Changes in spatio-temporal localization of tripeptidyl peptidase II (TPPII) in murine colon adenocarcinoma cells during aggresome formation: a microscopy study based on a novel fluorescent proteasome inhibitor.

Extralysosomal proteolysis is a multistep process involving the Ubiquitin- Proteasome System (UPS) and supplementary peptidases. Tripeptidyl peptidase II (TPPII) is the most extensively characterized enzyme, supplementing and sometimes substituting for proteasomal functions. In response to proteasome inhibition, polyubiquitinated proteins acting as proteasome substrates aggregate with proteasomes and form aggresomes. Several proteasome inhibitors are used as anti-cancer drugs. Thus, in our study, we used a novel fluorescent-tagged proteasome inhibitor BSc2118 to induce aggresome formation in C26 murine colon adenocarcinoma cells. It allowed us to obtain effective, inhibitor-based, proteasome staining in vivo. This method has been validated by standard post-fixed indirect immunostaining and also allowed co-immunodetection of TPPII and polyubiquitinated proteins under laser scanning confocal microscopy. We found that in the absence of the inhibitor, TPPII is diffusely dispersed within the cytoplasm of C26 cells. The proteasome and ubiquitin-rich perinuclear region failed to display enhanced TPPII staining. However, when proteasome function was impaired by the inhibitor, TPPII associated more closely with both the proteasome and polyubiquitinated proteins via TPPII recruitment to the perinuclear region and subsequently into emerging aggresomal structures. Furthermore, we have demonstrated the dynamic recruitment of TPPII into the developing aggresome: TPPII in the early aggresome was dispersed within the central part but subsequently aggregated on the surface of this structure. In the mature aggresome of C26 cells TPPII formed a spherical mantle, which surrounded the round core containing proteasomes and polyubiquitinated proteins. Our morphological data indicate that TPPII displays spatial localization with proteasomes especially upon proteasome inhibition in aggresomes of C26 cells.

Autoimmune reactivity against the 20S-proteasome includes immunosubunits LMP2 (beta1i), MECL1 (beta2i) and LMP7 (beta5i).

OBJECTIVES: Autoantibodies against the 20S-proteasome display a broad diversity with a remarkably low frequency of individual cross-reactivity against the different subunits of the proteasome. Although their pathogenic and diagnostic significance remains obscure, an involvement in the clearance of circulating proteasomes as well as an interaction with the activity of the proteolytic complex was assumed in previous studies. METHODS: To investigate the anti-proteasome response in more detail and to disclose reactivities against former neglected subunits, two-dimensional electrophoresis followed by immunoblotting was used. As a novel antigen source, the immunosubunits LMP2 (beta1i) and LMP7 (beta5i) were expressed as recombinant proteins and employed in ELISA. RESULTS: The subunits Iota (alpha1) and Zeta (alpha5) of the outer rings as well as the catalytic subunit Delta (beta1) and all three immunosubunits [MECL-1 (beta2i), LMP2 (beta1i) and LMP7 (beta5i)] of the inner rings of the proteasome were identified as autoantigens for the first time. Using a panel of anti-proteasome antibody-positive sera of patients with SLE, autoimmune myositis (PM/DM) and primary Sjögren's syndrome (pSS), an autoimmune response was documented against LMP2 (beta1i) and LMP7 (beta5i) in all three patient groups in ELISA. CONCLUSIONS: The frequent autoimmune response against LMP2 (beta1i) and LMP7 (beta5i) might indicate a role of inflammatory processes in the primary induction of the anti-proteasomal immune reaction, while the diversity of the humoral response against the proteasome system supports the assumption of a specific antigen-driven process leading to these extended autoimmune reactivities.

Anti-proteasome autoantibodies contribute to anti-nuclear antibody patterns on human larynx carcinoma cells.

BACKGROUND: Autoantibodies to the 20S proteasome represent an unspecific but common serological phenomenon in patients with systemic autoimmune diseases. Interestingly, a high prevalence of these antibodies have been observed in patients with connective tissue diseases, where anti-nuclear antibodies (ANAs) serve as an important diagnostic screening test. OBJECTIVE: To disclose interference of anti-proteasome antibodies with known ANA patterns. METHODS: Anti-proteasome antibodies were isolated for comprehensive immunofluorescence analyses. The immunofluorescence pattern of human anti-proteasome antibodies was compared with a panel of monoclonal and polyclonal reference antibodies, and colocalisation was analysed using confocal microscopy. RESULTS: Anti-proteasome antibodies clearly contributed to the ANA patterns of their respective serum samples from patients with different rheumatic disorders. In addition to the nuclear pattern, proteasomal staining was also detectable in the cytoplasm, at the endoplasmic reticulum and perinuclear regions showing features overlapping with other known autoantibodies such as those to mitochondria. The specificity of anti-proteasome antibodies was proved by competition experiments and by colocalisation with monoclonal reference antibodies in confocal microscopy. CONCLUSION: In ANA diagnostics, interference of anti-proteasome antibodies will have to be taken into account, especially in the differentiation of anti-cytoplasmatic autoantibodies.

Immunoproteasome subunit LMP2 expression is deregulated in Sjogren's syndrome but not in other autoimmune disorders.

BACKGROUND: The proteasome system has a pivotal role in the control of the immune response, which suggests that it might be involved in the pathogenesis of autoimmune disorders. OBJECTIVE: To investigate the expression profile of selected proteasomal genes in human peripheral blood mononuclear cells in patients with a variety of autoimmune diseases compared with healthy subjects. METHODS: Real time quantitative RT-PCR was used to analyse the mRNA expression pattern of the proteasome activator subunits PA28alpha and PA28beta and of constitutive proteasome and interferon-gamma-inducible immunoproteasome subunits in peripheral blood mononuclear cells. Simultaneously, protein expression of selected proteasome subunits was quantified by immunoblotting. RESULTS: Under systemic inflammatory conditions the proteasome subunits LMP2 (beta1i), LMP7 (beta5i), MECL1 (beta2i), and PA28alpha were expressed abundantly at the protein level in the vast majority of systemic autoimmune disorders. However, simultaneous mRNA and protein quantification showed a characteristic proteasome expression signature in primary Sjögren's syndrome. At the transcript level, the interferon-gamma-responsive subunits LMP2 (beta1i), MECL1 (beta2i), and the proteasome activator subunit PA28alpha were markedly up regulated. In contrast, LMP2 (beta1i) deficiency was evident at the protein level, indicating deregulation of proteasome expression in Sjögren's syndrome. CONCLUSIONS: These data provide evidence for a regulatory defect in the proteasome system in human autoimmune disorders, pointing to a unique role for LMP2 (beta1i) in the pathogenesis of primary Sjögren's syndrome.

The components of the proteasome system and their role in MHC class I antigen processing.

By generating peptides from intracellular antigens which are then presented to T cells, the ubiquitin/26S proteasome system plays a central role in the cellular immune response. The proteolytic properties of the proteasome are adapted to the requirements of the immune system by proteasome components whose synthesis is under the control of interferon-gamma. Among these are three subunits with catalytic sites that are incorporated into the enzyme complex during its de novo synthesis. Thus, the proteasome assembly pathway and the formation of immunoproteasomes play a critical regulatory role in the regulation of the proteasome's catalytic properties. In addition, interferon-gamma also induces the synthesis of the proteasome activator PA28 which, as part of the so-called hybrid proteasome, exerts a more selective function in antigen presentation. Consequently, the combination of a number of regulatory events tunes the proteasome system to gain maximal efficiency in the generation of peptides with regard to their quality and quantity.

The murine cytomegalovirus pp89 immunodominant H-2Ld epitope is generated and translocated into the endoplasmic reticulum as an 11-mer precursor peptide.

The 20S proteasome is involved in the processing of MHC class I-presented Ags. A number of epitopes is known to be generated as precursor peptides requiring trimming either before or after translocation into the endoplasmic reticulum (ER). In this study, we have followed the proteasomal processing and TAP-dependent ER translocation of the immunodominant epitope of the murine CMV immediate early protein pp89. For the first time, we experimentally linked peptide generation by the proteasome system and TAP-dependent ER translocation. Our experiments show that the proteasome generates both an N-terminally extended 11-mer precursor peptide as well as the correct H2-L(d) 9-mer epitope, a process that is accelerated in the presence of PA28. Our direct peptide translocation assays, however, demonstrate that only the 11-mer precursor peptide is transported into the ER by TAPs, whereas the epitope itself is not translocated. In consequence, our combined proteasome/TAP assays show that the 11-mer precursor is the immunorelevant peptide product that requires N-terminal trimming in the ER for MHC class I binding.

The effect of heat shock on 20S/26S proteasomes.

We have studied the consequences of heat shock on 20S/26S proteasome activity and activation, the proteasomal subunit composition, proteasome assembly, subunit mRNA stability as well as on the intracellular distribution of proteasomes. Our data show that heat shock locks 20S proteasomes in their latent inactive state and impairs further activation of the 26S proteasome by ATP. Proteasome mRNA levels are decreased after heat shock and the assembly of the proteasome complex is inhibited. Heat shock also induces a rapid reorganisation of the cellular distribution of the proteasome which appears to be connected with proteasome activity and the change of the cellular architecture after heat shock.

Diagnostic importance of anti-proteasome antibodies.

20S proteasome represents the proteolytic core complex for cytoplasmic protein degradation that is involved in the activation and regulation of the immune response. In this context, proteasome generates antigenic peptides for the MHC class I pathway and activates NF-kappaB. In a recent analysis, we could identify a frequent humoral autoimmune response directed against specific proteasomal subunits in patients with autoimmune myositis, systemic lupus erythematosus and primary Sjögren's syndrome. The outer ring subunit HC9(alpha3) was identified as the predominant target of the anti-proteasome response in these entities. In addition to the reactivity against HC9(alpha3), patients with primary Sjögren's syndrome expressed a more polyspecific recognition pattern of proteasomal subunits involving the active inner ring proteins. In follow-up analysis, anti-proteasome antibody titers revealed a correlation with disease activity in patients with autoimmune myositis and systemic lupus erythematosus. The current review summarizes recent data providing evidence that the 20S proteasome represents an important target of the humoral autoimmune response in systemic autoimmune diseases and extends insight into pathogenic aspects of these diseases.

Autoantibodies in primary Sjögren's syndrome are directed against proteasomal subunits of the alpha and beta type.

OBJECTIVE: The proteasome subunit HC9 (alpha3) has recently been identified as a major target of the humoral autoimmune response in patients with autoimmune myositis and systemic lupus erythematosus. Since B cell hyperreactivity is a common feature of systemic autoimmune diseases, patients with primary Sjögren's syndrome (SS) and other control groups were investigated to evaluate the significance of autoantibodies against the proteasome. METHODS: Analyses of autoantibodies directed against the 20S proteasome were performed using enzyme-linked immunosorbent assay, immunoblot, and 2-dimensional electrophoresis. Forty-three patients with primary SS, 47 patients with rheumatoid arthritis including 9 with secondary SS, 19 patients with gastrointestinal tumors, and 80 healthy controls were tested for antiproteasome antibodies. RESULTS: Antiproteasome antibodies were detected in 39% of patients (17 of 43) with primary SS. In contrast, only 1 of 47 patients with rheumatoid arthritis showed positive reactivity (P < 0.001). Serum samples from 19 tumor patients (P < 0.003) and 80 healthy controls (P < 0.001) were serologically negative. Moreover, immunoblotting and 2-dimensional analysis of the antiproteasome response revealed a polyspecific recognition pattern in 7 patients with primary SS. Different proteasomal subunits of the alpha and beta type, including subunits that carried the proteolytic active sites, were recognized by the patients' sera. CONCLUSION: The humoral antiproteasome response in primary SS, in contrast to its secondary form, is characterized by an extensive recognition pattern of several subunits, indicating a polyspecific B cell activation against the 20S proteasome. Moreover, proteolytically active beta-type subunits, which are important for the generation of major histocompatibility complex class I-restricted antigens, appear to be targets of the autoimmune response. The data indicate that the proteasome itself may stand on a cross point of pathways that links mechanisms of the immune defense with features of systemic autoimmunity.

Comparative resistance of the 20S and 26S proteasome to oxidative stress.

Oxidatively modified ferritin is selectively recognized and degraded by the 20S proteasome. Concentrations of hydrogen peroxide (H2O2) higher than 10 micromol/mg of protein are able to prevent proteolytic degradation. Exposure of the protease to high amounts of oxidants (H2O2, peroxynitrite and hypochlorite) inhibits the enzymic activity of the 20S proteasome towards the fluorogenic peptide succinyl-leucine-leucine-valine-tyrosine-methylcoumarylamide (Suc-LLVY-MCA), as well as the proteolytic degradation of normal and oxidant-treated ferritin. Fifty per cent inhibition of the degradation of the protein substrates was achieved using 40 micromol of H2O2/mg of proteasome. No change in the composition of the enzyme was revealed by electrophoretic analysis up to concentrations of 120 micromol of H2O2/mg of proteasome. In further experiments, it was found that the 26S proteasome, the ATP- and ubiquitin-dependent form of the proteasomal system, is much more susceptible to oxidative stress. Whereas degradation of the fluorogenic peptide, Suc-LLVY-MCA, by the 20S proteasome was inhibited by 50% with 12 micromol of H2O2/mg, 3 micromol of H2O2/mg was enough to inhibit ATP-stimulated degradation by the 26S proteasome by 50%. This loss in activity could be followed by the loss of band intensity in the non-denaturing gel. Therefore we concluded that the 20S proteasome was more resistant to oxidative stress than the ATP- and ubiquitin-dependent 26S proteasome. Furthermore, we investigated the activity of both proteases in K562 cells after H2O2 treatment. Lysates from K562 cells are able to degrade oxidized ferritin at a higher rate than non-oxidized ferritin, in an ATP-independent manner. This effect could be followed even after treatment of the cells with H2O2 up to a concentration of 2mM. The lactacystin-sensitive ATP-stimulated degradation of the fluorogenic peptide Suc-LLVY-MCA declined, after treatment of the cells with 1mM H2O2, to the same level as that obtained without ATP stimulation. Therefore, we conclude that the regulation of the 20S proteasome by various regulators takes place during oxidative stress. This provides further evidence for the role of the 20S proteasome in the secondary antioxidative defences of mammalian cells.

The sequence alteration associated with a mutational hotspot in p53 protects cells from lysis by cytotoxic T lymphocytes specific for a flanking peptide epitope.

A high proportion of tumors arise due to mutation of the p53 tumor suppressor protein. A p53 hotspot mutation at amino acid position 273 from R to H, flanking a peptide epitope that spans residues 264-272, renders cells resistant to killing by human histocompatibility leukocyte antigen (HLA)-A*0201-restricted cytotoxic T lymphocytes (CTLs) specific for this epitope. Acquisition of the R to H mutation at residue 273 of the human p53 protein promotes tumor growth in vivo by selective escape from recognition by p53.264-272 peptide-specific CTLs. Synthetic 27-mer p53 polypeptides covering the antigenic nonamer region 264-272 of p53 were used as proteasome substrates to investigate whether the R to H mutation at the P1' position of the COOH terminus of the epitope affects proteasome-mediated processing of the protein. Analysis of the generated products by tandem mass spectrometry and the kinetics of polypeptide processing in conjunction with CTL assays demonstrate that the R to H mutation alters proteasomal processing of the p53 protein by inhibiting proteolytic cleavage between residues 272 and 273. This prevents the release of the natural CTL epitope that spans flanking residues 264-272 as well as a putative precursor peptide. These results demonstrate that mutation of p53 not only leads to malignant transformation but may also, in some instances, affect immune surveillance and should be considered in the design of cancer vaccines.

Proteasome alpha-type subunit C9 is a primary target of autoantibodies in sera of patients with myositis and systemic lupus erythematosus.

Autoantibodies occur in low frequencies among patients with myositis characterizing only distinct subsets of this disease. Most of these known antibodies are directed to enzymatically active complexes. The 20S proteasome represents an essential cytoplasmatic protein complex for intracellular nonlysosomal protein degradation, and is involved in major histocompatibility complex class I restricted antigen processing. In this study we investigated whether the 20S proteasome complex is an antibody target in myositis and in other autoimmune diseases. 34 sera of poly/dermatomyositis patients were assayed for antiproteasomal antibodies using enzyme-linked immunosorbent assay, immunoblot, and two-dimensional non-equilibrium pH gradient electrophoresis (NEPHGE). Sera was from patients with systemic lupus erythematosus (SLE), mixed connective tissue disease, and rheumatoid arthritis; healthy volunteers served as controls. In 62% (21/34) of the cases sera from patients with myositis and in 58% (30/52) of the cases sera from patients with SLE reacted with the 20S proteasome. These frequencies exceeded those of sera from patients with mixed connective tissue disease, rheumatoid arthritis, and healthy controls. The alpha-type subunit C9 of the 20S proteasome was determined to be the predominant target of the autoimmune sera in myositis and SLE. Lacking other frequent autoantibodies in myositis, the antiproteasome antibodies are the most common humoral immune response so far detected in this disease entity.

20S proteasome from LMP7 knock out mice reveals altered proteolytic activities and cleavage site preferences.

20S proteasomes of tissues from LMP7 knock out mice which show reduced MHC class I restricted antigen presentation were analyzed with regard to their subunit composition, peptide hydrolyzing activity and their ability to cleave a synthetic 25-mer polypeptide. LMP7 deficiency results in an enhanced incorporation of subunit MB1 and in a 2-3.8-fold increase in Vmax for the Suc-LLVY-MCA hydrolyzing activity. Since LMP7 deficiency also affects the cleavage site preference of 20S proteasomes the reduced MHC class I antigen presentation of LMP7 knock out mice is most likely due to an impairment in peptide generation.

Incorporation of major histocompatibility complex--encoded subunits LMP2 and LMP7 changes the quality of the 20S proteasome polypeptide processing products independent of interferon-gamma.

The 20S proteasome is the enzyme complex responsible for the processing of antigens bound by major histocompatibility complex class I molecules. The role of the interferon-gamma (IFN-gamma)-inducible proteasome subunits LMP2 and LMP7 in this process is, however, still controversial. We have studied the effects of IFN-gamma-independent LMP incorporation on the quality of peptides processed from the murine cytomegalovirus IE pp89 25-mer polypeptide substrate through dual cleavages by 20S proteasomes. The incorporation of a single LMP subunit or both LMP2 and LMP7 induces changes in 20S proteasome subunit stoichiometry, alters its cleavage site preference and in consequence, the quality of the generated peptides. When the several hydrolytic activities are tested with short fluorogenic peptide substrates, the Vmax, S0.5 (Km), or both values of 20S proteasomes are altered, depending on the combination of LMP. There exists, however, no obvious correlation between the observed changes in hydrolytic activities against short fluorogenic peptides and the changes in dual cleavage site usage within the 25-mer polypeptide substrate. As judged from the calculated Hill coefficients, the presence of both LMP subunits induces a drastic increase in positive cooperativity between the proteasome subunits.

[Diagnosis of genetically determined diseases]. / Diagnostik genetisch bedingter Erkrankungen.

Enzymopathies of pyruvate kinase are caused by defects of structural genes forming stable and unstable mutants, respectively. Stable mutants of PK are characterized by high S0,5 PEP and nearly unchanged Vmax. A decrease of PEP affinity can be the result from a very high allosteric constant L0 or from an increased KPEP. From the pattern of PAGE can be concluded that stable PK mutants are tetraheteromers composed of two normal and two shortened polypeptide chains. We suppose that this is the result of a mutation of a codon which stops the polypeptide synthesis of PK earlier. Most PK mutants are unstable. They are characterized by low catalytic activity and high PEP affinity. The kinetic properties of unstable mutants are changed posttranslational by proteolytic modifications. Furthermore low S0,5 PEP values result from a persistence of the isoenzyme PK-K in reticulocytes and erythrocytes, respectively. A prenatal diagnosis of PK enzymopathies can be carried out with a very small blood volume by using the method of isoelectrophoretic focussing.

Glucose-6-phosphate dehydrogenase from Plasmodium berghei: kinetic and electrophoretic characterization.

Evidence is given for the existence of a parasite-specific glucose-6-phosphate dehydrogenase (G6PD) in Plasmodium berghei by characterization of its kinetic and electrophoretic properties. From infected rat erythrocytes the parasites were isolated, washed, and lysed. G6PD was purified by affinity chromatography with 2'5'-ADP-Sepharose 4B, although the separation of the malaria-specific enzyme from that of the host cell was not complete. Malarial G6PD significantly differed from the red cell enzyme with respect to its electrophoretic properties. In cellulose acetate electrophoresis, a band with catodic mobility was observed in addition to the anodically mobile host cell enzyme at pH 7.0. The subunits of the parasite-specific G6PD have a molecular weight of 55 kDa in contrast to 59 kDa of red cell G6PD subunits. The enzyme from P. berghei shows no cross-reactivity with polyclonal antibodies against G6PD from rat erythrocytes. Thus, a close evolutionary relationship between both proteins and the presence of proteolytic modifications could be excluded. The Km value for G6P of malarial G6PD is increased by one order of magnitude compared with the host cell enzyme.

Purification of pyruvate kinase from human erythrocytes by immunoaffinity chromatography.

An one-step-purification of native pyruvate kinase from human erythrocytes by immunoaffinity chromatography with monoclonal antibodies coupled on CNBr-activated Sepharose 4B is described. The recoveries of PK amounted up to 83%, with a 2400 fold purification of PK-R from human stroma free hemolysate.

Glucose-6-phosphate dehydrogenase from Plasmodium berghei: kinetic and electrophoretic characterization.

Evidence is given for the existence of a parasite-specific glucose-6-phosphate dehydrogenase in Plasmodium berghei by characterization of its kinetic and electrophoretic properties. After separating the parasites from infected RBC the G6PD was purified by affinity chromatography with 2'5'-ADP-Sepharose 4B. In cellulose acetate electrophoresis malarial G6PD significantly differs from the red cell enzyme. The subunits of the parasite-specific G6PD have a molecular weight of 55 kD in contrast to 59 kD of the RBC enzyme. G6PD from P. berghei shows no cross-reactivity with antibodies against G6PD from rat erythrocytes. The Km-value for G6P of malarial G6PD is increased by one order of magnitude compared with the host cell enzyme.

Heterogeneity of glucose-6-phosphate dehydrogenase enzymopathies in the GDR.

G6PD variants of 13 patients from 12 German families with different clinical symptoms have been characterized kinetically. Vmax G6PD was nearly zero in red blood cells of all carriers. Therefore G6PD variants were isolated from leucocytes, which proved to be a suitable source for analysis of instable G6PD variants. The testing program included KmG6P, KmNADP, Ki values of NADPH, ATP and 2,3 P2G, rate of utilization of dG6P, Gal6P, dNADP, NAD, and pH dependence. From the results obtained one can conclude that all analyzed G6PD variants represent individual mutations. The degree of metabolic dysregulation can be explained by the different kinetic and physico-chemical properties of these G6PD variants.