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

Heat-shock proteins in cardiovascular disease

Heat-shock proteins (HSPs) belong to a group of highly conserved families of proteins expressed by all cells and organisms and their expression may be constitutive or inducible. They are generally considered as protective molecules against different types of stress and have numerous intracellular functions. Secretion or release of HSPs has also been described, and potential roles for extracellular HSPs reported. HSP expression is modulated by different stimuli involved in all steps of atherogenesis including oxidative stress, proteolytic aggression, or inflammation. Also, antibodies to HSPs may be used to monitor the response to different types of stress able to induce changes in HSP levels. In the present review, we will focus on the potential implication of HSPs in atherogenesis and discuss the limitations to the use of HSPs and anti-HSPs as biomarkers of atherothrombosis. HSPs could also be considered as potential therapeutic targets to reinforce vascular defenses and delay or avoid clinical complications associated with atherothrombosis.

Muscle wasting in aged, sarcopenic rats is associated with enhanced activity of the ubiquitin proteasome pathway

Among the hallmarks of aged organisms are an accumulation of misfolded proteins and a reduction in skeletal muscle mass ("sarcopenia"). We have examined the effects of aging and dietary restriction (which retards many age-related changes) on components of the ubiquitin proteasome system (UPS) in muscle. The hindlimb muscles of aged (30 months old) rats showed a marked loss of muscle mass and contained 2-3-fold higher levels of 26S proteasomes than those of adult (4 months old) controls. 26S proteasomes purified from muscles of aged and adult rats showed a similar capacity to degrade peptides, proteins, and an ubiquitylated substrate, but differed in levels of proteasome-associated proteins (e.g. the ubiquitin ligase E6AP and deubiquitylating enzyme USP14). Also, the activities of many other deubiquitylating enzymes were greatly enhanced in the aged muscles. Nevertheless, their content of polyubiquitylated proteins was higher than in adult animals. The aged muscles contained higher levels of the ubiquitin ligase CHIP, involved in eliminating misfolded proteins, and MuRF1, which ubiquitylates myofibrillar proteins. These muscles differed from ones rapidly atrophying due to disease, fasting, or disuse in that Atrogin-1/MAFbx expression was low and not inducible by glucocorticoids. Thus, the muscles of aged rats showed many adaptations indicating enhanced proteolysis by the UPS, which may enhance their capacity to eliminate misfolded proteins and seems to contribute to the sarcopenia. Accordingly, dietary restriction decreased or prevented the aging-associated increases in proteasomes and other UPS components and reduced muscle wasting.

White muscle 20S proteasome activity is negatively correlated to growth rate at low temperature in the spotted wolffish Anarhichas minor

The effect of temperature and mass on specific growth rate (G) was examined in spotted wolffish Anarhichas minor of different size classes (ranging from 60 to 1500 g) acclimated at different temperatures (4, 8 and 12 degrees C). The relationship between G and 20S proteasome activity in heart ventricle, liver and white muscle tissue was then assessed in fish acclimated at 4 and 12 degrees C to determine if protein degradation via the proteasome pathway could be imposing a limitation on somatic growth. Cardiac 20S proteasome activity was not affected by acclimation temperature nor fish mass and had no correlation with G. Hepatic 20S proteasome activity was higher at 12 degrees C but did not show any relationship with G. Partial correlation analysis showed that white muscle 20S proteasome activity was negatively correlated to G (partial Pearson's r = -0.609) but only at cold acclimation temperature (4 degrees C). It is suggested that acclimation to cold temperature involves compensation of the mitochondrial oxidative capacity which would in turn lead to increased production of oxidatively damaged proteins that are degraded by the proteasome pathway and ultimately negatively affects G at cold temperature.

The ubiquitin-proteasome system is inhibited by p53 protein expression in human ovarian cancer cells

The ubiquitin-proteasome system (UPS) and autophagy provide major cellular pathways for protein degradation. Since the p53 pathway controls autophagy, we investigated whether p53 regulates UPS in ovarian tumour cell lines. A reporter cell line (SKOV3-EGFPu) was established to measure UPS function against a constant genetic background. Transient expression of either wild type or mutant p53 in SKOV3-EGFPu cells reduced UPS activity as compared to vector control. These results, together with those from endogenous p53 expression in seven ovarian cancer cell lines, suggest that expression of both wild-type and mutant p53 protein impairs UPS function. Thus, p53 expression may regulate protein homeostasis by down-regulating UPS function in response to cellular stress.

Aging and dietary restriction alter proteasome biogenesis and composition in the brain and liver

Interventions such as dietary restriction (DR) have been reported to ameliorate age-related proteasome inhibition in some tissues. Currently it is not known what effects aging and DR have on proteasome biogenesis in the liver and brain, nor have previous studies identified the links between changes in proteasome composition, biogenesis, and activity in the aging brain and liver. In the present study we demonstrate that the brain and liver exhibit age-dependent decreases in 26S and 20S proteasome activity. Additionally, our studies demonstrate that the brain and liver undergo selective changes in proteasome biology, including increases in proteasome biogenesis in response to aging and DR, with the liver exhibit more robust plasticity as compared to the brain. Lastly, studies demonstrated that aging and DR alter the interaction of Hsp90 with the 20S proteasome complex in the brain and liver. These studies affirm the dynamic nature of the proteasome complexes in both the liver and brain following aging and DR. Additionally, these data indicate that the relationship between proteasome composition/biogenesis and proteasome activity in tissues is extremely complex and tissue specific. These data have implications for understanding the effects of tissue specific effects of aging and DR on protein turnover and proteotoxicity.

Proteasomal degradation of beta-carotene metabolite--modified proteins

Free radical attack on beta-carotene results in the formation of high amounts of carotene breakdown products (CBPs) having biological activities. As several of the CBPs are reactive aldehydes, it has to be considered that these compounds are able to modify proteins. Therefore, the aim of the study was to investigate whether CBP-modification of proteins is leading to damaged proteins recognized and degraded by the proteasomal system. We used the model proteins tau and ferritin to test whether CBPs will modify them and whether such modifications lead to enhanced proteasomal degradation. To modify proteins, we used crude CBPs as a mixture obtained after hypochloric acid derived BC degradation, as well as several single compounds, as apo8'-carotenal, retinal, or beta-ionone. The majority of the CBPs found in our reaction mixture are well known metabolites as described earlier after BC degradation using different oxidants. CBPs are able to modify proteins, and in in vitro studies, we were able to demonstrate that the 20S proteasome is able to recognize and degrade CBP-modified proteins preferentially. In testing the proteolytic response of HT22 cells toward CBPs, we could demonstrate an enhanced protein turnover, which is sensitive to lactacystin. Interestingly, the proteasomal activity is resistant to treatment with CBP. On the other hand, we were able to demonstrate that supraphysiological levels of CBPs might lead to the formation of protein-CBP-adducts that are able to inhibit the proteasome. Therefore, the removal of CBP-modified proteins seems to be catalyzed by the proteasomal system and is effective, if the formation of CBPs is not overwhelming and leading to protein aggregates.

Aging: central role for autophagy and the lysosomal degradative system

The lysosomal network is the major intracellular proteolytic system accounting for more than 98% of long-lived bulk protein degradation and recycling particularly in tissues such as liver and muscles. Lysosomes are the final destination of intracellular damaged structures, identified and sequestered by the processes of macroautophagy and chaperone-mediated autophagy (CMA). In the process of macroautophagy, long-lived proteins and other macromolecular aggregates and damaged intracellular organelles are first engulfed by autophagosomes. Autophagosomes themselves have limited degrading capacity and rely on fusion with lysosomes. Unlike macroautophagy, CMA does not require intermediate vesicle formation and the cytosolic proteins recognized by this pathway are directly translocated to the lysosomal membrane. Aging is a universal phenomenon characterized by progressive deterioration of cells and organs due to accumulation of macromolecular and organelle damage. The continuous removal of worn-out components and replacement with newly synthesized ones ensures cellular homeostasis and delays the aging process. Growing evidence indicate that the rate of autophagosome formation and maturation and the efficiency of autophagosome/lysosome fusion decline with age. In addition, a progressive increase in intralysosomal concentration of free radicals and the age pigment lipofuscin further diminish the efficiency of lysosomal protein degradation. Therefore, integrity of the autophagosomal-lysosomal network appears to be critical in the progression of aging. Discovery of the genes involved in the process of autophagy has provided insight into the various molecular pathways that may be involved in aging and senescence. In this review, we discuss the cellular and molecular mechanisms involved in autophagy and the role of autophagosome/lysosome network in the aging process.

S-glutathionylation of the Rpn2 regulatory subunit inhibits 26 S proteasomal function

Although increased intracellular concentrations of hydrogen peroxide (H2O2) are associated with inhibition of 26 S proteasomal activity, the mechanisms responsible for such effects have not been well delineated. In the present studies, we found that direct exposure of purified 26 S proteasomes to H2O2 had negligible effects on their activity, whereas incubation with glutathione and H2O2 produced >80% decrease in chymotrypsin-like and trypsin-like activities. Rpn1 and Rpn2, which are subunits of the 19 S regulatory particle, undergo S-glutathionylation after exposure of purified 26 S proteasomes to glutathione and H2O2, as well as in HEK 293 cells and neutrophils incubated with H2O2. Increased oxidation of Rpn1 and Rpn2 cysteine thiols was also found in lung extracts from mice in which catalase was inactivated, a condition associated with augmented intracellular concentrations of H2O2 and diminished 26 S proteasomal activity. Although unoxidized Rpn2 enhanced 20 S proteolytic function in vitro, such potentiation was not found when the 20 S core particle was incubated with oxidized Rpn2. The composition of 26 S proteasomes was not altered after exposure to glutathione and H2O2, with similar amounts of Rpn1 and Rpn2 in control or oxidized 26 S proteasomal complexes. These findings identify S-glutathionylation of Rpn2 as a contributory mechanism for H2O2-induced inhibition of 26 S proteasomal function.

Protein stability and resistance to oxidative stress are determinants of longevity in the longest-living rodent, the naked mole-rat

The widely accepted oxidative stress theory of aging postulates that aging results from accumulation of oxidative damage. Surprisingly, data from the longest-living rodent known, naked mole-rats [MRs; mass 35 g; maximum lifespan (MLSP) > 28.3 years], when compared with mice (MLSP 3.5 years) exhibit higher levels of lipid peroxidation, protein carbonylation, and DNA oxidative damage even at a young age. We hypothesize that age-related changes in protein structural stability, oxidation, and degradation are abrogated over the lifespan of the MR. We performed a comprehensive study of oxidation states of protein cysteines [both reversible (sulfenic, disulfide) and indirectly irreversible (sulfinic/sulfonic acids)] in liver from young and old C57BL/6 mice (6 and 28 months) and MRs (2 and >24 years). Furthermore, we compared interspecific differences in urea-induced protein unfolding and ubiquitination and proteasomal activity. Compared with data from young mice, young MRs have 1.6 times as much free protein thiol groups and similar amounts of reversible oxidative damage to cysteine. In addition, they show less urea-induced protein unfolding, less protein ubiquitination, and higher proteasome activity. Mice show a significant age-related increase in cysteine oxidation and higher levels of ubiquitination. In contrast, none of these parameters were significantly altered over 2 decades in MRs. Clearly MRs have markedly attenuated age-related accrual of oxidation damage to thiol groups and age-associated up-regulation of homeostatic proteolytic activity. These pivotal mechanistic interspecies differences may contribute to the divergent aging profiles and strongly implicate maintenance of protein stability and integrity in successful aging.

Effects of proteasome inhibitor, MG132, on proteasome activity and oxidative status of rat liver

In vivo effects of N-benzyloxycarbonyl (Cbz)-Leu-Leu-leucinal (MG132) on chymotryptic-like (ChT-L), tryptic-like, and post-glutamyl peptide hydrolytic-like proteasome activities, protein oxidation, lipid peroxidation (LP), glutathione (GSH) level, as well as on the activity of antioxidant enzymes (superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GSH-Px), and glutathione-reductase) in the rat liver were studied. The possibility of MG132 provoking the formation of free oxygen radicals was also assayed in primary hepatocytes. The following results were obtained: (1) In vivo, MG132 did not change the spontaneous LP, but increased Fe-induced LP and the amount of oxidized proteins; it decreased the GSH level in liver. From the proteasome activities studied in liver cytosol only ChT-L activity was significantly decreased after MG132 administration. Furthermore, MG132 increased antioxidant enzyme activities of SOD, CAT, and GSH-Px. (2) In vitro, MG132 increased free radical oxygen species in hepatocytes; this effect disappeared in the presence of CAT or mannitol. In conclusion, since nowadays proteasome inhibitors are entering into the swing of laboratory and clinical practice, the present data could provide useful information for MG132 action. Consequently, future in vivo experiments with MG132 could highlight the possibility of its use at different pathological conditions.

CARF: an emerging regulator of p53 tumor suppressor and senescence pathway

Replicative senescence, a major outcome of normal cells with finite lifespan, is a widely accepted in vitro model for ageing studies. Limited repair and defense mechanisms of normal cells, in addition to DNA alterations and oncogene inductions under stress, are believed to result in senescence as a protective mechanism to prevent undesirable proliferation of cells. The ARF/p53/p21(cip1/waf1) tumor suppression pathway acts as a molecular sensor and regulator of cellular stress, senescence, and immortalization. Understanding the molecular regulation of this pathway by intrinsic and extrinsic signals is extremely important to address unsolved questions in senescence and cancer. CARF was first discovered as a binding partner of ARF and has since been shown to have both ARF-dependent and -independent functions that converge to regulate p53 pathway. CARF directly binds to p53 and HDM2, and functions in a negative feedback pathway. Whereas CARF transcriptionally represses HDM2 to increase p53 activity, HDM2 in return degrades CARF. Thus, CARF may act as a novel key regulator of the p53 pathway at multiple checkpoints. The aim of this article is to discuss the current knowledge about functions of CARF and its impact on p53 pathway in regulation of senescence and carcinogenesis.

Adaptation to chronic MG132 reduces oxidative toxicity by a CuZnSOD-dependent mechanism

To study whether and how cells adapt to chronic cellular stress, we exposed PC12 cells to the proteasome inhibitor MG132 (0.1 microM) for 2 weeks and longer. This treatment reduced chymotrypsin-like proteasome activity by 47% and was associated with protection against both 6-hydroxydopamine (6-OHDA; 100 microM) and higher dose MG132 (40 microM). Protection developed slowly over the course of the first 2 weeks of exposure and was chronic thereafter. There was no change in total GSH levels after MG132. Buthionine sulfoximine (100 microM) reduced GSH levels by 60%, but exacerbated 6-OHDA toxicity to the same extent in both MG132-treated and control cells and failed to reduce MG132-induced protection. Chronic MG132 resulted in elevated antioxidant proteins CuZn superoxide dismutase (SOD; +55%), MnSOD (+21%), and catalase (+15%), as well as chaperone heat-shock protein 70 (+42%). Examination of SOD enzyme activity revealed higher levels of CuZnSOD (+40%), with no change in MnSOD. We further assessed the mechanism of protection by reducing CuZnSOD levels with two independent siRNA sequences, both of which successfully attenuated protection against 6-OHDA. Previous reports suggested that artificial over-expression of CuZnSOD in dopaminergic cells is protective. Our data complement such observations, revealing that dopaminergic cells are also able to use endogenous CuZnSOD in self-defensive adaptations to chronic stress, and that they can even do so in the face of extensive GSH loss.

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.

Role of proteasomes in disease

A functional ubiquitin proteasome system is essential for all eukaryotic cells and therefore any alteration to its components has potential pathological consequences. Though the exact underlying mechanism is unclear, an age-related decrease in proteasome activity weakens cellular capacity to remove oxidatively modified proteins and favours the development of neurodegenerative and cardiac diseases. Up-regulation of proteasome activity is characteristic of muscle wasting conditions including sepsis, cachexia and uraemia, but may not be rate limiting. Meanwhile, enhanced presence of immunoproteasomes in aging brain and muscle tissue could reflect a persistent inflammatory defence and anti-stress mechanism, whereas in cancer cells, their down-regulation reflects a means by which to escape immune surveillance. Hence, induction of apoptosis by synthetic proteasome inhibitors is a potential treatment strategy for cancer, whereas for other diseases such as neurodegeneration, the use of proteasome-activating or -modulating compounds could be more effective. Publication history: Republished from Current BioData's Targeted Proteins database (TPdb; http://www.targetedproteinsdb.com).

Protein stress and stress proteins: implications in aging and disease

Environmental stress induces damage that activates an adaptive response in any organism. The cellular stress response is based on the induction of cytoprotective proteins,the so called stress or heat shock proteins. The stress response as well as stress proteins are ubiquitous,highly conserved mechanism, and genes, respectively, already present in prokaryotes. Chaperones protect the proteome against conformational damage, promoting the function of protein networks. Protein damage takes place during aging and in several degenerative diseases, and presents a threat to overload the cellular defense mechanisms. The preservation of a robust stress response and protein disposal is indispensable for health and longevity. This review summarizes the present knowledge of protein damage, turnover, and the stress response in aging and degenerative diseases.

Alterations of cerebral cortex and hippocampal proteasome subunit expression and function in a traumatic brain injury rat model

Following cellular stress or tissue injury, the proteasome plays a critical role in protein degradation and signal transduction. The present study examined the beta-subunit expression of constitutive proteasomes (beta1, beta2, and beta5), immunoproteasomes (beta1i, beta2i, and beta5i) and the 11S proteasome activator, PA28alpha, in the rat CNS after traumatic brain injury (TBI). Concomitant measures assessed changes in proteasome activities. Quantitative real time PCR results indicated that beta1 and beta2 mRNA levels were not changed, while beta5 mRNA levels were significantly decreased in injured CNS following TBI. However, beta1i, beta2i, beta5i, and PA28alpha mRNA levels were significantly increased in the injured CNS. Western blotting studies found that beta1, beta2, beta5, beta2i, and beta5i subunit protein levels remained unchanged in the injured CNS, but beta1i and PA28alpha protein levels were significantly elevated in ipsilateral cerebral cortex and hippocampus. Proteasome activity assays found that peptidyl glutamyl peptide hydrolase-like and chymotrypsin-like activity were significantly reduced in the CNS after TBI, and that trypsin-like proteasome activity was increased in the injured cerebral cortex. Our results demonstrated that both proteasome composition and function in the CNS were affected by trauma. Treatments that preserve proteasome function following CNS injury may be beneficial as an approach to cerebral neuroprotection.

Low-level caloric restriction rescues proteasome activity and Hsc70 level in liver of aged rats

Proteasome activity is known to decrease with aging in ad libitum (AL) fed rats. Severe caloric restriction (CR) significantly extends the maximum life-span of rats, and counteracts the age-associated decrease in liver proteasome activities. Since few investigations have explored whether lower CR diets might positively counteract the age associated decrease in proteasome activity, we then investigated the effects of a mild CR regimen on animal life-span, proteasome content and function. In addition, we addressed the question whether both CR regimens might also affect the expression of Hsc70 protein, a constitutive chaperone reported to share a role in the function of proteasome complex and in the repair of proteotoxic damage, and whose level decreased during aging. In contrast to severe CR, mild CR had a poor effect on life-span; however, it better counteracted the decrease of proteasome activities. Both regimens, however, maintain Hsc70 in liver of old rats at level comparable to that of young rats. Interestingly, the effects of aging and CRs on liver proteasome enzyme activities did not appear to be associated with parallel changes in the amount of proteasome proteins suggesting that the quality (molecular activity of the enzymes) rather than the quantity are likely to be modified with age. In conclusion, the results presented in this work show that a mild CR can have beneficial effects on liver function of aging rats because is adequate to counteract the decrease of proteasome function and Hsc70 chaperone level.

Proteasome and Photoaging: The Effects of UV Irradiation

Cellular aging is characterized by the accumulation of oxidatively modified proteins that result, at least in part, from impaired degradation of abnormal proteins. The proteasome is the major intracellular proteolytic system implicated in the removal of abnormal and oxidized proteins. In human epidermal cells, previous studies have evidenced that proteasome function is decreased during aging as well as upon UV irradiation, which is the main component of photoaging. The age-related decline of proteasome activity has been reported to be due to either or both decreased proteasome subunits expression and content, inactivation upon alteration of proteasome subunits, and accumulation of endogenous inhibitors, such as highly oxidized and cross-linked proteins. To gain further insight in the mechanisms that might be implicated in the decreased activity of the proteasome upon photoaging, purified 20S human proteasome has been exposed to UVA- and UVB-irradiation. The effect of such an irradiation on proteasome peptidase activities has been monitored and shown to promote a stimulation or an inhibition of the peptidase activities depending on whether the proteasome is under its latent or a nonphysiological active form. Analysis of the patterns of proteasome subunits by 2D gel electrophoresis has revealed modification for several subunits for UV-irradiated proteasome only in its irreversibly activated form, compared with nonirradiated and irradiated latent forms, indicating that the 20S proteasome is rather resistant to UV irradiation.

Redox regulation of the proteasome in T lymphocytes during aging

Proteasome is a major cellular organelle responsible for the regulated turnover of both normal and misfolded proteins. Recent reports from our laboratory have implicated lowered proteasomal chymotryptic activity to be responsible for decreased induction of the transcription factor NFkappaB in T lymphocytes during aging. In this study, we have further analyzed the basis for this decline in proteasomal function, by focusing on the role of oxidative stress. On exposure to the prooxidant BSO, both ATP-stimulatable 26S and ATP-independent 20S proteasomal catalytic activity could be down-regulated in T cells from young donors, mimicking the decline observed in T cells from the elderly. Loss in these catalytic activities, following exposure to prooxidant stimulus, also resulted in a decline in both activation-induced proliferation and degradation of the inhibitor IkappaBalpha, with concomitant increase in the accumulation of carbonylated proteins, mimicking responses seen in T cells from the elderly. Pretreatment with an antioxidant, NAC, could override prooxidant-mediated, but not age-associated, decrease in both 20S and 26S proteasomal activities. These results suggest that the decrease in proteasomal activities observed during aging may be secondary to oxidative stress and underlie immune senescence.

Maintenance of proteins and aging: the role of oxidized protein repair

According to the free radical theory of aging proposed by Denham Harman (Journal of Gerontology 1956, 11, pp. 298-300), the continuous oxidative damage to cellular components over an organism's life span is a causal factor of the aging process. The age-related build-up of oxidized protein is therefore resulting from increased protein oxidative damage and/or decreased elimination of oxidized proteins. In this mini-review, we will address the fate, during aging, of the protein maintenance systems that are involved in the degradation of irreversibly oxidized proteins and in the repair of reversible protein oxidative damage with a special focus on the methionine sulfoxide reductases system. Since these protein degradation and repair systems have been found to be impaired with age, it is proposed that not only failure of redox homeostasis but, as importantly, failure of protein maintenance are critical factors in the aging process.

Protein homeostasis and molecular chaperones in aging

Molecular chaperones are ubiquitous, highly conserved proteins responsible for the maintenance of protein folding homeostasis in cells. Environmental stress causes proteotoxic damage, which triggers chaperone induction and the subsequent reparation of cellular damage by chaperones, including disposing irreparable protein ensembles. We summarize the current view of protein damage, turnover, the stress response and chaperone function in aging, and review novel data showing that accumulation of misfolded proteins outcompete and overload the limited resources of the protein folding, maintenance and turnover system, compromising general protein homeastasis and cellular function. Possible involvement of chaperones and proteolysis in immunosenescence is highlighted. Defects in zinc metabolism are also addressed in relation to aging and changes in chaperone levels.

Effect of reduced culture temperature on antioxidant defences of mesenchymal stem cells

Mesenchymal stem cells (MSC) promise to be valuable therapeutic tools but, due to their low numbers, require considerable in vitro expansion before use. This leads to in vitro aging, the accumulation of intracellular oxidative damage, and subsequently a decreased potential for proliferation and differentiation. Optimised culture conditions might help to reduce oxidative damage in MSC in vitro, and therefore, as reduced temperature is known to reduce oxidative stress in other somatic cells, we have investigated the effect of reduced temperature on rat MSC viability, differentiation, and oxidative damage. Temperature reduction did not affect MSC viability but increased differentiation and reduced apoptosis. Oxidative-damage-related indices were improved; reactive oxide species, nitric oxide, thiobarbituric acid reactive substances, carbonyl, and lipofuscin levels were reduced and glutathione peroxidase and superoxide dimutase levels increased. Levels of antiapoptotic heat shock proteins (HSP-27, -70, and -90) were raised and levels of the proapoptotic HSP-60 reduced. These data demonstrate that culturing MSC at reduced temperature decreases the accumulation of oxidative damage and therefore would probably improve long-term viability and successful engraftment of MSC used for tissue engineering or cell therapeutic purposes.

The ubiquitin-proteasome system in cardiac physiology and pathology

The ubiquitin-proteasome system (UPS) is the major nonlysosomal pathway for intracellular protein degradation, generally requiring a covalent linkage of one or more chains of polyubiquitins to the protein intended for degradation. It has become clear that the UPS plays major roles in regulating many cellular processes, including the cell cycle, immune responses, apoptosis, cell signaling, and protein turnover under normal and pathological conditions, as well as in protein quality control by removal of damaged, oxidized, and/or misfolded proteins. This review will present an overview of the structure, biochemistry, and physiology of the UPS with emphasis on its role in the heart, if known. In addition, evidence will be presented supporting the role of certain muscle-specific ubiquitin protein ligases, key regulatory components of the UPS, in regulation of sarcomere protein turnover and cardiomyocyte size and how this might play a role in induction of the hypertrophic phenotype. Moreover, this review will present the evidence suggesting that proteasomal dysfunction may play a role in cardiac pathologies such as myocardial ischemia, congestive heart failure, and myofilament-related and idiopathic-dilated cardiomyopathies, as well as cardiomyocyte loss in the aging heart. Finally, certain pitfalls of proteasome studies will be described with the intent of providing investigators with enough information to avoid these problems. This review should provide current investigators in the field with an up-to-date analysis of the literature and at the same time provide an impetus for new investigators to enter this important and rapidly changing area of research.

Oxidized protein degradation and repair in ageing and oxidative stress

Cellular ageing is characterized by the accumulation of oxidatively modified proteins which may be due to increased protein damage and/or decreased elimination of oxidized protein. Since the proteasome is in charge of protein turnover and removal of oxidized protein, its fate during ageing and upon oxidative stress has received special attention, and evidence has been provided for an age-related impairment of proteasome function. However, proteins when oxidized at the level of sulfur-containing amino acids can also be repaired. Therefore, the fate of the methionine sulfoxide reductase system during ageing has also been addressed as well as its role in protection against oxidative stress.

Inactivation of the proteasome by 4-hydroxy-2-nonenal is site specific and dependant on 20S proteasome subtypes

The proteasome represents a major intracellular proteolytic system responsible for the degradation of oxidized and ubiquitinated proteins in both the nucleus and cytoplasm. We have previously reported that proteasome undergoes modification by the lipid peroxidation product 4-hydroxy-2-nonenal (HNE) and exhibits declines in peptidase activities during cardiac ischemia/reperfusion. This study was undertaken to characterize the effects of HNE on the structure and function of the 20S proteasome. To assess potential tissue-specific differences in the response to HNE, we utilized purified 20S proteasome from heart and liver, tissues that express different proteasome subtypes. Following incubation of heart and liver 20S proteasome with HNE, changes in the 2D gel electrophoresis patterns and peptidase activities of the proteasome were evaluated. Proteasome subunits were identified by mass spectrometry prior to and following treatment with HNE. Our results demonstrate that specific subunits of the 20S proteasome are targeted for modification by HNE and that modified proteasome exhibits selective alterations in peptidase activities. The results provide evidence for a likely mechanism of proteasome inactivation in response to oxidative stress particularly during cardiac ischemia/reperfusion.

Proteasome function in aging and oxidative stress: implications in protein maintenance failure

Damage to cellular components by reactive oxygen species is believed to be an important factor contributing to the aging process. Likewise, the progressive failure of maintenance and repair is believed to be a major cause of biological aging. Cellular aging is characterized by the accumulation of oxidatively modified proteins, a process that results, at least in part, from impaired protein turnover. Indeed, oxidized protein buildup with age may be due to increased protein damage, decreased elimination of oxidized protein (i.e., repair and degradation), or a combination of both mechanisms. Since the proteasome has been implicated in both general protein turnover and the removal of oxidized protein, the fate of the proteasome during aging has recently received considerable attention, and evidence has been provided for impaired proteasome function with age in different cellular systems. The present review will mainly address age-related changes in proteasome structure and function in relation to the impact of oxidative stress on the proteasome and the accumulation of oxidized protein. Knowledge of molecular mechanisms involved in the decline of proteasome function during aging and in oxidative stress is expected to provide new insight that will be useful in defining antiaging strategies aimed at preserving this critical function.

Redox modulation of cellular metabolism through targeted degradation of signaling proteins by the proteasome

Under conditions of oxidative stress, the 20S proteasome plays a critical role in maintaining cellular homeostasis through the selective degradation of oxidized and damaged proteins. This adaptive stress response is distinct from ubiquitin-dependent pathways in that oxidized proteins are recognized and degraded in an ATP-independent mechanism, which can involve the molecular chaperone Hsp90. Like the regulatory complexes 19S and 11S REG, Hsp90 tightly associates with the 20S proteasome to mediate the recognition of aberrant proteins for degradation. In the case of the calcium signaling protein calmodulin, proteasomal degradation results from the oxidation of a single surface exposed methionine (i.e., Met145); oxidation of the other eight methionines has a minimal effect on the recognition and degradation of calmodulin by the proteasome. Since cellular concentrations of calmodulin are limiting, the targeted degradation of this critical signaling protein under conditions of oxidative stress will result in the downregulation of cellular metabolism, serving as a feedback regulation to diminish the generation of reactive oxygen species. The targeted degradation of critical signaling proteins, such as calmodulin, can function as sensors of oxidative stress to downregulate global rates of metabolism and enhance cellular survival.

Algae extract-mediated stimulation and protection of proteasome activity within human keratinocytes exposed to UVA and UVB irradiation

Sun exposure is the major environmental influence for epidermal cells; the harmful effect of UV radiation on skin is related to the generation of reactive oxygen species that alter cellular components including proteins. It is now well established that the proteasome is responsible for the degradation of most of oxidized proteins and that impairment of proteasome function is a hallmark of cellular aging. In a previous study, we investigated the effects of UV irradiation on proteasomes in human keratinocyte cultures and showed that all three peptidase activities were decreased 24 h after irradiation of the cells. Increased levels of oxidatively modified proteins were observed in irradiated cells and were found to act as endogenous inhibitors of the proteasome. We report here on the stimulating and protective effects of an algae extract, prepared from Phaeodactylum tricornutum, on proteasome peptidase activities of human keratinocytes exposed to UVA and UVB irradiation. In addition, preserving proteasome function resulted in lowering the extent of the irradiation-induced protein oxidative damage, opening up new strategies for protection of epidermal cells against the detrimental effects of UV irradiation.

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 oxidation and degradation during postmitotic senescence

Oxidized and cross-linked proteinacious materials (lipofuscin, age pigments, ceroid, etc.) have long been known to accumulate in aging and in age-related diseases, and some studies have suggested that age-dependent inhibition of the proteasome and/or lysosomal proteases may contribute to this phenomenon. Cell culture studies trying to model these aging effects have almost all been performed with proliferating (divisionally competent) cell lines. There is little information on nondividing (postmitotic) cells; yet age-related accumulation of oxidized and cross-linked protein aggregates is most marked in postmitotic tissues such as brain, heart, and skeletal muscles. The present investigation was undertaken to test whether oxidized and cross-linked proteins generally accumulate in nondividing, IMR-90 and MRC-5, human cell lines, and whether such accumulation is associated with diminished proteolytic capacities. Since both protein oxidation and declining proteolytic activities might play major roles in the age-associated accumulation of intracellular oxidized materials, we tested for protein carbonyl formation, proteasomal activities, and lysosomal cathepsin activities. For these studies, confluent, postmitotic IMR-90 and MRC-5 fibroblasts (at various population doubling levels) were cultured under hyperoxic conditions to facilitate age-related oxidative senescence. Our results reveal marked decreases in the activity of both the proteasomal system and the lysosomal proteases during senescence of nondividing fibroblasts, but the peptidyl-glutamyl-hydrolyzing activity of the proteasome was particularly inhibited. This decline in proteolytic capacity was accompanied by an increased accumulation of oxidized proteins.

HDACs and the senescent phenotype of WI-38 cells

Background

Normal cells possess a limited proliferative life span after which they enter a state of irreversible growth arrest. This process, known as replicative senescence, is accompanied by changes in gene expression that give rise to a variety of senescence-associated phenotypes. It has been suggested that these gene expression changes result in part from alterations in the histone acetylation machinery. Here we examine the influence of HDAC inhibitors on the expression of senescent markers in pre- and post-senescent WI-38 cells.

Results

Pre- and post-senescent WI-38 cells were treated with the HDAC inhibitors butyrate or trichostatin A (TSA). Following HDAC inhibitor treatment, pre-senescent cells increased p21^WAF1 and β-galactosidase expression, assumed a flattened senescence-associated morphology, and maintained a lower level of proteasome activity. These alterations also occurred during normal replicative senescence of WI-38 cells, but were not accentuated further by HDAC inhibitors. We also found that HDAC1 levels decline during normal replicative senescence.

Conclusion

Our findings indicate that HDACs impact numerous phenotypic changes associated with cellular senescence. Reduced HDAC1 expression levels in senescent cells may be an important event in mediating the transition to a senescent phenotype.

Radical medicine: treating ageing to cure disease

The incidence of many diseases rises sharply with age. Although clearly separable, ageing and certain age-related diseases might share common mechanisms. Cellular metabolism and subsequent generation of reactive oxygen species might contribute both to the rate at which we age and to our susceptibility to numerous chronic diseases, therefore therapies that directly target the ageing process might provide new ways to treat human diseases.

Proteasomal degradation of mutant superoxide dismutases linked to amyotrophic lateral sclerosis

Mutations in copper-zinc superoxide dismutase cause the neurodegenerative disease amyotrophic lateral sclerosis. Many of the mutant proteins have increased turnover in vivo and decreased thermal stability. Here we show that purified, metal-free superoxide dismutases are degraded in vitro by purified 20 S proteasome in the absence of ATP and without ubiquitinylation, whereas their metal-bound counterparts are not. The rate of degradation by the proteasome varied among the mutants studied, and the rate correlated with the in vivo half-life. The monomeric forms of both mutant and wild-type superoxide dismutase are particularly susceptible to degradation by the proteasome. Exposure of hydrophobic regions as a consequence of decreased thermal stability may allow the proteasome to recognize these molecules as non-native.

The effects of acetaldehyde in vitro on proteasome activities and its potential involvement after alcoholization of rats by inhalation of ethanol vapours

BACKGROUND/AIMS

Some models of chronic ethanol administration resulted in decreased proteasome activities. The mechanisms still remain speculative. In the present study, we tested another model of alcoholization with high blood alcohol levels (BALs) and high acetaldehyde fluxes as well as the in vitro effect of acetaldehyde on proteasome. Methods/

RESULTS

Ethanol vapour chronically inhaled by adult Wistar rats up to a specific protocol, can reach high BALs (200 mg/dl) with significant circulating acetaldehyde levels. After 4 weeks of ethanol intoxication, although cytochrome CYP2E1 was increased, liver lipid peroxidation remained unchanged when protein carbonyls augmented selectively for high molecular weight with a decrease of the proteasome activities in ethanol rats. Several aldehydes inhibit proteasome function; we specifically explored the effects of acetaldehyde, the first alcohol metabolite. Adduction of acetaldehyde in vitro to cytosolic proteins inhibits proteasome in a dose-dependent manner. Acetaldehyde adducted to purified proteasome also exhibits a decrease in its activities. Furthermore, an acetaldehyde-adducted protein, i.e. bovine serum albumin (BSA) is less degraded than a native BSA by purified proteasome. These findings suggest that acetaldehyde, if overproduced, can inhibit proteasome activities and reduce the proteolysis of acetaldehyde-adducted proteins.

CONCLUSIONS

Our study, for the first time, provided the evidence that acetaldehyde by itself inhibits proteasome activities. As the chronic inhalation model used in this study is not associated with an overt lipid peroxidation, one can suggest that high BALs and their subsequent high acetaldehyde fluxes contribute to impairment of proteasome function and accumulation of carbonylated proteins. This early phenomenon may have relevance in experimental alcohol liver disease.

Single nucleotide polymorphisms of the heat shock protein 90 gene in varicocele-associated infertility

PURPOSE

Varicoceles are associated with impaired testicular function and male infertility, but the molecular mechanisms by which fertility is affected have not been satisfactorily explained. Spermatogenesis might be affected by increased scrotal temperature, such as that caused by varicocele. HSP90 is a molecular chaperone expressed in germ cells and is related to spermatogenesis, motility, and both heat and oxidative stress. Possible correlations between coding single region nucleotide polymorphisms (cSNPs) in the HSP90 gene in patients with varicocele associated with infertility were analyzed, and polymorphisms in these exons were characterized through DNA sequencing.

MATERIALS AND METHODS

PCR-SSCP and DNA sequencing were used to search for mutations in 18 infertile patients with varicocele, 11 patients with idiopathic infertility and 12 fertile men. DNA was extracted from leucocytes for PCR amplification and SSCP analysis. DNA from samples with an altered band pattern in the SSCP was then sequenced to search for polymorphisms.

RESULTS

Three silent polymorphisms that do not lead to amino acid substitutions were identified.

CONCLUSION

Mutations in the HSP90 gene do not appear to be a common cause of male factor infertility. The low incidence of gene variation, or SNPs, in infertile men demonstrates that this gene is highly conserved and thus confirms its key role in spermatogenesis and response to heat stress.

Proteasomal activity in brain differs between species and brain regions and changes with age

Age-related increase in protein oxidation in brain coupled to an impairment of proteasomal activity may underline neuronal loss but differences in susceptibility between species and brain regions remain unexplained. We now investigate differences in proteasomal activity, measured as chymotrypsin-, trypsin- and peptidylglutamyl-like hydrolysing activities between brain regions in rats, mice and common marmosets. In aged rats and mice, proteasomal activity was decreased in the cortex, striatum, cerebellum, globus pallidus and substantia nigra overall when compared to young animals. However, in the aged brain only chymotrypsin-like activity was decreased in the cortex and the globus pallidus while only trypsin-like activity was reduced in the cerebellum. In contrast, in the striatum, both chymotrypsin-like and trypsin-like activities were reduced and in the substantia nigra, all the three catalytic activities of proteasome were significantly impaired. Chymotrypsin-like and trypsin-like activities were significantly higher in all the brain regions of marmosets compared to those of mice and rats. Peptidylglutamyl-like activity was only significantly higher in the cerebellum and striatum of marmoset compared to rodents. The data suggest that there is higher proteasome activity in common marmoset brain compared to rat and mouse and that the basal ganglia are more prone to an age-related decrease in proteasomal activity. This may explain the involvement of altered ubiquitin-proteasome system activity in Parkinson's disease and the relationship to ageing.

Decreased proteolysis caused by protein aggregates, inclusion bodies, plaques, lipofuscin, ceroid, and 'aggresomes' during oxidative stress, aging, and disease

Protein aggregation seems to be a common feature of several neurodegenerative diseases and to some extent of physiological aging. It is not always clear why protein aggregation takes place, but a disturbance in the homeostasis between protein synthesis and protein degradation seems to be important. The result is the accumulation of modified proteins, which tend to form high molecular weight aggregates. Such aggregates are also called inclusion bodies, plaques, lipofuscin, ceroid, or 'aggresomes' depending on their location and composition. Such aggregates are not inert metabolic end products, but actively influence the metabolism of cells, in particular proteasomal activity and protein turnover. In this review we focus on the influence of oxidative stress on protein turnover, protein aggregate formation and the various interactions of protein aggregates with the proteasome. Furthermore, the formation and effects of protein aggregates during aging and neurodegeneration will be highlighted.

Mild heat stress stimulates 20S proteasome and its 11S activator in human fibroblasts undergoing aging in vitro

Repeated mild heat shock (RMHS) has been shown to have several beneficial hormetic effects on human skin fibroblast undergoing aging in vitro. Because an age-related decline in proteasome activity is 1 of the reasons for the accumulation of abnormal proteins during aging, we have investigated the effects of RMHS on the 20S proteasome, which is the major proteolytic system involved in the removal of abnormal and oxidatively damaged proteins. Serially passaged human skin fibroblasts exposed to RMHS at 41 degrees C for 60 minutes twice a week had increased 3 proteasomal activities by 40% to 95% in early- and midpassage cultures. RMHS-treated cells also contained a 2-fold higher amount of the proteasome activator 11S, and the extent of the bound activator was double in early- and midpassage cells only. Furthermore, there was no difference in the content of the 19S proteasome regulator in the stressed and the unstressed cells. Therefore, RMHS-induced proteasome stimulation in early- and midpassage fibroblasts appears to be due to an induction and enhanced binding of 11S proteasome activators. In contrast to this, the proteasomal system in late-passage senescent cells appears to be less responsive to the stimulatory effects of mild heat shock.

Neurodegenerative conditions associated with ageing: a molecular interplay?

The ageing process precipitates dramatic alterations in the physiology of all organisms, including reduced cellular function, compromised resistance to stress and pathological agents, and increased likelihood of developing age-related diseases. Among the most characteristic pathologies associated with old age are numerous late-onset neurodegenerative disorders such as Alzheimer's, Parkinson's and Huntington's diseases. In addition to stroke, which also inflicts loss of neuronal cells, these conditions account for ever-increasing debilitation among the elderly. Recent studies in model organisms such as the nematode Caenorhabditis elegans and the fruit fly Drosophila melanogaster, which offer the prowess of sophisticated genetic approaches, have uncovered significant, novel aspects of the molecular mechanisms that underlie both neurodegeneration and the ageing process. These advances hold promise that the intimate link between the aged state and the manifestation of several neurodegenerative diseases will be deciphered. Here, we discuss the mechanisms by which ageing interfaces with, and influences, the progression of neurodegeneration.

Quantitative proteomics analysis of specific protein expression and oxidative modification in aged senescence-accelerated-prone 8 mice brain

The senescence-accelerated mouse (SAM) is a murine model of accelerated senescence that was established using phenotypic selection. The SAMP series includes nine substrains, each of which exhibits characteristic disorders. SAMP8 is known to exhibit age-dependent learning and memory deficits. In our previous study, we reported that brains from 12-month-old SAMP8 have greater protein oxidation, as well as lipid peroxidation, compared with brains from 4-month-old SAMP8 mice. In order to investigate the relation between age-associated oxidative stress on specific protein oxidation and age-related learning and memory deficits in SAMP8, we used proteomics to identify proteins that are expressed differently and/or modified oxidatively in aged SAMP8 brains. We report here that in 12 month SAMP8 mice brains the expressions of neurofilament triplet L protein, lactate dehydrogenase 2 (LDH-2), heat shock protein 86, and alpha-spectrin are significantly decreased, while the expression of triosephosphate isomerase (TPI) is increased compared with 4-month-old SAMP8 brains. We also report that the specific protein carbonyl levels of LDH-2, dihydropyrimidinase-like protein 2, alpha-spectrin and creatine kinase, are significantly increased in the brain of 12-month-old SAMP8 mice when compared with the 4-month-old SAMP8 brain. These findings are discussed in reference to the effect of specific protein oxidation and changes of expression on potential mechanisms of abnormal alterations in metabolism and neurochemicals, as well as to the learning and memory deficits in aged SAMP8 mice.

Hsp90 enhances degradation of oxidized calmodulin by the 20 S proteasome

The 20 S proteasome has been suggested to play a critical role in mediating the degradation of abnormal proteins under conditions of oxidative stress and has been found in tight association with the molecular chaperone Hsp90. To elucidate the role of Hsp90 in promoting the degradation of oxidized calmodulin (CaM(ox)), we have purified red blood cell 20 S proteasomes free of Hsp90 and assessed their ability to degrade CaM(ox) in the absence or presence of Hsp90. Purified 20 S proteasome does not degrade CaM(ox) unless Hsp90 is added. CaM(ox) degradation is sensitive to both proteasome and Hsp90-specific inhibitors and is further enhanced in the presence of 2 mm ATP. Irrespective of the presence of Hsp90, we find that unoxidized CaM is not significantly degraded. Direct binding measurements demonstrate that Hsp90 selectively associates with CaM(ox); essentially no binding is observed between Hsp90 and unoxidized CaM. These results indicate that Hsp90 in association with the 20 S proteasome can selectively associate with oxidized and partially unfolded CaM to promote degradation by the proteasome.

Age-dependent protein modifications and declining proteasome activity in the human lens

The proteasome is known to be the main enzymatic complex responsible for the intracellular degradation of altered proteins, and the age-related accumulation of modified lens proteins is associated to the formation of cataracts. The aim of this study was to determine whether the human lens proteasome becomes functionally impaired with age. The soluble and insoluble protein fractions of human lenses corresponding to various age-groups were characterized in terms of their levels of glyco-oxidative damage and found to show increasing anti-carboxymethyl-lysine immunoreactivity with age. Concomitantly, decreasing proteasome contents and peptidase activities were observed in the water-soluble fraction. The fact that peptidylglutamyl-peptide hydrolase activity is most severely affected with age suggests that specific changes are undergone by the proteasome itself. In particular, increasing levels of carboxymethylation were observed with age in the proteasome. It was concluded that the lower levels of soluble active enzymatic complex present in elderly lenses and the post-translational modifications affecting the proteasome may at least partly explain the decrease in proteasome activity and the concomitant accumulation of carboxymethylated and ubiquitinated proteins which occur with age.

Free radicals and brain aging

We reviewed here the formation of free radicals and its effect physiologically. Studies mentioned above have indicated that free radical/ROS/RNS involvement in brain aging is direct as well as correlative. Increasing evidence demonstrates that accumulation of oxidation of DNA, proteins, and lipids by free radicals are responsible for the functional decline in aged brains. Also, lipid peroxidation products, such as MDA, HNE, and acrolein, were reported to react with DNA and proteins to produce further damage in aged brains. Therefore, the impact of free radicals on brain aging is pronounced. It has been estimated that 10,000 oxidative interactions occur between DNA and endogenously generated free radicals per human cell per day, and at least one of every three proteins in the cell of older animals is dysfunctional as an enzyme or structural protein, due to oxidative modification. Although these estimated numbers reveal that free radical-mediated protein and DNA modification play significant roles in the deterioration of aging brain, they do not imply that free radical damages are the only cause of functional decline in aged brain. Nevertheless,although other factors may be involved in the cascade of damaging effects in the brain, the key role of free radicals in this process cannot be underestimated. This article has examined the role and formation of free radicals in brain aging. We propose that free radicals are critical to cell damage in aged brain and endogenous, and that exogenous antioxidants, therefore, may play effective roles in therapeutic strategies for age-related neurodegenerative disorders.

Hydrogen peroxide induces the dissociation of GroEL into monomers that can facilitate the reactivation of oxidatively inactivated rhodanese

Although, several studies have been reported on the effects of oxidants on the structure and function of other molecular chaperones, no reports have been made so far for the chaperonin GroEL. The ability of GroEL to function under oxidative stress was investigated in this report by monitoring the effects of hydrogen peroxide (H(2)O(2)) on the structure and refolding activity of this protein. Using fluorescence spectroscopy and light scattering, we observed that GroEL showed increases in exposed hydrophobic sites and changes in tertiary and quaternary structure. Differential sedimentation, gel electrophoresis, and circular dichroism showed that H(2)O(2) treated GroEL underwent irreversible dissociation into monomers with partial loss of secondary structure. Relative to other proteins, GroEL was found to be highly resistant to oxidative damage. Interestingly, GroEL monomers produced under these conditions can facilitate the reactivation of H(2)O(2)-inactivated rhodanese but not urea-denatured rhodanese. Recovery of approximately 84% active rhodanese was obtained with either native or oxidized GroEL in the absence of GroES or ATP. In comparison, urea-denatured GroEL, BSA and the refolding mixture in the absence of proteins resulted in the recovery of 72, 50, and 49% rhodanese activity, respectively. Previous studies have shown that GroEL monomers can reactivate rhodanese. Here, we show that oxidized monomeric GroEL can reactivate oxidized rhodanese suggesting that GroEL retains the ability to protect proteins during oxidative stress.

Altered proteasome function and subunit composition in aged muscle

Myofibrillar protein degradation is mediated through the ubiquitin-proteasome pathway. To investigate if altered proteasome activity plays a role in age-related muscle atrophy, we examined muscle size and proteasome function in young and aged F344BN rats. Significant age-related muscle atrophy was confirmed by the 38% decrease in cross-sectional area of type 1 fibers in soleus muscle. Determination of proteasome function showed hydrolysis of fluorogenic peptides was equivalent between ages. However, when accounting for the 3-fold increase in content of the 20S catalytic core in aged muscle, the lower specific activity suggests a functional loss in individual proteins with aging. Comparing the composition of the catalytic beta-subunits showed an age-related 4-fold increase in the cytokine-inducible subunits, LMP2 and LMP7. Additionally, the content of the activating complexes, PA28 and PA700, relative to the 20S proteasome was reduced 50%. These results suggest significant alterations in the intrinsic activity, the percentage of immunoproteasome, and the regulation of the 20S proteasome by PA28 and PA700 in aged muscle.