Hemodynamic forces exerted by blood flow (cyclic strain, shear stress) affect the initiation and progression of angiogenesis; however, the precise signaling mechanism(s) involved are unknown. In this study, we examine the role of cyclic strain in regulating bovine aortic endothelial cell (BAEC) migration and tube formation, indices of angiogenesis. Considering their well-documented mechanosensitivity, functional inter-dependence, and involvement in angiogenesis, we hypothesized roles for matrix metalloproteinases (MMP-2/9), RGD-dependent integrins, and urokinase plasminogen activator (uPA) in this process. BAECs were exposed to equibiaxial cyclic strain (5% strain, 1 Hz for 24 h) before their migration and tube formation was assessed by transwell migration and collagen gel tube formation assays, respectively. In response to strain, both migration and tube formation were increased by 1.83 +/- 0.1- and 1.84 +/- 0.1-fold, respectively. Pertussis toxin, a Gi-protein inhibitor, decreased strain-induced migration by 45.7 +/- 32% and tube formation by 69.8 +/- 13%, whilst protein tyrosine kinase (PTK) inhibition with genistein had no effect. siRNA-directed attenuation of endothelial MMP-9 (but not MMP-2) expression/activity decreased strain-induced migration and tube formation by 98.6 +/- 41% and 40.7 +/- 31%, respectively. Finally, integrin blockade with cRGD peptide and siRNA-directed attenuation of uPA expression reduced strain-induced tube formation by 85.7 +/- 15% and 84.7 +/- 31%, respectively, whilst having no effect on migration. Conclusions: Cyclic strain promotes BAEC migration and tube formation in a Gi-protein-dependent PTK-independent manner. Moreover, we demonstrate for the first time a putative role for MMP-9 in both strain-induced events, whilst RGD-dependent integrins and uPA appear only to be involved in strain-induced tube formation. (c) 2005 Elsevier Inc. All rights reserved.