Serum lysophosphatidic acid is produced through diverse phospholipase pathways

Lysophosphatidic acid (LPA) is a lipid mediator with multiple biological activities that accounts for many biological properties of serum. LPA is thought to be produced during serum formation based on the fact that the LPA level is much higher in serum than in plasma. In this study, to better understand the pathways of LPA synthesis in serum, we evaluated the roles of platelets, plasma, and phospholipases by measuring LPA using a novel enzyme-linked fluorometric assay. First, examination of platelet-depleted rats showed that half of the LPA in serum is produced via a platelet-dependent pathway. However, the amount of LPA released from isolated platelets after they are activated by thrombin or calcium ionopbore accounted for only a small part of serum LPA. Most of the platelet-derived LPA was produced in a two-step process: lysophospholipids such as lysophosphatidylcholine (LPC), lysophospbatidylethanolamine, and lysophosphatidylserine, were released from activated rat platelets by the actions of two phospholipases, group IIA secretory phospholipase A₂ (sPLA₂-IIA) and phosphatidylserine-specific phospholipase A₁ (PS-PLA₁), which were abundantly expressed in the cells. Then these lysophospholipids were converted to LPA by the action of plasma lysophospholipase D (lysoPLD). Second, accumulation of LPA in incubated plasma was strongly accelerated by the addition of recombinant lysoPLD with a concomitant decrease in LPC accumulation, indicating that the enzyme produces LPA by hydrolyzing LPC produced during the incubation. In addition, incubation of plasma isolated from human subjects who were deficient in lecithin-cholesterol acyl. transferase (LCAT) did not result in increases of either LPC or LPA. The present study demonstrates multiple pathways for LPA production in serum and the involvement of several phospholipases, including PS-PLA₁, sPLA₂-IIA, LCAT, and lysoPLD.