Lipid Phosphate Phosphatases Regulate Lysophosphatidic Acid Production and Signaling in Platelets

Abstract

Blood platelets play an essential role in ischemic heart disease and stroke contributing to acute thrombotic events by release of potent inflammatory agents within the vasculature. Lysophosphatidic acid (LPA) is a bioactive lipid mediator produced by platelets and found in the blood and atherosclerotic plaques. LPA receptors on platelets, leukocytes, endothelial cells, and smooth muscle cells regulate growth, differentiation, survival, motility, and contractile activity. Definition of the opposing pathways of synthesis and degradation that control extracellular LPA levels is critical to understanding how LPA bioactivity is regulated. We show that intact platelets and platelet membranes actively dephosphorylate LPA and identify the major enzyme responsible as lipid phosphate phosphatase 1 (LPP1). Localization of LPP1 to the platelet surface is increased by exposure to LPA. A novel receptor-inactive sn-3-substituted difluoromethylenephosphonate analog of phosphatidic acid that is a potent competitive inhibitor of LPP1 activity potentiates platelet aggregation and shape change responses to LPA and amplifies LPA production by agonist-stimulated platelets. Our results identify LPP1 as a pivotal regulator of LPA signaling in the cardiovascular system. These findings are consistent with genetic and cell biological evidence implicating LPPs as negative regulators of lysophospholipid signaling and suggest that the mechanisms involve both attenuation of lysophospholipid actions at cell surface receptors and opposition of lysophospholipid production.Lysophosphatidic acid (1-acylglycerol-3-phosphate, LPA) 1 is the prototypic member of a class of receptor-active lipid phosphoric acid mediators that function as intercellular signaling molecules. LPA and the structurally related compound sphingosine 1-phosphate (S1P) are present in biological fluids at physiologically relevant concentrations and exert many of their effects on target cells by binding to specific heterotrimeric GTP-binding protein (G-protein)-coupled cell surface receptors. Responses to stimulation with LPA and S1P include growth, differentiation, chemotaxis, smooth muscle contraction, and changes in cell morphology and motility through alterations in organization of the actin cytoskeleton (1-3).Blood plasma contains physiologically relevant levels of LPA (4). Because of the multiple actions of LPA on cells of the vasculature, a growing body of evidence suggests that LPA plays an important role in blood clotting, wound healing, and tissue regeneration (3). The mitogenic and chemotactic actions of LPA may also participate in the recruitment of monocytes and macrophages and aberrant stimulation of endothelial and vascular muscle cell growth that underlie intimal hyperplasia, formation of an atherosclerotic plaque, and progression of atherosclerosis (5). Elevated plasma levels of LPA have also been associated with ovarian and certain other gynecological cancers, where LPA may participate in autocrine or paracrine stimulation of tumor cell growth (6).LPA is a major ...

Lack of sphingosine 1-phosphate-degrading enzymes in erythrocytes

“…Thus the lack of ceramide synthase and S1P lyase activities in erythrocytes would seem reasonable, since erythrocytes have no ER. The absence of S1P phosphohydrolase activity suggests, though, that erythrocytes contain neither SPP nor LPP members, which contrasts with platelets having at least LPP1 [26].…”

Binding of Autotaxin to Integrins Localizes Lysophosphatidic Acid Production to Platelets and Mammalian Cells

“…The time course of LPA in the absence of added ATX or in response to a maximally effective concentration of ATX in the absence of agonist was biphasic, which may reflect the previously reported ability of platelets to degrade LPA (Fig. 1, C and D) (30). The low concentration of delipidated BSA in these incubations contained trace levels of lysophospholipids, including LPC, that were not sufficient to sustain the quantities of LPA being formed.…”

Thematic Review Series: Phospholipases: Central Role in Lipid Signaling and Disease Autotaxin, a lysophospholipase D with pleomorphic effects in oncogenesis and cancer progression

“…In addition to synergizing with RAS signaling to promote tumor migration and metastatic dissemination in RAS-transformed cells ( 153 ), lysoPLD activity, likely mediated by ATX, has been proposed as one of the contributing factors in developing acquired resistance to the receptor tyrosine kinase inhibitor, sunitinib, in renal cell carcinoma ( 154 ) and carboplatin-induced implicated in driving the progression of atherosclerosis ( 83 ), neuropathic pain (84)(85)(86), and T cell homing ( 87 ), respectively. In contrast to ATX, there is no evidence indicating that lipid phosphate phosphatases, the main enzymes mediating LPA degradation (88)(89)(90)(91), have an equivalent mechanism for local action through binding to integrins or other extracellular receptors. Like other factors involved in development, fi ne spatiotemporal regulation of ATX expression and activity is essential not only for proper vasculogenesis but also for other key biological functions.…”