Negative regulation of TLR inflammatory signaling by the SUMO-deconjugating enzyme SENP6

SUMO and the robustness of cancer

Post-translational protein modification by small ubiquitin-like modifier (SUMO), termed sumoylation, is an important mechanism in cellular responses to stress and one that appears to be upregulated in many cancers. Here, we examine the role of sumoylation in tumorigenesis as a possibly necessary safeguard that protects the stability and functionality of otherwise easily misregulated gene expression programmes and signalling pathways of cancer cells.

Sub-cellular localization specific SUMOylation in the heart

Although the majority of SUMO substrates are localized in the nucleus, SUMOylation is not limited to nuclear proteins and can be also detected in extra-nuclear proteins. In this review, we will highlight and discuss how SUMOylation in different cellular compartments regulate biological processes. First, we will discuss the key role of SUMOylation of proteins in the extra-nuclear compartment in cardiomyocytes, which is overwhelmingly cardio-protective. On the other hand, SUMOylation of nuclear proteins is generally detrimental to the cardiac function mainly because of the trans-repressive nature of SUMOylation on many transcription factors. We will also discuss the potential role of SUMOylation in epigenetic regulation. In this review, we will propose a new concept that shuttling of SUMO proteases between the nuclear and extra-nuclear compartments without changing their enzymatic activity regulates the extent of SUMOylation in these compartments and determines the response and fate of cardiomyocytes after cardiac insults. Approaches focused specifically to inhibit this shuttling in cardiomyocytes will be necessary to understand the whole picture of SUMOylation and its pathophysiological consequences in the heart, especially after cardiac insults. This article is part of a Special Issue entitled: Genetic and epigenetic control of heart failure - edited by Jun Ren & Megan Yingmei Zhang.

SENP7 Potentiates cGAS Activation by Relieving SUMO-Mediated Inhibition of Cytosolic DNA Sensing

Cyclic GMP-AMP (cGAMP) synthase (cGAS, a.k.a. MB21D1), a cytosolic DNA sensor, catalyzes formation of the second messenger 2’3’-cGAMP that activates the stimulator of interferon genes (STING) signaling. How the cGAS activity is modulated remains largely unknown. Here, we demonstrate that sentrin/SUMO-specific protease 7 (SENP7) interacted with and potentiated cGAS activation. The small ubiquitin-like modifier (SUMO) was conjugated onto the lysine residues 335, 372 and 382 of cGAS, which suppressed its DNA-binding, oligomerization and nucleotidyl-transferase activities. SENP7 reversed this inhibition via catalyzing the cGAS de-SUMOylation. Consistently, silencing of SENP7 markedly impaired the IRF3-responsive gene expression induced by cGAS-STING axis. SENP7-knockdown mice were more susceptible to herpes simplex virus 1 (HSV-1) infection. SENP7 was significantly up-regulated in patients with SLE. Our study highlights the temporal modulation of the cGAS activity via dynamic SUMOylation, uncovering a novel mechanism for fine-tuning the STING signaling in innate immunity.

The Cyclic GMP-AMP (cGAMP) synthase (cGAS, a.k.a. MB21D1) is critical for monitoring the pathogen-derived DNA upon microbial infection. Its activity should be dynamically modulated in case the inadvertent recognition of the aberrant self nucleic acids in cytosol leads to severe autoimmune diseases. Protein posttranslational modifications dynamically shape the strength and duration of the immune signaling pathways. It is intriguing to explore whether SUMOylation could modulate the cGAS-initiated signaling. In this study, we characterized sentrin/SUMO-specific protease 7 (SENP7) to specifically potentiate the cGAS activation. Upon microbial DNA challenge, the small ubiquitin-like modifier (SUMO) was conjugated onto cGAS, which suppressed its DNA-binding, oligomerization and nucleotidyl-transferase activities. SENP7 reversed this inhibition via catalyzing the de-SUMOylation of cGAS. Our study sheds new light on the dynamic function of the SUMOylation in cytosolic DNAs-triggered innate immunity response.

Flow signaling and atherosclerosis

Atherosclerosis rarely develops in the region of arteries exposed to undisturbed flow (u-flow, unidirectional flow). Instead, atherogenesis occurs in the area exposed to disturbed flow (d-flow, multidirectional flow). Based on these general pathohistological observations, u-flow is considered to be athero-protective, while d-flow is atherogenic. The fact that u-flow and d-flow induce such clearly different biological responses in the wall of large arteries indicates that these two types of flow activate each distinct intracellular signaling cascade in vascular endothelial cells (ECs), which are directly exposed to blood flow. The ability of ECs to differentially respond to the two types of flow provides an opportunity to identify molecular events that lead to endothelial dysfunction and atherosclerosis. In this review, we will focus on various molecular events, which are differentially regulated by these two flow types. We will discuss how various kinases, ER stress, inflammasome, SUMOylation, and DNA methylation play roles in the differential flow response, endothelial dysfunction, and atherosclerosis. We will also discuss the interplay among the molecular events and how they coordinately regulate flow-dependent signaling and cellular responses. It is hoped that clear understanding of the way how the two flow types beget each unique phenotype in ECs will lead us to possible points of intervention against endothelial dysfunction and cardiovascular diseases.

Norcantharidin modulates miR-655-regulated SENP6 protein translation to suppresses invasion of glioblastoma cells

Norcantharidin (NCTD) is currently used as an anticancer drug for the treatment of some malignant cancers. However, whether it may have therapeutic effects on glioblastoma multiforme (GBM) remains unknown. Moreover, the underlying mechanisms have not been completely elucidated. Recently, SUMO-specific protease 6 (SENP6) has been shown as a tumor suppressor in some cancers. Nevertheless, whether it is involved in the pathogenesis of GBM has not been examined. Here, we studied the effects of NCTD on GBM cells. We found that NCTD dose-dependently increased SENP6 protein, but not messenger RNA (mRNA), in GBM cells, resulting in the suppression of cell invasion. Depletion of SENP6 in GBM cells significantly attenuated the NCTD-induced suppression of GBM cell invasion, while overexpression of SENP6 in GBM cells mimicked the effects of NCTD on cell invasion. Moreover, NCTD dose-dependently decreased the levels of microRNA-655 (miR-655), which bound to 3'-UTR of SENP6 mRNA to inhibit its translation. Overexpression of miR-655 decreased SENP6 in GBM cells, while depletion of miR-655 increased SENP6 protein in GBM cells. Taken together, our data demonstrates a previously unappreciated control of NCTD to suppress GBM cell invasion through modulation of miR-655-regulated SENP6 protein translation.

Keratin 8 limits TLR-triggered inflammatory responses through inhibiting TRAF6 polyubiquitination

Toll-like receptors (TLRs) have critical roles in innate immunity and inflammation and the detailed mechanisms by which TLR signaling is fine tuned remain unclear. Keratin 8 (CK8) belongs to the type II keratin family and is the major compontent of the intermediate filaments of simple or single-layered epithelia. Here we report that down-regulation of CK8 in mice enhanced TLR-mediated responses, rendering mice more susceptible to lipopolysaccharide (LPS)-induced endotoxin shock and Escherichia coli–caused septic peritonitis with reduced survival, elevated levels of inflammation cytokines and more severe tissue damage. We found that CK8 suppressed TLR-induced nuclear factor (NF)-κB activation and interacted with the adaptor tumor necrosis factor (TNF) receptor-associated factor 6 (TRAF6) to prevent its polyubiquitination. Our findings demonstrate a novel role of CK8 in negative regulation of TLR/NF-κB signaling and highlight a previously unidentified nonclassical function for CK8 in limiting inflammatory responses.

Kinase-SUMO networks in diabetes-mediated cardiovascular disease

Type II diabetes mellitus (DM) is a common comorbidity in patients with cardiovascular disease (CVD). Epidemiological studies including the Framingham, UKPDS, and MRFIT studies have shown diabetes to be an independent risk factor for cardiovascular disease associated with increased incidence of morbidity and mortality. However, major randomized controlled clinical trials including ADVANCE, VAD, and ACCORD have failed to demonstrate a significant reduction in CVD complications from longstanding DM with strict glycemic control. This suggests that despite the strong clinical correlation between DM and CVD, the precise mechanisms of DM-mediated CVD pathogenesis remain unclear. Signal transduction investigations have shed some light on this question with numerous studies demonstrating the role of kinase pathways in facilitating DM and CVD pathology. Abnormalities in endothelial, vascular smooth muscle, and myocardial function from the pathological insults of hyperglycemia and oxidative stress in diabetes are thought to accelerate the development of cardiovascular disease. Extensive interplay between kinase pathways that regulate the complex pathology of DM-mediated CVD is heavily regulated by a number of post-translational modifications (PTMs). In this review, we focus on the role of a dynamic PTM known as SUMOylation and its role in regulating these kinase networks to provide a mechanistic link between DM and CVD.

ER Adaptor SCAP Translocates and Recruits IRF3 to Perinuclear Microsome Induced by Cytosolic Microbial DNAs

Stimulator of interferon genes (STING, also known as MITA, ERIS or MPYS) induces the activation of TBK1 kinase and IRF3 transcription factor, upon sensing of microbial DNAs. How IRF3 is recruited onto the STING signalosome remains unknown. We report here that silencing of the ER adaptor SCAP markedly impairs the IRF3-responsive gene expression induced by STING. Scap knockdown mice are more susceptible to HSV-1 infection. Interestingly, SCAP translocates from ER, via Golgi, to perinuclear microsome in a STING-dependent manner. Mechanistically, the N-terminal transmembrane domain of SCAP interacts with STING, and the C-terminal cytosolic domain of SCAP binds to IRF3, thus recruiting IRF3 onto STING signalosome. Mis-localization of SCAP abolishes its antiviral function. Collectively, this study characterizes SCAP as an essential adaptor in the STING signaling pathway, uncovering a critical missing link in DNAs-triggered host antiviral responses.

The stimulator of interferon genes (STING/MITA/ERIS/MPYS) is characterized as the converging point of the cytosolic DNAs-triggered innate immune signaling, and its function has been well documented in mediating the production of type I interferon and other pro-inflammatory cytokines. It remains intriguing to address how IRF3 is recruited onto the STING signalosome. In this study, we have further identified and characterized the SREBP cleavage-activating protein (SCAP) as the long-sought-after adaptor of the STING signaling. Upon microbial DNA challenge, SCAP translocates from ER, via Golgi, to perinuclear microsome in a STING-dependent manner. SCAP thus serves as a scaffold adaptor to recruit IRF3 and facilitate its integration into the perinuclear microsomes. Our study reveals an important missing link in innate immunity, further highlighting the physical and/or functional links between innate immunity and metabolism.

CYLD-mediated signaling and diseases

The conserved cylindromatosis (CYLD) codes for a deubiquitinating enzyme and is a crucial regulator of diverse cellular processes such as immune responses, inflammation, death, and proliferation. It directly regulates multiple key signaling cascades, such as the Nuclear Factor kappa B [NFkB] and the Mitogen-Activated Protein Kinase (MAPK) pathways, by its catalytic activity on polyubiquitinated key intermediates. Several lines of emerging evidence have linked CYLD to the pathogenesis of various maladies, including cancer, poor infection control, lung fibrosis, neural development, and now cardiovascular dysfunction. While CYLD-mediated signaling is cell type and stimuli specific, the activity of CYLD is tightly controlled by phosphorylation and other regulators such as Snail. This review explores a broad selection of current and past literature regarding CYLD's expression, function and regulation with emerging reports on its role in cardiovascular disease.

The emerging roles of the STING adaptor protein in immunity and diseases

DNA that gains access to the cytoplasm generally serves as a danger signal for the hosts. An emerging paradigm for responding to cytosolic DNAs centres on the endoplasmic reticulum-resident protein stimulator of interferon genes (STING, also known as MITA, ERIS or MPYS), the hub adaptor of the recently identified DNA sensors. Dynamic regulations of STING action are critical for shaping innate immune responses against microbial infections, as well as for preventing autoimmune diseases. STING is also indispensable for the detection of immunogenic tumours. A deeper understanding of STING modulations could be instrumental for developing novel immunotherapeutic strategies against infectious, autoimmune and cancerous diseases. In this review, we summarize the latest advances on the role of STING in the DNA-triggered immune reactions, and underscore the critical issues that remain to be resolved in future studies.

Sumoylation coordinates the repression of inflammatory and anti-viral gene-expression programs during innate sensing

Innate sensing of pathogens initiates inflammatory cytokine responses that need to be tightly controlled. We found here that after engagement of Toll-like receptors (TLRs) in myeloid cells, deficient sumoylation caused increased secretion of transcription factor NF-κB-dependent inflammatory cytokines and a massive type I interferon signature. In mice, diminished sumoylation conferred susceptibility to endotoxin shock and resistance to viral infection. Overproduction of several NF-κB-dependent inflammatory cytokines required expression of the type I interferon receptor, which identified type I interferon as a central sumoylation-controlled hub for inflammation. Mechanistically, the small ubiquitin-like modifier SUMO operated from a distal enhancer of the gene encoding interferon-β (Ifnb1) to silence both basal and stimulus-induced activity of the Ifnb1 promoter. Therefore, sumoylation restrained inflammation by silencing Ifnb1 expression and by strictly suppressing an unanticipated priming by type I interferons of the TLR-induced production of inflammatory cytokines.

Chloroquine attenuates LPS-mediated macrophage activation through miR-669n-regulated SENP6 protein translation

Chloroquine (CQ) has been shown to inhibit Toll-like receptor 4 (TLR4)-mediated monocyte and macrophage activation induced by lipopolysaccharide (LPS). However, the underlying mechanisms have not been completely elucidated. Recently, SUMO-specific protease 6 (SENP6) has been reported to suppress LPS-induced activation of macrophages through deSUMOlation of NF-κB essential modifier (NEMO). Here, we studied whether this molecular pathway may also be involved in CQ/LPS model. We found that CQ dose-dependently increased SENP6 protein, but not mRNA, in mouse macrophages, RAW264.7 cells. Overexpression of SENP6 in RAW264.7 cells significantly decreased the LPS-induced release of pro-inflammatory proteins, TNF-α, IL-6 and IFN-γ, while depletion of SENP6 in RAW264.7 cells significantly increased these proteins. Moreover, in LPS-treated RAW264.7 cells, CQ dose-dependently decreased the levels of microRNA-669n (miR-669n), which bound to 3'-UTR of SENP6 mRNA to inhibit its translation. Overexpression of miR-669n decreased SENP6, resulting in increased production of TNF-α, IL-6 and IFN-γ in RAW264.7 cells, while depletion of miR-669n increased SENP6, resulting in decreased production of TNF-α, IL-6 and IFN-γ in RAW264.7 cells. In vivo, delivery of miR-669n plasmids augmented the effects of LPS, while delivery of antisense of miR-669n (as-miR-669n) plasmids abolished the effects of LPS. Taken together, our data demonstrate a previously unappreciated molecular control of LPS-induced macrophage activation by CQ, through miR-669n-regulated SENP6 protein translation.

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.

Orphan Nuclear Receptor ERRα Controls Macrophage Metabolic Signaling and A20 Expression to Negatively Regulate TLR-Induced Inflammation

The orphan nuclear receptor estrogen-related receptor α (ERRα; NR3B1) is a key metabolic regulator, but its function in regulating inflammation remains largely unknown. Here, we demonstrate that ERRα negatively regulates Toll-like receptor (TLR)-induced inflammation by promoting Tnfaip3 transcription and fine-tuning of metabolic reprogramming in macrophages. ERRα-deficient (Esrra(-/-)) mice showed increased susceptibility to endotoxin-induced septic shock, leading to more severe pro-inflammatory responses than control mice. ERRα regulated macrophage inflammatory responses by directly binding the promoter region of Tnfaip3, a deubiquitinating enzyme in TLR signaling. In addition, Esrra(-/-) macrophages showed an increased glycolysis, but impaired mitochondrial respiratory function and biogenesis. Further, ERRα was required for the regulation of NF-κB signaling by controlling p65 acetylation via maintenance of NAD(+) levels and sirtuin 1 activation. These findings unravel a previously unappreciated role for ERRα as a negative regulator of TLR-induced inflammatory responses through inducing Tnfaip3 transcription and controlling the metabolic reprogramming.

Suppression of Propionibacterium acnes Infection and the Associated Inflammatory Response by the Antimicrobial Peptide P5 in Mice

The cutaneous inflammation associated with acne vulgaris is caused by the anaerobic bacterium Propionibacterium acnes through activation of the innate immune system in the skin. Current standard treatments for acne have limitations that include adverse effects and poor efficacy in many patients, making development of a more effective therapy highly desirable. In the present study, we demonstrate the protective effects of a novel customized α-helical cationic peptide, P5, against P. acnes-induced inflammatory responses in vitro and in vivo. Application of P5 significantly reduced expression of two inflammatory cytokines IL-8 and TNF-α in P. acnes-treated primary human keratinocytes, where P5 appeared to act in part by binding to bacterial lipoteichoic acid, thereby suppressing TLR2-to-NF-κB signaling. In addition, in a mouse model of acne vulgaris, P5 exerted both anti-inflammatory and antimicrobial effects against P. acnes, but exerted no cytotoxic effects against skin cells. These results demonstrate that P5, and perhaps other cationic antimicrobial peptides, offer the unique ability to reduce numbers P. acnes cells in the skin and to inhibit the inflammation they trigger. This suggests these peptides could potentially be used to effectively treat acne without adversely affecting the skin.

Sensing and responding to cytosolic viruses invasions: An orchestra of kaleidoscopic ubiquitinations

Ubiquitin is a versatile molecular signature that modulates diverse cellular processes via proteasome-dependent and proteasome-independent mechanisms. The covalent and/or non-covalent binding of mono-ubiquitin and/or poly-ubiquitin chains to a target protein broadens the dynamic and functional spectra for signal integration. Different linkages of poly-ubiquitin chains determine specific physiological or pathological functions of target proteins. Accumulating evidences has revealed the essential roles of ubiquitination in orchestrating the host defenses against cytosolic RNA or DNA from viral infections. In this review, we summarize the current progress regarding the understanding of ubiquitin-mediated regulation of the RIG-I and STING antiviral signaling pathways and discuss certain critical issues that remain to be resolved in future studies.

Signaling mechanisms regulating B-lymphocyte activation and tolerance

It is becoming more and more accepted that, in addition to producing autoantibodies, B lymphocytes have other important functions that influence the development of autoimmunity. For example, autoreactive B cells are able to produce inflammatory cytokines and activate pathogenic T cells. B lymphocytes can react to extracellular signals with a range of responses from anergy to autoreactivity. The final outcome is determined by the relative contribution of signaling events mediated by activating and inhibitory pathways. Besides the B cell antigen receptor (BCR), several costimulatory receptors expressed on B cells can also induce B cell proliferation and survival, or regulate antibody production. These include CD19, CD40, the B cell activating factor receptor, and Toll-like receptors. Hyperactivity of these receptors clearly contributes to breaking B-cell tolerance in several autoimmune diseases. Inhibitors of these activating signals (including protein tyrosine phosphatases, deubiquitinating enzymes and several adaptor proteins) are crucial to control B-cell activation and maintain B-cell tolerance. In this review, we summarize the inhibitory signaling mechanisms that counteract B-cell activation triggered by the BCR and the coreceptors.

Microbial sensing by Toll-like receptors and intracellular nucleic acid sensors

Recognition of an invading pathogen is critical to elicit protective responses. Certain microbial structures and molecules, which are crucial for their survival and virulence, are recognized by different families of evolutionarily conserved pattern recognition receptors (PRRs). This recognition initiates a signaling cascade that leads to the transcription of inflammatory cytokines and chemokines to eliminate pathogens and attract immune cells, thereby perpetuating further adaptive immune responses. Considerable research on the molecular mechanisms underlying host-pathogen interactions has resulted in the discovery of multifarious PRRs. In this review, we discuss the recent developments in microbial recognition by Toll-like receptors (TLRs) and intracellular nucleic acid sensors and the signaling pathways initiated by them.

SUMO-specific proteases/isopeptidases: SENPs and beyond

We summarize the evolutionary relationship, structure and subcellular distribution of SUMO proteases (or SUMO isopeptidases). We also discuss their functions and allude to their involvement in human disease.

The role of ubiquitination and sumoylation in diabetic nephropathy

Diabetic nephropathy (DN) is a common and characteristic microvascular complication of diabetes; the mechanisms that cause DN have not been clarified, and the epigenetic mechanism was promised in the pathology of DN. Furthermore, ubiquitination and small ubiquitin-like modifier (SUMO) were involved in the progression of DN. MG132, as a ubiquitin proteasome, could improve renal injury by regulating several signaling pathways, such as NF-κB, TGF-β, Nrf2-oxidative stress, and MAPK. In this review, we summarize how ubiquitination and sumoylation may contribute to the pathology of DN, which may be a potential treatment strategy of DN.

MicroRNA-302b augments host defense to bacteria by regulating inflammatory responses via feedback to TLR/IRAK4 circuits

MicroRNAs (miRNAs) have been implicated in a spectrum of physiological and pathological conditions, including immune responses. miR-302b has been implicated in stem cell differentiation but its role in immunity remains unknown. Here we show that miR-302b is induced by TLR2 and TLR4 through ERK-p38-NF-κB signaling upon Gram-negative bacterium Pseudomonas aeruginosa infection. Suppression of inflammatory responses to bacterial infection is mediated by targeting IRAK4, a protein required for the activation and nuclear translocation of NF-κB. Through negative feedback, enforced expression of miR-302b or IRAK4 siRNA silencing inhibits downstream NF-κB signaling and airway leukocyte infiltration, thereby alleviating lung injury and increasing survival in P. aeruginosa-infected mice. In contrast, miR-302b inhibitors exacerbate inflammatory responses and decrease survival in P. aeruginosa-infected mice and lung cells. These findings reveal that miR-302b is a novel inflammatory regulator of NF-κB activation in respiratory bacterial infections by providing negative feedback to TLRs-mediated immunity.

Dynamic regulation of innate immunity by ubiquitin and ubiquitin-like proteins

Protein post-translational modifications (PTMs) are central to the host innate immune regulations. Dynamically, PTMs fine-tune the spatial and temporary responses of immune- and non-immune-cells, in accordance with extracellular and intracellular stresses. Ubiquitin and ubiquitin-like proteins (Ubls) are emerging as the important multi-functional signals, controlling the activation, stability, affinity and location of many signaling proteins. Recent investigations, at the molecular-cellular-animal models, have shed new light on the versatility of the ubiquitin, SUMO and ISG15, for shaping the strength and duration of the innate immune responses. This review summarizes our current knowledge on the functions and regulatory mechanisms of the ubiquitin and Ubls in the innate immunity, the first line of host defense against microbial infection.