The role of thrombin in angiogenesis was investigated in the chick chorioallantoic membrane (CAM) system. alpha-Thrombin promoted angiogenesis in a dose-dependent fashion and at 8.4 pmol/disk reached a maximum of 78% above the control. At a higher dose of alpha-thrombin (25 pmol/disk) the angiogenic effect declines and this can be explained by desensitization of the thrombin receptor. The promotion of angiogenesis by alpha-thrombin is specific as evidenced by the reversal of this effect by hirudin, which binds both the catalytic and the anion-binding exosite of thrombin or by heparin, which binds thrombin and accelerates its inactivation by antithrombin III. gamma-Thrombin, which is catalytically active but lacks the anion-binding exosite required for clotting activity, promotes angiogenesis in the CAM in the same fashion and to the same extent as alpha-thrombin, at doses up to 130 pmol/disk. Phenylalanyl-propyl-arginine chloromethyl ketone (P-PACK)-thrombin, the catalytically inactive analogue of alpha-thrombin that retains the anion-binding exosite, had no significant effect on angiogenesis in the CAM. When combined with alpha-thrombin, P-PACK-thrombin abolished the angiogenesis-promoting effect of alpha-thrombin. These results suggest that alpha-thrombin can promote angiogenesis in the CAM through interaction with its catalytic site without the requirement for fibrin formation.

. On the mechanism of thrombin-induced angiogenesis: involvement of ␣v␤3-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 ␣v␤3-integrin. This thrombin-mediated effect is specific, dose dependent, and requires the catalytic site of thrombin. In addition, thrombin interacts with ␣v␤3 as demonstrated by direct binding of ␣v␤3 protein to immobilized thrombin. This interaction of thrombin with ␣v␤3-integrin, which is an angiogenic marker in vascular tissue, is of functional significance. Immobilized thrombin promotes endothelial cells attachment, migration, and survival. Antibody to ␣v␤3 or a specific peptide antagonist to ␣v␤3 can abolish all these ␣v␤3-mediated effects. Furthermore, in the chick chorioallantoic membrane system, the antagonist peptide to ␣v␤3 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...

Clinical, laboratory, histopathological and pharmacological evidence support the notion that a systemic activation of blood coagulation is often present in cancer patients. Additionally, thrombin was shown to promote tumour progression and metastasis in animals, and epidemiological studies suggest an increased risk of cancer diagnosis after primary thromboembolism. We have proposed that the aforementioned results may be related to our finding that thrombin is a potent activator of angiogenesis. This is a thrombin receptor-mediated event (the receptor is referred to as protease-activate receptor) and is independent of fibrin formation. Many cellular effects of thrombin on endothelial cells can contribute to the angiogenic action of thrombin. (i) Exposure of endothelial cells to thrombin cause a time- and dose-dependent decrease in the attachment of these cells to basement membrane components, with a concomitant increase in matrix metalloproteinase 2 activation. (ii) Thrombin upregulates the expression of integrin alphavbeta3, the marker of the angiogenic phenotype of endothelial cells. (iii) Thrombin has chemotactic and aptotactic effects on endothelial cells and upregulates the expression of the vascular endothelial growth factor (VEGF) receptors (KDR and Flt1). Thus, thrombin synergizes with the key angiogenic factor VEGF in endothelial cell proliferation. Furthermore, thrombin enhances the secretion of VEGF and matrix metalloproteinase 9 of PC3 prostate cancer cells. These results can explain the angiogenic and tumour-promoting effect of thrombin and provide the basis for development of thrombin receptor mimetics or antagonists for therapeutic application.

Previous studies have suggested that thrombin interacts with integrins in endothelial cells through its RGD (Arg-187, Gly-188, Asp-189) sequence. All existing crystal structures of thrombin show that most of this sequence is buried under the 220-loop and therefore interaction via RGD implies either partial unfolding of the enzyme or its proteolytic digestion. Here, we demonstrate that surface-absorbed thrombin promotes attachment and migration of endothelial cells through interaction with ␣ v ␤ 3 and ␣ 5 ␤ 1 integrins. Using site-directed mutants of thrombin we prove that this effect is mediated by the RGD sequence and does not require catalytic activity. The effect is abrogated when residues of the RGD sequence are mutated to Ala and is not observed with proteases like trypsin and tissue-type plasminogen activator, unless the RGD sequence is introduced at position 187-189. The potent inhibitor hirudin does not abrogate the effect, suggesting that thrombin functions through its RGD sequence in a non-canonical conformation. A 1.9-Å resolution crystal structure of free thrombin grown in the presence of high salt (400 mM KCl) shows two molecules in the asymmetric unit, one of which assumes an unprecedented conformation with the autolysis loop shifted 20 Å away from its canonical position, the 220-loop entirely disordered, and the RGD sequence exposed to the solvent.

Many studies support the notion that protease-activated receptor (PAR)-1 plays a pivotal role in angiogenesis. However, direct evidence and understanding the molecular mechanisms involved were limited because PAR-1-specific antagonists have been developed only recently. In the present study, we evaluated the effects of two well characterized PAR-1 antagonists,, in the angiogenic cascade. These antagonists suppressed both the basic angiogenesis and that stimulated by thrombin in the chick chorioallantoic membrane model in vivo. PAR-1 antagonists also abrogated tube formation in the in vitro Matrigel system. These inhibitory effects were dose-dependent and well correlated with the inhibitory effects of SCH79797 and RWJ56110 on primary endothelial cell proliferation and on the initiation of apoptosis. PAR-1 blockage resulted in inhibition of endothelial cell growth by increasing the sub-G 0 /G 1 fraction and reducing the percentage of cells in the S phase. Consistent with this, PAR-1 antagonists reduced incorporation of [ 3 H]thymidine in endothelial cells and blocked the phosphorylation of extracellular signal-regulated kinases in a fashion depending specifically on PAR-1 activation. Analysis by annexin V/propidium iodide staining and poly(ADP-ribose) polymerase cleavage revealed that PAR-1 blockage increased apoptotic cell death by a mechanism involving caspases. These results provide further evidence that PAR-1 is a key receptor that mediates angiogenesis and suggest PAR-1 as target for developing antiangiogenic agents with potential therapeutic application in cancer and other angiogenesis-related diseases.