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Protective Effect of Low Concentrations of Oxidized Low-Density Lipoprotein on Endothelial Cell Integrity

Background: Increasing evidence suggests that oxidized low-density lipoprotein (LDL) is associated with the development of atherosclerosis in vivo. Because endothelial injury contributes to the development of the atherogenic plaque, we investigated the effect of oxidized LDL on the integrity of human umbilical cord endothelial cells (HUVECs) by analyzing cytotoxicity and cell detachment. Methods: The cellular integrity of cultured endothelial cells was evaluated using cytotoxicity and detachment assays of cells labeled with chromium-51. Results: Low concentrations of oxidized LDL (25–50 μg protein/ml) induced morphological changes (cell elongation and formation of gaps in the cobblestone monolayer), decreased cellular cytotoxicity (by 13% compared with control), and also decreased cell detachment (by 22% compared with control). At higher concentrations of oxidized LDL (100 μg protein/ml), however, increases in cytotoxicity (by up to 70%) and cellular detachment (by up to 90%) were demonstrated. Native LDL, which was not oxidized in our cell system, did not induce any changes either in cytotoxicity or in cell detachment. The protective effect of low oxidized LDL concentrations against endothelial cell cytotoxicity and detachment was abolished by indomethacin (2μM), indicating the involvement of prostaglandin synthesis in this protection. Conclusion: Our experiments suggest that oxidized LDL-induced alterations of endothelial cells involve a sequence of events leading from a non-cytotoxic protective stage to endothelial perturbation.