Selective killing of oncogenic transformed cells through ROS-mediated mechanism by β-phenylethyl isothiocyanates

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Therapeutic selectivity is important for anticancer drug development. Understanding the biological differences between normal and cancer cells is essential in designing strategies to preferentially kill cancer cells. Compared to normal cells, most cancer cells exhibit high metabolic activity, generate increased levels of reactive oxygen species (ROS), and are under oxidative stress. Elevated ROS renders the cancer cells more vulnerable to further ROS stress by exogenous ROS-generating agents. Here, we report that immortalization of ovarian epithelial cells with SV40 and hTERT followed by transformation with H-Ras^V12 oncogene rendered the cells tumorigenic, and exhibited significantly higher ROS generation compared to the non-tumorigenic parental cells. The malignant cells were highly vulnerable to further oxidative stress by β-phenylethyl isothiocyanates (PEITC), a dietary metabolic product of cruciferous vegetables. Treatment with 10 μM PEITC for 5 h preferentially induced ROS generation in (15-19 fold increase) and induced massive cell death (60-80%) within a relatively short period (5-24 h). In contrast, the parental non-tumorigenic cells without ras-transformation exhibited low basal ROS generation and better tolerated PEITC-induced ROS stress, as evidenced by only a modest ROS increase and less cell death (10-20%, 10 μM, 5-24 h). The in vitro therapeutic selectivity of PEITC against ovarian cancer cells was superior to cisplatin. Further studies revealed that PEITC caused a more severe depletion of glutathione in the malignant cells (93%, 5 μM, 3h) compared to the non-tumorigenic cells (52%), which appeared to be a biochemical mechanism for preferential increase of ROS in the malignant cells. Mechanistic studies demonstrated that PEITC induced cell death by causing a striking ROS-mediated oxidative damage to the mitochondrial membranes, and disrupting the redox-sensitive survival pathways such as NF-KB and Ras in the cancer cells. The action of PEITC was reversed by antioxidant N-acetylcysteine and ROS scavenger enzyme catalase. The promising therapeutic activity of PEITC was further demonstrated in animal models bearing the ras-transformed ovarian cancer cells, with an 84% increase in median animal survival time (25 vs 48 days, p<0.05). Our studies demonstrated that oxidative stress associated with oncogenic transformation may provide a biochemical basis for anticancer therapeutics through a ROS-mediated mechanism, and PEITC is potentially useful as a selective anticancer agent.