New Insights into the Mechanism of Action of Antioxidants
J. A. Milner
Nutrition Science Research Group
Division of Cancer Prevention,
National Cancer Institute
Bethesda, MD 20892
Free radicals are generated largely during the production of ATP in mitochondria. During this process, radicals leaking from the mitochondria form reactive oxygen species such as the superoxide anion and hydroxyl radicals. These species lead to the production of hydrogen peroxide from which further hydroxyl radicals are generated in a reaction that appear to depend on the presence of iron ions. These radicals have both beneficial and harmful actions in biological tissues. They are known to have a crucial role in stimulation of phagocytosis, induction of drug detoxification pathways and stimulation of signal –transduction pathways (Droge 2002, Salganik 2001). However, these same radicals can be potentially dangerous products of cellular metabolism in that they can directly influence cell growth and development, cell survival and likely increase the pathogenesis of atherosclerosis, cancer, aging and several other conditions, including inflammatory disease.
It has been suggested that the extent of damage caused by free radicals might be modified through three dietary intervention strategies: (a) caloric restriction and thus a depression in free radicals arising due to normal metabolism; (b) minimizing the intake of components that increase free radicals such as polyunsaturated fats; and (c) supplementation with one or more anti-oxidants. Several compounds with antioxidant properties including vitamin C, vitamin E, and carotenoids are found in various food products and have been reported to have varying effects on biomarkers of oxidative damage. The diet actually contains a host of bioactive compounds with antioxidant potential. While foods can been shown to vary in their antioxidant potential it remains less clear if this accounts for significant alterations in free radicals in in vivo situations (van den Berg et al. 2001).
Free radicals can unquestionably cause oxidative damage to DNA. Although there are methodological uncertainties about accurate measurement of oxidative DNA damage, the damage escaping repair appears to contribute to mutations. The observation that diets rich in fruits and vegetables can decrease both oxidative DNA damage and cancer incidence is consistent with a change in free radical homeostasis (Thompson et al. 1999). Support for this hypothesis comes from studies showing that increasing oxidative DNA damage is typically accompanied by an increase in cancer risk. In humans several fruits and vegetables have been shown to decrease oxidative DNA damage, but these have generally been for short durations and sometimes without adequate controls (Pool-Zobel et al. 1997). Overall these studies suggest several bioactive food components may be responsible for reducing in DNA damage.
Isoprostanes are prostaglandin-like compounds that are produced by free radical mediated peroxidation of polyunsaturated fatty acids. There is evidence that F2-isoprostanes may be a marker of lipid peroxidation because if their mechanism of their formation, chemical stability, and non-invasive method for evaluating. An altered generation of F2-isoprostanes has been found in a variety of pathological syndromes associated with oxidative stress. Evidence does suggest that increased fruits and vegetable consumption can alter F2-isoprostane release in urine (Thompson et al. 1999).
In vitro studies and experiments in animal models provide a large and compelling body of evidence that oxidation of low-density lipoprotein and/or related oxidative mechanisms play a critical role in the initiation and progression of atherogenesis (Blumberg 2002) and may be involved with the cancer process (Salganik 2001). However, large antioxidant intervention studies do not support a decreased cancer risk associated with antioxidant consumption. Likewise, recent clinical trials employing the principal lipid-soluble dietary antioxidant, vitamin E, have yielded mixed results in patients with cardiovascular disease (Blumberg 2002). While there is evidence that vitamin E supplementation in humans reduces prostate cancer, the response does not appear to occur in other tissues. Recent evidence suggests that modification of the vitamin E molecule such that antioxidant properties are no longer present does not prevent its ability to increase tumor apoptosis (Akazawa et al. 2002). Such evidence suggests that vitamin E may function by other mechanisms that by altering free radical homeostasis.
Green tea has surfaced as a rather potent antioxidant. In addition, recent evidence points to the ability of extracts of green tea to suppress an aggressive model for prostate cancer in rodents (TRAMP model) (Gupta et al. 2001). While the mechanism by which polyphenols in tea brings about these effects remains to be established there is strong evidence that perturbations occur in a variety of genes (Wang and Mukhtar 2002). Some of those involved with cell signaling or other key cellular events may account for the observed antioxidant properties.
It is suggested that free radicals and lipid peroxides can suppress the expression of the tumor suppressor gene Bcl-2, activate caspases and shorten telomere, and thus induce apoptosis of tumor cells. Ionizing radiation, anthracyclines, bleomycin and cytokines produce free radicals and thus are useful anti-cancer agents. It is difficult to make unequivocal conclusions about the effects of antioxidants due to many different forms and concentrations of antioxidants used, different experimental models and biomarkers, and the varying doses of the drug employed (Quiles et al. 2002). Regardless, antioxidants may protect against some oxidative injuries resulting from these agents, such as adriamycin, without attenuating their clinical efficacy. However, there is also evidence that antioxidants will reduce the efficacy of agents such as cisplatin, which is known to generate radicals (Salganik 2001).
Overall, while there remains considerable interest in the role of free radicals and antioxidants as modifiers of health, considerable confusion exists. Part of confusion likely reflects the belief that all antioxidants are created equal and under all circumstances. While radicals may be important in determining health it is certainly possible that antioxidants offer protection through several mechanisms. Finally, it may be that under some circumstances that free radicals should be maintained at higher concentrations (such as needed to induce apoptosis). Therefore it may be advisable to carefully regulate the use of antioxidants such that the desired outcome is achieved. Only through continued research will it be possible to identify mechanisms of action of antioxidants and ultimately to define who might benefit or be placed at increased risk from their use. Finally, additional carefully controlled preclinical and clinical studies are needed to determine what environmental and genetic factors influence the overall response to specific antioxidants.
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