Functional interactions of antiapoptotic proteins and tumor necrosis factor in the context of a replication-competent adenovirus

Clinical Trials of Oncolytic Viruses in Breast Cancer

Breast cancer is the second most common kind of cancer worldwide and oncolytic viruses may offer a new treatment approach. There are three different types of oncolytic viruses used in clinical trials; (i) oncolytic viruses with natural anti-neoplastic properties; (ii) oncolytic viruses designed for tumor-selective replication; (iii) oncolytic viruses modified to activate the immune system. Currently, fourteen different oncolytic viruses have been investigated in eighteen published clinical trials. These trials demonstrate that oncolytic viruses are well tolerated and safe for use in patients and display clinical activity. However, these trials mainly studied a small number of patients with different advanced tumors including some with breast cancer. Future trials should focus on breast cancer and investigate optimal routes of administration, occurrence of neutralizing antibodies, viral gene expression, combinations with other antineoplastic therapies, and identify subtypes that are particularly suitable for oncolytic virotherapy.

A bivalent live-attenuated vaccine candidate elicits protective immunity against human adenovirus types 4 and 7

Human adenovirus types 4 (HAdV4) and 7 (HAdV7) often lead to severe respiratory diseases and occur epidemically in children, adults, immune deficiency patients, and other groups, leading to mild or severe symptoms and even death. However, no licensed adenovirus vaccine has been approved in the market for general use. E3 genes of adenovirus are generally considered nonessential for virulence and replication; however, a few studies have demonstrated that the products of these genes are also functional. In this study, most of the E3 genes were deleted, and two E3-deleted recombinant adenoviruses (ΔE3-rAdVs) were constructed as components of the vaccine. After E3 deletion, the replication efficiencies and cytopathogenicity of ΔE3-rAdVs were reduced, indicating that ΔE3-rAdVs were attenuated after E3 genes deletion. Furthermore, single immunization with live-attenuated bivalent vaccine candidate protects mice against challenge with wild-type human adenovirus types 4 and 7, respectively. Vaccinated mice demonstrated remarkably decreased viral loads in the lungs and less lung pathology compared to the control animals. Taken together, our study confirms the possibility of the two live-attenuated viruses as a vaccine for clinic use and illustrates a novel strategy for the construction of an adenovirus vaccine.

The E1B19K-deleted oncolytic adenovirus mutant AdΔ19K sensitizes pancreatic cancer cells to drug-induced DNA-damage by down-regulating Claspin and Mre11

Adenovirus-mediated sensitization of cancer cells to cytotoxic drugs depends on simultaneous interactions of early viral genes with cell death and survival pathways. It is unclear what cellular factors mediate these interactions in the presence of DNA-damaging drugs. We found that adenovirus prevents Chk1-mediated checkpoint activation through inactivation of Mre11 and downregulation of the pChk1 adaptor-protein, Claspin, in cells with high levels of DNA-damage induced by the cytotoxic drugs gemcitabine and irinotecan. The mechanisms for Claspin downregulation involve decreased transcription and increased degradation, further attenuating pChk1-mediated signalling. Live cell imaging demonstrated that low doses of gemcitabine caused multiple mitotic aberrations including multipolar spindles, micro- and multi-nucleation and cytokinesis failure. A mutant virus with the anti-apoptotic E1B19K-gene deleted (AdΔ19K) further enhanced cell killing, Claspin downregulation, and potentiated drug-induced DNA damage and mitotic aberrations. Decreased Claspin expression and inactivation of Mre11 contributed to the enhanced cell killing in combination with DNA-damaging drugs. These results reveal novel mechanisms that are utilised by adenovirus to ensure completion of its life cycle in the presence of cellular DNA damage. Taken together, our findings reveal novel cellular targets that may be exploited when developing improved anti-cancer therapeutics.

Up-regulation of Mcl-1 and Bak by coronavirus infection of human, avian and animal cells modulates apoptosis and viral replication

Virus-induced apoptosis and viral mechanisms that regulate this cell death program are key issues in understanding virus-host interactions and viral pathogenesis. Like many other human and animal viruses, coronavirus infection of mammalian cells induces apoptosis. In this study, the global gene expression profiles are first determined in IBV-infected Vero cells at 24 hours post-infection by Affymetrix array, using avian coronavirus infectious bronchitis virus (IBV) as a model system. It reveals an up-regulation at the transcriptional level of both pro-apoptotic Bak and pro-survival myeloid cell leukemia-1 (Mcl-1). These results were further confirmed both in vivo and in vitro, in IBV-infected embryonated chicken eggs, chicken fibroblast cells and mammalian cells at transcriptional and translational levels, respectively. Interestingly, the onset of apoptosis occurred earlier in IBV-infected mammalian cells silenced with short interfering RNA targeting Mcl-1 (siMcl-1), and was delayed in cells silenced with siBak. IBV progeny production and release were increased in infected Mcl-1 knockdown cells compared to similarly infected control cells, while the contrary was observed in infected Bak knockdown cells. Furthermore, IBV infection-induced up-regulation of GADD153 regulated the expression of Mcl-1. Inhibition of the mitogen-activated protein/extracellular signal-regulated kinase (MEK/ERK) and phosphoinositide 3-kinase (PI3K/Akt) signaling pathways by chemical inhibitors and knockdown of GADD153 by siRNA demonstrated the involvement of ER-stress response in regulation of IBV-induced Mcl-1 expression. These results illustrate the sophisticated regulatory strategies evolved by a coronavirus to modulate both virus-induced apoptosis and viral replication during its replication cycle.

Oncolytic virotherapy in veterinary medicine: current status and future prospects for canine patients

Oncolytic viruses refer to those that are able to eliminate malignancies by direct targeting and lysis of cancer cells, leaving non-cancerous tissues unharmed. Several oncolytic viruses including adenovirus strains, canine distemper virus and vaccinia virus strains have been used for canine cancer therapy in preclinical studies. However, in contrast to human studies, clinical trials with oncolytic viruses for canine cancer patients have not been reported. An 'ideal' virus has yet to be identified. This review is focused on the prospective use of oncolytic viruses in the treatment of canine tumors - a knowledge that will undoubtedly contribute to the development of oncolytic viral agents for canine cancer therapy in the future.

Replication and virus-induced transcriptome of HAdV-5 in normal host cells versus cancer cells--differences of relevance for adenoviral oncolysis

Adenoviruses (Ads), especially HAdV-5, have been genetically equipped with tumor-restricted replication potential to enable applications in oncolytic cancer therapy. Such oncolytic adenoviruses have been well tolerated in cancer patients, but their anti-tumor efficacy needs to be enhanced. In this regard, it should be considered that cancer cells, dependent on their tissue of origin, can differ substantially from the normal host cells to which Ads are adapted by complex virus-host interactions. Consequently, viral replication efficiency, a key determinant of oncolytic activity, might be suboptimal in cancer cells. Therefore, we have analyzed both the replication kinetics of HAdV-5 and the virus-induced transcriptome in human bronchial epithelial cells (HBEC) in comparison to cancer cells. This is the first report on genome-wide expression profiling of Ads in their native host cells. We found that E1A expression and onset of viral genome replication are most rapid in HBEC and considerably delayed in melanoma cells. In squamous cell lung carcinoma cells, we observed intermediate HAdV-5 replication kinetics. Infectious particle production, viral spread and lytic activity of HAdV-5 were attenuated in melanoma cells versus HBEC. Expression profiling at the onset of viral genome replication revealed that HAdV-5 induced the strongest changes in the cellular transcriptome in HBEC, followed by lung cancer and melanoma cells. We identified prominent regulation of genes involved in cell cycle and DNA metabolism, replication and packaging in HBEC, which is in accord with the necessity to induce S phase for viral replication. Strikingly, in melanoma cells HAdV-5 triggered opposing regulation of said genes and, in contrast to lung cancer cells, no weak S phase induction was detected when using the E2F promoter as reporter. Our results provide a rationale for improving oncolytic adenoviruses either by adaptation of viral infection to target tumor cells or by modulating tumor cell functions to better support viral replication.

The oncolytic adenovirus AdΔΔ enhances selective cancer cell killing in combination with DNA-damaging drugs in pancreatic cancer models

Pancreatic adenocarcinomas are aggressive and frequently develop resistance to all current therapies. Replication-selective adenoviruses can overcome resistance to chemotherapeutics through their sensitizing effects on drug-induced cell killing. We previously found that adenovirus deleted in the anti-apoptotic E1B19K gene enhanced gemcitabine-induced apoptotis. Here we demonstrate that our engineered double-deleted AdΔΔ mutant (deleted in the pRb-binding E1ACR2 region and E1B19K) selectively replicates and enhances cell killing in combination with DNA-damaging cytotoxic drugs in pancreatic cancer cells. Combinations of AdΔΔ with gemcitabine, irinotecan or cisplatin resulted in two- to fourfold decreases in EC(50) (half maximal effective concentration) values and was more efficent than similar combinations with wild-type virus, the dl1520 (ONYX-015) and dl922-947 mutants. AdΔΔ replication was impaired in normal bronchial human epithelial cells and did not sensitize the cells to drugs. Gemcitabine-insensitive AsPC-1, BxPC-3 and PANC-1 cells were efficiently killed by irinotecan in combination with AdΔΔ. Suboptimal doses of AdΔΔ and gemcitabine significantly prolonged time to tumor progression in two human pancreatic tumor xenograft in vivo models, PT45 and SUIT-2. We conclude that AdΔΔ has low toxicity to normal cells while potently sensitizing pancreatic cancer cells to DNA-damaging drugs, and holds promise as an improved therapeutic strategy for pancreatic cancer.

Improved potency and selectivity of an oncolytic E1ACR2 and E1B19K deleted adenoviral mutant in prostate and pancreatic cancers

PURPOSE

Replication-selective oncolytic adenoviruses are a promising class of tumor-targeting agents with proven safety in hundreds of patients. However, clinical responses have been limited and viral mutants with higher potency are needed. Here, we report on the generation of a novel set of mutants with improved efficacy in prostate and pancreatic carcinoma models. Currently, no curative treatments are available for late-stage metastatic prostate or rapidly progressing pancreatic cancers.

EXPERIMENTAL DESIGN

Adenovirus type 5 mutants were created with deletions in the E1ACR2 region for tumor selectivity and/or the E1B19K gene for attenuated replication in vivo; all constructs retain the E3 genes intact. Cell-killing efficacy, replication, and cytotoxicity in combination with chemotherapeutics were investigated in normal cells (PrEC and NHBE), seven carcinoma cell lines, and human (PC3 and DU145) and murine (TRAMPC, CMT-64, and CMT-93) tumor models in vivo.

RESULTS

The double-deleted AdDeltaDelta (DeltaE1ACR2 and DeltaE1B19K) mutant had high cell-killing activity in prostate, pancreatic, and lung carcinomas. Replication was similar to wild-type in all tumor cells and was attenuated in normal cells to levels less than the single-deleted AdDeltaCR2 mutant. AdDeltaDelta combined with the chemotherapeutics docetaxel and mitoxantrone resulted in synergistically enhanced cell killing and greatly improved antitumor efficacy in prostate xenografts in vivo. In murine immunocompetent in vivo models efficacy was greater for mutants with the E3B genes intact even in the absence of viral replication, indicating attenuated macrophage-dependent clearance.

CONCLUSIONS

These data suggest that the novel oncolytic mutant AdDeltaDelta is a promising candidate for targeting of solid tumors specifically in combination with chemotherapeutics.

Oncolytic Viruses for Cancer Therapy: Overcoming the Obstacles

Targeted therapy of cancer using oncolytic viruses has generated much interest over the past few years in the light of the limited efficacy and side effects of standard cancer therapeutics for advanced disease. In 2006, the world witnessed the first government-approved oncolytic virus for the treatment of head and neck cancer. It has been known for many years that viruses have the ability to replicate in and lyse cancer cells. Although encouraging results have been demonstrated in vitro and in animal models, most oncolytic viruses have failed to impress in the clinical setting. The explanation is multifactorial, determined by the complex interactions between the tumor and its microenvironment, the virus, and the host immune response. This review focuses on discussion of the obstacles that oncolytic virotherapy faces and recent advances made to overcome them, with particular reference to adenoviruses.

Engineering oncolytic viruses to exploit tumor specific defects in innate immune signaling pathways

BACKGROUND

The use of oncolytic viruses for treatment of cancer marks a significant alteration in the battle between host and virus. Viruses are confronted by cellular innate immune responses and contain an armamentarium of immunomodulatory proteins that suppress innate immunity. Tumorigenesis can result in impairment of innate immune responses. Viruses engineered to be vulnerable to normal responses may mediate tumor-specific killing with minimal off-target toxicity.

OBJECTIVE

To examine the mechanisms by which mammalian cells respond to viral infections in normal versus cancer cells and how viruses overcome these responses and to illustrate how this knowledge is used to develop physiologically targeted oncolytic viruses.

METHODS

Literature describing studies investigating innate responses to virus infections, cancer-specific molecular defects, immunosuppressive viral products and design of oncolytic viruses is extensively reviewed, and pertinent concepts are distilled and developed.

RESULTS/CONCLUSION

Innate responses to viral infections are complex involving i) viral detection; ii) induction of interferon and other cytokines; and iii) establishment of an antiviral state. Oncolytic viruses are engineered to be susceptible to antiviral responses in normal cells. Cancers can be partially vulnerable to these viruses because they have defective antiviral responses but the antitumor potency of physiologically targeted viruses may be significantly diminished.

Adenoviral vector-based strategies for cancer therapy

Definitive treatment of cancer has eluded scientists for decades. Current therapeutic modalities like surgery, chemotherapy, radiotherapy and receptor-targeted antibodies have varied degree of success and generally have moderate to severe side effects. Gene therapy is one of the novel and promising approaches for therapeutic intervention of cancer. Viral vectors in general and adenoviral (Ad) vectors in particular are efficient natural gene delivery systems and are one of the obvious choices for cancer gene therapy. Clinical and preclinical findings with a wide variety of approaches like tumor suppressor and suicide gene therapy, oncolysis, immunotherapy, anti-angiogenesis and RNA interference using Ad vectors have been quite promising, but there are still many hurdles to overcome. Shortcomings like increased immunogenicity, prevalence of preexisting anti-Ad immunity in human population and lack of specific targeting limit the clinical usefulness of Ad vectors. In recent years, extensive research efforts have been made to overcome these limitations through a variety of approaches including the use of conditionally-replicating Ad and specific targeting of tumor cells. In this review, we discuss the potential strengths and limitations of Ad vectors for cancer therapy.

Oncolytic adenoviral gene therapy in ovarian cancer: why we are not wasting our time

Preclinical gene-therapy studies in the past 15 years have repeatedly raised hopes that we were about to enter a brave new era. However, many clinical trials have disappointed. For tumor types with poor response rates to first-line conventional cytotoxic chemotherapy and/or high rates of chemorefractory disease, there remain very few treatment options. In this article we review gene therapy within the context of ovarian cancer. We examine why clinical data have been discouraging and discuss how the lessons learned from earlier trials are being applied to current research.

Targeted genetic and viral therapy for advanced head and neck cancers

Head and neck cancers usually present with advanced disease and novel therapies are urgently needed. Genetic therapy aims at restoring malfunctioned tumor suppressor gene(s) or introducing proapoptotic genes. Oncolytic virotherapeutics induce multiple cycles of cancer-specific virus replication, followed by oncolysis, virus spreading and infection of adjacent cancer cells. Oncolytic viruses can also be armed to express therapeutic transgene(s). Recent advances in preclinical and clinical studies are revealing the potential of both therapeutic classes for advanced head and neck cancers, including the approval of two products (Gendicine and H101) by a governmental agency. This review summarizes the available clinical data to date and discusses the challenges and future directions.

Oncolytic adenoviral mutants with E1B19K gene deletions enhance gemcitabine-induced apoptosis in pancreatic carcinoma cells and anti-tumor efficacy in vivo

PURPOSE

Pancreatic adenocarcinoma is a rapidly progressive malignancy that is highly resistant to current chemotherapeutic modalities and almost uniformly fatal. We show that a novel targeting strategy combining oncolytic adenoviral mutants with the standard cytotoxic treatment, gemcitabine, can markedly improve the anticancer potency.

EXPERIMENTAL DESIGN

Adenoviral mutants with the E1B19K gene deleted with and without E3B gene expression (AdDeltaE1B19K and dl337 mutants, respectively) were assessed for synergistic interactions in combination with gemcitabine. Cell viability, mechanism of cell death, and antitumor efficacy in vivo were determined in the pancreatic carcinoma cells PT45 and Suit2, normal human bronchial epithelial cells, and in PT45 xenografts.

RESULTS

The DeltaE1B19K-deleted mutants synergized with gemcitabine to selectively kill cultured pancreatic cancer cells and xenografts in vivo with no effect in normal cells. The corresponding wild-type virus (Ad5) stimulated drug-induced cell killing to a lesser degree. Gemcitabine blocked replication of all viruses despite the enhanced cell killing activity due to gemcitabine-induced delay in G1/S-cell cycle progression, with repression of cyclin E and cdc25A, which was not abrogated by viral E1A-expression. Synergistic cell death occurred through enhancement of gemcitabine-induced apoptosis in the presence of both AdDeltaE1B19K and dl337 mutants, shown by increased cell membrane fragmentation, caspase-3 activation, and mitochondrial dysfunction.

CONCLUSIONS

Our data suggest that oncolytic mutants lacking the antiapoptotic E1B19K gene can improve efficacy of DNA-damaging drugs such as gemcitabine through convergence on cellular apoptosis pathways. These findings imply that less toxic doses than currently practiced in the clinic could efficiently target pancreatic adenocarcinomas when combined with adenoviral mutants.

Development of targeted oncolytic virotherapeutics through translational research

BACKGROUND

Oncolytic virotherapeutics is a promising platform for cancer treatment but the product class has yet been successful. The key to success is integration of bidirectional translational research to rapidly address issues encountered in the laboratory and the clinics.

OBJECTIVE

We highlight the hurdles identified for the targeted oncolytic virotherapy approach, specifically those identified in clinical trials with wild-type viruses and first-generation targeted agents. We also analyze the translational research and development that has been applied to overcome these hurdles, including virus engineering and design improvements for next-generation virotherapeutics.

RESULTS/CONCLUSION

The iterative loop between the clinic and the lab can function as a major driving force to optimize products from this platform.

Oncolytic virotherapy: molecular targets in tumor-selective replication and carrier cell-mediated delivery of oncolytic viruses

Tremendous advances have been made in developing oncolytic viruses (OVs) in the last few years. By taking advantage of current knowledge in cancer biology and virology, specific OVs have been genetically engineered to target specific molecules or signal transduction pathways in cancer cells in order to achieve efficient and selective replication. The viral infection and amplification eventually induce cancer cells into cell death pathways and elicit host antitumor immune responses to further help eliminate cancer cells. Specifically targeted molecules or signaling pathways (such as RB/E2F/p16, p53, IFN, PKR, EGFR, Ras, Wnt, anti-apoptosis or hypoxia) in cancer cells or tumor microenvironment have been studied and dissected with a variety of OVs such as adenovirus, herpes simplex virus, poxvirus, vesicular stomatitis virus, measles virus, Newcastle disease virus, influenza virus and reovirus, setting the molecular basis for further improvements in the near future. Another exciting new area of research has been the harnessing of naturally tumor-homing cells as carrier cells (or cellular vehicles) to deliver OVs to tumors. The trafficking of these tumor-homing cells (stem cells, immune cells and cancer cells), which support proliferation of the viruses, is mediated by specific chemokines and cell adhesion molecules and we are just beginning to understand the roles of these molecules. Finally, we will highlight some avenues deserving further study in order to achieve the ultimate goals of utilizing various OVs for effective cancer treatment.

Comparison of the E3 and L3 regions for arming oncolytic adenoviruses to achieve a high level of tumor-specific transgene expression

Arming oncolytic adenoviral vectors with anticancer transgenes that can be expressed in a tumor-selective manner may enable the engineering of vectors with increased potency, while retaining their safety profile. Armed oncolytic adenoviral vectors were constructed in which transgene expression has been linked via modified splice acceptor sequences that did not necessitate the deletion of any part of the adenoviral genome. Several oncolytic adenoviral vectors were compared in which the transgene was inserted in place of either the E3 or the L3 region. While all vectors had similar viral growth and cytotoxicity characteristics, the highest level of transgene expression was observed from a vector in which the transgene had been inserted downstream of the L3 23K protease gene, the Ad-23K-GM vector. Notably, no transgene expression occurred with this vector in the absence of DNA replication either in vitro or in vivo. In contrast, viruses in which the transgene was inserted into E3 locations exhibited a low level of transgene expression even in the absence of DNA replication. In summary, by utilizing the L3 region for arming oncolytic viruses, higher levels of tumor-specific transgene expression can be obtained without the need to delete any parts of the viral genome.

A phase I trial of intravenous infusion of ONYX-015 and enbrel in solid tumor patients

ONYX-015 is an attenuated chimeric human group C adenovirus, which preferentially replicates in and lyses tumor cells that are p53 negative. The purpose of this phase I, dose-escalation study was to determine the safety and feasibility of intravenous infusion with ONYX-015 in combination with enbrel in patients with advanced carcinoma. Enbrel is a recombinant dimer of human tumor-necrosis factor (TNF)-alpha receptor, previously shown to reduce the level of functional TNF. Nine patients, three in each cohort received multiple cycles of ONYX-015 infusion (1 x 10(10), 1 x 10(11) and 1 x 10(12) vp weekly for 4 weeks/cycle) in addition to subcutaneous enbrel (only during cycle 1) injections per FDA-indicated dosing. Of the nine patients, four had stable disease. No significant adverse events were attributed to the experimental regimen, confirming that enbrel can be safely administered along with oncolytic virotherapy. Two of the three patients in cohort 3 had detectable viral DNA at days 3 and 8 post-ONYX-015 infusion. Their detectable circulating viral DNA was markedly higher during cycle 1 (with enbrel coadministration) as compared with cycle 2 (without enbrel) at the same time points. Area under the curve determinations indicate a marked higher level of TNF-alpha induction and accelerated clearance at cycle 2 in the absence of enbrel. Further assessment is recommended.

Common effector processing mediates cell-specific responses to stimuli

The fundamental components of many signalling pathways are common to all cells. However, stimulating or perturbing the intracellular network often causes distinct phenotypes that are specific to a given cell type. This 'cell specificity' presents a challenge in understanding how intracellular networks regulate cell behaviour and an obstacle to developing drugs that treat signalling dysfunctions. Here we apply a systems-modelling approach to investigate how cell-specific signalling events are integrated through effector proteins to cause cell-specific outcomes. We focus on the synergy between tumour necrosis factor and an adenoviral vector as a therapeutically relevant stimulus that induces cell-specific responses. By constructing models that estimate how kinase-signalling events are processed into phenotypes through effector substrates, we find that accurate predictions of cell specificity are possible when different cell types share a common 'effector-processing' mechanism. Partial-least-squares regression models based on common effector processing accurately predict cell-specific apoptosis, chemokine release, gene induction, and drug sensitivity across divergent epithelial cell lines. We conclude that cell specificity originates from the differential activation of kinases and other upstream transducers, which together enable different cell types to use common effectors to generate diverse outcomes. The common processing of network signals by downstream effectors points towards an important cell biological principle, which can be applied to the understanding of cell-specific responses to targeted drug therapies.

Oncolytic HSV armed with platelet factor 4, an antiangiogenic agent, shows enhanced efficacy

Oncolytic herpes simplex viruses (HSV) have emerged as a promising platform for cancer therapy. However, efficacy as single agents has thus far been unsatisfactory. Tumor vasculature is critical in supporting tumor growth, but successful antiangiogenic approaches often require maintaining constant levels of antiangiogenic products. We hypothesized that oncolytic HSV has the potential to destroy tumor vasculature and that this effect can be enhanced by combination with antiangiogenic gene transfer. We examined the strategy of arming oncolytic HSV with an antiangiogenic transgene, platelet factor 4 (PF4). The PF4 transgene was inserted into oncolytic HSV G47Delta utilizing a bacterial artificial chromosome construction system. Whereas bG47Delta-empty showed robust cell killing and migration inhibition of proliferating endothelial cells (HUVEC and Py-4-1), the effect was further enhanced by PF4 expression. Importantly, enhanced potency did not impede viral replication. In vivo, bG47Delta-PF4 was more efficacious than its nonexpressing parent bG47Delta-empty at inhibiting tumor growth and angiogenesis in both human U87 glioma and mouse 37-3-18-4 malignant peripheral nerve sheath tumor models. Enhancing the antiangiogenic properties of oncolytic HSV through the expression of antiangiogenic factors such as PF4 is a powerful new strategy that targets both the tumor cells and tumor vasculature.

Mechanisms of apoptosis regulation by viral oncogenes in infection and tumorigenesis

Apoptosis mediated by the proapoptotic BCL-2 family members BCL-2-associated X-protein (BAX) and BCL-2 antagonist/killer (BAK) is part of the antiviral response at the cellular level to limit virus replication. Viruses, in turn, have evolved to encode antiapoptotic BCL-2 homologs (v-BCL-2s) to prevent the premature death of the infected host cell to sustain virus replication. These same v-BCL-2 proteins cooperate with loss of retinoblastoma protein and p53 tumor suppressor function, by inactivating the BAX and BAK apoptotic pathway to promote epithelial solid tumor growth and resistance to chemotherapy. Analogously to infected cells, failure of apoptosis in tumors permits the survival of abnormal, damaged cells displaying chromosome instability that may further promote tumor progression. Thus, both infected cells and tumor cells require inhibition of the apoptotic host defense mechanism, the insights from which can be exploited for therapy development.

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.

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.