The Key to Aging and Illness?

The Key to Aging and Illness?

Today, the New England Journal of Medicine published a lecture given by a scientist at MIT that goes a 

long way towards establishing a new approach to thinking about why and how we age, and how that process

is at the heart of most of the terrible afflictions that face us as we get older, namely:

dementia, heart disease, auto-immune disorders (such as arthritis), and cancer.

For most of us aging just seems to be the fault of time.  As time goes on, things wear out.  Why  would our

bodies be any different?  But the problem with that concept of aging is that bodies don't actually wear out.

The materials that make up our body are renewed constantly by the food we eat and the air we breathe.

For example, we have a new layer of skin created every three weeks.

Further, each species ages at a different rate.  If aging was just a matter of wear and tear, the bones of a dog and bear and human ought to wear out at the same rate, but they do not.  

And finally, there is the observation that all animals age in the same way.  All animals see their skin get wrinkly, their hair lose its pigment and shine, their joints get sore and inflamed, their hearts begin to fail, and cancers to appear.

A lot of research is now proving that aging is a program.  Our DNA and biology contains a program, like a piece of software, that dictates the decline in function that we call aging and its characteristic diseases.

Abundant examples now exist in animals and even humans that demonstrate if you change that program, you change the rate of aging.  And the center of control of the program we call aging is the part of the cell that creates all the cell's energy, a structures called the mitochondria.

It is now becoming clear that mitochondria set the pace and the program of aging, and changes to that program can change a person's experience of aging.  There are human populations whose mitochondria have a different approach to aging, and these populations live far longer with far less dementia, heart disease, cancer, and auto-immune disease.

Today's publication in the New England Journal reviews much of the science known about one set of genes that control this program we call aging.  The group of genes is called the sirtuins.  Dr. Gurante of MIT does a masterful job of reviewing what we know about the sirtuin genes and how they control aging.

The lecture is highly technical, so I apologize to those not so interested in such a technical paper, but even so, take a look at the section headings and marvel at the breadth of control the aging process commands:  with the right sirtuin genes animals, and likely humans, have less diabetes from obesity, less heart disease, less dementia, less cancer, and slower aging.

I share all this with you today, because I am convinced that the priorities of medical research need to shift.  Our usual approach to managing the illnesses of old age has been to have major research institutes devoted to researching each of these diseases.  These efforts should continue.  But with the emergence of knowledge about a very central control point on all these problems, the time has come for our science grants to push for a major effort to learn how to manage our aging software, and perhaps unlock a future where cancer, dementia, heart disease, arthritis are all held at bay as we enjoy a much longer and healthier life.

No research project aimed at a single disease can deliver this amazing goal, but it is increasingly likely that research aimed at our software for aging could.

To a good life,

Dr. Arthur Lavin

REVIEW ARTICLE

FRANKLIN H. EPSTEIN LECTURE

Sirtuins, Aging, and Medicine

Leonard Guarente, Ph.D.

N Engl J Med 2011; 364:2235-2244June 9, 2011

Article

Franklin H. Epstein, M.D., served the New England Journal of Medicine for more than 20 years. A keen clinician, accomplished researcher, and outstanding teacher, Dr. Epstein was Chair and Professor of Medicine at Beth Israel Deaconess Medical Center, Boston, where the Franklin H. Epstein, M.D., Memorial Lectureship in Mechanisms of Disease has been established in his memory.

Populations in developed countries continue to grow older, as medical advances allow baby boomers to march inexorably onward. Many of the most important diseases that lead to disability and death occur late in life, indicating that aging itself is a key risk factor. Recent research into the science of aging has identified genes and pathways that appear to control the aging process. This review describes one such family of antiaging genes, the sirtuins, and details progress in understanding the biology that undergirds their promise as therapeutic targets.

In the past century, medical research has led to the development of new therapeutic agents, including antibiotics and antiviral and anticancer drugs, as well as surgical interventions, such as coronary-artery bypass grafting, among others. Since many important diseases often occur later in life (e.g., diabetes, neurodegenerative diseases, cancer, cardiovascular disease, proinflammatory diseases, and osteoporosis), aging is an important risk factor for these conditions. During the past decade, research on aging, which began in simple laboratory organisms, has identified important genes and pathways that contribute to longevity. Included among these is the family of nicotinamide adenine dinucleotide (NAD)–dependent protein deacetylases termed sirtuins.1,2 These proteins can extend the life span in model organisms and are important in mediating the salutary antiaging effects of a low-calorie diet (calorie restriction). Mammals have seven sirtuin homologues, which perform nonredundant functions in adapting human physiology to environmental stressors, such as food scarcity. Small molecules have been identified that reportedly inhibit as well as activate sirtuins in vivo and in vitro. Whether such molecules could have antiaging properties is an interesting but open question.

Sir2 is one of a complex of proteins that mediate transcriptional silencing at selected regions of the yeast genome. Mutations that extend the replicative life span of yeast mother cells have been shown to increase the silencing activity of Sir2 at the ribosomal DNA repeats.3-5 Although the silencing of ribosomal DNA has turned out to be an idiosyncratic feature of aging in yeast, the role of Sir2-related gene products (sirtuins) in aging appears to be universal. Sir2 orthologues slow aging in the nematode Caenorhabditis elegans, in the fruit fly Drosophila melanogaster, and in mice.6-8 The sirtuins have been shown to have NAD-dependent protein deacetylase activity, which is associated with the splitting of NAD during each deacetylation cycle (Figure 1FIGURE 1Enzymatic Activity of Sirtuins.).9 Of the mammalian sirtuins, SIRT1, 2, 3, 4, 5, and 6 have been shown to have this activity.10Some SIRT family members (e.g., SIRT4 and SIRT6) also have ADP-ribosyltransferase activity.1,2,10

In mammals, the Sir2 orthologue SIRT1 is primarily a nuclear protein in most cell types and has evolved to deacetylate transcription factors and cofactors that govern many central metabolic pathways (Figure 2FIGURE 2Categories of SIRT1 Targets for Deacetylation and Their Associated Diseases.). Targets of SIRT1 include transcriptional proteins that are important in energy metabolism, such as nuclear receptors, peroxisome proliferator-activated receptor-γ coactivator 1α (PGC-1α), and forkhead box subgroup O (FOXO).11-14 SIRT1 also regulates components of the circadian clock, such as BMAL1 and PER2, which underscores the interconnectedness of protein acetylation, metabolism, circadian rhythm, and aging.15,16 SIRT1 is also closely coupled to AMP-kinase activity in a mutually enforcing mechanism that adjusts cellular physiology for conditions of energy limitation.17

Other categories of SIRT1 targets relate to stress tolerance (p53, hypoxia-inducible factor 1α and 2α, and heat-shock factor protein 1),18 DNA repair (NBS1, PARP1, Ku70, and WRN),18 and inflammation (nuclear factor κB).19 All told, the breadth of SIRT1 substrates indicates that its activity is poised to regulate metabolism and stress response.

One early indication that SIRT1 might be important in diseases of metabolism was the finding that the protein could influence differentiation and fat accumulation in the 3T3-L1 adipose-cell line and in primary preadipocytes in rats.20 In a second case, calorie restriction triggered a SIRT1–PGC-1α–dependent increase in muscle mitochondrial biogenesis21 and the activation of fatty acid oxidation by SIRT1 and peroxisome-proliferator–activated receptor α (PPAR-α),22 which together favor insulin sensitivity and evidently a slower rate of aging-related decline. In liver, SIRT1 has been found to govern two pathways with opposing effects on gluconeogenesis. On the one hand, the activation of PGC-1α and FOXO1 appears to favor glucose production,12 whereas the deacetylation and destabilization of the cyclic AMP response-element–binding (CREB) coactivator CRCT2 would suppress it.23 The relative importance of each pathway may switch as a function of the duration of fasting to fine-tune the magnitude of glucose production over time. In steady-state calorie reduction, the net effect of these pathways results in a mild elevation in glucose production.

The oral administration of the putative SIRT1-activating compounds has been shown to mitigate the prodiabetic effects of a high-fat diet without major toxic effects, which suggests that activation of SIRT1 is antidiabetic and not pharmacologically detrimental. This hypothesis was first shown with the natural product resveratrol24,25 and was subsequently corroborated with more selective synthetic-activating compounds.26 In the latter case, efficacy was also shown in a genetic model of murine obesity (the leptin-deficient ob/ob mouse) and in obese rats. Although these antidiabetic outcomes are consistent with known effects of SIRT1 on muscle, fat cells, and liver, recent studies have shown that this sirtuin also functions in the hypothalamus to control feeding behavior and energy expenditure.27,28

It has been argued that the SIRT1-activating compounds that have been described to date do not directly activate this sirtuin,29,30 a topic that is discussed in a later section. However, it is striking that several different genetic models of SIRT1 overexpression in transgenic mice also prevent diabetes. In one model, leanness and increased insulin sensitivity were shown to occur under standard laboratory conditions.31 In two other models, there was protection against metabolic decline induced by a high-fat diet or, more strikingly, by normal aging.32,33 In addition, SIRT1 activation, whether chemical or genetic, has been associated with changes in genomic transcriptional patterns normally induced by calorie restriction.34,35 All told, these genetic studies provide strong evidence that SIRT1 plays a central role in driving the phenotype of rodents toward metabolic fitness. Therefore, among the possibilities it seems most likely that the SIRT1-activating compounds work by the activation of SIRT1 in vivo (either directly or indirectly).

Studies of SIRT1 transgenic mice also suggest that activation may be protective against other diseases of aging (e.g., bone loss and an inflammation-induced model of liver cancer).8 The antiinflammatory effect of sirtuins may be much broader, since both SIRT1 and SIRT6 repress the activity of the major proinflammatory transcription factor, nuclear factor κB.19,36 In other murine studies, it was found that SIRT1 may be protective against colon cancer37 and breast cancer.38 The fact that SIRT1 represses hypoxia-inducible factor 1α furthers its candidacy as a tumor suppressor. However, the relationship between SIRT1 and cancer may not be totally straightforward, since other studies suggest that this sirtuin may also have oncogenic properties in certain contexts.39,40 The mitochondrial sirtuin SIRT3 (which is discussed below) has recently emerged as another interesting target for cancer therapy.

SIRT1 appears to possess cardiovascular protective properties beyond those deriving solely from metabolic fitness. For example, sirtuins may protect against hypertrophy in cardiac and smooth-muscle cells. Transgenic mice that overexpress SIRT1 in the heart are protected against pathological cardiac hypertrophy (Figure 3FIGURE 3Four Mechanisms of SIRT1 Protection against Cardiovascular Disease.).41 SIRT1 also protects smooth muscle by inhibiting the expression of the angiotensin receptor AT1.42,43 AT1^−/− mice produce lower levels of reactive oxygen species and live longer than normal mice.44 Further illustrating the functional interplay between SIRT1 and SIRT3, the reduction in reactive oxygen species in AT1^−/− mice appears to be mediated by elevated levels of SIRT3.

The earliest connection between SIRT1 and endothelial cells was the finding that SIRT1 deacetylates and activates endothelial nitric oxide synthase (eNOS).21 The activation of eNOS and repression of AT1 suggest that SIRT1 activity ought to curb high blood pressure. SIRT1 also inhibits the senescence of endothelial cells,45 and its salutary effect on these cells may mitigate atherosclerosis. Interestingly, calorie restriction is known to protect against atherosclerosis,46 and many of the physiological effects of calorie restriction are blunted in eNOS^−/−mice.21 These findings all indicate that SIRT1 helps facilitate the favorable effect of calorie restriction on cardiovascular function by its effects on eNOS, AT1, and perhaps other targets.

Another function of SIRT1 that may be critical in cardiovascular disease is its regulation of fat and cholesterol homeostasis. In addition to triggering β-oxidation of fatty acids in calorie restriction,22 SIRT1 also exerts two opposing effects on fat and cholesterol synthesis. SIRT1 deacetylates and activates the nuclear receptor liver X receptor (LXR), which up-regulates the ATP-binding cassette transporter A1 to facilitate reverse cholesterol transport from peripheral tissues.11 Likewise, SIRT1 was also shown to deacetylate and activate the nuclear bile acid receptor farnesoid X receptor (FXR) to increase its dimerization with its partner retinoid X receptor and its activity.47 Thus, LXR and FXR activation by SIRT1 has the potential to increase the production of high-density lipoprotein (HDL) cholesterol and protection against atherosclerosis by facilitating cholesterol removal.

But LXR also activates the gene encoding the sterol regulatory element-binding protein 1 (SREBP1) in liver to drive fat and cholesterol synthesis. Remarkably, LXR activation by SIRT1 in liver may be counteracted by an opposing SIRT1 activity, deacetylation of SREBP1 and repression of its activity.48,49 The subtlety in control of fat synthesis is reminiscent of the control of glucose synthesis discussed above; SIRT1^−/− mice were found to be prone to liver steatosis in some studies22,50,51 and protected in another,52 possibly because of differences in the experimental protocols.

Finally, the demonstrated protective effect of SIRT1 in the kidney would also be cardioprotective by aiding the control of blood pressure. In this organ, SIRT1 protects tubular epithelium53 and medullary cells54 and mediates kidney-protective effects of calorie restriction.55 Although not all of the effects of sirtuin on cardiovascular health are known, this is an area with much research potential.

As humans are living longer, neurodegenerative diseases have become a sobering obstacle to healthy aging. It is estimated that Alzheimer's disease alone will affect up to one third of those lucky enough to win the longevity lottery.56 Although methods to detect early stages of Alzheimer's disease have become ever more sensitive, treatment approaches remain elusive. Neuronal stress (e.g., in cultured neurons57 or in cyclin-dependent kinase 5 transgenic mice58) was mitigated by the overexpression of SIRT1, prompting the question as to whether this sirtuin might restrain Alzheimer's disease.

Indeed, SIRT1 overexpression in the brain has been shown to reduce the load of β-amyloid peptide, the toxic agent that is generated by proteolytic cleavage of amyloid precursor protein in mice that overexpress two human genes that predispose to early Alzheimer's disease (Figure 4FIGURE 4Two Mechanisms of SIRT1 Protection against Alzheimer's Disease.).59 By activating the gene encoding α-secretase, SIRT1 directed the processing of amyloid precursor protein along a pathway that avoided the production of β-amyloid peptide and thus protected against the disease. SIRT1 activated the α-secretase gene by deacetylating its transcriptional activator, the retinoic acid receptor β.

β-Amyloid peptide gives rise to a protein aggregate of plaques in the brain of patients with Alzheimer's disease. In mice overexpressing β-amyloid peptide, the level of plaques was reduced in mice that also overexpress SIRT1. Another hallmark of Alzheimer's disease in humans is an increase in the number of aggregates or tangles of the tau protein in neurons. Indeed, β-amyloid peptide and tau have been two leading candidates for the causal agent of Alzheimer's disease.56 In a separate study in mice,60 SIRT1 was shown to deacetylate tau protein to destabilize it and reduce tangles (Figure 4).

Beyond frank diseases, cognitive decline is another malady of aging. Recent studies suggested that SIRT1 also benefits learning and memory in mice61,62 by activating the gene for brain-derived neurotrophic factor (BDNF). SIRT1 appears to potentiate activity of the BDNF transcription factor CREB and may generally promote activation of CREB target genes in the brain. Important questions remain with regard to the potential of sirtuins as therapeutic targets in the brain. Can SIRT1 affect other neurodegenerative diseases? Might other sirtuins (SIRT2, 3, 4, 5, 6, and 7) play a role in combating brain diseases of aging? Finally, will sirtuin drugs that are being developed to protect against diabetes cross the blood–brain barrier and, if not, can such drugs be developed?

Studies of the mitochondrial sirtuins (SIRT3, 4, and 5) have suggested that sirtuins mediate physiologic adaptation to reduced energy consumption. All three of these sirtuins modify mitochondrial proteins governing metabolic pathways that are important in energy deprivation (Figure 5FIGURE 5Three Mitochondrial Matrix Sirtuins.). SIRT4 was shown to repress the enzyme glutamate dehydrogenase (by ADP-ribosylation), which determines the utilization of amino acids as energy sources.63 In beta cells or liver, SIRT4 activity declined during calorie restriction, which allowed glutamine to feed directly into catabolic metabolism. Thus, in SIRT4^−/− mice or in wild-type calorie-restricted mice, glutamine is a fuel source for glucose synthesis in liver and also drives insulin secretion by beta cells. Remarkably, SIRT4 depletion was also recently shown to increase fatty acid oxidation.64 SIRT5 was shown to deacetylate and activate carbamoyl-phosphate synthase 1, the first and limiting enzyme in the urea cycle.65,66 The demonstrated increase in SIRT5 activity during calorie restriction activates the urea cycle to facilitate the disposal of ammonia when amino acids are used as fuel sources.

SIRT3 was shown to deacetylate long-chain acyl dehydrogenase and other enzymes that are involved in β-oxidation of fatty acids, as well as the urea cycle.67,68 A recent study in C. elegans also revealed the close linkage of the sirtuin SIR-2.1, fatty acid oxidation, and longevity.69 It has been speculated that the production of ATP from catabolism of fat rather than carbohydrates may itself be protective against reactive oxygen species and aging.70

New findings more directly link SIRT3 and the production of reactive oxygen species. SIRT3^−/− cells produce increased levels of reactive oxygen species and have a concomitant reduction in ATP production.71,72 This finding suggests that SIRT3 may deacetylate components of the electron transport chain to render oxidative phosphorylation more efficient and less affected by reactive oxygen species. Indeed, SIRT3^−/− mitochondria show increased acetylation of electron transport chain (ETC) components and reduced activity of ETC complexes I and III in isolated mitochondria.71,73 Moreover, SIRT3 was also recently shown to deacetylate and activate mitochondrial superoxide dismutase 2 (SOD2)74,75 and isocitrate dehydrogenase 2 (Idh2),76the latter of which generates NADPH for the glutathione pathway of detoxification of reactive oxygen species in mice.

An example of the potential link between SIRT3 and aging was shown in a murine model of hearing loss.76 By 12 months of age, wild-type mice showed complete hearing loss, which is triggered by oxidative damage in the spiral ganglia neurons and sensory hair cells in the cochlea. Hearing loss and oxidative damage were completely prevented by calorie restriction in these mice. However, SIRT3^−/− mice were resistant to the protective effects of calorie restriction against hearing loss and oxidative damage. If this effect of SIRT3 extends to other neuronal types or more broadly to non-neuronal tissues, SIRT3 activators may be the simplest and most direct way to counteract aging itself by triggering mechanisms resembling those of calorie restriction. For example, SIRT3 was shown to protect cardiovascular function by suppressing the production of reactive oxygen species in endothelial and cardiac cells.77

Finally, the ability of SIRT3 to suppress reactive oxygen species suggests that it may be involved in tumor suppression. Indeed, SIRT3^−/− mice generate more reactive oxygen species and are more susceptible to mammary tumors than normal mice.72Recently, SIRT3 has been linked to the metabolic reprogramming in cancer cells termed the Warburg effect, which enforces the glycolytic production of ATP. The reactive oxygen species that are produced when SIRT3 activity is suppressed activate nuclear hypoxia-inducible factor 1α,78,79 which turns on genes for glycolysis and angiogenesis. Indeed, enforced SIRT3 expression reversed the Warburg effect in many tumor-cell lines.79 Moreover, a large percentage of human tumors (up to 40% of breast and ovarian cancers) show inactivation of SIRT3.72,79 It will be critical to further validate the possibility that the loss of SIRT3 is a major determinant of the Warburg effect and cancer.

This section will focus on small-molecule activators of SIRT1 rather than inhibitors, which have also been described.80 The first studies of chemical activators of sirtuins focused on SIRT1. The polyphenols, such as resveratrol, were identified by screening compounds for boosting the deacetylation of synthetic peptidyl substrates labeled with a chemical fluorophore group.81Subsequently, chemically distinct classes of SIRT1 activators that had higher potency than polyphenols were identified with the use of a different assay with fluorophore-containing peptides.26 Both polyphenols and newer activating compounds were shown to increase the binding affinity of SIRT1 for the peptide substrates used.

Questions were first raised regarding the mechanism of activation by resveratrol since there was a lack of activation with the use of nonfluorophore-containing peptides.29 It was subsequently reported that the nonpolyphenolic compounds also did not activate SIRT1 on peptide substrates lacking the fluorophore.30 These findings led to the proposal that the activating compounds interacted with the fluorophore and that their effects in vitro and in vivo were not mediated by SIRT1. A second school of thought proposed that activators such as resveratrol might target other proteins in cells (e.g., AMP kinase82) to affect SIRT1 indirectly. Relevant to this idea, resveratrol has been shown to inhibit mitochondrial ATP synthase,83,84 although such an effect may not occur at moderate yet efficacious concentrations. Finally, a more recent study responded to these challenges by showing that several classes of the nonpolyphenolic compounds can activate SIRT1 with the use of peptide substrates lacking any chemical modifications.85 This latter study reported a mechanism that was based on allosteric modulation of SIRT1 by the drug, which was dependent on the amino acid sequence of the protein substrate.

How can these seemingly disparate findings be reconciled? The model in which the compounds do not affect SIRT1 at all seems unlikely, given the congruence of their in vivo effects and genetic activation of SIRT1. It is also inconsistent with examples in which the effects of activators in cells or mice are SIRT1-dependent.86 To me, the most likely possibility seems to be that the compounds do bind directly to SIRT1 as described or to a component of a SIRT1 complex in cells. The activation mechanism in vivo could involve an increase in the binding affinity for protein substrates by the allosteric mechanism mentioned above. Alternatively, the activation mechanism could relate to other proteins in cells that bind to and modulate SIRT1 activity. One interesting example is the protein called deleted in breast cancer 1 (DBC1), which is a natural inhibitor of SIRT1 activity in many cell types.87,88 Pharmacologic activation may involve the binding of compounds to SIRT1 to release active enzyme from the SIRT1–DBC1 complex. Consistent with this idea, knocking out DBC1 increases deacetylation of SIRT1 targets in cells and elicits a phenotype resembling SIRT1 transgenic mice.50 In any case, the intense focus on SIRT1-activating compounds should clarify mechanisms of action conclusively.

Sirtuins were originally identified as antiaging proteins in model genetic organisms and have emerged as mediators of the beneficial effects of calorie restriction in mammals. The mammalian Sir2 orthologue, SIRT1, is an NAD-dependent deacetylase that is involved in many central pathways governing physiology and stress management. Genetic or pharmacologic activation of SIRT1 can benefit numerous diseases in murine models. Indeed, two different SIRT1-activating compounds are now in a diverse set of phase 1 or phase 2 human trials (ClinicalTrials.gov numbers, NCT00937326, NCT00964340, NCT01014117, NCT01018017, NCT01018628, NCT01262911, NCT01031108, and NCT01154101). Beyond SIRT1, there are six other mammalian sirtuins (SIRT2, 3, 4, 5, 6, and 7), and all may turn out to have therapeutic potential with the use of activators or inhibitors. Among these sirtuins, SIRT3 is extremely interesting, because it appears to suppress one of the contributing causes of aging itself, reactive oxygen species in mitochondria. Indeed, genetic polymorphisms in the SIRT3 promoter have been associated with extreme longevity in an Italian population,89,90 although these studies will have to be replicated in other groups. In conclusion, sirtuins are a unique class of proteins that link protein acetylation to metabolism and exert profound effects on mammalian physiology and diseases of aging. The development of drugs that target sirtuins to treat these diseases is ongoing.

Disclosure forms provided by the author are available with the full text of this article at NEJM.org.

From the Paul F. Glenn Lab and the Department of Biology, Massachusetts Institute of Technology, Cambridge, MA.

Address reprint requests to Dr. Guarente at the Department of Biology, Massachusetts Institute of Technology, 77 Massachusetts Ave., #68-280, Cambridge, MA 02139, or at leng@mit.edu.

References

1. 1
   Donmez G, Guarente L. Aging and disease: connections to sirtuins. Aging Cell 2010;9:285-290
   CrossRef | Web of Science | Medline
2. 2
   Haigis MC, Sinclair DA. Mammalian sirtuins: biological insights and disease relevance.Annu Rev Pathol 2010;5:253-295
   CrossRef | Web of Science | Medline
3. 3
   Kennedy BK, Gotta M, Sinclair DA, et al. Redistribution of silencing proteins from telomeres to the nucleolus is associated with extension of life span in S. cerevisiae. Cell 1997;89:381-391
   CrossRef | Web of Science | Medline
4. 4
   Sinclair DA, Guarente L. Extrachromosomal rDNA circles -- a cause of aging in yeast. Cell1997;91:1033-1042
   CrossRef | Web of Science | Medline
5. 5
   Kaeberlein M, McVey M, Guarente L. The SIR2/3/4 complex and SIR2 alone promote longevity in Saccharomyces cerevisiae by two different mechanisms. Genes Dev1999;13:2570-2580
   CrossRef | Web of Science | Medline
6. 6
   Tissenbaum HA, Guarente L. Increased dosage of a sir-2 gene extends lifespan in Caenorhabditis elegans. Nature 2001;410:227-230
   CrossRef | Web of Science | Medline
7. 7
   Rogina B, Helfand SL. Sir2 mediates longevity in the fly through a pathway related to calorie restriction. Proc Natl Acad Sci U S A 2004;101:15998-16003
   CrossRef | Web of Science | Medline
8. 8
   Herranz D, Munoz-Martin M, Canamero M, et al. Sirt1 improves healthy aging and protects from metabolic syndrome-associated cancer. Nat Commun 2010;1:3-3
   CrossRef
9. 9
   Imai S, Armstrong CM, Kaeberlein M, Guarente L. Transcriptional silencing and longevity protein Sir2 is an NAD-dependent histone deacetylase. Nature 2000;403:795-800
   CrossRef | Web of Science | Medline
10. 10
    Milne JC, Denu JM. The Sirtuin family: therapeutic targets to treat diseases of aging. Curr Opin Chem Biol 2008;12:11-17
    CrossRef | Web of Science | Medline
11. 11
    Li X, Zhang S, Blander G, Tse JG, Krieger M, Guarente L. SIRT1 deacetylates and positively regulates the nuclear receptor LXR. Mol Cell 2007;28:91-106
    CrossRef | Web of Science | Medline
12. 12
    Rodgers JT, Lerin C, Haas W, Gygi SP, Spiegelman BM, Puigserver P. Nutrient control of glucose homeostasis through a complex of PGC-1alpha and SIRT1. Nature 2005;434:113-118
    CrossRef | Web of Science | Medline
13. 13
    Motta MC, Divecha N, Lemieux M, et al. Mammalian SIRT1 represses forkhead transcription factors. Cell 2004;116:551-563
    CrossRef | Web of Science | Medline
14. 14
    Brunet A, Sweeney LB, Sturgill JF, et al. Stress-dependent regulation of FOXO transcription factors by the SIRT1 deacetylase. Science 2004;303:2011-2015
    CrossRef | Web of Science | Medline
15. 15
    Nakahata Y, Kaluzova M, Grimaldi B, et al. The NAD+-dependent deacetylase SIRT1 modulates CLOCK-mediated chromatin remodeling and circadian control. Cell2008;134:329-340
    CrossRef | Web of Science | Medline
16. 16
    Asher G, Gatfield D, Stratmann M, et al. SIRT1 regulates circadian clock gene expression through PER2 deacetylation. Cell 2008;134:317-328
    CrossRef | Web of Science | Medline
17. 17
    Canto C, Gerhart-Hines Z, Feige JN, et al. AMPK regulates energy expenditure by modulating NAD+ metabolism and SIRT1 activity. Nature 2009;458:1056-1060
    CrossRef | Web of Science | Medline
18. 18
    Nakagawa T, Guarente L. Sirtuins at a glance. J Cell Sci 2011;124:833-838
    CrossRef | Web of Science | Medline
19. 19
    Yeung F, Hoberg JE, Ramsey CS, et al. Modulation of NF-kappaB-dependent transcription and cell survival by the SIRT1 deacetylase. EMBO J 2004;23:2369-2380
    CrossRef | Web of Science | Medline
20. 20
    Picard F, Kurtev M, Chung N, et al. Sirt1 promotes fat mobilization in white adipocytes by repressing PPAR-gamma. Nature 2004;429:771-776[Erratum, Nature 2004;430:921.]
    CrossRef | Web of Science | Medline
21. 21
    Nisoli E, Tonello C, Cardile A, et al. Calorie restriction promotes mitochondrial biogenesis by inducing the expression of eNOS. Science 2005;310:314-317
    CrossRef | Web of Science | Medline
22. 22
    Purushotham A, Schug TT, Xu Q, Surapureddi S, Guo X, Li X. Hepatocyte-specific deletion of SIRT1 alters fatty acid metabolism and results in hepatic steatosis and inflammation. Cell Metab 2009;9:327-338
    CrossRef | Web of Science | Medline
23. 23
    Liu Y, Dentin R, Chen D, et al. A fasting inducible switch modulates gluconeogenesis via activator/coactivator exchange. Nature 2008;456:269-273
    CrossRef | Web of Science | Medline
24. 24
    Baur JA, Pearson KJ, Price NL, et al. Resveratrol improves health and survival of mice on a high-calorie diet. Nature 2006;444:337-342
    CrossRef | Web of Science | Medline
25. 25
    Lagouge M, Argmann C, Gerhart-Hines Z, et al. Resveratrol improves mitochondrial function and protects against metabolic disease by activating SIRT1 and PGC-1alpha. Cell2006;127:1109-1122
    CrossRef | Web of Science | Medline
26. 26
    Milne JC, Lambert PD, Schenk S, et al. Small molecule activators of SIRT1 as therapeutics for the treatment of type 2 diabetes. Nature 2007;450:712-716
    CrossRef | Web of Science | Medline
27. 27
    Ramadori G, Fujikawa T, Fukuda M, et al. SIRT1 deacetylase in POMC neurons is required for homeostatic defenses against diet-induced obesity. Cell Metab 2010;12:78-87
    CrossRef | Web of Science | Medline
28. 28
    Satoh A, Brace CS, Ben-Josef G, et al. SIRT1 promotes the central adaptive response to diet restriction through activation of the dorsomedial and lateral nuclei of the hypothalamus.J Neurosci 2010;30:10220-10232
    CrossRef | Web of Science | Medline
29. 29
    Kaeberlein M, McDonagh T, Heltweg B, et al. Substrate-specific activation of sirtuins by resveratrol. J Biol Chem 2005;280:17038-17045
    CrossRef | Web of Science | Medline
30. 30
    Pacholec M, Bleasdale JE, Chrunyk B, et al. SRT1720, SRT2183, SRT1460, and resveratrol are not direct activators of SIRT1. J Biol Chem 2010;285:8340-8351
    CrossRef | Web of Science | Medline
31. 31
    Bordone L, Cohen D, Robinson A, et al. SIRT1 transgenic mice show phenotypes resembling calorie restriction. Aging Cell 2007;6:759-767
    CrossRef | Web of Science | Medline
32. 32
    Banks AS, Kon N, Knight C, et al. SirT1 gain of function increases energy efficiency and prevents diabetes in mice. Cell Metab 2008;8:333-341
    CrossRef | Web of Science | Medline
33. 33
    Pfluger PT, Herranz D, Velasco-Miguel S, Serrano M, Tschop MH. Sirt1 protects against high-fat diet-induced metabolic damage. Proc Natl Acad Sci U S A 2008;105:9793-9798
    CrossRef | Web of Science | Medline
34. 34
    Barger JL, Kayo T, Vann JM, et al. A low dose of dietary resveratrol partially mimics caloric restriction and retards aging parameters in mice. PLoS ONE 2008;3:e2264-e2264
    CrossRef | Medline
35. 35
    Smith JJ, Kenney RD, Gagne DJ, et al. Small molecule activators of SIRT1 replicate signaling pathways triggered by calorie restriction in vivo. BMC Syst Biol 2009;3:31-31
    CrossRef | Medline
36. 36
    Kawahara TL, Michishita E, Adler AS, et al. SIRT6 links histone H3 lysine 9 deacetylation to NF-kappaB-dependent gene expression and organismal life span. Cell 2009;136:62-74
    CrossRef | Web of Science | Medline
37. 37
    Firestein R, Blander G, Michan S, et al. The SIRT1 deacetylase suppresses intestinal tumorigenesis and colon cancer growth. PLoS ONE 2008;3:e2020-e2020
    CrossRef | Medline
38. 38
    Wang RH, Zheng Y, Kim HS, et al. Interplay among BRCA1, SIRT1, and Survivin during BRCA1-associated tumorigenesis. Mol Cell 2008;32:11-20
    CrossRef | Web of Science | Medline
39. 39
    Lee H, Kim KR, Noh SJ, et al. Expression of DBC1 and SIRT1 is associated with poor prognosis for breast carcinoma. Hum Pathol 2011;42:204-213
    CrossRef | Web of Science | Medline
40. 40
    Chen WY, Wang DH, Yen RC, Luo J, Gu W, Baylin SB. Tumor suppressor HIC1 directly regulates SIRT1 to modulate p53-dependent DNA-damage responses. Cell 2005;123:437-448
    CrossRef | Web of Science | Medline
41. 41
    Planavila A, Iglesias R, Giralt M, Villarroya F. Sirt1 acts in association with PPAR{alpha} to protect the heart from hypertrophy, metabolic dysregulation, and inflammation. Cardiovasc Res 2010 December 22 (Epub ahead of print).
42. 42
    Li L, Gao P, Zhang H, et al. SIRT1 inhibits angiotensin II-induced vascular smooth muscle cell hypertrophy. Acta Biochim Biophys Sin (Shanghai) 2011;43:103-109
    CrossRef | Web of Science | Medline
43. 43
    Miyazaki R, Ichiki T, Hashimoto T, et al. SIRT1, a longevity gene, downregulates angiotensin II type 1 receptor expression in vascular smooth muscle cells. Arterioscler Thromb Vasc Biol 2008;28:1263-1269
    CrossRef | Web of Science | Medline
44. 44
    Benigni A, Corna D, Zoja C, et al. Disruption of the Ang II type 1 receptor promotes longevity in mice. J Clin Invest 2009;119:524-530
    CrossRef | Web of Science | Medline
45. 45
    Ota H, Eto M, Kano MR, et al. Cilostazol inhibits oxidative stress-induced premature senescence via upregulation of Sirt1 in human endothelial cells. Arterioscler Thromb Vasc Biol 2008;28:1634-1639
    CrossRef | Web of Science | Medline
46. 46
    Lefevre M, Redman LM, Heilbronn LK, et al. Caloric restriction alone and with exercise improves CVD risk in healthy non-obese individuals. Atherosclerosis 2009;203:206-213
    CrossRef | Web of Science | Medline
47. 47
    Kemper JK, Xiao Z, Ponugoti B, et al. FXR acetylation is normally dynamically regulated by p300 and SIRT1 but constitutively elevated in metabolic disease. Cell Metab 2009;10:392-404
    CrossRef | Web of Science | Medline
48. 48
    Walker AK, Yang F, Jiang K, et al. Conserved role of SIRT1 orthologs in fasting-dependent inhibition of the lipid/cholesterol regulator SREBP. Genes Dev 2010;24:1403-1417
    CrossRef | Web of Science | Medline
49. 49
    Ponugoti B, Kim DH, Xiao Z, et al. SIRT1 deacetylates and inhibits SREBP-1C activity in regulation of hepatic lipid metabolism. J Biol Chem 2010;285:33959-33970
    CrossRef | Web of Science | Medline
50. 50
    Escande C, Chini CC, Nin V, et al. Deleted in breast cancer-1 regulates SIRT1 activity and contributes to high-fat diet-induced liver steatosis in mice. J Clin Invest 2010;120:545-558
    CrossRef | Web of Science | Medline
51. 51
    Wang RH, Li C, Deng CX. Liver steatosis and increased ChREBP expression in mice carrying a liver specific SIRT1 null mutation under a normal feeding condition. Int J Biol Sci2010;6:682-690
    Web of Science | Medline
52. 52
    Chen D, Bruno J, Easlon E, et al. Tissue-specific regulation of SIRT1 by calorie restriction.Genes Dev 2008;22:1753-1757
    CrossRef | Web of Science | Medline
53. 53
    Hasegawa K, Wakino S, Yoshioka K, et al. Kidney-specific overexpression of SIRT1 protects against acute kidney injury by retaining peroxisome function. J Biol Chem2010;285:13045-13056
    CrossRef | Web of Science | Medline
54. 54
    He W, Wang Y, Zhang MZ, et al. Sirt1 activation protects the mouse renal medulla from oxidative injury. J Clin Invest 2010;120:1056-1068
    CrossRef | Web of Science | Medline
55. 55
    Kume S, Uzu T, Horiike K, et al. Calorie restriction enhances cell adaptation to hypoxia through Sirt1-dependent mitochondrial autophagy in mouse aged kidney. J Clin Invest2010;120:1043-1055
    CrossRef | Web of Science | Medline
56. 56
    Haass C, Selkoe DJ. Soluble protein oligomers in neurodegeneration: lessons from the Alzheimer's amyloid beta-peptide. Nat Rev Mol Cell Biol 2007;8:101-112
    CrossRef | Web of Science | Medline
57. 57
    Qin W, Yang T, Ho L, et al. Neuronal SIRT1 activation as a novel mechanism underlying the prevention of Alzheimer disease amyloid neuropathology by calorie restriction. J Biol Chem2006;281:21745-21754
    CrossRef | Web of Science | Medline
58. 58
    Kim D, Nguyen MD, Dobbin MM, et al. SIRT1 deacetylase protects against neurodegeneration in models for Alzheimer's disease and amyotrophic lateral sclerosis.EMBO J 2007;26:3169-3179
    CrossRef | Web of Science | Medline
59. 59
    Donmez G, Wang D, Cohen DE, Guarente L. SIRT1 suppresses beta-amyloid production by activating the alpha-secretase gene ADAM10. Cell 2010;142:320-332[Erratum, Cell 2010;142:494-5.]
    CrossRef | Web of Science | Medline
60. 60
    Min SW, Cho SH, Zhou Y, et al. Acetylation of tau inhibits its degradation and contributes to tauopathy. Neuron 2010;67:953-966[Erratum, Neuron 2010;68:801.]
    CrossRef | Web of Science | Medline
61. 61
    Gao J, Wang WY, Mao YW, et al. A novel pathway regulates memory and plasticity via SIRT1 and miR-134. Nature 2010;466:1105-1109
    CrossRef | Web of Science | Medline
62. 62
    Michan S, Li Y, Chou MM, et al. SIRT1 is essential for normal cognitive function and synaptic plasticity. J Neurosci 2010;30:9695-9707
    CrossRef | Web of Science | Medline
63. 63
    Haigis MC, Mostoslavsky R, Haigis KM, et al. SIRT4 inhibits glutamate dehydrogenase and opposes the effects of calorie restriction in pancreatic beta cells. Cell 2006;126:941-954
    CrossRef | Web of Science | Medline
64. 64
    Nasrin N, Wu X, Fortier E, et al. SIRT4 regulates fatty acid oxidation and mitochondrial gene expression in liver and muscle cells. J Biol Chem 2010;285:31995-32002
    CrossRef | Web of Science | Medline
65. 65
    Nakagawa T, Lomb DJ, Haigis MC, Guarente L. SIRT5 deacetylates carbamoyl phosphate synthetase 1 and regulates the urea cycle. Cell 2009;137:560-570
    CrossRef | Web of Science | Medline
66. 66
    Ogura M, Nakamura Y, Tanaka D, et al. Overexpression of SIRT5 confirms its involvement in deacetylation and activation of carbamoyl phosphate synthetase 1. Biochem Biophys Res Commun 2010;393:73-78
    CrossRef | Web of Science | Medline
67. 67
    Hirschey MD, Shimazu T, Goetzman E, et al. SIRT3 regulates mitochondrial fatty-acid oxidation by reversible enzyme deacetylation. Nature 2010;464:121-125
    CrossRef | Web of Science | Medline
68. 68
    Hallows WC, Yu W, Smith BC, et al. SIRT3 promotes the urea cycle and fatty acid oxidation during dietary restriction. Mol Cell 2011;41:139-149[Erratum, Mol Cell 2011;41:493.]
    CrossRef | Web of Science | Medline
69. 69
    Berdichevsky A, Nedelcu S, Boulias K, Bishop NA, Guarente L, Horvitz HR. 3-Ketoacyl thiolase delays aging of Caenorhabditis elegans and is required for lifespan extension mediated by sir-2.1. Proc Natl Acad Sci U S A 2010;107:18927-18932
    CrossRef | Web of Science | Medline
70. 70
    Guarente L. Mitochondria -- a nexus for aging, calorie restriction, and sirtuins? Cell2008;132:171-176
    CrossRef | Web of Science | Medline
71. 71
    Ahn BH, Kim HS, Song S, et al. A role for the mitochondrial deacetylase Sirt3 in regulating energy homeostasis. Proc Natl Acad Sci U S A 2008;105:14447-14452
    CrossRef | Web of Science | Medline
72. 72
    Kim HS, Patel K, Muldoon-Jacobs K, et al. SIRT3 is a mitochondria-localized tumor suppressor required for maintenance of mitochondrial integrity and metabolism during stress. Cancer Cell 2010;17:41-52
    CrossRef | Web of Science | Medline
73. 73
    Lombard DB, Alt FW, Cheng HL, et al. Mammalian Sir2 homolog SIRT3 regulates global mitochondrial lysine acetylation. Mol Cell Biol 2007;27:8807-8814
    CrossRef | Web of Science | Medline
74. 74
    Qiu X, Brown K, Hirschey MD, Verdin E, Chen D. Calorie restriction reduces oxidative stress by SIRT3-mediated SOD2 activation. Cell Metab 2010;12:662-667
    CrossRef | Web of Science | Medline
75. 75
    Tao R, Coleman MC, Pennington JD, et al. Sirt3-mediated deacetylation of evolutionarily conserved lysine 122 regulates MnSOD activity in response to stress. Mol Cell2010;40:893-904
    CrossRef | Web of Science | Medline
76. 76
    Someya S, Yu W, Hallows WC, et al. Sirt3 mediates reduction of oxidative damage and prevention of age-related hearing loss under caloric restriction. Cell 2010;143:802-812
    CrossRef | Web of Science | Medline
77. 77
    Sundaresan NR, Gupta M, Kim G, Rajamohan SB, Isbatan A, Gupta MP. Sirt3 blocks the cardiac hypertrophic response by augmenting Foxo3a-dependent antioxidant defense mechanisms in mice. J Clin Invest 2009;119:2758-2771
    Web of Science | Medline
78. 78
    Bell EL, Emerling BM, Ricoult SJ, Guarente L. SirT3 suppresses hypoxia inducible factor 1α and tumor growth by inhibiting mitochondrial ROS production. Oncogene 2011 February 28 (Epub ahead of print).
79. 79
    Finley LW, Carracedo A, Lee J, et al. SIRT3 opposes reprogramming of cancer cell metabolism through HIF1α destabilization. Cancer Cell 2011;19:416-428
    CrossRef | Web of Science | Medline
80. 80
    Alcaín FJ, Villalba JM. Sirtuin inhibitors. Expert Opin Ther Patents 2009:3:283-94.
81. 81
    Howitz KT, Bitterman KJ, Cohen HY, et al. Small molecule activators of sirtuins extend Saccharomyces cerevisiae lifespan. Nature 2003;425:191-196
    CrossRef | Web of Science | Medline
82. 82
    Ruderman NB, Xu XJ, Nelson L, et al. AMPK and SIRT1: a long-standing partnership? Am J Physiol Endocrinol Metab 2010;298:E751-E760
    CrossRef | Web of Science | Medline
83. 83
    Zheng J, Ramirez VD. Inhibition of mitochondrial proton F0F1-ATPase/ATP synthase by polyphenolic phytochemicals. Br J Pharmacol 2000;130:1115-1123
    CrossRef | Web of Science | Medline
84. 84
    Szkudelska K, Nogowski L, Szkudelski T. Resveratrol and genistein as adenosine triphosphate-depleting agents in fat cells. Metabolism 2010 September 15 (Epub ahead of print).
85. 85
    Dai H, Kustigian L, Carney D, et al. SIRT1 activation by small molecules: kinetic and biophysical evidence for direct interaction of enzyme and activator. J Biol Chem2010;285:32695-32703
    CrossRef | Web of Science | Medline
86. 86
    Boily G, He XH, Pearce B, Jardine K, McBurney MW. SirT1-null mice develop tumors at normal rates but are poorly protected by resveratrol. Oncogene 2009;28:2882-2893
    CrossRef | Web of Science | Medline
87. 87
    Kim JE, Chen J, Lou Z. DBC1 is a negative regulator of SIRT1. Nature 2008;451:583-586
    CrossRef | Web of Science | Medline
88. 88
    Zhao W, Kruse JP, Tang Y, Jung SY, Qin J, Gu W. Negative regulation of the deacetylase SIRT1 by DBC1. Nature 2008;451:587-590
    CrossRef | Web of Science | Medline
89. 89
    Rose G, Dato S, Altomare K, et al. Variability of the SIRT3 gene, human silent information regulator Sir2 homologue, and survivorship in the elderly. Exp Gerontol 2003;38:1065-1070
    CrossRef | Web of Science | Medline
90. 90
    Bellizzi D, Dato S, Cavalcante P, et al. Characterization of a bidirectional promoter shared between two human genes related to aging: SIRT3 and PSMD13. Genomics 2007;89:143-150
    CrossRef | Web of Science | Medline

---

*Disclaimer* The comments contained in this electronic source of information do not constitute and are not designed to imply that they constitute any form of individual medical advice. The information provided is purely for informational purposes only and not relevant to any person's particular medical condition or situation. If you have any medical concerns about yourself or your family please contact your physician immediately. In order to provide our patients the best uninfluenced information that science has to offer,we do not accept samples of drugs, advertising tchotchkes, money, food, or any item from outside vendors.