Altered drug interaction and regulation of topoisomerase II beta: Potential mechanisms governing sensitivity of HL-60 cells to amsacrine and etoposide

Topoisomerase II (topo II), an enzyme essential for cell viability, is pres ent in mammalian cells as the alpha- and beta-isoforms. In human leukemia H L-60/S or HL-60/doxorubicin (DOX)0.05 cells, the levels of topo II alpha- o r beta-protein were similar in either asynchronous exponential or synchroni zed cultures. Although topo II alpha was hypophosphorylated in HL-60/DOX0.0 5 compared with HL-60/S cells, both overall and site-specific hyperphosphor ylation of topo II beta was apparent in HL-60/DOX0.05 compared with HL-60/S cells. The phosphorylation of topo II alpha and not beta was enhanced in t he S and G(2) + M phases of HL-60/S cells. In contrast, an increase in the phosphorylation of topo II beta compared with a was apparent in the G(1) an d S phases of HL-60/DOX0.05 cells. The cytotoxicity and depletion of topo I I alpha or beta in cells treated with drug for 1 h revealed that mole-for-m ole, amsacrine was 2-fold more effective than etoposide in killing HL-60/S or HL-60/DOX0.05 cells and in depleting the beta versus alpha topo II prote in. Present results demonstrate that: 1) hyperphosphorylation of topo II be ta in HL-60/DOX0.05 cells may be a compensatory consequence of the hypophos phorylation of topo II alpha to maintain normal topo II function during pro liferation, and 2) enhanced sensitivity of HL-60/S or HL-60/DOX0.05 cells t o amsacrine may be due to the preferential interaction and depletion of top o II beta.