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

Viral Subversion of the Immune System

The continuous interactions between host and viruses during their co-evolution have shaped not only the immune system but also the countermeasures used by viruses. Studies in the last decade have described the diverse arrays of pathways and molecular targets that are used by viruses to elude immune detection or destruction, or both. These include targeting of pathways for major histocompatibility complex class I and class II antigen presentation, natural killer cell recognition, apoptosis, cytokine signalling, and complement activation. This paper provides an overview of the viral immune-evasion mechanisms described to date. It highlights the contribution of this field to our understanding of the immune system, and the importance of understanding this aspect of the biology of viral infection to develop efficacious and safe vaccines.

Development of packaging cell lines for generation of adeno-associated virus vectors by lentiviral gene transfer of trans-complementary components

Adeno-associated virus (AAV) vector system has several useful advantages with regard to in vitro and in vivo gene transfer. However, their usages have been limited by cumbersome and labor-intensive vector production in the traditional method. To overcome limitations in AAV production, in this report, we explored the possibility of generating AAV packaging cell line, 293T R/C.VA.E2A.E4. cells, by using lentivirus-mediated transduction of Rep/Cap gene of AAV-2, VA RNA, E2A, and E4 genes of Ad5 into 293T cells. In packaging cell lines, it is important that supply of the AAV vector can be stably performed for long time. We showed that the 293T R/C.VA.E2A.E4. cells have stably maintained the transduced components after more than 10 passages and yielded high-titer AAV vectors, and the titer of AAV vectors did not decline even if culture of the packaging cells was continued for long time. The Rep/Cap and E4 gene products caused no remarkable cytotoxicity. The 293T R/C.VA.E2A.E4. cells might be able to tolerate the Rep/Cap and E4 gene products, or have less copy numbers of the Rep/Cap and E4 genes than the traditional method. Moreover, we showed that the AAV vectors derived from 293T R/C.VA.E2A.E4. cells infected the primary human CD34+ haematopoietic progenitor cells with high efficiency (50-70%). In the 293T R/C.VA.E2A.E4. cells, the AAV vectors can be generated by the transfection of one AAV vector plasmid, and large-scale AAV production can be easily achieved. It is important that cumbersome, variable, and costly transfection is avoided.

Sequestration of p53 in the cytoplasm by adenovirus type 12 E1B 55-kilodalton oncoprotein is required for inhibition of p53-mediated apoptosis

The adenovirus E1B 55-kDa protein is a potent inhibitor of p53-mediated transactivation and apoptosis. The proposed mechanisms include tethering the E1B repression domain to p53-responsive promoters via direct E1B-p53 interaction. Cytoplasmic sequestration of p53 by the 55-kDa protein would impose additional inhibition on p53-mediated effects. To investigate further the role of cytoplasmic sequestration of p53 in its inhibition by the E1B 55-kDa protein we systematically examined domains in both the Ad12 55-kDa protein and p53 that underpin their colocalization in the cytoplasmic body and show that the N-terminal transactivation domain (TAD) of p53 is essential for retaining p53 in the cytoplasmic body. Deletion of amino acids 11 to 27 or even point mutation L22Q/W23S abolished the localization of p53 to the cytoplasmic body, whereas other parts of TAD and the C-terminal domain of p53 are dispensable. This cytoplasmic body is distinct from aggresome associated with overexpression of some proteins, since it neither altered vimentin intermediate filaments nor associated with centrosome or ubiquitin. Formation of this structure is sensitive to mutation of the Ad12 55-kDa protein. Strikingly, mutation S476/477A near the C terminus of the Ad12 55-kDa protein eliminated the formation of the cytoplasmic body. The equivalent residues in the Ad5 55-kDa protein were shown to be critical for its ability to inhibit p53. Indeed, Ad12 55-kDa mutants that cannot form a cytoplasmic body can no longer inhibit p53-mediated effects. Conversely, the Ad12 55-kDa protein does not suppress p53 mutant L22Q/W23S-mediated apoptosis. Finally, we show that E1B can still sequester p53 that contains the mitochondrial import sequence, thereby potentially preventing the localization of p53 to mitochondria. Thus, cytoplasmic sequestration of p53 by the E1B 55-kDa protein plays an important role in restricting p53 activities.

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.

Adenovirus E1B 55-kilodalton oncoprotein binds to Daxx and eliminates enhancement of p53-dependent transcription by Daxx

The adenovirus E1B 55-kDa protein impairs the p53 pathway and enhances transformation, although the underlying mechanisms remain to be defined. We found that Daxx binds to the E1B 55-kDa protein in a yeast two-hybrid screen. The two proteins interact through their C termini. Mutation of three potential phosphorylation sites (S489/490 and T494 to alanine) within the E1B 55-kDa protein did not affect its interaction with Daxx, although such mutations were previously shown to inhibit E1B's ability to repress p53-dependent transcription and to enhance transformation. In addition to their coimmunoprecipitation in 293 extracts, purified Daxx interacted with the E1B 55-kDa protein in vitro, indicating their direct interaction. In 293 cells, Daxx colocalized with the E1B 55-kDa protein within discrete nuclear dots, where p53 was also found. Such structures were distinct from PML (promyelocytic leukemia protein) bodies, and it appeared that Daxx was displaced from PML bodies. Thus, the Daxx concentration was diminished in dots with a prominent presence of PML and vice versa. Indeed, PML overexpression led to dramatic redistribution of Daxx from p53-E1B 55-kDa protein complexes to PML bodies. Additionally, expression of the E1B 55-kDa protein in Saos2 osteosarcoma cells reduced the number of PML bodies. Our data suggest that E1B and PML compete for available Daxx in the cell. Surprisingly, Daxx significantly augmented p53-mediated transcription and the E1B 55-kDa protein eliminated this effect. Thus, it is likely that the E1B 55-kDa protein sequesters Daxx and p53 in specific nuclear locations, where p53 cannot activate transcription. One consequence of the Daxx-E1B interaction might be an alteration of normal interactions of Daxx, PML, and p53, which may contribute to cell transformation.

A novel domain in adenovirus L4-100K is required for stable binding and efficient inhibition of human granzyme B: possible interaction with a species-specific exosite

Lymphocyte granule serine proteases (granzymes) play a critical role in protecting higher organisms against intracellular infections and cellular transformation. The proteases have also been implicated in the generation of tissue damage in a variety of chronic human conditions, including autoimmunity and transplant rejection. Granzyme B (GrB), one cytotoxic member of this family, achieves its effect through cleavage and activation of caspases as well as through caspase-independent proteolysis of cellular substrates. The 100,000-molecular-weight (100K) assembly protein of human adenovirus type 5 (Ad5-100K) was previously defined as a potent and specific inhibitor of human GrB. We now show that although human, mouse, and rat GrB proteases are well conserved in terms of structure, substrate specificity, and function, Ad5-100K inhibitory activity is directed exclusively against the human protease. Biochemical analysis demonstrates that the specificity of the 100K protein for human GrB resides in two distinct interactions with the protease: (i) a unique sequence within the reactive site loop (P(1))Asp(48)-(P(1'))Pro(49) in Ad5-100K which interacts with the active site and (ii) the presence of an additional inhibitor-enzyme interaction likely outside the enzyme catalytic site (i.e., an exosite). We have located this extended macromolecular interaction site in Ad5-100K within amino acids 688 to 781, and we have demonstrated that this region is essential for stable inhibitor-enzyme complex formation as well as efficient inhibition of human GrB. This novel component of the inhibitory mechanism of the 100K protein identifies a distinct target for selective inhibitor design, a finding which may be of benefit for diseases in which GrB plays a pathogenic role.

Regulation of mRNA production by the adenoviral E1B 55-kDa and E4 Orf6 proteins

The E1B 55-kDa and E4 Orf6 proteins of human subgroup C adenoviruses both counter host cell defenses mediated by the cellular p53 protein and regulate viral late gene expression. A complex containing the two proteins has been implicated in induction of selective export of viral late mRNAs from the nucleus to the cytoplasm, with concomitant inhibition of export of the majority of newly synthesized cellular mRNAs. The molecular mechanisms by which these viral proteins subvert cellular pathways of nuclear export are not yet clear. Here, we review recent efforts to identify molecular and biochemical functions of the E1B 55-kDa and E4 Orf6 proteins required for regulation of mRNA export, the several difficulties and discrepancies that have been encountered in studies of these viral proteins, and evidence indicating that the reorganization of the infected cell nucleus and production of viral late mRNA at specific intra-nuclear sites are important determinants of selective mRNA export in infected cells. In our view, it is not yet possible to propose a coherent molecular model for regulation of mRNA export by the E1B 55-kDa and E4 Orf6 proteins. However, it should now be possible to address specific questions about the roles of potentially relevant properties of these viral proteins.

Caspase cleavage of the nonstructural protein NS1 mediates replication of Aleutian mink disease parvovirus

Virus-induced apoptosis of infected cells can limit both the time and the cellular machinery available for virus replication. Hence, many viruses have evolved strategies to specifically inhibit apoptosis. However, Aleutian mink disease parvovirus (ADV) is the first example of a DNA virus that not only induces apoptosis but also utilizes caspase activity to facilitate virus replication. To determine the function of caspase activity during ADV replication, virus-infected cell lysates or purified ADV proteins were incubated with various purified caspases. Caspases cleaved the major nonstructural protein of ADV (NS1) at two caspase recognition sequences, whereas ADV structural proteins could not be cleaved. Importantly, the NS1 products could be identified in ADV-infected cells but were not present in infected cells pretreated with caspase inhibitors. By mutating putative caspase cleavage sites (D to E), we mapped the two cleavage sites to amino acid residues NS1:227 (INTD downward arrow S) and NS1:285 (DQTD downward arrow S). Replication of ADV containing either of these mutations was reduced 10(3)- to 10(4)-fold compared to that of wild-type virus, and a construct containing both mutations was replication defective. Immunofluorescent studies revealed that cleavage was required for nuclear localization of NS1. The requirement for caspase activity during permissive replication suggests that limitation of caspase activation and apoptosis in vivo may be a novel approach to restricting virus replication.

Human MI-ER1 alpha and beta function as transcriptional repressors by recruitment of histone deacetylase 1 to their conserved ELM2 domain

mi-er1 (previously called er1) was first isolated from Xenopus laevis embryonic cells as a novel fibroblast growth factor-regulated immediate-early gene. Xmi-er1 was shown to encode a nuclear protein with an N-terminal acidic transcription activation domain. The human orthologue of mi-er1 (hmi-er1) displays 91% similarity to the Xenopus sequence at the amino acid level and was shown to be upregulated in breast carcinoma cell lines and tumors. Alternative splicing at the 3' end of hmi-er1 produces two major isoforms, hMI-ER1alpha and hMI-ER1beta, which contain distinct C-terminal domains. In this study, we investigated the role of hMI-ER1alpha and hMI-ER1beta in the regulation of transcription. Using fusion proteins of hMI-ER1alpha or hMI-ER1beta tethered to the GAL4 DNA binding domain, we show that both isoforms, when recruited to the G5tkCAT minimal promoter, function to repress transcription. We demonstrate that this repressor activity is due to interaction and recruitment of a trichostatin A-sensitive histone deacetylase 1 (HDAC1). Furthermore, deletion analysis revealed that recruitment of HDAC1 to hMI-ER1alpha and hMI-ER1beta occurs through their common ELM2 domain. The ELM2 domain was first described in the Caenorhabditis elegans Egl-27 protein and is present in a number of SANT domain-containing transcription factors. This is the first report of a function for the ELM2 domain, highlighting its role in the regulation of transcription.

Gene therapy for pancreatic cancer targeting the genomic alterations of tumor suppressor genes using replication-selective oncolytic adenovirus

In order to develop an effective therapeutic intervention for patients with pancreatic cancer, we examined the genetic alternations of pancreatic cancer. Based on these results, we are developing a new gene therapy targeting the genetic character of pancreatic cancer using mutant adenoviruses selectively replication-competent in tumor cells. Loss of heterozygosity (LOH) of 30% or more were observed on chromosome arms 17p (47%), 9p (45%), 18q (43%), 12q (34%), and 6q (30%). LOH of 12q, 17p, and 18q showed the significant association with poor prognosis. These data strongly suggest that mutation of the putative suppressor genes, TP53 and SMAD4 play significant roles in the disease progression. Based on this rationale, we are developing a new gene therapy targeting tumors without normal TP53 function. E1B-55kDa-deleted adenovirus (AxE1AdB) can selectively replicate in TP53-deficient human tumor cells but not cells with functional TP53. We evaluated the therapeutic effect of this AxE1AdB on pancreatic cancer without normal TP53 function. The growth of human pancreatic tumor in SCID mice model was markedly inhibited by the consecutive injection of AxE1AdB. Furthermore, AxE1AdB is not only the strong weapon but also useful carrier of genes possessing anti-tumor activities as a virus vector specific to tumors without normal TP53 function. It was reported that uracil phosphoribosyl transferase (UPRT) overcomes 5FU resistance. UPRT catalyzes the synthesis of 5-fluorouridine monophosphate (FUMP) from Uracil and phosphoribosylpyrophosphate (PRPP). The antitumor effect of 5FU is enhanced by augmenting 5-fluorodeoxyuridine monophosphate (FdUMP) converted from FUMP, which inhibits Thymidylate Synthetase (TS). The therapeutic advantage of restricted replication competent adenovirus that expresses UPRT (AxE1AdB-UPRT) was evaluatedin an intra-peritoneal disseminated tumor model. To study the anti-tumor effect of AxE1AdB-UPRT/5FU, mice with disseminated AsPC-1 tumors were administered the adenovirus, followed by the 5FU treatment. It was shown that the treatment with AxE1AdB-UPRT/5FU caused a dramatic reduction of the disseminated tumor burden without toxicity in normal tissues. These results revealed thatthe AxE1AdB-UPRT/5FU system is a promising tool for intraperitoneal disseminated pancreatic cancer.

Adenoviral inhibitors of apoptotic cell death

Adenoviruses (Ads) are endemic in the human population and the well-studied group C Ads typically cause an acute infection in the respiratory epithelium. A growing body of evidence suggests that these viruses also establish a persistent infection. The Ad genome encodes several proteins that counteract the host anti-viral mechanisms, which function to limit viral infections. This review describes the adenovirus immuno-regulatory proteins and how they function to block apoptosis of infected cells. In addition to facilitating the successful completion of the viral replication cycle and spread of progeny virus, these functions may help maintain the virus in a persistent state.

Epidermal growth factor receptor targeting of replication competent adenovirus enhances cytotoxicity in bladder cancer

PURPOSE

We evaluated the delivery and oncolytic potential of targeted replication competent adenoviruses in bladder cancer lines.

MATERIALS AND METHODS

Seven established human bladder cancer tumor lines (5637, SW800, TCCsup, J82, Scaber, T24 and 253J) were studied for the expression of integrins alpha(v)beta3, alpha(v)beta5, Coxsackievirus and adenovirus receptor, epidermal growth factor receptor (EGF-R) and epithelial cell adhesion molecule antigens using flow cytometry analysis. Bispecific single chain Fv fragments were used to target replication deficient luciferase reporter adenovirus to EGF-R (425-s11) or to epithelial cell adhesion molecule (C28-s11) antigens. Moreover, a fiber modified adenovirus targeting alpha(v)-integrins was studied. Replication competent serotype-5 adenoviruses attenuated to replicate specifically in retinoblastoma pRb (Ad5-d24) or p53 deficient (Ad5-d55K) cells were tested in vitro for oncolytic properties.

RESULTS

Low to absent Coxsackievirus and adenovirus receptor expression was found in 5 of the 7 tumor lines (SW800, J82, T24, 5637 and Scaber). EGF-R expression was found in all cell lines, whereas elevated epithelial cell adhesion molecule expression was seen in 3 (5637, Scaber and TCCsup), alpha(v)beta3-integrin was found in 1 (Scaber) and alpha(v)beta5-integrin was found in 3 (TCCsup, 253J and T24). EGF-R targeting using 425-s11 improved transgene expression in all cell lines from 2.1 to 12.5 times over nontargeted viruses. Epithelial cell adhesion molecule and integrin targeting was inferior to EGF-R targeting with a maximal increase in transgene expression of 2 times for epithelial cell adhesion molecule in 5637cells and 1.6 times for integrin targeting in T24 cells. Comparison of the wild-type replication competent virus with conditionally replicating adenoviruses (Ad5-d55K and Ad5-d24) showed superior oncolytic activity for the latter 2 in all lines. Furthermore, improved cytotoxicity (29% to 33%) was obtained in 4 of the 7 lines after pre-incubation of Ad5-d24 with 425-s11.

CONCLUSIONS

EGF-R directed bispecific single chain antibodies enhance adenovirus mediated transgene expression and oncolysis in bladder cancer lines.

Epidermal growth factor receptor targeting of replication competent adenovirus enhances cytotoxicity in bladder cancer

PURPOSE

We evaluated the delivery and oncolytic potential of targeted replication competent adenoviruses in bladder cancer lines.

MATERIALS AND METHODS

Seven established human bladder cancer tumor lines (5637, SW800, TCCsup, J82, Scaber, T24 and 253J) were studied for the expression of integrins alpha(v)beta3, alpha(v)beta5, Coxsackievirus and adenovirus receptor, epidermal growth factor receptor (EGF-R) and epithelial cell adhesion molecule antigens using flow cytometry analysis. Bispecific single chain Fv fragments were used to target replication deficient luciferase reporter adenovirus to EGF-R (425-s11) or to epithelial cell adhesion molecule (C28-s11) antigens. Moreover, a fiber modified adenovirus targeting alpha(v)-integrins was studied. Replication competent serotype-5 adenoviruses attenuated to replicate specifically in retinoblastoma pRb (Ad5-d24) or p53 deficient (Ad5-d55K) cells were tested in vitro for oncolytic properties.

RESULTS

Low to absent Coxsackievirus and adenovirus receptor expression was found in 5 of the 7 tumor lines (SW800, J82, T24, 5637 and Scaber). EGF-R expression was found in all cell lines, whereas elevated epithelial cell adhesion molecule expression was seen in 3 (5637, Scaber and TCCsup), alpha(v)beta3-integrin was found in 1 (Scaber) and alpha(v)beta5-integrin was found in 3 (TCCsup, 253J and T24). EGF-R targeting using 425-s11 improved transgene expression in all cell lines from 2.1 to 12.5 times over nontargeted viruses. Epithelial cell adhesion molecule and integrin targeting was inferior to EGF-R targeting with a maximal increase in transgene expression of 2 times for epithelial cell adhesion molecule in 5637cells and 1.6 times for integrin targeting in T24 cells. Comparison of the wild-type replication competent virus with conditionally replicating adenoviruses (Ad5-d55K and Ad5-d24) showed superior oncolytic activity for the latter 2 in all lines. Furthermore, improved cytotoxicity (29% to 33%) was obtained in 4 of the 7 lines after pre-incubation of Ad5-d24 with 425-s11.

CONCLUSIONS

EGF-R directed bispecific single chain antibodies enhance adenovirus mediated transgene expression and oncolysis in bladder cancer lines.

A time to kill: viral manipulation of the cell death program

Many viruses have as part of their arsenal the ability to modulate the apoptotic pathways of the host. It is counter-intuitive that such simple organisms would be efficient at regulating this the most crucial pathway within the host, given the relative complexity of the host cells. Yet, viruses have the potential to initiate or stay the onset of programmed cell death through the manipulation of a variety of key apoptotic proteins. It is the intention of this review to provide an overview of viral gene products that are able to promote or inhibit apoptotic death of the host cell and to discuss their mechanisms of action. It is not until recently that the depth at which viruses exploit the apoptotic pathways of their host has been seen. This understanding may provide a great opportunity for future therapeutic ventures.

Selectively replicating adenoviruses targeting deregulated E2F activity are potent, systemic antitumor agents

We have engineered a human adenovirus, ONYX-411, that selectively replicates in human tumor cells, but not normal cells, depending upon the status of their retinoblastoma tumor suppressor protein (pRB) pathway. Early and late viral gene expression as well as DNA replication were significantly reduced in a functional pRB-pathway-dependent manner, resulting in a restricted replication profile similar to that of nonreplicating adenoviruses in normal cells both in vitro and in vivo. In contrast, the viral life cycle and tumor cell killing activity of ONYX-411 was comparable to that of wild-type adenovirus following infection of human tumor cells in vitro as well as after systemic administration in tumor-bearing animals.

Effects of mutations in the adenoviral E1B 55-kilodalton protein coding sequence on viral late mRNA metabolism

The human subgroup C adenoviral E1B 55-kDa protein cooperates with the viral E4 Orf6 protein to induce selective export of viral, late mRNAs from the nucleus to the cytoplasm. Previous studies have suggested that such preferential transport of viral mRNA and the concomitant inhibition of export of cellular mRNAs are the result of viral colonization of specialized microenvironments within the nucleus. However, neither the molecular basis of this phenomenon nor the mechanism by which the E1B 55-kDa protein acts has been elucidated. We therefore examined viral late mRNA metabolism in HeLa cells infected with a series of mutant viruses that carry insertions at various positions in the E1B protein coding sequence (P. R. Yew, C. C. Kao, and A. J. Berk, Virology 179:795-805, 1990). All the mutations examined impaired cytoplasmic accumulation of viral L2 mRNAs and reduced L2 mRNA export efficiency. However, in most cases these defects could be ascribed to reduced E1B 55-kDa protein concentration or the unexpected failure of the altered E1B proteins to enter the nucleus efficiently. The latter property, the pleiotropic defects associated with all the mutations that impaired nuclear entry of the E1B protein, and consideration of its primary sequence suggest that these insertions result in misfolding of the protein. Insertion of four amino acids at residue 143 also inhibited viral mRNA export but resulted in increased rather than decreased accumulation of the E1B 55-kDa protein in the nucleus. This mutation specifically impaired the previously described association of the E1B protein with intranuclear structures that correspond to sites of adenoviral DNA replication and transcription (D. Ornelles and T. Shenk, J. Virol. 65:424-439, 1991) and the colocalization of the E1B and E4 Orf6 proteins. As this insertion has been shown to inhibit the interaction of the E1B with the E4 Orf6 protein in infected cell extracts (S. Rubenwolf, H. Schütt, M. Nevels, H. Wolf, and T. Dobner, J. Virol. 71:1115-1123, 1997), these phenotypes provide direct support for the hypothesis that selective viral mRNA export is determined by the functional organization of the infected cell nucleus.

Adenovirus infection of primary malignant lymphoid cells

Adenovirus infection represents a cellular stress that induces host cell pro-apoptotic responses. To overcome this barrier to productive infection, viral polypeptides modulate a variety of host cell pathways. The interface of these early viral gene products with key cellular regulatory proteins has provided considerable information concerning basic cellular mechanisms operative in cell cycle regulation, transcriptional control and apoptosis. The overlap of these mechanisms with those impacted during oncogenesis provides the opportunity to use adenoviruses and adenovirus mutants to characterize the state of key regulatory pathways in specific malignant cells. For example, adenoviruses mediate cytotoxicity after infection of chronic lymphocytic leukemia (CLL) cells, mantle cell lymphoma (MCL) cells and multiple myeloma cell lines. Specific adenovirus mutants demonstrate enhanced cytotoxicity and, in many cases, apoptosis is not the primary mechanism of cell death. Analysis of these infections with respect to both the features of the primary malignant cell and the mechanisms of adenovirus-mediated cytotoxicity holds the prospect of providing novel insights into the status of key regulatory pathways in individual patient malignant cells. These studies also hold the prospect of supporting the development of specific attenuated adenoviruses as therapeutic agents with selective cytotoxicity for specific primary lymphoid malignancies.

Degradation of p53 by adenovirus E4orf6 and E1B55K proteins occurs via a novel mechanism involving a Cullin-containing complex

Although MDM2 plays a major role in regulating the stability of the p53 tumor suppressor protein, other poorly understood MDM2-independent pathways also exist. Human adenoviruses have evolved strategies to regulate p53 function and stability to permit efficient viral replication. One mechanism involves adenovirus E1B55K and E4orf6 proteins, which collaborate to target p53 for degradation. To determine the mechanism of this process, a multiprotein E4orf6-associated complex was purified and shown to contain a novel Cullin-containing E3 ubiquitin ligase that is (1) composed of Cullin family member Cul5, Elongins B and C, and the RING-H2 finger protein Rbx1(ROC1); (2) remarkably similar to the von Hippel-Lindau tumor suppressor and SCF (Skp1-Cul1/Cdc53-F-box) E3 ubiquitin ligase complexes; and (3) capable of stimulating ubiquitination of p53 in vitro in the presence of E1/E2 ubiquitin-activating and -conjugating enzymes. Cullins are activated by NEDD8 modification; therefore, to determine whether Cullin complexes are required for adenovirus-induced p53 degradation, studies were conducted in ts41 Chinese hamster ovary cells that are temperature sensitive for the NEDD8 pathway. E4orf6/E1B55K failed to induce the degradation of p53 at the nonpermissive temperature. Thus, our results identify a novel role for the Cullin-based machinery in regulation of p53.

Antiviral actions of interferons

Tremendous progress has been made in understanding the molecular basis of the antiviral actions of interferons (IFNs), as well as strategies evolved by viruses to antagonize the actions of IFNs. Furthermore, advances made while elucidating the IFN system have contributed significantly to our understanding in multiple areas of virology and molecular cell biology, ranging from pathways of signal transduction to the biochemical mechanisms of transcriptional and translational control to the molecular basis of viral pathogenesis. IFNs are approved therapeutics and have moved from the basic research laboratory to the clinic. Among the IFN-induced proteins important in the antiviral actions of IFNs are the RNA-dependent protein kinase (PKR), the 2',5'-oligoadenylate synthetase (OAS) and RNase L, and the Mx protein GTPases. Double-stranded RNA plays a central role in modulating protein phosphorylation and RNA degradation catalyzed by the IFN-inducible PKR kinase and the 2'-5'-oligoadenylate-dependent RNase L, respectively, and also in RNA editing by the IFN-inducible RNA-specific adenosine deaminase (ADAR1). IFN also induces a form of inducible nitric oxide synthase (iNOS2) and the major histocompatibility complex class I and II proteins, all of which play important roles in immune response to infections. Several additional genes whose expression profiles are altered in response to IFN treatment and virus infection have been identified by microarray analyses. The availability of cDNA and genomic clones for many of the components of the IFN system, including IFN-alpha, IFN-beta, and IFN-gamma, their receptors, Jak and Stat and IRF signal transduction components, and proteins such as PKR, 2',5'-OAS, Mx, and ADAR, whose expression is regulated by IFNs, has permitted the generation of mutant proteins, cells that overexpress different forms of the proteins, and animals in which their expression has been disrupted by targeted gene disruption. The use of these IFN system reagents, both in cell culture and in whole animals, continues to provide important contributions to our understanding of the virus-host interaction and cellular antiviral response.

SUMO-1 modification required for transformation by adenovirus type 5 early region 1B 55-kDa oncoprotein

SUMO-1 is a small ubiquitin-related modifier protein that is covalently linked to many cellular and viral protein targets. Modification by SUMO-1 is proposed to play a role in protein targeting and/or stability. We show here that adenovirus type 5 early region 1B 55-kDa (E1B-55kDa) oncoprotein can be covalently modified by SUMO-1 in vivo through a major attachment site comprising a single lysine residue at amino acid position 104. The sequence surrounding this lysine matches the proposed PsiKxE consensus motif required for SUMO-1 conjugation. A single mutation (K104R) that abolishes SUMOylation of E1B-55kDa dramatically reduces the ability of the adenovirus type 5 protein to transform primary baby rat kidney cells in cooperation with E1A and to inhibit p53-mediated transactivation. Overexpression of SUMO-1 in adenovirus type 5 E1A/E1B-55kDa-transformed baby rat kidney cells causes the relocalization of E1B-55kDa from the cytoplasm to the nucleus, where it accumulates with SUMO-1 in dot- or track-like structures. Significantly, when SUMO-1 is ectopically expressed in transformed rat cells no effect on the cytoplasmic localization of the E1B-K104R mutant protein is observed. Our results demonstrate that SUMO-1 modification is required for transformation by adenovirus type 5 E1B-55kDa and provide further evidence for the idea that this posttranslational modification plays a role in protein targeting to specific subcellular sites.

Identification of three functions of the adenovirus e4orf6 protein that mediate p53 degradation by the E4orf6-E1B55K complex

Complexes containing adenovirus E4orf6 and E1B55K proteins play critical roles in productive infection. Both proteins interact directly with the cellular tumor suppressor p53, and in combination they promote its rapid degradation. To examine the mechanism of this process, degradation of exogenously expressed p53 was analyzed in p53-null human cells infected with adenovirus vectors encoding E4orf6 and/or E1B55K. Coexpression of E4orf6 and E1B55K greatly reduced both the level and the half-life of wild-type p53. No effect was observed with the p53-related p73 proteins, which did not appear to interact with E4orf6 or E1B55K. Mutant forms of p53 were not degraded if they could not efficiently bind E1B55K, suggesting that direct interaction between p53 and E1B55K may be required. Degradation of p53 was independent of both MDM2 and p19ARF, regulators of p53 stability in mammalian cells, but required an extended region of E4orf6 from residues 44 to 274, which appeared to possess three separate biological functions. First, residues 39 to 107 were necessary to interact with E1B55K. Second, an overlapping region from about residues 44 to 218 corresponded to the ability of E4orf6 to form complexes with cellular proteins of 19 and 14 kDa. Third, the nuclear retention signal/amphipathic arginine-rich alpha-helical region from residues 239 to 253 was required. Interestingly, neither the E4orf6 nuclear localization signal nor the nuclear export signal was essential. These results suggested that if nuclear-cytoplasmic shuttling is involved in this process, it must involve another export signal. Degradation was significantly blocked by the 26S proteasome inhibitor MG132, but unlike the HPV E6 protein, E4orf6 and E1B55K were unable to induce p53 degradation in vitro in reticulocyte lysates. Thus, this study implies that the E4orf6-E1B55K complex may direct p53 for degradation by a novel mechanism.

Chemotherapeutic drug, adriamycin, restores the function of p53 protein in hepatitis B virus X (HBx) protein-expressing liver cells

Hepatitis B virus X (HBx) protein implicated in the development of liver cancer may inhibit the function of p53 tumor suppressor protein through cytoplasmic retention of p53 protein. Here, we attempt to investigate whether the functional inhibition of p53 protein by HBx protein is reversible. First, we provide the evidence for the association of endogenous p53 protein with HBx by co-immunoprecipitation in stable Chang cells that express HBx protein in an inducible manner (ChangX-34). By immunofluorescence microscopy, the major location of p53 protein of ChangX-34 cells was confirmed at the nuclear periphery as well as in the cytoplasm where HBx protein is mainly expressed. Surprisingly, anticancer drug, adriamycin induces the nuclear translocation of p53 protein sequestered in the cytoplasm. This change is accompanied by the restoration of p53 activity, which results in increased transcriptional activity at the p53-responsive DNA elements as well as increase of p21WAF1 mRNA expression. Further, we observed the induction of cell death and G1 arrest in these cells upon adriamycin treatment regardless of HBx expression. Together, we demonstrate that functional inhibition of p53 protein through its cytoplasmic retention by HBx protein is reversible. These results may be extended into other tumors of which p53 activity is modulated by viral oncoproteins.

Adenovirus E1B 55-kilodalton oncoprotein inhibits p53 acetylation by PCAF

The adenovirus E1B 55-kDa protein binds to cellular tumor suppressor p53 and inactivates its transcriptional transactivation function. p53 transactivation activity is dependent upon its ability to bind to specific DNA sequences near the promoters of its target genes. It was shown recently that p53 is acetylated by transcriptional coactivators p300, CREB bidning protein (CBP), and PCAF and that acetylation of p53 by these proteins enhances p53 sequence-specific DNA binding. Here we show that the E1B 55-kDa protein specifically inhibits p53 acetylation by PCAF in vivo and in vitro, while acetylation of histones and PCAF autoacetylation is not affected. Furthermore, the DNA-binding activity of p53 is diminished in cells expressing the E1B 55-kDa protein. PCAF binds to the E1B 55-kDa protein and to a region near the C terminus of p53 encompassing Lys-320, the specific PCAF acetylation site. We further show that the E1B 55-kDa protein interferes with the physical interaction between PCAF and p53, suggesting that the E1B 55-kDa protein inhibits PCAF acetylase function on p53 by preventing enzyme-substrate interaction. These results underscore the importance of p53 acetylation for its function and suggest that inhibition of p53 acetylation by viral oncoproteins prevent its activation, thereby contributing to viral transformation.

Ras regulates sympathetic neuron survival by suppressing the p53-mediated cell death pathway

In this report, we examine how the Ras protein regulates neuronal survival, focusing on sympathetic neurons. Adenovirus-expressed constitutively activated Ras (RasV12) enhanced survival and the phosphorylation of Akt (protein kinase B) and MAP kinase (MAPK), two targets of Ras activity. Functional inhibition of endogenous Ras by adenovirus-expressed dominant-inhibitory Ras (N17Ras) decreased nerve growth factor (NGF)-dependent survival and both Akt and MAPK phosphorylation as well. To determine the signaling pathways through which Ras mediates survival, we used Ras effector mutants and pharmacological inhibitors that selectively suppress phosphatidylinositol 3-kinase (PI3-K)/Akt or MAP kinase kinase (MEK)/MAPK pathways. The Ras effector mutant Ras(V12)Y40C, which selectively stimulates PI3-K and Akt, rescued survival in the absence of NGF, and the PI3-K inhibitor LY 294002 inhibited both Ras- and NGF-dependent survival. Ras(V12)T(35)S, which activates MEK/MAPK but not PI3-K/Akt, was less effective at rescuing survival, whereas the MEK inhibitor PD 098059 also partially suppressed Ras-dependent survival. To investigate the mechanisms by which Ras suppresses neuronal death, we examined whether Ras functions by inhibiting the proapoptotic p53 pathway (Jun-N-terminal kinase/p53/BAX) that is necessary for neuronal death after NGF withdrawal and p75NTR activation. We found that RasV12 suppressed c-jun, BAX, and p53 levels, whereas inhibition of NGF-induced Ras-survival activity via N17Ras increased the levels of these proteins. Furthermore, the E1B55K protein, which suppresses p53 activity, blocked N17Ras-induced neuronal death. Together, these results indicate that Ras is, in part, both necessary and sufficient for survival of sympathetic neurons and that this effect is mediated by activation of both the PI3-K- and MEK-signaling cascades, which in turn suppress a proapoptotic p53 pathway.

Ras regulates sympathetic neuron survival by suppressing the p53-mediated cell death pathway

In this report, we examine how the Ras protein regulates neuronal survival, focusing on sympathetic neurons. Adenovirus-expressed constitutively activated Ras (RasV12) enhanced survival and the phosphorylation of Akt (protein kinase B) and MAP kinase (MAPK), two targets of Ras activity. Functional inhibition of endogenous Ras by adenovirus-expressed dominant-inhibitory Ras (N17Ras) decreased nerve growth factor (NGF)-dependent survival and both Akt and MAPK phosphorylation as well. To determine the signaling pathways through which Ras mediates survival, we used Ras effector mutants and pharmacological inhibitors that selectively suppress phosphatidylinositol 3-kinase (PI3-K)/Akt or MAP kinase kinase (MEK)/MAPK pathways. The Ras effector mutant Ras(V12)Y40C, which selectively stimulates PI3-K and Akt, rescued survival in the absence of NGF, and the PI3-K inhibitor LY 294002 inhibited both Ras- and NGF-dependent survival. Ras(V12)T(35)S, which activates MEK/MAPK but not PI3-K/Akt, was less effective at rescuing survival, whereas the MEK inhibitor PD 098059 also partially suppressed Ras-dependent survival. To investigate the mechanisms by which Ras suppresses neuronal death, we examined whether Ras functions by inhibiting the proapoptotic p53 pathway (Jun-N-terminal kinase/p53/BAX) that is necessary for neuronal death after NGF withdrawal and p75NTR activation. We found that RasV12 suppressed c-jun, BAX, and p53 levels, whereas inhibition of NGF-induced Ras-survival activity via N17Ras increased the levels of these proteins. Furthermore, the E1B55K protein, which suppresses p53 activity, blocked N17Ras-induced neuronal death. Together, these results indicate that Ras is, in part, both necessary and sufficient for survival of sympathetic neurons and that this effect is mediated by activation of both the PI3-K- and MEK-signaling cascades, which in turn suppress a proapoptotic p53 pathway.

Viruses and apoptosis

Successful viral replication requires not only the efficient production and spread of progeny, but also evasion of host defense mechanisms that limit replication by killing infected cells. In addition to inducing immune and inflammatory responses, infection by most viruses triggers apoptosis or programmed cell death of the infected cell. This cell response often results as a compulsory or unavoidable by-product of the action of critical viral replicative functions. In addition, some viruses seem to use apoptosis as a mechanism of cell killing and virus spread. In both cases, successful replication relies on the ability of certain viral products to block or delay apoptosis until sufficient progeny have been produced. Such proteins target a variety of strategic points in the apoptotic pathway. In this review we summarize the great amount of recent information on viruses and apoptosis and offer insights into how this knowledge may be used for future research and novel therapies.

RBP1 recruits both histone deacetylase-dependent and -independent repression activities to retinoblastoma family proteins

Retinoblastoma (RB) tumor suppressor family proteins block cell proliferation in part by repressing certain E2F-specific promoters. Both histone deacetylase (HDAC)-dependent and -independent repression activities are associated with the RB "pocket." The mechanism by which these two repression functions occupy the pocket is unknown. A known RB-binding protein, RBP1, was previously found by our group to be an active corepressor which, if overexpressed, represses E2F-mediated transcription via its association with the pocket. We show here that RBP1 contains two repression domains, one of which binds all three known HDACs and represses them in an HDAC-dependent manner while the other domain functions independently of the HDACs. Thus, RB family members repress transcription by recruiting RBP1 to the pocket. RBP1, in turn, serves as a bridging molecule to recruit HDACs and, in addition, provides a second HDAC-independent repression function.

Corepressor required for adenovirus E1B 55,000-molecular-weight protein repression of basal transcription

Adenovirus E1B 55,000-molecular-weight protein (55K) binds to host cell p53, stabilizing it, greatly increasing its affinity for its cognate DNA-binding site, and converting it from a regulated activator to a constitutive repressor. Here we analyzed the mechanism of repression by the p53-E1B 55K complex. E1B 55K repression requires that 55K be tethered to the promoter by binding directly to DNA-bound p53. Transcription from an assembled, p53-activated preinitiation complex was not repressed by the subsequent addition of E1B 55K, suggesting that either sites of 55K interaction with p53 or targets of 55K in the preinitiation complex are blocked. Specific E1B 55K repression was observed in reactions lacking TFIIA and with recombinant TATA-binding protein in place of TFIID, conditions under which p53 does not activate transcription. Thus, E1B 55K does not simply inhibit a p53-specific activation mechanism but rather blocks basal transcription. As a consequence, E1B 55K may repress transcription from any promoter with an associated p53-binding site, no matter what other activators associate with the promoter. E1B 55K did not repress basal transcription in reactions with recombinant and highly purified general transcription factors and RNA polymerase II but rather required a corepressor that copurifies with the polymerase.

RBP1 induces growth arrest by repression of E2F-dependent transcription

Growth arrest and cell cycle progression are regulated by the retinoblastoma tumour suppressor pRB and related proteins p130 and p107 that bind to and inhibit the E2F family of transcription factors. Although the precise mechanism of this inhibition remains to be established, previous studies indicated the presence of transcriptional repression activity in the 'pocket' of RB family members. We show here that RBP1, a known pRB pocket-binding protein, possesses transcriptional repression activity and associates with p130-E2F and pRB-E2F complexes specifically during growth arrest. Overexpression of RBP1 both inhibited E2F-dependent gene expression and suppressed cell growth. Thus repression of E2F-dependent transcription by RBP1 via RB family members may play a central role in inducing growth arrest.

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.

Allosteric regulation of even-skipped repression activity by phosphorylation

The Drosophila homeodomain protein Even-skipped (Eve) is a well characterized transcriptional repressor. Here, we show that Eve's ability to function in vitro is negatively regulated by phosphorylation. DNA-binding activity was unaffected by phosphorylation, but phosphorylated Eve was unable to interact with the TATA-binding protein (TBP), a known target for repression. Unexpectedly, phosphorylation of the Eve N terminus, which is dispensable for repression and TBP binding, was necessary and sufficient to inactivate Eve. LiCl, which specifically inhibits glycogen synthase kinase-3 (GSK-3), reduced Eve phosphorylation in nuclear extract and blocked inhibition of repression. In addition, Eve was phosphorylated and inactivated by purified GSK-3 beta plus casein kinase II. Our results suggest a novel mechanism of transcriptional control involving phosphorylation-induced allosteric interference with a repressive protein-protein interaction.

p53 is essential for developmental neuron death as regulated by the TrkA and p75 neurotrophin receptors

Naturally occurring sympathetic neuron death is the result of two apoptotic signaling events: one normally suppressed by NGF/TrkA survival signals, and a second activated by the p75 neurotrophin receptor. Here we demonstrate that the p53 tumor suppressor protein, likely as induced by the MEKK-JNK pathway, is an essential component of both of these apoptotic signaling cascades. In cultured neonatal sympathetic neurons, p53 protein levels are elevated in response to both NGF withdrawal and p75NTR activation. NGF withdrawal also results in elevation of a known p53 target, the apoptotic protein Bax. Functional ablation of p53 using the adenovirus E1B55K protein inhibits neuronal apoptosis as induced by either NGF withdrawal or p75 activation. Direct stimulation of the MEKK-JNK pathway using activated MEKK1 has similar effects; p53 and Bax are increased and the subsequent neuronal apoptosis can be rescued by E1B55K. Expression of p53 in sympathetic neurons indicates that p53 functions downstream of JNK and upstream of Bax. Finally, when p53 levels are reduced or absent in p53+/- or p53-/- mice, naturally occurring sympathetic neuron death is inhibited. Thus, p53 is an essential common component of two receptor-mediated signal transduction cascades that converge on the MEKK-JNK pathway to regulate the developmental death of sympathetic neurons.

Phosphorylation of p53 serine 15 increases interaction with CBP

p53 exerts its cell cycle regulatory effects through its ability to function as a sequence-specific DNA binding transcription factor. CREB-binding protein (CBP)/p300, through its interaction with the N terminus of p53, acts as a coactivator for p53 and increases the sequence-specific DNA-binding activity of p53 by acetylating its C terminus. The same N-terminal domain of p53 has recently been shown to be phosphorylated at Ser15 in response to gamma-irradiation. Remarkably, we now demonstrate that phosphorylation of p53 at Ser15 increases its ability to recruit CBP/p300. The increase in CBP/p300 binding was followed by an increase in the overall level of acetylation of the C terminus of p53. These results provide a mechanism for the activation of p53-regulated genes following DNA damage, through a signaling pathway linking p53 N-terminal kinase and C-terminal acetyltransferase activities.

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.

Adenovirus E1B 55K represses p53 activation in vitro

Adenovirus E1B 55K protein cooperates with E1A gene products to induce cell transformation. E1B 55K mediates its effects by binding to and inhibiting the transcriptional activation and growth-suppression functions of the tumor suppressor p53. Previous studies in vivo have suggested that E1B 55K has an active role in repressing p53 transcriptional activation and that this repression function is directed to specific promoters through E1B 55K's interaction with DNA-bound p53. Flag-tagged E1B 55K (e55K) was expressed with the baculovirus expression system and immunopurified. Gel filtration, velocity sedimentation centrifugation, and glutaraldehyde cross-linking indicated that e55K is a dimer with a nonglobular conformation. e55K bound directly to purified p53, causing an approximately 10-fold increase in p53 affinity for tandem binding sites. Using in vitro transcription assays reconstituted with purified p53, e55K, and HeLa cell nuclear extracts, we found that e55K specifically repressed p53 activation. These results demonstrate that as postulated from earlier transient expression experiments, E1B 55K is a specific repressor of transcription from a promoter with bound p53. Since HeLa nuclear extracts contain little detectable histone protein, E1B 55K probably represses transcription through direct or indirect interactions with the RNA polymerase II transcription machinery.

Li-Fraumeni syndrome and the role of the p53 tumor suppressor gene in cancer susceptibility

Mutation of the tumor suppressor gene p53 is a molecular genetic event frequently observed in human cancer, and inactivating missense mutations usually are accompanied by the resultant overexpression of mutant p53 protein. In gynecologic cancers, p53 is also often altered; the frequency varies depending on types of cancers and where they develop. Further, human papillomavirus oncoproteins that inactivate p53 and Rb proteins play important roles in the development of several gynecologic cancers. Individuals who are heterozygous for germline mutations of the p53 gene are strongly predisposed to a variety of cancers. The identification of these individuals may have profound value in the future when therapies or chemopreventive agents specific for the p53 alteration are available. The role of p53 tumor suppressor gene in gynecologic cancers and heritable cancer susceptibility syndromes including Li-Fraumeni and Lynch II syndromes is an active and important area of study.

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.

Regulation of p53 levels by the E1B 55-kilodalton protein and E4orf6 in adenovirus-infected cells

The adenovirus type 5 243R E1A protein induces p53-dependent apoptosis in the absence of the 19- and 55-kDa E1B polypeptides. This effect appears to result from an accumulation of p53 protein and is unrelated to expression of E1B products. We now report that in the presence of the E1B 55-kDa polypeptide, the 289R E1A protein does not induce such p53 accumulation and, in fact, is able to block that induced by E1A 243R. This inhibition also requires the 289R-dependent transactivation of E4orf6 expression. E4orf6 is known to form complexes with the E1B 55-kDa protein and to function both in the transport and stabilization of viral mRNA and in shutoff of host cell protein synthesis. We demonstrated that the block in p53 accumulation is not due to the generalized shutoff of host cell metabolism. Rather, it appears to result from a mechanism targeted specifically to p53, most likely involving a decrease in the stability of p53 protein. The E1B 55-kDa protein is known to interact with both E4orf6 and p53, and as demonstrated recently by others, we showed that E4orf6 also binds directly to p53. Thus, multiple interactions between all three proteins may regulate p53 stability, resulting in the maintenance of low levels of p53 following virus infection.