Discovery Confirms Universal Mechanism for Stimulating Antiaging Pathway

Moreover, the researchers have discovered the molecular process for this interaction, revealing that a class of stronger drugs currently in clinical trials acts in a similar way. These findings not only answer questions about how resveratrol and related compounds work, but also will help scientists better engineer molecules that more precisely and effectively treat disease associated with aging.

The study was published in the March 8, 2013, issue of Science. The science of aging, for the past 10 years, has increasingly centered on sirtuins, a group of genes that are thought to protect many organisms, including mammals, against diseases of aging. Increasing evidence has demonstrated that resveratrol, a compound found in the skin of grapes as well as in peanuts and berries, increases the activity of a specific sirtuin, SIRT1, that protects the body from diseases by revving up the mitochondria.

Mice on resveratrol have twice the endurance and are relatively immune from effects of obesity and aging. In research attempts with nematodes, yeast, bees, flies and mice, lifespan has been extended. “In the history of pharmaceuticals, there has never been a drug that binds to a protein to make it run faster in the way that resveratrol activates SIRT1,” said Dr. David Sinclair, Harvard Medical School (HMS; Boston, MA, USA) professor of genetics and senior author on the paper. “Almost all drugs either slow or block them.”

In 2006, Dr. Sinclair’s group published a study showing that resveratrol could extend the lifespan of mice, and the company Sirtris Pharmaceuticals (Cambridge, MA, USA), which was started by HMS researchers, was founded to make drugs more potent than resveratrol.

Whereas ample studies, from Dr. Sinclair’s lab and elsewhere, highlighted a direct causal link between resveratrol and SIRT1, some scientists asserted that the studies were flawed. The contention lay in the way SIRT1 was examined in vitro, using a specific chemical group attached to the targets of SIRT1 that fluoresces more brightly as SIRT1 activity increases. This chemical group, however, is synthetic and does not exist in cells or in nature, and without it the experiments did not work. As a response to this, an article published in 2010 deduced that resveratrol’s activation of SIRT1 was an experimental artifact, one that existed in the lab, but not in an actual animal. SIRT1 activity in mice was, the article maintained, at most is an indirect result of resveratrol, and possibly even a pure coincidence.

As a result, controversy has exploded over the specific pathway that resveratrol and similar compounds affected. Does resveratrol directly activate SIRT1 or is the effect indirect? “We had six years of work telling us that this was most definitely not an artifact,” said Dr. Sinclair. “Still, we needed to figure out precisely how resveratrol works. The answer was extremely elegant.”

Dr. Sinclair and Basil Hubbard, then a doctoral student in the lab, teamed up with a group of researchers from both the US National Institutes of Health (Bethesda, MD, USA) and Sirtris Pharmaceuticals to address this question. First, the scientists addressed the difficulty of the fluorescent chemical group. Why was it required for resveratrol to rev up SIRT1 in the laboratory? Instead of rejecting the result as an artifact, the researchers theorized that the chemical might be mimicking molecules found naturally in the cell. These appeared to be a specific class of amino acid. In nature, there are three amino acids that resemble the fluorescent chemical group, one of which is tryptophan.

When researchers repeated the experiment, exchanging the fluorescing chemical group on the substrate with a tryptophan residue, resveratrol and similar molecules were once again able to activate SIRT1. “We discovered a signature for activation that is in fact found in the cell and doesn’t require these other synthetic groups,” said Dr. Hubbard, first author of the study. “This was a critical result, which allowed us to bridge the gap between our biochemical and physiological findings.”

“Next, we needed to identify precisely how resveratrol presses on SIRT1’s accelerator,” said Sinclair. The researchers tested approximately 2,000 mutants of the SIRT1 gene, eventually identifying one mutant that completely blocked resveratrol’s effect. The specific mutation resulted in the substitution of a single amino acid residue, out of the 747 that comprise SIRT1. The researchers also evaluated hundreds of other molecules from the Sirtris library, many of which are far more powerful than resveratrol, against this mutant SIRT1. All failed to activate it.

The authors propose a model for how resveratrol works: When the molecule binds, a hinge flips, and SIRT1 becomes hyperactive. Although these experiments occurred in a test tube, once the researchers identified the precise location of the accelerator pedal on SIRT1—and how to break it—they could test their ideas in a cell. They replaced the normal SIRT1 gene in muscle and skin cells with the accelerator-dead mutant. Now they could assess precisely whether resveratrol and the agents in development work by modifyingSIRT1 (in which case they would not work) or one of the thousands of other proteins in a cell (in which they would work). While resveratrol and the drugs tested revved up mitochondria in normal cells (an effect caused activating by SIRT1), the mutant cells were completely immune.

“This was the killer experiment,” concluded Dr. Sinclair. “There is no rational alternative explanation other than resveratrol directly activates SIRT1 in cells. Now that we know the exact location on SIRT1 where and how resveratrol works, we can engineer even better molecules that more precisely and effectively trigger the effects of resveratrol.”

The researchers plan on continuing academic-industry collaborations with the aim of bringing to market drugs that treat diseases associated with aging.

Related Links:
Harvard Medical School
Sirtris Pharmaceuticals