AN IMMUNOSUPPRESSIVE AGENT, FTY720, INCREASES INTRACELLULAR CONCENTRATION OF CALCIUM-ION AND INDUCES APOPTOSIS IN HL-60

We previously reported that FTY720 is an efficient inducer of apoptosi s in lymphocytes and cultured cell. lines. In the present study, HL-60 human promyerocytoma cells also induced apoptosis through in vitro tr eatment with the drug, demonstrating extensive DNA fragmentation 6 hr after incubation. The major target of FTY720 was the common signalling pathway of apoptosis, since a rapid (<1 min) increase in the intracel lular Ca2+ concentration ([Ca2+](i)) was found in the cells treated wi th the drug. Calcium chelation in the culture medium with EGTA did not affect the [Ca2+](i) mobilization. A phospholipase C inhibitor, U7312 2, inhibited the increase in [Ca2+](i) as well as the fragmentation of the nuclear DNA, whereas U73343. a non-effective analogue of U73122, had little effect. These results suggest that FTY72O-induced apoptosis is mediated through an activation of phospholipase C and the subseque nt release of Ca2+ from intracellular calcium pools. In addition, the treatment of HL-60 with pertussis toxin (PTX) did not inhibit Ca2+ mob ilization or apoptosis, suggesting that the activation of phospholipas e C is independent of PTX-sensitive G-proteins.