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Highlights: [•] Argpyrimidine and pentosidine accumulate in the skin of aged mice, whereas protein carbonylation is unchanged. [•] Collagen oxidation promotes collagenase degradation. [•] Enzymatic degradation of AGE-modified collagen is reduced. [•] Cross-linked collagen fibers are resistant to proteolytic degradation. [ABSTRACT FROM AUTHOR]

Abstract: Tau is the major protein exhibiting intracellular accumulation in Alzheimer disease. The mechanisms leading to its accumulation are not fully understood. It has been proposed that the proteasome is responsible for degrading tau but, since proteasomal inhibitors block both the ubiquitin-dependent 26S proteasome and the ubiqutin-independent 20S proteasome pathways, it is not clear which of these pathways is involved in tau degradation. Some involvement of the ubiquitin ligase, CHIP in tau degradation has also been postulated during stress. In the current studies, we utilized HT22 cells and tau-transfected E36 cells in order to test the relative importance or possible requirement of the ubiquitin-dependent 26S proteasomal system versus the ubiquitin-independent 20S proteasome, in tau degradation. By means of ATP-depletion, ubiquitinylation-deficient E36ts20 cells, a 19S proteasomal regulator subunit MSS1-siRNA approaches, and in vitro ubiquitinylation studies, we were able to demonstrate that ubiquitinylation is not required for normal tau degradation. [Copyright &y& Elsevier]

Oxidative stress in mammalian cells is an inevitable consequence of their aerobic metabolism. Oxidants produce modifications to proteins leading to loss of function (or gain of undesirable function) and very often to an enhanced degradation of the oxidized proteins. For several years it has been known that the proteasome is involved in the degradation of oxidized proteins. This review summarizes our knowledge about the recognition of oxidized protein substrates by the proteasome in in vitro systems and its applicability to living cells. The majority of studies in the field agree that the degradation of mildly oxidized proteins is an important function of the proteasomal system. The major recognition motif of the substrates seems to be hydrophobic surface patches that are recognized by the 20S ‘core’ proteasome. Such hydrophobic surface patches are formed by partial unfolding and exposure of hydrophobic amino acid residues during oxidation. Oxidized proteins appear to be relatively poor substrates for ubiquitination, and the ubiquitination system does not seem to be involved in the recognition or targeting of oxidized proteins. Heavily oxidized proteins appear to first aggregate (new hydrophobic and ionic bonds) and then to form covalent cross-links that make them highly resistant to proteolysis. The inability to degrade extensively oxidized proteins may contribute to the accumulation of protein aggregates during diseases and the aging process. [Copyright &y& Elsevier]

Generalized increases in protein oxidation and protein degradation in response to mild oxidative stress have been widely reported, but only a few individual proteins have actually been shown to undergo selective, oxidation-induced proteolysis. Our goal was to find such proteins in Clone 9 liver cells exposed to hydrogen peroxide. Using metabolic radiolabeling of intracellular proteins with [35S]cysteine/methionine, and analysis by two-dimensional polyacrylamide gel electrophoresis (2-D PAGE), we found at least three labeled proteins (“A,” “B,” and “C”) whose levels were decreased significantly more than the generalized protein loss after mild oxidative stress. “Protein C” was excised from 2-D PAGE and subjected to N-terminal amino acid microsequencing. “Protein C” was identified as Protein Disulfide Isomerase or PDI (E.C. 5.3.4.1), and this identity was reconfirmed by Western blotting with a C-terminal anti-PDI monoclonal antibody. A combination of quantitative radiometry and Western blotting in 2-D PAGE revealed that PDI was selectively degraded and then new PDI was synthesized, following H2O2 exposure. PDI degradation was blocked by inhibitors of the proteasome, and by cell treatment with proteasome C2 subunit antisense oligonucleotides, indicating that the proteasome was largely responsible for oxidation-induced PDI degradation. [Copyright &y& Elsevier]

Peroxynitrite is a strong oxidant and nitrating species that mediates certain biological effects of superoxide and nitrogen monoxide. These biological effects include oxidative damage to proteins as well as the formation of 3-nitrotyrosyl moieties in proteins. As a consequence, such proteins may lose their activity, gain altered function, or become prone to proteolytic degradation – resulting in modulation of cellular protein turnover and in the modulation of signaling cascades. In analogy to hydrogen peroxide, peroxynitrite may be scavenged by selenoproteins like glutathione peroxidase-1 (GPx-1) or by selenocompounds with a GPx-like activity, such as ebselen; in further analogy to H 2 O 2 , peroxiredoxins have also been established as contributors to peroxynitrite reduction. This review covers three aspects of peroxynitrite biochemistry, (i) the interaction of selenocompounds/-proteins with peroxynitrite, (ii) peroxynitrite-induced modulation of cellular proteolysis, and (iii) peroxynitrite-induced modulation of cellular signaling. [ABSTRACT FROM AUTHOR]

Abstract: The formation of oxidized proteins is one of the highlights of oxidative stress. In order not to accumulate such proteins have to be degraded. The major proteolytic system responsible for the removal of oxidized proteins is the proteasome. The proteasome is distributed throughout the cytosolic and nuclear compartment of mammalian cells, with high concentrations in the nucleus. On the other hand a major part of protein oxidation is taking place in the cytosol. The present review highlights the current knowledge on the intracellular distribution of oxidized proteins and put it into contrast with the concentration and distribution of the proteasome. [Copyright &y& Elsevier]

In living organisms a permanent oxidation of protein oxidation occurs. The degradation of intracellular oxidized proteins is intensively studied, but knowledge about the fate of oxidatively modified extracellular proteins is still limited. We studied the fate of exogenously added oxidized proteins in microglial cells. Both primary microglial cells and RAW cells are able to remove added oxidized laminin and myelin basic protein from the extracellular environment. Moderately oxidized proteins are degraded most efficiently, whereas strongly oxidized proteins are taken up by the microglial cells without an efficient degradation. Activation of microglial cells enhances the selective recognition and degradation of moderately oxidized protein substrates by proteases. Inhibitor studies also revealed an involvement of the lysosomal and the proteasomal system in the degradation of extracellular proteins. These studies let us conclude that microglial cells are able to remove oxidized proteins from the extracellular environment in the brain. [Copyright &y& Elsevier]

Abstract: Intracellular metabolism of 4-hydroxy-2-nonenal (HNE), a major product and mediator of oxidative stress and inflammation, is analyzed in resting and fMLP-stimulated human polymorphonuclear leukocytes (PMNL), where this compound is generated during activation of the respiratory burst. HNE consumption rate in PMNL is very low, if compared to other cell types (rat hepatocytes, rabbit fibroblasts), where HNE metabolism is always an important part of secondary antioxidative defense mechanisms. More than 98% of HNE metabolites are identified. The pattern of HNE intermediates is quite similar in stimulated and resting PMNL – except for higher water formation in resting PMNL – while the initial velocity of HNE degradation is somewhat higher in resting cells, 0.44 instead of 0.28nmol/(min×106 cells). The main products of HNE metabolism are 4-hydroxynonenoic acid (HNA), 1,4-dihydroxynonene (DHN) and the glutathione adducts with HNE, HNA, and DHN. Protein-bound HNE and water account for about 3–4% of the total HNE derivatives in stimulated cells, while in resting cells protein-bound HNE and water are 4% and 20%, respectively. Cysteinyl-glycine-HNE adduct and mercapturic acids contribute to about 5%. [ABSTRACT FROM AUTHOR]

Abstract: Aging is accompanied by an accumulation of oxidized proteins and cross-linked modified protein material. The intracellular formation and accumulation of highly oxidized and cross-linked proteins, the so-called lipofuscin, is a typical sign of senescence. However, little is known whether the lipofuscin accumulation during aging is related to environmental conditions, as oxidative stress, and whether the accumulation of oxidized proteins and lipofuscin is preferentially taking place in the cytosol or the nucleus and finally, what is the role of lysosomes in this process. Therefore, we investigated human skin fibroblasts in an early stage of proliferation (“young cells”) and in a late stage (“senescent cells”). Such cells were compared for the amount of protein carbonyls and lipofuscin and their distribution within the cytosol and the nucleus. Furthermore, cells were exposed to single and repeated doses of hydrogen peroxide and paraquat, measuring the same set of parameters. In addition to that the role of the proteasome to degrade oxidized proteins in young and senescent cells was tested. Furthermore, detailed microscopic analysis was performed testing the intracellular distribution of lipofuscin. The results clearly demonstrated that repeated/chronic oxidative stress induces a senescence–like phenotype of the distribution of oxidized proteins as well as of lipofuscin. It could be demonstrated that most of the lipofuscin is located in lysosomes and that senescent cells contain less lysosomes not lipofuscin-laden in comparison to young cells. [Copyright &y& Elsevier]

A redox steady state is important in maintaining vital cellular functions and is therefore homeostatically controlled by a number of antioxidative agents, the most important of which are enzymes. Oxidative Stress (OS) is associated with (or/and caused by) excessive production of damaging reactive oxygen and/or nitrogen species (ROS, RNS), which play a role in many pathologies. Because OS is a risk factor for many diseases, much effort (and money) is devoted to early diagnosis and treatment of OS. The desired benefit of the "identify (OS) and treat (by low molecular weight antioxidants, LMWA)" approach is to enable selective treatment of patients under OS. The present work aims at gaining understanding of the benefit of the antioxidants based on interrelationship between the concentration of different OS biomarkers and LMWA. Both the concentrations of a variety of biomarkers and of LMWA were previously determined and some analyses have been published by the MARK-AGE team. For the sake of simplicity, we assume that the concentration of an OS biomarker is a linear function of the concentration of a LMWA (if the association is due to causal relationship). A negative slope of this dependence (and sign of the correlation coefficient) can be intuitively expected for an antioxidant, a positive slope indicates that the LMWA is pro-oxidative, whereas extrapolation of the OS biomarker to [LMWA] = 0 is an approximation of the concentration of the OS biomarker in the absence of the LMWA. Using this strategy, we studied the effects of 12 LMWA (including tocopherols, carotenoids and ascorbic acid) on the OS status, as observed with 8 biomarkers of oxidative damage (including malondialdehyde, protein carbonyls, 3-nitrotyrosine). The results of this communication show that in a cross-sectional study the LMWA contribute little to the redox state and that different "antioxidants" are very different, so that single LMWA treatment of OS is not scientifically justified assuming our simple model. In view of the difficulty of quantitating the OS and the very different effects of various LMWA, the use of the "identify and treat" approach is questionable. • Antioxidants (LMWA) correlate differently with various oxidative stress (OS) biomarkers, association is weak even when highly statistically significant. • The variability of association of LMWA with biomarkers, accords with the existence of several types of OS. • Some LMWA correlate positively with OS biomarkers. • The use of "identify (people under OS) and treat (with LMWA)" approach is questionable. • The clinical use of the qualitatively-defined term "oxidative stress" requires quantitation. [ABSTRACT FROM AUTHOR]

Abstract: Oxidized proteins are recognized and degraded preferentially by the proteasome. This is true for numerous proteins including calmodulin (CaM). The degradation of CaM was investigated in a human fibroblast cell line under conditions of oxidative stress. Low molecular CaM fragments or peptides were found under such conditions. In in vitro experiments it was investigated whether this CaM breakdown product formation is induced by protein oxidation or is due to a limited proteolysis-derived degradation by the 20S proteasome. Native unoxidized CaM was not degraded by 20S proteasome, oxidized CaM was degraded in a time- and H2O2 concentration-dependent manner. Peptides of similar molecular weight were detected in isolated calmodulin as in oxidatively stressed fibroblasts. The peptides were identified using isolated calmodulin. Therefore, in oxidatively stressed fibroblasts and in vitro CaM is forming oxidation-driven fragments and proteasomal cleavage peptides of approximately 30 amino acids which undergo a slow or no degradation. [Copyright &y& Elsevier]

Abstract: Hyperammonemia, as a consequence of severe liver failure, is strongly associated with the neurological syndrome hepatic encephalopathy (HE) whereby excessive ammonia is metabolized by astrocytes, followed by cell and brain swelling in vivo. In the present study we were able to show that ammonia treatment of primary astrocytes in vitro is followed by cell swelling and a loss of cell viability at higher ammonia concentrations. Lower ammonia concentrations are accompanied by mitochondria-derived oxidative stress, as demonstrated by using inhibitors of mitochondrial glutaminase I, 143B-rho (0) cells and isolated mitochondria. The oxidative stress generated by mitochondria is accompanied by protein oxidation. In further studies we could show, that an activation of the proteasomal system takes place during ammonia exposure and protects cells. The proteasome acitvation can be blocked by antioxidants or by inhibitors of enzymes of glutamine metabolism. We conclude that oxidative stress-mediated proteasomal activation is important for survival of astroglial cells. [Copyright &y& Elsevier]

Substantial neurologic morbidity occurs in survivors of premature birth. Premature infants are exposed to partial oxygen pressures that are fourfold higher compared to intrauterine conditions, even if no supplemental oxygen is administered. Here we report that short exposures to nonphysiologic oxygen levels can trigger apoptotic neurodegeneration in the brains of infant rodents. Vulnerability to oxygen neurotoxicity is confined to the first 2 weeks of life, a period characterized by rapid growth, which in humans expands from the sixth month of pregnancy to the third year of life.Oxygen caused oxidative stress, decreased expression of neurotrophins, and inactivation of survival signaling proteins Ras, extracellular signal-regulated kinase (ERK 1/2), and protein kinase B (Akt). The synRas-transgenic mice overexpressing constitutively activated Ras and phosphorylated kinases ERK1/2 in the brain were protected against oxygen neurotoxicity. Our findings reveal a mechanism that could potentially damage the developing brain of human premature neonates. [Copyright &y& Elsevier]

Pigs with mutated calcium release channels are stress-susceptible and are known for increased postmortem metabolism which results in inferior meat quality. The lipid composition, lipid peroxidation, and oxidation of proteins of skeletal muscle homogenates from the longissimus muscle of pigs were investigated. Muscle tissue samples were obtained from malignant hyperthermia-resistant (MHR) and hyperthermia-susceptible (MHS) pigs in a postmortem time course. Differences in the n-3 polyunsaturated fatty acid composition with higher values in the susceptible pigs were detected (MHR versus MHS: 141±5 versus 116±4 μg/g wet wt.; P<0.05). This was accompanied by a dramatic 40% postmortem vitamin E loss in MHS muscles (22 h MHR versus 22 h MHS: 1.66±0.11 versus 1.06±0.16 μg/g wet wt.; P<0.05). Muscle tissue from MHS pigs oxidized at a significantly higher rate in comparison with tissue from MHR animals. This difference was also observed for spontaneous malondialdehyde formation as well as for oxidant-induced peroxidation rate (0.75 h MHR versus 0.75 h MHS: 0.81±0.09 versus 1.33±0.09 nmol/g wet wt.; P<0.05). No difference was observed in protein oxidation level in MHS and MHR animals. It is concluded that the enhanced lipid oxidation rate could contribute to the dramatic changes in meat quality from MHS pigs. [Copyright &y& Elsevier]