* This post is a companion to the previous one in which I talked about antioxidants and cancer in a more general manner. Here I will focus on one particular scientific article.
A study recently published in Science Translational Medicine shows that antioxidant supplementation speeds up lung cancer progression in mice (1). The researchers also suggest a potential molecular mechanism to explain how antioxidants may be detrimental in this particular setting.
N-acetylcysteine and vitamin E supplements accelerate lung tumor progression
In view of the popular belief that antioxidants are helpful in the fight against cancer, researchers from the University of Gothenburg in Sweden decided to look at the effect of dietary antioxidant supplementation on tumor growth in two mouse models of lung cancer. The models they used are based on genetically modified mice in which lung cancer can be induced by triggering the expression of a mutated gene in lung epithelial cells (in one model the mutated gene is Kras, in the other, Braf). At the same time as the expression of the mutated gene was switched on to initiate tumor formation, mice were put on antioxidant supplementation, either in the form of N-acetylcysteine (NAC) in their drinking water, or as vitamin E in their chow food. The amounts of NAC of vitamin E thus fed to the mice were comparable to what humans would get from supplement pills. Other mice were kept on a normal, non-supplemented diet as controls.
The researchers found that mice whose diet was supplemented with NAC or vitamin E had larger tumors and died faster than control mice. More precisely, the proportion of lung area occupied by tumor cells was about three times larger in supplemented mice than in control mice ten weeks after tumor initiation (done by activating the mutated genes). Survival analysis showed that only 50% of supplemented mice were still alive after about 10 weeks, and all were dead after about 15 weeks. By contrast, 50% of control mice were still alive after 15 weeks and all were dead by about 30 weeks. To give some perspective, an average lab mouse may live about 2 years or slightly more, so surviving for 2 or 3 more months is not negligible. (It is also perhaps important to mention that the numbers of mice in each group and the difference in survival observed were large enough to be statistically significant, that is, not due to chance.)
Antioxidants reduce levels of reactive oxygen species and DNA damage and promote tumor cell proliferation
Next, the researchers asked how two structurally different molecules, NAC and vitamin E, could have the same effect. Considering that both have antioxidant properties, it seemed logical that it could have something to do with oxidative stress. Looking at sections of tumor tissue, the researchers observed that the tumors from antioxidant-supplemented mice harbored lower levels of reactive oxygen species (ROS) and less ROS-induced DNA damage than tumors from control mice. There were also more proliferating cells in tumors from antioxidant-treated mice than in tumors from control mice.
Addition of NAC or of a water-soluble vitamin E analog to cells in vitro confirmed what was observed in vivo: cells made to express the mutated Kras or Braf genes that induced cancer in vivo (oncogenes) proliferated more and harbored lower levels of ROS when NAC or vitamin E was added to the cell cultures. By contrast, NAC and vitamin E did not change the proliferation rate of normal cells (cells not expressing the oncogenes). Of note, just the fact of making the cells express the oncogenes decreased the levels of ROS present (which in itself suggests that lower ROS levels are beneficial to tumor cells). However, the decrease was only transient, whereas addition of NAC or vitamin E to the cultures led to sustained lower ROS levels.
Tumor suppressor p53 involved?
Trying to go one step further in exploring the molecular mechanisms underlying the effect of antioxidants on tumor cells, the researchers looked at the levels of the p53 protein. p53 is a known tumor suppressor gene, it is involved in regulating cell proliferation and is activated by ROS and DNA damage. Protein level analysis revealed that both cells treated with NAC and vitamin E in vitro and tumor cells from antioxidant-treated mice had lower amounts of the p53 protein than controls.
In parallel, the researchers showed that, whereas treatment with NAC or vitamin E increased the proliferation of oncogene-expressing cells, it did not increase the proliferation of oncogene-expressing cells in which p53 had been inactivated. However, as the inactivation of p53 by itself increases the proliferation of the oncogene-expressing cells, it is difficult to interpret the lack of further increase in proliferation when adding NAC or vitamin E to the cell cultures: the authors suggest that it is because the effect of NAC and vitamin E is carried out via p53, however it could also be because the effect of p53 inactivation is itself so large that it masks any other effect that the antioxidants might have independently of p53.
Nevertheless, putting all their data together, the research team hypothesizes that antioxidants might support the growth of tumor cells by decreasing the levels of ROS and DNA damage, thereby removing stimuli that would otherwise activate p53, which would then keep tumor cell proliferation in check.
Similar effects of antioxidants on human lung cancer cell lines
Finally, the researchers repeated their experiments using human lung cancer cell lines (after all, the study was published in Science Translational Medicine, there had to be some sort of bench-to-bedside data). They observed that human lung cancer cells responded in the same way to vitamin E and NAC as oncogene-expressing mouse cells did: increased proliferation, decreased intracellular ROS levels and decreased p53 levels.
Limitations and relevance
Like all studies, this one has limitations. First, while the p53-involving mechanism hypothesized by the researchers to explain the effect of antioxidants on tumor growth is possible, the data presented in the study is not enough to establish it for sure, and other pathways may also be involved. Second, the two mouse models used only allow to study the impact of antioxidant supplementation on lung cancer progression, not on cancer initiation or prevention. Third, the models are based on specific mutated genes, it is therefore possible that antioxidants would not have the same effects in other types of cancers. Finally, two particular antioxidants were investigated, and since antioxidants are a group of molecules with many different structures and slightly different properties, the results of the study cannot be generalized blindly to every antioxidant.
Nevertheless, the study has undeniable positive points: first, it supports with experimental data what had been observed in human clinical association studies, namely that certain antioxidant supplements increase the risk of lung cancer in individuals already at risk (2-3); second, it provides clues as to what the biological mechanisms involved may be. Although the experimental data here is restricted to two particular antioxidants and two models of lung cancer, it may, for that same reason, be of particular relevance for people suffering from COPD (chronic obstructive pulmonary disease). Such people are indeed often smokers with increased risk of developing lung cancer and may take high amounts of NAC to help relieve mucus production. If the experimental findings in mice translate into humans, giving antioxidant supplementation to people with a high risk of developing lung cancer may have to be reconsidered.
1. Antioxidants accelerate lung cancer progression in mice. Sayin VI, Ibrahim MX, Larsson E, Nilsson JA, Lindahl P, Bergo MO. Sci Transl Med. 2014 Jan 29;6(221):221ra15. doi: 10.1126/scitranslmed.3007653
2. The Effect of Vitamin E and Beta Carotene on the Incidence of Lung Cancer and Other Cancers in Male Smokers. The Alpha-Tocopherol Beta Carotene Cancer Prevention Study Group. N Engl J Med 1994; 330:1029-1035. doi: 10.1056/NEJM199404143301501
3. Effects of a combination of beta-carotene and vitamin A on lung cancer and cardiovascular disease. Omenn GS, Goodman GE, Thornquist MD, et al. N Engl J Med 1996; 334:1150-55. doi: 10.1056/NEJM199605023341802
Sayin VI, Ibrahim MX, Larsson E, Nilsson JA, Lindahl P, & Bergo MO (2014). Antioxidants accelerate lung cancer progression in mice. Science translational medicine, 6 (221) PMID: 24477002