Protein oxidation and proteolysis in RAW264.7 macrophages: effects of PMA activation

Macrophages are stimulable cells able to increase the production of reactive oxygen and nitrogen species dramatically for a short period of time. Free radicals and other oxidants are able to oxidize the intracellular protein pool. These oxidized proteins are selectively recognized and degraded by the intracellular proteasomal system. We used the mouse macrophage-like cell line RAW264.7 to test whether macrophagial cells are able to increase their protein turnover after oxidative stress and whether this is accompanied by an increased protein oxidation. Macrophagial cells are particularly susceptible to bolus additions of hydrogen peroxide and peroxynitrite. In further experiments we activated RAW264.7 cells with PMA to test whether the production of endogenous oxidants has analogous effects. A clear dependence of the protein turnover and protein oxidation on the oxidative burst could be measured. In further experiments the role of the proteasomal system in the selective removal of oxidized proteins could be revealed exploring the proteasome specific inhibitor lactacystin. Therefore, although oxidants are able to attack the intracellular protein pool in macrophages, these cells are able to remove oxidized proteins selectively and protect the intracellular protein pool from oxidation.     

LPS-induced protein oxidation and proteolysis in BV-2 microglial cells

Exposure of proteins to oxidants leads to increased oxidation followed by preferential degradation by the proteasomal system. The role of the biological oxidant production in microglial BV-2 cells in the oxidation and turnover of endogenous proteins was measured. It could be demonstrated, that BV-2 cells are relatively resistant to fluxes of oxidants, but nevertheless protein oxidation occurs due to activation by LPS. This protein oxidation is followed by an enhanced degradation of endogenous proteins. Using PBN, a free radical scavenger and antioxidant, we could demonstrate the involvement of free radicals in the increased proteolysis in BV-2 cells after LPS-treatment. A slight but significant up-regulation of the proteasomal system after LPS activation takes place, indicating the importance of his proteolytic system in the maintenance of the protein pool of microglial cells.     

Ferritin oxidation in vitro: implication of iron release and degradation by the 20S proteasome

Ferritin, the major iron storage protein in mammalian cells, was treated with various concentrations of different oxidants: xanthine/xanthine oxidase, Sin-1 (3-morpholinosydnonimine, purchased from Alexis, Grunberg, Germany), DEA-NO (Diethylamine NONOate, purchased from Calblochem-Novabiochem, Schwalbach, Germany), and hydrogen peroxide. The proteolytic susceptibility towards the isolated 20S proteasome of untreated ferritin and oxidized ferritin was measured in parallel with the iron liberated by these oxidants. With increasing hydrogen peroxide, Sin-1, and xanthine oxidase concentrations, the measured proteasomal degradation of ferritin also increased. At higher oxidant concentrations, however, the proteolytic susceptibility began to decrease. The oxidation of ferritin by DEA-NO was accompanied by a lesser increase of proteolytic susceptibility in comparison with the effects of the other oxidants. Addition of DEA-NO to Sin-1 suppressed the increase in proteolytic susceptibility of ferritin, whereas adding xanthine/xanthine oxidase had no additional effect. Iron was liberated readily from ferritin as a result of the oxidation process, although the increase in proteolytic susceptibility was not always correlated to the iron release. In fact, the degradation of oxidatively damaged ferritin was not accompanied by a further increase of free iron. Therefore, we conclude that the proteasome is a secondary antioxidative defense system that degrades only nonfunctional ferritin.     

Degradation of hypochlorite-damaged glucose-6-phosphate dehydrogenase by the 20S proteasome.

Glucose-6-phosphate dehydrogenase (G6PD) was treated with various concentrations of hypochlorite, which is produced by myeloperoxidase and is one of the most important oxidants during inflammatory processes. Inhibition of enzymatic activity, protein fragmentation, and proteolytic susceptibility toward the isolated 20S proteasome of G6PD were investigated. With rising hypochlorite concentrations, an increased proteasomal degradation of G6PD was measured. This occurred at higher hypochlorite concentrations than G6PD inactivation and at lower levels than G6PD fragmentation. The proteolytic activities of the 20S proteasome itself was determined by degradation of oxidized model proteins and cleavage of the synthetic proteasome substrate suc-LLVY-MCA. Proteasome activities remained intact at hypochlorite concentrations in which G6PD is maximally susceptible to proteasomal degradation. Only higher hypochlorite concentrations could decrease the proteolytic activities of the proteasome, which was accompanied by disintegration and fragmentation of the proteasome and proteasome subunits. Therefore, we conclude that the 20S proteasome can degrade proteins moderately damaged by hypochlorite and could contribute to an increased protein turnover in cells exposed to inflammatory stress.     

Selective degradation of oxidatively modified protein substrates by the proteasome

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.     

Amino acid, peptide, and protein hydroperoxides and their decomposition products modify the activity of the 26S proteasome

Proteins are major biological targets for oxidative damage within cells because of their high abundance and rapid rates of reaction with radicals and singlet oxygen. These reactions generate high yields of hydroperoxides. The turnover of both native and modified/damaged proteins is critical for maintaining cell homeostasis, with this occurring via the proteasomal and endosomal-lysosomal systems; the former is of particular importance for intracellular proteins. In this study we have examined whether oxidation products generated on amino acids, peptides, and proteins modulate 26S proteasome activity. We show that oxidation products, and particularly protein hydroperoxides, are efficient inhibitors of the 26S proteasome tryptic and chymotryptic activities, with this depending, at least in part, on the presence of hydroperoxide groups. Removal of these species by reduction significantly reduces proteasome inhibition. This loss of activity is accompanied by a loss of thiol residues, but an absence of radical formation, consistent with molecular, rather than radical, reactions being responsible for proteasome inhibition. Aldehydes also seem to play a role in the inhibition of chymotryptic activity, with this prevented by treatment with NaBH(4), which reduces these groups. Inhibition occurred at hydroperoxide concentrations of ≥1μM for oxidized amino acids and peptides and ≥10μM for oxidized proteins, compared with ca. 100μM for H(2)O(2), indicating that H(2)O(2) is a much less effective inhibitor. These data indicate that the formation of oxidized proteins within cells may modulate cell function by interfering with the turnover of native proteins and the clearance of modified materials.     

The proteasomal system and HNE-modified proteins

Metabolic processes and environmental conditions cause the constant formation of oxidizing species over the lifetime of cells and organisms. This leads to a continuous oxidation of intracellular components, including lipids, DNA and proteins. During the extensively studied process of lipid peroxidation, several reactive low-molecular weight products are formed, including reactive aldehydes as 4-hydroxynonenal (HNE). These aldehydic lipid peroxidation products in turn are able to modify proteins. The degradation of oxidized and oxidatively modified proteins is an essential part of the oxidant defenses of cells. The major proteolytic system responsible for the removal of oxidized cytosolic and nuclear proteins is the proteasomal system. The proteasomal system by itself is a multicomponent system responsible for the degradation of the majority of intracellular proteins. It has been shown that some, mildly cross-linked, HNE-modified proteins are preferentially degraded by the proteasome, but extensive modification with this cross-linking aldehyde leads to the formation of protein aggregates, that can actually inhibit the proteasome. This review summarizes our knowledge of the interactions between lipid peroxidation products, proteins, and the proteasomal system.     

Regulation of connexin degradation as a mechanism to increase gap junction assembly and function

Connexins, the integral membrane protein constituents of gap junctions, are degraded at a rate (t(12) = 1.5-5 h) much faster than most other cell surface proteins. Although the turnover of connexins has been shown to be sensitive to inhibitors of either the lysosome or of the proteasome, how connexins are targeted for degradation and whether this process can be regulated to affect intercellular communication is unknown. We show here that reducing connexin degradation with inhibitors of the proteasome (but not with lysosomal blockers) is associated with a striking increase in gap junction assembly and intercellular dye transfer in cells inefficient in both processes under basal conditions. The effect of proteasome inhibitors on wild-type connexin stability, assembly, and function was mimicked by treatment of assembly-inefficient cells with inhibitors of protein synthesis such as cycloheximide. Sensitivity of connexin degradation to cycloheximide, but not to proteasome inhibitors, was abolished when connexins were rendered structurally abnormal by perturbation of essential disulfide bonds or by mutation. Our findings provide the first evidence that intercellular communication can be up-regulated at the level of connexin turnover and that a short-lived protein may be required for conformationally mature connexins to become substrates of proteasomal degradation.     

Blm10 protein promotes proteasomal substrate turnover by an active gating mechanism

For optimal proteolytic function, the central core of the proteasome (core particle (CP) or 20S) has to associate with activators. We investigated the impact of the yeast activator Blm10 on proteasomal peptide and protein degradation. We found enhanced degradation of peptide substrates in the presence of Blm10 and demonstrated that Blm10 has the capacity to accelerate proteasomal turnover of the unstructured protein tau-441 in vitro. Mechanistically, proteasome activation requires the opening of a closed gate, which allows passage of unfolded proteins into the catalytic chamber. Our data indicate that gate opening by Blm10 is achieved via engagement of its C-terminal segment with the CP. Crucial for this activity is a conserved C-terminal YYX motif, with the penultimate tyrosine playing a preeminent role. Thus, Blm10 utilizes a gate opening strategy analogous to the proteasomal ATPases HbYX-dependent mechanism. Because gating incompetent Blm10 C-terminal point mutants confers a loss of function phenotype, we propose that the cellular function of Blm10 is based on CP association and activation to promote the degradation of proteasome substrates.     

26S proteasome regulatory particle mutants have increased oxidative stress tolerance

The 26S proteasome (26SP) is a multi-subunit, multi-catalytic protease that is responsible for most of the cytosolic and nuclear protein turnover. The 26SP is composed of two sub-particles, the 19S regulatory particle (RP) that binds and unfolds protein targets, and the 20S core particle (20SP) that degrades proteins into small peptides. Most 26SP targets are conjugated to a poly-ubiquitin (Ub) chain that serves as a degradation signal. However, some targets, such as oxidized proteins, do not require a poly-Ub tag for proteasomal degradation, and recent studies have shown that the main protease in this Ub-independent pathway is free 20SP. It is currently unknown how the ratio of 26SP- to 20SP-dependent proteolysis is controlled. Here we show that loss of function of the Arabidopsis RP subunits RPT2a, RPN10 and RPN12a leads to decreased 26SP accumulation, resulting in reduced rates of Ub-dependent proteolysis. In contrast, all three RP mutants have increased 20SP levels and thus enhanced Ub-independent protein degradation. As a consequence of this shift in proteolytic activity, mutant seedlings are hypersensitive to stresses that cause protein misfolding, and have increased tolerance to treatments that promote protein oxidation. Taken together, the data show that plant cells increase 20SP-dependent proteolysis when 26SP activity is impaired.     

Proteasomes modulate conjugation to the ubiquitin-like protein,ISG15

ISG15 is a ubiquitin-like protein that is induced by interferon and microbial challenge. Ubiquitin-like proteins are covalently conjugated to cellular proteins and may intersect the ubiquitin-proteasome system via common substrates or reciprocal regulation. To investigate the relationship between ISG15 conjugation and proteasome function, we treated interferon-induced cells with proteasome inhibitors. Surprisingly, inhibition of proteasomal, but not lysosomal, proteases dramatically enhanced the level of ISG15 conjugates. The stimulation of ISG15 conjugates occurred rapidly in the absence of protein synthesis and was most dramatic in the cytoskeletal protein fraction. Inhibition of ISG15 conjugation by ATP depletion abrogated the proteasome inhibitor-dependent increase in ISG15 conjugates, suggesting that the effect was mediated by de novo conjugation, rather than protection from proteasomal degradation or inhibition of ISG15 deconjugating activity. The increase in ISG15 conjugates did not occur through a stabilization of the ISG15 E1 enzyme, UBE1L. Furthermore, simultaneous modification of proteins by both ISG15 and ubiquitin did not account for the proteasome inhibitor-dependent increase in ISG15 conjugates. These findings provide the first evidence for a link between ISG15 conjugation and proteasome function and support a model in which proteins destined for ISG15 conjugation are proteasome-regulated.     

Recent developments in the intracellular degradation of oxidized proteins

The accumulation of oxidized proteins in cells and tissues is a feature of a number of age-related diseases and may also occur as a result of the aging process itself. In this article we review recent advances in our understanding of the cellular degradation of oxidized proteins directing our attention primarily to information which directly bears on the behavior of intact eukaryotic cells. We summarize new work on the key intracellular degradative machineries, proteasomes and lysosomes and examine evidence implicating an increase in protein hydrophobicity as the primary signal to the proteasome to initiate degradation. The data identifying the proteasome as the main route of degradation of oxidized proteins is examined, as well as recent data investigating changes in proteasome function after exposure of cells to oxidants and the altered catabolism of oxidized proteins in aging cells. Evidence for the cooperation between the lysosomal and proteasomal systems in the degradation of oxidized proteins is discussed. We conclude that the cellular catabolism of oxidized proteins may be a more complex process than it first appeared and suggest key issues that need to be resolved to improve our understanding of this important process.     

Ubiquitin conjugation is not required for the degradation of oxidized proteins by proteasome

Oxidatively modified proteins that accumulate in aging and many diseases can form large aggregates because of covalent cross-linking or increased surface hydrophobicity. Unless repaired or removed from cells, these oxidized proteins are often toxic, and threaten cell viability. Most oxidatively damaged proteins appear to undergo selective proteolysis, primarily by the proteasome. Previous work from our laboratory has shown that purified 20 S proteasome degrades oxidized proteins without ATP or ubiquitin in vitro, but there have been no studies to test this mechanism in vivo. The aim of this study was to determine whether ubiquitin conjugation is necessary for the degradation of oxidized proteins in intact cells. We now show that cells with compromised ubiquitin-conjugating activity still preferentially degrade oxidized intracellular proteins, at near normal rates, and this degradation is still inhibited by proteasome inhibitors. We also show that progressive oxidation of proteins such as lysozyme and ferritin does not increase their ubiquitinylation, yet the oxidized forms of both proteins are preferentially degraded by proteasome. Furthermore, rates of oxidized protein degradation by cell lysates are not significantly altered by addition of ATP, excluding the possibility of an energy requirement for this pathway. Contrary to earlier popular belief that most proteasomal degradation is conducted by the 26 S proteasome with ubiquitinylated substrates, our work suggests that oxidized proteins are degraded without ubiquitin conjugation (or ATP hydrolysis) possibly by the 20 S proteasome, or the immunoproteasome, or both.     

Role of increased expression of the proteasome in the protective effects of sulforaphane against hydrogen peroxide-mediated cytotoxicity in murine neuroblastoma cells

The 26S proteasome is responsible for degradation of abnormal proteins and may play a role in cell survival upon oxidative stress. The indirect antioxidant sulforaphane (SFN) protects animal tissues from chemical toxicants by increasing the expression of several families of Nrf2-regulated genes. The role of induction of the 26S proteasome in cytoprotection by SFN was investigated in murine neuroblastoma Neuro2A cells. SFN enhanced the expression of the catalytic subunits of the proteasome, as well as proteasomal peptidase activities in these cells. Such treatment with SFN protected cells from hydrogen peroxide-mediated cytotoxicity in a manner dependent on proteasomal function. Inhibition of proteasome activities using pharmacological interventions significantly attenuated the protective effects of SFN against hydrogen peroxide cytotoxicity, as well as protein oxidation. Moreover, overexpression of the catalytic subunit PSMB5 enhanced proteasome function and led to elevated resistance against hydrogen peroxide toxicity and extent of protein oxidation compared to blank-plasmid-transfected cells. Pretreatment of PSMB5-overexpressing cells with SFN did not further enhance this resistance. Collectively, these results suggest that the cytoprotective effects of SFN against oxidative stress are in part due to up-regulation of the proteasome system. Therefore, inducers of proteasome expression may ameliorate the accumulation of damaged proteins associated with neurodegeneration and other diseases in whose etiologies protein oxidation plays a role.     

Aging and calorie restriction modulate yeast redox state, oxidized protein removal, and the ubiquitin-proteasome system

The ubiquitin-proteasome system governs the half-life of most cellular proteins. Calorie restriction (CR) extends the maximum life span of a variety of species and prevents oxidized protein accumulation. We studied the effects of CR on the ubiquitin-proteasome system and protein turnover in aging Saccharomyces cerevisiae. CR increased chronological life span as well as proteasome activity compared to control cells. The levels of protein carbonyls, a marker of protein oxidation, and those of polyubiquitinated proteins were modulated by CR. Controls, but not CR cells, exhibited a significant increase in oxidized proteins. In keeping with decreased proteasome activity, polyubiquitinated proteins were increased in young control cells compared to time-matched CR cells, but were profoundly decreased in aged control cells despite decreased proteasomal activity. This finding is related to a decreased polyubiquitination ability due to the impairment of the ubiquitin-activating enzyme in aged control cells, probably related to a more oxidative microenvironment. CR preserves the ubiquitin-proteasome system activity. Overall, we found that aging and CR modulate many aspects of protein modification and turnover.     

Ferritin oxidation and proteasomal degradation: Protection by antioxidants

The accumulation of oxidatively damaged proteins is a well-known hallmark of aging and several neurodegenerative diseases including Alzheimer's, Parkinson's and Huntigton's diseases. These highly oxidized protein aggregates are in general not degradable by the main intracellular proteolytic machinery, the proteasomal system. One possible strategy to reduce the accumulation of such oxidized protein aggregates is the prevention of the formation of oxidized protein derivatives or to reduce the protein oxidation to a degree that can be handled by the proteasome. To do so an antioxidative strategy might be successful. Therefore, we undertook the present study to test whether antioxidants are able to prevent the protein oxidation and to influence the proteasomal degradation of moderate oxidized proteins. As a model protein we choose ferritin. H₂O₂ induced a concentration dependent increase of protein oxidation accompanied by an increased proteolytic susceptibility. This increase of proteolytic susceptibility is limited to moderate hydrogen peroxide concentrations, whereas higher concentrations are accompanied by protein aggregate formation.

Protective effects of the vitamin E derivative Trolox, the pyridoindole derivative Stobadine and of the standardized extracts of flavonoids from bark of Pinus Pinaster Pycnogenol^® and from leaves of Ginkgo biloba (EGb 761) were studied on moderate damaged ferritin.     

Regulation of proteasome-mediated protein degradation during oxidative stress and aging

Protein degradation is a physiological process required to maintain cellular functions. There are distinct proteolytic systems for different physiological tasks under changing environmental and pathophysiological conditions. The proteasome is responsible for the removal of oxidatively damaged proteins in the cytosol and nucleus. It has been demonstrated that proteasomal degradation increases due to mild oxidation, whereas at higher oxidant levels proteasomal degradation decreases. Moreover, the proteasome itself is affected by oxidative stress to varying degrees. The ATP-stimulated 26S proteasome is sensitive to oxidative stress, whereas the 20S form seems to be resistant. Non-degradable protein aggregates and cross-linked proteins are able to bind to the proteasome, which makes the degradation of other misfolded and damaged proteins less efficient. Consequently, inhibition of the proteasome has dramatic effects on cellular aging processes and cell viability. It seems likely that during oxidative stress cells are able to keep the nuclear protein pool free of damage, while cytosolic proteins may accumulate. This is because of the high proteasome content in the nucleus, which protects the nucleus from the formation and accumulation of non-degradable proteins. In this review we highlight the regulation of the proteasome during oxidative stress and aging.     

Hydrogen peroxide-mediated protein oxidation in young and old human MRC-5 fibroblasts

It is suggested that the aging process is dependent on the action of free radicals. One of the highlights of age-related changes of cellular metabolism is the accumulation of oxidized proteins. The present investigation was undertaken to reveal the proliferation-related changes in the protein oxidation and proteasome activity during and after an acute oxidative stress. It could be demonstrated that the activity of the cytosolic proteasomal system declines during proliferative senescence of human MRC-5 fibroblasts and is not able to remove oxidized proteins in old cells efficiently. Whereas in young cells removal of oxidized proteins was accompanied by an increase in the overall protein turnover, this increase in protein turnover could not be seen in old MRC-5 fibroblasts. Therefore, our studies demonstrate that old fibroblasts are much more vulnerable to the accumulation of oxidized proteins after oxidative stress and are not able to remove these oxidized proteins as efficiently as young fibroblasts.