SUMO proteases: redox regulation and biological consequences

Sentrin-specific protease 1 maintains mitochondrial homeostasis through targeting the deSUMOylation of sirtuin-3 to alleviate oxidative damage induced by hepatic ischemia/reperfusion

Hepatic ischemia/reperfusion injury (HIRI) represents a prevalent pathophysiological process that imposes a substantial economic burden in clinical practice, especially in liver surgery. Sentrin-specific protease 1 (SENP1) is a crucial enzyme involved in the regulation of SUMOylation, and is related to various diseases. However, the role of SENP1 in HIRI remains unexplored. Here, we confirmed that SENP1 actively participated in modulating the oxidative damage induced by HIRI. Notably, SENP1 functioned by maintaining mitochondrial homeostasis. Further mechanistic exploration indicated that the protective mitochondrial protein sirtuin-3 (Sirt3) was inactivated by SUMOylation during HIRI, which was reversed by SENP1. Overexpression of SENP1 could restore mitochondrial function, mitigate oxidative stress and attenuated apoptosis through recovering the expression of Sirt3 during HIRI. Nevertheless, 3-TYP, an inhibitor of Sirt3, could eliminate the therapeutic effects brought by overexpression of SENP1. In conclusion, our findings demonstrated that SENP1 mediated the deSUMOylation of Sirt3 and maintained mitochondrial homeostasis, thus alleviating HIRI induced oxidative damage. SENP1 might be a promising therapeutic target for HIRI.

TFEB SUMOylation in macrophages accelerates atherosclerosis by promoting the formation of foam cells through inhibiting lysosomal activity

Atherosclerosis (AS) is a serious cardiovascular disease. One of its hallmarks is hyperlipidemia. Inhibiting the formation of macrophage foam cells is critical for alleviating AS. Transcription factor EB (TFEB) can limit the formation of macrophage foam cells by upregulating lysosomal activity. We examined whether TFEB SUMOylation is involved in this progress during AS. In this study, we investigated the role of TFEB SUMOylation in macrophages in AS using TFEB SUMOylation deficiency Ldlr-/- (TFEB-KR: Ldlr-/-) transgenic mice and TFEB-KR bone marrow-derived macrophages. We observed that TFEB-KR: Ldlr-/- atherosclerotic mice had thinner plaques and macrophages with higher lysosomal activity when compared to WT: Ldlr-/- mice. TFEB SUMOylation in macrophages decreased after oxidized low-density lipoprotein (OxLDL) treatment in vitro. Compared with wild type macrophages, TFEB-KR macrophages exhibited less lipid deposition after OxLDL treatment. Our study demonstrated that in AS, deSUMOylation of TFEB could inhibit the formation of macrophage foam cells through enhancing lysosomal biogenesis and autophagy, further reducing the accumulation of lipids in macrophages, and ultimately alleviating the development of AS. Thus, TFEB SUMOylation can be a switch to modulate macrophage foam cells formation and used as a potential target for AS therapy.

Crosstalk between SUMOylation and ubiquitylation controls DNA end resection by maintaining MRE11 homeostasis on chromatin

DNA end resection is delicately regulated through various types of post-translational modifications to initiate homologous recombination, but the involvement of SUMOylation in this process remains incompletely understood. Here, we show that MRE11 requires SUMOylation to shield it from ubiquitin-mediated degradation when resecting damaged chromatin. Upon DSB induction, PIAS1 promotes MRE11 SUMOylation on chromatin to initiate DNA end resection. Then, MRE11 is deSUMOylated by SENP3 mainly after it has moved away from DSB sites. SENP3 deficiency results in MRE11 degradation failure and accumulation on chromatin, causing genome instability. We further show that cancer-related MRE11 mutants with impaired SUMOylation exhibit compromised DNA repair ability. Thus, we demonstrate that MRE11 SUMOylation in coordination with ubiquitylation is dynamically controlled by PIAS1 and SENP3 to facilitate DNA end resection and maintain genome stability.

DNA end resection initiating DNA repair by homologous recombination needs to be delicately regulated. This study shows the interplay between SUMOylation and ubiquitylation maintains MRE11 homeostasis on chromatin, thus facilitating genome stability.

SENP1 promotes MCL pathogenesis through regulating JAK-STAT5 pathway and SOCS2 expression

Mantle cell lymphoma (MCL) is highly aggressive and its treatment remains challenging, understanding its pathogenesis is critical for future targeted therapy. SUMO specific proteases 1 (SENP1) is an important protein that regulates the balance between SUMOylation and deSUMOylation. We found that SENP1 was upregulated in MCL patient samples and cell lines. Knockdown of SENP1 could inhibit the proliferation and promote the apoptosis of MCL cells. We also found that SENP1 knockdown caused inhibition of the JAK-STAT5 pathway and upregulation of tumor suppressor cytokine signaling 2 (SOCS2). Moreover, MCL tumor growth in vivo was significantly suppressed after SENP1 knockdown in a xenograft nude mouse model. In summary, our results showed that SENP1 is involved in the pathogenesis of MCL and may be a potential therapeutic target.

A disease-causing mutation K240E disrupts ferroportin trafficking by SUMO (ferroportin SUMOylation)

Ferroportin (Fpn/IREG1/MTP1) is the only known transporter mediating iron efflux from epithelial cells and macrophages, and thus regulates how much iron is released into the circulation. Consequently, Fpn mutations are associated with haemochromatosis. Fpn itself is post-translationally regulated by hepcidin (Hepc) which induces its redistribution and degradation in a ubiquitin-dependent process. Together, the two proteins appear to be the nexus for iron homeostasis. Here we show that a rare gain-of-function mutation (K240E) that is associated with iron overload, impedes Fpn binding and subcellular trafficking by the small ubiquitin-like modifier (SUMO). Whereas wild-type Fpn is ensconced within vesicular bodies, the FpnK240E mutant appeared diffused within the cell when co-expressed with SUMO. Furthermore, compared with wild type Fpn, the sumoylation-defective mutant was constitutively-active, resulting in a lower intracellular labile iron pool than the former. These findings suggest that SUMO may regulate iron homeostasis by controlling Fpn trafficking.

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Ferroportin (Fpn) regulates iron efflux.

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A disease causing mutation (K240E) in a patient causes iron-overload.

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Fpn K240 is a SUMO conjugation site important for Fpn trafficking to endosomes by SUMO.

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The Fpn mutant K240E cannot be trafficked properly by SUMO and is a gain-of-function mutant that is constitutively active.

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FpnK240E effluxes more iron from intracellular stores than wild type Fpn.

Hypoxia protects H9c2 cells against Ferroptosis through SENP1-mediated protein DeSUMOylation

Hypoxia affects proliferation, differentiation, as well as death of cardiomyocyte, and plays an important role in the development of myocardial ischemia. However, the detailed mechanisms through which hypoxia regulates cardiomyocyte ferroptosis have not been explored. In this study, we revealed that hypoxia suppresses the proliferation, migration, and erastin-induced ferroptosis of H9c2 cells. First, we confirmed the upregulation of SENP1 in H9c2 cells cultured under hypoxic conditions. Through adenovirus-mediated SENP1 gene transfection, we demonstrated that SENP1 overexpression could enhance H9c2 cell proliferation and migration while also protecting H9c2 cells from erastin-induced ferroptosis. Furthermore, through immunoprecipitation and western blotting, we confirmed that SENP1 mediated deSUMOylation of HIF-1α and ACSL4 in H9c2 cells. In conclusion, this study describes the underlying mechanism through which hypoxia upregulates SENP1 expression, in turn protecting against ferroptosis via the regulation of HIF-1α and ACSL4 deSUMOylation. Our findings provide a theoretical foundation for the development of novel therapeutics for ischemic heart diseases.

SUMOylation, a multifaceted regulatory mechanism in the pancreatic beta cells

SUMOylation is an evolutionarily conserved post-translational modification (PTM) that regulates protein subcellular localization, stability, conformation, transcription and enzymatic activity. Recent studies indicate that SUMOylation plays a key role in insulin gene expression, glucose metabolism and insulin exocytosis under physiological conditions in the pancreatic beta cells. Furthermore, SUMOylation is implicated in beta cell survival and recovery following exposure to oxidative stress, ER stress and inflammatory mediators under pathological situations. SUMOylation is closely regulated by the cellular redox status, and it collaborates with other PTMs such as phosphorylation, ubiquitination, and NEDDylation, to maintain beta cellular homeostasis. We hereby provide an update on recent findings regarding the role of SUMOylation in the regulation of pancreatic beta cell viability and function, and discuss its potential implication in beta cell senescence and RNA processing (e.g., pre-mRNA splicing and mRNA methylation). Through which we intend to provide novel insights into this fundamental biological process regarding both maintenance of beta cell viability and functionality, and beta cell dysfunction in diabetes mellitus.

Roles and mechanisms of SUMOylation on key proteins in myocardial ischemia/reperfusion injury

Myocardial ischemia/reperfusion (MI/R) injury has a great influence on the prognosis of patients with acute coronary occlusion. The underlying mechanisms of MI/R injury are complex. While the incidence of MI/R injury is increasing every year, the existing therapies are not satisfactory. Recently, small ubiquitin-related modifier (SUMO), which is a post-translational modification and involved in many cell processes, was found to play remarkable roles in MI/R injury. Several proteins that can be SUMOylated were found to interfere with different mechanisms of MI/R injury. Sarcoplasmic reticulum Ca²⁺ ATPase pump SUMOylation alleviated calcium overload. Among the histone deacetylase (HDAC) members, SUMOylation of HDAC4 reduced reactive oxygen species generation, whereas Sirt1 played protective roles in the SUMOylated form. Dynamic-related protein 1 modified by different SUMO proteins exerted opposite effects on the function of mitochondria. SUMOylation of hypoxia-inducible factors was fundamental in oxygen homeostasis, while eukaryotic elongation factor 2 SUMOylation induced cardiomyocyte apoptosis. The impact of other SUMOylation substrates in MI/R injury remains unclear. Here we reviewed how these SUMOylated proteins alleviated or exacerbated myocardial impairments by effecting the MI/R injury mechanisms. This may suggest methods for relieving MI/R injury in clinical practice and provide a reference for further study of SUMOylation in MI/R injury.

The complex roles of STAT3 and STAT5 in maintaining redox balance: Lessons from STAT-mediated xCT expression in cancer cells

STAT3 and STAT5 mediate diverse cellular processes, transcriptionally regulating gene expression and interacting with cytoplasmic proteins. Their canonical activity is stimulated by cytokines/growth factors through JAK-STAT signaling. As targets of oncogenes with intrinsic tyrosine kinase activity, STAT3 and STAT5 become constitutively active in hematologic neoplasms and solid tumors, promoting cell proliferation and survival and modulating redox homeostasis. This review summarizes reactive oxygen species (ROS)-regulated STAT activation and how STATs influence ROS production. ROS-induced effects on post-translational modifications are presented, and STAT3/5-mediated regulation of xCT, a redox-sensitive target up-regulated in numerous cancers, is discussed with regard to transcriptional cross-talk.

Chronic Inhibition of STAT3/STAT5 in Treatment-Resistant Human Breast Cancer Cell Subtypes: Convergence on the ROS/SUMO Pathway and Its Effects on xCT Expression and System xc- Activity

Pharmacologically targeting activated STAT3 and/or STAT5 has been an active area of cancer research. The cystine/glutamate antiporter, system x_c^-, contributes to redox balance and export of intracellularly produced glutamate in response to up-regulated glutaminolysis in cancer cells. We have previously shown that blocking STAT3/5 using the small molecule inhibitor, SH-4-54, which targets the SH2 domains of both proteins, increases xCT expression, thereby increasing system x_c^- activity in human breast cancer cells. The current investigation demonstrates that chronic SH-4-54 administration, followed by clonal selection of treatment-resistant MDA-MB-231 and T47D breast cancer cells, elicits distinct subtype-dependent effects. xCT mRNA and protein levels, glutamate release, and cystine uptake are decreased relative to untreated passage-matched controls in triple-negative MDA-MB-231 cells, with the inverse occurring in estrogen-responsive T47D cells. This “ying-yang” effect is linked with a shifted balance between the phosphorylation status of STAT3 and STAT5, intracellular ROS levels, and STAT5 SUMOylation/de-SUMOylation. STAT5 emerged as a definitive negative regulator of xCT at the transcriptional level, while STAT3 activation is coupled with increased system x_c^- activity. We propose that careful classification of a patient’s breast cancer subtype is central to effectively targeting STAT3/5 as a therapeutic means of treating breast cancer, particularly given that xCT is emerging as an important biomarker of aggressive cancers.

SENP1-modulated sumoylation regulates retinoblastoma protein (RB) and Lamin A/C interaction and stabilization

The retinoblastoma tumor suppressor protein (RB) plays a critical role in cell proliferation and differentiation and its inactivation is a frequent underlying factor in tumorigenesis. While the regulation of RB function by phosphorylation is well studied, proteasome-mediated RB protein degradation is emerging as an important regulatory mechanism. Although our understanding of RB turnover is currently limited, there is evidence that the nuclear lamina filament protein Lamin A/C protects RB from proteasomal degradation. Here we show that SUMO1 conjugation of RB and Lamin A/C is modulated by the SUMO protease SENP1 and that sumoylation of both proteins is required for their interaction. Importantly, this SUMO1-dependent complex protects both RB and Lamin A/C from proteasomal turnover.

SENP1-Mediated Desumoylation of DBC1 Inhibits Apoptosis Induced by High Glucose in Bovine Retinal Pericytes

Pericyte loss is an early characteristic change in diabetic retinopathy, but its precise molecular mechanisms have not been elucidated. This study investigated the role of SENP1 in pericyte loss in diabetic retinopathy. We demonstrated that a high concentration of glucose inhibited the expression of the Sentrin/SUMO-specific protease 1 (SENP1), which resulted in an increase in DBC1 sumoylation in bovine retinal pericytes (BRPCs). Furthermore, SENP1 overexpression attenuated hyperemia-induced apoptosis of BPRCs, and SENP1 knockdown aggravated this effect. We also provide evidence that DBC1 sumoylation/desumoylation is involved in the SENP1-regulated apoptosis of BRPCs under high glucose conditions. Understanding the role of SENP1 in the pathogenesis of high glucose induced pericyte loss could help elucidate important targets for future pharmacological interventions.

PML nuclear bodies: assembly and oxidative stress-sensitive sumoylation

PML Nuclear Bodies (NBs) have fascinated cell biologists due to their exquisitely dynamic nature and their involvement in human diseases, notably acute promyelocytic leukemia. NBs, as well as their master organizer--the PML protein--exhibit multiple connections with stress responses. Initially viewed as a tumor suppressor, PML recently re-emerged as a multifaceted protein, capable of controlling numerous aspects of cellular homeostasis. NBs recruit many functionally diverse proteins and function as stress-regulated sumoylation factories. SUMO-initiated partner retention can subsequently facilitate a variety of other post-translational modifications, as well as partner degradation. With this newly elucidated central role of stress-enhanced sumoylation, it should now be possible to build a working model for the different NB-regulated cellular activities. Moreover, pharmacological manipulation of NB formation by interferons or oxidants holds the promise of clearing many undesirable proteins for clinical management of malignant, viral or neurodegenerative diseases.

Oxidative stress-induced assembly of PML nuclear bodies controls sumoylation of partner proteins

PML multimerization into nuclear bodies following its oxidation promotes sumoylation and sequestration of partner proteins in these structures.

The promyelocytic leukemia (PML) protein organizes PML nuclear bodies (NBs), which are stress-responsive domains where many partner proteins accumulate. Here, we clarify the basis for NB formation and identify stress-induced partner sumoylation as the primary NB function. NB nucleation does not rely primarily on intermolecular interactions between the PML SUMO-interacting motif (SIM) and SUMO, but instead results from oxidation-mediated PML multimerization. Oxidized PML spherical meshes recruit UBC9, which enhances PML sumoylation, allow partner recruitment through SIM interactions, and ultimately enhance partner sumoylation. Intermolecular SUMO–SIM interactions then enforce partner sequestration within the NB inner core. Accordingly, oxidative stress enhances NB formation and global sumoylation in vivo. Some NB-associated sumoylated partners also become polyubiquitinated by RNF4, precipitating their proteasomal degradation. As several partners are protein-modifying enzymes, NBs could act as sensors that facilitate and confer oxidative stress sensitivity not only to sumoylation but also to other post-translational modifications, thereby explaining alterations of stress response upon PML or NB loss.

SUMOylation in Trypanosoma brucei

Small ubiquitin like modifier (SUMO) proteins are involved in many processes in eukaryotes. We here show that Trypanosoma brucei SUMO (Tb927.5.3210) modifies many proteins. The levels of SUMOylation were unaffected by temperature changes but were increased by severe oxidative stress. We obtained evidence that trypanosome homologues of the SUMO conjugating enzyme Ubc9 (Tb927.2.2460) and the SUMO-specific protease SENP (Tb927.9.2220) are involved in SUMOylation and SUMO removal, respectively.

Biochemical analysis of protein SUMOylation

SUMOylation, the covalent attachment of Small Ubiquitin-like MOdifier (SUMO) polypeptides to other proteins, is among the most important post-translational modifications that regulate the functional properties of a large number of proteins. SUMOylation is broadly involved in cellular processes such as gene transcription, hormone response, signal transduction, DNA repair, and nuclear transport. SUMO modification has also been implicated in the pathogenesis of human diseases, such as cancer, neurodegenerative disorders, and viral infection. Attachment of a SUMO protein to another protein is carried out in multiple steps catalyzed by three enzymes. This unit describes and discusses the in vitro biochemical methods used for investigating each step of the SUMOylation process. In addition, a high-throughput screening protocol is included for the identification of inhibitors of SUMOylation.

Transcriptional Profile and Structural Conservation of SUMO-Specific Proteases in Schistosoma mansoni

Small ubiquitin-related modifier (SUMO) is involved in numerous cellular processes including protein localization, transcription, and cell cycle control. SUMOylation is a dynamic process, catalyzed by three SUMO-specific enzymes and reversed by Sentrin/SUMO-specific proteases (SENPs). Here we report the characterization of these proteases in Schistosoma mansoni. Using in silico analysis, we identified two SENPs sequences, orthologs of mammalian SENP1 and SENP7, confirming their identities and conservation through phylogenetic analysis. In addition, the transcript levels of Smsenp1/7 in cercariae, adult worms, and in vitro cultivated schistosomula were measured by qRT-PCR. Our data revealed upregulation of the Smsenp1/7 transcripts in cercariae and early schistosomula, followed by a marked differential gene expression in the other analyzed stages. However, no significant difference in expression profile between the paralogs was observed for the analyzed stages. Furthermore, in order to detect deSUMOylating capabilities in crude parasite extracts, SmSENP1 enzymatic activity was evaluated using SUMO-1-AMC substrate. The endopeptidase activity related to SUMO-1 precursor processing did not differ significantly between cercariae and adult worms. Taken together, these results support the developmentally regulated expression of SUMO-specific proteases in S. mansoni.

SUMO, a heavyweight player in plant abiotic stress responses

Protein post-translational modifications diversify the proteome and install new regulatory levels that are crucial for the maintenance of cellular homeostasis. Over the last decade, the ubiquitin-like modifying peptide small ubiquitin-like modifier (SUMO) has been shown to regulate various nuclear processes, including transcriptional control. In plants, the sumoylation pathway has been significantly implicated in the response to environmental stimuli, including heat, cold, drought, and salt stresses, modulation of abscisic acid and other hormones, and nutrient homeostasis. This review focuses on the emerging importance of SUMO in the abiotic stress response, summarizing the molecular implications of sumoylation and emphasizing how high-throughput approaches aimed at identifying the full set of SUMO targets will greatly enhance our understanding of the SUMO-abiotic stress association.

DeSUMOylation Controls Insulin Exocytosis in Response to Metabolic Signals

The secretion of insulin by pancreatic islet β-cells plays a pivotal role in glucose homeostasis and diabetes. Recent work suggests an important role for SUMOylation in the control of insulin secretion from β-cells. In this paper we discuss mechanisms whereby (de)SUMOylation may control insulin release by modulating β-cell function at one or more key points; and particularly through the acute and reversible regulation of the exocytotic machinery. Furthermore, we postulate that the SUMO-specific protease SENP1 is an important mediator of insulin exocytosis in response to NADPH, a metabolic secretory signal and major determinant of β-cell redox state. Dialysis of mouse β-cells with NADPH efficiently amplifies β-cell exocytosis even when extracellular glucose is low; an effect that is lost upon knockdown of SENP1. Conversely, over-expression of SENP1 itself augments β-cell exocytosis in a redox-dependent manner. Taken together, we suggest that (de)SUMOylation represents an important mechanism that acutely regulates insulin secretion and that SENP1 can act as an amplifier of insulin exocytosis.

Novel roles of SUMO in pancreatic β-cells: thinking outside the nucleus

The endocrine pancreas is critically important in the regulation of energy metabolism, with defective insulin secretion from pancreatic islet β-cells a major contributing factor to the development of type 2 diabetes. Small ubiquitin-like modifier (SUMO) proteins have been demonstrated to covalently modify a wide range of target proteins, mediating a broad range of cellular processes. While the effects of SUMOylation on β-cell gene transcription have been previously reviewed, recent reports indicate roles for SUMO outside of the nucleus. In this review we shall focus on the reported non-nuclear roles of SUMOylation in the regulation of β-cells, including SUMOylation as a novel signaling pathway in the acute regulation of insulin secretion.

Redox sensing by proteins: oxidative modifications on cysteines and the consequent events

SIGNIFICANCE

Reactive oxygen species (ROS) are not only essential for the cell's normal functions, but also mediate many pathological effects. When cells experience oxidative stress, proteins are modulated by redox changes and ultimately generate novel signaling patterns. It remains elusive how proteins are modulated, rather than simply damaged, by ROS and then mediate the diverse cellular responses.

RECENT ADVANCES

During the past decade, researchers frequently used "redox sensor" for proteins. However, the term "redox sensing" has not been clearly defined to date. Thiols of cysteines are subjected to oxidative modifications. The conformation changes and the various types of post-translational modifications (PTMs) may result from thiol oxidation of the same protein or other proteins. The molecular effects of redox sensing include changes in protein activity, abundance, localization, and interaction with other biomacromolecules.

CRITICAL ISSUES

We discuss the emerging concept of cysteine-based redox sensing, emphasizing "sensing redox changes by proteins using their thiols." ROS are an input, and the conformation changes and/or the other PTMs after thiol oxidation are the output of redox sensing. Among dozens of redox sensing proteins listed in this article, SENP3 and caspase-9, which have been investigated in our work, are given particular attention. We also introduce the notion of biphasic and compartment-specific redox sensing by nuclear factor kappa B.

FUTURE DIRECTIONS

Understanding chemical modifications and conformational changes following protein redox sensing requires more studies in mass spectrometry and crystallography. Redox-indicative probes in live cells and tissues will help monitor redox-related biological and pathological progresses.

Reconstitution of SUMO-dependent ubiquitylation in vitro

In eukaryotic cells, most soluble proteins are degraded via the ubiquitin proteasome system. The recognition signal for the proteasome consists of a lysine 48-linked ubiquitin chain which is posttranslationally conjugated to lysine residues in target proteins. This conjugation reaction is mediated by an enzymatic cascade consisting of specific E1, E2, and E3 enzymes. The small ubiquitin-related modifier (SUMO) is conjugated to target proteins via a similar cascade of SUMO-specific enzymes. Contrary to the long-standing assumption that SUMO does not participate in proteolytic targeting, proteasomal inhibition stabilizes both ubiquitin and SUMO conjugates (SCs). This led to the discovery of ubiquitin ligases for SUMO conjugates (ULS proteins or SUMO-targeted ubiquitin ligases) that target SUMOylated proteins for proteasomal degradation. The so far identified ULS proteins each contains a really interesting new gene domain with ubiquitin-E3 ligase activity and several SUMO interaction motifs that noncovalently bind SUMO. In order to identify ULS proteins and characterize their substrates, it is important to reconstitute this reaction in vitro. In this chapter, we describe step-by-step protocols for the production and purification of recombinant SUMOylated substrates as well as their in vitro ubiquitylation by ULS proteins.

Analysis of oxygen/glucose-deprivation-induced changes in SUMO3 conjugation using SILAC-based quantitative proteomics

Transient cerebral ischemia dramatically activates small ubiquitin-like modifier (SUMO2/3) conjugation. In cells exposed to 6 h of transient oxygen/glucose deprivation (OGD), a model of ischemia, SUMOylation increases profoundly between 0 and 30 min following re-oxygenation. To elucidate the effect of transient OGD on SUMO conjugation of target proteins, we exposed neuroblastoma B35 cells expressing HA-SUMO3 to transient OGD and used stable isotope labeling with amino acids in cell culture (SILAC) to quantify OGD-induced changes in levels of specific SUMOylated proteins. Lysates from control and OGD-treated cells were mixed equally, and HA-tagged proteins were immunoprecipitated and analyzed by 1D-SDS-PAGE-LC-MS/MS. We identified 188 putative SUMO3-conjugated proteins, including numerous transcription factors and coregulators, and PIAS2 and PIAS4 SUMO ligases, of which 22 were increased or decreased more than ±2-fold. In addition to SUMO3, the levels of protein-conjugated SUMO1 and SUMO2, as well as ubiquitin, were all increased. Importantly, protein ubiquitination induced by OGD was completely blocked by gene silencing of SUMO2/3. Collectively, these results suggest several mechanisms for OGD-modulated SUMOylation, point to a number of signaling pathways that may be targets of SUMO-based signaling and recovery from ischemic stress, and demonstrate a tightly controlled crosstalk between the SUMO and ubiquitin conjugation pathways.

Cardiac function and disease: emerging role of small ubiquitin-related modifier

Small ubiquitin-related modifiers, or SUMOs, have emerged as versatile regulators of many biological functions that do so by covalent attachment to a variety of substrates via enzymatic reactions. SUMO conjugation has also been shown to be involved in a number of human pathogenic processes. More recent advances in the SUMO field have indicated a potential role for SUMO conjugation pathway in cardiogenesis. This advanced review will describe the basic features of the SUMO conjugation pathway and will summarize the most recent studies implicating the influence of the sumoylation pathway in cardiac function under both physiological and pathological conditions. WIREs Syst Biol Med 2011 3 446-457 DOI: 10.1002/wsbm.130

SUMO2 and SUMO3 transcription is differentially regulated by oxidative stress in an Sp1-dependent manner

Protein SUMOylation (SUMO is small ubiquitin-related modifier) is a dynamic process that is strictly regulated under physiological and pathological conditions. However, little is known about how various intra- or extra-cellular stimuli regulate expression levels of components in the SUMO system. SUMO isoforms SUMO2 and SUMO3 can rapidly convert to be conjugated in response to a variety of cellular stresses. Owing to the limitations of sequence homology, SUMO2 and SUMO3 cannot be differentiated between and are thus referred to as SUMO2/3. Whether these two isoforms are regulated in distinct manners has never been addressed. In the present paper we report that the expression of SUMO3, but not SUMO2, can be down-regulated at the transcription level by cellular oxidative stress. In the present study, we checked SUMO2 and SUMO3 mRNA levels in cells exposed to various doses of H2O2 and in cells bearing different levels of ROS (reactive oxygen species). We found an inverse relationship between SUMO3 transcription and ROS levels. We characterized a promoter region specific for the mouse Sumo3 gene that is bound by the redox-sensitive transcription factor Sp1 (specificity protein 1) and demonstrated oxidation of Sp1, as well as suppression of Sp1–DNA binding upon oxidative stress. This revealed for the first time that the expression of SUMO2 and SUMO3 is regulated differently by ROS. These findings may enhance our understanding about the regulation of SUMOylation and also shed light on the functions of Sp1.

Expanding the substantial interactome of NEMO using protein microarrays

Signal transduction by the NF-kappaB pathway is a key regulator of a host of cellular responses to extracellular and intracellular messages. The NEMO adaptor protein lies at the top of this pathway and serves as a molecular conduit, connecting signals transmitted from upstream sensors to the downstream NF-kappaB transcription factor and subsequent gene activation. The position of NEMO within this pathway makes it an attractive target from which to search for new proteins that link NF-kappaB signaling to additional pathways and upstream effectors. In this work, we have used protein microarrays to identify novel NEMO interactors. A total of 112 protein interactors were identified, with the most statistically significant hit being the canonical NEMO interactor IKKbeta, with IKKalpha also being identified. Of the novel interactors, more than 30% were kinases, while at least 25% were involved in signal transduction. Binding of NEMO to several interactors, including CALB1, CDK2, SAG, SENP2 and SYT1, was confirmed using GST pulldown assays and coimmunoprecipitation, validating the initial screening approach. Overexpression of CALB1, CDK2 and SAG was found to stimulate transcriptional activation by NF-kappaB, while SYT1 overexpression repressed TNFalpha-dependent NF-kappaB transcriptional activation in human embryonic kidney cells. Corresponding with this finding, RNA silencing of CDK2, SAG and SENP2 reduced NF-kappaB transcriptional activation, supporting a positive role for these proteins in the NF-kappaB pathway. The identification of a host of new NEMO interactors opens up new research opportunities to improve understanding of this essential cell signaling pathway.

Sumoylation as a signal for polyubiquitylation and proteasomal degradation

The small ubiquitin-related modifier (SUMO) is a versatile cellular tool to modulate a protein's function. SUMO modification is a reversible process analogous to ubiquitylation. The consecutive actions of E1, E2 and E3 enzymes catalyze the attachment of SUMO to target proteins, while deconjugation is promoted by SUMO specific proteases. Contrary to the long-standing assumption that SUMO has no role in proteolytic targeting and rather acts as an antagonist of ubiquitin in some cases, it has recently been discovered that sumoylation itself can function as a secondary signal mediating ubiquitin-dependent degradation by the proteasome. The discovery of a novel family of RING finger ubiquitin ligases bearing SUMO interaction motifs implicated the ubiquitin system in the control of SUMO modified proteins. SUMO modification as a signal for degradation is conserved in eukaryotes and ubiquitin ligases that specifically recognize SUMO-modified proteins have been discovered in species ranging from yeasts to humans. This chapter summarizes what is known about these ligases and their role in controlling sumoylated proteins.