Effect of extracellular magnesium on topoisomerase II activity and expression in human leukemia HL-60 cells

Topoisomerase II (TopoII) is a Mg-dependent enzyme involved in topological modifications of DNA that are crucial to the regulation of cell proliferation and possibly differentiation. To investigate the role of Mg availability in the modulation of TopoII in whole cells, we studied enzyme activity and expression in HL-60 cells grown in the presence of decreasing amounts of extracellular Mg (0.5, 0.03, and 0.01 mM MgSO(4)). In comparison to cells grown in 0.5 mM Mg, cells grown in 0.03 mM Mg exhibited a decrease in TopoII activity, as evidenced by reduced induction of DNA/TopoII cleavable complexes and apoptosis by etoposide and teniposide. Enzyme activity was restored by the readdition of Mg (0.5 and 1.5 mM) in the incubation medium, confirming that this effect was indeed modulated by extracellular Mg. Restriction of Mg to 0.01 mM was associated with a dramatic decrease in TopoII activity resembling that observed in HL-60 cells differentiated by dimethyl sulfoxide treatment. The restriction of Mg, while decreasing enzyme activity, was found to upregulate TopoII protein expression, determined by Western blot analysis. The increase of TopoII protein levels was correlative with the degree of Mg deprivation. Collectively, these results indicate that extracellular levels of Mg may control availability of intracellular Mg, thus affecting the regulation of TopoII activity/expression and downstream processes of cell proliferation and/or differentiation.