title: Communication between multiple drug binding sites on P-glycoprotein.
abstract: P-glycoprotein, a member of the ATP-binding cassette transporter family, is able to confer resistance on tumors against a large number of functionally and chemically distinct cytotoxic compounds. Several recent investigations suggest that P-glycoprotein contains multiple drug binding sites rather than a single site of broad substrate specificity. In the present study, radioligand-binding techniques were used to directly characterize drug interaction sites on P-glycoprotein and how these multiple sites interact. The drugs used were classified as either 1) substrates, which are known to be transported by P-glycoprotein (e.g., vinblastine) or 2) modulators, which alter P-glycoprotein function but are not themselves transported by the protein (e.g., XR9576). Drug interactions with P-glycoprotein were either competitive, at a common site, or noncompetitive, and therefore at distinct sites. Based on these data, we can assign a minimum of four drug binding sites on P-glycoprotein. These sites fall into two categories: transport, at which translocation of drug across the membrane can occur, and regulatory sites, which modify P-glycoprotein function. Intriguingly, however, some modulators interact with P-glycoprotein at a transport site rather than a regulatory site. The pharmacological data also demonstrate that both transport and regulatory sites are able to switch between high- and low-affinity conformations. The multiple sites on P-glycoprotein display complex allosteric interactions through which interaction of drug at one site switches other sites between high- or low-affinity conformations. The data are discussed in terms of a model for the mechanism of transport by P-glycoprotein.
authors.affiliation: Department of Clinical Laboratory Sciences, John Radcliffe Hospital, University of Oxford, United Kingdom.
authors.forename: C
authors.initials: C
authors.lastname: Martin
authors.forename: G
authors.initials: G
authors.lastname: Berridge
authors.forename: C F
authors.initials: CF
authors.lastname: Higgins
authors.forename: P
authors.initials: P
authors.lastname: Mistry
authors.forename: P
authors.initials: P
authors.lastname: Charlton
authors.forename: R
authors.initials: R
authors.lastname: Callaghan
mesh_terms: D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1 / Q000187:drug effects / Q000378:metabolism*; D000166:Acridines / Q000494:pharmacology; D000494:Allosteric Regulation; D000818:Animals; D001562:Benzimidazoles / Q000494:pharmacology; D001597:Benzylidene Compounds / Q000494:pharmacology; D001667:Binding, Competitive; D016466:CHO Cells; D002462:Cell Membrane / Q000187:drug effects / Q000378:metabolism; D006224:Cricetinae; D004347:Drug Interactions; D007546:Isoquinolines / Q000494:pharmacology; D007700:Kinetics; D009529:Nicardipine / Q000494:pharmacology; D017239:Paclitaxel / Q000494:pharmacology; D011804:Quinolines / Q000494:pharmacology; D011869:Radioligand Assay; D019275:Radiopharmaceuticals / Q000494:pharmacology; D020112:Rhodamine 123 / Q000494:pharmacology; D044005:Tetrahydroisoquinolines; D014316:Tritium; D014747:Vinblastine / Q000494:pharmacology
chemical_list: D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1; D000166:Acridines; D001562:Benzimidazoles; D001597:Benzylidene Compounds; D007546:Isoquinolines; D011804:Quinolines; D019275:Radiopharmaceuticals; D044005:Tetrahydroisoquinolines; C118656:XR 9051; D014316:Tritium; D020112:Rhodamine 123; D014747:Vinblastine; D009529:Nicardipine; C402343:tariquidar; C083501:Elacridar; D017239:Paclitaxel; C017807:bisbenzimide ethoxide trihydrochloride