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Abstract
Endothelial cells respond to a large range of stimuli including circulating lipoproteins, growth factors and changes in haemodynamic mechanical forces to maintain the integrity of the blood vessels and control blood pressure. These adaptive responses can be either pro- or anti-atherosclerotic. The fundamental mechanisms of these cellular responses are complex and not well understood. Many endothelial cell responses are mediated by the plasma membrane of these cells and therefore dependent on its structure and composition. The aim was to determine the role of distinct membrane lipid domains (lipid rafts) in endothelial cell responses when subjected to changes in membrane lipid composition. Experimentally, membrane lipid composition can be manipulated by enrichment with cholesterol or treatment with the oxysterol 7-ketocholesterol, which depletes cholesterol from lipid rafts. For this study bovine aortic endothelial cells (BAEC) were treated with Chol (cholesterol/mβCD complex), 7KC (7-ketocholesterol/mβCD complex) or a 1:1 mixture of the two sterols (Ch:7KC). Endothelial cell function was assessed in terms of vascular endothelial cell growth factor (VEGF), high density lipoprotein (HDL) or shear stress-induced signalling and endothelial nitric oxide (eNOS) activity. Signal transduction processes initiated by these stimuli are mediated by ordered domains at focal adhesions (FA), lipid raft domains and caveolae membrane domains respectively. Hence the structure and composition of these specific domains is important for eNOS activation and signalling. Cholesterol enrichment had a dramatic effect on HDL-induced eNOS activation, while shear stress- and VEGF-induced eNOS activity was not significantly increased, suggesting that non-caveolar lipid rafts are 7KC-sensitive, while membrane domains at FA are not. Different membrane domains in endothelial cells are differentially modulated by cholesterol enrichment, initiating signalling pathways and eNOS activation. This research contributes to the understanding of the function of distinct endothelial cell membrane lipid domains in mediating signalling responses.