STUDIES ON THE ROLE OF PLATELET EICOSANOID METABOLISM AND INTEGRIN ALPHA-IIB-BETA-3 IN TUMOR-CELL-INDUCED PLATELET-AGGREGATION

Platelet eicosanoid metabolism resulting from tumor-cell-induced plate let aggregation (TCIPA) was examined in a homologous in vitro system. Rat Walker 256 carcinosarcoma cells induced the aggregation of rat pla telets via a thrombin-dependent mechanism with concomitant production of eicosanoid metabolites (e.g., 12-HETE, TXA2). TCIPA was dependent o n the concentration of tumor cells inducing aggregation, as well as cy clooxygenase and lipoxygenase products. Cyclooxygenase inhibitors, but not lipoxygenase inhibitors, blocked platelet aggregation induced in vitro by a low concentration of agonist. At a high agonist concentrati on, neither cyclooxygenase nor lipoxygenase inhibitors alone affected platelet aggregation; however, the combined inhibition of both the cyc looxygenase and lipoxygenase pathways resulted in subsequent inhibitio n of platelet aggregation regardless of agonist concentration. The ext ent of platelet TXA2 and 12-HETE biosynthesis was likewise dependent o n and correlated with agonist concentration. The inhibitors used in th is study did not significantly inhibit protein kinase C activity at th e doses tested. Platelet surface glycoproteins alpha(IIb)beta3 play an important role in platelet aggregation. The effect of platelet cycloo xygenase and lipoxygenase inhibition in regulating alpha(IIb)beta3 sur face expression was examined by flow cytometric analysis. Thrombin sti mulation of washed rat platelets resulted in significantly increased s urface expression of platelet alpha(IIb)beta3 integrin complex. The en hanced surface expression was not inhibited by a cyclooxygenase inhibi tor (aspirin), a thromboxane synthase inhibitor (CGS- 14854) or a thro mboxane receptor antagonist (SQ 29,548), nor was it stimulated by a th romboxane A2 MiMic (pinane-thromboxane A2). However, alpha(IIb)beta3 e xpression was blocked by lipoxygenase inhibition and stereospecificall y increased by the platelet lipoxygenase metabolite 12(S)-HETE. These results suggest that both the platelet lipoxygenase and cyclooxygenase pathways are important for TCIPA but that different mechanisms of act ion are involved.