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

BACKGROUND

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.

CONCLUSION

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.

CyA and OxLDL cause endothelial dysfunction in isolated arteries through endothelin-mediated stimulation of O(2)(-) formation

BACKGROUND

Cyclosporin A (CyA) and oxidized low-density lipoprotein (OxLDL) cause endothelial dysfunction, partly through stimulation of O(2)(-) formation (which can inactivate nitric oxide). We investigated whether CyA and OxLDL potentiate their influence on oxidative stress, whether endothelin (ET) is a mediator of CyA- and OxLDL-induced O(2)(-) formation, and whether enhanced oxidative stress results in further attenuation of endothelium-dependent vasodilation.

METHODS AND RESULTS

Human LDL was oxidized by Cu(++). O(2)(-) formation of isolated rat aortic rings was measured using a chemiluminescence assay. Incubation (60 min) of aortic rings with CyA (10 ng-10 microg/ml) or with OxLDL (300 microg/ml) caused a significant, dosedependent increase of the basal O(2)(-) formation. Pretreatment of the aortic rings with CyA (10 ng/ml) further enhanced the OxLDL-induced O(2)(-) formation by factor 1.9. The enhancement of the OxLDL-induced stimulation of O(2)(-) formation by CyA could be completely blocked by BQ123, a selective endothelin-1 (ET-1) receptor antagonist. Likewise, exogenously applied ET-1 (1 nM) potentiated the OxLDL-induced O(2)(-) formation by factor 1.8. Endothelium-dependent dilation was measured in isolated rings of rabbit aorta superfused with physiological salt solution in an organ bath. Incubation of the aortic rings with CyA (10 microg/ml, 60 min) or with OxLDL (300 microg/ml, 60 min) alone did not attenuate endothelium-dependent dilations. However, coincubation of the aortic rings with CyA+OxLDL in the presence of diethyl-dithio-carbamate, an inhibitor of the endogenous superoxide dismutase, caused a 60% inhibition of acetylcholine-induced dilator responses.

CONCLUSIONS

Coincubation of isolated aortic rings with CyA and OxLDL causes a potent enhancement of vascular O(2)(-) formation. ET-1 seems to be mediator of the CyA-induced O(2)(-) formation. Enhanced oxidative stress results in further attenuation of endothelium dependent vasodilation.

Oxidative stress mediates apoptosis induced by oxidized low-density lipoprotein and oxidized lipoprotein(a)

We measured oxidative stress caused by lipoproteins in human umbilical vein endothelial cells and rabbit aorta. Oxidized lipoprotein(a) was the more potent stimulus over oxidized low density lipoprotein, and we believe that both influence the pathogenesis of atherosclerosis.

Lp(a) and LDL induce apoptosis in human endothelial cells and in rabbit aorta: role of oxidative stress

BACKGROUND

Atherogenic lipoproteins cause injury to the vascular wall in the early phase of atherogenesis. We assessed the effects of native (nLDL) and oxidized (oxLDL) low-density lipoprotein (LDL) and lipoprotein (a) [Lp(a)] on O2- formation and cell death in cultured human umbilical vein endothelial cells (HUVECs) and rabbit aorta (RA).

METHODS AND RESULTS

O2- formation of HUVECs and RA segments was not influenced by nLDL, but was dose dependently increased by oxLDL and was moderately increased by nLp(a). oxLp(a) was the most potent stimulus for O2- formation, increasing it in HUVECs by 356% at 5 micrograms/ml and in RA by 294% at 100 micrograms/ml. Apoptosis was detected by DNA fragmentation and Annexin assay in HUVECs and by TUNEL staining in RA. Incubation of HUVECs and RA with oxLDL, but not nLDL, dose and time dependently induced apoptosis with only a minimal effect on necrosis. nLp(a) elicited a small but significant effect on apoptosis, whereas oxLp(a) induced apoptosis more potently than oxLDL in HUVECs and RA and caused necrotic cell death in HUVECs. Induction of apoptosis by oxLDL and oxLp(a) in RA was enhanced by the superoxide dismutase (SOD) inhibitor, diethyl-dithio-carbamate, and was blunted by SOD and catalase in HUVECs and RA, suggesting that O2- formation was involved. The concentration of lysophosphatidylcholine, a lipoprotein oxidation product and stimulus for O2- formation, was significantly enhanced by factor 5 in oxLDL and by factor 7 in oxLp(a) compared with native lipoproteins.

CONCLUSION

Atherogenic lipoproteins stimulate O2- formation and induction of apoptosis in HUVECs and RA, and may thereby influence the pathogenesis of atherosclerosis.

Lipids and progression of renal disease: role of modified low density lipoprotein and lipoprotein(a)

Atherogenic lipoproteins accumulate in the arterial wall as well as within the glomerulus and may accelerate vascular and glomerular injury. We therefore assessed whether oxidized low density lipoprotein (LDL) and lipoprotein(a) [Lp(a)] influence three major systems: (i) endothelium-dependent vasodilation, (ii) renin release of juxtaglomerular (JG) cells, and (iii) proliferation and viability of mesangial cells (MC). Lipoproteins were prepared from human plasma. Renal arteries were obtained from rabbits and JG as well as MC cells from mouse, rat and human kidneys. Dilator responses were detected in isolated arterial segments by a photoelectric device. Renin activity of JG cells was measured in culture supernatants and cells and DNA synthesis by 3H-thymidine incorporation in MC. Acetylcholine-induced, endothelium-dependent dilator responses of renal arteries were not significantly attenuated after incubation with native Lp(a). However, exposure to in vitro oxidized Lp(a) suppressed dilator responses in a dose-dependent manner. Using a chemiluminescence assay, we could detect increased O2- production by arteries pretreated with oxidized Lp(a), which suggested that enhanced nitric oxide (NO) inactivation by O2- might be the underlying mechanism of impairment of endothelium-dependent dilations. In general, oxidized Lp(a) was far more potent than oxidized LDL in this effect. In JG cells, both oxidized LDL and Lp(a) dose-dependently stimulated renin release. Coincubation with HDL significantly suppressed oxidized LDL and Lp(a) stimulated renin release and O2- production. In MC native and oxidized Lp(a) were poor ligands for the LDL receptor, but bound more tightly to extracellular matrix than native LDL. Native and oxidized Lp(a) elicited proliferation or toxicity of MC in a dose-dependent fashion. Stimulation of DNA synthesis in MC or renin release in JG cells was partly blunted or eliminated when cells were incubated with oxidized LDL and Lp(a) in the presence of superoxide dismutase and catalase, enzymes removing O2- and H2O2. These dat suggest a common underlying mechanism. Atherogenic lipoproteins induce formation of oxygen radicals not only in arteries, but also in glomeruli and JG cells, causing an inhibition of nitric oxide mediated vasodilation, stimulation of renin release, and modulation of mesangial cell growth and proliferation. The damaging effect of the lipoproteins can be prevented by antioxidative enzymes and HDL.

Effect of HDL and atherogenic lipoproteins on formation of O2- and renin release in juxtaglomerular cells

Atherogenic lipoproteins and reactive oxygen species stimulate renin release from isolated juxtaglomerular (JG) cells. Here we assessed whether stimulation of renin release is mediated by formation of superoxide anion (O2-), and whether the effects of oxidized lipoproteins, like in many other biological systems, can be prevented by the antiatherogenic high density lipoprotein (HDL). Lipoproteins were prepared from human plasma, and JG cells from mouse and rat kidneys. Basal renal activity of JG cells was measured in culture supernatants and cells, and was dose-dependently and significantly stimulated by oxidized LDL (50 and 300 micrograms/ml) and by oxidized Lp(a) (1, 10 and 30 micrograms/ml). Administration of HDL alone had no effect on renin release. However, coincubation with 100 micrograms/ml HDL significantly suppressed oxidized LDL- and oxidized Lp(a)-stimulated renin release. O2- production of JG cells was directly measured using a chemiluminescence assay. Stimulation with 10 micrograms/ml oxidized LDL and oxidized Lp(a) significantly increased the O2- generation of JG cells. In the presence of 5 micrograms/mL HDL, O2- production was reduced to control levels. These data indicate that stimulation of JG cells with oxidized LDL and Lp(a) induces formation of O2-, which may stimulate renin release in an autocrine fashion. Renin release can be prevented by HDL, presumably by preventing the formation of O2-.