New research evidence backs up previous findings that taking antioxidant supplements can speed up the spread of the deadliest form of skin cancer.

Researchers at the University of Gothenburg in Sweden have added new data to a study of the effects of antioxidants on malignant tumor development.

In earlier work with human cancer tissue and mouse models, the researchers demonstrated that antioxidants could hasten the development of lung cancer and accelerate the spread of malignant melanoma, an uncommon but invasive form of skin cancer.

Now, after investigating certain antioxidants that bind to mitochondria, the team has produced further evidence that the substances either do not affect tumor growth or speed it up. Mitochondria are tiny power plants inside cells that provide them with energy.

Many people believe that because antioxidants neutralize a class of compounds called free radicals that can harm cells, they can protect against cancer. This study, however, further indicates that this might not be the case.

Antioxidants protect cancer cells

Although it may be true that antioxidants protect healthy cells, a growing body of research shows that they also protect cancer cells.

"This is not the way to treat cancer," says Dr. Kristell Le Gal Beneroso, of the Sahlgrenska Academy at the University of Gothenburg, whose recently published doctoral thesis includes a detailed account of the new findings.

"In the best case, the treatment makes no difference, but it can also exacerbate the disease," she adds.

Of the cancers that start in the skin, melanoma is the "most dangerous." It is one of the least common types of skin cancer, but it accounts for the most deaths. Official estimates for the United States suggest that 91,270 people will discover that they have melanoma in 2018 and 9,320 will die of the disease.

Mitochondria-targeting antioxidants

In nearly all cases where doctors diagnose melanoma in the early stages, surgery can cure the disease.

The outlook is not so positive, however, once the cancer is metastatic. This means it has become invasive and spread to other parts of the body.

The most common destinations of metastatic melanoma are the brain, lungs, bones, and liver.

In their previous work, the researchers had found that the addition of some types of antioxidants sped up metastasis in lung cancer and malignant melanoma. This process leads to more secondary tumors, or metastases, in new sites elsewhere in the body.

The recent work took this research a step further and looked at how adding specific antioxidant compounds that bind to mitochondria affected the two cancers.

Mitochondria are the primary source of free radicals in cells. Free radicals can harm cells by damaging their DNA. This kind of damage can give rise to cancer.

As before, the investigation used human cancer cells and mouse models.

'Tumors grew significantly faster'

Dr. Le Gal Beneroso explains that they wanted to test the theory that reducing free radical production in cells by binding antioxidants to their mitochondria might protect against cancer.

The results, however, did not support the theory. On the contrary, the antioxidants "either had no effect or they made the situation worse," Dr. Le Gal Beneroso notes.

In mice with malignant melanoma, she continues, "tumors grew significantly faster than in the control animals that received no treatment."

The researchers conclude that their findings "show that dietary antioxidant supplementation increases metastasis in malignant melanoma," and that "mitochondria-targeted antioxidants do not inhibit cancer progression."

The team reaffirms the previous recommendation that people with cancer or who have a high risk of developing it "should avoid the use of antioxidant supplements."

'Not always healthful'

They suggest that further research into the mechanisms through which antioxidants influence tumor growth and spread is required.

One avenue that they think needs exploring is the idea that low levels of free radicals might spur the formation of blood vessels that feed tumors.

This could clarify, for instance, whether the "accelerated growth kinetics" that they observed in the treated mice was a result of "better vascularization" of the new tumor tissue rather than a consequence of "tumor cell proliferation."

In the meantime, Dr. Le Gal Beneroso urges people not to assume that taking antioxidant supplements is "always healthy for the body."

"Hypothetically, you might be helping your healthy cells, but there is no strong evidence that this is the case," she adds.

"On the other hand, we know that once you have cancer, even if it has not been diagnosed, antioxidants can contribute to increased tumor growth."

–Dr. Kristell Le Gal Beneroso

—Catharine Paddock PhD