Abstract
Platelet-activating factor (PAF) is a potent, bioactive phospholipid that acts on multiple cells and tissues through its G protein-coupled receptor (GPCR). PAF is not stored but is rapidly generated via enzymatic acetylation of the precursor 1-O-hexadecyl-2-hydroxy-sn-glycero-3-phosphocholine (lysoPAF). The bioactivity of PAF is effectively and tightly regulated by PAF acetylhydrolases, which convert PAF back to lysoPAF. Previous studies report that lysoPAF is an inactive precursor and metabolite of PAF. However, lysoPAF has not been carefully studied in its own context. Here we report that lysoPAF has an opposing effect of PAF in the activation of neutrophils and platelets. Whereas PAF potentiates neutrophil NADPH oxidase activation, lysoPAF dose-dependently inhibits this function. Inhibition by lysoPAF is not affected by the use of a PAF receptor antagonist or genetic deletion of the PAF receptor gene. The mechanism of lysoPAF-mediated inhibition of neutrophils involves an elevation in the intracellular cAMP level, and pharmacological blockade of adenylyl cyclase completely reverses the inhibitory effect of lysoPAF. In addition, lysoPAF increases intracellular cAMP levels in platelets and inhibits thrombin-induced platelet aggregation, which can be reversed by inhibition of protein kinase A. These findings identify lysoPAF as a bioactive lipid with opposing functions of PAF and suggest a novel and intrinsic regulatory mechanism for balance of the potent activity of PAF.
Footnotes
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This work was supported by the National Institutes of Health [Grants AI033503, HL077806] and by an American Heart Association Predoctoral Fellowship [Grant 0615496Z].
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ABBREVIATIONS: PAF, platelet-activating factor (1-O-hexadecyl-2-acetoyl-sn-glycero-3-phosphocholine); lysoPAF, 1-O-hexadecyl-2-hydroxy-sn-glycero-3-phosphocholine; GPCR, G-protein coupled receptor; PKA, protein kinase A; PAFR, platelet-activating factor receptor; fMLF, N-formyl-l-methionyl-l-leucyl-l-phenylalanine; HPMEC, human pulmonary microvessel endothelial cell; PAF-AH, platelet-activating factor acetylhydrolase; PAF-AT, platelet-activating factor acetyltransferase; TER, transendothelial electrical resistance; SQ22536, 9-(tetrahydro-2-furyl)-adenine; KT5720, (9S,10S,12R)-2,3,9,10,11,12-hexahydro-10-hydroxy-9-methyl-1-oxo-9,12-epoxy-1H-diindolo[1,2,3-fg:3′,2′,1′-kl]pyrrolo[3,4-i][1,6]-benzodiazocine-10-carboxylic acid hexyl ester; H-89, N-[2-(p-bromocinnamylamino)ethyl]-5-isoquinolinesulfonamide dihydrochloride CTx, cholera toxin; Rf, retardation factor; IBMX, 3-isobutyl-1-methylxanthine; HBSS, Hanks' balanced salt solution; PMN, polymorphonuclear neutrophil; AM, acetoxymethyl ester; FPR, formyl peptide receptor; AUC, area under the curve; SR27417, N-(2-dimethylaminoethyl)-N-(3-pyridinylmethyl)(4-[2,4,6-triisopropylphenyl]thoiazol-2-yl)amine.
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↵ The online version of this article (available at http://molpharm.aspetjournals.org) contains supplemental material.
- Received July 31, 2008.
- Accepted October 17, 2008.
- The American Society for Pharmacology and Experimental Therapeutics
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