. On the mechanism of thrombin-induced angiogenesis: involvement of ␣v3-integrin. Am J Physiol Cell Physiol 283: C1501-C1510, 2002. First published July 17, 2002 10.1152/ajpcell.00162.2002Thrombin has been reported to be a potent angiogenic factor both in vitro and in vivo, and many of the cellular effects of thrombin may contribute to activation of angiogenesis. In this report we show that thrombin-treatment of human endothelial cells increases mRNA and protein levels of ␣v3-integrin. This thrombin-mediated effect is specific, dose dependent, and requires the catalytic site of thrombin. In addition, thrombin interacts with ␣v3 as demonstrated by direct binding of ␣v3 protein to immobilized thrombin. This interaction of thrombin with ␣v3-integrin, which is an angiogenic marker in vascular tissue, is of functional significance. Immobilized thrombin promotes endothelial cells attachment, migration, and survival. Antibody to ␣v3 or a specific peptide antagonist to ␣v3 can abolish all these ␣v3-mediated effects. Furthermore, in the chick chorioallantoic membrane system, the antagonist peptide to ␣v3 diminishes both basal and the thrombin-induced angiogenesis. These results support the pivotal role of thrombin in activation of endothelial cells and angiogenesis and may be related to the clinical observation of neovascularization within thrombi. attachment; migration; apoptosis; reverse transcription-polymerase chain reaction THE FREQUENCY OF BLOOD COAGULATION in cancer patients, known for more than 130 years, is supported by clinical, laboratory, and histopathological evidence. This is explained at the molecular and cellular level by the thromboplastic activity of circulating tumor cells, the existence of "a cancer coagulative factor," the activation of factor X, the generation of prothrombinase by tumor cells, and the encircling of cancerous tissue by fibrin deposits (38,50). In addition, the possibility of a relation between blood clotting mechanisms and tumor progression and development of metastases was postulated as early as 1878 by Billroth (7) on the basis of the observation that cancer cells exist within thrombi. This finding was interpreted as evidence that tumor cells spread by thromboembolism. More recently, large epidemiological studies have provided evidence that the standardized incidence ratio for certain types of cancer is as high as 6.7 within a year following a thromboembolic episode (3, 41). These clinical data are in line with animal experiments where thrombin-treated B16 melanoma cells show a dramatic increase in their metastatic potential in the lung of rats (37). These observations have led to experimental use of heparin, aspirin, and warfarin for the prevention and treatment of tumors in animal models and humans (23, 50).We proposed earlier (33, 47) that the tumor-promoting effect of thrombin/thrombosis may be related to our finding that thrombin is a potent promoter of angiogenesis, a process essential for tumor growth and metastasis. The angiogenic action of thrombin was shown to b...