAb280 which recognizes SV40 small t antigen) grown on PC-polymer-coated, uncoated, and two commercially available tissue culture plates showed that PC-polymer-coated plates were more efficient than uncoated plates in sustaining cell growth and monoclonal antibody production/secretion as defined by growth assays and ELISA. Also, results demonstrated that PC-polymer-coated plates were able to perform better than commercially available plates. These observations suggest that PC polymers could be used as an alternative, efficient surface coating to grow hybridoma cell lines and allow detectable antibody secretion.

MDM2 is a target of simian virus 40 in cellular transformation and during lytic infection

Phosphopeptide analyses of the simian virus 40 (SV40) large tumor antigen (LT) in SV40-transformed rat cells, as well as in SV40 lytically infected monkey cells, showed that gel-purified LT that was not complexed to p53 (free LT) and p53-complexed LT differed substantially in their phosphorylation patterns. Most significantly, p53-complexed LT contained phosphopeptides not found in free LT. We show that these additional phosphopeptides were derived from MDM2, a cellular antagonist of p53, which coprecipitated with the p53-LT complexes, probably in a trimeric LT-p53-MDM2 complex. MDM2 also quantitatively bound the free p53 in SV40-transformed cells. Free LT, in contrast, was not found in complex with MDM2, indicating a specific targeting of the MDM2 protein by SV40. This specificity is underscored by significantly different phosphorylation patterns of the MDM2 proteins in normal and SV40-transformed cells. Furthermore, the MDM2 protein, like p53, becomes metabolically stabilized in SV40-transformed cells. This suggests the possibility that the specific targeting of MDM2 by SV40 is aimed at preventing MDM2-directed proteasomal degradation of p53 in SV40-infected and -transformed cells, thereby leading to metabolic stabilization of p53 in these cells.

Cooperation of simian virus 40 large and small T antigens in metabolic stabilization of tumor suppressor p53 during cellular transformation

Metabolic stabilization of the tumor suppressor p53 is a key event in cellular transformation by simian virus 40 (SV40). Expression of the SV40 large tumor antigen (large T) is necessary but not sufficient for this process, as metabolic stabilization of p53 complexed to large T in abortively SV40-infected cells strictly depends on the cellular systems analyzed (F. Tiemann and W. Deppert, J. Virol. 68:2869-2878, 1994). Comparative analyses of various cells differing in metabolic stabilization of p53 upon abortive infection with SV40 revealed that metabolic stabilization of p53 closely correlated with expression of the SV40 small t antigen (small t) in these cells: 3T3 cells do not express small t and do not stabilize p53 upon infection with wild-type SV40. However, ectopic expression of small t in 3T3 cells provided these cells with the capacity to stabilize p53 upon SV40 infection. Conversely, precrisis mouse embryo cells express small t and mediate metabolic stabilization of p53 upon infection with wild-type SV40. Infection of these cells with an SV40 small-t deletion mutant did not lead to metabolic stabilization of p53. Small-t expression and metabolic stabilization of p53 correlated with an enhanced transformation efficiency by SV40, supporting the conclusion that at least part of the documented helper effect of small t in SV40 transformation is its ability to promote metabolic stabilization of p53 complexed to large T.

Multiple, distinct trans-activation functions are encoded by the simian virus 40 large T and small t antigens, only some of which require the 82-residue amino-terminal common domain

Simian virus 40 (SV40) small t and large T antigens can each trans activate the adenovirus (Ad) E2A and the Ad VA-I promoters. The first 82 amino acids of large T and small t are identical. However, this large T-small t common domain between residues 1 and 82 does not trans activate, suggesting that large T and small t each encode separate trans-activation functions. To determine whether the large T or small t unique domains, which are required for trans activation of the E2A promoter, are sufficient for this activity, we have employed expression plasmids separately encoding the common and unique domains of large T and small t. Cotransfection of a large T unique domain expression plasmid efficiently trans activated the E2A promoter. Optimal trans activation by large T required the motif that binds cellular proteins such as the retinoblastoma gene product, which is located in the large T unique domain, and additional large T structures outside this motif. In contrast, the small t unique domain did not trans activate the E2A promoter. Experiments utilizing E2A promoter mutants containing only the ATF- or EIIF-binding sites demonstrated that trans activation by small t involves only the EIIF transcription factor and that this function requires both the common (residues 1 to 82) and the small t unique domains expressed as a colinear protein. trans activation by large T, in contrast, involves at least three mechanisms. There appear to be at least two mechanisms that involve the EIIF transcription factor, at least one of which does not require the common domain (residues 1 to 82) and one mechanism that involves the ATF factor and does require both the common and the large T unique domains.

pEXPRESS: a family of expression vectors containing a single transcription unit active in prokaryotes, eukaryotes and in vitro

We have constructed a family of expression vectors containing a single transcription unit that is active in Escherichia coli, eukaryotic cells, and in coupled in vitro transcription-translation systems. These vectors use the Rous sarcoma virus-long terminal repeat (RSV-LTR) as the promoter/enhancer for eukaryotic cells. In vitro transcription is made possible by inclusion of a bacteriophage T7 promoter. This same promoter is actively transcribed in E. coli that produce T7 RNA polymerase. Other features of this transcription unit include a high-efficiency eukaryotic translation start codon, a phage f1 origin of DNA replication for site-directed mutagenesis and a three-frame stop codon that facilitates C-terminal deletion mutagenesis. We term this vector family, pEXPRESS.

Protein phosphatases and DNA tumor viruses: transformation through the back door?

Cellular transformation by many oncogenic viruses is mediated by alterations in signal transduction pathways that control normal growth and proliferation. Common targets for many transforming viruses are pathways regulated by protein phosphorylation. The biochemical control of proteins in these pathways is a dynamic process that is regulated by the relative activities of protein kinases and phosphatases. Although there are numerous examples of viral oncogenes that encode protein kinases (Hunter, 1991), until recently there has been no evidence linking altered phosphatase activity to transformation. In this review we describe a novel mechanism, utilized by small DNA tumor viruses, in which viral oncogenes bind to and regulate a cellular protein serine/threonine phosphatase. The currently available evidence indicates that alteration of phosphatase activity and subsequent changes in phosphorylation levels is an important step in transformation by these viruses.

Modifications to Thylakoid Composition during Development of Maize Leaves at Low Growth Temperatures

The effects of reductions in growth temperature on the development of thylakoids of maize (Zea mays var LG11) leaves are examined. Thylakoids isolated from mesophyll cells of leaves grown at 17 degrees and 14 degrees C, compared with 25 degrees C, exhibited a decreased accumulation of many polypeptides, which was accompanied by a loss of activity of photosystems (PS) I and II. Probing the polypeptide profiles with a range of antibodies specific for thylakoid proteins demonstrated that a number of polypeptides encoded by the chloroplast genome failed to accumulate at low temperatures. Although thylakoid protein synthesis was reduced severely at 14 degrees C compared with 25 degrees C, major synthesis of both chloroplast and nuclear encoded polypeptides was detected. It is suggested that the lack of accumulation of some thylakoid proteins at low temperatures may be due to an inability to stabilize the proteins in the membranes. A number of thylakoid polypeptides were found to appear as the growth temperature was decreased. Analyses of pigments and polypeptides demonstrated that decreases in the photosystem reaction center core complexes occur relative to the light harvesting complex associated with PS II at reduced growth temperatures. Differential effects on the development of PSI and PSII were also observed, with PSII activity being preferentially reduced. Reductions in PSII content and activity occurred in parallel with decreases in the quantum yield and light-saturated rate of CO(2) assimilation. Fractionation of thylakoid pigment-protein complexes showed that the ratio of monomeric:oligomeric form of the light harvesting complex associated with PSII increased at low growth temperature, which is consistent with a chill-induced modification of thylakoid organization. Many, but not all, of the characteristic changes in thylakoid protein metabolism, which were observed when leaves were grown at low temperatures in controlled environments, were identified in leaves of a field maize crop during the early growing season when low temperatures were experienced by the crop. Chill-induced perturbations of thylakoid development can occur in the field in temperate regions and may have implications for the photosynthetic productivity of the crop.

Serotonin uptake, storage, and synthesis in an immortalized committed cell line derived from mouse teratocarcinoma

We report the isolation and characterization of a serotoninergic cell line, 1C11, derived from a mouse teratocarcinoma. The clone 1C11 was immortalized through the expression of the simian virus 40 oncogenes. 1C11 presents two states: an immature epithelial-like state (1C11 precursor) and a more differentiated state (1C11). After induction by dibutyryl cyclic AMP and cyclohexanecarboxylic acid, almost 100% of 1C11 cells continue to divide and have acquired a neural-like phenotype. 1C11* cells coexpress several neural markers, such as synaptophysin (the membrane constituent of synaptic vesicles), the neuropeptide [Met5]enkephalin, and the neurotransmitter serotonin. 1C11* cells store endogenous serotonin and are able to synthesize serotonin from L-tryptophan and to catabolize it by monoamine oxidase B. Moreover, the cells take up serotonin by a carrier-mediated mechanism very similar to that of serotoninergic neurons. The expression of the simian virus 40 oncogenes, which promoted immortalization, does not therefore prevent further differentiation. This inducible cell line constitutes a valuable model for cellular and molecular studies concerning the physiology and the pharmacological modulation of the serotoninergic phenotype.

An immortalized osteogenic cell line derived from mouse teratocarcinoma is able to mineralize in vivo and in vitro

The hybrid plasmid pK4 containing the early genes of the simian virus SV-40, under the control of the adenovirus type 5 E1a promoter, was introduced into the multipotent embryonal carcinoma (EC) 1003. Expression of the SV-40 oncogenes was observed at the EC cell stage, and this allowed the derivation of immortalized cells corresponding to early stages of differentiation. Among the immortalized mesodermal derivatives obtained, one clone, C1, is committed to the osteogenic pathway. C1 cells have a stable phenotype, synthesize type I collagen, and express alkaline phosphatase activity. Although immortalized and expressing the SV-40 T antigen, the cells continue to be able to differentiate in vivo and in vitro. In vivo, after injection into syngeneic mice, they produce osteosarcomas. In vitro, the cells form nodules and deposit a collagenous matrix that mineralizes, going to hydroxyapatite crystal formation, in the presence of beta-glycerophosphate. This clonal cell line, which originates from an embryonal carcinoma, therefore differentiates into osteogenic cells in vivo and in vitro. This immortalized cell line will be useful in identifying specific molecular markers of the osteogenic pathway, to investigate gene regulation during osteogenesis and to study the ontogeny of osteoblasts.

Monoclonal antibody analysis of simian virus 40 small t-antigen expression in infected and transformed cells

The monoclonal antibody PAb280 binds to small t antigen but not to large T antigen. Its binding site within the unique region of small t antigen was localized by studying its reaction with simian virus 40 mutants, other papovaviruses, and bacterial expression vectors coding for fragments of small t antigen. The antibody was used to define the cellular location of small t antigen by immunocytochemistry and by immunoprecipitation of subcellular extracts of infected cells. PAb280 reacts strongly with a cytoplasmic form of small t antigen that appears to be associated with the cytoskeleton and is not detected by antibodies directed to the common N terminus of small t and large T antigens. Immunoperoxidase staining of cells infected by the simian virus 40 defective strain SV402 with PAb280 and other anti-T antibodies demonstrated that this virus produced an N-terminal fragment of large T antigen as well as small t antigen. In cells infected by the virus, this fragment was located in the cell nucleus but was very unstable. These results suggest that the activity of the SV402 virus in transformation assays may not be entirely due to the action of small t antigen alone.

A mutant SV40 large T antigen interferes with nuclear localization of a heterologous protein

A mutant SV40 genome carrying a frameshift at the carboxyl terminus of the large T antigen failed to replicate SV40 DNA and to transform rat2 cells, although the altered region is known to be dispensable for these functions. The mutant T antigen also failed to localize normally in the nucleus and interfered with nuclear localization of at least one other nuclear protein, adenovirus fiber. A double mutant carrying an additional lesion in the nuclear localization signal was also localized in the cytoplasm, but regained the ability to transform rat2 cells and no longer affected the nuclear localization of fiber protein. We suggest that the frameshift T antigen may disrupt a mechanism required for nuclear localization of proteins.

Mutations in the phosphorylation sites of simian virus 40 (SV40) T antigen alter its origin DNA-binding specificity for sites I or II and affect SV40 DNA replication activity

A series of mutants of simian virus 40 was constructed by oligonucleotide-directed mutagenesis to study the role of phosphorylation in the functions of large T antigen. Each of the previously mapped phosphorylated serine and threonine residues in large T antigen was replaced by an alanine or cysteine residue or, in one case, by glutamic acid. Mutant DNAs were assayed for plaque-forming activity, viral DNA replication, expression of T antigen, and morphological transformation of rat cells. Viable mutants were isolated, suggesting that modification of some residues is not essential for the biological functions of T antigen. Two of these mutants replicated more efficiently than did the wild type. Seven mutants were partially or completely deficient in viral DNA replication but retained cell transformation activity comparable with that of the wild-type protein. Biochemical analysis of the mutant T antigens demonstrated novel origin DNA-binding properties of several mutant proteins. The results are consistent with the idea that differential phosphorylation defines several functional subclasses of T-antigen molecules.

Characterization of a 31 kDa polypeptide that accumulates in the light-harvesting apparatus of maize leaves during chilling

A 31 kDa polypeptide that accumulates in the thylakoids when maize leaves are chilled to 5°C in the light is characterized using monoclonal antibodies and analyses of chlorophyll-protein complexes. This polypeptide reacted with a monoclonal antibody, MLH2, that was specific for the 28 kDa polypeptide of the light-harvesting complex (LHCII) of pea leaves. On chilling leaves the appearance of a chlorophyll-protein complex having an apparent molecular weight of 31 kDa coincided with the appearance of a 31 kDa polypeptide and a decrease in the 29 kDa chlorophyll-protein, CP29. Returning the leaves to 25°C for 1 h produced a loss of both the 31 kDa chlorophyll-protein and 31 kDa polypeptide from the thylakoids, and an increase in the amount of CP29. Breakdown of the 31 kDa polypeptide in vitro was Mg(2+)-dependent and inhibited by EDTA and transition metal ions. It is suggested that the 31 kDa polypeptide may be a precursor of the apoprotein of CP29 and can bind chlorophyll. The appearance of the 31 kDa polypeptide correlated with a marked change in the 77 K fluorescence emission spectra of isolated LHCII particles, which did not revert with the disappearance of the 31 kDa on returning the leaves to 25°C for 1 h. The physiological significance of this spectral perturbation is discussed.

Structure and function of SV40 large-T antigen

The small eukaryotic DNA tumour virus, SV40, has long provided a very useful model for the study of eukaryotic DNA replication and cellular transformation. The viral gene product, large-tumour (large-T) antigen, is essential for the initiation of viral DNA replication and the initiation and maintenance of SV40-virus-mediated cellular transformation. The large-T antigen is a complex multifunctional protein, and to delineate its activity more precisely in viral DNA replication and cellular transformation, small functional domains of the protein have been expressed in Escherichia coli and analysed by using a very extensive library of anti-T monoclonal antibodies.

Immortalization of precursors of endodermal, neuroectodermal and mesodermal lineages, following the introduction of the simian virus (SV40) early region into F9 cells

F9 embryonal carcinoma cells were transfected with a hybrid plasmid containing the early genes of the simian virus SV40 under the control of the adenovirus type 5 E1A promoter [21]. These cells were induced to differentiate in aggregates in the presence of retinoic acid (RA). Unlike the derivatives of F9 that are usually obtained in this manner, the plasmid-containing cells were both programmed and immortalized; in addition, expression of the SV40 T antigen was now triggered. These immortalized cells could be separated into three classes: (1) extraembryonic derivatives, (2) embryonic differentiated tissues, (3) immature cells surrounding the differentiated cells. When injected into mice, the mixture of these cells gave rise to multipotential tumors. From the immature cells, committed precursors of the neuroectodermal, endodermal, and mesodermal pathways could be isolated by cloning and selection according to: (a) their specific pattern of differentiation in the tumors and (b) the occurrence of specific markers in the differentiated progeny. The isolation of stable immortalized cell lines corresponding to precursors of the three primitive germ layers and capable of differentiating reproducibly along a particular restricted pathway should facilitate molecular studies on early embryonic development in mouse.

SV40 small t antigen enhances the transformation activity of limiting concentrations of SV40 large T antigen

A murine recombinant Neo(r) retrovirus encoding the SV40 small t antigen was used to infect Balb/c 3T3 CIA31 cells. From analyses of G418-resistant clones containing at least as much intact t as Cos-1 cells, we found that t, alone, had no detectable A31 transforming activity. In contrast, we noted that SV40 large T promoted A31 agar colony formation when present over a 5- to 7.5-fold concentration range. However, at the low end of the spectrum, its transforming effect was manifest inefficiently except in the presence of t. Thus a major role for t in the SV40 transforming mechanism is to enhance directly or indirectly the transforming function of T.

The t-unique coding domain is important to the transformation maintenance function of the simian virus 40 small t antigen

The small t antigen (t) of simian virus 40, a 174-amino-acid-containing protein, when present together with the other early viral protein, large T antigen (T), plays an important role in the maintenance of simian virus 40-induced neoplastic phenotype in certain cells. Indeed, each protein functions in a complementary manner in this process. The t coding unit is composed of two segments, a 5' region of 246 nucleotides which is identical to that of the corresponding 5' region of the T coding unit and a 3' segment of 276 nucleotides which is unique. Two mutant, t-encoding genomes, one bearing a missense and the other a nonsense mutation at the same point in the t-unique coding region were constructed in vitro and found to be defective in their ability to dissolve the actin cytoskeleton of rat fibroblasts and to complement T in the growth of mouse fibroblasts in soft agar. Therefore, the unique segment of the t gene encodes a portion of the t molecule which is essential to its transformation maintenance function.

An SV40 mutant T antigen does not bind the SV40 viral origin

F8dl is an SV40 deletion mutant that lacks over 60% of the coding sequences for large T antigen and yet is able to immortalize early passage rat cells, to transform established cell lines, and to cause tumors in animals. We report here on the further characterization of this mutant and show that (a) transformation by F8dl is protein mediated but does not require the action of the SV40 small t antigen; (b) the F8dl T antigens have, or are associated with, an ATPase activity; (c) the 34-kDa mutant T antigen of F8dl is localized in nuclei and cell membranes of F8dl transformants and binds to double-stranded DNA; (d) the 20-25 kDa forms of the mutant T antigen are cytoplasmic; and (e) the F8dl T antigens do not bind with high affinity to the SV40 origin of viral DNA replication.

Cellular proteins which can specifically associate with simian virus 40 small t antigen

When crude, radiolabeled extracts of various cells were applied to homogeneous simian virus 40 small t antigen-Sepharose adsorbents, three cell proteins (57, 32, and 20 kilodaltons [kDa]) bound specifically. Each also bound to an insoluble, truncated t derivative composed of the COOH-terminal 123 residues of the protein. The binding of these proteins was greatly inhibited after reduction and alkylation of the t ligand. Therefore, some element of native conformation, but not all of the primary structure of t, is necessary for this binding property, which may constitute a discrete, in vitro biochemical function of this protein. Results of cell fractionation experiments suggested that the 57- and 32-kDa proteins are nonnuclear cell constituents, whereas the 20-kDa protein was closely associated with a detergent-washed nuclear fraction. Specific immunoblotting and comparative partial proteolytic digestion analyses indicated that the 57-kDa protein is tubulin, a major component of the cytoskeleton. In this regard, t and tubulin were observed to coimmunoprecipitate from crude cell extracts after incubation with monospecific anti-t antibody. Therefore, it is possible that t and tubulin interact in vivo.

Monoclonal antibody analysis of p53 expression in normal and transformed cells

The cellular phosphoprotein p53 binds tightly and specifically to simian virus 40 T antigen and the 58,000-molecular-weight adenovirus E1b protein. Many human and murine tumor cell lines contain elevated levels of the p53 protein even in the absence of these associated viral proteins. Recently the cloned p53 gene, linked to strong viral promoters, has been shown to complement activated ras genes in transformation of primary rodent cell cultures. Overexpression of the p53 gene alone rescues some primary rodent cell cultures from senescence. We isolated three new monoclonal antibodies to the p53 protein, designated PAb242, PAb246, and PAb248, and mapped the epitopes they recognized on p53 in comparison with other previously isolated antibodies. At least five sterically separate epitopes were defined on murine p53. One of the antibodies, PAb246, recognizes an epitope on p53 that is unstable in the absence of bound simian virus 40 T antigen. This effect is demonstrable in vivo and in newly developed in vitro assays of T-p53 complex formation. Using the panel of anti-p53 antibodies and sensitive immunocytochemical methods, we found that p53 has a predominantly nuclear location in established but not transformed cells as well as in the vast majority of transformed cell lines. Several monoclonal antibodies to p53 showed cross-reactions with non-p53 components in immunocytochemical staining.

The t-unique coding domain is important to the transformation maintenance function of the simian virus 40 small t antigen

The small t antigen (t) of simian virus 40, a 174-amino-acid-containing protein, when present together with the other early viral protein, large T antigen (T), plays an important role in the maintenance of simian virus 40-induced neoplastic phenotype in certain cells. Indeed, each protein functions in a complementary manner in this process. The t coding unit is composed of two segments, a 5' region of 246 nucleotides which is identical to that of the corresponding 5' region of the T coding unit and a 3' segment of 276 nucleotides which is unique. Two mutant, t-encoding genomes, one bearing a missense and the other a nonsense mutation at the same point in the t-unique coding region were constructed in vitro and found to be defective in their ability to dissolve the actin cytoskeleton of rat fibroblasts and to complement T in the growth of mouse fibroblasts in soft agar. Therefore, the unique segment of the t gene encodes a portion of the t molecule which is essential to its transformation maintenance function.

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.

Cellular targets for SV40 large T-antigen

SV40 virus infection is able to induce tumours in newborn hamsters and to transform a wide range of eukaryotic cells in in vitro culture. This is achieved by integration of the viral DNA into the host cell DNA and expression of the virus-encoded Large T-antigen. The expression of Large T, a 708 amino acid phosphoprotein, is required both to induce and maintain the transformed state. The Large T protein initiates viral DNA synthesis and regulates viral transcription, apparently by binding in a specific manner to viral DNA sequences at and near the viral origin of replication. SV40 Large T also affects cellular DNA synthesis and transcription and this may account for its oncogenic activity. A novel immunochemical procedure has permitted the isolation of cellular DNA sequences occupied by SV40 Large T in the chromatin of SV40 transformed cells. Some of the cellular sequences contain high affinity binding sites for SV40 Large T, and hybridize to messenger RNAs expressed in SV40 transformed but not in normal cells. A second type of cellular target for Large T is the cell coded p53 protein that it binds to and stabilizes. A range of monoclonal antibodies to p53 has been isolated and characterized. They demonstrate that p53 is in the cytoplasm of normal cells but is located in the nucleus of transformed cells. One of the antibodies recognizes an epitope on p53 that is stabilized or induced by binding to Large T. Further studies on the T-p53 protein complex have been facilitated by constructing bacterial plasmids that direct the synthesis of substantial quantities of Large T-beta-galactosidase and p53-beta-galactosidase fusion proteins in bacteria. The results are discussed in the context of our current knowledge of oncogene action.

Negative regulation of viral enhancers in undifferentiated embryonic stem cells

Many viral genomes, including those of SV40 and MuLV, are not efficiently expressed in undifferentiated embryonal carcinoma (EC) cells but are expressed in differentiated derivatives. This regulation appears to be at the level of transcription. We have used DNA-mediated gene transfer to analyze the function of several viral promoters in EC cells. We show that the SV40 early promoter works efficiently in an enhancer-independent fashion following transfection into undifferentiated cells. Strikingly, the promoter in the LTR of MSV does not function in such cells; but when upstream sequences, including the enhancer, are deleted expression ensues. Replacement of the SV40 enhancer by that of MSV results in inactivation of the SV40 early promoter in these cells. We propose that the undifferentiated cells contain a trans-acting regulatory factor (or factors) that reduces transcription by interacting with viral enhancers.