Caspase-8 Gene Therapy Using the Human Telomerase Reverse Transcriptase Promoter for Malignant Glioma Cells

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Abstract
Telomerase is a distinctive candidate for targeted gene therapy of malignant gliomas, because the vast majority of malignant gliomas express telomerase activity while normal brain tissues do not. Recently, we developed a telomerase-specific expression system of caspase-8 gene using the promoter of the human telomerase reverse transcriptase (hTERT) gene. However, the transcriptional activity of hTERT-181 promoter (a 181-base pair [bp] region upstream of the transcription start site) was relatively lower in malignant glioma cells than in other tumors such as prostate cancer cells. To establish the hTERT/caspase-8 construct as a novel therapy for malignant gliomas, we need to increase the transcriptional activity of the hTERT promoter in malignant glioma cells. In the present study, we demonstrate that the transcriptional activity of hTERT-378 promoter (a 378-bp region) was 2- to 40-fold higher in hTERT-positive malignant glioma cells (A172, GB-1, T98G, U87-MG, U251-MG, and U373-MG) than that of hTERT-181. We further demonstrate that by using the hTERT-378/caspase-8 construct, apoptosis was restricted to malignant glioma cells, and was not seen in astrocytes or fibroblasts lacking hTERT. Moreover, the growth of subcutaneously established U373-MG tumors in mice was significantly inhibited by seven daily intratumoral injections of hTERT-378/caspase-8 construct and its inhibitory effect persisted during 3 additional weeks without additional treatment. These results suggest that the telomerase-specific expression of caspase-8 under hTERT-378 promoter is a novel targeting approach for the treatment of telomerase-positive malignant gliomas.