Atherosclerotic lesions develop at arterial branches and curvatures, where the local flow is disturbed with low and oscillatory shear stress (OSS). We investigated the role of OSS in the endothelial cell (EC) activations of Smad1/5 and their regulation in OSS-induced EC proliferation. OSS (0.5±4 dynes/cm2) to human umbilical vein ECs induced sustained phosphorylations of Smad1/5. Smad1/5 activations were found only in the selective patches of ECs overlying foam cell lesions in human coronary arteries. These shear-activations of Smad1/5 were not inhibited by pre-treating ECs with Noggin, a specific antagonist of bone morphogenic proteins. Transfecting ECs with specific siRNA of BMPRII, BMPRIB, and v and β3 integrins inhibited OSS-induced Smad1/5 phosphorylations. OSS induced sustained increases in the association of vβ3 integrin with BMPRIB, which was inhibited by BMPRII-specific siRNA. OSS induced the activations of focal adhesion kinase/Shc/extracellular signal-regulated kinase, which are required for OSS-induced Smad1/5 phosphorylations. OSS induced increased expression of cyclin A and decreased expressions of p21CIP1 and p27KIP1 and hence proliferation in ECs. Transfecting ECs with Smad1/5-specific siRNA inhibited these OSS-induced responses in ECs. Our findings provide a molecular basis for the role of Smad1/5 in regulating EC proliferation in response to OSS.
