The proteasome: a protein-destroying machine

Proteasome inhibitors regulate tyrosine phosphorylation of IRS-1 and insulin signaling in adipocytes

Insulin rapidly stimulates the tyrosine kinase activity of its receptor, resulting in the phosphorylation of insulin receptor substrates (IRS), which in turn associates and activates PI 3-kinase, leading to an increase in glucose uptake. Phosphorylation of IRS proteins and activation of downstream kinases by insulin are transient and the mechanisms for the subsequent downregulation of their activity are largely unknown. We report here that the insulin-induced IRS-1 tyrosine phosphorylation and PI 3-kinase association to IRS-1 were strongly sustained by the proteasome inhibitors, MG132 and lactacystin. In contrast, no effect was detected on the insulin receptor and IRS-2 tyrosine phosphorylation. Interestingly, lactacystin also preserved PKB activation and insulin-induced glucose uptake. In contrast, calpeptin, a calpain inhibitor, was ineffective. Tyrosine phosphatase assays were also performed, showing that lactacystin was not functioning directly as a tyrosine phosphatase inhibitor "in vitro." In conclusion, proteasome inhibitors can regulate the tyrosine phosphorylation of IRS-1 and the downstream insulin signaling pathway, leading to glucose transport.

A novel link between the proteasome pathway and the signal transduction pathway of the bone morphogenetic proteins (BMPs)

BACKGROUND

The intracellular signaling events of the bone morphogenetic proteins (BMPs) involve the R-Smad family members Smad1, Smad5, Smad8 and the Co-Smad, Smad4. Smads are currently considered to be DNA-binding transcriptional modulators and shown to recruit the master transcriptional co-activator CBP/p300 for transcriptional activation. SNIP1 is a recently discovered novel repressor of CBP/p300. Currently, the detailed molecular mechanisms that allow R-Smads and Co-Smad to co-operatively modulate transcription events are not fully understood.

RESULTS

Here we report a novel physical and functional link between Smad1 and the 26S proteasome that contributes to Smad1- and Smad4-mediated transcriptional regulation. Smad1 forms a complex with a proteasome beta subunit HsN3 and the ornithine decarboxylase antizyme (Az). The interaction is enhanced upon BMP type I receptor activation and occur prior to the incorporation of HsN3 into the mature 20S proteasome. Furthermore, BMPs trigger the translocation of Smad1, HsN3 and Az into the nucleus, where the novel CBP/p300 repressor protein SNIP1 is further recruited to Smad1/HsN3/Az complex and degraded in a Smad1-, Smad4- and Az-dependent fashion. The degradation of the CBP/p300 repressor SNIP1 is likely an essential step for Smad1-, Smad4-mediated transcriptional activation, since increased SNIP1 expression inhibits BMP-induced gene responses.

CONCLUSIONS

Our studies thus add two additional important functional partners of Smad1 into the signaling web of BMPs and also suggest a novel mechanism for Smad1 and Smad4 to co-modulate transcription via regulating proteasomal degradation of CBP/p300 repressor SNIP1.

Regulation of Jak2 through the ubiquitin-proteasome pathway involves phosphorylation of Jak2 on Y1007 and interaction with SOCS-1

The family of cytoplasmic Janus (Jak) tyrosine kinases plays an essential role in cytokine signal transduction, regulating cell survival and gene expression. Ligand-induced receptor dimerization results in phosphorylation of Jak2 on activation loop tyrosine Y1007 and stimulation of its catalytic activity, which, in turn, results in activation of several downstream signaling cascades. Recently, the catalytic activity of Jak2 has been found to be subject to negative regulation through various mechanisms including association with SOCS proteins. Here we show that the ubiquitin-dependent proteolysis pathway is involved in the regulation of the turnover of activated Jak2. In unstimulated cells Jak2 was monoubiquitinated, and interleukin-3 or gamma interferon stimulation induced polyubiquitination of Jak2. The polyubiquitinated Jak2 was rapidly degraded through proteasomes. By using different Jak2 mutants we show that tyrosine-phosphorylated Jak2 is preferentially polyubiquitinated and degraded. Furthermore, phosphorylation of Y1007 on Jak2 was required for proteasomal degradation and for SOCS-1-mediated downregulation of Jak2. The proteasome inhibitor treatment stabilized the Jak2-SOCS-1 protein complex and inhibited the proteolysis of Jak2. In summary, these results indicate that the ubiquitin-proteasome pathway negatively regulates tyrosine-phosphorylated Jak2 in cytokine receptor signaling, which provides an additional mechanism to control activation of Jak2 and maintain cellular homeostasis.

Regulation of Jak2 through the ubiquitin-proteasome pathway involves phosphorylation of Jak2 on Y1007 and interaction with SOCS-1

The family of cytoplasmic Janus (Jak) tyrosine kinases plays an essential role in cytokine signal transduction, regulating cell survival and gene expression. Ligand-induced receptor dimerization results in phosphorylation of Jak2 on activation loop tyrosine Y1007 and stimulation of its catalytic activity, which, in turn, results in activation of several downstream signaling cascades. Recently, the catalytic activity of Jak2 has been found to be subject to negative regulation through various mechanisms including association with SOCS proteins. Here we show that the ubiquitin-dependent proteolysis pathway is involved in the regulation of the turnover of activated Jak2. In unstimulated cells Jak2 was monoubiquitinated, and interleukin-3 or gamma interferon stimulation induced polyubiquitination of Jak2. The polyubiquitinated Jak2 was rapidly degraded through proteasomes. By using different Jak2 mutants we show that tyrosine-phosphorylated Jak2 is preferentially polyubiquitinated and degraded. Furthermore, phosphorylation of Y1007 on Jak2 was required for proteasomal degradation and for SOCS-1-mediated downregulation of Jak2. The proteasome inhibitor treatment stabilized the Jak2-SOCS-1 protein complex and inhibited the proteolysis of Jak2. In summary, these results indicate that the ubiquitin-proteasome pathway negatively regulates tyrosine-phosphorylated Jak2 in cytokine receptor signaling, which provides an additional mechanism to control activation of Jak2 and maintain cellular homeostasis.

Proteasomes and proteasome inhibition in the central nervous system

Although the proteasome is responsible for the majority of intracellular protein degradation, and has been demonstrated to play a pivotal role in a diverse array of cellular activities, the role of the proteasome in the central nervous system is only beginning to be elucidated. Recent studies have demonstrated that proteasome inhibition occurs in numerous neurodegenerative conditions, and that proteasome inhibition is sufficient to induce neuron death, elevate intracellular levels of protein oxidation, and increase neural vulnerability to subsequent injury. The focus of this review is to describe what is currently known about proteasome biology in the central nervous system and to discuss the possible role of proteasome inhibition in the neurodegenerative process.

Review: prediction of in vivo fates of proteins in the era of genomics and proteomics

Even after a nascent protein emerges from the ribosome, its fate is still controlled by its own amino acid sequence information. Namely, it may be co-/posttranslationally modified (e.g., phosphorylated, N-/O-glycosylated, and lipidated); it may be inserted into the membrane, translocated to an organelle, or secreted to the outside milieu; it may be processed for maturation or selective degradation; finally, its fragment may be presented on the cell surface as an antigen. Here, prediction methods of such protein fates from their amino acid sequences are reviewed. In many cases, artificial neural network techniques have been effectively used. The prediction of in vivo fates of proteins will be useful for characterizing newly identified candidate genes in a genome or for interpreting multiple spots in proteome analyses.

Reactivity of anti-proliferating cell nuclear antigen (PCNA) murine monoclonal antibodies and human autoantibodies to the PCNA multiprotein complexes involved in cell proliferation

Proliferating cell nuclear Ag (PCNA) occurs as a component of multiprotein complexes during cell proliferation. We found the complexes to react with murine anti-PCNA mAbs, but not with anti-PCNA Abs in lupus sera. The complexes were purified from rabbit thymus extract by affinity chromatography using anti-PCNA mAbs (TOB7, TO17, and TO30) and analyzed by ELISA, immunoprecipitation, immunoblotting, and HPLC gel filtration. That PCNA was complexed with other proteins was demonstrated by its copurification with a group of proteins excluded by an HPLC G3000 SW column. Although immunoblot analysis showed the mAbs to react exclusively with the 34-kDa PCNA polypeptide, they nonetheless immunoprecipitated the same group of proteins, confirming the interaction of the isolated PCNA with other proteins. Anti-PCNA sera, including AK, which reacts with biologically functional sites on PCNA, did not react with complexed PCNA, but did react with it once it was dissociated from the complexes. PCNA complexes in turn reacted with murine anti-DNA mAbs, as well as with Abs against p21, replication protein A, DNA helicase II, cyclin-dependent kinases 4 and 5, and topoisomerase I. These findings suggest that the PCNA complexes purified using anti-PCNA mAbs comprise the "protein machinery" for DNA replication and cell cycle regulation. They also suggest that anti-PCNA mAbs are useful tools with which to characterize the protein-protein interactions within PCNA complexes, as well as the autoimmune responses to proteins interacting with PCNA, which may shed light on the mechanisms of autoantibody production in lupus patients.

Natural intramolecular isotope measurements in physiology: elements of the case for an effort toward high-precision position-specific isotope analysis

Chemical information available in organisms can be categorized into three major domains, macromolecular, small molecules, and isotope ratios. Information about physiological state is commonly obtained by qualitative and quantitative analysis in the macromolecular and small molecule domains. Genomics and proteomics are emerging approaches to analysis of macromolecules, and both areas yield definitive information on present physiological state. There is relatively little record of past physiological states of the individual available in these domains. Natural isotopic variability, particularly on an intramolecular level, is likely to retain more physiological history. Because of ubiquitous isotopic fractionation, every stereochemically unique position in every molecule has an isotope ratio that reflects the processes of synthesis and degradation. This fact highlights a vast amount of organismal chemical information that is essentially unstudied. Isotope measurements can be classified according to the chemical complexity of the analyte into bulk, compound-specific, and position-specific or intramolecular levels. Recent advances in analysis of isotope ratios are transforming natural science, and particularly answering questions about ecosystems using bulk methods; however, they have had relatively little impact on physiology. This may be because the vast complexities of physiological questions demand very selective information available in position-specific isotope analysis (PSIA). The relatively few high-precision PSIA studies, based on isotope ratio mass spectrometry (IRMS), have revealed intramolecular isotope ratio differences in pivotal physiological compounds including amino acids, glucose, glycerol, acetate, fatty acids, and purines. The majority of these analyses have been accomplished by laborious offline methods; however, recent advances in instrumentation presage rapid PSIA that will be necessary to attack real physiological problems. Gas-phase pyrolysis has been shown to be an effective method to determine (13)C/(12)C at high precision for molecular fragments, and technologies to extend C-based PSIA to N and other organic elements are emerging. Two related efforts are warranted, (a) development of rapid, convenient, and sensitive methods for high-precision PSIA, a necessary precursor to (b) a concerted investigation into the relationship of metabolic state to intramolecular isotope ratio. Inherent in this latter goal is the need to identify long-lived molecules in long-lived cells that retain a record of early isotopic conditions, as has been shown for post-mortem human neuronal DNA. Using known metabolic precursor-product relationships between intramolecular positions, future studies of physiological isotope fractionation should reveal the relationship of diet and environment to observed isotope ratio. This science of isotope physiology, or simply isotopics, should add an important tool for elucidation of early factors that effect later health, probably the most difficult class of biomedical issues.

Decreased levels of proteasome activity and proteasome expression in aging spinal cord

Neuron death and neuron degeneration occur in the CNS during the course of aging. Although multiple cellular alterations transpire during the aging process, those that mediate age-associated neuron death have not been identified. Recent evidence implicates oxidative stress as a possible means of neuron death and neuron degeneration during aging. In the present study, we demonstrate a marked decrease in multicatalytic proteasome activity in the spinal cord of Fisher 344 rats at 12, 24 and 28 months, compared with spinal cord tissue from 3-week- and 3-month-old animals. Application of oxidative injury (FeSO(4)) or the lipid peroxidation product 4-hydroxynonenal decreases multicatalytic proteasome activity in a time- and dose-dependent manner in a motor neuron cell line. Loss of multicatalytic proteasome activity occurs before the loss of multicatalytic proteasome immunoreactivity, with FeSO(4)- and 4-hydroxynonenal-mediated decreases ameliorated by the application of a cell permeable form of the antioxidant glutathione. Application of multicatalytic proteasome inhibitors, but not inhibitors of lysosomal proteases, induced neuron death that was attenuated by the caspase inhibitors benzyloxycarbonyl-Val-Ala-Asp-(O-methyl) fluoromethyl ketone or N-acetyl-Asp-Glu-Val-Asp-Cho (aldehyde). Together, these data suggest that multicatalytic proteasome inhibition occurs during aging of the spinal cord, possibly as the result of oxidative stress, and that multicatalytic proteasome inhibition may be causally related to neuron death.

Substrate binding is a prerequisite for stabilisation of mouse thymidine kinase in proliferating fibroblasts

Thymidine kinase (TK) expression in mammalian cells is strictly growth regulated, with high levels of the enzyme present in proliferating cells and low levels in resting cells. We have shown that mouse TK expressed from a constitutive promoter is still subject to this regulation. The drastic decline in TK enzyme levels in resting cells is largely due to a pronounced reduction in the half-life of the protein. Deletion of the 30 C-terminal amino acid residues from TK abrogates growth regulation, rendering the enzyme very stable. Moreover, the substrate thymidine was sufficient to stabilise the labile TK protein in quiescent cells. Here, we report that the ability of TK to bind substrates is essential for both growth-dependent regulation and stabilisation by the substrate. By mutation or elimination of the binding sites for either of the two substrates, ATP and thymidine, we expressed TK proteins lacking enzymatic activity which abolished growth-regulated expression in both cases. Mutant TK proteins impaired in substrate binding were subject to rapid degradation in exponentially growing cells and thymidine was no longer sufficient to inhibit this rapid decay. A C-terminal truncation known to stabilise the TK wild-type protein in resting cells did not affect the rapid turnover of enzymatically inactive TK proteins. Proteasome inhibitors also failed to stabilise these substrate-binding mutants. By cross-linking experiments, we show that TK proteins with mutated substrate-binding sites exist only as monomers, whereas active TK enzyme forms dimers and tetramers. Our data indicate that, In addition to the C terminus intact substrate-binding sites are required for growth-dependent regulation of TK protein stability.

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.

Possible involvement of proteasome inhibition in aging: implications for oxidative stress

Oxidative stress may contribute to the cellular alterations, which occur as the result of aging, and the nervous system is particularly vulnerable to aging associated oxidative injury. The multicatalytic proteasome (MCP) is responsible for the majority of protein degradation and is sensitive to oxidative stress. To determine if MCP activity is altered during aging, studies were conducted in multiple tissues from aged Fisher 344 rats. Analysis of heart, lung, kidney, and liver revealed decreased MCP activity in 12, 24, and 28 month old rats, compared with 3 week or 3 month old animals. The spinal cord, hippocampus, and cerebral cortex demonstrated age dependent decreases in MCP activity, but at no timepoint was MCP activity decreased in either the brain stem or cerebellum. Oxidative injury and the lipid oxidation product 4-hydroxynonenal caused decreased MCP activity in neural PC6 cells, while application of MCP inhibitors was sufficient to induce cell death in neural PC6 cells. Together, these data indicate a role for MCP inhibition in cellular dysfunction associated with aging and oxidative injury.

Mutational analysis of subunit i beta2 (MECL-1) demonstrates conservation of cleavage specificity between yeast and mammalian proteasomes

Proteasomes are the major protein-degrading complexes in the cytosol and regulate many cellular processes. To examine the functional importance of the MC14/MECL-1 proteasome active site subunits, cell lines expressing a catalytically inactive form of MECL-1 were established. Whereas mutant MECL-1 was readily incorporated into cytosolic proteasomes, replacing the constitutive MC14 subunit, removal of the prosequence was incomplete indicating that its processing required autocatalytic cleavage. Functional analyses showed that the absence of the MC14/MECL-1 active sites abrogated proteasomal trypsin-like activity, but did not affect other catalytic activities. Our data demonstrate a conservation of cleavage specificity between mammalian and yeast proteasomes.

The proteasome-dependent proteolytic system

The 20S proteasome is an intriguingly large complex that acts as a proteolytic catalytic machine. Accumulating evidence indicates the existence of multiple factors capable of regulating the proteasome function. They are classified into two different categories, one type of regulator is PA700 or PA28 that is reversibly associated with the 20S proteasome to form enzymatically active proteasomes and the other type including a 300-kDa modulator and PI31 indirectly influences proteasome activity perhaps by promoting or suppressing the assembly of the 20S proteasome with PA700 or PA28. Thus, there have been documented two types of proteasomes composed of a core catalytic proteasome and a pair of symmetrically disposed PA700 or PA28 regulatory particle. Moreover, the recently-identified proteasome containing both PA28 and PA700 appears to play a significant role in the ATP-dependent proteolytic pathway in cells, as can the 26S proteasome which is known as a eukaryotic ATP-dependent protease.