Oxidized Low-Density Lipoprotein Stimulates p53- Dependent

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Endocrinology 148(5):2085–2094 Copyright © 2007 by The Endocrine Society doi: 10.1210/en.2006-1709

Oxidized Low-Density Lipoprotein Stimulates p53Dependent Activation of Proapoptotic Bax Leading to Apoptosis of Differentiated Endothelial Progenitor Cells Jizhong Cheng, Ruwen Cui, Chu-Huang Chen, and Jie Du Department of Internal Medicine, Baylor College of Medicine, Houston, Texas 77030 Dyslipidemia increases the risks for atherosclerosis in part by impairing endothelial integrity; endothelial progenitor cells (EPCs) play a pivotal role in reendothelialization. In this study, we investigated the mechanism whereby oxidized lowdensity lipoprotein (oxLDL) affects the function of differentiated EPCs (EDCs). In EDCs expanded in vitro from EPCs isolated from human cord blood, we measured EDC responses to both copper-oxidized LDL and L5, an electronegative LDL minimally oxidized in vivo in patients with hypercholesterolemia. OxLDL induced apoptosis of EDCs and impaired their response to nitric oxide. We found that the key to oxLDLinduced apoptosis in both EDCs and endothelial cells is the induction of a conformational change of Bax, leading to Bax activation without altering its expression. The conformationally changed Bax translocated to the mitochondria and stimulated apoptosis, as Bax knockdown prevented oxLDL-induced apoptosis in EDCs. The activation of Bax is mediated by

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NDOTHELIAL REGENERATION PLAYS an important role in limiting smooth muscle cell proliferation and restoring vascular tone in response to vascular injury. Because of its importance, the mechanism underlying endothelial regeneration has been a focus of research in atherosclerosis. Mature endothelial cells have a very low regenerative capacity, compared with circulating endothelial progenitor cells (EPCs), which can proliferate, migrate, and differentiate into mature endothelial cells (1, 2). EPCs have been shown to enhance the formation of a new endothelium in animal models, in which vessel injury occurs after balloon injury, myocardial infarction, or heart transplantation. EPCdependent reendothelialization also contributes to diminished neointimal hyperplasia (3, 4). Thus, the presence of healthy EPCs is crucial to postevent vascular reconstruction. Clinical observations suggest an inverse correlation between First Published Online February 8, 2007 Abbreviations: DCF, 2⬘,7⬘-Dichlorofluorescein; DCFH-DA, 2⬘,7⬘-dichlorofluorescein diacetate; EDC, differentiated endothelial progenitor cell; EGM-2, endothelial growth medium-2; eNOS, endothelial nitric oxide synthase; EPC, endothelial progenitor cell; FACS, fluorescenceactivated cell sorter; HAEC, human aortic endothelial cell; LDL, lowdensity lipoprotein; NAC, N-acetyl-cysteine; OxLDL, oxidized low-density lipoprotein; PECAM, platelet endothelial cell adhesion molecule; ROS, reactive oxygen species; siRNA, small interfering RNA; SOD, superoxide dismutase; TBARS, thiobarbituric acid-reactive substance; TTFA, 4,4,4-trifluoro-1-(2-thienyl)-1,3-butanedione; TUNEL, transferase-mediated dUTP nick end labeling; vWF, von Willebrand factor. Endocrinology is published monthly by The Endocrine Society (http:// www.endo-society.org), the foremost professional society serving the endocrine community.

an increase in p53 and knockdown of p53 abolished oxLDLinduced activation of Bax and apoptosis. OxLDL activated p53 through production of mitochondria-derived reactive oxygen species. In EDCs treated with a recombinant adenovirus expressing superoxide dismutase or N-acetyl-cysteine (but not catalase), the p53-Bax pathway activated by oxLDL was blocked, and apoptosis was prevented. Of importance, treatment of EDC with low-concentration L5 stimulated superoxide dismutase expression, which significantly attenuated apoptosis in EDCs exposed to high-concentration L5. These findings suggest that exposure of EDCs and endothelial cells to either experimentally prepared or naturally occurring modified LDL results in an increased transfer of mitochondria-derived superoxide anion to p53, which stimulates a conformational change in Bax favoring its translocation to the mitochondria with resultant apoptosis of these cells. (Endocrinology 148: 2085–2094, 2007)

dyslipidemia and endothelial function. Functional impairment of the endothelium and the associated neointimal formation after percutaneous transluminal coronary angioplasty or coronary artery bypass graft surgery are accelerated in patients with diabetes or hyperlipidemia (5, 6). When the circulating level of oxidized low-density lipoprotein (oxLDL) is elevated, it represents an independent risk factor for acute cardiac events. Several groups have shown that oxLDL is likely an important factor that adversely influences the growth and bioactivity of EPCs (7–9). For example, oxLDL can act through its receptors, such as the scavenger receptor and the lectin-like oxidized lowdensity lipoprotein receptor-1 (10, 11) to cause apoptosis in both mature endothelial cells and EPCs. The processes associated with apoptosis are reported to involve down regulation of endothelial nitric oxide synthase (eNOS) (8), activation of Fas/ FasL (12), reactive oxygen species (ROS) (13), or mitochondrial depolarization releasing cytochrome c and activation of caspases (14, 15). Because EPCs are involved in endothelial regeneration and because oxLDL can inhibit the bioactivity of differentiated EPCs (EDCs) (7, 8), we decided to determine whether there is a defined pathway by which oxLDL impairs the function of well-differentiated EPCs. To achieve this, we isolated EPCs and expanded them into EDCs. The cells were then treated with either copper-oxLDL or an electronegative and minimally oxidized low-density lipoprotein (LDL) circulating in patients with hypercholesterolemia. After confirming their apoptotic response to these lipoproteins, we investigated the mechanism to define a specific signaling pathway mediating these effects.

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Materials and Methods Cell culture Mononuclear cells from human umbilical cord blood were isolated by density-gradient centrifugation with Lymphocyte-H buffer (Cedarlane Lab., Ontario, Canada). CD133⫹ cells were twice purified using a Direct CD133⫹ Progenitor Cell Isolation Kit (Miltenyi Biotec Inc., Bergisch Gladbach, Germany) according to the manufacturer’s instructions. Purity (routinely ⬎90%) was determined by fluorescence-activated cell sorter (FACS) analysis and immunostaining. Cells were cultured on fibronectin-coated dishes in endothelial growth medium-2 (EGM-2) supplemented with EGM SingleQuots (Cambrex, Inc., East Rutherford, NJ) for 5–7 d, and adherent cells were pooled and continuously cultured in EGM-2 (experiments conducted within passage 5). Control human aortic endothelial cells (HAECs) were grown in EGM-2 medium at 37 C with humidified 95% air/5% CO2.

Immunostaining Attached CD133⫹ cells were rinsed with Hanks’ buffered saline solution, fixed with 4% paraformaldehyde for 10 min at 37 C, and stained with antihuman antibodies recognizing VE-cadherin, platelet endothelial cell adhesion molecule (PECAM), or smooth muscle ␣-actin. After reacting with fluorescent dye-conjugated secondary antibodies, cells were visualized by fluorescence microscopy using FITC or rhodamine excitation/emission filter combinations.

Flow cytometry and apoptosis ⫹

Cultured CD133 cells and control HAECs were collected in Cell Dissociation Buffer (Invitrogen, Carlsbad, CA), washed twice in ice cold FACS buffer containing phosphate buffer (pH 7.2) with 5 mm EDTA and 5% FBS before being suspended, approximately 2 ⫻ 105 cells/100 ␮l, in FACS buffer. They were incubated with a primary antibody for 30 min at 4 C and, after two washes with a FACS buffer, the secondary antibody was added and incubated for 20 min at 4 C. After two washes with a FACS buffer, cells were suspended in a 500-␮l volume for flow cytometric analysis (FACSCalibur flow cytometer; Becton Dickinson, San Diego, CA). The expression of VE-cadherin, PECAM, CD34, and CD133 (antibodies from BD PharMingen, San Diego, CA) was detected by immunostaining or by FACS, using isotype-matched antibodies served as controls. Apoptosis was evaluated as annexin V-positive cells by FACS analysis according to the manufacturer’s protocol (Annexin V Kit; Roche, Indianapolis, IN).

Lipoprotein and oxLDL preparation Native LDL (density ⫽ 1.019 –1.063 g/ml) was separated from fresh normal human plasma by sequential ultracentrifugation as described (16). Briefly, LDL was isolated by ultracentrifugation at 290,000 ⫻ g for 4 h in a sodium bromide (NaBr) gradient, and the top layer (containing LDL) was collected. Protein concentration was measured using the ABC protein assay (Bio-Rad, Inc., Hercules, CA). The LDL was dialyzed for 24 h at 4 C against PBS containing 1 mm EDTA (pH 8.0) with three changes of the dialysate. EDTA was removed by chromatography (BioRad, Inc.) and LDL was oxidized by incubation in 5 ␮m CuSO4 for 4 h at 37 C. The extent of oxidation was assessed by the measuring thiobarbituric acid-reactive substances (TBARS) and by electrophoretic mobility on agarose gels in a barbital buffer (pH 8.6). The oxLDL we studied had a TBARS value of 20.6 –36.3 nmol/mg of LDL protein; native LDL had no detectable TBARS. To validate the observations made with copper-oxidized LDL, we performed additional experiments using L5, an electronegative and minimally oxidized LDL circulating in patients with hypercholesterolemia (17). Hypercholesterolemic (LDL cholesterol ⬎ 160 mg/dl) human LDL was divided into L1–L5 subfractions by fast protein liquid chromatography as previously described (17–19). Concentration- and timedependent effects of L5 on EDCs and endothelial cells were examined. L1, the nonelectronegative and nonoxidized subfraction, was used for comparison.

Cheng et al. • OxLDL and Apoptosis of EDCs

NO-dependent isometric studies of vessels seeded with EDCs EDCs and HAECs were labeled with PKH26 (Sigma-Aldrich, St. Louis, MO), a permanent red fluorescent dye that is retained in cells for up to 100 doublings (20). Labeled cells were trypsinized to a single-cell suspension, rinsed with PBS, and counted. Cells were gently resuspended at 2 ⫻ 106 cells/100 ␮l of diluent C (provided with the PKH26 dye). An equal volume of 40 ␮m PKH26 dye was added and incubated for 2–3 min with gentle agitation. Cell labeling was terminated by adding 2 vol of FCS and 7 ml of 0.1% BSA in PBS. The cells were layered onto 3 ml of FCS, centrifuged, rinsed, resuspended in fresh media, and incubated overnight at 37 C. To seed EDCs or control HAECs onto denuded vessels, the endothelium from freshly isolated sheep carotid artery strips was removed gently using cotton swabs, and 0.5 ml of 1 ⫻ 105 of cells prelabeled with PKH26 were pipetted onto a denuded vessel strip (1–1.5 cm ⫻ 3 cm) for a 24-h incubation. Unattached cells were removed by gentle washing. The amount of attached cells was adequate for complete covering of the denuded artery strip. The EDC- or HAEC-seeded carotid artery strips were suspended between two tungsten stirrups in an organ-chamber bath containing oxygenated Krebs buffer at 37 C. The vessels were allowed to relax for 1 h before isometric tension was measured (21). After stimulating contraction of the vessels with 0.4 ␮g/ml U46619 (SigmaAldrich) for 10 min, NO-mediated relaxation of the EDC-covered vessels was evaluated after adding the calcium ionophore, A23187 (SigmaAldrich) (1 mm). Matched, denuded vessel strips were used as a control. In some experiments, native LDL or oxLDL (100 ␮g/ml) was also added.

RT-PCR detection of p53 expression RNA was isolated using the Qiagen total RNA isolation kit (Qiagen Inc., Valencia, CA). cDNA was synthesized using SuperScript FirstStrand Synthesis System of RT for PCR kit (Invitrogen), and 1 ␮l cDNA was used as a template for PCR (Roche). Primers for amplifying p53 were: 5⬘-AACCTACCAGGGCAGCTACG-3⬘ (forward), 5⬘-TTCCTCTGTGCGCCGGTCTC-3⬘ (reverse), generating a 559-bp fragment; ␤-actin: 5⬘-GTC GAC AAC GGC TCC GGC AT-3⬘ (forward), 5⬘-GTC AGG TCC CGG CCA GCC AG-3⬘ (reverse), generating 533-bp fragment. A PCR profile with initial denaturation for 2 min at 94 C; followed by 16 cycles (p53) of 1 min at 94 C, 55 C, and 72 C; and a final extension step for 10 min at 72 C was performed on Genius thermal cycler (Techne Inc., Cambridge, UK); amplification products were separated on a 1.5% agarose gel and stained with ethidium bromide.

Transfection of small interfering RNAs (siRNAs) in EDCs Smart-pool siRNAs to human p53, Bax or control luciferase were purchased from Dharmacon (Chicago, IL). Cells were transfected with the siRNA using TransIT-TKO kit (Mirus Bio Cooperation, Madison, WI) according to the manufacturer’s protocol.

Intracellular ROS production and mitochondrial ROS localization analysis The oxidation of 2⬘,7⬘-dichlorofluorescein diacetate (DCFH-DA) to 2⬘,7⬘-dichlorofluorescein (DCF) (Sigma-Aldrich) was used to estimate the level of ROS (11). Confluent EDCs on coverslips were incubated with 2 ␮m DCFH-DA for 1 h in EGM-2 complete media before oxLDL was added. Cell fluorescence was analyzed with Metamorph Imagine Software. To assess the effect of antioxidants on ROS production, 2 mm N-acetyl-cysteine (NAC; Sigma-Aldrich) was added for 6 h before oxLDL treatment.

Imaging of DCF fluorescence The subcellular source of ROS production was assessed with a Zeiss LSM-510 inverted laser scanning confocal microscope. Cells were incubated with DCFH-DA (2 ␮m) for 60 min, and subsequently treated with various concentrations of oxLDL. Cells were also incubated with MitoTracker deep red 633 (Molecular Probes, Eugene, OR) (0.25 ␮m) for 15 min before being imaged at 1-min intervals. The green fluorescence of DCF-DA (excitation 488 nm, emission 505–530 nm) and the red fluo-


rescence of MitoTracker Red CMXRos (excitation 579 nm, emission 599 nm) were observed after excitation with an argon-krypton laser. Laser excitation was set (5–10%) to produce minimal photooxidation of the dye. Images were merged and processed using IPLab Spectrum and Adobe Photoshop (Adobe Systems) software. To examine the effects of mitochondrial complex inhibitor, cells were preincubated with 10 ␮m 4,4,4-trifluoro-1-(2-thienyl)-1,3-butanedione (TTFA) for 30 min before application of oxLDL.

Determination of changes in Bax conformation The conformational change in Bax that occurs after exposure to 100 ␮g/ml oxLDL was assessed as described (22). EDCs and HAECs were serum-starved and incubated with oxLDL for 24 h. They were lysed in CHAPS lysis buffer [10 mm HEPES (pH 7.4), 150 mm NaCl, 1% CHAPS, and 1 ␮g/ml protease inhibitors]. The lysates were centrifuged (15,000 ⫻ g, 4 C for 10 min) and 300 ␮g protein was incubated overnight at 4 C with monoclonal antibody 6A7 (Sigma-Aldrich). This antibody recognizes the buried epitope comprised of amino acids 13–19 after it is exposed by a conformational change in Bax. An immunoprecipitate of Bax was obtained by adding 50 ␮l antimouse, IgG1-conjugated, agarose beads (Sigma-Aldrich). The mixture was incubated overnight at 4 C. After extensive washing with CHAPS lysis buffer and boiling, released proteins were separated by SDS-PAGE (12% gel). To assess the level of activated Bax, we used a polyclonal antibody recognizing Bax (1:500; Santa Cruz Biotechnology Inc., Santa Cruz, CA) followed by incubation with fluorescent IRDye 800-conjugated antirabbit secondary antibody (Rockland, Gilbertsville, PA) and detected by laser scanner, Odyssey (Licor Inc., Lincoln, NE). Another portion of the cell lysate was used to assess total Bax level using the same polyclonal antibody. The membranes were reprobed with an antiactin antibody (Santa Cruz Biotechnology Inc.) for additional loading control.

Bax translocation to mitochondria EDCs were transfected with Bax-GFP expression plasmid using Amaxa Nucleofector endothelial transfection kit (Amaxa Inc., Gaithersburg, MD). Bax translocation to the mitochondria in EDCs was monitored by observing a Bax-GFP fusion protein using a confocal micro-


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scope. The Bax-GFP expression plasmid was kindly provided by Dr. H. Wang (University of South Florida, Tampa, FL).

DNA fragmentation and transferase-mediated dUTP nick end labeling (TUNEL) assays Histone-associated DNA fragments in EDCs were quantitated by Cell Death ELISA (Roche) according to manufacturer’s protocol. The TUNEL assay was performed using DeadEnd Fluorometric TUNEL System kit (Promega Corp., Madison, WI). Each experiment was carried out in triplicate and repeated independently at least three times.

Statistics All data are presented as mean ⫾ sem. Comparisons between groups were made using one-way ANOVA. A P ⬍ 0.05 was considered statistically significant.

Results Isolation and characterization of EDCs from cord blood CD133⫹ cells

The purity of isolated CD133⫹ mononuclear cells from human umbilical cord blood was over 90%. Adherent cell clusters appeared after 1 wk in culture. They took up Di-LDL (acetylated low-density lipoprotein labeled with 1,1⬘-dioctadecyl-3,3,3⬘,3⬘-tetramethylindo-carbocynanine perchlorate) and bound lectins (Fig. 1A). Immunostaining, FACS, and immunoblotting assays indicated positive signals for eNOS, PECAM, VE-cadherin, von Willebrand factor (vWF), and Flk-1 (Fig. 1, A–C), confirming their endothelial cell characteristics. They did not express smooth muscle ␣-actin (Fig. 1, A and C). FACS analysis demonstrated that early passage EDCs exhibited a lower level of CD34 and loss of CD133 expression, suggesting that EPCs had become well differen-

FIG. 1. Characterization of EDCs. A, CD133⫹ mononuclear cells were seeded on fibronectin-coated dishes and cultured for 2 wk. Cells were stained with Di-LDL, lectin, PECAM, VE-cadherin, vWF, and smooth muscle ␣-actin, and nuclei were counterstained with 4⬘,6-diamidino-2phenylindole (blue color). B, Representative FACS analysis of EDCs and HAECs after being stained with Flk-1, VE-cadherin, CD34, or CD133. The shaded area represents isotype controls. C, Protein levels of cell markers in EDCs, HAECs, and vascular smooth muscle cells (VSMC). D, EDC-seeded vessels regain their relaxation response to the calcium ionophore, A23187. Common carotid arteries were collected from sheep and denuded. EDCs were seeded onto the denuded vessel for 24 h. The vascular relaxation response to A23187 was detected in the presence of native LDL (nLDL, 100 ␮g/ml) or oxLDL (100 ␮g/ml) (*, P ⬍ 0.05, compared with each other; n ⫽ 3).

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tiated. By comparison, HAECs expressed the marker of endothelial cells but not CD34 or CD133 (Fig. 1, B and C). Effects of oxLDL on EDC bioactivities

We next evaluated the bioactivity of EDCs by examining NO-dependent vascular relaxation of an EDC-seeded, sheep carotid artery in response to oxLDL. EDCs or HAECs (1 ⫻ 105 cells) were seeded onto denuded arteries (1 ⫻ 3 cm) in 0.5 ml media. After 24 h, U46619, a thromboxane analog, was added and, after measuring contraction, the artery strips were exposed to the calcium ionophore, A23187, to induce endothelium-dependent NO arterial relaxation. A23187 induced a 40 ⫾ 7% relaxation of the contracted vessel vs. 17 ⫾ 3% relaxation by unseeded, denuded vessels (Fig. 1D). As a positive control, seeding of HAECs results in a similar endothelium-dependent relaxation (data not shown). In the presence of a pathological dose of oxLDL (100 ␮g/ml), the A23187-induced relaxation was completely abolished in EDC-seeded denuded vessel (Fig. 1D). Thus, the EDCs we isolated not only expressed endothelial cell markers but also exhibited functional characteristics of endothelial cells. OxLDL impaired such bioactivities of EDCs.


Proapoptotic effects of oxLDL and L5 on EDCs

Adding oxLDL to EDCs or control HAECs increased annexin V-positive cells (an early marker of apoptosis), as assessed by FACS analysis (oxLDL vs. native LDL-treated EDCs: 36 ⫾ 4.2 vs. 5.1 ⫾ 2.0%, n ⫽ 5; P ⬍ 0.001; Fig. 2A). A DNA fragmentation analysis showed that oxLDL induced apoptosis in both EDCs and HAECs in a concentration-dependent manner (100 ␮g/ml oxLDL increased DNA fragmentation 8.01 ⫾ 0.43- and 3.75 ⫾ 0.11-fold vs. nontreated controls, respectively; Fig. 2B). Exposing cells to the same concentrations of native LDL did not induce apoptosis. For subsequent experiments, 100 ␮g/ml oxLDL was used. Consistent with previously reported results for endothelial cells (17–19), incubation of EDCs with 50 ␮g/ml L5 resulted in marked DNA fragmentation analyzed by the TUNEL assay, whereas L1 had no effect (Fig. 2C). Moreover, L5 exhibited a concentration-dependent cytotoxic effect on EDCs similar to that of oxLDL. This was true under both serum-supplemented and serum-free conditions, although serum withdrawal enhanced the apoptotic effect. The increment of DNA fragmentation became significant at a concen-

FIG. 2. Proapoptotic effects of oxLDL and L5 on EDCs. A, Two-dimensional dot plots of copper-oxLDL-treated cells stained with annexin V and 7-amino-actinomycin D. B, DNA fragmentation analysis of oxLDL-induced apoptosis in both EDCs and HAECs. Cells were seeded into 24-well plates and incubated with/without 100 ␮g/ml copper oxLDL for 24 h. The amount of DNA fragmentation was detected with the Cell Death ELISA kit (*, P ⬍ 0.001, compared with untreated control; n ⫽ 5). C, TUNEL analysis of L5-treated EDCs. EDCs were seeded onto glass coverslips and incubated with L5 or L1 fraction (50 ␮g/ml) for 24 h, and the amount of DNA fragmentation was evaluated by TUNEL staining. TUNEL-positive and total cells were counted (shown on the top of bar graph is a representative of TUNEL staining). Four random areas were selected, and 100 –150 cells in each area were counted. Error bars indicate ⫾ SE of three independent experiments (*, P ⬍ 0.01, compared with control). D, Concentration-dependent analysis of DNA fragmentation in EDCs treated with L5 in both serum-containing and serum-free media. EDCs were seeded into 48-well plates and incubated with graded concentrations (0 –150 ␮g/ml) of L5 or L1 for 24 h. The amount of DNA fragmentation was detected with the Cell Death ELISA kit (*, P ⬍ 0.05, compared with untreated control; n ⫽ 3).



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tration of 50 ␮g/ml with serum withdrawal and further intensified at 100 ␮g/ml (Fig. 2D). Effects of oxLDL on conformational changes and mitochondrial translocation of Bax

A conformational change in cytosolic Bax precedes its translocation into the mitochondrial membrane, inducing release of cytochrome c and apoptosis (14, 15). In untreated cells, no activated Bax was detected (Fig. 3A), but oxLDL treatment of EDCs and HAECs led to a significant increase in activated Bax in these cells (Fig. 3A). Total Bax level was not changed in oxLDL-treated cells. The oxLDL-induced Bax mitochondrial translocation was examined in EDCs transfected with plasmid containing a Bax-GFP fusion protein gene. After transfection, EDCs were treated with native LDL or oxLDL for 12 h. Confocal microscopic imaging analysis showed that Bax-GFP fluorescence signal was evenly distributed in cytoplasm in native LDLtreated cells, but colocalized with the mitochondria to form a punctuated pattern after oxLDL exposure (Fig. 3B). Role of Bax activation in oxLDL-induced EDC apoptosis

To determine whether Bax activation is necessary for oxLDL-induced apoptosis in EDCs, we knocked down the level of Bax by transfecting EDCs with Bax siRNA. The Bax protein level was significantly reduced in EDCs transfected with Bax siRNA; Bax protein in control siRNA-transfected cells was unchanged (Fig. 3C, inset). We then determined the effect of knockdown Bax on oxLDL-induced apoptosis in EDCs. As shown in Fig. 3C, the percentage of apoptosis (TUNEL-positive cells) in oxLDL-treated EDCs transfected with control siRNA was significantly higher than that of native LDLtreated EDCs (13.6 ⫾ 2.5%, P ⬍ 0.01, n ⫽ 3). This oxLDLinduced increase in apoptosis was blocked (71 ⫾ 4% reduction) by Bax knockdown (Fig. 3C). Role of p53 in oxLDL-induced Bax mitochondrial translocation and apoptosis

We then examined whether oxLDL-induced Bax activation and apoptosis involved activation of p53. Treatment of EDCs with oxLDL increased p53 mRNA and protein expression (Fig. 4, A and B). To determine the role of oxLDL-induced p53 in Bax activation and apoptosis, we silenced p53 by transfection of EDCs with a p53-specific siRNA (Fig. 4C, inset). Down-regulation of p53 significantly attenuated oxLDL-induced apoptosis in EDCs as detected by a TUNEL analysis (Fig. 4C). OxLDL-induced Bax translocation to mitochondria was also abolished in EDCs transfected with p53 siRNA (Fig. 4D). These results indicate that p53 acts as an upstream mediator of oxLDL-induced apoptosis by activating Bax. Role of ROS as upstream mediators of oxLDL-induced apoptosis

In an effort to determine factors that activate p53, we exposed EDCs to 100 ␮g/ml oxLDL and discovered a significant increase in ROS production (Fig. 5A). Using confocal microscopy, we examined the source for ROS generation by observing oxidized DCF fluorescent-green product in cellu-

FIG. 3. OxLDL-induced apoptosis is Bax dependent. A, Induction of Bax conformational changes by oxLDL. Both EDCs and HAECs were cultured in the presence or absence of 100 ␮g/ml oxLDL. Activated Bax was detected by immunoprecipitation using monoclonal anti-Bax (Clone 6A7) antibodies and immunoblotting using anti-total Bax (shown in inset). Total Bax in cell lysates before immunoprecipitation was also detected. The bottom panel contains a quantitative analysis of activated Bax corrected by total Bax (mean ⫾ SE; *, P ⬍ 0.001, n ⫽ 3). B, Induction of Bax translocation to the mitochondria by oxLDL. EDCs were transfected with GFP-Bax expression plasmid, and were exposed to 100 ␮g/ml oxLDL for 12 h. The cells were then incubated with MitoTracker for 20 min. After fixation in 4% paraformaldehyde, the green (GFP-Bax) and red fluorescence (mitochondria) were observed under confocal microscope. C, Abrogation of oxLDL-induced apoptosis by siRNA-mediated Bax knockdown. Cells were seeded on a glass coverslip for 18 h and transfected with Bax or control luciferase siRNA (Luc siRNA) for 72 h. After exposure to 100 ␮g/ml OxLDL for another 24 h, cells were fixed with 4% paraformaldehyde and the TUNEL assay was performed. TUNEL-positive and total cells were counted. Four random areas were selected, and 100 –150 cells in each area were counted. Error bars indicate ⫾SE of three independent experiments (*, P ⬍ 0.01, compared with control). The inset shown on top of the bar graph is to demonstrate the knockdown of Bax; EDCs in six-well plates were transfected with Bax siRNA or control luciferase siRNA, and Western blots detected the total Bax level at 72 h; ␤-actin was used as loading control.

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FIG. 4. p53 mediates oxLDL-induced Bax activation and apoptosis. Cells treated with 100 ␮g/ml oxLDL for 3 (A) and 12 h (B), respectively, and total RNA and protein were used for RT-PCR (A) or Western blot (B). ␤-Actin was used as the internal quantitation standard (*, P ⬍ 0.05 vs. untreated control, n ⫽ 3). C, Inhibition of oxLDL-induced apoptosis by p53 knockdown. EDCs were seeded in 12-well plates and transfected with p53 or control siRNAs (to luciferase) followed by 24-h exposure to 100 ␮g/ml oxLDL, the cells were fixed, and TUNEL assay was performed. The percentage of apoptosis cells was calculated by counting the green cells in four random selected areas. Error bars indicate ⫾SE of three independent experiments (*, P ⬍ 0.05, compared with oxLDL-treated, luciferase siRNA-transfected, control, n ⫽ 3). The inset shown on the top of the bar graph is to demonstrate the knockdown of p53; cells were transfected with p53 siRNA or control luciferase siRNA, the p53 protein level was Western blot detected at 72 h. D, Inhibition of Bax translocation to the mitochondria by p53 knockdown. Cells were seeded on 12-well plates and transfected with Bax-GFP expression plasmid with p53 or control siRNAs. After 72 h, cells were treated with 100 ␮g/ml oxLDL and stained with 25 ␮g/ml MitoTracker red for 20 min. The cells were washed to remove nonincorporated MitoTracker dye, fixed, and visualized under confocal microscopy. Shown are representative pictures from three repeated experiments.

lar compartments. Although it is evident that the signals we obtained cannot be ascribed to a single mechanism, we were able to localize the source of DCF fluorescence within the cell. After 15 min of exposure to oxLDL, ROS was detected in the mitochondria within an additional 8 min (Fig. 5B). It has been shown that TTFA can quantitatively inhibit the mitochondrial component of ROS (23). When we treated EDCs with oxLDL in the presence of TTFA (10 ␮m), the increase in cellular ROS was eliminated (Fig. 5C). These results indicate that mitochondria are the primary site of oxLDL-induced ROS production. Addition of antioxidant NAC to the culture inhibited oxLDL-induced p53 expression; infection of EDCs with Adsuperoxide dismutase (SOD), a recombinant adenovirus expressing Cu-Zn SOD, resulted in similar inhibition (Fig. 5D). In contrast, expression of catalase (Ad-Cat) had no effect on p53 expression. Overexpression of Cu-Zn SOD in EDCs also suppressed oxLDL-mediated translocation of Bax to the mitochondria (Fig. 5E), as well as induction of DNA fragmentation (65 ⫾ 4%; Fig. 5F). These results suggest that oxLDLinduced EDC apoptosis is initiated by ROS-mediated p53 expression, leading to activation of the proapoptotic Bax. The negative response to catalase overexpression further suggests that superoxide anions but not H2O2 play a major role in oxLDL-induced apoptosis. It has been shown that preexposure to low-concentration

oxLDL may paradoxically protect endothelial cells against apoptosis provoked by high-concentration oxLDL (24). To examine whether LDL minimally modified in vivo has the same effect, we treated EDCs with 20 ␮g/ml L5 or L1 for 18 h, followed by 100 ␮g/ml L5 for an additional 24 h. Compared with L1, 20 ␮g/ml L5 significantly suppressed DNA fragmentation induced by the subsequent 100 ␮g/ml L5 (Fig. 5G). Further experiments showed that the protective effect of low-concentration L5 was associated with increased Cu-Zn SOD expression (Fig. 5H). Discussion

Accumulated evidence has strongly suggested a pivotal role of EPCs in postinjury regeneration of the endothelium (1, 2, 25). OxLDL reportedly inhibits the differentiation of stem cell to EPCs and induces apoptosis in EDCs. However, the mechanism by which oxLDL induces EDC apoptosis was not completely understood. We found evidence for a distinct pathway that explains how oxLDL mediates these responses. In both EDCs and HAECs, oxLDL stimulates superoxide anion production in the mitochondria to increase p53 expression and the subsequent activation of Bax. The activated Bax translocates into the mitochondria to release cytochrome c, eliciting the apoptotic reaction. It has been reported that that CD133⫹-expressing cells in



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FIG. 5. Stimulation of ROS generation in EDC mitochondria by oxLDL lead to Bax activation and apoptosis. A, Fluorescence detection of time-dependent ROS production induced by oxLDL. Confluent EDCs on coverslips were incubated with 10 ␮M DCFH-DA for 60 min in EGM-2 complete media before the addition of oxLDL. Cell fluorescence was recorded and analyzed with a Metamorph Imagine Software. B, Colocalization of ROS with mitochondria in oxLDL-treated EDCs. The fluorescence from untreated cells was the baseline; MitoTracker Red fluorescence (left panel) and DCF green fluorescence (middle panel) were recorded at 8 min, after exposure to 100 ␮g/ml oxLDL for 15 min. The colocalization of mitochondria and ROS fluorescence was shown by overlaying the confocal images (right panel). C, Effects of TTFA on oxLDL-induced ROS production. EDCs were pretreated with 2 ␮M DCFH-DA for 1 h and 10 ␮M TTFA for 30 min before being exposed to 100 ␮g/ml oxLDL. The green signals were acquired under fluorescence microscope. D, Inhibition of oxLDL-induced p53 overexpression by antioxidants. Cells were pretreated with 2 mM NAC or infected with Ad-SOD or Ad-Cat before being exposed to oxLDL. Western blot analysis was performed to detect p53 protein level. E, Confocal microscopic analysis of the effect of Ad-SOD on oxLDL induced mitochondrial translocation of Bax. EDCs were infected with Ad-SOD or control Ad-␤gal and transiently transfected with a GFP-Bax fusion plasmid for 24 h before treatment with oxLDL. Cells were fixed with 4% paraformaldehyde after incubation with 25 ␮g/ml MitoTracker for 20 min. Fluorescence was detected by confocal microscopy. Images are representative of three experiments. F, Differential counteracting effects of various antioxidants—SOD, NAC, catalase— on oxLDL-induced apoptosis. Cells were seeded in 24-well plates and infected with Ad-SOD, Ad-Cat, or Ad-␤-gal, or pretreated with 2 mM NAC for 6 h, followed by 100 ␮g/ml oxLDL treatment for 24 h. Cells were then lysed in lysis buffer and Death ELISA was performed (*, P ⬍ 0.01, n ⫽ 4; error bars are SEs). G, Differential protective effects of low-concentration L1 and L5 against apoptosis induced by high-concentration L5 in EDCs. Cells were seeded in 48-well plates and treated with 20 ␮g/ml L5 or L1 for 18 h, followed by 100 ␮g/ml L5 for 24 h. Apoptosis was analyzed by a Death ELISA (*, P ⬍ 0.05; n ⫽ 3). H, Effect of low-concentration L5 on Cu-Zn SOD expression in EDCs. Cells were treated with 5–25 ␮g/ml L5 for 18 h, and Cu-Zn SOD protein expression was assayed by Western blot analysis.

peripheral or umbilical cord blood contain EPCs (26). In our setting, CD133⫹ cells derived from the cord blood in VEGFcontaining media exhibited properties consistent with those of EDCs, including uptake of acetylated Di-LDL, binding of ulex-lextin, and expression of CD34, Flk1, VE-cadherin, and eNOS (27, 28). Similar to mature endothelial cells, these EDCs were also capable of inducing NO-dependent relaxation in denuded vessels. After culturing in vitro, these CD133⫹ cells

exhibited loss of CD133 and a lower level of CD34, indicating their transformation into more differentiated EPCs or EDCs. The proapoptotic effects of oxLDL on endothelial and vascular smooth muscle cells have been well documented (17, 29). Consistent with previous reports, our EDCs also underwent apoptosis when exposed to 100 ␮g/ml, but not lowconcentration (⬍25 ␮g/ml) oxLDL. Our data obtained from experiments using hypercholesterolemic L5 validates the

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findings made with copper-oxLDL. These findings also suggest that the effects induced by oxLDL was not a result of nonspecific cytotoxic properties of copper. OxLDL also impaired NO-dependent relaxation in denuded vessels. The pathway we identified acts through Bax, a member of multidomain Bcl-2 proapoptotic proteins. Bax has BH1, BH2, and BH3 domains, and its activation and translocation to the mitochondria has been shown to cause mitochondrial dysfunction and cell death by apoptosis (30). Similar cellular events occur in vivo. In a vascular injury model, the proportion of Bax-positive smooth muscle cells in the medial layer at 2 wk postinjury was increased markedly along with smooth muscle cell apoptosis (31). In EDCs and HAECs, we find that oxLDL did not increase the expression of Bax. Instead, oxLDL stimulated: 1) conformational and oligomerizational changes in Bax, and 2) mitochondrial translocation of Bax. Thus, Bax activation is a mediator in the pathway by which oxLDL induces apoptosis of EDCs. Our findings that Bax knockdown abolished oxLDL-induced EDC apoptosis further substantiated this mechanism. Whether circulating EPCs and EDCs may be exposed to oxidatively modified LDL in concentrations equivalent to those used in vitro by us and most other investigators is an important question. Unfortunately, a definite answer may be lacking on the basis of available data in the literature. As an attempt to answer this question, we may assume a plasma LDL-C range of 100 –200 mg/dl (1000 –2000 ␮g/ml) in a random cohort. A concentration of 50 ␮g/ml L5 would suggest a percentage of 2.5–5% of L5 in plasma LDL, an estimation close to the measured value in hypercholesterolemic human LDL (17, 19). Moreover, L5 may become more extensively oxidized when it is trapped in the subintimal area. Thus, we can further postulate that EPCs and EDCs are in contact with both minimally oxidized L5 in the plasma and extensively oxidized L5 in the subintima once they reside on the vascular bed. The postulated dual-exposure mechanism may potentially damage the tissue regenerative capacity of these cells. It has been reported that the mechanism underlying oxLDL-induced apoptosis in endothelial or EDCs involves the oxLDL scavenger receptor, lectin-like oxidized low-density lipoprotein receptor-1, activating cell signals that inhibit Akt activation and down-regulation of eNOS (7, 8, 32). It has also been shown that Bim activation plays a central role in oxidative stress-induced apoptosis in EPCs and EDCs (32). We have identified the downstream mediator of Akt or Bim, namely Bax (33, 34). Our finding that Bax mediates oxLDLinduced apoptosis incorporates both Akt and Bim signaling pathways in the ability of oxLDL to induce apoptosis in EDCs. OxLDL may also induce endothelial cell apoptosis in a Fas-dependent fashion (12). The pathway includes Fas engagement which results in cleavage of the Bcl-2 family protein, BID. This leads to Bax aggregation and mitochondrial translocation (30, 35, 36). Our results show that oxLDL activates Bax in EDCs and hence, this response may in part involve the downstream of the Fas ligand-activated death pathway. What is the signal by which oxLDL induces activation of Bax in EDCs? p53 can stimulate transcription of Bax and can activate Bax via a transcription-independent mechanism (37,


38). We found that oxLDL increased both p53 mRNA and protein levels. We also document the involvement of p53 in the pathway by showing that knockdown p53 significantly reduced oxLDL-induced Bax activation and apoptosis of EDCs (Fig. 3). Because oxLDL stimulated Bax activation without increasing Bax transcription, we conclude that oxLDL-mediated Bax activation results from a p53 transcription-independent mechanism. To determine how oxLDL induces the activation of p53/ Bax in EDCs, we examined the role of ROS. We found that antioxidant, NAC, prevented the effect of oxLDL on p53 and Bax. However, NAC could mediate an antioxidant effect that reacts with lipid mediators (39). To evaluate this possibility, we used a more specific antioxidant, a recombinant adenovirus expressing catalase or SOD. Because oxLDL-induced EDC apoptosis was prevented by SOD only, we conclude that superoxide anion is the ROS that mediate oxLDL-induced apoptosis. Moellering et al. (24) reported that lowconcentration oxLDL has a cytoprotective effect on apoptosis induced by high-concentration oxLDL in endothelial cells through induction of glutathione synthesis. In this study, we demonstrate a similar effect of circulating L5 on EDCs, which can probably be attributed to augmented expression of SOD, a result in accordance with that previously reported for oxLDL (40). This possible mechanism was further supported by the protective effects of adenovirus-mediated expression of SOD on EDCs. To identify the source of ROS, we turned to confocal florescent microscopy and found that the mitochondrial complex II inhibitor, TFAA, blocked oxLDL-induced mitochondrial production of ROS. This is consistent with the report that oxLDL causes mitochondrial production of ROS in endothelial cells and that this response is associated with apoptosis (23, 41, 42). The other possible sources of oxLDLinduced production of superoxide anions include uncoupled eNOS and increased NADPH oxidase (43– 47); however, their involvement in oxLDL-induced Bax activation and EDC apoptosis is unclear and remains to be determined. Our results extend the previous reports in two ways: 1) we identify that the superoxide anion but not hydrogen peroxide can activate p53 and Bax; and 2) we show that the superoxide anion regulation of Bax does not depend on an increase in Bax expression, but rather its activation. In conclusion, we have defined a signaling pathway that is activated by oxLDL in EDCs. The pathway incorporates both relationships between cell signaling elements described by others and our new information about redox-sensitive activation of p53 and Bax. The pathway that triggers apoptosis in both EDCs and endothelial cells proceeds from mitochondrial production of ROS which increases the expression of p53. Subsequently, p53 induces a conformational change of Bax, enabling its translocation to the mitochondria and results in apoptosis in EDCs. The antioxidants, NAC or SOD, but not catalase, reduces ROS and prevents oxLDLinduced expression of p53, Bax activation, and eventual apoptosis of EDCs. Our results suggest that EPCs may lose their full regenerative capacity in the presence of high elevated oxLDL, supporting the clinical observation that statin may facilitate EPC-dependent endothelial regeneration (48). Moreover, our results also suggest that manipulation of ROS-


p53-Bax pathways might be used to preserve EDCs and potentially improve endothelial function in atherosclerosis when circulating oxLDL, such as L5, is elevated. Acknowledgments


18.

19.

We are indebted to Dr. W. E. Mitch for insightful discussions and critical reading of this manuscript. Received December 19, 2006. Accepted January 17, 2007. Address all correspondence and requests for reprints to: Dr. Jie Du, Baylor College of Medicine, Division of Nephrology, One Baylor Plaza, BCM N-520, Houston, Texas 77030. E-mail: [email protected]. This project was supported by the National Institutes of Health through Grant RO1 HL 70762 and National Institutes of Health O’Brien Kidney Center P50-DK064233. Present address for R.C.: Department of Medicine, MRB9.130, University of Texas Medical Branch at Galveston, 301 University Boulevard, Galveston, Texas 77555. Disclosure Statement: The authors have nothing to disclose.

References 1. Asahara T, Murohara T, Sullivan A, Silver M, van der Zee R, Li T, Witzenbichler B, Schatteman G, Isner JM 1997 Isolation of putative progenitor endothelial cells for angiogenesis. Science 275:964 –967 2. Rabelink TJ, de Boer HC, de Koning EJ, van Zonneveld AJ 2004 Endothelial progenitor cells: more than an inflammatory response? Arterioscler Thromb Vasc Biol 24:834 – 838 3. Asahara T, Isner JM 2002 Endothelial progenitor cells for vascular regeneration. J Hematother Stem Cell Res 11:171–178 4. Hutter R, Carrick FE, Valdiviezo C, Wolinsky C, Rudge JS, Wiegand SJ, Fuster V, Badimon JJ, Sauter BV 2004 Vascular endothelial growth factor regulates reendothelialization and neointima formation in a mouse model of arterial injury. Circulation 110:2430 –2435 5. Schmidt-Lucke C, Rossig L, Fichtlscherer S, Vasa M, Britten M, Kamper U, Dimmeler S, Zeiher AM 2005 Reduced number of circulating endothelial progenitor cells predicts future cardiovascular events: proof of concept for the clinical importance of endogenous vascular repair. Circulation 111:2981–2987 6. Vasa M, Fichtlscherer S, Aicher A, Adler K, Urbich C, Martin H, Zeiher AM, Dimmeler S 2001 Number and migratory activity of circulating endothelial progenitor cells inversely correlate with risk factors for coronary artery disease. Circ Res 89:E1–E7 7. Imanishi T, Hano T, Sawamura T, Nishio I 2004 Oxidized low-density lipoprotein induces endothelial progenitor cell senescence, leading to cellular dysfunction. Clin Exp Pharmacol Physiol 31:407– 413 8. Ma FX, Zhou B, Chen Z, Ren Q, Lu SH, Sawamura T, Han ZC 2006 Oxidized low density lipoprotein impairs endothelial progenitor cells by regulation of endothelial nitric oxide synthase. J Lipid Res 47:1227–1237 9. Rosso A, Balsamo A, Gambino R, Dentelli P, Falcioni R, Cassader M, Pegoraro L, Pagano G, Brizzi MF 2006 p53 mediates the accelerated onset of senescence of endothelial progenitor cells in diabetes. J Biol Chem 281:4339 – 4347 10. Napoli C 2003 Oxidation of LDL, atherogenesis, and apoptosis. Ann NY Acad Sci 1010:698 –709 11. Cominacini L, Garbin U, Pasini AF, Davoli A, Campagnola M, Pastorino AM, Gaviraghi G, Lo Cascio V 1998 Oxidized low-density lipoprotein increases the production of intracellular reactive oxygen species in endothelial cells: inhibitory effect of lacidipine. J Hypertens 16:1913–1919 12. Sata M, Walsh K 1998 Oxidized LDL activates Fas-mediated endothelial cell apoptosis. J Clin Invest 102:1682–1689 13. Galle J, Schneider R, Heinloth A, Wanner C, Galle PR, Conzelmann E, Dimmeler S, Heermeier K 1999 Lp(a) and LDL induce apoptosis in human endothelial cells and in rabbit aorta: role of oxidative stress. Kidney Int 55: 1450 –1461 14. Wolter KG, Hsu YT, Smith CL, Nechushtan A, Xi XG, Youle RJ 1997 Movement of Bax from the cytosol to mitochondria during apoptosis. J Cell Biol 139:1281–1292 15. Chen J, Mehta JL, Haider N, Zhang X, Narula J, Li D 2004 Role of caspases in Ox-LDL-induced apoptotic cascade in human coronary artery endothelial cells. Circ Res 94:370 –376 16. Cominacini L, Pasini AF, Garbin U, Davoli A, Tosetti ML, Campagnola M, Rigoni A, Pastorino AM, Lo Cascio V, Sawamura T 2000 Oxidized low density lipoprotein (ox-LDL) binding to ox-LDL receptor-1 in endothelial cells induces the activation of NF-␬B through an increased production of intracellular reactive oxygen species. J Biol Chem 275:12633–12638 17. Chen CH, Jiang T, Yang JH, Jiang W, Lu J, Marathe GK, Pownall HJ, Ballantyne CM, McIntyre TM, Henry PD, Yang CY 2003 Low-density lipoprotein in hypercholesterolemic human plasma induces vascular endothe-

20. 21.

22. 23.

24.

25. 26. 27. 28. 29. 30.

31. 32.

33. 34. 35. 36. 37. 38. 39.

40. 41. 42.

2093

lial cell apoptosis by inhibiting fibroblast growth factor 2 transcription. Circulation 107:2102–2108 Yang CY, Chen HH, Huang MT, Raya JL, Yang JH, Chen CH, Gaubatz JW, Pownall HJ, Taylor AA, Ballantyne CM, Jenniskens FA, Smith CV 6 Oct 2006 Pro-apoptotic low-density lipoprotein subfractions in type II diabetes. Atherosclerosis 10.1016/j.atherosclerosis.2006.08.059 Yang CY, Raya JL, Chen HH, Chen CH, Abe Y, Pownall HJ, Taylor AA, Smith CV 2003 Isolation, characterization, and functional assessment of oxidatively modified subfractions of circulating low-density lipoproteins. Arterioscler Thromb Vasc Biol 23:1083–1090 Horan PK, Slezak SE 1989 Stable cell membrane labelling. Nature 340:167–168 Kaushal S, Amiel GE, Guleserian KJ, Shapira OM, Perry T, Sutherland FW, Rabkin E, Moran AM, Schoen FJ, Atala A, Soker S, Bischoff J, Mayer Jr JE 2001 Functional small-diameter neovessels created using endothelial progenitor cells expanded ex vivo. Nat Med 7:1035–1040 Du J, Wang X, Miereles C, Bailey JL, Debigare R, Zheng B, Price SR, Mitch WE 2004 Activation of caspase-3 is an initial step triggering accelerated muscle proteolysis in catabolic conditions. J Clin Invest 113:115–123 Zmijewski JW, Moellering DR, Le Goffe C, Landar A, Ramachandran A, Darley-Usmar VM 2005 Oxidized LDL induces mitochondrially associated reactive oxygen/nitrogen species formation in endothelial cells. Am J Physiol Heart Circ Physiol 289:H852–H861 Moellering DR, Levonen AL, Go YM, Patel RP, Dickinson DA, Forman HJ, Darley-Usmar VM 2002 Induction of glutathione synthesis by oxidized lowdensity lipoprotein and 1-palmitoyl-2-arachidonyl phosphatidylcholine: protection against quinone-mediated oxidative stress. Biochem J 362:51–59 Hristov M, Erl W, Weber PC 2003 Endothelial progenitor cells: mobilization, differentiation, and homing. Arterioscler Thromb Vasc Biol 23:1185–1189 Walter DH, Dimmeler S 2002 Endothelial progenitor cells: regulation and contribution to adult neovascularization. Herz 27:579 –588 Forraz N, Pettengell R, McGuckin CP 2004 Characterization of a lineagenegative stem-progenitor cell population optimized for ex vivo expansion and enriched for LTC-IC. Stem Cells 22:100 –108 Reyes M, Dudek A, Jahagirdar B, Koodie L, Marker PH, Verfaillie CM 2002 Origin of endothelial progenitors in human postnatal bone marrow. J Clin Invest 109:337–346 Kume N, Kita T 2004 Apoptosis of vascular cells by oxidized LDL: involvement of caspases and LOX-1 and its implication in atherosclerotic plaque rupture. Circ Res 94:269 –270 Wei MC, Zong WX, Cheng EH, Lindsten T, Panoutsakopoulou V, Ross AJ, Roth KA, MacGregor GR, Thompson CB, Korsmeyer SJ 2001 Proapoptotic BAX and BAK: a requisite gateway to mitochondrial dysfunction and death. Science 292:727–730 Dong C, Granville DJ, Tuffnel CE, Kenyon J, English D, Wilson JE, McManus BM 1999 Bax and apoptosis in acute and chronic rejection of rat cardiac allografts. Lab Invest 79:1643–1653 Urbich C, Knau A, Fichtlscherer S, Walter DH, Bruhl T, Potente M, Hofmann WK, de Vos S, Zeiher AM, Dimmeler S 2005 FOXO-dependent expression of the proapoptotic protein Bim: pivotal role for apoptosis signaling in endothelial progenitor cells. FASEB J 19:974 –976 Tong T, Ji J, Jin S, Li X, Fan W, Song Y, Wang M, Liu Z, Wu M, Zhan Q 2005 Gadd45a expression induces Bim dissociation from the cytoskeleton and translocation to mitochondria. Mol Cell Biol 25:4488 – 4500 Yamaguchi H, Wang HG 2001 The protein kinase PKB/Akt regulates cell survival and apoptosis by inhibiting Bax conformational change. Oncogene 20:7779 –7786 Clohessy JG, Zhuang J, de Boer J, Gil-Gomez G, Brady HJ 2006 Mcl-1 interacts with truncated Bid and inhibits its induction of cytochrome c release and its role in receptor-mediated apoptosis. J Biol Chem 281:5750 –5759 Luo X, Budihardjo I, Zou H, Slaughter C, Wang X 1998 Bid, a Bcl2 interacting protein, mediates cytochrome c release from mitochondria in response to activation of cell surface death receptors. Cell 94:481– 490 Yamaguchi H, Chen J, Bhalla K, Wang HG 2004 Regulation of bax activation and apoptotic response to microtubule-damaging agents by p53 transcriptiondependent and -independent pathways. J Biol Chem 279:39431–39437 Chipuk JE, Kuwana T, Bouchier-Hayes L, Droin NM, Newmeyer DD, Schuler M, Green DR 2004 Direct activation of Bax by p53 mediates mitochondrial membrane permeabilization and apoptosis. Science 303:1010 –1014 Van Antwerpen P, Boudjeltia KZ, Babar S, Legssyer I, Moreau P, Moguilevsky N, Vanhaeverbeek M, Ducobu J, Neve J 2005 Thiol-containing molecules interact with the myeloperoxidase/H2O2/chloride system to inhibit LDL oxidation. Biochem Biophys Res Commun 337:82– 88 Shatrov VA, Brune B 2003 Induced expression of manganese superoxide dismutase by non-toxic concentrations of oxidized low-density lipoprotein (oxLDL) protects against oxLDL-mediated cytotoxicity. Biochem J 374:505–511 Raha S, Robinson BH 2001 Mitochondria, oxygen free radicals, and apoptosis. Am J Med Genet 106:62–70 Sato T, Machida T, Takahashi S, Iyama S, Sato Y, Kuribayashi K, Takada K, Oku T, Kawano Y, Okamoto T, Takimoto R, Matsunaga T, Takayama T, Takahashi M, Kato J, Niitsu Y 2004 Fas-mediated apoptosome formation is dependent on reactive oxygen species derived from mitochondrial permeability transition in Jurkat cells. J Immunol 173:285–296

2094


43. Fleming I, Mohamed A, Galle J, Turchanowa L, Brandes RP, Fisslthaler B, Busse R 2005 Oxidized low-density lipoprotein increases superoxide production by endothelial nitric oxide synthase by inhibiting PKC␣. Cardiovasc Res 65:897–906 44. Satoh M, Fujimoto S, Haruna Y, Arakawa S, Horike H, Komai N, Sasaki T, Tsujioka K, Makino H, Kashihara N 2005 NAD(P)H oxidase and uncoupled nitric oxide synthase are major sources of glomerular superoxide in rats with experimental diabetic nephropathy. Am J Physiol Renal Physiol 288:F1144 –F1152 45. Landmesser U, Dikalov S, Price SR, McCann L, Fukai T, Holland SM, Mitch WE, Harrison DG 2003 Oxidation of tetrahydrobiopterin leads to uncoupling of endothelial cell nitric oxide synthase in hypertension. J Clin Invest 111:1201–1209 46. Vasquez-Vivar J, Kalyanaraman B, Martasek P 2003 The role of tetrahydro-


biopterin in superoxide generation from eNOS: enzymology and physiological implications. Free Radic Res 37:121–127 47. Rueckschloss U, Galle J, Holtz J, Zerkowski H-R, Morawietz H 2001 Induction of NAD(P)H oxidase by oxidized low-density lipoprotein in human endothelial cells: antioxidative potential of hydroxymethylglutaryl coenzyme A reductase inhibitor therapy. Circulation 104:1767–1772 48. Walter DH, Rittig K, Bahlmann FH, Kirchmair R, Silver M, Murayama T, Nishimura H, Losordo DW, Asahara T, Isner JM 2002 Statin therapy accelerates reendothelialization: a novel effect involving mobilization and incorporation of bone marrow-derived endothelial progenitor cells. Circulation 105:3017–3024

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