Targeting the cytotoxicity of fusogenic membrane glycoproteins in gliomas through protease-substrate interaction

Adenoviral Vectors Armed with Cell Fusion-Inducing Proteins as Anti-Cancer Agents

Cancer is a devastating disease that affects millions of patients every year, and causes an enormous economic burden on the health care system and emotional burden on affected families. The first line of defense against solid tumors is usually extraction of the tumor, when possible, by surgical methods. In cases where solid tumors can not be safely removed, chemotherapy is often the first line of treatment. As metastatic cancers often become vigorously resistant to treatments, the development of novel, more potent and selective anti-cancer strategies is of great importance. Adenovirus (Ad) is the most commonly used virus in cancer clinical trials, however, regardless of the nature of the Ad-based therapeutic, complete responses to treatment remain rare. A number of pre-clinical studies have shown that, for all vector systems, viral spread throughout the tumor mass can be a major limiting factor for complete tumor elimination. By expressing exogenous cell-fusion proteins, many groups have shown improved spread of Ad-based vectors. This review summarizes the research done to examine the potency of Ad vectors expressing fusogenic proteins as anti-cancer therapeutics.

Isolation of more potent oncolytic paramyxovirus by bioselection

Newcastle disease virus (NDV) is an oncolytic paramyxovirus with a nonsegmented single-stranded RNA genome. In this report, a recombinant oncolytic NDV was passaged in human tumor xenografts and reisolated and characterized after two rounds of bioselection. Several isolates could be recovered that differed from the parental virus with respect to virus spread in tumor cells and the ability to form syncytia in human tumor cells. Three isolates were identified that demonstrated superior oncolytic potency compared with the parental virus as measured by increased oncolytic potency in confluent tumor cell monolayers, in tumor cell spheroids and in a mouse xenograft tumor model. The surface proteins F and HN were sequence analyzed and characterized for fusogenicity. The present study demonstrates that in vivo NDV bioselection can enable the isolation of novel, oncolytic NDV and thus represents a powerful methodology for the development of highly potent oncolytic viruses.

Generation of optimized and urokinase-targeted oncolytic Sendai virus vectors applicable for various human malignancies

We previously reported the development of a prototype 'oncolytic Sendai virus (SeV) vector' formed by introducing two major genomic modifications to the original SeV, namely deletion of the matrix (M) gene to avoid budding of secondary viral particles and manipulation of the trypsin-dependent cleavage site of the fusion (F) gene to generate protease-specific sequences. As a result, the 'oncolytic SeV' that was susceptible to matrix metalloproteinases (MMPs) was shown to selectively kill MMP-expressing tumors through syncytium formation in vitro and in vivo. However, its efficacy has been relatively limited because of the requirement of higher expression of MMPs and smaller populations of MMP-expressing tumors. To overcome these limitations, we have designed an optimized and dramatically powerful oncolytic SeV vector. Truncation of 14-amino acid residues of the cytoplasmic domain of F protein resulted in dramatic enhancement of cell-killing activities of oncolytic SeV, and the combination with replacement of the trypsin cleavage site with the new urokinase type plasminogen activator (uPA)-sensitive sequence (SGRS) led a variety of human tumors, including prostate (PC-3), renal (CAKI-I), pancreatic (BxPC3) and lung (PC14) cancers, to extensive death through massive cell-to-cell spreading without significant dissemination to the surrounding noncancerous tissue in vivo. These results indicate a dramatic improvement of antitumor activity; therefore, extensive utility of the newly designed uPA-targeted oncolytic SeV has significant potential for treating patients bearing urokinase-expressing cancers in clinical settings.

Inhibition of NF-kappaB in fusogenic membrane glycoprotein causing HL-60 cell death: implications for acute myeloid leukemia

Viral fusogenic membrane glycoproteins (FMGs) are new therapeutic genes for the control of tumor growth, the cellular mechanisms mediating cell death is non-apoptotic. However, the precise molecular mechanism remains to be elucidated. Here, we showed overexpression of HSP70 in HL-60 cells mediated by Gibbon Ape leukemia virus hyperfusogenic envelope protein (GALV-FMG) inhibited the nuclear translocation of p65, the transcriptive activity of NF-kappaB and prevented the degradation of IkappaB. NF-kappaB may negatively regulate HSP70 expression, which made a positive feed back loop for expression of HSP70. FMG expression in HL-60 cells leaded to the formation of multinucleated syncytia and cell death, the main death mode of cells is necrosis. This form of cell death should be effective in vivo, gene therapy basing on FMG deserve further study for the treatment of AML.

Matrix metalloproteases: underutilized targets for drug delivery

Pathophysiological molecules in the extracellular environment offer excellent targets that can be exploited for designing drug targeting systems. Matrix metalloproteases (MMPs) are a family of extracellular proteolytic enzymes that are characterized by their overexpression or overactivity in several pathologies. Over the last two decades, the MMP literature reveals heightened interest in the research involving MMP biology, pathology and targeting. This review describes various strategies that have been designed to utilize MMPs for targeting therapeutic entities. Key factors that need to be considered in the successful design of such systems have been identified based on the analyses of these strategies. Development of targeted drug delivery using MMPs has been steadily pursued; however, drug delivery efforts using these targets need to be intensified and focused to realize the clinical application of the fast developing fundamental MMP research.

Recurrent glioblastoma multiforme: advances in treatment and promising drug candidates

Recurrent glioblastoma multiforme is a lethal disease with currently available treatment options having a limited impact on outcome. In this article, current and novel therapeutic approaches in the treatment of recurrent glioblastoma multiforme, including chemotherapy, targeted molecular agents, virotherapy/gene therapy and immunotherapy and challenges in developing novel therapeutic agents for glioblastoma multiforme will be discussed.

Targeted retroviral vectors displaying a cleavage site-engineered hemagglutinin (HA) through HA-protease interactions

We report here a targeting method that exploits the expression pattern of cell surface proteases to induce gene delivery to specific tissues. We describe retroviral vectors harboring modified surface glycoproteins derived from an avian influenza virus hemagglutinin (HA) for which the cell entry properties, dependent on HA cleavage by producer cells, were conditionally blocked at a postbinding step by insertion of matrix metalloproteinase (MMP) substrates. We demonstrate that such vectors induce gene transfer, both in vitro and in mice harboring human tumor xenografts, only through contact with target cells expressing MMPs that cleave the substrate introduced into the recombinant HA. This selective gene transfer in MMP-rich cells was specifically inhibited by 1,10-phenanthroline, a broad-range MMP inhibitor. Importantly, such MMP-activatable vectors selectively transduced MMP-rich cells in heterogeneous populations containing MMP-rich and MMP-poor cells. These vectors will allow useful gene transfer applications into target cells exhibiting specific protease activities.

Library-based selection of retroviruses selectively spreading through matrix metalloprotease-positive cells

Viruses conditionally replicating in cancer cells form an attractive novel class of antitumoral agents. To engineer such viruses infectivity can be coupled with proteolytic activity of the target cell by modifying the envelope (Env) protein of murine leukaemia virus (MLV) with blocking domains that prevent cell entry unless they are cleaved off by tumour-associated proteases like the matrix metalloproteases (MMP). Here we show that MLV variants selectively spreading through MMP-positive cells can be evolved from virus libraries, in which a standard MMP-2 substrate peptide connecting the blocking domain CD40L with the Env protein was diversified. Passaging the virus library on human fibrosarcoma or glioma cell lines resulted in the selection of about 10 virus clones, of which the three most frequent ones were shown to become activated by MMPs and to be replication competent on MMP-positive cells only. On these cells, the selected linker peptides improved the spreading by several orders of magnitude in vitro, as well as in tumour xenografts in vivo, approaching the kinetic of the unmodified wild-type virus. The data suggest that retroviral protease substrate libraries form a potent tool for the engineering of viruses conditionally replicating in a given cancer cell type of interest.

Generation of a recombinant Sendai virus that is selectively activated and lyses human tumor cells expressing matrix metalloproteinases

Malignant tumor cells often express matrix metalloproteinases (MMPs) at a high level to enable their dissemination and metastasis. Sendai virus (SeV), a nonsegmented negative strand RNA virus, spreads in the target tissues in vivo via cleavage activation of the viral fusion glycoprotein by a tissue-specific, trypsin-like enzyme. By deleting the viral matrix protein, we previously generated a recombinant SeV that does not bud to mature virions, but is highly fusogenic and spreads extensively from cell to cell in a trypsin-dependent manner. Here, we changed the tryptic cleavage site of the fusion glycoprotein of this virus to a site susceptible to MMPs. The resulting recombinant virus was no longer activated by trypsin but spread efficiently in cultured cells supplemented with MMP2 or MMP9 and in human tumor cell lines expressing these MMPs. Furthermore, the virus spread extensively in tumor cells xenotrasplanted to nude mice without disseminating to the surrounding normal cells, leading to the inhibition of the tumor growth in the mice. These results demonstrate the selective targeting and killing of human tumor cells by recombinant SeV technology and greatly advance the reemerging concept of oncolytic virotherapy, which currently appears to rely largely upon a natural preference of certain viruses for cancer cells.

Adenoviral vectors expressing fusogenic membrane glycoproteins activated via matrix metalloproteinase cleavable linkers have significant antitumor potential in the gene therapy of gliomas

BACKGROUND

Fusogenic membrane glycoproteins (FMG) such as the gibbon ape leukemia virus envelope (GALV) glycoprotein are potent therapeutic transgenes with potential utility in the gene therapy of gliomas. Transfection of glioma cell lines with FMG expression constructs results in fusion with massive syncytia formation followed by cytotoxic cell death. Nevertheless, ubiquitous expression of the GALV receptor, Pit-1, makes targeting desirable in order to increase the specificity of the observed cytopathic effect. Here we report on use of matrix metalloproteinase (MMP)-cleavable linkers to target the cytotoxicity of FMG-expressing adenoviral vectors against gliomas.

METHODS

Replication-defective adenoviruses (Ad) were constructed expressing the hyperfusogenic version of the GALV glycoprotein linked to a blocking ligand (C-terminal extracellular domain of CD40 ligand) through either an MMP-cleavable linker (AdM40) or a non-cleavable linker (AdN40). Both viruses also co-expressed the green fluorescent protein (GFP) via an internal ribosomal entry site.

RESULTS

The glioma cell lines U87, U118, and U251 characterized by zymography and MMP-2 activity assay as high, medium and low MMP expressors, respectively, the MMP-poor cell lines TE671 and normal human astrocytes were infected with AdM40 and AdN40 at different multiplicities of infection (MOIs) from 1-30. Fusion was quantitated by counting both number and size of syncytia. Infection of these cell lines with AdN40 did not result in fusion or cytotoxic cell death, despite the presence of infection, as demonstrated by GFP positivity, therefore indicating that the displayed CD40 ligand blocked GALV-induced fusion. Fusion was restored after infection of glioma cells with AdM40 at an MOI as low as 1 to an extent dependent on MMP expression and coxsackie adenovirus receptor (CAR) expression in the specific cell line. Western immunoblotting demonstrated the presence of the cleaved CD40 ligand in the supernatant of fused glioma cells. Use of the MMP inhibitors 1,10 phenanthroline and N-hydroxy-5,5-dimethylpiperazine-2-carboxamide completely abolished AdM40-induced fusion, while the non-specific serine protease inhibitor soybean trypsin inhibitor did not affect it, thus demonstrating specificity of the observed effect. Intratumoral treatment of BalbC/nude mice bearing subcutaneous U87 glioma xenografts with AdM40 at a total dose of 1.2 x 10(10) plaque-forming units (pfu) resulted in statistically significant tumor regression as compared with control animals either treated with AdN40 (p = 0.01) or untreated animals (p = 0.01). Treatment with AdM40 also resulted in survival improvement as compared with AdN40-treated animals (p = 0.006) or untreated animals (p = 0.001). Histopathologic examination of treated tumors demonstrated extensive syncytia formation.

CONCLUSIONS

Our data indicate that AdM40, a replication-defective adenovirus expressing the GALV fusogenic glycoprotein, attached to a blocking ligand via an MMP-cleavable linker, can target the cytotoxicity of GALV in MMP-overexpressing glioma lines and xenografts, and maintain significant antitumor activity both in vitro and in vivo. Given the high frequency of MMP overexpression in gliomas, AdM40 represents a potentially promising agent in the gene therapy of these tumors.

Transcriptionally targeted gene therapy to detect and treat cancer

The greatest challenge in cancer treatment is to achieve the highest levels of specificity and efficacy. Cancer gene therapy could be designed specifically to express therapeutic genes to induce cancer cell destruction. Cancer-specific promoters are useful tools to accomplish targeted expression; however, high levels of gene expression are needed to achieve therapeutic efficacy. Incorporating an imaging reporter gene in tandem with the therapeutic gene will allow tangible proof of principle that gene expression occurs at the correct location and at a sufficient level. Gene-based imaging can advance cancer detection and diagnosis. By combining the cancer-targeted imaging and therapeutic strategies, the exciting prospect of a 'one-two punch' to find hidden, disseminated cancer cells and destroy them simultaneously can potentially be realized.