The deubiquitinating enzyme USP11 controls an IkappaB kinase alpha (IKKalpha)-p53 signaling pathway in response to tumor necrosis factor alpha (TNFalpha)

Pparα knockout in mice increases the Th17 development by facilitating the IKKα/RORγt and IKKα/Foxp3 complexes

The helper CD4+ T cell-type 17 (Th17) cells and regulatory CD4+ T cells (Tregs) are balanced through numerous molecular regulators, particularly metabolic factors, and their alteration causes immune dysregulation. Herein, we report that peroxisome proliferator of activated receptor-alpha (Pparα), a lipid metabolism regulator, suppresses Th17 differentiation. We demonstrated that Pparα ablation improves Th17 and pro-Th17 factor HIF-1α by enhancing the expression and nuclear localization of NFκB-activator IκB kinase-alpha (IKKα). Unexpectedly, we found that IKKα directly interacts with RORγt and enhances the expression of Il17a gene. Meanwhile, IKKα also interacts with Foxp3, leading to the post-translational regulation of Foxp3 by elevating its proteasomal degradation, and influencing Th17 development. Pparα deficiency leads to enhanced Th17 development in vivo and is associated with enhanced pathology in a murine experimental autoimmune encephalomyelitis (EAE) model. Overall, our data indicate that Pparα may serve as a potential therapeutic target for autoimmune and inflammatory diseases.

USP11 plays a critical role in the onset and progression of acute graft-versus-host disease：Novel target for precision therapeutics

Acute graft-versus-host disease (aGvHD) is considered a result of "cytokine storm." Targeted therapeutic interventions on cytokines via ubiquitination regulatory pathways may provide a potential approach for aGvHD treatment. Ubiquitin-specific peptidase 11 (USP11) has been reported to play key roles in a variety of physiopathological processes by regulating the stability and function of several vital protein molecules. However, its role in aGvHD remains unclear. In this study, we identified USP11 was associated with aGvHD in patients. In the aGvHD mouse model, the colon and liver were more seriously affected in recipient mice who received USP11 wt bone marrow (BM) cells and eased after the donor was treated with a USP11 inhibitor or received USP11 ko BM cells. In mouse models, IL-6 was identified as a major effecter in accelerating aGvHD induced by USP11. In the cell model, IL-6 mRNA transcript was affected by USP11. In addition, USP11 also inhibited IL-6 degradation by affecting IL-6 ubiquitination. Furthermore, the positive correlation between USP11 and IL-6 was confirmed in the GvHD patients' samples. Collectively, all results indicated that USP11 played a critical role in the onset and progression of aGvHD. USP11 might be a potential target for aGvHD treatment.

Ubiquitin specific peptidase 11 as a novel therapeutic target for cancer management

Ubiquitination is a critical biological process in post-translational modification of proteins and involves multiple signaling pathways in protein metabolism, apoptosis, DNA damage, cell-cycle progression, and cancer development. Deubiquitinase, a specific enzyme that regulates the ubiquitination process, is also thought to be closely associated with the development and progression of various cancers. In this article, we systematically review the emerging role of the deubiquitinase ubiquitin-specific peptidase 11 (USP11) in many cancer-related pathways. The results show that USP11 promotes or inhibits the progression and chemoresistance of different cancers, including colorectal, breast, ovarian, and hepatocellular carcinomas, via deubiquitinating several critical proteins of cancer-related pathways. We initially summarize the role of USP11 in different cancers and further discuss the possibility of USP11 as a therapeutic strategy.

Redox regulation of DUBs and its therapeutic implications in cancer

Reactive oxygen species (ROS) act as a double-edged sword in cancer, where low levels of ROS are beneficial but excessive accumulation leads to cancer progression. Elevated levels of ROS in cancer are counteracted by the antioxidant defense system. An imbalance between ROS generation and the antioxidant system alters gene expression and cellular signaling, leading to cancer progression or death. Post-translational modifications, such as ubiquitination, phosphorylation, and SUMOylation, play a critical role in the maintenance of ROS homeostasis by controlling ROS production and clearance. Recent evidence suggests that deubiquitinating enzymes (DUBs)-mediated ubiquitin removal from substrates is regulated by ROS. ROS-mediated oxidation of the catalytic cysteine (Cys) of DUBs, leading to their reversible inactivation, has emerged as a key mechanism regulating DUB-controlled cellular events. A better understanding of the mechanism by which DUBs are susceptible to ROS and exploring the ways to utilize ROS to pharmacologically modulate DUB-mediated signaling pathways might provide new insight for anticancer therapeutics. This review assesses the recent findings regarding ROS-mediated signaling in cancers, emphasizes DUB regulation by oxidation, highlights the relevant recent findings, and proposes directions of future research based on the ROS-induced modifications of DUB activity.

C81-evoked inhibition of the TNFR1-NFκB pathway during inflammatory processes for stabilization of the impaired vascular endothelial barrier for leukocytes

Chronic inflammation-related diseases are characterized by persistent leukocyte infiltration into the underlying tissue. The vascular endothelium plays a major role in this pathophysiological condition. Only few therapeutic strategies focus on the vascular endothelium as a major target for an anti-inflammatory approach. In this study, we present the natural compound-derived carbazole derivative C81 as chemical modulator interfering with leukocyte-endothelial cell interactions. An in vivo assay employing intravital microscopy to monitor leukocyte trafficking after C81 treatment in postcapillary venules of a murine cremaster muscle was performed. Moreover, in vitro assays using HUVECs and monocytes were implemented. The impact of C81 on cell adhesion molecules and the NFκB signaling cascade was analyzed in vitro in endothelial cells. Effects of C81 on protein translation were determined by incorporation of a puromycin analog-based approach and polysome profiling. We found that C81 significantly reduced TNF-activated leukocyte trafficking in postcapillary venules. Similar results were obtained in vitro when C81 reduced leukocyte-endothelial cell interactions by down-regulating cell adhesion molecules. Focusing on the NFκB signaling cascade, we found that C81 reduced the activation on multiple levels of the cascade through promoted IκBα recovery by attenuation of IκBα ubiquitination and through reduced protein levels of TNFR1 caused by protein translation inhibition. We suggest that C81 might represent a promising lead compound for interfering with inflammation-related processes in endothelial cells by down-regulation of IκBα ubiquitination on the one hand and inhibition of translation on the other hand without exerting cytotoxic effects.

The Multifaceted Roles of USP15 in Signal Transduction

Ubiquitination and deubiquitination are protein post-translational modification processes that have been recognized as crucial mediators of many complex cellular networks, including maintaining ubiquitin homeostasis, controlling protein stability, and regulating several signaling pathways. Therefore, some of the enzymes involved in ubiquitination and deubiquitination, particularly E3 ligases and deubiquitinases, have attracted attention for drug discovery. Here, we review recent findings on USP15, one of the deubiquitinases, which regulates diverse signaling pathways by deubiquitinating vital target proteins. Even though several basic previous studies have uncovered the versatile roles of USP15 in different signaling networks, those have not yet been systematically and specifically reviewed, which can provide important information about possible disease markers and clinical applications. This review will provide a comprehensive overview of our current understanding of the regulatory mechanisms of USP15 on different signaling pathways for which dynamic reverse ubiquitination is a key regulator.

A pro-inflammatory mediator USP11 enhances the stability of p53 and inhibits KLF2 in intracerebral hemorrhage

Microglial cell activation and neuroinflammation after intracerebral hemorrhage (ICH) lead to secondary brain damage. Ubiquitin-specific protease 11 (USP11) has been correlated with ICH-induced neuron apoptosis. This study aims to explore the molecular mechanism of USP11 regulating neuroinflammation in ICH. First, an ICH rat model was developed by intracranial administration of collagenase. Silencing USP11 was found to alleviate nerve injury in rats with ICH-like symptoms. Then, through loss- and gain-of-function assays, USP11 knockdown was revealed to alleviate ICH-induced symptoms, corresponding to reduced modified neurological severity scores (mNSS) value, brain water content, blood-brain barrier permeability, neuron apoptosis, microglial cell activation, neutrophil infiltration, and inflammatory factor secretion. It was subsequently shown in microglial cells that USP11 stabilized p53 by deubiquitination and p53 targeted the Kruppel-like factor 2 (KLF2) promoter to repress KLF2 transcription, thereby activating the nuclear factor κB (NF-κB) pathway. Further, rescue experiments were conducted in vivo to validate the function of the USP11/p53/KLF2/NF-κB axis in ICH-induced inflammation, which confirmed that USP11 silencing blocked the release of pro-inflammatory cytokines following ICH by downregulating p53, thus protecting against neurological impairment. Hence silencing USP11 may be a novel anti-inflammatory method for ICH treatment.

Ursolic acid inhibits pigmentation by increasing melanosomal autophagy in B16F1 cells

Melanosomes are specialized membrane-bound organelles that are involved in melanin synthesis. Unlike melanosome biogenesis, the melanosome degradation pathway is poorly understood. Among the cellular processes, autophagy controls degradation of intracellular components by cooperating with lysosomes. In this study, we showed that ursolic acid inhibits skin pigmentation by promoting melanosomal autophagy, or melanophagy, in melanocytes. We found that B16F1 cells treated with ursolic acid suppressed alpha-melanocyte stimulating hormone (α-MSH) stimulated increase in melanin content and activated autophagy. In addition, we found that treatment with ursolic acid promotes melanosomal degradation, and bafilomycin A1 inhibition of autophagosome-lysosome fusion blocked the removal of melanosomes in α-MSH-stimulated B16F1 cells. Furthermore, depletion of the autophagy-related gene 5 (ATG5) resulted in significant suppression of ursolic acid-mediated anti-pigmentation activity and autophagy in α-MSH-treated B16F1 cells. Taken together, our results suggest that ursolic acid inhibits skin pigmentation by increasing melanosomal degradation in melanocytes.

Advances in Deubiquitinating Enzyme Inhibition and Applications in Cancer Therapeutics

Since the discovery of the ubiquitin proteasome system (UPS), the roles of ubiquitinating and deubiquitinating enzymes (DUBs) have been widely elucidated. The ubiquitination of proteins regulates many aspects of cellular functions such as protein degradation and localization, and also modifies protein-protein interactions. DUBs cleave the attached ubiquitin moieties from substrates and thereby reverse the process of ubiquitination. The dysregulation of these two paramount pathways has been implicated in numerous diseases, including cancer. Attempts are being made to identify inhibitors of ubiquitin E3 ligases and DUBs that potentially have clinical implications in cancer, making them an important target in the pharmaceutical industry. Therefore, studies in medicine are currently focused on the pharmacological disruption of DUB activity as a rationale to specifically target cancer-causing protein aberrations. Here, we briefly discuss the pathophysiological and physiological roles of DUBs in key cancer-related pathways. We also discuss the clinical applications of promising DUB inhibitors that may contribute to the development of DUBs as key therapeutic targets in the future.

Regulation of epidermal differentiation through KDF1-mediated deubiquitination of IKKα

Progenitor cells at the basal layer of skin epidermis play an essential role in maintaining tissue homeostasis and enhancing wound repair in skin. The proliferation, differentiation, and cell death of epidermal progenitor cells have to be delicately regulated, as deregulation of this process can lead to many skin diseases, including skin cancers. However, the underlying molecular mechanisms involved in skin homeostasis remain poorly defined. In this study, with quantitative proteomics approach, we identified an important interaction between KDF1 (keratinocyte differentiation factor 1) and IKKα (IκB kinase α) in differentiating skin keratinocytes. Ablation of either KDF1 or IKKα in mice leads to similar but striking abnormalities in skin development, particularly in skin epidermal differentiation. With biochemical and mouse genetics approach, we further demonstrate that the interaction of IKKα and KDF1 is essential for epidermal differentiation. To probe deeper into the mechanisms, we find that KDF1 associates with a deubiquitinating protease USP7 (ubiquitin-specific peptidase 7), and KDF1 can regulate skin differentiation through deubiquitination and stabilization of IKKα. Taken together, our study unravels an important molecular mechanism underlying epidermal differentiation and skin tissue homeostasis.

The deubiquitinating enzyme PSMD14 facilitates tumor growth and chemoresistance through stabilizing the ALK2 receptor in the initiation of BMP6 signaling pathway

BACKGROUND

Although bone morphogenetic protein 6 (BMP6) signaling pathway has been implicated in many types of cancer, its role of tumorigenesis seems to be controversial and its ubiquitin-modifying mechanisms have not been fully addressed. Our study was designed to investigate how BMP6 signaling pathway is regulated by ubiquitin-modifying systems and to address molecular and clinical significance in colorectal cancers.

METHODS

Human deubiquitnase (DUB) siRNA library was used to screen the specific DUB, named PSMD14, involved in BMP6 signaling pathway. Immunoblot, immunoprecipitation and ubiquitination assays were used to analyze targets of the PSMD14. A role of PSMD14-mediated BMP6 signaling pathway for malignant cancer progression was investigated using in vitro and in vivo model of colorectal cancers as well as clinical samples of colorectal cancer patients.

FINDINGS

The deubiquitinase PSMD14 acts as a positive regulator for the initiation of the BMP6 signaling pathway through deubiquitinating K48-linked ALK2 type I receptor ubiquitination mediated by Smurf1 E3 ligase, resulting in increased stability of the ALK2. This role of PSMD14 is independent of its intrinsic role in the 26S proteasome system. Furthermore, either PSMD14 or ALK2 depletion significantly decreases tumorigenesis of HCT116 colorectal cancer cells in a xenograft model as well as cancer stemness/chemoresistance, and expression of the PSMD14 and ALK2 gene are correlated with malignant progression and the survival of colorectal cancer patients.

INTERPRETATION

These findings suggest that the PSMD14-ALK2 axis plays an essential role in initiation of the BMP6 signaling pathway and contributes to tumorigenesis and chemoresistance of colorectal cancers.

Phosphorylated E2F1 is stabilized by nuclear USP11 to drive Peg10 gene expression and activate lung epithelial cells

Phosphorylation affects ubiquitination, stability, and activity of transcriptional factors, thus regulating various cellular functions. E2F transcriptional factor 1 (E2F1) regulates paternally expressed imprinted gene 10 (Peg10) expression, thereby promoting cell proliferation. However, the effect of E2F1 stability on Peg10 expression and the molecular regulation of E2F1 stability by its phosphorylation have not been well demonstrated. Here, we describe a new pathway in which phosphorylation of E2F1 by GSK3β increases E2F1 association with the deubiquitinating enzyme, ubiquitin-specific protease 11 (USP11), which removes K63-linked ubiquitin chains thereby preventing E2F1 degradation in the nuclei. Downregulation of USP11 increases E2F1 ubiquitination and reduces E2F1 stability and protein levels, thereby decreasing Peg10 mRNA levels. Physiologically, USP11 depletion suppresses cell proliferation and wound healing in lung epithelial cells, and these effects are reversed by E2F1 and PEG10 overexpression. Thus, our study reveals a new molecular model that phosphorylation promotes substrate stability through increasing its association with a deubiquitinating enzyme. The data suggest that GSK3β and USP11 act in concert to modulate E2F1 abundance and PEG10 expression in lung epithelial cells to affect cell wound healing. This study provides new therapeutic targets to lessen lung injury by improving lung epithelial cell repair and remodeling after injury.

TGF-β signaling pathway mediated by deubiquitinating enzymes

Ubiquitination is a reversible cellular process mediated by ubiquitin-conjugating enzymes, whereas deubiquitinating enzymes (DUBs) detach the covalently conjugated ubiquitin from target substrates to counter ubiquitination. DUBs play a crucial role in regulating various signal transduction pathways and biological processes including apoptosis, cell proliferation, DNA damage repair, metastasis, differentiation, etc. Since the transforming growth factor-β (TGF-β) signaling pathway participates in various cellular functions such as inflammation, metastasis and embryogenesis, aberrant regulation of TGF-β signaling induces abnormal cellular functions resulting in numerous diseases. This review focuses on DUBs regulating the TGF-β signaling pathway. We discuss the molecular mechanisms of DUBs involved in TGF-β signaling pathway, and biological and therapeutic implications for various diseases.

Ubiquitin-specific protease 11 serves as a marker of poor prognosis and promotes metastasis in hepatocellular carcinoma

Ubiquitin-specific protease 11 (USP11) is a deubiquitinating enzyme that exerts its biological functions by regulating multiple signaling pathways such as p53, NF-κB, TGF-β, and Hippo. A large body of evidence supports a link between UPS11 and tumorigenesis. However, the clinical significance and biological function of USP11 in hepatocellular carcinoma (HCC) remains unclear. Here, USP11 expression was assessed by immunohistochemistry in a pilot series of 71 HCC clinical samples, and the association between USP11 expression and clinicopathological features and overall survival time was analyzed. The cytoplasmic expression rate of USP11 was higher in non-cancerous tissue than that in cancer tissue (36.6 vs. 12.7%, P = 0.001), whereas the nuclear expression rate of USP11 was lower in non-cancerous tissue (5.6 vs. 69.0%, P < 0.001). USP11 expression level was higher in tumor than that in non-tumor tissue (P < 0.001). Chi-square analysis of variances suggested that USP11 expression was associated with vascular invasion (P = 0.033), differentiation (P = 0.027), tumor number (P = 0.009), and recurrence (P = 0.036). USP11 expression was also associated with shorter overall survival time (P = 0.001) by log-rank test. Unconditional logistic regression analysis with multiple covariates indicated that high USP11 expression was associated with a 2.96-fold increase in the risk of death compared with low USP11 levels (P = 0.041) and acted as an independent predictor of overall survival. HCC patients with simultaneously high USP11 and alpha-fetoprotein expression had an adjusted 5-fold higher risk of all-cause-related death (P = 0.006). Moreover, in vitro and in vivo experiments confirmed that USP11 could promote the migration and invasion of HCC cell. Overall, we suggest that USP11 promotes HCC cell metastasis, and we provide the first evidence of the prognostic significance of USP11 expression in HCC, which suggests that USP11 is a promising therapeutic target for the treatment of HCC.

p53 stability is regulated by diverse deubiquitinating enzymes

The tumor suppressor protein p53 has a variety of roles in responses to various stress signals. In such responses, p53 activates specific transcriptional targets that control cell cycle arrest, DNA repair, angiogenesis, autophagy, metabolism, migration, aging, senescence, and apoptosis. Since p53 has been identified as the most frequently altered gene in human cancers, regulation and stabilization of its normal functions are important. Stability of p53 is regulated by the ubiquitin-proteasome pathway (UPP). Furthermore, it is readjusted by deubiquitination via deubiquitinating enzymes (DUBs) that can eliminate ubiquitin from p53. Diverse DUBs directly or indirectly affect the ubiquitination of p53 and, consequently, regulate various cellular processes associated with p53. As maintenance of p53 is regulated by a variety of DUBs, the interaction of DUBs and p53 can affect diseases such as cancer. Currently, DUBs have a central role in our understanding of various cancers, and some have potential in the development of effective therapeutic strategies. This review summarizes the current knowledge of p53 and of the interconnection between p53 and DUBs.

Interplay between Inflammation and Stemness in Cancer Cells: The Role of Toll-Like Receptor Signaling

Cancer stem cells (CSCs) are a small population of cancer cells that exhibit stemness. These cells contribute to cancer metastasis, treatment resistance, and relapse following therapy; therefore, they may cause malignancy and reduce the success of cancer treatment. Nuclear factor kappa B- (NF-κB-) mediated inflammatory responses increase stemness in cancer cells, and CSCs constitutively exhibit higher NF-κB activation, which in turn increases their stemness. These opposite effects form a positive feedback loop that further amplifies inflammation and stemness in cancer cells, thereby expanding CSC populations in the tumor. Toll-like receptors (TLRs) activate NF-κB-mediated inflammatory responses when stimulated by carcinogenic microbes and endogenous molecules released from cells killed during cancer treatment. NF-κB activation by extrinsic TLR ligands increases stemness in cancer cells. Moreover, it was recently shown that increased NF-κB activity and inflammatory responses in CSCs may be caused by altered TLR signaling during the enrichment of stemness in cancer cells. Thus, the activation of TLR signaling by extrinsic and intrinsic factors drives a positive interplay between inflammation and stemness in cancer cells.

Targeting Deubiquitinating Enzymes in Glioblastoma Multiforme: Expectations and Challenges

Glioblastoma (GBM) is regarded as the most common primary intracranial neoplasm. Despite standard treatment with tumor resection and radiochemotherapy, the outcome remains gloomy. It is evident that a combination of oncogenic gain of function and tumor-suppressive loss of function has been attributed to glioma initiation and progression. The ubiquitin-proteasome system is a well-orchestrated system that controls the fate of most proteins by striking a dynamic balance between ubiquitination and deubiquitination of substrates, having a profound influence on the modulation of oncoproteins, tumor suppressors, and cellular signaling pathways. In recent years, deubiquitinating enzymes (DUBs) have emerged as potential anti-cancer targets due to their targeting several key proteins involved in the regulation of tumorigenesis, apoptosis, senescence, and autophagy. This review attempts to summarize recent studies of GBM-associated DUBs, their roles in various cellular processes, and discuss the relation between DUBs deregulation and gliomagenesis, especially how DUBs regulate glioma stem cells pluripotency, microenvironment, and resistance of radiation and chemotherapy through core stem-cell transcriptional factors. We also review recent achievements and progress in the development of potent and selective reversible inhibitors of DUBs, and attempted to find a potential GBM treatment by DUBs intervention.

Role of aquaporins in cell proliferation: What else beyond water permeability?

In addition to the extensive data demonstrating the importance of mammalian AQPs for the movement of water and some small solutes across the cell membrane, there is now a growing body of evidence indicating the involvement of these proteins in numerous cellular processes seemingly unrelated, at least some of them in a direct way, to their canonical function of water permeation. Here, we have presented a broad range of evidence demonstrating that these proteins have a role in cell proliferation by various different mechanisms, namely, by allowing fast cell volume regulation during cell division; by affecting progression of cell cycle and helping maintain the balance between proliferation and apoptosis, and by crosstalk with other cell membrane proteins or transcription factors that, in turn, modulate progression of the cell cycle or regulate biosynthesis pathways of cell structural components. In the end, however, after discussing all these data that strongly support a role for AQPs in the cell proliferation process, it remains impossible to conclude that all these other functions attributed to AQPs occur completely independently of their water permeability, and there is a need for new experiments designed specifically to address this interesting issue.

Evolution of the highly networked deubiquitinating enzymes USP4, USP15, and USP11

Background

USP4, USP15 and USP11 are paralogous deubiquitinating enzymes as evidenced by structural organization and sequence similarity. Based on known interactions and substrates it would appear that they have partially redundant roles in pathways vital to cell proliferation, development and innate immunity, and elevated expression of all three has been reported in various human malignancies. The nature and order of duplication events that gave rise to these extant genes has not been determined, nor has their functional redundancy been established experimentally at the organismal level.

Methods

We have employed phylogenetic and syntenic reconstruction methods to determine the chronology of the duplication events that generated the three paralogs and have performed genetic crosses to evaluate redundancy in mice.

Results

Our analyses indicate that USP4 and USP15 arose from whole genome duplication prior to the emergence of jawed vertebrates. Despite having lower sequence identity USP11 was generated later in vertebrate evolution by small-scale duplication of the USP4-encoding region. While USP11 was subsequently lost in many vertebrate species, all available genomes retain a functional copy of either USP4 or USP15, and through genetic crosses of mice with inactivating mutations we have confirmed that viability is contingent on a functional copy of USP4 or USP15. Loss of ubiquitin-exchange regulation, constitutive skipping of the seventh exon and neural-specific expression patterns are derived states of USP11. Post-translational modification sites differ between USP4, USP15 and USP11 throughout evolution.

Conclusions

In isolation sequence alignments can generate erroneous USP gene phylogenies. Through a combination of methodologies the gene duplication events that gave rise to USP4, USP15, and USP11 have been established. Although it operates in the same molecular pathways as the other USPs, the rapid divergence of the more recently generated USP11 enzyme precludes its functional interchangeability with USP4 and USP15. Given their multiplicity of substrates the emergence (and in some cases subsequent loss) of these USP paralogs would be expected to alter the dynamics of the networks in which they are embedded.

Electronic supplementary material

The online version of this article (doi:10.1186/s12862-015-0511-1) contains supplementary material, which is available to authorized users.

USP11, Deubiquitinating Enzyme, Associated with Neuronal Apoptosis Following Intracerebral Hemorrhage

Protein ubiquitination is a dynamic two-way process that can be reversed or regulated by deubiquitinating enzymes (DUB). USP11, located on the X chromosome, 6 is a member of USP subclass of the DUB family. Here, we demonstrate that USP11 may be involved in neuronal apoptosis in the processes of intracerebral hemorrhage (ICH). From the results of Western blot, immunohistochemistry, and immunofluorescence, we obtained a significant up-regulation of USP11 in neurons adjacent to the hematoma following ICH. Increasing USP11 level was found to be accompanied by the up-regulation of active caspase-3, Fas receptor (Fas), Fas ligand (FasL), and active caspase-8. Besides, USP11 co-localized well with active caspase-3 in neurons, indicating its potential role in neuronal apoptosis. What is more, knocking down USP11 by RNA-interference in PC12 cells reduced active caspase-3 expression. Thus, USP11 may play a role in promoting the brain secondary damage following ICH.

USP11 regulates p53 stability by deubiquitinating p53

The p53 tumor suppressor protein coordinates the cellular responses to a broad range of cellular stresses, leading to DNA repair, cell cycle arrest or apoptosis. The stability of p53 is essential for its tumor suppressor function, which is tightly controlled by ubiquitin-dependent degradation primarily through its negative regulator murine double minute 2 (Mdm2). To better understand the regulation of p53, we tested the interaction between p53 and USP11 using co-immunoprecipitation. The results show that USP11, an ubiquitin-specific protease, forms specific complexes with p53 and stabilizes p53 by deubiquitinating it. Moreover, down-regulation of USP11 dramatically attenuated p53 induction in response to DNA damage stress. These findings reveal that USP11 is a novel regulator of p53, which is required for p53 activation in response to DNA damage.

USP11-dependent selective cIAP2 deubiquitylation and stabilization determine sensitivity to Smac mimetics

Given their crucial role in apoptosis suppression, inhibitor of apoptosis proteins (IAPs) have recently become attractive targets for cancer therapy. Here, we report that cellular IAP2 (cIAP2) is specifically stabilized in several cancer cell lines, leading to resistance to Smac mimetics, such as BV6 and birinapant. In particular, our results showed that cIAP2 depletion, but not cIAP1 depletion, sensitized cancer cells to Smac mimetic-induced apoptosis. Ubiquitin-specific protease 11 (USP11) is a deubiquitylase that directly stabilizes cIAP2. USP11 overexpression is frequently found in colorectal cancer and melanoma and is correlated with poor survival. In our study, cancer cell lines expressing high levels of USP11 exhibited strong resistance to Smac mimetic-induced cIAP2 degradation. Furthermore, USP11 downregulation sensitized these cells to apoptosis induced by TRAIL and BV6 and suppressed tumor growth in a xenograft model. Finally, the TNFα/JNK pathway induced USP11 expression and maintained cIAP2 stability, suggesting an alternative TNFα-dependent cell survival pathway. Collectively, our data suggest that USP11-stabilized cIAP2 may serve as a barrier against IAP-targeted clinical approaches.

Cell death and deubiquitinases: perspectives in cancer

The process of cell death has important physiological implications. At the organism level it is mostly involved in maintenance of tissue homeostasis. At the cellular level, the strategies of cell death may be categorized as either suicide or sabotage. The mere fact that many of these processes are programmed and that these are often deregulated in pathological conditions is seed to thought. The various players that are involved in these pathways are highly regulated. One of the modes of regulation is via post-translational modifications such as ubiquitination and deubiquitination. In this review, we have first dealt with the different modes and pathways involved in cell death and then we have focused on the regulation of several proteins in these signaling cascades by the different deubiquitinating enzymes, in the perspective of cancer. The study of deubiquitinases is currently in a rather nascent stage with limited knowledge both in vitro and in vivo, but the emerging roles of the deubiquitinases in various processes and their specificity have implicated them as potential targets from the therapeutic point of view. This review throws light on another aspect of cancer therapeutics by targeting the deubiquitinating enzymes.

Structure and catalytic regulatory function of ubiquitin specific protease 11 N-terminal and ubiquitin-like domains

The ubiquitin specific protease 11 (USP11) is implicated in DNA repair, viral RNA replication, and TGFβ signaling. We report the first characterization of the USP11 domain architecture and its role in regulating the enzymatic activity. USP11 consists of an N-terminal "domain present in USPs" (DUSP) and "ubiquitin-like" (UBL) domain, together referred to as DU domains, and the catalytic domain harboring a second UBL domain. Crystal structures of the DU domains show a tandem arrangement with a shortened β-hairpin at the two-domain interface and altered surface characteristics compared to the homologues USP4 and USP15. A conserved VEVY motif is a signature feature at the two-domain interface that shapes a potential protein interaction site. Small angle X-ray scattering and gel filtration experiments are consistent with the USP11DU domains and full-length USP11 being monomeric. Unexpectedly, we reveal, through kinetic assays of a series of deletion mutants, that the catalytic activity of USP11 is not regulated through intramolecular autoinhibition or activation by the N-terminal DU or UBL domains. Moreover, ubiquitin chain cleavage assays with all eight linkages reveal a preference for Lys(63)-, Lys(6)-, Lys(33)-, and Lys(11)-linked chains over Lys(27)-, Lys(29)-, and Lys(48)-linked and linear chains consistent with USP11's function in DNA repair pathways that is mediated by the protease domain. Our data support a model whereby USP11 domains outside the catalytic core domain serve as protein interaction or trafficking modules rather than a direct regulatory function of the proteolytic activity. This highlights the diversity of USPs in substrate recognition and regulation of ubiquitin deconjugation.

The regulation of TGF-β/SMAD signaling by protein deubiquitination

Transforming growth factor-β (TGF-β) members are key cytokines that control embryogenesis and tissue homeostasis via transmembrane TGF-β type II (TβR II) and type I (TβRI) and serine/threonine kinases receptors. Aberrant activation of TGF-β signaling leads to diseases, including cancer. In advanced cancer, the TGF-β/SMAD pathway can act as an oncogenic factor driving tumor cell invasion and metastasis, and thus is considered to be a therapeutic target. The activity of TGF-β/SMAD pathway is known to be regulated by ubiquitination at multiple levels. As ubiquitination is reversible, emerging studies have uncovered key roles for ubiquitin-removals on TGF-β signaling components by deubiquitinating enzymes (DUBs). In this paper, we summarize the latest findings on the DUBs that control the activity of the TGF-β signaling pathway. The regulatory roles of these DUBs as a driving force for cancer progression as well as their underlying working mechanisms are also discussed.

The ubiquitin system in immune regulation

The ubiquitin system plays a pivotal role in the regulation of immune responses. This system includes a large family of E3 ubiquitin ligases of over 700 proteins and about 100 deubiquitinating enzymes, with the majority of their biological functions remaining unknown. Over the last decade, through a combination of genetic, biochemical, and molecular approaches, tremendous progress has been made in our understanding of how the process of protein ubiquitination and its reversal deubiquitination controls the basic aspect of the immune system including lymphocyte development, differentiation, activation, and tolerance induction and regulates the pathophysiological abnormalities such as autoimmunity, allergy, and malignant formation. In this review, we selected some of the published literature to discuss the roles of protein-ubiquitin conjugation and deubiquitination in T-cell activation and anergy, regulatory T-cell and T-helper cell differentiation, regulation of NF-κB signaling, and hematopoiesis in both normal and dysregulated conditions. A comprehensive understanding of the relationship between the ubiquitin system and immunity will provide insight into the molecular mechanisms of immune regulation and at the same time will advance new therapeutic intervention for human immunological diseases.

USP11 augments TGFβ signalling by deubiquitylating ALK5

The TGFβ receptors signal through phosphorylation and nuclear translocation of SMAD2/3. SMAD7, a transcriptional target of TGFβ signals, negatively regulates the TGFβ pathway by recruiting E3 ubiquitin ligases and targeting TGFβ receptors for ubiquitin-mediated degradation. In this report, we identify a deubiquitylating enzyme USP11 as an interactor of SMAD7. USP11 enhances TGFβ signalling and can override the negative effects of SMAD7. USP11 interacts with and deubiquitylates the type I TGFβ receptor (ALK5), resulting in enhanced TGFβ-induced gene transcription. The deubiquitylase activity of USP11 is required to enhance TGFβ-induced gene transcription. RNAi-mediated depletion of USP11 results in inhibition of TGFβ-induced SMAD2/3 phosphorylation and TGFβ-mediated transcriptional responses. Central to TGFβ pathway signalling in early embryogenesis and carcinogenesis is TGFβ-induced epithelial to mesenchymal transition. USP11 depletion results in inhibition of TGFβ-induced epithelial to mesenchymal transition.

USP-11 as a predictive and prognostic factor following neoadjuvant therapy in women with breast cancer

PURPOSE

USP-11, a member of the ubiquitin-specific protease family, has emerged as an essential regulator of double-strand break repair. Few studies have shown that silencing USP-11 led to hypersensitivity to poly(ADP-ribose) polymerase inhibition, ionizing radiation, and DNA-damaging agents. We sought to examine the predictive and prognostic relevance of USP-11 in patients treated with neoadjuvant systemic therapy (NST) for breast cancer.

METHODS

Fifty-six women who were treated with NST for breast cancer between 1999 and 2004 were included in the study. The Kaplan-Meier product-limit method was used to estimate disease-free survival and overall survival rates. Logistic regression models were fit to determine the associations between USP-11 status, pathological complete response (pCR), and survival.

RESULTS

Sixteen patients (29%) had high-USP-11-expressing tumors, and 40 (71%) patients had low-USP-11-expressing tumors. No significant differences were observed in pCR rates with respect to USP-11 status. At a median follow-up of 7.4 years, 33 patients (59%) experienced a disease recurrence or death. Patients with high-USP-11-expressing tumors had a higher risk of recurrence (odds ratio [OR], 3.87; 95% confidence interval [CI], 1.51-9.93; P = 0.005) and death (OR, 6.03; 95% CI, 2.00-18.17; P = 0.001) than those with low-USP-11-expressing tumors. Patients who did not achieve a pCR had an increased risk of recurrence (OR, 5.16; 95% CI, 1.16-23.07; P = 0.03).

CONCLUSIONS

Our data indicate that USP-11 is not a predictor of a pCR after anthracycline-taxane-containing NST for breast cancer. Low USP-11 expression was independently correlated with better survival outcomes.

The protein interaction landscape of the human CMGC kinase group

Cellular information processing via reversible protein phosphorylation requires tight control of the localization, activity, and substrate specificity of protein kinases, which to a large extent is accomplished by complex formation with other proteins. Despite their critical role in cellular regulation and pathogenesis, protein interaction information is available for only a subset of the 518 human protein kinases. Here we present a global proteomic analysis of complexes of the human CMGC kinase group. In addition to subgroup-specific functional enrichment and modularity, the identified 652 high-confidence kinase-protein interactions provide a specific biochemical context for many poorly studied CMGC kinases. Furthermore, the analysis revealed a kinase-kinase subnetwork and candidate substrates for CMGC kinases. Finally, the presented interaction proteome uncovered a large set of interactions with proteins genetically linked to a range of human diseases, including cancer, suggesting additional routes for analyzing the role of CMGC kinases in controlling human disease pathways.

Beyond NF-κB activation: nuclear functions of IκB kinase α

IκB kinase (IKK) complex, the master kinase for NF-κB activation, contains two kinase subunits, IKKα and IKKβ. In addition to mediating NF-κB signaling by phosphorylating IκB proteins during inflammatory and immune responses, the activation of the IKK complex also responds to various stimuli to regulate diverse functions independently of NF-κB. Although these two kinases share structural and biochemical similarities, different sub-cellular localization and phosphorylation targets between IKKα and IKKβ account for their distinct physiological and pathological roles. While IKKβ is predominantly cytoplasmic, IKKα has been found to shuttle between the cytoplasm and the nucleus. The nuclear-specific roles of IKKα have brought increasing complexity to its biological function. This review highlights major advances in the studies of the nuclear functions of IKKα and the mechanisms of IKKα nuclear translocation. Understanding the nuclear activity is essential for targeting IKKα for therapeutics.

Regulation of NF-κB by deubiquitinases

The nuclear factor-κB (NF-κB) pathway is a critical regulator of innate and adaptive immunity. Noncanonical K63-linked polyubiquitination plays a key regulatory role in NF-κB signaling pathways by functioning as a scaffold to recruit kinase complexes containing ubiquitin-binding domains. Ubiquitination is balanced by deubiquitinases that cleave polyubiquitin chains and oppose the function of E3 ubiquitin ligases. Deubiquitinases therefore play an important role in the termination of NF-κB signaling and the resolution of inflammation. In this review, we focus on NF-κB regulation by deubiquitinases with an emphasis on A20 and CYLD. Deubiquitinases and the ubiquitin/proteasome components that regulate NF-κB may serve as novel therapeutic targets for inflammatory diseases and cancer.

Lysine 92 amino acid residue of USP46, a gene associated with 'behavioral despair' in mice, influences the deubiquitinating enzyme activity

Deubiquitinating enzymes (DUBs) regulate diverse cellular functions by their activity of cleaving ubiquitin from specific protein substrates. Ubiquitin-Specific Protease 46 (USP46) has recently been identified as a quantitative trait gene responsible for immobility in the tail suspension test and forced swimming test in mice. Mice with a lysine codon (Lys 92) deletion in USP46 exhibited loss of ‘behavioral despair’ under inescapable stresses in addition to abnormalities in circadian behavioral rhythms and the GABAergic system. However, whether this deletion affects enzyme activity is unknown. Here we show that USP46 has deubiquitinating enzyme activity detected by USP cleavage assay using GST-Ub52 as a model substrate. Interestingly, compared to wild type, the Lys 92 deletion mutant resulted in a decreased deubiquitinating enzyme activity of 27.04%. We also determined the relative expression levels of Usp46 in rat tissues using real-time RT-PCR. Usp46 mRNA was expressed in various tissues examined including brain, with the highest expression in spleen. In addition, like rat USP46, both human and mouse USP46 are active toward to the model substrate, indicating the USP cleavage assay is a simple method for testing the deubiquitinating enzyme activity of USP46. These results suggest that the Lys 92 deletion of USP46 could influence enzyme activity and thereby provide a molecular clue how the enzyme regulating the pathogenesis of mental illnesses.

Deubiquitinases in cancer: new functions and therapeutic options

Deubiquitinases (DUBs) have fundamental roles in the ubiquitin system through their ability to specifically deconjugate ubiquitin from targeted proteins. The human genome encodes at least 98 DUBs, which can be grouped into 6 families, reflecting the need for specificity in their function. The activity of these enzymes affects the turnover rate, activation, recycling and localization of multiple proteins, which in turn is essential for cell homeostasis, protein stability and a wide range of signaling pathways. Consistent with this, altered DUB function has been related to several diseases, including cancer. Thus, multiple DUBs have been classified as oncogenes or tumor suppressors because of their regulatory functions on the activity of other proteins involved in tumor development. Therefore, recent studies have focused on pharmacological intervention on DUB activity as a rationale to search for novel anticancer drugs. This strategy may benefit from our current knowledge of the physiological regulatory mechanisms of these enzymes and the fact that growth of several tumors depends on the normal activity of certain DUBs. Further understanding of these processes may provide answers to multiple remaining questions on DUB functions and lead to the development of DUB-targeting strategies to expand the repertoire of molecular therapies against cancer.

The herpesvirus associated ubiquitin specific protease, USP7, is a negative regulator of PML proteins and PML nuclear bodies

The PML tumor suppressor is the founding component of the multiprotein nuclear structures known as PML nuclear bodies (PML-NBs), which control several cellular functions including apoptosis and antiviral effects. The ubiquitin specific protease USP7 (also called HAUSP) is known to associate with PML-NBs and to be a tight binding partner of two herpesvirus proteins that disrupt PML NBs. Here we investigated whether USP7 itself regulates PML-NBs. Silencing of USP7 was found to increase the number of PML-NBs, to increase the levels of PML protein and to inhibit PML polyubiquitylation in nasopharyngeal carcinoma cells. This effect of USP7 was independent of p53 as PML loss was observed in p53-null cells. PML-NBs disruption was induced by USP7 overexpression independently of its catalytic activity and was induced by either of the protein interaction domains of USP7, each of which localized to PML-NBs. USP7 also disrupted NBs formed from some single PML isoforms, most notably isoforms I and IV. CK2α and RNF4, which are known regulators of PML, were dispensable for USP7-associated PML-NB disruption. The results are consistent with a novel model of PML regulation where a deubiquitylase disrupts PML-NBs through recruitment of another cellular protein(s) to PML NBs, independently of its catalytic activity.

An atlas of altered expression of deubiquitinating enzymes in human cancer

Background

Deubiquitinating enzymes (DUBs) are proteases that process ubiquitin (Ub) or ubiquitin-like gene products, remodel polyubiquitin(-like) chains on target proteins, and counteract protein ubiquitination exerted by E3 ubiquitin-ligases. A wealth of studies has established the relevance of DUBs to the control of physiological processes whose subversion is known to cause cellular transformation, including cell cycle progression, DNA repair, endocytosis and signal transduction. Altered expression of DUBs might, therefore, subvert both the proteolytic and signaling functions of the Ub system.

Methodology/Principal Findings

In this study, we report the first comprehensive screening of DUB dysregulation in human cancers by in situ hybridization on tissue microarrays (ISH-TMA). ISH-TMA has proven to be a reliable methodology to conduct this kind of study, particularly because it allows the precise identification of the cellular origin of the signals. Thus, signals associated with the tumor component can be distinguished from those associated with the tumor microenvironment. Specimens derived from various normal and malignant tumor tissues were analyzed, and the “normal” samples were derived, whenever possible, from the same patients from whom tumors were obtained. Of the ∼90 DUBs encoded by the human genome, 33 were found to be expressed in at least one of the analyzed tissues, of which 22 were altered in cancers. Selected DUBs were subjected to further validation, by analyzing their expression in large cohorts of tumor samples. This analysis unveiled significant correlations between DUB expression and relevant clinical and pathological parameters, which were in some cases indicative of aggressive disease.

Conclusions/Significance

The results presented here demonstrate that DUB dysregulation is a frequent event in cancer, and have implications for therapeutic approaches based on DUB inhibition.

Ubiquitination and deubiquitination of NP protein regulates influenza A virus RNA replication

Influenza A virus RNA replication requires an intricate regulatory network involving viral and cellular proteins. In this study, we examined the roles of cellular ubiquitinating/deubiquitinating enzymes (DUBs). We observed that downregulation of a cellular deubiquitinating enzyme USP11 resulted in enhanced virus production, suggesting that USP11 could inhibit influenza virus replication. Conversely, overexpression of USP11 specifically inhibited viral genomic RNA replication, and this inhibition required the deubiquitinase activity. Furthermore, we showed that USP11 interacted with PB2, PA, and NP of viral RNA replication complex, and that NP is a monoubiquitinated protein and can be deubiquitinated by USP11 in vivo. Finally, we identified K184 as the ubiquitination site on NP and this residue is crucial for virus RNA replication. We propose that ubiquitination/deubiquitination of NP can be manipulated for antiviral therapeutic purposes.

Ubiquitin-specific proteases 7 and 11 modulate Polycomb regulation of the INK4a tumour suppressor

An important facet of transcriptional repression by Polycomb repressive complex 1 (PRC1) is the mono-ubiquitination of histone H2A by the combined action of the Posterior sex combs (Psc) and Sex combs extra (Sce) proteins. Here, we report that two ubiquitin-specific proteases, USP7 and USP11, co-purify with human PRC1-type complexes through direct interactions with the Psc orthologues MEL18 and BMI1, and with other PRC1 components. Ablation of either USP7 or USP11 in primary human fibroblasts results in de-repression of the INK4a tumour suppressor accompanied by loss of PRC1 binding at the locus and a senescence-like proliferative arrest. Mechanistically, USP7 and USP11 regulate the ubiquitination status of the Psc and Sce proteins themselves, thereby affecting their turnover and abundance. Our results point to a novel function for USPs in the regulation and function of Polycomb complexes.

TRE17/USP6 oncogene translocated in aneurysmal bone cyst induces matrix metalloproteinase production via activation of NF-kappaB

Aneurysmal bone cyst (ABC) is an aggressive, pediatric bone tumor characterized by extensive destruction of the surrounding bone. Though first described over 60 years ago, its molecular etiology remains poorly understood. Recent work revealed that ABCs harbor translocation of TRE17/USP6, leading to its transcriptional upregulation. TRE17 encodes a ubiquitin-specific protease (USP), and a TBC domain that mediates binding to the Arf6 GTPase. However, the mechanisms by which TRE17 overexpression contributes to tumor pathogenesis, and the role of its USP and TBC domains are unknown. ABCs are characterized by osteolysis, inflammatory recruitment, and extensive vascularization, processes in which matrix proteases play a prominent role. This led us to explore whether TRE17 regulates the production of matrix metalloproteinases (MMPs). In the current study, we demonstrate that TRE17 is sufficient to induce expression of MMP-9 and MMP-10, in a manner requiring its USP activity, but not its ability to bind Arf6. TRE17 induces transcription of MMP-9 through activation of NFκB, mediated in part by the GTPase RhoA and its effector kinase, ROCK. Furthermore, xenograft studies demonstrate that TRE17 induces formation of tumors that reproduce multiple features of ABC, including a high degree of vascularization, with an essential role for the USP domain. In sum, these studies reveal that TRE17 is sufficient to initiate tumorigenesis, identify MMPs as novel TRE17 effectors that likely contribute to ABC pathogenesis, and define the underlying signaling mechanism of their induction.

The regulatory crosstalk between kinases and proteases in cancer

Kinases and proteases are responsible for two fundamental regulatory mechanisms--phosphorylation and proteolysis--that orchestrate the rhythms of life and death in all organisms. Recent studies have highlighted the elaborate interplay between both post-translational regulatory systems. Many intracellular or pericellular proteases are regulated by phosphorylation, whereas multiple kinases are activated or inactivated by proteolytic cleavage. The functional consequences of this regulatory crosstalk are especially relevant in the different stages of cancer progression. What are the clinical implications derived from the fertile dialogue between kinases and proteases in cancer?

A new map to understand deubiquitination

Deubiquitination is a crucial mechanism in ubiquitin-mediated signalling networks. The importance of Dubs (deubiquitinating enzymes) as regulators of diverse cellular processes is becoming ever clearer as new roles are elucidated and new pathways are shown to be affected by this mechanism. Recent work, reviewed in the present paper, provides new perspective on the widening influence of Dubs and a new tool to focus studies of not only Dub interactions, but also potentially many more cellular systems.

USP11 negatively regulates TNFalpha-induced NF-kappaB activation by targeting on IkappaBalpha

IkappaBalpha serves as a central anchoring molecule in the sequestration of NF-kappaB transcription factor in the cytoplasm. Ubiquitination-mediated IkappaBalpha degradation immediately precedes and is required for NF-kappaB nuclear translocation and activation. However, the precise mechanism for the deubiquitination of IkappaBalpha is still not fully understood. Using a proteomic approach, we have identified Ubiquitin Specific Peptidase 11 (USP11) as an IkappaBalpha associated deubiquitinase. Overexpression of USP11 inhibits IkappaBalpha ubiquitination. Recombinant USP11 catalyzes deubiquitination of IkappaBalpha in vitro. Moreover, knockdown of USP11 expression enhances TNFalpha-induced IkappaBalpha ubiquitination and NF-kappaB activation. These data demonstrate that USP11 plays an important role in the downregulation of TNFalpha-mediated NF-kappaB activation through modulating IkappaBalpha stability. In addition, overexpression of a catalytically inactive USP11 mutant partially inhibits TNFalpha- and IKKbeta-induced NF-kappaB activation, suggesting that USP11 also exerts a non-catalytic function in its negative regulation of TNFalpha-mediated NF-kappaB activation. Thus, IkappaBalpha ubiquitination and deubiquitination processes function as a Yin-Yang regulatory mechanism on TNFalpha-induced NF-kappaB activation.

ATM augments nuclear stabilization of DYRK2 by inhibiting MDM2 in the apoptotic response to DNA damage

The tumor suppressor p53 is a transcription factor that regulates cell cycle, DNA repair, senescence, and apoptosis in response to DNA damage. Phosphorylation of p53 at Ser-46 is indispensable for the commitment to apoptotic cell death. A previous study has shown that upon exposure to genotoxic stress, DYRK2 translocates into the nucleus and phosphorylates p53 at Ser-46, thereby inducing apoptosis. However, less is known about mechanisms responsible for intracellular control of DYRK2. Here we show the functional nuclear localization signal at N-terminal domain of DYRK2. Under normal conditions, nuclear and not cytoplasmic DYRK2 is ubiquitinated by MDM2, resulting in its constitutive degradation. In the presence of proteasome inhibitors, we detected a stable complex of DYRK2 with MDM2 at the nucleus. Upon exposure to genotoxic stress, ATM phosphorylates DYRK2 at Thr-33 and Ser-369, which enables DYRK2 to escape from degradation by dissociation from MDM2 and to induce the kinase activity toward p53 at Ser-46 in the nucleus. These findings indicate that ATM controls stability and pro-apoptotic function of DYRK2 in response to DNA damage.

Ubiquitin Family Members in the Regulation of the Tumor Suppressor p53

It is commonly assumed that the p53 tumor suppressor pathway is deregulated in most if not all human cancers. Thus, the past two decades have witnessed intense efforts to identify and characterize the growth-suppressive properties of p53 as well as the proteins and mechanisms involved in regulating p53 activity. In retrospect, it may therefore not be surprising that p53 was one of the very first mammalian proteins that were identified as physiologically relevant substrate proteins of the ubiquitin-proteasome system. Since then, plenty of evidence has been accumulated that p53 is in part controlled by canonical (i.e., resulting in proteasome-mediated degradation) and noncanonical (i.e., nonproteolytic) ubiquitination and by modification with the ubiquitin family members SUMO-1 and NED 8. In this chapter, we will largely neglect the plethora of mechanisms that have been reported to be involved in the regulation of p53 ubiquitination but will focus on the enzymes and components of the respective conjugation systems that have been implicated in p53 modification and how the respective modifications (ubiquitin, SUMO-1, NED 8) may impinge on p53 activity.

Pim-1 controls NF-kappaB signalling by stabilizing RelA/p65

Post-translational modification and degradation of proteins by the ubiquitin-proteasome system are key regulatory mechanisms in cellular responses to various stimuli. The NF-kappaB signaling pathway is controlled by the ubiquitin-mediated proteolysis. RelA/p65, which is a main subunit of NF-kappaB, is ubiquitinated for degradation by SOCS-1, but the functional mechanism of its ubiquitination remains poorly understood. In this study we show that phosphorylation of RelA/p65 at Ser276 prevents its degradation by ubiquitin-mediated proteolysis. In contrast, impairment of Ser276 phosphorylation affects constitutive degradation of RelA/p65. Importantly, we identify Pim-1 as a further kinase responsible for the phosphorylation of RelA/p65 at Ser276. Depletion of Pim-1 hinders not only Ser276 phosphorylation but also transactivation of RelA/p65 target genes. We also show that Pim-1 contributes to recruitment of RelA/p65 to kappaB-elements to activate NF-kappaB signalling after TNF-alpha stimulation. In concert with these results, the knockdown of Pim-1 impairs IL-6 production and augments apoptosis by interfering RelA/p65 activation. These findings provide a model in which Pim-1 phosphorylation of RelA/p65 at Ser276 allows defense against ubiquitin-mediated degradation and whereby exerts activation of NF-kappaB signalling.

DNA damage signalling recruits RREB-1 to the p53 tumour suppressor promoter

Transcriptional regulation of the p53 tumour suppressor gene plays an important role in the control of the expression of various target genes involved in the DNA damage response. However, the molecular basis of this regulation remains obscure. In the present study we demonstrate that RREB-1 (Ras-responsive-element-binding protein-1) efficiently binds to the p53 promoter via the p53 core promoter element and transactivates p53 expression. Silencing of RREB-1 significantly reduces p53 expression at both the mRNA and the protein levels. Notably, disruption of RREB-1-mediated p53 transcription suppresses the expression of the p53 target genes. We also show that, upon exposure to genotoxic stress, RREB-1 controls apoptosis in a p53-dependent manner. These findings provide evidence that RREB-1 participates in modulating p53 transcription in response to DNA damage.