Arachidonic Acid in Platelet Microparticles Up-regulates Cyclooxygenase-2-dependent Prostaglandin Formation via a Protein Kinase C/Mitogen-activated Protein Kinase-dependent Pathway

Abstract

Activation of platelets by agonists, such as collagen and thrombin, results in shedding of membrane microparticles (MP) 1 from their surface (1). Formation of MP results from an exocytotic budding process (2, 3) and may include both procoagulant (4, 5) and anticoagulant (6, 7) proteins when shed from platelets. Increased circulating concentrations of MP have been detected in vivo in syndromes of platelet activation (8, 9). It is likely that the shear forces in areas of disordered flow would favor MP formation (10, 11).MP might themselves evoke cellular responses in the immediate microenvironment of their formation. For instance, activation of endothelial cells results in MP shedding, which, in turn, activates neutrophils, enhancing their propensity to adhere to endothelium (12). Recently, we have demonstrated that arachidonic acid (AA) in platelet MP can influence platelet activation in a PKC-dependent manner and endothelial cell COX-2 expression via transcellular lipid metabolism (13). The molecular mechanisms by which MP induce such cellular responses are, however, unknown. The present studies were designed to delineate the signaling pathways involved in MPinduced COX-2 activation and prostaglandin formation.COX is the first rate-limiting enzyme in the synthesis of prostacyclin, prostaglandins, and thromboxane from arachidonic acid (14). Two isozymes of COX have been described (15,16), and both catalyze the cyclooxygenase-dependent transformation of prostaglandin G 2 from AA and the subsequent peroxidation of prostaglandin G 2 to prostaglandin H 2 (17). While COX-1 is expressed constitutively in most tissues, COX-2 is usually induced as an immediate early gene by mitogenic or inflammatory stimuli as well as by ligands that act via G protein-and PKC-mediated pathways (18). More recent findings link the prostaglandin biosynthetic pathways with activation of MAPK signaling cascades (19 -21).The MAPK pathway, of which there are three subgroups, is a conserved eukaryotic signaling cascade that is responsible for mediating the effects of extracellular stimuli on a wide variety of biological processes. The extracellular signal-regulated kinases (also termed p42 MAPK and p44 MAPK) and the stressactivated protein kinases (also termed c-Jun NH 2 -terminal kinase (JNK)) and p38 (22) are distinguished by activating signals, substrate specificity, and cellular responses (23). While

IGF-II/IGF-I receptor pathway up-regulates COX-2 mRNA expression and PGE2 synthesis in Caco-2 human colon carcinoma cells

“…Concordingly, it has been demonstrated that arachidonic acid of platelet microparticles induces COX-2 expression through a PI3-kinase-dependent pathway in a human monocytoid cell line (Barry et al, 1999). Our data are also in agreement with those obtained in di erent experimental systems showing that a PI3-kinase-dependent pathway mediates the proliferative e ects of IGF-I receptor (Dufourny et al, 1997;Rubin and Baserga, 1995).…”

Induction of apoptosis in circulating angiogenic cells by microparticles

“…Arachidonic acid is a major mediator of the cellular effects of microparticles (32,(37)(38)(39). We previously demonstrated that microparticles shuttle arachidonic acid from leukocytes to fibroblasts (30).…”

Hydrogen peroxide generated at the level of mitochondria in response to peroxynitrite promotes U937 cell death via inhibition of the cytoprotective signalling mediated by cytosolic phospholipase A2

“…Our recent work 34 demonstrates that this is indeed the case, and future work will investigate the mechanism involved in this survivalpromoting response. The notion that AA causes translocation 35 and activation of protein kinase C, 36 as well as stimulation of extracellular signal-regulated kinases 1/2 phosphorylation, 37 is well established and these events were shown to prevent apoptosis. 38,39 Thus, the question to be answered is whether these pathways also mediate the protective effects of AA in the peroxynitrite-dependent necrotic response.…”