Plant-derived abrin-a induces apoptosis in cultured leukemic cell lines by different mechanisms

Abrin-a consists of A-chain with N-glycosidase activity, which inhibits protein synthesis, and lectin-like B-chain responsible for binding with cell-surface receptors and penetrating of abrin-a molecule into the cells. As a lectin component, the B-chain can also participate in cell signal transduction. It has been reported that abrin induces apoptosis, but the molecular mechanism(s) of this induction have been obscure and several alternative variants have been discussed. The present study demonstrates that abrin-a induces apoptosis in human cultured cell lines, derived from acute lymphoblastic leukemia (ALL) (Jurkat, CCRF-CEM, MOLT-4, HPB-ALL). The apoptosis was estimated by: phosphatidylserine (PSer) exposure at the cell surface, activation of caspase cascade, and DNA fragmentation. The penetrating of abrin-a into the cells was detected by fluorescent confocal microscopy, using fluorescein isothiocyanate (FITC) as a fluorescent marker. It was established that the effect of abrin-a on the apoptosis induction in leukemic cells was dose- and time-dependent. The process was initiated 1 h after abrin-a application (before its penetrating into the cells) and was characterized with PSer translocation from the inner to the outer monolayer of plasma membrane, caspase activation on the first to second hour after beginning of treatment, with maximum on the third to fourth hour, and DNA fragmentation on the fourth to sixth hour, depending of the cell line. The exposure of PSer on the cell surface was detected in Jurkat, CCRF-CEM, and MOLT-4 cells. In HPB-ALL, no significant changes in PSer exposure on the cell surface was observed. Activation of caspase-3, -8, and -9 was detected in Jurkat, MOLT-4, and HPB-ALL. Surprisingly, the activity of caspase-3 increased on the first hour after beginning of treatment, while the activity of caspase-8 and -9 began to increase on the second hour. In CCRF-CEM, activation of caspases was not measured, but the apoptosis progressed to DNA fragmentation in a dose- and time-dependent manner. DNA fragmentation was also detected in Jurkat, but not in MOLT-4 and HPB-ALL cells. It seems that the mechanisms of abrin-a-induced apoptosis are different and the progress of apoptosis depends of the cell line. There was a very good positive correlation between the agglutinating activity of abrin-a and development of apoptosis to DNA fragmentation. The time-dependent effects of abrin-a on apoptosis as well as its time-dependent penetration into the cells suggest that the B-chain probably triggers the apoptosis, while the A-chain and breakage of the disulfide bond are responsible for its progress.