Sirtuins and the circadian clock interplay in cardioprotection: focus on sirtuin 1

Abstract

Chronic disruption of circadian rhythms which include intricate molecular transcription–translation feedback loops of evolutionarily conserved clock genes has serious health consequences and negatively affects cardiovascular physiology. Sirtuins (SIRTs) are nuclear, cytoplasmic and mitochondrial histone deacetylases that influence the circadian clock with clock-controlled oscillatory protein, NAMPT, and its metabolite NAD⁺. Sirtuins are linked to the multi-organ protective role of melatonin, particularly in acute kidney injury and in cardiovascular diseases, where melatonin, via upregulation of SIRT1 expression, inhibits the apoptotic pathway. This review focuses on SIRT1, an NAD⁺-dependent class III histone deacetylase which counterbalances the intrinsic histone acetyltransferase activity of one of the clock genes, CLOCK. SIRT1 is involved in the development of cardiomyocytes, regulation of voltage-gated cardiac sodium ion channels via deacetylation, prevention of atherosclerotic plaque formation in the cardiovascular system, protection against oxidative damage and anti-thrombotic actions. Overall, SIRT1 has a see-saw effect on cardioprotection, with low levels being cardioprotective and higher levels leading to cardiac hypertrophy.

Introduction

Circadian rhythms, conserved in almost all known species, are an adaptation to the external light–dark cycle and other cyclic cues and have probably evolved as such. An endogenous clock governs behavioral, physiological, and metabolic processes. In mammals, the master clock is located in the suprachiasmatic nucleus (SCN), a paired group of about 20,000 intrinsically rhythmically active neurons in rats that is located in the anterior hypothalamus. Rhythmic output from the SCN is maintained by transcription–translation feedback loops and rhythmic neuronal activity [1]. Photic and non-photic input fine-tune this endogenous rhythm, allowing the SCN to provide a regulatory output that adjusts circadian rhythms to a 24-h periodicity [2, 3].

Sirtuins, a family of seven proteins encoded by Silent Information Regulator (SIR) genes, play an extensive role in promoting survival during unfavorable conditions [4, 5]. Sirtuins control various cellular and molecular processes via deacetylation [6]. Sirtuins are either cytoplasmic or nuclear (SIRT 1,2,6,7), and can also be directed to migrate to the mitochondria (SIRT 3,4,5), based on the targeting sequences they contain [7–10]. The nuclear/cytosolic sirtuins control cellular processes through the deacetylation of histones, NF-κB, P53, Forkhead box protein O (FOXO) proteins, peroxisome proliferator-activated receptor γ (PPARγ) coactivator 1α (PGC-1α) and others [11], and some energy metabolic process controlled through deacetylase (SIRT3) [7], auto-ADP-ribosyltransferases (SIRT6) or mono-ADP-ribosyltransferases (SIRT4) activity [12]. Their deacetylating and ADP-ribosylation properties confer upon them the ability to regulate mitochondrial biogenesis, insulin sensitivity, glucose and lipid metabolism, and antioxidant enzyme activity [11, 13, 14], urea cycle, cell cycle, DNA repair and rDNA transcription [5, 15]. Sirtuins are also linked to the multi-organ protective role of melatonin, particularly in acute kidney injury and in cardiovascular diseases (CVDs). Melatonin upregulates the expression of SIRT1, which in turn induces the expression of antioxidant enzymes and inhibits the apoptotic pathway [7, 16]. Among sirtuins, SIRT1 plays an important role in cardiac function by deacetylating the Na⁺ channel protein Na_V1.5, thus controlling the inward depolarizing sodium ion current, cardiac electrical activity and contractions [17]. SIRT1 improves vascular relaxation through activating nitric oxide synthase in the endothelium and inhibiting cardiac potassium channels [18], reducing antioxidant load, and activating anti-inflammatory and anti-apoptotic pathways [19]. Sirtuins also regulate the molecular circadian clock. SIRT1 controls the expression of circadian genes by deacetylating them (e.g. BMAL1 and PER2), thus potentially regulating the molecular transcription–translation feedback loop of the circadian clock [20, 21].

This review focuses on the intimate and intricate correlation that seems to exist between the mammalian circadian rhythms and sirtuins, particularly SIRT1, in mediating protection against cardiovascular disorders.

The circadian clock and its function

Natural light is the strongest synchronizer (“zeitgeber”) among the environmental photic (natural/artificial light) and non-photic (food, behavioral arousal, etc.) cues [22, 23]. Photic entrainment of the endogenous master circadian regulator, the SCN, occurs through the retinohypothalamic tract, via neurotransmitters that act as messengers to control the differential expression of clock genes and clock-controlled genes within SCN cells, and influences the observable output in the form of physiology and behavior [24]. The intricate molecular mechanisms of the circadian clock are conserved across species and include intracellular transcription–translation feedback loops [25]. In mammals, the genes Clock and Bmal1 are transcribed to form the transcription factors CLOCK and BMAL1, which form dimers through basic helix-loop-helix domains. The dimer then induces transcription of two other genes, Per and Cry, to produce the proteins PER and CRY, which in turn dimerize, and then decrease their own production by inhibiting CLOCK and BMAL1 expression. PER and CRY degrade over time, thereby restarting this loop (Fig. 1) [26, 27].

Fig. 1.

The mechanism of the feedback loop of the circadian clock. Basic helix-loop-helix (bHLH)-PER-ARNT-SIM transcription factors BMAL1 and CLOCK of Positive arms in the core network activate the per1, per2, cry1 and cry2 genes. The mRNA of per1, per2, cry1 and cry2 genes converted into protein and phosphorylated by CK1ε/δ and translocate to the nucleus as heterodimer complex, which suppresses their own transcription by inhibiting BMAL1-CLOCK complex. BMAL1 is repressed and activated by the ROR and REV-ERBα, respectively. Parallelly the stability of period (PER) and cryptochrome (CRY) is regulated by E3 ubiquitin ligase pathway. BMAL1 aryl hydrocarbon receptor nuclear translocator-like protein 1, CLOCK circadian locomotor output cycle protein kaput, PER1/2 period circadian protein homolog 1/2, CRY1/2 cryptochrome 1/2, P phosphate molecule, ROR retinoic acid receptor-related orphan receptor, REV-ERBα encoded by Nr1d1, RORE ROR/ REV-ERB response element, E-box enhancer box, Ccgs clock-controlled genes, Ub ubiquitin, CK1 casein kinase 1, 26S 26S proteasome

Per and Cry mRNAs peak in the SCN during mid to the later part of the day in both nocturnal and diurnal mammals [28]. Bmal1 mRNA peaks at approximately midnight, while Clock is constitutively expressed in the SCN [28]. PER and CRY bind to the E-box element of the promoter regions in Bmal1, Clock, Rev-Erbα and other clock-controlled genes (CCGs) to inhibit their expression through binding to the CLOCK/BMAL1 complex [29, 30]. PER and CRY are translocated to the nucleus after phosphorylation by casein kinase 1 ε/δ (CK1ε/δ) [29, 31]. Another regulatory loop consisting of the nuclear receptor REV-ERB and RORα (retinoid-related orphan receptor-α) provides additional modulation. Bmal1 is downregulated by REV-ERBα and upregulated by RORα by binding to the RORE (response element-binding site) sequence of the promoter region in Bmal1 [32]. BMAL1 and CLOCK transcription factors form a heterodimer and upregulate the expression of Per and Cry through binding to the E-box in the promoter of Per and Cry [33, 34]. PER and CRY stability is regulated by the phosphorylation state and ubiquitylation through the E3 ligase complex resulting in proteasomal degradation [27]. The complex interaction between the core clock genes and other several clock-controlled genes is overtly expressed in the form of physiological and behavioral processes (Fig. 1).

Disruptions of the circadian rhythm are detrimental to health. Chronic jet lag and shift work are correlated with heart disease [35, 36], impaired memory [37–40], disruptions in the timed release of hormones [41], diabetes [42–44], cancer due to disruption of p53 [45, 46], impaired reproductive health [47, 48], metabolic disorders [49, 50], and sleep disturbance [51] (Fig. 2). Since the secretion patterns of hormones are under circadian control, disruptions in the circadian rhythm dramatically affect the hypothalamic–pituitary–gonadal (HPG) axis, hypothalamic–pituitary–thyroid (HPT) and hypothalamic–pituitary–adrenal (HPA) axis [52]. The altered secretion of reproductive hormones such as luteinizing hormone (LH), follicle stimulating hormone (FSH) and testosterone leads to poor fertility and fecundity [52]. Disruption of clock function affects the feeding behavior and thus changes the lipid and glucose metabolism. Therefore, the metabolic imbalance leads to metabolic diseases such as non-alcoholic fatty liver disease (NAFLD) and non-alcoholic steatohepatitis (NASH) and diabetes [53, 54]. It is now clearly understood that disruption of clock genes functions hastens tumorigenesis [55, 56]. Chronic disruption of the circadian clock affects the sleep architecture which disturbed the neurogenesis [57]. Modulation of the molecular constituents of the circadian rhythms by chronotherapies used to ameliorate the ill-effects of circadian rhythm disorders and diseases with a circadian correlate is an area that has received the continuous attention by scientists.

Fig. 2.

Pathological conditions associated with circadian rhythm disruption. The diagram is showing examples of how circadian rhythm disruption adversely influences brain function (cognition), heart physiology, reproduction, metabolism, hormone secretion and cancer, diabetes, and sleep. The arrows indicate up and down-regulation. NAFLD non-alcoholic fatty liver disease, NASH non-alcoholic steatohepatitis, IR ischemia/reperfusion injury, HPA hypothalamic–pituitary–adrenal, HPG hypothalamic–pituitary–gonadal, HPT hypothalamic–pituitary–thyroid, LH luteinizing hormone, FSH follicle stimulating hormone

Circadian rhythm in cardiac physiology

Like other organs, the heart and its components, including endothelial cells, vascular smooth muscle cells and the aorta, have their intrinsic clocks (peripheral clocks), controlled by the master clock present in the SCN [58]. Microarray analysis suggested that 8–10% of the transcriptome exhibits circadian rhythmicity in the heart and liver [59]. However, a recent study using a diurnal primate, the baboon (Papio anubis), suggests that such rhythms may be highly species or activity-type related. In a very thorough analysis of 64 tissues, including 22 brain regions [60], reported that up to 82% of the protein encoding genes exhibited day:night fluctuation and, moreover, the rhythms were tissue specific. Neurotransmitters (norepinephrine), pyruvate dehydrogenase kinase, uncoupling protein-3, glucose transporter 1 and 4 transcripts, muscle-specific glycogen synthase, and atrial natriuretic peptide (ANP) also exhibit circadian patterns of expression [61]. The deletion of extended Per3 tandem repeat region increases the heart rate in humans [62]. The selective deletion of PPARγ, which is a putative activator of BMAL1 in the vasculature, leads to reduced diurnal variation in heart rate [63].

The heart exhibits diurnal variations in both heart rate and blood pressure [61]. In the rat heart, re-entrainment of clock gene oscillations after a 12 h shift in the ambient light/dark schedule takes five to eight days, while heart rate and blood pressure take a day or two to resynchronize. This indicates that within an organ, separate groups of cells may have a distinct circadian organization, allowing differential resynchronization of separate cellular subpopulations. After shifting of the light/dark cycle, the rate of resynchronization of the clock becomes tissue/organ-specific [64].

Acute and chronic circadian misalignment by jet lag or shift work leads to increased mean arterial pressure, decrease in sleep efficiency, and a complete inversion of the cortisol profile increasing the risk of developing hypertension [36]. Bmal1-knockout (KO) mice developed multiple organ dysfunction including dilated cardiomyopathy. Aged cardiomyocyte-specific Bmal1-KO mice exhibited reduced ejection fraction leading to heart failure [65]. Thus, circadian rhythms and cardiac functions are intricately connected, and disruptions of the circadian system lead to deleterious effects in the cardiac system.

Sirtuins: an overview

The first sirtuin gene, Sir2, was discovered in the yeast Saccharomyces cerevisiae [66, 67]. This is a highly conserved gene with its presence observed in species from bacteria to mammals [68]. Sirtuins are NAD-dependent deacetylating (HDAC) type-III enzymes [68, 69]. There are seven phylogenetically distinct members of the sirtuin protein family in mammals, SIRT1-SIRT7. These proteins are found in different subcellular locations, including the nucleus, cytosol, nucleolus and mitochondria [70]. SIRT1 is primarily nuclear in location but is also found in the cytoplasm under specific pharmacological circumstances [71, 72]. SIRT2 is a cytosolic protein and is located in the nucleus during the G2 to M phase transition of the cell division cycle [73]. SIRT3, 4, and 5 are found in mitochondria [74], SIRT6 is nuclear in distribution [75], and SIRT7 is usually found in the nucleolus [76]. Sirtuins have multiple functions in the regulation of physiological processes. SIRT1 has a role during heart development [77] and protects against different pathological conditions such as autophagy [78], age-related heart disease [79], atherosclerosis [80], and IR injury [81]. SIRT3 [82], SIRT5 [83], and SIRT6 [84] also having protective roles in IR injury. Similarly, SIRT1-SIRT7 having different roles in cardiac hypertrophy [85–91] and SIRT2 [92] and SIRT3 [93] having protective functions in atherosclerosis and heart failure, respectively. They interact with different substrates, and their upregulation and downregulation may contribute to the causal mechanisms of different diseases and syndromes, summarized in Table 1. All sirtuins have varied and essential roles at the cellular level, including cell differentiation of mesenchymal stem cells by deacetylation of different target proteins such as histone-H3K9 and non-histone-β-catenin [94, 95], Runx2 [96] and, PGC-1α [97].

Table 1.

The localization, biological functions, and associated diseases with different classes of sirtuins

Sirtuin	Class	Location	Role in cardiovascular system	References
SIRT1	I	Nucleus, cytosol	Developing heart	[71]
			Autophagy	[72]
			Age related cardiac diseases	[73]
			Atherosclerosis	[74]
			IR injury	[75]
			Cardiac hypertrophy	[79]
SIRT2	I	Cytosol	Atherosclerosis	[86]
			Cardiac hypertrophy	[80]
SIRT3	I	Mitochondria	Cardiac hypertrophy	[81]
			Heart failure	[87]
			IR injury	[76]
SIRT4	II	Mitochondria	Cardiac hypertrophy	[82]
SIRT5	III	Mitochondria	Cardiac hypertrophy	[83]
			IR injury	[77]
SIRT6	IV	Nucleus	IR injury	[78]
			Cardiac hypertrophy	[84]
SIRT7	IV	Nucleolus	Cardiac hypertrophy	[85]

SIRT1: a multifunctional protein

SIRT1 controls gene transcription by deacetylating both histone and non-histone targets [68]. Non-histone targets include P53 [98, 99], FOXO transcription factors and NF-kappa B [100–102], thus regulating stress responses, inflammation, cellular senescence and apoptosis [100, 101]. SIRT1 activity is dependent on the cofactor NAD⁺, and therefore it was initially considered to be an NAD⁺-dependent histone deacetylase [21, 99]. SIRT1 is involved in controlling glucose tolerance by overexpression of sirt1 which increases insulin sensitivity through deacetylating PGC-1α (a transcriptional coactivator) which controls glucose homeostasis at the transcriptional level [103, 104]. Overexpression of Sirt1 in the offspring of mice on a high-fat diet (C57BL/6) attenuates insulin resistance, increases glucose tolerance, prevents liver steatosis, and reduces ROS overproduction [105]. SIRT1 also participates in lipid metabolism by deacetylating the cholesterol-sensing nuclear receptor liver X receptor (LXR) which promotes ubiquitination, and regulating metabolism-controlling transcription factors (such as PPAR-α) and its coactivator (PGC1α) which are involved in metabolic adjustment [106–108].

Several studies show that SIRT1 has diverse roles in different signaling pathways involved in development [109], cognition impairment, heart disease, aging, cancer, and energy homeostasis, including lipid and glucose homeostasis [110]. SIRT1 has a potential role in the development of the hippocampus and cerebral cortex during both embryonic development (E) and postnatal stages (PN), since it is expressed differentially on embryonic day 18 (E18). The RNA or protein of other sirtuins are also expressed differentially in the brain during E18 (Sirt2, Sirt5) and between PN7 to PN21 (SIRT3) [111]. This differential expression in specific cellular lineages of the developing brain is a potential target for cellular subtype-specific strategies that seek to address developmental disorders.

SIRT1 may prevent neurodegenerative disease and reportedly increases lifespan [112]. In Alzheimer's disease, the upregulation of SIRT1 attenuates the elevation of β-amyloid deposition [113]. Overexpression of SIRT1 also has a beneficial role in Huntington and Parkinson diseases by reducing the acetylation of the SIRT1 substrate, FoxO3a, and suppressing the aggregation of α-synuclein by preventing the misfolding of α-synuclein protein respectively [114, 115]. Overexpression of SIRT1 maintains the thermal induction of heat shock protein (Hsp70) which is required for the proper folding of α-synuclein protein [115].

Additionally, SIRT1 has a complex role during aging. While SIRT1 levels increase during aging as a countermeasure against increased oxidative stress, its deacetylating, and consequently, gene-transcription regulating roles in decreases [116]. This is because deacetylation depends on additional post-translational changes such as sumoylation, methylation, nitrosylation, and phosphorylation, which may be differentially affected during aging [117].

Apart from these key roles, SIRT1 has an important role in the regulation of circadian rhythms through its interaction with core clock genes in the molecular mechanism of the endogenous clock. It thus forms a regulatory bridge between the metabolic and physiological processes and circadian rhythms [20, 21].

SIRT1 and the circadian clock

SIRT1 influences the circadian clock in both the brain and in peripheral tissues [118]. Sirt1-deficient mice exhibit alterations in the expression patterns of per1, per2, cry1, and cry2 circadian genes. Sirt1 and Per2 repress each other [119]. In the molecular loop of the circadian clock, CLOCK acetylates BMAL1 to upregulate the transcriptional activity of the CLOCK-BMAL1 heterodimeric complex, while BMAL1 is deacetylated on E-box by SIRT1 [21]. In the liver, PER2 is deacetylated and degraded by SIRT1. SIRT1 also binds to the CLOCK-BMAL1 complex in a circadian manner [20, 21], thus regulating circadian rhythms. The molecular circadian clock is, therefore, influenced by the acetylation and deacetylation of its components. In the SCN, NMDA receptors activate the histone acetyltransferase activity of CLOCK-BMAL1 complex, which helps in histone modification [120].

Sirtuins and circadian clock proteins together control oxidative metabolism, via responses to NAD⁺ and NADH. The heterodimer CLOCK-BMAL1, apart from activating the transcription of the clock genes Per and Cry, and other clock-controlled genes also control the activity of the gene, nicotinamide phosphoribosyltransferase (Nampt), which encodes the rate-limiting enzyme nicotinamide phosphoribosyltransferase, whose metabolite is NAD⁺. NAD⁺ synthesis exhibits a distinct circadian rhythm due to an oscillation in the levels of NAMPT. Distribution of nicotinamide adenine dinucleotide (NAD⁺) in the cytosol, nucleus and mitochondria maintain the cellular redox status, which in turn is essential for the normal functioning of the bioenergetic enzymatic machinery. The NAD⁺/NADH ratio varies among species, cell types and metabolic state, usually between 2–10:1. SIRT1 is sensitive to the cellular redox state as indicated by this NAD⁺/NADH ratio, particularly related to NADH. SIRT1 activity is upregulated by NAD⁺ and downregulated by NADH and nicotinamide (NAM) [121]. SIRT3, 4, and 5 also respond to changes in the NAD⁺/NADH ratio to modulate mitochondrial metabolic functions [122]. It has very recently been discovered by Levine and his Associates [123] that nicotinamide riboside (NR), the precursor to NAD⁺, can differentially affect the transcriptome, leading to de novo oscillatory expression (505 genes) or phase sifts in peak of expression (120 genes) in at least 625 genes and the suppression of oscillatory expression in 538 genes that previously had peak expression in the early subjective day or night. In the same study, NAD⁺ and SIRT1 were also found to control PER2 acetylation and, subsequently, the formation and dissociation of the PER-CRY heterodimer, leading to control of the repression of BMAL1 transcription. Thus, the core clock repressor proteins are controlled by NAD⁺ and SIRT1. In this study, SIRT1 knockout led to reduced levels of nuclear PER1, CRY1 and CRY2, but increased levels of nuclear PER2. In an advancement from earlier studies (20, 21), this study conclusively proves that NAD⁺ and SIRT1 drive circadian transcription through BMAL1 and PER2, and are not merely associated with and controlled by the circadian clock. Further, this study found an interesting development in aged mice: supplementation with NR led to restoration of BMAL1 chromatin binding, transcriptional oscillations, mitochondrial respiration rhythms and nocturnal activity. Thus, controlling the NAD⁺ SIRT1 pathway has implications in rescuing age-related dampening of the rhythms in the core circadian clock.

Since SIRT1 activity depends on the NAD⁺/ NADH ratio, the interaction between the CLOCK-BMAL1 complex and Nampt influences SIRT1 levels. In turn, SIRT1 controls the activity of CLOCK-BMAL1 complex and its interaction with the PER-CRY heterodimer [124], deacetylating BMAL1, PER2 and histone H3 at Lys9/lys14 [125]. This acetylation (CLOCK-dependent) and deacetylation (SIRT1-dependent) of histones has been linked to stable circadian clock function and is further controlled by several factors, including N-methyl-d-aspartate receptors (NMDAR)-dependent CLOCK histone acetyltransferase activity, mitogen-activated protein kinase, PKA, and PKC [120]. This evidence indicates that an intricate system of regulators, of which SIRT1 is an integral part, modulates the stability of the molecular circadian clock via multiple pathways.

Melatonin, an important rhythmic circadian neuroendocrine signal, modulates SIRT1 activity (Fig. 3) [126, 127]. Melatonin acts as a light-dependent interface between circadian clocks and multiple response pathways in the body. The SCN have melatonin receptors [128], and melatonin phase-shifts the circadian clock robustly [129], reducing PER1 and CLOCK, but not BMAL1 expression via the MT1 receptor [130]. An interconnected pathway of regulation thus exists between melatonin, circadian clocks and SIRT1. Experiments in prostate and human osteosarcoma cell lines have shown that melatonin inhibits SIRT1 signaling and expression, reducing its role in cell proliferation [131, 132]. The cardioprotective effect of melatonin during ischemia/reperfusion specifically involves SIRT1 signaling in antioxidant response pathways. Deacetylation by SIRT1 leads to the activation of forkhead box O1 (FOXO1), which in turn synthesizes the antioxidant enzymes manganese superoxide dismutase (MnSOD) and catalase. The presence of acetylated FOXO1 (Ac-FOXO1) promotes apoptosis. In a melatonin-treated group of myocardial ischemia/reperfusion (I/R) rats, SIRT1 and Ac-Foxo1 expression were significantly increased, and decreased, respectively, compared to the I/R + vehicle group where SIRT1 expression was reduced and Ac-FOXO1 expression was elevated significantly. In the same study, melatonin treatment ultimately increased the expression of the antiapoptotic gene Bcl-2, via upregulating SIRT1 and consequently reducing Ac-FOXO1. Thus, melatonin harnesses SIRT1 to reduce oxidative stress and inhibit apoptotic pathways in myocardial I/R [133].

Fig. 3.

Overview of signaling pathways and the protective function of SIRT1. SIRT1 directly interacts with core clock protein, which maintained the circadian rhythmicity. It reduced the oxidative stress via synthesizing antioxidant enzymes and maintained physiological homeostasis. Melatonin and resveratrol exert positive effects on SIRT1. CLOCK circadian locomotor output cycle protein kaput, + indicates positive effect, −AC deacetylation, BMAL1 aryl hydrocarbon receptor nuclear translocator-like protein 1, SIRT1 NAD-dependent protein deacetylase sirtuin-1, p53 tumor protein P53, NF-kB nuclear factor kappa-light-chain-enhancer of activated B cells, PGC-1α Peroxisome proliferator-activated receptor gamma coactivator 1-alpha, FOXO Forkhead box protein O, Bcl-2 B-cell lymphoma 2, Mn-SOD manganese superoxide dismutase

SIRT1 is also the effector behind melatonin’s protective role in renal function in severely burned rats through alleviating oxidative stress, controlling inflammatory responses, and by inhibiting apoptotic pathways [16]. SIRT1 is involved in the protective role of melatonin in sepsis after cecal ligation and puncture (CLP) in a C57BL/6J mice model [134]. Septic encephalopathy increases the load of neuroinflammatory and oxidative stress, which is alleviated by melatonin [135, 136]. A SIRT1 inhibitor (EX527) abolished this effect, suggesting that melatonin’s positive effect was via SIRT1 [134].

Other sirtuins and the circadian clock

Separately from the role of SIRT1, other sirtuins interact with the circadian clock to control metabolic pathways in the liver. SIRT6 has recently been found to regulate, in a circadian-dependent manner, a subset of genes distinct from those regulated by SIRT1. Particularly, fatty acid synthase, 3-hydroxy-3-methyl-glutaryl-CoA reductase, and lanosterol synthase were uniquely affected by SIRT6 knockout, suggesting that SIRT6 has a distinct role to play in fatty acid metabolism [137] by interacting with CLOCK and BMAL1. SIRT3, particularly during fasting, has a role to play in mitochondrial biogenesis by regulating fatty acid oxidation, dependent on NAD⁺ levels [138]. Thus, different sirtuins interact differentially with the circadian clock via regulation of the transcription–translation feedback loop through temporally differentiated histone-mediated exposure of the transcriptome.

SIRT1 function in cardiac physiology

Resveratrol (RSV; 3,5,4′-trihydroxystilbene) is a natural phytoalexin [139], found in red grape skins, red wine, and peanuts [140]. RSV induces SIRT1 activity and subsequently improves prognosis in cardiovascular diseases, neurological disorders, cancer, and aging [140]. RSV improves cardiac function after ischemia–reperfusion injury, decreases insulin resistance [141], and improves mitochondrial function. It also prevents the upregulation of the transcriptional activator and acetylator p300, reducing cardiomyocyte hypertrophy in cardiac dystrophy [85].

SIRT1 is predominantly present in the nucleus but can also be found in the cytosol. During stress conditions in cardiomyocytes, it shuttles between the nucleus and cytoplasm [71]. Its role in cardiovascular disease [142] is well-known. SIRT1 is expressed in the mammalian heart and modulates cellular processes by deacetylating histones and non-histone proteins [143]. The Sirt1 gene exhibits higher expression during embryonic days 10–12 during mouse embryonic development, suggesting a role in heart morphogenesis [109]. Binucleation of cardiomyocytes occurs as a transitory state during the differentiation and late fetal development of the heart. A recent study demonstrated the role of SIRT1 in retinoic acid (RA)-induced binucleation in H9c2 cells [77]. SIRT 1 inhibition by nicotinamide (NAM) in turn reduces RA induced binucleation. Thus, SIRT1 has a role to play in both the development and maintenance of the heart [77].

SIRT1 deacetylates voltage-gated cardiac Na⁺ channels (Na_v 1.5), which regulate normal cardiac electrical activity [17]. Sirtuins participate in the regulation of energy metabolism via controlling mitochondrial activity [144–146]. This is important in fine-tuning cardiac energy utilization during starvation [147], physiological and pathological conditions such as exercise, lack of nutrients or fasting, hypertension, and acute ischemic preconditioning, where SIRT1 is upregulated [142]. However, SIRT1 is downregulated during I/R injury. This upregulation and downregulation of SIRT1 reflects the cellular energy demand (metabolism) and redox balance [142], which aid the process of healing after I/R.

SIRT1 exhibits a see-saw effect in cardioprotection. Low (2.5) to moderate (7.5) levels of SIRT1 overexpression are cardioprotective and support the synthesis of antioxidant enzymes and inhibition of apoptotic pathways. Higher levels (12.5) of SIRT1 overexpression lead to cardiomyopathy due to elevated oxidative stress, less than optimal ATP, mitochondrial dysfunction and reduced expression of PGC-1α in cardiac tissue [116]. SIRT1 may cause cardiac hypertrophy by activating the Akt (Protein Kinase B) pathway. Akt (serine/threonine protein kinase), and its upstream kinase PDK1 (Pyruvate Dehydrogenase Kinase 1), are usually kept inactive via acetylation by acetyltransferase p300. SIRT1, through its greater deacetylating activity at higher levels of expression, allows activation of Akt and PDK1, facilitating the binding of phosphatidylinositol (3,4,5)-trisphosphate (PIP3) to the plasma membrane through its pleckstrin homology (PH) domain. PDK1 phosphorylates and activates Akt. This ultimately leads to activated-Akt-mediated hypertrophy of the heart [148].

An important function of SIRT1 in cardioprotection is its ability to mitigate atherosclerosis. One of its prominent effects is to reduce macrophage foam cell formation. Blood monocytes attracted by activated endothelial cells migrate to the vascular wall and develop into macrophages. These macrophages express scavenger receptors for oxidized low-density lipoproteins (oxLDL), such as Lox-1, which they use to consume oxLDL molecules and thus transform into foam cells which are major components of atherosclerotic plaques [149]. SIRT1 deacetylates RelA/p65-NF-kB in macrophages, downregulating Lox-1 expression, and in turn reducing foam cell formation. Additionally, SIRT1 influences Liver X-receptor (LXR) levels, helping move cholesterol out of plaque macrophages via ABCA1-driven reverse transport of cholesterol. SIRT1 also has an anti-thrombotic effect since it suppresses the expression of endothelial tissue factor (coagulation factor III) [149].

SIRT1 also provides cardioprotection from an ROS overload, inflammation and cardiomyocyte apoptosis. SIRT1 indirectly regulates Ca⁺ concentration in the cardiac sarcoplasmic reticulum through its action on the Ca⁺ channel known as sarco-endoplasmic reticulum Ca⁺-ATPase (SERCA2a). During heart failure (HF), acetylation at K492 increases by p300, downregulating SERCA2a activity. This acetylation by p300 is prevented by overexpression of SIRT1 by β-lapachone (a metabolic activator), which results in the restoration of the activity of SERCA2a. Thus, SIRT1 contributes in a variety of ways to enable cardioprotection, and is an attractive target in the treatment of cardiac disease. For example, timed administration of SIRT1 after heart failure or myocardial infarction might possibly lead to reduced cardiomyocyte loss and cardioinflammation, along with preventing further thrombosis, leading to better recovery from I/R injury and a better outcome [150].

Conclusion

In this review, we present three lines of evidence that suggest the involvement of sirtuins with cardiovascular health. First, sirtuins form key components of a system that regulate physiological processes at the time of reduced food availability. In particular, SIRT1 is a multifunctional protein that interfaces with histones to regulate gene transcription. Sirtuins regulate and are regulated by the circadian system. Second, insults to the circadian system, such as those experienced during circadian rhythm disruption produced by shift work and jet lag, may produce metabolic and cardiac imbalances, which might be addressed by sirtuins, since one of the major functions of SIRT1 is cardioprotection (Fig. 3). Third, both the circadian system and sirtuins contribute to cardiac well-being in a complex interplay of regulatory systems, chiefly by regulating metabolic and cell death/survival genes. We highlight the possibility of the regulation of cardiac physiology by the circadian system via sirtuins. Given that the NAD⁺/SIRT1 pathway controls the core circadian repressors, and NR supplementation can rescue dampened rhythms in aged mice, and also that SIRT1 has excellent cardioprotective capacity, it may be predicted that controlling the SIRT1 pathway may clinically help with age-related problems in circadian rhythms and cardiac health. Examining this pathway may reveal a way to reduce the ill-effects of circadian rhythm disruption on the cardiac system using sirtuins as a modulatory target. Controlled and timed administration of SIRT1 along with NR may be a useful strategy in cardioprotection and treatment of cardiac disease, leading to better recovery from I/R injury and an improvement in longevity through the modulation of inflammatory and apoptotic pathways in cardiomyocytes. There is justification for future basic and clinical research to investigate these possible interactions and treatment strategies.

Author contributions

SKS, PB, MS, SRP, DPC, RJR: concept development and study design; SKS, PB, MS: data acquisition, analysis, interpretation and preparation of the manuscript; RS, SRP, DPC, RJR: critical revision of the manuscript. SKS, PB: table and figure preparation; MS: funding acquisition and supervision. All authors read and approved the final version of the manuscript.

Compliance with ethical standards

Conflict of interest

The authors have read the journal’s policy and have the following potential conflicts: SRP is a stockholder and the President and Chief Executive Officer of Somnogen Canada Inc., a Canadian Corporation (non-financial relationship). He receives occasional royalties from Springer. He declares that he has no competing interests that might be perceived to influence the content of this article. This does not alter the authors’ adherence to all the journal policies. All remaining authors declare that they have no proprietary, financial, professional, nor any other personal interest of any nature or kind in any product or services and/or company that could be construed or considered to be a potential conflict of interest that might have influenced the views expressed in this manuscript.