DNA topoisomerase II isozymes involved in anticancer drug action and resistance

Abstract

DNA topoisomerase II is a major protein of the nuclear matrix. The enzyme appears to have a central role in both DNA organization and replication. The importance of nuclear matrix topoisomerase IIα as a target for certain anticancer agents was evaluated in CEM human leukemia cells. Studies were done to determine the extent to which the α (170 kDa) and β (180 kDa) isozymes of topoisomerase II form covalent enzyme-DNA complexes in whole cells and in the nuclear matrix and nonmatrix fractions of CEM cells that are either sensitive or resistant to topoisomerase II-active anticancer agents. Topoisomerase IIα was detected in both the high salt-soluble (nonmatrix) and matrix fractions of nuclei from parental CEM cells. Most of the matrix topoisomerase IIα was tightly bound to DNA in cells incubated with VM-26. In contrast, topoisomerase IIβ was detected only in the high salt-soluble (nonmatrix) fraction of the nucleus. The subnuclear distribution of the α and β topoisomerase II isozymes in CEM/VM-1 cells resistant to topoisomerase-active drugs was similar to that in drug-sensitive CEM cells. However, the amount and activity of topoisomerase IIα in nuclear matrices of CEM/VM-1 cells were decreased 3- to 6-fold relative to that of CEM cells. The differences observed in the subnuclear distribution and DNA binding pattern of the topoisomerase II isozymes support the hypotheses that each isozyme has a distinct cellular function. Furthermore, these results provide evidence that topoisomerase IIα is the nuclear matrix target for VM-26, and that depletion of the nuclear matrix isozyme contributes to cellular resistance to this anticancer agent.