Cell cycle arrest and protein kinase modulating effect of bufalin on human leukemia ML1 cells

Bufalin, an active principle of the traditional Chinese medicine chan' su, has been proved to be a potent differentiation inducer in human leukemia cells. To study the mechanism of the differentiation of human leukemia ML1 cells induced by bufalin, we measured the effect of 10 nM bufalin on cell growth, activities of various protein kinases, and cell cycle. The ML1 cell growth was inhibited significantly at 24 hr and the inhibiting effect persisted for 6 days. Activities of PKC, PKA, cdc2 kinase and CK II in ML1 cells were changed early by bufalin; PKA and PKC activities were inhibited, and cdc2 kinase and CK II activities were increased. These results suggest that bufalin induces differentiation of ML1 cells by modulating several protein kinase activities in a distinct way from RA and 1α, 25(OH) ₂D₃. Cell cycle changes, measured by flow cytometry, became evident at 12 hr after treatment of ML1 cells with bufalin and the cells were preferentially arrested in the G2/M phase. This effect of bufalin on the cell cycle of leukemia cells is similar to that of topoisomease inhibitors. Indeed, the activity of topoisomerase II but not topoisomerase I of ML1 cells was inhibited remarkably by the treatment of the cells with 10 nM bufalin.