23411

SIRT1

sirtuin 1;SIRT1;sirtuin 1

NAD-dependent deacetylase sirtuin-1|NAD-dependent protein deacetylase sirtuin-1|SIR2-like protein 1|SIR2alpha|regulatory protein SIR2 homolog 1|sir2-like 1|sirtuin type 1

10q21.3

protein-coding

The yeast Sir2 protein mediates chromatin silencing through an intrinsic NAD-dependent histone deacetylase activity. Sir2 is a conserved protein and was recently shown to regulate lifespan extension both in budding yeast and worms. Here, we show that SIRT1, the human Sir2 homolog, is recruited to the promyelocytic leukemia protein (PML) nuclear bodies of mammalian cells upon overexpression of either PML or oncogenic Ras (Ha-rasV12). SIRT1 binds and deacetylates p53, a component of PML nuclear bodies, and it can repress p53-mediated transactivation. Moreover, we show that SIRT1 and p53 co-localize in nuclear bodies upon PML upregulation. When overexpressed in primary mouse embryo fibroblasts (MEFs), SIRT1 antagonizes PML-induced acetylation of p53 and rescues PML-mediated premature cellular senescence. Taken together, our data establish the SIRT1 deacetylase as a novel negative regulator of p53 function capable of modulating cellular senescence.

OBJECTIVE: Cilostazol, a selective inhibitor of PDE3, has a protective effect on endothelium after ischemic vascular damage, through production of nitric oxide (NO). The purpose of the present study was to clarify the molecular mechanisms underlying the preventive effect of treatment with cilostazol on oxidative stress-induced premature senescence in human endothelial cells. METHODS AND RESULTS: Prematurely senescent human umbilical vein endothelial cells (HUVECs) were induced by treatment with hydrogen peroxide (H(2)O(2)) as judged by senescence-associated beta-galactosidase assay (SA-betagal), cell morphological appearance, and plasminogen activator inhibitor-1 (PAI-1) expression. Treatment with H(2)O(2) caused 93% of the cells to be SA-betagal positive, whereas 46% of cilostazol (100 micromol/L)-treated cells were positive. HUVECs treated with other cAMP-elevating agents and DETA-NO showed a reduction of SA-betagal-positive cells as well. Cilostazol increased phosphorylation of Akt at Ser(473) and of endothelial nitric oxide synthase (eNOS) at Ser(1177), with a dose-dependent increase in Sirt1 expression. Moreover, the effect of cilostazol on premature senescence was abrogated through inhibition of Sirt1. CONCLUSIONS: Our results indicated that cilostazol exerted protective effects against endothelial senescence and dysfunction, and enhancement of NO production is a key mediator in upregulation of Sirt1.

OBJECTIVE: Calorie restriction (CR) prolongs the lifespan of various species, ranging from yeasts to mice. In yeast, CR extends the lifespan by increasing the activity of silencing information regulator 2 (Sir2), an NAD(+)-dependent deacetylase. SIRT1, a mammalian homolog of Sir2, has been reported to downregulate p53 activity and thereby prolong the lifespan of cells. Although recent evidence suggests a link between SIRT1 activity and metabolic homeostasis during CR, its pathological role in human disease is not yet fully understood. METHODS AND RESULTS: Treatment of human endothelial cells with high glucose decreases SIRT1 expression and thus activates p53 by increasing its acetylation. This in turn accelerates endothelial senescence and induces functional abnormalities. Introduction of SIRT1 or disruption of p53 inhibits high glucose-induced endothelial senescence and dysfunction. Likewise, activation of Sirt1 prevents the hyperglycemia-induced vascular cell senescence and thereby protects against vascular dysfunction in mice with diabetes. CONCLUSIONS: These findings represent a novel mechanism of vascular cell senescence induced by hyperglycemia and suggest a protective role of SIRT1 in the pathogenesis of diabetic vasculopathy.

Prostate cancer (PCa), next only to skin cancer, is the most commonly occurring malignancy in men in the US. Aging is recognized as a major risk factor for this neoplasm as a mans chance for developing this disease significantly increases with increasing age. Because aging is inevitable, Americans are living longer, and the existing treatments have not been able to manage this neoplasm, novel mechanism-based approaches are needed. We have recently shown that Sirt1, a sirtuin class III histone deacetylases (HDACs) originally linked to aging and longevity in yeast, was overexpressed in human PCa cells and PCa tissues obtained from patients. We also found that chemical inhibition and/or genetic knockdown of Sirt1 caused a FoxO1-mediated inhibition in the growth and viability of human PCa cells. Since p53 is a target for deacetylation by Sirt1, we wanted to determine the involvement of p53 in Sirt1 inhibition mediated responses in PCa. To achieve our objective, we utilized a pair of isogenic PCa cell lines viz. PC3 and PC3-p53, which differ only in p53 status. Our data demonstrated that Sirt1 inhibition caused a decrease in cell growth, cell viability and the colony formation ability of both cell lines. Further, Sirt1 inhibition resulted in an increase in FoxO1 acetylation and subsequent transcriptional activation in both cell types regardless of p53 status. However, an interesting observation of our study was that Sirt1 inhibition resulted in an increase in senescence in PC3-p53 cells whereas it resulted in an increase in apoptosis in PC3 cells. The results of this study compliment our previous study and suggest that Sirt1 inhibition may have different downstream targets in cells with active p53 versus cells where p53 is inactive.

BACKGROUND: Aging is a major risk factor for the development of atherosclerosis and coronary artery disease. Through a microarray approach, we have identified a microRNA (miR-217) that is progressively expressed in endothelial cells with aging. miR-217 regulates the expression of silent information regulator 1 (SirT1), a major regulator of longevity and metabolic disorders that is progressively reduced in multiple tissues during aging. METHODS AND RESULTS: miR-217 inhibits SirT1 expression through a miR-217-binding site within the 3-UTR of SirT1. In young human umbilical vein endothelial cells, human aortic endothelial cells, and human coronary artery endothelial cells, miR-217 induces a premature senescence-like phenotype and leads to an impairment in angiogenesis via inhibition of SirT1 and modulation of FoxO1 (forkhead box O1) and endothelial nitric oxide synthase acetylation. Conversely, inhibition of miR-217 in old endothelial cells ultimately reduces senescence and increases angiogenic activity via an increase in SirT1. miR-217 is expressed in human atherosclerotic lesions and is negatively correlated with SirT1 expression and with FoxO1 acetylation status. CONCLUSIONS: Our data pinpoint miR-217 as an endogenous inhibitor of SirT1, which promotes endothelial senescence and is potentially amenable to therapeutic manipulation for prevention of endothelial dysfunction in metabolic disorders.

Radiotherapy is increasingly used in the treatment of joint diseases, but limited information is available on the effects of radiation on cartilage. Here, we characterize the molecular mechanisms leading to cellular senescence in irradiated primary cultured articular chondrocytes. Ionizing radiation (IR) causes activation of ERK, in turn generating intracellular reactive oxygen species (ROS) with induction of senescence-associated beta-galactosidase (SA-beta-gal) activity. ROS activate p38 kinase, which further promotes ROS generation, forming a positive feedback loop to sustain ROS-p38 kinase signaling. The ROS inhibitors, nordihydroguaiaretic acid and GSH, suppress phosphorylation of p38 and cell numbers positive for SA-beta-gal following irradiation. Moreover, inhibition of the ERK and p38 kinase pathways leads to blockage of IR-induced SA-beta-gal activity via reduction of ROS generation. Although JNK is activated by ROS, this pathway is not associated with cellular senescence of chondrocytes. Interestingly, IR triggers down-regulation of SIRT1 protein expression but not the transcript level, indicative of post-transcriptional cleavage of the protein. SIRT1 degradation is markedly blocked by SB203589 or MG132 after IR treatment, suggesting that cleavage occurs as a result of binding with p38 kinase, followed by processing via the 26 S proteasomal degradation pathway. Overexpression or activation of SIRT1 significantly reduces the IR-induced senescence phenotype, whereas inhibition of SIRT1 activity induces senescence. Based on these findings, we propose that IR induces cellular senescence of articular chondrocytes by negative post-translational regulation of SIRT1 via ROS-dependent p38 kinase activation.

RATIONALE: Endothelial senescence causes endothelial dysfunction, promotes atherogenesis and contributes to age-related vascular disorders. SIRT1 is a conserved NAD(+)-dependent deacetylase possessing beneficial effects against aging-related diseases, despite that the detailed functional mechanisms are largely uncharacterized. OBJECTIVE: The present study is designed to evaluate the protective effects of SIRT1 on endothelial senescence and to elucidate the underlying mechanisms. METHODS AND RESULTS: An in vitro senescence model was established by prolonged culture of primary endothelial cells isolated from porcine aorta. The freshly isolated "young" cells gradually underwent senescence during 1 month of repetitive passages. Both mRNA and protein expressions of SIRT1 were progressively decreased. In contrast, the protein levels of LKB1, a serine/threonine kinase and tumor suppressor, and the phosphorylation of its downstream target AMPK(Thr172) were dramatically increased in senescent cells. Overexpression of LKB1 promoted cellular senescence and retarded endothelial proliferation, which could be blocked by increasing SIRT1 levels. Knocking down of SIRT1 induced senescence and elevated the protein levels of LKB1 and phosphorylated AMPK(Thr172). Regardless of the nutritional status, hyperactivation of AMPK was able to induce endothelial senescence. SIRT1 antagonized LKB1-dependent AMPK activation through promoting the deacetylation, ubiquitination and proteasome-mediated degradation of LKB1. The survival signaling of Akt was also found to be modulated by SIRT1 and LKB1, and could cross-regulate AMPK activity. CONCLUSIONS: SIRT1 and LKB1/AMPK are the 2 key sensor systems for regulating endothelial cell survival, proliferation and senescence. The protective activities of SIRT1 may be achieved at least in part by fine tuning the acetylation/deacetylation status and stabilities of LKB1 protein.

Autophagy is a fundamental cellular process that eliminates long-lived proteins and damaged organelles through lysosomal degradation pathway. Cigarette smoke (CS)-mediated oxidative stress induces cytotoxic responses in lung cells. However, the role of autophagy and its mechanism in CS-mediated cytotoxic responses is not known. We hypothesized that NAD(+)-dependent deacetylase, sirtuin 1 (SIRT1) plays an important role in regulating autophagy in response to CS. CS exposure resulted in induction of autophagy in lung epithelial cells, fibroblasts and macrophages. Pretreatment of cells with SIRT1 activator resveratrol attenuated CS-induced autophagy whereas SIRT1 inhibitor, sirtinol, augmented CS-induced autophagy. Elevated levels of autophagy were induced by CS in the lungs of SIRT1 deficient mice. Inhibition of poly(ADP-ribose)-polymerase-1 (PARP-1) attenuated CS-induced autophagy via SIRT1 activation. These data suggest that the SIRT1-PARP-1 axis plays a critical role in the regulation of CS-induced autophagy and have important implications in understanding the mechanisms of CS-induced cell death and senescence.

High-affinity Na(+)-dependent dicarboxylate cotransporter (NaDC3) can transport Krebs cycle intermediates into cells. Our previous study has shown that NaDC3 promotes cellular senescence, but its mechanism is not clear. It is known that when the concentration of intermediates in Krebs cycle is increased, NAD(+)/NADH ratio will be decreased. NAD(+)-dependent histone deacetylase sirtuin1 (SIRT1) prolongs mammalian cellular lifespan. Therefore, we propose that NaDC3 accelerates cellular aging by inhibiting SIRT1. After NaDC3 was overexpressed in two human embryo lung fibroblastic cell lines, WI38 and MRC-5, we found that the cells displayed aging-related phenotypes in advance. Meanwhile, the level of SIRT1 activity was down-regulated. In WI38/hNaDC3 cells treated with the activators of SIRT1, aging-related phenotypes induced by NaDC3 were obviously improved. The NAD(+)/NADH ratio in WI38/hNaDC3 cells was also decreased. Further study found that enhanced intracellular NAD(+) level could attenuate the aging phenotypes induced by NaDC3. Thus, NaDC3 promotes cellular senescence probably by inhibiting NAD(+)-dependent SIRT1.

Hepatocellular carcinoma (HCC) is a highly malignant tumor with a poor prognosis. Treatment of HCC is complicated by the fact that the disease is often diagnosed at an advanced stage when it is no longer amenable to curative surgery, and current systemic chemotherapeutics are mostly inefficacious. Sirtuin 1 (SIRT1) is a class III histone deacetylase that is implicated in gene regulations and stress resistance. In this study, we found that SIRT1 is essential for the tumorigenesis of HCC. We showed that although SIRT1 was expressed at very low levels in normal livers, it was overexpressed in HCC cell lines and in a subset of HCC. Tissue microarray analysis of HCC and adjacent nontumoral liver tissues revealed a positive correlation between the expression levels of SIRT1 and advancement in tumor grades. Downregulation of SIRT1 consistently suppressed the proliferation of HCC cells via the induction of cellular senescence or apoptosis. SIRT1 silencing also caused telomere dysfunction-induced foci and nuclear abnormality that were clearly associated with reduced expressions of telomerase reverse transcriptase (TERT), and PTOP, which is a member of the shelter in complex. Ectopic expression of either TERT or PTOP in SIRT1-depleted cells significantly restored cell proliferation. There was also a positive correlation between the level of induction of SIRT1 and TERT [corrected] in human HCC. Finally, SIRT1-silencing sensitized HCC cells to doxorubicin treatment. Together, our findings reveal a novel function for SIRT1 in telomere maintenance of HCC, and they rationalize the clinical exploration of SIRT1 inhibitors for HCC therapy.

Human mesenchymal stem cells (MSCs) have therapeutic potential because of their ability to self-renew and differentiate into multiple tissues. However, senescence often occurs in MSCs when they are cultured in vitro and the molecular mechanisms underlying this effect remain unclear. In this study, we found that NAD-dependent protein deacetylase SIRT1 is differentially expressed in both human bone marrow-derived MSCs (B-MSCs) and adipose tissue-derived MSCs after increasing passages of cell culture. Using lentiviral shRNA we demonstrated that selective knockdown of SIRT1 in human MSCs at early passage slows down cell growth and accelerates cellular senescence. Conversely, overexpression of SIRT1 delays senescence in B-MSCs that have undergone prolonged in vitro culturing and the cells do not lose adipogenic and osteogenic potential. In addition, we found that the delayed accumulation of the protein p16 is involved in the effect of SIRT1. However, resveratrol, which has been used as an activator of SIRT1 deacetylase activity, only transiently promotes proliferation of B-MSCs. Our findings will help us understand the role of SIRT1 in the aging of normal diploid cells and may contribute to the prevention of human MSCs senescence thus benefiting MSCs-based tissue engineering and therapies.

Silent information regulator 1 (SIRT1) represents an NAD(+)-dependent deacetylase that inhibits proapoptotic factors including p53. Here we determined whether SIRT1 is downstream of the prototypic c-MYC oncogene, which is activated in the majority of tumors. Elevated expression of c-MYC in human colorectal cancer correlated with increased SIRT1 protein levels. Activation of a conditional c-MYC allele induced increased levels of SIRT1 protein, NAD(+), and nicotinamide-phosphoribosyltransferase (NAMPT) mRNA in several cell types. This increase in SIRT1 required the induction of the NAMPT gene by c-MYC. NAMPT is the rate-limiting enzyme of the NAD(+) salvage pathway and enhances SIRT1 activity by increasing the amount of NAD(+). c-MYC also contributed to SIRT1 activation by sequestering the SIRT1 inhibitor deleted in breast cancer 1 (DBC1) from the SIRT1 protein. In primary human fibroblasts previously immortalized by introduction of c-MYC, down-regulation of SIRT1 induced senescence and apoptosis. In various cell lines inactivation of SIRT1 by RNA interference, chemical inhibitors, or ectopic DBC1 enhanced c-MYC-induced apoptosis. Furthermore, SIRT1 directly bound to and deacetylated c-MYC. Enforced SIRT1 expression increased and depletion/inhibition of SIRT1 reduced c-MYC stability. Depletion/inhibition of SIRT1 correlated with reduced lysine 63-linked polyubiquitination of c-Myc, which presumably destabilizes c-MYC by supporting degradative lysine 48-linked polyubiquitination. Moreover, SIRT1 enhanced the transcriptional activity of c-MYC. Taken together, these results show that c-MYC activates SIRT1, which in turn promotes c-MYC function. Furthermore, SIRT1 suppressed cellular senescence in cells with deregulated c-MYC expression and also inhibited c-MYC-induced apoptosis. Constitutive activation of this positive feedback loop may contribute to the development and maintenance of tumors in the context of deregulated c-MYC.

Epidemiological and experimental studies indicate that early vascular dysfunction occurs in low-birth-weight subjects, especially preterm (PT) infants. We recently reported impaired angiogenic activity of endothelial colony-forming cells (ECFCs) in this condition. We hypothesized that ECFC dysfunction in PT might result from premature senescence and investigated the underlying mechanisms. Compared with ECFCs from term neonates (n = 18), ECFCs isolated from PT (n = 29) display an accelerated senescence sustained by growth arrest and increased senescence-associated beta-galactosidase activity. Increased p16(INK4a) expression, in the absence of telomere shortening, indicates that premature PT-ECFC aging results from stress-induced senescence. SIRT1 level, a nicotinamide adenine dinucleotide-dependent deacetylase with anti-aging activities, is dramatically decreased in PT-ECFCs and correlated with gestational age. SIRT1 deficiency is subsequent to epigenetic silencing of its promoter. Transient SIRT1 overexpression or chemical induction by resveratrol treatment reverses senescence phenotype, and rescues in vitro PT-ECFC angiogenic defect in a SIRT1-dependent manner. SIRT1 overexpression also restores PT-ECFC capacity for neovessel formation in vivo. We thus demonstrate that decreased expression of SIRT1 drives accelerated senescence of PT-ECFCs, and acts as a critical determinant of the PT-ECFC angiogenic defect. These findings lay new grounds for understanding the increased cardiovascular risk in individuals born prematurely and open perspectives for therapeutic strategy.