Concentration-dependent variable effects of etoposide on the cell cycle of CML cells

Background: Etoposide, a DNA-topoisomerase II inhibitor, is used for a broad spectrum of cancers with various therapeutic strategies. But the molecular mechanisms of its concentration-dependent effects are not clearly defined. Materials and Methods: Chronic myelogenous leukemia K562 cells were treated with low (5 muM) or high (100 muM) concentrations of this drug and the changes of cell cycle progression, expression of cell cycle regulating genes and cyclin B1-dependent histone H1 kinase activity were studied. Results: In the presence of 5 muM etoposide, K562 cells continued to synthesize DNA and most cells showed progress into G2 phase until 24 hours. In contrast, 100 muM etoposide rapidly inhibited DNA synthesis by around 6 hours and most cells remained in their initial phase, while the incorporation of bromodeoxyuridine was partially resumed from 12 hours. The histone H1 kinase activity was only down-regulated in the early phase of 100 muM treated cells. Among the cell cycle controlling genes, c-Myc and p21(Cip1/WAF1) showed impressive responses to the two etoposide concentrations. At 100 muM, c-Myc protein rapidly vanished at 3 hours while p21(Cip1/WAF1) was inversely induced from 3 hours. These changes were also observed at 5 muM, but they occurred slowly and weakly. Conclusion: The present findings indicate that two concentrations of etoposide functioned as an anticancer agent through modulating the genes related in cell cycle progression. Differing responses of c-Myc and p21(Cip1/WAF1) at two concentrations may govern the antiproliferative effects of etoposide.