|Antioxidants and Radiation Therapy1
Community Health and Family Medicine, Tufts University School of Medicine, Boston, MA 02111
2To whom correspondence should be addressed. E-mail: email@example.com.
Radiation therapy has been used in cancer treatment for many decades; it is used to eradicate cancer and as a palliative to relieve pain associated with metastases. In the course of treatment, radiation produces numerous biological perturbations in cells; because normal cell toxicity limits the doses used in effective treatment, approaches are designed to strike a balance between eliminating cancer cells and protecting normal tissues. The primary focus in radiotherapy is to increase DNA damage in tumor cells, as double strand breaks are important in cell death. Another course of action is to alter cellular homeostasis, modifying signal transduction pathways, redox state, and disposition to apoptosis. The cellular changes ideally would enhance the killing of tumor cells while reducing the probability of normal cell death.
Intracellular free radicals
Ionizing radiation consists of electromagnetic radiation (photons), including X-rays and gamma rays, and particulate radiation, such as electrons, protons, and neutrons. Clinical radiation oncology uses electromagnetic radiation and particulate radiation, mostly electrons and to a lesser extent neutrons and protons (1).
Radiation damages cells by direct ionization of DNA and other cellular targets and by indirect effect through ROS. Exposure to ionizing radiation produces oxygen-derived free radicals in the tissue environment; these include hydroxyl radicals (the most damaging), superoxide anion radicals and other oxidants such as hydrogen peroxide. Additional destructive radicals are formed through various chemical interactions.
About two-thirds of X-ray and gamma-ray damage is caused by indirect action; heavy particles, such as neutrons, act mostly by direct ionization. Although effective in killing tumor cells, ROS produced in radiotherapy threaten the integrity and survival of surrounding normal cells.
Cell response to radiation depends on the type and dose of radiation, inherent tissue sensitivity and repair, and modulating intracellular factors that include position in the cell cycle, oxygen concentration, and levels of thiols and other antioxidants. Intracellular oxygen determines the extent of DNA damage by X-rays and gamma-rays. To be effective, it must be present during radiation or at least during the lifetime of the free radical (10–5 s). Without oxygen, the indirect radiation damage can be repaired. Oxygen binds to (oxidation) short-lived free radical sites in DNA, "fixing" the damage. Thiols, e.g., glutathione, or other antioxidants compete with this oxidation, chemically reduce the free radicals, and repair the damage. Reduction of transient free radicals is one mechanism by which antioxidants influence the indirect action of radiation. The extent of danger to normal tissues in radiation therapy depends on the dose, tissue sensitivity and repair capacity, affected organs, and prevailing endogenous antioxidant defenses.
Mitotic death and apoptosis
Radiation induces mitotic cell death in dividing cells and activates pathways that lead to death by apoptosis in interphase cells and differentiated cells. Apoptosis is regulated by the Bcl-2 family of proteins that include the pro-apoptosis proteins Bax and others and the apoptosis-inhibitors Bcl-2, Bag, and others. Apoptosis is associated with characteristic morphological changes in cells and their DNA (2) that result largely from the action of activated cysteine proteinases (caspases). Apoptosis occurs through a mitochondrial-dependent pathway, with the release of cytochrome C, followed by activation of the caspase cascade, with caspase 3 leading cells to their death. Alternately, apoptosis proceeds via a mitochondrial-independent pathway, with the ligation of death receptors CD95 (Fas/Apo1/) and the subsequent recruitment of caspases (3).
Antioxidants and radiation
Antioxidant supplementation during radiation therapy poses a conundrum for the radiation oncologist, as antioxidants that protect normal cells from reactive oxygen species may provide the same benefits to cancer cells and reduce the efficacy of treatment. Short- and long-term injury to healthy cells, including tissue damage and increased risk of oncogenic transformation (4), can be prevented by antioxidants, as seen experimentally. New findings that antioxidants induce apoptosis in cancer cells and protect patients from painful side effects of radiation treatment may prove these compounds useful in future adjuvant therapy.
Radiation reduces tissue antioxidants
Radiation at doses used in therapy depletes cellular alpha-tocopherol in normal cells, thereby increasing their risk of damage; animal studies show that whole-body exposure to X-ray irradiation decreases tissue concentrations of vitamins C and E (6). A decline in tissue vitamins E and Se during radiation therapy for breast cancer and a fall in vitamins A, C, E, and Se during breast cancer treatment with ROS-producing adriamycine may increase normal tissue sensitivity to radiation damage.
Age-related antioxidant protection
Age plays a role in radiation response. Endogenous antioxidants decrease with age, as does DNA repair capacity; cellular and blood levels of antioxidants further decrease during exposure to ionizing radiation. Human plasma proteins that contain sulfhydryl moieties, as well as dietary antioxidants such as vitamins C and E, are radioprotective. Studies on people 30 to 80 y old show an inverse relationship between plasma radioprotective ability and age, suggesting an increased antioxidant requirement with age to halt oxidative damage (7).
Antioxidants differ in modulating radiation effects
Antioxidant action depends on the oxygen partial pressure in a tissue and the nature of the antioxidant. ß-carotene is an effective chain-breaking antioxidant at low PO2; at high oxygen pressure it is less efficient and may even act as a prooxidant due to autooxidation. In contrast, -tocopherol is an efficient antioxidant in cells with a high PO2, for example, in the lung. This is a point of consideration in antioxidant supplementation; radiation therapy aims to increase the oxygen content of tumors to enhance cell killing. As oxygen partial pressure differs among tumors, so does the modulating effect of radiation treatment by antioxidants, varying with antioxidant used and the tumor PO2 (8).
Antioxidant protection and apoptosis
Selenium. The micronutrient selenium is an integral part of glutathione peroxidase, which destroys peroxides. Selenium increases inherent levels of antioxidant enzymes in normal cells (5) but not in some cancer cells (e.g., leukemia); selenium inhibits radiogenic transformation (5) and can stimulate DNA repair in cells with functional p53 (normal cells). Selenium induces apoptosis in prostate cancer cells and protected normal human fibroblasts from single and multiple doses of radiation but not squamous cell carcinoma cells (9).
Vitamin E. Similar to selenium, with which it can act in synergistic fashion in protecting cells against radiogenic transformation (5), vitamin E has been shown to decrease radiation-induced chromosome damage in human tumor cells but not in normal cells and has an inhibitory effect on a variety of cancer cells (10). A combined treatment with vitamins E and C inhibits apoptosis in human endothelial cells more effectively than each alone, while increasing Bcl-2 and downregulating the pro-apoptotic Bax (11). By contrast, vitamin E induces apoptosis in human breast and prostate cancer cells as well as leukemia (12) and glioblastoma cells (13). Pretreatment of cells with vitamin E and selenium increases the levels of glutathione, glutathione peroxidase, and catalase, while doubling the breakdown of toxic peroxide and reducing transformation (5).
Vitamin E, neurons, and glioblastoma
Brain tissue is highly sensitive to free-radical damage because of its low level of endogenous antioxidants, notably vitamin E and superoxide dismutase. In addition, aging neuronal mitochondria are prone to oxidative damage. Vitamin E protects the integrity of acetyl choline receptors in normal neurons and prevents toxicity and apoptosis induced by ROS-producing amyloid ß peptides that increase in the brain with age and in dementia (14).
Glioblastoma multiforme is the most common and aggressive brain cancer in humans and resists all forms of therapy. Vitamin E (-tocopherol succinate) induces apoptosis in glioblastoma cells in a dose-related manner; we find that a 48-h exposure to 50 µmol/L vitamin E results in a 15% increase in apoptosis in the glioblastoma cells over control (13). Pretreatment with vitamin E may have a potential role in sensitizing glioblastoma to radiotherapy.
Antioxidants decrease painful side effects
Vitamin supplementation may help treat side effects of radiation therapy. Vitamin E (400 IU) and vitamin C (500 mg) have been shown to offer protection against proctitis, a painful chronic injury that affects 5–20% of people receiving radiation therapy for cervical and prostate cancer (15); a striking regression of chronic radiation-induced fibrosis was seen in a clinical trial that combined radiation treatment of head and neck cancer with vitamin E (1000 IU) and pentoxyfylline (0.8 g/d) supplementation (16).
A wide range of antioxidant phytochemicals, including flavonoids, polyphenols, carotenoids, and organosulfur compounds, are antioxidants and are radioprotective in experimental systems (17,18). Some phytochemicals induce apoptosis in cancer cells; for example, a water-soluble organosulfur compound, S-allylmercaptocysteine, unique to the supplement aged garlic extract, induces apoptosis in human prostate cancer cells and in breast cancer cells (19), as well as colon cancer cells (20). Additional research is needed to establish the role of phytochemicals in conjunction with cancer treatment by radiation.
Epidemiological studies link the intake of antioxidant-rich foods with a reduced risk of certain cancers. A large segment of the population in the United States consumes antioxidant supplements, sometimes in large doses, with the hope that, by protecting against excess ROS and oxidant stress, they will help stave off the disease. Yet many questions remain unanswered and require additional research (21). Some that stand out are: Which antioxidants are most protective? Are isolated antioxidants available as supplements as effective? What would be an optimal dose, under different circumstances of risk, in health and disease and in different populations? What is the role of antioxidants once cancer has been diagnosed? What is the risk/benefit relationship in taking antioxidants during radiation therapy?
Antioxidants do protect against radiation-induced oncogenic transformation in experimental systems; however, we do not have comparable human studies that show the same association. Antioxidants do reduce the painful side effects of radiation therapy, thus supporting the beneficial effects of antioxidants in protecting normal cells in radiation therapy and in being used in conjunction with treatment for certain cancers. When considering antioxidant supplementation during treatment, it is doubtful whether high doses of radiation given in certain treatments would be rendered less effective if patients took a daily supplement of antioxidants—for example, at RDA levels—yet, we do not know and more research is needed.
Experimental studies showing that antioxidants, including phytochemicals, induce apoptosis in cancer cells but not in normal cells are in vitro phenomena. At this point, one can only speculate on an in vivo correlation. More studies are needed when considering adjuvant therapy with radiation.
At present, with limited available data, most radiation oncologists counsel their patients to refrain from taking antioxidant supplements during radiation therapy. Others, however, consider the data and suggest that a cautious and judicious use of antioxidants that helps the patient maintain a good quality of life may be helpful in cancer treatment.
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