title: P-Glycoprotein (P-gp) expressed in a confluent monolayer of hMDR1-MDCKII cells has more than one efflux pathway with cooperative binding sites.
abstract: The multidrug resistance transporter P-glycoprotein (P-gp) effluxes a wide range of substrates and can be affected by a wide range of inhibitors or modulators. Many studies have presented classifications for these binding interactions, within either the context of equilibrium binding or the Michaelis-Menten enzyme analysis of the ATPase activity of P-gp. Our approach is to study P-gp transport and its inhibition using a physiologically relevant confluent monolayer of hMDR1-MDCKII cells. We measure the elementary rate constants for P-gp efflux of substrates and study inhibition using pairwise combinations with a different unlabeled substrate acting as the inhibitor. Our current kinetic model for P-gp has only a single binding site, because a previous study proved that the mass-action kinetics of efflux of a single substrate were not sensitive to whether there are one or more substrate-binding and efflux sites. In this study, using this one-site model, we found that, with "high" concentrations of either a substrate or an inhibitor, the elementary rate constants fitted independently for each of the substrates alone quantitatively predicted the efflux curves, simply applying the assumption that binding at the "one site" was competitive. On the other hand, at "low" concentrations of both the substrate and inhibitor, we found no inhibition of the substrate efflux, despite the fact that both the substrate and inhibitor were being well-effluxed. This was not an effect of excess "empty" P-gp molecules, because the competitive efflux model takes site occupancy into account. Rather, it is quantitative evidence that the substrate and inhibitor are being effluxed by multiple pathways within P-gp. Remarkably, increasing the substrate concentration above the "low" concentration, caused the inhibition to become competitive; i.e., the inhibitor became effective. These data and their analysis show that the binding of these substrates must be cooperative, either positive or negative.
authors.affiliation: Department of Bioscience and Biotechnology, Drexel University, Philadelphia, Pennsylvania 19104, USA.
authors.forename: Poulomi
authors.initials: P
authors.lastname: Acharya
authors.forename: Thuy T
authors.initials: TT
authors.lastname: Tran
authors.forename: Joseph W
authors.initials: JW
authors.lastname: Polli
authors.forename: Andrew
authors.initials: A
authors.lastname: Ayrton
authors.forename: Harma
authors.initials: H
authors.lastname: Ellens
authors.forename: Joe
authors.initials: J
authors.lastname: Bentz
mesh_terms: D018435:ATP Binding Cassette Transporter, Subfamily B; D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1 / Q000037:antagonists & inhibitors / Q000235:genetics* / Q000378:metabolism / Q000502:physiology; D000818:Animals; D001667:Binding, Competitive / Q000235:genetics; D001693:Biological Transport, Active / Q000187:drug effects / Q000235:genetics; D002219:Carbamates / Q000037:antagonists & inhibitors / Q000378:metabolism; D002460:Cell Line; D002463:Cell Membrane Permeability / Q000235:genetics; D004285:Dogs; D005663:Furans; D006801:Humans; D008139:Loperamide / Q000037:antagonists & inhibitors / Q000378:metabolism; D011485:Protein Binding / Q000235:genetics; D011802:Quinidine / Q000494:pharmacology; D015398:Signal Transduction / Q000235:genetics; D013379:Substrate Specificity / Q000187:drug effects / Q000235:genetics; D013449:Sulfonamides / Q000037:antagonists & inhibitors / Q000378:metabolism
chemical_list: C513055:ABCB1 protein, human; D018435:ATP Binding Cassette Transporter, Subfamily B; D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1; D002219:Carbamates; D005663:Furans; D013449:Sulfonamides; C095108:amprenavir; D008139:Loperamide; D011802:Quinidine