LMP2-specific inhibitors: chemical genetic tools for proteasome biology

Mapping the interplay of immunoproteasome and autophagy in different heart failure phenotypes

Proper protein degradation is required for cellular protein homeostasis and organ function. Particularly, in post-mitotic cells, such as cardiomyocytes, unbalanced proteolysis due to inflammatory stimuli and oxidative stress contributes to organ dysfunction. To ensure appropriate protein turnover, eukaryotic cells exert two main degradation systems, the ubiquitin-proteasome-system and the autophagy-lysosome-pathway. It has been shown that proteasome activity affects the development of cardiac dysfunction differently, depending on the type of heart failure. Studies analyzing the inducible subtype of the proteasome, the immunoproteasome (i20S), demonstrated that the i20S plays a double role in diseased hearts. While i20S subunits are increased in cardiac hypertrophy, atrial fibrillation and partly in myocarditis, the opposite applies to diabetic cardiomyopathy and ischemia/reperfusion injury. In addition, the i20S appears to play a role in autophagy modulation depending on heart failure phenotype. This review summarizes the current literature on the i20S in different heart failure phenotypes, emphasizing the two faces of i20S in injured hearts. A selection of established i20S inhibitors is introduced and signaling pathways linking the i20S to autophagy are highlighted. Mapping the interplay of the i20S and autophagy in different types of heart failure offers potential approaches for developing treatment strategies against heart failure.

Identification of a new class of proteasome inhibitors based on a naphthyl-azotricyclic-urea-phenyl scaffold

Proteasomes play an important role in protein degradation and regulation of many cellular pathways by maintaining protein balance. Inhibitors of proteasomes disrupt this balance affecting proteins that are key in malignancies and as such have found applications in the treatment of multiple myeloma and mantle cell lymphoma. However, resistance mechanisms have been reported for these proteasome inhibitors including mutations at the β5 site which necessitates the constant development of new inhibitors. In this work, we report the identification of a new class of proteasome inhibitors, polycyclic molecules bearing a naphthyl-azotricyclic-urea-phenyl scaffold, from screening of the ZINC library of natural products. The most potent of these compounds showed evidence of dose dependency through proteasome assays with IC50 values in the low micromolar range, and kinetic analysis revealed competitive binding at the β5c site with an estimated inhibition constant, K i, of 1.15 μM. Inhibition was also shown for the β5i site of the immunoproteasome at levels similar to those of the constitutive proteasome. Structure-activity relationship studies identified the naphthyl substituent to be crucial for activity and this was attributed to enhanced hydrophobic interactions within β5c. Further to this, halogen substitution within the naphthyl ring enhanced the activity and allowed for π-π interactions with Y169 in β5c and Y130 and F124 in β5i. The combined data highlight the importance of hydrophobic and halogen interactions in β5 binding and assist in the design of next generation inhibitors of proteasomes.

Targeting immunoproteasome in neurodegeneration: A glance to the future

The immunoproteasome is a specialized form of proteasome equipped with modified catalytic subunits that was initially discovered to play a pivotal role in MHC class I antigen processing and immune system modulation. However, over the last years, this proteolytic complex has been uncovered to serve additional functions unrelated to antigen presentation. Accordingly, it has been proposed that immunoproteasome synergizes with canonical proteasome in different cell types of the nervous system, regulating neurotransmission, metabolic pathways and adaptation of the cells to redox or inflammatory insults. Hence, studying the alterations of immunoproteasome expression and activity is gaining research interest to define the dynamics of neuroinflammation as well as the early and late molecular events that are likely involved in the pathogenesis of a variety of neurological disorders. Furthermore, these novel functions foster the perspective of immunoproteasome as a potential therapeutic target for neurodegeneration. In this review, we provide a brain and retina-wide overview, trying to correlate present knowledge on structure-function relationships of immunoproteasome with the variety of observed neuro-modulatory functions.

Site-Specific Proteasome Inhibitors

Proteasome is a multi-subunit protein degradation machine, which plays a key role in the maintenance of protein homeostasis and, through degradation of regulatory proteins, in the regulation of numerous cell functions. Proteasome inhibitors are essential tools for biomedical research. Three proteasome inhibitors, bortezomib, carfilzomib, and ixazomib are approved by the FDA for the treatment of multiple myeloma; another inhibitor, marizomib, is undergoing clinical trials. The proteolytic core of the proteasome has three pairs of active sites, β5, β2, and β1. All clinical inhibitors and inhibitors that are widely used as research tools (e.g., epoxomicin, MG-132) inhibit multiple active sites and have been extensively reviewed in the past. In the past decade, highly specific inhibitors of individual active sites and the distinct active sites of the lymphoid tissue-specific immunoproteasome have been developed. Here, we provide a comprehensive review of these site-specific inhibitors of mammalian proteasomes and describe their utilization in the studies of the biology of the active sites and their roles as drug targets for the treatment of different diseases.

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.

Non-selective proteasome inhibitors in multiple myeloma and future perspectives

INTRODUCTION

: The ubiquitination system is the most important cascade of protein degradation independently of lysosomal function. The proteasome system is actively involved in cell cycle regulation. Therefore, proteasome inhibition can lead to inhibition of tumor cell proliferation, and therefore it constitutes a potential therapeutic anticancer approach especially in the therapeutic algorithm of patients with multiple myeloma.

AREAS COVERED

Three different proteasome inhibitors are currently approved, bortezomib, carfilzomib and ixazomib, and they have been investigated in multiple myeloma and other hematological malignancies. Multiple myeloma cells are extremely sensitive to this inhibition which leads to accumulation of proteins and endoplasmic reticulum stress, leading finally to apoptosis. However, these agents lack specificity, since they target both the constitutive proteasome and the immunoproteasome. Targeting the constitutive proteasome is the main reason for side toxicity due to the effect on normal tissues. In contrary, immunoproteasome inhibition may reduce the adverse events while maintaining the therapeutic efficacy. In this review the authors present the role of the available proteasome inhibitors in myeloma therapeutics and future perspectives of both selective and non-selective proteasome inhibitors.

EXPERT OPINION

The available non-selective proteasome inhibitors have changed the therapeutics of multiple myeloma the last 10 years and have significantly improved the clinical outcomes of the patients. Furthermore, selective proteasome inhibitors are now under preclinical investigation and there is hope that their optimization will come with an improved safety profile with at least comparable efficacy.

A Nut for Every Bolt: Subunit-Selective Inhibitors of the Immunoproteasome and Their Therapeutic Potential

At the heart of the ubiquitin-proteasome system, the 20S proteasome core particle (CP) breaks down the majority of intracellular proteins tagged for destruction. Thereby, the CP controls many cellular processes including cell cycle progression and cell signalling. Inhibitors of the CP can suppress these essential biological pathways, resulting in cytotoxicity, an effect that is beneficial for the treatment of certain blood cancer patients. During the last decade, several preclinical studies demonstrated that selective inhibition of the immunoproteasome (iCP), one of several CP variants in mammals, suppresses autoimmune diseases without inducing toxic side effects. These promising findings led to the identification of natural and synthetic iCP inhibitors with distinct chemical structures, varying potency and subunit selectivity. This review presents the most prominent iCP inhibitors with respect to possible scientific and medicinal applications, and discloses recent trends towards pan-immunoproteasome reactive inhibitors that cumulated in phase II clinical trials of the lead compound KZR-616 for chronic inflammations.

Immunoproteasome Function in Normal and Malignant Hematopoiesis

The ubiquitin-proteasome system (UPS) is a central part of protein homeostasis, degrading not only misfolded or oxidized proteins but also proteins with essential functions. The fact that a healthy hematopoietic system relies on the regulation of protein homeostasis and that alterations in the UPS can lead to malignant transformation makes the UPS an attractive therapeutic target for the treatment of hematologic malignancies. Herein, inhibitors of the proteasome, the last and most important component of the UPS enzymatic cascade, have been approved for the treatment of these malignancies. However, their use has been associated with side effects, drug resistance, and relapse. Inhibitors of the immunoproteasome, a proteasomal variant constitutively expressed in the cells of hematopoietic origin, could potentially overcome the encountered problems of non-selective proteasome inhibition. Immunoproteasome inhibitors have demonstrated their efficacy and safety against inflammatory and autoimmune diseases, even though their development for the treatment of hematologic malignancies is still in the early phases. Various immunoproteasome inhibitors have shown promising preliminary results in pre-clinical studies, and one inhibitor is currently being investigated in clinical trials for the treatment of multiple myeloma. Here, we will review data on immunoproteasome function and inhibition in hematopoietic cells and hematologic cancers.

Covalent Small Molecule Immunomodulators Targeting the Protease Active Site

Cells of the immune system utilize multiple proteases to regulate cell functions and orchestrate innate and adaptive immune responses. Dysregulated protease activities are implicated in many immune-related disorders; thus, protease inhibitors have been actively investigated for pharmaceutical development. Although historically considered challenging with concerns about toxicity, compounds that covalently modify the protease active site represent an important class of agents, emerging not only as chemical probes but also as approved drugs. Here, we provide an overview of technologies useful for the study of proteases with the focus on recent advances in chemoproteomic methods and screening platforms. By highlighting covalent inhibitors that have been designed to target immunomodulatory proteases, we identify opportunities for the development of small molecule immunomodulators.

A new class of α-ketoamide derivatives with potent anticancer and anti-SARS-CoV-2 activities

Inhibitors of the proteasome have been extensively studied for their applications in the treatment of human diseases such as hematologic malignancies, autoimmune disorders, and viral infections. Many of the proteasome inhibitors reported in the literature target the non-primed site of proteasome’s substrate binding pocket. In this study, we designed, synthesized and characterized a series of novel α-keto phenylamide derivatives aimed at both the primed and non-primed sites of the proteasome. In these derivatives, different substituted phenyl groups at the head group targeting the primed site were incorporated in order to investigate their structure-activity relationship and optimize the potency of α-keto phenylamides. In addition, the biological effects of modifications at the cap moiety, P1, P2 and P3 side chain positions were explored. Many derivatives displayed highly potent biological activities in proteasome inhibition and anticancer activity against a panel of six cancer cell lines, which were further rationalized by molecular modeling analyses. Furthermore, a representative α-ketoamide derivative was tested and found to be active in inhibiting the cellular infection of SARS-CoV-2 which causes the COVID-19 pandemic. These results demonstrate that this new class of α-ketoamide derivatives are potent anticancer agents and provide experimental evidence of the anti-SARS-CoV-2 effect by one of them, thus suggesting a possible new lead to develop antiviral therapeutics for COVID-19.

Tradeoff between metabolic i-proteasome addiction and immune evasion in triple-negative breast cancer

In vitro studies have suggested proteasome inhibitors could be effective in triple-negative breast cancer (TNBC). We found that bortezomib and carfilzomib induce proteotoxic stress and apoptosis via the unfolded protein response (UPR) in TNBC cell lines, with sensitivity correlated with expression of immuno-(PSMB8/9/10) but not constitutive-(PSMB5/6/7) proteasome subunits. Equally, the transcriptomes of i-proteasome-high human TNBCs are enriched with UPR gene sets, and the genomic copy number landscape reflects positive selection pressure favoring i-proteasome activity, but in the setting of adjuvant treatment, this is actually associated with favorable prognosis. Tumor expression of PSMB8 protein (β5i) is associated with levels of MHC-I, interferon-γ-inducible proteasome activator PA28β, and the densities of stromal antigen-presenting cells and lymphocytes (TILs). Crucially, TILs were protective among TNBCs that maintain high β5i but did not stratify survival amongst β5i-low TNBCs. Moreover, β5i expression was lower in brain metastases than in patient-matched primary breast tumors (n = 34; P = 0.007), suggesting that suppression contributes to immune evasion and metastatic progression. Hence, inhibiting proteasome activity could be counterproductive in the adjuvant treatment setting because it potentiates anti-TNBC immunity.

[Research progress on selective immunoproteasome inhibitors]

Immunoproteasome is associated with various diseases such as hematologic malignancies, inflammatory, autoimmune and central nervous system diseases, and over expression of immunoproteasome is observed in all of these diseases. Immunoproteasome inhibitors can reduce the expression of immunoproteasome by inhibiting the production of related cell-inducing factors and the activity of T lymphocyte for treating related diseases. In order to achieve good efficacy and reduce the toxic effects, key for development of selective immunoproteasome inhibitors is the high selectivity and potent activity of the three active subunits of the proteasome. This review summarizes the structure and functions of immunoproteasome and the associated diseases. Besides, structure, activity and status of selective immunoproteasome inhibitors are also been highlighted.

Proteasome Inhibitors in Cancer Therapy and their Relation to Redox Regulation

Redox homeostasis is important for the maintenance of cell survival. Under physiological conditions, redox system works in a balance and involves activation of many signaling molecules. Regulation of redox balance via signaling molecules is achieved by different pathways and proteasomal system is a key pathway in this process. Importance of proteasomal system on signaling pathways has been investigated for many years. In this direction, many proteasome targeting molecules have been developed. Some of them are already in the clinic for cancer treatment and some are still under investigation to highlight underlying mechanisms. Although there are many studies done, molecular mechanisms of proteasome inhibitors and related signaling pathways need more detailed explanations. This review aims to discuss redox status and proteasomal system related signaling pathways. In addition, cancer therapies targeting proteasomal system and their effects on redox-related pathways have been summarized.

The Immunoproteasome: An Emerging Target in Cancer and Autoimmune and Neurological Disorders

The immunoproteasome (iCP) is an isoform of the 20S proteasome that is expressed when cells are stressed or receive an inflammatory signal. The primary role of the iCP is to hydrolyze proteins into peptides that are compatible with being loaded into a MHC-I complex. When the activity of the iCP is dysregulated or highly expressed, it can lead to unwanted cell death. Some cancer types express the iCP rather than the standard proteasome, and selective inhibitors have been developed to exploit this difference. Here, we describe diseases known to be influenced by iCP activity and the current status for targeting the iCP to elicit a therapeutic response. We also describe a variety of chemical tools that have been developed to monitor the activity of the iCP in cells. Finally, we present the future outlook for targeting the iCP in a variety of disease types and with mechanisms besides inhibition.

Immunoproteasome-selective inhibitors: An overview of recent developments as potential drugs for hematologic malignancies and autoimmune diseases

The immunoproteasome, a specialized form of proteasome, is mainly expressed in lymphocytes and monocytes of jawed vertebrates and responsible for the generation of antigenic peptides for cell-mediated immunity. Overexpression of immunoproteasome have been detected in a wide range of diseases including malignancies, autoimmune and inflammatory diseases. Following the successful approval of constitutive proteasome inhibitors bortezomib, carfilzomib and Ixazomib, and with the clarification of immunoproteasome crystal structure and functions, a variety of immunoproteasome inhibitors were discovered or rationally developed. Not only the inhibitory activities, the selectivities for immunoproteasome over constitutive proteasome are essential for the clinical potential of these analogues, which has been validated by the clinical evaluation of immunoproteasome-selective inhibitor KZR-616 for the treatment of systemic lupus erythematosus. In this review, structure, function as well as the current developments of various inhibitors against immunoproteasome are going to be summarized, which help to fully understand the target for drug discovery.

Argyrin B, a non-competitive inhibitor of the human immunoproteasome exhibiting preference for β1i

Inhibitors of the proteasome have found broad therapeutic applications; however, they show severe toxicity due to the abundance of proteasomes in healthy cells. In contrast, inhibitors of the immunoproteasome, which is upregulated during disease states, are less toxic and have increased therapeutic potential including against autoimmune disorders. In this project, we report argyrin B, a natural product cyclic peptide to be a reversible, non-competitive inhibitor of the immunoproteasome. Argyrin B showed selective inhibition of the β5i and β1i sites of the immunoproteasome over the β5c and β1c sites of the constitutive proteasome with nearly 20-fold selective inhibition of β1i over the homologous β1c. Molecular modelling attributes the β1i over β1c selectivity to the small hydrophobic S1 pocket of β1i and β5i over β5c to site-specific amino acid variations that enable additional bonding interactions and stabilization of the binding conformation. These findings facilitate the design of immunoproteasome selective and reversible inhibitors that may have a greater therapeutic potential and lower toxicity.

Development of Novel Epoxyketone-Based Proteasome Inhibitors as a Strategy To Overcome Cancer Resistance to Carfilzomib and Bortezomib

Over the past 15 years, proteasome inhibitors (PIs), namely bortezomib, carfilzomib (Cfz) and ixazomib, have significantly improved the overall survival and quality-of-life for multiple myeloma (MM) patients. However, a significant portion of MM patients do not respond to PI therapies. Drug resistance is present either de novo or acquired after prolonged therapy through mechanisms that remain poorly defined. The lack of a clear understanding of clinical PI resistance has hampered the development of next-generation PI drugs to treat MM patients who no longer respond to currently available therapies. Here, we designed and synthesized novel epoxyketone-based PIs by structural modifications at the P1' site. We show that a Cfz analog, 9, harboring a hydroxyl substituent at its P1' position was highly cytotoxic against cancer cell lines displaying de novo or acquired resistance to Cfz. These results suggest that peptide epoxyketones incorporating P1'-targeting moieties may have the potential to bypass resistance mechanisms associated with Cfz and to provide additional clinical options for patients resistant to Cfz.

Cellular Cullin RING Ubiquitin Ligases: Druggable Host Dependency Factors of Cytomegaloviruses

Human cytomegalovirus (HCMV) is a ubiquitous betaherpesvirus that frequently causes morbidity and mortality in individuals with insufficient immunity, such as transplant recipients, AIDS patients, and congenitally infected newborns. Several antiviral drugs are approved to treat HCMV infections. However, resistant HCMV mutants can arise in patients receiving long-term therapy. Additionally, side effects and the risk to cause birth defects limit the use of currently approved antivirals against HCMV. Therefore, the identification of new drug targets is of clinical relevance. Recent work identified DNA-damage binding protein 1 (DDB1) and the family of the cellular cullin (Cul) RING ubiquitin (Ub) ligases (CRLs) as host-derived factors that are relevant for the replication of human and mouse cytomegaloviruses. The first-in-class CRL inhibitory compound Pevonedistat (also called MLN4924) is currently under investigation as an anti-tumor drug in several clinical trials. Cytomegaloviruses exploit CRLs to regulate the abundance of viral proteins, and to induce the proteasomal degradation of host restriction factors involved in innate and intrinsic immunity. Accordingly, pharmacological blockade of CRL activity diminishes viral replication in cell culture. In this review, we summarize the current knowledge concerning the relevance of DDB1 and CRLs during cytomegalovirus replication and discuss chances and drawbacks of CRL inhibitory drugs as potential antiviral treatment against HCMV.

A Marine Diterpenoid Modulates the Proteasome Activity in Murine Macrophages Stimulated with LPS

The proteasome is an intracellular complex that degrades damaged or unfolded proteins and participates in the regulation of several processes. The immunoproteasome is a specialized form that is expressed in response to proinflammatory signals and is particularly abundant in immune cells. In a previous work, we found an anti-inflammatory effect in a diterpenoid extracted from the octocoral Pseudopterogorgia acerosa, here called compound 1. This compound prevented the degradation of inhibitor κB α (IκBα) and the subsequent activation of nuclear factor κB (NFκB), suggesting that this effect might be due to inhibition of the ubiquitin-proteasome system. Here we show that compound 1 inhibits the proteasomal chymotrypsin-like activity (CTL) of murine macrophages in the presence of lipopolysaccharide (LPS) but not in its absence. This effect might be due to the capacity of this compound to inhibit the activity of purified immunoproteasome. The compound inhibits the cell surface expression of major histocompatibility complex (MHC)-I molecules and the production of proinflammatory cytokines induced by LPS in vitro and in vivo, respectively. Molecular docking simulations predicted that compound 1 selectively binds to the catalytic site of immunoproteasome subunits β1i and β5i, which are responsible for the CTL activity. Taken together these findings suggest that the compound could be a selective inhibitor of the immunoproteasome, and hence could pave the way for its future evaluation as a candidate for the treatment of inflammatory disorders and autoimmune diseases.

(Immuno)proteasomes as therapeutic target in acute leukemia

The clinical efficacy of proteasome inhibitors in the treatment of multiple myeloma has encouraged application of proteasome inhibitor containing therapeutic interventions in (pediatric) acute leukemia. Here, we summarize the positioning of bortezomib, as first-generation proteasome inhibitor, and second-generation proteasome inhibitors in leukemia treatment from a preclinical and clinical perspective. Potential markers for proteasome inhibitor sensitivity and/or resistance emerging from leukemia cell line models and clinical sample studies will be discussed focusing on the role of immunoproteasome and constitutive proteasome (subunit) expression, PSMB5 mutations, and alternative mechanisms of overcoming proteolytic stress.

Impact of LMP7 (rs2071543) gene polymorphism in increasing cancer risk: evidence from a meta-analysis and trial sequential analysis

Genetic variant LMP7 (low molecular weight polypeptide 7) –145 C > A may influence the function of immune surveillance of an individual and lead to cancer development. Various studies have investigated the relevance of LMP7 –145 C > A gene polymorphism with cancer risk; but, their results are conflicting and inconsistent. To obtain a comprehensive conclusion, a meta-analysis was performed by including eight eligible published studies retrieved from PubMed (Medline), EMBASE and Google Scholar web search until December 2016. Individuals with AA genotype (AA vs CC: p = 0.001; OR = 2.602, 95% CI = 1.780 to 3.803) of LMP7 -145 C > A were found to have 2 folds higher risk of cancer than those with CC genotype. The recessive genetic model (AA vs AC + CC) also indicated that individuals with AA genotype have 2 folds higher cancer risk than AC and CC genotypes (p = 0.001; OR = 2.216, 95% CI = 1.525 to 3.221). Also, significant increased cancer risk was observed in Asians but not in Caucasians. No publication bias was observed during the analysis. Trial sequential analysis also strengthened our current findings. These results suggest that genetic variant LMP7–145 C > A has significant role in increasing cancer risk in overall and Asian population, and could be useful as a prognostic marker for early cancer predisposition.

Cadmium pyrithione suppresses tumor growth in vitro and in vivo through inhibition of proteasomal deubiquitinase

The ubiquitin-proteasome system (UPS) is indispensable to the protein quality control in eukaryotic cells. Due to the remarkable clinical success of using proteasome inhibitors for clinical treatment of multiple myeloma, it is anticipated that targeting the UPS upstream of the proteasome step be an effective strategy for cancer therapy. Deubiquitinases (DUB) are proteases that remove ubiquitin from target proteins and therefore regulate multiple cellular processes including some signaling pathways altered in cancer cells. Thus, targeting DUB is a promising strategy for cancer drug discovery. Previously, we have reported that metal complexes, such as copper and gold complexes, can disrupt the UPS via suppressing the activity of 19S proteasome-associated DUBs and/or of the 20S proteasomes, thereby inducing cancer cell death. In this study, we found that cadmium pyrithione (CdPT) treatment led to remarkable accumulation of ubiquitinated proteins in cultured cancer cells and primary leukemia cells. CdPT potently inhibited the activity of proteasomal DUBs (USP14 and UCHL5), but slightly inhibited 20S proteasome activity. The anti-cancer activity of CdPT was associated with triggering apoptosis via caspase activation. Moreover, treatment with CdPT inhibited proteasome function and repressed tumor growth in animal xenograft models. Our results show that cadmium-containing complex CdPT may function as a novel proteasomal DUB inhibitor and suggest appealing prospects for cancer treatment.

Immunoproteasome-selective and non-selective inhibitors: A promising approach for the treatment of multiple myeloma

The ubiquitin-proteasome system (UPS) is the major non-lysosomal proteolytic system for the degradation of abnormal or damaged proteins no longer required. The proteasome is involved in degradation of numerous proteins which regulate the cell cycle, indicating a role in controlling cell proliferation and maintaining cell survival. Defects in the UPS can lead to anarchic cell proliferation and to tumor development. For these reasons UPS inhibition has become a significant new strategy for drug development in cancer treatment. In addition to the constitutive proteasome, which is expressed in all cells and tissues, higher organisms such as vertebrates possess two immune-type proteasomes, the thymoproteasome and the immunoproteasome. The thymoproteasome is specifically expressed by thymic cortical epithelial cells and has a role in positive selection of CD8+ T cells, whereas the immunoproteasome is predominantly expressed in monocytes and lymphocytes and is responsible for the generation of antigenic peptides for cell-mediated immunity. Recent studies demonstrated that the immunoproteasome has a preservative role during oxidative stress and is up-regulated in a number of pathological disorders including cancer, inflammatory and autoimmune diseases. As a consequence, immunoproteasome-selective inhibitors are currently the focus of anticancer drug design. At present, the commercially available proteasome inhibitors bortezomib and carfilzomib which have been validated in multiple myeloma and other model systems, appear to target both the constitutive and immunoproteasomes, indiscriminately. This lack of specificity may, in part, explain some of the side effects of these agents, such as peripheral neuropathy and gastrointestinal effects, which may be due to targeting of the constitutive proteasome in these tissues. In contrast, by selectively inhibiting the immunoproteasome, it may be possible to maintain the antimyeloma and antilymphoma efficacy while reducing these toxicities, thereby increasing the therapeutic index. This review article will be focused on the discussion of the most promising immunoproteasome specific inhibitors which have been developed in recent years. Particular attention will be devoted to the description of their mechanism of action, their structure-activity relationship, and their potential application in therapy.

Learning from the Proteasome How To Fine-Tune Cancer Immunotherapy

Cancer immunotherapy has recently emerged as a forefront strategy to fight cancer. Key players in antitumor responses are CD8⁺ cytolytic T lymphocytes (CTLs) that can detect tumor cells that carry antigens, in other words, small peptides bound to surface major histocompatibility complex (MHC) class I molecules. The success and safety of cancer immunotherapy strategies depends on the nature of the antigens recognized by the targeted T cells, their strict tumor specificity, and whether tumors and antigen-presenting cells can efficiently process the peptide. We review here the nature of the tumor antigens and their potential for the development of immunotherapeutic strategies. We also discuss the importance of proteasome in the production of these peptides in the context of immunotherapy and therapeutic cancer vaccines.

Discovery of Highly Selective Inhibitors of the Immunoproteasome Low Molecular Mass Polypeptide 2 (LMP2) Subunit

Building upon the success of bortezomib (VELCADE) and carfilzomib (KYPROLIS), the design of a next generation of inhibitors targeting specific subunits within the immunoproteasome is of interest for the treatment of autoimmune disease. There are three catalytic subunits within the immunoproteasome (low molecular mass polypeptide-7, -2, and multicatalytic endopeptidase complex subunit-1; LMP7, LMP2, and MECL-1), and a campaign was undertaken to design a potent and selective LMP2 inhibitor with sufficient properties to allow for sustained inhibition in vivo. Screening a focused library of epoxyketones revealed a series of potent dipeptides that were optimized to provide the highly selective inhibitor KZR-504 (12).

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.

A Minimal β-Lactone Fragment for Selective β5c or β5i Proteasome Inhibitors

Broad-spectrum proteasome inhibitors are applied as anticancer drugs, whereas selective blockage of the immunoproteasome represents a promising therapeutic rationale for autoimmune diseases. We here aimed at identifying minimal structural elements that confer β5c or β5i selectivity on proteasome inhibitors. Based on the natural product belactosin C, we synthesized two β-lactones featuring a dimethoxybenzyl moiety and either a methylpropyl (pseudo-isoleucin) or an isopropyl (pseudo-valine) P1 side chain. Although the two compounds differ only by one methyl group, the isoleucine analogue is six times more potent for β5i (IC50=14 nM) than the valine counterpart. Cell culture experiments demonstrate the cell-permeability of the compounds and X-ray crystallography data highlight them as minimal fragments that occupy primed and non-primed pockets of the active sites of the proteasome. Together, these results qualify β-lactones as a promising lead-structure motif for potent nonpeptidic proteasome inhibitors with diverse pharmaceutical applications.

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.

Ubiquitin-independent proteosomal degradation of myelin basic protein contributes to development of neurodegenerative autoimmunity

Recent findings indicate that the ubiquitin–proteasome system is involved in the pathogenesis of cancer as well as autoimmune and several neurodegenerative diseases, and is thus a target for novel therapeutics. One disease that is related to aberrant protein degradation is multiple sclerosis, an autoimmune disorder involving the processing and presentation of myelin autoantigens that leads to the destruction of axons. Here, we show that brain-derived proteasomes from SJL mice with experimental autoimmune encephalomyelitis (EAE) in an ubiquitin-independent manner generate significantly increased amounts of myelin basic protein peptides that induces cytotoxic lymphocytes to target mature oligodendrocytes ex vivo. Ten times enhanced release of immunogenic peptides by cerebral proteasomes from EAE-SJL mice is caused by a dramatic shift in the balance between constitutive and β1i^high immunoproteasomes in the CNS of SJL mice with EAE. We found that during EAE, β1i is increased in resident CNS cells, whereas β5i is imported by infiltrating lymphocytes through the blood–brain barrier. Peptidyl epoxyketone specifically inhibits brain-derived β1i^high immunoproteasomes in vitro (k_obs/[I] = 240 M⁻¹s⁻¹), and at a dose of 0.5 mg/kg, it ameliorates ongoing EAE in vivo. Therefore, our findings provide novel insights into myelin metabolism in pathophysiologic conditions and reveal that the β1i subunit of the immunoproteasome is a potential target to treat autoimmune neurologic diseases.—Belogurov Jr., A., Kuzina, E., Kudriaeva, A., Kononikhin, A., Kovalchuk, S., Surina, Y., Smirnov, I., Lomakin, Y., Bacheva, A., Stepanov, A., Karpova, Y., Lyupina, Y., Kharybin, O., Melamed, D., Ponomarenko, N., Sharova, N., Nikolaev, E., Gabibov, A. Ubiquitin-independent proteosomal degradation of myelin basic protein contributes to development of neurodegenerative autoimmunity.

Toward understanding induction of oxidative stress and apoptosis by proteasome inhibitors

SIGNIFICANCE

Proteasome inhibitors (PIs) are used in the clinic for the treatment of hematopoietic malignancies. PI inhibitors induce endoplasmatic reticulum (ER) stress and oxidative stress, disruption of signaling pathways, mitochondrial dysfunction, and, eventually, cell death by apoptosis. PIs designated as clinical candidates include natural product derivatives and compounds developed by rational design and feature a wide diversity of structural elements. The vast amount of literature on this topic underscores PIs significance in driving basic research alongside therapeutic benefit.

RECENT ADVANCES

Research in recent years has brought an in-depth insight into the molecular mechanisms of PI-induced apoptosis. However, there are some paradoxes and controversies in the literature. In this review, the advances and uncertainties, in particular on the time course events that make cells commit to apoptosis, are discussed. In addition, some mechanisms of evolved PI resistance are presented, and speculations on the difference in sensitivity between cell or tumor types are brought forward. The review concludes by giving an outlook of recent methods that may be employed to describe the system biology of how PIs impact cell survival decisions.

CRITICAL ISSUES

The biology of ER stress, reactive oxygen species (ROS) production, and apoptosis as induced by PIs is not well understood. Absorbed by the strong focus on PIs, one might overlook the importance of proteasome activity activators or modulators and the study of enzymatic pathways that lie up- or downstream from the proteasome function.

FUTURE DIRECTIONS

An increased understanding of the systems biology at mRNA and protein levels and the kinetics behind the interaction between PIs and cells is imperative. The design and synthesis of subunit specific inhibitors for each of the seven known proteasome activities and for the enzymes associated to proteasomes will aid in unraveling biology of the ubiquitin-proteasome system in relation to ER stress, ROS production, and apoptosis and will generate leads for therapeutic intervention.

Targeting the ubiquitin-proteasome system in heart disease: the basis for new therapeutic strategies

SIGNIFICANCE

Novel therapeutic strategies to treat heart failure are greatly needed. The ubiquitin-proteasome system (UPS) affects the structure and function of cardiac cells through targeted degradation of signaling and structural proteins. This review discusses both beneficial and detrimental consequences of modulating the UPS in the heart.

RECENT ADVANCES

Proteasome inhibitors were first used to test the role of the UPS in cardiac disease phenotypes, indicating therapeutic potential. In early cardiac remodeling and pathological hypertrophy with increased proteasome activities, proteasome inhibition prevented or restricted disease progression and contractile dysfunction. Conversely, enhancing proteasome activities by genetic manipulation, pharmacological intervention, or ischemic preconditioning also improved the outcome of cardiomyopathies and infarcted hearts with impaired cardiac and UPS function, which is, at least in part, caused by oxidative damage.

CRITICAL ISSUES

An understanding of the UPS status and the underlying mechanisms for its potential deregulation in cardiac disease is critical for targeted interventions. Several studies indicate that type and stage of cardiac disease influence the dynamics of UPS regulation in a nonlinear and multifactorial manner. Proteasome inhibitors targeting all proteasome complexes are associated with cardiotoxicity in humans. Furthermore, the type and dosage of proteasome inhibitor impact the pathogenesis in nonuniform ways.

FUTURE DIRECTIONS

Systematic analysis and targeting of individual UPS components with established and innovative tools will unravel and discriminate regulatory mechanisms that contribute to and protect against the progression of cardiac disease. Integrating this knowledge in drug design may reduce adverse effects on the heart as observed in patients treated with proteasome inhibitors against noncardiac diseases, especially cancer.

Harnessing proteasome dynamics and allostery in drug design

SIGNIFICANCE

The proteasome is the essential protease that is responsible for regulated cleavage of the bulk of intracellular proteins. Its central role in cellular physiology has been exploited in therapies against aggressive cancers where proteasome-specific competitive inhibitors that block proteasome active centers are very effectively used. However, drugs regulating this essential protease are likely to have broader clinical usefulness. The non-catalytic sites of the proteasome emerge as an attractive alternative target in search of highly specific and diverse proteasome regulators.

RECENT ADVANCES

Crystallographic models of the proteasome leave the false impression of fixed structures with minimal molecular dynamics lacking long-distance allosteric signaling. However, accumulating biochemical and structural observations strongly support the notion that the proteasome is regulated by precise allosteric interactions arising from protein dynamics, encouraging the active search for allosteric regulators. Here, we discuss properties of several promising compounds that affect substrate gating and processing in antechambers, and interactions of the catalytic core with regulatory proteins.

CRITICAL ISSUES

Given the structural complexity of proteasome assemblies, it is a painstaking process to better understand their allosteric regulation and molecular dynamics. Here, we discuss the challenges and achievements in this field. We place special emphasis on the role of atomic force microscopy imaging in probing the allostery and dynamics of the proteasome, and in dissecting the mechanisms involving small-molecule allosteric regulators.

FUTURE DIRECTIONS

New small-molecule allosteric regulators may become a next generation of drugs targeting the proteasome, which is critical to the development of new therapies in cancers and other diseases.

Subunit specific inhibitors of proteasomes and their potential for immunomodulation

Specialized variants of the constitutive 20S proteasome in the immune system like the immunoproteasomes and the thymoproteasome contain active site-bearing subunits which differ in their cleavage priorities and substrate binding pockets. The immunoproteasome plays a crucial role in antigen processing and for the differentiation of pro-inflammatory T helper cells which are involved in the pathogenesis of autoimmunity. Selective inhibitors of the immunoproteasome and constitutive proteasome have recently been generated which interfere with the development and progression of autoimmune diseases. Here we describe these inhibitors and their therapeutic potential as predicted from preclinical models.

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.

Selective immunoproteasome inhibitors with non-peptide scaffolds identified from structure-based virtual screening

As a major component of the crucial nonlysosomal protein degradation pathway in the cells, the proteasome has been implicated in many diseases such as Alzheimer's disease, Huntington's disease, inflammatory bowel diseases, autoimmune diseases, multiple myeloma (MM) and other cancers. There are two main proteasome subtypes: the constitutive proteasome which is expressed in all eukaryotic cells and the immunoproteasome which is expressed in immune cells and can be induced in other cell types. Majority of currently available proteasome inhibitors are peptide backbone-based, having short half-lives in the body. It is highly desirable to identify novel, immunoproteasome-selective inhibitors with non-peptide scaffolds for development of novel therapeutics. Through combined virtual screening and experimental studies targeting the immunoproteasome, we have identified a set of novel immunoproteasome inhibitors with diverse non-peptide scaffolds. Some of the identified inhibitors have significant selectivity for the immunoproteasome over the constitutive proteasome. Unlike most of the currently available proteasome inhibitors, these new inhibitors lacking electrophilic pharmacophores are not expected to form a covalent bond with proteasome after the binding. These non-peptide scaffolds may provide a new platform for future rational drug design and discovery targeting the immunoproteasome.

Inhibitors of the immunoproteasome: current status and future directions

The ubiquitin-proteasome system (UPS) plays a vital role in maintaining protein homeostasis and regulating numerous cellular processes. The proteasome, a multi-protease complex, is the key component of the UPS and has been validated as a therapeutic target by the FDA's approval of bortezomib and carfilzomib. These proteasome inhibitor drugs have substantially improved outcomes in patients with hematological malignancies and are currently being investigated for other types of cancer as well as several other diseases. These approved proteasome inhibitors target the catalytic activity of both the constitutive proteasome and the immunoproteasome indiscriminately, and their inhibitory effects on the constitutive proteasome in normal cells are believed to contribute to unwanted side effects. In addition, selective immunoproteasome inhibition has been proposed to have unique effects on other diseases, including those involving aberrant immune function. Initially recognized for its role in the adaptive immune response, the immunoproteasome is often upregulated in disease states such as inflammatory diseases and cancer, suggesting functions beyond antigen presentation. In an effort to explore the immunoproteasome as a potential therapeutic target in these diseases, the development of immunoproteasome-specific inhibitors has become the focus of recent studies. Owing to considerable efforts by both academic and industry groups, immunoproteasome-selective inhibitors have now been identified and tested against several disease models. These inhibitors also provide a valuable set of chemical tools for investigating the biological function of the immunoproteasome. In this review, we will focus on the recent efforts towards the development of immunoproteasome-selective inhibitors.

Oxathiazolones Selectively Inhibit the Human Immunoproteasome over the Constitutive Proteasome

Selective inhibitors for the human immunoproteasome LMP7 (β5i) subunit over the constitutive proteasome hold promise for the treatment of autoimmune and inflammatory diseases and hematologic malignancies. Here we report that oxathiazolones inhibit the immunoproteasome β5i with up to 4700-fold selectivity over the constitutive proteasome, are cell permeable, and inhibit proteasomes inside cells.

Activity-based imaging probes of the proteasome

Over the years, the proteasome has been extensively investigated due to its crucial roles in many important signaling pathways and its implications in diseases. Two proteasome inhibitors--bortezomib and carfilzomib--have received FDA approval for the treatment of multiple myeloma, thereby validating the proteasome as a chemotherapeutic target. As a result, further research efforts have been focused on dissecting the complex biology of the proteasome to gain the insight required for developing next-generation proteasome inhibitors. It is clear that chemical probes have made significant contributions to these efforts, mostly by functioning as inhibitors that selectively block the catalytic activity of proteasomes. Analogues of these inhibitors are now providing additional tools for visualization of catalytically active proteasome subunits, several of which allow real-time monitoring of proteasome activity in living cells as well as in in vivo settings. These imaging probes will provide powerful tools for assessing the efficacy of proteasome inhibitors in clinical settings. In this review, we will focus on the recent efforts towards developing imaging probes of proteasomes, including the latest developments in immunoproteasome-selective imaging probes.

Anti-leukemic activity and mechanisms underlying resistance to the novel immunoproteasome inhibitor PR-924

PR-924 is a novel prototypic immunoproteasome inhibitor bearing markedly enhanced specificity for the β5i immunoproteasome subunit, compared to the classical proteasome inhibitor bortezomib. Here, we assessed the growth inhibitory potential of PR-924 in three human hematologic malignancy cell lines (CCRF-CEM, THP1, and 8226) and their bortezomib-resistant sublines. Parental cells displayed equal sensitivity to PR-924 (IC₅₀: 1.5-2.8 μM), whereas their bortezomib-resistant tumor lines displayed a 10-12 fold cross-resistance to PR-924. However, PR-924 cross-resistance factors for bortezomib-resistant sublines were markedly lower compared to the resistance factors to bortezomib. Proteasome inhibition experiments confirmed that PR-924 specifically inhibited β5i activity, even far below concentrations that exerted anti-proliferative activity. We further determined whether PR-924 activity might be compromised by acquisition of drug resistance phenomena. Indeed, CEM cells rendered stepwise resistant to 20 μM PR-924 (CEM/PR20) displayed 13-fold PR-924-resistance and 10-fold cross-resistance to bortezomib. CEM/PR20 cells were devoid of mutations in the PSMB8 gene (encoding β5i), but acquired Met45Ile mutation in the PSMB5 gene (encoding constitutive β5), consistent with β5 mutations observed in bortezomib-resistant cells. Furthermore, compared to parental CEM cells, CEM/PR20 cells exhibited 2.5-fold upregulation of constitutive proteasome subunit expression, whereas immunoproteasome subunit expression was 2-fold decreased. In conclusion, PR-924 displayed potent anti-leukemic activity including toward bortezomib-resistant leukemia cells. Despite the specificity of PR-924 to the β5i immunoproteasome subunit, its anti-leukemic effect required concentrations that blocked both β5 and β5i subunits. This is underscored by the emergence of mutations in PSMB5 rather than in PSMB8.

Inhibition of the immunoproteasome ameliorates experimental autoimmune encephalomyelitis

Multiple sclerosis (MS) is a chronic demyelinating immune mediated disease of the central nervous system. The immunoproteasome is a distinct class of proteasomes found predominantly in monocytes and lymphocytes. Recently, we demonstrated a novel function of immunoproteasomes in cytokine production and T cell differentiation. In this study, we investigated the therapeutic efficacy of an inhibitor of the immunoproteasome (ONX 0914) in two different mouse models of MS. ONX 0914 attenuated disease progression after active and passive induction of experimental autoimmune encephalomyelitis (EAE), both in MOG_35–55 and PLP_139–151-induced EAE. Isolation of lymphocytes from the brain or spinal cord revealed a strong reduction of cytokine-producing CD4⁺ cells in ONX 0914 treated mice. Additionally, ONX 0914 treatment prevented disease exacerbation in a relapsing-remitting model. An analysis of draining lymph nodes after induction of EAE revealed that the differentiation to Th17 or Th1 cells was strongly impaired in ONX 0914 treated mice. These results implicate the immunoproteasome in the development of EAE and suggest that immunoproteasome inhibitors are promising drugs for the treatment of MS.

Bortezomib

The ubiquitin-mediated degradation of proteins in numerous cellular processes, such as turnover and quality control of proteins, cell cycle and apoptosis, transcription and cell signaling, immune response and antigen presentation, and inflammation and development makes the ubiquitin-proteosome systems a very interesting target for various therapeutic interventions. Proteosome inhibitors were first synthesized as tools to probe the function and specificity of this particle's proteolytic activities. Most synthetic inhibitors rely on a peptide base, which mimics a protein substrate, attached at a COOH terminal "warhead." Notable warheads include boronic acids, such as bortezomib and epoxy ketones, such as carfilzomib. A variety of natural products also inhibit the proteosome that are not peptide-based, most notably lactacystin, that is related to NPI-0052, or salinosporamide A, another inhibitor in clinical trials. The possibility that proteosome inhibitors could be drug candidates was considered after studies showed that they induced apoptosis in leukemic cell lines. The first proteasome inhibitor in clinical application, bortezomib showed activity in non-small-cell lung and androgen-independent prostate carcinoma, as well as MM and mantle cell and follicular non-Hodgkin's lymphoma. It is now licensed for the treatment of newly diagnosed as well as relapsed/progressive MM and has had a major impact on the improvement in the treatment of MM in the last few years.

The unique functions of tissue-specific proteasomes

The 26S proteasome is the main protease in eukaryotes. Proteolysis occurs within the cylindrical 20S proteasome that is constitutively expressed in most tissues. However, three tissue-specific versions of the 20S proteasome have been discovered to date. The immunoproteasome is optimized to process antigens and it directs the differentiation of T helper (Th) cells. The thymoproteasome is selectively expressed in cortical epithelial cells of the thymus where it plays an essential role in the positive selection of T lymphocytes. Finally, the spermatoproteasome is found in the testes where it is required during spermatogenesis. Here, we outline how tissue-specific proteasomes adapt to functional needs in their respective tissues and how their selective inhibition may be used to interfere with autoimmune diseases and cancer.

PSMB9 codon 60 polymorphisms have no impact on the activity of the immunoproteasome catalytic subunit B1i expressed in multiple types of solid cancer

The proteasome is a key regulator of cellular protein homeostasis and is a clinically validated anticancer target. The immunoproteasome, a subtype of proteasome expressed mainly in hematopoietic cells, was initially recognized for its role in antigen presentation during the immune response. Recently, the immunoproteasome has been implicated in several disease conditions including cancer and autoimmune disorders, but many of the factors contributing to these pathological processes remain unknown. In particular, the codon 60 polymorphism of the PSMB9 gene encoding the β1i immunoproteasome catalytic subunit has been investigated in the context of a variety of diseases. Despite this, previous studies have so far reported inconsistent findings regarding the impact of this polymorphism on proteasome activity. Thus, we set out to investigate the impact of the PSMB9 codon 60 polymorphism on the expression and activity of the β1i immunoproteasome subunit in a panel of human cancer cell lines. The β1i-selective fluorogenic substrate Acetyl-Pro-Ala-Leu-7-amino-4-methylcoumarin was used to specifically measure β1i catalytic activity. Our results indicate that the codon 60 Arg/His polymorphism does not significantly alter the expression and activity of β1i among the cell lines tested. Additionally, we also examined the expression of β1i in clinical samples from colon and pancreatic cancer patients. Our immunohistochemical analyses showed that ∼70% of clinical colon cancer samples and ∼53% of pancreatic cancer samples have detectable β1i expression. Taken together, our results indicate that the β1i subunit of the immunoproteasome is frequently expressed in colon and pancreatic cancers but that the codon 60 genetic variants of β1i display similar catalytic activities and are unlikely to contribute to the significant inter-cell-line and inter-individual variabilities in the immunoproteasome activity.