Development of drug resistance in tumor cells is the major cause for failure of cancer chemotherapy. Although the mechanisms of drug action are quite diverse, the drugs generally kill tumor cells via apoptosis. A comparative study of a nearly isogenic pair of drug sensitive and resistant ovarian carcinoma lines led to identification of distinct cellular responses that may be causally linked to prevention of apoptosis in resistant cells. Many anticancer drugs and radiation induce reactive oxygen species (ROS) in the cells, presumably by damaging mitochondria. We observed that sub-toxic levels of ROS acted synergistically with drugs in inducing apoptosis in the resistant cells by (i) overriding the effects of overexpressing anti-apoptotic proteins, (ii) glutathione-S-transferase, (iii) increased repair of double strand breaks and (iv) induced nuclear accumulation of p53wt. The central hypothesis of this project is that apoptosis is triggered by chronic oxidative stress in two distinct ways: (i) by continuous damage to the mitochondria, resulting in glutathione (GSH) efflux and damage to mitochondrial membrane and DNA, (ii) by prevention of repair of DNA double strand breaks in the mitochondrial and nuclear DNA induced by DNA cross-linking agents and DNA topoisomerase inhibitor, etoposide. To test our hypothesis the specific aims are: (1) to examine whether depletion of GSH from the mitochondria is central to maintenance of chronic oxidative stress, (2) to test if chronic ROS affect efficiency and fidelity of mitochondria DNA repair induced by drug-treatment, (3) to determine how chronic ROS result in nuclear accumulation of p53 and inhibit nuclear DNA repair, (4) to test if chronic ROS affects DNA double strand break repair via DNA protein kinase pathway, and (5) to identify the signaling process by oxidative stress which induce apoptosis in resistant cells. These studies will utilize a variety of cellular and molecular approaches to determine the mechanism by which chronic oxidative stress renders drug resistant cells susceptible to apoptosis triggered by conventional drug treatment. The long-term objective of this translational research project is a comprehensive understanding of the multiple and interlocking molecular effects of ROS on drug resistance that should help develop new strategies to improve the therapeutic index of antitumor drugs, and which will have a significant impact on the treatment of cancer patients.