Studies on the role of platelet eicosanoid metabolism and integrin αIIbβ3 in tumor‐cell‐induced platelet aggregation

Abstract

Platelet eicosanoid metabolism resulting from tumor-cell-induced platelet aggregation (TCIPA) was examined in a homologous in vitro system. Rat Walker 256 carcinosarcoma cells induced the aggregation of rat platelets via a thrombin-dependent mechanism with concomitant production of eicosanoid metabolites (e.g., 12-HETE, TXA₂. TCIPA was dependent on the concentration of tumor cells inducing aggregation, as well as cyclooxygenase and lipoxygenase products. Cyclooxygen-ase inhibitors, but not lipoxygenase inhibitors, blocked platelet aggregation induced in vitro by a low concentration of agonist. At a high agonist concentration, neither cyclooxygenase nor lipoxygenase inhibitors alone affected platelet aggregation; however, the combined inhibition of both the cyclooxygenase and lipoxygenase pathways resulted in subsequent inhibition of platelet aggregation regardless of agonist concentration. The extent of platelet TXA₂ and 12-HETE biosynthesis was likewise dependent on and correlated with agonist concentration. The inhibitors used in this study did not significantly inhibit protein kinase C activity at the doses tested. Platelet surface glycoproteins α_IIbβ₃ play an important role in platelet aggregation. The effect of platelet cyclooxygenase and lipoxygenase inhibition in regulating α_IIbβ₃ surface expression was examined by flow cytometric analysis. Thrombin stimulation of washed rat platelets resulted in significantly increased surface expression of platelet α_IIbβ₃ integrin complex. The enhanced surface expression was not inhibited by a cyclooxygenase inhibitor (aspirin), a thromboxane synthase inhibitor (CGS-14854) or a throm-boxane receptor antagonist (SQ 29,548), nor was it stimulated by a thromboxane A₂ mimic (pinane-thromboxane A₂). However, α_IIbβ₃ expression was blocked by lipoxygenase inhibition and stereospecifically 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 action are involved.