Holotoxin A(1) Induces Apoptosis by Activating Acid Sphingomyelinase and Neutral Sphingomyelinase in K562 and Human Primary Leukemia Cells

Marine triterpene glycosides are attractive candidates for the development of anticancer agents. Holotoxin A(1) is a triterpene glycoside found in the edible sea cucumber, Apostichopus (Stichopus) japonicus. We previously showed that cladoloside C(2), the 25(26)-dihydro derivative of holotoxin A(1), induced apoptosis in human leukemia cells by activating ceramide synthase 6. Thus, we hypothesized that holotoxin A(1), which is structurally similar to cladoloside C(2), might induce apoptosis in human leukemia cells through the same molecular mechanism. In this paper, we compared holotoxin A(1) and cladoloside C(2) for killing potency and mechanism of action. We found that holotoxin A(1) induced apoptosis more potently than cladoloside C(2). Moreover, holotoxin A(1)-induced apoptosis in K562 cells by activating caspase-8 and caspase-3, but not by activating caspase-9. During holotoxin A(1) induced apoptosis, acid sphingomyelinase (SMase) and neutral SMase were activated in both K562 cells and human primary leukemia cells. Specifically inhibiting acid SMase and neutral SMаse with chemical inhibitors or siRNAs significantly inhibited holotoxin A(1)–induced apoptosis. These results indicated that holotoxin A(1) might induce apoptosis by activating acid SMase and neutral SMase. In conclusion, holotoxin A(1) represents a potential anticancer agent for treating leukemia. Moreover, the aglycone structure of marine triterpene glycosides might affect the mechanism involved in inducing apoptosis.