Resveratrol—Still the Antiaging King
How Does Resveratrol Work?
New research suggests that its supposed mediator
may work differently in the
brain than in other organs
By Will Block
Today if you’re not confused, you’re just not thinking clearly.
— Laurence J. Peter
f you watch enough TV, you know all about law and order, crime scene investigation, etc. Heck, you probably know enough to be a pretty good cop or lawyer yourself. The requirement for winning a criminal case against someone is, of course, that there must be proof of guilt beyond a reasonable doubt. That can be tricky. At what point does very strong evidence become proof? How do you define reasonable doubt? What’s reasonable to me may be unreasonable to you. Twelve jurors cannot always agree.
In civil court, winning a case is easier, because there is no burden of proof. All the plaintiff needs in order to sway the jury to the “right” conclusion is to convince them that a preponderance of the evidence supports the case. That’s probably why OJ Simpson lost the civil case against him after winning the criminal case.
Science Is More Civil than Criminal
But what does this have to do with science? A lot, actually, because there are strong parallels. At any given time, most open scientific questions are answered more in the manner of a civil case—by a preponderance of the evidence—than in the manner of a criminal case, with proof. In science, proof is a rare commodity except in the mathematically rigorous fields of physics and chemistry, where experiments may be so definitive as to leave no reasonable doubt regarding their validity.
That is not the case in biology (which is a science), let alone in medicine (which isn’t, although it’s based in science). In these disciplines, uncertainty is common, and there is seldom a definitive proof of anything. Most issues, therefore, are decided—if they’re decided at all—by consensus, the collective best judgment of experts in the field. (That’s true also in the physical sciences wherever open questions or controversial issues are concerned.) Sometimes there is no consensus, because the evidence is too inconclusive or contradictory, or because personal beliefs and biases get in the way of objective analysis.
The Scientific Enterprise—What a Mess!
In science, consensus does not come about through any semblance of a democratic process, with each scientist’s “vote” counting equally. On the contrary, science is a meritocracy in which the opinions of the most authoritative experts carry the greatest weight (as well they should). The foundation of any such consensus, though, rests on quicksand, and it can change substantially if new evidence warrants it. One of the beauties of science is that a single individual with a sufficiently compelling discovery or theory can overthrow an existing paradigm and force everyone to rethink—or abandon—their long-held beliefs. It has happened innumerable times throughout history.
Contrary to a popular misconception of the scientific enterprise as being an inexorably logical progression from one meticulously proven fact to the next, it is, in fact, a turbulent, messy, confusing affair, full of doubts and errors and passions and disputes (and even, occasionally, chicanery). The best one can hope for is that the scientific consensus, if there is one, is more or less correct. Although it usually is correct, it’s wrong often enough that all good scientists must remain inherently skeptical and keep an open mind, even when the consensus view is extremely convincing.
Resveratrol Is Not the Problem—SIRT1 Is
By now you must be nervous, because you realize that you’re being set up, in a sense, for some surprising news about the fabled red-wine compound resveratrol, and you might not like it. Please don’t fret. By a large body of objective evidence—and yes, by a consensus view among scientists—resveratrol remains the king of the hill in antiaging medicine. Its health benefits (in cell cultures and laboratory animals) against a broad range of degenerative diseases and its well documented longevity effects in a broad range of organisms are not in serious dispute.
Well, then, what’s the problem? The problem is that the role of SIRT1 in human physiology has become clouded in confusion and controversy. SIRT1 is the most important member of a family of seven mammalian proteins called sirtuins (sir·TOO·ins). It is widely believed to play a central role in the antiaging mechanisms of both caloric restriction (CR) and resveratrol supplementation, both of which increase its levels and activity. (CR is a long-term, ultralow-calorie but nutritious dietary regimen; it dramatically reduces the risk for degenerative diseases and substantially increases both average and maximum lifespan.)
Even though scientists now seem to
understand less than they thought
they had about the mechanism(s) by
which CR and resveratrol work, the
health and longevity benefits of CR
and resveratrol are not in question.
The two phenomena—CR and resveratrol supplementation—are remarkably similar in their effects. There is much evidence, obtained in experiments with laboratory animals, to suggest that resveratrol acts as a biochemical mimic of CR, providing many of the health and longevity benefits of eating a lot less (in caloric terms) every day without the actual necessity of eating a lot less every day. The results have been amazing. (For more on this, see
“Resveratrol Mimics Caloric Restriction” in the April 2008 issue.)
Return of the Hobgoblin
Resveratrol’s ability to extend lifespan has been demonstrated in a wide variety of organisms: brewer’s yeast (Saccharomyces cerevisiae), roundworms (Caenorhabditis elegans), fruit flies (Drosophila melanogaster), turquoise killifish (Nothobranchius furzeri), and mice (Mus musculus). In almost all experiments conducted thus far, resveratrol has extended lifespan, doing so even in obese mice that could reasonably have been expected to have shorter lifespans owing to their unhealthy condition. Curiously, however, a follow-up study by the same research group failed to show a longevity effect of resveratrol in healthy, normal-weight mice. (We reported on this study last month:
“Resveratrol Combats Age-Related Diseases,” September 2008.)
Now the “Resveratrol Hobgoblin” that we introduced in that article has struck again, albeit in a more indirect way. Considerable confusion has arisen over the role of SIRT1, especially in light of the fact that it seems to have potentially harmful as well as beneficial effects. This is unsettling, considering its central importance as the mediator of resveratrol’s effects.
But wait—does SIRT1 really mediate the effects of resveratrol in higher organisms, such as vertebrates? Perhaps not, in the view of some scientists, who have been openly skeptical since the beginning. (See the sidebar.) They point out that, even in lower organisms, such as yeasts, worms, and flies, the evidence regarding the role of Sir2 (the analogous sirtuin that exists in those critters) is somewhat contradictory and has proved difficult to replicate. In higher organisms, they say, it’s not at all certain that SIRT1 plays the central role that has been assumed, despite the evidence suggesting that it does. Some scientists question whether it even plays any important role.
A Skeptic Speaks
The two most prominent researchers in the field of sirtuin biology are Prof. Leonard Guarente of MIT and a former postdoctoral student of his, Prof. David Sinclair, now at Harvard Medical School. The work of both scientists as it relates to the health and longevity benefits of caloric restriction and resveratrol has been featured in this magazine numerous times. (Both scientists, by the way, have cofounded companies to exploit the commercial potential of their discoveries.)
The belief that the health and longevity effects of caloric restriction and resveratrol are mediated primarily by sirtuins has not met with universal acceptance, however. Perhaps the most authoritative and articulate critic of this idea is Prof. Matt Kaeberlein of the University of Washington, who was a graduate student of Guarente’s at MIT. His own research suggests that the effects in question may arise via other biochemical mechanisms. (Kaeberlein has cofounded a company too.)
In the same issue of Cell Metabolism in which the paper featured in the accompanying article appeared, Kaeberlein published an essay entitled “The Ongoing Saga of Sirtuins and Aging.” In it he discussed the implications of that paper and some of the key issues that have led to such confusion in the field of sirtuin biology, which he described as a “particularly complex” situation where mammals are concerned. He concluded with the following remarks:
So what’s the “take-home message” from all this? Is more SirT1 good, or is less SirT1 good? The answer, as is often the case in biology, is that there’s no simple answer. Activating SirT1 is probably a good thing in some cells under some conditions and is probably a bad thing in other cells under other conditions. SirT1 activators may be good for diabetes but may cause cancer due to p53 inhibition; SirT1 inhibitors may protect against cancer but cause metabolic disease; and there is evidence supporting the idea that both activators and inhibitors of SirT1 can confer protection against neurodegeneration in different contexts. The one thing that seems clear is that sirtuin activators are unlikely to be a “magic bullet” for aging.
A more realistic hope is that, as we continue to unravel the complexities of sirtuin biology, targeted activation or inhibition of SirT1—and perhaps other sirtuins as well—will prove therapeutically useful toward a subset of age-associated diseases. Such an achievement would be a huge step forward in the transition of aging-related science from the laboratory to the clinic, and we eagerly await the next chapter in the unfolding saga that is sirtuin biology.
- Kaeberlein M. The ongoing saga of sirtuins and aging. Cell Metab 2008;8:4-5.
In Rat Neurons, Less SIRT1 Is Better
Adding to the confusion regarding sirtuins—but also, of course, adding new and potentially important information that will ultimately help scientists sort things out—is a new study published in Cell Metabolism by researchers at the University of Southern California and the University of Ottawa. Building on previous work with lower organisms, and using neurons from the cerebral cortex of rats, they investigated the role of SIRT1 in affording resistance to oxidative stress. (The gene for SIRT1 is known to be expressed more strongly in the brain than elsewhere in the body, which suggests that the physiological role of SIRT1 may be more important there than in other organs and tissues.)
To induce oxidative stress in the cultured cortical neurons, the researchers treated them with hydrogen peroxide or menadione (vitamin K3, and yes, it’s a strong pro-oxidant). Predictably, their survival was drastically impaired, i.e., they tended to die off rapidly (the neurons, not the researchers).
The researchers then repeated the experiment, this time, however, after pretreating the neurons with two compounds that act as potent inhibitors of SIRT1 activity: nicotinamide (a form of vitamin B3) or the drug sirtinol. With SIRT1 activity thus suppressed, the neurons proved, surprisingly, to be more resistant to the oxidizing agents, and their survival rate was better than before! This suggests that inhibiting SIRT1 can protect against oxidative stress and provide an antiaging effect—at least in rat brains.
But what about mouse hearts?
In Mouse Hearts, More SIRT1 Is Better
(Up to a Point)
In another recent study, researchers at the University of Medicine & Dentistry of New Jersey, New Jersey Medical School, used transgenic mice that were genetically engineered to have different levels of overexpression of SIRT1. They found that low (2.5-fold) to moderate (7.5-fold) overexpression protected the mouse hearts against oxidative stress and provided a variety of cardiac antiaging effects. This is in stark conflict with the rat-brain results. It suggests, if nothing else, that the roles of SIRT1 (and perhaps, therefore, the effects of resveratrol) are different in different organs and tissues of the body.*
The New Jersey researchers also found that, at a higher level (12.5-fold) of overexpression of SIRT1, the cardiac benefits had turned around and headed south, so to speak, becoming pro-oxidative, proaging liabilities. In their words, “These results suggest that Sirt1 could retard aging and confer stress resistance to the heart in vivo, but these beneficial effects can be observed only at low to moderate doses (up to 7.5-fold) of Sirt1.” (The different use of capitalization is unimportant—Sirt1 and SIRT1 are the same thing. Also, the word “doses” may seem odd in this context; remember that SIRT1 is synthesized in the cells, not administered from outside.)
Should we be surprised by this turnaround in SIRT1’s effects? Not at all. It’s common in physiology (indeed, it’s the norm) for substances with beneficial effects within a certain range of concentrations to become ineffective or even harmful at higher concentrations. Thus it is always unwise, and often dangerous, to exceed the recommended levels of intake of any drug or supplement, thinking that more is better. (There is no evidence to suggest, by the way, that recommended intakes of resveratrol could activate SIRT1 to dangerous levels.)
From Rat Neurons to Mouse Brains—Confirmation
Getting back to the brain study, the USC/Ottawa researchers sought to determine whether SIRT1 (or rather, the lack of it) would have the same effect in living brains—of mice in this case—as in the cultured cortical rat neurons. (Why they switched rodents is not clear.) To this end, they used transgenic mice of the opposite kind from those used in the heart study: instead of genetically engineering the mice to overexpress SIRT1, they disabled (“knocked out”) the gene that codes for the protein so that the latter would not be produced. At 18 months of age, these knockout (KO) mice were killed and their brains examined for evidence of oxidative stress.
Sure enough, as with the rat cortical neurons, the brains of the KO mice had substantially less evidence of oxidative damage than those of normal mice, again indicating that the absence of SIRT1 had a protective effect. Put another way, the presence of SIRT1 appeared to sensitize the mouse brains to oxidative damage, suggesting that it has a proaging effect. Rats! That’s not what anyone wants to hear.
No SIRT1 = Early Death
But wait—how about actually measuring the lifespans of the mice with and without the normal complement of SIRT1? According to the results we’ve seen thus far, the KO mice should probably live longer than the normal control mice (whose lifespan is about 2 years), owing to their greater resistance to oxidative stress, right? Well, they didn’t. Quite the contrary: being a KO mouse in this study meant having a greatly increased risk of early death. Thus, the absence of SIRT1 appeared to have a proaging effect (or its presence had an antiaging effect, depending on how you want to look at it).
Having obtained this result with mice on a normal diet, the researchers did the same experiment with mice that were calorically restricted (by 40%), starting at 2 to 5 months of age. Here, the CR regimen dramatically extended the lifespan of the normal mice, as expected. Did it do the same for the KO mice? No, it did not: they died off early, just as in the other test.
Your Honor, I Can’t Help It that My Case Is a Turkey
Are you confused? If so, you’re just not thinking clearly! (According to Mr. Peter.) Imagine trying to present a case like this in a court of law. “Your Honor, SIRT1 is widely believed to have an antiaging effect in all organisms. In rat neurons and mouse brains, however, SIRT1 appears to be pro-oxidant and is therefore probably proaging—but not in mouse hearts, where it’s antiaging, except when the levels are too high and it becomes proaging. And in live mice, it’s presumably pro-oxidant in the brain but antioxidant elsewhere, and in any case it’s antiaging, regardless of diet. Of course, you would expect that . . . uh . . . Your Honor? You don’t look well. Can I get you a glass of wine?”
And that was just the simple version. In the course of their study, the researchers performed many other experiments, too complex to describe here, the results of which tended to support the conclusions stated above. In reporting the statistical techniques used for evaluating the data from the experiments, by the way, the authors cited “Turkey’s test,” not once but 31 times. This might have elicited gobbles from the test’s developer, the late, great Princeton mathematician John Tukey. (Wait a minute—what if there really is a Turkey test? Wattle they think of next?)
Experts Confused, but Resveratrol Works
So, what are we to make of all this? We’re just laymen, and even the experts are confused. One reason for that may be that they have only recently discovered that SIRT1 is localized differently in the brain cells of mammals (mice, anyway) from where it was thought to be. Evidence from many prior studies has shown that in most tissues, SIRT1 is found predominantly in the nucleus, where the DNA is. Recent work with brain cells, however (including the USC/Ottawa study discussed above), has shown that in the brain, it’s localized almost exclusively in the cytosol, the gel-like fluid that surrounds the nucleus and fills the cell. This implies that SIRT1’s roles in the brain may be very different from its roles throughout the rest of the body.
The authors stated (literature citations omitted),
Sirtuins, including Sir2 and SirT1, have been described as mediators of the effect of CR on longevity and are widely believed to protect against aging. However, others have proposed that sirtuins are not required for the effects of CR on life span. . . . Numerous studies point to SirT1 as a key regulator of cell survival in response to stress. It exhibits both pro- and antisurvival functions depending on the conditions. . . . Many factors may contribute to the seemingly contradictory effects of SirT1. . . . Yet we cannot conclude that SirT1 promotes oxidative damage in all organs in vivo, as SirT1 may play vastly different roles in various organs.
If “resveratrol” were substituted for “CR” in those statements, they would be equally valid. What’s most important, though, is this: even though scientists now seem to understand less than they thought they had about the mechanism(s) by which CR and resveratrol work, the health and longevity benefits of CR and resveratrol are not in question. What works, works, even if we don’t understand how. On that question, the jury has reached a verdict.
- Kaeberlein M. The ongoing saga of sirtuins and aging. Cell Metab 2008;
- Li Y, Xu W, McBurney MW, Longo VD. SirT1 inhibition reduces IGF-I/IRS-2/Ras/ERK1/2 signaling and protects neurons. Cell Metab 2008;8:38-48.
- Alcendor RR, Gao S, Zhai P, Zablocki D, Holle E, Yu X, Tian B, Wagner T, Vatner SF, Sadoshima J. Sirt1 regulates aging and resistance to oxidative stress in the heart. Circ Res 2007;100:1396-8.
Will Block is the publisher and editorial director of Life Enhancement magazine.