title: Mitoxantrone is expelled by the ABCG2 multidrug transporter directly from the plasma membrane.
abstract: ABC multidrug transporter proteins expel a wide variety of structurally unrelated, mostly hydrophobic compounds from cells. The special role of these transporters both at the physiological barriers and in cancer cells is based on their extremely broad substrate recognition. Since hydrophobic compounds are known to partition into the lipid bilayer and accumulate in membranes, the "classical pump" model for the mechanism of multidrug transporter proteins has been challenged, and alternative models suggesting substrate recognition within the lipid bilayer have been proposed. Although much effort has been made to validate this concept, unambiguous evidence for direct drug extrusion from the plasma membrane has not been provided yet. Here we show a detailed on-line microscopic analysis of cellular extrusion of fluorescent anti-cancer drugs, mitoxantrone and pheophorbide A, by a key human multidrug transporter, ABCG2. Using the fully active GFP-tagged ABCG2 and exploiting the special character of mitoxantrone that gains fluorescence in the lipid environment, we were able to determine transporter-modulated drug concentrations separately in the plasma membrane and the intracellular compartments. Different kinetic models describing the various transport mechanisms were generated and the experimental data were analyzed using these models. On the basis of the kinetic analysis, drug extrusion from the cytoplasm can be excluded, thus, our results indicate that ABCG2 extrudes mitoxantrone directly from the plasma membrane.
authors.affiliation: Membrane Biology Research Group, Semmelweis University, Hungarian Academy of Sciences, Diószegi u. 64, H-1113 Budapest, Hungary. homolya@biomembrane.hu
authors.forename: László
authors.initials: L
authors.lastname: Homolya
authors.forename: Tamás I
authors.initials: TI
authors.lastname: Orbán
authors.forename: László
authors.initials: L
authors.lastname: Csanády
authors.forename: Balázs
authors.initials: B
authors.lastname: Sarkadi
mesh_terms: D000070997:ATP Binding Cassette Transporter, Subfamily G, Member 2; D018528:ATP-Binding Cassette Transporters / Q000378:metabolism*; D000970:Antineoplastic Agents / Q000494:pharmacology; D001692:Biological Transport; D045744:Cell Line, Tumor; D002462:Cell Membrane / Q000378:metabolism*; D049452:Green Fluorescent Proteins / Q000737:chemistry; D006801:Humans; D007700:Kinetics; D008055:Lipids / Q000737:chemistry; D008942:Mitoxantrone / Q000737:chemistry* / Q000494:pharmacology; D015233:Models, Statistical; D009363:Neoplasm Proteins / Q000378:metabolism*; D009369:Neoplasms / Q000188:drug therapy; D013997:Time Factors; D014162:Transfection
chemical_list: C116614:ABCG2 protein, human; D000070997:ATP Binding Cassette Transporter, Subfamily G, Member 2; D018528:ATP-Binding Cassette Transporters; D000970:Antineoplastic Agents; D008055:Lipids; D009363:Neoplasm Proteins; D049452:Green Fluorescent Proteins; D008942:Mitoxantrone