Abstract 53: Ablation of Thrombin Signaling in Platelets but Not in Endothelial Cells Impairs Hemostasis in a Mouse Model of Spontaneous Gastrointestinal Bleeding

Human PAR1 is expressed in endothelial cells as well as in platelets where it facilitates the response to thrombin and platelet activation. Vorapaxar, a PAR1 antagonist, prevents myocardial infarction and stroke in patients with prior MI or peripheral arterial disease at a cost of increased bleeding risk. Par1 is also highly expressed in endothelial cells in mice, and Par1-deficiency is associated with bleeding in the mouse embryo at midgestation. Additionally, known actions of endothelial PAR1 activation suggest pro-hemostatic functions. This raises the question of whether inhibition of PAR1 function in endothelial cells (in addition to PAR1 inhibition in platelets) contributes to the bleeding risk associated with Vorapaxar treatment. Our previous work demonstrated that Par1 deficiency results in loss of thrombin signaling in mouse endothelial cells but not mouse platelets, while Par4 deficiency ablated thrombin-induced platelet activation in mice. Thus, mice allow us to separate loss of thrombin signaling in platelets from loss of thrombin signaling in endothelial cells. Accordingly, we used Apc min/+ mice, which develop intestinal polyposis and spontaneous GI bleeding, as a model to determine whether loss of thrombin signaling in platelets (Par4 KO) or endothelial and other cells (Par1 KO) exacerbates spontaneous bleeding. Hematocrit and other hematologic parameters were measured biweekly from 7 weeks through 15 weeks of age. Hematocrits in mice wild-type for Apc were stable over this period (41.48 ± 0.48 at 7 weeks; 40.48 ± 0.37 at 15 weeks, n=15). Hematocrits in Apc min/+ mice fell approximately linearly from 37.06 ± 0.82 at 7 weeks to 14.39 ± 1.12 at 15 weeks (n=15). Hematocrits in Par1-deficient Apc min/+ mice were indistinguishable from those in Apc min/+ without Par deficiency (14.39 ± 1.12 vs 14.47 ± 1.66 at 15 weeks; n=6-15). By contrast, Par4-deficient Apc min/+ mice were already severely anemic at 7 weeks compared to Apc min/+ mice (19 ± 2.0 vs 39 ± 3.6; p<0.01, n=4). Par-dependent differences in polyp count and size were not detected. Taken together, our results suggest that loss of thrombin signaling in platelets promotes spontaneous GI bleeding in the Apc min model while loss of thrombin signaling in endothelial cells is without effect in this system.