Effects of scalaradial, a type II phospholipase A2 inhibitor, on human neutrophil arachidonic acid mobilization and lipid mediator formation.

The role of scalaradial (SLD) (or its 12-epi analog), a marine natural product purified from sponge (Cacospongia mollior) in human neutrophil (polymorphonuclear leukocyte, PMN) arachidonic acid metabolism was studied. SLD potently inhibited human recombinant (rh) type II-14 kDa-phospholipase A2 (PLA2) (IC50 = 0.07 microM) but displayed weak inhibition of U937 cell, 85 kDa-PLA2 (IC50 = 20 microM). Sn-2 acylhydrolytic activity expressed in human PMN acid extract, that was completely neutralized by anti-rh type II-14 kDa-PLA2 monoclonal antibody, was inhibited by SLD in a concentration-dependent manner (IC50 = 35 microM). Exposure of human PMN to SLD resulted in a concentration-dependent inhibition of calcium ionophore (A23187)-induced leukotriene B4 release (IC50 = 0.1-0.6 microM). In contrast to the action of selective 5-lipoxygenase inhibitors (WY 50295 or zileuton), SLD decreased A23187-induced PMN liberation of arachidonic acid mass and platelet-activating factor biosynthesis (IC50 = 1-2 microM). PMN acetyltransferase activity was not significantly affected by SLD suggesting that platelet-activating factor inhibition was due, predominantly, to inactivation of PLA2 activity. In vivo, topical application of SLD on mouse ear treated with phorbol ester, not only inhibited edema formation but also the increase in myeloperoxidase activity (an index of cellular infiltration). SLD had little or no effect on arachidonic acid-induced ear edema or myeloperoxidase which is consistent with an action on PLA2. Take together these data suggest that the predominant mechanism of SLD is via inhibition of 14 kDa-like-PLA2 and suggests that this enzyme may participate in PMN arachidonic acid liberation and lipid mediator formation.