The aim of this study is to elucidate the role of integrins in transducing fluid shear stress into intracellular signals in vascular endothelial cells, a fundamental process in vascular biology. We demonstrated that shear stress activates specific integrins in endothelial cells plated on substrates containing the cognate extracellular matrix (ECM) ligands. The shear stress-induced mechanotransduction, as manifested by integrin–Shc association, was abolished when new integrin–ECM ligand interactions were prevented by either blocking the integrin-binding sites of ECM ligands or conjugating the integrins to immobilized antibodies. Our results indicate that the dynamic formation of new connections between integrins and their specific ECM ligands is critical in relaying the signals induced by shear stress to intracellular pathways.

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We studied interactions between integrins and Flk-1 in transducing the mechanical shear stress due to flow. This application of a step shear stress caused Flk-1 ⅐ Casitas B-lineage lymphoma (Cbl) activation (Flk-1 ⅐ Cbl association, tyrosine phosphorylation of the Cbl-bound Flk-1, and tyrosine phosphorylation of Cbl) in bovine aortic endothelial cells (BAECs). The activation of integrins by plating BAECs on vitronectin or fibronectin also induced this Flk-1 ⅐ Cbl activation. The shear-induced Flk-1 ⅐ Cbl activation was blocked by inhibitory antibodies for ␣ v␤3-or ␤1-integrin, suggesting that it is mediated by integrins. Inhibition of Flk-1 by SU1498 also abolished this shear-induced Flk-1 ⅐ Cbl activation. In contrast to the requirement of integrins for Flk-1 ⅐ Cbl activation, the Flk-1 blocker SU1498 had no detectable effect on the shear-induced integrin activation, suggesting that integrins and Flk-1 play sequential roles in the signal transduction hierarchy induced by shear stress. Integrins are essential for the mechanical activation of Flk-1 by shear stress but not for the chemical activation of Flk-1 by VEGF. mechanotransduction; vascular endothelial growth factor; Casitas B-lineage lymphoma MECHANICAL FORCES ARE KNOWN TO PLAY A KEY ROLE in many physiological processes, but how cells perceive mechanical stimuli and convert them into biochemical signaling pathways is not yet well understood. Shear stress, the tangential component of hemodynamic forces acting on the vessel wall, is an important modulator of vascular cell functions in normal and pathophysiological conditions, e.g., endothelial wound healing, atherosclerosis, and reperfusion injury (14,36,37,43). In vitro experiments using endothelial cells (ECs) cultured in flow channels have shown that the structure and function of ECs are modulated by shear stress (6). Recent studies indicate that the mechanochemical transduction in response to shear stress involves different types of receptors and multiple intracellular signaling pathways. Studies from several laboratories, including our own, have demonstrated that shear stress can activate integrins, VEGF receptor-2 (Flk-1), G protein-coupled receptors (GPCR), and ion channels and subsequently regulate mitogen-activated protein kinases and the NF-B pathway through Ras and Rho family GTPases (5,26,33,40). The available evidence indicates that the shear stress-induced signaling from different sensing elements can coordinate downstream signaling through convergent and divergent pathways. However, the possible interaction of these sensing elements in ECs in response to shear stress remains to be investigated.Most intracellular signals are not transduced along a simple linear path involving the interaction of only one molecule with the next. Proteins associate into networks, which are subject to control by many interdependent processes. Many different types of receptors exert mutual influences on each other (9,16,23,30). There is considerable evidence for the synergistic regulation of signals by integrins and...