Role of calcium ion in induction of apoptosis by etoposide in human leukemia HL-60 cells.

The epipodophyllotoxin derivative etoposide, an inhibitor of DNA topoisomerase II, has been found to induce internucleosomal DNA fragmentation, characteristic of apoptosis. In the present study, we examined the effect of Ca2+ depletion on VP-16-induced endonucleolytic DNA cleavage in HL-60 cells. VP-16-induced internucleosomal DNA fragmentation could not be blocked by the extracellular Ca2+ chelator EGTA. However, an intracellular Ca2+ chelator BAPTA-AM, which was added after treatment with VP-16, abolished both internucleosomal DNA fragmentation and the morphologic features of apoptosis. No significant increase of intracellular Ca2+ was found after VP-16 treatment. We demonstrate for the first time that preexisting intracellular Ca2+ plays an essential role in induction of apoptosis by VP-16 in HL-60 cells.