Transformation-defective mutant of adenovirus type 5 containing a single altered E1a mRNA species

Infection with E1B-mutant adenovirus stabilizes p53 but blocks p53 acetylation and activity through E1A

Wild-type adenovirus type 5 eliminates p53 through the E1B-55kDa and E4-34kDa gene products. Deletion or mutation of E1B-55kDa has long been thought to confer p53-selective replication of oncolytic viruses. We show here that infection with E1B-defective adenovirus mutants induces massive accumulation of p53, without obvious defects in p53 localization, phosphorylation, conformation and oligomerization. Nonetheless, p53 completely failed to induce its target genes in this scenario, for example, p21/CDKN1A, Mdm2 and PUMA. Two regions of the E1A gene products independently contributed to the suppression of p21 transcription. Depending on the E1A conserved region 3, E1B-defective adenovirus impaired the ability of the transcription factor Sp1 to bind the p21 promoter. Moreover, the amino terminal region of E1A, binding the acetyl transferases p300 and CREB-binding protein, blocked p53 K382 acetylation in infected cells. Mutating either of these E1A regions, in addition to E1B, partially restored p21 mRNA levels. Our findings argue that adenovirus attenuates p53-mediated p21 induction, through at least two E1B-independent mechanisms. Other virus species and cancer cells may employ analogous strategies to impair p53 activity.

The E1b promoter of Ad12 in mouse L tk- cells is activated by adenovirus region E1a

We have investigated the effect of the E1a region of adenovirus on the expression of the E1b transcriptional unit in mouse L tk- cells. To that end we have fused the promoter region of the E1b gene of Ad12 to the coding sequence of the thymidine kinase gene of herpes simplex virus type 1. We found that expression of this fusion gene is completely dependent on the presence of the E1a region. Sequences involved in this regulation were mapped between positions -135 and +11 from the E1b cap site. In addition, we found that the largest E1a protein is essential in this regulation process.

trans E1 component requirements for maximal replication of E1-defective recombinant adenovirus

Strategies that enable E1-defective recombinant adenoviruses to selectively undergo replication in neoplastic tissue may be useful for future investigations or therapies of malignancies. A growing body of evidence suggests that some molecular alterations commonly associated with malignancies, such as p53 mutations, can modify the specific E1 requirements for replication of human serotype adenoviruses. In the studies reported here, a panel of human non-small cell lung cancer cell lines with previously defined p53 status were characterized for basal interleukin-6 (IL-6) and bcl-2 content because previous studies have indicated both proteins can functionally substitute for the replication requirements provided by native E1 viral proteins. Cell lines were infected with E1-defective adenovirus 5 and simultaneously transfected with different combinations of E1 plasmids, or a bcl-2 expression plasmid, and adenovirus present in the cells was quantified 6 days later. These assays demonstrated that E1A with both 19- and 55-kDa E1B-encoding plasmids were required for maximal adenoviral replication, independent of the varying p53/IL-6/basal bcl-2 phenotypes of the host cell lines. E1A was required for maximal replication enablement, independent of the basal IL-6 content of these cell lines, and exogenous IL-6 also did not obviate the E1A requirement. Interestingly, the bcl-2 expression plasmid did not consistently substitute for the 19-kDa expression plasmid in the context of this replication complementation assay. These results suggest that (1) basal levels of IL-6 greater than that present in these cell lines are necessary for functional replacement of the E1A replication function and (2) bcl-2 does not predictably substitute for the 19-kDa E1B replication function in the context of trans complementation.

p300 binding by E1A cosegregates with p53 induction but is dispensable for apoptosis

E1A expression during adenovirus infection induces apoptosis. E1A expression causes accumulation of the p53 tumor suppressor protein, and E1A-induced apoptosis is p53 mediated in primary rodent cells, implying that p53 induction may be linked to apoptosis induction by E1A. Adenoviruses containing mutations in the E1A gene were tested for the ability to trigger both p53 accumulation and the appearance of enhanced cytopathy (cyt phenotype) and degradation of DNA (deg phenotype), indicative of apoptosis in infected HeLa cells. The adenoviruses had mutations which disrupted the pRb- and/or p300-binding activities of E1A so that the relationship between p53 induction and apoptosis and binding to these cellular proteins by E1A could be determined. An E1A mutation that specifically disrupted the p300-binding activity failed to induce p53 accumulation, whereas mutations in E1A which affected pRb binding induced p53 accumulation. Thus, p300 binding was required and pRb binding was dispensable for E1A-mediated accumulation of p53 in HeLa cells. All the E1A mutant viruses, regardless of the ability to induce p53 accumulation, induced the cyt and deg phenotypes, suggesting that p53 induction in infected HeLa cells was not essential for apoptosis, nor was binding of E1A to the pRb and/or p300 protein. The possibility that E1A induced a p53-independent apoptosis pathway was tested by analyzing the appearance of the cyt and deg phenotypes in Saos-2 cells, which were null for both alleles of p53, upon adenovirus infection. An adenovirus expressing wild-type 12S E1A induced both the cyt and deg phenotypes in Saos-2 cells, as did all the E1A mutant viruses. Thus, E1A expression during infection of human cells may trigger redundant p53-independent and -dependent apoptotic pathways.

Trans complementation of an E1A-deleted adenovirus with codelivered E1A sequences to make recombinant adenoviral producer cells

Replication-incompetent adenovirus is conventionally produced by cells that supply replication-enabling proteins from viral sequences present in trans. As an alternative means of recombinant adenovirus production, replication-enabling E1A sequences were cotransduced into human prostate carcinoma cells infected with an E1A-deleted adenovirus containing a luciferase expression cassette. The replication-enabling plasmid was cotransduced by ionic linkage to the recombinant adenovirus exterior. Cells cotransduced with the replication-enabling plasmid made new adenovirus with titers up to 8 x 10(6) in the supernatants 72-120 hr after transduction. Like the parent virus, the virus present in the cotransduced supernatants and lysates was capable of transferring luciferase activity to new cells. The virus present in the cotransduced cell supernatants was amplified and shown to be identical to the parent virus by genomic analysis. It was concluded that simultaneous addition of a replication-defective adenovirus and a replication-enabling gene sequence in a trans configuration converts some of the cotransduced cells into recombinant adenovirus-producing cells.

Identification of specific adenovirus E1A N-terminal residues critical to the binding of cellular proteins and to the control of cell growth

Adenovirus early region 1A (E1A) oncogene-encoded sequences essential for transformation- and cell growth-regulating activities are localized at the N terminus and in regions of highly conserved amino acid sequence designated conserved regions 1 and 2. These regions interact to form the binding sites for two classes of cellular proteins: those, such as the retinoblastoma gene product, whose association with the E1A products is specifically dependent on region 2, and another class which so far is known to include only a large cellular DNA-binding protein, p300, whose association with the E1A products is specifically dependent on the N-terminal region. Association between the E1A products and either class of cellular proteins can be disrupted by mutations in conserved region 1. While region 2 has been studied intensively, very little is known so far concerning the nature of the essential residues in the N-terminal region, or about the manner in which conserved region 1 participates in the binding of two distinct sets of cellular proteins. A combination of site-directed point mutagenesis and monoclonal antibody competition experiments reported here suggests that p300 binding is dependent on specific, conserved residues in the N terminus, including positively charged residues at positions 2 and 3 of the E1A proteins, and that p300 and pRB bind to distinct, nonoverlapping subregions within conserved region 1. The availability of precise point mutations disrupting p300 binding supports previous data linking p300 with cell cycle control and enhancer function.

Mechanism of induction of cellular DNA synthesis by the adenovirus E1A 12S cDNA product

The mechanism of induction of DNA synthesis in quiescent rat 3Y1 cells by the adenovirus E1A gene was investigated using the 3Y1 derivative cell lines g12-21, gn12RB1, and gn12RB2. The g12-21 cells express the E1A 12S cDNA and the latter two cells express both the E1A 12S cDNA and the human retinoblastoma susceptibility (Rb) gene at different levels in response to dexamethasone (dex). The cDNA sequences of E1A-inducible cell cycle-dependent genes, clone 3 and clone 16, were isolated by differential screening of a cDNA library constructed from dex-treated g12-21 cells. The quiescent 3Y1 cells induced c-fos and c-myc expression within 2 h after serum stimulation and expressed clone 16 and clone 3 transiently at around 8 h before the onset of DNA synthesis (10 h). In contrast, the quiescent g12-21 cells treated with dex expressed a high level of E1A at 6 to 8 h after treatment and expressed clone 16 and clone 3 at around 8 h without stimulation of c-fos and c-myc expression, suggesting that E1A bypasses the cell cycle early in G1. The half-maximal rate of DNA synthesis was reached in a much shorter time in dex-treated g12-21 cells (12 h) than in serum-treated 3Y1 cells (18 h), suggesting that E1A also bypasses the cell cycle at the G1/S boundary. The gn12RB1 and gn12RB2 cells were unable to induce DNA synthesis in response to dex presumably due to lower levels of E1A expression, although gn12RB2 but not gn12RB1 cells could express clone 16 and clone 3. These results suggest that the level of E1A required for bypass at the G1/S boundary is higher than that required early in G1.

E1A induces phosphorylation of the retinoblastoma protein independently of direct physical association between the E1A and retinoblastoma products

We have studied the initial effects of adenovirus E1A expression on the retinoblastoma (RB) gene product in normal quiescent cells. Although binding of the E1A products to pRB could, in theory, make pRB phosphorylation unnecessary for cell cycle progression, we have found that the 12S wild-type E1A product is capable of inducing phosphorylation of pRB in normal quiescent cells. The induction of pRB phosphorylation correlates with E1A-mediated induction of p34cdc2 expression and kinase activity, consistent with the possibility that p34cdc2 is a pRB kinase. Expression of simian virus 40 T antigen induces similar effects. Induction of pRB phosphorylation is independent of the pRB binding activity of the E1A products; E1A domain 2 mutants do not bind detectable levels of pRB but remain competent to induce pRB phosphorylation and to activate cdc2 protein kinase expression and activity. Although the kinetics of induction are slower, domain 2 mutants induce wild-type levels of pRB phosphorylation and host cell DNA synthesis and yet fail to induce cell proliferation. These results imply that direct physical interaction between the RB and E1A products does not play a required role in the early stages of E1A-mediated cell cycle induction and that pRB phosphorylation is not, of itself, sufficient to allow quiescent cells to divide. These results suggest that the E1A products do not need to bind pRB in order to stimulate resting cells to enter the cell cycle. Indeed, a more important role of the RB binding activity of the E1A products may be to prevent dividing cells from returning to G0.

E1A induces phosphorylation of the retinoblastoma protein independently of direct physical association between the E1A and retinoblastoma products

We have studied the initial effects of adenovirus E1A expression on the retinoblastoma (RB) gene product in normal quiescent cells. Although binding of the E1A products to pRB could, in theory, make pRB phosphorylation unnecessary for cell cycle progression, we have found that the 12S wild-type E1A product is capable of inducing phosphorylation of pRB in normal quiescent cells. The induction of pRB phosphorylation correlates with E1A-mediated induction of p34cdc2 expression and kinase activity, consistent with the possibility that p34cdc2 is a pRB kinase. Expression of simian virus 40 T antigen induces similar effects. Induction of pRB phosphorylation is independent of the pRB binding activity of the E1A products; E1A domain 2 mutants do not bind detectable levels of pRB but remain competent to induce pRB phosphorylation and to activate cdc2 protein kinase expression and activity. Although the kinetics of induction are slower, domain 2 mutants induce wild-type levels of pRB phosphorylation and host cell DNA synthesis and yet fail to induce cell proliferation. These results imply that direct physical interaction between the RB and E1A products does not play a required role in the early stages of E1A-mediated cell cycle induction and that pRB phosphorylation is not, of itself, sufficient to allow quiescent cells to divide. These results suggest that the E1A products do not need to bind pRB in order to stimulate resting cells to enter the cell cycle. Indeed, a more important role of the RB binding activity of the E1A products may be to prevent dividing cells from returning to G0.

Isolation and characterization of insertion mutants in E1A of adenovirus type 5

We have constructed a series of insertion mutations at 18 sites in the coding sequences of early region 1A (E1A) of human adenovirus type 5 (Ad5). At each site we have introduced three types of mutation: a 39-bp insertion specifying a 13-aa residue oligopeptide, a 39-bp insertion containing chain termination codons in all three reading frames, and a "collapsed" insert of 6-bp forming a conventional linker insertion mutation. All mutants were sequenced to determine the precise location, structure, and orientation of the inserts. The mutants were assayed for their abilities to trans-activate and to repress using transient expression assays in HeLa cells cotransfected with the E1A mutant plasmids and a reporter plasmid containing the bacterial beta-galactosidase (lac Z) gene under the control of Ad5 early promoters. The mutants were also tested for their ability to transform baby rat kidney cells in cooperation with either E1B or the ras oncogene. Each mutant was rescued into virus and infectivity was compared in HeLa and 293 cells. In addition, E1A protein synthesis was analyzed in cells infected with the mutant viruses and the insertions were found to have pronounced but unpredictable effects on electrophoretic mobility of E1A proteins in SDS-polyacrylamide gels. The results of functional assays indicated that only mutations mapping in, or deleting, the unique region of the 13 S mRNA product had any effect on ability to trans-activate and that a perfect correlation existed between ability of a mutant to trans-activate and to replicate efficiently in HeLa cells or to transform baby rat kidney cells in an E1A plus E1B mediated assay. In contrast, insertions near conserved region 2 of exon I and in the NH2-terminal portion of exon II significantly reduced repression activity but left transforming activity with E1B or with ras essentially unaffected suggesting that the repression function of E1A is separate from, or at least nonessential in, transformation.

Expression and interactions of human adenovirus oncoproteins

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Adenovirus infection of differentiated F9 cells results in a global shut-off of differentiation-induced gene expression

Previous experiments have demonstrated a link between transcriptional regulatory mechanisms acting during F9 cell differentiation and transcription control by the adenovirus E1A gene. We have isolated a number of differentiation-specific genes by cDNA cloning to determine if E1A exerts a coordinated control over differentiation specific gene expression. The mRNAs encoded by these cDNAs were undetectable or only barely detectable in undifferentiated cells but then rose in concentration upon differentiation. Analysis of transcription rates in isolated nuclei revealed that all but one of the genes was transcriptionally regulated during differentiation. Interestingly, alpha 2-type IV collagen expression was activated by a post-transcriptional mechanism since the gene was transcribed in both undifferentiated and differentiated cells whereas the cytoplasmic mRNA was undetectable in undifferentiated cells but rose in abundance in parallel with other regulated transcripts. Adenovirus infection of differentiated F9 cells reduced the cytoplasmic mRNA levels of each of the differentiation specific genes to near that found in the undifferentiated cell. Of those genes that were transcriptionally activated by differentiation, adenovirus infection specifically inhibited transcription. In contrast, although the alpha 2 collagen mRNA levels were reduced by adenovirus infection similar to the other mRNAs, the control was post-transcriptional since transcription of the gene was unaffected. Thus, the mechanism for loss of gene expression mediated by E1A reflects the mechanism by which the gene was activated during differentiation. Based on these results we suggest that E1A controls the expression of the F9 cell phenotype by targeting a regulatory activity acting early in the differentiation program.

Differential requirement for adenovirus type 12 E1A gene products in oncogenic transformation

During the early period of infection, adenovirus type 12 E1A gene is expressed as overlapping, spliced mRNAs of 12 and 13S, which encode in-frame proteins of 235 and 266 amino acid residues (235R and 266R), respectively. To define the functions of these related products in the infection of human cells and transformation of rodent cells, we created single T-to-C transitions at the second base of each mRNA intron which specifically prevent splicing of the respective mRNAs. Mutant pm712 expresses only the 13S mRNA and 266R protein, while pm713 expresses only the 12S mRNA and 235R protein. By using these mutants, we showed that only the larger product is required for growth in human cells, including growth-arrested W138 cells, that the capacity to activate other viral genes (in human cells, at least) lies primarily with that protein, and that the 266R product is not required for autoregulation of its own transcription. In the presence of the 266R protein the 235R product was not required for complete and efficient transformation of a variety of rodent cells or for direct induction of tumors in rats, whereas in its absence the smaller product was insufficient for transformation or tumor induction. Finally, we showed that transformants resulting from infection of rodent cells with pm712 possess a fully-transformed phenotype and are tumorigenic. Previous studies with group C adenoviruses led to the conclusion that both E1A products are required for complete transformation; we conclude that with oncogenic serotype 12, only the 266R product is required for this process.

Analysis of E1A-mediated growth regulation functions: binding of the 300-kilodalton cellular product correlates with E1A enhancer repression function and DNA synthesis-inducing activity

Adenovirus E1A transforming function requires two distinct regions of the protein. Transforming activity is closely linked with the presence of a region designated conserved domain 2 and the ability of this region to bind the product of the cellular retinoblastoma tumor suppressor gene. We have investigated the biological properties of the second transforming region of E1A, which is located near the N terminus. Transformation-defective mutants containing deletions in the N terminus (deletion of residues between amino acids 2 and 36) were deficient in the ability to induce DNA synthesis and repress insulin enhancer-stimulated activity. The function of the N-terminal region correlated closely with binding of the 300-kilodalton E1A-associated protein and not with binding of the retinoblastoma protein. These results indicate that transformation by E1A is mediated by two functionally independent regions of the protein which interact with different specific cellular proteins and suggest that the 300-kilodalton E1A-associated protein plays a major role in E1A-mediated cell growth control mechanisms.

Novel phenotype of C3H 10T1/2 fibroblasts cotransfected with the c-Ha-ras and adenovirus 5 E1A oncogenes

C3H 10T1/2 fibroblasts are converted to fully transformed phenotype following coexpression of an activated c-Ha-ras gene and either a constitutively expressed viral or cellular myc gene. In this report, we examined whether the early region 1A (E1A) of adenovirus 5, which synergizes with ras to convert primary embryonic cells to a transformed phenotype, can synergize with ras to transform the established mouse embryonic cell line, C3H 10T1/2. We demonstrate that coexpression of ras and E1A generated a transformed phenotype that could be scored by colony assays and by soft agarose assays but not by standard focus assays. The ras-E1A-transformed phenotype relies on sequences present in conserved regions 1 and 2 of the E1A proteins and, in part, on information encoded by the extreme carboxy terminus of E1A. The contrast between the transformed phenotypes generated following the transfection of C3H 10T1/2 cells with either ras and myc or ras and E1A suggests that myc and E1A cooperate with ras to transform C3H 10T1/2 cells by mechanisms that can be distinguished using this established cell line as a model system.

Genetic dissection of the transactivating domain of the E1a 289R protein of adenovirus type 2

A series of linker-scanning, deletion, and frameshift mutations were made in the pm975 variant of the adenovirus type 2 E1a gene, which expresses only the larger of the two major E1a proteins. Most of these were within the 46-amino-acid segment unique to the larger E1a protein product (the 289R protein), which confers on it the ability to activate in trans the expression of other genes. The mutations were recombined into virus and assayed by in vitro transcription in nuclei isolated from infected cells for their ability to activate the transcription of other viral early genes and of the endogenous hsp70 gene. Mutant E1a proteins from which the 289R-unique segment was removed by deletion or truncation did not completely lose the ability to transactivate by comparison with a virus which makes no E1a at all, indicating that sequences outside this domain are active in the positive regulation of transcription. The E1a mutations tested fell into several classes: those that increased transactivation of virtually all genes, those that severely depressed transactivation of all genes, and those that depressed transactivation only moderately. Each mutation had similar effects on the expression of all transcription units tested, indicating a common process in their transactivation. However, some mutants in the third category decreased transactivation of some induced genes more severely than of others. Such gene-specific defects suggest the existence of subclasses of E1a-responsive transcription units, consistent with the involvement of diverse proteins in the transactivation of different genes. Two specific structural components of the transactivating domain, a putative metal-binding element and a region with high potential for beta-sheet formation at its carboxy-terminus, appear to be important to the transactivation function.

Growth factor induction by the adenovirus type 5 E1A 12S protein is required for immortalization of primary epithelial cells

The 12S protein encoded by the adenovirus E1A region induces cellular DNA synthesis in and proliferation and immortalization of primary rat epithelial cells in the presence or absence of serum. It also induces the production of a growth factor(s) that stimulates epithelial cell proliferation. We have undertaken a mutational analysis of the 12S gene to determine the sequences required for these functions. We found that a region near the C-terminus of the 12S protein was required for growth factor induction. No activities have been defined previously for this region. Furthermore, we show that growth factor production was necessary for epithelial cells to survive past their normal life span in culture and to become immortalized. The ability to induce growth factor production required prior expression of E1A activities encoded by the N-terminus of the 12S protein, including activation of quiescent cells into the cell cycle, and an unknown activity that required expression of the first 13 amino acids of the gene. In addition, examination of the subcellular localization of mutant 12S polypeptides suggested new regions that affect the nuclear localization of E1A proteins.

Cellular targets for transformation by the adenovirus E1A proteins

Three cellular proteins, including species of 300,000 daltons and 107,000 daltons as well as p105-RB, the product of the retinoblastoma susceptibility gene, stably interact with the adenovirus E1A proteins. To help determine the functional basis of these interactions, the regions of E1A that participate in these interactions were mapped using a series of deletion mutants. The 300,000 dalton and the 107,000 dalton proteins interacted with sequences within amino acids 1 to 76 and 121 to 127, respectively. Interaction with the third cellular protein, p105-RB, required the presence of sequences from two noncontiguous regions of the E1A polypeptide chain, amino acids 30 to 60 and 121 to 127. The regions of E1A that are required for these interactions coincided precisely with the regions of E1A that are required for its transforming function. These results suggest that the interactions with these cellular proteins are fundamental to the transforming activity of E1A.

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.

Regulation of adenovirus and cellular gene expression and of cellular transformation by the E1B-encoded 175-amino-acid protein

Mutants of type 5 adenovirus that fail to express the E1B-gene-encoded 175-amino-acid (175R) protein are unable to morphologically transform primary or continuous cultures of rat embryo fibroblast cells. This phenotype could result from a direct effect of this E1B polypeptide (along with E1A polypeptides) on cellular gene expression resulting in a pathway leading to altered cell growth or from an indirect role of the 175R protein made possible by its ability to modulate viral early-gene (most likely E1A) expression. To distinguish between these two models, viruses were constructed that expressed the individual E1A 13S and 12S genes in the presence of either the E1B 175R or 495R protein. Regardless of the E1A gene product that was expressed, viruses that failed to express the E1B 175R protein were transformation defective. Additional studies suggest that the E1A 289R protein and E1B 495R protein function in a common pathway leading to the establishment of the transformed cell. We also observe that E3 gene expression by viruses that fail to express the E1A 289R protein affects the efficiency of focus formation. When tested in both nonpermissive CREF cells and permissive HeLa cells, the lack of 175R protein expression appeared to have no effect (a transient twofold decrease in E1A mRNA accumulation was observed in CREF cells) on viral early-gene expression. These results suggest that the initiation of the transformed cell phenotype occurs because of some interaction in a common pathway between the viral E1A proteins and E1B 175R protein. Furthermore, we have shown that the E1B 175R protein does not enhance the rate of transcription initiation from the mouse immunoglobulin heavy chain gene promoter when these sequences are localized on a viral genome, and it does not diminish the ability of the E1A proteins to decrease the rate of enhancer-dependent transcription.

The role of the two E1a mRNA products of subgroup B adenoviruses in the regulation of early promoters of subgroup C adenoviruses

HeLa cells were co-transfected with recombinant plasmids carrying adenovirus (Ad)2 or Ad3 E1a promoters fused to the chloramphenicol acetyl transferase gene (cat), and a plasmid encoding the Ad3 E1a promoter. Whereas no stimulating effect was observed on the Ad3 E1a promoter, the Ad2 promoter was inhibited. To determine which of the E1a gene products of Ad3 was responsible for the repressive effect, plasmids were constructed in which only the 13S or 12S mRNA product of Ad3 was expressed. Both the 12S and 13S mRNA products of Ad3 E1a were found to depress the transcription from the Ad2 E1a promoter. Each Ad3 E1a gene product was able to stimulate transcription from the Ad5 E2a early promoter in a manner similar to that of the Ad2 E1a gene products. In the case of the Ad5 E3 promoter, neither of the Ad3 E1a gene products stimulated transcription, but an inhibition was observed. These results suggest that both mRNA products of the Ad3 E1a region inhibit transcription at the TATA box transcription complex.

Growth factor induction by the adenovirus type 5 E1A 12S protein is required for immortalization of primary epithelial cells

The 12S protein encoded by the adenovirus E1A region induces cellular DNA synthesis in and proliferation and immortalization of primary rat epithelial cells in the presence or absence of serum. It also induces the production of a growth factor(s) that stimulates epithelial cell proliferation. We have undertaken a mutational analysis of the 12S gene to determine the sequences required for these functions. We found that a region near the C-terminus of the 12S protein was required for growth factor induction. No activities have been defined previously for this region. Furthermore, we show that growth factor production was necessary for epithelial cells to survive past their normal life span in culture and to become immortalized. The ability to induce growth factor production required prior expression of E1A activities encoded by the N-terminus of the 12S protein, including activation of quiescent cells into the cell cycle, and an unknown activity that required expression of the first 13 amino acids of the gene. In addition, examination of the subcellular localization of mutant 12S polypeptides suggested new regions that affect the nuclear localization of E1A proteins.

Interactions between cell growth-regulating domains in the products of the adenovirus E1A oncogene

Among the various biological activities expressed by the products of the adenovirus E1A gene are the abilities to induce cellular DNA synthesis and proliferation in quiescent primary baby rat kidney cells. The functional sites for these activities lie principally within two regions of the E1A proteins: an N-terminal region and a small second region of approximately 20 amino acids further downstream. To study the biological functions of the first domain, we constructed an in-frame deletion of amino acid positions 23 through 107 of the E1A products. This deletion did not impede the ability of the E1A products to transactivate the adenovirus early region 3 promoter in a transient-expression assay in HeLa cells. The ability to induce DNA synthesis in quiescent baby rat kidney cells was, however, lost in the absence of these sequences. Deletion of the small second region induced a form of S phase in which DNA synthesis occurred in the apparent absence of controls required for the cessation of DNA synthesis and progression through the remainder of the cell cycle. These cells did not appear to accumulate in or before G2, and many appeared to have a DNA content greater than that in G2. The functions of both domains are required for production of transformed foci in a ras cooperation assay. Focus formation occurred, however, even when the two domains were introduced on two separate plasmids. This complementation effect appeared to require expression of both of the mutant proteins and did not appear to result merely from recombination at the DNA level.

Use of recombinant retroviruses to study the regulation of integrated adenovirus early promoters

Adenovirus E1A gene products are capable of modulating the expression of a variety of integrated genes. To study the mechanisms by which this regulation occurs, recombinant retroviruses have been utilized to establish cell lines containing an integrated copy of either the adenovirus E2 or E3 promoter adjacent to the bacterial guanine phosphoribosyl transferase (GPT) gene. These cell lines have been characterized with respect to both basal and E1A-induced levels of GPT gene expression. Cell lines with low levels of GPT gene expression showed increased expression in the presence of E1A, whereas cell lines with high basal levels of GPT gene expression had decreased GPT RNA levels in the presence of E1A. Further characterization of these cell lines revealed E1A modulation of the accumulation of RNA initiating at a retrovirus promoter adjacent to the E2 or E3 promoter. The use of the GPT gene as a marker of E2 or E3 promoter activity has allowed the isolation of cell lines which have spontaneously increased their levels of GPT RNA. A preliminary characterization of four of these cell lines has indicated that GPT gene expression is increased as a result of cis activation of the E2 promoter.

Isolation and characterization of adenovirus type 12 E1 host-range mutants defective for growth in nontransformed human cells

In order to define the functions of human adenovirus type 12 (Ad12) early region 1 (E1) products in lytic infection and oncogenic transformation we have isolated and phenotypically characterized a set of host-range (hr) mutants of this serotype. These mutants grow efficiently upon HER3 cells, which contain and express type 12 E1 genes, but are restricted for growth upon A549 carcinoma and HeLa cells. Inter- and intratypic complementation analysis, marker-rescue mapping, and DNA sequence analysis have assigned some of the mutations to E1A sequence, and some to the reading frame encoding the E1B 54-kDa (482R) protein. Phenotypic analysis of the E1B mutants in particular has revealed some interesting, and in some cases surprising, findings relating to the roles of that protein in virus-cell interactions. This Ad12 gene product is required, either directly or indirectly, for efficient viral DNA replication in A549 and HeLa cells, unlike its counterpart in type 5 virus. Surprisingly, however, despite the severe defect in viral DNA replication, the synthesis of a few species of viral late proteins continues in cells infected by some of the E1B mutants. In contrast, none of these mutants brings about the inhibition of host-cell protein synthesis characteristic of wild-type virus infection, and with some E1B mutants no viral late proteins are made. Further, in a separate study reported elsewhere, we have demonstrated that the E1B 54-kDa product may also be involved, either directly or indirectly, in positive regulation of both E1A and E1B 19-kDa (163R) gene expression. The molecular and/or physiological bases for these various effects remain to be determined, but our initial results suggest that the E1B 54-kDa protein may carry out multiple regulatory functions during the viral life cycle.

Interactions between cell growth-regulating domains in the products of the adenovirus E1A oncogene

Among the various biological activities expressed by the products of the adenovirus E1A gene are the abilities to induce cellular DNA synthesis and proliferation in quiescent primary baby rat kidney cells. The functional sites for these activities lie principally within two regions of the E1A proteins: an N-terminal region and a small second region of approximately 20 amino acids further downstream. To study the biological functions of the first domain, we constructed an in-frame deletion of amino acid positions 23 through 107 of the E1A products. This deletion did not impede the ability of the E1A products to transactivate the adenovirus early region 3 promoter in a transient-expression assay in HeLa cells. The ability to induce DNA synthesis in quiescent baby rat kidney cells was, however, lost in the absence of these sequences. Deletion of the small second region induced a form of S phase in which DNA synthesis occurred in the apparent absence of controls required for the cessation of DNA synthesis and progression through the remainder of the cell cycle. These cells did not appear to accumulate in or before G2, and many appeared to have a DNA content greater than that in G2. The functions of both domains are required for production of transformed foci in a ras cooperation assay. Focus formation occurred, however, even when the two domains were introduced on two separate plasmids. This complementation effect appeared to require expression of both of the mutant proteins and did not appear to result merely from recombination at the DNA level.

Alternative mechanisms for activation of human immunodeficiency virus enhancer in T cells

The expression of human immunodeficiency virus (HIV) after T cell activation is regulated by NF-kappa B, an inducible DNA-binding protein that stimulates transcription. Proteins encoded by a variety of DNA viruses are also able to activate expression from the HIV enhancer. To determine how this activation occurs, specific genes from herpes simplex virus type 1 and adenovirus that activate HIV in T lymphoma cells have been identified. The cis-acting regulatory sequences in the HIV enhancer that mediate their effect have also been characterized. The relevant genes are those for ICP0-an immediate-early product of herpes simplex virus type 1-and the form of E1A encoded by the 13S messenger RNA of adenovirus. Activation of HIV by adenovirus E1A was found to depend on the TATA box, whereas herpesvirus ICP0 did not work through a single defined cis-acting element. These findings suggest multiple pathways that can be used to bypass normal cellular activation of HIV, and they raise the possibility that infection by herpes simplex virus or adenovirus may directly contribute to the activation of HIV in acquired immunodeficiency syndrome by mechanisms independent of antigenic stimulation in T cells.

Identification of factors that interact with the E1A-inducible adenovirus E3 promoter

We have investigated the E1A-inducible E3 promoter of adenovirus type 5 with respect to its ability to bind specific nuclear proteins. Four distinct nucleoprotein-binding sites were detected, located between positions-7 to -33, -44 to -68, -81 to -103, and -154 to -183, relative to the E3 cap site. These sites contain sequences previously shown to be functionally important for efficient E3 transcription. No major qualitative or quantitative differences were found in the binding pattern between nucleoprotein extracts prepared from uninfected or adenovirus-infected HeLa cells. Competition experiments suggest that the factors binding to the -154 to -183 and -81 to -103 sites are the previously identified nucleoproteins, NF1 and AP1, respectively. The factor binding to the -44 to -68 site, which we term ATF, also interacts with other E1A-inducible promoters and is very similar and probably identical to the factor that binds to the cAMP-responsive element of somatostatin. We have purified this factor, which is a protein of 43 kD in size.

Participation of two human cytomegalovirus immediate early gene regions in transcriptional activation of adenovirus promoters

The participation of human cytomegalovirus (HCMV) immediate early genes in the activation of the expression of adenovirus genes in trans (trans-activation) was examined. The initial strategy used was to determine the ability of HCMV genes to complement mutants of adenovirus E1a, an immediate early gene which encodes a trans-activator. The HCMV immediate early gene regions IE1 and IE2 complemented E1a-deficient mutants in three separate assays. IE1 and IE2 substituted for E1a in the synthesis of infectious adenovirus, late adenovirus RNA, and adenovirus DNA. Complementation by the IE2 gene region alone, but not by IE1 alone, was observed using the most discriminating assay, that for late adenovirus RNA synthesis. A role for both HCMV gene regions in positive transcriptional control was indicated by their ability to increase expression of chloramphenicol acetyltransferase (CAT) mediated by the adenovirus E2a promoter. The IE2 region alone activated CAT synthesis but IE1 alone had no detectable activity. Moreover, the activity of both gene regions was about 10-fold higher than that of IE2 alone. These data indicate that efficient complementation of E1a-deficient mutants and trans-activation of adenovirus early promoters involved the participation of both HCMV immediate early gene regions.

The adenovirus Ela gene induces differentiation of F9 teratocarcinoma cells

Undifferentiated F9 cells transfected with plasmids encoding adenovirus E1a gene products underwent radical morphological changes. They ceased to express the SSEA-1 stem cell marker antigen and started to express a number of the characteristics of the differentiated state that is induced in F9 cells by treatment with retinoic acid. In particular, they expressed keratin intermediate filaments and acquired the ability to synthesise simian virus 40 tumor antigens after virus infection. The transfected cells expressed the E1a proteins, and this expression was necessary to induce the phenotypic changes, since a coisogenic plasmid encoding only a truncated 70-amino-acid E1a polypeptide and the transfection procedure itself did not detectably after the morphology or marker expression of the F9 stem cells. The phenotypic change was induced by both 13S and 12S cDNA plasmids. We discuss these results in the context of known E1a functions and with reference to the other oncogenes and external factors that can cause F9 cell differentiation.

Mutational analysis of the adenovirus E1a gene: the role of transcriptional regulation in transformation

To determine whether the transcription regulatory activities of the adenoviral E1a gene play a role in its ability to transform primary cells we have constructed an extensive series of mutations within the E1a gene. The mutants have been characterized for their ability to transactivate the adenoviral early promoters, repress the transcriptional stimulation of the polyoma virus enhancer, establish primary baby rat kidney cells in culture and cooperate with the activated Ha-ras oncogene in morphologically transforming these cells. The mutant phenotypes reveal that: (i) the two transcription regulatory activities of E1a are separable since essential protein domains map within different regions of the protein; (ii) transactivation is unlikely to contribute significantly to E1a-mediated transformation since several isolated mutants lost the ability to transactivate but were nevertheless efficient at transformation; and (iii) both establishment and oncogene cooperation are linked to enhancer repression suggesting that E1a transforms cells by the repression of a cellular enhancer.

The adenovirus Ela gene induces differentiation of F9 teratocarcinoma cells

Undifferentiated F9 cells transfected with plasmids encoding adenovirus E1a gene products underwent radical morphological changes. They ceased to express the SSEA-1 stem cell marker antigen and started to express a number of the characteristics of the differentiated state that is induced in F9 cells by treatment with retinoic acid. In particular, they expressed keratin intermediate filaments and acquired the ability to synthesise simian virus 40 tumor antigens after virus infection. The transfected cells expressed the E1a proteins, and this expression was necessary to induce the phenotypic changes, since a coisogenic plasmid encoding only a truncated 70-amino-acid E1a polypeptide and the transfection procedure itself did not detectably after the morphology or marker expression of the F9 stem cells. The phenotypic change was induced by both 13S and 12S cDNA plasmids. We discuss these results in the context of known E1a functions and with reference to the other oncogenes and external factors that can cause F9 cell differentiation.

Different functional domains of the adenovirus E1A gene are involved in regulation of host cell cycle products

We have analyzed the cell cycle effects that different domains of the adenovirus E1A proteins have on quiescent primary BRK cells. Studies with deletion mutants that in combination removed all but the N-terminal 85 amino acids common to both the 12S and 13S proteins suggest that this region may be sufficient for the induction of synthesis of proliferating cell nuclear antigen and the stimulation of DNA synthesis. A second domain also common to the N-terminal exon of the 12S and 13S proteins was required for the induction of mitosis and stimulation of proliferation of primary BRK cells. A virus containing a mutation in this region was still able to stimulate DNA synthesis efficiently. A third domain, unique to the 13S protein, was required for the accelerated activation of the cellular thymidylate synthase gene in a manner similar to the 13S-dependent stimulation of adenovirus early region genes.

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.

Regulation of adenovirus gene expression in human WI38 cells by an E1B-encoded tumor antigen

Adenovirus mutants carrying alterations in the gene encoding the E1B 19-kilodalton tumor antigen (19K protein) cause enhanced cytopathic effect (cyt phenotype) and the degradation of host-cell chromosomal DNA (deg phenotype) upon infection of human HeLa or KB cells. Furthermore, E1B 19K gene mutant viruses are defective for cellular transformation. We report that these mutant viruses possess a host-range phenotype for growth in human cells. In human HeLa cells the mutant viruses grew to the same levels as the wild-type virus, but they were severely defective for growth in KB cells. In human WI38 cells, the E1B 19K gene mutant viruses had a substantial growth advantage over the wild-type virus, yielding 500-fold-higher titers. Viral DNA synthesis was reduced 10- to 20-fold in WI38 cells infected with the wild-type virus relative to that synthesized by the E1B mutant viruses. Viral early and late protein synthesis was similarly reduced in wild type- relative to mutant-infected cells. These reduced levels of early gene expression in wild-type virus-infected cells were paralleled by comparably reduced levels of early cytoplasmic mRNA. The primary cause of this host-range phenotype appeared at the level of early gene transcription, since transcription of viral early genes in the mutant-infected cells was substantially greater than levels found in cells infected with the wild-type virus. These results implicate the E1B 19K tumor antigen in the regulation of adenovirus early gene expression. Specifically, the E1B 19K protein directly or indirectly exerts a negative effect on early gene transcription accounting for efficient gene expression from the E1B mutant viruses in WI38 cells. Based on these findings it is probable that the cyt and deg phenotypes observed in mutant-infected HeLa and KB cells are the result of the pleiotropic effect of this altered gene regulation.

Identification of separate domains in the adenovirus E1A gene for immortalization activity and the activation of virus early genes

The transformation and early adenovirus gene transactivation functions of the E1A region were analyzed with deletion and point mutations. Deletion of amino acids from position 86 through 120 had little effect on the lytic or transforming functions of the E1A products, while deletion of amino acids from position 121 through 150 significantly impaired both functions. The sensitivity of the transformation function to alterations in the region from amino acid position 121 to 150 was further indicated by the impairment of transforming activity resulting from single amino acid substitutions at positions 124 and 135. Interestingly, conversion of a cysteine residue at position 124 to glycine severely impaired the transformation function without affecting the early adenovirus gene activating functions. Single amino acid substitutions in a different region of the E1A gene had the converse effect. All the mutants produced polypeptides of sufficient stability to be detected by Western immunoblot analysis. The single amino acid substitutions at positions 124 and 135, although impairing the transformation functions, did not detectably alter the formation of the higher-apparent-molecular-weight forms of the E1A products.

Mutations in the E1a gene of type 5 adenovirus result in oncogenic transformation of Fischer rat embryo cells

Transformation of a specific clone of Fischer rat embryo (CREF) cells with wild-type 5 adenovirus (Ad5) or the E1a plus E1b transforming gene regions of Ad5 results in epithelioid transformants that grow efficiently in agar but that do not induce tumors when inoculated into nude mice or syngeneic Fischer rats. In contrast, CREF cells transformed by a host-range Ad5 mutant, H5hrl, which contains a single base-pair deletion of nucleotide 1055 in E1a resulting in a 28-kd protein (calculated) in place of the wild-type 51-kd acidic protein, display a cold-sensitive transformation phenotype and an incomplete fibroblastic morphology but surprisingly do induce tumors in nude mice and syngeneic rats. Tumors develop in both types of animals following injection of CREF cells transformed by other cold-sensitive Ad5 E1a mutants (H5dl101 and H5in106), which contain alterations in their 13S mRNA and consequently truncated 289AA proteins. CREF cells transformed with only the E1a gene (0-4.5 m.u.) from H5hrl or H5dl101 also produce tumors in these animals. To directly determine the role of the 13S E1a encoded 289AA protein and the 12S E1a encoded 243AA protein in initiating an oncogenic phenotype in adenovirus-transformed CREF cells, we generated transformed cell lines following infection with the Ad2 mutant pm975, which synthesizes the 289AA E1a protein but not the 243AA protein, and the Ad5 mutant H5dl520 and the Ad2 mutant H2dl1500, which do not produce the 289AA E1a protein but synthesize the normal 243AA E1a protein. All three types of mutant adenovirus-transformed CREF cells induced tumors in nude mice and syngeneic rats. Tumor formation by these mutant adenovirus-transformed CREF cells was not associated with changes in the arrangement of integrated adenovirus DNA or in the expression of adenovirus early genes. These results indicate, therefore, that oncogenic transformation of CREF cells can occur in the presence of a wild-type 13S E1a protein or a wild-type 12S E1a protein when either protein is present alone, but does not occur when both wild-type E1a proteins are present.

Different functional domains of the adenovirus E1A gene are involved in regulation of host cell cycle products

We have analyzed the cell cycle effects that different domains of the adenovirus E1A proteins have on quiescent primary BRK cells. Studies with deletion mutants that in combination removed all but the N-terminal 85 amino acids common to both the 12S and 13S proteins suggest that this region may be sufficient for the induction of synthesis of proliferating cell nuclear antigen and the stimulation of DNA synthesis. A second domain also common to the N-terminal exon of the 12S and 13S proteins was required for the induction of mitosis and stimulation of proliferation of primary BRK cells. A virus containing a mutation in this region was still able to stimulate DNA synthesis efficiently. A third domain, unique to the 13S protein, was required for the accelerated activation of the cellular thymidylate synthase gene in a manner similar to the 13S-dependent stimulation of adenovirus early region genes.

Expression of adenovirus E1B mutant phenotypes is dependent on the host cell and on synthesis of E1A proteins

Adenovirus mutants containing genetic alterations in the gene encoding the E1B 19,000-molecular-weight (19K) tumor antigen induce the degradation of host cell chromosomal DNA (deg phenotype) and enhanced cytopathic effect (cyt phenotype) after infection of HeLa and KB cells. The deg and cyt phenotypes are a consequence of viral early gene expression in the absence of the E1B 19K protein. The role of the E1A proteins in induction of the cyt and deg phenotypes was investigated by constructing E1A-E1B double mutant viruses. Viruses were constructed to express the individual E1A 13S, 12S, or 9S cDNA genes in the presence of a mutation in the gene encoding the E1B 19K tumor antigen. Expression of either the 13S or 12S E1A proteins in the absence of functional E1B 19K protein produced the deg and cyt phenotypes. In contrast, a virus which expressed exclusively the 9S E1A gene product in the absence of the E1B 19K gene product did not induce the deg and cyt phenotypes, even at high multiplicities of infection. Therefore, both the 13S and 12S E1A gene products could directly or indirectly cause the deg and cyt phenotypes during infection of HeLa cells with an E1B 19K gene mutant virus. Furthermore, the deg phenotype was found to be host cell type specific, occurring in HeLa and KB cells but not in growth-arrested human WI38 cells. These results indicate that expression of the E1A trans-activating and transforming proteins is necessary for the induction of the cyt and deg phenotypes and that host cell factors also play a role.

A novel general approach to eucaryotic mutagenesis functionally identifies conserved regions within the adenovirus 13S E1A polypeptide

A new approach to the isolation of mutations in mammalian genes was developed which permits both the selection of infrequently occurring mutants that alter the cellular morphology of recipient cells and the rapid reisolation of the mutant gene. The adenovirus type 5 13S early region 1a (E1a) gene was mutagenized in vitro with sodium bisulfite and then efficiently transferred into cells with a retrovirus shuttle vector. Three classes of mutants of the 13S E1a gene product were isolated, each of which induced a distinct morphological alteration. The mutant E1a gene was reisolated from each cell line, and the precise nucleotide changes were determined. The E1a-induced morphological alterations were further examined by the construction of single and double point mutations within different regions of the polypeptides by utilizing the amino acid substitutions obtained from the original mutants. The results suggest that each of the three regions of highly conserved amino acids within the E1a 13S polypeptide has a distinct role in the alteration of cellular morphology and the activation of gene expression.

Downstream sequences affect transcription initiation from the adenovirus major late promoter

We analyzed a set of adenovirus-simian virus 40 (SV40) hybrids in which the SV40 T antigen coding sequences are inserted downstream from the adenovirus major late promoter within the first, second, and third segments of the tripartite leader. In infected cells, these viruses give rise to a matched set of hybrid SV40 mRNAs that differ only in the number of tripartite leader segments attached to the complete SV40 T antigen coding region. We found that the number of tripartite leader segments present at the 5' end of the hybrid SV40 mRNAs had little effect on the efficiency of T antigen translation. Surprisingly, insertion of SV40 sequences within the first leader segment, at +33 relative to the start of transcription, significantly reduced the frequency of transcription initiation from the major late promoter. The 3' boundary of this downstream transcriptional control element was mapped between +33 and +190 by showing that insertion of SV40 sequences within the intron after the first leader segment at +190 had very little effect on transcription initiation from the late promoter. A transient expression assay was used to show that the effect of downstream sequences on transcription initiation from the major late promoter is dependent on a trans-acting factor encoded or induced by adenovirus.

The E1A 13S product of adenovirus 5 activates transcription of the cellular human HSP70 gene

Expression of the human gene encoding the major heat shock protein, HSP70, was induced during cell growth by serum stimulation and after infection with adenovirus 5. In this study we showed that HSP70 gene expression could be induced by adenovirus 5 infection, even in the absence of exogenous serum factors. Whereas serum stimulation induced the expression of the endogenous HSP70 gene, it had no effect on early adenovirus promoters. However, expression of both the cellular HSP70 gene and the adenovirus E3 promoter were activated during adenovirus infection. By using a collection of reconstructed mutant viruses, we identified the 13S product of the E1A region as the specific transcriptional trans-activator of the HSP70 gene.

Expression of the human adenovirus E1a product in yeast

We synthesized the 13S mRNA-encoded protein of the early region 1a (E1a) of human adenovirus in Saccharomyces cerevisiae under the control of the yeast GAL7 gene promoter. Similar to the case in HeLa cells, the E1a protein in yeast was phosphorylated and formed multiple bands on sodium dodecylsulfate-polyacrylamide gel electrophoresis. These bands migrated more slowly than expected from the Mr calculated on the basis of the nucleotide sequence of the gene. Synthesis of the E1a protein caused induction of a specific family of heat-shock proteins (Hsp70), which, however, did not confer heat resistance to the yeast. In addition, the E1a production resulted in an elongation of the generation time of yeast from 2.4 h to 3.9 h, which was attributed specifically to elongation of the G1 interval in the cell cycle. In the light of these findings, we suggest that the E1a protein synthesized in yeast exerts a specific function.

Adenovirus E1A coding sequences that enable ras and pmt oncogenes to transform cultured primary cells

Plasmids expressing partial adenovirus early region 1A (E1A) coding sequences were tested for activities which facilitate in vitro establishment (immortalization) of primary baby rat kidney cells and which enable the T24 Harvey ras-related oncogene and the polyomavirus middle T antigen (pmt) gene to transform primary baby rat kidney cells. E1A cDNAs expressing the 289- and 243-amino acid proteins expressed both E1A transforming functions. Mutant hrA, which encodes a 140-amino acid protein derived from the amino-terminal domain shared by the 289- and 243-amino acid proteins, enabled ras (but not pmt) to transform and facilitated in vitro establishment to a low, but detectable, extent. These studies suggest that E1A functions which collaborate with ras oncogenes and those which facilitate establishment are linked. Furthermore, E1A transforming functions are not associated with activities of the 289-amino acid E1A proteins required for efficient transcriptional activation of viral early region promoters.

Regulation of adenovirus gene expression in human WI38 cells by an E1B-encoded tumor antigen

Adenovirus mutants carrying alterations in the gene encoding the E1B 19-kilodalton tumor antigen (19K protein) cause enhanced cytopathic effect (cyt phenotype) and the degradation of host-cell chromosomal DNA (deg phenotype) upon infection of human HeLa or KB cells. Furthermore, E1B 19K gene mutant viruses are defective for cellular transformation. We report that these mutant viruses possess a host-range phenotype for growth in human cells. In human HeLa cells the mutant viruses grew to the same levels as the wild-type virus, but they were severely defective for growth in KB cells. In human WI38 cells, the E1B 19K gene mutant viruses had a substantial growth advantage over the wild-type virus, yielding 500-fold-higher titers. Viral DNA synthesis was reduced 10- to 20-fold in WI38 cells infected with the wild-type virus relative to that synthesized by the E1B mutant viruses. Viral early and late protein synthesis was similarly reduced in wild type- relative to mutant-infected cells. These reduced levels of early gene expression in wild-type virus-infected cells were paralleled by comparably reduced levels of early cytoplasmic mRNA. The primary cause of this host-range phenotype appeared at the level of early gene transcription, since transcription of viral early genes in the mutant-infected cells was substantially greater than levels found in cells infected with the wild-type virus. These results implicate the E1B 19K tumor antigen in the regulation of adenovirus early gene expression. Specifically, the E1B 19K protein directly or indirectly exerts a negative effect on early gene transcription accounting for efficient gene expression from the E1B mutant viruses in WI38 cells. Based on these findings it is probable that the cyt and deg phenotypes observed in mutant-infected HeLa and KB cells are the result of the pleiotropic effect of this altered gene regulation.