Inhibition of PAF synthesis by stimulated human polymorphonuclear leucocytes with cloricromene, an inhibitor of phospholipase A(2) activation

1 A phospholipase A(2) (PLA(2)) represents the key enzyme in the remodelling pathway of platelet-activating factor (PAF) synthesis in human polymorphonuclear (PMN) leucocytes. 2 PLA(2) activation is also the rate-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 (8-monochloro-3-beta-diethylaminoethyl-4-methyl-7-ethoxy-carbonylmethoxy coumarin) is an antithrombotic coumarin derivative which inhibits platelet and leucocyte function and suppresses arachidonic acid liberation by interfering with PLA(2) activation. 4 The aim of the present study was to assess whether chloricromene inhibits PAF synthesis by stimulated human polymorphonuclear leucocytes (PMNs). 5 Cloricromene (50-500 mu M) inhibited in a concentration-dependent manner the release of PAF, as measured by h.p.l.c. bioassay, from A23187-stimulated PMNs. Significant inhibition (45%) of PAF-release was obtained with 50 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 incorporation 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 acetyltransferase or phosphocholine transferase. 8 Our data demonstrate that cloricromene, an inhibitor of PLA(2)-activation in human leucocytes, reduces the synthesis of PAF by stimulated PMNs. This finding has a twofold implication: the PLA(2)s (or the mechanisms that regulate their activation) involved in PAF synthesis and arachidonate release in human leucocytes are either identical or else indistinguishable by their sensitivity to cloricromene; the inhibition of PAF release by activated leucocytes may contribute to the antithrombotic and antiischaemic activities exerted by cloricromene.