Inverse agonist activity of selected ligands of platelet-activating factorreceptor

The receptor for platelet-activating factor (PAFR) is a member of the G pro tein-coupled receptor (GPCR) family. According to the allosteric ternary co mplex model, GPCRs exist in an equilibrium between different conformations. Agonist binding promotes and stabilizes the receptor in an active conforma tion. On the other hand, ligands that stabilize the inactive conformation a re known as inverse agonists. Due to the association of platelet-activating factor (PAF) with diverse physiological and pathological processes, consid erable efforts have been invested in the development of antagonists to PAFR . A large number of these molecules has been shown to specifically interact with PAFR but, surprisingly, little is known about their impact on the con formation of the receptor and its activity. By using a constitutively activ e mutant (L-231 R) of the human PAFR and by transiently coexpressing the wi ld-type (WT) receptor with the G alpha (q) subunit of the trimeric G protei n, we were able to address this issue with ligands of diverse structures su ch as phospholipids, benzodiazepines, furans, and others. We demonstrated t hat some of these molecules are potent inverse agonists. For example, when cells (WT PAFR + G alpha (q)) were exposed to WEB2086, SM10661, or alprazol am, the basal inositol phosphate production was reduced by 53 +/-6, 44 +/-3 , and 54 +/-4%, respectively. The decrease in basal inositol phosphate prod uction by WEB2086 was significantly inhibited by a more neutral antagonist BN52021, confirming the specificity of the reaction. We demonstrate here th at WEB2086 and other known ligands previously considered as antagonists can act as inverse agonists on the human PAF receptor.