COX-1+/−COX-2−/− genotype in mice is associated with shortened time to carotid artery occlusion through increased PAI-1

Summary. Background: We found a high incidence of thrombotic deaths in COX-1+/−COX-2−/− mice and sought to define the mechanism of these events. The cyclooxygenase products thromboxane A2 and prostacyclin are important in the regulation of coagulation but their role in fibrinolysis is largely unexplored. PAI-1 blocks fibrinolysis by inhibiting plasminogen activator. Aim: Our objective was to explain the mechanism of increased thrombosis associated with the COX-1+/−COX-2−/− genotype. Methods: Carotid artery occlusion times were measured after photochemical injury. PAI-1 levels were measured in the plasma by ELISA. PAI-1 levels in the aorta were measured by RT-PCR and Western blotting. Urinary metabolites of Thromboxane A2 and prostacyclin were measured by ELISA. Results: The COX-1+/−COX-2−/− genotype is associated with a decreased time to occlusion in the carotid artery thrombosis model (30 ± 5 minutes vs 60 ±  minutes in wild type, p<.001). The COX-1−/−COX-2+/+, COX-1+/−COX-2+/− and COX-1+/− COX-2+/+ all had occlusion times similar to wild type. COX-1+/+ COX-2−/− had a prolonged occlusion time. COX-1+/− COX-2−/− had increased PAI-1 levels in the plasma and aorta and with a prolonged euglobulin lysis time (37.4 ± 10.2 hours vs 15.6 ± 9.8 hours in wild type, p<.004). The decreased time to occlusion in the COX-1+/−COX2−/− mice was normalized by an inhibitory antibody to PAI-1 whereas the antibody had no effect on the time to occlusion in wild type mice. Conclusion: The COX-1+/−COX-2−/− genotype is associated with a shortened time to occlusion in the carotid thrombosis model and the shortened time to occlusion is mediated through increased PAI-1 levels resulting in decreased fibrinolysis.