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Chlorotyrosine is an oxidative product of hypochlorous acid and L-tyrosine, and is considered as a biomarker for oxidative stress and cardiovascular disease. However, it is not clear whether chlorotyrosine could directly contribute to vascular pathogenesis. In this study, we investigated the effect and potential mechanisms of chlorotyrosine on human aortic smooth muscle cell (AoSMC) migration. With Boyden chamber and wound healing assays, chlorotyrosine significantly increased AoSMC migration in a concentration- and time-dependent manner. In addition, chlorotyrosine significantly increased the expression of several key molecules related to cell migration including PDGF receptor-B (PDGFR-B), matrix metalloproteinases (MMP-1 and MMP-2) and integrins (α3, αV, and β3) in AoSMC at both mRNA and protein levels. Furthermore, chlorotyrosine also increased superoxide anion generation in AoSMC with the fluorescent dye dihydroethidium (DHE) staining. Activation of mitogen-activated protein kinases (MAPKs) was analyzed with Bio-Plex Luminex immunoassay and western blotting. Chlorotyrosine induced a transient phosphorylation of ERK1/2, but not JNK and p38 MAPKs. Antioxidants including selenomethionine (SeMet) and Mn (III) tetrakis (4-benzoic acid) porphyrin (MnTBAP) as well as ERK1/2 inhibitor PD98059 effectively blocked chlorotyrosine-induced AoSMC migration. Thus, these findings demonstrate new biological functions of chlorotyrosine in human SMC migration, which may play a crucial role in the vascular lesion formation.
Oxidative modification of proteins or amino acids is believed to be one of important pathogenic factors for many diseases including atherosclerosis . Myeloperoxidase (MPO), a major phagocyte secreted protein, has been implicated in oxidative damage at sites of inflammation. It catalyzes the two-electron peroxidation of chloride to generate hypochlorous acid (HOCl) , a potent chlorinating oxidant, capable of chlorinating tyrosine into chlorotyrosine. Levels of chlorotyrosine are increased in atherosclerotic tissues  and the blood of in patients with inflammatory conditions including atherosclerosis [4, 25, 26]. The chlorination of apolipoprotein A-I was found to impair its cardioprotective ability to remove excess cellular cholesterol from lipid-laden macrophages in the artery wall . Furthermore, increased chlorotyrosine was observed in the neointimal hyperplasia following vascular injury . Chlorotyrosine was considered as a biomarker for coronary artery disease. However, biological functions of chlorotyrosine in the vascular system are largely unknown.
Vascular smooth muscle cells (VSMC) in the tunica media can acquire the ability to proliferate, migrate, and accumulate within the intimal layer of arteries under the influence of chemotaxis-inducing chemokines, cytokines and growth factors . Migration and proliferation of VSMC are the key process of neointimal hyperplasia and atherosclerosis progression . Many growth factors such as platelet-derived growth factor-BB (PDGF-BB) play crucial roles in VSMC migration. Matrix metalloproteinases (MMPs) and integrins provide permissive effects for VSMC migration by breaking down major extracellular barriers such as basal membranes, interstitial collagens, and proteoglycans .
Reactive oxygen species (ROS) including superoxide anion are generated by a variety of extracellular and intracellular mechanisms and are involved in several signal transduction pathways such as mitogen-activated protein kinases (MAPKs) . ROS mediate cellular signaling pathways in VSMC proliferation and migration associated with atherosclerosis . Thus, increased ROS generation may contribute to cardiovascular diseases such as atherosclerosis, angina pectoris, and myocardial infarction . Antioxidants are believed to reduce the incidence of coronary artery disease . Mn (III) tetrakis (4-benzoic acid) porphyrin (MnTBAP) is a cell permeable superoxide dismutase (SOD) mimetic, which is an effective superoxide anion inhibitor . Seleno-l-methionine (SeMet) is an effective antioxidant by increasing the activity of glutathione peroxidase .
We hypothesized that chlorotyrosine may have directly effects on the vascular system. In the current study, we investigated the effect and potential mechanisms of chlorotyrosine on the migration of human aortic smooth muscle cells (AoSMC). The expression of PDGFR-B, MMPs and integrins were studied at mRNA and protein levels in chlorotyrosine-treated cells. We further investigated the involvement of superoxide anion generation and MAPK activation in chlorotyrosine-induced AoSMC migration. This study provides experimental evidence of biologic functions of chlorotyrosine in human VSMC, which may contribute to the vascular lesion formation.
L-Tyrosine, 3-chloro-L-tyrosine (MW 215.64), anti-human β-actin antibody and SeMet were obtained from Sigma-Aldrich (St. Louis, MO). Anti-human integrin β3 (CD61), integrin αV, and integrin α3 (CD49c) were purchased from BD Biosciences Pharmingen (San Diego, CA). Human platelet derived growth factor-BB (PDGF-BB), anti-human PDGFR-B antibody, and anti-human MMP1 were from R&D Systems, Inc. (Minneapolis, MN). Rabbit monoclonal antibodies specific for phosphor- and total ERK (p44/p42), JNK, and p38 were from Cell Signaling Technology, Inc. (Danvers, MA). Dihydroethidium (DHE) was obtained from Molecular Probes (Eugene, OR). MnTBAP was purchased from A.G. Scientific (San Diego, CA). Extracellular signal-regulated kinase (ERK) inhibitor (PD98059), JNK inhibitor (SB203580), p38 inhibitor (SP600125), and calcein-AM were obtained from Calbiochem Inc. (San Diego, CA). Trizol reagent was from Invitrogen (Carlsbad, CA).
AoSMC were purchased from Cambrex Bio Science Walkersvile, Inc. (Walkersville, MD). The cells were routinely cultured in Smooth Muscle Medium-2 (SmGM-2) with growth factors and antibiotic (SmGM-2 Bullet Kit) supplemented with 10% fetal calf serum (FCS). Prior to each experiment, AoSMC were placed in the SmGM-2 medium with 1% FCS, without addition of growth factors (basal medium) for 16 h (serum starvation). AoSMC were used between passages 3 and 7.
Cell migration was measured with a modified Boyden chamber assay including Fluoroblok transwell migration plates as described in our previous study . Briefly, serum-starved cells were trypsin-harvested. Cell suspension in basal medium (250 μl, 1 × 105 cells/well) seeded in the upper chamber (24-well plate). Then, 750 μl of basal medium with chlorotyrosine or L-tyrosine was added to the lower chamber and incubated for 4 to 24 h. After incubation, the cells were labeled by 50 nM calcein-AM, a fluorescence dye. The fluorescence of the cells migrated to the lower chamber was measured from the bottom using a fluorescence microplate reader at 485/535 nm wavelength. Each treatment was repeated in 4 independent transwells. The migrated cells in the filters were also observed under fluorescence microscope (Olympus; Tokyo, Japan).
When AoSMC have grown to 90% confluence in the six-well plate, a scratch was made with a sterile cell scraper as described in our previous study . The starting point was marked with a marker pen at the bottom of the plate. Cells were washed with basal medium twice and were incubated with chlorotyrosine (103 nM), L-tyrosine (103 nM), or PDGF-BB (10 ng/ml) in basal medium, and the cells were incubated for 16 h and stained with calcein-AM (50 nM). Photos were taken under fluorescence microscope.
Serum-starved AoSMC were treated with chlorotyrosin (103 nM) or L-tyrosine (103 nM) for 24 h, and total RNA was isolated for real time PCR analysis as described in our previous study . Briefly, primers for human PDGFR-B, MMPs, integrins and β-actin were designed using Beacon Designer software (Table 1). Sample cycle threshold (Ct) values were determined from plots of relative fluorescence units (RFU) versus PCR cycle number during exponential amplification so that sample measurement comparisons were possible. The mRNA levels of these molecules in each sample were calculated as 2(40 – Ct) and further normalized to β-actin expression as [2(Ct β-actin-Ct gene of interest)].
Protein levels of intergrin α3 and β3 of AoSMC were determined with flow cytometry analysis as previously described . Briefly, the cell suspension was immnunoblocked in 10% human serum in 4°C for 20 min. Similar numbers of cells (1 × 106) were immunostained with manufacturer-recommended concentrations of antibodies: anti-integrin α3 and β3 antibodies in 4°C for 30 min. Mouse IgG1 was used as an isotype control. PE-conjugated anti-mouse IgG1 was used as a secondary antibody. The cells were analyzed with FACScalibur flow cytometer (Becton Dickinson, San Jose, CA). Data are presented as the percentage of positive cells corresponding to the mean fluorescence intensity in each experiment.
Cultured cells were treated with chlorotyrosine (103 nM) or L-tyrosine (103 nM) for 60 min. Superoxide anion production was determined with DHE staining. DHE is freely permeable to cells. In the presence of superoxide anion, DHE is oxidized to ethidium bromide (EtBr) with red fluorescence, and it is trapped by intercalating with DNA. EtBr is excited at 488 nm with an emission spectrum of 610 nm. Thus, the amount of EtBr detected with fluorescence measurement is well correlated to the level of cellular superoxide anion. One milliliter of DHE (3 μM) in PBS was added into each well of six-well plates and incubated for 20 min at room temperature. DHE staining was analyzed with FACS Calibure flow cytometry.
The MAPK phosphorylation state of AoSMC lysates was analyzed with Bio-Plex phosphoprotein and total target assays (Bio-Rad) as described in our previous publication . Briefly, serum-starved AoSMC were treated with chlorotyrosine (103 nM) or L-tyrosine (103 nM), and the cell lysates were collected at different time points of 0, 5, 10, 20, 30, and 60 min. The phosphoprotein and total proteins of MAPKs were analyzed with a Luminex 100TM analyzer (Bio-Rad). Data are presented as the fold of phosphorylated to total MAPK proteins in each sample.
AoSMC were treated with chlorotyrosine (103 nM) and L-tyrosine (103 nM) for 24 h, and the cell lysates were collected. Total cellular proteins were separated with 10% SDS-polyacrylamide gel electrophoresis and then transblotted onto the Hybond-P PVDF membrane (Amersham, Piscataway, NJ). After blocking the membrane with 5% nonfat dried milk, the membrane was probed with the respective primary antibodies overnight at 4°C against human integrin αV, MMP1, and PDGFR-B, respectively. The membranes were then incubated in the horseradish peroxidase (HRP)–linked secondary antibody for 50 min. The immunoreactive bands were detected with enhanced chemiluminescent (ECL) plus reagent kit. The band density was measured with the use of AlphaEaseFC 3.1.2 software (Alpha Innotech Corporation, San Leandro, CA) and normalized to β-actin expression.
The MAPK phosphorylation state of AoSMC lysates was also analyzed with western blotting. The cell lysates were collected as described as above. The phosphor- and total proteins of ERK1/2, JNK, and p38 were recognized with rabbit anti-human phospho- and total p44/42 MAPK (ERK), SAPK/JNK, and p38 antibodies. Followed with HRP–linked secondary antibody, the immunoreactive bands were detected with enhanced chemiluminescence (ECL) plus reagent kit.
The MMP2 activity of AoSMC was determined with the EnzoLyte™ 490 MMP-2 Assay Kit from AnaSpec Co. (San Jose, CA). Briefly, the supernatants of AoSMC cultures were collected and concentrated in YM-10 columns. One hundred microgram protein in 50 μl of the concentrated supernatant was used for each assay. After activated by p-aminophenylmercuric acetate (APMA) for 15 min at 37°C, 50 μl of a fluorogenic [an EDANS/DABCYL fluorescence resonance energy transfer (FRET) peptide] MMP2 substrate solution was added and the reaction was incubated for 30 min at room temperature. The fluorescence intensity representing the MMP2 activity was measured at 485/535 nm wavelength.
Data are presented as mean±SEM in all experiments. Data from treated groups were compared with control groups with Student’s t-test (two tailed). A value of p < 0.05 was considered significant.
In order to investigate the effect of chlorotyrosine on AoSMC migration, cells were treated for 24 h with increasing concentrations of chlorotyrosine (0-104 nM) or L-tyrosine (104 nM) in the Boyden chamber. The cells migrated through a polystyrene-membrane with 8 μm-size pores were stained with calcein-AM, and measured with a fluorescence reader. As shown in Fig. 1A, L-tyrosine treatment had no effect on AoSMC migration compared with the no treatment group. However, we found that chlorotyrosine significantly (p < 0.01) increased AoSMC migration in a concentration-dependent manner (up to 149% compared with the L-tyrosine control). Chlorotyrosine at 103 nM was found to reach the maximal effect on AoSMC migration. As a positive control, PDGF-BB significantly increased AoSMC migration up to 200%. The cells that had migrated to the lower chamber were also visualized under fluorescence microscope (Fig. 1B). To confirm the above findings, we performed a time course study at 4, 8, 16 and 24 h, and observed a significant effect of chlorotyrosine (103 nM) on cell migration as early as 4 h after cell seeding (Fig. 1C).
To further confirm the effect of chlorotyrosine on AoSMC migration, we performed wound healing assay on single layered AoSMC and observed the cell morphology and migration behavior after treatment of chlorotyrosine (103 nM) or L-tyrosine (103 nM). As shown in Fig. 1D, chlorotyrosine substantially promoted cell migration during the wound healing process compared with the control.
PDGF-BB, mediated through its receptor PDGFR-B, is a major regulator of SMC proliferation and migration. MMPs and integrins play an essential permissive role in cell migration. Expression of PDGFR-B, MMPs and integrins in AoSMC with chlorotyrosine and L-tyrosine treatment were evaluated with real-time PCR. As shown in Fig. 2, chlorotyrosine significantly (p < 0.05) increased the mRNA levels of PDGFR-B, integrin β3, integrin α3, integrin αV, MMP1 and MMP2 up to 187%, 237%, 137%, 163%, 200% and 200%, respectively, compared with the L-tyrosine treated controls.(100%).
Western blot and quantitation data further demonstrated that chlorotyrosine increased protein levels of PDGFR-B, integrins (β3, α3, and αV), and MMP1 expression up to 245%, 238%, 185%, 198%, and 217% respectively, compared with the L-tyrosine treatment (100%, Fig. 2B and C). Furthermore, chlorotyrosine significantly (p < 0.01) increased MMP2 activity in a concentration-dependent manner with the maximal effect up to 213% compared with the L-tyrosine treated control (Fig. 2D). Thus, these data demonstrate that chlorotyrosine upregulates several key molecules related to SMC migration including PDGFR-B, integrins, and MMPs at both mRNA and protein levels.
ROS including superoxide anion mediate cellular signaling pathways in VSMC proliferation and migration associated with atherosclerosis (10). To explore whether superoxide anion could be involved in the molecular mechanism of chlorotyrosine-induced AoSMC migration, we examined the effects of chlorotyriosine on superoxide anion generation and antioxidant application. Superoxide anion levels were determined with fluorescence staining with DHE, a compound chemically oxidized to the fluorescent DNA intercalating agent ethidium in proportion to the amount of superoxide present . The fluorescence for superoxide anion was analyzed with flow cytometry analysis. As shown in Fig. 3A and B, chlorotyrosine (103 nM) significantly (p < 0.01) increased superoxide anion production up to 42% of cell population compared with L-tyrosine (103 nM) (26%) after 60 min treatment. These data indicate that chlorotyrosine can induce superoxide anion production, which might be involved in the signaling pathway of the chlorotyrosine-induced cell migration. Indeed, well characterized antioxidants including SeMet (20 μM) and MnTBAP (3 μM) were used in the AoSMC cultures with chlorotyrosine treatment, and effectively blocked chlorotyrosine-induced AoSMC migration (Fig. 3C and D). These data indicate that superoxide anion plays an important role in chlorotyrosine-induced AoSMC migration.
Since the activation of MAPKs can be triggered by oxidative stress  and chlorotyrosine can increase superoxide anion production in this study, we further investigated whether chlorotyrosine could activate MAPKs. Major MAPKs include ERK1/2, and the c-Jun N-terminal protein kinase (JNK), and p38. Serum-starved AoSMC were incubated chlorotyrosine (103 nM) or L-tyrosine (103 nM), and the cell lysates were collected at different time points. A Bio-Plex assay kit (Bio-Rad bioscience) was used to detect phosphoproteins and total proteins of ERK2, JNK and p38,. The ratio of phosphoproteins to total proteins at each time point was used to evaluate the phosphorylation levels of ERK2, JNK, and p38. As shown in Fig. 4A, chlorotyrosine (103 nM) substantially increased ERK2 phosphorylation with a peak up to 7-fold at 10 min, whereas L-tyrosine (103 nM) had very limited effects on ERK2 phosphorylation. This result was confirmed with western blot showed that increased phospho-ERK1/2 was detected from 5 to 20 minutes after chlorotyrosine treatment (Fig. 4B). However, chlorotyrosine had no significant effects on JNK and p38 phosphorylation (Fig. 4C, D, E, and F). These results suggest that chlorotyrosine can activate ERK1/2 signaling pathways on AoSMC.
To further test the impact of ERK1/2 activation on chlorotyrosine-induced AoSMC migration, we incubated AoSMC with an ERK1/2 specific inhibitor (PD98059, 10 μM), a JNK inhibitor (SP600125, 10 μM), or a p38 inhibitor (SB203580, 10 μM) for 1 h prior and during chlorotyrosine or L-tyrosine treatment. Cell migration was evaluated with the modified Boyden chamber assay. As shown in Fig. 5A, ERK1/2 inhibitor PD98059 completely blocked the promoting effect of chlorotyrosine on AoSMC migration (p < 0.01). However, JNK and p38 inhibitors did not block chlorotyrosine-induced AoSMC migration (Fig.5B). These findings suggest that chlorotyrosine promotes AoSMC migration through the ERK1/2 signaling pathway.
Chlorotyrosine is considered as a MPO-specific oxidant marker, while it is not clear whether chlorotyrosine is directly involved in vascular pathogenesis. In the current study, we demonstrated that chlorotyrosine promotes AoSMC migration in a concentration- and time-dependent manner. In addition, chlorotyrosine increases the expression of several key molecules for AoSMC migration including PDGFR-B, MMPs and integrins in AoSMC. Furthermore, we have found that chlorotyrosine induces superoxide anion production in AoSMC and activates ERK1/2 signal pathway. These findings provide a direct link between chlorotyrosine and AoSMC migration.
Current investigation of the biological functions of free chlorotyrosine in vitro may have clinical significance. Free chlorotyrosine has been used as a biomarker for MPO-catalyzed oxidation damage in vivo and in vitro besides the protein-bound chlorotyrosine. For example, human neutrophils convert free L-tyrosine to 3-chlorotyrosine by the MPO-hydrogen peroxide system using two independent models of phagocytosis . This study suggests that chlorotyrosine generated by phagocytes may be involved in the oxidative and cytotoxic reactions at sites of inflammation and vascular disease. Free 3-chlorotyrosine levels are elevated in peritoneal inflammatory fluid from septic wild-type mice but not in the septic MPO-deficient mice . Free 3-chlorotyrosine is the molecular fingerprint that implicates the MPO pathway in oxidative damage. In the current study, we have observed the direct effect of free 3-chlorotyrosine on AoSMC migration, which is the major mechanism of the vascular lesion formation. Therefore, we believe the current study is clinically relevant and increased levels of 3-chorotyrosine under inflammation conditions may contribute to vascular disease.
In the current study, we have confirmed the specific effects of 3-chlorotyrosine on AoSMC migration with two negative controls (L-tyrosine and no treatment) and one positive control (PDGF-BB). L-tyrosine at the concentration equivalent to 3-chlorotyrosine had no effects on AoSMC migration, which was similar to the no treatment group. However, 3-chlorotyrosine significantly increased AoSMC migration compared to L-tyrosine or no treatment. Current study design is consistent with the other studies on chlorotyrosine which are always compared with tyrosine in either plasmas or tissues [19, 20].
VSMC migration and proliferation play an important role in the vascular lesion formation including neointimal hyperplasia and atherosclerosis . Many molecules are involved in this process. Integrins are a family of heterodimeric transmembrane glycoproteins consisting of noncovalently associated α and β chains . The integrin complexes αVβ3 and αVβ5 have been shown to be expressed on VSMC and to regulate their migration through interactions with the extracellular matrix . In the current study, we found that chlorotyrosine not only directly promotes SMC migration but also upregulates the expression of intergrins (αV, β3 and β5), PDGFR-B, MMP1, and MMP2. These molecules could be responsible to the chlorotyrosine-induced AoSMC migration.
Oxidative stress is a state in which excess ROS overwhelms endogenous antioxidant systems. The effect of oxidative stress on endothelial cells and VSMC include direct oxidation of proteins and indirect modulation of kinase pathways such as PKC, ERK, Src, and Pho . Many factors such as cytokine, drugs, and extracellular inflammation factors could increase ROS generation to regulate cellular reactions including cell proliferation and migration . In the current study, we demonstrate that chlorotyrosine can increase the production of superoxide anion, a major ROS, in AoSMC after 60 min treatment. Functional significance of over production of superoxide anion is demonstrated by using antioxidants SeMet and MnTBAP, which completely blocked chlorotyrosine-induced AoSMC migration.
MAPK family members are key signaling molecules for many stimuli including regulation molecules and environmental factors, and ROS have been implicated in activation of ERK1/2 and p38 MAP kinase in VSMC . MAPKs are activated by growth factors and other signaling molecules, and are critical in signaling pathways mediating cellular proliferation and migration . Chlorotyrosine has been reported to be involved in ERK signaling pathway . To further investigate the molecular mechanisms of chlorotyrosine-induced AoSMC migration, we determined the involvement of signal transduction pathways such as ERK1/2, p38, and JNK using both a classic western blotting and a new Bio-Plex Luminex technology. We found that chlorotyrosine increased transient activation of ERK2, but not JNK and p38. In addition, a specific ERK1/2 inhibitor PD98059, but not JNK and p38 inhibitors, can completely inhibit the promoting effect of chlorotyrosine on AoSMC migration. It indicates that the activation of the ERK signaling pathway is essential for chlorotyrosine-induced AoSMC migration.
In conclusion, chlorotyrosine has direct effects on human AoSMC by promoting cell migration and upregulating the expression of PDGFR-B, integrins, and MMPs. These biological functions of chlorotyrosine may be mediated by increasing superoxide anion production and activating ERK1/2 signal pathway. Thus, current study provides direct evidence that chlorotyrosine is a biologic active molecule, which may contribute to the vascular lesion formation.
This work is partially supported by research grants from the National Institutes of Health (Peter Lin: HL076345; Qizhi Yao: DE15543 and AT003094; and Changyi Chen: HL65916, HL72716, EB-002436, and HL083471), by the Michael E. DeBakey VA Medical Center, and by the Michael E. DeBakey Department of Surgery, Baylor College of Medicine, Houston, Texas.
Authors have no conflict of interest.