The murine DUB-1 gene is specifically induced by the betac subunit of interleukin-3 receptor

USP17 is required for peripheral trafficking of lysosomes

Expression of the deubiquitinase USP17 is induced by multiple stimuli, including cytokines (IL‐4/6), chemokines (IL‐8, SDF1), and growth factors (EGF), and several studies indicate it is required for cell proliferation and migration. However, the mechanisms via which USP17 impacts upon these cellular functions are unclear. Here, we demonstrate that USP17 depletion prevents peripheral lysosome positioning, as well as trafficking of lysosomes to the cell periphery in response to EGF stimulation. Overexpression of USP17 also increases secretion of the lysosomal protease cathepsin D. In addition, USP17 depletion impairs plasma membrane repair in cells treated with the pore‐forming toxin streptolysin O, further indicating that USP17 is required for lysosome trafficking to the plasma membrane. Finally, we demonstrate that USP17 can deubiquitinate p62, and we propose that USP17 can facilitate peripheral lysosome trafficking by opposing the E3 ligase RNF26 to untether lysosomes from the ER and facilitate lysosome peripheral trafficking, lysosome protease secretion, and plasma membrane repair.

USP17-mediated de-ubiquitination and cancer: Clients cluster around the cell cycle

Eukaryotic cells perform a range of complex processes, some essential for life, others specific to cell type, all of which are governed by post-translational modifications of proteins. Among the repertoire of dynamic protein modifications, ubiquitination is arguably the most arcane and profound due to its complexity. Ubiquitin conjugation consists of three main steps, the last of which involves a multitude of target-specific ubiquitin ligases that conjugate a range of ubiquitination patterns to protein substrates with diverse outcomes. In contrast, ubiquitin removal is catalysed by a relatively small number of de-ubiquitinating enzymes (DUBs), which can also display target specificity and impact decisively on cell function. Here we review the current knowledge of the intriguing ubiquitin-specific protease 17 (USP17) family of DUBs, which are expressed from a highly copy number variable gene that has been implicated in multiple cancers, although available evidence points to conflicting roles in cell proliferation and survival. We show that key USP17 substrates populate two pathways that drive cell cycle progression and that USP17 activity serves to promote one pathway but inhibit the other. We propose that this arrangement enables USP17 to stimulate or inhibit proliferation depending on the mitogenic pathway that predominates in any given cell and may partially explain evidence pointing to both oncogenic and tumour suppressor properties of USP17.

The Role of Deubiquitinating Enzymes in Hematopoiesis and Hematological Malignancies

Hematopoietic stem cells (HSCs) are responsible for the production of blood cells throughout the human lifespan. Single HSCs can give rise to at least eight distinct blood-cell lineages. Together, hematopoiesis, erythropoiesis, and angiogenesis coordinate several biological processes, i.e., cellular interactions during development and proliferation, guided migration, lineage programming, and reprogramming by transcription factors. Any dysregulation of these processes can result in hematological disorders and/or malignancies. Several studies of the molecular mechanisms governing HSC maintenance have demonstrated that protein regulation by the ubiquitin proteasomal pathway is crucial for normal HSC function. Recent studies have shown that reversal of ubiquitination by deubiquitinating enzymes (DUBs) plays an equally important role in hematopoiesis; however, information regarding the biological function of DUBs is limited. In this review, we focus on recent discoveries about the physiological roles of DUBs in hematopoiesis, erythropoiesis, and angiogenesis and discuss the DUBs associated with common hematological disorders and malignancies, which are potential therapeutic drug targets.

Regulatory interplay between deubiquitinating enzymes and cytokines

Deubiquitinating enzymes (DUBs) are cysteine protease proteins that reverse the ubiquitination by removing ubiquitins from the target protein. With over 100 DUBs identified and categorized into at least 7 families, many DUBs interact with one or more cytokines, influencing cellular processes, such as antiviral responses, inflammatory responses, apoptosis, etc. While some DUBs influence cytokine pathway or production, some DUBs are cytokine-inducible. In this article, we summarize a list of DUBs, their interaction with cytokines, target proteins and mechanisms of action.

DUB3 deubiquitinates and stabilizes NRF2 in chemotherapy resistance of colorectal cancer

The transcription factor nuclear factor (erythroid-derived 2)-like 2 (NRF2) is one of the master regulators that control hundreds of genes containing antioxidant response elements (AREs). The NRF2-ARE pathway plays a complex role in colorectal cancer (CRC). NRF2 activity is known to be regulated by KEAP1-CUL3 E3 ligase-mediated ubiquitination, indicating the importance of deubiquitination regulation. However, the deubiquitinase (DUB) of NRF2 remains unknown. Here, by screening a DUB library, we identified DUB3 as a DUB that remarkably stabilized NRF2. Further experiments demonstrated that DUB3 promoted NRF2 stability and transcriptional activity by decreasing the K48-linked ubiquitination of NRF2. Coimmunoprecipitation studies revealed interactions between NRF2 and DUB3, as well as between KEAP1 and DUB3, indicating that NRF2, DUB3, and KEAP1 formed a large functional complex. Importantly, ectopic expression of DUB3 caused NRF2-dependent chemotherapy resistance in colon cancer cell lines. Thus, to the best of our knowledge, our findings are the first to identify DUB3 as a NRF2 DUB and may provide a new strategy against chemotherapy resistance in CRC and other NRF2-related diseases.

USP17 is required for trafficking and oncogenic signaling of mutant EGFR in NSCLC cells

Background

The deubiquitinase USP17 is overexpressed in NSCLC and has been shown to be required for the growth and motility of EGFR wild-type (WT) NSCLC cells. USP17 is also required for clathrin-mediated endocytosis of EGFR. Here, we examine the impact of USP17 depletion on the growth, as well as EGFR endocytosis and signaling, of EGFR mutant (MT) NSCLC cells. In particular, we examine NSCLC cells harboring an EGFR activating exon 19 deletion (HCC827), or both the L858R activating mutation and the T790M resistance gatekeeper mutation (H1975) which renders them resistant to EGFR tyrosine kinase inhibitors (TKIs).

Methods

MTT, trypan blue and clonogenic assays, confocal microscopy, Western blotting and cell cycle analysis were performed.

Results

USP17 depletion blocks the growth of EGFRMT NSCLC cells carrying either the EGFR exon 19 deletion, or L858R/T790M double mutation. In contrast to EGFRWT cells, USP17 depletion also triggers apoptosis of EGFRMT NSCLC cells. USP17 is required for clathrin-mediated endocytosis in these EGFRMT NSCLC cells, but it is not required for the internalization of the mutated EGFR receptors. Instead, USP17 depletion alters the localization of these receptors within the cell, and although it does not decrease basal EGFR activation, it potently reduces activation of Src, a key kinase in mutant EGFR-dependent tumorigenicity. Finally, we demonstrate that USP17 depletion can trigger apoptosis in EGFRWT NSCLC cells, when combined with the EGFR tyrosine kinase inhibitor (TKI) gefitinib.

Conclusions

Our data reveals that USP17 facilitates trafficking and oncogenic signaling of mutant EGFR and indicates targeting USP17 could represent a viable therapeutic strategy in NSCLC tumours carrying either an EGFR activating mutation, or a resistance gatekeeper mutation.

Stabilization of the transcription factors slug and twist by the deubiquitinase dub3 is a key requirement for tumor metastasis

The epithelial-mesenchymal transition (EMT) represents a cellular de-differentiation process that provides cells with the increased plasticity required during embryonic development, tissue remodeling, wound healing and metastasis. Slug and Twist are two key EMT transcription factors (EMT-TFs) that are tightly regulated via ubiquitination and degradation. How Slug and Twist escape degradation and become stabilized in cancer cells remains unclear. One plausible mechanism of Slug and Twist stabilization involves removal of ubiquitin by deubiquitinases (DUBs). In this study, we identified Dub3 as a novel DUB for both Slug and Twist. We further found that Dub3 overexpression increased Slug and Twist protein levels in a dose-dependent manner, whereas Dub3-knockdown decreased their protein levels. Of importance, Dub3 interacted with Slug and Twist and prevented them from degradation, thereby promoting migration, invasion, and cancer stem cell (CSC)-like properties of breast cancer cells. Intriguingly, Dub3 was identified as an early response gene that was upregulated after exposure to inflammatory cytokines such as IL-6, which plays a critical role in the growth and metastasis of breast cancer cells, as well as the maintenance of breast CSCs. We found that Dub3 played an essential role in IL-6 induced EMT through stabilization of Slug and Twist. Our study has uncovered an IL-6-Dub3-Slug/Twist signaling axis during EMT and suggests potential approaches that could target Dub3 to prevent metastatic breast tumor.

USP17 is required for clathrin mediated endocytosis of epidermal growth factor receptor

Previously we have shown that expression of the deubiquitinating enzyme USP17 is required for cell proliferation and motility. More recently we reported that USP17 deubiquitinates RCE1 isoform 2 and thus regulates the processing of 'CaaX' motif proteins. Here we now show that USP17 expression is induced by epidermal growth factor and that USP17 expression is required for clathrin mediated endocytosis of epidermal growth factor receptor. In addition, we show that USP17 is required for the endocytosis of transferrin, an archetypal substrate for clathrin mediated endocytosis, and that USP17 depletion impedes plasma membrane recruitment of the machinery required for clathrin mediated endocytosis. Thus, our data reveal that USP17 is necessary for epidermal growth factor receptor and transferrin endocytosis via clathrin coated pits, indicate this is mediated via the regulation of the recruitment of the components of the endocytosis machinery and suggest USP17 may play a general role in receptor endocytosis.

A novel RCE1 isoform is required for H-Ras plasma membrane localization and is regulated by USP17

Processing of the 'CaaX' motif found on the C-termini of many proteins, including the proto-oncogene Ras, requires the ER (endoplasmic reticulum)-resident protease RCE1 (Ras-converting enzyme 1) and is necessary for the proper localization and function of many of these 'CaaX' proteins. In the present paper, we report that several mammalian species have a novel isoform (isoform 2) of RCE1 resulting from an alternate splice site and producing an N-terminally truncated protein. We demonstrate that both RCE1 isoform 1 and the newly identified isoform 2 are required to reinstate proper H-Ras processing and thus plasma membrane localization in RCE1-null cells. In addition, we show that the deubiquitinating enzyme USP17 (ubiquitin-specific protease 17), previously shown to modulate RCE1 activity, can regulate the abundance and localization of isoform 2. Furthermore, we show that isoform 2 is ubiquitinated on Lys43 and deubiquitinated by USP17. Collectively, the findings of the present study indicate that RCE1 isoform 2 is required for proper 'CaaX' processing and that USP17 can regulate this via its modulation of RCE1 isoform 2 ubiquitination.

Molecular mechanisms and functions of cytokine-inducible deubiquitinating enzymes

Deubiquitinating enzymes (DUBs), a class of cysteine proteases which counteract the action of protein ubiquitination, hydrolyze ubiquitin from its specific targeted proteins. Approximately, 100 DUBs have been found from yeast to human, and they can be classified into at least 5 families based on their structures and functions. Most DUBs are involved in regulation of intracellular processes including cell cycle progression, apoptosis, immunity, reproduction, and target gene transcription. Recently, much progress has been made in understanding the physiological functions of cytokine-inducible DUBs such as DUB-1, DUB-2, and DUB-3/USP17, in regulation of cell proliferation and apoptosis in lymphocytes. Here, we have summarized the structure and functions of cytokine-inducible DUBs and their biological functions in regulating several interleukin-associated signaling pathways. Finally, we emphasize the importance of small molecules for cytokine-inducible DUBs for developing promising drug therapeutics for immune-related disorders.

Granulocyte colony-stimulating factor receptor signaling: implications for G-CSF responses and leukemic progression in severe congenital neutropenia

Following activation by their cognate ligands, cytokine receptors undergo intracellular routing toward lysosomes, where they are degraded. This review focuses on the signaling function of the G-CSFR in relation to the dynamics of endosomal routing of the G-CSFR. Mechanisms involving receptor lysine ubiquitination and redox-controlled phosphatase activities are discussed. Specific attention is paid to the consequences of G-CSFR mutations, acquired in patients with severe congenital neutropenias who receive G-CSF therapy, particularly in the context of leukemic transformation, a major clinical complication of the disease.

Embryonic demise caused by targeted disruption of a cysteine protease Dub-2

Background

A plethora of biological metabolisms are regulated by the mechanisms of ubiquitination, wherein this process is balanced with the action of deubiquitination system. Dub-2 is an IL-2-inducible, immediate-early gene that encodes a deubiquitinating enzyme with growth regulatory activity. DUB-2 presumably removes ubiquitin from ubiquitin-conjugated target proteins regulating ubiquitin-mediated proteolysis, but its specific target proteins are unknown yet.

Methodology/Principal Findings

To elucidate the functional role of Dub-2, we generated genetically modified mice by introducing neo cassette into the second exon of Dub-2 and then homologous recombination was done to completely abrogate the activity of DUB-2 proteins. We generated Dub-2+/− heterozygous mice showing a normal phenotype and are fertile, whereas new born mouse of Dub-2−/− homozygous alleles could not survive. In addition, Dub-2−/− embryo could not be seen between E6.5 and E12.5 stages. Furthermore, the number of embryos showing normal embryonic development for further stages is decreased in heterozygotes. Even embryonic stem cells from inner cell mass of Dub-2−/− embryos could not be established.

Conclusions

Our study suggests that the targeted disruption of Dub-2 may cause embryonic lethality during early gestation, possibly due to the failure of cell proliferation during hatching process.

Structural insights into functional modes of proteins involved in ubiquitin family pathways

The conjugation of ubiquitin and related modifiers to selected proteins represents a general mechanism to alter the function of these protein targets, thereby increasing the complexity of the cellular proteome. Ubiquitylation is catalyzed by a hierarchical enzyme cascade consisting of ubiquitin activating, ubiquitin conjugating, and ubiquitin ligating enzymes, and their combined action results in a diverse topology of ubiquitin-linkages on the modified proteins. Counteracting this machinery are various deubiquitylating enzymes while ubiquitin recognition in all its facets is accomplished by numerous ubiquitin-binding elements. In the following chapter, we attempt to provide an overview on enzymes involved in ubiquitylation as well as the removal of ubiquitin and proteins involved in the recognition and binding of ubiquitin from a structural biologist's perspective.

The deubiquitinating enzyme DUB2A enhances CSF3 signalling by attenuating lysosomal routing of the CSF3 receptor

Ubiquitination of the CSF3R [CSF3 (colony-stimulating factor 3) receptor] occurs after activated CSF3Rs are internalized and reside in early endosomes. CSF3R ubiquitination is crucial for lysosomal routing and degradation. The E3 ligase SOCS3 (suppressor of cytokine signalling 3) has been shown to play a major role in this process. Deubiquitinating enzymes remove ubiquitin moieties from target proteins by proteolytic cleavage. Two of these enzymes, AMSH [associated molecule with the SH3 domain of STAM (signal transducing adaptor molecule)] and UBPY (ubiquitin isopeptidase Y), interact with the general endosomal sorting machinery. Whether deubiquitinating enzymes control CSF3R trafficking from early towards late endosomes is unknown. In the present study, we asked whether AMSH, UBPY or a murine family of deubiquitinating enzymes could fulfil such a role. This DUB family (deubiquitin enzyme family) comprises four members (DUB1, DUB1A, DUB2 and DUB2A), which were originally described as being haematopoietic-specific and cytokine-inducible, but their function in cytokine receptor routing and signalling has remained largely unknown. We show that DUB2A expression is induced by CSF3 in myeloid 32D cells and that DUB2 decreases ubiquitination and lysosomal degradation of the CSF3R, leading to prolonged signalling. These results support a model in which CSF3R ubiquitination is dynamically controlled at the early endosome by feedback mechanisms involving CSF3-induced E3 ligase (SOCS3) and deubiquitinase (DUB2A) activities.

Evidence for OTUD-6B participation in B lymphocytes cell cycle after cytokine stimulation

Deubiquitinating enzymes (DUBs) are important regulators of cell proliferation. Here we identified a functional deubiquitinating enzyme, ovarian tumor domain-containing 6B (OTUD-6B). Mutation of the conserved Cys residue abolished its deubiquitinating activity in vitro. Otud-6b expression was induced with cytokine stimulation in both mouse Ba/F3 cells and primary B lymphocytes followed a rapid decrease. This rapid decrease was partially facilitated by tristetraprolin (TTP) destabilization of Otud-6b mRNA through AU-rich motifs. Enforced expression of OTUD-6B in Ba/F3 cells could block cell proliferation by arresting cells in G1 phase. In addition, cyclin D2 level was down-regulated when OTUD-6B WT was overexpressed. Therefore, down-regulation of Otud-6b expression after prolonged cytokine stimulation may be required for cell proliferation in B lymphocytes.

The role of deubiquitinating enzymes in apoptosis

It has become apparent that ubiquitination plays a critical role in cell survival and cell death. In addition, deubiquitinating enzymes (DUBs) have been determined to be highly important regulators of these processes. Cells can be subjected to various stresses and respond in a variety of different ways ranging from activation of survival pathways to the promotion of cell death, which eventually eliminates damaged cells. The regulatory mechanisms of apoptosis depend on the balanced action between ubiquitination and deubiquitination systems. There is a growing recognition that DUBs play essential roles in regulating several binding partners to modulate the process of apoptosis. Thus, the interplay between the timing of DUB activity and the specificity of ubiquitin attachment and removal from its substrates during apoptosis is important to ensure cellular homeostasis. This review discusses the role of a few ubiquitin-specific DUBs that are involved in either promoting or suppressing the process of apoptosis.

Ubiquitin-positive pneumocytes and inclusion bodies are present in secondary organizing pneumonia

OBJECTIVE

We previously reported that various types of interstitial pneumonia (IP) patterns contain intracytoplasmic eosinophilic inclusions or Mallory bodies (inclusions) that are ubiquitin positive (Ub+). In the present study, we examined tissues with the organizing pneumonia pattern (OP) to determine if they contain inclusions and Ub+ pneumocytes using lobectomized specimens.

METHODS

Tissues from 34 patients with secondary OP, which appeared in 33 carcinomas and 1 pulmonary abscess, were histologically evaluated for the type of intraluminal granulation tissue and the presence of erosions and inclusions. Granulation tissues were classified into polypoid, mural and occluded subtypes according to Basset's criteria and scored.

RESULTS

Inclusions were noted in 5.9% of the secondary OP cases with light microscope. Ub+ pneumocytes were detected after immunostaining and all inclusions were Ub+. Ub+ pneumocytes (inclusions) were noted in 14.7% of the secondary OP cases. OP contained pneumocyte erosions and inflammatory cell infiltration without a significant difference in the Ub+ and Ub- subgroups. Although there was no significant difference in the polypoid type of granulation tissue between the Ub+ and Ub- negative (Ub-) subgroups, the Ub+ subgroup had significant increases (p<0.05) in the mural-occluded type of granulation tissue (Ub+: 1.76±0.64, n=5 vs. Ub-: 0.72±0.87, n=29) as compared to the Ub- subgroup.

CONCLUSION

Some patients with secondary OP had Ub+ inclusions as pneumocyte injury.

The DUB/USP17 deubiquitinating enzymes: a gene family within a tandemly repeated sequence, is also embedded within the copy number variable beta-defensin cluster

Background

The DUB/USP17 subfamily of deubiquitinating enzymes were originally identified as immediate early genes induced in response to cytokine stimulation in mice (DUB-1, DUB-1A, DUB-2, DUB-2A). Subsequently we have identified a number of human family members and shown that one of these (DUB-3) is also cytokine inducible. We originally showed that constitutive expression of DUB-3 can block cell proliferation and more recently we have demonstrated that this is due to its regulation of the ubiquitination and activity of the 'CAAX' box protease RCE1.

Results

Here we demonstrate that the human DUB/USP17 family members are found on both chromosome 4p16.1, within a block of tandem repeats, and on chromosome 8p23.1, embedded within the copy number variable beta-defensin cluster. In addition, we show that the multiple genes observed in humans and other distantly related mammals have arisen due to the independent expansion of an ancestral sequence within each species. However, it is also apparent when sequences from humans and the more closely related chimpanzee are compared, that duplication events have taken place prior to these species separating.

Conclusions

The observation that the DUB/USP17 genes, which can influence cell growth and survival, have evolved from an unstable ancestral sequence which has undergone multiple and varied duplications in the species examined marks this as a unique family. In addition, their presence within the beta-defensin repeat raises the question whether they may contribute to the influence of this repeat on immune related conditions.

The deubiquitinating enzyme USP17 is highly expressed in tumor biopsies, is cell cycle regulated, and is required for G1-S progression

Ubiquitination is a reversible posttranslational modification that is essential for cell cycle control, and it is becoming increasingly clear that the removal of ubiquitin from proteins by deubiquitinating enzymes (DUB) is equally important. In this study, we have identified high levels of the DUB USP17 in several tumor-derived cell lines and primary lung, colon, esophagus, and cervix tumor biopsies. We also report that USP17 is tightly regulated during the cell cycle in all the cells examined, being abundantly evident in G(1) and absent in S phase. Moreover, regulated USP17 expression was necessary for cell cycle progression because its depletion significantly impaired G(1)-S transition and blocked cell proliferation. Previously, we have shown that USP17 regulates the intracellular translocation and activation of the GTPase Ras by controlling Ras-converting enzyme 1 (RCE1) activation. RCE1 also regulates the processing of other proteins with a CAAX motif, including Rho family GTPases. We now show that USP17 depletion blocks Ras and RhoA localization and activation. Moreover, our results confirm that USP17-depleted cells have constitutively elevated levels of the cyclin-dependent kinase inhibitors p21(cip1) and p27(kip1), known downstream targets of Ras and RhoA signaling. These observations clearly show that USP17 is tightly regulated during cell division and that its expression is necessary to coordinate cell cycle progression, and thus, it may be considered a promising novel cancer therapeutic target.

The Drosophila ubiquitin-specific protease dUSP36/Scny targets IMD to prevent constitutive immune signaling

Ubiquitin proteases remove ubiquitin monomers or polymers to modify the stability or activity of proteins and thereby serve as key regulators of signal transduction. Here, we describe the function of the Drosophila ubiquitin-specific protease 36 (dUSP36) in negative regulation of the immune deficiency (IMD) pathway controlled by the IMD protein. Overexpression of catalytically active dUSP36 ubiquitin protease suppresses fly immunity against Gram-negative pathogens. Conversely, silencing dUsp36 provokes IMD-dependent constitutive activation of IMD-downstream Jun kinase and NF-kappaB signaling pathways but not of the Toll pathway. This deregulation is lost in axenic flies, indicating that dUSP36 prevents constitutive immune signal activation by commensal bacteria. dUSP36 interacts with IMD and prevents K63-polyubiquitinated IMD accumulation while promoting IMD degradation in vivo. Blocking the proteasome in dUsp36-expressing S2 cells increases K48-polyubiquitinated IMD and prevents its degradation. Our findings identify dUSP36 as a repressor whose IMD deubiquitination activity prevents nonspecific activation of innate immune signaling.

Regulation and cellular roles of ubiquitin-specific deubiquitinating enzymes

Deubiquitinating enzymes (DUBs) are proteases that process ubiquitin or ubiquitin-like gene products, reverse the modification of proteins by a single ubiquitin(-like) protein, and remodel polyubiquitin(-like) chains on target proteins. The human genome encodes nearly 100 DUBs with specificity for ubiquitin in five gene families. Most DUB activity is cryptic, and conformational rearrangements often occur during the binding of ubiquitin and/or scaffold proteins. DUBs with specificity for ubiquitin contain insertions and extensions modulating DUB substrate specificity, protein-protein interactions, and cellular localization. Binding partners and multiprotein complexes with which DUBs associate modulate DUB activity and substrate specificity. Quantitative studies of activity and protein-protein interactions, together with genetic studies and the advent of RNAi, have led to new insights into the function of yeast and human DUBs. This review discusses ubiquitin-specific DUBs, some of the generalizations emerging from recent studies of the regulation of DUB activity, and their roles in various cellular processes.

DUB-1, a fate determinant of dynein heavy chain in B-lymphocytes, is regulated by the ubiquitin-proteasome pathway

Ubiquitination and deubiquitination of post-translational modification play counter roles in determining the fate of protein function in eukaryotic system for maintaining the cellular homeostasis. Even though novel family members of growth-regulating deubiquitinating enzymes (DUB-1 and DUB-2) have been identified, their target proteins and functions are poorly understood. Dub genes encoding DUB-1 and DUB-2 are immediate-early genes and are induced in response to cytokine stimuli rapidly and transiently. In order to explore the possible proteins regulated by DUB-1, we performed the matrix assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF-MS) analysis followed by immunoprecipitation. We confirmed that DUB-1 interacts with dynein heavy chain, which is known to regulate the movement of organelles and microtubule binding ability. In addition, structural and immunoprecipitation analyses revealed that DUB-1 contains a putative PEST motif and is polyubiquitinated, indicating that DUB-1 is also regulated by the ubiquitin-proteasome pathway.

USP17 regulates Ras activation and cell proliferation by blocking RCE1 activity

The proto-oncogene Ras undergoes a series of post-translational modifications at its carboxyl-terminal CAAX motif that are essential for its proper membrane localization and function. One step in this process is the cleavage of the CAAX motif by the enzyme Ras-converting enzyme 1 (RCE1). Here we show that the deubiquitinating enzyme USP17 negatively regulates the activity of RCE1. We demonstrate that USP17 expression blocks Ras membrane localization and activation, thereby inhibiting phosphorylation of the downstream kinases MEK and ERK. Furthermore, we show that this effect is caused by the loss of RCE1 catalytic activity as a result of its deubiquitination by USP17. We also show that USP17 and RCE1 co-localize at the endoplasmic reticulum and that USP17 cannot block proliferation or Ras membrane localization in RCE1 null cells. These studies demonstrate that USP17 modulates Ras processing and activation, at least in part, by regulating RCE1 activity.

De-ubiquitinating enzymes: intracellular signalling and disease

Ubiquitination is now accepted as an important process for regulating intracellular signalling and the realization that many known signalling molecules exhibit E3 ligase activity has led to great strides in our understanding of how these pathways are regulated. However, as most of the de-ubiquitinating enzymes have as yet no identified substrate, little is known about their potential role in the regulation of intracellular signalling. Here, we examine what is known about de-ubiquitinating enzymes and signalling, with particular emphasis on their role in the regulation of immune signalling and the initiation of DNA repair. In addition, we look at the evidence implicating these enzymes in the pathogenesis of diseases such as cancer and neurodegenerative diseases.

The DUB/USP17 deubiquitinating enzymes, a multigene family within a tandemly repeated sequence

Here we report the identification of 10 human, 1 murine, and 2 rat ORFs, all of which represent additional members of the DUB/USP17 family of deubiquitinating enzymes. In addition, we demonstrate that this family constitutes part of a tandemly repeated sequence conserved throughout humans, mice, and rats. Furthermore, upon examination of the known family members we have found that the multiple genes observed, in contrast to other gene families, have arisen due to the independent expansion of an ancestral sequence within each species. This premise is further strengthened by the observation that the murine and rat genes span two exons while their human counterparts have one. These observations, in conjunction with previous work demonstrating that the DUB/USP17's are cytokine inducible and that they regulate both cell growth and survival, suggest that the DUB/USP17's are a large highly conserved family of genes that may play an important role in controlling cell fate.

Essential regions of deubiquitinating enzyme activity and enhancer function for DUB-2A expressed in T-lymphocytes

Novel subfamily members of deubiquitinating enzyme gene, DUB-1 and DUB-2, are known to be immediate-early genes and are specifically expressed in B-lymphocytes and T-lymphocytes, respectively. Recently, another deubiquitinating enzyme gene DUB-2A, expressed also in T-lymphocytes, has been isolated and we, in this study, found that Dub-2A has isopeptidase activity and there are at least 4 conserved amino acids (Cys60, Asp133, His298, and His307) that are required for catalytic activity of the enzyme. Interestingly, the conserved Asp323 was not essential for the catalytic activity. In addition, transcriptional reporter assay showed that an Ets site and 2 AP-1 sites in the enhancer domain of this gene are required for cytokine inducibility. Taken all together, these results will be very helpful in designing activators or inhibitors of this enzyme in order to regulate its expression and catalytic activity in T-lymphocytes.

DUB-3, a Cytokine-inducible Deubiquitinating Enzyme That Blocks Proliferation

Previous studies have identified the DUB family of cytokine-regulated murine deubiquitinating enzymes, which play a role in the control of cell proliferation and survival. Through data base analyses and cloning, we have identified a human cDNA (DUB-3) that shows significant homology to the known murine DUB family members. Northern blotting has shown expression of this gene in a number of tissues including brain, liver, and muscle, with two transcripts being apparent (1.6 and 1.7 kb). In addition, expression was observed in cell lines including those derived from a number of hematopoietic tumors such as the Burkitt's lymphoma cell line RAJI. We have also demonstrated that DUB-3, which was shown to be an active deubiquitinating enzyme, is induced in response to interleukin-4 and interleukin-6 stimulation. Finally, we have demonstrated that constitutive expression of DUB-3 blocks proliferation and can initiate apoptosis in both IL-3-dependent Ba/F3 cells and NIH3T3 fibroblasts. These findings suggest that human DUB-3, like the murine DUB family members, is transiently induced in response to cytokines and can, when constitutively expressed, block growth factor-dependent proliferation.

Deubiquitinating enzymes: their roles in development, differentiation, and disease

The ubiquitin-mediated proteolysis pathway has come a long way in the past decade. At first thought to be an unglamorous garbage dump for damaged proteins, the ubiquitin pathway has been shown to regulate virtually everything that occurs in the cell. Deubiquitinating enzymes, which cleave ubiquitin-protein bonds, are the largest group of enzymes in the pathway, yet they are the least well understood. Deubiquitinating enzymes have two kinds of functions: housekeeping and regulatory. The housekeeping enzymes facilitate the proteolytic pathway. By contrast, the regulatory enzymes control the ubiquitination of specific protein substrates; their relationship to ubiquitination is analgous to that of phosphatases with respect to phosphorylation. Here, I review the current state of knowledge of the deubiquitinating enzymes. I focus particularly on the known regulatory enzymes, and also on the housekeeping enzymes that are implicated in development of disease.

Ubiquitin-dependent mechanism regulates rapid turnover of AU-rich cytokine mRNAs

An AU rich element (ARE) in the 3' noncoding region promotes the rapid degradation of mammalian cytokine and proto-oncogene mRNAs, such as tumor necrosis factor-alpha, granulocyte-macrophage colony-stimulating factor (GM-CSF) and c-fos. Destabilization of ARE-mRNAs involves the association of ARE-binding proteins tristetraprolin or AUF1 and proteasome activity, of which the latter has not been characterized. Here, we show that the stability of a model short-lived mRNA containing the GM-CSF ARE was regulated by the level of ubiquitin-conjugating activity in the cell, which links ARE-mRNA decay to proteasome activity. Increased expression of a cytokine-inducible deubiquitinating protein (DUB) that impairs addition of ubiquitin to proteins fully blocked ARE-mRNA decay, whereas increased expression of a DUB that promotes ubiquitin addition to proteins strongly accelerated ARE-mRNA decay. ARE-mRNA turnover was found to be activated by the ubiquitin-addition reaction and blocked by the ubiquitin-removal reaction. Saturation of the ARE-mRNA decay machinery by high levels of ARE-mRNA, which is well established but not understood, was found to be relieved by increased expression of a DUB that promotes ubiquitin addition to proteins. Finally, inhibition of proteasome activity also blocked accelerated ARE-mRNA decay that is mediated by increased ubiquitin recycling. These results demonstrate that both ubiquitinating activity and proteasome activity are essential for rapid turnover of a model cytokine ARE-mRNA containing the GM-CSF ARE.

Cloning and characterization of mouse UBPy, a deubiquitinating enzyme that interacts with the ras guanine nucleotide exchange factor CDC25(Mm)/Ras-GRF1

We used yeast "two-hybrid" screening to isolate cDNA-encoding proteins interacting with the N-terminal domain of the Ras nucleotide exchange factor CDC25(Mm). Three independent overlapping clones were isolated from a mouse embryo cDNA library. The full-length cDNA was cloned by RACE-polymerase chain reaction. It encodes a large protein (1080 amino acids) highly homologous to the human deubiquitinating enzyme hUBPy and contains a well conserved domain typical of ubiquitin isopeptidases. Therefore we called this new protein mouse UBPy (mUBPy). Northern blot analysis revealed a 4-kilobase mRNA present in several mouse tissues and highly expressed in testis; a good level of expression was also found in brain, where CDC25(Mm) is exclusively expressed. Using a glutathione S-transferase fusion protein, we demonstrated an "in vitro" interaction between mUBPy and the N-terminal half (amino acids 1-625) of CDC25(Mm). In addition "in vivo" interaction was demonstrated after cotransfection in mammalian cells. We also showed that CDC25(Mm), expressed in HEK293 cells, is ubiquitinated and that the coexpression of mUBPy decreases its ubiquitination. In addition the half-life of CDC25Mm protein was considerably increased in the presence of mUBPy. The specific function of the human homolog hUBPy is not defined, although its expression was correlated with cell proliferation. Our results suggest that mUBPy may play a role in controlling degradation of CDC25(Mm), thus regulating the level of this Ras-guanine nucleotide exchange factor.

A novel active site-directed probe specific for deubiquitylating enzymes reveals proteasome association of USP14

A C-terminally modified ubiquitin (Ub) derivative, ubiquitin vinyl sulfone (UbVS), was synthesized as an active site-directed probe that irreversibly modifies a subset of Ub C-terminal hydrolases (UCHs) and Ub-specific processing proteases (UBPs). Specificity of UbVS for deubiquitylating enzymes (DUBs) is demonstrated not only by inhibition of [(125)I]UbVS labeling with N-ethylmaleimide and Ub aldehyde, but also by genetic analysis. [(125)I]UbVS modifies six of the 17 known and putative yeast deubiquitylating enzymes (Yuh1p, Ubp1p, Ubp2p, Ubp6p, Ubp12p and Ubp15p), as revealed by analysis of corresponding mutant strains. In mammalian cells, greater numbers of polypeptides are labeled, most of which are likely to be DUBs. Using [(125)I]UbVS as a probe, we report the association of an additional DUB with the mammalian 26S proteasome. In addition to the 37 kDa enzyme reported to be part of the 19S cap, we identified USP14, a mammalian homolog of yeast Ubp6p, as being bound to the proteasome. Remarkably, labeling of 26S-associated USP14 with [(125)I]UbVS is increased when proteasome function is impaired, suggesting functional coupling between the activities of USP14 and the proteasome.

DUB-2A, a new member of the DUB subfamily of hematopoietic deubiquitinating enzymes

Protein ubiquitination is an important regulator of cytokine-activated signal transduction pathways and hematopoietic cell growth. Protein ubiquitination is controlled by the coordinate action of ubiquitin-conjugating enzymes and deubiquitinating enzymes. Recently a novel family of genes encoding growth-regulatory deubiquitinating enzymes (DUB-1 and DUB-2) has been identified. DUBs are immediate-early genes and are induced rapidly and transiently in response to cytokine stimuli. By means of polymerase chain reaction amplification with degenerate primers for the DUB-2 complementary DNA, 3 murine bacterial artificial chromosome (BAC) clones that contain DUB gene sequences were isolated. One BAC contained a novel DUB gene (DUB-2A) with extensive homology to DUB-2. Like DUB-1 and DUB-2, the DUB-2A gene consists of 2 exons. The predicted DUB-2A protein is highly related to other DUBs throughout the primary amino acid sequence, with a hypervariable region at its C-terminus. In vitro, DUB-2A had functional deubiquitinating activity; mutation of its conserved amino acid residues abolished this activity. The 5' flanking sequence of the DUB-2A gene has a hematopoietic-specific functional enhancer sequence. It is proposed that there are at least 3 members of the DUB subfamily (DUB-1, DUB-2, and DUB-2A) and that different hematopoietic cytokines induce specific DUB genes, thereby initiating a cytokine-specific growth response. (Blood. 2001;98:636-642)

Cloning and functional analysis of erythropoietin-, interleukin-3- and thrombopoietin-inducible genes

The receptor for thrombopoietin (TPO) is a member of the cytokine receptor superfamily. This superfamily also includes the receptors for erythropoietin (EPO), interleukin 3 (IL-3), GM-CSF and several other cytokines. Stimulation of cytokine receptors with their cognate ligands results in the activation of multiple signal transduction pathways and ultimately in the induction of new genes. The cloning of these specific genes provides one approach for analyzing cytokine-specific responses. In the current study, we have designed a strategy for isolating inducible genes. While the strategy has been used to identify EPO-specific and IL-3-specific inducible genes, the strategy can be extended to clone TPO-inducible genes.

Tissue-specificity, functional characterization and subcellular localization of a rat ubiquitin-specific processing protease, UBP109, whose mRNA expression is developmentally regulated

A cDNA encoding an ubiquitin-specific processing protease, UBP109, in rat skeletal muscle was cloned and its product was characterized. Northern analysis revealed that UBP109 mRNA is highly expressed in testis and spleen, compared with other tissues. Furthermore, in situ hybridization showed that the level of UBP109 mRNA in liver, spinal cord and brain dramatically changed during embryonic development, indicating that the expression of UBP109 mRNA is developmentally regulated. UBP109 was expressed in Escherichia coli and purified to apparent homogeneity using a (125)I-labelled ubiquitin-peptide fusion as a substrate. The purified enzyme cleaved at the C-terminus of the ubiquitin moiety in natural and engineered fusions irrespective of their sizes. UBP109 also released free ubiquitin from poly-His-tagged penta-ubiquitin. Moreover, it released free ubiquitin from poly-ubiquitinated protein conjugates of rabbit reticulocytes. In addition, UBP109 localized to both the cytoplasm and the nucleus and, among three putative nuclear localization sequences, only the one located near the C-terminus is responsible for nuclear localization. These results suggest that UBP109 may play an important role in generation of free ubiquitin from its precursors and its recycling from poly-ubiquitinated protein conjugates, and hence in regulation of ubiquitin-mediated cellular processes, particularly related to embryonic development.

Promoter analysis of the human ubiquitin-conjugating enzyme gene family UBE2L1-4, including UBE2L3 which encodes UbcH7

UbcH7 is a ubiquitin-conjugating enzyme mediating c-fos degradation, transcription factor NF-kappaB maturation, human papilloma virus-mediated p53 and Myc protein degradation, in vitro. Previously, we characterised a highly dispersed gene family, UBE2L1-UBE2L4, whose members could potentially encode different isoforms of the UbcH7 protein. UBE2L3, located at chromosome 22q11.2, is the only identified family member with introns and encodes a polypeptide sequence identical to that of UbcH7. Promoter characterisation of UBE2L1, UBE2L3 and UBE2L4 5'-upstream regions was performed to establish which are transcribed under normal physiological conditions and after heat shock. Promoter activity was observed only with the UBE2L3 construct, the minimal promoter lying within a region 100 bp upstream of the transcriptional start site. No evidence for the presence of UBE2L1 or UBE2L4 transcripts was observed in human or murine tissues and cell lines. These data strongly suggest that UBE2L1 and UBE2L4 are likely to encode pseudogenes. Sequencing revealed that the UBE2L3 promoter contained no TATA or CCAAT boxes. Protein:DNA interaction studies confirmed the presence of binding sites for the transcription factors AP2 and Sp1 in the UBE2L3 minimal promoter. Deletion of these binding sites indicated that these factors are crucial for transcription of this gene.

Analysis of cis-acting sequences and trans-acting factors regulating the interleukin-3 response element of the DUB-1 gene

The murine DUB-1 gene is a hematopoietic-specific, immediate-early gene that encodes a growth-regulatory deubiquitinating enzyme. DUB-1 contains an IL-3-inducible enhancer element that is activated in a JAK2-dependent, STAT5-independent manner. In this study, we have further characterized this novel IL-3 response element. Transcriptional reporter assays in Ba/F3 cells revealed that two AP-1 sites, a GATA motif, and an Ets site are required for induction of DUB-1 enhancer activity. Gel shift assays indicated that IL-3 activates the binding of an AP-1 complex containing JunD to the AP-1 sites and the binding of another protein complex to the Ets motif. The latter complex was not detectable in Ba/F3 cells stably transfected with a dominant-negative mutant of JAK2. As previously shown, these cells do not express DUB-1 mRNA or protein. Furthermore, we demonstrated that GATA-1 constitutively binds to the DUB-1 enhancer element. The involvement of GATA-1 may be important for the hematopoietic-restricted expression pattern of DUB-1. This combination of inducible and constitutive elements of the DUB-1 enhancer appears to account for the unique STAT-independent expression characteristics of DUB-1.

Role of STAT5 in interferon-alpha signal transduction in Ba/F3 cells

In interferon-alpha (IFN-alpha) signalling, the essential role of the transcription factors STAT1 and STAT2 is well established. In contrast, the involvement of other STAT proteins, including STAT5, is much less well understood. Here we show that, in IFN-alpha-responsive Ba/F3 cells, this cytokine stimulates the DNA-binding of STAT5A and B but that IL-3 is a much more potent activator of both STAT5 isoforms. A stably expressed dominant-negative mutant of JAK2 suppressed the IL-3- but not the IFN-alpha-dependent DNA binding of STAT5, suggesting independent mechanisms of its activation. Northern blots revealed that IL-3 strongly induced the expression of two STAT5-regulated genes, pim-1 and oncostatin-M, whereas IFN-alpha had a weak stimulatory effect on pim-1 expression only. In summary our results suggest that, despite the capability of IFN-alpha to stimulate DNA binding of STAT5, this transcription factor does not play a pivotal role in IFN-alpha signalling in Ba/F3 cells.

Deubiquitinating enzymes: a new class of biological regulators

Protein ubiquitination controls many intracellular processes, including cell cycle progression, transcriptional activation, and signal transduction. Like protein phosphorylation, protein ubiquitination is dynamic, involving enzymes that add ubiquitin (ubiquitin conjugating enzymes) and enzymes that remove ubiquitin (deubiquitinating enzymes). Considerable progress has been made in the understanding of ubiquitin conjugation and its role in regulating protein degradation. Recent studies have demonstrated that regulation also occurs at the level of deubiquitination. Deubiquitinating enzymes are cysteine proteases that specifically cleave ubiquitin from ubiquitin-conjugated protein substrates. Genome sequencing projects have identified many candidate deubiquitinating enzymes, making them the largest family of enzymes in the ubiquitin system. Deubiquitinating enzymes have significant sequence diversity and therefore may have a broad range of substrate specificities. Here we explore the structural and biochemical properties of deubiquitinating enzymes and their emerging roles as cellular switches.

Transcriptional activation of scavenger receptor expression in human smooth muscle cells requires AP-1/c-Jun and C/EBPbeta: both AP-1 binding and JNK activation are induced by phorbol esters and oxidative stress

Reactive oxygen species generated by treatment of smooth muscle cells (SMCs) with either phorbol 12-myristate 13-acetate or with the combination of H2O2 and vanadate strongly induce expression of the class A scavenger receptor (SR-A) gene. In the current studies, cis-acting elements in the proximal 245 bp of the SR-A promoter were shown to direct luciferase reporter expression in response to oxidative stress in both SMCs and macrophages. A composite activating protein-1 (AP-1)/ets binding element located between -67 and -50 bp relative to the transcriptional start site is critical for macrophage SR-A activity. Mutation of either the AP-1 or the ets component of this site also prevented promoter activity in SMCs. Mutation of a second site located between -44 and -21 bp, which we have identified as a CCAAT/enhancer binding protein (C/EBP) element, reduced the inducible activity of the promoter in SMCs by 50%, suggesting that combinatorial interactions between these sites are necessary for optimal gene induction. Interactions between SMC nuclear extracts and the SR-A promoter were analyzed by electrophoretic mobility shift assay. c-Jun/AP-1 binding activity, specific for the -67- to -50-bp site, was induced in SMCs by the same conditions that increased SR-A expression. Moreover, phorbol 12-myristate 13-acetate, H2O2, or the combination of H2O2 and sodium orthovanadate (vanadate) activated c-Jun-activating kinase. The binding activity within SMC extracts specific for the C/EBP site was shown to be C/EBPbeta in SMCs. Taken together, these findings demonstrate that reactive oxygen species regulate the interactions between c-Jun/AP-1 and C/EBPbeta in the SR-A promoter. Furthermore, induction of oxidative stress in THP-1 cells, with a combination of 10 micromol/L vanadate and 100 micromol/L H2O2, induced macrophage differentiation, adhesion, and SR activity. These data suggest that vascular oxidative stress may contribute to the induction of SR-A expression and thereby promote the uptake of oxidatively modified low density lipoprotein by both macrophage and SMCs to produce foam cells in atherosclerotic lesions.

Differential utilization of Ras signaling pathways by macrophage colony-stimulating factor (CSF) and granulocyte-macrophage CSF receptors during macrophage differentiation

Granulocyte-macrophage colony-stimulating factor (GM-CSF) and macrophage colony-stimulating factor (M-CSF) independently stimulate the proliferation and differentiation of macrophages from bone marrow progenitor cells. Although the GM-CSF and M-CSF receptors are unrelated, both couple to Ras-dependent signal transduction pathways, suggesting that these pathways might account for common actions of GM-CSF and M-CSF on the expression of macrophage-specific genes. To test this hypothesis, we have investigated the mechanisms by which GM-CSF and M-CSF regulate the expression of the macrophage scavenger receptor A (SR-A) gene. We demonstrate that induction of the SR-A gene by M-CSF is dependent on AP-1 and cooperating Ets domain transcription factors that bind to sites in an M-CSF-dependent enhancer located 4.1 to 4.5 kb upstream of the transcriptional start site. In contrast, regulation by GM-CSF requires a separate enhancer located 4.5 to 4.8 kb upstream of the transcriptional start site that confers both immediate-early and sustained transcriptional responses. Results of a combination of DNA binding experiments and functional assays suggest that immediate transcriptional responses are mediated by DNA binding proteins that are constitutively bound to the GM-CSF enhancer and are activated by Ras. At 12 to 24 h after GM-CSF treatment, the GM-CSF enhancer becomes further occupied by additional DNA binding proteins that may contribute to sustained transcriptional responses. In concert, these studies indicate that GM-CSF and M-CSF differentially utilize Ras-dependent signal transduction pathways to regulate scavenger receptor gene expression, consistent with the distinct functional properties of M-CSF- and GM-CSF-derived macrophages.

Expression of the peroxisome proliferator-activated receptor gamma (PPARgamma) in human atherosclerosis and regulation in macrophages by colony stimulating factors and oxidized low density lipoprotein

The peroxisome proliferator-activated receptor gamma (PPARgamma) is a ligand-dependent transcription factor that has been demonstrated to regulate fat cell development and glucose homeostasis. PPARgamma is also expressed in a subset of macrophages and negatively regulates the expression of several proinflammatory genes in response to natural and synthetic ligands. We here demonstrate that PPARgamma is expressed in macrophage foam cells of human atherosclerotic lesions, in a pattern that is highly correlated with that of oxidation-specific epitopes. Oxidized low density lipoprotein (oxLDL) and macrophage colony-stimulating factor, which are known to be present in atherosclerotic lesions, stimulated PPARgamma expression in primary macrophages and monocytic cell lines. PPARgamma mRNA expression was also induced in primary macrophages and THP-1 monocytic leukemia cells by the phorbol ester 12-O-tetradecanoylphorbol 13-acetate (TPA). Inhibition of protein kinase C blocked the induction of PPARgamma expression by TPA, but not by oxLDL, suggesting that more than one signaling pathway regulates PPARgamma expression in macrophages. TPA induced the expression of PPARgamma in RAW 264.7 macrophages by increasing transcription from the PPARgamma1 and PPARgamma3 promoters. In concert, these observations provide insights into the regulation of PPARgamma expression in activated macrophages and raise the possibility that PPARgamma ligands may influence the progression of atherosclerosis.

UBPY: a growth-regulated human ubiquitin isopeptidase

The ubiquitin pathway has been implicated in the regulation of the abundance of proteins that control cell growth and proliferation. We have identified and characterized a novel human ubiquitin isopeptidase, UBPY, which both as a recombinant protein and upon immunoprecipitation from cell extracts is able to cleave linear or isopeptide-linked ubiquitin chains. UBPY accumulates upon growth stimulation of starved human fibroblasts, and its levels decrease in response to growth arrest induced by cell-cell contact. Inhibition of UBPY accumulation by antisense plasmid microinjection prevents fibroblasts from entering S-phase in response to serum stimulation. By increasing or decreasing the cellular abundance of UBPY or by overexpressing a catalytic site mutant, we detect substantial changes in the total pattern of protein ubiquitination, which correlate stringently with cell proliferation. Our results suggest that UBPY plays a role in regulating the overall function of the ubiquitin-proteasome pathway. Affecting the function of a specific UBP in vivo could provide novel tools for controlling mammalian cell proliferation.

Genomic structure of Unp, a murine gene encoding a ubiquitin-specific protease

The murine Unp gene encodes a ubiquitin-specific protease, a member of a family of enzymes that includes the product of the human tre-2 oncogene. The Unp gene has previously been mapped to chromosome 9. We have cloned in bacteriophage a 50 kilobase region of chromosome 9 containing the Unp gene, and have determined the nucleotide sequence of the gene. The gene has 22 exons, distributed over 47.4 kb. A processed ribosomal S2 pseudogene was identified in the third intron of the Unp gene. Expression of Unp is driven by a GC-rich, 'housekeeping' type promoter.

The peroxisome proliferator-activated receptor-gamma is a negative regulator of macrophage activation

The peroxisome proliferator-activated receptor-gamma (PPAR-gamma) is a member of the nuclear receptor superfamily of ligand-dependent transcription factors that is predominantly expressed in adipose tissue, adrenal gland and spleen. PPAR-gamma has been demonstrated to regulate adipocyte differentiation and glucose homeostasis in response to several structurally distinct compounds, including thiazolidinediones and fibrates. Naturally occurring compounds such as fatty acids and the prostaglandin D2 metabolite 15-deoxy-delta prostaglandin J2 (15d-PGJ2) bind to PPAR-gamma and stimulate transcription of target genes. Prostaglandin D2 metabolites have not yet been identified in adipose tissue, but are major products of arachidonic-acid metabolism in macrophages, raising the possibility that they might serve as endogenous PPAR-gamma ligands in this cell type. Here we show that PPAR-gamma is markedly upregulated in activated macrophages and inhibits the expression of the inducible nitric oxide synthase, gelatinase B and scavenger receptor A genes in response to 15d-PGJ2 and synthetic PPAR-gamma ligands. PPAR-gamma inhibits gene expression in part by antagonizing the activities of the transcription factors AP-1, STAT and NF-kappaB. These observations suggest that PPAR-gamma and locally produced prostaglandin D2 metabolites are involved in the regulation of inflammatory responses, and raise the possibility that synthetic PPAR-gamma ligands may be of therapeutic value in human diseases such as atherosclerosis and rheumatoid arthritis in which activated macrophages exert pathogenic effects.

JAK2 is required for induction of the murine DUB-1 gene

Cytokine receptors activate multiple signal transduction pathways, resulting in the induction of specific target genes. We have recently identified a hematopoietic cell-specific immediate-early gene, DUB-1, that encodes a growth-regulatory deubiquitinating enzyme. The DUB-1 gene contains a 112-bp enhancer element that is specifically induced by the beta c subunit of the interleukin-3 (IL-3) receptor. To investigate the mechanism of DUB-1 induction, we examined the effects of dominant-negative forms of JAK kinases, STAT transcription factors, and Raf-1 in transient transfection assays. In Ba/F3 cells, IL-3 induced a dose-dependent activation of DUB-1-luciferase (luc) and GAS-luc reporter constructs. A dominant-negative form of JAK2 (truncated at amino acid 829) inhibited the induction of DUB-1-luc and GAS-luc by IL-3. A dominant-negative form of STAT5 (truncated at amino acid 650) inhibited the induction of GAS-luc but not DUB-1-luc. A dominant-negative form of Raf-1 inhibited the induction of DUB-1-luc but had no effect on the induction of GAS-luc by IL-3. The requirement for JAK2 in the stimulation of the DUB-1 enhancer was further supported by the suppression of DUB-1 induction in Ba/F3 cells stably expressing the dominant-negative JAK2 polypeptide. We hypothesize that IL-3 activates a JAK2/Raf-1 signaling pathway that is required for DUB-1 induction and is independent of STAT5.

DUB-2 is a member of a novel family of cytokine-inducible deubiquitinating enzymes

Cytokines regulate cell growth by inducing the expression of specific target genes. We have recently identified a cytokine-inducible, immediate-early gene, DUB-1, that encodes a deubiquitinating enzyme with growth regulatory activity. In the current study, we have isolated a highly related gene, DUB-2, that is induced by interleukin-2. The DUB-2 mRNA was induced in T cells as an immediate-early gene and was rapidly down-regulated. Like DUB-1, the DUB-2 protein had deubiquitinating activity in vitro. When a conserved cysteine residue of DUB-2, required for ubiquitin-specific thiol protease activity, was mutated to serine (C60S), deubiquitinating activity was abolished. DUB-1 and DUB-2 proteins are highly related throughout their primary amino acid sequence except for a hypervariable region at their COOH terminus. Moreover, the DUB genes co-localize to a region of mouse chromosome 7, suggesting that they arose by a tandem duplication of an ancestral DUB gene. Additional DUB genes co-localize to this region, suggesting a larger family of cytokine-inducible DUB enzymes. We propose that different cytokines induce specific DUB genes. Each induced DUB enzyme thereby regulates the degradation or the ubiquitination state of an unknown growth regulatory factor, resulting in a cytokine-specific growth response.