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

Identification and elimination of an aberrant splice product from cDNAs encoding the human adenovirus type 5 E4orf6 protein

Growing awareness of the central role of the E4orf6 protein in regulating the infectious cycle of human adenoviruses has led to greatly intensified efforts to define its functions and mechanisms of action. Many workers employ cDNAs to generate plasmid or viral vectors to express E4orf6 in the absence of other viral products. In addition to the normal 34-kDa product, such vectors consistently produce a polypeptide of about 8 kDa. In the present report we show that this protein is produced by an aberrant mRNA utilizing the 5' splice donor site used normally by the virus to produce the E4orf6/7 product, which shares 58 residues with E4orf6. This amino terminal coding sequence is linked to a 3' sequence via a novel splice acceptor site in an alternative reading frame of the E4orf6 cDNA. The 5' donor site was altered by PCR-directed mutagenesis to yield a construct that produces high levels of E4orf6 in the absence of the 8-kDa polypeptide. This construct should eliminate some of the problems encountered previously using the wild-type E4orf6 coding sequence.

A novel shuttling protein, 4E-T, mediates the nuclear import of the mRNA 5' cap-binding protein, eIF4E

The eukaryotic translation initiation factor 4E (eIF4E) plays an important role in the control of cell growth. eIF4E binds to the mRNA 5' cap structure m(7)GpppN (where N is any nucleotide), and promotes ribosome binding to the mRNA in the cytoplasm. However, a fraction of eIF4E localizes to the nucleus. Here we describe the cloning and functional characterization of a new eIF4E-binding protein, referred to as 4E-T (eIF4E-Transporter). We demonstrate that 4E-T is a nucleocytoplasmic shuttling protein that contains an eIF4E-binding site, one bipartite nuclear localization signal and two leucine-rich nuclear export signals. eIF4E forms a complex with the importin alphabeta heterodimer only in the presence of 4E-T. Overexpression of wild-type 4E-T, but not of a mutant defective for eIF4E binding, causes the nuclear accumulation of HA-eIF4E in cells treated with leptomycin B. Taken together, these results demonstrate that the novel nucleocytoplasmic shuttling protein 4E-T mediates the nuclear import of eIF4E via the importin alphabeta pathway by a piggy-back mechanism.

Two distinct activities contribute to the oncogenic potential of the adenovirus type 5 E4orf6 protein

Previous studies have shown that the adenovirus type 5 (Ad5) E4orf6 gene product displays features of a viral oncoprotein. It initiates focal transformation of primary rat cells in cooperation with Ad5 E1 genes and confers multiple additional transformed properties on E1-expressing cells, including profound morphological alterations and dramatically accelerated tumor growth in nude mice. It has been reported that E4orf6 binds to p53 and, in the presence of the Ad5 E1B-55kDa protein, antagonizes p53 stability by targeting the tumor suppressor protein for active degradation. In the present study, we performed a comprehensive mutant analysis to assign transforming functions of E4orf6 to distinct regions within the viral polypeptide and to analyze a possible correlation between E4orf6-dependent p53 degradation and oncogenesis. Our results show that p53 destabilization maps to multiple regions within both amino- and carboxy-terminal parts of the viral protein and widely cosegregates with E4orf6-dependent acceleration of tumor growth, indicating that both effects are related. In contrast, promotion of focus formation and morphological transformation require only a carboxy-terminal segment of the E4 protein. Thus, these effects are completely independent of p53 stability, but may involve other interactions with the tumor suppressor. Our results demonstrate that at least two distinct activities contribute to the oncogenic potential of Ad5 E4orf6. Although genetically separable, both activities are largely mediated through a novel highly conserved, cysteine-rich motif and a recently described arginine-faced amphipathic alpha helix, which resides within a carboxy-terminal "oncodomain" of the viral protein.

Genetic analysis of a potential zinc-binding domain of the adenovirus E4 34k protein

E4 34k, the product of adenovirus early region 4 (E4) open reading frame 6, modulates viral late gene expression, viral DNA replication, apoptosis, double strand break repair, and transformation through multiple interactions with components in infected and transformed cells. Conservation of several cysteine and histidine residues among E4 34k sequences from a variety of adenovirus serotypes suggests the presence of a zinc binding domain important for function. Consistent with the hypothesis that E4 34k is a zinc metalloprotein, zinc binding by baculovirus-expressed E4 34k protein was demonstrated in a zinc blotting assay. To investigate the relationship between the potential zinc-binding region and E4 34k function, a series of mutant genes containing single amino acid substitutions at each of the conserved cysteine and histidine residues in E4 34k were constructed. The mutant proteins were examined for the ability to complement the late protein synthetic defect of an E4 deletion mutant, to physically interact with the viral E1b 55-kDa protein (E1b 55k) and cellular p53 protein, to relocalize E1b 55k, and to destabilize the p53 protein. These analyses identified a subset of cysteine and histidine residues required for stimulation of late gene expression, physical interaction with E1b 55k, and p53 destabilization. These data suggest that a zinc-binding domain participates in the formation of the E4 34k-E1b 55k physical complex and that the complex is required in late gene expression and for p53 destabilization.

Human T-cell leukemia virus type 1 Tax shuttles between functionally discrete subcellular targets

Human T-cell leukemia virus type 1 (HTLV-1) Tax is a nuclear protein with striking pleiotropic functionality. We recently demonstrated that Tax localizes to a multicomponent nuclear speckled structure (Tax speckled structure [TSS]). Here, we examine these structures further and identify a partial overlap of TSS with transcription hot spots. We used a strategy of directed expression via fusion proteins to determine if these transcription sites are the subtargets within TSS required for Tax function. When fused to human immunodeficiency virus type 1 (HIV-1) Tat, the resulting Tat-Tax fusion protein displayed neither a Tat-like nor a Tax-like pattern but rather was targeted specifically to the transcription subsites. The Tat-Tax fusion was able to activate both the HIV-1 long terminal repeat (LTR) and the HTVL-1 LTR at the same level as the individual component; thus, targeting proteins to transcription hot spots was compatible with both Tax and Tat transcription function. In contrast, the fusion with HIV-1 Rev, Rev-Tax, resulted in a pattern of expression that was largely Rev-like (nucleolar and cytoplasmic). The reduced localization of Rev-Tax to transcription sites was reflected in a 10-fold drop in activation of the HTLV-1 LTR. However, there was no loss in the ability of Tax to activate via NF-kappaB. Thus, NF-kappaB-dependent Tax function does not require targeting of Tax to these transcription sites and suggests that activation via NF-kappaB is a cytoplasmic function. Selective mutation of the nuclear localization signal site in the Rev portion resulted in retargeting of Rev-Tax to TSS and subsequent restoration of transcription function, demonstrating that inappropriate localization preceded loss of function. Mutation of the nuclear export signal site in the Rev portion had no effect on transcription, although the relative amount of Rev-Tax in the cytoplasm was reduced. Finally, in explaining how Tax can occupy multiple subcellular sites, we show that Tax shuttles from the nucleus to the cytoplasm in a heterokaryon fusion assay. Thus, pleiotropic functionality by Tax is regulatable via shuttling between discrete subcellular compartments.

The adenovirus type 5 E1B-55K oncoprotein is a highly active shuttle protein and shuttling is independent of E4orf6, p53 and Mdm2

The E1B-55K and E4orf6 oncoproteins of adenovirus type 5 are involved in the export of viral mRNAs. Previously, it was suggested that a complex composed of E1B-55K and E4orf6 serves as a nucleocytoplasmic transporter for viral mRNAs in which the E4orf6 protein directs both nuclear import and export. We now demonstrate that the E1B-55K protein itself shuttles efficiently in the absence of E4orf6. In addition, E1B-55K trafficking was independent of the defined shuttle proteins Mdm2 or p53, which interacts with E1B-55K. The identified N-terminal E1B-55K leucine-rich nuclear-export signal (NES) conferred rapid nuclear export even in a heterologous system in contrast to the postulated E4orf6NES. Interestingly, although shuttling was blocked by inhibitors of the CRM1 mediated export pathway, E1B-55K inhibited neither the activity nor the trafficking of the retroviral shuttle proteins HIV-1 Rev and HTLV-1 Rex. In contrast, Rev or Rex blocked the nuclear export of E1B-55K, most likely by competing for essential export factors. Our results provide new insights into the regulation of the adenovirus mRNA export system and the processes of adenovirus mediated transformation. Oncogene (2000) 19, 850 - 857.

The nuclear export signal within the E4orf6 protein of adenovirus type 5 supports virus replication and cytoplasmic accumulation of viral mRNA

During the late phase of adenovirus infection, viral mRNA is efficiently transported from the nucleus to the cytoplasm while most cellular mRNA species are retained in the nucleus. Two viral proteins, E1B-55 kDa and E4orf6, are both necessary for these effects. The E4orf6 protein of adenovirus type 5 binds and relocalizes E1B-55 kDa, and the complex of the two proteins was previously shown to shuttle continuously between the nucleus and cytoplasm. Nucleocytoplasmic transport of the complex is achieved by a nuclear export signal (NES) within E4orf6. Mutation of this signal sequence severely reduces the ability of the E1B-55 kDa-E4orf6 complex to leave the nucleus. Here, we examined the role of functional domains within E4orf6 during virus infection. E4orf6 or mutants derived from it were transiently expressed, followed by infection with recombinant adenovirus lacking the E4 region and determination of virus yield. An arginine-rich putative alpha helix near the carboxy terminus of E4orf6 contributes to E1B-55 kDa binding and relocalization as well as to the synthesis of viral DNA, mRNA, and proteins. Further mutational analysis revealed that mutation of the NES within E4orf6 considerably reduces its ability to support virus production. The same effect was observed when nuclear export was blocked with a competitor. Further, a functional NES within E4orf6 contributed to the efficiency of late virus protein synthesis and viral DNA replication, as well as total and cytoplasmic accumulation of viral late mRNA. Our data support the view that NES-mediated nucleocytoplasmic shuttling strongly enhances most, if not all, intracellular activities of E4orf6 during the late phase of adenovirus infection.

E1B 55-kilodalton oncoproteins of adenovirus types 5 and 12 inactivate and relocalize p53, but not p51 or p73, and cooperate with E4orf6 proteins to destabilize p53

The p53 tumor suppressor protein represents a target for viral and cellular oncoproteins, including adenovirus gene products. Recently, it was discovered that several proteins with structural and functional homologies to p53 exist in human cells. Two of them were termed p51 and p73. We have shown previously that the E1B 55-kDa protein (E1B-55 kDa) of adenovirus type 5 (Ad5) binds and inactivates p53 but not p73. Further, p53 is rapidly degraded in the presence of E1B-55 kDa and the E4orf6 protein of this virus. Here, it is demonstrated that p51 does not detectably associate with E1B-55 kDa. While p53 is relocalized to the cytoplasm by E1B-55 kDa, p51's location is unaffected. Finally, p51 retains its full transcriptional activity in the presence of E1B-55 kDa. Apparently, p51 does not represent a target of Ad5 E1B-55 kDa, suggesting that the functions of p51 are distinct from p53-like tumor suppression. E1B-55 kDa from highly oncogenic adenovirus type 12 (Ad12) was previously shown to surpass the oncogenic activity of Ad5 E1B-55 kDa in various assay systems, raising the possibility that Ad12 E1B-55 kDa might target a broader range of p53-like proteins. However, we show here that Ad12 E1B-55 kDa also inhibits p53's transcriptional activity without measurably affecting p73 or p51. Moderate inhibition of p51's transcriptional activity was observed in the presence of the E4orf6 proteins from Ad5 and Ad12. p53 and Ad12-E1B-55 kDa colocalize in the nucleus and also in cytoplasmic clusters when transiently coexpressed. Finally, E1B-55 kDa and E4orf6 of Ad12 mediate rapid degradation of p53 with an efficiency comparable to that of the Ad5 proteins in human and rodent cells. Our results suggest that E1B-55 kDa of either virus type has similar effects on p53 but does not affect p73 and p51.

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

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

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

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

Adenovirus type 5 E4orf3 protein relieves p53 inhibition by E1B-55-kilodalton protein

The E1B-55-kDa protein of adenovirus type 5 and the p53 tumor suppressor gene product form a complex that localizes to the cytoplasm, thereby downregulating p53's transcriptional activity. The E4orf6 protein binds and relocalizes E1B-55-kDa, and the proteins act synergistically to inactivate p53. We show that another adenovirus E4 gene product, E4orf3, is also sufficient to relocalize E1B-55-kDa from the cytoplasm to the nucleus. Both proteins are then found in discrete nuclear structures (tracks) that are known to contain components of the promyelocytic leukemia-associated nuclear structure. Simultaneously, p53 is dissociated from E1B-55-kDa and is found evenly distributed over the nucleoplasm. In the presence of E4orf3, p53-dependent transcriptional activity is no longer repressed by E1B-55-kDa. When E1B-55-kDa is coexpressed with E4orf3 and E4orf6, E1B-55-kDa is found to colocalize with E4orf6 rather than E4orf3. In parallel, p53 is inhibited and degraded by the combination of E1B-55-kDa and E4orf6, regardless of coexpressed E4orf3. This suggests that the effects of E4orf6 on E1B-55-kDa overrule the actions of E4orf3. When cells are infected with virus expressing E4orf3 but not E4orf6, E1B is found in the cell nucleus and p53 enters the virus replication centers. After infection with wild-type adenovirus, E4orf3 is expressed before E4orf6 and E1B temporarily colocalizes with E4orf3 in nuclear tracks before associating with E4orf6. We propose that during adenovirus infection, the E4orf3 protein transiently liberates p53 from its association with E1B-55-kDa. Subsequently, p53 is inactivated and degraded by the combination of E1B-55-kDa and E4orf6.

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.

Analysis of synthesis, stability, phosphorylation, and interacting polypeptides of the 34-kilodalton product of open reading frame 6 of the early region 4 protein of human adenovirus type 5

The 34-kDa early-region 4 open reading frame 6 (E4orf6) product of human adenovirus type 5 forms complexes with both the cellular tumor suppressor p53 and the viral E1B 55-kDa protein (E1B-55kDa). E4orf6 can inhibit p53 transactivation activity, as can E1B-55kDa, and in combination these viral proteins cause the rapid turnover of p53. In addition, E4orf6-55kDa complexes play a critical role at later times in the regulation of viral mRNA transport and shutoff of host cell protein synthesis. In the present study, we have further characterized some of the biological properties of E4orf6. Analysis of extracts from infected cells by Western blotting indicated that E4orf6, like E1A and E1B products, is present at high levels until very late times, suggesting that it is available to act throughout the infectious cycle. This pattern is similar to that of E4orf4 but differs markedly from that of another E4 product, E4orf6/7, which is present only transiently. Synthesis of E4orf6 is maximal at early stages but ceases completely with the onset of shutoff of host protein synthesis; however, it was found that unlike E4orf6/7, E4orf6 is very stable, thus allowing high levels to be maintained even at late times. E4orf6 was shown to be phosphorylated at low levels. Coimmunoprecipitation studies in cells lacking p53 indicated that E4orf6 interacts with a number of other proteins. Five of these were shown to be viral or virally induced proteins ranging in size from 102 to 27 kDa, including E1B-55kDa. One such species, of 72 kDa, was shown not to represent the E2 DNA-binding protein and thus remains to be identified. Another appeared to be the L4 100-kDa nonstructural adenovirus late product, but it appeared to be present nonspecifically and not as part of an E4orf6 complex. Apart from p53, three additional cellular proteins, of 84, 19, and 14 kDa were detected by using an adenovirus vector that expresses only E4orf6. The 19-kDa species and a 16-kDa cellular protein were also shown to interact with E4orf6/7. It is possible that complex formation with these viral and cellular proteins plays a role in one or more of the biological activities associated with E4orf6 and E4orf6/7.

Definition of a major p53 binding site on Ad2E1B58K protein and a possible nuclear localization signal on the Ad12E1B54K protein

Previous studies have established that adenovirus 2/5 early region 1B (Ad E1B) 58K protein binds p53 strongly and co-localizes with it to cytoplasmic dense bodies whilst the homologous Ad12E1B54K protein binds only weakly and co-localizes primarily to the nucleus in Ad12E1 transformed cells. We have used these properties of the E1B proteins from different viral serotypes to map the p53 binding site on the Ad2/5 protein. A set of chimaeric genes was constructed containing different proportions of the Ad12 and Ad2E1B DNA. These, together with Ad12E1A and E1B19K DNA, were transfected into baby rat kidney cells and transformed lines isolated. From an examination of the properties of these Ad12/Ad2E1B fusion proteins in co-immunoprecipitation and subcellular localization experiments it has been concluded that the p53 binding site on Ad2E1B58K protein lies between amino acids 216 and 235 and that the homologous region on Ad12E1B54K protein also binds p53. In addition, a unique nuclear localization signal is located on Ad12E1B54K between residues 228 and 239. We suggest that primary structure differences in these regions of the Ad2 and Ad12E1B proteins are responsible for the different subcellular localizations in AdE1 transformants.

How do animal DNA viruses get to the nucleus?

Genome and pre-genome replication in all animal DNA viruses except poxviruses occurs in the cell nucleus (Table 1). In order to reproduce, an infecting virion enters the cell and traverses through the cytoplasm toward the nucleus. Using the cell's own nuclear import machinery, the viral genome then enters the nucleus through the nuclear pore complex. Targeting of the infecting virion or viral genome to the multiplication site is therefore an essential process in productive viral infection as well as in latent infection and transformation. Yet little is known about how infecting genomes of animal DNA viruses reach the nucleus in order to reproduce. Moreover, this nuclear locus for viral multiplication is remarkable in that the sizes and composition of the infectious particles vary enormously. In this article, we discuss virion structure, life cycle to reproduce infectious particles, viral protein's nuclear import signal, and viral genome nuclear targeting.

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.

Adenovirus E4orf6 oncoprotein modulates the function of the p53-related protein, p73

Recently, several proteins have been identified that are related in their sequence to the p53 tumor-suppressor protein. One of these proteins, which is termed p73, exhibits sequence homology to the p53 transcriptional activation, DNA binding, and oligomerization domains. The adenovirus E1B 55-kDa protein, the adenovirus E4orf6 protein, and SV40 T antigen each can bind to p53 and inhibit p53 function. Here we demonstrate that the adenovirus E4orf6 protein, but not the E1B 55-kDa protein or T antigen, interacts with p73. The E4orf6 protein inhibits p73-mediated transcriptional activation and cell killing in a manner similar to its effect on p53. Thus, only a subset of viral oncoproteins that antagonize p53 function also interacts with the related p73 protein.

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

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

The early region 4 orf4 protein of human adenovirus type 5 induces p53-independent cell death by apoptosis

Previous studies by our group showed that infection of human and rodent cells by human adenovirus type 5 (Ad5) results in the induction of p53-independent apoptosis and cell death that are dependent upon transactivation of early region 4 (E4). To identify which E4 products are involved, studies were conducted with p53-deficient human SAOS-2 cells infected with various Ad5 E4 mutants. An E4orf6-deficient mutant was defective in cell killing, whereas another that expressed only E4orf6 and E4orf4 killed like wild-type virus, suggesting that E4orf6 may be responsible for cytotoxicity; however, a mutant expressing only E4orf4 induced high levels of cell death, indicating that this E4 product may also be able to induce cytotoxicity. To define the E4 cell death-inducing functions more precisely, cDNAs encoding individual E4 products were introduced into cells by DNA transfection in the absence of other Ad5 proteins. In cotransfections with a cDNA encoding firefly luciferase, enzymatic activity was high in all cases except with E4orf4, where luciferase levels were less than 20% of those in controls. In addition, drug selection of several cell types following transfection with retroviral vector DNA encoding individual E4 products as well as puromycin resistance yielded a large number of cell colonies except when E4orf4 was expressed. These data demonstrated that E4orf4 is the only E4 product capable of independent cell killing. Cell death induced by E4orf4 was due to apoptosis, as evidenced by 4',6-diamidino-2-phenylindole (DAPI) staining of cell nuclei in E4orf4-expressing cells. Thus, although E4orf6 may play some role, these results suggested that E4orf4 may be the major E4 product responsible for induction of p53-independent apoptosis.

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.

A tandem array of minimal U1 small nuclear RNA genes is sufficient to generate a new adenovirus type 12-inducible chromosome fragile site

Infection of human cells with adenovirus serotype 12 (Ad12) induces metaphase fragility of four, and apparently only four, chromosomal loci. Surprisingly, each of these four loci corresponds to a cluster of genes encoding a small abundant structural RNA: the RNU1 and RNU2 loci contain tandemly repeated genes encoding U1 and U2 small nuclear RNAs (snRNAs), respectively; the PSU1 locus is a cluster of degenerate U1 genes; and the RN5S locus contains the tandemly repeated genes encoding 5S rRNA. These observations suggested that high local levels of transcription, in combination with Ad12 early functions, can interfere with metaphase chromatin packing. In support of this hypothesis, we and others found that an artificial tandem array of transcriptionally active, but not inactive, U2 snRNA genes would generate a novel Ad12-inducible fragile site. Although U1 and U2 snRNA are both transcribed by RNA polymerase II and share similar enhancer, promoter, and terminator signals, the human U1 promoter is clearly more complex than that of U2. In addition, the natural U1 tandem repeat unit exceeds 45 kb, whereas the U2 tandem repeat unit is only 6.1 kb. We therefore asked whether an artificial array of minimal U1 genes would also generate a novel Ad12-inducible fragile site. The exogenous U1 genes were marked by an innocuous U72C point mutation within the U1 coding region so that steady-state levels of U1 snRNA derived from the artificial array could be quantified by a simple primer extension assay. We found that the minimal U1 genes were efficiently expressed and were as effective as minimal U2 genes in generating a novel Ad12-inducible fragile site. Thus, despite significant differences in promoter architecture and overall gene organization, the active U1 transcription units suffice to generate a new virally inducible fragile site.

Adenovirus type 12-induced fragility of the human RNU2 locus requires p53 function

Adenovirus type 12 (Ad12) infection of human cells induces four chromosomal fragile sites corresponding to the U1 small nuclear RNA (snRNA) genes (the RNU1 locus), the U2 snRNA genes (RNU2), the U1 snRNA pseudogenes (PSU1), and the 5S rRNA genes (RN5S). Ad12-induced fragility of the RNU2 locus requires U2 snRNA transcriptional regulatory elements and viral early functions but not viral replication or integration, or chromosomal sequences flanking the RNU2 locus. We now show that Ad12 cannot induce the RNU1, RNU2, or PSU1 fragile sites in Saos-2 cells lacking the p53 and retinoblastoma (Rb) proteins but that viral induction of fragility is rescued in these cells when the expression of wild-type p53 or selected hot-spot mutants (i.e., V143A, R175H, R248W, and R273H) is restored by transient expression or stable retroviral transduction. We also observed weak constitutive fragility of the RNU1 and RNU2 loci in cells belonging to xeroderma pigmentosum complementation groups B and D (XPB and XPD) which are partially defective in the ERCC2 (XPD) and ERCC3 (XPB) helicase activities shared between the repairosome and the RNA polymerase H basal transcription factor TFIIH. We propose a model for Ad12-induced chromosome fragility in which interaction of p53 with the Ad12 E1B 55-kDa transforming protein (and possibly E4orf6) induces a p53 gain of function which ultimately perturbs the RNA polymerase II basal transcription apparatus. The p53 gain of function could interfere with chromatin condensation either by blocking mitotic shutdown of U1 and U2 snRNA transcription or by phenocopying global or local DNA damage. Specific fragilization of the RNU1, RNU2, and PSU1 loci could reflect the unusually high local concentration of strong transcription units or the specialized nature of the U1 and U2 snRNA transcription apparatus.

Nucleo-cytoplasmic shuttling of the hdm2 oncoprotein regulates the levels of the p53 protein via a pathway used by the human immunodeficiency virus rev protein

The hdm2 gene is overexpressed in a variety of human tumors. Its gene product localizes predominantly to the nucleus, where it acts as an inhibitor of the p53 tumor suppressor gene product. It is shown here that the hdm2 oncoprotein constantly shuttles between the nucleus and the cytoplasm. Shuttling of hdm2 does not depend on its interaction with p53. Nuclear export of hdm2 is mediated by a signal sequence similar to the nuclear export signal of the rev protein from human immunodeficiency virus and other lentiviruses. Mutation of this signal sequence abolishes detectable nucleo-cytoplasmic shuttling. When fused to a carrier protein, the hdm2 signal sequence can mediate nuclear export after intranuclear microinjection into HeLa cells. The export of hdm2 can be blocked by a competitive inhibitor of rev export, arguing that the export pathways for hdm2 and rev are either overlapping or identical. Inhibition of its export modifies the ability of hdm2 to block p53-mediated transcriptional activation, and hdm2's export function is required to accelerate the degradation of p53. Thus the rev nuclear export pathway may be used to regulate an oncogene product's activity and modulate cellular growth.

Activation of transgene expression by early region 4 is responsible for a high level of persistent transgene expression from adenovirus vectors in vivo

The persistence of transgene expression has become a hallmark for adenovirus vector evaluation in vivo. Although not all therapeutic benefit in gene therapy is reliant on long-term transgene expression, it is assumed that the treatment of chronic diseases will require significant persistence of expression. To understand the mechanisms involved in transgene persistence, a number of adenovirus vectors were evaluated in vivo in different strains of mice. Interestingly, the rate of vector genome clearance was not altered by the complete deletion of early region 4 (E4) in our vectors. The GV11 (E1- E4-) vector genome cleared with a similar kinetic profile as the GV10 (E1-) vector genome in immunocompetent and immunocompromised mice. These results suggest that the majority of adenovirus vector genomes are eliminated from transduced tissue via a mechanism(s) independent of T-cell, B-cell, and NK cell immune mechanisms. While the levels of persistence of transgene expression in liver or lung transduced with GV10 and GV11 vectors expressing beta-galactosidase, cystic fibrosis transmembrane conductance regulator, or secretory alkaline phosphatase were similar in immunocompetent mice, a marked difference was observed in immunocompromised animals. Levels of transgene expression initially from both GV10 and GV11 vectors were the same. However, GV11 transgene expression correlated with loss of vector genome, while GV10 transgene expression persisted at a high level. Coadministration and readministration of GV10 vectors showed that E4 provided in trans could activate transgene expression from the GV11 vector genome. While transgene expression activity per genome from the GV10 vector is clearly activated, expression from a cytomegalovirus promoter expression cassette in a GV11 vector appeared to be further inactivated as a function of time. Understanding the molecular mechanisms underlying these expression effects will be important for developing persistent adenovirus vectors for chronic applications.

p53/E1b58kDa complex regulates adenovirus replication

We have explored a role for the adenovirus (Ad5) E1b58kDa/p53 protein complex in adenovirus replication. This was done by using virus mutants containing different defects in the E1b58kDa gene and cell lines that express either a wild-type p53 protein or a mutant p53 protein. We find that infection of wild-type p53-containing cells with wild-type Ad5 causes a shutoff of p53 and alpha-actin protein synthesis by distinct mechanisms, but neither occurs in mutant p53 cells. Our data also indicate that the shutoff is dependent on formation of the p53/E1b complex and may also involve another virus protein, E4ORF6. Following from these observations we asked whether failure to form the complex resulted in impaired adenovirus replication. Our experiments showed that neither wild-type Ad5 nor the E1b mutant dl338 could replicate in cells expressing a mutant p53 protein, but that wild-type adenovirus replicated well in wild-type p53-expressing cells. Collectively, our data suggest that the interaction between p53 and the E1b58kDa protein is necessary for efficient adenovirus replication. This is the first time such a direct link between the complex and virus replication has been demonstrated. These data raise serious questions about the usefulness of E1b-defective viruses in tumor therapy.

Nuclear export of the E1B 55-kDa and E4 34-kDa adenoviral oncoproteins mediated by a rev-like signal sequence

The E1B 55-kDa and E4 34-kDa oncoproteins of adenovirus type 5 (abbreviated here as E1B-55kD and E4-34kD) promote the export of viral mRNA and inhibit the export of most cellular mRNA species. We show that the intracellular complex containing E1B-55kD and E4-34kD continuously shuttles between the nucleus and the cytoplasm, and may thus serve as a nucleocytoplasmic transporter for viral mRNA. We present evidence that within this complex, it is the E4-34kD protein that directs both nuclear import and nuclear export. E4-34kD contains a functional nuclear export signal similar to corresponding sequences found in the retroviral proteins rev and rex. This sequence element is required for nuclear export of the complex, and it can function autonomously when fused to a carrier protein and microinjected in HeLa cell nuclei. When E4-34kD is expressed alone, a portion of the protein that contains a predicted arginine-rich amphipathic alpha-helical structure mediates nuclear retention of the protein. This retention, however, can be abolished by the association with E1B-55kD or by a specific point mutation within the arginine-rich motif. The export of E4-34kD can be blocked by an HTLV-rex derived competitive inhibitor and overexpressed E4-34kD inhibits rev-mediated transport, suggesting that the export pathways accessed by the adenoviral and retroviral proteins share components. The interplay between two polypeptides as well as the involvement of a dominant nuclear retention domain are novel features that might contribute to the efficiency and regulation of the adenovirus export system.

Long-term gene delivery into the livers of immunocompetent mice with E1/E4-defective adenoviruses

We have compared the in vitro and in vivo behaviors of a set of isogenic E1- and E1/E4-defective adenoviruses expressing the lacZ gene of Escherichia coli from the Rous sarcoma virus long terminal repeat. Infection of tumor-derived established cell lines of human origin with the doubly defective adenoviruses resulted in (i) a lower replication of the viral backbone that correlated with reduced levels of E2A-specific RNA and protein, (ii) a significant shutoff of late gene and protein expression, and (iii) no apparent virus-induced cytotoxicity. Independently of the extent of the deletion, the additional inactivation of E4 from the viral backbone therefore drastically disabled the virus in vitro, with no apparent effect on transgene expression. A lacZ-transgenic model was used to compare the different recombinant adenoviruses in the livers of C57BL/6 mice. The immune response to the virally encoded beta-galactosidase was minimal in this model, as infusion of the E1-defective adenovirus resulted in a time course of transgene expression that mimicked that in immunodeficient (nu/nu) mice, with very little inflammation and necrosis in the liver. Administration of a doubly defective adenovirus to the transgenic animals led to long-term extrachromosomal persistence of viral DNA in the liver, with no detectable methylation of CpG dinucleotides. However, transient transgene expression was observed independently of the extent of the E4 deletion, suggesting that the choice of the promoter may be critical to maintain transgene expression from these attenuated adenovirus vectors.

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

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