Functions of the two adenovirus early E1A proteins and their conserved domains in cell cycle alteration, actin reorganization, and gene activation in rat cells

Cancer Treatment Goes Viral: Using Viral Proteins to Induce Tumour-Specific Cell Death

Cell death is a tightly regulated process which can be exploited in cancer treatment to drive the killing of the tumour. Several conventional cancer therapies including chemotherapeutic agents target pathways involved in cell death, yet they often fail due to the lack of selectivity they have for tumour cells over healthy cells. Over the past decade, research has demonstrated the existence of numerous proteins which have an intrinsic tumour-specific toxicity, several of which originate from viruses. These tumour-selective viral proteins, although from distinct backgrounds, have several similar and interesting properties. Though the mechanism(s) of action of these proteins are not fully understood, it is possible that they can manipulate several cell death modes in cancer exemplifying the intricate interplay between these pathways. This review will discuss our current knowledge on the topic and outstanding questions, as well as deliberate the potential for viral proteins to progress into the clinic as successful cancer therapeutics.

Dedifferentiation by adenovirus E1A due to inactivation of Hippo pathway effectors YAP and TAZ

In this study, Zemke et al. show that E1A inactivates the Hippo pathway-regulated TEAD coactivators YAP and TAZ by causing their sequestration in the cytoplasm. Their findings suggest that YAP/TAZ function in a developmental checkpoint controlled by signaling from the actin cytoskeleton that prevents differentiation of a progenitor cell until it is in the correct cellular and tissue environment.

Adenovirus transformed cells have a dedifferentiated phenotype. Eliminating E1A in transformed human embryonic kidney cells derepressed ∼2600 genes, generating a gene expression profile closely resembling mesenchymal stem cells (MSCs). This was associated with a dramatic change in cell morphology from one with scant cytoplasm and a globular nucleus to one with increased cytoplasm, extensive actin stress fibers, and actomyosin-dependent flattening against the substratum. E1A-induced hypoacetylation at histone H3 Lys27 and Lys18 (H3K27/18) was reversed. Most of the increase in H3K27/18ac was in enhancers near TEAD transcription factors bound by Hippo signaling-regulated coactivators YAP and TAZ. E1A causes YAP/TAZ cytoplasmic sequestration. After eliminating E1A, YAP/TAZ were transported into nuclei, where they associated with poised enhancers with DNA-bound TEAD4 and H3K4me1. This activation of YAP/TAZ required RHO family GTPase signaling and caused histone acetylation by p300/CBP, chromatin remodeling, and cohesin loading to establish MSC-associated enhancers and then superenhancers. Consistent results were also observed in primary rat embryo kidney cells, human fibroblasts, and human respiratory tract epithelial cells. These results together with earlier studies suggest that YAP/TAZ function in a developmental checkpoint controlled by signaling from the actin cytoskeleton that prevents differentiation of a progenitor cell until it is in the correct cellular and tissue environment.

Subversion of the actin cytoskeleton during viral infection

Viral infection converts the normal functions of a cell to optimize viral replication and virion production. One striking observation of this conversion is the reconfiguration and reorganization of cellular actin, affecting every stage of the viral life cycle, from entry through assembly to egress. The extent and degree of cytoskeletal reorganization varies among different viral infections, suggesting the evolution of myriad viral strategies. In this Review, we describe how the interaction of viral proteins with the cell modulates the structure and function of the actin cytoskeleton to initiate, sustain and spread infections. The molecular biology of such interactions continues to engage virologists in their quest to understand viral replication and informs cell biologists about the role of the cytoskeleton in the uninfected cell.

Adenovirus type 5 exerts multiple effects on the expression and activity of cytosolic phospholipase A2, cyclooxygenase-2, and prostaglandin synthesis

In this study, we examine how infection of murine and human fibroblasts by adenovirus (Ad) serotype 5 (Ad5) affects the expression and activity of cytosolic phospholipase A2 (cPLA2), cyclooxygenase-2 (COX-2), and production of PGs. Our experiments showed that infection with Ad5 is accompanied by the rapid activation of cPLA2 and the cPLA2-dependent release of [3H]arachidonic acid ([3H]AA). Increased expression of COX-2 was also observed after Ad infection, as was production of PGE2 and PGI2. Later, however, as the infection progressed, release of [3H]AA and production of PGs stopped. Late-stage Ad5-infected cells also did not release [3H]AA or PGs following treatment with a panel of biologically diverse agents. Experiments with UV-inactivated virus confirmed that Ad infection is accompanied by the activation of a host-dependent response that is later inhibited by the virus. Investigations of the mechanism of suppression of the PG pathway by Ad5 did not reveal major effects on the expression or activity of cPLA2 or COX-2. We did note a change in the intracellular position of cPLA2 and found that cPLA2 did not translocate normally in infected cells, raising the possibility that Ad5 interferes with the PG pathway by interfering with the intracellular movement of cPLA2. Taken together, these data reveal dynamic interactions between Ad5 and the lipid mediator pathways of the host and highlight a novel mechanism by which Ad5 evades the host immune response. In addition, our results offer insight into the inflammatory response induced by many Ad vectors lacking early region gene products.

Effect of hypoxia on Ad5 infection, transgene expression and replication

Oxygen deprivation (hypoxia) is a common feature of various human maladies, including cardiovascular diseases and cancer; however, the effect of hypoxia on Ad-based gene therapies has not been described. In this study, we evaluated how hypoxia (1% pO(2)) affects different aspects of Ad-based therapies, including attachment and uptake, transgene expression, and replication, in a series of cancer cell lines and primary normal cells. We found that hypoxia had no significant effect on the expression or function of the Ad5 attachment (Coxsackievirus and Adenovirus Receptor) and internalization (alpha(v) integrins) proteins, nor on the human cytomegalovirus-driven expression of an exogenous gene carried by a replication-incompetent Ad. Viral replication, however, was compromised by hypoxic conditions. Our studies revealed hypoxia-induced reductions in E1A levels that were mediated at the post-transcriptional level. E1A drives cells into the viral replication optimal S phase of the cell cycle; consequently, the combination of reduced E1A protein and hypoxia-induced G1 arrest of cells may be responsible for the lack of efficient viral replication under hypoxic conditions. Consequently, while traditional replication-incompetent Ad-based vectors appear to be viable delivery systems for hypoxia-associated disease indications, our studies suggest that Oncolytic Ads may need additional factors to efficiently treat hypoxic regions of human tumors.

G-protein-coupled receptor activation induces the membrane translocation and activation of phosphatidylinositol-4-phosphate 5-kinase I alpha by a Rac- and Rho-dependent pathway

Phosphatidylinositol 4,5-bisphosphate (PI4,5P(2)) mediates cell motility and changes in cell shape in response to extracellular stimuli. In platelets, it is synthesized from PI4P by PIP5K in response to stimulation of a G-protein-coupled receptor by an agonist, such as the thrombin. In the present study, we have addressed the pathway that induces PIP5K I alpha activation following the addition of thrombin. Under resting condition expressed PIP5K I alpha was predominantly localized in a perinuclear distribution. After stimulation of the thrombin receptor, PAR1, or overexpression of a constitutively active variant of G alpha(q), PIP5K I alpha translocated to the plasma membrane. Movement of PIP5K I alpha to the cell membrane was dependent on both GTP-bound Rac and Rho, but not Arf, because: 1) inactive GDP-bound variants of either Rac or Rho blocked the translocation induced by constitutively active G alpha(q), 2) constitutively GTP-bound active variants of Rac or Rho induced PIP5K I alpha translocation in the absence of other stimuli, and 3) constitutively active variants of Arf1 or Arf6 failed to induce membrane translocation of PIP5K I alpha. In addition, a dominant negative variant of Rho blocked the PIP5K I alpha membrane translocation induced by constitutively active Rac, whereas dominant negative variants of either Rac or Arf6 failed to block PIP5K I alpha membrane translocation induced by constitutively active Rho. This implies that the effect on PIP5K I alpha by Rac is indirect, and requires the activation of Rho. In contrast to the findings with PIP5K I alpha, the related lipid kinase PIP4K failed to undergo translocation after stimulation by small GTP-binding proteins Rac or Rho. We also tested whether membrane localization of PIP5K I alpha correlated with an increase in its lipid kinase activity and found that co-expressing of PIP5K I alpha with either constitutively active G alpha(q), Rac, or Rho led to a 5- to 7-fold increase in PIP5K I alpha activity. Thus, these findings suggest that stimulation of a G-protein-coupled receptor (PAR1) leads to the sequential activation of G alpha(q), Rac, Rho, and PIP5K I alpha. Once activated and translocated to the cell membrane, PIP5K I alpha becomes available to phosphorylate PI4P to generate PI4,5P(2) on the plasma membrane.

Accumulation of p53 induced by the adenovirus E1A protein requires regions involved in the stimulation of DNA synthesis

It has been known for some time that expression of the 243-residue (243R) human adenovirus type 5 (Ad5) early region 1A (E1A) protein causes an increase in the level of the cellular tumor suppressor p53 and induction of p53-dependent apoptosis. Deletion of a portion of conserved region 1 (CR1) had been shown to prevent apoptosis, suggesting that binding of p300 and/or the pRB retinoblastoma tumor suppressor and related proteins might be implicated. To examine the mechanism of the E1A-induced accumulation of p53, cells were infected with viruses expressing E1A-243R containing various deletions which have well-characterized effects on p300 and pRB binding. It was found that in human HeLa cells and rodent cells, complex formation with p300 but not pRB was required for the rise in p53 levels. However, in other human cell lines, including MRC-5 cells, E1A proteins which were able to form complexes with either p300 or pRB induced a significant increase in p53 levels. Only E1A mutants defective in binding both classes of proteins were unable to stimulate p53 accumulation. This same pattern was also apparent in p53-null mouse cells coinfected by Ad5 mutants and an adenovirus vector expressing either wild-type or mutant human p53 under a cytomegalovirus promoter, indicating that the difference in importance of pRB binding may relate to differences between rodent and human p53 expression. The increase in p53 levels correlated well with the induction of apoptosis and, as shown previously, with the stimulation of cellular DNA synthesis. Thus, it is possible that the accumulation of p53 is induced by the induction of unscheduled DNA synthesis by E1A proteins and that increased levels of p53 then activate cell death pathways.

Functional importance of complex formation between the retinoblastoma tumor suppressor family and adenovirus E1A proteins as determined by mutational analysis of E1A conserved region 2

Adenovirus early region 1A (E1A) products induce DNA synthesis, transform primary rodent cells, and activate transcription factor E2F through complex formation with an array of cellular proteins via the E1A amino terminus and conserved regions 1 and 2 (CR1 and CR2). Interactions with the retinoblastoma tumor suppressor, pRb, and related proteins p107 and p130 rely somewhat on CR1 but largely on CR2, which contains a core binding sequence Leu-122-X-Cys-X-Glu. We introduced point mutations in CR2 to define such interactions more precisely. In human cells, alteration of any of the conserved residues within the binding core eliminated complex formation with pRb. Conversion of nonconserved Thr-123 to Pro (but not to either Ala or Ser) disrupted binding of pRb, presumably because of conformational changes in the binding core. No single E1A point mutant was completely defective in binding p107, suggesting that molecular interactions between E1A proteins and p107 clearly differ from those with pRb and p130. In general, the patterns of complex formation by E1A mutants in rat, monkey, and human cells were quite similar. All mutants which failed to bind significant amounts of pRb also failed to transform primary rat cells. Several mutants demonstrated selective binding to pRb, p107, and p130, but transforming activity corresponded largely with complex formation with pRb, regardless of the levels of interactions with p107 and p130. Mutants defective for binding of both pRb and p107 failed to induce the activity of transcription factor E2F; however, quite high levels were activated by E1A mutants that interacted with p107 alone. These results suggested that both pRb and p107 are important regulators of E2F activity but that complex formation with and activation of E2F by p107 are insufficient for cell transformation.

The adenovirus E1A-associated kinase consists of cyclin E-p33cdk2 and cyclin A-p33cdk2

The adenovirus E1A oncoproteins form stable complexes with several cellular proteins. Association of E1A with these proteins has been shown to be important for the oncogenic potential of E1A. Several of these proteins have been identified and include the product of the retinoblastoma gene and a key cell cycle regulatory protein, cyclin A. E1A also associates with a potent histone H1 kinase. The two major components of this activity are the cyclin E-p33cdk2 and cyclin A-p33cdk2 complexes. Both the cyclin E-p33cdk2 and cyclin A-p33cdk2 complexes have been implicated in regulatory events controlling entry into or passage through DNA synthesis. Although the architecture of such interactions remains unclear, it is likely that by targeting such complexes, adenovirus is affecting some aspect of cell cycle control.

Immortalization of primary epithelial cells requires first- and second-exon functions of adenovirus type 5 12S

Immortalization of primary cells is a multistep process. The adenovirus E1A 12S gene product is a member of the class of oncoproteins that have the ability to establish primary cells as cell lines in culture. It is encoded by two exons. Extensive mutational analysis demonstrates that four regions of the E1A 12S gene, encoded by both exons, are necessary for immortalization of primary epithelial cells. Expression of two regions is necessary to activate quiescent cells into the cell cycle but is unable to extend the life span of these cells in culture and thus cannot immortalize them. These regions are encoded by the first exon. A third first-exon region, for which no function has yet been identified, is also required. These three regions are also required for 12S to cooperate with an activated ras gene to bring about tumorigenic transformation. The fourth region is required to maintain the cells in a proliferative mode, extend their life span in culture, and induce an autocrine growth factor. These functions are encoded by the second exon. The cells immortalized by wild-type 12S and immortalization-competent mutants retain their epithelial morphology and expression of keratin and vimentin intermediate filament proteins.

The adenovirus E1A 243R protein purified from Escherichia coli under nondenaturing conditions is found in association with dnaK

The adenovirus E1A 243R protein immortalizes primary cells in culture and induces part of the phenotypes required for transformation. It has also been shown to interact with a number of cellular polypeptides, including the product of the retinoblastoma gene. To understand more fully the molecular activities of the E1A 243R protein in association with these proteins as well as its role in the processes of cellular growth, we have developed a method for rapidly purifying this protein from genetically engineered Escherichia coli under nondenaturing conditions. The plasmid-encoded E1A protein, when expressed in a protease-deficient mutant, is found to have the same length and amino acid sequence as that which is produced in a mammalian cell. The procedure for purifying the E1A 243R protein from bacteria relies primarily upon immunoaffinity chromatography and the use of a peptide comprising the epitope recognized by an E1A-specific antibody. Elution of the E1A protein under this condition allows for gentle isolation and a purity that ranges from 90 to 96%. However, without the addition of micromolar amounts of ATP prior to its elution from the antibody column, the E1A protein is found in association with an E. coli protein of 70 kDa. Immunoblot analysis with a specific antibody showed that this bacterial protein was the heat shock protein dnaK, which is known to have extensive homology with the hsp-hsc70 family of proteins in mammalian cells. Recognition of E1A by the dnaK protein may very well reflect a situation that also occurs between the mammalian heat shock proteins and the E1A 243R protein after adenovirus infection.

Effects of Ad5 E1A mutant viruses on the cell cycle in relation to the binding of cellular proteins including the retinoblastoma protein and cyclin A

We have examined the ability of Ad5 E1A 12S viruses with deletions in E1A exon 1 to induce quiescent baby rat kidney cells to progress through the cell cycle and to undergo mitosis. Measurements of mitotic index and analyses by fluorescence activated cell sorting were correlated with the abilities of the mutant E1A proteins to bind to cellular proteins. All the mutants induced cells to leave G0/G1 and enter S phase, but two groups were defective at inducing mitosis, and cells infected with them appeared to be blocked between the S and M phases. The first group of mutants, with deletions in the regions of residues 4-25 and 30-60, bound p300 poorly or not at all and gave reduced numbers of mitoses. The second group, with deletions between residues 111 and 138 in CR2, failed to bind pRb and were completely defective at inducing mitosis. In this group, mutants lacking residues between 124 and 138 bound p107 and cyclin A at much reduced levels and induced cells to overreplicate their DNA. The site in E1A required to bind cyclin A extends from residue 124 to at least 127. Cyclin A binds to a 107-kDa cellular protein, which by peptide analysis appears identical to p107.

A protein kinase is present in a complex with adenovirus E1A proteins

A kinase activity can be immunoprecipitated in a complex that includes adenovirus E1A proteins. In vitro, this activity phosphorylated other E1A-associated proteins, as well as added E1A and histone H1 proteins. The E1A-associated kinase activity was cleared from extracts with an antibody to cyclin A, but not with antibody to cyclin B. The formation of a complex that included the kinase activity required amino acids 30-60 and 122-129 on the E1A proteins, sequences needed for association of E1A proteins with cyclin A and the retinoblastoma protein and implicated in control of cell growth. The complex of E1A-associated proteins included a 33-kDa ATP-binding protein, similar in size to a cyclin A-associated cdc2 kinase family member. Sucrose gradient analysis revealed two distinct E1A-containing complexes with the kinase activity. We suggest that E1A proteins may affect cellular proliferation by interacting with a member of the cdc2 kinase family and thereby influencing its activity.

The amino-terminal portion of CD1 of the adenovirus E1A proteins is required to induce susceptibility to tumor necrosis factor cytolysis in adenovirus-infected mouse cells

Previous work by our laboratory and others has shown that mouse cells normally resistant to tumor necrosis factor can be made sensitive to the cytokine by the expression of adenovirus E1A. The E1A gene can be introduced by either infection or transfection, and either of the two major E1A proteins, 289R or 243R, can induce this sensitivity. The E1A proteins are multifunctional and modular, with specific domains associated with specific functions. Here, we report that the CD1 domain of E1A is required to induce susceptibility to tumor necrosis factor cytolysis in adenovirus-infected mouse C3HA fibroblasts. Amino acids C terminal to residue 60 and N terminal to residue 36 are not necessary for this function. This conclusion is based on 51Cr-release assays for cytolysis in cells infected with adenovirus mutants with deletions in various portions of E1A. These E1A mutants are all in an H5dl309 background and therefore they lack the tumor necrosis factor protection function provided by the 14.7-kilodalton (14.7K) protein encoded by region E3. Western blot (immunoblot) analysis indicated that most of the mutant E1A proteins were stable in infected C3HA cells, although with certain large deletions the E1A proteins were unstable. The region between residues 36 and 60 is included within but does not precisely correlate with domains in E1A that have been implicated in nuclear localization, enhancer repression, cellular immortalization, cell transformation in cooperation with ras, induction of cellular DNA synthesis and proliferation, induction of DNA degradation, and binding to the 300K protein and the 105K retinoblastoma protein.

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

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

Transactivation of the p53 oncogene by E1a gene products

Infection of quiescent rat kidney cells with human adenovirus is shown to transcriptionally stimulate (transactivate) the p53 oncogene. The increased transcription results in an accumulation of p53-specific mRNA in parallel with an increase in p53 protein levels, although there is a considerable delay between transcriptional activation and the detection of stable p53 mRNA and protein. The induction of p53 is detectable with two monoclonal antibodies recognizing different epitopes. The induction of p53 by adenovirus is delayed compared to induction by serum, and it occurs after the onset of adenovirus-induced cellular DNA replication. Thus, adenovirus-induced DNA replication bypasses a G0/G1 control point. Experiments with hydroxyurea show that p53 activation does not require continued cell cycling and thus is likely to be a direct consequence of viral gene expression. Finally, the induction of p53 is shown to be dependent on expression of the 289-residue product encoded by the viral E1a gene.