SIRT1 deacetylates TopBP1 and modulates intra-S-phase checkpoint and DNA replication origin firing

A new role for human papillomavirus 16 E2: Mitotic activation of the DNA damage response to promote viral genome segregation

Human papillomaviruses (HPV) are causative agents in around 5% of all human cancers. To identify and develop new targeted HPV therapeutics we must enhance our understanding of the viral life cycle and how it interacts with the host. The HPV E2 protein dimerizes and binds to 12bp target sequences in the viral genome and segregates the viral genome during mitosis. In this function, E2 binds to the viral genome and the host chromatin simultaneously, ensuring viral genomes reside in daughter nuclei following cell division. We have demonstrated that a mitotic interaction between E2 and the DNA damage response (DDR) protein TOPBP1 is required for E2 segregation function. In non-infected cells, following DNA damage, TOPBP1 is recruited to the mitotic host genome via interaction with MDC1 and this interaction protects DNA integrity during mitosis. Recently we demonstrated that the E2-TOPBP1 interaction activates the DNA damage response (DDR) during mitosis independently from external stimuli, promoting TOPBP1 interaction with mitotic chromatin and therefore segregation of the viral genome. Therefore, the virus has hijacked an existing host mechanism in order to segregate the viral genome. This intricate E2 function will be described and discussed.

A human papillomavirus 16 E2-TopBP1 dependent SIRT1-p300 acetylation switch regulates mitotic viral and human protein levels and activates the DNA damage response

An interaction between human papillomavirus 16 (HPV16) E2 and the cellular proteins TopBP1 and BRD4 is required for E2 plasmid segregation function. The E2-TopBP1 interaction promotes increased mitotic E2 protein levels in U2OS and N/Tert-1 cells, as well as in human foreskin keratinocytes immortalized by HPV16 (HFK + HPV16). SIRT1 deacetylation reduces E2 protein stability and here we demonstrate that increased E2 acetylation occurs during mitosis in a TopBP1 interacting-dependent manner, promoting E2 mitotic stabilization. p300 mediates E2 acetylation and acetylation is increased due to E2 switching off SIRT1 function during mitosis in a TopBP1 interacting-dependent manner, confirmed by increased p53 stability and acetylation on lysine 382, a known target for SIRT1 deacetylation. SIRT1 can complex with E2 in growing cells but is unable to do so during mitosis due to the E2-TopBP1 interaction; SIRT1 is also unable to complex with p53 in mitotic E2 wild-type cells but can complex with p53 outside of mitosis. E2 lysines 111 and 112 are highly conserved residues across all E2 proteins and we demonstrate that K111 hyper-acetylation occurs during mitosis, promoting E2 interaction with Topoisomerase 1 (Top1). We demonstrate that K112 ubiquitination promotes E2 proteasomal degradation during mitosis. E2-TopBP1 interaction promotes mitotic acetylation of CHK2, promoting phosphorylation and activation of the DNA damage response (DDR). The results present a new model in which the E2-TopBP1 complex inactivates SIRT1 during mitosis, and activates the DDR. This is a novel mechanism of HPV16 activation of the DDR, a requirement for the viral life cycle.

IMPORTANCE

Human papillomaviruses (HPVs) are causative agents in around 5% of all human cancers. While there are prophylactic vaccines that will significantly alleviate HPV disease burden on future generations, there are currently no anti-viral strategies available for the treatment of HPV cancers. To generate such reagents, we must understand more about the HPV life cycle, and in particular about viral-host interactions. Here, we describe a novel mitotic complex generated by the HPV16 E2 protein interacting with the host protein TopBP1 that controls the function of the deacetylase SIRT1. The E2-TopBP1 interaction disrupts SIRT1 function during mitosis in order to enhance acetylation and stability of viral and host proteins. We also demonstrate that the E2-TopBP1 interaction activates the DDR. This novel complex is essential for the HPV16 life cycle and represents a novel anti-viral therapeutic target.

A human papillomavirus 16 E2-TopBP1 dependent SIRT1-p300 acetylation switch regulates mitotic viral and human protein levels

An interaction between human papillomavirus 16 (HPV16) E2 and the cellular proteins TopBP1 and BRD4 is required for E2 plasmid segregation function. The E2-TopBP1 interaction promotes increased mitotic E2 protein levels in U2OS and N/Tert-1 cells, as well as in human foreskin keratinocytes immortalized by HPV16 (HFK+HPV16). SIRT1 deacetylation reduces E2 protein stability and here we demonstrate that increased E2 acetylation occurs during mitosis in a TopBP1 interacting dependent manner, promoting E2 mitotic stabilization. p300 mediates E2 acetylation and acetylation is increased due to E2 switching off SIRT1 function during mitosis in a TopBP1 interacting dependent manner, confirmed by increased p53 stability and acetylation on lysine 382, a known target for SIRT1 deacetylation. SIRT1 can complex with E2 in growing cells but is unable to do so during mitosis due to the E2-TopBP1 interaction; SIRT1 is also unable to complex with p53 in mitotic E2 wild type cells but can complex with p53 outside of mitosis. E2 lysines 111 and 112 are highly conserved residues across all E2 proteins and we demonstrate that K111 hyper-acetylation occurs during mitosis, promoting E2 interaction with Topoisomerase 1 (Top1). We also demonstrate that K112 ubiquitination promotes E2 proteasomal degradation during mitosis. The results present a model in which the E2-TopBP1 complex inactivates SIRT1 during mitosis and E2 acetylation on K111 by p300 increases, promoting interaction with Top1 that protects K112 from ubiquitination and therefore E2 proteasomal degradation.

Importance

Human papillomaviruses are causative agents in around 5% of all human cancers. While there are prophylactic vaccines that will significantly alleviate HPV disease burden on future generations, there are currently no anti-viral strategies available for the treatment of HPV cancers. To generate such reagents, we must understand more about the HPV life cycle, and in particular about viral-host interactions. Here we describe a novel mitotic complex generated by the HPV16 E2 protein interacting with the host protein TopBP1 that controls the function of the deacetylase SIRT1. The E2-TopBP1 interaction disrupts SIRT1 function during mitosis in order to enhance acetylation and stability of viral and host proteins. This novel complex is essential for the HPV16 life cycle and represents a novel anti-viral therapeutic target.

In-depth urinary and exosome proteome profiling analysis identifies novel biomarkers for diabetic kidney disease

Diabetic kidney disease (DKD) is a major microvascular complication of type 2 diabetes mellitus (T2DM). Monitoring the early diagnostic period and disease progression plays a crucial role in treating DKD. In this study, to comprehensively elucidate the molecular characteristics of urinary proteins and urinary exosome proteins in type 2 DKD, we performed large-scale urinary proteomics (n=144) and urinary exosome proteomics (n=44) analyses on T2DM patients with albuminuria in varying degrees. The dynamics analysis of the urinary and exosome proteomes in our study provides a valuable resource for discovering potential urinary biomarkers in patients with DKD. A series of potential biomarkers, such as SERPINA1 and transferrin (TF), were detected and validated to be used for DKD diagnosis or disease monitoring. The results of our study comprehensively elucidated the changes in the urinary proteome and revealed several potential biomarkers reflecting the progression of DKD, which provide a reference for DKD biomarker screening.

SIRT1 regulates DNA damage signaling through the PP4 phosphatase complex

The Sirtuin family of NAD+-dependent enzymes plays an important role in maintaining genome stability upon stress. Several mammalian Sirtuins have been linked directly or indirectly to the regulation of DNA damage during replication through Homologous recombination (HR). The role of one of them, SIRT1, is intriguing as it seems to have a general regulatory role in the DNA damage response (DDR) that has not yet been addressed. SIRT1-deficient cells show impaired DDR reflected in a decrease in repair capacity, increased genome instability and decreased levels of γH2AX. Here we unveil a close functional antagonism between SIRT1 and the PP4 phosphatase multiprotein complex in the regulation of the DDR. Upon DNA damage, SIRT1 interacts specifically with the catalytical subunit PP4c and promotes its inhibition by deacetylating the WH1 domain of the regulatory subunits PP4R3α/β. This in turn regulates γH2AX and RPA2 phosphorylation, two key events in the signaling of DNA damage and repair by HR. We propose a mechanism whereby during stress, SIRT1 signaling ensures a global control of DNA damage signaling through PP4.

Human Papillomavirus 16 E2 Interaction with TopBP1 Is Required for E2 and Viral Genome Stability during the Viral Life Cycle

CK2 phosphorylation of HPV16 E2 at serine 23 promotes interaction with TopBP1, and this interaction is important for E2 plasmid segregation function. Here, we demonstrate that the E2-TopBP1 interaction is critical for E2 and viral genome stability during the viral life cycle. Introduction of the S23A mutation into the HPV16 genome results in a loss of E2 expression and viral genome integration during organotypic rafting. Coculture of N/Tert-1+E2-S23A cells with J2 fibroblasts results in E2-S23A degradation via the proteasome; wild-type E2 is not degraded. TopBP1 siRNA treatment of N/Tert-1+E2-WT cells results in E2 degradation only in the presence of J2 cells demonstrating the critical role for TopBP1 in maintaining E2 stability. The CK2 inhibitor CX4945 promotes E2-WT degradation in the presence of fibroblasts as it disrupts E2-TopBP1 interaction. siRNA targeting SIRT1 rescues E2-S23A stability in N/Tert-1 cells treated with J2 fibroblasts, with an increased E2-S23A acetylation. The results demonstrate that the E2-TopBP1 interaction is critical during the viral life cycle as it prevents fibroblast stimulated SIRT1 mediated deacetylation of E2 that promotes protein degradation. This means that the E2-TopBP1 complex maintains E2 and viral genome stability and that disruption of this complex can promote viral genome integration. Finally, we demonstrate that HPV11 E2 also interacts with TopBP1 and that this interaction is critical for HPV11 E2 stability in the presence of J2 cells. Treatment of N/Tert-1 + 11E2-WT cells with CX4945 results in 11E2 degradation. Therefore, CK2 inhibition is a therapeutic strategy for alleviating HPV11 diseases, including juvenile respiratory papillomatosis. IMPORTANCE Human papillomaviruses are pathogens that cause a host of diseases ranging from benign warts to cancers. There are no therapeutics available for combating these diseases that directly target viral proteins or processes; therefore, we must enhance our understanding of HPV life cycles to assist with identifying novel treatments. In this report, we demonstrate that HPV16 and HPV11 E2 protein expression is dependent upon TopBP1 interaction in keratinocytes interacting with fibroblasts, which recapitulate stromal interactions in culture. The degradation of 16E2 promotes HPV16 genome integration; therefore, the E2-TopBP1 interaction is critical during the viral life cycle. We demonstrate that the CK2 inhibitor CX4945 disrupts HPV11 interaction with TopBP1 and destabilizes HPV11 E2 protein in the presence of J2 fibroblasts; we propose that CX4945 could alleviate HPV11 disease burden.

Human papillomavirus 16 E2 interaction with TopBP1 is required for E2 and viral genome stability during the viral life cycle

CK2 phosphorylation of HPV16 E2 at serine 23 promotes interaction with TopBP1, and this interaction is important for E2 plasmid segregation function. Here we demonstrate that the E2-TopBP1 interaction is critical for E2 and viral genome stability during the viral life cycle. Introduction of the S23A mutation into the HPV16 genome results in a loss of E2 expression and viral genome integration during organotypic rafting. Co-culture of N/Tert-1+E2-S23A cells with J2 fibroblasts results in E2-S23A degradation via the proteasome, wild-type E2 is not degraded. TopBP1 siRNA treatment of N/Tert-1+E2-WT cells results in E2 degradation only in the presence of J2 cells demonstrating the critical role for TopBP1 in maintaining E2 stability. The CK2 inhibitor CX4945 promotes E2-WT degradation in the presence of fibroblasts as it disrupts E2-TopBP1 interaction. siRNA targeting SIRT1 rescues E2-S23A stability in N/Tert-1 cells treated with J2 fibroblasts, with an increased E2-S23A acetylation. The results demonstrate that the E2-TopBP1 interaction is critical during the viral life cycle as it prevents fibroblast stimulated SIRT1 mediated deacetylation of E2 that promotes protein degradation. This means that the E2-TopBP1 complex maintains E2 and viral genome stability and that disruption of this complex can promote viral genome integration. Finally, we demonstrate that HPV11 E2 also interacts with TopBP1 and that this interaction is critical for HPV11 E2 stability in the presence of J2 cells. Treatment of N/Tert-1+11E2-WT cells with CX4945 results in 11E2 degradation. Therefore, CK2 inhibition is a therapeutic strategy for alleviating HPV11 diseases, including juvenile respiratory papillomatosis.

Importance

Human papillomaviruses are pathogens that cause a host of diseases ranging from benign warts to cancers. There are no therapeutics available for combating these diseases that directly target viral proteins or processes, therefore we must enhance our understanding of HPV life cycles to assist with identifying novel treatments. In this report, we demonstrate that HPV16 and HPV11 E2 protein expression is dependent upon TopBP1 interaction in keratinocytes interacting with fibroblasts, which recapitulate stromal interactions in culture. The degradation of 16E2 promotes HPV16 genome integration, therefore the E2-TopBP1 interaction is critical during the viral life cycle. We demonstrate that the CK2 inhibitor CX4945 disrupts HPV11 interaction with TopBP1 and destabilizes HPV11 E2 protein in the presence of J2 fibroblasts; we propose that CX4945 could alleviate HPV11 disease burden.

p300 arrests intervertebral disc degeneration by regulating the FOXO3/Sirt1/Wnt/β-catenin axis

The transcription factor p300 is reportedly involved in age-associated human diseases, including intervertebral disc degeneration (IDD). In this study, we investigate the potential role and pathophysiological mechanism of p300 in IDD. Clinical tissue samples were collected from patients with lumbar disc herniation (LDH), in which the expression of p300, forkhead box O3 (FOXO3), and sirtuin 1 (Sirt1) was determined. Nucleus pulposus cells (NPCs) isolated from clinical degenerative intervertebral disc (IVD) tissues were introduced with oe-p300, oe-FOXO3, Wnt/β-catenin agonist 1, C646 (p300/CBP inhibitor), or si-p300 to explore the functional role of p300 in IDD and to characterize the relationship between p300 and the FOXO3/Sirt1/Wnt/β-catenin pathway. Also, we established a rat IDD model by inducing needle puncture injuries in the caudal IVDs for further verification of p300 functional role. We found that p300 was downregulated in the clinical tissues and NPCs of IDD. Overexpression of p300 promoted the proliferation and autophagy of NPCs while inhibiting cell apoptosis, which was associated with FOXO3 upregulation. p300 could increase the expression of FOXO3 by binding to the Sirt1 promoter, and thus, contributed to inactivation of the Wnt/β-catenin pathway. In vivo results further displayed that p300 slowed down the progression of IDD by disrupting the Wnt/β-catenin pathway through the FOXO3/Sirt1 axis. Taken together, we suggest that p300 can act to suppress IDD via a FOXO3-dependent mechanism, highlighting a potential novel target for treatment of IDD.

Convergence of SIRT1 and ATR signaling to modulate replication origin dormancy

During routine genome duplication, many potential replication origins remain inactive or 'dormant'. Such origin dormancy is achieved, in part, by an interaction with the metabolic sensor SIRT1 deacetylase. We report here that dormant origins are a group of consistent, pre-determined genomic sequences that are distinguished from baseline (i.e. ordinarily active) origins by their preferential association with two phospho-isoforms of the helicase component MCM2. During normal unperturbed cell growth, baseline origins, but not dormant origins, associate with a form of MCM2 that is phosphorylated by DBF4-dependent kinase (DDK) on serine 139 (pS139-MCM2). This association facilitates the initiation of DNA replication from baseline origins. Concomitantly, SIRT1 inhibits Ataxia Telangiectasia and Rad3-related (ATR)-kinase-mediated phosphorylation of MCM2 on serine 108 (pS108-MCM2) by deacetylating the ATR-interacting protein DNA topoisomerase II binding protein 1 (TOPBP1), thereby preventing ATR recruitment to chromatin. In cells devoid of SIRT1 activity, or challenged by replication stress, this inhibition is circumvented, enabling ATR-mediated S108-MCM2 phosphorylation. In turn, pS108-MCM2 enables DDK-mediated phosphorylation on S139-MCM2 and facilitates replication initiation at dormant origins. These observations suggest that replication origin dormancy and activation are regulated by distinct post-translational MCM modifications that reflect a balance between SIRT1 activity and ATR signaling.

Preventing excess replication origin activation to ensure genome stability

Cells activate distinctive regulatory pathways that prevent excessive initiation of DNA replication to achieve timely and accurate genome duplication. Excess DNA synthesis is constrained by protein-DNA interactions that inhibit initiation at dormant origins. In parallel, specific modifications of pre-replication complexes prohibit post-replicative origin relicensing. Replication stress ensues when the controls that prevent excess replication are missing in cancer cells, which often harbor extrachromosomal DNA that can be further amplified by recombination-mediated processes to generate chromosomal translocations. The genomic instability that accompanies excess replication origin activation can provide a promising target for therapeutic intervention. Here we review molecular pathways that modulate replication origin dormancy, prevent excess origin activation, and detect, encapsulate, and eliminate persistent excess DNA.

CK2 Phosphorylation of Human Papillomavirus 16 E2 on Serine 23 Promotes Interaction with TopBP1 and Is Critical for E2 Interaction with Mitotic Chromatin and the Viral Life Cycle

During the human papillomavirus 16 (HPV16) life cycle, the E2 protein interacts with host factors to regulate viral transcription, replication, and genome segregation/retention. Our understanding of host partner proteins and their roles in E2 functions remains incomplete. Here we demonstrate that CK2 phosphorylation of E2 on serine 23 promotes interaction with TopBP1 in vitro and in vivo and that E2 is phosphorylated on this residue during the HPV16 life cycle. We investigated the consequences of mutating serine 23 on E2 functions. E2-S23A (E2 with serine 23 mutated to alanine) activates and represses transcription identically to E2-WT (wild-type E2), and E2-S23A is as efficient as E2-WT in transient replication assays. However, E2-S23A has compromised interaction with mitotic chromatin compared with E2-WT. In E2-WT cells, both E2 and TopBP1 levels increase during mitosis compared with vector control cells. In E2-S23A cells, neither E2 nor TopBP1 levels increase during mitosis. Introduction of the S23A mutation into the HPV16 genome resulted in delayed immortalization of human foreskin keratinocytes (HFK) and higher episomal viral genome copy number in resulting established HFK. Remarkably, S23A cells had a disrupted viral life cycle in organotypic raft cultures, with a loss of E2 expression and a failure of viral replication. Overall, our results demonstrate that CK2 phosphorylation of E2 on serine 23 promotes interaction with TopBP1 and that this interaction is critical for the viral life cycle. IMPORTANCE Human papillomaviruses are causative agents in around 5% of all cancers, with no specific antiviral therapeutics available for treating infections or resultant cancers. In this report, we demonstrate that phosphorylation of HPV16 E2 by CK2 promotes formation of a complex with the cellular protein TopBP1 in vitro and in vivo. This complex results in stabilization of E2 during mitosis. We demonstrate that CK2 phosphorylates E2 on serine 23 in vivo and that CK2 inhibitors disrupt the E2-TopBP1 complex. Mutation of E2 serine 23 to alanine disrupts the HPV16 life cycle, hindering immortalization and disrupting the viral life cycle, demonstrating a critical function for this residue.

PHF8-promoted TOPBP1 demethylation drives ATR activation and preserves genome stability

The checkpoint kinase ATR [ATM (ataxia-telangiectasia mutated) and rad3-related] is a master regulator of DNA damage response. Yet, how ATR activity is regulated remains to be investigated. We report here that histone demethylase PHF8 (plant homeodomain finger protein 8) plays a key role in ATR activation and replication stress response. Mechanistically, PHF8 interacts with and demethylates TOPBP1 (DNA topoisomerase 2-binding protein 1), an essential allosteric activator of ATR, under unperturbed conditions, but replication stress results in PHF8 phosphorylation and dissociation from TOPBP1. Consequently, hypomethylated TOPBP1 facilitates RAD9 (RADiation sensitive 9) binding and chromatin loading of the TOPBP1-RAD9 complex to fully activate ATR and thus safeguard the genome and protect cells against replication stress. Our study uncovers a demethylation and phosphorylation code that controls the assembly of TOPBP1-scaffolded protein complex, and provides molecular insight into non-histone methylation switch in ATR activation.

Proteome dynamics at broken replication forks reveal a distinct ATM-directed repair response suppressing DNA double-strand break ubiquitination

Cells have evolved an elaborate DNA repair network to ensure complete and accurate DNA replication. Defects in these repair machineries can fuel genome instability and drive carcinogenesis while creating vulnerabilities that may be exploited in therapy. Here, we use nascent chromatin capture (NCC) proteomics to characterize the repair of replication-associated DNA double-strand breaks (DSBs) triggered by topoisomerase 1 (TOP1) inhibitors. We reveal profound changes in the fork proteome, including the chromatin environment and nuclear membrane interactions, and identify three classes of repair factors according to their enrichment at broken and/or stalled forks. ATM inhibition dramatically rewired the broken fork proteome, revealing that ataxia telangiectasia mutated (ATM) signalling stimulates DNA end resection, recruits PLK1, and concomitantly suppresses the canonical DSB ubiquitination response by preventing accumulation of RNF168 and BRCA1-A. This work and collection of replication fork proteomes provide a new framework to understand how cells orchestrate homologous recombination repair of replication-associated DSBs.

Using a Human Papillomavirus Model to Study DNA Replication and Repair of Wild Type and Damaged DNA Templates in Mammalian Cells

Human papillomaviruses have 8kbp DNA episomal genomes that replicate autonomously from host DNA. During initial infection, the virus increases its copy number to 20-50 copies per cell, causing torsional stress on the replicating DNA. This activates the DNA damage response (DDR) and HPV replicates its genome, at least in part, using homologous recombination. An active DDR is on throughout the HPV life cycle. Two viral proteins are required for replication of the viral genome; E2 binds to 12bp palindromic sequences around the A/T rich origin of replication and recruits the viral helicase E1 via a protein-protein interaction. E1 forms a di-hexameric complex that replicates the viral genome in association with host factors. Transient replication assays following transfection with E1-E2 expression plasmids, along with an origin containing plasmid, allow monitoring of E1-E2 replication activity. Incorporating a bacterial lacZ gene into the origin plasmid allows for the determination of replication fidelity. Here we describe how we exploited this system to investigate replication and repair in mammalian cells, including using damaged DNA templates. We propose that this system has the potential to enhance the understanding of cellular components involved in DNA replication and repair.

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.

Extra View: Sirt1 Acts As A Gatekeeper Of Replication Initiation To Preserve Genomic Stability

Since the discovery of a yeast gene silencing modifier (Silent Information Modifier 2, SIR2) and its role in maintaining genomic stability more than two decades ago, SIR2 homologs (sirtuins) were identified in diverse species. Sirtuins are protein deacetylases that play diverse roles in proper cellular metabolism including cell cycle progression and maintenance of genomic stability. In yeast, SIR2 interacts with replication origins and protein complexes that affect both replication origin usage and gene silencing. In metazoans, the largest SIR2 homolog, SIRT1, is implicated in epigenetic modifications, circadian signaling, DNA recombination and DNA repair. Until recently, very few studies investigated the role of mammalian SIRT1 in modulating DNA replication. We discuss a newly characterized interaction between human SIRT1 and the DNA replication machinery, reviewing data from recent studies that have investigated how complex signaling pathways that involve SIRT1 affect cellular growth regulatory circuits.

SIRT1 modulates cell cycle progression by regulating CHK2 acetylation-phosphorylation

Both the stress-response protein, SIRT1, and the cell cycle checkpoint kinase, CHK2, play critical roles in aging and cancer via the modulation of cellular homeostasis and the maintenance of genomic integrity. However, the underlying mechanism linking the two pathways remains elusive. Here, we show that SIRT1 functions as a modifier of CHK2 in cell cycle control. Specifically, SIRT1 interacts with CHK2 and deacetylates it at lysine 520 residue, which suppresses CHK2 phosphorylation, dimerization, and thus activation. SIRT1 depletion induces CHK2 hyperactivation-mediated cell cycle arrest and subsequent cell death. In vivo, genetic deletion of Chk2 rescues the neonatal lethality of Sirt1^−/− mice, consistent with the role of SIRT1 in preventing CHK2 hyperactivation. Together, these results suggest that CHK2 mediates the function of SIRT1 in cell cycle progression, and may provide new insights into modulating cellular homeostasis and maintaining genomic integrity in the prevention of aging and cancer.

Nuclear transport of nicotinamide phosphoribosyltransferase is cell cycle-dependent in mammalian cells, and its inhibition slows cell growth

Nicotinamide phosphoribosyltransferase (NAMPT) is located in both the nucleus and cytoplasm and has multiple biological functions including catalyzing the rate-limiting step in NAD synthesis. Moreover, up-regulated NAMPT expression has been observed in many cancers. However, the determinants and regulation of NAMPT's nuclear transport are not known. Here, we constructed a GFP-NAMPT fusion protein to study NAMPT's subcellular trafficking. We observed that in unsynchronized 3T3-L1 preadipocytes, 25% of cells had higher GFP-NAMPT fluorescence in the cytoplasm, and 62% had higher GFP-NAMPT fluorescence in the nucleus. In HepG2 hepatocytes, 6% of cells had higher GFP-NAMPT fluorescence in the cytoplasm, and 84% had higher GFP-NAMPT fluorescence in the nucleus. In both 3T3-L1 and HepG2 cells, GFP-NAMPT was excluded from the nucleus immediately after mitosis and migrated back into it as the cell cycle progressed. In HepG2 cells, endogenous, untagged NAMPT displayed similar changes with the cell cycle, and in nonmitotic cells, GFP-NAMPT accumulated in the nucleus. Similarly, genotoxic, oxidative, or dicarbonyl stress also caused nuclear NAMPT localization. These interventions also increased poly(ADP-ribosyl) polymerase and sirtuin activity, suggesting an increased cellular demand for NAD. We identified a nuclear localization signal in NAMPT and amino acid substitution in this sequence (⁴²⁴RSKK to ASGA), which did not affect its enzymatic activity, blocked nuclear NAMPT transport, slowed cell growth, and increased histone H₃ acetylation. These results suggest that NAMPT is transported into the nucleus where it presumably increases NAD synthesis required for cell proliferation. We conclude that specific inhibition of NAMPT transport into the nucleus might be a potential avenue for managing cancer.

Werner Helicase Control of Human Papillomavirus 16 E1-E2 DNA Replication Is Regulated by SIRT1 Deacetylation

Human papillomaviruses (HPV) are double-stranded DNA viruses causative in a host of human diseases, including several cancers. Following infection, two viral proteins, E1 and E2, activate viral replication in association with cellular factors and stimulate the DNA damage response (DDR) during the replication process. E1-E2 uses homologous recombination (HR) to facilitate DNA replication, but an understanding of host factors involved in this process remains incomplete. Previously, we demonstrated that the class III deacetylase SIRT1, which can regulate HR, is recruited to E1-E2-replicating DNA and regulates the level of replication. Here, we demonstrate that SIRT1 promotes the fidelity of E1-E2 replication and that the absence of SIRT1 results in reduced recruitment of the DNA repair protein Werner helicase (WRN) to E1-E2-replicating DNA. CRISPR/Cas9 editing demonstrates that WRN, like SIRT1, regulates the quantity and fidelity of E1-E2 replication. This is the first report of WRN regulation of E1-E2 DNA replication, or a role for WRN in the HPV life cycle. In the absence of SIRT1 there is an increased acetylation and stability of WRN, but a reduced ability to interact with E1-E2-replicating DNA. We present a model in which E1-E2 replication turns on the DDR, stimulating SIRT1 deacetylation of WRN. This deacetylation promotes WRN interaction with E1-E2-replicating DNA to control the quantity and fidelity of replication. As well as offering a crucial insight into HPV replication control, this system offers a unique model for investigating the link between SIRT1 and WRN in controlling replication in mammalian cells.IMPORTANCE HPV16 is the major viral human carcinogen responsible for between 3 and 4% of all cancers worldwide. Following infection, this virus activates the DNA damage response (DDR) to promote its life cycle and recruits DDR proteins to its replicating DNA in order to facilitate homologous recombination during replication. This promotes the production of viable viral progeny. Our understanding of how HPV16 replication interacts with the DDR remains incomplete. Here, we demonstrate that the cellular deacetylase SIRT1, which is a part of the E1-E2 replication complex, regulates recruitment of the DNA repair protein WRN to the replicating DNA. We demonstrate that WRN regulates the level and fidelity of E1-E2 replication. Overall, the results suggest a mechanism by which SIRT1 deacetylation of WRN promotes its interaction with E1-E2-replicating DNA to control the levels and fidelity of that replication.

The Cdk8/19-cyclin C transcription regulator functions in genome replication through metazoan Sld7

Accurate genome duplication underlies genetic homeostasis. Metazoan Mdm2 binding protein (MTBP) forms a main regulatory platform for origin firing together with Treslin/TICRR and TopBP1 (Topoisomerase II binding protein 1 (TopBP1)-interacting replication stimulating protein/TopBP1-interacting checkpoint and replication regulator). We report the first comprehensive analysis of MTBP and reveal conserved and metazoa-specific MTBP functions in replication. This suggests that metazoa have evolved specific molecular mechanisms to adapt replication principles conserved with yeast to the specific requirements of the more complex metazoan cells. We uncover one such metazoa-specific process: a new replication factor, cyclin-dependent kinase 8/19-cyclinC (Cdk8/19-cyclin C), binds to a central domain of MTBP. This interaction is required for complete genome duplication in human cells. In the absence of MTBP binding to Cdk8/19-cyclin C, cells enter mitosis with incompletely duplicated chromosomes, and subsequent chromosome segregation occurs inaccurately. Using remote homology searches, we identified MTBP as the metazoan orthologue of yeast synthetic lethal with Dpb11 7 (Sld7). This homology finally demonstrates that the set of yeast core factors sufficient for replication initiation in vitro is conserved in metazoa. MTBP and Sld7 contain two homologous domains that are present in no other protein, one each in the N and C termini. In MTBP the conserved termini flank the metazoa-specific Cdk8/19-cyclin C binding region and are required for normal origin firing in human cells. The N termini of MTBP and Sld7 share an essential origin firing function, the interaction with Treslin/TICRR or its yeast orthologue Sld3, respectively. The C termini may function as homodimerisation domains. Our characterisation of broadly conserved and metazoa-specific initiation processes sets the basis for further mechanistic dissection of replication initiation in vertebrates. It is a first step in understanding the distinctions of origin firing in higher eukaryotes.

Regulatory Effects of NAD+ Metabolic Pathways on Sirtuin Activity

NAD⁺ acts as a crucial regulator of cell physiology and as an integral participant in cellular metabolism. By virtue of a variety of signaling activities this central metabolite can exert profound effects on organism health status. Thus, while it serves as a well-known metabolic cofactor functioning as a redox-active substrate, it can also function as a substrate for signaling enzymes, such as sirtuins, poly (ADP-ribosyl) polymerases, mono (ADP-ribosyl) transferases, and CD38. Sirtuins function as NAD⁺-dependent protein deacetylases (deacylases) and catalyze the reaction of NAD⁺ with acyllysine groups to remove the acyl modification from substrate proteins. This deacetylation provides a regulatory function and integrates cellular NAD⁺ metabolism into a large spectrum of cellular processes and outcomes, such as cell metabolism, cell survival, cell cycle, apoptosis, DNA repair, mitochondrial homeostasis and mitochondrial biogenesis, and even lifespan. Increased attention to how regulated and pharmacologic changes in NAD⁺ concentrations can impact sirtuin activities has motivated openings of new areas of research, including investigations of how NAD⁺ levels are regulated at the subcellular level, and searches for more potent NAD⁺ precursors typified by nicotinamide riboside (NR) and nicotinamide mononucleotide (NMN). This review describes current results and thinking of how NAD⁺ metabolic pathways regulate sirtuin activities and how regulated NAD⁺ levels can impact cell physiology. In addition, NAD⁺ precursors are discussed, with attention to how these might be harnessed to generate novel therapeutic options to treat the diseases of aging.

Role of Sirtuin1-p53 regulatory axis in aging, cancer and cellular reprogramming

Regulatory role of Sirtuin 1 (SIRT1), one of the most extensively studied members of its kind in histone deacetylase family in governing multiple cellular fates, is predominantly linked to p53 activity. SIRT1 deacetylates p53 in a NAD+-dependent manner to inhibit transcription activity of p53, in turn modulate pathways that are implicated in regulation of tissue homoeostasis and many disease states. In this review, we discuss the role of SIRT1-p53 pathway and its regulatory axis in the cellular events which are implicated in cellular aging, cancer and reprogramming. It is noteworthy that these cellular events share few common regulatory pathways, including SIRT1-p53-LDHA-Myc, miR-34a,-Let7 regulatory network, which forms a positive feedback loop that controls cell cycle, metabolism, proliferation, differentiation, epigenetics and many others. In the context of aging, SIRT1 expression is reduced as a protective mechanism against oncogenesis and for maintenance of tissue homeostasis. Interestingly, its activation in aged cells is evidenced in response to DNA damage to protect the cells from p53-dependent apoptosis or senescence, predispose these cells to neoplastic transformation. Importantly, the dual roles of SIRT1-p53 axis in aging and tumourigenesis, either as tumour suppressor or tumour promoter are determined by SIRT1 localisation and type of cells. Conceptualising the distinct similarity between tumorigenesis and cellular reprogramming, this review provides a perspective discussion on involvement of SIRT1 in improving efficiency in the induction and maintenance of pluripotent state. Further research in understanding the role of SIRT1-p53 pathway and their associated regulators and strategies to manipulate this regulatory axis very likely foster the development of therapeutics and strategies for treating cancer and aging-associated degenerative diseases.

BRCA1 function in the intra-S checkpoint is activated by acetylation via a pCAF/SIRT1 axis

Breast cancer associated gene 1 (BRCA1) function has been shown to be regulated by phosphorylation but the role of acetylation has not been determined. Therefore, we tested whether BRCA1 can be acetylated by the acetyltransferases P300/CBP-associated factor (pCAF), GCN5, and p300. p300 exhibited the highest level of BRCA1 acetylation; however, there was also a decrease in the total level of BRCA1. Therefore, we focused on pCAF and GCN5 because they both acetylated BRCA1 without affecting BRCA1 expression. Further analysis indicated that the acetylated form of BRCA1 is deacetylated by wild-type (WT) SIRT1, but not deacetylase mutant SIRT1, suggesting that SIRT1 is a specific deacetylase of BRCA1. We demonstrated that lysine 830 of BRCA1 is a preferential acetylation site by pCAF and tested its function in embryonic stem (ES) cells by changing lysine 830 to arginine using a transcription activator-like effector nuclease (TALEN) system. After exposure to DNA damage-inducing UV radiation, the viability of BRCA1 K830R mutant cells is greater than the WT ES cells. Further analysis using additional cell lines indicated that the BRCA1 K830R mutation impairs the intra-S checkpoint. Also, checkpoint kinase 1 (CHK1) phosphorylation was less in K830R cells as compared with WT cells after UV exposure. These data suggest that acetylation of BRCA1 on lysine 830 activates BRCA1 function at the intra-S checkpoint after DNA damage.

SIRT6 Is Essential for Adipocyte Differentiation by Regulating Mitotic Clonal Expansion

Preadipocytes initiate differentiation into adipocytes through a cascade of events. Mitotic clonal expansion, as one of the earliest events, is essential for adipogenesis. However, the underlying mechanisms that regulate mitotic clonal expansion remain elusive. SIRT6 is a member of the evolutionarily conserved sirtuin family of nicotinamide adenine dinucleotide (NAD)+-dependent protein deacetylases. Here, we show that SIRT6 deficiency in preadipocytes blocks their adipogenesis. Analysis of gene expression during adipogenesis reveals that KIF5C, which belongs to the kinesin family, is negatively regulated by SIRT6. Furthermore, we show that KIF5C is a negative factor for adipogenesis through interacting with CK2α', a catalytic subunit of CK2. This interaction blocks CK2α' nuclear translocation and CK2 kinase activity and inhibits mitotic clonal expansion during adipogenesis. These findings reveal a crucial role of SIRT6 in adipogenesis and provide potential therapeutic targets for obesity.

Why Human Papillomaviruses Activate the DNA Damage Response (DDR) and How Cellular and Viral Replication Persists in the Presence of DDR Signaling

Human papillomaviruses (HPV) require the activation of the DNA damage response (DDR) in order to undergo a successful life cycle. This activation presents a challenge for the virus and the infected cell: how does viral and host replication proceed in the presence of a DDR that ordinarily arrests replication; and how do HPV16 infected cells retain the ability to proliferate in the presence of a DDR that ordinarily arrests the cell cycle? This raises a further question: why do HPV activate the DDR? The answers to these questions are only partially understood; a full understanding could identify novel therapeutic strategies to target HPV cancers. Here, we propose that the rapid replication of an 8 kb double stranded circular genome during infection creates aberrant DNA structures that attract and activate DDR proteins. Therefore, HPV replication in the presence of an active DDR is a necessity for a successful viral life cycle in order to resolve these DNA structures on viral genomes; without an active DDR, successful replication of the viral genome would not proceed. We discuss the essential role of TopBP1 in this process and also how viral and cellular replication proceeds in HPV infected cells in the presence of DDR signals.

Phosphorylated SIRT1 associates with replication origins to prevent excess replication initiation and preserve genomic stability

Chromatin structure affects DNA replication patterns, but the role of specific chromatin modifiers in regulating the replication process is yet unclear. We report that phosphorylation of the human SIRT1 deacetylase on Threonine 530 (T530-pSIRT1) modulates DNA synthesis. T530-pSIRT1 associates with replication origins and inhibits replication from a group of 'dormant' potential replication origins, which initiate replication only when cells are subject to replication stress. Although both active and dormant origins bind T530-pSIRT1, active origins are distinguished from dormant origins by their unique association with an open chromatin mark, histone H3 methylated on lysine 4. SIRT1 phosphorylation also facilitates replication fork elongation. SIRT1 T530 phosphorylation is essential to prevent DNA breakage upon replication stress and cells harboring SIRT1 that cannot be phosphorylated exhibit a high prevalence of extrachromosomal elements, hallmarks of perturbed replication. These observations suggest that SIRT1 phosphorylation modulates the distribution of replication initiation events to insure genomic stability.

Emerging roles of SIRT1 in fatty liver diseases

Fatty liver diseases, which are commonly associated with high-fat/calorie diet, heavy alcohol consumption and/or other metabolic disorder causes, lead to serious medical concerns worldwide in recent years. It has been demonstrated that metabolic homeostasis disruption is most likely to be responsible for this global epidemic. Sirtuins are a group of conserved nicotinamide adenine dinucleotide (NAD⁺) dependent histone and/or protein deacetylases belonging to the silent information regulator 2 (Sir2) family. Among seven mammalian sirtuins, sirtuin 1 (SIRT 1) is the most extensively studied one and is involved in both alcoholic and nonalcoholic fatty liver diseases. SIRT1 plays beneficial roles in regulating hepatic lipid metabolism, controlling hepatic oxidative stress and mediating hepatic inflammation through deacetylating some transcriptional regulators against the progression of fatty liver diseases. Here we summarize the latest advances of the biological roles of SIRT1 in regulating lipid metabolism, oxidative stress and inflammation in the liver, and discuss the potential of SIRT1 as a therapeutic target for treating alcoholic and nonalcoholic fatty liver diseases.

The Deacetylase SIRT1 Regulates the Replication Properties of Human Papillomavirus 16 E1 and E2

Human papillomaviruses (HPV) replicate their genomes in differentiating epithelium using the viral proteins E1 and E2 in association with host proteins. While the roles of E1 and E2 in this process are understood, the host factors involved and how they interact with and regulate E1-E2 are not. Our previous work identified the host replication and repair factor TopBP1 as an E2 partner protein essential for optimal E1-E2 replication and for the viral life cycle. The role of TopBP1 in host DNA replication is regulated by the class III deacetylase SIRT1; activation of the DNA damage response prevents SIRT1 deacetylation of TopBP1, resulting in a switch from DNA replication to repair functions for this protein and cell cycle arrest. Others have demonstrated an essential role for SIRT1 in regulation of the HPV31 life cycle; here, we report that SIRT1 can directly regulate HPV16 E1-E2-mediated DNA replication. SIRT1 is part of the E1-E2 DNA replication complex and is recruited to the viral origin of replication in an E1-E2-dependent manner. CRISPR/Cas9 was used to generate C33a clones with undetectable SIRT1 expression and lack of SIRT1 elevated E1-E2 DNA replication, in part due to increased acetylation and stabilization of the E2 protein in the absence of SIRT1. The results demonstrate that SIRT1 is a member of, and can regulate, the HPV16 replication complex. We discuss the potential role of this protein in the viral life cycle.IMPORTANCE HPV are causative agents in a number of human diseases, and currently only the symptoms of these diseases are treated. To identify novel therapeutic approaches for combating these diseases, the viral life cycle must be understood in more detail. This report demonstrates that a cellular enzyme, SIRT1, is part of the HPV16 DNA replication complex and is brought to the viral genome by the viral proteins E1 and E2. Using gene editing technology (CRISPR/Cas9), the SIRT1 gene was removed from cervical cancer cells. The consequence of this was that viral replication was elevated, probably due to a stabilization of the viral replication factor E2. The overall results demonstrate that an enzyme with known inhibitors, SIRT1, plays an important role in controlling how HPV16 makes copies of itself. Targeting this enzyme could be a new therapeutic approach for combating HPV spread and disease.

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.

HDACs and HDAC Inhibitors in Cancer Development and Therapy

Over the last several decades, it has become clear that epigenetic abnormalities may be one of the hallmarks of cancer. Posttranslational modifications of histones, for example, may play a crucial role in cancer development and progression by modulating gene transcription, chromatin remodeling, and nuclear architecture. Histone acetylation, a well-studied posttranslational histone modification, is controlled by the opposing activities of histone acetyltransferases (HATs) and histone deacetylases (HDACs). By removing acetyl groups, HDACs reverse chromatin acetylation and alter transcription of oncogenes and tumor suppressor genes. In addition, HDACs deacetylate numerous nonhistone cellular substrates that govern a wide array of biological processes including cancer initiation and progression. This review will discuss the role of HDACs in cancer and the therapeutic potential of HDAC inhibitors (HDACi) as emerging drugs in cancer treatment.

Negative reciprocal regulation between Sirt1 and Per2 modulates the circadian clock and aging

Sirtuin 1 (SIRT1) is involved in both aging and circadian-clock regulation, yet the link between the two processes in relation to SIRT1 function is not clear. Using Sirt1-deficient mice, we found that Sirt1 and Period 2 (Per2) constitute a reciprocal negative regulation loop that plays important roles in modulating hepatic circadian rhythmicity and aging. Sirt1-deficient mice exhibited profound premature aging and enhanced acetylation of histone H4 on lysine16 (H4K16) in the promoter of Per2, the latter of which leads to its overexpression; in turn, Per2 suppresses Sirt1 transcription through binding to the Sirt1 promoter at the Clock/Bmal1 site. This negative reciprocal relationship between SIRT1 and PER2 was also observed in human hepatocytes. We further demonstrated that the absence of Sirt1 or the ectopic overexpression of Per2 in the liver resulted in a dysregulated pace of the circadian rhythm. The similar circadian rhythm was also observed in aged wild type mice. The interplay between Sirt1 and Per2 modulates aging gene expression and circadian-clock maintenance.

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

The ataxia telangiectasia-mutated and Rad3-related (ATR) kinase checkpoint pathway maintains genome integrity; however, the role of the sirtuin 2 (SIRT2) acetylome in regulating this pathway is not clear. We found that deacetylation of ATR-interacting protein (ATRIP), a regulatory partner of ATR, by SIRT2 potentiates the ATR checkpoint. SIRT2 interacts with and deacetylates ATRIP at lysine 32 (K32) in response to replication stress. SIRT2 deacetylation of ATRIP at K32 drives ATR autophosphorylation and signaling and facilitates DNA replication fork progression and recovery of stalled replication forks. K32 deacetylation by SIRT2 further promotes ATRIP accumulation to DNA damage sites and binding to replication protein A-coated single-stranded DNA (RPA-ssDNA). Collectively, these results support a model in which ATRIP deacetylation by SIRT2 promotes ATR-ATRIP binding to RPA-ssDNA to drive ATR activation and thus facilitate recovery from replication stress, outlining a mechanism by which the ATR checkpoint is regulated by SIRT2 through deacetylation.

"DNA Binding Region" of BRCA1 Affects Genetic Stability through modulating the Intra-S-Phase Checkpoint

The breast cancer associated gene 1 (BRCA1) contains 3 domains: an N-terminal RING domain with ubiquitin E3 ligase activity, C-terminal BRCT protein interaction domain and a central region. RING and BRCT domains are well characterized, yet the function of the central region remains unclear. In this study, we identified an essential DNA binding region (DBR: 421-701 amino acids) within the central region of human BRCA1, and found that BRCA1 brings DNA together and preferably binds to splayed-arm DNA in a sequence-independent manner. To investigate the biological role of the DBR, we generated mouse ES cells, which lack the DBR (ΔDBR) by using the TALEN method. The ΔDBR cells exhibited decreased survival as compared to the wild type (WT) cells treated with a PARP inhibitor, however they have an intact ability to conduct DNA repair mediated by homologous recombination (HR). The ΔDBR cells continued to incorporate more EdU in the presence of hydroxyurea (HU), which causes replication stress and exhibited reduced viability than the WT cells. Moreover, phosphorylation of CHK1, which regulates the intra-S phase checkpoint, was moderately decreased in ΔDBR cells. These data suggest that DNA binding by BRCA1 affects the stability of DNA replication folks, resulting in weakened intra-S-phase checkpoint control in the ΔDBR cells. The ΔDBR cells also exhibited an increased number of abnormal chromosome structures as compared with WT cells, indicating that the ΔDBR cells have increased genetic instability. Thus, we demonstrated that the DBR of BRCA1 modulates genetic stability through the intra-S-phase checkpoint activated by replication stress.

The multifaceted functions of sirtuins in cancer

The sirtuins (SIRTs; of which there are seven in mammals) are NAD(+)-dependent enzymes that regulate a large number of cellular pathways and forestall the progression of ageing and age-associated diseases. In recent years, the role of sirtuins in cancer biology has become increasingly apparent, and growing evidence demonstrates that sirtuins regulate many processes that go awry in cancer cells, such as cellular metabolism, the regulation of chromatin structure and the maintenance of genomic stability. In this article, we review recent advances in our understanding of how sirtuins affect cancer metabolism, DNA repair and the tumour microenvironment and how activating or inhibiting sirtuins may be important in preventing or treating cancer.

Sirtuin regulation in aging and injury

Sirtuins or Sir2 family of proteins are a class of NAD(+) dependent protein deacetylases which are evolutionarily conserved from bacteria to humans. Some sirtuins also exhibit mono-ADP ribosyl transferase, demalonylation and desuccinylation activities. Originally identified in the yeast, these proteins regulate key cellular processes like cell cycle, apoptosis, metabolic regulation and inflammation. Humans encode seven sirtuin isoforms SIRT1-SIRT7 with varying intracellular distribution. Apart from their classic role as histone deacetylases regulating transcription, a number of cytoplasmic and mitochondrial targets of sirtuins have also been identified. Sirtuins have been implicated in longevity and accumulating evidence indicate their role in a spectrum of diseases like cancer, diabetes, obesity and neurodegenerative diseases. A number of studies have reported profound changes in SIRT1 expression and activity linked to mitochondrial functional alterations following hypoxic-ischemic conditions and following reoxygenation injury. The SIRT1 mediated deacetylation of targets such as PGC-1α, FOXO3, p53 and NF-κb has profound effect on mitochondrial function, apoptosis and inflammation. These biological processes and functions are critical in life-span determination and outcome following injury. Aging is reported to be characterized by declining SIRT1 activity, and its increased expression or activation demonstrated prolonged life-span in lower forms of animals. A pseudohypoxic state due to declining NAD(+) has also been implicated in aging. In this review we provide an overview of studies on the role of sirtuins in aging and injury.