17 June 2004. Sir Lancelot, for all his prowess protecting King Arthur’s court, couldn’t do one thing: prevent the death toll extracted by living high at the round table. Sir2 could have shown our legendary hero how to fight that battle gallantly. In today’s Sciencexpress, David Sinclair and colleagues at Harvard Medical School report that the mammalian protein SIRT1 promotes the survival of mammalian cells and suggest it may do so by preventing apoptosis. Like Sir2, SIRT1 is induced by caloric restriction (CR) and the actions of this protein open a door toward a better understanding of why CR delays the onset of numerous age-associated diseases, including Alzheimer’s disease, which has been linked to high caloric intake (see ARF related news story).
In yeast, caloric restriction leads to induction of the Sir2 histone deacetylase and subsequent gene silencing (see ARF related news story). To test if caloric restriction induces the Sir2 homolog SIRT1 in mammals, first author Haim Cohen and colleagues examined tissues extracted from year-old rats that had been fed either ad libitum, or a calorie-restricted diet (these animals ate about 40 percent less than their littermates). Cohen found that CR led to a greater abundance of SIRT1 in a variety of tissues including brain, liver, kidney and fat. In humans, CR may elicit similar changes because when Cohen grew human embryonic kidney cells (293T cells) in serum from CR rats, SIRT1 expression doubled.
So what does SIRT1 do in mammals? Work at Leonard Guarente’s lab at MIT has shown that SIRT1 can silence apoptotic forkhead transcription factors, such as Foxo3a (see Motta et al., 2004). But Sinclair and colleagues have found that SIRT1 can prevent apoptosis through an alternative pathway involving Bax, a protein which, when translocated to the outer mitochondrial membrane, triggers the release of cytochrome C, an apoptotic trigger. When Cohen examined 293T cells grown in serum from calorie-restricted rats, he found that they were less susceptible to Bax-mediated apoptosis.
Bax is normally sequestered in the cytosol by the protein Ku70. Acetylation of Ku70 releases Bax and pushes cells down the slippery apoptotic slope (see Cohen et al., 2004). Sinclair and colleagues wondered if the deacetylase activity of SIRT1 was the glue that holds Bax and Ku70 together. To test this, Cohen measured, in a test tube, deacetylation of Ku70 by SIRT1. Indeed, the authors found the SIRT1 could effectively remove acetyl groups from two Ku70 lysine residues that are essential for binding Bax. Similar results were obtained when Cohen measured acetylation of Ku70 in cells expressing normal or dominant-negative mutants of SIRT1.
All told, the experiments suggest that caloric restriction induces expression of the deacetylase, which then prevents acetylation of Ku70 and so retains Bax in the cytosol, where it does no damage. What component of rat CR-conditioned serum induces expression of SIRT1? While the scientists puzzle out this question, what’s a bon vivant to do? Very few people manage to stay on a caloric restriction for long, Mark Mattson being a famous exception from within the AD field. Maybe we can take one well-chosen leaf out of King Arthur’s book and live it up just a little. Recent findings from Sinclair’s group have shown the resveratrol, a polyphenol found in red wine, activates SIRT1. In yeast the polyphenol can increase life span by 70 percent (see Howitz et al., 2004). Other groups have sung resveratrol's praises as an antioxidant. A tankard, anyone?—Tom Fagan.
Cohen HY, Miller C, Bitterman KJ, Wall NR, Hekking B, Kessler B, Howitz KT, Gorospe M, de Cabo R, Sinclair DA. Calorie restriction promotes cell survival by inducing the SIRT1 deacetylase. Sciencexpress 2004 June 17. Abstract
Q&A with David Sinclair
Q: At first blush, Ku70 brings to mind DNA repair. Could SIRT1's
deacetylation of Ku70 exert its life-prolonging effect by helping the cell repair DNA breaks acquired with age?
A: It is very possible that Ku70 helps extend lifespan by controlling DNA repair and cell death. This would make a lot of sense. We are testing this possibility now.
Q: Korean scientists last year reported that caloric restriction
increased Ku activity in some aging tissues but not others (Um et al.,
2003), though they unfortunately did not report on brain. Did you see such differences?
A: I believe they tested DNA repair capacity, which we have not yet tested. We also don't know the effect of CR on Ku70's ability to prevent cell death in vivo yet. It's hard to test.
Q: Did you see any induction of chaperones with CR?
A: See deCabo et al., 2003. They showed that CR upregulates heat shock protein 70, a chaperone. This is also induced by CR serum, as we show in our paper.
Q: Caloric restriction by 40 percent makes for pretty austere living.
For those whose flesh is too weak for that, does cutting 10 or 20 percent help, also?
A: It depends on where you start. If you are eating a lean diet already, then a 10% drop might have some benefit, but not as great as 30-40 percent. Basically, for this to work, your body has to be in a state of hunger for most of the time.
Q: Does this depend on body weight? How about people who eat a lot but
A: This doesn't work. Just being thin is not severe enough to turn on the body's defenses.
Q: Do you have a specific research interest in neurodegeneration?
A: Not yet, but we will soon be looking at whether the SIRT1 enzyme plays a protective role in neurons and whether the SIRT1-activating molecules
(STACs) that we discovered last year can slow neurodegeneration in mice.