Functional association of type IIA secretory phospholipase A2 with the glycosylphosphatidylinositol-anchored heparan sulfate proteoglycan in the cyclooxygenase-2-mediated delayed prostanoid-biosynthetic pathway

Abstract

An emerging body of evidence suggests that type IIA secretory phospholipase A2 (sPLA2-IIA) participates in the amplification of the stimulus-induced cyclooxygenase (COX)-2-dependent delayed prostaglandin (PG)- biosynthetic response in several cell types. However, the biological importance of the ability of sPLA2-IIA to bind to heparan sulfate proteoglycan (HSPG) on cell surfaces has remained controversial. Here we show that glypican, a glycosylphosphatidylinositol (GPI)-anchored HSPG, acts as a physical and functional adaptor for sPLA2-IIA, sPLA2-IIA-dependent PGE2 generation by interleukin-1-stimulated cells was markedly attenuated by treatment of the cells with heparin, heparinase or GPI-specific phospholipase C, which solubilized the cell surface-associated sPLA2-IIA. Overexpression of glypican-1 increased the association of sPLA2-IIA with the cell membrane, and glypican-1 was coimmunoprecipitated by the antibody against sPLA2-IIA. Glypican-1 overexpression led to marked augmentation of sPLA2IIA-mediated arachidonic acid release, PGE2 generation, and COX-2 induction in interleukin-1-stimulated cells, particularly when the sPLA2-IIA expression level was suboptimal. Immunofluorescent microscopic analyses of cytokine- stimulated cells revealed that sPLA2-IIA was present in the caveolae, a microdomain in which GPI-anchored proteins reside, and also appeared in the perinuclear area in proximity to COX-2. We therefore propose that a GPI- anchored HSPG glypican facilitates the trafficking of sPLA2-IIA into particular subcellular compartments, and arachidonic acid thus released from the compartments may link efficiently to the downstream COX-2-mediated PG biosynthesis.