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

Polyamines and Their Role in Virus Infection

Polyamines are small, abundant, aliphatic molecules present in all mammalian cells. Within the context of the cell, they play a myriad of roles, from modulating nucleic acid conformation to promoting cellular proliferation and signaling. In addition, polyamines have emerged as important molecules in virus-host interactions. Many viruses have been shown to require polyamines for one or more aspects of their replication cycle, including DNA and RNA polymerization, nucleic acid packaging, and protein synthesis. Understanding the role of polyamines has become easier with the application of small-molecule inhibitors of polyamine synthesis and the use of interferon-induced regulators of polyamines. Here we review the diverse mechanisms in which viruses require polyamines and investigate blocking polyamine synthesis as a potential broad-spectrum antiviral approach.

Characterization of histone post-translational modifications during virus infection using mass spectrometry-based proteomics

Viruses are obligate intracellular parasites that necessarily rely on hijacking cellular resources to produce viral progeny. The success of viral infection requires manipulation of host chromatin in order to activate genes useful for production of viral proteins as well as to suppress antiviral responses. Host chromatin manipulation on a global level is likely reliant on modulation of post-translational modifications (PTMs) on histone proteins. Mass spectrometry (MS) is a powerful tool to quantify and identify novel histone PTMs, beyond the limitations of site-specific antibodies. Here, we employ MS to investigate global changes in histone PTM relative abundance in human cells during infection with adenovirus. Our method reveals several changes in histone PTM patterns during infection. We propose that this method can be used to uncover global changes in histone PTM patterns that are universally modulated by viruses to take over the cell.

Transactivation of the p53 oncogene by E1a gene products

Infection of quiescent rat kidney cells with human adenovirus is shown to transcriptionally stimulate (transactivate) the p53 oncogene. The increased transcription results in an accumulation of p53-specific mRNA in parallel with an increase in p53 protein levels, although there is a considerable delay between transcriptional activation and the detection of stable p53 mRNA and protein. The induction of p53 is detectable with two monoclonal antibodies recognizing different epitopes. The induction of p53 by adenovirus is delayed compared to induction by serum, and it occurs after the onset of adenovirus-induced cellular DNA replication. Thus, adenovirus-induced DNA replication bypasses a G0/G1 control point. Experiments with hydroxyurea show that p53 activation does not require continued cell cycling and thus is likely to be a direct consequence of viral gene expression. Finally, the induction of p53 is shown to be dependent on expression of the 289-residue product encoded by the viral E1a gene.

Polyamine metabolism and cell-cycle-dependent gene expression in IMR-90 human diploid fibroblasts during senescence in culture

Aging of IMR-90 human diploid fibroblasts in culture is accompanied by specific changes of polyamine metabolism including: (a) a fivefold decrease of serum-induced activity of ornithine decarboxylase (ODC1 EC 4.1.1.17); (b) a six to tenfold increase of polyamine catabolism; and (c) a reduction of putrescine uptake. These changes apparently led to a significant reduction of putrescine accumulation in senescent cells following serum stimulation. Since the induction of ODC is a mid-G1 event, the change of polyamine metabolism may be related to changes of expression of other cell-cycle-dependent genes during cellular aging. In addition to ODC gene, we have examined the expression of two early G1 genes, c-erbB and c-myc, and one late G1/S gene thymidine kinase, at mRNA levels, in both young and old IMR-90 cells. We have also compared the enzyme activities of two late G1/S genes, thymidine kinase and thymidylate synthetase, in young and old cells following serum stimulation. We did not observe significant changes of c-erbB, c-myc, and ODC mRNA levels during cellular senescence. However, we found that serum-induced mRNA level of thymidine kinase gene in old IMR-90 cells was significantly reduced compared to that in the young cells. Results also demonstrate that aging of IMR-90 cells was accompanied by significant decrease of both thymidine kinase and thymidylate synthetase activities. In view of the recognized importance of polyamines in growth regulation, it is possible that alteration of polyamine metabolism may contribute to the impairment of expression of some key G1/S genes and such impairment may contribute to the ultimate loss of dividing potential in senescent cells.

Functions of the two adenovirus early E1A proteins and their conserved domains in cell cycle alteration, actin reorganization, and gene activation in rat cells

Rat embryo cells were infected with adenovirus type 5 mutants that code for only one of the two early E1A proteins, mutants with defects in one of the two conserved regions common to the two proteins, or mutants with defects in the 46-amino-acid region unique to the 289-amino-acid E1A protein. Cells were scored for altered cell cycle progression, disruption of actin stress fibers, and activation of E2A expression. Mutants lacking either E1A protein were able to cause all of these effects; but mutants lacking a 243-amino-acid protein had less effect, and mutants lacking a 289-amino-acid protein much less effect, than wild-type virus. A mutation in any of the three conserved regions caused a defect in each E1A effect. To investigate the reported function of conserved domain 2 in mitosis, we monitored by fluorescence-activated cell sorter the reduction in Hoechst 33342 fluorescence that occurs when cells divide after undergoing a round of DNA replication in 5-bromodeoxyuridine. A smaller percentage of adenovirus-infected cells than mock-infected cells divided within a given period after completing a round of DNA replication. Viruses with mutations in conserved domain 2 were defective for initiation of cellular DNA replication, as were all other E1A mutants we have examined, but had no specific defect in cell division compared with wild-type virus. Thus, although there may be some specialization of function between the two E1A proteins and between their conserved domains, it was not apparent in the aspects of E1A function and the mutants that we examined.

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.

Control functions of adenovirus transformation region E1A gene products in rat and human cells

Altered control of the rat cell cycle induced by adenovirus requires expression of transformation region E1A, but not of E1B, E2A, E2B, or late genes. We show here that neither E3 nor E4 is required, so the effect results directly from an E1A product. Mutants with defects in the 289-amino-acid (aa) E1A product had little or no effect on the rat cell cycle even at 1,000 IU per cell. A mutant (pm975) lacking the 243-aa E1A product altered cell cycle progression, but less efficiently than did wild-type virus. The 289-aa E1A protein is therefore essential for cell cycle effects; the 243-aa protein is also necessary for the full effect but cannot act alone. Mutants with altered 289-aa E1A proteins showed different extents of leak expression of viral early region E2A as the multiplicity was increased; each leaked more in human than in rat cells. dl312, with no E1A products, failed to produce E2A mRNA or protein at 1,000 IU per cell in rat cells but did so in some experiments in human cells. There appears to be a very strict dependence of viral early gene expression on E1A in rat cells, whereas dependence on E1A is more relaxed in HeLa cells, perhaps due to a cellular E1A-like function. Altered cell cycle control is more dependent on E1A function than is early viral gene expression.

Adenovirus type 2 activates cell cycle-dependent genes that are a subset of those activated by serum

We have studied a panel of 10 genes and cDNA sequences that are expressed in a cell cycle-dependent manner in different types of cells from different species and that are inducible by different mitogens. These include five sequences (c-myc, 4F1, 2F1, 2A9, and KC-1) that are preferentially expressed in the early part of the G1 phase, three genes (ornithine decarboxylase, p53, and c-rasHa) preferentially expressed in middle or late G1, and two genes (thymidine kinase and histone H3) preferentially expressed in the S phase of the cell cycle. We have studied the expression of these genes in nonpermissive (tsAF8) and semipermissive (Swiss 3T3) cells infected with adenovirus type 2. Under the conditions of these experiments, adenovirus type 2 infection stimulates cellular DNA synthesis in both tsAF8 and 3T3 cells. However, four of the five early G1 genes (c-myc, 4F1, KC-1, and 2A9) and one of the late G1 genes (c-ras) are not induced by adenovirus infection, although they are strongly induced by serum. The other sequences (2F1, ornithine decarboxylase, p53, thymidine kinase, and histone H3) are activated by both adenovirus and serum. We conclude that the cell cycle-dependent genes activated by adenovirus 2 are a subset of the cell cycle-dependent genes activated by serum. The data suggest that the mechanisms by which serum and adenovirus induce cellular DNA synthesis are not identical.

Selective increase of c-myc mRNA levels by methylglyoxal-bis (guanylhydrazone) and novobiocin in serum-stimulated fibroblasts

We have studied the effect of methylglyoxal-bis (guanylhydrazone) (MGBG) and novobiocin on the accumulation of specific mRNAs in serum-stimulated ts13 cells (a temperature-sensitive mutant of the BHK cell line). The RNAs studied included: c-myc, v-ras, ornithine decarboxylase, beta-actin, histone H3, and those represented by clones p2F1 and p1B6 (Hirschhorn et al., Proc. Natl, Acad. Sci. USA, 81:6004, 1984) All these RNAs accumulated at higher levels when quiescent cells were serum stimulated for 16 h. Both MGBG (25 micronM and 100 micronM) and novobiocin (200 micrograms/ml) effectively prevented the transition from G0 to S phase. We found that 100 microM MGBG induced an overaccumulation of c-myc RNA while H3 RNA was decreased, and the steady-state levels of all other RNAs were the same as in cells stimulated without the drug. Novobiocin prevented the serum-induced increase in the amount of all RNAs, which remained at the same levels as in quiescent cells, with the exception of c-myc, which again accumulated at a higher level in drug-treated cells than in serum-stimulated untreated cells. The possible significance of these results is discussed.

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.

Adenovirus type 2 activates cell cycle-dependent genes that are a subset of those activated by serum

We have studied a panel of 10 genes and cDNA sequences that are expressed in a cell cycle-dependent manner in different types of cells from different species and that are inducible by different mitogens. These include five sequences (c-myc, 4F1, 2F1, 2A9, and KC-1) that are preferentially expressed in the early part of the G1 phase, three genes (ornithine decarboxylase, p53, and c-rasHa) preferentially expressed in middle or late G1, and two genes (thymidine kinase and histone H3) preferentially expressed in the S phase of the cell cycle. We have studied the expression of these genes in nonpermissive (tsAF8) and semipermissive (Swiss 3T3) cells infected with adenovirus type 2. Under the conditions of these experiments, adenovirus type 2 infection stimulates cellular DNA synthesis in both tsAF8 and 3T3 cells. However, four of the five early G1 genes (c-myc, 4F1, KC-1, and 2A9) and one of the late G1 genes (c-ras) are not induced by adenovirus infection, although they are strongly induced by serum. The other sequences (2F1, ornithine decarboxylase, p53, thymidine kinase, and histone H3) are activated by both adenovirus and serum. We conclude that the cell cycle-dependent genes activated by adenovirus 2 are a subset of the cell cycle-dependent genes activated by serum. The data suggest that the mechanisms by which serum and adenovirus induce cellular DNA synthesis are not identical.

Reduced microfilament organization in adenovirus type 5-infected rat embryo cells: a function of early region 1a

The actin microfilament organization in rat embryo cells was examined by fluorescence microscopy with 7-nitrobenz-2-oxa-1,3-diazole-phallacidin and by electron microscopy, after mock infection or infection with adenovirus type 5 (Ad5). Infected cells showed severely reduced numbers of actin microfilaments and stress fibers, detectable early after infection. Mutants defective in Ad5 early genes were used to show that reduced microfilament organization was a function of the Ad5 transformation early gene 1a (E1a) and did not require expression of any other viral gene. The product of the E1a 13s mRNA was essential for the effect, although the 12s mRNA product appeared to contribute. Ad5 infection of the cells had no observable effect on total cell actin levels or on the ratio of monomeric to polymeric actin. E1a, therefore, affected only the higher-order organization of actin.

Control functions of adenovirus transformation region E1A gene products in rat and human cells

Altered control of the rat cell cycle induced by adenovirus requires expression of transformation region E1A, but not of E1B, E2A, E2B, or late genes. We show here that neither E3 nor E4 is required, so the effect results directly from an E1A product. Mutants with defects in the 289-amino-acid (aa) E1A product had little or no effect on the rat cell cycle even at 1,000 IU per cell. A mutant (pm975) lacking the 243-aa E1A product altered cell cycle progression, but less efficiently than did wild-type virus. The 289-aa E1A protein is therefore essential for cell cycle effects; the 243-aa protein is also necessary for the full effect but cannot act alone. Mutants with altered 289-aa E1A proteins showed different extents of leak expression of viral early region E2A as the multiplicity was increased; each leaked more in human than in rat cells. dl312, with no E1A products, failed to produce E2A mRNA or protein at 1,000 IU per cell in rat cells but did so in some experiments in human cells. There appears to be a very strict dependence of viral early gene expression on E1A in rat cells, whereas dependence on E1A is more relaxed in HeLa cells, perhaps due to a cellular E1A-like function. Altered cell cycle control is more dependent on E1A function than is early viral gene expression.

Cytoskeletal events during calcium- or EGF-induced initiation of DNA synthesis in cultured cells. Role of protein phosphorylation and clues in the transformation process

The possible relationship between cytoskeletal events and growth regulation in response to stimulation by calcium and by growth factors such as EGF can be summarized as follows: An elaborate interaction exists between calcium and serum growth factors, such as EGF, in the initiation of DNA synthesis in quiescent cells. This implies that many processes between the external signals delivered at the cell surface and the sequential intracellular events that lead to chromosomal replication, and ultimately to cell division, must be coordinated in a reproducible manner. It is now apparent that because of its possible role as a dynamic integrator of the cytoplasm, the cytoskeleton could represent the coordinator of the events that lead to replication. Calcium (with its intracellular acceptor, calmodulin) and cAMP (which can act by opposing mechanisms) are extensively involved in the control of the integrity of the cytoskeleton. Distinct protein kinases are activated by calcium/calmodulin, EGF, and cAMP as aspects of the prereplicative response, and many of the substrates for phosphorylation are cytoskeletal proteins. The emerging picture seems to include a direct involvement of these protein kinases in the cascade of regulatory events that leads to the initiation of DNA synthesis. Thus, the cytoskeleton has a direct role in the transmission of proliferative signals from external receptor sites to the nucleus. A means by which neoplastic cells can bypass the normal regulatory pathways is proposed in the light of recent data showing that the product of oncogenes are protein kinases or proteins that intimately interact with cellular protein kinases.

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.

Transformation by human adenoviruses

When, approximately 10 years ago, it was shown that the functions essential for cell transformation were localized in a small region of the adenovirus genome, a DNA segment which at that time was thought to be capable of encoding two or three average-sized proteins at most, it seemed reasonable to hope that an understanding of the mechanisms by which adenoviruses transform cells might be quickly achieved. While such optimism might be forgiven, it was quite clearly naive in the extreme. As a consequence of mRNA splicing and the use of overlapping reading frames the number of proteins encoded within E1 is 2-3-times greater than would have been predicted a decade ago, and post-translational modifications may add another dimension of complexity. In fact it has taken nearly all of the past decade just to identify the proteins encoded in E1 and to characterize them in the most rudimentary way. However, we have now entered a period in which new information is accumulating at an extremely rapid rate as a result of several major technical and fundamental advances. Chief among these are the use of recombinant DNA techniques, particularly site-directed mutagenesis, which combined with methods for introducing mutations made in cloned sequences back into infectious virus, clearly represents a powerful approach to studying the functions of transforming proteins. In addition, the ability to express transforming proteins in bacteria and to produce large amounts of highly purified proteins which previously were only just detectable in infected and transformed cells is a major breakthrough. Advances in immunological techniques, particularly the development of monoclonal antibodies and antisera against synthetic peptides, have enormously simplified the task of detecting and characterizing E1 proteins. Finally, recent results suggesting that adenovirus transforming proteins may be functionally and structurally similar to other oncogenes brings a new perspective to the study of oncogenic transformation. Have all the proteins involved in transformation by adenoviruses been identified? It seems probable that all those virally coded proteins which play a major role are now known but of course minor players in the cast could still be waiting in the wings. We have pointed out that viral functions encoded outside region E1 may have some importance at least in initiation of transformation by virions and have speculated on the possibility that one or more of these may be involved in the integration of viral DNA into the host cell chromosome.(ABSTRACT TRUNCATED AT 400 WORDS)

An inhibitor of polyamine synthesis arrests cells at an earlier stage of G1 than does calcium deprivation

Methylglyoxal bis(guanylhydrazone) completely inhibits the induction of thymidine kinase after serum stimulation of quiescent fibroblasts only if added within 3 h after serum, whereas calcium deprivation blocks this induction up to 12 h after serum stimulation. Experiments in which one of these blocks was imposed as the other was released confirmed that cells blocked by methylglyoxal bis(guanylhydrazone) are arrested at an earlier stage in G1 than cells blocked by calcium deprivation.

An inhibitor of polyamine synthesis arrests cells at an earlier stage of G1 than does calcium deprivation

Methylglyoxal bis(guanylhydrazone) completely inhibits the induction of thymidine kinase after serum stimulation of quiescent fibroblasts only if added within 3 h after serum, whereas calcium deprivation blocks this induction up to 12 h after serum stimulation. Experiments in which one of these blocks was imposed as the other was released confirmed that cells blocked by methylglyoxal bis(guanylhydrazone) are arrested at an earlier stage in G1 than cells blocked by calcium deprivation.

Adenovirus type 5 induces progression of quiescent rat cells into S phase without polyamine accumulation

Adenovirus type 5 induces cellular DNA synthesis and thymidine kinase in quiescent rat cells but does not induce ornithine decarboxylase. We now show that unlike serum, adenovirus type 5 fails to induce S-adenosylmethionine decarboxylase or polyamine accumulation. The inhibition by methylglyoxal bis(guanylhydrazone) of the induction of thymidine kinase by adenovirus type 5 is probably unrelated to its effects on polyamine biosynthesis. Thus, induction of cellular thymidine kinase and DNA replication by adenovirus type 5 is uncoupled from polyamine accumulation.

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.

Adenovirus type 5 induces progression of quiescent rat cells into S phase without polyamine accumulation

Adenovirus type 5 induces cellular DNA synthesis and thymidine kinase in quiescent rat cells but does not induce ornithine decarboxylase. We now show that unlike serum, adenovirus type 5 fails to induce S-adenosylmethionine decarboxylase or polyamine accumulation. The inhibition by methylglyoxal bis(guanylhydrazone) of the induction of thymidine kinase by adenovirus type 5 is probably unrelated to its effects on polyamine biosynthesis. Thus, induction of cellular thymidine kinase and DNA replication by adenovirus type 5 is uncoupled from polyamine accumulation.

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.

Spermine and aminoguanidine protect cells from chromosome aberrations induced by adenovirus during the G2 phase of the cell cycle

Adenovirus uncouples DNA replication from polyamine biosynthesis and causes chromosome aberrations in rodent cells. Addition of polyamines protected infected cells from this chromosome damage. Spermine was the only individual polyamine which protected. The diamine oxidase inhibitor aminoguanidine also protected. Neither compound detectably reduced synthesis of viral early proteins. The protective effects of spermine and aminoguanidine were not additive. Maximal protection was obtained when the compounds were added 4.5 h before mitosis, but significant protection was observed up to 1.25 h before mitosis. This suggests that the compounds act in G2. In vitro, spermine bound strongly to DNA and protected it from mild endonuclease attack, but aminoguanidine did neither. We propose that viral infection causes a deficiency in spermine during a critical period G2, possibly accompanied by an increase in endonuclease activity. The resulting chromosome damage can be prevented by adding exogenous spermine, or by inhibiting the oxidative degradation of endogenous spermine.