Carta Acesso aberto Revisado por pares

NO Targets SIRT1

2008; Lippincott Williams & Wilkins; Volume: 28; Issue: 9 Linguagem: Estoniano

10.1161/atvbaha.108.173682

ISSN

1524-4636

Autores

Michael Potente, Stefanie Dimmeler,

Tópico(s)

Adipose Tissue and Metabolism

Resumo

HomeArteriosclerosis, Thrombosis, and Vascular BiologyVol. 28, No. 9NO Targets SIRT1 Free AccessEditorialPDF/EPUBAboutView PDFView EPUBSections ToolsAdd to favoritesDownload citationsTrack citationsPermissions ShareShare onFacebookTwitterLinked InMendeleyReddit Jump toFree AccessEditorialPDF/EPUBNO Targets SIRT1A Novel Signaling Network in Endothelial Senescence Michael Potente and Stefanie Dimmeler Michael PotenteMichael Potente From the Institute for Cardiovascular Regeneration (M.P., S.D.), Centre of Molecular Medicine, and the Department of Cardiology (M.P.), Internal Medicine III, Goethe University, Frankfurt, Germany. and Stefanie DimmelerStefanie Dimmeler From the Institute for Cardiovascular Regeneration (M.P., S.D.), Centre of Molecular Medicine, and the Department of Cardiology (M.P.), Internal Medicine III, Goethe University, Frankfurt, Germany. Originally published1 Sep 2008https://doi.org/10.1161/ATVBAHA.108.173682Arteriosclerosis, Thrombosis, and Vascular Biology. 2008;28:1577–1579Aging is considered to be a major risk factor for the development of cardiovascular diseases, the leading cause of morbidity and mortality in Western countries. Among the age-associated functional and structural changes in the vascular wall, particularly endothelial function declines during the ageing process. Impairment of endothelial function critically contributes to the pathogenesis of several cardiovascular diseases (eg, atherosclerosis) and is manifested in its earliest form as an attenuation of endothelium-dependent dilator responses as a consequence of an alteration in the expression or activity of the endothelial nitric oxide (NO) synthase (eNOS) and increased oxidative stress. Endothelial senescence appears to play a key role in the process of vascular aging, affecting vascular tone, blood vessel growth, and regeneration. Thus, identifying new molecular targets involved in senescence signaling offers opportunities to potentially improve endothelial cell function and cardiovascular disease progression.See accompanying article on page 1634In a study published in the present issue of Arteriosclerosis, Thrombosis, and Vascular Biology, Ota et al report that the PDE3 inhibitor cilostazol prevents endothelial premature senescence by a NO-dependent upregulation of SIRT1, a key regulator of ageing and longevity in lower organisms.1 Apart from the relevance of these findings for improving the understanding of vascular endothelial senescence pathways, they point to SIRT1 as an important modulator of signaling networks critical for maintaining vascular endothelial homeostasis and suggest novel therapeutic opportunities for the treatment of cardiovascular diseases.Sirtuins, Senescence, and AgingCellular senescence has been used as a model for mammalian aging. This process consists of a state of permanent cell cycle arrest associated with characteristic changes in cell morphology and gene expression. Most mammalian cells undergo a limited number of divisions in culture, eventually entering a state of cellular senescence. This process, also referred to as replicative senescence, is related to the attrition of telomeres after extended propagation in culture. In addition to the shortening of telomere ends, several cellular stressors, such as oxidative stress, induce a similar growth arrest within just a few days, referred to as stress-induced premature senescence. Although endothelial cells are rarely dividing under normal conditions and exhibit a turnover rate estimated in the range of years, endothelial cell senescence may play a critical role when endothelial proliferation and regeneration is essential to maintain the functional activity of the endothelial monolayer, such as after mechanical endothelial injury (eg, balloon dilatation) or continuous exposure to proapoptotic stimuli. In addition, the restoration of blood supply after ischemia or during wound healing requires extensive proliferative activity of endothelial cells during angiogenesis and tissue regeneration.Sirtuins (SIRTs) have been shown to regulate cellular senescence and are generally considered as longevity factors, based on the experimental observation that increased expression of Sir2 orthologs is sufficient to increase life span in lower organisms.2 Mammalian SIRTs are evolutionarily conserved and regulate a variety of physiological processes such as stress responses, genome maintenance, and metabolism (Figure 1). The sirtuin family consists of seven family members (SIRT1–7), each containing a conserved catalytic core domain. Mammalian sirtuins have diverse cellular localizations, modify multiple substrates, and affect numerous cellular functions.3 Most sirtuins (SIRT1, SIRT2, SIRT3, and SIRT5) catalyze the NAD+-dependent deacetylation, whereas SIRT4 and SIRT6 mediate the ADP-ribosylation of protein substrates. Among the sirtuin family, SIRT1 is the closest mammalian homologue of yeast Sir2, which has emerged as an important regulator of tissue homeostasis and stress responses. Though Sir2 is generally believed to protect cells against cell stress and to extend life span in response to caloric restriction in model organisms, it remains uncertain whether SIRT1 has a similar antiageing effect in mammals. Indeed, data from recent studies suggest that SIRT1 can have both pro- and antiageing roles. While inhibition of SIRT1 was overall associated with a shortened life span in mice, it also induced cellular phenotypes consistent with a slower aging phenotype in neurons.4 These opposing phenotypes are reminiscent of the controversial role of IGF1 in aging of model organisms and mammals, indicating that the regulation of ageing in mammals is more complex. With respect to SIRT1, the conflicting reports might be rationalized by the plethora of substrates SIRT1 targets for deacetylation. The physiological effects are mediated by SIRT1-dependent deacetylation of substrates, including histones, Foxos, NF-κB, PGC-1, LXR1, and p53.3 The tumor suppressor p53 was among the first nonhistone substrates shown to be functionally regulated by reversible acetylation, and SIRT1 promotes cell survival in response to cellular stress by deacetylating p53, which decreases p53 stability and activity.5,6 In fibroblasts, however, SIRT1 increases total p53 levels on conditions of chronic oxidative damage, causing cellular senescence.7Download figureDownload PowerPointFigure 1. Sirtuins and ageing. A, Sir-2.1 increases life span in C elegans by regulating the Foxo transcription factor DAF16. B, Signaling networks of SIRT1 in mammalian cells involved in the regulation of stress responses and metabolism. To which extent and by which molecular mechanisms SIRT1 affects ageing in mammals has yet to be determined. *It has been shown that SIRT1 can act as a repressor or activator of Foxo transcription factors depending on the target gene promoter.The SIRT1–eNOS Axis: A Key Pathway for Maintaining Vascular HomeostasisOta and coworkers now report that the PDE3 inhibitor, cilostazol, a vasodilating antiplatelet drug for peripheral artery disease, inhibited oxidative stress–induced premature senescence by enhancing the expression of SIRT1 in human endothelial cells in vitro and in vivo (Figure 2). The findings are based on a previous study by this group, which demonstrated that overexpression of SIRT1 prevented oxidative stress–induced endothelial senescence, whereas inhibition of SIRT1 activity or expression induced premature senescence-like phenotypes in endothelial cells.8 These studies suggested that SIRT1-activating drugs might exert protective effects on the vascular endothelium. Using H2O2-induced premature senescence in cultured endothelial cells or paraquat-treated mice as models for endothelial cell senescence, Ota et al show that cilostazol leads to the activation of eNOS through a cAMP/PKA- and PI3K/Akt-dependent pathway by inducing the phosphorylation of eNOS at Ser1177. Further detailed analysis revealed that cilostazol enhances the mRNA and protein expression of SIRT1, which requires the generation of nitric oxide by the endothelial nitric oxide synthase (Figure 2). Indeed, nitric oxide has been shown to activate the SIRT1 promoter in white adipose tissue, where caloric restriction, a dietary regimen known to extend life span, induces an increase in eNOS expression, which is in turn involved in both mitochondrial biogenesis and SIRT1 expression.9 Moreover, SIRT1 deacetylates and activates the eNOS enzyme,10 indicating that a positive feedback mechanism exists between these two key signaling molecules. Thus, activating SIRT1 through small molecules may help to reset the activity of eNOS during situations of endothelial dysfunction where nitric oxide availability is limited. Download figureDownload PowerPointFigure 2. Regulation of endothelial senescence by cilostazol. Cilostazol activates eNOS through a cAMP/PKA- and PI3K/Akt-dependent pathway by inducing the phosphorylation of eNOS at serine 1177 leading to enhanced NO generation. eNOS-derived NO enhances SIRT1 mRNA and protein expression by a yet unknown molecular mechanism. SIRT1, in turn, can activate the eNOS enzyme by deacetylating lysines 496 and 506 in the calmodulin-binding domain of eNOS leading to enhanced NO production.How does SIRT1 exert its beneficial effects on the vascular endothelium? The authors suggest that p53 might be of utmost importance. Indeed, p53 is itself acetylated in response to oxidative stress and has been identified as a key regulator of senescence signaling in different cell types.5,6 By assessing acetylation of p53 at lysine 373/382 in response to H2O2 treatment, the authors show that cilostazol decreased acetylation of p53 at SIRT1-targeted lysine residues. Obviously, these findings do not exclude the possibility that the protective effect of SIRT1 is also mediated by other SIRT1 targets known to affect senescence and endothelial cell biology. Indeed, SIRT1 has been shown to associate with Foxos in an acetylation-dependent manner, eg, in response to oxidative stress, thereby "tipping" Foxo responses away from apoptosis and toward stress resistance.11 Although an interaction between SIRT1 and Foxos in endothelial cells has so far only been documented for Foxo1 in the context of angiogenesis signaling,12 it is more than likely that the conserved SIRT1-Foxo interaction is also operational during stress-induced endothelial senescence.In addition to its cell-intrinsic function in endothelial cells, the well known effects of SIRT1 on cholesterol metabolism, glucose homeostasis, and insulin resistance might also beneficially affect vascular homeostasis and cardiovascular disease progression. Thus, these findings point to SIRT1 as a point of convergence of several signaling pathways critical for homeostatic endothelial functions and identify the cardiovascular system as an important target tissue for the direct and indirect actions of SIRT1.SIRT1: A Therapeutic Target in Cardiovascular DiseaseGiven the experimental observation that the effects of cilostazol were dependent on the functional activity of SIRT1, the study by Ota et al may also have implications for future design of novel cardiovascular drugs. Because SIRT1 has enzymatic activity, it has become an attractive target for drug development. Consistent with these considerations, recent studies reported that resveratrol, a polyphenolic activator of SIRT1, ameliorated insulin resistance and prolonged survival in mice fed a high-fat diet.13,14 Moreover, the finding that small molecule activators of SIRT1, which are 1000-fold more potent than resveratrol, induce many of the beneficial metabolic changes observed after resveratrol treatment,15 suggests that studying the regulation and function of SIRT1 may not only open new research avenues but might also offer novel opportunities for translational cardiovascular medicine.While these considerations imply that strategies to upregulate/enhance the activity of SIRT1 might serve as a potential therapy for preventing age-associated cardiovascular diseases, the rather ubiquitous expression and broad effects of SIRT1 might also pose significant hurdles with regard to specificity and side-effects. Therefore, indirect means to activate SIRT1 by stimulating its expression more specifically in endothelial cells, eg, by PDE inhibitors or activation of eNOS, might be alternative strategies to improve endothelial cell functions and antagonize endothelial cell aging.We apologize to those investigators whose work could not be cited in this article owing to space limitations.Sources of FundingThis work was supported by the DFG (PO1306/1-1 and Exc 147/1).DisclosuresNone.FootnotesCorrespondence to Stefanie Dimmeler, PhD, Institute for Cardiovascular Regeneration, Centre of Molecular Medicine, Goethe University, Theodor-Stern-Kai 7, D-60590 Frankfurt (Main), Germany. E-mail [email protected] References 1 Ota H, Eto M, Kano MR, Ogawa S, Iijima K, Akishita M, Ouchi Y. Cilostazol inhibits oxidative stress-induced premature senescence via upregulation of Sirt1 in human endothelial cells. Arterioscler Thromb Vasc Biol. 2008; 28: 1634–1639.LinkGoogle Scholar2 Guarente L, Picard F. Calorie restriction–the SIR2 connection. Cell. 2005; 120: 473–482.CrossrefMedlineGoogle Scholar3 Haigis MC, Guarente LP. Mammalian sirtuins–emerging roles in physiology, aging, and calorie restriction. Genes Dev. 2006; 20: 2913–2921.CrossrefMedlineGoogle Scholar4 Li Y, Xu W, McBurney MW, Longo VD. SirT1 Inhibition Reduces IGF-I/IRS-2/Ras/ERK1/2 Signaling and Protects Neurons. Cell Metab. 2008; 8: 38–48.CrossrefMedlineGoogle Scholar5 Vaziri H, Dessain SK, Ng Eaton E, Imai SI, Frye RA, Pandita TK, Guarente L, Weinberg RA. hSIR2(SIRT1) functions as an NAD-dependent p53 deacetylase. Cell. 2001; 107: 149–159.CrossrefMedlineGoogle Scholar6 Luo J, Nikolaev AY, Imai S, Chen D, Su F, Shiloh A, Guarente L, Gu W. Negative control of p53 by Sir2alpha promotes cell survival under stress. Cell. 2001; 107: 137–148.CrossrefMedlineGoogle Scholar7 Chua KF, Mostoslavsky R, Lombard DB, Pang WW, Saito S, Franco S, Kaushal D, Cheng HL, Fischer MR, Stokes N, Murphy MM, Appella E, Alt FW. Mammalian SIRT1 limits replicative life span in response to chronic genotoxic stress. Cell Metab. 2005; 2: 67–76.CrossrefMedlineGoogle Scholar8 Ota H, Akishita M, Eto M, Iijima K, Kaneki M, Ouchi Y. Sirt1 modulates premature senescence-like phenotype in human endothelial cells. J Mol Cell Cardiol. 2007; 43: 571–579.CrossrefMedlineGoogle Scholar9 Nisoli E, Tonello C, Cardile A, Cozzi V, Bracale R, Tedesco L, Falcone S, Valerio A, Cantoni O, Clementi E, Moncada S, Carruba MO. Calorie restriction promotes mitochondrial biogenesis by inducing the expression of eNOS. Science. 2005; 310: 314–317.CrossrefMedlineGoogle Scholar10 Mattagajasingh I, Kim CS, Naqvi A, Yamamori T, Hoffman TA, Jung SB, DeRicco J, Kasuno K, Irani K. SIRT1 promotes endothelium-dependent vascular relaxation by activating endothelial nitric oxide synthase. Proc Natl Acad Sci U S A. 2007; 104: 14855–14860.CrossrefMedlineGoogle Scholar11 Brunet A, Sweeney LB, Sturgill JF, Chua KF, Greer PL, Lin Y, Tran H, Ross SE, Mostoslavsky R, Cohen HY, Hu LS, Cheng HL, Jedrychowski MP, Gygi SP, Sinclair DA, Alt FW, Greenberg ME. Stress-dependent regulation of FOXO transcription factors by the SIRT1 deacetylase. Science. 2004; 303: 2011–2015.CrossrefMedlineGoogle Scholar12 Potente M, Ghaeni L, Baldessari D, Mostoslavsky R, Rossig L, Dequiedt F, Haendeler J, Mione M, Dejana E, Alt FW, Zeiher AM, Dimmeler S. SIRT1 controls endothelial angiogenic functions during vascular growth. Genes Dev. 2007; 21: 2644–2658.CrossrefMedlineGoogle Scholar13 Baur JA, Pearson KJ, Price NL, Jamieson HA, Lerin C, Kalra A, Prabhu VV, Allard JS, Lopez-Lluch G, Lewis K, Pistell PJ, Poosala S, Becker KG, Boss O, Gwinn D, Wang M, Ramaswamy S, Fishbein KW, Spencer RG, Lakatta EG, Le Couteur D, Shaw RJ, Navas P, Puigserver P, Ingram DK, de Cabo R, Sinclair DA. Resveratrol improves health and survival of mice on a high-calorie diet. Nature. 2006; 444: 337–342.CrossrefMedlineGoogle Scholar14 Lagouge M, Argmann C, Gerhart-Hines Z, Meziane H, Lerin C, Daussin F, Messadeq N, Milne J, Lambert P, Elliott P, Geny B, Laakso M, Puigserver P, Auwerx J. Resveratrol improves mitochondrial function and protects against metabolic disease by activating SIRT1 and PGC-1alpha. Cell. 2006; 127: 1109–1122.CrossrefMedlineGoogle Scholar15 Milne JC, Lambert PD, Schenk S, Carney DP, Smith JJ, Gagne DJ, Jin L, Boss O, Perni RB, Vu CB, Bemis JE, Xie R, Disch JS, Ng PY, Nunes JJ, Lynch AV, Yang H, Galonek H, Israelian K, Choy W, Iffland A, Lavu S, Medvedik O, Sinclair DA, Olefsky JM, Jirousek MR, Elliott PJ, Westphal CH. Small molecule activators of SIRT1 as therapeutics for the treatment of type 2 diabetes. Nature. 2007; 450: 712–716.CrossrefMedlineGoogle Scholar Previous Back to top Next FiguresReferencesRelatedDetailsCited By Chao J, Guo Y and Chao L (2018) Protective Role of Endogenous Kallistatin in Vascular Injury and Senescence by Inhibiting Oxidative Stress and Inflammation, Oxidative Medicine and Cellular Longevity, 10.1155/2018/4138560, 2018, (1-8), Online publication date: 2-Dec-2018. Guo Y, Chao L and Chao J (2018) Kallistatin attenuates endothelial senescence by modulating Let‐7g‐mediated miR‐34a‐ SIRT 1‐ eNOS pathway , Journal of Cellular and Molecular Medicine, 10.1111/jcmm.13734, 22:9, (4387-4398), Online publication date: 1-Sep-2018. Triggle C and Ding H (2016) Metformin is not just an antihyperglycaemic drug but also has protective effects on the vascular endothelium, Acta Physiologica, 10.1111/apha.12644, 219:1, (138-151), Online publication date: 1-Jan-2017. Li Z, Guo G, Wang H, Si X, Zhou G, Xiong Y, Li S, Dai R and Yang C (2017) TRPC5 channel modulates endothelial cells senescence, European Journal of Pharmacology, 10.1016/j.ejphar.2017.02.037, 802, (27-35), Online publication date: 1-May-2017. MA Y, GONG X, MO Y and WU S (2016)(2016) Polydatin inhibits the oxidative stress-induced proliferation of vascular smooth muscle cells by activating the eNOS/SIRT1 pathway, International Journal of Molecular Medicine, 10.3892/ijmm.2016.2554, 37:6, (1652-1660), Online publication date: 1-Jun-2016. Saboori S, Koohdani F, Nematipour E, Yousefi Rad E, Saboor-Yaraghi A, Javanbakht M, Eshraghian M, Ramezani A and Djalali M (2016) Beneficial effects of omega-3 and vitamin E coadministration on gene expression of SIRT1 and PGC1α and serum antioxidant enzymes in patients with coronary artery disease, Nutrition, Metabolism and Cardiovascular Diseases, 10.1016/j.numecd.2015.11.013, 26:6, (489-494), Online publication date: 1-Jun-2016. Martens C and Seals D (2016) Practical alternatives to chronic caloric restriction for optimizing vascular function with ageing, The Journal of Physiology, 10.1113/JP272348, 594:24, (7177-7195), Online publication date: 15-Dec-2016. Rubio-Ruiz M, Pérez-Torres I, Soto M, Pastelín G and Guarner-Lans V (2014) Aging in blood vessels. Medicinal agents FOR systemic arterial hypertension in the elderly, Ageing Research Reviews, 10.1016/j.arr.2014.10.001, 18, (132-147), Online publication date: 1-Nov-2014. Z. GAD M, A. ABU EL MAATY M, A. EL-MARAGHY S, T. FAHIM A and A. HAMDY M (2014) Investigating the Cardio-Protective Abilities of Supplemental L-Arginine on Parameters of Endothelial Function in a Hypercholesterolemic Animal Model, Journal of Nutritional Science and Vitaminology, 10.3177/jnsv.60.145, 60:3, (145-151), . Koka S, Aluri H, Xi L, Lesnefsky E and Kukreja R (2014) Chronic inhibition of phosphodiesterase 5 with tadalafil attenuates mitochondrial dysfunction in type 2 diabetic hearts: potential role of NO/SIRT1/PGC-1α signaling, American Journal of Physiology-Heart and Circulatory Physiology, 10.1152/ajpheart.00865.2013, 306:11, (H1558-H1568), Online publication date: 1-Jun-2014. Gong X, Ma Y, Ruan Y, Fu G and Wu S (2014) Long-term atorvastatin improves age-related endothelial dysfunction by ameliorating oxidative stress and normalizing eNOS/iNOS imbalance in rat aorta, Experimental Gerontology, 10.1016/j.exger.2014.01.015, 52, (9-17), Online publication date: 1-Apr-2014. Davis P, Pagnin E, Dal Maso L, Caielli P, Maiolino G, Fusaro M, Paolo Rossi G and Calò L (2013) SIRT1, heme oxygenase-1 and NO-mediated vasodilation in a human model of endogenous angiotensin II type 1 receptor antagonism: implications for hypertension, Hypertension Research, 10.1038/hr.2013.48, 36:10, (873-878), Online publication date: 1-Oct-2013. Bachschmid M, Schildknecht S, Matsui R, Zee R, Haeussler D, A. Cohen R, Pimental D and Loo B (2012) Vascular aging: Chronic oxidative stress and impairment of redox signaling—consequences for vascular homeostasis and disease, Annals of Medicine, 10.3109/07853890.2011.645498, 45:1, (17-36), Online publication date: 1-Feb-2013. Sundar I and Rahman I (2013) Reactive Oxygen Species, Kinase Signaling, and Redox Regulation of Epigenetics Oxidative Stress and Redox Regulation, 10.1007/978-94-007-5787-5_11, (309-342), . Sundar I, Yao H and Rahman I (2013) Oxidative Stress and Chromatin Remodeling in Chronic Obstructive Pulmonary Disease and Smoking-Related Diseases, Antioxidants & Redox Signaling, 10.1089/ars.2012.4863, 18:15, (1956-1971), Online publication date: 20-May-2013. Triggle C, Samuel S, Ravishankar S, Marei I, Arunachalam G and Ding H (2012) The endothelium: influencing vascular smooth muscle in many ways, Canadian Journal of Physiology and Pharmacology, 10.1139/y2012-073, 90:6, (713-738), Online publication date: 1-Jun-2012. Yao H and Rahman I (2011) Current concepts on oxidative/carbonyl stress, inflammation and epigenetics in pathogenesis of chronic obstructive pulmonary disease, Toxicology and Applied Pharmacology, 10.1016/j.taap.2009.10.022, 254:2, (72-85), Online publication date: 1-Jul-2011. de Kreutzenberg S, Ceolotto G, Papparella I, Bortoluzzi A, Semplicini A, Man C, Cobelli C, Fadini G and Avogaro A (2010) Downregulation of the Longevity-Associated Protein Sirtuin 1 in Insulin Resistance and Metabolic Syndrome: Potential Biochemical Mechanisms, Diabetes, 10.2337/db09-1187, 59:4, (1006-1015), Online publication date: 1-Apr-2010. Homma K, Sone M, Taura D, Yamahara K, Suzuki Y, Takahashi K, Sonoyama T, Inuzuka M, Fukunaga Y, Tamura N, Itoh H, Yamanaka S and Nakao K (2010) Sirt1 plays an important role in mediating greater functionality of human ES/iPS-derived vascular endothelial cells, Atherosclerosis, 10.1016/j.atherosclerosis.2010.04.021, 212:1, (42-47), Online publication date: 1-Sep-2010. Farsetti A, Grasselli A, Bacchetti S, Gaetano C and Capogrossi M (2009) The telomerase tale in vascular aging: regulation by estrogens and nitric oxide signaling, Journal of Applied Physiology, 10.1152/japplphysiol.91360.2008, 106:1, (333-337), Online publication date: 1-Jan-2009. López-Fernández-Sobrino R, Soliz-Rueda J, Ávila-Román J, Arola-Arnal A, Suárez M, Muguerza B and Bravo F (2021) Blood Pressure-Lowering Effect of Wine Lees Phenolic Compounds Is Mediated by Endothelial-Derived Factors: Role of Sirtuin 1, Antioxidants, 10.3390/antiox10071073, 10:7, (1073) Xing J, Liu H, Yang H, Chen R, Chen Y, Xu J and Zhao Y (2014) Upregulation of Unc-51-Like Kinase 1 by Nitric Oxide Stabilizes SIRT1, Independent of Autophagy, PLoS ONE, 10.1371/journal.pone.0116165, 9:12, (e116165) September 2008Vol 28, Issue 9 Advertisement Article InformationMetrics https://doi.org/10.1161/ATVBAHA.108.173682PMID: 18716320 Originally publishedSeptember 1, 2008 PDF download Advertisement

Referência(s)