title: Photoaffinity probes for the alpha 1-adrenergic receptor and the calcium channel bind to a common domain in P-glycoprotein.
abstract: P-glycoprotein is a 130-180-kDa integral membrane protein that is overproduced in multidrug-resistant cells. The protein appears to act as an energy-dependent drug efflux pump that has broad specificity for structurally diverse hydrophobic antitumor drugs. Many agents, such as the calcium channel blocker verapamil, reverse multidrug resistance and also interact with P-glycoprotein. The goal of this work was to determine if a common binding site participates in the transport of antitumor drugs and/or the reversal of drug resistance. This was done by comparing the peptide maps of P-glycoprotein (encoded by mdr1b) after it was labeled with a photoactive calcium channel blocker, [3H]azidopine, and a newly identified photoaffinity analog for P-glycoprotein 2-[4-(4-azido-3-[125I]iodobenzoyl) piperazin-1-yl]-4-amino-6,7-dimethoxyquinazoline [( 125I]iodoaryl azidoprazosin). [125I] Iodoaryl azidoprazosin, which classically has been used to identify the alpha 1-adrenergic receptor, bound to P-glycoprotein and was preferentially competed by vinblastine greater than actinomycin D greater than doxorubicin greater than colchicine. Peptide maps derived from P-glycoprotein labeled with [3H]azidopine or [125I]iodoaryl azidoprazosin were identical. After maximal digestion under conditions for Cleveland mapping, a single major 6-kDa fragment was obtained after digestion with V8 protease, whereas two major fragments, 6.5 and 5.5 kDa, were detected after digestion with chymotrypsin. The 6.0-kDa V8 fragment and the 6.5-kDa chymotrypsin fragment were both found when P-glycoprotein encoded by mdr1a and mdr1b was compared. Despite its specific interaction with P-glycoprotein, neither iodoaryl azidoprazosin nor prazosin markedly reversed resistance compared with verapamil or azidopine. Further, multidrug-resistant cells were 900-fold resistant to vinblastine but only 5-fold resistant to prazosin. These data demonstrate that structurally diverse reversal and/or antitumor agents are likely to have differential affinity for a small common domain of P-glycoprotein.
authors.affiliation: Department of Molecular Pharmacology, Albert Einstein College of Medicine, Bronx, New York 10461-1602.
authors.forename: L M
authors.initials: LM
authors.lastname: Greenberger
authors.forename: C P
authors.initials: CP
authors.lastname: Yang
authors.forename: E
authors.initials: E
authors.lastname: Gindin
authors.forename: S B
authors.initials: SB
authors.lastname: Horwitz
mesh_terms: D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1; D000345:Affinity Labels / Q000378:metabolism*; D000818:Animals; D001386:Azides / Q000378:metabolism; D001665:Binding Sites; D001667:Binding, Competitive; D015220:Calcium Channels / Q000378:metabolism*; D002460:Cell Line; D002462:Cell Membrane / Q000378:metabolism; D002918:Chymotrypsin; D004095:Dihydropyridines / Q000378:metabolism; D004351:Drug Resistance; D008264:Macrophages / Q000378:metabolism; D008562:Membrane Glycoproteins / Q000378:metabolism*; D051379:Mice; D010449:Peptide Mapping; D011224:Prazosin / Q000031:analogs & derivatives / Q000378:metabolism / Q000494:pharmacology; D011942:Receptors, Adrenergic, alpha / Q000378:metabolism*; D012689:Sequence Homology, Nucleic Acid; D014700:Verapamil / Q000494:pharmacology
chemical_list: D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1; D000345:Affinity Labels; D001386:Azides; D015220:Calcium Channels; D004095:Dihydropyridines; D008562:Membrane Glycoproteins; D011942:Receptors, Adrenergic, alpha; C041130:azidopine; C041552:azidoprazosin; D014700:Verapamil; D002918:Chymotrypsin; D011224:Prazosin