Reactive oxygen species (ROS) has been implicated in the development of atherosclerotic cardiovascular disease. In normal condition, ROS is continuously cleared by multiple antioxidant enzymes, such as superoxide dismutase (SOD), glutathione peroxidase (GPx) and glutathione reductase (GR), and several biological antioxidants including reduced glutathione (GSH). Low density lipoprotein (LDL) is a classical risk factor for atherosclerosis. Oxidation or glycation increases the atherogenicity of LDL. The present study examined the effects of oxidized LDL (ox-LDL) and glycated LDL (gly-LDL) on the levels of superoxide, hydrogen peroxide (H₂O₂), reduced/oxidized glutathione (GSH/GSSG), and the activities of SOD, GPx and GR in cultured human umbilical vein endothelial cells (HUVEC). LDL from healthy donors was oxidized with 5 *M cupric sulfate or glycated by 50 mM glucose for 2 weeks. The cells were treated with vehicle or 100 *g/mL of native LDL, ox-LDL or gly-LDL. Treatments with LDL or its modified forms for 24 h elevated the levels of superoxide released from EC compared to vehicle. Ox-LDL induced significantly greater increase in superoxide release than native or gly-LDL did. Generation of H₂O₂ was increased after 1-6 h of incubation with LDL or its modified forms with a peak around 2 h. The release of H2O2 was decreased following longer incubation up to 72 h. The treatment with LDL, ox-LDL or gly-LDL for 24 h increased the activities of SOD and GPx in EC. LDL reduced GSH/GSSG ratio in EC. Ox-LDL and gly-LDL induced greater decreases in GSH/GSSG in EC compared to LDL. The activity of GR in EC treated with LDL or ox-LDL for 24 h was significantly lower than that in control cultures. Gly-LDL induced greater decrease in GR activity than LDL. The findings indicated that LDL, particularly its oxidized or glycated forms, stimulated the generation of superoxide from EC, that was associated with up-regulation of the activity of SOD and GPx, the down-regulation of GR activity and GSH/GSSG ratio. The changes in ROS and glutathione redox system in EC may contribute to the development of endothelial dysfunction in patients with dyslipidemia and diabetes.

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