title: DNA strand breaks produced by etoposide (VP-16,213) in sensitive and resistant human breast tumor cells: implications for the mechanism of action.
abstract: Pleotropic resistant human breast cancer cells (MCF-7), selected for resistance to Adriamycin, were used to study the production of DNA strand breaks by etoposide (VP-16) and its relationship to drug cytotoxicity. It was shown that the resistant MCF-7 cell line was cross-resistant to VP-16, and the degree of resistance was found to be 125-200-fold. Alkaline elution studies indicated that the parental cell line was very sensitive to VP-16 which caused extensive DNA strand breakage. In contrast, little DNA strand breakage was detected in the resistant MCF-7 cells, even at very high drug concentrations, indicating a good agreement between strand breaks and cytotoxicity. Further studies indicated that the nuclei isolated from the parental cell line were more resistant to VP-16-induced DNA strand breaks than the intact cells, while the opposite was found in the resistant cell line. In addition, the alkaline elution studies in isolated nuclei showed only a 2-fold reduction of VP-16-induced DNA breaks in nuclei from the resistant cells. In agreement with this result, it was found that nuclear extract from the resistant cells produced 2-3-fold less VP-16-induced DNA breaks than that from the sensitive cells in 32P-end-labeled SV40 DNA. VP-16 uptake and efflux studies indicated that there was a 2-3-fold decrease in net cellular accumulation of VP-16 in the resistant cells. Although the reduced uptake of VP-16 and decreased drug sensitivity of topoisomerase II appear to contribute to the mechanism of action and the development of resistance to VP-16, they do not completely explain the degree of resistance to VP-16 in this multidrug-resistant MCF-7 cell line indicating that other biochemical factors, such as activation of VP-16, are also involved in drug resistance and suggesting that the resistance is multifactorial.
authors.affiliation: Clinical Pharmacology Branch, National Cancer Institute, Bethesda, Maryland 20892.
authors.forename: B K
authors.initials: BK
authors.lastname: Sinha
authors.forename: N
authors.initials: N
authors.lastname: Haim
authors.forename: L
authors.initials: L
authors.lastname: Dusre
authors.forename: D
authors.initials: D
authors.lastname: Kerrigan
authors.forename: Y
authors.initials: Y
authors.lastname: Pommier
mesh_terms: D001943:Breast Neoplasms / Q000235:genetics* / Q000473:pathology; D002460:Cell Line / Q000187:drug effects; D004249:DNA Damage; D004250:DNA Topoisomerases, Type II / Q000378:metabolism; D004273:DNA, Neoplasm / Q000187:drug effects*; D004351:Drug Resistance; D005047:Etoposide / Q000009:adverse effects*; D005260:Female; D006801:Humans; D013997:Time Factors
chemical_list: D004273:DNA, Neoplasm; D005047:Etoposide; D004250:DNA Topoisomerases, Type II