Tumor suppressor HIC1 directly regulates SIRT1 to modulate p53-dependent DNA-damage responses

Activation of p53 by SIRT1 inhibition enhances elimination of CML leukemia stem cells in combination with imatinib

BCR-ABL tyrosine kinase inhibitors (TKI) fail to eliminate quiescent leukemia stem cells (LSC) in chronic myelogenous leukemia (CML). Thus, strategies targeting LSC are required to achieve cure. We show that the NAD(+)-dependent deacetylase SIRT1 is overexpressed in human CML LSC. Pharmacological inhibition of SIRT1 or SIRT1 knockdown increased apoptosis in LSC of chronic phase and blast crisis CML and reduced their growth in vitro and in vivo. SIRT1 effects were enhanced in combination with the BCR-ABL TKI imatinib. SIRT1 inhibition increased p53 acetylation and transcriptional activity in CML progenitors, and the inhibitory effects of SIRT1 targeting on CML cells depended on p53 expression and acetylation. Activation of p53 via SIRT1 inhibition represents a potential approach to target CML LSC.

A small molecule Inauhzin inhibits SIRT1 activity and suppresses tumour growth through activation of p53

Although ∼50% of all types of human cancers harbour wild-type TP53, this p53 tumour suppressor is often deactivated through a concerted action by its abnormally elevated suppressors, MDM2, MDMX or SIRT1. Here, we report a novel small molecule Inauhzin (INZ) that effectively reactivates p53 by inhibiting SIRT1 activity, promotes p53-dependent apoptosis of human cancer cells without causing apparently genotoxic stress. Moreover, INZ stabilizes p53 by increasing p53 acetylation and preventing MDM2-mediated ubiquitylation of p53 in cells, though not directly in vitro. Remarkably, INZ inhibits cell proliferation, induces senescence and tumour-specific apoptosis, and represses the growth of xenograft tumours derived from p53-harbouring H460 and HCT116 cells without causing apparent toxicity to normal tissues and the tumour-bearing SCID mice. Hence, our study unearths INZ as a novel anti-cancer therapeutic candidate that inhibits SIRT1 activity and activates p53.

Hypermethylated in cancer 1 (HIC1) recruits polycomb repressive complex 2 (PRC2) to a subset of its target genes through interaction with human polycomb-like (hPCL) proteins

HIC1 (hypermethylated in cancer 1) is a tumor suppressor gene epigenetically silenced or deleted in many human cancers. HIC1 is involved in regulatory loops modulating p53- and E2F1-dependent cell survival, growth control, and stress responses. HIC1 is also essential for normal development because Hic1-deficient mice die perinatally and exhibit gross developmental defects throughout the second half of development. HIC1 encodes a transcriptional repressor with five C(2)H(2) zinc fingers mediating sequence-specific DNA binding and two repression domains: an N-terminal BTB/POZ domain and a central region recruiting CtBP and NuRD complexes. By yeast two-hybrid screening, we identified the Polycomb-like protein hPCL3 as a novel co-repressor for HIC1. Using multiple biochemical strategies, we demonstrated that HIC1 interacts with hPCL3 and its paralog PHF1 to form a stable complex with the PRC2 members EZH2, EED, and Suz12. Confirming the implication of HIC1 in Polycomb recruitment, we showed that HIC1 shares some of its target genes with PRC2, including ATOH1. Depletion of HIC1 by siRNA interference leads to a partial displacement of EZH2 from the ATOH1 promoter. Furthermore, in vivo, ATOH1 repression by HIC1 is associated with Polycomb activity during mouse cerebellar development. Thus, our results identify HIC1 as the first transcription factor in mammals able to recruit PRC2 to some target promoters through its interaction with Polycomb-like proteins.

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.

Analysis of 41 cancer cell lines reveals excessive allelic loss and novel mutations in the SIRT1 gene

SIRT1 is an evolutionarily conserved protein deacetylase that modulates stress response, cellular metabolism and aging in model organisms. While SIRT1 exerts beneficial effects in protecting against age-related diseases, the role of SIRT1 in cancer has been controversial. SIRT1 promotes cell survival by deacetylating, and thereby negatively regulating the activity of important tumor suppressors such as p53. In this regard, SIRT1 has been considered to be a potential oncogene, and SIRT1 inhibitors have been studied for possible anticancer therapeutic effects. In contrast, it has been shown that SIRT1 deficiency leads to increased genomic instability and tumorigenesis, and that overexpression of SIRT1 attenuates cancer formation in mice, suggesting it may also act as a tumor suppressor. Based on this evidence, SIRT1-activating molecules could act as candidate chemotherapeutic drugs. In order to gain insight into the role of SIRT1 in cancer, we performed a comprehensive resequencing analysis of the SIRT1 gene in 41 tumor cell lines and found an unusually excessive homozygosity, which was confirmed to be allelic loss by microsatellite analysis. Furthermore, we found two novel SIRT1 mutations (D739Y and R65_A72del) in addition to the known, rare non-synonymous variation resulting in I731V. In vitro assays using purified SIRT1 protein showed that these mutations do not alter SIRT1 deacetylase activity or telomerase activity, which was shown to be regulated by SIRT1. We conclude that allelic loss or mutations in the SIRT1 gene occur prevalently during tumorigenesis, supporting the assertion that SIRT1 may serve as a tumor suppressor.

Histone acetyltransferases and deacetylases: molecular and clinical implications to gastrointestinal carcinogenesis

Histone acetyltransferases and deacetylases are two groups of enzymes whose opposing activities govern the dynamic levels of reversible acetylation on specific lysine residues of histones and many other proteins. Gastrointestinal (GI) carcinogenesis is a major cause of morbidity and mortality worldwide. In addition to genetic and environmental factors, the role of epigenetic abnormalities such as aberrant histone acetylation has been recognized to be pivotal in regulating benign tumorigenesis and eventual malignant transformation. Here we provide an overview of histone acetylation, list the major groups of histone acetyltransferases and deacetylases, and cover in relatively more details the recent studies that suggest the links of these enzymes to GI carcinogenesis. As potential novel therapeutics for GI and other cancers, histone deacetylase inhibitors are also discussed.

Activation of stress response gene SIRT1 by BCR-ABL promotes leukemogenesis

The tyrosine kinase inhibitor imatinib is highly effective in the treatment of chronic myelogenous leukemia (CML), but primary and acquired resistance of CML cells to the drug offset its efficacy. Molecular mechanisms for resistance of CML to tyrosine kinase inhibitors are not fully understood. In the present study, we show that BCR-ABL activates the expression of the mammalian stress response gene SIRT1 in hematopoietic progenitor cells and that this involves STAT5 signaling. SIRT1 activation promotes CML cell survival and proliferation associated with deacetylation of multiple SIRT1 substrates, including FOXO1, p53, and Ku70. Imatinib-mediated inhibition of BCR-ABL kinase activity partially reduces SIRT1 expression and SIRT1 inhibition further sensitizes CML cells to imatinib-induced apoptosis. Knockout of SIRT1 suppresses BCR-ABL transformation of mouse BM cells and the development of a CML-like myeloproliferative disease, and treatment of mice with the SIRT1 inhibitor tenovin-6 deters disease progression. The combination of SIRT1 gene knockout and imatinib treatment further extends the survival of CML mice. Our results suggest that SIRT1 is a novel survival pathway activated by BCR-ABL expression in hematopoietic progenitor cells, which promotes oncogenic transformation and leukemogenesis. Our findings suggest further exploration of SIRT1 as a therapeutic target for CML treatment to overcome resistance.

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.

Loss of Hypermethylated in Cancer 1 (HIC1) in breast cancer cells contributes to stress-induced migration and invasion through β-2 adrenergic receptor (ADRB2) misregulation

The transcriptional repressor HIC1 (Hypermethylated in Cancer 1) is a tumor suppressor gene inactivated in many human cancers including breast carcinomas. In this study, we show that HIC1 is a direct transcriptional repressor of β-2 adrenergic receptor (ADRB2). Through promoter luciferase activity, chromatin immunoprecipitation (ChIP) and sequential ChIP experiments, we demonstrate that ADRB2 is a direct target gene of HIC1, endogenously in WI-38 cells and following HIC1 re-expression in breast cancer cells. Agonist-mediated stimulation of ADRB2 increases the migration and invasion of highly malignant MDA-MB-231 breast cancer cells but these effects are abolished following HIC1 re-expression or specific down-regulation of ADRB2 by siRNA treatment. Our results suggest that early inactivation of HIC1 in breast carcinomas could predispose to stress-induced metastasis through up-regulation of the β-2 adrenergic receptor.

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.

A systematic analysis on DNA methylation and the expression of both mRNA and microRNA in bladder cancer

BACKGROUND

DNA methylation aberration and microRNA (miRNA) deregulation have been observed in many types of cancers. A systematic study of methylome and transcriptome in bladder urothelial carcinoma has never been reported.

METHODOLOGY/PRINCIPAL FINDINGS

The DNA methylation was profiled by modified methylation-specific digital karyotyping (MMSDK) and the expression of mRNAs and miRNAs was analyzed by digital gene expression (DGE) sequencing in tumors and matched normal adjacent tissues obtained from 9 bladder urothelial carcinoma patients. We found that a set of significantly enriched pathways disrupted in bladder urothelial carcinoma primarily related to "neurogenesis" and "cell differentiation" by integrated analysis of -omics data. Furthermore, we identified an intriguing collection of cancer-related genes that were deregulated at the levels of DNA methylation and mRNA expression, and we validated several of these genes (HIC1, SLIT2, RASAL1, and KRT17) by Bisulfite Sequencing PCR and Reverse Transcription qPCR in a panel of 33 bladder cancer samples.

CONCLUSIONS/SIGNIFICANCE

We characterized the profiles between methylome and transcriptome in bladder urothelial carcinoma, identified a set of significantly enriched key pathways, and screened four aberrantly methylated and expressed genes. Conclusively, our findings shed light on a new avenue for basic bladder cancer research.

miR-486-5p induces replicative senescence of human adipose tissue-derived mesenchymal stem cells and its expression is controlled by high glucose

The reduction of adult stem cell self-renewal can be an important mechanism of aging. MicroRNAs have been reported to be involved in aging processes. Through a microarray approach, we have identified miR-486-5p, the expression of which is progressively expressed in human adipose tissue-derived mesenchymal stem cells (hAT-MSCs) with aging. Overexpression of miR-486-5p induces a premature senescence-like phenotype and inhibits proliferation of hAT-MSCs and inhibits adipogenic and osteogenic differentiation, whereas inhibition of miR-486-5p has the opposite effects. miR-486-5p regulates the expression of silent information regulator 1 (SIRT1), a major regulator of longevity and metabolic disorders. Decrease of SIRT1 deacetylase activity in hAT-MSCs is correlated with their passage number. miR-486-5p inhibits SIRT1 expression through a miR-486-5p binding site within the 3'-untranslated region of SIRT1. Overexpression of miR-486-5p inhibits SIRT1 deacetylase activity in hAT-MSCs, and transfection of miR-486-5p inhibitor shows the opposite effect. Downregulation of SIRT1 in hAT-MSCs induces senescence and inhibits cell proliferation. Exposure to high glucose increases miR-486-5p expression and inhibits SIRT1 expression in hAT-MSCs. Our data pinpoint miR-486-5p as an endogenous inhibitor of SIRT1 that promotes hAT-MSCs senescence and is potentially applicable to therapeutic manipulation of hAT-MSCs dysfunction in metabolic disorders.

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.

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.

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.

Interferon gamma (IFN-γ) disrupts energy expenditure and metabolic homeostasis by suppressing SIRT1 transcription

Chronic inflammation impairs metabolic homeostasis and is intimately correlated with the pathogenesis of type 2 diabetes. The pro-inflammatory cytokine IFN-γ is an integral part of the metabolic inflammation circuit and contributes significantly to metabolic dysfunction. The underlying mechanism, however, remains largely unknown. In the present study, we report that IFN-γ disrupts the expression of genes key to cellular metabolism and energy expenditure by repressing the expression and activity of SIRT1 at the transcription level. Further analysis reveals that IFN-γ requires class II transactivator (CIITA) to repress SIRT1 transcription. CIITA, once induced by IFN-γ, is recruited to the SIRT1 promoter by hypermethylated in cancer 1 (HIC1) and promotes down-regulation of SIRT1 transcription via active deacetylation of core histones surrounding the SIRT1 proximal promoter. Silencing CIITA or HIC1 restores SIRT1 activity and expression of metabolic genes in skeletal muscle cells challenged with IFN-γ. Therefore, our data delineate an IFN-γ/HIC1/CIITA axis that contributes to metabolic dysfunction by suppressing SIRT1 transcription in skeletal muscle cells and as such shed new light on the development of novel therapeutic strategies against type 2 diabetes.

POZ/BTB and AT-hook-containing zinc finger protein 1 (PATZ1) inhibits endothelial cell senescence through a p53 dependent pathway

Vascular cell senescence, induced by the DNA damage response or inflammatory stress, contributes to age-associated vascular disease. Using complementary DNA microarray technology, we found that the level of POZ/BTB and AT-hook-containing zinc finger protein 1 (PATZ1) is downregulated during endothelial cell (EC) senescence. PATZ1 may have an important role as a transcriptional repressor in chromatin remodeling and transcription regulation; however, the role of PATZ1 in EC senescence and vascular aging remains unidentified. Knockdown of PATZ1 in young cells accelerated premature EC senescence, which was confirmed by growth arrest, increased p53 protein level and senescence-associated β-galactosidase (SA-β-gal) activity, and repression of EC tube formation. In contrast, overexpression of PATZ1 in senescent cells reversed senescent phenotypes. Cellular senescence induced by PATZ1 knockdown in young cells was rescued by knockdown of p53, but not by knockdown of p16(INK4a). PATZ1 knockdown increased ROS levels, and pretreatment with N-acetylcysteine abolished EC senescence induced by PATZ1 knockdown. Notably, PATZ1 immunoreactivity was lower in ECs of atherosclerotic tissues than those of normal arteries in LDLR(-/-) mice, and immunoreactivity also decreased in ECs of old human arteries. These results suggest that PATZ1 may have an important role in the regulation of EC senescence through an ROS-mediated p53-dependent pathway and contribute to vascular diseases associated with aging.

Deacetylation of PGC-1α by SIRT1: importance for skeletal muscle function and exercise-induced mitochondrial biogenesis

Activation of peroxisome proliferator-activated receptor γ coactivator-1α (PGC-1α)-mediated transcription is important for both the determination of mitochondrial content and the induction of mitochondrial biogenesis in skeletal muscle. SIRT1 (silent mating type information regulator 2 homolog 1) deactetylation is proposed as a potential activator of PGC-1α transcriptional activity. The current review examines the importance of SIRT1 deacetylation of PGC-1α in skeletal muscle. Models of SIRT1 overexpression and pharmacological activation are examined, but changes in SIRT1 expression and deacetylase activity following acute and chronic contractile activity will be emphasized. In addition, potential mechanisms of SIRT1 activation in skeletal muscle will be examined. The importance of the PGC-1α acetyltransferase GCN5 will also be briefly discussed. The current evidence supports the contribution of SIRT1 deacetylation of PGC-1α to exercise-induced mitochondrial biogenesis. Further research examining exercise-mediated activation of SIRT1 and the role of GCN5 in regulating PGC-1α transcriptional activity in skeletal muscle is required.

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.

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 SIRT1 in gastric cardiac cancer and its clinicopathologic significance

SIRT1 is a deacetylase that modifies gene expression. Some researchers have found that SIRT1 is upregulated in malignant tumor tissues. Therefore, this study investigated the SIRT1 expression in gastric cardiac carcinoma and its correlation with clinicopathologic features and prognosis. Tissue microarray technique and immunohistochemical stains were used to detect the expression of SIRT1, p53, and Ki-67 in 176 gastric cardiac carcinoma tissues and 32 normal gastric cardiac region tissues. SIRT1 expression in gastric cardiac carcinoma was significantly higher than that in normal gastric cardiac tissues and was associated with lymphatic metastasis, TNM stage, survival rate and mean survival time. Expression of Ki-67 in the SIRT1 positive group was significantly higher than that in the negative group. In conclusion, high expression of SIRT1 in gastric cardiac carcinoma was correlated with lymphatic metastasis, TNM stage, proliferative status and prognosis. SIRT1 might be a biological parameter to evaluate malignant degree and prognosis of gastric cardiac carcinoma.

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.

Identification of oncogenic microRNA-17-92/ZBTB4/specificity protein axis in breast cancer

The human POK family members are transcription factors with a POZ domain and zinc-fingers that act primarily as transcriptional repressors. Several members of this family are involved in oncogenesis and this prompted us to assess whether expression levels of individual POK family members are associated with clinical outcomes in cancer. We have observed that ZBTB4 (zinc-finger and BTB domain containing 4) is downregulated in breast cancer patients, and that its expression is significantly correlated with relapse-free survival. Further integrative analysis of mRNA and microRNA (miR) expression data from the NCI-60 cell lines revealed an inverse correlation between ZBTB4 and oncogenic miRs derived from the miR-17-92 cluster and its paralogs. The experimental results using MDA-MB-231 and MCF-7 human breast cancer cells confirm that miRNAs derived from these clusters, containing miR-17-5p, miR-20a, miR-106a, miR-106b and miR-93, negatively regulate ZBTB4 expression. Overexpression of ZBTB4 or restoration of ZBTB4 by using an antagomir inhibit growth and invasion of breast cancer cells, and this effect is due, in part, to ZBTB4-dependent repression of the specificity protein 1 (Sp1), Sp3 and Sp4 genes, and subsequent downregulation of several Sp-dependent oncogenes, in part, through competition between ZBTB4 and Sp transcription factors for GC-rich promoter sequences. These results confirm that ZBTB4 functions as a novel tumor-suppressor gene with prognostic significance for breast cancer survival, and the oncogenic miR-17-92/ZBTB4/Sp axis may be a potential therapeutic target.

Nutrition, sirtuins and aging

Beyond our inherited genetic make-up environmental factors are central for health and disease and finally determine our life span. Amongst the environmental factors nutrition plays a prominent role in affecting a variety of degenerative processes that are linked to aging. The exponential increase of non-insulin-dependent diabetes mellitus in industrialized nations as a consequence of a long-lasting caloric supernutrition is an expression of this environmental challenge that also affects aging processes. The most consistent effects along the environmental factors that slow down aging - from simple organisms to rodents and primates - have been observed for caloric restriction. In the yeast Saccharomyces cerevisiae, the fruit fly Drosophila melanogaster and the nematode Caenorhabditis elegans, sirtuins (silencing information regulators) have been identified to mediate as "molecular sensors" the effects of caloric restriction on aging processes. Sirtuins are NAD(+)-dependent deacetylases that are activated when e.g. cell energy status is low and the NAD(+) over NADH ratio is high. As a consequence transcription rates of a variety of genes including that of the apoptosis inducing p(53) gene are reduced. Moreover, in C. elegans, sirtuins were shown to interact with proteins of the insulin/IGF-1 signaling cascade of which several members are known to extend life span of the nematodes when mutated. Downstream targets of this pathway include genes that encode antioxidative enzymes such as Superoxide dismutase (SOD) whose transcription is activated when receptor activation by insulin/IGF is low or when sirtuins are active and the ability of cells to resist oxidative damage appears to determine their life span. Amongst dietary factors that activate sirtuins are certain polyphenols such as quercetin and resveratrol. Whereas their ability to affect life span has been demonstrated in simple organisms, their efficacy in mammals awaits proof of principle.

Generation of two modified mouse alleles of the Hic1 tumor suppressor gene

HIC1 (hypermethylated in cancer 1) is a tumor suppressor gene located on chromosome 17p13.3, a region frequently hypermethylated or deleted in human neoplasias. In mouse, Hic1 is essential for embryonic development and exerts an antitumor role in adult animals. Since Hic1-deficient mice die perinatally, we generated a conditional Hic1 null allele by flanking the Hic1-coding region by loxP sites. When crossed to animals expressing Cre recombinase in a cell-specific manner, the Hic1 conditional mice will provide new insights into the function of Hic1 in developing and mature tissues. Additionally, we used gene targeting to replace sequence-encoding amino acids 186-893 of Hic1 by citrine fluorescent protein cDNA. We demonstrate that the distribution of Hic1-citrine fusion polypeptide corresponds to the expression pattern of wild-type Hic1. Consequently, Hic1-citrine "reporter" mice can be used to monitor the activity of the Hic1 locus using citrine fluorescence.

SirT1 brings stemness closer to cancer and aging

Sirtuin 1 acts in various cell processes, deacetylating both chromatin and non-histone proteins, and its role in cancer and aging has long been studied and debated. Here we discuss another aspect of SirT1 biology, its function as a stem cell pluripotency and differentiation regulator. We evaluate the implications of these findings in sirtuin inhibition-based cancer treatment and in the application of sirtuin activation for anti-aging therapy.

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.

miR-200a regulates SIRT1 expression and epithelial to mesenchymal transition (EMT)-like transformation in mammary epithelial cells

Evidence supports a critical role for microRNAs (miRNAs) in regulation of tissue-specific differentiation and development. Signifying a disruption of these programs, expression profiling has revealed extensive miRNA dysregulation in tumors compared with healthy tissue. The miR-200 family has been established as a key regulator of epithelial phenotype and, as such, is deeply involved in epithelial to mesenchymal transition (EMT) processes in breast cancer. However, the effects of the miR-200 family on transformation of normal mammary epithelial cells have yet to be fully characterized. By examining a TGF-β driven model of transformation of normal mammary epithelium, we demonstrate that the class III histone deacetylase silent information regulator 1 (SIRT1), a proposed oncogene in breast cancer, is overexpressed upon EMT-like transformation and that epigenetic silencing of miR-200a contributes at least in part to the overexpression of SIRT1. We have established the SIRT1 transcript as subject to regulation by miR-200a, through miR-200a targeting of SIRT1 3'-UTR. We also observed SIRT1 and miR-200a participation in a negative feedback regulatory loop. Restoration of miR-200a or the knockdown of SIRT1 prevented transformation of normal mammary epithelial cells evidenced by decreased anchorage-independent growth and decreased cell migration. Finally, we observed SIRT1 overexpression in association with decreased miR-200a in breast cancer patient samples. These observations provide further evidence for a critical tumor suppressive role of the miR-200 family in breast epithelium in addition to identifying a novel regulatory mechanism, which may contribute to SIRT1 up-regulation in breast cancer.

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.

Histone onco-modifications

Post-translational modification of histones provides an important regulatory platform for processes such as gene expression, DNA replication and repair, chromosome condensation and segregation and apoptosis. Disruption of these processes has been linked to the multistep process of carcinogenesis. We review the aberrant covalent histone modifications observed in cancer, and discuss how these epigenetic changes, caused by alterations in histone-modifying enzymes, can contribute to the development of a variety of human cancers. As a conclusion, a new terminology 'histone onco-modifications' is proposed to describe post-translational modifications of histones, which have been linked to cancer. This new term would take into account the active contribution and importance of these histone modifications in the development and progression of cancer.

CpG-island methylation study of liver fluke-related cholangiocarcinoma

BACKGROUND

Genetic changes have been widely reported in association with cholangiocarcinoma (CCA), while epigenetic changes are poorly characterised. We aimed to further evaluate CpG-island hypermethylation in CCA at candidate loci, which may have potential as diagnostic or prognostic biomarkers.

METHODS

We analysed methylation of 26 CpG-islands in 102 liver fluke related-CCA and 29 adjacent normal samples using methylation-specific PCR (MSP). Methylation of interest loci was confirmed using pyrosequencing and/or combined bisulfite restriction analysis, and protein expression by immunohistochemistry.

RESULTS

A number of CpG-islands (OPCML, SFRP1, HIC1, PTEN and DcR1) showed frequency of hypermethylation in >28% of CCA, but not adjacent normal tissues. The results showed that 91% of CCA were methylated in at least one CpG-island. The OPCML was the most frequently methylated locus (72.5%) and was more frequently methylated in less differentiated CCA. Patients with methylated DcR1 had significantly longer overall survival (Median; 41.7 vs 21.7 weeks, P=0.027). Low-protein expression was found in >70% of CCA with methylation of OPCML or DcR1.

CONCLUSION

Aberrant hypermethylation of certain loci is a common event in liver fluke-related CCA and may potentially contribute to cholangiocarcinogenesis. The OPCML and DcR1 might serve as methylation biomarkers in CCA that can be readily examined by MSP.

Sirtuin 1 in malignant transformation: friend or foe?

In the past decade, great interest has been shown in the role of sirtuin 1 (SIRT1) in tumorigenesis. The published data show SIRT1 can function as both a tumor promoter and tumor suppressor. In this review, we summarize the available data regarding the role of SIRT1 in tumorigenesis, with a focus on the potential mechanisms. The seemingly controversial role of SIRT1 in tumorigenesis suggests that SIRT1 might play a dual role in different tissue contexts depending on the temporal and special distribution of different SIRT1 upstream and downstream factors. Clearly, the role of SIRT1 in tumorigenesis is poorly understood and more research is necessary to elucidate its function in the malignant process.

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.

Short-term adenosine monophosphate-activated protein kinase activator 5-aminoimidazole-4-carboxamide-1-β-D-ribofuranoside treatment increases the sirtuin 1 protein expression in skeletal muscle

Adenosine monophosphate-activated protein kinase (AMPK) has been proposed to stimulate mitochondrial biogenesis and fat and glucose metabolism in skeletal muscle. Nicotinamide adenine dinucleotide-dependent histone deacetylase sirtuin 1 (SIRT1) is also thought to play a pivotal role for such metabolic adaptations. The purpose of the present study was to examine the effect of AMPK activation with the administration of AMPK activator 5-aminoimidazole-4-carboxamide-1-β-D-ribofuranoside (AICAR) to rats on skeletal muscle SIRT1 protein expression as well as peroxisome proliferator activated receptor γ coactivator-1α (PGC-1α) and glucose transporter 4 (GLUT4) protein expression and hexokinase activity. The AICAR promoted the phosphorylation of AMPK α-subunit (Thr¹⁷²) and acetyl-coenzyme A carboxylase (Ser⁷⁹) without any change of total AMPK α-subunit or acetyl-coenzyme A carboxylase protein levels in both the slow-twitch soleus and fast-twitch extensor digitorum longus (EDL) muscles. The SIRT1 protein expression increased at 24 hours after administration of AICAR in the EDL muscle but not in the soleus muscle. The PGC-1α protein expression increased in both the soleus and EDL muscles and GLUT4 did in the EDL muscle at 24 hours after an administration of AICAR. The hexokinase activity increased at 18 and 24 hours in the soleus and at 12, 18, and 24 hours in the EDL after an AICAR treatment. These results suggest that short-term AICAR treatment to rats promotes skeletal muscle AMPK phosphorylation and then coincidently increases the SIRT1 protein expression. In addition, such treatment also enhances the PGC-1α and GLUT4 protein contents and hexokinase activity in skeletal muscle.

Telomere dysfunction induces metabolic and mitochondrial compromise

Telomere dysfunction activates p53-mediated cellular growth arrest, senescence and apoptosis to drive progressive atrophy and functional decline in high-turnover tissues. The broader adverse impact of telomere dysfunction across many tissues including more quiescent systems prompted transcriptomic network analyses to identify common mechanisms operative in haematopoietic stem cells, heart and liver. These unbiased studies revealed profound repression of peroxisome proliferator-activated receptor gamma, coactivator 1 alpha and beta (PGC-1α and PGC-1β, also known as Ppargc1a and Ppargc1b, respectively) and the downstream network in mice null for either telomerase reverse transcriptase (Tert) or telomerase RNA component (Terc) genes. Consistent with PGCs as master regulators of mitochondrial physiology and metabolism, telomere dysfunction is associated with impaired mitochondrial biogenesis and function, decreased gluconeogenesis, cardiomyopathy, and increased reactive oxygen species. In the setting of telomere dysfunction, enforced Tert or PGC-1α expression or germline deletion of p53 (also known as Trp53) substantially restores PGC network expression, mitochondrial respiration, cardiac function and gluconeogenesis. We demonstrate that telomere dysfunction activates p53 which in turn binds and represses PGC-1α and PGC-1β promoters, thereby forging a direct link between telomere and mitochondrial biology. We propose that this telomere-p53-PGC axis contributes to organ and metabolic failure and to diminishing organismal fitness in the setting of telomere dysfunction.

Loss of a single Hic1 allele accelerates polyp formation in Apc(Δ716) mice

Adenomatous polyposis coli (APC) gene mutations have been implicated in familial and sporadic gastrointestinal (GI) cancers. APC mutations are associated with autosomal dominant inheritance of disease in humans. Similarly, mice that contain a single mutant APC gene encoding a protein truncated at residue 716 (Apc^Δ716) develop multiple polyps throughout the GI tract as early as 4 weeks after birth. Inactivation of another tumor suppressor gene, Hypermethylated in Cancer 1 (HIC1), often occurs in human colon cancers, among others, via CpG island hypermethylation. Homozygous deletion of Hic1 in mice results in major developmental defects and embryonic lethality. Hic1 heterozygotes have previously been shown to develop tumors of a variety of tissue types. We now report that loss of a single Hic1 allele can promote crypt hyperplasia and neoplasia of the GI tract, and Hic1^+/−, Apc^+/Δ716 double heterozygotes (DH) develop increased numbers of polyps throughout the GI tract at 60 days. Hic1 expression is absent in polyps from DH mice, with concomitant increased expression of two transcriptional repression targets of Hic1, Sirt1 and Sox9. Together, our data suggest that loss of a gene frequently silenced via epigenetic mechanisms, Hic1, can cooperate with loss of a gene mutated in GI cancer, Apc, to promote tumorigenesis in an in vivo model of multiple intestinal neoplasia.

Promoter hypermethylation of RASSF1A, MGMT, and HIC-1 genes in benign and malignant colorectal tumors

Hypermethylation at the promoter region is an important epigenetic mechanism underlying the inactivation of tumor suppressor genes and frequently occurs as an early event in the development of different types of cancer including colorectal carcinoma (CRC). The aim of the present study is the detection of methylation status for some tumor suppressor genes including RASSF1A, MGMT, and HIC-1 in both cancerous and precancerous lesions of colorectal mucosa to evaluate the possibility of developing epigenetic biomarker for early detection of Egyptian CRC. Tissue biopsy was collected from 72 patients (36 CRC, 17 adenomatous polyps, and 19 ulcerative colitis), and in addition, adjacent normal-appearing tissues were collected as control. Promoter hypermethylation status for RSSAF1A, MGMT, and HIC-1 genes was detected after isolation of genomic DNA from the tissues samples using methylation-specific PCR technique. High frequency of methylation at MGMT, RASSFA, and HIC-1 was detected in CRC patients (25%, 47.2%, and 41.7% respectively). The highest methylation detected in adenomatous polyps patients was in MGMT gene (47.1%) followed by 35.3% for HIC-1 and only 5.9% for RASSF1A gene. HIC-1 gene exhibited highest frequency of methylation in ulcerative colitis patients (57.8%) whereas it was 26.3% for both RASSF1A and MGMT genes. A nonsignificant association was recorded between the methylation status in different genes examined with the clinicopathological factors except the association between methylation at RASSF1A gene with gender (p=0.005), and it was significant. In conclusion, aberrant hypermethylation at promoter region of RASSFA, MGMT, and HIC-1 genes is involved in Egyptian CRCs. Hypermethylation of MGMT and HIC-1 genes plays an important role in the initiation of disease especially ulcerative colitis-carcinoma pathway.

Methyltransferase Set7/9 regulates p53 activity by interacting with Sirtuin 1 (SIRT1)

Numerous studies indicate that Sirtuin 1 (SIRT1), a mammalian nicotinamide adenine dinucleotide (NAD(+))-dependent histone deacetylase (HDAC), plays a crucial role in p53-mediated stress responses by deacetylating p53. Nevertheless, the acetylation levels of p53 are dramatically increased upon DNA damage, and it is not well understood how the SIRT1-p53 interaction is regulated during the stress responses. Here, we identified Set7/9 as a unique regulator of SIRT1. SIRT1 interacts with Set7/9 both in vitro and in vivo. In response to DNA damage in human cells, the interaction between Set7/9 and SIRT1 is significantly enhanced and coincident with an increase in p53 acetylation levels. Importantly, the interaction of SIRT1 and p53 is strongly suppressed in the presence of Set7/9. Consequently, SIRT1-mediated deacetylation of p53 is abrogated by Set7/9, and p53-mediated transactivation is increased during the DNA damage response. Of note, whereas SIRT1 can be methylated at multiple sites within its N terminus by Set7/9, a methylation-defective mutant of SIRT1 still retains its ability to inhibit p53 activity. Taken together, our results reveal that Set7/9 is a critical regulator of the SIRT1-p53 interaction and suggest that Set7/9 can modulate p53 function indirectly in addition to acting through a methylation-dependent mechanism.

Characterization of nuclear sirtuins: molecular mechanisms and physiological relevance

Sirtuins are protein deacetylases/mono-ADP-ribosyltransferases found in organisms ranging from bacteria to humans. This group of enzymes relies on nicotinamide adenine dinucleotide (NAD(+)) as a cofactor linking their activity to the cellular metabolic status. Originally found in yeast, Sir2 was discovered as a silencing factor and has been shown to mediate the effects of calorie restriction on lifespan extension. In mammals seven homologs (SIRT1-7) exist which evolved to have specific biological outcomes depending on the particular cellular context, their interacting proteins, and the genomic loci to where they are actively targeted. Sirtuins biological roles are highlighted in the early lethal phenotypes observed in the deficient murine models. In this chapter, we summarize current concepts on non-metabolic functions for sirtuins, depicting this broad family from yeast to mammals.

The role of mammalian sirtuins in the regulation of metabolism, aging, and longevity

Ever since the discovery of sirtuins a decade ago, interest in this family of NAD-dependent deacetylases has exploded, generating multiple lines of evidence implicating sirtuins as evolutionarily conserved regulators of lifespan. In mammals, it has been established that sirtuins regulate physiological responses to metabolism and stress, two key factors that affect the process of aging. Further investigation into the intimate connection among sirtuins, metabolism, and aging has implicated the activation of SIRT1 as both preventative and therapeutic measures against multiple age-associated disorders including type 2 diabetes and Alzheimer's disease. SIRT1 activation has clear potential to not only prevent age-associated diseases but also to extend healthspan and perhaps lifespan. Sirtuin activating compounds and NAD intermediates are two promising ways to achieve these elusive goals.

Epigenetic regulation of apoptosis and cell cycle in osteosarcoma

The role of genetic mutations in the development of osteosarcoma, such as alterations in p53 and Rb, is well understood. However, the significance of epigenetic mechanisms in the progression of osteosarcoma remains unclear and is increasingly being investigated. Recent evidence suggests that epigenetic alterations such as methylation and histone modifications of genes involved in cell cycle regulation and apoptosis may contribute to the pathogenesis of this tumor. Importantly, understanding the molecular mechanisms of regulation of these pathways may give insight into novel therapeutic strategies for patients with osteosarcoma. This paper serves to summarize the described epigenetic mechanisms in the tumorigenesis of osteosarcoma, specifically those pertaining to apoptosis and cell cycle regulation.