INHIBITION OF PAF SYNTHESIS BY STIMULATED HUMAN POLYMORPHONUCLEAR LEUKOCYTES WITH CLORICROMENE, AN INHIBITOR OF PHOSPHOLIPASE A(2) ACTIVATION

1 A phospholipase A(2) (PLA(2)) represents the key enzyme in the remod elling pathway of platelet-activating factor (PAF) synthesis in human polymorphonuclear (PMN) leucocytes. 2 PLA(2) activation is also the ra te-limiting step for the release of the arachidonic acid utilized for the synthesis of leukotrienes in stimulated leucocytes; however, it is unknown whether the PLA(2)s involved in the two biosynthetic pathways are identical. 3 Cloricromene iethylaminoethyl-4-methyl-7-ethoxy-carb onylmethoxy coumarin) is an antithrombotic coumarin derivative which i nhibits platelet and leucocyte function and suppresses arachidonic aci d liberation by interfering with PLA(2) activation. 4 The aim of the p resent study was to assess whether chloricromene inhibits PAF synthesi s by stimulated human polymorphonuclear leucocytes (PMNs). 5 Cloricrom ene (50-500 mu M) inhibited in a concentration-dependent manner the re lease of PAF, as measured by h.p.l.c. bioassay, from A23187-stimulated PMNs. Significant inhibition (45%) of PAF-release was obtained with 5 0 mu M cloricromene and the IC50 was 85 mu M. Mepacrine (500 mu M), a non-specific PLA(2) inhibitor, strikingly reduced PAF release. 6 The i ncorporation of [H-3]-acetate into [H-3]-PAF induced by serum-treated zymosan in human PMNs was also inhibited concentration-dependently by cloricromene, with an IC50 of 105 mu M. Mepacrine also suppressed [H-3 ]-acetate incorporation into [H-3]-PAF. 7 Cloricromene did not affect the activities of the enzymes involved in PAF-synthesis acetyltransfer ase or phosphocholine transferase. 8 Our data demonstrate that cloricr omene, an inhibitor of PLA(2)-activation in human leucocytes, reduces the synthesis of PAF by stimulated PMNs. This finding has a twofold im plication: the PLA(2)s (or the mechanisms that regulate their activati on) involved in PAF synthesis and arachidonate release in human leucoc ytes are either identical or else indistinguishable by their sensitivi ty to cloricromene; the inhibition of PAF release by activated leucocy tes may contribute to the antithrombotic and antiischaemic activities exerted by cloricromene.