Human platelets respond differentially to lysophosphatidic acids having a highly unsaturated fatty acyl group and alkyl ether-linked lysophosphatidic acids

Abstract

Lysophosphatidic acid (LPA) is a physiological agonist that is produced by lysophospholipase D, phospholipase A(1) and phospholipase A(2) in the blood of animals. It exerts diverse biological actions on a broad range of animal cells. Specific receptors for this important agonist have been characterized. In this investigation, for the first time we prepared LPAs having a highly unsaturated fatty acyl group, such as the eicosapentaenoyl or docosahexaenoyl residue, and their acetylated derivatives. Human platelets aggregated more potently in response to the highly unsaturated acyl-LPAs than to LPAs with a C(18) fatty acyl group, such as an oleoyl group, while alkyl ether-linked LPAs (alkyl-LPA) had much stronger aggregating activity. Two positional isomers of LPAs with an arachidonoyl, eicosapentaenoyl or docosahexaenoyl group had equipotent aggregatory activity as well as the positional isomers of their acetylated analogues, indicating that putative LPA receptors could not distinguish the difference between the positional isomers. We found that platelet preparations from two individuals showed no aggregatory response to alkyl-LPAs, although they contained mRNAs for known LPA receptors in the following order of expression level: endothelial differentiation gene (Edg)-4>Edg-7>Edg-2. We also obtained evidence that 2-(p-amylcinnamoyl)amino-4-chlorobenzoic acid (ONO-RS-082), a phospholipase A(2) inhibitor, potentiated alkyl-LPA-induced platelet aggregation, but inhibited highly unsaturated acyl-LPA-induced platelet aggregation. These results indicated that human platelets express acyl-LPA-selective and alkyl-LPA-selective receptors on their plasma membrane.

Lysophospholipid signaling: Beyond the EDGs

“…That report suggested the LPA-receptor activity in RH7777 cells pharmacologically resembled the mechanism that mediates platelet aggregation in response to LPA. Although the blebbing response in RH7777 cells occurs independently of LPA 1 , LPA 2 , and LPA 3 , and requires micromolar amounts of LPA for a robust response, the structure-activity relationship of blebbing we observed in RH7777 cells is not consistent with the structure-activity relationship reported by Tokumura et al for platelet aggregation [38]. LPA 18:0, AGP 16:0 and AGP 18:0 were ineffective at inducing membrane blebbing in our assays, whereas LPA 18:1 and AGP 18:1 were the most potent of the LPA species we tested.…”

“…The other unusual LPA response is observed in platelets. Platelets are 50 -100 times more responsive to AGP compared to LPA [35,38,52]. N-acetyl-serine phosphoric acid, a weak agonist of the EDG-family LPA GPCR [53], effectively inhibits platelet responses to LPA as well as the LPA-activated oscillatory chloride currents in Xenopus oocytes [54].…”

“…In contrast, Tokumura et al reported AGP 16:0, AGP 18:0, and LPA 16:0 were all more potent at inducing platelet aggregation than LPA 18:1. Tokumura et al posited the possible existence of at least two distinct novel receptors on platelets responsible for aggregation in response to LPA, one with a specificity for acyl-linked LPAs and one specific for alkyl-linked LPAs [38]. AGP appears to be the preferred ligand of LPA 5 (Yanagida, K., Ishii, S. and Shimizu, T., personal communication), and human platelets express abundant LPA 5 transcripts (Tigyi, G. and Siess, W., unpublished data), raising the intriguing possibility that a shared LPA 5 or a LPA 5 -like activity contributes to the LPA-responsiveness of both RH7777 cells and platelets.…”

The early- and late stages in phenotypic modulation of vascular smooth muscle cells: Differential roles for lysophosphatidic acid

“…The total amount of LPA in plasma is in the low nanomolar range [70][71][72][73][74], and the portion derived from isolated platelets remains only a fraction of that generated ex vivo during blood clotting [70].In the course of blood clotting, large amounts of LPA are generated, reaching concentrations as high as 5-10 µM [70,71,75,76], and the molecular species are dominated by the 18:2 and 20:4 unsaturated acyl species (20% and 34% of total LPA, respectively) [70]. LPA 20:4 is a more potent activator of platelets than other saturated or mono-unsaturated acyl species [77][78][79]. Among the acyl species, 20:4 and 18:2 acyl-LPA are the most potent activators of PPARγ and elicit neointima formation [55]).…”

Identification of Residues Responsible for Ligand Recognition and Regioisomeric Selectivity of Lysophosphatidic Acid Receptors Expressed in Mammalian Cells

“…Differences in ligand specificity probably introduce another level of signaling specificity. LPA is not a single molecular entity but a collection of endogenous structural analogs ranging from saturated to polyunsaturated fatty esters (6,7), to alkyl (8), and to alkenyl ether (9) analogs as well as 2,3-cyclic phosphates (10,11). Short chain phosphatidic acids (C 2 -C 8 ) are agonists of LPA receptors, and dioctylglycerol phosphate and pyrophosphate are competitive antagonists of the LPA 1/3 receptors (12).…”