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Biomedical subjects

Leonard Guarente

Publications and source records attributed to Leonard Guarente.

At least 19 recordsLinked to original sources
PubMed2007

SIR2: a potential target for calorie restriction mimetics.

Calorie restriction (CR) extends lifespan in a wide variety of species and mitigates diseases of aging in mammals. Here, we describe the evidence that the silent information regulator 2 (SIR2) gene, which encodes a nicotinamide adenine dinucleotide (NAD)-dependent deacetylase, regulates lifespan and mediates CR in lower species such as Saccharomyces cerevisiae and Caenorhabditis elegans. We discuss the emerging roles of mammalian SIR2 homologs in regulating physiological changes triggered by CR and their potential connections to diseases of aging. We conclude with the recent advances on small molecules that activate the enzymatic activity of SIR2 as potential CR mimetics. The SIR2 family represents an evolutionarily conserved lifespan regulator. Modulating the activity of SIR2 might provide effective CR mimetics to combat diseases of aging.

Aging↗
PubMed2006

Sirtuins as potential targets for metabolic syndrome.

Metabolic syndrome threatens health gains made during the past century. Physiological processes degraded by this syndrome are often oppositely affected by calorie restriction, which extends lifespan and prevents disease in rodents. Recent research in the field of ageing has begun to identify important mediators of calorie restriction, offering the hope of new drugs to improve healthspan. Moreover, if metabolic syndrome and calorie restriction are opposite extremes of the same metabolic spectrum, calorie restriction mimetics might provide another therapeutic approach to metabolic syndrome. Sirtuins and other important metabolic pathways that affect calorie restriction may serve as entry points for drugs to treat metabolic syndrome.

AMP-Activated Protein Kinases↗
PubMed2006

A stress response pathway involving sirtuins, forkheads and 14-3-3 proteins.

A conserved sir2 deacetylase gene can determine longevity of yeast, flies and worms. Recently we have reported a molecular mechanism of action of the C. elegans homologue sir-2.1. Our study revealed a novel stress-dependent pathway for lifespan determination in which SIR-2.1 binds to 14-3-3 proteins and a forkhead transcription factor DAF-16 to activate transcription of DAF-16 target genes. DAF-16 has long been known as a central protein in the regulation of lifespan that interfaces with multiple pathways. Recent studies by us and other laboratories suggest that DAF-16 requires co-factors for full activity. In this prospective we review recent literature highlighting the role of SIR-2.1, 14-3-3 and other DAF-16 co-factors in DAF-16 activation.

14-3-3 Proteins↗
PubMed2006

SIRT1 inhibits transforming growth factor beta-induced apoptosis in glomerular mesangial cells via Smad7 deacetylation.

SIRT1, a class III histone deacetylase, is considered a key regulator of cell survival and apoptosis through its interaction with nuclear proteins. In this study, we have examined the likelihood and role of the interaction between SIRT1 and Smad7, which mediates transforming growth factor beta (TGFbeta)-induced apoptosis in renal glomerular mesangial cells. Immunoprecipitation analysis revealed that SIRT1 directly interacts with the N terminus of Smad7. Furthermore, SIRT1 reversed acetyl-transferase (p300)-mediated acetylation of two lysine residues (Lys-64 and -70) on Smad7. In mesangial cells, the Smad7 expression level was reduced by SIRT1 overexpression and increased by SIRT1 knockdown. SIRT1-mediated deacetylation of Smad7 enhanced Smad ubiquitination regulatory factor 1 (Smurf1)-mediated ubiquitin proteasome degradation, which contributed to the low expression of Smad7 in SIRT1-overexpressing mesangial cells. Stimulation by TGFbeta or overexpression of Smad7 induced mesangial cell apoptosis, as assessed by morphological apoptotic changes (nuclear condensation) and biological apoptotic markers (cleavages of caspase3 and poly(ADP-ribose) polymerase). However, TGFbeta failed to induce apoptosis in Smad7 knockdown mesangial cells, indicating that Smad7 mainly mediates TGFbeta-induced apoptosis of mesangial cells. Finally, SIRT1 overexpression attenuated both Smad7- and TGFbeta-induced mesangial cell apoptosis, whereas SIRT1 knockdown enhanced this apoptosis. We have concluded that Smad7 is a new target molecule for SIRT1 and SIRT1 attenuates TGFbeta-induced mesangial cell apoptosis through acceleration of Smad7 degradation. Our results suggest that up-regulation of SIRT1 deacetylase activity is a potentially useful therapeutic strategy for prevention of TGFbeta-related kidney disease through its effect on cell survival.

Animals↗
PubMed2006

SIRT4 inhibits glutamate dehydrogenase and opposes the effects of calorie restriction in pancreatic beta cells.

Sir2 is an NAD-dependent deacetylase that connects metabolism with longevity in yeast, flies, and worms. Mammals have seven Sir2 homologs (SIRT1-7). We show that SIRT4 is a mitochondrial enzyme that uses NAD to ADP-ribosylate and downregulate glutamate dehydrogenase (GDH) activity. GDH is known to promote the metabolism of glutamate and glutamine, generating ATP, which promotes insulin secretion. Loss of SIRT4 in insulinoma cells activates GDH, thereby upregulating amino acid-stimulated insulin secretion. A similar effect is observed in pancreatic beta cells from mice deficient in SIRT4 or on the dietary regimen of calorie restriction (CR). Furthermore, GDH from SIRT4-deficient or CR mice is insensitive to phosphodiesterase, an enzyme that cleaves ADP-ribose, suggesting the absence of ADP-ribosylation. These results indicate that SIRT4 functions in beta cell mitochondria to repress the activity of GDH by ADP-ribosylation, thereby downregulating insulin secretion in response to amino acids, effects that are alleviated during CR.

Adenosine Diphosphate Ribose↗
PubMed2006

C. elegans SIR-2.1 interacts with 14-3-3 proteins to activate DAF-16 and extend life span.

The longevity of Caenorhabditis elegans is promoted by extra copies of the sir-2.1 gene in a manner dependent on the forkhead transcription factor DAF-16. We identify two C. elegans 14-3-3 proteins as SIR-2.1 binding partners and show that 14-3-3 genes are required for the life-span extension conferred by extra copies of sir-2.1. 14-3-3 proteins are also required for SIR-2.1-induced transcriptional activation of DAF-16 and stress resistance. Following heat stress, SIR-2.1 can bind DAF-16 in a 14-3-3-dependent manner. By contrast, low insulin-like signaling does not promote SIR-2.1/DAF-16 interaction, and sir-2.1 and the 14-3-3 genes are not required for the regulation of life span by the insulin-like signaling pathway. We propose the existence of a stress-dependent pathway in which SIR-2.1 and 14-3-3 act in parallel to the insulin-like pathway to activate DAF-16 and extend life span.

14-3-3 Proteins↗
PubMed2006

Mammalian Sir2 homolog SIRT7 is an activator of RNA polymerase I transcription.

We investigated the role of SIRT7, one of the seven members of the mammalian sirtuin family. We show that SIRT7 is a widely expressed nucleolar protein that is associated with active rRNA genes (rDNA), where it interacts with RNA polymerase I (Pol I) as well as with histones. Overexpression of SIRT7 increases Pol I-mediated transcription, whereas knockdown of SIRT7 or inhibition of the catalytic activity results in decreased association of Pol I with rDNA and a reduction of Pol I transcription. Depletion of SIRT7 stops cell proliferation and triggers apoptosis. Our findings suggest that SIRT7 is a positive regulator of Pol I transcription and is required for cell viability in mammals.

Animals↗
PubMed2006

Genomic instability and aging-like phenotype in the absence of mammalian SIRT6.

The Sir2 histone deacetylase functions as a chromatin silencer to regulate recombination, genomic stability, and aging in budding yeast. Seven mammalian Sir2 homologs have been identified (SIRT1-SIRT7), and it has been speculated that some may have similar functions to Sir2. Here, we demonstrate that SIRT6 is a nuclear, chromatin-associated protein that promotes resistance to DNA damage and suppresses genomic instability in mouse cells, in association with a role in base excision repair (BER). SIRT6-deficient mice are small and at 2-3 weeks of age develop abnormalities that include profound lymphopenia, loss of subcutaneous fat, lordokyphosis, and severe metabolic defects, eventually dying at about 4 weeks. We conclude that one function of SIRT6 is to promote normal DNA repair, and that SIRT6 loss leads to abnormalities in mice that overlap with aging-associated degenerative processes.

Aging↗
PubMed2005

Sirt1 regulates insulin secretion by repressing UCP2 in pancreatic beta cells.

Sir2 and insulin/IGF-1 are the major pathways that impinge upon aging in lower organisms. In Caenorhabditis elegans a possible genetic link between Sir2 and the insulin/IGF-1 pathway has been reported. Here we investigate such a link in mammals. We show that Sirt1 positively regulates insulin secretion in pancreatic beta cells. Sirt1 represses the uncoupling protein (UCP) gene UCP2 by binding directly to the UCP2 promoter. In beta cell lines in which Sirt1 is reduced by SiRNA, UCP2 levels are elevated and insulin secretion is blunted. The up-regulation of UCP2 is associated with a failure of cells to increase ATP levels after glucose stimulation. Knockdown of UCP2 restores the ability to secrete insulin in cells with reduced Sirt1, showing that UCP2 causes the defect in glucose-stimulated insulin secretion. Food deprivation induces UCP2 in mouse pancreas, which may occur via a reduction in NAD (a derivative of niacin) levels in the pancreas and down-regulation of Sirt1. Sirt1 knockout mice display constitutively high UCP2 expression. Our findings show that Sirt1 regulates UCP2 in beta cells to affect insulin secretion.

Adenosine Diphosphate↗
PubMed2005

Increase in activity during calorie restriction requires Sirt1.

Sir2 (silent information regulator 2) is a nicotinamide adenine dinucleotide-dependent deacetylase required for longevity due to calorie restriction in yeast and Drosophila. In mammals, calorie restriction induces a complex pattern of physiological and behavioral changes. Here we report that the mammalian Sir2 ortholog, Sirt1, is required for the induction of a phenotype by calorie restriction in mice.

Animals↗
PubMed2005

A novel telomere structure in a human alternative lengthening of telomeres cell line.

Cancer cells require mechanisms to maintain telomeres. Most use telomerase, but 5% to 20% of tumors use alternative lengthening of telomeres (ALT), a telomerase-independent mechanism that seems to depend on recombination. ALT is characterized by amplification of telomere TTAGGG repeats to lengths beyond 50 kb, by elevated rates of telomere recombination, and by nuclear structures called ALT-associated promyelocytic leukemia bodies. In Saccharomyces cerevisiae, survivors of telomerase inactivation also use recombination to maintain telomeres. There are two types of survivors, which differ in telomere structure. The first possesses telomere repeats and the Y' subtelomeric element amplified together as a tandem array at chromosome termini (type I), and the other possesses amplification of telomeric repeats alone (type II), similar to previously described human ALT cells. Here, we describe the first human ALT cell line having "tandem array" telomeres with a structure similar to that of type I yeast survivors. The chromosome termini consist of a repeat unit containing approximately 2.5 kb of SV40 DNA and a variable amount of TTAGGG sequence repeated in tandem an average of 10 to 20 times. Similar to previously described ALT cells, they show evidence of telomere recombination, but unlike standard ALT cells, they lack ALT-associated promyelocytic leukemia bodies and their telomeres are transcribed. These findings have implications for the pathogenesis and diagnosis of cancer.

Base Sequence↗
PubMed2005

Mouse Sir2 homolog SIRT6 is a nuclear ADP-ribosyltransferase.

Members of the Sir2 family of NAD-dependent protein deacetylases regulate diverse cellular processes including aging, gene silencing, and cellular differentiation. Here, we report that the distant mammalian Sir2 homolog SIRT6 is a broadly expressed, predominantly nuclear protein. Northern analysis of embryonic samples and multiple adult tissues revealed mouse SIRT6 (mSIRT6) mRNA peaks at day E11, persisting into adulthood in all eight tissues examined. At the protein level, mSIRT6 was readily detectable in the same eight tissue types, with the highest levels in muscle, brain, and heart. Subcellular localization studies using both C- and N-terminal green fluorescent protein fusion proteins showed mSIRT6-green fluorescent protein to be a predominantly nuclear protein. Indirect immunofluorescence using antibodies to two different mSIRT6 epitopes confirmed that endogenous mSIRT6 is also largely nuclear. Consistent with previous findings, we did not observe any NAD+-dependent protein deacetylase activity in preparations of mSIRT6. However, purified recombinant mSIRT6 did catalyze the robust transfer of radiolabel from [32P]NAD to mSIRT6. Two highly conserved residues within the catalytic core of the protein were required for this reaction. This reaction is most likely mono-ADP-ribosylation because only the modified form of the protein was recognized by an antibody specific to mono-ADP-ribose. Surprisingly, we observed that the catalytic mechanism of this reaction is intra-molecular, with individual molecules of mSIRT6 directing their own modification. These results provide the first characterization of a Sir2 protein from phylogenetic class IV.

ADP Ribose Transferases↗
PubMed2005

Calorie restriction--the SIR2 connection.

A nutritious diet low in calories improves the health and extends the life span of rodents. Recent studies identified a gene, SIR2, which encodes an NAD-dependent deacetylase and may mediate the effects of calorie restriction. In this review, we discuss SIR2 genes and calorie restriction in the lower organisms yeast and Drosophila. We then describe the physiological changes in mammals during calorie restriction and how they may lead to the observed health benefits. We summarize the roles of mammalian Sirt1 in mediating these changes in tissues and endocrine systems and propose that Sirt1 regulates calorie restriction by sensing low calories and triggering physiological changes linked to health and longevity.

Animals↗
PubMed2005

SIRT1 shows no substrate specificity in vitro.

SIR2 is a key regulator of the aging process in many model organisms. The human ortholog SIRT1 plays a pivotal role in the regulation of cellular differentiation, metabolism, cell cycle, and apoptosis. SIRT1 is an NAD(+)-dependent deacetylase, and its enzymatic activity may be regulated by cellular energy. There is a growing number of known SIRT1 substrates that contain epsilon-acetyl lysine but for which no obvious consensus sequence has been defined. In this study, we developed a novel unbiased method to identify deacetylase sequence specificity using oriented peptide libraries containing acetylated lysine. Following incubation with SIRT1, the subset of deacetylated peptides was selectively captured using a photocleavable N-hydroxysuccinimide (NHS)-biotin linker and streptavidin beads and analyzed using mass spectrometry and Edman degradation. These studies revealed that substrate recognition by SIRT1 does not depend on the amino acid sequence proximate to the acetylated lysine. This result brings us one step closer to understanding how SIRT1 and possibly other protein deacetylases chose their substrate.

Amino Acid Sequence↗
PubMed2005

A role for SIR-2.1 regulation of ER stress response genes in determining C. elegans life span.

C. elegans SIR-2.1, a member of the Sir-2 family of NAD(+)-dependent protein deacetylases, has been shown to regulate nematode aging via the insulin/IGF pathway transcription factor daf-16. Treatment of C. elegans with the small molecule resveratrol, however, extends life span in a manner fully dependent upon sir-2.1, but independent of daf-16. Microarray analysis of worms treated with resveratrol demonstrates the transcriptional induction of a family of genes encoding prion-like glutamine/asparagine-rich proteins involved in endoplasmic reticulum (ER) stress response to unfolded proteins. RNA interference of abu-11, a member of this ER stress gene family, abolishes resveratrol-mediated life span extension, and overexpression of abu-11 extends the life span of transgenic animals. Furthermore, SIR-2.1 normally represses transcription of abu-11 and other ER stress gene family members, indicating that resveratrol extends life span by inhibiting sir-2.1-mediated repression of ER stress genes. Our findings demonstrate that abu-11 and other members of its ER stress gene family are positive determinants of C. elegans life span.

Animals↗
PubMed2005

Calorie restriction and SIR2 genes--towards a mechanism.

Calorie restriction is the first and most compelling example of life extension in mammals. Much speculation about how CR works has focused on ideas of what causes aging. Since these causes themselves are much disputed, I have instead focused my thinking on lessons from simple model organisms, which have emerged from recent genetic studies. These findings can now be integrated with numerous, elegant studies on CR over the decades, which provide a treasure trove of information about physiological changes that are elicited by this regimen. In this paper, I present data showing that the SIR2 gene is a strong candidate to regulate CR in the simple model organisms, such as yeast and Drosophila. I then summarize what is known about the mammalian Sirt1 as it relates to physiological changes during CR, and discuss how this mechanism may impact on life span, as well as diseases of aging.

Aging↗