Abstract 8987: Prostacyclin Reduction Upregulates Tissue Factor and Predisposes COX-2 Knockout Mice to Thrombosis

Abstract

Selective inhibitors of cyclooxygenase-2 (COX-2) increase the risk of myocardial infarction and athero-thrombotic events, but the mechanisms responsible for these effects are not fully understood. To assess the role of COX-2 in arterial thrombosis, a well-studied model of ferric chloride-induced carotid artery injury was applied to COX-2 knockout (COX-2KO) mice and data were compared with those obtained in wild-type (WT) mice. We found that thrombus formation was significantly greater in COX-2-null compared to WT mice. We then explored the role of platelets in this model. Platelets isolated from blood of COX-2KO mice showed enhanced aggregation responses to collagen and ADP together with greater thromboxane production than WT platelets. Interestingly, cyclooxygenase-1 and thromboxane synthase protein levels were similar in both animal groups. In cross-transfusion experiments, we found that platelets from COX-2KO mice behaved similarly to WT platelets in terms of in vivo thrombus formation. Importantly, we observed that the activity of tissue factor (TF), initiator of blood coagulation, was elevated both in concentrated microparticles derived from plasma and in leukocytes or in the carotid arteries of COX-2KO mice. Furthermore, the arterial wall of COX-2KO mice expressed higher levels of TF mRNA compared to WT animals, and these levels correlated with both reduced prostacyclin synthase mRNA levels and decreased prostacyclin production. In addition, treatment with CA10441, a prostacyclin receptor (IP) antagonist, increased TF activity in WT mice. Conversely, carbaprostacyclin, a stable IP agonist, reduced TF activity in COX-2KO mice. Overall, these findings reveal for the first time that the propensity to thrombosis observed in association with COX-2 inhibition is consequent to the impairment in the generation of prostacyclin by vascular endothelium, which in turn results in increased TF expression and activity.