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Cancer prevention by dietary phytochemicals has been shown to involve decreased cell proliferation and cell cycle arrest in cancer cell lines. However, there is limited understanding of the mechanisms. Previously, we have shown that a common effect of phytochemicals investigated is to oxidize the intracellular glutathione (GSH) pool. Therefore, the objective of this study was to evaluate whether changes in the glutathione redox potential in response to select dietary phytochemicals was related to their induction of cell cycle arrest. Human colon carcinoma (HT29) cells were treated with benzyl isothiocyanate (BIT), diallyl disulfide (DADS), dimethyl fumarate (DMF), lycopene (LYC), sodium butyrate (NaB) or buthione sulfoxamine (BSO, a GSH synthesis inhibitor) at concentrations known to cause oxidation of the GSH: glutathione disulfide pool. After 16 h, cells were harvested, fixed in ethanol, centrifuged and ethanol removed by aspiration. Cell cycle analysis was performed using a Becton Dickinson FACS Calibur and data analyzed by ModFit LT™ software. The dietary phytochemicals resulted in increased percentage (ranging from 75% to 30% for benzyl isothiocyanate and lycopene, respectively) of cells at G2/M arrest (BIT > NaB > DADS > BSO >DMF > LYC) compared to control treatments (dimethylsulfoxide). Pretreatment for 6 h with the antioxidant N-acetylcysteine (NAC) resulted in a partial reversal of the G2/M arrest. Accompanying the increase in cells in G2/M, there was a decrease in the percentage of cells in G1 and S phases of the cell cycle in response to the phytochemical and BSO treatments. This was also partially reversed by pretreatment with NAC. As these phytochemical treatments cause GSH oxidation, reversible by NAC, we conclude that the cell cycle arrest is also mediated, in part, by intracellular oxidation. This conclusion is supported by the reversal in cell cycle arrest with treatment by the antioxidant, NAC. Thus, one potential mechanism for cancer prevention by dietary phytochemicals is inhibition of the growth of cancer cells through modulation of their intracellular redox environment. (Support: NIH MBRS/GM; RCMI RR033032)

98th AACR Annual Meeting-- Apr 14-18, 2007; Los Angeles, CA