Late viral RNA export, rather than p53 inactivation, determines ONYX-015 tumor selectivity

GC-selective DNA-binding antibiotic, mithramycin A, reveals multiple points of control in the regulation of Hdm2 protein synthesis

The primary role of the Hdm2/Mdm2 oncoprotein is to regulate the levels and activity of the transcription factor p53. Hdm2 synthesis is itself tightly controlled and, as demonstrated by a recently described SNP (SNP309) in the hdm2-P2 promoter, minor variations in Hdm2 expression have phenotypic consequences on radiation sensitivity and cancer predisposition. To further define mechanisms regulating Hdm2 expression, we have investigated the effects of the GC-selective DNA-binding drug, Mithramycin A (MA) on hdm2 mRNA transcription, trafficking, and translation. Firstly we show that the constitutive hdm2-P1 promoter is inhibited by MA. We define, for the first time, the minimal sequence elements that are required for P1-promoter activity and identify those which confer MA sensitivity. Secondly, MA induces p53-dependent transcription from the hdm2-P2 promoter. Thirdly, and critically, MA also inhibits Hdm2 synthesis at the post-transcriptional level, with negative effects on hdm2 mRNA nuclear export and translation. This study highlights the complex interplay between the pathways that regulate Hdm2 protein synthesis in cancer cells, and furthermore emphasizes the export of hdm2 mRNA from the nucleus to the cytoplasm as a key point of control in this process.

Genetic identification of adenovirus type 5 genes that influence viral spread

The mechanisms that control cell-to-cell spread of human adenoviruses (Ad) are not well understood. Two early viral proteins, E1B-19K and E3-ADP, appear to have opposing effects since viral mutants that are individually deficient in E1B-19K produce large plaques (G. Chinnadurai, Cell 33:759-766, 1983), while mutants deficient in E3-ADP produce small plaques (A. E. Tollefson et al., J. Virol. 70:2296-2306, 1996) on infected cell monolayers. We have used a genetic strategy to identify different viral genes that influence adenovirus type 5 (Ad5) spread in an epithelial cancer cell line. An Ad5 mutant (dl327; lacking most of the E3 region) with the restricted-spread (small-plaque) phenotype was randomly mutagenized with UV, and 27 large-plaque (lp) mutants were isolated. A combination of analyses of viral proteins and genomic DNA sequences have indicated that 23 mutants contained lesions in the E1B region affecting either 19K or both 19K and 55K proteins. Four other lp mutants contained lesions in early regions E1A and E4, in the early L1 region that codes for the i-leader protein, and in late regions that code for the viral structural proteins, penton base, and fiber. Our results suggest that the requirement of E3-ADP for Ad spread could be readily compensated for by abrogation of the functions of E1B-19K and provide genetic evidence that these two viral proteins influence viral spread in opposing manners. In addition to E1B and E3 proteins, other early and late proteins that regulate viral replication and infectivity also influence lateral viral spread. Our studies have identified novel mutations that could be exploited in designing efficient oncolytic Ad vectors.

Activity of the adenoviral E1A deletion mutant dl922-947 in ovarian cancer: comparison with E1A wild-type viruses, bioluminescence monitoring, and intraperitoneal delivery in icodextrin

The adenoviral mutant dl922-947 has potent activity in a variety of tumors. We investigated the efficacy of dl922-947 in ovarian carcinoma; compared its activity to wild-type adenovirus, dl309, and dl1520; and investigated the use of icodextrin to enhance activity in vivo. We also assessed the utility of luciferase bioluminescence imaging to quantify the response of human ovarian carcinoma xenografts to dl922-947. Ovarian carcinoma cell lines were transfected in vitro with dl922-947, adenovirus 5 wild-type (Ad5 WT), dl309, and dl1520 and monitored for S-phase induction, viral protein expression, replication, and overall survival. In vivo, the efficacy of dl922-947 when delivered in PBS or icodextrin to female nude mice bearing IGROV1 xenografts was determined. In vitro, dl922-947 induced lysis with greater efficacy than Ad5 WT, dl309, or dl1520 in all ovarian carcinoma cell lines tested, which was associated with earlier expression of viral proteins and S-phase induction. The lytic effect in immortalized ovarian surface epithelial cells confirmed that cellular retinoblastoma pathway status is a strong determinant of dl922-947 activity. In vivo, i.p. delivery of dl922-947 (5 x 10(9) particles daily x 5) increased median survival from 20 to 96 days (P < 0.0001) and delivery in icodextrin-enhanced survival further. However, delayed hepatic toxicity was evident in some dl922-947-treated mice, which was not dependent upon viral replication within tumor cells or the liver. dl922-947 has potency in ovarian carcinoma and i.p. delivery in icodextrin may enhance this activity. Immunocompetent models of ovarian carcinoma are required for further evaluation of hepatotoxicity.

Adenovirus E1B 55-kilodalton protein is required for both regulation of mRNA export and efficient entry into the late phase of infection in normal human fibroblasts

The human adenovirus type 5 (Ad5) E1B 55-kDa protein is required for selective nuclear export of viral late mRNAs from the nucleus and concomitant inhibition of export of cellular mRNAs in HeLa cells and some other human cell lines, but its contributions(s) to replication in normal human cells is not well understood. We have therefore examined the phenotypes exhibited by viruses carrying mutations in the E1B 55-kDa protein coding sequence in normal human fibroblast (HFFs). Ad5 replicated significantly more slowly in HFFs than it does in tumor cells, a difference that is the result of delayed entry into the late phase of infection. The A143 mutation, which specifically impaired export of viral late mRNAs from the nucleus in infected HeLa cells (R. A. Gonzalez and S. J. Flint, J. Virol. 76:4507-4519, 2002), induced a more severe defect in viral mRNA export in HFFs. This observation indicates that the E1B 55-kDa protein regulates mRNA export during the late phase of infection of normal human cells. Other mutants exhibited phenotypes not observed in HeLa cells. In HFFs infected by the null mutant Hr6, synthesis of viral late mRNAs and proteins was severely impaired. Such defects in late gene expression were the result of inefficient progression into the late phase of infection, for viral DNA synthesis was 10-fold less efficient in Hr6-infected HFFs than in cells infected by Ad5. Similar, but less severe, defects in viral DNA synthesis were induced by the insertion mutation H224, which has been reported to inhibit binding of the E1B 55-kDa protein to p53 (C. C. Kao, P. R. Yew, and A. J. Berk, Virology 179:806-814, 1990).

E1A genes of adenovirus type 2 and type 5 are expressed at different levels

Adenoviruses are an extensively studied system for modeling oncogenesis and for experimental cancer therapy. The most commonly analyzed virus types are 2 and 5, and little distinction has been made between them in past studies. Adenoviruses used for therapeutic purposes are frequently hybrids between these types, including the prototype dl1520/Onyx015. We tested the replication of the wild-type viruses WtD (a hybrid of the type 2 E1 region and type 5) and dl309 (type 5) in comparison with the mutants dl1520 (hybrid) and dl338 (type 5), the latter two lacking part of the E1B-55 kDa coding region. We found that the hybrid viruses replicated with considerably lower efficiency than their type 5 counterparts in H1299 cells (dl309:WtD = 3-4, dl338:dl1520 > 10). Moreover, adenovirus type 2 E1A expression from the hybrid viruses was strongly reduced in comparison to adenovirus type 5 E1A, as revealed by immunoblot analysis and RT-PCR, providing a potential explanation for the differences in virus yield. Differential E1A expression levels need to be taken into account for the construction of effective therapeutic viruses and when studying viral transformation.

Oncolytic virotherapy: approaches to tumor targeting and enhancing antitumor effects

The application of replicating viruses for the treatment of cancers represents a novel therapy that is distinct from traditional treatment modalities. It is apparent that the genetic changes that a virus produces within an infected cell in order to create an environment conducive to viral replication are often similar to the processes involved in cellular transformation. These include uncontrolled cellular proliferation, prevention of apoptosis, and resistance to host organism immune effector mechanisms. Deletions of viral genes involved in these processes have been exploited to produce viral mutants whose replication is selective for transformed cells. The use of tissue-specific transcriptional response or RNA stability elements to control the expression of critical viral genes has also resulted in targeted viruses. Work also is being undertaken to restrict or alter the tropism of viruses by altering their ability to infect certain cell types. Finally, the addition of exogenous genes can be used to increase the virus's lytic potential and/or bystander killing; to further induce the host's immune response against cancer cells; and/or to permit the controlled downregulation of viral replication if necessary. The combination of different tumor-targeting mutations in parallel with the expression of foreign genes has resulted in the evolution of second- and third-generation viruses that continue to become further distinct from their native parental strains. The movement of these viruses into the clinic has begun to demonstrate the potential of this approach in the treatment of cancers.

Cancer gene-therapy: clinical trials

The objective of gene therapy for the treatment of cancer is to kill tumour cells but preserve normal tissue; therefore, the ideal gene therapy agent would be targeted for specific transduction of tumour cells and have specificity in its cytotoxic action. A variety of strategies to achieve these aims have demonstrated promising results in the laboratory, including enzyme-pro-drug activating systems, correction of genetic mutations contributing to the malignant phenotype and stimulation of a T-cell-mediated anti-tumour immune response. The key to the success of all these strategies is an effective vector that can direct appropriate expression of the therapeutic gene. Viruses have many properties that can be adapted to achieve this therapeutic endpoint; furthermore, they can be engineered to replicate selectively in cancer cells and lyse them. The challenge now is to translate these features into effective therapies that can supplement or supplant existing treatments.

Meeting product development challenges in manufacturing clinical grade oncolytic adenoviruses

Oncolytic adenoviruses have been considered for use as anticancer therapy for decades, and numerous means of conferring tumor selectivity have been developed. As with any new therapy, the trip from the laboratory bench to the clinic has revealed a number of significant development hurdles. Viral therapies are subject to specific regulations and must meet a variety of well-defined criteria for purity, potency, stability, and product characterization prior to their use in the clinic. Published regulatory guidelines, although developed specifically for biotechnology-derived products, are applicable to the production of oncolytic adenoviruses and other cell-based products, and they should be consulted early during development. Most importantly, both the manufacturing process and the development of characterization and release assays should be science-driven, use the best available science and technology, and must consider the unique nature of the product: a living, and mutatable, virus. Potentially significant impacts on product quality and safety stem from the possibility of genetic instability related to over-engineering the viruses (as evidenced by their recombination and/or occasional reversion to wild-type virus during manufacturing). This report provides examples of some of the critical components affecting the development and production of clinical grade material and summarizes the significant progress made in recent years.

To replicate or not to replicate: achieving selective oncolytic virus replication in cancer cells through translational control

To ensure that their mRNAs are translated and that the viral proteins necessary for assembling the next generation of infectious progeny are produced, viruses must effectively seize control of the translational machinery within their host cells. In many cases, the ability to productively engage host translational components can determine if a given cell type can support viral replication, illustrating the critical importance of this task in the viral life cycle. Failure to interface properly with the host translational apparatus can compromise the productive growth cycle, resulting in an abortive infection and radically restricting viral replication. Not only have viruses become facile at commandeering this machinery, they are also particularly adept at manipulating cellular translation control pathways for their own ends. In this review, the mechanisms by which numerous viruses manipulate host translational control circuits are discussed. Furthermore, particular attention is devoted to understanding how interfering with the ability of a virus to properly regulate translation in its host can be exploited to generate oncolytic strains that selectively replicate in cancer cells.

Development of transcriptionally regulated oncolytic adenoviruses

Changes initiated at the cellular and systemic levels as a result of viral infection or neoplastic transformation share significant overlap. Therefore, the use of replicating viruses to treat tumors has long been postulated as a promising avenue for oncolytic therapy. Over the last 10 years, transcriptionally regulated adenoviruses have become a popular platform for the development of such oncolytic viruses. Placement of heterologous promoters in front of key adenoviral transcription units to achieve tumor- or tissue-specific viral replication is well documented. Various derivatives of this general strategy have led to considerable insight into its limitations, pitfalls, and potential. Although a general process can be described by which to develop transcriptionally regulated adenoviruses, it is apparent that few set rules can yet be defined as to what constitutes a safe, stable, and therapeutically effective vector. Clinical experiences to date suggest the short-term potential for this class of therapeutics lies in combination therapy regimens. Such lessons from the clinic suggest the next generation of transcriptionally regulated oncolytic adenoviruses take advantage of the ability of the platform to carry transgenes in order to deliver a multimodal therapy from a single agent. Beyond this 'arming' of the vectors lies the detargeting, retargeting, and coating of adenoviruses to improve the delivery of the agent to the treatment site(s). As a therapeutic platform, transcriptionally regulated adenoviruses are at an early stage of development with considerable opportunities for advancement.

Recent lessons in gene expression, cell cycle control, and cell biology from adenovirus

Adenovirus continues to be an important model system for investigating basic aspects of cell biology. Interactions of several cellular proteins with E1A conserved regions (CR) 1 and 2, and inhibition of apoptosis by E1B proteins are required for oncogenic transformation. CR2 binds RB family members, de-repressing E2F transcription factors, thus activating genes required for cell cycling. E1B-19K is a BCL2 homolog that binds and inactivates proapoptotic BAK and BAX. E1B-55K binds p53, inhibiting its transcriptional activation function. In productively infected cells, E1B-55K and E4orf6 assemble a ubiquitin ligase with cellular proteins Elongins B and C, Cullin 5 and RBX1 that polyubiquitinates p53 and one or more subunits of the MRN complex involved in DNA double-strand break repair, directing them to proteosomal degradation. E1A CR3 activates viral transcription by interacting with the MED23 Mediator subunit, stimulating preinitiation complex assembly on early viral promoters and probably also the rate at which they initiate transcription. The viral E1B-55K/E4orf6 ubiquitin ligase is also required for efficient viral late protein synthesis in many cell types, but the mechanism is not understood. E1A CR1 binds several chromatin-modifying complexes, but how this contributes to stimulation of cellular DNA synthesis and transformation is not clear. E1A CR4 binds the CtBP corepressor, but the mechanism by which this modulates the frequency of transformation remains to be determined. Clearly, adenovirus has much left to teach us about fundamental cellular processes.

Inactivating intracellular antiviral responses during adenovirus infection

DNA viruses promote cell cycle progression, stimulate unscheduled DNA synthesis, and present the cell with an extraordinary amount of exogenous DNA. These insults elicit vigorous responses mediated by cellular factors that govern cellular homeostasis. To ensure productive infection, adenovirus has developed means to inactivate these intracellular antiviral responses. Among the challenges to the host cell is the viral DNA genome, which is viewed as DNA damage and elicits a cellular response to inhibit replication. Adenovirus therefore encodes proteins that dismantle the cellular DNA damage machinery. Studying virus-host interactions has yielded insights into the molecular functioning of fundamental cellular mechanisms. In addition, it has suggested ways that viral cytotoxicity can be exploited to offer a selective means of restricted growth in tumor cells as a therapy against cancer. In this review, we discuss aspects of the intracellular response that are unique to adenovirus infection and how adenoviral proteins produced from the early region E4 act to neutralize antiviral defenses, with a particular focus on DNA damage signaling.

Unshackling the links between reovirus oncolysis, Ras signaling, translational control and cancer

Reovirus has an inherent preference for replicating in cells with dysregulated growth factor signaling cascades that comprise Ras activation. Precisely how reovirus exploits the host cell Ras pathway is unclear, but there is evidence suggesting that activated Ras signaling is important for efficient viral protein synthesis. Defining the molecular mechanism of reovirus oncolysis will shed light on reovirus replication and important aspects of cellular transformation, Ras signaling cascades and regulation of protein translation.

Recent progress in the battle between oncolytic viruses and tumours

In the past 5 years, the field of oncolytic virus research has matured significantly and is moving past the stage of being a laboratory novelty into a new era of preclinical and clinical trials. What have recent anticancer trials of oncolytic viruses taught us about this exciting new line of therapeutics?

Viruses – seeking and destroying the tumor program

DNA viruses have enormous utility in cancer research, both as tools for tumor target discovery as well as agents for lytic cancer therapies. This is because there is a profound functional overlap between the DNA viral and tumor cell programs. DNA viruses encode proteins that elicit growth deregulation in infected cells similar to that engendered by mutations in tumor cells. Evolution has refined viral proteins to target the critical cellular hubs that regulate growth. Thus, viral proteins are discriminating biochemical probes that can be used to identify and characterize novel tumor targets. Moreover, the overlap between the DNA viral and tumor programs can also be exploited for the development of lytic cancer therapies. Discovering whether tumor cells selectively complement the replication of viral mutants can reveal novel oncolytic viral therapies, as well as unexpected tumor properties. For example, altered RNA export was recently uncovered as a novel tumor cell property that underlies ONYX-015 replication, a promising oncolytic adenoviral therapy. A perspective is provided on how adenovirus could be systematically exploited to map the requisite role, or indeed the redundancy, of cellular pathways that act in an integrated program to elicit pathological replication. This knowledge has important applications for the rational design of the next generation of oncolytic viruses, as well as the discovery of efficacious combination cancer therapies.

Oncolytic viral therapies – the clinical experience

It has been 9 years since the beginning of the first clinical trial in which an oncolytic virus was administered to cancer patients. Since then, oncolytic viruses from five different species have been taken to phase I and II clinical trials in over 300 cancer patients. While additional studies will be required to ascertain if the efficacy of any of these agents is high enough to warrant adding them to the existing therapeutic regimen, it has been reassuring that DNA viruses engineered to achieve tumor selectivity and RNA viruses with relative inherent natural tumor selectivity have proven reasonably safe at the wide range of doses that were tested. Here, we review the biology and clinical results of these five species of viruses and discuss lessons learned and challenges for the future.

Future prospects for oncolytic therapy

Viruses have been engineered to replicate selectively in cancer cells, based on a number of innovative principles. Several of these viruses have entered clinical trials and have proven relatively safe, and have shown evidence of efficacy. However, further research is required to enable these agents to function systemically. This might involve attempts to suppress immune responses to virus antigens, and re-targeting of viruses to favor tumor infection and increased potency. When these barriers are overcome, oncolytic viruses could enter the mainstream of clinical oncology.

Adenovirus exploits the cellular aggresome response to accelerate inactivation of the MRN complex

Results reported here indicate that adenovirus 5 exploits the cellular aggresome response to accelerate inactivation of MRE11-RAD50-NBS1 (MRN) complexes that otherwise inhibit viral DNA replication and packaging. Aggresomes are cytoplasmic inclusion bodies, observed in many degenerative diseases, that are formed from aggregated proteins by dynein-dependent retrograde transport on microtubules to the microtubule organizing center. Viral E1B-55K protein forms aggresomes that sequester p53 and MRN in transformed cells and in cells transfected with an E1B-55K expression vector. During adenovirus infection, the viral protein E4orf3 associates with MRN in promyelocytic leukemia protein nuclear bodies before MRN is bound by E1B-55K. Either E4orf3 or E4orf6 is required in addition to E1B-55K for E1B-55K aggresome formation and MRE11 export to aggresomes in adenovirus-infected cells. Aggresome formation contributes to the protection of viral DNA from MRN activity by sequestering MRN in the cytoplasm and greatly accelerating its degradation by proteosomes following its ubiquitination by the E1B-55K/E4orf6/elongin BC/Cullin5/Rbx1 ubiquitin ligase. Our results show that aggresomes significantly accelerate protein degradation by the ubiquitin-proteosome system. The observation that a normal cellular protein is inactivated when sequestered into an aggresome through association with an aggresome-inducing protein has implications for the potential cytotoxicity of aggresome-like inclusion bodies in degenerative diseases.

Genetic strategies for brain tumor therapy

Gene therapy is a potentially useful approach in the treatment of human brain tumors, which are notoriously refractory to conventional approaches. Most human clinical trials to date have been unsuccessful in terms of improving patient outcome. Recent improvements in viral vectors, the development of stem cell technology, and increased understanding of the mechanism of action of therapeutic transgenes provide hope that the next generation of gene therapeutics may show increased efficacy in treatment of this devastating disease.

Heat shock phenocopies E1B-55K late functions and selectively sensitizes refractory tumor cells to ONYX-015 oncolytic viral therapy

ONYX-015 is an E1B-55K-deleted adenovirus that has promising clinical activity as a cancer therapy. However, many tumor cells fail to support ONYX-015 oncolytic replication. E1B-55K functions include p53 degradation, RNA export, and host protein shutoff. Here, we show that resistant tumor cell lines fail to provide the RNA export functions of E1B-55K necessary for ONYX-015 replication; viral 100K mRNA export is necessary for host protein shutoff. However, heat shock rescues late viral RNA export and renders refractory tumor cells permissive to ONYX-015. These data indicate that heat shock and late adenoviral RNAs may converge upon a common mechanism for their export. Moreover, these data suggest that the concomitant induction of a heat shock response could significantly improve ONYX-015 cancer therapy.

Adenoviral proteins mimic nutrient/growth signals to activate the mTOR pathway for viral replication

Like tumor cells, DNA viruses have had to evolve mechanisms that uncouple cellular replication from the many intra- and extracellular factors that normally control it. Here we show that adenovirus encodes two proteins that activate the mammalian target of rapamycin (mTOR) for viral replication, even under nutrient/growth factor-limiting conditions. E4-ORF1 mimics growth factor signaling by activating PI3-kinase, resulting in increased Rheb.GTP loading and mTOR activation. E4-ORF4 is redundant with glucose in stimulating mTOR, does not affect Rheb.GTP levels and is the major mechanism whereby adenovirus activates mTOR in quiescent primary cells. We demonstrate that mTOR is activated through a mechanism that is dependent on the E4-ORF4 protein phosphatase 2A-binding domain. We also show that mTOR activation is required for efficient S-phase entry, independently of E2F activation, in adenovirus-infected quiescent primary cells. These data reveal that adenovirus has evolved proteins that activate the mTOR pathway, irrespective of the cellular microenvironment, and which play a requisite role in viral replication.

DNA tumor viruses — the spies who lyse us

Identifying the molecular lesions that are 'mission critical' for tumorigenesis and maintenance is one of the burning questions in contemporary cancer biology. In addition, therapeutic strategies that trigger the lytic and selective death of tumor cells are the unfulfilled promise of cancer research. Fortunately, viruses can provide not only the necessary 'intelligence' to identify the critical players in the cancer cell program but also have great potential as lytic agents for tumor therapy. Recent studies with DNA viruses have contributed to our understanding of critical tumor targets (such as EGFR, PP2A, Rb and p53) and have an impact on the development of novel therapies, including oncolytic viral agents, for the treatment of cancer.