Purified herpes simplex virus thymidine kinase retroviral particles: III. Characterization of bystander killing mechanisms in transfected tumor cells

Neural Progenitor Cells Expressing Herpes Simplex Virus-Thymidine Kinase for Ablation Have Differential Chemosensitivity to Brivudine and Ganciclovir

Neural progenitor cell (NPC) transplants are a promising therapy for treating spinal cord injury (SCI), however, their long-term role after engraftment and the relative contribution to ongoing functional recovery remains a key knowledge gap. Selective human cell ablation techniques, currently being developed to improve the safety of progenitor cell transplant therapies in patients, may also be used as tools to probe the regenerative effects attributable to individual grafted cell populations. The Herpes Simplex Virus Thymidine Kinase (HSV-TK) and ganciclovir (GCV) system has been extensively studied in the context of SCI and broader CNS disease. However, the efficacy of brivudine (BVDU), another HSV-TK prodrug with potentially reduced bystander cytotoxic effects and in vivo toxicity, has yet to be investigated for NPC ablation. In this study, we demonstrate successful generation and in vitro ablation of HSV-TK-expressing human iPSC-derived NPCs with a >80% reduction in survival over controls. We validated an HSV-TK and GCV/BVDU synergistic system with iPSC-NPCs using an efficient gene-transfer method and in vivo ablation in a translationally relevant model of SCI. Our findings demonstrate enhanced ablation efficiency and reduced bystander effects when targeting all rapidly dividing cells with combinatorial GCV and BVDU treatment. However, for use in loss of function studies, BVDU alone is optimal due to reduced nonselective cell ablation.

Connexins in Cancer: Jekyll or Hyde?

The expression, localization, and function of connexins, the protein subunits that comprise gap junctions, are often altered in cancer. In addition to cell-cell coupling through gap junction channels, connexins also form hemichannels that allow communication between the cell and the extracellular space and perform non-junctional intracellular activities. Historically, connexins have been considered tumor suppressors; however, they can also serve tumor-promoting functions in some contexts. Here, we review the literature surrounding connexins in cancer cells in terms of specific connexin functions and propose that connexins function upstream of most, if not all, of the hallmarks of cancer. The development of advanced connexin targeting approaches remains an opportunity for the field to further interrogate the role of connexins in cancer phenotypes, particularly through the use of in vivo models. More specific modulators of connexin function will both help elucidate the functions of connexins in cancer and advance connexin-specific therapies in the clinic.

The Role of Oncolytic Viruses in the Treatment of Melanoma

PURPOSE OF REVIEW

Oncolytic virotherapy is a new approach to the treatment of cancer and its success in the treatment of melanoma represents a breakthrough in cancer therapeutics. This paper provides a review of the current literature on the use of oncolytic viruses (OVs) in the treatment of melanoma.

RECENT FINDINGS

Talimogene laherparepvec (T-VEC) is the first OV approved for the treatment of melanoma and presents new challenges as it enters the clinical setting. Several other OVs are at various stages of clinical and pre-clinical development for the treatment of melanoma. Reports from phase Ib-III clinical trials combining T-VEC with checkpoint blockade are encouraging and demonstrate potential added benefit of combination immunotherapy. OVs have recently emerged as a standard treatment option for patients with advanced melanoma. Several OVs and therapeutic combinations are in development. Immunooncolytic virotherapy combined with immune checkpoint inhibitors is promising for the treatment of advanced melanoma.

Microglia in Cancer: For Good or for Bad?

Glioblastoma is a malignant tumor of astrocytic origin that is highly invasive, proliferative and angiogenic. Despite current advances in multimodal therapies, such as surgery, radio- and chemotherapy, the outcome for patients with glioblastoma is nearly always fatal. The glioblastoma microenvironment has a tremendous influence over the tumor growth and spread. Microglia and macrophages are abundant cells in the tumor mass. Increasing evidence indicates that glioblastoma recruits these cell populations and signals in a way that microglia and macrophages are subverted to promote tumor progression. In this chapter, we discuss some aspects of the interaction between microglia and glioblastoma, consequences of this interaction for tumor progression and the possibility of microglial cells being used as therapeutic vectors, which opens up new alternatives for the development of GBM therapies targeting microglia.

Connexins in colorectal cancer pathogenesis

The connexins constitute a family of integral membrane proteins that form channels between adjacent cells. These channels are assembled in plasma membrane domains known as gap junctions and enable cells to directly exchange ions and small molecules. Intercellular communication via gap junctions plays important roles in regulating cell growth and differentiation and in maintaining tissue homeostasis. This type of cell communication is often impaired during cancer development, and several members of the connexin protein family have been shown to act as tumor suppressors. Emerging evidence suggests that the connexin protein family has important roles in colorectal cancer development. In the normal colonic epithelial tissue, three connexin isoforms, connexin 26 (Cx26), Cx32 and Cx43, have been shown to be expressed at the protein level. Colorectal cancer development is associated with loss of connexin expression or relocalization of connexins from the plasma membrane to intracellular compartments. Downregulation of connexins in colorectal carcinomas at the transcriptional level involves cancer-specific promoter hypermethylation. Recent studies suggest that Cx43 may constrain growth of colon cancer cells by interfering with the Wnt/β-catenin pathway. There is also increasing evidence that the connexins may have potential as prognostic markers in colorectal cancer. This review discusses the role of connexins in colorectal cancer pathogenesis, as well as their potential as prognostic markers and targets in the prevention and treatment of the disease.

Gap junctions propagate opposite effects in normal and tumor testicular cells in response to cisplatin

Gap junctions propagate toxic effects among tumor cells during chemotherapy, but could also enhance killing of normal cells by the same mechanism. We show that the effect of gap junctional intercellular communication (GJIC) on cisplatin toxicity differs between normal and tumor testicular cells. Downregulation of GJIC by each of several different manipulations (no cell contact, pharmacological inhibition, siRNA suppression) decreased cisplatin cytoxicity in tumor cells but enhanced it in normal cells. Enhanced toxicity due to GJIC downregulation in normal cells correlated with increased DNA interstrand crosslinks. Thus, GJIC protects normal cells from cisplatin toxicity while enhancing it in tumor cells, suggesting that enhancement/maintenance of GJIC increases therapeutic efficacy while decreasing off-target toxicity.

A heterotypic bystander effect for tumor cell killing after adeno-associated virus/phage-mediated, vascular-targeted suicide gene transfer

Suicide gene transfer is the most commonly used cytotoxic approach in cancer gene therapy; however, a successful suicide gene therapy depends on the generation of efficient targeted systemic gene delivery vectors. We recently reported that selective systemic delivery of suicide genes such as herpes simplex virus thymidine kinase (HSVtk) to tumor endothelial cells through a novel targeted adeno-associated virus/phage vector leads to suppression of tumor growth. This marked effect has been postulated to result primarily from the death of cancer cells by hypoxia following the targeted disruption of tumor blood vessels. Here, we investigated whether an additional mechanism of action is involved. We show that there is a heterotypic "bystander" effect between endothelial cells expressing the HSVtk suicide gene and tumor cells. Treatment of cocultures of HSVtk-transduced endothelial cells and non-HSVtk-transduced tumor cells with ganciclovir results in the death of both endothelial and tumor cells. Blocking of this effect by 18alpha-glycyrrhetinic acid indicates that gap junctions between endothelial and tumor cells are largely responsible for this phenomenon. Moreover, the observed bystander killing is mediated by connexins 43 and 26, which are expressed in endothelial and tumor cell types. Finally, this heterotypic bystander effect is accompanied by a suppression of tumor growth in vivo that is independent of primary gene transfer into host-derived tumor vascular endothelium. These findings add an alternative nonmutually exclusive and potentially synergistic cytotoxic mechanism to cancer gene therapy based on targeted adeno-associated virus/phage and further support the promising role of nonmalignant tumor stromal cells as therapeutic targets.

Antitumor therapy based on cellular competition

A major obstacle for the efficacy of cancer gene therapy is the need to transduce a high proportion of tumor cells with genes that directly or indirectly cause their death. During the formation of certain organs, cells compete among themselves to colonize the whole tissue. We reasoned that cell competition could be used to increase the proportion of cells that become transfected in a tumor. For this, a transgene that provides a selective advantage to the transfected cells should be used. If the same gene conferred a suicide mechanism the tumor could be eradicated after a period of selection. Bystander effect of transfected cells over neighboring nonmodified cells may eliminate tumors even with incomplete replacement of tumor cells. To test this strategy a competitive advantage was provided to colon cancer cells, using a gene encoding a fusion protein of dihydrofolate reductase (DHFR) and thymidine kinase (TK). DHFR confers resistance to methotrexate (MTX) and TK confers sensitivity to ganciclovir (GCV). Modified cells were also transduced with green fluorescent protein and parental cells with red fluorescent protein. In vitro and in vivo experiments were performed, using various proportions of modified cells and applying positive selection with MTX followed by negative selection with GCV. In vitro, cell competition was evident. Under MTX treatment, tumor cells transfected with the DHFR-TK fusion gene efficiently replaced the parental cells (from 0.1 to 90% in 35 days). After this positive selection period, negative selection with GCV eliminated the transfected cells. In vivo, positive selection was also achieved and resulted in a statistically significant therapeutic effect.

Acyclovir is activated into a HIV-1 reverse transcriptase inhibitor in herpesvirus-infected human tissues

For most viruses, there is a need for antimicrobials that target unique viral molecular properties. Acyclovir (ACV) is one such drug. It is activated into a human herpesvirus (HHV) DNA polymerase inhibitor exclusively by HHV kinases and, thus, does not suppress other viruses. Here, we show that ACV suppresses HIV-1 in HHV-coinfected human tissues, but not in HHV-free tissue or cell cultures. However, addition of HHV-6-infected cells renders these cultures sensitive to anti-HIV ACV activity. We hypothesized that such HIV suppression requires ACV phosphorylation by HHV kinases. Indeed, an ACV monophosphorylated prodrug bypasses the HHV requirement for HIV suppression. Furthermore, phosphorylated ACV directly inhibits HIV-1 reverse transcriptase (RT), terminating DNA chain elongation, and can trap RT at the termination site. These data suggest that ACV anti-HIV-1 activity may contribute to the response of HIV/HHV-coinfected patients to ACV treatment and could guide strategies for the development of new HIV-1 RT inhibitors.

ATRA enhances bystander effect of suicide gene therapy in the treatment of prostate cancer

OBJECTIVES

All-trans retinoic acid (ATRA) has been shown to inhibit the growth of many malignancies by altering gap junctional intercellular communication (GJIC) and the expression of connexin (Cx) 43. Here, we report that the alteration of GJIC by ATRA may directly enhance the bystander effect (BE) of suicide gene therapy against prostate cancer in vitro and in vivo.

METHODS

PC-3 cells were exposed to different concentrations of ATRA for varying lengths of time in culture. Flow cytometry was performed to measure Cx43-positive cells and the GJIC function of the cells was examined with the scrape-loading dye transfer assay. Cells were treated with ATRA in combination with an adenovirus/ganciclovir (Ad-TK/GCV) system encoding herpes simplex virus-thymidine kinase, and the BE was assessed in the treatment of androgen-independent prostate cancer both in vitro and in vivo. Semiquantitative reverse transcription polymerase chain reaction (RT-PCR) and immunohistochemistry were performed to assess the expression of Cx43 mRNA and protein in tumor tissues.

RESULTS

ATRA significantly increased the amount of Cx43-positive cells in a time- and dose-dependent manner (P < 0.05). GJIC functions were enhanced 3- to 5-fold in the presence of ATRA, although ATRA did not augment GCV toxicity of PC-3 cells. In the mixing assay, ATRA significantly increased cell killing when the ratio of TK-positive cells in the coculture ranged from 30% to 60% compared with ATRA-untreated cell (P < 0.05), and attained 50% cell killing cells when the ratio of TK-positive cell was 30%, but the same result did not appear until the ratio of TK-positive cell was up to 60% in the ATRA-untreated cell. Mice treated with a combination of ATRA and GCV had significantly smaller Ad-TK infected tumors than those treated with GCV or ATRA alone after 3-weeks of therapy (P < 0.05). However, from the fourth-week of therapy, there was no difference in tumor growth inhibition between GCV treatment and GCV + ATRA treatment (P > 0.05), as two tumors in the latter group started to grow more quickly than tumors in the control group. This phenomenon was not found in other groups.

CONCLUSIONS

ATRA could enhance the efficiency of cell killing in suicide gene therapy against prostate cancer by strengthening the BE in vitro and in vivo. Induction of Cxs and GJIC by ATRA might provide an element of selectivity to suicide gene therapy. Future studies should focus on safety and tailoring this cooperative therapy to the patient.

Microglia used as vehicles for both inducible thymidine kinase gene therapy and MRI contrast agents for glioma therapy

Microglia are phagocytic cells that are chemoattracted by brain tumors and can represent up to 70% of the tumor cell population. To get insight into gene therapy against glioma, we decided to take advantage of those microglia properties and to use those cells as vehicles to transport simultaneously a suicide gene (under the control of a heat-sensitive promoter) and contrast agents to localize them by magnetic resonance imaging before applying any therapeutic treatment. Thymidine kinase (TK) expression and its functionality after gancyclovir administration were investigated. After the heat shock (44 degrees C and 20 min), TK was expressed in 50% of the cells. However, after gancyclovir treatment, 90% of the cells died by apoptosis, showing an important bystander effect. Then, the cells were incubated with new lanthanide contrast agents to check both their potential toxicity and their MR properties. Results indicate that the nanoparticles did not induce any cell toxicity and yield a hypersignal on MR images at 4.7 T. These in vitro experiments indicate that microglia are good candidates as vectors in gene therapy against brain tumors. Finally, microglia containing gadolinium-grafted nanoparticles were injected in the close vicinity of C6 tumor, in a mouse. The hyperintensive signal obtained on in vivo images as well as its retention time show the potential of the novel contrast agents for cellular imaging.

HDAC inhibition amplifies gap junction communication in neural progenitors: potential for cell-mediated enzyme prodrug therapy

Enzyme prodrug therapy using neural progenitor cells (NPCs) as delivery vehicles has been applied in animal models of gliomas and relies on gap junction communication (GJC) between delivery and target cells. This study investigated the effects of histone deacetylase (HDAC) inhibitors on GJC for the purpose of facilitating transfer of therapeutic molecules from recombinant NPCs. We studied a novel immortalized midbrain cell line, NGC-407 of embryonic human origin having neural precursor characteristics, as a potential delivery vehicle. The expression of gap junction protein connexin 43 (Cx43) was analyzed by western blot and immunocytochemistry. While Cx43 levels were decreased in untreated differentiating NGC-407 cells, the HDAC inhibitor 4-phenylbutyrate (4-PB) increased Cx43 expression along with increased membranous deposition in both proliferating and differentiating cells. Simultaneously, Ser 279/282-phosphorylated form of Cx43 was declined in both culture conditions by 4-PB. The 4-PB effect in NGC-407 cells was verified by using HNSC.100 human neural progenitors and Trichostatin A. Improved functional GJC is of imperative importance for therapeutic strategies involving intercellular transport of low molecular-weight compounds. We show here an enhancement by 4-PB, of the functional GJC among NGC-407 cells, as well as between NGC-407 and human glioma cells, as indicated by increased fluorescent dye transfer.

Brainstem tumors: where are we today?

Brainstem tumors comprise 10-20% of all pediatric central nervous system tumors. The management of these tumors has evolved dramatically in the past century. Once considered uniformly fatal, it is now known that brainstem tumors have distinguishing characteristics and do not behave identically. The focality and location of the lesion is determined from the clinical history, presentation, and associated imaging. Based on these findings, it is possible to predict the behavior of the tumor and choose an appropriate intervention. Focal lesions have a good prognosis and are treated operatively while diffuse lesions have a poor prognosis and are managed medically. This article reviews the current classification of brainstem tumors, current management options and future directions for the treatment of these rare tumors.

Hydroxyurea induces bystander cytotoxicity in cocultures of herpes simplex virus thymidine kinase-expressing and nonexpressing HeLa cells incubated with ganciclovir

Suicide gene therapy with the herpes simplex virus thymidine kinase (HSV-TK) cDNA and ganciclovir can elicit cytotoxicity to transgene-expressing and nonexpressing bystander cells via transfer of ganciclovir phosphates through gap junctions. HeLa cells do not exhibit bystander cytotoxicity, although we showed recently that they transfer low levels of ganciclovir phosphates to bystander cells. Here, we attempted to induce bystander cytotoxicity using hydroxyurea, an inhibitor of ribonucleotide reductase, to decrease the endogenous dGTP pool, which should lessen competition with ganciclovir triphosphate for DNA incorporation. Addition of hydroxyurea to cocultures of HSV-TK-expressing and bystander cells synergistically increased ganciclovir-mediated cytotoxicity to both cell populations while producing primarily an additive effect in cultures of 100% HSV-TK-expressing cells. Whereas HSV-TK-expressing cells in coculture were approximately 50-fold less sensitive to ganciclovir compared with cultures of 100% HSV-TK-expressing cells, addition of hydroxyurea restored ganciclovir sensitivity. Quantification of deoxynucleoside triphosphate pools showed that hydroxyurea decreased dGTP pools without significantly affecting ganciclovir triphosphate levels. Although hydroxyurea significantly increased the ganciclovir triphosphate:dGTP value for 12 to 24 hours in HSV-TK-expressing and bystander cells from coculture (1.4- to 4.9-fold), this value was increased for <12 hours (2.5-fold) in 100% HSV-TK-expressing cells. These data suggest that the prolonged increase in the ganciclovir triphosphate:dGTP value in cells in coculture resulted in synergistic cytotoxicity. Compared with enhancement of bystander cytotoxicity through modulation of gap junction intercellular communication, this strategy is superior because it increased cytotoxicity to both HSV-TK-expressing and bystander cells in coculture. This approach may improve clinical efficacy.

Connexin over-expression differentially suppresses glioma growth and contributes to the bystander effect following HSV-thymidine kinase gene therapy

Neoplastic transformation is frequently associated with a loss of gap junctional intercellular communication and reduced expression of connexins. The introduction of connexin genes into tumor cells reverses the proliferative characteristics of such cells. However, there is very little comparative information on the effects of different connexins on cancer cell growth. We hypothesized that Cx26, Cx32, or Cx43 would display differential growth suppression of C6 glioma cells and uniquely modulate the bystander effect following transduction of C6 cells with HSVtk followed by suicide gene therapy. The bystander phenomenon is the death of a greater number of tumor cells than are expressing the HSVtk gene, presumably due to the passage of toxic molecules through gap junction channels. To test this hypothesis, we used retroviral vectors to infect C6 glioma cells producing connexin-expressing and HSVtk-expressing cell lines. All three connexin-expressing cell lines grew significantly slower than GFP-infected or native C6 cells. Cx32 and Cx26 were significantly more effective at mediating the bystander effect in cocultures of C6-connexin cells with C6-HSVtk cells. These studies indicate that connexins have unique properties that contribute to their tumor suppressive function.

Increased anti-tumor effect by a combination of HSV thymidine kinase suicide gene therapy and interferon-gamma/GM-CSF cytokine gene therapy in CT26 tumor model

The potential therapeutic benefit of introducing IFN-gamma and GM-CSF genes in combination with the HSVtk suicide gene into subcutaneously implanted CT26 tumor cells was compared with that from each treatment alone. Cells, unmodified or retrovirally transduced with HSVtk or IFN-gamma/GM-CSF genes, were inoculated subcutaneously into syngeneic BALB/c mice in various combinations. HSVtk gene, with intraperitoneal ganciclovir treatment, reduced tumor volume by 81% at locally inoculated tumor sites (p < 0.01) and by 25% at distantly inoculated tumor sites (p = 0.052). IFN-gamma/GM-CSF genes showed a 56% tumor volume reduction at local tumor sites (p < 0.01) and 15% volume reduction at remote tumor sites, although this was not statistically significant. The combination of HSVtk (with GCV) and IFN-gamma/GM-CSF genes showed an 81% volume reduction at local tumor sites (p < 0.01) and a 43% volume reduction at remote tumor sites (p < 0.01). Thus, the combination of HSVtk and IFN-gamma/GM-CSF gene therapy produced greater therapeutic efficacy than either treatment alone.

Gap junctions and the propagation of cell survival and cell death signals

Gap junctions are a unique type of intercellular channels that connect the cytoplasm of adjoining cells. Each gap junction channel is comprised of two hemichannels or connexons and each connexon is formed by the aggregation of six protein subunits known as connexins. Gap junction channels allow the intercellular passage of small (< 1.5 kDa) molecules and regulate essential processes during development and differentiation. However, their role in cell survival and cell death is poorly understood. We review experimental data that support the hypothesis that gap junction channels may propagate cell death and survival modulating signals. In addition, we explore the hypothesis that hemichannels (or unapposed connexons) might be used as a paracrine conduit to spread factors that modulate the fate of the surrounding cells. Finally, direct signal transduction activity of connexins in cell death and survival pathways is addressed.

Migratory neural stem cells for improved thymidine kinase-based gene therapy of malignant gliomas

Gene therapy of glioma based on viral delivery of herpes simplex virus type I thymidine kinase (HSV-TK) has failed in the clinic because of low transduction efficacy. To circumvent this problem, this study evaluated highly migratory HSV-TK-transduced neural stem cells (NSC) for their ability to kill untransduced glioma cells by a gap junction-mediated bystander effect. The admixture of HSV-TK-transduced NSC to U87MG and LN-18 human malignant glioma cell lines at ratios of 1:10 or 1:1 eliminated more than 50% or 90% of glioma cells in the presence of ganciclovir (25 microM). Glioma cell cytotoxicity required cell-cell contact. Similarly, tumor cell cytotoxicity was observed in two of three primary glioblastoma cell cultures, and the presence of this bystander effect correlated with the expression of connexin 43 in the untransduced glioma target cells. In conclusion, we delineate a role for migratory HSV-transfected NSC to eliminate glioma cells purely by means of the bystander effect.

Role of connexin-based gap junction channels and hemichannels in ischemia-induced cell death in nervous tissue

Gap junction channels and hemichannels formed of connexin subunits are found in most cell types in vertebrates. Gap junctions connect cells via channels not open to the extracellular space and permit the passage of ions and molecules of approximately 1 kDa. Single connexin hemichannels, which are connexin hexamers, are present in the surface membrane before docking with a hemichannel in an apposed membrane. Because of their high conductance and permeability in cell-cell channels, it had been thought that connexin hemichannels remained closed until docking to form a cell-cell channel. Now it is clear that at least some hemichannels can open to allow passage of molecules between the cytoplasm and extracellular space. Here we review evidence that gap junction channels may allow intercellular diffusion of necrotic or apoptotic signals, but may also allow diffusion of ions and substances from healthy to injured cells, thereby contributing to cell survival. Moreover, opening of gap junction hemichannels may exacerbate cell injury or mediate paracrine or autocrine signaling. In addition to the cell specific features of an ischemic insult, propagation of cell damage and death within affected tissues may be affected by expression and regulation of gap junction channels and hemichannels formed by connexins.

Photochemically enhanced gene transfection increases the cytotoxicity of the herpes simplex virus thymidine kinase gene combined with ganciclovir

Tumor targeting is an important issue in cancer gene therapy. We have developed a gene transfection method, based on light-inducible photochemical internalization (PCI) of a transgene, to improve gene delivery and expression selectively in illuminated areas, for example, in tumors. In the present work, we demonstrate that PCI improved the nonviral vector polyethylenimine (PEI)-mediated transfection of a therapeutic gene, the 'suicide' gene encoding herpes simplex virus thymidine kinase (HSVtk). In U87MG glioblastoma cells in vitro, the photochemical treatment stimulated expression of the HSVtk transgene, and, consequently, enhanced cell killing by the subsequent treatment with the prodrug ganciclovir (GCV). When relatively low doses of DNA (1 microg/ml) and the PEI vector (N/P 4) were used, HSVtk gene transfection followed by the GCV treatment did not have an effect on cell survival unless the photochemical treatment was performed, which potentiated the cytotoxicity to 90%. These findings indicate that photochemical transfection allows: (i) selective enhancement in gene expression and gene-mediated biological effects (cell killing by the Hsvtk/GCV approach) in response to illumination; (ii) the use of low, suboptimal for the nonviral transfection methods without PCI, doses of both DNA and the vector, which may be relevant and advantageous for therapeutic gene transfer in vivo.

Herpes simplex virus thymidine kinase and granulocyte macrophage colony-stimulating factor combination gene therapy in a murine CT26 cell colon cancer model

We evaluated the antitumor effects of combination gene therapy on CT26 mouse colon cancer cells, using the genes for herpes simplex virus thymidine kinase gene HSV-TK combined with granulocyte macrophage colony-stimulating factor (GM-CSF) compared with HSV-TK alone. Cells, unmodified or retrovirally transduced with HSV-TK or GM-CSF, were inoculated subcutaneously into syngeneic BALB/c mice in various combinations. HSV-TK and GM-CSF were also delivered using different routes (in separate cells vs doubly transfected single cells). Both HSV-TK (with i.p. ganciclovir - GCV - treatment) and GM-CSF genes had independent antitumor effects, and given together they caused significant reduction in tumor volumes compared with the HSV-TK gene alone (P < 0.001). Following GCV treatment, however, the treated/control ratios for tumor volumes were not different between tumors containing either HSV-TK alone or both genes (0.27 vs 0.25, respectively). Thus, the presence of GM-CSF did not increase the bystander effect of HSV-TK. Tumors receiving genes transferred in separate cells tended to be more consistently suppressed after GCV treatment than when both genes were transferred in the same cells, although this was not statistically significant. Thus, combination GM-CSF and HSV-TK gene therapy produced greater therapeutic efficacy than HSV-TK alone, but the bystander effect was not enhanced by GM-CSF.

The role of altered cell-cell communication in melanoma progression

Under normal homeostasis, melanocyte growth and behaviour is tightly controlled by the surrounding keratinocytes. Keratinocytes regulate melanocyte behaviour through a complex system of paracrine growth factors and cell-cell adhesion molecules. Pathological changes, leading to development of malignant melanoma, upset this delicate homeostatic balance and can lead to altered expression of cell-cell adhesion and cell-cell communication molecules. In particular, there is a switch from the E-cadherin-mediated keratinocyte-melanocyte partnership to the N-cadherin-mediated melanoma-melanoma and melanoma-fibroblast interaction. Other changes include the alteration in the gap junctions formed between the melanocyte and keratinocyte. Changes in the connexin expression, in particular the loss of connexin 43, may result in a reduction or a loss of gap junctional activity, which is thought to contribute towards tumour progression. In the current review we describe the alterations in cell-cell adhesion and communication associated with melanoma development and progression, and discuss how a greater understanding of these processes may aid the future therapy of this disease.

Clinical trials with retrovirus mediated gene therapy--what have we learned?

Retrovirus (RV) has been one of the earliest recombinant vectors to be investigated in the context of cancer gene therapy. Experiments in cell culture and in animal brain tumor models have demonstrated the feasibility of RV mediated gene transduction and killing of glioma cells by toxicity generating transgenes. Phase I and II clinical studies in patients with recurrent malignant glioma have shown a favorable safety profile and some efficacy of RV mediated gene therapy. On the other hand, a prospective randomized phase III clinical study of RV gene therapy in primary malignant glioma failed to demonstrate significant extension of the progression-free or overall survival times in RV treated patients. The failure of this RV gene therapy study may be due to the low tumor cell transduction rate observed in vivo. The biological effects of the treatment may also heavily depend on the choice of transgene/prodrug system and on the vector delivery methods. Retrovirus clinical trials in malignant glioma have nevertheless produced a substantial amount of data and have contributed toward the identification of serious shortcomings of the non-replicating virus vector gene therapy strategy. Novel types of therapeutic virus vector systems are currently being designed and new clinical protocols are being created based on the lessons learned from the RV gene therapy trials in patients with malignant brain tumors.

Do there exist synergistic antitumor effects by coexpression of herpes simplex virus thymidine kinase with cytokine genes on human gastric cancer cell line SGC7901?

AIM: To evaluate the synergistic antitumor effects of herpes simplex virus thymidine kinase (HSV-TK) together with tumor necrosis factor alpha (TNF-α) or interleukin-2 (IL-2) gene expression on gastric cancer cell line SGC7901.

METHODS: Recombinant vectors pL(TT)SN and pL(TI)SN, which express TK-IRES-TNF-α and TK-IRES-IL-2 genes separately, as well as the control plasmids pL(TK)SN and pLXSN were employed to transfect PA317 cells respectively to generate the viruses that can stably express the objective genes through G418 selection. The gastric cancer cells were then transfected by the retroviral serum from the package cells and maintained in culture to determine the cell growth and apoptosis. The cytotoxic effects of HSV-TK together with TNF-α or IL-2 gene expression on the transfected cancer cells were evaluated by the cell viability and bystander effects in the presence of GCV supplemented in the cultural medium.

RESULTS: Expression of recombinant proteins including TNF-α and IL-2 by stable transfectants was confirmed by Western blotting. The percentage of cell apoptosis in the SGC/0, SGC/TK-TNF-α, SGC/TK-IL-2 and SGC/TK clone was 2.3%, 12.3%, 11.1% and 10.9% respectively at 24 h post-transfection. Cell growth status among all the experimental groups as judged by cell absorbance (A) at 570nm did not exhibit any significant difference (P > 0.05); although it was noted to be slightly lower in the SGC/TT group. Cell survival rate in SGC/TI, SGC/TT and SGC/TK group was significantly decreased in a dose-dependent manner of GCV compared with that of the SGC/0 group (P < 0.05-0.01). Among all studied cells, the SGC/TT was shown most sensitive to GCV with a half lethal dose of 0.5 mg·L^-1. In contrast, the survival rate of SGC/0 cells was not affected by the presence of GCV with the doses less than 10 mg·L^-1. The half lethal dose of GCV for SGC/0 cells was more than 100 mg·L^-1. Marked bystander effect induced by SGC/TI, SGC/TT and SGC/TK cells was confirmed by the fact that 20% of these stable transfectants could kill 50% of the co-cultured cells, in which the most prominent bystander effect was found in the circumstance of SGC/TT presence. However, no significant difference of these variables was found among SGC/TI, SGC/TT and SGC/TK cells (P > 0.05).

CONCLUSION: The synergistic antitumor effects produced by the co-expression of HSV-TK with TNF-α or IL-2 genes were not present in the transfected SGC7901 cells. The mechanism underlying these phenomena was not known.

Phase 1 trial of FVIII gene transfer for severe hemophilia A using a retroviral construct administered by peripheral intravenous infusion

In a phase 1 dose escalation study, 13 subjects with hemophilia A received by peripheral intravenous infusion a retroviral vector carrying a B-domain-deleted human factor VIII (hFVIII) gene. Infusions were well tolerated. Tests for replication competent retrovirus have been negative. Polymerase chain reaction (PCR) analyses demonstrate the persistence of vector gene sequences in peripheral blood mononuclear cells in 3 of 3 subjects tested. Factor VIII was measured in serial samples using both a one-stage clotting assay and a chromogenic assay. While no subject had sustained FVIII increases, 9 subjects had FVIII higher than 1% on at least 2 occasions 5 or more days after infusion of exogenous FVIII, with isolated levels that ranged from 2.3% to 19%. Pharmacokinetic parameters of exogenous FVIII infused into subjects 13 weeks after vector infusion showed an increased half-life (T1/2; P <.02) and area under the curve (AUC, P <.04) compared with prestudy values. Bleeding frequency decreased in 5 subjects compared with historical rates. These results demonstrate that this retroviral vector (hFVIII(V)) is safe and, in some subjects, persists more than a year in peripheral blood mononuclear cells, with measurable factor VIII levels and with increased available FVIII activity (increased T1/2 and AUC) after infusion of exogenous FVIII concentrate.

Gap junction hemichannels in astrocytes of the CNS

Connexins are protein subunits that oligomerize into hexamers called connexons, gap junction hemichannels or just hemichannels. Because some gap junction channels are permeable to negatively and/or positively charged molecules up to approximately 1kDa in size, it was thought that hemichannels should not open to the extracellular space. A growing amount of evidence indicates that opening of hemichannels does occur under both physiological and pathological conditions in astrocytes and other cell types. Electrophysiological studies indicate that hemichannels have a low open probability under physiological conditions but may have a much higher open probability under certain pathological conditions. Some of the physiological behaviours of astrocytes that have been attributed to gap junctions may, in fact, be mediated by hemichannels. Hemichannels constituted of Cx43, the main connexin expressed by astrocytes, are permeable to small physiologically significant molecules, such as ATP, NAD+ and glutamate, and may mediate paracrine as well as autocrine signalling. Hemichannels tend to be closed by negative membrane potentials, high concentrations of extracellular Ca2+ and intracellular H+ ions, gap junction blockers and protein phosphorylation. Hemichannels tend to be opened by positive membrane potentials and low extracellular Ca2+, and possibly by as yet unidentified cytoplasmic signalling molecules. Exacerbated hemichannel opening occurs in metabolically inhibited cells, including cortical astrocytes, which contributes to the loss of chemical gradients across the plasma membrane and speeds cell death.

Clinical development of gene therapy for colorectal cancer

Colorectal cancer (CRC) is the second most common type of malignancy in Western nations. Improvements in surgical and radiotherapeutic techniques and the increased availability of new cytotoxic drugs have improved outcome, but 50% of patients still die from recurrent or metastatic disease. Several features of its natural history render CRC a good candidate for gene therapy. Techniques include gene replacement, virus-directed enzyme-prodrug therapy, immune manipulation and virotherapy, all of which have entered clinical trials.

Gamma interferon transduced 9L gliosarcoma. Cytokine gene therapy and its relevance to cellular therapy with alloreactive cytotoxic T lymphocytes

In earlier studies, we demonstrated that intratumoral infusions of alloreactive cytotoxic T lymphocytes (aCTL), sensitized to the major histocompatibility complex (MHC) antigens of the host, effectively retarded the intracranial growth of Fischer 9L gliosarcoma. We further demonstrated that continuous in vitro exposure to gamma-interferon (gammaIFN) upregulates MHC on 9L gliosarcoma cells and that they were better targets of anti-Fischer aCTL. We hypothesized that the efficacy of cellular therapy with aCTL could be further improved by in situ transduction of the tumor with retroviral vectors coding for gammaIFN, which would generate continuous secretion of the cytokine and maintain upregulated MHC expression by the tumor cells. 9L gliosarcoma and Herpes simplex virus thymidine kinase (tk) transductants of those cells were transduced with a retrovirus carrying the murine gammaIFN gene. By limiting dilution, clones of these cells, designated 9Lgamma 7, 9Lgamma tk8, and 9Lgamma tk10, which produced similar levels of gammaIFN (383-411 ng gammaIFN/10(6) cells/24 h) were isolated. The production of gammaIFN by one clone, 9Lgamma 7, was stable when monitored over 6 weeks in vitro. The clones also demonstrated upregulated MHC class I expression, and the tk-transduced clones maintained their sensitivity to ganciclovir. Compared to the wildtype cells, 9Lgamma 7 had approximate 6- and 1.5-fold increases in the relative antigen densities of MHC I and II, respectively. Addition of exogenous gammaIFN to 9Lgamma 7 cultures did not significantly increase the MHC expression. In cytotoxicity assays, 9Lgamma 7 cells, or 9Lgamma 7 incubated with exogenous gammaIFN, were better targets of aCTL than the parental 9L cells. The growth rate of 9Lgamma-transduced cells was decreased compared to the wildtype cells both in vitro and in vivo. Proliferation studies with transwell plated 9L, 9Lgamma 7, and 9Lgamma tk10 cells in various combinations revealed that the secreted cytokine itself caused a decrease in proliferation. However, the transduced cells exhibited a much reduced growth rate, which likely was a consequence of redirected metabolic activity of the cells. In vivo growth of the 9L and 9Lgamma 7 tumors in rat brains given identical inoculums similarly demonstrated significantly reduced 9Lgamma 7 tumor volumes at various timepoints, indicative of slower growth of the gammaIFN-producing tumors.

Experimental study of thymidine kinase gene therapy of neuroblastoma in vitro and in vivo

Neuroblastoma arises as a direct result of genetic disorder; therefore, it should be well treated and conquered by gene therapy in future. In this study, neuroblastoma cell line SH-SY5Y experiments, in vitro and in nude mice in vivo, were subjected to research thymidine kinase suicide gene to treat neuroblastoma. The plasmid LXpsp-hytk and a plasmid LXSH were transduced separately by lipofectin into human neuroblastoma cell line SH-SY5Y. SH-SY5Y-hy and SH-SY5Y-hytk were selected by hygromycin B. Different ganciclovir (GCV) concentration was given to SH-SY5Y-hytk to determine optimal GCV concentration. The cytotoxic effect of GCV on SH-SY5Y-hytk, SH-SY5Y-hy, and SH-SY5Y cells was determined. Scapular subcutaneous tumors were established in nude mice by inoculating 2.5 x 10(6) SH-SY5Y-hytk on their left sides and 2.5 x 10(6) SH-SY5Y-hy cells on their right sides for every mouse of treatment group and control group, respectively. After 1 week, mass grew in both sides of all the mice, and from the eighth day on, every mouse in treatment group received daily intraperitoneal injection of GCV 50 mg/kg body weight for 14 days; every mouse in control group received daily intraperitoneal injection of 1 ml saline for 14 days. On day 22 tumors were excised and weighed on the left and right sides, respectively, and apoptosis was detected by TUNEL method. Apoptotic index was calculated on the left and on the right sides, respectively, for every mouse in treatment group and control group. The lowest concentration of hygromycin B was 60 microg/ml. The cytotoxic effect of GCV on SH-SY5Y-hytk cells was obvious (IC(50)=0.03 microM), whereas GCV showed almost no cytotoxic effect on SH-SY5Y and SH-SY5Y-hy cells (IC(50)>400 microM). SH-SY5Y-hytk was killed by concentrations of 30 microM GCV effectively and it obviously showed the bystander effect, when SH-SY5Y-hytk remained at least 18% in the mixture culture cells. The tumor on the left side was much smaller than that of the right side in control group (p<0.05), and apoptotic index of the left was higher than that of the right in control group (p<0.01). SH-SY5Y-hytk has the bystander effect over 18% SH-SY5Y-hytk of the mixture culture cells at the concentration of 30 microM GCV. The HSV-tk/GCV system was effective in treating SH-SY5Y neuroblastoma cell line in vivo as well. Our findings suggest that thymidine kinase gene therapy could be a potential method for treating neuroblastoma in the future.