Up-regulation of endothelial cyclooxygenase-2 and prostanoid synthesis by platelets. Role of thromboxane A₂

Platelet-vascular endothelial cell interactions are central to the maintenance of vascular homeostasis. Thromboxane A₂ (TXA ₂) and prostacyclin (prostaglandin (PG) I₂) are the major products of cyclooxygenase (COX) metabolism by platelets and the vascular endothelium, respectively. Here we report the effects of platelet-endothelial interactions on human umbilical vein endothelial cells (HUVECs) COX-2 expression and prostanoid synthesis. Co-incubation of platelets with HUVECs resulted in a dose-dependent induction in COX-2 expression. This was accompanied by a relatively small increase in thromboxane B₂ synthesis (2 ng) by comparison to the production of 6-keto-PGF _1α and PGE₂, which increased by ∼14 and 12 ng, respectively. Abrogation of platelet-HUVEC interactions excluded direct cell-cell contact as a required event. Preincubation of HUVECs with SQ29548, a TXA₂ receptor antagonist, dose-dependently inhibited platelet-induced COX-2 expression and prostanoid synthesis. Similarly, if platelet TXA₂ synthesis was inhibited no induction of COX-2 was observed. Furthermore, a TXA₂ analog, carbocyclic TXA₂, induced HUVEC COX-2 expression and the synthesis of 6-keto-PGF _1α and PGE₂. This was also associated with an increase in the expression and activity of PGI synthase and PGE synthase but not TX synthase. Platelet co-incubation (or TXA₂) also selectively activated the p44/42 mitogen-activated protein kinase pathway to regulate HUVEC COX-2 expression. Thus it seems that platelet-derived TXA₂ can act in a paracrine manner to up-regulate endothelial COX-2 expression and PGI ₂ synthesis. These observations are of particular importance given the recent observations regarding selective COX-2 inhibitors and the suppression of PGI₂ synthesis.