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 O2-, and OxLDL stimulates vascular O2- formation. Here we studied the role of NADPH oxidase, a potential source for O2- formation after OxLDL stimulation, in cell proliferation, and we investigated whether OxLDL influences anti-apoptotic genes in cultured human umbilical vein endothelial cells (HUVEC). Methods and Results. OxLDL dose-dependently (10 to 300 microg/mL) stimulated O2- formation in HUVEC (detected by cytochrome c assay and by chemiluminescence of lucigenin). Low OxLDL concentrations (5 to 10 microg/mL) induced proliferation (detected by 3H-thymidine incorporation), while higher concentrations (50 to 300 microg/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 micromol/L), an inhibitor of the O2- generating NADPH oxidase. In addition, cells transfected with antisense oligonucleotides for NADPH oxidase showed a significantly reduced O2- 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 O2- formation, with NADPH oxidase being a major source for O2-. Thus, OxLDL contributes importantly to vascular cellular turnover through the induction of oxidative stress.