Hindawi Publishing Corporation International Journal of Cell Biology Volume 2009, Article ID 532432, 9 pages doi:10.1155/2009/532432 Research Article Lowering Caveolin-1 Expression in Human Vascular Endothelial Cells Inhibits Signal Transduction in Response to Shear Stress A. D. van der Meer, 1 M. M. J. Kamphuis, 1, 2 A. A. Poot, 1 J. Feijen, 1 and I. Vermes 1, 2 1 Institute for Biomedical Technology and Department of Polymer Chemistry and Biomaterials, Faculty of Science and Technology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands 2 Department of Clinical Chemistry, Medical Spectrum Twente, Hospital Group, P.O. Box 50000, 7500 KA Enschede, The Netherlands Correspondence should be addressed to I. Vermes, i.vermes@ziekenhuis-mst.nl Received 18 July 2008; Accepted 19 October 2008 Recommended by Rony Seger Vascular endothelial cells have an extensive response to physiological levels of shear stress. There is evidence that the protein caveolin-1 is involved in the early phase of this response. In this study, caveolin-1 was downregulated in human endothelial cells by RNAi. When these cells were subjected to a shear stress of 15dyn/cm 2 for 10 minutes, activation of Akt and ERK1/2 was significantly lower than in control cells. Moreover, activation of Akt and ERK1/2 in response to vascular endothelial growth factor was significantly lower in cells with low levels of caveolin-1. However, activation of integrin-mediated signaling during cell adhesion onto fibronectin was not hampered by lowered caveolin-1 levels. In conclusion, caveolin-1 is an essential component in the response of endothelial cells to shear stress. Furthermore, the results suggest that the role of caveolin-1 in this process lies in facilitating efficient VEGFR2-mediated signaling. Copyright © 2009 A. D. van der Meer et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. 1. Introduction Vascular endothelial cells (ECs) are constantly subjected to shear stress caused by the flow of blood. ECs are highly responsive to changes in this shear stress. They are able to convert these mechanical stimuli into relevant biological sig- nals by a process that is known as mechanotransduction [1]. The best-known elements in the early stages of this response are the cell-anchoring integrins [2] and certain membrane- associated receptors, such as the vascular endothelial growth factor receptor 2 (VEGFR2) [3, 4] and G-protein coupled receptors [5]. After the initial activation of these molecules, the biological signal is transmitted into the cell by activation of major signal transduction pathways, such as mitogen- activated protein kinase (MAPK) pathways, the protein kinase B (PKB/Akt) pathway, and the endothelial nitric oxide synthase (eNOS) signaling route. These events lead to a functional response of the cell, influencing rate of apoptosis and proliferation [1, 6], sensitivity to inflammation [7], and cytoskeletal remodeling [8]. Studies have shown that 50-nanometer, omega-shaped membrane invaginations, known as caveolae, are linked to mechanotransduction. The majority of membrane- associated proteins that are phosphorylated in response to shear stress localize to these domains [9, 10]. Also, as ECs are subjected to shear stress, the density of caveolae in the cell membrane increases, modulating the activation of signaling pathways [11, 12]. Moreover, mice that lack a structural protein of the caveolar domain, caveolin-1, have an abnormal vascular response when shear stress is altered [13]. In order to gain more mechanistic insight into the exact role of caveolae in the EC response to shear stress, in vitro studies were carried out to interfere directly with caveolar function. In some studies, caveolae were disrupted by cholesterol extraction [9, 14, 15], and in one study caveolar functioning was inhibited by introducing blocking antibodies to caveolin-1 [16]. These studies have shown that interfering with caveolar function causes an impaired response to shear stress, as characterized by a lowered activation of MAPKs [9, 14, 16], Akt [15], and eNOS [10]. An important molecular biological tool to study the role of proteins in cellular processes is RNA interference (RNAi). By transfecting cells with short interfering RNA (siRNA) molecules with a sequence that is complementary to the