Age-related decline in ubiquitin conjugation in response to oxidative stress in the lens

Proteostasis in aging-associated ocular disease

Vision impairment has devastating consequences for the quality of human life. The cells and tissues associated with the visual process must function throughout one's life span and maintain homeostasis despite exposure to a variety of insults. Maintenance of the proteome is termed proteostasis, and is vital for normal cellular functions, especially at an advanced age. Here we describe basic aspects of proteostasis, from protein synthesis and folding to degradation, and discuss the current status of the field with a particular focus on major age-related eye diseases: age-related macular degeneration, cataract, and glaucoma. Our intent is to allow vision scientists to determine where and how to harness the proteostatic machinery for extending functional homeostasis in the aging retina, lens, and trabecular meshwork. Several common themes have emerged despite these tissues having vastly different metabolisms. Continued exposure to insults, including chronic stress with advancing age, increases proteostatic burden and reduces the fidelity of the degradation machineries including the ubiquitin-proteasome and the autophagy-lysosome systems that recognize and remove damaged proteins. This "double jeopardy" results in an exponential accumulation of cytotoxic proteins with advancing age. We conclude with a discussion of the challenges in maintaining an appropriate balance of protein synthesis and degradation pathways, and suggest that harnessing proteostatic capacities should provide new opportunities to design interventions for attenuating age-related eye diseases before they limit sight.

Aged Nrf2-Null Mice Develop All Major Types of Age-Related Cataracts

Purpose

Age-related cataracts affect the majority of older adults and are a leading cause of blindness worldwide. Treatments that delay cataract onset or severity have the potential to delay cataract surgery, but require relevant animal models that recapitulate the major types of cataracts for their development. Unfortunately, few such models are available. Here, we report the lens phenotypes of aged mice lacking the critical antioxidant transcription factor Nfe2l2 (designated as Nrf2 −/−).

Methods

Three independent cohorts of Nrf2 −/− and wild-type C57BL/6J mice were evaluated for cataracts using combinations of slit lamp imaging, photography of freshly dissected lenses, and histology. Mice were fed high glycemic diets, low glycemic diets, regular chow ad libitum, or regular chow with 30% caloric restriction.

Results

Nrf2 −/− mice developed significant opacities between 11 and 15 months and developed advanced cortical, posterior subcapsular, anterior subcapsular, and nuclear cataracts. Cataracts occurred similarly in male mice fed high or low glycemic diets, and were also observed in 21-month male and female Nrf2 −/− mice fed ad libitum or 30% caloric restriction. Histological observation of 18-month cataractous lenses revealed significant disruption to fiber cell architecture and the retention of nuclei throughout the cortical region of the lens. However, fiber cell denucleation and initiation of lens differentiation was normal at birth, with the first abnormalities observed at 3 months.

Conclusions

Nrf2 −/− mice offer a tool to understand how defective antioxidant signaling causes multiple forms of cataract and may be useful for screening drugs to prevent or delay cataractogenesis in susceptible adults.

ube3d, a New Gene Associated with Age-Related Macular Degeneration, Induces Functional Changes in Both In Vivo and In Vitro Studies

Neovascular age-related macular degeneration (AMD) is characterized by the formation of choroidal neovascularization, which is responsible for more than 80% of cases of severe vision loss. Ubiquitin protein ligase E3D (UBE3D) gene missense has been proven to be associated with neovascular AMD in the East Asian population based on our previous study. In vivo, we explored the role of ube3d in eye development and the mechanisms underlying the development of neovascular AMD in a zebrafish model. In vitro, we investigated the function and mechanism of ube3d in oxidative damage in human retinal pigment epithelium (hRPE) cells. The ube3d gene was knocked down in zebrafish in our experiments, and rescue of ube3d morphants was also performed. We observed the zebrafish model at the molecular level and functional and morphological changes in vivo. Lentivirus-based gene transfer technology was used to overexpress/knockdown ube3d expression in hRPE cells in vitro. hRPE oxidative damage was induced by tert-butyl hydroperoxide (t-TBH). Cell proliferation and migration were assessed. Quantitative real-time PCR and western blot were used to measure the expression levels of UBE3D and CyclinB1. Abnormal eye development was found in zebrafish in this study, including small eyes, delayed retinal development, delayed retrograde melanosome transport, and reduced dark-induced hyper-locomotor activity under light-off conditions. In addition, increased angiogenesis was observed in ube3d morphants. A negative correlation between UBE3D and CyclinB1 was observed. Low UBE3D expression can promote oxidative damage and inflammatory reactions. UBE3D and autophagy have a synergetic effect on anti-oxidative damage. These findings indicate that ube3d may play an important role in the pathogenesis of AMD by affecting retinal development, oxidative damage, and autophagy.

Inhibition of Proteasome Activity Upregulates IL-6 Expression in RPE Cells through the Activation of P38 MAPKs

Purpose

As far as we know, during the development of age-related macular degeneration (AMD), the activity of proteasome in retinal pigment epithelium cells (RPE) gradually decreases. And a lot of research has shown that age-related macular degeneration is closely related to inflammation and autoimmune. Moreover, there are many cytokines (CKs) involved in the course of inflammation. In this study, we are going to investigate how the decrease of proteasome activity affects the production of interleukin-6 (IL-6) in human retinal pigment epithelium cells (ARPE-19).

Methods

Cultured ARPE-19 was treated with or without MG132, a proteasome inhibitor, and the levels of IL-6 mRNA (messenger ribonucleic acid) in RPE at 1 h, 4 h, 8 h, and IL-6 protein in the culture medium at 2 h, 4 h, 6 h, 8 h, 10 h, and 12 h were measured by real-time polymerase chain reaction (real-time PCR) and enzyme-linked immunosorbent assay (ELISA). The protein levels of MCP-1 (monocyte chemoattractant protein-1) in the culture medium at 2 h, 4 h, 6 h, 8 h, 10 h, and 12 h were also measured by ELISA. Then we tested which of cell signal pathways regulating the production of IL-6 were activated when we added MG132 into the medium by Western blot and electrophoretic mobility shift assays (EMSA). After that, we put the inhibitors of these activated cell signal pathways into the medium individually to see which inhibitor can counteract the effect of upregulating the levels of IL-6 in the culture medium of RPE.

Results

MG132 decreased the secretion of MCP-1 in the culture medium of RPE, but it increased the expression of IL-6 mRNA in RPE and IL-6 protein level in the culture medium of RPE. MG132 treatment was also found to enhance the level of phosphorylated p38 mitogen-activated protein kinases (MAPKs) and c-Jun N-terminal Kinase (JNK) by Western blotting. More importantly, the effect of MG132 on upregulating the levels of IL-6 was inhibited by SB203580, an inhibitor of P38 MAP kinases. But the JNK inhibitor, SP600125, cannot prevent the effect of upregulating the levels of IL-6 by MG132 in the RPE culture medium.

Conclusions

We concluded that the proteasome inhibitor, MG132, upregulates IL-6 production in RPE cells through the activation of P38 MAPKs.

Protective effects of Cassia tora leaves in experimental cataract by modulating intracellular communication, membrane co-transporters, energy metabolism and the ubiquitin-proteasome pathway

CONTEXT

Cataract is the clouding of eye lens which causes impairment in vision and accounts for the leading factor of global blindness. Functional food-based prevention of cataract finds application in vision research because of its availability and easy access to all classes of the society. Cassia tora Linn. (Caesalpinaceae) is an edible plant mentioned in the traditional systems of medicine for whole body health, especially to the eyes.

OBJECTIVE

The present study evaluates the potential of ethyl acetate fraction of Cassia tora leaves (ECT) on experimental cataract.

MATERIALS AND METHODS

Cataract was induced by a single subcutaneous injection of sodium selenite (4 μg/g body weight) on 10th day. ECT was supplemented orally from 8th day up to 12th day at a concentration of 5 μg/g body weight and marker parameters were evaluated after 30 days.

RESULTS

The production of MPO and the activation of calpain were reduced 52.17% and 36.67% by ECT in lens tissue, respectively. It modulated the energy status by significantly increasing the activity of CCO 1 (55.56%) and ATP production (41.88%). ECT maintained the ionic balance in the lens by reducing the level of sodium (50%) and increasing the level of potassium (42.5%). It also reduced cell junction modifications and preserved a functional ubiquitin-proteasome pathway.

DISCUSSION AND CONCLUSION

The results reinforce the growing attention on wild plant food resources for preventive protection against cataract. The data suggest the value of Cassia tora leaves as a functional food for ameliorating cataract pathology.

Molecular Mechanisms Responsible for Increased Vulnerability of the Ageing Oocyte to Oxidative Damage

In their midthirties, women experience a decline in fertility, coupled to a pronounced increase in the risk of aneuploidy, miscarriage, and birth defects. Although the aetiology of such pathologies are complex, a causative relationship between the age-related decline in oocyte quality and oxidative stress (OS) is now well established. What remains less certain are the molecular mechanisms governing the increased vulnerability of the aged oocyte to oxidative damage. In this review, we explore the reduced capacity of the ageing oocyte to mitigate macromolecular damage arising from oxidative insults and highlight the dramatic consequences for oocyte quality and female fertility. Indeed, while oocytes are typically endowed with a comprehensive suite of molecular mechanisms to moderate oxidative damage and thus ensure the fidelity of the germline, there is increasing recognition that the efficacy of such protective mechanisms undergoes an age-related decline. For instance, impaired reactive oxygen species metabolism, decreased DNA repair, reduced sensitivity of the spindle assembly checkpoint, and decreased capacity for protein repair and degradation collectively render the aged oocyte acutely vulnerable to OS and limits their capacity to recover from exposure to such insults. We also highlight the inadequacies of our current armoury of assisted reproductive technologies to combat age-related female infertility, emphasising the need for further research into mechanisms underpinning the functional deterioration of the ageing oocyte.

Effects of microRNA-211 on proliferation and apoptosis of lens epithelial cells by targeting SIRT1 gene in diabetic cataract mice

Our study aimed at exploring the effects of miR-211 on the proliferation and apoptosis of lens epithelial cells in diabetic cataract mice by targetting NAD+-dependent histone deacetylase sirtulin 1 (SIRT1). Healthy male mice were assigned into normal and diabetic cataract groups. Blood glucose, lens turbidity, and apoptosis were measured. Lens epithelial cells were classified into the normal, blank, negative control (NC), miR-211 mimics, miR-211 inhibitors, siRNA-SIRT1, and miR-211 inhibitors + siRNA-SIRT1 groups. MiR-211, Bcl-2, Bax, p53, and SIRT1 expressions of each group were detected. Cell proliferation, cycle and apoptosis were tested by MTT assay and flow cytometry. MiR-211 can specifically bind to SIRT1 according to the luciferase system. SIRT1 protein concentration was strongly positive in normal mice and weakly positive in diabetic cataract mice. Apoptosis index of diabetic cataract mice was higher than the normal mice. Compared with normal mice, the expressions of miR-211, Bax, and p53 increased in diabetic cataract mice, while the Bcl-2 and SIRT1 expressions decreased. In comparison with the blank and NC groups, the expressions of miR-211, Bax, and p53 increased, while Bcl-2 and SIRT1 expressions decreased, and the proliferation decreased and apoptosis rate increased in the miR-211 mimics and siRNA-SIRT1 groups; the results were contradicting for the miR-211 inhibitor group. MiR-211 could promote apoptosis and inhibit proliferation of lens epithelial cells in diabetic cataract mice by targetting SIRT1.

The oxidized thiol proteome in aging and cataractous mouse and human lens revealed by ICAT labeling

Age‐related cataractogenesis is associated with disulfide‐linked high molecular weight (HMW) crystallin aggregates. We recently found that the lens crystallin disulfidome was evolutionarily conserved in human and glutathione‐depleted mouse (LEGSKO) cataracts and that it could be mimicked by oxidation in vitro (Mol. Cell Proteomics, 14, 3211‐23 (2015)). To obtain a comprehensive blueprint of the oxidized key regulatory and cytoskeletal proteins underlying cataractogenesis, we have now used the same approach to determine, in the same specimens, all the disulfide‐forming noncrystallin proteins identified by ICAT proteomics. Seventy‐four, 50, and 54 disulfide‐forming proteins were identified in the human and mouse cataracts and the in vitro oxidation model, respectively, of which 17 were common to all three groups. Enzymes with oxidized cysteine at critical sites include GAPDH (hGAPDH, Cys247), glutathione synthase (hGSS, Cys294), aldehyde dehydrogenase (hALDH1A1, Cys126 and Cys186), sorbitol dehydrogenase (hSORD, Cys140, Cys165, and Cys179), and PARK7 (hPARK7, Cys46 and Cys53). Extensive oxidation was also present in lens‐specific intermediate filament proteins, such as BFSP1 and BFSP12 (hBFSP1 and hBFSP12, Cys167, Cys65, and Cys326), vimentin (mVim, Cys328), and cytokeratins, as well as microfilament and microtubule filament proteins, such as tubulin and actins. While the biological impact of these modifications for lens physiology remains to be determined, many of these oxidation sites have already been associated with either impaired metabolism or cytoskeletal architecture, strongly suggesting that they have a pathogenic role in cataractogenesis. By extrapolation, these findings may be of broader significance for age‐ and disease‐related dysfunctions associated with oxidant stress.

Inhibition of Protein Ubiquitination by Paraquat and 1-Methyl-4-Phenylpyridinium Impairs Ubiquitin-Dependent Protein Degradation Pathways

Intracytoplasmic inclusions of protein aggregates in dopaminergic cells (Lewy bodies) are the pathological hallmark of Parkinson's disease (PD). Ubiquitin (Ub), alpha (α)-synuclein, p62/sequestosome 1, and oxidized proteins are the major components of Lewy bodies. However, the mechanisms involved in the impairment of misfolded/oxidized protein degradation pathways in PD are still unclear. PD is linked to mitochondrial dysfunction and environmental pesticide exposure. In this work, we evaluated the effects of the pesticide paraquat (PQ) and the mitochondrial toxin 1-methyl-4-phenylpyridinium (MPP(+)) on Ub-dependent protein degradation pathways. No increase in the accumulation of Ub-bound proteins or aggregates was observed in dopaminergic cells (SK-N-SH) treated with PQ or MPP(+), or in mice chronically exposed to PQ. PQ decreased Ub protein content, but not its mRNA transcription. Protein synthesis inhibition with cycloheximide depleted Ub levels and potentiated PQ-induced cell death. The inhibition of proteasomal activity by PQ was found to be a late event in cell death progression and had neither effect on the toxicity of either MPP(+) or PQ, nor on the accumulation of oxidized sulfenylated, sulfonylated (DJ-1/PARK7 and peroxiredoxins), and carbonylated proteins induced by PQ. PQ- and MPP(+)-induced Ub protein depletion prompted the dimerization/inactivation of the Ub-binding protein p62 that regulates the clearance of ubiquitinated proteins by autophagy. We confirmed that PQ and MPP(+) impaired autophagy flux and that the blockage of autophagy by the overexpression of a dominant-negative form of the autophagy protein 5 (dnAtg5) stimulated their toxicity, but there was no additional effect upon inhibition of the proteasome. PQ induced an increase in the accumulation of α-synuclein in dopaminergic cells and membrane-associated foci in yeast cells. Our results demonstrate that the inhibition of protein ubiquitination by PQ and MPP(+) is involved in the dysfunction of Ub-dependent protein degradation pathways.

Nutritional modulation of cataract

Lens opacification or cataract reduces vision in over 80 million people worldwide and blinds 18 million. These numbers will increase dramatically as both the size of the elderly demographic and the number of those with carbohydrate metabolism-related problems increase. Preventative measures for cataract are critical because the availability of cataract surgery in much of the world is insufficient. Epidemiologic literature suggests that the risk of cataract can be diminished by diets that are optimized for vitamin C, lutein/zeaxanthin, B vitamins, omega-3 fatty acids, multivitamins, and carbohydrates: recommended levels of micronutrients are salutary. The limited data from intervention trials provide some support for observational studies with regard to nuclear - but not other types of - cataracts. Presented here are the beneficial levels of nutrients in diets or blood and the total number of participants surveyed in epidemiologic studies since a previous review in 2007.

The ubiquitin-proteasome system in retinal health and disease

The ubiquitin-proteasome system (UPS) is the main intracellular pathway for modulated protein turnover, playing an important role in the maintenance of cellular homeostasis. It also exerts a protein quality control through degradation of oxidized, mutant, denatured, or misfolded proteins and is involved in many biological processes where protein level regulation is necessary. This system allows the cell to modulate its protein expression pattern in response to changing physiological conditions and provides a critical protective role in health and disease. Impairments of UPS function in the central nervous system (CNS) underlie an increasing number of genetic and idiopathic diseases, many of which affect the retina. Current knowledge on the UPS composition and function in this tissue, however, is scarce and dispersed. This review focuses on UPS elements reported in the retina, including ubiquitinating and deubiquitinating enzymes (DUBs), and alternative proteasome assemblies. Known and inferred roles of protein ubiquitination, and of the related, SUMO conjugation (SUMOylation) process, in normal retinal development and adult homeostasis are addressed, including modulation of the visual cycle and response to retinal stress and injury. Additionally, the relationship between UPS dysfunction and human neurodegenerative disorders affecting the retina, including Alzheimer's, Parkinson's, and Huntington's diseases, are dealt with, together with numerous instances of retina-specific illnesses with UPS involvement, such as retinitis pigmentosa, macular degenerations, glaucoma, diabetic retinopathy (DR), and aging-related impairments. This information, though still basic and limited, constitutes a suitable framework to be expanded in incoming years and should prove orientative toward future therapy design targeting sight-affecting diseases with a UPS underlying basis.

Enhancement of ubiquitin conjugation activity reduces intracellular aggregation of V76D mutant γD-crystallin

PURPOSE

The ubiquitin-proteasome pathway (UPP) is an important protein quality control mechanism for selective degradation of abnormal proteins. The objective of this study was to test the hypothesis that enhancement of the UPP capacity could attenuate the accumulation and aggregation of misfolded proteins using V76D-γD-crystallin as a model substrate.

METHODS

Wild type (wt) and V76D mutant γD-crystallin were fused to red fluorescence protein (RFP) and expressed in human lens epithelial cells. The cellular distribution of the expressed proteins was compared by fluorescence microscopy. The solubility of wt- and V76D-γD-crystallin was determined by cellular fractionation and Western blotting. Wt-γD-RFP and V76D-γD-RFP were also cotransfected along with a ubiquitin ligase (CHIP) or a ubiquitin-conjugating enzyme (Ubc5) into cells. Levels of wt- and V76D-γD-crystallin, the percentage of transfected cells with aggregates, and aggregate size were quantified and compared among different groups.

RESULTS

Wt-γD-crystallin was evenly distributed in cells, whereas V76D-γD-crystallin formed intracellular aggregates. Eighty percent of wt-γD-crystallin was detected in the soluble fraction, whereas only 7% of V76D-γD-crystallin was soluble. CHIP or Ubc5 coexpression reduced the protein level of V76D-γD and concomitantly its aggregation in transfected cells; these effects could be attenuated by proteasome inhibitor. Mutant CHIP with defect TPR (tetratricopeptide repeat) or U-box domain failed to reduce levels of V76D-γD-crystallin.

CONCLUSIONS

Enhancing ubiquitin conjugation activity reduces accumulation and aggregation of V76D-γD-crystallin by promoting its degradation. Upregulation of ubiquitin-conjugating activity could be an effective strategy to maintain lens transparency by eliminating other forms of misfolded proteins.

Roles for the ubiquitin-proteasome pathway in protein quality control and signaling in the retina: implications in the pathogenesis of age-related macular degeneration

The accumulation of damaged or postsynthetically modified proteins and dysregulation of inflammatory responses and angiogenesis in the retina/RPE are thought be etiologically related to formation of drusen and choroidal neovascularization (CNV), hallmarks of age-related macular degeneration (AMD). The ubiquitin-proteasome pathway (UPP) plays crucial roles in protein quality control, cell cycle control and signal transduction. Selective degradation of aberrant proteins by the UPP is essential for timely removal of potentially cytotoxic damaged or otherwise abnormal proteins. Proper function of the UPP is thought to be required for cellular function. In contrast, age--or stress induced--impairment the UPP or insufficient UPP capacity may contribute to the accumulation of abnormal proteins, cytotoxicity in the retina, and AMD. Crucial roles for the UPP in eye development, regulation of signal transduction, and antioxidant responses are also established. Insufficient UPP capacity in retina and RPE can result in dysregulation of signal transduction, abnormal inflammatory responses and CNV. There are also interactions between the UPP and lysosomal proteolytic pathways (LPPs). Means that modulate the proteolytic capacity are making their way into new generation of pharmacotherapies for delaying age-related diseases and may augment the benefits of adequate nutrition, with regard to diminishing the burden of AMD.

Oligomerization with wt αA- and αB-crystallins reduces proteasome-mediated degradation of C-terminally truncated αA-crystallin

PURPOSE

We previously demonstrated that the ubiquitin-proteasome pathway (UPP) is a general protein quality control system that selectively degrades damaged or abnormal lens proteins, including C-terminally truncated αA-crystallin. The objective of this work was to determine the effects of wt αA- and αB-crystallins on the degradation of C-terminally truncated αA-crystallin (αA(1-162)) and vice versa.

METHODS

Recombinant wt αA, αB, and αA(1-162) were expressed in Escherichia coli and purified to homogeneity by chromatography. Subunit exchange and oligomerization were detected by fluorescence resonance energy transfer (FRET), multiangle-light scattering and coprecipitation assays. Protein substrates were labeled with (125)I and lens epithelial cell lysates were used as the source of the UPP for degradation assays.

RESULTS

FRET, multiangle light scattering, and coprecipitation assays showed that αA(1-162) exchanged subunits with wt αA- or wt αB- crystallin to form hetero-oligomers. αA(1-162) was more susceptible than wt αA-crystallin to degradation by the UPP. When mixed with wt αA-crystallin at 1:1 or 1:4 (αA(1-162) : wt) ratios to form hetero-oligomers, the degradation of αA(1-162) was significantly decreased. Conversely, formation of hetero-oligomers with αA(1-162) enhanced the degradation of wt αA-crystallin. The presence of αA(1-162), but not wt αA-crystallin, decreased the degradation of wt αB-crystallin.

CONCLUSIONS

αA(1-162) forms hetero-oligomers with wt αA- and αB-crystallins. Oligomerization with wt αA- or αB-crystallins reduces the susceptibility of αA(1-162) to degradation by the UPP. In addition, the presence of αA(1-162) in the hetero-oligomers also affects the degradation of wt αA- and αB-crystallins.

Role of the ubiquitin-proteasome in protein quality control and signaling: implication in the pathogenesis of eye diseases

The ubiquitin-proteasome pathway (UPP) plays important roles in many cellular functions, such as protein quality control, cell cycle control, and signal transduction. The selective degradation of aberrant proteins by the UPP is essential for the timely removal of potential cytotoxic damaged or otherwise abnormal proteins. Conversely, accumulation of the cytotoxic abnormal proteins in eye tissues is etiologically associated with many age-related eye diseases such as retina degeneration, cataract, and certain types of glaucoma. Age- or stress-induced impairment or overburdening of the UPP appears to contribute to the accumulation of abnormal proteins in eye tissues. Cell cycle and signal transduction are regulated by the conditional UPP-dependent degradation of the regulators of these processes. Impairment or overburdening of the UPP could also result in dysregulation of cell cycle control and signal transduction. The consequences of the improper cell cycle and signal transduction include defects in ocular development, wound healing, angiogenesis, or inflammatory responses. Methods that enhance or preserve UPP function or reduce its burden may be useful strategies for preventing age-related eye diseases.

Lens fibre cell differentiation and organelle loss: many paths lead to clarity

The programmed removal of organelles from differentiating lens fibre cells contributes towards lens transparency through formation of an organelle-free zone (OFZ). Disruptions in OFZ formation are accompanied by the persistence of organelles in lens fibre cells and can contribute towards cataract. A great deal of work has gone into elucidating the nature of the mechanisms and signalling pathways involved. It is apparent that multiple, parallel and redundant pathways are involved in this process and that these pathways form interacting networks. Furthermore, it is possible that the pathways can functionally compensate for each other, for example in mouse knockout studies. This makes sense given the importance of lens clarity in an evolutionary context. Apoptosis signalling and proteolytic pathways have been implicated in both lens fibre cell differentiation and organelle loss, including the Bcl-2 and inhibitor of apoptosis families, tumour necrosis factors, p53 and its regulators (such as Mdm2) and proteolytic enzymes, including caspases, cathepsins, calpains and the ubiquitin-proteasome pathway. Ongoing approaches being used to dissect the molecular pathways involved, such as transgenics, lens-specific gene deletion and zebrafish mutants, are discussed here. Finally, some of the remaining unresolved issues and potential areas for future studies are highlighted.

Ubiquitin-proteasome pathway and cellular responses to oxidative stress

The ubiquitin-proteasome pathway (UPP) is the primary cytosolic proteolytic machinery for the selective degradation of various forms of damaged proteins. Thus, the UPP is an important protein quality control mechanism. In the canonical UPP, both ubiquitin and the 26S proteasome are involved. Substrate proteins of the canonical UPP are first tagged by multiple ubiquitin molecules and then degraded by the 26S proteasome. However, in noncanonical UPP, proteins can be degraded by the 26S or the 20S proteasome without being ubiquitinated. It is clear that a proteasome is responsible for selective degradation of oxidized proteins, but the extent to which ubiquitination is involved in this process remains a subject of debate. Whereas many publications suggest that the 20S proteasome degrades oxidized proteins independent of ubiquitin, there is also solid evidence indicating that ubiquitin and ubiquitination are involved in degradation of some forms of oxidized proteins. A fully functional UPP is required for cells to cope with oxidative stress and the activity of the UPP is also modulated by cellular redox status. Mild or transient oxidative stress up-regulates the ubiquitination system and proteasome activity in cells and tissues and transiently enhances intracellular proteolysis. Severe or sustained oxidative stress impairs the function of the UPP and decreases intracellular proteolysis. Both the ubiquitin-conjugating enzymes and the proteasome can be inactivated by sustained oxidative stress, especially the 26S proteasome. Differential susceptibilities of the ubiquitin-conjugating enzymes and the 26S proteasome to oxidative damage lead to an accumulation of ubiquitin conjugates in cells in response to mild oxidative stress. Thus, increased levels of ubiquitin conjugates in cells seem to be an indicator of mild oxidative stress.

Glyoxalase I activity and immunoreactivity in the aging human lens

Glyoxalase I (GLOI) is the first enzyme of the glyoxalase system that catalyzes the metabolism of reactive dicarbonyls, such as methylglyoxal (MGO). During aging and cataract development, human lens proteins are chemically modified by MGO, which is likely due to inadequate metabolism of MGO by the glyoxalase system. In this study, we have determined the effect of aging on GLOI activity and the immunoreactivity and morphological distribution of GLOI in the human lens. A monoclonal antibody was developed against human GLOI. GLOI immunoreactivity was strongest in the anterior epithelial cells and weaker in rest of the lens. Cultured human lens epithelial cells showed immunostaining throughout the cytoplasm. In the human lens, GLOI activity and immunoreactivity both decreased with age. We believe that this would lead to promotion of MGO-modification in aging lens proteins.

Post-translational modification of cellular proteins by ubiquitin and ubiquitin-like molecules: role in cellular senescence and aging

Ubiquitination ofendogenous proteins is one of the key regulatory steps that guides protein degradation through regulation of proteasome activity. During the last years evidence has accumulated that proteasome activity is decreased during the aging process in various model systems and that these changes might be causally related to aging and age-associated diseases. Since in most instances ubiquitination is the primary event in target selection, the system ofubiquitination and deubiquitination might be of similar importance. Furthermore, ubiquitination and proteasomal degradation are not completely congruent, since ubiquitination confers also functions different from targeting proteins for degradation. Depending on mono- and polyubiquitination and on how ubiquitin chains are linked together, post-translational modifications of cellular proteins by covalent attachment of ubiquitin and ubiquitin-like proteins are involved in transcriptional regulation, receptor internalization, DNA repair, stabilization of protein complexes and autophagy. Here, we summarize the current knowledge regarding the ubiquitinome and the underlying ubiquitin ligases and deubiquitinating enzymes in replicative senescence, tissue aging as well as in segmental progeroid syndromes and discuss potential causes and consequences for aging.

Proteasome inactivation promotes p38 mitogen-activated protein kinase-dependent phosphatidylinositol 3-kinase activation and increases interleukin-8 production in retinal pigment epithelial cells

Oxidative stress and inflammation are implicated in the pathogenesis of many age-related diseases. We have demonstrated previously that oxidative inactivation of the proteasome is a molecular link between oxidative stress and overexpression of interleukin (IL)-8. Here, we elucidated a novel signaling cascade that leads to up-regulation of IL-8 in response to proteasome inactivation. The sequence of events in this cascade includes proteasome inactivation, activation of mitogen-activated protein kinase kinase (MKK)3/MKK6, activation of p38 mitogen-activated protein kinase (MAPK), epidermal growth factor receptor phosphorylation, phosphatidylinositol 3-kinase (PI3K) activation and increased IL-8 expression. Blocking any of these signaling pathways abolished the up-regulation of IL-8 induced by proteasome inhibition. Although Akt is also activated in response to proteasome inactivation, we found that the PI3K-dependent up-regulation of IL-8 is independent of 3-phosphoinositide-dependent protein kinase (PDK)1 and Akt. Inhibition of PDK1 and Akt with chemical inhibitors or expression of constitutive active Akt had little effects on IL-8 expression in response to proteasome inactivation. In contrast, inhibition of interleukin 2-inducible T cell kinase, a kinase downstream of PI3K, significantly reduced the expression and secretion of IL-8 in response to proteasome inactivation. Together, these data elucidate a novel signaling network that leads to increased IL-8 production in response to proteasome inactivation.

Lens aging: effects of crystallins

The primary function of the eye lens is to focus light on the retina. The major proteins in the lens--alpha, beta, and gamma-crystallins--are constantly subjected to age-related changes such as oxidation, deamidation, truncation, glycation, and methylation. Such age-related modifications are cumulative and affect crystallin structure and function. With time, the modified crystallins aggregate, causing the lens to increasingly scatter light on the retina instead of focusing light on it and causing the lens to lose its transparency gradually and become opaque. Age-related lens opacity, or cataract, is the major cause of blindness worldwide. We review deamidation, and glycation that occur in the lenses during aging keeping in mind the structural and functional changes that these modifications bring about in the proteins. In addition, we review proteolysis and discuss recent observations on how crystallin fragments generated in vivo, through their anti-chaperone activity may cause crystallin aggregation in aging lenses. We also review hyperbaric oxygen treatment induced guinea pig and 'humanized' ascorbate transporting mouse models as suitable options for studies on age-related changes in lens proteins.

Increased age reduces DAF-16 and SKN-1 signaling and the hormetic response of Caenorhabditis elegans to the xenobiotic juglone

Cells adapt to stressors by activating mechanisms that repair damage and protect them from further injury. Stress-induced damage accumulates with age and contributes to age associated diseases. Increased age attenuates the ability to mount a stress response, but little is known about the mechanisms by which this occurs. To begin addressing this problem, we studied hormesis in the nematode Caenorhabditis elegans. When exposed to a low concentration of the xenobiotic juglone, young worms mount a robust hormetic stress response and survive a subsequent exposure to a higher concentration of juglone that is normally lethal to naïve animals. Old worms are unable to mount this adaptive response. Microarray and RNAi analyses demonstrate that an altered transcriptional response to juglone is responsible in part for the reduced adaptation of old worms. Many genes differentially regulated in young versus old animals are known or postulated to be regulated by the FOXO homologue DAF-16 and the Nrf2 homologue SKN-1. Activation of these pathways is greatly reduced in juglone stressed old worms. DAF-16- and SKN-1-like transcription factors play highly conserved roles in regulating stress resistance and longevity genes. Our studies provide a foundation for developing a molecular understanding of how age affects cytoprotective transcriptional pathways.

Glutathione and its transporters in ocular surface defense

Glutathione (GSH) is an abundant antioxidant ubiquitous in nearly all cell types. Deficiency of GSH has been linked to ocular disease and viral infection. Other established vital roles of GSH include detoxification and immunoprotection. Endogenous GSH plays a protagonist's role in safeguarding active transport processes compartmentalized at the interface between conjunctival mucosa and the tear film. Optimal electrokinetic transport across the conjunctival epithelium requires the mucosal presence of GSH. Glutathione is the most abundant known endogenous antioxidant molecule in tear fluid, mainly derived from conjunctival secretion. Conjunctival GSH transport, a major kinetic component of GSH turnover, occurs through multiple functionally distinct mechanisms. Cell membrane potential regulates conjunctival GSH efflux, while conjunctival GSH uptake requires extracellular Na(+). Significant modulation of GSH, its constituent amino acids, and functions of associated transporters occurs in the conjunctival epithelium with viral inflammatory disease. Topical conjunctival delivery of GSH, its metabolic precursors, or pharmacologic stimulation of endogenous conjunctival GSH secretion carry potential in alleviating viral-inflammatory conjunctivitis.

Cell cycle arrest by kynurenine in lens epithelial cells

PURPOSE

Indolemine 2,3-dioxygenase (IDO)-mediated oxidation of tryptophan produces kynurenines (KYNs), which may play a role in cataract formation. The molecular mechanisms by which KYNs cause cellular changes are poorly understood. The effects of KYNs on mouse lens epithelial cells by overexpression of human IDO were investigated.

METHODS

Lens epithelial cells (mLECs) derived from human IDO-overexpressing hemizygous transgenic (hemTg) and wild-type (Wt) mice were used. IDO activity was measured by quantifying kynurenine (KYN) by HPLC. KYN-mediated protein modifications were detected by immunocytochemistry and measured by ELISA. Cell proliferation and apoptosis were measured with commercially available kits. Cell distribution between cell cycle phases was examined with flow cytometric analysis. Immunoprecipitation followed by LC/MS was used to identify kynurenine-modified proteins.

RESULTS

mLECs derived from hemTg animals exhibited considerable IDO immunoreactivity and enzyme activity, which were barely detectable in Wt mLECs. KYN and KYN-mediated protein modification were detected in hemTg but not in Wt mLECs; the modified proteins were myosin II and alpha/gamma-actin. HemTg mLECs displayed reduced viability and proliferation. Cell cycle analysis of hemTg mLEC cultures showed approximately a twofold increase in cells at G(2)/M or in both phases, relative to Wt mLECs. Blocking IDO activity with 1-methyl-d,l-tryptophan in hemTg mLECs prevented KYN formation, KYN-mediated protein modification, and G(2)/M arrest.

CONCLUSIONS

Excess IDO activity in mLECs results in KYN production, KYN-mediated modification of myosin II and alpha/gamma-actin, and cell cycle perturbation. Modification of myosin II and gamma-actin by KYN may interfere with cytokinesis, leading to defective epithelial cell division and thus a decreased number of fiber cells.

Oxidative inactivation of the proteasome in retinal pigment epithelial cells. A potential link between oxidative stress and up-regulation of interleukin-8

Oxidative stress and inflammation are implicated in the pathogenesis of many age-related diseases. Stress-induced overproduction of inflammatory cytokines, such as interleukin-8 (IL-8), is one of the early events of inflammation. The objective of this study was to elucidate mechanistic links between oxidative stress and overproduction of IL-8 in retinal pigment epithelial (RPE) cells. We found that exposure of RPE cells to H(2)O(2), paraquat, or A2E-mediated photooxidation resulted in increased expression and secretion of IL-8. All of these oxidative stressors also inactivated the proteasome in RPE cells. In contrast, tert-butylhydroperoxide (TBH), a lipophilic oxidant that did not stimulate IL-8 production, also did not inactivate the proteasome. Moreover, prolonged treatment of RPE cells with proteasome-specific inhibitors recapitulated the stimulation of IL-8 production. These data suggest that oxidative inactivation of the proteasome is a potential mechanistic link between oxidative stress and up-regulation of the proinflammatory IL-8. The downstream signaling pathways that govern the production of IL-8 include NF-kappaB and p38 MAPK. Proteasome inhibition both attenuated the activation and delayed the turnoff of NF-kappaB, resulting in biphasic effects on the production of IL-8. Prolonged proteasome inhibition (>2 h) resulted in activation of p38 MAPK via activation of MKK3/6 and increased the production of IL-8. Chemically inhibiting the p38 MAPK blocked the proteasome inhibition-induced up-regulation of IL-8. Together, these data indicate that oxidative inactivation of the proteasome and the related activation of the p38 MAPK pathway provide a potential link between oxidative stress and overproduction of proinflammatory cytokines, such as IL-8.

Oxidative stress and the eye

Oxidative and particularly photo-oxidative processes are critical factors many ocular conditions but are often poorly recognized by those investigating ocular disease. The author discusses oxidative stress in inflammatory processes of the conjunctiva, cornea, and uvea; in cataract formation in the lens; in retinal degeneration; and in optic nerve pathologic conditions, inflammatory in optic neuritis and degenerative in glaucoma.

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

PURPOSE

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Degradation of C-terminal truncated alpha A-crystallins by the ubiquitin-proteasome pathway

PURPOSE

Calpain-mediated C-terminal cleavage of alpha A-crystallins occurs during aging and cataractogenesis. The objective of the present study was to explore the role of the ubiquitin-proteasome pathway (UPP) in degrading C-terminal truncated alpha A-crystallins.

METHODS

Recombinant wild-type (wt) alpha A-crystallin and C-terminal truncated alpha A(1-168)-, alpha A(1-163)-, and alpha A(1-162)-crystallins were expressed in Escherichia coli and purified to homogeneity. The wt and truncated alpha A-crystallins were labeled with (125)I, and proteolytic degradation was determined using both lens fiber lysate and reticulocyte lysate as sources of ubiquitinating and proteolytic enzymes. Far UV circular dichroism, tryptophan fluorescence intensity, and binding to the hydrophobic fluorescence probe Bis-ANS were used to characterize the wt and truncated alpha A-crystallins. Oligomer sizes of these crystallins were determined by multiangle light-scattering.

RESULTS

Whereas wt alpha A-crystallin was degraded moderately in both lens fiber and reticulocyte lysates, alpha A(1-168)-crystallin was resistant to degradation. The susceptibility of alpha A(1-163)-crystallin to degradation was comparable to that of wt alpha A-crystallin. However, alpha A(1-162)-crystallin was much more susceptible than wt alpha A-crystallin to degradation in both lens fiber and reticulocyte lysates. The degradation of both wt and C-terminal truncated alpha A(1-162)-crystallins requires adenosine triphosphate (ATP) and was stimulated by addition of a ubiquitin-conjugating enzyme, Ubc4. The degradation was substantially inhibited by the proteasome inhibitor MG132 and a dominant negative mutant of ubiquitin, K6W-Ub, indicating that at least part of the proteolysis was mediated by the UPP. Spectroscopic analyses of wt and C-terminal truncated alpha A-crystallins revealed that C-terminal truncation of alpha A-crystallin resulted in only subtle changes in secondary structures. However, C-terminal truncations resulted in significant changes in surface hydrophobicity and thermal stability. Thus, these conformational changes may reveal or mask the signals for the ubiquitin-dependent degradation.

CONCLUSIONS

The present data demonstrate that C-terminal cleavage of alpha A-crystallin not only alters its conformation and thermal stability, but also its susceptibility to degradation by the UPP. The rapid degradation of alpha A(1-162) by the UPP may prevent its accumulation in the lens.

Diaphragmatic proteasome function is maintained in the ageing Fisher 344 rat

The diaphragm is the most important inspiratory muscle in mammals and is essential for normal ventilation. Therefore, maintenance of diaphragm function is critical to overall health throughout the lifespan. Evidence indicates that the ubiquitin proteasome pathway (UPP) function is diminished in locomotor skeletal muscle of ageing animals, but the function of the UPP in the senescent diaphragm has not yet been studied. Diaphragms were harvested from 6- and 24- to 26-month-old Fisher 344 rats (n = 8 per group), and a comprehensive assessment of key components of the UPP, proteasome activity and ubiquitin-conjugating enzyme activity was performed. Gene expression and diaphragmatic protein levels of several key proteasome components are not altered in the diaphragm by ageing. Furthermore and most importantly, the senescent diaphragm exhibited no age-related changes in the content of endogenous ubiquitin-protein conjugates or 20S proteasome activity. In conclusion, in contrast to locomotor skeletal muscle, proteasome function and ubiquitin-conjugating enzyme activity are preserved during senescence in diaphragm. A more thorough understanding of the divergent molecular mechanisms and pathways regulating the UPP in different skeletal muscles could lead to the enhancement of therapeutic strategies aimed at improving morbidity and mortality outcomes in different clinical populations.

Aging and the ubiquitinome: traditional and non-traditional functions of ubiquitin in aging cells and tissues

Ubiquitination of endogenous proteins is one of the key regulatory steps of protein degradation followed by regulation of proteasome activity. During the last years evidence has increased that proteasome activity is decreased during the aging process in various model systems and that these changes might be causally related to aging and aging associated diseases. Since in most instances ubiquitination is the primary event in target selection, the system of ubiquitination and deubiquitination might be of similar importance. Furthermore, ubiquitination and proteasomal degradation are not completely congruent, since ubiquitination also confers functions different from giving "the kiss of death" to proteins. Depending on mono- and polyubiquitination and on how ubiquitin chains are linked together, ubiquitination is involved in transcriptional regulation, receptor internalization, DNA repair, and stabilization of protein complexes. This review is therefore the first to summarize the current knowledge regarding the ubiquitinome and the underlying ubiquitin ligases and deubiquitinating enzymes in replicative senescence, tissue aging as well as in segmental progeroid syndromes and to discuss potential causes and consequences for aging.

Proteasome-dependent regulation of signal transduction in retinal pigment epithelial cells

As in many other types of cells, retinal pigment epithelial (RPE) cells have an active ubiquitin-proteasome pathway (UPP). However, the function of the UPP in RPE remains to be elucidated. The objective of this study is to determine the role of the UPP in controlling the levels and activities of transcription factors hypoxia-inducible factor (HIF) and NF-kappaB. We inhibited the UPP with proteasome-specific inhibitors and determined the activation of HIF and NF-kappaB as well as the expression and secretion of pro-angiogenic factors. HIF-1alpha was not detectable in ARPE-19 cells under normal culture conditions. However, when proteasome activity was inhibited, HIF-1alpha accumulated in RPE in a time-dependent manner. Consistent with accumulation of HIF-1alpha in the cells, levels of mRNA for vascular endothelial growth factor (VEGF) and angiopoietin-2 (Ang-2) in RPE were up to 7-fold higher upon inhibition of the proteasome. Proteasome inhibition was also associated with a 2-fold increase in levels of mRNA for angiopoietin-1 (Ang-1). ARPE-19 cells secrete significant levels of VEGF under normal culture conditions. Inhibition of proteasome activity increased the secretion of VEGF by 2-fold. In contrast to the increase in HIF activity, NF-kappaB activation was reduced by proteasome inhibition. In addition, the expression and secretion of monocyte chemoattractant protein-1 (MCP-1) by RPE were substantially attenuated by the inhibition of proteasome activity. These data demonstrate that the UPP plays an important role in modulating the activities of HIF and NF-kappaB in the RPE. Consequences of an impairment of the UPP include accumulation of HIF-1alpha and diminished NF-kappaB activation, which lead to enhanced expression and secretion of pro-angiogenic factors and attenuated expression of MCP-1. Taken together, these data predict that the impairment of the UPP could lead to the development of AMD-related phenotypes.

Glutathiolation enhances the degradation of gammaC-crystallin in lens and reticulocyte lysates, partially via the ubiquitin-proteasome pathway

PURPOSE

S-glutathiolated proteins are formed in the lens during aging and cataractogenesis. The objective of this work was to explore the role of the ubiquitin-proteasome pathway in eliminating S-glutathiolated gammaC-crystallin.

METHODS

Recombinant human gammaC-crystallin was mixed with various concentrations of glutathione (GSH) and diamide at 25 degrees C for 1 hour. The extent of glutathiolation of the gammaC-crystallin was determined by mass spectrometry. Native and S-glutathiolated gammaC-crystallins were labeled with (125)I, and proteolytic degradation was determined using both lens fiber lysate and reticulocyte lysate as sources of ubiquitinating and proteolytic enzymes. Far UV circular dichroism, tryptophan fluorescence intensity, and binding to the hydrophobic fluorescence probe 4,4'-dianilino-1,1'-binaphthalene-5,5'-disulfonic acid (Bis-ANS), were used to characterize the native and glutathiolated gammaC-crystallins.

RESULTS

On average, two and five of the eight cysteines in gammaC-crystallin were glutathiolated when molar ratios of gammaC-crystallin-GSH-diamide were 1:2:5 and 1:10:25, respectively. Native gammaC-crystallin was resistant to degradation in both lens fiber lysate and reticulocyte lysate. However, glutathiolated gammaC-crystallin showed a significant increase in proteolytic degradation in both lens fiber and reticulocyte lysates. Proteolysis was stimulated by addition of adenosine triphosphate (ATP) and Ubc4 and was substantially inhibited by the proteasome inhibitor MG132 and a dominant negative form of ubiquitin, indicating that at least part of the proteolysis was mediated by the ubiquitin-proteasome pathway. Spectroscopic analyses of glutathiolated gammaC-crystallin revealed conformational changes and partial unfolding, which may provide a signal for the ubiquitin-dependent degradation.

CONCLUSIONS

The present data demonstrate that oxidative modification by glutathiolation can render lens proteins more susceptible to degradation by the ubiquitin-proteasome pathway. Together with previous results, these data support the concept that the ubiquitin-proteasome pathway serves as a general protein quality-control mechanism.

Selective downregulation of ubiquitin conjugation cascade mRNA occurs in the senescent rat soleus muscle

Aging-related alterations of the ubiquitin proteasome pathway (UPP) have been reported in locomotor skeletal muscle. Specifically, declines in proteasome activity have been observed in the soleus of senescent animals compared to the soleus of young controls. However, the influence of aging on the mRNA levels of key components within the ubiquitin conjugation cascade (UCC) remains unknown. We hypothesized that aged soleus muscle would exhibit downregulated expression of select UCC mRNA and decreased levels of ubiquitin-protein conjugates. To test this postulate, we harvested soleus muscles from 6 and 24-26 month old Fisher 344 rats. Aging resulted in a decline in mRNA expression of two key UCC components in soleus muscle; ubiquitin conjugating enzyme E2(14k) (E2(14k)) and muscle ring finger-1 (MuRF1). Surprisingly, no age-related differences existed in the total content of endogenous ubiquitin-protein conjugates in the soleus muscle. Nonetheless, a selective decrease in the level of ubiquitin-protein conjugates ( approximately 30kDa) was detected in the soleus of senescent animals. These results indicate that the soleus muscle displays a differential mRNA response of select UCC components to aging. Furthermore, the decline in E2(14k) and MuRF1 mRNA levels may contribute to altered substrate degradation by the UCC in the soleus muscle of senescent rats.

The ubiquitin-proteasome system in cardiac physiology and pathology

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

Dysregulation of the ubiquitin-proteasome system in human carotid atherosclerosis

OBJECTIVE

The ubiquitin-proteasome system is the principal degradation route of intracellular and oxidized proteins, thus regulating many cellular processes conceivably important for atherosclerosis. The aim of this study was to evaluate the activity of ubiquitin-proteasome system in human carotid artery plaques in relation to oxidative stress and clinical manifestation.

METHODS AND RESULTS

In carotid endarterectomy specimens from 83 asymptomatic and 94 symptomatic patients, content of ubiquitin, ubiquitin conjugates, matrix metalloproteases, and NADPH-oxidase-p67 was evaluated by immunoblotting; proteolytic proteasome activity by fluorometric assay; single and double immunostaining for ubiquitin conjugates, 3-nitrotyrosine, apoptosis, smooth muscle alpha-actin, and macrophage CD-68, as well as Sirius Red staining for collagen were performed. Compared with asymptomatic patients, symptomatic patients showed a more unstable plaque phenotype, an increased degree of apoptosis, a significantly higher ubiquitin conjugates content (17.72+/-1.36 versus 10.99+/-1.04; P<0.001), and lower proteasome activity (5.01+/-0.70 versus 9.41+/-1.19 nmol AMC/mg protein/min; P<0.01). Ubiquitin conjugates content was directly correlated to NADPH-p67 and degree of apoptosis. Immunostaining revealed colocalization of ubiquitin conjugates and 3-nitrotyrosine, and accumulation of ubiquitin conjugates in smooth muscle cells and macrophages.

CONCLUSIONS

In human carotid plaques increased oxidative stress is associated with inhibition of the proteasome activity and accumulation of ubiquitin conjugates, particularly in symptomatic patients. These results suggest a possible role of the ubiquitin-proteasome system in influencing plaque stability.

Oxidative damage and age-related functional declines

Most organisms experience progressive declines in physiological function as they age. Since this senescence of function is thought to underlie the decrease in quality of life in addition to the increase in susceptibility to disease and death associated with aging, identifying the mechanisms involved would be highly beneficial. One of the leading mechanistic theories for aging is the oxidative damage hypothesis. A number of studies in a variety of species support a strong link between oxidative damage and life span determination. The role of oxidative damage in functional senescence has also been investigated, albeit not as comprehensively. Here, we review these investigations. Several studies show that the age-related loss of a number of functions is associated with an accrual of oxidative damage in the tissues mediating those functions. Additionally, treatments that increase the accumulation of oxidative damage with age frequently exacerbate functional losses. Moreover, treatments that reduce the accumulation of oxidative damage often attenuate or delay the loss of function associated with aging. These data provide the foundation for a link between oxidative damage and functional senescence, thereby supporting the oxidative damage hypothesis of aging within the context of age-related functional decline.

Proteases in eye development and disease

The eye is one of the classical systems in developmental biology. Furthermore, diseases of the eye, many of which have a developmental basis, have devastating effects that often result in blindness. Proteases have diverse roles in ocular physiology and pathophysiology. Here, a broad overview is provided of the recent literature pertaining to the involvement of proteases in various aspects of eye development and disease: lens development (focusing on apoptosis and lens fiber cell denucleation and organelle loss) and cataract progression, cornea development and disease, retina development and degeneration, sclera development and myopia, and the trabecular meshwork and glaucoma. Proteases discussed include caspases, calpains, matrix metalloproteases (MMPs), a disintegrin and metalloproteinases (ADAMs) and ADAM with thrombospondin motifs (ADAMTS), the ubiquitin-proteasome pathway (UPP), tissue plasminogen activator (tPA), and secretases. It is clear that proteases have diverse and important roles in ocular development and disease, and represent, in many cases, useful therapeutic targets for treating ocular conditions, which would otherwise lead to visual impairment.

Proteasome dysfunction in mammalian aging: Steps and factors involved

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

Selectivity of the ubiquitin pathway for oxidatively modified proteins: relevance to protein precipitation diseases

There is now consensus that the accumulation of oxidatively modified proteins is cytotoxic and causally related to several age-related diseases, including the amyloid diseases and age-related cataracts. There is also general agreement that proteolytic pathways provide a quality control mechanism to limit accumulation of damaged proteins. Although many researchers assume that the ubiquitin pathway is involved in recognition and proteolytic removal of oxidatively modified proteins, which are produced upon cellular stress, there has been no direct evidence to support this hypothesis. In this work, we used a novel proteolysis-resistant ubiquitin variant to demonstrate that ubiquitin conjugates isolated from oxidatively stressed mammalian cells are enriched 3.3-15-fold for oxidatively modified proteins and that failure to execute ubiquitin-dependent proteolysis renders various cell types more susceptible to oxidative stress-related cytotoxicity. These results were corroborated using several inhibitors of the ubiquitin proteasome pathway, including PS-341, an anticancer drug in clinical use. Taken together the data indicate that the ubiquitin proteolytic pathway recognizes and removes oxidatively modified proteins, and that failure of this system, as occurs upon aging or stress, may be involved in and exacerbate cytotoxicity and age-related syndromes in which accumulation of ubiquitinated and oxidatively modified proteins has an etiologic role.

Lys6-modified ubiquitin inhibits ubiquitin-dependent protein degradation

Ubiquitin plays essential roles in various cellular processes; therefore, it is of keen interest to study the structure-function relationship of ubiquitin itself. We investigated the modification of Lys(6) of ubiquitin and its physiological consequences. Mass spectrometry-based peptide mapping and N-terminal sequencing demonstrated that, of the 7 Lys residues in ubiquitin, Lys(6) was the most readily labeled with sulfosuccinimidobiotin. Lys(6)-biotinylated ubiquitin was incorporated into high molecular mass ubiquitin conjugates as efficiently as unmodified ubiquitin. However, Lys(6)-biotinylated ubiquitin inhibited ubiquitin-dependent proteolysis, as conjugates formed with Lys(6)-biotinylated ubiquitin were resistant to proteasomal degradation. Ubiquitins with a mutation of Lys(6) had similar phenotypes as Lys(6)-biotinylated ubiquitin. Lys(6) mutant ubiquitins (K6A, K6R, and K6W) also inhibited ATP-dependent proteolysis and caused accumulation of ubiquitin conjugates. Conjugates formed with K6W mutant ubiquitin were also resistant to proteasomal degradation. The dominant-negative effect of Lys(6)-modified ubiquitin was further demonstrated in intact cells. Overexpression of K6W mutant ubiquitin resulted in accumulation of intracellular ubiquitin conjugates, stabilization of typical substrates for ubiquitin-dependent proteolysis, and enhanced susceptibility to oxidative stress. Taken together, these results show that Lys(6)-modified ubiquitin is a potent and specific inhibitor of ubiquitin-mediated protein degradation.

Age-associated oxidative damage leads to absence of gamma-cystathionase in over 50% of rat lenses: relevance in cataractogenesis

Oxidative damage to lens proteins and glutathione depletion play a major role in the development of senile cataract. We previously found that a deficiency in gamma-cystathionase activity may be responsible for glutathione depletion in old lenses. The aims of this study were: (1) to investigate the mechanism that causes the age-related deficiency in gamma-cystathionase activity in the eye lens, and (2) to determine the role of gamma-cystathionase deficiency in cataractogenesis. Two populations of old rats were found, one (56%) whose lenses lacked gamma-cystathionase activity and the rest that exhibited detectable enzyme activity. gamma-Cystathionase protein was absent in lenses from old rats without gamma-cystathionase activity. Oxidative stress targeted gamma-cystathionase in the eye lens upon aging, since the enzyme contained more carbonyl groups in old lenses than in young ones. gamma-Cystathionase mRNA was also markedly reduced in old lenses, thus contributing to the age-associated deficiency in gamma-cystathionase. Inhibition of gamma-cystathionase activity caused glutathione depletion in lenses and led to cataractogenesis in vitro. In conclusion, the lack of gamma-cystathionase activity in over 50% of old lenses is due to decreased gene expression and proteolytic degradation of the oxidized enzyme. This results in a high risk for the development of senile cataract.

Differential regulation of components of the ubiquitin-proteasome pathway during lens cell differentiation

PURPOSE

To investigate the role for the ubiquitin-proteasome pathway in controlling lens cell proliferation and differentiation and the regulation of the ubiquitin conjugation machinery during the differentiation process.

METHODS

bFGF-induced lens cell proliferation and differentiation was monitored in rat lens epithelial explants by bromodeoxyuridine (BrdU) incorporation and expression of crystallins and other differentiation markers. Levels of typical substrates for the ubiquitin-proteasome pathway, p21(WAF) and p27(Kip), were monitored during the differentiation process, as were levels and activities of the enzymes involved in ubiquitin conjugation.

RESULTS

Explants treated with bFGF initially underwent enhanced proliferation as indicated by BrdU incorporation. Then they withdrew from the cell cycle as indicated by diminished BrdU incorporation and accumulation of p21(WAF) and p27(Kip). bFGF-induced cell proliferation was prohibited or delayed by proteasome inhibitors. Lens epithelial explants treated with bFGF for 7 days displayed characteristics of lens fibers, including expression of large quantities of crystallins. Whereas levels of E1 remained constant during the differentiation process, the levels of ubiquitin-conjugating enzyme (Ubc)-1 increased approximately twofold, and the thiol ester form of Ubc1 increased approximately threefold on 7 days of bFGF treatment. Levels of Ubc2 increased moderately on bFGF treatment, and most of the Ubc2 was found in the thiol ester form. Although levels of total Ubc3 and -7 remained unchanged, the proportions of Ubc3 and -7 in the thiol ester form were significantly higher in the bFGF-treated explants. Levels of Ubc4/5 and -9 also increased significantly on treatment with bFGF, and more than 90% of Ubc9 was found in the thiol ester form in the bFGF-treated explants. In contrast, levels of Cul1, the backbone of the SCF type of E3s, decreased 50% to 70% in bFGF-treated explants.

CONCLUSIONS

The data show that proteolysis through the ubiquitin-proteasome pathway is required for bFGF-induced lens cell proliferation and differentiation. Various components of the ubiquitin-proteasome pathway are differentially regulated during lens cell differentiation. The downregulation of Cul1 appears to contribute to the accumulation of p21(WAF) and p27(Kip), which play an important role in establishing a differentiated phenotype.

Function of the ubiquitin proteolytic pathway in the eye

The ubiquitin pathway (UP) is involved in regulation of many essential cellular processes usually by the degradation of regulators of these processes. For example the UP is involved in regulation of cell cycle, proliferation, differentiation, organogenesis, development, and signal transduction in the lens and retina. A functional UP has also been documented in the cornea. Upon aging or exposure to stress there is an accumulation of damaged proteins, including ubiquitinated proteins, in the lens and retina. Some of these proteins may be cytotoxic. Thus, an active UP may be required to avoid such age and disease-related accumulation of damaged proteins. In this review we will explain the biochemistry of the UP and we will document the most important studies regarding UP function in the lens, retina and cornea.

The Ubiquitin Proteasome System in Neurodegenerative Diseases

The ubiquitin-proteasome system targets numerous cellular proteins for degradation. In addition, modifications by ubiquitin-like proteins as well as proteins containing ubiquitin-interacting and -associated motifs modulate many others. This tightly controlled process involves multiple specific and general enzymes of the system as well as many modifying and ancillary proteins. Thus, it is not surprising that ubiquitin-mediated degradation/processing/modification regulates a broad array of basic cellular processes. Moreover, aberrations in the system have been implicated, either as a primary cause or secondary consequence, in the pathogenesis of both inherited and acquired neurodegenerative diseases. Recent findings indicate that the system is involved in the pathogenesis of Parkinson's, Alzheimer's, Huntington's, and Prion diseases as well as amyotrophic lateral sclerosis. This raises hopes for a better understanding of the pathogenetic mechanisms involved in these diseases and for the development of novel, mechanism-based therapeutic modalities.