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J. Biol. Chem., Vol. 281, Issue 14, 9001-9004, April 7, 2006
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1


2
From the
Department of Medicine, University of Washington, Seattle, Washington 98195, the
Children's Hospital Oakland Research Institute, Oakland, California 94609, and the ¶Department of Microbiology and Immunology, Weill Medical College of Cornell University, New York, New York 10021
Received for publication, January 26, 2006 , and in revised form, February 22, 2006.
| ABSTRACT |
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| INTRODUCTION |
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70% of the total protein in HDL, promotes cholesterol and phospholipid efflux largely by an active transport process mediated by ATP-binding cassette transporter A1 (ABCA1). However, oxidation of apoA-I severely impairs cholesterol efflux by the ABCA1 pathway (35). Moreover, HDL is chlorinated in human atherosclerotic lesions and the blood of subjects with established coronary artery disease, suggesting that apoA-I oxidation might promote atherogenesis.
One potential pathway for apoA-I oxidation involves myeloperoxidase (MPO), a heme protein expressed by macrophages in human atherosclerotic tissue (68). MPO secreted by phagocytes uses hydrogen peroxide (H2O2) and chloride (Cl) to generate the powerful oxidant hypochlorous acid (HOCl). HOCl converts tyrosine to 3-chlorotyrosine (810), and MPO is the only known source of this halogenated amino acid during acute inflammation in mice (11), indicating that MPO oxidizes HDL in vivo. We previously showed that MPO or HOCl targets tyrosine residue 192 (Tyr-192) when it chlorinates apoA-I, regardless of whether the protein is free or associated with HDL (12, 13). Moreover, apoA-I loses its ability to remove cholesterol from cells as it becomes oxidized in this manner (35, 13), indicating that Tyr chlorination might be important for this impaired activity.
The pathways that promote the MPO-dependent oxidation of specific residues in apoA-I and loss of its ABCA1 activity are controversial. The mechanism we have proposed is based on the observation that Tyr-192 lies in an YXXK motif (Y = Tyr, K = Lys, X = unreactive amino acid) and therefore is adjacent to a Lys residue on the same face of an amphipathic
-helix (12, 14). Moreover, HOCl reacts rapidly with the
amino group of lysine to form long-lived chloramines (8, 12). Using synthetic peptides, we demonstrated that lysine residues can direct the regiospecific chlorination of Tyr residues by a pathway involving chloramine formation. In this model, the site-specific chlorination of Tyr-192 in apoA-I requires the participation of a nearby lysine residue (12).
An alternative proposal is that MPO must bind directly to the region of apoA-I containing Tyr-192 to promote site-specific chlorination of the residue (5). Based on studies of mutated apoA-I, it was also concluded that tyrosine chlorination is not a prerequisite for loss of ABCA1 transport activity when MPO oxidizes apoA-I (15).
Methionine sulfoxide (Met(O)) has also been detected in circulating HDL (16). ApoA-I can reduce lipid hydroperoxides to alcohols in concert with oxidation of Met residues to Met(O) (17, 18). HDL is the major carrier of lipid hydroperoxides in the blood of humans (19), suggesting that this pathway may be physiologically relevant. The alkyl thiol of Met is the most HOCl-reactive moiety in the 20 common amino acids (20, 21), but the role of oxidation of Met residues in the impaired cholesterol efflux activity of MPO-oxidized apoA-I has received surprisingly little attention.
Met(O) residues in proteins are reduced to methionine by a family of intracellular enzymes termed methionine sulfoxide reductases (22). In the current studies, we used mutated apoA-I and methionine sulfoxide reductase to probe the role of specific amino acids residues in Tyr chlorination and the loss of ABCA1-cholesterol efflux activity that occurs when apoA-I is exposed to MPO.
| EXPERIMENTAL PROCEDURES |
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Oxidation and Methionine Sulfoxide Reduction ReactionsApoA-I was dialyzed against 10 mM sodium phosphate buffer (pH 7.4). Oxidation reactions were carried out at 37 °C for 1 h in 10 mM sodium phosphate buffer (pH 7.4) containing 100 µM diethylenetriaminepentaacetic acid (27). For the MPO-H2O2-Cl system, the reaction mixture was supplemented with 50 nM MPO and 100 mM NaCl. Oxidized apoA-I (6 µM) was incubated with PilB (4.5:1, apoA-I/enzyme, w/w) for 2 h at 37°C in Tris-HCl buffer (25 mM (pH 7.4)) containing 15 mM dithiothreitol (28).
Liquid Chromatography Electrospray Ionization Mass Spectrometry (LC-ESI-MS)ApoA-I was incubated overnight at 37 °C with sequencing grade modified trypsin (20:1, protein/enzyme, w/w) or with endoproteinase Glu-C (Staphylococcus aureus V8; 10:1, protein/enzyme, w/w) in 100 mM NH4HCO3 (pH 7.8) (13, 29). Digestion was halted by acidification (pH 23). LC-ESI-MS analyses were performed in the positive ion mode with a Finnigan Mat LCQ ion trap instrument (San Jose, CA) coupled to a Waters 2690 HPLC system (13, 30). Peptide digests were separated on a reverse-phase column (Vydac C18 MS column) and subjected to MS and MS/MS analysis as described previously (13, 29).
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| RESULTS |
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As with apoA-I isolated from human HDL (12, 13), Tyr-192 was the major site of chlorination in recombinant wild-type apoA-I (Fig. 1A). In the K195R mutant, Tyr-192 chlorination was markedly decreased, suggesting that Lys-195 plays a critical role in directing the chlorination of Tyr-192. Tyr-166, which does not reside in a YXXK motif in apoA-I, was chlorinated in low yield when the protein was exposed to HOCl or the complete MPO system (Fig. 1B). However, when Glu-169 was mutated to lysine, chlorination of Tyr-166 increased 20-fold (Fig. 1B). These results demonstrate that lysine residues located in the YXXK motif can direct tyrosine chlorination in apoA-I.
Methionine Residues in the MXXK Motif Inhibit Tyrosine Chlorination Protein-bound Met residues have been proposed to act as local scavengers of H2O2 (32). To determine whether Met residues might similarly scavenge oxidants derived from HOCl, we mutated glutamic acid 198 (which lies 2 residues away from the Lys-195 that directs chlorination of Tyr-192) to Met. In this mutant, Tyr-192 chlorination was dramatically decreased (Fig. 1C), suggesting that Met residues can inhibit Tyr chlorination by scavenging lysine chloramines.
Tyr-115, which also resides in a YXXK motif in apoA-I, was chlorinated poorly (Fig. 1D) by HOCl or the complete MPO system. When we mutated Met-112 to Ala, however, HOCl generated a high yield of chlorinated Try-115 (data not shown). Thus, chlorination of Tyr-115 appears to be inhibited by the adjacent Met residue in wild type apoA-I. When Met-112 was mutated to Lys, chlorination of Tyr-115 also increased markedly (Fig. 1D). Collectively, these observations suggest that protein-bound Met residues that reside in MXXYor KXXM motifs can inhibit Tyr chlorination by scavenging lysine chloramines.
Methionine Sulfoxide Reductase Converts Met(O) to Methionine in Oxidized ApoA-IApoA-I contains 3 Met residues, but it is unknown how oxidizing them with HOCl affects apoA-I-mediated cholesterol efflux by the ABCA1 pathway. When we exposed apoA-I to HOCl or the MPO system and then digested it with trypsin or Glu-C, LC-ESI-MS/MS analysis showed that each Met had been targeted for oxidation to Met(O) (Fig. 2, A and B). In contrast, methionine sulfone was not detected in oxidized apoA-I. When apoA-I was first exposed to the complete MPO system and then incubated with the methionine sulfoxide reductase PilB, Met(O) was converted back to methionine (Fig. 2C).
Chlorination of Tyr192 and Oxidation of Met Residues Play Critical Roles in the Oxidative Inactivation of the Cholesterol Efflux Activity of ApoA-ITo test the role of tyrosine chlorination of apoA-I in the loss of ABCA1 activity, we engineered the Tyr-192
Phe mutant of apoA-I. LC-ESI-MS analysis confirmed that HOCl fails to chlorinate Phe-192 of this mutant protein. We then investigated the ability of apoA-I and its Tyr-192
Phe mutant form to promote cholesterol efflux from ABCA1-transfected BHK cells. The rates of cholesterol efflux were virtually identical as concentrations of the native and Tyr-192
Phe mutant proteins increased (Fig. 3A), indicating that substituting Phe for Tyr had little effect on the biological activity of the apolipoprotein.
We next compared the effects of oxidation on the ability of apoA-I and Tyr-192
Phe apoA-I to remove cholesterol from cells by the ABCA1 pathway. Whereas H2O2 alone had no effect, oxidation by the complete MPO system significantly decreased the cholesterol efflux promoted by apoA-I or Tyr-192
Phe apoA-I (Fig. 3B and Fig. 4A). However, the mutant protein appeared somewhat resistant to oxidative inactivation by MPO when the concentration of H2O2 was high. Similar results were observed when apoA-I was directly oxidized with HOCl (Fig. 4B). After apoA-I was exposed to the complete MPO system with increasing H2O2 concentrations (Fig. 4A) or increasing HOCl concentrations (Fig. 4B), incubating apoA-I with PilB to reduce Met(O) back to Met partially restored its ability to promote cholesterol efflux at all oxidant concentrations. Similar partial protective effects of the Tyr-192
Phe substitution or methionine sulfoxide reduction were observed when cellular cholesterol efflux was monitored over a range of apoA-I concentrations (Fig. 3B). Thus, neither inhibition of Tyr-192 chlorination nor reduction of Met(O) alone markedly protected apoA-I from oxidative inactivation.
We next determined the effect of a combination of Met(O) reduction and the Tyr-192
Phe mutation on the ability of oxidized apoA-I to promote cholesterol efflux by the ABCA1 pathway. Remarkably, when the mutant protein was first exposed to increasing H2O2 concentrations in the complete MPO system and then incubated with PilB, its ability to promote cholesterol efflux was nearly the same as that of native apoA-I (Fig. 4A). This was true even at low concentrations of apoA-I (Fig. 3B). We obtained similar results after exposing Tyr-192
Phe to HOCl (Fig. 4B). Our observations strongly suggest that two oxidation events, chlorination of Tyr-192 and oxidation of Met residues, account for most of the decrease in the ability of apoA-I to promote cholesterol efflux by the ABCA1 pathway.
| DISCUSSION |
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Arg mutation and chlorinated normally resistant Tyr residues by creating the KXXY motif in the protein. Virtually identical results were observed with either the complete MPO system or reagent HOCl. These observations strongly support the proposed role of the YXXK motif (12) and argue against the hypothesis that the selective chlorination of Tyr-192 requires MPO to interact directly with the apolipoprotein (5). We previously noted that two other Tyr residues in apoA-I (Tyr-115 and Tyr-236) that reside in YXXK motifs are resistant to chlorination (13). Importantly, Tyr-115 lies close to Met-112 in a YXXM motif. When we mutated Met-112 to Ala, which does not react with HOCl, the yield of chlorinated Tyr-115 in apoA-I exposed to MPO increased. When we introduced a Met residue 2 residues away from the Lys residue in apoA-I, Tyr-192 chlorination was inhibited. Our results strongly support the hypothesis that protein-bound Met residues act as local antioxidants (32) by scavenging chlorinating intermediates.
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-helical structure of apoA-I depends critically upon hydrophobic amino acids, which form the lipid-associating face of the amphipathic helix (14). Oxygenation markedly decreases the hydrophobicity of Met. By disrupting the secondary structure of the apolipoprotein, Met oxidation could alter the ability of nearby Lys residues located in the YXXK motif to promote the site specific chlorination of apoA-I.
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To address this issue, we measured the cholesterol efflux activity of apoA-I containing a Phe substituted for Tyr-192, which makes this residue completely resistant to chlorination. In contrast to the previous report with murine macrophages (15), we found using ABCA1-transfected BHK cells that this mutation had a small but reproducible protective effect against apoA-I inactivation by either HOCl or the MPO system. The difference between our study and the previous one may reflect the fact that our transfected BHK cells express much higher ABCA1 levels than murine macrophages, allowing us to detect modest changes in apoA-I activity.
Our observation that protein-bound Met is a potent local scavenger of HOCl led us to investigate the role of Met oxidation in the biological activity of apoA-I. When we exposed apoA-I to HOCl or to H2O2 with the complete MPO system, we observed near-quantitative oxidation of the 3 Met residues in apoA-I. Met oxidation was completely reversed by treatment with PilB, a methionine sulfoxide reductase that reduces both the (R) and (S) epimers of Met(O) (22). Remarkably, when the Tyr-192
Phe mutant was exposed to HOCl or the complete MPO system and then incubated with the methionine reductase PilB, its ability to promote cholesterol efflux by the ABCA1 pathway was almost completely restored. These observations indicate that Met oxidation and Tyr chlorination together, in contrast to Met oxidation alone (33), are necessary for depriving apoA-I of its cholesterol efflux activity.
This synergism between the Tyr-192
Phe mutation and Met(O) reduction implies that the functional defect induced by oxidation of apoA-I arises from a cooperative interaction between oxidation of Tyr-192 and one or more Met residues. Alternatively, it is possible that other structural modifications in oxidized apoA-I are involved in the loss of ABCA1 activity. It is noteworthy that Tyr-192 resides in the middle of the random coil loop that changes its conformation when it encounters lipid, driving the remodeling of apoA-I to its lipid-associated form (34). Chlorination of this Tyr residue, in concert with Met oxidation, may stabilize a conformational variant that fails to either associate strongly with lipid or interact productively with ABCA1.
In conclusion, oxidation by MPO impairs the ability of apoA-I to promote cholesterol efflux by the ABCA1 pathway, suggesting that this oxidative process might contribute to foam cell formation and atherogenesis. Studies with mutant forms of apoA-I strongly support the proposal that specific amino acid sequences direct the regioselective chlorination of Tyr. Moreover, oxidation of Met and Tyr residues plays a critical role in the oxidative inactivation of the protein. Our observations raise the possibility that modified forms of apoA-I that are resistant to oxidation might be especially anti-atherogenic in vivo.
| FOOTNOTES |
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1 Present address: Dept. of Internal Medicine, University of California, Davis, CA 95616. ![]()
2 To whom correspondence should be addressed: Division of Metabolism, Endocrinology and Nutrition, Box 356426, University of Washington, Seattle, WA 98195. E-mail: heinecke{at}u.washington.edu.
3 The abbreviations used are: HDL, high density lipoproteins; ABCA1, ATP-binding cassette transporter A1; apoA-I, apolipoprotein A-I; ESI, electrospray ionization; LC, liquid chromatography; Met(O), methionine sulfoxide; MPO, myeloperoxidase; MS, mass spectrometry; PilB, methionine sulfoxide reductase; BHK, baby hamster kidney. ![]()
| REFERENCES |
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