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, mon ocyte, and vascular endothelial cell function, both by direct effects of MP arachidonic acid (AA) and by its metabolism to bioactive prostanoids. We h ave previously reported that platelet MP induce expression of cyclooxygenas e (COX)-2 and prostacyclin production in monocytes and endothelial cells. T o elucidate further the molecular mechanisms that underlie RIP-induced upre gulation of COX-2 expression, we investigated the response of a human monoc ytoid (U-937) cell line to platelet MP stimulation, In U-937 cells, RIP-induced COX-2 expression and eicosanoid formation is pr evented by pharmacological inhibitors of protein kinase C (PKC), PI 3-kinas e, mitogen-activated protein kinase (MAPK)/extracellular signal-regulated k inase, and p38 kinase, Treatment with the PI 3-kinase inhibitors wortmannin and LY294002 also blocked RIP-induced p42/p44 MAPK, p38, and JNK1 phosphor ylation. Conversely, platelet RIP stimulation of U-937 cells results in dir ect activation of PKC, p42/p44 MAPK, p38 kinase, and c-Jun N-terminal kinas e (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 bet a), novel (PKC delta) and atypical (PKC zeta and PKC lambda) isozymes of PK C from the cytosol to the membrane, with concomitant activation of downstre am MAPK. While RIP-induced activation of p42/p44 MAPK and p38 kinase is tra nsient, a sustained activation of JNK1 was observed. Although PKC activatio n is required for RIP-induced p42/p44 MAPK, activation of the stress kinase s p38 and JNK1 was PKC-independent, The fatty acid fraction of the MP accou nted for these effects, which were mimicked by MP Ak Rather than acting dir ectly 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 ac tivation of COX-2 as well as c-Jun and Elk-1.