Integration of the ubiquitin-proteasome pathway with a cytosolic oligopeptidase activity

An adaptive stress response that confers cellular resilience to decreased ubiquitination

Ubiquitination is a post-translational modification initiated by the E1 enzyme UBA1, which transfers ubiquitin to ~35 E2 ubiquitin-conjugating enzymes. While UBA1 loss is cell lethal, it remains unknown how partial reduction in UBA1 activity is endured. Here, we utilize deep-coverage mass spectrometry to define the E1-E2 interactome and to determine the proteins that are modulated by knockdown of UBA1 and of each E2 in human cells. These analyses define the UBA1/E2-sensitive proteome and the E2 specificity in protein modulation. Interestingly, profound adaptations in peroxisomes and other organelles are triggered by decreased ubiquitination. While the cargo receptor PEX5 depends on its mono-ubiquitination for binding to peroxisomal proteins and importing them into peroxisomes, we find that UBA1/E2 knockdown induces the compensatory upregulation of other PEX proteins necessary for PEX5 docking to the peroxisomal membrane. Altogether, this study defines a homeostatic mechanism that sustains peroxisomal protein import in cells with decreased ubiquitination capacity.

Modulation of protease expression by the transcription factor Ptx1/PITX regulates protein quality control during aging

Protein quality control is important for healthy aging and is dysregulated in age-related diseases. The autophagy-lysosome and ubiquitin-proteasome are key for proteostasis, but it remains largely unknown whether other proteolytic systems also contribute to maintain proteostasis during aging. Here, we find that expression of proteolytic enzymes (proteases/peptidases) distinct from the autophagy-lysosome and ubiquitin-proteasome systems declines during skeletal muscle aging in Drosophila. Age-dependent protease downregulation undermines proteostasis, as demonstrated by the increase in detergent-insoluble poly-ubiquitinated proteins and pathogenic huntingtin-polyQ levels in response to protease knockdown. Computational analyses identify the transcription factor Ptx1 (homologous to human PITX1/2/3) as a regulator of protease expression. Consistent with this model, Ptx1 protein levels increase with aging, and Ptx1 RNAi counteracts the age-associated downregulation of protease expression. Moreover, Ptx1 RNAi improves muscle protein quality control in a protease-dependent manner and extends lifespan. These findings indicate that proteases and their transcriptional modulator Ptx1 ensure proteostasis during aging.

Nilabh Shastri - Towards understanding classical and non-classical MHC-I antigen processing and presentation

Major histocompatibility complex (MHC-I) peptide antigen processing and presentation has experienced a revived interest in the context of immuno oncology, immune surveillance escape by pathogen mutations and technical advances that accelerate vaccine design. This sheds new light on the discoveries made by Nilabh Shastri and colleagues that includes the characterisation of cryptic MHC-I peptide antigen epitopes derived from untranslated regions and the N-terminal trimming of peptide antigen precursors by the aminopeptidase ERAAP (ERAP1/2 / ARTS1/LRAP) in the endoplasmic reticulum (ER) prior to the complete assembly of MHC-I complexes and their subsequent exposure to the cell surface. These scientific findings have important implications for developing novel therapeutic approaches in immunotherapy and modern vaccine design.

Contribution of proteases to the hallmarks of aging and to age-related neurodegeneration

Protein quality control ensures the degradation of damaged and misfolded proteins. Derangement of proteostasis is a primary cause of aging and age-associated diseases. The ubiquitin-proteasome and autophagy-lysosome play key roles in proteostasis but, in addition to these systems, the human genome encodes for ~600 proteases, also known as peptidases. Here, we examine the role of proteases in aging and age-related neurodegeneration. Proteases are present across cell compartments, including the extracellular space, and their substrates encompass cellular constituents, proteins with signaling functions, and misfolded proteins. Proteolytic processing by proteases can lead to changes in the activity and localization of substrates or to their degradation. Proteases cooperate with the autophagy-lysosome and ubiquitin-proteasome systems but also have independent proteolytic roles that impact all hallmarks of cellular aging. Specifically, proteases regulate mitochondrial function, DNA damage repair, cellular senescence, nutrient sensing, stem cell properties and regeneration, protein quality control and stress responses, and intercellular signaling. The capacity of proteases to regulate cellular functions translates into important roles in preserving tissue homeostasis during aging. Consequently, proteases influence the onset and progression of age-related pathologies and are important determinants of health span. Specifically, we examine how certain proteases promote the progression of Alzheimer's, Huntington's, and/or Parkinson's disease whereas other proteases protect from neurodegeneration. Mechanistically, cleavage by proteases can lead to the degradation of a pathogenic protein and hence impede disease pathogenesis. Alternatively, proteases can generate substrate byproducts with increased toxicity, which promote disease progression. Altogether, these studies indicate the importance of proteases in aging and age-related neurodegeneration.

Proteasome stress in skeletal muscle mounts a long-range protective response that delays retinal and brain aging

Neurodegeneration in the central nervous system (CNS) is a defining feature of organismal aging that is influenced by peripheral tissues. Clinical observations indicate that skeletal muscle influences CNS aging, but the underlying muscle-to-brain signaling remains unexplored. In Drosophila, we find that moderate perturbation of the proteasome in skeletal muscle induces compensatory preservation of CNS proteostasis during aging. Such long-range stress signaling depends on muscle-secreted Amyrel amylase. Mimicking stress-induced Amyrel upregulation in muscle reduces age-related accumulation of poly-ubiquitinated proteins in the brain and retina via chaperones. Preservation of proteostasis stems from the disaccharide maltose, which is produced via Amyrel amylase activity. Correspondingly, RNAi for SLC45 maltose transporters reduces expression of Amyrel-induced chaperones and worsens brain proteostasis during aging. Moreover, maltose preserves proteostasis and neuronal activity in human brain organoids challenged by thermal stress. Thus, proteasome stress in skeletal muscle hinders retinal and brain aging by mounting an adaptive response via amylase/maltose.

Alternative Antigen Processing for MHC Class I: Multiple Roads Lead to Rome

The well described conventional antigen-processing pathway is accountable for most peptides that end up in MHC class I molecules at the cell surface. These peptides experienced liberation by the proteasome and transport by the peptide transporter TAP. However, there are multiple roads that lead to Rome, illustrated by the increasing number of alternative processing pathways that have been reported during last years. Interestingly, TAP-deficient individuals do not succumb to viral infections, suggesting that CD8 T cell immunity is sufficiently supported by alternative TAP-independent processing pathways. To date, a diversity of viral and endogenous TAP-independent peptides have been identified in the grooves of different MHC class I alleles. Some of these peptides are not displayed by normal TAP-positive cells and we therefore called them TEIPP, for “T-cell epitopes associated with impaired peptide processing.” TEIPPs are hidden self-antigens, are derived from normal housekeeping proteins, and are processed via unconventional processing pathways. Per definition, TEIPPs are presented via TAP-independent pathways, but recent data suggest that part of this repertoire still depend on proteasome and metalloprotease activity. An exception is the C-terminal peptide of the endoplasmic reticulum (ER)-membrane-spanning ceramide synthase Trh4 that is surprisingly liberated by the signal peptide peptidase (SPP), the proteolytic enzyme involved in cleaving leader sequences. The intramembrane cleaving SPP is thereby an important contributor of TAP-independent peptides. Its family members, like the Alzheimer’s related presenilins, might contribute as well, according to our preliminary data. Finally, alternative peptide routing is an emerging field and includes processes like the unfolded protein response, the ER-associated degradation, and autophagy-associated vesicular pathways. These data convince us that there is a world to be discovered in the field of unconventional antigen processing.

The stability of NR2B in the nucleus accumbens controls behavioral and synaptic adaptations to chronic stress

BACKGROUND

The nucleus accumbens (NAc) is closely correlated with depression. It has been demonstrated that the glutamatergic system in NAc plays an important role in the reward pathway, dysfunction of which would cause anhedonia, a core symptom of depression. We therefore tested whether N-methyl-D-aspartate receptors and the synaptic plasticity in the NAc are regulated by chronic stress and the relevance to depression.

METHODS

We applied behavioral tests (n = 12, each group) of social interaction and sucrose preference tests to identify the susceptibility of mice to chronic social defeat stress. We then tested N-methyl-D-aspartate receptor-long-term depression at cortico-accumbal synapse to determine the relationship between the susceptibility and changes in synaptic plasticity (n = 8, each group). We further investigated whether restoration of these changes could produce antidepressant effects (n = 10).

RESULTS

We found that chronic stress induced selective downregulation of N-methyl-D-aspartate receptor NR2B subunits in the confined surface membrane pool of NAc neurons. Remarkably, the loss of synaptic NR2B was a long-lived event and further translated to the significant modulation of synaptic plasticity in the form of long-term depression. We further observed that the stress-induced changes were restored by fluoxetine and that resilient mice-those resistant to chronic stress-showed patterns of molecular regulation in the NAc that overlapped dramatically with those seen with fluoxetine treatment. Behaviorally, restoration of NR2B loss prevented the behavioral sensitization of mice to chronic stress.

CONCLUSIONS

Our results identify NR2B in the NAc as a key regulator in the modulation of persistent psychomotor plasticity in response to chronic stress.

Tripeptidyl peptidase II regulates sperm function by modulating intracellular Ca(2+) stores via the ryanodine receptor

Recent studies have identified Ca²⁺ stores in sperm cells; however, it is not clear whether these Ca²⁺ stores are functional and how they are mobilized. Here, in vitro and in vivo, we determined that tripeptidyl peptidase II antagonists strongly activated the cAMP/PKA signaling pathway that drives sperm capacitation-associated protein tyrosine phosphorylation. We demonstrated that in the absence of Ca²⁺, TPIII antagonists elevated the intracellular Ca²⁺ levels in sperm, resulting in a marked improvement in sperm movement, capacitation, acrosome reaction, and the in vitro fertilizing ability. This antagonist-induced release of intracellular Ca²⁺ could be blocked by the inhibitors of ryanodine receptors (RyRs) which are the main intracellular Ca²⁺ channels responsible for releasing stored Ca²⁺. Consistent with these results, indirect immunofluorescence assay using anti-RyR antibodies further validated the presence of RyR₃ in the acrosomal region of mature sperm. Thus, TPPII can regulate sperm maturation by modulating intracellular Ca²⁺ stores via the type 3 RyR.

Short peptide tools for monitoring caspase and proteasome activities in embryonal and adult rat brain lysates: an approach for the differential identification of proteases

The numerous caspase-like activities present in nervous tissue can be investigated with labelled peptides. However, the cross-reactivities of peptides with both proteasomes and caspases complicate the analysis of protease activity. The pharmacological features of substrates and inhibitors specific for either caspases or proteasome caspase-like proteases in rat brain lysates were similar or identical to the profiles of commercially purified proteasome preparations. Caspase inhibitors bind directly to active proteasome centres, thus competing with selective antagonists of proteasomes. Separation of lysates by molecular weight does not separate active caspases from proteasomes because these enzymes co-localize under native electrophoresis. The addition of ATP or its analogues is associated with the differential modulation of proteasomal activity, which also leads to ambiguity in the data. However, induced caspase activity could be successfully differentiated from proteasome activity in embryonal brain lysates with the non-selective caspase inhibitors Z-VAD-FMK and Q-VD-OPh and the proteasome inhibitor AdaAhx(3)L(3)VS that are not cross-reactive. This strategy is proposed for the simultaneous examination of caspases and proteasomes using proteolysis experiments. The present study reveals that all of the caspase-like activities in the tissue lysates of non-injured adult rat brains were related to proteasomal caspase-like activities.

Tripeptidyl Peptidase II Is Required for c-MYC-Induced Centriole Overduplication and a Novel Therapeutic Target in c-MYC-Associated Neoplasms

Centrosome aberrations are frequently detected in c-MYC-associated human malignancies. Here, we show that c-MYC-induced centrosome and centriole overduplication critically depend on the protease tripeptidyl peptidase II (TPPII). We found that TPPII localizes to centrosomes and that overexpression of TPPII, similar to c-MYC, can disrupt centriole duplication control and cause centriole multiplication, a process during which maternal centrioles nucleate the formation of more than a single daughter centriole. We report that inactivation of TPPII using chemical inhibitors or siRNA-mediated protein knockdown effectively reduced c-MYC-induced centriole overduplication. Remarkably, the potent and selective TPPII inhibitor butabindide not only potently suppressed centriole aberrations but also caused significant cell death and growth suppression in aggressive human Burkitt lymphoma cells with c-MYC overexpression. Taken together, these results highlight the role of TPPII in c-MYC-induced centriole overduplication and encourage further studies to explore TPPII as a novel antineoplastic drug target.

The Enigma of Tripeptidyl-Peptidase II: Dual Roles in Housekeeping and Stress

The tripeptidyl-peptidase II complex consists of repeated 138 kDa subunits, assembled into two twisted strands that form a high molecular weight complex (>5 MDa). TPPII, like many other cytosolic peptidases, plays a role in the ubiquitin-proteasome pathway downstream of the proteasome as well as in the production and destruction of MHC class I antigens and degradation of neuropeptides. Tripeptidyl-peptidase II activity is increased in cells with an increased demand for protein degradation, but whether degradation of cytosolic peptides is the only cell biological role for TPPII has remained unclear. Recent data indicated that TPPII translocates into the nucleus to control DNA damage responses in malignant cells, supporting that cytosolic “housekeeping peptidases” may have additional roles in cell biology, besides their contribution to protein turnover. Overall, TPPII has an emerging importance in several cancer-related fields, such as metabolism, cell death control, and control of genome integrity; roles that are not understood in detail. The present paper reviews the cell biology of TPPII and discusses distinct roles for TPPII in the nucleus and cytosol.

Accumulation of polyubiquitylated proteins in response to Ala-Ala-Phe-chloromethylketone is independent of the inhibition of Tripeptidyl peptidase II

In the present study we have addressed the issue of proteasome independent cytosolic protein degradation. Tripeptidyl peptidase II (TPPII) has been suggested to compensate for a reduced proteasome activity, partly based on evidence using the inhibitor Ala-Ala-Phe-chloromethylketone (AAF-cmk). Here we show that AAF-cmk induces the formation of polyubiquitin-containing accumulations in osteosarcoma and Burkitt's lymphoma cell lines. These accumulations meet many of the landmarks of the aggresomes that form after proteasome inhibition. Using a combination of experiments with chemical inhibitors and interference of gene expression, we show that TPPII inhibition is not responsible for these accumulations. Our evidence suggests that the relevant target(s) is/are in the ubiquitin-proteasome pathway, most likely upstream the proteasome. We obtained evidence supporting this model by inhibition of Hsp90, which also acts upstream the proteasome. Although our data suggest that Hsp90 is not a target of AAF-cmk, its inhibition resulted in accumulations similar to those obtained with AAF-cmk. Therefore, our results question the proposed role for TPPII as a prominent alternative to the proteasome in cellular proteolysis.

Proteasomes are not a target for doxorubicin in feline injection-site sarcoma

The potent anti-cancer agent doxorubicin (DOX) induces apoptosis of rapidly proliferating cells by inhibiting cellular proteasomes. The aim of the present study was to determine whether DOX modulates the level of expression and function of proteasomes in feline injection-site sarcoma (FISS). Tissue extracts from primary sarcoma lesions and the related healthy subcutis of 18 cats affected by FISS were investigated. Nine of these cats had received neoadjuvant DOX treatment and nine cats did not receive this therapy. There was enhanced proteasome expression in FISS, but this was not affected by administration of DOX. This finding may account for the low clinical effectiveness of DOX therapy in FISS and provides the rationale for developing new therapeutic protocols aimed at achieving better proteasomal inhibition in FISS and other tumours that respond poorly to DOX therapy.

Proteases in MHC class I presentation and cross-presentation

Cells that have mutated their genes or are virally infected are a potential threat to a host. Consequently, the immune system has evolved mechanisms for CD8 T lymphocytes to identify such cells and eliminate them. The generation of CD8 T cell responses occurs in two phases, both of which critically involve the process of Ag presentation. In the first phase, sentinel cells gather Ags present in tissues and then present them to naive CD8 T cells in ways that stimulate their maturation into effectors. In the second phase, these effector cells seek out and eliminate the pathological cells. The abnormal cells are identified through their presentation of immunogenic Ags that they are producing. The Ag presentation mechanisms used by the sentinel cells can be different from those in other cells. This article will review these mechanisms with a focus in each case on how antigenic peptides are generated for presentation.

Analysis of the role of tripeptidyl peptidase II in MHC class I antigen presentation in vivo

Previous experiments using enzyme inhibitors and RNA interference in cell lysates and cultured cells have suggested that tripeptidyl peptidase II (TPPII) plays a role in creating and destroying MHC class I-presented peptides. However, its precise contribution to these processes has been controversial. To elucidate the importance of TPPII in MHC class I Ag presentation, we analyzed TPPII-deficient gene-trapped mice and cell lines from these animals. In these mice, the expression level of TPPII was reduced by >90% compared with wild-type mice. Thymocytes from TPPII gene-trapped mice displayed more MHC class I on the cell surface, suggesting that TPPII normally limits Ag presentation by destroying peptides overall. TPPII gene-trapped mice responded as well as did wild-type mice to four epitopes from lymphocytic choriomeningitis virus. The processing and presentation of peptide precursors with long N-terminal extensions in TPPII gene-trapped embryonic fibroblasts was modestly reduced, but in vivo immunization with recombinant lentiviral or vaccinia virus vectors revealed that such peptide precursors induced an equivalent CD8 T cell response in wild-type and TPPII-deficient mice. These data indicate that while TPPII contributes to the trimming of peptides with very long N-terminal extensions, TPPII is not essential for generating most MHC class I-presented peptides or for stimulating CTL responses to several Ags in vivo.

Chronic ethanol feeding affects proteasome-interacting proteins

Studies on alcoholic liver injury mechanisms show a significant inhibition of the proteasome activity. To investigate this phenomenon, we isolated proteasome complexes from the liver of rats fed ethanol chronically, and from the liver of their pair-fed controls, using a non-denaturing multiple centrifugations procedure to preserve proteasome-interacting proteins (PIPs). ICAT and MS/MS spectral counting, further confirmed by Western blot, showed that the levels of several PIPs were significantly decreased in the isolated ethanol proteasome fractions. This was the case of PA28alpha/beta proteasome activator subunits, and of three proteasome-associated deubiquitinases, Rpn11, ubiquitin C-terminal hydrolase 14, and ubiquitin carboxyl-terminal hydrolase L5. Interestingly, Rpn13 C-terminal end was missing in the ethanol proteasome fraction, which probably altered the linking of ubiquitin carboxyl-terminal hydrolase L5 to the proteasome. 20S proteasome and most 19S subunits were however not changed but Ecm29, a protein known to stabilize the interactions between the 20S and its activators, was decreased in the isolated ethanol proteasome fractions. It is proposed that ethanol metabolism causes proteasome inhibition by several mechanisms, including by altering PIPs and proteasome regulatory complexes binding to the proteasome.

Molecular mechanisms of MHC class I-antigen processing: redox considerations

Major histocompatibility complex (MHC) class I molecules present antigenic peptides to the cell surface for screening by CD8(+) T cells. A number of ER-resident chaperones assist the assembly of peptides onto MHC class I molecules, a process that can be divided into several steps. Early folding of the MHC class I heavy chain is followed by its association with beta(2)-microglobulin (beta(2)m). The MHC class I heavy chain-beta(2)m heterodimer is incorporated into the peptide-loading complex, leading to peptide loading, release of the peptide-filled MHC class I molecules from the peptide-loading complex, and exit of the complete MHC class I complex from the ER. Because proper antigen presentation is vital for normal immune responses, the assembly of MHC class I molecules requires tight regulation. Emerging evidence indicates that thiol-based redox regulation plays critical roles in MHC class I-restricted antigen processing and presentation, establishing an unexpected link between redox biology and antigen processing. We review the influences of redox regulation on antigen processing and presentation. Because redox signaling pathways are a rich source of validated drug targets, newly discovered redox biology-mediated mechanisms of antigen processing may facilitate the development of more selective and therapeutic drugs or vaccines against immune diseases.

Is there an alternative to the proteasome in cytosolic protein degradation?

While it is clear that the proteasome is the major player in degradative proteolysis in the nucleus and cytosol, there is a lack of complete agreement on whether there are alternative proteolytic pathways or activities responsible for a significant degradation of cytosolic/nuclear substrates. Particularly relevant is the case of the aminopeptidase TPPII (tripeptidyl peptidase II), which has been suggested to be able to perform some of the proteasome functions. However, the current evidence seems to support only a limited role for these cytosolic alternatives. On the other hand, there is evidence of an alternative, autophagy, a pathway involving the delivery of cytosolic substrates to the lysosome for degradation.

Molecular basis of bortezomib resistance: proteasome subunit beta5 (PSMB5) gene mutation and overexpression of PSMB5 protein

The proteasome inhibitor bortezomib is a novel anticancer drug that has shown promise in the treatment of refractory multiple myeloma. However, its clinical efficacy has been hampered by the emergence of drug-resistance phenomena, the molecular basis of which remains elusive. Toward this end, we here developed high levels (45- to 129-fold) of acquired resistance to bortezomib in human myelomonocytic THP1 cells by exposure to stepwise increasing (2.5-200 nM) concentrations of bortezomib. Study of the molecular mechanism of bortezomib resistance in these cells revealed (1) an Ala49Thr mutation residing in a highly conserved bortezomib-binding pocket in the proteasome beta5-subunit (PSMB5) protein, (2) a dramatic overexpression (up to 60-fold) of PSMB5 protein but not of other proteasome subunits including PSMB6, PSMB7, and PSMA7, (3) high levels of cross-resistance to beta5 subunit-targeted cytotoxic peptides 4A6, MG132, MG262, and ALLN, but not to a broad spectrum of chemotherapeutic drugs, (4) no marked changes in chymotrypsin-like proteasome activity, and (5) restoration of bortezomib sensitivity in bortezomib-resistant cells by siRNA-mediated silencing of PSMB5 gene expression. Collectively, these findings establish a novel mechanism of bortezomib resistance associated with the selective overexpression of a mutant PSMB5 protein.

Tripeptidyl peptidase II is dispensable for the generation of both proteasome-dependent and proteasome-independent ligands of HLA-B27 and other class I molecules

A significant fraction of the HLA-B27-bound peptide repertoire is resistant to proteasome inhibitors. The possible implication of tripeptidyl peptidase II (TPPII) in generating this subset was analyzed by quantifying the surface re-expression of HLA-B*2705 after acid stripping in the presence of two TPPII inhibitors, butabindide and Ala-Ala-Phe-chloromethylketone. Neither decreased HLA-B27 re-expression under conditions in which TPPII activity was largely inhibited. This was in contrast to a significant effect of the proteasome inhibitor epoxomicin. The failure of TPPII inhibition to decrease surface re-expression was not limited to HLA-B27, since it was also observed in several HLA-B27-negative cell lines, including Mel JuSo. Actually, HLA class I re-expression in Mel JuSo cells increased as a function of butabindide concentration, which is consistent with an involvement of TPPII in destroying HLA class I ligands. Inhibition of TPPII with small interfering RNA also failed to decrease the surface expression of HLA class I molecules on 143B cells. Our results indicate that TPPII is dispensable for the generation of proteasome-dependent HLA class I ligands and, without excluding its role in producing some individual epitopes, this enzyme is not involved to any quantitatively significant extent, in generating the proteasome-independent HLA-B27-bound peptide repertoire.

Activation of cellular death programs associated with immunosenescence-like phenotype in TPPII knockout mice

The giant cytosolic protease tripeptidyl peptidase II (TPPII) has been implicated in the regulation of proliferation and survival of malignant cells, particularly lymphoma cells. To address its functions in normal cellular and systemic physiology we have generated TPPII-deficient mice. TPPII deficiency activates cell type-specific death programs, including proliferative apoptosis in several T lineage subsets and premature cellular senescence in fibroblasts and CD8(+) T cells. This coincides with up-regulation of p53 and dysregulation of NF-kappaB. Prominent degenerative alterations at the organismic level were a decreased lifespan and symptoms characteristic of immunohematopoietic senescence. These symptoms include accelerated thymic involution, lymphopenia, impaired proliferative T cell responses, extramedullary hematopoiesis, and inflammation. Thus, TPPII is important for maintaining normal cellular and systemic physiology, which may be relevant for potential therapeutic applications of TPPII inhibitors.

Increased expression and altered subunit composition of proteasomes induced by continuous proteasome inhibition establish apoptosis resistance and hyperproliferation of Burkitt lymphoma cells

The proteasome is the main protease for extralysosomal protein degradation in eukaryotic cells, and constitutes a sophisticated high molecular mass proteinase complex underlying a tightly coordinated expression and assembly of multiple subunits and subcomplexes. Here we show that continuous inhibition of proteasomal chymotrypsin-like peptidase activity by the proteasome inhibitor bortezomib induces in human Namalwa Burkitt lymphoma cells increased de novo biogenesis of proteasomes accompanied by increased expression of the proteasome maturation protein POMP, increased expression of 19S-20S-19S proteasomes, and abrogation of expression of beta 1i, beta 2i and beta 5i immunosubunits and PA28 in favor of increased expression of constitutive proteolytic beta1, beta2 and beta 5 subunits and 19S regulatory complexes. These alterations of proteasome expression and subunit composition are accompanied by an increase in proteasomal caspase-like, trypsin-like and chymotrypsin-like peptidase activities, not inhibitable by high doses of bortezomib. Cells harboring these proteasomal alterations display rapid proliferation and cell cycle progression, and acquire resistance to apoptosis induced by proteasome inhibitors, gamma-irradiation and staurosporine. This acquired apoptosis resistance is accompanied by de novo expression of anti-apoptotic Hsp27 protein and the loss of ability to accumulate and stabilize pro-apoptotic p53 protein. Thus, increased expression, altered subunit composition and increased activity of proteasomes constitute a hitherto unknown adaptive and autoregulatory feedback mechanism to allow cells to survive the lethal challenge of proteasome inhibition and to establish a hyperproliferative and apoptosis-resistant phenotype.

A tale of two giant proteases

The 26S proteasome and tripeptidyl peptidase II (TPPII) are two exceptionally large eukaryotic protein complexes involved in intracellular proteolysis, where they exert their function sequentially: the proteasome, a multisubunit complex of 2.5 MDa, acts at the downstream end of the ubiquitin pathway and degrades ubiquitinylated proteins into small oligopeptides. Such oligopeptides are substrates for TPPII, a 6-MDa homooligomer, which releases tripeptides from their free N-terminus. Both 26S and TPPII are very fragile complexes refractory to crystallization and in their fully assembled native form have been visualized only by electron microscopy. Here, we will discuss the structural features of the two complexes and their functional implications.

Adaptive modification and flexibility of the proteasome system in response to proteasome inhibition

The highly conserved ubiquitin-proteasome system is the principal machinery for extralysosomal protein degradation in eukaryotic cells. The 26S proteasome, a large multicatalytic multisubunit protease that processes cell proteins by limited and controlled proteolysis, constitutes the central proteolytic component of the ubiquitin-proteasome system. By processing cell proteins essential for development, differentiation, proliferation, cell cycling, apoptosis, gene transcription, signal transduction, senescence, and inflammatory and stress response, the 26S proteasome plays a key role in the regulation and maintenance of basic cellular processes. Various synthetic and biologic inhibitors with different inhibitory profiles towards the proteolytic activities of the 26S proteasome have been identified recently. Such proteasome inhibitors induce apoptosis and cell cycle arrest preferentially in neoplastic cells. Based on these findings proteasome inhibitors became useful in cancer therapy. However, under the pressure of continuous proteasome inhibition, eukaryotic cells can develop complex adaptive mechanisms to subvert the lethal attack of proteasome inhibitors. Such mechanisms include the adaptive modification of the proteasome system with increased expression, enhanced proteolytic activity and altered subcomplex assembly and subunit composition of proteasomes as well as the expression of a giant oligomeric protease complex, tripeptidyl peptidase II, which partially compensates for impaired proteasome function. Here we review the adaptive mechanisms developed by eukaryotic cells in response to proteasome inhibition. These mechanisms reveal enormous flexibility of the proteasome system and may have implications in cancer biology and therapy.

Tripeptidyl-peptidase II Controls DNA Damage Responses and In vivo γ-Irradiation Resistance of Tumors

Cellular responses to gamma-irradiation exposure are controlled by phosphatidylinositol 3-kinase-related kinases (PIKK) in the nucleus, and in addition, cytosolic PIKKs may have a role in such responses. Here, we show that the expression of tripeptidyl-peptidase II (TPPII), a high molecular weight cytosolic peptidase, required PIKK signaling and that TPPII was rapidly translocated into the nucleus of gamma-irradiated cells. These events were dependent on mammalian target of rapamycin, a cytosolic/mitochondrial PIKK that is activated by gamma-irradiation. Lymphoma cells with inhibited expression of TPPII failed to efficiently stabilize p53 and had reduced ability to arrest proliferation in response to gamma-irradiation. We observed that TPPII contains a BRCA COOH-terminal-like motif, contained within sequences of several proteins involved in DNA damage signaling pathways, and this motif was important for nuclear translocation of TPPII and stabilization of p53. Novel tripeptide-based inhibitors of TPPII caused complete in vivo tumor regression in mice in response to relatively low doses of gamma-irradiation (3-4 Gy/wk). This was observed with established mouse and human tumors of diverse tissue backgrounds, with no tumor regrowth after cancellation of treatment. These TPPII inhibitors had minor effects on tumor growth as single agent and had low cellular toxicity. Our data indicated that TPPII connects signaling by cytosolic/mitochondrial and nuclear PIKK-dependent pathways and that TPPII can be targeted for inhibition of tumor therapy resistance.

Novel tetra-acridine derivatives as dual inhibitors of topoisomerase II and the human proteasome

Acridine derivatives, such as amsacrine, represent a well known class of multi-targeted anti-cancer agents that generally interfere with DNA synthesis and inhibit topoisomerase II. But in addition, these tricyclic molecules often display secondary effects on other biochemical pathways including protein metabolism. In order to identify novel anti-cancer drugs, we evaluated the mechanism of action of a novel series of bis- and tetra-acridines. As expected, these molecules were found to interact with DNA and inhibit the topoisomerase II-mediated DNA decatenation. Interestingly when tested on human tumour cells either sensitive (HL-60) or resistant (HL-60/MX2) to topoisomerase II inhibitors, these molecules proved equicytotoxic against the two cell lines, suggesting that they do not only rely on topoisomerase II inhibition to exert their cytotoxic effects. In order to identify alternative targets, we tested the capacity of acridines 1-9 to inhibit the proteasome machinery. Four tetra-acridines inhibited the proteasome in vitro, with IC(50) values up to 40 times lower than that of the reference proteasome inhibitor lactacystin. Moreover, unlike peptide aldehydes used as reference inhibitors for the proteasome, these new acridine compounds demonstrated a good selectivity towards the proteasome, when tested against four unrelated proteases. A cellular assay based on the degradation of a proteasome protein substrate indicated that at least two of the tetra-acridines maintained this proteasome inhibition activity in a cellular context. This is the first report of tetra-acridines that demonstrate dual topoisomerase II and proteasome inhibition properties. This new dual activity could represent a novel anti-cancer approach to circumvent certain forms of tumour resistance.

No essential role for tripeptidyl peptidase II for the processing of LCMV-derived T cell epitopes

The proteasome is critically involved in the production of MHC class I-restricted T cell epitopes. Approximately 20% of all peptides generated by the proteasome are too large for direct presentation by MHC class I molecules. Reits et al. (Immunity 2004. 20: 495-506) suggested that a major portion of proteasomal products are larger than 15 amino acids and require further degradation by the tripeptidyl peptidase II (TPPII) before becoming ligands of MHC class I molecules. Using the well-characterized lymphocytic choriomeningitis virus (LCMV) model, the role of TPPII in the processing of several LCMV-derived T cell epitopes was investigated. In contrast to Reits' proposal, TPPII inhibition and TPPII overexpression experiments revealed that five out of six LCMV-derived CD8(+) T cell epitopes were not affected by inhibition of TPPII, while one epitope (GP276) was slightly reduced upon TPPII overexpression. Additionally, we demonstrated that the processing of two epitopes derived from ovalbumin and murine cytomegalovirus were not altered by TPPII inhibition. We propose that TPPII is not generally required for the production of MHC class I peptides, but the presentation of some peptides can be negatively affected by TPPII.

Characterization of the promoter of the gene encoding human tripeptidyl-peptidase II and identification of upstream silencer elements

Tripeptidyl-peptidase II (TPP II) is one of the many proteases involved in the important process of intracellular proteolysis. The widespread distribution and broad substrate specificity suggest that TPP II is encoded by a "house-keeping gene". However, both TPP II protein and mRNA levels vary in different cells. To investigate whether these variations are due to regulation on a genetic level, the promoter of the TPP2 gene has previously been identified. The promoter contains two inverted CCAAT-boxes and an E-box. By means of reporter assays and electrophoretic mobility shift assays the promoter has now been further characterized. It could be concluded that USF-1 (upstream stimulatory factor-1) binds to the E-box in the promoter. The transcription factors NF-Y and USF-1 are present in protein-DNA complexes of different sizes. Mutation of the E-box had no effect, indicating that only binding of NF-Y to the two CCAAT-boxes was important for activation of transcription. However, this does not exclude the possibility that USF-1 can play an important role in transcription in other types of cells. Furthermore, the region upstream of the promoter was investigated due to its ability to inhibit transcription. Several silencer elements were identified and we also showed that Oct-1 binds to one of these elements. Thus, this investigation reveals that TPP II expression could be regulated through both positive and negative regulatory elements. Further studies are required to establish the involvement of different genetic elements, and how the interplay between different transcription factors will affect the transcriptional rate in vivo.

Down-regulation of proteolytic complexes following EBV activation in BL cells

In Burkitt's lymphoma cells, Epstein Barr virus (EBV) latency products interact with the ubiquitin-proteasome system to promote episomal maintenance and immunological evasion while the tripeptidylpeptidase II (TPPII) functions as an alternative protease. In the present study, we have examined the activities and levels of the proteasome and TPPII complex in Raji and in Akata cells after induction of EBV lytic cycle. The results show that the chymotrypsin-like and caspase-like activities of the proteasome were substantially reduced in Raji and Akata cells. Similarly, TPPII activity was diminished in both cell lines but was recovered in Akata cells at longer time after induction. Protein levels of the alpha/beta subunits of the 20S proteasome and TPPII concentration decreased to different extents after EBV activation, whereas the ubiquitin binding S6' subunit of the 19S regulatory complex increased three to fourfold along with the levels of ubiquitin-conjugates. Collectively, these observations demonstrate impairment of two major cellular proteolytic systems at the onset of EBV lytic infection.

Need for tripeptidyl-peptidase II in major histocompatibility complex class I viral antigen processing when proteasomes are detrimental

CD8(+) T lymphocytes recognize infected cells that display virus-derived antigenic peptides complexed with major histocompatibility complex class I molecules. Peptides are mainly byproducts of cellular protein turnover by cytosolic proteasomes. Cytosolic tripeptidyl-peptidase II (TPPII) also participates in protein degradation. Several peptidic epitopes unexpectedly do not require proteasomes, but it is unclear which proteases generate them. We studied antigen processing of influenza virus nucleoprotein epitope NP(147-155), an archetype epitope that is even destroyed by a proteasome-mediated mechanism. TPPII, with the assistance of endoplasmic reticulum trimming metallo-aminopeptidases, probably ERAAP (endoplasmic reticulum aminopeptidase associated with antigen processing), was crucial for nucleoprotein epitope generation both in the presence of functional proteasomes and when blocked by lactacystin, as shown with specific chemical inhibitors and gene silencing. Different protein contexts and subcellular targeting all allowed epitope processing by TPPII as well as trimming. The results show the plasticity of the cell's assortment of proteases for providing ligands for recognition by antiviral CD8(+) T cells. Our observations identify for the first time a set of proteases competent for antigen processing of an epitope that is susceptible to destruction by proteasomes.

Size Matters for the Tripeptidylpeptidase II Complex from Drosophila

Tripeptidylpeptidase II (TPP II) is an exopeptidase of the subtilisin type of serine proteases, a key component of the protein degradation cascade in many eukaryotes, which cleaves tripeptides from the N terminus of proteasome-released products. The Drosophila TPP II is a large homooligomeric complex (approximately 6 MDa) that is organized in a unique repetitive structure with two strands each composed of ten stacked homodimers; two strands intertwine to form a spindle-shaped structure. We report a novel procedure of preparing an active, structurally homogeneous TPP II holo-complex overexpressed in Escherichia coli. Assembly studies revealed that the specific activity of TPP II increases with oligomer size, which in turn is strongly concentration-dependent. At a TPP II concentration such as prevailing in Drosophila, equilibration of size and activity proceeds on a time scale of hours and leads to spindle formation at a TPP II concentration of > or =0.03 mg/ml. Before equilibrium is reached, activation lags behind assembly, suggesting that activation occurs in a two-step process consisting of (i) assembly and (ii) a subsequent conformational change leading to a switch from basal to full activity. We propose a model consistent with the hyperbolic increase of activity with oligomer size. Spindle formation by strand pairing causes both significant thermodynamic and kinetic stabilization. The strands inherently heterogeneous in length are thus locked into a discrete oligomeric state. Our data indicate that the unique spindle form of the holo-complex represents an assembly motif stabilizing a highly active state.

Structure of aminopeptidase N from Escherichia coli suggests a compartmentalized, gated active site

Aminopeptidase N from Escherichia coli is a major metalloprotease that participates in the controlled hydrolysis of peptides in the proteolytic pathway. Determination of the 870-aa structure reveals that it has four domains similar to the tricorn-interacting factor F3. The thermolysin-like active site is enclosed within a large cavity with a volume of 2,200 A(3), which is inaccessible to substrates except for a small opening of approximately 8-10 A. The substrate-based inhibitor bestatin binds to the protein with minimal changes, suggesting that this is the active form of the enzyme. The previously described structure of F3 had three distinct conformations that were described as "closed," "intermediate," and "open." The structure of aminopeptidase N from E. coli, however, is substantially more closed than any of these. Taken together, the results suggest that these proteases, which are involved in intracellular peptide degradation, prevent inadvertent hydrolysis of inappropriate substrates by enclosing the active site within a large cavity. There is also some evidence that the open form of the enzyme, which admits substrates, remains inactive until it adopts the closed form.

Tripeptidyl Peptidase II Is the Major Peptidase Needed to Trim Long Antigenic Precursors, but Is Not Required for Most MHC Class I Antigen Presentation

Recent reports concluded that tripeptidyl peptidase (TPPII) is essential for MHC class I Ag presentation and that the proteasome in vivo mainly releases peptides 16 residues or longer that require processing by TPPII. However, we find that eliminating TPPII from human cells using small interfering RNA did not decrease the overall supply of peptides to MHC class I molecules and reduced only modestly the presentation of SIINFEKL from OVA, while treatment with proteasome inhibitors reduced these processes dramatically. Purified TPPII digests peptides from 6 to 30 residues long at similar rates, but eliminating TPPII in cells reduced the processing of long antigenic precursors (14-17 residues) more than short ones (9-12 residues). Therefore, TPPII appears to be the major peptidase capable of processing proteasome products longer than 14 residues. However, proteasomes in vivo (like purified proteasomes) release relatively few such peptides, and these peptides processed by TPPII require further trimming in the endoplasmic reticulum (ER) by ER aminopeptidase 1 for presentation. Taken together, these observations demonstrate that TPPII plays a specialized role in Ag processing and one that is not essential for the generation of most presented peptides. Moreover, these findings reveal that three sequential proteolytic steps (by proteasomes, TPPII, and then ER aminopepsidase 1) are required for the generation of a subset of epitopes.

TPPII promotes genetic instability by allowing the escape from apoptosis of cells with activated mitotic checkpoints

Overexpression of TPPII correlates with accelerated growth and the appearance of centrosome and chromosome aberrations, suggesting that the activity of this enzyme may be critical for the induction and/or maintenance of genetic instability in malignant cells. We now find that the length of mitosis and of the entire cell cycle is significantly reduced in TPPII overexpressing HEK293 cells compared to untransfected and control transfected cells. Functional TPPII knockdown by shRNA interference caused a significant slowdown in cell growth and the accumulation of cells that delayed or failed to complete mitosis. TPPII overexpressing cells evade mitotic arrest induced by spindle poisons and display high levels of polyploidy despite the constitutively high expression of major components of the spindle checkpoint. TPPII overexpression correlated with upregulation of IAPs and with resistance to mitochondria dependent apoptosis induced by p53 stabilization. Thus, TPPII appears to promote malignant cell growth by allowing exit from mitosis and the survival of cells with severe mitotic spindle damage.

Re-evaluating the Generation of a “Proteasome-Independent” MHC Class I-Restricted CD8 T Cell Epitope

The proteasome is primarily responsible for the generation of MHC class I-restricted CTL epitopes. However, some epitopes, such as NP(147-155) of the influenza nucleoprotein (NP), are presented efficiently in the presence of proteasome inhibitors. The pathways used to generate such apparently "proteasome-independent" epitopes remain poorly defined. We have examined the generation of NP(147-155) and a second proteasome-dependent NP epitope, NP(50-57), using cells adapted to growth in the presence of proteasome inhibitors and also through protease overexpression. We observed that: 1) Ag processing and presentation proceeds in proteasome-inhibitor adapted cells but may become more dependent, at least in part, on nonproteasomal protease(s), 2) tripeptidyl peptidase II does not substitute for the proteasome in the generation of NP(147-155), 3) overexpression of leucine aminopeptidase, thymet oligopeptidase, puromycin-sensitive aminopeptidase, and bleomycin hydrolase, has little impact on the processing and presentation of NP(50-57) or NP(147-155), and 4) proteasome-inhibitor treatment altered the specificity of substrate cleavage by the proteasome using cell-free digests favoring NP(147-155) epitope preservation. Based on these results, we propose a central role for the proteasome in epitope generation even in the presence of proteasome inhibitors, although such inhibitors will likely alter cleavage patterns and may increase the dependence of the processing pathway on postproteasomal enzymes.

Characterization of endoplasmic reticulum-associated degradation of a protein S mutant identified in a family of quantitative protein S deficiency

INTRODUCTION

Misfolded and unassembled glycoproteins are eliminated from the endoplasmic reticulum (ER) lumen by the ER-associated degradation (ERAD). We previously identified a Tyr595Cys (Y595C) mutation of protein S (PS) in a family of a quantitative PS deficiency. The mutation causes intracellular degradation and decreased secretion of the Y595C mutant PS. The aim of the present study was to further characterize the molecular basis of the intracellular degradation of the mutant.

MATERIALS AND METHODS

We stably expressed the mutant in mammalian cells, and analyzed the intracellular localization of the protein. The intracellular degradation pathway was determined by pulse-chase analyses in the presence of various inhibitors of ERAD.

RESULTS AND CONCLUSIONS

Endoglycosidase H digestion and immunofluorescence staining revealed the mutant being retained in the ER. Epoxomicin, a potent and specific proteasome inhibitor, and Ala-Ala-Phe-CH(2)Cl (AAF), an inhibitor of tripeptidyl peptidase II (TPPII), suppressed the intracellular degradation of the mutant by about 65% and 50%, respectively. When epoxomicin was combined with AAF, the inhibitory effect was substantially enhanced. Although castanospermine, an inhibitor of glucosidases I and II, did not affect the degradation, kifunensine, an inhibitor of ER mannosidase I, suppressed it. Thus, it appears that the Y595C mutant is degraded through more than one pathway of ERAD, including the proteasome-dependent pathway and an alternate proteasome-independent pathway where proteases such as TPPII may be involved. Production of the critical B isoform of Man(8)GlcNAc(2) targets the mutant for ERAD, however, the interaction with calnexin/calreticulin through monoglucosylated oligosaccharides may not be required for the degradation of the mutant.

Complexity, contradictions, and conundrums: studying post-proteasomal proteolysis in HLA class I antigen presentation

The vast majority of the peptides produced during protein degradation by the cytosolic proteasome-ubiquitin system are consecutively hydrolyzed to single amino acids by multiple cytosolic peptidases preferring intermediate length or short substrates. The small fraction of peptides surviving the aggressive cytosolic environment can be recruited for presentation by major histocompatibility complex (MHC) class I molecules. However, such peptides may frequently have to be adapted to the strict MHC class I-binding requirements by one or several N-terminal-trimming steps. A recent model proposes that an initial step, in which peptides of 15 or more residues are shortened by cytosolic tripeptidylpeptidase II, is followed by additional trimming by cytosolic or endoplasmic reticulum (ER) aminopeptidases. In humans, at least two ER resident aminopeptidases, ERAP1 and ERAP2, contribute to trimming of human leukocyte antigen class I ligands. These interferon-gamma-regulated metallopeptidases show distinct substrate preferences and may have to act in a concerted fashion to remove some complex or longer N-terminal extensions and to trim the full spectrum of precursor peptides. This task is likely facilitated by the formation of presumably heterodimeric ERAP1-2 complexes. RNA interference experiments suggest that both enzymes are important for normal antigen presentation, but precise determination of the extent and the cellular context of their requirement will be left to future experimentation.

Tripeptidyl-peptidase II: A multi-purpose peptidase

Tripeptidyl-peptidase II is a high-molecular weight peptidase with a widespread distribution in eukaryotic cells. The enzyme sequentially removes tripeptides from a free N-terminus of longer peptides and also displays a low endopeptidase activity. A role for tripeptidyl-peptidase II in the formation of peptides for antigen presentation has recently become evident, and the enzyme also appears to be important for the degradation of some specific substrates, e.g. the neuropeptide cholecystokinin. However, it is likely that the main biological function of tripeptidyl-peptidase II is to participate in a general intracellular protein turnover. This peptidase may act on oligopeptides generated by the proteasome, or other endopeptidases, and the tripeptides formed would subsequently be good substrates for other exopeptidases. The fact that tripeptidyl-peptidase II activity is increased in sepsis-induced muscle wasting, a situation of enhanced protein turnover, corroborates this biological role.

Molecular machines for protein degradation

One of the most precisely regulated processes in living cells is intracellular protein degradation. The main component of the degradation machinery is the 20S proteasome present in both eukaryotes and prokaryotes. In addition, there exist other proteasome-related protein-degradation machineries, like HslVU in eubacteria. Peptides generated by proteasomes and related systems can be used by the cell, for example, for antigen presentation. However, most of the peptides must be degraded to single amino acids, which are further used in cell metabolism and for the synthesis of new proteins. Tricorn protease and its interacting factors are working downstream of the proteasome and process the peptides into amino acids. Here, we summarise the current state of knowledge about protein-degradation systems, focusing in particular on the proteasome, HslVU, Tricorn protease and its interacting factors and DegP. The structural information about these protein complexes opens new possibilities for identifying, characterising and elucidating the mode of action of natural and synthetic inhibitors, which affects their function. Some of these compounds may find therapeutic applications in contemporary medicine.

Molecular architecture and assembly mechanism of Drosophila tripeptidyl peptidase II

In eukaryotes, tripeptidyl peptidase II (TPPII) is a crucial component of the proteolytic cascade acting downstream of the 26S proteasome in the ubiquitin-proteasome pathway. It is an amino peptidase belonging to the subtilase family removing tripeptides from the free N terminus of oligopeptides. The 150-kDa subunits of Drosophila TPPII assemble into a giant proteolytic complex of 6 MDa with a remarkable architecture consisting of two segmented and twisted strands that form a spindle-shaped structure. A refined 3D model has been obtained by cryoelectron microscopy, which reveals details of the molecular architecture and, in conjunction with biochemical data, provides insight into the assembly mechanism. The building blocks of this complex are apparently dimers, within which the 150-kDa monomers are oriented head to head. Stacking of these dimers leads to the formation of twisted single strands, two of which comprise the fully assembled spindle. This spindle also forms when TPPII is heterologously expressed in Escherichia coli, demonstrating that no scaffolding protein is required for complex formation and length determination. Reciprocal interactions of the N-terminal part of subunits from neighboring strands are probably involved in the formation of the native quaternary structure, lending the TPPII spindle a stability higher than that of single strands.

The ubiquitin-proteasome system as a drug target in cerebrovascular disease: therapeutic potential of proteasome inhibitors

Proteasomes are large, multi-catalytic protease complexes that are found in the cytosol and in the nucleus of eukaryotic cells with a central role in cellular protein turnover. The ubiquitin-proteasome system (UPS) is the predominant non-lysosomal protein degradation pathway that ensures the viability, proliferation and signaling of eukaryotic organisms. Overwhelming data exist implicating a critical role for the UPS in cerebral ischemic injury. Ischemic and hypoxic trauma, and their associated oxidative, nitrosylative and energetic stress, underlie neurodegeneration following stroke, and evoke a discreet set of transcriptional events which have a complex and interdependent relationship with proteasomal function. Rapid elimination of denatured, misfolded and damaged proteins by the proteasome becomes a critical determinant of cell fate. Proof-of-principle has been obtained from animal models of cerebral ischemia, in which proteasome inhibitors reduce neuronal and astrocytic degeneration, cortical infarct volume, infarct neutrophil infiltration. and nuclear factor kappaB immunoreactivity. This neuroprotective efficacy has also been observed when proteasome inhibitors have been used 6 h after ischemic insult. Strategies aimed at effecting long-lasting changes in proteasomal function are not recommended, given the growing body of evidence implicating long-term proteasomal dysfunction in chronic neurodegenerative disease. These effects are likely due to the fact that the UPS is also essential for cellular growth, metabolism and repair, and untoward effects of proteasomal inhibition indicate that the development of short-lived proteasome inhibitors, or compounds which can spatially and temporally regulate the UPS, is a desirable clinical target. Studies in animal models indicate that the use of specific proteasome inhibitors may be beneficial in treating a host of acute neurological disorders, including ischemic stroke.

Mitotic Infidelity and Centrosome Duplication Errors in Cells Overexpressing Tripeptidyl-Peptidase II

The oligopeptidase tripeptidyl-peptidase II (TPP II) is up-regulated Burkitt's lymphoma (BL) cells that overexpress the c-myc proto-oncogene and is required for their growth and survival. Here we show that overexpression of TPP II induces accelerated growth and resistance to apoptosis in human embryonic kidney 293 cells. This correlates with the appearance of multiple chromosomal aberrations, numerical and structural centrosome abnormalities, and multipolar cell divisions. Similar mitotic aberrations were also observed in a panel of BL lines and were suppressed, in parallel with TPP II down-regulation, upon reversion of BL-like characteristics in EBV-immortalized B lymphocytes carrying a tetracycline-regulated c-myc. Functional TPP II knockdown by small interfering RNA expression in BL cells caused the appearance of giant polynucleated cells that failed to complete cell division. Collectively, these data point to a role of TPP II in the regulation of centrosome homeostasis and mitotic fidelity suggesting that this enzyme may be a critical player in the induction and/or maintenance of genetic instability in malignant cells.

Identification and characterization of the promoter for the gene encoding human tripeptidyl-peptidase II

Tripeptidyl-peptidase II (TPP II) is a ubiquitously expressed exopeptidase. The expression of this enzyme is increased, e.g. in some tumor cells, but the regulation of the expression of the gene has not been investigated previously. The gene encoding human TPP II (TPP2) is 82 kb and consists of 30 exons. An 8 kb NcoI fragment covering the 5'-flanking region of the TPP2 gene, including the initiation codon, was cloned into a luciferase-containing reporter vector. Human embryonic kidney cells (HEK-293 cells) and murine fibroblasts (NIH3T3 cells) were transiently transfected with the construct. Through sequential deletions and analysis of short PCR-fragments, the promoter could be localized to a 215 bp fragment upstream of the initiation codon. This region is GC-rich, lacks a TATA-box and contains two inverted CCAAT-boxes and a GC-box. Electrophoretic mobility shift assays showed that nuclear proteins bind to the promoter fragment. The 85 bp 5'-end of the promoter fragment is essential for transcriptional activation. Out of this a 44 bp fragment suffices to compete with binding of nuclear proteins to the 215 bp fragment. Supershift assays demonstrated that the CCAAT-binding factor (CBF; NF-Y) is involved in the formation of a complex with the 215 bp fragment. Although Sp1 binds to the promoter fragment in vitro, it was found to bind to the 3'-end of the 215 bp fragment which is not essential for transcription. The potential role of Sp1 in transcription of TPP2 therefore remains to be established.

Trafficking of exogenous peptides into proteasome-dependent major histocompatibility complex class I pathway following enterotoxin B subunit-mediated delivery

The B-subunit component of Escherichia coli heat-labile enterotoxin (EtxB), which binds to cell surface GM1 ganglioside receptors, was recently shown to be a highly effective vehicle for delivery of conjugated peptides into the major histocompatibility complex (MHC) class I pathway. In this study we have investigated the pathway of epitope delivery. The peptides used contained the epitope either located at the C terminus or with a C-terminal extension. Pretreatment of cells with cholesterol-disrupting agents blocked transport of EtxB conjugates to the Golgi/endoplasmic reticulum, but did not affect EtxB-mediated MHC class I presentation. Under these conditions, EtxB conjugates entered EEA1-positive early endosomes where peptides were cleaved and translocated into the cytosol. Endosome acidification was required for epitope presentation. Purified 20 S immunoproteasomes were able to generate the epitope from peptides in vitro, but 26 S proteasomes were not. Only presentation from the C-terminal extended peptide was proteasome-dependent in cells, and this was found to be significantly slower than presentation from peptides with the epitope at the C terminus. These results implicate the proteasome in the generation of the correct C terminus of the epitope and are consistent with proteasome-independent N-terminal trimming. Epitope presentation was blocked in a TAP-deficient cell line, providing further evidence that conjugated peptides enter the cytosol as well as demonstrating a requirement for the peptide transporter. Our findings demonstrate the utility of EtxB-mediated peptide delivery for rapid and efficient loading of MHC class I epitopes in several different cell types. Conjugated peptides are released from early endosomes into the cytosol where they gain access to proteasomes and TAP in the "classical" pathway of class I presentation.

Amino acids at the N- and C-termini of human glutamate carboxypeptidase II are required for enzymatic activity and proper folding

Human glutamate carboxypeptidase II (GCPII) is a co-catalytic metallopeptidase and its putative catalytic domain is homologous to the aminopeptidases from Vibrio proteolyticus and Streptomyces griseus. In humans, the enzyme is expressed predominantly in the nervous system and the prostate. The prostate form, termed prostate-specific membrane antigen, is overexpressed in prostate cancer and is used as a diagnostic marker of the disease. Inhibition of the form of GCPII expressed in the central nervous system has been shown to protect against ischemic injury in experimental animal models. Human GCPII consists of 750 amino acids, and six individual domains were predicted to constitute the protein structure. Here, we report the analysis of the contribution of these putative domains to the structure/function of recombinant human GCPII. We cloned 13 mutants of human GCPII that are truncated or extended at one or both the N- and C-termini of the GCPII sequence. The clones were used to generate stably transfected Drosophila Schneider's cells, and the expression and carboxypeptidase activities of the individual protein products were determined. The extreme C-terminal region of human GCPII was found to be critical for the hydrolytic activity of the enzyme. The deletion of as few as 15 amino acids from the C-terminus was shown to completely abolish the enzymatic activity of GCPII. Furthermore, the GCPII carboxypeptidase activity was abrogated upon removal of more than 60 amino acid residues from the N-terminus of the protein. Overall, these results clearly show that amino acid segments at the N- and C-termini of the ectodomain of GCPII are essential for its carboxypeptidase activity and/or proper folding.