Vol. 56, Issue 6, 1340-1345, December 1999

Altered Drug Interaction and Regulation of Topoisomerase II: Potential Mechanisms Governing Sensitivity of HL-60 Cells to Amsacrine and Etoposide

Dale R. Grabowski, Katherine A. Holmes, Masako Aoyama, Ying Ye, Lisa A. Rybicki, Ronald M. Bukowski, Mahrukh K. Ganapathi, Ian D. Hickson, and Ram Ganapathi

Experimental Therapeutics Program, Taussig Cancer Center, Cleveland Clinic Foundation, Cleveland, Ohio; and the Molecular Oncology Laboratory, Imperial Cancer Research Fund Unit, Institute of Molecular Medicine, John Radcliffe Hospital, Oxford, United Kingdom

Topoisomerase II (topo II), an enzyme essential for cell viability, is present in mammalian cells as the - and -isoforms. In human leukemia HL-60/S or HL-60/doxorubicin (DOX)0.05 cells, the levels of topo II- or -protein were similar in either asynchronous exponential or synchronized cultures. Although topo II was hypophosphorylated in HL-60/DOX0.05 compared with HL-60/S cells, both overall and site-specific hyperphosphorylation of topo II was apparent in HL-60/DOX0.05 compared with HL-60/S cells. The phosphorylation of topo II and not  was enhanced in the S and G₂ + M phases of HL-60/S cells. In contrast, an increase in the phosphorylation of topo II compared with  was apparent in the G₁ and S phases of HL-60/DOX0.05 cells. The cytotoxicity and depletion of topo II or  in cells treated with drug for 1 h revealed that mole-for-mole, amsacrine was 2-fold more effective than etoposide in killing HL-60/S or HL-60/DOX0.05 cells and in depleting the  versus  topo II protein. Present results demonstrate that: 1) hyperphosphorylation of topo II in HL-60/DOX0.05 cells may be a compensatory consequence of the hypophosphorylation of topo II to maintain normal topo II function during proliferation, and 2) enhanced sensitivity of HL-60/S or HL-60/DOX0.05 cells to amsacrine may be due to the preferential interaction and depletion of topo II.