Transformation of rodent cells by DNA extracted from transformation-defective adenovirus mutants

Expression and interactions of human adenovirus oncoproteins

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Separate regions of an adenovirus E1B protein critical for different biological functions

The E1B region of Ad12 encodes two major proteins, the 482R (55K) and 163R (19K). In this report we showed that the E1B 482R is multifunctional, in that its structure may somehow contribute to its own stability, in viral DNA and virus replication, in transformation of primary cells, and in tumorigenicity. Deletion of the first 24 amino acids and of aa residues 114-155 (dl42) results in an instability of the 55K protein. The N-terminal 24 aa residues (pm1852) or amino acids residues 80-96 (dl17) are not required for viral DNA or virus replication, whereas amino acid residues between 114 and 155 (dl42) are absolutely necessary for viral DNA synthesis. Deletion of amino acid residues 1-24, 80-96, and 114-155 (dl42) greatly reduces the transforming ability of both virus and plasmids containing any one of these deletions. One of the critical regions for tumorigenicity residues within amino acid residues 80-96, since cells transformed by this plasmid are nontumorigenic. On the other hand the region bounded by amino acid residues 114-155 (dl42) is not required for tumorigenicity in immunocompetent animals.

The effect of E1 mutations on biochemical transformation by an adenovirus carrying the herpes simplex virus thymidine kinase gene in region E3

A series of human adenovirus type 5 (Ad5) vectors has been constructed in which a vector containing the human herpes simplex virus thymidine kinase (TK) gene has been recombined with several Ad5 early region 1 (E1) mutants. The resulting viruses were used to study host-virus interactions in TK- rat cells and to examine the importance of E1 functions in a biochemical transformation assay. One of the most important parameters affecting transformation efficiency in this system was the cytotoxicity of the transforming virus. Ad5 viruses expressing the E1a 289 amino acid protein were all highly cytotoxic and induced significantly fewer colonies than did less cytotoxic mutants which were defective in expression of the 289 amino acid product. When correction was made for differential cell viability the variation in transformation efficiencies was considerably reduced although some E1a mutants still demonstrated an enhanced ability to transform in comparison to wt virus. The significance of these results to morphological transformation by adenoviruses is discussed.

Host range mutants of adenovirus type 12 E1 defective for lytic infection, transformation, and oncogenicity

The human adenovirus type 12 (H12) E1A region encodes two early proteins of 266 amino acid residues (266R) and 235R whilst the H12 E1B promoter directs the synthesis of two major proteins of 163R and 482R. To determine the functions of E1A and E1B in lytic infection and oncogenic transformation we have isolated and characterized a series of H12 E1 mutants. Mutant H12 hr 700 contains a point mutation in exon 1 that alters a single amino acid common to both the 266 and 235R proteins. This mutant synthesized reduced levels of E1 and structural proteins at delayed times in HEK cells, transformed BRK cells, and induced tumors in newborn rats at reduced efficiency compared to wild-type virus. The mutation in H12 in 600 truncates the 266R protein in its unique sequences but this mutant synthesized the 235R, E1B, and structural proteins at delayed times in HEK cells. H12 in 600 was nontransforming but induced rare tumors in newborn rats. A third E1A mutant H12 in 601 synthesized no E1A proteins, reduced levels of E1B and structural proteins at delayed times in lytic infections, and was not a transforming or oncogenic virus. Three E1B mutants were studied in detail. Both H12 hr 703 and H12 in 602 encode N-terminal truncated 482R proteins whereas H12 del 620 encodes an in-frame internally deleted 482R protein. All three synthesized reduced amounts of E1A proteins and the E1B 163R protein, identifying a regulatory function for the 482R protein. None of the E1B mutants could transform and only H12 del 620 could induce rare tumors in newborn rats. These results show that H12 oncogenesis requires the coordinated expression of the E1 proteins.

Requirements for immortalization of primary mouse embryo fibroblasts probed with mutants bearing deletions in the 3' end of SV40 gene A

The influence of specific contiguous stretches of amino acids predominantly in the carboxy terminal third of the SV40 large T antigen on the immortalization of cells in culture was investigated. Mutants that bear either small in-phase or frameshift deletions in the large T antigen coding sequence were transfected into primary mouse embryo fibroblasts of C57Bl/6 origin (B6/MEF). The frequency of immortalization was determined as the number of colonies that developed from cells escaping senescence. The results indicated that the terminal 81 amino acids of large T antigen are not needed for efficient immortalization or tumorigenicity. In contrast removal of as few as three amino acids encoded in the vicinity of the Dde-1 site at 0.234 map units (m.u.) severely restricted immortalization, suggesting that this region of the coding sequence either structurally or functionally is essential to at least one parameter of the transformed cell phenotype. The T antigen produced by dlA2433 which bears a deletion of nine nucleotides at 0.234 m.u. fails to associate stably with the cellular protein p53. The results showed that the addition of long stretches of amino acids (96 or 97 residues) from the open reading frame at the 3' end of the early region inactivated immortalizing functions, although the addition of as many as 18 amino acids from other reading frames was not detrimental. The evidence presented also confirmed that wild-type levels of ATPase activity are not necessary for immortalization or tumorigenicity of B6/MEF. Finally, we show that one of the mutants that immortalized primary cells did not produce dense foci on a cell monolayer. This last result indicated that independent functions are required for these two parameters of the transformed cell phenotype.

Transformation of differentiated rat hepatocytes with adenovirus and adenovirus DNA

Primary cultures of hepatocytes isolated by collagenase perfusion of adult rats were transformed by infection with adenovirus type 5 or transfection with adenovirus DNA. Total virion DNA or recombinant plasmid DNA containing the adenovirus E1A and E1B genes transformed hepatocytes at comparable frequencies. No foci of replicating hepatocytes were detected after transfection with a plasmid containing the E1A gene alone. The frequency of transformation by the adenovirus E1A and E1B genes was dependent on the composition of the culture medium. Transformation occurred at a low frequency when the transfected hepatocytes were maintained in a chemically defined medium (CDM), but the frequency was enhanced 8- to 10-fold when the cells were maintained in (i) serum-supplemented medium or (ii) CDM supplemented with epidermal growth factor. Cell lines derived from the adenovirus-transformed colonies of hepatocytes expressed adenovirus E1A and E1B RNAs. When hepatocytes were maintained in CDM supplemented with dimethyl sulfoxide and transfected with plasmids containing the E1A and E1B genes, it was possible to derive cell lines that retained the ability to express several liver-specific genes, including albumin, transferrin, hemopexin, and the third component of complement. The amount of albumin secreted per cell varied from 1 to 5 pg per cell per 24 h, and in one cell line it was below detectable levels by passage 9. Adenovirus-transformed hepatocytes were not tumorigenic when inoculated subcutaneously into neonatal syngeneic rats. We conclude that the adenovirus E1A and E1B genes are capable of transforming adult rat hepatocytes, a differentiated epithelial cell type.

Morphological transformation of established rodent cell lines by high-level expression of the adenovirus type 2 E1a gene

When a strong promoter derived from the mouse metallothionein gene was substituted for the homologous adenovirus type 2 E1a promoter, leading to enhanced levels of E1a RNAs and proteins in cells transfected with the chimeric gene, the E1a gene alone was able to induce in established cell lines alterations in cellular morphology and growth properties similar to those produced by the combined action of E1a and E1b genes. The qualitative effects of E1a gene expression upon cellular properties thus depend on the level of expression of the E1a gene. Furthermore, E1a may be the primary transforming gene of adenoviruses, since it produced many of the characteristics of transformed cells that had previously been attributed to E1b.

Adenovirus proteins from both E1B reading frames are required for transformation of rodent cells by viral infection and DNA transfection

To determine the requirements for the individual Ad2 E1B proteins during the transformation of rodent cells, viral mutants were constructed with genetic lesions disrupting the coding sequence of either the 175 amino acid residue (175R) or the 495 amino acid residue (495R) E1B proteins. Point mutations generating stop codons very early in the coding sequences were constructed to prevent the expression of amino-terminal protein fragments which might have biological activity. Mutant virus pm1722 contains a point mutation that terminates translation of the 175R protein after three amino acids. It was completely defective for transformation of CREF cells in virion- and DNA-mediated assays. In HeLa cells, pm1722 replicated as well as wild-type virus but produced an extreme cytopathic effect and fragmentation of host-cell DNA. Nonetheless, we provide evidence that the observed transformation defect is not due to the death of transformed cells. The mutant virus dl1520, a double mutant unable to synthesize the 495R protein, was also extremely defective for the transformation of CREF cells in virion- and viral DNA-mediated assays. This result is in contrast to studies with other Ad5 mutants with lesions in the equivalent protein. Possible explanations for this difference are discussed. Replication of dl1520 in HeLa cells was significantly reduced compared to wild-type. Studies with a third mutant virus, pm2022, which contains a stop codon after the second codon of the 495R protein, suggest that very low levels of 495R protein activity are sufficient for a productive infection and significant transforming activity.

An adenovirus 2-coded tumor antigen located on the endoplasmic reticulum and nuclear envelope is required for growth of transformed cells in Ca2+-deficient media

Rat embryo cell lines containing the adenovirus 2 E1a region together with normal or mutant forms of the N-terminal half of the E1b region (HindIII G fragment) were generated by using a dominant selection marker, neo. Biochemically transformed cells containing a nonmutated HindIII G fragment proliferated more rapidly in Ca2+-deficient media, whereas cells containing a specific deletion within the E1b-encoded, 175-amino-acid (175R) (19-kilodalton) T-antigen gene and nontransformed cells grew at a slower rate. Furthermore, transformed cells that did not express the 175R T antigen and untransformed cells could not replicate their DNA efficiently in low-Ca2+ medium. Our results suggest that Ca2+ ions may provide an important stimulus for cell proliferation in adenovirus-transformed cells through a mechanism that involves the functions of the 175R T antigen.

Expression of the adenovirus E1A oncogene during cell transformation is sufficient to induce susceptibility to lysis by host inflammatory cells

Mammalian cells transformed by nononcogenic human adenoviruses exhibit high susceptibility to destruction by host mononuclear inflammatory cells. We have analyzed the viral gene regulation of the susceptibility of transformed cells to lysis by natural killer cells and activated macrophages. Comparisons of target cell lines transformed by overlapping segments of the adenovirus E1-transforming gene region revealed that isolated expression of a single oncogene, E1A, was sufficient to cause increased cytolytic susceptibility in the absence of detectable transformed cell-surface expression of viral transplantation antigens and irrespective of histocompatibility antigen identity between killer cells and target cells. These results suggest that oncogene functions that are not linked to the expression of previously recognized cell-surface target structures may actively induce neoplastic cell elimination by components of the host immune surveillance system.

Role of the adenovirus early region 1B tumor antigens in transformation and lytic infection

We have investigated the contribution of each of the two adenovirus type 5 (Ad5) major early region 1b (E1b) proteins in cell transformation and in lytic infection. An Ad5 E1 plasmid, in which the reading frame for the 19-kDa E1b protein was abolished by a stop codon close to the initiation codon, transformed primary baby rat kidney (BRK) cells with an efficiency of about half of that of a wild type Ad5 E1 plasmid, whereas a plasmid with a mutation in the gene for the 58-kDa E1b protein transformed the same primary cells with only one-third of the wild type efficiency. Plasmids containing region E1a only or a plasmid carrying mutations in the genes for major E1b proteins all transformed primary cells with an efficiency of approximately 5% of wild type. To test the effect of the E1b mutations in virion-mediated cell transformation, the mutant E1b regions were introduced into intact viral genomes by overlap recombination and were subsequently used in a transformation assay on BRK cells. The 19 and 58-kDa mutant viruses were found to transform BRK cells with 11 and 25% of the efficiency of wild type virus, respectively. These results suggest that the 19-kDa E1b protein is essential for virus-mediated cell transformation, in agreement with results of others, but not for plasmid-mediated cell transformation. In lytic infection, the 19-kDa mutant virus was some 30-fold reduced in yield on HeLa cells, whereas the 58-kDa mutant virus was 3000-fold reduced in its ability to grow on HeLa cells at low multiplicity of infection, but showed a marked multiplicity-dependent leakiness. The 58-kDa mutant virus was not defective when its growth was assayed on human embryonic kidney (HEK) cells. This may indicate that cellular proteins are expressed in HEK cells that are functionally homologous to the 58-kDa E1b protein.

Morphological transformation of established rodent cell lines by high-level expression of the adenovirus type 2 E1a gene

When a strong promoter derived from the mouse metallothionein gene was substituted for the homologous adenovirus type 2 E1a promoter, leading to enhanced levels of E1a RNAs and proteins in cells transfected with the chimeric gene, the E1a gene alone was able to induce in established cell lines alterations in cellular morphology and growth properties similar to those produced by the combined action of E1a and E1b genes. The qualitative effects of E1a gene expression upon cellular properties thus depend on the level of expression of the E1a gene. Furthermore, E1a may be the primary transforming gene of adenoviruses, since it produced many of the characteristics of transformed cells that had previously been attributed to E1b.

Mutations in the E1a gene of adenovirus type 5 alter the tumorigenic properties of transformed cloned rat embryo fibroblast cells

The adenovirus type 5 mutants H5hr1 and H5dl101 contain modifications in the E1a gene affecting the 13S mRNA-encoded 289-amino acid polypeptide and exhibit a cold-sensitive transformation phenotype upon infection of cloned rat embryo fibroblast (CREF) cells. Transformed cell lines expressing solely E1a or E1a and E1b gene products derived from these viruses display enhanced anchorage-independent growth at 37 degrees C versus 32 degrees C and display a cytoskeletal architecture resembling untransformed fibroblastic CREF cells. In contrast, CREF cells transformed by H5wt or the E1a and E1b region of H5wt grow with similar efficiency in agar at 37 degrees C or 32 degrees C and exhibit an epithelioid morphology that is associated with an altered cytoskeleton. Regardless of the expression or presence of other viral early regions, including E1b, E2a, and E4 genes, specific CREF cell lines expressing an altered 289-amino acid protein and a wild-type 12S mRNA-encoded 243-amino acid protein were capable of inducing tumors in nude mice and in immunocompetent syngeneic Fischer rats. In sharp contrast, cells expressing a wild-type 289-amino acid protein were unable to induce tumors in either nude mice or syngeneic rats. The ability to induce tumors did not correlate with alterations in the pattern of viral DNA integration or differential expression of the E1a and E1b genes, nor was the tumor induction a consequence of unique properties of the immortal parental CREF cell line.

The adenovirus type 12 early-region 1B 58,000-Mr gene product is required for viral DNA synthesis and for initiation of cell transformation

An E1B 58K mutant of adenovirus type 12 (Ad12), dl207, was constructed by the deletion of 852 base pairs in the E1B 58K coding region. The mutant could grow efficiently in 293E1 cells but not in HeLa, KB, or human embryo kidney (HEK) cells. Viral DNA replication of dl207 was not detected in HeLa and KB cells and was seldom detected in HEK cells. Analysis of viral DNA synthesis in vitro showed that the Ad12-DNA-protein complex replicated by using the nuclear extract from Ad12 wild-type (WT)-infected HeLa cells but not by using the nuclear extract from dl207-infected cells. In dl207-infected HeLa and KB cells, early mRNAs were detected, but late mRNAs were not detected. The mutant induced fewer transformed foci than the WT in rat 3Y1 cells. Cells transformed by dl207 could grow efficiently in fluid medium, form colonies in soft agar culture, and induce tumors in rats transplanted with the transformed cells at the same efficiency as WT-transformed cells. Tumors were induced in hamsters injected with WT virions but were not induced in hamsters injected with dl207 virions. The results indicate that the E1B 58K protein is required both for viral DNA replication in productive infection and for initiation of cell transformation, but not for maintenance of the transformed phenotype.

An adenovirus 2-coded tumor antigen located on the endoplasmic reticulum and nuclear envelope is required for growth of transformed cells in Ca2+-deficient media

Rat embryo cell lines containing the adenovirus 2 E1a region together with normal or mutant forms of the N-terminal half of the E1b region (HindIII G fragment) were generated by using a dominant selection marker, neo. Biochemically transformed cells containing a nonmutated HindIII G fragment proliferated more rapidly in Ca2+-deficient media, whereas cells containing a specific deletion within the E1b-encoded, 175-amino-acid (175R) (19-kilodalton) T-antigen gene and nontransformed cells grew at a slower rate. Furthermore, transformed cells that did not express the 175R T antigen and untransformed cells could not replicate their DNA efficiently in low-Ca2+ medium. Our results suggest that Ca2+ ions may provide an important stimulus for cell proliferation in adenovirus-transformed cells through a mechanism that involves the functions of the 175R T antigen.

Effect on transformation of mutations in the early region 1b-encoded 21- and 55-kilodalton proteins of adenovirus 5

It is well established that the adenovirus 5 genes responsible for the initiation and maintenance of the transformed cell reside in the early region 1a and 1b genes, but it remains unclear how the polypeptides encoded in these genes mediate their functions. To probe the function of the early region 1b-encoded 55- and 21-kilodalton (kd) polypeptides during this process, a series of viral mutants was engineered so that they contained deletions or insertions at 5.4, 5.7, 7.9, or 9.6 map units. By means of either an overlap recombination procedure involving H5dl314 (delta 3.7 to 4.6 map units) cleaved with ClaI, or a marker rescue procedure involving H5dl312 (delta 1.2 to 3.8 map units), viral mutants were isolated by their ability to produce plaques on KB cell line 18 cells, which constitutively express only viral early region 1b functions. DNA sequence analysis confirmed that the series of mutants generated differed in their abilities to express the 21- or the 55-kd polypeptides, or both. Upon infection of cloned rat embryo fibroblast cells with viruses containing mutations affecting the 55-kd protein, the transformation frequency decreased as the size of the predicted truncated polypeptide decreased. Although all of the foci generated by the 55-kd protein mutants were indistinguishable from the foci induced by wild-type virus, they displayed an inefficient ability to grow in soft agar, again in relation to the size of the truncated polypeptide. In contrast, if cloned rat embryo fibroblast cells were transfected with viral DNA, the defectiveness in transformation observed after infection with virions was not as dramatic. However, all of the viruses containing 21-kd mutations were transformation defective, regardless of the mode by which the viral nucleic acid was introduced into the cell.

Mutations in the gene encoding the adenovirus early region 1B 19,000-molecular-weight tumor antigen cause the degradation of chromosomal DNA

The adenovirus mutant Ad2ts111 has been previously shown to contain a mutation in the early region 2A gene encoding the single-stranded-DNA-binding protein that results in thermolabile replication of virus DNA and a mutation in early region 1 that causes degradation of intracellular DNA. A recombinant virus, Ad2cyt106, has been constructed which contains the Ad2ts111 early region 1 mutation and the wild-type early region 2A gene from adenovirus 5. This virus, like its parent Ad2ts111, has two temperature-independent phenotypes; first, it has the ability to cause an enhanced and unusual cytopathic effect on the host cell (cytocidal [cyt] phenotype) and second, it induces degradation of cell DNA (DNA degradation [deg] phenotype). The mutation responsible for these phenotypes is a single point mutation in the gene encoding the adenovirus early region 1B (E1B) 19,000-molecular-weight (19K) tumor antigen. This mutation causes a change from a serine to an asparagine in the 20th amino acid from the amino terminus of the protein. Three other mutants that affect the E1B 19K protein function have been examined. The mutants Ad2lp5 and Ad5dl337 have both the cytocidal and DNA degradation phenotypes (cyt deg), whereas Ad2lp3 has only the cytocidal phenotype and does not induce degradation of cell DNA (cyt deg+). Thus, the DNA degradation is not caused by the altered cell morphology. Furthermore, the mutant Ad5dl337 does not make any detectable E1B 19K protein product, suggesting that the absence of E1B 19K protein function is responsible for the mutant phenotypes. A fully functional E1B 19K protein is not absolutely required for lytic growth of adenovirus 2 in HeLa cells, and its involvement in transformation of nonpermissive cells to morphological variants is discussed.

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.

Early region 1B of adenovirus 2 encodes two coterminal proteins of 495 and 155 amino acid residues

Partial sequence analysis of tryptic peptides has identified the E1B-495R (E1b-57K) (early transcription region 1B of 495 amino acid residues, with an approximate molecular weight of 57,000) protein of adenovirus 2 as encoded by the 495 amino acid open reading frame located in the adenovirus 2 DNA sequence between nucleotides 2016 and 3500. Additional proteins of 16,000 Mr and 18,000 Mr that are related to the E1B-495R protein were identified by cell-free translation of hybridization-selected mRNA. Analysis of [35S]methionine-containing amino terminal tryptic peptides by thin-layer chromatography showed that the E1B-495R, E1B-18K, and E1B-16K proteins all begin at the same initiation codon. The E1B-495R protein from 293 cells also has the same initial tryptic peptide, acetyl-methionyl-glutamyl-arginine. Sequence analysis of E1B-18K tryptic peptides indicated that this protein also has the same carboxy terminus as the E1B-495R protein and that it is derived from an mRNA that is spliced to remove sequences between nucleotides 2250 and 3269, resulting in a protein product of 155 amino acid residues. Analysis of E1B-16K tryptic peptides has not yet revealed the carboxy terminal structure of this protein. Both the E1B-495R and the E1B-155R (E1B-18K) proteins, as well as the E1B-16K protein, were precipitated from cell-free translations and from extracts of infected cells by antiserum against an amino terminal nonapeptide common to these proteins.

Adenovirus type 5 early region 1b gene product is required for efficient shutoff of host protein synthesis

To determine the role adenovirus 5 early region 1b-encoded 21- and 55-kilodalton proteins play in adenovirus productive infection, mutants have been isolated which were engineered to contain small deletions or insertions at 5.8, 7.9, or 9.6 map units. By using an overlap recombination procedure involving H5dl314 (delta 3.7 to 4.6 map units) DNA cleaved at 2.6 map units with ClaI and the adenovirus 5 XhoI-C (0 to 15.5 map units) fragment containing the desired mutation, viral mutants were isolated by their ability to produce plaques on KB cell line 18, which constitutively expresses only viral early region 1b functions (Babiss et al., J. Virol. 46:454-465, 1983). DNA sequence analysis of the viral mutants isolated (H5dl118, H5dl110, H5in127, and H5dl163) indicates that all of the viruses contain mutations which affect the 55-kilodalton protein, whereas dl118 should also produce a truncated form of the 21-kilodalton protein. When analyzed for their replication characteristics in HeLa cells, all of the mutant viruses exhibited extended eclipse periods and effected yields that were reduced to 10% or less of that produced by H5sub309 (parent virus of the mutants which is phenotypically identical to wild-type adenovirus 5). When compared with characteristics of sub309, the early and late transcription and DNA replication of the mutants were similar, but synthesis of late polypeptides and late cytoplasmic mRNAs was greatly reduced. Quantitation of mutant virus-specific late mRNAs associated with polysomes revealed a threefold reduction when compared with that of sub309. Analysis of infected cell extracts further revealed that these mutants were incapable of efficiently shutting off host cell protein synthesis, suggesting that the 55-kilodalton protein plays a role in this process. These data suggest that early region 1b products may function by interacting with additional viral or host cell macromolecules to modulate host cell shutoff or that some late viral mRNA or polypeptide may potentiate this reaction.

Large-T-antigen-p53 complex formation is not cold sensitive in a cold-sensitive transformant induced by simian virus 40 mutant tsA1499

F111 rat cells transformed by simian virus 40 mutant tsA1499 are cold sensitive for the expression of transformation. Yet, unlike F111 cells transformed by tsA58, they do not lose the ability to stabilize the transformation-associated host cell protein p53 at the temperature at which transformation is extinguished.

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)

Intracellular localization of adenovirus type 5 tumor antigens in productively infected cells

The intracellular localization of tumor antigens of human adenovirus type 5 (Ad5) during lytic infection of KB cells has been studied. The cells were pulse labeled with [35S]methionine early after infection and early proteins of 58,000 D (58K), 44K, 19K, 18.5K, and 14K detectable by immunoprecipitation with hamster antitumor serum were assayed for association with cytoplasm, nucleoplasm, chromatin, cytosol, cytoskeleton, and membranes. The 44,000 D (44K) tumor antigen encoded in early region 1A (E1A: 0-4.4%) was recovered in approximately equal amounts from cytoplasmic and nucleoplasmic fractions of pulse-labeled cells and within the cytoplasmic compartment was found in the cytosol as well as associated with the cytoskeleton. The E1B-58K (E1B: 4.5-11.2%) antigen was also found to be associated with the cytoplasmic and nucleoplasmic fractions in approximately equal amounts but unlike the E1A-44K showed no affinity for cytoskeletons. Pulse-chase and immunofluorescence experiments suggested the 58K antigen accumulated in the nucleus late in infection. The E1B-19K antigen was found almost exclusively associated with the membrane fraction of infected KB cells and was resolved in polyacrylamide gels into two related species of 18.5K and 19K. Immunofluorescence studies on the E1B 18.5-19K doublet suggested that within a population of infected HeLa cells a small minority seemed to be expressing copious amounts of stainable antigen. Cell fractionation and immunofluorescence studies showed that the E4-14K antigen was a nuclear protein and the only antigen in this study which showed a significant association with a nuclear subfraction composed almost entirely of histones. The implications of these findings for the roles of the Ad5 tumor antigens in lytic infection and transformation are discussed.