Cardioprotective Effects of SIRT6 in a Mouse Model of Transverse Aortic Constriction-Induced Heart Failure

Systemic aging fuels heart failure: Molecular mechanisms and therapeutic avenues

Systemic aging influences various physiological processes and contributes to structural and functional decline in cardiac tissue. These alterations include an increased incidence of left ventricular hypertrophy, a decline in left ventricular diastolic function, left atrial dilation, atrial fibrillation, myocardial fibrosis and cardiac amyloidosis, elevating susceptibility to chronic heart failure (HF) in the elderly. Age-related cardiac dysfunction stems from prolonged exposure to genomic, epigenetic, oxidative, autophagic, inflammatory and regenerative stresses, along with the accumulation of senescent cells. Concurrently, age-related structural and functional changes in the vascular system, attributed to endothelial dysfunction, arterial stiffness, impaired angiogenesis, oxidative stress and inflammation, impose additional strain on the heart. Dysregulated mechanosignalling and impaired nitric oxide signalling play critical roles in the age-related vascular dysfunction associated with HF. Metabolic aging drives intricate shifts in glucose and lipid metabolism, leading to insulin resistance, mitochondrial dysfunction and lipid accumulation within cardiomyocytes. These alterations contribute to cardiac hypertrophy, fibrosis and impaired contractility, ultimately propelling HF. Systemic low-grade chronic inflammation, in conjunction with the senescence-associated secretory phenotype, aggravates cardiac dysfunction with age by promoting immune cell infiltration into the myocardium, fostering HF. This is further exacerbated by age-related comorbidities like coronary artery disease (CAD), atherosclerosis, hypertension, obesity, diabetes and chronic kidney disease (CKD). CAD and atherosclerosis induce myocardial ischaemia and adverse remodelling, while hypertension contributes to cardiac hypertrophy and fibrosis. Obesity-associated insulin resistance, inflammation and dyslipidaemia create a profibrotic cardiac environment, whereas diabetes-related metabolic disturbances further impair cardiac function. CKD-related fluid overload, electrolyte imbalances and uraemic toxins exacerbate HF through systemic inflammation and neurohormonal renin-angiotensin-aldosterone system (RAAS) activation. Recognizing aging as a modifiable process has opened avenues to target systemic aging in HF through both lifestyle interventions and therapeutics. Exercise, known for its antioxidant effects, can partly reverse pathological cardiac remodelling in the elderly by countering processes linked to age-related chronic HF, such as mitochondrial dysfunction, inflammation, senescence and declining cardiomyocyte regeneration. Dietary interventions such as plant-based and ketogenic diets, caloric restriction and macronutrient supplementation are instrumental in maintaining energy balance, reducing adiposity and addressing micronutrient and macronutrient imbalances associated with age-related HF. Therapeutic advancements targeting systemic aging in HF are underway. Key approaches include senomorphics and senolytics to limit senescence, antioxidants targeting mitochondrial stress, anti-inflammatory drugs like interleukin (IL)-1β inhibitors, metabolic rejuvenators such as nicotinamide riboside, resveratrol and sirtuin (SIRT) activators and autophagy enhancers like metformin and sodium-glucose cotransporter 2 (SGLT2) inhibitors, all of which offer potential for preserving cardiac function and alleviating the age-related HF burden.

Exploring Sirtuins: New Frontiers in Managing Heart Failure with Preserved Ejection Fraction

With increasing research, the sirtuin (SIRT) protein family has become increasingly understood. Studies have demonstrated that SIRTs can aid in metabolism and affect various physiological processes, such as atherosclerosis, heart failure (HF), hypertension, type 2 diabetes, and other related disorders. Although the pathogenesis of HF with preserved ejection fraction (HFpEF) has not yet been clarified, SIRTs have a role in its development. Therefore, SIRTs may offer a fresh approach to the diagnosis, treatment, and prevention of HFpEF as a novel therapeutic intervention target.

SIRT6 in Regulation of Mitochondrial Damage and Associated Cardiac Dysfunctions: A Possible Therapeutic Target for CVDs

Cardiovascular diseases (CVDs) can be described as a global health emergency imploring possible prevention strategies. Although the pathogenesis of CVDs has been extensively studied, the role of mitochondrial dysfunction in CVD development has yet to be investigated. Diabetic cardiomyopathy, ischemic-reperfusion injury, and heart failure are some of the CVDs resulting from mitochondrial dysfunction Recent evidence from the research states that any dysfunction of mitochondria has an impact on metabolic alteration, eventually causes the death of a healthy cell and therefore, progressively directing to the predisposition of disease. Cardiovascular research investigating the targets that both protect and treat mitochondrial damage will help reduce the risk and increase the quality of life of patients suffering from various CVDs. One such target, i.e., nuclear sirtuin SIRT6 is strongly associated with cardiac function. However, the link between mitochondrial dysfunction and SIRT6 concerning cardiovascular pathologies remains poorly understood. Although the Role of SIRT6 in skeletal muscles and cardiomyocytes through mitochondrial regulation has been well understood, its specific role in mitochondrial maintenance in cardiomyocytes is poorly determined. The review aims to explore the domain-specific function of SIRT6 in cardiomyocytes and is an effort to know how SIRT6, mitochondria, and CVDs are related.

NAD in pathological cardiac remodeling: Metabolic regulation and beyond

Nicotinamide adenine dinucleotide (NAD) coenzymes are carriers of high energy electrons in metabolism and also play critical roles in numerous signaling pathways. NAD metabolism is decreased in various cardiovascular diseases. Importantly, stimulation of NAD biosynthesis protects against heart disease under different pathological conditions. In this review, we describe pathways for both generation and catabolism of NAD coenzymes and the respective changes of these pathways in the heart under cardiac diseases, including pressure overload, myocardial infarction, cardiometabolic disease, cancer treatment cardiotoxicity, and heart failure. We next provide an update on the strategies and treatments to increase NAD levels, such as supplementation of NAD precursors, in the heart that prevent or reverse cardiomyopathy. We also introduce the approaches to manipulate NAD consumption enzymes to ameliorate cardiac disease. Finally, we discuss the mechanisms associated with improvements in cardiac function by NAD coenzymes, differentiating between mitochondria-dependent effects and those independent of mitochondrial metabolism.

The role of structural biology in the design of sirtuin activators

Sirtuins are NAD+-dependent protein lysine deacylases and mono-ADP-ribosylases whose activity regulates different pathways, including DNA damage repair, cell survival and metabolism, reactive oxygen species (ROS) detoxification, inflammation, cardiac function, and neuronal signaling. Considering the beneficial effects of specific sirtuin isoforms on health and lifespan, the past two decades have seen a mounting interest in the development of sirtuin activators. The availability of enzyme-activator co-crystal structures has proven significant throughout the years for elucidating the mechanisms of action of activators and designing more potent and selective molecules. In this review, we highlight the most interesting examples of sirtuin activators and provide comprehensive coverage of the role that structural biology played in their discovery and characterization.

The role of mammalian Sirtuin 6 in cardiovascular diseases and diabetes mellitus

Cardiovascular diseases are severe diseases posing threat to human health because of their high morbidity and mortality worldwide. The incidence of diabetes mellitus is also increasing rapidly. Various signaling molecules are involved in the pathogenesis of cardiovascular diseases and diabetes. Sirtuin 6 (Sirt6), which is a class III histone deacetylase, has attracted numerous attentions since its discovery. Sirt6 enjoys a unique structure, important biological functions, and is involved in multiple cellular processes such as stress response, mitochondrial biogenesis, transcription, insulin resistance, inflammatory response, chromatin silencing, and apoptosis. Sirt6 also plays significant roles in regulating several cardiovascular diseases including atherosclerosis, coronary heart disease, as well as cardiac remodeling, bringing Sirt6 into the focus of clinical interests. In this review, we examine the recent advances in understanding the mechanistic working through which Sirt6 alters the course of lethal cardiovascular diseases and diabetes mellitus.

Activation of the sirtuin silent information regulator 1 pathway inhibits pathological myocardial remodeling

Myocardial remodeling refers to structural and functional disorders of the heart caused by molecular biological changes in the cardiac myocytes in response to neurological and humoral factors. A variety of heart diseases, such as hypertension, coronary artery disease, arrhythmia, and valvular heart disease, can cause myocardial remodeling and eventually lead to heart failure. Therefore, counteracting myocardial remodeling is essential for the prevention and treatment of heart failure. Sirt1 is a nicotinamide adenine dinucleotide⁺-dependent deacetylase that plays a wide range of roles in transcriptional regulation, energy metabolism regulation, cell survival, DNA repair, inflammation, and circadian regulation. It positively or negatively regulates myocardial remodeling by participating in oxidative stress, apoptosis, autophagy, inflammation, and other processes. Taking into account the close relationship between myocardial remodeling and heart failure and the involvement of SIRT1 in the development of the former, the role of SIRT1 in the prevention of heart failure via inhibition of myocardial remodeling has received considerable attention. Recently, multiple studies have been conducted to provide a better understanding of how SIRT1 regulates these phenomena. This review presents the progress of research involving SIRT1 pathway involvement in the pathophysiological mechanisms of myocardial remodeling and heart failure.

The sirtuin family in health and disease

Sirtuins (SIRTs) are nicotine adenine dinucleotide(+)-dependent histone deacetylases regulating critical signaling pathways in prokaryotes and eukaryotes, and are involved in numerous biological processes. Currently, seven mammalian homologs of yeast Sir2 named SIRT1 to SIRT7 have been identified. Increasing evidence has suggested the vital roles of seven members of the SIRT family in health and disease conditions. Notably, this protein family plays a variety of important roles in cellular biology such as inflammation, metabolism, oxidative stress, and apoptosis, etc., thus, it is considered a potential therapeutic target for different kinds of pathologies including cancer, cardiovascular disease, respiratory disease, and other conditions. Moreover, identification of SIRT modulators and exploring the functions of these different modulators have prompted increased efforts to discover new small molecules, which can modify SIRT activity. Furthermore, several randomized controlled trials have indicated that different interventions might affect the expression of SIRT protein in human samples, and supplementation of SIRT modulators might have diverse impact on physiological function in different participants. In this review, we introduce the history and structure of the SIRT protein family, discuss the molecular mechanisms and biological functions of seven members of the SIRT protein family, elaborate on the regulatory roles of SIRTs in human disease, summarize SIRT inhibitors and activators, and review related clinical studies.

SIRT6 in Aging, Metabolism, Inflammation and Cardiovascular Diseases

As an important NAD⁺-dependent enzyme, SIRT6 has received significant attention since its discovery. In view of observations that SIRT6-deficient animals exhibit genomic instability and metabolic disorders and undergo early death, SIRT6 has long been considered a protein of longevity. Recently, growing evidence has demonstrated that SIRT6 functions as a deacetylase, mono-ADP-ribosyltransferase and long fatty deacylase and participates in a variety of cellular signaling pathways from DNA damage repair in the early stage to disease progression. In this review, we elaborate on the specific substrates and molecular mechanisms of SIRT6 in various physiological and pathological processes in detail, emphasizing its links to aging (genomic damage, telomere integrity, DNA repair), metabolism (glycolysis, gluconeogenesis, insulin secretion and lipid synthesis, lipolysis, thermogenesis), inflammation and cardiovascular diseases (atherosclerosis, cardiac hypertrophy, heart failure, ischemia-reperfusion injury). In addition, the most recent advances regarding SIRT6 modulators (agonists and inhibitors) as potential therapeutic agents for SIRT6-mediated diseases are reviewed.

SIRT6 Mitigates Heart Failure With Preserved Ejection Fraction in Diabetes

BACKGROUND

Heart failure with preserved ejection fraction (HFpEF) is a growing health problem without effective therapies. Epidemiological studies indicate that diabetes is a strong risk factor for HFpEF, and about 45% of patients with HFpEF are suffering from diabetes, yet the underlying mechanisms remain elusive.

METHODS

Using a combination of echocardiography, hemodynamics, RNA-sequencing, molecular biology, in vitro and in vivo approaches, we investigated the roles of SIRT6 (sirtuin 6) in regulation of endothelial fatty acid (FA) transport and HFpEF in diabetes.

RESULTS

We first observed that endothelial SIRT6 expression was markedly diminished in cardiac tissues from heart failure patients with diabetes. We then established an experimental mouse model of HFpEF in diabetes induced by a combination of the long-term high-fat diet feeding and a low-dose streptozocin challenge. We also generated a unique humanized SIRT6 transgenic mouse model, in which a single copy of human SIRT6 transgene was engineered at mouse Rosa26 locus and conditionally induced with the Cre-loxP technology. We found that genetically restoring endothelial SIRT6 expression in the diabetic mice ameliorated diastolic dysfunction concurrently with decreased cardiac lipid accumulation. SIRT6 gain- or loss-of-function studies showed that SIRT6 downregulated endothelial FA uptake. Mechanistically, SIRT6 suppressed endothelial expression of PPARγ through SIRT6-dependent deacetylation of histone H3 lysine 9 around PPARγ promoter region; and PPARγ reduction mediated SIRT6-dependent inhibition of endothelial FA uptake. Importantly, oral administration of small molecule SIRT6 activator MDL-800 to diabetic mice mitigated cardiac lipid accumulation and diastolic dysfunction.

CONCLUSIONS

The impairment of endothelial SIRT6 expression links diabetes to HFpEF through the alteration of FA transport across the endothelial barrier. Genetic and pharmacological strategies that restored endothelial SIRT6 function in mice with diabetes alleviated experimental HFpEF by limiting FA uptake and improving cardiac metabolism, thus warranting further clinical evaluation.

Damage of uremic myocardium by p-cresyl sulfate and the ameliorative effect of Klotho by regulating SIRT6 ubiquitination

Uremic cardiomyopathy (UCM) is a common complication in patients with chronic kidney disease (CKD) and an important risk factor for death. P-Cresyl sulfate (PCS) is a damaging factor in UCM, and Klotho is a protective factor. However, the molecular mechanisms of Klotho and PCS in UCM and the relationship between PCS and Klotho are unclear. In vitro, Klotho treatment inhibited PCS-induced cardiomyocyte hypertrophy and apoptosis by blocking mTOR phosphorylation and inhibiting DNA double-strand breaks (DSBs), respectively. Moreover, PCS increased SIRT6 protein ubiquitination and downregulated SIRT6 protein expression, while Klotho inhibited SIRT6 protein ubiquitination and upregulated SIRT6 protein expression. In a mouse model of 5/6 nephrectomy (5/6Nx)-induced UCM, the expression of Klotho in the kidney and serum was decreased, and the expression of SIRT6 protein in myocardial tissues was lower. PCS further reduced Klotho and SIRT6 expression, aggravated heart structure and function abnormalities, and increased myocardial cell apoptosis in UCM mice. Administration of Klotho protein inhibited the downregulation of SIRT6 protein expression and improved cardiac structure and function. Furthermore, serum PCS level was associated with the left ventricular mass (LVM) and left ventricular mass index (LVMI) in hemodialysis patients. In conclusion, the uremic toxin PCS injures cardiomyocytes via mTOR phosphorylation and DSBs, and Klotho antagonizes the damaging effects of PCS. Moreover, the SIRT6 protein plays an important role in UCM, and Klotho suppresses SIRT6 ubiquitination induced by PCS, further improves cardiac structure and function in UCM and exerts protective effects.

Limonin stabilises SIRT6 by activating USP10 in cardiac hypertrophy

BACKGROUND AND PURPOSE

Limonin, a natural tetracyclic triterpenoid extract, exerts extensive pharmacological effects; however, its role in cardiac hypertrophy remains to be elucidated. We investigated the beneficial effects of limonin on cardiac hypertrophy and explored the potential mechanisms.

EXPERIMENTAL APPROACH

C57/BL6 male mice were subjected to aortic banding (AB) surgery and neonatal rat cardiac myocytes (NRCMs) were stimulated with phenylephrine (PE) to evaluate the effects of limonin on cardiac hypertrophy.

KEY RESULTS

Limonin markedly improved the cardiac function and heart weight in AB operation mice. In addition, limonin-treated mice and NRCMs produced fewer cardiac hypertrophy markers than those treated with the vehicle in hypertrophic groups. Sustained AB- or PE-stimulation impaired cardiac sirtuin 6 (SIRT6) protein levels, which were partially rescued by limonin and subsequently enhanced the activity of PPARα, and Sirt6 siRNA inhibited the anti-hypertrophic effects of limonin in vitro. Interestingly, limonin did not influence Sirt6 mRNA levels, but controlled its ubiquitin levels. Thus, the protein biosynthesis inhibitor, cycloheximide (CHX), and proteasome inhibitor, MG-132, were used to determine SIRT6 protein expression levels. Under PE stimulation, limonin increased SIRT6 protein levels in the presence of CHX, but it didn't influence SIRT6 expression in the presence of MG-132, suggesting that limonin promotes SIRT6 abundance by inhibiting its ubiquitination degradation. Furthermore, limonin inhibited the degradation of SIRT6 by activating ubiquitin-specific peptidase (Cuspidi et al.)-10, while USP10 siRNA abrogated the beneficial effects of limonin.

CONCLUSION AND IMPLICATIONS

Limonin mediates the ubiquitination and degradation of SIRT6 by activating USP10, providing an attractive therapeutic target for cardiac hypertrophy.

Harnessing NAD+ Metabolism as Therapy for Cardiometabolic Diseases

PURPOSE OF THE REVIEW

This review summarizes current understanding on the roles of nicotinamide adenine dinucleotide (NAD⁺) metabolism in the pathogeneses and treatment development of metabolic and cardiac diseases.

RECENT FINDINGS

NAD⁺ was identified as a redox cofactor in metabolism and a co-substrate for a wide range of NAD⁺-dependent enzymes. NAD⁺ redox imbalance and depletion are associated with many pathologies where metabolism plays a key role, for example cardiometabolic diseases. This review is to delineate the current knowledge about harnessing NAD⁺ metabolism as potential therapy for cardiometabolic diseases. The review has summarized how NAD⁺ redox imbalance and depletion contribute to the pathogeneses of cardiometabolic diseases. Therapeutic evidence involving activation of NAD⁺ synthesis in pre-clinical and clinical studies was discussed. While activation of NAD⁺ synthesis shows great promise for therapy, the field of NAD⁺ metabolism is rapidly evolving. Therefore, it is expected that new mechanisms will be discovered as therapeutic targets for cardiometabolic diseases.

Nicotinamide Adenine Dinucleotide in the Development and Treatment of Cardiac Remodeling and Aging

Background:

Recently, the beneficial effects of nicotinamide adenine dinucleotide (NAD+) as an antiaging and antioxidant molecule have become a focus of research. However, the mechanisms by which NAD+ supplementation affects the associated metabolites under physiological conditions remain unclear. Specifically, although NAD+ is involved in several processes that are dysregulated in cardiovascular diseases, some effects of NAD+ precursors and NAD+ on cardiac diseases have started to gain recognition only recently.

Objective:

To discuss the influence of NAD+ supplementation on adverse cardiac remodeling and aging.

Results:

Supplementation with NAD+ precursors or nicotinamide riboside, which enhances or supplements the NAD+ metabolome, might have a protective effect on the heart. NAD+ can alleviate chronic heart failure via a mitochondrial oxidation–reduction (redox) state mechanism. Furthermore, NAD+ replenishment can improve the life span of mice.

Conclusion:

NAD+ exerts considerable antiaging and antioxidant effects with promising therapeutic effects. However, its effect in humans and use as a dietary supplement need to be studied further.

BaZiBuShen alleviates cognitive deficits and regulates Sirt6/NRF2/HO-1 and Sirt6/P53-PGC-1α-TERT signaling pathways in aging mice

ETHNOPHARMACOLOGICAL RELEVANCE

BaZiBuShen formula (BZBS) is clinically used to counteract mental fatigue and to retard the aging process. Brain aging echoes in major risks of human sufferings and has become one of the main challenges to our societies and the health-care systems.

AIM OF THE STUDY

To investigate the effect and mode of action of BZBS on aging-associated cognitive impairments.

MATERIALS AND METHODS

BZBS was orally administered to D-galactose and NaNO2-induced aging mice. Premature senescence was assessed using the Morris water maze, step-down type passive avoidance, and pole-climbing tests. Telomere length was examined by qPCR analysis. Telomerase activity was assessed using PCR ELISA assay. Mitochondrial complex IV activity was examined by biochemical test. The levels of redox and immune status were determined by ELISA or biochemical assay. The expressions of sirtuin 6 (Sirt6), peroxisome proliferator-activated receptor γ coactivator-1α (PGC-1α), P53, telomerase reverse transcriptase (TERT), heme oxygenase-1 (HO-1), phospho(p)-nuclear factor erythroid-2 related factor 2 (NRF2), caspase-3, Bcl-2 associated x (Bax), and B-cell lymphoma-2 (Bcl-2) in the cerebral cortex were examined by Western blot and/or immunohistochemical staining.

RESULTS

BZBS intervention ameliorated reduced brain performances in aging mice, including memory, cognitive, and motor functions. In addition, BZBS administration to aging mice preserved redox homeostasis, attenuated immunosenescence, and maintained telomerase activity and telomere length. Moreover, BZBS treatment were associated with a declines in P53, caspase-3, Bax expressions and an increase in Sirt6, p-HO-1, p-NRF2, PGC-1α, and Bcl-2 expressions in the brains of this rapid aging mouse.

CONCLUSIONS

BZBS attenuates premature senescence possibly via the preservation of redox homeostasis and telomere integrity, and inhibition of apoptosis in rapid aging mouse. The mechanism governing the alterations may be associated with through the activation of Sirt6/NRF2/HO-1 and Sirt6/P53-PGC-1α-TERT signaling pathways. The results suggest that BZBS may provide a novel strategy for confronting aging and age-associated diseases.

Therapeutic DNA vaccine encoding CEMIP (KIAA1199) ameliorates kidney fibrosis in obesity through inhibiting the Wnt/β-catenin pathway

BACKGROUND

CEMIP is a novel risk factor of various cancers through activating Wnt/β-catenin /epithelial-mesenchymal transition between epithelial cells and stroma. The chronic fibrosis commonly contributes renal carcinogenesis in patients with obesity. As there have very few choices of medicines targeting CEMIP. This study intended to design therapeutic DNA vaccines for nephropathy in obesity, through diminishing the CEMIP/Wnt1/β-catenin pathway.

METHOD

In an 8-week experiment, plasmid-encoding CEMIP was vaccinated into high-fat diet (HFD) or obesity mice in the first 4 weeks, and then vaccination was stopped for at least 4 weeks. Then, plasma and spleens were harvested to evaluate anti-CEMIP antibody synthesis and T-helper type 1 and 2 activation after vaccination. Kidneys were collected to investigate efficacy of CEMIP DNA vaccine on inhibiting HFD and obesity-induced fibrosis and Wnt1/β-catenin pathway. To confirm that CEMIP crucially contributed towards fibrotic formation, CEMIP gene or siRNA transfection was performed in HK-2 cells under VLDL stimulation, or not.

RESULTS

At the end point, anti-CEMIP antibody was successfully produced in the pcDNA 3.1-CEMIP vaccinated group, while Wnt1/β-catenin signaling and fibrosis was inactive. Through VLDL stimulation and CEMIP overexpression, Wnt1/β-catenin signaling and fibrosis significantly presented in vitro. Otherwise, anti-sera of CEMIP-vaccinated mice could inhibit the VLDL-induced Wnt1/β-catenin/fibrosis pathway in HK-2 cells. Similarly, the silencing of CEMIP by siRNA ameliorated the Wnt1/β-catenin pathway and fibrogenesis under VLDL stimulation.

CONCLUSION

DNA vaccine targeting CEMIP/Wnt1/β-catenin pathway plays a novel strategy in nephropathy.

GENERAL SIGNIFICANCE

Immune therapy might provide a new therapeutic option on nephropathy of obesity.

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.

The nuclear sirtuin SIRT6 protects the heart from developing aging-associated myocyte senescence and cardiac hypertrophy

Sirtuins have been shown to regulate the aging process. We have previously demonstrated that Sirt6 blocks the pressure overload-induced cardiac hypertrophy in mice. Here, we show that Sirt6 can also mitigate aging-induced cardiomyocyte senescence and cardiac hypertrophy. We found that aging is associated with altered Sirt6 activity along with development of cardiac hypertrophy and fibrosis. Compared to young mice (4-months), the hearts of aged mice (24-months) showed increased levels of mitochondrial DNA damage, shortened telomere length, and increased accumulation of 8-oxo-dG adducts, which are hallmarks of aging. The aged hearts also showed reduced levels of NAD⁺ and altered levels of mitochondrial fusion-fission proteins. Similar characteristics were observed in the hearts of Sirt6 deficient mice. Additionally, we found that doxorubicin (Dox) induced cardiomyocyte senescence, as measured by expression of p16^INK4a, p53, and β-galactosidase, was associated with loss of Sirt6. However, Sirt6 overexpression protected cardiomyocytes from developing Dox-induced senescence. Further, compared to wild-type mice, the hearts of Sirt6.Tg mice showed reduced expression of aging markers, and the development of aging-associated cardiac hypertrophy and fibrosis. Our data suggest that Sirt6 is a critical anti-aging molecule that regulates various cellular processes associated with aging and protects the heart from developing aging-induced cardiac hypertrophy and fibrosis.

SIRT6 in Senescence and Aging-Related Cardiovascular Diseases

SIRT6 belongs to the nicotinamide adenine dinucleotide (NAD⁺)-dependent deacetylases and has established diverse roles in aging, metabolism and disease. Its function is similar to the Silent Information Regulator 2 (SIR2), which prolongs lifespan and regulates genomic stability, telomere integrity, transcription, and DNA repair. It has been demonstrated that increasing the sirtuin level through genetic manipulation extends the lifespan of yeast, nematodes and flies. Deficiency of SIRT6 induces chronic inflammation, autophagy disorder and telomere instability. Also, these cellular processes can lead to the occurrence and progression of cardiovascular diseases (CVDs), such as atherosclerosis, hypertrophic cardiomyopathy and heart failure. Herein, we discuss the implications of SIRT6 regulates multiple cellular processes in cell senescence and aging-related CVDs, and we summarize clinical application of SIRT6 agonists and possible therapeutic interventions in aging-related CVDs.

Nicotinamide adenine dinucleotide: Biosynthesis, consumption and therapeutic role in cardiac diseases

Nicotinamide adenine dinucleotide (NAD) is an abundant cofactor that plays crucial roles in several cellular processes. NAD can be synthesized de novo starting with tryptophan, or from salvage pathways starting with NAD precursors like nicotinic acid (NA), nicotinamide (NAM) or nicotinamide riboside (NR), referred to as niacin/B₃ vitamins, arising from dietary supply or from cellular NAD catabolism. Given the interconversion between its oxidized (NAD⁺ ) and reduced form (NADH), NAD participates in a wide range of reactions: regulation of cellular redox status, energy metabolism and mitochondrial biogenesis. Plus, NAD acts as a signalling molecule, being a cosubstrate for several enzymes such as sirtuins, poly-ADP-ribose-polymerases (PARPs) and some ectoenzymes like CD38, regulating critical biological processes like gene expression, DNA repair, calcium signalling and circadian rhythms. Given the large number of mitochondria present in cardiac tissue, the heart has the highest NAD levels and is one of the most metabolically demanding organs. In several models of heart failure, myocardial NAD levels are depressed and this depression is caused by mitochondrial dysfunction, metabolic remodelling and inflammation. Emerging evidence suggests that regulating NAD homeostasis by NAD precursor supplementation has therapeutic efficiency in improving myocardial bioenergetics and function. This review provides an overview of the latest understanding of the different NAD biosynthesis pathways, as well as its role as a signalling molecule particularly in cardiac tissue. We highlight the significance of preserving NAD equilibrium in various models of heart diseases and shed light on the potential pharmacological interventions aiming to use NAD boosters as therapeutic agents.

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.

Emerging roles of SIRT6 in human diseases and its modulators

The biological functions of sirtuin 6 (SIRT6; e.g., deacetylation, defatty-acylation, and mono-ADP-ribosylation) play a pivotal role in regulating lifespan and several fundamental processes controlling aging such as DNA repair, gene expression, and telomeric maintenance. Over the past decades, the aberration of SIRT6 has been extensively observed in diverse life-threatening human diseases. In this comprehensive review, we summarize the critical roles of SIRT6 in the onset and progression of human diseases including cancer, inflammation, diabetes, steatohepatitis, arthritis, cardiovascular diseases, neurodegenerative diseases, viral infections, renal and corneal injuries, as well as the elucidation of the related signaling pathways. Moreover, we discuss the advances in the development of small molecule SIRT6 modulators including activators and inhibitors as well as their pharmacological profiles toward potential therapeutics for SIRT6-mediated diseases.

SIRT6 enhances telomerase activity to protect against DNA damage and senescence in hypertrophic ligamentum flavum cells from lumbar spinal stenosis patients

Lumbar spinal stenosis (LSS) is a condition wherein patients exhibit age-related fibrosis, elastin-to-collagen ratio reductions, and ligamentum flavum hypertrophy. This study was designed to assess the relationship between SIRT6 and telomerase activity in hypertrophic ligamentum flavum (LFH) cells from LSS patients. We observed significant reductions in SIRT6, TPP1, and POT1 protein levels as well as increases in telomerase reverse transcriptase (TERT) levels and telomerase activity in LFH tissues relative to non- hypertrophic ligamentum flavum (LFN) tissues. When SIRT6 was overexpressed in these LFH cells, this was associated with significant increases in telomerase activity and a significant reduction in fibrosis-related protein expression. These effects were reversed, however, when telomerase activity was inactivated by hTERT knockdown in these same cells. SIRT6 overexpression was further found to reduce the frequency of senescence-associated β-galactosidase (SA-β-Gal)-positive LFH cells and to decrease p16, MMP3, and L1 mRNA levels and telomere dysfunction-induced foci (TIFs) in LFH cells. In contrast, hTERT knockdown-induced telomerase inactivation eliminated these SIRT6-dependent effects. Overall, our results indicate that SIRT6 functions as a key protective factor that prevents cellular senescence and telomere dysfunction in ligamentum flavum cells, with this effect being at least partially attributable to SIRT6-dependent telomerase activation.

Melatonin attenuates diabetic cardiomyopathy and reduces myocardial vulnerability to ischemia-reperfusion injury by improving mitochondrial quality control: Role of SIRT6

Targeting mitochondrial quality control with melatonin has been found promising for attenuating diabetic cardiomyopathy (DCM), although the underlying mechanisms remain largely undefined. Activation of SIRT6 and melatonin membrane receptors exerts cardioprotective effects while little is known about their roles during DCM. Using high-fat diet-streptozotocin-induced diabetic rat model, we found that prolonged diabetes significantly decreased nocturnal circulatory melatonin and heart melatonin levels, reduced the expressions of cardiac melatonin membrane receptors, and decreased myocardial SIRT6 and AMPK-PGC-1α-AKT signaling. 16 weeks of melatonin treatment inhibited the progression of DCM and the following myocardial ischemia-reperfusion (MI/R) injury by reducing mitochondrial fission, enhancing mitochondrial biogenesis and mitophagy via re-activating SIRT6 and AMPK-PGC-1α-AKT signaling. After the induction of diabetes, adeno-associated virus carrying SIRT6-specific small hairpin RNA or luzindole was delivered to the animals. We showed that SIRT6 knockdown or antagonizing melatonin receptors abolished the protective effects of melatonin against mitochondrial dysfunction as evidenced by aggravated mitochondrial fission and reduced mitochondrial biogenesis and mitophagy. Additionally, SIRT6 shRNA or luzindole inhibited melatonin-induced AMPK-PGC-1α-AKT activation as well as its cardioprotective actions. Collectively, we demonstrated that long-term melatonin treatment attenuated the progression of DCM and reduced myocardial vulnerability to MI/R injury through preserving mitochondrial quality control. Melatonin membrane receptor-mediated SIRT6-AMPK-PGC-1α-AKT axis played a key role in this process. Targeting SIRT6 with melatonin treatment may be a promising strategy for attenuating DCM and reducing myocardial vulnerability to ischemia-reperfusion injury in diabetic patients.

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.

Sirtuin 6: A potential therapeutic target for cardiovascular diseases

Cardiovascular diseases (CVDs) are serious diseases endangering human health due to high morbidity and mortality worldwide, and numerous signal molecules are involved in this pathological process. As a member of the Sirtuin family NAD ⁺-dependent deacetylases, indeed, Sirtuin 6 (SIRT6) plays an important role in regulating biological homeostasis, longevity, and various diseases. More importantly, SIRT6 performs as an indispensable role in glucose and lipid metabolism, inflammation and genomic stability for the occurrence and development of various CVDs. Recent advances: among sirtuins, SIRT6 was frequently unveiled thanks for its protective roles against heart failure, cardiovascular remodeling and atherosclerosis, and identified as an essential intervention target of CVDs, bringing SIRT6 into the focus of clinical interest. Herein, we provide an overview of the current molecular mechanism through which SIRT6 regulates CVDs, and we highlight a potential therapeutic target for CVDs.

Biological and catalytic functions of sirtuin 6 as targets for small-molecule modulators

Sirtuin 6 (SIRT6) is a nuclear NAD⁺-dependent deacetylase of histone H3 that regulates genome stability and gene expression. However, nonhistone substrates and additional catalytic activities of SIRT6, including long-chain deacylation and mono-ADP-ribosylation of other proteins, have also been reported, but many of these noncanonical roles remain enigmatic. Genetic studies have revealed critical homeostatic cellular functions of SIRT6, underscoring the need to better understand which catalytic functions and molecular pathways are driving SIRT6-associated phenotypes. At the physiological level, SIRT6 activity promotes increased longevity by regulating metabolism and DNA repair. Recent work has identified natural products and synthetic small molecules capable of activating the inefficient in vitro deacetylase activity of SIRT6. Here, we discuss the cellular functions of SIRT6 with a focus on attributing its catalytic activity to its proposed biological functions. We cover the molecular architecture and catalytic mechanisms that distinguish SIRT6 from other NAD⁺-dependent deacylases. We propose that combining specific SIRT6 amino acid substitutions identified in enzymology studies and activity-selective compounds could help delineate SIRT6 functions in specific biological contexts and resolve the apparently conflicting roles of SIRT6 in processes such as tumor development. We further highlight the recent development of small-molecule modulators that provide additional biological insight into SIRT6 functions and offer therapeutic approaches to manage metabolic and age-associated diseases.

SIRT6 participates in the quality control of aged oocytes via modulating telomere function

It has been well recognized that oocyte quality declines in aging animals. However, to date, the underlying mechanism remains to be explored. In the present study, we report that oocytes and embryos from aged mice (42-45 weeks old) display the reduced expression of SIRT6 protein, accompanying with telomere shortening and DNA lesions. Moreover, we demonstrate that specific depletion of SIRT6 in oocytes induces dysfunctional telomeres and apoptosis of the resultant early embryos, leading to the developmental delay and cytoplasmic fragmentation. Importantly, we further find that overexpression of SIRT6 in aged oocytes promotes the telomere elongation in 2-cell embryos and lowers the incidence of apoptotic blastomeres. In summary, our data indicate a role for SIRT6 in modulating telomere function during oocyte maturation and embryonic development, and discover that SIRT6 reduction is an important point connecting maternal aging and quality control of oocyte/embryos.

CEMIP regulates the proliferation and migration of vascular smooth muscle cells in atherosclerosis through the WNT–beta-catenin signaling pathway

In this study we investigated the regulatory role of cell-migration-inducing and hyaluronan-binding protein (CEMIP) in the proliferation and migration of vascular smooth muscle cells (VSMCs). The mRNA and protein levels of CEMIP were upregulated in the plasma samples from patients with atherosclerosis, and in VSMCs stimulated with platelet-derived growth factor-BB (PDGF-BB), compared with plasma from healthy subjects and untreated VSMCs. Silencing CEMIP suppressed PDGF-BB-induced cell migration and proliferation in VSMCs, as determined using a Cell Counting Kit-8 assays, 5-ethynyl-2′-deocyuridine (EDU) assays, flow cytometry, wound healing assays, and Transwell assays. Overexpression of CEMIP promoted the proliferation and migration of VSMCs via activation of the Wnt–β-catenin signaling pathway and the upregulation of its target genes, including matrix metalloproteinase-2, matrix metalloproteinase-7, cyclin D1, and c-myc, whereas CEMIP deficiency showed the opposite effects. The knockdown of CEMIP in ApoE−/− mice by intravenous injection of lentiviral vector expressing si-CEMIP protected against high-fat-diet-induced atherosclerosis, as shown by the reduced aortic lesion areas, aortic sinus lesion areas, and the concentration of blood lipids compared with mice normally expressing CEMIP. These results demonstrated that CEMIP regulates the proliferation and migration of VSMCs in atherosclerosis by activating the WNT–β-catenin signaling pathway, which suggests the therapeutic potential of CEMIP for the management of atherosclerosis.

Sirt6 attenuates hypoxia-induced tubular epithelial cell injury via targeting G2/M phase arrest

Acute kidney injury (AKI) is a condition that has a high incidence and death rate. Unfortunately, the kidney may not recover completely after AKI, which then develops to chronic kidney disease (CKD). Therefore, it is necessary to identify potential curative targets to avoid its development to CKD. As an NAD⁺ -dependent deacetylase, sirtuin 6 (Sirt6) has been linked to different types of biological processes. In the present work, our group investigated the role of Sirt6 in tubular epithelial cells (TECs) under hypoxic stress. Sirt6 expression was examined in mouse kidney following ischemia/reperfusion (IR) injury and hypoxia-challenged TECs. Using Sirt6 plasmid and small interfering RNA, we also investigated how, in regard to inflammation and epithelial-to-mesenchymal transition, Sirt6 affects hypoxia-triggered injury. In addition, cell cycle was detected in hypoxia-challenged TECs. Sirt6 was downregulated in the kidney of mice with IR injury and hypoxia-challenged TECs. Consequently, Sirt6 depletion aggravated hypoxia-induced injury and G2/M phase arrest. Sirt6 overexpression attenuated hypoxia-triggered damage and G2/M phase arrest in TECs. Sirt6 prevented hypoxia-triggered TEC damage via suppressing G2/M phase arrest. Thus, Sirt6 is a possible candidate for alleviating the effects of kidney injury.

Sirtuins and Accelerated Aging in Scleroderma

PURPOSE OF REVIEW

Premature activation of aging-associated molecular mechanisms is emerging as an important contributor to many diseases, including scleroderma. Among central regulators of the aging process are a group of histone deacetylases called sirtuins (SIRTs). Recent findings implicate these molecules as pathophysiological players in scleroderma skin and lung fibrosis. The goal of this article is to review recent studies on the involvement of SIRTs in scleroderma from the perspective of aging-related molecular mechanisms.

RECENT FINDINGS

Despite a degree of controversy in this rapidly developing field, the majority of data suggest that SIRT levels are decreased in tissues from patients with scleroderma compared to healthy controls as well as in animal models of scleroderma. Molecular studies reveal several mechanisms through which declining SIRT levels contribute to fibrosis, with the most attention given to modulation of the TGF-β signaling pathway. Activation of SIRTs in cell culture and in animal models elicits antifibrotic effects. Declining SIRT levels and activity are emerging as pathophysiological contributors to scleroderma. Restoration of SIRTs may be therapeutic in patients with scleroderma.

Sirtuins in the Cardiovascular System: Potential Targets in Pediatric Cardiology

Cardiovascular diseases represent a major cause of death and morbidity. Cardiac and vascular pathologies develop predominantly in the aged population in part due to lifelong exposure to numerous risk factors but are also found in children and during adolescence. In comparison to adults, much has to be learned about the molecular pathways driving cardiovascular diseases in the pediatric population. Sirtuins are highly conserved enzymes that play pivotal roles in ensuring cardiac homeostasis under physiological and stress conditions. In this review, we discuss novel findings about the biological functions of these molecules in the cardiovascular system and their possible involvement in pediatric cardiovascular diseases.

Emerging potential benefits of modulating NAD+ metabolism in cardiovascular disease

Nicotinamide adenine dinucleotide (NAD⁺) and related metabolites are central mediators of fuel oxidation and bioenergetics within cardiomyocytes. Additionally, NAD⁺ is required for the activity of multifunctional enzymes, including sirtuins and poly(ADP-ribose) polymerases that regulate posttranslational modifications, DNA damage responses, and Ca²⁺ signaling. Recent research has indicated that NAD⁺ participates in a multitude of processes dysregulated in cardiovascular diseases. Therefore, supplementation of NAD⁺ precursors, including nicotinamide riboside that boosts or repletes the NAD⁺ metabolome, may be cardioprotective. This review examines the molecular physiology and preclinical data with respect to NAD⁺ precursors in heart failure-related cardiac remodeling, ischemic-reperfusion injury, and arrhythmias. In addition, alternative NAD⁺-boosting strategies and potential systemic effects of NAD⁺ supplementation with implications on cardiovascular health and disease are surveyed.