08 Jul 2008

From yeast to mammals, the realization that calorie restriction (CR) increases lifespan led to the mantra “eat less—live longer.” Soon after, it became the tongue-in-cheek “drink more—live longer” with the discovery that resveratrol, a substance in red wine, could boost longevity without the calorie-cutting. We can now merge the mantras, thanks to a pair of recent studies reporting that resveratrol, an activator of the SIRT1 histone deacetylase, slows age-associated maladies in healthy mice through tissue-specific transcriptional changes markedly similar to those induced by eating less. These findings would seem to nudge scientists a step closer to the dream of an anti-aging pill that mimics the effects of calorie restriction. Three other papers hot off the presses offer new insight into SIRT1’s roles in metabolism and neuroprotection, but at the same time highlight the need for context-specific analysis of this jack-of-all-trades protein. And if all the news on SIRT1 and resveratrol hasn’t sapped your mental bandwidth, another paper published last week offers additional food for thought on the anti-aging front—the use of combined treatment with statins and aminobisphosphonates (isoprenylation-blocking drugs) to extend lifespan and stave off aging-like phenotypes in mice.

Previous gene expression profiling studies from the lab of David Sinclair at Harvard Medical School, Boston, had shown that resveratrol opposes most transcriptional changes in the liver of mice fed a high-calorie diet (Baur and Sinclair, 2006). In mammals, both resveratrol and CR activate SIRT1, which regulates cell physiology in various contexts, including glucose metabolism, DNA repair, and apoptosis. SIRT1 has been shown to curb neurodegeneration in mouse models of Alzheimer disease (Kim et al., 2007), apparently by enhancing the non-amyloidogenic arm of amyloid precursor protein (APP) processing mediated by α-secretase cleavage (Qin et al., 2006 and ARF related news story). Having demonstrated resveratrol’s prowess at treating the effects of high-calorie diet in mice as well as its protective role in mouse models of AD, “the question was, are these effects related to the CR effect, or not?” Sinclair told ARF. He and Rafael de Cabo, at the National Institute on Aging in Baltimore, Maryland, are senior authors of a paper that addresses this question in the July issue of Cell Metabolism.

The new study—using freshly isolated RNA, as well as a different microarray platform and newer analysis software—confirms the 2006 liver data and extends it to three additional tissues. First author Kevin Pearson and colleagues studied the effects of resveratrol on mice fed, beginning at 12 months of age, the following diets: standard, every-other-day (EOD) feeding, and high-calorie. The researchers found directionally correlated changes of gene expression in 82 percent (liver), 76 percent (skeletal muscle), 96 percent (adipose), and 64 percent (heart) of functional pathways affected by the resveratrol and EOD regimens. Furthermore, the resveratrol-induced transcriptional changes correlated with functional benefits. “The effects we saw in our mice were just what we’d expect to see if you’re slowing aging—less heart disease, fewer cataracts, greater mobility,” Sinclair said. “These animals were on a healthy diet, but we made them even healthier with resveratrol.”

Working independently and publishing 4 June in PLoS ONE, researchers led by Tomas Prolla at the University of Wisconsin, Madison, report similar results in their microarray analysis comparing transcription profiles induced by CR and resveratrol. First author Jamie Barger and colleagues fed mice from middle age (14 months) to old age (30 months) a control diet, CR diet, or resveratrol-supplemented control diet. The researchers report a “striking transcriptional overlap” of CR and resveratrol (99.7 percent of gene expression changes correlating by direction) in heart, skeletal muscle, and brain (neocortex), and show that both regimens prevent age-related cardiac problems.

Leonard Guarante, an MIT scientist among those who originally proposed that SIRT1 would mediate the benefits of CR eight or nine years ago, is delighted by the new findings. “The notion that there will be mimetics of caloric restriction that people can take in pill form is becoming a reality,” he told ARF in a phone interview. “That’s a big deal. It will have implications for major diseases.”

One such disease—a cluster of aging-related conditions known as the metabolic syndrome, which has been linked to dementia (see ARF related news story), was the focus of a mouse study published 3 July in PNAS online by researchers led by Matthias Tschöp at the University of Cincinnati College of Medicine, Ohio, with collaborators at the Spanish National Cancer Research Center in Madrid. To test the effect of SIRT1 on metabolic damage resulting from a prolonged high-fat diet, lead author Paul Pfluger and colleagues engineered mice with an additional transgenic copy of the entire SIRT1 gene, expressed from its own promoter within its natural genomic context. In these mice, SIRT1 was overexpressed two- to fourfold in a variety of tissues including liver, brown fat, and muscle. This moderate overexpression of SIRT1 protected the mice from lipid-induced inflammation, glucose intolerance, and fatty liver (hepatic steatosis) inflicted by high-fat diet, the researchers report.

Guarente sees these findings as additional support for the notion that activating SIRT1 can mimic the effects of CR. “There are no drugs at all—just a SIRT1 transgenic mouse,” he said, noting “significant overlap in the phenotype of these animals and those with resveratrol treatment.”

Tschöp and colleagues’ data suggest that SIRT1 confers protection from metabolic harm by inducing expression of antioxidant enzymes manganese superoxide dismutase (MnSOD) and nuclear respiratory factor 1 (Nrf1) and reducing expression of proinflammatory cytokines tumor necrosis factor α and interleukin 6.

Valter Longo at the University of Southern California, Los Angeles, cautions that the observed protection may not reflect lasting benefit. “Just because you have more antioxidant enzymes in certain conditions doesn’t mean the mouse has adopted a long-term chronic protective mode,” he told ARF.

As an illustration of this principle, a paper in this month’s Cell Metabolism by Longo’s group and Canadian collaborators at the University of Ottawa, shows apparent contradictions in short-term, context-specific versus long-term, systemic effects of SIRT1 in neurons. Prior work in the Longo lab had shown that deletion of Sir2—the yeast homolog of mammalian SIRT1—increases stress resistance (Fabrizio et al., 2005). But not much was understood about the mechanisms behind these effects and, in particular, whether SIRT1 regulates oxidative stress in the brain, where SIRT1 expression is highest in humans (Michishita et al., 2005). To address these issues, first author Ying Li and colleagues subjected primary rat cortical neurons and other cultured cells to conditions that induce oxidative stress (treatment with hydrogen peroxide or menadione). The researchers found that preincubating the rat neurons with SIRT1 inhibitors (nicotinamide or sirtinol) significantly increased cell survival under both stress-inducing conditions in a dose-dependent manner. Further experiments revealed that the neuroprotective effects of SIRT1 inhibition are linked to increased acetylation and decreased phosphorylation of insulin receptor substrate (IRS)-2, and reduced activation of the Ras-ERK1/2 pathway.

To extend the neuroprotection results in vivo, the researchers examined brains of 18-month-old SIRT1 knockout mice for markers of oxidative damage—protein carbonyl content and lipid peroxidation. They found 17 and 20 percent decreases in each marker, respectively, in SIRT1 knockouts compared with wild-type mice. However, the SIRT1-deficient mice had reduced lifespan under both normal and calorie-restricted conditions. “We’re showing that when you reduce SIRT1 activity in neurons, you have protection, but if you take away all the SIRT1, then you have a negative effect,” Longo said, noting that SIRT1 knockout mice are smaller and have developmental defects in addition to a shorter lifespan. To further dissect SIRT1’s neuroprotective role from its broader systemic effects, he has generated brain-specific SIRT1 knockout mice. Longo told ARF that his lab is doing behavioral studies on these animals, and may cross them with Frank LaFerla's triple transgenic 3xTg-AD line (expressing mutated human APP, presenilin 1, and tau) to see if SIRT1 deficiency in the brain can rescue AD phenotypes.

A paper by Guarente and colleagues in the 1 July issue of Genes and Development adds to the idea that SIRT1-targeting strategies may not always produce the expected changes across different organs. By assaying levels of SIRT1 and its small-molecule regulators NAD and NADH, and assessing phenotypes of a liver-specific SIRT1 knockout mouse on various diets, first author Danica Chen and colleagues report this surprising finding: in the liver, SIRT1 is reduced by CR and activated by a high-calorie diet. The finding suggests that the effects of targeting SIRT1 may be more complex than previously thought.

The effects of targeting SIRT1 may also manifest themselves differently depending on when, during an animal’s lifespan, the treatment is started. In the study led by Sinclair and de Cabo, resveratrol and calorie restriction begun in mice at midlife (12 months) did not affect longevity. Sinclair told Alzforum that his group is beginning new studies in which the dietary regimens are begun at six weeks of age.

In the human testing arena, Sirtris Pharmaceuticals, Inc.—a company Sinclair cofounded in 2004 and that was acquired last month by GlaxoSmithKline—has developed a version of resveratrol with increased stability and bioavailability for treatment of aging-related diseases. This liquid compound is in Phase 2 clinical trials for type 2 diabetes. Other Sirtris small-molecule SIRT1 activators that are unrelated to resveratrol but have 1,000 times its potency (Milne et al., 2007) have recently entered the clinic in Phase 1 human safety trials. The longest any of these compounds have been in humans is three months, a senior director at Sirtris told ARF.

In other age-related news published 29 June in Nature Medicine online, researchers led by Carlos López-Otín at Universidad de Oviedo, Madrid, have extended longevity in a mouse model of premature aging using statins and aminobisphosphonates. Their strategy is intriguing in light of prior studies suggesting that statins—well-known as cholesterol-lowering agents—might help prevent AD by reducing synthesis of isoprenoids (see ARF related news story and ARF news story). In the new work, first author Ignacio Varela and colleagues show that combined treatment with statins and aminobisphosphonates improved age-related defects—including growth retarding, weight loss, and defects of fat metabolism, hair loss, and bone—in Zmpste24-/- mice with a premature aging phenotype. The drugs conferred these benefits presumably by blocking farnesylation and geranylgeranylation.—Esther Landhuis

Comments

1. • Sam Gandy
     Icahn School of Medicine at Mount Sinai
   • Posted: 07 Jul 2008
   • Paper: Combined treatment with statins and aminobisphosphonates extends longevity in a mouse model of human premature aging.

   The possible role for isoprenoids in modulating certain aging-related phenotypes
   is emphasized by this new work demonstrating that the combination of statins
   and bisphosphonates appears to mitigate these phenotypes, presumably via
   blockade of both farnesylation and geranylgeranylation. Farnesyl transferase
   inhibitors (FTIs) had been reported to have such properties, but
   those observations were apparently less robust in subsequent studies. Another
   aging-related phenomenon, accumulation of amyloid-β peptide in the brains of
   amyloid-depositing transgenic mice, is robustly modulated by statins, yet
   statins have failed in clinical trials to modulate clinical outcome. The new data
   suggest that human Alzheimer trials of statins might be revisited: perhaps
   a clinical trial of statins plus bisphosphonates will reveal efficacy where statins
   alone have failed.

   View all comments by Sam Gandy

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References

News Citations

1. Aging, Acetate, and Aβ: Sirtuins Regulate Metabolism and More 28 Jun 2006
2. Study Links Metabolic Syndrome, Inflammation to Cognitive Decline 18 Nov 2004
3. Statins and AD—What Role Isoprenoids? 28 Feb 2005
4. Statins Boost α-Secretase, but Not Through Cholesterol 13 Jan 2005

Paper Citations

1. Baur JA, Sinclair DA. Therapeutic potential of resveratrol: the in vivo evidence. Nat Rev Drug Discov. 2006 Jun;5(6):493-506. PubMed.
2. Kim D, Nguyen MD, Dobbin MM, Fischer A, Sananbenesi F, Rodgers JT, Delalle I, Baur JA, Sui G, Armour SM, Puigserver P, Sinclair DA, Tsai LH. SIRT1 deacetylase protects against neurodegeneration in models for Alzheimer's disease and amyotrophic lateral sclerosis. EMBO J. 2007 Jul 11;26(13):3169-79. PubMed.
3. Qin W, Yang T, Ho L, Zhao Z, Wang J, Chen L, Zhao W, Thiyagarajan M, MacGrogan D, Rodgers JT, Puigserver P, Sadoshima J, Deng H, Pedrini S, Gandy S, Sauve AA, Pasinetti GM. Neuronal SIRT1 activation as a novel mechanism underlying the prevention of Alzheimer disease amyloid neuropathology by calorie restriction. J Biol Chem. 2006 Aug 4;281(31):21745-54. PubMed.
4. Fabrizio P, Gattazzo C, Battistella L, Wei M, Cheng C, McGrew K, Longo VD. Sir2 blocks extreme life-span extension. Cell. 2005 Nov 18;123(4):655-67. PubMed.
5. Michishita E, Park JY, Burneskis JM, Barrett JC, Horikawa I. Evolutionarily conserved and nonconserved cellular localizations and functions of human SIRT proteins. Mol Biol Cell. 2005 Oct;16(10):4623-35. PubMed.
6. 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 Nov 29;450(7170):712-6. PubMed.

Other Citations

1. 3xTg-AD line

External Citations

1. Sirtris Pharmaceuticals, Inc.

Further Reading

Papers

1. Weiss EP, Villareal DT, Racette SB, Steger-May K, Premachandra BN, Klein S, Fontana L. Caloric restriction but not exercise-induced reductions in fat mass decrease plasma triiodothyronine concentrations: a randomized controlled trial. Rejuvenation Res. 2008 Jun;11(3):605-9. PubMed.

News

1. Who Says Chivalry is Dead?—Sir2 Fights Against Aging in Mammals 17 Jun 2004
2. Sirtuins Link Stress, Autophagy 22 Feb 2008
3. Research Brief: Redox Redact—Sirt1 Seals Non-neuronal Fate 21 Mar 2008
4. Study Links Metabolic Syndrome, Inflammation to Cognitive Decline 18 Nov 2004
5. Statins and AD—What Role Isoprenoids? 28 Feb 2005
6. Aging, Acetate, and Aβ: Sirtuins Regulate Metabolism and More 28 Jun 2006
7. Statins Boost α-Secretase, but Not Through Cholesterol 13 Jan 2005