The relationship between calcium, MAP kinase, and DNA synthesis in the sea urchin egg at fertilization

Fertilization releases the brake on the cell cycle and the egg completes meiosis and enters into S phase of the mitotic cell cycle. The MAP kinase pathway has been implicated in this process, but the precise role of MAP kinase in meiosis and the first mitotic cell cycle remains unknown and may differ according to species. Unlike the eggs of most animals, sea urchin eggs have completed meiosis prior to fertilization and are arrested at the pronuclear stage. Using both phosphorylation-state-specific antibodies and a MAP kinase activity assay, we observe that MAP kinase is phosphorylated and active in unfertilized sea urchin eggs and then dephosphorylated and inactivated by 15 min postinsemination. Further, Ca(2+) was both sufficient and necessary for this MAP kinase inactivation. Treatment of eggs with the Ca(2+) ionophore A23187 caused MAP kinase inactivation and triggered DNA synthesis. When the rise in intracellular Ca(2+) was inhibited by injection of a chelator, BAPTA or EGTA, the activity of MAP kinase remained high. Finally, inhibition of the MAP kinase signaling pathway by the specific MEK inhibitor PD98059 triggered DNA synthesis in unfertilized eggs. Thus, whenever MAP kinase activity is retained, DNA synthesis is inhibited while inactivation of MAP kinase correlates with initiation of DNA synthesis.

Host range analysis of simian virus 40, BK virus and chimaeric SV40/BKV: relative expression of large T-antigen and Vp1 in infected and transformed cells

Simian virus 40 (SV40) persists in Rhesus monkeys and productively infects cultured simian kidney cells. In contrast to the closely related human virus BKV, SV40 is known to propagate inefficiently in human embryonic kidney (HEK) cells and human fibroblasts (HFF). We examined the growth of SV40, BKV and the chimaeric genome virus, SV40/RFV, in several types of human cells. We analysed replication, expression of T-Ag and Vp1 capsid proteins, and cytopathic effects (CPE). We also compared T-Ag and Vp1 expression in infected versus transformed HFF cells. Although SV40 DNA replicated in HFF and in one subtype of HEK cells, viral DNA accumulated slowly and did not reach high levels until six to eight weeks after transfection. In HFF or HEK cells there was little T-Ag produced but Vp1 was produced in significant amounts in HFF cells. In HFF cells the Vp1/T-Ag ratio was approximately 200: 1, and expression of the viral late region appeared to inhibit expression of the T-Ag gene. In contrast, BKV and the SV40/RFV hybrid propagated well in HEK and HFF cells. The Vp1/T-Ag ratios were also high in BKV and SV40/RFV infected HFF cells but more T-Ag was produced with BKV and SV40/RFV Because SV40/RFV contained the RFV capsid genes but a SV40 T-Ag gene and regulatory region, the human versus simian host range of SV40 was controlled by the viral late region, or one or more capsid proteins. This suggested that the production of small amounts of T-Ag could not by itself account for poor growth of SV40 in HFF cells and that very small, barely detectable amounts of T-Ag were sufficient to activate Vp1 gene expression. Also, although some feature of the SV40 late region prevented rapid growth of the virus in HFF cells, poor virus growth could not be explained by the inability to produce a significant amount of Vp1. Although little T-Ag accumulated in SV40 infected HFF and HEK cells, transformants contained large amounts of T-Ag. In transformants there was a reversal of the Vp1/T-Ag ratio, such that T-Ag was now in 10-20 fold greater amount than Vp1. The relatively large amount of T-Ag in transformants could be accounted for by the relative absence of Vp1, which may inhibit T-Ag production, or by integration of the T-Ag gene at a site in the cell DNA which allows for elevated T-Ag gene expression.

DNA sequences outside the simian virus 40 early region cause downregulation of T-antigen production in permissive simian cells

Using a series of modified wtSV40 and early region SV40 DNAs we assayed the effect of viral late region sequences on T-antigen production by the SV40 early region. We found that SV40 late region (L-SV40) DNA sequences reduced T-antigen (T-Ag) production by the SV40 early region (E-SV40) when both viral regions were linked as they are in wtSV40 DNA. This was demonstrated by Western analysis which showed that E-SV40 DNA produced 10 times more T-Ag than wtSV40 DNA L-SV40, with its own promoter but unlinked to E-SV40 DNA, also greatly inhibited T-Ag production when it was contrasfected with E-SV40. Therefore, L-SV40 DNA inhibited T-Ag production by E-SV40 DNA when present in cis or in trans. We have shown that expression of the SV40 late transcription unit dominated that of the early (T-Ag gene) transcription unit because late region RNA accumulated to much higher levels than early viral RNA. However, in contrasfected cells L-SV40 DNA did not replicate to higher levels than E-SV40 DNA. We offer a model for control of T-Ag expression in which a relatively small amount of T-Ag activates late transcription at the expense of T-Ag gene transcription and that this represents a switch from early to late viral gene expression. We suggest that when activation of the late transcription unit occurs at the late promoter, expression of the T-Ag gene is greatly reduced. The L-SV40 promoter may inhibit T-Ag gene transcription by sequestering cellular factors required for early transcription, factors which may be present in limited amounts. We suggest further that activation of late transcription allows for the necessary production of large amounts of capsomeres and virions and downregulation of early transcription prevents the early region from interfering with capsid synthesis. We tested the model using a construct with a wild-type T-Ag gene but with mutations in the SV40 major late promoter which prevent the promoter from being bound by cellular repressors of late transcription. We found that this construct, which overproduces late SV40 RNA, was defective for T-Ag production. This indicates that activation of the late promoter results in repression of T-Ag gene expression.

Mutations of pma-1, the gene encoding the plasma membrane H+-ATPase of Neurospora crassa, suppress inhibition of growth by concanamycin A, a specific inhibitor of vacuolar ATPases

Concanamycin A (CCA), a specific inhibitor of vacuolar ATPases, inhibited growth of Neurospora crassa in medium adjusted to pH 7 or above. Mutant strains were selected for growth on medium containing 1.0 microM CCA. Sixty-four (of 66) mutations mapped in the region of the pma1 locus, which encodes the plasma membrane H+-ATPase. Analysis of V-ATPase activity in isolated vacuolar membranes from the mutant strains showed wild-type activity and sensitivity to CCA. In contrast, plasma membrane H+-ATPase activity in isolated plasma membranes from the mutants was reduced as compared with wild-type, and in four strains the activity showed increased resistance to vanadate. The most interesting change in the plasma membrane H+-ATPase was in kinetic behavior. The wild-type enzyme showed sigmoid dependence on MgATP concentration with a Hill number of 2.0, while the seven mutants tested exhibited hyperbolic kinetics with a Hill number of 1.0. One interpretation of these data was that the enzyme had changed from a functional dimer to a functional monomer. Mutation of the plasma membrane H+-ATPase did not confer resistance by preventing uptake of CCA. In the presence of CCA both wild-type and mutant strains were unable to accumulate arginine, failed to concentrate chloroquine in acidic vesicles, and exhibited gross alterations in hyphal morphology, indicating that the CCA had entered the cells and inactivated the V-ATPase. Instead, we hypothesize that the mutations conferred resistance because the altered plasma membrane H+-ATPase could more efficiently rid the cell of toxic levels of Ca2+ or protons or other ions accumulated in the cytoplasm following inactivation of the V-ATPase by CCA.

Identification of p53 unbound to T-antigen in human cells transformed by simian virus 40 T-antigen

In several clones of SV40-transformed human cells, we investigated the relative amounts of large T-Antigen (T-Ag) and p53 proteins, both unbound and associated within complexes, with the goal of identifying changes associated with transformation and immortalization. Cells were transformed by wild type (wt) T-Ag, a functionally temperature sensitive T-Ag (tsA58) and other T-Ag variants. Western analysis showed that while most of the T-Ag was ultimately bound by p53, most of the p53 remained unbound to T-Ag. Unbound p53 remained in the supernatant after a T-Ag immunoprecipitation and p53 was present in two to fourfold excess of T-Ag. In one transformant there was five to tenfold more p53 than T-Ag. p53 was present in transformants in amounts at least 200-fold greater than in untransformed human cells. In wt and variant T-Ag transformants, including those generated with tsA58 T-Ag, large amounts of unbound p53 were present in both pre-crisis and immortal cells and when the cells were grown at permissive or non-permissive temperatures. We also found that in transformants produced by tsA58, an SV40/JCV chimeric T-Ag and other variants, T-Ag appeared to form a complex with p53 slowly perhaps because one or both proteins matured slowly. The presence in transformed human cells of large amounts of unbound p53 and in excess of T-Ag suggests that sequestration of p53 by T-Ag, resulting from complex formation, is required neither for morphological transformation nor immortalization of human cells. Rather, these results support the proposal that high levels of p53, the T-Ag/p53 complexes, or other biochemical event(s), lead to transformation and immortalization of human cells by T-Ag.

Immortalization of human cells by mutant and chimeric primate polyomavirus T-antigen genes

Human fibroblasts were morphologically transformed with wild type and mutant SV40 T-antigens (T-Ags) and with SV40/JCV and SV40/BKV chimeric T-Ags. The transformants were then assayed for the attainment of immortal cell growth. Several observations relating T-Ag and T-Ag domains to immortalization were made. Approximately 10% of SV40-transformants became immortal. Transformants generated by transfection or infection of cells with C-terminal T-Ag deletion mutants of SV40 did not immortalize. SV40/JCV and SV40/BKV chimeric T-Ags, containing C-terminal sequences from JCV or BKV, immortalized cells more efficiently than did the intact SV40 T-Ag, suggesting that the C-termini of the JCV and BKV T-Ags contain an enhanced immortalization function. However, chimeras in which the N-terminal or proximal-central portions of T-Ag were composed of JCV sequences failed to immortalize but did induce transformation. Constructs in which the JCV T-Ag Rb binding domain was replaced with SV40 sequences transformed human cells, but again the cells failed to immortalize. Transformants and immortalized cell lines produced by some SV40/JCV chimeras, contained p53 which was unbound by T-Ag. This occurred under conditions where p53 from SV40 and SV40/BKV transformants was bound to T-Ag. This may reflect the reduced stability of the SV40/JCV T-Ags.

Persistence of the SV40 early region without expression in permissive simian cells

SV40 containing recombinant vectors were introduced into permissive simian, non-permissive rodent and semi-permissive human cell lines, and assayed for transformation. All mouse and human cell clones expressed T-antigen (T-Ag) and were morphologically transformed when they contained only the wt T-Ag gene (E-SV40) or the entire wt viral genome with an interrupted late region. However, of 63 simian clones with these recombinant vectors, none became morphologically transformed and T-Ag containing cells were rare or absent. Nearly all simian cell lines made either no detectable early SV40 RNA or only small amounts of viral RNA but contained viral DNA restriction fragments similar to those in the original recombinant vectors. Functional T-Ag genes were recoverable from several cell clones and used to regenerate infectious virus. Hence, T-Ag gene expression had been suppressed. We found two conditions where T-Ag expression was activated. In a BSC-1 cell line containing E-SV40 DNA, subsequent introduction of a vector with a functional viral late coding region (L-SV40) resulted in the appearance of T-Ag and transformation. These findings suggest that L-SV40 sequences activate or enhance T-Ag expression and that this activation requires a functional Vpl gene. We found also, that vectors with E-SV40 DNA from the bipartite variant EL-SV40 consistently transformed simian CV-1 cells. Transformation was shown to be effected by the multiple alterations present in the regulatory region of this variant.

Reconstitution of wild type viral DNA in simian cells transfected with early and late SV40 defective genomes

The DNAs of polyomaviruses ordinarily exist as a single circular molecule of approximately 5000 base pairs. Variants of SV40, BKV and JCV have been described which contain two complementing defective DNA molecules. These defectives, which form a bipartite genome structure, contain either the viral early region or the late region. The defectives have the unique property of being able to tolerate variable sized reiterations of regulatory and terminus region sequences, and portions of the coding region. They can also exchange coding region sequences with other polyomaviruses. It has been suggested that the bipartite genome structure might be a stage in the evolution of polyomaviruses which can uniquely sustain genome and sequence diversity. However, it is not known if the regulatory and terminus region sequences are highly mutable. Also, it is not known if the bipartite genome structure is reversible and what the conditions might be which would favor restoration of the monomolecular genome structure. We addressed the first question by sequencing the reiterated regulatory and terminus regions of E- and L-SV40 DNAs. This revealed a large number of mutations in the regulatory regions of the defective genomes, including deletions, insertions, rearrangements and base substitutions. We also detected insertions and base substitutions in the T-antigen gene. We addressed the second question by introducing into permissive simian cells, E- and L-SV40 genomes which had been engineered to contain only a single regulatory region. Analysis of viral DNA from transfected cells demonstrated recombined genomes containing a wild type monomolecular DNA structure. However, the complete defectives, containing reiterated regulatory regions, could often compete away the wild type genomes. The recombinant monomolecular genomes were isolated, cloned and found to be infectious. All of the DNA alterations identified in one of the regulatory regions of E-SV40 DNA were present in the recombinant monomolecular genomes. These and other findings indicate that the bipartite genome state can sustain many mutations which wtSV40 cannot directly sustain. However, the mutations can later be introduced into the wild type genomes when the E- and L-SV40 DNAs recombine to generate a new monomolecular genome structure.

Host range analysis of a chimeric simian virus 40 genome containing the BKV capsid genes

Simian virus 40 (SV40) propagates poorly in cells from human embryonic kidney (HEK) and human fetal fibroblasts (HFF) while BK virus grows well in many human cell types. It has been suggested that sequences within the SV40 late region but not within the BKV late region may act to inhibit growth of virus in HEK and HFF cells. In order to test this and to identify a late region host range function, we have replaced the late region of wtSV40 DNA with the late region of RFV (a variant of BKV) to produce an intermolecular hybrid or chimera. The constructed SV40/RFV chimeric genome contained approx. 5900 base pairs, more than 650 base pairs greater than wtSV40. Nevertheless, when introduced by transfection the chimera appeared to be infectious. Three chimeric genomes were recovered from infected cells and all contained deletions of nearly 600 base pairs, exclusively at the region of the 3' terminal junction. Since all three chimeras propagated in human HFF and HEK cells, the RFV late region and not the RFV regulatory region possesses a host range function required for growth in human cells. Analysis of T-antigen gene expression suggests that the replacement of the SV40 late region with the BKV late region leads to full expression of the SV40 early region in human cells. Two chimeras exhibited a BKV-like host range and the third exhibited both a BKV and an SV40-like host range. We determined precisely which sequences were deleted in each chimera and we exchanged 3' terminal junction fragments containing these deletions, between two chimeras with different host ranges. From these experiments we demonstrated that: (1) The 3' terminus of the SV40 large T-antigen gene is required for growth of SV40/RFV in TC-7 and CV-1 simian cells but not for growth in human cells; (2) while the SV40 late region is refractory for growth in human cells, the RFV late region is not refractory for growth in simian cells; (3) the 3' terminus of the RFV T-antigen gene is not required for growth in human cells. The results of the 3' terminal junction exchanges and studies of early gene expression also demonstrate that BKV and SV40 can penetrate human and simian cells, even when they failed to grow in one cell type.

Host range determinant in the late region of SV40 and RF virus affecting growth in human cells

WtSV40 and its variant EL-SV40 (contains two complementing defective genomes) fail to productively infect human embryonic kidney cells or human fibroblasts. However, early SV40 (E-SV40) genomes can propagate in human cells when complemented by a particular late RF virus (L-RFV) genome or the closely related wtBKV genome. The L-RFV genome (L-RFV clone H) contains a deleted early region, a complete set of BKV capsid genes, and a single SV40 regulatory region (acquired by recombination). In contrast, it was not possible to make the reciprocal genome cross in human cells; late SV40 genomes containing a deleted early region do not complement early RFV or early BKV DNAs. The L-RFV clone H genome was also shown to complement wtSV40 in human cells. However, wtSV40 DNA was rapidly lost and replaced by a defective SV40 genome. The SV40 defective (E-SV40 alpha) contained a deletion of the late region, an intact early region, and paired with L-RFV clone H DNA to form a new hybrid virus. In human cells wtSV40 was also complemented by wtBKV DNA, but after two serial passages SV40 DNA disappeared. These findings indicate that SV40 late or capsid gene sequences, but not SV40 early sequences, generate a block to SV40 growth in human cells. When the SV40 late region is replaced by a RFV or a BKV late region, E-SV40 DNA propagates efficiently in human cells and in some cases more rapidly than wtBKV. Northern blot hybridization indicates that SV40 DNA is poorly transcribed in human cells when the SV40 late region is present.

Transformation of human cells by a polyomavirus containing complementing JCV and RFV genomes

Molecularly cloned viral DNA from late RFV (L-RFV) and early JCV (E-JCV) were transfected into human fetal brain (HFB) cells and complementation was demonstrated. A new infectious virus (E-JCV/L-RFV) was produced. Infection resulted in partial transformation of HFB and human embryonic kidney cells. No transformation was observed with EL-JCV or wtJCV. The transformants contained T-antigen and had a lifespan similar to SV40-transformed human cells but failed to express some phenotypes of transformation. All transformants contained E-JCV viral DNA, usually both integrated and episomal. Although no L-RFV DNA was present in the transformants, L-RFV appears to play a role in the initiation of transformation.

Late coding region sequences required for competition by SV40 defectives

SV40 defectives containing the complete early coding region (E-SV40) or the complete late region (L-SV40) were separately transfected into green monkey cells. They were analyzed for their ability to compete with wtSV40 (introduced by infection) or to undergo replication in the presence of constitutively produced SV40 T-antigen. L-SV40 competed very strongly. It appeared rapidly in infected cells, overgrowing wt genomes by at least 10:1. In addition, it slowed the growth of wt virus and reduced its ability to kill cells. L-SV40 DNA, as expected, replicated continuously in Cosl cells. E-SV40 genomes were poor competitors. They appeared slowly and by themselves did not overgrow wtSV40. When transfected into Cosl cells, E-SV40 genomes replicated efficiently for the first few days and disappeared within a week. Deletion or insertion mutations were introduced into a molecular clone of L-SV40, within the Vp1 gene or the Vp2 gene. All mutants were unable to form infectious virus in two different assays. The mutants were then assayed for competition against wtSV40 and for replication in Cosl cells. The Vp1 mutants competed very poorly with wt genomes and were rapidly lost from coinfected cells. These mutants, like E-SV40, replicated for only a few days in Cosl cells. In contrast, the Vp2 mutant competed with wtSV40 nearly as well as L-SV40. It also replicated continuously rather than transiently in Cosl cells. Next, we determined whether L-SV40 could effectively compete with other evolved SV40 defectives, not containing the late region but containing up to nine SV40 origin regions. We have shown that within five serial passages, L-SV40 became the predominant viral DNA species and the other defectives were lost. Although the Vp1 mutants and E-SV40 were weak competitors, they were shown to recombine with wtSV40 genomes to generate new L-SV40 genomes which again became the predominant species of viral DNA. These results demonstrate that L-SV40 is a potent competitor and that the Vp1 gene or a part of Vp1 plays an important role in this extraordinary competition. We suggest that the Vp1 gene functions to allow L-SV40 genomes to persist rather than generating a product which directly interferes with wtSV40 replication.

Complementation between SV40 and RFV defectives and acquisition of SV40 origins by late RFV genomes

EL SV40 and RFV are variants of SV40 and BKV which contain bipartite or dual genomes. One molecule contains all the early viral sequences (E-SV40, E-RFV) and the other all the late viral sequences (L-SV40, L-RFV). Early and late genomes complement one another during productive infection. Experiments were designed to determine if E-genomes of one virus could complement L-genomes of another virus. If complementation did occur, intermolecular recombination events which lead to a more efficient infection or an altered host range might occur, and the sequences involved could than be identified. Two combinations were generated by direct transfection of BSC-1 green monkey cells. E-RFV and L-SV40 DNA complementation resulted in hybrid virus growth and cell killing. The hybrid demonstrated a narrow host range. Following serial passage, some E-RFV genomes contained SV40 origin region sequences but these recombinants did not overgrow prototype E-RFV genomes, even after many virus passages. In addition, no significant alterations in host range could be detected. Complementation between E-SV40 and L-RFV yielded a virus with a relatively wider host range. Virus growth and cell killing appeared very slowly at first. However, with each passage of E-SV40/L-RFV, cell killing occurred progressively more rapidly, until passage 7 when it became extensive in 7 days rather than 6-8 weeks. Infected cells contained 10-20 times more E-SV40 than L-RFV DNA during the first passage. However, by passage 7, both genomes were equally represented. During serial passage, L-RFV DNA acquired SV40 sequences from around the origin and the terminus of replication, such that recombinant (r) L-RFV genomes contained three SV40 origins [corrected] (including the 72-bp repeat) and 2 termini, and prototype L-RFV DNA was lost. E-SV40/rL-RFV demonstrated an altered host range propagating in some cell lines which did not support E-SV40/L-RFV growth. Both the host range change and the increased growth of rL-RFV genomes were shown to be at least partly caused by the acquisition of the SV40 sequences.

Isolation of a papovavirus with a bipartite genome containing unlinked SV40 and BKV sequences

Wild-type (wt) BK virus was introduced into permissive BSC-1 cells along with either early or late defective SV40 genomes. The defectives contained all of the late (L-SV40) or all of the early (E-SV40) coding sequences. Persistently infected (PI) BSC-1 cultures were established and contained wt BKV DNA and E- or L-SV40 DNA in Hirt supernatants. Each of the BKV/SV40 combinations could be serially passed in BSC-1 cells. Also, DNase I digestion of virus stocks from BKV/E-SV40 infections did not eliminate E-SV40. This suggested that (1) E-SV40 genomes could be packaged in BKV capsids and (2) BKV T antigen acted to stimulate the growth of L-SV40 genomes. During continuous culture of PI BSC-1 cells containing BKV and L-SV40, wt BKV genomes were lost and replaced by a BKV defective. The BKV defective (E-BKV) contained a deletion in the late region, an intact early region, and a duplication of the origin. This combination represents a new papovavirus with a bipartite genome in which the early region is derived from BKV and the late region from SV40, and both are present in separate molecules. The BKV and SV40 defectives complement each other for infectivity. Infectious virus is formed with the E-BKV genomes packaged in SV40 capsids. It is hypothesized that this kind of recombination (reassortment) is a way in which papovaviruses may generate variation. The host range for the new BKV/SV40 is narrow. It propagates well in BSC-1 cells, relatively poorly in fetal human brain cells, and not at all in green monkey TC-7 or human embryonic kidney cells. However, it transforms fetal human brain cells at a frequency 25-50 times greater than wt BKV does.

In vitro growth control phenotypes of transformed rodent cells prior to and following tumorigenesis

A number of virus and chemical carcinogen-transformed cell lines were generated in tissue culture and analyzed for growth control phenotypes prior to and following tumorigenesis in appropriate hosts. The cell lines include those of mouse, rat, human, and Syrian hamster, transformed by papovaviruses and adenoviruses (DNA) or murine (RNA) tumor viruses. Cell lines were assayed for: (a) multinucleation or uncontrolled nuclear division (UND+) and uncontrolled DNA synthesis in cytochalasin B (CB) medium; and (b) the continuation of DNA synthesis in media containing reduced (0.5%) amounts of serum. All or nearly all lines of DNA virus transformants exhibited UND+ and high frequencies of DNA-synthetic cells in CB medium. Two lines of SV40-transformed hamster cells also showed UND+ following tumorigenesis in weaning hamsters. In addition, DNA virus transformants showed the ability to continue DNA synthesis unabated in low-serum medium. In contrast, the mouse sarcoma virus (MSV)-transformed lines exhibited varying degrees of controlled nuclear division and reduced DNA synthesis in CB medium, both prior to and following tumorigenesis. However, the reduction in DNA-synthetic cells was often not as great as that found in untransformed cells. Results similar to the RNA virus transformants were observed with hamster cells transformed by chemical carcinogens. Nearly all of the MSV-transformed lines showed significantly reduced levels of DNA synthesis in low-serum medium as was found in untransformed cells. One cell line, KA31, was followed through three consecutive in vivo tumorigenic passages, but these cells did not acquire UND+ or the ability to continue DNA synthesis in low-serum medium. These results suggest that many MSV- and carcinogen-transformed rodent cells exhibit transformation phenotypes at levels barely above those of normal cells and markedly less than those of DNA virus transformants, and yet they are tumorigenic.

Isolation and characterization of defective simian virus 40 genomes which complement for infectivity

A new variant of simian virus 40 (EL SV40), containing the complete viral DNA separated into two molecules, was isolated. One DNA species contains nearly all of the early (E) SV40 sequences, and the other DNA contains nearly all of the late (L) viral sequences. Each genome was encircled by reiterated viral origins and termini and migrated in agarose gels as covalently closed supercoiled circles. EL SV40 or its progenitor appears to have been generated in human A172 glioblastoma cells, as defective interfering genomes during acute lytic infections, but was selected during the establishment of persistently infected (PI) green monkey cells (TC-7). PI TC-7/SV40 cells contained EL SV40 as the predominant SV40 species. EL SV40 propagated efficiently and rapidly in BSC-1, another line of green monkey cells, where it also formed plaques. EL SV40 stocks generated in BSC-1 cells were shown to be free of wild-type SV40 by a number of criteria. E and L SV40 genomes were also cloned in the bacterial plasmid pBR322. When transfected into BSC-1 cell monolayers, only the combination of E and L genomes produced a lytic infection, followed by the synthesis of EL SV40. However, transfection with E SV40 DNA alone did produce T-antigen, although at reduced frequency.

Temperature dependency for maintenance of transformation in mouse cells transformed by simian virus 40 tsA mutants

Mouse embryo fibroblasts and 3T3 cells were transformed by wild-type, tsB4, tsA7, tsA58, and tsA209 simian virus 40. Clones of transformants were generated both in soft agar and in liquid medium by focus formation and at both high and relatively low multiplicities of infection. All transformants were assayed for three phenotypes of transformation: (i) the ability to form highly multinucleated cells in cytochalasin B-supplemented medium, i.e., uncontrolled nuclear division; (ii) the capacity to continue DNA synthesis at increasing cell density; and (iii) the ability to form colonies in soft agar. The great majority of mouse embryo fibroblast transformants generated with tsA mutant virus were temperature sensitive for transformation in all three assays, regardless of the input multiplicity or whether they were generated in liquid medium or soft agar. These transformants exhibited a normal or near-normal phenotype at the nonpermissive temperature of 40 degrees C. All but one of the transformants which appeared transformed at both temperatures were in the A209 group. In contrast to mouse embryo fibroblasts, transformants generated with 3T3 cells and tsA virus were often not temperature sensitive, exhibiting the transformation phenotypes at both temperatures. This phenomenon was more often observed when 3T3 transformants were generated in soft agar. These results, along with other published data, suggest that uncontrolled nuclear division and uncontrolled DNA synthesis are a function of the simian virus 40 A gene. Finally, with the 3T3 transformants, there was often discordance in the expression of transformation among the three phenotypes. Some tsA transformants were temperature sensitive in one of two assays but were transformed at both 33 and 40 degrees C in the remaining assay(s). Other transformants exhibited a normal cytochalasin B response at either temperature but were temperature sensitive in the other assays.

Prolongation of herpes simplex virus latency in cultured human cells by temperature elevation

Treatment of herpes simplex virus type 2 (HSV-2)-infected human fibroblast cells with cytosine arabinoside (ara-C) at 25 microgram/ml resulted in complete inhibition of virus replication. Removal of ara-C after 7 days of treatment ultimately resulted in renewed virus replication, but after a delay of at least 5 days. If however, the temperature was elevated from 37 degrees C to 39.5 to 40 degrees C at the time of ara-C reversal, infectious HSV-2 did not reappear. As long as the cultures were maintained at 39.5 to 40 degrees C (up to at least 128 days), HSV-2 was latent and infectious virus was undetectable. If the temperature was reduced to 37 degrees C at any time during the latent period, infectious virus was always reactivated, but only after a period of incubation at 37 degrees C of a least 11 days. Infectious-center assays performed with latent cultures indicated that only a very small fraction of cells could reactivate virus. The infectious-center titer did not show significant changes during much of the period of latency. This seemed to argue against the possibility that the latent cultures were synthesizing very small amounts of infectious virus. Additional studies were aimed at determining the minimum incubation period at 37 degrees C required to reactivate infectious HSV-2. Latent cultures reduced from 39.5 to 40 degrees C to 37 degrees C for less than 96 h did not yield infectious HSV-2, but those incubated at 37 degrees C for 96 h or more did.

Loss of controlled nuclear division in BHK21 cells passed in vivo

Low-passage BHK21/C13 cells respond to cytochalasin B (CB) by undergoing limited or controlled nuclear division. These cells respond to CB as normal cells do since nuclear division usually occurs only once. Premature chromosome condensation, a result of mitoses in highly multinucleate cells, occurs in less than 0.5% of the cells. When they are inoculated into weanling hamsters, s. c. tumors appear within 3 to 4 weeks with as few as 10(3) cells. When these cells are returned to cell culture they respond to CB with uncontrolled nuclear division and premature chromosome condensation. All cultured tumors respond in this manner regardless of the number of cells originally inoculated into animals. This suggests at least two possibilities: (a) that loss of controlled nuclear division in BHK cells is closely associated with or required for tumorigenicity, with in vivo passage selecting for a rare tumorigenic variant or (b) that loss of controlled nuclear division is secondary to tumorigenicity and results when cells are passed in vivo, i.e., in vivo passage has the direct effect of causing cells to lose controlled nuclear division. If the first possibility is operating, then it would be expected that a very small fraction of BHK21/C13 cells show uncontrolled nuclear division (approximately 1 of 1000 CB-treated cells). Also, clones of C13 should be nontumorigenic if only 1 of 1000 cells is tumorigenic. Extensive examination of CB-treated C13 cells shows 1 of 1250 cells to be highly multinucleated although not as highly multinucleated as tumor cells. This provides some evidence in support of the first possibility. However, three separate clones of C13 cells were found to be tumorigenic providing evidence supporting the second possibility. BHK21/C13 and various BHK21 tumors all appeared to grow to concentration densities markedly higher than hamster embryo fibroblasts. However, the tumor cells usually grew to the same density as did BHK21/C13 or only slightly higher. This suggests that loss of contact inhibition is not sufficient for loss of controlled nuclear division. It also suggests that the hypothetical 1 of 1000 tumorigenic cells and 1 of 1250 cells with uncontrolled nuclear division do not overgrow the "normal" cells because all cells grow to similar densities. The relationship between the ability to grow in agar and uncontrolled nuclear division was also examined. Approximately 1 of 2500 C13 cells were able to form colonies in agar. All three colonies isolated showed normal control of nuclear division. These results show that the ability to grow in agar may be separate from the expression of uncontrolled nuclear division. They also suggest a fundamental difference between tumor cells and cells that have been grown in agar.

High frequency variation in mammary tumor virus expression in cell culture

Clonal derivatives of C3HMT murine mammary cell lines in culture demonstrate conversion of mammary tumor virus (MMTV) expression at a rate of appriximately 6 per 100 clones. This alteration is largely unidirectional from a relatively high level (MMTV(H)) to a 10 fold lower level (MMTV(L)). This high rate of MMTV(L) variant conversion is in apparent contrast to the presumably mutational rate (approximately 3 per million cells) that governs development of resistance to 6-thioguanine (TG) in the same mammary cells. In somatic cell hybrids between different MMTV TGr clones and mouse or hamster TK- cells, expression of constitutive levels of MMTV and responsiveness to dexamethasone induction is dominant. Thus MMTV expression is regulated by at least two levels of positive control, constitutive expression and glucocorticoid stimulation, but the former is subject to a high rate of variant formation.

Limitation of multinucleation by dibutyryl adenosine 3',5'-cyclic monophosphate in tumor cells treated with cytochalasin B

Hamster BHK21 tumor cells and human rhabdomyosarcoma (RD) cells responded to cytochalasin B (CB) by becoming highly multinucleated. Nearly 80% of the BHK cells contained four or more nuclei after 7 days of exposure to CB, and 35% of these had at least seven nuclei. Similar frequencies of multinucleation were obtained with RD cells. Administration of 10(-3) M theophylline and dibutyryl adenosine 3',5'-cyclic monophosphate (Bu2cAMP) at 5X10(-4) M or 10(-3) M to CB-treated RD or BHK cells greatly reduced the frequency of cells with five or more nuclei. The frequency of cells with seven or more nuclei was reduced to less than 5%. Along with this reduction in highly multinucleated cells was an increase in the incidence of binucleated cells. Bu2cAMP was toxic and caused many CB-treated BHK cells to detach from the culture surface, but not all cells were killed. Bu2cAMP had little toxic effect on CB-treated RD cells. These observations indicated that the inhibition of high degrees of multinucleation were not the result of nonspecific toxic effects of Bu2cAMP but that nuclear division was limited by it. The effect of Bu2cAMP on density-dependent inhibition of growth was also studied. Addition of only theophylline and Bu2cAMP to either BHK or RD cells resulted in growth to significantly lower saturation densities. The toxicity of Bu2cAMP on cells in crowded cultures apparently caused the limited propagation. Bu2cAMP resulted in significant cell killing or detachment but, once the lower saturation densities were reached, cell death was minimized. Thus Bu2cAMP did not restore contact inhibition per se. It was also found that untreated RD cells grew to lower concentration densities than expected from the microscopic inspection of cells in situ. Microscopic inspection revealed high concentration densities and numerous mitoses. This apparent contradiction was due to the ability of RD cells to fuse upon the attainment of confluence and to produce multinucleated cells without the aid of CB.

Uncontrolled nuclear division in murine cells abortively transformed by simian virus 40

Cytochalasin B (CB) prevents cytoplasmic cleavage without directly affecting nuclear division. Secondary cultures of mouse embryo fibroblasts or 3T3 cells show controlled nuclear division when treated with CB: only binucleated cells are formed. Many CB-treated transformed cells show uncontrolled nuclear division and become highly multinucleated. When CB-treated normal cells are concurrently infected with high inputs of SV40, many of these cells become highly multinucleated. It is suggested that these highly multinucleated cells represent abortively transformed cells since the actual number of transforming units (focus-forming units) of simian virus 40 (SV40) is too low to account for the appearance of these cells. Also, if the CB treatment is begun 6 days after SV40 inoculation, the large increase in highly multinucleated cells is not observed. Most cells stably transformed by SV40 or adenovirus show uncontrolled nuclear division when treated with CB. However, 3T3 cells transformed by SV40 or adenovirus and analyzed shortly after transformation are an exception and show controlled nuclear division. This property of 3T3 cells is apparently overcome by high inputs of SV40 since abortively transformed cells become highly multinucleated.

Selective destruction of cultured tumor cells with uncontrolled nuclear division by cytochalasin B and cytosine arabinoside

Cultures of normal human and hamster and malignant human and hamster cells respond to cytochalasin B (CB) differently. The neoplastic cells become highly multinucleated with continuous nuclear division, while cytoplasmic division is prevented. These cells exhibit uncontrolled nuclear division. The normal cells show control of nuclear division, since CB treatment results in only binucleation, although cytoplasmic division is prevented. The CB-treated normal cells also show reduced incorporation of [3H]thymidine. When rapidly growing normal or neoplastic cells of either species are treated with cytosine arabinoside (ara-C), all cells are killed within 10 to 20 days. If the normal cells are treated with ara-C in the presence of CB, the cells survive for at least 35 to 40 days, suggesting that CB can protect normal cells from the destructive effects of ara-C. However, if malignant cells are treated with ara-C in the presence of CB, all cells are destroyed in 25 days. Although CB affords some protection to tumor cells, it is relatively small and appears only to effect a delay in ultimate cell death. Treatment of any of the cell lines with CB alone results in some loss of viability, but the drug is reversible although less so for the malignant cells, In additional experiments, are-C plus CB-treated normal and neoplastic cells were reversed from the drugs and propagated in normal medium at various periods after the initiation of ara-C treatment. ara-C plus CB-treated normal cells could be freely reversed at any time after the start of ara-C treatment up to at least 35 days. After reversal, cell growth resumed, and the total number of cells returned to that present prior to drug treatment. The tumor cells could not be freely reversed from ara-C plus CB, even when the ara-C was present for only 5 days. In this case the cells seemed to be destroyed at the same rate as in cultures where both drugs were kept in the medium. These results suggest that CB can protect normal but not malignant cells from the toxic effects of ara-C and offer a method to specifically destroy tumor cells.

Differential response to cytochalasin B among cells transformed by DNA and RNA tumor viruses

Mouse, hamster, rat, human, and chick cells were transformed by RNA and DNA tumor viruses: simian virus 40, adenovirus type 7, Kirsten mouse sarcoma virus (Ki-MuSV), Moloney mouse sarcoma virus, and Rous sarcoma virus. All cultures of transformed cells grew to high concentration densities. Normal and transformed cells were treated with cytochalasin B (CB) at concentrations preventing cytoplasmic cleavage. Cells altered by DNA tumor viruses responded to CB with numerous nuclear divisions resulting in highly multinucleated cells. All but one line of cells transformed by RNA tumor viruses responded to CB with usually only one and occasionally two nuclear divisions. Only binucleated cells were formed. One clone of CB-treated BALB/c mouse embryo fibroblasts transformed by Ki-MuSV showed numerous cells with four and five nuclei. HOWEVER, IN CONTRASt to cells transformed by DNA viruses, few cells had seven or more nuclei. These results suggest that, in the presence of CB, cells transformed by DNA tumor viruses show uncontrolled nuclear division, whereas cells tranformed by RNA tumor viruses show controlled nuclear division.

Control of nuclear division in sv40 and adenovirus type 12 transformed mouse 3t3 cells

Untransformed, non-tumorigenic mouse cells respond to cytochalasin B (CB) with limited nuclear division. BALB/c mouse embryo fibroblasts (MEF) and both BALB/c 3T3 and Swiss 3T3 cells become binucleated in the presence of CB and cells with three or more nuclei are very rare or undetectable. MEF are diploid (40 chromosomes) and 3T3 cells are subtetraploid (74-76 chromosomes). Transformation of MEF by SV40 produces a dramatic change in response to CB. These cells, which contain SV40 T-antigen, become highly multinucleated in the presence of CB. More than 20% of the cells have at least seven nuclei. Also premature chromosome condensation (PCC), an abnormality rare in CB-treated normal cells but one which is common to highly multinucleated cells, is frequent and occurs in at least 10% of mitoses. SV40-transformed MEF have 40 or 80 chromosomes, e.g. are diploid or tetraploid. Transformation of 3T3 by SV40 or adenovirus type 12 does not result in a marked change in the response to CB. Although some trinucleate and tetranucleate cells are formed, cells with more nuclei are undetectable or rare. PCC is also rare. These cells show chromosome numbers somewhat lower than their untransformed parents and in one line the chromosome number appears to decrease with passage of the cells. This failure to undergo a marked change in responsiveness to CB following transformation is not a characteristic of all transformed 3T3 cells. SVT2, a line of 3T3 which was transformed by SV40 prior to its establishment as a continuous line, responds to CB with a high degree of multinucleation. These cells are diploid. These results suggests that 3T3 cells are constitutive for controlled nuclear division and that this feature may be related to chromosome constitution.

Specific C band patterns in continuous human lymphoblastoid cell lines

Analysis of centromeric heterochromatin in five human lymphoblastoid cell lines is described utilizing the C banding technique. Two lines, LK 60 and NC 37 showed a polymorphism for the size of the band on chromosome 1. LK 60 also showed accentuation or stretching of the secondary constriction on No. 1 and in almost all cells studied the affected homolog was also the one with the large C band. Another line SKL-1, also showed an accentuated constriction on chromosome 1 but did not have a detectable polymorphism. NC 37 did not show a constriction. In LK 60 the stretching of the constriction always appeared within the boundaries of the constitutive heterochromatin, regardless of the degree of stretching.SKL-1 and RPMI 6410 showed marker chromosomes with double C bands. One such chromosome appeared in SKL-1 and the bands were relatively widely spaced. However, analysis of this chromosome with standard staining procedures showed that one band was only rarely associated with a constriction while the other band, nearest the telomere, always showed a constriction. In RPMI 6410 two such markers were apparent. In one, the bands were well spaced, to allow an analysis for association with constrictions. In this case one band was always associated with a constriction while the other band showed a constriction in most of the cells. The possibility that these chromosomes are dicentric is discussed.

Hybridization of Burkitt lymphoblastoid cells

Burkitt lymphoblastoid cell lines have been fused to mouse and human cell lines with the use of inactivated Sendai virus. The heterokaryons have developed into somatic cell hybrids of both parental cell types. Chromosome analyses confirm that cells now growing in selective medium are hybrids. Initial observations of preparations of the hybrid cells reveal that 5'-iododeoxyuridine can induce continued synthesis of Epstein-Barr virus antigens by these hybrid cells.

Synergistic effect of herpes simplex virus and cytosine arabinoside on human chromosomes

The combined treatment of cultures of human embryonic lung cells with herpes simplex virus type 2 and cytosine arabinoside produced a significantly increased number of cells containing multiple chromatid and chromosome breaks. The incidence of such cells was found to be approximately two and one half times greater than the additive effects of virus and cytosine arabinoside induced separately and is therefore synergistic.