Inositol 1,3,4-trisphosphate 5/6-kinase associates with the COP9 signalosome by binding to CSN1

USP48 and A20 synergistically promote cell survival in Helicobacter pylori infection

The human pathogen Helicobacter pylori represents a risk factor for the development of gastric diseases including cancer. The H. pylori-induced transcription factor nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) is involved in the pro-inflammatory response and cell survival in the gastric mucosa, and represents a trailblazer of gastric pathophysiology. Termination of nuclear NF-κB heterodimer RelA/p50 activity is regulated by the ubiquitin-RING-ligase complex elongin-cullin-suppressor of cytokine signalling 1 (ECS^SOCS1), which leads to K48-ubiquitinylation and degradation of RelA. We found that deubiquitinylase (DUB) ubiquitin specific protease 48 (USP48), which interacts with the COP9 signalosome (CSN) subunit CSN1, stabilises RelA by deubiquitinylation and thereby promotes the transcriptional activity of RelA to prolong de novo synthesis of DUB A20 in H. pylori infection. An important role of A20 is the suppression of caspase-8 activity and apoptotic cell death. USP48 thus enhances the activity of A20 to reduce apoptotic cell death in cells infected with H. pylori. Our results, therefore, define a synergistic mechanism by which USP48 and A20 regulate RelA and apoptotic cell death in H. pylori infection.

Cullin-RING Ligase Regulation by the COP9 Signalosome: Structural Mechanisms and New Physiologic Players

The Cullin-RING E3 ligases (CRLs) are major ubiquitylation machineries regulated by reversible cycles of neddylation and deneddylation. The deneddylase COP9 Signalosome (CSN) terminates CRL catalytic cycle. CSN also provides a docking platform for several kinases and deubiquitinases that might play a role in regulating CRL. Recently, remarkable progress has been made in elucidating the biochemical principles and physiological implications of such exquisite regulation. The cryo-EM structures of CRL-CSN complexes provide the biochemical basis of their cognate interactions and reveal potential regulatory mechanisms during complex disassembly. Moreover, novel players beyond the canonical eight subunits of CSN were identified. This includes CSNAP, a potential 9th CSN subunit with regulatory functions, and the metabolite inositol hexakisphosphate (IP₆), which enhances CRL-CSN complex formation, with IP₆-metabolizing enzymes possibly instilling dynamics to the CRL-CSN system. Here, we review and summarize these new mechanistic insights along with progress in understanding CSN biology based on model organisms with genetically edited CSN subunits.

Are Inositol Polyphosphates the Missing Link in Dynamic Cullin RING Ligase Regulation by the COP9 Signalosome?

The E3 ligase activity of Cullin RING Ligases (CRLs) is controlled by cycles of neddylation/deneddylation and intimately regulated by the deneddylase COP9 Signalosome (CSN), one of the proteasome lid-CSN-initiation factor 3 (PCI) domain-containing “Zomes” complex. Besides catalyzing the removal of stimulatory Cullin neddylation, CSN also provides a docking platform for other proteins that might play a role in regulating CRLs, notably protein kinases and deubiquitinases. During the CRL activity cycle, CRL–CSN complexes are dynamically assembled and disassembled. Mechanisms underlying complex dynamics remain incompletely understood. Recently, the inositol polyphosphate metabolites (IP6, IP7) and their metabolic enzymes (IP5K, IP6K) have been discovered to participate in CRL–CSN complex formation as well as stimulus-dependent dissociation. Here we discuss these mechanistic insights in light of recent advances in elucidating structural basis of CRL–CSN complexes.

COP9 Signalosome Interaction with UspA/Usp15 Deubiquitinase Controls VeA-Mediated Fungal Multicellular Development

COP9 signalosome (CSN) and Den1/A deneddylases physically interact and promote multicellular development in fungi. CSN recognizes Skp1/cullin-1/Fbx E3 cullin-RING ligases (CRLs) without substrate and removes their posttranslational Nedd8 modification from the cullin scaffold. This results in CRL complex disassembly and allows Skp1 adaptor/Fbx receptor exchange for altered substrate specificity. We characterized the novel ubiquitin-specific protease UspA of the mold Aspergillusnidulans, which corresponds to CSN-associated human Usp15 and interacts with six CSN subunits. UspA reduces amounts of ubiquitinated proteins during fungal development, and the uspA gene expression is repressed by an intact CSN. UspA is localized in proximity to nuclei and recruits proteins related to nuclear transport and transcriptional processing, suggesting functions in nuclear entry control. UspA accelerates the formation of asexual conidiospores, sexual development, and supports the repression of secondary metabolite clusters as the derivative of benzaldehyde (dba) genes. UspA reduces protein levels of the fungal NF-kappa B-like velvet domain protein VeA, which coordinates differentiation and secondary metabolism. VeA stability depends on the Fbx23 receptor, which is required for light controlled development. Our data suggest that the interplay between CSN deneddylase, UspA deubiquitinase, and SCF-Fbx23 ensures accurate levels of VeA to support fungal development and an appropriate secondary metabolism.

Lsi1 modulates the antioxidant capacity of rice and protects against ultraviolet-B radiation

Silicon (Si) enhances the resistance of rice to biotic and abiotic stress. In rice, the accumulation of Si is controlled by the low silicon rice 1 (Lsi1) gene; overexpression of Lsi1 (Lsi1-OX) increases Si uptake and accumulation, while the reverse is observed in Lsi1-RNA interference (Lsi1-RNAi) transgenic rice. When the two transgenic rice lines and wild-type (WT) rice were exposed to ultraviolet (UV)-B radiation, the Lsi1-OX or Lsi1-RNAi rice showed differential microRNA (miRNA) expression, compared to WT rice. These miRNAs were predicted to target genes involved in light signal transduction and cell detoxification. The greatest capacities of ascorbate peroxidase, superoxide dismutase, peroxidase, and phenylalanine ammonia lyase (PAL) and highest contents of phenolics, flavonoids, and proline were found in Lsi1-OX rice, followed by WT rice and Lsi1-RNAi transgenic rice. A further comparison of the transcript levels of individual PAL genes revealed that the expression of PAL2-2 (Os02g0626400) was positively regulated by Lsi1. Our results demonstrate that Lsi1 overexpression or interference causes changes in both miRNA expression and antioxidant capacity in rice, and therefore modulates rice tolerance to UV-B radiation. Furthermore, we demonstrated that PAL2-2 was positively regulated by Lsi1 during this process.

Corrected and Republished from: The COP9 Signalosome Interacts with and Regulates Interferon Regulatory Factor 5 Protein Stability

The transcription factor interferon regulatory factor 5 (IRF5) exerts crucial functions in the regulation of host immunity against extracellular pathogens, DNA damage-induced apoptosis, death receptor signaling, and macrophage polarization. Tight regulation of IRF5 is thus warranted for an efficient response to extracellular stressors and for limiting autoimmune and inflammatory responses. Here we report that the COP9 signalosome (CSN), a general modulator of diverse cellular and developmental processes, associates constitutively with IRF5 and promotes its protein stability. The constitutive CSN/IRF5 interaction was identified using proteomics and confirmed by endogenous immunoprecipitations. The CSN/IRF5 interaction occurred on the carboxyl and amino termini of IRF5; a single internal deletion (Δ455-466) was found to significantly reduce IRF5 protein stability. CSN3 was identified as a direct interacting partner of IRF5, and knockdown of this subunit with small interfering RNAs (siRNAs) resulted in enhanced degradation. Degradation was further augmented by knockdown of CSN1 and CSN3 together. The ubiquitin E1 inhibitor UBEI-41 or the proteasome inhibitor MG132 prevented IRF5 degradation, supporting that its stability is regulated by the ubiquitin-proteasome system. Importantly, activation of IRF5 by the death receptor ligand tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) resulted in enhanced degradation via loss of the CSN/IRF5 interaction. This study defines the CSN as a new interacting partner of IRF5 that controls its stability.

COP9 signalosome subunits protect Capicua from MAPK-dependent and -independent mechanisms of degradation

The COP9 signalosome removes Nedd8 modifications from the Cullin subunits of ubiquitin ligase complexes, reducing their activity. Here, we show that mutations in the Drosophila COP9 signalosome subunit 1b (CSN1b) gene increase the activity of ubiquitin ligases that contain Cullin 1. Analysis of CSN1b mutant phenotypes revealed a requirement for the COP9 signalosome to prevent ectopic expression of Epidermal growth factor receptor (EGFR) target genes. It does so by protecting Capicua, a transcriptional repressor of EGFR target genes, from EGFR pathway-dependent ubiquitylation by a Cullin 1/SKP1-related A/Archipelago E3 ligase and subsequent proteasomal degradation. The CSN1b subunit also maintains basal Capicua levels by protecting it from a separate mechanism of degradation that is independent of EGFR signaling. As a suppressor of tumor growth and metastasis, Capicua may be an important target of the COP9 signalosome in cancer.

Cul3 neddylation is crucial for gradual lipid droplet formation during adipogenesis

Cullin 3 (Cul3) belongs to the family of cullins (Cul1-7) providing the scaffold for cullin-RING ubiquitin (Ub) ligases (CRLs), which are activated by neddylation and represent essential E3 ligases of the Ub proteasome system. During adipogenic differentiation neddylated Cul3 accumulates in LiSa-2 preadipocytes. Downregulation of Cul3 and inhibition of neddylation by MLN4924 blocks the formation of lipid droplets (LDs), the lipid storage organelles and markers of adipogenesis. Neddylation of Cul3 coincides with an increase of Rab18, a GTPase associated with LDs. Immunoprecipitation and confocal fluorescence microscopy revealed physical association of Cul3 and Rab18 at the membrane of LDs. RhoA, a suppressor of adipogenesis decreased during differentiation. Our results in LiSa-2 cells, but also mouse embryonic fibroblasts revealed a connection between Cul3, Rab18 and RhoA. Downregulation of Cul3 led to a marked increase in RhoA protein expression after 6days of LiSa-2 cell differentiation, suggesting that Cul3 is involved in the regulation of RhoA stability.

Roles of Multifunctional COP9 Signalosome Complex in Cell Fate and Implications for Drug Discovery

The eight subunits containing COP9 signalosome (CSN) complex, is highly conserved among eukaryotes. CSN, identified as a negative regulator of photomorphogenesis, has also been demonstrated to be important in proteolysis, cellular signal transduction and cell cycle regulation in various eukaryotic organisms. This review mainly summarizes the roles of CSN in cell cycle regulation, signal transduction and apoptosis, and its potential as diagnostic biomarkers, drug targets for cancer and infectious diseases. J. Cell. Physiol. 232: 1246-1253, 2017. © 2016 Wiley Periodicals, Inc.

The devil is in the details: comparison between COP9 signalosome (CSN) and the LID of the 26S proteasome

The COP9 signalosome (CSN) and the proteasomal LID are conserved macromolecular complexes composed of at least eight subunits with molecular weights of approximately 350 kDa. CSN and LID are part of the ubiquitin–proteasome pathway and cleave isopeptide linkages of lysine side chains on target proteins. CSN cleaves the isopeptide bond of ubiquitin-like protein Nedd8 from cullins, whereas the LID cleaves ubiquitin from target proteins sentenced for degradation. CSN and LID are structurally and functionally similar but the order of the assembly pathway seems to be different. The assembly differs in at least the last subunit joining the pre-assembled subcomplex. This review addresses the similarities and differences in structure, function and assembly of CSN and LID.

Inositol hexakisphosphate (IP6) generated by IP5K mediates cullin-COP9 signalosome interactions and CRL function

The family of cullin-RING E3 Ligases (CRLs) and the constitutive photomorphogenesis 9 (COP9) signalosome (CSN) form dynamic complexes that mediate ubiquitylation of 20% of the proteome, yet regulation of their assembly/disassembly remains poorly understood. Inositol polyphosphates are highly conserved signaling molecules implicated in diverse cellular processes. We now report that inositol hexakisphosphate (IP6) is a major physiologic determinant of the CRL-CSN interface, which includes a hitherto unidentified electrostatic interaction between the N-terminal acidic tail of CSN subunit 2 (CSN2) and a conserved basic canyon on cullins. IP6, with an EC50 of 20 nM, acts as an intermolecular "glue," increasing cullin-CSN2 binding affinity by 30-fold, thereby promoting assembly of the inactive CRL-CSN complexes. The IP6 synthase, Ins(1,3,4,5,6)P5 2-kinase (IPPK/IP5K) binds to cullins. Depleting IP5K increases the percentage of neddylated, active Cul1 and Cul4A, and decreases levels of the Cul1/4A substrates p27 and p21. Besides dysregulating CRL-mediated cell proliferation and UV-induced apoptosis, IP5K depletion potentiates by 28-fold the cytotoxic effect of the neddylation inhibitor MLN4924. Thus, IP5K and IP6 are evolutionarily conserved components of the CRL-CSN system and are potential targets for cancer therapy in conjunction with MLN4924.

The Role of the COP9 Signalosome and Neddylation in DNA Damage Signaling and Repair

The maintenance of genomic integrity is an important process in organisms as failure to sense and repair damaged DNA can result in a variety of diseases. Eukaryotic cells have developed complex DNA repair response (DDR) mechanisms to accurately sense and repair damaged DNA. Post-translational modifications by ubiquitin and ubiquitin-like proteins, such as SUMO and NEDD8, have roles in coordinating the progression of DDR. Proteins in the neddylation pathway have also been linked to regulating DDR. Of interest is the COP9 signalosome (CSN), a multi-subunit metalloprotease present in eukaryotes that removes NEDD8 from cullins and regulates the activity of cullin-RING ubiquitin ligases (CRLs). This in turn regulates the stability and turnover of a host of CRL-targeted proteins, some of which have established roles in DDR. This review will summarize the current knowledge on the role of the CSN and neddylation in DNA repair.

Diversity of COP9 signalosome structures and functional consequences

The COP9 signalosome (CSN) is a regulator of the ubiquitin (Ub) proteasome system (UPS). It interacts with hundreds of cullin-RING ubiquitin E3 ligases (CRLs) and regulates their activity by removing the Ub-like protein Nedd8 from cullins. In mammalian cells 7 different cullins exist which form CRLs with adaptor proteins and with a large number of substrate recognition subunits such as F-box and BTB proteins. This large variety of CRL-complexes is deneddylated by the CSN. The capacity of the CSN to interact with numerous types of CRL complexes can be explained by its structural diversity, which allows different CSN variants to interact with different binding partners and substrates and enables different subunit expression profiles. Diversity of CSN complexes presumably occurs by: (1) flexibility of CSN holo complex structure; (2) formation of CSN mini complexes and free CSN subunits and (3) generation of CSN variants via integration of CSN subunit isoforms. In this review we will discuss the structural diversity of the CSN complex and possible functional consequences.

Inositol hexakisphosphate kinase-1 mediates assembly/disassembly of the CRL4-signalosome complex to regulate DNA repair and cell death

Inositol polyphosphates containing an energetic pyrophosphate bond are formed primarily by a family of three inositol hexakisphosphate (IP6) kinases (IP6K1-3). The Cullin-RING ubiquitin ligases (CRLs) regulate diverse biological processes through substrate ubiquitylation. CRL4, comprising the scaffold Cullin 4A/B, the E2-interacting Roc1/2, and the adaptor protein damage-specific DNA-binding protein 1, is activated by DNA damage. Basal CRL4 activity is inhibited by binding to the COP9 signalosome (CSN). UV radiation and other stressors dissociate the complex, leading to E3 ligase activation, but signaling events that trigger signalosome dissociation from CRL4 have been unclear. In the present study, we show that, under basal conditions, IP6K1 forms a ternary complex with CSN and CRL4 in which IP6K1 and CRL4 are inactive. UV dissociates IP6K1 to generate IP7, which then dissociates CSN-CRL4 to activate CRL4. Thus, IP6K1 is a novel CRL4 subunit that transduces UV signals to mediate disassembly of the CRL4-CSN complex, thereby regulating nucleotide excision repair and cell death.

Global Human-Kinase Screening Identifies Therapeutic Host Targets against Influenza

During viral infection of human cells, host kinases mediate signaling activities that are used by all viruses for replication; therefore, targeting of host kinases is of broad therapeutic interest. Here, host kinases were globally screened during human influenza virus (H1N1) infection to determine the time-dependent effects of virus infection and replication on kinase function. Desthiobiotin-labeled analogs of adenosine triphosphate and adenosine diphosphate were used to probe and covalently label host kinases in infected cell lysates, and probe affinity was determined. Using infected human A549 cells, we screened for time-dependent signal changes and identified host kinases whose probe affinities differed significantly when compared to uninfected cells. Our screen identified 10 novel host kinases that have not been previously shown to be involved with influenza virus replication, and we validated the functional importance of these novel kinases during infection using targeted small interfering RNAs (siRNAs). The effects of kinase-targeted siRNA knockdowns on replicating virus levels were measured by quantitative reverse-transcription PCR and cytoprotection assays. We identified several novel host kinases that, when knocked down, enhanced or reduced the viral load in cell culture. This preliminary work represents the first screen of the changing host kinome in influenza virus-infected human cells.

Differential isolation and identification of PI(3)P and PI(3,5)P2 binding proteins from Arabidopsis thaliana using an agarose-phosphatidylinositol-phosphate affinity chromatography

A phosphatidylinositol-phosphate affinity chromatographic approach combined with mass spectrometry was used in order to identify novel PI(3)P and PI(3,5)P2 binding proteins from Arabidopsis thaliana suspension cell extracts. Most of the phosphatidylinositol-phosphate interacting candidates identified from this differential screening are characterized by lysine/arginine rich patches. Direct phosphoinositide binding was identified for important membrane trafficking regulators as well as protein quality control proteins such as the ATG18p orthologue involved in autophagosome formation and the lipid Sec14p like transfer protein. A pentatricopeptide repeat (PPR) containing protein was shown to directly bind to PI(3,5)P2 but not to PI(3)P. PIP chromatography performed using extracts obtained from high salt (0.4M and 1M NaCl) pretreated suspensions showed that the association of an S5-1 40S ribosomal protein with both PI(3)P and PI(3,5)P2 was abolished under salt stress whereas salinity stress induced an increase in the phosphoinositide association of the DUF538 domain containing protein SVB, associated with trichome size. Additional interacting candidates were co-purified with the phosphoinositide bound proteins. Binding of the COP9 signalosome, the heat shock proteins, and the identified 26S proteasomal subunits, is suggested as an indirect effect of their interaction with other proteins directly bound to the PI(3)P and the PI(3,5)P2 phosphoinositides.

BIOLOGICAL SIGNIFICANCE

PI(3,5)P2 is of special interest because of its low abundance. Furthermore, no endogenous levels have yet been detected in A. thaliana (although there is evidence for its existence in plants). Therefore the isolation of novel interacting candidates in vitro would be of a particular importance since the future study and localization of the respective endogenous proteins may indicate possible targeted compartments or tissues where PI(3,5)P2 could be enriched and thereafter identified. In addition, PI(3,5)P2 is a phosphoinositide extensively studied in mammalian and yeast systems. However, our knowledge of its role in plants as well as a list of its effectors from plants is very limited.

Crystal structure and versatile functional roles of the COP9 signalosome subunit 1

The constitutive photomorphogenesis 9 (COP9) signalosome (CSN) plays key roles in many biological processes, such as repression of photomorphogenesis in plants and protein subcellular localization, DNA-damage response, and NF-κB activation in mammals. It is an evolutionarily conserved eight-protein complex with subunits CSN1 to CSN8 named following the descending order of molecular weights. Here, we report the crystal structure of the largest CSN subunit, CSN1 from Arabidopsis thaliana (atCSN1), which belongs to the Proteasome, COP9 signalosome, Initiation factor 3 (PCI) domain containing CSN subunit family, at 2.7 Å resolution. In contrast to previous predictions and distinct from the PCI-containing 26S proteasome regulatory particle subunit Rpn6 structure, the atCSN1 structure reveals an overall globular fold, with four domains consisting of helical repeat-I, linker helix, helical repeat-II, and the C-terminal PCI domain. Our small-angle X-ray scattering envelope of the CSN1-CSN7 complex agrees with the EM structure of the CSN alone (apo-CSN) and suggests that the PCI end of each molecule may mediate the interaction. Fitting of the CSN1 structure into the CSN-Skp1-Cul1-Fbox (SCF) EM structure shows that the PCI domain of CSN1 situates at the hub of the CSN for interaction with several other subunits whereas the linker helix and helical repeat-II of CSN1 contacts SCF using a conserved surface patch. Furthermore, we show that, in human, the C-terminal tail of CSN1, a segment not included in our crystal structure, interacts with IκBα in the NF-κB pathway. Therefore, the CSN complex uses multiple mechanisms to hinder NF-κB activation, a principle likely to hold true for its regulation of many other targets and pathways.

CAND1-dependent control of cullin 1-RING Ub ligases is essential for adipogenesis

Cullin-RING ubiquitin (Ub) ligases (CRLs) are responsible for ubiquitinylation of approximately 20% of all proteins degraded by the Ub proteasome system (UPS). CRLs are regulated by the COP9 signalosome (CSN) and by Cullin-associated Nedd8-dissociated protein 1 (CAND1). The CSN is responsible for removal of Nedd8 from cullins inactivating CRLs. CAND1 modulates the assembly of F-box proteins into cullin 1-RING Ub ligases (CRL1s). We show that CAND1 preferentially blocks the integration of Skp2 into CRL1s. Suppression of CAND1 expression in HeLa cells leads to an increase of the Skp2 assembly into CRL1s and to significant reduction of the cyclin-dependent kinase (CDK) inhibitor p27. In contrary, CAND1 overexpression causes elevation of p27. The observed CAND1-dependent effects and CAND1 expression are independent of the CSN as demonstrated in CSN1 knockdown cells. Increase of p27 is a hallmark of preadipocyte differentiation to adipocytes (adipogenesis). We demonstrate that the accumulation of p27 is associated with an increase of CAND1 and a decrease of Skp2 during adipogenesis of human LiSa-2 preadipocytes. CAND1 knockdown reduces p27 and blocks adipogenesis. Due to the impact of CAND1 on Skp2 control, CAND1 could represent an important effector molecule in adipogenesis, but also in cancer development.

The COP9 signalosome interacts with and regulates interferon regulatory factor 5 protein stability

The transcription factor interferon regulatory factor 5 (IRF5) exerts crucial functions in the regulation of host immunity against extracellular pathogens, DNA damage-induced apoptosis, death receptor signaling, and macrophage polarization. Tight regulation of IRF5 is thus warranted for an efficient response toward extracellular stressors and for limiting autoimmune and inflammatory responses. Here we report that the COP9 signalosome (CSN), a general modulator of diverse cellular and developmental processes, associates constitutively with IRF5 and promotes its protein stability. The constitutive CSN/IRF5 interaction was identified using proteomics and confirmed by endogenous immunoprecipitations. The CSN/IRF5 interaction occurred on the carboxyl and amino termini of IRF5; a single internal deletion from amino acids 455 to 466 (Δ455-466) was found to significantly reduce IRF5 protein stability. CSN subunit 3 (CSN3) was identified as a direct interacting partner of IRF5, and knockdown of this subunit with small interfering RNAs resulted in enhanced degradation. Degradation was further augmented by knockdown of CSN1 and CSN3 together. The ubiquitin E1 inhibitor UBEI-41 or the proteasome inhibitor MG132 prevented IRF5 degradation, supporting the idea that its stability is regulated by the ubiquitin-proteasome system. Importantly, activation of IRF5 by the death receptor ligand tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) resulted in enhanced degradation via loss of the CSN/IRF5 interaction. This study defines CSN to be a new interacting partner of IRF5 that controls its stability.

Structural basis for a reciprocal regulation between SCF and CSN

Skp1-Cul1-Fbox (SCF) E3 ligases are activated by ligation to the ubiquitin-like protein Nedd8, which is reversed by the deneddylating Cop9 signalosome (CSN). However, CSN also promotes SCF substrate turnover through unknown mechanisms. Through biochemical and electron microscopy analyses, we determined molecular models of CSN complexes with SCF(Skp2/Cks1) and SCF(Fbw7) and found that CSN occludes both SCF functional sites-the catalytic Rbx1-Cul1 C-terminal domain and the substrate receptor. Indeed, CSN binding prevents SCF interactions with E2 enzymes and a ubiquitination substrate, and it inhibits SCF-catalyzed ubiquitin chain formation independent of deneddylation. Importantly, CSN prevents neddylation of the bound cullin, unless binding of a ubiquitination substrate triggers SCF dissociation and neddylation. Taken together, the results provide a model for how reciprocal regulation sensitizes CSN to the SCF assembly state and inhibits a catalytically competent SCF until a ubiquitination substrate drives its own degradation by displacing CSN, thereby promoting cullin neddylation and substrate ubiquitination.

Regulation of inositol 1,3,4-trisphosphate 5/6-kinase (ITPK1) by reversible lysine acetylation

The enzyme inositol 1,3,4-trisphosphate 5/6-kinase (ITPK1) catalyzes the rate-limiting step in the formation of higher phosphorylated forms of inositol in mammalian cells. Because it sits at a key regulatory point in the inositol metabolic pathway, its activity is likely to be regulated. We have previously shown that ITPK1 is phosphorylated, a posttranslational modification used by cells to regulate enzyme activity. We show here that ITPK1 is modified by acetylation of internal lysine residues. The acetylation sites, as determined by mass spectrometry, were found to be lysines 340, 383, and 410, which are all located on the surface of this protein. Overexpression of the acetyltransferases CREB-binding protein or p300 resulted in the acetylation of ITPK1, whereas overexpression of mammalian silent information regulator 2 resulted in the deacetylation of ITPK1. Functionally, ITPK1 acetylation regulates its stability. CREB-binding protein dramatically decreased the half-life of ITPK1. We further found that ITPK1 acetylation down-regulated its enzyme activity. HEK293 cells stably expressing acetylated ITPK1 had reduced levels of the higher phosphorylated forms of inositol, compared with the levels seen in cells expressing unacetylated ITPK1. These results demonstrate that lysine acetylation alters both the stability as well as the activity of ITPK1 in cells.

Characterization of an inositol 1,3,4-trisphosphate 5/6-kinase gene that is essential for drought and salt stress responses in rice

Drought and salt stresses are major limiting factors for crop production. To identify critical genes for stress resistance in rice (Oryza sativa L.), we screened T-DNA mutants and identified a drought- and salt-hypersensitive mutant dsm3. The mutant phenotype was caused by a T-DNA insertion in a gene encoding a putative inositol 1,3,4-trisphosphate 5/6-kinase previously named OsITPK2 with unknown function. Under drought stress conditions, the mutant had significantly less accumulation of osmolytes such as proline and soluble sugar and showed significantly reduced root volume, spikelet fertility, biomass, and grain yield; however, malondialdehyde level was increased in the mutant. Interestingly, overexpression of DSM3 (OsITPK2) in rice resulted in drought- and salt-hypersensitive phenotypes and physiological changes similar to those in the mutant. Inositol trisphosphate (IP3) level was decreased in the overexpressors under normal condition and drought stress. A few genes related to osmotic adjustment and reactive oxygen species scavenging were down-regulated in the mutant and overexpression lines. The expression level of DSM3 promoter-driven β-glucuronidase (GUS) reporter gene in rice was induced by drought, salt and abscisic acid. Protoplast transient expression assay indicated that DSM3 is an endoplasmic reticulum protein. Sequence analysis revealed six putative ITPKs in rice. Transcript level analysis of OsITPK genes revealed that they had different tempo-spatial expression patterns, and the responses of DSM3 to abiotic stresses, including drought, salinity, cold, and high temperature, were distinct from the other five members in rice. These results together suggest that DSM3/OsITPK2 is an important member of the OsITPK family for stress responses, and an optimal expression level is essential for drought and salt tolerance in rice.

Function of COP9 signalosome in regulation of mouse oocytes meiosis by regulating MPF activity and securing degradation

The COP9 (constitutive photomorphogenic) signalosome (CSN), composed of eight subunits, is a highly conserved protein complex that regulates processes such as cell cycle progression and kinase signalling. Previously, we found the expression of the COP9 constitutive photomorphogenic homolog subunit 3 (CSN3) and subunit 5 (CSN5) changes as oocytes mature for the first time, and there is no report regarding roles of COP9 in the mammalian oocytes. Therefore, in the present study, we examined the effects of RNA interference (RNAi)-mediated transient knockdown of each subunit on the meiotic cell cycle in mice oocytes. Following knockdown of either CSN3 or CSN5, oocytes failed to complete meiosis I. These arrested oocytes exhibited a disrupted meiotic spindle and misarranged chromosomes. Moreover, down-regulation of each subunit disrupted the activity of maturation-promoting factor (MPF) and concurrently reduced degradation of the anaphase-promoting complex/cyclosome (APC/C) substrates Cyclin B1 and Securin. Our data suggest that the CSN3 and CSN5 are involved in oocyte meiosis by regulating degradation of Cyclin B1 and Securin via APC/C.

Roles of COP9 signalosome in cancer

The constitutive photomorphogenesis 9 signalosome (COP9 or CSN) is an evolutionarily conserved multiprotein complex found in plants and animals. Because of the homology between the COP9 signalosome and the 19S lid complex of the proteosome, COP9 has been postulated to play a role in regulating the degradation of polyubiquitinated proteins. Many tumor suppressor and oncogene products are regulated by ubiquitination- and proteosome-mediated protein degradation. Therefore, it is conceivable that COP9 plays a significant role in cancer, regulating processes relevant to carcinogenesis and cancer progression (e.g., cell cycle control, signal transduction and apoptosis). In mammalian cells, it consists of eight subunits (CSN1 to CSN8). The relevance and importance of some subunits of COP9 to cancer are emerging. However, the mechanistic regulation of each subunit in cancer remains unclear. Among the CSN subunits, CSN5 and CSN6 are the only two that each contain an MPN (Mpr1p and Pad1p N-terminal) domain. The deneddylation activity of an MPN domain toward cullin-RING ubiquitin ligases (CRL) may coordinate CRL-mediated ubiquitination activity. More recent evidence shows that CSN5 and CSN6 are implicated in ubiquitin-mediated proteolysis of important mediators in carcinogenesis and cancer progression. Here, we discuss the mechanisms by which some CSN subunits are involved in cancer to provide a much needed perspective regarding COP9 in cancer research, hoping that these insights will lay the groundwork for cancer intervention.

CSN1 inhibits c-Jun phosphorylation and down-regulates ectopic expression of JNK1

CSN1 is a component of the COP9 signalosome (CSN), a conserved protein complex with pleiotropic functions in many organs and cell types. CSN regulates ubiquitinproteasome dependent protein degradation via the deneddylation and the associated deubiquitination activities. In addition, CSN associates with protein kinases and modulates cell signaling, particularly the activator protein 1 (AP-1) pathway. We have shown previously that CSN1 suppresses AP-1 transcription activity and inhibits ultraviolet (UV) and serum activation of c-fos expression. Here we show that CSN1 can inhibit phosphorylation of proto-oncogene c-Jun product and repress c-Jun dependent transcription. Further, CSN1 dramatically downregulates ectopic expression of c-Jun N-terminal kinase 1 (JNK1) in cultured cells. The decline in JNK1 is not caused by excessive proteolysis or by 3' UTR-dependent mRNA instability, but by CSN1-dependent repression of one or multiple steps in transcriptional and posttranscriptional mechanisms. Thus, in contrast to CSN5/Jab1, which promotes AP-1 activity, CSN1 displays a negative effect on the AP-1 pathway. Finally, we discuss about the dynamic equilibrium of the CSN complexes in regulation of the AP-1 pathway.

Post-transcriptional fine-tuning of COP9 signalosome subunit biosynthesis is regulated by the c-Myc/Lin28B/let-7 pathway

The COP9 signalosome (CSN) complex controls protein degradation via the ubiquitin (Ub) proteasome system (UPS) in eukaryotes. In mammalian cells, the multimeric CSN is composed of eight subunits (CSN1 - CSN8). It regulates cullin-RING Ub ligases (CRLs), which target essential regulatory proteins for ubiquitination and subsequent degradation. Thereby, the CSN cooperates with the UPS in a variety of essential cellular functions, including DNA repair, cell cycle and differentiation. Although functions of the CSN have been elucidated, mechanisms and regulatory principles of its de novo formation are completely unknown. Here, we show that there is a fundamental mechanism that allows a coordinated expression of all CSN subunits, a prerequisite for CSN assembly. CSN subunit mRNAs are targets of miRNAs of the let-7 family suppressing CSN subunit expression in human cells. Factors that reduce or block let-7 miRNAs induce the coordinated expression of CSN subunits. For instance, over-expression of CSN1 specifically traps let-7a-1 miRNA and elevates CSN subunit levels by two- to fourfold in a coordinated manner. CSN subunit expression is also increased by specific miRNA inhibitors or by interferon (IFN)-mediated induction of STAT1 and c-Myc reducing levels of let-7 miRNAs. Activation of STAT1 by IFNα or IFNγ induces c-Myc, which increases CSN subunit expression via the Lin28B/let-7 regulatory pathway. By contrast, a let-7a-1 mimic reduces CSN subunit expression. Our data show that let-7 miRNAs control the fine-tuning and coordinated expression of subunits for CSN de novo formation, presumably a general regulatory principle for other Zomes complexes as well.

COP9 signalosome function in the DDR

The COP9 signalosome (CSN) is a platform for protein communication in eukaryotic cells. It has an intrinsic metalloprotease that removes the ubiquitin (Ub)-like protein Nedd8 from cullins. CSN-mediated deneddylation regulates culling-RING Ub ligases (CRLs) and controls ubiquitination of proteins involved in DNA damage response (DDR). CSN forms complexes with CRLs containing cullin 4 (CRL4s) which act on chromatin playing crucial roles in DNA repair, checkpoint control and chromatin remodeling. Furthermore, via associated kinases the CSN controls the stability of DDR effectors such as p53 and p27 and thereby the DDR outcome. DDR is a protection against cancer and deregulation of CSN function causes cancer making it an attractive pharmacological target. Here we review current knowledge on CSN function in DDR.

JAB1/CSN5: a new player in cell cycle control and cancer

c-Jun activation domain-binding protein-1 (Jab1) acts as a modulator of intracellular signaling and affects cellular proliferation and apoptosis, through its existence as a monomer or as the fifth component of the constitutive photomorphogenic-9 signalosome (CSN5). Jab1/CSN5 is involved in transcription factor specificity, deneddylation of NEDD8, and nuclear-to-cytoplasmic shuttling of key molecules. Jab1/CSN5 activities positively and negatively affect a number of pathways, including integrin signaling, cell cycle control, and apoptosis. Also, more recent studies have demonstrated the intriguing roles of Jab1/CSN5 in regulating genomic instability and DNA repair. The effects of Jab1/CSN5's multiple protein interactions are generally oncogenic in nature, and overexpression of Jab1/CSN5 in cancer provides evidence that it is involved in the tumorigenic process. In this review, we highlight our current knowledge of Jab1/CSN5 function and the recent discoveries in dissecting the Jab1 signaling pathway. Further, we also discuss the regulation of Jab1/CSN5 in cancers and its potential as a therapeutic target.

The COP9 signalosome mediates transcriptional and metabolic response to hormones, oxidative stress protection and cell wall rearrangement during fungal development

The COP9 signalosome complex (CSN) is a crucial regulator of ubiquitin ligases. Defects in CSN result in embryonic impairment and death in higher eukaryotes, whereas the filamentous fungus Aspergillus nidulans survives without CSN, but is unable to complete sexual development. We investigated overall impact of CSN activity on A. nidulans cells by combined transcriptome, proteome and metabolome analysis. Absence of csn5/csnE affects transcription of at least 15% of genes during development, including numerous oxidoreductases. csnE deletion leads to changes in the fungal proteome indicating impaired redox regulation and hypersensitivity to oxidative stress. CSN promotes the formation of asexual spores by regulating developmental hormones produced by PpoA and PpoC dioxygenases. We identify more than 100 metabolites, including orsellinic acid derivatives, accumulating preferentially in the csnE mutant. We also show that CSN is required to activate glucanases and other cell wall recycling enzymes during development. These findings suggest a dual role for CSN during development: it is required early for protection against oxidative stress and hormone regulation and is later essential for control of the secondary metabolism and cell wall rearrangement.

Control of Deneddylation by the COP9 Signalosome

The interplay between ubiquitin (Ub) family modifiers creates a regulatory network of Ub family proteins which is essential for cell growth and differentiation. One of the best studied crosstalks between Ub family modifiers is the stimulation of ubiquitination by Nedd8 (neural precursor cell expressed developmentally down regulated 8) modification. The neddylation-deneddylation pathway controls the selective ubiquitination of important cellular regulators targeted for proteolysis by the Ub proteasome system (UPS). In this process the cullin scaffolds of cullin-RING Ub ligases (CRLs) are neddylated, which allosterically activates the transfer of Ub to substrates of the CRLs. A major reaction of the regulatory network is the removal of Nedd8 by the COP9 signalosome (CSN), which converts CRLs into an inactive state. The CSN is a conserved protein complex that interacts with CRLs and possesses an intrinsic metalloprotease with a Jab1/Pad1/MPN+ (JAMM) motif responsible for deneddylation.In the present chapter we focus on the CSN-mediated deneddylation and its biological significance. We summarize latest developments on the mechanism of the CSN and its association with supercomplexes. In addition, data on the regulation of CSN-mediated deneddylation are described. Moreover, dysfunctions of the CSN and their implication in the pathogenesis of diseases are discussed.

Crosstalk between the NF-kappaB activating IKK-complex and the CSN signalosome

A great variety of signalling pathways regulating inflammation, cell development and cell survival require NF-κB transcription factors, which are normally inactive due to binding to inhibitors, such as IκBα. The canonical activation pathway of NF-κB is initiated by phosphorylation of the inhibitor by an IκB kinase (IKK) complex triggering ubiquitination of IκB molecules by SCF-type E3-ligase complexes and rapid degradation by 26S-proteasomes. The ubiquitination machinery is regulated by the COP9 signalosome (CSN). We show that IκB kinases interact with the CSN-complex, as well as the SCF-ubiquitination machinery, providing an explanation for the rapid signalling-induced ubiquitination and degradation of IκBα. Furthermore, we reveal that IKK’s phosphorylate not only IκBα, but also the CSN-subunit Csn5/JAB1 (c-Jun activation domain binding protein-1) and that IKK2 influences ubiquitination of Csn5/JAB1. Our observations imply that the CSN complex acts as an inhibitor of constitutive NF-κB activity in non-activated cells. Knock-down of Csn5/JAB1 clearly enhanced basal NF-κB activity and improved cell survival under stress. The inhibitory effect of Csn5/JAB1 requires a functional MPN⁺ metalloprotease domain, which is responsible for cleaving ubiquitin-like Nedd8-modifications. Upon activation of cells with tumour necrosis factor-α, the CSN complex dissociates from IKK’s allowing full and rapid activation of the NF-κB pathway by the concerted action of interacting protein complexes.

Characterization of the human COP9 signalosome complex using affinity purification and mass spectrometry

The COP9 signalosome (CSN) is a multiprotein complex that plays a critical role in diverse cellular and developmental processes in various eukaryotic organisms. Despite of its significance, current understanding of the biological functions and regulatory mechanisms of the CSN complex is still very limited. To unravel these molecular mechanisms, we have performed a comprehensive proteomic analysis of the human CSN complex using a new purification method and quantitative mass spectrometry. Purification of the human CSN complex from a stable 293 cell line expressing N-terminal HBTH-tagged CSN5 subunit was achieved by high-affinity streptavidin binding with TEV cleavage elution. Mass spectrometric analysis of the purified CSN complex has revealed the identity of its composition as well as N-terminal modification and phosphorylation of the CSN subunits. N-terminal modifications were determined for seven subunits, six of which have not been reported previously, and six novel phosphorylation sites were also identified. Additionally, we have applied the newly developed MAP-SILAC and PAM-SILAC methods to decipher the dynamics of the human CSN interacting proteins. A total of 52 putative human CSN interacting proteins were identified, most of which are reported for the first time. In comparison to PAM-SILAC results, 20 proteins were classified as stable interactors, whereas 20 proteins were identified as dynamic ones. This work presents the first comprehensive characterization of the human CSN complex by mass spectrometry-based proteomic approach, providing valuable information for further understanding of CSN complex structure and biological functions.

Roles for CSN5 in control of p53/MDM2 activities

The 5th subunit of COP9 signalosome (CSN5, also known as Jab1 or COPS5) is implicated in regulating p53 activity and is overexpressed in various tumors. However, the precise roles of CSN5 in p53 network and tumorigenesis are not well characterized. Here we show that CSN5 is a critical regulator of both p53 and MDM2. We show that curcumin, an important inhibitor of CSN-associated kinases, can downregulate not only CSN5 but also MDM2, which results in p53 stabilization. Importantly, CSN5 interacts with p53. CSN5 expression leads to p53 degradation, facilitating MDM2-mediated p53 ubiquitination, and promoting p53 nuclear export. Additionally, CSN5 expression results in stabilization of MDM2 through reducing MDM2 self-ubiquitination and decelerating turnover rate of MDM2. Significantly, we further show that CSN5 antagonizes the transcriptional activity of p53. These results demonstrate that CSN5 is a pivotal regulator for both p53 and MDM2. Our studies may pave the way for targeting CSN5 for anti-cancer drug development.

Shuttling imbalance of MLF1 results in p53 instability and increases susceptibility to oncogenic transformation

Myeloid leukemia factor 1 (MLF1) stabilizes the activity of the tumor suppressor p53 by suppressing its E3 ubiquitin ligase, COP1, through a third component of the COP9 signalosome (CSN3). However, little is known about how MLF1 functions upstream of the CSN3-COP1-p53 pathway and how its deregulation by the formation of the fusion protein nucleophosmin (NPM)-MLF1, generated by t(3;5)(q25.1;q34) chromosomal translocation, leads to leukemogenesis. Here we show that MLF1 is a cytoplasmic-nuclear-shuttling protein and that its nucleolar localization on fusing with NPM prevents the full induction of p53 by both genotoxic and oncogenic cellular stress. The majority of MLF1 was located in the cytoplasm, but the treatment of cells with leptomycin B rapidly induced a nuclear accumulation of MLF1. A mutation of the nuclear export signal (NES) motif identified in the MLF1 sequence enhanced the antiproliferative activity of MLF1. The fusion of MLF1 with NPM translocated MLF1 to the nucleolus and abolished the growth-suppressing activity. The introduction of NPM-MLF1 into early-passage murine embryonic fibroblasts allowed the cells to escape from cellular senescence at a markedly earlier stage and induced neoplastic transformation in collaboration with the oncogenic form of Ras. Interestingly, disruption of the MLF1-derived NES sequence completely abolished the growth-promoting activity of NPM-MLF1 in murine fibroblasts and hematopoietic cells. Thus, our results provide important evidence that the shuttling of MLF1 is critical for the regulation of cell proliferation and a disturbance in the shuttling balance increases the cell's susceptibility to oncogenic transformation.

Downregulation of COP9 signalosome subunits differentially affects the CSN complex and target protein stability

BACKGROUND

The COP9 signalosome (CSN) is a conserved protein complex in eukaryotic cells consisting of eight subunits (CSN1 to CSN8). Recent data demonstrate that the CSN is a regulator of the ubiquitin (Ub) proteasome system (UPS). It controls substrate ubiquitination by cullin-RING Ub ligases (CRLs), a process that determines substrate specificity of the UPS. The intrinsic deneddylating activity localized to CSN5 as well as the associated kinases and deubiquitinating activity are involved in the regulatory function of CSN. The exact mechanisms are unclear. In this study we knocked down CSN1 (siCSN1), CSN3 (siCSN3) and CSN5 (siCSN5) by specific siRNA oligos permanently expressed in HeLa cells. The analysis and comparison of siRNA cells revealed differential impact of individual subunits on CSN structure and function.

RESULTS

Permanent knockdowns of CSN1 and CSN3 led to a reduction of the subunits to approximately 40%, which is accompanied by a proportional decrease of the CSN holocomplex. In contrast, downregulation of CSN5 in HeLa cells reduced the CSN5 protein below 20% without significant effects on the remaining complex. The CRL component Rbx1 was characterized by accelerated proteolysis in siCSN1 and siCSN3 and also in siCSN5 cells, however, with lesser extent. Immunoprecipitated CSN complex from siCSN5 cells was less effective in phosphorylating c-Jun and p27. Accelerated degradation of c-Jun in siCSN5 cells was rescued by overexpression of CSN5 as well as of the deneddylation mutant CSN5D151N. Overexpression of CSN5 cannot rescue c-Jun destabilization in siCSN1.

CONCLUSION

There exists a coordinated downregulation of CSN subunits in the CSN1 and CSN3 knockdowns. The underlying regulatory mechanisms are obscure. CSN5 seems to possess a specific status in HeLa cells. Its reduction is not connected with coordinated downregulation of other subunits. CSN knockdowns confirm that the stabilization of the CRL component Rbx1 is a major CSN function. In addition, downregulation of CSN subunits influences the stability of important cellular regulators such as c-Jun and p27.

GLH-1, the C. elegans P granule protein, is controlled by the JNK KGB-1 and by the COP9 subunit CSN-5

The GLHs (germline RNA helicases) are constitutive components of the germline-specific P granules in the nematode Caenorhabditis elegans and are essential for fertility, yet how GLH proteins are regulated remains unknown. KGB-1 and CSN-5 are both GLH binding partners, previously identified by two-hybrid interactions. KGB-1 is a MAP kinase in the Jun N-terminal kinase (JNK) subfamily, whereas CSN-5 is a subunit of the COP9 signalosome. Intriguingly, although loss of either KGB-1 or CSN-5 results in sterility, their phenotypes are strikingly different. Whereas csn-5 RNA interference (RNAi) results in under-proliferated germlines, similar to glh-1/glh-4(RNAi), the kgb-1(um3) loss-of-function mutant exhibits germline over-proliferation. When kgb-1(um3) mutants are compared with wild-type C. elegans, GLH-1 protein levels are as much as 6-fold elevated and the organization of GLH-1 in P granules is grossly disrupted. A series of additional in vivo and in vitro tests indicates that KGB-1 and CSN-5 regulate GLH-1 levels, with GLH-1 targeted for proteosomal degradation by KGB-1 and stabilized by CSN-5. We propose the ;good cop: bad cop' team of CSN-5 and KGB-1 imposes a balance on GLH-1 levels, resulting in germline homeostasis. In addition, both KGB-1 and CSN-5 bind Vasa, a Drosophila germ granule component; therefore, similar regulatory mechanisms might be conserved from worms to flies.

Curcumin as an inhibitor of angiogenesis

Angiogenesis, the formation of new blood vessels from host vasculature, is critical for tumor growth and metastases. -Curcumin, a novel small-molecular-weight compound, has been shown to inhibit carcinogenesis in different organs and the common link between these actions is its antiangiogenic effect. Curcumin is a direct inhibitor of angiogenesis and also downregulates various proangiogenic proteins like vascular endothelial growth factor and basic fibroblast growth factor. Curcumin's antiangiogenic effect is also in part due to its inhibitory effect on signal transduction pathways, including those involving protein kinase C and the transcription factors NF-kappaB and AP-1. Curcumin has an inhibitory effect on two groups of proteinases involved in angiogenesis that are the members of the matrix metalloproteinase family and the urokinase plasminogen activator family. Cell adhesion molecules are upregulated in active angiogenesis and curcumin can block'this effect, adding further dimensions to curcumin's antiangiogenic effect. Curcumin shows a dose-dependent inhibition on tumor necrosis factor, a versatile cytokine, which has its effect on angiogenesis through the signal transduction pathways, expression of proangiogenic factors, and cell adhesion molecules. Curcumin's effect on the overall process of angiogenesis compounds its enormous potential as an antiangiogenic drug.

Ubiquitin-dependent Proteolysis of the Microtubule End-binding Protein 1, EB1, Is Controlled by the COP9 Signalosome: Possible Consequences for Microtubule Filament Stability

The COP9 signalosome (CSN) is a regulatory particle of the ubiquitin (Ub) proteasome system (UPS) consisting of eight subunits (CSN1-CSN8). We show that the CSN stabilizes the microtubule end-binding protein 1 (EB1) towards degradation by the UPS. EB1, the master regulator of microtubule plus ends, controls microtubule growth and dynamics. Therefore, regulation of EB1 stability by the CSN has consequences for microtubule function. EB1 binds the CSN via subunit CSN5. The C terminus of EB1 is sufficient for interaction with the CSN. Dimerization of EB1 is a prerequisite for complex association and subsequent CSN-mediated phosphorylation, as revealed by studies with the EB1I224A mutant, which is unable to dimerize. In cells, EB1 and CSN co-localize to the centrosome, as demonstrated by confocal fluorescence microscopy. EB1 is ubiquitinated and its proteolysis can be inhibited by MG132, demonstrating that it is a substrate of the UPS. Its degradation is accelerated by inhibition of CSN-associated kinases. HeLa cells permanently expressing siRNAs against CSN1 (siCSN1) or CSN3 (siCSN3) exhibit reduced levels of the CSN complex accompanied by lower steady-state concentrations of EB1. In siCSN1 cells, EB1 is less phosphorylated as compared with control cells, demonstrating that the protein is most likely protected towards the UPS by CSN-mediated phosphorylation. The CSN-dependent EB1 stabilization is not due to the CSN-associated deubiquitinating enzyme USP15. Treatment with nocodazole revealed a significantly increased sensitivity of siCSN1 and siCSN3 cells towards the microtubule depolymerizing drug accompanied by a collapse of microtubule filaments. A nocodazole-induced cell-cycle arrest was partially rescued by CSN1 or EB1. These data demonstrate that the CSN-dependent protection of EB1 is important for microtubule function.

The subunit CSN6 of the COP9 signalosome is cleaved during apoptosis

The COP9 signalosome is a large multiprotein complex that consists of eight subunits termed CSN1-CSN8. The diverse functions of the COP9 complex include regulation of several important intracellular pathways, including the ubiquitin/proteasome system, DNA repair, cell cycle, developmental changes, and some aspects of immune responses. Nod1 is also thought to be an important cytoplasmic receptor involved in innate immune responses. It detects specific motifs of bacterial peptidoglycan, and this results in activation of multiple signaling pathways and changes in cell function. In this report, we performed a yeast two-hybrid screening and discovered that Nod1 interacts with several components of the COP9 signalosome through its CARD domain. Moreover, we observed that activation of the Nod1 apoptotic pathway leads to specific cleavage of the subunit CSN6. This cleavage is concomitant with caspase processing and generates a short amino-terminal peptide of 3 kDa. A complete inhibition of this cleavage was achieved in the presence of the broad spectrum pharmacological inhibitor of apoptosis, Z-VAD. Furthermore, overexpression of CLARP, a specific caspase 8 inhibitor, completely blocked cleavage of CSN6. Taken together, these results suggest a critical role of caspase 8 in the processing of CSN6. Moreover, these findings suggest that CSN6 cleavage may result in modifications of functions of the COP9 complex that are involved in apoptosis.

Ankyrin repeat and SOCS box containing protein 4 (Asb-4) interacts with GPS1 (CSN1) and inhibits c-Jun NH2-terminal kinase activity

Asb-4 is a gene that is specifically expressed in the hypothalamic energy homeostasis-associated areas and is down-regulated in the arcuate nucleus of fasted Sprague Dawley and obese Zucker rats. It has two functional domains, the ankyrin repeat and the SOCS box. The function of Asb-4 is unclear. We used yeast two hybridization to search for protein(s) that interact with Asb-4. With Asb-4 minus its SOCS box (Asb-4/Deltasb) as a bait, we screened mouse testis and arcuate nucleus cDNA libraries and identified G-protein pathway suppressor 1 (GPS1, also known as CSN1) as an Asb-4 interacting protein. GPS1 co-immunoprecipitated with Asb-4 both in vitro and in human HEK293 cells. When Asb-4 and GPS1 were co-transfected into HEK293 cells, expression of Asb-4 reduced the protein level of GPS1. Deletion of the SOCS box (Asb4/Deltasb) did not abolish the inhibitory effect of Asb-4 on GPS1, indicating that the SOCS box was not needed for its inhibitory effect. In NIH 3T3 L1 cells, expression of GPS1 enhanced c-Jun NH2-terminal kinase (JNK) activity. Co-expression of Asb-4 with GPS1 inhibited JNK activity. Treatment of the cells with insulin (20 nM) stimulated JNK activity. Expression of GPS1 potentiated the stimulatory effect of insulin, whereas co-expression of Asb-4 along with GPS1 inhibited JNK activity. In HEK293 cells expression of GPS1 elevated phosphorylation of insulin receptor substrate 1 (IRS-1) at serine307, co-expression of Asb-4 with GPS1 reduced the IRS-1ser307 phosphorylation. The present study demonstrates that Asb-4 interacts with GPS1 and inhibits JNK activity.

COPing with hypoxia

To understand how cells respond to altered oxygenation, a frequent experimental paradigm is to isolate known components of bona fide oxygen responsive proteins. Recent studies have shown that a protein known as CSN5 or JAB1 interacts with both the HIF-1alpha oxygen-responsive transcription factor and its oxygen-dependent regulator, the Von Hippel-Lindau (pVHL) tumor suppressor. CSN5 is a component of the COP9 Signalosome (CSN) which is a multi-subunit protein that has high homology to the lid of the 19S lid of 26S proteasome. The exact function of the CSN5 interaction with pVHL and HIF-1alpha remains to be fully elucidated, but it is clear that the interaction is both oxygen dependent and that CSN5 may play different roles under oxic and hypoxic responses. Further, evidence has also been published indicating that pVHL can be potentially post-translationally modified by CSN5 (de-neddylation) and that CSN5 transcription is regulated by hypoxia as are many of the key pVHL/HIF-1alpha regulatory genes such as the PHDs and OS-9. This review will give a broad overview of known CSN5 and COP9 Signalosome functions and how these functions impact the pVHL/HIF-1alpha signaling complex and potentially other oxygen-sensitive response networks.

Targeted ablation of the chromogranin a (Chga) gene: normal neuroendocrine dense-core secretory granules and increased expression of other granins

Chromogranin A (CgA), originally identified in adrenal chromaffin cells, is a member of the granin family of acidic secretory glycoproteins that are expressed in endocrine cells and neurons. CgA has been proposed to play multiple roles in the secretory process. Intracellularly, CgA may control secretory granule biogenesis and target neurotransmitters and peptide hormones to granules of the regulated pathway. Extracellularly, peptides formed as a result of proteolytic processing of CgA may regulate hormone secretion. To investigate the role of CgA in the whole animal, we created a mouse mutant null for the Chga gene. These mice are viable and fertile and have no obvious developmental abnormalities, and their neural and endocrine functions are not grossly impaired. Their adrenal glands were structurally unremarkable, and morphometric analyses of chromaffin cells showed vesicle size and number to be normal. However, the excretion of epinephrine, norepinephrine, and dopamine was significantly elevated in the Chga null mutants. Adrenal medullary mRNA and protein levels of other dense-core secretory granule proteins including chromogranin B, and secretogranins II to VI were up-regulated 2- to 3-fold in the Chga null mutant mice. Hence, the increased expression of the other granin family members is likely to compensate for the Chga deficiency.

Apical localization of ITPK1 enhances its ability to be a modifier gene product in a murine tracheal cell model of cystic fibrosis

A new aspect of research into the pathogenesis of cystic fibrosis (CF) is a genetics-based search for ;modifier genes' that may affect the severity of CF lung disease. Using an alternative, cell biological approach, we show that ITPK1 should be considered a modifier gene. ITPK1 synthesizes an intracellular signal, inositol (3,4,5,6)-tetrakisphosphate [Ins(3,4,5,6)P4]. A bio-activatable, cell-permeable analogue of Ins(3,4,5,6)P4 inhibited Ca2+-dependent secretion of Cl- from polarized monolayers of immortalized mouse tracheal epithelial cells (MTEs). Analysis by high-pressure liquid chromatography showed endogenous Ins(3,4,5,6)P4 levels in CF MTEs were approximately 60% below those in wild-type MTEs (P<0.03). This adaptation, which improves purinergic activation of Ca2+-dependent Cl- secretion in CF MTEs, was exceptionally specific; there was no effect upon the cellular levels of all the other inositol phosphate signals. Real-time PCR provided the explanation: the level of ITPK1 expression in wild-type MTEs was twice as high as that in CF MTEs (P<0.002). The biological impact of this differential gene expression is amplified by ITPK1 being concentrated at the apical membrane of MTEs, which we discovered following confocal immunofluorescence microscopy. Compartmentalization of Ins(3,4,5,6)P4 synthesis adjacent to its site of action will enhance its regulatory capacity.