Regulation of cellular glutathione modulates nuclear accumulation of daunorubicin in human MCF7 cells overexpressing multidrug resistance associated protein

Multidrug resistance (MDR) is frequently associated with the overexpression of P-glycoprotein (Pgp) and/or multidrug resistance associated protein (MR P1), both members of the ABC superfamily of transporters. Pgp and MRP1 func tion as ATP-dependent efflux pumps that extrude cytotoxic drugs from tumour cells. Glutathione (GSH) has been considered to play an important role in the MRP1-mediated MDR. In our study, we examined the effects of buthionine sulphoximine (BSO), an inhibitor of GSH biosynthesis, on the nuclear accumu lation of daunorubicin (DNR). in etoposide (VP16) and doxorubicin (ADR) res istant MCF7 cell lines, overexpressing respectively MRP1 (MCF7/VP) and Pgp (MCF7/ADR). The study of DNR transport was carried out using scanning confo cal microspectrofluorometry. This technique allows the determination of the nuclear accumulation of anthracyclines in single living tumour cells. Trea tment of MCF7/VP cells with BSO increased the sensitivity of these cells to DNR whilst the cytotoxicity of the drug in MCF7/ADR cells remained unchang ed. In MCF7 resistant cells treated with BSO, their GSH level decreased as observed by confocal microscopy. DNR nuclear accumulation in MCF7/VP cells was increased by BSO whereas in MCF7/ADR cells BSO was unable to significan tly increase the DNR nuclear accumulation. These data suggest a requirement for GSH in MRP1-mediated resistance whilst the nuclear efflux of GSH conju gates is probably not the primary mechanism of Pgp-mediated MDR. Finally, B SO might be a useful agent in clinical assays for facilitating detection of MRP1 expression. (C) 2000 Published by Elsevier Science Ltd. All rights re served.