Fibronectin and F-actin redistribution in cultured endothelial cells exposed to shear stress.

Cultured endothelial cells exposed to shear stresses in vitro undergo a reorganization of their F-actin-containing cytoskeletons which culminates in realignment with flow direction. Since a close transmembrane association exists between actin microfilaments and extracellular fibronectin, this study was undertaken to examine whether the actin reorganization induced by shear stress is accompanied by perturbations in the underlying fibronectin matrix. In a closed circulatory loop, bovine endothelial monolayers were exposed to steady, laminar flows corresponding to shear stress levels of 6 and 26 dynes/cm2 for 2, 6, 12, and 24 hours. The co-distribution of fibronectin and F-actin was determined in specimens which were double-labeled with antiserum to fibronectin and rhodamine phalloidin, respectively. Under the influence of shear stress, cells underwent coordinate shape changes resulting in varying degrees of alignment with flow direction. Reorientation at these shear stress levels was dependent on both the time of exposure and the magnitude of shear stress and was accompanied by a reorganization in cellular fibronectin and F-actin. In controls (no flow) correspondence between the two proteins was limited to similarly arranged, radial foci of fibronectin and F-actin filaments at the basal cell surfaces. In flow specimens, coincidence was detected only between occasional fibronectin fibrils and F-actin stress fibers. As a consequence of shear stress, fibronectin became more uniformly distributed beneath monolayers and frequently was organized into bands of densely packed fibrils. Despite this extensive reorganization, rearrangement of fibronectin did not result in the formation of identical, linear structures with F-.