Antioxidants Suppress Apoptosis
Summary: 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. The study observed a reduction in brain tumor size in a mouse model, which spontaneously develops brain cancer, when these mice were fed diets depleted of antioxidants; there was enhanced apoptosis within tumors
EXPANDED ABSTRACT
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 <30% of the number of lung metastatic tumor foci compared with mice fed the control diet.
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.