Nitroxides as Protectors Against Oxidative Stress

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Extensive studies in our laboratory have demonstrated that nitroxides (such as tempol) are effective antioxidants and protectors against ionizing radiation damage. We have previously demonstrated that the potential mechanism(s) for nitroxide-mediated protection against oxidative stress include superoxide dismutase- and catalase-like activity and radical-radical reactions. However, more recent studies have focused on whether nitroxide treatment impacts gene expression. Cells treated with non-toxic concentrations of several different nitroxide analogues, including a nitroxide incapable of cellular entry (and protection) all exhibit similar patterns of gene expression. Prominent genes upregulated by nitroxide treatment include the heat shock protein (HSP) family, reductive enzyme genes, and genes associated with the Wnt/beta-catenin pathway. These studies will hopefully enable us to better understand the complex cellular/molecular mechanisms of nitroxides that trigger responses we have observed in animals. For example, long-term administration of tempol (in the food or drinking water) results in dramatic weight reduction and a decrease in spontaneous tumor incidence in mice. Animals maintained on tempol exhibit increased expression in mitochondrial uncoupling protein 2 (UCP2) and HSP in skeletal muscle. Preliminary transient transfection studies using UCP2 in breast cancer cells indicate a protective role for this protein with respect to agents imposing oxidative stress. Further we have treated ATM-deficient mice with continuous administration of tempol. The ATM-deficient mouse is a model for a human cancer prone syndrome ataxia-telangiectasia which displays oxidative stress and damage, as well as DNA damage induced cell cycle checkpoints. Remarkably, tempol treatment resulted in a dramatically increased lifespan of these mice by prolonging the latency to thymic lymphomas. Tempol treatment reduced the elevated level of reactive oxygen species, tissue oxidative damage and stress, and radiation induced DNA double strand breaks. In addition, tempol induced a dose-dependent prolongation of S-phase in the checkpoint defective ATM-deficient mice. These experiments provide strong evidence that nitroxide antioxidants can be used as novel chemopreventative agents in cancer prone syndromes. Lastly, we are continuing our studies on the differential radioprotection of tempol toward normal tissues as opposed to tumor. Recent studies indicate that tempol administered 10 min prior to fractionated radiation treatment does not protect SCC murine tumor growth. Studies are currently underway to evaluate normal tissues such as lung, skin, and kidney. Since nitroxides readily penetrate cell membranes and are potent antioxidants, they may be of use in other areas of medical research such as ischemia/reperfusion injury studies, prevention of cataracts, inflammatory processes, and aging.