20S proteasomal degradation of ornithine decarboxylase is regulated by NQO1

NQO1 protein expression predicts poor prognosis of non-small cell lung cancers

BACKGROUND

High-level expression of

NAD(P)H

quinoneoxidoreductase 1 (NQO1) has been correlated with many types of human cancers, suggesting that NQO1 plays important roles in tumor occurrence and progression. This study attempted to explore the role of NQO1 in tumor progression and prognostic evaluation of non-small cell lung cancer (NSCLC).

METHODS

Total 164 tissue samples, including 150 NSCLC paired with the adjacent non-tumor tissues and 14 normal lung tissues, were picked-up for immunohistochemical (IHC) staining of the NQO1 protein, and immunofluorescence (IF) staining was also performed to detect the subcellular localization of the NQO1 protein in A549 human lung cancer cells. The correlation between NQO1 expression and clinicopathological characteristics were evaluated by Chi-square test and Fisher's exact tests. The disease-free survival (DFS) and overall survival (OS) rates of NSCLC patients were calculated by the Kaplan-Meier method, and univariate and multivariate analyses were performed using the Cox proportional hazards regression model.

RESULTS

The NQO1 protein showed a mainly cytoplasmic staining pattern in lung cancer cells, including adenocarcinoma and squamous cell carcinoma (SCC). Both positive rate and strongly positive rate of NQO1 protein expression were significantly higher in NSCLC (59.3% and 28.0%) than that in adjacent non tumor (8.0% and 1.3%) and normal lung tissues (0%). The positive rate of NQO1 was related with clinical stage and lymph node metastasis, and the strongly positive rate of NQO1 protein was significantly correlated with tumor size, poor differentiation, advanced clinical stage and lymph node metastasis in NSCLC. Additionally, survival analyses showed that the patients with NQO1 positive expression had lower OS rates compared with those with NQO1 negative expression in the groups of T1-2, T3-4, without LN metastasis and stage I-II of NSCLC, respectively; however, in the groups of patients with LN metastasis or III-IV stages, OS rate was not correlated with NQO1 expression status. Moreover, multivariate analysis suggested that NQO1 emerged as a significant independent prognostic factor along with tumor size, differentiation, lymph node metastasis and clinical stage in patients with NSCLC.

CONCLUSIONS

NQO1 is upregulated in NSCLC, and it may be a useful poor prognostic biomarker and a potential therapeutic target for patients with NSCLC.

Regulating the 20S proteasome ubiquitin-independent degradation pathway

For many years, the ubiquitin-26S proteasome degradation pathway was considered the primary route for proteasomal degradation. However, it is now becoming clear that proteins can also be targeted for degradation by the core 20S proteasome itself. Degradation by the 20S proteasome does not require ubiquitin tagging or the presence of the 19S regulatory particle; rather, it relies on the inherent structural disorder of the protein being degraded. Thus, proteins that contain unstructured regions due to oxidation, mutation, or aging, as well as naturally, intrinsically unfolded proteins, are susceptible to 20S degradation. Unlike the extensive knowledge acquired over the years concerning degradation by the 26S proteasome, relatively little is known about the means by which 20S-mediated proteolysis is controlled. Here, we describe our current understanding of the regulatory mechanisms that coordinate 20S proteasome-mediated degradation, and highlight the gaps in knowledge that remain to be bridged.

Rapid proteasomal degradation of posttranscriptional regulators of the TIS11/tristetraprolin family is induced by an intrinsically unstructured region independently of ubiquitination

The TIS11/tristetraprolin (TTP) CCCH tandem zinc finger proteins are major effectors in the destabilization of mRNAs bearing AU-rich elements (ARE) in their 3' untranslated regions. In this report, we demonstrate that the Drosophila melanogaster dTIS11 protein is short-lived due to its rapid ubiquitin-independent degradation by the proteasome. Our data indicate that this mechanism is tightly associated with the intrinsically unstructured, disordered N- and C-terminal domains of the protein. Furthermore, we show that TTP, the mammalian TIS11/TTP protein prototype, shares the same three-dimensional characteristics and is degraded by the same proteolytic pathway as dTIS11, thereby indicating that this mechanism has been conserved across evolution. Finally, we observed a phosphorylation-dependent inhibition of dTIS11 and TTP degradation by the proteasome in vitro, raising the possibility that such modifications directly affect proteasomal recognition for these proteins. As a group, RNA-binding proteins (RNA-BPs) have been described as enriched in intrinsically disordered regions, thus raising the possibility that the mechanism that we uncovered for TIS11/TTP turnover is widespread among other RNA-BPs.

FAD binding overcomes defects in activity and stability displayed by cancer-associated variants of human NQO1

NAD(P)H quinone oxidoreductase 1 is involved in antioxidant defence and protection from cancer, stabilizing the apoptosis regulator p53 towards degradation. Here, we studied the enzymological, biochemical and biophysical properties of two cancer-associated variants (p.R139W and p.P187S). Both variants (especially p.187S) have lower thermal stability and greater susceptibility to proteolysis compared to the wild-type. p.P187S also has reduced activity due to a lower binding affinity for the FAD cofactor as assessed by activity measurements and direct titrations. Native gel electrophoresis and dynamic light scattering also suggest that p.P187S has a higher tendency to populate unfolded states under native conditions. Detailed thermal stability studies showed that all variants irreversibly denature causing dimer dissociation, while addition of FAD restores the stability of the polymorphic forms to wild-type levels. The kinetic destabilization induced by polymorphisms as well as the kinetic protection exerted by FAD was confirmed by measuring denaturation kinetics at temperatures close to physiological. Our data suggest that the main molecular mechanisms associated with these cancer-related variants are their low binding affinity for FAD and/or kinetic instability. Thus, pharmacological chaperones may be useful in the treatment of patients bearing these polymorphisms.

Regulation of ubiquitin-proteasome and autophagy pathways after acute LPS and epoxomicin administration in mice

Background

The ubiquitin-proteasome pathway (UPP) is a major protein degradation pathway that is activated during sepsis and has been proposed as a therapeutic target for preventing skeletal muscle loss due to cachexia. Although several studies have investigated the modulation of proteasome activity in response to LPS administration, none have characterized the overall UPP response to LPS administration in the fate of proteasome inhibition.

Methods

Here, we determined the modulation pattern of the main key components of the UPP in the gastrocnemius (GAS) of mice during the acute phase of lipopolysaccharide (LPS)-mediated endotoxemia (7.5 mg/kg – 8 h) by measuring all three β1, β2 and β5 activites of the 20S and 26S proteasomes, the levels of steady state polyubiquitinated proteins, mRNA levels of muscle ligases, as well as signaling pathways regulating the UPP. Another goal was to assess the effects of administration of a specific proteasome inhibitor (epoxomicin, 0.5 mg/kg) on UPP response to sepsis.

Results

The acute phase of LPS-induced endotoxemia lowered GAS/body weight ratio and increased MuRF1 and MAFbx mRNA concomitantly to an activation of the pathways known to regulate their expression. Unexpectedly, we observed a decrease in all 20S and 26S proteasome activities measured in GAS, which might be related to oxidative stress, as oxidized proteins (carbonyl levels) increase with LPS. While significantly inhibiting 20S and 26S proteasome β5 activities in heart and liver, epoxomicin did not lower proteasome activity in GAS. However, the increase in mRNA expression of the muscle ligases MuRF1 and MAFbx were partially rescued without affecting the other investigated signaling pathways. LPS also strongly activated autophagy, which could explain the observed GAS atrophy with LPS-induced reduction of proteasome activity.

Conclusions

Our results highlight an opposite regulation of UPP in the early hours of LPS-induced muscle atrophy by showing reduced proteasome activities and increased mRNA expression of muscle specific ligases. Furthermore, our data do not support any preventive effect of epoxomicin in muscle atrophy due to acute cachexia since proteasome activities are not further repressed.

Paradigms of protein degradation by the proteasome

The proteasome is the main proteolytic machine in the cytosol and nucleus of eukaryotic cells where it degrades hundreds of regulatory proteins, removes damaged proteins, and produces peptides that are presented by MHC complexes. New structures of the proteasome particle show how its subunits are arranged and provide insights into how the proteasome is regulated. Proteins are targeted to the proteasome by tags composed of several ubiquitin moieties. The structure of the tags tunes the order in which proteins are degraded. The proteasome itself edits the ubiquitin tags and drugs that interfere in this process can enhance the clearance of toxic proteins from cells. Finally, the proteasome initiates degradation at unstructured regions within its substrates and this step contributes to substrate selection.

Systematic nucleo-cytoplasmic trafficking of proteins following exposure of MCF7 breast cancer cells to estradiol

We have used a proteomics subcellular spatial razor approach to look at changes in total protein abundance and in protein distribution between the nucleus and cytoplasm following exposure of MCF7 breast cancer cells to estradiol. The dominant response of MCF7 cells to estrogen stimulation involves dynamic changes in protein subcellular spatial distribution rather than changes in total protein abundance. Of the 3604 quantitatively monitored proteins, only about 2% show substantial changes in total abundance (>2-fold), whereas about 20% of the proteins show substantial changes in local abundance and/or redistribution of their subcellular location, with up to 16-fold changes in their local concentration in the nucleus or the cytoplasm. We propose that dynamic redistribution of the subcellular location of multiple proteins in response to stimuli is a fundamental characteristic of cells and suggest that perturbation of cellular spatial control may be an important feature of cancer.

Ubiquitin-independent proteasomal degradation

Most proteasome substrates are marked for degradation by ubiquitin conjugation, but some are targeted by other means. The properties of these exceptional cases provide insights into the general requirements for proteasomal degradation. Here the focus is on three ubiquitin-independent substrates that have been the subject of detailed study. These are Rpn4, a transcriptional regulator of proteasome homeostasis, thymidylate synthase, an enzyme required for production of DNA precursors and ornithine decarboxylase, the initial enzyme committed to polyamine biosynthesis. It can be inferred from these cases that proteasome association and the presence of an unstructured region are the sole prerequisites for degradation. Based on that inference, artificial substrates have been designed to test the proteasome's capacity for substrate processing and its limitations. Ubiquitin-independent substrates may in some cases be a remnant of the pre-ubiquitome world, but in other cases could provide optimized regulatory solutions. This article is part of a Special Issue entitled: Ubiquitin-Proteasome System. Guest Editors: Thomas Sommer and Dieter H. Wolf.

The protein level of PGC-1α, a key metabolic regulator, is controlled by NADH-NQO1

PGC-1α is a key transcription coactivator regulating energy metabolism in a tissue-specific manner. PGC-1α expression is tightly regulated, it is a highly labile protein, and it interacts with various proteins--the known attributes of intrinsically disordered proteins (IDPs). In this study, we characterize PGC-1α as an IDP and demonstrate that it is susceptible to 20S proteasomal degradation by default. We further demonstrate that PGC-1α degradation is inhibited by NQO1, a 20S gatekeeper protein. NQO1 binds and protects PGC-1α from degradation in an NADH-dependent manner. Using different cellular physiological settings, we also demonstrate that NQO1-mediated PGC-1α protection plays an important role in controlling both basal and physiologically induced PGC-1α protein level and activity. Our findings link NQO1, a cellular redox sensor, to the metabolite-sensing network that tunes PGC-1α expression and activity in regulating energy metabolism.

Absolute quantification of NAD(P)H:quinone oxidoreductase 1 in human tumor cell lines and tissues by liquid chromatography-mass spectrometry/mass spectrometry using both isotopic and non-isotopic internal standards

NAD(P)H:quinone oxidoreductase 1 (NQO1, DT-diaphorase) is a prognostic biomarker and a potential therapeutic target for various tumors. Therefore, it is of significance to develop a robust method for the absolute quantification of NQO1. This study aimed to develop and validate a LC-MS/MS based method and to test the appropriateness of using non-isotopic analog peptide as the internal standard (IS) by comparing with a stable isotope labeled (SIL) peptide. The chromatographic performance and mass spectra between the selected signature peptide of NQO1 and the non-isotopic peptide were observed to be very similar. The use of the two internal standards was validated appropriate for the absolute quantification of NQO1, as evidenced by satisfactory validation results over a concentration range of 1.62-162 fmol μL(-1). This method has been successfully applied to the absolute quantification of NQO1 expression in various tumor cell lines and tissues. NQO1 expression in human tumor tissues is much higher than that in the neighboring normal tissues in both the cases of lung and colon cancer. The quantitative results obtained from the isotopic and non-isotopic methods are quite similar, further supporting that the use of non-isotopic analog peptide as internal standard is appropriate and feasible for the quantification of NQO1. By comparing with a classical isotopic IS, the present study indicates that the use of a non-isotopic peptide analog to the proteotypic peptide as the internal standard can get equal accuracy and preciseness in measuring NQO1. The universal applicability of the non-isotopic IS approach for the quantification of proteins warrants further research.

NAD(P)H:quinone oxidoreductase 1 (NQO1) localizes to the mitotic spindle in human cells

NAD(P)H:quinone oxidoreductase 1 (NQO1) is an FAD containing quinone reductase that catalyzes the 2-electron reduction of a broad range of quinones. The 2-electron reduction of quinones to hydroquinones by NQO1 is believed to be a detoxification process since this reaction bypasses the formation of the highly reactive semiquinone. NQO1 is expressed at high levels in normal epithelium, endothelium and adipocytes as well as in many human solid tumors. In addition to its function as a quinone reductase NQO1 has been shown to reduce superoxide and regulate the 20 S proteasomal degradation of proteins including p53. Biochemical studies have indicated that NQO1 is primarily located in the cytosol, however, lower levels of NQO1 have also been found in the nucleus. In these studies we demonstrate using immunocytochemistry and confocal imaging that NQO1 was found associated with mitotic spindles in cells undergoing division. The association of NQO1 with the mitotic spindles was observed in many different human cell lines including nontransformed cells (astrocytes, HUVEC) immortalized cell lines (HBMEC, 16HBE) and cancer (pancreatic adenocarcinoma, BXPC3). Confocal analysis of double-labeling experiments demonstrated co-localization of NQO1with alpha-tubulin in mitotic spindles. In studies with BxPc-3 human pancreatic cancer cells the association of NQO1 with mitotic spindles appeared to be unchanged in the presence of NQO1 inhibitors ES936 or dicoumarol suggesting that NQO1 can associate with the mitotic spindle and still retain catalytic activity. Analysis of archival human squamous lung carcinoma tissue immunostained for NQO1 demonstrated positive staining for NQO1 in the spindles of mitotic cells. The purpose of this study is to demonstrate for the first time the association of the quinone reductase NQO1 with the mitotic spindle in human cells.

Non-canonical ubiquitin-based signals for proteasomal degradation

Regulated cellular proteolysis is mediated largely by the ubiquitin-proteasome system (UPS). It is a highly specific process that is time- (e.g. cell cycle), compartment- (e.g. nucleus or endoplasmic reticulum) and substrate quality- (e.g. denatured or misfolded proteins) dependent, and allows fast adaptation to changing conditions. Degradation by the UPS is carried out through two successive steps: the substrate is covalently tagged with ubiquitin and subsequently degraded by the 26S proteasome. The accepted 'canonical' signal for proteasomal recognition is a polyubiquitin chain that is anchored to a lysine residue in the target substrate, and is assembled through isopeptide bonds involving lysine 48 of ubiquitin. However, several 'non-canonical' ubiquitin-based signals for proteasomal targeting have also been identified. These include chains anchored to residues other than internal lysine in the substrates, chains assembled through linking residues other than lysine 48 in ubiquitin, and mixed chains made of both ubiquitin and a ubiquitin-like protein. Furthermore, some proteins can be degraded following modification by a single ubiquitin (monoubiquitylation) or multiple single ubiquitins (multiple monoubiquitylation). Finally, some proteins can be proteasomally degraded without prior ubiquitylation (the process is also often referred to as ubiquitination). In this Commentary, we describe these recent findings and discuss the possible physiological roles of these diverse signals. Furthermore, we discuss the possible impact of this signal diversity on drug development.

ATP independent proteasomal degradation of NQO1 in BL cell lines

Human NAD(P)H: quinone oxidoreductase 1 (NQO1) catalyzes the obligatory two-electron reduction of quinones. For this peculiar catalytic mechanism, the enzyme is considered an important cytoprotector. The NQO1 gene is expressed in all human tissues, unless a polymorphism due to C609T point mutation is present. This polymorphism produces a null phenotype in the homozygous condition and reduced enzyme activity in the heterozygous one. We previously demonstrated that two cell lines of haematopoietic origin, HL60 and Raji cells, possess the same heterozygous genotype, but different phenotypes; as expected for a heterozygous condition the HL60 cell line showed a low level of enzyme activity, while the Raji cell line appeared as null phenotype. The level of NQO1 mRNA was similar in the two cell lines and the different phenotype was not due to additional mutations or to expression of alternative splicing products. Here we show that in Raji BL cell line with heterozygous genotype the null NQO1 phenotype is due to 20S proteasome degradation of wild type and mutant protein isoforms and is not directly linked to C609T polymorphism. This finding may have important implications in B-cell differentiation, in leukaemia risk evaluation and in chemotherapy based on proteasome inhibitors.

Chemically induced oxidative stress increases polyamine levels by activating the transcription of ornithine decarboxylase and spermidine/spermine-N1-acetyltransferase in human hepatoma HUH7 cells

Biogenic polyamines spermine and spermidine participate in numerous cellular processes including transcription, RNA processing and translation. Specifically, they counteract oxidative stress, an alteration of cell redox balance involved in generation and progression of various pathological states including cancer. Here, we investigated how chemically induced oxidative stress affects polyamine metabolism, specifically the expression and activities of enzymes catalyzing polyamine synthesis (ornithine decarboxylase; ODC) and degradation (spermidine/spermine-N(1)-acetyltransferase; SSAT), in human hepatoma cells. Oxidative stress induced the up-regulation of ODC and SSAT gene transcription mediated by Nrf2, and in case of SSAT, also by NF-κB transcription factors. Activation of transcription led to the elevated intracellular activities of both enzymes. The balance in antagonistic activities of ODC and SSAT in the stressed hepatoma cells was shifted towards polyamine biosynthesis, which resulted in increased intracellular levels of putrescine, spermidine, and spermine. Accumulation of putrescine is indicating for accelerated degradation of polyamines by SSAT - acetylpolyamine oxidase (APAO) pathway generating toxic products that promote carcinogenesis, whereas accelerated polyamine synthesis via activation of ODC is favorable for proliferation of cells including those sub-lethally damaged by oxidative stress.

Two novel mutations in the POU1F1 gene generate null alleles through different mechanisms leading to combined pituitary hormone deficiency

BACKGROUND

  Mutations in the POU1F1 gene severely affect the development and function of the anterior pituitary gland and lead to combined pituitary hormone deficiency (CPHD).

OBJECTIVE

  The clinical and genetic analysis of a patient presenting with CPHD and functional characterization of identified mutations.

PATIENT

  We describe a male patient with extreme short stature, learning difficulties, anterior pituitary hypoplasia, secondary hypothyroidism and undetectable prolactin, growth hormone (GH) and insulin-like growth factor 1 (IGF1), with normal random cortisol.

DESIGN

  The POU1F1 coding region was amplified by PCR and sequenced; the functional consequence of the mutations was analysed by cell transfection and in vitro assays.

RESULTS

  Genetic analysis revealed compound heterozygosity for two novel putative loss of function mutations in POU1F1: a transition at position +3 of intron 1 [IVS1+3nt(A>G)] and a point mutation in exon 6 resulting in a substitution of arginine by tryptophan (R265W). Functional analysis revealed that IVS1+3nt(A>G) results in a reduction in the correctly spliced POU1F1 mRNA, which could be corrected by mutations of the +4, +5 and +6 nucleotides. Analysis of POU1F1(R265W) revealed complete loss of function resulting from severely reduced protein stability.

CONCLUSIONS

  Combined pituitary hormone deficiency in this patient is caused by loss of POU1F1 function by two novel mechanisms, namely aberrant splicing (IVS1+3nt (A>G) and protein instability (R265W). Identification of the genetic basis of CPHD enabled the cessation of hydrocortisone therapy without the need for further assessment for evolving endocrinopathy.

Actin cytoskeleton remodeling by the alternatively spliced isoform of PDLIM4/RIL protein

RIL (product of PDLIM4 gene) is an actin-associated protein that has previously been shown to stimulate actin bundling by interacting with actin-cross-linking protein α-actinin-1 and increasing its affinity to filamentous actin. Here, we report that the alternatively spliced isoform of RIL, denoted here as RILaltCterm, functions as a dominant-negative modulator of RIL-mediated actin reorganization. RILaltCterm is regulated at the level of protein stability, and this protein isoform accumulates particularly in response to oxidative stress. We show that the alternative C-terminal segment of RILaltCterm has a disordered structure that directs the protein to rapid degradation in the core 20 S proteasomes. Such degradation is ubiquitin-independent and can be blocked by binding to NAD(P)H quinone oxidoreductase NQO1, a detoxifying enzyme induced by prolonged exposure to oxidative stress. We show that either overexpression of RILaltCterm or its stabilization by stresses counteracts the effects produced by full-length RIL on organization of actin cytoskeleton and cell motility. Taken together, the data suggest a mechanism for fine-tuning actin cytoskeleton rearrangement in response to stresses.

Trans-plasma membrane electron transport in mammals: functional significance in health and disease

Trans-plasma membrane electron transport (t-PMET) has been established since the 1960s, but it has only been subject to more intensive research in the last decade. The discovery and characterization at the molecular level of its novel components has increased our understanding of how t-PMET regulates distinct cellular functions. This review will give an update on t-PMET, with particular emphasis on how its malfunction relates to some diseases, such as cancer, abnormal cell death, cardiovascular diseases, aging, obesity, neurodegenerative diseases, pulmonary fibrosis, asthma, and genetically linked pathologies. Understanding these relationships may provide novel therapeutic approaches for pathologies associated with unbalanced redox state.

α-Tocotrienol quinone modulates oxidative stress response and the biochemistry of aging

We report that α-tocotrienol quinone (ATQ3) is a metabolite of α-tocotrienol, and that ATQ3 is a potent cellular protectant against oxidative stress and aging. ATQ3 is orally bioavailable, crosses the blood-brain barrier, and has demonstrated clinical response in inherited mitochondrial disease in open label studies. ATQ3 activity is dependent upon reversible 2e-redox-cycling. ATQ3 may represent a broader class of unappreciated dietary-derived phytomolecular redox motifs that digitally encode biochemical data using redox state as a means to sense and transfer information essential for cellular function.

BIMEL, an intrinsically disordered protein, is degraded by 20S proteasomes in the absence of poly-ubiquitylation

BIM-extra long (BIMEL), a pro-apoptotic BH3-only protein and part of the BCL-2 family, is degraded by the proteasome following activation of the ERK1/2 signalling pathway. Although studies have demonstrated poly-ubiquitylation of BIMEL in cells, the nature of the ubiquitin chain linkage has not been defined. Using ubiquitin-binding domains (UBDs) specific for defined ubiquitin chain linkages, we show that BIMEL undergoes K48-linked poly-ubiquitylation at either of two lysine residues. Surprisingly, BIMELΔKK, which lacks both lysine residues, was not poly-ubiquitylated but still underwent ERK1/2-driven, proteasome-dependent turnover. BIM has been proposed to be an intrinsically disordered protein (IDP) and some IDPs can be degraded by uncapped 20S proteasomes in the absence of poly-ubiquitylation. We show that BIMEL is degraded by isolated 20S proteasomes but that this is prevented when BIMEL is bound to its pro-survival target protein MCL-1. Furthermore, knockdown of the proteasome cap component Rpn2 does not prevent BIMEL turnover in cells, and inhibition of the E3 ubiquitin ligase β-TrCP, which catalyses poly-Ub of BIMEL, causes Cdc25A accumulation but does not inhibit BIMEL turnover. These results provide new insights into the regulation of BIMEL by defining a novel ubiquitin-independent pathway for the proteasome-dependent destruction of this highly toxic protein.

Genetic polymorphism in metabolism and host defense enzymes: implications for human health risk assessment

Genetic polymorphisms in xenobiotic metabolizing enzymes can have profound influence on enzyme function, with implications for chemical clearance and internal dose. The effects of polymorphisms have been evaluated for certain therapeutic drugs but there has been relatively little investigation with environmental toxicants. Polymorphisms can also affect the function of host defense mechanisms and thus modify the pharmacodynamic response. This review and analysis explores the feasibility of using polymorphism data in human health risk assessment for four enzymes, two involved in conjugation (uridine diphosphoglucuronosyltransferases [UGTs], sulfotransferases [SULTs]), and two involved in detoxification (microsomal epoxide hydrolase [EPHX1], NADPH quinone oxidoreductase I [NQO1]). This set of evaluations complements our previous analyses with oxidative and conjugating enzymes. Of the numerous UGT and SULT enzymes, the greatest likelihood for polymorphism effect on conjugation function are for SULT1A1 (*2 polymorphism), UGT1A1 (*6, *7, *28 polymorphisms), UGT1A7 (*3 polymorphism), UGT2B15 (*2 polymorphism), and UGT2B17 (null polymorphism). The null polymorphism in NQO1 has the potential to impair host defense. These highlighted polymorphisms are of sufficient frequency to be prioritized for consideration in chemical risk assessments. In contrast, SNPs in EPHX1 are not sufficiently influential or defined for inclusion in risk models. The current analysis is an important first step in bringing the highlighted polymorphisms into a physiologically based pharmacokinetic (PBPK) modeling framework.

Cutting Edge: Selective role of ubiquitin in MHC class I antigen presentation

The importance of ubiquitination in MHC class I-restricted Ag processing remains unclear. To address this issue, we overexpressed wild-type and dominant-negative lysineless forms of ubiquitin (Ub) in mammalian cells using an inducible vaccinia virus system. Overexpression of the lysineless Ub nearly abrogated polyubiquitination and potently inhibited epitope presentation from a cytosolic N-end rule substrate as well as endoplasmic reticulum (ER)-targeted model Ags. In contrast, there was little impact on Ag presentation from cytosolic proteins. These trends were location dependent; redirecting cytosolic Ag to the ER rendered presentation lysineless Ub-sensitive, whereas retargeting exocytic Ag to the cytosol had the inverse effect. This dichotomy was further underscored by small interfering RNA knockdown of the ER-associated Ub ligase Hrd1. Thus, Ub-dependent degradation appears to play a major role in the MHC class I-restricted processing of ER-targeted proteins and a more restricted role in the processing of cytosolic proteins.

Benzene toxicity: The role of the susceptibility factor NQO1 in bone marrow endothelial cell signaling and function

The homozygous NQO1*2 polymorphism results in a null NQO1 phenotype and is a susceptibility factor for occupational benzene poisoning. NQO1 plays an important role in detoxification of benzene-derived quinones but plays a role in numerous other non-metabolic cellular functions. NQO1 is expressed in endothelial cells of bone marrow which form the vascular stem cell niche important in stem cell homing and mobilization. We therefore employed a transformed human bone marrow endothelial cell (HBMEC) line to define the effects of compromising NQO1 on endothelial function. Either inhibition or knockdown of NQO1 led to decreased expression of the adhesion molecules E-selectin, VCAM-1 and ICAM-1 and decreased functional adhesion of CD34+ progenitor cells after TNFα stimulation. Suicide inhibition or knockdown of NQO1 decreased NFκB p105 precursor and NFκB p50 subunit levels as well as leading to decreased nuclear levels of NFκB phospho-p65. An additional function of endothelial cells is tube formation and angiogenesis which was inhibited by the benzene metabolite hydroquinone suggesting that endothelial function may be affected at multiple levels after exposure of NQO1*2 polymorphic individuals to benzene. These data demonstrate that NQO1 plays an upstream role in NFκB signaling and adhesion molecule expression in HBMEC and that NQO1 has important regulatory effects in its own right in addition to being a marker for Nrf-2 activation. Metabolic susceptibility factors such as NQO1 have roles in addition to detoxification of reactive intermediates and interrogation of these novel roles can inform both mechanisms of toxicity and human risk assessment.

NAD(P)H:quinone acceptor oxidoreductase 1 (NQO1), a multifunctional antioxidant enzyme and exceptionally versatile cytoprotector

NAD(P)H:quinone acceptor oxidoreductase 1 (NQO1) is a widely-distributed FAD-dependent flavoprotein that promotes obligatory 2-electron reductions of quinones, quinoneimines, nitroaromatics, and azo dyes, at rates that are comparable with NADH or NADPH. These reductions depress quinone levels and thereby minimize opportunities for generation of reactive oxygen intermediates by redox cycling, and for depletion of intracellular thiol pools. NQO1 is a highly-inducible enzyme that is regulated by the Keap1/Nrf2/ARE pathway. Evidence for the importance of the antioxidant functions of NQO1 in combating oxidative stress is provided by demonstrations that induction of NQO1 levels or their depletion (knockout, or knockdown) are associated with decreased and increased susceptibilities to oxidative stress, respectively. Furthermore, benzene genotoxicity is markedly enhanced when NQO1 activity is compromised. Not surprisingly, human polymorphisms that suppress NQO1 activities are associated with increased predisposition to disease. Recent studies have uncovered protective roles for NQO1 that apparently are unrelated to its enzymatic activities. NQO1 binds to and thereby stabilizes the important tumor suppressor p53 against proteasomal degradation. Indeed, NQO1 appears to regulate the degradative fate of other proteins. These findings suggest that NQO1 may exercise a selective "gatekeeping" role in regulating the proteasomal degradation of specific proteins, thereby broadening the cytoprotective role of NQO1 far beyond its highly effective antioxidant functions.

Physiological levels of ATP negatively regulate proteasome function

Intracellular protein degradation by the ubiquitin-proteasome system is ATP dependent, and the optimal ATP concentration to activate proteasome function in vitro is ∼100 μM. Intracellular ATP levels are generally in the low millimolar range, but ATP at a level within this range was shown to inhibit proteasome peptidase activities in vitro. Here, we report new evidence that supports a hypothesis that intracellular ATP at the physiological levels bidirectionally regulates 26S proteasome proteolytic function in the cell. First, we confirmed that ATP exerted bidirectional regulation on the 26S proteasome in vitro, with the optimal ATP concentration (between 50 and 100 μM) stimulating proteasome chymotrypsin-like activities. Second, we found that manipulating intracellular ATP levels also led to bidirectional changes in the levels of proteasome-specific protein substrates in cultured cells. Finally, measures to increase intracellular ATP enhanced, while decreasing intracellular ATP attenuated the ability of proteasome inhibition to induce cell death. These data strongly suggest that endogenous ATP within the physiological concentration range can exert a negative impact on proteasome activities, allowing the cell to rapidly upregulate proteasome activity on ATP reduction under stress conditions.

NAD(P)H quinone oxidoreductase protects TAp63gamma from proteasomal degradation and regulates TAp63gamma-dependent growth arrest

Background

p63 is a member of the p53 transcription factor family. p63 is expressed from two promoters resulting in proteins with opposite functions: the transcriptionally active TAp63 and the dominant-negative ΔNp63. Similar to p53, the TAp63 isoforms induce cell cycle arrest and apoptosis. The ΔNp63 isoforms are dominant-negative variants opposing the activities of p53, TAp63 and TAp73. To avoid unnecessary cell death accompanied by proper response to stress, the expression of the p53 family members must be tightly regulated. NAD(P)H quinone oxidoreductase (NQO1) has recently been shown to interact with and inhibit the degradation of p53. Due to the structural similarities between p53 and p63, we were interested in studying the ability of wild-type and polymorphic, inactive NQO1 to interact with and stabilize p63. We focused on TAp63γ, as it is the most potent transcription activator and it is expected to have a role in tumor suppression.

Principal Findings

We show that TAp63γ can be degraded by the 20S proteasomes. Wild-type but not polymorphic, inactive NQO1 physically interacts with TAp63γ, stabilizes it and protects it from this degradation. NQO1-mediated TAp63γ stabilization was especially prominent under stress. Accordingly, we found that downregulation of NQO1 inhibits TAp63γ-dependant p21 upregulation and TAp63γ-induced growth arrest stimulated by doxorubicin.

Conclusions/Significance

Our report is the first to identify this new mechanism demonstrating a physical and functional relationship between NQO1 and the most potent p63 isoform, TAp63γ. These findings appoint a direct role for NQO1 in the regulation of TAp63γ expression, especially following stress and may therefore have clinical implications for tumor development and therapy.

Pharmacological inhibitors of NAD(P)H quinone oxidoreductase, NQO1: structure/activity relationships and functional activity in tumour cells

NAD(P)H quinone oxidoreductase (NQO1) has multiple functions in the cell including an ability to act as a detoxifying enzyme and as a protein chaperone. The latter property is particularly important in oncology as one of the client proteins of NQO1 is p53. The inhibitor, dicoumarol, is classically used to probe the biological properties of NQO1, but interpretation of enzyme function is compromised by the multiple "off-target" effects of this agent. Coumarin-based compounds that are more potent than dicoumarol as inhibitors of recombinant human NQO1 have been identified (Nolan et al., J Med Chem 2009;52:7142-56) The purpose of the work reported here is to demonstrate the functional activity of these agents for inhibiting NQO1 in cells. To do this, advantage was taken of the NQO1-mediated toxicity of the chemotherapeutic drug EO9 (Apaziquone). The toxicity of this drug is substantially reduced when the function of NQO1 is inhibited and many of the coumarin-based compounds are more efficient than dicoumarol for inhibiting EO9 toxicity. The ability to do this appears to be related to their capacity to inhibit NQO1 in cell free systems. In conclusion, agents have been identified that may be more pharmacologically useful than dicoumarol for probing the function of NQO1 in cells and tissues.

c-Fos proteasomal degradation is activated by a default mechanism, and its regulation by NAD(P)H:quinone oxidoreductase 1 determines c-Fos serum response kinetics

The short-lived proto-oncoprotein c-Fos is a component of the activator protein 1 (AP-1) transcription factor. A large region of c-Fos is intrinsically unstructured and susceptible to a recently characterized proteasomal ubiquitin-independent degradation (UID) pathway. UID is active by a default mechanism that is inhibited by NAD(P)H:quinone oxidoreductase 1 (NQO1), a 20S proteasome gatekeeper. Here, we show that NQO1 binds and induces robust c-Fos accumulation by blocking the UID pathway. c-Jun, a partner of c-Fos, also protects c-Fos from proteasomal degradation by default. Our findings suggest that NQO1 protects monomeric c-Fos from proteasomal UID, a function that is fulfilled later by c-Jun. We show that this process regulates c-Fos homeostasis (proteostasis) in response to serum stimulation, phosphorylation, nuclear translocation, and transcription activity. In addition, we show that NQO1 is important to ensure immediate c-Fos accumulation in response to serum, since a delayed response was observed under low NQO1 expression. These data suggest that in vivo, protein unstructured regions determine the kinetics and the homeostasis of regulatory proteins. Our data provide evidence for another layer of regulation of key regulatory proteins that functions at the level of protein degradation and is designed to ensure optimal formation of functional complexes such as AP-1.

Tau protein degradation is catalyzed by the ATP/ubiquitin-independent 20S proteasome under normal cell conditions

Tau is the major protein exhibiting intracellular accumulation in Alzheimer disease. The mechanisms leading to its accumulation are not fully understood. It has been proposed that the proteasome is responsible for degrading tau but, since proteasomal inhibitors block both the ubiquitin-dependent 26S proteasome and the ubiqutin-independent 20S proteasome pathways, it is not clear which of these pathways is involved in tau degradation. Some involvement of the ubiquitin ligase, CHIP in tau degradation has also been postulated during stress. In the current studies, we utilized HT22 cells and tau-transfected E36 cells in order to test the relative importance or possible requirement of the ubiquitin-dependent 26S proteasomal system versus the ubiquitin-independent 20S proteasome, in tau degradation. By means of ATP-depletion, ubiquitinylation-deficient E36ts20 cells, a 19S proteasomal regulator subunit MSS1-siRNA approaches, and in vitro ubiquitinylation studies, we were able to demonstrate that ubiquitinylation is not required for normal tau degradation.

Genetic traits for the persistence of Helicobacter pylori infection

Helicobacter pylori infection elevates the risk of gastric diseases, including peptic ulcer and gastric cancer. Persistent infection is the first step to induce H. pylori-induced multistage diseases. Although the roles of genetic traits on persistent infection have not yet been elucidated, some individuals escape from persistent infection. Possible favorable conditions for H. pylori seem to be low acid secretion, reduced innate immune responses, and easier binding to gastric epithelial cells. IL-1β and TNF-α inhibit acid secretion. The genetic polymorphisms associated with both molecules have the potential to be the genetic traits underlying persistent infection. Functional polymorphisms associated with innate immune responses could also be involved with the genetic traits, but no polymorphisms with consistent associations have been identified so far. The polymorphisms associated with molecules for adhesion to epithelial cells are candidates of genetic traits, but more research is needed.

NAD(P)H quinone-oxydoreductase 1 protects eukaryotic translation initiation factor 4GI from degradation by the proteasome

The eukaryotic translation initiation factor 4GI (eIF4GI) serves as a central adapter in cap-binding complex assembly. Although eIF4GI has been shown to be sensitive to proteasomal degradation, how the eIF4GI steady-state level is controlled remains unknown. Here, we show that eIF4GI exists in a complex with NAD(P)H quinone-oxydoreductase 1 (NQO1) in cell extracts. Treatment of cells with dicumarol (dicoumarol), a pharmacological inhibitor of NQO1 known to preclude NQO1 binding to its protein partners, provokes eIF4GI degradation by the proteasome. Consistently, the eIF4GI steady-state level also diminishes upon the silencing of NQO1 (by transfection with small interfering RNA), while eIF4GI accumulates upon the overexpression of NQO1 (by transfection with cDNA). We further reveal that treatment of cells with dicumarol frees eIF4GI from mRNA translation initiation complexes due to strong activation of its natural competitor, the translational repressor 4E-BP1. As a consequence of cap-binding complex dissociation and eIF4GI degradation, protein synthesis is dramatically inhibited. Finally, we show that the regulation of eIF4GI stability by the proteasome may be prominent under oxidative stress. Our findings assign NQO1 an original role in the regulation of mRNA translation via the control of eIF4GI stability by the proteasome.

Triazoloacridin-6-ones as novel inhibitors of the quinone oxidoreductases NQO1 and NQO2

A range of triazoloacridin-6-ones functionalized at C5 and C8 have been synthesized and evaluated for ability to inhibit NQO1 and NQO2. The compounds were computationally docked into the active site of NQO1 and NQO2, and calculated binding affinities were compared with IC(50) values for enzyme inhibition. Excellent correlation coefficients were demonstrated suggesting a predictive QSAR model for this series of structurally similar analogues. From this we have identified some of these triazoloacridin-6-ones to be the most potent NQO2 inhibitors so far reported.

Commitment of 1-methyl-4-phenylpyrinidinium ion-induced neuronal cell death by proteasome-mediated degradation of p35 cyclin-dependent kinase 5 activator

The dysfunction of proteasomes and mitochondria has been implicated in the pathogenesis of Parkinson disease. However, the mechanism by which this dysfunction causes neuronal cell death is unknown. We studied the role of cyclin-dependent kinase 5 (Cdk5)-p35 in the neuronal cell death induced by 1-methyl-4-phenylpyrinidinium ion (MPP+), which has been used as an in vitro model of Parkinson disease. When cultured neurons were treated with 100 microM MPP+, p35 was degraded by proteasomes at 3 h, much earlier than the neurons underwent cell death at 12-24 h. The degradation of p35 was accompanied by the down-regulation of Cdk5 activity. We looked for the primary target of MPP+ that triggered the proteasome-mediated degradation of p35. MPP+ treatment for 3 h induced the fragmentation of the mitochondria, reduced complex I activity of the respiratory chain without affecting ATP levels, and impaired the mitochondrial import system. The dysfunction of the mitochondrial import system is suggested to up-regulate proteasome activity, leading to the ubiquitin-independent degradation of p35. The overexpression of p35 attenuated MPP+-induced neuronal cell death. In contrast, depletion of p35 with short hairpin RNA not only induced cell death but also sensitized to MPP+ treatment. These results indicate that a brief MPP+ treatment triggers the delayed neuronal cell death by the down-regulation of Cdk5 activity via mitochondrial dysfunction-induced up-regulation of proteasome activity. We propose a role for Cdk5-p35 as a survival factor in countering MPP+-induced neuronal cell death.

Impact of NAD(P)H:quinone oxidoreductase-1 on pigmentation

We obtained metastasized melanoma tissue from a primary acral lentiginous melanoma (ALM) patient and established a melanoma cell line named primary culture of melanoma cell derived from lymph node (PML)-1. PML-1 cells had a light brown color and decreased the expression of melanogenesis markers, including tyrosinase (TYR), microphthalmia-associated transcription factor, and tyrosinase-related protein-1. To identify genes differentially regulated in PML-1 melanoma cells, we performed DNA microarray and two-dimensional matrix-assisted laser desorption ionization-time of flight mass spectrometry analyses. Among the candidate genes identified, we chose NAD(P)H:quinone oxidoreductase-1 (NQO1) for further study. Reverse transcription-PCR and western blot analyses showed that NQO1 was markedly decreased in PML-1 cells and in several amelanotic melanoma cell lines. To investigate whether NQO1 affects the melanogenesis, we treated the cultured normal human melanocytes (NHMC) and zebrafish with NQO1 inhibitors, ES936 and dicoumarol. Interestingly, melanogenesis was significantly decreased by the addition of NQO1 inhibitors in both NHMC and zebrafish models. In contrast, overexpression of NQO1 using a recombinant adenovirus clearly induced melanogenesis, concomitantly with an increase of TYR protein level. These results suggest that NQO1 is a positive regulator of the pigmentation process.

Misfolding of proteins with a polyglutamine expansion is facilitated by proteasomal chaperones

Deposition of misfolded proteins with a polyglutamine expansion is a hallmark of Huntington disease and other neurodegenerative disorders. Impairment of the proteolytic function of the proteasome has been reported to be both a cause and a consequence of polyglutamine accumulation. Here we found that the proteasomal chaperones that unfold proteins to be degraded by the proteasome but also have non-proteolytic functions co-localized with huntingtin inclusions both in primary neurons and in Huntington disease patients and formed a complex independently of the proteolytic particle. Overexpression of Rpt4 or Rpt6 facilitated aggregation of mutant huntingtin and ataxin-3 without affecting proteasomal degradation. Conversely, reducing Rpt6 or Rpt4 levels decreased the number of inclusions in primary neurons, indicating that endogenous Rpt4 and Rpt6 facilitate inclusion formation. In vitro reconstitution experiments revealed that purified 19S particles promote mutant huntingtin aggregation. When fused to the ornithine decarboxylase destabilizing sequence, proteins with expanded polyglutamine were efficiently degraded and did not aggregate. We propose that aggregation of proteins with expanded polyglutamine is not a consequence of a proteolytic failure of the 20S proteasome. Rather, aggregation is elicited by chaperone subunits of the 19S particle independently of proteolysis.

Contrasting proteome biology and functional heterogeneity of the 20 S proteasome complexes in mammalian tissues

The 20 S proteasome complexes are major contributors to the intracellular protein degradation machinery in mammalian cells. Systematic administration of proteasome inhibitors to combat disease (e.g. cancer) has resulted in positive outcomes as well as adversary effects. The latter was attributed to, at least in part, a lack of understanding in the organ-specific responses to inhibitors and the potential diversity of proteomes of these complexes in different tissues. Accordingly, we conducted a proteomic study to characterize the 20 S proteasome complexes and their postulated organ-specific responses in the heart and liver. The cardiac and hepatic 20 S proteasomes were isolated from the same mouse strain with identical genetic background. We examined the molecular composition, complex assembly, post-translational modifications and associating partners of these proteasome complexes. Our results revealed an organ-specific molecular organization of the 20 S proteasomes with distinguished patterns of post-translational modifications as well as unique complex assembly characteristics. Furthermore, the proteome diversities are concomitant with a functional heterogeneity of the proteolytic patterns exhibited by these two organs. In particular, the heart and liver displayed distinct activity profiles to two proteasome inhibitors, epoxomicin and Z-Pro-Nle-Asp-H. Finally, the heart and liver demonstrated contrasting regulatory mechanisms from the associating partners of these proteasomes. The functional heterogeneity of the mammalian 20 S proteasome complexes underscores the concept of divergent proteomes among organs in the context of an identical genome.

Ubiquitin-independent degradation of proteins by the proteasome

The proteasome is the main proteolytic machinery of the cell and constitutes a recognized drugable target, in particular for treating cancer. It is involved in the elimination of misfolded, altered or aged proteins as well as in the generation of antigenic peptides presented by MHC class I molecules. It is also responsible for the proteolytic maturation of diverse polypeptide precursors and for the spatial and temporal regulation of the degradation of many key cell regulators whose destruction is necessary for progression through essential processes, such as cell division, differentiation and, more generally, adaptation to environmental signals. It is generally believed that proteins must undergo prior modification by polyubiquitin chains to be addressed to, and recognized by, the proteasome. In reality, however, there is accumulating evidence that ubiquitin-independent proteasomal degradation may have been largely underestimated. In particular, a number of proto-oncoproteins and oncosuppressive proteins are privileged ubiquitin-independent proteasomal substrates, the altered degradation of which may have tumorigenic consequences. The identification of ubiquitin-independent mechanisms for proteasomal degradation also poses the paramount question of the multiplicity of catabolic pathways targeting each protein substrate. As this may help design novel therapeutic strategies, the underlying mechanisms are critically reviewed here.

NF-kappaB dictates the degradation pathway of IkappaBalpha

IkappaB proteins are known as the regulators of NF-kappaB activity. They bind tightly to NF-kappaB dimers, until stimulus-responsive N-terminal phosphorylation by IKK triggers their ubiquitination and proteasomal degradation. It is known that IkappaBalpha is an unstable protein whose rapid degradation is slowed upon binding to NF-kappaB, but it is not known what dynamic mechanisms control the steady-state level of total IkappaBalpha. Here, we show clearly that two degradation pathways control the level of IkappaBalpha. Free IkappaBalpha degradation is not controlled by IKK or ubiquitination but intrinsically, by the C-terminal sequence known as the PEST domain. NF-kappaB binding to IkappaBalpha masks the PEST domain from proteasomal recognition, precluding ubiquitin-independent degradation; bound IkappaBalpha then requires IKK phosphorylation and ubiquitination for slow basal degradation. We show the biological requirement for the fast degradation of the free IkappaBalpha protein; alteration of free IkappaBalpha degradation dampens NF-kappaB activation. In addition, we find that both free and bound IkappaBalpha are similar substrates for IKK, and the preferential phosphorylation of NF-kappaB-bound IkappaBalpha is due to stabilization of IkappaBalpha by NF-kappaB.

NAD(P)H quinone oxidoreductase 1 inhibits the proteasomal degradation of the tumour suppressor p33(ING1b)

The tumour suppressor p33(ING1b) ((ING1b) for inhibitor of growth family, member 1b) is important in cellular stress responses, including cell-cycle arrest, apoptosis, chromatin remodelling and DNA repair; however, its degradation pathway is still unknown. Recently, we showed that genotoxic stress induces p33(ING1b) phosphorylation at Ser 126, and abolishment of Ser 126 phosphorylation markedly shortened its half-life. Therefore, we suggest that Ser 126 phosphorylation modulates the interaction of p33(ING1b) with its degradation machinery, stabilizing this protein. Combining the use of inhibitors of the main degradation pathways in the nucleus (proteasome and calpains), partial isolation of the proteasome complex, and in vitro interaction and degradation assays, we set out to determine the degradation mechanism of p33(ING1b). We found that p33(ING1b) is degraded in the 20S proteasome and that NAD(P)H quinone oxidoreductase 1 (NQO1), an oxidoreductase previously shown to modulate the degradation of p53 in the 20S proteasome, inhibits the degradation of p33(ING1b). Furthermore, ultraviolet irradiation induces p33(ING1b) phosphorylation at Ser 126, which, in turn, facilitates its interaction with NQO1.

Stabilization of RelB requires multidomain interactions with p100/p52

The NF-kappaB family member RelB has many properties not shared by other family members such as restricted subunit association and lack of regulation by the classical IkappaB proteins. We show that the protein level of RelB is significantly reduced in the absence of p100 and reduced even more when both p100 and p105 are absent. RelB stabilizes itself by directly interacting with p100, p105, and their processed products. However, RelB forms complexes with its partners using different interaction modes. Although the C-terminal ankyrin repeat domain of p105 is not involved in the RelB-p105 complex formation, all domains and flexible regions of each protein are engaged in the RelB-p100 complex. In several respects the RelB-p52 and RelB-p100 complexes are unique in the NF-kappaB family. The N-terminal domain of p100/p52 interacts with RelB but not RelA. The transcriptional activation domain of RelB, but not RelA, directly interacts with the processing region of p100. These unique protein-protein contacts explain why RelB prefers p52 as its dimeric partner for transcriptional activity and is retained in the cytoplasm as an inhibited complex by p100. This association-mediated stabilization of RelB implies a possible role for RelB in the processing of p100 into p52.

Role of proteasomes in disease

A functional ubiquitin proteasome system is essential for all eukaryotic cells and therefore any alteration to its components has potential pathological consequences. Though the exact underlying mechanism is unclear, an age-related decrease in proteasome activity weakens cellular capacity to remove oxidatively modified proteins and favours the development of neurodegenerative and cardiac diseases. Up-regulation of proteasome activity is characteristic of muscle wasting conditions including sepsis, cachexia and uraemia, but may not be rate limiting. Meanwhile, enhanced presence of immunoproteasomes in aging brain and muscle tissue could reflect a persistent inflammatory defence and anti-stress mechanism, whereas in cancer cells, their down-regulation reflects a means by which to escape immune surveillance. Hence, induction of apoptosis by synthetic proteasome inhibitors is a potential treatment strategy for cancer, whereas for other diseases such as neurodegeneration, the use of proteasome-activating or -modulating compounds could be more effective. Publication history: Republished from Current BioData's Targeted Proteins database (TPdb; http://www.targetedproteinsdb.com).