Recent Advances and Future Perspectives of Noncompetitive Proteasome Inhibitors

The proteasome regulates intracellular processes, maintains biological homeostasis, and has shown great significance in the study of various diseases, such as neurodegenerative diseases, immune-related diseases, and cancer, especially in hematologic malignancies such as multiple myeloma (MM) and mantle cell lymphoma (MCL). All clinically used proteasome inhibitors bind to the active site of the proteasome and thus exhibit a competitive mechanism. The development of resistance and intolerance during treatment drives the search for inhibitors with different mechanisms of action. In this review, we provide an overview of noncompetitive proteasome inhibitors, including their mechanisms of action, function, possible applications, and their advantages and disadvantages compared with competitive inhibitors.

An Analysis Regarding the Association Between Proteasome (PSM) and Hepatocellular Carcinoma (HCC)

Background

The Proteasome (PSM) is a large multi-catalytic protease complex consisting of a 20S core particle and a 19S regulatory particle whose main function is to accept and degrade ubiquitinated substrates, are now considered as one of the potential regulators of tumor proliferation, and stemness maintenance. However, to date, studies on the relationship between PSM and hepatocellular carcinoma (HCC) are limited.

Methods

This study used a bioinformatics approach combining validation experiments to investigate the biological mechanisms that may be related with PSM. A series of experiments in vivo and in vitro were performed to explore the function of the 26S proteasome non-ATPase regulatory subunit 13 (PSMD13) in HCC.

Results

HCC patients can be divided into two clusters. Cluster 1 (C1) patients having a significantly worse prognosis than Cluster (C2). Two subtypes had significant differences in proliferation-related signaling. In particular, the frequency of TP53 mutation was significantly higher in C1 than in C2. In addition, PSM-associated genes were highly consistent with the expression of DNA repair-related signatures, suggesting a potential link between PSM and genomic instability. We also found that downregulation of PSMD13 expression significantly inhibited stemness of tumor cells and impaired the Epithelial mesenchymal transition (EMT) process. Finally, the correlation between the PSMD13 and Ki67 was found to be strong.

Conclusion

PSM is a valid predictor of prognosis and therapeutic response in patients with HCC disease. Furthermore, PSMD13 may be a potential therapeutic target.

Integration of Mass Spectrometry Data for Structural Biology

Mass spectrometry (MS) is increasingly being used to probe the structure and dynamics of proteins and the complexes they form with other macromolecules. There are now several specialized MS methods, each with unique sample preparation, data acquisition, and data processing protocols. Collectively, these methods are referred to as structural MS and include cross-linking, hydrogen-deuterium exchange, hydroxyl radical footprinting, native, ion mobility, and top-down MS. Each of these provides a unique type of structural information, ranging from composition and stoichiometry through to residue level proximity and solvent accessibility. Structural MS has proved particularly beneficial in studying protein classes for which analysis by classic structural biology techniques proves challenging such as glycosylated or intrinsically disordered proteins. To capture the structural details for a particular system, especially larger multiprotein complexes, more than one structural MS method with other structural and biophysical techniques is often required. Key to integrating these diverse data are computational strategies and software solutions to facilitate this process. We provide a background to the structural MS methods and briefly summarize other structural methods and how these are combined with MS. We then describe current state of the art approaches for the integration of structural MS data for structural biology. We quantify how often these methods are used together and provide examples where such combinations have been fruitful. To illustrate the power of integrative approaches, we discuss progress in solving the structures of the proteasome and the nuclear pore complex. We also discuss how information from structural MS, particularly pertaining to protein dynamics, is not currently utilized in integrative workflows and how such information can provide a more accurate picture of the systems studied. We conclude by discussing new developments in the MS and computational fields that will further enable in-cell structural studies.

Conformational maps of human 20S proteasomes reveal PA28- and immuno-dependent inter-ring crosstalks

Hydrogen-Deuterium eXchange coupled to Mass Spectrometry (HDX-MS) is now common practice in structural biology. However, it is most of the time applied to rather small oligomeric complexes. Here, we report on the use of HDX-MS to investigate conformational differences between the human standard 20S (std20S) and immuno 20S (i20s) proteasomes alone or in complex with PA28αβ or PA28γ activators. Their solvent accessibility is analyzed through a dedicated bioinformatic pipeline including stringent statistical analysis and 3D visualization. These data confirm the existence of allosteric differences between the std20S and i20S at the surface of the α-ring triggered from inside the catalytic β-ring. Additionally, binding of the PA28 regulators to the 20S proteasomes modify solvent accessibility due to conformational changes of the β-rings. This work is not only a proof-of-concept that HDX-MS can be used to get structural insights on large multi-protein complexes in solution, it also demonstrates that the binding of the std20S or i20S subtype to any of its PA28 activator triggers allosteric changes that are specific to this 20S/PA28 pair.

Ubiquitination in rheumatoid arthritis

Rheumatoid arthritis is a chronic, inflammatory joint disease leading to inflammation of synovial membrane that lines the joints. This inflammation further progresses and results in destruction of joints and surrounding cartilages. The underlying factors can be oxidative stress, pro-inflammatory mediators, imbalance and attenuation between various enzymes and proteins (like nuclear factor erythroid 2 related factor 2/Nrf2 and ubiquitin). Protein degradation pathways comprises of lysosomal, proteasomal pathway, and autophagosome (that are carried out in mammalian cells) are regulated through ubiquitin. Ubiquitin proteasomal system is dominating pathway for carrying out non-lysosomal proteolysis of intracellularly proteins. Fundamental processes including cell cycle progression, process of division, apoptosis, modulation of immune responses and cell trafficking are regulated by process of ubiquitination. Ubiquitin proteasomal pathway (UPP) includes ubiquitin moieties which are covalently attached to proteins and guides them proteasome for degradation. Misfolded, oxidized and damaged proteins which are responsible for critical processes, are major targets of degradation process. Any alteration in this system leads to dysregulated cellular homeostasis; progressively leading to numerous diseases including rheumatoid arthritis. Factors including TAK1, TRAF6 undergo are required for the progression of disease and thus contributes towards pathology of inflammatory disorders such as rheumatoid arthritis. This review will include all linked aspects which contribute its major role in rheumatoid arthritis.

The activation of the chymotrypsin-like activity of the proteasome is regulated by soluble adenyl cyclase/cAMP/protein kinase A pathway and required for human sperm capacitation

One of the first events of mammalian sperm capacitation is the activation of the soluble adenyl cyclase/cAMP/protein kinase A (SACY/cAMP/PKA) pathway. Here, we evaluated whether the increase in PKA activity at the onset of human sperm capacitation is responsible for the activation of the sperm proteasome and whether this activation is required for capacitation progress. Viable human sperm were incubated with inhibitors of the SACY/cAMP/PKA pathway. The chymotrypsin-like activity of the sperm proteasome was evaluated using a fluorogenic substrate. Sperm capacitation status was evaluated using the chlortetracycline assay and tyrosine phosphorylation. To determine whether proteasomal subunits were phosphorylated by PKA, the proteasome was immunoprecipitated and tested on a western blot using an antibody against phosphorylated PKA substrates. Immunofluorescence microscopy analysis and co-immunoprecipitation (IPP) were used to investigate an association between the catalytic subunit alpha of PKA (PKA-Cα) and the proteasome. The chymotrypsin-like activity of the sperm proteasome significantly increased after 5 min of capacitation (P < 0.001) and remained high for the remaining incubation time. Treatment with H89, KT5720 or KH7 significantly decreased the chymotrypsin-like activity of the proteasome (P < 0.001). IPP experiments indicated that PKA inhibition significantly modified phosphorylation of proteasome subunits. In addition, PKA-Cα colocalized with the proteasome in the equatorial segment and in the connecting piece, and co-immunoprecipitated with the proteasome. This is the first demonstration of sperm proteasome activity being directly regulated by SACY/PKA-Cα. This novel discovery extends our current knowledge of sperm physiology and may be used to manage sperm capacitation during assisted reproductive technology procedures.

Genome-wide suppressor screen identifies USP35/USP38 as therapeutic candidates for ciliopathies

The ciliopathies are a group of phenotypically overlapping disorders caused by structural or functional defects in the primary cilium. Although disruption of numerous signaling pathways and cellular trafficking events have been implicated in ciliary pathology, treatment options for affected individuals remain limited. Here, we performed a genome-wide RNAi (RNA interference) screen to identify genetic suppressors of BBS4, one of the genes mutated in Bardet-Biedl syndrome (BBS). We discovered 10 genes that, when silenced, ameliorate BBS4-dependent pathology. One of these encodes USP35, a negative regulator of the ubiquitin proteasome system, suggesting that inhibition of a deubiquitinase, and subsequent facilitation of the clearance of signaling components, might ameliorate BBS-relevant phenotypes. Testing of this hypothesis in transient and stable zebrafish genetic models showed this posit to be true; suppression or ablation of usp35 ameliorated hallmark ciliopathy defects including impaired convergent extension (CE), renal tubule convolution, and retinal degeneration with concomitant clearance of effectors such as β-catenin and rhodopsin. Together, our findings reinforce a direct link between proteasome-dependent degradation and ciliopathies and suggest that augmentation of this system might offer a rational path to novel therapeutic modalities.

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.

Proteasome Activation as a New Therapeutic Approach To Target Proteotoxic Disorders

Proteasomes are multienzyme complexes that maintain protein homeostasis (proteostasis) and important cellular functions through the degradation of misfolded, redundant, and damaged proteins. It is well established that aging is associated with the accumulation of damaged and misfolded proteins. This phenomenon is paralleled by declined proteasome activity. When the accumulation of redundant proteins exceed degradation, undesirable signaling and/or aggregation occurs and are the hallmarks of neurodegenerative diseases and many cancers. Thus, increasing proteasome activity has been recognized as a new approach to delay the onset or ameliorate the symptoms of neurodegenerative and other proteotoxic disorders. Enhancement of proteasome activity has many therapeutic potentials but is still a relatively unexplored field. In this perspective, we review current approaches, genetic manipulation, posttranslational modification, and small molecule proteasome agonists used to increase proteasome activity, challenges facing the field, and applications beyond aging and neurodegenerative diseases.

Functioning of Proteasomes in Lymphogenic Metastasizing of Non-Small-Cell Lung Cancer

Chymotrypsin- and caspase-like activities and expression of the total proteasome pool subunits (α1α2α3α5α6α7) were studied in patients with non-small-cell lung cancer. Proteasome activities were higher in the primary tumors and lymphogenic metastases than in corresponding adjacent lung tissue. The content of α1α2α3α5α6α7 subunits decreased in metastases and remained unchanged in primary tumor tissue. The development of metastatic process in non-small-cell lung cancer was associated with nonlinear changes in proteasome activities and content in primary tumor tissue and lymphogenic metastases.

O-GlcNAcylation Signal Mediates Proteasome Inhibitor Resistance in Cancer Cells by Stabilizing NRF1

Cancer cells often heavily depend on the ubiquitin-proteasome system (UPS) for their growth and survival. Irrespective of their strong dependence on the proteasome activity, cancer cells, except for multiple myeloma, are mostly resistant to proteasome inhibitors.

Cancer cells often heavily depend on the ubiquitin-proteasome system (UPS) for their growth and survival. Irrespective of their strong dependence on the proteasome activity, cancer cells, except for multiple myeloma, are mostly resistant to proteasome inhibitors. A major cause of this resistance is the proteasome bounce-back response mediated by NRF1, a transcription factor that coordinately activates proteasome subunit genes. To identify new targets for efficient suppression of UPS, we explored, using immunoprecipitation and mass spectrometry, the possible existence of nuclear proteins that cooperate with NRF1 and identified O-linked N-acetylglucosamine transferase (OGT) and host cell factor C1 (HCF-1) as two proteins capable of forming a complex with NRF1. O-GlcNAcylation catalyzed by OGT was essential for NRF1 stabilization and consequent upregulation of proteasome subunit genes. Meta-analysis of breast and colorectal cancers revealed positive correlations in the relative protein abundance of OGT and proteasome subunits. OGT inhibition was effective at sensitizing cancer cells to a proteasome inhibitor both in culture cells and a xenograft mouse model. Since active O-GlcNAcylation is a feature of cancer metabolism, our study has clarified a novel linkage between cancer metabolism and UPS function and added a new regulatory axis to the regulation of the proteasome activity.

Proteasome dysregulation in human cancer: implications for clinical therapies

Cancer cells show heightened dependency on the proteasome for their survival, growth, and spread. Proteasome dysregulation is therefore commonly selected in favor of the development of many types of cancer. The vast abnormalities in a cancer cell, on top of the complexity of the proteasome itself, have enabled a plethora of mechanisms gearing the proteasome to the oncogenic process. Here, we use selected examples to highlight some general mechanisms underlying proteasome dysregulation in cancer, including copy number variations, transcriptional control, epigenetic regulation, and post-translational modifications. Research in this field has greatly advanced our understanding of proteasome regulation and will shed new light on proteasome-based combination therapies for cancer treatment.

Phosphatase UBLCP1 controls proteasome assembly

Ubiquitin-like domain-containing C-terminal domain phosphatase 1 (UBLCP1), an FCP/SCP phosphatase family member, was identified as the first proteasome phosphatase. UBLCP1 binds to proteasome subunit Rpn1 and dephosphorylates the proteasome in vitro. However, it is still unclear which proteasome subunit(s) are the bona fide substrate(s) of UBLCP1 and the precise mechanism for proteasome regulation remains elusive. Here, we show that UBLCP1 selectively binds to the 19S regulatory particle (RP) through its interaction with Rpn1, but not the 20S core particle (CP) or the 26S proteasome holoenzyme. In the RP, UBLCP1 dephosphorylates the subunit Rpt1, impairs its ATPase activity, and consequently disrupts the 26S proteasome assembly, yet it has no effects on the RP assembly from precursor complexes. The Rpn1-binding and phosphatase activities of UBLCP1 are essential for its function on Rpt1 dephosphorylation and proteasome activity both in vivo and in vitro. Our study establishes the essential role of the UBLCP1/Rpn1/Rpt1 complex in regulating proteasome assembly.

Protein Quality Control by Molecular Chaperones in Neurodegeneration

Protein homeostasis (proteostasis) requires the timely degradation of misfolded proteins and their aggregates by protein quality control (PQC), of which molecular chaperones are an essential component. Compared with other cell types, PQC in neurons is particularly challenging because they have a unique cellular structure with long extensions. Making it worse, neurons are postmitotic, i.e., cannot dilute toxic substances by division, and, thus, are highly sensitive to misfolded proteins, especially as they age. Failure in PQC is often associated with neurodegenerative diseases, such as Huntington's disease (HD), Alzheimer's disease (AD), Parkinson's disease (PD), and prion disease. In fact, many neurodegenerative diseases are considered to be protein misfolding disorders. To prevent the accumulation of disease-causing aggregates, neurons utilize a repertoire of chaperones that recognize misfolded proteins through exposed hydrophobic surfaces and assist their refolding. If such an effort fails, chaperones can facilitate the degradation of terminally misfolded proteins through either the ubiquitin (Ub)-proteasome system (UPS) or the autophagy-lysosome system (hereafter autophagy). If soluble, the substrates associated with chaperones, such as Hsp70, are ubiquitinated by Ub ligases and degraded through the proteasome complex. Some misfolded proteins carrying the KFERQ motif are recognized by the chaperone Hsc70 and delivered to the lysosomal lumen through a process called, chaperone-mediated autophagy (CMA). Aggregation-prone misfolded proteins that remain unprocessed are directed to macroautophagy in which cargoes are collected by adaptors, such as p62/SQSTM-1/Sequestosome-1, and delivered to the autophagosome for lysosomal degradation. The aggregates that have survived all these refolding/degradative processes can still be directly dissolved, i.e., disaggregated by chaperones. Studies have shown that molecular chaperones alleviate the pathogenic symptoms by neurodegeneration-causing protein aggregates. Chaperone-inducing drugs and anti-aggregation drugs are actively exploited for beneficial effects on symptoms of disease. Here, we discuss how chaperones protect misfolded proteins from aggregation and mediate the degradation of terminally misfolded proteins in collaboration with cellular degradative machinery. The topics also include therapeutic approaches to improve the expression and turnover of molecular chaperones and to develop anti-aggregation drugs.

Spatiotemporal progression of ubiquitin-proteasome system inhibition after status epilepticus suggests protective adaptation against hippocampal injury

BACKGROUND

The ubiquitin-proteasome-system (UPS) is the major intracellular pathway leading to the degradation of unwanted and/or misfolded soluble proteins. This includes proteins regulating cellular survival, synaptic plasticity and neurotransmitter signaling; processes controlling excitability thresholds that are altered by epileptogenic insults. Dysfunction of the UPS has been reported to occur in a brain region- and cell-specific manner and contribute to disease progression in acute and chronic brain diseases. Prolonged seizures, status epilepticus, may alter UPS function but there has been no systematic attempt to map when and where this occurs in vivo or to determine the consequences of proteasome inhibition on seizure-induced brain injury.

METHOD

To determine whether seizures lead to an impairment of the UPS, we used a mouse model of status epilepticus whereby seizures are triggered by an intra-amygdala injection of kainic acid. Status epilepticus in this model causes cell death in selected brain areas, in particular the ipsilateral CA3 subfield of the hippocampus, and the development of epilepsy after a short latent period. To monitor seizure-induced dysfunction of the UPS we used a UPS inhibition reporter mouse expressing the ubiquitin fusion degradation substrate ubiquitin^G76V-green fluorescent protein. Treatment with the specific proteasome inhibitor epoxomicin was used to establish the impact of proteasome inhibition on seizure-induced pathology.

RESULTS AND CONCLUSIONS

Our studies show that status epilepticus induced by intra-amygdala kainic acid causes select spatio-temporal UPS inhibition which is most evident in damage-resistant regions of the hippocampus, including CA1 pyramidal and dentate granule neurons then appears later in astrocytes. In support of this exerting a beneficial effect, injection of mice with the proteasome inhibitor epoxomicin protected the normally vulnerable hippocampal CA3 subfield from seizure-induced neuronal death in the model. These studies reveal brain region- and cell-specific UPS impairment occurs after seizures and suggest UPS inhibition can protect against seizure-induced brain damage. Identifying networks or pathways regulated through the proteasome after seizures may yield novel target genes for the treatment of seizure-induced cell death and possibly epilepsy.

APEH Inhibition Affects Osteosarcoma Cell Viability via Downregulation of the Proteasome

The proteasome is a multienzymatic complex that controls the half-life of the majority of intracellular proteins, including those involved in apoptosis and cell-cycle progression. Recently, proteasome inhibition has been shown to be an effective anticancer strategy, although its downregulation is often accompanied by severe undesired side effects. We previously reported that the inhibition of acylpeptide hydrolase (APEH) by the peptide SsCEI 4 can significantly affect the proteasome activity in A375 melanoma or Caco-2 adenocarcinoma cell lines, thus shedding new light on therapeutic strategies based on downstream regulation of proteasome functions. In this work, we investigated the functional correlation between APEH and proteasome in a panel of cancer cell lines, and evaluated the cell proliferation upon SsCEI 4-treatments. Results revealed that SsCEI 4 triggered a proliferative arrest specifically in osteosarcoma U2OS cells via downregulation of the APEH–proteasome system, with the accumulation of the typical hallmarks of proteasome: NF-κB, p21^Waf1, and polyubiquitinylated proteins. We found that the SsCEI 4 anti-proliferative effect involved a senescence-like growth arrest without noticeable cytotoxicity. These findings represent an important step toward understanding the mechanism(s) underlying the APEH-mediated downregulation of proteasome in order to design new molecules able to efficiently regulate the proteasome system for alternative therapeutic strategies.

TRIM21: a cytosolic Fc receptor with broad antibody isotype specificity

Antibodies are key molecules in the fight against infections. Although previously thought to mediate protection solely in the extracellular environment, recent research has revealed that antibody-mediated protection extends to the cytosolic compartment of cells. This postentry viral defense mechanism requires binding of the antibody to a cytosolic Fc receptor named tripartite motif containing 21 (TRIM21). In contrast to other Fc receptors, TRIM21 shows remarkably broad isotype specificity as it does not only bind IgG but also IgM and IgA. When viral pathogens coated with these antibody isotypes enter the cytosol, TRIM21 is rapidly recruited and efficient neutralization occurs before the virus has had the time to replicate. In addition, inflammatory signaling is induced. As such, TRIM21 acts as a cytosolic sensor that engages antibodies that have failed to protect against infection in the extracellular environment. Here, we summarize our current understanding of how TRIM21 orchestrates humoral immunity in the cytosolic environment.

Bufalin derivative BF211 inhibits proteasome activity in human lung cancer cells in vitro by inhibiting β1 subunit expression and disrupting proteasome assembly

AIM

Bufalin is one of the active components in the traditional Chinese medicine ChanSu that is used to treat arrhythmia, inflammation and cancer. BF211 is a bufalin derivative with stronger cytotoxic activity in cancer cells. The aim of this study was to identify the putative target proteins of BF211 and the signaling pathways in cancer cells.

METHODS

A549 human lung cancer cells were treated with BF211. A SILAC-based proteomic analysis was used to detect the protein expression profiles of BF211-treated A549 cells. Cellular proteasome activities were examined using fluorogenic peptide substrates, and the binding affinities of BF211 to recombinant proteasome subunit proteins were evaluated using the Biacore assay. The expression levels of proteasome subunits were determined using RT-PCR and Western blotting, and the levels of the integral 26S proteasome were evaluated using native PAGE analysis.

RESULTS

The proteomic analysis revealed that 1282 proteins were differentially expressed in BF211-treated A549 cells, and the putative target proteins of BF211 were associated with various cellular functions, including transcription, translation, mRNA splicing, ribosomal protein synthesis and proteasome function. In A549 cells, BF211 (5, 10, and 20 nmol/L) dose-dependently inhibited the enzymatic activities of proteasome. But BF211 displayed a moderate affinity in binding to proteasome β1 subunit and no binding affinity to the β2 and β5 subunits. Moreover, BF211 (0.1, 1, and 10 nmol/L) did not inhibit the proteasome activities in the cell lysates. BF211 (5, 10, and 20 nmol/L) significantly decreased the expression level of proteasome β1 subunit and the levels of integral 26S proteasome in A549 cells. Similarly, knockdown of the β1 subunit with siRNA in A549 cells significantly decreased integral 26S proteasome and proteasome activity.

CONCLUSION

BF211 inhibits proteasome activity in A549 cells by decreasing β1 subunit expression and disrupting proteasome assembly.

The ubiquitin proteasomal system: a potential target for the management of Alzheimer's disease

The cellular quality control system degrades abnormal or misfolded proteins and consists of three different mechanisms: the ubiquitin proteasomal system (UPS), autophagy and molecular chaperones. Any disturbance in this system causes proteins to accumulate, resulting in neurodegenerative diseases such as amyotrophic lateral sclerosis, Alzheimer's disease (AD), Parkinson's disease, Huntington's disease and prion or polyglutamine diseases. Alzheimer's disease is currently one of the most common age-related neurodegenerative diseases. However, its exact cause and pathogenesis are unknown. Currently approved medications for AD provide symptomatic relief; however, they fail to influence disease progression. Moreover, the components of the cellular quality control system represent an important focus for the development of targeted and potent therapies for managing AD. This review aims to evaluate whether existing evidence supports the hypothesis that UPS impairment causes the early pathogenesis of neurodegenerative disorders. The first part presents basic information about the UPS and its molecular components. The next part explains how the UPS is involved in neurodegenerative disorders. Finally, we emphasize how the UPS influences the management of AD. This review may help in the design of future UPS-related therapies for AD.

Glucocorticoids and Skeletal Muscle

Glucocorticoids are known to regulate protein metabolism in skeletal muscle, producing a catabolic effect that is opposite that of insulin. In many catabolic diseases, such as sepsis, starvation, and cancer cachexia, endogenous glucocorticoids are elevated contributing to the loss of muscle mass and function. Further, exogenous glucocorticoids are often given acutely and chronically to treat inflammatory conditions such as asthma, chronic obstructive pulmonary disease, and rheumatoid arthritis, resulting in muscle atrophy. This chapter will detail the nature of glucocorticoid-induced muscle atrophy and discuss the mechanisms thought to be responsible for the catabolic effects of glucocorticoids on muscle.

Modulatory effects of bortezomib on host immune cell functions

Bortezomib is an inhibitor of the ubiquitin-proteasome proteolytic pathway responsible for intracellular protein turnover. Cellular proteins controlled by this pathway represent a diverse group of potential therapeutic targets, particularly in cancer cells, which exploit this proteasomal pathway to promote their growth and diminish apoptosis. Along with inhibiting the proteasome and thus sensitizing tumor cells to apoptosis, bortezomib may also have multiple effects on the host immune responses. This review summarizes the effects that bortezomib may play on immune cell subsets in various disease states in modifying lymphocyte receptors, ligands, the expression of various cytokines and chemokines and their downstream signaling. We also propose steps that can be taken to refine combinatorial strategies that include bortezomib to improve current immunotherapeutic approaches.

Effective delivery of a rationally designed intracellular peptide drug with gold nanoparticle-peptide hybrids

A novel gold nanoparticle-peptide hybrid strategy was developed to intracellularly deliver a potent PKCδ inhibitor peptide for the treatment of acute lung injury. The gold nanoparticle-peptide hybrids showed good stability with high uptake, and demonstrated in vitro and in vivo efficacy. Our formulation strategy shows great promise in intracellular delivery of peptides.

Dynamic aberrant NF-κB spurs tumorigenesis: a new model encompassing the microenvironment

Recently it was discovered that a transient activation of transcription factor NF-κB can give cells properties essential for invasiveness and cancer initiating potential. In contrast, most oncogenes to date were characterized on the basis of mutations or by their constitutive overexpression. Study of NF-κB actually leads to a far more dynamic perspective on cancer: tumors caused by diverse oncogenes apparently evolve into cancer after loss of feedback regulation for NF-κB. This event alters the cellular phenotype and the expression of hormonal mediators, modifying signals between diverse cell types in a tissue. The result is a disruption of stem cell hierarchy in the tissue, and pervasive changes in the microenvironment and immune response to the malignant cells.

A variant of PSMD6 is associated with the therapeutic efficacy of oral antidiabetic drugs in Chinese type 2 diabetes patients

The PSMD6 variant rs831571 has been identified as a susceptibility locus for type 2 diabetes mellitus (T2DM). This study aimed to investigate the association of this variant with therapeutic effects of oral antidiabetic drugs in Chinese T2DM patients. 209 newly diagnosed T2DM patients were randomly assigned to treatment with repaglinide or rosiglitazone for 48 weeks, and the therapeutic effects were compared. In the rosiglitazone cohort, rs831571 showed significant associations with fasting plasma glucose (FPG), 2-h glucose and decrement of glycated haemoglobin (HbA1c) levels after 24 weeks of treatment (P = 0.0368, 0.0468 and 0.0247, respectively). The C allele was significantly associated with a better attainment of FPG at 24 and 32 weeks (P = 0.0172 and 0.0257, respectively). Survival analyses showed CC homozygotes were more likely to attain a standard FPG level (P = 0.0654). In the repaglinide cohort, rs831571 was significantly associated with decreased HbA1c levels after 24 weeks of treatment, the homeostatic model assessment of insulin resistance and fasting insulin level after 48 weeks of treatment with repaglinide (P = 0.0096, 0235 and 0.0212, respectively). In conclusion, we observed that the PSMD6 variant rs831571 might be associated with the therapeutic effects of rosiglitazone and repaglinide in Chinese T2DM patients. However, these findings need to be confirmed in the future.

Adenylate kinase 2 deficiency limits survival and regulates various genes during larval stages of Drosophila melanogaster

Adenylate kinase isozyme 2 (AK2) is located in mitochondrial intermembrane space and regulates energy metabolism by reversibly converting ATP and AMP to 2 ADPs. We previously demonstrated that disruption of the Drosophila melanogaster AK2 gene (Dak2) resulted in growth arrest during the larval stage and subsequent death. Two other groups found that human AK2 mutations cause reticular dysgenesis, a form of severe combined immunodeficiency (SCID) that is associated with severe hematopoietic defects and sensorineural deafness. However, the mechanisms underlying differential outcomes of AK2 deficiency in Drosophila and human systems remain unknown. In this study, effects of tissue-specific inactivation of the Dak2 gene on Drosophila development were analyzed using RNAi-mediated gene knockdown. In addition, to investigate the roles of AK2 in the regulation of gene expression during development, microarray analysis was performed using RNA from first and second instar larvae of Dak2-deficient mutant and wild-type D. melanogaster. Knockdown of Dak2 in all germ layers caused cessation of growth and subsequent death of flies. Microarray analysis revealed that Dak2 deficiency downregulates various genes, particularly those involved in the proteasomal function and in mitochondrial translation machinery. These data indicate that adenine nucleotide interconversion by Dak2 is crucial for developmental processes of Drosophila melanogaster.

Profiling human protein degradome delineates cellular responses to proteasomal inhibition and reveals a feedback mechanism in regulating proteasome homeostasis

Global change in protein turnover (protein degradome) constitutes a central part of cellular responses to intrinsic or extrinsic stimuli. However, profiling protein degradome remains technically challenging. Recently, inhibition of the proteasome, e.g., by using bortezomib (BTZ), has emerged as a major chemotherapeutic strategy for treating multiple myeloma and other human malignancies, but systematic understanding of the mechanisms for BTZ drug action and tumor drug resistance is yet to be achieved. Here we developed and applied a dual-fluorescence-based Protein Turnover Assay (ProTA) to quantitatively profile global changes in human protein degradome upon BTZ-induced proteasomal inhibition. ProTA and subsequent network analyses delineate potential molecular basis for BTZ action and tumor drug resistance in BTZ chemotherapy. Finally, combined use of BTZ with drugs targeting the ProTA-identified key genes or pathways in BTZ action reduced BTZ resistance in multiple myeloma cells. Remarkably, BTZ stabilizes proteasome subunit PSMC1 and proteasome assembly factor PSMD10, suggesting a previously under-appreciated mechanism for regulating proteasome homeostasis. Therefore, ProTA is a novel tool for profiling human protein degradome to elucidate potential mechanisms of drug action and resistance, which might facilitate therapeutic development targeting proteostasis to treat human disorders.

Melatonin as a proteasome inhibitor. Is there any clinical evidence?

Proteasome inhibitors and melatonin are both intimately involved in the regulation of major signal transduction proteins including p53, cyclin p27, transcription factor NF-κB, apoptotic factors Bax and Bim, caspase 3, caspase 9, anti-apoptotic factor Bcl-2, TRAIL, NRF2 and transcription factor beta-catenin. The fact that these factors are shared targets of the proteasome inhibitor bortezomib and melatonin suggests the working hypothesis that melatonin is a proteasome inhibitor. Supporting this hypothesis is the fact that melatonin shares with bortezomib a selective pro-apoptotic action in cancer cells. Furthermore, both bortezomib and melatonin increase the sensitivity of human glioma cells to TRAIL-induced apoptosis. Direct evidence for melatonin inhibition of the proteasome was recently found in human renal cancer cells. We raise the issue whether melatonin should be investigated in combination with proteasome inhibitors to reduce toxicity, to reduce drug resistance, and to enhance efficacy. This may be particularly valid for hematological malignancies in which proteasome inhibitors have been shown to be useful. Further studies are necessary to determine whether the actions of melatonin on cellular signaling pathways are due to a direct inhibitory effect on the catalytic core of the proteasome, due to an inhibitory action on the regulatory particle of the proteasome, or due to an indirect effect of melatonin on phosphorylation of signal transducing factors.

Melatonin and ubiquitin: what's the connection?

Melatonin has been widely studied for its role in photoperiodism in seasonal breeders; it is also a potent antioxidant. Ubiquitin, a protein also widespread in living cells, contributes to many cellular events, although the most well known is that of tagging proteins for destruction by the proteasome. Herein, we suggest a model in which melatonin interacts with the ubiquitin-proteasome system to regulate a variety of seemingly unrelated processes. Ubiquitin, for example, is a major regulator of central activity of thyroid hormone type 2 deiodinase; the subsequent regulation of T3 may be central to the melatonin-induced changes in seasonal reproduction and seasonal changes in metabolism. Both melatonin and ubiquitin also have important roles in protecting cells from oxidative stress. We discuss the interaction of melatonin and the ubiquitin-proteasome system in oxidative stress through regulation of the ubiquitin-activating enzyme, E1. Previous reports have shown that glutathiolation of this enzyme protects proteins from unnecessary degradation. In addition, evidence is discussed concerning the interaction of ubiquitin and melatonin in activation of the transcription factor NF-κB as well as modulating cellular levels of numerous signal transducing factors including the tumor suppressor, p53. Some of the actions of melatonin on the regulatory particle of the proteasome appear to be related to its inhibition of the calcium-dependent calmodulin kinase II, an enzyme which reportedly copurifies with proteasomes. Many of the actions of melatonin on signal transduction are similar to those of a proteasome inhibitor. While these actions of melatonin could be explained by a direct inhibitory action on the catalytic core particle of the proteasome, this has not been experimentally verified. If our hypothesis of melatonin as a general inhibitor of the ubiquitin-proteasome system is confirmed, it is predicted that more examples of this interaction will be demonstrated in a variety of tissues in which ubiquitin and melatonin co-exist. Furthermore, the hypothesis of melatonin as an inhibitor of the ubiquitin-proteasome system will be a very useful model for clinical testing of melatonin.

Proteasome activation delays aging in vitro and in vivo

Aging is a natural biological process that is characterized by a progressive accumulation of macromolecular damage. In the proteome, aging is accompanied by decreased protein homeostasis and function of the major cellular proteolytic systems, leading to the accumulation of unfolded, misfolded, or aggregated proteins. In particular, the proteasome is responsible for the removal of normal as well as damaged or misfolded proteins. Extensive work during the past several years has clearly demonstrated that proteasome activation by either genetic means or use of compounds significantly retards aging. Importantly, this represents a common feature across evolution, thereby suggesting proteasome activation to be an evolutionarily conserved mechanism of aging and longevity regulation. This review article reports on the means of function of these proteasome activators and how they regulate aging in various species.

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.

Organizing principles of mammalian nonsense-mediated mRNA decay

Cells use messenger RNAs (mRNAs) to ensure the accurate dissemination of genetic information encoded by DNA. Given that mRNAs largely direct the synthesis of a critical effector of cellular phenotype, i.e., proteins, tight regulation of both the quality and quantity of mRNA is a prerequisite for effective cellular homeostasis. Here, we review nonsense-mediated mRNA decay (NMD), which is the best-characterized posttranscriptional quality control mechanism that cells have evolved in their cytoplasm to ensure transcriptome fidelity. We use protein quality control as a conceptual framework to organize what is known about NMD, highlighting overarching similarities between these two polymer quality control pathways, where the protein quality control and NMD pathways intersect, and how protein quality control can suggest new avenues for research into mRNA quality control.

Nuclear import factor Srp1 and its associated protein Sts1 couple ribosome-bound nascent polypeptides to proteasomes for cotranslational degradation

Cotranslational protein degradation plays an important role in protein quality control and proteostasis. Although ubiquitylation has been suggested to signal cotranslational degradation of nascent polypeptides, cotranslational ubiquitylation occurs at a low level, suggesting the existence of an alternative route for delivery of nascent polypeptides to the proteasome. Here we report that the nuclear import factor Srp1 (also known as importin α or karyopherin α) is required for ubiquitin-independent cotranslational degradation of the transcription factor Rpn4. We further demonstrate that cotranslational protein degradation is generally impaired in the srp1-49 mutant. Srp1 binds nascent polypeptides emerging from the ribosome. The association of proteasomes with polysomes is weakened in srp1-49. The interaction between Srp1 and the proteasome is mediated by Sts1, a multicopy suppressor of srp1-49. The srp1-49 and sts1-2 mutants are hypersensitive to stressors that promote protein misfolding, underscoring the physiological function of Srp1 and Sts1 in degradation of misfolded nascent polypeptides. This study unveils a previously unknown role for Srp1 and Sts1 in cotranslational protein degradation and suggests a novel model whereby Srp1 and Sts1 cooperate to couple proteasomes to ribosome-bound nascent polypeptides.

Role of the ubiquitin-proteasome system in brain ischemia: friend or foe?

The ubiquitin-proteasome system (UPS) is a catalytic machinery that targets numerous cellular proteins for degradation, thus being essential to control a wide range of basic cellular processes and cell survival. Degradation of intracellular proteins via the UPS is a tightly regulated process initiated by tagging a target protein with a specific ubiquitin chain. Neurons are particularly vulnerable to any change in protein composition, and therefore the UPS is a key regulator of neuronal physiology. Alterations in UPS activity may induce pathological responses, ultimately leading to neuronal cell death. Brain ischemia triggers a complex series of biochemical and molecular mechanisms, such as an inflammatory response, an exacerbated production of misfolded and oxidized proteins, due to oxidative stress, and the breakdown of cellular integrity mainly mediated by excitotoxic glutamatergic signaling. Brain ischemia also damages protein degradation pathways which, together with the overproduction of damaged proteins and consequent upregulation of ubiquitin-conjugated proteins, contribute to the accumulation of ubiquitin-containing proteinaceous deposits. Despite recent advances, the factors leading to deposition of such aggregates after cerebral ischemic injury remain poorly understood. This review discusses the current knowledge on the role of the UPS in brain function and the molecular mechanisms contributing to UPS dysfunction in brain ischemia with consequent accumulation of ubiquitin-containing proteins. Chemical inhibitors of the proteasome and small molecule inhibitors of deubiquitinating enzymes, which promote the degradation of proteins by the proteasome, were both shown to provide neuroprotection in brain ischemia, and this apparent contradiction is also discussed in this review.

Assembly of the 20S proteasome

The proteasome is a cellular protease responsible for the selective degradation of the majority of the intracellular proteome. It recognizes, unfolds, and cleaves proteins that are destined for removal, usually by prior attachment to polymers of ubiquitin. This macromolecular machine is composed of two subcomplexes, the 19S regulatory particle (RP) and the 20S core particle (CP), which together contain at least 33 different and precisely positioned subunits. How these subunits assemble into functional complexes is an area of active exploration. Here we describe the current status of studies on the assembly of the 20S proteasome (CP). The 28-subunit CP is found in all three domains of life and its cylindrical stack of four heptameric rings is well conserved. Though several CP subunits possess self-assembly properties, a consistent theme in recent years has been the need for dedicated assembly chaperones that promote on-pathway assembly. To date, a minimum of three accessory factors have been implicated in aiding the construction of the 20S proteasome. These chaperones interact with different assembling proteasomal precursors and usher subunits into specific slots in the growing structure. This review will focus largely on chaperone-dependent CP assembly and its regulation. This article is part of a Special Issue entitled: Ubiquitin-Proteasome System. Guest Editors: Thomas Sommer and Dieter H. Wolf.

Microarray analysis of isolated human islet transcriptome in type 2 diabetes and the role of the ubiquitin-proteasome system in pancreatic beta cell dysfunction

To shed light on islet cell molecular phenotype in human type 2 diabetes (T2D), we studied the transcriptome of non-diabetic (ND) and T2D islets to then focus on the ubiquitin-proteasome system (UPS), the major protein degradation pathway. We assessed gene expression, amount of ubiquitinated proteins, proteasome activity, and the effects of proteasome inhibition and prolonged exposure to palmitate. Microarray analysis identified more than one thousand genes differently expressed in T2D islets, involved in many structures and functions, with consistent alterations of the UPS. Quantitative RT-PCR demonstrated downregulation of selected UPS genes in T2D islets and beta cell fractions, with greater ubiquitin accumulation and reduced proteasome activity. Chemically induced reduction of proteasome activity was associated with lower glucose-stimulated insulin secretion, which was partly reproduced by palmitate exposure. These results show the presence of many changes in islet transcriptome in T2D islets and underline the importance of the association between UPS alterations and beta cell dysfunction in human T2D.

AAA ATPase p97/VCP is essential for TRIM21-mediated virus neutralization

Tripartite motif-containing 21 (TRIM21) is a cytosolic IgG receptor that mediates intracellular virus neutralization by antibody. TRIM21 targets virions for destruction in the proteasome, but it is unclear how a substrate as large as a viral capsid is degraded. Here, we identify the ATPase p97/valosin-containing protein (VCP), an enzyme with segregase and unfoldase activity, as a key player in this process. Depletion or catalytic inhibition of VCP prevents capsid degradation and reduces neutralization. VCP is required concurrently with the proteasome, as addition of inhibitor after proteasomal degradation has no effect. Moreover, our results suggest that it is the challenging nature of virus as a substrate that necessitates involvement of VCP, since intracellularly expressed IgG Fc is degraded in a VCP-independent manner. These results implicate VCP as an important host factor in antiviral immunity.

Inhibitor mediated protein degradation

The discovery of drugs that cause the degradation of their target proteins has been largely serendipitous. Here we report that the tert-butyl carbamate-protected arginine (Boc(3)Arg) moiety provides a general strategy for the design of degradation-inducing inhibitors. The covalent inactivators ethacrynic acid and thiobenzofurazan cause the specific degradation of glutathione-S-transferase when linked to Boc(3)Arg. Similarly, the degradation of dihydrofolate reductase is induced when cells are treated with the noncovalent inhibitor trimethoprim linked to Boc(3)Arg. Degradation is rapid and robust, with 30%-80% of these abundant target proteins consumed within 1.3-5 hr. The proteasome is required for Boc(3)Arg-mediated degradation, but ATP is not necessary and the ubiquitin pathways do not appear to be involved. These results suggest that the Boc(3)Arg moiety may provide a general strategy to construct inhibitors that induce targeted protein degradation.

[Proteasome activity in tumors of female reproductive system]

Proteasomes (multiproteinase protein complexes) are known to play an important role in cancer pathogenesis, however, few information about their activity in human tumor tissues is available so far. We studied chymotrypsin-like activity of proteasomes in tissues of breast cancer (BC) and endometrial cancer (EC). The chymotrypsin-like total proteasome activity and the 20S and 26S proteasome activity in malignant tissues were shown to be significantly higher in malignant tumors than in normal tissues. No increase in proteasome activity was registered with larger tumor size in both BC and EC, whereas proteasome activity was changed with respect to the extent of tumor involvement. In breast cancer tissues, significant reductions in the total and the 26S proteaome activities were observed in tumors with regional lymph node metastases as compared to tumors without metastases. In endometrial cancer tissues, the total proteasome activity and the 20S and 26S proteasome activities were increased as the depth of myometrial invasion. The data obtained indicate that the proteasome acyivity is significantly changed in the process of cancerogenesis and further study is needed to develop new additional prognostic criteria and effective anti-tumor agents in molecular-directed therapy.

Involvement of the Nrf2-proteasome pathway in the endoplasmic reticulum stress response in pancreatic β-cells

The ubiquitin-proteasome system plays a central role in protein quality control through endoplasmic reticulum (ER)-associated degradation (ERAD) of unfolded and misfolded proteins. NF-E2-related factor 2 (Nrf2) is a transcription factor that controls the expression of an array of phase II detoxification and antioxidant genes. Nrf2 signaling has additionally been shown to upregulate the expression of the proteasome catalytic subunits in several cell types. Here, we investigated the role of Nrf2 in tunicamycin-induced ER stress using a murine insulinoma β-cell line, βTC-6. shRNA-mediated silencing of Nrf2 expression in βTC-6 cells significantly increased tunicamycin-induced cytotoxicity, elevated the expression of the pro-apoptotic ER stress marker Chop10, and inhibited tunicamycin-inducible expression of the proteasomal catalytic subunits Psmb5 and Psmb6. The effects of 3H-1,2-dithiole-3-thione (D3T), a small molecule Nrf2 activator, on ER stress were also examined in βTC-6 cells. D3T pretreatment reduced tunicamycin cytotoxicity and attenuated the tunicamycin-inducible Chop10 and protein kinase RNA-activated-like ER kinase (Perk). The protective effect of D3T was shown to be associated with increased ERAD. D3T increased the expression of Psmb5 and Psmb6 and elevated chymotrypsin-like peptidase activity; proteasome inhibitor treatment blocked D3T effects on tunicamycin cytotoxicity and ER stress marker changes. Similarly, silencing of Nrf2 abolished the protective effect of D3T against ER stress. These results indicate that the Nrf2 pathway contributes to the ER stress response in pancreatic β-cells by enhancing proteasome-mediated ERAD.

Protein kinase cδ in apoptosis: a brief overview

Protein kinase C-delta (PKCδ), a member of the lipid-regulated serine/threonine PKC family, has been implicated in a wide range of important cellular processes. In the past decade, the critical role of PKCδ in the regulation of both intrinsic and extrinsic apoptosis pathways has been widely explored. In most cases, over-expression or activation of PKCδ results in the induction of apoptosis. The phosphorylations and multiple cell organelle translocations of PKCδ initiate apoptosis by targeting multiple downstream effectors. During apoptosis, PKCδ is proteolytically cleaved by caspase-3 to generate a constitutively activated catalytic fragment, which amplifies apoptosis cascades in nucleus and mitochondria. However, PKCδ also exerts its anti-apoptotic and pro-survival roles in some cases. Therefore, the complicated role of PKCδ in apoptosis appears to be stimulus and cell type dependent. This review is mainly focused on how PKCδ gets activated in diverse ways in response to apoptotic signals and how PKCδ targets different downstream regulators to sponsor or restrain apoptosis induction.

Association of growth factors, HIF-1 and NF-κB expression with proteasomes in endometrial cancer

Insulin-like growth factors (IGFs), vascular endothelial growth factor (VEGF), hypoxia-inducible factor-1 (HIF-1), and nuclear factor kappa-B (NF-κB) are known to play an important role in endometrial cancer pathogenesis. However, the proteolytic regulation of these factors is still poorly understood. We studied the correlation between chymotrypsin-like activity of proteasomes and IGF-I, IGF-II, VEGF, HIF-1, and NF-κB levels in endometrial cancer tissues. It was shown that the total activity of proteasomes and the activity of the 20S and 26S proteasomes in malignant tumors were significantly higher than those observed in the normal endometrium. Negative relationships between the proteasome activity and IGF-I, HIF-1, and NF-κB p50 expressions were found. High 20S proteasome activity was associated with increase of HIF-1 level. Positive relationships between IGF-I expression and two classic forms of NF-κB p50 and p65 in endometrial cancer were revealed. The data obtained indicate the possible proteasomal regulation of growth and transcription factors. The major pool of IGF-I is located in the extracellular space, and it is likely that extracellular proteasomes also take part in the regulation of the IGF-I content. The present data show the evidence of proteasome regulation of growth and nuclear factors that can play an important role in cancer pathogenesis.

Ubiquitination and proteolysis in acute lung injury

Ubiquitination is a posttranslational modification that regulates a variety of cellular functions depending on timing, subcellular localization, and type of tagging, as well as modulators of ubiquitin binding leading to proteasomal or lysosomal degradation or nonproteolytic modifications. Ubiquitination plays an important role in the pathogenesis of acute lung injury (ALI) and other lung diseases with pathologies secondary to inflammation, mechanical ventilation, and decreased physical mobility. Particularly, ubiquitination has been shown to affect alveolar epithelial barrier function and alveolar edema clearance by targeting the Na,K-ATPase and epithelial Na(+) channels upon lung injury. Notably, the proteasomal system also exhibits distinct functions in the extracellular space, which may contribute to the pathogenesis of ALI and other pulmonary diseases. Better understanding of these mechanisms may ultimately lead to novel therapeutic modalities by targeting elements of the ubiquitination pathway.

Proteasome inhibition is partially effective in attenuating pre-existing immunity against recombinant adeno-associated viral vectors

Pre-existing immunity against adeno-associated virus (AAV) remains a major challenge facing the clinical use of systemic administration of recombinant AAV vectors for the treatment of genetic and acquired diseases using gene therapy. In this study, we evaluated the potential of bortezomib (marketed under trade name Velcade) to abrogate a pre-existing immunity to AAV in mice, thereby allowing subsequent transduction by a recombinant AAV vector of the same serotype. We demonstrate that bortezomib efficiently reduces AAV-specific IgG titres and moderates the cytotoxic T cell response in mice that have a pre-existing immunity to AAV2/8. Significant depletion of AAV2/8-specific IgG-producing plasma cells in secondary lymphoid organs and bone marrow was observed. However, this inhibition of the immune response by bortezomib was insufficient to allow subsequent re-infection with a recombinant AAV vector of a similar serotype. We show that this shortcoming is probably due to the combination of residual antibody levels and the inability of bortezomib to completely deplete the memory B cells that are re-activated in response to a repeated infection with a recombinant AAV vector. Taken together, the results of this study argue for the use of immunosuppressive therapies that target both plasma and memory B cells for the efficient elimination of pre-existing immunity against AAV2/8 vectors.