Enhancement of manumycin A-induced apoptosis by methoxyamine in myeloid leukemia cells

Farnesyltransferase inhibitors (FTIs) are currently under investigation for leukemia treatment. We evaluated the FTI manumycin A (manumycin) in two myeloid leukemia cell lines (U937 and HL-60). Manumycin induced nitric oxide production and apoptosis of the leukemia cells. Nitric oxide or other reactive oxygen species may induce oxidative DNA damage, and the number of apurinic sites increased after manumycin treatment, which was reversed by concurrent treatment with N-acetyl-L-cysteine. Since repair of DNA damage is important to cell survival, we hypothesized that methoxyamine, an inhibitor of base-excision repair, would enhance the antineoplastic effect of manumycin. The combination of manumycin and methoxyamine resulted in enhanced apoptosis by six criteria - increased annexin V binding, release of mitochondrial cytochrome c into the cytosol, activation of caspase-9, activation of caspose-3, specific cleavage of poly-adenosyl ribose polymerase, and increase in the sub-G1 cell cycle fraction. The drug combination enhanced inhibition on the soft agar clonogenic assay and on the formazan dye cell viability assay. The effects of manumycin or manumycin plus methoxyamine on apoptosis were blocked by N-acetyl-L-cysteine, and partially by nitric oxide synthase inhibitors or scavenger of peroxide. We conclude that methoxyamine enhances manumycin-induced apoptosis in myeloid leukemia cells.