SUMO-specific protease 2 in Mdm2-mediated regulation of p53

SUMO-specific proteases: SENPs in oxidative stress-related signaling and diseases

Oxidative stress is employed to depict a series of responses detrimental to normal cellular functions resulting from an imbalance between intracellular oxidants, mainly reactive oxygen species and antioxidant defenses. Oxidative stress often contributes to the development of various diseases, including cancer, cardiovascular diseases, and neurodegenerative diseases. In this process, the relationship between small ubiquitin-like modifier (SUMO) and oxidative stress has garnered significant attention, with its posttranslational modification (PTM) frequently serving as a marker of oxidative stress status. Sentrin/SUMO-specific proteases (SENPs), affected by alternative splicing, PTMs such as phosphorylation and ubiquitination, and various protein interactions, are crucial molecules in the SUMO process. The human SENP family has six members (SENP1-3, SENP5-7), which are classified into two categories based on sequence similarity, substrate specificity, and subcellular location. They have two core functions in the human body: first, by cleaving the precursor SUMO and exposing the C-terminal glycine, they initiate the SUMO process; second, they can specifically recognize and dissociate SUMO proteins bound to substrates, a process known as deSUMOylation. However, the connection between deSUMOylation and oxidative stress remains a relatively unexplored area despite their strong association with oxidative diseases such as cancer and cardiovascular disease. This article aims to illustrate the significant contribution of SENPs to the oxidative stress pathway through deSUMOylation by reviewing their structure and classification, their roles in oxidative stress, and the changes in their expression and activity in several typical oxidative stress-related diseases.

Sumo-regulatory SENP2 controls the homeostatic squamous mitosis-differentiation checkpoint

Squamous or epidermoid cancer arises in stratified epithelia but also is frequent in the non-epidermoid epithelium of the lung by unclear mechanisms. A poorly studied mitotic checkpoint drives epithelial cells bearing irreparable genetic damage into epidermoid differentiation. We performed an RNA-sequencing gene search to target unknown regulators of this response and selected the SUMO regulatory protein SENP2. Alterations of SENP2 expression have been associated with some types of cancer. We found the protein to be strongly localised to mitotic spindles of freshly isolated human epidermal cells. Primary cells rapidly differentiated after silencing SENP2 with specific shRNAs. Loss of SENP2 produced in synchronised epithelial cells delays in mitotic entry and exit and defects in chromosomal alignment. The results altogether strongly argue for an essential role of SENP2 in the mitotic spindle and hence in controlling differentiation. In addition, the expression of SENP2 displayed an inverse correlation with the immuno-checkpoint biomarker PD-L1 in a pilot collection of aggressive lung carcinomas. Consistently, metastatic head and neck cancer cells that do not respond to the mitosis-differentiation checkpoint were resistant to depletion of SENP2. Our results identify SENP2 as a novel regulator of the epithelial mitosis-differentiation checkpoint and a potential biomarker in epithelial cancer.

Unlocking the Gateway: The Spatio-Temporal Dynamics of the p53 Family Driven by the Nuclear Pores and Its Implication for the Therapeutic Approach in Cancer

The p53 family remains a captivating focus of an extensive number of current studies. Accumulating evidence indicates that p53 abnormalities rank among the most prevalent in cancer. Given the numerous existing studies, which mostly focus on the mutations, expression profiles, and functional perturbations exhibited by members of the p53 family across diverse malignancies, this review will concentrate more on less explored facets regarding p53 activation and stabilization by the nuclear pore complex (NPC) in cancer, drawing on several studies. p53 integrates a broad spectrum of signals and is subject to diverse regulatory mechanisms to enact the necessary cellular response. It is widely acknowledged that each stage of p53 regulation, from synthesis to degradation, significantly influences its functionality in executing specific tasks. Over recent decades, a large body of data has established that mechanisms of regulation, closely linked with protein activation and stabilization, involve intricate interactions with various cellular components. These often transcend canonical regulatory pathways. This new knowledge has expanded from the regulation of genes themselves to epigenomics and proteomics, whereby interaction partners increase in number and complexity compared with earlier paradigms. Specifically, studies have recently shown the involvement of the NPC protein in such complex interactions, underscoring the further complexity of p53 regulation. Furthermore, we also discuss therapeutic strategies based on recent developments in this field in combination with established targeted therapies.

SUMOylation effects on neural stem cells self-renewal, differentiation, and survival

SUMO (small ubiquitin-like modifier) conjugation or SUMOylation, a post-translational modification, is a crucial regulator of protein function and cellular processes. In the context of neural stem cells (NSCs), SUMOylation has emerged as a key player, affecting their proliferation, differentiation, and survival. By modifying transcription factors, such as SOX1, SOX2, SOX3, SOX6, Bmi1, and Nanog, SUMOylation can either enhance or impair their transcriptional activity, thus impacting on NSCs self-renewal. Moreover, SUMOylation regulates neurogenesis and neuronal differentiation by modulating key proteins, such as Foxp1, Mecp2, MEF2A, and SOX10. SUMOylation is also crucial for the survival and proliferation of NSCs in both developing and adult brains. By regulating the activity of transcription factors, coactivators, and corepressors, SUMOylation acts as a molecular switch, inducing cofactor recruitment and function during development. Importantly, dysregulation of NSCs SUMOylation has been implicated in various disorders, including embryonic defects, ischemic cerebrovascular disease, glioma, and the harmful effects of benzophenone-3 exposure. Here we review the main findings on SUMOylation-mediated regulation of NSCs self-renewal, differentiation and survival. Better understanding NSCs SUMOylation mechanisms and its functional consequences might provide new strategies to promote neuronal differentiation that could contribute for the development of novel therapies targeting neurodegenerative diseases.

USP51 facilitates colorectal cancer stemness and chemoresistance by forming a positive feed-forward loop with HIF1A

In the current study, we have shown that USP51 promotes colorectal cancer stemness and chemoresistance, and high expression of USP51 predicts survival disadvantage in colorectal cancer patients. Mechanically, USP51 directly binds to Elongin C (ELOC) and forms a larger functional complex with VHL E3 ligase (USP51/VHL/CUL2/ELOB/ELOC/RBX1) to regulate the ubiquitin-dependent proteasomal degradation of HIF1A. USP51 efficiently deubiquitinates HIF1A and activates hypoxia-induced gene transcription. Conversely, the activation of HIF1A under hypoxia transcriptionally upregulates the expression of USP51. Thus, USP51 and HIF1A form a positive feedback loop. Further, we found that the SUMOylation of ELOC at K32 inhibits its binding to USP51. SUMO-specific protease 1 (SENP1) mediates the deSUMOylation of ELOC, promoting the binding of USP51 to ELOC and facilitating the deubiquitination and stabilization of HIF1A by USP51. Importantly, USP51 plays a crucial role in promoting the HIF1A and SENP1-dependent proliferation, migration, stemness, and chemoresistance under hypoxia in colorectal cancer. Together, our data revealed that USP51 is an oncogene stabilizing the pro-survival protein HIF1A, offering a potential therapeutic target for colorectal cancer.

C9orf72 poly-PR helps p53 escape from the ubiquitin-proteasome system and promotes its stability

C9orf72-derived dipeptide repeats (DPRs) proteins have been regarded as the pathogenic cause of neurodegeneration in amyotrophic lateral sclerosis and frontotemporal dementia (C9-ALS/FTD). As the most toxic DPRs in C9-ALS/FTD, poly-proline-arginine (poly-PR) is associated with the stability and accumulation of p53, which consequently induces neurodegeneration. However, the exact molecular mechanism via which C9orf72 poly-PR stabilizes p53 remains unclear. In this study, we showed that C9orf72 poly-PR induces not only neuronal damage but also p53 accumulation and p53 downstream gene activation in primary neurons. C9orf72 (PR)₅₀ also slows down p53 protein turnover without affecting the p53 transcription level and thus promotes its stability in N2a cells. Interestingly, the ubiquitin-proteasome system but not the autophagy function was impaired in (PR)₅₀ transfected N2a cells, resulting in defective p53 degradation. Moreover, we found that (PR)₅₀ induces mdm2 mistranslocation from the nucleus to the cytoplasm and competitively binds to p53, reducing mdm2-p53 interactions in the nucleus in two different (PR)₅₀ transfected cells. Our data strongly indicate that (PR)₅₀ reduces mdm2-p53 interactions and causes p53 to escape from the ubiquitin-proteasome system, promoting its stability and accumulation. Inhibiting or at least downregulating (PR)₅₀ binding with p53 may be therapeutically exploited for the treatment of C9-ALS/FTD.

Targeting the p53 signaling pathway in cancers: Molecular mechanisms and clinical studies

Tumor suppressor p53 can transcriptionally activate downstream genes in response to stress, and then regulate the cell cycle, DNA repair, metabolism, angiogenesis, apoptosis, and other biological responses. p53 has seven functional domains and 12 splice isoforms, and different domains and subtypes play different roles. The activation and inactivation of p53 are finely regulated and are associated with phosphorylation/acetylation modification and ubiquitination modification, respectively. Abnormal activation of p53 is closely related to the occurrence and development of cancer. While targeted therapy of the p53 signaling pathway is still in its early stages and only a few drugs or treatments have entered clinical trials, the development of new drugs and ongoing clinical trials are expected to lead to the widespread use of p53 signaling-targeted therapy in cancer treatment in the future. TRIAP1 is a novel p53 downstream inhibitor of apoptosis. TRIAP1 is the homolog of yeast mitochondrial intermembrane protein MDM35, which can play a tumor-promoting role by blocking the mitochondria-dependent apoptosis pathway. This work provides a systematic overview of recent basic research and clinical progress in the p53 signaling pathway and proposes that TRIAP1 is an important therapeutic target downstream of p53 signaling.

Protein sumoylation in normal and cancer stem cells

Stem cells with the capacity of self-renewal and differentiation play pivotal roles in normal tissues and malignant tumors. Whereas stem cells are supposed to be genetically identical to their non-stem cell counterparts, cell stemness is deliberately regulated by a dynamic network of molecular mechanisms. Reversible post-translational protein modifications (PTMs) are rapid and reversible non-genetic processes that regulate essentially all physiological and pathological process. Numerous studies have reported the involvement of post-translational protein modifications in the acquirement and maintenance of cell stemness. Recent studies underscore the importance of protein sumoylation, i.e., the covalent attachment of the small ubiquitin-like modifiers (SUMO), as a critical post-translational protein modification in the stem cell populations in development and tumorigenesis. In this review, we summarize the functions of protein sumoylation in different kinds of normal and cancer stem cells. In addition, we describe the upstream regulators and the downstream effectors of protein sumoylation associated with cell stemness. We also introduce the translational studies aiming at sumoylation to target stem cells for disease treatment. Finally, we propose future directions for sumoylation studies in stem cells.

Overview of the regulation of the class IA PI3K/AKT pathway by SUMO

The phosphatidylinositol-3-kinase (PI3K)/AKT pathway is a major regulator of metabolism, migration, survival, proliferation, and antiviral immunity. Both an overactivation and an inhibition of the PI3K/AKT pathway are related to different pathologies. Activation of this signaling pathway is tightly controlled through a multistep process and its deregulation can be associated with aberrant post-translational modifications including SUMOylation. Here, we review the complex modulation of the PI3K/AKT pathway by SUMOylation and we discuss its putative incvolvement in human disease.

Targeting the MDM2-p53 pathway in dedifferentiated liposarcoma

Dedifferentiated liposarcoma (DDLPS) is an aggressive adipogenic cancer with poor prognosis. DDLPS tumors are only modestly sensitive to chemotherapy and radiation, and there is a need for more effective therapies. Genetically, DDLPS is characterized by a low tumor mutational burden and frequent chromosomal structural abnormalities including amplification of the 12q13-15 chromosomal region and the MDM2 gene, which are defining features of DDLPS. The MDM2 protein is an E3 ubiquitin ligase that targets the tumor suppressor, p53, for proteasomal degradation. MDM2 amplification or overexpression in human malignancies is associated with cell-cycle progression and worse prognosis. The MDM2-p53 interaction has thus garnered interest as a therapeutic target for DDLPS and other malignancies. MDM2 binds p53 via a hydrophobic protein interaction that is easily accessible with synthetic analogues. Multiple agents have been developed, including Nutlins such as RG7112 and small molecular inhibitors including SAR405838 and HDM201. Preclinical in vitro and animal models have shown promising results with MDM2 inhibition, resulting in robust p53 reactivation and cancer cell death. However, multiple early-phase clinical trials have failed to show a benefit with MDM2 pathway inhibition for DDLPS. Mechanisms of resistance are being elucidated, and novel inhibitors and combination therapies are currently under investigation. This review provides an overview of these strategies for targeting MDM2 in DDLPS.

Pepper SUMO E3 ligase CaDSIZ1 enhances drought tolerance by stabilizing the transcription factor CaDRHB1

Small ubiquitin-like modifier (SUMO) conjugation (SUMOylation) is a reversible post-translational modification associated with protein stability and activity, and modulates hormone signaling and stress responses in plants. Previously, we reported that the pepper dehydration-responsive homeobox domain transcription factor CaDRHB1 acts as a positive modulator of drought response. Here, we show that CaDRHB1 protein stability is enhanced by SUMO E3 ligase Capsicum annuum DRHB1-interacting SAP and Miz domain (SIZ1) (CaDSIZ1)-mediated SUMOylation in response to drought, thereby positively modulating abscisic acid (ABA) signaling and drought responses. Substituting lysine (K) 138 of CaDRHB1 with arginine reduced CaDSIZ1-mediated SUMOylation, indicating that K138 is the principal site for SUMO conjugation. Virus-induced silencing of CaDSIZ1 promoted CaDRHB1 degradation, suggesting that CaDSIZ1 is involved in drought-induced SUMOylation of CaDRHB1. CaDSIZ1 interacted with and facilitated SUMO conjugation of CaDRHB1. CaDRHB1, mainly localized in the nucleus, but also in the cytoplasm in the SUMOylation mimic state, suggesting that SUMOylation of CaDRHB1 promotes its nuclear export, leading to cytoplasmic accumulation. Moreover, CaDSIZ1-silenced pepper plants were less sensitive to ABA and considerably sensitive to drought stress, whereas CaDSIZ1-overexpressing plants displayed ABA-hypersensitive and drought-tolerant phenotypes. Collectively, our data indicate that CaDSIZ1-mediated SUMOylation of CaDRHB1 functions in ABA-mediated drought tolerance.

SUMO-specific protease SENP3 enhances MDM2-mediated ubiquitination of PARIS/ZNF746 in HeLa cells

The transcriptional repressor PARIS, a substrate of the ubiquitin E3 ligase parkin, represses the expression of the transcriptional co-activator, PGC-1α gene, and is involved in several pathological processes, including neurodegenerative disease and cancers. We have previously shown that SUMOylation of PARIS play an important role in its transcriptional repression activity. In addition, RNF4-mediated ubiquitination of SUMO2/3-conjugated PARIS is required for the control of PARIS-mediated transcriptional repression in HeLa cells that lack parkin expression. However, little is known about how PARIS ubiquitination and degradation are regulated in parkin-deficient cells. Here, we report that the deSUMOylase SENP3 interacted with PARIS and enhanced the ubiquitination of PARIS independently of its SUMOylation in HeLa cells. SENP3-enhanced PARIS ubiquitination mainly contributed to its proteasomal degradation, and required the oncogenic E3 ubiquitin ligase MDM2. MDM2 knockdown by small interfering RNA or expression of a dominant-negative MDM2 mutant inhibited the ubiquitination of PARIS. We further found that MDM2 activation via the PI3K/AKT pathway was involved in PARIS ubiquitination. Taken together, these results suggest that PARIS ubiquitination through SENP3-mediated MDM2 activation may control its functions in parkin-deficient cells.

SUMOylation in peripheral tissues under low perfusion-related pathological states

SUMOylation is described as a posttranslational protein modification (PTM) that is involved in the pathophysiological processes underlying several conditions related to ischemia- and reperfusion-induced damage. Increasing evidence suggests that, under low oxygen levels, SUMOylation might be part of an endogenous mechanism, which is triggered by injury to protect cells within the central nervous system. However, the role of ischemia-induced SUMOylation in the periphery is still unclear. This article summarizes the results of recent studies regarding SUMOylation profiles in several diseases characterized by impaired blood flow to the cardiorenal, gastrointestinal, and respiratory systems. Our review shows that although ischemic injury per se does not always increase SUMOylation levels, as seen in strokes, it seems that in most cases the positive modulation of protein SUMOylation after peripheral ischemia might be a protective mechanism. This complex relationship warrants further investigation, as the role of SUMOylation during hypoxic conditions differs from organ to organ and is still not fully elucidated.

Small-molecule inhibitors targeting small ubiquitin-like modifier pathway for the treatment of cancers and other diseases

SUMOylation is a key post-translational modification that involves the covalent attachment of small ubiquitin-like modifier (SUMO) to the lysine residues of target proteins. The well-balanced SUMOylation is essential for normal cellular behaviors, while disturbance of SUMOylation is associated with various cancers and other diseases. Herein, we summarize the structures and biological functions of proteins involved in the SUMOylation process, their dysregulation in human diseases, and the discovery of small-molecular inhibitors targeting this pathway. In addition, we highlight the emerging trends in this field.

Therapeutic Potential of Targeting the SUMO Pathway in Cancer

SUMOylation is a dynamic and reversible post-translational modification, characterized more than 20 years ago, that regulates protein function at multiple levels. Key oncoproteins and tumor suppressors are SUMO substrates. In addition to alterations in SUMO pathway activity due to conditions typically present in cancer, such as hypoxia, the SUMO machinery components are deregulated at the genomic level in cancer. The delicate balance between SUMOylation and deSUMOylation is regulated by SENP enzymes possessing SUMO-deconjugation activity. Dysregulation of SUMO machinery components can disrupt the balance of SUMOylation, contributing to the tumorigenesis and drug resistance of various cancers in a context-dependent manner. Many molecular mechanisms relevant to the pathogenesis of specific cancers involve SUMO, highlighting the potential relevance of SUMO machinery components as therapeutic targets. Recent advances in the development of inhibitors targeting SUMOylation and deSUMOylation permit evaluation of the therapeutic potential of targeting the SUMO pathway in cancer. Finally, the first drug inhibiting SUMO pathway, TAK-981, is currently also being evaluated in clinical trials in cancer patients. Intriguingly, the inhibition of SUMOylation may also have the potential to activate the anti-tumor immune response. Here, we comprehensively and systematically review the recent developments in understanding the role of SUMOylation in cancer and specifically focus on elaborating the scientific rationale of targeting the SUMO pathway in different cancers.

SENP Proteases as Potential Targets for Cancer Therapy

Simple Summary

Post-translational modification—the biochemical addition of functional groups or proteins—occurs following protein biosynthesis and contributes to an increase in the functional diversity of the proteome. Post-translational modifications include SUMOylation—the covalent attachment of small ubiquitin-related modifier (SUMO) proteins to substrate proteins. SUMOylation is a reversible modification, which is erased by SUMO-specific proteases (SENPs). Deregulation of SENPs leads to cellular dysfunction and is associated with various diseases, including cancer. The role of SENPs in cancer pathogenesis is expected, and thus these proteins are considered promising targets for drug design and development. In this review, we will discuss the role of SENPs, focusing on DNA repair and the cell cycle—cellular pathways malfunctioning in most cancer cells—and provide an update on advances in the development of SENP-oriented inhibitors.

Abstract

SUMOylation is a reversible post-translational modification (PTM) involving a covalent attachment of small ubiquitin-related modifier (SUMO) proteins to substrate proteins. SUMO-specific proteases (SENPs) are cysteine proteases with isopeptidase activity facilitating the de-conjugation of SUMO proteins and thus participating in maintaining the balance between the pools of SUMOylated and unSUMOylated proteins and in SUMO recycling. Several studies have reported that SENPs’ aberrant expression is associated with the development and progression of cancer. In this review, we will discuss the role of SENPs in the pathogenesis of cancer, focusing on DNA repair and the cell cycle—cellular pathways malfunctioning in most cancer cells. The plausible role of SENPs in carcinogenesis resulted in the design and development of their inhibitors, including synthetic protein-based, peptide-based, and small molecular weight inhibitors, as well as naturally occurring compounds. Computational methods including virtual screening have been implemented to identify a number of lead structures in recent years. Some inhibitors suppressed the proliferation of prostate cancer cells in vitro and in vivo, confirming that SENPs are suitable targets for anti-cancer treatment. Further advances in the development of SENP-oriented inhibitors are anticipated toward SENP isoform-specific molecules with therapeutic potential.

Sumoylation as an Emerging Target in Therapeutics against Cancer

Sumoylation is the Post-translational modification gaining most of the research interest recently. Sumoylation is involved in various crucial functions of the cell such as regulation of cell cycle, DNA damage repair, apoptosis, etc. Oncology is advancing in radiotherapy, targeted chemotherapy, various forms of immunotherapy and targeted gene therapy. Researches are being conducted to prove its connotation with a variety of cancers and inhibitors are being developed to obstruct the fatal effect caused by misbalance of the SUMO-catalytic cycle. It has been shown that up-regulation of certain enzymes of Sumoylation correlates with cancer incidence in most of the cases. However, in some cases, down-regulation also associates with cancer invasion such as underexpression of UBC9 in initial stage breast cancer. This can aid in future study, treatment, and diagnosis of a variety of cancers including breast cancer, prostate cancer, lung adenocarcinoma, melanoma, multiple myeloma, etc. Various mechanistic assays are being developed and used to identify potential inhibitors against the dysregulated proteins of Sumoylation. This review summarizes the normal roles of the enzymes involved in the SUMOcatalytic cycle, their misbalanced regulation leading to tumorigenesis and nearly all the potent inhibitors identified to date, while after detailed studied it was observed that ML-792 could be a promising inhibitor in treating cancers by inhibiting Sumoylation enzymes.

The Role of Sumoylation in the Response to Hypoxia: An Overview

Sumoylation is the covalent attachment of the small ubiquitin-related modifier (SUMO) to a vast variety of proteins in order to modulate their function. Sumoylation has emerged as an important modification with a regulatory role in the cellular response to different types of stress including osmotic, hypoxic and oxidative stress. Hypoxia can occur under physiological or pathological conditions, such as ischemia and cancer, as a result of an oxygen imbalance caused by low supply and/or increased consumption. The hypoxia inducible factors (HIFs), and the proteins that regulate their fate, are critical molecular mediators of the response to hypoxia and modulate procedures such as glucose and lipid metabolism, angiogenesis, erythropoiesis and, in the case of cancer, tumor progression and metastasis. Here, we provide an overview of the sumoylation-dependent mechanisms that are activated under hypoxia and the way they influence key players of the hypoxic response pathway. As hypoxia is a hallmark of many diseases, understanding the interrelated connections between the SUMO and the hypoxic signaling pathways can open the way for future molecular therapeutic interventions.

The SUMO-specific protease SENP1 deSUMOylates p53 and regulates its activity

The stability and activity of the p53 tumor suppressor protein are tightly regulated by various posttranslational modifications, including SUMOylation. p53 can be modified by both SUMO1 and SUMO2, although how SUMOylation regulates p53 activity is still obscure. Whether p53 activity is directly regulated by deSUMOylation is also unclear. Here, we show that SENP1, a SUMO-specific protease implicated in pro-oncogenic roles, is a p53 deSUMOylating enzyme. SENP1 interacts with p53 and deSUMOylates p53 in cells and in vitro. Knockdown of SENP1 markedly induced p53 transactivation activity. We further show that SENP1 depletion synergizes with DNA damage-inducing agent etoposide to induce p53 activation and the expression of p21, leading to synergistic growth inhibition of cancer cells. Our results reveal that SENP1 is a critical p53 deSUMOylating enzyme and a promising therapeutic target in wild-type p53 containing cancer cells.

The requirement of SUMO2/3 for SENP2 mediated extraembryonic and embryonic development

Small ubiquitin‐related modifier (SUMO)‐specific protease 2 (SENP2) is essential for the development of healthy placenta. The loss of SENP2 causes severe placental deficiencies and leads to embryonic death that is associated with heart and brain deformities. However, tissue‐specific disruption of SENP2 demonstrates its dispensable role in embryogenesis and the embryonic defects are secondary to placental insufficiency. SENP2 regulates SUMO1 modification of Mdm2, which controls p53 activities critical for trophoblast cell proliferation and differentiation. Here we use genetic analyses to examine the involvement of SUMO2 and SUMO3 for SENP2‐mediated placentation. The results indicate that hyper‐SUMOylation caused by SENP2 deficiency can be compensated by reducing the level of SUMO modifiers. The placental deficiencies caused by the loss of SENP2 can be alleviated by the inactivation of gene encoding SUMO2 or SUMO3. Our findings demonstrate that SENP2 genetically interacts with SUMO2 and SUMO3 pivotal for the development of three major trophoblast layers. The alleviation of placental defects in the SENP2 knockouts further leads to the proper formation of the heart structures, including atrioventricular cushion and myocardium. SUMO2 and SUMO3 modifications regulate placentation and organogenesis mediated by SENP2.

Genetic analyses reveal that hyper sumoylation caused by SENP2 deficiency can be compensated by reducing the level of SUMO modifiers. The placental deficiencies caused by the loss of SENP2 can be alleviated by the inactivation of gene encoding SUMO2 or SUMO3. SENP2 genetically interacts with SUMO2 and SUMO3 pivotal for the development of three major trophoblast layers. The alleviation of placental defects in the SENP2 knockouts further leads to the proper formation of the heart structures, including atrioventricular cushion and myocardium. Protein modification by SUMO2 and SUMO3 is essential for SENP2‐mediated placentation and organogenesis.

Chlorogenic acid effectively treats cancers through induction of cancer cell differentiation

Rationale: Inducing cancer differentiation is a promising approach to treat cancer. Here, we identified chlorogenic acid (CA), a potential differentiation inducer, for cancer therapy, and elucidated the molecular mechanisms underlying its differentiation-inducing effects on cancer cells.

Methods: Cancer cell differentiation was investigated by measuring malignant behavior, including growth rate, invasion/migration, morphological change, maturation, and ATP production. Gene expression was analyzed by microarray analysis, qRT-PCR, and protein measurement, and molecular biology techniques were employed for mechanistic studies. LC/MS analysis was the method of choice for chemical detection. Finally, the anticancer effect of CA was evaluated both in vitro and in vivo.

Results: Cancer cells treated with CA showed reduced proliferation rate, migration/invasion ability, and mitochondrial ATP production. Treating cancer cells with CA resulted in elevated SUMO1 expression through acting on its 3'UTR and stabilizing the mRNA. The increased SUMO1 caused c-Myc sumoylation, miR-17 family downregulation, and p21 upregulation leading to G₀/G₁ arrest and maturation phenotype. CA altered the expression of differentiation-related genes in cancer cells but not in normal cells. It inhibited hepatoma and lung cancer growth in tumor-bearing mice and prevented new tumor development in naïve mice. In glioma cells, CA increased expression of specific differentiation biomarkers Tuj1 and GFAP inducing differentiation and reducing sphere formation. The therapeutic efficacy of CA in glioma cells was comparable to that of temozolomide. CA was detectable both in the blood and brain when administered intraperitoneally in animals. Most importantly, CA was safe even at very high doses.

Conclusion: CA might be a safe and effective differentiation-inducer for cancer therapy. “Educating” cancer cells to differentiate, rather than killing them, could be a novel therapeutic strategy for cancer.

SENP2 exerts an anti‑tumor effect on chronic lymphocytic leukemia cells through the inhibition of the Notch and NF‑κB signaling pathways

Chronic lymphocytic leukemia (CLL) is one of the most often diagnosed hematological malignant tumors in the Western world and a type of inert B-cell lymphoma that commonly attacks the elderly. Small ubiquitin related modifier (SUMO)-specific protease 2 (SENP2) can act as a suppressor in various types of cancer by regulating the stability of β-catenin to affect the Notch signaling pathway; however, it has a low expression level in CLL cells. In this study, we firstly used western blot analysis and RT-qPCR to detect the protein and mRNA expression levels of SENP2 in the peripheral blood of patients with CLL and healthy volunteers. Secondly, we overexpressed or knocked down the expression of SENP2 in CLL cells and then determined the cell invasive and chemotactic ability in a Transwell assay and chemotaxis assay. We examined the sensitivity of the cells to cytarabine and dexamethasone via a CCK-8 assay and determined the cell apoptotic condition and the expression of the Notch signaling pathway using flow cytometry and western blot analysis. The results demonstrated that the patients with CLL had relatively low expression levels of SENP2. The overexpression of SENP2 in the CLL cells decreased their invasive and proliferative ability, as well as their chemotactic response and enhanced their sensitivity to cytarabine and dexamethasone, while it promoted cell apoptosis. The silencing of SENP2 in the CLL cells generally produced the opposite results. We thus hypothesized that the overexpression of SENP2 downregulated β-catenin expression, thus inhibiting the Notch signaling pathway in CLL cells. Moreover, the nuclear factor (NF)-κB signaling pathway was also regulated by the overexpression of SENP2. On the whole, the findings of this study indicate tha SENP2 can act as a tumor suppressor in CLL cells, and may thus prove to be a novel target for CLL treatment in clinical practice.

The Ubiquitin E3 Ligase RHA2b Promotes Degradation of MYB30 in Abscisic Acid Signaling

The phytohormone abscisic acid (ABA) is critical for plants encountering abiotic stress. We reported previously that the Arabidopsis (Arabidopsis thaliana) transcription factor MYB30 participates in ABA responses via SUMO ligase SAP-MIZ Domain-Containing SIZ1-mediated sumoylation. Here, we show that the RING-type ubiquitin E3 ligase RHA2b, which positively regulates ABA signaling, interacted with and ubiquitinated MYB30 to modulate MYB30 stability through the 26S proteasome pathway. The degradation rate of MYB30 was repressed significantly in the rha2b-1 mutant. Phenotypic analyses showed that MYB30 acts genetically downstream of RHA2b in ABA signaling. Substitutions of lysine-283 (K283) and K165 blocked ubiquitination, suggesting that these residues are sites of ubiquitination. K283 residue substitution significantly inhibited the degradation of MYB30 induced by ABA. The K165 site functioned additively with K283 in ABA-induced MYB30 degradation and ABA responses. At the same time, sumoylation protected MYB30 from degradation under cycloheximide and ABA treatment. Compared with MYB30, overexpression of MYB30-SUMO1 partially recovered the ABA sensitivity of siz1-2 But MYB30-SUMO1 exhibited similar localization with MYB30 in nuclei. Overall, our results suggest that RHA2b targets MYB30 for degradation to modulate ABA signaling. Considering that the K283 residue also is the major site for sumoylation, we propose that sumoylation and ubiquitination act antagonistically in the ABA response to regulate the stability of MYB30 by occupying the same residue.

Silencing SUMO2 promotes protection against degradation and apoptosis of nucleus pulposus cells through p53 signaling pathway in intervertebral disc degeneration

Objective: Intervertebral disc degeneration (IDD), as a common cause of back pain, is related to the promotion of cellular senescence and reduction in proliferation. Based on recent studies, small ubiquitin-related modifier (SUMO) proteins have been implicated in various biological functions. Therefore, in the present study, we investigated the effects of SUMO2 on proliferation and senescence of nucleus pulposus cells (NPCs) via mediation of p53 signaling pathway in rat models of IDD.

Methods: After the establishment of rat models of IDD for the measurement of positive expression of SUMO2/3 protein, the mRNA and protein levels of SUMO2, molecular phenotype [matrix metalloproteinase-2 (MMP-2) and hypoxia-inducible factor-1α (HIF-1α)] and p53 signaling pathway-related genes [p21, murine double minute-2 (MDM2), growth arrest and DNA-damage-inducible protein 45 α (GADD45α), cyclin-dependent kinase 2/4 (CDK2/4), and CyclinB1] were determined, followed by the detection of cell proliferation, cell cycle, apoptosis, and cell senescence.

Results: The rat models of IDD were successfully constructed. The results obtained showed that there was a higher positive expression of SUMO2/3 protein in IDD rats. Moreover, the silencing of the SUMO2 gene decreases the levels of SUMO2, p53, p21, MDM2, GADD45α, MMP-2, and HIF-1α expressions and p53 phosphorylation level while it increases the levels of CDK2/4 and CyclinB1 expressions. In addition, SUMO2 gene silencing enhances proliferation and suppresses apoptosis and cell senescence of NPCs.

Conclusion: In conclusion, SUMO2 gene silencing promotes proliferation, and inhibits the apoptosis and senescence of NPCs in rats with IDD through the down-regulation of the p53 signaling pathway. Thus, SUMO2 is a potential target in the treatment of IDD.

SUMO1/sentrin/SMT3 specific peptidase 2 modulates target molecules and its corresponding functions

Small ubiquitin-like modifier (SUMOylation) is a reversible post-translational modification, which plays important roles in numerous biological processes. SUMO could be covalently attached to target proteins in an isopeptide bond manner that occurs via a lysine ε-amino group on the target proteins and the glycine on SUMO C-terminus. This covalent binding could affect the subcellular localization and stability of target proteins. SUMO modification can be reversed by members of the Sentrin/SUMO-specific proteases (SENPs) family, which are highly evolutionarily conserved from yeast to human. SENP2, a member of the SENPs family, mainly plays a physiological function in the nucleus. SENP2 can promote maturity of the SUMO and deSUMOylate for single-SUMO modified or poly-SUMO modified proteins. SENP2 can affect the related biological processes through its peptidase activity or the amino terminal transcriptional repression domain. It plays important roles by inhibiting or activating some molecular functions. Therefore, the research achievements of SENP2 are reviewed in order to understand its related functions and the underlying molecular mechanisms and provide a clue for future research on SENP2.

SUMO-specific proteases and isopeptidases of the SENP family at a glance

The ubiquitin-related SUMO system controls many cellular signaling networks. In mammalian cells, three SUMO forms (SUMO1, SUMO2 and SUMO3) act as covalent modifiers of up to thousands of cellular proteins. SUMO conjugation affects cell function mainly by regulating the plasticity of protein networks. Importantly, the modification is reversible and highly dynamic. Cysteine proteases of the sentrin-specific protease (SENP) family reverse SUMO conjugation in mammalian cells. In this Cell Science at a Glance article and the accompanying poster, we will summarize how the six members of the mammalian SENP family orchestrate multifaceted deconjugation events to coordinate cell processes, such as gene expression, the DNA damage response and inflammation.

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.

A systems medicine approach for finding target proteins affecting treatment outcomes in patients with non-Hodgkin lymphoma

Autoantibody profiling with a systems medicine approach can help identify critical dysregulated signaling pathways (SPs) in cancers. In this way, immunoglobulins G (IgG) purified from the serum samples of 92 healthy controls, 10 pre-treated (PR) non-Hodgkin lymphoma (NHL) patients, and 20 NHL patients who underwent chemotherapy (PS) were screened with a phage-displayed random peptide library. Protein-protein interaction networks of the PR and PS groups were analyzed and visualized by Gephi. The results indicated AXIN2, SENP2, TOP2A, FZD6, NLK, HDAC2, HDAC1, and EHMT2, in addition to CAMK2A, PLCG1, PLCG2, GRM5, GRIN2B, GRIN2D, CACNA2D3, and SPTAN1 as hubs in 11 and 7 modules of PR and PS networks, respectively. PR- and PS-specific hubs were evaluated in the Kyoto Encyclopedia of Genes and Genomes (KEGG) and Reactome databases. The PR-specific hubs were involved in Wnt SP, signaling by Notch1 in cancer, telomere maintenance, and transcriptional misregulation. In contrast, glutamate receptor SP, Fc receptor-related pathways, growth factors-related SPs, and Wnt SP were statistically significant enriched pathways, based on the pathway analysis of PS hubs. The results revealed that the most PR-specific proteins were associated with events involved in tumor development, while chemotherapy in the PS group was associated with side effects of drugs and/or cancer recurrence. As the findings demonstrated, PR- and PS-specific proteins in this study can be promising therapeutic targets in future studies.

Rap1b Is an Effector of Axin2 Regulating Crosstalk of Signaling Pathways During Skeletal Development

Recent identification and isolation of suture stem cells capable of long-term self-renewal, clonal expanding, and differentiating demonstrate their essential role in calvarial bone development, homeostasis, and injury repair. These bona fide stem cells express a high level of Axin2 and are able to mediate bone regeneration and repair in a cell autonomous fashion. The importance of Axin2 is further demonstrated by its genetic inactivation in mice causing skeletal deformities resembling craniosynostosis in humans. The fate determination and subsequent differentiation of Axin2+ stem cells are highly orchestrated by a variety of evolutionary conserved signaling pathways including Wnt, FGF, and BMP. These signals are often antagonistic of each other and possess differential effects on osteogenic and chondrogenic cell types. However, the mechanisms underlying the interplay of these signaling transductions remain largely elusive. Here we identify Rap1b acting downstream of Axin2 as a signaling interrogator for FGF and BMP. Genetic analysis reveals that Rap1b is essential for development of craniofacial and body skeletons. Axin2 regulates Rap1b through modulation of canonical BMP signaling. The BMP-mediated activation of Rap1b promotes chondrogenic fate and chondrogenesis. Furthermore, by inhibiting MAPK signaling, Rap1b mediates the antagonizing effect of BMP on FGF to repress osteoblast differentiation. Disruption of Rap1b in mice not only enhances osteoblast differentiation but also impairs chondrocyte differentiation during intramembranous and endochondral ossifications, respectively, leading to severe defects in craniofacial and body skeletons. Our findings reveal a dual role of Rap1b in development of the skeletogenic cell types. Rap1b is critical for balancing the signaling effects of BMP and FGF during skeletal development and disease. © 2017 American Society for Bone and Mineral Research.

Therapeutic potential of mitotic interaction between the nucleoporin Tpr and aurora kinase A

Spindle poles are defined by centrosomes; therefore, an abnormal number or defective structural organization of centrosomes can lead to loss of spindle bipolarity and genetic integrity. Previously, we showed that Tpr (translocated promoter region), a component of the nuclear pore complex (NPC), interacts with Mad1 and dynein to promote proper chromosome segregation during mitosis. Tpr also associates with p53 to induce autophagy. Here, we report that Tpr depletion induces mitotic catastrophe and enhances the rate of tetraploidy and polyploidy. Mechanistically, Tpr interacts, via its central domain, with Aurora A but not Aurora B kinase. In Tpr-depleted cells, the expression levels, centrosomal localization and phosphorylation of Aurora A were all reduced. Surprisingly, an Aurora A inhibitor, Alisertib (MLN8237), also disrupted centrosomal localization of Tpr and induced mitotic catastrophe and cell death in a time- and dose-dependent manner. Strikingly, over-expression of Aurora A disrupted Tpr centrosomal localization only in cells with supernumerary centrosomes but not in bipolar cells. Our results highlight the mutual regulation between Tpr and Aurora A and further confirm the importance of nucleoporin function in spindle pole organization, bipolar spindle assembly, and mitosis; functions that are beyond the conventional nucleocytoplasmic transport and NPC structural roles of nucleoporins. Furthermore, the central coiled-coil domain of Tpr binds to and sequesters extra Aurora A to safeguard bipolarity. This Tpr domain merits further investigation for its ability to inhibit Aurora kinase and as a potential therapeutic agent in cancer treatment.

Extraembryonic but not embryonic SUMO-specific protease 2 is required for heart development

SUMO-specific protease 2 (SENP2) activities to remove SUMO from its substrates is essential for development of trophoblast stem cells, niches and lineages. Global deletion of SENP2 leads to midgestation lethality, and causes severe defects in the placenta which is accompanied by embryonic brain and heart abnormalities. Because of the placental deficiencies, the role of SENP2 in development of the embryonic tissues has not been properly determined. The brain and heart abnormalities may be secondary to placental insufficiency. Here we have created a new mouse strain permitting conditional inactivation of SENP2. Mice homozygous for germline deletion of the conditional allele exhibit trophoblast defects and embryonic abnormalities resembling the global SENP2 knockout. However, tissue-specific disruptions of SENP2 demonstrate its dispensable role in embryogenesis. Placental expression of SENP2 is necessary and sufficient for embryonic heart and brain development. Using a protease deficient model, we further demonstrate the requirement of SENP2-dependent SUMO modification in development of all major trophoblast lineages. SENP2 regulates sumoylation of Mdm2 which controls p53 activities critical for G-S transition of mitotic division and endoreduplication in trophoblast proliferation and differentiation, respectively. The differentiation of trophoblasts is also dependent on SENP2-mediated activation of p57^Kip2, a CDK-specific inhibitor required for endoreduplication.

Regulators in the DNA damage response

Maintenance of genome integrity is essential for the proper function of all cells and organisms. In response to both endogenous and exogenous DNA damaging agents, mammalian cells have evolved a delicate system to sense DNA damage, stop cell cycle progression, modulate cell metabolism, repair damaged DNA, and induce programmed cell death if the damage is too severe. This coordinated global signaling network, namely the DNA damage response (DDR), ensures the genome stability under DNA damaging stress. A variety of regulators have been shown to modulate the activity and levels of key proteins in the DDR, including kinases, phosphatases, ubiquitin ligases, deubiquitinases, and other protein modifying enzymes. Epigenetic regulators, particularly microRNAs and long noncoding RNAs, have been emerging as an important payer of regulation in addition to canonical DNA damage signaling proteins. In this review, we will discuss the functional interaction between the regulators and their targets in the DDR.

DeSUMOylation: An Important Therapeutic Target and Protein Regulatory Event

The discovery of the process of small ubiquitin-like modifier (SUMO)-mediated post-translational modification of targets (SUMOylation) in early 1990s proved to be a significant step ahead in understanding mechanistic regulation of proteins and their functions in diverse life processes at the cellular level. The critical step in reversing the SUMOylation pathway is its ability to be dynamically deSUMOylated by SUMO/sentrin-specific protease (SENP). This review is intended to give a brief introduction about the process of SUMOylation, different mammalian deSUMOylating enzymes with special emphasis on their regulation of ribosome biogenesis at the molecular level, and its emerging roles in mitochondrial dynamics that might reveal usefulness of SENPs for therapeutic applications.

Advances in the development of SUMO specific protease (SENP) inhibitors

Sumoylation is a reversible post-translational modification that involves the covalent attachment of small ubiquitin-like modifier (SUMO) proteins to their substrate proteins. Prior to their conjugation, SUMO proteins need to be proteolytically processed from its precursor form to mature or active form. SUMO specific proteases (SENPs) are cysteine proteases that cleave the pro or inactive form of SUMO at C-terminus using its hydrolase activity to expose two glycine residues. SENPs also catalyze the de-conjugation of SUMO proteins using their isopeptidase activity, which is crucial for recycling of SUMO from substrate proteins. SENPs are important for maintaining the balance between sumoylated and unsumoylated proteins required for normal cellular physiology. Several studies reported the overexpression of SENPs in disease conditions and highlighted their role in the development of various diseases, especially cancer. In this review, we will address the current biological understanding of various SENP isoforms and their role in the pathogenesis of different cancers and other diseases. We will then discuss the advances in the development of protein-based, peptidyl and small molecule inhibitors of various SENP isoforms. Finally, we will summarize successful examples of computational screening that allowed the identification of SENP inhibitors with therapeutic potential.

Disturbed flow-activated p90RSK kinase accelerates atherosclerosis by inhibiting SENP2 function

Disturbed blood flow (d-flow) causes endothelial cell (EC) dysfunction, leading to atherosclerotic plaque formation. We have previously shown that d-flow increases SUMOylation of p53 and ERK5 through downregulation of sentrin/SUMO-specific protease 2 (SENP2) function; however, it is not known how SENP2 itself is regulated by d-flow. Here, we determined that d-flow activated the serine/threonine kinase p90RSK, which subsequently phosphorylated threonine 368 (T368) of SENP2. T368 phosphorylation promoted nuclear export of SENP2, leading to downregulation of eNOS expression and upregulation of proinflammatory adhesion molecule expression and apoptosis. In an LDLR-deficient murine model of atherosclerosis, EC-specific overexpression of p90RSK increased EC dysfunction and lipid accumulation in the aorta compared with control animals; however, these pathologic changes were not observed in atherosclerotic mice overexpressing dominant negative p90RSK (DN-p90RSK). Moreover, depletion of SENP2 in these mice abolished the protective effect of DN-p90RSK overexpression. We propose that p90RSK-mediated SENP2-T368 phosphorylation is a master switch in d-flow-induced signaling, leading to EC dysfunction and atherosclerosis.

Inhibition of p53 deSUMOylation exacerbates puromycin aminonucleoside-induced apoptosis in podocytes

Apoptosis is a major cause of reduced podocyte numbers, which leads to proteinuria and/or glomerulosclerosis. Emerging evidence has indicated that deSUMOylation, a dynamic post-translational modification that reverses SUMOylation, is involved in the apoptosis of Burkitt’s lymphoma cells and cardiomyocytes; however, the impact of deSUMOylation on podocyte apoptosis remains unexplored. The p53 protein plays a major role in the pathogenesis of podocyte apoptosis, and p53 can be SUMOylated. Therefore, in the present study, we evaluated the effect of p53 deSUMOylation, which is regulated by sentrin/SUMO-specific protease 1 (SENP1), on podocyte apoptosis. Our results showed that SENP1 deficiency significantly increases puromycin aminonucleoside (PAN)-induced podocyte apoptosis. Moreover, SENP1 knockdown results in the accumulation of SUMOylated p53 protein and the increased expression of the p53 target pro-apoptotic genes, BAX, Noxa and PUMA, in podocytes during PAN stimulation. Thus, SENP1 may be essential for preventing podocyte apoptosis, at least partly through regulating the functions of p53 protein via deSUMOylation. The regulation of deSUMOylation may provide a novel strategy for the treatment of glomerular disorders that involve podocyte apoptosis.

Disruption of SUMO-specific protease 2 induces mitochondria mediated neurodegeneration

Post-translational modification of proteins by small ubiquitin-related modifier (SUMO) is reversible and highly evolutionarily conserved from yeasts to humans. Unlike ubiquitination with a well-established role in protein degradation, sumoylation may alter protein function, activity, stability and subcellular localization. Members of SUMO-specific protease (SENP) family, capable of SUMO removal, are involved in the reversed conjugation process. Although SUMO-specific proteases are known to reverse sumoylation in many well-defined systems, their importance in mammalian development and pathogenesis remains largely elusive. In patients with neurodegenerative diseases, aberrant accumulation of SUMO-conjugated proteins has been widely described. Several aggregation-prone proteins modulated by SUMO have been implicated in neurodegeneration, but there is no evidence supporting a direct involvement of SUMO modification enzymes in human diseases. Here we show that mice with neural-specific disruption of SENP2 develop movement difficulties which ultimately results in paralysis. The disruption induces neurodegeneration where mitochondrial dynamics is dysregulated. SENP2 regulates Drp1 sumoylation and stability critical for mitochondrial morphogenesis in an isoform-specific manner. Although dispensable for development of neural cell types, this regulatory mechanism is necessary for their survival. Our findings provide a causal link of SUMO modification enzymes to apoptosis of neural cells, suggesting a new pathogenic mechanism for neurodegeneration. Exploring the protective effect of SENP2 on neuronal cell death may uncover important preventive and therapeutic strategies for neurodegenerative diseases.

SENP2 regulated the stability of β-catenin through WWOX in hepatocellular carcinoma cell

SUMOylation and deSUMOylation are dynamic mechanisms regulating a spectrum of protein activities. The SUMO proteases (SENP) remove SUMO conjugate from proteins, and their expression is deregulated in cancers. SENP2 has been reported to play a critical role in the control of hepatocellular carcinoma (HCC) cell growth by modulating the stability of β-catenin. However, the underlying mechanism remains largely unknown. Here, we show that the WW domain-containing oxidoreductase (WWOX), a novel inhibitor of the Wnt/β-catenin pathway, is required for stabilization of β-catenin regulated by SENP2 in HCC cells. The transcriptional level of WWOX is tightly regulated by SENP2. Moreover, knockdown of WWOX by siRNA attuned SENP2-induced β-catenin degradation and decreased SENP2-mediated HCC cell proliferation arrest. Taken together, our data suggested that WWOX is a key downstream modulator of the SENP2 tumor suppressor function in HCC cell.

Shear stress and atherosclerosis

Hemodynamic shear stress, the frictional force acting on vascular endothelial cells, is crucial for endothelial homeostasis under normal physiological conditions. When discussing blood flow effects on various forms of endothelial (dys)function, one considers two flow patterns: steady laminar flow and disturbed flow because endothelial cells respond differently to these flow types both in vivo and in vitro. Laminar flow which exerts steady laminar shear stress is atheroprotective while disturbed flow creates an atheroprone environment. Emerging evidence has provided new insights into the cellular mechanisms of flow-dependent regulation of vascular function that leads to cardiovascular events such as atherosclerosis, atherothrombosis, and myocardial infarction. In order to study effects of shear stress and different types of flow, various models have been used. In this review, we will summarize our current views on how disturbed flow-mediated signaling pathways are involved in the development of atherosclerosis.

Deubiquitylating enzymes and DNA damage response pathways

Covalent post-translational modification of proteins by ubiquitin and ubiquitin-like factors has emerged as a general mechanism to regulate myriad intra-cellular processes. The addition and removal of ubiquitin or ubiquitin-like proteins from factors has recently been demonstrated as a key mechanism to modulate DNA damage response (DDR) pathways. It is thus, timely to evaluate the potential for ubiquitin pathway enzymes as DDR drug targets for therapeutic intervention. The synthetic lethal approach provides exciting opportunities for the development of targeted therapies to treat cancer: most tumours have lost critical DDR pathways, and thus rely more heavily on the remaining pathways, while normal tissues are still equipped with all DDR pathways. Here, we review key deubiquitylating enzymes (DUBs) involved in DDR pathways, and describe how targeting DUBs may lead to selective therapies to treat cancer patients.

Identification of 1,2,5-oxadiazoles as a new class of SENP2 inhibitors using structure based virtual screening

Small ubiquitin like modifier (SUMO) specific proteases (SENPs) are cysteine proteases that carry out the proteolytic processing of SUMO from its pro form as well as the deconjugation of SUMO from substrate proteins. SENPs are attractive targets for drug discovery due to their crucial role in the development of various diseases. However, the SENPs inhibitor discovery efforts were limited, and only a few inhibitors or activity based probes have been identified until now. Here, we report a new class of SENP2 inhibitors identified by a combination of structure based virtual screening and quantitative FRET based assay. Our virtual screening protocol initially involves the identification of small molecules that have similar shape and electrostatic properties with the conjugate of SUMO1 C-terminal residues and substrate lysine. Molecular docking was then used to prioritize these small molecules for a FRET based assay that quantifies their SENP2 endopeptidase activity. The initial round of virtual screening followed by FRET based assay has enabled the identification of eight compounds with >40% SENP2 inhibition at 30 μM compound concentration. Five of these compounds belong to two scaffolds containing a 1,2,5-oxadiazole core that represent a novel class of SENP2 inhibitors. To improve the inhibitory potency and explore the structure-activity relationship of these two 1,2,5-oxadiazole scaffolds, structurally related compounds were identified in another round of virtual screening. The biological assay results confirmed SENP2 inhibitory activity of these two scaffolds. The most potent compound of each scaffold showed an IC50 of 5.9 and 3.7 μM. Most of the compounds also inhibited closely related isoform SENP1, while no detectable inhibition on other proteases, such as papain and trypsin, was observed. Our study suggests that 1,2,5-oxadiazoles could be used as a starting point for the development of novel therapeutic agents against various diseases targeting SENPs.

Targeting MDM2-p53 interaction for cancer therapy: are we there yet?

Inactivation of the tumor suppressor p53 and/or overexpression of the oncogene MDM2 frequently occur in human cancers, and are associated with poor prognosis, advanced forms of the disease, and chemoresistance. MDM2, the major negative regulator of p53, induces p53 degradation and inactivates its tumor suppressing activity. In turn, p53 regulates MDM2 expression. This MDM2-p53 negative feedback loop has been widely studied and presents an attractive target for cancer therapy, with a few of the inhibitors of this interaction already having advanced into clinical trials. Additionally, there is an increasing interest in understanding MDM2's p53-independent activities in carcinogenesis and cancer progression, which may also have implications for cancer therapy. This review aims to highlight the various roles that the MDM2-p53 interaction plays in cancer, the p53 independent oncogenic activities of MDM2 and the various strategies that may be used to target MDM2 and the MDM2-p53 interaction. We will summarize the major preclinical and clinical evidences of MDM2 inhibitors for human cancer treatment and make suggestions to further improve efficacy and safety of this interesting class of cancer therapeutics.

The deSUMOylase SENP7 promotes chromatin relaxation for homologous recombination DNA repair

SUMO conjugation is known to occur in response to double-stranded DNA breaks in mammalian cells, but whether SUMO deconjugation has a role remains unclear. Here, we show that the SUMO/Sentrin/Smt3-specific peptidase, SENP7, interacts with the chromatin repressive KRAB-associated protein 1 (KAP1) through heterochromatin protein 1 alpha (HP1α). SENP7 promotes the removal of SUMO2/3 from KAP1 and regulates the interaction of the chromatin remodeler CHD3 with chromatin. Consequently, in the presence of CHD3, SENP7 is required for chromatin relaxation in response to DNA damage, for homologous recombination repair and for cellular resistance to DNA-damaging agents. Thus, deSUMOylation by SENP7 is required to promote a permissive chromatin environment for DNA repair.

Integrative genomics analysis identifies candidate drivers at 3q26-29 amplicon in squamous cell carcinoma of the lung

PURPOSE

Chromosome 3q26-29 is a critical region of genomic amplification in lung squamous cell carcinomas (SCC). Identification of candidate drivers in this region could help uncover new mechanisms in the pathogenesis and potentially new targets in SCC of the lung.

EXPERIMENTAL DESIGN

We conducted a meta-analysis of seven independent datasets containing a total of 593 human primary SCC samples to identify consensus candidate drivers in 3q26-29 amplicon. Through integrating protein-protein interaction network information, we further filtered for candidates that may function together in a network. Computationally predicted candidates were validated using RNA interference (RNAi) knockdown and cell viability assays. Clinical relevance of the experimentally supported drivers was evaluated in an independent cohort of 52 lung SCC patients using survival analysis.

RESULTS

The meta-analysis identified 20 consensus candidates, among which four (SENP2, DCUN1D1, DVL3, and UBXN7) are involved in a small protein-protein interaction network. Knocking down any of the four proteins led to cell growth inhibition of the 3q26-29-amplified SCC. Moreover, knocking down of SENP2 resulted in the most significant cell growth inhibition and downregulation of DCUN1D1 and DVL3. Importantly, a gene expression signature composed of SENP2, DCUN1D1, and DVL3 stratified patients into subgroups with different response to adjuvant chemotherapy.

CONCLUSION

Together, our findings show that SENP2, DCUN1D1, and DVL3 are candidate driver genes in the 3q26-29 amplicon of SCC, providing novel insights into the molecular mechanisms of disease progression and may have significant implication in the management of SCC of the lung.

Role of SUMO-specific protease 2 in reprogramming cellular glucose metabolism

Most cancer cells exhibit a shift in glucose metabolic strategy, displaying increased glycolysis even with adequate oxygen supply. SUMO-specific proteases (SENPs) de-SUMOylate substrates including HIF1α and p53,two key regulators in cancer glucose metabolism, to regulate their activity, stability and subcellular localization. However, the role of SENPs in tumor glucose metabolism remains unclear. Here we report that SUMO-specific protease 2 (SENP2) negatively regulates aerobic glycolysis in MCF7 and MEF cells. Over-expression of SENP2 reduces the glucose uptake and lactate production, increasing the cellular ATP levels in MCF7 cells, while SENP2 knockout MEF cells show increased glucose uptake and lactate production along with the decreased ATP levels. Consistently, the MCF7 cells over-expressing SENP2 exhibit decreased expression levels of key glycolytic enzymes and an increased rate of glucose oxidation compared with control MCF7 cells, indicating inhibited glycolysis but enhanced oxidative mitochondrial respiration. Moreover, SENP2 over-expressing MCF7 cells demonstrated a reduced amount of phosphorylated AKT, whereas SENP2 knockout MEFs exhibit increased levels of phosphorylated AKT. Furthermore, inhibiting AKT phosphorylation by LY294002 rescued the phenotype induced by SENP2 deficiency in MEFs. In conclusion, SENP2 represses glycolysis and shifts glucose metabolic strategy, in part through inhibition of AKT phosphorylation. Our study reveals a novel function of SENP2 in regulating glucose metabolism.

Gpr177, a novel locus for bone mineral density and osteoporosis, regulates osteogenesis and chondrogenesis in skeletal development

Human genetic analysis has recently identified Gpr177 as a susceptibility locus for bone mineral density and osteoporosis. Determining the unknown function of this gene is therefore extremely important to furthering our knowledge base of skeletal development and disease. The protein encoded by Gpr177 exhibits an ability to modulate the trafficking of Wnt, similar to the Drosophila Wls/Evi/Srt. Because it plays a critical role in Wnt regulation, Gpr177 might be required for several key steps of skeletogenesis. To overcome the early lethality associated with the inactivation of Gpr177 in mice, conditional gene deletion is used to assess its functionality. Here we report the generation of four different mouse models with Gpr177 deficiency in various skeletogenic cell types. The loss of Gpr177 severely impairs development of the craniofacial and body skeletons, demonstrating its requirement for intramembranous and endochondral ossifications, respectively. Defects in the expansion of skeletal precursors and their differentiation into osteoblasts and chondrocytes suggest that Wnt production and signaling mediated by Gpr177 cannot be substituted. Because the Gpr177 ablation impairs Wnt secretion, we therefore identify the sources of Wnt proteins essential for osteogenesis and chondrogenesis. The intercross of Wnt signaling between distinct cell types is carefully orchestrated and necessary for skeletogenesis. Our findings lead to a proposed mechanism by which Gpr177 controls skeletal development through modulation of autocrine and paracrine Wnt signals in a lineage-specific fashion.

Gpr177 deficiency impairs mammary development and prohibits Wnt-induced tumorigenesis

Aberrant regulation of the Wnt pathway, essential for various developmental processes, is tightly linked to human breast cancers. By hijacking this evolutionary conserved signaling pathway, cancer cells acquire sustaining proliferation ability, leading to modification of physiologic properties necessary for tumor initiation and progression. An enormous wealth of knowledge on the importance of Wnt signaling in breast development and cancer has been obtained, but the cell types responsible for production of this proliferative signal operating within normal and malignant tissues remains poorly understood. Here we report that Wnt production mediated by Gpr177 is essential for mammary morphogenesis. The loss of Gpr177 interferes with mammary stem cells, leading to deficiencies in cell proliferation and differentiation. Genetic analysis further demonstrates an indispensable role of Gpr177 in Wnt-induced tumorigenesis. The Gpr177-deficiency mice are resistant to malignant transformation. This study not only demonstrates the necessity of Wnt in mammary organogenesis but also provides a proof-of-principle for targeting of Gpr177 as a potential new treatment for human diseases with aberrant Wnt stimulation.

De-SUMOylation enzyme of sentrin/SUMO-specific protease 2 regulates disturbed flow-induced SUMOylation of ERK5 and p53 that leads to endothelial dysfunction and atherosclerosis

RATIONALE

Disturbed flow induces proinflammatory and apoptotic responses in endothelial cells, causing them to become dysfunctional and subsequently proatherogenic.

OBJECTIVE

Although a possible link between SUMOylation of p53 and ERK5 detected during endothelial apoptosis and inflammation has been suggested, the mechanistic insights, especially under the proatherogenic flow condition, remain largely unknown.

METHODS AND RESULTS

SUMOylation of p53 and ERK5 was induced by disturbed flow but not by steady laminar flow. To examine the role of the disturbed flow-induced p53 and ERK5 SUMOylation, we used de-SUMOylation enzyme of sentrin/Small Ubiquitin-like MOdifier (SUMO)-specific protease 2 deficiency (Senp2(+/-)) mice and observed a significant increase in endothelial apoptosis and adhesion molecule expression both in vitro and in vivo. These increases, however, were significantly inhibited in endothelial cells overexpressing p53 and ERK5 SUMOylation site mutants. Senp2(+/-) mice exhibited increased leukocyte rolling along the endothelium, and accelerated formation of atherosclerotic lesions was observed in Senp2(+/-)/Ldlr(-/-), but not in Senp2(+/+)/Ldlr(-/-), mice fed a high-cholesterol diet. Notably, the extent of lesion size in the aortic arch of Senp2(+/-)/Ldlr(-/-) mice was much larger than that in the descending aorta, also suggesting a crucial role of the disturbed flow-induced SUMOylation of proteins, including p53 and ERK5 in atherosclerosis formation.

CONCLUSIONS

These data show the unique role of sentrin/SUMO-specific protease 2 on endothelial function under disturbed flow and suggest that SUMOylation of p53 and ERK5 by disturbed flow contributes to the atherosclerotic plaque formation. Molecules involved in this newly discovered signaling will be useful targets for controlling endothelial cells dysfunction and consequently atherosclerosis formation.