Localization and roles in fertilization of sperm proteasomes in the ascidian Halocynthia roretzi

We previously reported that sperm proteasome is responsible for degradation of the ubiquitinated vitelline-coat during fertilization in the ascidian Halocynthia roretzi. Here, we report the roles in fertilization and localization in the sperm cell surface of H. roretzi sperm proteasome. An anti-proteasome antibody, as well as the proteasome inhibitors MG115 and MG132, inhibited the fertilization, indicating that the sperm proteasome functions extracellularly in ascidian fertilization. In order to further assess this issue, the sperm surface proteasome activity was labeled with a cell-impermeable labeling reagent, NHS-LC-biotin, extracted with 0.1% CHAPS, and was subjected to a pull-down assay with avidin-agarose beads. It was found that a substantial amount of sperm proteasome is exposed to the cell surface. Partition analysis with Triton X-114 also revealed that a considerable amount of the sperm proteasome activity is partitioned into a lipid layer. Localization of the proteasome activity was investigated by fluorescence microscopy with succinyl-Leu-Leu-Val-Tyr-4-methylcoumaryl-7-amide as a substrate. The sperm proteasome activity was specifically detected in the sperm head region, and it was markedly activated upon sperm activation. The membrane-associated proteasome was purified from the membrane fraction of H. roretzi sperm by affinity chromatography using an anti-20S proteasome antibody-immobilized Sepharose column. SDS-PAGE of the purified preparation showed a similar pattern of subunit composition to that of the 26S proteasome of mammalian origin. Taken together, these results indicate that H. roretzi sperm has the membrane-associated proteasome on its head, which is activated upon sperm activation, and that sperm proteasome plays an essential role in H. roretzi fertilization.     

Proteasome inhibitors in cancer therapy

The ubiquitin proteasome pathway plays a critical role in regulating many processes in the cell which are important for tumour cell growth and survival. Inhibition of proteasome function has emerged as a powerful strategy for anti-cancer therapy. Clinical validation of the proteasome as a therapeutic target was achieved with bortezomib and has prompted the development of a second generation of proteasome inhibitors with improved pharmacological properties. This review summarises the main mechanisms of action of proteasome inhibitors in cancer, the development of proteasome inhibitors as therapeutic agents and the properties and progress of next generation proteasome inhibitors in the clinic.     

Impaired proteasome function in Alzheimer's disease

Inhibition of proteasome activity is sufficient to induce neuron degeneration and death; however, altered proteasome activity in a neurodegenerative disorder has not been demonstrated. In the present study, we analyzed proteasome activity in short-postmortem-interval autopsied brains from 16 Alzheimer's disease (AD) and nine age- and sex-matched controls. A significant decrease in proteasome activity was observed in the hippocampus and parahippocampal gyrus (48%), superior and middle temporal gyri (38%), and inferior parietal lobule (28%) of AD patients compared with controls. In contrast, no significant decrease in proteasome activity was observed in either the occipital lobe or the cerebellum. The loss of proteasome activity was not associated with a decrease in proteasome expression, suggesting that the proteasome may become inhibited in AD by a posttranslational modification. Together, these data indicate a possible role for proteasome inhibition in the neurodegeneration associated with AD.     

Impaired methylation as a novel mechanism for proteasome suppression in liver cells

The proteasome is a multi-catalytic protein degradation enzyme that is regulated by ethanol-induced oxidative stress; such suppression is attributed to CYP2E1-generated metabolites. However, under certain conditions, it appears that in addition to oxidative stress, other mechanisms are also involved in proteasome regulation. This study investigated whether impaired protein methylation that occurs during exposure of liver cells to ethanol, may contribute to suppression of proteasome activity. We measured the chymotrypsin-like proteasome activity in Huh7CYP cells, hepatocytes, liver cytosols and nuclear extracts or purified 20S proteasome under conditions that maintain or prevent protein methylation. Reduction of proteasome activity of hepatoma cell and hepatocytes by ethanol or tubercidin was prevented by simultaneous treatment with S-adenosylmethionine (SAM). Moreover, the tubercidin-induced decline in proteasome activity occurred in both nuclear and cytosolic fractions. In vitro exposure of cell cytosolic fractions or highly purified 20S proteasome to low SAM:S-adenosylhomocysteine (SAH) ratios in the buffer also suppressed proteasome function, indicating that one or more methyltransferase(s) may be associated with proteasomal subunits. Immunoblotting a purified 20S rabbit red cell proteasome preparation using methyl lysine-specific antibodies revealed a 25 kDa proteasome subunit that showed positive reactivity with anti-methyl lysine. This reactivity was modified when 20S proteasome was exposed to differential SAM:SAH ratios. We conclude that impaired methylation of proteasome subunits suppressed proteasome activity in liver cells indicating an additional, yet novel mechanism of proteasome activity regulation by ethanol.     

Inactivation of the proteasome by 4-hydroxy-2-nonenal is site specific and dependant on 20S proteasome subtypes

The proteasome represents a major intracellular proteolytic system responsible for the degradation of oxidized and ubiquitinated proteins in both the nucleus and cytoplasm. We have previously reported that proteasome undergoes modification by the lipid peroxidation product 4-hydroxy-2-nonenal (HNE) and exhibits declines in peptidase activities during cardiac ischemia/reperfusion. This study was undertaken to characterize the effects of HNE on the structure and function of the 20S proteasome. To assess potential tissue-specific differences in the response to HNE, we utilized purified 20S proteasome from heart and liver, tissues that express different proteasome subtypes. Following incubation of heart and liver 20S proteasome with HNE, changes in the 2D gel electrophoresis patterns and peptidase activities of the proteasome were evaluated. Proteasome subunits were identified by mass spectrometry prior to and following treatment with HNE. Our results demonstrate that specific subunits of the 20S proteasome are targeted for modification by HNE and that modified proteasome exhibits selective alterations in peptidase activities. The results provide evidence for a likely mechanism of proteasome inactivation in response to oxidative stress particularly during cardiac ischemia/reperfusion.     

Cell cycle-dependent change of proteasome distribution during embryonic development of the ascidian Halocynthia roretzi.

The proteasome is a multicatalytic proteinase complex composed of nonidentical subunits. By immunocytochemical analysis using monoclonal antibody raised against the egg proteasome, we demonstrate that the proteasome undergoes changes in its subcellular distribution, depending on the cell division cycle during embryonic development of the ascidian Halocynthia roretzi. During interphase, the proteasome is localized in the nucleus, i.e., in the nucleoplasm and along the nuclear membrane. The proteasome disappears from the nucleoplasm in prophase and from the nuclear envelope in prometaphase. During early metaphase, the proteasome is detectable in the chromosomes and, at late stages of metaphase, the immunoreactivity also occurs in the peripheral region of each spindle pole and at the mitotic spindle. In anaphase, however, the staining disappears in the mitotic apparatus. In telophase, the proteasome is again localized in the newly formed nucleus. In addition to the localization in the nucleus and around the mitotic apparatus, the proteasome shows cytoplasmic localization throughout the cell division cycle. Such a change of subcellular distribution of the proteasome is clearly demonstrated in the synchronously dividing blastomeres and also is believed to occur in the postcleavage embryos. These observations suggest that the proteasome may play a key role in the progression of cell division cycle.     

Distinct specificities of Mycobacterium tuberculosis and mammalian proteasomes for N-acetyl tripeptide substrates

The proteasome of Mycobacterium tuberculosis (Mtb) is a validated and drug-treatable target for therapeutics. To lay ground-work for developing peptide-based inhibitors with a useful degree of selectivity for the Mtb proteasome over those of the host, we used a library of 5,920 N-acetyl tripeptide-aminomethylcoumarins to contrast the substrate preferences of the recombinant Mtb proteasome wild type and open gate mutant, the Rhodococcus erythropolis proteasome, and the bovine proteasome with activator PA28. The Mtb proteasome was distinctive in strictly preferring P1 = tryptophan, particularly in combination with P3 = glycine, proline, lysine or arginine. Screening results were validated with Michalis-Menten kinetic analyses of 21 oligopeptide aminomethyl-coumarin substrates. Bortezomib, a proteasome inhibitor in clinical use, and 17 analogs varying only at P1 were used to examine the differential impact of inhibitors on human and Mtb proteasomes. The results with the inhibitor panel confirmed those with the substrate panel in demonstrating differential preferences of Mtb and mammalian proteasomes at the P1 amino acid. Changing P1 in bortezomib from Leu to m-CF(3)-Phe led to a 220-fold increase in IC(50) against the human proteasome, whereas changing a P1 Ala to m-F-Phe decreased the IC(50) 400-fold against the Mtb proteasome. The change of a P1 Ala to m-Cl-Phe led to an 8000-fold shift in inhibitory potency in favor of the Mtb proteasome, resulting in 8-fold selectivity. Combinations of preferred amino acids at different sites may thus improve the species selectivity of peptide-based inhibitors that target the Mtb proteasome.     

A ubiquitin stress response induces altered proteasome composition

Ubiquitin-dependent protein degradation is essential for cells to survive many environmental stresses. Thus, it may be necessary to buffer ubiquitin and proteasome pools against fluctuation. Proteasome levels are tightly regulated, and proteasome deficiency stimulates a stress response. Here we report a novel pathway of cellular response to ubiquitin depletion. Unlike proteasome stress, ubiquitin stress does not upregulate proteasome abundance. Instead, ubiquitin stress alters proteasome composition. The proteasome-associated deubiquitinating enzyme Ubp6, which spares ubiquitin from proteasomal degradation, is induced by ubiquitin deficiency. This enhances loading of proteasomes with Ubp6, thereby altering proteasome function. A catalytically inactive mutant of Ubp6 fails to recycle ubiquitin and also inhibits proteasome function directly, thus inducing both ubiquitin stress and proteasome stress. These results show that homeostatic control of the ubiquitin-proteasome pathway can be achieved through signal-dependent, subunit-specific regulation of the proteasome, and indicate a dual role of Ubp6 in regulating ubiquitin levels and proteasome function.     

C termini of proteasomal ATPases play nonequivalent roles in cellular assembly of mammalian 26 S proteasome

The 26 S proteasome comprises two multisubunit subcomplexes as follows: 20 S proteasome and PA700/19 S regulatory particle. The cellular mechanisms by which these subcomplexes assemble into 26 S proteasome and the molecular determinants that govern the assembly process are poorly defined. Here, we demonstrate the nonequivalent roles of the C termini of six AAA subunits (Rpt1-Rpt6) of PA700 in 26 S proteasome assembly in mammalian cells. The C-terminal HbYX motif (where Hb is a hydrophobic residue, Y is tyrosine, and X is any amino acid) of each of two subunits, Rpt3 and Rpt5, but not that of a third subunit Rpt2, was essential for assembly of 26 S proteasome. The C termini of none of the three non-HbYX motif Rpt subunits were essential for cellular 26 S proteasome assembly, although deletion of the last three residues of Rpt6 destabilized the 20 S-PA700 interaction. Rpt subunits defective for assembly into 26 S proteasome due to C-terminal truncations were incorporated into intact PA700. Moreover, intact PA700 accumulated as an isolated subcomplex when cellular 20 S proteasome content was reduced by RNAi. These results indicate that 20 S proteasome is not an obligatory template for assembly of PA700. Collectively, these results identify specific structural elements of two Rpt subunits required for 26 S proteasome assembly, demonstrate that PA700 can be assembled independently of the 20 S proteasome, and suggest that intact PA700 is a direct intermediate in the cellular pathway of 26 S proteasome assembly.     

Identification of Nitric Oxide as an Endogenous Inhibitor of 26S Proteasomes in Vascular Endothelial Cells

The 26S proteasome plays a fundamental role in almost all eukaryotic cells, including vascular endothelial cells. However, it remains largely unknown how proteasome functionality is regulated in the vasculature. Endothelial nitric oxide (NO) synthase (eNOS)-derived NO is known to be essential to maintain endothelial homeostasis. The aim of the present study was to establish the connection between endothelial NO and 26S proteasome functionality in vascular endothelial cells. The 26S proteasome reporter protein levels, 26S proteasome activity, and the O-GlcNAcylation of Rpt2, a key subunit of the proteasome regulatory complex, were assayed in 26S proteasome reporter cells, human umbilical vein endothelial cells (HUVEC), and mouse aortic tissues isolated from 26S proteasome reporter and eNOS knockout mice. Like the other selective NO donors, NO derived from activated eNOS (by pharmacological and genetic approach) increased O-GlcNAc modification of Rpt2, reduced proteasome chymotrypsin-like activity, and caused 26S proteasome reporter protein accumulation. Conversely, inactivation of eNOS reversed all the effects. SiRNA knockdown of O-GlcNAc transferase (OGT), the key enzyme that catalyzes protein O-GlcNAcylation, abolished NO-induced effects. Consistently, adenoviral overexpression of O-GlcNAcase (OGA), the enzyme catalyzing the removal of the O-GlcNAc group, mimicked the effects of OGT knockdown. Finally, compared to eNOS wild type aortic tissues, 26S proteasome reporter mice lacking eNOS exhibited elevated 26S proteasome functionality in parallel with decreased Rpt2 O-GlcNAcylation, without changing the levels of Rpt2 protein. In conclusion, the eNOS-derived NO functions as a physiological suppressor of the 26S proteasome in vascular endothelial cells.     

The capture proteasome assay: A method to measure proteasome activity in vitro

Because of its crucial role in various cellular processes, the proteasome is the focus of intensive research for the development of proteasome inhibitors to treat cancer and autoimmune diseases. Here, we describe a new and easy assay to measure the different proteasome activities in vitro (chymotrypsin-like, caspase-like, and trypsin-like) based on proteasome capture on antibody-coated plates, namely the capture proteasome assay (CAPA). Applying the CAPA to lysates from cells expressing standard proteasome, immunoproteasome, or intermediate proteasomes β5i or β1i–β5i, we can monitor the activity of the four proteasome subtypes. The CAPA provided similar results as the standard whole-cell proteasome–Glo assay without the problem of contaminating proteases requiring inhibitors. However, the profile of trypsin-like activity differed between the two assays. This could be partly explained by the presence of MgSO₄ in the proteasome–Glo buffer, which inhibits the trypsin-like activity of the proteasome. The CAPA does not need MgSO₄ and, therefore, provides a more precise measurement of the trypsin-like activity. The CAPA provides a quick and accurate method to measure proteasome activity in vitro in a very specific manner and should be useful for the development of proteasome inhibitors.     

Characterization of the proteasome from the extremely halophilic archaeon Haloarcula marismortui

A 20S proteasome, comprising two subunits alpha and beta, was purified from the extreme halophilic archaeon Haloarcula marismortui, which grows only in saturated salt conditions. The three-dimensional reconstruction of the H. marismortui proteasome (Hm proteasome), obtained from negatively stained electron micrographs, is virtually identical to the structure of a thermophilic proteasome filtered to the same resolution. The stability of the Hm proteasome was found to be less salt-dependent than that of other halophilic enzymes previously described. The proteolytic activity of the Hm proteasome was investigated using the malate dehydrogenase from H. marismortui (HmMalDH) as a model substrate. The HmMalDH denatures when the salt concentration is decreased below 2 M. Under these conditions, the proteasome efficiently cleaves HmMalDH during its denaturation process, but the fully denatured HmMalDH is poorly degraded. These in vitro experiments show that, at low salt concentrations, the 20S proteasome from halophilic archaea eliminates a misfolded protein.     

Comparison of rat liver and brain proteasomes for oxidative stress-induced inactivation: Influence of ageing and dietary restriction

The present study examined brain and liver derived proteasome complexes to elucidate if there is a differential susceptibility in proteasome complexes from these tissues to undergo inactivation following exposure to oxidative stressors. It then examined the influence of ageing and dietary restriction (DR) on the observed proteasome inactivation. Studies used a filtration based methodology that allows for enrichment of proteasome complexes with less tissue than is required for traditional chromatography procedures. The results indicate that the brain has much lower levels of overall proteasome activity and exhibits increased sensitivity to hydrogen peroxide mediated inactivation as compared to proteasome complexes derived from the liver. Interestingly, the brain proteasome complexes did not appear to have increased susceptibility to 4-hydroxynonenal (HNE)-induced inactivation. Surprisingly, ageing and DR induced minimal effects on oxidative stress mediated proteasome inhibition. These results indicate that the brain not only has lower levels of proteasome activity compared to the liver, but is also more susceptible to inactivation following exposure to some (but certainly not all) oxidative stressors. This data also suggest that ageing and DR may not significantly modulate the resistance of the proteasome to inactivation in some experimental settings.     

泛素/蛋白酶体途径及其在高等植物有性生殖中的作用

泛素／蛋白酶体途径是真核细胞内蛋白质降解的主要途径之一,涉及细胞内一系列基本生理过程。本文就蛋白泛素／酶体途径的组成及生化特性,泛素／蛋白酶体途径的一些生物学功能,研究泛素／蛋白酶体途径的一些技术和方法,泛素／蛋白酶体途径与高等植物有性生殖等方面作了较全面论述。     

Lactacystin augments the sulindac-induced apoptosis in HT-29 cells

The present study was conducted to explore the potential role of proteasome pathway in NSAIDs-induced apoptosis. We employed sulindac as a NSAID, and chose the lactacystin for inhibition of proteasome activity. Assessment of apoptosis and proteasome activity assay were undertaken. We demonstrated that sulindac treatment resulted in a decrease of proteasome activity, and that the co-treatment of a proteasome inhibitor lactacystin potentiated the extent of sulindac-induced apoptosis in HT-29 cells by augmentation of the decrease in proteasome activity. Elucidation of the mechanism underlying the regression of colon cancers by combinations of sulindac and lactacystin seems to be an immediate challenge for the near future.     

Proteasome regulation,plant growth and stress tolerance

Plant cells contain a mixture of 26S and 20S proteasomes that mediate ubiquitin-dependent and ubiquitin-independent proteolysis, respectively. The 26S proteasome contains the 20S proteasome and one or two regulatory particles that are required for ubiquitin-dependent degradation. Comparative analyses of Arabidopsis proteasome mutants revealed that a decrease in 26S proteasome biogenesis causes heat shock hypersensitivity and reduced cell division rates that are compensated by increased cell expansion. Loss of 26S proteasome function also leads to an increased 20S proteasome biogenesis, which in turn enhances the cellular capacity to degrade oxidized proteins and thus increases oxidative stress tolerance. These findings suggest the intriguing possibility that 26S and 20S proteasome activities are regulated to control plant development and stress responses. This mini-review highlights some of the recent studies on proteasome regulation in plants.Key words: proteasome, cell division, ubiquitin-dependent proteolysis, ubiquitin-independent proteolysis, stress responses