DeSUMOylation: An Important Therapeutic Target and Protein Regulatory Event

cccDNA epigenetic regulator as target for therapeutical vaccine development against hepatitis B

Chronic hepatitis B virus infection (CHB) remains a global health concern, with currently available antiviral therapies demonstrating limited effectiveness in preventing hepatocellular carcinoma (HCC) development. Two primary challenges in CHB treatment include the persistence of the minichromosome, covalently closed circular DNA (cccDNA) of the hepatitis B virus (HBV), and the failure of the host immune response to eliminate cccDNA. Recent findings indicate several host and HBV proteins involved in the epigenetic regulation of cccDNA, including HBV core protein (HBc) and HBV x protein (HBx). Both proteins might contribute to the stability of the cccDNA minichromosome and interact with viral and host proteins to support transcription. One potential avenue for CHB treatment involves the utilization of therapeutic vaccines. This paper explores HBV antigens suitable for epigenetic manipulation of cccDNA, elucidates their mechanisms of action, and evaluates their potential as key components of epigenetically-driven vaccines for CHB therapy. Molecular targeted agents with therapeutic vaccines offer a promising strategy for addressing CHB by targeting the virus and enhancing the host's immunological response. Despite challenges, the development of these vaccines provides new hope for CHB patients by emphasizing the need for HBV antigens that induce effective immune responses without causing T cell exhaustion.

SENP3 attenuates foam cell formation by deSUMOylating NLRP3 in macrophages stimulated with ox-LDL

SUMO-specific protease 3 (SENP3) participates in the removal of SUMOylation and maintains the balance of the SUMO system, which ensures normal functioning of substrates and cellular activities. In the present study, we found that SENP3 expression was significantly reduced in ox-LDL-stimulated macrophages. SENP3 overexpression suppressed and SENP3 knockdown promoted macrophage foam cell formation. Moreover, SENP3 inhibited cholesterol uptake, CD36 expression, and NOD-like receptor thermal protein domain-associated protein 3 (NLRP3) inflammasome activation in ox-LDL-stimulated macrophages. Ox-LDL-stimulated NLRP3 SUMOylation was reduced by SENP3. Blocking NLRP3 SUMOylation inhibited foam cell formation and NLRP3 inflammasome activation. Thus, this study revealed that SENP3 inhibits macrophage foam cell formation by deSUMOylating NLRP3 and regulating NLRP3 inflammasome activation, which may provide a potentially innovative approach to treatment of atherosclerosis.

Exome sequencing links the SUMO protease SENP7 with fatal arthrogryposis multiplex congenita, early respiratory failure and neutropenia

BACKGROUND

SUMOylation involves the attachment of small ubiquitin-like modifier (SUMO) proteins to specific lysine residues on thousands of substrates with target-specific effects on protein function. Sentrin-specific proteases (SENPs) are proteins involved in the maturation and deconjugation of SUMO. Specifically, SENP7 is responsible for processing polySUMO chains on targeted substrates including the heterochromatin protein 1α (HP1α).

METHODS

We performed exome sequencing and segregation studies in a family with several infants presenting with an unidentified syndrome. RNA and protein expression studies were performed in fibroblasts available from one subject.

RESULTS

We identified a kindred with four affected subjects presenting with a spectrum of findings including congenital arthrogryposis, no achievement of developmental milestones, early respiratory failure, neutropenia and recurrent infections. All died within four months after birth. Exome sequencing identified a homozygous stop gain variant in SENP7 c.1474C>T; p.(Gln492*) as the probable aetiology. The proband's fibroblasts demonstrated decreased mRNA expression. Protein expression studies showed significant protein dysregulation in total cell lysates and in the chromatin fraction. We found that HP1α levels as well as different histones and H3K9me3 were reduced in patient fibroblasts. These results support previous studies showing interaction between SENP7 and HP1α, and suggest loss of SENP7 leads to reduced heterochromatin condensation and subsequent aberrant gene expression.

CONCLUSION

Our results suggest a critical role for SENP7 in nervous system development, haematopoiesis and immune function in humans.

The emerging era of lactate: A rising star in cellular signaling and its regulatory mechanisms

Cellular metabolites are ancient molecules with pleiotropic implications in health and disease. Beyond their cognate roles, they have signaling functions as the ligands for specific receptors and the precursors for epigenetic or posttranslational modifications. Lactate has long been recognized as a metabolic waste and fatigue product mainly produced from glycolytic metabolism. Recent evidence however suggests lactate is an unique molecule with diverse signaling attributes in orchestration of numerous biological processes, including tumor immunity and neuronal survival. The copious metabolic and non-metabolic functions of lactate mediated by its bidirectional shuttle between cells or intracellular organelles lead to a phenotype called "lactormone." Importantly, the mechanisms of lactate signaling, via acting as a molecular sensor and a regulator of NAD+ metabolism and AMP-activated protein kinase signaling, and via the newly identified lactate-driven lactylation, have been discovered. Further, we include a brief discussion about the autocrine regulation of efferocytosis by lactate in Sertoli cells which favoraerobic glycolysis. By emphasizing a repertoire of the most recent discovered mechanisms of lactate signaling, this review will open tantalizing avenues for future investigations cracking the regulatory topology of lactate signaling covered in the veil of mystery.

SENP2 knockdown in human adipocytes reduces glucose metabolism and lipid accumulation, while increases lipid oxidation

Adipose tissue is one of the main regulative sites for energy metabolism. Excess lipid storage and expansion of white adipose tissue (WAT) is the primary contributor to obesity, a strong predisposing factor for development of insulin resistance. Sentrin-specific protease (SENP) 2 has been shown to play a role in metabolism in murine fat and skeletal muscle cells, and we have previously demonstrated its role in energy metabolism of human skeletal muscle cells. In the present work, we have investigated the impact of SENP2 on fatty acid and glucose metabolism in primary human fat cells by using cultured primary human adipocytes to knock down the SENP2 gene. Glucose uptake and oxidation, as well as accumulation and distribution of oleic acid into complex lipids were decreased, while oleic acid oxidation was increased in SENP2-knockdown cells compared to control adipocytes. Furthermore, lipogenesis was reduced by SENP2-knockdown in adipocytes. Although TAG accumulation relative to total uptake was unchanged, there was increased mRNA expression of metabolically relevant genes such as UCP1 and PPARGC1A and mRNA and proteomic data revealed increased levels of mRNA and proteins related to mitochondrial function by SENP2-knockdown. In conclusion, SENP2 is an important regulator of energy metabolism in primary human adipocytes and its knockdown reduce glucose metabolism and lipid accumulation, while increasing lipid oxidation in human adipocytes.

SUMO control of nervous system development

In the last decades, the post-translational modification system by covalent attachment of the SUMO polypeptide to proteins has emerged as an essential mechanism controlling virtually all the physiological processes in the eukaryotic cell. This includes vertebrate development. In the nervous system, SUMO plays crucial roles in synapse establishment and it has also been linked to a variety of neurodegenerative diseases. However, to date, the involvement of the modification of specific targets in key aspects of nervous system development, like patterning and differentiation, has remained largely elusive. A number of recent works confirm the participation of target-specific SUMO modification in critical aspects of nervous system development. Here, we review pioneering and new findings demonstrating the essential role SUMO plays in neurogenesis and other facets of neurodevelopment, which will help to precisely understand the variety of mechanisms SUMO utilizes to control most fundamental processes in the cell.

Posttranslational control of lipogenesis in the tumor microenvironment

Metabolic reprogramming of cancer cells within the tumor microenvironment typically occurs in response to increased nutritional, translation and proliferative demands. Altered lipid metabolism is a marker of tumor progression that is frequently observed in aggressive tumors with poor prognosis. Underlying these abnormal metabolic behaviors are posttranslational modifications (PTMs) of lipid metabolism-related enzymes and other factors that can impact their activity and/or subcellular localization. This review focuses on the roles of these PTMs and specifically on how they permit the re-wiring of cancer lipid metabolism, particularly within the context of the tumor microenvironment.

Cancer-Associated Dysregulation of Sumo Regulators: Proteases and Ligases

SUMOylation is a post-translational modification that has emerged in recent decades as a mechanism involved in controlling diverse physiological processes and that is essential in vertebrates. The SUMO pathway is regulated by several enzymes, proteases and ligases being the main actors involved in the control of sumoylation of specific targets. Dysregulation of the expression, localization and function of these enzymes produces physiological changes that can lead to the appearance of different types of cancer, depending on the enzymes and target proteins involved. Among the most studied proteases and ligases, those of the SENP and PIAS families stand out, respectively. While the proteases involved in this pathway have specific SUMO activity, the ligases may have additional functions unrelated to sumoylation, which makes it more difficult to study their SUMO-associated role in cancer process. In this review we update the knowledge and advances in relation to the impact of dysregulation of SUMO proteases and ligases in cancer initiation and progression.

FOXM1 Promotes Malignant Proliferation of Esophageal Squamous Cell Carcinoma Through Transcriptional Activating CDC6

Forkhead box M1 (FOXM1) is a proliferative transcription factor and plays a vital role in many cancers. However, the function and molecular mechanism of FOXM1 in esophageal squamous cell carcinoma (ESCC) remain poorly understood. Hence, we aim to clarify the molecular basis of FOXM1-mediated ESCC progression. In this study, bioinformatics analysis showed that FOXM1 was mainly involved in key signal pathways, including cell proliferation, cell cycle, and homologous recombination in ESCC, and predicted that CDC6 might be a potential regulatory target gene of FOXM1. The results revealed that FOXM1 and CDC6 were significantly overexpressed in ESCC tissue and cell line, and their expression was positively correlated. Further studies showed that FOXM1 directly transcriptionally activated CDC6 by binding to its promoter region in ESCC cells. Moreover, FOXM1 mediated ESCC cell proliferation by regulating CDC6 expression, which may be related to promoting G1-S phase transition of cell cycle. Taken together, FOXM1-CDC6 axis mediates ESCC malignant proliferation and may serve as a potential biological target for ESCC treatment.

SUMOylation in the control of cholesterol homeostasis

SUMOylation—protein modification by the small ubiquitin-related modifier (SUMO)—affects several cellular processes by modulating the activity, stability, interactions or subcellular localization of a variety of substrates. SUMO modification is involved in most cellular processes required for the maintenance of metabolic homeostasis. Cholesterol is one of the main lipids required to preserve the correct cellular function, contributing to the composition of the plasma membrane and participating in transmembrane receptor signalling. Besides these functions, cholesterol is required for the synthesis of steroid hormones, bile acids, oxysterols and vitamin D. Cholesterol levels need to be tightly regulated: in excess, it is toxic to the cell, and the disruption of its homeostasis is associated with various disorders like atherosclerosis and cardiovascular diseases. This review focuses on the role of SUMO in the regulation of proteins involved in the metabolism of cholesterol.

SUMO1 SUMOylates and SENP3 deSUMOylates NLRP3 to orchestrate the inflammasome activation

The NLRP3 inflammasome regulates innate immune and inflammatory responses by promoting caspase1-dependent induction of pro-inflammatory cytokines. However, aberrant inflammasome activation causes diverse diseases, and thus inflammasome activity must be tightly controlled. Here, we reveal a molecular mechanism underlying the regulation of NLRP3 inflammasome. NLRP3 interacts with SUMO-conjugating enzyme (UBC9), which subsequently promotes small ubiquitin-like modifier 1 (SUMO1) to catalyze NLRP3 SUMOylation at residue Lys²⁰⁴. SUMO1-catalyzed SUMOylation of NLRP3 facilitates ASC oligomerization, inflammasome activation, and interleukin-1β secretion. Moreover, this study also reveals that SUMO-specific protease 3 (SENP3) is required for the deSUMOylation of NLRP3. Interestingly, SENP3 deSUMOylates NLRP3 to attenuate ASC recruitment and speck formation, the NLRP3 inflammasome activation, as well as IL-1β cleavage and secretion. In conclusion, we reveal that SUMO1-catalyzed SUMOylation and SENP3-mediated deSUMOylation of NLRP3 orchestrate the inflammasome activation.

Regulation of the Expression of DAPK1 by SUMO Pathway

Death Associated Protein Kinase 1 (DAPK1) is an important signaling kinase mediating the biological effect of multiple natural biomolecules such as IFN-γ, TNF-α, curcumin, etc. DAPK1 is degraded through both ubiquitin-proteasomal and lysosomal degradation pathways. To investigate the crosstalk between these two DAPK1 degradation pathways, we carried out a screen using a set of ubiquitin E2 siRNAs at the presence of Tuberous Sclerous 2 (TSC2) and identified that the small ubiquitin-like molecule (SUMO) pathway is able to regulate the protein levels of DAPK1. Inhibition of the SUMO pathway enhanced DAPK1 protein levels and the minimum domain of DAPK1 protein required for this regulation is the kinase domain, suggesting that the SUMO pathway regulates DAPK1 protein levels independent of TSC2. Suppression of the SUMO pathway did not enhance DAPK1 protein stability. In addition, mutation of the potential SUMO conjugation sites on DAPK1 kinase domain did not alter its protein stability or response to SUMO pathway inhibition. These data suggested that the SUMO pathway does not regulate DAPK1 protein degradation. The exact molecular mechanism underlying this regulation is yet to be discovered.

Lack of androgen receptor SUMOylation results in male infertility due to epididymal dysfunction

Androgen receptor (AR) is regulated by SUMOylation at its transactivation domain. In vitro, the SUMOylation is linked to transcriptional repression and/or target gene-selective regulation. Here, we generated a mouse model (ArKI) in which the conserved SUMO acceptor lysines of AR are permanently abolished (Ar^{K381R, K500R}). ArKI males develop normally, without apparent defects in their systemic androgen action in reproductive tissues. However, the ArKI males are infertile. Their spermatogenesis appears unaffected, but their epididymal sperm maturation is defective, shown by severely compromised motility and fertilization capacity of the sperm. Fittingly, their epididymal AR chromatin-binding and gene expression associated with sperm maturation and function are misregulated. AR is SUMOylated in the wild-type epididymis but not in the testis, which could explain the tissue-specific response to the lack of AR SUMOylation. Our studies thus indicate that epididymal AR SUMOylation is essential for the post-testicular sperm maturation and normal reproductive capability of male mice.

SUMOylation is known to regulate androgen receptor (AR) activity in cultured cells. Here, using SUMOylation-deficient AR knock-in mice, the authors demonstrate that SUMOylation is required for AR-related gene expression specifically in the epididymal tissues, but not the testis.

The SUMO protease SENP1 and the chromatin remodeler CHD3 interact and jointly affect chromatin accessibility and gene expression

The small ubiquitin-like modifier (SUMO) post-translationally modifies lysine residues of transcription factors and co-regulators and thereby contributes to an important layer of control of the activities of these transcriptional regulators. Likewise, deSUMOylation of these factors by the sentrin-specific proteases (SENPs) also plays a role in gene regulation, but whether SENPs functionally interact with other regulatory factors that control gene expression is unclear. In the present work, we focused on SENP1, specifically, on its role in activation of gene expression investigated through analysis of the SENP1 interactome, which revealed that SENP1 physically interacts with the chromatin remodeler chromodomain helicase DNA-binding protein 3 (CHD3). Using several additional methods, including GST pulldown and co-immunoprecipitation assays, we validated and mapped this interaction, and using CRISPR-Cas9-generated CHD3- and SENP1-KO cells (in the haploid HAP1 cell line), we investigated whether these two proteins are functionally linked in regulating chromatin remodeling and gene expression. Genome-wide ATAC-Seq analysis of the CHD3- and SENP1-KO cells revealed a large degree of overlap in differential chromatin openness between these two mutant cell lines. Moreover, motif analysis and comparison with ChIP-Seq profiles in K562 cells pointed to an association of CHD3 and SENP1 with CCCTC-binding factor (CTCF) and SUMOylated chromatin-associated factors. Lastly, genome-wide RNA-Seq also indicated that these two proteins co-regulate the expression of several genes. We propose that the functional link between chromatin remodeling by CHD3 and deSUMOylation by SENP1 uncovered here provides another level of control of gene expression.

SENP2 alleviates CCl4-induced liver fibrosis by promoting activated hepatic stellate cell apoptosis and reversion

SUMOylation and deSUMOylation, a dynamic process, is proved to be involved in various fibrotic diseases. Here, we found SENP2, one of deSUMOylation protease family member, was decreased in CCl₄-induced mice fibrotic liver tissues, primary HSCs and restored after spontaneously recovery. In addition, HSC-T6 cells with TGF-β1 treatment resulted in a significant reduction of SENP2. Ectopic expression of SENP2 hindered cells activation and proliferation induced by TGF-β1 while knockdown of SENP2 showed an opposite effect. Importantly, SENP2 promoted apoptosis of HSC-T6 cells activated by TGF-β1. Furthermore, restoration of SENP2 was observed in inactivated HSCs after adipogenic differentiation mixture (MDI) treatment. Inadequate SENP2 inhibited the reversion of HSC-T6 cells, featured as aberrant expressions of α-SMA and col1a1, two markers of liver fibrosis. It has been reported SENP2 was a suppressant regulator of Wnt/β-catenin signal pathway. Similarly, we found SENP2 has a negative effect on β-catenin as well as its downstream genes C-myc and CyclinD1 in liver fibrosis. Collectively, our data indicated SENP2 may be involved in HSCs apoptosis and reversion in liver fibrosis.

The post-translational modification, SUMOylation, and cancer (Review)

SUMOylation is a reversible post-translational modification which has emerged as a crucial molecular regulatory mechanism, involved in the regulation of DNA damage repair, immune responses, carcinogenesis, cell cycle progression and apoptosis. Four SUMO isoforms have been identified, which are SUMO1, SUMO2/3 and SUMO4. The small ubiquitin-like modifier (SUMO) pathway is conserved in all eukaryotes and plays pivotal roles in the regulation of gene expression, cellular signaling and the maintenance of genomic integrity. The SUMO catalytic cycle includes maturation, activation, conjugation, ligation and de-modification. The dysregulation of the SUMO system is associated with a number of diseases, particularly cancer. SUMOylation is widely involved in carcinogenesis, DNA damage response, cancer cell proliferation, metastasis and apoptosis. SUMO can be used as a potential therapeutic target for cancer. In this review, we briefly outline the basic concepts of the SUMO system and summarize the involvement of SUMO proteins in cancer cells in order to better understand the role of SUMO in human disease.

Deubiquitylating enzymes and drug discovery: emerging opportunities

More than a decade after a Nobel Prize was awarded for the discovery of the ubiquitin-proteasome system and clinical approval of proteasome and ubiquitin E3 ligase inhibitors, first-generation deubiquitylating enzyme (DUB) inhibitors are now approaching clinical trials. However, although our knowledge of the physiological and pathophysiological roles of DUBs has evolved tremendously, the clinical development of selective DUB inhibitors has been challenging. In this Review, we discuss these issues and highlight recent advances in our understanding of DUB enzymology and biology as well as technological improvements that have contributed to the current interest in DUBs as therapeutic targets in diseases ranging from oncology to neurodegeneration.

SUMO Signaling by Hypoxic Inactivation of SUMO-Specific Isopeptidases

Post-translational modification of proteins with ubiquitin-like SUMO modifiers is a tightly regulated and highly dynamic process. The SENP family of SUMO-specific isopeptidases comprises six cysteine proteases. They are instrumental in counterbalancing SUMO conjugation, but their regulation is not well understood. We demonstrate that in hypoxic cell extracts, the catalytic activity of SENP family members, in particular SENP1 and SENP3, is inhibited in a rapid and fully reversible process. Comparative mass spectrometry from normoxic and hypoxic cells defines a subset of hypoxia-induced SUMO1 targets, including SUMO ligases RanBP2 and PIAS2, glucose transporter 1, and transcriptional regulators. Among the most strongly induced targets, we identified the transcriptional co-repressor BHLHE40, which controls hypoxic gene expression programs. We provide evidence that SUMOylation of BHLHE40 is reversed by SENP1 and contributes to transcriptional repression of the metabolic master regulator gene PGC-1α. We propose a pathway that connects oxygen-controlled SENP activity to hypoxic reprogramming of metabolism.

Hypothermia exerts early neuroprotective effects involving protein conjugation of SUMO‑2/3 in a rat model of middle cerebral artery occlusion

How hypothermia serves an early protective role against cerebral ischemia remains to be determined. The small ubiquitin-related modifier protein (SUMO) functions as a post-translational modification system and SUMO-2/3 subtypes are often activated in early stress. The present study investigated changes in the protein level of SUMO using western blotting and immunocytochemistry when neurons were exposed to oxygen-glucose deprivation (OGD) in vitro, as well as in a rat model of middle cerebral artery occlusion (MCAO) in vivo. The results demonstrated that a large number of proteins were conjugated to SUMO-2/3 in OGD-injured neurons (within 10 min, peaking at 12 h), and was markedly enhanced under conditions of hypothermia (33°C). Concordantly, lactate dehydrogenase (LDH) release and the apoptosis rate, as determined by LDH and TUNEL assays, respectively, were lower in hypothermia-treated neurons. Similar results were obtained in a rat model of MCAO. Neurological deficit scores were lower in the hypothermia group than in the sham group in the early stage of cerebral ischemia (P<0.05). However, no significant differences in neurological deficit scores were detected between the hypothermia group and the sham group in the late stage of ischemia (21 days; P>0.05). This study implicates a role for SUMO-2/3 in early hypothermia-induced neuroprotection against stroke. The development of small molecule therapeutics based on SUMO-2/3 may benefit patients with cerebral ischemia.

The cohesion stabilizer sororin favors DNA repair and chromosome segregation during mouse oocyte meiosis

Maintenance and timely termination of cohesion on chromosomes ensures accurate chromosome segregation to guard against aneuploidy in mammalian oocytes and subsequent chromosomally abnormal pregnancies. Sororin, a cohesion stabilizer whose relevance in antagonizing the anti-cohesive property of Wings-apart like protein (Wapl), has been characterized in mitosis; however, the role of Sororin remains unclear during mammalian oocyte meiosis. Here, we show that Sororin is required for DNA damage repair and cohesion maintenance on chromosomes, and consequently, for mouse oocyte meiotic program. Sororin is constantly expressed throughout meiosis and accumulates on chromatins at germinal vesicle (GV) stage/G2 phase. It localizes onto centromeres from germinal vesicle breakdown (GVBD) to metaphase II stage. Inactivation of Sororin compromises the GVBD and first polar body extrusion (PBE). Furthermore, Sororin inactivation induces DNA damage indicated by positive γH2AX foci in GV oocytes and precocious chromatin segregation in MII oocytes. Finally, our data indicate that PlK1 and MPF dissociate Sororin from chromosome arms without affecting its centromeric localization. Our results define Sororin as a determinant during mouse oocyte meiotic maturation by favoring DNA damage repair and chromosome separation, and thereby, maintaining the genome stability and generating haploid gametes.

The role of SUMOylation in ageing and senescent decline

Posttranslational protein modifications are playing crucial roles in essential cellular mechanisms. SUMOylation is a reversible posttranslational modification of specific target proteins by the attachment of a small ubiquitin-like protein. Although the mechanism of conjugation of SUMO to proteins is analogous to ubiquitination, it requires its own, specific set of enzymes. The consequences of SUMOylation are widely variable, depending on the physiological state of the cell and the attached SUMO isoform. Accumulating recent findings have revealed a prominent role of SUMOylation in molecular pathways that govern senescence and ageing. Here, we review the link between SUMO attachment events and cellular processes that influence senescence and ageing, including promyelocytic leukaemia (PML) nuclear body and telomere function, autophagy, reactive oxygen species (ROS) homeostasis and growth factor signalling.

SUMO-specific protease 3 is a key regulator for hepatic lipid metabolism in non-alcoholic fatty liver disease

Non-alcoholic fatty liver disease (NAFLD) is characterized by excessive lipid accumulation in hepatocytes. The role of SENP3 in lipid metabolism, particularly NAFLD, is unclear. Our results showed that hepatic SENP3 was up-regulated in NAFLD patients and an animal model in vivo and after loading hepatocytes with free fatty acids (FFA) in vitro. Intracellular lipid accumulation was determined in SENP3 silenced or overexpressed hepatocytes with/without FFA in vitro. Confirming a role for SENP3, gene silencing was associated in vitro with amelioration of lipid accumulation and overexpression with enhancement of lipid accumulation. SENP3 related genes in NAFLD were determined in vitro using RNA-Seq. Eleven unique genes closely associated with lipid metabolism were generated using bioinformatics. Three selected genes (apoe, a2m and tnfrsf11b) were verified in vitro, showing apoe, a2m and tnfrsf11b were regulated by SENP3 with FFA stimulation. Intrahepatic and circulating APOE, A2M and TNFRSF11B were elevated in NAFLD compared with controls. These data demonstrate the important role of SENP3 in lipid metabolism during the development of NAFLD via downstream genes, which may be useful information in the development of NAFLD. The precise role of SENP3 in NAFLD will be investigated using liver-specific conditional knockout mice in future studies.

Mitochondrial E3 ubiquitin ligase 1: A key enzyme in regulation of mitochondrial dynamics and functions

Mitochondrial E3 ubiquitin ligase 1 (Mul1) is a multifunctional mitochondrial membrane protein with its RING domain exposed to the cytoplasm. On the one hand, Mul1 functions as a ubiquitin-ligase to ubiquitinate a bunch of signal molecules, such as mitofusin2 (Mfn2), Akt, p53 and ULK1, through its RING finger domain, leading to proteins degradation. On the other hand, Mul1 acts as a small ubiquitin-like modifiers (SUMO) E3 ligase to sumoylate certain proteins, such as dynamin-related protein 1 (Drp1), enhancing protein stabilization. Through the dual functions of ubiquitination and SUMOylation, Mul1 involves in regulation of many physiological and pathological processes, such as mitochondrial dynamics, cell growth, apoptosis and mitophagy. In addition, Mul1 can also directly activate or interact with some proteins, such as NF-κB and JNK, to take part in the regulation of cellular apoptosis. This review summarizes recent progress in relevant studies on the physiological and pathological functions of Mul1 and pays special attention to its role in regulation of mitochondrial dynamics.

Protein Modifications as Manifestations of Hyperglycemic Glucotoxicity in Diabetes and Its Complications

Diabetes and its complications are hyperglycemic toxicity diseases. Many metabolic pathways in this array of diseases become aberrant, which is accompanied with a variety of posttranslational protein modifications that in turn reflect diabetic glucotoxicity. In this review, we summarize some of the most widely studied protein modifications in diabetes and its complications. These modifications include glycation, carbonylation, nitration, cysteine S-nitrosylation, acetylation, sumoylation, ADP-ribosylation, O-GlcNAcylation, and succination. All these posttranslational modifications can be significantly attributed to oxidative stress and/or carbon stress induced by diabetic redox imbalance that is driven by activation of pathways, such as the polyol pathway and the ADP-ribosylation pathway. Exploring the nature of these modifications should facilitate our understanding of the pathological mechanisms of diabetes and its associated complications.