Failure of simian virus 40 small t antigen to disorganize actin cables in nonpermissive cell lines

SV40 reporter viruses

Three simian virus 40 (SV40) reporter viruses were constructed in this study. One expresses the green fluorescent protein (GFP) as a fusion protein with the first exon of large-T (LT) antigen and is useful for live-cell imaging. A second reporter virus has a FLAG epitope tag at the C-terminus of large-T antigen (vC-LT(FLAG)), and a third has the FLAG tag at the N-terminus of LT (vN-LT(FLAG)). The vC-LT(FLAG) construct grows to titers near those of wild-type (WT) virus and functions well as a reporter virus for SV40 infection. The vN-LT(FLAG) construct, while viable, has a defect in the production and spread of infectious particles. All three viruses are useful in detecting superinfecting virus in cells in which nuclear LT is already present, such as persistently infected human mesothelial cells.

Simian virus 40 late proteins possess lytic properties that render them capable of permeabilizing cellular membranes

Many nonenveloped viruses have evolved an infectious cycle that culminates in the lysis or permeabilization of the host to enable viral release. How these viruses initiate the lytic event is largely unknown. Here, we demonstrated that the simian virus 40 progeny accumulated at the nuclear envelope prior to the permeabilization of the nuclear, endoplasmic reticulum, and plasma membranes at a time which corresponded with the release of the progeny. The permeabilization of these cellular membranes temporally correlated with late protein expression and was not observed upon the inhibition of their synthesis. To address whether one or more of the late proteins possessed an inherent capacity to induce membrane permeabilization, we examined the permeability of Escherichia coli that separately expressed the late proteins. VP2 and VP3, but not VP1, caused the permeabilization of bacterial membranes. Additionally, VP3 expression resulted in bacterial cell lysis. These findings demonstrate that VP3 possesses an inherent lytic property that is independent of eukaryotic signaling or cell death pathways.

Activation of the tumor-specific death effector apoptin and its kinase by an N-terminal determinant of simian virus 40 large T antigen

Apoptin, a viral death protein derived from chicken anemia virus, displays a number of tumor-specific behaviors. In particular, apoptin is phosphorylated, translocates to the nucleus, and induces apoptosis specifically in tumor or transformed cells, whereas it is nonphosphorylated and remains primarily inactive in the cytoplasm of nontransformed normal cells. Here, we show that in normal cells apoptin can also be activated by the transient transforming signals conferred by ectopically expressed simian virus 40 (SV40) large T antigen (LT), which rapidly induces apoptin's phosphorylation, nuclear accumulation, and the ability to induce apoptosis. Further analyses with mutants of LT showed that the minimum domain capable of inducing all three of apoptin's tumor-specific properties resided in the N-terminal J domain, a sequence which is largely shared by SV40 small t antigen (st). Interestingly, the J domain in st, which lacks its own nuclear localization signal (NLS), required nuclear localization to activate apoptin. These results reveal the existence of a cellular pathway shared by conditions of transient transformation and the stable cancerous or precancerous state, and they support a model whereby a transient transforming signal confers on apoptin both the upstream activity of phosphorylation and the downstream activity of nuclear accumulation and apoptosis induction. Such a pathway may reflect a general lesion contributing to human cancers.

SV40 17KT antigen complements dnaj mutations in large T antigen to restore transformation of primary human fibroblasts

Transformation of human cells requires both SV40 large T and small t antigens. Plasmids that contained mutations in the amino-terminal dnaJ domain of the early region fail to transform human diploid fibroblasts. However, large T dnaJ mutants can be rescued by plasmids that express early region products other than large T antigen. The protein found to be responsible for such complementation was the third early region product, 17KT. Similar to large T, this protein reduces levels of the retinoblastoma-related protein, p130, and stimulates cell-cycle progression of quiescent fibroblasts, two activities of large T that are disrupted by dnaJ mutations.

Simian virus 40 small tumor antigen induces deregulation of the actin cytoskeleton and tight junctions in kidney epithelial cells

There is increasing evidence that the transforming DNA tumor virus simian virus 40 (SV40) is associated with human malignancies. SV40 small tumor antigen (small t) interacts with endogenous serine/threonine protein phosphatase 2A (PP2A) and is required for the transforming activity of SV40 in epithelial cells of the lung and kidney. Here, we show that expression of SV40 small t in epithelial MDCK cells induces acute morphological changes and multilayering. Significantly, it also causes severe defects in the biogenesis and barrier properties of tight junctions (TJs) but does not prevent formation of adherens junctions. Small t-induced TJ defects are associated with a loss of PP2A from areas of cell-cell contact; altered distribution and reduced amounts of the TJ proteins ZO-1, occludin, and claudin-1; and marked disorganization of the actin cytoskeleton. Small t-mediated F-actin rearrangements encompass increased Rac-induced membrane ruffling and lamellipodia, Cdc42-initiated filopodia, and loss of Rho-dependent stress fibers. Indeed, these F-actin changes coincide with elevated levels of Rac1 and Cdc42 and decreased amounts of RhoA in small t-expressing cells. Notably, these cellular effects of small t are dependent on its interaction with endogenous PP2A. Thus, our findings provide the first evidence that, in polarized epithelial cells, expression of small t alone is sufficient to induce deregulation of Rho GTPases, F-actin, and intercellular adhesion, through interaction with endogenous PP2A. Because defects in the actin cytoskeleton and TJ disruption have been linked to loss of cell polarity and tumor invasiveness, their deregulation by PP2A and small t likely contributes to the role of SV40 in epithelial cell transformation.

Regions and activities of simian virus 40 T antigen that cooperate with an activated ras oncogene in transforming primary rat embryo fibroblasts

Prolonged expression of a ras oncogene in primary cells accelerates the natural process of senescence. This ras-induced permanent growth arrest is bypassed in cells expressing the simian virus 40 large T antigen. Previously we showed that two regions of T antigen, a region consisting of the N-terminal 147 amino acids and a region consisting of amino acids 251 to 708 (T251-708), independently overcome ras-induced senescence. Coexpression of either T-antigen fragment and Ras results in the appearance of dense foci of transformed cells. Using a series of mutants that produce shorter T-antigen fragments, we show that the C-terminal limit of the N-terminal T-antigen fragment that cooperates with Ras lies between amino acids 83 and 121. The N-terminal limit of the C-terminal T-antigen fragment lies between amino acids 252 and 271. In addition, we present evidence that cooperation between the N-terminal fragment and Ras depends upon an intact T-antigen J domain and the ability of the T antigen to bind and inactivate the growth-suppressive effect of the tumor suppressor Rb. Introduction of specific amino acid substitutions surrounding residue 400 into T251-708 prevented the fragment from cooperating with Ras. T251-708 proteins with these same substitutions inhibited the transcriptional transactivating potential of p53 as effectively as did the wild-type protein. Thus, at least one activity contained within T251-708, other than inactivating p53 as a transcriptional transactivator, is likely to be required to bypass Ras-induced senescence.

The role of the SV40 ST antigen in cell growth promotion and transformation

The simian virus 40 small-t (ST) antigen plays a key role in permissive and nonpermissive infections, increasing virus yields in lytic cycles of primate cells and enhancing the ability of large-T (LT) to transform rodent or even human cells. In the absence of ST, tumors in rodent model systems appear primarily in lymphoid and other proliferative tissues and transformation is reduced in several in vitro systems. The functions of ST largely reflect its binding and inhibition of protein phosphatase 2A, although a recently described dnaJ domain also contributes to its biology. The dnaJ domain is present in LT and a third early gene product, the 17kT protein, for which a potential role in transformation deserves further evaluation.

A novel simian virus 40 early-region domain mediates transactivation of the cyclin A promoter by small-t antigen and is required for transformation in small-t antigen-dependent assays

At least three regions of the simian virus 40 small-t antigen (small-t) contribute to the protein's ability to enhance cellular transformation. As we showed previously for rat F111 cells, one region includes sequences from residues 97 to 103 that are involved in the binding and inhibition of protein phosphatase 2A. In the present study, the role of the protein phosphatase 2A binding region was confirmed in two additional small-t-dependent transformation systems. Second, small-t was found to provide a function previously identified as a large-T transformation domain. Mutations in residues 19 to 28 of large-T affected its transforming ability, but these mutations were complemented by a wild-type small-t. A third region of small-t was also required for efficient transformation. This region, the 42-47 region, is shared by large-T and small-t and contains a conserved HPDKGG hexapeptide. The 42-47 region function could be provided by either small-t or large-T in small-t-dependent systems. Mutations in the 42-47 region reduced the ability of small-t to transactivate the cyclin A promoter, of interest because small-t increased endogenous cyclin A mRNA levels in both human and monkey cells, as well as transactivating the promoter in transient assays.

The block of adipocyte differentiation by a C-terminally truncated, but not by full-length, simian virus 40 large tumor antigen is dependent on an intact retinoblastoma susceptibility protein family binding domain

Simian virus 40 (SV40) can promote cell transformation and suppress differentiation. It does this partly by targeting tumor suppressors such as p53 and members of the retinoblastoma susceptibility protein (Rb) family. This work concentrates on mechanisms by which SV40 large tumor antigen (SVLT) suppresses adipocyte differentiation. We created cell lines derived from murine 3T3-L1 preadipocytes expressing different versions of SV40 early-region sequences. SVLT-expressing cells failed to exhibit adipocyte morphology, to induce glycerophosphate dehydrogenase activity, and to induce differentiation-dependent mRNA for adipocyte P2. SVLT alone was sufficient, in the absence of SV40 small tumor antigen, to inhibit differentiation. A truncated SVLT containing only the N-terminal 121 amino acids (SVLT1-121) blocked differentiation, thus mapping at least one differentiation blocking function to the N-terminal region. K1 (Glu-107-->Lys) point mutants of SVLT, which are unable to bind to the Rb protein family or induce neoplastic transformation, are defective for blocking differentiation in the case of SVLT1-121 but retain the ability to block differentiation in the case of full-length SVLT. This finding demonstrates that Rb family proteins are important in regulating adipocyte differentiation but that other functions of full-length SVLT can block adipocyte differentiation independently of RB family binding and transformation.

Mutations which affect the inhibition of protein phosphatase 2A by simian virus 40 small-t antigen in vitro decrease viral transformation

Three independent point mutations within residues 97 to 103 of the simian virus 40-small-t antigen (small-t) greatly reduced the ability of purified small-t to inhibit protein phosphatase 2A in vitro. These mutations affected the interaction of small-t antigen with the protein phosphatase 2A A subunit translated in vitro, and a peptide from the region identified by these mutations released the A subunit from immune complexes. When introduced into virus, the mutations eliminated the ability of small-t to enhance viral transformation of growth-arrested rat F111 cells. In contrast, the mutant small-t antigens were unimpaired in the transactivation of the adenovirus E2 promoter, an activity which was reduced by a double mutation in small-t residues 43 and 45.

An amino-terminal fragment of SV40 T antigen transforms REF52 cells

An SV40 mutant, T147D, encodes only the amino-terminal 147 amino acids of large T antigen and does not make small t antigen. We show here that a retrovirus which expresses this mutant T antigen transforms rat REF52 cells as efficiently as a retrovirus that expresses both the wild-type large and small T antigens. This cell line had previously been refractory to transformation by mutants that make short, amino-terminal fragments of T antigen.

Modulation of the human papillomavirus type 16 induced transformation and transcription by deletion of loci on the short arm of human chromosome 11 can be mimicked by SV40 small t

The human papillomavirus (HPV) type 16 enhancer-promoter has been shown to be active in human fibroblasts with a deletion on the short arm of one chromosome 11 (karyotype 46,del(11)(p11.11p15.1)) but is virtually inactive in diploid human fibroblasts (Smits, Smits, Jebbink, and ter Schegget, 1990b, Virology, 176, 158-165). In diploid human embryonic fibroblasts, activation of the HPV16 enhancer-promoter could be achieved by expression of the SV40 small t. By cotransfecting SV40 small t cDNA together with HPV16 DNA into diploid cells, it was possible to increase the transforming activity of HPV16 by 10- 15-fold. Furthermore, SV40 small t was essential for the SV40 large T-induced morphological transformation of human diploid fibroblasts, whereas SV40 small t was dispensable for transformation of del-11 cells. We propose that, as a result of the deletion of loci on the short arm of chromosome 11 in del-11 cells, functions are expressed that mimic those of SV40 small t in transformation and trans-activation.

Effect of zinc ions on the biochemical behavior of simian virus 40 small-t antigen expressed in bacteria

The simian virus 40 small-t antigen contains 10 cysteine residues, 6 of which are organized in two CysXCysXXCys clusters. Mutation of individual Cys residues in the two clusters or mutation of specific residues found between these clusters causes pronounced instability of the protein in animal cells. Protein instability correlates with failure of the bacterially expressed mutant proteins to bind zinc ions, an interaction which allows purification of large amounts of small-t antigen in monomeric form.

Simian virus 40 small-t does not transactivate RNA polymerase II promoters in virus infections

Transcriptional stimulatory properties of virus-encoded transactivators appear to be critical for viral gene expression and may be linked to cellular transformation in certain cases. Recently, the simian virus 40 (SV40) 17-kDa small-t antigen was shown to stimulate transcription of polymerase II and III genes in transient transfection assays. In experiments performed in our laboratory, two of the polymerase II promoters of the adenovirus genome, namely, the EII-early and EIII promoters, were transactivation, we examined the transient transfection assays. To further elucidate the mechanism of this transactivation, we examined the ability of small-t to transactivate the adenovirus type 5 EII-early and EIII promoters in CV-1 cells under conditions in which the small-t gene or the reporter genes were introduced into the cells through transfection and other routes. In one approach, we used established CV-1 cell lines which constitutively express the small-t gene, and study of the EII-early promoter was afforded by infection of an EIA-negative adenovirus type 5 variant. For the second approach, a recombinant adenovirus was constructed in which small-t was expressed from a replication origin-negative SV40 early promoter in the EIA region of an adenovirus vector (Ad-SV-t). The effect of small-t on adenovirus EII-early and EIII promoter expression was studied in coinfection or single-infection experiments. In both cases, transcription of the adenovirus early promoters was not stimulated by small-t. These and other results indicate that transactivation of polymerase II promoters by small-t occurs only when the target gene is in a transiently transfected state. Thus, small-t-mediated transactivation of polymerase II promoters is dependent on the type of assay system used and may be mechanistically different from that of the widely studied EIA.

Characterization of human vascular smooth muscle cells transformed by the early genetic region of SV40 virus

Human arterial smooth muscle cells transfected with the plasmid pSV3-neo, which contains the SV40 virus early region and the neor gene, developed colonies of morphologically transformed cells. Five cell strains were initiated from these colonies and could be subcultivated for up to 9 months before entering a stage of crisis that ended their life span. Deoxyribonucleic acid (DNA) molecules containing viral sequences were found free and integrated in the transformed cells. The intranuclear SV40 large T antigen and the p53 cellular protein were expressed in the transformed cells. Most of the transformed cells were spindle shaped but some were large and multinucleated. The modal chromosome numbers were in the triploid range, and aberrations, particularly dicentrics, were common. The transcripts for smooth muscle actins were significantly reduced and there were less alpha-actin filaments detected by immunofluorescence. Cytochemical staining disclosed a large accumulation of lipid droplets in the transformed cells incubated with rabbit hypercholesterolemic beta-very-low-density lipoprotein. Chemical analysis showed that cholesteryl esters were significantly elevated in these cells. Phenotypic changes induced in human vascular smooth muscle cells by SV40 early genes are similar to those found in smooth muscle cells from atherosclerotic lesions and may indicate common pathogenetic mechanisms.

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.

Association of simian virus 40 small-t antigen with the 61-kilodalton component of a cellular protein complex

Two cellular proteins, 61 and 37 kDa, are found in association with the simian virus 40 (SV40) small-t antigen. Fractionation in standard chromatography systems showed that these proteins were associated with one another in uninfected cells, suggesting that the small-t antigen may bind the complex as a whole and not each individual protein independently. In the presence of N-ethylmaleimide, the 37-kDA protein was selectively released from immune complexes, leaving the small-t antigen and 61-kDa protein in association. This result suggests that the small-t antigen may bind only the 61-kDa protein and that the 37-kDa protein may be associated with immune complexes by virtue of its association with the 61-kDa cellular protein.

Mutational analysis of simian virus 40 small-t antigen

Several point mutations in the simian virus 40 (SV40) small-t antigen have been analyzed for their effects on protein stability, transformation, transactivation, and binding of two cellular proteins. All mutations which affected cysteine residues in two cysteine clusters produced highly unstable small-t antigens. Four point mutations outside these clusters and one in-frame deletion mutant, dl890, produced stable proteins but reduced transformation efficiency. These were able to transactivate the EII promoter and bind the cellular proteins, suggesting that these activities are not sufficient for small-t-mediated enhancement of transformation.

Transformation of rabbit vascular smooth muscle cells by transfection with the early region of SV40 DNA

Rabbit aortic smooth muscle cells (SMC) and Rb-1 cells, a continuous line of the same origin, were transformed by transfection with pSV3-neo DNA, a plasmid containing the SV40 early region linked to the neoR resistance gene. Transformed clones were selected in G418-containing medium at a rate of 10(-4) per cell. All transformed clones were immortalized and contained in the early passages two free recombined plasmids derived from pSV3-neo. At advanced passages pSV3-neo sequences were found integrated in the cellular genome. Transformed cells had an altered morphology and growth pattern that differed among clones. Some clones reached high density in low-serum medium. All the clones stained positively for the intranuclear T antigen. Some clones had distinct transcripts for the large T and small t antigens, while in others only larger or truncated transcripts were found. Alpha-actin filaments were visualized by immunofluorescent staining in all the clones, but Northern blot analysis revealed a significant reduction in transcripts for this actin. All the transformed clones accumulated, to a variable extent, cholesteryl esters after incubation with beta very low-density lipoprotein. Six of the eight transformed clones maintained a diploid chromosome number, but there was an increase in structural chromosome aberrations, predominantly dicentrics. Transfection of pSV3-neo into rabbit vascular SMCs is an efficient model for obtaining transformed clonal populations. These clones show some phenotypic changes that may be relevant to the study of atherogenesis.

The large tumor antigen of simian virus 40 encodes at least two distinct transforming functions

The large tumor antigen (T antigen) of simian virus 40 is necessary and sufficient for the neoplastic transformation of a number of established cell lines. Mutational analysis has revealed that a biochemical activity residing within the amino-terminal 121 amino acids of T antigen is sufficient to induce the transformation of some cell lines, such as C3H10T1/2. The same domain of the molecule also encodes the transactivation function of T antigen and the ability to complex with the retinoblastoma susceptibility gene product. However, the transformation of other lines, such as REF52, requires an additional activity that is affected by mutations in other portions of the molecule.