Deoxyribonucleic Acid Replication in Simian Virus 40-Infected Cells: III. Comparison of Simian Virus 40 Lytic Infection in Three Different Monkey Kidney Cell Lines

A comparative study of simian virus 40 (SV40) lytic infection in three different monkey cell lines is described. The results demonstrate that viral deoxyribonucleic acid (DNA) synthesis and infectious virus production begin some 10 to 20 hr earlier in CV-1 cells and primary African green monkey kidney (AGMK) cells than in BSC-1 cells. Induction of cellular DNA synthesis by SV40 was observed in CV-1 and AGMK cells but not with BSC-1 cells. Excision of large molecular weight cellular DNA to smaller fragments was easily detectable late in infection of AGMK cells. Little or no excision was observed at comparable times after infection of CV-1 and BSC-1 cells. The different kinds of responses of these three monkey cell lines during SV40 lytic infection suggest the involvement of cellular functions in the virus-directed induction of cellular DNA synthesis and the excision of this DNA from the genome.     

Negative regulation of mitotic promoting factor by the checkpoint kinase chk1 in simian virus 40 lytic infection

Lytic infection of African green monkey kidney (CV-1) cells by simian virus 40 (SV40) is characterized by stimulation of DNA synthesis leading to bypass of mitosis and replication of cellular and viral DNA beyond a 4C DNA content. To define mechanisms underlying the absence of mitosis, the expression levels of upstream regulatory molecules of mitosis-promoting factor (MPF) were compared in parallel synchronized cultures of SV40-infected and uninfected CV-1 cells. The DNA replication/damage checkpoint kinase Chk1 was phosphorylated in both uninfected and SV40-infected cultures arrested at G(1)/S by mimosine, consistent with checkpoint activation. Following release of uninfected cultures from G(1)/S, Chk1 phosphorylation was lost even though Chk1 protein levels were retained. In contrast, G(1)/S-released SV40-infected cultures exhibited dephosphorylation of Chk1 in S phase, followed by an increase in Chk1 phosphorylation coinciding with entry of infected cells into >G(2). Inhibitors of Chk1, UCN-01 and caffeine, induced mitosis and abnormal nuclear condensation and increased the protein kinase activity of MPF in SV40-infected CV-1 cells. These results demonstrate that SV40 lytic infection triggers components of a DNA damage checkpoint pathway. In addition, chemical inhibition of Chk1 activity suggests that Chk1 contributes to the absence of mitosis during SV40 lytic infection.     

Simian Virus 40-Host Cell Interaction During Lytic Infection

Both exponentially growing and serum-arrested subcloned CV-1 cell cultures were infected with simian virus 40 (SV40). By 24 h after infection 96% of the nuclei of these permissive cells contained SV40 T-antigen. Analysis of the average DNA content per cell at various times after infection indicated that by 24 h most of the cells contained amounts of DNA similar to those normally found in G(2) cells. Analysis of cell cycle distributions indicated that a G(2) DNA complement was maintained by over 90% of the cells in the infected populations 24 to 48 h postinfection. Cells continued to synthesize SV40 DNA during the first 50 h after infection, and cytopathic effect was first observed 60 h after inoculation. After infection the number of mitotic cells that could be recovered by selective detachment decreased precipitously and was drastically reduced by 24 h. A study of the kinetics of decline in the number of mitotic cells suggests that this decline is related to an event during the cell cycle at or near the G(1)-S-phase border upon which commencement of SV40 DNA replication apparently depends. It was concluded that after SV40 infection, stationary cells are induced to cycle, and cycling cells complete one round of cellular DNA synthesis but do not divide. Although the infected cells continue to synthesize viral DNA, they do not appear able to reinitiate cellular DNA replication units. These results imply that the abundance of T-antigen (produced independently of cell cycle phase) in the presence of the enzymes required for continued DNA synthesis is not sufficient for reinitiation of cellular DNA synthesis.     

Histone synthesis during infection of monkey kidney cells with Simian Virus 40.

The synthesis of histones during lytic infection of BSC-1 (African Green Monkey kidney) cells with SV40 has been investigated. The synthesis of all five classes of histones was stimulated, and all classes appeared to be stimulated to the same extent. The increase in rate of histone synthesis in response to SV40 infection was detectable several hours before SV40 DNA synthesis was measureable, and the rate of histone synthesis decreased at a time when SV40 DNA synthesis was occuring at a maximal or relatively high rate. In addition, the changes in rates of histone synthesis did not correlate well with the rates of host DNA synthesis during infection. Thus it appears that DNA synthesis and histone synthesis may not be strictly coupled in SV40 infected cells.     

Temperature-Sensitive Simian Virus 40 Mutant Defective in a Late Function

A temperature-sensitive simian virus 40 (SV40) mutant, tsTNG-1, has been isolated from nitrosoguanidine-treated and SV40-infected African green monkey kidney (CV-1) cultures. Replication of virus at the nonpermissive temperature (38.7 C) was 3,000-fold less than at the permissive temperature (33.5 C). Plaque formation by SV40tsTNG-1 deoxyribonucleic acid (DNA) on CV-1 monolayers occurred normally at 33.5 C but was grossly inhibited at 38.7 C. The time at which virus replication was blocked at 38.7 C was determined by temperature-shift experiments. In shift-up experiments, cultures infected for various times at 33.5 C were shifted to 38.7 C. In shift-down experiments, cultures infected for various times at 38.7 C were shifted to 33.5 C. All cultures were harvested at 96 hr postinfection (PI). No virus growth occurred when the shift-up occurred before 40 hr PI. Maximum virus yields were obtained at 96 hr PI when the shift-down occurred at 66 hr, but only about 15% of the maximum yield was obtained when the shift-down occurred at 76 hr PI. These results indicate that SV40tsTNG-1 contains a conditional lethal mutation in a late viral gene function. Mutant SV40tsTNG-1 synthesized T antigen, viral capsid antigens, and viral DNA, and induced thymidine kinase activity at either 33.5 or 38.7 C. The properties of the SV40 DNA synthesized in mutant-infected CV-1 cells at 33.5 or 38.7 C were very similar to those of SV40 DNA made in parental virus-infected cells, as determined by nitrocellulose column chromatography, cesium-chloride-ethidium bromide equilibrium centrifugation, and by velocity centrifugation in neutral sucrose gradients. Mutant SV40tsTNG-1 enhanced cellular DNA synthesis in primary cultures of mouse kidney cells at 33.5 and 38.7 C and also transformed mouse kidney cultures at 36.5 C. SV40tsTNG-1 was recovered from clonal lines of transformed cells after fusion with susceptible CV-1 cells and incubation of heterokaryons at 33.5 C, but not at 38.7 C.     

Simian Virus 40 Deoxyribonucleic Acid Replication: I. Effect of Cycloheximide on the Replication of SV40 Deoxyribonucleic Acid in Monkey Kidney Cells and in Heterokaryons of SV40-transformed and Susceptible Cells

Infectious deoxyribonucleic acid (DNA) was extracted from green monkey kidney (CV-1) cultures at various times after the cultures were infected with simian virus 40 (SV40) at input multiplicities of 0.01 and 0.1 plaque-forming unit (PFU) per cell. A pronounced decrease in infectious DNA was observed from 3 to 16 hr after virus infection, suggesting that structurally altered intracellular forms may have been generated early in infection. Evidence is also presented that SV40 DNA synthesis requires concurrent protein synthesis. DNA replication was studied in the presence and absence of cycloheximide in: (i) SV40-infected and uninfected cultures of CV-1 cells; (ii) cultures synchronized with 1-β-d-arabinofuranosylcytosine (ara-C) for 24 to 30 hr prior to the addition of cycloheximide; and (iii) in heterokaryons of SV40-transformed hamster and susceptible monkey kidney cells. DNA synthesis was determined by pulse-labeling the cultures with ³H-thymidine at various times from 24 to 46 hr after infection. In addition, the total infectious SV40 DNA was measured. Addition of cycloheximide, even after early proteins had been induced, grossly inhibited both SV40 and cellular DNA syntheses. The activities of thymidine kinase, DNA polymerase, deoxycytidylate deaminase, and thymidylate kinase were measured; these enzyme activities remained high for at least 9 hr in the presence of cycloheximide. SV40 DNA prelabeled with ³H-thymidine before the addition of cycloheximide was also relatively stable during the time required for cycloheximide to inhibit further DNA replication.     

Simian virus 40 host range/helper function mutations cause multiple defects in viral late gene expression.

Simian virus 40 (SV40) deletion mutants dlA2459 and dlA2475 express T antigens that lack the normal carboxy terminus. These mutants are called host range/helper function (hr/hf) mutants because they form plaques at 37 degrees C on BSC-1 and Vero monkey kidney cell lines but not on CV-1p monkey kidney cells. Wild-type SV40 can provide a helper function to permit growth of human adenoviruses in monkey kidney cells; the hr/hf mutants cannot. Progeny yields of hr/hf mutants are also cold sensitive in all cell lines tested. Patterns of viral macromolecular synthesis in three cell lines (Vero, BSC-1, and CV-1) at three temperatures (40, 37, and 32 degrees C) were examined to determine the nature of the growth defect of hr/hf mutants. Mutant viral DNA replication was similar to that of the wild type in all three cell lines, indicating that the mutations affect late events in the viral lytic cycle. In mutant-infected Vero cells, in which viral yields were highest, late mRNA levels were similar to those observed during wild-type infection. Levels of viral late mRNA from mutant-infected CV-1 and BSC-1 cells at 32 and 37 degrees C were reduced relative to those of wild-type-infected cells. The steady-state level of the major viral capsid protein, VP1, in mutant-infected CV-1 cells was reduced to the same extent as was late mRNA. The synthesis of agnoprotein could not be detected in mutant-infected CV-1 cells but was readily detected in CV-1 cells infected by wild-type SV40. Primer extension analyses indicated that most late mRNAs from mutant-infected CV-1 cells utilize start sites downstream from the major wild-type cap site (nucleotide 325) and the agnoprotein initiation codon (nucleotide 335). These results indicate that deletion of the carboxyl-terminal domain of T antigen affects viral late mRNA production, both quantitatively and qualitatively. The agnoprotein is detected late in the wild-type SV40 lytic cycle and is thought to play a role in the assembly or maturation of virions. Reduced hr/hf progeny yields could result from decreased capsid protein synthesis and, in the absence of detectable levels of agnoprotein, from inefficient use of available capsid proteins.     

Stimulation of non-histone chromosomal protein synthesis in simian virus 40-infected simian cells.

The pattern of synthesis of non-histone chromosomal proteins in simian virus (SV) 40-infected African green monkey kidney cells was analyzed by polyacryl-amide gel electrophoresis to see whether the changes in chromosomal protein metabolism are involved in the viral-induced synthesis of cellular DNA and mRNA. During the prereplicative phase of infection, the rate of histone synthesis was decreased until 15 h postinfection, whereas that of non-histone protein synthesis was increased after 5 h postinfection and reached a maximum at 10 to 15 h postinfection when viral-induced synthesis of cellular DNA and mRNA began to be observed. Stimulation of non-histone protein synthesis was also observed in the infected cells treated with cytosine arabinoside and was dependent on the multiplicity of infection. Stimulation occurred in almost all species of non-histone proteins. These results suggest that the stimulation of non-histone protein synthesis is caused by an early SV40 function and occurs prior to the viral-induced synthesis of cellular DNA and mRNA. During the replicative phase of infection, a marked increase in the rate of synthesis was observed in the non-histone proteins with molecular weights of about 48,000, 35,000, and 23,000, which were subsequently found to be SV40 capsid proteins.     

On the functional roles of simian virus 40 large and small T-antigen in the induction of a mitotic host response.

The early gene of wild-type (wt) SV40 specifies two related proteins, referred to as large (Mr 88,000) and small (Mr 19,000) T-antigen. Infection with wt SV40 of Go/G1-arrested monkey kidney and CV-1 cell cultures induced in virtually 100% of the cells T-antigen synthesis, followed by a mitotic reaction and the production of SV40 DNA. Parallel cultures were infected with SV40 deletion mutants that produce either no small T-antigen (d1883) or only trace amounts of a truncated form (d1891). Kinetics of synthesis and accumulation of large T-antigen was closely similar to that observed with wtSV40 whereas apparently only 50-60% of the cells participated in the mitotic reaction and the production of viral DNA. These results and those obtained from a comparative study on the abortive (transforming) infection in Go-arrested mouse tissue culture cells indicate that synthesis of large T-antigen alone is sufficient to trigger in 50-60% of the infected cells a mitotic reaction.     

Pattern of Protein Synthesis in Monkey Cells Infected by Simian Virus 40

After infection of several permanent monkey cell lines by simian virus 40 (SV40), four additional protein bands can be detected by simple sodium dodecyl sulfate-polyacrylamide gel electrophoresis of whole-cell extracts. These bands appear only after the onset of viral deoxyribonucleic acid (DNA) synthesis, and inhibitors of DNA synthesis prevent their appearance. Three of them correspond to three previously identified capsid components, VP1, VP2, and VP3. The fourth protein band, which does not correspond to a previously identified virion component, is induced by SV40 infection of CV-1 and BSC-1 cultures but not by infection of MA-134 cultures.     

Simian virus 40 prevents activation of M-phase-promoting factor during lytic infection.

Simian virus 40 (SV40) infection stimulates confluent cultures of monkey kidney cells into successive rounds of cellular DNA synthesis without intervening mitosis. As an initial step in defining the mechanisms responsible for viral inhibition of mitosis, M-phase-promoting factor (MPF) was examined in SV40-infected CV-1 cells passing from G2 phase into a second S phase. MPF is a serine-threonine protein kinase that is essential for mitosis in eukaryotic cells. In SV40-infected cells exiting G2 phase, there was a reduced amount of MPF-associated H1 kinase activity relative to that of uninfected cells passing through mitosis. Both subunits of MPF, cyclin B and the p34cdc2 catalytic subunit, were present and in a complex in infected cells. In uninfected cultures, passage through mitosis was associated with the dephosphorylation of the p34cdc2 subunit, which is characteristic of MPF activation. In contrast, the p34cdc2 subunit remained in the tyrosine-phosphorylated, inactive form in SV40-infected cells passing from G2 phase into a second S phase. These results suggest that although the MPF complex is assembled and modified normally, SV40 interferes with pathways leading to MPF activation.     

Virogenic Properties of Bromodeoxyuridine-sensitive and Bromodeoxyuridine-resistant Simian Virus 40-transformed Mouse Kidney Cells

When simian virus 40 (SV40)-transformed mouse kidney cells (mKS) were grown in the presence of susceptible indicator cells, SV40 was readily recovered from: (i) 15 transformed cell lines, (ii) transformed cells subcultured 45 times over a 7-month period in medium containing antiviral serum and bromodeoxyuridine (dBU), (iii) 45 of 46 clonal lines isolated in the presence of antiviral serum, (iv) 19 of 19 secondary clones isolated from two clonal lines, and (v) dBU-resistant transformed cell lines. dBU-resistant SV40-transformed mouse kidney cell lines were selected and shown to contain the T antigen and to have normal levels of thymidylate kinase and deoxyribonucleic acid (DNA) polymerase, but to be deficient in thymidine (dT) kinase. Radioautographic and biochemical experiments demonstrated that very little (3)H-dT was incorporated into DNA of dBU-resistant cells during a 6-hr labeling period. After infection of dT kinase-deficient mKS cells with vaccinia virus, high levels of dT kinase were induced. The properties of SV40 recovered from dBU-sensitive and dBU-resistant cells were studied. SV40 recovered from transformed cells was shown to express in CV-1 cells at least six functions characteristic of parental virus: synthesis of capsid antigen, synthesis of T antigen, synthesis of viral DNA, induction of dT kinase, induction of DNA polymerase, and induction of host cell DNA synthesis. In addition, SV40 recovered from the transformed cells induced T antigen, dT kinase, deoxycytidylate deaminase, thymidylate kinase, and DNA polymerase in abortively infected mouse kidney cultures, and the virus was also capable of transforming primary cultures of mouse kidney cells.     

Histone gene expression remains coupled to DNA synthesis during in vitro cellular senescence

Despite a decrease in the extent to which confluent monolayers of late compared to early passage CF3 human diploid fibroblasts can be stimulated to proliferate, the time course of DNA synthesis onset is similar regardless of the in vitro age of the cells. A parallel and stoichiometric relationship is maintained between the rate of DNA synthesis and the cellular levels of histone mRNA independent of the age of the cell cultures. Furthermore, DNA synthesis and cellular histone mRNA levels decline in a coordinate manner after inhibition of DNA replication by hydroxyurea treatment. These results indicate that while the proliferative activity of human diploid fibroblasts decreases with passage in culture, those cells that retain the ability to proliferate continue to exhibit a tight coupling of DNA replication and histone gene expression.     

SV40 small T antigen enhances progression to >G2 during lytic infection.

The infection of monkey kidney (CV-1) cells with simian virus 40 (SV40) stimulates the cells into successive rounds of DNA synthesis without an intervening mitosis, leading to the acquisition of a >G2 DNA content. To elucidate the role of small t antigen in cell cycle progression and in viral replication during infection, studies were performed using an SV40 mutant (dl888) that lacks the ability to produce small t. Initially dl888-infected cells move through the first S phase at roughly the same rate as wild-type infected cells. Upon reaching G2, however, the dl888-infected cells progressed to >G2 at a reduced rate relative to wild-type. The slower rate of entry into >G2 of dl888-infected cells is associated with a decrease in total pRb and an increase in the ratio of hypophosphorylated to hyperphosphorylated pRb. The expression of cyclin D1 and p27(kip1) were elevated in dl888-infected cells compared to wild-type-infected CV-1 cells. Taken together, these results indicate that small t antigen plays a role in stimulating entry into >G2 in SV40-infected CV-1 cells, possibly by affecting the regulation of key cell cycle proteins.     

Involvement of eucaryotic deoxyribonucleic acid polymerases alpha and gamma in the replication of cellular and viral deoxyribonucleic acid.

In an effort to identify the deoxyribonucleic acid (DNA) polymerase activities responsible for mammalian viral and cellular DNA replication, the effect of DNA synthesis inhibitors on isolated DNA polymerases was compared with their effects on viral and cellular DNA replication in vitro. DNA polymerase alpha, simian virus 40 (SV40) DNA replication in nuclear extracts, and CV-1 cell (the host for SV40) DNA replication in isolated nuclei all responded to DNA synthesis inhibitors in a quantitatively similar manner: they were relatively insensitive to 2',3'-dideoxythymidine 5'-triphosphate (d2TTP), but completely inhibited by aphidicolin, 1-beta-D-arabinofuranosylcytosine 5'-triphosphate (araCTP), and N-ethylmaleimide. In comparison, DNA polymerases beta and gamma were inhibited by d2TTP but insensitive to aphidicolin and 20--30 times less sensitive to araCTP than DNA polymerase alpha. Herpes simplex virus type 1 (HSV-1) DNA polymerase and DNA polymerase alpha were the only enzymes tested that were relatively insensitive to d2TTP; DNA polymerases beta and gamma, phage T4 and T7 DNA polymerases, and Escherichia coli DNA polymerase I were 100--250 times more sensitive. The results with d2TTP were independent of enzyme concentration, primer-template concentration, primer-template choice, and the labeled dNTP. A specific requirement for DNA polymerase alpha in the replication of SV40 DNA was demonstrated by the fact that DNA polymerase alpha was required, in addition to other cytosol proteins, to reconstitute SV40 DNA replication activity in N-ethylmaleimide-inactivated nuclear extracts containing replicating SV40 chromosomes. DNA polymerases beta and gamma did not substitute for DNA polymerase alpha. In contrast to SV40 and CV-1 DNA replication, adenovirus type 2 (Ad-2) DNA replication in isolated nuclei was inhibited by d2TTP to the same extent as gamma-polymerase. Ad-2 DNA replication was also inhibited by aphidicolin to the same extent as alpha-polymerase. Synthesis of CV-1 DNA, SV40 DNA, and HSV-1 DNA in intact CV-1 cells was inhibited by aphidicolin. Ad-2 DNA replication was also inhibited, but only at a 100-fold higher concentration. We found no effect of 2'-3'-dideoxythymidine (d2Thd) on cellular or viral DNA replication in spite of the fact that Ad-2 DNA replication in isolated nuclei was inhibited 50% by a ratio of d2TTP/dTTP of 0.02. This was due to the inability of CV-1 and Hela cells to phosphorylate d2Thd to d2TTP. These data are consistent with the hypothesis that DNA polymerase alpha is the only DNA polymerase involved in replicating SV40 DNA and CV-1 DNA and that Ad-2 DNA replication involves both DNA polymerases gamma and alpha.     

DNA polymerase and simian virus 40 infection of resting monkey cells: induction of aphidicolin resistant alpha-polymerase.

Tightly confluent monkey cell lines BSC-1 and CV-1 held in stale medium for several days exhibited an extremely low level of thymidine incorporation into cellular DNA. Yet, these cells contained a level of alpha-polymerase equal to about 15% of the level in rapidly dividing cells, and they still were capable of supporting replication of SV40 DNA. SV40 infection and culture in stale medium resulted in a four-fold induction of alpha-polymerase in CV-1 cells, whereas no change in alpha-polymerase level was observed in BSC-1 cells. Characterization of alpha-polymerase partially purified from infected CV-1 cells revealed that 80-90% of the enzyme activity was aphidicolin resistant. SV40 DNA replication in resting CV-1 cells, however, was aphidicolin sensitive. SV40 infection of resting CV-1 cells may induce an aphidicolin-resistant enzyme or lead to a modified alpha-polymerase species.     

Characterization of simian virus 40 transformed African green monkey cells (CV-1). I. Defective virion and viral genome.

Several clones of SV40 transformed CV-1 cells have been characterized for the production of T- and V-antigens and for the state of viral genome. The transformed CV-1 cells failed to produce infectious virions as assayed after sonication or cocultivation and fusion with normal CV-1 cells, and were resistant to super-infection by SV40. Some clones of the transformed cells contained V-antigens. The population of V-antigen positive cells varied from 0 to 100% depending on the passage number while the T-antigen positive cells were always 100%. The virions isolated from the transformed cells were similar in morphology to complete SV40, but lighter in density than complete SV40. In one clone, a small amount of SV40 DNA was detectable in a free state while a large proportion of the DNA hybridizable with SV40 3H cRNA was linearly integrated into the cell DNA. The free SV40 DNA was noninfectious, closed circular DNA with a size smaller than infectious SV40 DNA component I. Since the cell extracts of the transformed cells contained an agent(s) which induced T- and V-antigens in normal CV-1 cells, it was suggested that the SV40 transformed CV-1 cells contained free as well as integrated defective SV40 genomes responsible for the synthesis of T- and V-antigens.