Dichotomous Sirtuins: Implications for Drug Discovery in Neurodegenerative and Cardiometabolic Diseases

Semaglutide ameliorates cognition and glucose metabolism dysfunction in the 3xTg mouse model of Alzheimer's disease via the GLP-1R/SIRT1/GLUT4 pathway

Disorders of brain glucose metabolism is known to affect brain activity in neurodegenerative diseases including Alzheimer's disease (AD). Furthermore, recent evidence has shown an association between AD and type 2 diabetes. Numerous reports have found that glucagon-like peptide-1 (GLP-1) receptor agonists improve the cognitive behavior and pathological features in AD patients and animals, which may be related to the improvement of glucose metabolism in the brain. However, the mechanism by which GLP-1 agonists improve the brain glucose metabolism in AD patients remains unclear. In this study, we found that SIRT1 is closely related to expression of GLP-1R in hippocampus of 3xTg mice. Therefore, we used semaglutide, a novel GLP-1R agonist currently undergoing two phase 3 clinical trials in AD patients, to observe the effect of SIRT1 after semaglutide treatment in 3XTg mice and HT22 cells, and to explore the mechanism of SIRT1 in the glucose metabolism disorders of AD. The mice were injected with semaglutide on alternate days for 30 days, followed by behavioral experiments including open field test, new object recognition test, and Y-maze. The content of glucose in the brain was also measured by using 18FDG-PET-CT scans. We measured the expression of Aβ and tau in the hippocampus, observed the expression of GLUT4 which is downstream of SIRT1, and tested the Glucose oxidase assay (GOD-POD) and Hexokinase (HK) in HT22 cells. Here, we found in the 3xTg mouse model of AD and in cultured HT22 mouse neurons that SIRT1 signaling is involved in the impairment of glucose metabolism in AD. Semaglutide can increased the expression levels of SIRT1 and GLUT4 in the hippocampus of 3xTg mice, accompanied by an improvement in learning and memory, decreased in Aβ plaques and neurofibrillary tangles. In addition, we further demonstrated that semaglutide improved glucose metabolism in the brain of 3xTg mice in vitro, semaglutide promoted glycolysis and improved glycolytic disorders, and increased the membrane translocation of GLUT4 in cultured HT22 cells. These effects were blocked by the SIRT1 inhibitor (EX527). These findings indicate that semaglutide can regulate the expression of GLUT4 to mediate glucose transport through SIRT1, thereby improving glucose metabolism dysfunction in AD mice and cells. The present study suggests that SIRT1/GLUT4 signaling pathway may be an important mechanism for GLP-1R to promote glucose metabolism in the brain, providing a reliable strategy for effective therapy of AD.

Proteomic profiling reveals the potential mechanisms and regulatory targets of sirtuin 4 in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced Parkinson's mouse model

Introduction

Parkinson's disease (PD), as a common neurodegenerative disease, currently has no effective therapeutic approaches to delay or stop its progression. There is an urgent need to further define its pathogenesis and develop new therapeutic targets. An increasing number of studies have shown that members of the sirtuin (SIRT) family are differentially involved in neurodegenerative diseases, indicating their potential to serve as targets in therapeutic strategies. Mitochondrial SIRT4 possesses multiple enzymatic activities, such as deacetylase, ADP ribosyltransferase, lipoamidase, and deacylase activities, and exhibits different enzymatic activities and target substrates in different tissues and cells; thus, mitochondrial SIRT4 plays an integral role in regulating metabolism. However, the role and mechanism of SIRT4 in PD are not fully understood. This study aimed to investigate the potential mechanism and possible regulatory targets of SIRT4 in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced PD mice.

Methods

The expression of the SIRT4 protein in the MPTP-induced PD mouse mice or key familial Parkinson disease protein 7 knockout (DJ-1 KO) rat was compared against the control group by western blot assay. Afterwards, quantitative proteomics and bioinformatics analyses were performed to identify altered proteins in the vitro model and reveal the possible functional role of SIRT4. The most promising molecular target of SIRT4 were screened and validated by viral transfection, western blot assay and reverse transcription quantitative PCR (RT-qPCR) assays.

Results

The expression of the SIRT4 protein was found to be altered both in the MPTP-induced PD mouse mice and DJ-1KO rats. Following the viral transfection of SIRT4, a quantitative proteomics analysis identified 5,094 altered proteins in the vitro model, including 213 significantly upregulated proteins and 222 significantly downregulated proteins. The results from bioinformatics analyses indicated that SIRT4 mainly affected the ribosomal pathway, propionate metabolism pathway, peroxisome proliferator-activated receptor (PPAR) signaling pathway and peroxisome pathway in cells, and we screened 25 potential molecular targets. Finally, only fatty acid binding protein 4 (FABP4) in the PPAR signaling pathway was regulated by SIRT4 among the 25 molecules. Importantly, the alterations in FABP4 and PPARγ were verified in the MPTP-induced PD mouse model.

Discussion

Our results indicated that FABP4 in the PPAR signaling pathway is the most promising molecular target of SIRT4 in an MPTP-induced mouse model and revealed the possible functional role of SIRT4. This study provides a reference for future drug development and mechanism research with SIRT4 as a target or biomarker.

Insights on the Modulation of SIRT5 Activity: A Challenging Balance

SIRT5 is a member of the Sirtuin family, a class of deacetylating enzymes consisting of seven isoforms, involved in the regulation of several processes, including gene expression, metabolism, stress response, and aging. Considering that the anomalous activity of SIRT5 is linked to many pathological conditions, we present herein an overview of the most interesting modulators, with the aim of contributing to further development in this field.

Monoterpenes as Sirtuin-1 Activators: Therapeutic Potential in Aging and Related Diseases

Sirtuin 1 (SIRT) is a class III, NAD+-dependent histone deacetylase that also modulates the activity of numerous non-histone proteins through deacylation. SIRT1 plays critical roles in regulating and integrating cellular energy metabolism, response to stress, and circadian rhythm by modulating epigenetic and transcriptional regulation, mitochondrial homeostasis, proteostasis, telomere maintenance, inflammation, and the response to hypoxia. SIRT1 expression and activity decrease with aging, and enhancing its activity extends life span in various organisms, including mammals, and improves many age-related diseases, including cancer, metabolic, cardiovascular, neurodegenerative, respiratory, musculoskeletal, and renal diseases, but the opposite, that is, aggravation of various diseases, such as some cancers and neurodegenerative diseases, has also been reported. Accordingly, many natural and synthetic SIRT1 activators and inhibitors have been developed. Known SIRT1 activators of natural origin are mainly polyphenols. Nonetheless, various classes of non-polyphenolic monoterpenoids have been identified as inducers of SIRT1 expression and/or activity. This narrative review discusses current information on the evidence that supports the role of those compounds as SIRT1 activators and their potential both as tools for research and as pharmaceuticals for therapeutic application in age-related diseases.

SIRT2 Deficiency Exacerbates Hepatic Steatosis via a Putative Role of the ER Stress Pathway

Nonalcoholic fatty liver disease (NAFLD), a condition strongly associated with obesity and insulin resistance, is characterized by hepatic lipid accumulation and activation of the endoplasmic reticulum (ER) stress response. The sirtuin 2 (SIRT2) protein deacetylase is emerging as a new player in metabolic homeostasis, but its role in the development of hepatic steatosis and its link with ER stress activation remains unknown. SIRT2-knockout (SIRT2-KO) and wild-type mice were fed either a control or a high-fat diet (HFD) for 4 weeks. Genetic manipulation of SIRT2 levels was performed in human hepatic cells. Although apparently normal under a control diet, SIRT2-KO mice showed accelerated body weight gain and adiposity on a HFD, accompanied by severe insulin resistance. Importantly, SIRT2-KO mice exhibited worsened hepatic steatosis independently from diet, consistent with upregulated gene expression of lipogenic enzymes and increased expression of ER stress markers. Exposure of hepatic cells to palmitate induced lipid accumulation, increased ER stress, and decreased SIRT2 expression. Moreover, SIRT2-silenced cells showed enhanced lipid accumulation and ER stress activation under basal conditions, whereas SIRT2 overexpression abrogated palmitate-induced lipid deposition and ER stress activation. Our findings reveal a role for SIRT2 in the regulation of hepatic lipid homeostasis, potentially through the ER stress response, suggesting that SIRT2 activation might constitute a therapeutic strategy against obesity and its metabolic complications.

Inhibition of Sirt2 Alleviates Fibroblasts Activation and Pulmonary Fibrosis via Smad2/3 Pathway

Idiopathic pulmonary fibrosis (IPF) is a fatal disease with unknown cause and limited treatment options. Its mechanism needs to be further explored. Sirtuin2 (Sirt2), a nicotinamide adenine dinucleotide (NAD)-dependent deacetylase, has been proved to be involved in the fibrosis and inflammation in the liver, kidney and heart. In this study, we aimed to evaluate the role of Sirt2 in pulmonary fibrosis. We found that Sirt2 expression was upregulated in transforming growth factor-β1 (TGF-β1) treated human embryonic lung fibroblasts. Sirt2 inhibitor AGK2 or the knockdown of Sirt2 expression by targeting small interfering RNA (siRNA) suppressed the fibrogenic gene α-SMA and Fibronectin expression in TGF-β1 treated fibroblasts and primary lung fibroblasts derived from patients with IPF. In addition, Sirt2 inhibition suppresses the phosphorylation of Smad2/3. Co-immunoprecipitation (Co-IP) showed that there is interaction between Sirt2 and Smad3 in the TGF-β1 treated lung fibroblasts. In bleomycin-induced pulmonary fibrosis in mice, AGK2 treatment significantly mitigated the degree of fibrosis and decreased the phosphorylation of Smad2/3. These data suggest that Sirt2 may participate in the development of IPF via regulating the Smad2/3 pathway. Inhibition of Sirt2 would provide a novel therapeutic strategy for this disease.

Sirtuin Modulators in Cellular and Animal Models of Human Diseases

Sirtuins use NAD⁺ to remove various acyl groups from protein lysine residues. Through working on different substrate proteins, they display many biological functions, including regulation of cell proliferation, genome stability, metabolism, and cell migration. There are seven sirtuins in humans, SIRT1-7, each with unique enzymatic activities, regulatory mechanisms, subcellular localizations, and substrate scopes. They have been indicated in many human diseases, including cancer, neurodegeneration, microbial infection, metabolic and autoimmune diseases. Consequently, interests in development of sirtuin modulators have increased in the past decade. In this brief review, we specifically summarize genetic and pharmacological modulations of sirtuins in cancer, neurological, and cardiovascular diseases. We further anticipate this review will be helpful for scrutinizing the significance of sirtuins in the studied diseases.

From Hit Seeking to Magic Bullets: The Successful Union of Epigenetic and Fragment Based Drug Discovery (EPIDD + FBDD)

We review progress in the application of fragment-based drug discovery (FBDD) to epigenetic drug discovery (EPIDD) targeted at epigenetic writer and eraser enzymes as well as reader domains over the last 15 years. The greatest successes to date are in prospecting for bromodomain binding ligands. From a diverse array of fragment hits, multiple potent and selective compounds ensued, including the oncology clinical candidates mivebresib, ABBV-744, pelabresib, and PLX51107.

The role of microRNA-34 family in Alzheimer's disease: A potential molecular link between neurodegeneration and metabolic disorders

Growing evidence indicates that overexpression of the microRNA-34 (miR-34) family in the brain may play a crucial role in Alzheimer's disease (AD) pathogenesis by targeting and downregulating genes associated with neuronal survival, synapse formation and plasticity, Aβ clearance, mitochondrial function, antioxidant defense system, and energy metabolism. Additionally, elevated levels of the miR-34 family in the liver and pancreas promote the development of metabolic syndromes (MetS), such as diabetes and obesity. Importantly, MetS represent a well-documented risk factor for sporadic AD. This review focuses on the recent findings regarding the role of the miR-34 family in the pathogenesis of AD and MetS, and proposes miR-34 as a potential molecular link between both disorders. A comprehensive understanding of the functional roles of miR-34 family in the molecular and cellular pathogenesis of AD brains may lead to the discovery of a breakthrough treatment strategy for this disease.

Elucidation of the Mechanism Underlying the Anti-Inflammatory Properties of (S)-(+)-Carvone Identifies a Novel Class of Sirtuin-1 Activators in a Murine Macrophage Cell Line

The signaling pathways involved in age-related inflammation are increasingly recognized as targets for the development of preventive and therapeutic strategies. Our previous study elucidated the structure-activity relationship of monoterpene compounds derived from p-menthane as potential anti-inflammatory drugs and identified (S)-(+)-carvone as the most potent among the compounds tested. This study aims at identifying the molecular mechanism underlying the anti-inflammatory properties of (S)-(+)-carvone. The murine macrophage cell line, Raw 264.7, was stimulated with bacterial lipopolysaccharide (LPS) to simulate inflammation. Western blot was used to assess protein levels and post-translational modifications. The subcellular localization of NF-κB/p65 was visualized by immunocytochemistry. An in vitro fluorometric assay was used to measure Sirtuin-1 (SIRT1) activity. (S)-(+)-carvone inhibited LPS-induced JNK1 phosphorylation, but not that of p38 and ERK1/2 and also did not affect the phosphorylation and degradation of the NF-κB inhibitor, IκB-α. Accordingly, (S)-(+)-carvone did not affect LPS-induced phosphorylation of NF-κB/p65 on Ser536 and its nuclear translocation, but it significantly decreased LPS-induced IκB-α resynthesis, a NF-κB-dependent process, and NF-κB/p65 acetylation on lysine (Lys) 310. Deacetylation of that Lys residue is dependent on the activity of SIRT1, which was found to be increased by (S)-(+)-carvone, while its protein levels were unaffected. Taken together, these results show that (S)-(+)-carvone is a new SIRT1 activator with the potential to counteract the chronic low-grade inflammation characteristic of age-related diseases.

A New Vision of Mitochondrial Unfolded Protein Response to the Sirtuin Family

Mitochondrial damage is involved in many pathophysiological processes, such as tumor development, metabolism, and neurodegenerative diseases. The mitochondrial unfolded protein response (mtUPR) is the first stress-protective response initiated by mitochondrial damage, and it repairs or clears misfolded proteins to alleviate this damage. Studies have confirmed that the sirtuin family is essential for the mitochondrial stress response; in particular, SIRT1, SIRT3, and SIRT7 participate in the mtUPR in different axes. This article summarizes the associations of sirtuins with the mtUPR as well as specific molecular targets related to the mtUPR in different disease models, which will provide new inspiration for studies on mitochondrial stress, mitochondrial function protection, and mitochondria-related diseases, such as neurodegenerative diseases.

Sirtuin Acetylation and Deacetylation: a Complex Paradigm in Neurodegenerative Disease

Sirtuins are the class III of histone deacetylases that depend on nicotinamide adenine dinucleotide for their activity. Sirtuins can influence the progression of neurodegenerative disorders by switching between deacetylation and acetylation processes. Histone acetylation occurs when acetyl groups are added to lysine residues on the N-terminal part of histone proteins. Deacetylation, on the other hand, results in the removal of acetyl groups. Pharmacological modulation of sirtuin activity has been shown to influence various neurodegenerative disorders including Alzheimer's disease, Parkinson's disease, Huntington's disease, stroke, and amyotrophic lateral sclerosis. In this review, mechanistic perspective of sirtuins has been discussed in anti-inflammatory, antiapoptotic, and neuroprotective effects in various disorders. We have discussed the structure, neurobiology, and physiology of sirtuins in neurodegenerative disease. Recent preclinical and clinical studies and their outcome have also been elucidated. The aim of this review is to fill in the gaps in our understanding of sirtuins' role in histone acetylation and deacetylation in all neurodegenerative diseases. Here, we emphasized on reviewing all the studies carried out in various labs depicting the role of sirtuin modulators in neuroprotection and highlighted the ideas that can be considered for future perspectives. Taken together, sirtuins may serve as a promising therapeutic target for the treatment of neurodegenerative disorders.

Neuroprotective Effect of Astragaloside IV on Cerebral Ischemia/Reperfusion Injury Rats Through Sirt1/Mapt Pathway

Background: Ischemic stroke is a common disease with poor prognosis, which has become one of the leading causes of morbidity and mortality worldwide. Astragaloside IV (AS-IV) is the main bioactive ingredient of Astragali Radix (which has been used for ischemic stroke for thousands of years) and has been found to have multiple bioactivities in the nervous system. In the present study, we aimed to explore the neuroprotective effects of AS-IV in rats with cerebral ischemia/reperfusion (CIR) injury targeting the Sirt1/Mapt pathway. Methods: Sprague-Dawley rats (male, 250-280 g) were randomly divided into the Sham group, middle cerebral artery occlusion/reperfusion (MCAO/R) group, AS-IV group, MCAO/R + EX527 (SIRT1-specific inhibitor) group, and AS-IV + EX527 group. Each group was further assigned into several subgroups according to ischemic time (6 h, 1 d, 3 d, and 7 days). The CIR injury was induced in MCAO/R group, AS-IV group, MCAO/R + EX527 group, and AS-IV + EX527 group by MCAO surgery in accordance with the modified Zea Longa criteria. Modified Neurological Severity Scores (mNSS) were used to evaluate the neurological deficits; TTC (2,3,5-triphenyltetrazolium chloride) staining was used to detect cerebral infarction area; Western Blot was used to assess the protein levels of SIRT1, acetylated MAPT (ac-MAPT), phosphorylated MAPT (p-MAPT), and total MAPT (t-MAPT); Real-time Quantitative Polymerase Chain Reaction (qRT-PCR) was used in the detection of Sirt1 and Mapt transcriptions. Results: Compared with the MCAO/R group, AS-IV can significantly improve the neurological dysfunction (p < 0.05), reduce the infarction area (p < 0.05), raise the expression of SIRT1 (p < 0.05), and alleviate the abnormal hyperacetylation and hyperphosphorylation of MAPT (p < 0.05). While compared with the AS-IV group, AS-IV + EX527 group showed higher mNSS scores (p < 0.05), more severe cerebral infarction (p < 0.05), lower SIRT1 expression (p < 0.01), and higher ac-MAPT and p-MAPT levels (p < 0.05). Conclusion: AS-IV can improve the neurological deficit after CIR injury in rats and reduce the cerebral infarction area, which exerts neuroprotective effects probably through the Sirt1/Mapt pathway.

Hit-to-lead optimization on aryloxybenzamide derivative virtual screening hit against SIRT

Sirtuins (SIRTs) are a class of nicotinamide adenine dinucleotide (NAD⁺)-dependent protein histone deacetylases (HDACs) that are evolutionarily conserved from bacteria to mammals. This group of enzymes catalyses the reversible deacetylation of lysine residues in the histones or non-histone substrates using NAD⁺ as a cosubstrate. Numerous studies have demonstrated that the aberrant enzymatic activity of SIRTs has been linked to various diseases like diabetes, cancer, and neurodegenerative disorders. Previously, we performed a pharmacophore-based virtual screening campaign and an aryloxybenzamide derivative (1) displaying SIRT1/2 inhibitory effect was identified as a hit compound. In the current study, the hit-to-lead optimization on the hit compound was explored in order to improve the SIRT binding and inhibition. Fourteen compounds, ten of which were new, have been synthesized and subjected to in vitro biological evaluation for their inhibitory activity against SIRT1-3. By the structural modifications performed, a significant improvement was observed in selective SIRT1 inhibition for ST01, ST02, and ST11 compared to that of the hit compound. The highest SIRT2 inhibitory activity was observed for ST14, which was designed according to compatibility with pharmacophore model developed for SIRT2 inhibitors and thus, providing the interactions required with key residues in SIRT2 active site. Furthermore, ST01, ST02, ST11, and ST14 were subjected to in vitro cytotoxicity assay against MCF-7 human breast cancer cell line to determine the influence of the improvement in SIRT1/2 inhibition along with the structural modifications on the cytotoxic properties of the compounds. The cytotoxicity of the compounds was found to be correlated with their SIRT inhibitory profiles indicating the effects of SIRT1/2 inhibition on cancer cell viability. Overall, this study provides structural insights for further inhibitor improvement.

Sirtuins as Important Factors in Pathological States and the Role of Their Molecular Activity Modulators

Sirtuins (SIRTs), enzymes from the family of NAD⁺-dependent histone deacetylases, play an important role in the functioning of the body at the cellular level and participate in many biochemical processes. The multi-directionality of SIRTs encourages scientists to undertake research aimed at understanding the mechanisms of their action and the influence that SIRTs have on the organism. At the same time, new substances are constantly being sought that can modulate the action of SIRTs. Extensive research on the expression of SIRTs in various pathological conditions suggests that regulation of their activity may have positive results in supporting the treatment of certain metabolic, neurodegenerative or cancer diseases or this connected with oxidative stress. Due to such a wide spectrum of activity, SIRTs may also be a prognostic markers of selected pathological conditions and prove helpful in assessing their progression, especially by modulating their activity. The article presents and discusses the activating or inhibiting impact of individual SIRTs modulators. The review also gathered selected currently available information on the expression of SIRTs in individual disease cases as well as the biological role that SIRTs play in the human organism, also in connection with oxidative stress condition, taking into account the progress of knowledge about SIRTs over the years, with particular reference to the latest research results.

Role of Sirtuins in Modulating Neurodegeneration of the Enteric Nervous System and Central Nervous System

Neurodegeneration of the central and enteric nervous systems is a common feature of aging and aging-related diseases, and is accelerated in individuals with metabolic dysfunction including obesity and diabetes. The molecular mechanisms of neurodegeneration in both the CNS and ENS are overlapping. Sirtuins are an important family of histone deacetylases that are important for genome stability, cellular response to stress, and nutrient and hormone sensing. They are activated by calorie restriction (CR) and by the coenzyme, nicotinamide adenine dinucleotide (NAD+). Sirtuins, specifically the nuclear SIRT1 and mitochondrial SIRT3, have been shown to have predominantly neuroprotective roles in the CNS while the cytoplasmic sirtuin, SIRT2 is largely associated with neurodegeneration. A systematic study of sirtuins in the ENS and their effect on enteric neuronal growth and survival has not been conducted. Recent studies, however, also link sirtuins with important hormones such as leptin, ghrelin, melatonin, and serotonin which influence many important processes including satiety, mood, circadian rhythm, and gut homeostasis. In this review, we address emerging roles of sirtuins in modulating the metabolic challenges from aging, obesity, and diabetes that lead to neurodegeneration in the ENS and CNS. We also highlight a novel role for sirtuins along the microbiota-gut-brain axis in modulating neurodegeneration.

Common risk factors and therapeutic targets in obstructive sleep apnea and osteoarthritis: An unexpectable link?

Osteoarthritis (OA) and Obstructive Sleep Apnea (OSA) are two highly prevalent chronic diseases for which effective therapies are urgently needed. Recent epidemiologic studies, although scarce, suggest that the concomitant occurrence of OA and OSA is associated with more severe manifestations of both diseases. Moreover, OA and OSA share risk factors, such as aging and metabolic disturbances, and co-morbidities, including cardiovascular and metabolic diseases, sleep deprivation and depression. Whether this coincidental occurrence is fortuitous or involves cause-effect relationships is unknown. This review aims at collating and integrating present knowledge on both diseases by providing a brief overview of their epidemiology and pathophysiology, analyzing current evidences relating OA and OSA and discussing potential common mechanisms by which they can aggravate each other. Such mechanisms constitute potential therapeutic targets whose pharmacological modulation may provide more efficient ways of reducing the consequences of OA and OSA and, thus, lessen the huge individual and social burden that they impose.

Sirtuins and Their Implications in Neurodegenerative Diseases from a Drug Discovery Perspective

Sirtuins are class III histone deacetylase (HDAC) enzymes that target both histone and non-histone substrates. They are linked to different brain functions and the regulation of different isoforms of these enzymes is touted to be an emerging therapy for the treatment of neurodegenerative diseases (NDs), including Parkinson's disease (PD), Alzheimer's disease (AD), and amyotrophic lateral sclerosis (ALS). The level of sirtuins affects brain health as many sirtuin-regulated pathways are responsible for the progression of NDs. Certain sirtuins are also implicated in aging, which is a risk factor for many NDs. In addition to SIRT1-3, it has been suggested that the less studied sirtuins (SIRT4-7) also play critical roles in brain health. This review delineates the role of each sirtuin isoform in NDs from a disease centric perspective and provides an up-to-date overview of sirtuin modulators and their potential use as therapeutics in these diseases. Furthermore, the future perspectives for sirtuin modulator development and their therapeutic application in neurodegeneration are outlined in detail, hence providing a research direction for future studies.

Sirt6 Deacetylase: A Potential Key Regulator in the Prevention of Obesity, Diabetes and Neurodegenerative Disease

Sirtuins, NAD + dependent proteins belonging to class III histone deacetylases, are involved in regulating numerous cellular processes including cellular stress, insulin resistance, inflammation, mitochondrial biogenesis, chromatin silencing, cell cycle regulation, transcription, and apoptosis. Of the seven mammalian sirtuins present in humans, Sirt6 is an essential nuclear sirtuin. Until recently, Sirt6 was thought to regulate chromatin silencing, but new research indicates its role in aging, diabetes, cardiovascular disease, lipid metabolism, neurodegenerative diseases, and cancer. Various murine models demonstrate that Sirt6 activation is beneficial in alleviating many disease conditions and increasing lifespan, showing that Sirt6 is a critical therapeutic target in the treatment of various disease conditions in humans. Sirt6 also regulates the pathogenesis of multiple diseases by acting on histone proteins and non-histone proteins. Endogenous and non-endogenous modulators regulate both activation and inhibition of Sirt6. Few Sirt6 specific non-endogenous modulators have been identified. Hence the identification of Sirt6 specific modulators may have potential therapeutic roles in the diseases described above. In this review, we describe the development of Sirt6, the role it plays in the human condition, the functional role and therapeutic importance in disease processes, and specific modulators and molecular mechanism of Sirt6 in the regulation of metabolic homeostasis, cardiovascular disease, aging, and neurodegenerative disease.

RTN4B-mediated suppression of Sirtuin 2 activity ameliorates β-amyloid pathology and cognitive impairment in Alzheimer's disease mouse model

Sirtuin 2 (SIRT2) is an NAD+ dependent deacetylase that is the most abundant sirtuin protein in the brain. Accumulating evidence revealed the role of SIRT2 in a wide range of biological processes and age-related diseases. However, the pivotal mechanism of SIRT2 played in Alzheimer's disease (AD) remains unknown. Here, we report that pharmacological inactivation of SIRT2 has a beneficial effect in AD. The deacetylase inhibitor of SIRT2 rescued the cognitive impairment in amyloid precursor protein/presenilin 1 transgenic mouse (APP/PS1 mouse), and the BACE1 cleavage was weakened to reduce the β-amyloid (Aβ) production in the hippocampus. Moreover, we firstly identified that Reticulon 4B (RTN4B) played a crucial role between SIRT2/BACE1 regulation in AD. RTN4B, as a deacetylation substrate for SIRT2, the deacetylation by SIRT2 drived the ubiquitination and degradation of RTN4B and then the disturbed RTN4B interacted with and influenced the expression of BACE1. When we overexpressed RTN4B in neurons of the hippocampus in the AD mouse model, the abnormal Aβ accumulation and cognitive impairment were ameliorated, consistent with the results of SIRT2 inhibition in vivo. Moreover, we showed that the regulatory effect of SIRT2 on BACE1 is dependent on RTN4B. When RTN4B was knocked down, the effects of SIRT2 inhibition on the BACE1 level, Aβ pathology, and AD-liked behaviors were also blocked. Collectively, we provide evidence that SIRT2 may be a potential target for AD; the new found SIRT2/RTN4B/BACE1 pathological pathway is one of the critical mechanisms for the improvement of SIRT2 on AD.

Will Sirtuins Be Promising Therapeutic Targets for TBI and Associated Neurodegenerative Diseases?

Traumatic brain injury (TBI), a leading cause of morbidity worldwide, induces mechanical, persistent structural, and metabolic abnormalities in neurons and other brain-resident cells. The key pathological features of TBI include neuroinflammation, oxidative stress, excitotoxicity, and mitochondrial dysfunction. These pathological processes persist for a period of time after TBIs. Sirtuins are evolutionarily conserved nicotinamide-adenine dinucleotide (NAD+)-dependent deacetylases and mono-ADP-ribosyl transferases. The mammalian sirtuin family has seven members, referred to as Sirtuin (SIRT) 1-7. Accumulating evidence suggests that SIRT1 and SIRT3 play a neuroprotective role in TBI. Although the evidence is scant, considering the involvement of SIRT2, 4-7 in other brain injury models, they may also intervene in similar pathophysiology in TBI. Neurodegenerative diseases are generally accepted sequelae of TBI. It was found that TBI and neurodegenerative diseases have many similarities and overlaps in pathological features. Besides, sirtuins play some unique roles in some neurodegenerative diseases. Therefore, we propose that sirtuins might be a promising therapeutic target for both TBI and associated neurodegenerative diseases. In this paper, we review the neuroprotective effects of sirtuins on TBI as well as related neurodegeneration and discuss the therapeutic potential of sirtuin modulators.

miR-138-5p targets sirtuin1 to regulate acute lung injury by regulation of the NF-κB signaling pathway

Acute lung injury (ALI), a disease with a high mortality rate, is a noncardiogenic pulmonary inflammatory response and characterized by damage to the pulmonary system. In this study, we explored the mechanism of the occurrence and development of ALI. It was firstly found that miR-138-5p could inhibit the expression of sirtuin1 (SIRT1), and we further demonstrated that miR-138-5p targets directly SIRT1 through the luciferase assay, while the latter negatively regulated the expression of NF-κB. A549 cells were treated with lipopolysaccharide in vitro to simulate ALI cells and induce ALI in the model mice. The results showed that inhibiting the expression of miR-138-5p could effectively increase the viability of damaged cells, promote cell proliferation, reduce apoptosis, inhibit the inflammatory response, reduce oxidative stress, and then relieve ALI symptoms. Collectively, our results suggested that miR-138-5p can inhibit SIRT1 expression and indirectly activate the NF-κB signaling pathway, thus regulating the development of ALI.

Trimetazidine Inhibits Renal Tubular Epithelial Cells to Mesenchymal Transition in Diabetic Rats via Upregulation of Sirt1

Trimetazidine (TMZ), as a metabolic regulator, is effective in treatment of coronary atherosclerotic heart disease with rare side effects in the clinic for long years. Interestingly, studies have shown that TMZ protects against several acute kidney injuries (AKI). However, the effect of TMZ on chronic kidney diseases (CKD) remains unknown. This study aimed to investigate the role of TMZ in diabetic nephropathy (DN) and its potential mechanisms. A rat model of DN was established in male Sprague-Dawley rats by streptozotocin (STZ) intraperitoneal injection. Experimental rats were separated into three groups: control, DN and DN + TMZ treatment. Metabolic parameters, pathological features and renal function markers were evaluated after 20 weeks of diabetes induction. In vitro experiments, the effect of TMZ on high fat and high glucose (HFG) induced or TGFβ1-induced epithelial-to-mesenchymal transition (EMT) was examined in HK-2 cells. Our results showed that TMZ could maintain renal function without affecting hemodynamic and plasma metabolic levels in diabetic rats. The effect was associated with a reversion of pathological progression of DN, especially for tubulointerstitial fibrosis. EMT is an important contributor to renal fibrosis. In this study, we investigated the role of TMZ in the process of EMT in DN. Mechanistically; TMZ attenuated HFG-induced EMT by relieving oxidative stress via deacetylation forkhead box O1 (FoxO1) in a Sirt1-dependent pathway. And it suppressed TGFβ1-induced EMT by deacetylating Smd4 in a Sirt1-dependent manner. Moreover, our study found that TMZ upregulated Sirt1 expression by increasing the expression of nicotinamide phosphoribosyl transferase (Nampt), which is a rate limiting enzyme for nicotinamide adenine dinucleotide (NAD⁺) generation by salvage pathway. And the increased NAD⁺ promoted Sirt1 expression. In conclusion, TMZ can prevent renal dysfunction and pathogenesis of tubulointerstitial fibrosis in DN, partly by inhibition of EMT via FoxO1/ROS pathway and TGFβ/Smad pathway in a Nampt/NAD⁺/Sirt1 dependent manner.