Antioxidants have long been thought to have anti-aging properties, primarily by protecting a person's genetic material from damaging chemicals. The story, however, now appears to be much more complicated.

National Institutes of Health researchers from two institutes and one center have demonstrated that some antioxidants damage DNA and kill cells instead of protecting them. The findings, published in the Proceedings of the National Academy of Sciences on March 19, 2012, also suggest that this surprising capability may be good for treating cancer, but may prove cautionary when using antioxidant-based medicines to treat other disorders, such as diabetes.

"It's an unexpected discovery," says Kyungjae Myung, Ph.D., a senior investigator in the Genetics and Molecular Biology Branch of the National Human Genome Research Institute (NHGRI), and the senior author on the report. "It may have important clinical applications in treating people with cancer, especially if they have failed previous treatments."

Many people attempt to boost their levels of antioxidants by eating fruits and vegetables, nuts and grains, or by taking vitamins such as A, C, E and beta-carotene, among others. Some research suggests that antioxidants soak up compounds called free radicals produced by burning oxygen during normal respiration. Free radicals cause random chemical reactions that can damage cellular components, including DNA, leading to disease. By adding antioxidants to the diet, many people hope to slow down the process that some believe contributes to the normal process of aging.

Dr. Myung did not set out to challenge this anti-aging strategy, and the new findings may not fundamentally alter the approach; much more study will be needed. Instead, his lab studies DNA repair, the enzyme systems within a cell that fix mistakes and other damage that routinely accumulate in DNA as cells simply live and divide to make daughter cells. Researchers know that naturally occurring defects in DNA repair can lead to a number of disorders, including cancer.

To study DNA repair, Dr. Myung's group sought a new way to easily identify chemicals that damage DNA and then use those chemicals to study cellular repair mechanisms, a basic research question. Using a laboratory grown cell line from human kidneys, the NHGRI team, which included Jennifer Fox, Ph.D., lead author and post-doctoral fellow, developed a novel laboratory test, or assay, that readily shows when a chemical exposure damages DNA.

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With the test developed, Dr. Myung's team formed collaborations with two other NIH research groups: The first was with what is now the NIH National Center for Advancing Translational Sciences (NCATS). Over the last several years, a team led by Christopher Austin, M.D., head of the NCATS laboratories, has developed high-throughput chemical screening systems using robotics. Dr. Austin agreed to use Dr. Myung's assay to rapidly test thousands of chemicals for their ability to damage DNA. But what chemicals should they test?

In 2008, the NIH Chemical Genomics Center, then part of NHGRI and now at NCATS, the National Institute of Environmental Health Sciences (NIEHS) and the U.S. Environmental Protection Agency (EPA) formed an initiative called Tox21 to develop high-throughput screening tests that measure cellular harm caused by environmental chemicals. The Tox21 team created a library of some 2,000 compounds and agreed to test them against Dr. Myung's assay. The NHGRI researchers also added a commercially available chemical collection to the screening runs for a total of some 4,000 chemicals.

The screening runs produced surprises, identifying 22 antioxidants that damaged DNA. Three of the antioxidants -- resveratrol, genistein and baicalein -- are currently used -- or being studied -- to treat several disorders, including heart disease, type 2 diabetes, osteopenia and osteoporosis and chronic hepatitis, as well as serving as an anti-aging treatment.

Not only did the antioxidants damage the DNA, the researchers found, but also, in dividing cells (such as in tumors), the antioxidants can be lethal, killing the disease-causing cells.

"This is what's cool about biology," Dr. Austin said. "Just when we think we understand something, it turns out to be more complex than we thought. Not only did the NHGRI team produce a novel way to measure DNA damage, but their test has given us insights into the effects of chemical compounds that were not seen in more conventional strategies."

The discovery opens up several new lines of research. As a first step, the collaborators are dramatically expanding the number of compounds -- more than 300,000 -- that will be tested with the new assay. The Tox21 team also has decided to include the NHGRI test in its standard screen for biological harm produced by environmental chemicals.

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The clinical implications for these findings are more complicated. This initial discovery is only in lab-grown cell lines, not even in intact organisms. The relevance for humans has yet to be demonstrated.

Still, there is plenty of work already underway. Other researcher teams had launched various studies of these DNA-damaging antioxidants in various diseases. For example, 44 studies are currently listed in www.clinicaltrials.gov for resveratrol, which is found in many foods, including red grapes and wine, peanuts and chocolate. The studies focus on treating Alzheimer's disease, type 2 diabetes, obesity, inflammation, colon cancer, multiple myeloma, and testing other anti-aging strategies, among others. The newly reported study does not suggest that resveratrol in red wine is harmful; the dose is probably too low to be significant, Dr. Myung said.

Researchers also have launched 43 studies on genistein, including trials to treat cancers of the prostate, pancreas, bladder, breast, kidney and skin (metastatic melanoma) and as adjunct treatments for rare diseases such as cystic fibrosis.

Even though the antioxidants damaged the DNA, the researchers reported that the chemicals did not cause genetic mutations, another surprise. "Because they don't cause genetic mutations, antioxidants may be useful for treating cancer," Dr. Myung said. "Standard chemotherapy mutates the tumor's DNA, speeding its evolution and sometimes allowing it to escape the toxic treatment intended to kill it. This leads to multi-drug resistance in some cancer patient's disease."

To test whether the antioxidants might help, the NHGRI team borrowed some multi-drug resistant cancer cells from Dr. Michael Gottesman, a National Cancer Institute researcher and NIH Deputy Director for Intramural Research. Although these cells are very resistant to anti-cancer drugs, treatment with resveratrol appeared to sensitize the cancer cells, leading to their death. "Resveratrol," Dr. Myung said, "could prove useful in treating multi-drug resistant cancers."

The findings do raise concerns about using antioxidants to treat disorders, as treatment with high doses may cause unexpected DNA damage that leads to other problems. "Clearly," Dr. Myung said, "much more study will be needed."