title: Multidrug-resistance P-glycoprotein (MDR1) secretes platelet-activating factor.
abstract: The human multidrug-resistance (MDR1) P-glycoprotein (Pgp) is an ATP-binding-cassette transporter (ABCB1) that is ubiquitously expressed. Often its concentration is high in the plasma membrane of cancer cells, where it causes multidrug resistance by pumping lipophilic drugs out of the cell. In addition, MDR1 Pgp can transport analogues of membrane lipids with shortened acyl chains across the plasma membrane. We studied a role for MDR1 Pgp in transport to the cell surface of the signal-transduction molecule platelet-activating factor (PAF). PAF is the natural short-chain phospholipid 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine. [(14)C]PAF synthesized intracellularly from exogenous alkylacetylglycerol and [(14)C]choline became accessible to albumin in the extracellular medium of pig kidney epithelial LLC-PK1 cells in the absence of vesicular transport. Its translocation across the apical membrane was greatly stimulated by the expression of MDR1 Pgp, and inhibited by the MDR1 inhibitors PSC833 and cyclosporin A. Basolateral translocation was not stimulated by expression of the basolateral drug transporter MRP1 (ABCC1). It was insensitive to the MRP1 inhibitor indomethacin and to depletion of GSH which is required for MRP1 activity. While efficient transport of PAF across the apical plasma membrane may be physiologically relevant in MDR1-expressing epithelia, PAF secretion in multidrug-resistant tumours may stimulate angiogenesis and thereby tumour growth.
authors.affiliation: Department of Cell Biology and Histology, Academic Medical Center, University of Amsterdam, Meibergdreef 15, P.O. Box 22700, 1100 DE Amsterdam, The Netherlands.
authors.forename: R J
authors.initials: RJ
authors.lastname: Raggers
authors.forename: I
authors.initials: I
authors.lastname: Vogels
authors.forename: G
authors.initials: G
authors.lastname: van Meer
mesh_terms: D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1 / Q000037:antagonists & inhibitors / Q000378:metabolism*; D018528:ATP-Binding Cassette Transporters / Q000378:metabolism; D000818:Animals; D001692:Biological Transport / Q000502:physiology; D002462:Cell Membrane / Q000187:drug effects / Q000378:metabolism; D002478:Cells, Cultured; D003524:Cyclosporins / Q000494:pharmacology; D005995:Glyceryl Ethers / Q000378:metabolism*; D006801:Humans; D010713:Phosphatidylcholines / Q000378:metabolism; D010972:Platelet Activating Factor / Q000378:metabolism*; D012710:Serum Albumin, Bovine; D013552:Swine; D014162:Transfection
chemical_list: D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1; D018528:ATP-Binding Cassette Transporters; D003524:Cyclosporins; D005995:Glyceryl Ethers; D010713:Phosphatidylcholines; D010972:Platelet Activating Factor; D012710:Serum Albumin, Bovine; C070272:valspodar