HERC2-USP20 axis regulates DNA damage checkpoint through Claspin

Single-molecule characterization of SV40 replisome and novel factors: human FPC and Mcm10

The simian virus 40 (SV40) replisome only encodes for its helicase; large T-antigen (L-Tag), while relying on the host for the remaining proteins, making it an intriguing model system. Despite being one of the earliest reconstituted eukaryotic systems, the interactions coordinating its activities and the identification of new factors remain largely unexplored. Herein, we in vitro reconstituted the SV40 replisome activities at the single-molecule level, including DNA unwinding by L-Tag and the single-stranded DNA-binding protein Replication Protein A (RPA), primer extension by DNA polymerase δ, and their concerted leading-strand synthesis. We show that RPA stimulates the processivity of L-Tag without altering its rate and that DNA polymerase δ forms a stable complex with L-Tag during leading-strand synthesis. Furthermore, similar to human and budding yeast Cdc45-MCM-GINS helicase, L-Tag uses the fork protection complex (FPC) and the mini-chromosome maintenance protein 10 (Mcm10) during synthesis. Hereby, we demonstrate that FPC increases this rate, and both FPC and Mcm10 increase the processivity by stabilizing stalled replisomes and increasing their chances of restarting synthesis. The detailed kinetics and novel factors of the SV40 replisome establish it as a closer mimic of the host replisome and expand its application as a model replication system.

Autophagy dysregulation via the USP20-ULK1 axis in the HERC2-related neurodevelopmental disorder

Sequence variants in the HERC2 gene are associated with a significant reduction in HERC2 protein levels and cause a neurodevelopmental disorder known as the HERC2-related disorder, which shares clinical features with Angelman syndrome, including global developmental delay, intellectual disability, autism, and movement disorders. Remarkably, the HERC2 gene is commonly deleted in individuals with Angelman syndrome, suggesting a potential contribution of HERC2 to the pathophysiology of this disease. Given the known critical role of autophagy in brain development and its implication in neurodevelopmental diseases, we undertook different experimental approaches to monitor autophagy in fibroblasts derived from individuals affected by the HERC2-related disorder. Our findings reveal alterations in the levels of the autophagy-related protein LC3. Furthermore, experiments with lysosomal inhibitors provide confirmation of an upregulation of the autophagy pathway in these patient-derived cells. Mechanistically, we corroborate an interaction between HERC2 and the deubiquitylating enzyme USP20; and demonstrate that HERC2 deficiency leads to increased USP20 protein levels. Notably, USP20 upregulation correlates with enhanced stability of the autophagy initiating kinase ULK1, highlighting the role of HERC2 as an autophagy regulator factor through the USP20-ULK1 axis. Moreover, we show that p38 acts as a modulator of this pathway, since p38 activation disrupts HERC2-USP20 interaction, leading to increased USP20 and LC3-II protein levels. Together, these findings uncover a previously unknown role for HERC2 in autophagy regulation and provide insights into the pathomolecular mechanisms underlying the HERC2-related disorder and Angelman syndrome.

Ubiquitin-specific proteases: From biological functions to potential therapeutic applications in gastric cancer

Deubiquitination, a post-translational modification regulated by deubiquitinases, is essential for cancer initiation and progression. Ubiquitin-specific proteases (USPs) are essential elements of the deubiquitinase family, and are overexpressed in gastric cancer (GC). Through the regulation of several signaling pathways, such as Wnt/β-Catenin and nuclear factor-κB signaling, and the promotion of the expression of deubiquitination- and stabilization-associated proteins, USPs promote the proliferation, metastasis, invasion, and epithelial-mesenchymal transition of GC. In addition, the expression of USPs is closely related to clinicopathological features, patient prognosis, and chemotherapy resistance. USPs therefore could be used as prognostic biomarkers. USP targeting small molecule inhibitors have demonstrated strong anticancer activity. However, they have not yet been tested in the clinic. This article provides an overview of the latest fundamental research on USPs in GC, aiming to enhance the understanding of how USPs contribute to GC progression, and identifying possible targets for GC treatment to improve patient survival.

The Emerging Role of Deubiquitinases in Radiosensitivity

Radiation therapy is a primary treatment for cancer, but radioresistance remains a significant challenge in improving efficacy and reducing toxicity. Accumulating evidence suggests that deubiquitinases (DUBs) play a crucial role in regulating cell sensitivity to ionizing radiation. Traditional small-molecule DUB inhibitors have demonstrated radiosensitization effects, and novel deubiquitinase-targeting chimeras (DUBTACs) provide a promising strategy for radiosensitizer development by harnessing the ubiquitin-proteasome system. This review highlights the mechanisms by which DUBs regulate radiosensitivity, including DNA damage repair, the cell cycle, cell death, and hypoxia. Progress on DUB inhibitors and DUBTACs is summarized, and their potential radiosensitization effects are discussed. Developing drugs targeting DUBs appears to be a promising alternative approach to overcoming radioresistance, warranting further research into their mechanisms.

How CLSPN could demystify its prognostic value and potential molecular mechanism for hepatocellular carcinoma: A crosstalk study

BACKGROUND & AIMS

CLSPN, a critical component of the S-phase checkpoint in response to DNA replication stress, has been implicated in the pathogenesis of multiple tumor types. The rising incidence of hepatocellular carcinoma (HCC) poses a significant challenge to global public health. Despite this, the specific functions of CLSPN in the development of HCC remain poorly understood.

METHODS

We systematically evaluated the expression of CLSPN, prognosis and immune infiltration in patients with HCC and identified a competing endogenous RNA (ceRNA) network by using public database. The RT-qPCR, western blot, CCK8, transwell, flow cytometry, animal experiments, proteasome inhibition experiment, Co-IP assay and mass spectrometry were applied to explore its biological functions, post-transcriptional modifications and potential molecular mechanisms of CLSPN in HCC.

RESULTS

We verified the expression of CLSPN, and its high expression is an independent prognostic factor in HCC. The expression of CLSPN is also associated with the immune microenvironment of HCC. CLSPN silencing inhibited the proliferation, migration, invasion and cell cycle progression of HCC cells. We established a PSMA3-AS1/hsa-miR-101-3p/CLSPN regulator axis in HCC. CLSPN was influenced by ubiquitination and was involved in the Wnt/β-catenin pathway to regulate HCC progression.

CONCLUSIONS

It was the first time to comprehensively discover and identify the expression, prognosis, immunotherapy, RNAs regulator, posttranscriptional modification, and molecular mechanisms of CLSPN in HCC. These novel insights have the potential to expedite the development of personalized treatment strategies and translational medicine approaches for HCC patients.

Friend or foe? Reciprocal regulation between E3 ubiquitin ligases and deubiquitinases

Protein ubiquitination is a post-translational modification that entails the covalent attachment of the small protein ubiquitin (Ub), which acts as a signal to direct protein stability, localization, or interactions. The Ub code is written by a family of enzymes called E3 Ub ligases (∼600 members in humans), which can catalyze the transfer of either a single ubiquitin or the formation of a diverse array of polyubiquitin chains. This code can be edited or erased by a different set of enzymes termed deubiquitinases (DUBs; ∼100 members in humans). While enzymes from these distinct families have seemingly opposing activities, certain E3-DUB pairings can also synergize to regulate vital cellular processes like gene expression, autophagy, innate immunity, and cell proliferation. In this review, we highlight recent studies describing Ub ligase-DUB interactions and focus on their relationships.

USP50 suppresses alternative RecQ helicase use and deleterious DNA2 activity during replication

Mammalian DNA replication employs several RecQ DNA helicases to orchestrate the faithful duplication of genetic information. Helicase function is often coupled to the activity of specific nucleases, but how helicase and nuclease activities are co-directed is unclear. Here we identify the inactive ubiquitin-specific protease, USP50, as a ubiquitin-binding and chromatin-associated protein required for ongoing replication, fork restart, telomere maintenance and cellular survival during replicative stress. USP50 supports WRN:FEN1 at stalled replication forks, suppresses MUS81-dependent fork collapse and restricts double-strand DNA breaks at GC-rich sequences. Surprisingly we find that cells depleted for USP50 and recovering from a replication block exhibit increased DNA2 and RECQL4 foci and that the defects in ongoing replication, poor fork restart and increased fork collapse seen in these cells are mediated by DNA2, RECQL4 and RECQL5. These data define a novel ubiquitin-dependent pathway that promotes the balance of helicase: nuclease use at ongoing and stalled replication forks.

ATR-dependent ubiquitin-specific protease 20 phosphorylation confers oxaliplatin and ferroptosis resistance

Oxaliplatin (OXA) resistance is a major clinic challenge in hepatocellular carcinoma (HCC). Ferroptosis is a kind of iron-dependent cell death. Triggering ferroptosis is considered to restore sensitivity to chemotherapy. In the present study, we found that USP20 was overexpressed in OXA-resistant HCC cells. High expression of USP20 in HCC was associated with poor prognosis. USP20 contributes OXA resistance and suppress ferroptosis in HCC. Pharmacological inhibition or knockdown of USP20 triggered ferroptosis and increased the sensitivity of HCC cells to OXA both in vitro and in vivo. Coimmunoprecipitation results revealed that the UCH domain of USP20 interacted with the N terminal of SLC7A11. USP20 stabilized SLC7A11 via removing K48-linked polyubiquitination of SLC7A11 protein at K30 and K37. Most importantly, DNA damage-induced ATR activation was required for Ser132 and Ser368 phosphorylation of USP20. USP20 phosphorylation at Ser132 and Ser368 enhanced its stability and thus conferred OXA and ferroptosis resistance of HCC cells. Our study reveals a previously undiscovered association between OXA and ferroptosis and provides new insight into mechanisms regarding how DNA damage therapies always lead to therapeutic resistance. Therefore, targeting USP20 may mitigate the development of drug resistance and promote ferroptosis of HCC in patients receiving chemotherapy.

Phosphorylation of USP20 on Ser334 by IRAK1 promotes IL-1β-evoked signaling in vascular smooth muscle cells and vascular inflammation

Reversible lysine-63 (K63) polyubiquitination regulates proinflammatory signaling in vascular smooth muscle cells (SMCs) and plays an integral role in atherosclerosis. Ubiquitin-specific peptidase 20 (USP20) reduces NFκB activation triggered by proinflammatory stimuli, and USP20 activity attenuates atherosclerosis in mice. The association of USP20 with its substrates triggers deubiquitinase activity; this association is regulated by phosphorylation of USP20 on Ser334 (mouse) or Ser333 (human). USP20 Ser333 phosphorylation was greater in SMCs of atherosclerotic segments of human arteries as compared with nonatherosclerotic segments. To determine whether USP20 Ser334 phosphorylation regulates proinflammatory signaling, we created USP20-S334A mice using CRISPR/Cas9-mediated gene editing. USP20-S334A mice developed ∼50% less neointimal hyperplasia than congenic WT mice after carotid endothelial denudation. WT carotid SMCs showed substantial phosphorylation of USP20 Ser334, and WT carotids demonstrated greater NFκB activation, VCAM-1 expression, and SMC proliferation than USP20-S334A carotids. Concordantly, USP20-S334A primary SMCs in vitro proliferated and migrated less than WT SMCs in response to IL-1β. An active site ubiquitin probe bound to USP20-S334A and USP20-WT equivalently, but USP20-S334A associated more avidly with TRAF6 than USP20-WT. IL-1β induced less K63-linked polyubiquitination of TRAF6 and less downstream NFκB activity in USP20-S334A than in WT SMCs. Using in vitro phosphorylation with purified IRAK1 and siRNA-mediated gene silencing of IRAK1 in SMCs, we identified IRAK1 as a novel kinase for IL-1β-induced USP20 Ser334 phosphorylation. Our findings reveal novel mechanisms regulating IL-1β-induced proinflammatory signaling: by phosphorylating USP20 Ser334, IRAK1 diminishes the association of USP20 with TRAF6 and thus augments NFκB activation, SMC inflammation, and neointimal hyperplasia.

Population structure, genetic diversity and prolificacy in pishan red sheep under an extreme desert environment

Extreme environmental conditions are a major challenge for livestock production. Changes in climate conditions, especially those that lead to extreme weather, can reduce livestock production. The screening of genes and molecular markers is of great significance to explore the genetic mechanism of sheep prolificacy traits in Taklimakan Desert environment. We selected healthy adult Pishan Red Sheep (PRS) and Qira Black Sheep (QR) which live in Taklimakan Desert environment, collected blood from jugular vein, extracted DNA, and prepared Illumina Ovine SNP50 chip. For PRS, linkage disequilibrium (LD) was calculated using the ovine SNP50 Beadchip and the effective population size (Ne) was estimated using SMC++. The genetic characteristics of PRS were analyzed by integrated haplotype score (iHS) and fixation index (F _ST ). The result showed that r ² of PRS was 0.233 ± 0.280 in the range of 0-10 Kb and decreased with increasing distances. SMC++ tested that the Ne of PRS remained at 236.99 in recent generations. 184 genes were screened out under iHS 1% threshold, and 1148 genes were screened out with F _ST under the 5% threshold, and 29 genes were obtained from the intersection of the two gene sets. In this study, the genetic characteristics of PRS and QR were compared by ovine genome chip, and the related excellent genes were searched, providing reference for the protection of sheep germplasm resources and molecular breeding in a desert environment.

The intricate interplay between HIFs, ROS, and the ubiquitin system in the tumor hypoxic microenvironment

Alterations in protein ubiquitination and hypoxia-inducible factor (HIF) signaling both contribute to tumorigenesis and tumor progression. Ubiquitination is a dynamic process that is coordinately regulated by E3 ligases and deubiquitinases (DUBs), which have emerged as attractive therapeutic targets. HIF expression and transcriptional activity are usually increased in tumors, leading to poor clinical outcomes. Reactive oxygen species (ROS) are upregulated in tumors and have multiple effects on HIF signaling and the ubiquitin system. A growing body of evidence has shown that multiple E3 ligases and UBDs function synergistically to control the expression and activity of HIF, thereby allowing cancer cells to cope with the hypoxic microenvironment. Conversely, several E3 ligases and DUBs are regulated by hypoxia and/or HIF signaling. Hypoxia also induces ROS production, which in turn modulates the stability or activity of HIF, E3 ligases, and DUBs. Understanding the complex networks between E3 ligase, DUBs, ROS, and HIF will provide insights into the fundamental mechanism of the cellular response to hypoxia and help identify novel molecular targets for cancer treatment. We review the current knowledge on the comprehensive relationship between E3 ligase, DUBs, ROS, and HIF signaling, with a particular focus on the use of E3 ligase or DUB inhibitors in cancer.

HERC2 deficiency activates C-RAF/MKK3/p38 signalling pathway altering the cellular response to oxidative stress

HERC2 gene encodes an E3 ubiquitin ligase involved in several cellular processes by regulating the ubiquitylation of different protein substrates. Biallelic pathogenic sequence variants in the HERC2 gene are associated with HERC2 Angelman-like syndrome. In pathogenic HERC2 variants, complete absence or marked reduction in HERC2 protein levels are observed. The most common pathological variant, c.1781C > T (p.Pro594Leu), encodes an unstable HERC2 protein. A better understanding of how pathologic HERC2 variants affect intracellular signalling may aid definition of potential new therapies for these disorders. For this purpose, we studied patient-derived cells with the HERC2 Pro594Leu variant. We observed alteration of mitogen-activated protein kinase signalling pathways, reflected by increased levels of C-RAF protein and p38 phosphorylation. HERC2 knockdown experiments reproduced the same effects in other human and mouse cells. Moreover, we demonstrated that HERC2 and RAF proteins form molecular complexes, pull-down and proteomic experiments showed that HERC2 regulates C-RAF ubiquitylation and we found out that the p38 activation due to HERC2 depletion occurs in a RAF/MKK3-dependent manner. The displayed cellular response was that patient-derived and other human cells with HERC2 deficiency showed higher resistance to oxidative stress with an increase in the master regulator of the antioxidant response NRF2 and its target genes. This resistance was independent of p53 and abolished by RAF or p38 inhibitors. Altogether, these findings identify the activation of C-RAF/MKK3/p38 signalling pathway in HERC2 Angelman-like syndrome and highlight the inhibition of RAF activity as a potential therapeutic option for individuals affected with these rare diseases.

Regulation of ATR-CHK1 signaling by ubiquitination of CLASPIN

DNA replication forks are frequently forced into stalling by persistent DNA aberrations generated from endogenous or exogenous insults. Stalled replication forks are catastrophic for genome integrity and cell survival if not immediately stabilized. The ataxia-telangiectasia and RAD3-related kinase (ATR)-CLASPIN-checkpoint kinase 1 (CHK1) signaling cascade is a pivotal mechanism that initiates cell-cycle checkpoints and stabilizes stalled replication forks, assuring the faithful duplication of genomic information before entry into mitosis. The timely recovery of checkpoints after stressors are resolved is also crucial for normal cell proliferation. The precise activation and inactivation of ATR-CHK1 signaling are usually efficiently regulated by turnover and the cellular re-localization of the adaptor protein CLASPIN. The ubiquitination-proteasome-mediated degradation of CLASPIN, driven by APC/CCDH1 and SCFβTrCP, results in a cell-cycle-dependent fluctuation pattern of CLASPIN levels, with peak levels seen in S/G2 phase when it functions in the DNA replisome or as an adaptor protein in ATR-CHK1 signaling under replication stress. Deubiquitination mediated by a series of ubiquitin-specific protease family proteins releases CLASPIN from proteasome-dependent destruction and activates the ATR-CHK1 checkpoint to overcome replication stress. Moreover, the non-proteolytic ubiquitination of CLASPIN also affects CHK1 activation by regulating CLASPIN localization. In this review, we discuss the functions of CLASPIN ubiquitination with specific linkage types in the regulation of the ATR-CHK1 signaling pathway. Research in this area is progressing at pace and provides promising chemotherapeutic targets.

USP17L2-SIRT7 axis regulates DNA damage repair and chemoresistance in breast cancer cells

PURPOSE

Sirtuin7 (SIRT7), as a member of the sirtuin and NAD⁺-dependent protein-modifying enzyme family, plays an important role in regulating cellular metabolism, stress responses, tumorigenesis, and aging. Ubiquitination and deubiquitination are reversible post-translational modifications that regulate protein stability, enzyme activity, protein-protein interactions, and cellular signaling transduction. However, whether SIRT7 is regulated by deubiquitination signaling is unclear. This study aims to elucidate the molecular mechanism of SIRT7 via deubiquitination signaling.

METHODS

USP17L2 or SIRT7-targeting shRNAs were used to deplete USP17L2 or SIRT7. Western blot was applied to assess the effects of USP17L2 or SIRT7 depletion. A co-immunoprecipitation assay was used to detect the interaction relationship. Cell Counting Kit-8 assays were applied to assess the viability of breast cancer cells. An immunohistochemistry assay was employed to detect the protein level in samples from breast cancer patients, and the TCGA database was applied to analyze the survival rate of breast cancer patients. Statistical analyses were performed with the Student's t test (two-tailed unpaired) and χ² test.

RESULTS

We find that the deubiquitinase USP17L2 interacts with and deubiquitinates SIRT7, thereby increasing SIRT7 protein stability. In addition, USP17L2 regulates DNA damage repair through SIRT7. Furthermore, SIRT7 polyubiquitination is increased by knocking down of USP17L2, which leads to cancer cells sensitizing to chemotherapy. In breast cancer patient samples, high expression of USP17L2 is correlated with increased levels of SIRT7 protein. In conclusion, our study demonstrates that the USP17L2-SIRT7 axis is the new regulator in DNA damage response and chemo-response, suggesting that USP17L2 may be a prognostic factor and a potential therapeutic target in breast cancer.

CONCLUSION

Our results highlighted that USP17L2 regulates the chemoresistance of breast cancer cells in a SIRT7-dependent manner. Moreover, the role of USP17L2 as a potential therapeutic target in breast cancer and a prognostic factor for patients was elucidated.

TRIM21 suppresses CHK1 activation by preferentially targeting CLASPIN for K63-linked ubiquitination

Expression of the E3 ligase TRIM21 is increased in a broad spectrum of cancers; however, the functionally relevant molecular pathway targeted by TRIM21 overexpression remains largely unknown. Here, we show that TRIM21 directly interacts with and ubiquitinates CLASPIN, a mediator for ATR-dependent CHK1 activation. TRIM21-mediated K63-linked ubiquitination of CLASPIN counteracts the K6-linked ubiquitination of CLASPIN which is essential for its interaction with TIPIN and subsequent chromatin loading. We further show that overexpression of TRIM21, but not a TRIM21 catalytically inactive mutant, compromises CHK1 activation, leading to replication fork instability and tumorigenesis. Our findings demonstrate that TRIM21 suppresses CHK1 activation by preferentially targeting CLASPIN for K63-linked ubiquitination, providing a potential target for cancer therapy.

Sleeve gastrectomy improves lipid dysmetabolism by downregulating the USP20-HSPA2 axis in diet-induced obese mice

INTRODUCTION

Obesity is a metabolic disease accompanied by abnormalities in lipid metabolism that can cause hyperlipidemia, non-alcoholic fatty liver disease, and artery atherosclerosis. Sleeve gastrectomy (SG) is a type of bariatric surgery that can effectively treat obesity and improve lipid metabolism. However, its specific underlying mechanism remains elusive.

METHODS

We performed SG, and sham surgery on two groups of diet-induced obese mice. Histology and lipid analysis were used to evaluate operation effect. Immunohistochemistry, immunoblotting, real-time quantitative PCR, immunoprecipitation, immunofluorescence and mass spectrometry were used to reveal the potential mechanisms of SG.

RESULTS

Compared to the sham group, the SG group displayed a downregulation of deubiquitinase ubiquitin-specific peptidase 20 (USP20). Moreover, USP20 could promote lipid accumulation in vitro. Co-immunoprecipitation and mass spectrometry analyses showed that heat-shock protein family A member 2 (HSPA2) potentially acts as a substrate of USP20. HSPA2 was also downregulated in the SG group and could promote lipid accumulation in vitro. Further research showed that USP20 targeted and stabilized HSPA2 via the ubiquitin-proteasome pathway.

CONCLUSION

The downregulation of the USP20-HSPA2 axis in diet-induced obese mice following SG improved lipid dysmetabolism, indicating that USP20-HSPA2 axis was a noninvasive therapeutic target to be investigated in the future.

USP20 mitigates ischemic stroke in mice by suppressing neuroinflammation and neuron death via regulating PTEN signal

Ischemic stroke is a leading cause of death worldwide. The lack of effective pharmacotherapies for ischemic stroke is mainly attributed to the incomplete understanding of its pathogenesis. Deubiquitinase ubiquitin-specific protease 20 (USP20) plays an important role in regulating multiple cellular processes. However, its effects on cerebral ischemic stroke still remain unknown. In the present study, we found that USP20 expression was markedly increased in the early phase of ischemic stroke in mice with middle cerebral artery occlusion (MCAO) operation, and were then considerably decreased in mice with ischemia reperfusion (I/R) injury. Double immunofluorescence staining showed USP20 abundance in both microglial cells and neurons. We then found that promoting USP20 expression remarkably ameliorated MCAO-induced ischemic brain injury, along with significantly reduced infarct volume, neurological scores and brain water contents. In addition, cognitive impairments in MCAO-operated mice were considerably alleviated by USP20 over-expression. Furthermore, USP20 over-expression dramatically restrained microglial activation, inflammatory response and neuronal death in mice with ischemic stroke. Moreover, our results indicated that phosphatase and tensin homolog (PTEN) expression was highly decreased in the infarct areas of MCAO-treated mice, while being greatly rescued by USP20 over-expression. All these effects mediated by USP20 during cerebral I/R injury were confirmed in the cultured primary microglial cells and cortical neurons stimulated by oxygen-glucose deprivation and reoxygenation (OGD/R). Mechanistically, we found that USP20 directly interacted with PTEN. Notably, suppressing PTEN with its specific inhibitor dramatically abolished the function of USP20 to ameliorate neuroinflammation and neuron death induced by OGD/R. Collectively, our results illustrated that USP20 could effectively mitigate the severity of cerebral ischemic stroke and improve behavior deficits in MCAO-operated mice, and identified the USP20/PTEN axis as a promising therapeutic target for ischemic stroke treatment.

Damsel in distress calling on her knights: Illuminating the pioneering role of E3 ubiquitin ligases in guarding the genome integrity

The maintenance of genomic integrity is of utmost importance for the organisms to survive and to accurately inherit traits to their progenies. Any kind of DNA damage either due to defect in DNA duplication and/ or uncontrolled cell division or intracellular insults or environment radiation can result in gene mutation, chromosomal aberration and ultimately genomic instability, which may cause several diseases including cancers. Therefore, cells have evolved machineries for the surveillance of genomic integrity. Enormous exciting studies in the past indicate that ubiquitination (a posttranslational modification of proteins) plays a crucial role in maintaining the genomic integrity by diverse ways. In fact, various E3 ubiquitin ligases catalyse ubiquitination of key proteins to control their central role during cell cycle, DNA damage response (DDR) and DNA repair. Some E3 ligases promote genomic instability while others prevent it, deregulation of both of which leads to several malignancies. In this review, we consolidate the recent findings wherein the role of ubiquitination in conferring genome integrity is highlighted. We also discuss the latest discoveries on the mechanisms utilized by various E3 ligases to preserve genomic stability, with a focus on their actions during cell cycle progression and different types of DNA damage response as well as repair pathways.

The deubiquitinating enzyme USP37 enhances CHK1 activity to promote the cellular response to replication stress

The deubiquitinating enzyme USP37 is known to contribute to timely onset of S phase and progression of mitosis. However, it is not clear if USP37 is required beyond S-phase entry despite expression and activity of USP37 peaking within S phase. We have utilized flow cytometry and microscopy to analyze populations of replicating cells labeled with thymidine analogs and monitored mitotic entry in synchronized cells to determine that USP37-depleted cells exhibited altered S-phase kinetics. Further analysis revealed that cells depleted of USP37 harbored increased levels of the replication stress and DNA damage markers γH2AX and 53BP1 in response to perturbed replication. Depletion of USP37 also reduced cellular proliferation and led to increased sensitivity to agents that induce replication stress. Underlying the increased sensitivity, we found that the checkpoint kinase 1 is destabilized in the absence of USP37, attenuating its function. We further demonstrated that USP37 deubiquitinates checkpoint kinase 1, promoting its stability. Together, our results establish that USP37 is required beyond S-phase entry to promote the efficiency and fidelity of replication. These data further define the role of USP37 in the regulation of cell proliferation and contribute to an evolving understanding of USP37 as a multifaceted regulator of genome stability.

Circadian Rhythm of NER and ATR Pathways

Genomic integrity is constantly insulted by solar ultraviolet (UV) radiation. Adaptative cellular mechanisms called DNA damage responses comprising DNA repair, cell cycle checkpoint, and apoptosis, are believed to be evolved to limit genomic instability according to the photoperiod during a day. As seen in many other key cellular metabolisms, genome surveillance mechanisms against genotoxic UV radiation are under the control of circadian clock systems, thereby exhibiting daily oscillations in their catalytic activities. Indeed, it has been demonstrated that nucleotide excision repair (NER), the sole DNA repair mechanism correcting UV-induced DNA photolesions, and ataxia–telangiectasia-mutated and Rad3-related (ATR)-mediated cell cycle checkpoint kinase are subjected to the robust control of the circadian clock. The molecular foundation for the circadian rhythm of UV-induced DNA damage responses in mammalian cells will be discussed.

Exploring the Roles of HERC2 and the NEDD4L HECT E3 Ubiquitin Ligase Subfamily in p53 Signaling and the DNA Damage Response

Cellular homeostasis is governed by the precise expression of genes that control the translation, localization, and termination of proteins. Oftentimes, environmental and biological factors can introduce mutations into the genetic framework of cells during their growth and division, and these genetic abnormalities can result in malignant transformations caused by protein malfunction. For example, p53 is a prominent tumor suppressor protein that is capable of undergoing more than 300 posttranslational modifications (PTMs) and is involved with controlling apoptotic signaling, transcription, and the DNA damage response (DDR). In this review, we focus on the molecular mechanisms and interactions that occur between p53, the HECT E3 ubiquitin ligases WWP1, SMURF1, HECW1 and HERC2, and other oncogenic proteins in the cell to explore how irregular HECT-p53 interactions can induce tumorigenesis.

Role of deubiquitinating enzymes in DNA double-strand break repair

DNA is the hereditary material in humans and almost all other organisms. It is essential for maintaining accurate transmission of genetic information. In the life cycle, DNA replication, cell division, or genome damage, including that caused by endogenous and exogenous agents, may cause DNA aberrations. Of all forms of DNA damage, DNA double-strand breaks (DSBs) are the most serious. If the repair function is defective, DNA damage may cause gene mutation, genome instability, and cell chromosome loss, which in turn can even lead to tumorigenesis. DNA damage can be repaired through multiple mechanisms. Homologous recombination (HR) and non-homologous end joining (NHEJ) are the two main repair mechanisms for DNA DSBs. Increasing amounts of evidence reveal that protein modifications play an essential role in DNA damage repair. Protein deubiquitination is a vital post-translational modification which removes ubiquitin molecules or polyubiquitinated chains from substrates in order to reverse the ubiquitination reaction. This review discusses the role of deubiquitinating enzymes (DUBs) in repairing DNA DSBs. Exploring the molecular mechanisms of DUB regulation in DSB repair will provide new insights to combat human diseases and develop novel therapeutic approaches.

HERC Ubiquitin Ligases in Cancer

HERC proteins are ubiquitin E3 ligases of the HECT family. The HERC subfamily is composed of six members classified by size into large (HERC1 and HERC2) and small (HERC3-HERC6). HERC family ubiquitin ligases regulate important cellular processes, such as neurodevelopment, DNA damage response, cell proliferation, cell migration, and immune responses. Accumulating evidence also shows that this family plays critical roles in cancer. In this review, we provide an integrated view of the role of these ligases in cancer, highlighting their bivalent functions as either oncogenes or tumor suppressors, depending on the tumor type. We include a discussion of both the molecular mechanisms involved and the potential therapeutic strategies.

Identification of long non-coding RNA HERC2P2 as a tumor suppressor in glioma

Long non-coding RNAs (lncRNAs) have been reported to play important roles in glioma; however, most of them promote glioma progression. We constructed a competing endogenous (ceRNA) network based on the Chinese Glioma Genome Atlas dataset, and lncRNA hect domain and RLD 2 pseudogene 2 (HERC2P2) is the core of this network. Highly connected genes in the ceRNA network classified the glioma patients into three clusters with significantly different survival rates. The expression of HERC2P2 is positively correlated with survival and negatively correlated with clinical grade. Cell colony formation, Transwell and cell scratch tests were performed to evaluate the role of HERC2P2 in glioblastoma growth. Furthermore, we overexpressed HERC2P2 in U87 cells and established a mouse intracranial glioma model to examine the function of HERC2P2 in vivo. In conclusion, we identified a lncRNA with tumor suppressor functions in glioma that could be a potential biomarker for glioma patients.

The Deubiquitinating Enzyme USP20 Regulates the TNFα-Induced NF-κB Signaling Pathway through Stabilization of p62

p62/sequestosome-1 is a scaffolding protein involved in diverse cellular processes such as autophagy, oxidative stress, cell survival and death. It has been identified to interact with atypical protein kinase Cs (aPKCs), linking these kinases to NF-κB activation by tumor necrosis factor α (TNFα). The diverse functions of p62 are regulated through post-translational modifications of several domains within p62. Among the enzymes that mediate these post-translational modifications, little is known about the deubiquitinating enzymes (DUBs) that remove ubiquitin chains from p62, compared to the E3 ligases involved in p62 ubiquitination. In this study, we first demonstrate a role of ubiquitin-specific protease USP20 in regulating p62 stability in TNFα-mediated NF-κB activation. USP20 specifically binds to p62 and acts as a positive regulator for NF-κB activation by TNFα through deubiquitinating lysine 48 (K48)-linked polyubiquitination, eventually contributing to cell survival. Furthermore, depletion of USP20 disrupts formation of the atypical PKCζ-RIPK1-p62 complex required for TNFα-mediated NF-κB activation and significantly increases the apoptosis induced by TNFα plus cycloheximide or TNFα plus TAK1 inhibitor. These findings strongly suggest that the USP20-p62 axis plays an essential role in NF-κB-mediated cell survival induced by the TNFα-atypical PKCζ signaling pathway.

Role of Deubiquitinases in Human Cancers: Potential Targeted Therapy

Deubiquitinases (DUBs) are involved in various cellular functions. They deconjugate ubiquitin (UBQ) from ubiquitylated substrates to regulate their activity and stability. Studies on the roles of deubiquitylation have been conducted in various cancers to identify the carcinogenic roles of DUBs. In this review, we evaluate the biological roles of DUBs in cancer, including proliferation, cell cycle control, apoptosis, the DNA damage response, tumor suppression, oncogenesis, and metastasis. This review mainly focuses on the regulation of different downstream effectors and pathways via biochemical regulation and posttranslational modifications. We summarize the relationship between DUBs and human cancers and discuss the potential of DUBs as therapeutic targets for cancer treatment. This review also provides basic knowledge of DUBs in the development of cancers and highlights the importance of DUBs in cancer biology.

Posttranscriptional control of the replication stress response via TTP-mediated Claspin mRNA stabilization

ATR and CHK1 play key roles in the protection and recovery of the stalled replication forks. Claspin, an adaptor for CHK1 activation, is essential for DNA damage signaling and efficient replication fork progression. Here, we show that tristetraprolin (TTP), an mRNA-binding protein, can modulate the replication stress response via stabilization of Claspin mRNA. TTP depletion compromised specifically in the phosphorylation of CHK1, but not p53 or H2AX among other ATR substrates, and produced CHK1-defective replication phenotypes including accumulation of stalled replication forks. Importantly, the expression of siRNA-resistant TTP in TTP-deficient cells restored CHK1 phosphorylation and reduced the number of stalled replication forks as close to the control cells. Besides, we found that TTP was required for efficient replication fork progression even in the absence of exogenous DNA damage in a Claspin-dependent manner. Mechanistically, TTP was able to bind to the 3'-untranslated region of Claspin mRNA to increase the stability of Claspin mRNA which eventually contributed to the subsequent ATR-CHK1 activation upon DNA damage. Taken together, our results revealed an intimate link between TTP-dependent Claspin mRNA stability and ATR-CHK1-dependent replication fork stability to maintain replication fork integrity and chromosomal stability.

The role of the redox/miR-6855-3p/PRDX5A axis in reversing SLUG-mediated BRCA2 silencing in breast cancer cells

BACKGROUND

We have previously shown that the zinc finger transcription repressor SNAI2 (SLUG) represses tumor suppressor BRCA2-expression in non-dividing cells by binding to the E2-box upstream of the transcription start site. However, it is unclear how proliferating breast cancer (BC) cells that has higher oxidation state, overcome this repression. In this study, we provide insight into the mechanism of de-silencing of BRCA2 gene expression by PRDX5A, which is the longest member of the peroxiredoxin5 family, in proliferating breast cancer cells.

METHODS

We used cell synchronization and DNA affinity pulldown to analyze PRDX5A binding to the BRCA2 silencer. We used oxidative stress and microRNA (miRNA) treatments to study nuclear localization of PRDX5A and its impact on BRCA2-expression. We validated our findings using mutational, reporter assay, and immunofluorescence analyses.

RESULTS

Under oxidative stress, proliferating BC cells express PRDX5 isoform A (PRDX5A). In the nucleus, PRDX5A binds to the BRCA2 silencer near the E2-box, displacing SLUG and enhancing BRCA2-expression. Nuclear PRDX5A is translated from the second AUG codon in frame to the first AUG codon in the PRDX5A transcript that retains all exons. Mutation of the first AUG increases nuclear localization of PRDX5A in MDA-MB-231 cells, but mutation of the second AUG decreases it. Increased mitronic hsa-miRNA-6855-3p levels under oxidative stress renders translation from the second AUG preferable. Mutational analysis using reporter assay uncovered a miR-6855-3p binding site between the first and second AUG codon in the PRDX5A transcript. miR-6855-3p mimic increases accumulation of nuclear PRDX5A and inhibits reporter gene translation.

CONCLUSION

Oxidative stress increases miR-6855-3p expression and binding to the inter-AUG sequence of the PRDX5A transcript, promoting translation of nuclear PRDX5A. Nuclear PRDX5A relieves SLUG-mediated BRCA2 silencing, resulting in increased BRCA2-expression.

Beyond reversal: ubiquitin and ubiquitin-like proteases and the orchestration of the DNA double strand break repair response

The cellular response to genotoxic DNA double strand breaks (DSBs) uses a multitude of post-translational modifications to localise, modulate and ultimately clear DNA repair factors in a timely and accurate manner. Ubiquitination is well established as vital to the DSB response, with a carefully co-ordinated pathway of histone ubiquitination events being a central component of DSB signalling. Other ubiquitin-like modifiers (Ubl) including SUMO and NEDD8 have since been identified as playing important roles in DSB repair. In the last five years ∼20 additional Ub/Ubl proteases have been implicated in the DSB response. The number of proteases identified highlights the complexity of the Ub/Ubl signal present at DSBs. Ub/Ubl proteases regulate turnover, activity and protein-protein interactions of DSB repair factors both catalytically and non-catalytically. This not only ensures efficient repair of breaks but has a role in channelling repair into the correct DSB repair sub-pathways. Ultimately Ub/Ubl proteases have essential roles in maintaining genomic stability. Given that deficiencies in many Ub/Ubl proteases promotes sensitivity to DNA damaging chemotherapies, they could be attractive targets for cancer treatment.

DUBs, Hypoxia, and Cancer

Alterations in protein ubiquitylation and hypoxia are commonly associated with cancer. Ubiquitylation is carried out by three sequentially acting ubiquitylating enzymes and can be opposed by deubiquitinases (DUBs), which have emerged as promising drug targets. Apart from protein localization and activity, ubiquitylation regulates degradation of proteins, among them hypoxia-inducible factors (HIFs). Thereby, various E3 ubiquitin ligases and DUBs regulate HIF abundance. Conversely, several E3s and DUBs are regulated by hypoxia. While hypoxia is a powerful HIF regulator, less is known about hypoxia-regulated DUBs and their impact on HIFs. Here, we review current knowledge about the relationship of E3s, DUBs, and hypoxia signaling. We also discuss the reciprocal regulation of DUBs by hypoxia and use of DUB-specific drugs in cancer.

HERC2 regulates RPA2 by mediating ATR-induced Ser33 phosphorylation and ubiquitin-dependent degradation

Replication protein A (RPA) binds to and stabilizes single-stranded DNA and is essential for the genome stability. We reported that an E3 ubiquitin ligase, HERC2, suppresses G-quadruplex (G4) DNA by regulating RPA-helicase complexes. However, the precise mechanism of HERC2 on RPA is as yet largely unknown. Here, we show essential roles for HERC2 on RPA2 status: induction of phosphorylation and degradation of the modified form. HERC2 interacted with RPA through the C-terminal HECT domain. Ubiquitination of RPA2 was inhibited by HERC2 depletion and rescued by reintroduction of the C-terminal fragment of HERC2. ATR-mediated phosphorylation of RPA2 at Ser33 induced by low-level replication stress was inhibited by depletion of HERC2. Contrary, cells lacking HERC2 catalytic residues constitutively expressed an increased level of Ser33-phosphorylated RPA2. HERC2-mediated ubiquitination of RPA2 was abolished by an ATR inhibitor, supporting a hypothesis that the ubiquitinated RPA2 is a phosphorylated subset. Functionally, HERC2 E3 activity has an epistatic relationship with RPA in the suppression of G4 when judged with siRNA knockdown experiments. Together, these results suggest that HERC2 fine-tunes ATR-phosphorylated RPA2 levels through induction and degradation, a mechanism that could be critical for the suppression of secondary DNA structures during cell proliferation.

HERCing: Structural and Functional Relevance of the Large HERC Ubiquitin Ligases

Homologous to the E6AP carboxyl terminus (HECT) and regulator of chromosome condensation 1 (RCC1)-like domain-containing proteins (HERCs) belong to the superfamily of ubiquitin ligases. HERC proteins are divided into two subfamilies, Large and Small HERCs. Despite their similarities in terms of both structure and domains, these subfamilies are evolutionarily very distant and result from a convergence phenomenon rather than from a common origin. Large HERC genes, HERC1 and HERC2, are present in most metazoan taxa. They encode very large proteins (approximately 5,000 amino acid residues in a single polypeptide chain) that contain more than one RCC1-like domain as a structural characteristic. Accumulating evidences show that these unusually large proteins play key roles in a wide range of cellular functions which include neurodevelopment, DNA damage repair, and cell proliferation. To better understand the origin, evolution, and function of the Large HERC family, this minireview provides with an integrated overview of their structure and function and details their physiological implications. This study also highlights and discusses how dysregulation of these proteins is associated with severe human diseases such as neurological disorders and cancer.

GSK3-β Stimulates Claspin Degradation via β-TrCP Ubiquitin Ligase and Alters Cancer Cell Survival

Claspin is essential for activating the DNA damage checkpoint effector kinase Chk1, a target in oncotherapy. Claspin functions are tightly correlated to Claspin protein stability, regulated by ubiquitin-dependent proteasomal degradation. Here we identify Glycogen Synthase Kinase 3-β (GSK3-β) as a new regulator of Claspin stability. Interestingly, as Chk1, GSK3-β is a therapeutic target in cancer. GSK3-β inhibition or knockdown stabilizes Claspin, whereas a GSK3-β constitutively active form reduces Claspin protein levels by ubiquitination and proteasome-mediated degradation. Our results also suggest that GSK3-β modulates the interaction of Claspin with β-TrCP, a critical E3 ubiquitin ligase that regulates Claspin stability. Importantly, GSK3-β knock down increases Chk1 activation in response to DNA damage in a Claspin-dependent manner. Therefore, Chk1 activation could be a pro-survival mechanism that becomes activated upon GSK3-β inhibition. Importantly, treating triple negative breast cancer cell lines with Chk1 or GSK3-β inhibitors alone or in combination, demonstrates that Chk1/GSK3-β double inhibition restrains cell growth and triggers more apoptosis compared to individual treatments, thereby revealing novel possibilities for a combination therapy for cancer.

Involvement of E3 Ligases and Deubiquitinases in the Control of HIF-α Subunit Abundance

The ubiquitin and hypoxia-inducible factor (HIF) pathways are cellular processes involved in the regulation of a variety of cellular functions. Enzymes called ubiquitin E3 ligases perform protein ubiquitylation. The action of these enzymes can be counteracted by another group of enzymes called deubiquitinases (DUBs), which remove ubiquitin from target proteins. The balanced action of these enzymes allows cells to adapt their protein content to a variety of cellular and environmental stress factors, including hypoxia. While hypoxia appears to be a powerful regulator of the ubiquitylation process, much less is known about the impact of DUBs on the HIF system and hypoxia-regulated DUBs. Moreover, hypoxia and DUBs play crucial roles in many diseases, such as cancer. Hence, DUBs are considered to be promising targets for cancer cell-specific treatment. Here, we review the current knowledge about the role DUBs play in the control of HIFs, the regulation of DUBs by hypoxia, and their implication in cancer progression.

USP20 Promotes Cellular Antiviral Responses via Deconjugating K48-Linked Ubiquitination of MITA

Mediator of IRF3 activation ([MITA] also known as STING) is a direct sensor of cyclic dinucleotide and critically mediates cytoplasmic DNA--triggered innate immune signaling. The activity of MITA is extensively regulated by ubiquitination and deubiquitination. In this study, we report that USP20 interacts with and removes K48-linked ubiquitin chains from MITA after HSV-1 infection, thereby stabilizing MITA and promoting cellular antiviral responses. Deletion of USP20 accelerates HSV-1-induced degradation of MITA and impairs phosphorylation of IRF3 and IκBα as well as subsequent induction of type I IFNs and proinflammatory cytokines after HSV-1 infection or cytoplasmic DNA challenge. Consistently, Usp20 ^-/- mice produce decreased type I IFNs and proinflammatory cytokines, exhibit increased susceptibility to lethal HSV-1 infection, and aggravated HSV-1 replication compared with Usp20 ^+/+ mice. In addition, complement of MITA into Usp20 ^-/- cells fully restores HSV-1-triggered signaling and inhibits HSV-1 infection. These findings suggest a crucial role of USP20 in maintaining the stability of MITA and promoting innate antiviral signaling.

HERC2 Facilitates BLM and WRN Helicase Complex Interaction with RPA to Suppress G-Quadruplex DNA

BLM and WRN are RecQ DNA helicasesessential for genomic stability. Here, we demonstrate that HERC2, a HECT E3 ligase, is critical for their functions to suppress G-quadruplex (G4) DNA. HERC2 interacted with BLM, WRN, and replication protein A (RPA) complexes during the S-phase of the cell cycle. Depletion of HERC2 dissociated RPA from BLM and WRN complexes and significantly increased G4 formation. Triple depletion revealed that HERC2 has an epistatic relationship with BLM and WRN in their G4-suppressing function. In vitro, HERC2 released RPA onto single-stranded DNA (ssDNA) rather than anchoring onto RPA-coated ssDNA. CRISPR/Cas9-mediated deletion of the catalytic ubiquitin-binding site of HERC2 inhibited ubiquitination of RPA2, caused RPA accumulation in the helicase complexes, and increased G4, indicating an essential role for E3 activity in the suppression of G4. Both depletion of HERC2 and inactivation of E3 sensitized cells to the G4-interacting compounds telomestatin and pyridostatin. Overall, these results indicate that HERC2 is a master regulator of G4 suppression that affects the sensitivity of cells to G4 stabilizers. Given that HERC2 expression is frequently reduced in many types of cancers, G4 accumulation as a result of HERC2 deficiency may provide a therapeutic target for G4 stabilizers.Significance: HERC2 is revealed as a master regulator of G-quadruplex, a DNA secondary structure that triggers genomic instability and may serve as a potential molecular target in cancer therapy.Graphical Abstract: http://cancerres.aacrjournals.org/content/canres/78/22/6371/F1.large.jpg Cancer Res; 78(22); 6371-85. ©2018 AACR.

Clinicopathological significance of claspin overexpression and its association with spheroid formation in gastric cancer

Gastric cancer (GC) is one of the leading causes of cancer-related death worldwide. Spheroid colony formation is a useful method to identify cancer stem cells (CSCs). The aim of this study was to identify a novel prognostic marker or therapeutic target for GC using a method to identify CSCs. We analyzed the microarray data in spheroid body-forming and parental cells and focused on the CLSPN gene because it is overexpressed in the spheroid body-forming cells in both the GC cell lines MKN-45 and MKN-74. Quantitative reverse-transcription polymerase chain reaction analysis revealed that CLSPN messenger RNA expression was up-regulated in GC cell lines MKN-45, MKN-74, and TMK-1. Immunohistochemistry of claspin showed that 94 (47%) of 203 GC cases were positive. Claspin-positive GC cases were associated with higher T and N grades, tumor stage, lymphatic invasion, and poor prognosis. In addition, claspin expression was coexpressed with CD44, human epidermal growth factor receptor type 2, and p53. CLSPN small interfering RNA treatment decreased GC cell proliferation and invasion. These results indicate that the expression of claspin might be a key regulator in the progression of GC and might play an important role in CSCs of GC.

USP20 positively regulates tumorigenesis and chemoresistance through β-catenin stabilization

β-catenin is a major transcriptional activator of the canonical Wnt/β-catenin signaling pathway. It is important for a series of biological processes including tissue homeostasis, and embryonic development and is involved in various human diseases. Elevated oncogenic activity of β-catenin is frequently observed in cancers, which contributes to survival, metastasis and chemo-resistance of cancer cells. However, the mechanism of β-catenin overexpression in cancers is not well defined. Here we demonstrate that the deubiquitination enzyme USP20 is a new regulator of the Wnt/β-catenin signaling pathway. Mechanistically, USP20 regulates the deubiquitination of β-catenin to control its stability, thereby inducing proliferation, invasion and migration of cancer cells. High expression of USP20 correlates with increased β-catenin protein level in multiple cancer cell lines and patient samples. Moreover, knockdown of USP20 increases β-catenin polyubiquitination, which enhances β-catenin turnover and cell sensitivity to chemotherapy. Collectively, our results establish the USP20-β-catenin axis as a critical regulatory mechanism of canonical Wnt/β-catenin signaling pathway with an important role in tumorigenesis and chemo response in human cancers.

The HNF-1β-USP28-Claspin pathway upregulates DNA damage-induced Chk1 activation in ovarian clear cell carcinoma

Transcription factor hepatocyte nuclear factor 1-beta (HNF-1β) enhances checkpoint kinase 1 (Chk1) activation and promotes G2/M cell cycle progression in ovarian clear cell carcinoma (CCC) following exposure to diverse genotoxic agents including bleomycin. However, the underlying mechanism leading to checkpoint activation of HNF-1β still remains largely unknown. To clarify the effects of HNF-1β on cell cycle checkpoints, human CCC cell lines were transfected with siRNAs targeting HNF-1β, Claspin, USP28, or a control vector. Ubiquitination and stabilization of Claspin protein by HNF-1β was assessed by immunoprecipitation. Loss-of-function studies using RNAi-mediated gene silencing indicated that HNF-1β facilitated the Claspin expression after treatment with a genotoxic agent bleomycin, resulting in accumulation of phosphorylated Chk1 (p-Chk1) and promotion of survival in CCC cell lines. This study showed for the first time that USP28, a de-ubiquitinase crucial for Claspin expression, is one target gene of HNF-1β. Knockdown of endogenous USP28 suppressed the Claspin expression and p-Chk1 activation and cell viability. Our findings identify a novel pathway of the HNF-1β-USP28-Claspin-Chk1 axis in checkpoint signal amplification in response to DNA damage. Targeting this pathway may represent a putative, novel, anticancer strategy in ovarian CCC.

Network Analysis of UBE3A/E6AP-Associated Proteins Provides Connections to Several Distinct Cellular Processes

Perturbations in activity and dosage of the UBE3A ubiquitin-ligase have been linked to Angelman syndrome and autism spectrum disorders. UBE3A was initially identified as the cellular protein hijacked by the human papillomavirus E6 protein to mediate the ubiquitylation of p53, a function critical to the oncogenic potential of these viruses. Although a number of substrates have been identified, the normal cellular functions and pathways affected by UBE3A are largely unknown. Previously, we showed that UBE3A associates with HERC2, NEURL4, and MAPK6/ERK3 in a high-molecular-weight complex of unknown function that we refer to as the HUN complex (HERC2, UBE3A, and NEURL4). In this study, the combination of two complementary proteomic approaches with a rigorous network analysis revealed cellular functions and pathways in which UBE3A and the HUN complex are involved. In addition to finding new UBE3A-associated proteins, such as MCM6, SUGT1, EIF3C, and ASPP2, network analysis revealed that UBE3A-associated proteins are connected to several fundamental cellular processes including translation, DNA replication, intracellular trafficking, and centrosome regulation. Our analysis suggests that UBE3A could be involved in the control and/or integration of these cellular processes, in some cases as a component of the HUN complex, and also provides evidence for crosstalk between the HUN complex and CAMKII interaction networks. This study contributes to a deeper understanding of the cellular functions of UBE3A and its potential role in pathways that may be affected in Angelman syndrome, UBE3A-associated autism spectrum disorders, and human papillomavirus-associated cancers.

The deubiquitinating enzyme USP20 stabilizes ULK1 and promotes autophagy initiation

Autophagy begins with the formation of autophagosomes, a process that depends on the activity of the serine/threonine kinase ULK1 (hATG1). Although earlier studies indicated that ULK1 activity is regulated by dynamic polyubiquitination, the deubiquitinase involved in the regulation of ULK1 remained unknown. In this study, we demonstrate that ubiquitin-specific protease 20 (USP20) acts as a positive regulator of autophagy initiation through stabilizing ULK1. At basal state, USP20 binds to and stabilizes ULK1 by removing the ubiquitin moiety, thereby interfering with the lysosomal degradation of ULK1. The stabilization of basal ULK1 protein levels is required for the initiation of starvation-induced autophagy, since the depletion of USP20 by RNA interference inhibits LC3 puncta formation, a marker of autophagic flux. At later stages of autophagy, USP20 dissociates from ULK1, resulting in enhanced ULK1 degradation and apoptosis. Taken together, our findings provide the first evidence that USP20 plays a crucial role in autophagy initiation by maintaining the basal expression level of ULK1.

Claspin functions in cell homeostasis-A link to cancer?

Cancer remains one of the leading causes of mortality worldwide. Most cancers present high degrees of genomic instability. DNA damage and replication checkpoints function as barriers to halt cell cycle progression until damage is resolved, preventing the perpetuation of errors. Activation of these checkpoints is critically dependent on Claspin, an adaptor protein that mediates the phosphorylation of the effector kinase Chk1 by ATR. However, Claspin also performs other roles related to the protection and maintenance of cell and genome integrity. For instance, following DNA damage and checkpoint activation, Claspin bridges checkpoint responses to DNA repair or to apoptosis. During DNA replication, Claspin acts a sensor and couples DNA unwinding to strand polymerization, and may also indirectly regulate replication initiation at firing origins. As Claspin participates in several processes that are vital to maintenance of cell homeostasis, its function is tightly regulated at multiple levels. Nevertheless, little is known about its role in cancer. Accumulating evidence suggests that Claspin inactivation could be an essential event during carcinogenesis, indicating that Claspin may function as a tumour suppressor. In this review, we will examine the functions of Claspin and how its deregulation may contribute to cancer initiation and progression. To conclude, we will discuss means by which Claspin can be targeted for cancer therapy.

Ataxin-3 promotes genome integrity by stabilizing Chk1

The Chk1 protein is essential for genome integrity maintenance and cell survival in eukaryotic cells. After prolonged replication stress, Chk1 can be targeted for proteasomal degradation to terminate checkpoint signaling after DNA repair finishes. To ensure proper activation of DNA damage checkpoint and DNA repair signaling, a steady-state level of Chk1 needs to be retained under physiological conditions. Here, we report a dynamic signaling pathway that tightly regulates Chk1 stability. Under unperturbed conditions and upon DNA damage, ataxin-3 (ATX3) interacts with Chk1 and protects it from DDB1/CUL4A- and FBXO6/CUL1-mediated polyubiquitination and subsequent degradation, thereby promoting DNA repair and checkpoint signaling. Under prolonged replication stress, ATX3 dissociates from Chk1, concomitant with a stronger binding between Chk1 and its E3 ligase, which causes Chk1 proteasomal degradation. ATX3 deficiency results in pronounced reduction of Chk1 abundance, compromised DNA damage response, G2/M checkpoint defect and decreased cell survival after replication stress, which can all be rescued by ectopic expression of ATX3. Taken together, these findings reveal ATX3 to be a novel deubiquitinase of Chk1, providing a new mechanism of Chk1 stabilization in genome integrity maintenance.

NEURL4 regulates the transcriptional activity of tumor suppressor protein p53 by modulating its oligomerization

p53 is a transcription factor that regulates important cellular processes related to tumor suppression, including induction of senescence, apoptosis, and DNA repair as well as the inhibition of angiogenesis and cell migration. Therefore, it is critical to understand the molecular mechanism that regulates it. p53 tetramerization is a key step in its activation process and the regulation of this oligomerization, an important control point. The E3 ubiquitin ligase HERC2 controls the p53 transcriptional activity by regulation of its oligomerization state. HERC2-interacting proteins such as the adaptor-like protein with six neuralized domains NEURL4 are also candidates to regulate p53 activity. Here, we demonstrate the existence of an interaction network between NEURL4, HERC2 and p53 proteins. We report a functional interaction between NEURL4 and p53, involving the C-terminal region of p53 and the neuralized domains 3 and 4 of NEURL4. Through this interaction, NEURL4 regulates the transcriptional activity of p53. Thus, NEURL4 depletion reduced the transcriptional activity whereas NEURL4 overexpression increased it. In both cases, p53 stability was not affected. Although NEURL4 may interact with p53 independently of the E3 ubiquitin ligase HERC2, we observed that both proteins are needed to regulate the transcriptional activity of p53. Clonogenic assays confirmed the functional relevance of this interaction observing a decrease in cell growth by NEURL4 overexpression correlated to the increase of cellular cycle inhibitor p21 by p53 activation. Under these conditions, NEURL4 activated p53 oligomerization. All these findings identify NEURL4 as a novel regulator of the p53’s signaling.

HERC4 Is Overexpressed in Hepatocellular Carcinoma and Contributes to the Proliferation and Migration of Hepatocellular Carcinoma Cells

Recently, more and more evidences unveiled that ubiquitin-proteasome system (UPS) makes an important contribution to the occurrence and development of cancer. HERC4 is one identified Ubiqutin ligase E3, a member of UPS. Although some studies showed that HERC4 abnormally expresses in many cancer cells, till now, nothing has been reported about the function of HERC4 in the development of hepatoma carcinoma. To this end, in this study, we studied the function of HERC4 for the first time in hepatoma carcinoma cells. We detected the expression of HERC4 in tumor and normal tissues, and in hepatoma carcinoma cell lines by using qRT-PCR, Western blot, immunohistochemistry, and immunofluorescence. The data showed that tumor tissues expressed higher HERC4 than normal ones. HERC4 was expressed, although to a different extent, in hepatoma carcinoma cell lines. Colony formation assay, CCK-8 assay, EdU assay, wound healing assay, and FACS indicated that HERC4 plays a role in cell proliferative and migration ability. HERC4 overexpression increases the proliferative and migration ability and reduces apoptosis of hepatoma carcinoma cells; in contrast, knockdown of HERC4 decreases the proliferative and migration ability and increases the apoptosis rate of hepatoma carcinoma cells. Taken together, our findings showed that HERC4 has an effect on the occurrence and development of hepatoma carcinoma by promoting hepatoma carcinoma cell proliferation and migration, and by reducing cell apoptosis, further providing another therapeutic target for the intervention of related diseases.

Deubiquitinating Enzyme USP20 Regulates Extracellular Signal-Regulated Kinase 3 Stability and Biological Activity

Extracellular signal-regulated kinase 3 (ERK3) is an atypical mitogen-activated protein kinase (MAPK) whose regulatory mechanisms and biological functions remain superficially understood. Contrary to most protein kinases, ERK3 is a highly unstable protein that is subject to dynamic regulation by the ubiquitin-proteasome system. However, the effectors that control ERK3 ubiquitination and degradation are unknown. In this study, we carried out an unbiased functional loss-of-function screen of the human deubiquitinating enzyme (DUB) family and identified ubiquitin-specific protease 20 (USP20) as a novel ERK3 regulator. USP20 interacts with and deubiquitinates ERK3 both in vitro and in intact cells. The overexpression of USP20 results in the stabilization and accumulation of the ERK3 protein, whereas USP20 depletion reduces the levels of ERK3. We found that the expression levels of ERK3 correlate with those of USP20 in various cellular contexts. Importantly, we show that USP20 regulates actin cytoskeleton organization and cell migration in a manner dependent on ERK3 expression. Our results identify USP20 as a bona fide regulator of ERK3 stability and physiological functions.

Deubiquitinating enzyme USP20 is a positive regulator of Claspin and suppresses the malignant characteristics of gastric cancer cells

The aim of the present study was to investigate the clinical significance, the biological function and the mechanisms of USP20 in gastric cancer. The expression of USP20 in 89 pairs of primary gastric cancer and peritumoral gastric tissues specimens were measured by immunohistochemistry. The correlation of USP20 expression with the survival and the clinicopathological characteristics of patients were analyzed. Moreover, the underlying mechanisms of ectopic USP20 expression and its impact on GC cells were also investigated. We found that the expression of USP20 is relatively low in GC tissues and negatively correlated with tumor size, tumor invasion and TNM staging. High expression of USP20 in GC predicted longer survival. Experimentally, small interfering RNA-mediated knockdown of USP20 expression significantly promoted cell proliferation, accelerated G1-S phase transition and attenuated the autophagy activity. Overexpression of USP20 led to the inhibition of proliferation, G1-S cell cycle transition delay and autophagy activation. Mechanistically, we confirmed that silencing the expression of USP20 in GC cells could reduce Claspin protein levels without altering Claspin mRNA levels, which is involved in the antitumor activity of USP20. Furthermore, the expression level of Claspin was relatively higher in peritumoral tissue than that of GC tissues and higher expression of Claspin in GC was also correlated with good prognosis of patients. Given its pivotal role in gastric tumorigenesis and progression, USP20 functioned as the tumor suppressor in GC and possessed promising value to be a therapeutic target for GC.

Proteasome-associated deubiquitinase ubiquitin-specific protease 14 regulates prostate cancer proliferation by deubiquitinating and stabilizing androgen receptor

Androgen receptor (AR) is frequently over-expressed and plays a critical role in the growth and progression of human prostate cancer. The therapy attempting to target AR signalling was established in decades ago but the treatment of prostate cancer is far from being satisfactory. The assignable cause is that our understanding of the mechanism of AR regulation and re-activation remains incomplete. Increasing evidence suggests that deubiquitinases are involved in the regulation of cancer development and progression but the specific underlying mechanism often is not elucidated. In the current study, we have identified ubiquitin-specific protease 14 (USP14) as a novel regulator of AR, inhibiting the degradation of AR via deubiquitinating this oncoprotein in the androgen-responsive prostate cancer cells. We found that (i) USP14 could bind to AR, and additionally, both genetic and pharmacological inhibition of USP14 accelerated the ubiquitination and degradation of AR; (ii) downregulation or inhibition of USP14 suppressed cell proliferation and colony formation of LNcap cells and, conversely, overexpression of USP14 promoted the proliferation; and (iii) reduction or inhibition of USP14 induced G0/G1 phase arrest in LNcap prostate cancer cells. Hence, we conclude that USP14 promotes prostate cancer progression likely through stabilization of AR, suggesting that USP14 could be a promising therapeutic target for prostate cancer.