Health benefits of red-wine chemical unclear
Sceptics continue to ask whether resveratrol really can delay the effects of ageing.
Five years ago it seemed that resveratrol, a compound present most famously in red wine, could slow down the ageing process. But a study published on 8 January in The Journal of Biological Chemistry1 deepens the divide between those who are confident in its potential and those who think it is too good to be true.
Resveratrol's health benefits are thought to result from its activation of enzymes called sirtuins, which were linked to longevity 10 years ago when Leonard Guarente from the Massachusetts Institute of Technology in Cambridge found that yeast with additional copies of the gene that encodes sirtuin, called sir2, lived significantly longer than did those that had the usual two copies2. Four years later, Guarente's former post-doc David Sinclair published work showing that resveratrol activated sirtuins in yeast and extended the organism's lifespan3. Sinclair later went on to show that resveratrol fed to worms and flies lengthened lifespan by acting through the sirtuins4.
Then in 2007, Sinclair and Sirtris Pharmaceuticals — a company Sinclair had co-founded with venture capitalist Christoph Westphal in Cambridge, Massachusetts, to develop sirtuin activators — screened a large selection of small molecules to look for activators of SIRT1, the mammalian version of the yeast Sir2 enzyme. Their study, which was published in Nature5 found three compounds that were 1,000-fold more potent than resveratrol at activating the enzyme5. Furthermore, one of these compounds made obese mice and rats more sensitive to insulin, suggesting that the substances could be used to treat type 2 diabetes — a disease that gets more common with age. Less than a year later, the British pharmaceutical giant GlaxoSmithKline bought Sirtris for US$720 million. Two Sirtris drugs are already in Phase II clinical trials — one for cancer, and both for the treatment of type 2 diabetes.
But the excitement over the potential health benefits was tempered by reports concluding that resveratrol does not activate SIRT1 directly6,7 and that it works only when the substrates are linked to a fluorophore, as they were in the screen and in the study that first showed resveratrol activated SIRT1. Now, researchers led by biochemist Kay Ahn of Pfizer in Groton, Connecticut, have shown that the Sirtris compounds do not seem to activate SIRT1 when not attached to fluorophores either1.
"We can only replicate the Nature findings by using a fluorescent peptide, which [Sinclair and his colleagues] had used exclusively," says Ahn.
Only in real life
However, Guarente, who is now a scientific adviser to Sirtris, says that the latest findings are neither surprising nor worrisome. The compounds may work only with fluorophore-conjugated peptides in vitro, says Guarente, but the situation is different in cells and in animals. The Nature paper, among others, went beyond the test tube and indicated that SIRT1 was more active in cells and in animals after application of the Sirtris compounds. Furthermore, resveratrol administration made no difference to the lifespan of yeast that did not have Sir23, indicating that the compound's action depends on this gene.
According to a statement from GlaxoSmithKline, Ahn's conclusion "ignores any possibility of direct activation of SIRT1 that may occur in a cellular environment that is not reproduced in vitro".
But some remain unconvinced. Another former member of Guarente's lab, Brian Kennedy, now at the University of Washington in Seattle, points out that cell-based assays are difficult to interpret, particularly because resveratrol is thought to interact with many enzymes.
"It's very nonspecific," says Kennedy, who in 2005 was the first to report that resveratrol activates SIRT1 in vitro only when the substrates are conjugated to fluorophores. Although resveratrol does seem to affect animals, he says, "It's still highly unclear what the targets are that lead to those activities. I remain sceptical that SIRT1 is a key target."
In the second portion of the latest study, Ahn tried, unsuccessfully, to replicate Sirtris' findings that the compounds reduced the blood-glucose levels of obese mice. A few of the mice even died, despite receiving the same dose as that given in the Nature paper. But Ahn is quick to point out that "every in vivo experiment is a little bit different". "Under our conditions we didn't see beneficial effects, but we don't want to make a big conclusion out of those results."
A possible explanation for the discrepancy, says Sinclair, is that Ahn and her colleagues did not provide information on the characterization of the compounds, which they synthesized themselves. So there is no way of knowing how pure they were or whether they're the same as those made by Sirtris. "The fact that mice died indicates that there may be an issue with purity," says Sinclair.
Sceptics of Sinclair's findings continue to voice doubt. "The enthusiasm for resveratrol and for Sirtris activators seems to have been premature," says Richard Miller at the University of Michigan Geriatrics Center in Ann Arbor, who has found that activating a different pathway extends lifespan in mammals8. "They may have health benefits, but the early evidence doesn't seem very strong and all the newer evidence suggests that the system may be more complicated."
But because of the growing number of studies showing beneficial effects of sirtuins and resveratrol, no one is willing to write them off quite yet. "If I were asked to list ten proteins that deserve a lot of attention in ageing in mammals, the sirtuins would be on that list," says Miller. "They just wouldn't be at the top of the list."
- Pacholec, M. et al. J. Biol. Chem. advance online publication doi:10.1074/jbc.M109.088682 (2010).
- Kaeberlein, M., McVey, M. & Guarente, L. Genes Dev. 13, 2570-2580 (1999).
- Howitz, K. T. et al. Nature 425, 191-196 (2003).
- Wood, J. G. et al. Nature 430, 686-689 (2004).
- Milne, J. C. et al. Nature 450, 712-716 (2007).
- Kaeberlein, M. et al. J. Biol. Chem. 280, 17038-17045 (2005).
- Beher, D. et al. Chem. Biol. Drug Des. 74, 619-624 (2009).
- Harrison, D. E. et al. Nature 460, 392-395 (2009).