Cell cycle-regulated ubiquitination of tankyrase 1 by RNF8 and ABRO1/BRCC36 controls the timing of sister telomere resolution

ADP-ribosylation, a multifaceted modification: Functions and mechanisms in aging and aging-related diseases

Aging, a complex biological process, plays key roles the development of multiple disorders referred as aging-related diseases involving cardiovascular diseases, stroke, neurodegenerative diseases, cancers, lipid metabolism-related diseases. ADP-ribosylation is a reversible modification onto proteins and nucleic acids to alter their structures and/or functions. Growing evidence support the importance of ADP-ribosylation and ADP-ribosylation-associated enzymes in aging and age-related diseases. In this review, we summarized ADP-ribosylation-associated proteins including ADP-ribosyl transferases, the ADP-ribosyl hydrolyses and ADP-ribose binding domains. Furthermore, we outlined the latest knowledge about regulation of ADP-ribosylation in the pathogenesis and progression of main aging-related diseases, organism aging and cellular senescence, and we also speculated the underlying mechanisms to better disclose this novel molecular network. Moreover, we discussed current issues and provided an outlook for future research, aiming to revealing the unknown bio-properties of ADP-ribosylation, and establishing a novel therapeutic perspective in aging-related diseases and health aging via targeting ADP-ribosylation.

CAMK2D serves as a molecular scaffold for RNF8-MAD2 complex to induce mitotic checkpoint in glioma

MAD2 is a spindle assembly checkpoint protein that participates in the formation of mitotic checkpoint complex, which blocks mitotic progression. RNF8, an established DNA damage response protein, has been implicated in mitotic checkpoint regulation but its exact role remains poorly understood. Here, RNF8 proximity proteomics uncovered a role of RNF8-MAD2 in generating the mitotic checkpoint signal. Specifically, RNF8 competes with a small pool of p31comet for binding to the closed conformer of MAD2 via its RING domain, while CAMK2D serves as a molecular scaffold to concentrate the RNF8-MAD2 complex via transient/weak interactions between its p-Thr287 and RNF8's FHA domain. Accordingly, RNF8 overexpression impairs glioma stem cell (GSC) mitotic progression in a FHA- and RING-dependent manner. Importantly, low RNF8 expression correlates with inferior glioma outcome and RNF8 overexpression impedes GSC tumorigenicity. Last, we identify PLK1 inhibitor that mimics RNF8 overexpression using a chemical biology approach, and demonstrate a PLK1/HSP90 inhibitor combination that synergistically reduces GSC proliferation and stemness. Thus, our study has unveiled a previously unrecognized CAMK2D-RNF8-MAD2 complex in regulating mitotic checkpoint with relevance to gliomas, which is therapeutically targetable.

High-throughput screen to identify compounds that prevent or target telomere loss in human cancer cells

Chromosome instability (CIN) is an early step in carcinogenesis that promotes tumor cell progression and resistance to therapy. Using plasmids integrated adjacent to telomeres, we have previously demonstrated that the sensitivity of subtelomeric regions to DNA double-strand breaks (DSBs) contributes to telomere loss and CIN in cancer. A high-throughput screen was created to identify compounds that affect telomere loss due to subtelomeric DSBs introduced by I-SceI endonuclease, as detected by cells expressing green fluorescent protein (GFP). A screen of a library of 1832 biologically-active compounds identified a variety of compounds that increase or decrease the number of GFP-positive cells following activation of I-SceI. A curated screen done in triplicate at various concentrations found that inhibition of classical nonhomologous end joining (C-NHEJ) increased DSB-induced telomere loss, demonstrating that C-NHEJ is functional in subtelomeric regions. Compounds that decreased DSB-induced telomere loss included inhibitors of mTOR, p38 and tankyrase, consistent with our earlier hypothesis that the sensitivity of subtelomeric regions to DSBs is a result of inappropriate resection during repair. Although this assay was also designed to identify compounds that selectively target cells experiencing telomere loss and/or chromosome instability, no compounds of this type were identified in the current screen.

Functional roles of ADP-ribosylation writers, readers and erasers

ADP-ribosylation is a reversible post-translational modification (PTM) tightly regulated by the dynamic interplay between its writers, readers and erasers. As an intricate and versatile PTM, ADP-ribosylation plays critical roles in various physiological and pathological processes. In this review, we discuss the major players involved in the ADP-ribosylation cycle, which may facilitate the investigation of the ADP-ribosylation function and contribute to the understanding and treatment of ADP-ribosylation associated disease.

Identification and Verification of the Ability of Cdk5 to Phosphorylate Deubiquitinating Enzyme BRCC3 In Vitro

It is known that the expression of the deubiquitinating enzyme BRCA1-BRCA2-containing complex subunit 3 (BRCC3) and cyclin-dependent protein kinase 5 (Cdk5) is increased in Parkinson's disease (both are involved in neuroinflammatory response). However, the regulatory mechanism of Cdk5 on the post-translational modification of BRCC3 remains unclear. Here we studied whether Cdk5 phosphorylates BRCC3. Phosphorylation of BRCC3 by Cdk5 was predicted by GPS 5.0 software. His-BRCC3 plasmid was constructed by cloning the BRCC3 gene into pGEX-6P-1 vector, and then His-BRCC3 fusion protein was induced with isopropyl β-d-1-thiogalactopyranoside and purified using His-Tag affinity chromatography purification agarose. Phosphorylation of BRCC3 fusion protein by Cdk5 in vitro was detected by mass spectrometry and Western blotting. The results showed that multiple phosphorylation sites were predicted by GPS 5.0, and the His-BRCC3 fusion protein was successfully induced and purified. In vitro kinase assay, Western blotting, and mass spectrometry showed that Cdk5 can phosphorylate BRCC3. It has been demonstrated that protein kinase Cdk5 can phosphorylate the deubiquitinating enzyme BRCC3 in vitro, which provides new data for further study on the mechanism of neurodegeneration.

RNF8 up-regulates AR/ARV7 action to contribute to advanced prostate cancer progression

Androgen receptor (AR) signaling drives prostate cancer (PC) progression. Androgen deprivation therapy (ADT) is temporally effective, whereas drug resistance inevitably develops. Abnormal expression of AR/ARV7 (the most common AR splicing variant) is critical for endocrine resistance, while the detailed mechanism is still elusive. In this study, bioinformatics and immunohistochemical analyses demonstrate that RNF8 is high expressed in PC and castration-resistant PC (CRPC) samples and the expression of RNF8 is positively correlated with the Gleason score. The high expression of RNF8 in PCs predicts a poor prognosis. These results provide a potential function of RNF8 in PC progression. Furthermore, the mRNA expression of RNF8 is positively correlated with that of AR in PC. Mechanistically, we find that RNF8 upregulates c-Myc-induced AR transcription via altering histone modifications at the c-Myc binding site within the AR gene. RNF8 also acts as a co-activator of AR, promoting the recruitment of AR/ARV7 to the KLK3 (PSA) promoter, where RNF8 modulates histone modifications. These functions of RNF8 are dependent on its E3 ligase activity. RNF8 knockdown further reduces AR transactivation and PSA expression in CRPC cells with enzalutamide treatment. RNF8 depletion restrains cell proliferation and alleviates enzalutamide resistance in CRPC cells. Our findings indicate that RNF8 may be a potential therapeutic target for endocrine resistance in PC.

Functions of ADP-ribose transferases in the maintenance of telomere integrity

The ADP-ribose transferase (ART) family comprises 17 enzymes that catalyze mono- or poly-ADP-ribosylation, a post-translational modification of proteins. Present in all subcellular compartments, ARTs are implicated in a growing number of biological processes including DNA repair, replication, transcription regulation, intra- and extra-cellular signaling, viral infection and cell death. Five members of the family, PARP1, PARP2, PARP3, tankyrase 1 and tankyrase 2 are mainly described for their crucial functions in the maintenance of genome stability. It is well established that the most describedrole of PARP1, 2 and 3 is the repair of DNA lesions while tankyrases 1 and 2 are crucial for maintaining the integrity of telomeres. Telomeres, nucleoprotein complexes located at the ends of eukaryotic chromosomes, utilize their unique structure and associated set of proteins to orchestrate the mechanisms necessary for their own protection and replication. While the functions of tankyrases 1 and 2 at telomeres are well known, several studies have also brought PARP1, 2 and 3 to the forefront of telomere protection. The singular quality of the telomeric environment has highlighted protein interactions and molecular pathways distinct from those described throughout the genome. The aim of this review is to provide an overview of the current knowledge on the multiple roles of PARP1, PARP2, PARP3, tankyrase 1 and tankyrase 2 in the maintenance and preservation of telomere integrity.

Small-Molecule Inhibitors of Tankyrases as Prospective Therapeutics for Cancer

Tankyrases are multifunctional poly(adenosine diphosphate-ribose) polymerases that regulate diverse biological processes including telomere maintenance and cellular signaling. These processes are often implicated in a number of human diseases, with cancer being the most prevalent example. Accordingly, tankyrase inhibitors have gained increasing attention as potential therapeutics. Since the discovery of XAV939 and IWR-1 as the first tankyrase inhibitors over two decades ago, tankyrase-targeted drug discovery has made significant progress. This review starts with an introduction of tankyrases, with emphasis placed on their cancer-related functions. Small-molecule inhibitors of tankyrases are subsequently delineated based on their distinct modes of binding to the enzymes. In addition to inhibitors that compete with oxidized nicotinamide adenine dinucleotide (NAD⁺) for binding to the catalytic domain of tankyrases, non-NAD⁺-competitive inhibitors are detailed. This is followed by a description of three clinically trialled tankyrase inhibitors. To conclude, some of challenges and prospects in developing tankyrase-targeted cancer therapies are discussed.

BRCC36 promotes intestinal mucosal barrier injury caused by BMP2 after ischemia reperfusion via inhibiting PPARγ signaling

As one of the most common pathological changes in trauma and surgery practice, intestinal ischemia-reperfusion (I/R) injury is regarded as a major precipitating factor in the occurrence and development of fatal diseases. BRCA1-BRCA2-containing complex subunit 36 (BRCC36), a deubiquitinase, has been proved important in a variety of pathophysiological processes such as DNA repair, cell cycle regulation, tumorigenesis, and inflammatory response. However, the effect of BRCC36 on intestinal mucosal barrier injury after I/R has not been fully elucidated. Our research found that BRCC36 aggravated intestinal mucosal barrier injury caused by bone morphogenetic protein 2 after I/R by downregulating peroxisome proliferator-activated receptor-γ (PPARγ) signaling. These results suggested that BRCC36/PPARγ axis might serve as a potential therapeutic target for preventing intestinal mucosal barrier injury after I/R.

Noninvasive Analysis Using Data-Independent Acquisition Mass Spectrometry: New Epidermal Proteins That Reveal Sex Differences in the Aging Process

The development of mass spectrometry has provided a method with extremely high sensitivity and selectivity that can be used to identify protein biomarkers. Epidermal proteins, lipids, and cornified envelopes are involved in the formation of the skin epidermal barrier. The epidermal protein composition changes with age. Therefore, quantitative proteomic changes may be indicative of skin aging. We sought to utilize data-independent acquisition mass spectrometry for noninvasive analysis of epidermal proteins in healthy Chinese individuals of different age groups and sexes. In our study, we completed high-throughput protein detection, analyzed protein differences with MaxQuant software, and performed statistical analyses of the proteome. We obtained interesting findings regarding ceruloplasmin (CP), which exhibited significant differences and is involved in ferroptosis, a signaling pathway significantly associated with aging. There were also several proteins that differed between sexes in the younger group, but the sex differences disappeared with aging. These proteins, which were associated with both aging processes and sex differences, are involved in signaling pathways such as apoptosis, oxidative stress, and genomic stability and can serve as candidate biomarkers for sex differences during aging. Our approach for noninvasive detection of epidermal proteins and its application to accurately quantify protein expression can provide ideas for future epidermal proteomics studies.

Tankyrase inhibitors as antitumor agents: a patent update (2013 - 2020)

INTRODUCTION

Tankyrase inhibitors gained significant attention as therapeutic targets in oncology because of their potency. Their primary role in inhibiting the Wnt signaling pathway makes them an important class of compounds with the potential to be used as a combination therapy in future treatments of colorectal cancer.

AREAS COVERED

This review describes pertinent work in the development of tankyrase inhibitors with a great emphasis on the recently patented TNKS inhibitors published from 2013 to 2020. This article also highlights a couple of promising candidates having tankyrase inhibitory effects and are currently undergoing clinical trials.

EXPERT OPINION

Following the successful clinical applications of PARP inhibitors, tankyrase inhibition has gained significant attention in the research community as a target with high therapeutic potential. The ubiquitous role of tankyrase in cellular homeostasis and Wnt-dependent tumor proliferation brought difficulties for researchers to strike the right balance between potency and on-target toxicity. The need for novel tankyrase inhibitors with a better ADMET profile can introduce an additional regimen in treating various malignancies in monotherapy or adjuvant therapy. The development of combination therapies, including tankyrase inhibitors with or without PARP inhibitory properties, can potentially benefit the larger population of patients with unmet medical needs.

RNA-Binding RING E3-Ligase DZIP3/hRUL138 Stabilizes Cyclin D1 to Drive Cell-Cycle and Cancer Progression

DZIP3/hRUL138 is a poorly characterized RNA-binding RING E3-ubiquitin ligase with functions in embryonic development. Here we demonstrate that DZIP3 is a crucial driver of cancer cell growth, migration, and invasion. In mice and zebrafish cancer models, DZIP3 promoted tumor growth and metastasis. In line with these results, DZIP3 was frequently overexpressed in several cancer types. Depletion of DZIP3 from cells resulted in reduced expression of Cyclin D1 and a subsequent G1 arrest and defect in cell growth. Mechanistically, DZIP3 utilized its two different domains to interact and stabilize Cyclin D1 both at mRNA and protein levels. Using an RNA-binding lysine-rich region, DZIP3 interacted with the AU-rich region in 3' untranslated region of Cyclin D1 mRNA and stabilized it. Using a RING E3-ligase domain, DZIP3 interacted and increased K63-linked ubiquitination of Cyclin D1 protein to stabilize it. Remarkably, DZIP3 interacted with, ubiquitinated, and stabilized Cyclin D1 predominantly in the G1 phase of the cell cycle, where it is needed for cell-cycle progression. In agreement with this, a strong positive correlation of mRNA expression between DZIP3 and Cyclin D1 in different cancer types was observed. Additionally, DZIP3 regulated several cell cycle proteins by modulating the Cyclin D1-E2F axes. Taken together, this study demonstrates for the first time that DZIP3 uses a unique two-pronged mechanism in its stabilization of Cyclin D1 to drive cell-cycle and cancer progression. SIGNIFICANCE: These findings show that DZIP3 is a novel driver of cell-cycle and cancer progression via its control of Cyclin D1 mRNA and protein stability in a cell-cycle phase-dependent manner. GRAPHICAL ABSTRACT: http://cancerres.aacrjournals.org/content/canres/81/2/315/F1.large.jpg.

ABRO1 stabilizes the deubiquitinase BRCC3 through inhibiting its degradation mediated by the E3 ubiquitin ligase WWP2

BRCA1/BRCA2-containing complex subunit 3 (BRCC3) is a lysine 63-specific deubiquitinase involved in multiple biological processes, such as DNA repair and immune responses. However, the regulation mechanism for BRCC3 protein stability is still unknown. Here, we demonstrate that BRCC3 is mainly degraded through the ubiquitin-proteasome pathway. The HECT-type E3 ubiquitin ligase WWP2 modulates BRCC3 ubiquitination and degradation. ABRO1, a subunit of the BRCC36 isopeptidase complex (BRISC), competes with WWP2 to bind to BRCC3, thereby preventing WWP2-mediated BRCC3 ubiquitination and enhancing BRCC3 stability. Functionally, we show that lentivirus-mediated overexpression of WWP2 in murine macrophages inhibits NLRP3 inflammasome activation by decreasing BRCC3 protein level. This study provides the first insights into the regulation of BRCC3 stability and expands our knowledge about the physiological function of WWP2.

BRCA1-A and BRISC: Multifunctional Molecular Machines for Ubiquitin Signaling

The K63-linkage specific deubiquitinase BRCC36 forms the core of two multi-subunit deubiquitination complexes: BRCA1-A and BRISC. BRCA1-A is recruited to DNA repair foci, edits ubiquitin signals on chromatin, and sequesters BRCA1 away from the site of damage, suppressing homologous recombination by limiting resection. BRISC forms a complex with metabolic enzyme SHMT2 and regulates the immune response, mitosis, and hematopoiesis. Almost two decades of research have revealed how BRCA1-A and BRISC use the same core of subunits to perform very distinct biological tasks.

Transcriptional regulation of human abraxas brother protein 1 expression by yin yang 1

Abraxas brother protein 1 (ABRO1) is a subunit of the deubiquitinating enzyme BRCC36-containing isopeptidase complex and plays important roles in cellular responses to stress by interacting with its binding partners, such as ubiquitin-specific peptidase 7, p53, activating transcription factor 4, THAP-domain containing 5, and serine hydroxymethyltransferase. However, the transcriptional regulation of ABRO1 remains unexplored. In this study, we identified and characterized the core regulatory elements of the human ABRO1 gene and mapped them to the ABRO1 promoter region. Additionally, 5' rapid amplification of cDNA ends revealed that the transcriptional start site (TSS) was located -13 bp upstream from the start codon. Reporter gene, chromatin immunoprecipitation, and electrophoretic mobility shift assays demonstrated that ABRO1 transcription was regulated through cis-acting elements located in the region -89 to -59 bp upstream of the ABRO1 TSS and that these elements were targeted by yin yang 1 transcription factor (YY1). Moreover, YY1 overexpression increased human ABRO1 mRNA and protein expression, and small-interfering RNA-mediated downregulation of YY1 attenuated ABRO1 expression. These results suggested that YY1 positively regulated human ABRO1 expression by binding to cis-acting elements located in the ABRO1 TSS.

BRCC36 functions noncatalytically to promote antiviral response by maintaining STAT1 protein stability

Viral infection is a serious threat to both normal population and clinical patients. STAT1 plays central roles in host defense against viral infection. How STAT1 protein maintains stable in different conditions remains largely unknown. Here, we identified BRCC36 as a potent regulator of STAT1 protein stability. Mechanistically, BRCC36 maintains STAT1 levels by utilizing USP13 to form a balanced complex for antagonizing Smurf1-mediated degradation. Importantly, cellular BRCC36 deficiency results in rapid downregulation of STAT1 during viral infection, whereas a supplement of BRCC36 maintains STAT1 protein levels and host antiviral immunity in vivo. Moreover, we revealed that BRCC36 expression was downregulated in allogeneic HSC transplantation (allo-HSCT) mice that showed increased susceptibility to viral infection. Supplementing BRCC36 enhanced antiviral response of allo-HSCT mice by maintaining STAT1 stability. This study uncovers a critical role of BRCC36 in STAT1 protein stability and could provide potential strategies for enhancing clinical antiviral therapy.

Nuclear PARPs and genome integrity

Effective maintenance and stability of our genomes is essential for normal cell division, tissue homeostasis, and cellular and organismal fitness. The processes of chromosome replication and segregation require continual surveillance to insure fidelity. Accurate and efficient repair of DNA damage preserves genome integrity, which if lost can lead to multiple diseases, including cancer. Poly(ADP-ribose) a dynamic and reversible posttranslational modification and the enzymes that catalyze it (PARP1, PARP2, tankyrase 1, and tankyrase 2) function to maintain genome stability through diverse mechanisms. Here we review the role of these enzymes and the modification in genome repair, replication, and resolution in human cells.

How to Inactivate Human Ubiquitin E3 Ligases by Mutation

E3 ubiquitin ligases are the ultimate enzymes involved in the transfer of ubiquitin to substrate proteins, a process that determines the fate of the modified protein. Numerous diseases are caused by defects in the ubiquitin-proteasome machinery, including when the activity of a given E3 ligase is hampered. Thus, inactivation of E3 ligases and the resulting effects at molecular or cellular level have been the focus of many studies during the last few years. For this purpose, site-specific mutation of key residues involved in either protein interaction, substrate recognition or ubiquitin transfer have been reported to successfully inactivate E3 ligases. Nevertheless, it is not always trivial to predict which mutation(s) will block the catalytic activity of a ligase. Here we review over 250 site-specific inactivating mutations that have been carried out in 120 human E3 ubiquitin ligases. We foresee that the information gathered here will be helpful for the design of future experimental strategies.

Poly(ADP-ribose) polymerase enzymes and the maintenance of genome integrity

DNA damage response (DDR) relies on swift and accurate signaling to rapidly identify DNA lesions and initiate repair. A critical DDR signaling and regulatory molecule is the posttranslational modification poly(ADP-ribose) (PAR). PAR is synthesized by a family of structurally and functionally diverse proteins called poly(ADP-ribose) polymerases (PARPs). Although PARPs share a conserved catalytic domain, unique regulatory domains of individual family members endow PARPs with unique properties and cellular functions. Family members PARP-1, PARP-2, and PARP-3 (DDR-PARPs) are catalytically activated in the presence of damaged DNA and act as damage sensors. Family members tankyrase-1 and closely related tankyrase-2 possess SAM and ankyrin repeat domains that regulate their diverse cellular functions. Recent studies have shown that the tankyrases share some overlapping functions with the DDR-PARPs, and even perform novel functions that help preserve genomic integrity. In this review, we briefly touch on DDR-PARP functions, and focus on the emerging roles of tankyrases in genome maintenance. Preservation of genomic integrity thus appears to be a common function of several PARP family members, depicting PAR as a multifaceted guardian of the genome.

The role of DUBs in the post-translational control of cell migration

Cell migration is a multifactorial/multistep process that requires the concerted action of growth and transcriptional factors, motor proteins, extracellular matrix remodeling and proteases. In this review, we focus on the role of transcription factors modulating Epithelial-to-Mesenchymal Transition (EMT-TFs), a fundamental process supporting both physiological and pathological cell migration. These EMT-TFs (Snail1/2, Twist1/2 and Zeb1/2) are labile proteins which should be stabilized to initiate EMT and provide full migratory and invasive properties. We present here a family of enzymes, the deubiquitinases (DUBs) which have a crucial role in counteracting polyubiquitination and proteasomal degradation of EMT-TFs after their induction by TGFβ, inflammatory cytokines and hypoxia. We also describe the DUBs promoting the stabilization of Smads, TGFβ receptors and other key proteins involved in transduction pathways controlling EMT.

Mitotic functions of poly(ADP-ribose) polymerases

Mitosis ensures accurate segregation of duplicated DNA through tight regulation of chromosome condensation, bipolar spindle assembly, chromosome alignment in the metaphase plate, chromosome segregation and cytokinesis. Poly(ADP-ribose) polymerases (PARPs), in particular PARP1, PARP2, PARP3, PARP5a (TNKS1), as well as poly(ADP-ribose) glycohydrolase (PARG), regulate different mitotic functions, including centrosome function, mitotic spindle assembly, mitotic checkpoints, telomere length and telomere cohesion. PARP depletion or inhibition give rise to various mitotic defects such as centrosome amplification, multipolar spindles, chromosome misalignment, premature loss of cohesion, metaphase arrest, anaphase DNA bridges, lagging chromosomes, and micronuclei. As the mechanisms of PARP1/2 inhibitor-mediated cell death are being progressively elucidated, it is becoming clear that mitotic defects caused by PARP1/2 inhibition arise due to replication stress and DNA damage in S phase. As it stands, entrapment of inactive PARP1/2 on DNA phenocopies replication stress through accumulation of unresolved replication intermediates, double-stranded DNA breaks (DSBs) and incorrectly repaired DSBs, which can be transmitted from S phase to mitosis and instigate various mitotic defects, giving rise to both numerical and structural chromosomal aberrations. Cancer cells have increased levels of replication stress, which makes them particularly susceptible to a combination of agents that compromise replication fork stability. Indeed, combining PARP1/2 inhibitors with genetic deficiencies in DNA repair pathways, DNA-damaging agents, ATR and other cell cycle checkpoint inhibitors has yielded synergistic effects in killing cancer cells. Here I provide a comprehensive overview of the mitotic functions of PARPs and PARG, mitotic phenotypes induced by their depletion or inhibition, as well as the therapeutic relevance of targeting mitotic cells by directly interfering with mitotic functions or indirectly through replication stress.

The Functions of DNA Damage Factor RNF8 in the Pathogenesis and Progression of Cancer

The really interesting new gene (RING) finger protein 8 (RNF8) is a central factor in DNA double strand break (DSB) signal transduction. DSB damage is the most toxic type of DNA damage to cells and is related to genomic instability. Multiple roles for RNF8 have been identified in DNA damage response as well as in other functions, such as telomere protection, cell cycle control and transcriptional regulation. These functions are closely correlated to tumorigenesis and cancer progression. Indeed, deficiency of RNF8 caused spontaneous tumorigenesis in a mouse model. Deciphering these mechanisms of RNF8 may shed light on strategies for cancer treatment. In this review, we summarize the current understanding of both classical and nonclassical functions of RNF8, and discuss its roles in the pathogenesis and progression of tumor.

ABRO1 promotes NLRP3 inflammasome activation through regulation of NLRP3 deubiquitination

Deubiquitination of NLRP3 has been suggested to contribute to inflammasome activation, but the roles and molecular mechanisms are still unclear. We here demonstrate that ABRO1, a subunit of the BRISC deubiquitinase complex, is necessary for optimal NLRP3-ASC complex formation, ASC oligomerization, caspase-1 activation, and IL-1β and IL-18 production upon treatment with NLRP3 ligands after the priming step, indicating that efficient NLRP3 activation requires ABRO1. Moreover, we report that ABRO1 deficiency results in a remarkable attenuation in the syndrome severity of NLRP3-associated inflammatory diseases, including MSU- and Alum-induced peritonitis and LPS-induced sepsis in mice. Mechanistic studies reveal that LPS priming induces ABRO1 binding to NLRP3 in an S194 phosphorylation-dependent manner, subsequently recruiting the BRISC to remove K63-linked ubiquitin chains of NLRP3 upon stimulation with activators. Furthermore, deficiency of BRCC3, the catalytically active component of BRISC, displays similar phenotypes to ABRO1 knockout mice. Our findings reveal an ABRO1-mediated regulatory signaling system that controls activation of the NLRP3 inflammasome and provide novel potential targets for treating NLRP3-associated inflammatory diseases.

MERIT40-dependent recruitment of tankyrase to damaged DNA and its implication for cell sensitivity to DNA-damaging anticancer drugs

Tankyrase, a member of the poly(ADP-ribose) polymerase (PARP) family, regulates various intracellular responses, such as telomere maintenance, Wnt/β-catenin signaling and cell cycle progression through its interactions with multiple target proteins. Tankyrase contains a long stretch of 24 ankyrin repeats that are further divided into five subdomains, called ANK repeat clusters (ARCs). Each ARC works as an independent ligand-binding unit, which implicates tankyrase as a platform for multiple protein-protein interactions. Furthermore, tankyrase distributes to various intracellular loci, suggesting potential distinct but yet unidentified physiological functions. To explore the novel functions of tankyrase, we performed liquid chromatography-mass spectrometry analysis and identified the BRE-BRCC36-MERIT40 complex, a regulator of homologous recombination, as tankyrase-binding proteins. Among the complex components, MERIT40 was directly associated with tankyrase via a tankyrase-binding consensus motif, as previously reported. In X-ray-irradiated non-small cell lung cancer cells, tankyrase localized to DNA double-stranded break sites in a MERIT40-dependent manner. MERIT40 knockdown increased the cell sensitivity to X-ray, whereas the wild-type, but not the tankyrase-unbound mutant, MERIT40 rescued the phenotype of the knockdown cells. Tankyrase inhibitors, such as G007-LK and XAV939, increased the cellular sensitivity to X-ray irradiation and anticancer drugs that induce DNA double-stranded breaks. These observations suggest that tankyrase plays a role in the DNA damage repair response and implicates a potential therapeutic utility of tankyrase inhibitors in combination treatments with DNA-damaging anticancer drugs.

Novel insight into the function of tankyrase

Tankyrases are multifunctional poly(ADP-ribose) polymerases that regulate a variety of cellular processes, including Wnt signaling, telomere maintenance and mitosis regulation. Tankyrases interact with target proteins and regulate their interactions and stability through poly(ADP-ribosyl) ation. In addition to their roles in telomere maintenance and regulation of mitosis, tankyrase proteins regulate tumor suppressors, including AXIN, phosphatase and tensin homolog and angiomotin. Therefore, tankyrases may be effective targets for cancer treatment. Tankyrase inhibitors could affect a variety of carcinogenic pathways that promote uncontrolled proliferation, including Wnt, AKT, yes-associated protein, telomere maintenance and mitosis regulation. Recently, novel aspects of the function and mechanism of tankyrases have been reported, and a number of tankyrase inhibitors have been identified. A combination of conventional chemotherapy agents with tankyrase inhibitors may have synergistic anticancer effects. Therefore, it is expected that more advanced and improved tankyrase inhibitors will be developed, enabling novel therapeutic strategies against cancer and other tankyrase-associated diseases. The present review discusses tankyrase function and the role of tankyrase inhibitors in the treatment of cancer.

Stretching, scrambling, piercing and entangling: Challenges for telomeres in mitotic and meiotic chromosome segregation

The consequences of telomere loss or dysfunction become most prominent when cells enter the nuclear division stage of the cell cycle. At this climactic stage when chromosome segregation occurs, telomere fusions or entanglements can lead to chromosome breakage, wreaking havoc on genome stability. Here we review recent progress in understanding the mechanisms of detangling and breaking telomere associations at mitosis, as well as the unique ways in which telomeres are processed to allow regulated sister telomere separation. Moreover, we discuss unexpected roles for telomeres in orchestrating nuclear envelope breakdown and spindle formation, crucial processes for nuclear division. Finally, we discuss the discovery that telomeres create microdomains in the nucleus that are conducive to centromere assembly, cementing the unexpectedly influential role of telomeres in mitosis.

Whole proteome analysis of human tankyrase knockout cells reveals targets of tankyrase-mediated degradation

Tankyrase 1 and 2 are poly(ADP-ribose) polymerases that function in pathways critical to cancer cell growth. Tankyrase-mediated PARylation marks protein targets for proteasomal degradation. Here, we generate human knockout cell lines to examine cell function and interrogate the proteome. We show that either tankyrase 1 or 2 is sufficient to maintain telomere length, but both are required to resolve telomere cohesion and maintain mitotic spindle integrity. Quantitative analysis of the proteome of tankyrase double knockout cells using isobaric tandem mass tags reveals targets of degradation, including antagonists of the Wnt/β-catenin signaling pathway (NKD1, NKD2, and HectD1) and three (Notch 1, 2, and 3) of the four Notch receptors. We show that tankyrases are required for Notch2 to exit the plasma membrane and enter the nucleus to activate transcription. Considering that Notch signaling is commonly activated in cancer, tankyrase inhibitors may have therapeutic potential in targeting this pathway.

Tankyrase 1 and 2 are poly(ADP-ribose) polymerases that mark proteins for degradation, but there is a current lack of knowledge about their distinct functions and substrates. Here, the authors elucidate the cellular roles and substrates of these polymerases using comparative functional and proteomics analyses of tankyrase knockout cell lines.

Radiosensitization with an inhibitor of poly(ADP-ribose) glycohydrolase: A comparison with the PARP1/2/3 inhibitor olaparib

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PARG and PARP inhibition both radiosensitize.

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PARP and PARG inhibition both alter the DNA damage response following irradiation (IR).

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PARP and PARG inhibition both alter homologous recombination following IR.

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Only PARG inhibition induces rapid activation of non-homologous end-joining post-IR.

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Only inhibition of PARG causes accumulation of cells in metaphase post-IR.

Upon DNA binding the poly(ADP-ribose) polymerase family of enzymes (PARPs) add multiple ADP-ribose subunits to themselves and other acceptor proteins. Inhibitors of PARPs have become an exciting and real prospect for monotherapy and as sensitizers to ionising radiation (IR). The action of PARPs are reversed by poly(ADP-ribose) glycohydrolase (PARG). Until recently studies of PARG have been limited by the lack of an inhibitor. Here, a first in class, specific, and cell permeable PARG inhibitor, PDD00017273, is shown to radiosensitize. Further, PDD00017273 is compared with the PARP1/2/3 inhibitor olaparib. Both olaparib and PDD00017273 altered the repair of IR-induced DNA damage, resulting in delayed resolution of RAD51 foci compared with control cells. However, only PARG inhibition induced a rapid increase in IR-induced activation of PRKDC (DNA-PK) and perturbed mitotic progression. This suggests that PARG has additional functions in the cell compared with inhibition of PARP1/2/3, likely via reversal of tankyrase activity and/or that inhibiting the removal of poly(ADP-ribose) (PAR) has a different consequence to inhibiting PAR addition. Overall, our data are consistent with previous genetic findings, reveal new insights into the function of PAR metabolism following IR and demonstrate for the first time the therapeutic potential of PARG inhibitors as radiosensitizing agents.

Cell cycle-regulated ubiquitination of tankyrase 1 by RNF8 and ABRO1/BRCC36 controls the timing of sister telomere resolution

Timely resolution of sister chromatid cohesion in G2/M is essential for genome integrity. Resolution at telomeres requires the poly(ADP-ribose) polymerase tankyrase 1, but the mechanism that times its action is unknown. Here, we show that tankyrase 1 activity at telomeres is controlled by a ubiquitination/deubiquitination cycle depending on opposing ubiquitin ligase and deubiquitinase activities. In late S/G2 phase, the DNA damage-responsive E3 ligase RNF8 conjugates K63-linked ubiquitin chains to tankyrase 1, while in G1 phase such ubiquitin chains are removed by BRISC, an ABRO1/BRCC36-containing deubiquitinase complex. We show that K63-linked ubiquitin chains accumulate on tankyrase 1 in late S/G2 to promote its stabilization, association with telomeres, and resolution of cohesion. Timing of this posttranslational modification coincides with the ATM-mediated DNA damage response that occurs on functional telomeres following replication in G2. Removal of ubiquitin chains is controlled by ABRO1/BRCC36 and occurs as cells exit mitosis and enter G1, ensuring that telomere cohesion is not resolved prematurely in S phase. Our studies suggest that a cell cycle-regulated posttranslational mechanism couples resolution of telomere cohesion with completion of telomere replication to ensure genome integrity.