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Circulation Research. 2006;98:785-792
Published online before print March 9, 2006, doi: 10.1161/01.RES.0000216288.93234.c3
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(Circulation Research. 2006;98:785.)
© 2006 American Heart Association, Inc.

Molecular Medicine

Desensitization of Platelet-Derived Growth Factor Receptor-ß by Oxidized Lipids in Vascular Cells and Atherosclerotic Lesions

Prevention by Aldehyde Scavengers

Cecile Vindis, Isabelle Escargueil-Blanc, Meyer Elbaz, Bertrand Marcheix, Marie-Helene Grazide, Koji Uchida, Robert Salvayre, Anne Nègre-Salvayre

From the INSERM U-466 and Biochemistry Department (C.V., I. E.-B., M.E., B.M., M.-H.G., R.S., A.N.-S.), IFR-31, CHU Rangueil, Toulouse, France; and Laboratory of Food and Biodynamics (K.U.), Graduate School of Bioagricultural Sciences, Nagoya University, Japan.

Correspondence to Professor R. Salvayre, Biochimie, INSERM U466, IFR-31, CHU Rangueil, 1, Avenue Jean Poulhès, TSA-50032-31059 Toulouse Cedex 9, France. E-mail salvayre{at}toulouse.inserm.fr


*    Abstract
up arrowTop
 *Abstract
down arrowIntroduction
 down arrowMaterials and Methods
 down arrowResults
 down arrowDiscussion
 down arrowReferences
 
The platelet-derived growth factor receptor-ß (PDGFRß) signaling pathway regulates smooth muscle cell (SMC) migration and proliferation and plays a role in the vascular wall response to injury. Oxidized low-density lipoprotein (oxLDL) in atherosclerotic lesions can activate the PDGFRß pathway, but the long-term effects of oxLDL on PDGFRß function are not well understood. We found that oxLDL induced a dual effect on PDGFRß signaling. Initial activation of the PDGFR was followed by desensitization of the receptor. PDGFRß desensitization was not attributable to PDGFRß degradation or changes in localization to the caveolae but instead resulted from decreased PDGF binding and inhibition of PDGFRß tyrosine kinase activity. This inhibition was associated with formation of (4HNE)– and acrolein–PDGFRß adducts and was mimicked by preincubation of cells with 4HNE. These PDGFRß adducts were also detected in aortae of apolipoprotein-deficient mice and hypercholesterolemic rabbits and in human carotid plaques. The aldehyde scavengers DNPH and Hydralazine prevented both oxLDL- and 4HNE-induced structural modification and PDGFRß signaling dysfunction in cells and in vivo. OxLDL inhibition of PDGF signaling may contribute to defective SMC proliferation and decrease the stability of a vulnerable plaque.


Key Words: atherosclerosis • oxidized LDL • PDGF receptor • cell proliferation • signaling


*    Introduction
up arrowTop
 up arrowAbstract
 *Introduction
down arrowMaterials and Methods
 down arrowResults
 down arrowDiscussion
 down arrowReferences
 
During atherogenesis, the formation of fibroatheroma plaques involves a complex sequence of events, including endothelial activation, transendothelial migration of mononuclear cells, lipoprotein oxidation, lipid accumulation in macrophagic cells, smooth muscle cell (SMC) migration and proliferation, and local inflammatory response.1,2

Low-density lipoproteins (LDLs) are atherogenic3 after undergoing oxidative modifications.4 Oxidized LDLs (oxLDLs), present in atherosclerotic areas, exert various biological effects potentially involved in atherogenesis, including lipid accumulation in macrophages (foam cells), changes in gene expression of adhesion molecules, cytokines, growth factors and coagulation proteins, cell migration, cell proliferation, apoptosis, and local inflammatory/immune response.4,5

SMC migration and proliferation play a critical role in the formation of fibrous cap of atherosclerotic plaques, whereas toxic events are potentially involved in endothelial cell injury, necrotic core formation, and plaque rupture.1,2,6 Excessive SMC proliferation and extracellular matrix biosynthesis may lead to occlusive lesions.1 In contrast, the "thin cap fibroatheroma" containing inflammatory cells and a lipid-rich core is generally a vulnerable plaque prone to atherothrombotic events.7–9

Proliferation of SMC is regulated by numerous growth factors and cytokines8,10 and by oxLDL.11,12 Platelet-derived growth factor (PDGF) plays a major role in the fibroproliferative response during atherogenesis and restenosis.1,13 PDGF-induced signaling is mediated by PDGF receptor-{alpha} (PDGFR{alpha}) and PDGFRß, which belong to the large family of receptor tyrosine kinases.14 PDGFR{alpha}, which binds A and B PDGF isoforms, is involved in SMC hypertrophy. PDGFRß, which binds only PDGF B-chain, is implicated in the migration and proliferation of SMCs.14,16 On ligand binding, the PDGFRß tyrosine kinase is activated and creates phosphotyrosine residues that act as docking sites for Src homology 2–containing proteins, including Src, phospholipase C-{gamma}1, phosphatidylinositol-3'-kinase, SH2 domain containing rynosine phosphatase protein tyrosine phosphatase, GTPase-activating protein of p21ras, Nck, Grb2, and Shc.14–16

We recently reported that PDGFRß, like epidermal growth factor receptor (EGFR), acts as a sensor for both oxidized lipids and oxidative stress and that 4-hydroxynonenal (4HNE) contained in mildly oxidized LDL induces both adduct formation and activation of these receptor tyrosine kinases.17,18 Because oxLDL and PDGF are present in atherosclerotic lesions,1,2 we evaluated their respective contribution to SMC proliferation. Preliminary experiments showed unexpected results and led us to investigate this paradoxical effect.

The reported data show that after moderate but long-lasting PDGFRß activation, oxLDL (as well as 4HNE) induced HNE–PDGFRß adduct formation associated with desensitization of PDGFRß. This was characterized by a loss of PDGFRß signaling and mitogenic effect induced by PDGF. Moreover, HNE–PDGFRß adducts were observed in atherosclerotic plaques of humans and animal models and was prevented by aldehyde scavenger treatment.


*    Materials and Methods
up arrowTop
 up arrowAbstract
 up arrowIntroduction
 *Materials and Methods
down arrowResults
 down arrowDiscussion
 down arrowReferences
 
An extensive Materials and Methods section is available in the online supplement at http://circres.ahajournals.org.

Chemicals and Biological Reagents
For chemicals and biological reagents, see the online supplement.

Cell Culture
Rabbit arterial SMCs were grown and starved under the conditions used previously.18 For extensive information, see the online supplement.

Animal Models: Apolipoprotein E–Deficient Mice and Rabbits
Apolipoprotein E–deficient (apoE–/–) and wild-type mice were from Transgenic Alliance (IFFA Credo; Charles River; Les Oncins, France). Mice, housed under specific pathogen-free conditions, were fed a standard diet.

New Zealand white male rabbits (2.5 kg) were randomly assigned to regular (n=4) and hypercholesterolemic (2% cholesterol for 12 weeks) diets, associated (n=6) or not (n=6) to Hydralazine treatment (10 mg/kg per day for 12 weeks). After euthanasia (by pentobarbital; 50 mg/kg), thoracic aortas were removed, cleaned of excess adventicial tissues, opened longitudinally to obtain an en face preparation, and photographed (Nikon Coolpix 995). Fatty streaks were stained with Sudan IV. The extent of atherosclerosis was assessed using computer-aided planimetry (Cyberview 3.0 software), and the lesion areas covering the aortic surface were quantitated.

Experiments were performed according to French accreditation for laboratory animal care.

Western Blot Analysis and Determination of PDGFRß Free Amino Groups
Cells or tissue samples were lysed, PDGFRß was immunoprecipitated, and Western blots were performed under conditions described previously.17,18 The free amino group content of PDGFRß was evaluated after immunoprecipitation using [3H]NSP ([3H]N-succinimidyl propionate; Amersham) labeling.17

Detergent-Free Purification of Caveolae/Raft Membrane Fractions
Lipid raft-enriched domains were purified using a modified carbonate method.19 For extensive information see the online supplement.

Evaluation of DNA Synthesis, Cell Proliferation, and Viability
DNA synthesis was evaluated by [3H]thymidine incorporation as described.11 Alternatively, proliferating cells were detected by immunocytochemistry using anti–proliferating-cell nuclear antigen (PCNA) antibody and visualized using a Zeiss LSM 510 fluorescence confocal microscope.20 Cell viability was evaluated by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) assay.20

Flow Cytofluometry
Cell surface PDGFR determination was performed by flow cytofluometry according to Okumura.21 For extensive information see the online supplement.

Binding Experiments
[125I]PDGF binding was performed according to Escargueil-Blanc et al.18 Cells incubated±oxLDL (75 µg/mL; 24 hours) were incubated with [125I]PDGF (70 000 cpm/mL; 30 pmol/L), then washed in PBS containing 0.5% BSA, and the cell-associated radioactivity was determined (Minaxi; Packard). Nonspecific binding was determined on the basis of excess unlabeled PDGF (5 nmol/L). Cells were washed twice in PBS containing 0.5% BSA and once in PBS alone, and the radioactivity was counted (Minaxi Gamma Packard Counter).

Determination of Enzyme Activities
Proteasome activity and PDGFRß tyrosine kinase activities were determined using conditions described previously.18,22

Immunohistochemistry
Frozen sections of aortic sinus from apoE–/– and wild-type mice were labeled by antiacrolein antibody,23 and aortas from cholesterol-fed rabbits, treated or not by Hydralazine, were labeled by anti–4-HNE monoclonal antibody and revealed by a peroxidase-conjugated secondary antibody (immunoperoxidase kit; Dako; StrepABComplex/HRP Duet). For each detection, a control was done by omitting the primary antibody to determine the nonspecific binding. Nuclei were counterstained with hematoxylin.

Statistical Analysis
Data are given as means±SEM. Estimates of statistical significance were performed by ANOVA (Tukey test; SigmaStat software). Values of P<0.05 were considered significant.


*    Results
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 up arrowAbstract
 up arrowIntroduction
 up arrowMaterials and Methods
 *Results
down arrowDiscussion
 down arrowReferences
 
Preincubation of Cells With oxLDL Inhibits the Mitogenic Effect of PDGF and PDGFRß Signaling
Mitogenic concentration (75 µg apoB/mL) of oxLDL moderately stimulated DNA synthesis, whereas, as expected, PDGF was a more potent mitogen (Figure 1A, top). However, unexpectedly, in cells preincubated with nontoxic oxLDL concentrations, the mitogenic response to PDGF was strongly reduced. Inhibition of DNA synthesis induced by oxLDL pretreatment was not attributable to cell loss, as assessed by MTT assay (Figure 1A, bottom) but rather to cell cycle inhibition, as shown by the nuclear expression of PCNA, a marker of cell cycle progression. PCNA labeling was intense in the nucleus of PDGF-treated cells but not in cells incubated with oxLDL or oxLDL+PDGF (Figure 1B). Interestingly, preincubation of cells with oxLDL did not inhibit the mitogenic effect of FCS (Figure 1A), indicating that under our conditions, all the mitogenic pathways were not blocked by oxLDL preincubation. We thus hypothesized that oxLDL could inhibit an early step of PDGF signaling.


Figure 1
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  		Figure 1. Preincubation of SMCs with oxLDL inhibits the mitogenic effect of PDGF and PDGFRß signaling pathway. A, Preincubation of SMCs with oxLDL prevents the mitogenic effect of PDGF. SMCs were preincubated with or without UV-oxLDL (75 µg apoB/mL) for 20 hours then 5 ng/mL PDGF (P) or FCS (10%; S) were added. DNA synthesis ([3H]thymidine incorporation) and toxicity measured by the MTT assay with low concentrations of UV-oxLDL (L; 75 µg apoB/mL), high concentrations of UV-oxLDL (H; 300 µg apoB/mL), and taxol (1 µmol/L) were evaluated after 24 hours under standard conditions. B, Immunofluorescence of PCNA in SMCs treated as in A. For the quantitative analyses, PCNA-positive cells were evaluated by counting 100 nuclei in three random fields from three independent experiments. C, Time course of PDGFRß tyrosine phosphorylation in cells incubated with UV-oxLDL or native LDL (nL; 75 µg apoB/mL for the indicated time) or with PDGF (5 ng/mL; 20 minutes). PDGFRß was immunoprecipitated, and Western blots revealed by antiphosphotyrosine (anti-PY) and anti-PDGFRß antibodies. D and E, Activation of PDGFRß (D) and ERK1/2 (E) in cells preincubated with or without UV-oxLDL (UV) or with cell-oxLDL (cell) and treated with PDGF, as in A. Western blots were revealed by antiphosphotyrosine and anti-PDGFRß antibodies (D) and by antiactivated ERK1/2 and total ERK1/2 (E). A and B, Results are expressed as mean±SEM of four experiments. *P<0.05. Western blots are representative of three to five experiments.

The mitogenic effect of oxLDL was associated with PDGFRß autophosphorylation,18 which was sustained for 16 to 20 hours and then declined toward the baseline at 24 hours (Figure 1C). In contrast, native LDL did not trigger PDGFRß activation up to 4 hours (Figure 1C). Interestingly, in cells preincubated with oxLDL for 24 hours, addition of PDGF did not induce PDGFRß activation nor extracellular signal-regulated kinase 1/2 (ERK1/2) signaling (Figure 1D and 1E). Of note, ultraviolet (UV)-oxLDL and cell oxLDL gave similar results (Figure 1D and 1E). We preferred to use UV-oxLDL because cell–oxLDL preparations may contain potentially interfering bioactive compounds (released from endothelial cell vein-304 cells).

These data suggest that preincubating cells with oxLDL induces a decrease of PDGF-induced PDGFRß activation. We next investigated the mechanism of this inhibitory effect.

A Role for 4HNE in the OxLDL-Induced PDGFR Inhibition
In contrast to native LDL, oxLDL induced aldehyde–protein adduct formation, as shown by Western blots labeled with anti-4HNE and antiacrolein antibodies, and by determining free amino groups content (Figure 2 A through 2C).


Figure 2
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  		Figure 2. Aldehyde–protein adduct formation and inhibition by 4HNE of PDGF-induced PDGFRß signaling and mitogenic effect. A, Preincubation of SMCs with UV-oxLDL induces HNE– and acrolein–protein adduct formation. Cells were incubated with or without UV-oxLDL (75 µg apoB/mL), and cell extracts were resolved by SDS-PAGE and blotted by anti-4HNE and antiacrolein antibodies. B, Determination of free [3H]NSP-reactive amino groups in cells treated with or without UV-oxLDL (75 µg apoB/mL) for 16 hours, 4HNE (50 µmol/L) for 1 hour, or PDGF (5 ng/mL PDGF) for 30 minutes. C, Western blots of aldehyde–protein adducts in PDGFRß immunoprecipitates from cells treated for 6 hours with 75 µg apoB/mL UV-oxLDL or native LDL (nLDL) or 5 ng/mL PDGF or 25 µmol/L 4HNE. The graph represents values of acrolein adducts band intensity after normalization for PDGFR by densitometry. D, Preincubation of cells with 4HNE inhibits PDGF-induced PDGFRß autophosphorylation. Cells were preincubated with 4HNE (25 µmol/L) for 24 hours, then stimulated by 5 ng/mL PDGF for 20 or 30 minutes. E, Preincubation of cells with 4HNE inhibits subsequent ERK1/2 activation. Cells were preincubated with 4HNE (25 µmol/L) for 24 hours then stimulated by 5 ng/mL PDGF for 30 minutes. F, Preincubation of SMCs with 4HNE prevents the mitogenic effect of PDGF. SMCs were preincubated with 4HNE (25 µmol/L) for 24 hours, for 16 hours in FCS-free medium, then 5 ng/mL PDGF were added, and after 24-hour incubation, DNA synthesis ([3H]thymidine incorporation) was evaluated. G, Preincubation of cells with CHP inhibits PDGF-induced PDGFRß autophosphorylation. Cells were preincubated with CHP (50 µmol/L) and 4HNE (25 µmol/L) for 24 hours, then stimulated by 5 ng/mL PDGF for 20 minutes. PDGFRß was immunoprecipitated for Western blotting as in Figure 1D. In B and F, mean±SEM of four experiments. *P<0.05. Western blots are representative of three to five experiments.

Preincubation of cells with 4HNE mimicked the effect of oxLDL, as assessed by the loss of [3H]NSP-reactive free amino groups (Figure 2B) and the formation of 4HNE–PDGFR adducts (Figure 2C). These 4HNE-induced structural modifications (Figure 2C) were associated with a loss of PDGF-induced PDGFRß activation and subsequent decrease of both ERK1/2 activation and DNA synthesis (Figure 2D through 2F). Acrolein gave similar results on PDGFRß signaling (data not shown) but was not used in the following experiments because of its volatility (making it difficult to perform a dose-effect study) and toxicity (potentially interfering in signaling studies).

These data support the hypothesis that 4HNE and acrolein mediate, at least in part, the structural modification of PDGFRß induced by oxLDL.

Because cumene hydroperoxide (CHP) has been shown to inhibit cell proliferation, similarly to 4HNE,24 we investigated whether CHP could also inactivate PDGFRß. Cells were treated with the relatively hydrosoluble CHP (used at 50 µmol/L, a subtoxic concentration). CHP treatment induced a significant decrease in PDGF-induced PDGFRß activation (Figure 2G) with no detectable level of cellular lipid peroxidation or thiobarbituric acid reactive substances (data not shown), similarly to oxLDL under the used nontoxic conditions (oxLDLs induce the peroxidation of cellular lipids only at toxic concentration).

Mechanisms of oxLDL-Induced PDGFRß Dysfunction
Because PDGF stimulation is known to induce subcellular relocation of PDGFRß and its degradation by proteasome or lysosomes,25,26 we investigated whether oxLDLs were able to induce these quantitative or topological changes. OxLDL stimulated proteasome activity (Figure 3A), but neither oxLDL nor 4HNE induced loss of total cellular PDGFRß (Figure 3B). Then, we evaluated the level of PDGFRß exposed at the cell surface by flow cytometry of nonpermeabilized cells labeled with anti-PDGFRß antibody. After 24-hour preincubation with oxLDL or 4HNE, cell surface level of PDGFRß was decreased by 19±6% and 15±4%, respectively (Figure 3C), whereas, as expected,24 the cell surface level of PDGFRß was dramatically decreased in cells stimulated by PDGF (Figure 3C).


Figure 3
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  		Figure 3. Mechanisms implicated in oxLDL-induced PDGFRß downregulation. A, Time course of proteasome activity in cells with UV-oxLDL (75 µg apoB/mL). B, Evaluation of the whole level of PDGFRß in cells preincubated for the indicated time with UV-oxLDL (oxL) or native (nL; 75 µg apoB/mL), or 4HNE (25 µmol/L) or PDGF (5 ng/mL; PDGF). The graph represents values of PDGFR band intensity after normalization for ß-actin by densitometry. C, Evaluation of PDGFRß level exposed at the cell surface. Cells were preincubated with oxLDL, 4HNE, or PDGF under conditions in B, harvested by trypsin/EDTA treatment, labeled with anti-PDGFR primary antibody (and fluorescein isothiocyanate–labeled secondary antibody) and propidium iodide (only intact cells, ie, non–IP-labeled, were taken into account) and analyzed by flow cytometry. D, Caveolae/rafts localization of PDGFRß. Cells incubated for 24 hours±oxLDL were lysed and subjected to sucrose gradient sedimentation, as described in Materials and Methods. Then, 50 µL of each fraction was used for the detection of PDGFRß, CD55 (raft marker), and caveolin-1 (caveolae marker). E, Binding of [131I]PDGF-BB to SMCs preincubated for 16 hours with 75 µg apoB/mL of UV-oxLDL or native LDL (nL) or for 6 hours with 4HNE (25 µmol/L). F, PDGF-induced activation of PDGFRß tyrosine kinase after UV-oxLDL, nLDL, and 4HNE treatment. Cells were pretreated under conditions of E, PDGFRß was immunoprecipitated and tyrosine kinase activity was determined. In A, C, E, and F, data are expressed as mean±SEM of three or four experiments. *P<0.05. Western blots are representative of three to five experiments.

Because PDGF signaling may also depend on cholesterol-rich microdomains (rafts and caveolae),27 and extensively, oxLDLs have been shown to alter microdomains integrity,28 we investigated whether the location of PDGFRß in caveolae/raft microdomains was altered in cells preincubated with mildly oxidized LDL. As shown in Figure 3D, oxLDL induced no major change in the distribution of PDGFRß between raft and nonraft membranes.

Altogether, these data suggest that part of the inhibitory effect of oxLDL or 4HNE results from a decrease of PDGFRß located at the cell surface.

Because this only partial internalization of PDGFRß probably does not account for the complete inhibition of PDGFRß by oxLDL, we investigated whether oxLDL and 4HNE altered the intrinsic functional properties of the receptor. Preincubation of cells with oxLDL or 4HNE reduced the binding of PDGF by 46% and 27%, respectively (Figure 3E). The tyrosine kinase activity of PDGFRß was more strongly inhibited by 82% and 71%, respectively (Figure 3F), thus explaining the severe reduction of PDGF-induced PDGFRß activation elicited by oxLDL or 4HNE.

DNPH and Hydralazine Prevent the oxLDL-Induced PDGFRß Dysfunction In Vitro
Because oxLDL-induced PDGFRß dysfunction results, at least in part, from the formation of 4HNE–PDGFRß adducts, we hypothesized that aldehyde scavengers may prevent both protein modification and subsequent dysfunction. For this purpose, we used the prototypic hydrazine compound DNPH, which can be used on cultured cells but not in vivo because of its toxicity, and Hydralazine, another hydrazine compound, compatible with in vivo studies.

As reported in Figure 4 A and 4B, DNPH and Hydralazine prevented oxLDL-induced 4HNE–PDGFRß adduct formation and rescued in part PDGF-induced PDGFRß activation. Concomitantly, DNPH prevented oxLDL-induced inhibition of the mitogenic effect of PDGF. In contrast, the protective effect of Hydralazine on the mitogenic effect of PDGF could not be tested because Hydralazine by itself inhibited the mitogenic effect of PDGF through an unknown mechanism (data not shown).


Figure 4
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  		Figure 4. DNPH and Hydralazine prevent oxLDL-induced PDGFRß inhibition in cultured cells. A, Formation of 4HNE–protein adducts in cells treated for 16 hours with 75 µg apoB/mL UV-oxLDL and DNPH (500 µmol/L) or Hydralazine (100 µmol/L). B, PDGF-induced activation of PDGFRß under the conditions of A. C, PDGF-induced DNA synthesis ([3H]thymidine incorporation) in cells treated under the conditions of A. Western blots (A and B) are representative of three experiments. In C, data are expressed as mean±SEM of four experiments. *P<0.05.

PDGFRß Modification in Atherosclerotic Lesions: Protection by Hydralazine
We next evaluated whether 4HNE– or acrolein–PDGFRß adducts were detected in atherosclerotic areas and whether aldehyde scavengers were able to prevent PDGFRß modification.

As shown in Figure 5 A and 5B, PDGFRß modification was clearly visualized by antiacrolein and by anti-4HNE antibodies on Western blots of aortic atherosclerotic lesions from apoE–/– mice and in human carotid advanced plaques. In contrast, normal arteries were not labeled. It may be noted that atherosclerotic subendothelial areas of aortic sinus from apoE–/– mice were stained by antiacrolein antibodies, in contrast to aortic sinus of control mice (Figure 5A).


Figure 5
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  		Figure 5. Formation of 4HNE and acrolein adducts in apoE–/– mice and human atherosclerotic lesions. A, Top, Acrolein–PDGFRß adducts in PDGFR immunoprecipitated from aortas of apoE+/+ and apoE–/– mice. The graph represents values of acrolein band intensity after normalization for PDGFR by densitometry. Bottom, Immunohistochemistry of acrolein adducts in aortic sinus from apoE+/+ and apoE–/– mice. B, Acrolein– and 4HNE–PDGFRß adducts in human carotids. Western blots of PDGFR immunoprecipitated from homogenates of atherosclerotic and nonatherosclerotic areas. The graph represents values of acrolein and 4-HNE adducts band intensity after normalization for PDGFR by densitometry. A and B are representative of three experiments.

Cholesterol-fed rabbits exhibited hypercholesterolemia (23±4 g/L) and large fatty streaks in aorta (supplemental Figure IA, available in the online supplement; additional data available in the online supplement), whereas chow diet–fed rabbits have low cholesterol levels (1.1±0.2 g/L) and no fatty streak in aortas (data not shown). As reported in Western blots of Figure 6A, high levels of 4HNE–PDGFRß adducts were detected in aortic atherosclerotic lesions of hypercholesterolemic rabbits, whereas PDGFRß was only very slightly labeled by anti-4HNE antibody in normocholesterolemic controls. It may be noted that PDGFRß was also labeled by antiubiquitin antibody in atherosclerotic lesions but not in control aortas. Hydralazine was effective in preventing both 4HNE– and ubiquitin–PDGFRß adduct formation in aortas of hypercholesterolemic rabbits (Figure 6A). It may be noted that the extension of atherosclerotic plaques was lesser in Hydralazine-treated cholesterol-fed rabbits than in the untreated cholesterol fed rabbits, as shown by en face aorta photographs (supplemental Figure IA; additional data available in the online supplement) and by quantitative evaluation of atherosclerotic lesion area (supplemental Figure IB; additional data available in the online supplement). In the aortic intima of cholesterol-fed rabbits, fatty streaks areas contained areas of 4HNE–protein adduct formation (Figure 6B, arrows). In contrast, anti-4HNE labeling was much lower in the intima of aorta of Hydralazine-treated cholesterol-fed rabbits (Figure 6B, arrows).


Figure 6
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  		Figure 6. Formation of 4HNE adducts in hypercholesterolemic rabbits. Prevention by Hydralazine. A, 4HNE– and ubiquitin–PDGFRß adducts in aortas of control and hypercholesterolemic rabbits treated or not by Hydralazine. The graph represents values of 4-HNE adducts and ubiquitin band intensity after normalization for PDGFR by densitometry. B, Immunohistochemistry of 4HNE adducts in aortic atherosclerotic plaques from cholesterol-fed rabbits, either untreated or Hydralazine treated. A and B are representative of three experiments.


*    Discussion
up arrowTop
 up arrowAbstract
 up arrowIntroduction
 up arrowMaterials and Methods
 up arrowResults
 *Discussion
down arrowReferences
 
The reported data show that: (1) long-term incubation of SMCs with mildly oxidized LDL reduced PDGF-induced PDGFRß activation and subsequent mitogenic effect; (2) oxLDL- and 4HNE-induced structural modification and desensitization of PDGFRß resulted mainly from inhibition of PDGF binding and PDGFRß–tyrosine kinase; and (3) PDGFRß downregulation was prevented in part by aldehyde scavengers, such as DNPH and Hydralazine, both in cell culture and in vivo in animal models.

We recently reported that short-term incubation of cells with oxLDL elicited the activation of EGFR17 and PDGFRß18 by at least two mechanisms: one involving 4HNE–protein adduct formation and the second involving the generation of reactive oxygen species and subsequent PTPase inhibition.18 This partial PTPase inhibition may explain in part the sustained (20 hours) tyrosine phosphorylation of PDGFRß observed during incubation with oxLDL. However, after 24-hour incubation with oxLDL, PDGFRß was dephosphorylated but was insensitive to PDGF stimulation. Moreover, preliminary experiments showed that the mitogenic effect of oxLDL and of PDGF were not additive, which led us to investigate the mechanisms by which oxLDLs inhibit PDGF-induced PDGFRß activation.

A classical mechanism of PDGFRß downregulation after PDGF activation consists in the internalization of the complex ligand-receptor into endosomes.25 Then the complex dissociates and the receptor recycles to the cell surface, or the complex is degraded on fusion of endosomes with lysosomes.15 Alternatively, PDGFRß may undergo ubiquitination and degradation in the proteasome.26 Because oxLDL-induced PDGFRß downregulation occurred after a moderate but sustained activation of PDGFRß,18 and because oxLDL induced proteasome activation, we investigated whether oxLDLs were able to trigger PDGFRß degradation. Under our conditions, oxLDL induced no change in the whole level of PDGFRß, whereas, as expected,15 PDGFRß level was dramatically reduced after 8-hour stimulation by PDGF. This is consistent with the fact that oxLDL-induced PDGFRß activation does not involve autocrine secretion of PDGF.18 In the same way, the inhibitory effect of oxLDL cannot be explained by the minor internalization of cell surface PDGFRß.

Because PDGFRß signaling is dependent on caveolae-associated signalosome27 and because extensively oxLDL can extract cholesterol from caveolae,28 thereby altering caveolae structure and potentially changing PDGFRß localization, we hypothesized that oxLDL may induce PDGFRß relocalization and subsequent function alteration. Because we observed no major change in PDGFRß distribution between raft and nonraft membranes, it was therefore excluded that the severe inhibition of PDGFRß induced by oxLDL may result from degradation or internalization of the receptor. This led us to hypothesize that peroxidation products of oxLDL (4HNE or acrolein) might alter the structure and function of the receptor.

4HNE and acrolein are lipid peroxidation products of polyunsaturated fatty acids, formed during the oxidation of LDL or membranes.29–31 These aldehydes can react with thiols and amino groups of cell proteins and apoB, thereby altering their structure and functional properties.30 Incubation of cells with oxLDL elicits the formation of 4HNE–cell protein adducts,17,18,23 and experiments with [3H]4HNE-labeled oxLDL have shown that a small part of [3H]4HNE of oxLDL is transferred to PDGFRß (and other cell proteins).18 How 4HNE transfers from oxLDL to cell proteins is only partly understood because a larger part of 4HNE is covalently bound to amino groups of oxLDL and because the minor part of free 4HNE is probably dissolved in the phospholipid phase.30 The transfer of 4HNE from oxLDL to cellular proteins is energy dependent (inhibited at 4°C) and blocked by proton pump inhibitors,18 thus suggesting an active metabolic process.

Moreover, we examined whether oxLDL may trigger oxidative stress18,32 and thereby oxidize cellular lipids and generate 4HNE.31 Under nontoxic conditions used here, oxLDL induced no detectable peroxidation of cellular lipids (thus probably generating not 4HNE). This was confirmed by using CHP, which can induce DNA synthesis inhibition24 and cellular lipid peroxidation.33 In our experimental system, CHP (used at nontoxic concentration) was able to inhibit PDGFRß, independently of any peroxidation of cellular lipids (thus independently of 4HNE formation). It is therefore unlikely that in our experiments, 4HNE may result from cellular lipid peroxidation.

Another crucial question was understanding whether oxLDL-induced PDGFRß inactivation was causally related to 4HNE–PDGFRß adducts formation. This was not a trivial question because our previous reports demonstrated that short incubation of PDGFRß or EGFR with 4HNE induced a moderate but sustained activation of these receptors.17,18 Our data suggest that incubation of cells with oxLDL triggers a biphasic effect, the initial activation (sustained for 16 hours) being followed by a late inhibition, associating dephosphorylation of PDGFRß with a marked resistance to PDGF stimulation. These effects were mimicked by preincubation of cells with 4HNE, suggesting that the formation of 4HNE–PDGFRß adducts mediates the insensitivity of PDGFRß to PDGF. The mechanism of this inhibitory effect was relatively complex because it resulted from decreased binding of PDGF and inhibition of PDGFRß tyrosine kinase activity. These data are consistent with previous reports indicating that extensive 4HNE adduct formation is often associated with a loss of function of several enzymes,31 probably by altering the protein conformation or the active site.

The causal relationship between aldehyde–PDGFRß adduct formation and PDGFRß inhibition induced by oxLDL was supported by the protective effect of aldehydes scavengers DNPH and Hydralazine, which prevented concomitantly adduct formation and receptor inhibition.

4HNE– and acrolein–PDGFRß adducts were found in aortic atherosclerotic lesions of two animal models, cholesterol-fed rabbits and apoE–/– mice, suggesting that PDGFRß modification by lipid peroxidation products occurs in vivo, like modification of other proteins (apoB) visualized by antibodies recognizing oxidation-specific epitopes in atherosclerotic plaques.34,35 Oxidation-specific modifications of PDGFRß were associated with increased ubiquitination of the receptor, in agreement with that observed in cultured cells treated with oxLDL.22 This demonstrates that modified PDGFRß is detected and labeled by the ubiquitination system in vivo and confirms that the observed modification is not an artifactual postmortem modification.

From a (patho)physiological point of view, PDGFRß is implicated in angiogenesis, neointima formation, fibrous cap thickening, and restenosis.1,13 Atherosclerotic plaques displaying thick fibrous cap are generally stable and may lead to occlusive lesions, whereas plaques with thin fibrous cap and lipid-rich core (containing oxidized lipids) are generally unstable, prone to rupture and initiate frequently atherothrombotic events.8,9

The presence of modified and ubiquitinated PDGFRß in atherosclerotic lipid-rich lesions suggests that PDGFRß could be locally downregulated and refractory to PDGF and could participate in forming a thin fibrous cap, characteristic of vulnerable lipid-rich plaques.8,9 Moreover, oxidized lipids can trigger local inflammatory response,36 metalloproteinase overexpression,37 and apoptosis,6,38 events converging toward triggering plaque disruption and subsequent atherothrombotic events. Hydralazine that acts in vitro as a carbonyl scavenger like DNPH is also active in vivo in preventing aldehyde–PDGFRß adduct formation. However, it may be noted that Hydralazine is a well-known antihypertensive agent and is able to inhibit oxidizing enzymes such as NADPH oxidase and cytochrome P450.39 The antihypertensive effect is probably not involved in the inhibition of 4HNE–PDGFRß adduct formation because we did not observe any major change in arterial blood pressure in Hydralazine-treated rabbits. However, it cannot be excluded that the protective effect of Hydralazine may be mediated by the inhibitory effect on oxidant enzymes because oxidative stress is thought to play a role in atherogenesis by inducing endothelial dysfunction, inflammatory response40 and LDL oxidation.4 All these mechanisms of action of Hydralazine converge to prevent the formation of aldehyde–protein and aldehyde–PDGFRß adducts in vivo, thus suggesting that it may prevent the PDGFRß dysfunction and subsequent events potentially involved in plaque formation and instability.


*    Acknowledgments
 
This work was supported by grants from INSERM, Université Paul Sabatier-Toulouse III to U-466.


*    Footnotes
 
Original received August 31, 2005; revision received February 16, 2006; accepted February 23, 2006.


*    References
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 *References
 

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