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

Omega-3 polyunsaturated fatty acids alleviate hepatic steatosis-induced inflammation through Sirt1-mediated nuclear translocation of NF-κB p65 subunit in hepatocytes of large yellow croaker (Larmichthys crocea)

Hepatic steatosis induced inflammation is becoming increasingly prevalent in farmed fish. This study was conducted to investigate the protective effects of omega-3 polyunsaturated fatty acids (ω-3 PUFAs) against hepatic steatosis-induced inflammation and its potential molecular mechanisms in hepatocyte of large yellow croaker (Larmichthys crocea). We found that the hepatic steatosis-induced inflammation was relieved by ω-3 PUFAs, meanwhile, the Sirt1 activity and transcript expression was increased by ω-3 PUFAs. The increased Sirt1 activity can decrease the hepatic steatosis-induced inflammation. The protective effects of ω-3 PUFAs against hepatic steatosis-induced inflammation was reversed by the treatment with Sirt1 inhibitor EX-527. The nuclear translocation of nuclear transcription factor kappa-B (NF-κB) p65 was significantly decreased after ω-3 PUFAs treatments compared to the palmitic acid stimulation group. The ω-3 PUFAs induced cytoplasm translocation of NF-κB p65 was reversed by EX-527. Together, ω-3 PUFAs alleviate hepatic steatosis-induced inflammation through Sirt1-mediated nuclear translocation of NF-κB p65 subunit in hepatocytes of large yellow croaker. The present study provides important insight into the mechanisms of the protective effects of ω-3 PUFAs, providing theory bases for alleviating the hepatic steatosis induced inflammation of farmed fish, thereby offering great benefits to the aquaculture industry and fish consumers.

Sirt1 Protects Endothelial Cells against LPS-Induced Barrier Dysfunction

Sepsis is a threatening health problem and characterized by microvascular dysfunction. In this study, we verified that LPS caused the downregulation of Sirt1 and the hyperpermeability of endothelial cells. Inhibition of Sirt1 with ex527 or Sirt1 siRNA displayed a higher permeability, while activation of Sirt1 with SRT1720 reversed the LPS-induced hyperpermeability, formation of fiber stress, and disruption of VE-cadherin distribution. In pulmonary microvascular vein endothelial cells isolated from wild-type mice, Sirt1 was attenuated upon LPS, while Sirt1 was preserved in a receptor of advanced glycation end product-knockout mice. The RAGE antibody could also diminish the downregulation and ubiquitination of Sirt1 in LPS-exposed human umbilical vein endothelial cells. An LPS-induced decrease in Sirt1 activity was attenuated by the RAGE antibody and TLR4 inhibitor. In vivo study also demonstrated the attenuating role of Sirt1 and RAGE knockout in LPS-induced increases in dextran leakage of mesenteric venules. Furthermore, activation of Sirt1 prevented LPS-induced decreases in the activity and expression of superoxide dismutase 2, as well as the increases in NADPH oxidase 4 and reactive oxygen species, while inhibition of Sirt1 aggravated the SOD2 decline. It also demonstrated that Sirt1-deacetylated p53 is required for p53 inactivation, which reversed the downregulation of β-catenin caused by LPS.

Carbon Monoxide Potentiation of L-Type Ca2+ Channel Activity Increases HIF-1α-Independent VEGF Expression via an AMPKα/SIRT1-Mediated PGC-1α/ERRα Axis

AIMS

The heme oxygenase-1 (HO-1)/carbon monoxide (CO) pathway induced in astrocytes after ischemic brain injury promotes vascular endothelial growth factor (VEGF) expression to maintain and repair neurovascular function. Although HO-1-derived CO has been shown to induce hypoxia-inducible factor-1α (HIF-1α)-dependent VEGF expression, the underlying mechanism independent of HIF-1α remains to be elucidated.

RESULTS

HO-1 and VEGF were coexpressed in astrocytes of ischemic mouse brain tissues. Experiments with specific siRNAs and pharmacological activators/inhibitors of various target genes demonstrated that astrocytes pre-exposed to the CO-releasing compound, CORM-2, or transfected with HO-1 increased HIF-1α-independent VEGF expression via sequential activation of the following signal cascades; Ca2+/calmodulin-dependent protein kinase kinase β-mediated AMP-activated protein kinase (AMPK)α activation, AMPKα-induced increases in nicotinamide phosphoribosyltransferase (NAMPT) expression and cellular NAD+ level, sirtuin 1 (SIRT1)-dependent peroxisome proliferator-activated receptor-γ coactivator-1α (PGC-1α) stabilization and activation, and PGC-1α/estrogen-related receptor (ERR)α-mediated VEGF expression. All of these sequential events were blocked by an L-type voltage-gated Ca2+ channel inhibitor and Ca2+ chelators, but not by other Ca2+ channel inhibitors.

INNOVATION

HO-1-derived CO elicits Ca2+ influx by activating L-type Ca2+ channels, which is a key player in HIF-1α-independent VEGF expression by activating the AMPKα-NAMPT-SIRT1-PGC-1α/ERRα pathway.

CONCLUSION

Our results provide new mechanistic insight into the possible role for L-type Ca2+ channels in HO-1/CO-induced angiogenesis. Antioxid. Redox Signal. 27, 21-36.

The role of «metabolic memory» mechanisms in the development and progression of vascular complications of diabetes mellitus

The study of diabetes mellitus (DM), its complications and related pathologies has been continuously performed for many years; however, despite the substantial work and outstanding achievements in studying the mechanisms of DM development and the success of new medicinal products for controlling glycaemia, the problems associated with the late complications of DM continue to increase. The importance of glycaemic control in the early stages of DM for the development of complications is seen only after a sufficiently long period of observation. Such a delayed effect of primary good or unsatisfactory metabolic control, which shapes the patients clinical fate to a greater extent, is termed metabolic memory. The disorders developed under the influence of hyperglycaemia persist for long periods after the normalisation of carbohydrate metabolism; moreover, the effect of previous hyperglycaemia extends over the next 20 and even 30 years. Current research is focused on the possible mechanisms of metabolic memory development, including oxidative stress, advanced glycation end products and epigenetic mechanisms. This research will provide insight into potential markers for the early development and progression of vascular complications and new therapeutic possibilities for the future. However, determining the probable point of no return is more important, which implies that a point exists; after this point is crossed, the progression of vascular complications associated with DM cannot be prevented or reversed. The results of numerous experimental studies demonstrate that the prerequisite components of metabolic memory can be used as potential markers of the progression of DM complications, and may be potential therapeutic targets.

Sirt1 Inhibits Oxidative Stress in Vascular Endothelial Cells

The vascular endothelium is a layer of cells lining the inner surface of vessels, serving as a barrier that mediates microenvironment homeostasis. Deterioration of either the structure or function of endothelial cells (ECs) results in a variety of cardiovascular diseases. Previous studies have shown that reactive oxygen species (ROS) is a key factor that contributes to the impairment of ECs and the subsequent endothelial dysfunction. The longevity regulator Sirt1 is a NAD⁺-dependent deacetylase that has a potential antioxidative stress activity in vascular ECs. The mechanisms underlying the protective effects involve Sirt1/FOXOs, Sirt1/NF-κB, Sirt1/NOX, Sirt1/SOD, and Sirt1/eNOs pathways. In this review, we summarize the most recent reports in this field to recapitulate the potent mechanisms involving the protective role of Sirt1 in oxidative stress and to highlight the beneficial effects of Sirt1 on cardiovascular functions.

Posttranslational Modifications and Plant-Environment Interaction

Posttranslational modifications (PTMs) of proteins such as phosphorylation and ubiquitination are crucial for controlling protein stability, localization, and conformation. Genetic information encoded in DNA is transcribed, translated, and increases its complexity by multiple PTMs. Conformational change introduced by PTMs affects interacting partners of each proteins and their downstream signaling; therefore, PTMs are the major level of modulations of total outcome of living cells. Plants are living in harsh environment that requires unremitting physiological modulation to survive, and the plant response to various environment stresses is regulated by PTMs of proteins. This review deals with the novel knowledge of PTM-focused proteomic studies on various life conditions. PTMs are focused that mediate plant-environment interaction such as stress perception, protein homeostasis, control of energy shift, and defense by immune system. Integration of diverse signals on a protein via multiple PTMs is discussed as well, considering current situation where signal integration became an emerging area approached by systems biology into account.

miR-23b-3p induces the cellular metabolic memory of high glucose in diabetic retinopathy through a SIRT1-dependent signalling pathway

AIMS/HYPOTHESIS

The mechanisms underlying the cellular metabolic memory induced by high glucose remain unclear. Here, we sought to determine the effects of microRNAs (miRNAs) on metabolic memory in diabetic retinopathy.

METHODS

The miRNA microarray was used to examine human retinal endothelial cells (HRECs) following exposure to normal glucose (N) or high glucose (H) for 1 week or transient H for 2 days followed by N for another 5 days (H→N). Levels of sirtuin 1 (SIRT1) and acetylated-nuclear factor κB (Ac-NF-κB) were examined following transfection with miR-23b-3p inhibitor or with SIRT1 small interfering (si)RNA in the H→N group, and the apoptotic HRECs were determined by flow cytometry. Retinal tissues from diabetic rats were similarly studied following intravitreal injection of miR-23b-3p inhibitor. Chromatin immunoprecipitation (ChIP) analysis was performed to detect binding of NF-κB p65 to the potential binding site of the miR-23b-27b-24-1 gene promoter in HRECs.

RESULTS

High glucose increased miR-23b-3p expression, even after the return to normal glucose. Luciferase assays identified SIRT1 as a target mRNA of miR-23b-3p. Reduced miR-23b-3p expression inhibited Ac-NF-κB expression by rescuing SIRT1 expression and also relieved the effect of metabolic memory induced by high glucose in HRECs. The results were confirmed in the retina using a diabetic rat model of metabolic memory. High glucose facilitated the recruitment of NF-κB p65 and promoted transcription of the miR-23b-27b-24-1 gene, which can be suppressed by decreasing miR-23b-3p expression.

CONCLUSIONS/INTERPRETATION

These studies identify a novel mechanism whereby miR-23b-3p regulates high-glucose-induced cellular metabolic memory in diabetic retinopathy through a SIRT1-dependent signalling pathway.

Sirtuin1 (SIRT1) in the Acetylation of Downstream Target Proteins

Acetylation has been shown to be an important posttranslational modification (PTM) of both histone and nonhistone proteins with particular implications in cell signaling and transcriptional regulation of gene expression. Many studies have already demonstrated that SIRT1 is able to deacetylate histones and lead to gene silencing. It can also regulate the function of tumor suppressors including FOXO proteins and p53 by deacetylation. Here, we describe three experimental approaches for studying the modulation of the acetylation status of some of the known downstream targets of SIRT1.

Dysregulation of SIRT-1 in aging mice increases skeletal muscle fatigue by a PARP-1-dependent mechanism

Accumulation of reactive oxygen species (ROS) in skeletal muscles and the resulting decline in muscle performance are hallmarks of sarcopenia. However, the precise mechanism by which ROS results in a decline in muscle performance is unclear. We demonstrate that isometric-exercise concomitantly increases the activities of Silent information regulator 1 (SIRT-1) and Poly [ADP-ribose] polymerase (PARP-1), and that activated SIRT-1 physically binds with and inhibits PARP-1 activity by a deacetylation dependent mechanism in skeletal muscle from young mice. In contrast, skeletal muscle from aged mice displays higher PARP-1 activity and lower SIRT-1 activity due to decreased intracellular NAD⁺ content, and as a result reduced muscle performance in response to exercise. Interestingly, injection of PJ34, a PARP-1 inhibitor, in aged mice increased SIRT-1 activity by preserving intracellular NAD⁺ content, which resulted in higher skeletal muscle mitochondrial biogenesis and performance. We found that the higher activity of PARP-1 in H₂O₂-treated myotubes or in exercised-skeletal muscles from aged mice is due to an elevated level of PARP-1 acetylation by the histone acetyltransferase General control of amino acid synthesis protein 5-like 2 (GCN-5). These results suggest that activation of SIRT-1 and/or inhibition of PARP-1 may ameliorate skeletal muscle performance in pathophysiological conditions such as sarcopenia and disuse-induced atrophy in aging.

Hugan Qingzhi Exerts Anti-Inflammatory Effects in a Rat Model of Nonalcoholic Fatty Liver Disease

Ethnopharmacological Relevance. The Hugan Qingzhi tablet (HQT) is a traditional Chinese medicine used for treating NAFLD (nonalcoholic fatty liver disease). The present study evaluated the anti-inflammatory effects of HQT in rats with NAFLD. Materials and Methods. HQT was administered daily to the NAFLD experimental groups. Biochemical markers, histopathological data, and oxidative stress/antioxidant biomarkers were determined. Proinflammatory cytokines interleukin-1β (IL-1β), tumor necrosis factor α (TNF-α), and interleukin-6 (IL-6) were detected by enzyme-linked immunoassay. Expressions of silent information regulator 1 (SIRT1) and acetylated-nuclear-factor kappaB-p65 (Ac-NF-κB-p65) were performed by western blotting. Results. At high and moderate doses, HQT was highly effective in decreasing serum alanine aminotransferase (P < 0.01), aspartate aminotransferase (P < 0.01), hepatic total cholesterol (P < 0.01), triglycerides (P < 0.01), and free fatty acid levels (P < 0.01). Moreover, high and moderate doses of HQT reduced hepatic levels of the proinflammatory cytokines TNF-α (P < 0.01), IL-1β (P < 0.01), and IL-6 (P < 0.01), enhanced SIRT1 expression, and depressed Ac-NF-κB-p65 expression at protein level. Conclusions. In our NAFLD rat model, HQT exerted substantial anti-inflammatory and antioxidant activities, possibly involving the regulation of SIRT1 and Ac-NF-κB-p65 expression.

Mechanistic Role of MicroRNAs in Coupling Lipid Metabolism and Atherosclerosis

MicroRNAs (miRNAs, miRs) represent a group of powerful and versatile posttranscriptional regulators of gene expression being involved in the fine control of a plethora of physiological and pathological processes. Besides their well-established crucial roles in the regulation of cell cycle, embryogenesis or tumorigenesis, these tiny molecules have also been shown to participate in the regulation of lipid metabolism. In particular, miRs orchestrate cholesterol and fatty acids synthesis, transport, and degradation and low-density and high-density lipoprotein (LDL and HDL) formation. It is thus not surprising that they have also been reported to affect the development and progression of several lipid metabolism-related disorders including liver steatosis and atherosclerosis. Mounting evidence suggests that miRs might represent important "posttranscriptional hubs" of lipid metabolism, which means that one miR usually targets 3'-untranslated regions of various mRNAs that are involved in different steps of one precise metabolic/signaling pathway, e.g., one miR targets mRNAs of enzymes important for cholesterol synthesis, degradation, and transport. Therefore, changes in the levels of one key miR affect various steps of one pathway, which is thereby promoted or inhibited. This makes miRs potent future diagnostic and even therapeutic tools for personalized medicine. Within this chapter, the most prominent microRNAs involved in lipid metabolism, e.g., miR-27a/b, miR-33/33*, miR-122, miR-144, or miR-223, and their intracellular and extracellular functions will be extensively discussed, in particular focusing on their mechanistic role in the pathophysiology of atherosclerosis. Special emphasis will be given on miR-122, the first microRNA currently in clinical trials for the treatment of hepatitis C and on miR-223, the most abundant miR in lipoprotein particles.

High-fat diet-induced impairment of skeletal muscle insulin sensitivity is not prevented by SIRT1 overexpression

Skeletal muscle sirtuin 1 (SIRT1) expression is reduced under insulin-resistant conditions, such as those resulting from high-fat diet (HFD) feeding and obesity. Herein, we investigated whether constitutive activation of SIRT1 in skeletal muscle prevents HFD-induced muscle insulin resistance. To address this, mice with muscle-specific overexpression of SIRT1 (mOX) and wild-type (WT) littermates were fed a control diet (10% calories from fat) or HFD (60% of calories from fat) for 12 wk. Magnetic resonance imaging and indirect calorimetry were used to measure body composition and energy expenditure, respectively. Whole body glucose metabolism was assessed by oral glucose tolerance test, and insulin-stimulated glucose uptake was measured at a physiological insulin concentration in isolated soleus and extensor digitorum longus muscles. Although SIRT1 was significantly overexpressed in muscle of mOX vs. WT mice, body weight and percent body fat were similarly increased by HFD for both genotypes, and energy expenditure was unaffected by diet or genotype. Importantly, impairments in glucose tolerance and insulin-mediated activation of glucose uptake in skeletal muscle that occurred with HFD feeding were not prevented in mOX mice. In contrast, mOX mice showed enhanced postischemic cardiac functional recovery compared with WT mice, confirming the physiological functionality of the SIRT1 transgene in this mouse model. Together, these results demonstrate that activation of SIRT1 in skeletal muscle alone does not prevent HFD-induced glucose intolerance, weight gain, or insulin resistance.

AAV8-mediated Sirt1 gene transfer to the liver prevents high carbohydrate diet-induced nonalcoholic fatty liver disease

Nonalcoholic fatty liver disease (NAFLD) is the most common hepatic disease worldwide, and evidence suggests that it promotes insulin resistance and type 2 diabetes. Caloric restriction (CR) is the only available strategy for NAFLD treatment. The protein deacetylase Sirtuin1 (SIRT1), which is activated by CR, increases catabolic metabolism and decreases lipogenesis and inflammation, both involved in the development of NAFLD. Here we show that adeno-associated viral vectors of serotype 8 (AAV8)-mediated liver-specific Sirt1 gene transfer prevents the development of NAFLD induced by a high carbohydrate (HC) diet. Long-term hepatic SIRT1 overexpression led to upregulation of key hepatic genes involved in β-oxidation, prevented HC diet-induced lipid accumulation and reduced liver inflammation. AAV8-Sirt1-treated mice showed improved insulin sensitivity, increased oxidative capacity in skeletal muscle and reduced white adipose tissue inflammation. Moreover, HC feeding induced leptin resistance, which was also attenuated in AAV8-Sirt1-treated mice. Therefore, AAV-mediated gene transfer to overexpress SIRT1 specifically in the liver may represent a new gene therapy strategy to counteract NAFLD and related diseases such as type 2 diabetes.

Ketogenic diet in neuromuscular and neurodegenerative diseases

An increasing number of data demonstrate the utility of ketogenic diets in a variety of metabolic diseases as obesity, metabolic syndrome, and diabetes. In regard to neurological disorders, ketogenic diet is recognized as an effective treatment for pharmacoresistant epilepsy but emerging data suggests that ketogenic diet could be also useful in amyotrophic lateral sclerosis, Alzheimer, Parkinson's disease, and some mitochondriopathies. Although these diseases have different pathogenesis and features, there are some common mechanisms that could explain the effects of ketogenic diets. These mechanisms are to provide an efficient source of energy for the treatment of certain types of neurodegenerative diseases characterized by focal brain hypometabolism; to decrease the oxidative damage associated with various kinds of metabolic stress; to increase the mitochondrial biogenesis pathways; and to take advantage of the capacity of ketones to bypass the defect in complex I activity implicated in some neurological diseases. These mechanisms will be discussed in this review.

Redox control of platelet functions in physiology and pathophysiology

SIGNIFICANCE

An imbalance between the production and the detoxification of reactive oxygen species and reactive nitrogen species (ROS/RNS) can be implicated in many pathological processes. Platelets are best known as primary mediators of hemostasis and can be either targets of ROS/RNS or generate radicals during cell activation. These conditions can dramatically affect platelet physiology, leading even, as an ultimate event, to the cell number modification. In this case, pathological conditions such as thrombocytosis (promoted by increased cell number) or thrombocytopenia and myelodysplasia (promoted by cell decrease mediated by accelerated apoptosis) can occur.

RECENT ADVANCES

Usually, in peripheral blood, ROS/RNS production is balanced by the rate of oxidant elimination. Under this condition, platelets are in a nonadherent "resting" state. During endothelial dysfunction or under pathological conditions, ROS/RNS production increases and the platelets respond with specific biochemical and morphologic changes. Mitochondria are at the center of these processes, being able to both generate ROS/RNS, that drive redox-sensitive events, and respond to ROS/RNS-mediated changes of the cellular redox state. Irregular function of platelets and enhanced interaction with leukocytes and endothelial cells can contribute to pathogenesis of atherosclerotic and thrombotic events.

CRITICAL ISSUES

The relationship between oxidative stress, platelet death, and the activation-dependent pathways that drive platelet pro-coagulant activity is unclear and deserves to be explored.

FUTURE DIRECTIONS

Expanding knowledge about how platelets can mediate hemostasis and modulate inflammation may lead to novel and effective therapeutic strategies for the long and growing list of pathological conditions that involve both thrombosis and inflammation.

Sirt1, a negative regulator of matrix metalloproteinase-9 in diabetic retinopathy

PURPOSE

In the pathogenesis of diabetic retinopathy, matrix metalloproteinase (MMP)-9 damages retinal mitochondria, activating the apoptotic machinery. Transcription of MMP-9 is regulated by nuclear factor kappa B (NF-κB), and the activation of NF-κB is modulated by the acetylation of its p65 subunit. Sirtuin 1 (Sirt1), a deacetylase, plays an important role in the acetylation-deacetylation of p65. The goal of this study is to investigate the role of Sirt1 in the activation of MMP-9 in diabetic retinopathy.

METHODS

The effect of hyperglycemia and Sirt1 activator, resveratrol, on acetylation of p65 and its binding at MMP-9 promoter-and mitochondrial damage and apoptosis-was assessed in the retinal endothelial cells. Role of oxidative stress in the regulation of Sirt1 was evaluated in the cells incubated in H2O2. The results were confirmed in the retina from diabetic mice with Sod2 or MMP-9 gene manipulated.

RESULTS

High glucose decreased Sirt1 activity and increased p65 acetylation, and resveratrol prevented increase in p65 acetylation, binding of p65 at MMP-9 promoter and MMP-9 activation, mitochondria damage, and cell apoptosis. While Sirt1 was decreased by H2O2, MMP-9 was significantly increased. Retina from wild-type diabetic mice presented similar decrease in Sirt1, and diabetic mice with Sod2 overexpression or MMP-9 deletion had normal retinal Sirt1. Retinal microvasculature from human donors with established diabetic retinopathy also had decreased Sirt1.

CONCLUSIONS

Thus, in diabetes, increase in oxidative stress inhibits Sirt1 and p65 is hyperacetylated, increasing the binding of p65 at MMP-9 promoter. Prevention of Sirt1 inhibition, via modulating acetylation of p65, should protect activation of MMP-9 and inhibit the development of diabetic retinopathy.

Histone/protein deacetylase SIRT1 is an anticancer therapeutic target

SIRT1, a member of the NAD(+)-dependent histone/protein deacetylase family, is involved in chromatin remodeling, DNA repair, and stress response and is a potential drug target. 5-fluorouracil (FU) and the SN1-type DNA methylating agent temozolomide (TMZ) are anticancer agents. In this study, we demonstrate that sirt1 knockout mouse embryonic fibroblast cells are more sensitive to FU and DNA methylating agents than normal cells. Based on these findings, the chemotherapy efficacy of SIRT1 inhibitors in combination with FU or TMZ were tested with human breast cancer cells. We found that treatments combining SIRT1 inhibitors with FU or TMZ show synergistic reduction of cell viability and colony formation of breast cancer cells. Thus, inhibition of SIRT1 activity provides a novel anticancer strategy.

Nicotine inhibits the proliferation by upregulation of nitric oxide and increased HDAC1 in mouse neural stem cells

Cigarette smoking (CS) is considered one of the major risk factors to cause neurodegenerative disorders. Nicotine is the main chemical in CS which is responsible for dysfunction of the brain as a neuroteratogen. Also, nicotine dependency is a real mental illness and disease. Recently, chronic nicotine exposure has been shown to cause oxidative/nitrosative stress leading to a deleterious condition to cellular death in different brain regions. However, little is known about the effects of nicotine on mouse neural stem cells (mNSCs). The aim of this study is to investigate the effects of nicotine on mNSCs and elucidate underlying mechanisms involved in expression of a diversity of genes regulated by nicotine. When mNSCs were isolated from the whole brain of embryonic day 16 mice treated with nicotine at vehicle, 100, 400, and 800 μM for 5 d, nicotine significantly decreased the number and size of neurospheres. In immunocytochemistry, nicotine-exposed mNSCs expressing nestin showed the shortened filaments and condensed nuclei. In RT-PCR, messenger RNA (mRNA) levels of proliferating cell nuclear antigen (PCNA) and sirtuin1 (SIRT1) were significantly decreased, while the production of nitric oxide and mRNA levels of cyclooxygenase2 (COX-2), tumor necrosis factor-alpha TNF-α, and histone deacetylase 1 (HDAC1) were increased in a dose-dependent manner. In addition, sodium butyrate and valproic acid, HDAC inhibitors, partially rescue proliferation of mNSCs via inhibition of HDAC1 expression and NO production. Taken together, these data demonstrate that prolonged exposure of nicotine decreased proliferation of mNSCs by increased NO and inflammatory cytokine through increased HDAC1. Furthermore, this study could help in the development of a therapy for nicotine-induced neurodegenerative disorder and drug abuse.

Histone deacetylase SIRT1 modulates and deacetylates DNA base excision repair enzyme thymine DNA glycosylase

TDG (thymine DNA glycosylase) is an essential multifunctional enzyme involved in DNA base excision repair, DNA demethylation and transcription regulation. TDG is the predominant enzyme that removes thymine from T/G mispair, which arises due to deamination of 5-methyl-cytosine at the CpG dinucleotide, thereby preventing C to T mutations. SIRT1 is a member of class III NAD+-dependent histone/protein deacetylases. In the present study, we demonstrate that SIRT1 interacts with residues 67-110 of hTDG (human TDG). In addition, SIRT1 enhances TDG glycosylase activity and deacetylates acetylated TDG. TDG acetylation weakens its interaction with SIRT1. Although acetylated TDG has reduced glycosylase activity towards T/G, 5-formylcytosine/G and 5-carboxylcytosine/G, it has a stronger activity towards a 5-fluorouracil/G substrate as compared with unmodified TDG. SIRT1 weakly stimulates acetylated hTDG activity towards T/G, 5-formylcytosine/G and 5-carboxylcytosine/G as compared with control hTDG. Sirt1-knockout mouse embryonic fibroblast cells have higher levels of TDG expression and acetylation. The physical and functional interactions between SIRT1 and TDG may mediate DNA repair, gene expression and FU (5-fluorouracil)-mediated cytotoxicity.

Oligodendrogenesis after cerebral ischemia

Neural stem cells in the subventricular zone (SVZ) of the lateral ventricle of adult rodent brain generate oligodendrocyte progenitor cells (OPCs) that disperse throughout the corpus callosum and striatum where some of OPCs differentiate into mature oligodendrocytes. Studies in animal models of stroke demonstrate that cerebral ischemia induces oligodendrogenesis during brain repair processes. This article will review evidence of stroke-induced proliferation and differentiation of OPCs that are either resident in white matter or are derived from SVZ neural progenitor cells and of therapies that amplify endogenous oligodendrogenesis in ischemic brain.

SIRT1 deacetylates FOXA2 and is critical for Pdx1 transcription and β-cell formation

Pancreas duodenum homeobox 1 (PDX1) is essential for pancreas development and β-cell formation; however more studies are needed to clearly illustrate the precise mechanism regarding spatiotemporal regulation of Pdx1 expression during β-cell formation and development. Here, we demonstrate that SIRT1, FOXA2 and a number of proteins form a protein complex on the promoter of the Pdx1 gene. SIRT1 and PDX1 are expressed in the same set of cells during β-cell differentiation and maturation. Pancreas-specific disruption of SIRT1 diminished PDX1 expression and impaired islet development. Consequently, SIRT1 mutant mice develop progressive hyperglycemia, glucose intolerance, and insulin insufficiency, which directly correlate with the extent of SIRT1 deletion. We further show that SIRT1 interacts with and deacetylates FOXA2 on the promoter of the Pdx1gene, and positively regulates its transcription. These results uncover an essential role of SIRT1 in β-cell formation by maintaining expression of PDX1 and its downstream genes, and identify pancreas-specific SIRT1 mutant mice as a relevant model for studying insulin insufficiency.

Nutritional models of foetal programming and nutrigenomic and epigenomic dysregulations of fatty acid metabolism in the liver and heart

Barker's concept of 'foetal programming' proposes that intrauterine growth restriction (IUGR) predicts complex metabolic diseases through relationships that may be further modified by the postnatal environment. Dietary restriction and deficit in methyl donors, folate, vitamin B12, and choline are used as experimental conditions of foetal programming as they lead to IUGR and decreased birth weight. Overfeeding and deficit in methyl donors increase central fat mass and lead to a dramatic increase of plasma free fatty acids (FFA) in offspring. Conversely, supplementing the mothers under protein restriction with folic acid reverses metabolic and epigenomic phenotypes of offspring. High-fat diet or methyl donor deficiency (MDD) during pregnancy and lactation produce liver steatosis and myocardium hypertrophy that result from increased import of FFA and impaired fatty acid β-oxidation, respectively. The underlying molecular mechanisms show dysregulations related with similar decreased expression and activity of sirtuin 1 (SIRT1) and hyperacetylation of peroxisome proliferator-activated receptor-γ coactivator 1α (PGC-1α). High-fat diet and overfeeding impair AMPK-dependent phosphorylation of PGC-1α, while MDD decreases PGC-1α methylation through decreased expression of PRMT1 and cellular level of S-adenosyl methionine. The visceral manifestations of metabolic syndrome are under the influence of endoplasmic reticulum (ER) stress in overnourished animal models. These mechanisms should also deserve attention in the foetal programming effects of MDD since vitamin B12 influences ER stress through impaired SIRT1 deacetylation of HSF1. Taken together, similarities and synergies of high-fat diet and MDD suggest, therefore, considering their consecutive or contemporary influence in the mechanisms of complex metabolic diseases.

Skeletal muscle-specific overexpression of SIRT1 does not enhance whole-body energy expenditure or insulin sensitivity in young mice

AIMS/HYPOTHESIS

The NAD(+)-dependent protein deacetylase sirtuin (SIRT)1 is thought to be a key regulator of skeletal muscle metabolism. However, its precise role in the regulation of insulin sensitivity is unclear. Accordingly, we sought to determine the effect of skeletal muscle-specific overexpression of SIRT1 on skeletal muscle insulin sensitivity and whole-body energy metabolism.

METHODS

At 10 weeks of age, mice with muscle-specific overexpression of SIRT1 and their wild-type littermates were fed a standard diet with free access to chow or an energy-restricted (60% of standard) diet for 20 days. Energy expenditure and body composition were measured by indirect calorimetry and magnetic resonance imaging, respectively. Skeletal muscle insulin-stimulated glucose uptake was measured ex vivo in soleus and extensor digitorum longus muscles using a 2-deoxyglucose uptake technique with a physiological insulin concentration of 360 pmol/l (60 μU/ml).

RESULTS

Sirt1 mRNA and SIRT1 protein levels were increased by approximately 100- and 150-fold, respectively, in skeletal muscle of mice with SIRT1 overexpression compared with wild-type mice. Despite this large-scale overexpression of SIRT1, body composition, whole-body energy expenditure, substrate oxidation and voluntary activity were comparable between genotypes. Similarly, skeletal muscle basal and insulin-stimulated glucose uptake were unaltered with SIRT1 overexpression. Finally, while 20 days of energy restriction enhanced insulin-stimulated glucose uptake in skeletal muscles of wild-type mice, no additional effect of SIRT1 overexpression was observed.

CONCLUSIONS/INTERPRETATION

These results demonstrate that upregulation of SIRT1 activity in skeletal muscle does not affect whole-body energy expenditure or enhance skeletal muscle insulin sensitivity in young mice on a standard diet with free access to chow or in young mice on energy-restricted diets.

Resveratrol activates SIRT1 in a Lamin A-dependent manner

Human sirtuin1 (SIRT1), the closest homolog of the yeast sir2 protein, functions as an NAD+-dependent histone and non-histone protein deacetylase in several cellular processes, like energy metabolism, stress responses, aging, etc. In our recent study, we have shown that lamin A (a major nuclear matrix protein) directly binds with and activates SIRT1. Resveratrol, a natural phenol, has long been known as an activator of SIRT1. However, resveratrol's direct activation of SIRT1 has been refuted several times. In our study, we have provided a mechanistic explanation to this question, and have shown that resveratrol activates SIRT1 by increasing its binding with lamin A, thus aiding in the nuclear matrix (NM) localization of SIRT1. We have also shown that rescue of adult stem cell (ASC) decline in laminopathy-based premature aging mice by resveratrol is SIRT1-dependent. Further, resveratrol's ameliorating effects on progeria and its capacity to extend lifespan in progeria mice has been established. Here we have summarized these findings and their probable implications on other aspects, like chromatin remodeling, stem cell therapy, DNA damage responses, etc.

SIRT1 inhibits NADPH oxidase activation and protects endothelial function in the rat aorta: implications for vascular aging

Vascular aging is characterized by up-regulation of NADPH oxidase, oxidative stress and endothelial dysfunction. Previous studies demonstrate that the activity of the evolutionarily conserved NAD(+)-dependent deacetylase SIRT1 declines with age and that pharmacological activators of SIRT1 confer significant anti-aging cardiovascular effects. To determine whether dysregulation of SIRT1 promotes NADPH oxidase-dependent production of reactive oxygen species (ROS) and impairs endothelial function we assessed the effects of three structurally different inhibitors of SIRT1 (nicotinamide, sirtinol, EX527) in aorta segments isolated from young Wistar rats. Inhibition of SIRT1 induced endothelial dysfunction, as shown by the significantly reduced relaxation to the endothelium-dependent vasodilators acetylcholine and the calcium ionophore A23187. Endothelial dysfunction induced by SIRT1 inhibition was prevented by treatment of the vessels with the NADPH oxidase inhibitor apocynin or superoxide dismutase. Inhibition of SIRT1 significantly increased vascular superoxide production, enhanced NADPH oxidase activity, and mRNA expression of its subunits p22(phox) and NOX4, which were prevented by resveratrol. Peroxisome proliferator-activated receptor-α (PPARα) activation mimicked the effects of resveratrol while PPARα inhibition prevented the effects of this SIRT1 activator. SIRT1 co-precipitated with PPARα and nicotinamide increased the acetylation of the PPARα coactivator PGC-1α, which was suppressed by resveratrol. In conclusion, impaired activity of SIRT1 induces endothelial dysfunction and up-regulates NADPH oxidase-derived ROS production in the vascular wall, mimicking the vascular aging phenotype. Moreover, a new mechanism for controlling endothelial function after SIRT1 activation involves a decreased PGC-1α acetylation and the subsequent PPARα activation, resulting in both decreased NADPH oxidase-driven ROS production and NO inactivation.

Crosstalk between Oxidative Stress and SIRT1: Impact on the Aging Process

Increased oxidative stress has been associated with the aging process. However, recent studies have revealed that a low-level oxidative stress can even extend the lifespan of organisms. Reactive oxygen species (ROS) are important signaling molecules, e.g., being required for autophagic degradation. SIRT1, a class III protein deacetylase, is a crucial cellular survival protein, which is also involved in combatting oxidative stress. For instance, SIRT1 can stimulate the expression of antioxidants via the FoxO pathways. Moreover, in contrast to ROS, SIRT1 inhibits NF-κB signaling which is a major inducer of inflammatory responses, e.g., with inflammasome pathway. Recent studies have demonstrated that an increased level of ROS can both directly and indirectly control the activity of SIRT1 enzyme. For instance, ROS can inhibit SIRT1 activity by evoking oxidative modifications on its cysteine residues. Decreased activity of SIRT1 enhances the NF-κB signaling, which supports inflammatory responses. This crosstalk between the SIRT1 and ROS signaling provokes in a context-dependent manner a decline in autophagy and a low-grade inflammatory phenotype, both being common hallmarks of ageing. We will review the major mechanisms controlling the signaling balance between the ROS production and SIRT1 activity emphasizing that this crosstalk has a crucial role in the regulation of the aging process.

A high-confidence interaction map identifies SIRT1 as a mediator of acetylation of USP22 and the SAGA coactivator complex

Although many functions and targets have been attributed to the histone and protein deacetylase SIRT1, a comprehensive analysis of SIRT1 binding proteins yielding a high-confidence interaction map has not been established. Using a comparative statistical analysis of binding partners, we have assembled a high-confidence SIRT1 interactome. Employing this method, we identified the deubiquitinating enzyme ubiquitin-specific protease 22 (USP22), a component of the deubiquitinating module (DUBm) of the SAGA transcriptional coactivating complex, as a SIRT1-interacting partner. We found that this interaction is highly specific, requires the ZnF-UBP domain of USP22, and is disrupted by the inactivating H363Y mutation within SIRT1. Moreover, we show that USP22 is acetylated on multiple lysine residues and that alteration of a single lysine (K129) within the ZnF-UBP domain is sufficient to alter interaction of the DUBm with the core SAGA complex. Furthermore, USP22-mediated recruitment of SIRT1 activity promotes the deacetylation of individual SAGA complex components. Our results indicate an important role of SIRT1-mediated deacetylation in regulating the formation of DUBm subcomplexes within the larger SAGA complex.

Sirt1 inhibits the transcription factor CREB to regulate pituitary growth hormone synthesis

Growth hormone (GH) is a major anabolic hormone and the primary regulator of organism growth. Its transcription is triggered by GH-releasing hormone (GHRH) through the transcription factor cAMP response element-binding protein (CREB) and by caloric intake. In contrast, the deacetylase Sirt1 is activated by caloric restriction. Therefore, the present study investigates how Sirt1 affects CREB function and GH synthesis. Sirt1 pharmacological activation with resveratrol (IC₅₀=87 μM) suppressed GHRH-induced GH secretion from rat anterior pituitary cells in vivo and in vitro, while vehicle controls showed no effect. Resveratrol's effects were abolished after knocking down Sirt1 with RNA interference, but not in control scrambled siRNA-transfected rat somatotrophs, confirming the Sirt1 specificity. Sirt1 activation and overexpression suppressed forskolin-induced CREB-Ser(133) phosphorylation, but no effect was seen with vehicle and empty plasmid controls. The deacetylase-dead mutant Sirt1 retained CREB-Ser(133) phosphorylation by keeping protein phosphatase protein phosphatase 1 activity low. Sirt1 activation suppressed glycogen synthase kinase 3 β acetylation, and a mutation on the GSK3β-Lys(205) residue mimicking a hypoacetylated form revealed increased activity. In summary, this is a novel mechanism through which Sirt1 intercepts the cAMP pathway by suppressing CREB transcriptional activation, resulting in decreased GH synthesis.

Retinoic acid as target for local pharmacokinetic interaction with modafinil in neural cells

While the biological importance of the cytochrome P450 system in the liver is well established, much less is known about its role in the brain and drug interactions at the level of brain cells have hardly been investigated. Here, we show that modafinil, a well-known inducer of hepatic CYP enzymes, also increases CYP3A4 expression in human-derived neuron-like SH-SY5Y cells. Upregulation of CYP3A4 by modafinil was associated with increased retinoic acid (RA) degradation, which could be blocked by specific CYP3A4 inhibitor erythromycin. In turn, reduced RA levels in culture medium during modafinil treatment resulted in decreased neuronal differentiation of SH-SY5Y cells as assessed by intracellular neurotransmitter concentrations and proliferative activity. Again, this differentiation-impeding effect of modafinil on SH-SY5Y cells was antagonized by erythromycin. Similarly, modafinil treatment of the murine GL261 glioma cell line resulted in increased proliferative activity. This was associated with upregulation of RA-degrading CYP26A1 in GL261 cells. Taken together, our results indicate that psychopharmacological agents such as modafinil may directly act on CYP enzymes in neural tissue. These kinds of drug effects may become highly relevant especially in the context of biomolecules such as RA whose local metabolism in brain is under tight spatial and temporal control.

Leucine supplementation increases SIRT1 expression and prevents mitochondrial dysfunction and metabolic disorders in high-fat diet-induced obese mice

Leucine supplementation has been shown to prevent high-fat diet (HFD)-induced obesity, hyperglycemia, and dyslipidemia in animal models, but the underlying mechanisms are not fully understood. Recent studies suggest that activation of Sirtuin 1 (SIRT1) is an important mechanism to maintain energy and metabolic homeostasis. We therefore examined the involvement of SIRT1 in leucine supplementation-prevented obesity and insulin resistance. To accomplish this goal, male C57BL/6J mice were fed normal diet or HFD, supplemented with or without leucine. After 2 mo of treatment, alterations in SIRT1 expression, insulin signaling, and energy metabolism were analyzed. Eight weeks of HFD induced obesity, fatty liver, mitochondrial dysfunction, hyperglycemia, and insulin resistance in mice. Addition of leucine to HFD correlated with increased expression of SIRT1 and NAMPT (nicotinamide phosphoribosyltransferase) as well as higher intracellular NAD(+) levels, which decreased acetylation of peroxisome proliferator-activated receptor-γ coactivator 1α (PGC1α) and forkhead box O1 (FoxO1). The deacetylation of PGC1α may contribute to upregulation of genes controlling mitochondrial biogenesis and fatty acid oxidation, thereby improving mitochondrial function and preventing HFD-induced obesity in mice. Moreover, decreased acetylation of FoxO1 was accompanied by decreased expression of pseudokinase tribble 3 (TRB3) and reduced the association between TRB3 and Akt, which enhanced insulin sensitivity and improved glucose metabolism. Finally, transfection of dominant negative AMPK prevented activation of SIRT1 signaling in HFD-Leu mice. These data suggest that increased expression of SIRT1 after leucine supplementation may lead to reduced acetylation of PGC1α and FoxO1, which is associated with attenuation of HFD-induced mitochondrial dysfunction, insulin resistance, and obesity.

Synthetic condensed 1,4-naphthoquinone derivative shifts neural stem cell differentiation by regulating redox state

Naphthoquinones are bioactive compounds widespread in nature that impact on several cellular pathways, including cell proliferation and survival, by acting as prooxidants and electrophiles. We have previously described the role of the synthetic isoxazole condensed 1,4-naphthoquinone derivative 1a in preventing apoptosis induced by distinct stimuli in several cell models. In addition, apoptosis regulators and executioners may control neural stem cell (NSC) fate, without involving cell death per se. Here, we hypothesize that 1a might also play a role in NSC fate decision. We found that exposure to 1a shifts NSC differentiation potential from neurogenic to gliogenic lineage and involves the generation of reactive oxygen species, without increasing cell death. Modulation of caspases and calpains, using cysteine protease inhibitors, failed to mimic 1a effects. In addition, incubation with the naphthoquinone derivative resulted in upregulation and nuclear translocation of antioxidant responsive proteins, Nrf2 and Sirt1, which in turn may mediate 1a-directed shift in NSC differentiation. In fact, antioxidants halted the shift in NSC differentiation potential from neurogenic to gliogenic lineage, while strongly reducing reactive oxygen species generation and Nrf2 and Sirt1 nuclear translocation in NSC exposed to 1a. Collectively, these data support a new role for a specific naphthoquinone derivative in NSC fate decision and underline the importance of redox environment control.

Metabolic memory and chronic diabetes complications: potential role for epigenetic mechanisms

Recent estimates indicate that diabetes mellitus currently affects more than 10 % of the world's population. Evidence from both the laboratory and large scale clinical trials has revealed that prolonged hyperglycemia induces chronic complications which persist and progress unimpeded even when glycemic control is pharmaceutically achieved via the phenomenon of metabolic memory. The epigenome is comprised of all chromatin modifications including post translational histone modification, expression control via miRNAs and the methylation of cytosine within DNA. Modifications of these epigenetic marks not only allow cells and organisms to quickly respond to changing environmental stimuli but also confer the ability of the cell to "memorize" these encounters. As such, these processes have gained much attention as potential molecular mechanisms underlying metabolic memory and chronic diabetic complications. Here we present a review of the very recent literature published pertaining to this subject.

Sirtuin 1 regulates SREBP-1c expression in a LXR-dependent manner in skeletal muscle

Sirtuin 1 (SIRT1), a NAD⁺-dependent protein deacetylase, has emerged as a main determinant of whole body homeostasis in mammals by regulating a large spectrum of transcriptional regulators in metabolically relevant tissue such as liver, adipose tissue and skeletal muscle. Sterol regulatory element binding protein (SREBP)-1c is a transcription factor that controls the expression of genes related to fatty acid and triglyceride synthesis in tissues with high lipid synthesis rates such as adipose tissue and liver. Previous studies indicate that SIRT1 can regulate the expression and function of SREBP-1c in liver. In the present study, we determined whether SIRT1 regulates SREBP-1c expression in skeletal muscle. SREBP-1c mRNA and protein levels were decreased in the gastrocnemius muscle of mice harboring deletion of the catalytic domain of SIRT1 (SIRT1^Δex4/Δex4 mice). By contrast, adenoviral expression of SIRT1 in human myotubes increased SREBP-1c mRNA and protein levels. Importantly, SREBP-1c promoter transactivation, which was significantly increased in response to SIRT1 overexpression by gene electrotransfer in skeletal muscle, was completely abolished when liver X receptor (LXR) response elements were deleted. Finally, our in vivo data from SIRT1^Δex4/Δex4 mice and in vitro data from human myotubes overexpressing SIRT1 show that SIRT1 regulates LXR acetylation in skeletal muscle cells. This suggests a possible mechanism by which the regulation of SREBP-1c gene expression by SIRT1 may require the deacetylation of LXR transcription factors.

Rodent models and contemporary molecular techniques: notable feats yet incomplete explanations of Parkinson's disease pathogenesis

Rodent models and molecular tools, mainly omics and RNA interference, have been rigorously used to decode the intangible etiology and pathogenesis of Parkinson's disease (PD). Although convention of contemporary molecular techniques and multiple rodent models paved imperative leads in deciphering the role of putative causative factors and sequential events leading to PD, complete and clear-cut mechanisms of pathogenesis are still hard to pin down. The current article reviews the implications and pros and cons of rodent models and molecular tools in understanding the molecular and cellular bases of PD pathogenesis based on the existing literature. Probable rationales for short of comprehensive leads and future possibilities in spite of the extensive applications of molecular tools and rodent models have also been discussed.

Are sirtuins viable targets for improving healthspan and lifespan?

Although the increased lifespan of our populations illustrates the success of modern medicine, the risk of developing many diseases increases exponentially with old age. Caloric restriction is known to retard ageing and delay functional decline as well as the onset of disease in most organisms. Studies have implicated the sirtuins (SIRT1-SIRT7) as mediators of key effects of caloric restriction during ageing. Two unrelated molecules that have been shown to increase SIRT1 activity in some settings, resveratrol and SRT1720, are excellent protectors against metabolic stress in mammals, making SIRT1 a potentially appealing target for therapeutic interventions. This Review covers the current status and controversies surrounding the potential of sirtuins as novel pharmacological targets, with a focus on SIRT1.

SIRT1 negatively regulates the activities, functions, and protein levels of hMOF and TIP60

SIRT1 is a NAD(+)-dependent histone H4K16 deacetylase that controls several different normal physiologic and disease processes. Like most histone deacetylases, SIRT1 also deacetylates nonhistone proteins. Here, we show that two members of the MYST (MOZ, Ybf2/Sas3, Sas2, and TIP60) acetyltransferase family, hMOF and TIP60, are SIRT1 substrates. SIRT1 deacetylation of the enzymatic domains of hMOF and TIP60 inhibits their acetyltransferase activity and promotes ubiquitination-dependent degradation of these proteins. Importantly, immediately following DNA damage, the binding of SIRT1 to hMOF and TIP60 is transiently interrupted, with corresponding hMOF/TIP60 hyperacetylation. Lysine-to-arginine mutations in SIRT1-targeted lysines on hMOF and TIP60 repress DNA double-strand break repair and inhibit the ability of hMOF/TIP60 to induce apoptosis in response to DNA double-strand break. Together, these findings uncover novel pathways in which SIRT1 dynamically interacts with and regulates hMOF and TIP60 through deacetylation and provide additional mechanistic insights by which SIRT1 regulates DNA damage response.

Sirtuin biology and relevance to diabetes treatment

Sirtuins are a group of NAD(+)-dependent enzymes that post-translationally modify histones and other proteins. Among seven mammalian sirtuins, SIRT1 has been the most extensively studied and has been demonstrated to play a critical role in all major metabolic organs and tissues. SIRT1 regulates glucose and lipid homeostasis in the liver, modulates insulin secretion in pancreatic islets, controls insulin sensitivity and glucose uptake in skeletal muscle, increases adiponectin expression in white adipose tissue and controls food intake and energy expenditure in the brain. Recently, SIRT3 has been demonstrated to modulate insulin sensitivity in skeletal muscle and systemic metabolism, and Sirt3-null mice manifest characteristics of metabolic syndrome on a high-fat diet. Thus, it is reasonable to believe that enhancing the activities of SIRT1 and SIRT3 may be beneficial for Type 2 diabetes. Although it is controversial, the SIRT1 activator SRT1720 has been reported to be effective in improving glucose metabolism and insulin sensitivity in animal models. More research needs to be conducted so that we can better understand the physiological functions and molecular mechanisms of sirtuins in order to therapeutically target these enzymes for diabetes treatment.

Regulation of SIRT1 activity by genotoxic stress

SIRT1 regulates a variety of cellular functions, including cellular stress responses and energy metabolism. SIRT1 activity is negatively regulated by DBC1 (Deleted in Breast Cancer 1) through direct binding. However, how the DBC1-SIRT1 interaction is regulated remains unclear. We found that the DBC1-SIRT1 interaction increases following DNA damage and oxidative stress. The stress-induced DBC1-SIRT1 interaction requires the ATM-dependent phosphorylation of DBC1 at Thr 454, which creates a second binding site for SIRT1. Finally, we showed that the stress-induced DBC1-SIRT1 interaction is important for cell fate determination following genotoxic stress. These results revealed a novel mechanism of SIRT1 regulation during genotoxic stress.

Three novel acetylation sites in the Foxp3 transcription factor regulate the suppressive activity of regulatory T cells

The Foxp3 transcription factor is the master regulator of regulatory T cell (Treg) differentiation and function. Its activity is regulated by reversible acetylation. Using mass spectrometry of immunoprecipitated proteins, we identify three novel acetylation sites in murine Foxp3 (K31, K262, and K267) and the corresponding sites in human FOXP3 proteins. Newly raised modification-specific Abs against acetylated K31 and K267 confirm acetylation of these residues in murine Tregs. Mutant Foxp3 proteins carrying arginine substitutions at the three acetylation sites (3KR) accumulate in T cells to higher levels than wild-type Foxp3 and exert better suppressive activity in coculture experiments. Acetylation and stability of wild-type, but not mutant, Foxp3 is enhanced when cells are treated with Ex-527, an inhibitor of the NAD(+)-dependent deacetylase SIRT1. Treatment with Ex-527 promotes Foxp3 expression during induced Treg differentiation, enhances Foxp3 levels in natural Tregs, and prevents loss of Foxp3 expression in adoptively transferred Tregs in mice. Our data identify SIRT1 as a negative regulator of Treg function via deacetylation of three novel target sites in Foxp3. SIRT1 inhibitors strengthen the suppressive activity of Tregs and may be useful in enhancing Treg-based therapeutic approaches to autoimmune diseases or graft rejections.

Sirtuin 1-mediated cellular metabolic memory of high glucose via the LKB1/AMPK/ROS pathway and therapeutic effects of metformin

Cellular metabolic memory occurs in diabetic microvascular and macrovascular complications, but the underlying mechanisms remain unclear. Here, we investigate the role of sirtuin 1 (SIRT1) and metformin in this phenomenon. In bovine retinal capillary endothelial cells (BRECs) and retinas of diabetic rats, the inflammatory gene, nuclear factor-κB (NF-κB), and the proapoptotic gene, Bax, induced by hyperglycemia, remained elevated after returning to normoglycemia. BRECs with small interfering RNA-mediated SIRT1 knockdown had increased sensitivity to hyperglycemia stress, whereas SIRT1 overexpression or activation by metformin inhibited the increase of mitochondrial reactive oxygen species-mediated glyceraldehyde-3-phosphate dehydrogenase by poly (ADP-ribose) polymerase (PARP) activity through the upregulation of liver kinase B1/AMP-activated protein kinase (LKB1/AMPK), ultimately suppressing NF-κB and Bax expression. Furthermore, we showed that hyperglycemia led to PARP activation, which in turn may have downregulated SIRT1. Of importance, this study also demonstrated that metformin suppressed the "memory" of hyperglycemia stress in the diabetic retinas, which may be involved in the SIRT1/LKB1/AMPK pathway. Our data suggest that SIRT1 is a potential therapeutic target for the treatment of the cellular metabolic memory, and the use of metformin specifically for such therapy may be a new avenue of investigation in the diabetes field.