Efficient lysis of B-chronic lymphocytic leukemia cells by the plant-derived sesquiterpene alcohol α-bisabolol, a dual proapoptotic and antiautophagic agent

The sesquiterpene α-bisabolol (α-BSB) is a cytotoxic agent against acute leukemia and chronic myeloid leukemia cells. Here the profile of α-BSB citotoxicity was evaluated ex vivo in primary mononuclear blood cells isolated from 45 untreated B-chronic lymphocytic leukemia (B-CLL) patients. We studied the effects of α-BSB by flow cytometric and western blotting techniques with the following findings: (1) α-BSB was an effective proapoptotic agent against B-CLL cells (IC50 42 ± 15 μM). It was also active, but to a lesser extent, on normal residual B cells and monocytes (IC50 68 ± 34 and 74 ± 28 μM, respectively; p < 0.01), while T-cells, though not achieving IC50, were nevertheless decreased. (2) Lipid raft content positively correlated with α-BSB cell sensitivity, while neither the phenotype of B-CLL cells nor the disease clinical stage did affect the sensitivity to α-BSB. (3) Flow cytometry analysis evidenced the induction of pores in mitochondrial and lysosomal membrane after 3- to 5-hour exposure of B-CLL cells to α-BSB, leading to apoptosis; in contrast, western blotting analysis showed inhibition of the autophagic flux. Therefore, according to cellular selectivity, α-BSB is a cytotoxic agent preferentially active against leukemic cells, while its lower activity on normal B cells, monocytes and T cells may account for an additive anti-inflammatory effect targeting the leukemia-associated pro-inflammatory microenvironment. Consistent with the observed effects on intracellular processes, α-BSB should be regarded as a dual agent, both activating mitochondrial-based apoptosis and inhibiting autophagy by disrupting lysosomes.