DIRECT INHIBITION OF CYCLOOXYGENASE-1 AND CYCLOOXYGENASE-2 BY THE KINASE INHIBITORS SB-203580 AND PD-98059 - SB-203580 ALSO INHIBITS THROMBOXANE SYNTHASE

The kinase inhibitors SE 203580 and PD 98059 have been reported to be specific inhibitors of the 38- and 42/44-kDa mitogen-activated protein kinase (MAPK) pathways, respectively. In this study, the two inhibito rs were found to decrease platelet aggregation induced by low concentr ations of arachidonic acid, suggesting that they also interfere with t he metabolism of arachidonic acid to thromboxane A(2). In support of t his, SE 203580 and PD 98059 inhibited the conversion of exogenous [H-3 ]arachidonic acid to [H-3]thromboxane in intact platelets. Measurement of platelet cyclooxygenase-1 activity following immunoprecipitation r evealed that SE 203580 and PD 98059 are direct inhibitors of this enzy me. Both compounds were shown to inhibit purified cyclooxygenase-1 and -2 by a reversible mechanism, In addition, SE 203580 (but not PD 9805 9) inhibited platelet aggregation induced by prostaglandin H-2 and the conversion of prostaglandin H-2 to thromboxane A(2) in intact platele ts. SE 203580 also inhibited this pathway in platelet microsome prepar ations, suggesting a direct inhibitory effect on thromboxane synthase. These results demonstrate that direct effects of the two kinase inhib itors on active arachidonic acid metabolites have to be excluded befor e using these compounds for the investigation of MAPKs in signal trans duction pathways. This is of particular relevance to studies on the re gulation of cytosolic phospholipase A(2) as these two MAPKs are capabl e of phosphorylating cytosolic phospholipase A(2), thereby increasing its intrinsic activity.