Mouse group X secretory phospholipase A2 induces a potent release of arachidonic acid from spleen cells and acts as a ligand for the phospholipase A2 receptor

Group X secretory phospholipase A(2) (sPLA(2)-X) has recently been shown to possess a powerful potency for releasing arachidonic acid from cell membrane phospholipids, Here, we report the purification of mouse pro- and mature forms of sPLA(2)-X, as well as its expression and biological functions. Purified pro-sPLA(2)-X was found to possess a propeptide of 11 amino acid residues attached at the NH2-terminals of the mature protein, and showed as little as 8% of the PLA(2) activity of the mature form. Limited proteolysis of pro-sPLA(2)-X with trypsin resulted in the appearance of the mature form with a concomitant increase in PLA(2) activity, suggesting a requirement of proteolytic removal of the propeptide for the optimal activity. The expression of sPLA(2)-X mRNA was detected in various tissues including the lung, thymus, and spleen, and immunohistochemical analysis revealed its expression in splenic macrophages, In the spleen cells, mature sPLA(2)-X elicited a prompt release of arachidonic acid with significant production of prostaglandin E-2 more efficiently than group IB and IIA sPLA(2)s. In addition, sPLA(2)-X was identified as a high-affinity ligand for both native and recombinant form of mouse PLA(2) receptor (PLA(2)R), However, there was no significant difference in the sPLA(2)-X-induced arachidonic acid release responses in the spleen cells between wild-type and PLA(2)R-deficient mice. These findings strongly suggest that sPLA(2)-X possesses two distinct biological functions in mice: it elicits a marked release of arachidonic acid from membrane phospholipids leading to the production of lipid mediators based on its enzymatic potency, and it acts as a natural ligand for the PLA(2)R that has been shown to play a critical role in the production of inflammatory cytokines during endotoxic shock. (C) 2000 Academic Press.