Vascular endothelial cells (ECs) exposed to fluid shear stress (FSS) become elongated and aligned to the direction of flow. However, the process of morphological change in individual cells is different depending of their initial shape. Rac1 and RhoA, members of the family of Rho GTPases, play important roles in cellular morphological changes but are thought to be activated differently in the process. Here, we measured changes in Rac1 and RhoA activities with a focus on the effect of cell orientation when exposed to FSS. In ECs initially oriented parallel to the direction of flow, RhoA and Rac1 were activated primarily in the upstream and the downstream regions of cells, respectively, accompanied by the formation of lamellipodia in the direction of flow. On the other hand, in cells oriented perpendicular to the direction of flow, FSS caused RhoA activation in the upstream region but did not change Rac1 activity. Furthermore, treatment with cytochalasin D inhibited the localization of Rac1 activation and suppressed RhoA activation by FSS. These results indicate that cell orientation affects the local activation of Rac1 and RhoA when induced by forces transmitted through the actin cytoskeleton under a FSS condition.
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