The cytoplasmic trafficking of DNA topoisomerase IIα correlates with etoposide resistance in human myeloma cells

In this study we have investigated the role of topoisomerase (topo) IIα trafficking in cellular drug resistance. To accomplish this, it was necessary to separate the influence of cell cycle, drug uptake, topo protein levels, and enzyme trafficking on drug sensitivity. Thus, we developed a cell model (called accelerated plateau) using human myeloma H929 cells that reproducibly translocates topo IIα to the cytoplasm. Compared to log-phase cells, the cytoplasmic redistribution of topo IIα in plateau-phase cells correlated with a 10-fold resistance to VP-16 and a 40–60% reduction in the number of drug-induced double-strand DNA breaks. In addition, 7-fold more VP-16 was necessary to achieve 50% topo IIα band depletion, suggesting that there are fewer drug-induced topo–DNA complexes formed in quiescent cells than in log-phase cells. The total cellular amount of topo IIα and topo IIβ protein in log- and plateau-phase cells was similar as determined by Western blot analysis. There was a 25% reduction in S-phase cell number in plateau cells (determined by bromodeoxyuridine (BrdU) incorporation), while there was no significant difference in the equilibrium concentrations of [ 3H]-VP-16 when log cells were compared with plateau cells. Furthermore, the nuclear/cytoplasmic ratio of topo IIα is increased 58-fold in accelerated-plateau H929 cells treated with leptomycin B (LMB) when compared to untreated cells. It appears that the nuclear-cytoplasmic shuttling of topo IIα, which decreases the amount of nuclear target enzyme, is a major mechanism of drug resistance to topo II inhibitors in plateau-phase myeloma cells.