Background-Selective inhibitors of cyclooxygenase (COX)-2 increase the risk of myocardial infarction and thrombotic events, but the responsible mechanisms are not fully understood. Methods and Results-We found that ferric chloride-induced arterial thrombus formation was significantly greater in COX-2 knockout compared with wild-type mice. Cross-transfusion experiments excluded the likelihood that COX-2 knockout platelets, despite enhanced aggregation responses to collagen and thrombin, are responsible for increased arterial thrombus formation in COX-2 knockout mice. Importantly, we observed that COX-2 deletion decreased prostacyclin synthase and production and peroxisome proliferator-activated receptor-and sirtuin-1 (SIRT1) expression, with consequent increased upregulation of tissue factor (TF), the primary initiator of blood coagulation. Treatment of wild-type mice with a prostacyclin receptor antagonist or a peroxisome proliferator-activated receptor-␦ antagonist, which predisposes to arterial thrombosis, decreased SIRT1 expression and increased TF activity. Conversely, exogenous prostacyclin or peroxisome proliferator-activated receptor-␦ agonist completely reversed the thrombotic phenotype in COX-2 knockout mice, restoring normal SIRT1 levels and reducing TF activity. Furthermore, inhibition of SIRT1 increased TF expression and activity and promoted generation of occlusive thrombi in wild-type mice, whereas SIRT1 activation was sufficient to decrease abnormal TF activity and prothrombotic status in COX-2 knockout mice. Conclusions-Modulation of SIRT1 and hence TF by prostacyclin/peroxisome proliferator-activated receptor-␦ pathways not only represents a new mechanism in controlling arterial thrombus formation but also might be a useful target for therapeutic intervention in the atherothrombotic complications associated with COX-2 inhibitors. (Circulation. 2012; 126:1373-1384.)Key Words: blood coagulation Ⅲ carotid arteries Ⅲ prostacyclin Ⅲ signal transduction Ⅲ thrombosis P rostanoids are a family of bioactive lipid mediators formed from arachidonic acid by cyclooxygenases (COXs; COX-1 and COX-2). 1 Prostanoids are involved in a variety of physiological activities, including platelet aggregation, vasorelaxation and vasoconstriction, local inflammatory response, and leukocyte-endothelial cell adhesion. 2,3 In this context, prostanoids modulate the pathogenesis of vascular diseases such as arterial thrombosis and atherosclerosis. 4 The role of COX-2 in atherothrombosis is complex. This enzyme generates not only proinflammatory, but also anti-inflammatory prostanoids, such as prostacyclin (PGI 2 ), which, under certain conditions, may counteract platelet-derived thromboxane (TXA 2 ). 5 PGI 2 is a potent vasodilator and an inhibitor of platelet aggregation, leukocyte adhesion, and vascular smooth muscle cell proliferation. 6 These actions of PGI 2 are mediated through specific cell surface receptors known as PGI 2 receptors (IPs). Each IP is a 7-membrane-spanning G-protein-coupled receptor that elicits cAMP p...