TRANSFECTION OF A HUMAN TOPOISOMERASE II-ALPHA GENE INTO ETOPOSIDE-RESISTANT HUMAN BREAST-TUMOR CELLS SENSITIZES THE CELLS TO ETOPOSIDE

The etoposide-resistant human breast cancer cell line MDA-VP was deriv ed from MDA-parent cells by sequential selection in increasing concent rations of etoposide. MDA-VP cells express a lower amount of topoisome rase II alpha mRNA than the MDA-parent does, have mutations in topoiso merase II alpha (topo II) cDNA, and show cross-resistance to doxorubic in and amsacrine. We investigated whether transfer of a normal human t opoisomerase II alpha (H-topo II) gene into MDA-VP cells could overcom e their resistance to etoposide. H-topo II in a mammalian expression v ector containing a glucocorticoid-inducible mouse mammary tumor virus (MMTV) promoter (pMAMneo) was transfected into MDA-VP cells (MDA-VP-hT OP2MAM). These H-topo II-transfected cells showed increased H-topo II mRNA expression and protein levels compared with MDA-VP parental cells or with MDA-VP cells transfected with the control pMAM vector (MDA-VP -MAM). Following cell exposure to dexamethasone, DNA-protein cleavable complex formation and cytotoxicity induced by etoposide, doxorubicin, and amsacrine were increased in the MDA-VP-hTOP2MAM cells compared wi th MDA-VP-MAM cells. However, these changes were short-lived, and by 2 4 h, cytotoxicity, cleavable DNA-protein complex formation, and H-topo II protein levels returned to baseline values. These results indicate that sensitivity of MDA-VP cells correlated with changes in cellular H-topo II. The gene transfer of a normal H-topo II gene can sensitize MDA-VP cells to the actions of multiple antineoplastic agents that tar get topo II.