Increased bax expression is associated with cell death induced by ganciclovir in a herpes thymidine kinase gene-expressing glioma cell line

Smart nanocarriers for enzyme-activated prodrug therapy

Exogenous enzyme-activated prodrug therapy (EPT) is a potential cancer treatment strategy that delivers non-human enzymes into or on the surface of the cell and subsequently converts a non-toxic prodrug into an active cytotoxic substance at a specific location and time. The development of several pharmacological pairs based on EPT has been the focus of anticancer research for more than three decades. Numerous of these pharmacological pairs have progressed to clinical trials, and a few have achieved application in specific cancer therapies. The current review highlights the potential of enzyme-activated prodrug therapy as a promising anticancer treatment. Different microbial, plant, or viral enzymes and their corresponding prodrugs that advanced to clinical trials have been listed. Additionally, we discuss new trends in the field of enzyme-activated prodrug nanocarriers, including nanobubbles combined with ultrasound (NB/US), mesoscopic-sized polyion complex vesicles (PICsomes), nanoparticles, and extracellular vesicles (EVs), with special emphasis on smart stimuli-triggered drug release, hybrid nanocarriers, and the main application of nanotechnology in improving prodrugs.

Fluoxetine enhances the antitumor effect of olfactory ensheathing cell-thymidine kinase/ganciclovir gene therapy in human glioblastoma multiforme cells through upregulation of Connexin43 levels

Glioblastoma multiforme (GBM) is the most invasive form of primary brain astrocytoma, resulting in poor clinical outcomes. Herpes simplex virus thymidine kinase/ganciclovir (HSV-TK/GCV) gene therapy is considered a promising strategy for GBM treatment. Since Connexin43 (Cx43) expression is reduced in GBM cells, increasing Cx43 levels could enhance the effectiveness of gene therapy. The present study aims to examine the impact of fluoxetine on HSV-TK/GCV gene therapy in human GBM cells using human olfactory ensheathing cells (OECs) as vectors. The effect of fluoxetine on Cx43 levels was assessed using the western blot technique. GBM-derived astrocytes and OECs-TK were Cocultured, and the effect of fluoxetine on the Antitumor effect of OEC-TK/GCV gene therapy was evaluated using MTT assay and flow cytometry. Our results showed that fluoxetine increased Cx43 levels in OECs and GBM cells and augmented the killing effect of OECs-TK on GBM cells. Western blot data revealed that fluoxetine enhanced the Bax/Bcl2 ratio and the levels of cleaved caspase-3 in the coculture of OECs-TK and GBM cells. Moreover, flow cytometry data indicated that fluoxetine increased the percentage of apoptotic cells in the coculture system. This study suggests that fluoxetine, by upregulating Cx43 levels, could strengthen the Antitumor effect of OEC-TK/GCV gene therapy on GBM cells.

β2-Adrenergic receptor agonist enhances the bystander effect of HSV-TK/GCV gene therapy in glioblastoma multiforme via upregulation of connexin 43 expression

Glioblastoma multiforme (GBM) is the most invasive form of primary brain astrocytoma. Gene therapy using the herpes simplex virus thymidine kinase/ganciclovir (HSV-TK/GCV) is a new strategy for GBM treatment. As the connexin 43 (Cx43) levels are downregulated in GBM cells, it seems that the upregulation of Cx43 could improve the efficacy of the gene therapy. This study aims to evaluate the effect of clenbuterol hydrochloride (Cln) as a β2-adrenergic receptor agonist on HSV-TK/GCV gene therapy efficacy in human GBM cells using olfactory ensheathing cells (OECs) as vectors. The lentivirus containing the thymidine kinase gene was transduced to OECs and the effective dose of GCV on cells was measured by MTT assay. We found that Cln upregulated Cx43 expression in human GBM cells and OECs and promoted the cytotoxic effect of GCV on the co-culture cells. Western blot results showed that Cln increased the cleaved caspase-3 expression and the Bax/Bcl2 ratio in the co-culture of GBM cells and OEC-TK. Also, the flow cytometry results revealed that Cln increased apoptosis in the co-culture of GBM cells and OEC-TK cells. This study showed that Cln via upregulation of Cx43 expression could enhance the bystander effect of HSVTK-GCV gene therapy in human GBM cells.

Adenovirus-mediated herpes simplex virus thymidine kinase gene therapy combined with ganciclovir induces hepatoma cell apoptosis

The present study aimed to examine the apoptotic effects of adenovirus (ADV)-mediated herpes simplex virus thymidine kinase (ADV-TK) combined with ganciclovir (GCV) in tissues obtained from patients with hepatocellular carcinoma in order to provide a theoretical basis for the development of this gene therapy program. Apoptosis detection was conducted using the terminal deoxynucleotidyl-transferase-mediated dUTP nick end labelling assay and the apoptosis index was compared between the experimental; and control groups. Furthermore, the protein expression levels of caspase-3, B-cell lymphoma-2 (Bcl-2), Bcl-2-assoicated protein X (Bax) and nuclear factor (NF)-κB were examined in pathological specimens using immunohistochemical staining. The Bax/Bcl-2 ratio and the release of cytochrome c were examined using western blot analysis. Results indicated that combined ADV-TK and GCV treatment significantly increased the number of apoptotic cells compared with the control group (P<0.05). Immunohistological analysis revealed that ADV-TK and GCV treatment significantly increased the number of caspase-3-positive cells, reduced the Bax/Bcl-2 ratio and NF-κB expression levels and promoted the release of cytochrome c compared with the control group (P<0.01). In conclusion, the present results suggest that combined ADV-TK and GCV treatment exerts its effect through the apoptotic signaling pathway.

The Use of Normal Stem Cells and Cancer Stem Cells for Potential Anti-Cancer Therapeutic Strategy

BACKGROUND

Despite recent advance in conventional cancer therapies including surgery, radiotherapy, chemotherapy, and immunotherapy to reduce tumor size, unfortunately cancer mortality and metastatic cancer incidence remain high. Along with a deeper understanding of stem cell biology, cancer stem cell (CSC) is important in targeted cancer therapy. Herein, we review representative patents using not only normal stem cells as therapeutics themselves or delivery vehicles, but also CSCs as targets for anti-cancer strategy.

METHODS

Relevant patent literatures published between 2005 and 2017 are discussed to present developmental status and experimental results on using normal stem cells and CSCs for cancer therapy and explore potential future directions in this field.

RESULTS

Stem cells have been considered as important element of regenerative therapy by promoting tissue regeneration. Particularly, there is a growing trend to use stem cells as a target drug-delivery system to reduce undesirable side effects in non-target tissues. Noteworthy, studies on CSC-specific markers for distinguishing CSCs from normal stem cells and mature cancer cells have been conducted as a selective anti-cancer therapy with few side effects. Many researchers have also reported the development of various substances with anticancer effects by targeting CSCs from cancer tissues.

CONCLUSION

There has been a continuing increase in the number of studies on therapeutic stem cells and CSC-specific markers for selective diagnosis and therapy of cancer. This review focuses on the current status in the use of normal stem cells and CSCs for targeted cancer therapy. Future direction is also proposed.

Gene therapy in pancreatic cancer

Pancreatic cancer (PC) is a highly lethal disease and notoriously difficult to treat. Only a small proportion of PC patients are eligible for surgical resection, whilst conventional chemoradiotherapy only has a modest effect with substantial toxicity. Gene therapy has become a new widely investigated therapeutic approach for PC. This article reviews the basic rationale, gene delivery methods, therapeutic targets and developments of laboratory research and clinical trials in gene therapy of PC by searching the literature published in English using the PubMed database and analyzing clinical trials registered on the Gene Therapy Clinical Trials Worldwide website (http://www. wiley.co.uk/genmed/ clinical). Viral vectors are main gene delivery tools in gene therapy of cancer, and especially, oncolytic virus shows brighter prospect due to its tumor-targeting property. Efficient therapeutic targets for gene therapy include tumor suppressor gene p53, mutant oncogene K-ras, anti-angiogenesis gene VEGFR, suicide gene HSK-TK, cytosine deaminase and cytochrome p450, multiple cytokine genes and so on. Combining different targets or combination strategies with traditional chemoradiotherapy may be a more effective approach to improve the efficacy of cancer gene therapy. Cancer gene therapy is not yet applied in clinical practice, but basic and clinical studies have demonstrated its safety and clinical benefits. Gene therapy will be a new and promising field for the treatment of PC.

Inhibition of ATP-sensitive potassium channels increases HSV-tk/GCV bystander effect in U373 human glioma cells by enhancing gap junctional intercellular communication

It is well known that the efficiency of Herpes simplex virus thymidine kinase gene/ganciclovir (HSV-tk/GCV) therapy is improved by the bystander effect, which mainly relies on gap junctional intercellular communication (GJIC). Malignant gliomas communicate poorly through gap junctions, consequently, agents with the ability to increase GJIC are good candidates to improve the efficiency of this therapy. Since we previously showed that the inhibition of ATP-sensitive potassium (KATP) channels promoted by tolbutamide increased GJIC in rat C6 glioma cells, we have investigated whether tolbutamide could increase the bystander effect in HSV-tk/GCV therapy against human glioma cells. We found that tolbutamide increased GJIC in U373 human glioma cells, an effect that was due to the up-regulation of connexin43, a protein that forms gap junctions channels. More interestingly, our results show that tolbutamide increased the efficiency of HSV-tk/GCV in co-cultures containing U373 cells and U373 cells transfected with HSV-tk. This effect was impaired in the presence of carbenoxolone, an inhibitor of GJIC. Furthermore, tolbutamide did not enhance the bystander effect in connexin43-silenced co-cultures. Together our results reveal that the inhibition of KATP channels promoted by tolbutamide enhances the bystander effect in HSV-tk/GCV therapy by increasing connexin43-mediated gap junctional intercellular communication in U373 human glioma cells.

Effectiveness of HSV-tk suicide gene therapy driven by the Grp78 stress-inducible promoter in esophagogastric junction and gastric adenocarcinomas

BACKGROUND

The thymidine kinase gene of the herpes simplex virus (HSV-tk) is a suicide gene when administrated with the prodrug ganciclovir (GCV). This study investigated the effectiveness of HSV-tk activation as gene therapy for gastroesophageal junction and gastric adenocarcinomas using either the stress-inducible Grp78 promoter or the murine leukemia virus long-terminal repeat (LTR) promoter.

METHODS

The HSV-tk gene, controlled by either the Grp78 promoter or the LTR promoter, was transduced into the gastroesophageal junction adenocarcinoma cell line SK-GT-5 and the gastric adenocarcinoma cell line MKN-74. Cell viability after exposure to varying concentrations of GCV was compared. The same cell lines were used to develop a nude mouse model for studies of the HSV-tk/GCV effect in vivo. The effect of intraperitoneal GCV injection on growth of the subcutaneous tumors was measured. HSV-TK expression was measured by Western blot and reverse transcription polymerase chain reaction.

RESULTS

Cell viability in vitro was significantly lower in the HSV-tk expressing (HSV-tk+) cells compared to control (no HSV-tk) cells after exposure to GCV. MKN-74tk+ cells were more sensitive to GCV killing than SK-GT-5tk+ cells. After culture with 1 microg/ml GCV for 10 days, MKN-74/tk cells were totally killed, whereas most SK-GT-5/tk cells survived. Cell viability was significantly lower under glucose starvation conditions when HSV-tk expression was regulated by the Grp78 promoter compared with the LTR promoter. MKN-74 tumors formed with HSV-tk+ cells in nude mice were eliminated after administration of GCV for 3 weeks, but GCV had no effect on tumors formed from HSV-tk- cells. Eradication of tumor formed with Grp78-tk cells was faster than that with LTR-tk cells. HSV-TK protein and mRNA were expressed in the transduced, but not the non-transduced tumors.

CONCLUSION

HSV-tk xwith ganciclovir suicide gene therapy results in significant cell killing in gastroesophageal junction and gastric adenocarcinoma cells both in vitro and in vivo, but complete tumor elimination only occurred with the gastric adenocarcinoma cell tumors. The most effective approach in this study used the Grp78 promoter in glucose-starvation stress conditions.

Biological approaches to therapy of pancreatic cancer

Pancreatic cancer is a lethal disease and notoriously difficult to treat. Only a small proportion is curative by surgical resection, whilst standard chemotherapy for patients with advanced disease has only modest effect with substantial toxicity. Clearly there is a need for the continual development of novel therapeutic agents to improve the current situation. Improvement of our understanding of the disease has generated a large number of studies on biological approaches targeting the molecular abnormalities of pancreatic cancer, including gene therapy and signal transduction inhibition, antiangiogenic and matrix metalloproteinase inhibition, oncolytic viral therapy and immunotherapy. This article provides a review of these approaches, both investigated in the laboratories and in subsequent clinical trials.

The role of herpes simplex virus thymidine kinase in the treatment of solid tumours

Suicide gene therapy is the approach whereby the genetic alteration of a cell renders it susceptible to an otherwise non-toxic prodrug. Suicide gene therapy for solid tumours has progressed rapidly since the concept was originally described: nearly all tumour types have been explored, with some, such as glioma, melanoma and colon cancer frequently used experimentally. The exciting aspect of suicide gene therapy is the bystander effect, the phenomenon whereby there is extended tumour death when only a small fraction is transfected with the suicide gene. This phenomenon implies that there is a reduced need to target specifically all tumour cells, as the effect mechanism itself carries out this function. The bystander effect mode of action has not yet been fully characterised, but the role of gap junctions and the immune system are implicated as the main instruments in its potentiation. This approach is also amenable to pharmacological intervention, which may help to optimise parameters prior to commencing suicide gene therapy. Clinical trails have already commenced using this form of treatment and results are eagerly awaited.

Dexamethasone inhibits the HSV-tk/ ganciclovir bystander effect in malignant glioma cells

BACKGROUND

HSV-tk/ ganciclovir (GCV) gene therapy has been extensively studied in the setting of brain tumors and largely relies on the bystander effect. Large studies have however failed to demonstrate any significant benefit of this strategy in the treatment of human brain tumors. Since dexamethasone is a frequently used symptomatic treatment for malignant gliomas, its interaction with the bystander effect and the overall efficacy of HSV-TK gene therapy ought to be assessed.

METHODS

Stable clones of TK-expressing U87, C6 and LN18 cells were generated and their bystander effect on wild type cells was assessed. The effects of dexamethasone on cell proliferation and sensitivity to ganciclovir were assessed with a thymidine incorporation assay and a MTT test. Gap junction mediated intercellular communication was assessed with microinjections and FACS analysis of calcein transfer. The effect of dexamethasone treatment on the sensitivity of TK-expressing to FAS-dependent apoptosis in the presence or absence of ganciclovir was assessed with an MTT test. Western blot was used to evidence the effect of dexamethasone on the expression of Cx43, CD95, CIAP2 and BclXL.

RESULTS

Dexamethasone significantly reduced the bystander effect in TK-expressing C6, LN18 and U87 cells. This inhibition results from a reduction of the gap junction mediated intercellular communication of these cells (GJIC), from an inhibition of their growth and thymidine incorporation and from a modulation of the apoptotic cascade.

CONCLUSION

The overall efficacy of HSV-TK gene therapy is adversely affected by dexamethasone co-treatment in vitro. Future HSV-tk/ GCV gene therapy clinical protocols for gliomas should address this interference of corticosteroid treatment.

Herpes simplex virus thymidine kinase/ganciclovir system in multicellular tumor spheroids

We have developed multicellular spheroids (MCS) established from LM05e and LM3 spontaneous Balb/c-murine mammary adenocarcinoma and B16 C57-murine melanoma derived cell lines as an in vitro model to study the efficacy of the herpes simplex virus thymidine kinase/ganciclovir (HSVtk/GCV) suicide system. We demonstrated for the first time that HSVtk-expressing cells assembled as MCS manifested a GCV resistance phenotype compared to the same cells grown as sparse monolayers. HSVtk-expressing LM05e, LM3 and B16 spheroids were 16-, three- and nine-fold less sensitive to GCV than their respective monolayers, even though they could express transgenes 10-, eight- and five-fold more efficiently. Mixed populations of HSVtk- and their respective beta gal-expressing cells displayed a cell-type specific bystander effect that was higher in monolayers than in MCS. However, HSVtk-expressing cells in two- or three-dimensional cultures were always significantly more sensitive to GCV than the beta gal-expressing counterparts, supporting the feasibility of this suicide approach in vivo. We present evidence showing that HSVtk-expressing tumor cells, when transferred from monolayers to MCS, displayed: (i) lower GCV cytotoxic activity and bystander effect; (ii) higher and efficient expression of genes transferred as lipoplexes; (iii) lower cell proliferation rates; and (iv) changes in intracellular Bax/Bcl-xL rheostat of mitochondria-mediated apoptosis.

Escherichia coli-nitroreductase suicide gene control of human telomerase reverse transcriptase-transduced minor histocompatibility antigen-specific cytotoxic T cells

Adoptive immunotherapy with ex vivo generated cytotoxic T lymphocytes (CTLs) is applied for the treatment of leukemia relapses or viral infections after allogeneic stem cell transplantation. A common problem of adoptive immunotherapy strategies is the ex vivo expansion of the generated T cells to sufficient numbers. CTLs can be efficiently expanded by ectopic expression of the human telomerase gene (hTert). However, hTert transduction may also increase the chance for malignant transformation. Therefore, we explored the feasibility of suicide gene control of ex vivo generated CTLs expanded through the ectopic expression of hTert. To this end, we compared the efficacy of the new Escherichia coli-nitroreductase (E. coli-Ntr) suicide gene with the well-known herpes simplex virus-thymidine kinase (HSV-Tk). Introduction of hTert provided the transduced CTLs with a distinct growth advantage over the nontransduced CTLs. The hTert-E. coli-Ntr double-transduced CTLs retained their antigen-specific functions. Treatment of hTert-E. coli-Ntr double-transduced CTLs with metronidazole significantly inhibited the proliferation to a similar extent to the treatment of hTert-HSV-Tk double-transduced CTLs with ganciclovir. This is the first application of the E. coli-nitroreductase gene for the elimination of human T cells with metronidazole.

Herpes simplex virus thymidine kinase and ganciclovir suicide gene therapy for human pancreatic cancer

AIM: To investigate the in vitro effects of suicide gene therapy system of herpes simplex virus thymidine kinase gene (HSV-TK) in combination with the treatment of nucleotide analog-ganciclovir (GCV) on human pancreatic cancer, and to provide a novel clinical therapeutic method for human pancreatic cancer.

METHODS: We used a replication defective recombinant retrovirus vector GINaTK (bearing HSV-TK gene) to make packaging cell PA317 produce progeny virions. We then transferred the HSV-TK gene to target cells SW1990 using these progeny virions, and treated these gene-modified tumor cells with GCV to study the sensitivity of the cells to GCV and their bystander effects by routine MTT-method.

RESULTS: Packaging cell PA317/TK was successfully constructed, and we acquired SW1990/TK through virus progeny infection. These gene-modified pancreatic cancer cells were sensitive to the treatment of GCV compared with unmodified tumor cells (t = 4.15, n = 10, P < 0.0025). We also observed a remarkable bystander effect by mixing two kinds of cells at different ratio.

CONCLUSION: Our data demonstrate that HSV-TK/GCV suicide gene therapy system is effective for treating experimental human pancreatic cancer, which is largely resistant to the common therapies, so the suicide gene therapy system may be a potential treatment approach for pancreatic cancer.

Enhancement of thymidine kinase-mediated killing of malignant glioma by BimS, a BH3-only cell death activator

Herpes simplex virus thymidine kinase (HSV-tk)/gancyclovir (GCV) therapy has the ability to inhibit tumor formation in animal models but the results of clinical trials have been disappointing. To improve the performance of tk/GCV therapy, we tried combination therapy designed to enhance its cytotoxic effects by introducing genes that induce apoptosis of the tumor cells through different pathways. We concentrated our efforts on the use of Bim, a BH3-only member of death activators in the Bcl-2 superfamily, because Bim is not involved in the pathways through which HSV-tk/GCV therapy induces apoptosis in malignant glioma cells. Among three alternative splicing variants, BimEL, BimL, and BimS, BimS lacks the binding domain for the dynein light chain LC8, which negatively regulates the proapoptotic function of BimEL and BimL. All four malignant glioma cell lines, U251, A172, T-430, and U373 underwent cell death after transfer of BimS using an adenovirus vector (AVC2). Intriguingly, combination of AVC2-BimS with AVC2-tk markedly increased the sensitivity of U251 cells to GCV both in vitro and in vivo. In contrast, AVC2-BimL did not induce significant cell death. These results indicated that BimS had the ability to improve the efficiency of HSV-tk/GCV therapy in the treatment of malignant glioma and suggested that the targeting of different proapoptotic pathways may be a useful strategy for the development of an effective gene therapy approach to treatment.

Transcriptionally mediated gene targeting of gas1 to glioma cells elicits growth arrest and apoptosis

Induction of growth arrest-specific genes (gas1) prevents cell proliferation and/or leads to apoptosis in different cell types. In neurons, it has been recently reported that mild excitotoxic neuronal death is associated with gas1 induction, and that overexpression of Gas1 induces apoptosis in terminally differentiated neurons or in proliferating neuroblastoma cells. In the present study, we have analysed the effects of the transcriptionally mediated targeting of gas1 to C6 rat glioma cells. Expression of Gas1 decreased glial proliferation and induced C6 cell apoptosis. While the identity of the caspase(s) responsible for Gas1-induced apoptosis in neurons has remained elusive, in C6 glioma cells, overexpression of Gas1 reproducibly activated caspase-3. Our results support the concept of targeted expression of gas1 as a potentially useful gene therapy strategy in the treatment of human gliomas.

Ganciclovir-induced apoptosis in HSV-1 thymidine kinase expressing cells: critical role of DNA breaks, Bcl-2 decline and caspase-9 activation

Although ganciclovir (GCV) is most often used in suicide anticancer gene therapy, the mechanism of GCV-induced cell killing and apoptosis is not fully understood. We analysed the mechanism of apoptosis triggered by GCV using a model system of CHO cells stably transfected with HSV-1 thymidine kinase (HSVtk). GCV-induced apoptosis is due to incorporation of the drug into DNA resulting in replication-dependent formation of DNA double-strand breaks and, at later stages, S and G2/M arrest. GCV-provoked DNA instability was likely to be responsible for the observed initial decline in Bcl-2 level and caspase-9/-3 activation. Further decline in the Bcl-2 level was due to cleavage of the protein by caspase-9, as demonstrated by use of caspase inhibitors and transfection with trans-dominant negative caspase expression vectors. Bcl-2 cleavage resulted in the appearance of a pro-apoptotic 23 kDa Bcl-2 fragment and in excessive cytochrome c release, dephosphorylation of BAD, cleavage of PARP and finally DNA degradation. Since Fas/CD95 and caspase-8 were only slightly activated we conclude GCV-induced apoptosis to occur in this cell system mainly by activating the mitochondrial damage pathway. This process is independent of p53 for which the cells are mutated. Caspase-9 mediated cleavage of Bcl-2 accelerates the apoptotic process and may explain the high potential of GCV to induce apoptosis. Data are also discussed as to implications for HSVtk gene therapy utilizing GCV.

Ganciclovir and penciclovir, but not acyclovir, induce apoptosis in herpes simplex virus thymidine kinase-transformed baby hamster kidney cells

The efficacies of ganciclovir (GCV), penciclovir (PCV) and acyclovir (ACV) in inducing cell death in the herpes simplex virus thymidine kinase (HSVTK) system were compared. HSVTK-transformed baby hamster kidney cells treated with GCV, PCV or ACV were monitored for growth by viable count, and for death by TUNEL assay, propidium iodide staining, detection of phosphatidyl serine translocation and detection of DNA laddering. All compounds delayed growth or reduced viability of HSVTK-transformed cells. Drug treatment reduced levels of cyclin B1 message (which normally peaks in G2/M-phase of the cell cycle) and induced a four- to fivefold upregulation of GADD45 message. Treatment with GCV or PCV induced rapid accumulation of cells in S-phase and apoptotic death. Treatment with ACV, however, was associated with sustained S-phase arrest. GCV (and to a lesser extent PCV) increased phosphatidyl serine translocation, induced positive TUNEL results with alterations in cell morphology, caused marked propidium iodide staining and induced DNA laddering. By contrast, up to 7 days' exposure to ACV did not induce DNA laddering, with very little TUNEL staining. ACV treatment had little effect on phosphatidyl serine translocation and propidium iodide staining was markedly reduced compared with treatment with the other compounds. Thus, by all criteria, GCV was the most potent inducer of cell death. The current theories regarding apoptosis or necrosis as the preferred form of cell death in prodrug gene therapy are considered and the suitability of PCV or ACV as potential alternatives to GCV in the HSVTK system is discussed.

BCR-ABL-Expressing Cells Transduced with the HSV-tk Gene Die by Apoptosis upon Treatment with Ganciclovir

The potential efficacy of prodrug activation of a transduced suicide gene in a cancer cell may be impaired or enhanced by oncoproteins produced by that cell. In the context of a gene therapy protocol for chronic myeloid leukemia (CML) we examined whether the Bcr-Abl fusion protein would have either of these effects. Thus, the mechanism of cell killing by transfer of herpes simplex virus thymidine kinase (HSV-tk) and subsequent ganciclovir (GCV) treatment was examined in pre-B (TonB210.1) cells and myeloid cells (32D) and in their BCR-ABL-expressing counterparts. HSV-tk-transduced cell lines, either in the presence or in the absence of BCR-ABL expression, became susceptible to GCV at concentrations which were nontoxic to the nontransduced cells. This susceptibility was represented by apoptotic cell death in all cases. Apoptosis was observed after 24 h of treatment with GCV in the tk-transduced parental cells and in the BCR-ABL-expressing TonB210.1 cells but only after a delay of more than 24 h in the 32Dp210 cells compared to 32D. Cell death in the BCR-ABL-expressing clones was preceded by S- and G2/M-phase cell cycle arrest. Activation of FAS/APO-1 and caspase-8 was observed in all the tk-transduced cell lines after GCV treatment. However, the caspase-8 inhibitor Z-IETD-FMK only partially abrogated tk/GCV-induced apoptosis. A possible role for inhibition of Bcl-2 or Bcl-x(L) expression in the apoptosis induced by GCV was observed in the tk-transduced TonB210.1 cells but not in the 32D or 32Dp210 cells. The data demonstrate that expression of the Bcr-Abl oncoprotein does not block the apoptosis induced by the HSV-tk/GCV system, suggesting that this suicide gene therapy strategy could be considered for the treatment of CML in blast crisis.

Death receptor-independent cytochrome c release and caspase activation mediate thymidine kinase plus ganciclovir-mediated cytotoxicity in LN-18 and LN-229 human malignant glioma cells

Suicide gene therapy using viral transfer of herpes simplex virus type I (HSV-1) thymidine kinase (TK) and subsequent ganciclovir (GCV) chemotherapy was the first approach used in clinical trials of somatic gene therapy for glioblastoma. The molecular pathways mediating TK/GCV-induced cell death remain to be elucidated. Here, we report that adenoviral (Ad)-TK/GCV-induced death is p53-independent and does not involve altered CD95 or CD95L expression. Ectopic expression of the preferential caspase 8 inhibitor, crm-A, inhibits Ad-CD95L-induced cell death but has no effect on TK/GCV cytotoxicity. LN-18 glioma cells selected for resistance to death receptor-mediated cell death do not acquire cross-resistance to TK/GCV. TK/GCV triggers mitochondrial cytochrome c release and activation of caspases 3, 7, 8 and 9 in a death receptor-independent manner. These events are associated with the loss of BCL-X(L). Forced expression of a BCL-X(L) transgene, or co-exposure to a pseudosubstrate caspase inhibitor, zVAD-fmk, inhibit TK/GCV cytotoxicity. Double-transfected cell lines expressing crm-A and enhanced green fluorescent protein (eGFP) show that the bystander effect in vitro is also death receptor- and caspase 8-independent. TK/GCV therapy does not kill glioma cells in synergy with cancer chemotherapy drugs, including lomustine, temozolomide and topotecan. In contrast, there is strong synergy of TK/GCV and CD95L. Thus, TK/GCV-induced cell death involves a mitochondria-dependent loop of caspase acvtivation that can be synergistically enhanced by death receptor agonists such as CD95L. TK/GCV-mediated sensitization of glioma cells to CD95L expressed on immune effector cells or parenchymal brain cells might account for the immune system's and bystander effects of TK/GCV therapy observed in rodent glioma models in vivo.

Inhibition of caspase-1 induces cell death in pancreatic carcinoma cells and potentially modulates expression levels of bcl-2 family proteins

Caspase-1 (interleukin-1beta-converting enzyme) is reported to play an important role in the regulation of apoptosis. We investigated the inhibition of caspase-1 by the cell permeable caspase-1 inhibitor Ac-AAVALLPAVLLALLAP-YVAD.CHO in pancreatic carcinoma cells. Inhibition of caspase-1 induced a non-apoptotic/"necrotic-like" cell death in AsPC-1, BxPC-3, MiaPaCa-2 and Panc-1 cells. Expression levels of bcl-2 and bax were up-regulated in caspase-1 inhibitor-treated cells while that of bcl-x(L) remained unaltered. Our observations support our previous findings that caspase-1 is potentially involved in anti-apoptotic processes in pancreatic carcinoma.

Transduction of a fiber-mutant adenovirus for the HSVtk gene highly augments the cytopathic effect towards gliomas

Suicide gene therapy utilizing the herpes simplex thymidine kinase (HSVtk) / ganciclovir (GCV) system has been performed to kill cancer cells. However, the low transduction efficiency of HSVtk gene into cancer cells critically limits its efficacy in cancer treatment in clinical situations. To improve delivery of the HSVtk gene into cancer cells, we transduced U-87MG and U-373MG glioma cells with adenovirus (Adv) vectors with a fiber mutant, F / K20, which has a stretch of 20 lysine residues added at the C-terminus of the fiber, for the HSVtk gene (Adv-TK-F / K20), and compared the cytopathic effect of Adv-TK-F / K20 with that of the Adv for HSVtk with wild-type fiber (Adv-TK). The cytopathic effect of Adv-TK-F / K20 in U-87MG and U-373MG cells was approximately 140 and 40 times, respectively, stronger than that of Adv-TK. At the same multiplicity of infection (MOI) in each cell line, Adv-TK-F / K20 induced a higher degree of apoptosis (U-87MG, 35%; U-373MG, 77%) than Adv-TK (U-87MG, 0.11%; U-373MG, 27%) in U-87MG (MOI 0.03) and U-373MG cells (MOI 0.1). Cleavage of poly(ADP-ribose)polymerase (PARP) was more marked in the cells that were infected with Adv-TK-F / K20 than in cells that were infected with Adv-TK. These results indicate that gene therapy utilizing Adv-TK-F / K20 may be a promising therapeutic modality for the treatment of gliomas.

Potential applications of gene therapy in the patient with cancer

The elderly population has much to gain from the advances of molecular medicine, although at present genetic pharmacology remains mostly at the conceptual level. Cancer, in particular, is an increasing health burden and the majority (over 70%) of gene therapy trials are aimed at tackling this problem. Available strategies employ both viral and synthetic vectors with the selective delivery and expression of therapeutic genes a pivotal requirement. Clinical trials are now in progress with a view to modulating disease at many different levels, including the direct replacement of abnormal genes. suicide-gene formulations, and the delivery of 'gain of function' genes, which seek to alter the malignant phenotype by indirect means, such as, immunopotentiation and stromal reorganisation. Early data from these studies is tantalising and we must remain optimistic that gene therapy will benefit the patient with cancer by both reducing morbidity and extending life.

Pharmacological modulation of the bystander effect in the herpes simplex virus thymidine kinase/ganciclovir gene therapy system: effects of dibutyryl adenosine 3',5'-cyclic monophosphate, alpha-glycyrrhetinic acid, and cytosine arabinoside

The herpes simplex virus type 1 thymidine kinase (HSV1-tk) suicide gene/ganciclovir system was first applied to the treatment of glioblastoma tumors, but was hampered by the low gene transfection yield. Fortunately, the gap junction-dependent diffusion of phosphorylated ganciclovir metabolites from transfected cells to their neighbors proved to enhance the overall benefit of this strategy. However, as tumor cells are often gap junction-deficient, we sought to restore this property pharmacologically and hence to improve the efficacy of the treatment. We demonstrated that this approach was feasible in glioblastoma cells using dibutyryl adenosine 3',5'-cyclic monophosphate (cAMP) (100 microM) as a pharmacological inducer of gap junctions. alpha-Glycyrrhetinic acid (25 microM), on the other hand, strongly inhibited both gap junction-mediated intercellular communication and the bystander effect, thus confirming the role of gap junctions in HSV-tk-mediated bystander killing. Using cytosine arabinoside as a growth inhibitor, we underlined the role of tumor cell proliferation in the sensitivity of HSV-tk-transfected cells to ganciclovir and demonstrated its correlation with the importance of the bystander effect.

Bcl-2 expression in higher-grade human glioma: a clinical and experimental study

Bcl-2 protein plays an important role in inhibiting apoptosis and protecting normal and neoplastic cells from toxicity. Bcl-2 overexpression in malignant tumors, on the other hand, may cause resistance against adjuvant treatment. Since there are subpopulations of patients with glioma that differ considerably in their treatment benefit, it is important to identify prognostic factors for outcome and to tailor adjuvant protocols in accordance with specific biological features of the respective tumor. The present study aimed at investigating the role of bcl-2 expression in higher-grade glioma (WHO grade III and IV). Bcl-2 expression was correlated with clinical and paraclinical parameters, and evaluated in univariate and multivariate statistical models. In addition, bcl-2-overexpressing human glioma cells in culture were used for modeling the in vivo findings and for investigating the importance of bcl-2 for tumor resistance against cytotoxic treatment. A group of 86 patients with higher-grade glioma were investigated. Anaplastic astrocytoma (AA; WHO G III, n = 29) showed bcl-2 expression in 48% of the cases, and immunohistochemical positivity was associated with a significantly shorter survival time (p = 0.0068). In glioblastoma patients (GBM; WHO G IV, n = 57), 51% of tumors were bcl-2 positive, but bcl-2 expression did not correlate significantly with survival (p = 0.39). In a Cox proportional hazards regression model, bcl-2 positivity was confirmed as a negative prognostic parameter in AA, but not in GBM. Bcl-2 overexpressing and control human glioma cell clones (T98MG line) were treated in culture with the cytotoxic drugs carmustine (BCNU), paclitaxel, vincristine, and doxorubicin. In addition, bcl-2-overexpressing and control cells were infected with a retrovirus carrying the herpes-simplex-virus thymidine kinase gene (HSV-tk), and then treated with ganciclovir (GCV). Bcl-2 overexpression significantly increased tumor cell resistance against all of the above cytotoxic drugs, and also against HSV-TK/GCV mediated gene therapy.