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Sirtuin

Sirtuins are a highly conserved family of NAD⁺-dependent protein deacetylases, classified as class III histone deacetylases, that catalyze the removal of acetyl groups from residues on proteins using (NAD⁺) as a cofactor, thereby linking cellular to and other processes. These enzymes are present across all domains of life, from and to eukaryotes, with the founding member, Sir2, discovered in the budding Saccharomyces cerevisiae in the 1970s for its role in transcriptional silencing at mating-type loci and later shown to extend replicative lifespan when overexpressed. In humans, the sirtuin family consists of seven members (SIRT1–SIRT7), each localized to specific cellular compartments—such as the (SIRT1, SIRT6, SIRT7), mitochondria (SIRT3, SIRT4, SIRT5), or (SIRT2)—and exhibiting distinct substrate specificities beyond deacetylation, including and defatty-acylation activities. Sirtuins function as metabolic sensors, with their activity modulated by NAD⁺ levels, which fluctuate in response to nutritional status and energy demands, thereby integrating environmental cues with intracellular signaling. They regulate a wide array of biological processes, including DNA repair, chromatin remodeling, circadian rhythms, inflammation, oxidative stress response, and apoptosis, often promoting cellular homeostasis and resilience. Notably, sirtuins have been implicated in aging and longevity; for instance, increased Sir2 activity extends lifespan in yeast, and mammalian orthologs like SIRT1 mediate calorie restriction benefits by enhancing mitochondrial function and stress resistance. In health and disease contexts, sirtuins influence metabolic disorders, neurodegeneration, cardiovascular conditions, and cancer, where their dysregulation—often due to declining NAD⁺ levels with age—contributes to , positioning them as potential therapeutic targets for interventions like NAD⁺ boosters or sirtuin activators. For example, SIRT1 deacetylates key transcription factors such as and FOXO to suppress and promote survival under stress, while mitochondrial sirtuins (SIRT3–5) fine-tune energy metabolism and reactive oxygen species management. Overall, the sirtuin family's evolutionary conservation underscores their fundamental role in adapting cellular function to metabolic challenges, with ongoing research exploring their modulation for extending healthy lifespan.

Discovery and History

Discovery

The sirtuin family traces its origins to genetic studies in the budding yeast , where the foundational member, Sir2, was identified as a key regulator of . In 1979, Amar J. S. Klar and colleagues isolated a in a initially designated (mating-type regulator 1), which disrupted the silent state of the cryptic mating-type loci HML and HMR, leading to defects in mating-type switching. This discovery highlighted MAR1's role in maintaining transcriptional repression at these loci to ensure proper cellular identity during mating. Subsequent research in 1987 by Jasper Rine and Ira Herskowitz clarified the function of this , renaming it SIR2 (silent regulator 2) as part of a of four genes (SIR1–SIR4) responsible for position-effect variegation at the silent mating-type cassettes. These studies established Sir2 as essential for -mediated silencing, preventing ectopic of mating-type . Further experiments around the same period revealed Sir2's involvement in maintenance, where it contributes to the repression of genes near chromosomal ends, thereby stabilizing telomeric and preventing genomic instability. A pivotal advance came in 1999 when Roy A. Frye linked Sir2 to NAD⁺ dependency, showing that Sir2 and its homologs consume NAD⁺ in a reaction involving , which was soon refined to reveal their role as NAD⁺-dependent deacetylases. This enzymatic characterization connected sirtuins to broader epigenetic regulation. In 2000, Frye formalized the nomenclature by coining "sirtuins" for the Sir2-like protein family and demonstrated its evolutionary conservation from to eukaryotes through phylogenetic analysis, identifying homologs in diverse species including humans. Key experiments in further illuminated Sir2's biological impact, particularly its role in lifespan . Overexpression of SIR2 was found to extend replicative lifespan by up to 30%, an effect mimicked by caloric restriction through elevated NAD⁺ levels that activate Sir2, thereby reducing rDNA recombination and promoting without altering silencing at mating loci or telomeres. This established sirtuins as conserved mediators linking to aging across eukaryotes.

Historical Milestones

The mammalian sirtuin homologs (SIRT1–SIRT7) were identified starting in 1999, expanding the understanding of sirtuin functions beyond , with early studies in 2001 demonstrating that SIRT1 deacetylates to suppress . This work, building on the initial cloning of SIRT1-5 in 1999, established sirtuins as key regulators in mammalian cellular processes. By 2003, research linked SIRT1 to mammalian aging and , proposing that sirtuin activation mimics the longevity benefits of dietary limitation through metabolic reprogramming. In , the discovery of SIRT6 revealed its role in maintaining genomic stability via and modulating metabolic pathways, marking a toward sirtuins' involvement in nuclear functions. During the 2010s, investigations into mitochondrial sirtuins SIRT3, SIRT4, and SIRT5 highlighted their critical contributions to , including regulation of oxidation and detoxification. From 2020 to 2024, numerous sirtuin modulator compounds were patented, targeting therapeutic applications in aging-related conditions through activation or inhibition of specific isoforms. In 2025, the initiation of clinical trials for novel SIRT3 activators in represented a major advancement, aiming to enhance mitochondrial function and .

Structure and Classification

Molecular Structure

Sirtuins possess a highly conserved catalytic domain spanning approximately 250-300 , which is responsible for their NAD+-dependent enzymatic activity. This is structurally divided into two : a larger Rossmann-fold domain that binds NAD+ through a characteristic involving a conserved Gly-X-Gly sequence for recognition, and a smaller zinc-binding domain that stabilizes the overall fold via four conserved residues coordinating a Zn²⁺ ion in a Cys-X-X-Cys-X15–20-Cys-X-X-Cys . The Rossmann-fold , typical of many NAD+-binding proteins, consists of a central β-sheet flanked by α-helices, forming an where the and cofactor interact during . Flanking the catalytic core are variable N- and C-terminal extensions that vary in length and sequence among sirtuin isoforms, influencing their stability, regulation, and subcellular localization. For instance, the extended of SIRT1 facilitates its predominant localization, enabling interactions with chromatin-associated proteins, while SIRT3's N-terminal mitochondrial targeting sequence directs it to the . These extensions can also serve as regulatory domains, modulating access to the or mediating protein-protein interactions without altering the core's fundamental architecture. The molecular structure of sirtuins was first elucidated through of prokaryotic and eukaryotic homologs in the early 2000s, providing foundational insights into their conserved fold. The initial structure was determined for the archaeal Sir2 homolog AfSir2 in complex with NAD+ in 2001, revealing the bipartite domain organization and NAD+-binding geometry. Subsequent structures included the bacterial sirtuin CobB from in 2004, confirming evolutionary conservation across domains of life, and the yeast homolog Hst2 (a Sir2 paralog) in ternary complex with and in 2002, which highlighted domain closure upon binding. For human sirtuins, the catalytic domain of SIRT1 was resolved in 2013, exposing allosteric sites in the N-terminal extension that influence conformational dynamics and activator binding, such as those targeted by small-molecule modulators. Within the cleft formed by the two subdomains, several residues are highly conserved across sirtuins and essential for . A key residue, such as His-116 in Sir2 or equivalent positions in homologs, serves as a general base to deprotonate the 2'-hydroxyl of the NAD+ , facilitating nucleophilic attack and subsequent to generate the ADP-ribosyl intermediate. An adjacent conserved aspartate, like Asp-101 in some structures, contributes to substrate positioning by forming bonds or salt bridges that orient the acetyl-lysine for transfer. These residues ensure precise coordination of NAD+ and transfer, underscoring the mechanistic uniformity of the sirtuin family.

Types of Sirtuins

Sirtuins in mammals are represented by seven isoforms, SIRT1 through SIRT7, which are distinguished by their subcellular localization, preferred substrates, and specialized functions within cellular processes such as and stress response. These isoforms share a conserved NAD⁺-dependent catalytic domain but exhibit diversity due to variable N- and C-terminal regions that dictate their targeting and specificity. SIRT1, the longest mammalian sirtuin isoform at 747 amino acids, primarily resides in the but can translocate to the under certain conditions. It functions as a deacetylase targeting histones to modulate structure, the tumor suppressor to influence , and PGC-1α to regulate and energy . SIRT2 is predominantly cytoplasmic, with nuclear translocation occurring during the G2/M phase of the . It deacetylates α-tubulin to affect stability and participates in cell cycle regulation by modulating and progression through G2/M checkpoints. SIRT3 localizes to the mitochondria, where it acts as a metabolic sensor by deacetylating key enzymes such as superoxide dismutase 2 () to enhance defense and isocitrate dehydrogenase 2 (IDH2) to boost NADPH production and reductive stress resistance. SIRT4 is also mitochondrial and exhibits activity rather than robust deacetylation, primarily targeting to inhibit and thereby regulate and insulin secretion. SIRT5 resides in the mitochondria and specializes in desuccinylation and demalonylation, modifying proteins involved in the such as synthetase 1 (CPS1) to support detoxification and metabolic . SIRT6 is nuclear and chromatin-associated, performing both deacetylation of at lysine 9 and mono- to promote pathways and maintain genomic stability. SIRT7 localizes to the and regulates rRNA processing and transcription by interacting with , influencing nucleolar function and .

Mechanisms of Action

Enzymatic Activities

Sirtuins function primarily as NAD⁺-dependent deacetylases, catalyzing the removal of from the ε-amino group of residues on target proteins. This reaction consumes one molecule of NAD⁺ per transferred, distinguishing sirtuins from other classes that do not require this cofactor. The core deacetylation mechanism proceeds in two steps: first, the acetylated attacks the C1' position of the in NAD⁺, releasing and forming a covalent acyl-peptidyl ; second, the 2'-hydroxyl group of the ADP- moiety attacks the , yielding the deacetylated protein and 2'-O-acetyl-ADP- as the modified byproduct. The overall reaction can be represented as: \text{Protein-Lys(acetyl)} + \text{NAD}^+ \rightarrow \text{Protein-Lys} + \text{[nicotinamide](/page/Nicotinamide)} + 2'\text{-O-acetyl-ADP-[ribose](/page/Ribose)} Beyond deacetylation, sirtuins display diverse acyl-processing activities tailored to specific family members. SIRT4 and SIRT6 exhibit ADP-ribosyltransferase activity, transferring the ADP-ribose moiety from NAD⁺ onto acceptor proteins or , often with weaker deacylase functions. SIRT5 specializes in desuccinylation, efficiently removing succinyl groups from residues, and also catalyzes demalonylation and deglutarylation. Additionally, SIRT1, SIRT2, SIRT3, and SIRT6 perform defatty-acylation, hydrolyzing long-chain fatty acyl modifications such as myristoyl and palmitoyl from side chains. Sirtuin enzymatic activity is tightly coupled to cellular NAD⁺ availability, which acts as an ; elevated NAD⁺ levels enhance , while depletion during nutrient scarcity inhibits activity to conserve energy. For SIRT1, the Michaelis constant (Kₘ) for NAD⁺ typically ranges from 20 to 100 μM, reflecting its sensitivity to physiological fluctuations in cofactor concentration.

Species Distribution and Evolution

Sirtuins represent an ancient family of NAD+-dependent enzymes with broad phylogenetic distribution across the domains of life, including , , and eukaryotes, reflecting their fundamental role in cellular since early evolutionary history. In prokaryotes, sirtuin homologs such as the bacterial CobB protein in function in metabolic regulation, including deacetylation of acetyl-coenzyme A synthetase to enhance utilization under . Archaeal sirtuins, often classified in IIIb or U1, are present in most and contribute to responses, though some lineages may lack them due to incomplete genome sequencing or secondary loss. In eukaryotes, sirtuin diversity arose through gene duplications, with yeast serving as a key model where the founding member Sir2 maintains genomic stability and regulates lifespan, complemented by three main paralogs—Hst1, , and Hst3—plus Hst4, totaling five homologs that collectively influence silencing and . Mammalian genomes expanded this repertoire to seven isoforms (SIRT1–SIRT7) via duplications traceable to the last common ancestor of vertebrates approximately 500 million years ago, enabling specialized functions in complex multicellular organisms. Non-mammalian eukaryotes illustrate conserved yet adapted roles; for instance, in the , the SIR-2.1 homolog promotes lifespan extension when overexpressed, dependent on the DAF-16/FOXO to modulate stress resistance and metabolism. Similarly, in the fruit fly , the dSir2 sirtuin in the regulates lipid metabolism and systemic insulin signaling, influencing organismal longevity under dietary variations. Evolutionary adaptations include the emergence of mitochondrial-localized sirtuins (SIRT3, SIRT4, and SIRT5) in metazoans, coinciding with the evolution of ; these class II and III members, absent in many like , fine-tune mitochondrial and management to support aerobic metabolism in higher animals.

Physiological Roles

In Cellular Metabolism

Sirtuins play a pivotal role in regulating cellular metabolism by sensing availability and modulating through NAD⁺-dependent deacetylation or deacylation of key metabolic enzymes and transcription factors. In particular, SIRT1, a and cytoplasmic sirtuin, deacetylates the transcriptional coactivator PGC-1α during fasting states, thereby enhancing its activity to promote in the liver and fatty acid β-oxidation in , which collectively support adaptive responses to deprivation. This deacetylation occurs at specific residues on PGC-1α in an NAD⁺-dependent manner, linking cellular energy status directly to metabolic . Mitochondrial sirtuins, such as SIRT3, further contribute to energy production by deacetylating enzymes involved in oxidative metabolism. SIRT3 activates through deacetylation, facilitating the conversion of to for entry into the tricarboxylic acid cycle and subsequent ATP generation via , particularly under conditions of high energy demand like . This regulation helps maintain mitochondrial function and bioenergetic efficiency, preventing the accumulation of acetylated, inactive forms of metabolic proteins. Caloric restriction, a dietary intervention that extends lifespan in various organisms, activates sirtuins by elevating NAD⁺ levels, mimicking fasting-induced catabolic shifts to enhance and glucose utilization while suppressing anabolic processes. Sirtuins integrate with other nutrient-sensing pathways, such as AMPK and , to coordinate metabolic adaptation; for instance, AMPK activation under low energy boosts NAD⁺ availability for SIRT1, which in turn inhibits signaling to favor over growth. This crosstalk ensures balanced responses to fluctuating nutrient levels, prioritizing energy conservation and . Recent investigations have highlighted SIRT5's involvement in ammonia detoxification, where it desuccinylates carbamoyl phosphate synthetase 1 (CPS1) in the , activating the enzyme to convert toxic into during metabolic stress. Studies from 2024 confirm that SIRT5 deficiency elevates CPS1 succinylation, impairing this process and leading to accumulation, underscoring SIRT5's role in mitochondrial .

In Aging Processes

Sirtuins play a central role in modulating aging processes across species by influencing lifespan extension and mitigating age-related decline through NAD⁺-dependent deacetylation activities. In model organisms, overexpression of Sir2, the yeast ortholog of mammalian sirtuins, has been shown to extend replicative lifespan in Saccharomyces cerevisiae by approximately 30% and chronological lifespan by up to 50%, primarily by promoting genomic stability and silencing extraneous ribosomal DNA circles. Similarly, in Caenorhabditis elegans, overexpression of the Sir2 homolog sir-2.1 increases mean lifespan by 30-50% via enhanced stress resistance and metabolic regulation, establishing sirtuins as conserved longevity factors. In mammals, sirtuin activation mimics the beneficial effects of caloric restriction, a well-established intervention that extends lifespan. , a compound, has been shown to partially recapitulate the effects of caloric restriction, potentially through indirect of SIRT1 activity via pathways such as AMPK, enhancing mitochondrial and insulin , thereby delaying age-related metabolic deterioration in mice. Conversely, SIRT6 deficiency in mice leads to accelerated aging phenotypes, including severe metabolic defects, lymphopenia, and premature death within weeks, underscoring SIRT6's essential role in maintaining systemic during aging. Key mechanisms underlying sirtuins' anti-aging effects include the maintenance of structure and suppression of chronic . SIRT1 and SIRT6 promote formation by deacetylating histones such as H3K9 and H4K16, which silences transposable elements and preserves genomic integrity against age-related instability. Additionally, sirtuins attenuate by deacetylating the subunit p65 () at lysine 310, reducing pro-inflammatory and preventing the chronic low-grade associated with aging. Recent advancements as of 2025 highlight therapeutic potential for sirtuin modulation in aging. As of 2025, novel activators of SIRT3, a mitochondrial sirtuin, are advancing toward clinical trials to address age-related mitochondrial dysfunction, showing promise in enhancing deacetylation of respiratory chain proteins to improve in elderly populations. Downregulation of SIRT1 has been implicated in ovarian aging, contributing to diminished and accelerated reproductive in mammalian models. Despite these insights, controversies persist regarding sirtuins' direct versus indirect contributions to . While genetic variations in sirtuin genes correlate with exceptional in cohorts, debates center on whether sirtuins primarily extend lifespan through direct enzymatic actions or indirectly via metabolic sensing pathways like caloric restriction mimicry. Further human studies are needed to resolve these discrepancies and validate sirtuins as viable targets.

In DNA Repair

Sirtuins play a crucial role in maintaining genomic stability by modulating key pathways, particularly through their NAD⁺-dependent deacetylase and ADP-ribosyltransferase activities. , a sirtuin, enhances (NHEJ) repair of double-strand breaks (DSBs) by deacetylating NBS1, a component of the MRN complex, thereby promoting its and activation at damage sites. Similarly, SIRT1 deacetylates Ku70, a core NHEJ factor, which increases its binding affinity to DNA ends and stimulates overall repair efficiency following genotoxic stress. These modifications ensure timely resolution of DSBs, preventing chromosomal aberrations. SIRT6 further contributes to DNA repair by facilitating (BER) and DSB repair through interactions with . Specifically, SIRT6 mono-ADP-ribosylates at lysine 521, activating its poly-ADP-ribosylation activity to recruit repair factors and promote relaxation for access during BER of oxidative damage. In DSB contexts, SIRT6 recruits the remodeler SNF2H to deacetylate H3K56, facilitating mobilization and repair factor assembly. Laboratory studies reveal that SIRT6-deficient cells exhibit markedly impaired DSB repair, with persistent γ-H2AX foci indicating roughly doubled levels of unresolved breaks compared to wild-type cells, underscoring SIRT6's essential role in genomic integrity. The NAD⁺ dependency of sirtuins couples to cellular metabolic states, as fluctuating NAD⁺ levels—derived from sensing and —directly influence sirtuin activity at damage sites. In , the ortholog Sir2 aids maintenance by deacetylating at lysine 56, compacting telomeric to suppress aberrant recombination and promote , a mechanism conserved in higher eukaryotes for preventing replicative instability. Recent advances highlight SIRT7's involvement in (HR) repair under replication stress, where it deacetylates K18 to modulate accessibility and factor recruitment, reducing fork stalling and collapse. This function positions SIRT7 as a key guardian against replication-associated DSBs, with implications for cancer susceptibility when dysregulated.

In Tissue Homeostasis

Sirtuins play a critical role in maintaining by counteracting fibrotic processes that disrupt (ECM) balance and integrity. Specifically, SIRT1 exerts anti-fibrotic effects in the lungs and kidneys by inhibiting transforming growth factor-β (TGF-β) signaling, which reduces excessive deposition and activation. In models, such as those induced by silica exposure, SIRT1 activation promotes deacetylation of Smad3, a key mediator of TGF-β/Smad signaling, thereby attenuating and fibrotic remodeling. Similarly, in renal fibroblasts, SIRT1 activation diminishes TGF-β1-induced production by suppressing Smad3 activity, preventing interstitial progression. Depletion of SIRT1 has been associated with enhanced fibrotic responses in tissues, underscoring its protective function in preserving pulmonary architecture. In the heart, SIRT3 contributes to tissue by mitigating oxidative stress-driven . SIRT3 attenuates (ROS) production in cardiomyocytes, thereby inhibiting TGF-β1 signaling and reducing fibrotic accumulation following angiotensin II stimulation. This protection involves SIRT3-mediated deacetylation of targets that suppress , maintaining cardiac tissue integrity against oxidative insults. At the mechanistic level, sirtuins regulate through targeted deacetylation of transcription factors like Smad3 and , which suppresses activation and overproduction. SIRT1 deacetylates Smad3 to disrupt its interaction with TGF-β receptors, inhibiting downstream fibrogenic in multiple tissues. Likewise, SIRT1 and SIRT3 promote deacetylation, reducing its and activity to prevent profibrotic signaling cascades that drive differentiation. Experimental models highlight these roles; for instance, SIRT1 depletion exacerbates bleomycin-induced by amplifying TGF-β/Smad-mediated collagen deposition and inflammatory responses. A 2025 study further revealed SIRT6's involvement in ovarian tissue , where its dysregulation promotes through ECM remodeling, potentially predisposing to tumorigenesis by altering stromal .

Roles in Disease

In Cancer

Sirtuins exhibit a dual role in cancer, functioning as both tumor suppressors and oncogenes depending on the cellular context, cancer type, and specific isoform involved. This ambivalence arises from their regulation of key processes such as , , and genomic stability, which can either inhibit or promote tumorigenesis. For instance, while certain sirtuins like SIRT6 suppress tumor initiation by modulating metabolic reprogramming, others such as SIRT7 can drive in specific malignancies. In their tumor-suppressive capacity, sirtuins counteract oncogenic pathways. SIRT1 deacetylates p53 at lysine 382, which in some contexts enhances p53's mitochondrial translocation and promotes apoptosis in response to DNA damage, thereby preventing cancer progression. Similarly, SIRT6 acts as a metabolic regulator by deacetylating HIF-1α and inhibiting the expression of glycolytic genes, thereby suppressing the Warburg effect—a hallmark of cancer cell metabolism that supports rapid proliferation. Loss of SIRT6 has been linked to increased tumor growth in models of pancreatic and colon cancer. Conversely, sirtuins can exhibit oncogenic properties. Loss of SIRT2 leads to centrosome amplification and mitotic aberrations, promoting genomic instability and tumorigenesis, as observed in mouse models of and . SIRT7 overexpression is prevalent in , where it inactivates the tumor suppressor p21 by deacetylating H3K18, facilitating cell cycle progression and tumor growth. In , a 2024 review highlights that SIRT1 upregulation correlates with advanced TNM stage, metastasis, and poor , partly through interactions with HIF-1α that enhance epithelial-mesenchymal transition and immune evasion. Therapeutically, targeting sirtuins exploits their dual nature, with context-dependent outcomes. SIRT1 inhibitors, such as EX-527, sensitize cancer cells to by restoring activity and overcoming resistance in colorectal and other cancers. This context-dependency underscores sirtuins' pro-survival role under cellular stress (e.g., ) versus pro-apoptotic effects in irreparable damage, guiding isoform-specific interventions to tilt the balance toward tumor suppression.

In Neurodegenerative Disorders

Sirtuins play protective roles in neurodegenerative disorders by modulating , mitochondrial function, and neuronal survival pathways, counteracting mechanisms of protein misfolding and in conditions such as (AD), (HD), and (PD). SIRT1, in particular, exerts neuroprotective effects through its deacetylase activity, which influences key pathological proteins in these diseases. In , SIRT1 deacetylates , promoting its degradation and reducing formation and aggregate accumulation in affected brain regions. Similarly, in , SIRT1 mitigates mutant toxicity by deacetylating downstream targets such as PGC-1α and TORC1, thereby enhancing and transcriptional responses that limit and neuronal damage. For , inhibition of SIRT2 has been shown to alleviate α-synuclein-mediated toxicity by increasing α-tubulin acetylation, which modifies inclusion body morphology and enhances , thereby rescuing dopaminergic neuron survival in cellular and animal models. Preclinical studies using transgenic mouse models, such as APP/PS1 mice that recapitulate amyloid-beta plaque formation and synaptic deficits in AD, demonstrate that SIRT1 overexpression rescues neuronal loss, reduces amyloid pathology, and improves cognitive performance by activating pathways like MAPK/ERK1/2 signaling. A parallel decline in NAD+ levels in aging brains further impairs sirtuin activity, exacerbating vulnerability to neurodegeneration through diminished deacetylation capacity and mitochondrial dysfunction. Emerging therapeutic strategies target sirtuin modulation for . For example, a computationally discovered SIRT3 activator, compound 5689785, nearly doubles SIRT3 activity under low NAD+ conditions and shows potential for enhancing mitochondrial protection and reducing in AD models, with plans announced for clinical trials in 2025.

In Cardiovascular Conditions

Sirtuins play critical roles in maintaining vascular integrity and cardiac function, with SIRT1 emerging as a key protector of endothelial cells. SIRT1 enhances endothelial (eNOS) activity through deacetylation at specific residues, thereby increasing (NO) production and promoting while reducing and in the vasculature. This mechanism underlies SIRT1's protective effects against , a hallmark of cardiovascular pathologies. Additionally, SIRT1 deacetylates FoxO1 transcription factors in cardiomyocytes, suppressing pathological by modulating related to and stress responses. Overexpression of SIRT1 has been shown to attenuate cardiac in pressure-overload models, highlighting its therapeutic potential in preventing maladaptive remodeling. SIRT3 contributes to cardioprotection by regulating mitochondrial function in cardiomyocytes. As a mitochondrial deacetylase, SIRT3 promotes biogenesis through of PGC-1α and upregulation of enzymes like , thereby mitigating oxidative damage during stress conditions such as ischemia. Deficiency in SIRT3 exacerbates (ROS) accumulation and impairs mitochondrial integrity, leading to increased cardiomyocyte and contractile dysfunction. These actions position SIRT3 as a guardian against oxidative insults that drive chronic cardiac injury. In atherosclerosis, SIRT1 deficiency accelerates plaque formation by impairing endothelial barrier function and promoting adhesion and accumulation in arterial walls. Studies in E-deficient mice demonstrate that heterozygous SIRT1 enhances lesion size and vulnerability, underscoring its anti-atherogenic role via regulation of and LXR pathways. Similarly, in ischemia-reperfusion injury, SIRT1 preconditioning—achieved through pharmacological activation or overexpression—reduces infarct size by up to 40% in murine models, primarily by activating FoxO-dependent antioxidants and inhibiting pro-apoptotic Bax translocation. This preconditioning effect preserves myocardial viability post-reperfusion, offering a basis for sirtuin-targeted interventions in acute coronary syndromes.

Regulation and Modulators

Natural Regulators

Sirtuins are a family of NAD+-dependent deacetylases whose enzymatic activity relies on (NAD+) as an essential cofactor, which binds to the catalytic domain and facilitates the deacetylation of substrates by transferring an to the ribose moiety of NAD+, producing and O-acetyl-ADP-ribose.00755-1) This cofactor requirement links sirtuin function directly to cellular NAD+ levels, which fluctuate in response to metabolic states and environmental cues. NAD+ biosynthesis occurs primarily through the salvage pathway, where (NAMPT) catalyzes the conversion of to (NMN), the rate-limiting step that is upregulated during cellular stress conditions such as nutrient deprivation or oxidative damage to replenish NAD+ pools and sustain sirtuin activity. This stress-induced elevation of NAMPT expression ensures that sirtuins, particularly SIRT1, can maintain metabolic by deacetylating key regulators like PGC-1α in response to energy deficits. Post-translational modifications further fine-tune sirtuin activity; for instance, (AMPK), a central , phosphorylates SIRT1 at serine 434, enhancing its deacetylase function and promoting interactions with substrates involved in stress resistance. This phosphorylation event integrates AMPK signaling with sirtuin pathways, amplifying adaptive responses to low-energy states without altering NAD+ dependency. In circadian regulation, SIRT1 deacetylates the core clock proteins CLOCK and BMAL1 within the , thereby modulating their transcriptional activity to entrain daily rhythms and coordinate peripheral clocks with environmental light-dark cycles.00837-4) By rhythmically influencing the status of these heterodimers, SIRT1 helps sustain oscillatory patterns essential for temporal control of . Dietary polyphenols, such as found in grapes and berries, indirectly enhance sirtuin activity by stimulating the NAD+ salvage pathway through upregulation of NAMPT expression, thereby increasing intracellular NAD+ availability without directly binding the enzyme. This mechanism allows polyphenols to boost SIRT1-mediated deacetylation in a nutrient-responsive manner, mimicking caloric restriction effects on pathways. A key regulatory feedback loop involves , the end product of sirtuin-catalyzed deacetylation, which acts as a noncompetitive by binding to the enzyme's catalytic site and promoting a base-exchange reaction that reverses deacetylation, thus preventing overactivation and maintaining balanced NAD+ consumption. This inhibitory feedback ensures that sirtuin activity scales with NAD+ levels, integrating it into broader cellular and metabolic networks.

Synthetic Inhibitors

Synthetic inhibitors of sirtuins are small molecules designed to suppress the enzymatic activity of these NAD+-dependent deacetylases, primarily by targeting the NAD+ or the substrate-binding cleft in a competitive manner. This inhibition disrupts sirtuin-mediated deacetylation of key proteins involved in cellular processes such as , stress response, and , holding potential for therapeutic applications in diseases where sirtuin hyperactivity promotes , including certain cancers. However, many inhibitors exhibit off-target effects, such as unintended interactions with other deacetylases or cellular pathways, which can limit their specificity and clinical translation. A prominent example of a selective SIRT1 is EX-527, also known as selisistat, which exhibits an of 38 nM against SIRT1 in cell-free assays and demonstrates over 200-fold selectivity relative to SIRT2. EX-527 functions by exploiting the unique NAD+-dependent mechanism of sirtuins, forming a covalent with the enzyme that blocks deacetylation activity. It advanced to clinical trials for , where it was tested in phase II studies for potential neuroprotective effects by modulating SIRT1's role in neuronal survival; however, development was discontinued in 2013 due to insufficient efficacy signals despite a favorable safety profile. For SIRT2, AGK2 serves as a selective with an of 3.5 μM, showing minimal activity against SIRT1 ( >30 μM) or SIRT3 ( >91 μM). In preclinical models of , AGK2 provides by reducing α-synuclein toxicity and inclusion formation in neurons, as well as attenuating microglial activation and through downregulation of pathways like AKT/FOXO3a and MAPK. This selectivity highlights AGK2's utility in targeting SIRT2's role in neurodegeneration without broadly affecting other sirtuins. Pan-sirtuin inhibitors, such as the nicotinamide analog sirtinol, target multiple isoforms including SIRT1 and SIRT2 with IC50 values in the micromolar range (e.g., 38-131 μM for SIRT1/2). Sirtinol competitively inhibits sirtuin activity by binding near the NAD+ site, leading to accumulation of acetylated substrates like p53 and promoting apoptosis in cancer cells; however, its lack of isoform selectivity contributes to off-target effects, including iron chelation and interference with unrelated cellular processes. Recent advancements include the development of selective SIRT1/2 inhibitors like the tenovins, a class of small molecules that inhibit both isoforms at low micromolar concentrations, inducing -dependent and reducing tumor growth in xenograft models of various cancers, including and gastric . Tenovins exemplify mechanism-based inhibition by stabilizing acetylated and disrupting sirtuin-mediated suppression of tumor suppressors. Recent patents have been filed for novel selective SIRT1/2 inhibitors targeted at cancer therapy. These compounds underscore the preclinical promise of sirtuin inhibition for , though challenges with selectivity and persist.

Synthetic Activators

Synthetic activators of sirtuins are small molecules designed to enhance the enzymatic activity of these NAD+-dependent deacetylases, primarily through allosteric mechanisms or by increasing availability, offering potential therapeutic avenues for age-related diseases. Unlike natural regulators, these compounds are pharmacologically optimized for selectivity and potency, with ongoing research addressing challenges in isoform specificity to minimize off-target effects. A more potent synthetic analog of natural activators, SRT2104, has advanced to clinical testing; in a Phase II randomized trial involving patients with moderate-to-severe , doses up to 2 g daily for 12 weeks demonstrated tolerability and modest improvements in disease activity, attributed to SIRT1-mediated effects. SIRT3 activators target mitochondrial deacetylation to mitigate and neurodegeneration. Emerging compounds, such as optimized SIRT3-selective molecules entering Phase I trials as of early 2025, show greater efficacy than NAD+ precursors like in boosting mitochondrial and reducing neuronal in preclinical Alzheimer's models. Pan-sirtuin activators often function indirectly by elevating NAD+ levels, the common cosubstrate for all sirtuins. (NR) and (NMN), as NAD+ precursors, increase cellular NAD+ pools by 2- to 5-fold in and tissues, thereby potentiating sirtuin activity across isoforms without direct binding, as evidenced in metabolic and studies. Recent advances from 2024-2025 highlight computational modeling for novel activators, including molecular studies showing quercetin binds SIRT1 with high affinity ( ≈ -26 kcal/mol), stabilizing the enzyme-substrate complex and suggesting potential for dietary-derived pharmaceuticals in neurodegeneration. Ongoing structural studies using cryo-EM and are guiding isoform-specific designs to improve therapeutic windows.