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From the Department of Medicine* (Division of Pulmonary Medicine) and Pediatrics, and the Microchemical and Proteomics Facility, Emory University, Atlanta, Georgia
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Abstract |
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 Top Abstract Materials and MethodsResultsDiscussionReferences |
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The thioredoxin-1 (Trx1) system plays a key role in modulating redox signaling pathways regulating physiological as well as pathophysiological processes such as atherosclerosis development.1,2 Trx1 is a major substrate for thioredoxin reductase-1 (TrxR1), serving as an electron carrier to reduce peroxiredoxins, redox factor-1 (Ref1), and other proteins. A dithiol (Cys-32, Cys-35) in the active site of Trx1 undergoes reversible oxidation to the disulfide during the transfer of reducing equivalents. In addition to the redox regulatory function of the active-site cysteines (Cys-32, Cys-35), post-translational modifications of the other cysteines by oxidation (Cys-62, Cys-69), S-nitrosylation (Cys-69), and glutathionylation (Cys-73) have a significant effect on Trx1 function.3-5 Modification of thiols in Trx1 interrupts signaling mechanisms involved in cell growth, proliferation, and apoptosis.3,4
Previous studies show that common products of lipid peroxidation, including acrolein and 4-hydroxy-2-nonenal (HNE) react with and modify functions of various cellular proteins. Acrolein and HNE are electrophilic lipids that target a number of redox-sensitive proteins, inducing multiple cellular responses through several mechanisms.6 The cytoprotective role of acrolein and HNE in cellular mechanisms has been demonstrated at low concentrations. For example, HNE induces cytoprotective antioxidants, HO-1 and glutathione (GSH), in bovine aortic endothelial cells (BAECs)7 and activates thioredoxin reductase (TrxR)-1 via Nrf2 activation.8 Many cytotoxic effects are mainly attributed to the alkylating properties and strong reactivity of both acrolein and HNE, resulting from the conjugated double-bond system (CH2=CH-CHO). Yang and colleagues9 demonstrated that acrolein inhibited Trx1 activity without affecting total cellular Trx1 protein amount in A594 cells. However, the mechanism of acrolein-induced inhibition of cellular Trx1 is currently unknown, and a specific role of acrolein-modified Trx1 in cardiovascular disease has not been identified. HNE, a product of lipid peroxidation, modified Cys32 and Cys35 of Escherichia coli Trx and inhibited catalytic activity.10 Increased HNE concentrations can be a potent biomarker for numerous diseases and conditions, including atherosclerosis, ischemia, diabetes, inflammation, and neurodegenerative diseases such as Alzheimer’s disease and Parkinson’s disease. Accumulation of HNE and acrolein under pathological conditions are toxic and can modify cellular proteins and DNA.10-12
The present study shows that acrolein and HNE inhibit Trx1 activity through direct modification of a nonactive site thiol. Acrolein reacts with the thiol residue Cys-73 in Trx1, which was identified by mass spectrometry and Western blot analyses under native conditions,5,13 with modification of Trx1 occurring at lower concentration in endothelial cells. Microinjection of acrolein- or HNE-modified Trx1 into endothelial cells elevated monocyte binding to endothelium, showing that modification of Trx1 by reactive aldehydes is sufficient to stimulate critical early events of atherosclerosis.
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Materials and Methods |
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TopAbstractMaterials and MethodsResultsDiscussionReferences |
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BAECs purchased from American Type Culture Collection, Bethesda, MD, were maintained with 10% fetal bovine serum in Dulbecco’s modified Eagle’s medium. Confluent BAECs were treated with acrolein or HNE for 1 hour, washed with Hanks’ balanced salt solution three times, replenished with fresh Dulbecco’s modified Eagle’s medium including 10% fetal bovine serum for 5 hours, and then used for each experiment. 6-Carboxy-2',7'-dichlorofluorescein diacetate, calcein AM, and Alexa Fluor 568 were purchased from Molecular Probes (Eugene, OR), acrolein and iodoacetic acid (IAA) were from Sigma-Aldrich (St. Louis, MO), and HNE was from Cayman Chemical (Ann Arbor, MI). Purified human Trx1 and TrxR1 were obtained from Labfrontier (Seoul, Korea) and American Diagnostica (Greenwich, CT), respectively. Mutant human Trx1 proteins (C32,35S and C62,69,73S) were obtained as reported previously.5
Antibodies were from BD Pharmingen (San Diego, CA) for Trx1; Santa Cruz Biotechnology (Santa Cruz, CA) for phospho-I
B
, ICAM1, and P-selectin; and Cell Signaling Technology (Beverly, MA) for phospho-cJun. To examine protein expression level, cells treated with or without acrolein were washed with ice-cold phosphate-buffered saline (PBS) three times, lysed with ice-cold buffer (10 mmol/L Tris, pH 7.4, 100 mmol/L NaCl, 1 mmol/L ethylenediaminetetraacetic acid, 2 mmol/L Na3VO4, 1% Triton X-100, 0.1% sodium dodecyl sulfate, 10% glycerol, and 0.5% deoxycholate), sonicated for 10 seconds on ice, and heated at 95°C. Fifty µg of protein were analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (PAGE), followed by Western blot with an antibody described above and an Alexa Fluor 680 or 800 secondary antibody (Molecular Probes).5,13
Corresponding protein bands were visualized using an Odyssey scanner using the Odyssey 2.1 software (LI-COR, Lincoln, NE) and quantified by densitometry. For measurement of gene expression, total cell mRNA was isolated and used for real-time polymerase chain reaction (PCR).14
Determination of Modifications of Trx1 by Western Blotting under Native Condition
To examine thiol modifications in Trx1, BAECs and purified human Trx1 proteins (WT and mutants) treated with acrolein, HNE, dithiothreitol, or H2O2 were prepared following the procedures as described.5 In brief, cells or purified proteins suspended in the buffer including 6 mol/L guanidine and 50 mmol/L IAA (pH 8.3) were incubated at 37°C for 30 minutes (note: excess dithiothreitol was always removed by G-25 spin column before adding IAA) followed by G-25 column process. The modified forms of cellular and purified Trx1 were analyzed by native PAGE followed by Western blot with mouse anti-human Trx1 and an Alexa Fluor 680 anti-mouse secondary antibody for Trx1 (Molecular Probes).5,13 Bands corresponding to Trx1 were visualized using an Odyssey scanner using the Odyssey 2.1 software (LI-COR).
Preparation of Acrolein- and HNE-Modified Trx1 [Wild Type (WT), Mutant (C62,69,73S)] and Activity Assays for Trx1 (WT, Mutant) and TrxR1
Acrolein- or HNE-modified Trx1 was prepared fresh for each experiment. Purified human Trx1 WT and mutant proteins (mutation of Cys 62,69,73 to Ser), fully reduced by TCEP or dithiothreitol, were purified using Microspin G-25 columns (GE Health Care, Little Chalfont, Buckinghamshire, UK) before further treatment with acrolein or HNE. After 1 hour, activated thiol-Sepharose (Sigma-Aldrich) was added to remove unreacted acrolein and HNE, samples were centrifuged, and the supernatant fractions, including acrolein- or HNE-modified Trx1, were collected for additional experiments including activity assays, mass spectrometry analysis, and microinjection. Trx1 and TrxR1 activities in a cell-free system were measured by the insulin reduction assay.5,15
Mass Spectrometry
For matrix-assisted laser desorption ionization/time of flight (MALDI-TOF)-mass spectrometry (MS), purified Trx1 (WT, mutant) proteins were prepared by tryptic digestion (Promega, Madison, WI) and desalting with Ziptip (Millipore, Bedford, MA). Positive-ion MALDI-TOF MS analysis was performed using Reflex III delayed-extraction MALDI-TOF mass spectrometer (Bruker Daltonics, Billerica, MA) equipped with a 337-nm nitrogen laser; -cyano-4-hydroxycinnamic acid (Agilent Technology, Santa Clara, CA) was used as the matrix. MALDI-TOF/TOF MS/MS was performed on a 4700 Proteomics Analyzer (Applied Biosystems, Foster City, CA) MALDI-TOF/TOF instrument. The instrument was operated in the positive ion mode. Laser intensity was adjusted manually to obtain the optimal fragmentation.
Monocyte Adhesion and Reactive Oxygen Species Measurement
Confluent BAECs, plated in six-well plates were exposed to acrolein for 1 hour, washed, incubated in fresh culture medium at 37°C for 5 hours, and used for monocyte adhesion studies. THP1 monocyte cells (5 x 105/plate) were prelabeled with green calcein AM and then coincubated with BAECs for 30 minutes. Plates were washed three times with PBS to remove unbound THP1 cells and observed at x20 magnification of fluorescent microscope. For quantification of monocyte binding, BAECs in 96-well plates were treated with acrolein or HNE and prepared as described above. Attached THP1 were quantified by measuring fluorescence intensity on a M2 plate reader (Molecular Devices, Sunnyvale, CA). Reactive oxygen species (ROS) detection was determined by dichlorofluorescein oxidation of BAECs plated onto 96-well plates incubated with acrolein or HNE for 1 hour and assayed as previously described.14
Glutathione Analysis and Redox Potential (Eh) Calculation
GSH and GSSG were quantified by high-performance liquid chromatography with fluorescence detection, expressed as molar concentrations based on cell volume, which were used to calculate the steady-state redox potential values using the Nernst equation.16
Determination of Gene Expression Levels by Real-Time RT-PCR
Total cellular mRNA was isolated from BAECs 5 hours after being treated, washed, and incubated with fresh medium. An equal amount of mRNA from each sample was used to generate cDNAs by reverse transcription (BD Bioscience, Franklin Lakes, NJ). For quantitative PCR, amplication was performed in triplicate on an iCycler IQ Multicolor RT-PCR detection system (Bio-Rad, Hercules, CA) for 35 cycles as follows: 95°C for 30 seconds, 62°C for 30 seconds, and 72°C for 1 minute. Quantification and melting curves were analyzed with iCycler software relative to a standard curve for each gene. The primers used for real-time PCR are as follows: ICAM forward, 5'-CTGAGGTCTCAGAATGGACTACTG-3'; ICAM reverse, 5'-GCCACGTCCAGTTTCCCGGGCAAT-3'; PECAM-1 forward, 5'-GTCCAACGTGGAGTCCTCCAGACC-3'; PECAM-1 reverse, 5'-GCCTCACTGCTGAGGCTGTTGTGT-3'; P-selectin forward, 5'-GACATGTCCTGCAGCAAGCAAGGA-3'; P-selectin reverse, 5'-GCTGCAGCTGGACTGGTGCTGGAA-3'; and MCP-1 forward, 5'-AACAGCTTCCCGCTGAAAC-3'; MCP-1 reverse, 5'-TCTGCACATAACTCCTTGCC-3'.
Microinjection
BAECs grown in 35-mm glass-bottom dishes were coinjected with Trx1 or acrolein- or HNE-modified Trx1 and cell membrane impermeable Alexa Fluor 568. Microinjection (Eppendorf FemtoJet with glass microcapillary) was performed under a fluorescence microscope (Ultraview ERS; Perkin-Elmer, Emeryville, CA) enclosed in a heated chamber with CO2 perfusion (5% CO2 at 37°C). Injection parameters were Pi: 45 hPa, Ti: 0.5 s, Pc: 15 hPa. BAECs were microinjected with modified or unmodified Trx1 (Trx1, Acr-Trx1, HNE-Trx1) and with the red fluorescent Alexa Fluor 568 as injection control. Cultures were then incubated for 5 hours at 37°C and 5% CO2, before the monocyte adhesion assay.
Electrophoretic Mobility Shift Assay
Cells (1 x 107) with or without acrolein treatment were prepared for nuclear extraction using a nuclear extraction kit (Panomics, Fremont, CA) following the manufacturer’s protocol. To examine NF-B activity, electrophoretic mobility shift assay was performed using an electrophoretic mobility shift assay gel-shift kit (Panomics) by incubating a biotin-labeled or unlabeled probe containing an NF-B DNA-binding consensus sequence (5'-AGTTGAGGGGACTTTCCCAGGC-3') with a nuclear extract (5 µg) for 30 minutes at 20°C. The samples were analyzed by a 6% nondenaturing PAGE, electroblotted for 30 minutes, and signals were detected by chemiluminescence imaging according to the manufacturer’s protocol (Panomics).
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Results |
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TopAbstractMaterials and MethodsResultsDiscussionReferences |
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Previously, Yang and colleague9
showed that acrolein inhibited Trx1 and TrxR1 activities using cell lysates extracted from acrolein-treated human lung adenocarcinoma cell line A549. Another recent study demonstrated that HNE inhibits E. coli Trx activity examined by using purified E. coli Trx protein.10
We examined whether acrolein or HNE reacted with purified human Trx1 and caused inhibition of Trx1 activity (Figure 1A)
. Purified human Trx1 protein was reduced by dithiothreitol (2 mmol/L) and excess dithiothreitol was removed by G-25 column before incubation with each aldehyde for 1 hour at room temperature. Remaining aldehyde was removed by thiol-conjugated agarose beads before performing Trx1 activity assay. Addition of acrolein or HNE to purified Trx1 at molar ratios of 1:1 (Trx1/aldehyde) showed complete inhibition of activity (Figure 1A
, Acr-Trx1 and HNE-Trx1) compared with untreated, control Trx1, indicating a high specificity of reaction, probably with a critical residue. Figure 1B
shows that mutation of the nonactive site disulfide of Trx1 (C62,69,73S) inhibits catalytic activity of Trx1 (WT, 100 ± 3.79; C62,69,73S, 55.7 ± 7.59) consistent with our previous report demonstrating inhibitory function of C62,69S.5
To examine whether modified Trx1 by acrolein or HNE inhibits TrxR1 function, we measured TrxR1 activity in a cell-free system under conditions with excess unmodified Trx1.15
As shown in Figure 1C
both acrolein- and HNE-modified Trx1 significantly inhibited TrxR1 activity by 26% and 18%, respectively [TrxR1 activity, 73.9 ± 3.8 (by acr-Trx1); 81.8 ± 6.1 (by HNE-Trx1); 100 ± 2.2 (by Trx1 control)].
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To identify acrolein- or HNE-modified residue(s) of Trx1, purified Trx1 and mutant Trx1 were reduced with Tris-(2-carboxyethyl)phosphine (TCEP) or dithiothreitol, purified with a G-25 column to remove excess dithiothreitol, and incubated at various ratios of acrolein or HNE to Trx1. After 1 hour, any excess free aldehyde was removed with activated thiol-containing agarose beads. Collected acrolein-Trx1 (WT, mutant) or HNE-Trx1 adduct was digested with trypsin and analyzed by MALDI-TOF mass spectrometry. At the bottom of Tables 1 and 2
, predicted peptides are listed and labeled T1 through T12, with five cysteine (Cys) residues for Trx1 (WT) and two Cys and three serine (Ser) residues for the mutant (C62,69,73S) indicated by italicized and underlined letters, respectively. The mass spectrum shown in Figure 2A
displays the tryptic fragments (T3 to T7) of control Trx1 (WT) larger than 1000 Da. All predicted peptides more than 1000 m/z were identified in the spectra including T4, T6, and T7, which contain all five cysteines, and T3 and T5, which do not contain cysteine residues (Table 1)
. Figure 2, B and C
, shows mass spectra used to identify acrolein-modified thiol residues of Trx1. No m/z values were obtained corresponding to the acrolein modification of Cys-32 or Cys-35 in T4 or Cys-62 or Cys-69 in T6 of Trx1 under all experimental conditions by varying ratios of acrolein to Trx1 as analyzed by MALDI-TOF mass spectrometry (Trx1/Acr, 1:5, 1:10; data not shown). However, modification of Trx1 by low concentrations of acrolein (Trx1/Acr, 1:1) was observed as a new peak at m/z 1186 (Figure 2B
, right), which was not detected in Trx1 controls (Figure 2B
, left). The mass increase in formation of the acrolein-T7 adduct was 38, which was 18 less than expected from addition of the molecular weight of acrolein (56) and is probably attributable to water elimination. The T7 peptide contains both a thiol and free amine on the lysine residue, either of which could react with acrolein. However, if the lysines were modified by acrolein, trypsin would not hydrolyze the adjacent peptide bond connecting T7 and T8. Therefore, the 1186 m/z seemed to be attributable to reaction of acrolein with Trx1 Cys-73, with rearrangement and elimination of water on tryptic digestion. We also examined the possibility of acrolein modification of His residues. One His residue is contained in T5 peptide of Trx1. If His is modified by acrolein, an adduct should appear at 1519.7 m/z. The mass of this peak was not detected in MS data of acrolein-Trx1 (Table 1)
, suggesting that His is not modified by acrolein at the conditions used for the current study. In the present study, we have not investigated detailed chemical identification. Therefore, whether acrolein-modified Trx1 adducts are a result from the Michael addition or from the Schiff base is unknown. We conclude that Trx1 adduct formation based on expected masses and observed masses is through the acrolein modification adduct, which was formed after reaction with acrolein (56 Da) and subsequent elimination of H2O (18 Da) (Table 1)
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shows MS/MS fragmentation data confirming that the cysteine residue (C-73) of T7 was modified by acrolein and resulted in 38 m/z increase in m/z 1148 of T7. Taken together, at the low 1:1 ratio of acrolein: Trx1, MALDI TOF/TOF and MS/MS analyses strongly suggest that none of the other Cys are modified except Cys-73. To support Cys-73 modification by acrolein, we also performed an experiment by utilization of Trx1 mutant protein, in which three regulatory Cys residues including Cys-73 were mutated to Ser (Table 2)
. All predicted peptides more than 1000 m/z were identified in the spectra including T4 containing two active cysteines, T6 containing two serines, T7 containing one serine, and T3 and T5, which do not contain cysteine residues (Table 2)
. The increase in mass after formation of the acrolein-T7 adduct (m/z 1186) shown from Trx1 (WT) with acrolein was not observed in the Trx1 mutant (expected mass, 1186.5 m/z; observed mass, ND; Table 2
). However, a new peak (m/z 1700.8) was detected that was not observed in samples in which Trx1 (WT) was treated with acrolein. This is possibly an acrolein-T4 adduct involving the dithiol in the active site (C32,C35) because of the absence of thiol residues in T7 (C73S) and T6 (C62,69S) in the Trx1 mutant. The mass is consistent with addition of acrolein to each of the active site thiols, after elimination of water. However, this adduct needs further study for chemical identification.
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Reactive Aldehyde-Induced Irreversible Thiol Modification of Trx1 in Endothelial Cells
Acrolein- and HNE-induced thiol modifications in Trx1 were further examined by native and nonreducing PAGE combining Western blot analysis [Figure 3
, A (left), B, and C]. Extracts are treated with IAA to introduce negative charges to thiols, allowing separation of thiol and disulfide forms of Trx1 by native gel electrophoresis.5
As previously shown for control Trx1,5
Trx1 pretreated with dithiothreitol before the addition of IAA had greater mobility than Trx1 pretreated with H2O2 (Figure 3A
, left). H2O2-treated active site mutant Trx1 (C32,35S) had an identical mobility to that of WT Trx1 (Figure 3A
, right; purified Trx1). However, with dithiothreitol treatment before IAA, the C32,35S-Trx1 had an intermediate mobility (arrow with asterisk). This result shows that C32,35S mutation affects Trx1 redox state examined by native, nonreducing PAGE combining Western blot analysis.
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, left). With 1 µmol/L acrolein, Trx1 moved with slower mobility than that of Trx1 fully reduced form (Trx1Red). However, at 5 and 10 µmol/L, the majority of Trx1 migrated more slowly, indicating a dose-dependent modification (Figure 3A
, left). The slower mobility of acrloein-Trx1 in cells was the same as seen for treatment of purified reduced Trx1 with acrolein (Figure 1A
, top).
To examine if acrolein-induced Trx1 modification is irreversible, BAECs were treated with acrolein for 1 hour, washed, lysed, and incubated with dithiothreitol for 30 minutes. This treatment results in reduction of all disulfides in Trx1. Thus, acrolein-modified Trx1 (Acr-Trx1) can be resolved from unmodified Trx1 by the same procedure used above. As shown in Figure 3B
, acrolein-modified and an irreversible form of Trx1 (Acr-Trx1) were observed after dithiothreitol treatment, whereas the oxidized, disulfide form of Trx1 was not shown any longer. The modification of Trx1 was still observed at 1 µmol/L or greater after dithiothreitol treatment (data not shown) suggesting acrolein-induced changes in Trx1 mobility represent irreversible modification. Analyses of HNE-treated BAECs showed that HNE similarly modified Trx1 (Figure 3C)
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Acrolein and HNE Enhance Monocyte Adhesion to Endothelium
To examine acrolein or HNE stimulation of monocyte adhesion to the endothelium, we used an in vitro cell model consisting of THP1 cells and confluent BAECs. Confluent BAECs were exposed to 1, 5, or 10 µmol/L of acrolein or HNE in Hanks’ balanced salt solution for 1 hour, washed with Hanks’ balanced salt solution three times, and incubated in fresh culture medium (10% fetal bovine serum in Dulbecco’s modified Eagle’s medium) for 5 hours before monocyte addition. BAECs treated with either acrolein (Figure 4
, middle) or HNE (Figure 4
, bottom) showed a significantly higher degree of monocyte adhesion as determined by phase-contrast (Ph) and fluorescence (Fl) microscopy. Quantification of monocyte adhesion was obtained by measuring the fluorescence of attached monocytes labeled with calcein AM to BAECs with a fluorescence plate reader. Results showed that monocyte binding was increased in acrolein-treated (1 µmol/L, 116 ± 4.2; 5 µmol/L, 134 ± 5.3; 10 µmol/L, 137 ± 3.7) and HNE-treated (1 µmol/L, 125 ± 2.2; 5 µmol/L, 151 ± 5.7; 10 µmol/L, 155 ± 5.4) cells (Figure 4
, bar graphs). We also examined if acrolein-induced cytotoxic effects were reversible. BAECs were incubated with complete media for 1 day after an initial 1-hour acrolein treatment. If acrolein-induced inflammatory effects are reversible in BAECs, increased monocyte adhesion should not be observed. The results showed that BAECs treated with 10 µmol/L acrolein increased cell death and loss of Trx1 protein significantly after 24 hours. Because of these results, cells with 10 µmol/L acrolein were not applicable for cell adhesion assay 24 hours after acrolein treatment. Cells treated with 1 or 5 µmol/L acrolein showed significant increase in cell adhesion examined 24 hours after acrolein exposure (data not shown), suggesting that acrolein-induced toxicity (cell death) and cell adhesion are not reversible.
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Because acrolein can have multiple cellular targets, the downstream effects that result directly from Trx1 modification cannot be determined by treating cells with acrolein or HNE. To specifically address Trx1 modification-related effects on monocyte adhesion, BAECs were grown in 35-mm plates and microinjected with Trx1, acrolein-modified Trx1, or HNE-modified Trx1, along with cell membrane-impermeant red fluorescent molecule (Alexa Fluor 568; Fl) to identify injected cells. After injection, cells were incubated at 37°C, 5% CO2 for 5 hours and then analyzed for monocyte adhesion assay as described above. THP1 monocytes (5 x 105) labeled by calcein AM (green-fluorescence after deacetylation) were added to BAEC monolayer cultures for 1 hour and then washed twice in the PBS to remove unbound THP1. Attached monocytes were observed using fluorescence microscopy (Olympus 1 x 81 with x200 magnification). Red-fluorescent cells in Figure 5
are BAECs injected with acrolein-modified Trx1 (Figure 5A
, Acr-Trx1, Fl), HNE-modified Trx1 (Figure 5B
, HNE-Trx1, Fl), and Trx1 (Figure 5C
, Trx1, Fl). THP1 (green) attached to the BAECs injected with acrolein-modified Trx1 and HNE-modified Trx1 are shown (Figure 5, A and B
; top). The same images were taken to visualize all cells, including injected and uninjected cells, by phase-contrast microscopy (Ph; Figure 5
, bottom). Monocytes attached to the injected BAECs with Trx1, Trx1-Acr, Trx1-HNE, or red-fluorescent dye alone were quantified as shown in Figure 5D
. BAECs injected with acrolein-modified Trx1 (Acr-Trx1) and HNE-modified Trx1 (HNE-Trx1) showed between three and seven times more attached monocytes, respectively, than those in fluorescent dye alone (Alexa Fluor 568) or with injected Trx1 (Trx1).
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Previous studies have shown that acrolein and HNE stimulate peroxide production in lung epithelial and lung endothelial cells by depletion of cellular thiols such as GSH.17-19
To determine whether acrolein and HNE stimulate ROS generation in BAECs, cultures were treated with acrolein (5 µmol/L) or HNE (5 µmol/L) for 1 hour, and dichlorofluorescein fluorescence was used to measure ROS (Figure 6A)
. ROS levels were elevated by both acrolein (152 ± 11.5%) and HNE (173 ± 9.8%) treatment compared with the control (Figure 6A)
. Amounts of reduced (GSH) and oxidized (GSSG) glutathione (Figure 6B
: bottom left, acrolein; bottom right, HNE) and correlating redox potentials were measured in cells treated with acrolein and HNE (Figure 6B
, top: filled circle, HNE; opened circle, acrolein). Oxidation of GSH/GSSG redox potential was significantly increased at 5 µmol/L or higher concentration of acrolein but not at 1 µmol/L. Oxidation of GSH/GSSG redox state was observed at 5 µmol/L acrolein, resulting in a 32 mV increase compared with control. Interestingly, there was no significant effect of HNE on the GSH/GSSG redox state.
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Increased cell adhesion in atherosclerosis is mediated by an increased expression of cell adhesion molecules in endothelial cells. Abundance of mRNA for members of the cell adhesion molecules and the selectin family, including ICAM-1, PECAM-1, P-selectin, E-selectin, and monocyte chemoattractant protein (MCP-1), was examined using real-time PCR. Expression levels for ICAM-1, PECAM-1, P-selectin, and MCP-1 were significantly increased in cells exposed to acrolein, even at 1 µmol/L (Figure 7A)
. The maximal increase in gene expression was 
5.5-fold for PECAM-1, 1.7-fold for ICAM-1, 18-fold for MCP-1, and 2.2-fold for P-selectin by 10 µmol/L acrolein treatment, whereas mRNA for E-selectin was decreased by acrolein (data not shown). BAECs pre-exposed to acrolein for 1 hour followed by an additional 5-hour incubation with acrolein-free media enhanced gene expression levels as well as protein concentrations for cell adhesion molecules. To examine whether protein levels were elevated, P-selectin and ICAM-1 proteins were examined by immunoblotting (Figure 7A)
. Although fold increases in protein levels were not the same as mRNA, significant increases were observed in both ICAM-1 and P-selectin proteins (1.8- and 3.0-fold, respectively; Figure 7A
). Collectively, the data show that exposure of endothelial cells to acrolein increased gene expression for cell-cell adhesion molecules, even at a concentration of acrolein (1 µmol/L) that did not cause detectable change in GSH/GSSG.
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B but Stimulated c-Jun Phosphorylation
NF-B is a redox-sensitive transcription factor that functions in regulation of adhesion molecule expression on oxidative stimuli. To monitor the effect of acrolein on NF-B activity, nuclear fractions of BAECs treated with acrolein for 1 hour were used for electrophoretic mobility shift assay (Panomics) (Figure 7B
, left). Background level of binding activity of NF-B to the biotin-labeled probe is shown and indicated as NF-B-bound probe (Figure 7B
, left, lane 3; acrolein, 0 µmol/L). Increasing concentrations of acrolein inhibited NF-B binding to the probe, showing that acrolein inhibited rather than stimulated NF-B function (Figure 7B
, left; lanes 3 to 5).
As an independent approach to monitor effects of acrolein treatment on the NF-B signaling, Western blot analysis was performed with an antibody specific to phosphorylated IB (Figure 7B
, right). Phosphorylation of IB under control condition (acrolein: 0 µmol/L) was decreased by acrolein treatment in a dose-dependent pattern (Figure 7B
, right; 1 µmol/L, 52.2 ± 5.8; 5 µmol/L, 34.7 ± 3.9; 10 µmol/L, 29.8 ± 12.7). Thus, the combination of approaches suggests that the activation of adhesion molecule expression (Figure 7A)
in response to acrolein is not regulated through NF-B activation.
AP-1 also mediates transcription of cell adhesion molecules, and acrolein has been shown to activate c-Jun in rat vascular smooth muscle cells.20
Consistent with this result, acrolein also increased phosphorylation of c-Jun in BAECs in a concentration-dependent manner (fold stimulation relative to control; 1 µmol/L,1.98 ± 0.16; 5 µmol/L, 6.5 ± 0.4; 10 µmol/L, 10.7 ± 0.2) (Figure 7C)
. Thus, the results suggest that acrolein-increased monocyte adhesion to vascular endothelial cells is associated with increased expression of adhesion molecules regulated by an AP-1-dependent mechanism.
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Discussion |
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TopAbstractMaterials and MethodsResultsDiscussionReferences |
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The level of acrolein in the sera of healthy persons was measured as high as 50 µmol/L and was measured at as high as 80 µmol/L in respiratory tract lining fluids in cigarette smokers.24 Acrolein and HNE are conjugated aldehydes10,25 that react with cellular thiols and primary amines.26 Acrolein found as protein conjugates in the plasma of patients was 180 µmol/L.24 HNE concentrations in the sera of healthy people were in the range 70 to 100 nmol/L,27 and liver tissue HNE levels were in the range 0.5 to 10 µmol/L.28 It is thought that the function of some proteins would be lost through the covalent linkage with these reactive aldehydes. Most proteins contain thiols and primary amines so that elucidation of critical targets of modification has proven difficult. However, thioredoxins and related proteins are common targets. Because Trx1 plays a central role in cell growth and survival signaling, modification and inhibition of Trx1 could have important pathological consequences. To support the present study, research is needed to determine whether acrolein- or HNE-modified Trx1 is actually present in diseased vascular tissues. Our initial studies of tissue samples show that the procedures described here should be suitable to detect such modifications when they occur. The present study shows that acrolein and HNE modify Trx1 in endothelial cells and stimulate inflammatory signaling events, including ROS generation, GSH/GSSG redox oxidation, elevated mRNA of cell adhesion molecules, and increased monocyte adhesion. Importantly, microinjection of acrolein- or HNE-modified Trx1 into the endothelial cells stimulated monocyte adhesion. We also performed experiments to test whether microinjection of reduced Trx1 can block acrolein-stimulated inflammatory signaling. Confluent BAECs pretreated with acrolein (1 µmol/L, 37°C) for 1 hour were used for microinjection of Trx1 in the complete medium after removal of acrolein. However, the experimental conditions combining acrolein treatment and microinjection (injection of reduced Trx1 protein plus dye or dye alone) stimulated cell death after an additional 5 hours of incubation at 37°C, 5% CO2 (95% of cell death in cells pretreated with acrolein followed by microinjection, data not shown). Because of the large amount of cell death, there were too few cells remaining to determine whether microinjection of Trx1 protected against acrolein-induced monocyte adhesion.
The active site Cys-32 residue of all thioredoxins including E. coli Trx has a low pKa and is very reactive under physiological conditions,29
but other Cys residues found in human Trx1, including Cys-62, -69, and -73, are also structurally and functionally important.5
In addition to a recent report suggesting that nitration of Tyr-49 is a novel posttranslational modification that inhibits Trx1 function,23
modification of cysteines by oxidation,5
S-nitrosylation (Cys-69),4,21
glutathionylation (Cys-73),3
or the experimental anticancer drug PX-12 (Cys-73) also inhibited activity.30,31
In addition to the previous study showing HNE-induced catalytic inhibition of E. coli Trx by modification of active site cysteines,10
we also examined E. coli Trx and found that Cys-32/Cys-35 were modified by acrolein (data not shown), which is consistent with the previous report by Fang and Holmgren.10
E. coli Trx is different from mammalian Trx1 structurally and functionally because E. coli Trx contains two cysteines in the catalytic site and does not have the other three regulatory cysteines observed in mammalian Trx1. We also examined mitochondrial Trx2, which only has active site cysteines, and found that this is also modified at the active site cysteines (data not shown). The present study demonstrates that low concentrations of acrolein and HNE preferentially modify Cys-73 of Trx1 rather than Cys-32 or Cys-35 in the active site, or the regulatory Cys-62 or Cys-69 sites. This may be attributable to Cys-73 being more accessible than Cys residues in active site, based on Trx1 structure.32
Previous reports showed that glutathionylation and dimerization of Trx1 occurs through Cys-73.33
The nature of the Cys-73 product detected by mass spectrometry is not clear but is likely to be derived from an initial acrolein-induced modification of the thiol group followed by reaction with the nucleophilic -amino group on Cys-73, which becomes available for reaction after tryptic hydrolysis of the Lys-72-Cys-73 peptide bond. In addition to the spectral data, Western blots to detect chemical modifications by native PAGE showed that Trx1 is extensively modified and more sensitive than glutathione (Figure 6B)
or mitochondrial Trx2 (data not shown).
Trx1 showed decreased electrophoretic migration at higher concentrations of acrolein (5 and 10 µmol/L) than at lower concentrations (1 µmol/L) as separated by native, nonreducing PAGE combining Western blot analysis, suggesting that the number of free thiols was decreased by increasing acrolein concentration in BAECs (Figure 3A)
. Mass spectrometry data using purified Trx1 protein showed that Cys-73 undergoes selective thiol modification and that this modification is independent of acrolein concentration throughout the range studied (Trx1/acrolein, 1:10; data not shown). Thus, additional low-mobility Trx1 bands at higher concentration of acrolein could be from disulfide formation resulting from increased ROS and oxidation of Trx1. This result could be attributable to inhibition of TrxR1 activity directly by acrolein and/or by acrolein-modified Trx1 as shown in Figure 1
, which then results in Trx1 oxidation. This interpretation is consistent with earlier research showing that TrxR1 activity in human umbilical vein endothelial cells was significantly inhibited by acrolein treatment.34
Inhibition of TrxR1 activity has also been observed by HNE-induced modification of Cys-496/Sec-497 in the active site of TrxR1 in HeLa cells.10
A Trx1-regulated redox mechanism for activation of transcriptional factors, including NF-B and AP-1, has been well studied. These transcription factors are redox-sensitive, regulate cell adhesion molecules, and are controlled by Trx1. In this study we found differential effects of acrolein on transcriptional factors such as inhibition of NF-B and stimulation of c-Jun phosphorylation as a subunit of AP-1. Our data suggest that elevated expression of cell adhesion molecules (ICAM, PECAM, and P-selectin) and MCP-1 by acrolein is through an NF-B-independent mechanism. Previous reports are somewhat confusing because AP-1 regulation by acrolein examined with c-Jun phosphorylation was controversial depending on cell types and exposure times.20,35
The present study shows that acrolein-induced c-Jun phosphorylation was increased in endothelial cells, suggesting that the acrolein-dependent monocyte adhesion is mediated by activation of AP-1.
In summary, stoichiometric amounts of acrolein or HNE selectively modified the nonactive site Cys-73 of Trx1. The modification resulted in loss of catalytic activity and inhibition of TrxR1. It seems that modification by acrolein or HNE is more complicated than oxidation or nitration because modified Trx1 not only lost its normal antioxidant function but also gained a biological function that has an adverse impact on monocyte-endothelial interaction, ie, stimulating monocyte binding. In cells, modification of Trx1 was detectable at 1 µmol/L whereas oxidation of GSH redox state was detectable at 5 µmol/L or higher concentrations. Elevated ROS levels by acrolein and HNE were also observed, indicating dysfunction of antioxidant systems and increased oxidative stress by these reactive aldehydes. Microinjection studies show that Cys-73 modification is sufficient to stimulate monocyte adhesion to endothelial cells. Therefore, chemical modification of Trx1 by reactive conjugated aldehydes may contribute to early events of atherosclerosis.
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Supported by the National Institutes of Health (grants ES011195 and ES09047).
Accepted for publication July 19, 2007.
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