Efflux kinetics and intracellular distribution of daunorubicin are not affected by major vault protein/lung resistance-related protein (vault) expression

Vaults may contribute to multidrug resistance by transporting drugs away from their subcellular targets. To study the involvement of vaults in the extrusion of anthracyclines from the nucleus, we investigated the handling of daumorubicin by drug-sensitive and drug-resistant non-small lung cancer cells, including a green fluorescent protein (GFP)-tagged major vault protein (MVP)-overexpressing transfectant (SW1573/MVPGFP). Cells were exposed to 1 mum daumorubicin for 60 min, after which the cells were allowed to efflux the accumulated drug. No significant differences in datmorubicin efflux kinetics were observed between the sensitive SW1573 and SW1573/MVP-GFP transfectant, whereas the drug-resistant SW1573/2R120 cells clearly demonstrated an increased efflux rate. It was noted that the redistribution of daunorubicin from the nucleus into distinct vesicular structures in the cytoplasm was not accompanied by changes in the intracellular localization of vaults. Similar experiments were performed using mouse embryonic fibroblasts derived from wildtype and MVP knockout mice, which were previously shown to be devoid of vault particles. Both cell lines showed comparable drug efflux rates, and the intracellular distribution of datmorubicin in time was identical. Reintroduction of a human MVP tagged with GFP in the MVP-/- cells results in the formation of vault particles but did not give rise an altered daunorubicin handling compared with MVP-/- cells expressing GFP. Our results indicate that vaults are not directly involved in the sequestration of anthracyclines in vesicles nor in their efflux from the nucleus.