Association of a murine 53,000-dalton phosphoprotein with simian virus 40 large-T antigen in transformed cells

The SV40 virus enhancer functions as a somatic hypermutation-targeting element with potential oncogenic activity

Simian virus 40 (SV40) is a monkey virus associated with several types of human cancers. SV40 is most frequently detected in mesotheliomas, brain and bone tumors and lymphomas, but the mechanism for SV40 tumorigenesis in humans is not clear. SV40 relative Merkel cell polyomavirus (MCPyV) causes Merkel cell carcinoma (MCC) in humans by expressing truncated large tumor antigen (LT) caused by APOBEC cytidine deaminase family enzymes induced mutations. AID (activation-induced cytidine deaminase), a member of the APOBEC family, is the initiator of the antibody diversification process known as somatic hypermutation (SHM) and its aberrant expression and targeting is a frequent source of lymphomagenesis. In this study, we investigated whether AID-induced mutations could cause truncation of SV40 LT. We demonstrate that the SV40 enhancer has strong SHM targeting activity in several cell types and that AID-induced mutations accumulate to SV40 LT in B cells and kidney cells and cause truncated LT expression in B cells. Our results argue that the ability of the SV40 enhancer to target SHM to LT is a potential source of LT truncation events in various cell types that could contribute to carcinogenesis.

Fifty Years of JC Polyomavirus: A Brief Overview and Remaining Questions

In the fifty years since the discovery of JC polyomavirus (JCPyV), the body of research representing our collective knowledge on this virus has grown substantially. As the causative agent of progressive multifocal leukoencephalopathy (PML), an often fatal central nervous system disease, JCPyV remains enigmatic in its ability to live a dual lifestyle. In most individuals, JCPyV reproduces benignly in renal tissues, but in a subset of immunocompromised individuals, JCPyV undergoes rearrangement and begins lytic infection of the central nervous system, subsequently becoming highly debilitating-and in many cases, deadly. Understanding the mechanisms allowing this process to occur is vital to the development of new and more effective diagnosis and treatment options for those at risk of developing PML. Here, we discuss the current state of affairs with regards to JCPyV and PML; first summarizing the history of PML as a disease and then discussing current treatment options and the viral biology of JCPyV as we understand it. We highlight the foundational research published in recent years on PML and JCPyV and attempt to outline which next steps are most necessary to reduce the disease burden of PML in populations at risk.

The biology of JC polyomavirus

JC polyomavirus (JCPyV) is the causative agent of a fatal central nervous system demyelinating disease known as progressive multifocal leukoencephalopathy (PML). PML occurs in people with underlying immunodeficiency or in individuals being treated with potent immunomodulatory therapies. JCPyV is a DNA tumor virus with a double-stranded DNA genome and encodes a well-studied oncogene, large T antigen. Its host range is highly restricted to humans and only a few cell types support lytic infection in vivo or in vitro. Its oncogenic potential in humans has not been firmly established and the international committee on oncogenic viruses lists JCPyV as possibly carcinogenic. Significant progress has been made in understanding the biology of JCPyV and here we present an overview of the field and discuss some important questions that remain unanswered.

A genetically engineered mouse model developing rapid progressive pancreatic ductal adenocarcinoma

The premalignant lesions of pancreatic cancer, pancreatic intraepithelial neoplasia (PanIN), have a high frequency of mutations in Kirsten rat sarcoma viral oncogene homologue (KRAS), and genetic alterations in the retinoblastoma (Rb)-E2 factor (E2F) and transformed 3T3 cell double minute 2 (MDM2)-p53 pathways accelerate development of pancreatic ductal adenocarcinoma. The viral oncoprotein SV40 large T antigen (TAg) can inhibit the effects of the Rb family of molecules and of p53 on these pathways, and targeted expression of TAg in mouse pancreas is associated with the development of endocrine or acinar cell tumours. In this study, to determine whether the viral oncoprotein promotes pancreatic duct carcinogenesis initiated by oncogenic KRAS, we generated mice expressing temperature-sensitive SV40 large T antigen (tsTAg) on pancreatic epithelial cells in the presence or absence of Kras(G12D) . Mice with pancreas-specific tsTAg expression developed acinar cell dysplasia by 22 weeks without PanIN formation, while mice expressing both tsTAg and Kras(G12D) developed highly aggressive adenocarcinoma with a ductal cell phenotype within a short period, and died within 3 weeks. The tumours resembled human pancreatic ductal adenocarcinoma (PDAC) at the histological level, and oncogenic Kras and tsTAg synergistically activated E2f and Sre transcription in established PDAC cell lines. These results suggest that tsTAg synergistically promotes Kras(G12D) -associated PDAC formation, and our study identifies a new mouse model of PDAC that may allow a better understanding of the mechanism of carcinogenesis in pancreatic carcinoma, which shows a catastrophic clinical course.

Common threads in persistent viral infections

Most viral infections are self-limiting, resulting in either clearance of the pathogen or death of the host. However, a subset of viruses can establish permanent infection and persist indefinitely within the host. Even though persisting viruses are derived from various viral families with distinct replication strategies, they all utilize common mechanisms for establishment of long-lasting infections. Here, we discuss the commonalities between persistent infections with herpes-, retro-, flavi-, arena-, and polyomaviruses that distinguish them from acutely infecting viral pathogens. These shared strategies include selection of cell subsets ideal for long-term maintenance of the viral genome, modulation of viral gene expression, viral subversion of apoptotic pathways, and avoidance of clearance by the immune system.

Interactions between adenovirus proteins and the p53 pathway: the development of ONYX-015

dl1520 (ONYX-015) is an adenovirus mutant that lacks the E1b 55K gene. As a result it cannot neutralize p53. Therefore dl1520 should only grow in cells that lack p53, and should replicate selectively in cancer cells. However, there is no correlation between replication and p53 status, in cancer cells. This is for two reasons: (1) E1B 55K has additional functions, that are necessary in some tumor cells and not in others. (2) p53 function can be lost by alternative mechanisms such as loss of p14ARF. In normal cells, dl1520 induces p53, and is generally strongly attenuated for replication. ONYX-015 is currently being tested in clinical trials, and is a promising new therapeutic agent in cancer.

Three unrelated viral transforming proteins (vIRF, EBNA2, and E1A) induce the MYC oncogene through the interferon-responsive PRF element by using different transcription coadaptors

Kaposi sarcoma-associated herpesvirus vIRF is a viral transcription factor that inhibits interferon signaling and transforms NIH 3T3 cells, but does not bind interferon-stimulated response element (ISRE) DNA sequences. Here we show that induction of the MYC protooncogene is required for cell transformation by vIRF, and that vIRF increases MYC transcription up to 15-fold through specific promoter interactions at an ISRE sequence called the plasmacytoma repressor factor (PRF) element. These effects are resistant to cycloheximide but are inhibited by a dominant-negative ISRE-binding protein, indicating that vIRF acts together with a cellular cofactor at the PRF element to directly transactivate MYC. The coadaptor CREB-binding protein (CBP) binds vIRF and synergizes transactivation of MYC, but, unexpectedly, closely related histone acetyltransferases p300 and P/CAF potently suppress vIRF transactivation. On the basis of the prediction that other interferon-inhibiting viral transforming proteins behave similarly, we found that Epstein-Barr virus-induced nuclear antigen 2 (EBNA2) also binds p300/CBP, and that both EBNA2 and adenovirus E1A transactivate MYC through the PRF element. For E1A, P/CAF coactivates MYC, whereas both p300 and CBP suppress E1A transactivation. For EBNA2, both P/CAF and CBP coactivate the MYC promoter, whereas p300 suppresses EBNA2 transactivation. These findings demonstrate that viral transforming proteins can activate as well as inhibit transcription through coadaptor interactions. At some promoters CBP and p300 have previously unrecognized, competitive antagonism to each other. While all three viral proteins target the same promoter element, each has a different coadaptor use profile. These findings are consistent with cellular MYC repression playing a role in innate immunity as well as in control of cell proliferation.

Alterations of the p53, Rb and MDM2 genes in osteosarcoma

Molecular defects affecting tumor-suppressor genes are an important step in the genesis of sarcomas. For example, inheritance of a defective Rb or p53 gene predisposes the carrier to develop osteosarcoma, among other malignancies. In this study, we have assessed the occurrence of p53, Rb and MDM2 alterations in the same samples of osteosarcomas, along with representative samples of various other sarcomas. Point mutations of the p53 gene were found in 13 of 42 osteosarcomas and 1 of 8 leiomyosarcomas, and gross rearrangement of the p53 gene was demonstrated in 5 of 37 osteosarcomas. The retinoblastoma susceptibility gene (Rb) was either rearranged or deleted in 7 of 37 osteosarcomas, 1 of 7 soft-tissue sarcomas and 1 of 4 Ewing sarcomas. Remarkably, 5 of the osteosarcomas having Rb alterations also had p53 mutations. Amplification and overexpression of the MDM2 oncogene may lead to increased MDM2-p53 binding resulting in inactivation of p53 function. A two- to threefold increase in the copy number of MDM2 was detected in 7 of 37 samples, 5 of which were osteosarcomas. Amplification of the MDM2 gene occurred independently of p53 mutation; one sample having threefold amplification of MDM2 also had a p53 mutation. In summary, 34 alterations of the p53, Rb and MDM2 genes were found in 26 of 42 (62%) osteosarcomas.

Analysis of genomic instability in Li-Fraumeni fibroblasts with germline p53 mutations

Germline p53 mutations are frequently observed in the normal DNA of cancer-prone patients with Li-Fraumeni syndrome (LFS). Fibroblasts from LFS patients develop chromosomal aberrations, loss of cell cycle control, and spontaneous immortalization. We transfected four different mutant p53 genes into human skin fibroblasts from normal donors with two copies of wild-type p53 (p53(wt/wt)). Each mutant p53 expression-plasmid induced genomic instability equivalent to that seen in LFS cells. To test the role of wild-type and mutant p53 alleles in DNA replication and fidelity in LFS cells, we analysed the replication of the SV40-based shuttle vector pZ189 in four types of cells. We used p53(wt/mut) and p53(mut/-) LFS fibroblasts, and p53(-/-) non-LFS cells. Replication of pZ189 in vivo was significantly reduced by the presence of a p53(wt) allele. To show that this was not just due to inhibition of the function of T-antigen in SV40-based replication, we constructed a shuttle vector, pZ402, that contains a mutation in SV40 T-antigen which blocks its ability to interact with p53. Replication of pZ402 in LFS cells was also reduced by the presence of p53(wt), indicating that p53 can inhibit replication by interacting with proteins within the cellular replication machinery. Replicative errors in this shuttle vector are detected as mutations in a marker gene, supF. In addition to supF mutations, we observed deletion of a portion of the SV40 T-antigen gene in 100% of replicated plasmid pZ189 mutants (supF-) from the p53(wt/mut) fibroblasts and in 88% of the supF mutants from the p53(mut/-) (amino acid 175 arg to his) LFS cells. In one cell strain of immortal LFS cells, P53(mut/-) , containing a p53 frameshift mutation at amino acid 184, pZ189 replication yielded very few of these deleted shuttle vector plasmids (15%). These large deletions were not detected in plasmids replicated in p53(-/-) non-LFS cells, Saos-2 cells. Replicated plasmids with a normal supF gene were never found to have this large deletion regardless of the cell from which they were derived. Because the supF gene is not in the same region of the shuttle vector as the T-antigen gene it appears that second, independent gene deletions are frequent when replicative errors in supF occur in cells with a mutant p53. We conclude, therefore, that p53(wt/mut) LFS cells contain an activity that promotes mutations. Such an activity, which is likely to be due to the p53(mut), could result in the high rate of chromosomal instability and allelic loss of the wild-type p53 observed as these cells spontaneously immortalize.

Biochemical characterisation of purified human wild-type p53 overexpressed in insect cells

Conditions for the overexpression of human wild-type p53 using a baculovirus construct were optimised in insect cells which produced up to 20 mg p53/1 culture. Milligram amounts of p53 were purified to apparent homogeneity using chromatography on double-stranded DNA-cellulose (approximately 58% yield) followed by immunoaffinity chromatography with an epitope elution step (up to 48% yields) at 4 degrees C. The M(r) of extracted p53 both from insect cell lysates and after purification was 54,000 by SDS/PAGE. Isoelectric focusing showed recombinant p53 to be an acidic protein, focusing at pI 6.0 under non-denaturing conditions. Expressed p53 at all stages of purification reacted by immunoblotting with specific p53 monoclonal antibodies, indicating the presence of intact epitopes at the C-terminus, N-terminus and central region of the protein. From ultracentrifugation studies, pure p53 exhibited significant oligomerisation, and sedimented broadly within the 7-12-S region of sucrose gradients. Pure p53 slowly precipitated out of solution at concentrations between 1-6 mg/ml even in the presence of 1% detergent. Using metal affinity chromatography, we have established that pure p53 binds the immobilised divalent ions Zn2+, Ni2+ and Co2+ with high affinity.

Overproduction of Rb protein after the G1/S boundary causes G2 arrest

The Rb protein is known to exert its activity at decision points in the G1 phase of the cell cycle. To investigate whether it may also play some role(s) at later points in the cell cycle, we used a system of rapid inducible gene amplification to conditionally overexpress Rb protein during G2 phase. A cell line expressing a temperature-sensitive simian virus 40 large T antigen (T-Ag) was stably transfected with plasmids containing the Rb cDNA linked to the simian virus 40 origin of replication: pRB-wt, pRB-fs, and pRB-Dra, carrying wild-type murine Rb cDNA, a frameshift mutation close to the beginning of the Rb coding region, and a single-amino-acid deletion in the E1A/T-Ag binding pocket, respectively. Numerous independent cell lines were isolated at the nonpermissive temperature; cell lines displaying a high level of episomal amplification of an intact Rb expression cassette following shiftdown to the permissive temperature were chosen for further analysis. Plasmid pRB-fs did not express detectable Rb antigen, while pRB-Dra expressed full-length Rb protein. The Dra mutation has previously been shown to abrogate phosphorylation as well as T-Ag binding. Fluorescence-activated cell sorting (FACS) analysis revealed that cultures induced to overexpress either wild-type or Dra mutant Rb proteins were significantly enriched for cells with a G2 DNA content. Cultures that amplified pRB-fs or rearranged pRB-wt and did not express Rb protein had normal cell cycle profiles. Double-label FACS analysis showed that cells overexpressing Rb or Rb-Dra proteins were uniformly accumulating in G2, whereas cells expressing endogenous levels of Rb were found throughout the cell cycle. These results indicate that Rb protein is interacting with some component(s) of the cell cycle-regulatory machinery during G2 phase.

Overproduction of Rb protein after the G1/S boundary causes G2 arrest

The Rb protein is known to exert its activity at decision points in the G1 phase of the cell cycle. To investigate whether it may also play some role(s) at later points in the cell cycle, we used a system of rapid inducible gene amplification to conditionally overexpress Rb protein during G2 phase. A cell line expressing a temperature-sensitive simian virus 40 large T antigen (T-Ag) was stably transfected with plasmids containing the Rb cDNA linked to the simian virus 40 origin of replication: pRB-wt, pRB-fs, and pRB-Dra, carrying wild-type murine Rb cDNA, a frameshift mutation close to the beginning of the Rb coding region, and a single-amino-acid deletion in the E1A/T-Ag binding pocket, respectively. Numerous independent cell lines were isolated at the nonpermissive temperature; cell lines displaying a high level of episomal amplification of an intact Rb expression cassette following shiftdown to the permissive temperature were chosen for further analysis. Plasmid pRB-fs did not express detectable Rb antigen, while pRB-Dra expressed full-length Rb protein. The Dra mutation has previously been shown to abrogate phosphorylation as well as T-Ag binding. Fluorescence-activated cell sorting (FACS) analysis revealed that cultures induced to overexpress either wild-type or Dra mutant Rb proteins were significantly enriched for cells with a G2 DNA content. Cultures that amplified pRB-fs or rearranged pRB-wt and did not express Rb protein had normal cell cycle profiles. Double-label FACS analysis showed that cells overexpressing Rb or Rb-Dra proteins were uniformly accumulating in G2, whereas cells expressing endogenous levels of Rb were found throughout the cell cycle. These results indicate that Rb protein is interacting with some component(s) of the cell cycle-regulatory machinery during G2 phase.

Phosphorylation at Ser-15 and Ser-392 in mutant p53 molecules from human tumors is altered compared to wild-type p53

The product of the p53 gene suppresses cell growth and plays a critical role in suppressing development of human tumors. p53 protein binds DNA, activates transcription, and can be phosphorylated at N- and C-terminal sites. Previously, wild-type p53 was shown to be hyperphosphorylated compared to mutant p53 during p53-mediated growth arrest in vivo. Here we show that Ser-15 and Ser-9 in the N-terminal transactivation domain of wild-type human p53 are phosphorylated in vivo in cells derived from the human glioblastoma line T98G. In [Ile237]p53 and [Ala143]p53, two natural p53 mutants from human tumors that are defective for activation of transcription, phosphorylation at Ser-15 was reduced and phosphorylation at Ser-392 was increased compared to wild-type p53. No change was observed at Ser-9. [His273]p53, a third mutant, had a phosphorylation state similar to that of wild-type p53. We suggest that phosphorylation of Ser-15 may depend on the ability of p53 to adopt a wild-type conformation and may contribute to p53's ability to block cell growth.

Wild-type p53 is not a negative regulator of simian virus 40 DNA replication in infected monkey cells

To analyze the proposed growth-inhibitory function of wild-type p53, we compared simian virus 40 (SV40) DNA replication in primary rhesus monkey kidney (PRK) cells, which express wild-type p53, and in the established rhesus monkey kidney cell line LLC-MK2, which expresses a mutated p53 that does not complex with large T antigen. SV40 DNA replication proceeded identically in both cell types during the course of infection. Endogenously expressed wild-type p53 thus does not negatively modulate SV40 DNA replication in vivo. We suggest that inhibition of SV40 DNA replication by wild-type p53 in in vitro replication assays is due to grossly elevated ratios of p53 to large T antigen, thus depleting the replication-competent free large T antigen in the assay mixtures by complex formation. In contrast, the ratio of p53 to large T antigen in in vivo replication is low, leaving the majority of large T antigen in a free, replication-competent state.

Stability of p53 protein in rat cells transformed by various viral transforming genes

To clarify the correlation between metabolic stabilization of p53 protein and cellular transformation, we transformed the normal rat cell line F2408 with various viral transforming genes and examined the expression and stability of p53 protein in these transformed cells by pulse-chase immunoprecipitation experiments. As expected, the level of p53 in the SV40-transformed rat cell line was higher than that in the normal cell line and its stability was also increased. In contrast, in cells transformed with the E7 and E6 genes of human papillomavirus type 16, the level and stability of p53 were similar to those in the normal cells. In cells transformed by the middle T or large T of polyomavirus, v-K-ras, and v-src, the levels and stabilities of p53 were also not elevated, although the level of p53 was increased in activated c-H-ras-transformed cells without an increase in its stability. These results show that increased stability and expression of the p53 protein are not common events in viral transformation of the rat cell line. In addition, we demonstrated with a monoclonal antibody specific for the mutant form of p53 that cellular transformation by viral transforming genes does not involve mutational activation of p53 to an oncogenic form.

Protein-protein interactions in high molecular weight forms of the transformation-related phosphoprotein p53

The transformation-related cellular phosphoprotein p53 interacts with a variety of viral and cellular proteins and with itself to form high molecular weight complexes. The formation of high molecular weight complexes correlates with the transformed morphology of the cells whereas in non-transformed cells low molecular weight forms are predominant. Thus, aggregation seems to be involved in the regulation of biological functions of p53. Analyzing wild-type and mutant p53 in the same cellular environment i.e. after an in vitro transcription/translation reaction in rabbit reticulocytes we found high molecular weight forms for wild-type and mutant p53. The sedimentation profile resembled the profile obtained for mutant p53 from transformed cells. As shown by dilution experiments, aggregation of p53 was not due to high p53 protein concentrations. Although p53 is known to bind RNA, treatment with RNAse did not change the aggregation state of p53 suggesting that RNA may not contribute to the quaternary structure of p53. High molecular weight aggregates of p53 were resistant to treatment with 1 M NaCl and also stable in weak acidic conditions. Alkaline pH as well as treatment with 3.5 M NaCl led to a disaggregation of high molecular weight complexes of p53. This treatment resulted in low molecular weight forms consisting probably of dimers to tetramers whereas monomers of p53 are hardly detectable. A nearly complete disaggregation was obtained only with the ionic denaturing detergent sodium dodecyl sulfate. Therefore, one has to assume different types of protein-protein interactions leading to the various quaternary structures of p53.

Diagnostic utility of oncogenes and their products in human cancer

The first clear cut association of an oncogene with a specific cancer is the c-abl translocation in chronic myelogenous leukemia and acute lymphocytic leukemia; it has been observed in 90% of CML cases examined. This is the major contributing factor to its being the target of the first oncogene-based FDA-approved diagnostic test. Although the role of the abl translocation in the tumorigenic process is not yet understood, it is clear that somehow it must be causally related to the disease, and thus is an ideal target for a diagnostic test. The association of this oncogene with a specific cancer is the model on which all others may be based in the future. Second generation tests could easily include PCR on mRNA, and/or in situ hybridization, both of which could be performed using blood samples. Both methods would provide a faster means of testing a large number of cells, however, the methodologies must be improved through automation and computer-aided image analysis, respectively, in order to become useful routine tests. Both neu and epidermal growth factor receptor (EGFR) appear to have a close correlation between overexpression of the gene product and outcome of disease in breast cancer; valuable information for prognosis of the disease. And again, although the actual mechanism of action of these molecules and how this relates to the tumorigenic process is not yet known, it is believed from the very nature of the molecules that they must in some way contribute to the progression of the disease. In both cases, the protein products are overexpressed in tissue, and in the case of Neu, it appears as through at least some of the patients have a Neu-related protein in their serum. These molecules present relatively easy targets for the development of diagnostic/prognostic assays, as antibodies are easily made and can be incorporated into a variety of assay formats. Current assays available, an ELISA for Neu and a radio-ligand binding assay for EGFR, are highly sensitive, reproducible and relatively easy to perform. Only the ELISA is commercially available, however, and hence allows for easy comparison between laboratories. An abvious step towards the routine measurement of EGFR then is the development of a comparable commercially available test. An improvement for both types of assay would be the incorporation of an internal control to gauge the cellular component of the tissue samples that are tested. The outcome of the applications of myc and ras to cancer diagnostics is not so easily predictable, with a couple of exceptions.(ABSTRACT TRUNCATED AT 400 WORDS)

Tumor suppressor genes

For the past decade, cellular oncogenes have attracted the attention of biologists intent on understanding the molecular origins of cancer. As the present decade unfolds, oncogenes are yielding their place at center stage to a second group of actors, the tumor suppressor genes, which promise to teach us equally important lessons about the molecular mechanisms of cancer pathogenesis.

Oncogenes: a review of their clinical application

One objective of this review is to sort through and collate the recent data that suggest that human cellular oncogenes, which have been implicated as the etiologic agents in both animal and human malignancies, have also the potential to be employed as clinical tools in the struggle against cancer. For nearly 10 years, reports have been suggesting that advantage can be taken of cellular oncogenes as to their use as diagnostic and prognostic indicators of cancer and eventually as therapeutic cancer agents. It is also the purpose of this review to give an objective evaluation of these predictions. Moreover, this review will try to highlight some of the significant advances in this most rapidly evolving field of biology. Although the enormity of what has been learned about cellular oncogenes is nothing less than impressive, it is the view here that the routine implementation of oncogenes into the clinical setting will not become evident as early as the many predictions had purported.

Genetic characterization of adenovirus transformed cell revertants resistant to methylglyoxal bis(guanylhydrazone): evidence for the involvement of three genetic loci

Five new independent rat somatic cell mutants resistant to the antileukemic drug methylglyoxal bis(guanylhydrazone) (MGBG) were isolated after mutagen treatment. The mutants were 7- to 10-fold more resistant to MGBG than were the parental wild-type cells. When the MGBG-resistant (MGR) mutants were exposed to the drug in the presence of Tween-80, a nonionic detergent, they became as sensitive (MGS) to MGBG as the wild-type cells, indicating that they were probably permeability mutants. Genetic analysis of hybrids between MGR mutants and wild-type cells showed that MGR and the nontransformed alleles to be recessive to the MGS (wild-type) and transformed phenotype, respectively. Complementation analysis of the seven mutants revealed three functional genetic units or loci responsible not only for the MGR phenotype but also for tumorigenicity as determined in nude mice. Only the MGS hybrids produced tumors in the nude mice, whereas the MGR hybrids and mutants did not. Our results suggest the existence of cellular membrane components that are responsible both for cellular tumorigenicity and resistance to MGBG.

Expression and interactions of human adenovirus oncoproteins

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Stable T-p53 complexes are not required for replication of simian virus 40 in culture or for enhanced phosphorylation of T antigen and p53

We generated a number of simian virus 40 (SV40) mutants with single amino acid substitutions in T antigen between residues 388 and 411. All but one mutant (398LV) replicated like wild-type SV40 and gave rise to normal-size plaques. Three different mutations at residue 402 (Asp to Glu, Asn, or His) totally prevented the formation of stable complexes with the cellular protein p53 in monkey cells but had no effect on virus replication. Only one other mutation in this region, involving residue 401 (Met to Thr), slightly inhibited the formation of T-monkey p53 complexes. The three mutant T antigens with substitutions at residue 402 also formed no stable complexes with human p53 but generated low levels of complexes with mouse p53. These results indicate that residue 402 is critical for binding to monkey and human p53 proteins and is important for binding to mouse p53. We suggest that it is one of several points of contact. In cells infected with any one of the three residue 402 mutant viruses. T antigen and p53 became increasingly phosphorylated, as they were in cells infected with wild-type virus. Our data therefore show that stable T-p53 complexes are not required for replication of SV40 in culture or for enhanced phosphorylation of either protein.

Viral and cellular oncogene expression during progressive malignant transformation of SV40 transformed human fibroblasts

In vitro investigation of the multistep neoplastic progression which occurs during transformation of human cells has been hindered by resistance of human cells to both immortalization and tumorigenicity (Mut. Res. 199; 273, 1988). Previously our laboratory established a cell line, HSF4-T12, by transfection of normal human foreskin fibroblasts with the plasmid pSV3-neo which contains the early genes of simian virus 40 (SV40). A multistep progression in karyotypic alterations and transformed phenotype occurred resulting in a neoplastic cell line that was immortal, transformed, and tumorigenic. We have examined changes in the SV40 proteins, large T (T-antigen) and small t (t-antigen) antigens, and in the cellular protein, p53, during progressive transformation of these cells. Total viral protein expression relative to total cellular protein increased following immortalization of HSF4-T12 as did the ratio of T-antigen to t-antigen. Interestingly, no significant change in DNA content accompanied immortalization. However, during the progressive in vitro transformation of HSF4-T12 which occurred primarily post-immortalization, DNA index increased to 1.6 but only small additional increases in T-antigen expression were seen. No consistent or critical role for t-antigen in development of the tumorigenic phenotype was found in this system.

Temperature-dependent switching between "wild-type" and "mutant" forms of p53-Val135

The p53 gene is a suppressor of abnormal cell growth but is also subject to oncogenic activation by mutation. The mutant allele p53-Val135, has recently been discovered to be temperature-sensitive and functions as an oncogene at 37 degrees C and as a tumor suppressor at 32.5 degrees C. In order to investigate the molecular mechanism underlying the temperature sensitivity of p53-Val135 rabbit reticulocyte lysate was used to translate the p53 mRNAs in vitro at 37 degrees C and at 30 degrees C. The immunoreactivity and T antigen binding of wild-type protein p53-Ala135 were unaffected by temperature and were similar to wild-type p53 expressed in vivo. In contrast, the mutant p53-Val135 protein was markedly affected by temperature. At 37 degrees C p53-Val135 showed reduced T antigen binding and did not react with monoclonal antibodies PAb246 and PAb1620. At 30 degrees C, p53-Val135 behaved as the wild-type p53. Temperature also exerted a post-translational effect on p53-Val135 with complete conversion from wild-type to mutant phenotype within two minutes of temperature shift from 30 degrees C to 37 degrees C. There was incomplete conversion from mutant to wild-type phenotype when the temperature was shifted down from 37 degrees C to 30 degrees C. We propose that the temperature dependent forms of p53-Val135 represent conformational variants of the p53 protein with opposing functions in cell growth control.

Dissociation of Rb-binding and anchorage-independent growth from immortalization and tumorigenicity using SV40 mutants producing N-terminally truncated large T antigens

The large T antigen of SV40 is both necessary and sufficient for conversion of primary mouse cells to cells with fully transformed phenotype. In this investigation, the influence of the N-terminal portion of T antigen on individual transformed cell characteristics was probed by using mutants bearing deletions in the 5'T antigen coding sequence. Specifically, DNA constructs expected to produce T antigens missing the first 109, 127, 150, or 176 amino acids or internal amino acid segments between 117 and 250 were tested for the ability to immortalize C57Bl/6 mouse embryo fibroblasts. The transformed cell properties displayed by clonally derived cell lines were then examined. The results indicated that neither the first 127 amino acids nor amino acids 127-250 of T antigen were necessary for efficient immortalization of primary cells or for their tumorigenicity. Functions mapped within these regions, including binding of the retinoblastoma susceptibility gene product (Rb) and transactivation of heterologous promoters, therefore, were not required to confer either of these growth properties. In addition the results showed that anchorage-independent growth was separable genetically from tumorigenicity and that removal of amino acids within the first 250 residues of T antigen compromised other transformed cell growth properties.

SV40 immortalizes myogenic cells: DNA synthesis and mitosis in differentiating myotubes

Primary skeletal muscle myoblasts have a limited proliferative capacity in cell culture and cease to proliferate after several passages. We examined the effects of several oncogenes on the immortalization and differentiation of primary cultures of rat skeletal muscle myoblasts. Retroviruses containing a SV40 large T antigen (LT) gene very efficiently immortalize myogenic cells. The immortalized cell lines retain a very high differentiation capacity and form, in the appropriate culture conditions, a very dense network of muscle fibers. As in primary culture, cell fusion is associated with the synthesis of large amounts of muscle-specific proteins. However, unlike normal myoblasts (and previously established myogenic cell lines), nuclei in the multinucleated fibers of SV40-immortalized cells synthesize DNA and enter mitosis. Thus, withdrawal from DNA synthesis is not obligatory for cell fusion and biochemical differentiation. Using a retrovirus coding for a temperature-sensitive SV40 LT, myogenic cell lines were produced in which the SV40 LT could be inactivated by a shift from 33 degrees C to 39 degrees C. The inactivation of LT induced massive cell fusion and synthesis of muscle proteins. The nuclei in those fibers did not synthesize DNA, nor did they undergo mitosis. This approach enabled the reproducible establishment of myogenic cell lines from very small populations of myoblasts or single primary myogenic clones. Activated p53 also readily immortalized cells in primary muscle cultures, however the cells of eight out of the nine cell lines isolated had a fibroblastic morphology and could not be induced to form multinucleated fibers.

Transformation by simian virus 40 does not involve the mutational activation of p53 to an oncogenic form

We investigated whether the p53 protein of SV40-transformed mouse cells reacted with the conformation-dependent monoclonal antibody pAb246. This antibody can usually distinguish between a p53 with anti-proliferative activity like the wild-type protein (pAb246+) and a mutated form of p53 with oncogenic activity (pAb246-). Of the 13 cell lines that were screened, 12 contained the pAb246+ form of p53 and one had the pAb246- form. We showed that SV40 did not induce an activating mutation in the p53 of this latter cell line, because the cells from which it was derived were also pAb246-. Cascade immunoprecipitation experiments demonstrated that in three SV40-transformed cell lines that were examined, all of the p53 was of the pAb246+ form making it unlikely that small amounts of pAb246- p53 were responsible for the transformation properties of these cells. We therefore concluded that SV40-mediated transformation of murine cells is not dependent on the activation of their p53 to an oncogenic form, and that, in all probability, transformation is allowed to occur in part because the anti-proliferative activity of p53 is blocked by SV40 T antigen.

The retinoblastoma susceptibility gene product undergoes cell cycle-dependent dephosphorylation and binding to and release from SV40 large T

Synchronized monkey cells pulse-labeled with [35S]-methionine and chased for various lengths of time were extracted, and immunoprecipitations were performed using monoclonal antibodies directed against the retinoblastoma protein (RB) and SV40 T antigen (T). By following a discrete population of these two proteins through the cell cycle, the following information was obtained. RB, which is wholly unphosphorylated in G1, became phosphorylated at the beginning of S and remained phosphorylated through S and G2. RB was, then, completely dephosphorylated between the end of G2 and the beginning of G1. Second, while all of the detectable unphosphorylated RB can be found complexed with T, these complexes present during G1 dissociated in S and reformed again in M or early G1. Finally, T molecules appeared to oligomerize prior to binding RB. Thus, complex formation between T and RB may be regulated in part by the cell cycle-dependent phosphorylation and dephosphorylation of RB and by the quaternary structure of T.

The cellular chromatin is an important target for SV40 large T antigen in maintaining the transformed phenotype

To identify cellular targets of simian virus 40 large T antigen (SV40 large T) important for the maintenance of cellular transformation, we have compared biological properties of SV40 tsA58 mutant large T antigens expressed in cells of a matched pair of SV40 tsA58 N-type (temperature-sensitive) and A-type (temperature-insensitive) transformants of the normal rat fibroblast line F111 (D. Pintel et al., J. Virol. 38, 518-528, 1981). Characterization of the selected cell lines demonstrated that cells of the N-type transformant [FR(tsA58)A] exhibited properties similar to those of the corresponding SV40 wild-type transformant [FR(wt648)] at the permissive growth temperature (32 degrees ), but reverted to a phenotype indistinguishable from the parental F111 cells at the nonpermissive growth temperature (39 degrees). At both growth temperatures, cells of the A-type transformant [FR(tsA58)57] were very similar to FR(wt648) cells in all properties analyzed. Both mutant-transformed cell lines expressed authentic tsA58 mutant large T antigens at comparable steady-state levels. Analysis of the subnuclear distribution of large T antigens in wild-type and in mutant-transformed cells kept at permissive or at nonpermissive growth temperature, respectively, revealed an important biological difference between the mutant T antigens in N- and A-type transformants: Whereas the subnuclear distribution of wild-type large T in FR(wt648) cells remained unchanged at both growth temperatures, mutant large T in FR(tsA58)A cells (N-type transformant) already 1 day after the shift to the nonpermissive growth temperature no longer stably associated with nuclear substructures, notably the cellular chromatin. In contrast, mutant large T in FR(tsA58)57 cells (A-type transformant) retained this ability. The ability (or inability) of the mutant T antigens to associate with the cellular chromatin in vivo was paralleled by different DNA binding properties of the mutant large T antigens in vitro. Large T in FR(tsA58)A cells no longer bound to the SV40 ORI in vitro after the shift to the nonpermissive growth temperature, whereas large T in FR(tsA58)57 cells at the elevated growth temperature had preserved this activity to a degree similar to its ability to associate with the cellular chromatin. We suggest that in the system of matched pairs of N- and A-type transformants analyzed in this study, expression of the transformed phenotype in FR(tsA58)57 (A-type) cells at the nonpermissive growth temperature is due to the preservation of a biologically active conformation of the mutant large T, allowing it to maintain its interaction with specific targets at the cellular chromatin.

Simian virus 40 large T antigen induces or activates a protein kinase which phosphorylates the transformation-associated protein p53

The cellular phosphoprotein p53 is presumably involved in simian virus 40 (SV40)-induced transformation. We have monitored changes in the state of phosphorylation of p53 from normal versus SV40-infected or -transformed cells. In normal cells, p 53 was hardly phosphorylated. Upon infection or transformation, a quantitative and qualitative increase in p53 phosphorylation was observed as revealed by two-dimensional phosphopeptide analysis. This increase was dependent on a functional large T antigen. In rat cells, enhanced phosphorylation of p53 resulted in conversion to a second, electrophoretically distinct form. In cells transformed with transformation-defective mutants, phosphorylation of p53 was reduced and conversion to form 2 was inefficient. These data suggest (i) that SV40 large T antigen induces or activates a protein kinase, one substrate of which is p53, (ii) that transformation-defective mutants are impaired in kinase induction, and (iii) that either a certain phosphorylation state of p53 or the SV40-induced kinase is critical for efficient transformation.

Functional interaction of nuclear transport-defective simian virus 40 large T antigen with chromatin and nuclear matrix

We analyzed the subcellular distribution of nuclear transport-defective simian virus 40 Lys-128-mutant (cT-3 [R. E. Lanford and J. S. Butel, Cell 37:801-813, 1984] and d10 [D. Kalderon, W. D. Richardson, A. F. Markham, and A. E. Smith, Nature (London) 311:33-38, 1984]) large T antigens in various Lys-128-mutant-transformed rodent cells and in Lys-128-mutant d10-infected TC7 cells. Small but significant amounts of the mutant large T antigens were found in association with nuclear substructures, both in mutant-transformed and in mutant-infected cells. Experiments with TC7 cells made incompetent for cell division by 60Co irradiation supported the assumption that Lys-128-mutant large T antigen did not associate with nuclear components during mitosis but most likely was transported into the nucleus because the Lys-128 mutation was leaky for nuclear transport. Low-level simian virus 40 DNA replication and production of infectious mutant virus progeny in TC7 cells indicated that the association of Lys-128-mutant large T antigen with nuclear substructures is functional.

Human papilloma virus DNAs immortalize normal human mammary epithelial cells and reduce their growth factor requirements

Human papilloma virus (HPV) types 16 and 18 are most commonly associated with cervical carcinoma in patients and induce immortalization of human keratinocytes in culture. HPV has not been associated with breast cancer. This report describes the immortalization of normal human mammary epithelial cells (76N) by plasmid pHPV18 or pHPV16, each containing the linearized viral genome. Transfectants were grown continuously for more than 60 passages, whereas 76N cells senesce after 18-20 passages. The transfectants also differ from 76N cells in cloning in a completely defined medium called D2 and growing in a minimally supplemented defined medium (D3) containing epidermal growth factor. All transfectants tested contain integrated HPV DNA, express HPV RNA, and produce HPV E7 protein. HPV transfectants do not form tumors in a nude mouse assay. It is concluded that products of the HPV genome induce immortalization of human breast epithelial cells and reduce their growth factor requirements. This result raises the possibility that HPV might be involved in breast cancer. Furthermore, other tissue-specific primary epithelial cells that are presently difficult to grow and investigate may also be immortalized by HPV.

Complex formation of human papillomavirus E7 proteins with the retinoblastoma tumor suppressor gene product

The E7 proteins encoded by the human papillomaviruses (HPVs) associated with anogenital lesions share significant amino acid sequence homology. The E7 proteins of these different HPVs were assessed for their ability to form complexes with the retinoblastoma tumor suppressor gene product (p105-RB). Similar to the E7 protein of HPV-16, the E7 proteins of HPV-18, HBV-6b and HPV-11 were found to associate with p105-RB in vitro. The E7 proteins of HPV types associated with a high risk of malignant progression (HPV-16 and HPV-18) formed complexes with p105-RB with equal affinities. The E7 proteins encoded by HPV types 6b and 11, which are associated with clinical lesions with a lower risk for progression, bound to p105-RB with lower affinities. The E7 protein of the bovine papillomavirus type 1 (BPV-1), which does not share structural similarity in the amino terminal region with the HPV E7 proteins, was unable to form a detectable complex with p105-RB. The amino acid sequences of the HPV-16 E7 protein involved in complex formation with p105-RB in vitro have been mapped. Only a portion of the sequences that are conserved between the HPV E7 proteins and AdE1A were necessary for association with p105-RB. Furthermore, the HPV-16 E7-p105-RB complex was detected in an HPV-16-transformed human keratinocyte cell line.

Transformed cells overexpress a cytosolic nucleic acid binding protein

A protein blotting technique was used to identify a 57,000 dalton cytosolic nucleic-acid-binding protein found in neoplastically transformed cell lines. Specifically, greater amounts of this protein were found in Kirsten Murine Sarcoma Virus-, Simian Virus 40-, and methylcholanthrene-transformed Balb 3T3 cells than in comparable untransformed cells. An analogous protein was identified in other transformed mammalian cells. Increased levels of the DNA binding protein in sarcoma virus transformants were shown to be dependent on the continued maintenance of the transformed phenotype. The properties of this protein are compared to those of other previously reported nucleic acid binding proteins.

The cellular 107K protein that binds to adenovirus E1A also associates with the large T antigens of SV40 and JC virus

The association between the retinoblastoma protein (p105-RB) and either the large T antigen of SV40 or the E1A proteins of adenovirus is thought to be an important step in transformation by these viral oncogenes. E1A and large T antigen share a small region of amino acid homology that is necessary for high affinity binding with p105-RB. Mutations of this homology region were shown to reduce drastically the frequency of transformation mediated by the E1A or large T oncogenes. Previously, this small region in E1A was shown to be sufficient for interaction with a second cellular protein of 107,000 daltons (107K). Here we show that in human cells, the large T antigens of SV40 or JC virus also form complexes with 107K. Demonstration of complexes between 107K and the large T antigens of SV40 and JC virus suggests that these associations may represent another component of a common mechanism for transformation between adenoviruses and polyoma viruses.

Nuclear subcompartmentalization of simian virus 40 large T antigen: evidence for in vivo regulation of biochemical activities

Simian virus 40 large T antigen (large T) in the early and the late phases of infection differs significantly in its sequence-specific DNA-binding and ATPase activities, indicating that different large-T populations participate in virus-specific events at various stages of the infectious cycle. To further characterize these large-T populations, we have analyzed nuclear subclasses of large T, isolated from their in vivo location, for their biochemical activities. We show that chromatin- and nuclear matrix-associated large-T molecules exhibit different simian virus 40 control region (ORI) DNA-binding and ATPase activities. The association of large T with a certain nuclear substructure, therefore, subcompartmentalizes large-T molecules exerting different biochemical activities. Nuclear subcompartmentalization thus may provide a higher-order level for the regulation of biochemical activities of large T in vivo.

Correlation of DNA content, p53, T antigen, and V antigen in simian virus 40-infected human diploid cells

Human diploid fibroblasts (HDF) have a finite life span in cell culture which can be extended when transformed with simian virus 40 (SV40). Flow cytometric analysis of SV40-HDF transformation allowed DNA content changes to be correlated with the appearance, quantity, and distribution of T antigen, p53, and V antigen, three proteins associated with this process. These studies demonstrated a shift in the DNA content to tetraploidy, which was correlated with the age of the SV40-HDF but not the time of infection. A significant increase of the epitope recognized by PAb122 to host p53 and the epitope PAb101 to SV40 T antigen occurred at the same time the tetraploid population appeared. However, an antigen reactive with SV40 V antibody was present at high levels in most of the population early after infection, but the levels declined with time. The percentage of PAb101-T antigen-positive cells increased more rapidly in cells infected at a late passage, and this was concomitant with the shift in DNA content to tetraploid. Analysis of the mean fluorescence of total, gated populations (G1, G2, and greater than G2) demonstrated that a threshold level of p53 and T antigen was reached in each compartment of the cell cycle. As the transformed phenotype appeared, a population of cells was continually released into the supernatant, and although these cells had a DNA pattern similar to the monolayer cells, the T antigen and p53 levels were 3-5 times higher in the tetraploid G2 cells. These studies correlated the expression of proteins associated with viral transformation in HDF which vary with time and shift in DNA content.

p59v-rel, the transforming protein of reticuloendotheliosis virus, is complexed with at least four other proteins in transformed chicken lymphoid cells

Previous studies have identified the protein product of v-rel, the oncogene carried by reticuloendotheliosis virus (REV), as a 59,000-dalton phosphoprotein located predominantly in the cytosol of transformed chicken lymphoid cells. In immune precipitates of p59v-rel, there is a closely associated protein kinase activity. In chicken lymphoid cells that do not contain REV, p68c-rel is found free in the cytosol not associated with other proteins and not detectably phosphorylated. In this study, we found that immune precipitates of 59v-rel from REV-transformed cells contain at least four other proteins, of approximate molecular weights 124, 115, 68, and 36 kilodaltons (kDa). The 124-, 115-, and 36-kDa proteins are apparently unrelated to p59v-rel in sequence, and their coprecipitation suggests that they are complexed with p59v-rel. The coprecipitating 68-kDa protein was found to be p68c-rel, which, like the other three proteins, precipitates by virtue of its association with p59v-rel. Glycerol gradient analysis suggested the presence of more than one type of complex: one containing p115, p68c-rel, p59v-rel, and p36, and another containing p124, p115, p59v-rel, and possibly p68c-rel. In vitro kinase activity was found in all size classes, coinciding with the distribution of p115 and p59v-rel. The complex(es) was stable under a variety of conditions, including a wide range of ionic strengths, chelators, and detergents, and through multiple cycles of immune precipitation and elution. This suggests a specific and functionally significant interaction among the members that may be of direct relevance to the mechanism of REV-induced transformation.

Oligomerization of oncoprotein p53

Cellular phosphoprotein p53, which seems to be a multifunctional protein, may be assigned to different structural subclasses. Recently established immortalized or transformed cell lines that overexpress p53 allowed us to perform a detailed analysis of the quaternary structure of p53. By means of sucrose density gradient centrifugation, we found in simian virus 40-transformed cells that overexpress p53, in addition to high-molecular-weight T-p53 complexes, low-molecular-weight forms. The level of T-p53 complexes within simian virus 40-transformed cells seemed to be determined by the intracellular concentration of p53. However, the presence of uncomplexed T antigen and p53 indicated that an appropriate modification of at least one of the two proteins appears to be necessary for complex formation. Using different monoclonal antibodies that distinguish between (i) p53 associated with T antigen or heat shock proteins and (ii) p53 in apparently free form, we found p53 from transformed cells always in high-molecular-weight forms. p53 from normal and immortalized cells, however, was found mainly in low-molecular-weight forms. Pulse-labeling experiments revealed that oligomerization of p53 is a very rapid process. Monomeric forms of p53 which could be detected only by 2 min of pulse-labeling were rapidly converted to stable, high-molecular-weight oligomers. Furthermore, our data indicate a correlation between the occurrence of p53 in high-molecular-weight forms and the transformation state of the cell.

Expression of simian virus 40 T antigen in insect cells using a baculovirus expression vector

Simian virus 40 (SV40) large T and small t antigens were synthesized in insect cells using the baculovirus Autographa californica as an expression vector. A recombinant virus containing a genomic copy of the SV40 early region expressed high levels of small t antigen but only low levels of large T antigen. However, very high levels of T antigen synthesis were observed when viruses were constructed with a cDNA copy of the large T antigen mRNA. Insect cells were capable of modifying T antigen by phosphorylation, palmitylation, glycosylation, and oligomerization. Functional assays demonstrated that the origin-specific DNA binding, ATPase, and helicase activities of insect cell-derived T antigen were comparable to T antigen synthesized in mammalian cells. Use of the baculovirus vector system to produce T antigen should facilitate future investigations requiring large quantities of T antigen.

Expression and complex formation of simian virus 40 large T antigen and mouse p53 in insect cells

Recombinant baculoviruses were constructed which express simian virus 40 large T antigen (SVT-Ag) or murine p53 to high levels in infected insect cells. Characterization of the expressed proteins revealed that they display many properties of the corresponding mammalian-derived proteins. Both proteins are of wild-type size, localize to the nucleus, are recognized by several SVT-Ag- or p53-specific monoclonal antibodies, and are phosphorylated in this system. Complexes are formed between baculovirus-derived SVT-Ag and p53 after coinfection of insect cells with both recombinant viruses. After infection of insect cells with either virus individually, each protein can self-associate to form a variety of oligomeric species. Pulse-chase experiments indicated that both SVT-Ag and p53 are highly stable in insect cells, even in the absence of complex formation.

Association between an oncogene and an anti-oncogene: the adenovirus E1A proteins bind to the retinoblastoma gene product

One of the cellular targets implicated in the process of transformation by the adenovirus E1A proteins is a 105K cellular protein. Previously, this protein had been shown to form stable protein/protein complexes with the E1A polypeptides but its identity was unknown. Here, we demonstrate that it is the product of the retinoblastoma gene. The interaction between E1A and the retinoblastoma gene product is the first demonstration of a physical link between an oncogene and an anti-oncogene.

Intranuclear distribution of SV40 large T-antigen and transformation-related protein p53 in abortively infected cells

The intranuclear localization of SV40 T-antigen (T-Ag) and the cellular protein p53 was studied in SV40 abortively infected baby mouse kidney cells using two complementary methods of ultrastructural immunocytochemistry in combination with preferential staining of nuclear RNP components and electron microscope autoradiography. Both proteins were revealed in association with peri- and interchromatin RNP fibrils containing the newly synthesized hnRNA. In addition, T-Ag and p53 remained bound, at least in part, to the residual internal nuclear matrix following nuclease and salt extractions of infected cells. The localization of T-Ag was different in SV40 lytically infected monkey kidney cells since, in addition to hnRNP fibrils, the viral protein was also associated with cellular chromatin. However, when lytic infection was performed in conditions of blocked viral DNA replication, T-Ag was no longer associated with the cellular chromatin but remained bound to the hnRNP fibrils. We conclude that the transforming and lytic functions of T-Ag can be distinguished by different subnuclear distributions. The significance of the association of T-Ag and p53 with hnRNP fibrils and the internal nuclear matrix is discussed in relation to the role of these structures in the control of cellular mRNA biogenesis.

Variable expression of SV40 large T antigen in CV1 cell clones

Using immunofluorescence and immunoadsorption, CV1 cell clones MA2, V4, USA3, TR7 and P3 infected with SV40 were found to express variably SV40 large T antigen. The monoclonal antibody used was Pab 419. The results indicate that P3 cells express T antigen to a considerable level as early as 10 h post-infection, while that of TR7 and USA3 cells is minute as judged from their positive nuclei. MA2 and V4 cells did not show any positive nuclei over this period of infection. At 20 h post-infection MA2, V4 and USA3 cells developed a considerable amount of fluorescence in their nuclei while TR7 and P3 cells produced high values. By immunoadsorption of cell extracts for the same periods of infection, similar results were obtained on the electrophoretograms. We also relate these findings with those from induction of heatshock proteins by SV40 infection.

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.