Molecular biology of proteasomes

More than 20 years of the discovery of Park2

Discovery of Park2 is our finding of a family of young onset parkinsonism, in which this family was thought to be associated with a polymorphism of the manganese superoxide gene. The gene locus of the manganese superoxide dismutase has been known. We were able to pick up a gene for this family and related families in the close approximate position at the long arm of chromosome 6. The gene for this disease has a ubiquitin-like motif in the N-terminus and two RING finger structures. It was shown that this gene had a ubiquitin-protein ligase activity. But it is not elucidated the substrate of this enzyme. Meanwhile, it has become clear that PINK1 and Parkin work together to remove the mitochondria of the lowered membrane potential in the autophagosomes (mitophagy). Now that the molecular mechanisms of mitophagy is under investigation. In addition, many hot topics are going on such as Lewy body in Park2, single heterozygotes, rare clinical manifestations, and so on.

Effects of tris (2-chloroethyl) phosphate (TCEP) on growth, reproduction and gene transcription in the protozoan Tetrahymena thermophila

As a typical organophosphorus flame retardant, tris (2-chloroethyl) phosphate (TCEP) has been widely detected in various environmental media. Toxicity of TCEP to vertebrates have been investigated, but potential effects on lower trophic level species were unknown to date. In this study, toxic effects and molecular mechanisms of toxic actions of TCEP on the aquatic protozoan Tetrahymena thermophila were evaluated by use of phenotypic observations, transcriptome sequencing analysis and real-time quantitative PCR detection. Exposure to 0.044, 0.411 or 4.26 mg/L TCEP for 5 days decreased the theoretical population, cell viability, number of cilia and cell size of Tetrahymena thermophila in a time- and dose-dependent manner. Meanwhile, RNA-Seq analysis indicated that exposure to 4.26 mg/L TCEP significantly changed expression of 2932 genes (up-regulation: 1228; down-regulation: 1704). Of these, expressions of 9, 10 and 17 genes that were enriched in soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) interaction in vesicular transport, proteasome and endocytosis pathway respectively were down-regulated. Data collected during this study suggested that exposure to high concentrations of TCEP might affect growth and reproduction of Tetrahymena thermophila through down-regulating transcriptional levels of genes encoding proteins associated with vesicle trafficking, proteasome and endocytosis.

Twenty years since the discovery of the parkin gene

Nearly 20 years have passed since we identified the causative gene for a familial Parkinson's disease, parkin (now known as PARK2), in 1998. PARK2 is the most common gene responsible for young-onset Parkinson's disease. It codes for the protein Parkin RBR E3 ubiquitin-protein ligase (PARK2), which directly links to the ubiquitin-proteasome as a ubiquitin ligase. PARK2 is involved in mitophagy, which is a type of autophagy, in collaboration with PTEN-induced putative kinase 1 (PINK1). The PINK1 gene (previously known as PARK6) is also a causative gene for young-onset Parkinson's disease. Both gene products may be involved in regulating quality control within the mitochondria. The discovery of PARK2 as a cause of young-onset Parkinson's disease has had a major impact on other neurodegenerative diseases. The involvement of protein degradation systems has been implicated as a common mechanism for neurodegenerative diseases in which inclusion body formation is observed. The discovery of the involvement of PARK2 in Parkinson's disease focused attention on the involvement of protein degradation systems in neurodegenerative diseases. In this review, we focus on the history of the discovery of PARK2, the clinical phenotypes of patients with PARK2 mutations, and its functional roles.

Interferon-dependent immunoproteasome activity during mouse adenovirus type 1 infection

The immunoproteasome is an inducible host mechanism that aids in the clearance of damaged proteins. The immunoproteasome also influences immune function by enhancing peptide presentation by MHC class I and promotes inflammation via IκB degradation and activation of NF-κB. We used mouse adenovirus type 1 (MAV-1) to characterize the role of the immunoproteasome in adenovirus pathogenesis. Following intranasal infection of mice, immunoproteasome activity in the heart and lung was significantly increased in an IFN-γ-dependent manner. Absence of the β5i immunoproteasome subunit and pharmacological inhibition of β5i activity had minimal effects on viral replication, virus-induced cellular inflammation, or induction of cytokine expression. Likewise, the establishment of protective immunity following primary infection was not significantly altered by β5i deficiency. Thus, although immunoproteasome activity is robustly induced during acute infection with MAV-1, our data suggest that other mechanisms are capable of compensating for immunoproteasome activity to maintain antiviral immunity and appropriate inflammatory responses.

Analysis of Differentially Expressed Genes Associated With Alzheimer's Disease Based on Bioinformatics Methods

OBJECTIVE

To screen differentially expressed genes (DEGs) of Alzheimer's disease (AD).

METHODS

The gene expression profile (GSE26972) of AD was downloaded from Gene Expression Omnibus database. The DEGs were mapped to protein-protein interaction (PPI) data for acquiring the potential PPI relationship. The coexpressed significance of a gene pair in AD was determined. Then significantly enriched Gene Ontology (GO) terms and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways of DEGs were analyzed based on database for annotation visualization and integrated discovery tool.

RESULTS

The PPI network showed 7 upregulated genes and 4 downregulated genes that might play meaningful functional roles in AD. Meanwhile, 3 significantly enriched KEGG pathways as well as several significant GO terms (included α-actinin binding, interleukin 33 receptor activity, and telethonin binding) were identified.

CONCLUSIONS

The screened DEGs have the potential to become candidate target molecules to monitor, diagnose, and treat AD.

New Insights into the Function of the Immunoproteasome in Immune and Nonimmune Cells

The immunoproteasome is a highly efficient proteolytic machinery derived from the constitutive proteasome and is abundantly expressed in immune cells. The immunoproteasome plays a critical role in the immune system because it degrades intracellular proteins, for example, those of viral origin, into small proteins. They are further digested into short peptides to be presented by major histocompatibility complex (MHC) class I molecules. In addition, the immunoproteasome influences inflammatory disease pathogenesis through its ability to regulate T cell polarization. The immunoproteasome is also expressed in nonimmune cell types during inflammation or neoplastic transformation, supporting a role in the pathogenesis of autoimmune diseases and neoplasms. Following the success of inhibitors of the constitutive proteasome, which is now an established treatment modality for multiple myeloma, compounds that selectively inhibit the immunoproteasome are currently under active investigation. This paper will review the functions of the immunoproteasome, highlighting areas where novel pharmacological treatments that regulate immunoproteasome activity could be developed.

The immunoproteasome and viral infection: a complex regulator of inflammation

During viral infection, proper regulation of immune responses is necessary to ensure successful viral clearance with minimal host tissue damage. Proteasomes play a crucial role in the generation of antigenic peptides for presentation on MHC class I molecules, and thus activation of CD8 T cells, as well as activation of the NF-κB pathway. A specialized type of proteasome called the immunoproteasome is constitutively expressed in hematopoietic cells and induced in non-immune cells during viral infection by interferon signaling. The immunoproteasome regulates CD8 T cell responses to many viral epitopes during infection. Accumulating evidence suggests that the immunoproteasome may also contribute to regulation of proinflammatory cytokine production, activation of the NF-κB pathway, and management of oxidative stress. Many viruses have mechanisms of interfering with immunoproteasome function, including prevention of transcriptional upregulation of immunoproteasome components as well as direct interaction of viral proteins with immunoproteasome subunits. A better understanding of the role of the immunoproteasome in different cell types, tissues, and hosts has the potential to improve vaccine design and facilitate the development of effective treatment strategies for viral infections.

Activators of G protein signaling in the kidney

Heterotrimeric G proteins play a crucial role in regulating signal processing to maintain normal cellular homeostasis, and subtle perturbations in its activity can potentially lead to the pathogenesis of renal disorders or diseases. Cell-surface receptors and accessory proteins, which normally modify and organize the coupling of individual G protein subunits, contribute to the regulation of heterotrimeric G protein activity and their convergence and/or divergence of downstream signaling initiated by effector systems. Activators of G protein signaling (AGS) are a family of accessory proteins that intervene at multiple distinct points during the activation-inactivation cycle of G proteins, even in the absence of receptor stimulation. Perturbations in the expression of individual AGS proteins have been reported to modulate signal transduction pathways in a wide array of diseases and disorders within the brain, heart, immune system, and more recently, the kidney. This review will provide an overview of the expression profile, localization, and putative biologic role of the AGS family in the context of normal and diseased states of the kidney.

Molecular characterization and expressional affirmation of the beta proteasome subunit cluster in rock bream immune defense

Immunoproteasomes are primarily induced upon infection and formed by replacing constitutive beta subunits with inducible beta subunits which possess specific cleavage properties that aid in the release of peptides necessary for MHC class I antigen presentation. In this study, we report the molecular characterization and expression analysis of the inducible immunosubunits PSMB8, PSMB9, PSMB9-L, and PSMB10 from rock bream, Oplegnathus fasciatus. The three subunits shared common active site residues and were placed in close proximity to fish homologues in the reconstructed phylogenetic tree, in which the mammalian homologues formed separate clades, indicating a common ancestral origin. The rock bream immunosubunits possessed higher identity and similarity with the fish homologues. RbPSMB8, RbPSMB9, RbPSMB9-L, and RbPSMB10 were multi-exonic genes with 6, 6, 7 and 8 exons, respectively. These four genes were constitutively expressed in all the examined tissues. Immunostimulants such as lipopolysaccharide and poly I:C induced RbPSMB8, RbPSMB9, RbPSMB9-L, and RbPSMB10 in liver and head kidney, suggesting their possible involvement in immune defense in rock bream.

Correlation between circulating proteasome activity, total protein and c-reactive protein levels following burn in children

AIM OF THE STUDY

To characterize burn-induced changes following burn in children by analyzing circulating proteasome (c-proteasome) activity in the plasma in correlation with total protein and c-reactive protein levels in the plasma, and the severity of the burn.

METHODS

Fifty consecutive children scalded by hot water who were managed at the Department of Pediatric Surgery after primarily presenting with burns in 4-20% TBSA were included into the study. The children were aged 9 months up to 14 years (mean age 2.5±1 years). Patients were divided into groups according to the pediatric injury severity score used by American Burns Association. Plasma proteasome activity was assessed using Suc-Leu-Leu-Val-Tyr-AMC peptide substrate, 2-6 h, 12-16 h, 3 days, 5 days, and 7 days after injury. 20 healthy children consecutively admitted for planned inguinal hernia repair served as controls.

RESULTS

Statistically significant elevation of plasma c-proteasome activity was noted in all groups of burned children 12-16 h after the injury. We found a strong negative correlation of c-proteasome activity with total protein levels, and positive correlation with CRP levels 12-16 h after burn. We also found stronger correlation between c-proteasome activity and severity of burn, than CRP level and severity of burn 12-16 h, and 3 days after the burn. Correlations were statistically significant.

CONCLUSIONS

This study characterized circulating 20S proteasome activity levels after burn. C-proteasome activity elevate after burn and correlate negatively with plasma total protein level, thus plasma 20S proteasome activity could be additional biomarker of tissue damage in burn in pediatric population.

Interactions of SARS coronavirus nucleocapsid protein with the host cell proteasome subunit p42

Background

Severe acute respiratory syndrome-associated coronavirus (SARS-CoV) spreads rapidly and has a high case-mortality rate. The nucleocapsid protein (NP) of SARS-CoV may be critical for pathogenicity. This study sought to discover the host proteins that interact with SARS-CoV NP.

Results

Using surface plasmon resonance biomolecular interaction analysis (SPR/BIA) and matrix-assisted laser desorption/ionization time of flight (MALDI-TOF) mass spectrometry, we found that only the proteasome subunit p42 from human fetal lung diploid fibroblast (2BS) cells bound to SARS-CoV NP. This interaction was confirmed by the glutathione S-transferase (GST) fusion protein pulldown technique. The co-localization signal of SARS-CoV NP and proteasome subunit p42 in 2BS cells was detected using indirect immunofluorescence and confocal microscopy. p42 is a subunit of the 26S proteasome; this large, multi-protein complex is a component of the ubiquitin-proteasome pathway, which is involved in a variety of basic cellular processes and inflammatory responses.

Conclusion

To our knowledge, this is the first report that SARS-CoV NP interacts with the proteasome subunit p42 within host cells. These data enhance our understanding of the molecular mechanisms of SARS-CoV pathogenicity and the means by which SARS-CoV interacts with host cells.

Differential display identifies genes in Chinese hamster ovary cells sensitive to elevated ammonium

Ammonium is a toxic waste product that has been reported to negatively inhibit cell growth and recombinant glycosylation in Chinese hamster ovary (CHO) cells; however, the effect of this toxicity on intracellular gene expression has received only limited investigation. We used a differential display method to identify genes in CHO cells that were affected by ammonium stress. Eight genes whose mRNA levels significantly changed in response to elevated ammonium were isolated and identified. Five of the genes were identified as having lower expression under the ammonium stress, whereas three genes were identified as having higher expression. Sequence homology with other mammalian organisms was used to attribute function to these newly identified genes. The identified ammonium-sensitive genes were grouped into three broad functional groups: cellular processes, energy metabolism, and genetic-information processing. The three cellular process-related genes had lower expression (anaphase-promoting complex subunit 5, eukaryotic initiation factor 5A II, KIAA1091 protein). The two energy-related genes had higher expression under ammonium stress (adenosine triphosphate synthase subunit C and mitofusin 1). Both of the genetic information-processing genes (endoplasmic reticulum [ER]-resident protein ERdj5 and structure-specific recognition protein 1) had lower expression under the ammonium stress, whereas the 26S proteasome subunit adenosine triphosphatase 3 gene had higher expression. These preliminary results indicate that ammonium stress lowers expression of genes controlling cell cycle, protein folding, and quality and raises genes that control energy metabolism and degradation. Our findings demonstrate the usefulness of mRNA differential-display techniques for the detection of CHO cell genes affected by ammonium stress.

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.

Binding of aflatoxins to the 20S proteasome: effects on enzyme functionality and implications for oxidative stress and apoptosis

Aflatoxins (AF) are contaminants of improperly stored foods; they are potent genotoxic and carcinogenic compounds, exerting their effects through damage to DNA. They can also induce mutations that increase oxidative damage. The goal of this study was to evaluate the possibility that a third mechanism could be involved in the carcinogenic action of aflatoxins, namely, direct binding to key enzymes involved in the regulatory pathways of the cell cycle, thereby modulating enzyme functionality. The 20S constitutive and immunoproteasome peptidase and proteolytic activities were assayed in the presence of aflatoxins B1, G1 and M1. All three toxins activated multiple peptidase activities of the proteasome. Aflatoxin (AF) M1 was the most potent activator of proteasome activity, while the constitutive 20S proteasome was specifically stimulated by AFG1. Furthermore, the effects of AFB1 on cultured hepatoma cells were investigated and the various proteasomal activities determined with cell lysates were differently affected. Taking into account the key role of the proteasome in cellular defense against oxidative stress, the carbonyl group content and the activities of antioxidant enzymes in cell lysates were analyzed. The proapoptotic effect of AFB1 was also investigated by measuring caspase-3 activity and cellular levels of p27 and IkappaBalpha.

Comprehensive mass spectrometric analysis of the 20S proteasome complex

The 20S proteasome is a multicatalytic protein complex that plays an important role in intracellular protein degradation from archaebacteria to eukaryotes. This complex is made up of two copies each of seven different alpha (alpha) and seven different beta (beta) subunits arranged into four stacked rings (alpha7beta7beta7alpha7). Although the proteasome's cylindrical structure is conserved, the subunit composition of the 20S protein complex varies during the evolution, and the number of subunits increases from archaebacteria to mammals. To fully characterize the 20S proteasome subunit composition and understand the subunit functions, we, the authors of this chapter, have developed and employed various mass spectrometry (MS)-based approaches to generate a comprehensive profile of the 20S proteasomes from rat liver and Tropanosoma brucei. We have identified 7 alpha and 10 beta subunits, including 7 essential and 3 nonessential beta subunits from rat 20S proteasome complex using two-dimensional (2-D) gel electrophoresis and tandem MS (MS/MS). In addition, multiple isoforms of most of the subunits were determined; indicating the composition of rat 20S proteasome complex was much more complicated than expected. Further analysis of the intact protein molecular weight of each subunit using LC-MS confirmed the heterogeneous population of the 20S proteasome and revealed that many of the experimental measured molecular weights do not correspond well with the theoretical values deduced from the sequences in protein databases. This finding is mostly due to the sequence errors in the protein databases and possible posttranslational modifications. Although the protein sequences of rat 20S proteasome are present in the databases, the sequences of the 20S proteasome from T. brucei were not available at the time when the analysis was carried out. To determine the subunit composition of the 20S proteasome from T. brucei, we developed a homology-based database searching tool to identify unknown proteins based on the novel sequences determined by de novo sequencing using MS/MS. As a result, 14 subunits (7 alpha and 7 beta) were identified on the 2-D gel, which was later confirmed by the full-length sequences. Using the same approach, we also identified and characterized an activator protein, PA26, from T. brucei. The purified recombinant PA26 self-assembles into a heptamer ring, which can bind and activate the 20S proteasome from T. brucei as well as rat. Compared to the human PA28 complex, PA26 may be the prototype activator protein involved in proteasomal protein degradation. Therefore, the MS-based strategy developed here for identification of the known and unknown protein complexes can be generalized for the study of other protein complexes.

Ubiquitin-proteasome-dependent proteolytic activity remains elevated after zymosan-induced sepsis in rats while muscle mass recovers

We studied the role of the ubiquitin-proteasome system in rat skeletal muscle during sepsis and subsequent recovery. Sepsis was induced with intraperitoneal zymosan injections. This model allows one to study a sustained and reversible catabolic phase and mimics the events that prevail in septic and subsequently recovering patients. In addition, the role of the ubiquitin-proteasome system during muscle recovery is poorly documented. There was a trend for increased ubiquitin-conjugate formation in the muscle wasting phase, which was abolished during the recovery phase. The trypsin- and chymotrypsin-like peptidase activities of the 20S proteasome peaked at day 6 following zymosan injection (i.e. when both muscle mass and muscle fiber cross-sectional area were reduced the most), but remained elevated when muscle mass and muscle fiber cross-sectional area were recovering (11 days). This clearly suggests a role for the ubiquitin-proteasome pathway in the muscle remodeling and/or recovery process. Protein levels of 19S complex and 20S proteasome subunits did not increase throughout the study, pointing to alternative mechanisms regulating proteasome activities. Overall these data support a role for ubiquitin-proteasome dependent proteolysis in the zymosan septic model, in both the catabolic and muscle recovery phases.

The proteasome inhibitor MG-132 sensitizes PC-3 prostate cancer cells to ionizing radiation by a DNA-PK-independent mechanism

BACKGROUND

By modulating the expression levels of specific signal transduction molecules, the 26S proteasome plays a central role in determining cell cycle progression or arrest and cell survival or death in response to stress stimuli, including ionizing radiation. Inhibition of proteasome function by specific drugs results in cell cycle arrest, apoptosis and radiosensitization of many cancer cell lines. This study investigates whether there is also a concomitant increase in cellular radiosensitivity if proteasome inhibition occurs only transiently before radiation. Further, since proteasome inhibition has been shown to activate caspase-3, which is involved in apoptosis, and caspase-3 can cleave DNA-PKcs, which is involved in DNA-double strand repair, the hypothesis was tested that caspase-3 activation was essential for both apoptosis and radiosensitization following proteasome inhibition.

METHODS

Prostate carcinoma PC-3 cells were treated with the reversible proteasome inhibitor MG-132. Cell cycle distribution, apoptosis, caspase-3 activity, DNA-PKcs protein levels and DNA-PK activity were monitored. Radiosensitivity was assessed using a clonogenic assay.

RESULTS

Inhibition of proteasome function caused cell cycle arrest and apoptosis but this did not involve early activation of caspase-3. Short-time inhibition of proteasome function also caused radiosensitization but this did not involve a decrease in DNA-PKcs protein levels or DNA-PK activity.

CONCLUSION

We conclude that caspase-dependent cleavage of DNA-PKcs during apoptosis does not contribute to the radiosensitizing effects of MG-132.

The proteolytic processing of the amyloid precursor protein gene family members APLP-1 and APLP-2 involves alpha-, beta-, gamma-, and epsilon-like cleavages: modulation of APLP-1 processing by n-glycosylation

Amyloid precursor protein (APP) processing is of major interest in Alzheimer's disease research, since sequential cleavages by beta- and gamma-secretase lead to the formation of the 4-kDa amyloid Abeta protein peptide that accumulates in Alzheimer's disease brain. The processing of APP involves proteolytic conversion by different secretases leading to alpha-, beta-, gamma-, delta-, and epsilon-cleavages. Since modulation of these cleavages represents a rational therapeutic approach to control amyloid formation, its interference with the processing of the members of the APP gene family is of considerable importance. By using C-terminally tagged constructs of APLP-1 and APLP-2 and the untagged proteins, we have characterized their proteolytic C-terminal fragments produced in stably transfected SH-SY5Y cells. Pharmacological manipulation with specific protease inhibitors revealed that both homologues are processed by alpha- and gamma-secretase-like cleavages, and that their intracellular domains can be released by cleavage at epsilon-sites. APLP-2 processing appears to be the most elaborate and to involve alternative cleavage sites. We show that APLP-1 is the only member of the APP gene family for which processing can be influenced by N-glycosylation. Additionally, we were able to detect p3-like fragments of APLP-1 and p3-like and Abeta-like fragments of APLP-2 in the media of stably transfected SH-SY5Y cells.

The diverse roles of ubiquitin and the 26S proteasome in the life of plants

A tightly regulated and highly specific system for the degradation of individual proteins is essential for the survival of all organisms. In eukaryotes, this is achieved by the tagging of proteins with ubiquitin and their subsequent recognition and degradation by the 26S proteasome. In plants, genetic analysis has identified many genes that regulate developmental pathways. Subsequent analysis of these genes has implicated ubiquitin and the 26S proteasome in the control of diverse developmental processes, and indicates that proteolysis is a crucial regulatory step throughout the life cycle of plants.

Assessment of proteasome activity in cell lysates and tissue homogenates using peptide substrates

The ubiquitin-proteasome pathway is a major route of degradation of cell proteins. It also plays an essential role in maintaining cell homeostasis by degrading many rate-limiting enzymes and critical regulatory proteins. Alterations in proteasome activity have been implicated in a number of pathologies including Parkinson's disease, Alzheimer's disease and diabetes. The eukaryotic proteasome is a multicatalytic protease characterized by three activities with distinct specificities against peptide substrates. Although substrates were identified which could selectively measure the individual activities in the purified proteasome little data is available on how specific those substrates are for proteasomal activity when used with biological samples which may contain many other active peptidases. Here we examine the three major peptidase activities in lysates of two cell types and in a liver cytosol fraction in the presence of specific proteasome inhibitors and after fractionation by gel permeation chromatography. We demonstrate that other proteinases present in these preparations can degrade the commonly used proteasome substrates under the standard assay conditions. We develop a simple method for separating the proteasome from the lower molecular weight proteases using a 500kDa molecular weight cut-off membrane. This allows proteasome activity to be accurately measured in crude biological samples and may have quite broad applicability. We also identify low molecular weight tryptic activity in both the cell and tissue preparations which could not be inhibited by the proteasome inhibitor epoxomycin but was inhibitable by two cysteine proteinase inhibitors and by lactacystin suggesting that lactacystin may not be completely proteasome specific.

N-acetyl-L-cysteine inhibits 26S proteasome function: implications for effects on NF-kappaB activation

Ionizing radiation shares with cytokines, such as TNF-alpha, an ability to generate free radicals in cells and activate downstream proinflammatory responses through NF-kappaB-dependent signal transduction pathways. Support for the role of free radicals in triggering such responses comes from the use of free radical scavengers like N-acetyl-L-cysteine (NAC). The nature of the link between free radical generation and NF-kappaB activation is, however, unclear. In this study, we explore the possibility that scavenging of free radicals by NAC might not be the mechanism by which it inhibits NF-kappaB activation, but rather that NAC acts through inhibition of proteasome function. The effect of NAC on the chymotryptic function of the 26s and 20s proteasome complex was measured in extracts from EVC 304 bladder carcinoma cells by assessing degradation of fluorogenic substrates. NAC inhibited 26s but not 20s proteasome activity, suggesting that it interferes with 19s regulatory subunit function. NAC blocked radiation-induced NF-kappaB activity in ECV 304 cells and RAW 264.7 macrophages, as measured by a gel shift assay, at doses that inhibited proteasome activity. This provides a possible mechanism whereby NAC could block NF-kappaB activation and affect the expression of other molecules that are dependent on the ubiquitin/proteasome system for their degradation, other than by scavenging free radicals.

The proteasome in cancer biology and treatment

During the last 30 years, investigation of the transcriptional and translational mechanisms of gene regulation has been a major focus of molecular cancer biology. More recently, it has become evident that cancer-related mutations and cancer-related therapies also can affect post-translational processing of cellular proteins and that control exerted at this level can be critical in defining both the cancer phenotype and the response to therapeutic intervention. One post-translational mechanism that is receiving considerable attention is degradation of intracellular proteins through the multicatalytic 26S proteasome. This follows growing recognition of the fact that protein degradation is a well-regulated and selective process that can differentially control intracellular protein expression levels. The proteasome is responsible for the degradation of all short-lived proteins and 70-90% of all long-lived proteins, thereby regulating signal transduction through pathways involving factors such as AP1 and NFKB, and processes such as cell cycle progression and arrest, DNA transcription, DNA repair/misrepair, angiogenesis, apoptosis/survival, growth and development, and inflammation and immunity, as well as muscle wasting (e.g. in cachexia and sepsis). In this review, we discuss the potential involvement of the proteasome in both cancer biology and cancer treatment.

The expression of pregnane X receptor and its target gene, cytochrome P450 3A1, in perinatal mouse

Recently, pregnane X receptor (PXR) has been described to mediate the genomic effects of several steroid hormones, such as progesterone (P), glucocorticoid (Dex), pregnenolone (Preg), and xenobiotics through the cytochrome P-450 3A gene family (CYP3A), which are monooxygenases, responsible for the oxidative metabolism of some endogenous substrates and xenobiotics. In the present study, we used a transient transfection reporter gene expression assay of COS-7 cells to demonstrate that P, Dex and Preg significantly stimulate PXR-mediated transcription at relatively high concentration comparable with that of progesterone near term pregnancy. In yeast two-hybrid protein interaction assay, PXR interacted with nuclear receptor coactivator proteins, SRC1, RIP140, and SUG1 in a ligand-dependent manner. The expression of PXR mRNA was observed in the liver, intestine, uterus, ovary and placenta. The expressions of PXR mRNA in the liver and ovary increased towards term about fifty-fold compared with that of non-pregnancy and decreased postpartum. Its expression in the placenta was not drastically changed towards term. CYP3A, a target gene of PXR, was also expressed in the liver, ovary, and placenta. The expressions of CYP3A mRNA as well as PXR in the liver and ovary increased about 20-fold during prenatal period. These data suggest that PXR may play certain roles in perinatal period, possibly in the protection of the feto-maternal system from the toxic effect of endogenous steroids and foreign substrates.

Muscle cachexia: current concepts of intracellular mechanisms and molecular regulation

OBJECTIVE

To review present knowledge of intracellular mechanisms and molecular regulation of muscle cachexia.

SUMMARY BACKGROUND DATA

Muscle cachexia, mainly reflecting degradation of myofibrillar proteins, is an important clinical feature in patients with severe injury, sepsis, and cancer. The catabolic response in skeletal muscle may result in muscle wasting and weakness, delaying or preventing ambulation and rehabilitation in these patients and increasing the risk for pulmonary complications.

RESULTS

Muscle cachexia, induced by severe injury, sepsis, and cancer, is associated with increased gene expression and activity of the calcium/calpain- and ubiquitin/proteasome-proteolytic pathways. Calcium/calpain-regulated release of myofilaments from the sarcomere is an early, and perhaps rate-limiting, component of the catabolic response in muscle. Released myofilaments are ubiquitinated in the N-end rule pathway, regulated by the ubiquitin-conjugating enzyme E2(14k) and the ubiquitin ligase E3 alpha, and degraded by the 26S proteasome.

CONCLUSIONS

An understanding of the mechanisms regulating muscle protein breakdown is important for the development of therapeutic strategies aimed at treating or preventing muscle cachexia in patients with severe injury, sepsis, cancer, and perhaps other catabolic conditions as well.

Tissue and cell distribution of a mammalian proteasomal ATPase, MSS1, and its complex formation with the basal transcription factors

The proteasome is an eukaryotic multi-subunit protease complex composed of one 20S core component and two 19S regulatory complexes. The regulatory complex contains 6 putative ATPases. We investigated tissue and cell distribution of one of these ATPases, MSS1 (mammalian suppressor of sgv1). MSS1 was ubiquitously present in rat tissues as was the 20S core component of proteasome. However, the ratio of MSS1 to 20S varied greatly among tissues and MSS1 was concentrated in the thymus. Glycerol gradient sedimentation analysis revealed that MSS1 is included in protein complexes whose density is lighter than that of the proteasome. MSS1 was distributed in mammalian cells ubiquitously, while proteasome was rather concentrated in the nuclei. Hence, a novel molecular status of MSS1 distinct from proteasome is implicated. Interestingly, multiple basal transcription factors for RNA polymerase II, including TBP, TFIIB, TFIIH, and TFIIF, were found to be associated with MSS1. These results suggest that MSS1, in addition to proteolysis, plays a role in DNA metabolism including transcriptional regulation.

Endocrine disrupting chemicals, phthalic acid and nonylphenol, activate Pregnane X receptor-mediated transcription

Recently, Pregnane X receptor (PXR), a new member of the nuclear receptor superfamily, was shown to mediate the effects of several steroid hormones, such as progesterone, glucocorticoid, pregnenolone, and xenobiotics on cytochrome P450 3A genes (CYP3A) through the specific DNA sequence for CYP3A, suggesting that PXR may play a role in steroid hormone metabolism. In this paper, we demonstrated that phthalic acid and nonylphenol, endocrine-disrupting chemicals (EDCs), stimulated PXR-mediated transcription at concentrations comparable to those at which they activate estrogen receptor-mediated transcription using a transient reporter gene expression assay in COS-7 cells. However, bisphenol A, another EDC, had no effect on PXR-mediated transcription, although this chemical significantly enhanced ER-mediated transcription. In the yeast two-hybrid protein interaction assay, PXR interacted with two nuclear receptor coactivator proteins, steroid hormone receptor coactivator-1 and receptor interacting protein 140, in the presence of phthalic acid or nonylphenol. Thus, EDC-occupied PXR may regulate its specific gene expression through the receptor-coactivator interaction. In contrast, these EDCs had no effect on the interaction between PXR and suppressor for gal 1, a component of proteasome. Finally, the expression of CYP3A1 mRNA in the liver of rats exposed to phthalic acid or nonylphenol markedly increased compared with that in rats treated with estradiol, bisphenol A, or ethanol as assessed by competitive RT-PCR. These data suggest that EDCs may affect endocrine functions by altering steroid hormone metabolism through PXR.

Multiple mammalian proteasomal ATPases, but not proteasome itself, are associated with TATA-binding protein and a novel transcriptional activator, TIP120

BACKGROUND

SUG1 belongs to proteasomal ATPase. Previous studies have demonstrated that SUG1 is associated with TBP. It is assumed to be involved in transcriptional regulation in addition to proteolysis. In this study, we investigated the association of mammalian SUG1 with TBP in more detail.

RESULTS

Pull-down experiments with TBP revealed multiple TBP-interacting proteins (TIPs) that were recovered dependent upon the presence of C-terminal conserved domain of TBP. By 2-D electrophoresis, we identified SUG1 in TIPs. By using far-Western analysis, we identified two proteins that could directly bind to TBP: SUG1 and another proteasomal ATPase (S4). Protein microsequencing and Western blotting identified all the remaining proteasomal ATPases (MSS1, TBP1, TBP7, and SUG2) in the TIP preparations. We present evidence that TBP and at least SUG1, MSS1, and S4 form a complex in the cell. However, no evidence of association of TBP with the 26S proteasome or its 19S regulatory unit was obtained. The molecular mass of the TBP/ATPases-complex, which also included a novel transcription regulatory factor, TIP120, was estimated to be approximately 800 kDa.

CONCLUSION

These results suggest that there is a novel multisubunit complex containing TBP and proteasomal ATPases. Based on our findings, we hypothesize that proteasomal ATPases are involved in transcriptional regulation in addition to proteolysis.

Intracellular proteinases of invertebrates: calcium-dependent and proteasome/ubiquitin-dependent systems

Cytosolic proteinases carry out a variety of regulatory functions by controlling protein levels and/or activities within cells. Calcium-dependent and ubiquitin/proteasome-dependent pathways are common to all eukaryotes. The former pathway consists of a diverse group of Ca(2+)-dependent cysteine proteinases (CDPs; calpains in vertebrate tissues). The latter pathway is highly conserved and consists of ubiquitin, ubiquitin-conjugating enzymes, deubiquitinases, and the proteasome. This review summarizes the biochemical properties and genetics of invertebrate CDPs and proteasomes and their roles in programmed cell death, stress responses (heat shock and anoxia), skeletal muscle atrophy, gametogenesis and fertilization, development and pattern formation, cell-cell recognition, signal transduction and learning, and photoreceptor light adaptation. These pathways carry out bulk protein degradation in the programmed death of the intersegmental and flight muscles of insects and of individuals in a colonial ascidian; molt-induced atrophy of crustacean claw muscle; and responses of brine shrimp, mussels, and insects to environmental stress. Selective proteolysis occurs in response to specific signals, such as in modulating protein kinase A activity in sea hare and fruit fly associated with learning; gametogenesis, differentiation, and development in sponge, echinoderms, nematode, ascidian, and insects; and in light adaptation of photoreceptors in the eyes of squid, insects, and crustaceans. Proteolytic activities and specificities are regulated through proteinase gene expression (CDP isozymes and proteasomal subunits), allosteric regulators, and posttranslational modifications, as well as through specific targeting of protein substrates by a diverse assemblage of ubiquitin-conjugases and deubiquitinases. Thus, the regulation of intracellular proteolysis approaches the complexity and versatility of transcriptional and translational mechanisms.

Altered properties of the branched chain amino acid-preferring activity contribute to increased cleavages after branched chain residues by the "immunoproteasome"

The multicatalytic proteinase complex (MPC, proteasome) is assembled from 14 nonidentical protein subunits. It expresses five distinct proteolytic activities, including a chymotrypsin-like activity, cleaving after hydrophobic residues, and a branched chain amino acid-preferring component (BrAAP), cleaving preferentially after branched chain residues. Exposure of cells to interferons leads to replacement of the X, Y, and Z subunits by the LMP2, LMP7, and MECL1 subunits. This "immunoproteasome" is critical to processing of certain antigens. The enzymatic basis for enhanced antigen processing has not been determined. To gain insight into this question, we examined sites and relative rates of cleavage of bonds in denatured, reduced, carboxyamidomethylated lysozyme, a 129-amino acid protein, by MPC from bovine spleen, in which the X, Y, and Z subunits are replaced by LMP2, LMP7, and MECL1. We compared cleavages to those catalyzed by MPC from bovine pituitary, which contains only the X, Y, and Z subunits. We found marked increases in the rates and number of cleavages after branched chain residues in reduced, carboxyamidomethylated lysozyme by the spleen MPC. This was largely due to accelerated cleavages of bonds after a Phi-X-Br motif, where Phi is a hydrophobic residue, X is a small neutral or polar residue, and Br is a branched chain residue. Inhibitors with these structural properties were selective and potent inhibitors of the BrAAP activity of the spleen MPC. The above findings indicate that alterations in activity and substrate specificity of the BrAAP activity are important factors underlying the altered cleavages after hydrophobic residues associated with incorporation of interferon-inducible subunits. The potential relevance of the findings to antigen processing functions of MPC is discussed.

Age-related changes in the 20S and 26S proteasome activities in the liver of male F344 rats

Accumulation of altered proteins in old animals has been ascribed to slower turnover of proteins. Since proteasomes can be regarded as the major proteolytic enzymes responsible for the degradation of the majority of cellular proteins, we examined age-related changes of 20S and 26S proteasomes in the liver of young (8-10-month-old), middle-aged (15-18-month-old) and old (25-28-month-old) Fischer 344 male rats. The two forms of proteasomes were separated by glycerol gradient centrifugation. Fluorogenic peptides were used as substrates to evaluate three types of peptidase activities. The ratio of peptidase activities in the 20S proteasome vs. those in the 26S form did not appear to change with age. Unstimulated chymotrypsin-like activity found only in the 26S form decreased by 30% in the old rats as compared with that in the young ones, while no change in the activity was observed during aging when stimulated by sodium dodecyl sulfate. The trypsin-like activity declined significantly by 17% to an apparently similar extent in both 20S and 26S forms. The peptidylglutamyl peptide hydrolyzing activity exhibited gradual decrease with age, resulting in 60% lower value in the old rats as compared with the young animals. These changes are considered to account for the age-related extension of half-life of proteins. Since the amount of total proteasomes measured by immunoblot did not appear to change with age, posttranslational modifications or subunit replacement is possibly responsible for the decrease in the activities.

Molecular cloning and testicular expression of the gene transcripts encoding the murine multiubiquitin-chain-binding protein (Mcb1)

A murine cDNA encoding a homolog of the human multiubiquitin-chain-binding protein Mcb1 was isolated and sequenced from a mouse testis cDNA library. The encoded Mcb1 protein is highly conserved between mouse and human. Northern hybridisation showed expression of Mcb1 transcripts in all examined mouse tissues. In the testis, however, there is additionally a second, longer Mcb1 transcript in wild-type mice that is absent in the azoospermic W/Wv mutant mice, suggesting expression of this transcript in association with germ cell differentiation.

Sepsis-induced increase in muscle proteolysis is blocked by specific proteasome inhibitors

Recent studies suggest that sepsis stimulates ubiquitin-dependent protein breakdown in skeletal muscle. The 20S proteasome is the catalytic core of the ubiquitin-dependent proteolytic pathway. We tested the effects in vitro of the proteasome inhibitors N-acetyl-L-leucinyl-L-leucinal-L-norleucinal (LLnL) and lactacystin on protein breakdown in incubated muscles from septic rats. LLnL resulted in a dose- and time-dependent inhibition of protein breakdown in muscles from septic rats. Lactacystin blocked both total and myofibrillar muscle protein breakdown. In addition to inhibiting protein breakdown, LLnL reduced muscle protein synthesis and increased ubiquitin mRNA levels, probably reflecting inhibited proteasome-associated ribonuclease activity. Inhibited muscle protein breakdown caused by LLnL or lactacystin supports the concept that the ubiquitin-proteasome pathway plays a central role in sepsis-induced muscle proteolysis. The results suggest that muscle catabolism during sepsis may be inhibited by targeting specific molecular mechanisms of muscle proteolysis.

Mts4, a non-ATPase subunit of the 26 S protease in fission yeast is essential for mitosis and interacts directly with the ATPase subunit Mts2

We have isolated a fission yeast gene, mts4(+), by complementation of a temperature-sensitive mutation and show that it encodes subunit 2 (S2) of the 19 S regulatory complex of the 26 S protease. mts4(+) is an essential gene, and we show that loss of this subunit causes cells to arrest in metaphase, illustrating the importance of S2 for mitosis. The Mts4 protein is 48% identical to S2 of the human 26 S protease, and the lethal phenotype of the null mts4 allele can be rescued by the human cDNA encoding S2. We provide genetic and physical evidence to suggest that the Mts4 protein interacts with the product of the mts2(+) gene, an ATPase which has previously been shown to be subunit 4 of the 26 S protease.

HEC binds to the seventh regulatory subunit of the 26 S proteasome and modulates the proteolysis of mitotic cyclins

A newly identified nuclear protein rich in leucine heptad repeats called HEC is important for mitosis. To elucidate its mechanism of action, the region containing leucine heptad repeats was used to identify cellular proteins that potentially interact with HEC. Complementary DNAs encoding several proteins including MSS1, p45, Nek2, and Smc1/Smc2, known to be important for G2/M progression, were identified. The interaction between HEC and MSS1, the seventh regulatory subunit of the 26 S proteasome, was further demonstrated by in vitro GST pull-down assays. HEC is not a part of the 26 S proteasome and interacts with MSS1 only when it is dissociated from the complex during M phase. Purified MSS1 specifically hydrolyzes ATP, an activity inhibited by HEC. In addition, HEC inhibits the proteolysis of mitotic cyclin B in vitro. Consistent with this biochemical activity, ectopic expression of HEC inhibits the degradation of mitotic cyclins after telophase, resulting eventually in cell death. These results show that HEC is a negative regulator of MSS1 and suggest that it may modulate M phase progression, in part, through the regulation of proteasome-mediated degradation of cell cycle regulatory proteins.

A protein associated with the manchette during rat spermiogenesis is encoded by a gene of the TBP-1-like subfamily with highly conserved ATPase and protease domains

We have used a rat pachytene spermatocyte cDNA expression library to clone TBP-1 (for tat-binding protein-1; designated rat testis TBP-1 [rtTBP-1]), a new member of the family of putative ATPases associated with the 26S proteasome complex. The 1.63 kb rtTBP-1 cDNA encodes a 49 kDa protein with 99% amino acid identity to human TBP-1 protein. rtTBP-1 protein contains a heptad repeat of six leucine-type zipper fingers at the amino terminal end and highly conserved ATPase and DNA/RNA helicase motifs towards the carboxyl terminal region. Chromatofocusing fractionation of rat testis sucrose extracts demonstrates that the encoded product, recognized by an antiserum raised to the first 196 amino acids of human TBP-1, consists of a protein triplet with a molecular mass range of 52-48 kDa and acidic pI (5.0-5.9). An identical immunoreactive triplet was detected by immunoblotting in extracts of fractionated pachytene spermatocytes, round spermatids and epididymal sperm. In situ hybridization using digoxigenin-labeled antisense RNA probes shows a predominant distribution of specific mRNA in the seminiferous epithelial region occupied by elongating spermatids and primary spermatocytes. Indirect immunofluorescence and immunogold electron microscopy studies show that rtTBP-1 immunoreactive sites colocalize with alpha-tubulin-decorated manchettes of elongating spermatids. In addition, rtTBP-1 immunoreactivity was detected in fibrillar and granular cytoplasmic bodies typically observed in spermatocytes and spermatids as well as in association with paraaxonemal mitochondria and outer dense fibers of the developing spermatid tail. Results of this study indicate that rtTBP-1 is a member of the highly evolutionary conserved TBP-1-like subfamily of putative ATPases, sharing regions of identity-including ATP-binding sites-with several subunits of the 26S proteasome, known to be involved in the ATP-dependent degradation of ubiquitin-conjugated proteins.

Expression and subcellular localization of mouse 20S proteasome activator complex PA28

We have cloned the mouse PA28 proteasome activator cDNAs. Northern blot demonstrates high PA28 mRNA levels in liver, kidney and lung. mRNA levels are low in thymus, spleen and brain. In contrast, PA28 protein levels vary little between these tissues. Immunocytological analysis and cell fractionation experiments demonstrate that both subunits are almost equally distributed between the cytoplasm and the nucleus. Interestingly, PA28alpha spares nucleoli, while PA28beta is strongly enhanced in the nucleolus. This indicates for the first time that the PA28alpha and PA28beta subunits may serve nuclear functions which may be different from and independent of each other.

The heat-shock protein HslVU from Escherichia coli is a protein-activated ATPase as well as an ATP-dependent proteinase

HslVU in Escherichia coli a new two-component ATP-dependent protease composed of two heat-shock proteins, the HslU ATPase and the HslV peptidase which is related to proteasome beta-type subunits. Here we show that the reconstituted HslVU enzyme degrades not only certain hydrophobic peptides but also various polypeptides, including insulin B-chain, casein, and carboxymethylated lactalbumin. Maximal proteolytic activity was obtained with a 1:2 molar ratio of HslV (a 250-kDa complex) to HslU (a 450-kDa complex). By itself, HslV could slowly hydrolyze these polypeptides, but its activity was stimulated 20-fold by HslU in the presence of ATP. The ATPase activity of HslU was stimulated up to 50% by the protein substrates, but not by nonhydrolyzed proteins, and this stimulation further increased 2-3-fold in the presence of HslV. Concentrations of insulin B-chain that maximally stimulated the ATPase allowed maximal rates of the B-chain hydrolysis. Furthermore, addition of increasing amounts of ADP or N-ethylmaleimide reduced ATP and protein or peptide hydrolysis in parallel. Thus, HslVU is a protein-activated ATPase as well as an ATP-dependent proteinase, and these processes appear linked. Surprisingly, the protein and peptide substrates do not compete with each other for hydrolysis. Lactacystin strongly inhibits protein degradation, but has little effect on peptide hydrolysis, while the peptide aldehydes are potent inhibitors of hydrolysis of small peptides, but have little effect on proteins. Thus, the functional requirements for ATP-dependent hydrolysis of peptides and proteins appear different.

Specific increase in amyloid beta-protein 42 secretion ratio by calpain inhibition

Cerebral deposition of amyloid beta-protein (Abeta) as senile plaques is a pathological hallmark of Alzheimer's disease (AD). Abeta falls into two major subspecies defined by their C-termini, Abeta40 and Abeta42, ending in Val-40 and Ala-42, respectively. Although Abeta42 accounts for only approximately 10% of secreted Abeta, Abeta42 is the predominant species accumulated in senile plaques in AD brain and appears to be the initially deposited species. Its secretion level has recently been reported to be increased in the plasma or culture media of fibroblasts from patients affected by any of early-onset familial AD (FAD). Thus, inhibition of Abeta42 production would be one of the therapeutic targets for AD. However, there is little information about the cleavage mechanism via which Abeta40 and Abeta42 are generated and its relationship to intracellular protease activity. Here, we examined by well-characterized enzyme immunoassay the effects of calpain and proteasome inhibitors on the levels of Abeta40 and Abeta42 secretion by cultured cells. A calpastatin peptide homologous to the inhibitory domain of calpastatin, an endogenous calpain specific inhibitor, induced a specific increase in secreted Abeta42 relative to the total secreted Abeta level, a characteristic of the cultured cells transfected with FAD-linked mutated genes, while a proteasome specific inhibitor, lactacystin, showed no such effect. These findings suggest that the Abeta42 secretion ratio is modulated by the calpain-calpastatin system and may point to the possibility of exploring particular compounds that inhibit Abeta42 secretion through this pathway.

Delivery of antigens to the MHC class I pathway using bacterial toxins

Cytotoxic T lymphocytes (CTL) recognize antigens derived from endogenously expressed proteins presented on the cell surface in the context of major histocompatibility complex (MHC) class I molecules. Because CTL are effective in antiviral and antitumor responses, the delivery of antigens to the class I pathway has been the focus of numerous efforts. Generating CTL by immunization with exogenous proteins is often ineffective because these antigens typically enter the MHC class II pathway. This review focuses on the usefulness of bacterial toxins for delivering antigens to the MHC class I pathway. Several toxins naturally translocate into the cytosol, where they mediate their cytopathic effects, and the mechanisms by which this occurs has been elucidated. Molecular characterization of these toxins identified the functional domains and enabled the generation of modified proteins that were no longer toxic but retained the ability to translocate into the cytosol. Thus, these modified toxins could be examined for their ability to carry peptides or whole proteins into the cytosolic processing pathway. Of the toxins studied-diphtheria, pertussis, Pseudomonas, and anthrax-the anthrax toxin appears the most promising in its ability to deliver large protein antigens and its efficiency of translocation.

The proteasome

The proteasome is a macromolecular assembly that is designed to confine proteolytic activity to an inner cavity. Access to the central proteolytic nanocompartment is restricted to unfolded proteins, which necessitates a functional coupling of the 20S proteasome to a substrate-recognition and unfolding machinery. Significant progress has been made during the past two years in elucidating the structural principles and the enzymatic mechanism of the 20S proteasome.

Subunit arrangement in the human 20S proteasome

In human 20S proteasomes two copies of each of seven different alpha-type and seven different beta-type subunits are assembled to form a stack of four seven-membered rings, giving the general structure alpha(1-7), beta(1-7), beta(1-7), alpha(1-7). By means of immunoelectron microscopy and chemical crosslinking of neighboring subunits, we have determined the positions of the individual subunits in the proteasome. The topography shows that for the trypsin-like, the chymotrypsin-like, and the postglutamyl cleaving activities, the pairs of beta type subunits, which are thought to form active sites, are nearest neighbors.

The 26S proteasome: subunits and functions

The 26S proteasome is an eukaryotic ATP-dependent, dumbbell-shaped protease complex with a molecular mass of approximately 2000 kDa. It consists of a central 20S proteasome, functioning as a catalytic machine, and two large V-shaped terminal modules, having possible regulatory roles, composed of multiple subunits of 25-110 kDa attached to the central portion in opposite orientations. The primary structures of all the subunits of mammalian and yeast 20S proteasomes have been determined by recombinant DNA techniques, but structural analyses of the regulatory subunits of the 26S proteasome are still in progress. The regulatory subunits are classified into two subgroups, a subgroup of at least 6 ATPases that constitute a unique multi-gene family encoding homologous polypeptides conserved during evolution and a subgroup of approximately 15 non-ATPase subunits, most of which are structurally unrelated to each other.

Eubacterial proteasomes

Proteasomes are large, multisubunit proteases with highly conserved structures. The 26S proteasome of eukaryotes is an ATP-dependent enzyme of about 2 MDa, which acts as the central protease of the ubiquitin-dependent pathway of protein degradation. The core of the 26S complex is formed by the 20S proteasome, an ATP-independent, barrel-shaped protease of about 700 kDa, which has also been detected in archaebacteria and, more recently, in eubacteria. Currently, the distribution of 20S proteasomes in eubacteria appears limited to the actinomycetes, while most other eubacteria contain a related complex of simpler structure.

The ubiquitin-proteasome pathway: review of a novel intracellular mechanism of muscle protein breakdown during sepsis and other catabolic conditions

SUMMARY BACKGROUND DATA

Patients with sepsis and other catabolic conditions, such as severe trauma, cancer, and fasting, suffer significant loss of body protein, the majority of which originates from skeletal muscle. Recent evidence suggests that muscle protein breakdown during sepsis is caused by upregulated activity in the ubiquitin-proteasome pathway and is associated with increased expression of the ubiquitin gene.

PURPOSE

The purpose of the study was to review the role of the ubiquitin-proteasome pathway in the regulation of muscle proteolysis during sepsis and other catabolic conditions.

REVIEW

Proteins that are degraded by the ubiquitin-proteasome mechanism are first conjugated to ubiquitin, a 76-amino-acid, highly conserved residue. Ubiquitinated proteins are recognized by the 26S proteasome, which is a large proteolytic complex consisting of the 19S cap complex and the 20S proteasome. The 20S proteasome is a cylindrical particle composed of four stacked rings, making it look like a barrel. The rings form a "tunnel" in which the target proteins are hydrolyzed, after which ubiquitin is released to be reused in the proteolytic pathway. A unique feature of the ubiquitin-proteasome proteolytic pathway is its energy dependency.

CONCLUSIONS

An understanding of the molecular regulation of protein metabolism in patients with sepsis and other catabolic conditions is important because it may form the basis for improved treatment in the future.

Prosomes (proteasomes) changes during differentiation are related to the type of inducer

The core of the 26S proteasome, the 20S prosome, is a highly organized multi-protein complex found in large amount in malignant cells. Differentiation of several cell lines, including the monoblastic U937 and the lymphoblastoid CCRF-CEM, is accompanied by a general decrease in the prosome concentration when phorbol-myrirtic-acetate (PMA) and retinoic acid plus dihydroxyvitamine D3 (RA+VD) are used. Incubation of U937 cells for three days with PMA or RA+VD causes differentiation, but the resulting patterns of prosome labeling in the cell and on the plasma membrane are not the same. In contrast, the same kind of prosome changes occur in U937 and CCRF-CEM cells when PMA is used as inducer. The intracellular distribution of prosomes is also linked to malignancy and differentiation. Prosomes are found in the nucleus and the cytoplasm of cancer cells; and treatment with RA+VD decreases the prosomes in the nucleus whereas PMA causes various prosome proteins changes. These results indicate that prosomes are important in cell regulation and in the expression of malignancy.

Proteasome and myogenesis

In this report, we examine the involvement of the ubiquitin-proteasome pathway during fusion and differentiation of myoblast primary cell cultures. Up-regulation of proteasome was observed at the maximum fusion rate and was preceded by an increase of unidentified ubiquitin-conjugates. Cell permeable proteasome inhibitors prevent fusion as do antisense oligodesoxyribonucleotides targetted to three proteasome subunits. Identical results were obtained using E3 ubiquitin-ligases dipeptide inhibitor. Involvement of the ubiquitin-proteasome pathway in the regulation of myogenic factors was hypothesized.