Caloric restriction, SIRT1 and longevity

Interference between PARPs and SIRT1: a novel approach to healthy ageing?

Poly(ADP-ribosyl) polymerases (PARPs) have traditionally been linked to chromosome maintenance and DNA repair. Recent findings identify PARPs as key modulators of metabolism through their influence on SIRT1 activity, hinting to a possible role of PARPs as longevity regulators.

Free will and the obesity epidemic

The increase in body weight in the USA over the past several decades is now commonly referred to as the 'obesity epidemic'. An empirical analysis of the literature suggests that the increased weight can be accounted for by an increase in food intake. The solution to the obesity epidemic, therefore, must centre on a reduction in food consumption, a position well accepted by the American population who think that they, as individuals, are responsible for their adiposity by holding the belief that the decision as to what and how much to eat is determined by their own free will. The evidence demonstrates, however, that this is not true. Variables such as portion size, variety of foods offered, fat content of the diet, the number of people eating, the location where eating occurs and even watching food advertisements act as 'food primes' causing individuals to increase their energy intake. Despite the plethora of diets, weight-loss clubs, drugs and mechanical devices available to facilitate weight loss, once treatment is terminated and people return to the 'free' environment, their weight returns to pre-treatment levels. Only when individuals are protected from environmental variables by gastric surgery or limited to consume only portion-controlled meals can they successfully maintain a reduced weight. Combining the technique of daily weight monitoring with accepting that our eating behaviour is not determined totally by our free choice, we may be able to curb the obesity epidemic.

Wld(S) protects against peripheral neuropathy and retinopathy in an experimental model of diabetes in mice

AIMS/HYPOTHESIS

We aimed to evaluate the effect of the mutant Wld(S) (slow Wallerian degeneration; also known as Wld) gene in experimental diabetes on early experimental peripheral diabetic neuropathy and diabetic retinopathy.

METHODS

The experiments were performed in four groups of mice: wild-type (WT), streptozotocin (STZ)-induced diabetic WT, C57BL/Wld(S) and STZ-induced diabetic C57BL/Wld(S). In each group, intraperitoneal glucose and insulin tolerance tests were performed; blood glucose, glycated haemoglobin and serum insulin were monitored. These mice were also subjected to the following behavioural tests: grasping test, hot-plate test and von Frey aesthesiometer test. For some animals, sciatic-tibial motor nerve conduction velocity, tail sensory nerve conduction velocity and eye pattern electroretinogram were measured. At the end of the experiments, islets were isolated to detect glucose-stimulated insulin secretion, ATP content and extent of apoptosis. The NAD/NADH ratio in islets and retinas was evaluated. Surviving retinal ganglion cells were estimated by immunohistochemistry.

RESULTS

We found that the Wld(S) gene is expressed in islets and protects beta cells against multiple low doses of STZ by increasing the NAD/NADH ratio, maintaining the ATP concentration, and reducing apoptosis. Consistently, significantly higher insulin concentrations, lower blood glucose concentrations, and better glucose tolerance were observed in Wld(S) mice compared with WT mice after STZ treatment. Furthermore, Wld(S) alleviated abnormal sensory responses, nerve conduction, retina dysfunction and reduction of surviving retinal ganglion cells in STZ-induced diabetic models.

CONCLUSIONS/INTERPRETATION

We provide the first evidence that expression of the Wld(S) gene decreases beta cell destruction and preserves islet function in STZ-induced diabetes, thus revealing a novel protective strategy for diabetic models.

Kidney aging--inevitable or preventable?

The aging process affects all organs, including the kidneys. As part of this process, progressive scarring and a measurable decline in renal function occur in most people over time. The improved understanding of the processes that can lead to and/or hasten scarring and loss of renal function over time parallels advances in our understanding of the aging process. Clinical factors, including hypertension, diabetes mellitus, obesity, abnormal lipid levels and vitamin D deficiency, have been associated with increasing renal sclerosis with age. In addition, tissue factors such as angiotensin II, advanced glycation end products, oxidative stress and Klotho are associated with renal aging. These associations and possible interventions, including the control of blood pressure, blood sugar, weight, diet and calorie restriction might make renal aging more preventable than inevitable.

Regulation and protection of mitochondrial physiology by sirtuins

The link between sirtuin activity and mitochondrial biology has recently emerged as an important field. This conserved family of NAD(+)-dependent deacetylase proteins has been described to be particularly involved in metabolism and longevity. Recent studies on protein acetylation have uncovered a high number of acetylated mitochondrial proteins indicating that acetylation/deacetylation processes may be important not only for the regulation of mitochondrial homeostasis but also for metabolic dysfunction in the context of various diseases such as metabolic syndrome/diabetes and cancer. The functional involvement of sirtuins as sensors of the redox/nutritional state of mitochondria and their role in mitochondrial protection against stress are hereby described, suggesting that pharmacological manipulation of sirtuins is a viable strategy against several pathologies.

SIRT1 controls lipolysis in adipocytes via FOXO1-mediated expression of ATGL

Recent studies have established SIRT1 as an important regulator of lipid metabolism, although the mechanism of its action at the molecular level has not been revealed. Here, we show that knockdown of SIRT1 with the help of small hairpin RNA decreases basal and isoproterenol-stimulated lipolysis in cultured adipocytes. This effect is attributed, at least in part, to the suppression of the rate-limiting lipolytic enzyme, adipose triglyceride lipase (ATGL), at the level of transcription. Mechanistically, SIRT1 controls acetylation status and functional activity of FoxO1 that directly binds to the ATGL promoter and regulates ATGL gene transcription. We have also found that depletion of SIRT1 decreases AMP-dependent protein kinase (AMPK) activity in adipocytes. To determine the input of AMPK in regulation of lipolysis, we have established a stable adipose cell line that expresses a dominant-negative α1 catalytic subunit of AMPK under the control of the inducible TET-OFF lentiviral expression vector. Reduction of AMPK activity does not have a significant effect on the rates of lipolysis in this cell model. We conclude, therefore, that SIRT1 controls ATGL transcription primarily by deacetylating FoxO1.

Sirt1's systemic protective roles and its promise as a target in antiaging medicine

Silent information regulator 2 (Sir2/Sirt1), a member of the sirtuin family of class III histone deacetylases, has been implicated extensively in lifespan extension and is a prominent drug target in antiaging medicine. The mammalian Sirt1 has multiple targets, which include histones, transcription factors, and other molecules that collectively modulate energy metabolism, stress response, and cell/tissue survival. Several of Sirt1's substrates regulate key metabolic processes, and Sirt1 activation may underlie the lifespan prolonging effect of caloric restriction. Recent studies have also identified multifaceted protective roles for Sirt1 against cellular senescence and stress in the neural, cardiovascular, and renal systems. Sirt1's activity in multiple tissues may decline with aging, and sustaining or reactivating this activity seems invariably beneficial. Several studies also point towards a general tumor suppressive role for Sirt1, at least in the context of certain human cancers. Development of Sirt1-based therapeutic interventions against systemic aging and aging-associated diseases will benefit from a thorough understanding of underlying pathological mechanisms of diseases as well as metabolic connections between different tissues and organs.

Food scarcity, neuroadaptations, and the pathogenic potential of dieting in an unnatural ecology: binge eating and drug abuse

In the laboratory, food restriction has been shown to induce neuroadaptations in brain reward circuitry which are likely to be among those that facilitate survival during periods of food scarcity in the wild. However, the upregulation of mechanisms that promote foraging and reward-related learning may pose a hazard when food restriction is self-imposed in an ecology of abundant appetitive rewards. For example, episodes of loss of control during weight-loss dieting, use of drugs with addictive potential as diet aids, and alternating fasting with alcohol consumption in order to avoid weight gain, may induce synaptic plasticity that increases the risk of enduring maladaptive reward-directed behavior. In the present mini-review, representative basic research findings are outlined which indicate that food restriction alters the function of mesoaccumbens dopamine neurons, potentiates cellular and behavioral responses to D-1 and D-2 dopamine receptor stimulation, and increases stimulus-induced synaptic insertion of AMPA receptors in nucleus accumbens. Possible mechanistic underpinnings of increased drug reward magnitude, drug-seeking, and binge intake of sucrose in food-restricted animal subjects are discussed and possible implications for human weight-loss dieting are considered.

Mammalian circadian clock and metabolism - the epigenetic link

Circadian rhythms regulate a wide variety of physiological and metabolic processes. The clock machinery comprises complex transcriptional-translational feedback loops that, through the action of specific transcription factors, modulate the expression of as many as 10% of cellular transcripts. This marked change in gene expression necessarily implicates a global regulation of chromatin remodeling. Indeed, various descriptive studies have indicated that histone modifications occur at promoters of clock-controlled genes (CCGs) in a circadian manner. The finding that CLOCK, a transcription factor crucial for circadian function, has intrinsic histone acetyl transferase (HAT) activity has paved the way to unraveling the molecular mechanisms that govern circadian chromatin remodeling. A search for the histone deacetylase (HDAC) that counterbalances CLOCK activity revealed that SIRT1, a nicotinamide adenin dinucleotide (NAD(+))-dependent HDAC, functions in a circadian manner. Importantly, SIRT1 is a regulator of aging, inflammation and metabolism. As many transcripts that oscillate in mammalian peripheral tissues encode proteins that have central roles in metabolic processes, these findings establish a functional and molecular link between energy balance, chromatin remodeling and circadian physiology. Here we review recent studies that support the existence of this link and discuss their implications for understanding mammalian physiology and pathology.

Molecular inflammation as an underlying mechanism of the aging process and age-related diseases

Aging is a biological process characterized by time-dependent functional declines that are influenced by changes in redox status and by oxidative stress-induced inflammatory reactions. An organism's pro-inflammatory status may underlie the aging process and age-related diseases. In this review, we explore the molecular basis of low-grade, unresolved, subclinical inflammation as a major risk factor for exacerbating the aging process and age-related diseases. We focus on the redox-sensitive transcription factors, NF-κB and FOXO, which play essential roles in the expression of pro-inflammatory mediators and anti-oxidant enzymes, respectively. Major players in molecular inflammation are discussed with respect to the age-related up-regulation of pro-inflammatory cytokines and adhesion molecules, cyclo-oxygenase-2, lipoxygenase, and inducible nitric oxide synthase. The molecular inflammation hypothesis proposed by our laboratory is briefly described to give further molecular insights into the intricate interplay among redox balance, pro-inflammatory gene activation, and chronic age-related inflammatory diseases. The final section discusses calorie restriction as an aging-retarding intervention that also exhibits extraordinarily effective anti-inflammatory activity by modulating GSH redox, NF-κB, SIRT1, PPARs, and FOXOs.

SIRT1 contributes to telomere maintenance and augments global homologous recombination

SIRT1 is a positive regulator of telomere length and attenuates age-associated telomere shortening.

Yeast Sir2 deacetylase is a component of the silent information regulator (SIR) complex encompassing Sir2/Sir3/Sir4. Sir2 is recruited to telomeres through Rap1, and this complex spreads into subtelomeric DNA via histone deacetylation. However, potential functions at telomeres for SIRT1, the mammalian orthologue of yeast Sir2, are less clear. We studied both loss of function (SIRT1 deficient) and gain of function (SIRT1^super) mouse models. Our results indicate that SIRT1 is a positive regulator of telomere length in vivo and attenuates telomere shortening associated with aging, an effect dependent on telomerase activity. Using chromatin immunoprecipitation assays, we find that SIRT1 interacts with telomeric repeats in vivo. In addition, SIRT1 overexpression increases homologous recombination throughout the entire genome, including telomeres, centromeres, and chromosome arms. These findings link SIRT1 to telomere biology and global DNA repair and provide new mechanistic explanations for the known functions of SIRT1 in protection from DNA damage and some age-associated pathologies.

The other lives of ribosomal proteins

Despite the fact that ribosomal proteins are the constituents of an organelle that is present in every cell, they show a surprising level of regulation, and several of them have also been shown to have other extra-ribosomal functions, such in replication, transcription, splicing or even ageing. This review provides a comprehensive summary of these important aspects.

Novel avenues of drug discovery and biomarkers for diabetes mellitus

Globally, developed nations spend a significant amount of their resources on health care initiatives that poorly translate into increased population life expectancy. As an example, the United States devotes 16% of its gross domestic product to health care, the highest level in the world, but falls behind other nations that enjoy greater individual life expectancy. These observations point to the need for pioneering avenues of drug discovery to increase life span with controlled costs. In particular, innovative drug development for metabolic disorders such as diabetes mellitus becomes increasingly critical given that the number of diabetic people will increase exponentially over the next 20 years. This article discusses the elucidation and targeting of novel cellular pathways that are intimately tied to oxidative stress in diabetes mellitus for new treatment strategies. Pathways that involve wingless, β-nicotinamide adenine dinucleotide (NAD(+)) precursors, and cytokines govern complex biological pathways that determine both cell survival and longevity during diabetes mellitus and its complications. Furthermore, the role of these entities as biomarkers for disease can further enhance their utility irrespective of their treatment potential. Greater understanding of the intricacies of these unique cellular mechanisms will shape future drug discovery for diabetes mellitus to provide focused clinical care with limited or absent long-term complications.

Stress resistance and longevity are not directly linked to levels of enzymatic antioxidants in the ponerine ant Harpegnathos saltator

Background

The molecular mechanisms of variations in individual longevity are not well understood, even though longevity can be increased substantially by means of diverse experimental manipulations. One of the factors supposed to be involved in the increase of longevity is a higher stress resistance. To test this hypothesis in a natural system, eusocial insects such as bees or ants are ideally suited. In contrast to most other eusocial insects, ponerine ants show a peculiar life history that comprises the possibility to switch during adult life from a normal worker to a reproductive gamergate, therewith increasing their life expectancy significantly.

Results

We show that increased resistance against major stressors, such as reactive oxygen species and infection accompanies the switch from a life-history trait with normal lifespan to one with a longer life expectancy. A short period of social isolation was sufficient to enhance stress resistance of workers from the ponerine ant species Harpegnathos saltator significantly. All ant groups with increased stress resistances (reproducing gamergates and socially isolated workers) have lower catalase activities and glutathione levels than normal workers. Therewith, these ants resemble the characteristics of the youngest ants in the colony.

Conclusions

Social insects with their specific life history including a switch from normal workers to reproducing gamergates during adult life are well suited for ageing research. The regulation of stress resistance in gamergates seemed to be modified compared to foraging workers in an economic way. Interestingly, a switch towards more stress resistant animals can also be induced by a brief period of social isolation, which may already be associated with a shift to a reproductive trajectory. In Harpegnathos saltator, stress resistances are differently and potentially more economically regulated in reproductive individuals, highlighting the significance of reproduction for an increase in longevity in social insects. As already shown for other organisms with a long lifespan, this trait is not directly coupled to higher levels of enzymatic and non-enzymatic antioxidants.

Hormonal regulation of gluconeogenic gene transcription in the liver

Glucose homeostasis in mammals is achieved by the actions of counterregulatory hormones, namely insulin, glucagon and glucocorticoids. Glucose levels in the circulation are regulated by the liver, the metabolic centre which produces glucose when it is scarce in the blood. This process is catalysed by two rate-limiting enzymes, phosphoenolpyruvate carboxykinase (PEPCK) and glucose-6-phosphatase (G6Pase) whose gene expression is regulated by hormones. Hormone response units (HRUs) present in the two genes integrate signals from various signalling pathways triggered by hormones. How such domains are arranged in the regulatory region of these two genes, how this complex regulation is accomplished and the latest advancements in the field are discussed in this review.

Circadian clocks in fuel harvesting and energy homeostasis

Circadian systems have evolved in plants, eubacteria, neurospora, and the metazoa as a mechanism to optimize energy acquisition and storage in synchrony with the rotation of the Earth on its axis. In plants, circadian clocks drive the expression of genes involved in oxygenic photosynthesis during the light and nitrogen fixation during the dark, repeating this cycle each day. In mammals, the core clock in the suprachiasmatic nucleus (SCN) functions to entrain extra-SCN and peripheral clocks to the light cycle, including regions central to energy homeostasis and sleep, as well as peripheral tissues involved in glucose and lipid metabolism. Tissue-specific gene targeting has shown a primary role of clock genes in endocrine pancreas insulin secretion, indicating that local clocks play a cell-autonomous role in organismal homeostasis. A present focus is to dissect the consequences of clock disruption on modulation of nuclear hormone receptor signaling and on posttranscriptional regulation of intermediary metabolism. Experimental genetic studies have pointed toward extensive interplay between circadian and metabolic systems and offer a means to dissect the impact of local tissue molecular clocks on fuel utilization across the sleep-wake cycle.

NAD+ as a signaling molecule modulating metabolism

The ability of NAD(+) to act as a metabolic cofactor and as a rate-limiting cosubstrate for many enzymes, particularly the sirtuins, has led to the identification of a pivotal role of NAD(+) levels in the control of whole-body metabolic homeostasis. Bioavailability and compartmentalization of NAD(+) have become highly relevant issues that we need to understand in order to elucidate how NAD(+) acts both as a readout of the metabolic milieu and as an effector triggering appropriate cellular adaptations.

Translating cell survival and cell longevity into treatment strategies with SIRT1

The sirtuin SIRT1, a class III NAD(+)-dependent protein histone deacetylase, is present throughout the body that involves cells of the central nervous system, immune system, cardiovascular system, and the musculoskeletal system. SIRT1 has broad biological effects that affect cellular metabolism as well as cellular survival and longevity that can impact both acute and chronic disease processes that involve neurodegenerative disease, diabetes mellitus, cardiovascular disease, and cancer. Given the intricate relationship SIRT1 holds with a host of signal transduction pathways ranging from transcription factors, such as forkhead, to cytokines and growth factors, such as erythropoietin, it becomes critical to elucidate the cellular pathways of SIRT1 to safely and effectively develop and translate novel avenues of treatment for multiple disease entities.

Disruption of adaptive energy metabolism and elevated ribosomal p-S6K1 levels contribute to INCL pathogenesis: partial rescue by resveratrol

The infantile neuronal ceroid lipofuscinosis (INCL) is a devastating neurodegenerative lysosomal storage disease. Despite our knowledge that palmitoyl-protein thioesterase-1 (PPT1)-deficiency causes INCL, the molecular mechanism(s) of neurodegeneration and the drastically reduced lifespan of these patients remain poorly understood. Consequently, an effective treatment for this disease is currently unavailable. We previously reported that oxidative stress-mediated abnormality in mitochondria activates caspases-9 pathway of apoptosis in INCL fibroblasts and in neurons of Ppt1-knockout (Ppt1-KO) mice, which mimic INCL. Since mitochondria play critical roles in maintaining cellular energy homeostasis, we hypothesized that oxidative stress-mediated disruption of energy metabolism and homeostasis may contribute to INCL pathogenesis. We report here that, in cultured INCL fibroblasts and in the brain tissues of Ppt1-KO mice, the NAD(+)/NADH ratio, the levels of phosphorylated-AMPK (p-AMPK), peroxisome proliferator-activated receptor-γ (PPARγ) coactivator-1α (PGC-1α) and Silent Information Regulator T1 (SIRT1) are markedly down-regulated. This suggested an abnormality in AMPK/SIRT1/PGC-1α signaling pathway of energy metabolism. Moreover, we found that, in INCL fibroblasts and in the Ppt1-KO mice, phosphorylated-S6K-1 (p-S6K1) levels, which inversely correlate with lifespan, are markedly elevated. Most importantly, resveratrol (RSV), an antioxidant polyphenol, elevated the NAD(+)/NADH ratio, levels of ATP, p-AMPK, PGC-1α and SIRT1 while decreasing the level of p-S6K1 in both INCL fibroblasts and in Ppt1-KO mice, which showed a modest increase in lifespan. Our results show that disruption of adaptive energy metabolism and increased levels of p-S6K1 are contributing factors in INCL pathogenesis and provide the proof of principle that small molecules such as RSV, which alleviate these abnormalities, may have therapeutic potential.

Controlling SIRT1 expression by microRNAs in health and metabolic disease

SIRT1 is a NAD^+-dependent deacetylase implicated in longevity and diverse physiological processes. SIRT1, as a key mediator of beneficial effects of caloric restriction, regulates lipid and glucose metabolism by deacetylating metabolic regulators, as well as histones, in response to nutritional deprivation. Here we discuss how SIRT1 levels are regulated by microRNAs (miRs) which are emerging as important metabolic regulators; the recently identified nuclear receptor FXR/SHP cascade pathway that controls the expression of miR-34a and its target SIRT1; and a FXR/SIRT1 positive feedback regulatory loop, which is deregulated in metabolic disease states. The FXR/miR-34a pathway and other miRs controlling SIRT1 may be useful therapeutic targets for age-related diseases, including metabolic disorders.

Effect of feeding regimens on circadian rhythms: implications for aging and longevity

Increased longevity and improved health can be achieved in mammals by two feeding regimens, caloric restriction (CR), which limits the amount of daily calorie intake, and intermittent fasting (IF), which allows the food to be available ad libitum every other day. The precise mechanisms mediating these beneficial effects are still unresolved. Resetting the circadian clock is another intervention that can lead to increased life span and well being, while clock disruption is associated with aging and morbidity. Currently, a large body of evidence links circadian rhythms with metabolism and feeding regimens. In particular, CR, and possibly also IF, can entrain the master clock located in the suprachiasmatic nuclei (SCN) of the brain hypothalamus. These findings raise the hypothesis that the beneficial effects exerted by these feeding regimens could be mediated, at least in part, through resetting of the circadian clock, thus leading to synchrony in metabolism and physiology. This hypothesis is reinforced by a transgenic mouse model showing spontaneously reduced eating alongside robust circadian rhythms and increased life span. This review will summarize recent findings concerning the relationships between feeding regimens, circadian rhythms, and metabolism with implications for ageing attenuation and life span extension.

Sirt1’s beneficial roles in neurodegenerative diseases - a chaperonin containing TCP-1 (CCT) connection?

Sir2 ⁄ Sirt1 and its orthologues are known lifespan extension factors in several aging models from yeast to invertebrates. Sirt1 activation is also known to be beneficial and protective in both invertebrate and mammalian models of neurodegenerative disease. Sirt1’s lifespan extension effect, as well as the beneficial outcome of its activation in models of aging-associated diseases, is often attributed to its ability to instill a gene expression profile that is pro-survival and antiaging. A recent report from Nyström and colleagues showed that the yeast Sir2p affects the function of the polarisome in segregation and retrograde transport of damaged and aggregated proteins from the bud to the mother cell, thereby ensuring the generation of a 'rejuvenated' daughter cell. Interestingly, the role of Sir2p in this case involves deacetylation and activation of cytoplasmic chaperonin containing TCP-1 (CCT, or TriC), thereby enhancing actin folding and polymerization. In view of a previously documented role of CCT in modulating polyglutamine-containing protein aggregation and toxicity, we hypothesized that CCT deacetylation may also underlie Sirt1’s beneficial effects in several neurodegenerative diseases precipitated by toxic aggregates. Other than alterations in gene expression profile, another major way whereby Sirt1 activation may counter neural aging could be to promote neuronal survival via prevention of toxic aggregate formation through CCT.

Effect of a 21 day Daniel Fast on metabolic and cardiovascular disease risk factors in men and women

Background

Dietary modification via caloric restriction is associated with multiple effects related to improved metabolic and cardiovascular health. However, a mandated reduction in kilocalories is not well-tolerated by many individuals, limiting the long-term application of such a plan. The Daniel Fast is a widely utilized fast based on the Biblical book of Daniel. It involves a 21 day ad libitum food intake period, devoid of animal products and preservatives, and inclusive of fruits, vegetables, whole grains, legumes, nuts, and seeds. The purpose of the present study was to determine the efficacy of the Daniel Fast to improve markers of metabolic and cardiovascular disease risk.

Methods

43 subjects (13 men; 30 women; 35 ± 1 yrs; range: 20-62 yrs) completed a 21 day period of modified food intake in accordance with detailed guidelines provided by investigators. All subjects purchased and prepared their own food. Following initial screening, subjects were given one week to prepare for the fast, after which time they reported to the lab for their pre-intervention assessment (day 1). After the 21 day fast, subjects reported to the lab for their post-intervention assessment (day 22). For both visits, subjects reported in a 12 hr fasted state, performing no strenuous physical activity during the preceding 24-48 hrs. At each visit, mental and physical health (SF-12 form), resting heart rate and blood pressure, and anthropometric variables were measured. Blood was collected for determination of complete blood count, metabolic panel, lipid panel, insulin, HOMA-IR, and C-reactive protein (CRP). Subjects' self-reported compliance, mood, and satiety in relation to the fast were also recorded. Diet records were maintained by all subjects during the 7 day period immediately prior to the fast (usual intake) and during the final 7 days of the fast.

Results

Subjects' compliance to the fast was 98.7 ± 0.2% (mean ± SEM). Using a 10 point scale, subjects' mood and satiety were both 7.9 ± 0.2. The following variables were significantly (p < 0.05) lower following the fast as compared to before the fast: white blood cell count (5.68 ± 0.24 vs. 4.99 ± 0.19 10³·μL^-1), blood urea nitrogen (13.07 ± 0.58 vs. 10.14 ± 0.59 mg·dL^-1), blood urea nitrogen/creatinine (14.74 ± 0.59 vs. 11.67 ± 0.68), protein (6.95 ± 0.07 vs. 6.77 ± 0.06 g·dL^-1), total cholesterol (171.07 ± 4.57 vs. 138.69 ± 4.39 mg·dL^-1), LDL-C (98.38 ± 3.89 vs. 76.07 ± 3.53 mg·dL^-1), HDL-C (55.65 ± 2.50 vs. 47.58 ± 2.19 mg·dL^-1), SBP (114.65 ± 2.34 vs. 105.93 ± 2.12 mmHg), and DBP (72.23 ± 1.59 vs. 67.00 ± 1.43 mmHg). Insulin (4.42 ± 0.52 vs. 3.37 ± 0.35 μU·mL^-1; p = 0.10), HOMA-IR (0.97 ± 0.13 vs.0.72 ± 0.08; p = 0.10), and CRP (3.15 ± 0.91 vs. 1.60 ± 0.42 mg·L^-1; p = 0.13), were lowered to a clinically meaningful, albeit statistically insignificant extent. No significant difference was noted for any anthropometric variable (p > 0.05). As expected, multiple differences in dietary intake were noted (p < 0.05), including a reduction in total kilocalorie intake (2185 ± 94 vs. 1722 ± 85).

Conclusion

A 21 day period of modified dietary intake in accordance with the Daniel Fast is 1) well-tolerated by men and women and 2) improves several risk factors for metabolic and cardiovascular disease. Larger scale, randomized studies, inclusive of a longer time period and possibly a slight modification in food choice in an attempt to maintain HDL cholesterol, are needed to extend these findings.

Therapeutic potential of activators and inhibitors of sirtuins

Sirtuins are evolutionary conserved NAD(+)-dependent acetyl-lysine deacetylases and ADP ribosyltransferases dual-function enzymes involved in the regulation of metabolism and lifespan. Sirtuins are also implicated in determining the balance between apoptosis, cell survival, and cell proliferation. In humans, seven sirtuins isoforms (SIRT₁₋₇) have been identified that localize either in the nucleus, cytoplasm, or mitochondria. The genetic demonstration that increasing gene dosage of sirtuin orthologs in eukaryotes, including yeast and multicellular Caenorhabditis elegans and Drosophila melanogaster, leads to prolonged lifespan induced considerable interest toward the discovery of sirtuin-activating molecules, on the ground that the phenomenon of sirtuin-induced lifespan prolongation-which is consequential to improved metabolic control-can be exploited therapeutically to counteract insulin resistance and diabetes. Conversely, ample evidence that either pharmacological inhibition or activation of sirtuin isoforms is potentially beneficial in study models of cancer and neurodegenerative diseases have been obtained. Here, we (i) survey the key roles of sirtuin isoforms and discuss the evidence in favor of activatory versus inhibitory targeting of sirtuins, (ii) discuss some of the inhibitors and activators of the sirtuin family members that have been described in the literature, (iii) review model systems in which these molecules have proved to exert therapeutic effects, and (iv) discuss the outcome of pharmacokinetic studies and phase I and II clinical trials employing sirtuin modulators.

Progeria syndromes and ageing: what is the connection?

One of the many debated topics in ageing research is whether progeroid syndromes are really accelerated forms of human ageing. The answer requires a better understanding of the normal ageing process and the molecular pathology underlying these rare diseases. Exciting recent findings regarding a severe human progeria, Hutchinson-Gilford progeria syndrome, have implicated molecular changes that are also linked to normal ageing, such as genome instability, telomere attrition, premature senescence and defective stem cell homeostasis in disease development. These observations, coupled with genetic studies of longevity, lead to a hypothesis whereby progeria syndromes accelerate a subset of the pathological changes that together drive the normal ageing process.

dSir2 and longevity in Drosophila

The silent information regulator 2 (Sir2 or Sirtuin) family of proteins is highly conserved and has been implicated in the extension of longevity for several species. Mammalian Sirtuins have been shown to affect various aspects of physiology including metabolism, the stress response, cell survival, replicative senescence, inflammation, the circadian rhythm, neurodegeneration, and even cancer. Evidence in Drosophila implicates Sir2 in at least some of the beneficial effects of caloric restriction (CR). CR delays age-related pathology and extends life span in a wide variety of species. Here we will review the evidence linking Drosophila Sir2 (dSir2) to longevity regulation and the pathway associated with CR in Drosophila, as well as the effects of the Sir2 activator resveratrol and potential interactions between dSir2 and p53.

NAD: a master regulator of transcription

Cellular processes such as proliferation, differentiation and death are intrinsically dependent upon the redox status of a cell. Among other indicators of redox flux, cellular NAD(H) levels play a predominant role in transcriptional reprogramming. In addition to this, normal physiological functions of a cell are regulated in response to perturbations in NAD(H) levels (for example, due to alterations in diet/metabolism) to maintain homeostatic conditions. Cells achieve this homeostasis by reprogramming various components that include changes in chromatin structure and function (transcription). The interdependence of changes in gene expression and NAD(H) is evolutionarily conserved and is considered crucial for the survival of a species (by affecting reproductive capacity and longevity). Proteins that bind and/or use NAD(H) as a co-substrate (such as, CtBP and PARPs/Sirtuins respectively) are known to induce changes in chromatin structure and transcriptional profiles. In fact, their ability to sense perturbations in NAD(H) levels has been implicated in their roles in development, stress responses, metabolic homeostasis, reproduction and aging or age-related diseases. It is also becoming increasingly clear that both the levels/activities of these proteins and the availability of NAD(H) are equally important. Here we discuss the pivotal role of NAD(H) in controlling the functions of some of these proteins, the functional interplay between them and physiological implications during calorie restriction, energy homeostasis, circadian rhythm and aging.

Early apoptotic vascular signaling is determined by Sirt1 through nuclear shuttling, forkhead trafficking, bad, and mitochondrial caspase activation

Complications of diabetes mellitus (DM) weigh heavily upon the endothelium that ultimately affect multiple organ systems. These concerns call for innovative treatment strategies that employ molecular pathways responsible for cell survival and longevity. Here we show in a clinically relevant model of DM with elevated D-glucose that endothelial cell (EC) SIRT1 is vital for the prevention of early membrane apoptotic phosphatidylserine externalization and subsequent DNA degradation supported by studies with modulation of SIRT1 activity and gene knockdown of SIRT1. Furthermore, during elevated D-glucose exposure, we show that SIRT1 is sequestered in the cytoplasm of ECs, but specific activation of SIRT1 shuttles the protein to the nucleus to allow for cytoprotection. The ability of SIRT1 to avert apoptosis employs the activation of protein kinase B (Akt1), the post-translational phosphorylation of the forkhead member FoxO3a, the blocked trafficking of FoxO3a to the nucleus, and the inhibition of FoxO3a to initiate a "pro-apoptotic" program as shown by complimentary gene knockdown studies of FoxO3a. Vascular apoptotic oversight by SIRT1 extends to the direct modulation of mitochondrial membrane permeability, cytochrome c release, Bad activation, and caspase 1 and 3 activation, since inhibition of SIRT1 activity and gene knockdown of SIRT1 significantly accentuate cascade progression while SIRT1 activation abrogates these apoptotic elements. Our work identifies vascular SIRT1 and its control over early apoptotic membrane signaling, Akt1 activation, post-translational modification and trafficking of FoxO3a, mitochondrial permeability, Bad activation, and rapid caspase induction as new avenues for the treatment of vascular complications during DM.

Metabolic networks of longevity

Molecular and cellular networks implicated in aging depend on a multitude of proteins that collectively mount adaptive and contingent metabolic responses to environmental challenges. Here, we discuss the intimate links between metabolic regulation and longevity and outline new approaches for analyzing and manipulating such links to promote human health span.

Signaling pathways in mitochondrial dysfunction and aging

Mitochondria are central players in the determination of cell life and death. They are essential for energy production, since most cellular ATP is produced in their matrix by the oxidative phosphorylation pathway. At the same time, mitochondria are the main regulators of apoptotic cell death, mediating both extrinsic (cell-surface receptor mediated) and intrinsic apoptotic pathways. Reactive oxygen species (ROS) accumulate as side products of the electron transport chain, causing mitochondrial damage. Non-functional mitochondria accumulate in aged individuals, and cell homeostasis is maintained by removing damaged mitochondria by an autophagic process called "mitophagy". In addition, mitochondrial ROS represent signaling molecules leading to autophagy, consisting in the bulk degradation of cytosolic portions. When cell homeostasis is perturbed, and cytosolic components are damaged, autophagy represents a defense mechanism aimed at removing non-functional proteins and organelles. If this is not sufficient, cell death occurs with distinct morphological hallmarks from apoptosis. This binary choice integrates a number of critical information converging on a number of common regulatory elements. In this review, the focus will be placed on the central role of mitochondria in the cross-talk between autophagy and apoptosis, highlighting the signaling pathways and molecular machinery impinging on these organelles.

AMP-activated protein kinase and its downstream transcriptional pathways

The AMP-activated protein kinase (AMPK) is a key regulator of catabolic versus anabolic processes. Its properties as an energy sensor allow it to couple the energy status of the cell to the metabolic environment. These adaptations not only take place through the acute modulation of key metabolic enzymes via direct phosphorylation, but also through a slower transcriptional adaptative response. The question of how AMPK regulates the expression of a number of gene sets, such as those related to mitochondrial biogenesis, energy production and oxidative protection, is only beginning to be elucidated, and still many questions remain to be answered. In this review we will try to integrate our current knowledge on how AMPK regulates transcription in muscle and liver, which will serve as examples to illustrate the major advances in the field and the key challenges ahead.

Sirtuin activators: designing molecules to extend life span

Resveratrol (RESV) exerts important pharmacological effects on human health: in addition to its beneficial effects on type 2 diabetes and cardiovascular diseases, it also modulates neuronal energy homeostasis and shows antiaging properties. Although it clearly has free radical scavenger properties, the mechanisms involved in these beneficial effects are not fully understood. In this regard, one area of major interest concerns the effects of RESV on the activity of sirtuin 1 (SIRT1), an NAD(+)-dependent histone deacetylase that has been implicated in aging. Indeed, the role of SIRT1 is currently the subject of intense research due to the antiaging properties of RESV, which increases life span in various organisms ranging from yeast to rodents. In addition, when RESV is administered in experimental animal models of neurological disorders, it has similar beneficial effects to caloric restriction. SIRT1 activation could thus constitute a potential strategic target in neurodegenerative diseases and in disorders involving disturbances in glucose homeostasis, as well as in dyslipidaemias or cardiovascular diseases. Therefore, small SIRT1 activators such as SRT501, SRT2104, and SRT2379, which are currently undergoing clinical trials, could be potential drugs for the treatment of type 2 diabetes, obesity, and metabolic syndrome, among other disorders. This review summarises current knowledge about the biological functions of SIRT1 in aging and aging-associated diseases and discusses its potential as a pharmacological target.

Diabetes mellitus: channeling care through cellular discovery

Diabetes mellitus (DM) impacts a significant portion of the world's population and care for this disorder places an economic burden on the gross domestic product for any particular country. Furthermore, both Type 1 and Type 2 DM are becoming increasingly prevalent and there is increased incidence of impaired glucose tolerance in the young. The complications of DM are protean and can involve multiple systems throughout the body that are susceptible to the detrimental effects of oxidative stress and apoptotic cell injury. For these reasons, innovative strategies are necessary for the implementation of new treatments for DM that are generated through the further understanding of cellular pathways that govern the pathological consequences of DM. In particular, both the precursor for the coenzyme beta-nicotinamide adenine dinucleotide (NAD(+)), nicotinamide, and the growth factor erythropoietin offer novel platforms for drug discovery that involve cellular metabolic homeostasis and inflammatory cell control. Interestingly, these agents and their tightly associated pathways that consist of cell cycle regulation, protein kinase B, forkhead transcription factors, and Wnt signaling also function in a broader sense as biomarkers for disease onset and progression.

Dietary manipulation and caloric restriction in the development of mouse models relevant to neurological diseases

Manipulation of diet such as increasing the level of fat or inducing insulin resistance has been shown to exacerbate the pathology in several animal models of neurological disease. Caloric restriction, however, has been demonstrated to extend the life span of many organisms. Reduced calorie consumption appears to increase the resistance of neurons to intracellular and extracellular stress and consequently improves the behavioural phenotype in animal models of neurological diseases, such as Alzheimer's disease. We review the evidence from a variety of mouse models that diet is a risk factor that can significantly contribute to the development of neurological diseases.

SIRT1 and p53, effect on cancer, senescence and beyond

NAD(+)-dependent Class III histone deacetylase SIRT1 is a multiple function protein critically involved in stress responses, cellular metabolism and aging through deacetylating a variety of substrates including p53, forkhead-box transcription factors, PGC-1alpha, NF-kappaB, Ku70 and histones. The first discovered non-histone target of SIRT1, p53, is suggested to play a central role in SIRT1-mediated functions in tumorigenesis and senescence. SIRT1 was originally considered to be a potential tumor promoter since it negatively regulates the tumor suppressor p53 and other tumor suppressors. There is new evidence that SIRT1 acts as a tumor suppressor based on its role in negatively regulating beta-catenin and survivin. This review provides an overview of current knowledge of SIRT1-p53 signaling and controversies regarding the functions of SIRT1 in tumorigenesis.

SirT1 in muscle physiology and disease: lessons from mouse models

Sirtuin 1 (SirT1) is the largest of the seven members of the sirtuin family of class III nicotinamide adenine dinucleotide (NAD(+))-dependent protein deacetylases, whose activation is beneficial for metabolic, neurodegenerative, inflammatory and neoplastic diseases, and augments life span in model organisms (Finkel et al., 2009; Lavu et al., 2008). In vitro studies show that SirT1 protects genome integrity and is involved in circadian physiological rhythms (Asher et al., 2008; Nakahata et al., 2008; Oberdoerffer et al., 2008). In the last few years, a fundamental role for SirT1 in the metabolism and differentiation of skeletal muscle cells has been uncovered (Fulco et al., 2003), and the use of specific transgenic or knockout SirT1 mouse models implicates it in the protection of heart muscle from oxidative and hypertrophic stresses (Alcendor et al., 2007). In this Perspective, we review the recent exciting findings that have established a key role for the 'longevity' protein SirT1 in skeletal and heart muscle physiology and disease. Furthermore, given the multiple biological functions of SirT1, we discuss the unique opportunities that SirT1 mouse models can offer to improve our integrated understanding of the metabolism, as well as the regeneration and aging-associated changes in the circadian function, of skeletal and heart muscle.

Interactions between light, mealtime and calorie restriction to control daily timing in mammals

Daily variations in behaviour and physiology are controlled by a circadian timing system consisting of a network of oscillatory structures. In mammals, a master clock, located in the suprachiasmatic nuclei (SCN) of the hypothalamus, adjusts timing of other self-sustained oscillators in the brain and peripheral organs. Synchronisation to external cues is mainly achieved by ambient light, which resets the SCN clock. Other environmental factors, in particular food availability and time of feeding, also influence internal timing. Timed feeding can reset the phase of the peripheral oscillators whilst having almost no effect in shifting the phase of the SCN clockwork when animals are exposed (synchronised) to a light-dark cycle. Food deprivation and calorie restriction lead not only to loss of body mass (>15%) and increased motor activity, but also affect the timing of daily activity, nocturnal animals becoming partially diurnal (i.e. they are active during their usual sleep period). This change in behavioural timing is due in part to the fact that metabolic cues associated with calorie restriction affect the SCN clock and its synchronisation to light.

CYB5R3: a key player in aerobic metabolism and aging?

Aging results from a complex and not completely understood chain of processes that are associated with various negative metabolic consequences and ultimately leads to senescence and death. The intracellular ratio of pyridine nucleotides (NAD⁺/NADH), has been proposed to be at the center stage of age-related biochemical changes in organisms, and may help to explain the observed influence of calorie restriction and energy-sensitive proteins on lifespan in model organisms. Indeed, the NAD⁺/NADH ratios affect the activity of a number of proteins, including sirtuins, which have gained prominence in the aging field as potential mediators of the beneficial effects of calorie restriction and mediating lifespan. Here we review the activities of a redox enzyme (NQR1 in yeast and CYB5R3 in mammals) that also influences the NAD⁺/NADH ratio and may play a regulatory role that connects aerobic metabolism with aging.

Protein deacetylation by SIRT1: an emerging key post-translational modification in metabolic regulation

The biological function of most proteins relies on reversible post-translational modifications, among which phosphorylation is most prominently studied and well recognized. Recently, a growing amount of evidence indicates that acetylation-deacetylation reactions, when applied to crucial mediators, can also robustly affect the function of target proteins and thereby have wide-ranging physiological impacts. Sirtuin 1 (SIRT1), which functions as a nicotinamide adenine dinucleotide (NAD(+))-dependent protein deacetylase, deacetylates a wide variety of metabolic molecules in response to the cellular energy and redox status and as such causes significant changes in metabolic homeostasis. This review surveys the evidence for the emerging role of SIRT1-mediated deacetylation in the control of metabolic homeostasis.

Is systemic activation of Sirt1 beneficial for ageing-associated metabolic disorders?

Sir2/Sirt1, a mediator of longevity in several animal models, is a member of the sirtuin family of type III histone deacetylases. Its non-histone substrates include a group of regulatory molecules that modulate energy metabolism, such as peroxisome proliferator-activated receptor-gamma (PPARgamma), and its transcriptional coactivator, PPARgammacoactivator-1alpha (PGC-1alpha). Sirt1's activity on these substrates may underlie its connection with the metabolic changes brought about by caloric restriction (CR). Recent studies have elucidated new substrates for Sirt1 that are involved in metabolic regulation, and have further delineated Sirt1's functional associations with other metabolic regulators like AMP-activated kinase (AMPK). Perplexingly, manipulations that either increase or decrease Sirt1 activity have both been associated with a beneficial effect in animal models of ageing-associated disorders, such as neurodegenerative diseases. Sirt1's activation patterns and roles in energy metabolism appear to have tissue specific differences. A deeper understanding of the mechanistic underpinnings of Sirt1's metabolic functions is necessary to effectively design Sirt1-based therapeutic interventions for metabolic disorders.