Functions of the sirtuin deacylase SIRT5 in normal physiology and pathobiology

Mammalian Sirtuins and Their Relevance in Vascular Calcification

Cardiovascular diseases are a group of diseases with high morbidity and mortality that affect millions of people each year. Vascular calcification (VC) is an active process that involves the mineral deposition of calcium-phosphate complexes. VC is closely related to cardiovascular diseases, such as hypertension, heart failure, and calcific aortic stenosis, and is a type of ectopic calcification that occurs in the vessel walls. The sirtuins (silent mating-type information regulation 2; SIRTs), are a family of histone deacetylases whose function relies on nicotinamide adenine dinucleotide (NAD+). They have non-negligible functions in the regulation of energy metabolism, senescence, apoptosis, and other biological processes. Sirtuins have important effects on bone homeostasis and VC processes that share many similarities with bone formation. Sirtuins have been confirmed to deacetylate a variety of target proteins related to the occurrence and development of VC, thereby affecting the process of VC and providing new possibilities for the prevention and treatment of cardiovascular diseases. To facilitate the understanding of vascular calcification and accelerate the development of cardiovascular drugs, we reviewed and summarized recent research progress on the relationship between different types of sirtuins and VC.

Investigation of Carboxylic Acid Isosteres and Prodrugs for Inhibition of the Human SIRT5 Lysine Deacylase Enzyme

Sirtuin 5 (SIRT5) is a protein lysine deacylase enzyme that regulates diverse biology by hydrolyzing ϵ-N-carboxyacyllysine posttranslational modifications in the cell. Inhibition of SIRT5 has been linked to potential treatment of several cancers but potent compounds with activity in cells have been lacking. Here we developed mechanism-based inhibitors that incorporate isosteres of a carboxylic acid residue that is important for high-affinity binding to the enzyme active site. By masking of the tetrazole moiety of the most potent candidate from our initial SAR study, we achieved potent and cytoselective growth inhibition for the treatment of SIRT5-dependent leukemic cancer cell lines in culture. Thus, we provide an efficient, cellularly active small molecule that targets SIRT5, which can help elucidate its function and potential as a future drug target. This work shows that masked isosteres of carboxylic acids are viable chemical motifs for the development of inhibitors that target mitochondrial enzymes, which may have applications beyond the sirtuin field.

Sirtuin modulators: past, present, and future perspectives

Sirtuins are NAD⁺-dependent protein lysine deacylase and mono-ADP ribosylases present in both prokaryotes and eukaryotes. The sirtuin family comprises seven isoforms in mammals, each possessing different subcellular localization and biological functions. Sirtuins have received increasing attention in the past two decades given their pivotal functions in a variety of biological contexts, including cytodifferentiation, transcriptional regulation, cell cycle progression, apoptosis, inflammation, metabolism, neurological and cardiovascular physiology and cancer. Consequently, modulation of sirtuin activity has been regarded as a promising therapeutic option for many pathologies. In this review, we provide an up-to-date overview of sirtuin biology and pharmacology. We examine the main features of the most relevant inhibitors and activators, analyzing their structure-activity relationships, applications in biology, and therapeutic potential.

SIRT5 is involved in the proliferation and metastasis of breast cancer by promoting aerobic glycolysis

OBJECTIVE

Breast cancer (BC) is the most commonly diagnosed cancer among females and has a poor prognosis, breast invasive ductal carcinoma is the most common histological type. The occurrence and development of BC is closely related to aberrant glucose metabolism. In the hyperglycemic environment caused by abnormal glucose metabolism, hypoxia-inducible factor-1 alpha (HIF-1α) enables tumor cells to absorb large amounts of glucose and enhance glycolysis by inducing the expression of glucose transporter type1 (GLUT1) and glycolysis genes, thus promoting tumor cell proliferation and metastasis. Mitochondrial Sirtuin5 (SIRT5) plays a role in the rewiring of glucose metabolism during the progression of cancers. Thus, we aimed to elucidate whether SIRT5 promotes BC proliferation and metastasis by facilitating aerobic glycolysis in BC.

METHODS

The expression of SIRT5 in breast carcinoma tissue and cells was evaluated using immunohistochemical staining, western blot and quantitative reverse transcription-polymerase chain reaction (qRT-PCR) analysis to confirm the biological role of SIRT5 in breast carcinoma. We established a stable cell line with SIRT5 knockdown using lentiviral transduction in T47D cells to reduce SIRT5 expression and then evaluated the effect of SIRT5 on cells cultured in the presence of high glucose (4500 mg/L) and normal glucose (2000 mg/L) concentrations. Cell proliferation was detected using the CCK-8 assay, the cell cycle and cell apoptosis were measured using flow cytometry and Annexin V staining, and cell migration was tested by performing Celigo scratch and Transwell assays. The expression of PKM2, HK2, mTOR and HIF-1α, which play roles in aerobic glycolysis, was investigated using western blot.

RESULTS

SIRT5 was overexpressed in BC tissues compared with paired normal tissues. Prognostic and OS analyses showed that the SIRT5 expression level was an individual prognostic factor for patients with BC. SIRT5 knockdown inhibited proliferation and metastasis and slightly increased apoptosis in T47D cells under high-glucose conditions. Furthermore, the downregulation of HK2 and HIF-1α caused by SIRT5 knockdown was a high glucose-dependent process, while the downregulation of PKM2 was mediated by a high glucose-independent process.

CONCLUSIONS

SIRT5 is an independent prognostic factor for BC and contributes to cell proliferation and metastasis in a high glucose-dependent manner to some degree, which might be mediated by promoting aerobic glycolysis.

Molecular and cellular regulatory roles of sirtuin protein

Sirtuins (SIRT) are unique posttranslational modification enzymes that utilize NAD + as co-substrate to remove acyl groups from lysine residues. SIRT act on variety of substrates and impact major metabolic process. All seven members of SIRT family are unique and targets wide range of cellular proteins in nucleus, cytoplasm, and mitochondria for post-translational modification by acetylation (SIRT1, 2, 3, and 5) or ADP-ribosylation (SIRT4 and 6). Each member of SIRT family is distinct. SIRT2 was first to be discovered that incited research on mammalian SIRT. Enzymatic activities of SIRT 4 are yet to be elucidated while only SIRT7 is localized in nucleoli that govern the transcription of RNA polymerase I. SIRT 5 and 6 exhibit weakest deacetylase activity. Out of all SIRT analogs, SIRT1 is identified as nutrient sensor. Increased expression of only SIRT3 is linked with longevity in humans. Since SIRT is regulated by the bioenergetic state of the cell, nutrition impacts it but very few studies about diet-mediated effect on SIRT are reported. The present review elaborates distribution, specific biological role and prominent effect of all SIRT on vital human tissue along with highlighting need to trace molecular mechanisms and identifying foods that may augment it beneficially.

2,3,5,4'-tetrahydroxy-stilbene-2-O-β-D-glucoside ameliorates NAFLD via attenuating hepatic steatosis through inhibiting mitochondrial dysfunction dependent on SIRT5

BACKGROUND

Non-alcoholic fatty liver disease (NAFLD) is becoming more and more common in clinic in the world, and the study on its mechanism and treatment strategy has already been a research hotspot. Natural chemical compound 2,3,5,4'-tetrahydroxy-stilbene-2-O-β-d-glucoside (TSG) is isolated from Polygonum multiflorum Thunb. that has already been reported to have the lipid-lowering activity.

PURPOSE

The purpose of this research was to observe the improvement of TSG on methionine and choline deficient (MCD) diet-induced NAFLD in mice and to further elucidate its engaged mechanism.

METHODS

NAFLD was induced in mice fed by MCD diet for 6 weeks. The accumulation of lipids in hepatocytes was induced by 0.5 mM non-esterified fatty acid (NEFA). Biochemical parameters in serum or livers from mice were tested. Protein and mRNA expression and stability were measured. Mitochondrial dysfunction was analyzed both in vivo and in vitro. The Label-free quantitative proteomic analysis was used to find potential involved key molecules.

RESULTS

TSG attenuated hepatic parenchymal cells injury, liver inflammatory responses and hepatic fibrosis, and markedly ameliorated liver steatosis in mice from MCD group. In vitro results indicated that TSG reduced the accumulation of cellular lipids in hepatocytes induced by NEFA. TSG reduced reactive oxygen species (ROS) formation and attenuated mitochondrial dysfunction both in vivo and in vitro. The label-free quantitative proteomic analysis predicted the crucial participation of NAD-dependent protein deacylase sirtuin-5 (SIRT5). Next experimental results further evidenced that TSG enhanced SIRT5 expression in mitochondria both in vitro and in vivo. The TSG-supplied inhibition on ROS formation and mitochondrial dysfunction in hepatocytes was disappeared after the application of SIRT5 siRNA. TSG increased the expression and enzymatic activity of carnitine palmitoyltransferase 1A (CPT1A), but this enhance was diminished in hepatocytes transfected with SIRT5 siRNA. Additionally, the TSG-provided inhibition on cellular lipids accumulation was also disappeared in hepatocytes transfected with SIRT5 siRNA. Further results demonstrated that TSG increased SIRT5 expression by regulating its mRNA stability through enhancing the binding of SIRT5 mRNA with serine/arginine-rich splicing factor 2 (SRSF2), which is an RNA-binding protein (RBP).

CONCLUSION

TSG attenuated liver steatosis and inhibited NAFLD progression through preventing oxidative stress injury and improving mitochondrial dysfunction, and SIRT5 played a key role in this process.

Biochemical Functions and Clinical Characterizations of the Sirtuins in Diabetes-Induced Retinal Pathologies

Diabetic retinopathy (DR) is undoubtedly one of the most prominent causes of blindness worldwide. This pathology is the most frequent microvascular complication arising from diabetes, and its incidence is increasing at a constant pace. To date, the insurgence of DR is thought to be the consequence of the intricate complex of relations connecting inflammation, the generation of free oxygen species, and the consequent oxidative stress determined by protracted hyperglycemia. The sirtuin (SIRT) family comprises 7 histone and non-histone protein deacetylases and mono (ADP-ribosyl) transferases regulating different processes, including metabolism, senescence, DNA maintenance, and cell cycle regulation. These enzymes are involved in the development of various diseases such as neurodegeneration, cardiovascular pathologies, metabolic disorders, and cancer. SIRT1, 3, 5, and 6 are key enzymes in DR since they modulate glucose metabolism, insulin sensitivity, and inflammation. Currently, indirect and direct activators of SIRTs (such as antagomir, glycyrrhizin, and resveratrol) are being developed to modulate the inflammation response arising during DR. In this review, we aim to illustrate the most important inflammatory and metabolic pathways connecting SIRT activity to DR, and to describe the most relevant SIRT activators that might be proposed as new therapeutics to treat DR.

Roles of sirtuin family members in chronic obstructive pulmonary disease

The globally increasing annual incidence of chronic obstructive pulmonary disease (COPD), a common chronic disease, poses a serious risk to public health. Although the exact mechanism underlying the pathogenesis of COPD remains unclear, a large number of studies have shown that its pathophysiology and disease course are closely related to oxidative stress, inflammation, apoptosis, autophagy, and aging. The key players involved in COPD include the sirtuin family of NAD-dependent deacetylases that comprise seven members (SIRT1-7) in mammals. Sirtuins play an important role in metabolic diseases, cell cycle control, proliferation, apoptosis, and senescence. Owing to differences in subcellular localization, sirtuins exhibit anisotropy. In this narrative review, we discuss the roles and molecular pathways of each member of the sirtuin family involved in COPD to provide novel insights into the prevention and treatment of COPD and how sirtuins may serve as adjuvants for COPD treatment.

Overexpression of SLC25A51 promotes hepatocellular carcinoma progression by driving aerobic glycolysis through activation of SIRT5

Solute carrier family 25 member 20 (SLC25A51) is a newly identified mammalian mitochondrial NAD⁺ transporter. However, the clinicopathological and biological significance of SLC25A51 in human cancers, including hepatocellular carcinoma (HCC), remains unclear. The aim of this study was to define the role of SLC25A51 in HCC progression. Here we demonstrate that SLC25A51 is significantly overexpressed in human HCC specimens and cell lines, caused by, at least in partial, the decrease of miR-212-3p. SLC25A51 overexpression is positively correlated with the clinicopathological characteristics of vascular invasion and tumor diameter, as well as poor survival in patients with HCC. Knockdown of SLC25A51 attenuated, while overexpression of SLC25A51 enhanced the growth and metastasis of HCC cells both in vitro and in vivo. Mechanistically, glucose metabolism reprogramming from oxidative phosphorylation to glycolysis by activation of mitochondrial sirtuin 5 (SIRT5) was found to contribute to the promotion of growth and metastasis by SLC25A51 in HCC cells. Together, these findings reveal important roles of SLC25A51 in HCC tumorigenesis and suggest SLC25A51 as a promising prognostic marker and therapeutic target for treating HCC.

Hesperetin derivative-16 attenuates CCl4-induced inflammation and liver fibrosis by activating AMPK/SIRT3 pathway

Liver fibrosis, a chronic inflammatory healing reaction, progresses to hepatocirrhosis without effective intervention. Hesperetin derivative (HD-16), a monomer compound derived from hesperitin, exerts anti-inflammatory and hepatoprotective effects against a spectrum of liver diseases. However, the anti-fibrotic potential of HD-16 in liver fibrosis and its underlying mechanism have not yet been elucidated. In this study, we investigated the anti-fibrotic effect of HD-16 on mouse liver fibrosis induced by CCl4 and on LX-2 cells (human immortalized HSCs) stimulated by TGF-β1, in vivo and in vitro. HD-16 exerted an anti-fibrotic effect via regulation of the AMPK/SIRT3 pathway. Pharmacodynamic results showed that HD-16 alleviated the degree of injury and inflammation in CCl4-induced mouse liver fibrosis. Consistently, HD-16 also effectively inhibited the expression of α-SMA, Col1α1, Col3α1, and TIMP-1 in TGF-β1-activated LX-2 cells. Mechanistically, HD-16 promoted SIRT3 expression and activity in fibrotic liver and activated LX-2 cells. Furthermore, SIRT3 depletion attenuated the anti-fibrotic effects of HD-16. Intriguingly, HD-16 increased AMPK phosphorylation, whereas inhibition of SIRT3 expression did not affect AMPK phosphorylation. In contrast, AMPK silencing suppressed SIRT3 expression, suggesting that SIRT3 is a downstream target of AMPK in liver fibrosis. Overall, HD-16 attenuated CCl4-induced liver inflammation and fibrosis by activating the AMPK/SIRT3 pathway, and HD-16 may be a potential anti-fibrotic compound in the treatment of liver fibrosis.

Mitochondrial sirtuins genetic variations and gastric cancer risk: Evidence from retrospective observational study

AIMS

The purpose of the present study was to analyze the role of selected polymorphisms of SIRT3 and SIRT5 in gastric carcinogenesis.

METHODS

For this study, 500 blood samples of GC patients and 500 blood samples of healthy individuals were collected. Six selected polymorphisms of mitochondrial sirtuins were analyzed for analysis using Tetra-Arms PCR followed by DNA sequencing.

RESULTS

Mutant allele frequencies of selected polymorphisms [rs3782116 (p < 0.0001), rs6598072 (p < 0.0001) and rs11246020 (p < 0.0001), rs938222 (p = 0.0136), rs3757261 (p = 0.0005) and rs2841511 (p = 0.0015)] were observed significant higher in GC patients vs controls. Haplotype analysis was performed, and 51 haplotypes were generated using haploview software. Among these haplotypes, eleven haplotypes were found associated with a significantly increased risk of GC. Furthermore, SNP-SNP interaction showed a significant correlation between studied SNPs and GC risk. Kaplan Meier analysis showed that mutant allele frequencies of selected polymorphisms are linked with a significant decrease in survival of GC patients CONCLUSIONS: It can be concluded that selected SNPs may be associated with enhanced risk of GC and hence can be potential prognostic markers for prognosis and predisposition of GC.

An expanding repertoire of protein acylations

Protein post-translational modifications play key roles in multiple cellular processes by allowing rapid reprogramming of individual protein functions. Acylation, one of the most important post-translational modifications, is involved in different physiological activities including cell differentiation and energy metabolism. In recent years, the progression in technologies, especially the antibodies against acylation and the highly sensitive and effective mass spectrometry–based proteomics, as well as optimized functional studies, greatly deepen our understanding of protein acylation. In this review, we give a general overview of the 12 main protein acylations (formylation, acetylation, propionylation, butyrylation, malonylation, succinylation, glutarylation, palmitoylation, myristoylation, benzoylation, crotonylation, and 2-hydroxyisobutyrylation), including their substrates (histones and nonhistone proteins), regulatory enzymes (writers, readers, and erasers), biological functions (transcriptional regulation, metabolic regulation, subcellular targeting, protein–membrane interactions, protein stability, and folding), and related diseases (cancer, diabetes, heart disease, neurodegenerative disease, and viral infection), to present a complete picture of protein acylations and highlight their functional significance in future research.

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Provide a general overview of the 12 main protein acylations.

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Acylation of viral proteins promotes viral integration and infection.

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Hyperacylation of histone has antitumous and neuroprotective effects.

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MS is widely used in the identification of acylation but has its challenges.

SIRT5 is a proviral factor that interacts with SARS-CoV-2 Nsp14 protein

SARS-CoV-2 non-structural protein Nsp14 is a highly conserved enzyme necessary for viral replication. Nsp14 forms a stable complex with non-structural protein Nsp10 and exhibits exoribonuclease and N7-methyltransferase activities. Protein-interactome studies identified human sirtuin 5 (SIRT5) as a putative binding partner of Nsp14. SIRT5 is an NAD-dependent protein deacylase critical for cellular metabolism that removes succinyl and malonyl groups from lysine residues. Here we investigated the nature of this interaction and the role of SIRT5 during SARS-CoV-2 infection. We showed that SIRT5 stably interacts with Nsp14, but not with Nsp10, suggesting that SIRT5 and Nsp10 are parts of separate complexes. We found that SIRT5 catalytic domain is necessary for the interaction with Nsp14, but that Nsp14 does not appear to be directly deacylated by SIRT5. Furthermore, knock-out of SIRT5 or treatment with specific SIRT5 inhibitors reduced SARS-CoV-2 viral levels in cell-culture experiments. SIRT5 knock-out cells expressed higher basal levels of innate immunity markers and mounted a stronger antiviral response. Our results indicate that SIRT5 is a proviral factor necessary for efficient viral replication, which opens novel avenues for therapeutic interventions.

Redox Homeostasis in Cardiovascular Disease: The Role of Mitochondrial Sirtuins

Cardiovascular disease (CVD) is still the leading cause of death worldwide. Despite successful advances in both pharmacological and lifestyle strategies to fight well-established risk factors, the burden of CVD is still increasing. Therefore, it is necessary to further deepen our knowledge of the pathogenesis of the disease for developing novel therapies to limit even more its related morbidity and mortality. Oxidative stress has been identified as a common trait of several manifestations of CVD and could be a promising target for innovative treatments. Mitochondria are a major source of oxidative stress and sirtuins are a family of enzymes that generate different post-translational protein modifications, thus regulating important cellular processes, including cell cycle, autophagy, gene expression, and others. In particular, three sirtuins, SIRT3, SIRT4, and SIRT5 are located within the mitochondrial matrix where they regulate energy production and antioxidant pathways. Therefore, these sirtuins are strongly involved in the balance between oxidant and antioxidant mechanisms. In this review, we summarize the activities of these sirtuins with a special focus on their role in the control of oxidative stress, in relation to energy metabolism, atherosclerosis, and CVD.

Sirtuin 5 is Dispensable for CD8+ T Cell Effector and Memory Differentiation

CD8⁺ T cell effector and memory differentiation is tightly controlled at multiple levels including transcriptional, metabolic, and epigenetic regulation. Sirtuin 5 (SIRT5) is a protein deacetylase mainly located at mitochondria, but it remains unclear whether SIRT5 plays key roles in regulating CD8⁺ T cell effector or memory formation. Herein, with adoptive transfer of Sirt5^+/+ or Sirt5^−/− OT-1 cells and acute Listeria monocytogenes infection model, we demonstrate that SIRT5 deficiency does not affect CD8⁺ T cell effector function and that SIRT5 is not required for CD8⁺ T cell memory formation. Moreover, the recall response of SIRT5 deficient memory CD8⁺ T cells is comparable with Sirt5^+/+ memory CD8⁺ T cells. Together, these observations suggest that SIRT5 is dispensable for the effector function and memory differentiation of CD8⁺ T cells.

Discovery of new human Sirtuin 5 inhibitors by mimicking glutaryl-lysine substrates

Human sirtuin 5 (SIRT5) plays pivotal roles in metabolic pathways and other biological processes, and is involved in several human diseases including cancer. Development of new potent and selective SIRT5 inhibitors is currently desirable to provide potential therapeutics for related diseases. Herein, we report a series of new 3-thioureidopropanoic acid derivatives, which were designed to mimic the binding features of SIRT5 glutaryl-lysine substrates. Structure-activity relationship studies revealed several compounds with low micromolar inhibitory activities to SIRT5. Computational and biochemical studies indicated that these compounds exhibited competitive SIRT5 inhibition with respect to the glutaryl-lysine substrate rather than nicotinamide adenine dinucleotide cofactor. Moreover, they showed high selectivity for SIRT5 over SIRT1-3 and 6 and could stabilize SIRT5 proteins as revealed by thermal shift analyses. This work provides an effective substrate-mimicking strategy for future inhibitor design, and offers new inhibitors to investigate their therapeutic potentials in SIRT5-associated disease models.

Sirt5 Deficiency Causes Posttranslational Protein Malonylation and Dysregulated Cellular Metabolism in Chondrocytes Under Obesity Conditions

OBJECTIVE

Obesity accelerates the development of osteoarthritis (OA) during aging and is associated with altered chondrocyte cellular metabolism. Protein lysine malonylation (MaK) is a posttranslational modification (PTM) that has been shown to play an important role during aging and obesity. The objective of this study was to investigate the role of sirtuin 5 (Sirt5) in regulating MaK and cellular metabolism in chondrocytes under obesity-related conditions.

METHODS

MaK and SIRT5 were immunostained in knee articular cartilage of obese db/db mice and different aged C57BL6 mice with or without destabilization of the medial meniscus surgery to induce OA. Primary chondrocytes were isolated from 7-day-old WT and Sirt5^-/- mice and treated with varying concentrations of glucose and insulin to mimic obesity. Sirt5-dependent effects on MaK and metabolism were evaluated by western blot, Seahorse Respirometry, and gas/chromatography-mass/spectrometry (GC-MS) metabolic profiling.

RESULTS

MaK was significantly increased in cartilage of db/db mice and in chondrocytes treated with high concentrations of glucose and insulin (Glu^hiIns^hi). Sirt5 was increased in an age-dependent manner following joint injury, and Sirt5 deficient primary chondrocytes had increased MaK, decreased glycolysis rate, and reduced basal mitochondrial respiration. GC-MS identified 41 metabolites. Sirt5 deficiency altered 13 distinct metabolites under basal conditions and 18 metabolites under Glu^hiIns^hi treatment. Pathway analysis identified a wide range of Sirt5-dependent altered metabolic pathways that include amino acid metabolism, TCA cycle, and glycolysis.

CONCLUSION

This study provides the first evidence that Sirt5 broadly regulates chondrocyte metabolism. We observed changes in SIRT5 and MaK levels in cartilage with obesity and joint injury, suggesting that the Sirt5-MaK pathway may contribute to altered chondrocyte metabolism that occurs during OA development.

Human Sirtuin Regulators: The "Success" Stories

The human sirtuins are a group of NAD⁺-dependent protein deacylases. They “erase” acyl modifications from lysine residues in various cellular targets including histones, transcription factors, and metabolic enzymes. Through these far-reaching activities, sirtuins regulate a diverse array of biological processes ranging from gene transcription to energy metabolism. Human sirtuins have been intensely pursued by both academia and industry as therapeutic targets for a broad spectrum of diseases such as cancer, neurodegenerative diseases, and metabolic disorders. The last two decades have witnessed a flood of small molecule sirtuin regulators. However, there remain relatively few compounds targeting human sirtuins in clinical development. This reflects the inherent issues concerning the development of isoform-selective and potent molecules with good drug-like properties. In this article, small molecule sirtuin regulators that have advanced into clinical trials will be discussed in details as “successful” examples for future drug development. Special attention is given to the discovery of these compounds, the mechanism of action, pharmacokinetics analysis, formulation, as well as the clinical outcomes observed in the trials.

Impact of Dietary Modifications on Plasma Sirtuins 1, 3 and 5 in Older Overweight Individuals Undergoing 12-Weeks of Circuit Training

Sirtuins are nicotinamide adenine dinucleotide (NAD+)-dependent deacetylases that regulate numerous pathways such as mitochondrial energy metabolism in the human body. Lower levels of these enzymes were linked to diseases such as diabetes mellitus and were also described as a result of aging. Sirtuins were previously shown to be under the control of exercise and diet, which are modifiable lifestyle factors. In this study, we analyzed SIRT1, SIRT3 and SIRT5 in blood from a subset of healthy elderly participants who took part in a 12-week randomized, controlled trial during which they performed, twice-weekly, resistance and aerobic training only (EX), the exercise routine combined with dietary counseling in accordance with the guidelines of the German Nutrition Society (EXDC), the exercise routine combined with intake of 2 g/day oil from Calanus finmarchicus (EXCO), or received no treatment and served as the control group (CON). In all study groups performing exercise, a significant increase in activities of SIRT1 (EX: +0.15 U/mg (+0.56/−[−0.16]), EXDC: +0.25 U/mg (+0.52/−0.06), EXCO: +0.40 U/mg (+0.88/−[−0.12])) and SIRT3 (EX: +0.80 U/mg (+3.18/−0.05), EXDC: 0.95 U/mg (+3.88/−0.55), EXCO: 1.60 U/mg (+2.85/−0.70)) was detected. Group comparisons revealed that differences in SIRT1 activity in EXCO and EXDC differed significantly from CON (CON vs. EXCO, p = 0.003; CON vs. EXDC, p = 0.010). For SIRT3, increases in all three intervention groups were significantly different from CON (CON vs. EX, p = 0.007; CON vs. EXDC, p < 0.001, CON vs. EXCO, p = 0.004). In contrast, differences in SIRT5-activities were less pronounced. Altogether, the analyses showed that the activity of SIRT1 and SIRT3 increased in response to the exercise intervention and that this increase may potentially be enhanced by additional dietary modifications.

The selective degradation of sirtuins via macroautophagy in the MPP+ model of Parkinson's disease is promoted by conserved oxidation sites

The sirtuin (SIRT) protein family has been of major research interest over the last decades because of their involvement in aging, cancer, and cell death. SIRTs have been implicated in gene and metabolic regulation through their capacity to remove acyl groups from lysine residues in proteins in an NAD⁺-dependent manner, which may alter individual protein properties as well as the histone–DNA interaction. Since SIRTs regulate a wide range of different signaling cascades, a fine-tuned homeostasis of these proteins is imperative to guarantee the function and survival of the cell. So far, however, how exactly this homeostasis is established has remained unknown. Here, we provide evidence that neuronal SIRT degradation in Parkinson’s disease (PD) models is executed by autophagy rather than the proteasome. In neuronal Lund human mesencephalic (LUHMES) cells, all seven SIRTs were substrates for autophagy and showed an accelerated autophagy-dependent degradation upon 1-methyl-4-phenylpyridinium (MPP⁺) mediated oxidative insults in vitro, whereas the proteasome did not contribute to the removal of oxidized SIRTs. Through blockade of endogenous H₂O₂ generation and supplementation with the selective radical scavenger phenothiazine (PHT), we could identify H₂O₂-derived species as the responsible SIRT-oxidizing agents. Analysis of all human SIRTs suggested a conserved regulatory motif based on cysteine oxidation, which may have triggered their degradation via autophagy. High amounts of H₂O₂, however, rapidly carbonylated selectively SIRT2, SIRT6, and SIRT7, which were found to accumulate carbonylation-prone amino acids. Our data may help in finding new strategies to maintain and modify SIRT bioavailability in neurodegenerative disorders.

Sirtuin 5 promotes arterial thrombosis by blunting the fibrinolytic system

AIMS

Arterial thrombosis as a result of plaque rupture or erosion is a key event in acute cardiovascular events. Sirtuin 5 (SIRT5) belongs to the lifespan-regulating sirtuin superfamily and has been implicated in acute ischaemic stroke and cardiac hypertrophy. This project aims at investigating the role of SIRT5 in arterial thrombus formation.

METHODS AND RESULTS

Sirt5 transgenic (Sirt5Tg/0) and knock-out (Sirt5-/-) mice underwent photochemically induced carotid endothelial injury to trigger arterial thrombosis. Primary human aortic endothelial cells (HAECs) were treated with SIRT5 silencing-RNA (si-SIRT5) as well as peripheral blood mononuclear cells from acute coronary syndrome (ACS) patients and non-ACS controls (case-control study, total n = 171) were used to increase the translational relevance of our data. Compared to wild-type controls, Sirt5Tg/0 mice displayed accelerated arterial thrombus formation following endothelial-specific damage. Conversely, in Sirt5-/- mice, arterial thrombosis was blunted. Platelet function was unaltered, as assessed by ex vivo collagen-induced aggregometry. Similarly, activation of the coagulation cascade as assessed by vascular and plasma tissue factor (TF) and TF pathway inhibitor expression was unaltered. Increased thrombus embolization episodes and circulating D-dimer levels suggested augmented activation of the fibrinolytic system in Sirt5-/- mice. Accordingly, Sirt5-/- mice showed reduced plasma and vascular expression of the fibrinolysis inhibitor plasminogen activator inhibitor (PAI)-1. In HAECs, SIRT5-silencing inhibited PAI-1 gene and protein expression in response to TNF-α. This effect was mediated by increased AMPK activation and reduced phosphorylation of the MAP kinase ERK 1/2, but not JNK and p38 as shown both in vivo and in vitro. Lastly, both PAI-1 and SIRT5 gene expressions are increased in ACS patients compared to non-ACS controls after adjustment for cardiovascular risk factors, while PAI-1 expression increased across tertiles of SIRT5.

CONCLUSION

SIRT5 promotes arterial thrombosis by modulating fibrinolysis through endothelial PAI-1 expression. Hence, SIRT5 may be an interesting therapeutic target in the context of atherothrombotic events.

Post-translational lysine ac(et)ylation in health, ageing and disease

The acetylation/acylation (ac(et)ylation) of lysine side chains is a dynamic post-translational modification (PTM) regulating fundamental cellular processes with implications on the organisms' ageing process: metabolism, transcription, translation, cell proliferation, regulation of the cytoskeleton and DNA damage repair. First identified to occur on histones, later studies revealed the presence of lysine ac(et)ylation in organisms of all kingdoms of life, in proteins covering all essential cellular processes. A remarkable finding showed that the NAD⁺-dependent sirtuin deacetylase Sir2 has an impact on replicative lifespan in Saccharomyces cerevisiae suggesting that lysine acetylation has a direct role in the ageing process. Later studies identified sirtuins as mediators for beneficial effects of caloric/dietary restriction on the organisms' health- or lifespan. However, the molecular mechanisms underlying these effects are only incompletely understood. Progress in mass-spectrometry, structural biology, synthetic and semi-synthetic biology deepened our understanding of this PTM. This review summarizes recent developments in the research field. It shows how lysine ac(et)ylation regulates protein function, how it is regulated enzymatically and non-enzymatically, how a dysfunction in this post-translational machinery contributes to disease development. A focus is set on sirtuins and lysine acyltransferases as these are direct sensors and mediators of the cellular metabolic state. Finally, this review highlights technological advances to study lysine ac(et)ylation.

Preparation of Affinity Purified Antibodies against ε-Glutaryl-Lysine Residues in Proteins for Investigation of Glutarylated Proteins in Animal Tissues

The glutarylation of lysine residues in proteins attracts attention as a possible mechanism of metabolic regulation, perturbed in pathologies. The visualization of protein glutarylation by antibodies specific to ε-glutaryl-lysine residues may be particularly useful to reveal pathogenic mutations in the relevant enzymes. We purified such antibodies from the rabbit antiserum, obtained after sequential immunization with two artificially glutarylated proteins, using affinity chromatography on ε-glutaryl-lysine-containing sorbents. Employing these anti(ε-glutaryl-lysine)-antibodies for the immunoblotting analysis of rat tissues and mitochondria has demonstrated the sample-specific patterns of protein glutarylation. The study of the protein glutarylation in rat tissue homogenates revealed a time-dependent fragmentation of glutarylated proteins in these preparations. The process may complicate the investigation of potential changes in the acylation level of specific protein bands when studying time-dependent effects of the acylation regulators. In the rat brain, the protein glutarylation, succinylation and acetylation patterns obtained upon the immunoblotting of the same sample with the corresponding antibodies are shown to differ. Specific combinations of molecular masses of major protein bands in the different acylation patterns confirm the selectivity of the anti(ε-glutaryl-lysine)-antibodies obtained in this work. Hence, our affinity-purified anti(ε-glutaryllysine)-antibodies provide an effective tool to characterize protein glutarylation, revealing its specific pattern, compared to acetylation and succinylation, in complex protein mixtures.

Sirtuins and Renal Oxidative Stress

Renal failure is a major health problem that is increasing worldwide. To improve clinical outcomes, we need to understand the basic mechanisms of kidney disease. Aging is a risk factor for the development and progression of kidney disease. Cells develop an imbalance of oxidants and antioxidants as they age, resulting in oxidative stress and the development of kidney damage. Calorie restriction (CR) is recognized as a dietary approach that promotes longevity, reduces oxidative stress, and delays the onset of age-related diseases. Sirtuins, a type of nicotinamide adenine dinucleotide (NAD)-dependent histone deacetylase, are considered to be anti-aging molecules, and CR induces their expression. The sirtuin family consists of seven enzymes (Sirt1–7) that are involved in processes and functions related to antioxidant and oxidative stress, such as DNA damage repair and metabolism through histone and protein deacetylation. In fact, a role for sirtuins in the regulation of antioxidants and redox substances has been suggested. Therefore, the activation of sirtuins in the kidney may represent a novel therapeutic strategy to enhancing resistance to many causative factors in kidney disease through the reduction of oxidative stress. In this review, we discuss the relationship between sirtuins and oxidative stress in renal disease.

The role of SIRT5 and p53 proteins in the sensitivity of colon cancer cells to chemotherapeutic agent 5-Fluorouracil

BACKGROUND

In the treatment of colorectal cancer, it is important to develop drug combinations that will increase the effectiveness of chemotherapy and to determine the molecular targets of the drugs. Therefore, combined therapies that can increase the sensitivity of 5-Fluorouracil (5-FU) and the molecular pathways involved in this process are important in the treatment of the disease. Here we examined the SIRT5 (Resveratrol and Suramin) and p53 (Nutlin3a) modulators alone or in combination with 5-FU on the proliferation of colon cancer cells and effect of 5-FU on the SIRT5 and FOXO3a protein expressions whether p53 dependent or independent manner. METHODS AND RESULTS: According to our MTT assay results, Resveratrol (RSV), Nutlin3a and Suramin was found to be more effective in HCT-116 p53+/+ cells and these differences were evaluated together with the effect of 5-FU on the SIRT5, FOXO3a and Bim protein expressions in HCT-116 p53 +/+ and HCT-116 p53 -/- cells. SIRT5 is known to deacetylate FOXO3a which plays roles in the induction of apoptosis via Bim protein. Our western blot experiment results showed that while Suramin decreased SIRT5 and RSV decreased FOXO3a protein expressions significantly in HCT-116 p53 -/- cells, 5-FU decreased significantly SIRT5 and FOXO3a protein expressions in a p53 independent manner.

CONCLUSIONS

In this study, the effect of 5-FU on SIRT5 and FOXO 3a proteins was determined for the first time in HCT-116 p53 +/+ and HCT-116 p53 -/- cells. These results may help the discovery of new markers in colon cancer treatment.

Mitochondrial Sirtuins in Reproduction

Mitochondria act as hubs of numerous metabolic pathways. Mitochondrial dysfunctions contribute to altering the redox balance and predispose to aging and metabolic alterations. The sirtuin family is composed of seven members and three of them, SIRT3-5, are housed in mitochondria. They catalyze NAD+-dependent deacylation and the ADP-ribosylation of mitochondrial proteins, thereby modulating gene expression and activities of enzymes involved in oxidative metabolism and stress responses. In this context, mitochondrial sirtuins (mtSIRTs) act in synergistic or antagonistic manners to protect from aging and aging-related metabolic abnormalities. In this review, we focus on the role of mtSIRTs in the biological competence of reproductive cells, organs, and embryos. Most studies are focused on SIRT3 in female reproduction, providing evidence that SIRT3 improves the competence of oocytes in humans and animal models. Moreover, SIRT3 protects oocytes, early embryos, and ovaries against stress conditions. The relationship between derangement of SIRT3 signaling and the imbalance of ROS and antioxidant defenses in testes has also been demonstrated. Very little is known about SIRT4 and SIRT5 functions in the reproductive system. The final goal of this work is to understand whether sirtuin-based signaling may be taken into account as potential targets for therapeutic applications in female and male infertility.

Progress on mitochondrial silence information regulator family in epilepsy

SIRT3, SIRT4 and SIRT5 are located in mitochondria and also known as mitochondrial sirtuins. They play important roles in regulating many cellular functions including cell survival, cell cycle or apoptosis, DNA repair and metabolism. Mitochondrial sirtuins are involved in the protection of mitochondrial integrity and energy metabolism under stress regulating the expression of neurotransmitter receptors, neurotrophins, extracellular matrix proteins and various transcription factors, thus involved in epileptogenesis triggered by both genetic or acquired factors. Here we review research progress on the actions of mitochondrial sirtuin in epilepsy; and discuss the challenges and perspectives of mitochondrial sirtuin as a potential therapeutic target for epilepsy.

Functions and Mechanisms of Lysine Glutarylation in Eukaryotes

Lysine glutarylation (Kglu) is a newly discovered post-translational modification (PTM), which is considered to be reversible, dynamic, and conserved in prokaryotes and eukaryotes. Recent developments in the identification of Kglu by mass spectrometry have shown that Kglu is mainly involved in the regulation of metabolism, oxidative damage, chromatin dynamics and is associated with various diseases. In this review, we firstly summarize the development history of glutarylation, the biochemical processes of glutarylation and deglutarylation. Then we focus on the pathophysiological functions such as glutaric acidemia 1, asthenospermia, etc. Finally, the current computational tools for predicting glutarylation sites are discussed. These emerging findings point to new functions for lysine glutarylation and related enzymes, and also highlight the mechanisms by which glutarylation regulates diverse cellular processes.

The deacylase SIRT5 supports melanoma viability by influencing chromatin dynamics

Cutaneous melanoma remains the most lethal skin cancer, and ranks third among all malignancies in terms of years of life lost. Despite the advent of immune checkpoint and targeted therapies, only roughly half of patients with advanced melanoma achieve a durable remission. Sirtuin 5 (SIRT5) is a member of the sirtuin family of protein deacylases that regulates metabolism and other biological processes. Germline Sirt5 deficiency is associated with mild phenotypes in mice. Here we showed that SIRT5 was required for proliferation and survival across all cutaneous melanoma genotypes tested, as well as uveal melanoma, a genetically distinct melanoma subtype that arises in the eye and is incurable once metastatic. Likewise, SIRT5 was required for efficient tumor formation by melanoma xenografts and in an autochthonous mouse Braf Pten-driven melanoma model. Via metabolite and transcriptomic analyses, we found that SIRT5 was required to maintain histone acetylation and methylation levels in melanoma cells, thereby promoting proper gene expression. SIRT5-dependent genes notably included MITF, a key lineage-specific survival oncogene in melanoma, and the c-MYC proto-oncogene. SIRT5 may represent a druggable genotype-independent addiction in melanoma.

The Emerging Role of HDACs: Pathology and Therapeutic Targets in Diabetes Mellitus

Diabetes mellitus (DM) is one of the principal manifestations of metabolic syndrome and its prevalence with modern lifestyle is increasing incessantly. Chronic hyperglycemia can induce several vascular complications that were referred to be the major cause of morbidity and mortality in DM. Although several therapeutic targets have been identified and accessed clinically, the imminent risk of DM and its prevalence are still ascending. Substantial pieces of evidence revealed that histone deacetylase (HDAC) isoforms can regulate various molecular activities in DM via epigenetic and post-translational regulation of several transcription factors. To date, 18 HDAC isoforms have been identified in mammals that were categorized into four different classes. Classes I, II, and IV are regarded as classical HDACs, which operate through a Zn-based mechanism. In contrast, class III HDACs or Sirtuins depend on nicotinamide adenine dinucleotide (NAD+) for their molecular activity. Functionally, most of the HDAC isoforms can regulate β cell fate, insulin release, insulin expression and signaling, and glucose metabolism. Moreover, the roles of HDAC members have been implicated in the regulation of oxidative stress, inflammation, apoptosis, fibrosis, and other pathological events, which substantially contribute to diabetes-related vascular dysfunctions. Therefore, HDACs could serve as the potential therapeutic target in DM towards developing novel intervention strategies. This review sheds light on the emerging role of HDACs/isoforms in diabetic pathophysiology and emphasized the scope of their targeting in DM for constituting novel interventional strategies for metabolic disorders/complications.

Demalonylation of DDX3 by Sirtuin 5 promotes antiviral innate immune responses

Rationale: Hosts defend against viral infection by sensing viral pathogen-associated molecular patterns and activating antiviral innate immunity through TBK1-IRF3 signaling. However, the underlying molecular mechanism remains unclear.

Methods: SiRNAs targeting Sirt1-7 were transfected into macrophages to screen the antiviral function. Sirt5 deficient mice or macrophages were subjected to viral infection to assess in vivo and in vitro function of Sirt5 by detecting cytokines, viral replicates and survival rate. Immunoprecipitation, WesternBlot and luciferase reporter assay were used to reveal molecular mechanism.

Results: In this study, we functionally screened seven Sirtuin family members, and found that Sirtuin5 (Sirt5) promotes antiviral signaling and responses. Sirt5 deficiency leads to attenuated antiviral innate immunity in vivo and in vitro upon viral infection by decreasing TBK1-IRF3 activation and type I IFN production. Sirt5 overexpression increased antiviral innate immunity. Mechanism investigation revealed that Sirt5 interacts with DDX3 and demalonylates DDX3, which is critical for TBK1-IRF3 activation. Mutation of the demalonylation lysine sites (K66, K130, and K162) of DDX3 increased ifnβ transcription. Furthermore, the acetylation on lysine 118 of DDX3 positively regulated ifnβ transcription, whereas Sirt5 could not deacetylate this site.

Conclusion: Sirt5 promotes anti- RNA and DNA virus innate immune responses by increasing TBK1 signaling through demalonylating DDX3, which identifies a novel regulatory pathway of antiviral innate immune response.

Sirtuins as molecular targets, mediators, and protective agents in metal-induced toxicity

Metal dyshomeostasis, and especially overexposure, is known to cause adverse health effects due to modulation of a variety of metabolic pathways. An increasing body of literature has demonstrated that metal exposure may affect SIRT signaling, although the existing data are insufficient. Therefore, in this review we discuss the available data (PubMed-Medline, Google Scholar) on the influence of metal overload on sirtuin (SIRT) signaling and its association with other mechanisms involved in metal-induced toxicity. The existing data demonstrate that cadmium (Cd), mercury (Hg), arsenic (As), lead (Pb), aluminium (Al), hexavalent chromium (Cr^VI), manganese (Mn), iron (Fe), and copper (Cu) can inhibit SIRT1 activity. In addition, an inhibitory effect of Cd, Pb, As, and Fe on SIRT3 has been demonstrated. In turn, metal-induced inhibition of SIRT was shown to affect deacetylation of target proteins including FOXO, PGC1α, p53 and NF-kB. Increased acetylation downregulates PGC1α signaling pathway, resulting in cellular altered redox status and increased susceptibility to oxidative stress, as well as decreased mitochondrial biogenesis. Lower rates of LKB1 deacetylation may be responsible for metal-induced decreases in AMPK activity and subsequent metabolic disturbances. A shift to the acetylated FOXO results in increased expression of pro-apoptotic genes which upregulates apoptosis together with increased p53 signaling. Correspondingly, decreased NF-kB deacetylation results in upregulation of target genes of proinflammatory cytokines, enzymes, and cellular adhesion molecules thus promoting inflammation. Therefore, alterations in sirtuin activity may at least partially mediate metal-induced metabolic disturbances that have been implicated in neurotoxicity, nephrotoxicity, cardiotoxicity, and other toxic effects of heavy metals.

Dietary fatty acids as nutritional modulators of sirtuins: a systematic review

CONTEXT

The sirtuins (SIRT1 to SIRT7) constitute a family of highly conserved nicotinamide adenine dinucleotide-dependent proteins. When activated, sirtuins control essential cellular processes to maintain metabolic homeostasis, while lack of expression of sirtuins has been related to chronic disease.

OBJECTIVE

The aim of this systematic review is to analyze the role of fat consumption as a modulator of human sirtuins.

DATA SOURCES

This review was conducted according to PRISMA guidelines. Studies were identified by searches of the electronic databases PubMed/MEDLINE, Scopus, and Web of Science.

STUDY SELECTION

Randomized clinical trials assessing the effect of fatty acid consumption on sirtuin mRNA expression, sirtuin protein expression, or sirtuin protein activity were eligible for inclusion.

DATA EXTRACTION

Two authors screened and determined the quality of the studies; disagreements were resolved by the third author. All authors compared the compiled data.

RESULTS

Seven clinical studies with 3 different types of interventions involving healthy and nonhealthy participants were selected. Only SIRT1 and SIRT3 were evaluated. Overall, the evidence from clinical studies to date is insufficient to understand how lipid consumption modulates sirtuins in humans. The best-characterized mechanism highlights oleic acid as a natural activator of SIRT1.

CONCLUSION

These results draw attention to a new field of interest in nutrition science. The possible activation of sirtuins by dietary fat manipulation may represent an important nutritional strategy for management of chronic and metabolic disease.

SYSTEMATIC REVIEW REGISTRATION

PROSPERO registration number CRD42018114456.

Short-term fasting reshapes fat tissue

Intermittent fasting, which can effectively reduce obesity and improve the related metabolic syndrome has become an exciting research area in recent years. Adipose tissue is pivotal in regulating the metabolism and determining the occurrence of obesity. In the current study, we aimed to investigate the effects of acute fasting (AF) on fat tissue. Mice were subjected to AF for 36 h, receiving normal chow (low-fat diet [LFD]) or a high-fat diet (HFD), with free ad libitum access to drinking water, and those fed on free-diet counterparts without fasting serveding as controls. We found that AF obviously reshaped the morphology of fat tissue (WAT) and promoted the beiging of white adipose tissue in both LFD- and HFD-fed mice. AF principally improved the lipid metabolism, and increased the M2- polarization of macrophages in WAT white fat tissue of HFD-fed mice. Interestingly, we found that AF dramatically upregulated Sirt5 expression levels and fat tissue succinylation, suggesting that AF-induced beneficial effects on fat might occur via the regulation of Sirt5 levels and altered succinylation in fatty tissues. Our study clearly showed the remodeling function of adipose tissue during AF; in terms of mechanism, the regulation of succinylation levels by AF might provide new insights into the mechanism(s) underlying the improvement in fat metabolism by energy restriction.

Sirtuins: To Be or Not To Be in Diabetic Cardiomyopathy

Diabetic cardiomyopathy is the leading cause of death among people with diabetes. Despite its severity and poor prognosis, there are currently no approved specific drugs to prevent or even treat diabetic cardiomyopathy. There is a need to understand the pathogenic mechanisms underlying the development of diabetic cardiomyopathy to design new therapeutic strategies. These mechanisms are complex and intricate and include metabolic dysregulation, inflammation, oxidative stress, fibrosis, and apoptosis. Sirtuins, a group of deacetylase enzymes, play an important role in all these processes and are, therefore, potential molecular targets for treating this disease. In this review, we discuss the role of sirtuins in the heart, focusing on their contribution to the pathogenesis of diabetic cardiomyopathy and how their modulation could be therapeutically useful.

A SARS-CoV-2 -human metalloproteome interaction map

The recent pandemic caused by the novel coronavirus resulted in the greatest global health crisis since the Spanish flu pandemic of 1918. There is limited knowledge of whether SARS-CoV-2 is physically associated with human metalloproteins. Recently, high-confidence, experimentally supported protein-protein interactions between SARS-CoV-2 and human proteins were reported. In this work, 58 metalloproteins among these human targets have been identified by a structure-based approach. This study reveals that most human metalloproteins interact with the recently discovered SARS-CoV-2 orf8 protein, whose antibodies are one of the principal markers of SARS-CoV-2 infections. Furthermore, this work provides sufficient evidence to conclude that Zn²⁺ plays an important role in the interplay between the novel coronavirus and humans. First, the content of Zn-binding proteins in the involved human metalloproteome is significantly higher than that of the other metal ions. Second, a molecular linkage between the identified human Zn-binding proteome with underlying medical conditions, that might increase the risk of severe illness from the SARS-CoV-2 virus, has been found. Likely perturbations of host cellular metal homeostasis by SARS-CoV-2 infection are highlighted.

Is nuclear sirtuin SIRT6 a master regulator of immune function?

The ability to ward off pathogens with minimal damage to the host determines the immune system's robustness. Multiple factors, including pathogen processing, identification, secretion of mediator and effector molecules, and immune cell proliferation and differentiation into various subsets, constitute the success of mounting an effective immune response. Cellular metabolism controls all of these intricate processes. Cells utilize diverse fuel sources and switch back and forth between different metabolic pathways depending on their energy needs. The three most critical metabolic pathways on which immune cells depend to meet their energy needs are oxidative metabolism, glycolysis, and glutaminolysis. Dynamic switching between these metabolic pathways is needed for optimal function of the immune cells. Moreover, switching between these metabolic pathways needs to be tightly regulated to achieve the best results. Immune cells depend on the Warburg effect for their growth, proliferation, secretory, and effector functions. Here, we hypothesize that the sirtuin, SIRT6, could be a negative regulator of the Warburg effect. We also postulate that SIRT6 could act as a master regulator of immune cell metabolism and function by regulating critical signaling pathways.

The Pleiotropic Function of Human Sirtuins as Modulators of Metabolic Pathways and Viral Infections

Sirtuins (SIRTs) are nicotinamide adenine dinucleotide-dependent histone deacetylases that incorporate complex functions in the mechanisms of cell physiology. Mammals have seven distinct members of the SIRT family (SIRT1-7), which play an important role in a well-maintained network of metabolic pathways that control and adapt the cell to the environment, energy availability and cellular stress. Until recently, very few studies investigated the role of SIRTs in modulating viral infection and progeny. Recent studies have demonstrated that SIRT1 and SIRT2 are promising antiviral targets because of their specific connection to numerous metabolic and regulatory processes affected during infection. In the present review, we summarize some of the recent progress in SIRTs biochemistry and their emerging function as antiviral targets. We also discuss the potential of natural polyphenol-based SIRT modulators to control their functional roles in several diseases including viral infections.

Succinylation Regulators Promote Clear Cell Renal Cell Carcinoma by Immune Regulation and RNA N6-Methyladenosine Methylation

Succinylation is a newly discovered and multienzyme-regulated post-translational modification (PTM) that is associated with the initiation and progression of cancer. Currently, no systematic analyses on the role of succinylation regulators in tumors have been reported. In this study, we performed a comprehensive pan-cancer analysis on four well-known succinylation regulators (CPT1A, KAT2A, SIRT5, and SIRT7). We found that these regulators played specific and critical roles in the prognosis of clear cell renal cell carcinoma (ccRCC). We constructed a risk score (RS) based on two independent prognostic prediction factors, CPT1A and KAT2A, and subsequently developed a nomogram model containing the RS, which showed good accuracy in the prediction of overall survival (OS) in ccRCC patients. Furthermore, we used the similar expression pattern of four succinylation regulators according to consensus clustering analysis to divide the patients into three clusters that exhibited prominently different OS as well as clinicopathological characteristics. Differently expressed genes (DEGs) and pathway enrichment analyses of three clusters indicated that succinylation regulators might promote malignant progression of ccRCC by regulating the infiltration of immune cells and RNA N6-methyladenosine (m6A) methylation. Importantly, our data suggest that CPT1A and SIRT5 might up-regulate and down-regulate the expression of LRPPRC and EIF3B, respectively. Our study systematically analyzed the prognostic predictive values of four succinylation regulators and revealed their potential mechanisms in ccRCC aggressiveness. These data provide new insight into the understanding of succinylation modification and present clinical evidence for its role in ccRCC treatments.

Substrate-Dependent Sensitivity of SIRT1 to Nicotinamide Inhibition

SIRT1 is the most extensively studied human sirtuin with a broad spectrum of endogenous targets. It has been implicated in the regulation of a myriad of cellular events, such as gene transcription, mitochondria biogenesis, insulin secretion as well as glucose and lipid metabolism. From a mechanistic perspective, nicotinamide (NAM), a byproduct of a sirtuin-catalyzed reaction, reverses a reaction intermediate to regenerate NAD+ through "base exchange", leading to the inhibition of the forward deacetylation. NAM has been suggested as a universal sirtuin negative regulator. Sirtuins have evolved different strategies in response to NAM regulation. Here, we report the detailed kinetic analysis of SIRT1-catalyzed reactions using endogenous substrate-based synthetic peptides. A novel substrate-dependent sensitivity of SIRT1 to NAM inhibition was observed. Additionally, SIRT1 demonstrated pH-dependent deacetylation with normal solvent isotope effects (SIEs), consistent with proton transfer in the rate-limiting step. Base exchange, in contrast, was insensitive to pH changes with no apparent SIEs, indicative of lack of proton transfer in the rate-limiting step. Consequently, NAM inhibition was attenuated at a high pH in proteated buffers. Our study provides new evidence for "activation by de-repression" as an effective sirtuin activation strategy.

MiR-140-3p inhibits the cell viability and promotes apoptosis of synovial fibroblasts in rheumatoid arthritis through targeting sirtuin 3

Background

Synovial fibroblasts (SFs) with the abnormal expressions of miRNAs are the key regulator in rheumatoid arthritis (RA). Low-expressed miR-140-3p was found in RA tissues. Therefore, we attempted to investigate the effect of miR-140-3p on SFs of RA.

Methods

RA and normal synovial fibrous tissue were gathered. The targets of miR-140-3p were found by bioinformatics and luciferase analysis. Correlation between the expressions of miR-140-3p with sirtuin 3 (SIRT3) was analyzed by Pearson correlation analysis. After transfection, cell viability and apoptosis were detected by cell counting kit-8 and flow cytometry. The expressions of miR-140-3p, SIRT3, Ki67, Bcl-2, Bax, and cleaved Caspase-3 were detected by RT-qPCR or western blot.

Results

Low expression of miR-140-3p and high expression of SIRT3 were found in RA synovial fibrous tissues. SIRT3 was a target of miR-140-3p. SIRT3 expression was negatively correlated to the expression of miR-140-3p. MiR-140-3p mimic inhibited the MH7A cell viability and the expressions of SIRT3, Ki67, and Bcl-2 and promoted the cell apoptosis and the expressions of Bax and cleaved Caspase-3; miR-140-3p inhibitor showed an opposite effect to miR-140-3p mimic on MH7A cells. SIRT3 overexpression not only promoted the cell viability and inhibited cell apoptosis of MH7A cells but also reversed the effect of miR-140-3p mimic had on MH7A cells.

Conclusions

The results in this study revealed that miR-140-3p could inhibit cell viability and promote apoptosis of SFs in RA through targeting SIRT3.

Histone Deacetylases in the Pathogenesis of Diabetic Cardiomyopathy

The global burden of diabetes mellitus and its complications are currently increasing. Diabetic cardiomyopathy (DCM) is the main cause of diabetes mellitus associated morbidity and mortality; therefore, a comprehensive understanding of DCM development is required for more effective treatment. A disorder of epigenetic posttranscriptional modification of histones in chromatin has been reported to be associated with the pathology of DCM. Recent studies have implicated that histone deacetylases could regulate cardiovascular and metabolic diseases in cellular processes including cardiac fibrosis, hypertrophy, oxidative stress and inflammation. Therefore in this review, we summarized the roles of histone deacetylases in the pathogenesis of DCM, aiming to provide insights into exploring potential preventative and therapeutic strategies of DCM.

Non-histone substrates of histone deacetylases as potential therapeutic targets in epilepsy

Introduction: Epilepsy is a network-level neurological disorder characterized by unprovoked recurrent seizures and associated comorbidities. Aberrant activity and localization of histone deacetylases (HDACs) have been reported in epilepsy and HDAC inhibitors (HDACi) have been used for therapeutic purposes. Several non-histone targets of HDACs have been recognized whose reversible acetylation can modulate protein functions and can contribute to disease pathology. Areas covered: This review provides an overview of HDACs in epilepsy and reflects its action on non-histone substrates involved in the pathogenesis of epilepsy and explores the effectiveness of HDACi as anti-epileptic drugs (AEDs). It also covers the efforts undertaken to target the interaction of HDACs with their substrates. We have further discussed non-deacetylase activity possessed by specific HDACs that might be essential in unraveling the molecular mechanism underlying the disease. For this purpose, relevant literature from 1996 to 2020 was derived from PubMed. Expert opinion: The interaction of HDACs and their non-histone substrates can serve as a promising therapeutic target for epilepsy. Pan-HDACi offers limited benefits to the epileptic patients. Thus, identification of novel targets of HDACs contributing to the disease and designing inhibitors targeting these complexes would be more effective and holds a greater potential as an anti-epileptogenic therapy.

Sirtuins in female meiosis and in reproductive longevity

Transmission of genetic material through high-quality gametes to progeny requires accurate homologous chromosome recombination and segregation during meiosis. A failure to accomplish these processes can have major consequences in reproductive health, including infertility, and development disorders in offspring. Sirtuins, a family of NAD+ -dependent protein deacetylases and ADP-ribosyltransferases, play key roles in genome maintenance, metabolism, and aging. In recent years, Sirtuins have emerged as regulators of several reproductive processes and interventions aiming to target Sirtuin activity are of great interest in the reproductive biology field. Sirtuins are pivotal to protect germ cells against oxidative stress, a major determinant influencing ovarian aging and the quality of gametes. Sirtuins also safeguard the integrity of the genome through epigenetic programs required for regulating gene repression, DNA repair, and chromosome segregation, among others. Although these functions are relatively well characterized in many somatic tissues, how they contribute to reproductive functions is not well understood. This review summarizes our current knowledge on the role of Sirtuins in female reproductive systems and discusses the underlying molecular pathways. In addition, we highlight the importance of Sirtuins as antiaging factors in the ovary and summarize current preclinical efforts to identify treatments to extend female reproductive longevity.

Sirtuins' control of autophagy and mitophagy in cancer

Mammalian cells use a specialized and complex machinery for the removal of altered proteins or dysfunctional organelles. Such machinery is part of a mechanism called autophagy. Moreover, when autophagy is specifically employed for the removal of dysfunctional mitochondria, it is called mitophagy. Autophagy and mitophagy have important physiological implications and roles associated with cellular differentiation, resistance to stresses such as starvation, metabolic control and adaptation to the changing microenvironment. Unfortunately, transformed cancer cells often exploit autophagy and mitophagy for sustaining their metabolic reprogramming and growth to a point that autophagy and mitophagy are recognized as promising targets for ongoing and future antitumoral therapies. Sirtuins are NAD+ dependent deacylases with a fundamental role in sensing and modulating cellular response to external stresses such as nutrients availability and therefore involved in aging, oxidative stress control, inflammation, differentiation and cancer. It is clear, therefore, that autophagy, mitophagy and sirtuins share many common aspects to a point that, recently, sirtuins have been linked to the control of autophagy and mitophagy. In the context of cancer, such a control is obtained by modulating transcription of autophagy and mitophagy genes, by post translational modification of proteins belonging to the autophagy and mitophagy machinery, by controlling ROS production or major metabolic pathways such as Krebs cycle or glutamine metabolism. The present review details current knowledge on the role of sirtuins, autophagy and mitophagy in cancer to then proceed to discuss how sirtuins can control autophagy and mitophagy in cancer cells. Finally, we discuss sirtuins role in the context of tumor progression and metastasis indicating glutamine metabolism as an example of how a concerted activation and/or inhibition of sirtuins in cancer cells can control autophagy and mitophagy by impinging on the metabolism of this fundamental amino acid.

Cyclic Tripeptide-based Potent and Selective Human SIRT5 Inhibitors

BACKGROUND

SIRT5 is one of the seven members (SIRT1-7) of the mammalian sirtuin family of protein acyl-lysine deacylase enzymes. In recent years, important regulatory roles of SIRT5 in (patho)physiological conditions (e.g. metabolism and cancer) have been increasingly demonstrated. For a better biological understanding and therapeutic exploitation of the SIRT5- catalyzed deacylation reaction, more effort on identifying potent and selective SIRT5 inhibitors beyond those currently known would be rewarding.

OBJECTIVE

In the current study, we would like to see if it would be possible to develop potent and selective SIRT5 inhibitory lead compounds with a novel structural scaffold than those of the currently known potent and selective SIRT5 inhibitors.

METHODS

In the current study, six N-terminus-to-side chain cyclic tripeptides (i.e. 8-13) each harboring the thiourea-type catalytic mechanism-based SIRT5 inhibitory warhead Nε-carboxyethylthiocarbamoyl- lysine as the central residue were designed, synthesized by the Nα-9- fluorenylmethoxycarbonyl (Fmoc) chemistry-based solid phase peptide synthesis (SPPS) on the Rink amide 4-methylbenzhydrylamine (MBHA) resin, purified by the semi-preparative reversedphase high performance liquid chromatography (RP-HPLC), characterized by the high-resolution mass spectrometry (HRMS); and were evaluated by the in vitro sirtuin inhibition assay and the in vitro proteolysis assay.

RESULTS

Among the cyclic tripeptides 8-13, we found that 10 exhibited a potent (IC50 ~2.2 μM) and selective (≥60-fold over the SIRT1/2/3/6-catalyzed deacylation reactions) inhibition against the SIRT5-catalyzed desuccinylation reaction. Moreover, 10 was found to exhibit a ~42.3-fold stronger SIRT5 inhibition and a greater proteolytic stability than its linear counterpart 14.

CONCLUSION

With a novel and modular structural scaffold as compared with those of all the currently reported potent and selective SIRT5 inhibitors, 10 could be also a useful and feasible lead compound for the quest for superior SIRT5 inhibitors as potential chemical/pharmacological probes of SIRT5 and therapeutics for human diseases in which SIRT5 desuccinylase activity is upregulated.

The possible role of sirtuin 5 in the pathogenesis of apical periodontitis

OBJECTIVES

We investigated the relation between expression of sirtuin 5 (SIRT5) in osteoblastic cells and progression of apical periodontitis. The role of SIRT5 in hypoxia-induced reactive oxygen species (ROS) formation and osteoblast apoptosis was also examined.

MATERIALS AND METHODS

Progression of rat apical periodontitis was monitored by conventional radiography and microcomputed tomography. SIRT5 and oxidative stress biomarker 8-OHdG in bone-lining cells were assessed by immunohistochemistry. Terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling was used to demonstrate apoptosis. In primary human osteoblasts cultured under hypoxia, Western blot was used to analyze SIRT5 expression and cleavage of pro-caspase 3 and poly(ADP-ribose) polymerase (PARP). SIRT5 was overexpressed through lentiviral technique. ROS formation and mitochondrial membrane potential changes were assessed by MitoSOX-Red and JC-1 fluorescence, respectively. Immunofluorescence microscope was used to evaluate mitochondrial release of cytochrome c.

RESULTS

In rat apical periodontitis, disease progression was accompanied by decreased expression of SIRT5, increased oxidative stress, and enhanced apoptosis in bone-lining cells. SIRT5 was suppressed in cultured osteoblasts under hypoxia. SIRT5 overexpression ameliorated hypoxia-enhanced ROS formation, mitochondrial depolarization, cytochrome c leakage, activation of caspase-3, and PARP fragmentation.

CONCLUSIONS

SIRT5 is able to alleviate hypoxia-enhanced osteoblast apoptosis. SIRT5 augmentation may have therapeutic potential for apical periodontitis.