Immunoproteasome β5i-Selective Dipeptidomimetic Inhibitors

Targeting Microglial Immunoproteasome: A Novel Approach in Neuroinflammatory-Related Disorders

It is widely acknowledged that the aging process is linked to the accumulation of damaged and misfolded proteins. This phenomenon is accompanied by a decrease in proteasome (c20S) activity, concomitant with an increase in immunoproteasome (i20S) activity. These changes can be attributed, in part, to the chronic neuroinflammation that occurs in brain tissues. Neuroinflammation is a complex process characterized by the activation of immune cells in the central nervous system (CNS) in response to injury, infection, and other pathological stimuli. In certain cases, this immune response becomes chronic, contributing to the pathogenesis of various neurological disorders, including chronic pain, Alzheimer's disease, Parkinson's disease, brain traumatic injury, and others. Microglia, the resident immune cells in the brain, play a crucial role in the neuroinflammatory response. Recent research has highlighted the involvement of i20S in promoting neuroinflammation, increased activity of which may lead to the presentation of self-antigens, triggering an autoimmune response against the CNS, exacerbating inflammation, and contributing to neurodegeneration. Furthermore, since i20S plays a role in breaking down accumulated proteins during inflammation within the cell body, any disruption in its activity could lead to a prolonged state of inflammation and subsequent cell death. Given the pivotal role of i20S in neuroinflammation, targeting this proteasome subtype has emerged as a potential therapeutic approach for managing neuroinflammatory diseases. This review delves into the mechanisms of neuroinflammation and microglia activation, exploring the potential of i20S inhibitors as a promising therapeutic strategy for managing neuroinflammatory disorders.

Discovery of Highly Selective Inhibitors of the Human Constitutive Proteasome β5c Chymotryptic Subunit

We describe our discovery and development of potent and highly selective inhibitors of human constitutive proteasome chymotryptic activity (β5c). Structure-activity relationship studies of the novel class of inhibitors focused on optimization of N-cap, C-cap, and side chain of the chemophore asparagine. Compound 32 is the most potent and selective β5c inhibitor in this study. A docking study provides a structure rationale for potency and selectivity. Kinetic studies show a reversible and noncompetitive inhibition mechanism. It enters the cells to engage the proteasome target, potently and selectively kills multiple myeloma cells, and does so by synergizing with a β5i-selective inhibitor.

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.

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.

Structure-Based Optimization and Discovery of M3258, a Specific Inhibitor of the Immunoproteasome Subunit LMP7 (β5i)

Proteasomes are broadly expressed key components of the ubiquitin-dependent protein degradation pathway containing catalytically active subunits (β1, β2, and β5). LMP7 (β5i) is a subunit of the immunoproteasome, an inducible isoform that is predominantly expressed in hematopoietic cells. Clinically effective pan-proteasome inhibitors for the treatment of multiple myeloma (MM) nonselectively target LMP7 and other subunits of the constitutive proteasome and immunoproteasome with comparable potency, which can limit the therapeutic applicability of these drugs. Here, we describe the discovery and structure-based hit optimization of novel amido boronic acids, which selectively inhibit LMP7 while sparing all other subunits. The exploitation of structural differences between the proteasome subunits culminated in the identification of the highly potent, exquisitely selective, and orally available LMP7 inhibitor 50 (M3258). Based on the strong antitumor activity observed with M3258 in MM models and a favorable preclinical data package, a phase I clinical trial was initiated in relapsed/refractory MM patients.

Immunoproteasome and Non-Covalent Inhibition: Exploration by Advanced Molecular Dynamics and Docking Methods

The selective inhibition of immunoproteasome is a valuable strategy to treat autoimmune, inflammatory diseases, and hematologic malignancies. Recently, a new series of amide derivatives as non-covalent inhibitors of the β1i subunit with K_i values in the low/submicromolar ranges have been identified. Here, we investigated the binding mechanism of the most potent and selective inhibitor, N-benzyl-2-(2-oxopyridin-1(2H)-yl)propanamide (1), to elucidate the steps from the ligand entrance into the binding pocket to the ligand-induced conformational changes. We carried out a total of 400 ns of MD-binding analyses, followed by 200 ns of plain MD. The trajectories clustering allowed identifying three representative poses evidencing new key interactions with Phe31 and Lys33 together in a flipped orientation of a representative pose. Further, Binding Pose MetaDynamics (BPMD) studies were performed to evaluate the binding stability, comparing 1 with four other inhibitors of the β1i subunit: N-benzyl-2-(2-oxopyridin-1(2H)-yl)acetamide (2), N-cyclohexyl-3-(2-oxopyridin-1(2H)-yl)propenamide (3), N-butyl-3-(2-oxopyridin-1(2H)-yl)propanamide (4), and (S)-2-(2-oxopyridin-1(2H)-yl)-N,4-diphenylbutanamide (5). The obtained results in terms of free binding energy were consistent with the experimental values of inhibition, confirming 1 as a lead compound of this series. The adopted methods provided a full dynamic description of the binding events, and the information obtained could be exploited for the rational design of new and more active inhibitors.

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.

Noncytotoxic Inhibition of the Immunoproteasome Regulates Human Immune Cells In Vitro and Suppresses Cutaneous Inflammation in the Mouse

Inhibitors of the immunoproteasome (i-20S) have shown promise in mouse models of autoimmune diseases and allograft rejection. In this study, we used a novel inhibitor of the immunoproteasome, PKS3053, that is reversible, noncovalent, tight-binding, and highly selective for the β5i subunit of the i-20S to evaluate the role that i-20S plays in regulating immune responses in vitro and in vivo. In contrast to irreversible, less-selective inhibitors, PKS3053 did not kill any of the primary human cell types tested, including plasmacytoid dendritic cells, conventional dendritic cells, macrophages, and T cells, all of which expressed genes encoding both the constitutive proteasome (c-20S) and i-20S. PKS3053 reduced TLR-dependent activation of plasmacytoid dendritic cells, decreasing their maturation and IFN-α response and reducing their ability to activate allogenic T cells. In addition, PKS3053 reduced T cell proliferation directly and inhibited TLR-mediated activation of conventional dendritic cells and macrophages. In a mouse model of skin injury that shares some features of cutaneous lupus erythematosus, blocking i-20S decreased inflammation, cellular infiltration, and tissue damage. We conclude that the immunoproteasome is involved in the activation of innate and adaptive immune cells, that their activation can be suppressed with an i-20S inhibitor without killing them, and that selective inhibition of β5i holds promise as a potential therapy for inflammatory skin diseases such as psoriasis, cutaneous lupus erythematosus, and systemic sclerosis.

Mitigated viral myocarditis in A/J mice by the immunoproteasome inhibitor ONX 0914 depends on inhibition of systemic inflammatory responses in CoxsackievirusB3 infection

A preclinical model of troponin I-induced myocarditis (AM) revealed a prominent role of the immunoproteasome (ip), the main immune cell-resident proteasome isoform, in heart-directed autoimmunity. Viral infection of the heart is a known trigger of cardiac autoimmunity, with the ip enhancing systemic inflammatory responses after infection with a cardiotropic coxsackievirusB3 (CV). Here, we used ip-deficient A/J-LMP7-/- mice to investigate the role of ip-mediated effects on adaptive immunity in CV-triggered myocarditis and found no alteration of the inflammatory heart tissue damage or cardiac function in comparison to wild-type controls. Aiming to define the impact of the systemic inflammatory storm under the control of ip proteolysis during CV infection, we targeted the ip in A/J mice with the inhibitor ONX 0914 after the first cycle of infection, when systemic inflammation has set in, well before cardiac inflammation. During established acute myocarditis, the ONX 0914 treatment group had the same reduction in cardiac output as the controls, with inflammatory responses in heart tissue being unaffected by the compound. Based on these findings and with regard to the known anti-inflammatory role of ONX 0914 in CV infection, we conclude that the efficacy of ip inhibitors for CV-triggered myocarditis in A/J mice relies on their immunomodulatory effects on the systemic inflammatory reaction.

Structure-Activity Relationships of Noncovalent Immunoproteasome β5i-Selective Dipeptides

The immunoproteasome (i-20S) has emerged as a therapeutic target for autoimmune and inflammatory disorders and hematological malignancies. Inhibition of the chymotryptic β5i subunit of i-20S inhibits T cell activation, B cell proliferation, and dendritic cell differentiation in vitro and suppresses immune responses in animal models of autoimmune disorders and allograft rejection. However, cytotoxicity to immune cells has accompanied the use of covalently reactive β5i inhibitors, whose activity against the constitutive proteasome (c-20S) is cumulative with the time of exposure. Herein, we report a structure-activity relationship study of a class of noncovalent proteasome inhibitors with picomolar potencies and 1000-fold selectivity for i-20S over c-20S. Furthermore, these inhibitors are specific for β5i over the other five active subunits of i-20S and c-20S, providing useful tools to study the functions of β5i in immune responses. The potency of these compounds in inhibiting human T cell activation suggests that they may have therapeutic potential.

ONX 0914 Lacks Selectivity for the Cardiac Immunoproteasome in CoxsackievirusB3 Myocarditis of NMRI Mice and Promotes Virus-Mediated Tissue Damage

: Inhibition of proteasome function by small molecules is highly efficacious in cancer treatment. Other than non-selective proteasome inhibitors, immunoproteasome-specific inhibitors allow for specific targeting of the proteasome in immune cells and the profound anti-inflammatory potential of such compounds revealed implications for inflammatory scenarios. For pathogen-triggered inflammation, however, the efficacy of immunoproteasome inhibitors is controversial. In this study, we investigated how ONX 0914, an immunoproteasome-selective inhibitor, influences CoxsackievirusB3 infection in NMRI mice, resulting in the development of acute and chronic myocarditis, which is accompanied by formation of the immunoproteasome in heart tissue. In groups in which ONX 0914 treatment was initiated once viral cytotoxicity had emerged in the heart, ONX 0914 had no anti-inflammatory effect in the acute or chronic stages. ONX 0914 treatment initiated prior to infection, however, increased viral cytotoxicity in cardiomyocytes, promoting infiltration of myeloid immune cells into the heart. At this stage, ONX 0914 completely inhibited the β5 subunit of the standard cardiac proteasome and less efficiently blocked its immunoproteasome counterpart LMP7. In conclusion, ONX 0914 unselectively perturbs cardiac proteasome function in viral myocarditis of NMRI mice, reduces the capacity of the host to control the viral burden and promotes cardiac inflammation.

[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.

A focused structure-activity relationship study of psoralen-based immunoproteasome inhibitors

The immunoproteasome is a multicatalytic protease that is predominantly expressed in cells of hematopoietic origin. Its elevated expression has been associated with autoimmune diseases, various types of cancer, and inflammatory diseases. The development of immunoproteasome-selective inhibitors with non-peptidic scaffolds remains a challenging task. Here, we describe a focused series of psoralen-based inhibitors of the β5i subunit of the immunoproteasome with different substituents placed at position 4'. The most promising compound was further evaluated through changes at position 3 of the psoralen ring. Despite a small decrease in the inhibitory potency in comparison with the parent compound, we were able to improve the selectivity against other subunits of both the immunoproteasome and the constitutive proteasome. The most potent compounds discriminated between both proteasome types in cell lysates and also showed a decrease in cytokine secretion in peripheral blood mononuclear cells.

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.

Improvement of Asparagine Ethylenediamines as Anti-malarial Plasmodium-Selective Proteasome Inhibitors

The Plasmodium proteasome (Pf20S) emerged as a target for antimalarials. Pf20S inhibitors are active at multiple stages of the parasite life cycle and synergize with artemisinins, suggesting that Pf20S inhibitors have potential to be prophylactic, therapeutic, and transmission blocking as well as are useful for combination therapy. We recently reported asparagine ethylenediamines (AsnEDAs) as immunoproteasome inhibitors and modified AsnEDAs as selective Pf20S inhibitors. Here, we report further a structure-activity relationship study of AsnEDAs for selective inhibition of Pf20S over human proteasomes. Additionally, we show new mutation that conferred resistance to AsnEDAs and collateral sensitivity to an inhibitor of the Pf20S β2 subunit, the same as previously identified resistant mutation. This resistance could be overcome through the use of the structure-guided inhibitor design. Collateral sensitivity to inhibitors among respective proteasome subunits underscores the potential value of treating malaria with combinations of inhibitors of different proteasome subunits to minimize the emergence of drug resistance.

Antimalarial proteasome inhibitor reveals collateral sensitivity from intersubunit interactions and fitness cost of resistance

We describe noncovalent, reversible asparagine ethylenediamine (AsnEDA) inhibitors of the Plasmodium falciparum proteasome (Pf20S) β5 subunit that spare all active subunits of human constitutive and immuno-proteasomes. The compounds are active against erythrocytic, sexual, and liver-stage parasites, against parasites resistant to current antimalarials, and against P. falciparum strains from patients in Africa. The β5 inhibitors synergize with a β2 inhibitor in vitro and in mice and with artemisinin. P. falciparum selected for resistance to an AsnEDA β5 inhibitor surprisingly harbored a point mutation in the noncatalytic β6 subunit. The β6 mutant was resistant to the species-selective Pf20S β5 inhibitor but remained sensitive to the species-nonselective β5 inhibitors bortezomib and carfilzomib. Moreover, resistance to the Pf20S β5 inhibitor was accompanied by increased sensitivity to a Pf20S β2 inhibitor. Finally, the β5 inhibitor-resistant mutant had a fitness cost that was exacerbated by irradiation. Thus, used in combination, multistage-active inhibitors of the Pf20S β5 and β2 subunits afford synergistic antimalarial activity with a potential to delay the emergence of resistance to artemisinins and each other.

A patent review of immunoproteasome inhibitors

INTRODUCTION

The ubiquitin-proteasome system is responsible for maintaining protein homeostasis and regulating a variety of cellular processes. The constitutive proteasome is expressed in all cells while the immunoproteasome (IP) is predominantly found in cells of hematopoietic origin. In other cells, the expression of IP can be induced under the influence of cytokines released by T cells during acute immune and stress responses. Inhibitors of IP are of significant interest, because it is expected that selective inhibition of the IP would cause fewer adverse effects.

AREAS COVERED

There is a considerable interest on patenting IP-specific inhibitors. Relevant patents and patent applications disclosing IP inhibitors are summarized and divided into two parts according to the chemical characteristics of compounds. We also briefly report on the biochemical methods used in the patents to profile the characteristics of IP inhibitors.

EXPERT OPINION

Several selective inhibitors of IP with a promising ability to address autoimmune and inflammatory diseases are being developed. Peptidic compounds are prevalent and the most advanced IP-selective compounds to date, ONX-0914 and KZR-616, are tripeptide epoxyketone-based molecules. However, some patents disclose that IP-selective inhibition is possible with compounds possessing non-peptidic scaffolds indicating countless possibilities to address inhibition of IP in the future.

Blockade of the malignant phenotype by β-subunit selective noncovalent inhibition of immuno- and constitutive proteasomes

A structure-based virtual screening of over 400,000 small molecules against the constitutive proteasome activity followed by in vitro assays led to the discovery of a family of proteasome inhibitors with a sulfonyl piperazine scaffold. Some members of this family of small non-peptidic inhibitors were found to act selectively on the β2 trypsin-like catalytic site with a preference for the immunoproteasome β2i over the constitutive proteasome β2c, while some act on the β5 site and post-acid site β1 of both, the immunoproteasome and the constitutive proteasome. Anti-proliferative and anti-invasive effects on tumor cells were investigated and observed for two compounds. We report novel chemical inhibitors able to interfere with the three types of active centers of both, the immuno- and constitutive proteasomes. Identifying and analyzing a novel scaffold with decorations able to shift the binders’ active site selectivity is essential to design a future generation of proteasome inhibitors able to distinguish the immunoproteasome from the constitutive proteasome.

Structure-based design of human immuno- and constitutive proteasomes inhibitors

Starting from the X-ray structure of our previous tripeptidic linear mimics of TMC-95A in complex with yeast 20S proteasome, we introduced new structural features to induce a differential inhibition between human constitutive and immunoproteasome 20S particles. Libraries of 24 tripeptidic and 6 dipeptidic derivatives were synthesized. The optimized preparation of 3-hydroxyoxindolyl alanine residues from tryptophan and their incorporation in peptides were described. Several potent inhibitors of human constitutive proteasome and immunoproteasome acting at the nanomolar level (IC₅₀ = 7.1 nM against the chymotrypsin-like activity for the best inhibitor) were obtained. A cytotoxic effect at the submicromolar level was observed against 6 human cancer cell lines.

Amelioration of autoimmunity with an inhibitor selectively targeting all active centres of the immunoproteasome

BACKGROUND AND PURPOSE

Multicatalytic endopeptidase complex-like-1 (β2i), low molecular mass polypeptide (LMP) 2 (β1i) and LMP7 (β5i) are the proteolytically active subunits of the immunoproteasome, a special type of proteasome mainly expressed in haematopoietic cells. Targeting LMP7 has been shown to be therapeutically effective in preclinical models of autoimmune diseases. In this study, we investigated the selectivity and biological activity of LU-005i, a recently described inhibitor of the immunoproteasome.

EXPERIMENTAL APPROACH

The specificity of LU-005i and other immunoproteasome-selective inhibitors was characterized using fluorogenic peptide substrates. The effect of proteasome inhibition on cytokine release was investigated in endotoxin-stimulated mouse splenocytes or human peripheral blood mononuclear cells (PBMCs). The effect of proteasome inhibition on inflammatory bowel disease in the dextran sulfate sodium (DSS)-induced colitis model was assessed by measuring weight loss and colon length.

KEY RESULTS

LU-005i is the first human and mouse immunoproteasome-selective inhibitor that targets all three proteolytically active immunoproteasome subunits. LU-005i inhibited cytokine secretion from endotoxin-stimulated mouse splenocytes or human PBMCs. Furthermore, differentiation of naïve T helper cells to T helper 17 cells was impaired in the presence of LU-005i. Additionally, LU-005i ameliorated DSS-induced colitis.

CONCLUSION AND IMPLICATIONS

This study with a novel pan-immunoproteasome inhibitor substantiates that the immunoproteasome is a promising drug target for the treatment of inflammatory diseases and that exclusive inhibition of LMP7 is not necessary for therapeutic effectiveness. Our results will promote the design of new generations of immunoproteasome inhibitors with optimal therapeutic efficacy for clinical use in the treatment of autoimmunity and cancer.

Structure of human immunoproteasome with a reversible and noncompetitive inhibitor that selectively inhibits activated lymphocytes

Proteasome inhibitors benefit patients with multiple myeloma and B cell-dependent autoimmune disorders but exert toxicity from inhibition of proteasomes in other cells. Toxicity should be minimized by reversible inhibition of the immunoproteasome β5i subunit while sparing the constitutive β5c subunit. Here we report β5i-selective inhibition by asparagine-ethylenediamine (AsnEDA)-based compounds and present the high-resolution cryo-EM structural analysis of the human immunoproteasome. Despite inhibiting noncompetitively, an AsnEDA inhibitor binds the active site. Hydrophobic interactions are accompanied by hydrogen bonding with β5i and β6 subunits. The inhibitors are far more cytotoxic for myeloma and lymphoma cell lines than for hepatocarcinoma or non-activated lymphocytes. They block human B-cell proliferation and promote apoptotic cell death selectively in antibody-secreting B cells, and to a lesser extent in activated human T cells. Reversible, β5i-selective inhibitors may be useful for treatment of diseases involving activated or neoplastic B cells or activated T cells.

Structural Basis for the Species-Selective Binding of N,C-Capped Dipeptides to the Mycobacterium tuberculosis Proteasome

The Mycobacterium tuberculosis (Mtb) 20S proteasome is vital for the pathogen to survive under nitrosative stress in vitro and to persist in mice. To qualify for drug development, inhibitors targeting Mtb 20S must spare both the human constitutive proteasome (c-20S) and immunoproteasome (i-20S). We recently reported members of a family of noncovalently binding dipeptide proteasome inhibitors that are highly potent and selective for Mtb 20S over human c-20S and i-20S. To understand the structural basis of their potency and selectivity, we have studied the structure-activity relationship of six derivatives and solved their cocrystal structures with Mtb 20S. The dipeptide inhibitors form an antiparallel β-strand with the active site β-strands. Selectivity is conferred by several features of Mtb 20S relative to its mouse counterparts, including a larger S1 pocket, additional hydrogen bonds in the S3 pocket, and hydrophobic interactions in the S4 pocket. Serine-20 and glutamine-22 of Mtb 20S interact with the dipeptides and confer Mtb-specific inhibition over c-20S and i-20S. The Mtb 20S and mammalian i-20S have a serine-27 that interacts strongly with the dipeptides, potentially explaining the higher inhibitory activity of the dipeptides toward i-20S over c-20S. This detailed structural knowledge will aid in optimizing the dipeptides as anti-tuberculosis drugs.

Brief treatment with a highly selective immunoproteasome inhibitor promotes long-term cardiac allograft acceptance in mice

Constitutive proteasomes (c-20S) are ubiquitously expressed cellular proteases that degrade polyubiquitinated proteins and regulate cell functions. An isoform of proteasome, the immunoproteasome (i-20S), is highly expressed in human T cells, dendritic cells (DCs), and B cells, suggesting that it could be a potential target for inflammatory diseases, including those involving autoimmunity and alloimmunity. Here, we describe DPLG3, a rationally designed, noncovalent inhibitor of the immunoproteasome chymotryptic subunit β5i that has thousands-fold selectivity over constitutive β5c. DPLG3 suppressed cytokine release from blood mononuclear cells and the activation of DCs and T cells, diminished accumulation of effector T cells, promoted expression of exhaustion and coinhibitory markers on T cells, and synergized with CTLA4-Ig to promote long-term acceptance of cardiac allografts across a major histocompatibility barrier. These findings demonstrate the potential value of using brief posttransplant immunoproteasome inhibition to entrain a long-term response favorable to allograft survival as part of an immunomodulatory regimen that is neither broadly immunosuppressive nor toxic.