Regulation of cytosolic phospholipase A₂ activation and cyclooxygenase 2 expression in macrophages by the β-glucan receptor

Phagocytosis of non-opsonized microorganisms bymacrophages initiates innate immune responses for host defense against infection. Cytosolic phospholipase A₂ is activated during phagocytosis, releasing arachidonic acid for production of eicosanoids, which initiate acute inflammation. Our objective was to identify pattern recognition receptors that stimulate arachidonic acid release and cyclooxygenase 2 (COX2) expression in macrophages by pathogenic yeast and yeast cell walls. Zymosan- and Candida albicans-stimulated arachidonic acid release from resident mouse peritoneal macrophages was blocked by soluble glucan phosphate. In RAW264.7 cells arachidonic acid release, COX2 expression, and prostaglandin production were enhanced by overexpressing the β-glucan receptor, dectin-1, but not dectin-1 lacking the cytoplasmic tail. Pure particulate (1, 3)-β-D-glucan stimulated arachidonic acid release and COX2 expression, which were augmented in a Toll-like receptor 2 (TLR2)-dependent manner by macrophage-activating lipopeptide-2. However, arachidonic acid release and leukotriene C₄ production stimulated by zymosan and C. albicans were TLR2-independent, whereas COX2 expression and prostaglandin production were partially blunted in TLR2^-/- macrophages. Inhibition of Syk tyrosine kinase blocked arachidonic acid release and COX2 expression in response to zymosan, C. albicans, and particulate (1, 3)-β-D-glucan. The results suggest that cytosolic phospholipase A₂ activation triggered by the β-glucan component of yeast is dependent on the immunoreceptor tyrosine-based activation motif-like domain of dectin-1 and activation of Syk kinase, whereas both TLR2 and Syk kinase regulate COX2 expression.