The plant alkaloid usambarensine intercalates into DNA and induces apoptosis in human HL60 leukemia cells

Usambarensine is a plant alkaloid isolated from the roots of Strychnos usam barensis collected in Central Africa. This bis-indole compound displays pot ent antiamoebic activities and shows antigardial, antimalarial and cytotoxi c effects. Usamba-rensine is highly toxic to B16 melanoma cells and inhibit s the growth of leukemia and carcinoma cells. To date the molecular basis f or its diverse biological effects remains totally unknown. However; its cap acity to inhibit nucleic acids synthesis in melanoma cells, on the one hand , and its structural analogy with DNA-binding pyridoindole plant alkaloids recently studied (cryptolepine and matadine), on the other hand, suggested that usambarensine could also bind to DNA. Consequently we studied the stre ngth and mode of binding to DNA of usambarensine by means of absorption, ci rcular and lineal dichroism. The results of the optical measurements indica te that the alkaloid effectively: binds to DNA and behaves as a typical int ercalating agent. Biochemical experiments indicated that, in contrast to cr yptolepine and matadine, usambarensine does not interfere with the catalyti c activity of topoisomerase II. Human HL60 leukemia cells were risen to ass ess the cytotoxicity of the alkaloid and its effect on the cell cycle. Usam barensine treatment is associated with a loss of cells in the G1 phase acco mpanied with a large increase in the sub-G1 region which is characteristic of apoptotic cells. The DNA of usambarensine-treated cells was severely fra gmented and the proteolytic activity of DEVD-caspases is enhanced. Usambare nsine is thus characterized as DNA intercalator inducing apoptosis in leuke mia cells.