Negative regulation of the deacetylase SIRT1 by DBC1

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.

DBIRD complex integrates alternative mRNA splicing with RNA polymerase II transcript elongation

Alternative messenger RNA splicing is the main reason that vast mammalian proteomic complexity can be achieved with a limited number of genes. Splicing is physically and functionally coupled to transcription, and is greatly affected by the rate of transcript elongation. As the nascent pre-mRNA emerges from transcribing RNA polymerase II (RNAPII), it is assembled into a messenger ribonucleoprotein (mRNP) particle; this is the functional form of the nascent pre-mRNA and determines the fate of the mature transcript. However, factors that connect the transcribing polymerase with the mRNP particle and help to integrate transcript elongation with mRNA splicing remain unclear. Here we characterize the human interactome of chromatin-associated mRNP particles. This led us to identify deleted in breast cancer 1 (DBC1) and ZNF326 (which we call ZNF-protein interacting with nuclear mRNPs and DBC1 (ZIRD)) as subunits of a novel protein complex--named DBIRD--that binds directly to RNAPII. DBIRD regulates alternative splicing of a large set of exons embedded in (A + T)-rich DNA, and is present at the affected exons. RNA-interference-mediated DBIRD depletion results in region-specific decreases in transcript elongation, particularly across areas encompassing affected exons. Together, these data indicate that the DBIRD complex acts at the interface between mRNP particles and RNAPII, integrating transcript elongation with the regulation of alternative splicing.

Uncovering the molecular machinery of the human spindle--an integration of wet and dry systems biology

The mitotic spindle is an essential molecular machine involved in cell division, whose composition has been studied extensively by detailed cellular biology, high-throughput proteomics, and RNA interference experiments. However, because of its dynamic organization and complex regulation it is difficult to obtain a complete description of its molecular composition. We have implemented an integrated computational approach to characterize novel human spindle components and have analysed in detail the individual candidates predicted to be spindle proteins, as well as the network of predicted relations connecting known and putative spindle proteins. The subsequent experimental validation of a number of predicted novel proteins confirmed not only their association with the spindle apparatus but also their role in mitosis. We found that 75% of our tested proteins are localizing to the spindle apparatus compared to a success rate of 35% when expert knowledge alone was used. We compare our results to the previously published MitoCheck study and see that our approach does validate some findings by this consortium. Further, we predict so-called "hidden spindle hub", proteins whose network of interactions is still poorly characterised by experimental means and which are thought to influence the functionality of the mitotic spindle on a large scale. Our analyses suggest that we are still far from knowing the complete repertoire of functionally important components of the human spindle network. Combining integrated bio-computational approaches and single gene experimental follow-ups could be key to exploring the still hidden regions of the human spindle system.

Sirtuins as regulators of metabolism and healthspan

Since the beginning of the century, the mammalian sirtuin protein family (comprising SIRT1-SIRT7) has received much attention for its regulatory role, mainly in metabolism and ageing. Sirtuins act in different cellular compartments: they deacetylate histones and several transcriptional regulators in the nucleus, but also specific proteins in other cellular compartments, such as in the cytoplasm and in mitochondria. As a consequence, sirtuins regulate fat and glucose metabolism in response to physiological changes in energy levels, thereby acting as crucial regulators of the network that controls energy homeostasis and as such determines healthspan.

Silent information regulator 2 (Sir2) and Forkhead box O (FOXO) complement mitochondrial dysfunction and dopaminergic neuron loss in Drosophila PTEN-induced kinase 1 (PINK1) null mutant

PTEN-induced kinase 1 (PINK1), which is associated with early onset Parkinson disease, encodes a serine-threonine kinase that is critical for maintaining mitochondrial function. Moreover, another Parkinson disease-linked gene, parkin, functions downstream of PINK1 in protecting mitochondria and dopaminergic (DA) neuron. In our fly genetic screening, knockdown of Sir2 blocked PINK1 overexpression-induced phenotypes. Consistently, ectopic expression of Sir2 successfully rescued mitochondrial defects in PINK1 null mutants, but unexpectedly, failed in parkin mutants. In further genetic analyses, deletion of FOXO nullified the Sir2-induced mitochondrial restoration in PINK1 null mutants. Moreover, overexpression of FOXO or its downstream target gene such as SOD2 or Thor markedly ameliorated PINK1 loss-of-function defects, suggesting that FOXO mediates the mitochondrial protecting signal induced by Sir2. Consistent with its mitochondria-protecting role, Sir2 expression prevented the DA neuron loss of PINK1 null mutants in a FOXO-dependent manner. Loss of Sir2 or FOXO induced DA neuron degeneration, which is very similar to that of PINK1 null mutants. Furthermore, PINK1 deletion had no deleterious effect on the DA neuron loss in Sir2 or FOXO mutants, supporting the idea that Sir2, FOXO, and PINK1 protect DA neuron in a common pathway. Overall, these results strongly support the role of Sir2 and FOXO in preventing mitochondrial dysfunction and DA neuron loss, further suggesting that Sir2 and FOXO function downstream of PINK1 and independently of Parkin.

Resveratrol promotes expression of SIRT1 and StAR in rat ovarian granulosa cells: an implicative role of SIRT1 in the ovary

BACKGROUND

Resveratrol is a natural polyphenolic compound known for its beneficial effects on energy homeostasis, and it also has multiple properties, including anti-oxidant, anti-inflammatory, and anti-tumor activities. Recently, silent information regulator genes (Sirtuins) have been identified as targets of resveratrol. Sirtuin 1 (SIRT1), originally found as an NAD+-dependent histone deacetylase, is a principal modulator of pathways downstream of calorie restriction, and the activation of SIRT1 ameliorates glucose homeostasis and insulin sensitivity. To date, the presence and physiological role of SIRT1 in the ovary are not known. Here we found that SIRT1 was localized in granulosa cells of the human ovary.

METHODS

The physiological roles of resveratrol and SIRT1 in the ovary were analyzed. Immunohistochemistry was performed to localize the SIRT1 expression. SIRT1 protein expression of cultured cells and luteinized human granulosa cells was investigated by Western blot. Rat granulosa cells were obtained from diethylstilbestrol treated rats. The cells were treated with increasing doses of resveratrol, and subsequently harvested to determine mRNA levels and protein levels. Cell viability was tested by MTS assay. Cellular apoptosis was analyzed by caspase 3/7 activity test and Hoechst 33342 staining.

RESULTS

SIRT1 protein was expressed in the human ovarian tissues and human luteinized granulosa cells. We demonstrated that resveratrol exhibited a potent concentration-dependent inhibition of rat granulosa cells viability. However, resveratrol-induced inhibition of rat granulosa cells viability is independent of apoptosis signal. Resveratrol increased mRNA levels of SIRT1, LH receptor, StAR, and P450 aromatase, while mRNA levels of FSH receptor remained unchanged. Western blot analysis was consistent with the results of quantitative real-time RT-PCR assay. In addition, progesterone secretion was induced by the treatment of resveratrol.

CONCLUSIONS

These results suggest a novel mechanism that resveratrol could enhance progesterone secretion and expression of luteinization-related genes in the ovary, and thus provide important implications to understand the mechanism of luteal phase deficiency.

Sirtuins, metabolism, and cancer

More than a decade ago, sirtuins were discovered as a highly conserved family of NAD⁺-dependent enzymes that extend lifespan in lower organisms. In mammals, sirtuins are key regulators of stress responses and metabolism, influencing a range of diseases, including diabetes, neurodegeneration, and cancer. In recent years, new functions of sirtuins have been characterized, uncovering the underlying mechanisms of their multifaceted role in metabolism. Here, we specifically review recent progress on the role of sirtuins in DNA repair and energy metabolism, further discussing the implication of sirtuins in the biology of cancer.

Thiosuccinyl peptides as Sirt5-specific inhibitors

Sirtuins, a class of enzymes known as nicotinamide adenine dinucleotide-dependent deacetylases, have been shown to regulate a variety of biological processes, including aging, transcription, and metabolism. Sirtuins are considered promising targets for treating several human diseases. There are seven sirtuins in humans (Sirt1-7). Small molecules that can target a particular human sirtuin are important for drug development and fundamental studies of sirtuin biology. Here we demonstrate that thiosuccinyl peptides are potent and selective Sirt5 inhibitors. The design of these inhibitors is based on our recent discovery that Sirt5 prefers to catalyze the hydrolysis of malonyl and succinyl groups, rather than an acetyl group, from lysine residues. Furthermore, among the seven human sirtuins, Sirt5 is the only one that has this unique acyl group preference. This study demonstrates that the different acyl group preferences of different sirtuins can be conveniently utilized to develop small molecules that selectively target different sirtuins.

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.

SIRT1 inhibits the mouse intestinal motility and epithelial proliferation

Sirtuin 1 (SIRT1), a NAD(+)-dependent histone deacetylase, is involved in a wide array of cellular processes including glucose homeostasis, energy metabolism, proliferation and apoptosis, and immune response. However, it is unknown whether SIRT1 plays any physiological role in the regulation of intestinal homeostasis and motility. Thus the aim was to define SIRT1 expression and function in the gastrointestinal (GI) tract under physiological conditions. Forty 12-14-wk-old SIRT1 knockout (KO) and wild-type (WT) mice were fasted 21 h and/or refed 3 h. Fasted mice were injected intraperitoneally with bromodeoxyuridine (120 mg/kg body wt) 2 h before euthanasia. SIRT1 protein was localized to gastric and intestinal epithelial nuclei and was responsive to the nutritional status. SIRT1 was required for intestinal epithelial homeostasis. The SIRT1 KO mice showed enhanced crypt proliferation and suppressed villous apoptosis, resulting in increased intestinal villous height. In the SIRT1 KO intestine, the abundance of Forkhead box protein O1 and p53 protein decreased, whereas the subcellular localization of β-catenin protein accumulated mainly in the crypts. The SIRT1 KO mice showed accelerated gastric emptying rate with increased abundance of ghrelin mRNA and protein in the stomach. Moreover, the SIRT1 KO mouse intestine showed enhanced ex vivo spontaneous contraction. We concluded that, SIRT1 plays a critical role in the control of intestinal homeostasis (by promoting apoptosis and inhibiting proliferation) and gastrointestinal motility (by reducing gastric emptying and intestinal contractile activity), implicating a novel role for SIRT1.

Modeling miRNA regulation in cancer signaling systems: miR-34a regulation of the p53/Sirt1 signaling module

MicroRNAs (miRNAs) are a family of small regulatory RNAs whose function is to regulate the activity and stability of specific messenger RNA targets through posttranscriptional regulatory mechanisms. Most of the times signaling systems involving miRNA modulation are not linear pathways in which a certain transcription factor activate the expression of miRNAs that posttranscriptionally represses targeting proteins, but complex regulatory structures involving a variety of feedback-loop architectures.In this book chapter, we define, discuss, and apply a Systems Biology approach to investigate dynamical features of miRNA regulation, based on the integration of experimental evidences, hypotheses, and quantitative data through mathematical modeling. We further illustrate the approach using as case study the signaling module composed by the proteins p53, Sirt1, and the regulatory miRNA miR-34a. The model was used not only to investigate different possible designs of the silencing mechanism exerted by miR-34a on Sirt1 but also to simulate the dynamics of the system under conditions of (pathological) deregulation of its compounds.

Peptide switch is essential for Sirt1 deacetylase activity

In mammals, the Sirtuins are composed of seven Sir2 orthologs (Sirt1-7) with a conserved deacetylase core that utilizes NAD(+) as a cofactor. Interestingly, the deacetylase core of Sirt1 by itself has no catalytic activity. We found within the C-terminal domain a 25 aa sequence that is essential for Sirt1 activity (ESA). Our results indicate that the ESA region interacts with and functions as an "on switch" for the deacetylase core. The endogenous Sirt1 inhibitor DBC1, which also binds to the deacetylase core, competes with and inhibits the ESA region from interacting with the deacetylase core. We discovered an ESA mutant peptide that can bind to the deacetylase core and inhibit Sirt1 in trans. By using this mutant peptide, we were able to inhibit Sirt1 activity and to increase the chemosensitivity of androgen-refractory prostate cancer cells. Therefore, the ESA region is a potential target for development of therapies to regulate Sirt1.

SIRT1 activates MAO-A in the brain to mediate anxiety and exploratory drive

SIRT1 is a NAD(+)-dependent deacetylase that governs a number of genetic programs to cope with changes in the nutritional status of cells and organisms. Behavioral responses to food abundance are important for the survival of higher animals. Here we used mice with increased or decreased brain SIRT1 to show that this sirtuin regulates anxiety and exploratory drive by activating transcription of the gene encoding the monoamine oxidase A (MAO-A) to reduce serotonin levels in the brain. Indeed, treating animals with MAO-A inhibitors or selective serotonin reuptake inhibitors (SSRIs) normalized anxiety differences between wild-type and mutant animals. SIRT1 deacetylates the brain-specific helix-loop-helix transcription factor NHLH2 on lysine 49 to increase its activation of the MAO-A promoter. Both common and rare variations in the SIRT1 gene were shown to be associated with risk of anxiety in human population samples. Together these data indicate that SIRT1 mediates levels of anxiety, and this regulation may be adaptive in a changing environment of food availability.

A biotin switch-based proteomics approach identifies 14-3-3ζ as a target of Sirt1 in the metabolic regulation of caspase-2

While lysine acetylation in the nucleus is well characterized, comparatively little is known about its significance in cytoplasmic signaling. Here we show that inhibition of the Sirt1 deacetylase, which is primarily cytoplasmic in cancer cell lines, sensitizes these cells to caspase-2-dependent death. To identify relevant Sirt1 substrates, we developed a proteomics strategy, enabling the identification of a range of putative substrates, including 14-3-3ζ, a known direct regulator of caspase-2. We show here that inhibition of Sirtuin activity accelerates caspase activation and overrides caspase-2 suppression by nutrient abundance. Furthermore, 14-3-3ζ is acetylated prior to caspase activation, and supplementation of Xenopus egg extract with glucose-6-phosphate, which promotes caspase-2/14-3-3ζ binding, enhances 14-3-3ζ-directed Sirtuin activity. Conversely, inhibiting Sirtuin activity promotes14-3-3ζ dissociation from caspase-2 in both egg extract and human cultured cells. These data reveal a role for Sirt1 in modulating apoptotic sensitivity, in response to metabolic changes, by antagonizing 14-3-3ζ acetylation.

The c-MYC oncoprotein, the NAMPT enzyme, the SIRT1-inhibitor DBC1, and the SIRT1 deacetylase form a positive feedback loop

Silent information regulator 1 (SIRT1) represents an NAD(+)-dependent deacetylase that inhibits proapoptotic factors including p53. Here we determined whether SIRT1 is downstream of the prototypic c-MYC oncogene, which is activated in the majority of tumors. Elevated expression of c-MYC in human colorectal cancer correlated with increased SIRT1 protein levels. Activation of a conditional c-MYC allele induced increased levels of SIRT1 protein, NAD(+), and nicotinamide-phosphoribosyltransferase (NAMPT) mRNA in several cell types. This increase in SIRT1 required the induction of the NAMPT gene by c-MYC. NAMPT is the rate-limiting enzyme of the NAD(+) salvage pathway and enhances SIRT1 activity by increasing the amount of NAD(+). c-MYC also contributed to SIRT1 activation by sequestering the SIRT1 inhibitor deleted in breast cancer 1 (DBC1) from the SIRT1 protein. In primary human fibroblasts previously immortalized by introduction of c-MYC, down-regulation of SIRT1 induced senescence and apoptosis. In various cell lines inactivation of SIRT1 by RNA interference, chemical inhibitors, or ectopic DBC1 enhanced c-MYC-induced apoptosis. Furthermore, SIRT1 directly bound to and deacetylated c-MYC. Enforced SIRT1 expression increased and depletion/inhibition of SIRT1 reduced c-MYC stability. Depletion/inhibition of SIRT1 correlated with reduced lysine 63-linked polyubiquitination of c-Myc, which presumably destabilizes c-MYC by supporting degradative lysine 48-linked polyubiquitination. Moreover, SIRT1 enhanced the transcriptional activity of c-MYC. Taken together, these results show that c-MYC activates SIRT1, which in turn promotes c-MYC function. Furthermore, SIRT1 suppressed cellular senescence in cells with deregulated c-MYC expression and also inhibited c-MYC-induced apoptosis. Constitutive activation of this positive feedback loop may contribute to the development and maintenance of tumors in the context of deregulated c-MYC.

The role of SIRT1 in tumorigenesis

NAD-dependent Class III histone deacetylase SIRT1 is a multiple functional protein and has been demonstrated critically involved in stress response, cellular metabolism and aging through deacetylating variety of substrates including p53, forkhead transcription factors, PGC-1α, NF-κB, Ku70 and histones. Increasing evidences indicate that SIRT1 plays a complex role in tumorigenesis with functions in both tumor promoting and tumor suppressing. This review provides an overview of current knowledge of SIRT1 and its controversies regarding the functions of SIRT1 in tumorigenesis.

SIRT1 and the clock gene machinery in colorectal cancer

SIRT1 and the clock genes are involved in carcinogenesis. We evaluated SIRT1 expression in 19 human colorectal cancer (CRC) specimens and clock gene expression in SIRT1-overexpressing CaCo2 and SW480 cells. In CRC, SIRT1 mean expression level was decreased. Compared to CaCo2 cells, SW480 cells displayed lower levels of SIRT1 and PER3 and higher levels of ARNTL1, CLOCK, PER1, PER2, CRY1, TIPIN, and CSNKIE. SIRT1 overexpression induced PER1 upregulation in CaCo2 and downregulation in SW480 cells. SIRT1 expression was heterogeneous in human CRC and in CRC cell lines. These results might have relevant implications for a better understanding of colorectal carcinogenesis.

Proatherogenic abnormalities of lipid metabolism in SirT1 transgenic mice are mediated through Creb deacetylation

Dyslipidemia and atherosclerosis are associated with reduced insulin sensitivity and diabetes, but the mechanism is unclear. Gain of function of the gene encoding deacetylase SirT1 improves insulin sensitivity and could be expected to protect against lipid abnormalities. Surprisingly, when transgenic mice overexpressing SirT1 (SirBACO) are placed on atherogenic diet, they maintain better glucose homeostasis, but develop worse lipid profiles and larger atherosclerotic lesions than controls. We show that transcription factor cAMP response element binding protein (Creb) is deacetylated in SirBACO mice. We identify Lys136 is a substrate for SirT1-dependent deacetylation that affects Creb activity by preventing its cAMP-dependent phosphorylation, leading to reduced expression of glucogenic genes and promoting hepatic lipid accumulation and secretion. Expression of constitutively acetylated Creb (K136Q) in SirBACO mice mimics Creb activation and abolishes the dyslipidemic and insulin-sensitizing effects of SirT1 gain of function. We propose that SirT1-dependent Creb deacetylation regulates the balance between glucose and lipid metabolism, integrating fasting signals.

Sirtuin 1 in immune regulation and autoimmunity

The NAD-dependent histone deacetylase sirtuin (Sirt)1 is implicated in a wide variety of physiological processes, ranging from tumorigenesis to mitochondrial biogenesis to neuronal development. Recent studies indicate that Sirt1 is a critical regulator of both the innate and adaptive immune response in mice and its altered functions are likely involved in autoimmune diseases. Small molecules that modulate Sirt1 functions are potential therapeutic reagents for autoimmune inflammatory diseases. In this review, we highlight the functions of Sirt1 in the immune system focusing on the underlying molecular mechanisms, and the potential of Sirt1 as a therapeutic target for autoimmune diseases.

The nucleoporin Nup358/RanBP2 promotes nuclear import in a cargo- and transport receptor-specific manner

In vertebrates, the nuclear pore complex (NPC), the gate for transport of macromolecules between the nucleus and the cytoplasm, consists of approximately 30 different nucleoporins (Nups). The Nup and SUMO E3-ligase Nup358/RanBP2 are the major components of the cytoplasmic filaments of the NPC. In this study, we perform a structure-function analysis of Nup358 and describe its role in nuclear import of specific proteins. In a screen for nuclear proteins that accumulate in the cytoplasm upon Nup358 depletion, we identified proteins that were able to interact with Nup358 in a receptor-independent manner. These included the importin α/β-cargo DBC-1 (deleted in breast cancer 1) and DMAP-1 (DNA methyltransferase 1 associated protein 1). Strikingly, a short N-terminal fragment of Nup358 was sufficient to promote import of DBC-1, whereas DMAP-1 required a larger portion of Nup358 for stimulated import. Neither the interaction of RanGAP with Nup358 nor its SUMO-E3 ligase activity was required for nuclear import of all tested cargos. Together, Nup358 functions as a cargo- and receptor-specific assembly platform, increasing the efficiency of nuclear import of proteins through various mechanisms.

Targeting sirtuin 1 to improve metabolism: all you need is NAD(+)?

Sirtuin 1 (SIRT1) is an evolutionarily conserved NAD(+)-dependent deacetylase that is at the pinnacle of metabolic control, all the way from yeast to humans. SIRT1 senses changes in intracellular NAD(+) levels, which reflect energy level, and uses this information to adapt the cellular energy output such that it matches cellular energy requirements. The changes induced by SIRT1 activation are generally (but not exclusively) transcriptional in nature and are related to an increase in mitochondrial metabolism and antioxidant protection. These attractive features have validated SIRT1 as a therapeutic target in the management of metabolic disease and prompted an intensive search to identify pharmacological SIRT1 activators. In this review, we first give an overview of the SIRT1 biology with a particular focus on its role in metabolic control. We then analyze the pros and cons of the current strategies used to activate SIRT1 and explore the emerging evidence indicating that modulation of NAD(+) levels could provide an effective way to achieve such goals.

The dual role of sirtuins in cancer

Among the greatest challenges facing organisms is that of detecting and effectively responding to life-threatening environmental changes that are intimately associated with metabolic fluctuations and certain forms of stress. These conditions have been linked to the onset of many human pathologies, including cancer. Over the past decade, members of the Sir2 family, or sirtuins, have been described as major players in sensing and coordinating stress response. Evidence has imputed mammalian sirtuins in carcinogenesis, although the mechanisms involved seem to be more diverse and complex than previously anticipated. Some sirtuins, such as SirT2 and SirT6, seem to work as tumor suppressors, but others, such as SirT1, are apparently bifunctional: operating as both tumor suppressors and oncogenic factors depending on the context and the study conditions. The mechanisms underlying these apparently contradictory activities are not well understood, although recent findings suggest that they might actually be two sides of the same coin. In this review, the authors summarize current knowledge on the functional implications of sirtuins in cancer and discuss possible explanations for their functional duality.

SIRT1 is essential for oncogenic signaling by estrogen/estrogen receptor α in breast cancer

The NAD-dependent histone deacetylase silent information regulator 1 (SIRT1) is overexpressed and catalytically activated in a number of human cancers, but recent studies have actually suggested that it may function as a tumor suppressor and metastasis inhibitor in vivo. In breast cancer, SIRT1 stabilization has been suggested to contribute to the oncogenic potential of the estrogen receptor α (ERα), but SIRT1 activity has also been associated with ERα deacetylation and inactivation. In this study, we show that SIRT1 is critical for estrogen to promote breast cancer. ERα physically interacted and functionally cooperated with SIRT1 in breast cancer cells. ERα also bound to the promoter for SIRT1 and increased its transcription. SIRT1 expression induced by ERα was sufficient to activate antioxidant and prosurvival genes in breast cancer cells, such as catalase and glutathione peroxidase, and to inactivate tumor suppressor genes such as cyclin G2 (CCNG2) and p53. Moreover, SIRT1 inactivation eliminated estrogen/ERα-induced cell growth and tumor development, triggering apoptosis. Taken together, these results indicated that SIRT1 is required for estrogen-induced breast cancer growth. Our findings imply that the combination of SIRT1 inhibitors and antiestrogen compounds may offer more effective treatment strategies for breast cancer.

Protective effects and mechanisms of sirtuins in the nervous system

Silent information regulator two proteins (sirtuins or SIRTs) are a group of histone deacetylases whose activities are dependent on and regulated by nicotinamide adenine dinucleotide (NAD(+)). They suppress genome-wide transcription, yet upregulate a select set of proteins related to energy metabolism and pro-survival mechanisms, and therefore play a key role in the longevity effects elicited by calorie restriction. Recently, a neuroprotective effect of sirtuins has been reported for both acute and chronic neurological diseases. The focus of this review is to summarize the latest progress regarding the protective effects of sirtuins, with a focus on SIRT1. We first introduce the distribution of sirtuins in the brain and how their expression and activity are regulated. We then highlight their protective effects against common neurological disorders, such as cerebral ischemia, axonal injury, Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, and multiple sclerosis. Finally, we analyze the mechanisms underlying sirtuin-mediated neuroprotection, centering on their non-histone substrates such as DNA repair enzymes, protein kinases, transcription factors, and coactivators. Collectively, the information compiled here will serve as a comprehensive reference for the actions of sirtuins in the nervous system to date, and will hopefully help to design further experimental research and expand sirtuins as therapeutic targets in the future.

Antagonists of anaphase-promoting complex (APC)-2-cell cycle and apoptosis regulatory protein (CARP)-1 interaction are novel regulators of cell growth and apoptosis

CARP-1/CCAR1, a perinuclear phosphoprotein, is a regulator of cell growth and apoptosis signaling. Although CARP-1 is a regulator of chemotherapy-dependent apoptosis, it is also a part of the NF-κB proteome and a co-activator of steroid/thyroid nuclear receptors as well as β-catenin signaling. Our yeast two-hybrid screen revealed CARP-1 binding with the anaphase-promoting complex/cyclosome E3 ubiquitin ligase component APC-2 protein. CARP-1 also binds with anaphase-promoting complex/cyclosome co-activators Cdc20 and Cdh1. Following mapping of the minimal epitopes involved in CARP-1 binding with APC-2, a fluorescence polarization assay was established that indicated a dissociation constant (K(d)) of 480 nm for CARP-1/APC-2 binding. Fluorescence polarization assay-based high throughput screening of a chemical library yielded several small molecule antagonists of CARP-1/APC-2 binding, termed CARP-1 functional mimetics. CFM-4 (1(2-chlorobenzyl)-5'-phenyl-3'H-spiro[indoline-3,2'-[1,3,4]thiadiazol]-2-one), a lead compound, binds with and stimulates CARP-1 expression. CFM-4 prevents CARP-1 binding with APC-2, causes G(2)M cell cycle arrest, and induces apoptosis with an IC(50) range of 10-15 μm. Apoptosis signaling by CFM-4 involves activation of caspase-8 and -9 and caspase-mediated ubiquitin-proteasome pathway-independent loss of cyclin B1 and Cdc20 proteins. Depletion of CARP-1, however, interferes with CFM-4-dependent cell growth inhibition, activation of caspases, and apoptosis. Because CFM-4 also suppresses growth of drug-resistant human breast cancer cells without affecting the growth of human breast epithelial MCF-10A cells, elevating CARP-1 by CFM-4 and consequent apoptosis could in principle be exploited to further elucidate, and perhaps effectively target, often deregulated cell cycle pathways in pathological conditions, including cancer.

Sirtuins: molecular traffic lights in the crossroad of oxidative stress, chromatin remodeling, and transcription

Transcription is regulated by acetylation/deacetylation reactions of histone and nonhistone proteins mediated by enzymes called KATs and HDACs, respectively. As a major mechanism of transcriptional regulation, protein acetylation is a key controller of physiological processes such as cell cycle, DNA damage response, metabolism, apoptosis, and autophagy. The deacetylase activity of class III histone deacetylases or sirtuins depends on the presence of NAD⁺ (nicotinamide adenine dinucleotide), and therefore, their function is closely linked to cellular energy consumption. This activity of sirtuins connects the modulation of chromatin dynamics and transcriptional regulation under oxidative stress to cellular lifespan, glucose homeostasis, inflammation, and multiple aging-related diseases including cancer. Here we provide an overview of the recent developments in relation to the diverse biological activities associated with sirtuin enzymes and stress responsive transcription factors, DNA damage, and oxidative stress and relate the involvement of sirtuins in the regulation of these processes to oncogenesis. Since the majority of the molecular mechanisms implicated in these pathways have been described for Sirt1, this sirtuin family member is more extensively presented in this paper.

Expression of DBC1 is associated with nuclear grade and HER2 expression in breast cancer

DBC1/KIAA1967 (deleted in breast cancer 1) is a putative tumor-suppressor gene cloned from breast cancer specimens and is reported to regulate p53-dependent apoptosis through its specific inhibition of SIRT1 deacetylase. Although SIRT1 plays a pivotal role in carcinogenesis by regulating cellular proliferation, survival and death, its role in breast cancer remains controversial. Therefore, we aimed to investigate the expression status and clinicopathological significance of DBC1 and SIRT1 in breast cancer tissues. We evaluated the expression of DBC1 and SIRT1 in breast core-needle biopsy specimens from 48 primary breast cancer patients between 2005 and 2008. These patients were treated with primary systemic chemotherapy and subsequent surgical resection of the lesions. Immunohistochemical expression scores of DBC1 and SIRT1 were evaluated, and the relationship between their expression levels and clinicopathological features of breast cancer was analyzed. The expression was observed exclusively in the nuclei of normal and neoplastic ductal cells. In breast biopsy specimens, positive expression of DBC1 and SIRT1 was noted in 85 and 98% of patients, respectively. Expression of DBC1 was significantly associated with the tumor nuclear grade (P=0.019). DBC1 and SIRT1 expression was inversely correlated with HER2 expression (P=0.026 and 0.003, respectively). Lower expression of DBC1 and SIRT1 indicated a tendency for a favorable pathological response to chemotherapy, although this was not statistically significant. Our results reveal that the expression of DBC1 and SIRT1 in breast tissues is associated with tumor characteristics.

Cellular and molecular effects of sirtuins in health and disease

Sirtuins are NAD+-dependent protein deacetylases that are broadly conserved from bacteria to humans. Because sirtuins extend the lifespan of yeast, worms and flies, much attention has been paid to their mammalian homologues. Recent studies have revealed diverse physiological functions of sirtuins that are essentially similar to those of their yeast homologue, Sir2 (silent information regulator 2). Sirtuins are implicated in the pathology of many diseases, for which sirtuin activators such as resveratrol have great promise as potential treatments. In the present review, we describe the functions of sirtuins in cell survival, inflammation, energy metabolism, cancer and differentiation, and their impact on diseases. We also discuss the organ-specific functions of sirtuins, focusing on the brain and blood vessels.

The impact of acetylation and deacetylation on the p53 pathway

The p53 tumor suppressor is a sequence-specific transcription factor that undergoes an abundance of post-translational modifications for its regulation and activation. Acetylation of p53 is an important reversible enzymatic process that occurs in response to DNA damage and genotoxic stress and is indispensible for p53 transcriptional activity. p53 was the first non-histone protein shown to be acetylated by histone acetyl transferases, and a number of more recent in vivo models have underscored the importance of this type of modification for p53 activity. Here, we review the current knowledge and recent findings of p53 acetylation and deacetylation and discuss the implications of these processes for the p53 pathway.

SIRT1 promotes N-Myc oncogenesis through a positive feedback loop involving the effects of MKP3 and ERK on N-Myc protein stability

The N-Myc oncoprotein is a critical factor in neuroblastoma tumorigenesis which requires additional mechanisms converting a low-level to a high-level N-Myc expression. N-Myc protein is stabilized when phosphorylated at Serine 62 by phosphorylated ERK protein. Here we describe a novel positive feedback loop whereby N-Myc directly induced the transcription of the class III histone deacetylase SIRT1, which in turn increased N-Myc protein stability. SIRT1 binds to Myc Box I domain of N-Myc protein to form a novel transcriptional repressor complex at gene promoter of mitogen-activated protein kinase phosphatase 3 (MKP3), leading to transcriptional repression of MKP3, ERK protein phosphorylation, N-Myc protein phosphorylation at Serine 62, and N-Myc protein stabilization. Importantly, SIRT1 was up-regulated, MKP3 down-regulated, in pre-cancerous cells, and preventative treatment with the SIRT1 inhibitor Cambinol reduced tumorigenesis in TH-MYCN transgenic mice. Our data demonstrate the important roles of SIRT1 in N-Myc oncogenesis and SIRT1 inhibitors in the prevention and therapy of N-Myc–induced neuroblastoma.

The class III histone deacetylase SIRT1 is repressed by tumor suppressor genes and exerts divergent effects on tumorigenesis depending on its down-stream targets. Small molecule SIRT1 inhibitors have shown promising anti-cancer effects both in vitro and in vivo. Here we identified SIRT1 as a gene directly up-regulated by N-Myc and identified SIRT1-mediated transcriptional repression as a novel mechanism responsible for maintaining N-Myc oncoprotein stability. Moreover, SIRT1 contributed to N-Myc–induced cell proliferation, and preventative treatment with the SIRT1 inhibitor Cambinol reduced tumorigenesis in N-Myc transgenic mice. Our data identify SIRT1 as an important co-factor for N-Myc oncogenesis and provide important evidence for the potential application of SIRT1 inhibitors in the prevention and therapy of N-Myc–induced neuroblastoma.

A SIRT1-LSD1 corepressor complex regulates Notch target gene expression and development

Epigenetic regulation of gene expression by histone-modifying corepressor complexes is central to normal animal development. The NAD(+)-dependent deacetylase and gene repressor SIRT1 removes histone H4K16 acetylation marks and facilitates heterochromatin formation. However, the mechanistic contribution of SIRT1 to epigenetic regulation at euchromatic loci and whether it acts in concert with other chromatin-modifying activities to control developmental gene expression programs remain unclear. We describe here a SIRT1 corepressor complex containing the histone H3K4 demethylase LSD1/KDM1A and several other LSD1-associated proteins. SIRT1 and LSD1 interact directly and play conserved and concerted roles in H4K16 deacetylation and H3K4 demethylation to repress genes regulated by the Notch signaling pathway. Mutations in Drosophila SIRT1 and LSD1 orthologs result in similar developmental phenotypes and genetically interact with the Notch pathway in Drosophila. These findings offer new insights into conserved mechanisms of epigenetic gene repression and regulation of development by SIRT1 in metazoans.

Reciprocal roles of DBC1 and SIRT1 in regulating estrogen receptor α activity and co-activator synergy

Estrogen receptor α (ERα) plays critical roles in development and progression of breast cancer. Because ERα activity is strictly dependent upon the interaction with coregulators, coregulators are also believed to contribute to breast tumorigenesis. Cell Cycle and Apoptosis Regulator 1 (CCAR1) is an important co-activator for estrogen-induced gene expression and estrogen-dependent growth of breast cancer cells. Here, we identified Deleted in Breast Cancer 1 (DBC1) as a CCAR1 binding protein. DBC1 was recently shown to function as a negative regulator of the NAD-dependent protein deacetylase SIRT1. DBC1 associates directly with ERα and cooperates synergistically with CCAR1 to enhance ERα function. DBC1 is required for estrogen-induced expression of a subset of ERα target genes as well as breast cancer cell proliferation and for estrogen-induced recruitment of ERα to the target promoters in a gene-specific manner. The mechanism of DBC1 action involves inhibition of SIRT1 interaction with ERα and of SIRT1-mediated deacetylation of ERα. SIRT1 also represses the co-activator synergy between DBC1 and CCAR1 by binding to DBC1 and disrupting its interaction with CCAR1. Our results indicate that DBC1 and SIRT1 play reciprocal roles as major regulators of ERα activity, by regulating DNA binding by ERα and by regulating co-activator synergy.

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.

Mammalian Sirt1: insights on its biological functions

Sirt1 (member of the sirtuin family) is a nicotinamide adenosine dinucleotide (NAD)-dependent deacetylase that removes acetyl groups from various proteins. Sirt1 performs a wide variety of functions in biological systems. The current review focuses on the biological functions of Sirt1 in obesity-associated metabolic diseases, cancer, adipose tissue, aging, cellular senescence, cardiac aging and stress, prion-mediated neurodegeneration, inflammatory signaling in response to environmental stress, development and placental cell survival.

Sirt1's systemic protective roles and its promise as a target in antiaging medicine

Silent information regulator 2 (Sir2/Sirt1), a member of the sirtuin family of class III histone deacetylases, has been implicated extensively in lifespan extension and is a prominent drug target in antiaging medicine. The mammalian Sirt1 has multiple targets, which include histones, transcription factors, and other molecules that collectively modulate energy metabolism, stress response, and cell/tissue survival. Several of Sirt1's substrates regulate key metabolic processes, and Sirt1 activation may underlie the lifespan prolonging effect of caloric restriction. Recent studies have also identified multifaceted protective roles for Sirt1 against cellular senescence and stress in the neural, cardiovascular, and renal systems. Sirt1's activity in multiple tissues may decline with aging, and sustaining or reactivating this activity seems invariably beneficial. Several studies also point towards a general tumor suppressive role for Sirt1, at least in the context of certain human cancers. Development of Sirt1-based therapeutic interventions against systemic aging and aging-associated diseases will benefit from a thorough understanding of underlying pathological mechanisms of diseases as well as metabolic connections between different tissues and organs.

Sirtuin 1 in lipid metabolism and obesity

Sirtuin 1 (SIRT1), the mammalian ortholog of yeast Sir2, is a highly conserved NAD(+)-dependent protein deacetylase that has emerged as a key metabolic sensor that directly links environmental nutrient signals to animal metabolic homeostasis. SIRT1 is known to be involved in gluconeogenesis in the liver, fat mobilization in white adipose tissue, and insulin secretion in the pancreas. Recent studies have shown SIRT1 to regulate fatty acid oxidation in the liver, sense nutrient availability in the hypothalamus, influence obesity-induced inflammation in macrophages, and modulate the activity of the circadian clock in metabolic tissues. The activity of SIRT1 also appears to be under the control of AMPK and adiponectin. This review focuses on the involvement of SIRT1 in regulating metabolic diseases associated with obesity. It includes brief overviews of sirtuin signaling, with emphasis on SIRT1's role in the liver, macrophage, brain, and adipose tissue as it relates to obesity.

A nutrient-sensitive interaction between Sirt1 and HNF-1α regulates Crp expression

Silent information regulator 2 (Sir2) orthologs are an evolutionarily conserved family of NAD-dependent protein deacetylases that regulate aging and longevity in model organisms. The mammalian Sir2 ortholog Sirt1 regulates metabolic and stress responses through the deacetylation of many transcriptional regulatory factors. To elucidate the mechanism by which Sirt1 controls gene expression in response to nutrient availability, we devised a bioinformatic screen combining gene expression analysis with phylogenetic footprinting to identify transcription factors as new candidate partners of Sirt1. One candidate target was HNF-1α, a homeodomain transcription factor that regulates pancreatic β-cell and hepatocyte functions and is commonly mutated in patients with maturity-onset diabetes of the young (MODY). Interestingly, Sirt1 physically interacts with HNF-1αin vitro but does so in vivo only in nutrient-restricting conditions. This interaction requires 12-24 h of nutrient restriction and is dependent on protein synthesis. Both nutrient restriction and Sirt1 suppress HNF-1α transcriptional activity and the expression of one of its target genes, C-reactive protein (Crp), in mouse primary hepatocytes. Pharmacological inhibition of Sirt1 blocks the suppression of Crp by nutrient restriction. Similarly, Crp expression is also suppressed in fasted and diet-restricted liver. Furthermore, Sirt1 and HNF-1α co-localize on two HNF-1α binding sites on the Crp promoter, leading to decreased acetylation of lysine 16 of histone H4 at these sites only in response to nutrient restriction. These findings reveal a novel nutrient-dependent interaction between Sirt1 and HNF-1α and provide important insight into the molecular mechanism by which Sirt1 mediates the anti-aging effects of diet restriction.

p53 post-translational modification: deregulated in tumorigenesis

The p53 tumor suppressor protein has well-established roles in monitoring various types of stress signals by activating specific transcriptional targets that control cell cycle arrest and apoptosis, although some activities are also mediated in a transcription-independent manner. Here, we review the recent advances in our understanding of the wide spectrum of post-translational modifications that act as epigenetic-like codes for modulating specific functions of p53 in vivo and how deregulation of these modifications might contribute to tumorigenesis. We also discuss future research priorities to further understand p53 post-translational modifications and the interpretation of genetic data in appreciation of the increasing evidence that p53 regulates cellular metabolism, autophagy and many unconventional tumor suppressor activities.

Melatonin, a novel Sirt1 inhibitor, imparts antiproliferative effects against prostate cancer in vitro in culture and in vivo in TRAMP model

We recently demonstrated that Sirt1, a NAD(+) -dependent histone deacetylase, was overexpressed in prostate cancer (PCa) and its inhibition resulted in a significant antiproliferative response in human PCa cells. Studies have suggested a link between Sirt1 and circadian rhythms, the disruption of which has been linked to cancer. Interestingly, a decreased production of the pineal melatonin has been shown to deregulate the circadian rhythm machinery and increase cancer risk. Furthermore, disruption in melatonin production and circadian rhythmicity has been associated with aging. Here, we challenged our hypothesis that melatonin will impart antiproliferative response against PCa via inhibiting Sirt1. We demonstrated that melatonin significantly inhibited Sirt1 protein and activity in vitro in multiple human PCa cell lines, and melatonin-mediated Sirt1 inhibition was accompanied with a significant decrease in the proliferative potential of PCa cells, but not of normal cells. Forced overexpression of Sirt1 partially rescued the PCa cells from melatonin's antiproliferative effects, suggesting that Sirt1 is a direct target of melatonin. Employing transgenic adenocarcinoma of mouse prostate (TRAMP) mice, we also demonstrated that oral administration of melatonin, at human-achievable doses, significantly inhibited PCa tumorigenesis as shown by decreases in (i) prostate and genitourinary weight, (ii) serum insulin-like growth factor-1 (IGF-1)/IGF-binding protein-3 (IGFBP3) ratio, (iii) mRNA and protein levels of the proliferation markers (PCNA, Ki-67). This anti-PCa response was accompanied with a significant decrease in Sirt1 in TRAMP prostate. Our data identified melatonin as a novel inhibitor of Sirt1 and suggest that melatonin can inhibit PCa growth via Sirt1 inhibition.

NAD(+) -dependent histone deacetylases (sirtuins) as novel therapeutic targets

Histone deacetylases (HDACs) are enzymes that cleave off acetyl groups from acetyl-lysine residues in histones and various nonhistone proteins. Four different classes of HDACs have been identified in humans so far. Although classes I, II, and IV are zinc-dependent amidohydrolases, class III HDACs depend on nicotinamide adenine dinucleotide (NAD(+)) for their catalytic activity. According to their homology to Sir2p, a yeast histone deacetylase, the class III is also termed sirtuins. Seven members have been described in humans so far. As sirtuins are involved in many physiological and pathological processes, their activity has been associated with the pathogenesis of cancer, HIV, metabolic, or neurological diseases. Herein, we present an overview over sirtuins including their biology, targets, inhibitors, and activators and their potential as new therapeutic agents.

RNA content in the nucleolus alters p53 acetylation via MYBBP1A

A number of external and internal insults disrupt nucleolar structure, and the resulting nucleolar stress stabilizes and activates p53. We show here that nucleolar disruption induces acetylation and accumulation of p53 without phosphorylation. We identified three nucleolar proteins, MYBBP1A, RPL5, and RPL11, involved in p53 acetylation and accumulation. MYBBP1A was tethered to the nucleolus through nucleolar RNA. When rRNA transcription was suppressed by nucleolar stress, MYBBP1A translocated to the nucleoplasm and facilitated p53-p300 interaction to enhance p53 acetylation. We also found that RPL5 and RPL11 were required for rRNA export from the nucleolus. Depletion of RPL5 or RPL11 blocked rRNA export and counteracted reduction of nucleolar RNA levels caused by inhibition of rRNA transcription. As a result, RPL5 or RPL11 depletion inhibited MYBBP1A translocation and p53 activation. Our observations indicated that a dynamic equilibrium between RNA generation and export regulated nucleolar RNA content. Perturbation of this balance by nucleolar stress altered the nucleolar RNA content and modulated p53 activity.

Catalysis and mechanistic insights into sirtuin activation

SIRT1 is a member of the Sir2 family of NAD(+)-dependent protein deacetylases. The central role of SIRT1 in multiple metabolic and age-related pathways has pushed SIRT1 to the forefront to discover small-molecule activators. Promising compounds, including resveratrol and SRT1720 have been reported, however, whether these compounds are direct activators and the mechanism by which they activate remains poorly defined. This review examines the current debate surrounding purported activators, and will focus on the assays used in screening compounds, sirtuin catalysis, and the mechanistic basis for their actions. We discuss the potential pathways of SIRT1 activation that could be exploited for the development of novel therapeutics for treating type II diabetes, neurodegeneration, and diseases associated with aging.