Signalling to Activation of Cytosolic Phospholipase A2 and Arachidonate Release in Macrophages

Eicosanoids include prostaglandins, thromboxanes and leukotrienes, which are arachidonic acid oxygenation products. These lipids make important contributions to the signs and symptoms of inflammatory diseases. The level of free arachidonic acid seems to be a rate-limiting factor in the synthesis of eicosanoids and the enzyme cytosolic phospholipase A2 (cPLA2) has been shown to preferentially liberate arachidonic acid from phospholipids in the cell membranes of macrophages. Stimulation of mouse peritoneal macrophages with Prevotella intermedia, a bacterium involved in periodontitis, led to activation of cPLA2 and release of arachidonate. The use of the inhibitors of mitogen-activated protein kinase (MAP kinase) kinase (PD 98059) and p38 (SB 203580) revealed that both the MAP kinases ERK-1/2 and p38 were regulating the bacteria- or zymosan-induced activation of cPLA2. Another kinase, phosphatidylinositol 3-kinase (PtdIns 3-kinase) was found to be involved in the activation of ERK-1/2 and p38 and thereby also in the activation of cPLA2 and the subsequent release of arachidonate. However, lipopolysaccharide induced a PtdIns 3-kinase-independent activation of cPLA2. Pretreatment of the cells with an inhibitor of PtdIns 3-kinase (wortmannin) decreased the zymosan-induced translocation of phospholipase C-gamma2 (PLC-gamma2) to the cytoskeleton. Both zymosan and bacteria stimulation induced a tyrosine phosphoprylation of PLC-gamma2. In contrast, stimulation with LPS failed to induce any tyrosine phosphorylation of the enzyme. The synthetic glucocorticoid dexamethasone (dex) was found to reduce the mRNA expression and activation of cPLA2 The effects of dex on cPLA2 activation were not exerted to any major extent at the MAP kinase level. In conclusion, bacteria- and zymosan-induced signalling to activation of cPLA2 and release of arachidonate is mediated via PtdIns 3-kinase, ERK-1/2 and p38.