Apoptosis induction with 5-fluorocytosine/cytosine deaminase gene therapy for human malignant glioma cells mediated by adenovirus

Previously, we evaluated the therapeutic efficacy of the adenovirus-mediated transduction of the cytosine deaminase (CD) gene and 5-fluorocytosine (5-FC) for malignant gliomas. However, the molecular pathways that mediate the 5-FC/CD gene therapy-induced cell death remains to be elucidated. In this study, we examined the induction of apoptosis and the role of caspases in 5-FC/CD gene therapy using human malignant glioma cells [Gli36Δ5 (mutated p53) and U87MG (wild p53)]. The treatment with 5-FC/CD gene-therapy-induced apoptosis both in Gli36Δ5 cells and in U87MG cells according to flow cytometric analysis. Immunoblot analysis revealed that caspases 3 and 9 were processed in response to 5-FC/CD in a concentration- and time-dependent manner, but caspase 8 was not. Each caspase 3 and 9 inhibitor significantly reduced apoptosis triggered by 5-FC/CD, but the caspase 8 inhibitor did not affect apoptosis induction. 5-FC/CD significantly promoted the release of cytochorme c from mitochondria in a concentration-dependent manner. These results indicate that 5-FC/CD gene therapy induces apoptosis in human malignant glioma cells and that the apoptotic cell death is mediated by the activation of mitochondrial caspase cascades involving caspases 3 and 9. This is the first report concerning the apoptotic mechanism of 5-FC/CD gene therapy, and these findings could be used to increase the efficacy of suicide gene therapy systems for the treatment of malignant glioma.