Glutathiolation of the proteasome is enhanced by proteolytic inhibitors

Proteasome Biology: Chemistry and Bioengineering Insights

Proteasomal degradation provides the crucial machinery for maintaining cellular proteostasis. The biological origins of modulation or impairment of the function of proteasomal complexes may include changes in gene expression of their subunits, ubiquitin mutation, or indirect mechanisms arising from the overall impairment of proteostasis. However, changes in the physico-chemical characteristics of the cellular environment might also meaningfully contribute to altered performance. This review summarizes the effects of physicochemical factors in the cell, such as pH, temperature fluctuations, and reactions with the products of oxidative metabolism, on the function of the proteasome. Furthermore, evidence of the direct interaction of proteasomal complexes with protein aggregates is compared against the knowledge obtained from immobilization biotechnologies. In this regard, factors such as the structures of the natural polymeric scaffolds in the cells, their content of reactive groups or the sequestration of metal ions, and processes at the interface, are discussed here with regard to their influences on proteasomal function.

The Aspergillus fumigatus transcription factor RglT is important for gliotoxin biosynthesis and self-protection, and virulence

Aspergillus fumigatus is an opportunistic fungal pathogen that secretes an array of immune-modulatory molecules, including secondary metabolites (SMs), which contribute to enhancing fungal fitness and growth within the mammalian host. Gliotoxin (GT) is a SM that interferes with the function and recruitment of innate immune cells, which are essential for eliminating A. fumigatus during invasive infections. We identified a C6 Zn cluster-type transcription factor (TF), subsequently named RglT, important for A. fumigatus oxidative stress resistance, GT biosynthesis and self-protection. RglT regulates the expression of several gli genes of the GT biosynthetic gene cluster, including the oxidoreductase-encoding gene gliT, by directly binding to their respective promoter regions. Subsequently, RglT was shown to be important for virulence in a chemotherapeutic murine model of invasive pulmonary aspergillosis (IPA). Homologues of RglT and GliT are present in eurotiomycete and sordariomycete fungi, including the non-GT-producing fungus A. nidulans, where a conservation of function was described. Phylogenetically informed model testing led to an evolutionary scenario in which the GliT-based resistance mechanism is ancestral and RglT-mediated regulation of GliT occurred subsequently. In conclusion, this work describes the function of a previously uncharacterised TF in oxidative stress resistance, GT biosynthesis and self-protection in both GT-producing and non-producing Aspergillus species.

Proteasome Inhibitors in Cancer Therapy and their Relation to Redox Regulation

Redox homeostasis is important for the maintenance of cell survival. Under physiological conditions, redox system works in a balance and involves activation of many signaling molecules. Regulation of redox balance via signaling molecules is achieved by different pathways and proteasomal system is a key pathway in this process. Importance of proteasomal system on signaling pathways has been investigated for many years. In this direction, many proteasome targeting molecules have been developed. Some of them are already in the clinic for cancer treatment and some are still under investigation to highlight underlying mechanisms. Although there are many studies done, molecular mechanisms of proteasome inhibitors and related signaling pathways need more detailed explanations. This review aims to discuss redox status and proteasomal system related signaling pathways. In addition, cancer therapies targeting proteasomal system and their effects on redox-related pathways have been summarized.

Elastic titin properties and protein quality control in the aging heart

Cardiac aging affects the heart on the functional, structural, and molecular level and shares characteristic hallmarks with the development of chronic heart failure. Apart from age-dependent left ventricular hypertrophy and fibrosis that impairs diastolic function, diminished activity of cardiac protein-quality-control systems increases the risk of cytotoxic accumulation of defective proteins. Here, we studied the impact of cardiac aging on the sarcomeric protein titin by analyzing titin-based cardiomyocyte passive tension, titin modification and proteasomal titin turnover. We analyzed left ventricular samples from young (6 months) and old (20 months) wild-type mice and healthy human donor patients grouped according to age in young (17-50 years) and aged hearts (51-73 years). We found no age-dependent differences in titin isoform composition of mouse or human hearts. In aged hearts from mice and human we determined altered titin phosphorylation at serine residues S4010 and S4099 in the elastic N2B domain, but no significant changes in phosphorylation of S11878 and S12022 in the elastic PEVK region. Importantly, overall titin-based cardiomyocyte passive tension remained unchanged. In aged hearts, the calcium-activated protease calpain-1, which provides accessibility to ubiquitination by releasing titin from the sarcomere, showed decreased proteolytic activity. In addition, we observed a reduction in the proteasomal activities. Taken together, our data indicate that cardiac aging does not affect titin-based passive properties of the cardiomyocytes, but impairs protein-quality control, including titin, which may result in a diminished adaptive capacity of the aged myocardium.

Reactive Oxygen Species in Metabolic and Inflammatory Signaling

Reactive oxygen species (ROS) are well known for their role in mediating both physiological and pathophysiological signal transduction. Enzymes and subcellular compartments that typically produce ROS are associated with metabolic regulation, and diseases associated with metabolic dysfunction may be influenced by changes in redox balance. In this review, we summarize the current literature surrounding ROS and their role in metabolic and inflammatory regulation, focusing on ROS signal transduction and its relationship to disease progression. In particular, we examine ROS production in compartments such as the cytoplasm, mitochondria, peroxisome, and endoplasmic reticulum and discuss how ROS influence metabolic processes such as proteasome function, autophagy, and general inflammatory signaling. We also summarize and highlight the role of ROS in the regulation metabolic/inflammatory diseases including atherosclerosis, diabetes mellitus, and stroke. In order to develop therapies that target oxidative signaling, it is vital to understand the balance ROS signaling plays in both physiology and pathophysiology, and how manipulation of this balance and the identity of the ROS may influence cellular and tissue homeostasis. An increased understanding of specific sources of ROS production and an appreciation for how ROS influence cellular metabolism may help guide us in the effort to treat cardiovascular diseases.

Causative Links between Protein Aggregation and Oxidative Stress: A Review

Compelling evidence supports a tight link between oxidative stress and protein aggregation processes, which are noticeably involved in the development of proteinopathies, such as Alzheimer’s disease, Parkinson’s disease, and prion disease. The literature is tremendously rich in studies that establish a functional link between both processes, revealing that oxidative stress can be either causative, or consecutive, to protein aggregation. Because oxidative stress monitoring is highly challenging and may often lead to artefactual results, cutting-edge technical tools have been developed recently in the redox field, improving the ability to measure oxidative perturbations in biological systems. This review aims at providing an update of the previously known functional links between oxidative stress and protein aggregation, thereby revisiting the long-established relationship between both processes.

Regulation of Proteasome Activity by (Post-)transcriptional Mechanisms

Intracellular protein synthesis, folding, and degradation are tightly controlled processes to ensure proper protein homeostasis. The proteasome is responsible for the degradation of the majority of intracellular proteins, which are often targeted for degradation via polyubiquitination. However, the degradation rate of proteins is also affected by the capacity of proteasomes to recognize and degrade these substrate proteins. This capacity is regulated by a variety of proteasome modulations including (1) changes in complex composition, (2) post-translational modifications, and (3) altered transcription of proteasomal subunits and activators. Various diseases are linked to proteasome modulation and altered proteasome function. A better understanding of these modulations may offer new perspectives for therapeutic intervention. Here we present an overview of these three proteasome modulating mechanisms to give better insight into the diversity of proteasomes.

Mutations of Cys and Ser residues in the α5-subunit of the 20S proteasome from Saccharomyces cerevisiae affects gating and chronological lifespan

In addition to autophagy, proteasomes are critical for regulating intracellular protein levels and removing misfolded proteins. The 20S proteasome (20SPT), the central catalytic unit, is sometimes flanked by regulatory units at one or both ends. Additionally, proteosomal activation has been associated with increased lifespan in many organisms. Our group previously reported that the gating (open/closed) of the free 20S proteasome is redox controlled, and that S-glutathionylation of two Cys residues (Cys76 and Cys221) in the α5 subunit promotes gate opening. The present study constructed site-directed mutants of these Cys residues, and evaluated the effects these mutations have on proteosome gate opening and yeast cell survival. Notably, the double mutation of both Cys residues (Cys76 and Cys221) rendered the cells nonviable, whereas the lifespan of the yeast carrying the single mutations (α5-C76S or α5-C221S) was attenuated when compared to the wild type counterpart. Furthermore, it was found that α5-C76S or α5-C221S 20SPT were more likely to be found with the gate in a closed conformation. In contrast, a random α5-subunit double mutation (S35P/C221S) promoted gate opening, increased chronological lifespan and provided resistance to oxidative stress. The 20SPT core particle purified from the long-lived strain degraded model proteins (e.g., α-synuclein) more efficiently than preparations obtained from the wild-type counterpart, and also displayed an increased chymotrypsin-like activity. Mass spectrometric analyses of the C76S, C221S, S35P/C221S, S35P and S35P/C76S mutants provided evidence that the highly conserved Cys76 residue of the α5-subunit is the key determinant for gate opening and cellular survival. The present study reveals a sophisticated regulatory mechanism that controls gate opening, which appears to be based on the interactions among multiple residues within the α5-subunit, and consequently impacts the lifespan of yeast.

Heterologous expression of an alternative oxidase from Moniliophthora perniciosa in Saccharomyces cerevisiae: Antioxidant function and in vivo platform for the study of new drugs against witches' broom disease

The fungus Moniliophthora perniciosa is the causal agent of witches' broom disease (WBD), one of the most devastating diseases of cacao, the chocolate tree. Many strategies to control WBD have been tested so far, including the use of agrochemicals such as the strobilurins. Strobilurins are fungicides of the QoI family, and they are used in the control of a wide array of fungal diseases in many different crops, including cereals, field crops, fruits, tree nuts, and vegetables. These drugs act by specifically inhibiting fungal respiration at the Qo site of complex III, which is a component of the main mitochondrial respiratory chain. However, M. perniciosa is resistant to this family of chemicals. It has been postulated that this resistant phenotype is, at least in part, a result of the strong ability of this fungus to counteract the oxidative stress generated by the impairment of the main mitochondrial respiratory chain, through the activation of an alternative oxidase (Mp-AOX). To test this hypothesis, we expressed functional mitochondria-localized Mp-AOX in the model yeast Saccharomyces cerevisiae. We demonstrated that heterologous expression of Mp-AOX strongly inhibits hydrogen peroxide production by mitochondria. It also diminishes the total cell amount of oxidized glutathione (GSSG), resulting in a fifty-fold higher GSH/GSSG ratio in cells expressing Mp-AOX than in wild type cells. In addition, Mp-AOX activity decreases yeast growth rate and leads to low biomass production. Therefore, we propose the use of this heterologous expression system to direct the development of new inhibitors of fungal AOX by comparing the differences in optical density of Mp-AOX-expressing cells in the presence and absence of potential AOX inhibitors. Together, our results confirm the antioxidant role of Mp-AOX and provide an in vivo platform to be used in the screening of new fungicides based on Mp-AOX inhibition.

An adverse outcome pathway for parkinsonian motor deficits associated with mitochondrial complex I inhibition

Epidemiological studies have observed an association between pesticide exposure and the development of Parkinson's disease, but have not established causality. The concept of an adverse outcome pathway (AOP) has been developed as a framework for the organization of available information linking the modulation of a molecular target [molecular initiating event (MIE)], via a sequence of essential biological key events (KEs), with an adverse outcome (AO). Here, we present an AOP covering the toxicological pathways that link the binding of an inhibitor to mitochondrial complex I (i.e., the MIE) with the onset of parkinsonian motor deficits (i.e., the AO). This AOP was developed according to the Organisation for Economic Co-operation and Development guidelines and uploaded to the AOP database. The KEs linking complex I inhibition to parkinsonian motor deficits are mitochondrial dysfunction, impaired proteostasis, neuroinflammation, and the degeneration of dopaminergic neurons of the substantia nigra. These KEs, by convention, were linearly organized. However, there was also evidence of additional feed-forward connections and shortcuts between the KEs, possibly depending on the intensity of the insult and the model system applied. The present AOP demonstrates mechanistic plausibility for epidemiological observations on a relationship between pesticide exposure and an elevated risk for Parkinson's disease development.

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.

Metabolic Syndrome, Redox State, and the Proteasomal System

SIGNIFICANCE

Since the metabolic syndrome (MS) and pathologies associated with/resulting from metabolic dysregulations became a worldwide spreading and growing problem, the mechanisms mediating the according cellular changes got into a focus of interest. The ubiquitin-proteasomal system (UPS) is the main regulator of both the functional and dysfunctional protein pool of (not only) mammalian cells-thus, it is obvious that an impact on this system may also affect cellular functionality that directly depends on permanent regulation/adaption of the cell's proteostasis. However, the according research is still at the beginning. Recent Advances: It was also recently shown that maintaining a highly functional UPS positively correlates with increased health or even life span, thus modulation or restoration of UPS function may be an effective approach alleviating or even preventing MS detrimental consequences.

CRITICAL ISSUES

Even if many consequences of metabolic dysregulation such as a slight but chronic redox shift to a more oxidative state (i.e., a low-grade systemic inflammation that increases reactive oxygen species formation, lipid peroxidation, protein oxidation, formation of advanced glycation end products, glycosylation, S-glutathionylation, redox shifts, endoplasmic reticulum stress, unfolded protein response, expression of transcription factors, and release of cytokines) are already known to affect the highly redox-regulated UPS, experimental data about UPS changes that are directly mediated by glucotoxic and/or lipotoxic stress are still rarely published.

FUTURE DIRECTIONS

It may be taken into account that many MS-related pathologic changes result from UPS dysfunction or dysregulation. In this review, the main interface between MS effects and their impact on the UPS are highlighted since they may direct to new therapeutic approaches. Antioxid. Redox Signal. 25, 902-917.

The Oxygen Paradox, oxidative stress, and ageing

Professor Helmut Sies is being lauded in this special issue of Archives of Biochemistry & Biophysics, on the occasion of his retirement as Editor-in-Chief. There is no doubt that Helmut has exerted an enormously positive influence on this journal, the fields of Biochemistry & Biophysics in general, and the areas of free radical and redox biology & medicine in particular. Helmut Sies' many discoveries about peroxide metabolism, glutathione, glutathione peroxidases, singlet oxygen, carotenoids in general and lycopene in particular, and flavonoids, fill the pages of his more than 600 publications. In addition, he will forever be remembered for coining the term 'oxidative stress' that is so widely used (and sometimes abused) by most of his colleagues.

A Bowman-Birk inhibitor induces apoptosis in human breast adenocarcinoma through mitochondrial impairment and oxidative damage following proteasome 20S inhibition

Proteasome inhibitors are emerging as a new class of chemopreventive agents and have gained huge importance as potential pharmacological tools in breast cancer treatment. Improved understanding of the role played by proteases and their specific inhibitors in humans offers novel and challenging opportunities for preventive and therapeutic intervention. In this study, we demonstrated that the Bowman-Birk protease inhibitor from Vigna unguiculata seeds, named black-eyed pea trypsin/chymotrypsin Inhibitor (BTCI), potently suppresses human breast adenocarcinoma cell viability by inhibiting the activity of proteasome 20S. BTCI induced a negative growth effect against a panel of breast cancer cells, with a concomitant cytostatic effect at the G2/M phase of the cell cycle and an increase in apoptosis, as observed by an augmented number of cells at the sub-G1 phase and annexin V-fluorescin isothiocyanate (FITC)/propidium iodide (PI) staining. In contrast, BTCI exhibited no cytotoxic effect on normal mammary epithelial cells. Moreover, the increased levels of intracellular reactive oxygen species (ROS) and changes in the mitochondrial membrane potential in cells treated with BTCI indicated mitochondrial damage as a crucial cellular event responsible for the apoptotic process. The higher activity of caspase in tumoral cells treated with BTCI in comparison with untreated cells suggests that BTCI induces apoptosis in a caspase-dependent manner. BTCI affected NF-kB target gene expression in both non invasive and invasive breast cancer cell lines, with the effect highly pronounced in the invasive cells. An increased expression of interleukin-8 (IL-8) in both cell lines was also observed. Taken together, these results suggest that BTCI promotes apoptosis through ROS-induced mitochondrial damage following proteasome inhibition. These findings highlight the pharmacological potential and benefit of BTCI in breast cancer treatment.

The proteasome inhibitor lactacystin enhances GSH synthesis capacity by increased expression of antioxidant components in an Nrf2-independent, but p38 MAPK-dependent manner in rat colorectal carcinoma cells

Proteasome inhibitors may induce ER stress and oxidative stress, disrupt signaling pathways, and trigger apoptosis in several cancer cells. However, they are also reported to increase glutathione (GSH) synthesis and protect cells from oxidative stress. In the present study, we showed that the proteasome inhibitor lactacystin increased reactive oxygen species (ROS) and GSH levels after the treatment of HT-29 colorectal cancer cells. The increased GSH depended upon the activity of glutamate cysteine ligase (GCL), uptake of cystine/cysteine via the cystine/glutamate transporter [Formula: see text], and the activity of γ-glutamyltransferase (GGT). Increased transcription levels of the catalytic subunit of glutamate cysteine ligase (GCLC), the catalytic subunit xCT of [Formula: see text], and GGT were induced by lactacystin, although with different kinetics and stoichiometry. Lactacystin treatment also augmented protein levels of GCLC, xCT, and GGT, but significant levels were not detected until 48 h after initiation of lactacystin treatment. These increases in protein levels were dependent on the p38 MAPK pathway. Studies in cells transfected with siRNA against the transcription factor Nrf2 demonstrated that the promoter activities of xCT and GCLC, but not of GGT, depended on Nrf2. However, depletion of Nrf2 had no effect on lactacystin-induced upregulation of the GGT, GCLC, and xCT mRNA levels. Taken together, our results suggest that oxidative stress provoked by proteasomal inhibition results in the elevation of cellular GSH levels due to increased synthesis of GSH and uptake of cystine/cysteine. Following treatment with lactacystin, enhanced expression of antioxidant components involved in GSH homeostasis is p38 MAPK-dependent, but Nrf2-independent, resulting in increased GSH synthesis capacity.

The Keap1-Nrf2-antioxidant response element pathway: a review of its regulation by melatonin and the proteasome

Both melatonin and proteasome inhibitors upregulate antioxidant enzymes including superoxide dismutase (SOD), glutathione peroxidase (GP), hemoxygenase 1 (HO-1), and NADPH:quinone oxidoreductase (NQO1). Recent evidence suggests that the antioxidant action of both melatonin and proteasome inhibitors involves the Keap1-ARE (Keap1 antioxidant response element) pathway via the upregulation of Nrf2. Melatonin and proteasome inhibitors suppress the degradation of Nrf2 and also enhance its nuclear translocation. In the nucleus Nrf2, together with a cofactor, stimulates the transcription of antioxidant enzymes and detoxifying enzymes. The ligase (E3) complex (Keap1-Cul3-Rbx1) responsible for ubiquitinating Nrf2, prior to proteasomal degradation, also ubiquitinates IkB kinase and the antiapoptotic factor Bcl-2, and possibly additional proteins. In various systems, NF-κB, which is inhibited by IkBα, is downregulated by proteasome inhibitors as well as by melatonin. Similarly in leukemic cells, Bcl-2 is down-regulated by the proteasome inhibitor, bortezomib, and also by melatonin. Thus melatonin administration modulates the activity of three separate substrates of the Keap1-Cul3-Rbx1 ubiquitin ligase. These facts could be accounted for by the hypothesis that melatonin interacts with the ubiquitin ligase complex or, more likely, by the hypothesis that melatonin acts as a proteasome inhibitor. A recent study documented that melatonin acts as a proteasome inhibitor in cancer cells as well as inhibiting chymotrypsin-like activity in cell-free systems of these cells. Further studies, however, are needed to clarify the interaction of melatonin and the ubiquitin-proteasome system as they relate to oxidative stress.

Emerging Therapeutic Strategies for Overcoming Proteasome Inhibitor Resistance

The debut of the proteasome inhibitor bortezomib (Btz; Velcade®) radically and immediately improved the treatment of multiple myeloma (MM), an incurable malignancy of the plasma cell. Therapeutic resistance is unavoidable, however, and represents a major obstacle to maximizing the clinical potential of the drug. To address this challenge, studies have been conducted to uncover the molecular mechanisms driving Btz resistance and to discover new targeted therapeutic strategies and combinations that restore Btz activity. This review discusses the literature describing molecular adaptations that confer Btz resistance with a primary disease focus on MM. Also discussed are the most recent advances in therapeutic strategies that overcome resistance, approaches that include redox-modulating agents, murine double minute 2 inhibitors, therapeutic monoclonal antibodies, and new epigenetic-targeted drugs like bromodomain and extra terminal domain inhibitors.

Targeting the ubiquitin-proteasome system in heart disease: the basis for new therapeutic strategies

SIGNIFICANCE

Novel therapeutic strategies to treat heart failure are greatly needed. The ubiquitin-proteasome system (UPS) affects the structure and function of cardiac cells through targeted degradation of signaling and structural proteins. This review discusses both beneficial and detrimental consequences of modulating the UPS in the heart.

RECENT ADVANCES

Proteasome inhibitors were first used to test the role of the UPS in cardiac disease phenotypes, indicating therapeutic potential. In early cardiac remodeling and pathological hypertrophy with increased proteasome activities, proteasome inhibition prevented or restricted disease progression and contractile dysfunction. Conversely, enhancing proteasome activities by genetic manipulation, pharmacological intervention, or ischemic preconditioning also improved the outcome of cardiomyopathies and infarcted hearts with impaired cardiac and UPS function, which is, at least in part, caused by oxidative damage.

CRITICAL ISSUES

An understanding of the UPS status and the underlying mechanisms for its potential deregulation in cardiac disease is critical for targeted interventions. Several studies indicate that type and stage of cardiac disease influence the dynamics of UPS regulation in a nonlinear and multifactorial manner. Proteasome inhibitors targeting all proteasome complexes are associated with cardiotoxicity in humans. Furthermore, the type and dosage of proteasome inhibitor impact the pathogenesis in nonuniform ways.

FUTURE DIRECTIONS

Systematic analysis and targeting of individual UPS components with established and innovative tools will unravel and discriminate regulatory mechanisms that contribute to and protect against the progression of cardiac disease. Integrating this knowledge in drug design may reduce adverse effects on the heart as observed in patients treated with proteasome inhibitors against noncardiac diseases, especially cancer.

Role of the ubiquitin-proteasome system in cardiac dysfunction of adipose triglyceride lipase-deficient mice

Systemic deletion of the gene encoding for adipose triglyceride lipase (ATGL) in mice leads to severe cardiac dysfunction due to massive accumulation of neutral lipids in cardiomyocytes. Recently, impaired peroxisome proliferator-activated receptor α (PPARα) signaling has been described to substantially contribute to the observed cardiac phenotype. Disturbances of the ubiquitin-proteasome system (UPS) have been implicated in numerous cardiac diseases including cardiomyopathy, ischemic heart disease, and heart failure. The objective of the present study was to investigate the potential role of UPS in cardiac ATGL deficiency. Our results demonstrate prominent accumulation of ubiquitinated proteins in hearts of ATGL-deficient mice, an effect that was abolished upon cardiomyocyte-directed overexpression of ATGL. In parallel, cardiac protein expression of the ubiquitin-activating enzyme E1a, which catalyzes the first step of the ubiquitination cascade, was significantly upregulated in ATGL-deficient hearts. Dysfunction of the UPS was accompanied by activation of NF-κB signaling. Moreover, the endoplasmic reticulum (ER)-resident chaperon protein disulfide isomerase was significantly upregulated in ATGL-deficient hearts. Chronic treatment of ATGL-deficient mice with the PPARα agonist Wy14,643 improved proteasomal function, prevented NF-κB activation and decreased oxidative stress. In summary, our data point to a hitherto unrecognized link between proteasomal function, PPARα signaling and cardiovascular disease.

20S proteasome activity is modified via S-glutathionylation based on intracellular redox status of the yeast Saccharomyces cerevisiae: implications for the degradation of oxidized proteins

Protein S-glutathionylation is a post-translational modification that controls many cellular pathways. Recently, we demonstrated that the α5-subunit of the 20S proteasome is S-glutathionylated in yeast cells grown to the stationary phase in rich medium containing glucose, stimulating 20S core gate opening and increasing the degradation of oxidized proteins. In the present study, we evaluated the correlation between proteasomal S-glutathionylation and the intracellular redox status. The redox status was controlled by growing yeast cells in distinct carbon sources which induced respiratory (glycerol/ethanol) or fermentative (glucose) metabolism. Cells grown under glycerol/ethanol displayed higher reductive power when compared to cells grown under glucose. When purified from cells grown in glucose, 20S proteasome α5-subunit exhibited an intense anti-glutathione labeling. A higher frequency of the open catalytic chamber gate was observed in the S-glutathionylated preparations as demonstrated by transmission electron microscopy. Therefore, cells that had been grown in glucose displayed an increased ability to degrade oxidized proteins. The results of the present study suggest that 20S proteasomal S-glutathionylation is a relevant adaptive response to oxidative stress that is capable to sense the intracellular redox environment, leading to the removal of oxidized proteins via a process that is not dependent upon ubiquitylation and ATP consumption.

Peptides that activate the 20S proteasome by gate opening increased oxidized protein removal and reduced protein aggregation

The proteasome is a multicatalytic protease that is responsible for the degradation of the majority of intracellular proteins. Its role is correlated with several major regulatory pathways that are involved in cell cycle control, signaling, and antigen presentation, as well as in the removal of oxidatively damaged proteins. Although several proteasomal catalytic inhibitors have been described, very few activators have been reported to date. Some reports in the literature highlight the cellular protective effects of proteasome activation against oxidative stress and its effect on increased life span. In this work, we describe a peptide named proteasome-activating peptide 1 (PAP1), which increases the chymotrypsin-like proteasomal catalytic activity and, consequently, proteolytic rates both in vitro and in culture. PAP1 proteasomal activation is mediated by the opening of the proteasomal catalytic chamber. We also demonstrate that the observed proteasomal activation protected cells from oxidative stress; further, PAP1 prevented protein aggregation in a cellular model of amyotrophic lateral sclerosis. The role of 20SPT gate opening underlying protection against oxidative stress was also explored in yeast cells. The present data indicate the importance of proteasomal activators as potential drugs for the treatment of pathologies associated with the impaired removal of damaged proteins, which is observed in many neurodegenerative diseases.

Effects of an anticarcinogenic Bowman-Birk protease inhibitor on purified 20S proteasome and MCF-7 breast cancer cells

Proteasome inhibitors have been described as an important target for cancer therapy due to their potential to regulate the ubiquitin-proteasome system in the degradation pathway of cellular proteins. Here, we reported the effects of a Bowman-Birk-type protease inhibitor, the Black-eyed pea Trypsin/Chymotrypsin Inhibitor (BTCI), on proteasome 20S in MCF-7 breast cancer cells and on catalytic activity of the purified 20S proteasome from horse erythrocytes, as well as the structural analysis of the BTCI-20S proteasome complex. In vitro experiments and confocal microscopy showed that BTCI readily crosses the membrane of the breast cancer cells and co-localizes with the proteasome in cytoplasm and mainly in nucleus. Indeed, as indicated by dynamic light scattering, BTCI and 20S proteasome form a stable complex at temperatures up to 55°C and at neutral and alkaline pHs. In complexed form, BTCI strongly inhibits the proteolytic chymotrypsin-, trypsin- and caspase-like activities of 20S proteasome, indicated by inhibition constants of 10⁻⁷ M magnitude order. Besides other mechanisms, this feature can be associated with previously reported cytostatic and cytotoxic effects of BTCI in MCF-7 breast cancer cells by means of apoptosis.

Redox regulation of the proteasome via S-glutathionylation

The proteasome is a multimeric and multicatalytic intracellular protease responsible for the degradation of proteins involved in cell cycle control, various signaling processes, antigen presentation, and control of protein synthesis. The central catalytic complex of the proteasome is called the 20S core particle. The majority of these are flanked on one or both sides by regulatory units. Most common among these units is the 19S regulatory unit. When coupled to the 19S unit, the complex is termed the asymmetric or symmetric 26S proteasome depending on whether one or both sides are coupled to the 19S unit, respectively. The 26S proteasome recognizes poly-ubiquitinylated substrates targeted for proteolysis. Targeted proteins interact with the 19S unit where they are deubiquitinylated, unfolded, and translocated to the 20S catalytic chamber for degradation. The 26S proteasome is responsible for the degradation of major proteins involved in the regulation of the cellular cycle, antigen presentation and control of protein synthesis. Alternatively, the proteasome is also active when dissociated from regulatory units. This free pool of 20S proteasome is described in yeast to mammalian cells. The free 20S proteasome degrades proteins by a process independent of poly-ubiquitinylation and ATP consumption. Oxidatively modified proteins and other substrates are degraded in this manner. The 20S proteasome comprises two central heptamers (β-rings) where the catalytic sites are located and two external heptamers (α-rings) that are responsible for proteasomal gating. Because the 20S proteasome lacks regulatory units, it is unclear what mechanisms regulate the gating of α-rings between open and closed forms. In the present review, we discuss 20S proteasomal gating modulation through a redox mechanism, namely, S-glutathionylation of cysteine residues located in the α-rings, and the consequence of this post-translational modification on 20S proteasomal function.

Oxidative stress, neurodegeneration, and the balance of protein degradation and protein synthesis

Oxidative stress occurs in a variety of disease settings and is strongly linked to the development of neuron death and neuronal dysfunction. Cells are equipped with numerous pathways to prevent the genesis, as well as the consequences, of oxidative stress in the brain. In this review we discuss the various forms and sources of oxidative stress in the brain and briefly discuss some of the complexities in detecting the presence of oxidative stress. We then focus the review on the interplay between the diverse cellular proteolytic pathways and their roles in regulating oxidative stress in the brain. Additionally, we discuss the involvement of protein synthesis in regulating the downstream effects of oxidative stress. Together, these components of the review demonstrate that the removal of damaged proteins by effective proteolysis and the synthesis of new and protective proteins are vital in the preservation of brain homeostasis during periods of increased levels of reactive oxygen species. Last, studies from our laboratory and others have demonstrated that protein synthesis is intricately linked to the rates of protein degradation, with impairment of protein degradation sufficient to decrease the rates of protein synthesis, which has important implications for successfully responding to periods of oxidative stress. Specific neurodegenerative diseases, including Alzheimer disease, Parkinson disease, amyotrophic lateral sclerosis, and stroke, are discussed in this context. Taken together, these findings add to our understanding of how oxidative stress is effectively managed in the healthy brain and help elucidate how impairments in proteolysis and/or protein synthesis contribute to the development of neurodegeneration and neuronal dysfunction in a variety of clinical settings.

Nuclear glutathione

Glutathione (GSH) is a linchpin of cellular defences in plants and animals with physiologically-important roles in the protection of cells from biotic and abiotic stresses. Moreover, glutathione participates in numerous metabolic and cell signalling processes including protein synthesis and amino acid transport, DNA repair and the control of cell division and cell suicide programmes. While it is has long been appreciated that cellular glutathione homeostasis is regulated by factors such as synthesis, degradation, transport, and redox turnover, relatively little attention has been paid to the influence of the intracellular partitioning on glutathione and its implications for the regulation of cell functions and signalling. We focus here on the functions of glutathione in the nucleus, particularly in relation to physiological processes such as the cell cycle and cell death. The sequestration of GSH in the nucleus of proliferating animal and plant cells suggests that common redox mechanisms exist for DNA regulation in G1 and mitosis in all eukaryotes. We propose that glutathione acts as "redox sensor" at the onset of DNA synthesis with roles in maintaining the nuclear architecture by providing the appropriate redox environment for the DNA replication and safeguarding DNA integrity. In addition, nuclear GSH may be involved in epigenetic phenomena and in the control of nuclear protein degradation by nuclear proteasome. Moreover, by increasing the nuclear GSH pool and reducing disulfide bonds on nuclear proteins at the onset of cell proliferation, an appropriate redox environment is generated for the stimulation of chromatin decompaction. This article is part of a Special Issue entitled Cellular functions of glutathione.

Redox control of 20S proteasome gating

The proteasome is the primary contributor in intracellular proteolysis. Oxidized or unstructured proteins can be degraded via a ubiquitin- and ATP-independent process by the free 20S proteasome (20SPT). The mechanism by which these proteins enter the catalytic chamber is not understood thus far, although the 20SPT gating conformation is considered to be an important barrier to allowing proteins free entrance. We have previously shown that S-glutathiolation of the 20SPT is a post-translational modification affecting the proteasomal activities.

AIMS

The goal of this work was to investigate the mechanism that regulates 20SPT activity, which includes the identification of the Cys residues prone to S-glutathiolation.

RESULTS

Modulation of 20SPT activity by proteasome gating is at least partially due to the S-glutathiolation of specific Cys residues. The gate was open when the 20SPT was S-glutathiolated, whereas following treatment with high concentrations of dithiothreitol, the gate was closed. S-glutathiolated 20SPT was more effective at degrading both oxidized and partially unfolded proteins than its reduced form. Only 2 out of 28 Cys were observed to be S-glutathiolated in the proteasomal α5 subunit of yeast cells grown to the stationary phase in glucose-containing medium.

INNOVATION

We demonstrate a redox post-translational regulatory mechanism controlling 20SPT activity.

CONCLUSION

S-glutathiolation is a post-translational modification that triggers gate opening and thereby activates the proteolytic activities of free 20SPT. This process appears to be an important regulatory mechanism to intensify the removal of oxidized or unstructured proteins in stressful situations by a process independent of ubiquitination and ATP consumption. Antioxid. Redox Signal. 16, 1183-1194.

Physiological and pathological role of the ubiquitin-proteasome system in the vascular smooth muscle cell

Vascular smooth muscle cell (VSMC) plasticity implies a capacity for rapid change and adaptability through processes requiring protein turnover. The ubiquitin-proteasome system (UPS) is at the core of protein turnover as the main pathway for the degradation of proteins related to cell-cycle regulation, signalling, apoptosis, and differentiation. This review briefly addresses some structural UPS aspects under the perspective of VSMC (patho)biology. The UPS loss-of-function promotes direct cell effects and many indirect effects related to the adaptation to apoptosis/survival signalling, oxidative stress, and endoplasmic reticulum stress. The UPS regulates redox homeostasis and is redox-regulated. Also, the UPS closely interacts with endoplasmic reticulum (ER) homeostasis as the effector of un/misfolded protein degradation, and ER stress is strongly involved in atherosclerosis. Inhibition of cell cycle-controlling ubiquitin ligases or the proteasome reduces VSMC proliferation and prevents modulation of their synthetic phenotype. Proteasome inhibition also strongly promotes VSMC apoptosis and reduces neointima. In atherosclerosis models, proteasome inhibitors display vasculoprotective effects and reduce inflammation. However, worsening of atherosclerosis or vascular dysfunction has also been reported. Proteasome inhibitors sensitize VSMC to increased ER stress-mediated cell death and suppress unfolded protein response signalling. Taken together, these observations show that the UPS has powerful effects in the control of VSMC phenotype and survival signalling. However, more profound knowledge of mechanisms is needed in order to render the UPS an operational therapeutic target.

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.

Oxidative stress-mediated regulation of proteasome complexes

Oxidative stress has been implicated in aging and many human diseases, notably neurodegenerative disorders and various cancers. The reactive oxygen species that are generated by aerobic metabolism and environmental stressors can chemically modify proteins and alter their biological functions. Cells possess protein repair pathways to rescue oxidized proteins and restore their functions. If these repair processes fail, oxidized proteins may become cytotoxic. Cell homeostasis and viability are therefore dependent on the removal of oxidatively damaged proteins. Numerous studies have demonstrated that the proteasome plays a pivotal role in the selective recognition and degradation of oxidized proteins. Despite extensive research, oxidative stress-triggered regulation of proteasome complexes remains poorly defined. Better understanding of molecular mechanisms underlying proteasome function in response to oxidative stress will provide a basis for developing new strategies aimed at improving cell viability and recovery as well as attenuating oxidation-induced cytotoxicity associated with aging and disease. Here we highlight recent advances in the understanding of proteasome structure and function during oxidative stress and describe how cells cope with oxidative stress through proteasome-dependent degradation pathways.

Thiol-protease oxidation in age-related neuropathology

Increased oxidative stress is a hallmark of every major neurodegenerative disease that has been studied. Numerous biomarkers of oxidative stress have been found, indicating that waves of oxidation had, at one time or another, overwhelmed antioxidant defenses, leaving behind a host of oxidized DNA, lipids, and proteins in their path. Although some level of oxidation may be beneficial, perhaps mediated by a hormetic response, the extent and types of oxidation detected in neuropathological states would suggest that oxidative stress contributes to a loss of homeostasis and cellular dysfunction. Although there are many targets of oxidants, this review emphasizes protein oxidation with a focus on an important group of redox-sensitive enzymes, the thiol-proteases. Both the direct and the indirect effects of oxidation and their potential importance in neurodegeneration are considered.

S-glutathionylation: from molecular mechanisms to health outcomes

Redox homeostasis governs a number of critical cellular processes. In turn, imbalances in pathways that control oxidative and reductive conditions have been linked to a number of human disease pathologies, particularly those associated with aging. Reduced glutathione is the most prevalent biological thiol and plays a crucial role in maintaining a reduced intracellular environment. Exposure to reactive oxygen or nitrogen species is causatively linked to the disease pathologies associated with redox imbalance. In particular, reactive oxygen species can differentially oxidize certain cysteine residues in target proteins and the reversible process of S-glutathionylation may mitigate or mediate the damage. This post-translational modification adds a tripeptide and a net negative charge that can lead to distinct structural and functional changes in the target protein. Because it is reversible, S-glutathionylation has the potential to act as a biological switch and to be integral in a number of critical oxidative signaling events. The present review provides a comprehensive account of how the S-glutathionylation cycle influences protein structure/function and cellular regulatory events, and how these may impact on human diseases. By understanding the components of this cycle, there should be opportunities to intervene in stress- and aging-related pathologies, perhaps through prevention and diagnostic and therapeutic platforms.

Regulatory functions of glutathione S-transferase P1-1 unrelated to detoxification

Glutathione S-transferase P1-1 (GSTP) is one member of the family of GSTs and is ubiquitously expressed in human tissues. The literature is replete with reports of high levels of GSTP linked either with cancer incidence or drug resistance, and yet no entirely cogent explanation for these correlations exists. The catalytic detoxification properties of the GST isozyme family have been a primary research focus for the last four decades. However, it has become apparent that they have undergone structural and functional convergence where evolutionary selective pressures have favored the emergence of noncatalytic properties of GSTP that has imbued this isozyme with expanded biological importance. For example, GSTP has now been linked with two cell-signaling functions that are critical to survival. Through protein:protein interactions, GSTP can sequester c-jun N-terminal kinase (JNK) and act as a negative regulator of this stress kinase. Pharmacologically, this activity has been linked with the activity of GSTP inhibitors in stimulating myeloproliferation. In addition, GSTP is linked with the forward S-glutathionylation reaction, a post-translational modification that impacts the function/activity of a number of proteins. Catalytic reversal of S-glutathionylation is well characterized, but the role of GSTP in catalyzing the forward reaction contributes to the "glutathionylation cycle." Moreover, GSTP is itself susceptible to S-glutathionylation, providing an autoregulatory loop for the cycle. Because oxidative stress regulates both S-glutathionylation and JNK-signaling pathways, such links may help to explain the aberrant patterns of GSTP expression in the cancer phenotype. As such, there is an ongoing preclinical and clinical platform of drug discovery and development around GSTP.

Role of Glutathione in the Multidrug Resistance in Cancer

Multidrug resistance is the main problem in anticancer therapy. Cancer cells use many defense strategies in order to survive chemotherapy. Among known multidrug resistance mechanisms the most important are: drug detoxification inside the cell using II phase detoxifying enzymes and active transport of the drug to the extracellular environment. Cancer cells may be also less sensitive to proapoptotic signals and have different intracellular drug distribution, which makes them more resistant to anticancer drugs. Role of glutathione in multidrug resistance is the object of interest of many scientists, however, defining it’s function in these processes still remains a challenge. In this paper, properties of glutathione and it’s role in multidrug resistance in cancer cells were described.

Changes in Activity and Kinetic Properties of the Proteasome in Different Rat Organs during Development and Maturation

The proteasome is considered the most important proteolytic system for removal of damaged proteins with aging. Using fluorogenic peptide substrates, the chymotrypsin-like, the trypsin-like, and the peptidylglutamyl peptidase activities of the proteasome were measured in the soluble fractions of liver, brain, and lens rat homogenates. Specific activity was significantly decreased in liver and brain homogenates with maturation of the animal, that is, from newborn (7 days old) to fertile rats (2–4 months old). Rat lens homogenate exhibited an increase in activity with maturation and also with aging. Chymotrypsin-like activity was stimulated by calcium and this proteolytic activity was significantly decreased with maturation of the rat brain. The Michaelis-Menten constant (K_m) increased with age in rat liver and lens, indicating a loss of affinity for its substrates by the proteasome in the animal with maturation and aging. The present data suggest that the loss of function of the proteasome with maturation may be due to structural changes of the proteasome or a decreased content of regulatory components.

In AbetaPP-overexpressing cultured human muscle fibers proteasome inhibition enhances phosphorylation of AbetaPP751 and GSK3beta activation: effects mitigated by lithium and apparently relevant to sporadic inclusion-body myositis

Muscle fiber degeneration in sporadic inclusion-body myositis (s-IBM) is characterized by accumulation of multiprotein aggregates, including aggregated amyloid-beta (Abeta)-precursor protein 751 (AbetaPP751), Abeta, phosphorylated tau, and other 'Alzheimer-characteristic' proteins. Proteasome inhibition is an important component of the s-IBM pathogenesis. In brains of Alzheimer's disease (AD) patients and AD transgenic-mouse models, phosphorylation of neuronal AbetaPP695 (p-AbetaPP) on Thr668 (equivalent to T724 of AbetaPP751) is considered detrimental because it increases generation of cytotoxic Abeta and induces tau phosphorylation. Activated glycogen synthase kinase3beta (GSK3beta) is involved in phosphorylation of both AbetaPP and tau. Lithium, an inhibitor of GSK3beta, was reported to reduce levels of both the total AbetaPP and p-AbetaPP in AD animal models. In relation to s-IBM, we now show for the first time that (1) In AbetaPP-overexpressing cultured human muscle fibers (human muscle culture IBM model: (a) proteasome inhibition significantly increases GSK3beta activity and AbetaPP phosphorylation, (b) treatment with lithium decreases (i) phosphorylated-AbetaPP, (ii) total amount of AbetaPP, (iii) Abeta oligomers, and (iv) GSK3beta activity; and (c) lithium improves proteasome function. (2) In biopsied s-IBM muscle fibers, GSK3beta is significantly activated and AbetaPP is phosphorylated on Thr724. Accordingly, treatment with lithium, or other GSK3beta inhibitors, might benefit s-IBM patients.

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.

S-glutathionylation: indicator of cell stress and regulator of the unfolded protein response

The specific posttranslational modification of protein cysteine residues by the addition of the tripeptide glutathione is termed S-glutathionylation. This process is promoted by oxidative and nitrosative stress but also occurs in unstressed cells. Altered levels of S-glutathionylation in some proteins have been associated with numerous pathologies, many of which have been linked to redox stress in the endoplasmic reticulum (ER). Proper protein folding is dependent upon controlled redox conditions within the ER, and it seems that ER conditions can in turn affect rates of S-glutathionylation. This article seeks to bring together the ways through which these processes are interrelated and considers the implications of these interrelationships upon therapeutic approaches to disease.

The proteasome and intracellular redox status: implications for apoptotic regulation in lens epithelial cells

PURPOSE

This study aimed to investigate redox regulation of the proteasome as well as the effect of proteasome inhibition on intracellular oxidative status and apoptosis.

METHODS

Oxidative stress was induced in cultured human lens epithelial cells (HLECs) and intact mouse lenses by 100 microM H2O2. HLECs were also exposed to the reduced and the oxidized forms of glutathione (GSH/GSSG) and the reducing agent dithiotreitol (DTT). The chymotrypsin-like, the trypsin-like, and the peptidylglutamyl peptidase activities of the proteasome were measured using synthetic fluorogenic substrates. Superoxide as well as peroxide production, mitochondrial membrane potential, and the level of GSH was measured in HLECs after proteasome inhibition by MG-132 or lactacystin. Apoptosis was determined by measuring caspase-3 activation and by studying apoptotic nuclei after staining with Hoechst 33342.

RESULTS

All three peptidase activities of the proteasome were inhibited by 100 microM H2O2 and by the oxidized form of glutathione (GSSG), whereas the reduced form (GSH) stimulated chymotrypsin-like and peptidylglutamyl peptidase activities in HLECs lysates. Intact mouse lenses exposed to 100 microM H2O2 exhibited loss of transparency and trends of decreased chymotrypsin-like proteasome activity as well as decreased GSH levels. Inhibition of the proteasome in cultured HLECs caused significant increase in apoptosis and disturbed intracellular redox balance. Simultaneous addition of exogenous GSH completely abolished the increased apoptosis seen after MG-132 treatment.

CONCLUSIONS

This study supports the hypothesis that intracellular proteolytic and oxidative regulatory systems are tightly coupled. The current data also indicate that apoptosis by proteasome inhibition is mediated through oxidative mechanisms.

Role of proteasomes in cellular regulation

The 26S proteasome is the key enzyme of the ubiquitin-dependent pathway of protein degradation. This energy-dependent nanomachine is composed of a 20S catalytic core and associated regulatory complexes. The eukaryotic 20S proteasomes demonstrate besides several kinds of peptidase activities, the endoribonuclease, protein-chaperone and DNA-helicase activities. Ubiquitin-proteasome pathway controls the levels of the key regulatory proteins in the cell and thus is essential for life and is involved in regulation of crucial cellular processes. Proteasome population in the cell is structurally and functionally heterogeneous. These complexes are subjected to tightly organized regulation, particularly, to a variety of posttranslational modifications. In this review we will summarize the current state of knowledge regarding proteasome participation in the control of cell cycle, apoptosis, differentiation, modulation of immune responses, reprogramming of these particles during these processes, their heterogeneity and involvement in the main levels of gene expression.

Protective role for nitric oxide during the endoplasmic reticulum stress response in pancreatic beta-cells

Higher requirements for disulfide bond formation in professional secretory cells may affect intracellular redox homeostasis, particularly during an endoplasmic reticulum (ER) stress response. To assess this hypothesis, we investigated the effects of the ER stress response on the major redox couple (GSH/GSSG), endogenous ROS production, expression of genes involved in ER oxidative protein folding, general antioxidant defense, and thiol metabolism by use of the well-validated MIN6 beta-cell as a model and mouse islets. The data revealed that glucose concentration-dependent decreases in the GSH/GSSG ratio were further decreased significantly by ER-derived oxidative stress induced by inhibiting ER-associated degradation with the specific proteasome inhibitor lactacystin (10 microM) in mouse islets. Notably, minimal cell death was observed during 12-h treatments. This was likely attributed to the upregulation of genes encoding the rate limiting enzyme for glutathione synthesis (gamma-glutamylcysteine ligase), as well as genes involved in antioxidant defense (glutathione peroxidase, peroxiredoxin-1) and ER protein folding (Grp78/BiP, PDI, Ero1). Gene expression and reporter assays with a NO synthase inhibitor (Nomega-nitro-L-arginine methyl ester, 1-10 mM) indicated that endogenous NO production was essential for the upregulation of several ER stress-responsive genes. Specifically, gel shift analyses demonstrate NO-independent binding of the transcription factor NF-E2-related factor to the antioxidant response element Gclc-ARE4 in MIN6 cells. However, endogenous NO production was necessary for activation of Gclc-ARE4-driven reporter gene expression. Together, these data reveal a distinct protective role for NO during the ER stress response, which helps to dissipate ROS and promote beta-cell survival.

Age-dependent inhibition of proteasome chymotrypsin-like activity in the retina

The proteasome plays a fundamental role in processes essential for cell viability. A loss in proteasome function has been associated with aging, as well as a number of age-related diseases. Defining the mechanism(s) behind this loss in function will add important information regarding the molecular basis for aging. In the current study, we performed an age-based comparison of proteasome function and composition of subunits and regulatory proteins in the neural retina and retinal pigment epithelium (RPE) in Fischer 344 rats. In the RPE, there was no age-dependent difference in activity, subunit composition, or content of proteasome regulators, PA28 and PA700. In contrast, the aged neural retina demonstrated a significant reduction in the chymotrypsin-like activity and decreased degradation of both casein and casein modified by 4-hydroxynonenal. This loss in function could not be explained by differences in subunit composition, content of PA28 and PA700, or reversible modification of cysteine residues. To begin investigating the molecular basis for the age-associated decrement in proteasome function, we modified the cysteine residues in proteasome from young rats with the sulfhydryl-reactive chemical N-ethylmaleimide. We observed inhibition of the chymotrypsin-like activity and decreased degradation of casein that was comparable to that seen in aged retinas. Thus, chemical modification of cysteine provides an in vitro method that partially recapitulates aging proteasome. Further studies are required to confirm irreversible modification of functionally significant cysteine as a potential mechanism behind the age-related loss in proteasome function.

Proteasome inhibition and aggresome formation in sporadic inclusion-body myositis and in amyloid-beta precursor protein-overexpressing cultured human muscle fibers

The 26S proteasome system is involved in eliminating various proteins, including ubiquitinated misfolded/unfolded proteins, and its inhibition results in cellular accumulation of protein aggregates. Intramuscle-fiber ubiquitinated multiprotein-aggregates are characteristic of sporadic inclusion-body myositis (s-IBM) muscle fibers. Two major types of aggregates exist, containing either amyloid-beta (Abeta) or phosphorylated tau (p-tau). We have now asked whether abnormalities of the 26S proteasome contribute to s-IBM pathogenesis and whether the multiprotein aggregates have features of aggresomes. Using cultured human muscle fibers we also studied the effect of amyloid-beta precursor protein (AbetaPP) overexpression on proteasome function and the influence of proteasome inhibition on aggresome formation. We report that in s-IBM muscle biopsies 26S proteasome subunits were immunodetected in the gamma-tubulin-associated aggresomes, which also contained Abeta, p-tau, ubiquitin, and HSP70. In addition, a) expression of proteasome subunits was greatly increased, b) the 20Salpha proteasome subunit co-immunoprecipitated with AbetaPP/Abeta, and c) the three major proteasomal proteolytic activities were reduced. In cultured muscle fibers, AbetaPP-overexpressing fibers displayed diminished proteasomal proteolytic activities, and addition of proteasome inhibitor strikingly increased aggresome formation. Accordingly, proteasome dysfunction in s-IBM muscle fibers may play a role in accumulation of misfolded, potentially cytotoxic proteins and may be induced by increased intracellular AbetaPP/Abeta.

Proteasome structures affected by ionizing radiation

Exposure of cells to ionizing radiation slows the rate of degradation of substrates through the proteasome. Because the 26S proteasome degrades most short-lived cellular proteins, changes in its activity might significantly, and selectively, alter the life span of many signaling proteins and play a role in promoting the biological consequences of radiation exposure, such as cell cycle arrest, DNA repair, and apoptosis. Experiments were therefore undertaken to identify the radiation target that is associated with the proteasome. Regardless of whether they were irradiated before or after extraction and purification from human prostate cancer PC3 cells, 26S proteasomes remained intact but showed a rapid 30% to 50% dose-independent decrease in their three major enzymatic activities following exposure to 1 to 20 Gy. There was no effect on 20S proteasomes, suggesting that the radiation-sensitive target is located in the 19S cap of the 26S proteasome, rather than in the enzymatically active core. Because the base of the 19S cap contains an ATPase ring that mediates substrate unfolding, pore opening, and translocation of substrates into the catalytic chamber, we examined whether the ATPase activity of purified 26S proteasomes was affected. In fact, in vitro irradiation of proteasomes enhanced their ATPase activity. Furthermore, pretreatment with low concentrations of the free radical scavenger tempol was able to prevent both the radiation-induced decrease in proteolytic activity and the increase in ATP utilization, indicating that free radicals are mediators of these radiation-induced phenomena. Finally, we have shown that cell irradiation results in the accumulation of proteasome substrates: polyubiquitinated proteins and ornithine decarboxylase, indicating that the observed decrease in proteasome function is physiologically relevant.

Stress-induced protein S-glutathionylation in Arabidopsis

S-Glutathionylation (thiolation) is a ubiquitous redox-sensitive and reversible modification of protein cysteinyl residues that can directly regulate their activity. While well established in animals, little is known about the formation and function of these mixed disulfides in plants. After labeling the intracellular glutathione pool with [35S]cysteine, suspension cultures of Arabidopsis (Arabidopsis thaliana ecotype Columbia) were shown to undergo a large increase in protein thiolation following treatment with the oxidant tert-butylhydroperoxide. To identify proteins undergoing thiolation, a combination of in vivo and in vitro labeling methods utilizing biotinylated, oxidized glutathione (GSSG-biotin) was developed to isolate Arabidopsis proteins/protein complexes that can be reversibly glutathionylated. Following two-dimensional polyacrylamide gel electrophoresis and matrix-assisted laser desorption/ionization time of flight mass spectrometry proteomics, a total of 79 polypeptides were identified, representing a mixture of proteins that underwent direct thiolation as well as proteins complexed with thiolated polypeptides. The mechanism of thiolation of five proteins, dehydroascorbate reductase (AtDHAR1), zeta-class glutathione transferase (AtGSTZ1), nitrilase (AtNit1), alcohol dehydrogenase (AtADH1), and methionine synthase (AtMetS), was studied using the respective purified recombinant proteins. AtDHAR1, AtGSTZ1, and to a lesser degree AtNit1 underwent spontaneous thiolation with GSSG-biotin through modification of active-site cysteines. The thiolation of AtADH1 and AtMetS required the presence of unidentified Arabidopsis proteins, with this activity being inhibited by S-modifying agents. The potential role of thiolation in regulating metabolism in Arabidopsis is discussed and compared with other known redox regulatory systems operating in plants.

Catalytic site-specific inhibition of the 20S proteasome by 4-hydroxynonenal

The proteasome is responsible for most intracellular protein degradation and is essential for cell survival. Previous research has shown that the proteasome can be inhibited by a number of oxidants, including 4-hydroxynonenal (HNE). The present study demonstrates that HNE rapidly inhibits the chymotrypsin-like activity of the 20S proteasome purified from liver. Subunits containing HNE-adducts were identified following 2D gel electrophoresis, Western immunoblotting, and analysis by MALDI-TOF MS. At a time when only the chymotrypsin-like activity was inhibited, the alpha 6/C2 subunit was uniquely modified. These results provide important molecular details regarding the catalytic site-specific inhibition of proteasome by HNE.