Serum ubiquitin levels in patients with alcoholic liver disease

Interferon regulatory factor 1‐triggered free ubiquitin protects the intestines against radiation‐induced injury via CXCR4/FGF2 signaling

Radiation‐induced intestinal injury is a serious concern during abdominal and pelvic cancers radiotherapy. Ubiquitin (Ub) is a highly conserved protein found in all eukaryotic cells. This study aims to explore the role and mechanism of free Ub against radiogenic intestinal injury. We found that free Ub levels of irradiated animals and human patients receiving radiotherapy were upregulated. Radiation‐induced Ub expression was associated with the activation of interferon regulatory factor 1 (IRF1). Intraperitoneal injection of free Ub significantly reduced the mortality of mice following 5–9 Gy total body irradiation (TBI) through the Akt pathway. Free Ub facilitates small intestinal regeneration induced by TBI or abdominal irradiation. At the cellular level, free Ub or its mutants significantly alleviated cell death and enhanced the survival of irradiated intestinal epithelial cells. The radioprotective role of free Ub depends on its receptor CXCR4. Mechanistically, free Ub increased fibroblast growth factor‐2 (FGF2) secretion and consequently activated FGFR1 signaling following radiation in vivo and in vivo. Thus, free Ub confers protection against radiation‐induced intestinal injury through CXCR4/Akt/FGF2 axis, which provides a novel therapeutic option.

Ubiquitin pathways regulate the pathogenesis of chronic liver disease

Chronic liver disease (CLD) is considered the leading cause of global mortality. In westernized countries, increased consumption of alcohol and overeating foods with high fat/ high glucose promote progression of CLD such as alcoholic liver disease (ALD) and non-alcoholic liver disease (NAFLD). Accumulating evidence and research suggest that ubiquitin, a 75 amino acid protein, plays crucial role in the pathogenesis of CLD through dynamic post-translational modifications (PTMs) exerting diverse cellular outcomes such as protein degradation through ubiquitin-proteasome system (UPS) and autophagy, and regulation of signal transduction. In this review, we present the function of ubiquitination and latest findings on diverse mechanism of PTMs, UPS and autophagy which significantly contribute to the pathogenesis of alcoholic liver disease (ALD), non-alcoholic fatty liver disease (NAFLD), cirrhosis, and HCC. Despite its high prevalence, morbidity, and mortality, there are only few FDA approved drugs that could be administered to CLD patients. The goal of this review is to present a variety of pathways and therapeutic targets involving ubiquitination in the pathogenesis of CLD. Further, this review summarizes collective views of pharmaceutical inhibition or activation of recent drugs targeting UPS and autophagy system to highlight potential targets and new approaches to treat CLD.

Electrochemical sensing of blood proteins for mild traumatic brain injury (mTBI) diagnostics and prognostics: towards a point-of-care application

Traumatic Brain Injury (TBI) being one of the principal causes of death and acquired disability in the world imposes a large burden on the global economy. Mild TBI (mTBI) is particularly challenging to assess due to the frequent lack of well-pronounced post-injury symptoms. However, if left untreated mTBI (especially when repetitive) can lead to serious long-term implications such as cognitive and neuropathological disorders. Computer tomography and magnetic resonance imaging commonly used for TBI diagnostics require well-trained personnel, are costly, difficult to adapt for on-site measurements and are not always reliable in identifying small brain lesions. Thus, there is an increasing demand for sensitive point-of-care (POC) testing tools in order to aid mTBI diagnostics and prediction of long-term effects. Biomarker quantification in body fluids is a promising basis for POC measurements, even though establishing a clinically relevant mTBI biomarker panel remains a challenge. Actually, a minimally invasive, rapid and reliable multianalyte detection device would allow the efficient determination of injury biomarker release kinetics and thus support the preclinical evaluation and clinical validation of a proposed biomarker panel for future decentralized in vitro diagnostics. In this respect electrochemical biosensors have recently attracted great attention and the present article provides a critical study on the electrochemical protocols suggested in the literature for detection of mTBI-relevant protein biomarkers. The authors give an overview of the analytical approaches for transduction element functionalization, review recent technological advances and highlight the key challenges remaining in view of an eventual integration of the proposed concepts into POC diagnostic solutions.

Multi-Omics Integration Highlights the Role of Ubiquitination in CCl4-Induced Liver Fibrosis

Liver fibrosis is the excessive accumulation of extracellular matrix proteins that occurs in chronic liver disease. Ubiquitination is a post-translational modification that is crucial for a plethora of physiological processes. Even though the ubiquitin system has been implicated in several human diseases, the role of ubiquitination in liver fibrosis remains poorly understood. Here, multi-omics approaches were used to address this. Untargeted metabolomics showed that carbon tetrachloride (CCl₄)-induced liver fibrosis promotes changes in the hepatic metabolome, specifically in glycerophospholipids and sphingolipids. Gene ontology analysis of public deposited gene array-based data and validation in our mouse model showed that the biological process “protein polyubiquitination” is enriched after CCl₄-induced liver fibrosis. Finally, by using transgenic mice expressing biotinylated ubiquitin (^bioUb mice), the ubiquitinated proteome was isolated and characterized by mass spectrometry in order to unravel the hepatic ubiquitinated proteome fingerprint in CCl₄-induced liver fibrosis. Under these conditions, ubiquitination appears to be involved in the regulation of cell death and survival, cell function, lipid metabolism, and DNA repair. Finally, ubiquitination of proliferating cell nuclear antigen (PCNA) is induced during CCl₄-induced liver fibrosis and associated with the DNA damage response (DDR). Overall, hepatic ubiquitome profiling can highlight new therapeutic targets for the clinical management of liver fibrosis.

Exogenous ubiquitin reduces inflammatory response and preserves myocardial function 3 days post-ischemia-reperfusion injury

β-Adrenergic receptor (β-AR) stimulation increases extracellular levels of ubiquitin (UB) in myocytes, and exogenous UB decreases β-AR-stimulated myocyte apoptosis and myocardial fibrosis. Here, we hypothesized that exogenous UB modulates the inflammatory response, thereby playing a protective role in cardiac remodeling after ischemia-reperfusion (I/R) injury. C57BL/6 mice infused with vehicle or UB (1 μg·g^-1·h^-1) were subjected to myocardial I/R injury. Functional and biochemical parameters of the heart were examined 3 days post-I/R. Heart weight-to-body weight ratios were similarly increased in I/R and UB + I/R groups. The area at risk and infarct size were significantly lower in UB + I/R versus I/R groups. Measurement of heart function using echocardiography revealed that I/R decreases percent fractional shortening and percent ejection fraction. However, the decrease in fractional shortening and ejection fraction was significantly lower in the UB + I/R group. The UB + I/R group displayed a significant decrease in inflammatory infiltrates, neutrophils, and macrophages versus the I/R group. Neutrophil activity was significantly lower in the UB + I/R group. Analysis of the concentration of a panel of 23 cytokines/chemokines in the serum using a Bio-Plex assay revealed a significantly lower concentration of IL-12 subunit p40 in the UB + I/R versus I/R group. The concentration of monocyte chemotactic protein-1 was lower, whereas the concentration of macrophage inflammatory protein-1α was significantly higher, in the UB+I/R group versus the sham group. Expression of matrix metalloproteinase (MMP)-2 and activity of MMP-9 were higher in the UB + I/R group versus the I/R group. Levels of ubiquitinated proteins and tissue inhibitor of metalloproteinase 2 expression were increased to a similar extent in both I/R groups. Thus, exogenous UB plays a protective role in myocardial remodeling post-I/R with effects on cardiac function, area at risk/infarct size, the inflammatory response, levels of serum cytokines/chemokines, and MMP expression and activity. NEW & NOTEWORTHY Stimulation of β-adrenergic receptors increases extracellular levels of ubiquitin (UB) in myocytes, and exogenous UB decreases β-adrenergic receptor-stimulated myocyte apoptosis and myocardial fibrosis. Here, we provide evidence that exogenous UB decreases the inflammatory response and preserves heart function 3 days after myocardial ischemia-reperfusion injury. Further identification of the molecular events involved in the anti-inflammatory role of exogenous UB may provide therapeutic targets for patients with ischemic heart disease.

Extracellular ubiquitin modulates cardiac fibroblast phenotype and function via its interaction with CXCR4

AIMS

β-adrenergic receptor (β-AR) stimulation increases extracellular levels of ubiquitin (UB), and exogenous UB plays an important role in β-AR-stimulated myocardial remodeling with effects on heart function, fibrosis and myocyte apoptosis. Cardiac fibroblasts are vital for maintaining the normal function of the heart, and in the structural remodeling of the heart in response to injury. Here we hypothesized that extracellular UB modulates cardiac fibroblast phenotype and function via its interaction with CXC chemokine receptor type 4 (CXCR4).

MAIN METHODS

Serum starved adult cardiac fibroblasts were used to identify CXCR4 as a receptor for UB. Fluorescent microscopy, co-immunoprecipitation, western blot, proliferation, migration and collagen contraction assays were performed to investigate the role of UB/CXCR4 axis on cell signaling, and modulation of fibroblast phenotype and function.

KEY FINDINGS

Using fluorescent microscopy and co-immunoprecipitation assay, we provide evidence that extracellular UB interacts with CXCR4. CXCR4 antagonist, AMD3100, inhibited interaction of UB with CXCR4. UB activated ERK1/2, not Akt. It enhanced VEGF-A expression, while decreasing β3 integrins expression. Two mutated UB proteins (V70A and F4A; unable to interact with CXCR4) failed to affect the expression of VEGF-A and β3 integrins. UB treatment inhibited migration of cells into the wound and FBS-stimulated cell proliferation. UB enhanced expression of α-smooth muscle actin (marker of myofibroblast differentiation) and contraction of fibroblast-populated collagen gel pads. Most of the effects of UB were negated by AMD3100.

SIGNIFICANCE

The data presented here suggest that UB interacts with CXCR4, and UB/CXCR4 interaction affects intracellular signaling, and modulates fibroblast phenotype and function.

Extracellular Ubiquitin: Role in Myocyte Apoptosis and Myocardial Remodeling

Ubiquitin (UB) is a highly conserved low molecular weight (8.5 kDa) protein. It consists of 76 amino acid residues and is found in all eukaryotic cells. The covalent linkage of UB to a variety of cellular proteins (ubiquitination) is one of the most common posttranslational modifications in eukaryotic cells. This modification generally regulates protein turnover and protects the cells from damaged or misfolded proteins. The polyubiquitination of proteins serves as a signal for degradation via the 26S proteasome pathway. UB is present in trace amounts in body fluids. Elevated levels of UB are described in the serum or plasma of patients under a variety of conditions. Extracellular UB is proposed to have pleiotropic roles including regulation of immune response, anti-inflammatory, and neuroprotective activities. CXCR4 is identified as receptor for extracellular UB in hematopoietic cells. Heart failure represents a major cause of morbidity and mortality in western society. Cardiac remodeling is a determinant of the clinical course of heart failure. The components involved in myocardial remodeling include-myocytes, fibroblasts, interstitium, and coronary vasculature. Increased sympathetic nerve activity in the form of norepinephrine is a common feature during heart failure. Acting via β-adrenergic receptor (β-AR), norepinephrine is shown to induce myocyte apoptosis and myocardial fibrosis. β-AR stimulation increases extracellular levels of UB in myocytes, and UB inhibits β-AR-stimulated increases in myocyte apoptosis and myocardial fibrosis. This review summarizes intracellular and extracellular functions of UB with particular emphasis on the role of extracellular UB in cardiac myocyte apoptosis and myocardial remodeling.

Extracellular ubiquitin increases expression of angiogenic molecules and stimulates angiogenesis in cardiac microvascular endothelial cells

Extracellular Ub is an immune modulator that plays a role in suppression of inflammation, organ injury, myocyte apoptosis, and fibrosis. The purpose of this study was to investigate the effects of extracellular Ub on the process of cardiac angiogenesis. CMECs and aortic tissue were isolated from rats to measure changes in angiogenic protein levels and to assess angiogenic responses to extracellular Ub. In CMECs, extracellular Ub increased protein levels of VEGF-A and MMP-2, known angiogenesis regulators. CMECs demonstrated enhanced rearrangement of fibrillar actin and migration in response to Ub treatment. Ub-treated CMECs demonstrated an increase in tube network formation which was inhibited by the CXCR4 receptor antagonist, AMD3100. Methylated Ub, unable to form polyubiquitin chains, enhanced tube network formation. Aortic ring sprouting assays demonstrated that Ub increases microvessel sprouting in the Matrigel. The results of our study suggest a novel role for extracellular Ub in cardiac angiogenesis, providing evidence that extracellular Ub, at least in part acting via the CXCR4 receptor, has the potential to facilitate the process of angiogenesis in myocardial endothelial cells.

Exogenous ubiquitin modulates chronic β-adrenergic receptor-stimulated myocardial remodeling: role in Akt activity and matrix metalloproteinase expression

β-Adrenergic receptor (β-AR) stimulation increases extracellular ubiquitin (UB) levels, and extracellular UB inhibits β-AR-stimulated apoptosis in adult cardiac myocytes. This study investigates the role of exogenous UB in chronic β-AR-stimulated myocardial remodeling. l-Isoproterenol (ISO; 400 μg·kg(-1)·h(-1)) was infused in mice in the presence or absence of UB (1 μg·g(-1)·h(-1)). Left ventricular (LV) structural and functional remodeling was studied 7 days after infusion. UB infusion enhanced serum UB levels. In most parts, UB alone had no effect on morphometric or functional parameters. Heart weight-to-body weight ratios were increased to a similar extent in the ISO and UB + ISO groups. Echocardiographic analyses showed increased percent fractional shortening, ejection fraction, and LV circumferential stress and fiber-shortening velocity in the ISO group. These parameters were significantly lower in UB + ISO vs. ISO. Isovolumic contraction and relaxation times and ejection time were significantly lower in ISO vs. UB + ISO. The increase in the number of TUNEL-positive myocytes and fibrosis was significantly higher in ISO vs. UB + ISO. Activation of Akt was higher, whereas activation of GSK-3β and JNKs was lower in UB + ISO vs ISO. Expression of MMP-2, MMP-9, and TIMP-2 was higher in UB + ISO vs ISO. In isolated cardiac fibroblasts, UB enhanced expression of MMP-2 and TIMP-2 in the presence of ISO. Neutralizing UB antibodies negated the effects of UB on MMP-2 expression, whereas recombinant UB enhanced MMP-2 expression. UB activated Akt, and inhibition of Akt inhibited UB + ISO-mediated increases in MMP-2 expression. Thus, exogenous UB plays an important role in β-AR-stimulated myocardial remodeling with effects on LV function, fibrosis, and myocyte apoptosis.

Extracellular ubiquitin: immune modulator and endogenous opponent of damage-associated molecular pattern molecules

Ubiquitin is a post-translational protein modifier and plays essential roles in all aspects of biology. Although the discovery of ubiquitin introduced this highly conserved protein as a molecule with extracellular actions, the identification of ubiquitin as the ATP-dependent proteolysis factor 1 has focused subsequent research on its important intracellular functions. Little attention has since been paid to its role outside of the cell. During recent years, multiple observations suggest that extracellular ubiquitin can modulate immune responses and that exogenous ubiquitin has therapeutic potential to attenuate exuberant inflammation and organ injury. These observations have not been integrated into a comprehensive assessment of its possible role as an endogenous immune modulator. This review recapitulates the current knowledge about extracellular ubiquitin and discusses an emerging facet of its role in biology during infectious and noninfectious inflammation. The synopsis of these data along with the recent identification of ubiquitin as a CXCR4 agonist suggest that extracellular ubiquitin may have pleiotropic roles in the immune system and functions as an endogenous opponent of DAMPs. Functions of extracellular ubiquitin could constitute an evolutionary conserved control mechanism aimed to balance the immune response and prevent exuberant inflammation. Further characterization of its mechanism of action and cellular signaling pathways is expected to provide novel insights into the regulation of the innate immune response and opportunities for therapeutic interventions.

CXC chemokine receptor 4 is a cell surface receptor for extracellular ubiquitin

Ubiquitin is one of the most highly conserved proteins in eukaryotes and plays major biological roles as a post-translational protein modifier. Ubiquitin is also a natural constituent of plasma, and several lines of evidence suggest that extracellular ubiquitin is an immune modulator with anti-inflammatory properties. In addition, ubiquitin treatment has been shown to limit inflammation and reduce organ injury in various disease models and species in vivo. However, its mechanism of action is unknown. Here we show that extracellular ubiquitin is a natural CXC chemokine receptor 4 (CXCR4 and CD184) agonist. Extracellular ubiquitin promotes intracellular Ca(2+) flux and reduces cAMP levels through a G protein-coupled receptor that signals via a Galpha(i/o) protein in THP1 cells. Toll-like receptor 4 stimulation reduces ubiquitin-binding sites, which enabled identification of four Galpha(i/o) PCRs as ubiquitin receptor candidates. Overexpression of candidate genes in HEK293 cells, gene silencing in THP1 cells, competition binding, and signaling studies with the CXCR4 agonist stromal cell-derived factor-1alpha (chemokine (CXC motif) ligand 12) and inhibitor AMD3100 identify CXCR4 as a functional ubiquitin receptor. Our finding uncovers a fundamentally new aspect of the role of ubiquitin in biology, has implications for the understanding of CXCR4-mediated events, and is expected to facilitate development of new therapeutic avenues for a variety of diseases.

Extracellular ubiquitin inhibits beta-AR-stimulated apoptosis in cardiac myocytes: role of GSK-3beta and mitochondrial pathways

AIMS

Beta-adrenergic receptor (beta-AR) stimulation induces apoptosis in adult rat ventricular myocytes (ARVMs) via the activation of glycogen synthase kinase-3beta (GSK-3beta) and mitochondrial pathways. However, beta-AR stimulation induces apoptosis only in a fraction ( approximately 15-20%) of ARVMs. We hypothesized that ARVMs may secrete/release a survival factor(s) which protects 80-85% of cells from apoptosis.

METHODS AND RESULTS

Using two-dimensional gel electrophoresis followed by MALDI TOF and MS/MS, we identified ubiquitin (Ub) in the conditioned media of ARVMs treated with beta-AR agonist (isoproterenol). Western blot analysis confirmed increased Ub levels in the conditioned media 3 and 6 h after beta-AR stimulation. Inhibition of beta1-AR and beta2-AR subtypes inhibited beta-AR-stimulated increases in extracellular levels of Ub, whereas activation of adenylyl cyclase using forskolin mimicked the effects of beta-AR stimulation. Incubation of cells with exogenous biotinylated Ub followed by western blot analysis of the cell lysates showed uptake of extracellular Ub into cells, which was found to be higher after beta-AR stimulation (1.9 +/- 0.4-fold; P < 0.05 vs. control, n = 6). Pre-treatment with Ub inhibited beta-AR-stimulated increases in apoptosis. Inhibition of phosphoinositide 3-kinase using wortmannin and LY-294002 prevented anti-apoptotic effects of extracellular Ub. Ub pre-treatment inhibited beta-AR-stimulated activation of GSK-3beta and c-Jun N-terminal kinase (JNK) and increases in the levels of cytosolic cytochrome c. The use of methylated Ub suggested that the anti-apoptotic effects of extracellular Ub are mediated via monoubiquitination.

CONCLUSION

beta-AR stimulation increases levels of Ub in the conditioned media. Extracellular Ub plays a protective role in beta-AR-stimulated apoptosis, possibly via the inactivation of GSK-3beta/JNK and mitochondrial pathways.

Chronic ethanol feeding affects proteasome-interacting proteins

Studies on alcoholic liver injury mechanisms show a significant inhibition of the proteasome activity. To investigate this phenomenon, we isolated proteasome complexes from the liver of rats fed ethanol chronically, and from the liver of their pair-fed controls, using a non-denaturing multiple centrifugations procedure to preserve proteasome-interacting proteins (PIPs). ICAT and MS/MS spectral counting, further confirmed by Western blot, showed that the levels of several PIPs were significantly decreased in the isolated ethanol proteasome fractions. This was the case of PA28alpha/beta proteasome activator subunits, and of three proteasome-associated deubiquitinases, Rpn11, ubiquitin C-terminal hydrolase 14, and ubiquitin carboxyl-terminal hydrolase L5. Interestingly, Rpn13 C-terminal end was missing in the ethanol proteasome fraction, which probably altered the linking of ubiquitin carboxyl-terminal hydrolase L5 to the proteasome. 20S proteasome and most 19S subunits were however not changed but Ecm29, a protein known to stabilize the interactions between the 20S and its activators, was decreased in the isolated ethanol proteasome fractions. It is proposed that ethanol metabolism causes proteasome inhibition by several mechanisms, including by altering PIPs and proteasome regulatory complexes binding to the proteasome.

Extracellular, circulating proteasomes and ubiquitin - incidence and relevance

The ubiquitin-proteasome system is the major pathway for intracellular protein degradation and is also deeply involved in the regulation of most basic cellular processes. Its proteolytic core, the 20S proteasome, has found to be attached also to the cell plasma membrane and certain observations are interpreted as to suggest that they may be released into the extracellular medium, e.g. in the alveolar lining fluid, epididymal fluid and possibly during the acrosome reaction. Proteasomes have also been detected in normal human blood plasma and designated circulating proteasomes; these have a comparatively low specific activity, a distinct pattern of subtypes and their exact origin is still enigmatic. In patients suffering from autoimmune diseases, malignant myeloproliferative syndromes, multiple myeloma, acute and chronic lymphatic leukaemia, solid tumour, sepsis or trauma, respectively, the concentration of circulating proteasomes has been found to be elevated, to correlate with the disease state and has even prognostic significance. Similarly, ubiquitin has been discovered as a normal component of human blood and seminal plasma and in ovarian follicular fluid. Increased concentrations were measured in diverse pathological situations, not only in blood plasma but also in cerebrospinal fluid, where it may have neuroprotective effects. As defective spermatozoa are covered with ubiquitin in the epididymal fluid, extracellular ubiquitination is proposed to be a mechanism for quality control in spermatogenesis. Growing evidence exists also for a participation of extracellular proteasomes and ubiquitin in the fertilization process.

Systemic ubiquitin release after blunt trauma and burns: association with injury severity, posttraumatic complications, and survival

BACKGROUND

Recent data suggest that ubiquitin (Ub) is systemically released after trauma, has pleiotropic effects on host defense mechanisms, and that Ub administration reduces fluid shifts into tissues during inflammation. Ub release after burns (B) has not been studied and its association with injury severity and outcome after blunt trauma (T) is unknown. Thus, we evaluated Ubs association with injury severity and outcomes after B and T.

METHODS

Injury severity was assessed with the Injury Severity Score (ISS) in T and burn size (% total body surface area, %TBSA) in B. A total of 129 T (ISS: 26 +/- 13) and 55 B (46% +/- 18% TBSA) were observed for sepsis/multiple organ failure (MOF) and survival. In B, sequential organ failure assessment scores were documented daily. Fifty volunteers served as controls (C) Ub serum levels were measured on day 0 (admission), 1, 3, 5, and 7 by enzyme-linked immunosorbent assay. Data were analyzed using bivariate or partial correlation analyses, t test, and analysis of variance with Tukey post-hoc test for multiple comparisons (two-tailed p < 0.05).

RESULTS

Ub was significantly elevated in patients. Peak levels (ng/mL) were detectable on day 0 (C: 118 +/- 76; T: 359 +/- 205; B: 573 +/- 331) and increased with increased ISS, %TBSA, and presence of inhalation injury. In T, Ub normalized by day 3, but remained elevated in B. In B, Ub correlated significantly negative with sequential organ failure assessment scores (r: -0.143; p = 0.0147), sepsis/MOF development (r: -0.363; p = 0.001), and survival (r: -0.231; p = 0.009). Compared with B who recovered uneventfully, Ub levels were significantly lower on days 1 to 7 and on days 5/7 in B who developed sepsis/MOF or died, respectively.

CONCLUSION

Ub concentrations reflect the extent of tissue damage. Along with Ubs previously described anti- inflammatory properties, this study suggests that its systemic release is protective, that burn patients who develop sepsis/MOF have a relative Ub deficiency and that Ub could play an important role during the physiologic response to burn injury.

Ubiquitin reduces contusion volume after controlled cortical impact injury in rats

Recent data suggest that ubiquitin has anti-inflammatory properties and therapeutic potential after severe trauma and brain injuries. However, direct evidence for its neuroprotective effects has not yet been provided. We hypothesized that ubiquitin treatment is neuroprotective, and thus reduces brain edema formation and cortical contusion volume after closed traumatic brain injuries. To test this hypothesis, a focal cortical contusion was induced using a controlled cortical impact (CCI) model in Sprague-Dawley rats. Animals (n = 27) were randomized to either 1.5 mg/kg ubiquitin or vehicle (placebo) intravenously within 5 min after CCI. Blood pressure, arterial blood gases (ABG) and intracranial pressure (ICP) were monitored. Ubiquitin serum and cerebrospinal fluid levels were measured by ELISA. Brain water content was quantified gravimetrically after 24 h and cerebral contusion volume was determined in triphenyltetrazolium-chloride stained brains after 7 days. All animals recovered to normal activity. ICP and cerebral perfusion pressures were normal at the end of the observation period. Ubiquitin serum and CSF levels at 24 h and 7 days after CCI were similar in both groups. With ubiquitin brain water content of the injured hemisphere was slightly lower (n = 6/group; 79.97 +/- 0.29% vs. 81.11 +/- 0.52%; p = 0.08). Cortical contusion volume was significantly lower with ubiquitin (n = 7-8/group; 32.88 +/- 2.1 mm(3) vs. 43.96 +/- 4.56 mm(3); p = 0.025). This study shows that ubiquitin treatment after brain injury has direct neuroprotective effects, as demonstrated by improved brain morphology 7 days after brain injury. In connection with its beneficial effects in our previous studies, these data suggest ubiquitin as a promising candidate protein therapeutic for the treatment of brain injuries.

Role of the proteasome in ethanol-induced liver pathology

The ubiquitin-proteasome system has come to be known as a vital constituent of mammalian cells. The proteasome is a large nonlysosomal enzyme that acts in concert with an 8.5 kDa polypeptide called ubiquitin and a series of conjugating enzymes, known as E1, E2 and E3, that covalently bind multiple ubiquitin moieties in a polyubiquitin chain to protein substrates in a process called ubiquitylation. The latter process targets protein substrates for unfolding and degradation by the 26S proteasome. This enzyme system specifically recognizes and degrades polyubiquitylated proteins, many of which are key proteins involved in cell cycle regulation, apoptosis, signal transduction, and antigen presentation. The 26S proteasome contains a cylinder-shaped 20S catalytic core that, itself, degrades proteins in an ATP- and ubiquitin-independent manner. The 20S form is actually the predominant enzyme form in mammalian cells. Proteolysis by the constitutive 20S proteasome is vital in removing oxidized, misfolded and otherwise modified proteins. Such degradation is critical as a means of cellular detoxification, as intracellular accumulation of damaged and misfolded proteins is potentially lethal. Studies have shown that inhibition of proteasome activity can lead to cell death. Ethanol and its metabolism cause partial inhibition of the proteasome. This leads to a number of pleiotropic effects that can affect a variety of cellular processes. This critical review describes important aspects of ethanol metabolism and its influence on the proteasome. The review will summarize recent findings on: (1) the interactions between the proteasome and the ethanol metabolizing enzyme, CYP2E1; (2) the dynamics of proteasome inhibition by ethanol in animal models and cultured cells; (3) ethanol-elicited suppression of proteasome activity and its effect on signal transduction; (4) The role of proteasome inhibition in cytokine production by liver cells; and (5) ethanol elicited suppression of peptide hydrolysis and the potential effects on antigen presentation. While the principal focus is on alcohol-induced liver injury, the authors foresee that the findings presented in this review will prompt further research on the role of this proteolytic system in other tissues injured by excessive alcohol consumption.

Prolongation of skin graft survival by exogenous ubiquitin

Recently, it was shown that exogenous ubiquitin has anti-inflammatory actions in vivo and that the ubiquitin-decapeptide 50-59 has immunosuppressive effects similar to cyclosporine. Immunosuppressive effects of the native ubiquitin molecule and its therapeutic potential in transplantation are unknown. We tested the hypothesis that ubiquitin inhibits alloreactivity and increases allograft survival in a murine model of skin transplantation in fully mismatched strain combinations (C3H/HEJ-DBA2). Ubiquitin dose-dependently inhibited mixed leukocyte reaction in C3H/HEJ splenocytes in vitro. Intraperitoneal ubiquitin administration (25 microg/h for 14 days) was well-tolerated, dose-dependently increased ubiquitin serum concentrations and median allograft survival from 10 days (with albumin; control) to 17 days in DBA2 mice (survival ratio: 1.7, 95% confidence interval: 1.266-2.134; P=0.0005). The in vivo effects in this study combined with our previous work strongly indicate that ubiquitin is a potent immune modulator with broad therapeutic potential. Ubiquitin treatment could be a novel strategy to improve immunosuppressive regimens in transplantation.

Dysregulated cytokine metabolism, altered hepatic methionine metabolism and proteasome dysfunction in alcoholic liver disease

Alcoholic liver disease (ALD) remains an important complication and cause of morbidity and mortality from alcohol abuse. Major developments in our understanding of the mechanisms of ALD over the past decade are now being translated into new forms of therapy for this disease process which currently has no FDA approved treatment. Cytokines are low molecular weight mediators of cellular communication, and the pro-inflammatory cytokine tumor necrosis factor (TNF) has been shown to play a pivotal role in the development of experimental ALD. Similarly, TNF levels are elevated in the serum of alcoholic hepatitis patients. Abnormal methionine metabolism is well documented in patients with ALD, with patients having elevated serum methionine levels, but low S-adenosylmethionine levels in the liver. On the other hand, S-adenosylhomocysteine and homocysteine levels are elevated in ALD. Recent studies have documented potential interactions between homocysteine and S-adenosylhomocysteine with TNF in the development of ALD. Altered proteasome function also is now well documented in ALD, and decreased proteasome function can cause hepatocyte apoptosis. Recently it has been shown that decreased proteasome function can also act synergistically to enhance TNF hepatotoxicity. Hepatocytes dying of proteasome dysfunction release pro-inflammatory cytokines such as Interleukin-8 to cause sustained inflammation. This article reviews the interactions of cytokines, altered methionine metabolism, and proteasome dysfunction in the development of ALD.

Targeting the monocytic ubiquitin system with extracellular ubiquitin

Recent findings suggest that extracellular ubiquitin has pleiotropic effects on host defence mechanisms, but its cellular mechanism of action is not yet understood. Using fluorescence and in vivo confocal microscopy, we observed uptake of N-terminal fluorescein-labelled ubiquitin into human PBMC and MonoMac 6 cells. Immunoblotting experiments indicated that extracellular ubiquitin is then rapidly conjugated to a multitude of intracellular proteins. LPS and lipoteichoic acid significantly increased uptake and subsequent conjugation to intracellular proteins dose dependently. This mechanism showed saturation kinetics with a K(d) value for ubiquitin in the low nanomolar range (<10 nmol/L) and a B(max) value of 0.14-0.27 micromol ubiquitin/mg protein. These results suggest that the monocytic ubiquitin system can be targeted with physiologically relevant concentrations of extracellular ubiquitin during inflammation. This concept could provide a simple explanation for a multitude of extracellular ubiquitin's actions and open up new strategies to influence ubiquitin-dependent intracellular processes.

Ubiquitin reduces fluid shifts after traumatic brain injury

BACKGROUND

Ubiquitin has well-described intracellular properties. Recent data also suggest pleiotropic effects of extracellular ubiquitin, including induction of apoptosis, regulation of immune functions, and therapeutic potential during fluid resuscitation from severe trauma. However, the actions of exogenous ubiquitin after traumatic brain injury (TBI) are unknown.

METHODS

Series 1: Thirty-five minutes after TBI and hemorrhage, 1.5 mg ubiquitin/kg (n = 5) or albumin (n = 5) intravenous was followed by fluid resuscitation to maintain mean arterial and cerebral perfusion pressure. Series 2: Ubiquitin (n = 5) or vehicle (n = 6) was administered after TBI only. Ubiquitin was measured with enzyme-linked immunosorbent assay in serum, urine (series 1), and cerebrospinal fluid (series 2) for 300 minutes.

RESULTS

Series 1: After intravenous bolus, serum ubiquitin peaked at t = 45 minutes with a half-life of 54 minutes. Recovery in urine was 10%. With albumin versus ubiquitin, 85% more resuscitation fluid was required to stabilize systemic and cerebral hemodynamics (P < .05 for t = 150 to 300 minutes), but hematocrit was similar. With albumin there were progressive increases in intracranial pressure, peak inspiratory pressure, and decreases in oxygenation. All were significantly attenuated by ubiquitin (all P < .05 vs albumin). Series 2: Intravenous ubiquitin altered cerebrospinal fluid ubiquitin with an increased time to peak (t = 88 +/- 13 min vs 45 +/- 7 min, P < .05) and area under the concentration-time curve (82 +/- 22 vs 23 +/- 11 microg/min(1)/mL(-1), P < .05).

CONCLUSIONS

After TBI, intravenous ubiquitin crossed the blood-brain barrier and significantly reduced third spacing of fluid into the brain and lung during resuscitation.

Role of the ubiquitin-proteasome pathway in the diagnosis of human diseases

The ubiquitin-proteasome pathway constitutes the major system for nuclear and extralysosomal cytosolic protein degradation in eukaryotic cells. A plethora of cell proteins implicated in the maintenance and regulation of essential cellular processes undergoes processing and functional modification by proteolytic degradation via the ubiquitin-proteasome pathway. Deregulations of the pathway have been shown to contribute to the pathogenesis of several human diseases, such as cancer, neurodegenerative, autoimmune, genetic and metabolic disorders, most of them exhibiting abnormal accumulation and altered composition of components of the pathway that is suitable for diagnostic proceedings. While the ubiquitin-proteasome pathway is currently exploited to develop novel therapeutic strategies, it is less regarded as a diagnostic area. Future research should lead to an improved understanding of the pathophysiology of the ubiquitin-proteasome pathway with the aim of allowing the development of subtle diagnostic strategies.

Effects of exogenous ubiquitin in lethal endotoxemia

BACKGROUND

Recent data indicated a potential role for extracellular ubiquitin in hematopoiesis and inflammation. The biological significance and therapeutic potential of these findings in vivo are unknown. Based on its in vitro abilities to inhibit endotoxin-stimulated tumor necrosis factor alpha (TNFalpha) production, we hypothesized that exogenous ubiquitin has salutary effects on sequelae caused by endotoxin in vivo.

METHODS

Anesthetized and mechanically ventilated swine were infused with endotoxin for 3 hours. Ubiquitin was administered intravenously either 15 minutes before or 45 minutes after the endotoxin infusion was started. Albumin was administered to a control group. An additional control group received only ubiquitin. Ex vivo endotoxin evoked TNFalpha production was measured using a whole blood assay. Ubiquitin and TNFalpha concentrations were determined by enzyme-linked immunosorbent assay.

RESULTS

Ubiquitin reduced mortality (P <.05), prevented development of pulmonary failure (P <.05), reduced fluid requirements (P <.05), and diminished erythema and edema formation. Ubiquitin pretreatment was more effective than treatment 45 minutes after an endotoxin infusion was started. In vivo ubiquitin administration alone inhibited ex vivo endotoxin-evoked TNFalpha secretion, but had no effect on TNFalpha serum levels after endotoxin infusion.

CONCLUSION

In vivo ubiquitin administration has salutary actions during lethal endotoxemia and inhibits ex vivo whole blood TNFalpha production upon endotoxin stimulation. The clinical appearance after ubiquitin treatment in endotoxemia indicates the endothelium as another potential target cell population for interactions with ubiquitin. A novel therapeutic approach to a broad variety of diseases, in which endotoxin triggers immune activation, is suggested.

Inhibition of proteasome function leads to NF-kappaB-independent IL-8 expression in human hepatocytes

Breakdown of cellular proteins is a highly regulated process, and the ubiquitin-proteasome pathway is the major proteolytic system in the cell. It regulates the levels of numerous proteins that control gene expression and cell division, as well as responses to stress and inflammation. Recent studies have reported abnormalities in proteasome function in alcoholic liver disease (ALD). Moreover, a direct relation has been reported between impaired proteasome function and oxidative stress in experimental models of ALD. Neutrophil infiltration is a hallmark of ALD, and activated neutrophils are thought to play a role in the pathology of ALD. As a potent neutrophil chemoattractant and activator, interleukin 8 (IL-8) likely plays a key mechanistic role in many forms of liver injury. In this study, we evaluated the effects of inhibition of proteasome function on expression and release of IL-8 by human fetal hepatocytes and hepatoma cells. Our data demonstrate that inhibition of proteasome function in hepatocytes leads to apoptotic cell death. Decreased hepatocyte survival coincides with enhanced expression of IL-8, both at the protein and the messenger RNA (mRNA) levels. This increase in IL-8 is independent of nuclear factor kappaB (NF-kappaB) activation and is associated with an increase in c-Jun N-terminal kinase (JNK) and activator protein-1 (AP-1) activity. In conclusion, hepatocytes dying because of inhibition of proteasome function produce massive quantities of the proinflammatory chemokine IL-8, possibly resulting in neutrophil infiltration, increased inflammation, and liver injury.

Proteasome inhibition potentiates CYP2E1-mediated toxicity in HepG2 cells

Chronic ethanol consumption causes increased oxidative damage in the liver. Induction of CYP2E1 is one pathway involved in how ethanol produces oxidative stress. Ethanol can cause protein accumulation, decreased proteolysis, and decreased proteasome activity. The objective of this study was to investigate the effect of inhibition of the proteasome activity on CYP2E1-dependent toxicity. HepG2 cells over-expressing CYP2E1 (E47 cells) were treated with arachidonic acid (AA) plus iron, agents important in development of alcoholic liver injury and which are toxic to E47 cells by a mechanism dependent on CYP2E1, oxidative stress, and lipid peroxidation. Addition of various proteasome inhibitors was associated with significant potentiation of the loss of cell viability caused by AA plus iron. Potentiation of toxicity was associated with increased oxidative damage as reflected by an increase in lipid peroxidation and accumulation of oxidized and nitrated proteins in E47 cells and an enhanced decline in mitochondrial membrane potential. Antioxidants prevented the loss of viability and the potentiation of this loss of viability by proteasome inhibition. CYP2E1 levels were elevated about 3-fold by the proteasome inhibitors. Inhibition of proteasome activity also potentiated toxicity of AA alone and toxicity after treatment to remove glutathione (GSH). Similar results were found in hepatocytes from pyrazole-treated rats with high levels of CYP2E1. In conclusion, proteasome activity plays an important role in modulating CYP2E1-mediated toxicity in HepG2 cells by regulating CYP2E1 levels and by removal of oxidized proteins. Such interactions may be important in CYP2E1-catalyzed toxicity of hepatotoxins and in alcohol-induced liver injury.

Extracellular ubiquitin inhibits the TNF-alpha response to endotoxin in peripheral blood mononuclear cells and regulates endotoxin hyporesponsiveness in critical illness

Ubiquitin is suggested to play a key role in essential intracellular functions, such as heat shock response, protein breakdown, and regulation of immune responses. Ubiquitin has also been detected in the extracellular space, but the function and biologic significance is unclear. We describe a new function of extracellular ubiquitin and show that extracellular ubiquitin specifically inhibits ex vivo secretion of tumor necrosis factor-alpha (TNF-alpha) and TNF-alpha mRNA expression from peripheral blood mononuclear cells (PBMNCs) in response to endotoxin in a dose-dependent manner. In contrast, the TNF-alpha response to zymosan or Staphylococcus aureus as well as the interleukin-6 (IL-6) and IL-8 responses to endotoxin were unaffected by ubiquitin. Measurement of serum ubiquitin levels showed a significant 5- to 7-fold increase in sepsis and trauma patients, to the level required for inhibition of the PBMNC TNF-alpha response to endotoxin by ubiquitin. Elevated ubiquitin levels in serum were significantly correlated with a reduced TNF-alpha production. Antibodies to ubiquitin were able to (1) significantly increase (2- to 5-fold) the TNF-alpha response to endotoxin in whole blood from trauma and sepsis patients, (2) completely neutralize the inhibitory effect of trauma patients' serum on healthy donors' TNF-alpha production, and (3) partially neutralize the inhibitory effect of sepsis patients' serum on healthy donors' TNF-alpha production. Ubiquitin-depleted serum from trauma patients lost the inhibitory activity for TNF-alpha production, whereas extracted endogenous ubiquitin exerts the inhibitory activity. The results demonstrate that extracellular ubiquitin acts as a cytokinelike protein with anti-inflammatory properties and indicate that extracellular ubiquitin is involved in the regulation of immunodepression in critical illness.

The ubiquitin-proteasome system and its role in ethanol-induced disorders

Some of the most fundamental yet important cellular activities such as cell division and gene expression are controlled by short-lived regulatory proteins. The levels of these proteins are controlled by their rates of degradation. Similarly, protein catabolism plays a crucial role in prolonging cellular life by destroying damaged proteins that are potentially cytotoxic. A major player in these catabolic reactions is the ubiquitin-proteasome system, a novel proteolytic system that has become the primary proteolytic pathway in eukaryotic cells. Ubiquitin-mediated proteolysis is now regarded as the major pathway by which most intracellular proteins are destroyed. Equally important, from a toxicological standpoint, is that the ubiquitin-proteasome system is also widely considered to be a cellular defense mechanism, since it is involved in the removal of damaged proteins generated by adduct formation and oxidative stress. This review describes the history and the components of the ubiquitin-proteasome system, its regulation and its role in pathological states, with the major emphasis on ethanol-induced organ injury. The available literature cited here deals mainly with the effects of ethanol consumption on the ubiquitin-proteasome pathway in the liver. However, since this proteolytic system is an essential pathway in all cells it is an attractive experimental model and therapeutic target in extrahepatic organs such as the brain and heart that are also affected by excessive alcohol consumption.