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(Stroke. 2004;35:1443.)
© 2004 American Heart Association, Inc.
Original Contributions |
From the Department of Neurosurgery, Department of Neurology and Neurological Sciences, and Program in Neurosciences, Stanford University School of Medicine, Stanford, Calif.
Correspondence to Dr Pak H. Chan, Neurosurgical Laboratories, Stanford University, 1201 Welch Rd, MSLS, #P314, Stanford, CA 94305-5487. E-mail phchan{at}stanford.edu
| Abstract |
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Methods We used a tFCI model of SOD1 transgenic mice and wild-type littermates to examine the expression of XIAP and Smac/DIABLO by Western blotting and the interaction of XIAP with Smac/DIABLO (XIAP/Smac) or caspase-9 (XIAP/caspase-9) by coimmunoprecipitation. The direct oxidation of carbonyl groups, an indication of oxidative injury to total and individual proteins caused by tFCI, was examined using a 2,4-dinitrophenylhydrazone reaction assay.
Results Direct oxidative injury to cytosolic and mitochondrial proteins was reduced by SOD1 after tFCI. The individual oxidized carbonyls in XIAP, mitochondrial Smac/DIABLO, and caspase-9 were also reduced by SOD1. Expression of XIAP and XIAP/caspase-9 was promoted, whereas translocation of Smac/DIABLO and XIAP/Smac was reduced, by SOD1 after tFCI.
Conclusions These results suggest that overexpression of SOD1 may affect the XIAP pathway after tFCI by reducing the direct oxidative reaction to XIAP regulators after reperfusion injury.
Key Words: cerebral ischemia superoxide dismutase apoptosis
| Introduction |
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The IAP family includes cIAP1/2, X chromosome-linked IAP (XIAP), NIAP, and survivin.12 These proteins bind to and inhibit both initiator caspases such as caspase-9 and effector caspases.13 All IAPs contain baculovirus IAP repeat (BIR) domains.14 Among the IAPs, XIAP is the most potent inhibitor of caspases.12 The third BIR domain of XIAP in particular inhibits cleaved caspase-9.15,16 We have shown that the interaction of XIAP with Smac/DIABLO and the caspases plays a critical role in the regulation of apoptotic neuronal cell death after in vivo cerebral ischemia.11
Reactive oxygen species have been implicated in the mechanism of reperfusion injury after cerebral ischemia.17,18 The electron flow in isolated brain mitochondria produces superoxide anions, which are scavenged by superoxide dismutase (SOD).19 We have shown that copper/zinc-SOD (SOD1), a cytosolic isoenzyme, is highly protective against ischemia and reperfusion injury after ischemia.20,21 Our studies have demonstrated that SOD1 affected the early release of cytochrome c and Smac/DIABLO from mitochondria in ischemia models in vivo.10,22 However, whether SOD1 affects IAP reaction remains unknown. The present study was designed to clarify the role of SOD1 in the XIAP pathway after transient focal cerebral ischemia (tFCI).
| Materials and Methods |
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Focal Cerebral Ischemia
Adult male mice (3 months old, 35 to 40 g) were subjected to tFCI by intraluminal middle cerebral artery blockade with a 6-0 nylon suture as described previously.18,20 The mice were anesthetized with 1.5% isoflurane in 30% oxygen and 70% nitrous oxide using a face mask. After 60 minutes of middle cerebral artery occlusion, blood flow was restored by withdrawal of the nylon suture.
Western Blot Analysis
Protein extraction was performed to obtain the mitochondrial and cytosolic fractions as described previously.16 Equal amounts of the samples were loaded per lane. The primary antibodies were 1:1000 dilution of antibody against Smac/DIABLO (Chemicon International), 1:1000 dilution of antibody against XIAP (BD Transduction Laboratories), or 1:10 000 dilution of antiß-actin monoclonal antibody (Sigma Chemical Co). Western blots were performed with horseradish peroxidase-conjugated immunoglobulin G (Cell Signaling Technology) with the use of enhanced chemiluminescence detection reagents (Amersham International).
Coimmunoprecipitation
Protein extraction and immunoprecipitation were performed as described previously.24,25 Two hundred micrograms of the protein sample were incubated with protein G-Sepharose (Amersham Biosciences) and with an anti-XIAP antibody (BD Transduction Laboratories) for 3 hours at 4°C. The pellets were used as the samples and were immunoblotted with anti-Smac/DIABLO antibody (Chemicon) and anticaspase-9 antibody (Santa Cruz Biotechnology) as described in the Western blot method.
Detection of Oxidized Carbonyls in Protein
A commercial detection assay was used with 2,4-dinitrophenylhydrazone (DNP) to examine the direct oxidative injury to proteins (Chemicon). The manufacturers methodological protocols were followed. DNP specifically reacted with the oxidized carbonyl residues on the protein samples.26 The DNP binding sites of the oxidized proteins were specifically detected using an anti-DNP antibody by Western blot or coimmunoprecipitation.26,27 We analyzed the density of each lane by the total expression of DNP.
Quantification and Statistical Analysis
The data are expressed as mean±SD. We performed 1-way analysis of variance followed by Fisher protected least-significant difference test for multiple-group analysis (SigmaStat software; Jandel Corporation). Comparisons between 2 groups were achieved using Students t test. Significance was accepted with P<0.05.
| Results |
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Direct Oxidative Injury to Cytosolic and Mitochondrial Proteins After tFCI
We used Western blot analysis of DNP to examine oxidative injury to cytosolic and mitochondrial proteins by detecting the DNP binding sites. Total oxidative injury to cytosolic carbonyl proteins was reduced in the SOD1 Tg mice compared with the wild-type mice 1, 4, and 24 hours after tFCI (Figure 2A; 1 hour: n=4, 28.1%±13.7%, *P<0.05; 4 hours: n=4, 28.9%±9.6%, *P<0.05; 24 hours: n=4, 40.2%±9.3%, *P<0.05). Total oxidative injury to mitochondrial proteins was reduced in the SOD1 Tg mice compared with the wild-type mice 1, 4, and 24 hours after tFCI (Figure 2B; 1 hour: n=4, 31.2%±8.2%, *P<0.05; 4 hours: n=4, 32.9%±12.1%, *P<0.05; 24 hours: n=4, 59.0%±6.0%, *P<0.05).
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Direct Oxidative Injury to the Individual Regulators of the XIAP/Smac Pathway
We examined oxidative injury to the individual proteins, indicated by oxidation of the carbonyl groups, with the use of samples precipitated by the DNP antibody after DNP binding. XIAP expression in the proteins bound to DNP (XIAP/DNP) remarkably increased 1 and 2 hours after tFCI (Figure 3A and B; n=4, *P<0.05). Smac/DIABLO in the DNP-bound proteins (Smac/DNP) was not detected in the cytosolic proteins after tFCI (data not shown); however, in the mitochondrial fraction, Smac/DNP was detected and increased 1 and 2 hours after tFCI (Figure 3A and B; n=4, *P<0.05). Caspase-9 in the DNP-bound proteins (caspase-9/DNP) significantly increased 8 hours after tFCI (Figure 3A and B; n=4, *P<0.05).
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SOD1 Overexpression Reduced Oxidative Injury to the Individual Binding Proteins of the XIAP Pathway
Cytosolic XIAP/DNP (n=4, 63.7%±15.4%, *P<0.05) and mitochondrial Smac/DNP (n=4, 73.8%±15.5%, *P<0.05) were both significantly reduced in the SOD1 Tg mice compared with the wild-type mice 1 hour after tFCI (Figure 4A and B). Caspase-9/DNP was also reduced in the SOD1 Tg mice compared with the wild-type mice 8 hours after tFCI (Figure 4C; n=4, 75.5%±32.5%, *P<0.05).
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| Discussion |
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Our previous studies demonstrated that SOD1 overexpression has protective effects against ischemic damage.18,2022 In this study, we examined the role of SOD1 in the XIAP pathway after tFCI by focusing on the formation of oxidized carbonyls in the individual proteins. Superoxide production was detected 1 hour after reperfusion in our model.18 The total cytosolic and mitochondrial oxidized carbonyl proteins were shown as smear bands, the expression of which began to increase concurrently with superoxide production, which matches results of our previous report.25 Each band changed in a variety of ways, some being time-dependent and others not. These results suggest that total oxidation of proteins may accumulate in a time-dependent manner; however, oxidative stress may not injure each protein uniformly and the injuries may progress at various time points. Apoptotic cell death induced by oxidative injury is known to proceed to chain reactions. Our results suggest that not only an upstream substrate but also individual downstream substrates might be directly injured by oxidative stress and that each individual protein injured by oxidative stress might be rescued by SOD. Protein oxidation was quantitatively analyzed by measuring the amount of DNP, and it caused an enzymatic dysfunction of glutamine synthetase after ischemia/reperfusion injury in vivo.29 Oxidative stress caused structural changes in intracellular proteins as well as membranous lipids after reperfusion injury.30 The significant difference in protein expression between the SOD1 Tg mice and the wild-type mice in our study was in accord with the results of oxidized carbonyl protein expression by DNP assay. These results suggest that superoxide may directly or indirectly cause protein oxidation and that it may also affect the binding of proteins to other proteins.
Neuronal mitochondria are a major source of superoxide, and excessive superoxide production after tFCI may cause mitochondrial oxidative injury that leads to the release of cytochrome c from the intermembrane space.22 Accumulation of cytosolic Smac/DIABLO and release of cytochrome c may be controlled by continuous mitochondrial oxidative stress after reperfusion injury.10 In our study, Smac/DIABLO had less oxidative injury in the cytoplasm, but more in mitochondria. This result suggests that mitochondrial oxidative injury may play an important role in the accumulation and the translocation of Smac/DIABLO after reperfusion. Overexpression of cytosolic SOD could reduce both cytosolic and mitochondrial oxidative injury. There are reports that SOD1 may exist in mitochondrial space as well in cytosolic space.31,32 Recent experiments revealed that voltage-dependent anion channels control the balance of superoxide between the mitochondrial and cytosolic space, and that overproduction of superoxide in the mitochondrial space spreads into the cytosolic space.33 This suggests that cytosolic SOD1 can contribute to the reduction of overproduced superoxide, not only in the cytosol but also in the mitochondrial space.33 However, the precise mechanism remains unknown and further studies are needed to clarify the machinery of SOD in mitochondrial injury.
In conclusion, SOD1 contributes to the inhibition of direct oxidation of carbonyls on binding-protein regulators of the XIAP pathway. The sensitivity of XIAP to oxidation during ischemia and reperfusion may provide a molecular target for therapeutic intervention.
| Acknowledgments |
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Received December 16, 2003; revision received February 27, 2004; accepted March 10, 2004.
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