Our efforts to treat malignant gliomas are focused on methods to transfer the caspase genes to tumors. Because the apoptotic pathway may be disrupted in tumors and caspases are the mainstay of apoptosis programs, this approach is one of the most promising strategies for cancer gene therapy. However, if caspases were transduced to normal brain cells, they will undergo apoptosis. To restrict induction of apoptosis to tumor cells, we need to establish a tumor specific expression system of caspases. Telomerase is a particularly attractive target for the tumor-targeting system. It is because a vast majority of malignant gliomas have telomerase activity, while most normal brain cells do not. Activation of telomerase is tightly regulated at the transcriptional level of the telomerase catalytic subunit (hTERT). Therefore, we hypothesize that by using the hTERT promoter-driven vector system, the expression of caspases can be restricted to telomerase-positive malignant gliomas. In preliminary studies, we constructed the caspase-8 (initiator caspase) or rev-caspase-6 (executioner caspase) expression vector with the hTERT promoter (hTERT/caspase-8 or rev-caspase-6) and demonstrated that each construct induced apoptosis in telomerase-positive malignant glioma cells, but not in cultured astrocytes lacking telomerase. Furthermore, the growth of subcutaneous tumors in nude mice was significantly suppressed by the treatment with the hTERT/rev-caspase-6. Additionally, the antitumor effect of hTERT/caspase-8 or rev-caspase-6 was enhanced by the combination with anticancer drug, cisplatin. The goal of this proposal is to investigate whether treatment with the hTERT/caspase-8 or rev-caspase-6 construct is an effective approach for telomerase-positive malignant gliomas using an experimental model of human malignant gliomas.
The specific aims are: 1) to select tumor model systems for the in vivo treatment with hTERT/caspase-8 or rev-caspase-6 construct, 2) to investigate the antitumor effect of the hTERT/caspase-8 or rev-caspase-6 construct on intracranial tumors, 3) to investigate their in vitro and in vivo antitumor efficacy combined with conventional therapy (cisplatin, temozolomide, or gamma-irradiation), and 4) to investigate the molecular mechanisms underlying the effect of the hTERT/caspase-8 or rev-caspase-6. A unique focus of this work is the emphasis on the telomerase-specific gene transfer of caspases. We anticipate that the studies described in the current proposal will lead to a novel and promising targeting approach for malignant gliomas expressing telomerase activity.
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