Telomerase activity as a measure for monitoring radiocurability of tumor cells

Radiotherapy plays a key role in the treatment of many tumors. It is diffic ult to determine what fraction of tumor cells survives after treatment with ionizing radiation. A convenient and sensitive biochemical assay could be efficacious in determining the potential success of radiotherapy. Since tel omerase activity is frequently associated with the malignant phenotype, we sought to determine whether a correlation existed between ionizing radiatio n-induced cell killing and telomerase activity. We evaluated telomerase act ivity in two telomerase-positive and one telomerase-negative human cell lin e exposed to ionizing radiation. Telomerase activity was determined using a PCR-based telomeric repeat amplification protocol coupled with ELISA We fo und ionizing radiation treatment to decrease the telomerase activity (in pl ateau phase cells of RKO, HeLa; and growing cells of RKO) in a dose-depende nt manner, which correlated with cell death in in vitro tests as well as du ring tumor regression in nude mice. In contrast, growing HeLa cells after 2 4 h postradiation treatment showed an increase in telomerase activity, but there was no increase in the levels of mRNA of hTERT. To assess the sensiti vity of the telomerase activity assay, we performed mixing experiments of H eLa and AG1522 cell. extracts. These studies showed that telomerase activit y could be detected in lysate equal to a single HeLa cell when mixed with 1 0,000 AG1522 cells. Our results indicate that even a few surviving neoplast ic cells can be detected by telomerase activity assay. Therefore, detection of telomerase activity may be a useful monitor of radiotherapeutic efficac y and an early predictor of outcome.