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Department of Cell Biology, Harvard Medical School, Boston, Massachusetts 02115Harvard School of Engineering and Applied Sciences, Cambridge, Massachusetts 02138
* This work was supported, in whole or in part, by National Institutes of Health Grant AG032375 (to M. C. H.), Grants DK088190 and GM093072 (to R. M.), and Training Grant T32 DK007260 (to F. K. S.). This is the fourth article in the Thematic Minireview Series on Sirtuins: From Biochemistry to Health and Disease. 1 Both authors contributed equally to this work. 2 Recipient of a Beatriu de Pinós postdoctoral fellowship.
Originally rising to notoriety for their role in the regulation of aging, sirtuins are a family of NAD+-dependent enzymes that have been connected to a steadily growing set of biological processes. In addition to regulating aging, sirtuins play key roles in the maintenance of organismal metabolic homeostasis. These enzymes also have primarily protective functions in the development of many age-related diseases, including cancer, neurodegeneration, and cardiovascular disease. In this minireview, we provide an update on the known roles for each of the seven mammalian sirtuins in these areas.
Over three-quarters of a century ago, Clive McCay and colleagues first noted that rats kept on a calorie-restricted diet lived longer than freely fed controls (
). Despite the length of time that has elapsed since this discovery, the molecular mechanism that drives this life span extension has remained elusive. Originally described as a silencing factor in yeast (silencing information regulator), the protein Sir2 came out on top in a screen for modulators of yeast life span (
). Sirtuins are found in different subcellular locations, including the nucleus (SIRT1, SIRT6, and SIRT7), cytosol (SIRT2), and mitochondria (SIRT3–5), although in some studies, SIRT1 has been found to possess cytosolic activities, and SIRT2 has been found to associate with nuclear proteins. Sirtuins have important functions in a diverse yet interrelated set of physiological processes. In this minireview, we discuss research done in the past four years featuring their roles in aging, metabolism, cancer biology, cardiovascular disease, inflammation, and brain pathology.
Aging
Following the first publication describing a role for yeast Sir2 in promoting longevity (
), many laboratories focused on elucidating whether sirtuins might play similar roles in other organisms. Sirtuins have been shown to regulate life span in lower organisms, including yeast, nematodes, and fruit flies (
), a dietary intervention that robustly extends life span across numerous species. However, whole-body overexpression of SIRT1 in mice does not affect life span (
The strongest link between mammalian sirtuins and the anti-aging effects of CR comes from SIRT3, which mediates the prevention of age-related hearing loss by CR (
). Hearing loss is a hallmark of mammalian aging and is characterized by a gradual loss of spiral ganglion neurons and sensory hair cells in the cochlea of the inner ear, which is triggered by oxidative damage in these cells (
). Remarkably, CR prevents hearing loss and oxidative damage in wild-type mice, whereas Sirt3-deficient mice are resistant to the effects of CR. In addition, SIRT3 is required for the CR-mediated reduction of oxidative damage in multiple tissues via regulation of the glutathione antioxidant system (
), this is the first demonstration that a mammalian sirtuin positively regulates life span. Notably, the observed life span extension seems to be modest (the median life span increases between 10 and 15%) and gender-specific (affecting males only) for reasons that remain unclear. Despite reduced levels of insulin-like growth factor 1 (IGF-1) in serum, a phenotype previously associated with life span regulation, the “anti-aging” effect of SIRT6 could be explained by SIRT6 acting as a tumor suppressor (as described below) (
). These results provide encouraging evidence that mammalian sirtuins may in fact represent critical modulators of life span and age-related diseases.
Metabolism
Because sirtuin enzymatic activity is dependent upon the presence of NAD+, sirtuin activity is directly linked to the metabolic state of the cell. Indeed, nearly every sirtuin has been shown to play a role in regulating metabolism and energy homeostasis, often in roles that help the cell adapt to periods of low-energy input (Fig. 1). Indeed, sirtuins are involved in multiple metabolic pathways, as described below.
FIGURE 1Metabolic functions of mammalian sirtuins in different tissues. For specific molecular targets, see text. PGC1α, PPARγ coactivator 1α; PEPCK, phosphoenolpyruvate carboxykinase; LCAD, long-chain acyl-CoA dehydrogenase; OTC, ornithine transcarbamoylase.
) and responds to the organism's need for energy by stimulating lipid breakdown through the transcription factor FoxO1, which directly induces expression of the rate-limiting lipolytic enzyme adipose triglyceride lipase (
). SIRT1 also inhibits cells' ability to synthesize fat by deacetylating the lipogenic activator SREBP-1c, preventing this factor from binding the promoters of lipogenic genes (
). Notably, when SIRT1 is absent from specific tissues, mice have significant defects in their ability to metabolize lipids normally, leading to fat accumulation (
Despite these seemingly clear results, further study has shown that the effects of SIRT1 may in fact be dose- or context-dependent. Although SIRT1-overexpressing mice were protected from hepatic steatosis when placed on a high-fat diet in one study (
), attributing the outcome to SIRT1 inhibition of cAMP-responsive element-binding protein, a transcription factor that normally activates fatty acid metabolism and gluconeogenesis. Additionally, overexpression of SIRT1 in the forebrain of female mice led to increased fat mass and increased expression of adipogenic genes in white adipose tissue (
). Taken together, these studies suggest that whether SIRT1 promotes lipid catabolism or instead promotes adipogenesis may be tissue- and expression level-dependent.
The metabolic effects of SIRT3 appear thus far to be more straightforward (Fig. 1). SIRT3 is up-regulated by fasting in liver and brown adipose tissue and promotes mitochondrial oxidative metabolism via deacetylation of numerous metabolic enzymes (
). The importance of this activity is demonstrated by an abnormal accumulation of fatty acid oxidation intermediates in Sirt3 knock-out mice. When fasted, these mice also show reduced ATP production and are intolerant to cold. Conversely, Sirt3 knock-out mice are unable to cope with a high-fat diet and gain more weight than their wild-type counterparts, developing hepatic steatosis and exhibiting signs of inflammation (
). Furthermore, wild-type animals on a high-fat diet have reduced hepatic SIRT3 activity and increased protein acetylation relative to those on a control diet (
), suggesting that a feedback mechanism may be involved in the interplay between SIRT3 activity and diet.
SIRT6 promotes fatty acid oxidation as well, interestingly through a mechanism involving another sirtuin. SIRT1 and the transcription factors FoxO3a and NRF1 form a complex on the Sirt6 promoter, leading to SIRT6 expression and promotion of fat oxidation in wild-type mice (
). Mice with a liver-specific deletion of Sirt6 develop a fatty liver, and primary hepatocytes from these mice show relatively low levels of fatty acid oxidation. By contrast, mice with transgenic overexpression of SIRT6 show down-regulation of genes associated with lipid storage and are somewhat protected against the accumulation of visceral fat when placed on a high-fat diet (
). These studies cast Sirt6 as an important promoter of fat utilization.
Roles are beginning to be described for the other sirtuins in lipid metabolism and adipose biology. SIRT2 seems to fall in line with other family members described above: it inhibits adipocyte differentiation (
Dietary obesity-associated Hif1α activation in adipocytes restricts fatty acid oxidation and energy expenditure via suppression of the Sirt2-NAD+ system.
). However, not all sirtuins act in the same direction: knocking down Sirt4 increases fatty acid oxidation, suggesting that this sirtuin opposes its mitochondrial counterpart SIRT3 (
). This induction goes away when Sirt1 is knocked down at the same time, indicating that there may be cross-talk between these sirtuins. The precise molecular mechanisms for such cross-regulation remain unclear.
Glucose Metabolism
Most of the work done so far on how sirtuins regulate glucose metabolism has focused on SIRT1, which is known to modulate gluconeogenesis in liver mainly through its substrate peroxisome proliferator-activated receptor (PPAR)-γ coactivator 1α (
). Recently, other sirtuins have appeared on the stage as key regulators of glucose homeostasis. SIRT6 has come to the fore as an important regulator of glucose uptake and metabolism (Fig. 1). Sirt6 knock-out mice die early in life from a fatal hypoglycemia. These animals exhibit increased expression of the glucose transporter GLUT1, leading to increased glucose uptake in skeletal muscle and brown adipose tissue (
). Sirt6-deficient embryonic stem cells similarly show lower oxygen consumption and greater lactate production than controls. In part, this phenotype appears to be related to the ability of SIRT6 to lower expression of glycolytic genes, such as Pfk1 and Glut1, through deacetylation of histone H3K9. In this context, SIRT6 works as a co-repressor of HIF-1α, and the increased glucose uptake seen in Sirt6 knock-outs is reversed by treatment with a HIF-1α inhibitor (
). These results indicate that SIRT6 may function as a critical modulator of glucose homeostasis. Recently, SIRT2 has also been found to take part in glucose metabolism by promoting gluconeogenesis through deacetylation and stabilization of the rate-limiting enzyme phosphoenolpyruvate carboxykinase (
), adding this protein to the list of sirtuins that control glucose homeostasis.
Other Metabolic Processes
Sirtuins have important functions in additional metabolic pathways that promote adaptation to periods of low-energy input. For example, SIRT3 activates ketone body synthesis via HMGCS2 (
). SIRT5 enhances the urea cycle through CPS1 (carbamoyl phosphate synthetase 1), the first step of the cycle. Indeed, Sirt5 knock-out mice show reduced basal CPS1 activity and lack the increase in activity normally seen with fasting (
). Suggestively, in the absence of SIRT5, the levels of succinylation of CPS1 were increased, indicating that SIRT5 may function as a bona fide deacylase. All together, these studies highlight that we are yet learning about the enzymatic functions of sirtuins, and future studies will surely investigate the mechanisms through which sirtuins might work together to coordinate metabolic responses.
Cancer
Tumorigenesis is a multistep process that involves the acquisition of several mutations leading to cell transformation and cancer initiation. Among the hallmarks of cancer cells, metabolic reprogramming is also an important regulator of tumor growth, allowing tumor cells to fulfill their energetic and anabolic demands (
). In addition to their role in regulating metabolism, sirtuins are important in the regulation of genomic stability, making them excellent candidates to control tumorigenesis (Fig. 2).
FIGURE 2Mammalian sirtuins in carcinogenesis. The roles of sirtuins in initiation and tumor progression are summarized. For molecular targets, see text. ERα, estrogen receptor-α.
). However, the ability of SIRT1 to promote or suppress tumorigenesis seems to depend on the specific tumor type, cellular context, and signaling pathway affected. Several studies support a role for SIRT1 as a tumor suppressor. SIRT1 opposes Myc-dependent transformation by interacting with and deacetylating this proto-oncogene (
). Furthermore, overexpression of SIRT1 in vivo protects against metabolic syndrome-associated liver cancer by reducing DNA damage and inflammation. Importantly, a moderate increase of SIRT1 in these mice protects from spontaneous and aging-associated cancers (
). Moreover, recent work has demonstrated that SIRT1 interacts with HIF-1α and represses its activity, thereby inhibiting growth and angiogenesis of xenograft tumors (
). Although the influence of SIRT1 and SIRT2 in modulating this histone mark specifically in tumors has not been addressed, it is tempting to speculate that decreased levels or activity of these sirtuins could have an impact on tumorigenesis by increasing the levels of H3K56 acetylation. Notably, H3K56 acetylation has been shown to be a primary substrate for SIRT6, thereby raising the possibility that the modulation of this particular histone mark in tumors may depend on this chromatin deacetylase rather than SIRT1/2, as proposed.
In contrast with the above observations, recently published data have added evidence for a tumor-promoting function of SIRT1. A new positive feedback loop involving N-Myc and SIRT1 has been described to promote tumorigenesis in a mouse model of neuroblastoma (
). N-Myc induces the expression of Sirt1, which in turn deacetylates and stabilizes N-Myc, thereby promoting tumor growth. Furthermore, a complex between SIRT1 and estrogen receptor-α has been found to be essential to promote the expression of prosurvival genes and to inactivate tumor suppressor genes in breast cancer cells (
). Sirt1 expression also appears to be up-regulated in hepatocellular carcinomas, where it inhibits senescence and apoptosis and leads to tumor growth (
). However, the precise molecular mechanism through which SIRT1 exerts this tumor-promoting function remains unclear.
As mentioned above, metabolic reprogramming is a key feature of cancer cells. Although differentiated cells under normal conditions obtain energy by oxidizing fuels such as glucose through mitochondrial oxidative phosphorylation, Warburg observed that rapidly proliferating cancer cells often up-regulate glycolysis, which we now understand allows cells to generate macromolecules needed for cellular proliferation. Recent studies reported that SIRT3 normally counteracts this metabolic switch by destabilizing HIF-1α through down-regulation of ROS (
), suggesting that SIRT3 may be a tumor suppressor. Consistent with this role, Sirt3 expression is down-regulated in human breast cancers, and Sirt3 knock-out mice have a higher incidence of spontaneous mammary tumors (
). As noted above, SIRT6 is a key regulator of glucose metabolism, and lack of this chromatin factor leads to a phenotype that is reminiscent of the Warburg effect (
). Mechanistically, SIRT6 suppresses aerobic glycolysis and Myc-dependent ribosome biosynthesis, and lack of this sirtuin leads to robust metabolic reprogramming, sufficient to promote tumorigenesis. In line with this, Sirt6 expression is down-regulated in human pancreatic and colorectal cancers, and conditional deletion of Sirt6 promotes intestinal tumorigenesis in vivo (
). However, the mechanism and physiological relevance of this observation remain as yet unexplored.
The roles of other sirtuins in cancer development have just started to emerge (Fig. 2). Because SIRT2 regulates normal mitotic progression by controlling the activity of the anaphase-promoting complex/cyclosome (
), SIRT2 was proposed to be a tumor suppressor by preventing chromosomal instability during mitosis. Indeed, Sirt2-deficient mice develop spontaneous and gender-specific tumors, with females primarily developing mammary tumors, whereas hepatocellular carcinomas arise in males (
). Finally, a recent study has demonstrated that SIRT7 functions as an H3K18 deacetylase, repressing transcription of multiple genes involved in anchorage-independent growth and contact inhibition (
). In this context, SIRT7 depletion markedly reduced tumorigenicity of cancer cells, suggesting that SIRT7 may play a critical role in maintaining oncogenic transformation.
Inflammation
Inflammation plays a central role in the pathogenesis of many diseases, including cancer, diabetes, and cardiac disease. Current research points to an anti-inflammatory role for sirtuins, especially SIRT1 and SIRT3. Several studies have shown SIRT1 to be protective against inflammation (e.g. Ref.
Other sirtuins may also negatively regulate inflammation. SIRT2, for instance, may down-regulate the immune response through deacetylation of the NF-κB subunit p65 at Lys-310 (
). Lys-310 is hyperacetylated in Sirt2 knock-out mouse embryonic fibroblasts following TNF-α stimulation, leading to an increase in the expression of NF-κB target genes. These results suggest that SIRT2 is a negative regulator of NF-κB gene expression and therefore potentially of inflammation as well. SIRT6 binds many of the same promoters in the mouse genome as NF-κB subunit RelA (
). Like SIRT2, SIRT6 may also have an anti-inflammatory role through suppression of NF-κB target gene expression. However, Sirt6-deficient T cells exhibit reduced interferon-γ secretion upon activation (
), so the picture is not entirely clear. Finally, in one study, Sirt7 knock-out mice had elevated myocardial levels of several cytokines compared with controls, demonstrating that SIRT7 may help fight inflammation (
Cardiovascular disease is one of the leading causes of death worldwide and involves the deterioration of heart and blood vessel function. Remarkably, early studies demonstrated that CR protects from cardiovascular disease by improving both endothelial and heart function (
). Given the role of sirtuins in mediating, at least in part, some of the effects of CR, multiple studies have explored their function in cardiovascular disease. Most of this work described the role of SIRT1 in endothelial function, vessel inflammation, vascularization, and cholesterol metabolism and has been reviewed previously (
). Here, we summarize recent findings regarding the role of sirtuins in cardiac function.
SIRT1, SIRT7, and, more recently, SIRT3 and SIRT6 have been described as key regulators of cardiac hypertrophy, one of the main causes leading to heart failure. Early studies demonstrated that SIRT1 and SIRT7 play a protective role against cardiac hypertrophy by deacetylating and modulating p53 activity (
Silent information regulator 2α, a longevity factor and class III histone deacetylase, is an essential endogenous apoptosis inhibitor in cardiac myocytes.
Poly(ADP-ribose) polymerase-1-dependent cardiac myocyte cell death during heart failure is mediated by NAD+ depletion and reduced Sir2α deacetylase activity.
). A normal healthy heart utilizes both fatty acids and glucose simultaneously to obtain energy. However, cardiac hypertrophy is accompanied by a metabolic shift similar to the Warburg effect that favors glycolysis and impairs fatty acid oxidation (
). Mechanistically, SIRT1 binds to PPARα, favoring the deacetylation of PPARγ coactivator 1α and preventing the down-regulation of fatty acid oxidation genes (
). In line with a protective role for SIRT1 in cardiovascular disease, it also protects mice from hyperglycemia-induced endothelial dysfunction by inhibiting the expression of p66Shc (
). Mice deficient in p66Shc have increased resistance to oxidative stress and improved endothelial function and are protected against vascular and cardiac diseases (
). A similar phenotype was observed in a different line of SIRT1 transgenic mice, where SIRT1 overexpression reduced cardiac function and was associated with impaired mitochondria (
). In response to a hypertrophic stimulus, Sirt1-deficient mice failed to develop cardiac hypertrophy, a phenotype associated with impaired Akt signaling. Moreover, hearts of mice overexpressing SIRT1 exhibited increased phosphorylation and deacetylation of Akt and developed cardiac hypertrophy under basal conditions (
), suggesting that a beneficial effect of SIRT1 in the context of cardiac function, like the topic areas covered above, may be confined to a window of optimal activity.
Due to its mitochondrial localization and ability to regulate cellular ROS levels, SIRT3 has also emerged as a critical regulator of cardiac function (
). Moreover, SIRT3-dependent deacetylation of FoxO3a leads to its nuclear localization and enhanced expression of antioxidant genes, such as manganese superoxide dismutase, reducing cellular ROS levels (
). Thus, by regulating these two pathways, SIRT3 exerts a key antihypertrophic function in cardiomyocytes.
SIRT6 is also involved in cardiac hypertrophy and myocardial infarction. In models of cardiac hypertrophy, Sirt6 expression is up-regulated, yet its deacetylase activity is reduced (
). However, in contrast with these results, inhibition of nicotinamide phosphoribosyltransferase, an enzyme involved in NAD synthesis, reduces myocardial infarction by inhibiting neutrophil infiltration and ROS production within the infarcted hearts (
). Interestingly, the same protective phenotype was observed when Sirt6 expression was silenced, implying that although SIRT6 may protect against cardiac hypertrophy, lower SIRT6 levels may be beneficial in the context of myocardial infarction.
Brain
Important roles have been elucidated for SIRT1 and SIRT2 in the brain. SIRT1 in particular has shown itself to be beneficial in multiple models of neuropathology. Alzheimer disease is characterized by the accumulation of the β-amyloid peptide and the aggregation of tau protein (
). In a mouse model of Alzheimer disease, brain-specific knock-out of Sirt1 increased levels of β-amyloid plaque formation, whereas overexpression of SIRT1 decreased the plaque levels (
). SIRT1 may contribute to this effect because it activates transcription of the ADAM10 gene, which encodes α-secretase, an enzyme whose activity helps the brain avoid plaque formation. SIRT1 also deacetylates and destabilizes tau protein, thereby suppressing its aggregation (
Immunoelectron-microscopic demonstration of NACP/α-synuclein-epitopes on the filamentous component of Lewy bodies in Parkinson's disease and in dementia with Lewy bodies.
). One possible mechanism is that SIRT1 deacetylates HSF1 to increase levels of HSP70, a chaperone that could prevent aggregation by helping with protein folding. SIRT1 overexpression improved motor function in a mouse model of Huntington disease (
). Sirt1 knock-out mice also have defective short- and long-term memory despite seemingly normal gross brain anatomy, an effect possibly due to a decrease in hippocampal synaptic plasticity (
By contrast, models of neuropathology show protective effects when Sirt2 is knocked down, where inhibition of SIRT2 lowered toxicity in a striatal neuron model of Huntington disease (
). Thus, although SIRT1 is protective against brain pathology, SIRT2 may have opposing effects.
SIRT1 and SIRT2 are also important for differentiation and migration of certain types of brain cells. In primary neurons, SIRT1 promotes neurite outgrowth and increases cell survival, and it also decreases mTOR signaling (
Sirtuin 2, a mammalian homolog of yeast silent information regulator-2 longevity regulator, is an oligodendroglial protein that decelerates cell differentiation through deacetylating α-tubulin.
Little is known about the activity of the remaining sirtuins in the brain. SIRT3 was important for neuron viability in a primary cell model that used NMDA to deplete NAD and induce excitotoxic injury (
). Rather, they initially exhibited growth retardation, with lower levels of pituitary growth hormone and IGF-1, ultimately becoming obese, likely secondary to altered neuropeptide levels.
Future Perspectives
In the past few years, the function of mammalian sirtuins has been investigated in greater detail than ever before, and we now have a much better molecular understanding of the multiple roles that this unique family of enzymes plays in seemingly every biological process. There is little doubt that sirtuins have emerged as critical modulators of metabolic adaptive responses, and their activities have been linked to multiple diseases, including metabolic abnormalities, cancer, inflammation, cardiac hypertrophy, and neurodegeneration. However, key questions will keep investigators busy in the coming years. We still have a poor understanding of the molecular mechanisms regulating sirtuins' expression and activity and of the precise stimuli that regulate these proteins. Are the activities of different sirtuins regulated in a coordinated fashion? In other words, is there cross-talk between sirtuins? Will future studies cement the argument that sirtuins are indeed critical modulators of life span? The future “sirtainly” looks promising.
REFERENCES
McCay C.M.
Crowell M.F.
Maynard L.A.
The effect of retarded growth upon the length of life and upon ultimate size.
Dietary obesity-associated Hif1α activation in adipocytes restricts fatty acid oxidation and energy expenditure via suppression of the Sirt2-NAD+ system.
Silent information regulator 2α, a longevity factor and class III histone deacetylase, is an essential endogenous apoptosis inhibitor in cardiac myocytes.
Poly(ADP-ribose) polymerase-1-dependent cardiac myocyte cell death during heart failure is mediated by NAD+ depletion and reduced Sir2α deacetylase activity.
Immunoelectron-microscopic demonstration of NACP/α-synuclein-epitopes on the filamentous component of Lewy bodies in Parkinson's disease and in dementia with Lewy bodies.
Sirtuin 2, a mammalian homolog of yeast silent information regulator-2 longevity regulator, is an oligodendroglial protein that decelerates cell differentiation through deacetylating α-tubulin.
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