Objective: Previously we have demonstrated that porcine aortic endothelial cells (PAEC) released fibroblast growth factor-2 (FGF-2) under shear stress in an integrin avb3 dependent manner. In this study we investigated the signalling process of shear stress induced integrin activation. Methods and results: Integrin activation of PAEC was assessed as the interaction of the integrin avb3 and its adaptor protein Shc by coimmunoprecipitation. Shear stress (16 dynes/cm²) induced integrin activation within 60 minutes (n = 4). This activation was blocked by the specific elastase inhibitor (MeOSuc-Ala-Ala-Pro-Val- CHCl₂). Furthermore, sheared cells showed a significant higher elastolytic activity in their supernatants than static control cells (static 0.0619 U/ml ± 0.0206, shear 0.2036 U/ml ± 0.0577, n = 6). Blockade of the integrin avb3 (Abciximab) had no influence on the shear stress induced elastase release. The application of exogenous (pancreatic) elastase (0.5 U/ml) activated avb3 in a similar way than shear stress. However, PCR revealed no evidence for the presence of already characterised elastases (pancreatic and neutrophil). Immunofluorescence staining demonstrated that shear stress induced a restructuring of the fibronectin matrix, an effect that could be mimicked by application of exogenous, pancreatic elastase. In further experiments we could show that this elastase induced the degradation of fibronectin, vitronectin and laminin, known ligands of the integrin vb3, but not of collagen. Conclusion: Our results demonstrate that shear stress induced activation of integrin vb3 is mediated by the release of a novel endothelium derived elastase. Integrin activation may therefore either be induced by the release of biologically active fibronectin fragments or by the associated matrix restructuring, leading to an altered clustering of integrins.