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Putative role of SUMOylation in controlling the activity of deubiquitinating enzymes in cancer Katarzyna C Masoumi1, Gemma Marfany2,3,4, Yingli Wu5 & Ramin Massoumi*,1

Deubiquitinating enzymes (DUBs) are specialized proteins that can recognize ubiquitinated proteins, and after direct interaction, deconjugate monomeric or polymeric ubiquitin chains, thus changing the fate of the substrates. This process is instrumental in mediating or changing downstream signaling pathways. Beside mutations and alterations in their expression levels, the activity and stability of deubiquitinating enzymes is vital for their function. SUMOylations consist of the conjugation of the small peptide SUMO to protein substrates which is very similar to ubiquitination in the mechanistic and machinery required. In this review, we will focus on how SUMOylation can regulate DUB enzymatic activity, stability or DUB interaction with partners and substrates, in cancer. Furthermore, we will discuss the impact of these recent findings in the identification of new potential tools for efficient anticancer treatment strategies. First draft submitted: 8 September 2015; Accepted for publication: 6 November 2015; Published online: 18 January 2018 Deubiquitination & SUMOylation machinery Post-translational modifications (PTMs) of proteins such as ubiquitination and SUMOylation can affect downstream signaling inside the cells and quickly switch on or off different cellular processes, including survival, proliferation and differentiation. Ubiquitination and SUMOylation of target substrates are facilitated through a hierarchical interplay between specific enzymes in the ubiquitin and SUMO pathways, respectively. Ubiquitination is the process of covalent conjugation of ubiquitin or ubiquitin chains to specific lysine residues of protein substrates. Generally, polyubiquitination through the ubiquitin of lysines 11, 29 and 48 are involved in substrate degradation via proteasome, whereas lysine 63-linked polyubiquitin chains are mainly involved in DNA repair, transcription and recombination, protein trafficking, and the activation of kinases [1–3] . Ubiquitination of substrates is a reversible process, and deubiquitinating enzymes (DUBs) can bind directly to the targeted substrate and release ubiquitin or ubiquitin chains from substrates [4–6] . The human genome encodes approximately 90 putative DUBs, which are classified into cysteine and metalloproteases subfamilies. Alterations in the function of this family of enzymes have been discovered in many pathologies, such as cancer and neurodegeneration [7–10] . Cysteine proteases are divided into four subclasses: Machado-Joseph disease (MJD) proteases, ubiquitin-specific Department of Laboratory Medicine, Medicon Village, Lund University, 22381 Lund, Sweden Departament de Genètica, Facultat de Biologia, Universitat de Barcelona, 08028 Barcelona, Spain 3 Institut de Biomedicina (IBUB), Universitat de Barcelona, 08007 Barcelona, Spain 4 CIBERER, Instituto de Salud Carlos III, Barcelona, Spain 5 Department of Pathophysiology, Chemical Biology Division of Shanghai Universities E-Institutes, Shanghai Jiao Tong University School of Medicine, Shanghai 200025, China *Author for correspondence: Tel.: +46 768 890 264; [email protected]

Keywords

• ataxin-3 • CYLD • deubiquitination • SUMOylation • USP25 • USP28 • USP39

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Review  Masoumi, Marfany, Wu & Massoumi proteases (USP), OTU proteases and ubiquitin C-terminal hydrolases (UCH), while there is a single metalloprotease class that groups the enzymes with the ubiquitin protease domain JAMM (JAB1/MPN/Mov34 metalloenzyme) [11] . MJDs consist of ataxin-3 and Josd proteins and their functions are best studied in neurodegenerative disorders [12] although they have also been involved in DNA repair lately [13] . The structure of the catalytic (Josephin) domain of ataxin-3 shows a typical UCH domain and it has been demonstrated that ataxin-3 preferably removes lysine 63 ubiquitin chains from substrates  [14] via its ubiquitin-interacting motifs, which were required for binding to ubiquitin and ubiquitin cleavage [14] . USPs are the largest of the DUB families and these DUBs can process both small and larger substrates [10,15] . In general, cysteine and histidine boxes are the two well-conserved catalytic domains of USPs and mutations of these domains render the enzyme catalytically inactive [4] . Although the functional data are scarce for USPs, some members have been more deeply studied, such as CYLD, USP25, USP28 and USP39. SUMOylation is also a PTM that plays a major role in different cellular processes [16] . There are four different isoforms of SUMO protein (SUMO-1, SUMO-2, SUMO-3 and SUMO-4). Similarities between ubiquitination and SUMOylation are unique: the SUMO and ubiquitin polypeptides share an amino acid identity of 18 %; a substrate can undergo either mono- or poly-SUMOylation, similarly to mono- or poly-ubiquitination; also, similarly to the ubiquitination pathway, the SUMOylation of any substrate needs a hierarchical interplay of three enzymatic steps including SUMOactivation by enzyme E1 (SAE1/2), SUMOconjugation by enzyme E2 (UBC9) and SUMOligation by enzyme E3 (among them the PIAS family and RanBP2, but the number of reported E3 ligases in mammals has increased up to 15 members [17]); and the two processes are reversible, DUBs reverse ubiquitination, whereas deSUMOylation enzymes, namely SUMOspecific proteases, deconjugate SUMO from the substrates [16,18–20] . Current studies highlight the interplay between the SUMO and ubiquitin pathways. SUMOylation of a given substrate in the same lysine residues where it can be ubiquitinated can prevent polyubiquitination and proteasome mediated-degradation. In the NF-κB signaling

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pathway, ubiquitination of inhibitor of nuclear factor-κB (IκBα) on lysine 21 and 22 promotes the degradation of IκBα, whereas SUMOylation on lysine 21 prevents ubiquitination and degradation of IκBα [21] . Furthermore, SUMOylation and ubiquitination can alter the substrate subcellular localization. Monoubiquitination of p53 promotes SUMOylation and nuclear export of p53 [22] . However, a ubiquitin-independent and SUMO-dependent pathway of p53 nuclear export has been discovered recently [23] . In the regulation of NF-κB signaling, SUMOylation of NEMO on lysines 277 and 309 facilitates the translocation of NEMO to the nucleus, while ubiquitination of the same residues holds NEMO in the cytoplasm compartment [24] . Many DUBs can undergo PTMs including phosphorylation and ubiquitination, which can lead to inactivation, activation or the degradation of these DUBs. SUMOylation of several DUBs, including ataxin-3, CYLD, USP25, USP28 and USP39, which has been discovered recently, is the focus of this review. Ataxin-3 Ataxin-3 belongs to the MJD class of cysteine proteases that binds poly-ubiquitinated proteins and has ubiquitin protease activity [25] . Ataxin-3 has many isoforms, and the largest displays three ubiquitin-interacting motifs (UIMs), one SUMO-interacting motif (SIM) and a tandem domain array that has been suggested to act as a receptor of SUMO-ubiquitin hybrid chains with putative functions in regulation of cell cycle and DNA repair [26] . The DUB activity of ataxin-3 is higher toward cleavage of longer lysine 63 poly-ubiquitin chains compared with other lysines  [14] . Close to the UIM domains, a polyglutamine stretch can be identified. Mutations expanding the polyglutamine stretch cause a neurodegenerative disorder, namely spinocerebellar ataxia type 3/MJD [27] , which is the most common dominantly inherited ataxia [28] . Ataxin proteins, including ataxin 1 [29] , ataxin 7 [30] and the deubiquitinating enzyme, ataxin-3  [31,32] have been shown to undergo SUMOylation. The interaction between ataxin-3 and SUMO was first identified through yeast two-hybrid screening by using ataxin-3 as bait and identifying SUMO-1 as an interactor partner at the N-terminal of the protein [31] at lysine 166 [32] . This SUMOylation did not influence the subcellular localization or ubiquitination of ataxin-3, but instead increased its

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SUMOylation in controlling the activity of DUBs  stability  [31] . Mutation of lysine 166 reduced cell death in HEK293 cells, suggesting that SUMOylation of ataxin-3 promotes cell apoptosis  [32] . SUMOylation of ataxin-3 was confirmed through immunofluorescence staining in neuroblastoma cancer cells, where ataxin-3 and SUMO co-localized in the nucleus of the cells  [31] . In an independent study, lysine 356, instead of lysine 166, was identified as the main SUMOylation site of ataxin-3. SUMOylation at lysine 356 affected the interaction between ataxin-3 and p97 [33] . The protein p97 is an AAA+ hexameric ring-shaped ATPase involved in the degradation of soluble and membrane proteins by the ubiquitin-proteasome system [34,35] . Since SUMOylation of ataxin-3 increased the affinity for binding to the protein p97, this resulted in decreasing the formation of amyloid fibrils [33] . It has to be noted that in cancer, elevated levels of p97 have been correlated with tumor aggressiveness [36] ; however, the precise underlying molecular mechanisms are unknown. Remarkably, ataxin-3 activity is allosterically regulated by monoubiquitination in lysine 117, which results in a conformational change that locks the enzyme in a locked activated state [37] . Therefore, ataxin-3 can bind both SUMO and ubiquitin moieties, and it is also post-translationally modified by SUMO (increase of stability) and ubiquitin (increase of the enzymatic activity), setting the grounds for an interesting cross-talk between the two PTMs. Several additional pieces of evidence have associated ataxin-3 with cancer. Recent results suggest that ataxin-3 plays a role in DNA repair, since mutant ataxin-3 cells show increased DNA damage and apoptosis via the activation of several tumor suppressor pathways, such as p53 and the ataxia-telangiectasia-mutated signaling pathways [26] . Besides, ataxin-3 negatively regulates the transcription of the well-known tumor suppressor PTEN in lung cancer cells [38] . All of these results provide an interesting link between neurodegeneration, DNA damage and cancer. CYLD Originally, the CYLD gene was discovered in patients suffering from a benign skin cancer, cylindromatosis. Linkage analysis in multiple cylindromas mapped the susceptibility gene CYLD to a single locus on chromosome 16q12–13 in these families [39] . The CYLD protein, which is encoded by a single gene in

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humans and mouse, is ubiquitously expressed and highly conserved. CYLD protein contains three cytoskeletal-associated protein-glycineconserved domains. The cytoskeletal-associated protein-glycine-conserved domains of CYLD are important for the re-localization of CYLD or binding to substrates and functioning as a scaffold [40–43] . The DUB function of CYLD is through the C-terminal domain, which encodes an ubiquitin carboxyl-terminal UCH domain. In general, CYLD recognizes and cleaves K63-linked and linear ubiquitin structures following interaction with the substrate [44,45] . Modification/removal of ubiquitin chains from specific substrates allows CYLD to negatively regulate multiple signaling pathways including NF-kB  [9,45–46] . TRAF2 and TRAF6 were the first CYLD substrates to be identified. Dissociation of K63linked polyubiquitin chains from these proteins by CYLD blocked the classical NF-kB signaling cascade [47–49] . In neuroblastoma, a childhood cancer, force differentiation of these cells with all-trans retinoic acid (ATRA) leads to an increase in the expression of SUMO1 and SUMO2 proteins as well as activation of the NF-κB signaling pathway. Prolonged treatment with ATRA for more than 6 days reduces NF-κB signaling and instead facilitates cell death [50] . ATRA treatment of neuroblastoma also causes a transient SUMOylation of CYLD and reduction in the DUB activity of CYLD, as well as activation of NF-κB signaling via ubiquitination of TRAF2/TRAF6 (Figure 1A) . Lysine 40 was the SUMO binding site located at the N-terminal part of CYLD [50] . Mutation of this lysine inhibited removal of the lysine 63-linked poly-ubiquitin chain from TRAF6 and TRAF2. Since prolonged ATR A treatment reduced CYLD SUMOylation and promoted cell death, it was suggested that the balance between nonSUMOylated and SUMOylated CYLD can direct neuroblastoma cancer cells against differentiation or cell death via regulation of NF-κB signaling [50] . USP25 USP25 is a DUB encoded by a gene located in the gene-poor region 21q11.2 [51] , whose function is still not well characterized. This gene has been long associated with immune response, inflammation and cancer, and several evidences have been gathered that support the involvement

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RA

S

UU

63

USP25

USP25

SUMOylation

U IκBα U U

Enzymatic activityn

Ubiquitination USP25

5 p5 0

U

U

U

S

p6

S

C In YLD ac tiv e

Ac t CY ive LD

TRAF2/6

Enzymatic activityn

Phosphorylation

Proteasomal degradation

p6 5 p5 0

Enzymatic activityn

Repressing the expression by miR-200a

P USP25

USP25

Cell differentiation SUMO sites USP39SUMO RS

ZnP

UCH1

UCH2

ZnP

UCH1

UCH2

SENP1 USP39

RS

Recruitment of tri-snRNP Proliferation

Figure 1. The impact of SUMOylated deubiquitinating enzymes in cancer. (A) Model of ATRA-mediated regulation of NF-κB signaling by CYLD in neuroblastoma cells. SUMOylation of CYLD induced by ATRA inhibits DUB activity of CYLD and prevents removal of lysine 63-linked polyubiquitin chain from TRAF2 and TRAF6. This leads to activation of NF-κB (p65 and p50) via proteasomal degradation of IκBα and differentiation of neuroblastoma cells. (B) Pressumptive targets for decreasing USP25 expression in cancer treatment, emphasizing post-translational modifications and post-transcriptional repression. A red arrow indicates the pathway that could be favored (lower activity of the enzyme) in contrast to the red cross that indicates the pathway that increases the activity of the enzyme. (C) Model of how deSUMOylation of USP39 via SENP1 affects the recruitment of tri-snRNP leading to an increase in the proliferation rate of prostate cancer cells. 48: Lysine 48 ubiquitin chains; 63: Lysine 63 ubiquitin chains; ATRA: All-trans retinoic acid; DUB: Deubiquitinating enzyme; P: Phosphate; RA: Retinoic acid; S: SUMO; U: Ubiquitin.

of this DUB in these processes [52–54] . Moreover, some authors have reported that USP25 is involved in the ER stress produced by misfolded mutant proteins that aggregate, particularly with amyloid precursor protein (APP) [55–57] . USP25, a large DUB that displays one SIM domain intercalated in tandem to several ubiquitin-binding domains (one ubiquitin associated domain [UBA] and two UIMs) at the N-terminus  [58,59] , is the target of a variety of PTMs including phosphorylation, acetylation, SUMOylation and ubiquitination. Similarly to ataxin-3, USP25 can recognize and bind both

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SUMO and ubiquitin moieties via its SIM and ubiquitin-binding domains. In fact the catalytic activity of the enzyme is regulated by SUMO or ubiquitin conjugation. In USP25, the same lysine residue, lysine 99 (K99), can be either SUMOylated [58] or ubiquitinated [59] , and these mutually exclusive modifications have opposite effects on USP25 activity. SUMOylation at K99 and K141, both located within the UIM domains, inhibits the catalytic activity of USP25 by reducing its binding and hydrolysis of ubiquitin chains [58] . Conversely, ubiquitination at K99 activates the deubiquitinating activity,

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SUMOylation in controlling the activity of DUBs  probably by preventing SUMOylation at the same residue [59] Since USP25 is capable of autodeubiquitination, a self-regulatory model for post-translational regulation of USP25 activity has been proposed, a mechanism that may well apply to the regulation of the enzymatic activity of other DUBs [59] . USP25 has been shown to undergo other PTMs that may regulate their activity or their contribution into a particular cell signaling pathway, for example, USP25 is phosphorylated by SYK tyrosine kinase, which does not affect the DUB enzymatic activity but decreases its protein levels [60] , and also by vaccinia-related kinase 2 that inactivates USP25 enzymatic activity [57] . On the other hand, USP25 has been repeatedly associated with cancer. For instance, higher levels of USP25 have been reported in breast [61] and non-small-cell lung cancer (NSCLC) [54] patients. Furthermore, miR-200c is considered a tumor suppressor gene because it represses the levels of USP25, which when overexpressed, favors tumor cell migration, invasion and metastasis  [54] . These incidental results are strongly suggestive of presumptive targets for novel drug therapy in particular cancer types where overexpression of USP25 is instrumental for carcinogenesis and metastasis. For instance, an inhibition of USP25 enzymatic activity could be achieved by facilitating its SUMOylation and/or preventing ubiquitination to favor SUMOylation (Figure 1B) . Other anticancer treatments might increase USP25 phosphorylation or facilitate miR-200c-mediated repression of USP25 (Figure 1B) . Nonetheless, further characterization of USP25-dependent pathways is still missing and any anticancer treatment designed to decrease USP25 activity should be carefully pondered, particularly since USP25 stabilizes some chaperonins (TRiC/CCT) against proteasome degradation [57] ; is also involved in endoplasmic reticulum stress related to aggregated proteins in metabolic and neurodegenerative disorders [55,56] ; and knockout UPS25-/- mice showed an exacerbated inflammatory response and were much more susceptible to develop ­autoimmune diseases  [52] . USP28 USP28 belongs to the USP family and was originally identified through its homology to USP25, since both of them are evolutionarily derived from a common ancestor [62] . Similar to USP25, USP28 shows a SIM domain intercalated amid

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a tandem array of one UBA and two UIM domains. USP28 binds to 53BP protein and is involved in double-strand break repair mechanism  [63] . However, USP binding to 53BP1 seems not to be crucial for the 53BP1-dependent DNA damage response in vitro or in vivo using USP28-deficient mice [64,65] . Irradiation of cells and activation of ataxia-telangiectasia mutated enzyme causes the phosphorylation of USP28 (Serine 67 and Serine 714) [63] and USP28 has been implicated in the stabilization of claspin to maintain the cells at G2 cell cycle arrest [66] . USP28 has been shown to rescue substrates and prevent their proteasomal degradation. However, in vitro, USP28 is unable to distinguish between lysine-11, -48 and -63-linked poly-ubiquitin chains. This could be explained by results showing that the deubiquitinating activity of USP28 was not affected by deletion of the UBA, UIM or SIM domains in vitro [67] , which suggests that USP28 might be more selective in vivo, probably due to the proteins that it binds to and/or the PTMs that this enzyme undergoes. Some of the substrates that USP28 can rescue from degradation include Myc, c-Jun, Notch-1 and cyclin E. These substrates undergo ubiquitin-mediated degradation by the action of Fbw7, which is an E3 ubiquitin ligase [68,69] . The best known and validated substrate for USP28 is MYC. USP28 does not bind to MYC directly, but can interact through its association via Fbw7. Dissociation of USP28 from the Fbw7 was necessary for MYC-mediated ubiquitination and degradation via the proteasome by Fbw7 upon DNA damage [70] . USP28 level has been shown to be upregulated in colon adenocarcinomas and invasive ductal breast carcinomas compared with normal colon or breast tissues. In colon cancer cells, overexpression of the wildtype, but not the catalytically mutant form, of USP28 stabilized the MYC protein and promoted an increase in the proliferation rate of cells [68] . This increased cell proliferation in the presence of USP28 has also been shown in NSCLC cells, where high USP28 expression inversely correlated with s­urvival rate in NSCLC patients [71] . The N-terminal of USP28 attracts the attachment of SUMO; more precisely, at lysine 99 located in the UIM domain [67] . SUMO conjugation to USP28 inhibited the DUB activity of USP28, without any preferences against ubiquitin conjugation against lysine 48 or lysine 63 [67] . Although alternative and mutually exclusive ubiquitination has not been demonstrated so far,

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Review  Masoumi, Marfany, Wu & Massoumi based on this reported finding, inactivation of USP28 via SUMO binding and SUMOylation would lead to increased levels of Myc and the progression of different types of cancer. USP39 USP39 encodes a conserved protein termed Sad1p, which was originally identified in a screening of mutations blocking the assembly of newly synthesized U4 snRNA into a functional snRNP in Saccharomyces cerevisiae  [72] . Sad1p localizes to the nucleus and is not stably associated with any of the U snRNAs. Sad1p contains a zinc finger and is highly conserved, with homologs identified in humans, Caenorhabditis elegans, Arabidopsis and Drosophila. Sad1p is involved in splicing in vivo and in vitro and in the assembly of U4 snRNP to U6 snRNP. In 2001, Makarova et al. identified USP39, the homolog of Sad1p in humans through SDSPAGE of purified HeLa cell U4/U6.U5 trisnRNPs, followed by EST database analysis [73] . USP39 shares 65 % amino acid similarity with yeast Sad1 and contains the N-terminal RS-like (rich in arginine, serine and glutamic acid and resembles the RS domain of SR-related proteins), zinc finger and two ubiquitin C-terminal hydrolase (UCH-1 and UCH-2) domains. Although both Sad1p and USP39 contain Znf-UBP and UCH, they cannot bind ubiquitin nor do they have hydrolase activity [74,75] . This might be due to a functional critical cysteine residue in the catalytic center of UCHs which is replaced by aspartate. Moreover, the two histidine residues, which are important for UCHs, are absent from the UCH-2-like domain in USP39. USP39 is

essential for recruitment of the tri-snRNP to the prespliceosome and is known as a tri-snRNPspecific protein. Furthermore, the depletion of USP39 causes defect in spindle checkpoint function and cytokinesis through regulating of Aurora B mRNA splicing in mammalian cells  [75] and zebrafish USP39 mutation leads to rb1 splicing defect and pituitary lineage expansion [76] . Accumulating data indicate that USP39 plays a critical role in cancer development. Overexpression of USP39 was observed in breast cancer and hepatocellular cancer cells, while silencing of USP39 significantly inhibited cell proliferation and increased the population of apoptotic cells [77–79] . Consistent with these reports, overexpression of USP39 was recently found to enhance the proliferation of the androgen-independent PC3 and androgen-dependent LNCaP cells [80] . Furthermore, USP39 can undergo SUMOylation and inhibition of the SUMOylation of USP39 will further increase the proliferation of PC3 cells (Figure 1C) [80] . All five sumo sites of USP39 (K6, K16, K29, K51, K73) are located within the RS-like domain. Since the RS domain in SR proteins is essential to recruit the tri-snRNP to the prespliceosome, SUMOylation of USP39 could affect the recruitment of tri-snRNP. Conclusion In summary, further studies are still needed to find the consequences and outcomes of SUMOylated DUBs in cancer. Among the cellular pool of any particular protein, the percentage of SUMO-modified proteins is assumed

Table 1. The action of ubiquitination is reversible by deubiquitinating enzymes and dysregulation of deubiquitinating enzyme function has been connected to the etiopathogenesis of human diseases, including cancer. Gene

DUB family SUMO site/s

Consequence

Cancer

Ataxin-3 

MJD

K166 or K356

Lung cancer

[38]

CYLD

USP

K40

Neuroblastoma

[50]

USP25

USP

K99

Stability or binding to p97 and PTEN Inactivation of DUB activity Inactivation of DUB activity Inactivation of DUB activity de-SUMOylation of USP39 enhances cell proliferation

Breast cancer Non-small-cell lung cancer ?

[54]

Prostate cancer

[80]

USP28

USP

K99

USP39

USP

K6, K16, K29, K51, K73

Ref.

Several DUBs including ataxin-3, CYLD, USP25, USP28 and USP39 can undergo SUMOylation, which in turn leads to inactivation/ activation, changing the associated partner or degradation of these DUBs. ?: Not known; DUB: Deubiquitinating enzyme; MJD: Machado-Joseph disease; USP: Ubiquitin-specific proteases.

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[61] [67]

SUMOylation in controlling the activity of DUBs  to be limited; therefore, any small shift in the equilibrium of PTMs for a given DUB might be of extreme significance to the initiation, progression and eventual outcome of the disease. In this aspect, global proteomics analyses of SUMO modifications in DUBs, as a means to obtain a comprehensive view of PTMs in DUBs and their regulation, will certainly provide important information connected to identifying activation of specific cancer pathways as well as to defining the tumor stage, both being particular poignant issues in cancer where targeted and timely therapeutic intervention is decisive for tumor remission and patient survival. Drugs targeting proteins of the ubiquitin/proteasome and SUMO pathways, such as DUBs and SUMO metabolism enzymes, are either on the way or already have entered clinical trials in cancer therapy. A better understanding of the cross-talk and interplay between these two pathways can lead to the identification of novel anticancer tools for treating diseases in which SUMOylation plays a major role. Future perspective The human genome encodes a large number of putative ubiquitin ligases and deubiquitination enzymes. Mounting evidence indicates that

Review

most of these enzymes regulate a large number of proteins whose expression levels, activation or stability are crucial for maintaining normal cellular function. Interfering with ubiquitination machinery and the regulation of these signaling key molecules leads to pathogenesis. Obviously, dysregulation in the function of DUBs is considered a hallmark of many human diseases, such as neurodegeneration and cancer. The function of DUBs can be modulated through PTM and SUMOylation is now emerging as the main process involved in the aetiopathogenesis and progression of human disease states, particularly in cancer (see Table 1). The SUMOylation of certain DUBs has been shown to directly affect tumor cell growth and promote insensitiveness to cell death signals, through modification of the signaling pathways by translocation of the proteins into different subcellular compartments, altering the interaction and recruitment of partners in protein complexes, and changing the half-life/ stability of proteins, thereby shifting the scales toward cell proliferation and survival. In this context, recent reports on combined ubiquitin and SUMO modifications in DUBs [81,82] , the presumed role for some DUBs to act as co-receptors for doubly modified SUMO and ubiquitin protein substrates [26] , and the merging link

Executive summary Ataxin-3 SUMOylation at different sites of the protein with different outcomes ●●

Ataxin-3 SUMOylation leads to an increase in the stability of ataxin-3 for mediating cell death.

●●

Ataxin-3 SUMOylation promotes interaction between ataxin-3 and p97 for reducing the formation of amyloid fibrils.

In neuroblastoma, CYLD shifts between the non-SUMO & the SUMO-conjugated state ●●

T ransient SUMOylation of CYLD reduces its deubiquitinating enzyme (DUB) activity and promotes differentiation of cancer cells.

●●

Non-SUMOylated CYLD promotes cancer cell death.

USP25 activity is controlled by SUMO & ubiquitin conjugation ●●

SUMOylated USP25 is inactive, whereas ubiquitination enhances USP25 activity.

●●

Attachment of SUMO to USP28 could lead to increased levels of Myc and progression of different types of cancer.

Inhibition of USP39 SUMOylation affects the growth of cancer cells ●●

Non-SUMOylated USP39 increases the proliferation rate of prostate cancer cells.

●●

Non-SUMOylated USP39 could be implicated in cancer development through affecting the recruitment of tri-snRNP.

Conclusion ●●

SUMOylation of DUBs can change the cellular response, thereby affecting the development or progression of cancer.

●●

T he proteomics analyses of SUMO modifications for DUBs will result in prior important information about the posttranslational modification state of the cell DUBs, and provide valuable insight into how to target key proteins in canceraltered pathways and identify novel effective drugs for treating SUMO-mediated cancer-related diseases.

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Review  Masoumi, Marfany, Wu & Massoumi between ubiquitin/SUMO PTMs controlling the nucleocytoplasmic shuttling of some cancer proteins [83] are intriguing new venues to explore. Acknowledgements G Marfany acknowledges all the present and past members of her research group for long-standing support and helpful discussions.

Financial & competing interests disclosure R Massoumi has been funded by Swedish Cancer Foundation and by funding from the European Research

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Council (ERC), under the European Union’s Seventh Framework Programme for Research and Technology Development, grant agreement no. 260460. G Marfany has been funded by BFU2010-15656 and SAF201349069-C2-1-R (Ministerio de Ciencia e Innovación) and SGR2014-0932 (Generalitat de Catalunya). The authors have no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript apart from those disclosed. No writing assistance was utilized in the production of this manuscript.

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Putative role of SUMOylation in controlling the activity of deubiquitinating enzymes in cancer.

Deubiquitinating enzymes (DUBs) are specialized proteins that can recognize ubiquitinated proteins, and after direct interaction, deconjugate monomeri...
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