THE EFFECTS OF THE PHOSPHOLIPASE-A2 INHIBITORS ARISTOLOCHIC ACID AND PGBX ON A23187-STIMULATED MOBILIZATION OF ARACHIDONATE IN HUMAN NEUTROPHILS ARE OVERCOME BY DIACYLGLYCEROL OR PHORBOL ESTER

Aristolochic acid and PGBx, two structurally unrelated, protein-targeted inhibitors of isolated phospholipases A2, are effective antagonists of calcium ionophore A23187-stimulated mobilization of [H-3]arachidonate from human neutrophils. We now report that preincubation of neutrophils with oleoylacetylglycerol (OAG, 15-mu-M) substantially reverses the inhibitory effect of 200-mu-M aristolochic acid (from 70 to 24% inhibition). Similarly, OAG increases the IC50 for PGBx from 2.5 to > 20-mu-M. The effects of OAG on inhibition by either aristolochic acid or PGBx are dose-dependent, with an ED50 of 2.5-mu-M. Protection against inhibition by either aristolochic acid or PGBx is also observed with phorbol myristate acetate (PMA, ED50 3 nM), but not 4-alpha-phorbol didecanoate. Aristolochic acid and PGBx do not inhibit PMA-stimulated superoxide generation, and are thus not protein kinase C inhibitors. Furthermore, neither aristolochic acid nor PGBx inhibit diglyceride generation through the phospholipase D/phosphatidate phosphohydrolase pathway. A23187-stimulated [H-3]arachidonate mobilization is increased by 20-50% when neutrophils are preincubated with OAG or PMA. The present results indicate that OAG and PMA also modulate the A23187-stimulated [H-3]arachidonate mobilization so as to render it less sensitive to inhibitors of phospholipase A2.