Arachidonic acid in platelet microparticles up-regulates cyclooxygenase-2-dependent prostaglandin formation via a protein kinase C mitogen-activated protein kinase-dependent pathway

Activation of platelets results in shedding of membrane microparticles (RIP) with potentially bioactive properties. Platelet MP modulate platelet, monocyte, and vascular endothelial cell function, both by direct effects of MP arachidonic acid (AA) and by its metabolism to bioactive prostanoids. We have previously reported that platelet MP induce expression of cyclooxygenase (COX)-2 and prostacyclin production in monocytes and endothelial cells. To elucidate further the molecular mechanisms that underlie RIP-induced upregulation of COX-2 expression, we investigated the response of a human monocytoid (U-937) cell line to platelet MP stimulation, In U-937 cells, RIP-induced COX-2 expression and eicosanoid formation is prevented by pharmacological inhibitors of protein kinase C (PKC), PI 3-kinase, mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase, and p38 kinase, Treatment with the PI 3-kinase inhibitors wortmannin and LY294002 also blocked RIP-induced p42/p44 MAPK, p38, and JNK1 phosphorylation. Conversely, platelet RIP stimulation of U-937 cells results in direct activation of PKC, p42/p44 MAPK, p38 kinase, and c-Jun N-terminal kinase (JNK) as well as activation of the transcription factors c-Jun and Elk-l, However, MP failed to activate the cAMP response element. Activation of U-937 cells by MP induces translocation of classical (PKC beta), novel (PKC delta) and atypical (PKC zeta and PKC lambda) isozymes of PKC from the cytosol to the membrane, with concomitant activation of downstream MAPK. While RIP-induced activation of p42/p44 MAPK and p38 kinase is transient, a sustained activation of JNK1 was observed. Although PKC activation is required for RIP-induced p42/p44 MAPK, activation of the stress kinases p38 and JNK1 was PKC-independent, The fatty acid fraction of the MP accounted for these effects, which were mimicked by MP Ak Rather than acting directly via nuclear receptors, MP AA activates COX-2-dependent prostaglandin production by a PKC/p42/p44 MAPK/p38 kinase-sensitive pathway in which PI 3-kinase plays a significant role. RIP AA also stimulates transcriptional activation of COX-2 as well as c-Jun and Elk-1.