SIRT6 Promotes Hepatic Beta-Oxidation via Activation of PPARα.

The pro-longevity enzyme SIRT6 regulates various metabolic pathways. Gene expression analyses in SIRT6 heterozygotic mice identify significant decreases in PPARα signaling, known to regulate multiple metabolic pathways. SIRT6 binds PPARα and its response element within promoter regions and activates gene transcription. Sirt6+/- results in significantly reduced PPARα-induced ß-oxidation and its metabolites and reduced alanine and lactate levels, while inducing pyruvate oxidation. Reciprocally, starved SIRT6 transgenic mice show increased pyruvate, acetylcarnitine, and glycerol levels and significantly induce ß-oxidation genes in a PPARα-dependent manner. Furthermore, SIRT6 mediates PPARα inhibition of SREBP-dependent cholesterol and triglyceride synthesis. Mechanistically, SIRT6 binds PPARα coactivator NCOA2 and decreases liver NCOA2 K780 acetylation, which stimulates its activation of PPARα in a SIRT6-dependent manner. These coordinated SIRT6 activities lead to regulation of whole-body respiratory exchange ratio and liver fat content, revealing the interactions whereby SIRT6 synchronizes various metabolic pathways, and suggest a mechanism by which SIRT6 maintains healthy liver.

Breaking the Ceiling of Human Maximal Life span.

While average human life expectancy has increased dramatically in the last century, the maximum life span has only modestly increased. These observations prompted the notion that human life span might have reached its maximal natural limit of ~115 years. To evaluate this hypothesis, we conducted a systematic analysis of all-cause human mortality throughout the 20th century. Our analyses revealed that, once cause of death is accounted for, there is a proportional increase in both median age of death and maximum life span. To examine whether pathway targeted aging interventions affected both median and maximum life span, we analyzed hundreds of interventions performed in multiple organisms (yeast, worms, flies, and rodents). Three criteria: median, maximum, and last survivor life spans were all significantly extended, and to a similar extent. Altogether, these findings suggest that targeting the biological/genetic causes of aging can allow breaking the currently observed ceiling of human maximal life span.

Sirt6 alters adult hippocampal neurogenesis.

Sirtuins are pleiotropic NAD+ dependent histone deacetylases involved in metabolism, DNA damage repair, inflammation and stress resistance. SIRT6, a member of the sirtuin family, regulates the process of normal aging and increases the lifespan of male mice over-expressing Sirt6 by 15%. Neurogenesis, the formation of new neurons within the hippocampus of adult mammals, involves several complex stages including stem cell proliferation, differentiation, migration and network integration. During aging, the number of newly generated neurons continuously declines, and this is correlated with a decline in neuronal plasticity and cognitive behavior. In this study we investigated the involvement of SIRT6 in adult hippocampal neurogenesis. Mice over-expressing Sirt6 exhibit increased numbers of young neurons and decreased numbers of mature neurons, without affecting glial differentiation. This implies of an involvement of SIRT6 in neuronal differentiation and maturation within the hippocampus. This work adds to the expanding body of knowledge on the regulatory mechanisms underlying adult hippocampal neurogenesis, and describes novel roles for SIRT6 as a regulator of cell fate during adult hippocampal neurogenesis.

New Role for Interleukin-13 Receptor α1 in Myocardial Homeostasis and Heart Failure.

BACKGROUND: The immune system plays a pivotal role in myocardial homeostasis and response to injury. Interleukins-4 and -13 are anti-inflammatory type-2 cytokines, signaling via the common interleukin-13 receptor α1 chain and the type-2 interleukin-4 receptor. The role of interleukin-13 receptor α1 in the heart is unknown. METHODS AND RESULTS: We analyzed myocardial samples from human donors (n=136) and patients with end-stage heart failure (n=177). We found that the interleukin-13 receptor α1 is present in the myocardium and, together with the complementary type-2 interleukin-4 receptor chain Il4ra, is significantly downregulated in the hearts of patients with heart failure. Next, we showed that Il13ra1-deficient mice develop severe myocardial dysfunction and dyssynchrony compared to wild-type mice (left ventricular ejection fraction 29.7±9.9 versus 45.0±8.0; P=0.004, left ventricular end-diastolic diameter 4.2±0.2 versus 3.92±0.3; P=0.03). A bioinformatic analysis of mouse hearts indicated that interleukin-13 receptor α1 regulates critical pathways in the heart other than the immune system, such as extracellular matrix (normalized enrichment score=1.90; false discovery rate q=0.005) and glucose metabolism (normalized enrichment score=-2.36; false discovery rate q=0). Deficiency of Il13ra1 was associated with reduced collagen deposition under normal and pressure-overload conditions. CONCLUSIONS: The results of our studies in humans and mice indicate, for the first time, a role of interleukin-13 receptor α1 in myocardial homeostasis and heart failure and suggests a new therapeutic target to treat heart disease.

Characterization of physiological defects in adult SIRT6-/- mice.

The NAD+-dependent SIRT6 deacetylase was shown to be a major regulator of lifespan and healthspan. Mice deficient for SIRT6 develop a premature aging phenotype and metabolic defects, and die before four weeks of age. Thus, the effect of SIRT6 deficiency in adult mice is unknown. Here we show that SIRT6-/- mice in mixed 129/SvJ/BALB/c background reach adulthood, allowing examination of SIRT6-related metabolic and developmental phenotypes in adult mice. In this mixed background, at 200 days of age, more than 80% of the female knock-out mice were alive whereas only 10% of male knock-out mice survived. In comparison to their wild-type littermates, SIRT6 deficient mice have reduced body weight, increased glucose uptake and exhibit an age-dependent progressive impairment of retinal function accompanied by thinning of retinal layers. Together, these results demonstrate a role for SIRT6 in metabolism and age-related ocular changes in adult mice and suggest a gender specific regulation of lifespan by SIRT6.

SIRT6 Overexpression Improves Various Aspects of Mouse Healthspan.

The extension in human lifespan in the last century results in a significant increase in incidence of age related diseases. It is therefore crucial to identify key factors that control elderly healthspan. Similar to dietary restriction, mice overexpressing the NAD+ dependent protein deacylase SIRT6 (MOSES) live longer and have reduced IGF-1 levels. However, it is as yet unknown whether SIRT6 also affects various healthspan parameters. Here, a range of age related phenotypes was evaluated in MOSES mice. In comparison to their wild-type (WT) littermates, old MOSES mice showed amelioration of a variety of age-related disorders, including: improved glucose tolerance, younger hormonal profile, reduced age-related adipose inflammation and increased physical activity. The increased activity was accompanied with increased muscle AMP-activated protein kinase (AMPK) activity. Altogether, these results indicate that overexpression of SIRT6 in mice retards important aspects of the aging process and suggest SIRT6 to be a potential therapeutic target for the treatment of a set of age-related disorders.

Reciprocal Regulation between SIRT6 and miR-122 Controls Liver Metabolism and Predicts Hepatocarcinoma Prognosis.

Mice overexpressing the longevity protein SIRT6 or deficient for the liver's most prevalent microRNA miR-122 display a similar set of phenotypes, including improved lipid profile and protection against damage linked to obesity. Here, we show that miR-122 and SIRT6 negatively regulate each other's expression. SIRT6 downregulates miR-122 by deacetylating H3K56 in the promoter region. MiR-122 binds to three sites on the SIRT6 3' UTR and reduces its levels. The interplay between SIRT6 and miR-122 is manifested in two physiologically relevant ways in the liver. First, they oppositely regulate a similar set of metabolic genes and fatty acid ß-oxidation. Second, in hepatocellular carcinoma patients, loss of a negative correlation between SIRT6 and miR-122 expression is significantly associated with better prognosis. These findings show that SIRT6 and miR-122 negatively regulate each other to control various aspects of liver physiology and SIRT6-miR-122 correlation may serve as a biomarker for hepatocarcinoma prognosis.

Sirtuin 6 protects the heart from hypoxic damage.

Sirtuin 6 (SIRT6) is a protein associated with prolonged life expectancy. We investigated whether life extension is associated with cardioprotection against hypoxia. The proposed study is to develop approaches to reduce hypoxic damage through the use of the sirtuin pathway and to elucidate the mechanism involved. For that purpose we subjected cardiomyocytes from transgenic mice (TG) with over-expression of SIRT6, to hypoxic stress in cell cultures. We hypothesized that cardiomyocytes from transgenic mice subjected to prolonged hypoxia may release survival factors or fewer damage markers to protect them from hypoxic stress compared with wild type (WT) mice. Lactate dehydrogenase (LDH) and creatine kinase (CK) released to the medium and propidium iodide (PI) binding, were markedly decreased following hypoxia in TG cardiomyocytes. The protective mechanism of SIRT6 over-expression includes the activation of pAMPKα pathway, the increased protein level of B-cell lymphoma 2 (Bcl2), the inhibition of nuclear factor kappa-light-chain-enhancer of activated B cells (NFκB), the decrease of reactive oxygen species (ROS) and the reduction in the protein level of phospho-protein kinase B (pAkt) during hypoxia. Together, all these processes impede the necrosis/apoptosis pathways leading to the improved survival of cardiomyocytes following hypoxia, which might explain life extension.

Role of sirtuins in lifespan regulation is linked to methylation of nicotinamide.

Sirtuins, a family of histone deacetylases, have a fiercely debated role in regulating lifespan. In contrast with recent observations, here we find that overexpression of sir-2.1, the ortholog of mammalian SirT1, does extend Caenorhabditis elegans lifespan. Sirtuins mandatorily convert NAD(+) into nicotinamide (NAM). We here find that NAM and its metabolite, 1-methylnicotinamide (MNA), extend C. elegans lifespan, even in the absence of sir-2.1. We identify a previously unknown C. elegans nicotinamide-N-methyltransferase, encoded by a gene now named anmt-1, to generate MNA from NAM. Disruption and overexpression of anmt-1 have opposing effects on lifespan independent of sirtuins, with loss of anmt-1 fully inhibiting sir-2.1-mediated lifespan extension. MNA serves as a substrate for a newly identified aldehyde oxidase, GAD-3, to generate hydrogen peroxide, which acts as a mitohormetic reactive oxygen species signal to promote C. elegans longevity. Taken together, sirtuin-mediated lifespan extension depends on methylation of NAM, providing an unexpected mechanistic role for sirtuins beyond histone deacetylation.

Multiple regulatory layers of SREBP1/2 by SIRT6.

The NAD(+)-dependent protein deacetylase SIRT6 regulates genome stability, cancer, and lifespan. Mice overexpressing SIRT6 (MOSES) have lower low-density lipoprotein cholesterol levels and are protected against the physiological damage of obesity. Here, we examined the role of SIRT6 in cholesterol regulation via the lipogenic transcription factors SREBP1 and SREBP2, and AMP-activated protein kinase (AMPK). We show that SIRT6 represses SREBP1 and SREBP2 by at least three mechanisms. First, SIRT6 represses the transcription levels of SREBP1/SREBP2 and that of their target genes. Second, SIRT6 inhibits the cleavage of SREBP1/SREBP2 into their active forms. Third, SIRT6 activates AMPK by increasing the AMP/ATP ratio, which promotes phosphorylation and inhibition of SREBP1 by AMPK. Reciprocally, the expression of miR33a and miR33b from the introns of SREBP2 and SREBP1, respectively, represses SIRT6 levels. Together, these findings explain the mechanism underlying the improved cholesterol homeostasis in MOSES mice, revealing a relationship between fat metabolism and longevity.

SIRT6 exhibits nucleosome-dependent deacetylase activity.

The SIRT6 deacetylase is a key regulator of mammalian genome stability, metabolism and lifespan. Previous studies indicated that SIRT6 exhibits poor deacetylase activity in vitro. Here, we explored the specific conditions that allow SIRT6 to function as a significant deacetylase. We show that SIRT6 associates with the nucleosome and deacetylates histones H3 and H4 when they are packaged as nucleosomes, but not as free histones. In contrast, SIRT1 shows the opposite characteristics. Thus, our results show that SIRT6 activity is nucleosome dependent, and suggest that its binding to the nucleosome might convert it into an active structure.

The sirtuin SIRT6 regulates lifespan in male mice.

The significant increase in human lifespan during the past century confronts us with great medical challenges. To meet these challenges, the mechanisms that determine healthy ageing must be understood and controlled. Sirtuins are highly conserved deacetylases that have been shown to regulate lifespan in yeast, nematodes and fruitflies. However, the role of sirtuins in regulating worm and fly lifespan has recently become controversial. Moreover, the role of the seven mammalian sirtuins, SIRT1 to SIRT7 (homologues of the yeast sirtuin Sir2), in regulating lifespan is unclear. Here we show that male, but not female, transgenic mice overexpressing Sirt6 (ref. 4) have a significantly longer lifespan than wild-type mice. Gene expression analysis revealed significant differences between male Sirt6-transgenic mice and male wild-type mice: transgenic males displayed lower serum levels of insulin-like growth factor 1 (IGF1), higher levels of IGF-binding protein 1 and altered phosphorylation levels of major components of IGF1 signalling, a key pathway in the regulation of lifespan. This study shows the regulation of mammalian lifespan by a sirtuin family member and has important therapeutic implications for age-related diseases.

SIRT6 protects against pathological damage caused by diet-induced obesity.

The NAD+-dependent SIRT6 deacetylase is a therapeutic candidate against the emerging metabolic syndrome epidemic. SIRT6, whose deficiency in mice results in premature aging phenotypes and metabolic defects, was implicated in a calorie restriction response that showed an opposite set of phenotypes from the metabolic syndrome. To explore the role of SIRT6 in metabolic stress, wild type and transgenic (TG) mice overexpressing SIRT6 were fed a high fat diet. In comparison to their wild-type littermates, SIRT6 TG mice accumulated significantly less visceral fat, LDL-cholesterol, and triglycerides. TG mice displayed enhanced glucose tolerance along with increased glucose-stimulated insulin secretion. Gene expression analysis of adipose tissue revealed that the positive effect of SIRT6 overexpression is associated with down regulation of a selective set of peroxisome proliferator-activated receptor-responsive genes, and genes associated with lipid storage, such as angiopoietin-like protein 4, adipocyte fatty acid-binding protein, and diacylglycerol acyltransferase 1, which were suggested as potential targets for drugs to control metabolic syndrome. These results demonstrate a protective role for SIRT6 against the metabolic consequences of diet-induced obesity and suggest a potentially beneficial effect of SIRT6 activation on age-related metabolic diseases.

Regulation of SIRT1 protein levels by nutrient availability.

The mammalian NAD+ dependent deacetylase, SIRT1, was shown to be a key protein in regulating glucose homeostasis, and was implicated in the response to calorie restriction. We show here that levels of SIRT1 increased in response to nutrient deprivation in cultured cells, and in multiple tissues of mice after fasting. The increase in SIRT1 levels was due to stabilization of SIRT1 protein, and not an increase in SIRT1 mRNA. In addition, p53 negatively regulated SIRT1 levels under normal growth conditions and is also required for the elevation of SIRT1 under limited nutrient conditions. These results have important implications on the relationship between sirtuins, nutrient availability and aging.

Regulation of SIRT6 protein levels by nutrient availability.

Sirtuins have been shown to regulate life-span in response to nutritional availability. We show here that levels of the mammalian sirtuin, SIRT6, increased upon nutrient deprivation in cultured cells, in mice after fasting, and in rats fed a calorie-restricted diet. The increase in SIRT6 levels is due to stabilization of SIRT6 protein, and not via an increase in SIRT6 transcription. In addition, p53 positively regulates SIRT6 protein levels under standard growth conditions but has no role in the nutrient-dependent regulation of SIRT6. These observations imply that at least two sirtuins are involved in regulation of life-span by nutrient availability.