Telomerase Activity in Leukemic Cells Is Not Related to Puromycin- and Actinomycin D-Induced Apoptosis

Abstract

Apoptosis and cellular senescence are two forms of irreversible growth arrest which control the excessive cell growth. Deregulation of apoptosis or an escape from cellular senescence (which is an activation of telomerase) will be the crucial event driving the cells to neoplasia. It remains unclear whether there is a direct linkage between apoptosis and telomerase activity particularly in cancer cells. In the present study, we investigated telomerase activities in three leukemic cell lines (HL60, U937 and K562) after treating the cells with various doses of antitumor drugs, puromycin and actinomycin D. Our results show that the cells underwent apoptosis rapidly when treated with 20 Ìg/ml of puromycin and 5 Ìg/ml of actinomycin D, that is, about 60% of HL60 cells became apoptotic at 6 h and almost all the cells at 12 h. But telomerase activities detected by TRAP assay in these apoptotic cells remained elevated as those found in the untreated cells. However, if these cells were treated with lower dose of the drugs, i.e. 0.5–1.5 Ìg/ml of puromycin and 0.01–0.5 Ìg/ml of actinomycin D, a decrease in telomerase activity was observed at 24–48 h after addition of the drugs, and this activity could hardly be detected after 72 h. This decrease in telomerase activity was in a dose-dependent manner. Analysis of the cell cycle of these drugtreated cells by flow cytometry shows that the drugs were unselectively induced apoptosis in all phases of the cell cycle and that changes of the telomerase activity were not related to the cell cycle. Similar results were also observed in U937 and K562 cells except that K562 cells underwent apoptosis more slower than the former two cell lines. Our results suggest that apoptosis induced by puromycin and actinomycin in these three leukemic cell lines was not related to telomerase activity. The suppression of telomerase activity by low doses of the drugs is probably due to the inhibitory effect of the drugs to protein translation and RNA transcription rather than to the direct inhibition of the telomerase synthesis.