The response of BHK21 cells to infection with type 12 adenovirus. II. Relationship of virus-stimulated DNA synthesis to other viral functions

Epigenetic mechanisms in human adenovirus type 12 oncogenesis

For the past 30 years, my laboratory has concentrated its work on demonstrating that the epigenetic consequences of foreign DNA insertion into established mammalian genomes – de novo DNA methylation of the integrate and alterations of methylation patterns across the recipient genome – are essential elements in setting the stage towards oncogenic transformation. We have primarily studied human adenovirus type 12 (Ad12) which induces undifferentiated tumors in Syrian hamsters (Mesocricetus auratus) either at the site of subcutaneous Ad12 injection or intraperitoneally upon intramuscular injection. Up to 90% of the hamsters injected with Ad12 develop tumors within 3–6 weeks. Integration of foreign DNA, its de novo methylation, and the consequences of insertion on the cellular methylation and transcription profiles have been studied in detail. While viral infections are a frequent source of foreign genomes entering mammalian and other hosts and often their genomes, we have also pursued the fate of food-ingested foreign DNA in the mouse organism. The persistence of this DNA in the animals is transient and there is no evidence for the expression or germ line fixation of foreign DNA. Nevertheless, the occasional cell that carries integrated genomes from that foreign source deserves the oncologist's sustained interest.

Human CAR gene expression in nonpermissive hamster cells boosts entry of type 12 adenovirions and nuclear import of viral DNA

Adenovirus type 12 (Ad12) propagation in hamster BHK21 cells is blocked prior to viral DNA replication. The amounts of Ad12 DNA in the nuclei or cytoplasm of hamster cells are about 2 orders of magnitude (2 h postinfection [p.i.]) and 4 to 5 orders of magnitude (48 h p.i.) lower than in permissive human cells. Cell line BHK21-hCAR is transgenic for and expresses the human coxsackie- and adenovirus receptor (hCAR) gene. Nuclear uptake of Ad12 DNA in BHK21-hCAR cells is markedly increased compared to that in naïve BHK21 cells. Ad12 elicits a cytopathic effect in BHK21-hCAR cells but not in BHK21 cells. Quantitative PCR or [(3)H]thymidine labeling followed by zone velocity sedimentation fails to detect Ad12 DNA replication in BHK21 or BHK21-hCAR cells. Newly assembled Ad12 virions cannot be detected. Thus, the block in Ad12 DNA replication in hamster cells is not released by enhanced nuclear import of Ad12 DNA.

Cellular and early viral factors in the interaction of adenovirus type 12 with hamster cells: the abortive response

The interaction of human adenovirus type 12 (Ad12) with Syrian hamster cells is remarkable in that there is a block of viral DNA replication and late gene transcription. We have screened several cellular factors known to play a role in adenovirus replication for their possible contributions to the interactions of Ad12 in the abortive BHK21 hamster cell system. (1) Western blot analyses of total protein extracts from Ad12- or Ad2-infected BHK21 cells do not reveal a significant difference in the accumulation of NFIII protein at different times after infection. Transcriptional levels of the NFIII gene in BHK21 cells are not altered upon the abortive infection with Ad12 or the productive infection with Ad2. The amount of NFIII protein is markedly reduced in nuclear extracts from BHK21 cells as compared with extracts from C131 hamster cells or human HeLa cells. A presumptive defect in NFIII transport to the nuclei rather than overall reduced NFIII gene transcription might explain the low abundance of NFIII in the nuclei of uninfected or Ad12-infected BHK21 cells. The productive infection of BHK21 or C131 cells with Ad2 leads to an increase in the NFIII concentration in the nuclei of infected cells, late after infection to a decrease; (2) NFI levels in the nuclei of mock-infected or Ad2- or Ad12-infected BHK21 cells are comparable with those in HeLa or in C131 cells. Thus, deficiencies in NFI may not play a role in the abortive system; (3) The absence of morphological alterations in PML protein domains from globular to track-like structures in the nuclei of Ad12-infected hamster cells correlates with the inability of Ad12 DNA to replicate in BHK21 cells. In BHK21 cells, the E4-ORF3 of Ad12 DNA is only weakly transcribed and only small amounts of the gene product are synthesized. In Ad12-infected C131 cells, which allow the replication of Ad12 DNA, the E4-ORF3 of Ad12 DNA is expressed, and track-like PML protein structures are observed. Transfection of the 12-E4-ORF3-EGFP construct leads to the expression of both the green fluorescent protein (GFP) and of the 12-E4-ORF3 gene product in 20-30% of the transfected BHK21 cells and elicits the morphological reorganization of the PML protein structures in the successfully transfected BHK21 cells. Similar results are obtained upon transfection of the 2-E4-ORF3 construct. Untransfected cells or cells transfected with the empty pIRES2-pEGFP vector carry the globular PML protein phenotype; (4) The expression of the 12-E4-ORF3-EGFP and/or of the NFIII-EGFP constructs upon transfection following Ad12-infection of BHK21 cells fails to promote Ad12 DNA replication. Hence, the formation of track-like PML protein structures in BHK21 cells by itself is not a sufficient precondition for Ad12 DNA replication in this abortive system. The data demonstrate that the expression of NFI, NFIII, and/or the conversion of the PML domains do not suffice to elicit Ad12 DNA replication in the abortive hamster cell system.

Overexpression of the adenovirus type 12 (Ad12) pTP or E1A gene facilitates Ad12 DNA replication in nonpermissive BHK21 hamster cells

In the adenovirus type 12 (Ad12) hamster cell system, abortive virus infection is one of the factors associated with the highly efficient oncogenesis in newborn Syrian hamsters. We have shown earlier that the replication and efficient late transcription of the Ad12 genome are blocked in Syrian hamster cells. Some of the early Ad12 functions are transcribed in these cells, although at a minimal rate. In the present study, we demonstrate that low expression levels of the E1A and precursor to terminal protein (pTP) genes of Ad12 seem to be responsible for the lack of Ad12 DNA replication in hamster cells. The Ad12 genes for the E1A functions or for pTP were tethered to the strong early promoter of the human cytomegalovirus and transfected into BHK21 cells. Subsequently, these cells were infected with Ad12 virions. In Ad12-infected BHK21 cells, which overexpressed pTP or E1A, full-length Ad12 DNA was de novo synthesized, as documented by metabolic labeling with [3H]thymidine and by zone velocity sedimentation in alkaline sucrose gradients followed by gel electrophoresis of the 3H-labeled DNA and Southern blot hybridization to 32P-labeled Ad12 DNA. Transfection of the cloned E1A region of Ad2 yielded similar results. The newly synthesized Ad12 DNA was covalently linked to pTP. The Ad12 DNA binding protein (DBP) and DNA polymerase (pol) genes were transcribed at levels similar to those in merely Ad12-infected cells. In pTP or E1A gene-transfected and Ad12-infected BHK21 cells, marginal levels of late Ad12 mRNA were detectable. Late Ad12 proteins were, however, not synthesized. Apparently, Ad12 DNA replication in hamster cells is rendered impossible due to insufficient threshold levels of the viral E1A and/or pTP.

Human cytomegalovirus infection inhibits cell cycle progression at multiple points, including the transition from G1 to S

Human cytomegalovirus inhibits the growth of human foreskin fibroblast cells by 12 h after infection. Analysis of the cellular DNA content of infected cells by flow cytometry demonstrated that cytomegalovirus does not arrest cell cycle progression at a single point. At least two blockages occur, one of which is in the G1 phase of the cell cycle. The G1 arrest introduced by cytomegalovirus infection blocks S-phase entry after serum stimulation.

Suppression of block to entry into S phase in cell-cycle mutants of rat 3Y1 fibroblasts after transformation by adenovirus type 12

Four temperature-sensitive (ts) mutants of rat 3Y1 fibroblasts, belonging to separate complementation groups, cease to proliferate in the G1 phase of the cell cycle at a restrictive temperature (39.8 degrees). These ts mutants were transformed at a permissive temperature with adenovirus type 12 (Ad12), its E1A gene, or in203S mutant of Ad12 which has a mutation in the E1A 13 S mRNA unique region. We examined whether the proliferation of the transformed cells would be blocked in the G1 phase, at 39.8 degrees. One mutant did not cease to proliferate at 39.8 degrees after transformation with either Ad12 or E1A. In two other mutants, Ad12-transformed cells did not cease to proliferate at 39.8 degrees, whereas E1A-transformed cells did not survive at 39.8 degrees, though they did continue to enter the S phase. Analysis of transcription of the viral early genes in the transformants of one of the latter two mutants suggests that the expression of other viral early genes, in addition to E1A, is required for cell proliferation, in addition to entry into S phase. In the fourth mutant, both Ad12- and E1A-transformed sublines did not cease to enter the S phase but cells readily detached from the dishes. These results suggest that (1) function(s) of the E1A gene alone is sufficient for Ad12 to suppress the inhibition of the initiation of cellular DNA synthesis caused by four different cellular ts defects, (2) functions of Ad12 early genes other than, or in addition to, E1A are necessary for suppression of the inhibition of cell proliferation (and/or for survival) in two of the four ts mutants, and (3) in the case of one other ts mutant, E1A alone overcomes the ts defect and allows for the entire cell proliferation.

Induction of cellular DNA synthesis by adenovirus type 12 in a set of temperature-sensitive mutants of rat 3Y1 fibroblasts blocked in G1 phase

Four temperature-sensitive (ts) mutants of rat 3Y1 fibroblasts, which represent separate complementation groups, cease to proliferate predominantly with a 2C DNA content, either at 39.8 degrees (temperature arrest), or at 33.8 degrees at a confluent cell density (density arrest). When infected at 39.8 degrees with adenovirus type 12 (Ad12), cells of all four ts mutants in both arrest states entered the S phase, thereby suggesting that Ad12 overcomes the four independent functional blocks to cellular entry into S phase. Results of experiments using Ad12 E1-region mutants suggest that the E1A gene product(s) is indispensable to overcoming the ts block, whereas the E1B product(s) may be dispensable. The cell killing observed in 3Y1 cells infected with wild-type Ad12 did not occur in infection with one of the E1-region mutants with a 6-bp insertion in the E1A 13 S mRNA unique region. When infected with this mutant at 39.8 degrees, two ts mutants of 3Y1 (3Y1tsF121 and 3Y1tsG125) in both arrested states proliferated through at least one generation. Another mutant (3Y1tsD123) was accelerated to die following entry into the S phase. In the other mutant (3Y1tsH203), the cell number was either unchanged (temperature arrest) or was increased less than twofold and then decreased (density arrest). The findings with the latter two mutant lines suggest that induction of cellular DNA synthesis is not sufficient for the subsequent proliferation of the infected cells, and that the Ad12 gene function(s) does not directly rescue the primary lesions in these ts mutants but does overcome some of the blocks to concomitantly occurring events. In the former two mutant lines, however, Ad12 gene function(s) may directly rescue the ts lesions. We propose that the Ad12 gene product(s) can overcome blocks to the initiation of cellular DNA synthesis but cannot overcome blocks to events related to cell survival.

Adenovirus E1A 12S protein induces DNA synthesis and proliferation in primary epithelial cells in both the presence and absence of serum

Infection of primary baby rat kidney (BRK) cells with an adenovirus that carries an E1A 12S cDNA in place of the normal E1A region (adenovirus 5 [Ad5] 12S) resulted in the induction of cellular DNA synthesis and proliferation of the epithelial cells in the population, even in the absence of serum. Increased cellular DNA synthesis was first detectable by 12 h after infection and was maintained at a 10- to 20-fold higher level than in mock-infected cells. By 5 days after infection there was a 10-fold-greater number of 12S virus-infected BRK cells. These infected BRK cells retained many of their normal epithelial cell characteristics and were not transformed. The expression of the E1A 12S protein(s) occurred early after infection. There was no induction of adenoviral gene expression or viral DNA replication in these cells. The early effects of a fully transforming gene product(s) were also examined. The Ad5-simian virus 40 hybrid virus, Ad5.SVR4, in which the early region of simian virus 40 has replaced the E1 region of Ad5, was used to infect BRK cells. The kinetics of expression of the T antigens were similar to those of the 12S polypeptides. Infection with Ad5.SV4 also resulted in the induction of cellular DNA synthesis and cell proliferation at levels similar to those observed with the 12S virus. However, infection with Ad5.SVR4 resulted in cells that had lost some of their epithelial cell characteristics and were fully transformed. Thus, although the early cellular events induced by the two genes were similar, they did not yield the same final cellular phenotype.

Restricted replication of adenovirus type 2 in mouse Balb/3T3 cells

The nature of the infection of mouse B3T3 cells by adenovirus type 2 (Ad2) has been studied in vitro. Following infection with an adsorbed MOI of 225, more than 90 percent of the cells synthesized both early and late virus-specific antigens. In contrast, the yield of progeny virus varied from only 2 X 10(4) to 2 X 10(6) FFU/2 X 10(5) cells. The range in yields was related, in part, to the number of cell generations from the time of the initial subcloning, the yield increasing with passage level. Infectious center analysis suggested that fewer than 0.5 percent of infected cells synthesized progeny virus. Analysis of DNA synthesis in infected multiplying B3T3 cells demonstrated that cellular DNA synthesis began to be shut off at 12 hours p.i., a time when viral DNA synthesis was beginning. The maximum rate of viral DNA synthesis was approximately 12 percent of that in infected human cells. In contrast to infected multiplying cells, infection of quiescent B3T3 cell cultures resulted in the induction of cellular, along with viral, DNA synthesis. Analysis of late gene expression detected synthesis of most viral polypeptides, but revealed greater than 90 percent reductions in the rate of synthesis of polypeptides II, III, IV, and IX, as compared with infected human cells.

Induction of cellular DNA synthesis by purified adenovirus E1A proteins

The purified Escherichia coli-expressed products of the human subgroup-C adenovirus E1A 13 S and 12 S mRNAs are shown to induce cellular DNA synthesis when introduced by microinjection into quiescent, G0-arrested mammalian cells from immortalized cell lines. The E1A proteins stimulated cellular DNA synthesis in mouse Swiss 3T3 cells, when microinjected either individually or in combination. A truncated E1A protein, in which 169 carboxyl terminal residues of the 289-amino acid E1A 13 S mRNA product are deleted, was unable to induce cellular DNA synthesis in these cells. Our results provide evidence that E1A proteins can function, independent of other viral functions, in the stimulation of cellular DNA synthesis in certain cell types. The present results are consistent with the E1A gene products acting to modulate the expression of the cellular genes which control cell cycle progression into S phase.

Control of cellular and viral transcription during adenovirus infection

The control of transcription initiation is an issue central to the regulation of eukaryotic gene expression, and as such, the elucidation of the mechanisms of control of initiation frequency is critical. The study of adenovirus transcription control has provided insights into these mechanisms. Transcription of the early viral genes is activated by the product of the viral E1A gene. Possibly of greater importance is the fact that this activation does not appear to be "viral specific". Rather, the E1A protein effects a general activation of transcription in the cell, resulting in the stimulation of transcription of at least one cellular gene in addition to the viral genes. Furthermore, there appears to be a cellular activity that functions in a manner analogous to E1A. Recent experiments also suggest a role for E1A in negative regulation of transcription, mediated through enhancer elements, that may be one aspect of gene control during cellular differentiation. Therefore, the study of E1A action may well contribute to an understanding of cellular transcription control. Finally, other mechanisms of transcription control in adenovirus infected cells such as genome replication-dependent gene activation and transcription termination control will likely contribute to the overall understanding of the control of mammalian cell gene expression.

Infection and transformation of mouse cells by human adenovirus type 2

The susceptibilities of C57Bl/6J mouse embryo fibroblasts (MEF) and baby mouse kidney (BMK) cells to infection or transformation by adenovirus type 2 have been compared to those of rat embryo fibroblasts (REF). Both MEF and BMK cells were some 10-fold less permissive to replication of the virus than were REF cells, even though similar fractions of all three cell types, a maximum of 50-60%, produced viral tumor and structural antigens. This observation suggests that a very late step in adenovirus production, such as assembly or maturation, occurs much less efficiently in mouse cells than it does in rat cells. No significant differences in the frequencies of transformation, as assayed by the appearance of foci of morphologically transformed cells, were observed following transfection of adenovirus type 2 or type 5 DNA into the three cell types. However, it proved extremely difficult to establish permanent lines of adenovirus-transformed mouse cells: only 2 of more than 100 attempts were successful, compared to a success rate of close to 100% with adenovirus type 2-transformed REF or SV40-transformed MEF or BMK cells. The two lines of type 2 adenovirus-transformed MEF that were established have been shown to retain and express viral genetic information.

The release of growth arrest by microinjection of adenovirus E1A DNA

The induction of DNA synthesis in growth-arrested mouse fibroblasts (NIH 3T3) was studied by microinjection of different constructs of adenovirus DNA using SV40 DNA and plasmid DNA as positive and negative controls. The E1A region of adenovirus types 2 and 12 appears to be sufficient to induce cellular DNA synthesis after growth arrest in approximately 30% of the cells and both 13S and 12S cDNA constructs mediate this effect. The presence of the E1A protein products as assayed by immunofluorescence does not strictly correlate with the induction of DNA synthesis in microinjected cells in contrast to the SV40 large T-antigen. Microinjection of truncated fragments of the Ad12 E1A region suggests, however, that the protein products of 12S and 13S may be involved in the induction process. A sequence comparison of the SV40 T-antigen and the adenovirus E1A products identified a region of significant homology providing a basis for a hypothesis concerning the evolution of T-antigen genes in DNA viruses.

Adenovirus types 2 and 5 functions elicit replication and late expression of adenovirus type 12 DNA in hamster cells

Human adenovirus type 12 (Ad12) cannot replicate in hamster cells. There is a complete block of viral DNA replication and of the expression of late viral genes. Early viral functions are expressed. In contrast, hamster cells are permissive for human adenovirus type 2 (Ad2). Some of the Ad12-specific functions are insufficient to support viral replication in hamster cells, or else cellular functions are missing or inhibitory for Ad12 replication. It was shown that the block in the replication and late expression of the Ad12 genome in hamster cells could, at least in part, be complemented by Ad2 and adenovirus type 5 (Ad5) functions. When hamster cells were coinfected with Ad2 (or Ad5) and Ad12, both Ad2 (Ad5) and Ad12 DNA replicated. Ad2 (Ad5) virions were produced in double-infected hamster cells. The assembly of intact Ad12 virions was not detectable by the techniques used here. The analysis was further refined by Ad12 superinfecting Ad2- or Ad5-transformed cells which carried in an integrated form defined fragments of the Ad2 or Ad5 genome. Persistence and continued expression of the left terminus of the Ad2 or Ad5 DNA in these cells has been documented and helped to support replication and late expression of Ad12 DNA. It remains to be determined which of the E1 functions of Ad2 or Ad5 were responsible for the helper effect. Investigations on the biochemical mechanism of this complementation will entail studies on very complex viral and possibly cellular functions involved in the control of viral gene expression.

Cell cycle control of the human HSP70 gene: implications for the role of a cellular E1A-like function

The gene encoding the human 70-kilodalton heat shock protein (HSP70) is subject to activation by the adenovirus E1A gene product and appears to be regulated in the absence of heat shock by a cellular activity similar to E1A. Given the relation of E1A to alteration of growth control, we have investigated the expression of the HSP70 gene during the cell cycle. Assay of mRNA levels after release from a thymidine-aphidicolin block revealed a 20-fold increase in mRNA abundance, reaching a peak level in the post-S-phase period. Upon reaching this peak level, the abundance of the mRNA then declined as the cells entered the next cycle. Control of the abundance of the mRNA during the cell cycle appeared to be primarily at the level of transcription as measured in nuclear runoff assays. Very similar results were obtained by analyzing the expression of the HSP70 gene in the adenovirus-transformed 293 cell line. Furthermore, the E1A gene was also found to be cell cycle regulated; the activation and peak level of the E1A mRNA occurred at an earlier time than those of the heat shock mRNA, consistent with, but not proof of, the hypothesis that E1A is responsible for the cell cycle control of the HSP70 expression. We therefore suggest that the E1A-like cellular activity may govern certain aspects of cell cycle transcription.

Cell cycle control of the human HSP70 gene: implications for the role of a cellular E1A-like function

The gene encoding the human 70-kilodalton heat shock protein (HSP70) is subject to activation by the adenovirus E1A gene product and appears to be regulated in the absence of heat shock by a cellular activity similar to E1A. Given the relation of E1A to alteration of growth control, we have investigated the expression of the HSP70 gene during the cell cycle. Assay of mRNA levels after release from a thymidine-aphidicolin block revealed a 20-fold increase in mRNA abundance, reaching a peak level in the post-S-phase period. Upon reaching this peak level, the abundance of the mRNA then declined as the cells entered the next cycle. Control of the abundance of the mRNA during the cell cycle appeared to be primarily at the level of transcription as measured in nuclear runoff assays. Very similar results were obtained by analyzing the expression of the HSP70 gene in the adenovirus-transformed 293 cell line. Furthermore, the E1A gene was also found to be cell cycle regulated; the activation and peak level of the E1A mRNA occurred at an earlier time than those of the heat shock mRNA, consistent with, but not proof of, the hypothesis that E1A is responsible for the cell cycle control of the HSP70 expression. We therefore suggest that the E1A-like cellular activity may govern certain aspects of cell cycle transcription.

Adenoviruses have homology with a reiterated sequence in genomic DNA

The DNAs of human adenovirus types 2 and 5 (Ad2, Ad5) hybridize to genomic DNAs from human and chicken tissue. The pattern of Southern blot hybridization to genomic DNA digested with Eco RI suggests that sequences homologous to cloned Ad2 DNA are reiterated in genomic DNA. This was confirmed by liquid hybridization which shows that the hybrids have a low C0t 1/2 value. Finally, we show that total genomic human DNA probes hybridize to short fragments of adenoviral DNA in early-region genes near the ends of the viral genome.

Infection of eucaryotic cells by helper-independent recombinant adenoviruses: early region 1 is not obligatory for integration of viral DNA

Recombinant viral genomes carrying a selectable drug resistance marker have been constructed by insertion of a hybrid gene for neomycin resistance into the helper-independent adenovirus vector, delta E1/X. The hybrid gene consists of sequences coding for the aminoglycoside 3'-phosphotransferase II from Tn5, under the control of the simian virus 40 early promoter, and renders mammalian cells resistant to the neomycin analog, G-418. Most of adenovirus early region 1 is deleted from delta E1/X (nucleotides 455 to 3330), and recombinant viral genomes carry the hybrid gene in its place. The large and small XbaI fragments of delta E1/X were ligated to the hybrid gene, and the mixture was transfected into 293 cells. Single plaques were isolated and subsequently passaged in 293 cells to produce virus stocks. The recombinant viruses efficiently rendered cultured rat (Rat2) and simian (CV1) cells resistant to G-418. Cloned cell lines selected for resistance to G-418 contained viral DNA integrated into the host cell genome, demonstrating that early region 1 is not essential for integration of the viral genome. Southern transfer experiments revealed that (i) the sites of integration in the host genome were not unique; (ii) in general, transformed CV1 cell lines contained single-copy, full-length viral genomes, colinear with the infecting virus; (iii) transformed Rat2 cell lines generally contained one to several copies of full-length viral genomes integrated colinearly with the infecting viral DNA; and (iv) three of these five lines of transformed Rat2 cell lines contained tandemly repeated viral DNA sequences in which the right and left ends of the viral genome were joined to each other.

Genetic analysis of control of proliferation in fibroblastic cells in culture. II. Alteration in proliferative and survival phenotypes in a set of temperature-sensitive mutants of rat 3Y1 cells after infection or transformation with simian virus 40

Mutants of rat 3Y1 fibroblasts, temperature sensitive for proliferation or survival and which represent each of eight complementation groups, were examined to determine whether cells made quiescent at confluence at 33.8 degrees C (permissive temperature) can be stimulated to enter S phase at 39.8 degrees C (nonpermissive temperature) by 20% serum or by infection with simian virus 40 (SV40). Three mutants with a short survival at 39.8 degrees C did not enter S phase at 39.8 degrees C under either condition. The remaining five entered S at 39.8 degrees C by infection with SV40. However, only one of these five entered S in response to high serum. After transformation with SV40, three mutants accumulating at 39.8 degrees C with a predominantly 2n (G1) DNA content did not proliferate, there was a rapid and extensive cell death, and the cells had a DNA content similar to that seen in randomly proliferating populations. The other two mutants, accumulating at 39.8 degrees C with a predominantly 2n or 2n X 4n DNA content, proliferated at this temperature after transformation with SV40. These results clearly indicate that SV40 interacts closely with cellular ts lesions related to control of proliferation and cell survival.

Adenovirus-induced alterations of the cell growth cycle: a requirement for expression of E1A but not of E1B

Mutants dl312, dl314, hr1, and hr3 with mutations in region E1A of adenovirus type 5 were defective for the induction of cell cycle abnormalities detectable by flow cytometry, cell DNA replication, thymidine kinase production, and chromosome aberrations and did not synthesize the viral DNA-binding protein (E2A) in rat cells. dl311, a leaky E1A mutant, induced cell cycle effects at high multiplicity in only one of three experiments, and synthesized the DNA-binding protein. hr7 (E1B) gave a wild-type response in all tests. dl313 was also positive in all tests, although it induced fewer polyploid cells than did wild-type virus, probably because of the leftward extension of the dl313 E1B deletion into E1A. sub315 and sub316, with mutations which also span the E1A-E1B border, synthesized DNA-binding protein, but caused no cell cycle alterations detectable by flow cytometry in rat or mouse cells. Although the participation of other viral early regions cannot be completely excluded, our results suggest that alteration of cell cycle progression is a direct effect of E1A unrelated to its control of other viral early regions, and may be the function of E1A in transformation.

Adenovirus-induced alterations of the cell growth cycle: effects of mutations in early regions E2A and E2B

Mutants ts125 (E2A) and ts36 (E2B) of adenovirus type 5 induced alterations to cell cycle progression at the nonpermissive temperature which were detectable by flow cytometry. Thus neither E2A, nor gene N in E2B, is required for these effects. Whereas the wild-type virus induced cells with aneuploid (between 4n and 8n) DNA contents, as did ts125 at the permissive temperature, ts125 induced peaks of cells with 8n, 16n, and 32n DNA contents at the nonpermissive temperature. This was probably due to the failure of regulation of E1A by E2.

Altered cell cycle progression and aberrant mitosis in adenovirus-infected rodent cells

Actively growing mouse or rat embryo cells suffered structural chromosome damage, mitotic anomalies, and polyploidy after infection by human adenovirus type 5. Chromosome damage required expression of one or more early viral genes and showed regular periodicity in its frequency. The growth cycle time of some of the infected cells was reduced by about 5 hours due to a decrease in G1, and the interval between successive waves of chromosome damage corresponded to this reduced cycle time. After infection there was a decrease in cells with G1 DNA content and an increase in cells with G2 diploid, aneuploid, and polyploid DNA contents. We suggest these effects are due to the expression in semipermissive cells fo early viral gene(s), whose function in productive infection in vivo is to alter cell cycle controls in order to maximize the number of cells able to replicate viral DNA and the time such cells spend in DNA replication.

A biochemical investigation of the adenovirus-induced G1 to S phase progression: thymidine kinase, ornithine decarboxylase, and inhibitors of polyamine biosynthesis

Biochemical events were investigated in the G1 to S phase progression induced in quiescent rodent cells by human adenovirus type 5 (Ad5) and by serum. Thymidine kinase activity increased after infection of cells with Ad5 or addition of 10% serum. These stimulations were additive. An early viral gene was responsible for induction by Ad5, but the early mutants ts36, ts37, and ts125 induced thymidine kinase at the permissive and nonpermissive temperatures. Several differences were found between cells stimulated by serum compared with Ad5. Induction of thymidine kinase was delayed in Ad5-infected cells, insensitive to 0.01 microgram/ml actinomycin D and relatively resistant to reduced Ca2+ compared with induction by serum. Ornithine decarboxylase was induced by serum, but not by Ad5, alpha-Methylornithine had little effect on the induction of thymidine kinase by Ad5, but reduced the induction of thymidine kinase by serum, suggesting that Ad5-induced entry into S phase is uncoupled from polyamine biosynthesis. Methylglyoxal bis(guanylhydrazone), however, prevented the induction of thymidine kinase by both serum and Ad5. Adenovirus infection appears to induce cellular DNA synthesis and thymidine kinase in G1-arrested cells by a mechanism different from serum, and bypasses events in the normal G1 to S phase progression.

Alterations to controls of cellular DNA synthesis by adenovirus infection

Human adenovirus type 5 and temperature-sensitive mutants ts36, ts37, and ts125 induced cellular DNA synthesis in quiescent rodent cells at both permissive and nonpermissive temperatures. Cellular DNA synthesis induced by adenovirus type 5 or by serum required protein synthesis for both initiation and continuation, whereas viral DNA synthesis was not dependent upon continued protein synthesis once it was initiated. Both cellular and viral DNA replication was induced in adenovirus type 5-infected cells in the presence of dibutyryl cyclic AMP at concentrations which inhibited induction by serum which suggested that some of the controls of DNA synthesis in serum-treated and virus-infected cells are different. After adenovirus infection of quiescent cells, there was a decrease in the number of cells with G1 DNA content and an increase in cells with G2 diploid and greater DNA contents. Thus, adenovirus type 5 induces a complete round of cellular DNA replication, but in some cells, it induces a second round without completion of a normal mitosis. These results suggest that adenovirus type 5 is able to alter cell growth cycle controls in a way which may be related to its ability to transform cells.

Intracellular forms of adenovirus DNA. V. Viral DNA sequences in hamster cells abortively infected and transformed with human adenovirus type 12

The persistence of viral DNA in BHK-21 cells abortively infected with human adenovirus type 12 has been investigated using reassociation kinetics. No indication of an increase in the amount of viral DNA per cell has been found. On the contrary, the amount of intracellular viral DNA sequences decreases rapidly after infection. Thus, free adenovirus type 12 DNA does not replicate in BHK-21 cells. The influence of the multiplicity of infection on the amount of persisting adenovirus type 12 DNA has also been explored. The viral DNA sequences persisting in four lines of hamster cells transformed in vitro by adenovirus type 12 at various multiplicities of infection have been quantitated and mapped by reassociation kinetics experiments using restriction endonuclease fragments of 3H-labeled adenovirus type 12 DNA. All the EcoRI restriction nuclease fragments of the adenovirus type 12 genome are represented in each of the four cell lines. Individual fragments of the viral genome are represented in multiple copies in non-equimolar amounts.

Transcription of the genome of adenovirus type 12. Viral mRNA in productively infected KB cells

In human KB cells productively infected with adenovirus type 12, viral DNA replication starts between 12 and 14h postinfection. Virus-specific, polysome-associated mRNA was investigated early (6-8h) and late (26-28h) after infection. Most of the viral mRNA was polyadenylated and accounted for 0.46% and 24.1% of the mRNA synthesized early and late postinfection, respectively. The viral-specific mRNA isolated both early and late after infection falls into several distinct size-classes, ranging in molecular weights between 0.3X10(6) and 1.5X10(6) for the early RNA and between 0.6X10(6) and 2.3X10(6) for the RNA synthesized late in the infection.

Transcription of the genome of adenovirus type 12. I. Viral mRNA in abortively infected and transformed cells

In baby hamster kidney (BKH-21) cells abortively infected with adenovirus type 12, polysome-associated, virus-specific RNA could be detected starting 5 to 7 h after infection. The amount of this RNA reached a maximum between 10 to 12 h after infection and continued to be synthesized at a reduced level until late in infection (48 to 50 h.). In BHK-21 cells transformed by adenovirus type 12 (HB cells), 0.26% of the polysome-associated mRNA was virus specific. The size of the virus-specific mRNA isolated from polysomes of BHK-21 cells abortively infected with, or transformed by adenovirus type 12 was determined by electrophoresis in polyacrylamide gels in 98% formamide, i.e., under conditions which eliminated secondary structure or aggregation of RNA. In abortively infected hamster cells viral mRNA size classes of molecular weights 0.9 times 10-6 and 0.65 times 10-6 to 0.67 times 10-6 were predominant. A minor fraction of 1.5 times 10-6 daltons was consistently found and increased with time after infection. Late after infection (24 to 26 h), viral mRNA of 1.9 times 10-6 daltons was also observed. The size distribution of adenovirus type 12-specific mRNA from transformed hamster cells (HB line) was very similar to that in abortively infected cells, except that the relative amount of the viral mRNA fraction of 1.5 times 10-6 daltons was much higher. It is uncertain whether the viral mRNA of high-molecular-weight represents mixed transcripts derived from integrated viral genomes and adjacent host genes.

Transformation potentials of the noninfectious (defective) component in pools of adenoviruses type 12 and simian adenovirus 7

Pools of adenovirus 12 and simian adenovirus 7 were separated into four or five fractions by density gradient centrifugation in cesium chloride. Each fraction was analyzed for total in vitro infectivity units, total transformation activity, and for total virus particle (VP) content. Two major subpopulations were separated with mean densities of 1.30 +/- 0.02 and 1.34 +/- 0.02 g/ml, respectively. Virions in the 1.34 g/ml range were highly infectious (10(2) to 10(3) VP per infectivity unit) in contrast to virions at 1.30 g/ml density (10(4) to 10(5) VP per infectivity units). Transformation capacity was evenly distributed throughout fractions of both viruses, indicating that genetically incomplete or defective virus particles were not deficient in their ability to induce transformation. The average VP per transformation unit for adenovirus 12 (2.85 x 10(6)) and for simian adenovirus 7 (4.00 x 10(6)) did not vary significantly from fraction to fraction. These values were obtained with optimal input multiplicities of 16 to 64 VP per cell. At higher multiplicities the apparent increase in VP per transformation unit was attributable to the viral cytocidal effect on hamster cells. These studies revealed that quantitation of in vitro transformation based on VP multiplicities was more reliable than on the basis of infectious units. These estimates were independent of method of virus production, extraction, and purification.