title: Relationships between rhodamine 123 transport, cell volume, and ion-channel function of P-glycoprotein.
abstract: The P-glycoprotein (Pgp), a plasma membrane protein overexpressed in multidrug-resistant tumor cells, is thought to be both an ATPase that actively exports cytotoxic drugs and a Cl- channel activated by cell swelling. The partial reversal of multidrug resistance by Cl- transport blockers suggests a possible role for Cl- in Pgp-mediated drug transport. We used multidrug-resistant Chinese hamster fibroblasts and human breast cancer cells expressing Pgp to study the roles of Cl- (and also Na+ and HCO3-/CO2) on Pgp-mediated efflux of the fluorescent dye rhodamine 123 (R123). In Pgp-expressing Chinese hamster fibroblasts, exposed to isosmotic solutions, the unidirectional efflux of R123 was not measurably changed by a approximately 60-min removal of Cl- (or by exposure to Na(+)-free, or nominally HCO3-/CO2-free medium); short term (2-3 min) ion substitutions were also ineffective. In human breast cancer cells transfected with human mdr1 cDNA, hyposmotic solutions activated a Cl- current but had no effect on the Pgp-mediated unidirectional efflux of R123. Additionally, in human breast cancer cells, the intracellular presence of R123 did not prevent activation of the Cl- current by hyposmotic solution. The lack of detectable effect of removal of Cl-, Na+, or HCO3- on Pgp-mediated R123 transport rules out direct coupling between substrate transport and transport of either of these ions by Pgp. The persistence of Pgp-mediated R123 efflux in osmotically swollen cells indicates that activation of the Pgp-associated Cl- current does not hinder the Pgp pump function. The lack of effect of R123 on swelling-activated Cl- current denotes that Pgp-mediated transport of organic substrates and Pgp-associated Cl- currents can occur at the same time in a single cell. These results underscore the dissociation between Pgp-mediated active drug transport and electrodiffusive Cl- transport.
authors.affiliation: Department of Physiology and Biophysics, University of Texas Medical Branch, Galveston 77555-0641.
authors.forename: G A
authors.initials: GA
authors.lastname: Altenberg
authors.forename: C G
authors.initials: CG
authors.lastname: Vanoye
authors.forename: E S
authors.initials: ES
authors.lastname: Han
authors.forename: J W
authors.initials: JW
authors.lastname: Deitmer
authors.forename: L
authors.initials: L
authors.lastname: Reuss
mesh_terms: D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1; D000818:Animals; D001692:Biological Transport; D002352:Carrier Proteins / Q000378:metabolism*; D002460:Cell Line; D048429:Cell Size; D006224:Cricetinae; D003412:Cricetulus; D018076:DNA, Complementary; D004351:Drug Resistance; D004573:Electrolytes / Q000378:metabolism; D006801:Humans; D007473:Ion Channels / Q000378:metabolism*; D008562:Membrane Glycoproteins / Q000378:metabolism*; D020112:Rhodamine 123; D012235:Rhodamines / Q000378:metabolism*; D014162:Transfection; D014407:Tumor Cells, Cultured
chemical_list: D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1; D002352:Carrier Proteins; D018076:DNA, Complementary; D004573:Electrolytes; D007473:Ion Channels; D008562:Membrane Glycoproteins; D012235:Rhodamines; D020112:Rhodamine 123