Receptor-mediated activation of phospholipase A2 and arachidonic acid release in signal transduction.

Neurotransmitters, hormones, cytokines, growth factors and other ligands transmit their specific messages by binding to receptor molecules on the cell surface. The ligand-bound receptors activate effector systems such as adenylate cyclase, cyclic nueleotide phosphodiesterase, ion channels, and phospholipases via membrane-associated transducing systems to generate second messengers. Receptors activate effector systems via signal transducing elements called GTPbinding proteins (G-proteins) which are heterotrimers composed of a-, Band y-subunits. When a receptor binds ligands, the G-protein disassociates into the aand Bysubunits [ 11. The disassociation of subunits is caused by the exchange of GTP for GDP which is bound to the a-subunit. Specific a-subunits are associated with the inhibition of adenylate cyclase and activation of cyclic GMP phosphodiesterase, voltage-sensitive ion channels, adenylate cyclase and phospholipase C. The a-subunits have endogenous GTPase activity which hydrolyses the bound GTP to GDP, resulting in the reassociation of the a and By-subunits and subsequent termination of the signal [ 11. Phospholipase A, as an effector enzyme can mediate receptor-induced arachidonic acid release regulated by an unknown G-protein [2]. Phospholipase A, hydrolyses arachidonic acid-containing phospholipids, such as phosphatidylcholine, phosphatidylethanolamine, phosphatidylserinc, phosphatidylinositides, phosphatidic acid and plasmalogcns, to generate free arachidonic acid and lysophospholipids. The arachidonic acid is further metabolized by cyclo-oxygenase to prostaglandin and thromboxanes; by lipoxygenase to leukotricnes and hydroxyeicosatetraenoic acids ( HETEs), and by epoxygenases to epoxides. Metabolites of arachidonic acid havc many important biological actions [3, 41. Our intcrest in phospholipase A, as a receptor-mediated effector enzyme stemmed from the initial observation that the chcmotactic peptide, fMetLeuPhe, liberated free arachidonic acid from neutrophils prelabeled with IT-arachidonic acid 151. A direct association between the amount o f arachidonic acid released by NetLeuPhe and the extent of chemotaxis was also demonstrated. Several pathways exist for the receptor-mediated generation o f arachidonic acid. The most direct involves coupling o f the receptor protein with phospholipasc A, activation through a G-protein of unknown character 161. Indirect routes havc been demonstrated involving the releasc of arachidonic acid from diacylglycerol by diglyceridc lipase 171. Diacylglycerol can be generated from the action of phospholipasc C on several phospholipids such as phosphatidylcholine and phosphatidylinositol. Alternatively, arachidonic acid can he released from phosphatidic acid by diglyceride lipase. Phosphatidic acid can be produced from the hydrolysis o f phospholipids by phospholipase D 181. In many studies, receptor-mediated release of arachidonic acid was found to be calcium dependent, while phospholipase C and D are partially or insensitive to calcium [9, lo].