Improved gene transfer selectivity to hepatocarcinoma cells by retrovirus vector displaying single-chain variable fragment antibody against c-Met

TRYBE®: an Fc-free antibody format with three monovalent targeting arms engineered for long in vivo half-life

TrYbe® is an Fc-free therapeutic antibody format, capable of engaging up to three targets simultaneously, with long in vivo half-life conferred by albumin binding. This format is shown by small-angle X-ray scattering to be conformationally flexible with favorable 'reach' properties. We demonstrate the format's broad functionality by co-targeting of soluble and cell surface antigens. The benefit of monovalent target binding is illustrated by the lack of formation of large immune complexes when co-targeting multivalent antigens. TrYbes® are manufactured using standard mammalian cell culture and protein A affinity capture processes. TrYbes® have been formulated at high concentrations and have favorable drug-like properties, including stability, solubility, and low viscosity. The unique functionality and inherent developability of the TrYbe® makes it a promising multi-specific antibody fragment format for antibody therapy.

The Evolution of Gene Therapy in the Treatment of Metabolic Liver Diseases

Monogenic metabolic disorders of hepatic origin number in the hundreds, and for many, liver transplantation remains the only cure. Liver-targeted gene therapy is an attractive treatment modality for many of these conditions, and there have been significant advances at both the preclinical and clinical stages. Viral vectors, including retroviruses, lentiviruses, adenovirus-based vectors, adeno-associated viruses and simian virus 40, have differing safety, efficacy and immunogenic profiles, and several of these have been used in clinical trials with variable success. In this review, we profile viral vectors and non-viral vectors, together with various payloads, including emerging therapies based on RNA, that are entering clinical trials. Genome editing technologies are explored, from earlier to more recent novel approaches that are more efficient, specific and safe in reaching their target sites. The various curative approaches for the multitude of monogenic hepatic metabolic disorders currently at the clinical development stage portend a favorable outlook for this class of genetic disorders.

Anti-MET VHH Pool Overcomes MET-Targeted Cancer Therapeutic Resistance

Receptor tyrosine kinase MET and its ligand hepatocyte growth factor (HGF) play crucial roles in many human malignancies. Numerous drugs have been developed against kinase center of MET or HGF-mediated activation; however, the outcomes in patients are not so promising. Increasing evidence show that MET has kinase-independent effects on tumorigenesis and dissemination, which explains the low efficacy in kinase inhibition-based strategy. VHH is the recombinant variable region of Camelid heavy-chain antibody. As a nanoscale antigen-binding unit, VHH has become an appealing drug candidate in cancer therapy. In our study, we choose a novel strategy to construct an anti-MET VHH pool against the whole ecto-domain of MET. Comparing to monoclonal antibody or single VHH, the anti-MET VHH pool strongly promotes MET degradation through Clathrin-dependent endo-lysosomal pathway. Thus, the anti-MET VHH pool not only blocks kinase activity of MET, but also reduces protein level of MET. As a consequence, anti-MET VHH pool dramatically suppresses cancer cell proliferation, viability, and colony formation in vitro, and inhibits tumorigenesis and growth in mice. Taken together, VHH pool-based strategy greatly improves MET-targeted therapeutic effects on cancer.

A novel bispecific c-MET/PD-1 antibody with therapeutic potential in solid cancer

The bispecific antibody is a novel antibody, which can target two different antigens and mediate specific killing effects by selectively redirecting effector cells to the target cells. Here, we designed and synthesized a bispecific antibody (BsAb) that can bind cellular-mesenchymal to epithelial transition factor (c-MET, overexpressed in several human solid tumor), and programmed death-1 (PD-1, involved in cancer cell immune evasion) with high affinity and specificity. We found that BsAb can induce the degradation of c-MET protein in cancer cells, including MKN45, a gastric cancer cell line, and A549, a lung cancer cell line. BsAb inhibited hepatocyte growth factor (HGF)-mediated proliferation, migration, and antiapoptosis, and downregulated HGF-stimulated phosphorylation of c-MET, protein kinase B (AKT), and extracellular signal-regulated kinase (ERK1/2). BsAb can also rescue T cell activation. Furthermore, xenograft analysis revealed that BsAb markedly inhibits the growth of subcutaneously implanted tumors and chronic inflammation. On the basis of these results, we have identified a potential bispecific drug, which can effectively target c-MET and PD-1 for the treatment of human solid cancers.

Targeting blockage of STAT3 inhibits hepatitis B virus-related hepatocellular carcinoma

Hepatitis B virus (HBV) infection is a significant cause of liver disease pathogenesis, which results in the development of hepatic dysfunction, cirrhosis and hepatocellular carcinoma (HCC). Our previous studies showed that oncogene STAT3 might be an ideal target for HCC therapy. Here, we investigated whether targeting blockage of STAT3 signaling is efficient for HBV-related HCC. Based on the refractory of HCC and the persistence of HBV, in this study, we designed shRNAs targeting STAT3. The results showed that blocking STAT3 signaling by shRNAs could promote HBV positive HCC cell apoptosis and induce cell cycle arrest, resulting in HCC cell growth inhibition in vitro. Importantly, STAT3-shRNAs efficiently suppressed HBV replication, which would reduce HBV-derived stimulation to STAT3 signaling and augment STAT3-shRNAs-mediated anti-HCC effect. Finally, STAT3-shRNAs-mediated anti-HBV positive HCC effect was confirmed in xenograft nude mice. This study suggested that targeting STAT3 therapies such as STAT3-shRNAs may be an efficacious strategy for HBV-related HCC.

Histidine-rich stabilized polyplexes for cMet-directed tumor-targeted gene transfer

Overexpression of the hepatocyte growth factor receptor/c-Met proto oncogene on the surface of a variety of tumor cells gives an opportunity to specifically target cancerous tissues. Herein, we report the first use of c-Met as receptor for non-viral tumor-targeted gene delivery. Sequence-defined oligomers comprising the c-Met binding peptide ligand cMBP2 for targeting, a monodisperse polyethylene glycol (PEG) for polyplex surface shielding, and various cationic (oligoethanamino) amide cores containing terminal cysteines for redox-sensitive polyplex stabilization, were assembled by solid-phase supported syntheses. The resulting oligomers exhibited a greatly enhanced cellular uptake and gene transfer over non-targeted control sequences, confirming the efficacy and target-specificity of the formed polyplexes. Implementation of endosomal escape-promoting histidines in the cationic core was required for gene expression without additional endosomolytic agent. The histidine-enriched polyplexes demonstrated stability in serum as well as receptor-specific gene transfer in vivo upon intratumoral injection. The co-formulation with an analogous PEG-free cationic oligomer led to a further compaction of pDNA polyplexes with an obvious change of shape as demonstrated by transmission electron microscopy. Such compaction was critically required for efficient intravenous gene delivery which resulted in greatly enhanced, cMBP2 ligand-dependent gene expression in the distant tumor.

Combining onartuzumab with erlotinib inhibits growth of non-small cell lung cancer with activating EGFR mutations and HGF overexpression

Erlotinib, a tyrosine kinase inhibitor of the epidermal growth factor receptor (EGFR-TKI), benefits survival of patients with non-small cell lung cancer (NSCLC) who harbor activating EGFR mutations. However, elevated expression of hepatocyte growth factor (HGF), a ligand of the MET receptor tyrosine kinase, causes erlotinib resistance. Because onartuzumab, a monovalent antibody to MET, blocks HGF-induced MET activation, the addition of onartuzumab to erlotinib may improve therapeutic efficacy. We engineered the human NSCLC cell line PC-9 (MET-positive cells harboring an exon 19 deletion of EGFR) to overexpress hHGF and evaluated the effects of an onartuzumab and erlotinib combination in vitro and in vivo in xenograft models. A stable clone of PC-9/hHGF was less sensitive to erlotinib than the parental PC-9, and the addition of onartuzumab to erlotinib suppressed the proliferation of these cells in vitro. In PC-9/hHGF xenograft tumors, onartuzumab or erlotinib alone minimally inhibited tumor growth; however, combining onartuzumab and erlotinib markedly suppressed tumor growth. The total MET protein level was decreased in PC-9/hHGF cells, because MET is constitutively phosphorylated by autocrine HGF, leading to its ubiquitination and degradation. Onartuzumab reduced phospho-MET levels, inhibited MET ubiquitination, and consequently restored MET protein levels. Moreover, in NSCLC clinical specimens harboring activating EGFR mutations, more than 30% of patients expressed high levels of HGF. Our findings raised the possibility that patients with NSCLC with EGFR mutations who express high levels of HGF may benefit from onartuzumab and erlotinib combination therapy, and that HGF can be a novel biomarker for selecting such patients.

An update in the use of antibodies to treat glioblastoma multiforme

Glioblastoma is a deadly brain disease and modest improvement in survival has been made. At initial diagnosis, treatment consists of maximum safe surgical resection, followed by temozolomide and chemoirradiation or adjuvant temozolomide alone. However, these treatments do not improve the prognosis and survival of patients. New treatment strategies are being sought according to the biology of tumors. The epidermal growth factor receptor has been considered as the hallmark in glioma tumors; thereby, some antibodies have been designed to bind to this receptor and block the downstream signaling pathways. Also, it is known that vascularization plays an important role in supplying new vessels to the tumor; therefore, new therapy has been guided to inhibit angiogenic growth factors in order to limit tumor growth. An innovative strategy in the treatment of glial tumors is the use of toxins produced by bacteria, which may be coupled to specific carrier-ligands and used for tumoral targeting. These carrier-ligands provide tumor-selective properties by the recognition of a cell-surface receptor on the tumor cells and promote their binding of the toxin-carrier complex prior to entry into the cell. Here, we reviewed some strategies to improve the management and treatment of glioblastoma and focused on the use of antibodies.

Construction, expression, and characterization of an anti-tumor immunotoxin containing the human anti-c-Met single-chain antibody and PE38KDEL

Recombinant immunotoxins consisting of small antibody fragments fused to cytotoxic moieties are being evaluated for use in prospective antibody-targeted cancer therapies. A receptor tyrosine kinase known as c-Met is overexpressed in a vast range of human malignancies, making it an ideal target for antibody-mediated delivery of numerous cytotoxic agents. A single Fab molecule capable of binding to human c-Met with high affinity and specificity was previously identified using antibody phage-display technology. In order to develop a molecule to increase both the cytotoxicity and anti-tumor activity of the anti-c-Met molecule, a recombinant immunotoxin anti-c-Met/PE38KDEL was constructed and expressed by fusing the human anti-c-Met single-chain variable fragment (ScFv) with a modified Pseudomonas exotoxin A (PE38KDEL). Purified anti-c-Met/PE38KDEL was demonstrated to specifically bind to cells of c-Met-positive human hepatoma cell lines, causing a proliferation defect by inducing caspase-3/8-mediated apoptosis, as observed by in vitro assays. Furthermore, anti-c-Met/PE38KDEL administration was shown to inhibit the growth of hepatocellular carcinoma xenografts in vivo through suppression of Ki-67 expression and enhancement of tumor cell apoptosis rates. Cumulatively, the current findings demonstrate the successful construction of a recombinant immunotoxin capable of accurately targeting c-Met-positive human hepatoma cell lines both in vitro and in vivo, providing a novel compound with potential for applications as an alternative therapy for c-Met-positive cancer management.

In the clinic: ongoing clinical trials evaluating c-MET-inhibiting drugs

The c-MET (mesenchymal-epithelial transition factor) pathway is dysregulated in many human cancers and promotes tumor growth, invasion and dissemination. The c-MET receptor tyrosine kinase can be activated via gene mutation, gene amplification, protein overexpression and/or a ligand-dependent autocrine/paracrine loop. Abnormalities in c-MET signaling have been reported to correlate with poor clinical outcomes and drug resistance in patients with cancer. Significant progress has been made in advancement of c-MET pathway inhibitors through to clinical trials. A robust pipeline of high-quality inhibitors targeting different aspects of c-MET activation is currently being explored in phase I, II and III clinical trials across multiple tumor types. Preliminary data demonstrate promising clinical activity with these agents, along with an acceptable toxicity profile. In this manuscript, the pharmacological profile of drugs targeting the c-MET pathway and available data from ongoing clinical trials of these drugs are discussed.

Enhancement of antitumor activity of gammaretrovirus carrying IL-12 gene through genetic modification of envelope targeting HER2 receptor: a promising strategy for bladder cancer therapy

The objective of this study was to develop an HER2-targeted, envelope-modified Moloney murine leukemia virus (MoMLV)-based gammaretroviral vector carrying interleukin (IL)-12 gene for bladder cancer therapy. It displayed a chimeric envelope protein containing a single-chain variable fragment (scFv) antibody to the HER2 receptor and carried the mouse IL-12 gene. The fragment of anti-erbB2scFv was constructed into the proline-rich region of the viral envelope of the packaging vector lacking a transmembrane subunit of the carboxyl terminal region of surface subunit. As compared with envelope-unmodified gammaretroviruses, envelope-modified ones had extended viral tropism to human HER2-expressing bladder cancer cell lines, induced apoptosis, and affected cell cycle progression despite lower viral titers. Moreover, animal studies showed that envelope-modified gammaretroviruses carrying IL-12 gene exerted higher antitumor activity in terms of retarding tumor growth and prolonging the survival of tumor-bearing mice than unmodified ones, which were associated with enhanced tumor cell apoptosis as well as increased intratumoral levels of IL-12, interferon-gamma, IL-1beta, and tumor necrosis factor-alpha proteins. Therefore, the antitumor activity of gammaretroviruses carrying the IL-12 gene was enhanced through genetic modification of the envelope targeting HER2 receptor, which may be a promising strategy for bladder cancer therapy.

Novel therapeutic inhibitors of the c-Met signaling pathway in cancer

A wide variety of human malignancies exhibit sustained c-Met stimulation, overexpression, or mutation, including carcinomas of the breast, liver, lung, ovary, kidney, and thyroid. Notably, activating mutations in c-Met have been positively identified in patients with a particular hereditary form of papillary renal cancer, directly implicating c-Met in human tumorigenesis. Aberrant signaling of the c-Met signaling pathway due to dysregulation of the c-Met receptor or overexpression of its ligand, hepatocyte growth factor (HGF), has been associated with an aggressive phenotype. Extensive evidence that c-Met signaling is involved in the progression and spread of several cancers and an enhanced understanding of its role in disease have generated considerable interest in c-Met and HGF as major targets in cancer drug development. This has led to the development of a variety of c-Met pathway antagonists with potential clinical applications. The three main approaches of pathway-selective anticancer drug development have included antagonism of ligand/receptor interaction, inhibition of the tyrosine kinase catalytic activity, and blockade of the receptor/effector interaction. Several c-Met antagonists are now under clinical investigation. Preliminary clinical results of several of these agents, including both monoclonal antibodies and small-molecule tyrosine kinase inhibitors, have been encouraging. Several multitargeted therapies have also been under investigation in the clinic and have demonstrated promise, particularly with regard to tyrosine kinase inhibition.

A strategy for systemic delivery of the oncolytic herpes virus HSV1716: redirected tropism by antibody-binding sites incorporated on the virion surface as a glycoprotein D fusion protein

We report on the ability of single-chain variable fragment (scFv) incorporated into the viral envelope to alter the tropism of herpes simplex virus (HSV) 1716. Using recombinant viruses expressing fusion proteins comprising cell-surface antigen-specific scFvs N terminus linked to amino acids 274-393 of gD, we demonstrated that the tropism of these HSV1716 variants was modified such that infection was mediated by the cognate antigen. Thus, an HSV1716 variant that expressed an anti-CD55 scFv targeting moiety linked to these gD residues was able to infect non-permissive Chinese hamster ovary cells expressing CD55 and this infection was specifically blocked by an anti-CD55 monoclonal antibody. Similarly, the infection efficiency of an HSV1716 variant for semi-permissive human leukaemic, CD38-positive cell lines was greatly improved by an anti-CD38 scFv targeting moiety linked to gD residues 274-393, and this enhanced infectivity was abrogated specifically by an anti-CD38 monoclonal antibody. Finally, intravenous/intraperitoneal injection of an HSV1716 variant displaying an anti-epidermal growth factor receptor (EGFR) scFv linked to residues 274-393 of gD enhanced destruction of subcutaneous EGFR-positive tumours in nude mice compared to unmodified HSV1716. Therefore, targeting of HSV1716 oncolysis to specific cell types through the display of entry mediating scFv/gD fusion proteins represents an efficient route for systemic delivery.

Downstream signalling and specific inhibition of c-MET/HGF pathway in small cell lung cancer: implications for tumour invasion

The c-MET receptor can be overexpressed, amplified, or mutated in solid tumours including small cell lung cancer (SCLC). In c-MET-overexpressing SCLC cell line NCI-H69, hepatocyte growth factor (HGF) dramatically induced c-MET phosphorylation at phosphoepitopes pY1230/1234/1235 (catalytic tyrosine kinase), pY1003 (juxtamembrane), and also of paxillin at pY31 (CRKL-binding site). We utilised a global proteomics phosphoantibody array approach to identify further c-MET/HGF signal transduction intermediates in SCLC. Strong HGF induction of specific phosphorylation sites in phosphoproteins involved in c-MET/HGF signal transduction was detected, namely adducin-alpha [S724], adducin-gamma [S662], CREB [S133], ERK1 [T185/Y187], ERK1/2 [T202/Y204], ERK2 [T185/Y187], MAPKK (MEK) 1/2 [S221/S225], MAPKK (MEK) 3/6 [S189/S207], RB [S612], RB1 [S780], JNK [T183/Y185], STAT3 [S727], focal adhesion kinase (FAK) [Y576/S722/S910], p38alpha-MAPK [T180/Y182], and AKT1[S473] and [T308]. Conversely, inhibition of phosphorylation by HGF in protein kinase C (PKC), protein kinase R (PKR), and also CDK1 was identified. Phosphoantibody-based immunohistochemical analysis of SCLC tumour tissue and microarray established the role of c-MET in SCLC biology. This supports a role of c-MET activation in tumour invasive front in the tumour progression and invasion involving FAK and AKT downstream. The c-MET serves as an attractive therapeutic target in SCLC, as shown through small interfering RNA (siRNA) and selective prototype c-MET inhibitor SU11274, inhibiting the phosphorylation of c-MET itself and its downstream molecules such as AKT, S6 kinase, and ERK1/2. Investigation of mechanisms of invasion and, ultimately, metastasis in SCLC would be very useful with these signal transduction molecules.

A Novel One-Armed Anti-c-Met Antibody Inhibits Glioblastoma GrowthIn vivo

PURPOSE

Expression of the receptor tyrosine kinase c-Met and its ligand scatter factor/hepatocyte growth factor (SF/HGF) are strongly increased in glioblastomas, where they promote tumor proliferation, migration, invasion, and angiogenesis. We used a novel one-armed anti-c-Met antibody to inhibit glioblastoma growth in vivo.

EXPERIMENTAL DESIGN

U87 glioblastoma cells (c-Met and SF/HGF positive) or G55 glioblastoma cells (c-Met positive and SF/HGF negative) were used to generate intracranial orthotopic xenografts in nude mice. The one-armed 5D5 (OA-5D5) anti-c-Met antibody was infused intratumorally using osmotic minipumps. Following treatment, tumor volumes were measured and tumors were analyzed histologically for extracellular matrix (ECM) components and proteases relevant to tumor invasion. Microarray analyses were done to determine the effect of the antibody on invasion-related genes.

RESULTS

U87 tumor growth, strongly driven by SF/HGF, was inhibited > 95% with OA-5D5 treatment. In contrast, G55 tumors, which are not SF/HGF driven, did not respond to OA-5D5, suggesting that the antibody can have efficacy in SF/HGF-activated tumors. In OA-5D5-treated U87 tumors, cell proliferation was reduced > 75%, microvessel density was reduced > 90%, and apoptosis was increased > 60%. Furthermore, OA-5D5 treatment decreased tumor cell density > 2-fold, with a consequent increase in ECM deposition and increased immunoreactivity for laminin, fibronectin, and tenascin. Microarray studies showed no increase in these ECM factors, rather down-regulation of urokinase-type plasminogen activator and matrix metalloproteinase 16 in glioblastoma cells treated with OA-5D5.

CONCLUSIONS

Local treatment with OA-5D5 can almost completely inhibit intracerebral glioblastoma growth when SF/HGF is driving tumor growth. The mechanisms of tumor inhibition include antiproliferative, antiangiogenic, and proapoptotic effects.

Gene therapy of liver cancer

The application of gene transfer technologies to the treatment of cancer has led to the development of new experimental approaches like gene directed enzyme/pro-drug therapy (GDEPT), inhibition of oncogenes and restoration of tumor-suppressor genes. In addition, gene therapy has a big impact on other fields like cancer immunotherapy, anti-angiogenic therapy and virotherapy. These strategies are being evaluated for the treatment of primary and metastatic liver cancer and some of them have reached clinical phases. We present a review on the basis and the actual status of gene therapy approaches applied to liver cancer.

Efficient ex vivo gene transfer into non-human primate hepatocytes using HIV-1 derived lentiviral vectors

BACKGROUND/AIMS

Lentivirus-mediated ex vivo gene therapy is becoming a promising approach for the treatment of liver metabolic disorders. However, the feasibility of this approach needs to be studied in large animal models. The purpose of this study was to evaluate the efficacy of ex vivo gene transfer into Macaca hepatocytes with two different HIV-1 derived lentiviral vectors.

METHODS

A self-inactivating lentivector was constructed to express GFP under the control of the hepatic apolipoprotein A-II promoter. Freshly isolated and thawed hepatocytes were transduced in suspension with lentiviral vectors expressing the GFP gene under the control of a ubiquitous promoter (EF1-alpha) and the apolipoprotein A-II promoter. Transduced thawed hepatocytes were transplanted into the spleen of newborn mice, and livers analyzed 4 and 12 weeks after transplantation.

RESULTS

We show that lentivectors are efficient in transducing hepatocytes in suspension either freshly isolated or cryopreserved. We also show that thawed and transduced hepatocytes engrafted and participated in liver growth after transplantation into newborn mice and that the apolipoprotein A-II promoter is functional.

CONCLUSIONS

Our data show that transplantation of transduced hepatocytes into monkeys should allow to evaluate the fate of transplanted cells and transgene expression in a pre-clinical model of ex vivo gene therapy.

An adenovirus vector with a chimeric fiber incorporating stabilized single chain antibody achieves targeted gene delivery

Adenovirus (Ad) vectors are of utility for many therapeutic applications. Strategies have been developed to alter adenoviral tropism to achieve a cell-specific gene delivery capacity employing fiber modifications allowing genetic incorporation of targeting motifs. In this regard, single chain antibodies (scFv) represent potentially useful agents to achieve targeted gene transfer. However, the distinct biosynthetic pathways that scFv and Ad capsid proteins are normally routed through have thus far been problematic with respect to scFv incorporation into the Ad capsid. Utilization of stable scFv, which also maintain correct folding and thus functionality under intracellular reducing conditions, could overcome this restriction. We genetically incorporated a stable scFv into a de-knobbed, fibritin-foldon trimerized Ad fiber and demonstrated selective targeting to the cognate epitope expressed on the membrane surface of cells. We have shown that the scFv employed in this study retains functionality and that stabilizing the targeting molecule, per se, is critical to allow retention of antigen recognition in the adenovirus capsid-incorporated context.

Histone deacetylase inhibitors for treatment of hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is one of the most common cancers in the world. Surgical resection has been considered the optimal treatment approach, but only a small proportion of patients are suitable candidates for surgery, and the relapse rate is high. Approaches to prevent recurrence, including chemoembolization before and adjuvant therapy after surgery, have proven to have a limited benefit; liver transplantation is successful in treating limited-stage HCC because only a minority of patients qualify for transplantation. Therefore, new therapeutic strategies are urgently needed. Because in addition to the classical genetic mechanisms of deletion or inactivating point mutations, epigenetic alterations, such as hyperacetylation of the chromatin-associated histones (responsible for gene silencing), are believed to be involved in the development and progression of HCC, novel compounds endowed with a histone deacetylase (HDAC) inhibitory activity are an attractive therapeutic approach. In particular, pre-clinical results obtained using HA-But, an HDAC inhibitor in which butyric acid residues are esterified to a hyaluronic acid backbone and characterized by a high affinity for the membrane receptor CD44, indicated that this class of compounds may represent a promising approach for hepatocellular carcinoma treatment.

Retroviral display of urokinase-binding domain fused to amphotropic envelope protein

Tumors frequently express urokinase (uPA) receptor (uPAR). To investigate whether uPAR can efficiently target cancerous cells using amphotropic retroviral vectors, we generated a retrovirus displaying the amino-terminal fragment (ATF) of uPA as an N-terminal extension of viral envelope protein. We also made use of a "two-step strategy" by inserting a uPA cleavage site between the ATF moiety and the envelope. We measured the ability of ATF-bearing chimeric envelopes to infect huPAR-overexpressing Madin-Darby canine kidney (MDCK) and control MDCK II cells. The ATF-viruses infected both MDCK cell lines with an equivalent efficiency, suggesting that the chimeric viruses were not sequestered by uPAR and infect cells preferentially via the Pit-2 receptor. The addition of a uPA cleavage site increased the infection level of huPAR-MDCK cells by 2-fold when uPA was present in the infection medium. Surprisingly, ATF-env viruses infected huPAR-MDCK cells 5.5-fold more efficiently in the presence of exogenous uPA. This stimulatory effect of uPA on infection of huPAR-MDCK cells by the ATF-env virus was completely abolished by methyl-beta-cyclodextrin, suggesting that this effect involves the caveolar endocytosis pathway.

Therapeutic potential of retroviral RNAi vectors

The ability of small interfering RNA (siRNA) to mediate gene-specific post-transcriptional silencing in mammalian cells will undoubtedly revolutionise functional genomics, as well as drug target identification and validation. Furthermore, there is widespread excitement that siRNA itself might prove useful in the clinical setting. For those wishing to develop siRNA as a therapeutic agent, the most difficult obstacle to overcome will be delivery. Recently, several breakthroughs have highlighted viruses as excellent vehicles for siRNA delivery. Retroviruses, the transgene-delivery vector of choice for many experimental gene therapy studies, have been engineered to deliver and stably express therapeutic siRNA within cells, both in vitro and in vivo. These findings are important milestones for the development of siRNA as a gene therapy for treatment of viral infections, cancer, autoimmune syndromes and numerous genetic disorders. This review describes the development of retroviral siRNA vectors, highlights proof-of-concept experiments demonstrating their therapeutic efficacy and explores therapeutic targets particularly suitable for retroviral-mediated gene silencing.

Gene transfer of constitutively active caspase-3 induces apoptosis in a human hepatoma cell line

BACKGROUND

The caspase-3 gene is expressed at significantly lower levels in human hepatocellular carcinomas than in normal hepatocytes. Gene transfer technologies offer the possibility to restore caspase-3 gene expression. We explored the interest for cancer gene therapy of a constitutively active recombinant caspase-3 (RevCasp3) obtained by rearranging its subunits.

METHODS

An amphotropic retroviral vector was used to express the RevCasp3 gene. HuH7 cells were infected 1 and 2 days after plating. Caspase-3 activity was measured every 24 h for the following 6 days. The level of poly (ADP-ribose) polymerase cleavage induced by caspase-3 was measured by Western blot. The percentage of apoptotic cells was estimated after Hoechst-acridine orange and TUNEL stainings.

RESULTS

Caspase-3 activity significantly increased from days 4 to 7 after infection. Caspase-3 activity peaked on day 7, and was 5.4-fold higher in RevCasp3-transduced HuH7 cells than in control cells. Poly (ADP-ribose) polymerase cleavage was first detected 6 days after the first infection. Hoechst-acridine orange and TUNEL stainings showed that most infected HuH7 cells were apoptotic.

CONCLUSIONS

Apoptosis was selectively induced following infection of HuH7 cells with RevCasp3, demonstrating that retroviruses expressing RevCasp3 are of potential interest for the treatment of hepatocellular carcinomas and other tumour types.