title: Multidrug resistance-associated protein gene overexpression and reduced drug sensitivity of topoisomerase II in a human breast carcinoma MCF7 cell line selected for etoposide resistance.
abstract: 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 II alpha or II beta. 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.
authors.affiliation: Medicine Branch, National Cancer Institute, Bethesda, Maryland 20892.
authors.forename: E
authors.initials: E
authors.lastname: Schneider
authors.forename: J K
authors.initials: JK
authors.lastname: Horton
authors.forename: C H
authors.initials: CH
authors.lastname: Yang
authors.forename: M
authors.initials: M
authors.lastname: Nakagawa
authors.forename: K H
authors.initials: KH
authors.lastname: Cowan
mesh_terms: D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1; D001943:Breast Neoplasms / Q000201:enzymology*; D002352:Carrier Proteins / Q000032:analysis; D004264:DNA Topoisomerases, Type I / Q000032:analysis; D004250:DNA Topoisomerases, Type II / Q000032:analysis / Q000235:genetics / Q000378:metabolism*; D004305:Dose-Response Relationship, Drug; D004351:Drug Resistance / Q000235:genetics; D005047:Etoposide / Q000378:metabolism* / Q000494:pharmacology; D005260:Female; D006801:Humans; D008562:Membrane Glycoproteins / Q000032:analysis; D012333:RNA, Messenger / Q000032:analysis; D013713:Teniposide / Q000378:metabolism* / Q000494:pharmacology; D014407:Tumor Cells, Cultured
chemical_list: D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1; D002352:Carrier Proteins; D008562:Membrane Glycoproteins; D012333:RNA, Messenger; D005047:Etoposide; D013713:Teniposide; D004264:DNA Topoisomerases, Type I; D004250:DNA Topoisomerases, Type II