Heterocyclic organobismuth(III) induces apoptosis of human promyelocytic leukemic cells through activation of caspases and mitochondrial perturbation

We have synthesized novel heterocyclic organobismuth compounds that have potent antibacterial properties. In this study, we examined their anticancer activity and addressed the cellular mechanisms involved. Heterocyclic organobismuth compounds showed anticancer activities in various human cancer cell lines. These compounds have particularly potent anticancer activities against leukemia cell lines. One of them, bi-chlorodibenzo [c,f][1,5] thiabismocine (compound 3), inhibited the growth of the human promyelocytic leukemia cell line HL-60 at a concentration of 0.22 μM. Low concentrations of compound 3 (0.22–0.44 μM) induced apoptosis, whereas at a higher concentration (>1.1 μM) it causes acute necrosis. During the apoptosis, caspase-3, -8, and -9 were activated but caspase-12 was not. A broad caspase inhibitor (z-VAD-fmk), and caspase-3 (z-DEVD-fmk) and caspase-9 (z-LEHD-fmk) inhibitors suppressed the compound 3-induced apoptosis, but a caspase-8 inhibitor (z-IETD-fmk) was less effective, suggesting that the caspase-8 activity only partially participates in the apoptosis. In the apoptotic cells, cytochrome c was released from mitochondria to cytosol and a loss of mitochondrial transmembrane potential (ΔΨm) was detected. Compound 3-induced apoptosis was associated with enhanced generation of intracellular reactive oxygen species (ROS). Pretreatment of the cells with N-acetyl-l-cysteine or catalase suppressed the apoptosis. On the other hand, buthionine sulfoximine enhanced the compound 3-induced collapse of ΔΨm and apoptosis. Taken together, these results indicate that compound 3 is a potent inducer of apoptosis, triggering a caspase-3-mediated mechanism via the generation of ROS and release of cytochrome c from mitochondria, suggesting a potential mechanism for the anticancer activity of compound 3.