SIRT1-deficient Mice Research Articles

Microglia Sirt6 modulates the transcriptional activity of NRF2 to ameliorate high-fat diet-induced obesity.

Microglia play a pivotal role in neuroinflammation, while obesity triggers hypothalamic microglia activation and inflammation. Sirt6 is an important regulator of energy metabolism in many peripheral tissues and hypothalamic anorexic neurons. However, the exact mechanism for microglia Sirt6 in controlling high-fat diet-induced obesity remain unknown. Microglia Sirt6 expression levels under various nutritional conditions were measured in the hypothalamus of mice. Also, microglia Sirt6-deficient mice were provided various diets to monitor metabolic changes and hypothalamic inflammatory response. Besides, RNA-seq and Co-IP of microglia with Sirt6 alterations were conducted to further investigate the detailed mechanism by which Sirt6 modulated microglia activity. We found that Sirt6 was downregulated in hypothalamic microglia in mice given a high-fat diet (HFD). Additionally, knockout of microglia Sirt6 exacerbated high-fat diet-induced hypothalamic microglial activation and inflammation. As a result, mice were more prone to obesity, exhibiting a decrease in energy expenditure, impaired glucose tolerance, insulin and leptin resistance, and increased food intake. In vitro, Sirt6 overexpression in BV2 cells displayed protective effects against oleic acid and palmitic acid treatment-derived inflammatory response. Mechanically, Sirt6 deacetylated and stabilised NRF2 to increase the expression of anti-oxidative genes and defend against reactive oxygen species overload. Pharmacological inhibition of NRF2 eliminated the beneficial modulating effects of Sirt6 on microglial activity. Collectively, our results revealed that microglial Sirt6 was a primary contributor of microglial activation in the central regulation of obesity. Thus, microglial Sirt6 may be an important therapeutic target for obesity.

Cartilage-specific Sirt6 deficiency represses IGF-1 and enhances osteoarthritis severity in mice

ObjectivesPrior studies noted that chondrocyte SIRT6 activity is repressed in older chondrocytes rendering cells susceptible to catabolic signalling events implicated in osteoarthritis (OA). This study aimed to define the effect...

Pachymic acid modulates sirtuin 6 activity to alleviate lipid metabolism disorders.

Pachymic acid (Pac), a major bioactive constituent of Poria cocos, is an antioxidant that inhibits triglyceride (TG) accumulation. To the best of our knowledge, the present study investigated for the first time whether Pac activated sirtuin 6 (SIRT6) signaling to alleviate oleic acid (OA)-palmitic acid (PA)-induced lipid metabolism disorders in mouse primary hepatocytes (MPHs). In the present study, MPHs challenged with Pac were used to test the effects of Pac on intracellular lipid metabolism. Molecular docking studies were performed to explore the potential targets of Pac in defending against lipid deposition. MPHs isolated from liver-specific SIRT6-deficient mice were subjected to OA + PA incubation and treated with Pac to determine the function and detailed mechanism. It was revealed that Pac activated SIRT6 by increasing its expression and deacetylase activity. Pa prevented OA + PA-induced lipid deposition in MPHs in a dose-dependent manner. Pac (50 µM) administration significantly reduced TG accumulation and increased fatty acid oxidation rate in OA + PA-incubated MPHs. Meanwhile, as per the results of molecular docking and relative mRNA levels, Pac activated SIRT6 and increased SIRT6 deacetylation levels. Furthermore, SIRT6 deletions in MPHs abolished the protective effects of Pac against OA + PA-induced hepatocyte lipid metabolism disorders. The present study demonstrated that Pac alleviates OA + PA-induced hepatocyte lipid metabolism disorders by activating SIRT6 signaling. Overall, SIRT6 signaling increases oxidative stress burden and promotes hepatocyte lipolysis.

Sirt6 mediates antioxidative functions by increasing Nrf2 abundance

Oxidative stress caused by excess ROS often leads to cellular macromolecule damage and eventually causes various biological catastrophes. Sirt6, a member of the mammalian homolog family of yeast Sir2 NAD+-dependent histone deacetylases, regulates multiple biological processes. Sirt6 exerts antioxidative functions by enhancing DNA repair and DNA end resection. In our study, we found that Sirt6 expression was induced by H2O2 and paraquat (PQ) in cells. When exposed to PQ, the Sirt6+/− C57BL/6 mice showed more serious liver damage and lower survival rate than the Sirt6+/+ mice. The Nrf2 protein levels and the mRNA levels of its target genes in mouse tissues were decreased by Sirt6 deficiency, and Sirt6 overexpression increased the Nrf2 protein content. Moreover, the endogenous H2O2 levels were increased in the tissues of Sirt6-deficient mice and were decreased in Sirt6 overexpression cells. Then, we found that Nrf2 was degraded faster in the Sirt6-deficient mouse embryonic fibroblasts (MEFs) than in the wild type MEFs and that Sirt6 enhanced the protein accumulation of Nrf2 in the nucleus. Lastly, we found that Sirt6 interacted with Nrf2 in co-IP and GST pull-down assays and that Sirt6 overexpression decreased the binding of Nrf2 to Keap1. Taken together, the results of the present study suggest that Sirt6 exerts antioxidative functions by increasing the Nrf2 protein level via Keap1-mediated regulation.

Sirtuin 6 inhibition protects against glucocorticoid-induced skeletal muscle atrophy by regulating IGF/PI3K/AKT signaling

Chronic activation of stress hormones such as glucocorticoids leads to skeletal muscle wasting in mammals. However, the molecular events that mediate glucocorticoid-induced muscle wasting are not well understood. Here, we show that SIRT6, a chromatin-associated deacetylase indirectly regulates glucocorticoid-induced muscle wasting by modulating IGF/PI3K/AKT signaling. Our results show that SIRT6 levels are increased during glucocorticoid-induced reduction of myotube size and during skeletal muscle atrophy in mice. Notably, overexpression of SIRT6 spontaneously decreases the size of primary myotubes in a cell-autonomous manner. On the other hand, SIRT6 depletion increases the diameter of myotubes and protects them against glucocorticoid-induced reduction in myotube size, which is associated with enhanced protein synthesis and repression of atrogenes. In line with this, we find that muscle-specific SIRT6 deficient mice are resistant to glucocorticoid-induced muscle wasting. Mechanistically, we find that SIRT6 deficiency hyperactivates IGF/PI3K/AKT signaling through c-Jun transcription factor-mediated increase in IGF2 expression. The increased activation, in turn, leads to nuclear exclusion and transcriptional repression of the FoxO transcription factor, a key activator of muscle atrophy. Further, we find that pharmacological inhibition of SIRT6 protects against glucocorticoid-induced muscle wasting in mice by regulating IGF/PI3K/AKT signaling implicating the role of SIRT6 in glucocorticoid-induced muscle atrophy.

The nuclear sirtuin SIRT6 protects the heart from developing aging-associated myocyte senescence and cardiac hypertrophy.

Sirtuins have been shown to regulate the aging process. We have previously demonstrated that Sirt6 blocks the pressure overload-induced cardiac hypertrophy in mice. Here, we show that Sirt6 can also mitigate aging-induced cardiomyocyte senescence and cardiac hypertrophy. We found that aging is associated with altered Sirt6 activity along with development of cardiac hypertrophy and fibrosis. Compared to young mice (4-months), the hearts of aged mice (24-months) showed increased levels of mitochondrial DNA damage, shortened telomere length, and increased accumulation of 8-oxo-dG adducts, which are hallmarks of aging. The aged hearts also showed reduced levels of NAD+ and altered levels of mitochondrial fusion-fission proteins. Similar characteristics were observed in the hearts of Sirt6 deficient mice. Additionally, we found that doxorubicin (Dox) induced cardiomyocyte senescence, as measured by expression of p16INK4a, p53, and β-galactosidase, was associated with loss of Sirt6. However, Sirt6 overexpression protected cardiomyocytes from developing Dox-induced senescence. Further, compared to wild-type mice, the hearts of Sirt6.Tg mice showed reduced expression of aging markers, and the development of aging-associated cardiac hypertrophy and fibrosis. Our data suggest that Sirt6 is a critical anti-aging molecule that regulates various cellular processes associated with aging and protects the heart from developing aging-induced cardiac hypertrophy and fibrosis.

"Cell Membrane Theory of Senescence" and the Role of Bioactive Lipids in Aging, and Aging Associated Diseases and Their Therapeutic Implications.

Lipids are an essential constituent of the cell membrane of which polyunsaturated fatty acids (PUFAs) are the most important component. Activation of phospholipase A2 (PLA2) induces the release of PUFAs from the cell membrane that form precursors to both pro- and ant-inflammatory bioactive lipids that participate in several cellular processes. PUFAs GLA (gamma-linolenic acid), DGLA (dihomo-GLA), AA (arachidonic acid), EPA (eicosapentaenoic acid) and DHA (docosahexaenoic acid) are derived from dietary linoleic acid (LA) and alpha-linolenic acid (ALA) by the action of desaturases whose activity declines with age. Consequently, aged cells are deficient in GLA, DGLA, AA, AA, EPA and DHA and their metabolites. LA, ALA, AA, EPA and DHA can also be obtained direct from diet and their deficiency (fatty acids) may indicate malnutrition and deficiency of several minerals, trace elements and vitamins some of which are also much needed co-factors for the normal activity of desaturases. In many instances (patients) the plasma and tissue levels of GLA, DGLA, AA, EPA and DHA are low (as seen in patients with hypertension, type 2 diabetes mellitus) but they do not have deficiency of other nutrients. Hence, it is reasonable to consider that the deficiency of GLA, DGLA, AA, EPA and DHA noted in these conditions are due to the decreased activity of desaturases and elongases. PUFAs stimulate SIRT1 through protein kinase A-dependent activation of SIRT1-PGC1α complex and thus, increase rates of fatty acid oxidation and prevent lipid dysregulation associated with aging. SIRT1 activation prevents aging. Of all the SIRTs, SIRT6 is critical for intermediary metabolism and genomic stability. SIRT6-deficient mice show shortened lifespan, defects in DNA repair and have a high incidence of cancer due to oncogene activation. SIRT6 overexpression lowers LDL and triglyceride level, improves glucose tolerance, and increases lifespan of mice in addition to its anti-inflammatory effects at the transcriptional level. PUFAs and their anti-inflammatory metabolites influence the activity of SIRT6 and other SIRTs and thus, bring about their actions on metabolism, inflammation, and genome maintenance. GLA, DGLA, AA, EPA and DHA and prostaglandin E2 (PGE2), lipoxin A4 (LXA4) (pro- and anti-inflammatory metabolites of AA respectively) activate/suppress various SIRTs (SIRt1 SIRT2, SIRT3, SIRT4, SIRT5, SIRT6), PPAR-γ, PARP, p53, SREBP1, intracellular cAMP content, PKA activity and peroxisome proliferator-activated receptor γ coactivator 1-α (PGC1-α). This implies that changes in the metabolism of bioactive lipids as a result of altered activities of desaturases, COX-2 and 5-, 12-, 15-LOX (cyclo-oxygenase and lipoxygenases respectively) may have a critical role in determining cell age and development of several aging associated diseases and genomic stability and gene and oncogene activation. Thus, methods designed to maintain homeostasis of bioactive lipids (GLA, DGLA, AA, EPA, DHA, PGE2, LXA4) may arrest aging process and associated metabolic abnormalities.

Protection against Doxorubicin‐Related Cardiotoxicity by Jaceosidin Involves the Sirt1 Signaling Pathway

The clinical use of doxorubicin (DOX) is largely limited by its cardiotoxicity. Previous studies have shown that jaceosidin has many biological activities. However, little is known about whether jaceosidin can attenuate DOX-related acute cardiotoxicity. Here, we investigated the therapeutic effects of jaceosidin on DOX-induced acute cardiotoxicity. Mice were intraperitoneally injected with a single dose of DOX to establish an acute cardiac injury model. To explore the protective effects, mice were orally administered jaceosidin daily for 7 days, with dosing beginning 2 days before DOX injection. The results demonstrated that jaceosidin dose-dependently reduced free radical generation, inflammation accumulation, and cell loss induced by DOX in cardiomyocytes. Further studies showed that jaceosidin treatment inhibited myocardial oxidative damage and the inflammatory response and attenuated myocardial apoptotic death, thus improving cardiac function in mice injected with DOX. The inhibitory effects of jaceosidin on DOX-related acute cardiotoxicity were mediated by activation of the sirtuin1 (Sirt1) signaling pathway. Jaceosidin lost its protective effect against DOX-related injury in Sirt1-deficient cardiomyocytes and mice. In conclusion, jaceosidin has protective potential in treating DOX-related cardiac injury through activation of the Sirt1 signaling pathway.

SIRT1 Alleviates Aldosterone-Induced Podocyte Injury by Suppressing Mitochondrial Dysfunction and NLRP3 Inflammasome Activation

Background: Podocyte injury contributes to progressive glomerulosclerosis. Previously, we demonstrated the important role of the NLR family pyrin domain containing 3 (NLRP3) inflammasome in mediating the podocyte injury induced by aldosterone. Silent mating type information regulation 2 homolog 1 (SIRT1) is an NAD+-dependent deacetylase that is associated with the regulation of cellular inflammation. However, whether the activation of the NLRP3 inflammasome in podocytes is regulated by SIRT1, and the mechanism involved, remains unknown. Methods: In this study, we detected SIRT1 expression in patients with podocyte disease and performed an aldosterone infusion model in podocyte-specific Sirt1 knockout mice. In cultured podocytes, we used plasmids to overexpress SIRT1 and treated the podocyte with aldosterone. Results: SIRT1 was significantly decreased in the glomeruli of patients with podocyte disease. Sirt1-deficient mice showed significant urinary albumin excretion after aldosterone infusion, and the severity of the glomerular injury was significantly greater in podocyte-specific Sirt1 knockout mice than in the wild-type mice. Moreover, podocyte conditional Sirt1 knockout aggravated NLRP3 inflammasome activation and mitochondrial dysfunction (MtD). In vitro, overexpression of SIRT1 inhibited NLRP3 activation, protected against MtD and podocyte injury. Conclusion: Taken together, these findings revealed a novel regulatory mechanism of the NLRP3 inflammasome by SIRT1 by promoting mitochondrial function, which provides some potential targets for the treatment of glomerular diseases.

Epigenetic regulation of chondrocyte hypertrophy and apoptosis through Sirt1/P53/P21 pathway in surgery-induced osteoarthritis

Sirt1 involved in cellular aging and aging-related diseases, including osteoarthritis (OA). Our previous study showed Sirt1 played a role in the pathogenesis of OA, however, the underlying mechanisms are still poorly elicited. In this study, we investigated the role of Sirt1 in epigenetically regulating P53/P21 pathway in a Sirt1 loss model. Sirt1 deletion male mice (n = 10) with destabilization of the medial meniscus (DMM) were used to observe its role on OA development. Then, the relationships between SIRT1 and P53 were detected by Coimmunoprecipitation (CoIP), and the gain-off function of P53 gene was indicated by P53 activators and inhibitors in vitro. Finally, human cartilage samples from patients with OA were collected. Sirt1 deletion mice displayed a spontaneous OA development, manifesting severe chondrocytes hypertrophy markers MMP13 and ADAMTS5, highly expressed P53 and P21. Strikingly, surgery-induced meniscus injury promoted the OA pathogenesis and apoptosis in Sirt1 deficient mice. Ultimately, our CoIP data demonstrated that Sirt1 directly interacted with P53 in vitro. However inhibition of P53 alleviated OA progression. We also observed that chondrocyte apoptosis and P53 increased in osteoarthritis (OA) progression with a declining expression of Sirt1 in human cartilage. Loss of Sirt1 in cartilage led to accelerated OA pathogenesis via aberrant activation of p53/p21 mediated senescence associated secretory phenotype, hypertrophy and apoptosis.

Sexually Dimorphic Responses to a High-Refined Carbohydrate Diet in a Nonalcoholic Fatty Liver Disease Mouse Model

Abstract Objectives Nonalcoholic fatty liver disease (NAFLD) incidence and prevalence have been reported to be higher in men than women, however, the effects of sexual dimorphism on NAFLD risk and progression have not been adequately examined. Our lab has previously shown that a liquid high-refined carbohydrate diet (HRCD) induced more severe hepatic steatosis compared to an isocaloric high fat diet in male mice. Also, HRCD-induced reduction in sirtuin 1 (SIRT1), an NAD-dependent deacetylase protein, has previously been implicated in NAFLD pathogenesis. Therefore, we investigated whether there were sexually dimorphic responses to a liquid high-refined carbohydrate diet (HRCD) in male and female, wildtype and SIRT1-deficient mice. Methods Male and female 10–12-week-old wildtype (SIRT1 +/+: n = 12; M = 6, F = 6) and mice carrying a heterozygous H355Y SIRT1 point mutation (SIRT1 +/y: n = 14; M = 7, F = 7) were both fed a HRCD (Lieber-DeCarli liquid diet supplemented with maltose dextrin; 47% energy from refined carbohydrate, Dyets, #710,260) for 5 weeks and 9 weeks. Hepatic gene expression was examined using qRT-PCR. Plasma ALT (alanine transaminase) and hepatic MDA (malondialdehyde) levels were determined using colorimetric assay kits. Hepatic steatosis scoring was conducted by analyzing Hematoxylin and Eosin (H&E) stains. Results 9 weeks of HRCD induced significantly less hepatic steatosis in female mice irrespective of genotype compared to male mice as determined by grading of H&E stains (P < 0.05). Furthermore, liver expression of several fatty acid oxidation genes (CPT1, ACOX1) was significantly higher in females (P < 0.05), which potentially suggests increased fatty acid oxidation. Additionally, female mice had significantly increased antioxidant gene expression (GPX4, SOD1, SOD2, Catalase) and significantly lower hepatic MDA (P < 0.05), which indicate an increased capacity to mitigate oxidative stress. Lastly, plasma ALT levels were significantly lower in females compared to males after 9 weeks of HRCD (P < 0.05). Conclusions Collectively, these data indicate that female mice are moderately protected against HRCD-induced NAFLD compared to male mice, potentially through increased hepatic fatty acid oxidation and superior mitigation of oxidative stress due to increased antioxidant system gene expression in the liver. Funding Sources HNRCA, USDA/ARS Grants.

Sirt6 deficiency impairs corneal epithelial wound healing

Corneal transparency, dependent on the integrity of epithelial cells, is essential for vision. Corneal epithelial damage is one of the most commonly observed ocular conditions and proper wound healing is necessary for corneal transparency. Sirt6, a histone deacetylase, has been shown to regulate many cellular events including aging and inflammation. However, its specific role in corneal epithelial wound healing remains unknown. Here we demonstrated that Sirt6 was expressed in corneal epithelial cells and its expression decreased with age. In an in vivo corneal epithelial wound healing model, Sirt6 deficiency resulted in delayed and incomplete wound healing and was associated excessive inflammation in the corneal stroma and dysfunction of Notch signaling, leading to keratinization of the corneal epithelium and corneal opacity. Aging Sirt6-deficient mice spontaneously developed corneal keratitis with extensive infiltration of inflammatory cells into the cornea. In vitro experiments demonstrated that primary corneal epithelial cells with Sirt6 downregulation expressed increased basal levels of inflammatory genes and exhibited hyper-inflammatory reactivity to IL-1β and TNFα treatment. These results provide compelling evidence that Sirt6 is a critical regulator of inflammation in the cornea, and is responsible for corneal epithelial wound healing, thus contributing to the maintenance of epithelial integrity and corneal transparency.

Haploinsufficiency of Trp53 dramatically extends the lifespan of Sirt6-deficient mice.

Mammalian sirtuin 6 (Sirt6) is a conserved NAD+-dependent deacylase and mono-ADP ribosylase that is known to be involved in DNA damage repair, metabolic homeostasis, inflammation, tumorigenesis, and aging. Loss of Sirt6 in mice results in accelerated aging and premature death within a month. Here, we show that haploinsufficiency (i.e., heterozygous deletion) of Trp53 dramatically extends the lifespan of both female and male Sirt6-deficient mice. Haploinsufficiency of Trp53 in Sirt6-deficient mice rescues several age-related phenotypes of Sirt6-deficient mice, including reduced body size and weight, lordokyphosis, colitis, premature senescence, apoptosis, and bone marrow stem cell decline. Mechanistically, SIRT6 deacetylates p53 at lysine 381 to negatively regulate the stability and activity of p53. These findings establish that elevated p53 activity contributes significantly to accelerated aging in Sirt6-deficient mice. Our study demonstrates that p53 is a substrate of SIRT6, and highlights the importance of SIRT6-p53 axis in the regulation of aging.

SIRT6-mediated transcriptional suppression of Txnip is critical for pancreatic beta cell function and survival in mice.

Better understanding of how genetic and epigenetic components control beta cell differentiation and function is key to the discovery of novel therapeutic approaches to prevent beta cell dysfunction and failure in the progression of type 2 diabetes. Our goal was to elucidate the role of histone deacetylase sirtuin 6 (SIRT6) in beta cell development and homeostasis. Sirt6 endocrine progenitor cell conditional knockout and beta cell-specific knockout mice were generated using the Cre-loxP system. Mice were assayed for islet morphology, glucose tolerance, glucose-stimulated insulin secretion and susceptibility to streptozotocin. Transcriptional regulatory functions of SIRT6 in primary islets were evaluated by RNA-Seq analysis. Reverse transcription-quantitative (RT-q)PCR and immunoblot were used to verify and investigate the gene expression changes. Chromatin occupancies of SIRT6, H3K9Ac, H3K56Ac and active RNA polymerase II were evaluated by chromatin immunoprecipitation. Deletion of Sirt6 in pancreatic endocrine progenitor cells did not affect endocrine morphology, beta cell mass or insulin production but did result in glucose intolerance and defective glucose-stimulated insulin secretion in mice. Conditional deletion of Sirt6 in adult beta cells reproduced the insulin secretion defect. Loss of Sirt6 resulted in aberrant upregulation of thioredoxin-interacting protein (TXNIP) in beta cells. SIRT6 deficiency led to increased acetylation of histone H3 lysine residue at 9 (H3K9Ac), acetylation of histone H3 lysine residue at 56 (H3K56Ac) and active RNA polymerase II at the promoter region of Txnip. SIRT6-deficient beta cells exhibited a time-dependent increase in H3K9Ac, H3K56Ac and TXNIP levels. Finally, beta cell-specific SIRT6-deficient mice showed increased sensitivity to streptozotocin. Our results reveal that SIRT6 suppresses Txnip expression in beta cells via deacetylation of histone H3 and plays a critical role in maintaining beta cell function and viability. Sequence data have been deposited in the National Institutes of Health (NIH) Gene Expression Omnibus (GEO) with the accession code GSE104161.

SIRT6 promotes osteogenic differentiation of mesenchymal stem cells through BMP signaling

SIRT6 has been identified as an H3K9 deacetylase and a critical regulator of genome stability, telomere integrity, and metabolic homeostasis. Sirt6-deficient mice displayed dramatic phenotypes including profound lymphopenia, loss of subcutaneous fat, lordokyphosis and low bone marrow density. Here, we report that SIRT6 regulates osteogenic differentiation independent of its deacetylase activity in vitro. Further mechanistic studies showed that SIRT6 involves the cell fate determination by modulating bone morphogenetic protein (BMP) signaling. Unexpectedly, this modulation depends upon P300/CBP-associated factor (PCAF). In addition, we observed impaired SIRT6 expression in bone marrow mesenchymal stem cells and in bone sections of ovariectomized mice. Taken together, our present study provide new insights into mechanisms of SIRT6-regulated MSC function beyond its H3K9 deacetylase activity.

Retinoic acid ameliorates high-fat diet-induced liver steatosis through sirt1.

In this study, treatment of C57BL/6J (wild type, WT) mice fed a high-fat diet (HFD) with retinoic acid (RA) decreased body weight and subcutaneous and visceral fat content, reversed the apparent hepatosteatosis, and reduced hepatic intracellular triglyceride and serum alanine transaminase (ALT) and aspartate aminotransferase (AST) concentrations. Moreover, RA treatment improved glucose tolerance and insulin sensitivity in WT mice fed a HFD. However, these RA-induced effects in WT mice fed a HFD were alleviated in liver specific Sirtuin 1 (Sirt1) deficient (LKO) mice fed a HFD. Furthermore, RA also could not improve glucose tolerance and insulin sensitivity in LKO mice fed a HFD. The mechanism studies indicated that RA indeed increased the expression of hepatic Sirt1 and superoxide dismutase 2 (Sod2), and inhibited the expression of sterol regulatory element binding protein 1c (Srebp-1c) in WT mice in vivo and in vitro. RA decreased mitochondrial reactive oxygen species (ROS) production in WT primary hepatocytes and increased mitochondrial DNA (mtDNA) copy number in WT mice liver. However, these RA-mediated molecular effects were also abolished in the liver and primary hepatocytes from LKO mice. In summary, RA protected against HFD-induced hepatosteatosis by decreasing Srebp-1c expression and improving antioxidant capacity through a Sirt1-mediated mechanism.

Is SIRT6 Activity Neuroprotective and How Does It Differ from SIRT1 in This Regard?

The NAD+-dependent type III histone deacetylases, or Sirtuins, have differing cellular localizations, and can have nuclear (SIRT1 and SIRT6), cytoplasmic (SIRT2), mitochondrial (SIRT3, SIRT4, and SIRT5) or nucleolar (SIRT7) localization. The founding member of the Sirtuin family, S. cerevisiae Sir2p, and its orthologs have important roles in senescence and aging, with manipulation of their activities resulting in modest lifespan extension, as verified in several model organisms. Sirtuins have a wide range of nuclear and cytoplasmic substrates, and are implicated in the transcriptional and epigenetic regulations of cellular and systemic processes of energy metabolism, stress response, death and survival, as well as pathological conditions such as malignancy (Chalkiadaki and Guarente, 2015) and neurodegeneration (Herskovits and Guarente, 2013). Perhaps the most extensively studied member of the mammalian Sirtuin paralogs is SIRT1, but the cellular and systemic roles of other paralogs have also been intensively investigated in recent years. SIRT6, like SIRT1, is nuclear localized. Acting primarily as a histone deacetylase, earlier studies have delineated its roles in telomere maintenance (Michishita et al., 2008), DNA repair (Mao et al., 2011) and transcription regulation of glucose homeostasis (Zhong et al., 2010). The importance of SIRT6 in mammals is illustrated by the fact that SIRT6 deficient mice have retarded postnatal development associated with cellular genomic instability, systemic metabolic defects, and exhibited premature senescence and aging, with early death occurring at around 4 weeks after birth (Mostoslavsky et al., 2006). In the past several years, many other emerging physiological as well as pathological functions for SIRT6 in multiple tissue contexts have also been reported (Tasselli et al., 2017). These include the regulation of the circadian clock (Masri et al., 2014), tumor suppression (Kugel et al., 2016) as well as lifespan extension in mammals (Kanfi et al., 2012), among others. In many of these cases, SIRT6 functions at first glance appear to have significant overlaps with that of SIRT1. Unlike SIRT1, which has been extensively studied with regards to its roles in neurons and the central nervous system (CNS) (Herskovits and Guarente, 2014; Ng et al., 2015), the function of SIRT6 in neural tissues has been less well explored. Like SIRT1, SIRT6 appears to have anti-aging activities through the regulation of inflammation (Kawahara et al., 2009), the expression of antioxidant genes (Pan et al., 2016) as well as proteasomal degradation of senescence factors (Zhao et al., 2016), all of which are related to neuroprotection. Like SIRT1, SIRT6 activity may therefore be largely neuroprotective, and this expectation appears to have been borne out by several recent reports, which suggest that loss of SIRT6 during aging and neuropathological settings contributes to neurodegenerative processes. However, elevated SIRT6 level has also been noted to be undesirable under certain conditions. SIRT6 may also differ from SIRT1 in terms of enzymatic activity and mechanism of action. In the paragraphs below, I shall explore current evidence (as well as counterevidence) for the neuroprotective potential of SIRT6, and draw comparisons with that of SIRT1.

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 deficiency results in progression of glomerular injury in the kidney.

Aging is associated with an increased incidence and prevalence of renal glomerular diseases. Sirtuin (Sirt) 6, a nicotinamide adenine dinucleotide (NAD)-dependent histone deacetylase, has been shown to protect against multiple age-associated phenotypes; however it is unknown whether Sirt6 has a direct pathophysiologic role in the kidney. In the present study, we demonstrate that Sirt6 is expressed in the kidney and aging Sirt6-deficient mice exhibit renal hypertrophy with glomerular enlargement. Sirt6 deletion induces podocyte injury, including decreases in slit diaphragm proteins, foot process effacement, and cellular loss, resulting in proteinuria. Knockdown of Sirt6 in cultured primary murine podocytes induces shape changes with loss of process formation and cell apoptosis. Moreover, Sirt6 deficiency results in progressive renal inflammation and fibrosis. Collectively, these data provide compelling evidence that Sirt6 is important for podocyte homeostasis and maintenance of glomerular function, and warrant further investigation into the role of Sirt6 in age-associated kidney dysfunction.

Neuroprotective Functions for the Histone Deacetylase SIRT6

The histone deacetylase SIRT6 promotes DNA repair, but its activity declines with age with a concomitant accumulation of DNA damage. Furthermore, SIRT6 knockout mice exhibit an accelerated aging phenotype and die prematurely. Here, we report that brain-specific SIRT6-deficient mice survive but present behavioral defects with major learning impairments by 4months of age. Moreover, the brains of these mice show increased signs of DNA damage, cell death, and hyperphosphorylated Tau-a critical mark in several neurodegenerative diseases. Mechanistically, SIRT6 regulates Tau protein stability and phosphorylation through increased activation of the kinase GSK3α/β. Finally, SIRT6 mRNA and protein levels are reduced in patients with Alzheimer's disease. Taken together, our results suggest that SIRT6 is critical to maintain genomic stability in the brain and that its loss leads to toxic Tau stability and phosphorylation. Therefore, SIRT6 and its downstream signaling could be targeted in Alzheimer's disease and age-related neurodegeneration.