ATRIP Deacetylation by SIRT2 Drives ATR Checkpoint Activation by Promoting Binding to RPA-ssDNA

BubR1 and SIRT2: Insights into aneuploidy, aging, and cancer

Aging is a significant risk factor for cancer which is due, in part, to heightened genomic instability. Mitotic surveillance proteins such as BubR1 play a pivotal role in ensuring accurate chromosomal segregation and preventing aneuploidy. BubR1 levels have been shown to naturally decline with age and its loss is associated with various age-related pathologies. Sirtuins, a class of NAD+-dependent deacylases, are implicated in cancer and genomic instability. Among them, SIRT2 acts as an upstream regulator of BubR1, offering a critical pathway that can potentially mitigate age-related diseases, including cancer. In this review, we explore BubR1 as a key regulator of cellular processes crucial for aging-related phenotypes. We delve into the intricate mechanisms through which BubR1 influences genomic stability and cellular senescence. Moreover, we highlight the role of NAD+ and SIRT2 in modulating BubR1 expression and function, emphasizing its potential as a therapeutic target. The interaction between BubR1 and SIRT2 not only serves as a fundamental regulatory pathway in cellular homeostasis but also represents a promising avenue for developing targeted therapies against age-related diseases, particularly cancer.

SIRT2 promotes base excision repair by transcriptionally activating OGG1 in an ATM/ATR-dependent manner

Sirtuin 2 (SIRT2) regulates the maintenance of genome integrity by targeting pathways of DNA damage response and homologous recombination repair. However, whether and how SIRT2 promotes base excision repair (BER) remain to be determined. Here, we found that independent of its catalytic activity SIRT2 interacted with the critical glycosylase OGG1 to promote OGG1 recruitment to its own promoter upon oxidative stress, thereby enhancing OGG1 promoter activity and increasing BER efficiency. Further studies revealed that SIRT2 was phosphorylated on S46 and S53 by ATM/ATR upon oxidative stress, and SIRT2 phosphorylation enhanced the SIRT2-OGG1 interaction and mediated the stimulatory effect of SIRT2 on OGG1 promoter activity. We also characterized 37 cancer-derived SIRT2 mutants and found that 5 exhibited the loss of the stimulatory effects on OGG1 transcription. Together, our data reveal that SIRT2 acts as a tumor suppressor by promoting OGG1 transcription and increasing BER efficiency in an ATM/ATR-dependent manner.

Age-Associated Changes of Sirtuin 2 Expression in CNS and the Periphery

Sirtuin 2 (SIRT2), one of the seven members of the sirtuin family, has emerged as a potential regulator of aging and age-related pathologies since several studies have demonstrated that it shows age-related changes in humans and different animal models. A detailed analysis of the relevant works published to date addressing this topic shows that the changes that occur in SIRT2 with aging seem to be opposite in the brain and in the periphery. On the one hand, aging induces an increase in SIRT2 levels in the brain, which supports the notion that its pharmacological inhibition is beneficial in different neurodegenerative diseases. However, on the other hand, in the periphery, SIRT2 levels are reduced with aging while keeping its expression is protective against age-related peripheral inflammation, insulin resistance, and cardiovascular diseases. Thus, systemic administration of any known modulator of this enzyme would have conflicting outcomes. This review summarizes the currently available information on changes in SIRT2 expression in aging and the underlying mechanisms affected, with the aim of providing evidence to determine whether its pharmacological modulation could be an effective and safe pharmacological strategy for the treatment of age-related diseases.

Exome sequencing identifies breast cancer susceptibility genes and defines the contribution of coding variants to breast cancer risk

Linkage and candidate gene studies have identified several breast cancer susceptibility genes, but the overall contribution of coding variation to breast cancer is unclear. To evaluate the role of rare coding variants more comprehensively, we performed a meta-analysis across three large whole-exome sequencing datasets, containing 26,368 female cases and 217,673 female controls. Burden tests were performed for protein-truncating and rare missense variants in 15,616 and 18,601 genes, respectively. Associations between protein-truncating variants and breast cancer were identified for the following six genes at exome-wide significance (P < 2.5 × 10-6): the five known susceptibility genes ATM, BRCA1, BRCA2, CHEK2 and PALB2, together with MAP3K1. Associations were also observed for LZTR1, ATR and BARD1 with P < 1 × 10-4. Associations between predicted deleterious rare missense or protein-truncating variants and breast cancer were additionally identified for CDKN2A at exome-wide significance. The overall contribution of coding variants in genes beyond the previously known genes is estimated to be small.

DNA-PK is activated by SIRT2 deacetylation to promote DNA double-strand break repair by non-homologous end joining

DNA-dependent protein kinase (DNA-PK) plays a critical role in non-homologous end joining (NHEJ), the predominant pathway that repairs DNA double-strand breaks (DSB) in response to ionizing radiation (IR) to govern genome integrity. The interaction of the catalytic subunit of DNA-PK (DNA-PKcs) with the Ku70/Ku80 heterodimer on DSBs leads to DNA-PK activation; however, it is not known if upstream signaling events govern this activation. Here, we reveal a regulatory step governing DNA-PK activation by SIRT2 deacetylation, which facilitates DNA-PKcs localization to DSBs and interaction with Ku, thereby promoting DSB repair by NHEJ. SIRT2 deacetylase activity governs cellular resistance to DSB-inducing agents and promotes NHEJ. SIRT2 furthermore interacts with and deacetylates DNA-PKcs in response to IR. SIRT2 deacetylase activity facilitates DNA-PKcs interaction with Ku and localization to DSBs and promotes DNA-PK activation and phosphorylation of downstream NHEJ substrates. Moreover, targeting SIRT2 with AGK2, a SIRT2-specific inhibitor, augments the efficacy of IR in cancer cells and tumors. Our findings define a regulatory step for DNA-PK activation by SIRT2-mediated deacetylation, elucidating a critical upstream signaling event initiating the repair of DSBs by NHEJ. Furthermore, our data suggest that SIRT2 inhibition may be a promising rationale-driven therapeutic strategy for increasing the effectiveness of radiation therapy.

A DNA damage-induced phosphorylation circuit enhances Mec1ATR Ddc2ATRIP recruitment to Replication Protein A

The cell cycle checkpoint kinase Mec1ATR and its integral partner Ddc2ATRIP are vital for the DNA damage and replication stress response. Mec1-Ddc2 "senses" single-stranded DNA (ssDNA) by being recruited to the ssDNA binding Replication Protein A (RPA) via Ddc2. In this study, we show that a DNA damage-induced phosphorylation circuit modulates checkpoint recruitment and function. We demonstrate that Ddc2-RPA interactions modulate the association between RPA and ssDNA and that Rfa1-phosphorylation aids in the further recruitment of Mec1-Ddc2. We also uncover an underappreciated role for Ddc2 phosphorylation that enhances its recruitment to RPA-ssDNA that is important for the DNA damage checkpoint in yeast. The crystal structure of a phosphorylated Ddc2 peptide in complex with its RPA interaction domain provides molecular details of how checkpoint recruitment is enhanced, which involves Zn2+. Using electron microscopy and structural modeling approaches, we propose that Mec1-Ddc2 complexes can form higher order assemblies with RPA when Ddc2 is phosphorylated. Together, our results provide insight into Mec1 recruitment and suggest that formation of supramolecular complexes of RPA and Mec1-Ddc2, modulated by phosphorylation, would allow for rapid clustering of damage foci to promote checkpoint signaling.

DNA Damage-Mediated Neurotoxicity in Parkinson’s Disease

Parkinson’s disease (PD) is the second most common neurodegenerative disease around the world; however, its pathogenesis remains unclear so far. Recent advances have shown that DNA damage and repair deficiency play an important role in the pathophysiology of PD. There is growing evidence suggesting that DNA damage is involved in the propagation of cellular damage in PD, leading to neuropathology under different conditions. Here, we reviewed the current work on DNA damage repair in PD. First, we outlined the evidence and causes of DNA damage in PD. Second, we described the potential pathways by which DNA damage mediates neurotoxicity in PD and discussed the precise mechanisms that drive these processes by DNA damage. In addition, we looked ahead to the potential interventions targeting DNA damage and repair. Finally, based on the current status of research, key problems that need to be addressed in future research were proposed.

The "extreme phenotype approach" applied to male breast cancer allows the identification of rare variants of ATR as potential breast cancer susceptibility alleles

In oncogenetics, some patients could be considered as "extreme phenotypes", such as those with very early onset presentation or multiple primary malignancies, unusually high numbers of cancers of the same spectrum or rare cancer types in the same parental branch. For these cases, a genetic predisposition is very likely, but classical candidate gene panel analyses often and frustratingly remains negative. In the framework of the EX2TRICAN project, exploring unresolved extreme cancer phenotypes, we applied exome sequencing on rare familial cases with male breast cancer, identifying a novel pathogenic variant of ATR (p.Leu1808*). ATR has already been suspected as being a predisposing gene to breast cancer in women. We next identified 3 additional ATR variants in a cohort of both male and female with early onset and familial breast cancers (c.7762-2A>C; c.2078+1G>A; c.1A>G). Further molecular and cellular investigations showed impacts on transcripts for variants affecting splicing sites and reduction of ATR expression and phosphorylation of the ATR substrate CHEK1. This work further demonstrates the interest of an extended genetic analysis such as exome sequencing to identify very rare variants that can play a role in cancer predisposition in extreme phenotype cancer cases unexplained by classical cancer gene panels testing.

DNA replication: Mechanisms and therapeutic interventions for diseases

Accurate and integral cellular DNA replication is modulated by multiple replication-associated proteins, which is fundamental to preserve genome stability. Furthermore, replication proteins cooperate with multiple DNA damage factors to deal with replication stress through mechanisms beyond their role in replication. Cancer cells with chronic replication stress exhibit aberrant DNA replication and DNA damage response, providing an exploitable therapeutic target in tumors. Numerous evidence has indicated that posttranslational modifications (PTMs) of replication proteins present distinct functions in DNA replication and respond to replication stress. In addition, abundant replication proteins are involved in tumorigenesis and development, which act as diagnostic and prognostic biomarkers in some tumors, implying these proteins act as therapeutic targets in clinical. Replication-target cancer therapy emerges as the times require. In this context, we outline the current investigation of the DNA replication mechanism, and simultaneously enumerate the aberrant expression of replication proteins as hallmark for various diseases, revealing their therapeutic potential for target therapy. Meanwhile, we also discuss current observations that the novel PTM of replication proteins in response to replication stress, which seems to be a promising strategy to eliminate diseases.

SAMHD1 deacetylation by SIRT1 promotes DNA end resection by facilitating DNA binding at double-strand breaks

Sterile alpha motif and HD domain-containing protein 1 (SAMHD1) has a dNTPase-independent function in promoting DNA end resection to facilitate DNA double-strand break (DSB) repair by homologous recombination (HR); however, it is not known if upstream signaling events govern this activity. Here, we show that SAMHD1 is deacetylated by the SIRT1 sirtuin deacetylase, facilitating its binding with ssDNA at DSBs, to promote DNA end resection and HR. SIRT1 complexes with and deacetylates SAMHD1 at conserved lysine 354 (K354) specifically in response to DSBs. K354 deacetylation by SIRT1 promotes DNA end resection and HR but not SAMHD1 tetramerization or dNTPase activity. Mechanistically, K354 deacetylation by SIRT1 promotes SAMHD1 recruitment to DSBs and binding to ssDNA at DSBs, which in turn facilitates CtIP ssDNA binding, leading to promotion of genome integrity. These findings define a mechanism governing the dNTPase-independent resection function of SAMHD1 by SIRT1 deacetylation in promoting HR and genome stability.

Sirtuin5 protects colorectal cancer from DNA damage by keeping nucleotide availability

In our previous study, we reported that sirtuin5 (SIRT5), a member of the NAD⁺-dependent class III histone deacetylase family, is highly expressed in colorectal cancer (CRC). Herein we show that SIRT5 knockdown impairs the production of ribose-5-phosphate, which is essential for nucleotide synthesis, resulting in continuous and irreparable DNA damage and consequently leading to cell cycle arrest and enhanced apoptosis in CRC cells. These SIRT5 silencing-induced effects can be reversed by nucleoside supplementation. Mechanistically, SIRT5 activates transketolase (TKT), a key enzyme in the non-oxidative pentose phosphate pathway, in a demalonylation-dependent manner. Furthermore, TKT is essential for SIRT5-induced malignant phenotypes of CRC both in vivo and in vitro. Altogether, SIRT5 silencing induces DNA damage in CRC via post-translational modifications and inhibits tumor growth, suggesting that SIRT5 can serve as a promising target for CRC treatment.

Multiple Roles of SIRT2 in Regulating Physiological and Pathological Signal Transduction

Sirtuin 2 (SIRT2), as a member of the sirtuin family, has representative features of evolutionarily highly conserved nicotinamide adenine dinucleotide (NAD+)-dependent deacetylase activity. In addition, SIRT2, as the only sirtuin protein colocalized with tubulin in the cytoplasm, has its own functions and characteristics. In recent years, studies have increasingly shown that SIRT2 can participate in the regulation of gene expression and regulate signal transduction in the metabolic pathway mainly through its post-translational modification of target genes; thus, SIRT2 has become a key centre in the metabolic pathway and participates in the pathological process of metabolic disorder-related diseases. In this paper, it is discussed that SIRT2 can regulate all aspects of gene expression, including epigenetic modification, replication, transcription and translation, and post-translational modification, which enables SIRT2 to participate in energy metabolism in life activities, and it is clarified that SIRT2 is involved in metabolic process-specific signal transduction mechanisms. Therefore, SIRT2 can be involved in metabolic disorder-related inflammation and oxidative stress, thereby triggering the occurrence of metabolic disorder-related diseases, such as neurodegenerative diseases, tumours, diabetes, and cardiovascular diseases. Currently, although the role of SIRT2 in some diseases is still controversial, given the multiple roles of SIRT2 in regulating physiological and pathological signal transduction, SIRT2 has become a key target for disease treatment. It is believed that with increasing research, the clinical application of SIRT2 will be promoted.

Molecular Link between DNA Damage Response and Microtubule Dynamics

Microtubules are major components of the cytoskeleton that play important roles in cellular processes such as intracellular transport and cell division. In recent years, it has become evident that microtubule networks play a role in genome maintenance during interphase. In this review, we highlight recent advances in understanding the role of microtubule dynamics in DNA damage response and repair. We first describe how DNA damage checkpoints regulate microtubule organization and stability. We then highlight how microtubule networks are involved in the nuclear remodeling following DNA damage, which leads to changes in chromosome organization. Lastly, we discuss how microtubule dynamics participate in the mobility of damaged DNA and promote consequent DNA repair. Together, the literature indicates the importance of microtubule dynamics in genome organization and stability during interphase.

Fueling genome maintenance: On the versatile roles of NAD+ in preserving DNA integrity

NAD+ is a versatile biomolecule acting as a master regulator and substrate in various cellular processes, including redox regulation, metabolism, and various signaling pathways. In this article, we concisely and critically review the role of NAD+ in mechanisms promoting genome maintenance. Numerous NAD+-dependent reactions are involved in the preservation of genome stability, the cellular DNA damage response, and other pathways regulating nucleic acid metabolism, such as gene expression and cell proliferation pathways. Of note, NAD+ serves as a substrate to ADP-ribosyltransferases, sirtuins, and potentially also eukaryotic DNA ligases, all of which regulate various aspects of DNA integrity, damage repair, and gene expression. Finally, we critically analyze recent developments in the field as well as discuss challenges associated with therapeutic actions intended to raise NAD+ levels.

Quantitative proteomic analysis of the lysine acetylome reveals diverse SIRT2 substrates

Sirtuin 2 (SIRT2) is a NAD+-dependent deacetylase, which regulates multiple biological processes, including genome maintenance, aging, tumor suppression, and metabolism. While a number of substrates involved in these processes have been identified, the global landscape of the SIRT2 acetylome remains unclear. Using a label-free quantitative proteomic approach following enrichment for acetylated peptides from SIRT2-depleted and SIRT2-overexpressing HCT116 human colorectal cancer cells, we identified a total of 2,846 unique acetylation sites from 1414 proteins. 896 sites from 610 proteins showed a > 1.5-fold increase in acetylation with SIRT2 knockdown, and 509 sites from 361 proteins showed a > 1.5-fold decrease in acetylation with SIRT2 overexpression, with 184 proteins meeting both criteria. Sequence motif analyses identified several site-specific consensus sequence motifs preferentially recognized by SIRT2, most commonly KxxxxK(ac). Gene Ontology, KEGG, and MetaCore pathway analyses identified SIRT2 substrates involved in diverse pathways, including carbon metabolism, glycolysis, spliceosome, RNA transport, RNA binding, transcription, DNA damage response, the cell cycle, and colorectal cancer. Collectively, our findings expand on the number of known acetylation sites, substrates, and cellular pathways targeted by SIRT2, providing support for SIRT2 in regulating networks of proteins in diverse pathways and opening new avenues of investigation into SIRT2 function.

Emerging roles of Sirtuin 2 in cardiovascular diseases

Sirtuins are a family of NAD⁺ -dependent deacetylases implicated in a wide variety of age-associated pathologies, including cardiovascular disorders. Among the seven mammalian sirtuins, SIRT2 modulates various cellular processes through the deacetylation or deacylation of their target proteins. Notably, the levels of SIRT2 in the heart decline with age and other pathological conditions, leading to cardiovascular dysfunction. In the present review, we discuss the emerging roles of SIRT2 in cardiovascular dysfunction and heart failure associated with factors like age, hypertension, oxidative stress, and diabetes. We also discuss the potential of using inhibitors to study the unexplored role of SIRT2 in the heart. While SIRT2 undoubtedly plays a crucial role in the cardiovascular system, its functions are only beginning to be understood, making it an attractive candidate for further research in the field.

Understanding the Function of Mammalian Sirtuins and Protein Lysine Acylation

Protein lysine acetylation is an important posttranslational modification that regulates numerous biological processes. Targeting lysine acetylation regulatory factors, such as acetyltransferases, deacetylases, and acetyl-lysine recognition domains, has been shown to have potential for treating human diseases, including cancer and neurological diseases. Over the past decade, many other acyl-lysine modifications, such as succinylation, crotonylation, and long-chain fatty acylation, have also been investigated and shown to have interesting biological functions. Here, we provide an overview of the functions of different acyl-lysine modifications in mammals. We focus on lysine acetylation as it is well characterized, and principles learned from acetylation are useful for understanding the functions of other lysine acylations. We pay special attention to the sirtuins, given that the study of sirtuins has provided a great deal of information about the functions of lysine acylation. We emphasize the regulation of sirtuins to illustrate that their regulation enables cells to respond to various signals and stresses.

SIRT2 promotes BRCA1-BARD1 heterodimerization through deacetylation

The breast cancer type I susceptibility protein (BRCA1) and BRCA1-associated RING domain protein I (BARD1) heterodimer promote genome integrity through pleiotropic functions, including DNA double-strand break (DSB) repair by homologous recombination (HR). BRCA1-BARD1 heterodimerization is required for their mutual stability, HR function, and role in tumor suppression; however, the upstream signaling events governing BRCA1-BARD1 heterodimerization are unclear. Here, we show that SIRT2, a sirtuin deacetylase and breast tumor suppressor, promotes BRCA1-BARD1 heterodimerization through deacetylation. SIRT2 complexes with BRCA1-BARD1 and deacetylates conserved lysines in the BARD1 RING domain, interfacing BRCA1, which promotes BRCA1-BARD1 heterodimerization and consequently BRCA1-BARD1 stability, nuclear retention, and localization to DNA damage sites, thus contributing to efficient HR. Our findings define a mechanism for regulation of BRCA1-BARD1 heterodimerization through SIRT2 deacetylation, elucidating a critical upstream signaling event directing BRCA1-BARD1 heterodimerization, which facilitates HR and tumor suppression, and delineating a role for SIRT2 in directing DSB repair by HR.

Minten et al. show that SIRT2, a sirtuin deacetylase and tumor suppressor protein, promotes BRCA1-BARD1 heterodimerization through deacetylation of BARD1 at conserved lysines within its RING domain. These findings elucidate a critical upstream signaling event directing BRCA1-BARD1 heterodimerization, which facilitates HR and tumor suppression.

Human SIRT2 and SIRT3 deacetylases function in DNA homologous recombinational repair

SIRT2 and SIRT3 protein deacetylases maintain genome integrity and stability. However, their mechanisms for maintaining the genome remain unclear. To examine the roles of SIRT2 and SIRT3 in DSB repair, I-SceI-based GFP reporter assays for HR, single-strand annealing (SSA) and nonhomologous end joining (NHEJ) repair were performed under SIRT2- or SIRT3-depleted conditions. SIRT2 or SIRT3 depletion inhibited HR repair equally to RAD52 depletion, but did not affect SSA and NHEJ repairs. SIRT2 or SIRT3 depletion disturbed the recruitment of RAD51 to DSB sites, an essential step for RAD51-dependent HR repair, but not directly through RAD52 deacetylation. SIRT2 or SIRT3 depletion decreased the colocalization of γH2AX foci with RPA1, and thus, they might be involved in initiating DSB end resection for the recruitment of RAD51 to DSB sites at an early step in HR repair. These results show the novel underlying mechanism of the SIRT2 and SIRT3 functions in HR for genome stability.

Fight to the bitter end: DNA repair and aging

DNA carries the genetic information that directs complex biological processes; thus, maintaining a stable genome is critical for individual growth and development and for human health. DNA repair is a fundamental and conserved mechanism responsible for mending damaged DNA and restoring genomic stability, while its deficiency is closely related to multiple human disorders. In recent years, remarkable progress has been made in the field of DNA repair and aging. Here, we will extensively discuss the relationship among DNA damage, DNA repair, aging and aging-associated diseases based on the latest research. In addition, the possible role of DNA repair in several potential rejuvenation strategies will be discussed. Finally, we will also review the emerging methods that may facilitate future research on DNA repair.

ROS-Induced SIRT2 Upregulation Contributes to Cisplatin Sensitivity in Ovarian Cancer

Cisplatin resistance remains a significant obstacle for improving the clinical outcome of ovarian cancer patients. Recent studies have demonstrated that cisplatin is an important inducer of intracellullar reactive oxygen species (ROS), triggering cancer cell death. Sirtuin 2 (SIRT2), a member of class III NAD⁺ dependent histone deacetylases (HDACs), has been reported to be involved in regulating cancer hallmarks including drug response. In this study, we aimed to identify the role of SIRT2 in oxidative stress and cisplatin response in cancer. Two ovarian cancer cell lines featuring different sensitivities to cisplatin were used in this study. We found different expression patterns of SIRT2 in cisplatin-sensitive (A2780/S) and cisplatin-resistant (A2780/CP) cancer cells with cisplatin treatment, where SIRT2 expression was augmented only in A2780/S cells. Furthermore, cisplatin-induced ROS generation was responsible for the upregulation of SIRT2 in A2780/S cells, whereas overexpression of SIRT2 significantly enhanced the sensitivity of cisplatin-resistant counterpart cells to cisplatin. Our study proposes that targeting SIRT2 may provide new strategies to potentiate platinum-based chemotherapy in ovarian cancer patients.

Loss of Slug Compromises DNA Damage Repair and Accelerates Stem Cell Aging in Mammary Epithelium

DNA damage activates checkpoints that limit the replicative potential of stem cells, including differentiation. These checkpoints protect against cancer development but also promote tissue aging. Because mice lacking Slug/Snai2 exhibit limited stem cell activity, including luminobasal differentiation, and are protected from mammary cancer, we reasoned that Slug might regulate DNA damage checkpoints in mammary epithelial cells. Here, we show that Slug facilitates efficient execution of RPA32-mediated DNA damage response (DDR) signaling. Slug deficiency leads to delayed phosphorylation of ataxia telangiectasia mutated and Rad3-related protein (ATR) and its effectors RPA32 and CHK1. This leads to impaired RAD51 recruitment to DNA damage sites and persistence of unresolved DNA damage. In vivo, Slug/Snai2 loss leads to increased DNA damage and premature aging of mammary epithelium. Collectively, our work demonstrates that the mammary stem cell regulator Slug controls DDR checkpoints by dually inhibiting differentiation and facilitating DDR repair, and its loss causes unresolved DNA damage and accelerated aging.

The Clinical Significance of SIRT2 in Malignancies: A Tumor Suppressor or an Oncogene?

Sirtuin 2 (SIRT2) is a member of the sirtuin protein family. It is a Class III histone deacetylase (HDACs) and predominantly localized to the cytosol. SIRT2 deacetylates histones and a number of non-histone proteins and plays a pivotal role in various physiologic processes. Previously, SIRT2 has been considered indispensable during carcinogenesis; however, there is now a significant controversy regarding whether SIRT2 is an oncogene or a tumor suppressor. The purpose of this review is to summarize the physiological functions of SIRT2 and its mechanisms in cancer. We will focus on five malignancies (breast cancer, non-small cell lung cancer, hepatocellular carcinoma, colorectal cancer, and glioma) to describe the current status of SIRT2 research and discuss the clinical evaluation of SIRT2 expression and the use of SIRT2 inhibitors.

The role of the α-tubulin acetyltransferase αTAT1 in the DNA damage response

Lysine 40 acetylation of α-tubulin (Ac-α-tubulin), catalyzed by the acetyltransferase αTAT1, marks stabilized microtubules. Recently, there is growing evidence to suggest crosstalk between the DNA damage response (DDR) and microtubule organization; we therefore investigated whether αTAT1 is involved in the DDR. Following treatment with DNA-damaging agents, increased levels of Ac-α-tubulin were detected. We also observed significant induction of Ac-α-tubulin after depletion of DNA repair proteins, suggesting that αTAT1 is positively regulated in response to DNA damage. Intriguingly, αTAT1 depletion decreased DNA damage-induced replication protein A (RPA) phosphorylation and foci formation. Moreover, DNA damage-induced cell cycle arrest was significantly delayed in αTAT1-depleted cells, indicating defective checkpoint activation. The checkpoint defects seen upon αTAT1 deficiency were restored by expression of wild-type αTAT1, but not by αTAT1-D157N (a catalytically inactive αTAT1), indicating that the role of αTAT1 in the DDR is dependent on enzymatic activity. Furthermore, αTAT1-depleted direct repeat GFP (DR-GFP) U2OS cells had a significant decrease in the frequency of homologous recombination repair. Collectively, our results suggest that αTAT1 may play an essential role in DNA damage checkpoints and DNA repair through its acetyltransferase activity.

DNA Damage Response and Metabolic Reprogramming in Health and Disease

Nuclear DNA damage contributes to cellular malfunction and the premature onset of age-related diseases, including cancer. Until recently, the canonical DNA damage response (DDR) was thought to represent a collection of nuclear processes that detect, signal and repair damaged DNA. However, recent evidence suggests that beyond nuclear events, the DDR rewires an intricate network of metabolic circuits, fine-tunes protein synthesis, trafficking, and secretion as well as balances growth with defense strategies in response to genotoxic insults. In this review, we discuss how the active DDR signaling mobilizes extranuclear and systemic responses to promote cellular homeostasis and organismal survival in health and disease.

Tubastatin A inhibits HDAC and Sirtuin activity rather than being a HDAC6-specific inhibitor in mouse oocytes

Tubastatin A (TubA) is a highly selective histone deacetylase 6 (HDAC6) inhibitor. As expected, mouse germinal vesicle oocytes fail to extrude the first polar body following TubA treatment. However, a previous study demonstrated that homozygous Hdac6 knockout (KO) mice can be viable and fertile. Therefore, we asked whether TubA is indeed a specific inhibitor of HDAC6 activity. RNA-sequencing and in silico analysis demonstrated that the TubA-treated group presented significant changes in the expression of Hdac subfamily genes such as Hdac6, 10, and 11, and Sirtuin 2, 5, 6, and 7. Additionally, gene expression related to the p53, MAPK, Wnt, and Notch signaling pathways in the TubA-treated group were increased significantly; in contrast, gene expression related to metabolism, DNA replication, and oxidative phosphorylation was decreased significantly. Furthermore, gene expression related to cell cycle, cell structure, pyrimidine metabolism, pentose phosphate pathway, mitochondrial activation, proteasome pathway, RNA polymerase, DNA replication, cyclin-dependent kinase, nucleolar activity, and MI arrest were significantly decreased, indicating that TubA-induced abnormal meiotic maturation and oocyte senescence may be due to the combined effects of HDAC and Sirtuin inhibition, and not HDAC6 inhibition alone. Thus, we believed that this system could provide a model for monitoring the effects of TubA on mouse oocytes.

SIRT2 Affects Primary Cilia Formation by Regulating mTOR Signaling in Retinal Pigmented Epithelial Cells

SIRT2, a member of the Class III HDAC family, participates in diverse cellular processes and regulates several pathological conditions. Although a few reports show that SIRT2 regulates the cell cycle, the causes and outcomes of SIRT2-dependent cell proliferation remain unclear. Here, we examined the effects of SIRT2 suppression in human RPE1 cells using siRNA targeting SIRT2, and AK-1, a SIRT2-specific inhibitor. The number of primary cilia in SIRT2-suppressed cells increased under serum-present conditions. Suppressing SIRT2 induced cell cycle arrest at G0/G1 phase by inactivating mammalian target of rapamycin (mTOR) signaling, possibly through mTORC1. Treatment with torin 1, an inhibitor of mTORC1/mTORC2, yielded results similar to those observed after SIRT2 suppression. However, SIRT2 suppression did not affect primary cilia formation or mTOR signaling following serum starvation. This suggests that SIRT2 acts as a critical sensor that links growth factor-dependent signal transduction and primary cilia formation by regulating the cell cycle.

SIRT6 is a DNA double-strand break sensor

DNA double-strand breaks (DSB) are the most deleterious type of DNA damage. In this work, we show that SIRT6 directly recognizes DNA damage through a tunnel-like structure that has high affinity for DSB. SIRT6 relocates to sites of damage independently of signaling and known sensors. It activates downstream signaling for DSB repair by triggering ATM recruitment, H2AX phosphorylation and the recruitment of proteins of the homologous recombination and non-homologous end joining pathways. Our findings indicate that SIRT6 plays a previously uncharacterized role as a DNA damage sensor, a critical factor in initiating the DNA damage response (DDR). Moreover, other Sirtuins share some DSB-binding capacity and DDR activation. SIRT6 activates the DDR before the repair pathway is chosen, and prevents genomic instability. Our findings place SIRT6 as a sensor of DSB, and pave the road to dissecting the contributions of distinct DSB sensors in downstream signaling.

DNA is a double-stranded molecule in which the two strands run in opposite directions, like the lanes on a two-lane road. Also like a road, DNA can be damaged by use and adverse conditions. Double-strand breaks – where both strands of DNA snap at once – are the most dangerous type of DNA damage, so cells have systems in place to rapidly detect and repair this kind of damage.

There are three confirmed sensors for double-strand break in human cells. A fourth protein, known as SIRT6, arrives within five seconds of DNA damage, and was known to make the DNA more accessible so that it can be repaired. However, it was unclear whether SIRT6 could detect the double-strand break itself, or whether it was recruited to the damage by another double-strand break sensor.

To address this issue, Onn et al. blocked the three other sensors in human cells and watched the response to DNA damage. Even when all the other sensors were inactive, SIRT6 still arrived at damaged DNA and activated the DNA damage response. To find out how SIRT6 sensed DNA damage, Onn et al. examined how purified SIRT6 interacts with different kinds of DNA. This revealed that SIRT6 sticks to broken DNA ends, especially if the end of one strand slightly overhangs the other – a common feature of double-strand breaks. A closer look at the structure of the SIRT6 protein revealed that it contains a narrow tube, which fits over the end of one broken DNA strand. When both strands break at once, two SIRT6 molecules cap the broken ends, joining together to form a pair. This pair not only protects the open ends of the DNA from further damage, it also sends signals to initiating repairs. In this way, SIRT6 could be thought of acting like a paramedic who arrives first on the scene of an accident and works to treat the injured while waiting for more specialized help to arrive.

Understanding the SIRT6 sensor could improve knowledge about how cells repair their DNA. SIRT6 arrives before the cell chooses how to fix its broken DNA, so studying it further could reveal how that critical decision happens. This is important for medical research because DNA damage builds up in age-related diseases like cancer and neurodegeneration. In the long term, these findings can help us develop new treatments that target different types of DNA damage sensors.

SIRT2 Contributes to the Regulation of Intestinal Cell Proliferation and Differentiation

BACKGROUND AND AIMS

Intestinal mucosa undergoes a continual process of proliferation, differentiation, and apoptosis. Disruption of this homeostasis is associated with disorders such as inflammatory bowel disease (IBD). We investigated the role of Sirtuin 2 (SIRT2), a NAD-dependent protein deacetylase, in intestinal epithelial cell (IEC) proliferation and differentiation and the mechanism by which SIRT2 contributes to maintenance of intestinal cell homeostasis.

METHODS

IECs were collected from SIRT2-deficient mice and patients with IBD. Expression of SIRT2, differentiation markers (mucin2, intestinal alkaline phosphatase, villin, Na,K-ATPase, and lysozyme) and Wnt target genes (EPHB2, AXIN2, and cyclin D1) was determined by western blot, real-time RT-PCR, or immunohistochemical (IHC) staining. IECs were treated with TNF or transfected with siRNA targeting SIRT2. Proliferation was determined by villus height and crypt depth, and Ki67 and cyclin D1 IHC staining. For studies using organoids, intestinal crypts were isolated.

RESULTS

Increased SIRT2 expression was localized to the more differentiated region of the intestine. In contrast, SIRT2 deficiency impaired proliferation and differentiation and altered stemness in the small intestinal epithelium ex vivo and in vivo. SIRT2-deficient mice showed decreased intestinal enterocyte and goblet cell differentiation but increased the Paneth cell lineage and increased proliferation of IECs. Moreover, we found that SIRT2 inhibits Wnt/β-catenin signaling, which critically regulates IEC proliferation and differentiation. Consistent with a distinct role for SIRT2 in maintenance of gut homeostasis, intestinal mucosa from IBD patients exhibited decreased SIRT2 expression.

CONCLUSION

We demonstrate that SIRT2, which is decreased in intestinal tissues from IBD patients, regulates Wnt-β-catenin signaling and is important for maintenance of IEC proliferation and differentiation.

SIRT2 deacetylates GRASP55 to facilitate post-mitotic Golgi assembly

Sirtuin 2 (SIRT2) is an NAD-dependent sirtuin deacetylase that regulates microtubule and chromatin dynamics, gene expression and cell cycle progression, as well as nuclear envelope reassembly. Recent proteomic analyses have identified Golgi proteins as SIRT2 interactors, indicating that SIRT2 may also play a role in Golgi structure formation. Here, we show that SIRT2 depletion causes Golgi fragmentation and impairs Golgi reassembly at the end of mitosis. SIRT2 interacts with the Golgi reassembly stacking protein GRASP55 (also known as GORASP2) in mitosis when GRASP55 is highly acetylated on K50. Expression of wild-type and the K50R acetylation-deficient mutant of GRASP55, but not the K50Q acetylation-mimetic mutant, in GRASP55 and GRASP65 (also known as GORASP1) double-knockout cells, rescued the Golgi structure and post-mitotic Golgi reassembly. Acetylation-deficient GRASP55 exhibited a higher self-interaction efficiency, a property required for Golgi structure formation. These results demonstrate that SIRT2 regulates Golgi structure by modulating GRASP55 acetylation levels.

SIRT2: Controversy and multiple roles in disease and physiology

Sirtuin 2 (SIRT2) is an NAD+-dependent deacetylase that was under studied compared to other sirtuin family members. SIRT2 is the only sirtuin protein which is predominantly found in the cytoplasm but is also found in the mitochondria and in the nucleus. Recently, accumulating evidence has uncovered a growing number of substrates and additional detailed functions of SIRT2 in a wide range of biological processes, marking its crucial role. Here, we give a comprehensive profile of the crucial physiological functions of SIRT2 and its role in neurological diseases, cancers, and other diseases. This review summarizes the functions of SIRT2 in the nervous system, mitosis regulation, genome integrity, cell differentiation, cell homeostasis, aging, infection, inflammation, oxidative stress, and autophagy. SIRT2 inhibition rescues neurodegenerative disease symptoms and hence SIRT2 is a potential therapeutic target for neurodegenerative disease. SIRT2 is undoubtedly dysfunctional in cancers and plays a dual-faced role in different types of cancers, and although its mechanism is unresolved, SIRT2 remains a promising therapeutic target for certain cancers. In future, the continued rapid growth in SIRT2 research will help clarify its role in human health and disease, and promote the progress of this target in clinical practice.

To control or to be controlled? Dual roles of CDK2 in DNA damage and DNA damage response

CDK2 (cyclin-dependent kinase 2), a member of the CDK family, has been shown to play a role in many cellular activities including cell cycle progression, apoptosis and senescence. Recently, accumulating evidence indicates that CDK2 is involved in DNA damage and DNA repair response (DDR). When DNA is damaged by internal or external genotoxic stresses, CDK2 activity is required for proper DNA repair in vivo and in vitro, whereas inactivation of CDK2 by siRNA techniques or by inhibitors could result in DNA damage and stimulate DDR. Hence, CDK2 seems to play dual roles in DNA damage and DDR. On one aspect, it is activated and stimulates DDR to repair DNA damage when DNA damage occurs; on the other hand, its inactivation directly leads to DNA damage and evokes DDR. Here, we describe the roles of CDK2 in DNA damage and DDR, and discuss the potential application of CDK2 inhibitors as anti-cancer agents.

Acetylation regulates ribonucleotide reductase activity and cancer cell growth

Ribonucleotide reductase (RNR) catalyzes the de novo synthesis of deoxyribonucleoside diphosphates (dNDPs) to provide dNTP precursors for DNA synthesis. Here, we report that acetylation and deacetylation of the RRM2 subunit of RNR acts as a molecular switch that impacts RNR activity, dNTP synthesis, and DNA replication fork progression. Acetylation of RRM2 at K95 abrogates RNR activity by disrupting its homodimer assembly. RRM2 is directly acetylated by KAT7, and deacetylated by Sirt2, respectively. Sirt2, which level peak in S phase, sustains RNR activity at or above a threshold level required for dNTPs synthesis. We also find that radiation or camptothecin-induced DNA damage promotes RRM2 deacetylation by enhancing Sirt2-RRM2 interaction. Acetylation of RRM2 at K95 results in the reduction of the dNTP pool, DNA replication fork stalling, and the suppression of tumor cell growth in vitro and in vivo. This study therefore identifies acetylation as a regulatory mechanism governing RNR activity.

DNA Repair: Translation to the Clinic

It has been well established that an accumulation of mutations in DNA, whether caused by external sources (e.g. ultraviolet light, radioactivity) or internal sources (e.g. metabolic by-products, such as reactive oxygen species), has the potential to cause a cell to undergo carcinogenesis and increase the risk for the development of cancer. Therefore, it is critically important for a cell to have the capacity to properly respond to and repair DNA damage as it occurs. The DNA damage response (DDR) describes a collection of DNA repair pathways that aid in the protection of genomic integrity by detecting myriad types of DNA damage and initiating the correct DNA repair pathway. In many instances, a deficiency in the DDR, whether inherited or spontaneously assumed, can increase the risk of carcinogenesis and ultimately tumorigenesis through the accumulation of mutations that fail to be properly repaired. Interestingly, although disruption of the DDR can lead to the initial genomic instability that can ultimately cause carcinogenesis, the DDR has also proven to be an invaluable target for anticancer drugs and therapies. Making matters more complicated, the DDR is also involved in the resistance to first-line cancer therapy. In this review, we will consider therapies already in use in the clinic and ongoing research into other avenues of treatment that target DNA repair pathways in cancer.

Updates on the epigenetic roles of sirtuins

Sirtuins are a class of enzyme with NAD⁺-dependent protein lysine deacylase activities. They were initially discovered to regulate transcription and life span via histone deacetylase activities. Later studies expanded their activities to other proteins and acyl lysine modifications. Through deacylating various substrate proteins, they regulate many biological processes, including transcription, DNA repair and genome stability, metabolism, and signal transduction. Here, we review recent understandings of the epigenetic functions (broadly defined to include transcriptional, post-transcriptional regulation, and DNA repair) of mammalian sirtuins. Because of the important functions of sirtuins, their own regulation is of great interest and is also discussed.

The signaling pathways that mediate the anti-cancer effects of caloric restriction

Caloric restriction (CR) has been shown to promote longevity and ameliorate aging-associated diseases, including cancer. Extensive research over recent decades has revealed that CR reduces IGF-1/PI3K/AKT signaling and increases sirtuin signaling. We recently found that CR also enhances ALDOA/DNA-PK/p53 signaling. In the present review, we summarize the molecular mechanisms underlying the modulation of the IGF-1/PI3K/AKT pathway, sirtuin signaling, and the ALDOA/DNA-PK/p53 pathway by CR. We also summarize the evidence concerning the roles of these signaling pathways in carcinogenesis, and discuss how they are regulated by CR. Finally, we discuss the crosstalk between these signaling pathways.

SAMHD1 Promotes DNA End Resection to Facilitate DNA Repair by Homologous Recombination

DNA double-strand break (DSB) repair by homologous recombination (HR) is initiated by CtIP/MRN-mediated DNA end resection to maintain genome integrity. SAMHD1 is a dNTP triphosphohydrolase, which restricts HIV-1 infection, and mutations are associated with Aicardi-Goutières syndrome and cancer. We show that SAMHD1 has a dNTPase-independent function in promoting DNA end resection to facilitate DSB repair by HR. SAMHD1 deficiency or Vpx-mediated degradation causes hypersensitivity to DSB-inducing agents, and SAMHD1 is recruited to DSBs. SAMHD1 complexes with CtIP via a conserved C-terminal domain and recruits CtIP to DSBs to facilitate end resection and HR. Significantly, a cancer-associated mutant with impaired CtIP interaction, but not dNTPase-inactive SAMHD1, fails to rescue the end resection impairment of SAMHD1 depletion. Our findings define a dNTPase-independent function for SAMHD1 in HR-mediated DSB repair by facilitating CtIP accrual to promote DNA end resection, providing insight into how SAMHD1 promotes genome integrity.

The SIRT2 Deacetylase Stabilizes Slug to Control Malignancy of Basal-like Breast Cancer

Overabundance of Slug protein is common in human cancer and represents an important determinant underlying the aggressiveness of basal-like breast cancer (BLBC). Despite its importance, this transcription factor is rarely mutated in BLBC, and the mechanism of its deregulation in cancer remains unknown. Here, we report that Slug undergoes acetylation-dependent protein degradation and identify the deacetylase SIRT2 as a key mediator of this post-translational mechanism. SIRT2 inhibition rapidly destabilizes Slug, whereas SIRT2 overexpression extends Slug stability. We show that SIRT2 deacetylates Slug protein at lysine residue K116 to prevent Slug degradation. Interestingly, SIRT2 is frequently amplified and highly expressed in BLBC. Genetic depletion and pharmacological inactivation of SIRT2 in BLBC cells reverse Slug stabilization, cause the loss of clinically relevant pathological features of BLBC, and inhibit tumor growth. Our results suggest that targeting SIRT2 may be a rational strategy for diminishing Slug abundance and its associated malignant traits in BLBC.

The essential kinase ATR: ensuring faithful duplication of a challenging genome

One way to preserve a rare book is to lock it away from all potential sources of damage. Of course, an inaccessible book is also of little use, and the paper and ink will continue to degrade with age in any case. Like a book, the information stored in our DNA needs to be read, but it is also subject to continuous assault and therefore needs to be protected. In this Review, we examine how the replication stress response that is controlled by the kinase ataxia telangiectasia and Rad3-related (ATR) senses and resolves threats to DNA integrity so that the DNA remains available to read in all of our cells. We discuss the multiple data that have revealed an elegant yet increasingly complex mechanism of ATR activation. This involves a core set of components that recruit ATR to stressed replication forks, stimulate kinase activity and amplify ATR signalling. We focus on the activities of ATR in the control of cell cycle checkpoints, origin firing and replication fork stability, and on how proper regulation of these processes is crucial to ensure faithful duplication of a challenging genome.

Role of specialized DNA polymerases in the limitation of replicative stress and DNA damage transmission

Replication stress is a strong and early driving force for genomic instability and tumor development. Beside replicative DNA polymerases, an emerging group of specialized DNA polymerases is involved in the technical assistance of the replication machinery in order to prevent replicative stress and its deleterious consequences. During S-phase, altered progression of the replication fork by endogenous or exogenous impediments induces replicative stress, causing cells to reach mitosis with genomic regions not fully duplicated. Recently, specific mechanisms to resolve replication intermediates during mitosis with the aim of limiting DNA damage transmission to daughter cells have been identified. In this review, we detail the two major actions of specialized DNA polymerases that limit DNA damage transmission: the prevention of replicative stress by non-B DNA replication and the recovery of stalled replication forks.

Sirtuin 2 mutations in human cancers impair its function in genome maintenance

mutations in genome maintenance and tumor suppression.

Sirtuins and DNA damage repair: SIRT7 comes to play

Aging is characterized by a cumulative loss of genome integrity, which involves chromatin reorganization, transcriptional dysregulation and the accumulation of DNA damage. Sirtuins participate in the protection against these aging processes by promoting genome homeostasis in response to cellular stress. We recently reported that SirT7^−/− mice suffer from partial embryonic lethality and a progeroid like phenotype. At the cellular level, SIRT7 depletion results in the impaired repair of DNA double-strand breaks (DSBs), one the most dangerous DNA lesions, leading to genome instability. SIRT7 is recruited to DSBs, where it specifically deacetylates histone H3 at lysine 18 and affects the focal accumulation of the DNA damage response factor 53BP1, thus influencing the efficiency of repair. Here, we integrate our findings with the current knowledge on the mode of action of other sirtuin family members in DNA repair. We emphasize their capacity to regulate chromatin structure as a response to DNA damage within the constraints imposed by cellular status.

The Current State of NAD+ -Dependent Histone Deacetylases (Sirtuins) as Novel Therapeutic Targets

Sirtuins are NAD⁺ -dependent protein deacylases that cleave off acetyl, as well as other acyl groups, from the ε-amino group of lysines in histones and other substrate proteins. Seven sirtuin isotypes (Sirt1-7) have been identified in mammalian cells. As sirtuins are involved in the regulation of various physiological processes such as cell survival, cell cycle progression, apoptosis, DNA repair, cell metabolism, and caloric restriction, a dysregulation of their enzymatic activity has been associated with the pathogenesis of neoplastic, metabolic, infectious, and neurodegenerative diseases. Thus, sirtuins are promising targets for pharmaceutical intervention. Growing interest in a modulation of sirtuin activity has prompted the discovery of several small molecules, able to inhibit or activate certain sirtuin isotypes. Herein, we give an update to our previous review on the topic in this journal (Schemies, 2010), focusing on recent developments in sirtuin biology, sirtuin modulators, and their potential as novel therapeutic agents.

Sirtuins in metabolism, DNA repair and cancer

The mammalian sirtuin family has attracted tremendous attention over the past few years as stress adaptors and post-translational modifier. They have involved in diverse cellular processes including DNA repair, energy metabolism, and tumorigenesis. Notably, genomic instability and metabolic reprogramming are two of characteristic hallmarks in cancer. In this review, we summarize current knowledge on the functions of sirtuins mainly regarding DNA repair and energy metabolism, and further discuss the implication of sirtuins in cancer specifically by regulating genome integrity and cancer-related metabolism.

SIRT2 regulates nuclear envelope reassembly through ANKLE2 deacetylation

Sirtuin 2 (SIRT2) is an NAD-dependent deacetylase known to regulate microtubule dynamics and cell cycle progression. SIRT2 has also been implicated in the pathology of cancer, neurodegenerative diseases and progeria. Here, we show that SIRT2 depletion or overexpression causes nuclear envelope reassembly defects. We link this phenotype to the recently identified regulator of nuclear envelope reassembly ANKLE2. ANKLE2 acetylation at K302 and phosphorylation at S662 are dynamically regulated throughout the cell cycle by SIRT2 and are essential for normal nuclear envelope reassembly. The function of SIRT2 therefore extends beyond the regulation of microtubules to include the regulation of nuclear envelope dynamics.