Stability of the nuclear protein turnover during cellular senescence of human fibroblasts

Proteostasis, oxidative stress and aging

The production of reactive species is an inevitable by-product of metabolism and thus, life itself. Since reactive species are able to damage cellular structures, especially proteins, as the most abundant macromolecule of mammalian cells, systems are necessary which regulate and preserve a functional cellular protein pool, in a process termed “proteostasis”. Not only the mammalian protein pool is subject of a constant turnover, organelles are also degraded and rebuild. The most important systems for these removal processes are the “ubiquitin-proteasomal system” (UPS), the central proteolytic machinery of mammalian cells, mainly responsible for proteostasis, as well as the “autophagy-lysosomal system”, which mediates the turnover of organelles and large aggregates.

Many age-related pathologies and the aging process itself are accompanied by a dysregulation of UPS, autophagy and the cross-talk between both systems. This review will describe the sources and effects of oxidative stress, preservation of cellular protein- and organelle-homeostasis and the effects of aging on proteostasis in mammalian cells.

Endoplasmic reticulum stress and proteasomal system in amyotrophic lateral sclerosis

Protein processing including folding, unfolding and degradation is involved in the mechanisms of many diseases. Unfolded protein response and/or endoplasmic reticulum stress are accepted to be the first steps which should be completed via protein degradation. In this direction, proteasomal system and autophagy play important role as the degradation pathways and controlled via complex mechanisms. Amyotrophic lateral sclerosis is a multifactorial neurodegenerative disease which is also known as the most catastrophic one. Mutation of many different genes are involved in the pathogenesis such as superoxide dismutase 1, chromosome 9 open reading frame 72 and ubiquilin 2. These genes are mainly related to the antioxidant defense systems, endoplasmic reticulum stress related proteins and also protein aggregation, degradation pathways and therefore mutation of these genes cause related disorders.This review focused on the role of protein processing via endoplasmic reticulum and proteasomal system in amyotrophic lateral sclerosis which are the main players in the pathology. In this direction, dysfunction of endoplasmic reticulum associated degradation and related cell death mechanisms that are autophagy/apoptosis have been detailed.

The amazing ubiquitin-proteasome system: structural components and implication in aging

Proteome quality control (PQC) is critical for the maintenance of cellular functionality and it is assured by the curating activity of the proteostasis network (PN). PN is constituted of several complex protein machines that under conditions of proteome instability aim to, firstly identify, and then, either rescue or degrade nonnative polypeptides. Central to the PN functionality is the ubiquitin-proteasome system (UPS) which is composed from the ubiquitin-conjugating enzymes and the proteasome; the latter is a sophisticated multi-subunit molecular machine that functions in a bimodal way as it degrades both short-lived ubiquitinated normal proteins and nonfunctional polypeptides. UPS is also involved in PQC of the nucleus, the endoplasmic reticulum and the mitochondria and it also interacts with the other main cellular degradation axis, namely the autophagy-lysosome system. UPS functionality is optimum in the young organism but it is gradually compromised during aging resulting in increasing proteotoxic stress; these effects correlate not only with aging but also with most age-related diseases. Herein, we present a synopsis of the UPS components and of their functional alterations during cellular senescence or in vivo aging. We propose that mild UPS activation in the young organism will, likely, promote antiaging effects and/or suppress age-related diseases.

Vascular aging: chronic oxidative stress and impairment of redox signaling-consequences for vascular homeostasis and disease

Characteristic morphological and molecular alterations such as vessel wall thickening and reduction of nitric oxide occur in the aging vasculature leading to the gradual loss of vascular homeostasis. Consequently, the risk of developing acute and chronic cardiovascular diseases increases with age. Current research of the underlying molecular mechanisms of endothelial function demonstrates a duality of reactive oxygen and nitrogen species in contributing to vascular homeostasis or leading to detrimental effects when formed in excess. Furthermore, changes in function and redox status of vascular smooth muscle cells contribute to age-related vascular remodeling. The age-dependent increase in free radical formation causes deterioration of the nitric oxide signaling cascade, alters and activates prostaglandin metabolism, and promotes novel oxidative posttranslational protein modifications that interfere with vascular and cell signaling pathways. As a result, vascular dysfunction manifests. Compensatory mechanisms are initially activated to cope with age-induced oxidative stress, but become futile, which results in irreversible oxidative modifications of biological macromolecules. These findings support the 'free radical theory of aging' but also show that reactive oxygen and nitrogen species are essential signaling molecules, regulating vascular homeostasis.

Protein damage, repair and proteolysis

Proteins are continuously affected by various intrinsic and extrinsic factors. Damaged proteins influence several intracellular pathways and result in different disorders and diseases. Aggregation of damaged proteins depends on the balance between their generation and their reversal or elimination by protein repair systems and degradation, respectively. With regard to protein repair, only few repair mechanisms have been evidenced including the reduction of methionine sulfoxide residues by the methionine sulfoxide reductases, the conversion of isoaspartyl residues to L-aspartate by L-isoaspartate methyl transferase and deglycation by phosphorylation of protein-bound fructosamine by fructosamine-3-kinase. Protein degradation is orchestrated by two major proteolytic systems, namely the lysosome and the proteasome. Alteration of the function for both systems has been involved in all aspects of cellular metabolic networks linked to either normal or pathological processes. Given the importance of protein repair and degradation, great effort has recently been made regarding the modulation of these systems in various physiological conditions such as aging, as well as in diseases. Genetic modulation has produced promising results in the area of protein repair enzymes but there are not yet any identified potent inhibitors, and, to our knowledge, only one activating compound has been reported so far. In contrast, different drugs as well as natural compounds that interfere with proteolysis have been identified and/or developed resulting in homeostatic maintenance and/or the delay of disease progression.

Redox control of the ubiquitin-proteasome system: from molecular mechanisms to functional significance

In their natural environments, cells are regularly exposed to oxidizing conditions that may lead to protein misfolding. If such misfolded proteins are allowed to linger, they may form insoluble aggregates and pose a serious threat to the cell. Accumulation of misfolded, oxidatively damaged proteins is characteristic of many diseases and during aging. To counter the adverse effects of oxidative stress, cells can initiate an antioxidative response in an attempt to repair the damage, or rapidly channel the damaged proteins for degradation by the ubiquitin-proteasome system (UPS). Recent studies have shown that elements of the oxidative stress response and the UPS are linked on many levels. To manage the extra burden of misfolded proteins, the UPS is induced by oxidative stress, and special proteasome subtypes protect cells against oxidative damage. In addition, the proteasome is directly associated with a thioredoxin and other cofactors that may adjust the particle's response during an oxidative challenge. Here, we give an overview of the UPS and a detailed description of the degradation of oxidized proteins and of the crosstalk between oxidative stress and protein degradation in health and disease.

Ex vivo detection of histone H1 modified with advanced glycation end products

A number of oxidative stress agents cause DNA and protein damage, which may compromise genomic integrity. Whereas oxidant-induced DNA damage has been extensively studied, much less is known concerning the occurrence and fate of nuclear protein damage, particularly of proteins involved in the regulation and maintenance of chromatin structure. Protein damage may be caused by the formation of reactive carbonyl species such as glyoxal, which forms after lipid peroxide degradation. It may also result from degradation of early protein glycation adducts and from methylglyoxal, formed in the process of glycolytic intermediate degradation. Major adducts indicative of protein damage include the advanced glycation end product (AGE) carboxymethyllysine (CML) and argpyrimidine protein adducts. Thus, the formation of CML and argpyrimidine protein adducts represents potential biomarkers for nuclear protein damage deriving from a variety of sources. The purpose of this study was to identify and quantify AGE adducts formed in vivo in a nuclear protein, specifically histone H1, using CML and argpyrimidine as biomarkers. Histone H1 was isolated from calf thymus collected immediately after slaughter under conditions designed to minimize AGE formation before isolation. Using antibodies directed against oxidative protein adducts, we identified CML, argpyrimidine, and protein crosslinks present in the freshly isolated histone H1. Detailed mass spectroscopy analysis of histone H1 revealed the presence of two specific lysine residues modified by CML adducts. Our results strongly suggest that glycation of important nuclear protein targets such as histone H1 occurs in vivo and that these oxidative changes may alter chromatin structure, ultimately contributing to chronic changes associated with aging and diseases such as diabetes.

Age-related loss of stress-induced nuclear proteasome activation is due to low PARP-1 activity

Changes in protein turnover are among the dominant metabolic changes during aging. Of special importance is the maintenance of nuclear protein homeostasis to ensure a coordinated cellular metabolism. Therefore, in the nucleus a special PARP-1-mediated mechanism of proteasomal activation exists to ensure a rapid degradation of oxidized nuclear proteins. It was already demonstrated earlier that the cytosolic proteasomal system declines dramatically with aging, whereas the nuclear proteasome remains less affected. We demonstrate here that the stress-mediated proteasomal activation in the nucleus declines during replicative senescence of human fibroblasts. Furthermore, we clearly show that this decline in the PARP-1-mediated proteasomal activation is due to a decline in the expression and activity of PARP-1 in senescent fibroblasts. In a final study we show that this process also happens in vivo, because the protein expression level of PARP-1 is significantly lower in the skin of aged donors compared to that of young ones. Therefore, we conclude that the rate-limiting factor in poly(ADP-ribose)-mediated proteasomal activation in oxidative stress is PARP-1 and not the nuclear proteasome itself.

Age-related differences in oxidative protein-damage in young and senescent fibroblasts

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.

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.

The proteasome: a target of oxidative damage in cultured human retina pigment epithelial cells

PURPOSE

Dysfunction of the ubiquitin-proteasome pathway (UPP) is associated with several age-related degenerative diseases. The objective of this study was to investigate the effect of oxidative stress on the UPP in cultured human retina pigment epithelial cells.

METHODS

To mimic physiological oxidative stress, ARPE-19 cells were exposed to continuously generated H2O(2) or A2E-mediated photooxidation. Proteasome activity was monitored using fluorogenic peptides as substrates. The ubiquitin conjugation activity and activities of E1 and E2 were determined by the thiolester assays. Levels of ubiquitin and ubiquitin conjugates were determined by Western blotting.

RESULTS

Exposure of ARPE-19 cells to 40 to 50 microM H2O(2) for 4 hours resulted in a 30% to 50% reduction in all three peptidase activities of the proteasome. Similarly, exposure of A2E-loaded ARPE-19 cells to blue light resulted in a 40% to 60% reduction in proteasome activity. Loading of A2E or exposure to blue light alone had little effect on proteasome activity. In contrast, exposure of ARPE-19 to low levels of H2O(2) (10 microM) stimulated ubiquitin conjugation activity. Loading of A2E, with or without exposure to blue light, upregulated the levels of ubiquitin-activating enzyme and increased conjugation activity. Exposure to H2O(2) or A2E-mediated photooxidation also resulted in a twofold to threefold increase in levels of endogenous ubiquitin conjugates.

CONCLUSIONS

These data show that the proteasome in ARPE-19 is susceptible to oxidative inactivation, whereas activities of the ubiquitin-conjugating enzymes are more resistant to oxidative stress. Oxidative inactivation of the proteasome appears to be one of the mechanisms underlying stress-induced accumulation of ubiquitin conjugates in the cells.

The proteasome and its role in nuclear protein maintenance

The cellular proteome is in a dynamic state of synthesis and degradation. Degradation of extracellular proteins is mainly mediated non-specifically by the lysosomes or due to released proteases, while the proteolysis of intracellular including nuclear proteins is catalyzed by the ubiquitin-proteasome pathway. Furthermore, the proteasomal system is largely responsible for the removal of unfolded and oxidatively damaged proteins. Taking into account the role of ubiquitin and proteasome system in protein metabolism, studies of its spatial organization within the cell are of great importance. For the understanding of cellular, including nuclear, protein maintenance the distribution of the proteasomes in both the nucleus and the cytosol and their response upon oxidative stress is of great interest. Although, the functional diversity of the cells is ensured by the three dimensional organization of the nucleus, nuclear proteins are also prone to oxidation and have to be removed from the cellular environment by the nuclear proteasome. Interestingly, nuclear proteins are partly degraded within the nucleus, whereas some are exported from the nucleus to the cytosol. Proteasomes are transported unidirectionally from the cytoplasm to the nucleus with a possible countervail during mitosis. This review is focused largely on the specifics of cellular proteasome distribution and on nuclear protein maintenance under physiological and oxidative stress conditions.

Nitrotyrosine and protein carbonyls are equally distributed in HT22 cells after nitrosative stress

The generation of reactive oxygen and nitrogen species is an inevitable result of cellular metabolism and environmental influence. Such oxidation processes are always combined with the formation of various protein oxidation products. Environmental oxidants might either be activated inside the cell or act by themselves. Therefore, differences in the localization of oxidant formation might change the major compartment of oxidant action. Therefore, we employed NO donors (SNOC, DETA/NO, and Spe/NO) alone or in combination with the redox-cycling bipyridinium compound paraquat, the superoxide- and NO-releasing compound SIN-1, the relatively more lipophilic oxidants tert-butyl and cumene hydroperoxide, and peroxynitrite itself to test the ability of these compounds to generate oxidized and nitrated proteins in various cellular compartments. Combined treatment with oxidants and nitrating compounds led to the formation of protein carbonyls and nitrotyrosine with a severalfold higher concentration in the cytosol, compared to the nucleus. In fluorescence microscopy studies, the resulting protein modifications show a similar distribution of oxidized proteins and nitrotyrosine with highest concentrations in the perinuclear area. Studying the time- and concentration-dependent formation and degradation of protein carbonyls and nitrated proteins large similarities could be measured. Therefore, it can be concluded that formation, localization, and kinetics of protein carbonyl and nitrotyrosine formation parallel each other depending on the stress-inducing agent.

Oxidized proteins: intracellular distribution and recognition by the proteasome

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.

Oxidative modification of proteins: age-related changes

Aging is a complex biological phenomenon which involves progressive loss of different physiological functions of various tissues of living organisms. It is the inevitable fate of life and is a major risk factor for death and different pathological disorders. Based on a wide variety of studies performed in humans as well as in various animal models and microbial systems, reactive oxygen species (ROS) are believed to play a key role in the aging process. The production of ROS is influenced by cellular metabolic activities as well as environmental factors. ROS can react with all major biological macromolecules such as carbohydrates, nucleic acids, lipids, and proteins. Since, in general, proteins are the key molecules that play the ultimate role in various structural and functional aspects of living organisms, this review will focus on the age-related oxidative modifications of proteins as well as on mechanism for removal or repair of the oxidized proteins. The topics covered include protein oxidation as a marker of oxidative stress, experimental evidence indicating the role of ROS in protein oxidation, protein carbonyl content, enzymatic degradation of oxidized proteins, and effects of caloric restriction on protein oxidation in the context of aging. Finally, we will discuss different strategies which have been or can be undertaken to slow down the oxidative damage of proteins and the aging process.

Protein oxidation and proteolysis

One of the hallmarks of chronic or severe oxidative stress is the accumulation of oxidized proteins, which tend to form high-molecular-weight aggregates. The major proteolytic system responsible for the removal of oxidized cytosolic and nuclear proteins is the proteasome. This complicated proteolytic system contains a core proteasomal form (20S proteasome) and several regulators. All of these components are affected by oxidative stress to various degrees. The ATP-stimulated 26S proteasome is sensitive to oxidative stress, whereas the 20S form seems to be more resistant. The nuclear proteasome selectively degrades oxidatively damaged histones in the nuclei of mammalian cells, where it is activated and regulated by automodified PARP-1 after oxidative challenge. In this brief review we highlight the proteolysis and its regulatory effects during oxidative stress.

The nuclear proteasome and the degradation of oxidatively damaged proteins

The accumulation of oxidized proteins is known to be linked to some severe neurodegenerative diseases like Alzheimer's, Parkinson's and Huntington's disease. Furthermore, the aging process is also accompanied by an ongoing aggregation of misfolded and damaged proteins. Therefore, mammalian cells have developed potent degradation systems, which selectively degrade damaged and misfolded proteins. The proteasomal system is largely responsible for the removal of oxidatively damaged proteins form the cellular environment. Not only cytosolic proteins are prone to oxidative stress, also nuclear proteins are readily oxidized. The nuclear proteasomal system is responsible for the degradation of these proteins. This review is focused on the specific degradation of oxidized nuclear proteins, the role of the proteasome in this process and the regulation of the nuclear proteasomal system under oxidative conditions.

Protein expression dynamics during replicative senescence of endothelial cells studied by 2-D difference in-gel electrophoresis

Endothelial senescence contributes to endothelium dysfunctionality and is thereby linked to vascular aging. A dynamic proteomic study on human umbilical vein endothelial cells, isolated from three umbilical cords, was performed. The cells were cultured towards replicative senescence and whole cell lysates were subjected to 2-D difference gel electrophoresis (DIGE). Despite the biological variability of the three independent isolations, a set of proteins was found that showed senescence-dependent expression patterns in all isolations. We focused on those proteins that showed significant changes, with a paired analysis of variance (RM-ANOVA) p-value of < or =0.05. Thirty-five proteins were identified with LC-Fourier transform MS, and functional annotation revealed that endothelial replicative senescence is accompanied by increased cellular stress, protein biosynthesis and reduction in DNA repair and maintenance. Nuclear integrity becomes affected and cytoskeletal structure is also changed. Such important changes in the cell infrastructure might accelerate endothelium dysfunctionality. This study provides biological information that will initiate studies to further unravel endothelial senescence and gain more knowledge about the consequences of this process in the in vivo situation.

Intracellular distribution of oxidized proteins and proteasome in HT22 cells during oxidative stress

The production of free radicals and the resulting oxidative damage of cellular structures are always connected with the formation of oxidized proteins. The 20S proteasome is responsible for recognition and degradation of oxidatively damaged proteins. No detailed studies on the intracellular distribution of oxidized proteins during oxidative stress and on the distribution of the proteasome have been performed until now. Therefore, we used immunocytochemical methods to measure protein carbonyls, a form of protein oxidation products, and proteasome distribution within cells. Both immunocytochemical methods of measurement are semiquantitative and the load of oxidized proteins is increased after various oxidative stresses explored, with the highest increase in the perinuclear region of the cell. Distribution of the proteasome and the total protein content revealed the highest concentration of both in the nucleus. No redistribution of the proteasome during oxidative stress occurs. The normalized ratio of protein carbonyls to protein content was formed, indicating the highest concentration of oxidized proteins in the cytosolic region near the cell membrane. By forming the protein oxidation-to-proteasome ratio it was concluded that the highest load of oxidized proteins to the proteasome takes place in the cytosol, independent of the oxidant explored.

Proteasome dysfunction in mammalian aging: Steps and factors involved

Mammalian aging is a natural biological process, determined by both genetic and environmental/stochastic factors, that results in the gradual decline of physiological function and the eventual failure of organism homeostasis. The proteasome is one of the major proteolytic systems of mammalian cells. It is responsible for the degradation of normal proteins as well as of abnormal proteins (like misfolded and oxidized proteins) that tend to accumulate during aging. Impaired proteasome function has been tightly correlated with aging both in vivo and in vitro. Given the fundamental function of proteasome for retaining cellular homeostasis, this review article examines the steps and the factors involved in proteasome dysfunction during mammalian aging. We discuss the proteasome structural organization, its activities and biosynthesis during aging and senescence as well as the genetic and environmental causes of its age-dependent alterations. Finally, we provide insights on the possibilities of proteasome activation that may retard the appearance of the senescent phenotype.

Protein maintenance in aging and replicative senescence: a role for the peptide methionine sulfoxide reductases

Cellular aging is characterized by the build-up of oxidatively modified protein that results, at least in part, from impaired redox homeostasis associated with the aging process. Protein degradation and repair are critical for eliminating oxidized proteins from the cell. Oxidized protein degradation is mainly achieved by the proteasomal system and it is now well established that proteasomal function is generally impaired with age. Specific enzymatic systems have been identified which catalyze the regeneration of cysteine and methionine following oxidation within proteins. Protein-bound methionine sulfoxide diastereoisomers S and R are repaired by the combined action of the enzymes MsrA and MsrB that are subsequently regenerated by thioredoxin/thioredoxin reductase. Importantly, the peptide methionine sulfoxide reductase system has been implicated in increased longevity and resistance to oxidative stress in different cell types and model organisms. In a previous study, we reported that peptide methionine sulfoxide reductase activity as well as gene and protein expression of MsrA are decreased in various organs as a function of age. More recently, we have shown that gene expression of both MsrA and MsrB2 (Cbs-1) is decreased during replicative senescence of WI-38 fibroblasts, and this decline is associated with an alteration in catalytic activity and the accumulation of oxidized protein. In this review, we will address the importance of protein maintenance in the aging process as well as in replicative senescence, with a special focus on regulation of the peptide methionine sulfoxide reductase systems.

Proteasome inhibition increases HuR level, restores heat-inducible HSP72 expression and thermotolerance in WI-38 senescent human fibroblasts

At the end of their replicative potential in vitro, late passage WI-38 human diploid fibroblasts (HDF) have a low basal expression of heat shock protein 72 (HSP72) and an attenuated ability to induce it in response to heat shock. The transient exposure to the specific and reversible proteasome inhibitor MG132 during a mild heat shock induced late passage HDF to synthesize and accumulate high levels of HSP72. This HSP72 expression was long-lasting and appeared to result from both increased cytoplasmic levels and enhanced translation of HSP72 mRNA. The level of HuR, a stabilizing mRNA-binding protein, increased following the MG132 treatment. This result is consistent with the proposed role of HuR in assisting mRNA export to the cytoplasm and in antagonizing its degradation. Furthermore, the previous exposure of late passage HDF to a mild heat shock in the presence of MG132 protected these cells against the otherwise lethal effect of a subsequent severe heat shock. This acquisition of thermotolerance appeared to be correlated with the level of HSP72.