Generally, NAD-linked dehydrogenase catalyzes oxidoreduction reactions in the oxidative pathways of metabolism, particularly in glycolysis, in the citric acid cycle, and in the respiratory chain of mitochondria. NAD⁺ not only acts as a coenzyme for oxidoreductases, but also as a donator of ADP-ribose in some specific reactions. Three major families of enzymes can cleave NAD⁺ in mammals: sirtuins; ADP-ribose transferases, including poly(ADP-ribose) polymerases (PARPs); and cyclic ADP (cADP)-ribose synthases.15 In addition to de novo synthesis from tryptophan, the major pool of NAD⁺ is from salvage pathways, which require the uptake of other NAD⁺ precursors. Dietary niacin, consisting of nicotinic acid (NA), nicotinamide (NAM), and NAM riboside (NR), can serve as an NAD⁺ precursor by means of the salvage pathways (Fig. 2).5 In mammals, NAM phosphoribosyltransferase (NAMPT) converts NAM to NAM mononucleotide (NMN), an enzymatic activity that has not been described for lower organisms.5 There are two isoforms of NAMPT, intracellular and extracellular forms (iNAMPT and eNAMPT, respectively). NAMPT, which is the rate-limiting step of this part of the pathway, is highly expressed in brown adipose tissue and liver and is undetectable in brain and pancreas.22 By virtue of its function in the conversion of NAM to NAD⁺, hence lowering NAM levels and increasing NAD⁺, NAMPT is considered an important regulatory enzyme with respect to the NAD⁺ consumers, notably the aging-associated histone deacetylase Sirt1. In human vascular smooth muscle cells, reduced NAMPT expression resulted in premature senescence, whereas a significant delay in senescence was observed upon overexpression of NAMPT.22,23 NAMPT, also known as visfatin, has been reported to have an insulin mimetic effect.15 However, further studies are required to verify this insulin mimetic effect. Among de novo biosynthetic pathway and salvage pathways, it is yet to be determined which pathway is more important for the cellular concentration of NAD and Sirt1 activation.

A protein called DBC1 (deleted in breast cancer 1) has been shown to inhibit Sirt1 deacetylation of its well-established substrate, p53.24 Stress-inducing agents such as UV radiation and hydrogen peroxide induce the desumoylase SENP1 to interact with Sirt1. This association caused the desumoylation and consequent inactivation of Sirt1.25 A nuclear protein, formerly identified as a ribosomal S19-binding protein with unknown function, was shown to enhance Sirt1 activity and was thereby named active regulator of Sirt1 (AROS). The expression profile of AROS is remarkably similar to that of Sirt1 in multiple cell lines. AROS specifically interacts with Sirt1 but not with other sirtuins, and its binding promoted Sirt1 to deacetylate p53.26

Mammalian AMPK is a Ser/Thr kinase that is activated upon alterations in the cellular AMP/ATP ratio. Hence, perturbations in this ratio due to either defects in energy production or increased energy consumption will lead to the activation of AMPK.27 AMPK as a metabolic switch activates catabolic pathways to produce ATP while simultaneously shutting down ATP-consuming anabolic processes. AMPK mediates these effects through the rapid phosphorylation of metabolic enzymes, such as acetyl CoA carboxylase and hydroxymethylglutaryl-CoA reductase, two rate-limiting enzymes for fatty acid oxidation and cholesterol synthesis, respectively.28 If ATP stores remain depleted, AMPK can induce the phosphorylation of transcription factors and co-activators that regulate gene expression, including FOXO3, PGC1α, and HNF4. Interestingly, many of these transcription factors are also regulated by Sirt1.29 In addition to having common activation events (glucose restriction, fasting, chronic calorie restriction, oxidative stress, and exercise), the AMPK and Sirt1 signaling pathways have similar effects on life span, aging, and metabolism.29 Like Sirt1, AMPK has been proposed to be one of several molecules involved in regulating mammalian longevity. Chronic activation of AMPK occurs on a calorically restricted diet and has been proposed as a longevity strategy for mammals. Consistent with this idea, additional copies of the AMPK gene are sufficient to extend lifespan in C. elegans.30 The aging process itself has been described to be associated with a decline in the activity of both Sirt1 and AMPK.

AMPK can phosphorylate PGC-1α directly,31 whereas Sirt1 deacetylates and activates PGC-1α.29 The connection between the two pathways was revealed in a recent excellent study.32 AMPK regulates the expression of mitochondrial and lipid metabolism genes through the modulation of PGC-1α activity. This activation of PGC-1α involves both direct phosphorylation of PGC-1α by AMPK and deacetylation of PGC-1α by Sirt1. It seems that AMPK indirectly activates Sirt1 by altering the intracellular NAD⁺/NADH ratio through an increase in mitochondrial fatty acid oxidation (Fig. 3).32

Resveratrol (3, 5, 4'-trihydroxystilbene) (Fig. 4) belongs to the large group of polyphenols found in different plant species. Recently, resveratrol has been in the center of the Sirt1 research field after becoming known as a Sirt1 activator. Although the richest natural source of resveratrol is Polygonum cuspidatum, a plant root extract that has been used in Oriental folk medicine, grapes, grape juice, and red wine are more well-known sources because of the "French paradox," i.e., low mortality due to coronary heart disease as a result of moderate consumption of red wine.1 There is an enormous amount of data about resveratrol and Sirt1. However, data concerning the potential beneficial effects of resveratrol in humans are still very limited.

Long-term rodent studies provided relatively convincing evidence that resveratrol exerts favorable effects in high-fat-fed animals. Resveratrol (22.4 mg/kg/day) treatment of 1-year-old middle-aged mice on a high-fat diet extends their life span, enhances insulin sensitivity, and increases hepatic mitochondrial content similar to calorie-restricted animals with greater Sirt1 expression.13 The other important issue is that in contrast to the effect in high-fat-fed mice, mice consuming a standard diet did not live longer when treated with resveratrol.13 A high dose of resveratrol (400 mg/kg/day) significantly increases the aerobic capacity and the endurance during exercise test by promoting oxidative phosphorylation and mitochondrial biogenesis of skeletal muscle in high-fat-fed or control mice and decreases total body fat content in high-fat-fed mice.14 However, a lower dose of resveratrol (10 mg/kg body weight; admi-nistered for 8 weeks) was shown to be ineffective in reducing body weight gain in obese Zucker rats, although a slight decrease in body fat content was found.33 I also observed that resveratrol therapy (30 mg/kg daily for 3 weeks) did not decrease body weight significantly in mice on a high-fat diet (manuscript in preparation). Even a smaller dose of resveratrol (4.9 mg/kg daily) in aging mice was reported to be effective in preventing cardiac and skeletal muscle dysfunctions induced by ageing and attenuating age-related changes in gene expressions.34 Effects caused by resveratrol paralleled those found in mice on a calorie-restricted diet or evoked by every-other-day feeding.35 It is also known that CR increases the expression of Sirt1, which provided further evidence supporting the importance of Sirt1 activation in the mechanism of resveratrol's action.35 Interestingly, AMPK activity is also increased by resveratrol therapy in rats fed a high-fat diet.13 Intensive research is ongoing to identify more potent, and more bioavailable, activators of Sirt1 than resveratrol. Recently, several small molecule activators of Sirt1 have been developed. They are structurally unrelated to and 1000-fold more potent than resveratrol and were shown to improve insulin sensitivity, lower plasma glucose, and increase mitochondrial function in insulin-resistant obese mice models.36