SIRT3 deacetylates mitochondrial 3-hydroxy-3-methylglutaryl CoA synthase 2 and regulates ketone body production

Sirtuins mediate mammalian metabolic responses to nutrient availability

Metabolic diseases are an increasing threat in developed countries. Dysregulation of metabolic pathways, caused by imbalances in energy homeostasis, leads to obesity, diabetes and cardiovascular disease with devastating results for both individuals and societies. Sirtuins, a conserved family of NAD(+)-dependent deacetylase enzymes found in many species, regulate various metabolic pathways and have emerged as important sensors of energy status in mammals. The nuclear sirtuins, SIRT1, SIRT6 and SIRT7, regulate the activity of key transcription factors and cofactors of numerous metabolic pathways in almost all tissues by linking nutrient signals with the cellular responses to energy demands. The mitochondrial sirtuins, SIRT3, SIRT4 and SIRT5, regulate the activity of important mitochondrial enzymes and drive metabolic cycles in response to fasting and calorie restriction. Accumulating evidence indicates that sirtuins can be beneficial in the prevention of metabolic and age-related diseases and suggests that they can be pharmacologically activated to ameliorate such diseases. This Review describes the latest advances in the understanding of the function of sirtuins as regulators of mammalian metabolism and focuses on the role of these enzymes as mediators of nutrient availability.

SIRT3 deficiency and mitochondrial protein hyperacetylation accelerate the development of the metabolic syndrome

Acetylation is increasingly recognized as an important metabolic regulatory posttranslational protein modification, yet the metabolic consequence of mitochondrial protein hyperacetylation is unknown. We find that high-fat diet (HFD) feeding induces hepatic mitochondrial protein hyperacetylation in mice and downregulation of the major mitochondrial protein deacetylase SIRT3. Mice lacking SIRT3 (SIRT3KO) placed on a HFD show accelerated obesity, insulin resistance, hyperlipidemia, and steatohepatitis compared to wild-type (WT) mice. The lipogenic enzyme stearoyl-CoA desaturase 1 is highly induced in SIRT3KO mice, and its deletion rescues both WT and SIRT3KO mice from HFD-induced hepatic steatosis and insulin resistance. We further identify a single nucleotide polymorphism in the human SIRT3 gene that is suggestive of a genetic association with the metabolic syndrome. This polymorphism encodes a point mutation in the SIRT3 protein, which reduces its overall enzymatic efficiency. Our findings show that loss of SIRT3 and dysregulation of mitochondrial protein acetylation contribute to the metabolic syndrome.

Regulation of glycolytic enzyme phosphoglycerate mutase-1 by Sirt1 protein-mediated deacetylation

Emerging proteomic evidence suggests that acetylation of metabolic enzymes is a prevalent post-translational modification. In a few recent reports, acetylation down-regulated activity of specific enzymes in fatty acid oxidation, urea cycle, electron transport, and anti-oxidant pathways. Here, we reveal that the glycolytic enzyme phosphoglycerate mutase-1 (PGAM1) is negatively regulated by Sirt1, a member of the NAD(+)-dependent protein deacetylases. Acetylated PGAM1 displays enhanced activity, although Sirt1-mediated deacetylation reduces activity. Acetylation sites mapped to the C-terminal "cap," a region previously known to affect catalytic efficiency. Overexpression of a constitutively active variant (acetylated mimic) of PGAM1 stimulated flux through glycolysis. Under glucose restriction, Sirt1 levels dramatically increased, leading to PGAM1 deacetylation and attenuated activity. Previously, Sirt1 has been implicated in the adaptation from glucose to fat burning. This study (i) demonstrates that protein acetylation can stimulate metabolic enzymes, (ii) provides biochemical evidence that glycolysis is modulated by reversible acetylation, and (iii) demonstrates that PGAM1 deacetylation and activity are directly controlled by Sirt1.

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.

The role of SIRT3 in mitochondrial homeostasis and cardiac adaptation to hypertrophy and aging

Although acetyl-modification of protein lysine residues has been recognized for many decades, the appreciation that this post-translational modification is highly prevalent in mitochondria and plays a pivotal regulatory role in mitochondrial function has only become apparent since 2006. The classical biological stressors that modulate mitochondrial protein acetylation include alterations in caloric levels and redox signaling and the major enzyme orchestrating deacetylation is the mitochondrial enriched sirtuin SIRT3. Overall the action of SIRT3 modulates mitochondrial homeostasis and SIRT3 target proteins include mediators of energy metabolism and mitochondrial redox stress adaptive program proteins. Given these effects, it is not surprising that the role of SIRT3 has begun to be implicated in cardiac biology. This review gives a brief overview of sirtuin biology and then focuses on the role of the SIRT3 regulatory program in the control of cardiac hypertrophy and aging. This article is part of a Special Section entitled "Post-translational Modification."

Sirtuin 1 (SIRT1): the misunderstood HDAC

The sirtuin family of NAD-dependent histone deacetylases (HDACs) consists of seven mammalian proteins, SIRT1-7. Many of the sirtuin isoforms also deacetylate nonhistone substrates, such as p53 (SIRT1) and α-tubulin (SIRT2). The sirtuin literature focuses on pharmacological activators of SIRT1 (e.g., resveratrol, SRT1720), proposed as therapeutics for diabetes, neurodegeneration, inflammation, and others. However, many of the SIRT1 activator results may have been due to artifacts in the assay methodology (i.e., use of fluorescently tagged substrates). A biological role for SIRT1 in cancer has been given less scrutiny but is no less equivocal. Although proposed initially as an oncogene, we present herein compelling data suggesting that SIRT1 is indeed a context-specific tumor suppressor. For oncology, SIRT1 inhibitors (dual SIRT1/2) are indicated as potential therapeutics. A number of sirtuin inhibitors have been developed but with mixed results in cellular systems and animal models. It is unclear whether this has been due to poorly understood model systems, signalling redundancy, and/or inadequately potent and selective tool compounds. This review provides an overview of recent developments in the field of SIRT1 function. While focusing on oncology, it aims to shed light on new concepts of expanding the selectivity spectrum, including other sirtuins such as SIRT2.

SIRT1 and SIRT3 deacetylate homologous substrates: AceCS1,2 and HMGCS1,2

SIRT1 and SIRT3 are NAD+-dependent protein deacetylases that are evolutionarily conserved across mammals. These proteins are located in the cytoplasm/nucleus and mitochondria, respectively. Previous reports demonstrated that human SIRT1 deacetylates Acetyl-CoA Synthase 1 (AceCS1) in the cytoplasm, whereas SIRT3 deacetylates the homologous Acetyl-CoA Synthase 2 (AceCS2) in the mitochondria. We recently showed that 3-hydroxy-3-methylglutaryl CoA synthase 2 (HMGCS2) is deacetylated by SIRT3 in mitochondria, and we demonstrate here that SIRT1 deacetylates the homologous 3-hydroxy-3-methylglutaryl CoA synthase 1 (HMGCS1) in the cytoplasm. This novel pattern of substrate homology between cytoplasmic SIRT1 and mitochondrial SIRT3 suggests that considering evolutionary relationships between the sirtuins and their substrates may help to identify and understand the functions and interactions of this gene family. In this perspective, we take a first step by characterizing the evolutionary history of the sirtuins and these substrate families.

Reversible acetylation of metabolic enzymes celebration: SIRT2 and p300 join the party

Compelling evidence suggests that metabolic pathways are coordinated through reversible acetylation of metabolic enzymes in response to nutrient availability. In this issue of Molecular Cell, Jiang et al. (2011) show that the rate-limiting enzyme in gluconeogenesis, phosphoenolpyruvate carboxykinase 1, is regulated through reversible acetylation by SIRT2 and p300.

The SirT3 divining rod points to oxidative stress

Sirtuins are NAD(+) dependent deacetylases that counter aging and diseases of aging. Sirtuin research has focused on SirT1, which deacetylates transcription factors and cofactors in the nucleus. More recent findings highlight SirT3 as a mitochondrial sirtuin that regulates metabolism and oxidative stress. This review focuses on new data linking SirT3 to management of reactive oxygen species from mitochondria, which may have profound implications for aging and late-onset diseases.

Succinate dehydrogenase is a direct target of sirtuin 3 deacetylase activity

Background

Sirtuins (SIRT1-7) are a family of NAD-dependent deacetylases and/or ADP-ribosyltransferases that are involved in metabolism, stress responses and longevity. SIRT3 is localized to mitochondria, where it deacetylates and activates a number of enzymes involved in fuel oxidation and energy production.

Methodology/Principal Findings

In this study, we performed a proteomic screen to identify SIRT3 interacting proteins and identified several subunits of complex II and V of the electron transport chain. Two subunits of complex II (also known as succinate dehydrogenase, or SDH), SDHA and SDHB, interacted specifically with SIRT3. Using mass spectrometry, we identified 13 acetylation sites on SDHA, including six novel acetylated residues. SDHA is hyperacetylated in SIRT3 KO mice and SIRT3 directly deacetylates SDHA in a NAD-dependent manner. Finally, we found that SIRT3 regulates SDH activity both in cells and in murine brown adipose tissue.

Conclusions/Significance

Our study identifies SDHA as a binding partner and substrate for SIRT3 deacetylase activity. SIRT3 loss results in decreased SDH enzyme activity, suggesting that SIRT3 may be an important physiological regulator of SDH activity.

Proteome-wide mapping of the Drosophila acetylome demonstrates a high degree of conservation of lysine acetylation

Posttranslational modification of proteins by acetylation and phosphorylation regulates most cellular processes in living organisms. Surprisingly, the evolutionary conservation of phosphorylated serine and threonine residues is only marginally higher than that of unmodified serines and threonines. With high-resolution mass spectrometry, we identified 1981 lysine acetylation sites in the proteome of Drosophila melanogaster. We used data sets of experimentally identified acetylation and phosphorylation sites in Drosophila and humans to analyze the evolutionary conservation of these modification sites between flies and humans. Site-level conservation analysis revealed that acetylation sites are highly conserved, significantly more so than phosphorylation sites. Furthermore, comparison of lysine conservation in Drosophila and humans with that in nematodes and zebrafish revealed that acetylated lysines were significantly more conserved than were nonacetylated lysines. Bioinformatics analysis using Gene Ontology terms suggested that the proteins with conserved acetylation control cellular processes such as protein translation, protein folding, DNA packaging, and mitochondrial metabolism. We found that acetylation of ubiquitin-conjugating E2 enzymes was evolutionarily conserved, and mutation of a conserved acetylation site impaired the function of the human E2 enzyme UBE2D3. This systems-level analysis of comparative posttranslational modification showed that acetylation is an anciently conserved modification and suggests that phosphorylation sites may have evolved faster than acetylation sites.

Regulation and protection of mitochondrial physiology by sirtuins

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

Fine tuning our cellular factories: sirtuins in mitochondrial biology

Sirtuins have emerged in recent years as critical regulators of metabolism, influencing numerous facets of energy and nutrient homeostasis. Here, we review recent advances on the role of this fascinating family of mammalian proteins and their well-orchestrated function in modulating mitochondrial activity.

Hepatic fatty acid partitioning

PURPOSE OF REVIEW

A net retention of triacylglycerol within the liver is a prerequisite for the development of nonalcoholic fatty liver disease. The accumulation of liver fat reflects an imbalance between fatty acid input and disposal. Here we summarize recent research into understanding the fate of fatty acids within the hepatocyte.

RECENT FINDINGS

Several recent studies have elucidated the contribution of different sources of fatty acids to liver fat and to plasma triacylglycerol. Some recent studies have suggested that, contrary to expectations, hepatic fatty acid oxidation is upregulated in insulin-resistant individuals. A recent observation shows the potential importance of fatty acid transformation, especially desaturation, to determination of metabolic fate. These studies highlight our lack of understanding of the regulation of metabolic partitioning of fatty acids within the human liver.

SUMMARY

The regulation of hepatic fatty acid partitioning involves many factors; not least insulin. Insulin undoubtedly regulates the supply of fatty acids to the liver from adipose tissue; however, whether insulin has a direct intrahepatic effect on hepatic fatty acid partitioning, in humans, remains unclear. The transformation of fatty acids, by desaturases, may have an important role in aiding the disposal of saturated fatty acids via oxidative pathways. Factors that upregulate hepatic fatty acid oxidation need to be elucidated.

Mammalian sirtuins and energy metabolism

Sirtuins are highly conserved NAD⁺-dependent protein deacetylases and/or ADP-ribosyltransferases that can extend the lifespan of several lower model organisms including yeast, worms and flies. The seven mammalian sirtuins, SIRT1 to SIRT7, have emerged as key metabolic sensors that directly link environmental signals to mammalian metabolic homeostasis and stress response. Recent studies have shed light on the critical roles of sirtuins in mammalian energy metabolism in response to nutrient signals. This review focuses on the involvement of two nuclear sirtuins, SIRT1 and SIRT6, and three mitochondrial sirtuins, SIRT3, SIRT4, and SIRT5, in regulation of diverse metabolic processes.

SIRT3 and cancer: tumor promoter or suppressor?

Sirtuins (SIRT1-7), the mammalian homologues of the Sir2 gene in yeast, have emerging roles in age-related diseases, such as cardiac hypertrophy, diabetes, obesity, and cancer. However, the role of several sirtuin family members, including SIRT1 and SIRT3, in cancer has been controversial. The aim of this review is to explore and discuss the seemingly dichotomous role of SIRT3 in cancer biology with particular emphasis on its potential role as a tumor promoter and tumor suppressor. This review will also discuss the potential role of SIRT3 as a novel therapeutic target to treat cancer.

Hepatic FoxOs regulate lipid metabolism via modulation of expression of the nicotinamide phosphoribosyltransferase gene

FoxO transcription factors have been implicated in lipid metabolism; however, the underlying mechanisms are not well understood. Here, in an effort to elucidate such mechanisms, we examined the phenotypic consequences of liver-specific deletion of three members of the FoxO family: FoxO1, FoxO3, and FoxO4. These liver-specific triply null mice, designated LTKO, exhibited elevated triglycerides in the liver on regular chow diet. More remarkably, LTKO mice developed severe hepatic steatosis following placement on a high fat diet. Further analyses revealed that hepatic NAD(+) levels and Sirt1 activity were decreased in the liver of the LTKO mice relative to controls. At the mechanistic level, expression profile analyses showed that LTKO livers had significantly down-regulated expression of the nicotinamide phosphoribosyltransferase (Nampt) gene encoding the rate-limiting enzyme in the salvage pathway of NAD(+) biosynthesis. Luciferase reporter assays and chromatin immunoprecipitation analyses demonstrated that Nampt is a transcriptional target gene of FoxOs. Significantly, overexpression of Nampt gene reduced, whereas knockdown increased, hepatic triglyceride levels in vitro and in vivo. Thus, FoxOs control the Nampt gene expression and the NAD(+) signaling in the regulation of hepatic triglyceride homeostasis.

Caloric excess or restriction mediated modulation of metabolic enzyme acetylation-proposed effects on cardiac growth and function

Caloric excess has been postulated to disrupt cardiac function via (i) the generation of toxic intermediates, (ii) via protein glycosylation and (iii) through the generation of reactive oxygen species. It is now increasingly being recognized that the nutrient intermediates themselves may modulate metabolic pathways through the post-translational modifications of metabolic enzymes. In light of the high energy demand of the heart, these nutrient mediated modulations in metabolic pathway functioning may play an important role in cardiac function and in the capacity of the heart to adapt to biomechanical stressors. In this review the role of protein acetylation and deacetylation in the control of metabolic programs is explored. Although not extensively investigated directly in the heart, the emerging data support that these nutrient mediated post-translational regulatory events (i) modulate cardiac metabolic pathways, (ii) integrate nutrient flux mediated post-translational effects with cardiac function and (iii) may be important in the development of cardiac pathology. Areas of investigation that need to be explored are highlighted. This article is part of a Special Issue entitled: Mitochondria and Cardioprotection.

Mitochondrial sirtuins in the regulation of mitochondrial activity and metabolic adaptation

In eukaryotes, mitochondria carry out numerous functions that are central to cellular and organismal health. How mitochondrial activities are regulated in response to differing environmental conditions, such as variations in diet, remains an important unsolved question in biology. Here, we review emerging evidence suggesting that reversible acetylation of mitochondrial proteins on lysine residues represents a key mechanism by which mitochondrial functions are adjusted to meet environmental demands. In mammals, three members of the sirtuin class of NAD(+)-dependent deacetylases - SIRT3, SIRT4, and SIRT5 - localize to mitochondria and regulate targets involved in a diverse array of biochemical pathways. The importance of this activity is highlighted by recent studies of SIRT3 indicating that this protein suppresses the emergence of diverse age-related pathologies: hearing loss, cardiac fibrosis, and malignancy. Together, these findings argue that mitochondrial protein acetylation represents a central means by which mammals regulate mitochondrial functions to maintain cellular and organismal homeostasis.