Limited but durable changes to cellular gene expression in a model of latent adenovirus infection are reflected in childhood leukemic cell lines

Mucosal lymphocytes support latent infections of species C adenoviruses. Because infected lymphocytes resist re-infection with adenovirus, we sought to identify changes in cellular gene expression that could inhibit the infectious process. The expression of over 30,000 genes was evaluated by microarray in persistently infected B-and T-lymphocytic cells. BBS9, BNIP3, BTG3, CXADR, SLFN11 and SPARCL1 were the only genes differentially expressed between mock and infected B cells. Most of these genes are associated with oncogenesis or cancer progression. Histone deacetylase and DNA methyltransferase inhibitors released the repression of some of these genes. Cellular and viral gene expression was compared among leukemic cell lines following adenovirus infection. Childhood leukemic B-cell lines resist adenovirus infection and also show reduced expression of CXADR and SPARCL. Thus adenovirus induces limited changes to infected B-cell lines that are similar to changes observed in childhood leukemic cell lines.

Adenovirus DNA is detected at increased frequency in Guthrie cards from children who develop acute lymphoblastic leukaemia

Epidemiological evidence suggests that childhood acute lymphoblastic leukaemia (ALL) may be initiated by an in infection in utero. Adenovirus DNA was detected in 13 of 49 neonatal blood spots from ALL patients but only in 3 of 47 controls (P=0.012) suggesting a correlation between prenatal adenovirus infection and the development of ALL

Roles for the E4 orf6, orf3, and E1B 55-kilodalton proteins in cell cycle-independent adenovirus replication

Adenoviruses bearing lesions in the E1B 55-kDa protein (E1B 55-kDa) gene are restricted by the cell cycle such that mutant virus growth is most impaired in cells infected during G(1) and least restricted in cells infected during S phase (F. D. Goodrum and D. A. Ornelles, J. Virol. 71:548-561, 1997). A similar defect is reported here for E4 orf6-mutant viruses. An E4 orf3-mutant virus was not restricted for growth by the cell cycle. However, orf3 was required for enhanced growth of an E4 orf6-mutant virus in cells infected during S phase. The cell cycle restriction may be linked to virus-mediated mRNA transport because both E1B 55-kDa- and E4 orf6-mutant viruses are defective at regulating mRNA transport at late times of infection. Accordingly, the cytoplasmic-to-nuclear ratio of late viral mRNA was reduced in G(1) cells infected with the mutant viruses compared to that in G(1) cells infected with the wild-type virus. By contrast, this ratio was equivalent among cells infected during S phase with the wild-type or mutant viruses. Furthermore, cells infected during S phase with the E1B 55-kDa- or E4 orf6-mutant viruses synthesized more late viral protein than did cells infected during G(1). However, the total amount of cytoplasmic late viral mRNA was greater in cells infected during G(1) than in cells infected during S phase with either the wild-type or mutant viruses, indicating that enhanced transport of viral mRNA in cells infected during S phase cannot account for the difference in yields in cells infected during S phase and in cells infected during G(1). Thus, additional factors affect the cell cycle restriction. These results indicate that the E4 orf6 and orf3 proteins, in addition to the E1B 55-kDa protein, may cooperate to promote cell cycle-independent adenovirus growth.

An arginine-faced amphipathic alpha helix is required for adenovirus type 5 e4orf6 protein function

A region in the carboxy terminus of the protein encoded by open reading frame 6 in early region 4 (E4orf6) of adenovirus type 5 was determined to be required for directing nuclear localization of the E1B 55-kDa protein and for efficient virus replication. A peptide encompassing this region, corresponding to amino acids 239 through 255 of the E4orf6 protein, was analyzed by circular dichroism spectroscopy. The peptide showed evidence of self-interaction and displayed the characteristic spectra of an amphipathic alpha helix in the helix-stabilizing solvent trifluoroethanol. Disrupting the integrity of this alpha helix in the E4orf6 protein by proline substitutions or by removing amino acids 241 through 250 abolished its ability to direct the E1B 55-kDa protein to the nucleus when both proteins were transiently expressed in HeLa cells. Expression of E4orf6 variants that failed to direct nuclear localization of the E1B 55-kDa protein failed to enhance replication of the E4 mutant virus, dl1014, whereas expression of the wild-type E4orf6 protein restored growth of dl1014 to near-wild-type levels. These results suggest that the E4orf6 protein contains an arginine-faced, amphipathic alpha helix that is critical for a functional interaction with the E1B 55-kDa protein in the cell and for the function of the E4orf6 protein during a lytic infection.

p53 status does not determine outcome of E1B 55-kilodalton mutant adenovirus lytic infection

The ability of the adenovirus type 5 E1B 55-kDa mutants dl1520 and dl338 to replicate efficiently and independently of the cell cycle, to synthesis viral DNA, and to lyse infected cells did not correlate with the status of p53 in seven cell lines examined. Rather, cell cycle-independent replication and virus-induced cell killing correlated with permissivity to viral replication. This correlation extended to S-phase HeLa cells, which were more susceptible to virus-induced cell killing by the E1B 55-kDa mutant virus than HeLa cells infected during G1. Wild-type p53 had only a modest effect on E1B mutant virus yields in H1299 cells expressing a temperature-sensitive p53 allele. The defect in E1B 55-kDa mutant virus replication resulting from reduced temperature was as much as 10-fold greater than the defect due to p53 function. At 39 degreesC, the E1B 55-kDa mutant viruses produced wild-type yields of virus and replicated independently of the cell cycle. In addition, the E1B 55-kDa mutant viruses directed the synthesis of late viral proteins to levels equivalent to the wild-type virus level at 39 degreesC. We have previously shown that the defect in mutant virus replication can also be overcome by infecting HeLa cells during S phase. Taken together, these results indicate that the capacity of the E1B 55-kDa mutant virus to replicate independently of the cell cycle does not correlate with the status of p53 but is determined by yet unidentified mechanisms. The cold-sensitive nature of the defect of the E1B 55-kDa mutant virus in both late gene expression and cell cycle-independent replication leads us to speculate that these functions of the E1B 55-kDa protein may be linked.

The early region 1B 55-kilodalton oncoprotein of adenovirus relieves growth restrictions imposed on viral replication by the cell cycle

The E1B 55-kDa oncoprotein of adenovirus enables the virus to overcome restrictions imposed on viral replication by the cell cycle. Approximately 20% of HeLa cells infected with an E1B 55-kDa mutant adenovirus produced virus when evaluated by electron microscopy or by assays for infectious centers. By contrast, all HeLa cells infected with a wild-type adenovirus produced virus. The yield of E1B mutant virus from randomly cycling HeLa cells correlated with the fraction of cells in S phase at the time of infection. In synchronously growing HeLa cells, approximately 75% of the cells infected during S phase with the E1B mutant virus produced virus, whereas only 10% of the cells infected during G1 produced virus. The yield of E1B mutant virus from HeLa cells infected during S phase was sevenfold greater than that of cells infected during G1 and threefold greater than that of cells infected during asynchronous growth. Cells infected during S phase with the E1B mutant virus exhibited severe cytopathic effects, whereas cells infected with the E1B mutant virus during G1 exhibited a mild cytopathic effect. Viral DNA synthesis appeared independent of the cell cycle because equivalent amounts of viral DNA were synthesized in cells infected with either wild-type or E1B mutant virus. The inability of the E1B mutant virus to replicate was not mediated by the status of p53. These results define a novel property of the large tumor antigen of adenovirus in relieving growth restrictions imposed on viral replication by the cell cycle.

Adenovirus early region 4 34-kilodalton protein directs the nuclear localization of the early region 1B 55-kilodalton protein in primate cells

The localization of the adenovirus type 5 34-kDa E4 and 55-kDa E1B proteins was determined in the absence of other adenovirus proteins. When expressed by transfection in human, monkey, hamster, rat, and mouse cell lines, the E1B protein was predominantly cytoplasmic and typically was excluded from the nucleus. When expressed by transfection, the E4 protein accumulated in the nucleus. Strikingly, when coexpressed by transfection in human, monkey, or baby hamster kidney cells, the E1B protein colocalized in the nucleus with the E4 protein. A complex of the E4 and E1B proteins was identified by coimmunoprecipitation in transfected HeLa cells. By contrast to the interaction observed in primate and baby hamster kidney cells, the E4 protein failed to direct the E1B protein to the nucleus in rat and mouse cell lines as well as CHO and V79 hamster cell lines. This failure of the E4 protein to direct the nuclear localization of the E1B protein in REF-52 rat cells was overcome by fusion with HeLa cells. Within 4 h of heterokaryon formation and with protein synthesis inhibited, a portion of the E4 protein present in the REF-52 nuclei migrated to the HeLa nuclei. Simultaneously, the previously cytoplasmic E1B protein colocalized with the E4 protein in both human and rat cell nuclei. These results suggest that a primate cell-specific factor mediates the functional interaction of the E1B and E4 proteins of adenovirus.

The adenovirus L3 23-kilodalton proteinase cleaves the amino-terminal head domain from cytokeratin 18 and disrupts the cytokeratin network of HeLa cells

Immunofluorescence studies revealed that adenovirus induces a reorganization of the cytokeratin system in lytically infected HeLa cells. At 24 h postinfection, the cytokeratin network began to disassemble into prominent spheroid globules. By 36 h postinfection, host cell lysis occurred, accompanied by the formation of perinuclear cytokeratin clumps and additional spheroid globules. Immunoblots detected 41- and 44-kDa fragments of cytokeratin 18 and reduced levels of cytokeratin 7 at 24 and 36 h postinfection. Cytokeratin proteins isolated from HeLa cells at 36 h postinfection were deficient in filament polymerization. The 41-kDa proteolytic cytokeratin 18-specific fragment was purified, and its amino-terminal sequence was determined to be GGIQNEKETM. These residues correspond to amino acids 74 through 83 of cytokeratin 18, identifying a cleavage site at the junction of the globular head domain and the alpha-helical rod domain. Moreover, this truncation event occurs at a consensus cleavage site for the adenovirus L3 23-kDa proteinase. The temperature-sensitive mutant H2-ts1, which contains a mutation in the proteinase, neither induced cleavage of cytokeratin 18 nor precipitated the formation of spheroid globules during lytic infection at the nonpermissive temperature. The active proteinase is therefore required for cleavage of cytokeratin 18 and morphological rearrangement of the cytokeratins. We suggest that disruptions in the cytokeratin system weaken the mechanical integrity of the cell, thus promoting host cell lysis and release of progeny virions.

Adenovirus L1 52- and 55-kilodalton proteins are present within assembling virions and colocalize with nuclear structures distinct from replication centers

Analysis of a temperature-sensitive mutant, Ad5ts369, had indicated that the adenovirus L1 52- and 55-kDa proteins (52/55-kDa proteins) are required for the assembly of infectious virions. By using monoclonal antibodies directed against bacterially produced L1 52-kDa protein, the L1 52/55-kDa proteins were found to be differentially phosphorylated forms of a single 48-kDa polypeptide. Both phosphoforms were shown to be present within all suspected virus assembly intermediates (empty capsids, 50 to 100 molecules; young virions, 1 to 2 molecules) but not within mature virions. The mobilities of these proteins in polyacrylamide gels were affected by reducing agents, indicating that the 52/55-kDa proteins may exist as homodimers within the cell and within assembling particles. Immunofluorescence analysis revealed that the 52/55-kDa proteins localize to regions within the infected nucleus that are distinct from viral DNA replication centers, indicating that replication and assembly of viral components likely occur in separate nuclear compartments. Immunoelectron microscopic studies determined that the 52/55-kDa proteins are found in close association with structures that appear to contain assembling virions. These results are consistent with an active but transient role for the L1 products in assembly of the adenovirus particle, perhaps as scaffolding proteins.

Localization of the adenovirus early region 1B 55-kilodalton protein during lytic infection: association with nuclear viral inclusions requires the early region 4 34-kilodalton protein

The distribution of the adenovirus early region 1B 55-kDa protein (E1B-55kDa) in lytically infected HeLa cells was determined. At the time of infection, when the E1B-55kDa protein facilitates the cytoplasmic accumulation of viral mRNA while simultaneously restricting the accumulation of most cellular mRNA, five distinct intracellular localizations of the protein were observed. Only one of these was disrupted when cells were infected with a mutant virus that fails to produce a second viral protein encoded by early region 4 (E4-34kDa). This protein normally forms a complex with the E1B-55kDa polypeptide, enabling it to influence RNA metabolism. This key localization of the E1B protein was within and about the periphery of nuclear viral inclusion bodies believed to be the site of viral DNA replication and transcription. In the absence of the E4-34kDa protein, the coincidence of E1B-55kDa-specific immunofluorescence and phase-dense viral inclusions was reduced compared with that in a wild-type infection. Similarly, by immunoelectron microscopy, the relative number of E1B-55kDa-specific immunogold particles associated with the clear fibrillar inclusion bodies was reduced. However, the E4-34kDa protein was not required for the close association of the early region 2A DNA binding protein with the viral inclusions. We propose that the viral 55-kDa-34-kDa protein complex interacts with a cellular factor required for cytoplasmic accumulation of mRNAs and directs it to the periphery of the transcriptionally active viral inclusion bodies. This model provides an explanation for the ability of these viral proteins to simultaneously enhance accumulation of viral mRNAs and inhibit accumulation of cellular mRNAs.

The adenovirus E4 17-kilodalton protein complexes with the cellular transcription factor E2F, altering its DNA-binding properties and stimulating E1A-independent accumulation of E2 mRNA

E2F is a cellular DNA-binding factor. Its binding activity is changed within adenovirus-infected cells so that it binds cooperatively to pairs of properly spaced and oriented E2F recognition sites. In the work described in this report, the conversion to cooperative binding was shown to require the adenovirus E4 17-kilodalton (kDa) polypeptide. Mutant viruses carrying alterations within the E4 17-kDa coding region failed to generate the infection-specific, cooperatively binding form of E2F. It was possible to alter E2F from uninfected cells so that it bound cooperatively by incubation with a partially purified fraction obtained from infected cells. The E4 17-kDa protein copurified with this activity and was also found to be present in a complex containing E2F. Consistent with its ability to alter the binding of E2F to its recognition sites within the E2 promoter, the E4 17-kDa polypeptide contributed to maximal expression of E2 mRNAs in some cell types. Its ability to enhance E2 transcription did not require expression of the E1A transactivator protein. These results are consistent with a model which proposes that the E4 17-kDa polypeptide binds to the cellular E2F factor, altering its binding behavior and thereby enhancing its ability to stimulate transcription.

Prompt heat-shock and heat-shifted proteins associated with the nuclear matrix-intermediate filament scaffold in Drosophila melanogaster cells

Elevated temperatures induced the synthesis of several new proteins in Drosophila melanogaster cells. Besides the conventional heat shock (HS) proteins, another set of temperature-induced proteins has been found. These latter resemble the prompt HS proteins of mammalian cells. The prompt HS proteins of Drosophila differ from the well-known conventional HS proteins in the following properties: (1) synthesis of the prompt HS proteins is insensitive to the transcription inhibitor actinomycin D, which blocks the appearance of conventional HS proteins; (2) induction of the prompt HS proteins requires a significantly higher temperature than conventional HS proteins; (3) prompt HS proteins associate strictly with the nuclear matrix-intermediate filament complex (NM-IF), while the conventional HS proteins are found in all subcellular fractions; (4) prompt HS proteins of Drosophila are induced by high temperature alone while the conventional HS proteins are also produced by a variety of stress conditions. Resinless-section electron micrographs show an altered nuclear matrix morphology in heat-shocked cells. The nuclear matrix fibers are altered in spatial distribution and have much additional electron-dense material. This added material probably reflects the soluble proteins shifted into the nuclear matrix at high temperature. The prompt HS proteins can be distinguished clearly from heat-shifted proteins by several criteria. Also, the prompt HS proteins are distinct from the heat-insensitive viral proteins of a persistent virus (HPS-1).

Adenovirus L1 52- and 55-kilodalton proteins are required for assembly of virions

A variant of adenovirus type 5 that contained a mutation within the L1 52- and 55-kilodalton (52/55K) protein-coding region was isolated. The mutant, termed ts369, produced L1 52/55K proteins with a two-amino-acid substitution and was temperature sensitive. Temperature-shift experiments indicated that the ts369 defect was late in the viral growth cycle. DNA replication and synthesis of late proteins occurred normally in ts369-infected cells at the nonpermissive temperature, but mature virions were not produced. Rather, capsidlike particles associated with the left-terminal region of the viral chromosome accumulated. These incomplete particles could not be chased into mature virions when the infected cells were shifted to the permissive temperature. However, previously synthesized proteins could be assembled into virions in the presence of a protein synthesis inhibitor upon shiftdown from the nonpermissive temperature, suggesting that the inactivation of the L1 52/55K proteins was reversible. These results indicate that the adenovirus L1 52/55K proteins play a role in the assembly of infectious virus particles.

Cytochalasin releases mRNA from the cytoskeletal framework and inhibits protein synthesis

Cytochalasin D was shown to be a reversible inhibitor of protein synthesis in HeLa cells. The inhibition was detectable at drug levels typically used to perturb cell structure and increased in a dose-dependent manner. The drug also released mRNA from the cytoskeletal framework in direct proportion to the inhibition of protein synthesis. The released mRNA was unaltered in its translatability as measured in vitro but was no longer translated in the cytochalasin-treated HeLa cells. The residual protein synthesis occurred on polyribosomes that were reduced in amount but displayed a normal sedimentation distribution. The results support the hypothesis that mRNA binding to the cytoskeletal framework is necessary although not sufficient for translation. Analysis of the cytoskeletal framework, which binds the polyribosomes, revealed no alterations in composition or amount of protein as a result of treatment with cytochalasin D. Electron microscopy with embedment-free sections shows the framework in great detail. The micrographs revealed the profound reorganization effected by the drug but did not indicate substantial disaggregation of the cytoskeletal elements.

Association of RNA with the cytoskeleton and the nuclear matrix

Heteronuclear RNA (hnRNA) is preferentially associated (76%) with the nuclear matrix in mammalian cells. Active mRNA, in the form of polyribosomes, is associated (greater than 97%) with the cytoskeletal framework. In this report, we present evidence that the association of both hnRNA and mRNA with structural networks of the cell may be essential features of gene expression. To study the association of polyribosomes with the cytoskeletal framework, cytochalasin D was used to release mRNA from the cytoskeletal framework. Protein synthesis was inhibited by cytochalasin D in direct proportion to the release of mRNA. The released mRNA is unaltered in its translatability as measured in vitro but is no longer translated in the cytochalasin-treated HeLa cells. The residual protein synthesis occurs on polyribosomes that are reduced in amount but display a normal sedimentation distribution. The results support the hypothesis that mRNA binding to the cytoskeletal framework is necessary, though not sufficient, for translation. Further fractionation of the cytoskeletal framework separates nuclear constituents into three distinct protein fractions. Chromatin proteins and 94% of the DNA are released by 0.25 M-ammonium sulphate after inter-nucleosomal DNA is cut with DNase I. The resulting structure retains 76% of the hnRNA in the form of ribonucleoprotein and is designated the RNP-containing nuclear matrix. The proteins of hnRNP complex are those associated with the nucleus only if RNA is intact. These proteins and 97% of the hnRNA are released after brief digestion with RNase A. Visualizing the nuclear matrix using resinless sections shows that nuclear RNA plays an important role in the organization of the nuclear matrix. Electron micrographs of resinless sections show the interior of the matrix to be a three-dimensional network of thick filaments bounded by the nuclear lamina. The filaments are densely covered with 20-30 nm electron-dense particles, which may contain the hnRNA. The RNP-depleted matrix is disordered and the interior fibres aggregated. These results suggest that hnRNA is involved in the spatial organization of the interior of the nuclear matrix.