Original Publication: Chang H-C, Guarente L. SIRT1 and other sirtuins in Metabolism. Trends in endocrinology and metabolism: TEM. 2014;25(3):138-145. doi:10.1016/j.tem.2013.12.001.
- Sirtuins respond to energy level changes and execute salutary effects resembling calorie restriction (CR)
- Sirtuins mediate CR effects in various cellular compartments and are crucial metabolic regulators in multiple tissues.
- Small molecules that enhance sirtuin activities, including CR mimetics and NAD+ precursors, are promising strategies to ameliorate age-related diseases.
Sirtuins such as SIRT1 are conserved protein NAD+-dependent deacylases and thus their function is intrinsically linked to cellular metabolism. Over the past two decades, accumulating evidence has indicated that sirtuins are not only important energy status sensors but also protect cells against metabolic stresses. Sirtuins regulate the aging process and are themselves regulated by diet and environmental stress. The versatile functions of sirtuins including, more specifically, SIRT1 are supported by their diverse cellular location allowing cells to sense changes in energy levels in the nucleus, cytoplasm, and mitochondrion. SIRT1 plays a critical role in metabolic health by deacetylating many target proteins in numerous tissues, including liver, muscle, adipose tissue, heart, and endothelium. This sirtuin also exerts important systemic effects via the hypothalamus. This review will cover these topics and suggest that strategies to maintain sirtuin activity may be on the horizon to forestall diseases of aging.
Sirtuins: indispensable energy sensors
Sirtuins are class III histone deacylases that consume one molecule of NAD+ (see Glossary) during each deacylation cycle . The first identified sirtuin protein was silent information regulator 2 (SIR2) from Saccharomyces cerevisiae. SIR2 was originally characterized as a chromatin-silencing component that repressed gene transcription at selected loci . Soon after the discovery that SIR2 extended the replicative lifespan of yeast [3, 4], the orthologs of SIR2 were proposed to carry out the same lifespan-prolonging effects in Caenorhabditis elegans [5, 6] and in Drosophila melanogaster , and to mediate beneficial effects of calorie restriction (CR) on health and longevity[7, 8, 9, 10]. These findings were challenged in 2011 by a study suggesting that SIR2 orthologs in worms and flies did not mediate increases in lifespan . As discussed in the next section, more recent studies in many organisms have now confirmed the original hypothesis that sirtuins are conserved, diet-sensitive, antiaging proteins.
In mammals, the antiaging functions of sirtuins are conserved [12, 13]. There are seven mammalian sirtuins, SIRT1–7, which function to regulate metabolism in nonredundant ways in many tissues. Because sirtuins are located in distinct cellular compartments, they can coordinate cellular responses to CR throughout the organism. SIRT1, SIRT6, and SIRT7 are localized in the nucleus, where they function to deacetylate histones thereby influencing gene expression epigenetically . SIRT1 also deacetylates specific transcription factors and enzymes to influence their activities, as described below. SIRT2 was originally described as a cytosolic sirtuin; however, recent data show that SIRT2 is also found in the nucleus where it functions to modulate cell cycle control [15, 16, 17]. SIRT3, SIRT4, and SIRT5 are localized in mitochondria, and regulate the activities of metabolic enzymes and moderate oxidative stress in this organelle . In general, SIRT3–5 respond to CR by switching cells to favor mitochondrial oxidative metabolism, along with the induction of accompanying stress tolerance.
In this review, we focus our attention on SIRT1, the most studied sirtuin, but also touch briefly on other mammalian paralogs of SIRT1. We focus on the metabolic functions of SIRT1 and other sirtuins in critical tissues to mediate physiological adaptability to diets. We also discuss briefly some of the challenges and controversies that have emerged about the role of sirtuins in CR, and critically assess new findings that have begun to resolve these differences. Although we will not cover the large body of data on sirtuins and diabetes and neurodegenerative diseases, we will address the relationship between sirtuins and cancer. Finally, we will consider emerging findings on the importance of the sirtuin co-substrate NAD+ in aging and disease.
The evolving role of sirtuins in CR and aging
The finding that sirtuins are NAD+-dependent deacetylases  prompted the suggestion that they helped mediate the effects of CR in an active process. This idea contrasted with earlier proposals that CR extended lifespan by passive mechanisms, such as lowering the production of reactive oxygen species. In model organisms, nutrient limitation was shown to extend the lifespan via sirtuins in yeast, Drosophila, and C. elegans . However, some laboratories observed lifespan extension by nutrient limitation that was independent of SIR2 orthologs [11, 20, 21]. Part of the difficulty in interpreting these data is that laboratories may use a variety of protocols to limit nutrients. Another potential problem is differences in strain backgrounds among laboratories. Because several other nutrient sensors besides sirtuins exist, such as insulin signaling , target of rapamycin (TOR) , and AMP-activated protein kinase (AMPK) , varied experimental conditions between different laboratories may activate different nutrient-sensing pathways. In the lower organisms, it therefore seems extremely likely that multiple pathways, including sirtuins, can elicit the benefits of nutrient limitation. In mice, the same murine strains are used under the same limitation of food of roughly the same composition. The lines of evidence that sirtuins mediate effects of CR in mammals are numerous and are outlined below.
First, the non-histone proteins targeted for deacetylation by sirtuins closely define those pathways involved in metabolic adaptation to CR, for example oxidative metabolism in mitochondria (Figure 1) . Second, CR induces the expression of the sirtuins: SIRT1 , SIRT3 , and SIRT5  in mice and SIRT1 in humans . Conversely, a high-fat diet can trigger the loss of SIRT1 in mice via proteolysis , and obesity can reduce the expression of SIRT1 in humans [30, 31]. Third, loss of function mutations in specific sirtuin genes can reduce specific outputs of CR. For example, SIRT1 knockout mice do not show the usual increase in physical activity induced by this diet . In addition, brain-specific knockout of SIRT1 in mice does not show the characteristic changes in the somatotropic axis [growth hormone/insulin-like growth factor 1 (IGF-1)] induced by CR . Most revealingly, SIRT1 knockout mice do not live longer on a CR diet [34, 35]. As for other sirtuins, knocking out the mitochondrial SIRT3 prevents the protective effect of CR against neuronal degeneration, leading to hearing loss . In this case, SIRT3 is required to reduce oxidative damage in crucial hair cell neurons of the cochlea. Deletion of the mitochondrial SIRT5 prevents the upregulation of the urea cycle, which is required to reduce blood ammonia when amino acids serve as energy sources . SIRT3 null mice also show a defect in regulation of the urea cycle .
A number of reports have also demonstrated that overexpression of SIRT1 in transgenic mice can mitigate disease syndromes much like CR, including diabetes, neurodegenerative diseases, liver steatosis, bone loss, and inflammation [38, 39, 40, 41, 42]. Tissues responsible for these effects are shown in detail in Figure 1. Conversely, compromised sirtuin activity contributes to metabolic syndrome and diabetes, and exacerbates the effects of a high-fat diet in mice and humans [29, 43]. In addition, SIRT1 activators like resveratrol  and newer sirtuin-activating compounds (STACs)  exert effects that are similar to those of CR, as revealed by measures of whole animal physiology  or by transcriptional profiling . Importantly, a recent study strongly suggests that the effects of resveratrol and all 117 described STACs are direct and not artifacts of assays using fluorophore-containing peptides. STACs target an allosteric site in SIRT1, which is separate from the catalytic domain, and thus activate the deacetylation of substrates containing lysines with nearby hydrophobic residues . In toto these data comprise a compelling set of evidence that suggests sirtuins are fundamentally involved in mediating effects of CR.
Although a paper by Burnett et al.  did not find lifespan extension in SIR2 ortholog transgenic lines of C. elegans or Drosophila, many other studies have established such a connection [5, 6, 7, 10, 49, 50, 51]. During the past year, new studies were reported confirming the importance of SIR2 orthologs in slowing aging and extending lifespan in yeast , C. elegans [53, 54], and Drosophila . Extension of murine lifespan has also been reported for transgenic lines of SIRT6  or SIRT1 . Thus, there is also compelling evidence that sirtuins regulate aging, which is consistent with their important role in CR.
Metabolic regulation in the liver
Whole body glucose homeostasis is critically regulated by the liver. When blood glucose levels are low, due to fasting or CR, hepatic metabolism immediately shifts to glycogen breakdown and then gluconeogenesis to ensure glucose supply and ketone body production to bridge energy deficits. Fasting also activates muscle and liver oxidation of fatty acids produced by lipolysis in white adipose tissue (WAT). Several transcription factors are involved in a sophisticated switch to adapt to energy deprivation, and SIRT1 mediates the metabolic switch during fasting (Figure 1) . During the initial (post-glycogen breakdown) phase of fasting, pancreatic alpha cells produce glucagon to activate gluconeogenesis systemically in the liver via the cyclic AMP response-element-binding protein (CREB) and its coactivator: CREB-regulated transcription coactivator 2 (CRTC2). During more prolonged fasting, however, this effect is cancelled by SIRT1-mediated CRTC2 deacetylation, which targets the coactivator for ubiquitin/proteasome–mediated destruction . SIRT1 then triggers the next stage of gluconeogenesis by deacetylating and activating the peroxisome proliferator-activated receptor (PPAR)γ coactivator 1α (PGC-1α), a coactivator for forkhead box O1 (FOXO1) . Besides supporting gluconeogenesis, PGC-1α is also important for mitochondrial biogenesis, which assists the liver in accommodating the reduced energy status. Meanwhile, to increase energy production, SIRT1 stimulates fatty acid oxidation by deacetylating and activating the nuclear receptor, PPARα . SIRT1 can also shut down the production of energy via glycolysis by deacetylating and repressing glycolytic enzymes, for example phosphoglycerate mutase-1 (PGAM-1) . Interestingly, another nuclear sirtuin, SIRT6, has also been reported to repress glycolysis by serving as a co-repressor for hypoxia-inducible factor 1α (HIF-1α) . Because SIRT6 itself is transcriptionally activated by SIRT1, sirtuins might regulate cellular physiology in a coordinated way to determine the duration of each phase of fasting .