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

Cell transformation by the adenovirus oncogenes E1 and E4

Extensive studies on viral-mediated oncogenic transformation by human adenoviruses have revealed much of our current understanding on the molecular mechanisms that are involved in the process. To date, these studies have shown that cell transformation is a multistep process regulated by the cooperation of several adenoviral gene products encoded in the early regions 1 (E1) and 4 (E4). Early region 1A immortalizes primary rodent cells, whereas co-expression of early region protein 1B induces full manifestation of the transformed phenotype. Beside E1 proteins, also some E4 proteins have partial transforming activities through regulating many cellular pathways. Here, we summarize recent data of how adenoviral oncoproteins may contribute to viral transformation and discuss the challenge of pinpointing the underlying mechanisms.

Impact of E1 and Cre on adenovirus vector amplification: developing MDCK CAV-2-E1 and E1-Cre transcomplementing cell lines

Adenovirus vectors have been extensively studied through the manipulation of viral genome. However, little attention is being paid to their producer cell-lines; cells are selected according to virus yields, neglecting the expression profile of transcomplementing gene products underlying cell performance. This work evaluates the impact of E1 (E1A and E1B) and Cre recombinase levels in the production of E1-deleted and helper-dependent canine adenovirus type 2 (CAV-2) vectors using MDCK cells. E1A and E1B gene expression and Cre activity were evaluated in different cell clones and compared with the corresponding cell productivity and susceptibility to oxidative stress injury. CAV-2 production was proportional to E1A expression (the highest levels of E1A corresponding to productivities of 3000–5000 I.P./cell), while E1B prolonged host cell viability after infection, conferring protection against apoptosis. Cre recombinase counteracted E1B anti-apoptotic properties, however viral production was maintained under high levels of Cre. Yet, Cre recombinase side effects can be reduced using cell lines with lower Cre-activities, without compromising the excision efficiency of helper vector packaging signal. These results highlight the influence of transcomplementing gene products on CAV-2 producer cell line performance, and the ability to express high levels of E1A and E1B as an important feature for cell line establishment and high adenovirus titers.

The adenoviral E1B 55-kilodalton protein controls expression of immune response genes but not p53-dependent transcription

The human adenovirus type 5 (Ad5) E1B 55-kDa protein modulates several cellular processes, including activation of the tumor suppressor p53. Binding of the E1B protein to the activation domain of p53 inhibits p53-dependent transcription. This activity has been correlated with the transforming activity of the E1B protein, but its contribution to viral replication is not well understood. To address this issue, we used microarray hybridization methods to examine cellular gene expression in normal human fibroblasts (HFFs) infected by Ad5, the E1B 55-kDa-protein-null mutant Hr6, or a mutant carrying substitutions that impair repression of p53-dependent transcription. Comparison of the changes in cellular gene expression observed in these and our previous experiments (D. L. Miller et al., Genome Biol. 8:R58, 2007) by significance analysis of microarrays indicated excellent reproducibility. Furthermore, we again observed that Ad5 infection led to efficient reversal of the p53-dependent transcriptional program. As this same response was also induced in cells infected by the two mutants, we conclude that the E1B 55-kDa protein is not necessary to block activation of p53 in Ad5-infected cells. However, groups of cellular genes that were altered in expression specifically in the absence of the E1B protein were identified by consensus k-means clustering of the hybridization data. Statistical analysis of the enrichment of genes associated with specific functions in these clusters established that the E1B 55-kDa protein is necessary for repression of genes encoding proteins that mediate antiviral and immune defenses.

Modification of the Rb-binding domain of replication-competent adenoviral vector enhances cytotoxicity against human esophageal cancers via NF-kappaB activity

A replication-competent adenoviral vector deficient for expression of the early E1B55K protein has been applied in clinical studies. The vector, however, was not fully effective for the treatment of human cancer. In this study, the E1A gene (which encodes an Rb-binding domain protein) of the adenoviral vector AxE1AdB was further engineered with a point mutation designed to abolish binding to Rb protein (pRb) and arrest the cell cycle (AxdAdB-3). The difference in the cytotoxicity of these vectors in two cancer cell lines was observed in association with differences in replication, infection efficiency, and expression levels of adenovirus receptors. Relative to the parent vector (AxE1AdB), which worked in a manner similar to ONYX-015, AxdAdB-3 with the mutated pRb-binding motif demonstrated increased cytotoxicity against p53-mutant human esophageal cancer cell lines EC-GI-10 and T.Tn. AxdAdB-3 showed a greater oncolytic effect than AxE1AdB in vivo despite almost the same replication efficiency in vitro. Unexpectedly, cell cycle arrest in AxdAdB-3-infected cells was less efficient than that in cell lines infected with AxE1AdB. However, AxdAdB-3 strongly reduced NF-kappaB activity and thereby enhanced apoptosis more than AxE1AdB did. These data demonstrate that the Rb-binding domain of E1A can regulate NF-kappaB activity and that modifications to this domain may lead to advances in gene therapies for the treatment of human cancers.

Bcl-2 antisense oligonucleotide overcomes resistance to E1A gene therapy in a low HER2-expressing ovarian cancer xenograft model

We are currently conducting clinical trials of E1A gene therapy for patients with ovarian cancer. The adenovirus type 5 E1A gene suppresses growth of ovarian cancer cells that overexpress HER-2/neu (HER2) and growth of some--but not all--that express low HER2. In HER2-overexpressing cells, suppression by E1A is predominantly by down-regulation of HER2, but the mechanism in low HER2-expressing cells is not fully understood. The adenoviral E1B protein has sequential and functional homology to Bcl-2 and prolongs the viability of adenovirus host cells by inhibiting E1A-induced apoptosis. Bcl-2 is overexpressed in ovarian cancer and participates in chemoresistance; we hypothesized that Bcl-2 inhibits E1A-induced apoptosis leading to resistance to E1A gene therapy. E1A suppressed colony formation of ovarian cancer cells that express low levels of Bcl-2 and HER2 (OVCAR-3 and OVCA 433), but enhanced colony formation in low HER2-, high Bcl-2-expressing ovarian cancer cells (2774 and HEY). Treating 2774 or HEY cells with antisense oligonucleotide Bcl-2 (Bcl-2-ASO) did not reduce cell viability. E1A combined with Bcl-2-ASO led to significant decreases in cell viability resulting from increased apoptosis relative to cells treated with E1A alone (P < 0.05). The increase in apoptosis was partly due to cytochrome c release and subsequently caspase-9 activation by Bcl-2-ASO. Finally, in an ovarian cancer xenograft model, treatment with Bcl-2-ASO did not prolong survival, but E1A plus Bcl-2-ASO did (P < 0.001). In conclusion, ovarian tumors overexpressing Bcl-2 may not respond well to E1A gene therapy, but treatment with a combination of E1A and Bcl-2-ASO may overcome this resistance.

Cell transformation by human adenoviruses

The last 40 years of molecular biological investigations into human adenoviruses have contributed enormously to our understanding of the basic principles of normal and malignant cell growth. Much of this knowledge stems from analyses of their productive infection cycle in permissive host cells. Also, initial observations concerning the carcinogenic potential of human adenoviruses subsequently revealed decisive insights into the molecular mechanisms of the origins of cancer, and established adenoviruses as a model system for explaining virus-mediated transformation processes. Today it is well established that cell transformation by human adenoviruses is a multistep process involving several gene products encoded in early transcription units 1A (E1A) and 1B (E1B). Moreover, a large body of evidence now indicates that alternative or additional mechanisms are engaged in adenovirus-mediated oncogenic transformation involving gene products encoded in early region 4 (E4) as well as epigenetic changes resulting from viral DNA integration. In particular, detailed studies on the tumorigenic potential of subgroup D adenovirus type 9 (Ad9) E4 have now revealed a new pathway that points to a novel, general mechanism of virus-mediated oncogenesis. In this chapter, we summarize the current state of knowledge about the oncogenes and oncogene products of human adenoviruses, focusing particularly on recent findings concerning the transforming and oncogenic properties of viral proteins encoded in the E1B and E4 transcription units.

The replicative capacities of large E1B-null group A and group C adenoviruses are independent of host cell p53 status

Recent reports suggest that an early region 1B (E1B) 55, 000-molecular-weight polypeptide (55K)-null adenovirus type 5 (Ad5) mutant (dl1520) can replicate to the same extent as wild-type (wt) Ad5 in cells either deficient or mutated in p53, implicating p53 in limiting viral replication in vivo. In contrast, we show here that the replicative capacity of Ad5 dl1520 is wholly independent of host cell p53 status, as is the replicative capacity of comparable Ad12 E1B 54K-null adenoviruses (Ad12 dl620 and Ad12 hr703). Furthermore, we show that there is no requirement for complex formation between p53 and Ad5 E1B 55K or Ad12 E1B 54K for a productive infection, such that wt Ad5 and wt Ad12 will both replicate in cells which are null for p53. In addition, we find that these Ad5 and Ad12 mutant viruses induce S phase irrespective of the p53 status of the cell and that, therefore, S-phase induction does not correlate with the replicative capacity of the virus. Interestingly, the replicative capacities of the large E1B-null adenoviruses correlated positively with the ability to express E1B 19K and were related to the ability to repress premature adenovirus-induced apoptosis. Infection of primary human cells indicated that Ad5 dl1520, wt Ad5, and wt Ad12 replicated better in cycling normal human skin fibroblasts (HSFs) than in quiescent HSFs. Thus, the cell cycle status of the host cell, upon infection, also influences viral yield.

The adenovirus E4orf6 protein contributes to malignant transformation by antagonizing E1A-induced accumulation of the tumor suppressor protein p53

The adenovirus type 5 (Ad5) E4orf6 protein promotes focus formation of primary baby rat kidney (BRK) cells in cooperation with Ad5 E1 proteins. This activity is most likely related to the ability of the E4orf6 protein to bind to p53 and modulate its tumor suppressor functions. In this study we report that transformed BRK cells that stably express E4orf6 in addition to E1A and E1B (ABS cells) displayed multiple additional properties commonly associated with a high grade of oncogenic transformation compared to cells expressing only E1A and E1B (AB cells). These properties included morphological alterations, markedly enhanced growth rates and growth to much higher saturation densities. Following injection into nude mice ABS-derived tumors exhibited accelerated growth and, based on histopathological criteria, proofed to be much more malignant compared to tumors generated by AB cells. Interestingly, these highly transformed properties of ABS cells correlated with a dramatic reduction of p53 steady-state levels which inversely correlated with E4orf6 expression. From these results we conclude that expression of the Ad5 E4orf6 protein (i) confers additional transformed in vitro properties to primary rat cells expressing the Ad5 E1 proteins, and (ii) increases the tumorigenic and malignant potential of these cells in vivo. Our data suggest that the Ad5 E4orf6 protein enhances the intrinsic ability of E1-transformed rat cells to grow in a neoplastic state by completely inactivating p53 tumor suppressor function in combination with the E1A and E1B proteins.

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

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

Replication of ONYX-015, a potential anticancer adenovirus, is independent of p53 status in tumor cells

The 55-kDa E1B protein of adenovirus, which binds to and inactivates the tumor suppressor protein p53, is not expressed in the adenoviral mutant termed ONYX-015 (i.e., dl1520). It was reported that the mutant virus due to a deletion in E1B is able to replicate only in cells deficient for wild-type p53. Accordingly, dl1520 is currently being evaluated as a potential tool in the therapy of p53 deficient cancers. In contrast, we report here that dl1520 replicates independently of the p53 status in various tumor cell lines (U87, RKO, A549, H1299, and U373). In addition, the inhibition of p53-mediated transcriptional activation in wild-type p53 containing U2OS cells, by overexpression of a transdominant negative p53 mutant, did not render the cells permissive for dl1520 replication. Finally, we show that, depending on the multiplicity of infection, the deleted virus is able to replicate in and to kill primary human cells. Thus, the molecular basis for the growth differences of dl1520 within different cell types remains to be determined.

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.

Regulation of p53-dependent apoptosis, transcriptional repression, and cell transformation by phosphorylation of the 55-kilodalton E1B protein of human adenovirus type 5

The adenovirus type 5 55-kDa E1B protein (E1B-55kDa) cooperates with E1A gene products to induce cell transformation. E1A proteins stimulate DNA synthesis and cell proliferation; however, they also cause rapid cell death by p53-dependent and p53-independent apoptosis. It is believed that the role of the E1B-55kDa protein in transformation is to protect against p53-dependent apoptosis by binding to and inactivating p53. It has been shown previously that the 55-kDa polypeptide abrogates p53-mediated transactivation and that mutants defective in p53 binding are unable to cooperate with E1A in transformation. We have previously mapped phosphorylation sites near the carboxy terminus of the E1B-55kDa protein at Ser-490 and Ser-491, which lie within casein kinase II consensus sequences. Conversion of these sites to alanine residues greatly reduced transforming activity, and although the mutant 55-kDa protein was found to interact with p53 at normal levels, it was somewhat defective for suppression of p53 transactivation activity. We now report that a nearby residue, Thr-495, also appears to be phosphorylated. We demonstrate directly that the wild-type 55-kDa protein is able to block E1A-induced p53-dependent apoptosis, whereas cells infected by mutant pm490/1/5A, which contains alanine residues at all three phosphorylation sites, exhibited extensive DNA fragmentation and classic apoptotic cell death. The E1B-55kDa product has been shown to exhibit intrinsic transcriptional repression activity when localized to promoters, such as by fusion with the GAL4 DNA-binding domain, even in the absence of p53. Such repression activity was totally absent with mutant pm490/1/5A. These data suggested that inhibition of p53-dependent apoptosis may depend on the transcriptional repression function of the 55-kDa protein, which appears to be regulated be phosphorylation at the carboxy terminus.

Adenovirus type 5 early region 4 is responsible for E1A-induced p53-independent apoptosis

In the absence of E1B, the 289- and 243-residue E1A products of human adenovirus type 5 induce p53-dependent apoptosis. However, our group has shown recently that the 289-residue E1A protein is also able to induce apoptosis by a p53-independent mechanism (J. G. Teodoro, G. C. Shore, and P. E. Branton, Oncogene 11:467-474, 1995). Preliminary results suggested that p53-independent cell death required expression of one or more additional adenovirus early gene products. Here we show that both the E1B 19-kDa protein and cellular Bcl-2 inhibit or significantly delay p53-independent apoptosis. Neither early region E2 or E3 appeared to be necessary for such cell death. Analysis of a series of E1A mutants indicated that mutations in the transactivation domain and other regions of E1A correlated with E1A-mediated transactivation of E4 gene expression. Furthermore, p53-deficient human SAOS-2 cells infected with a mutant which expresses E1B but none of the E4 gene products remained viable for considerably longer times than those infected with wild-type adenovirus type 5. In addition, an adenovirus vector lacking both E1 and E4 was unable to induce DNA degradation and cell killing in E1A-expressing cell lines. These data showed that an E4 product is essential for E1A-induced p53-independent apoptosis.

Adenovirus E1A243 disrupts the ATF/CREB-YY1 complex at the mouse c-fos promoter

The adenovirus E1A243 protein can activate transcription of the mouse c-fos gene in a manner that depends on treatment of cells with inducers or analogs of cyclic AMP (cAMP). Activation requires conserved region 1 and the N-terminal domain of E1A243 and is mediated by a 22-bp E1A response element containing a cAMP response element (CRE) at -67 and a binding site for transcription factor YY1 at -54. In the absence of E1A243, YY1 represses CRE-dependent transcription of c-fos by physically interacting with ATF/CREB proteins bound to the -67 CRE. Here we present evidence that expression of E1A243 leads to relief of YY1-mediated repression by a disruption of the ATF/CREB-YY1 complex. Addition of E1A243 to in vitro binding assays prevented binding of ATF-2 to glutathione S-transferase-YY1. Similarly, expression of E1A243 in HeLa cells prevented the association of a YY1-VP16 fusion protein with endogenous ATF/CREB proteins bound to the -67 CRE of a transfected c-fosCAT reporter plasmid. In each case, the N-terminal domain of E1A243, which mediates a direct interaction with YY1, was responsible for disruption of the ATF/CREB-YY1 complex. On the basis of these and previously published results, we present a model for the synergistic transcriptional activation of the c-fos gene by E1A243 and cAMP.

Viral proteins E1B19K and p35 protect sympathetic neurons from cell death induced by NGF deprivation

To study molecular mechanisms underlying neuronal cell death, we have used sympathetic neurons from superior cervical ganglia which undergo programmed cell death when deprived of nerve growth factor. These neurons have been microinjected with expression vectors containing cDNAs encoding selected proteins to test their regulatory influence over cell death. Using this procedure, we have shown previously that sympathetic neurons can be protected from NGF deprivation by the protooncogene Bcl-2. We now report that the E1B19K protein from adenovirus and the p35 protein from baculovirus also rescue neurons. Other adenoviral proteins, E1A and E1B55K, have no effect on neuronal survival. E1B55K, known to block apoptosis mediated by p53 in proliferative cells, failed to rescue sympathetic neurons suggesting that p53 is not involved in neuronal death induced by NGF deprivation. E1B19K and p35 were also coinjected with Bcl-Xs which blocks Bcl-2 function in lymphoid cells. Although Bcl-Xs blocked the ability of Bcl-2 to rescue neurons, it had no effect on survival that was dependent upon expression of E1B19K or p35.

Abrogation of oncogene-associated apoptosis allows transformation of p53-deficient cells

p53-deficient mouse embryonic fibroblasts were used to establish a direct mechanism of tumor suppression by p53 involving the destruction of oncogene-expressing cells by apoptosis. The absence of p53 enhanced cell growth, appeared sufficient for immortalization, and allowed a single oncogene [adenovirus early region 1A (E1A)] to transform cells to a tumorigenic state. p53 suppressed transformation of E1A-expressing cells by apoptosis. Apoptosis was associated with p53 stabilization and was triggered by environmental signals that normally suppress cell growth. Absence of even a single p53 allele significantly enhanced cell growth and survival. Although abrogation of apoptosis allowed transformation by E1A alone, escape from apoptosis susceptibility was not a prerequisite for tumor growth. Consequently, p53 mutation could enhance the survival of malignant cells expressing oncogenes activated early in tumor progression.

Phosphorylation at the carboxy terminus of the 55-kilodalton adenovirus type 5 E1B protein regulates transforming activity

The 55-kDa product of early region 1B (E1B) of human adenoviruses is required for viral replication and participates in cell transformation through complex formation with and inactivation of the cellular tumor suppressor p53. We have used both biochemical and genetic approaches to show that this 496-residue (496R) protein of adenovirus type 5 is phosphorylated at serine and threonine residues near the carboxy terminus within sequences characteristic of substrates of casein kinase II. Mutations which converted serines 490 and 491 to alanine residues decreased viral replication and greatly reduced the efficiency of transformation of primary baby rat kidney cells. Such mutant 496R proteins interacted with p53 at efficiencies similar to those of wild-type 496R but only partially inhibited p53 transactivation activity. These results indicated that phosphorylation at these carboxy-terminal sites either regulates the inhibition of p53 or regulates some other 496R function required for cell transformation.

Absence of an essential regulatory influence of the adenovirus E1B 19-kilodalton protein on viral growth and early gene expression in human diploid WI38, HeLa, and A549 cells

Mutations in the gene encoding the adenovirus (Ad) early region 1B 19-kDa protein (the 19K gene) result in multiple phenotypic effects upon infection of permissive human cells. It has been reported, for example, that Ad type 2 (Ad2) and Ad5 with mutations in the 19K gene (19K-defective mutants) have a marked growth advantage compared with wild-type virus in human diploid WI38 cells (E. White, B. Faha, and B. Stillman, Mol. Cell. Biol. 6:3763-3773, 1986), and it was proposed that this host range phenotype stems from the large increase in viral early gene expression reported to occur in the mutant-infected cells. These observations gave rise to the hypothesis that the 19-kDa protein (the 19K protein) normally functions as a negative regulator of Ad early gene expression and growth. We have tested this hypothesis and find that Ad5 and Ad12 wild-type viruses grow as efficiently as their respective 19K-defective mutants, in1 and dl337 and pm700 and in700, in WI38 and other human cell types. Neither the accumulation of E1A cytoplasmic mRNAs nor the synthesis of E1A and other viral early proteins in these cells is altered as a result of these mutations in the 19K gene, and we conclude that the 19K protein does not play an essential role in regulating viral early gene expression or viral growth in human cells.

Stabilization of the p53 tumor suppressor is induced by adenovirus 5 E1A and accompanies apoptosis

Oncogenic transformation by human adenoviruses requires early regions 1A and 1B (E1A and E1B) and provides a model of multistep carcinogenesis. This study shows that the metabolic stabilization of p53 observed in adenovirus 5 (Ad5)-transformed cells can occur in untransformed cells expressing E1A alone. Stabilized p53 was localized to the nucleus and was indistinguishable from wild-type p53 with respect to its interactions with hsc70, PAb420, Ad5 p55E1B, and SV40 large T antigen. Moreover, binding of Ad5 p55E1B or SV40 large T antigen had no additional effect on p53 levels or turnover. Higher levels of p53 were also induced in a variety of cell types within 40 hr after transferring E1A genes. E1A also caused cells to lose viability by a process resembling apoptosis. The apoptosis appeared to involve p53, because p53 levels reverted to normal in surviving cells that had lost E1A, and E1B protected cells from the toxic effects of E1A. These results suggest that (1) the involvement of p53 in tumor suppression and/or apoptosis can be regulated at the level of protein turnover, and (2) a major oncogenic role for E1B is to counter cellular responses to E1A (i.e., stabilization of p53 and associated apoptosis) that preclude transformation by E1A alone. This represents the first physiological setting in which high levels of endogenous p53 are induced in response to an oncogenic challenge, with the apparent consequence of suppressing transformation.

The E1B 19-kilodalton protein is not essential for transformation of rodent cells in vitro by adenovirus type 5

The newly constructed adenovirus type 5 mutant in1 carries a single AT base pair insertion immediately after nucleotide position 1715 in the E1B gene sequence which destroys the proximal AUG normally present in E1B messages and prevents production of intact E1B 19-kDa protein in infected cells. We have used in1, variants of in1 containing mutant alleles of viral genes known to enhance transformation frequency, and adenovirus type 5 mutant dl337 (S. Pilder, J. Logan, and T. Shenk, J. Virol. 52:664-671, 1984), in which the sequence between nucleotides 1770 and 1916 within the 19-kDa reading frame is deleted, to test the generally accepted hypothesis that this E1B protein is essential for the transformation of rodent cells and maintenance of the transformed phenotype. We find that these mutants transform rat embryo cells, rat kidney and mouse kidney primary cells, and cells of the 3Y1 rat line with decreased frequencies only when virus is added to these various cells at high input multiplicities of infection. In contrast, when lower doses of virus are used, the mutants transform with wild-type frequencies. Cells infected with higher doses of mutant virus show increased levels of DNA degradation and cell killing compared with those of cells infected with the same levels of wild-type virus, and these effects most likely contribute to the decreased transformation frequencies observed. On the basis of these results and the results of phenotypic analyses of numerous transformants, we propose that the E1B 19-kDa protein is not required for induction and/or maintenance of transformed-cell characteristics in rodent cells infected with adenovirus type 5.

Mutational analysis of the transforming and apoptosis suppression activities of the adenovirus E1B 175R protein

The role of the adenovirus-2 E1B 19-kDa (175R) T antigen in E1a-cooperative transformation was determined by cotransfection of plasmids expressing E1A or E1B 175R T antigens into primary rat kidney (BRK) cells. Transformed cells were selected by virtue of their resistance to the antibiotic Geneticin (G418) conferred by neo gene co-expression from plasmids coding for 175R. 175R cooperated efficiently with genomic E1a and specifically with the 289R protein coded by the 13S mRNA in the transformation of primary BRK cells. Mutational analysis of the 175R protein revealed that the N terminus and the C-terminal 30 amino acids are not essential for E1a-cooperative transformation. Several conserved sequences located in the middle of the 175R protein are essential for transformation. The effect of various mutants to suppress apoptosis (programmed cell death) induced by an anti-cancer agent, cisplatin, was examined in cells producing the E1A and E1B 175R proteins. Apoptosis was measured by flow cytometric analysis and indicates that the 175R protein efficiently prevents cisplatin-induced apoptosis. This suggests that the 175R function involved in transformation segregates with its ability to suppress cisplatin-induced apoptosis.

Adenovirus type 12 early region 1B proteins and metabolism of early viral mRNAs

Early region 1B (E1B) of human adenoviruses encodes two major proteins. The 19-kDa polypeptide appears to prevent E1A-induced cytolysis and DNA degradation. The larger E1B product of approximately 55 kDa, which is essential for viral replication, plays a role in the accumulation and stability of viral mRNAs and the late shutoff of host metabolism. For serotype 12 (Ad12), this 482-residue (482R) protein is essential for viral DNA replication. In the present report we have used a series of mutants to examine the roles of Ad12 482R and the 19-kDa, 163R protein in the metabolism of early viral mRNAs. No specific effects on the accumulation of early (or late) mRNAs were detected with any of the mutants affecting 163R. With mutant dl42, which encodes an altered 482R product that lacks residues 114-155, both viral DNA replication and late viral protein synthesis were defective. Accumulation of E1A transcripts in the nucleus and cytoplasm resembled wt. The levels of mRNAs from early regions E1B, E2A and E3 at later times during infection were somewhat lower than those of wt, but this decrease may have been due to the absence of progeny viral DNA in dl42-infected cells. However, the accumulation of both E2B and E4 mRNAs at all times was severely reduced. These data suggested that the requirement of 482R for Ad12 DNA replication may be related to its specific role in the metabolism of E2B and E4 mRNAs that encode products necessary for viral DNA synthesis.

The adenovirus E1A proteins induce apoptosis, which is inhibited by the E1B 19-kDa and Bcl-2 proteins

Cooperation between the adenovirus E1A and E1B oncogenes is required for transformation of primary quiescent rodent cells. Although expression of E1A alone will stimulate cell proliferation sufficient to initiate transformed focus formation, proliferation fails to be sustained and foci degenerate. Coexpression of either the 19-kDa or 55-kDa E1B oncoproteins with E1A permits high-frequency transformation by overcoming this cytotoxic response. Without E1B 19-kDa protein expression, however, transformants remain susceptible to induction of cell death. Rapid loss of viability is coincident with nucleolytic cleavage of DNA in intranucleosomal regions and chromatin condensation, hallmarks of programmed cell death (apoptosis). Furthermore, overexpression of a known suppressor of apoptosis, the Bcl-2 protooncogene, can rescue E1A-induced focus degeneration. Thus E1A-dependent stimulation of cell proliferation is accompanied by apoptosis and thereby insufficient to singly induce transformation. High-frequency transformation requires a second function encoded by the E1B 19-kDa protein to block apoptosis.

The 19-kilodalton adenovirus E1B transforming protein inhibits programmed cell death and prevents cytolysis by tumor necrosis factor alpha

The adenovirus E1A and E1B proteins are required for transformation of primary rodent cells. When expressed in the absence of the 19,000-dalton (19K) E1B protein, however, the E1A proteins are acutely cytotoxic and induce host cell chromosomal DNA fragmentation and cytolysis, analogous to cells undergoing programmed cell death (apoptosis). E1A alone can efficiently initiate the formation of foci which subsequently undergo abortive transformation whereby stimulation of cell growth is counteracted by continual cell death. Cell lines with an immortalized growth potential eventually arise with low frequency. Coexpression of the E1B 19K protein with E1A is sufficient to overcome abortive transformation to produce high-frequency transformation. Like E1A, the tumoricidal cytokine tumor necrosis factor alpha (TNF-alpha) evokes a programmed cell death response in many tumor cell lines by inducing DNA fragmentation and cytolysis. Expression of the E1B 19K protein by viral infection, by transient expression, or in transformed cells completely and specifically blocks this TNF-alpha-induced DNA fragmentation and cell death. Cosegregation of 19K protein transforming activity with protection from TNF-alpha-mediated cytolysis demonstrates that both activities are likely the consequence of the same function of the protein. Therefore, we propose that by suppressing an intrinsic cell death mechanism activated by TNF-alpha or E1A, the E1B 19K protein enhances the transforming activity of E1A and enables adenovirus to evade TNF-alpha-dependent immune surveillance.

The 19-kilodalton adenovirus E1B transforming protein inhibits programmed cell death and prevents cytolysis by tumor necrosis factor alpha

The adenovirus E1A and E1B proteins are required for transformation of primary rodent cells. When expressed in the absence of the 19,000-dalton (19K) E1B protein, however, the E1A proteins are acutely cytotoxic and induce host cell chromosomal DNA fragmentation and cytolysis, analogous to cells undergoing programmed cell death (apoptosis). E1A alone can efficiently initiate the formation of foci which subsequently undergo abortive transformation whereby stimulation of cell growth is counteracted by continual cell death. Cell lines with an immortalized growth potential eventually arise with low frequency. Coexpression of the E1B 19K protein with E1A is sufficient to overcome abortive transformation to produce high-frequency transformation. Like E1A, the tumoricidal cytokine tumor necrosis factor alpha (TNF-alpha) evokes a programmed cell death response in many tumor cell lines by inducing DNA fragmentation and cytolysis. Expression of the E1B 19K protein by viral infection, by transient expression, or in transformed cells completely and specifically blocks this TNF-alpha-induced DNA fragmentation and cell death. Cosegregation of 19K protein transforming activity with protection from TNF-alpha-mediated cytolysis demonstrates that both activities are likely the consequence of the same function of the protein. Therefore, we propose that by suppressing an intrinsic cell death mechanism activated by TNF-alpha or E1A, the E1B 19K protein enhances the transforming activity of E1A and enables adenovirus to evade TNF-alpha-dependent immune surveillance.

A block in release of progeny virus and a high particle-to-infectious unit ratio contribute to poor growth of enteric adenovirus types 40 and 41 in cell culture

The fastidious enteric adenovirus (FEAd) types 40 (Ad40) and 41 (Ad41) are found in stool specimens of infants and young children in association with gastroenteritis. Although they can be isolated routinely from clinical specimens by using 293 cells, they are propagated with variable success in cell lines which support the replication of other adenovirus serotypes. HeLa cells are generally considered to be nonpermissive for the replication of FEAds, but in this study, Ad40 and Ad41 grew to comparable titers in individual 293 and HeLa cells. However, virus was not efficiently released from infected HeLa cells and thus did not undergo multiple cycles of infection in HeLa cell cultures. The block in virus release was not overcome in KB18 cells which, like 293 cells, constitutively express proteins encoded by the E1B region of a subgroup C adenovirus (in this case Ad2). Moreover, it was apparent from these studies that Ad40 and Ad41 have particle-to-infectious unit ratios several orders of magnitude greater than that for Ad5, even in 293 cells which express the E1A and E1B proteins of Ad5 and are considered to be permissive for replication of the FEAds. Neither the block in release of progeny virus nor the high particle-to-infectious unit ratio is explained solely by the defect in expression of the E1B 55K protein identified by Mautner et al. (V. Mautner, N. MacKay, and V. Steinthorsdottir, Virology 171:619-622, 1989; V. Mautner, N. MacKay, and K. Morris, Virology 179:129-138, 1990).

Overexpression of the E1B 55-kilodalton (482R) protein of human adenovirus type 12 appears to permit efficient transformation of primary baby rat kidney cells in the absence of the E1B 19-kilodalton protein

To analyze the structure and function of the E1B 19,000-molecular-weight protein (19K protein) (163R) of human adenovirus type 12, mutants were produced at various positions across the 163R-coding sequence. Viruses bearing mutations within the first 100 or so amino acids yielded unstable 163R-related products, induced DNA degradation and enhanced cytopathic effect (cyt/deg phenotype) in KB cells, and transformed primary rodent cells at much lower efficiencies than wild-type (wt) virus. Deletion of the final 16 residues at the carboxy terminus had no phenotypic effect. Alteration of residue 105 reduced transforming efficiency significantly, suggesting that this region of 163R is functionally important. Disruption of the AUG initiation codon at nucleotide 1542 blocked production of 163R completely but resulted in higher levels of E1B 55K-482R protein synthesis and a transforming efficiency similar to that of wt virus. These data suggested that while 163R is of some importance, normal transforming efficiencies can be obtained in its absence if 482R is overexpressed.

The E1B 19,000-molecular-weight protein of group C adenoviruses prevents tumor necrosis factor cytolysis of human cells but not of mouse cells

Tumor necrosis factor (TNF) is a multifunctional immunoregulatory protein that is secreted by activated macrophages and is believed to have antiviral activities. We reported earlier that when mouse C3HA fibroblasts are infected with human adenoviruses, the 289R and 243R proteins encoded by region E1A render the cells susceptible to lysis by TNF, and a 14,700-molecular-weight protein (14.7K protein) encoded by region E3 protects the cells against lysis by TNF. We now report that the 19,000-molecular-weight (19K) (176R) protein encoded by the E1B transcription unit can protect human HEL-299 fibroblasts and human ME-180 cervical carcinoma cells against lysis by TNF. This was determined by infecting cells with adenovirus double mutants that lack region E3 and do or do not express the E1B-19K protein and by measuring cytolysis by using a short-term (18-h) 51Cr-release assay. Under these assay conditions, the 51Cr release was specific to TNF and was not a consequence of the cyt phenotype associated with E1B-19K protein-negative mutants. Also, by using virus double mutants that lack E3 in combination with other early regions, we found that E1A, the E1B-55K protein-encoding gene, E3, and E4 are not required to protect HEL-299 cells against TNF cytolysis. Three additional human cancer cell lines (HeLa, HCT8, and RC29) and a simian virus 40-transformed WI38 cell line (VA-13) also required E1B for protection against TNF cytolysis, indicating that the E1B-19K protein is required to protect many if not all human cell types against lysis by TNF when infected by adenovirus. The E1B-19K protein was not able to protect six different adenovirus-infected mouse cell lines against TNF lysis, even though the protein was shown to be efficiently expressed in one of the cell lines. HEL-299 or ME-180 cells infected by a mutant that lacks the E1B-19K protein but retains region E3 were not lysed by TNF, indicating that one or more of the E3 proteins can protect these cells against TNF lysis in the absence of the E1B-19K protein. Thus, the E3-14.7K but not the E1B-19K protein can protect adenovirus-infected mouse cells against TNF cytolysis, whereas the E1B-19K protein as well as one or more of the E3 proteins can protect adenovirus-infected human cells against TNF cytolysis.

Enteric adenovirus type 40: expression of E1B mRNA and proteins in permissive and nonpermissive cells

The enteric adenovirus type 40 (strain Dugan) grows well in tissue culture only when the E1B 55K protein of Ad5 or Ad12 is supplied in trans, either constitutively expressed in an established cell line or by coinfection with an appropriate helper virus (V. Mautner, N. Mackay, and V. Steinthorsdottir, 1989, Virology 171, 619-622). The synthesis of Ad40 E1B mRNAs and proteins has been examined under permissive and nonpermissive conditions: At late times postinfection in permissive cells, E1B-specific mRNA species of 22 and 13-14 S are made, as well as 15 and 9 S messages for the late IVa2 and ppIX proteins. None of these are detected before the onset of DNA replication and none of them accumulate in the presence of a cytosine arabinoside block to DNA replication. The failure to detect cytoplasmic mRNAs as early times cannot be attributed to a failure of mRNA transport from the nucleus as there is no accumulation of nuclear E1 RNA. In nonpermissive Hela cells only traces of E1B- and ppIX-specific mRNAs are detectable, at very late times postinfection. Antibodies raised to synthetic oligopeptides corresponding to the N- and C-terminal domains of the putative E1B 19K and 55K proteins show a high titer against the cognate peptide by ELISA, but only the E1B 19K C-terminus-specific sera have detected a unique polypeptide in Ad40-infected cells, at late times postinfection. There is no shut-off of host protein synthesis in permissive cells, despite the expression of Ad2 55K protein.

Role of adenovirus E1B proteins in transformation: altered organization of intermediate filaments in transformed cells that express the 19-kilodalton protein

Cooperation of the nuclear oncogene E1A with the E1B oncogene is required for transformation of primary cells. Expression vectors were constructed to produce the 19-kilodalton (19K) and 55K E1B proteins under the direction of heterologous promoters in order to investigate the role of individual E1B proteins in transformation. Coexpression of E1A and either the 19K or 55K E1B gene products was sufficient for the formation of transformed foci in primary rat cells at half the frequency of an intact E1B gene, suggesting that the 19K and 55K proteins function via independent pathways in transformation. Furthermore, the effects of Ha-ras and the E1B 19K gene product were additive when cotransfected with E1A, suggesting that the 19K protein functions in transformation by a mechanism independent from that of ras as well. Although expression of E1A and either E1B protein was sufficient for the subsequent growth of cells in long-term culture, the 19K protein was required to support growth in semisolid media. As the 19K protein has been shown to associate with and disrupt intermediate filaments (IFs) when transiently expressed with plasmid vectors (E. White and R. Cipriani, Proc. Natl. Acad. Sci. USA, 86:9886-9890, 1989), the organization of IFs in transformed cells was investigated. Primary rat cells transformed by plasmids encoding E1A plus the E1B 19K protein showed gross perturbations of IFs, whereas cell lines transformed by plasmids encoding E1A plus the E1B 55K protein or E1A plus Ha-ras did not. These results suggest that an intact IF cytoskeleton may inhibit anchorage-independent growth and that the E1B 19K protein can overcome this inhibition by disrupting the IF cytoskeleton.

The adenovirus E1B 19-kilodalton protein stimulates gene expression by increasing DNA levels

In transient expression assays, the adenovirus E1B 19-kilodalton (19K) tumor antigen increases expression from viral promoters and the promoter for the cellular 70-kilodalton heat shock protein (hsp70). To study the mechanism of this effect, we constructed HeLa cell lines that contain stably integrated copies of the 19K gene. Compared with a 19K- control cell line, 19K+ cells produced a significantly higher level of expression from every promoter introduced into the cells by transfection. The 19K protein also increased expression of an RNA polymerase III-transcribed gene but did not affect the level of expression of the endogenous hsp70 gene. The rate of transcription from transfected promoters, as measured by a nuclear run-on assay, was higher in the 19K+ cells than in the 19K- control cells. Furthermore, the level of plasmid DNA remained higher in the 19K+ cell line, suggesting that the 19K protein stabilizes transfected plasmid DNA. The elevated DNA levels seemed to account in full for the increased transcription. The role of the 19K protein in increasing gene expression during viral infection was found to be due to a replication-dependent increase in viral DNA levels. Thus, the 19K protein activates transcription indirectly by producing a higher level of viral or plasmid DNA. The DNA stabilization function of the 19K protein is probably related to the protective role of the 19K protein during viral infection and represents the first example of a viral oncogene product that modulates gene expression by regulating viral and plasmid DNA levels.

Role of adenovirus E1B proteins in transformation: altered organization of intermediate filaments in transformed cells that express the 19-kilodalton protein

Cooperation of the nuclear oncogene E1A with the E1B oncogene is required for transformation of primary cells. Expression vectors were constructed to produce the 19-kilodalton (19K) and 55K E1B proteins under the direction of heterologous promoters in order to investigate the role of individual E1B proteins in transformation. Coexpression of E1A and either the 19K or 55K E1B gene products was sufficient for the formation of transformed foci in primary rat cells at half the frequency of an intact E1B gene, suggesting that the 19K and 55K proteins function via independent pathways in transformation. Furthermore, the effects of Ha-ras and the E1B 19K gene product were additive when cotransfected with E1A, suggesting that the 19K protein functions in transformation by a mechanism independent from that of ras as well. Although expression of E1A and either E1B protein was sufficient for the subsequent growth of cells in long-term culture, the 19K protein was required to support growth in semisolid media. As the 19K protein has been shown to associate with and disrupt intermediate filaments (IFs) when transiently expressed with plasmid vectors (E. White and R. Cipriani, Proc. Natl. Acad. Sci. USA, 86:9886-9890, 1989), the organization of IFs in transformed cells was investigated. Primary rat cells transformed by plasmids encoding E1A plus the E1B 19K protein showed gross perturbations of IFs, whereas cell lines transformed by plasmids encoding E1A plus the E1B 55K protein or E1A plus Ha-ras did not. These results suggest that an intact IF cytoskeleton may inhibit anchorage-independent growth and that the E1B 19K protein can overcome this inhibition by disrupting the IF cytoskeleton.

The adenovirus E1B 19-kilodalton protein stimulates gene expression by increasing DNA levels

In transient expression assays, the adenovirus E1B 19-kilodalton (19K) tumor antigen increases expression from viral promoters and the promoter for the cellular 70-kilodalton heat shock protein (hsp70). To study the mechanism of this effect, we constructed HeLa cell lines that contain stably integrated copies of the 19K gene. Compared with a 19K- control cell line, 19K+ cells produced a significantly higher level of expression from every promoter introduced into the cells by transfection. The 19K protein also increased expression of an RNA polymerase III-transcribed gene but did not affect the level of expression of the endogenous hsp70 gene. The rate of transcription from transfected promoters, as measured by a nuclear run-on assay, was higher in the 19K+ cells than in the 19K- control cells. Furthermore, the level of plasmid DNA remained higher in the 19K+ cell line, suggesting that the 19K protein stabilizes transfected plasmid DNA. The elevated DNA levels seemed to account in full for the increased transcription. The role of the 19K protein in increasing gene expression during viral infection was found to be due to a replication-dependent increase in viral DNA levels. Thus, the 19K protein activates transcription indirectly by producing a higher level of viral or plasmid DNA. The DNA stabilization function of the 19K protein is probably related to the protective role of the 19K protein during viral infection and represents the first example of a viral oncogene product that modulates gene expression by regulating viral and plasmid DNA levels.

Mapping of a phosphorylation site in the 176R (19 kDa) early region 1B protein of human adenovirus type 5

The 176-residue (176R) early region 1B (E1B) protein of human adenovirus type 5 (Ad5) was shown to be phosphorylated at serine in lytically infected KB cells at a level estimated to be about one phosphate group per 28 176R molecules. Through the analysis of peptides generated by cleavage with cyanogen bromide and Staphylococcus aureus V-8 protease the phosphorylation site was mapped to Ser-164. Using site-directed mutagenesis, a mutant was produced in which the codon for Ser-164 was changed to that of asparagine while leaving the coding sequence for the overlapping 496R protein unchanged. This virus, which replicated well on human KB cells, produced normal levels of 176R, but in an unphosphorylated form. The mutant transformed baby rat kidney cells in cooperation with E1A at an efficiency about one-half that obtained with wt E1B. These data therefore gave little indication that phosphorylation is essential for the function of 176R.

A cell cycle G0-ts mutant, tsJT60, becomes lethal at the nonpermissive temperature after transformation with adenovirus 12 E1B 19K mutant

tsJT60, a temperature-sensitive (ts) cell-cycle mutant of Fischer rats, is viable at both the permissive (34 degrees C) and nonpermissive (40 degrees C) temperatures. The cells grow normally in exponential growth phase at both temperatures, but when stimulated with serum from G0 phase they enter S phase at 34 degrees C but not at 40 degrees C. tsJT60 cells transformed with human adenovirus (Ad) 12 dl205, which lacks the E1B 19-kDa polypeptide gene, were lethal at 40 degrees C, whereas tsJT60 cells transformed with Ad12 wt, dl207, which lacks E1B 58-kDa protein gene, or in206B, which produces 19- to 58- kDa fused protein, were viable. Degradation of cell DNA occurred in dl205-transformed tsJT60 cultured at both 34 degrees C and 40 degrees C. Neither cytocidal phenotype nor degradation of DNA occurred in 3Y1 cells (a parental line of tsJT60) transformed with dl205. These results suggest that the lethal phenotype and degradation of DNA are related to the ts mutation in tsJT60 and also to the lack of Ad12 E1B 19kDa polypeptide.

Role of the adenovirus E1B 19,000-dalton tumor antigen in regulating early gene expression

Mutations in the adenovirus gene encoding the E1B 19-kilodalton protein (the 19K protein) result in pleiotropic phenotypes that affect the host cell and virus growth. Examination of viral gene expression in HeLa cells infected with E1B 19K mutant viruses revealed synthesis and accumulation of E1A proteins to higher steady-state levels than those proteins synthesized during infection with the wild-type virus. As a consequence of elevated E1A levels, another early gene product, the 72K DNA-binding protein, accumulated earlier in mutant-infected cells. In a 12S E1A cDNA virus background, E1B 19K gene mutations had a more profound effect. Larger amounts of the 12S E1A product were present in E1B mutant-infected cells. A deletion mutation that eliminated expression of the 19K protein was also responsible for a 200-fold increased plaque-forming efficiency of the 12S cDNA virus in HeLa cells and an increased rate of virus production. Therefore, the E1B 19K tumor antigen may function to down-regulate virus replication by repressing E1A-dependent gene transcription. Eliminating expression of the E1A 13S and 12S gene products by substitution of an E1A 9S cDNA gene, however, uncovered a stimulatory effect of the E1B 19K protein on early gene expression and virus replication. An E1A 9S virus with a wild-type gene encoding the E1B 19K protein displayed increased early gene transcription, synthesized more 72K DNA-binding protein, and replicated more efficiently than an E1A 9S virus containing a mutation that eliminated expression of the 19K protein. Therefore, the E1B 19K protein has both positive and negative effects on early gene expression and virus replication. In the presence of functional E1A gene products, the 19K protein repressed E1A-dependent gene expression, but in the absence of E1A, the 19K protein stimulated viral gene expression and DNA synthesis. This raises the possibility that the E1B 19K protein functions to repress transcription by modifying the activity of the E1A proteins. Independent of E1A, however, the E1B 19K protein can increase viral gene expression and DNA synthesis, which then leads to increased virus replication.

Adenovirus type 12 E1B 19-kilodalton protein is not required for oncogenic transformation in rats

The adenovirus type 12 mutants in700 and pm700 carry site-specific mutations within the reading frame encoding the E1B 19-kilodalton protein (19K protein) which prevent the production of the intact 19K protein. In cultures of human A549 cells, these mutants grow just as well as the wild-type virus does, but they display a large-plaque (lp), cytocidal (cyt) phenotype. DNA in these infected cells is not degraded, but at late times in human KB cells infected by the mutants, the mutants display a DNA degradation (deg) phenotype. The transformation phenotype of these mutants is also host range. Although the mutants are defective for transformation of the 3Y1 rat cell line, they transform rat and mouse primary kidney cells in vitro at wild-type efficiency and are capable of inducing tumors in rats. These results support the view that the type 12 E1B 19K protein is not obligatory for oncogenic transformation.

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.

Identification of adenovirus type 2 early region 1B proteins that share the same amino terminus as do the 495R and 155R proteins

Adenovirus type 2 early region 1B (E1B) proteins synthesized in vitro were fractionated chromatographically and characterized by peptide and sequence analysis and by reaction with peptide-specific antisera targeted to either the N or C terminus of either of two overlapping E1B reading frames (175 or 495 codons). In addition to the previously identified E1B-495R, E1B-175R, and E1B-155R species, two other E1B proteins of similar electrophoretic mobility to the 175R protein were identified. E1B-82R is an abundant product in vitro and in vivo that has the same N terminus as that of the 495R and 155R proteins but a different C terminus. The structure of 82R is predicted by the structure of the abundant 13S (1.02-kilobase) E1B mRNA. E1B-168R is a novel minor species consisting of the 24 amino-terminal residues of the 495R protein fused to the entire polypeptide IX sequence. An additional, minor 16,000-molecular-weight polypeptide was detected that may correspond to a predicted 92R E1B protein, but definitive identification was not possible. These observations establish that the leftmost portion (78 codons) of the 495-codon reading frame, which overlaps the right half of the 175-codon reading frame, is expressed as an abundant protein that does not contain other 495R sequences. This region, which may participate in the regulation of region E1A expression, may thus constitute a functional domain distinct from the rightward portion of the 495R protein.

Adenovirus transforming 19-kD T antigen has an enhancer-dependent trans-activation function and relieves enhancer repression mediated by viral and cellular genes

The adenovirus E1b region codes for two major tumor antigens of 19 kD and 55 kD, which are important for cell transformation. Our results indicate that the 19-kD T antigen possesses two enhancer-regulatory functions. It can trans-activate enhancer-linked promoters and relieve enhancer repression mediated by viral and cellular repressors. The 19-kD activation function enhances expression from different promoters linked to SV40, Py, Ela, and immunoglobulin heavy-chain enhancers. Enhancer activation by the 19-kD protein appears to be cell type-specific, since the heavy-chain and SV40 enhancers were not trans-activated in myeloma cells whereas the same enhancers were trans-activated in fibroblasts. The 19-kD enhancer activation function appears to be dominant over the enhancer repression function of E1a, since in cells expressing the 19-kD protein there is no significant repression despite a large increase in E1a expression. The 19-kD T antigen activates the Py enhancer in undifferentiated F9 cells indicating that the activation function of E1b masks enhancer repression by an "E1a-like" cellular gene product. The enhancer activation function of the 19-kD T antigen may be important for cell transformation and cell differentiation.

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.

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.