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Plant polyphenols are important components of human diet and a number of them are considered to possess chemopreventive and therapeutic properties against cancer. They are recognized as naturally occurring antioxidants but also act as prooxidants catalyzing DNA degradation in the presence of transition metal ions such as copper. Using human peripheral lymphocytes and Comet assay we have previously confirmed that resveratrol-Cu(II) is indeed capable of causing DNA degradation in cells. In this paper we show that the polyphenols alone (in the absence of added copper) are also capable of causing DNA breakage in cells. Incubation of lymphocytes with neocuproine inhibited the DNA degradation confirming that Cu(I) is an intermediate in the DNA cleavage reaction. Further, we have also shown that polyphenols generate oxidative stress in lymphocytes which is inhibited by scavengers of reactive oxygen species and neocuproine. These results are in further support of our hypothesis that anticancer mechanism of plant polyphenols involves mobilization of endogenous copper, possibly chromatin bound copper, and the consequent prooxidant action.

Resveratrol (3,40 ,5-trihydroxy stilbene), a plant derived polyphenol found in mulberries, grapes and red wine is considered to possess chemopreventive properties against cancer. It is recognized as a naturally occurring antioxidant but also catalyzes oxidative DNA degradation in vitro in the presence of transition metal ions such as copper. Using a cellular system of lymphocytes isolated from human peripheral blood and Comet assay, we have confirmed that resveratrol-Cu(II) system is indeed capable of causing DNA degradation in cells such as lymphocytes. Also, trans-stilbene, which does not have any hydroxyl groups, is inactive in the lymphocyte system. Pre-incubation of lymphocytes with resveratrol indicates that it is capable of either traversing the cell membrane or binding to it. Our results are in partial support of our hypothesis that anticancer properties of various plant derived polyphenols may involve mobilization of endogenous copper and the consequent prooxidant action.

It was earlier proposed that an important anti-cancer mechanism of plant polyphenols may involve mobilization of endogenous copper ions, possibly chromatin-bound copper and the consequent pro-oxidant action. This paper shows that plant polyphenols are able to mobilize nuclear copper in human lymphocytes, leading to degradation of cellular DNA. A cellular system of lymphocytes isolated from human peripheral blood and comet assay was used for this purpose. Incubation of lymphocytes with neocuproine (a cell membrane permeable copper chelator) inhibited DNA degradation in intact lymphocytes. Bathocuproine, which is unable to permeate through the cell membrane, did not cause such inhibition. This study has further shown that polyphenols are able to degrade DNA in cell nuclei and that such DNA degradation is inhibited by neocuproine as well as bathocuproine (both of which are able to permeate the nuclear pore complex), suggesting that nuclear copper is mobilized in this reaction. Pre-incubation of lymphocyte nuclei with polyphenols indicates that it is capable of traversing the nuclear membrane. This study has also shown that polyphenols generate oxidative stress in lymphocyte nuclei which is inhibited by scavengers of reactive oxygen species (ROS) and neocuproine. These results indicate that the generation of ROS occurs through mobilization of nuclear copper resulting in oxidatively generated DNA breakage.

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