title: Inverse agonist-induced signaling and down-regulation of the platelet-activating factor receptor.
abstract: Platelet-activating factor (PAF) is a potent phospholipid mediator involved in several diseases such as allergic asthma, atherosclerosis and psoriasis. The human PAF receptor (PAFR) is a member of the G-protein-coupled receptor family. Following stimulation, PAFR becomes rapidly desensitized; this refractory state is dependent on PAFR phosphorylation, internalization and down-regulation. In this report, we show that the PAFR inverse agonist, WEB2086, can induce phosphorylation and down-regulation of PAFR. Using selective inhibitors, we determined that the agonist, PAF, and WEB2086 could induce phosphorylation of PAFR by PKC. Moreover, dominant-negative (DN) mutant of PKC isoforms beta inhibited WEB2086-stimulated PAFR phosphorylation, whereas PAF-stimulated phosphorylation was inhibited by DN PKCalpha and delta. WEB2086 also induced PAFR down-regulation which could be blocked by PKC inhibitors and by DN PKCbeta. WEB2086-induced down-regulation was dynamin-dependent but arrestin-independent. Unlike PAF, WEB2086-stimulated intracellular trafficking of PAFR was independent of Rab5. Specific inhibitors of lysosomal proteases and of proteasomes were both effective in reducing WEB2086-induced PAFR down-regulation, indicating the importance of receptor targeting to both lysosomes and proteasomes in long-term cell desensitization to WEB2086. These results indicate that although both agonists and inverse agonists induce receptor PAFR down-regulation, this may be accomplished through different signal transduction and trafficking pathways.
authors.affiliation: Immunology Division, Department of Pediatrics, Faculty of Medicine, Université de Sherbrooke, Sherbrooke, 3001 12th Avenue North, Sherbrooke, Quebec, Canada.
authors.forename: Denis J
authors.initials: DJ
authors.lastname: Dupré
authors.forename: Charles
authors.initials: C
authors.lastname: Thompson
authors.forename: Zhangguo
authors.initials: Z
authors.lastname: Chen
authors.forename: Simon
authors.initials: S
authors.lastname: Rollin
authors.forename: Jean-François
authors.initials: JF
authors.lastname: Larrivée
authors.forename: Christian
authors.initials: C
authors.lastname: Le Gouill
authors.forename: Marek
authors.initials: M
authors.lastname: Rola-Pleszczynski
authors.forename: Jana
authors.initials: J
authors.lastname: Stanková
mesh_terms: D000818:Animals; D001381:Azepines / Q000494:pharmacology*; D019556:COS Cells; D002522:Chlorocebus aethiops; D015536:Down-Regulation; D008247:Lysosomes / Q000187:drug effects; D010972:Platelet Activating Factor / Q000494:pharmacology; D010980:Platelet Membrane Glycoproteins / Q000819:agonists / Q000037:antagonists & inhibitors* / Q000378:metabolism; D061988:Proteasome Inhibitors; D011493:Protein Kinase C / Q000378:metabolism; D021381:Protein Transport / Q000187:drug effects; D043562:Receptors, G-Protein-Coupled / Q000819:agonists / Q000037:antagonists & inhibitors* / Q000378:metabolism; D015398:Signal Transduction; D014230:Triazoles / Q000494:pharmacology*
chemical_list: D001381:Azepines; D010972:Platelet Activating Factor; D010980:Platelet Membrane Glycoproteins; D061988:Proteasome Inhibitors; D043562:Receptors, G-Protein-Coupled; D014230:Triazoles; C039351:platelet activating factor receptor; C052518:WEB 2086; D011493:Protein Kinase C