Biochemical characterization,localization, and tissue distribution of the longer form of mouse SIRT3

SIRT3 is a key mitochondrial protein deacetylase proposed to play key roles in regulating mitochondrial metabolism but there has been considerable debate about its actual size, the sequences required for activity, and its subcellular localization. A previously cloned mouse SIRT3 has high sequence similarity with the C‐terminus of human SIRT3 but lacks an N‐terminal mitochondrial targeting sequence and has no detectable deacetylation activity in vitro. Using 5′ rapid amplification of cDNA ends, we cloned the entire sequence of mouse SIRT3, as well as rat and rabbit SIRT3. Importantly, we find that full‐length SIRT3 protein localizes exclusively to the mitochondria, in contrast to reports of SIRT3 localization to the nucleus. We demonstrate that SIRT3 has no deacetylation activity in vitro unless the protein is truncated, consistent with human SIRT3. In addition, we determined the inhibition constants and mechanism of action for nicotinamide and a small molecule SIRT3 inhibitor against active mouse SIRT3 and show that the mechanisms are different for the two compounds with respect to peptide substrate and NAD⁺. Thus, identification and characterization of the actual SIRT3 sequence should help resolve the debate about the nature of mouse SIRT3 and identify new mechanisms to modulate enzymatic activity.     

SIRT3 deregulation is linked to mitochondrial dysfunction in Alzheimer's disease

Alzheimer's disease (AD) is the leading cause of dementia in the elderly. Despite decades of study, effective treatments for AD are lacking. Mitochondrial dysfunction has been closely linked to the pathogenesis of AD, but the relationship between mitochondrial pathology and neuronal damage is poorly understood. Sirtuins (SIRT, silent mating type information regulation 2 homolog in yeast) are NAD‐dependent histone deacetylases involved in aging and longevity. The objective of this study was to investigate the relationship between SIRT3 and mitochondrial function and neuronal activity in AD. SIRT3 mRNA and protein levels were significantly decreased in AD cerebral cortex, and Ac‐p53 K320 was significantly increased in AD mitochondria. SIRT3 prevented p53‐induced mitochondrial dysfunction and neuronal damage in a deacetylase activity‐dependent manner. Notably, mitochondrially targeted p53 (mito‐p53) directly reduced mitochondria DNA‐encoded ND2 and ND4 gene expression resulting in increased reactive oxygen species (ROS) and reduced mitochondrial oxygen consumption. ND2 and ND4 gene expressions were significantly decreased in patients with AD. p53‐ChIP analysis verified the presence of p53‐binding elements in the human mitochondrial genome and increased p53 occupancy of mitochondrial DNA in AD. SIRT3 overexpression restored the expression of ND2 and ND4 and improved mitochondrial oxygen consumption by repressing mito‐p53 activity. Our results indicate that SIRT3 dysfunction leads to p53‐mediated mitochondrial and neuronal damage in AD. Therapeutic modulation of SIRT3 activity may ameliorate mitochondrial pathology and neurodegeneration in AD.     

Emerging role of SIRT3 in mitochondrial dysfunction and cardiovascular diseases

As a nicotinamide adenine dinucleotide (NAD)⁺-dependent protein deacetylase, SIRT3 is highly expressed in tissues with high metabolic turnover and mitochondrial content. It has been demonstrated that SIRT3 plays a critical role in maintaining normal mitochondrial biological function through reversible protein lysine deacetylation. SIRT3 has a variety of substrates that are involved in mitochondrial biological processes such as energy metabolism, reactive oxygen species production and clearance, electron transport chain flux, mitochondrial membrane potential maintenance, and mitochondrial dynamics. In the suppression of SIRT3, functional deficiencies of mitochondria contribute to the development of various cardiovascular disorders. Activation of SIRT3 may represent a promising therapeutic strategy for the improvement of mitochondrial function and the treatment of relevant cardiovascular disorders. In the current review, we discuss the emerging roles of SIRT3 in mitochondrial derangements and subsequent cardiovascular malfunctions, including cardiac hypertrophy and heart failure, ischemia-reperfusion injury, and endothelial dysfunction in hypertension and atherosclerosis.     

The human SIRT3 protein deacetylase is exclusively mitochondrial

It has recently been suggested that perhaps as many as 20% of all mitochondrial proteins are regulated through lysine acetylation while SIRT3 has been implicated as an important mitochondrial protein deacetylase. It is therefore of crucial importance that the mitochondrial localization of potential protein deacetylases is unambiguously established. Although mouse SIRT3 was recently shown to be mitochondrial, HsSIRT3 (human SIRT3) was reported to be both nuclear and mitochondrial and to relocate from the nucleus to the mitochondrion upon cellular stress. In the present study we show, using various HsSIRT3 expression constructs and a combination of immunofluorescence and careful subcellular fractionation, that in contrast with earlier reports HsSIRT3 is exclusively mitochondrial. We discuss possible experimental explanations for these discrepancies. In addition we suggest, on the basis of the analysis of public genome databases, that the full-length mouse SIRT3 protein is a 37 kDa mitochondrial precursor protein contrary to the previously suggested 29 kDa protein.     

Mammalian Sir2 homolog SIRT3 regulates global mitochondrial lysine acetylation

Homologs of the Saccharomyces cerevisiae Sir2 protein, sirtuins, promote longevity in many organisms. Studies of the sirtuin SIRT3 have so far been limited to cell culture systems. Here, we investigate the localization and function of SIRT3 in vivo. We show that endogenous mouse SIRT3 is a soluble mitochondrial protein. To address the function and relevance of SIRT3 in the regulation of energy metabolism, we generated and phenotypically characterized SIRT3 knockout mice. SIRT3-deficient animals exhibit striking mitochondrial protein hyperacetylation, suggesting that SIRT3 is a major mitochondrial deacetylase. In contrast, no mitochondrial hyperacetylation was detectable in mice lacking the two other mitochondrial sirtuins, SIRT4 and SIRT5. Surprisingly, despite this biochemical phenotype, SIRT3-deficient mice are metabolically unremarkable under basal conditions and show normal adaptive thermogenesis, a process previously suggested to involve SIRT3. Overall, our results extend the recent finding of lysine acetylation of mitochondrial proteins and demonstrate that SIRT3 has evolved to control reversible lysine acetylation in this organelle.     

SIRT3 attenuates palmitate-induced ROS production and inflammation in proximal tubular cells

Free fatty acid (FFA)-mediated renal lipotoxicity is associated with the progression of tubulointerstitial inflammation in proteinuric kidney disease. SIRT3 is an antiaging molecule regulated by calorie restriction and mitochondria-localized NAD⁺-dependent deacetylase. In this study, we investigated whether SIRT3 reversed renal lipotoxicity-mediated ROS and inflammation. In the kidney of the FFA-bound BSA-overloaded mouse, which is a well-established experimental model of FFA-associated tubulointerstitial inflammation, mRNA expression of SIRT3 was significantly decreased and negatively correlated with mRNA expression of an inflammatory cytokine, monocyte chemoattractant protein-1 (MCP-1). In cultured proximal tubular (mProx) cells, the saturated FFA palmitate stimulated ROS accumulation and expression of MCP-1. These effects were ameliorated by retrovirus-mediated overexpression of SIRT3, whereas they were exacerbated by either overexpression of a dominant-negative form of SIRT3(N87A) lacking deacetylase activity or knockdown of SIRT3 by siRNA transfection. Furthermore, we showed that SIRT3 positively regulated both mitochondrial oxidative capacity and antioxidant gene expression, thereby reducing ROS accumulation in mProx cells, which suggests a mechanism that underlies SIRT3-mediated reversal of palmitate-induced inflammation. In conclusion, these results highlight a new role for SIRT3 in lipotoxicity/ROS-related inflammation, reveal a new molecular mechanism underlying calorie restriction-mediated antioxidant and anti-inflammatory effects, and could aid in the design of new therapies for the prevention of tubulointerstitial lesions in proteinuric kidney disease.     

Emerging characterization of the role of SIRT3-mediated mitochondrial protein deacetylation in the heart

Studies to quantify the protein acetylome show that lysine-residue acetylation rivals phosphorylation in prevalence as a posttranslational modification. Interesting, this posttranslational modification is modified by nutrient flux and by redox stress and targets the vast majority of metabolic pathway proteins in the mitochondria. Furthermore, the mitochondrial deacetylase enzyme SIRT3 appears to be regulated by exercise in skeletal muscle and in response to pressure overload in the heart. The alteration of protein lysine residues by acetylation and the enzymes controlling deacetylation are beginning to be explored as important regulatory events in the control of mitochondrial function and homeostasis. This review focuses on the mitochondrial targets of SIRT3 that are functionally implicated in heart biology and pathology and on the direct cardiac consequences of the genetic manipulation of SIRT3. As therapeutic modulators of other SIRT isoforms have been identified, the longer-term objective of our understanding of this biology would be to identify SIRT3 modulators as putative cardiac therapeutic agents.     

SIRT3, a mitochondrial sirtuin deacetylase, regulates mitochondrial function and thermogenesis in brown adipocytes

SIRT3 is one of the seven mammalian sirtuin homologs of the yeast Sir2 gene, which mediates the effect of caloric restriction on life span extension in yeast and Caenorhabditis elegans. Because adipose tissue is essential in energy homeostasis and also plays a role in life span determination, we decided to investigate the function of sirtuin members in fat. We report here that murine SIRT3 is expressed in brown adipose tissue and is localized on the mitochondria inner membrane. Caloric restriction activates SIRT3 expression in both white and brown adipose. Additionally, cold exposure up-regulates SIRT3 expression in brown fat, whereas elevated climate temperature reduces the expression. Enforced expression of SIRT3 in the HIB1B brown adipocytes enhances the expression of the uncoupling protein PGC-1alpha, UCP1, and a series of mitochondria-related genes. Both ADP-ribosyltransferase and deacetylase activities of SIRT3 are required for this action. Furthermore, the SIRT3 deacetylase mutant exhibits a dominant negative effect by inhibiting UCP1 expression. This inhibitive effect can be abolished by the coexpression of PGC-1alpha, indicating a major role of PGC-1alpha in the SIRT3 action. In addition, SIRT3 stimulates CREB phosphorylation, which reportedly activates PGC-1alpha promoter directly. Functionally, sustained expression of SIRT3 decreases membrane potential and reactive oxygen species production while increasing cellular respiration. Finally, SIRT3, along with genes related to mitochondrial function, is down-regulated in the brown adipose tissue of several genetically obese mice. In summary, our results demonstrate that SIRT3 activates mitochondria functions and plays an important role in adaptive thermogenesis in brown adipose.     

SIRT1 attenuates high glucose-induced insulin resistance via reducing mitochondrial dysfunction in skeletal muscle cells

Insulin resistance is often characterized as the most critical factor contributing to the development of type 2 diabetes mellitus (T2DM). Sustained high glucose is an important extracellular environment that induces insulin resistance. Acquired insulin resistance is associated with reduced insulin-stimulated mitochondrial activity as a result of increased mitochondrial dysfunction. Silent information regulator 1 (SIRT1) is one member of the SIRT2 (Sir2)-like family of proteins involved in glucose homeostasis and insulin secretion in mammals. Although SIRT1 has a therapeutic effect on metabolic deterioration in insulin resistance, it is still not clear how SIRT1 is involved in the development of insulin resistance. Here, we demonstrate that pcDNA3.1 vector-mediated overexpression of SIRT1 attenuates insulin resistance in the high glucose-induced insulin-resistant skeleton muscle cells. These beneficial effects were associated with ameliorated mitochondrial dysfunction. Further studies have demonstrated that SIRT1 restores mitochondrial complex I activity leading to decreased oxidative stress and mitochondrial dysfunction. Furthermore, SIRT1 significantly elevated the level of another SIRT which is named SIRT3, and SIRT3 siRNA-suppressed SIRT1-induced mitochondria complex activity increments. Taken together, these results showed that SIRT1 improves insulin sensitivity via the amelioration of mitochondrial dysfunction, and this is achieved through the SIRT1–SIRT3–mitochondrial complex I pathway.     

SIRT3, a Mitochondrial NAD+-Dependent Deacetylase,Is Involved in the Regulation of Myoblast Differentiation

Sirtuin 3 (SIRT3), one of the seven mammalian sirtuins, is a mitochondrial NAD⁺-dependent deacetylase known to control key metabolic pathways. SIRT3 deacetylases and activates a large number of mitochondrial enzymes involved in the respiratory chain, in ATP production, and in both the citric acid and urea cycles. We have previously shown that the regulation of myoblast differentiation is tightly linked to mitochondrial activity. Since SIRT3 modulates mitochondrial activity, we decide to address its role during myoblast differentiation. For this purpose, we first investigated the expression of endogenous SIRT3 during C2C12 myoblast differentiation. We further studied the impact of SIRT3 silencing on both the myogenic potential and the mitochondrial activity of C2C12 cells. We showed that SIRT3 protein expression peaked at the onset of myoblast differentiation. The inhibition of SIRT3 expression mediated by the stable integration of SIRT3 short inhibitory RNA (SIRT3shRNA) in C2C12 myoblasts, resulted in: 1) abrogation of terminal differentiation - as evidenced by a marked decrease in the myoblast fusion index and a significant reduction of Myogenin, MyoD, Sirtuin 1 and Troponin T protein expression - restored upon MyoD overexpression; 2) a decrease in peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α) and citrate synthase protein expression reflecting an alteration of mitochondrial density; and 3) an increased production of reactive oxygen species (ROS) mirrored by the decreased activity of manganese superoxide dismutase (MnSOD). Altogether our data demonstrate that SIRT3 mainly regulates myoblast differentiation via its influence on mitochondrial activity.     

SIRT3 is a stress-responsive deacetylase in cardiomyocytes that protects cells from stress-mediated cell death by deacetylation of Ku70

There are seven SIRT isoforms in mammals, with diverse biological functions including gene regulation, metabolism, and apoptosis. Among them, SIRT3 is the only sirtuin whose increased expression has been shown to correlate with an extended life span in humans. In this study, we examined the role of SIRT3 in murine cardiomyocytes. We found that SIRT3 is a stress-responsive deacetylase and that its increased expression protects myocytes from genotoxic and oxidative stress-mediated cell death. We show that, like human SIRT3, mouse SIRT3 is expressed in two forms, a ~44-kDa long form and a ~28-kDa short form. Whereas the long form is localized in the mitochondria, nucleus, and cytoplasm, the short form is localized exclusively in the mitochondria of cardiomyocytes. During stress, SIRT3 levels are increased not only in mitochondria but also in the nuclei of cardiomyocytes. We also identified Ku70 as a new target of SIRT3. SIRT3 physically binds to Ku70 and deacetylates it, and this promotes interaction of Ku70 with the proapoptotic protein Bax. Thus, under stress conditions, increased expression of SIRT3 protects cardiomyocytes, in part by hindering the translocation of Bax to mitochondria. These studies underscore an essential role of SIRT3 in the survival of cardiomyocytes in stress situations.     

Localization of mouse mitochondrial SIRT proteins: shift of SIRT3 to nucleus by co-expression with SIRT5

Yeast silent information regulator 2 (SIR2) is involved in extension of yeast longevity by calorie restriction, and SIRT3, SIRT4, and SIRT5 are mammalian homologs of SIR2 localized in mitochondria. We have investigated the localization of these three SIRT proteins of mouse. SIRT3, SIRT4, and SIRT5 proteins were localized in different compartments of the mitochondria. When SIRT3 and SIRT5 were co-expressed in the cell, localization of SIRT3 protein changed from mitochondria to nucleus. These results suggest that the SIRT3, SIRT4, and SIRT5 proteins exert distinct functions in mitochondria. In addition, the SIRT3 protein might function in nucleus.     

SIRT3-dependent deacetylation exacerbates acetaminophen hepatotoxicity

Acetaminophen/paracetamol-induced liver failure--which is induced by the binding of reactive metabolites to mitochondrial proteins and their disruption--is exacerbated by fasting. As fasting promotes SIRT3-mediated mitochondrial-protein deacetylation and acetaminophen metabolites bind to lysine residues, we investigated whether deacetylation predisposes mice to toxic metabolite-mediated disruption of mitochondrial proteins. We show that mitochondrial deacetylase SIRT3(-/-) mice are protected from acetaminophen hepatotoxicity, that mitochondrial aldehyde dehydrogenase 2 is a direct SIRT3 substrate, and that its deacetylation increases acetaminophen toxic-metabolite binding and enzyme inactivation. Thus, protein deacetylation enhances xenobiotic liver injury by modulating the binding of a toxic metabolite to mitochondrial proteins.     

SIRT3-SOD2-mROS-dependent autophagy in cadmium-induced hepatotoxicity and salvage by melatonin

Cadmium is one of the most toxic metal compounds found in the environment. It is well established that Cd induces hepatotoxicity in humans and multiple animal models. Melatonin, a major secretory product of the pineal gland, has been reported to protect against Cd-induced hepatotoxicity. However, the mechanism behind this protection remains to be elucidated. We exposed HepG2 cells to different concentrations of cadmium chloride (2.5, 5, and 10 μM) for 12 h. We found that Cd induced mitochondrial-derived superoxide anion-dependent autophagic cell death. Specifically, Cd decreased SIRT3 protein expression and activity and promoted the acetylation of SOD2, superoxide dismutase 2, mitochondrial, thus decreasing its activity, a key enzyme involved in mitochondrial ROS production, although Cd did not disrupt the interaction between SIRT3 and SOD2. These effects were ameliorated by overexpression of SIRT3. However, a catalytic mutant of SIRT3 (SIRT3^H248Y) lacking deacetylase activity lost the capacity to suppress Cd-induced autophagy. Notably, melatonin treatment enhanced the activity but not the expression of SIRT3, decreased the acetylation of SOD2, inhibited mitochondrial-derived O₂^•− production and suppressed the autophagy induced by 10 μM Cd. Moreover, 3-(1H-1,2,3-triazol-4-yl)pyridine, a confirmed selective SIRT3 inhibitor, blocked the melatonin-mediated suppression of autophagy by inhibiting SIRT3-SOD2 signaling. Importantly, melatonin suppressed Cd-induced autophagic cell death by enhancing SIRT3 activity in vivo. These results suggest that melatonin exerts a hepatoprotective effect on mitochondrial-derived O₂^•−-stimulated autophagic cell death that is dependent on the SIRT3/SOD2 pathway.     

SIRT4 is the last puzzle of mitochondrial sirtuins

Sirtuins are recently redefined as a family of nicotinamide adenine dinucleotide (NAD)-dependent deacylases. Sirtuins in mammals including human have seven members, which are SIRT1-7. Compared to other sirtuin members, not much study is focused on mitochondrial sirtuins (SIRT3-5). In mitochondrial sirtuins, SIRT4 was the last of less well-understood mitochondrial sirtuins especially for its robust enzymatic activity. This makes SIRT4 become the last puzzle of mitochondrial sirtuins, and thus brings some obstacles for studying SIRT4 biological functions or developing SIRT4 modulators. In this review, we will summarize and discuss the current findings for substrates, biological functions and possible enzymatic activities of SIRT4. The purpose of this review is to facilitate in discovering the robust enzymatic activity of SIRT4 and eventually finish this last puzzle of mitochondrial sirtuins.     

The Protein Deacetylase SIRT3 Prevents Oxidative Stress-induced Keratinocyte Differentiation

Keratinocyte differentiation is a key process in the formation and maintenance of the protective skin barrier. Dysregulation in the balance of reactive oxygen species homeostasis may play a role in keratinocyte differentiation. We have identified the mitochondrial deacetylase SIRT3 as a key regulator of mitochondrial reactive oxygen species in keratinocytes. Our studies demonstrate that SIRT3 expression is down-regulated during keratinocyte differentiation, consistent with an increase in mitochondrial superoxide levels. Importantly, loss of SIRT3 expression in keratinocytes increased superoxide levels and promoted the expression of differentiation markers, whereas overexpression decreased superoxide levels and reduced the expression of differentiation markers. These findings identify a new role for SIRT3 in the suppression of epidermal differentiation via lowering oxidative stress.