The proteasome and the degradation of oxidized proteins: Part I-structure of proteasomes. Redox Biol. 2013;1:178-182. [PMID: 24024151 PMCID: PMC3757679 DOI: 10.1016/j.redox.2013.01.004]

Glitches in the brain: the dangerous relationship between radiotherapy and brain fog

Up to approximately 70% of cancer survivors report persistent deficits in memory, attention, speed of information processing, multi-tasking, and mental health functioning, a series of symptoms known as "brain fog." The severity and duration of such effects can vary depending on age, cancer type, and treatment regimens. In particular, every year, hundreds of thousands of patients worldwide undergo radiotherapy (RT) for primary brain tumors and brain metastases originating from extracranial tumors. Besides its potential benefits in the control of tumor progression, recent studies indicate that RT reprograms the brain tumor microenvironment inducing increased activation of microglia and astrocytes and a consequent general condition of neuroinflammation that in case it becomes chronic could lead to a cognitive decline. Furthermore, radiation can induce endothelium reticulum (ER) stress directly or indirectly by generating reactive oxygen species (ROS) activating compensatory survival signaling pathways in the RT-surviving fraction of healthy neuronal and glial cells. In particular, the anomalous accumulation of misfolding proteins in neuronal cells exposed to radiation as a consequence of excessive activation of unfolded protein response (UPR) could pave the way to neurodegenerative disorders. Moreover, exposure of cells to ionizing radiation was also shown to affect the normal proteasome activity, slowing the degradation rate of misfolded proteins, and further exacerbating ER-stress conditions. This compromises several neuronal functions, with neuronal accumulation of ubiquitinated proteins with a consequent switch from proteasome to immunoproteasome that increases neuroinflammation, a crucial risk factor for neurodegeneration. The etiology of brain fog remains elusive and can arise not only during treatment but can also persist for an extended period after the end of RT. In this review, we will focus on the molecular pathways triggered by radiation therapy affecting cognitive functions and potentially at the origin of so-called "brain fog" symptomatology, with the aim to define novel therapeutic strategies to preserve healthy brain tissue from cognitive decline.

Oxidative damage in neurodegeneration: roles in the pathogenesis and progression of Alzheimer disease

Alzheimer disease (AD) is associated with multiple etiologies and pathological mechanisms, among which oxidative stress (OS) appears as a major determinant. Intriguingly, OS arises in various pathways regulating brain functions, and it seems to link different hypotheses and mechanisms of AD neuropathology with high fidelity. The brain is particularly vulnerable to oxidative damage, mainly because of its unique lipid composition, resulting in an amplified cascade of redox reactions that target several cellular components/functions ultimately leading to neurodegeneration. The present review highlights the "OS hypothesis of AD," including amyloid beta-peptide-associated mechanisms, the role of lipid and protein oxidation unraveled by redox proteomics, and the antioxidant strategies that have been investigated to modulate the progression of AD. Collected studies from our groups and others have contributed to unraveling the close relationships between perturbation of redox homeostasis in the brain and AD neuropathology by elucidating redox-regulated events potentially involved in both the pathogenesis and progression of AD. However, the complexity of AD pathological mechanisms requires an in-depth understanding of several major intracellular pathways affecting redox homeostasis and relevant for brain functions. This understanding is crucial to developing pharmacological strategies targeting OS-mediated toxicity that may potentially contribute to slow AD progression as well as improve the quality of life of persons with this severe dementing disorder.

Ubiquitin-proteasome pathway in skeletal muscle atrophy

Skeletal muscles underpin myriad human activities, maintaining an intricate balance between protein synthesis and degradation crucial to muscle mass preservation. Historically, disruptions in this balance-where degradation overshadows synthesis-have marked the onset of muscle atrophy, a condition diminishing life quality and, in grave instances, imperiling life itself. While multiple protein degradation pathways exist-including the autophagy-lysosome, calcium-dependent calpain, and cysteine aspartate protease systems-the ubiquitin-proteasome pathway emerges as an especially cardinal avenue for intracellular protein degradation, wielding pronounced influence over the muscle atrophy trajectory. This paper ventures a panoramic view of predominant muscle atrophy types, accentuating the ubiquitin-proteasome pathway's role therein. Furthermore, by drawing from recent scholarly advancements, we draw associations between the ubiquitin-proteasome pathway and specific pathological conditions linked to muscle atrophy. Our exploration seeks to shed light on the ubiquitin-proteasome pathway's significance in skeletal muscle dynamics, aiming to pave the way for innovative therapeutic strategies against muscle atrophy and affiliated muscle disorders.

Proteostasis in T cell aging

Aging leads to a decline in immune cell function, which leaves the organism vulnerable to infections and age-related multimorbidities. One major player of the adaptive immune response are T cells, and recent studies argue for a major role of disturbed proteostasis contributing to reduced function of these cells upon aging. Proteostasis refers to the state of a healthy, balanced proteome in the cell and is influenced by synthesis (translation), maintenance and quality control of proteins, as well as degradation of damaged or unwanted proteins by the proteasome, autophagy, lysosome and cytoplasmic enzymes. This review focuses on molecular processes impacting on proteostasis in T cells, and specifically functional or quantitative changes of each of these upon aging. Importantly, we describe the biological consequences of compromised proteostasis in T cells, which range from impaired T cell activation and function to enhancement of inflamm-aging by aged T cells. Finally, approaches to improve proteostasis and thus rejuvenate aged T cells through pharmacological or physical interventions are discussed.

Inhibition of the Proteasome Regulator PA28 Aggravates Oxidized Protein Overload in the Diabetic Rat Brain

Oxidized protein overloading caused by diabetes is one accelerating pathological pathway in diabetic encephalopathy development. To determine whether the PA28-regulated function of the proteasome plays a role in diabetes-induced oxidative damaged protein degradation, brain PA28α and PA28β interference experiments were performed in a high-fat diet (HFD) and streptozotocin (STZ)-induced rat model. The present results showed that proteasome activity was changed in the brains of diabetic rats, but the constitutive subunits were not. In vivo PA28α and PA28β inhibition via adeno-associated virus (AAV) shRNA infection successfully decreased PA28 protein levels and further exacerbated oxidized proteins load by regulating proteasome catalytic activity. These findings suggest that the proteasome plays a role in the elimination of oxidized proteins and that PA28 is functionally involved in the regulation of proteasome activity in vivo. This study suggests that abnormal protein turbulence occurring in the diabetic brain could be explained by the proteasome-mediated degradation pathway. Changes in proteasome activity regulator PA28 could be a reason to induce oxidative aggregation in diabetic brain. Proteasome regulator PA28 inhibition in vivo by AAV vector injection could aggravate oxidized proteins abundance in brain of HFD-STZ diabetic rat model.

Targeted Protein Degradation: Principles and Applications of the Proteasome

The proteasome is a multi-catalytic protease complex that is involved in protein quality control via three proteolytic activities (i.e., caspase-, trypsin-, and chymotrypsin-like activities). Most cellular proteins are selectively degraded by the proteasome via ubiquitination. Moreover, the ubiquitin-proteasome system is a critical process for maintaining protein homeostasis. Here, we briefly summarize the structure of the proteasome, its regulatory mechanisms, proteins that regulate proteasome activity, and alterations to proteasome activity found in diverse diseases, chemoresistant cells, and cancer stem cells. Finally, we describe potential therapeutic modalities that use the ubiquitin-proteasome system.

Distinguishing between DNA-Loaded Full and Empty Capsids of Adeno-Associated Virus with Atomic Force Microscopy Imaging

Recently, miraculous therapy approaches involving adeno-associated virus (AAV) for incurable diseases such as spinal muscular atrophy and inherited retinal dysfunction have been introduced. Nonreplicative, nonpathogenic, low rates of chromosome insertional properties and the existence of neutralizing antibodies are main safety reasons why the FDA approved its use in gene delivery. To date, AAV production always results in a mixture of nontherapeutic (empty) and therapeutic (DNA-loaded) full capsids (10-98%). Such existence of empty viral particles inevitably increases viral doses to human. Thus, the rapid monitoring of empty capsids and reducing the empty-to-full ratio are critical in AAV science. However, transmission electron microscopy (TEM) is the primary tool for distinguishing between empty and full capsids, which creates a research bottleneck because of instrument accessibility and technical difficulty. Herein, we demonstrate that atomic force microscopy (AFM) can be an alternative tool to TEM. The simple, noncontact-mode imaging of AAV particles allows the distinct height difference between full capsids (∼22 nm) and empty capsids (∼16 nm). The sphere-to-ellipsoidal morphological distortion observed for empty AAV particles clearly distinguishes them from full AAV particles. Our study indicates that AFM imaging can be an extremely useful, quality-control tool in AAV particle monitoring, which is beneficial for the future development of AAV-based gene therapy.

Link between Energy Investment in Biosynthesis and Proteostasis: Testing the Cost-Quality Hypothesis in Insects

The energy requirement for biosynthesis plays an important role in an organism's life history, as it determines growth rate, and tradeoffs with the investment in somatic maintenance. This energetic trait is different between painted lady (Vanessa cardui) and Turkestan cockroach (Blatta lateralis) due to the different life histories. Butterfly caterpillars (holometabolous) grow 30-fold faster, and the energy cost of biosynthesis is 20 times cheaper, compared to cockroach nymphs (hemimetabolous). We hypothesize that physiologically the difference in the energy cost is partially attributed to the differences in protein retention and turnover rate: Species with higher energy cost may have a lower tolerance to errors in newly synthesized protein. Newly synthesized proteins with errors are quickly unfolded and refolded, and/or degraded and resynthesized via the proteasomal system. Thus, much protein output may be given over to replacement of the degraded new proteins, so the overall energy cost on biosynthesis is high. Consequently, the species with a higher energy cost for biosyntheses has better proteostasis and cellular resistance to stress. Our study found that, compared to painted lady caterpillars, the midgut tissue of cockroach nymphs has better cellular viability under oxidative stresses, higher activities of proteasome 20S, and a higher RNA/growth ratio, supporting our hypothesis. This comparative study offers a departure point for better understanding life history tradeoffs between somatic maintenance and biosynthesis.

Proteomic Analysis Reveals a Critical Role of the Glycosyl Hydrolase 17 Protein in Panax ginseng Leaves under Salt Stress

Ginseng, an important crop in East Asia, exhibits multiple medicinal and nutritional benefits because of the presence of ginsenosides. On the other hand, the ginseng yield is severely affected by abiotic stressors, particularly salinity, which reduces yield and quality. Therefore, efforts are needed to improve the ginseng yield during salinity stress, but salinity stress-induced changes in ginseng are poorly understood, particularly at the proteome-wide level. In this study, we report the comparative proteome profiles of ginseng leaves at four different time points (mock, 24, 72, and 96 h) using a label-free quantitative proteome approach. Of the 2484 proteins identified, 468 were salt-responsive. In particular, glycosyl hydrolase 17 (PgGH17), catalase-peroxidase 2, voltage-gated potassium channel subunit beta-2, fructose-1,6-bisphosphatase class 1, and chlorophyll a-b binding protein accumulated in ginseng leaves in response to salt stress. The heterologous expression of PgGH17 in Arabidopsis thaliana improved the salt tolerance of transgenic lines without compromising plant growth. Overall, this study uncovers the salt-induced changes in ginseng leaves at the proteome level and highlights the critical role of PgGH17 in salt stress tolerance in ginseng.

High glucose leads to redistribution of the proteasomal system

The impact of high glucose on the cellular redox state, causing both induction of antioxidative systems and also enhanced protein oxidation is discussed for a long time. It is established that elevated glucose levels are disrupting the cellular proteostasis and influencing the proteasomal system. However, it is still unresolved whether this is due to a reaction of the cellular proteasomal system towards the high glucose or whether this is a secondary reaction to inflammatory stimuli. Therefore, we used a dermal fibroblast cell line exposed to high glucose in order to reveal whether a response of the proteasomal system takes place. We investigated the α4 and the inducible iβ5 subunits of the 20S proteasome, as well as the Rpn1-subunit of the 19S proteasomal regulator complex, measured activity of the 20S, 20S1, and 26S proteasome and detected as well changes in expression as a redistribution into the nucleus. Interestingly, while the activity of the proteasomal forms rather decreased under high glucose treatment; higher expression levels of components of the proteasomal system and higher concentrations of protein-bound 3-nitrotyrosine and Nrf2 (nuclear factor [erythroid-derived 2]-like 2) were detected. However, no change in the cytosol-nucleus distribution could be detected for most of the quantified parameters. We concluded that high glucose alone, without additional inflammatory stimuli, provokes a regulatory response on the ubiquitin-proteasomal system.

Natural Agents as Novel Potential Source of Proteasome Inhibitors with Anti-Tumor Activity: Focus on Multiple Myeloma

Multiple myeloma (MM) is an aggressive and incurable disease for most patients, characterized by periods of treatment, remission and relapse. The introduction of new classes of drugs, such as proteasome inhibitors (PIs), has improved survival outcomes in these patient populations. The proteasome is the core of the ubiquitin-proteasome system (UPS), a complex and conserved pathway involved in the control of multiple cellular processes, including cell cycle control, transcription, DNA damage repair, protein quality control and antigen presentation. To date, PIs represent the gold standard for the treatment of MM. Bortezomib was the first PI approved by the FDA, followed by next generation of PIs, namely carfilzomib and ixazomib. Natural agents play an important role in anti-tumor drug discovery, and many of them have recently been reported to inhibit the proteasome, thus representing a new potential source of anti-MM drugs. Based on the pivotal biological role of the proteasome and on PIs' significance in the management of MM, in this review we aim to briefly summarize recent evidence on natural compounds capable of inhibiting the proteasome, thus triggering anti-MM activity.

The Link between Oxidative Stress, Mitochondrial Dysfunction and Neuroinflammation in the Pathophysiology of Alzheimer's Disease: Therapeutic Implications and Future Perspectives

Alzheimer's disease (AD), the most common form of dementia, has increasing incidence, increasing mortality rates, and poses a huge burden on healthcare. None of the currently approved drugs for the treatment of AD influence disease progression. Many clinical trials aiming at inhibiting amyloid plaque formation, increasing amyloid beta clearance, or inhibiting neurofibrillary tangle pathology yielded inconclusive results or failed. Meanwhile, research has identified many interlinked vicious cascades implicating oxidative stress, mitochondrial dysfunction, and chronic neuroinflammation, and has pointed to novel therapeutic targets such as improving mitochondrial bioenergetics and quality control, diminishing oxidative stress, or modulating the neuroinflammatory pathways. Many novel molecules tested in vitro or in animal models have proven efficient, but their translation into clinic needs further research regarding appropriate doses, delivery routes, and possible side effects. Cell-based therapies and extracellular vesicle-mediated delivery of messenger RNAs and microRNAs seem also promising strategies allowing to target specific signaling pathways, but need further research regarding the most appropriate harvesting and culture methods as well as control of the possible tumorigenic side effects. The rapidly developing area of nanotechnology could improve drug delivery and also be used in early diagnosis.

Candida albicans Hyphal Extracellular Vesicles Are Different from Yeast Ones, Carrying an Active Proteasome Complex and Showing a Different Role in Host Immune Response

Candida albicans is the principal causative agent of lethal fungal infections, predominantly in immunocompromised hosts. Extracellular vesicles (EVs) have been described as crucial in the interaction of microorganisms with their host. Since the yeast-to-hypha transition is an important virulence trait with great impact in invasive candidiasis (IC), we have addressed the characterization of EVs secreted by hyphal cells (HEVs) from C. albicans, comparing them to yeast EVs (YEVs). YEVs comprised a larger population of bigger EVs with mainly cell wall proteins, while HEVs were smaller, in general, and had a much higher protein diversity. YEVs were able to rescue the sensitivity of a cell wall mutant against calcofluor white, presumably due to the larger amount of cell wall proteins they contained. On the other hand, HEVs also contained many cytoplasmic proteins related to protein metabolism and intracellular protein transport and the endosomal sorting complexes required for transport (ESCRT) pathway related to exosome biogenesis, pointing to an intracellular origin of HEVs. Interestingly, an active 20S proteasome complex was secreted exclusively in HEVs. Moreover, HEVs contained a greater number of virulence-related proteins. As for their immunogenic role, both types of EV presented immune reactivity with human sera from patients suffering invasive candidiasis; however, under our conditions, only HEVs showed a cytotoxic effect on human macrophages and could elicit the release of tumor necrosis factor alpha (TNF-α) by these macrophages. IMPORTANCE This first analysis of HEVs of C. albicans has shown clear differences between them and the YEVs of C. albicans, showing their relevance and possible use in the discovery of new diagnostic markers and treatment targets against C. albicans infections. The data obtained point to different mechanisms of biogenesis of YEVs and HEVs, as well as different involvements in cell biology and host interaction. YEVs played a more relevant role in cell wall maintenance, while HEVs were more closely related to virulence, as they had greater effects on human immune cells. Importantly, an active 20S proteosome complex was described as a fungal-EV cargo. A deeper study of its role and those of many other proteins exclusively detected in HEVs and involved in different relevant biological processes of this fungus could open up interesting new areas of research in the battle against C. albicans.

Exploring the Role of Ubiquitin-Proteasome System in Parkinson's Disease

Over the last decade, researchers have discovered that  a group of apparently unrelated neurodegenerative disorders, such as Parkinson's disease, have remarkable cellular and molecular biology similarities. Protein misfolding and aggregation are involved in all of the neurodegenerative conditions; as a result, inclusion bodies aggregation starts in the cells. Chaperone proteins and ubiquitin (26S proteasome's proteolysis signal), which aid in refolding misfolded proteins, are frequently found in these aggregates. The discovery of disease-causing gene alterations that code for multiple ubiquitin-proteasome pathway proteins in Parkinson's disease has strengthened the relationship between the ubiquitin-proteasome system and neurodegeneration. The specific molecular linkages between these systems and pathogenesis, on the other hand, are unknown and controversial. We outline the current level of knowledge in this article, focusing on important unanswered problems.

Differential oxidative stress responses and tobacco-specific nitrosamine accumulation in two burley varieties

Tobacco-specific nitrosamines (TSNAs) are carcinogens that accumulate in tobacco leaves during curing, storage, and processing, and their amounts in processed tobacco vary dependent on several intrinsic and extrinsic factors. Here, we assessed the hypothesis that there is a link between reactive oxygen species levels in leaves and TSNA formation during curing. First, we show that burley varieties KT 204LC and NCBH 129LC accumulate TSNAs to different levels but not as a result of a variety-specific abundance of TSNA precursors. Next, we measured the levels of reactive oxygen species, and we show that the variety that accumulates more TSNAs, NCBH 129LC, had significantly higher levels of hydrogen peroxide than KT 204LC. The NCBH 129LC also has more oxidatively damaged and glutathionylated proteins. Finally, we analyzed the antioxidant levels in KT 204LC and NCBH 129LC and their tolerance to oxidative stress. NCBH 129LC contained more of the essential antioxidant glutathione and was more tolerant to the oxidative stress-generating compound paraquat. Collectively, our data suggest that there is indeed a link between foliar oxidative stress parameters and the extent to which TSNAs accumulate in cured tobacco leaves.

Identifying Genes Devoted to the Cell Death Process in the Gene Regulatory Network of Ustilago maydis

Cell death is a process that can be divided into three morphological patterns: apoptosis, autophagy and necrosis. In fungi, cell death is induced in response to intracellular and extracellular perturbations, such as plant defense molecules, toxins and fungicides, among others. Ustilago maydis is a dimorphic fungus used as a model for pathogenic fungi of animals, including humans, and plants. Here, we reconstructed the transcriptional regulatory network of U. maydis, through homology inferences by using as templates the well-known gene regulatory networks (GRNs) of Saccharomyces cerevisiae, Aspergillus nidulans and Neurospora crassa. Based on this GRN, we identified transcription factors (TFs) as hubs and functional modules and calculated diverse topological metrics. In addition, we analyzed exhaustively the module related to cell death, with 60 TFs and 108 genes, where diverse cell proliferation, mating-type switching and meiosis, among other functions, were identified. To determine the role of some of these genes, we selected a set of 11 genes for expression analysis by qRT-PCR (sin3, rlm1, aif1, tdh3 [isoform A], tdh3 [isoform B], ald4, mca1, nuc1, tor1, ras1, and atg8) whose homologues in other fungi have been described as central in cell death. These genes were identified as downregulated at 72 h, in agreement with the beginning of the cell death process. Our results can serve as the basis for the study of transcriptional regulation, not only of the cell death process but also of all the cellular processes of U. maydis.

An update in toxicology of ageing

The field of ageing research has been rapidly advancing in recent decades and it had provided insight into the complexity of ageing phenomenon. However, as the organism-environment interaction appears to significantly affect the organismal pace of ageing, the systematic approach for gerontogenic risk assessment of environmental factors has yet to be established. This puts demand on development of effective biomarker of ageing, as a relevant tool to quantify effects of gerontogenic exposures, contingent on multidisciplinary research approach. Here we review the current knowledge regarding the main endogenous gerontogenic pathways involved in acceleration of ageing through environmental exposures. These include inflammatory and oxidative stress-triggered processes, dysregulation of maintenance of cellular anabolism and catabolism and loss of protein homeostasis. The most effective biomarkers showing specificity and relevancy to ageing phenotypes are summarized, as well. The crucial part of this review was dedicated to the comprehensive overview of environmental gerontogens including various types of radiation, certain types of pesticides, heavy metals, drugs and addictive substances, unhealthy dietary patterns, and sedentary life as well as psychosocial stress. The reported effects in vitro and in vivo of both recognized and potential gerontogens are described with respect to the up-to-date knowledge in geroscience. Finally, hormetic and ageing decelerating effects of environmental factors are briefly discussed, as well.

Nucleoredoxin Knockdown in SH-SY5Y Cells Promotes Cell Renewal

Nucleoredoxin (NXN) is a redox regulator of Disheveled and thereby of WNT signaling. Deficiency in mice leads to cranial dysmorphisms and defects of heart, brain, and bone, suggesting defects of cell fate determination. We used shRNA-mediated knockdown of NXN in SH-SY5Y neuroblastoma cells to study its impact on neuronal cells. We expected that shNXN cells would easily succumb to redox stress, but there were no differences in viability on stimulation with hydrogen peroxide. Instead, the proliferation of naïve shNXN cells was increased with a higher rate of mitotic cells in cell cycle analyses. In addition, basal respiratory rates were higher, whereas the relative change in oxygen consumption upon mitochondrial stressors was similar to control cells. shNXN cells had an increased expression of redox-sensitive heat shock proteins, Hsc70/HSPA8 and HSP90, and autophagy markers suggested an increase in autophagosome formation upon stimulation with bafilomycin and higher flux under low dose rapamycin. A high rate of self-renewal, autophagy, and upregulation of redox-sensitive chaperones appears to be an attractive anti-aging combination if it were to occur in neurons in vivo for which SH-SY5Y cells are a model.

Lipotoxicity dysregulates the immunoproteasome in podocytes and kidneys in type 2 diabetes

Palmitic acid (PA) leads to lipotoxicity in type 2 diabetes and induces oxidative stress in podocytes. Oxidized cellular proteins are degraded by proteasomes. The role of proteasomes in PA- or oxidative stress-induced podocyte injury and pathogenesis of diabetic nephropathy (DN) is unknown. We investigated the effects of PA on expression of 20S and 26S proteasomes, proteasome activator 28 (PA28) regulators, and the immunoproteasome in cultured podocytes and renal cortical tissues of db/db and db/m mice using Western blot analysis. Glomerular areas and glomerular basement membrane (GBM) widths of db/db and db/m mice were examined using morphometry. Short-term incubation of PA or low levels of H₂O₂ upregulated only the immunoproteasome in cultured podocytes. Long-term exposure of podocytes to PA ultimately downregulated the immunoproteasome as with other proteasomes, whereas oleic acid (OA) or eicosapentaenoic acid (EPA) restored the PA-induced decreased protein levels. In db/db mice, renal cortical immunoproteasome expression with PA28α was significantly decreased compared with db/m mice, and glomerular areas and GBM widths were significantly increased compared with db/m mice. Feeding of an OA-rich olive oil or EPA-rich fish oil protected db/db mice against the reduced renal cortical immunoproteasome expression, glomerular enlargement, and GBM thickening. These results demonstrate that lipotoxicity downregulates the immunoproteasome in podocytes and kidneys in type 2 diabetes and that OA and EPA protected type 2 diabetic mice against decreased renal cortical immunoproteasome expression and the progression of DN. Given this, lipotoxicity-induced podocyte injury with impaired immunoproteasome expression appears to play an important role in the pathogenesis of DN.NEW & NOTEWORTHY In podocytes, PA rapidly induced immunoproteasome expression but ultimately decreased it, while OA and EPA restored the decreased immunoproteasome levels. In the renal cortex of type 2 diabetic mice, immunoproteasome expression was significantly decreased, whereas feeding of OA-rich olive oil or EPA-rich fish oil diets protected them against the reduced immunoproteasome expression and progression of diabetic nephropathy. Thus, lipotoxicity-induced podocyte injury with impaired immunoproteasome expression may be related to the pathogenesis of diabetic nephropathy.

The Ubiquitin-Proteasome System in Immune Cells

The ubiquitin–proteasome system (UPS) is the major intracellular and non-lysosomal protein degradation system. Thanks to its unique capacity of eliminating old, damaged, misfolded, and/or regulatory proteins in a highly specific manner, the UPS is virtually involved in almost all aspects of eukaryotic life. The critical importance of the UPS is particularly visible in immune cells which undergo a rapid and profound functional remodelling upon pathogen recognition. Innate and/or adaptive immune activation is indeed characterized by a number of substantial changes impacting various cellular processes including protein homeostasis, signal transduction, cell proliferation, and antigen processing which are all tightly regulated by the UPS. In this review, we summarize and discuss recent progress in our understanding of the molecular mechanisms by which the UPS contributes to the generation of an adequate immune response. In this regard, we also discuss the consequences of UPS dysfunction and its role in the pathogenesis of recently described immune disorders including cancer and auto-inflammatory diseases.

Methods for the discovery of small molecules to monitor and perturb the activity of the human proteasome

Regulating protein production and degradation is critical to maintaining cellular homeostasis. The proteasome is a key player in keeping proteins at the proper levels. However, proteasome activity can be altered in certain disease states, such as blood cancers and neurodegenerative diseases. Cancers often exhibit enhanced proteasomal activity, as protein synthesis is increased in these cells compared with normal cells. Conversely, neurodegenerative diseases are characterized by protein accumulation, leading to reduced proteasome activity. As a result, the proteasome has emerged as a target for therapeutic intervention. The potential of the proteasome as a therapeutic target has come from studies involving chemical stimulators and inhibitors, and the development of a suite of assays and probes that can be used to monitor proteasome activity with purified enzyme and in live cells.

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.

Accumulation of polyubiquitinated proteins: A consequence of early inactivation of the 26S proteasome

The proteasomal degradation system is one of the most important protein degradation systems in the cytosol and nucleus. This system is present in two major forms: the ATP-stimulated 26S/30 S proteasome or the ATP-independent 20S core proteasome. While the first recognize ubiquitin-tagged target proteins and degrade them, the 20S proteasome works also independent from ATP, but requires partially unfolded substrates. While the role of the proteasome in the selective removal of oxidized proteins is undoubted, the debate about a selective ubiquitination of oxidized proteins is still ongoing. Here we demonstrate, that under some conditions of oxidative stress an accumulation of oxidized and of K48-ubiquitinated proteins occurs. However, the removal of oxidized proteins seems not to be linked to ubiquitination. In further experiments, we could show that the accumulation of ubiquitinated proteins under certain oxidative stress conditions is rather a result of a different sensitivity of the 26S proteasome and the ubiquitination machinery towards oxidants.

Proteostasis-associated aging: lessons from a Drosophila model

As cells age, they lose their ability to properly fold proteins, maintain protein folding, and eliminate misfolded proteins, which leads to the accumulation of abnormal protein aggregates and loss of protein homeostasis (proteostasis). Loss of proteostasis can accelerate aging and the onset of neurodegenerative diseases such as Alzheimer's disease and Parkinson's disease. Mechanisms exist to prevent the detrimental effects of abnormal proteins that incorporate chaperones, autophagy, and the ubiquitin-proteasome system. These mechanisms are evolutionarily conserved across various species. Therefore, the effect of impaired proteostasis on aging has been studied using model organisms that are appropriate for aging studies. In this review, we focus on the relationship between proteostasis and aging, and factors that affect proteostasis in Drosophila. The manipulation of proteostasis can alter lifespan, modulate neurotoxicity, and delay the onset of neurodegeneration, indicating that proteostasis may be a novel pharmacological target for the development of treatments for various age-associated diseases.

Proteasome, a Promising Therapeutic Target for Multiple Diseases Beyond Cancer

Proteasome is vital for intracellular protein homeostasis as it eliminates misfolded and damaged protein. Inhibition of proteasome has been validated as a powerful strategy for anti-cancer therapy, and several drugs have been approved for treatment of multiple myeloma. Recent studies indicate that proteasome has potent therapeutic effects on a variety of diseases besides cancer, including parasite infectious diseases, bacterial/fungal infections diseases, neurodegenerative diseases and autoimmune diseases. In this review, recent developments of proteasome inhibitors for various diseases and related structure activity relationships are going to be summarized.

The ubiquitin-proteasome system and its crosstalk with mitochondria as therapeutic targets in medicine

The ubiquitin-proteasome system constitutes a major pathway for protein degradation in the cell. Therefore the crosstalk of this pathway with mitochondria is a major topic with direct relevance to many mitochondrial diseases. Proteasome dysfunction triggers not only protein toxicity, but also mitochondrial dysfunction. The involvement of proteasomes in the regulation of protein transport into mitochondria contributes to an increase in mitochondrial function defects. On the other hand, mitochondrial impairment stimulates reactive oxygen species production, which increases protein damage, and protein misfolding and aggregation leading to proteasome overload. Concurrently, mitochondrial dysfunction compromises cellular ATP production leading to reduced protein ubiquitination and proteasome activity. In this review we discuss the complex relationship and interdependence of the ubiquitin-proteasome system and mitochondria. Furthermore, we describe pharmacological inhibition of proteasome activity as a novel strategy to treat a group of mitochondrial diseases.

The co-existence of cold activity and thermal stability in an Antarctic GH42 β-galactosidase relies on its hexameric quaternary arrangement

To survive in cold environments, psychrophilic organisms produce enzymes endowed with high specific activity at low temperature. The structure of these enzymes is usually flexible and mostly thermolabile. In this work, we investigate the structural basis of cold adaptation of a GH42 β-galactosidase from the psychrophilic Marinomonas ef1. This enzyme couples cold activity with astonishing robustness for a psychrophilic protein, for it retains 23% of its highest activity at 5 °C and it is stable for several days at 37 °C and even 50 °C. Phylogenetic analyses indicate a close relationship with thermophilic β-galactosidases, suggesting that the present-day enzyme evolved from a thermostable scaffold modeled by environmental selective pressure. The crystallographic structure reveals the overall similarity with GH42 enzymes, along with a hexameric arrangement (dimer of trimers) not found in psychrophilic, mesophilic, and thermophilic homologues. In the quaternary structure, protomers form a large central cavity, whose accessibility to the substrate is promoted by the dynamic behavior of surface loops, even at low temperature. A peculiar cooperative behavior of the enzyme is likely related to the increase of the internal cavity permeability triggered by heating. Overall, our results highlight a novel strategy of enzyme cold adaptation, based on the oligomerization state of the enzyme, which effectively challenges the paradigm of cold activity coupled with intrinsic thermolability. DATABASE: Structural data are available in the Protein Data Bank database under the accession number 6Y2K.

Targeting Mitochondria during Cold Storage to Maintain Proteasome Function and Improve Renal Outcome after Transplantation

Kidney transplantation is the preferred treatment for end-stage kidney disease (ESKD). Compared to maintenance dialysis, kidney transplantation results in improved patient survival and quality of life. Kidneys from living donors perform best; however, many patients with ESKD depend on kidneys from deceased donors. After procurement, donor kidneys are placed in a cold-storage solution until a suitable recipient is located. Sadly, prolonged cold storage times are associated with inferior transplant outcomes; therefore, in most situations when considering donor kidneys, long cold-storage times are avoided. The identification of novel mechanisms of cold-storage-related renal damage will lead to the development of new therapeutic strategies for preserving donor kidneys; to date, these mechanisms remain poorly understood. In this review, we discuss the importance of mitochondrial and proteasome function, protein homeostasis, and renal recovery during stress from cold storage plus transplantation. Additionally, we discuss novel targets for therapeutic intervention to improve renal outcomes.

Design of Fluorescent Probes for Bioorthogonal Labeling of Carbonylation in Live Cells

With the rapid development of chemical biology, many diagnostic fluorophore-based tools were introduced to specific biomolecules by covalent binding. Bioorthogonal reactions have been widely utilized to manage challenges faced in clinical practice for early diagnosis and treatment of several tumor samples. Herein, we designed a small molecule fluorescent-based biosensor, 2Hydrazine-5nitrophenol (2Hzin5NP), which reacts with the carbonyl moiety of biomolecules through bioorthogonal reaction, therefore can be utilized for the detection of biomolecule carbonylation in various cancer cell lines. Our almost non-fluorescent chemical probe has a fast covalent binding with carbonyl moieties at neutral pH to form a stable fluorescent hydrazone product leading to a spectroscopic alteration in live cells. Microscopic and fluorometric analyses were used to distinguish the exogenous and endogenous ROS induced carbonylation profile in human dermal fibroblasts along with A498 primary site and ACHN metastatic site renal cell carcinoma (RRC) cell lines. Our results showed that carbonylation level that differs in response to exogenous and endogenous stress in healthy and cancer cells can be detected by the newly synthesized bioorthogonal fluorescent probe. Our results provide new insights into the development of novel bioorthogonal probes that can be utilized in site-specific carbonylation labeling to enhance new diagnostic approaches in cancer.

Oxidized protein aggregates: Formation and biological effects

The study of protein aggregates has a long history. While in the first decades until the 80ies of the 20th century only the observation of the presence of such aggregates was reported, later the biochemistry of the formation and the biological effects of theses aggregates were described. This review focusses on the complexity of the biological effects of protein aggregates and its potential role in the aging process.

Irreversible oxidative post-translational modifications in heart disease

Introduction: Development of specific biomarkers aiding early diagnosis of heart failure is an ongoing challenge. Biomarkers commonly used in clinical routine usually act as readouts of an already existing acute condition rather than disease initiation. Functional decline of cardiac muscle is greatly aggravated by increased oxidative stress and damage of proteins. Oxidative post-translational modifications occur already at early stages of tissue damage and are thus regarded as potential up-coming disease markers. Areas covered: Clinical practice regarding commonly used biomarkers for heart disease is briefly summarized. The types of oxidative post-translational modification in cardiac pathologies are discussed with a special focus on available quantitative techniques and characteristics of individual modifications with regard to their stability and analytical accessibility. As irreversible oxidative modifications trigger protein degradation pathways or cause protein aggregation, both influencing biomarker abundance, a chapter is dedicated to their regulation in the heart.

Differences in plasma and peritoneal fluid proteomes identifies potential biomarkers associated with survival following strangulating small intestinal disease

BACKGROUND

Strangulating small intestinal disease (SSID) carries a poor prognosis for survival in comparison to other types of colic, particularly if resection is required. Identification of markers which aid early diagnosis may prevent the need for resection, assist with more accurate prognostication and/or support the decision on whether surgical intervention is likely to be successful, would be of significant welfare benefit.

OBJECTIVES

To apply an unbiased methodology to investigate the plasma and peritoneal fluid proteomes in horses diagnosed with SSID requiring resection, to identify novel biomarkers which may be of diagnostic or prognostic value.

STUDY DESIGN

Prospective clinical study.

METHODS

Plasma and peritoneal fluid from horses presented with acute abdominal signs consistent with SSID was collected at initial clinical examination. Samples from eight horses diagnosed with SSID at surgery in which resection of affected bowel was performed and four control horses subjected to euthanasia for orthopaedic conditions were submitted for liquid chromatography tandem mass spectrometry. Protein expression profiles were determined using label-free quantification. Data were analysed using analysis of variance to identify differentially expressed proteins between control and all SSID horses and SSID horses which survived to hospital discharge and those which did not. Significance was assumed at P≤0.05.

RESULTS

A greater number of proteins were identified in peritoneal fluid than plasma of both SSID cases and controls, with 123 peritoneal fluid and 13 plasma proteins significantly differentially expressed (DE) between cases and controls (P<0.05, ≥2 fold change). Twelve peritoneal fluid proteins (P<0.036) and four plasma proteins (P<0.05) were significantly DE between SSID horses which survived and those which did not.

MAIN LIMITATIONS

A low number of samples were analysed, there was variation in duration and severity of SSID and only short-term outcome was considered.

CONCLUSIONS

Changes in peritoneal fluid proteome may provide a sensitive indicator of small intestinal strangulation and provide biomarkers relevant to prognosis.

Impaired proteostasis during skeletal muscle aging

Aging is a complex phenomenon that has detrimental effects on tissue homeostasis. The skeletal muscle is one of the earliest tissues to be affected and to manifest age-related changes such as functional impairment and the loss of mass. Common to these alterations and to most of tissues during aging is the disruption of the proteostasis network by detrimental changes in the ubiquitin-proteasomal system (UPS) and the autophagy-lysosomal system (ALS). In fact, during aging the accumulation of protein aggregates, a process mainly driven by increased levels of oxidative stress, has been observed, clearly demonstrating UPS and ALS dysregulation. Since the UPS and ALS are the two most important pathways for the removal of misfolded and aggregated proteins and also of damaged organelles, we provide here an overview on the current knowledge regarding the connection between the loss of proteostasis and skeletal muscle functional impairment and also how redox regulation can play a role during aging. Therefore, this review serves for a better understanding of skeletal muscle aging in regard to the loss of proteostasis and how redox regulation can impact its function and maintenance.

Non-enzymatic cleavage of Hsp90 by oxidative stress leads to actin aggregate formation: A novel gain-of-function mechanism

Aging is accompanied by the accumulation of oxidized proteins. To remove them, cells employ the proteasomal and autophagy-lysosomal systems; however, if the clearance rate is inferior to its formation, protein aggregates form as a hallmark of proteostasis loss. In cells, during stress conditions, actin aggregates accumulate leading to impaired proliferation and reduced proteasomal activity, as observed in cellular senescence. The heat shock protein 90 (Hsp90) is a molecular chaperone that binds and protects the proteasome from oxidative inactivation. We hypothesized that in oxidative stress conditions a malfunction of Hsp90 occurs resulting in the aforementioned protein aggregates.

Here, we demonstrate that upon oxidative stress Hsp90 loses its function in a highly specific non-enzymatic iron-catalyzed oxidation event and its breakdown product, a cleaved form of Hsp90 (Hsp90cl), acquires a new function in mediating the accumulation of actin aggregates. Moreover, the prevention of Hsp90 cleavage reduces oxidized actin accumulation, whereas transfection of the cleaved form of Hsp90 leads to an enhanced accumulation of oxidized actin.

This indicates a clear role of the Hsp90cl in the aggregation of oxidized proteins.

The multicatalytic compartment of propionyl-CoA synthase sequesters a toxic metabolite

Cells must cope with toxic or reactive intermediates formed during metabolism. One coping strategy is to sequester reactions that produce such intermediates within specialized compartments or tunnels connecting different active sites. Here, we show that propionyl-CoA synthase (PCS), an ∼ 400-kDa homodimer, three-domain fusion protein and the key enzyme of the 3-hydroxypropionate bi-cycle for CO₂ fixation, sequesters its reactive intermediate acrylyl-CoA. Structural analysis showed that PCS forms a multicatalytic reaction chamber. Kinetic analysis suggested that access to the reaction chamber and catalysis are synchronized by interdomain communication. The reaction chamber of PCS features three active sites and has a volume of only 33 nm³. As one of the smallest multireaction chambers described in biology, PCS may inspire the engineering of a new class of dynamically regulated nanoreactors.

Ubiquitin-proteasome system and oxidative stress in liver transplantation

A major issue in organ transplantation is the development of a protocol that can preserve organs under optimal conditions. Damage to organs is commonly a consequence of flow deprivation and oxygen starvation following the restoration of blood flow and reoxygenation. This is known as ischemia-reperfusion injury (IRI): a complex multifactorial process that causes cell damage. While the oxygen deprivation due to ischemia depletes cell energy, subsequent tissue oxygenation due to reperfusion induces many cascades, from reactive oxygen species production to apoptosis initiation. Autophagy has also been identified in the pathogenesis of IRI, although such alterations and their subsequent functional significance are controversial. Moreover, proteasome activation may be a relevant pathophysiological mechanism. Different strategies have been adopted to limit IRI damage, including the supplementation of commercial preservation media with pharmacological agents or additives. In this review, we focus on novel strategies related to the ubiquitin proteasome system and oxidative stress inhibition, which have been used to minimize damage in liver transplantation.

Precoce and opposite response of proteasome activity after acute or chronic exposure of C. elegans to γ-radiation

Species are chronically exposed to ionizing radiation, a natural phenomenon which can be enhanced by human activities. The induced toxicity mechanisms still remain unclear and seem depending on the mode of exposure, i.e. acute and chronic. To better understand these phenomena, studies need to be conducted both at the subcellular and individual levels. Proteins, functional molecules in organisms, are the targets of oxidative damage (especially via their carbonylation (PC)) and are likely to be relevant biomarkers. After exposure of Caenorhabditis elegans to either chronic or acute γ rays we showed that hatching success is impacted after acute but not after chronic irradiation. At the molecular level, the carbonylated protein level in relation with dose was slightly different between acute and chronic exposure whereas the proteolytic activity is drastically modified. Indeed, whereas the 20S proteasome activity is inhibited by acute irradiation from 0.5 Gy, it is activated after chronic irradiation from 1 Gy. As expected, the 20S proteasome activity is mainly modified by irradiation whereas the 26S and 30S activity are less changed. This study provides preliminaries clues to understand the role of protein oxidation and proteolytic activity in the radiation-induced molecular mechanisms after chronic versus acute irradiation in C. elegans.

Proteasome 20S in multiple myeloma: comparison of concentration and chymotrypsin-like activity in plasma and serum

The ubiquitin-proteasome system is relevant in the pathobiology of many haematological malignancies, including multiple myeloma. The assessment of proteasome concentration and chymotrypsin-like (ChT-L) activity might constitute a new approach to diagnosis, prognosis and monitoring of anticancer treatment of patients with haematological malignancies and other diseases. The aim of our study was to determine which material, plasma or serum, is better for measuring chymotrypsin-like (ChT-L) activity and proteasome concentration. We analysed proteasome concentration and chymotrypsin-like (ChT-L) activity in 70 plasma and serum samples drawn from 28 patients at different treatment stages for multiple myeloma (MM) and 31 healthy volunteers. Proteasome ChT-L activity and concentration in multiple myeloma patients were significantly higher in plasma compared to serum. In this group we observed significant and positive correlations both between the plasma and serum proteasome ChT-L activity and plasma and serum proteasome concentration. The higher values of proteasome concentration and ChT-L activity in plasma than in serum and their better correlations with parameters of tumour load and prognosis suggest that plasma constitutes a better biological material for measuring ChT-L activity and proteasome concentration than serum in multiple myeloma patients.

Early cysteine-dependent inactivation of 26S proteasomes does not involve particle disassembly

Under oxidative stress 26S proteasomes suffer reversible disassembly into its 20S and 19S subunits, a process mediated by HSP70. This inhibits the degradation of polyubiquitinated proteins by the 26S proteasome and allows the degradation of oxidized proteins by a free 20S proteasome. Low fluxes of antimycin A-stimulated ROS production caused dimerization of mitochondrial peroxiredoxin 3 and cytosolic peroxiredoxin 2, but not peroxiredoxin overoxidation and overall oxidation of cellular protein thiols. This moderate redox imbalance was sufficient to inhibit the ATP stimulation of 26S proteasome activity. This process was dependent on reversible cysteine oxidation. Moreover, our results show that this early inhibition of ATP stimulation occurs previous to particle disassembly, indicating an intermediate step during the redox regulation of the 26S proteasome with special relevance under redox signaling rather than oxidative stress conditions.

"Cyt/Nuc," a Customizable and Documenting ImageJ Macro for Evaluation of Protein Distributions Between Cytosol and Nucleus

Large amounts of data from multi-channel, high resolution, fluorescence microscopic images require tools that provide easy, customizable, and reproducible high-throughput analysis. The freeware "ImageJ" has become one of the standard tools for scientific image analysis. Since ImageJ offers recording of "macros," even a complex multi-step process can be easily applied fully automated to large numbers of images, saving both time and reducing human subjective evaluation. In this work, we present "Cyt/Nuc," an ImageJ macro, able to recognize and to compare the nuclear and cytosolic areas of tissue samples, in order to investigate distributions of immunostained proteins between both compartments, while it documents in detail the whole process of evaluation and pattern recognition. As practical example, the redistribution of the 20S proteasome, the main intracellular protease in mammalian cells, is investigated in NZO-mouse liver after feeding the animals different diets. A significant shift in proteasomal distribution between cytosol and nucleus in response to metabolic stress was revealed using "Cyt/Nuc" via automatized quantification of thousands of nuclei within minutes. "Cyt/Nuc" is easy to use and highly customizable, matches the precision of careful manual evaluation and bears the potential for quick detection of any shift in intracellular protein distribution.

Oxidised protein metabolism: recent insights

The ‘oxygen paradox’ arises from the fact that oxygen, the molecule that aerobic life depends on, threatens its very existence. An oxygen-rich environment provided life on Earth with more efficient bioenergetics and, with it, the challenge of having to deal with a host of oxygen-derived reactive species capable of damaging proteins and other crucial cellular components. In this minireview, we explore recent insights into the metabolism of proteins that have been reversibly or irreversibly damaged by oxygen-derived species. We discuss recent data on the important roles played by the proteasomal and lysosomal systems in the proteolytic degradation of oxidatively damaged proteins and the effects of oxidative damage on the function of the proteolytic pathways themselves. Mitochondria are central to oxygen utilisation in the cell, and their ability to handle oxygen-derived radicals is an important and still emerging area of research. Current knowledge of the proteolytic machinery in the mitochondria, including the ATP-dependent AAA+ proteases and mitochondrial-derived vesicles, is also highlighted in the review. Significant progress is still being made in regard to understanding the mechanisms underlying the detection and degradation of oxidised proteins and how proteolytic pathways interact with each other. Finally, we highlight a few unanswered questions such as the possibility of oxidised amino acids released from oxidised proteins by proteolysis being re-utilised in protein synthesis thus establishing a vicious cycle of oxidation in cells.

Antioxidants and HNE in redox homeostasis

Under physiological conditions, cells are in a stable state known as redox homeostasis, which is maintained by the balance between continuous ROS/RNS generation and several mechanisms involved in antioxidant activity. ROS overproduction results in alterations in the redox homeostasis that promote oxidative damage to major components of the cell, including the biomembrane phospholipids. Lipid peroxidation subsequently generates a diverse set of products, including α,β-unsaturated aldehydes. Of these products, 4-hydroxy-2-nonenal (HNE) is the most studied aldehyde on the basis of its involvement in cellular physiology and pathology. This review summarizes the current knowledge in the field of HNE generation, metabolism, and detoxification, as well as its interactions with various cellular macromolecules (protein, phospholipid, and nucleic acid). The formation of HNE-protein adducts enables HNE to participate in multi-step regulation of cellular metabolic pathways that include signaling and transcription of antioxidant enzymes, pro-inflammatory factors, and anti-apoptotic proteins. The most widely described roles for HNE in the signaling pathways are associated with its activation of kinases, as well as transcription factors that are responsible for redox homeostasis (Ref-1, Nrf2, p53, NFκB, and Hsf1). Depending on its level, HNE exerts harmful or protective effects associated with the induction of antioxidant defense mechanisms. These effects make HNE a key player in maintaining redox homeostasis, as well as producing imbalances in this system that participate in aging and the development of pathological conditions.

Interplay between recombinant Hsp70 and proteasomes: proteasome activity modulation and ubiquitin-independent cleavage of Hsp70

The heat shock protein 70 (Hsp70, human HSPA1A) plays indispensable roles in cellular stress responses and protein quality control (PQC). In the framework of PQC, it cooperates with the ubiquitin-proteasome system (UPS) to clear damaged and dysfunctional proteins in the cell. Moreover, Hsp70 itself is rapidly degraded following the recovery from stress. It was demonstrated that its fast turnover is mediated via ubiquitination and subsequent degradation by the 26S proteasome. At the same time, the effect of Hsp70 on the functional state of proteasomes has been insufficiently investigated. Here, we characterized the direct effect of recombinant Hsp70 on the activity of 20S and 26S proteasomes and studied Hsp70 degradation by the 20S proteasome in vitro. We have shown that the activity of purified 20S proteasomes is decreased following incubation with recombinant human Hsp70. On the other hand, high concentrations of Hsp70 activated 26S proteasomes. Finally, we obtained evidence that in addition to previously reported ubiquitin-dependent degradation, Hsp70 could be cleaved independent of ubiquitination by the 20S proteasome. The results obtained reveal novel aspects of the interplay between Hsp70 and proteasomes.

SIPS as a model to study age-related changes in proteolysis and aggregate formation

Aging is accompanied by the accumulation of cellular damage over time in response to stress, lifestyle and environmental factors ultimately leading to age-related diseases and death. Additionally, the number of senescent cells increases with age. Senescence is most likely not a static endpoint, it represents a series of hallmarks including morphological changes, alterations in protein turnover and accumulation of protein aggregates. The importance of protein oxidation and aggregate accumulation in the progression of aging is not yet fully understood and research to what extent the accumulation of oxidized proteins has an effect on senescence and the aging process is still ongoing. To study the mechanisms of aging, the impact of senescence and the role of protein aggregates on the aging process, cell culture models are useful tools. Most notably stress induced premature senescence (SIPS) models have contributed to the identification of mechanisms involved in the aging process and helped unravel the age-related changes in proteolysis and the importance of protein aggregation. Here we review characteristics of replicative and premature senescence, how to induce most frequently used senescence models and gained knowledge on age-related changes in the major proteolytic systems.

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.

Modulation of proteostasis by transcription factor NRF2 and impact in neurodegenerative diseases

Neurodegenerative diseases are linked to the accumulation of specific protein aggregates, suggesting an intimate connection between injured brain and loss of proteostasis. Proteostasis refers to all the processes by which cells control the abundance and folding of the proteome thanks to a wide network that integrates the regulation of signaling pathways, gene expression and protein degradation systems. This review attempts to summarize the most relevant findings about the transcriptional modulation of proteostasis exerted by the transcription factor NRF2 (nuclear factor (erythroid-derived 2)-like 2). NRF2 has been classically considered as the master regulator of the antioxidant cell response, although it is currently emerging as a key component of the transduction machinery to maintain proteostasis. As we will discuss, NRF2 could be envisioned as a hub that compiles emergency signals derived from misfolded protein accumulation in order to build a coordinated and perdurable transcriptional response. This is achieved by functions of NRF2 related to the control of genes involved in the maintenance of the endoplasmic reticulum physiology, the proteasome and autophagy.

Happily (n)ever after: Aging in the context of oxidative stress, proteostasis loss and cellular senescence

Aging is a complex phenomenon and its impact is becoming more relevant due to the rising life expectancy and because aging itself is the basis for the development of age-related diseases such as cancer, neurodegenerative diseases and type 2 diabetes. Recent years of scientific research have brought up different theories that attempt to explain the aging process. So far, there is no single theory that fully explains all facets of aging. The damage accumulation theory is one of the most accepted theories due to the large body of evidence found over the years. Damage accumulation is thought to be driven, among others, by oxidative stress. This condition results in an excess attack of oxidants on biomolecules, which lead to damage accumulation over time and contribute to the functional involution of cells, tissues and organisms. If oxidative stress persists, cellular senescence is a likely outcome and an important hallmark of aging. Therefore, it becomes crucial to understand how senescent cells function and how they contribute to the aging process. This review will cover cellular senescence features related to the protein pool such as morphological and molecular hallmarks, how oxidative stress promotes protein modifications, how senescent cells cope with them by proteostasis mechanisms, including antioxidant enzymes and proteolytic systems. We will also highlight the nutritional status of senescent cells and aged organisms (including human clinical studies) by exploring trace elements and micronutrients and on their importance to develop strategies that might increase both, life and health span and postpone aging onset.