Increase in ubiquitin conjugates dependent on ischemic damage

Proteasome Stress Triggers Death of SH-SY5Y and T98G Cells via Different Cellular Mechanisms

Overload or dysfunction of ubiquitin-proteasome system (UPS) is implicated in mechanisms of neurodegeneration associated with neurodegenerative diseases, e.g. Parkinson and Alzheimer disease, and ischemia-reperfusion injury. The aim of this study was to investigate the possible association between viability of neuroblastoma SH-SY5Y and glioblastoma T98G cells treated with bortezomib, inhibitor of 26S proteasome, and accumulation of ubiquitin-conjugated proteins with respect to direct cytotoxicity of aggregates of ubiquitin-conjugated proteins. Bortezomib-induced death of SH-SY5Y cells was documented after 24 h of treatment while death of T98G cells was delayed up to 48 h. Already after 4 h of treatment of both SH-SY5Y and T98G cells with bortezomib, increased levels of both ubiquitin-conjugated proteins with molecular mass more than 150 kDa and Hsp70 were observed whereas Hsp90 was elevated in T98G cells and decreased in SH-SY5Y cells. With respect to the cell death mechanism, we have documented bortezomib-induced activation of caspase 3 in SH-SY5Y cells that was probably a result of increased expression of pro-apoptotic proteins, PUMA and Noxa. In T98G cells, bortezomib-induced expression of caspase 4, documented after 24 h of treatment, with further activation of caspase 3, observed after 48 h of treatment. The delay in activation of caspase 3 correlated well with the delay of death of T98G cells. Our results do not support the possibility about direct cytotoxicity of aggregates of ubiquitin-conjugated proteins. They are more consistent with a view that proteasome inhibition is associated with both transcription-dependent and -independent changes in expression of pro-apoptotic proteins and consequent cell death initiation associated with caspase 3 activation.

Characterization of the ubiquitin-modified proteome regulated by transient forebrain ischemia

Ubiquitylation is a posttranslational protein modification that modulates various cellular processes of key significance, including protein degradation and DNA damage repair. In animals subjected to transient cerebral ischemia, ubiquitin-conjugated proteins accumulate in Triton-insoluble aggregates. Although this process is widely considered to modulate the fate of postischemic neurons, few attempts have been made to characterize the ubiquitin-modified proteome in these aggregates. We performed proteomics analyses to identify ubiquitylated proteins in postischemic aggregates. Mice were subjected to 10 minutes of forebrain ischemia and 4 hours of reperfusion. The hippocampi were dissected, aggregates were isolated, and trypsin-digested after spiking with GG-BSA as internal standard. K-ɛ-GG-containing peptides were immunoprecipitated and analyzed by label-free quantitative liquid chromatography tandem mass spectrometry (LC-MS/MS) analysis. We identified 1,664 peptides to 520 proteins containing at least one K-ɛ-GG. Sixty-six proteins were highly ubiquitylated, with 10 or more K-ɛ-GG peptides. Based on selection criteria of greater than fivefold increase and P<0.001, 763 peptides to 272 proteins were highly enriched in postischemic aggregates. These included proteins involved in important neuronal functions and signaling pathways that are impaired after ischemia. Results of this study could serve as an important platform to uncover the mechanisms linking insoluble ubiquitin aggregates to the functions of postischemic neurons.

Reperfusion rather than ischemia drives the formation of ubiquitin aggregates after middle cerebral artery occlusion

BACKGROUND AND PURPOSE

Cerebral ischemia leads to accumulation of ubiquitinated protein aggregates. However, the factors triggering ubiquitination and their impact on the outcome of cerebral ischemia remain poorly understood. Here we investigate the relationship between ubiquitin aggregation and duration of ischemia/reperfusion, infarct volume, and proteasomal activity in a mouse model of focal ischemia.

METHODS

Free ubiquitin and ubiquitin aggregate levels were examined by Western blotting in the mouse neocortex and striatum after different periods of ischemia/reperfusion and permanent ischemia induced by middle cerebral artery occlusion. Infarct volumes were measured in thionin-stained brain sections. Proteasome activity was studied by fluorometric peptidase activity assay.

RESULTS

Following transient ischemia, ubiquitin aggregates were detected in the ipsilateral neocortex and, to a lesser extent, striatum only after induction of reperfusion. In permanent ischemia, no ubiquitin aggregates were found. Shorter ischemic periods producing no or minimal tissue damage (10-15 minutes) resulted in ubiquitin aggregate levels similar to those produced by ischemia resulting in substantial infarction (30 minutes). Proteasomal impairment was greatest in ischemia without reperfusion, in which no ubiquitin aggregates were detected.

CONCLUSIONS

The data demonstrate that reperfusion rather than ischemia leads to the appearance of ubiquitinated aggregates, which form in the absence of major tissue damage and are not correlated with decreased proteasomal peptidase activity. Ubiquitin aggregates may form in potentially viable brain tissue, which may be later recruited into infarction by factors independent of ubiquitination.

Ubiquitin and ubiquitin-conjugated protein expression in the rat cerebral cortex and hippocampus following traumatic brain injury (TBI)

Regulation of protein turnover is essential to the survival of eukaryotic cells. This important cellular process is partly regulated by the ubiquitin-proteasome system through posttranslational modification by the conjugation of ubiquitin chains to proteins targeted for degradation by proteasomes. The present study examined ubiquitin mRNA and protein expression in the CNS of rats that sustained traumatic brain injury (TBI). Quantitative real-time polymerase chain reaction results indicated that mRNA levels of ubA52, ubB and ubC in the ipsilateral cerebral cortex were significantly decreased on Day 1 post-TBI, that ubC mRNA levels also were significantly lower than control on Day 3 post-TBI, but that by Day 7 post-TBI, ubA52, ubB and ubC mRNA levels had all returned to control levels. In the ipsilateral hippocampus, ubA52 mRNA levels were significantly lower on Days 1-7 post-TBI, while ubB and ubC mRNA levels were less only on Day 1 post-TBI. Western blotting found that free ubiquitin protein levels were significantly reduced in both ipsilateral cerebral cortex and hippocampus on Days 1-7 post-TBI, while there was markedly increased ubiquitin-conjugated protein in ipsilateral cerebral cortex on Day 7 and in hippocampus on Days 3-7 post-TBI. Our study suggests that altered ubiquitin system function in the CNS contributes to the pathological outcomes of TBI.

Irreversible translation arrest in the reperfused brain

Irreversible translation arrest occurs in reperfused neurons that will die by delayed neuronal death. It is now recognized that suppression of protein synthesis is a general response of eukaryotic cells to exogenous stressors. Indeed, stress-induced translation arrest can be viewed as a component of cell stress responses, and consists of initiation, maintenance, and termination phases that work in concert with stress-induced transcriptional mechanisms. Within this framework, we review translation arrest in reperfused neurons. This framework provides a basis to recognize that phosphorylation of the alpha subunit of eukaryotic initiation factor 2 is the initiator of translation arrest, and a key marker indicating activation of neuronal stress responses. However, eIF2 alpha phosphorylation is reversible. Other phases of stress-induced translation arrest appear to contribute to irreversible translation arrest specifically in ischemic vulnerable neuron populations. We detail two lines of evidence supporting this view. First, ischemia, as a stress stimulus, induces irreversible co-translational protein misfolding and aggregation after 4 to 6 h of reperfusion, trapping protein synthesis machinery into functionally inactive protein aggregates. Second, ischemia and reperfusion leads to modifications of stress granules (SGs) that sequester functionally inactive 48S preinitiation complexes to maintain translation arrest. At later reperfusion durations, these mechanisms may converge such that SGs become sequestered in protein aggregates. These mechanisms result in elimination of functionally active ribosomes and preclude recovery of protein synthesis in selectively vulnerable neurons. Thus, recognizing translation arrest as a component of endogenous cellular stress response pathways will aid in making sense of the complexities of postischemic translation arrest.

Killer proteases and little strokes--how the things that do not kill you make you stronger

The phenomenon of ischemic preconditioning was initially observed over 20 years ago. The basic tenant is that if stimuli are applied at a subtoxic level, cells upregulate endogenous protective mechanisms to block injury induced by subsequent stress. Since this discovery, many conserved signaling mechanisms that contribute to activation of this potent protective program have been identified in the brain. A clinical correlate of this basic research finding can be found in patients with a history of transient ischemic attack (TIA), who have a decreased morbidity after stroke. In spite of multidisciplinary efforts to design safer, more effective stroke therapies, we have thus far failed to translate our understanding of endogenous protective pathways to treatments for neurodegeneration. This review is designed to provide clinicians and basic scientists with an overview of stress biology after TIA and preconditioning, discuss new therapeutic strategies to target the protein dysfunction that follows ischemic injury, and propose enhanced biochemical profiling to identify individuals at risk of stroke after TIA. We pay particular attention to the unanticipated consequences of overly aggressive intervention after TIA in which we have found that traditional cytotoxic agents such as free radicals and apoptosis associated proteases is essential for neuroprotection and communication in the stressed brain. These data emphasize the importance of understanding the complex interplay between chaperones, apoptotic proteases including caspases, and the proteolytic degradation machinery in adaptation to neurological injury.

Cardiovascular disease could be contained based on currently available data!

Largely due to better control of infectious diseases and significant advances in biomedical research, life expectancy worldwide has increased dramatically in the last three decades. However, as the average age of the population has risen, the incidence of chronic age-related diseases such as arthritis, Alzheimer's, Parkinson's, cardiovascular disease, cancer, osteoporosis, benign prostatic hyperplasia, and late-onset diabetes have increased and have become serious public health problem, as well. The etiology of these disorders is still incompletely understood, therefore, neither preventive strategies nor long-term effective treatment modalities are available for these disorders. In keeping with the aforementioned, the ultimate goal in cardiovascular research is to prevent the onset of cardiovascular episodes and thereby allow successful ageing without morbidity and cognitive decline. Herein, I argue that cardiovascular episodes could be contained with relatively simple approaches. Cardiovascular disorder is characterized by cellular and molecular changes that are commonplace in age-related diseases in other organ system, such alterations include increased level of oxidative stress, perturbed energy metabolism, and "horror autotoxicus" largely brought about by the perturbation of ubiquitin -proteasome system, and excessive oxidative stress damage to the cardiac muscle cells and tissues, and cross-reactions of specific antibodies against human heat shock protein 60 with that of mycobacterial heat shock protein 65. "Horror autotoxicus", a Latin expression, is a term coined by Paul Ehrlich at the turn of the last century to describe autoimmunity to self, or the attack of "self" by immune system, which ultimately results to autoimmune condition. Based on the currently available data, the risk of cardiovascular episodes and several other age-related disorders, including cancer, Alzheimer's disease and diabetes, is known to be influenced by the nature and level of food intake. Now, a wealth of scientific data from studies of rodents and monkeys has documented the significant beneficial effects of calorie restriction (CR) or dietary restriction (DR), and multiple antioxidant agents in extending life span and reducing the incidence of progeroid-related diseases. Reduced levels of cellular oxidative stress, protection of genome from deleterious damage, detoxification of toxic molecules, and enhancement of energy homeostasis, contribute to the beneficial effects of dietary restriction and multiple antioxidant agents. Recent findings suggest that employment of DR and multiple antioxidant agents (including, catalase, glutathione peroxidase, CuZn superoxide dismutase, and Mn superoxide dismutase = enzymes forming the primary defense against oxygen toxicity), and ozone therapy may mount an effective resistance to pathogenic factors relevant to the pathogenesis of cardiovascular episodes. Hence, while further studies will be needed to establish the extent to which CR and multiple antioxidant agents will reduce incidence of cardiovascular episodes in humans, it would seem prudent to recommend CR and multiple antioxidant agents as widely applicable preventive approach for cardiovascular disorders and other progeroid-related disorders.

Postischemic Alterations of BDNF, NGF, HSP 70 and Ubiquitin Immunoreactivity in the Gerbil Hippocampus: Pharmacological Approach

1. We investigated the immunohistochemical alterations of BDNF, NGF, HSP 70 and ubiquitin in the hippocampus 1 h to 14 days after transient cerebral ischemia in gerbils. We also examined the effect of 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase inhibitor pitavastatin against the changes of BDNF, NGF, HSP 70 and ubiquitin in the hippocampus after cerebral ischemia in the hippocampus after ischemia. 2. The transient cerebral ischemia was carried out by clamping the carotid arteries with aneurismal clips for 5 min. 3. In the present study, the alteration of HSP 70 and ubiquitin immunoreactivity in the hippocampal CA1 sector was more pronounced than that of BDNF and NGF immunoreactivity after transient cerebral ischemia. In double-labeled immunostainings, BDNF, NGF and ubiquitin immunostaining was observed both in GFAP-positive astrocytes and MRF-1-positive microglia in the hippocampal CA1 sector after ischemia. Furthermore, prophylactic treatment with pitavastatin prevented the damage of neurons with neurotrophic factor and stress proteins in the hippocampal CA1 sector after ischemia. 4. These findings suggest that the expression of stress protein including HSP 70 and ubiquitin may play a key role in the protection against the hippocampal CA1 neuronal damage after transient cerebral ischemia in comparison with the expression of neurotrophic factor such as BDNF and NGF. The present findings also suggest that the glial BDNF, NGF and ubiquitin may play some role for helping surviving neurons after ischemia. Furthermore, our present study indicates that prophylactic treatment with pitavastatin can prevent the damage of neurons with neurotrophic factor and stress proteins in the hippocampal CA1 sector after transient cerebral ischemia. Thus our study provides further valuable information for the pathogenesis after transient cerebral ischemia.

Effects of chronic administration with nilvadipine against immunohistochemical changes related to aging in the mouse hippocampus

We investigated the effect of Ca2+ antagonist nilvadipine on age-related immunohistochemical alterations in ubiquitin and S100beta protein of the hippocampal CA1 sector in mice using 8-, 18-, 40-, and 59-week-old mice. No significant changes in the number of neuronal cells were observed in the hippocampal CA1 sector up to 59 weeks after birth. The administration of nilvadipine did not affect the number of the hippocampal CA1 cells of 40-week-old mice. Age-dependent increases in ubiquitin immunoreactivity were observed in the hippocampal CA1 neurons up to 59 weeks after birth. The administration of nilvadipine prevented dose-dependently the increases in the number of ubiquitin-immunoreactive neurons in the hippocampal CA1 sector of 40-week-old mice. S100,beta immunoreactivity was unchanged in the hippocampal CA1 sector up to 40 weeks after birth. In 59-week-old mice, the level of staining of S100beta-immunoreactive cells increased significantly in the hippocampal CA1 sector. The administration of nilvadipine decreased dose-dependently the number of S 100beta-immunoreactive cells in the hippocampal CA1 sector of 40-week-old mice. The present study demonstrates that age-related increases in ubiquitin system may play a pivotal role in protecting neuronal cell damage during aging. In contrast, our results suggest that expression of S 100beta protein in the hippocampal CA1 sector may play an exacerbating factor in some neuronal cells damaged by aging. Our results also demonstrate that nilvadipine, a dihydropyridine-type calcium channel blocker, can prevent dose-dependently the increases in the ubiquitin immunoreactive neurons and decrease the number of S100beta immunoreactive cells in the hippocampal CA1 neurons of aged mice. These results suggest that nilvadipine may offer a new approach for the treatment of neuronal dysfunction in aged humans.

Acute ischemia/hypoxia in rat hippocampal neurons activates nuclear ubiquitin and alters both chromatin and DNA

We investigated early alterations in rat neurons after experimental ischemic stress. Transient ischemia was generated by bilateral occlusion of the carotids after hypoxia. Data show a relevant increase of the nuclear level of ubiquitin 2 h post-stress as evaluated by immuno-cytolocalization. Ubiquitin returns to normal levels after 6 h. The increase in ischemic/hypoxic rats was localized preferentially in nuclei of hippocampal neurons, although some augmentation was also shown essentially in dendrites. The activation of ubiquitin system is related to a defective homeostasis and might trigger different degenerative processes. With respect to this, we observed chromatin alterations by densitometric analysis. The shown extensive DNA degeneration is consistent with the occurrence of necrotic phenomena at an early stage. However the parallel internucleosomal specific DNA fragmentation, strongly suggests that apoptotic events also occur. In any case both necrosis and apoptosis are likely to occur at same time, although apoptosis is less extensive, and the two phenomena take place in different neural cells.

The N- and C-terminal fragments of ubiquitin are important for the antimicrobial activities

Secretory granules of chromaffin cells contain catecholamines and several antimicrobial peptides derived from chromogranins and proenkephalin-A. These peptides are secreted in the extracellular medium following exocytosis. Here, we show that ubiquitin is stored in secretory chromaffin granules and released into the circulation upon stimulation of chromaffin cells. We also show that the C-terminal fragment (residues 65-76) of ubiquitin displays, at the micromolar range, a lytic antifungal activity. Using confocal laser scan microscopy and rhodamine-labeled synthetic peptides, we could demonstrate that the C-terminal peptide (residues 65-76) is able to cross the cell wall and the plasma membrane of fungi and to accumulate in fungi, whereas the N-terminal peptide (residues 1-34) is stopped at the fungal wall level. Furthermore, these two peptides act synergistically to kill filamentous fungi. Because of the interaction of the C-terminal sequence of ubiquitin with calmodulin, the synthetic peptide (residues 65-76) was tested in vitro against calmodulin-dependent calcineurin, an enzyme crucial for fungal growth. This peptide was found to inhibit the phosphatase activity of calcineurin. Our data show a new property of ubiquitin C-terminal-derived peptide (65-76) that could be used with N-terminal peptide (1-34) as a new potent antifungal agent.

Isolation of ubiquitin-E2 (ubiquitin-conjugating enzyme) complexes from erythroleukaemia cells using immunoaffinity techniques

A variety of ubiquitin-associated (or conjugated) proteins, including substrates and enzymes for the ubiquitin system, are present in eukaryotic cells. In the present study we developed a simple method for their isolation, consisting of immunoaffinity chromatography using the monoclonal antibody FK2, which recognizes the conjugated ubiquitin molecule. Using this method followed by gel filtration, we isolated multi-ubiquitinated proteins with high molecular masses (>30 kDa) and also ubiquitinthioester-linked and mono-ubiquitinated forms of ubiquitin-conjugating (E2) enzymes, UbcH7 and UBE2N, together with mono-, di- and tri-ubiquitin molecules, from the cytoplasmic extract of heat-shock-treated K562 erythroleukaemia cells. We also demonstrated that the FK2 antibody was capable of precipitating a ubiquitin-UbcH7 thioester, but not free UbcH7, which enabled the measurement of the respective cellular levels separately. The immunoprecipitable ubiquitin-UbcH7 thioester was found only when the cells were treated with heat-shock. These results suggest the usefulness of the immunoaffinity techniques for identifying and analysing the cellular enzyme/protein-ubiquitin complexes.

Ubiquitin dependent proteolysis is activated in apoptotic fibroblasts in culture

The ubiquitin mediated pathway constitutes an early response in cultured cells where apoptosis, assessed by internucleosomal specific DNA fragmentation, was induced by serum withdrawal. Data demonstrate that nuclear ubiquitin proteolytic system, but not cytoplasmic, is activated. This activation is paralleled by a substantial chromatin de-condensation. We suggest that chromatin relaxation is causative of the fragmentation since it exposes the DNA to nucleolytic attack. Finally, maintenance of homeostasis and induction of apoptosis seem to undergo a parallel contemporary pathway with a possible mutual feedback.

Relationship between electroneurographic changes and serum ubiquitin levels in patients with type 2 diabetes

OBJECTIVE

The aim of the present study was to investigate any relationship between serum ubiquitin levels and electroneurographic changes in peripheral nerves for patients with type 2 diabetes.

RESEARCH DESIGN AND METHODS

The study involved 34 patients (19 men, 15 women; mean age 46 +/- 13 years) with type 2 diabetes. Serum ubiquitin values were measured by sandwich enzyme-linked immunosorbent assay Measurement of nerve conduction velocity (NCV) was performed on three motor (median, tibial, and peroneal) and three sensory (median, ulnar, and sural) nerves. The value of motor compound muscle action potential (CMAP) was obtained from the sum of median, tibial, and peroneal motor nerve amplitudes, and sensory compound nerve action potential (CNAP) was computed as the sum of median and ulnar sensory nerve amplitudes.

RESULTS

Patients with diabetes were divided into three groups: group 1 (n = 8) had normal electroneurography results, group 2 (n = 8) had slowed NCV, and group 3 (n = 18) had low values of motor CMAP and/or sensory CNAP as well as slowed NCV. Mean ubiquitin level in group 3 (20.4 +/- 2.9 ng/dl) was significantly higher than that in group 1 (11.2 +/- 1.1 ng/dl, t = 11.5, P < 0.0001) and group 2 (13.2 +/- 2.7 ng/dl, t = 5.9, P < 0.0001). Serum ubiquitin levels were inversely correlated with motor CMAP (r = -0.68) and sensory CNAP (r = -0.61) values.

CONCLUSIONS

The results of this study indicate that there could be a relationship between the diminished amplitudes of axons of the peripheral nerve and the increase in serum ubiquitin levels in patients with type 2 diabetes. Further studies are required to confirm this relationship.

Ubiquitin stress response in postischemic hippocampal neurons under nontolerant and tolerant conditions

Ubiquitin, an essential protein in nonlysosomal proteolytic system, is expressed after metabolic stress to the cell. The authors investigated stress response of ubiquitin in the hippocampus of the Mongolian gerbil after forebrain ischemia. The level of hippocampal ubiquitin was compared with that under ischemic tolerance induced by ischemic preconditioning. The authors also studied ubiquitin gene expression using in situ hybridization method. Transient ischemia resulted in consumption of free ubiquitin and an increase of multiubiquitin chains. These changes were transient in the hippocampus outside of the CA1 region where neurons survived, whereas it was persistent in the CA1 region where neurons were destined to die after ischemia. Under tolerant condition, subsequent ischemia provoked rapid recovery and further increase of free ubiquitin. The signal of ubiquitin messenger ribonucleic acid was continuously detected after ischemia, not only under tolerant conditions, but without tolerance induced by preconditioning. Thus, ubiquitin stress response takes place, at least at a transcriptional level, in dying CA1 neurons. Under tolerant conditions, however, subsequent ischemia in the CA1 region induces the stress response of ubiquitin up to the translational level, leading to the rapid restoration of protein synthesis and to eventual neuronal survival.

Proteasome inhibitors which induce neurite outgrowth from PC12h cells cause different subcellular accumulations of multi-ubiquitin chains

The effects of two proteasome inhibitors on neurite outgrowth from PC12h cells were investigated in terms of the mean length of the neurites and the frequency of occurrence of cells with long neurites. Benzyloxycarbonyl-leucyl-leucyl-leucinal (ZLLLal) and benzyloxycarbonyl-isoleucyl-t-butyl-glutamyl-leucinal (PSI) caused a significant elongation of PC12h cell neurites. Since ZLLLal is known to inhibit both calpain and proteasome activity, we examined the effects ofbenzyloxycarbonyl-leucyl-leucinal (ZLLal) which inhibits calpain activity to the same degree as ZLLLal, but which inhibits proteasome activity only weakly. ZLLal did not induce the significant elongation of neurites at any of the concentrations we studied. These results show that the inhibition of proteasome activity causes neurite elongation. We also quantified subcellular levels of multi-ubiquitin chains and free ubiquitin after treatments with PSI, ZLLLal and ZLLal. Treatment with ZLLal had no effects on levels of water- and urea-soluble multi-ubiquitin chains or of free ubiquitin either in the nucleus or in the cytoplasm. PSI and ZLLLal induced a large accumulation of water- and urea-soluble multi-ubiquitin chains and free ubiquitin in the nucleus. Similarly, PSI and ZLLLal increased cytoplasmic levels of urea-soluble multi-ubiquitin chains. On the contrary, PSI and ZLLLal had no effect on levels of water-soluble multi-ubiquitin chains or free ubiquitin in the cytoplasm. This is the first study to demonstrate subcellular differences in the accumulation of multi-ubiquitin chains and free ubiquitin during the neurite elongation induced by proteasome inhibitors.

Regulation of ubiquitin-conjugating enzymes by glutathione following oxidative stress

Upon oxidative stress cells show an increase in the oxidized glutathione (GSSG) to reduced glutathione (GSH) ratio with a concomitant decrease in activity of the ubiquitinylation pathway. Because most of the enzymes involved in the attachment of ubiquitin to substrate proteins contain active site sulfhydryls that might be covalently modified (thiolated) upon enhancement of GSSG levels (glutathiolation), it appeared plausible that glutathiolation might alter ubiquitinylation rates upon cellular oxidative stress. This hypothesis was explored using intact retina and retinal pigment epithelial (RPE) cell models. Exposure of intact bovine retina and RPE cells to H2O2 (0.1-1.7 micromol/mg) resulted in a dose-dependent increase in the GSSG:GSH ratio and coincident dose-dependent reductions in the levels of endogenous ubiquitin-activating enzyme (E1)-ubiquitin thiol esters and endogenous protein-ubiquitin conjugates and in the ability to form de novo retinal protein-125I-labeled ubiquitin conjugates. Oxidant-induced decrements in ubiquitin conjugates were associated with 60-80% reductions in E1 and ubiquitin-conjugating enzyme (E2) activities as measured by formation of ubiquitin thiol esters. When GSH levels in RPE cells recovered to preoxidation levels following H2O2 removal, endogenous E1 activity and protein-ubiquitin conjugates were restored. Evidence that S thiolation of E1 and E2 enzymes is the biochemical link between cellular redox state and E1/E2 activities includes: (i) 5-fold increases in levels of immunoprecipitable, dithiothreitol-labile 35S-E1 adducts in metabolically labeled, H2O2-treated, RPE cells; (ii) diminished formation of E1- and E2-125I-labeled ubiquitin thiol esters, oligomerization of E225K, and coincident reductions in protein-125I-labeled ubiquitin conjugates in supernatants from nonstressed retinas upon addition of levels of GSSG equivalent to levels measured in oxidatively stressed retinas; and (iii) partial restoration of E1 and E2 activities and levels of protein-125I-labeled ubiquitin conjugates in supernatants from H2O2-treated retinas when GSSG:GSH ratios were restored to preoxidation levels by the addition of physiological levels of GSH. These data suggest that the cellular redox status modulates protein ubiquitinylation via reversible S thiolation of E1 and E2 enzymes, presumably by glutathione.

Serum concentrations of free ubiquitin and multiubiquitin chains

Ubiquitin, which can conjugate with cellular proteins, is classified into two forms: free ubiquitin and multiubiquitin chains. The latter is active as a signal for degradation of the targeted proteins. We found both forms in human serum and, using two immunoassays, quantitated them in sera from healthy subjects and patients with some diseases. Because of putative leakage of erythrocyte ubiquitin, hemolytic serum and serum obtained after long incubation (&gt;1–2 h) of blood at room temperature were excluded. Serum concentrations of multiubiquitin chains and free ubiquitin were substantially higher in rheumatoid arthritis and hemodialysis patients, respectively, than healthy subjects. Additionally, in acute viral hepatitis, serum multiubiquitin chain concentrations were increased in the acute phase, decreased in the recovery phase, and correlated with alanine and aspartate aminotransferase activities (r = 0.676 and 0.610, P &lt;0.0001 and &lt;0.001, respectively). Therefore, serum ubiquitin may have prognostic value.

Ubiquitin-mediated stress response in a rat model of brain transient ischemia/hypoxia

Ubiquitin (Ub) is a small 76-residue protein, involved in intracellular protein degradation through a specific ATP-dependent system, which uses Ub as a tag to label proteins committed to be hydrolyzed by a specific 26 S protease. PGP-9.5 is another important component of the Ub system, i.e. a neuron-specific carboxyl-terminal hydrolase, which recycles Ub from Ub-polypeptide complexes. We have investigated the expression of Ub and PGP-9.5 in rat hippocampal neurons in an early phase of reperfusion in a model of transient global brain ischemia/hypoxia (bilateral occlusion of common carotid arteries for 10 min accompanied by mild hypoxia-15% O2-for 20 min), by means of immunohistochemical methods using light and electron microscopy. The intensity of Ub and PGP-9.5 immunoreactivity was evaluated by image analysis. We have detected a marked increase of Ub immunoreactivity (UIR) in neurons of CA1, CA2, CA3, CA4, and dentate gyrus subfields 1 hr after ischemia/hypoxia (but not after hypoxia only), statistically significant as confirmed by image analysis. Such increase in immunoreactivity in ischemic/hypoxic rats was localized essentially in the nuclei of hippocampal neurons. There were no changes in PGP-9.5 immunoreactivity. The data suggest that in the present model of rat brain ischemia/hypoxia Ub is involved in the neuronal stress response.

Ubiquitin gene expression following transient forebrain ischemia

Ubiquitin gene expression following transient forebrain ischemia in the rat was analyzed by three probes which were specific for UbC, UbB and UbS30 mRNA. According to the in situ hybridization studies, each type of ubiquitin gene expression decreased at 30 min of reperfusion following 20 min of forebrain ischemia, thereafter increased, and then reached a peak at 4-6 h, both in the cortex and hippocampus. These changes returned to the control level after 24-48 h of recirculation. Among the three ubiquitin transcripts, changes in UbC expression were more marked in the hippocampus, and persistent expression of UbC transcripts in the CA1 and CA3 regions was observed at 24 h of reperfusion. With dot-blot analysis, significant increases in the UbC transcripts were noted at 4 h of reperfusion in the hippocampus, and at 6 h in the cortex following 20 min of ischemia. These results suggest that changes in UbC expression might be a good indicator of ischemic stress.

Changes in proteasome activity following transient ischemia

Succinyl-Leu-Leu-Val-Tyr-4-methylcoumaryl-7-amide (Suc-LLVY-MCA) hydrolyzing activities of the 20S and 26S proteasomes in the gerbil cortex following transient forebrain ischemia were examined. Using extraction solutions without ATP, only 20S proteasome activity was noted after separation with glycerol gradient centrifugation. When these extracts were incubated with ATP and an ATP-regenerating system prior to glycerol gradient separation, both 20S and 26S proteasome activities were detected. Following 10 min of ischemia, the activity of the 26S proteasomes decreased, whereas the 20S proteasome activity increased after 30 min of reperfusion. These changes returned to the control level after 1 h. The active 26S proteasomes were formed with ATP-dependent association with the 20S proteasomes and several subunits and the 26S proteasomes degraded ubiquitin-protein conjugates. These results indicate that proteasome activity might not be irreversibly impaired after transient ischemia. However, transient inhibition of ATP-dependent conversion of 20S to 26S proteasomes in vitro must be one of the causes of the accumulation of the ubiquitin-protein conjugates in the early reperfusion period.

Lack of effect of transient ischemia on ubiquitin conjugation

Ubiquitin-conjugating activities in the soluble fractions of gerbil cortex and hippocampus following transient ischemia were examined in vitro. Ten minutes of ischemia did not affect the ubiquitination of heat-denatured lysozyme both in the cortex and in the hippocampus. No reduction of the conjugating activities following ischemia was also confirmed using the partially purified ubiquitin conjugating enzymes from the cortex. These results indicate that protein ubiquitination might not be impaired following transient ischemia.

Production and characterization of monoclonal antibodies specific to multi-ubiquitin chains of polyubiquitinated proteins

Polyubiquitinated proteins tagged with multi-ubiquitin chains are substrates preferred by the 26 S proteasome (a ubiquitin/ATP-dependent proteolytic complex). Here, we developed a simple method for the efficient preparation of polyubiquitinated proteins which are degraded by the 26 S proteasome in an ATP-dependent manner. Our efficient method enabled us to produce ten monoclonal antibodies that recognized the multi-ubiquitin chains of the polyubiquitinated proteins, but not free ubiquitin or the protein moieties. Eight of the antibodies recognized only the multi-ubiquitin chains of the polyubiquitinated proteins, while the other two antibodies cross-reacted with mono-ubiquitin and methyl-ubiquitin, both of which are linked to proteins via an isopeptide bond, as well as with the multi-ubiquitin chains. Thus these antibodies are novel and useful tools for the identification and quantification of polyubiquitinated proteins in various cells and tissues under physiological and pathological conditions.