Vitamins and other supplements may not always help prevent cancer. Source: http://commons.wikimedia.org/wiki/File:B_vitamin _supplement_tablets.jpg
Antioxidants are often hailed for their cancer-fighting effects for their ability to neutralize free radicals, reactive particles that can damage DNA. However, Swedish researchers have found that two such antioxidants, n-acetylcysteine (NAC) and vitamin E, can help cancer cells and tumors progress by affecting the work of p53, a protein that helps suppress tumors (1).
Past research has shown that people who consume fruits and vegetables have lower rates of cancer. This observation led to the conclusion that antioxidants within these foods must be thwarting cancer’s development (2). The food supplement industry’s promotion of these studies began a boom in antioxidant supplements such as vitamins (1).
Now, a host of studies have shown that such supplements may do the opposite and promote some cancers. In the 1980s, some studies had hinted at this. Two studies had looked at how beta-carotene, the precursor to vitamin A, affected cancer, but one had to be stopped when there were increased rates of lung cancer in the participants taking beta-carotene. Later, another study on the effects of vitamin E and selenium on prostate cancer stopped when those participants taking the vitamin ended up with higher rates of the cancer (2).
Unlike in these previous studies, the Swedish researchers have not only found increased rates of lung cancer tumors in mice but have also come up with a possible biological explanation. They chose to test the effects of two biologically different antioxidants on mice that had been mutated so as to be more susceptible to lung cancer. Vitamin E is fat soluble and helps regulate enzymes while NAC dissolves in water and is often given to those with lung disease to thin mucus (1,2).
Although the two antioxidants are functionally unrelated, both led to the susceptible mice having not only more tumors but tumors that were more aggressive than those in the control mice that were merely mutated but not given any antioxidants (2). This result implies that the effects of vitamin E and NAC converge, mostly likely in how they act as antioxidants (1).
As suspected, the antioxidants did their jobs in reducing the number of free radicals and DNA damage in the lung cells. However, the researchers found that both antioxidants also led to a suppression of the genes already present in cells to reduce free radicals. These genes were suppressed in normal tissue as well as the cancerous ones. The results imply a feedback mechanism, where the cells reduce expression of their natural defenses when there are fewer free radicals present due to the antioxidants (1).
One of the natural defense genes that was affected was p53. This gene helps regulate cell division, stopping cells so that they can repair their DNA or undergo apoptosis, or cell suicide, if there is too much damage. P53 is often mutated in late stage tumors that have grown substantially. The researchers found that the antioxidants had no effect on how fast the tumors grew if p53 had already been knocked out, by mutation or by interfering RNA. A similar result was found in human lung cancer cell lines as well (1).
These results imply an indirect and possibly complex role of antioxidants affecting the genes naturally present to combat free radicals and cancer. Their experiment was also run on cells that were already mutated and predisposed to cancer. Additional studies could be done about how they affect tumor initiation. Some other studies have already shown that other antioxidants may lead to tumor formation (1).
The researchers say that the implications are not that clear but that those who are susceptible to lung cancer, like smokers, or those in the early stages of the disease, may be harmed by heightened levels of these antioxidants (1). More studies on vitamin E, NAC, and other antioxidants that are supplemented are needed, for they all have their own effects on different cells. In the meantime, however, due to other benefits, the researchers and others do not advise abandoning your fruits and vegetables just yet (2).
References:
1. V. Sayin et al., Sci. Transl. Med. 6, 221 (2014).
2. J. Kaiser, Antioxidants Could Spur Tumors by Acting on Cancer Gene (31 January 2014). Available at http://www.sciencemag.org/content/343/6170/477.full (8 February 2014).