The metabolic abnormalities caused by abdominal obesity can be further aggravated by additional factors including advancing age. Typically, aging is accompanied by gradual weight gain, and changes in the levels and distribution of fat (3). In the past, humans have shown increases in visceral fat of roughly 1 lb per year as they progress through adulthood (3). However, modern developments in agriculture and technologies have promoted the intake of high-calorie diets and sedentary lifestyles, causing accelerated weight gain in both children and adults. In obese individuals, excess adipose tissue releases increased levels of free fatty acids (NEFA). These excess fatty acids overload other metabolic tissues such as liver, muscle, and pancreatic β-cells with lipid, resulting in atherogenic dyslipidemia, insulin resistance, and hyperinsulinaemia (4). While the physiological implications of obesity remain an area of intense research, recent evidence has shown that the fat-storage depot white adipose tissue (WAT), has other functions besides fat storage (5). WAT is an endocrine organ, which produces hormones such as resistin, adiponectin, and leptin that are active throughout the entire body (6, 7). WAT can also become a source of inflammation through secretion of adipokines, and accumulation of cytokine producing adipose tissue macrophages (ATMs) (8, 9). In obese people, increased production of inflammatory cytokines and resistin seem to have a role in pro-inflammatory state and insulin resistance. Excess obesity also results in low adiponectin release from WAT which has been implicated in insulin resistance and fatty liver (4).

As a member of class III family of histone deacetylase (HDACs), SIRT1 deacetylates histone proteins in addition to transcription factors and cofactors. As SIRT1 lacks a DNA-binding domain, it is recruited to target promoters by sequence-specific transcription factors to induce chromatin remodeling and subsequently regulation of gene expression (16). SIRT1 is also known to associate with heterochromatin regions and promote deacetylation of histone tails, recruitment and deacetylation of histone H1, and spreading of hypomethylated H3-K79 (17). These epigenetic modifications mediated by SIRT1 generally result in a silencing of gene transcription and are thought to play an integral role in organismal health and ultimately lifespan (18). Given its multiple functions, such as deacetylation, epigenetic modifications, and transcription factor modulation, SIRT1 have been proposed to provide a molecular link between cellular metabolic status, and adaptive transcriptional responses (3).

Not surprisingly, the activity of SIRT1 is tightly controlled in response to different environmental cues. One feeding regime that is thought to heighten sirtuin activation is Caloric Restriction (CR), which is a 20–40% reduction in calories consumed below ad libitum intake without malnutrition. CR has been shown to extend the median and maximum life span of numerous organisms including yeast, flies, worms, fish, and rodents and mammals (19). In mammals, CR can also ameliorate many of the pathologies associated with obesity and metabolic syndrome, such as reduction of body fat, lowering serum triglycerides and LDL cholesterol, increase of HDL cholesterol, and improvement of insulin sensitivity (20, 21). Sirtuins have been implicated as important mediators of CR-mediated lifespan extension. For example, in a number of lower model organisms, CR extends lifespan through activation of sirtuins (14, 22). In mice, SIRT1 regulates energy metabolism and physical responses to CR (23-25). SIRT1 has also been shown to participate in autophagy (26-29), which is a major mechanism by which a starving cell reallocates nutrients from unnecessary processes to more-essential processes (30, 31)

Whether sirtuins mediate the life-extending effects of CR in mammals is currently an area of intense research. A recent study reported that behavior associated with caloric restriction did not occur when SIRT1 knockout mice were put on a calorie restricted diet, implying that SIRT1 is necessary for mediating the effects of caloric restriction (23). Although biochemical parameters thought to mediate the lifespan extending effects of calorie restriction (reduced insulin, IGF1 and fasting glucose) were essentially the same in control mice as those lacking SIRT1, CR did not extend lifespan of full-body SIRT1 knockout mice (24). Moreover, transgenic mice overexpressing SIRT1 are leaner than controls, more metabolically active, and have reduced serum levels of cholesterol, adipokines, insulin, and glucose (32-34). Furthermore, activation of SIRT1 by the polyphenol resveratrol and several synthetic pharmacologic activators has been shown to protect against high-fat induced obesity and metabolic derangements (35-37).

Consistent with above observations, SIRT1 protein levels have been shown to be elevated during CR in the brain, WAT, muscles, liver, and kidney (38, 39). However, cellular NAD⁺ levels or the NAD⁺/NADH ratio, which have been claimed as the primary mechanisms regulating SIRT1 activity, fluctuate depending on tissue type during CR (40-42), suggesting that the activity of SIRT1 changes in different directions in different tissues during CR. Additional stimuli also modulate SIRT1 activity by altering cellular NAD⁺ levels or the NAD⁺/NADH ratio. For example, in skeletal muscle C2C12 cells, SIRT1 activity is enhanced by AMP-activated protein kinase (AMPK) and increasing cellular NAD⁺ levels (43, 44). SIRT1 activity in C2C12 cells also appears to be under control of adiponectin through Ca²⁺ signaling and changes of the NAD⁺/NADH ratio (45). What drives cellular NAD⁺/NADH levels and SIRT1 activity during different physiological conditions, and in different tissues remains unclear. For instance, it has been reported that cellular NAD⁺ levels increase in the liver upon starvation/fasting, but decrease upon refeeding (46). However, a more recent report by Escande et al. analyzed the levels of both SIRT1 protein and NAD⁺ concentration in the liver following a standard diet, starvation, and HFD feeding (47). Their data show that neither NAD⁺ levels nor SIRT1 protein levels fluctuate in the liver regardless of feeding patterns, which suggests that factors other than NAD⁺ changes, such as post-translational modifications or protein-protein interactions, regulate SIRT1 activity (47).

The biological consequence of SIRT1's ability to control hepatic lipid metabolism is evidenced in several liver-specific transgenic mouse studies. Using an adenovirus expressing short hairpin RNA to knock down SIRT1, Rogers et al. showed that adenoviral knockdown of SIRT1 reduces expression of fatty acid β-oxidation genes in the liver of fasted mice (59). Using liver-specific knockout mice (SIRT1 LKO), our group showed that deletion of SIRT1 in the liver impairs PPARα signaling and decreases fatty acid β-oxidation, whereas over-expression of SIRT1 induces expression of PPARα target genes (53). As a result, SIRT1 LKO mice are susceptible to high-fat diet induced dyslipidemia and hepatic steatosis (53). Our data further suggest that SIRT1 likely modulates PPARα activity through PGC-1α, a key coactivator for PPARα signaling and a direct target of SIRT1 (46, 60). SIRT1 activates PGC-1α primarily though its ability to deacetylate this coactivator.

In addition to fatty acid metabolism, SIRT1 has also been shown to regulate cholesterol metabolism through deacetylation of LXRs (55). Activation of LXR is beneficial in that it not only inhibits intestinal cholesterol uptake and promotes reverse cholesterol transport but also exerts potent anti-inflammatory effects that involve transrepression (61). SIRT1 can directly deacetylate LXRs, resulting in increased LXR turnover and target gene expression (55). Disruptions in LXR signaling are observed in whole-body SIRT1 deficient mice, which have lower levels of HDL cholesterol, while low-density lipoprotein (LDL) cholesterol is unaffected (55). Since cholesterol efflux is reduced in macrophages and hepatocytes from these mice, it is likely that SIRT1 positively controls LXR activity to stimulate reverse cholesterol transport in vivo. An independent study from Rodgers et al. has shown that another LXR target gene CYP7A1 is reduced in the absence of hepatic SIRT1. This evidence suggests that bile acid synthesis, in addition to cholesterol efflux, is enhanced by SIRT1 (59). Interestingly, the regulation of CYP7A1 was also found to be dependent on the coregulator PGC-1α.

FXR is also a target of hepatic SIRT1 in metabolic regulation (56). FXR is an important regulator of cholesterol and bile acid homeostasis that regulates expression of numerous bile acid-responsive genes in the liver and intestine (57). Acetylation of FXR inhibits its activity and is dynamically regulated by p300 and SIRT1 under normal conditions (56). Downregulation of hepatic SIRT1 increases FXR acetylation, causing deleterious metabolic outcomes. Additionally, FXR acetylation levels were shown to be elevated in two mouse models of metabolic disorders, ob/ob mice and mice chronically fed a western-style diet. Treatment of mice with resveratrol or overexpression of SIRT1 substantially reduced FXR acetylation levels in these disease models (56). These findings provide an intriguing correlation between elevated FXR acetylation and decreased SIRT1 activity, and provide further evidence of benefits for therapeutic targeting of SIRT1.

Another means by which SIRT1 may regulate hepatic lipid metabolism is through deacetylation of the sterol regulatory element binding protein (SREBP) family of transcription factors. SREBPs are critical regulators of lipid metabolism, which promote expression of lipogenic and cholesterolgenic genes involved in lipid storage (62). Two recent reports revealed that SIRT1 can directly deacetylate SREBP, and that SIRT1 activity is important in the fasting-dependent attenuation of SREBP (63, 64). In addition, chemical activators of SIRT1 inhibit SREBP target gene expression in vitro and in vivo, correlating with decreased hepatic lipid and cholesterol levels and attenuated liver steatosis in diet-induced and genetically obese mice. In summary, these findings imply that hepatic SIRT1 plays a critical role in metabolic regulation and activation of SIRT1 in the liver may prove beneficial in treating obesity-associates diseases.

Again, studies on the function of SIRT1 in the liver have yielded controversial and even contradictory observations. For example, we have observed that SIRT1 LKO mice gain weight and develop hepatic steatosis when fed a high-fat high-cholesterol diet (53). However, using a similar hepatic-specific knockout mouse model, Chen et al. (42) reported a reduction in weight gain and liver fat accumulation in LKO mice when fed a western-style diet. One possible factor contributing to the discrepancy between our observations and those of Chen et al. may be the difference in age of animals at which the feeding was initiated and data were collected. The varied responses of SIRT1 LKO mice to a western-style diet at different ages raises the possibility that hepatic SIRT1 may selectively regulate alternative metabolic pathways at multiple stages of development. An inducible SIRT1 knockout model will be helpful to dissect age-dependent effects of SIRT1.

Over the past few years, sirtuins have been identified as important regulators of the immune system, with several studies showing that SIRT1 can repress inflammation in multiple tissues including the macrophage (71-74). Some reports have demonstrated that artificial overexpression of SIRT1 leads to suppression of the inflammatory response, whereas deletion of SIRT1 in hepatocytes results in increased local inflammation (34, 53). It appears that in immune signaling SIRT1's inhibitory actions work through at least two mechanistically distinct pathways. On one hand, sirtuins may diminish histone acetylation by inactivating histone acetyltransferase (HAT) enzymatic activity. In support of this notion, SIRT1 directly interacts with p300, the CREB-binding protein (CBP), and other HATs to inhibit the acetylation status of these enzymes (75). Alternatively, SIRT1 has recently been reported to deacetylate the RelA/p65 subunit of NF-κB at lysine 310 in vitro using overexpression systems (75). Deacetylation of K310 of RelA/p65 leads to decreases in NF-κB transcription activity, reducing production of proinflammatory cytokines and anti-apoptotic genes (75). In addition, it was recently reported that moderate overexpression of SIRT1 in mice leads to down-regulated NF-κB activity (34).

Obese individuals have an increase in quantity and size of adipocytes, whereas individuals under dietary restriction display decrease in body weight and WAT mass. The repression of PPARγ activity by SIRT1 in WAT therefore implies that SIRT1 may regulate adipocyte formation and function in response to dietary restriction (38). As a reduction in adipose tissue is sufficient to extend murine lifespan (81), these findings provide a possible molecular pathway connecting calorie restriction to life extension in mammals.

The anorexigenic POMC expressing neurons and the orexigenic agouti-related protein (AgRP) expressing neurons of the hypothalamus are the major regulators of feeding and energy expenditure (85). The POMC neurons produce satiety peptides thereby inhibiting food intake after feeding, while the AgRP neurons promote feeding in response to fasting and CR. Studies have shown that CR and fasting enhance SIRT1 expression and activity in the hypothalamus (86, 87). However, in contrast to its anti-obesity functions in the periphery tissues, increased hypothalamic SIRT1 appears to associate with increased food intake and subsequent body-weight gain. A study by Çakir et al. revealed that ablation or knockdown of SIRT1 in the hypothalamic region results in decreased food intake and body weight gain (Figure 6, (86)). Moreover, fasting increases hypothalamic NAD⁺ levels and SIRT1 protein content, while inhibition of hypothalamic SIRT1 activity reverses the fasting induced decrease of FOXO1 acetylation, resulting in increased POMC and decreased AgRP expressions (86). Furthermore, recent studies have demonstrated that brain-specific SIRT1 transgenic mice displays enhanced neural activity in hypothalamus (87), and AgRP neurons-specific deletion of SIRT1 decreases AgRP neuronal activity, thereby alleviating the inhibitory tone on the POMC neurons, resulting in decreased food intake and body weight (88). However, the apparent dichotomy of role of SIRT1 action in the periphery vs the hypothalamus is not all together surprising, given the fact that both fasting and CR induce fatty acid oxidation in periphery tissues but feeding behavior through central regulation. These observations confirm that SIRT1 is an essential element in the periphery-central feedback circuits that mediate normal responses to nutrient deprivation.

In addition, the role of SIRT1 in the central control of whole body energy homeostasis appears to be complicated. A recent study has revealed that POMC neurons-specific deletion of SIRT1 in mice causes hypersensitivity to diet-induced obesity due to a blunted response to leptin signaling and reduced energy expenditure (83). Importantly, deletion of SIRT1 in POMC neurons does not lead to overt dysfunctions of the CNS melanocortin system, and the reduced energy expenditure is not associated with changes in food intake (83). Therefore, it is possible that chronic losses of SIRT1 in different neurons lead to different compensation patterns in the hypothalamus. An inducible cell type-specific SIRT1 knockout model will be helpful to further dissect the roles of SIRT1 in regulating central adaptive response to nutrient signals.

Recent studies have linked SIRT1 function to the regulation of the circadian rhythm. The mammalian clock is controlled by negative-feedback loops mediated by the heterodimeric transcription factors CLOCK-BMAL1 and their transcriptional targets, including the PER and CRY proteins that directly repress CLOCK-BMAL1 activity, as well as REV-ERB and ROR nuclear receptors that control BMAL1 expression (95). Interestingly, the CLOCK protein itself is a transcriptional activator that functions as a histone acetyltransferase (HAT) (96), indicating that chromatin remodeling plays an important role in the regulation of circadian gene expression. Two earlier studies have shown that SIRT1 interacts with CLOCK-BMAL1 to directly regulate the amplitude of circadian clock-controlled gene expression through deacetylation of PER2 and/or BMAL1 (97, 98). However, whether it is the amount of SIRT1 or its activity that is cyclically regulated by circadian clock remains under debate. Two recent studies appear to favor the circadian regulation of SIRT1 activity (99, 100). The expression of NAMPT, an enzyme that controls a rate-limiting step in NAD⁺ biosynthesis, is directly controlled by CLOCK-BMAL1 (Figure 7). It has been proposed that this regulation leads to circadian oscillation of cellular NAD⁺ levels, resulting in a cyclical regulation of SIRT1 activity (99, 100). Together, these studies add a new feedback loop in the circadian clock that involves CLOCK-BMAL1, NAMPT, NAD⁺, and SIRT1 (Figure 7), and provide an important link between the circadian clock and cellular metabolism.