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1 Department of Ophthalmic Research, Cole Eye Institute, Cleveland Clinic Foundation, Cleveland, Ohio 44195, USA
2 Department of Cell Biology, Lerner Research Institute, Cleveland Clinic Foundation, Cleveland, Ohio 44195, USA
Reprint requests to: John W. Crabb, Ph.D., Cole Eye Institute (i31), Cleveland Clinic Foundation, 9500 Euclid Avenue, Cleveland, OH 44195, USA; e-mail: crabbj{at}ccf.org; fax: (216) 445-3670 or Henry F. Hoff, Ph.D., Department of Cell Biology (NC-10), Lerner Research Institute, Cleveland Clinic Foundation, Cleveland, OH 44195, USA; e-mail: hoffh{at}ccf.org; fax: (216) 444-9404.
Article and publication are at http://www.proteinscience.org/cgi/doi/10.1110/ps.4400102.
| Abstract |
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76% of its protease activity and also developed immunoreactivity with antibodies to HNE adducts in Western blot analysis. After stabilization of the potential Michael adducts by sodium borohydride reduction, modified amino acids were localized within the bovine cathepsin B protein structure by mass spectrometric analysis of tryptic peptides. Michael adducts were identified by tandem mass spectrometry at cathepsin B active site residues Cys 29 (mature A chain) and His 150 (mature B chain). Thus, covalent interaction between HNE and critical active site residues inactivates cathepsin B. These results support the hypothesis that the accumulation of undegraded macromolecules in lysosomes after oxidative damage are caused in part by direct protease inactivation by adduct formation with lipid peroxidation products such as HNE. Keywords: Oxidative damage; hydroxynonenal; cathepsin B; Michael adducts; mass spectrometry
Abbreviations: BHT, butylated hydroxytoluene BSA, bovine serum albumin CLN, N
-CBZ-l-lysine p-nitrophenyl ester DTT, dithiothreitol ESMS, electrospray mass spectrometry HNE, 4-hydroxy-2-nonenal LC ESMS, liquid chromatography electrospray mass spectrometry LDL, low-density lipoprotein ox-LDL, oxidized LDL PBS, phosphate-buffered saline TCA, trichloroacetic acid
| Introduction |
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Oxidation of LDL leads to multiple changes in the composition of both the lipid and protein constituents of LDL (Esterbauer et al. 1991; Witztum and Steinberg 1991). Free radicalinduced formation of hydroperoxides in the unsaturated fatty acids of cholesterol and phospholipid esters (Esterbauer et al. 1991) results in alkoxyl radical formation and subsequent scission to form a variety of reactive lipid oxidation products including HNE. HNE is created by cleavage of oxidized arachidonic and linoleic esters (Fig. 1
) and forms Schiff base linkages with primary amino groups and Michael adducts with Lys, His, and Cys residues and also forms fluorescent cross-links (Esterbauer et al. 1991; Xu et al. 2000). HNE modification induces LDL aggregation (Hoff et al. 1992) and impairs proteasome function via cross-linking and other undetermined mechanisms (Friguet and Szweda 1997; Okada et al. 1999). Previously, we reported that a reversal of Michael additions of HNE to lysine may occur at the low pH that exists in lysosomes after receptor-mediated endocytosis of oxLDL (O'Neil et al. 1997). We hypothesized that HNE, still tethered to apo B by a Schiff base linkage also may form Michael additions with particularly vulnerable proteins such as the lysosomal thiol protease cathepsin B (O'Neil et al. 1997). With or without cross-links, Michael additions from HNE may help explain why oxLDL-treated macrophages show reduced activity of cathepsin B but not reduced activity of cathepsin D, an aspartate protease (Hoppe et al. 1994).
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| Results |
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35% and 65%, respectively, relative to the untreated control cells (Fig. 2a
40% reduction in activity relative to untreated cells (Fig. 2b
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30 kD, near the intact mass of active bovine cathepsin B (Fig. 3
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75% of the protein sequence (Table 1
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| Discussion |
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4 that facilitates subsequent deacylation (Storer and Menard 1994). HNE modification of either Cys 29 or His 150 would disrupt this enzyme reaction mechanism. Under the pH 4.5 reaction conditions used in this study, we speculate that HNE reacts first with Cys 29, which causes a significant decrease in the pKa of His 150, rendering His 150 also susceptible to electrophilic attack by HNE. Whether both Cys 29 and His 150 are modified in the same cathespsin B molecule is not clear as unmodified peptides containing these residues were detected (Table 1
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In summary, HNE or oxLDL treatment of macrophages inhibits intracellular degradation of macromolecues and reduces cathepsin B activity. HNE treatment of purified bovine cathepsin B results in selective modification of active site residues Cys 29 (A chain) and His 150 (B chain), with the generation of Michael adducts and significantly reduced enzyme activity. Notably, others have established the presence of HNE Michael adducts in oxLDL (Esterbauer et al. 1991; Bolgar et al. 1996). Overall, these results support the hypothesis that specific lysosomal proteases are inactivated by HNE generated during oxidation of LDL.
| Materials and methods |
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-CBZ-L-lysine p-nitrophenyl ester, fatty acidfree bovine serum albumin, butylated hydroxytoluene, trichloroacetic acid, and dithiothreitol were purchased from Sigma Chemical Co. Carrier-free Na125I and 14C-adenine were from ICN, Roswell Park Memorial Institute media (RPMI-1640) was from Whittaker Bioproducts Inc., and fetal calf serum was from Gibco Laboratories. Tissue culture plates were obtained from Costar; C57BL/6 mice (1620 weeks of age) from Charles River Laboratory; and polyvinylidene difluoride (PVDF) membrane was from Amersham-Pharmacia Biotech. HNE and rabbit polyclonal antiserum to HNE were kind gifts from Drs. Lawrence M. Sayre and Luke I. Szweda, respectively, Case Western Reserve University, Cleveland, OH.
Degradation of maleyl bovine serum albumin by macrophages in culture
LDL was isolated from fresh plasma obtained from the Cleveland Clinic Blood Bank and oxidized with CuSO4 as described elsewhere (Hoppe et al. 1994). BSA was subjected to maleylation (Haberland and Fogelman 1985), labeled with Na125I, and used as a ligand for receptor-mediated endocytosis. Mouse peritoneal macrophages were harvested 23 days after thioglycollate stimulation of female C57BL/6 mice by peritoneal lavage with ice-cold phosphate-buffered saline (PBS). Primary cultures were prepared at a density of 106 cells/16-mm-diameter well in RPMI-1640 containing 10% fetal calf serum and used 48 h after plating (Hoppe at al. 1994). Cells were treated with HNE or oxLDL for 3 h, washed two times with PBS to remove oxLDL or HNE, then incubated with 125I-maleyl BSA and media assayed for trichloroacetic acidsoluble, noniodine degradation products essentially as described elsewhere (Hoppe et al. 1994). Cells were washed three times with PBS, dissolved in 0.1 N NaOH, and assayed for protein content using the bicinchoninic acid assay and BSA as a standard (Pierce Chemicals). Cytotoxicity was determined by measuring the incorporation of 35S-methionine into proteins, and by determining the release of 14C-adenine from cells (Coffey et al. 1995). In the latter technique, macrophages were first incubated with media containing 14C-adenine (0.2 µ Ci/mL) for 12 h, washed with media, and then incubated with HNE or oxLDL for 3 h at 37°C. The amount of radiolabel present in the conditioned media as a result of release from cells then was measured.
Preparation of macrophage cell extracts
Mouse peritoneal macrophages were assayed for cathepsin B activity as previously described (Hoppe et al. 1994). Briefly, after incubation with HNE or oxLDL, macrophage monolayers were washed twice with PBS, scraped with a rubber policeman, pelleted by centrifugation at 2000g for 10 min and resuspended in 1 mL of buffer. The cells then were ruptured by passing 10 times through a 30-gauge needle, followed by 10 cycles of freezing and thawing, and finally by two 15-sec bursts of sonication (Hoppe et al. 1994). The homogenates were spun at 5000g to remove particulate matter, and supernatant fractions were collected and defined as cell extracts.
Enzyme assay for cathepsin B activity
The thiol protease activity of cell extracts and purified cathepsin B preparations was assayed by the N
-CBZ-L-lysine p-nitrophenyl ester (CLN) procedure (Hoppe et al. 1994). Briefly, bovine cathepsin B was activated by incubating the proenzyme for 15 min at 37°C in 0.2 M Na acetate at pH 4.5 containing 2 mM DTT-EDTA. The activated enzyme was treated with 15 µM HNE for periods of up to 60 min. An aliquot of the mixture of activated enzyme with or without HNE was adjusted to 1 mL with the same buffer, and the enzyme reaction was initiated by addition of CLN to a final concentration of 0.5 mM. Cathepsin B activity at 37°C was expressed as change in absorbance at 326 nm wavelength per min because of hydrolysis of CLN.
HNE modification of bovine cathepsin B, sodium borohydride reduction, alkylation, and proteolytic digestions
Purified and activated bovine cathepsin B (
8 nmole based on vendor quantification) in 250 µL of 100 mM sodium acetate at pH 4.5 and 1 mM DTT-EDTA was added to 1.6 µmole HNE dissolved in 25 µL of ethanol. After an 8-min incubation at 37°C, 8 M urea in 200 mM ammonium bicarbonate at pH 8.0 (1 mL) was added to stop autoproteolysis, and the preparation was frozen. To stabilize Michael adducts and reduce disulfide bonds, we added 1 M sodium borohydride in 0.1 M NaOH to the HNE-modified protein to a final concentration of 50 mM, and the EDTA concentration was adjusted to 1 mM. After 30 min incubation at room temperature in a fume hood, hydrogen gas bubbling was stopped and
2.8-fold molar excess of iodoacetamide was added relative to the cysteine content. After a 15-min incubation under argon, the alkylation reaction was stopped by the addition of 1 µmole DTT. The modified cathepsin B was dialyzed exhaustively into 15 mM N-ethylmorpholine acetate at pH 8.0, 40% acetonitrile using 1000-MW cutoff Spectrapor dialysis tubing. After dialysis, the modified cathepsin B was quantified by amino acid analysis and recovery determined to be
109 µg (
4 nmole). HNE-modified and S-carboxyamidomethylated cathepsin B (2.1 nmole) was digested with 1.8 µg of trypsin (Promega) in 15 mM N-ethylmorpholine acetate at pH 8.0, 40% acetonitrile at 37°C for 16 h. Subfragmentation of tryptic peptides with V8 protease or chymotrypsin (0.1 µg protease) was in 10 mM ammonium bicarbonate containing 20% acetonitrile at 25°C for 12 h (Crabb et al. 1986).
Amino acid analysis, electrophoresis, and Western blot analysis
Phenylthiocarbamyl amino acid analysis was performed using an Applied Biosystems model 420H/130/920 automated analysis system (Crabb et al. 1998a). SDS-PAGE was performed on 12% acrylamide gels using a Mini-Protein II slab gel system (Bio-Rad) and visualized by Coomassie blue staining. For Western analyses, the gels were blotted to PVDF membranes and probed with rabbit polyclonal antibodies to Michael adducts (Uchida et al. 1993). Immunoreactivity was detected by chemiluminescence (Amersham-Pharmacia Biotech).
Mass spectrometry
ESMS and liquid chromatography ESMS (LC ESMS) were performed with a Perkin-Elmer Sciex API 3000 triple quadrupole mass spectrometer equipped with an ionspray source (Crabb et al. 1998b). Nitrogen was used as the nebulization gas (at 40 p.s.i.) and curtain gas and was supplied from a nitrogen dewar. For LC ESMS, the cathepsin B tryptic digest was diluted twofold with 0.1% trifluoroacetic acid and
1 nmole (
27 µg) chromatographed on a 5 µm Vydac C18 column (1.0 x 150 mm) using an Applied Biosystems model 140D HPLC system, aqueous acetonitrile/propanol/trifluoroacetic acid solvents and a linear gradient of 2% 80% acetonitrile over 60 min at a flow rate of 40 µL/min. The column eluant was split with 25% directed to the mass spectrometer. The remainder of the eluant was collected manually in fractions by monitoring the total ion current. A scan range of m/z 3502200 in the positive ion mode was used with 0.2-a.m.u. steps, 0.5-msec dwell time per step, 70-V orifice potential, and 4900-V ionspray.
Nanoelectrospray MS/MS was performed with a nanoelectrospray interface (MDS Protana) attached to the Perkin-Elmer Sciex API 3000 triple quadrupole mass spectrometer. Select chromatography fractions from LC ESMS (
5 µL) were placed in gold-coated glass capillaries (4-µm outlet; New Objectives, Inc.), and the samples were infused into the mass spectrometer with 10001500 V applied to the capillary and orifice potential set at 40 V. Precursor ions were selected in the first quadrupole (Q1) and collision-induced dissociation effected with nitrogen gas in the second quadrupole (Q2). Product ion spectra were obtained by scanning the third quadrupole (Q3) using a step size of 0.2 Dalton and 0.5 msec dwell time per step in the positive ion mode. The Q1 resolution was lowered to allow transmission of M+1 precursor ions into Q2; Q3 was kept at unit mass resolution.
| Acknowledgments |
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The publication costs of this article were defrayed in part by payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 USC section 1734 solely to indicate this fact.
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