gamma-Interferon decreases the level of 26 S proteasomes and changes the pattern of phosphorylation

The Functional and Mechanistic Roles of Immunoproteasome Subunits in Cancer

Cell-mediated immunity is driven by antigenic peptide presentation on major histocompatibility complex (MHC) molecules. Specialized proteasome complexes called immunoproteasomes process viral, bacterial, and tumor antigens for presentation on MHC class I molecules, which can induce CD8 T cells to mount effective immune responses. Immunoproteasomes are distinguished by three subunits that alter the catalytic activity of the proteasome and are inducible by inflammatory stimuli such as interferon-γ (IFN-γ). This inducible activity places them in central roles in cancer, autoimmunity, and inflammation. While accelerated proteasomal degradation is an important tumorigenic mechanism deployed by several cancers, there is some ambiguity regarding the role of immunoproteasome induction in neoplastic transformation. Understanding the mechanistic and functional relevance of the immunoproteasome provides essential insights into developing targeted therapies, including overcoming resistance to standard proteasome inhibition and immunomodulation of the tumor microenvironment. In this review, we discuss the roles of the immunoproteasome in different cancers.

Proteasome regulation by reversible tyrosine phosphorylation at the membrane

Reversible phosphorylation has emerged as an important mechanism for regulating 26S proteasome function in health and disease. Over 100 phospho-tyrosine sites of the human proteasome have been detected, and yet their function and regulation remain poorly understood. Here we show that the 19S subunit Rpt2 is phosphorylated at Tyr439, a strictly conserved residue within the C-terminal HbYX motif of Rpt2 that is essential for 26S proteasome assembly. Unexpectedly, we found that Y439 phosphorylation depends on Rpt2 membrane localization mediated by its N-myristoylation. Multiple receptors tyrosine kinases can trigger Rpt2-Y439 phosphorylation by activating Src, a N-myristoylated tyrosine kinase. Src directly phosphorylates Rpt2-Y439 in vitro and negatively regulates 26S proteasome activity at cellular membranes, which can be reversed by the membrane-associated isoform of protein tyrosine phosphatase nonreceptor type 2 (PTPN2). In H1975 lung cancer cells with activated Src, blocking Rpt2-Y439 phosphorylation by the Y439F mutation conferred partial resistance to the Src inhibitor saracatinib both in vitro and in a mouse xenograft tumor model, and caused significant changes of cellular responses to saracatinib at the proteome level. Our study has defined a novel mechanism involved in the spatial regulation of proteasome function and provided new insights into tyrosine kinase inhibitor-based anticancer therapies.

PA28αβ overexpression enhances learning and memory of female mice without inducing 20S proteasome activity

Background

The proteasome system plays an important role in synaptic plasticity. Induction and maintenance of long term potentiation is directly dependent on selective targeting of proteins for proteasomal degradation. The 20S proteasome activator PA28αβ activates hydrolysis of small nonubiquitinated peptides and possesses protective functions upon oxidative stress and proteinopathy. The effect of PA28αβ activity on behavior and memory function is, however, not known. We generated a mouse model that overexpresses PA28α (PA28αOE) to understand PA28αβ function during healthy adult homeostasis via assessment of physiological and behavioral profiles, focusing on female mice.

Results

PA28α and PA28β protein levels were markedly increased in all PA28αOE tissues analyzed. PA28αOE displayed reduced depressive-like behavior in the forced swim test and improved memory/learning function assessed by intersession habituation in activity box and shuttle box passive avoidance test, with no significant differences in anxiety or general locomotor activity. Nor were there any differences found when compared to WT for body composition or immuno-profile. The cognitive effects of PA28αOE were female specific, but could not be explained by alterations in estrogen serum levels or hippocampal regulation of estrogen receptor β. Further, there were no differences in hippocampal protein expression of neuronal or synaptic markers between PA28αOE and WT. Biochemical analysis of hippocampal extracts demonstrated that PA28α overexpression did not increase PA28–20S peptidase activity or decrease K48-polyubiquitin levels. Instead, PA28αOE exhibited elevated efficiency in preventing aggregation in the hippocampus.

Conclusions

This study reveals, for the first time, a connection between PA28αβ and neuronal function. We found that PA28α overexpressing female mice displayed reduced depressive-like behavior and enhanced learning and memory. Since the positive effects of PA28α overexpression arose without an activation of 20S proteasome capacity, they are likely independent of PA28αβ’s role as a 20S proteasome activator and instead depend on a recognized chaperone-like function. These findings suggest that proteostasis in synaptic plasticity is more diverse than previously reported, and demonstrates a novel function of PA28αβ in the brain.

Electronic supplementary material

The online version of this article (10.1186/s12868-018-0468-2) contains supplementary material, which is available to authorized users.

Islet stress, degradation and autoimmunity

β-cell destruction in type 1 diabetes (T1D) results from the effect of inflammation and autoimmunity. In response to inflammatory signals, islet cells engage adaptive mechanisms to restore and maintain cellular homeostasis. Among these mechanisms, the unfolded protein response (UPR) leads to a reduction of the general protein translation rate, increased production of endoplasmic reticulum chaperones and the initiation of degradation by activation of the ER associated degradation pathway (ERAD) in which newly synthetized proteins are ubiquitinylated and processed through the proteasome. This adaptive phase is also believed to play a critical role in the development of autoimmunity by the generation of neoantigens. While we have previously investigated the effect of stress on transcription, translation and post-translational events as possible source for neoantigens, the participation of the degradation machinery, yet crucial in the generation of antigenic peptides, remains to be investigated in the context of T1D pathology. In this review, we will describe the relation between the unfolded protein response and the Ubiquitin Proteasome System (UPS) and address the role of the cellular degradation machinery in the generation of antigens. Learning from tumour immunology, we propose how these processes may unmask β-cells by triggering the generation of aberrant peptides recognized by the immune cells.

Reversible phosphorylation of the 26S proteasome

The 26S proteasome at the center of the ubiquitin-proteasome system (UPS) is essential for virtually all cellular processes of eukaryotes. A common misconception about the proteasome is that, once made, it remains as a static and uniform complex with spontaneous and constitutive activity for protein degradation. Recent discoveries have provided compelling evidence to support the exact opposite insomuch as the 26S proteasome undergoes dynamic and reversible phosphorylation under a variety of physiopathological conditions. In this review, we summarize the history and current understanding of proteasome phosphorylation, and advocate the idea of targeting proteasome kinases/phosphatases as a new strategy for clinical interventions of several human diseases.

The Immunoproteasome in oxidative stress, aging, and disease

The Immunoproteasome has traditionally been viewed primarily for its role in peptide production for antigen presentation by the major histocompatibility complex, which is critical for immunity. However, recent research has shown that the Immunoproteasome is also very important for the clearance of oxidatively damaged proteins in homeostasis, and especially during stress and disease. The importance of the Immunoproteasome in protein degradation has become more evident as diseases characterized by protein aggregates have also been linked to deficiencies of the Immunoproteasome. Additionally, there are now diseases defined by mutations or polymorphisms within Immunoproteasome-specific subunit genes, further suggesting its crucial role in cytokine signaling and protein homeostasis (or "proteostasis"). The purpose of this review is to highlight our growing understanding of the importance of the Immunoproteasome in the management of protein quality control, and the detrimental impact of its dysregulation during disease and aging.

Multiple-embryo transfer for studying very early maternal-embryo interactions in cattle

In the present paper, we highlight the need to study very early maternal-embryo interactions and discuss how these interactions can be addressed. Bovine species normally carry one or, less frequently, two embryos to term; there are very rare cases of triplets or higher-order multiple pregnancies in which all the offspring are born alive. Multiple-embryo transfer (MET) in cattle allows for the detection of endometrial responses in scenarios where single-embryo transfer would not. Although MET is non-physiological, the present study shows that at the very early embryonic stages, a uterus carrying zona-enclosed embryos does not exhibit non-physiological reactions. On the contrary, MET should be considered the sum of multiple individual effects triggered by developing embryos. We provide arguments to support our hypothesis that describe a rationale for current work with MET, and we discuss alternative hypotheses. Using cattle as a model, we describe how technical approaches to analyzing zona-enclosed early embryo-maternal interactions (i.e., transcriptomics, proteomics, and endometrial cell culture) can help identify molecular changes that may be difficult to observe when only a single embryo is present. We conclude that MET can be used for studying very early maternal-embryo interactions in vivo in monotocous species. Free Spanish abstract: A Spanish translation of this abstract is freely available at http://www.reproduction-online.org/content/150/2/R35/suppl/DC1.

PA28αβ: the enigmatic magic ring of the proteasome?

PA28αβ is a γ-interferon-induced 11S complex that associates with the ends of the 20S proteasome and stimulates in vitro breakdown of small peptide substrates, but not proteins or ubiquitin-conjugated proteins. In cells, PA28 also exists in larger complexes along with the 19S particle, which allows ATP-dependent degradation of proteins; although in vivo a large fraction of PA28 is present as PA28αβ-20S particles whose exact biological functions are largely unknown. Although several lines of evidence strongly indicate that PA28αβ plays a role in MHC class I antigen presentation, the exact molecular mechanisms of this activity are still poorly understood. Herein, we review current knowledge about the biochemical and biological properties of PA28αβ and discuss recent findings concerning its role in modifying the spectrum of proteasome's peptide products, which are important to better understand the molecular mechanisms and biological consequences of PA28αβ activity.

Genomic structural characterization and transcriptional expression analysis of proteasome activator PA28α and PA28β subunits from Oplegnathus fasciatus

Proteasomes are multicatalytic subunit complexes involved in the degradation of cytosolic proteins and antigen presentation. In this study, we have characterized the alpha and beta subunits of proteasome activator complex from rock bream at the molecular level. RbPA28α and RbPA28β possessed the characteristic features of the subunits identified from mammals and teleosts. The RbPA28α and RbPA28β proteasome subunits contained a proline-rich motif (Region A), subunit-specific insert in the region corresponding to the KEKE motif of the known PA28α (Region B), conserved activation loop (Region C), a potential protein kinase C recognition site (Region D) and a highly homologous C-terminal region (Region E) among all three PA28 subunits. Multiple sequence alignment and pairwise alignment revealed that RbPA28α and RbPA28β proteins shared high homology with the teleosts and mammals. RbPA28α and RbPA28β genome possessed 11 exons interrupted by 10 introns. In silico promoter analysis of RbPA28α and RbPA28β revealed various transcription factor-binding sites displaying their regulation under various stress conditions. Tissue distribution profiling showed a higher expression in blood and gills. Transcriptional expression analysis of RbPA28α and RbPA28β showed up-regulation in the immune tissues following LPS and poly I:C challenges, providing further evidence for the immunological role of RbPA28α and RbPA28β.

Immunoproteasomes: structure, function, and antigen presentation

Immunoproteasomes contain replacements for the three catalytic subunits of standard proteasomes. In most cells, oxidative stress and proinflammatory cytokines are stimuli that lead to elevated production of immunoproteasomes. Immune system cells, especially antigen-presenting cells, express a higher basal level of immunoproteasomes. A well-described function of immunoproteasomes is to generate peptides with a hydrophobic C terminus that can be processed to fit in the groove of MHC class I molecules. This display of peptides on the cell surface allows surveillance by CD8 T cells of the adaptive immune system for pathogen-infected cells. Functions of immunoproteasomes, other than generating peptides for antigen presentation, are emerging from studies in immunoproteasome-deficient mice, and are complemented by recently described diseases linked to mutations or single-nucleotide polymorphisms in immunoproteasome subunits. Thus, this growing body of literature suggests a more pleiotropic role in cell function for the immunoproteasome.

Lipopolysaccharide-induced neuroinflammation leads to the accumulation of ubiquitinated proteins and increases susceptibility to neurodegeneration induced by proteasome inhibition in rat hippocampus

Background

Neuroinflammation and protein accumulation are characteristic hallmarks of both normal aging and age-related neurodegenerative diseases. However, the relationship between these factors in neurodegenerative processes is poorly understood. We have previously shown that proteasome inhibition produced higher neurodegeneration in aged than in young rats, suggesting that other additional age-related events could be involved in neurodegeneration. We evaluated the role of lipopolysaccharide (LPS)-induced neuroinflammation as a potential synergic risk factor for hippocampal neurodegeneration induced by proteasome inhibition.

Methods

Young male Wistar rats were injected with 1 μL of saline or LPS (5 mg/mL) into the hippocampus to evaluate the effect of LPS-induced neuroinflammation on protein homeostasis. The synergic effect of LPS and proteasome inhibition was analyzed in young rats that first received 1 μL of LPS and 24 h later 1 μL (5 mg/mL) of the proteasome inhibitor lactacystin. Animals were sacrificed at different times post-injection and hippocampi isolated and processed for gene expression analysis by real-time polymerase chain reaction; protein expression analysis by western blots; proteasome activity by fluorescence spectroscopy; immunofluorescence analysis by confocal microscopy; and degeneration assay by Fluoro-Jade B staining.

Results

LPS injection produced the accumulation of ubiquitinated proteins in hippocampal neurons, increased expression of the E2 ubiquitin-conjugating enzyme UB2L6, decreased proteasome activity and increased immunoproteasome content. However, LPS injection was not sufficient to produce neurodegeneration. The combination of neuroinflammation and proteasome inhibition leads to higher neuronal accumulation of ubiquitinated proteins, predominant expression of pro-apoptotic markers and increased neurodegeneration, when compared with LPS or lactacystin (LT) injection alone.

Conclusions

Our results identify neuroinflammation as a risk factor that increases susceptibility to neurodegeneration induced by proteasome inhibition. These results highlight the modulation of neuroinflammation as a mechanism for neuronal protection that could be relevant in situations where both factors are present, such as aging and neurodegenerative diseases.

Differential roles of proteasome and immunoproteasome regulators Pa28αβ, Pa28γ and Pa200 in the degradation of oxidized proteins

The response and functions of proteasome regulators Pa28αβ (or 11S), Pa28γ and Pa200 in oxidative-stress adaptation (also called hormesis) was studied in murine embryonic fibroblasts (MEFs), using a well-characterized model of cellular adaptation to low concentrations (1.0-10.0 μM) of hydrogen peroxide (H(2)O(2)), which alter gene expression profiles, increasing resistance to higher levels of oxidative-stress. Pa28αβ bound to 20S proteasomes immediately upon H(2)O(2)-treatment, whereas 26S proteasomes were disassembled at the same time. Over the next 24h, the levels of Pa28αβ, Pa28γ and Pa200 proteasome regulators increased during H(2)O(2)-adaptation, whereas the 19S regulator was unchanged. Purified Pa28αβ, and to a lesser extent Pa28γ, significantly increased the ability of purified 20S proteasome to selectively degrade oxidized proteins; Pa28αβ also increased the capacity of purified immunoproteasome to selectively degrade oxidized proteins but Pa28γ did not. Pa200 regulator actually decreased 20S proteasome and immunoproteasome's ability to degrade oxidized proteins but Pa200 and poly-ADP ribose polymerase may cooperate in enabling initiation of DNA repair. Our results indicate that cytoplasmic Pa28αβ and nuclear Pa28γ may both be important regulators of proteasome's ability to degrade oxidatively-damaged proteins, and induced-expression of both 20S proteasome and immunoproteasome, and their Pa28αβ and Pa28γ regulators are important for oxidative-stress adaptation.

Circadian regulation of cellular homeostasis--implications for cell metabolism and clinical diseases

The major pathways involving nutrient and energy metabolism including cellular homeostasis are profoundly impacted by the circadian clock, which orchestrates diurnal rhythms in physiology and behavior. While the links between circadian and metabolic rhythms are unclear, recent studies imply a close link between the two with one feeding back on the other. In this discussion, we present the hypothesis that circadian clocks likely contribute to cellular homeostasis, especially proteostasis, through regulation of metabolic rhythms, which in turn feed-back on circadian oscillators. The disruption of circadian clocks leads to altered metabolic rhythms and metabolic disease states as a result of altered cellular homeostasis.

Immunoproteasomes preserve protein homeostasis upon interferon-induced oxidative stress

Interferon (IFN)-induced immunoproteasomes (i-proteasomes) have been associated with improved processing of major histocompatibility complex (MHC) class I antigens. Here, we show that i-proteasomes function to protect cell viability under conditions of IFN-induced oxidative stress. IFNs trigger the production of reactive oxygen species, which induce protein oxidation and the formation of nascent, oxidant-damaged proteins. We find that the ubiquitylation machinery is concomitantly upregulated in response to IFNs, functioning to target defective ribosomal products (DRiPs) for degradation by i-proteasomes. i-proteasome-deficiency in cells and in murine inflammation models results in the formation of aggresome-like induced structures and increased sensitivity to apoptosis. Efficient clearance of these aggregates by the enhanced proteolytic activity of the i-proteasome is important for the preservation of cell viability upon IFN-induced oxidative stress. Our findings suggest that rather than having a specific role in the production of class I antigens, i-proteasomes increase the peptide supply for antigen presentation as part of a more general role in the maintenance of protein homeostasis.

Phosphorylation and methylation of proteasomal proteins of the haloarcheon Haloferax volcanii

Proteasomes are composed of 20S core particles (CPs) of alpha- and beta-type subunits that associate with regulatory particle AAA ATPases such as the proteasome-activating nucleotidase (PAN) complexes of archaea. In this study, the roles and additional sites of post-translational modification of proteasomes were investigated using the archaeon Haloferax volcanii as a model. Indicative of phosphorylation, phosphatase-sensitive isoforms of alpha1 and alpha2 were detected by 2-DE immunoblot. To map these and other potential sites of post-translational modification, proteasomes were purified and analyzed by tandem mass spectrometry (MS/MS). Using this approach, several phosphosites were mapped including alpha1 Thr147, alpha2 Thr13/Ser14 and PAN-A Ser340. Multiple methylation sites were also mapped to alpha1, thus, revealing a new type of proteasomal modification. Probing the biological role of alpha1 and PAN-A phosphorylation by site-directed mutagenesis revealed dominant negative phenotypes for cell viability and/or pigmentation for alpha1 variants including Thr147Ala, Thr158Ala and Ser58Ala. An H. volcanii Rio1p Ser/Thr kinase homolog was purified and shown to catalyze autophosphorylation and phosphotransfer to alpha1. The alpha1 variants in Thr and Ser residues that displayed dominant negative phenotypes were significantly reduced in their ability to accept phosphoryl groups from Rio1p, thus, providing an important link between cell physiology and proteasomal phosphorylation.

Persistent hijacking of brain proteasomes in HIV-associated dementia

Immunoproteasome induction sustains class 1 antigen presentation and immunological vigilance against HIV-1 in the brain. Investigation of HIV-1-associated alterations in brain protein turnover by the ubiquitin-proteasome system was performed by (1) determining proteasome subunit changes associated with persistent brain inflammation due to HIV-1; (2) determining whether these changes are related to HIV-1 neurocognitive disturbances, encephalitis, and viral loads; and (3) localizing proteasome subunits in brain cells and synapses. On the basis of neurocognitive performance, virological, and immunological measurements obtained within 6 months before death, 153 autopsy cases were selected. Semiquantitative immunoblot analysis performed in the dorsolateral prefrontal cortex revealed up to threefold induction of immunoproteasome subunits LMP7 and PA28alpha in HIV-1-infected subjects and was strongly related to diagnoses of neuropsychological impairment and HIV encephalitis. Low performance on neurocognitive tests specific for dorsolateral prefrontal cortex functioning domains was selectively correlated with immunoproteasome induction. Immunohistochemistry and laser confocal microscopy were then used to localize immunoproteasome subunits to glial and neuronal elements including perikarya, dystrophic axons, and synapses. In addition, HIV loads in brain tissue, cerebrospinal fluid, and blood plasma were robustly correlated to immunoproteasome levels. This persistent "hijacking" of the proteasome by HIV-1-mediated inflammatory response and immunoproteasome induction in the brain is hypothesized to impede turnover of folded proteins in brain cells. This would disrupt neuronal and synaptic protein dynamics, contributing to HIV-1 neurocognitive disturbances.

Aging and dietary restriction alter proteasome biogenesis and composition in the brain and liver

Interventions such as dietary restriction (DR) have been reported to ameliorate age-related proteasome inhibition in some tissues. Currently it is not known what effects aging and DR have on proteasome biogenesis in the liver and brain, nor have previous studies identified the links between changes in proteasome composition, biogenesis, and activity in the aging brain and liver. In the present study we demonstrate that the brain and liver exhibit age-dependent decreases in 26S and 20S proteasome activity. Additionally, our studies demonstrate that the brain and liver undergo selective changes in proteasome biology, including increases in proteasome biogenesis in response to aging and DR, with the liver exhibit more robust plasticity as compared to the brain. Lastly, studies demonstrated that aging and DR alter the interaction of Hsp90 with the 20S proteasome complex in the brain and liver. These studies affirm the dynamic nature of the proteasome complexes in both the liver and brain following aging and DR. Additionally, these data indicate that the relationship between proteasome composition/biogenesis and proteasome activity in tissues is extremely complex and tissue specific. These data have implications for understanding the effects of tissue specific effects of aging and DR on protein turnover and proteotoxicity.

Two-dimensional electrophoresis-based characterization of post-translational modifications of mammalian 20S proteasome complexes

PTMs serve as key regulatory mechanisms for 20S proteasome functions. Alterations in 20S PTMs have been previously observed with changes in modified protein degradation patterns and altered cellular phenotypes. Despite decades of investigation, our knowledge pertaining to the various PTMs of 20S complexes and their biological significance remain limited. In this investigation, we show that 2-DE offers an analytical tool with high resolution and reproducibility. Accordingly, it has been applied for the characterization of PTMs including glycosylation, phosphorylation, oxidation, and nitrosylation. The PTMs of murine cardiac 20S proteasomes and their associating proteins were examined. Our 2-DE analyses displayed over 25 spots for the 20S complexes (17 subunits), indicating multiply modified subunits of cardiac proteasomes. The identification of specific PTM sites subsequent to 2-DE was supported by MS. These PTMs included phosphorylation and oxidation. Most of the PTMs occurred in low stoichiometry and required enrichment to enhance the detection sensitivity. In conclusion, our studies support 2-DE as a central tool in the analyses of 20S proteasome PTMs. The approaches utilized in this investigation demonstrate their application in mapping the PTMs of the 20S proteasomes in cardiac tissue, which are applicable to other samples and biological conditions.

Epidermal growth factor receptor vIII expression in U87 glioblastoma cells alters their proteasome composition, function, and response to irradiation

Little is known about the factors that influence the proteasome structures in cells and their activity, although this could be highly relevant to cancer therapy. We have previously shown that, within minutes, irradiation inhibits substrate degradation by the 26S proteasome in most cell types. Here, we report an exception in U87 glioblastoma cells transduced to express the epidermal growth factor receptor vIII (EGFRvIII) mutant (U87EGFRvIII), which does not respond to irradiation with 26S proteasome inhibition. This was assessed using either a fluorogenic substrate or a reporter gene, the ornithine decarboxylase degron fused to ZsGreen (cODCZsGreen), which targets the protein to the 26S proteasome. To elucidate whether this was due to alterations in proteasome composition, we used quantitative reverse transcription-PCR to quantify the constitutive (X, Y, Z) and inducible 20S subunits (Lmp7, Lmp2, Mecl1), and 11S (PA28alpha and beta) and 19S components (PSMC1 and PSMD4). U87 and U87EGFRvIII significantly differed in expression of proteasome subunits, and in particular immunosubunits. Interestingly, 2 Gy irradiation of U87 increased subunit expression levels by 16% to 324% at 6 hours, with a coincident 30% decrease in levels of the proteasome substrate c-myc, whereas they changed little in U87EGFRvIII. Responses similar to 2 Gy were seen in U87 treated with a proteasome inhibitor, NPI0052, suggesting that proteasome inhibition induced replacement of subunits independent of the means of inhibition. Our data clearly indicate that the composition and function of the 26S proteasome can be changed by expression of the EGFRvIII. How this relates to the increased radioresistance associated with this cell line remains to be established.

Role of proteasomes in cellular regulation

The 26S proteasome is the key enzyme of the ubiquitin-dependent pathway of protein degradation. This energy-dependent nanomachine is composed of a 20S catalytic core and associated regulatory complexes. The eukaryotic 20S proteasomes demonstrate besides several kinds of peptidase activities, the endoribonuclease, protein-chaperone and DNA-helicase activities. Ubiquitin-proteasome pathway controls the levels of the key regulatory proteins in the cell and thus is essential for life and is involved in regulation of crucial cellular processes. Proteasome population in the cell is structurally and functionally heterogeneous. These complexes are subjected to tightly organized regulation, particularly, to a variety of posttranslational modifications. In this review we will summarize the current state of knowledge regarding proteasome participation in the control of cell cycle, apoptosis, differentiation, modulation of immune responses, reprogramming of these particles during these processes, their heterogeneity and involvement in the main levels of gene expression.

MDM2 promotes proteasome-dependent ubiquitin-independent degradation of retinoblastoma protein

Inactivation of retinoblastoma protein (Rb) plays a critical role in the development of human malignancies. It has been shown that Rb is degraded through a proteasome-dependent pathway, yet the mechanism is largely unclear. MDM2 is frequently found amplified and overexpressed in a variety of human tumors. In this study, we find that MDM2 promotes Rb degradation in a proteasome-dependent and ubiquitin-independent manner. We show that Rb, MDM2, and the C8 subunit of the 20S proteasome interact in vitro and in vivo and that MDM2 promotes Rb-C8 interaction. Expression of wild-type MDM2, but not the mutant MDM2 defective either in Rb interaction or in RING finger domain, promotes cell cycle S phase entry independent of p53. Furthermore, MDM2 ablation results in Rb accumulation and inhibition of DNA synthesis. Taken together, these findings demonstrate that MDM2 is a critical negative regulator for Rb and suggest that MDM2 overexpression contributes to cancer development by destabilizing Rb.

Proteasomes from Structure to Function: Perspectives from Archaea

Insight into the world of proteolysis has expanded considerably over the past decade. Energy-dependent proteases, such as the proteasome, are no longer viewed as nonspecific degradative enzymes associated solely with protein catabolism but are intimately involved in controlling biological processes that span life to death. The proteasome maintains this exquisite control by catalyzing the precisely timed and rapid turnover of key regulatory proteins. Proteasomes also interplay with chaperones to ensure protein quality and to readjust the composition of the proteome following stress. Archaea encode proteasomes that are highly related to those of eukaryotes in basic structure and function. Investigations of archaeal proteasomes coupled with those of eukaryotes has greatly facilitated our understanding of the molecular mechanisms that govern regulated protein degradation by this elaborate nanocompartmentalized machine.

Mild heat stress stimulates 20S proteasome and its 11S activator in human fibroblasts undergoing aging in vitro

Repeated mild heat shock (RMHS) has been shown to have several beneficial hormetic effects on human skin fibroblast undergoing aging in vitro. Because an age-related decline in proteasome activity is 1 of the reasons for the accumulation of abnormal proteins during aging, we have investigated the effects of RMHS on the 20S proteasome, which is the major proteolytic system involved in the removal of abnormal and oxidatively damaged proteins. Serially passaged human skin fibroblasts exposed to RMHS at 41 degrees C for 60 minutes twice a week had increased 3 proteasomal activities by 40% to 95% in early- and midpassage cultures. RMHS-treated cells also contained a 2-fold higher amount of the proteasome activator 11S, and the extent of the bound activator was double in early- and midpassage cells only. Furthermore, there was no difference in the content of the 19S proteasome regulator in the stressed and the unstressed cells. Therefore, RMHS-induced proteasome stimulation in early- and midpassage fibroblasts appears to be due to an induction and enhanced binding of 11S proteasome activators. In contrast to this, the proteasomal system in late-passage senescent cells appears to be less responsive to the stimulatory effects of mild heat shock.

Phosphorylation of 20S proteasome alpha subunit C8 (alpha7) stabilizes the 26S proteasome and plays a role in the regulation of proteasome complexes by gamma-interferon

In animal cells there are several regulatory complexes which interact with 20S proteasomes and give rise to functionally distinct proteasome complexes. gamma-Interferon upregulates three immuno beta catalytic subunits of the 20S proteasome and the PA28 regulator, and decreases the level of 26S proteasomes. It also decreases the level of phosphorylation of two proteasome alpha subunits, C8 (alpha7) and C9 (alpha3). In the present study we have investigated the role of phosphorylation of C8 by protein kinase CK2 in the formation and stability of 26S proteasomes. An epitope-tagged C8 subunit expressed in mammalian cells was efficiently incorporated into both 20S proteasomes and 26S proteasomes. Investigation of mutants of C8 at the two known CK2 phosphorylation sites demonstrated that these are the two phosphorylation sites of C8 in animal cells. Although phosphorylation of C8 was not absolutely essential for the formation of 26S proteasomes, it did have a substantial effect on their stability. Also, when cells were treated with gamma-interferon, there was a marked decrease in phosphorylation of C8, a decrease in the level of 26S proteasomes, and an increase in immunoproteasomes and PA28 complexes. These results suggest that the down-regulation of 26S proteasomes after gamma-interferon treatment results from the destabilization that occurs after dephosphorylation of the C8 subunit.

MEKK3 interacts with the PA28 gamma regulatory subunit of the proteasome

The proteasome is a multisubunit proteolytic enzyme comprising activator complexes bound to the 20 S catalytic core. The functions of the proteasomal activator (PA) 700 in ubiquitin/ATP-dependent protein degradation and of the PA28 alpha/beta activators in antigen presentation are well defined. However, the function of a third PA, PA28 gamma, remains elusive. We now show that mitogen-activated protein kinase (MAPK)/extracellular-signal-regulated kinase (ERK) kinase kinase 3 (MEKK3), a MAPK kinase kinase (MAPKKK) involved in MAPK kinase 7 (MKK7)-c-Jun N-terminal kinase ('JNK') and MKK6-p38 signalling, can bind PA28 gamma but not PA28 alpha. In contrast, B-Raf, a MAPKKK specific for the MAPK/ERK kinase ('MEK')-ERK module, binds PA28 gamma and alpha. The PA28 gamma-binding domain of MEKK3 is located within its N-terminal regulatory domain (amino acids 1-178). Expression of MEKK3 in Cos-7 cells led to an increase in endogenous and co-expressed PA28 gamma protein levels, whereas kinase-deficient MEKK3 had no effect on PA28 gamma expression. Furthermore, in vitro assays indicated that PA28 gamma was a MEKK3 substrate. MEKK3 represents the first protein kinase capable of binding and phosphorylating a PA, and provides a potential mechanism to link stress-activated protein kinase signalling with the PA28 gamma-dependent proteasome.

Cellular senescence in human keratinocytes: unchanged proteolytic capacity and increased protein load

In order to assess the activity of cellular proteasome, we developed a method to permeabilize keratinocyte monolayers and measure proteasome activities intracellularly, using fluorogenic peptide substrates. The observed K(m) did not differ significantly in situ and in soluble extracts, and the K(i) of proteasome inhibitor MG132 was slightly higher in situ (34nM instead of 4nM). Inhibition studies in permeabilized cells showed that MG132 followed competitive inhibition patterns, and clasto-lactacystin beta-lactone non-competitive patterns, as expected. The observed velocities in situ (500pmoles/min/mg protein) were comparable to the best values of proteasome activity in crude cellular extracts. These features altogether allowed to identify the in situ activity as that of proteasome. To characterize proteasome complexes present in human keratinocytes, we analyzed cellular lysates by ultracentrifugation and gel filtration: most proteasome activity was associated with PA700-bound, presumably 26S, particles. PA28 activator was detected only when cells were treated by gamma interferon. Proteasome activities were determined using the in situ method in keratinocytes at different stages of replicative senescence. Only a slight decrease of proteasome activity per cell was seen at intermediate passages, followed by a slight increase in senescent cells. In the same time, the amount of total proteins increased notably with cellular ageing. Thus, proteasome activity decreased relatively to total proteins, but not relatively to cell numbers. Flow cytometry confirmed that the size of aged keratinocytes increased with the ageing marker beta-galactosidase.

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.

Targeted disruption of specific steps of the ubiquitin-proteasome pathway by oxidation in lens epithelial cells

Several steps in the ubiquitin-proteasome pathway have been shown to be inhibited in models of oxidative stress and aging. We have designed similar models of aging and oxidation in the HLE B-3 human lens epithelial cell line. Following hydrogen peroxide (H2O2) treatment, B-3 cells exhibited an expected activation of c-fos. The effect of these same and similar treatments on the lens proteasome system was unexpected. The 2D gel pattern and the chymotrypsin-like activity of the 20S core were unaffected by this H2O2 treatment, contrary to previous experience in other culture systems. The critical role of proteolysis in the aging lens, and the strong tie between oxidation and proteasome changes, urged us to further model lens oxidation and investigate several steps of the ubiquitin-proteasome pathway with an alternative agent: the thiol-specific oxidant, diamide. The 20S core proteasome, de-ubiquitinating, and ATP-dependent 26S proteasome activities all showed decreases 10 min after diamide was applied, and recovered to near normal within 1h. The higher, 300 microM dose inhibited the 20S by 43%, the de-ubiquitinating activity by 17% and the 26S by 31%. The comparable susceptibility of the 20S activity and the 26S activity differs from several previously published models. Such differences may be the result of tissue or cell line-specific variants in either the components of the ubiquitin-proteasome pathway or in their modification by intracellular oxidants or reductants.

Assays of proteasome activity in relation to aging

Proteasomes play a major role in intracellular protein turnover. They exist in cells in several different molecular forms including 20S proteasomes, 26S proteasomes and PA28-20S proteasome complexes. In this study we have compared the properties of these purified proteasome complexes to try to design assays that will distinguish between the different complexes (26S proteasome, 20S proteasome, PA28-20S proteasome) in cell extracts. Although the different purified complexes were found to have differences in stability, and in their sensitivity to low concentrations of SDS and salt, the results suggest that it is not straightforward to assay selectively for each type of complex in cell extracts. The relative contribution of different proteasome complexes varies in different cell types and there may be other proteases present which hydrolyse the chosen substrate. Proteasome assays carried out under defined conditions allow comparisons of activity in cell extracts as a function of age, but separation by gel filtration on a Superose 6 column was found to be a useful method for determining the level of different proteasome related complexes.

Functional regulation of immunoproteasomes and transporter associated with antigen processing

The central event in the cellular immune response to invading pathogens is the presentation of non-self antigenic peptides by major histocompatibility complex (MHC) class I molecules to cytotoxic T lymphocytes (CTLs). As peptide binding and transport proteins, MHC class I molecules have evolved distinct biochemical and cellular strategies for acquiring antigenic peptides, providing CTLs an extracellular representation of the intracellular antigen content. Whereas efficient generation of MHC class I binding peptides depends on the intracellular, immunoproteasome-mediated proteolysis machinery, translocation of peptides into the lumen of the endoplasmic reticulum requires the endoplasmic reticulum-resident, adenosine 5'-triphosphate (ATP) binding cassette transporter associated with antigen processing (TAP). Here we show, for the first time, that immunoproteasomes, TAP complexes, and MHC class I molecules are physically associated, providing an effective means of transporting MHC class I binding peptides from their sites of generation into the lumen of the endoplasmic reticulum for loading onto MHC class I molecules. In this review, we assess the current understanding of the functional regulation of immunoproteasomes and transporter associated with antigen processing.

Treatment of COS-7 cells with proteasome inhibitors or gamma-interferon reduces the increase in caspase 3 activity associated with staurosporine-induced apoptosis

Proteasomes play a major role in intracellular protein degradation and have been implicated in apoptosis. In this study we have investigated proteasome activity and the effects of inhibition of proteasomes or modulation of proteasome complexes on staurosporine-induced apoptosis in COS-7 cells. Staurosporine treatment of COS-7 cells had little direct effect on proteasome activity and did not cause dissociation of 26S proteasomes. There was also no major redistribution of proteasomes accompanying apoptosis in COS-7 cells. However, when the cells were pretreated with proteasome inhibitors, both the caspase 3 activity of the cells and the percentage of apoptotic cells measured by the TUNEL assay were reduced compared to staurosporine-treated cells, which had no inhibitor added. Proteasome inhibitors were also found to reduce the activation of caspase 3 in living cells which was assayed using a FRET-based method. However, proteasome inhibitors did not prevent some of the morphological changes associated with staurosporine-induced apoptosis. Pretreatment of cells with gamma-interferon, which increases immunoproteasomes and PA28 complexes and reduces 26S proteasome levels, had an antiapoptotic effect. These results are consistent with a role for 26S proteasomes in regulating the activation of caspase 3 through the degradation of key regulatory proteins.

Regulation of proteasome complexes by gamma-interferon and phosphorylation

Proteasomes play a major role in non-lysosomal proteolysis and also in the processing of proteins for presentation by the MHC class I pathway. In animal cells they exist in several distinct molecular forms which contribute to the different functions. 26S proteasomes contain the core 20S proteasome together with two 19S regulatory complexes. Alternatively, PA28 complexes can bind to the ends of the 20S proteasome to form PA28-proteasome complexes and PA28-proteasome-19S hybrid complexes have also been described. Immunoproteasome subunits occur in 26S proteasomes as well as in PA28-proteasome complexes. We have found differences in the subcellular distribution of the different forms of proteasomes. The gamma-interferon inducible PA28 alpha and beta subunits are predominantly located in the cytoplasm, while 19S regulatory complexes (present at significant levels only in 26S complexes) are present in the nucleus as well as in the cytoplasm. Immunoproteasomes are greatly enriched at the endoplasmic reticulum (ER) where they may facilitate the generation of peptides for transport into the lumen of the ER. We have also investigated the effects of gamma-interferon on the levels and subcellular distribution of inducible subunits and regulator subunits. In each case gamma-interferon was found to increase the level but not to alter the distribution. Several subunits of proteasomes are phosphorylated including alpha subunits C8 (alpha7) and C9 (alpha3), and ATPase subunit S4 (rpt2). Our studies have shown that gamma-interferon treatment decreases the level of phosphorylation of proteasomes. We have investigated the role of phosphorylation of C8 by casein kinase II by site directed mutagenesis. The results demonstrate that phosphorylation at either one of the two sites is essential for the association of 19S regulatory complexes and that the ability to undergo phosphorylation at both sites gives the most efficient incorporation of C8 into the 26S proteasome.