November 27, 2004
Antioxidants Suppress Apoptosis1,2
KEY WORDS: * antioxidants * apoptosis * cancer therapy
There is a well-documented association between increased consumption of antioxidants and decreased incidence of cancer (1- 4). These epidemiological studies are supported by animal-model and cell-culture studies correlating oxidative DNA damage to the process of carcinogenesis (5,6). For these reasons, antioxidant supplements are often recommended as part of a cancer prevention diet (7,8). However, the generation of excess levels of reactive oxygen species is important for activation of internal cell programs for cell suicide (apoptosis) that are important protection mechanisms that kill cancer cells (9,10). Also, this mechanism is critical for effective cancer chemotherapy and radiation treatment (10,11). Perhaps, before cancer patients supplement their diets, suppression of apoptosis by antioxidants needs to be considered.
Apoptosis occurs when internal monitors recognize damage or malfunction and initiate signaling cascades that eventually activate caspases and endonucleases that kill the cell (12-15). One of the important functions of apoptosis is the elimination of preneoplastic and neoplastic cells (16-18). In most forms of cell suicide, the signaling cascade utilizes reactive oxygen species as essential intermediate messenger molecules (19-23). This is the reason that antioxidants are capable of inhibiting apoptosis. Antioxidants such as α-tocopherol, which partition into the lipid compartment of cells, or N-acetylcysteine, a free radical scavenger that partitions into the aqueous phase of the cytosol, can delay or inhibit apoptosis (24,25). Thus, it is reasonable to suggest that removal of antioxidants from the diet might enhance apoptosis, and thereby inhibit tumor growth.
We observed a reduction in brain tumor size in the TgT (121) transgenic mouse model, which spontaneously develops brain cancer, when these mice were fed diets depleted of antioxidants; there was enhanced apoptosis within tumors (26). Recently, colleagues extended this observation to another cancer type, breast cancer (27). Using a transgenic mouse model of mammary tumorigenesis with defined rates of tumor growth and lung-targeted metastasis, they determined that dietary antioxidant depletion inhibited tumor growth and diminished metastasis. Compared with control mice fed a standard diet, mice fed an antioxidant-depleted diet exhibited tumor-targeted generation of reactive oxygen species; the number of apoptotic cells in tumors increased 5-fold, and the percentage of tumor cells undergoing mitosis decreased by half. The mice fed the antioxidant-depleted diet had more small primary tumors and fewer large primary tumors than did controls, and they also had
Cells contain endogenous antioxidant enzymes (e.g., catalase, superoxide dismutase, and glutathione peroxidase), and many, but not all, human cancer cell types have decreased antioxidant enzyme levels compared to their normal tissue counterparts (28-30). The concentrations of free oxygen radicals are reportedly higher in malignant cells than in normal cells (31,32). Thus, some cancer cells may be more sensitive to generated reactive oxygen species, and this may be a useful difference that can be exploited when seeking to kill cancer cells but spare normal cells. Even a moderate increase in the accumulation of oxygen radicals in malignant cells of animals fed an antioxidant-poor diet could increase reactive oxygen species to the critical level required for progression of apoptosis (21-23). Conversely, even modest quenching of oxygen radicals by dietary antioxidants could block completion of apoptosis.
Antioxidants, by preventing oxidant-mediated damage to diverse targets (DNA, RNA, proteins, and lipids), may play a protective role in healthy individuals with no existing cancer cells that must be eliminated; however, by inhibiting apoptosis, these same antioxidants may exert a cancer-promoting effect in cancer patients and in individuals with precancerous DNA changes. Inhibition of apoptosis by antioxidants may explain why, in several studies in heavy smokers, vitamin E and β-carotene enhanced carcinogenesis in the lung (33) (where, presumably, precancerous lesions caused by smoking predated antioxidant treatment) but decreased carcinogenesis in the prostate (34) (where, presumably, smoking had not caused precancerous lesions that predated antioxidant treatment). Thus, though early administration of antioxidants may prevent the initiation and progression of cancer by quenching the action of potentially mutagenic reactive free radicals, administration of antioxidants subsequent to a mutagenic event may effectively intercept free radicals that are critical in promoting apoptosis. This imbalance may allow the rate of proliferation in tumors to exceed the capacity for apoptosis. It seems reasonable to suggest that the potential risks and benefits of high-dose antioxidants need to be considered on a case-to-case basis, and indiscriminate use of antioxidant dietary supplements should be avoided.
1 Presented as part of the conference "Free Radicals: The Pros and Cons of Antioxidants," held June 26-27 in Bethesda, MD. This conference was sponsored by the Division of Cancer Prevention (DCP) and the Division of Cancer Treatment and Diagnosis, National Cancer Institute, NIH, Department of Health and Human Services (DHHS); the National Center for Complementary and Alternative Medicine (NCCAM), NIH, DHHS; the Office of Dietary Supplements (CDS), NIH, DHHS; the American Society for Nutritional Science; and the American Institute for Cancer Research and supported by the DCP, NCCAM, and ODS. Guest editors for the supplement publication were Harold E. Seifried, National Cancer Institute, NIH; Barbara Sorkin, NCCAM, NIH; and Rebecca Costello, ODS, NIH.
2 This work was supported by National Institutes of Health Grants DK55865, AG09525, and DK56350.
1. Steinmetz, K. A. & Potter, J. D. (1996) Vegetables, fruit, and cancer prevention: a review. J. Am. Diet Assoc. 96: 1027-1039.
2. Willett, W. C. & Trichopoulos, D. (1996) Nutrition and cancer: a summary of the evidence. Cancer Causes Control 7: 178-180.
3. Taylor, P. R., Qiao, Y. L., Abnet, C. C., Dawsey, S. M., Yang, C. S., Gunter, E. W., Wang, W., Blot, W. J., Dong, Z. W. & Mark, S. D. (2003) Prospective study of serum vitamin E levels and esophageal and gastric cancers. J. Natl. Cancer Inst. 95: 1414-1416.
4. Seifried, H. E., McDonald, S. S., Anderson, D. E., Greenwald, P. & Milner, J. A. (2003) The antioxidant conundrum in cancer. Cancer Res. 63: 4295-4298.
5. Cooke, M. S., Evans, M. D., Dizdaroglu, M. & Lunec, J. (2003) Oxidative DNA damage: mechanisms, mutation, and disease. FASEB J. 17: 1195-1214.
6. Cerutti, P., Shah, G., Peskin, A. & Amstad, P. (1992) Oxidant carcinogenesis and antioxidant defense. Ann. N.Y. Acad. Sci. 663: 158-166.
7. Ames, B. N. (1999) Micronutrient deficiencies. A major cause of DNA damage. Ann. N.Y. Acad. Sci. 889: 87-106.
8. Prasad, K. N., Kumar, A., Kochupillai, V. & Cole, W. C. (1999) High doses of multiple antioxidant vitamins: essential ingredients in improving the efficacy of standard cancer therapy. J. Am. Coll. Nutr. 18: 13-25.
9. Weijl, N. I., Cleton, F. J. & Osanto, S. (1997) Free radicals and anti-oxidants in chemotherapy-induced toxicity. Cancer Treat. Rev. 23: 209-240.
10. Kuipers, G. K. & Lafleur, M. V. (1998) Characterization of DNA damage induced by gamma-radiation-derived water radicals, using DNA repair enzymes. Int. J. Radial. Biol. 74: 511-519.
11. Blumenthal, R. D., Lew, W., Reising, A., Soyne, D., Osorio, L., Ying, Z. & Goldenberg, D. M. (2000) Anti-oxidant vitamins reduce normal tissue toxicity induced by radio-immunotherapy. Int. J. Cancer 86: 276-280.
12. Kokileva, L (1994) Multi-step chromatin degradation in apoptosis. Int. Arch. Allergy Immunol. 105: 339-343.
13. Zhivotovsky, B., Wade, D., Nicotera, P. & Orrenius, S. (1994) Role of nucleases in apoptosis. Int. Arch. Allergy Immunol. 105: 333- 338.
14. Wyllie, A. H. (1987) Cell death. Int. Rev. Cytol. 17 (suppl.): 755-785.
15. Arends, M. J., Morris, R. G. & Wyllie, A. H. (1990) Apoptosis. The role of the endonuclease. Am. J. Pathol. 136: 593- 608.
16. Lowe, S. W., Ruley, H. E., Jacks, T. & Housman, D. E. (1993) p53Dependent apoptosis modulates the cytotoxicity of anticancer agents. Cell 34: 957-967.
17. Thompson, C. B. (1995) Apoptosis in the pathogenesis and treatment of disease. Science 267: 1456-1462.
18. Tomlinson, I.P.M. & Bodmer, W. F. (1995) Failure of programmed cell death and differentiation as causes of tumors: Some simple mathematical models. Proc. Natl. Acad. Sci. U.S.A. 92: 11130- 11134.
19. Albright, C. D., Salganik, R. I., Craciunescu, C. N., Mar, M. H. & Zeisel, S. H. (2003) Mitochondrial and microsomal derived reactive oxygen species mediate apoptosis induced by transforming growth factor-beta1 in immortalized rat hepatocytes. J. Cell Biochem. 89: 254-261.
20. Vrablic, A. S., Albright, C. D., Craciunescu, C. N., Salganik, R. I. & Zeisel, S. H. (2001) Altered mitochondrial function and overgeneration of reactive oxygen species precede the induction of apoptosis by 1-O-octadecyl-2-methylrac-glyce\ro-3- phosphocholine in p53-defective hepatocytes. FASEB J. 15: 1739- 1744.
21. Slater, A. F., Nobel, C. S. & Orrenius, S. (1995) The role of intracellular oxidants in apoptosis. Biochim. Biophys. Acta 1271: 59- 62.
22. Johnson, T. M., Yu, Z. X., Ferrans, V. J., Lowenstein, R. A. & Finkel, T. (1996) Reactive oxygen species are downstream mediators of p53-dependent apoptosis. Proc. Natl. Acad. Sci. U.S.A. 93: 11848- 11852.
23. Sugiyama, H., Kashihara, N., Makino, H., Yamasaki, Y. & Ota, Z. (1996) Reactive oxygen spesies induce apoptosis in cultured human mesangial cells. J. Amer. Soc. Nephrol. 7: 2357-2363.
24. Hawkins, R. A., Sangster, K. & Arends, M. J. (1998) Apoptotic death of pancreatic cancer cells induced by polyunsaturated fatty acids varies with double bond number and involves an oxidative mechanism. J. Pathol. 185: 61-70.
25. Takahashi, H., Kosaka, N. & Nakagawa, S. (1998) Alpha- tocopherol protects PC12 cells from hyperoxia-induced apoptosis. J. Neurosci. Res. 52: 184-191.
26. Salganik, R. I., Albright, C. D., Rodgers, J., Kim, J., Zeisel, S. H., Sivashinskiy, M. S. & Van Dyke, T. A. (2000) Dietary antioxidant depletion: enhancement of tumor apoptosis and inhibition of brain tumor growth in transgenic mice. Carcinogenesis 21: 909- 914.
27. Albright, C. D., Salganik, R. I. & Van Dyke, T. (2004) Dietary depletion of vitamin E and vitamin A inhibits mammary tumor growth and metastasis in transgenic mice. J. Nutr. 134: 1139-1144.
28. Coursin, D. B., Cihla, H. P., Sempf, J., Oberley, T. D. & Oberley, L. W. (1996) An immunohistochemical analysis of antioxidant and glutathione S-transferase enzyme levels in normal and neoplastic human lung. Histol. Histopathol. 11: 851-860.
29. Oberley, T. D., Sempf, J. M. & Oberley, L. W. (1996) Immunogold analysis of antioxidant enzymes in common renal cancers. Histol. Histopathol. 11 : 153-160.
30. Oberley, T. D. & Oberley, L. W. (1997) Antioxidant enzyme levels in cancer. Histol. Histopathol. 12: 525-535.
31. Sztarovski, T. R. & Nathan, C. F. (1991) Production of large amounts of hydrogen peroxide by human tumor cells. Cancer Res. 51: 794-798.
32. Toyokuni, S., Okamoto, K., Yodoi, J. & Hiai, H. (1995) Hypothesis: persistent oxidative stress in cancer. FEBS Lett. 358: 1- 3.
33. De Luca, L. M. & Ross, S. A. (1996) Beta-carotene increases lung cancer incidence in cigarette smokers. Nutr. Rev. 54: 178-180.
34. Heinonen, O. P., Albanes, D., Virtamo, J., Taylor, P. R., Huttunen, J. K., Hartman, A. M., Haapakoski, J., Malila, N., Rautalahti, M., et al. (1998) Prostate cancer and supplementation with alpha-tocopherol and beta-carotene: incidence and mortality in a controlled trial. J. Natl. Cancer Inst. 90: 440-446.
Steven H. Zeisel3
Department of Nutrition, University of North Carolina-Chapel Hill, Chapel Hill, NC 27599-7461
3 To whom correspondence should be addressed. E-mail: [email protected]
Copyright American Institute of Nutrition Nov 2004