Dual effect of oxidized LDL on cell cycle in human endothelial cells through oxidative stress

Dual effect of oxidized LDL on cell cycle in human endothelial cells through oxidative stress. Oxidized low-density lipoprotein (OxLDL) exerts proliferation and apoptosis in vascular cells, depending on its concentration and the exposure time. Various steps in the cell cycle and in the apoptotic signaling cascade are modulated by O−2, and OxLDL stimulates vascular O−2 formation. Here we studied the role of NADPH oxidase, a potential source for O−2 formation after OxLDL stimulation, in cell proliferation, and we investigated whether OxLDL influences anti-apoptotic genes in cultured human umbilical vein endothelial cells (HUVEC). OxLDL dose-dependently (10 to 300 μg/mL) stimulated O−2 formation in HUVEC (detected by cytochrome c assay and by chemiluminescence of lucigenin). Low OxLDL concentrations (5 to 10 μg/mL) induced proliferation (detected by3H-thymidine incorporation), while higher concentrations (50 to 300 μg/mL) induced apoptotic cell death (detected by Annexin assay and DNA fragmentation). Proliferation was blocked by the antioxidants SOD and catalase and by diphenyleneiodonium (10 μmol/L), an inhibitor of the O−2 generating NADPH oxidase. In addition, cells transfected with antisense oligonucleotides for NADPH oxidase showed a significantly reduced O−2 formation after stimulation with OxLDL. The OxLDL effect on apoptosis was also blocked by antioxidants. Since endothelial cells are protected against apoptosis through anti-apoptotic genes, we investigated whether OxLDL overcomes protection against apoptosis through suppression of the anti-apoptotic gene A20, a zinc-finger protein. OxLDL suppressed the expression of A20 in a dose-dependent manner. These data indicate that OxLDL has a dual effect on cell cycle in HUVEC, inducing proliferation at low and apoptosis at higher concentrations. Both effects are mediated by O−2 formation, with NADPH oxidase being a major source for O−2. Thus, OxLDL contributes importantly to vascular cellular turnover through the induction of oxidative stress.