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SIRTUIN 1: Regulating the regulator
Biochemical and Biophysical Research Communications 376 (2008) 251–255
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Biochemical and Biophysical Research Communications journal homepage: www.elsevier.com/locate/ybbrc
Mini Review
SIRTUIN 1: Regulating the regulator Barbara Zschoernig, Ulrich Mahlknecht * Saarland University Medical Center, Department of Internal Medicine, Division of Immunotherapy and Gene Therapy, José Carreras Center for Immunotherapy and Gene Therapy, Kirrberger Strasse, Building 40, D-66421 Homburg/Saar, Germany
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Article history: Received 21 August 2008 Available online 5 September 2008 Keywords: Sirtuins SIRT1 Histone deacetylases Cancer Metabolic homeostasis Differentiation
a b s t r a c t Earlier analyses on the sirtuin family of histone deacetylases and its well-known member SIRT1 had their primary focus mostly on the identification of cellular targets exploring molecular mechanisms and functional networks in the control of metabolic homeostasis, differentiation, apoptosis and cell survival. However, only little is known about the regulation of SIRT1 itself, so far. Presently, SIRT1 is gaining increasing importance in the development of innovative treatment strategies for cancer, neurodegenerative disorders and metabolic disease. Based on differences in their catalytic activities, SIRT1 and the sirtuins in general, are insensitive to the classical class I and II HDAC inhibitors which are increasingly becoming part of treatment regimens for solid tumors and hematological malignancies. In this review we outline recent research advances on the regulation of SIRT1 which may provide the basis for the development of therapeutic inhibitors with improved specificity. Ó 2008 Elsevier Inc. All rights reserved.
The class III histone deacetylase SIRT1—a therapeutic target The class III histone deacetylases (HDACs) (‘‘sirtuins”), received their name on the basis of their homology with the yeast silent information regulator 2 (SIR2) protein, which is an NAD+-dependent HDAC [1,2] in contrast to the class I, II, and IV HDACs, which are Zn2+-dependent hydrolases. The requirement of NAD+ as a cofactor for enzymatic activity suggested the sirtuins to be crucial in the regulation of energy dependent transcription. Indeed, SIRT1 [3,4], has been reported to play a key role in a variety of physiological processes such as metabolism, neurogenesis and cell survival due to its ability to deacetylate both histone and numerous nonhistone substrates [5,6]. Several lines of evidence indicate that SIRT1 is crucial in the pathophysiology of metabolic disease, neurodegenerative disorders, cancer and aging. SIRT1 has therefore become an important target for innovative molecular treatment strategies. First, metabolic homeostasis is controlled by SIRT1mediated deacetylation and thus activation of the peroxisome proliferation activating receptor (PPAR)-gamma co-activator-1a (PGC-1a) [7], which stimulates mitochondrial activity and subsequently increases glucose metabolism, which in turn improves insulin sensitivity [8]. Secondly, the activation of SIRT1 appears to be neuroprotective in animal models for Alzheimer’s disease and amyotrophic lateral sclerosis as well as optic neuritis mainly due to decreased deacetylation of the tumor suppressor p53 and PGC-1a [9,10]. Thirdly, siRNA-mediated SIRT1 knockdown induces growth arrest and apoptosis in human epithelial cancer cells, while * Corresponding author. Fax: +49 6841 1621389. E-mail address: [email protected] (U. Mahlknecht). 0006-291X/$ - see front matter Ó 2008 Elsevier Inc. All rights reserved. doi:10.1016/j.bbrc.2008.08.137
inhibition of SIRT1 deacetylase activity is known to reverse epigenetic silencing which leads to the re-expression of silenced tumor suppressor genes such as p53, which results in increased cell survival and therefore raises SIRT1, next to p53, which is known as the guardian of cellular integrity and which is mutated in more than half of all human cancers, to a novel target in the development of innovative cancer therapeutics [11–16]. Consequently, several pharmacological agents have been identified to be useful in the modulation of SIRT1 activity both positively and negatively albeit with moderate specificity [9,17–19]. In this review, we summarize recent studies on the regulation of SIRT1, as elucidating the underlying mechanisms of SIRT1 biology might lead to the development of more specific inhibitors (see Figs. 1 and 2).
SIRT1 regulation at the transcriptional level Oxidative stress as well as calorie restriction has been identified to modulate SIRT1 protein levels. Furthermore, elevated levels of SIRT1 have been detected in a number of different types of cancer. In a number of studies an increase of SIRT1 protein expression was reported to be—at least in part—mediated on the transcriptional level [20–23]. Under conditions of oxidative stress, two transcription factors have been identified to modulate SIRT1 expression: E2F1 and HIC1 [20,23]. While the cycle regulator E2F1, which directly binds to the SIRT1 promoter at a consensus site located at bp position 65 appears to regulate the basal expression level of SIRT1. Oxidative stress is known to stabilize and therefore to activate E2F1 leading to increased SIRT1 transcription [23]. Such elevated levels of SIRT1 open out into a negative feedback loop
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Fig. 1. Overview of mechanisms that might be responsible for the regulation of SIRT1. All known processes discussed in this review are marked in bold. For reasons of simplicity, post-translational modifications and complex formation are shown in the cytoplasm although they occur in the nucleus.
Fig. 2. Transcriptional and post-transcriptional regulatory mechanisms of SIRT1. (A) Transcriptional control of SIRT1. Three transcription factors (E2F1, HIC1, and p53) have been identified to modulate SIRT1 expression under oxidative stress/DNA-damage conditions and/or nutrient deprivation. Arrow: increased SIRT1 transcription; Square: inhibited SIRT1 transcription. (B) Post-transcriptional control of SIRT1. Upon oxidative stress, SIRT1 mRNA is degraded due to a checkpoint kinase 2 (Chk2)-mediated dissociation of the RNA binding protein HuR. 50 UTR, 50 untranslated region; 30 UTR, 30 untranslated region.
B. Zschoernig, U. Mahlknecht / Biochemical and Biophysical Research Communications 376 (2008) 251–255
where E2F1 activity is inhibited through SIRT1-mediated deacetylation. Conversely, the tumor suppressor HIC1 (hypermethylated in cancer 1) appeared to be a negative regulator of SIRT1 expression through formation of a transcriptional repression complex with SIRT1 via its N-terminal POZ domain thereby inhibiting SIRT1mediated p53 inactivation [20]. Two HIC1 binding sites have been assigned to base pair positions 1116 and 1039 within the SIRT1 promoter. Despite its inactivating function under oxidative stress, HIC1 is also able to induce SIRT1 transcription upon calorie restriction [22]. Increased SIRT1 protein levels may be the consequence of reduced recruitment of the transcriptional co-repressor CtBP, since complex formation of HIC1 and CtBP significantly contributes to the transcriptional repression activity of HIC1 [24]. Furthermore, two functional p53 binding sites within the SIRT1 promoter (178 bp and 168 bp) have been identified which normally repress SIRT1 expression [21]. In the absence of nutrients, SIRT1 transcription is induced through nuclear translocation of FOXO3a, which interacts with p53 and thereby inhibits p53 suppressive activity. SIRT1 regulation at the post-transcriptional level Apart from mechanisms that interfere with transcriptional control, post-transcriptional processing of mRNA transcripts such as the regulation of intracellular transport, silencing or mRNA stability has been demonstrated to be major determinants in the regulation of protein abundance [25]. These processes are governed by specific RNA binding proteins or microRNAs which interact with defined mRNA sequences. HUR, a ubiquitously expressed RNA binding protein [26], has been demonstrated to associate with the 30 untranslated region of the SIRT1 mRNA under physiological conditions [27]. This interaction leads to increased SIRT1mRNA stability and thus in elevated protein levels. By contrast, the HUR–SIRT1mRNA complex is being disrupted upon oxidative stress, which finally results in decreased mRNA stability and therefore decreased SIRT protein levels. The HUR binding activity to the SIRT1mRNA has been shown to depend on its phosphorylation, which is being controlled by the checkpoint kinase 2 (CHK2), which in turn, is activated through oxidative stress and explains how the HUR binding affinity to SIRT1mRNA is reduced through hyperphosphorylation. SIRT1 regulation through post-translational modification Sumoylation The small ubiquitin-like modifier (SUMO-1) is a protein that is covalently attached to lysine residues being located in the common consensus sequence W-K-X-D/E of its target proteins (where W is a large hydrophobic amino acid, K is the lysine conjugated to SUMO, X is any amino acid and D or E are acidic residues). The reaction is mediated by a multi-enzyme process, which is similar to ubiquitinylation. Conversely, desumoylation is catalyzed by a family of specific isopeptidases called the ‘‘SENP” desumoylases [28]. While ubiquitinylation primarily targets proteins for proteasomal degradation, sumoylation has been implicated to exert a regulatory influence on its target proteins such as altered subcellular localization, protein–protein interactions and enzymatic activity [29]. In a recent study, the ‘molecular switch’ between sumoylation and desumoylation of SIRT1 has been identified as a regulatory mechanism in the control of SIRT1 enzymatic activity in response to genotoxic stress [30]. In the absence of DNA-damage, Lys734, which is located within the carboxy-terminal domain of SIRT1, is targeted by SUMO-1 both in vitro and in vivo, whereas the presence of genotoxic stress leads to SENP-1-mediated desumoylation of SIRT1.
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Sumoylation of Lys734 significantly contributes to the enzymatic activity of SIRT1 since abrogation of sumoylation by site-directed mutagenesis revealed an impaired deacetylase activity in vitro and in vivo towards known substrates such as p53 (Lys382) and histone H4 (Lys16). Notably, the nuclear localization of SIRT1 was not affected. As a physiological outcome, desumoylation of SIRT1 during genotoxic stress counteracts its enzymatic activity thereby promoting the activity of its apoptotic substrates which tips the balance towards increased cell death. Phosphorylation Reversible protein phosphorylation is arguably the most common post-translational modification that functions as a ‘molecular switch’ in the concerted control of biological systems. Large-scale mass spectrometry studies on HeLa nuclear cell extracts identified Ser27 and Ser47 to be potential phosphorylation sites which are located within the amino-terminal domain of SIRT1 [31,32]. Up to now, neither a function nor a phosphorylating kinase has yet been identified to target any of these potential phosphorylation sites. Only one study indirectly suggested a physiological function for the phosphorylation of murine SIRT1 (SIR2alpha), since the treatment with LY294002, an inhibitor of phosphoinositide 3-OH kinase (PI3K), affected the subcellular localization of an EGFP-SIRT1 fusion protein in undifferentiated C2C12 cells [33]. So far, there is no experimental evidence of a direct phosphorylation of either the murine and/or the human SIRT1 protein by PI3K or any other kinase. In consideration of the demonstrated impact of reversible phosphorylation on the regulation of class I and II HDAC proteins (for review see [34]) and most recently on SIRT2 [35–38], the analysis of SIRT1 phosphorylation as a potential post-translational regulatory mechanism is of particular importance. SIRT1 regulation via its subcellular localization SIRT1 has initially been thought to represent an exclusively nuclear protein that is distributed over the whole cell during mitosis [39]. However, depending on cell lines and organisms examined, a partial or temporary cytoplasmic localization was observed in murine pancreatic beta cells [40], neonatal rat cardiomyocytes [41], and for Drosophila SIR2 (dSIR2) [42]. Since SIRT1 deacetylates both, Histones and non-histone proteins such as a number of transcription factors, changes in subcellular localization are likely to play a role in the regulation of its function. Two independent studies confirmed that murine SIRT1, which shares about 84% homology with human SIRT1, is indeed subjected to nucleo-cytoplasmic shuttling upon oxidative stress [33,43]. Both studies demonstrate that a cytoplasmic accumulation of SIRT1 sequesters it away from its nuclear substrates rendering SIRT1 unable to exert its inactivating function on its predominantly anti-apoptotic substrates. Consequently, the cytoplasmic localization of SIRT1 appears to sensitize cells to oxidative stress-mediated apoptosis. As shown by analyses with EGFP-SIRT1 deletion constructs [33,43] as well as heterokaryon shuttling assays [33], the cytoplasmic localization of SIRT1 is regulated by CRM1-mediated nuclear export. Using site-directed mutagenesis, Tanno and coworkers [33] identified two functional nuclear localization signals (NLS) NLS1 and NLS2 (spanning amino acids 31–38 and 223–230, respectively) as well as two nuclear export signals (NES) NES1 and NES2 (spanning amino acids 138–145 and 425–431). In agreement with this observation, EGFP-SIRT1 deletion construct analyses of Jin and coworkers [43] indicated the existence of a NES in the region spanning SIRT1 amino acids 238–517. Despite the fact that the NES and NLS sequences of both murine and human SIRT1 are highly conserved, the functional significance of nucleo-cytoplasmic
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shuttling on the regulation of human SIRT1 requires further investigation. SIRT1 regulation via protein–protein interactions Regulation of protein function has been well-established as a key regulatory mechanism for class I and II HDAC proteins (for review see [34]). So far, two proteins have been identified to regulate the SIRT1 activity both positively and negatively through complex formation in the context of the cellular stress response. The first direct regulator of SIRT1 which has initially been identified by yeast two-hybrid screening was the active regulator of SIRT1 (AROS) [44], which was then further confirmed by GST-pulldown analyses and co-immunoprecipitation experiments. Deletion construct analyses identified the AROS protein region spanning amino acids 114–217 to directly interact with the amino-terminal domain of SIRT1. The AROS protein is known to significantly enhance the activity of SIRT1 on acetylated p53 both in vitro and in cell lines thereby promoting the inhibitory effect of SIRT1 on p53-mediated transcriptional activity of pro-apoptotic genes (e.g. Bax and p21Waf-1) under conditions of DNA-damage. Remarkably, the activating effect of AROS is more pronounced in vivo which implies that another cellular factor, either a co-repressor or co-activator, may be involved in the regulation of SIRT1. While AROS acts as a positive regulator of SIRT1, complex formation of DBC-1 (deleted in breast cancer-1) with SIRT1 has most recently been identified to negatively regulate SIRT1 activity by a mechanism, which is currently poorly understood [45–48]. Deletion construct analyses of both SIRT1 and DBC-1 assigned the region of interaction to a leucine zipper motif within the amino-terminal domain of DBC-1 (aa positions 243–264) and the SIRT1 deacetylase domain. DBC-1 interacts with SIRT1 but not with other members of the sirtuin family (SIRT2–SIRT7), which implies that the underlying regulatory mechanism is highly specific. Unlike the complex formation with AROS, DBC-1 inhibits the activity of SIRT1 both in vitro and in vivo in the context of cellular stress and may be explained either on the basis of a deacetylation-mediated transcriptional activation of its anti-apoptotic targets (MnSOD and Gadd45) or transcriptional repression of its pro-apoptotic substrates such as BIM [46], which in the end lead to the sensitization of cells for cell death. The mechanism by which DBC-1 inhibits SIRT1 activity is likely to be based on direct substrate competition, since (a) DBC-1-binding defective SIRT1 mutants are not affected by DBC-1 enzymatic activity and (b) the binding of SIRT1 to its substrates such as p53, or FOXO3 is significantly abrogated by co-expression of DBC-1. The insights into the molecular mechanisms underlying SIRT1 activation by AROS and SIRT1 inactivation through DBC-1 are currently poor and remain to be further elucidated. Conclusions SIRT1 is an emerging molecular target in the development of innovative treatment strategies for cancer, neurodegenerative disorders and metabolic disease. Combined with the lack of sensitivity for the known class I and II HDAC inhibitors, research on the regulation of SIRT1 is increasingly attracting attention to the field. In this regard, it is interesting to note that many of the regulatory processes described so far, depend on the unique amino- and carboxy-terminal extensions of SIRT1. In contrast to the highly conserved deacetylase domain, these extensions exhibit a remarkable divergence among sirtuin family members and are therefore ascribed to mediate the specific functions of every sirtuin. While we discussed the different mechanisms of SIRT1 regulation separately for reasons of clarity, they are actually most likely to be interconnected as demonstrated in the case of PI3K-mediated
phosphorylation of murine SIRT1 which affects its subcellular localization. Furthermore, the partial opposite regulation of SIRT1 under the same cellular conditions, arguing for the importance of SIRT1 for the maintenance of the cellular homeostasis, requires further investigation. However, studies of SIRT1 regulation warrant further analysis especially with regard to the relevance of reversible phosphorylation which has already been demonstrated to be a key regulatory mechanism for class I, II, and IV HDACs. Acknowledgments We apologize to all investigators whose relevant work was not discussed or cited directly in this manuscript due to space limitations. This work was supported by grants from the Deutsche José Carreras-Leukämie Stiftung e.V. (DJCLS R 05/11) and the Deutsche Forschungsgemeinschaft (DFG, MA 2057/2-4). References [1] J. Landry, A. Sutton, S.T. Tafrov, R.C. Heller, J. Stebbins, L. Pillus, R. Sternglanz, The silencing protein SIR2 and its homologs are NAD-dependent protein deacetylases, Proc. Natl. Acad. Sci. USA 97 (2000) 5807–5811. [2] K.G. Tanner, J. Landry, R. Sternglanz, J.M. Denu, Silent information regulator 2 family of NAD-dependent histone/protein deacetylases generates a unique product 1-O-acetyl-ADP-ribose, Proc. Natl. Acad. Sci. USA 97 (2000) 14178– 14182. [3] R.A. Frye, Characterization of five human cDNAs with homology to the yeast SIR2 gene: Sir2-like proteins (sirtuins) metabolize NAD and may have protein ADP-ribosyltransferase activity, Biochem. Biophys. Res. Commun. 260 (1999) 273–279. [4] R.A. Frye, Phylogenetic classification of prokaryotic and eukaryotic Sir2-like proteins, Biochem. Biophys. Res. Commun. 273 (2000) 793–798. [5] S. Michan, D. Sinclair, Sirtuins in mammals: insights into their biological function, Biochem. J. 404 (2007) 1–13. [6] A.A. Sauve, C. Wolberger, V.L. Schramm, J.D. Boeke, The biochemistry of sirtuins, Annu. Rev. Biochem. (2006). [7] J.T. Rodgers, C. Lerin, W. Haas, S.P. Gygi, B.M. Spiegelman, P. Puigserver, Nutrient control of glucose homeostasis through a complex of PGC-1alpha and SIRT1, Nature 434 (2005) 113–118. [8] M. Lagouge, C. Argmann, Z. Gerhart-Hines, H. Meziane, C. Lerin, F. Daussin, N. Messadeq, J. Milne, P. Lambert, P. Elliott, B. Geny, M. Laakso, P. Puigserver, J. Auwerx, Resveratrol improves mitochondrial function and protects against metabolic disease by activating SIRT1 and PGC-1alpha, Cell 127 (2006) 1109– 1122. [9] K.S. Shindler, E. Ventura, T.S. Rex, P. Elliott, A. Rostami, SIRT1 activation confers neuroprotection in experimental optic neuritis, Invest. Ophthalmol. Vis. Sci. 48 (2007) 3602–3609. [10] D. Kim, M.D. Nguyen, M.M. Dobbin, A. Fischer, F. Sananbenesi, J.T. Rodgers, I. Delalle, J.A. Baur, G. Sui, S.M. Armour, P. Puigserver, D.A. Sinclair, L.H. Tsai, SIRT1 deacetylase protects against neurodegeneration in models for Alzheimer’s disease and amyotrophic lateral sclerosis, EMBO J. 26 (2007) 3169–3179. [11] J. Ford, M. Jiang, J. Milner, Cancer-specific functions of SIRT1 enable human epithelial cancer cell growth and survival, Cancer Res. 65 (2005) 10457– 10463. [12] C.S. Lim, SIRT1: tumor promoter or tumor suppressor?, Med Hypotheses 67 (2006) 341–344. [13] E. Langley, M. Pearson, M. Faretta, U.M. Bauer, R.A. Frye, S. Minucci, P.G. Pelicci, T. Kouzarides, Human SIR2 deacetylates p53 and antagonizes PML/p53induced cellular senescence, EMBO J. 21 (2002) 2383–2396. [14] J. Luo, A.Y. Nikolaev, S. Imai, D. Chen, F. Su, A. Shiloh, L. Guarente, W. Gu, Negative control of p53 by Sir2alpha promotes cell survival under stress, Cell 107 (2001) 137–148. [15] K. Pruitt, R.L. Zinn, J.E. Ohm, K.M. McGarvey, S.H. Kang, D.N. Watkins, J.G. Herman, S.B. Baylin, Inhibition of SIRT1 reactivates silenced cancer genes without loss of promoter DNA hypermethylation, PLoS Genet. 2 (2006) e40. [16] H. Vaziri, S.K. Dessain, E. Ng Eaton, S.I. Imai, R.A. Frye, T.K. Pandita, L. Guarente, R.A. Weinberg, hSIR2(SIRT1) functions as an NAD-dependent p53 deacetylase, Cell 107 (2001) 149–159. [17] Z.Z. Chong, S.H. Lin, F. Li, K. Maiese, The sirtuin inhibitor nicotinamide enhances neuronal cell survival during acute anoxic injury through AKT BAD PARP and mitochondrial associated anti-apoptotic pathways, Curr. Neurovasc. Res. 2 (2005) 271–285. [18] J.C. Milne, P.D. Lambert, S. Schenk, D.P. Carney, J.J. Smith, D.J. Gagne, L. Jin, O. Boss, R.B. Perni, C.B. Vu, J.E. Bemis, R. Xie, J.S. Disch, P.Y. Ng, J.J. Nunes, A.V. Lynch, H. Yang, H. Galonek, K. Israelian, W. Choy, A. Iffland, S. Lavu, O. Medvedik, D.A. Sinclair, J.M. Olefsky, M.R. Jirousek, P.J. Elliott, C.H. Westphal, Small molecule activators of SIRT1 as therapeutics for the treatment of type 2 diabetes, Nature 450 (2007) 712–716.
B. Zschoernig, U. Mahlknecht / Biochemical and Biophysical Research Communications 376 (2008) 251–255 [19] H. Ota, E. Tokunaga, K. Chang, M. Hikasa, K. Iijima, M. Eto, K. Kozaki, M. Akishita, Y. Ouchi, M. Kaneki, Sirt1, inhibitor, Sirtinol induces senescence-like growth arrest with attenuated Ras-MAPK signaling in human cancer cells, Oncogene 25 (2006) 176–185. [20] W.Y. Chen, D.H. Wang, R.C. Yen, J. Luo, W. Gu, S.B. Baylin, Tumor suppressor HIC1 directly regulates SIRT1 to modulate p53-dependent DNA-damage responses, Cell 123 (2005) 437–448. [21] S. Nemoto, M.M. Fergusson, T. Finkel, Nutrient availability regulates SIRT1 through a forkhead-dependent pathway, Science 306 (2004) 2105–2108. [22] X. Zhang, Y. Ozawa, H. Lee, Y.D. Wen, T.H. Tan, B.E. Wadzinski, E. Seto, Histone deacetylase 3 (HDAC3) activity is regulated by interaction with protein serine/ threonine phosphatase 4, Genes Dev. 19 (2005) 827–839. [23] C. Wang, L. Chen, X. Hou, Z. Li, N. Kabra, Y. Ma, S. Nemoto, T. Finkel, W. Gu, W.D. Cress, J. Chen, Interactions between E2F1 and SirT1 regulate apoptotic response to DNA damage, Nat. Cell Biol. 8 (2006) 1025–1031. [24] N. Stankovic-Valentin, S. Deltour, J. Seeler, S. Pinte, G. Vergoten, C. Guerardel, A. Dejean, D. Leprince, An acetylation/deacetylation–SUMOylation switch through a phylogenetically conserved psiKXEP motif in the tumor suppressor HIC1 regulates transcriptional repression activity, Mol. Cell. Biol. 27 (2007) 2661– 2675. [25] C. Cheadle, J. Fan, Y.S. Cho-Chung, T. Werner, J. Ray, L. Do, M. Gorospe, K.G. Becker, Stability regulation of mRNA and the control of gene expression, Ann. NY Acad. Sci. 1058 (2005) 196–204. [26] C.M. Brennan, J.A. Steitz, HuR and mRNA stability, Cell. Mol. Life Sci. 58 (2001) 266–277. [27] K. Abdelmohsen, R. Pullmann Jr., A. Lal, H.H. Kim, S. Galban, X. Yang, J.D. Blethrow, M. Walker, J. Shubert, D.A. Gillespie, H. Furneaux, M. Gorospe, Phosphorylation of HuR by Chk2 regulates SIRT1 expression, Mol. Cell 25 (2007) 543–557. [28] E.T. Yeh, L. Gong, T. Kamitani, Ubiquitin-like proteins: new wines in new bottles, Gene 248 (2000) 1–14. [29] A. Verger, J. Perdomo, M. Crossley, Modification with SUMO. A role in transcriptional regulation, EMBO Rep. 4 (2003) 137–142. [30] Y. Yang, W. Fu, J. Chen, N. Olashaw, X. Zhang, S.V. Nicosia, K. Bhalla, W. Bai, SIRT1 sumoylation regulates its deacetylase activity and cellular response to genotoxic stress, Nat. Cell Biol. 9 (2007) 1253–1262. [31] S.A. Beausoleil, M. Jedrychowski, D. Schwartz, J.E. Elias, J. Villen, J. Li, M.A. Cohn, L.C. Cantley, S.P. Gygi, Large-scale characterization of HeLa cell nuclear phosphoproteins, Proc. Natl. Acad. Sci. USA 101 (2004) 12130–12135. [32] S.A. Beausoleil, J. Villen, S.A. Gerber, J. Rush, S.P. Gygi, A probability-based approach for high-throughput protein phosphorylation analysis and site localization, Nat. Biotechnol. 24 (2006) 1285–1292. [33] M. Tanno, J. Sakamoto, T. Miura, K. Shimamoto, Y. Horio, Nucleocytoplasmic shuttling of the NAD+dependent histone deacetylase SIRT1, J. Biol. Chem. 282 (2007) 6823–6832.
255
[34] N. Sengupta, E. Seto, Regulation of histone deacetylase activities, J. Cell. Biochem. 93 (2004) 57–67. [35] R. Pandithage, R. Lilischkis, K. Harting, A. Wolf, B. Jedamzik, J. Luscher-Firzlaff, J. Vervoorts, E. Lasonder, E. Kremmer, B. Knoll, B. Luscher, The regulation of SIRT2 function by cyclin-dependent kinases affects cell motility, J. Cell Biol. 180 (2008) 915–929. [36] B.J. North, E. Verdin, Mitotic regulation of SIRT2 by cyclin-dependent kinase 1-dependent phosphorylation, J. Biol. Chem. 282 (2007) 19546– 19555. [37] F. Nahhas, S.C. Dryden, J. Abrams, M.A. Tainsky, Mutations in SIRT2 deacetylase which regulate enzymatic activity but not its interaction with HDAC6 and tubulin, Mol. Cell. Biochem. 303 (2007) 221–230. [38] S.C. Dryden, F.A. Nahhas, J.E. Nowak, A.S. Goustin, M.A. Tainsky, Role for human SIRT2 NAD-dependent deacetylase activity in control of mitotic exit in the cell cycle, Mol. Cell. Biol. 23 (2003) 3173–3185. [39] E. Michishita, J.Y. Park, J.M. Burneskis, J.C. Barrett, I. Horikawa, Evolutionarily conserved and nonconserved cellular localizations and functions of human SIRT proteins, Mol. Biol. Cell 16 (2005) 4623–4635. [40] K.A. Moynihan, A.A. Grimm, M.M. Plueger, E. Bernal-Mizrachi, E. Ford, C. CrasMeneur, M.A. Permutt, S. Imai, Increased dosage of mammalian Sir2 in pancreatic beta cells enhances glucose-stimulated insulin secretion in mice, Cell Metab. 2 (2005) 105–117. [41] I.Y. Chen, J. Lypowy, J. Pain, D. Sayed, S. Grinberg, R.R. Alcendor, J. Sadoshima, M. Abdellatif, Histone H2A.z is essential for cardiac myocyte hypertrophy but opposed by silent information regulator 2alpha, J. Biol. Chem. 281 (2006) 19369–19377. [42] M.I. Rosenberg, S.M. Parkhurst, Drosophila Sir2 is required for heterochromatic silencing and by euchromatic Hairy/E(Spl) bHLH repressors in segmentation and sex determination, Cell 109 (2002) 447–458. [43] Q. Jin, T. Yan, X. Ge, C. Sun, X. Shi, Q. Zhai, Cytoplasm-localized SIRT1 enhances apoptosis, J. Cell. Physiol. 213 (2007) 88–97. [44] E.J. Kim, J.H. Kho, M.R. Kang, S.J. Um, Active regulator of SIRT1 cooperates with SIRT1 and facilitates suppression of p53 activity, Mol. Cell 28 (2007) 277–290. [45] W. Zhao, J.P. Kruse, Y. Tang, S.Y. Jung, J. Qin, W. Gu, Negative regulation of the deacetylase SIRT1 by DBC1, Nature 451 (2008) 587–590. [46] J.E. Kim, J. Chen, Z. Lou, DBC1 is a negative regulator of SIRT1, Nature 451 (2008) 583–586. [47] M. Hamaguchi, J.L. Meth, C. von Klitzing, W. Wei, D. Esposito, L. Rodgers, T. Walsh, P. Welcsh, M.C. King, M.H. Wigler, DBC2, a candidate for a tumor suppressor gene involved in breast cancer, Proc. Natl. Acad. Sci. USA 99 (2002) 13647–13652. [48] R. Sundararajan, G. Chen, C. Mukherjee, E. White, Caspase-dependent processing activates the proapoptotic activity of deleted in breast cancer-1 during tumor necrosis factor-alpha-mediated death signaling, Oncogene 24 (2005) 4908–4920.
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Mini Review
SIRTUIN 1: Regulating the regulator Barbara Zschoernig, Ulrich Mahlknecht * Saarland University Medical Center, Department of Internal Medicine, Division of Immunotherapy and Gene Therapy, José Carreras Center for Immunotherapy and Gene Therapy, Kirrberger Strasse, Building 40, D-66421 Homburg/Saar, Germany
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Article history: Received 21 August 2008 Available online 5 September 2008 Keywords: Sirtuins SIRT1 Histone deacetylases Cancer Metabolic homeostasis Differentiation
a b s t r a c t Earlier analyses on the sirtuin family of histone deacetylases and its well-known member SIRT1 had their primary focus mostly on the identification of cellular targets exploring molecular mechanisms and functional networks in the control of metabolic homeostasis, differentiation, apoptosis and cell survival. However, only little is known about the regulation of SIRT1 itself, so far. Presently, SIRT1 is gaining increasing importance in the development of innovative treatment strategies for cancer, neurodegenerative disorders and metabolic disease. Based on differences in their catalytic activities, SIRT1 and the sirtuins in general, are insensitive to the classical class I and II HDAC inhibitors which are increasingly becoming part of treatment regimens for solid tumors and hematological malignancies. In this review we outline recent research advances on the regulation of SIRT1 which may provide the basis for the development of therapeutic inhibitors with improved specificity. Ó 2008 Elsevier Inc. All rights reserved.
The class III histone deacetylase SIRT1—a therapeutic target The class III histone deacetylases (HDACs) (‘‘sirtuins”), received their name on the basis of their homology with the yeast silent information regulator 2 (SIR2) protein, which is an NAD+-dependent HDAC [1,2] in contrast to the class I, II, and IV HDACs, which are Zn2+-dependent hydrolases. The requirement of NAD+ as a cofactor for enzymatic activity suggested the sirtuins to be crucial in the regulation of energy dependent transcription. Indeed, SIRT1 [3,4], has been reported to play a key role in a variety of physiological processes such as metabolism, neurogenesis and cell survival due to its ability to deacetylate both histone and numerous nonhistone substrates [5,6]. Several lines of evidence indicate that SIRT1 is crucial in the pathophysiology of metabolic disease, neurodegenerative disorders, cancer and aging. SIRT1 has therefore become an important target for innovative molecular treatment strategies. First, metabolic homeostasis is controlled by SIRT1mediated deacetylation and thus activation of the peroxisome proliferation activating receptor (PPAR)-gamma co-activator-1a (PGC-1a) [7], which stimulates mitochondrial activity and subsequently increases glucose metabolism, which in turn improves insulin sensitivity [8]. Secondly, the activation of SIRT1 appears to be neuroprotective in animal models for Alzheimer’s disease and amyotrophic lateral sclerosis as well as optic neuritis mainly due to decreased deacetylation of the tumor suppressor p53 and PGC-1a [9,10]. Thirdly, siRNA-mediated SIRT1 knockdown induces growth arrest and apoptosis in human epithelial cancer cells, while * Corresponding author. Fax: +49 6841 1621389. E-mail address: [email protected] (U. Mahlknecht). 0006-291X/$ - see front matter Ó 2008 Elsevier Inc. All rights reserved. doi:10.1016/j.bbrc.2008.08.137
inhibition of SIRT1 deacetylase activity is known to reverse epigenetic silencing which leads to the re-expression of silenced tumor suppressor genes such as p53, which results in increased cell survival and therefore raises SIRT1, next to p53, which is known as the guardian of cellular integrity and which is mutated in more than half of all human cancers, to a novel target in the development of innovative cancer therapeutics [11–16]. Consequently, several pharmacological agents have been identified to be useful in the modulation of SIRT1 activity both positively and negatively albeit with moderate specificity [9,17–19]. In this review, we summarize recent studies on the regulation of SIRT1, as elucidating the underlying mechanisms of SIRT1 biology might lead to the development of more specific inhibitors (see Figs. 1 and 2).
SIRT1 regulation at the transcriptional level Oxidative stress as well as calorie restriction has been identified to modulate SIRT1 protein levels. Furthermore, elevated levels of SIRT1 have been detected in a number of different types of cancer. In a number of studies an increase of SIRT1 protein expression was reported to be—at least in part—mediated on the transcriptional level [20–23]. Under conditions of oxidative stress, two transcription factors have been identified to modulate SIRT1 expression: E2F1 and HIC1 [20,23]. While the cycle regulator E2F1, which directly binds to the SIRT1 promoter at a consensus site located at bp position 65 appears to regulate the basal expression level of SIRT1. Oxidative stress is known to stabilize and therefore to activate E2F1 leading to increased SIRT1 transcription [23]. Such elevated levels of SIRT1 open out into a negative feedback loop
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Fig. 1. Overview of mechanisms that might be responsible for the regulation of SIRT1. All known processes discussed in this review are marked in bold. For reasons of simplicity, post-translational modifications and complex formation are shown in the cytoplasm although they occur in the nucleus.
Fig. 2. Transcriptional and post-transcriptional regulatory mechanisms of SIRT1. (A) Transcriptional control of SIRT1. Three transcription factors (E2F1, HIC1, and p53) have been identified to modulate SIRT1 expression under oxidative stress/DNA-damage conditions and/or nutrient deprivation. Arrow: increased SIRT1 transcription; Square: inhibited SIRT1 transcription. (B) Post-transcriptional control of SIRT1. Upon oxidative stress, SIRT1 mRNA is degraded due to a checkpoint kinase 2 (Chk2)-mediated dissociation of the RNA binding protein HuR. 50 UTR, 50 untranslated region; 30 UTR, 30 untranslated region.
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where E2F1 activity is inhibited through SIRT1-mediated deacetylation. Conversely, the tumor suppressor HIC1 (hypermethylated in cancer 1) appeared to be a negative regulator of SIRT1 expression through formation of a transcriptional repression complex with SIRT1 via its N-terminal POZ domain thereby inhibiting SIRT1mediated p53 inactivation [20]. Two HIC1 binding sites have been assigned to base pair positions 1116 and 1039 within the SIRT1 promoter. Despite its inactivating function under oxidative stress, HIC1 is also able to induce SIRT1 transcription upon calorie restriction [22]. Increased SIRT1 protein levels may be the consequence of reduced recruitment of the transcriptional co-repressor CtBP, since complex formation of HIC1 and CtBP significantly contributes to the transcriptional repression activity of HIC1 [24]. Furthermore, two functional p53 binding sites within the SIRT1 promoter (178 bp and 168 bp) have been identified which normally repress SIRT1 expression [21]. In the absence of nutrients, SIRT1 transcription is induced through nuclear translocation of FOXO3a, which interacts with p53 and thereby inhibits p53 suppressive activity. SIRT1 regulation at the post-transcriptional level Apart from mechanisms that interfere with transcriptional control, post-transcriptional processing of mRNA transcripts such as the regulation of intracellular transport, silencing or mRNA stability has been demonstrated to be major determinants in the regulation of protein abundance [25]. These processes are governed by specific RNA binding proteins or microRNAs which interact with defined mRNA sequences. HUR, a ubiquitously expressed RNA binding protein [26], has been demonstrated to associate with the 30 untranslated region of the SIRT1 mRNA under physiological conditions [27]. This interaction leads to increased SIRT1mRNA stability and thus in elevated protein levels. By contrast, the HUR–SIRT1mRNA complex is being disrupted upon oxidative stress, which finally results in decreased mRNA stability and therefore decreased SIRT protein levels. The HUR binding activity to the SIRT1mRNA has been shown to depend on its phosphorylation, which is being controlled by the checkpoint kinase 2 (CHK2), which in turn, is activated through oxidative stress and explains how the HUR binding affinity to SIRT1mRNA is reduced through hyperphosphorylation. SIRT1 regulation through post-translational modification Sumoylation The small ubiquitin-like modifier (SUMO-1) is a protein that is covalently attached to lysine residues being located in the common consensus sequence W-K-X-D/E of its target proteins (where W is a large hydrophobic amino acid, K is the lysine conjugated to SUMO, X is any amino acid and D or E are acidic residues). The reaction is mediated by a multi-enzyme process, which is similar to ubiquitinylation. Conversely, desumoylation is catalyzed by a family of specific isopeptidases called the ‘‘SENP” desumoylases [28]. While ubiquitinylation primarily targets proteins for proteasomal degradation, sumoylation has been implicated to exert a regulatory influence on its target proteins such as altered subcellular localization, protein–protein interactions and enzymatic activity [29]. In a recent study, the ‘molecular switch’ between sumoylation and desumoylation of SIRT1 has been identified as a regulatory mechanism in the control of SIRT1 enzymatic activity in response to genotoxic stress [30]. In the absence of DNA-damage, Lys734, which is located within the carboxy-terminal domain of SIRT1, is targeted by SUMO-1 both in vitro and in vivo, whereas the presence of genotoxic stress leads to SENP-1-mediated desumoylation of SIRT1.
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Sumoylation of Lys734 significantly contributes to the enzymatic activity of SIRT1 since abrogation of sumoylation by site-directed mutagenesis revealed an impaired deacetylase activity in vitro and in vivo towards known substrates such as p53 (Lys382) and histone H4 (Lys16). Notably, the nuclear localization of SIRT1 was not affected. As a physiological outcome, desumoylation of SIRT1 during genotoxic stress counteracts its enzymatic activity thereby promoting the activity of its apoptotic substrates which tips the balance towards increased cell death. Phosphorylation Reversible protein phosphorylation is arguably the most common post-translational modification that functions as a ‘molecular switch’ in the concerted control of biological systems. Large-scale mass spectrometry studies on HeLa nuclear cell extracts identified Ser27 and Ser47 to be potential phosphorylation sites which are located within the amino-terminal domain of SIRT1 [31,32]. Up to now, neither a function nor a phosphorylating kinase has yet been identified to target any of these potential phosphorylation sites. Only one study indirectly suggested a physiological function for the phosphorylation of murine SIRT1 (SIR2alpha), since the treatment with LY294002, an inhibitor of phosphoinositide 3-OH kinase (PI3K), affected the subcellular localization of an EGFP-SIRT1 fusion protein in undifferentiated C2C12 cells [33]. So far, there is no experimental evidence of a direct phosphorylation of either the murine and/or the human SIRT1 protein by PI3K or any other kinase. In consideration of the demonstrated impact of reversible phosphorylation on the regulation of class I and II HDAC proteins (for review see [34]) and most recently on SIRT2 [35–38], the analysis of SIRT1 phosphorylation as a potential post-translational regulatory mechanism is of particular importance. SIRT1 regulation via its subcellular localization SIRT1 has initially been thought to represent an exclusively nuclear protein that is distributed over the whole cell during mitosis [39]. However, depending on cell lines and organisms examined, a partial or temporary cytoplasmic localization was observed in murine pancreatic beta cells [40], neonatal rat cardiomyocytes [41], and for Drosophila SIR2 (dSIR2) [42]. Since SIRT1 deacetylates both, Histones and non-histone proteins such as a number of transcription factors, changes in subcellular localization are likely to play a role in the regulation of its function. Two independent studies confirmed that murine SIRT1, which shares about 84% homology with human SIRT1, is indeed subjected to nucleo-cytoplasmic shuttling upon oxidative stress [33,43]. Both studies demonstrate that a cytoplasmic accumulation of SIRT1 sequesters it away from its nuclear substrates rendering SIRT1 unable to exert its inactivating function on its predominantly anti-apoptotic substrates. Consequently, the cytoplasmic localization of SIRT1 appears to sensitize cells to oxidative stress-mediated apoptosis. As shown by analyses with EGFP-SIRT1 deletion constructs [33,43] as well as heterokaryon shuttling assays [33], the cytoplasmic localization of SIRT1 is regulated by CRM1-mediated nuclear export. Using site-directed mutagenesis, Tanno and coworkers [33] identified two functional nuclear localization signals (NLS) NLS1 and NLS2 (spanning amino acids 31–38 and 223–230, respectively) as well as two nuclear export signals (NES) NES1 and NES2 (spanning amino acids 138–145 and 425–431). In agreement with this observation, EGFP-SIRT1 deletion construct analyses of Jin and coworkers [43] indicated the existence of a NES in the region spanning SIRT1 amino acids 238–517. Despite the fact that the NES and NLS sequences of both murine and human SIRT1 are highly conserved, the functional significance of nucleo-cytoplasmic
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shuttling on the regulation of human SIRT1 requires further investigation. SIRT1 regulation via protein–protein interactions Regulation of protein function has been well-established as a key regulatory mechanism for class I and II HDAC proteins (for review see [34]). So far, two proteins have been identified to regulate the SIRT1 activity both positively and negatively through complex formation in the context of the cellular stress response. The first direct regulator of SIRT1 which has initially been identified by yeast two-hybrid screening was the active regulator of SIRT1 (AROS) [44], which was then further confirmed by GST-pulldown analyses and co-immunoprecipitation experiments. Deletion construct analyses identified the AROS protein region spanning amino acids 114–217 to directly interact with the amino-terminal domain of SIRT1. The AROS protein is known to significantly enhance the activity of SIRT1 on acetylated p53 both in vitro and in cell lines thereby promoting the inhibitory effect of SIRT1 on p53-mediated transcriptional activity of pro-apoptotic genes (e.g. Bax and p21Waf-1) under conditions of DNA-damage. Remarkably, the activating effect of AROS is more pronounced in vivo which implies that another cellular factor, either a co-repressor or co-activator, may be involved in the regulation of SIRT1. While AROS acts as a positive regulator of SIRT1, complex formation of DBC-1 (deleted in breast cancer-1) with SIRT1 has most recently been identified to negatively regulate SIRT1 activity by a mechanism, which is currently poorly understood [45–48]. Deletion construct analyses of both SIRT1 and DBC-1 assigned the region of interaction to a leucine zipper motif within the amino-terminal domain of DBC-1 (aa positions 243–264) and the SIRT1 deacetylase domain. DBC-1 interacts with SIRT1 but not with other members of the sirtuin family (SIRT2–SIRT7), which implies that the underlying regulatory mechanism is highly specific. Unlike the complex formation with AROS, DBC-1 inhibits the activity of SIRT1 both in vitro and in vivo in the context of cellular stress and may be explained either on the basis of a deacetylation-mediated transcriptional activation of its anti-apoptotic targets (MnSOD and Gadd45) or transcriptional repression of its pro-apoptotic substrates such as BIM [46], which in the end lead to the sensitization of cells for cell death. The mechanism by which DBC-1 inhibits SIRT1 activity is likely to be based on direct substrate competition, since (a) DBC-1-binding defective SIRT1 mutants are not affected by DBC-1 enzymatic activity and (b) the binding of SIRT1 to its substrates such as p53, or FOXO3 is significantly abrogated by co-expression of DBC-1. The insights into the molecular mechanisms underlying SIRT1 activation by AROS and SIRT1 inactivation through DBC-1 are currently poor and remain to be further elucidated. Conclusions SIRT1 is an emerging molecular target in the development of innovative treatment strategies for cancer, neurodegenerative disorders and metabolic disease. Combined with the lack of sensitivity for the known class I and II HDAC inhibitors, research on the regulation of SIRT1 is increasingly attracting attention to the field. In this regard, it is interesting to note that many of the regulatory processes described so far, depend on the unique amino- and carboxy-terminal extensions of SIRT1. In contrast to the highly conserved deacetylase domain, these extensions exhibit a remarkable divergence among sirtuin family members and are therefore ascribed to mediate the specific functions of every sirtuin. While we discussed the different mechanisms of SIRT1 regulation separately for reasons of clarity, they are actually most likely to be interconnected as demonstrated in the case of PI3K-mediated
phosphorylation of murine SIRT1 which affects its subcellular localization. Furthermore, the partial opposite regulation of SIRT1 under the same cellular conditions, arguing for the importance of SIRT1 for the maintenance of the cellular homeostasis, requires further investigation. However, studies of SIRT1 regulation warrant further analysis especially with regard to the relevance of reversible phosphorylation which has already been demonstrated to be a key regulatory mechanism for class I, II, and IV HDACs. Acknowledgments We apologize to all investigators whose relevant work was not discussed or cited directly in this manuscript due to space limitations. This work was supported by grants from the Deutsche José Carreras-Leukämie Stiftung e.V. (DJCLS R 05/11) and the Deutsche Forschungsgemeinschaft (DFG, MA 2057/2-4). References [1] J. Landry, A. Sutton, S.T. Tafrov, R.C. Heller, J. Stebbins, L. Pillus, R. Sternglanz, The silencing protein SIR2 and its homologs are NAD-dependent protein deacetylases, Proc. Natl. Acad. Sci. USA 97 (2000) 5807–5811. [2] K.G. Tanner, J. Landry, R. Sternglanz, J.M. Denu, Silent information regulator 2 family of NAD-dependent histone/protein deacetylases generates a unique product 1-O-acetyl-ADP-ribose, Proc. Natl. Acad. Sci. USA 97 (2000) 14178– 14182. [3] R.A. Frye, Characterization of five human cDNAs with homology to the yeast SIR2 gene: Sir2-like proteins (sirtuins) metabolize NAD and may have protein ADP-ribosyltransferase activity, Biochem. Biophys. Res. Commun. 260 (1999) 273–279. [4] R.A. Frye, Phylogenetic classification of prokaryotic and eukaryotic Sir2-like proteins, Biochem. Biophys. Res. Commun. 273 (2000) 793–798. [5] S. Michan, D. Sinclair, Sirtuins in mammals: insights into their biological function, Biochem. J. 404 (2007) 1–13. [6] A.A. Sauve, C. Wolberger, V.L. Schramm, J.D. Boeke, The biochemistry of sirtuins, Annu. Rev. Biochem. (2006). [7] J.T. Rodgers, C. Lerin, W. Haas, S.P. Gygi, B.M. Spiegelman, P. Puigserver, Nutrient control of glucose homeostasis through a complex of PGC-1alpha and SIRT1, Nature 434 (2005) 113–118. [8] M. Lagouge, C. Argmann, Z. Gerhart-Hines, H. Meziane, C. Lerin, F. Daussin, N. Messadeq, J. Milne, P. Lambert, P. Elliott, B. Geny, M. Laakso, P. Puigserver, J. Auwerx, Resveratrol improves mitochondrial function and protects against metabolic disease by activating SIRT1 and PGC-1alpha, Cell 127 (2006) 1109– 1122. [9] K.S. Shindler, E. Ventura, T.S. Rex, P. Elliott, A. Rostami, SIRT1 activation confers neuroprotection in experimental optic neuritis, Invest. Ophthalmol. Vis. Sci. 48 (2007) 3602–3609. [10] D. Kim, M.D. Nguyen, M.M. Dobbin, A. Fischer, F. Sananbenesi, J.T. Rodgers, I. Delalle, J.A. Baur, G. Sui, S.M. Armour, P. Puigserver, D.A. Sinclair, L.H. Tsai, SIRT1 deacetylase protects against neurodegeneration in models for Alzheimer’s disease and amyotrophic lateral sclerosis, EMBO J. 26 (2007) 3169–3179. [11] J. Ford, M. Jiang, J. Milner, Cancer-specific functions of SIRT1 enable human epithelial cancer cell growth and survival, Cancer Res. 65 (2005) 10457– 10463. [12] C.S. Lim, SIRT1: tumor promoter or tumor suppressor?, Med Hypotheses 67 (2006) 341–344. [13] E. Langley, M. Pearson, M. Faretta, U.M. Bauer, R.A. Frye, S. Minucci, P.G. Pelicci, T. Kouzarides, Human SIR2 deacetylates p53 and antagonizes PML/p53induced cellular senescence, EMBO J. 21 (2002) 2383–2396. [14] J. Luo, A.Y. Nikolaev, S. Imai, D. Chen, F. Su, A. Shiloh, L. Guarente, W. Gu, Negative control of p53 by Sir2alpha promotes cell survival under stress, Cell 107 (2001) 137–148. [15] K. Pruitt, R.L. Zinn, J.E. Ohm, K.M. McGarvey, S.H. Kang, D.N. Watkins, J.G. Herman, S.B. Baylin, Inhibition of SIRT1 reactivates silenced cancer genes without loss of promoter DNA hypermethylation, PLoS Genet. 2 (2006) e40. [16] H. Vaziri, S.K. Dessain, E. Ng Eaton, S.I. Imai, R.A. Frye, T.K. Pandita, L. Guarente, R.A. Weinberg, hSIR2(SIRT1) functions as an NAD-dependent p53 deacetylase, Cell 107 (2001) 149–159. [17] Z.Z. Chong, S.H. Lin, F. Li, K. Maiese, The sirtuin inhibitor nicotinamide enhances neuronal cell survival during acute anoxic injury through AKT BAD PARP and mitochondrial associated anti-apoptotic pathways, Curr. Neurovasc. Res. 2 (2005) 271–285. [18] J.C. Milne, P.D. Lambert, S. Schenk, D.P. Carney, J.J. Smith, D.J. Gagne, L. Jin, O. Boss, R.B. Perni, C.B. Vu, J.E. Bemis, R. Xie, J.S. Disch, P.Y. Ng, J.J. Nunes, A.V. Lynch, H. Yang, H. Galonek, K. Israelian, W. Choy, A. Iffland, S. Lavu, O. Medvedik, D.A. Sinclair, J.M. Olefsky, M.R. Jirousek, P.J. Elliott, C.H. Westphal, Small molecule activators of SIRT1 as therapeutics for the treatment of type 2 diabetes, Nature 450 (2007) 712–716.
B. Zschoernig, U. Mahlknecht / Biochemical and Biophysical Research Communications 376 (2008) 251–255 [19] H. Ota, E. Tokunaga, K. Chang, M. Hikasa, K. Iijima, M. Eto, K. Kozaki, M. Akishita, Y. Ouchi, M. Kaneki, Sirt1, inhibitor, Sirtinol induces senescence-like growth arrest with attenuated Ras-MAPK signaling in human cancer cells, Oncogene 25 (2006) 176–185. [20] W.Y. Chen, D.H. Wang, R.C. Yen, J. Luo, W. Gu, S.B. Baylin, Tumor suppressor HIC1 directly regulates SIRT1 to modulate p53-dependent DNA-damage responses, Cell 123 (2005) 437–448. [21] S. Nemoto, M.M. Fergusson, T. Finkel, Nutrient availability regulates SIRT1 through a forkhead-dependent pathway, Science 306 (2004) 2105–2108. [22] X. Zhang, Y. Ozawa, H. Lee, Y.D. Wen, T.H. Tan, B.E. Wadzinski, E. Seto, Histone deacetylase 3 (HDAC3) activity is regulated by interaction with protein serine/ threonine phosphatase 4, Genes Dev. 19 (2005) 827–839. [23] C. Wang, L. Chen, X. Hou, Z. Li, N. Kabra, Y. Ma, S. Nemoto, T. Finkel, W. Gu, W.D. Cress, J. Chen, Interactions between E2F1 and SirT1 regulate apoptotic response to DNA damage, Nat. Cell Biol. 8 (2006) 1025–1031. [24] N. Stankovic-Valentin, S. Deltour, J. Seeler, S. Pinte, G. Vergoten, C. Guerardel, A. Dejean, D. Leprince, An acetylation/deacetylation–SUMOylation switch through a phylogenetically conserved psiKXEP motif in the tumor suppressor HIC1 regulates transcriptional repression activity, Mol. Cell. Biol. 27 (2007) 2661– 2675. [25] C. Cheadle, J. Fan, Y.S. Cho-Chung, T. Werner, J. Ray, L. Do, M. Gorospe, K.G. Becker, Stability regulation of mRNA and the control of gene expression, Ann. NY Acad. Sci. 1058 (2005) 196–204. [26] C.M. Brennan, J.A. Steitz, HuR and mRNA stability, Cell. Mol. Life Sci. 58 (2001) 266–277. [27] K. Abdelmohsen, R. Pullmann Jr., A. Lal, H.H. Kim, S. Galban, X. Yang, J.D. Blethrow, M. Walker, J. Shubert, D.A. Gillespie, H. Furneaux, M. Gorospe, Phosphorylation of HuR by Chk2 regulates SIRT1 expression, Mol. Cell 25 (2007) 543–557. [28] E.T. Yeh, L. Gong, T. Kamitani, Ubiquitin-like proteins: new wines in new bottles, Gene 248 (2000) 1–14. [29] A. Verger, J. Perdomo, M. Crossley, Modification with SUMO. A role in transcriptional regulation, EMBO Rep. 4 (2003) 137–142. [30] Y. Yang, W. Fu, J. Chen, N. Olashaw, X. Zhang, S.V. Nicosia, K. Bhalla, W. Bai, SIRT1 sumoylation regulates its deacetylase activity and cellular response to genotoxic stress, Nat. Cell Biol. 9 (2007) 1253–1262. [31] S.A. Beausoleil, M. Jedrychowski, D. Schwartz, J.E. Elias, J. Villen, J. Li, M.A. Cohn, L.C. Cantley, S.P. Gygi, Large-scale characterization of HeLa cell nuclear phosphoproteins, Proc. Natl. Acad. Sci. USA 101 (2004) 12130–12135. [32] S.A. Beausoleil, J. Villen, S.A. Gerber, J. Rush, S.P. Gygi, A probability-based approach for high-throughput protein phosphorylation analysis and site localization, Nat. Biotechnol. 24 (2006) 1285–1292. [33] M. Tanno, J. Sakamoto, T. Miura, K. Shimamoto, Y. Horio, Nucleocytoplasmic shuttling of the NAD+dependent histone deacetylase SIRT1, J. Biol. Chem. 282 (2007) 6823–6832.
255
[34] N. Sengupta, E. Seto, Regulation of histone deacetylase activities, J. Cell. Biochem. 93 (2004) 57–67. [35] R. Pandithage, R. Lilischkis, K. Harting, A. Wolf, B. Jedamzik, J. Luscher-Firzlaff, J. Vervoorts, E. Lasonder, E. Kremmer, B. Knoll, B. Luscher, The regulation of SIRT2 function by cyclin-dependent kinases affects cell motility, J. Cell Biol. 180 (2008) 915–929. [36] B.J. North, E. Verdin, Mitotic regulation of SIRT2 by cyclin-dependent kinase 1-dependent phosphorylation, J. Biol. Chem. 282 (2007) 19546– 19555. [37] F. Nahhas, S.C. Dryden, J. Abrams, M.A. Tainsky, Mutations in SIRT2 deacetylase which regulate enzymatic activity but not its interaction with HDAC6 and tubulin, Mol. Cell. Biochem. 303 (2007) 221–230. [38] S.C. Dryden, F.A. Nahhas, J.E. Nowak, A.S. Goustin, M.A. Tainsky, Role for human SIRT2 NAD-dependent deacetylase activity in control of mitotic exit in the cell cycle, Mol. Cell. Biol. 23 (2003) 3173–3185. [39] E. Michishita, J.Y. Park, J.M. Burneskis, J.C. Barrett, I. Horikawa, Evolutionarily conserved and nonconserved cellular localizations and functions of human SIRT proteins, Mol. Biol. Cell 16 (2005) 4623–4635. [40] K.A. Moynihan, A.A. Grimm, M.M. Plueger, E. Bernal-Mizrachi, E. Ford, C. CrasMeneur, M.A. Permutt, S. Imai, Increased dosage of mammalian Sir2 in pancreatic beta cells enhances glucose-stimulated insulin secretion in mice, Cell Metab. 2 (2005) 105–117. [41] I.Y. Chen, J. Lypowy, J. Pain, D. Sayed, S. Grinberg, R.R. Alcendor, J. Sadoshima, M. Abdellatif, Histone H2A.z is essential for cardiac myocyte hypertrophy but opposed by silent information regulator 2alpha, J. Biol. Chem. 281 (2006) 19369–19377. [42] M.I. Rosenberg, S.M. Parkhurst, Drosophila Sir2 is required for heterochromatic silencing and by euchromatic Hairy/E(Spl) bHLH repressors in segmentation and sex determination, Cell 109 (2002) 447–458. [43] Q. Jin, T. Yan, X. Ge, C. Sun, X. Shi, Q. Zhai, Cytoplasm-localized SIRT1 enhances apoptosis, J. Cell. Physiol. 213 (2007) 88–97. [44] E.J. Kim, J.H. Kho, M.R. Kang, S.J. Um, Active regulator of SIRT1 cooperates with SIRT1 and facilitates suppression of p53 activity, Mol. Cell 28 (2007) 277–290. [45] W. Zhao, J.P. Kruse, Y. Tang, S.Y. Jung, J. Qin, W. Gu, Negative regulation of the deacetylase SIRT1 by DBC1, Nature 451 (2008) 587–590. [46] J.E. Kim, J. Chen, Z. Lou, DBC1 is a negative regulator of SIRT1, Nature 451 (2008) 583–586. [47] M. Hamaguchi, J.L. Meth, C. von Klitzing, W. Wei, D. Esposito, L. Rodgers, T. Walsh, P. Welcsh, M.C. King, M.H. Wigler, DBC2, a candidate for a tumor suppressor gene involved in breast cancer, Proc. Natl. Acad. Sci. USA 99 (2002) 13647–13652. [48] R. Sundararajan, G. Chen, C. Mukherjee, E. White, Caspase-dependent processing activates the proapoptotic activity of deleted in breast cancer-1 during tumor necrosis factor-alpha-mediated death signaling, Oncogene 24 (2005) 4908–4920.