Sirt1 S-nitrosylation induces acetylation of HMGB1 in LPS-activated RAW264.7 cells and endotoxemic mice

Inhibition of Sirtuin Deacylase Activity by Peroxynitrite

Sirtuins are a class of enzymes that deacylate protein lysine residues using NAD+ as a cosubstrate. Sirtuin deacylase activity has been historically regarded as protective; loss of sirtuin deacylase activity potentially increases susceptibility to aging-related disease development. However, which factors may inhibit sirtuins during aging or disease is largely unknown. Increased oxidant and inflammatory byproduct production damages cellular proteins. Previously, we and others found that sirtuin deacylase activity is inhibited by the nitric oxide (NO)-derived cysteine post-translational modification S-nitrosation. However, the comparative ability of the NO-derived oxidant peroxynitrite (ONOO-) to affect human sirtuin activity had not yet been assessed under uniform conditions. Here, we compare the ability of ONOO- (donated from SIN-1) to post-translationally modify and inhibit SIRT1, SIRT2, SIRT3, SIRT5, and SIRT6 deacylase activity. In response to SIN-1 treatment, inhibition of SIRT1, SIRT2, SIRT3, SIRT5, and SIRT6 deacylase activity correlated with increased tyrosine nitration. Mass spectrometry identified multiple novel tyrosine nitration sites in SIRT1, SIRT3, SIRT5, and SIRT6. As each sirtuin isoform has at least one tyrosine nitration site within the catalytic core, nitration may result in sirtuin inhibition. ONOO- can also react with cysteine residues, resulting in sulfenylation; however, only SIRT1 showed detectable peroxynitrite-mediated cysteine sulfenylation. While SIRT2, SIRT3, SIRT5, and SIRT6 showed no detectable sulfenylation, SIRT6 likely undergoes transient sulfenylation, quickly resolving into an intermolecular disulfide bond. These results suggest that the aging-related oxidant peroxynitrite can post-translationally modify and inhibit sirtuins, contributing to susceptibility to aging-related disease.

The Multifaceted Role of Endothelial Sirt1 in Vascular Aging: An Update

NAD+-dependent deacetylase sirtuin-1 (Sirt1) belongs to the sirtuins family, known to be longevity regulators, and exerts a key role in the prevention of vascular aging. By aging, the expression levels of Sirt1 decline with a severe impact on vascular function, such as the rise of endothelial dysfunction, which in turn promotes the development of cardiovascular diseases. In this context, the impact of Sirt1 activity in preventing endothelial senescence is particularly important. Given the key role of Sirt1 in counteracting endothelial senescence, great efforts have been made to deepen the knowledge about the intricate cross-talks and interactions of Sirt1 with other molecules, in order to set up possible strategies to boost Sirt1 activity to prevent or treat vascular aging. The aim of this review is to provide a proper background on the regulation and function of Sirt1 in the vascular endothelium and to discuss the recent advances regarding the therapeutic strategies of targeting Sirt1 to counteract vascular aging.

Ox-LDL promotes M1-like polarization of macrophages through the miR-21-5p/SKP2/EP300 pathway

Oxidized low-density lipoprotein (ox-LDL) mediated inflammatory damage, which possibly induces atherosclerosis (AS); however, the role of miRNA in this process has rarely been reported. In this paper, we study the ox-LDL-related endothelial cell damage and changes of macrophages. The bioinformatics method was used to analyze the expression changes of miRNA in AS patients, luciferase assay was used to study the interaction of protein and miRNA, and co-IP and ubiquitination experiments were used to analyze protein interaction. Flow cytometry was used to detect the polarization of macrophages. Database analysis showed that the expression of miR-21-5p was upregulated in AS patients. Luciferase assay showed that miR-21-5p can bind to SKP2 and subsequently influence ubiquitination of EP300. Overexpression of EP300 strengthens the HMGB1-induced acetylation and subsequently mediates the dissociation of HMGB1 from SIRT1, and thus HMGB1 could be secreted outside the cell. The HMGB1 released from endothelial cells can promote macrophage M1 polarization. This study shows that ox-LDL activates the SKP2/EP300 pathway through promoting upregulation of miR-21-5p, thereby acetylating and secreting HMGB1 outside the endothelium, subsequently enhancing macrophage polarization to further stabilize the inflammation situation.

Sirtuins in macrophage immune metabolism: A novel target for cardiovascular disorders

Sirtuins (SIRT1-SIRT7), as a family of NAD+-dependent protein modifying enzymes, have various catalytic functions, such as deacetylases, dealkalylases, and deribonucleases. The Sirtuins family is directly or indirectly involved in pathophysiological processes such as glucolipid metabolism, oxidative stress, DNA repair and inflammatory response through various pathways and assumes an important role in several cardiovascular diseases such as atherosclerosis, myocardial infarction, hypertension and heart failure. A growing number of studies supports that metabolic and bioenergetic reprogramming directs the sequential process of inflammation. Failure of homeostatic restoration leads to many inflammatory diseases, and that macrophages are the central cells involving the inflammatory response and are the main source of inflammatory cytokines. Regulation of cellular metabolism has emerged as a fundamental process controlling macrophage function, but its exact signaling mechanisms remain to be revealed. Understanding the precise molecular basis of metabolic control of macrophage inflammatory processes may provide new approaches for targeting immune metabolism and inflammation. Here, we provide an update of studies in cardiovascular disease on the function and role of sirtuins in macrophage inflammation and metabolism, as well as drug candidates that may interfere with sirtuins, pointing to future prospects in this field.

Activated SIRT1 contributes to DPT-induced glioma cell parthanatos by upregulation of NOX2 and NAT10

Parthanatos is a type of programmed cell death dependent on hyper-activation of poly (ADP-ribose) polymerase 1 (PARP-1). SIRT1 is a highly conserved nuclear deacetylase and often acts as an inhibitor of parthanatos by deacetylation of PARP1. Our previous study showed that deoxypodophyllotoxin (DPT), a natural compound isolated from the traditional herb Anthriscus sylvestris, triggered glioma cell death via parthanatos. In this study, we investigated the role of SIRT1 in DPT-induced human glioma cell parthanatos. We showed that DPT (450 nmol/L) activated both PARP1 and SIRT1, and induced parthanatos in U87 and U251 glioma cells. Activation of SIRT1 with SRT2183 (10 μmol/L) enhanced, while inhibition of SIRT1 with EX527 (200 μmol/L) or knockdown of SIRT1 attenuated DPT-induced PARP1 activation and glioma cell death. We demonstrated that DPT (450 nmol/L) significantly decreased intracellular NAD+ levels in U87 and U251 cells. Further decrease of NAD+ levels with FK866 (100 μmol/L) aggravated, but supplement of NAD+ (0.5, 2 mmol/L) attenuated DPT-induced PARP1 activation. We found that NAD+ depletion enhanced PARP1 activation via two ways: one was aggravating ROS-dependent DNA DSBs by upregulation of NADPH oxidase 2 (NOX2); the other was reinforcing PARP1 acetylation via increase of N-acetyltransferase 10 (NAT10) expression. We found that SIRT1 activity was improved when being phosphorylated by JNK at Ser27, the activated SIRT1 in reverse aggravated JNK activation via upregulating ROS-related ASK1 signaling, thus forming a positive feedback between JNK and SIRT1. Taken together, SIRT1 activated by JNK contributed to DPT-induced human glioma cell parthanatos via initiation of NAD+ depletion-dependent upregulation of NOX2 and NAT10.

INTERLEUKIN-35 DOWNREGULATES THE IMMUNE RESPONSE OF EFFECTOR CD4 + T CELLS VIA RESTRICTING HIGH MOBILITY GROUP BOX-1 PROTEIN-DEPENDENT AUTOPHAGY IN SEPSIS

ABSTRACT

Background: Immunosuppression is critically involved in the development of sepsis and is closely associated with poor outcomes. The novel role of the anti-inflammatory cytokine IL-35 in sepsis was examined. Methods: Sepsis was induced by in C57BL/6 mice cecal ligation and puncture (CLP). The impacts of IL-35 on effector CD4 + T cells were investigated by examining cell proliferation and the Th1/Th2 ratio in the presence of recombinant IL-35 (rIL-35) or anti-IL-35 (EBI3). The regulatory effect of IL-35 on autophagy was evaluated by measuring autophagy markers and autophagic flux in CLP mice in vivo and in activated effector CD4 + T cells in vitro . Results: IL-35 levels were significantly increased in the serum and spleens of septic mice. rIL-35 administration after CLP further decreased proliferation and the Th1/Th2 ratio in effector CD4 + T cells and significantly shortened the survival time. Sepsis-induced autophagy activation was protective in effector CD4 + T cells and was blocked by rIL-35. The inhibitory effect of IL-35 on autophagy was observed in activated effector CD4 + T cells in vitro , and this effect was mediated by restricting high mobility group box-1 protein (HMGB1) translocation. Conclusion: IL-35 is an immunosuppressive cytokine that impairs CD4 + T-cell proliferation and differentiation in sepsis, and the effect might be mediated by reducing HMGB1-dependent autophagy.

Pillars and Gaps of S-Nitrosylation-Dependent Epigenetic Regulation in Physiology and Cancer

Nitric oxide (NO) is a diffusible signaling molecule produced by three isoforms of nitric oxide synthase, which release NO during the metabolism of the amino acid arginine. NO participates in pathophysiological responses of many different tissues, inducing concentration-dependent effect. Indeed, while low NO levels generally have protective effects, higher NO concentrations induce cytotoxic/cytostatic actions. In recent years, evidences have been accumulated unveiling S-nitrosylation as a major NO-dependent post-translational mechanism ruling gene expression. S-nitrosylation is a reversible, highly regulated phenomenon in which NO reacts with one or few specific cysteine residues of target proteins generating S-nitrosothiols. By inducing this chemical modification, NO might exert epigenetic regulation through direct effects on both DNA and histones as well as through indirect actions affecting the functions of transcription factors and transcriptional co-regulators. In this light, S-nitrosylation may also impact on cancer cell gene expression programs. Indeed, it affects different cell pathways and functions ranging from the impairment of DNA damage repair to the modulation of the activity of signal transduction molecules, oncogenes, tumor suppressors, and chromatin remodelers. Nitrosylation is therefore a versatile tool by which NO might control gene expression programs in health and disease.

Sirtuin Oxidative Post-translational Modifications

Increased sirtuin deacylase activity is correlated with increased lifespan and healthspan in eukaryotes. Conversely, decreased sirtuin deacylase activity is correlated with increased susceptibility to aging-related diseases. However, the mechanisms leading to decreased sirtuin activity during aging are poorly understood. Recent work has shown that oxidative post-translational modification by reactive oxygen (ROS) or nitrogen (RNS) species results in inhibition of sirtuin deacylase activity through cysteine nitrosation, glutathionylation, sulfenylation, and sulfhydration as well as tyrosine nitration. The prevalence of ROS/RNS (e.g., nitric oxide, S-nitrosoglutathione, hydrogen peroxide, oxidized glutathione, and peroxynitrite) is increased during inflammation and as a result of electron transport chain dysfunction. With age, cellular production of ROS/RNS increases; thus, cellular oxidants may serve as a causal link between loss of sirtuin activity and aging-related disease development. Therefore, the prevention of inhibitory oxidative modification may represent a novel means to increase sirtuin activity during aging. In this review, we explore the role of cellular oxidants in inhibiting individual sirtuin human isoform deacylase activity and clarify the relevance of ROS/RNS as regulatory molecules of sirtuin deacylase activity in the context of health and disease.

Endothelial SIRT1 as a Target for the Prevention of Arterial Aging: Promises and Challenges

ABSTRACT

SIRT1, a member of the sirtuin family of longevity regulators, possesses potent activities preventing vascular aging. The expression and function of SIRT1 in endothelial cells are downregulated with age, in turn causing early vascular aging and predisposing various vascular abnormalities. Overexpression of SIRT1 in the vascular endothelium prevents aging-associated endothelial dysfunction and senescence, thus the development of hypertension and atherosclerosis. Numerous efforts have been directed to increase SIRT1 signaling as a potential strategy for different aging-associated diseases. However, the complex mechanisms underlying the regulation of SIRT1 have posed a significant challenge toward the design of specific and effective therapeutics. This review aimed to provide a summary on the regulation and function of SIRT1 in the vascular endothelium and to discuss the different approaches targeting this molecule for the prevention and treatment of age-related cardiovascular and cerebrovascular diseases.

Evaluation of the Molecular Mechanisms of Sepsis Using Proteomics

Sepsis is a complex syndrome promoted by pathogenic and host factors; it is characterized by dysregulated host responses and multiple organ dysfunction, which can lead to death. However, its underlying molecular mechanisms remain unknown. Proteomics, as a biotechnology research area in the post-genomic era, paves the way for large-scale protein characterization. With the rapid development of proteomics technology, various approaches can be used to monitor proteome changes and identify differentially expressed proteins in sepsis, which may help to understand the pathophysiological process of sepsis. Although previous reports have summarized proteomics-related data on the diagnosis of sepsis and sepsis-related biomarkers, the present review aims to comprehensively summarize the available literature concerning “sepsis”, “proteomics”, “cecal ligation and puncture”, “lipopolysaccharide”, and “post-translational modifications” in relation to proteomics research to provide novel insights into the molecular mechanisms of sepsis.

Baicalin Ameliorates Cognitive Impairment and Protects Microglia from LPS-Induced Neuroinflammation via the SIRT1/HMGB1 Pathway

Systemic inflammation often induces neuroinflammation and disrupts neural functions, ultimately causing cognitive impairment. Furthermore, neuronal inflammation is the key cause of many neurological conditions. It is particularly important to develop effective neuroprotectants to prevent and control inflammatory brain diseases. Baicalin (BAI) has a wide variety of potent neuroprotective and cognitive enhancement properties in various models of neuronal injury through antioxidation, anti-inflammation, anti-apoptosis, and stimulating neurogenesis. Nevertheless, it remains unclear whether BAI can resolve neuroinflammation and cognitive decline triggered by systemic or distant inflammatory processes. In the present study, intraperitoneal lipopolysaccharide (LPS) administration was used to establish neuroinflammation to evaluate the potential neuroprotective and anti-inflammatory effects of BAI. Here, we report that BAI activated silent information regulator 1 (SIRT1) to deacetylate high-mobility group box 1 (HMGB1) protein in response to acute LPS-induced neuroinflammation and cognitive deficits. Furthermore, we demonstrated the anti-inflammatory and cognitive enhancement effects and the underlying molecular mechanisms of BAI in modulating microglial activation and systemic cytokine production, including tumor necrosis factor- (TNF-) α and interleukin- (IL-) 1β, after LPS exposure in mice and in the microglial cell line, BV2. In the hippocampus, BAI not only reduced reactive microglia and inflammatory cytokine production but also modulated SIRT1/HMGB1 signaling in microglia. Interestingly, pretreatment with SIRT1 inhibitor EX-527 abolished the beneficial effects of BAI against LPS exposure. Specifically, BAI treatment inhibited HMGB1 release via the SIRT1/HMGB1 pathway and reduced the nuclear translocation of HMGB1 in LPS-induced BV2 cells. These effects were reversed in BV2 cells by silencing endogenous SIRT1. Taken together, these findings indicated that BAI reduced microglia-associated neuroinflammation and improved acute neurocognitive deficits in LPS-induced mice via SIRT1-dependent downregulation of HMGB1, suggesting a possible novel protection against acute neurobehavioral deficits, such as delayed neurocognitive recovery after anesthesia and surgery challenges.

Platycodon grandiflorus Fermented Extracts Attenuate Endotoxin-Induced Acute Liver Injury in Mice

Endotoxin-induced acute liver injury is mediated by an excessive inflammatory response, hepatocellular oxidative stress, and apoptosis. Traditional medicinal plants have been used to treat various disorders. Platycodon grandifloras (PG) has been shown to be beneficial in relieving cough and asthma and to have anti-tumor, anti-inflammatory, anti-diabetic activities. The pharmacological action of PG is mainly due to saponins, flavonoids, phenolic, and other compounds. However, raw PG exhibits some side effects at high doses. Here, we extracted raw PG with varying fermentation methods and examined its anti-inflammatory effect and associated signaling kinases in Raw264.7 cells. Then, we investigated the effect of fermented black PG (FBPG) on endotoxin-induced liver injury. Mice were administered FBPG orally at 1 h before the lipopolysaccharide and D-galactosamine (LPS/GalN) injection and sacrificed after 5 h. Black PG (BPG) and FBPG showed a significant reduction in pro-inflammatory cytokines and extracellular nitric oxide (NO); p-38 and ERK signaling was involved in reducing inducible NO synthase in Raw264.7 cells. Consistently, FBPG attenuates LPS/GalN-induced liver injury; plasma ALT and AST, hepatic necrosis, pro-inflammatory cytokines, apoptosis, and lipid peroxidation were all reduced. In conclusion, PG extracts, particularly FBPG, play anti-inflammatory, antioxidant, and anti-apoptotic roles, alleviating endotoxin-induced acute liver injury. Processing raw PG into FBPG extract may be clinically useful by improving the pharmacologically active ingredients and reducing the required dosage.

AMPK Ameliorates Tau Acetylation and Memory Impairment Through Sirt1

Alzheimer's disease (AD) is the most common neurodegenerative disease, but its underlying mechanism is still unclear and the identities of drugs for AD also lack. Tau acetylation has become potentially important post-translational modification of tau. Levels of tau acetylation are significantly enhanced in AD patients and transgenic mouse models of AD, but the underlying mechanism and roles of tau hyperacetylation in AD onset maintain elusive. In the current study, we found that tau acetylation is obviously enhanced and the activities of AMP-activated protein kinase (AMPK) and sirtuin1 (Sirt1) are significantly decreased in APP/PS1 and streptozotocin (STZ) mice and high glucose (HG)-treated cells. Moreover, we demonstrated that activation of AMPK reduces the level of tau acetylation and ameliorates memory impairment, and its mechanism is associated with activation of Sirt1. Taken together, AMPK might be a crucial upstream molecular to regulate acetylation of tau and become a new target for AD therapy in the future.

Human sirtuins are differentially sensitive to inhibition by nitrosating agents and other cysteine oxidants

Sirtuins (e.g. human Sirt1-7) catalyze the removal of acyl groups from lysine residues in proteins in an NAD⁺-dependent manner, and loss of sirtuin deacylase activity correlates with the development of aging-related diseases. Although multiple reports suggest that sirtuin activity is regulated by oxidative post-translational modifications of cysteines during inflammation and aging, no systematic comparative study of potential direct sirtuin cysteine oxidative modifications has been performed. Here, using IC₅₀ and k _inact/K_I analyses, we quantified the ability of nitrosothiols (S-nitrosoglutathione and S-nitroso-N-acetyl-d,l-penicillamine), nitric oxide, oxidized GSH, and hydrogen peroxide to post-translationally modify and inhibit the deacylase activity of Sirt1, Sirt2, Sirt3, Sirt5, and Sirt6. The inhibition was correlated with cysteine modification and assessed with chemical-probe and blot-based assays for cysteine S-nitrosation, sulfenylation, and glutathionylation. We show that the primarily nuclear sirtuins Sirt1 and Sirt6, as well as the primarily cytosolic sirtuin Sirt2, are modified and inhibited by cysteine S-nitrosation in response to exposure to both free nitric oxide and nitrosothiols (k _inact/K_I ≥ 5 m^-1 s^-1), which is the first report of Sirt2 and Sirt6 inhibition by S-nitrosation. Surprisingly, the mitochondrial sirtuins Sirt3 and Sirt5 were resistant to inhibition by cysteine oxidants. Collectively, these results suggest that nitric oxide-derived oxidants may causatively link nuclear and cytosolic sirtuin inhibition to aging-related inflammatory disease development.

The Role of HMGB1, a Nuclear Damage-Associated Molecular Pattern Molecule, in the Pathogenesis of Lung Diseases

Significance: High-mobility group protein box 1 (HMGB1), a ubiquitous nuclear protein, regulates chromatin structure and modulates the expression of many genes involved in the pathogenesis of lung cancer and many other lung diseases, including those that regulate cell cycle control, cell death, and DNA replication and repair. Extracellular HMGB1, whether passively released or actively secreted, is a danger signal that elicits proinflammatory responses, impairs macrophage phagocytosis and efferocytosis, and alters vascular remodeling. This can result in excessive pulmonary inflammation and compromised host defense against lung infections, causing a deleterious feedback cycle. Recent Advances: HMGB1 has been identified as a biomarker and mediator of the pathogenesis of numerous lung disorders. In addition, post-translational modifications of HMGB1, including acetylation, phosphorylation, and oxidation, have been postulated to affect its localization and physiological and pathophysiological effects, such as the initiation and progression of lung diseases. Critical Issues: The molecular mechanisms underlying how HMGB1 drives the pathogenesis of different lung diseases and novel therapeutic approaches targeting HMGB1 remain to be elucidated. Future Directions: Additional research is needed to identify the roles and functions of modified HMGB1 produced by different post-translational modifications and their significance in the pathogenesis of lung diseases. Such studies will provide information for novel approaches targeting HMGB1 as a treatment for lung diseases.