Neurofilaments are the major neuronal target of hydroxynonenal-mediated protein cross-links

Lipid peroxidation generates reactive aldehydes, most notably hydroxynonenal (HNE), which covalently binds amino acid residue side chains leading to protein inactivation and insolubility. Specific adducts of lipid peroxidation have been demonstrated to be intimately associated with pathological lesions of Alzheimer's disease (AD), suggesting that oxidative stress is a major component in the disease. Here, we examined the HNE-cross-linking modifications by using an antibody specific for a lysine-lysine cross-link. Since in a prior study we noted no immunolabeling of neuritic plaques or neurofibrillary tangles but instead found strong labeling of axons, we focused this study on axons. Axonal labeling was examined in mouse sciatic nerve, and immunoblotting showed the cross-link was restricted to neurofilament heavy and medium subunits, which while altering migration, did not indicate larger NF aggregates, indicative of intermolecular cross-links. Examination of mice at various ages showed the extent of modification remaining relatively constant through the life span. These findings demonstrate lipid-cross-linking peroxidation primarily involves lysine-rich neurofilaments and is restricted to intramolecular cross-links.

Identification of ATP synthase as a lipid peroxide protein adduct in pancreatic islets from humans with and without type 2 diabetes mellitus

CONTEXT

Most current knowledge of pancreatic islet pathophysiology in diabetes mellitus has come from animal models. Even though islets from humans are readily available, only a few come from diabetic donors. We had the uncommon opportunity to acquire islets from humans with type 2 diabetes and used it to perform a study not previously done with human or animal islets.

OBJECTIVES

Oxidative stress has been proposed as a mechanism for impaired β-cell function in type 2 diabetes. Lipid peroxides caused by reactive oxygen species are damaging to body tissues. The objective was to determine whether lipid peroxide-protein adducts occur in pancreatic islets of humans with type 2 diabetes.

DESIGN

Immunoblots with two antibodies to hydroxynonenal and 2 other antibodies we generated against reactive small aliphatic compounds were used to detect lipid peroxide-protein adducts in islets of patients with type 2 diabetes and controls.

RESULTS

The antibodies reacted strongly to ≥5 islet proteins. The major hydroxynonenal adduct in the islets of type 2 diabetes patients was a 52-kDa protein seen with all 4 antibodies that was also seen in islets of nondiabetic humans, rat islets, and insulinoma cells and in mitochondria of various rat tissues. Nano-LC-MS/MS (liquid chromatography-tandem mass spectrometry) and MALDI-TOF (matrix-assisted laser desorption/ionization-time of flight) analysis identified the protein as the β-chain of the mitochondrial F-ATP synthase, an enzyme responsible for 95% of ATP formed in tissues.

CONCLUSIONS

Lipid peroxide-protein adducts occur in β-cells in the nondiabetic state and in diabetes. Lipid peroxidation is thought to be damaging to tissues. Analogous to various other unhealthy characteristics, the presence in nondiabetic individuals of lipid peroxide-protein adducts does not necessarily indicate they are not detrimental.

Oxidative Modification and Inactivation of the Proteasome during Coronary Occlusion/Reperfusion

Restoration of blood flow to ischemic myocardial tissue results in an increase in the production of oxygen radicals. Highly reactive, free radical species have the potential to damage cellular components. Clearly, maintenance of cellular viability is dependent, in part, on the removal of altered protein. The proteasome is a major intracellular proteolytic system which degrades oxidized and ubiquitinated forms of protein. Utilizing an in vivo rat model, we demonstrate that coronary occlusion/reperfusion resulted in declines in chymotrypsin-like, peptidylglutamyl-peptide hydrolase, and trypsin-like activities of the proteasome as assayed in cytosolic extracts. Analysis of purified 20 S proteasome revealed that declines in peptidase activities were accompanied by oxidative modification of the protein. We provide conclusive evidence that, upon coronary occlusion/reperfusion, the lipid peroxidation product 4-hydroxy-2-nonenal selectively modifies 20 S proteasome alpha-like subunits iota, C3, and an isoform of XAPC7. Occlusion/reperfusion-induced declines in trypsin-like activity were largely preserved upon proteasome purification. In contrast, loss in chymotrypsin-like and peptidylglutamyl-peptide hydrolase activities observed in cytosolic extracts were not evident upon purification. Thus, decreases in proteasome activity are likely due to both direct oxidative modification of the enzyme and inhibition of fluorogenic peptide hydrolysis by endogenous cytosolic inhibitory protein(s) and/or substrate(s). Along with inhibition of the proteasome, increases in cytosolic levels of oxidized and ubiquitinated protein(s) were observed. Taken together, our findings provide insight into potential mechanisms of coronary occlusion/reperfusion-induced proteasome inactivation and cellular consequences of these events.

The glial glutamate transporter, GLT-1, is oxidatively modified by 4-hydroxy-2-nonenal in the Alzheimer's disease brain: the role of Abeta1-42

Glutamate transporters are involved in the maintenance of synaptic glutamate concentrations. Because of its potential neurotoxicity, clearance of glutamate from the synaptic cleft may be critical for neuronal survival. Inhibition of glutamate uptake from the synapse has been implicated in several neurodegenerative disorders. In particular, glutamate uptake is inhibited in Alzheimer's disease (AD); however, the mechanism of decreased transporter activity is unknown. Oxidative damage in brain is implicated in models of neurodegeneration, as well as in AD. Glutamate transporters are inhibited by oxidative damage from reactive oxygen species and lipid peroxidation products such as 4-hydroxy-2-nonenal (HNE). Therefore, we have investigated a possible connection between the oxidative damage and the decreased glutamate uptake known to occur in AD brain. Western blots of immunoprecipitated HNE-immunoreactive proteins from the inferior parietal lobule of AD and control brains suggest that HNE is conjugated to GLT-1 to a greater extent in the AD brain. A similar analysis of beta amyloid (Abeta)-treated synaptosomes shows for the first time that Abeta1-42 also increases HNE conjugation to the glutamate transporter. Together, our data provide a possible link between the oxidative damage and neurodegeneration in AD, and supports the role of excitotoxicity in the pathogenesis of this disorder. Furthermore, our data suggests that Abeta may be a possible causative agent in this cascade.

Modulation of mitochondrial function by hydrogen peroxide

During normal cellular metabolism, mitochondrial electron transport results in the formation of superoxide anion (O(2)) and subsequently hydrogen peroxide (H(2)O(2)). Because H(2)O(2) increases in concentration under certain physiologic and pathophysiologic conditions and can oxidatively modify cellular components, it is critical to understand the response of mitochondria to H(2)O(2). In the present study, treatment of isolated rat heart mitochondria with H(2)O(2) resulted in a decline and subsequent recovery of state 3 NADH-linked respiration. Alterations in NADH levels induced by H(2)O(2) closely paralleled changes in the rate of state 3 respiration. Assessment of electron transport chain complexes and Krebs cycle enzymes revealed that alpha-ketoglutarate dehydrogenase (KGDH), succinate dehydrogenase (SDH), and aconitase were susceptible to H(2)O(2) inactivation. Of particular importance, KGDH and SDH activity returned to control levels, concurrent with the recovery of state 3 respiration. Inactivation is not because of direct interaction of H(2)O(2) with KGDH and SDH. In addition, removal of H(2)O(2) alone is not sufficient for reactivation. Enzyme activity does not recover unless mitochondria remain intact. The sensitivity of KGDH and SDH to H(2)O(2)-mediated inactivation and the reversible nature of inactivation suggest a potential role for H(2)O(2) in the regulation of KGDH and SDH.

Free radicals alter maximal diaphragmatic mitochondrial oxygen consumption in endotoxin-induced sepsis

Recent studies indicate that sepsis is associated with enhanced generation of several free radical species (nitric oxide, superoxide, hydrogen peroxide) in skeletal muscle. While studies suggest that free radical generation causes uncoupling of oxidative phosphorylation in sepsis, no previous report has examined the role of free radicals in modulating skeletal muscle oxygen consumption during State 3 respiration or inhibiting the electron transport chain in sepsis. The purpose of the present study was to examine the effects of endotoxin-induced sepsis on State 3 diaphragm mitochondrial oxygen utilization and to determine if inhibitors/scavengers of various free radical species would protect against these effects. We also examined mitochondrial protein electrophoretic patterns to determine if observed endotoxin-related physiological derangements were accompanied by overt alterations in protein composition. Studies were performed on: (a) control animals, (b) endotoxin-treated animals, (c) animals given endotoxin plus PEG-SOD, a superoxide scavenger, (d) animals given endotoxin plus L-NAME, a nitric oxide synthase inhibitor, (e) animals given only PEG-SOD or L-NAME, (f) animals given endotoxin plus D-NAME, and (g) animals given endotoxin plus denatured PEG-SOD. We found: (a) no alteration in maximal State 3 mitochondrial oxygen consumption rate at 24 h after endotoxin administration, but (b) a significant reduction in oxygen consumption rate at 48 h after endotoxin, (c) no effect of endotoxin to induce uncoupling of oxidative phosphorylation, (d) either PEG-SOD or L-NAME (but neither denatured PEG-SOD nor D-NAME) prevented endotoxin-mediated reductions in State 3 respiration rates, (e) some mitochondrial proteins underwent tyrosine nitrosylation at 24 h after endotoxin administration, and (f) SDS-page electrophoresis of mitochondria from endotoxin-treated animals revealed a selective depletion of several proteins at 48 h after endotoxin administration (but not at 24 h); (g) administration of L-NAME or PEG-SOD prevented this protein depletion. These data provide the first evidence that endotoxin-induced reductions in State 3 mitochondrial oxygen consumption are free radical-mediated.

Caloric restriction of rhesus monkeys lowers oxidative damage in skeletal muscle

In laboratory rodents, caloric restriction (CR) retards several age-dependent physiological and biochemical changes in skeletal muscle, including increased steady-state levels of oxidative damage to lipids, DNA, and proteins. We used immunogold electron microscopic (EM) techniques with antibodies raised against 4-hydroxy-2-nonenal (HNE) -modified proteins, dinitrophenol, and nitrotyrosine to quantify and localize the age-dependent accrual of oxidative damage in rhesus monkey vastus lateralis skeletal muscle. Using immunogold EM analysis of muscle from rhesus monkeys ranging in age from 2 to 34 years old, a fourfold maximal increase in levels of HNE-modified proteins was observed. Likewise, carbonyl levels increased approximately twofold with aging. Comparing 17- to 23-year-old normally fed to age-matched monkeys subjected to CR for 10 years, levels of HNE-modified proteins, carbonyls, and nitrotyrosine in skeletal muscle from the CR group were significantly less than control group values. Oxidative damage largely localized to myofibrils, with lesser labeling in other subcellular compartments. Accumulation of lipid peroxidation-derived aldehydes, such as malondialdehyde and 4-hydroxy-2-alkenals, and protein carbonyls were measured biochemically and confirmed the morphological data. Our study is the first to quantify morphologically and localize the age-dependent accrual of oxidative damage in mammalian skeletal muscle and to demonstrate that oxidative damage in primates is lowered by CR.

Comparative immunoreactivity of anti-trifluoroacetyl (TFA) antibody and anti-lipoic acid antibody in primary biliary cirrhosis: searching for a mimic

Previous studies documenting the existence of cross-reactivity between the lipoated (but not unlipoated) forms of the inner lipoyl domain (E2L2) of PDC-E2 [the major autoantigen in Primary biliary cirrhosis (PBC)] and trifluoroacetylated (TFA) proteins, led us to hypothesize that PBC may be due to an initial insult with an environmental agent that cross-reacts with TFA. Therefore, we performed a comparative study of the reactivity of rabbit anti-TFA antibody and anti-lipoic acid (LA) antibody against the mitochondrial autoantigens of human PBC and various TFA and LA conjugated proteins. Whereas both anti-TFA and anti-LA reacted with PDC-E2, the wild-type lipoated form of E2L2, OGDC-E2, E3-BP and LA-KLH, neither reacted with BCOADC-E2 or the non-lipoated form of E2L2. Of interest was that while anti-TFA reacted with PDC-E2, TFA-RSA and LA-KLH, it failed to inhibit PDC-E2 enzyme function. In contrast, anti-LA demonstrated cytoplasmic and mitochondrial staining, and inhibited PDC enzyme activity. Hence, although considerable cross reactivity exists between anti-TFA and anti-LA, the molecular nature of the interaction is clearly different. One of 14 PBC sera reacted weakly with TFA-albumin, whereas four of 14 PBC sera reacted with LA-KLH. Immunohistochemically, both anti-TFA and anti-LA antibodies reacted focally with periportal hepatocytes and bile ducts in both PBC and controls. However, anti-LA produced much stronger focalized staining of the bile ducts of diseased liver. This study suggests that while anti-TFA antibody recognizes lipoic acid-linked enzymes and proteins, the epitope recognized differs from that of anti-LA antibody and PBC autoantibodies. It is unlikely that a response to TFA is the triggering event in PBC. Anti-LA antibodies share a higher degree of similarity to PBC sera providing suggestive evidence that anti-LA antibodies or anti-LA like antibodies (mimotopes) may help define the initiator of the autoimmune response.

Localization of antioxidant enzymes and oxidative damage products in normal and malignant prostate epithelium

BACKGROUND

The risk for prostate cancer seems to be reduced by certain antioxidant compounds (vitamins E and A, and selenium).

METHODS

Antioxidant enzymes and oxidative damage products were localized in normal prostatic epithelium and malignant glands in primary and metastatic prostatic adenocarcinomas, using well-characterized antibodies and immunoperoxidase techniques.

RESULTS

Antioxidant enzymes and four markers of oxidative damage were compared in basal and secretory cells of normal prostatic epithelium and prostate adenocarcinoma cells, and each cell type had unique patterns of enzymes and oxidative damage products. One marker of oxidative damage, a fluorophore derived from 4-hydroxy-2-nonenal-lysine adduction, was found in secretory cells of normal but not malignant epithelium, demonstrating a different oxidative metabolism in normal vs. malignant prostate epithelium. Metastatic lesions from primary prostate cancer had higher levels of manganese superoxide dismutase and nuclear oxidative damage products than did primary tumors.

CONCLUSIONS

Antioxidant enzymes and oxidative damage products are modulated in metastatic compared to primary prostate cancer.

Free radicals and lipid peroxidation do not mediate beta-amyloid-induced neuronal cell death

"beta Amyloid (Abeta)-induced free radical-mediated neurotoxicity" is a leading hypothesis as a cause of Alzheimer's disease (AD). Abeta increased free radical production and lipid peroxidation in PC12 nerve cells, leading to increased 4-hydroxy-2-nonenal (HNE) production and modification of specific mitochondrial target proteins, apoptosis and cell death. Pretreatment of the cells with isolated ginkgolides, the anti-oxidant component of Ginkgo biloba leaves, or vitamin E, prevented the Abeta-induced increase of reactive oxygen species (ROS). Ginkgolides, but not vitamin E, inhibited the Abeta-induced HNE modification of mitochondrial proteins. However, treatment with these anti-oxidants did not rescue the cells from Abeta-induced apoptosis and cell death. These results indicate that free radicals and lipid peroxidation may not mediate Abeta-induced neurotoxicity.

Localization of hydroxynonenal protein adducts in normal human kidney and selected human kidney cancers

Both polyclonal and monoclonal antibodies to 4-hydroxy-2-nonenal (HNE) protein adducts were used to identify lipid peroxidation products in normal human kidney and in selected human kidney cancers using immunoperoxidase techniques at the light microscopic level and immunogold techniques at the ultrastructural level. HNE protein adducts were detected in most cell types in normal kidney, although in highly variable amounts. All six morphologic types of renal tumors examined showed some staining with antibodies to HNE protein adducts, although the intensity of staining varied considerably depending on tumor type. Renal oncocytoma and the granular cell variant of renal adenocarcinoma both showed greater cytoplasmic staining for HNE protein adducts than the other tumors examined; these tumors both contain high numbers of mitochondria and suggest that mitochondria are a major source of lipid peroxidation products. To test this possibility, immunogold ultrastructural analysis was performed. HNE protein adducts were identified in nuclei and mitochondria in both normal proximal tubule and three types of renal carcinoma examined; these results localize oxidative damage at the subcellular level in both benign and malignant epithelium to nuclei and mitochondria. In conclusion, HNE protein adducts occur in kidneys in both normal and tumor cells, although immunomorphologic analyses suggest less HNE protein adducts in tumor cells.

Declines in mitochondrial respiration during cardiac reperfusion: age-dependent inactivation of alpha-ketoglutarate dehydrogenase

We previously reported that cardiac reperfusion results in declines in mitochondrial NADH-linked respiration. The degree of inactivation increased with age and was paralleled by modification of protein by the lipid peroxidation product 4-hydroxy-2-nonenal. To gain insight into potential sites of oxidative damage, the present study was undertaken to identify specific mitochondrial protein(s) inactivated during ischemia and reperfusion and to determine which of these losses in activity are responsible for observed declines in mitochondrial respiration. Using a Langendorff rat heart perfusion protocol, we observed age-dependent inactivation of complex I during ischemia and complex IV and alpha-ketoglutarate dehydrogenase during reperfusion. Although losses in complex I and IV activities were found not to be of sufficient magnitude to cause declines in mitochondrial respiration, an age-related decrease in complex I activity during ischemia may predispose old animals to more severe oxidative damage during reperfusion. It was determined that inactivation of alpha-ketoglutarate dehydrogenase is responsible, in large part, for observed reperfusion-induced declines in NADH-linked respiration. alpha-Ketoglutarate dehydrogenase is highly susceptible to 4-hydroxy-2-nonenal inactivation in vitro. Thus, our results suggest a plausible mechanism for age-dependent, reperfusion-induced declines in mitochondrial function and identify alpha-ketoglutarate dehydrogenase as a likely site of free radical-mediated damage.

Tissue-destructive macrophages in giant cell arteritis

Giant cell arteritis (GCA) is an inflammatory vasculopathy in which T cells and macrophages infiltrate the wall of medium and large arteries. Clinical consequences such as blindness and stroke are related to arterial occlusion. Formation of aortic aneurysms may result from necrosis of smooth muscle cells and fragmentation of elastic membranes. The molecular mechanisms of arterial wall injury in GCA are not understood. To identify mechanisms of arterial damage, gene expression in inflamed and unaffected temporal artery specimens was compared by differential display polymerase chain reaction. Genes differentially expressed in arterial lesions included 3 products encoded by the mitochondrial genome. Immunohistochemistry with antibodies specific for a 65-kDa mitochondrial antigen revealed that increased expression of mitochondrial products was characteristic of multinucleated giant cells and of CD68+ macrophages that cluster in the media and at the media-intima junction. 4-Hydroxy-2-nonenal adducts, products of lipid peroxidation, were detected on smooth muscle cells and on tissue infiltrating cells, in close proximity to multinucleated giant cells and CD68+ macrophages. Also, giant cells and macrophages with overexpression of mitochondrial products were able to synthesize metalloproteinase-2. Our data suggest that in the vascular lesions characteristic for GCA, a subset of macrophages has the potential to support several pathways of arterial injury, including the release of reactive oxygen species and the production of metalloproteinase-2. This macrophage subset is topographically defined and is also identified by overexpression of mitochondrial genes. Because these macrophages have a high potential to promote several mechanisms of arterial wall damage, they should be therapeutically targeted to prevent blood vessel destruction.

Localization of 4-hydroxy-2-nonenal-modified proteins in kidney following iron overload

Intraperitoneal (IP) injection of ferric nitrilotriacetate (Fe-NTA) to rats and mice results in iron-induced free radical injury and cancer in kidneys. We sought to clarify the exact localization of acute oxidative damage in Fe-NTA-induced nephrotoxicity by performing immunogold light and electron microscopic (EM) techniques using an antibody against 4-hydroxy-2-nonenal (HNE)-modified proteins. Biochemical assays were done to provide complementary quantitative data. Renal accumulation of lipid peroxidation-derived aldehydes, such as malondialdehyde (MDA) and 4-hydroxy-2-alkenals (4-HDA), increased in parallel with protein carbonyl content, an indicator of protein oxidation, 30 min after administration of Fe-NTA. Immunogold light microscopy showed that HNE-modified proteins increased at 30 min with positivity localized to proximal tubular cells. Immunogold EM demonstrated that HNE-modified proteins were mainly in the mitochondria and nuclei of the proximal tubular epithelium. The intensity of labeling at both the light and EM levels increased together with levels of biochemically measured lipid peroxidation products and protein carbonyl content. Our data suggest that the mechanism of acute nephrotoxicity of Fe-NTA involves mitochondrial and nuclear oxidative damage, findings that may help to define the mechanisms of iron-induced cell injury.

Aldose reductase functions as a detoxification system for lipid peroxidation products in vasculitis

Giant cell arteritis (GCA) is a systemic vasculitis preferentially affecting large and medium-sized arteries. Inflammatory infiltrates in the arterial wall induce luminal occlusion with subsequent ischemia and degradation of the elastic membranes, allowing aneurysm formation. To identify pathways relevant to the disease process, differential display-PCR was used. The enzyme aldose reductase (AR), which is implicated in the regulation of tissue osmolarity, was found to be upregulated in the arteritic lesions. Upregulated AR expression was limited to areas of tissue destruction in inflamed arteries, where it was detected in T cells, macrophages, and smooth muscle cells. The production of AR was highly correlated with the presence of 4-hydroxynonenal (HNE), a toxic aldehyde and downstream product of lipid peroxidation. In vitro exposure of mononuclear cells to HNE was sufficient to induce AR production. The in vivo relationship of AR and HNE was explored by treating human GCA temporal artery-severe combined immunodeficiency (SCID) mouse chimeras with the AR inhibitors Sorbinil and Zopolrestat. Inhibition of AR increased HNE adducts twofold and the number of apoptotic cells in the arterial wall threefold. These data demonstrate that AR has a tissue-protective function by preventing damage from lipid peroxidation. We propose that AR is an oxidative defense mechanism able to neutralize the toxic effects of lipid peroxidation and has a role in limiting the arterial wall injury mediated by reactive oxygen species.

Manganese superoxide dismutase protects mitochondrial complex I against adriamycin-induced cardiomyopathy in transgenic mice

Adriamycin (ADR) is a potent anticancer drug that causes severe cardiomyopathy. We have previously demonstrated that ADR-induced ultrastructural mitochondrial injury in the heart was attenuated in manganese superoxide dismutase (MnSOD) transgenic mice. To further investigate the biochemical mechanisms by which MnSOD protected mitochondria against ADR-induced damage, cardiac mitochondrial function and activities were evaluated. The results showed that ADR caused significant decrease in state 3 respiration and respiratory control ratio using both complex I and II substrates in nontransgenic mice. In transgenic mice, state 3 respiration for complex I substrates remained unaffected by ADR, but was reduced for complex II substrate. Complex I activity was significantly decreased in nontransgenic, but not in transgenic mice after ADR treatment, suggesting that mitochondrial complex I is sensitive to inactivation by superoxide radicals. The activities of complex II and mitochondrial creatine kinase were decreased by ADR in both nontransgenic and transgenic mice. These results support our previous observations on the protective role of MnSOD on the ultrastructural damage of the heart after ADR treatment and extend the understanding of its mechanisms in mitochondria.

Selective inactivation of alpha-ketoglutarate dehydrogenase and pyruvate dehydrogenase: reaction of lipoic acid with 4-hydroxy-2-nonenal

Previous research has established that 4-hydroxy-2-nonenal (HNE), a highly toxic product of lipid peroxidation, is a potent inhibitor of mitochondrial respiration. HNE exerts its effects on respiration by inhibiting alpha-ketoglutarate dehydrogenase (KGDH). Because of the central role of KGDH in metabolism and emerging evidence that free radicals contribute to mitochondrial dysfunction associated with numerous diseases, it is of great interest to further characterize the mechanism of inhibition. In the present study, treatment of rat heart mitochondria with HNE resulted in the selective inhibition of KGDH and pyruvate dehydrogenase (PDH), while other NADH-linked dehydrogenases and electron chain complexes were unaffected. KGDH and PDH are structurally and catalytically similar multienzyme complexes, suggesting a common mode of inhibition. To determine the mechanism of inhibition, the effects of HNE on purified KGDH and PDH were examined. These studies revealed that inactivation by HNE was greatly enhanced in the presence of substrates that reduce the sulfur atoms of lipoic acid covalently bound to the E2 subunits of KGDH and PDH. In addition, loss of enzyme activity induced by HNE correlated closely with a decrease in the availability of lipoic acid sulfhydryl groups. Use of anti-lipoic acid antibodies indicated that HNE modified lipoic acid in both purified enzyme preparations and mitochondria and that this modification was dependent upon the presence of substrates. These results therefore identify a potential mechanism whereby free radical production and subsequent lipid peroxidation lead to specific modification of KGDH and PDH and inhibition of NADH-linked mitochondrial respiration.

Structural characterization and immunochemical detection of a fluorophore derived from 4-hydroxy-2-nonenal and lysine

Aging and the progression of certain degenerative diseases are accompanied by increases in intracellular fluorescent material, termed "lipofuscin" and ceroid, respectively. These pigments are observed within granules composed, in part, of damaged protein and lipid. Modification of various biomolecules by aldehyde products of lipid peroxidation is believed to contribute to lipofuscin and ceroid formation. However, little direct evidence currently exists because the structures responsible for the fluorescent, cross-linked nature of this material are not well characterized. In this study, we have identified a fluorescent product formed in the reaction of Nalpha-acetyllysine and 4-hydroxy-2-nonenal (HNE), a major product of lipid peroxidation and the most reactive of these compounds under physiological conditions [Esterbauer, H., Shaur, R. J. & Zollner, H. (1991) Free Radical Biol. Med. 11, 81-128]. This fluorescent compound, characterized as a 2-hydroxy-3-imino-1,2-dihydropyrrol derivative, appears to form upon oxidative cyclization of the nonfluorescent 2:1 lysine-HNE Michael adduct-Schiff base cross-link. Polyclonal antibody was raised to the Nalpha-acetyllysine-HNE fluorophore and found to be highly specific to the chromophore structure of the compound. This antibody has been used to conclusively demonstrate that the lysine-HNE derivative of this fluorophore forms on protein upon exposure to HNE. The results of this study therefore provide the basis for future investigations on the contribution(s) of HNE-derived fluorophore formation to lipofuscin and ceroid accumulation.

Prevention of mitochondrial injury by manganese superoxide dismutase reveals a primary mechanism for alkaline-induced cell death

Alkalosis is a clinical complication resulting from various pathological and physiological conditions. Although it is well established that reducing the cellular proton concentration is lethal, the mechanism leading to cell death is unknown. Mitochondrial respiration generates a proton gradient and superoxide radicals, suggesting a possible link between oxidative stress, mitochondrial integrity, and alkaline-induced cell death. Manganese superoxide dismutase removes superoxide radicals in mitochondria, and thus protects mitochondria from oxidative injury. Cells cultured under alkaline conditions were found to exhibit elevated levels of mitochondrial membrane potential, reactive oxygen species, and calcium which was accompanied by mitochondrial damage, DNA fragmentation, and cell death. Overexpression of manganese superoxide dismutase reduced the levels of intracellular reactive oxygen species and calcium, restored mitochondrial transmembrane potential, and prevented cell death. The results suggest that mitochondria are the primary target for alkaline-induced cell death and that free radical generation is an important and early event conveying cell death signals under alkaline conditions.

Cardiac reperfusion injury: aging, lipid peroxidation, and mitochondrial dysfunction

Cardiac reperfusion and aging are associated with increased rates of mitochondrial free radical production. Mitochondria are therefore a likely site of reperfusion-induced oxidative damage, the severity of which may increase with age. 4-Hydroxy-2-nonenal (HNE), a major product of lipid peroxidation, increases in concentration upon reperfusion of ischemic cardiac tissue, can react with and inactivate enzymes, and inhibits mitochondrial respiration in vitro. HNE modification of mitochondrial protein(s) might, therefore, be expected to occur during reperfusion and result in loss in mitochondrial function. In addition, this process may be more prevalent in aged animals. To begin to test this hypothesis, hearts from 8- and 24-month-old rats were perfused in Langendorff fashion and subjected to periods of ischemia and/or reperfusion. The rate of state 3 respiration of mitochondria isolated from hearts exposed to ischemia (25 min) was approximately 25% less than that of controls, independent of age. Reperfusion (40 min) caused a further decline in the rate of state 3 respiration in hearts isolated from 24- but not 8-month-old rats. Furthermore, HNE modification of mitochondrial protein (approximately 30 and 44 kDa) occurred only during reperfusion of hearts from 24-month-old rats. Thus, HNE-modified protein was present in only those mitochondria exhibiting reperfusion-induced declines in function. These studies therefore identify mitochondria as a subcellular target of reperfusion damage and a site of age-related increases in susceptibility to injury.

Inhibition of NADH-linked mitochondrial respiration by 4-hydroxy-2-nonenal

During the progression of certain degenerative conditions, including myocardial ischemia-reperfusion injury, mitochondria are a source of increased free-radical generation and exhibit declines in respiratory function(s). It has therefore been suggested that oxidative damage to mitochondrial components plays a critical role in the pathology of these processes. Polyunsaturated fatty acids of membrane lipids are prime molecular targets of free-radical damage. A major product of lipid peroxidation, 4-hydroxy-2-nonenal (HNE), is highly cytotoxic and can readily react with and damage protein. In this study, the effects of HNE on intact cardiac mitochondria were investigated to gain insight into potential mechanisms by which free radicals mediate mitochondrial dysfunction. Exposure of mitochondria to micromolar concentrations of HNE caused rapid declines in NADH-linked but not succinate-linked state 3 and uncoupled respiration. The activity of complex I was unaffected by HNE under the conditions of our experiments. Loss of respiratory activity reflected the inability of HNE-treated mitochondria to meet NADH demand during maximum rates of O2 consumption. HNE exerted its effects on intact mitochondria by inactivating alpha-ketoglutarate dehydrogenase. These results therefore identify a potentially important mechanism by which free radicals bring about declines in mitochondrial respiration.

Rapamycin inhibits proteasome activator expression and proteasome activity

Rapamycin (RAPA) is a potent immunosuppressive drug, and certain of its direct or indirect targets might be of vital importance to the regulation of an immune response. In this study, we used differential hybridization to search for human genes whose expression was sensitive to RAPA. Seven RAPA-sensitive genes were found and one of them encoded a protein with high homology to the alpha subunit of a proteasome activator (PA28 alpha). This gene was later found to code for the beta subunit of the proteasome activator (PA28 beta). Activated T and B cells had up-regulated PA28 beta expression at the mRNA level. Such up-regulation could be suppressed by RAPA, FK506, and cyclosporin A. RAPA and FK506 also repressed the up-regulated PA28 alpha messages in phytohemagglutinin (PHA)-stimulated T cells. At the protein level, RAPA inhibited PA28 alpha and PA28 beta in the activated T cells according to immunoblotting and confocal microscopy. Probably as a consequence, there was a fourfold increase of proteasome activities in the peripheral blood mononuclear cell lysate after the PHA activation. RAPA could inhibit the enhanced part of the proteasome activity. Considering the critical role played by the proteasome in degrading regulatory proteins, our data suggest that the proteasome activator is a relevant and important downstream target of rapamycin, and that the immune response could be modulated through the activity of the proteasome.

Inhibition of the multicatalytic proteinase (proteasome) by 4-hydroxy-2-nonenal cross-linked protein

Oxidative modification of glucose-6-phosphate dehydrogenase (Glu-6-PDH), as observed for other proteins, increases the susceptibility of the protein to degradation by the multicatalytic proteinase/proteasome (MCP). Oxidized Glu-6-PDH is, however, more prone to cross-linking reactions by the lipid peroxidation product 4-hydroxy-2-nonenal (HNE), processes which render the protein resistant to proteolysis. In addition, HNE cross-linked protein inhibits the degradation of oxidatively modified glutamine synthetase by the MCP. In contrast to oxidized Glu-6-PDH, which inhibits the proteolysis of GS in a competitive manner, HNE cross-linked protein acts as a noncompetitive inhibitor. As judged by binding of the hydrophobic fluorescent probe 8-anilino-1-naphthalenesulfonic acid, a common structural feature of both macromolecular substrates and inhibitors of the MCP is an increased accessibility of hydrophobic regions on the protein.

Age-related decline of rat liver multicatalytic proteinase activity and protection from oxidative inactivation by heat-shock protein 90

To test whether an observed age-related increase in the level of oxidized protein in rat liver is due to a decrease in the activity of the multicatalytic proteinase (MCP), this protease was isolated from liver of young (8-month-old) and old (24-month-old) male Fischer 344 rats. Three peptidase activities of the MCP were assayed using fluorogenic peptides: trypsin-like, chymotrypsin-like, and peptidylglutamyl-peptide hydrolase. Only peptidylglutamyl-peptide hydrolase activity declined with age, with protease from old animals exhibiting approximately 50% of the activity of that from young animals. Bidimensional gel electrophoresis and thermostability studies did not reveal age-related structural modifications of the MCP subunits. Peptidylglutamyl-peptide hydrolase activity and trypsin-like activity were sensitive to metal-catalyzed oxidation. In some preparations, a 95-kDa protein that has been identified as the heat shock protein 90 copurified with the MCP. In the presence of HSP 90, trypsin-like activity is protected from oxidative inactivation and chymotrypsin-like activity is slightly activated. Peptidylglutamyl-peptide hydrolase activity remained sensitive to oxidation in protease isolated from young rats, but that from old rats was resistant to oxidative inactivation. Furthermore, addition of rat HSP 90 to rat liver MCP (purified from 8-month-old animals and free of contaminating HSP 90) was found to protect trypsin-like activity from oxidative inactivation.

Chemical characterization of a protein-4-hydroxy-2-nonenal cross-link: immunochemical detection in mitochondria exposed to oxidative stress

We have previously shown that incubation of the model protein glucose-6-phosphate dehydrogenase (Glu-6-PDH) from the bacterium Leuconostoc mesenteroides with 4-hydroxy-2-nonenal (HNE), a major product of lipid peroxidation, results in the formation of cross-linked protein. HNE-modified protein is resistant to proteolytic degradation and acts as an inhibitor of the multicatalytic proteinase. It was therefore important to establish the chemistry of the cross-linking reaction. The formation of cross-linked Glu-6-PDH is associated with the nearly exclusive loss of lysine residues. For this reason the reaction of N-acetyllysine with HNE has been investigated. The epsilon-amino group of lysine reacts with the double bond (C3) and the carbonyl (C1) functions of HNE via Michael addition and Schiff base formation resulting in the production of a 2:1 amino acid-HNE cross-link. Chromatographic detection of this adduct in the acid hydrolysate of HNE-treated Glu-6-PDH reveals that this chemistry is responsible for the formation of cross-linked protein. Antibody to the reduced form of the 2:1 lysine-HNE adduct was prepared. The antibody was used to demonstrate that exposure of isolated liver mitochondria to oxidative stress led to the formation of intra- and intermolecular protein-HNE cross-links. The results of the present study indicate that modifications to protein by lipid peroxidation products may be physiologically relevant and could contribute to the disease- and age-related buildup of damaged protein.

Aging of the liver: proteolysis of oxidatively modified glutamine synthetase

During aging cells accumulate altered forms of proteins, notably oxidatively modified proteins. The multicatalytic protease selectively degrades oxidized proteins, suggesting that the age-related accumulation of oxidized proteins might be a consequence of decreased activity of this protease. The protease activity of liver homogenates was assayed with an improved fluorimetric method, using oxidatively modified glutamine synthetase as substrate. Application of this assay to extracts from liver of Fischer 344 rats from both Japan and the United States demonstrated a marked preference for the oxidized substrates, as expected. Extracts from animals ages 8 to 26 months maintain both total proteolytic activity and the ability to distinguish between native and oxidized substrates. Oxidatively modified hepatocyte extracts were also employed as substrate, and older animals again maintained proteolytic activity. The multicatalytic protease was purified from liver of young and old rats, and the specific activity of the preparations were comparable when assayed with oxidatively modified glutamine synthetase. We conclude that the intrinsic neutral or alkaline proteolytic activity of rat liver is maintained during aging.

Modification of glucose-6-phosphate dehydrogenase by 4-hydroxy-2-nonenal. Formation of cross-linked protein that inhibits the multicatalytic protease

Incubation of glucose-6-phosphate dehydrogenase (Glu-6-PDH) from Leuconostoc mesenteroides with the lipid peroxidation product 4-hydroxy-2-nonenal leads to the formation of cross-linked protein. This is accompanied by the appearance of protein-associated fluorescence with excitation and emission maxima of 340 and 415 nm, respectively, and with the disappearance of histidine and lysine residues. Cross-linked protein is less susceptible than native Glu-6-PDH to proteolysis by the multicatalytic protease, a multienzymic proteolytic complex involved in the intracellular degradation of damaged proteins. In addition, 4-hydroxy-2-nonenal-modified Glu-6-PDH inhibits the multicatalytic protease and can therefore prevent the efficient degradation of oxidized protein. These findings may have important implications for the accumulation of altered protein and fluorescent material in vivo, processes that are believed to be involved in age- and disease-related impairment of cellular function.

Susceptibility of glucose-6-phosphate dehydrogenase modified by 4-hydroxy-2-nonenal and metal-catalyzed oxidation to proteolysis by the multicatalytic protease

Glucose-6-phosphate dehydrogenase from Leuconostoc mesenteroides is inactivated when exposed to metal-catalyzed oxidation or when modified by the lipid peroxidation product, 4-hydroxy-2-nonenal (HNE). Although in each case inactivation appears to be the result of the selective modification of an active site lysine residue, only the oxidized enzyme becomes more susceptible to proteolysis by purified rat liver multicatalytic protease, a multienzymatic proteolytic complex involved in the intracellular degradation of damaged proteins. The HNE-treated enzyme remains as resistant to proteolysis by the multicatalytic protease as the native enzyme. In contrast to the HNE-treated Glu-6-PDH, enzyme modified by Fe2+ and citrate is more thermolabile and exhibits increased binding of the hydrophobic probe 8-anilino-1-naphthalene sulfonic acid (ANSA). Heat inactivation is characterized, in part, by dissociation of the dimer to inactive subunits. No change in the secondary structure and only small variations in the fluorescence and circular dichroism of the aromatic residues are observed for the two modified forms of the enzyme as compared with the native enzyme. The increased heat sensitivity, ANSA binding, and proteolytic susceptibility are likely related to a decrease in the structural stability of oxidatively modified Glu-6-PDH. Conversely, modification of Glu-6-PDH with HNE has no apparent effect on its structural stability or proteolytic susceptibility. This finding may have important implications for the accumulation of altered protein in vivo, a process that is believed to be involved in age- and disease-related impairment of cellular function.

Age-related increase in liver retinyl palmitate. Relationship to lipofuscin

Lipofuscin is a general term assigned to fluorescent material that accumulates in cells as they age. It is apparent from this study that the fluorescent intensity of detergent-solubilized liver from Fisher-344 rats increased as a function of age. The fluorophore responsible for this increase was extracted with methanol and could be resolved from other cellular components when the methanol extracts were chromatographed over a reverse phase column. This compound was identified as retinyl palmitate and was found to increase throughout the entire life of the Fisher-344 rat (2-24 months), from a value of 0.26-1.77 mg/g of liver. In addition, the results presented here demonstrate that concentration, time between extraction and analysis, exposure to light, and degree of purity affect the observed fluorescent properties of retinyl palmitate. These factors affect many fluorophores and are likely to be, at least in part, responsible for the multiplicity of reported properties of lipofuscin. As has been reported for lipofuscin, retinyl palmitate accumulates in intracellular granules and exhibits fluorescence between 450 and 600 nm. Due to these similarities, the relationship between retinyl palmitate and lipofuscin warrants further investigation.

Immunochemical detection of 4-hydroxynonenal protein adducts in oxidized hepatocytes

We report here the development of an immunochemical procedure that uses an antibody specific to the 4-hydroxynonenal (HNE) moiety for the detection of HNE-protein adducts. The HNE-specific antibody was prepared by immunizing rabbits with a HNE-keyhole limpet hemocyanin conjugate and purifying the rabbit serum on an affinity gel prepared by covalent attachment of a HNE-conjugated heptapeptide. When various preparations of glyceraldehyde-3-phosphate dehydrogenase containing 0-7.0 equivalent of HNE-histidine residues per subunit were obtained by incubating samples of glyceraldehyde-3-phosphate dehydrogenase with increased amounts of HNE and subjected to immunoblotting with the HNE-specific antibody, the intensities of the blots were directly proportional to the number of HNE-histidine adducts as measured directly by amino acid analysis. Binding of the HNE-conjugated glyceraldehyde-3-phosphate dehydrogenase to the HNE-specific antibody could be completely inhibited by HNE-N-acetylhistidine, HNE-N-acetyllysine, or HNE-glutathione, suggesting that the antigenic determinant recognized by the antibody is the HNE moiety, not the HNE-amino acid conjugates, such as HNE-histidine, HNE-lysine, and HNE-cysteine. The utility of the HNE-specific antibody was demonstrated by its ability to react selectively with a number of HNE-protein adducts in immunoblot analyses of crude homogenates of rat liver hepatocytes that had been exposed to HNE or oxidative stresses with tert-butylhydroperoxide or metal-ion-catalyzed oxidation systems.

Oxidative modification of glucose-6-phosphate dehydrogenase from Leuconostoc mesenteroides by an iron(II)-citrate complex

Incubation of glucose-6-phosphate dehydrogenase from Leuconostoc mesenteroides with Fe2+ and citrate results in rapid O2-dependent inactivation of the enzyme. Maximal rate of inactivation occurred at equimolar concentrations of Fe2+ and citrate. Loss of enzyme activity appears to be the result of selective oxidative modification, as evidenced by a corresponding increase in protein carbonyl content. Partially inactivated enzyme remained predominantly in the dimeric form with no change in the apparent affinity of the remaining active subunits for substrate. Modified Glu-6-PDH was, however, more susceptible to heat denaturation. Our results suggest that the Fe(2+)-citrate complex binds to the glucose 6-phosphate binding site and then undergoes reaction with H2O2 formed in solution leading to the oxidative modification of amino acids essential for enzyme activity.

Inactivation of glucose-6-phosphate dehydrogenase by 4-hydroxy-2-nonenal. Selective modification of an active-site lysine

Incubation of glucose-6-phosphate dehydrogenase from Leuconostoc mesenteroides with 4-hydroxy-2-nonenal (HNE) results in a pseudo first-order loss of enzyme activity. The pH dependence of the inactivation rate exhibits an inflection around pH 10, and the enzyme is protected from inactivation by glucose 6-phosphate. Loss of enzyme activity corresponds with the formation of one carbonyl function per enzyme subunit and the appearance of a lysine-HNE adduct. The data presented in this paper are consistent with the view that the epsilon-amino group of a lysine residue in the glucose 6-phosphate-binding site reacts with the double bond (C3) of HNE, resulting in the formation of a stable secondary amine derivative and loss of enzyme activity. We have described a mechanism by which HNE may, in part, mediate free radical damage. In addition, a method for the detection of the lysine-HNE adduct is introduced.

Iron-catalyzed oxidative modification of glucose-6-phosphate dehydrogenase from Leuconostoc mesenteroides. Structural and functional changes

As a variety of eukaryotic cells age, the specific activity of glucose-6-phosphate dehydrogenase (Glu-6-PDH) declines as much as 50%. Because of the central role of this enzyme in metabolism, it is important to define factors responsible for this loss in enzyme activity. We report that Glu-6-PDH from Leuconostoc mesenteroides is rapidly inactivated by micromolar concentrations of Fe2+ and H2O2. Inactivation correlated with the formation of one carbonyl functionality/enzyme subunit, indicating that inactivation is the result of site-specific oxidative modification. Our results suggest that Fe2+ binds to the glucose 6-phosphate binding site and that interaction of the enzyme-bound Fe2+ with H2O2 leads to the oxidative modification of amino acids essential for enzyme activity. Partially inactivated enzyme remained predominantly in the dimeric form, and no change in the apparent affinity of the remaining active subunits for substrate was observed. Partial inactivation did, however, lead to a decrease in the thermal stability of the remaining activity. This decrease in thermal stability could be largely overcome by the addition of glucose 6-phosphate. Thus, although exposure to H2O2 and Fe2+ results in the irreversible inactivation of Glu-6-PDH, the resulting modification is selective, leads to the formation of heterodimers of both active and inactive subunits, and does not appear to cause large scale structural changes. Our results demonstrate the inherent susceptibility of Glu-6-PDH from L. mesenteroides to modification by an oxidation system known to exist in vivo. An assessment of the physiological significance of Fe(2+)-catalyzed oxidation of Glu-6-PDH awaits extension of these studies to mammalian sources known to accumulate less active or inactive forms of the enzyme as a function of age.

Response of rat liver glutaminase to pH, ammonium, and citrate. Possible regulatory role of glutaminase in ureagenesis

Liver glutaminase is stimulated by an increase in NH4+ concentration and NH4+ is an absolute requirement for activity at approximate physiological concentrations of phosphate and glutamine. Increases in the concentration of NH4+ cannot, however, overcome the inhibitory effect of a decrease in pH. In addition, the concentration of NH4+ required for half-maximal rate decreases as pH increases. This decrease is the result of two factors: a direct effect of pH on the apparent affinity of the enzyme for NH4+, and an indirect effect of pH brought about by an increase in the apparent affinity of the enzyme for phosphate which results in a further decrease in the M0.5 for NH4+. In addition, liver glutaminase responds strongly to the concentration of citrate over a physiologically relevant range at approximate physiological concentrations of NH4+, phosphate, and glutamine. An increase in citrate concentration stimulates glutaminase by increasing the affinity of the enzyme for glutamine. The apparent affinity of the enzyme for citrate increases as pH increases. The strong response of liver glutaminase to pH, NH4+, and citrate and the fact that the hydrolysis of glutamine can supply metabolites and effectors for urea synthesis suggest a possible regulatory role of glutaminase in ureagenesis.

Response of rat liver glutaminase to magnesium ion

The activity of rat liver glutaminase from sedimented fractions of freeze-thawed mitochondria is strongly affected by variation in the Mg2+ concentration within the approximate physiological range of activators. A rise in the Mg2+ concentration stimulates glutaminase by increasing the apparent affinity of the enzyme for its positive modifier phosphate. With the addition of 4 mM Mg2+ the M0.5 for phosphate activation decreased from 18 to 9.5 mM at pH 7.1, 10 to 5.8 mM at pH 7.4 and 6.4 to 4.0 mM at pH 7.7. The result is an increase in the apparent affinity of the enzyme for glutamine. With the addition of 4 mM Mg2+ the S0.5 of glutaminase for glutamine decreased from 24 to 13 mM at pH 7.1, 14 to 9.6 mM at pH 7.4, and remained unchanged at 8.2 mM at pH 7.7. Since Mg2+ stimulates glutaminase, as does a rise in pH (Szweda, L.I. and Atkinson, D.E. (1989) J. Biol. Chem. 264, 15357-15360), by increasing the apparent affinity of the enzyme for phosphate, it reduces the inhibitory effect of a decrease in pH and/or phosphate concentration over a physiologically relevant range.

Response of rat liver glutaminase to pH. Mediation by phosphate and ammonium ions

The activity of rat liver glutaminase from sedimented fractions of freeze-thawed mitochondria is strongly affected by variation in pH over a physiologically relevant range at approximate physiological concentrations of activators. As pH increases from 7.1 to 7.7 at 0.7 mM ammonium and 10 mM phosphate, the S0.5 for glutamine decreases 3.5-fold, from 38 to 11 mM. This results in an 8-fold increase in reaction velocity at 10 mM glutamine. In addition, the M0.5 for phosphate activation decreases from 21 to 8.9 mM as pH increases from 7.1 to 7.7. This apparent effect of pH on the affinity of glutaminase for phosphate is similar to previous reports of the pH effect on activation by ammonium (Verhoeven, A. J., Van Iwaarden, J. F., Joseph, S. K., and Meijer, A. J. (1983) Eur. J. Biochem. 133, 241-244; McGivan, J. D., and Bradford, N. M. (1983) Biochim. Biophys. Acta 159, 296-302). Glutaminase does not respond to variation in pH between 7.1 and 7.7 when phosphate and ammonium are saturating. The effects of the two modifiers are additive. Each is still effective, as is pH, when the other is saturating. Therefore, it appears that the effects of pH on the apparent affinity of the enzyme for ammonium and phosphate account for the enzyme's response to pH. These results may help explain previous reports of minimal effects of pH on glutaminase at saturating concentrations of related substances (McGivan, J. D., Lacey, J. H., and Joseph, K. (1980) Biochim. J. 192, 537-542; Horowitz, M. L., and Knox, W. E. (1968) Enzymol. Biol. Clin. 9, 241-255; McGivan, J. D., and Bradford, N. M. (1983) Biochim. Biophys. Acta 759, 296-302). Glutaminase binds glutamine cooperatively with Hill coefficients ranging from 1.7 to 2.2, which suggests at least two and probably three or more interacting binding sites for glutamine. The strong response of liver glutaminase to pH and the fact that the reaction can supply metabolites for urea synthesis suggest a possible regulatory role of glutaminase in ureagenesis.