Keywords: Hemangiosarcoma, Angiosarcoma, Tissue Factor Hemangiosarcoma (HSA) and angiosarcoma (AS) are malignant endothelial neoplasms in dogs and human beings, respectively. Canine patients suffering from hemangiosarcoma often succumb to concurrent hemostatic disorders such as disseminated intravascular coagulopathy (DIC); human beings with AS are also prone to DIC. The cause of DIC in affected patients is unknown. Tissue factor (TF), a 47-kDa membrane-bound glycoprotein, is the main initiator of the coagulation cascade. Tissue factor has been found to be upregulated in some canine epithelial and mesenchymal cancer cells, but we currently do not know how TF is expressed in HSA and AS. Endothelial cells do not normally express TF or bind coagulation factor VII (FVII), the enzymatic binding partner for TF in plasma. Our global hypothesis is that TF contributes to the aggressive biological behavior of canine hemangiosarcoma, including the initiation of thrombin formation as seen in DIC. We hypothesize that hemangiosarcoma, a malignant endothelial neoplasm, is procoagulant due to the aberrant expression of TF. We found that HSA cell lines derived from primary tumors in dogs expressed TF mRNA and cell surface protein as determined by quantitative RT-PCR, and flow cytometry, respectively. In addition these cells produced thrombin as shown by calibrated automated thrombography (CAT) in canine plasma. Using human plasma deficient in specific factors and protein inhibitors, we found that thrombin generation by HSA cells is mediated by the extrinsic pathway of coagulation, requiring FVII and phosphatidylserine (PS), and is independent of contact activation. Conclusion: Our results show that HSA cells are procoagulant and this is most likely due to TF expression. Procoagulant TF expression could explain the pathogenesis of DIC in dogs with HSA. Future studies will examine if an AS-derived cell line has similar activity to establish canine HSA as a model for human AS. Tissue factor's interaction with protease-activated receptor 2 is involved several signaling pathways that promote hemostasis, repair, cell survival, and inflammation. These signaling ii pathways can be subverted during cancer to promote tumor growth, angiogenesis, tumor survival and metastasis. We have developed a TF knockdown in a HSA cell line and plan to assess changes in cell growth, anchorage independence, and doxorubicin-sensitivity. This research could lead to future diagnostics for hypercoagulability associated with DIC and therapeutics for treating dogs with HSA. iii