A human breast cancer cell line (MCF7/WT) was selected for resistance to etoposide (VP-16) by stepwise exposure to 2-fold increasing concentrations of this agent. The resulting cell line (MCF7/VP) was 28-, 21-, and 9-fold resistant to VP-16, VM-26, and doxorubicin, respectively. MCF7/VP cells also exhibited low-level cross-resistance to 4′-(9-acridinylamino)-methanesulfon-m-anisidide, mitoxantrone, and vincristine and no cross-resistance to genistein and camptothecin. Furthermore, these cells were collaterally sensitive to the alkylating agents melphalan and chlorambucil. DNA topoisomerase II levels were similar in both wild-type MCF7/WT and drug-resistant MCF7/VP cells. In contrast, topoisomerase II from MCF7/VP cells appeared to be 7-fold less sensitive to drug-induced cleavable complex formation in whole cells and 3-fold less sensitive in nuclear extracts than topoisomerase II from MCF7/WT cells. Although this suggested that the resistant cells may contain a qualitatively altered topoisomerase II, no mutations were detected in either the ATP-binding nor the putative breakage/resealing regions of either DNA topoisomerase IIα or IIβ. In addition, the steady-state intracellular VP-16 concentration was reduced by 2-fold in the resistant cells, in the absence of detectable mdr1/P-gp expression and without any change in drug efflux. In contrast, expression of the gene encoding the MRP was increased at least 10-fold in resistant MCF7/VP cells as compared to sensitive MCF7/WT cells. These results suggest that resistance to epipodophyllotoxins in MCF7/VP cells is multifactorial, involving a reduction in intracellular drug concentration, possibly as a consequence of MRP overexpression, and an altered DNA topoisomerase II drug sensitivity.
The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.