Interplay between integrins and FLK-1 in shear stress-induced signaling

Blood flow can modulate vascular cell functions. 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·Casitas B-lineage lymphoma (Cbl) activation (Flk-1·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·Cbl activation. The shear-induced Flk-1·Cbl activation was blocked by inhibitory antibodies for α_vβ₃- or β₁-integrin, suggesting that it is mediated by integrins. Inhibition of Flk-1 by SU1498 also abolished this shear-induced Flk-1·Cbl activation. In contrast to the requirement of integrins for Flk-1·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.