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

Abstract

Topoisomerase II (topo II), an enzyme essential for cell viability, is present in mammalian cells as the α- and β-isoforms. In human leukemia HL-60/S or HL-60/doxorubicin (DOX)0.05 cells, the levels of topo IIα- or β-protein were similar in either asynchronous exponential or synchronized cultures. Although topo IIα was hypophosphorylated in HL-60/DOX0.05 compared with HL-60/S cells, both overall and site-specific hyperphosphorylation of topo IIβ was apparent in HL-60/DOX0.05 compared with HL-60/S cells. The phosphorylation of topo IIα and not β was enhanced in the S and G₂ + M phases of HL-60/S cells. In contrast, an increase in the phosphorylation of topo IIβ compared with α was apparent in the G₁ and S phases of HL-60/DOX0.05 cells. The cytotoxicity and depletion of topo IIα or β 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 β versus α topo II protein. Present results demonstrate that: 1) hyperphosphorylation of topo IIβ in HL-60/DOX0.05 cells may be a compensatory consequence of the hypophosphorylation of topo IIα 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 IIβ.