Apoptotic cell death induced by taxol is inhibited by nitric oxide in human leukemia HL-60 cells

Taxol, an antineoplastic drug, increases the fraction of cells in G2/M phases of cell cycle, induces apoptosis of leukemic cells, and activates macrophages to produce nitric oxide (NO) in response to interferon-gamma. NO has been found to play roles as pro-apoptotic or anti-apoptotic effector molecules. In this study, we investigate effects of NO on taxol-induced apoptosis in human myeloid leukemia cell, HL-60. Incubation of the cells with taxol for 24 hr induced marked DNA fragmentation of HL-60 cells. Treatment of the cells with S-nitrosogluthathione (GSNO), a NO-generating agent, protected the cells against taxol-induced apoptosis. Cell cycle analysis showed that treatment of the cells with 100 nM taxol for 12 hr rendered the cells to be accumulated in G2/M phase, but the cotreatment of the cells with taxol and 0.1 mM GSNO decreased the accumulation of the cell in G2/M phases, suggesting that NO might interfere entering of taxol-treated cells into G2/M phases. Deferoxamine or mimosine, which can arrest cells mainly at G1/S phases, also decreased taxol-induced apoptosis and reduced the number of the taxol-treated cells arresting in G2/M phases. Thus, we conclude that a protective effect of NO on taxol-treated cells from apoptosis may be partially caused by interfering entering of the taxol-treated cells into G2/M phases.