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The primary purpose of this research is to investigate the effects of 2,3,7,8-tetrachlorodibenzo- p-dioxin (TCDD) pretreatment and estrogen receptors-alpha (ER alpha) status on the induction of DNA damage by 17beta-estradiol (E 2) in human ER alpha(+)/MCF-7 and ER alpha(-)/MDA-MB-231 breast cancer cells. Results indicated that E 2 (0.1-100 nM) alone induced significant increases in cytotoxic response, reactive oxygen species (ROS) generation, and glutathione depletion in MDA-MB-231 cells but not in MCF-7 cells. At noncytotoxic concentrations, E 2 induced dose-related reduction in intracellular NAD(P)H in MDA-MB-231 cells through decreases in intracellular NAD (+) mediated by poly(ADP-ribose) polymerase-1 (PARP-1) activation as determined by detection of the presence of polymers of ADP-ribose-modified PARP-1 using Western blotting. Further investigation using the single-cell gel electrophoresis (Comet) assay confirmed that the PARP-1 activation induced by estrogen in MDA-MB-231 was due to increases in the number of DNA strand breaks. This evidence indicates that E 2 induces decreases in intracellular NAD(P)H and NAD (+) in MDA-MB-231 cells through PARP-1 activation mediated by the formation of DNA strand breaks. Further investigation indicated that the cytotoxic and DNA-damaging effects induced by E 2 in MDA-MB-231 cells were completely blocked by pretreatment of TCDD (10 nM for 72 h). In contrast, with TCDD pretreatment, significant increases in cytotoxic response, ROS generation, glutathione (GSH) depletion, DNA strand breaks, and PARP-1 activation were detected in MCF-7 cells exposed to E 2. We demonstrated that TCDD modulated the differential induction of DNA damage by estrogen in MDA-MB-231 and MCF-7 cells primarily through the inducibility of cytochrome P450 1A1 and 1B1 expression. Overall, this evidence suggests that TCDD is capable of inducing imbalances in the expression of enzymes responsible for the bioactivation of estrogen leading to the subsequent accumulation of DNA damage and initiation of DNA repair in MDA-MB-231 and MCF-7 cells. Furthermore, we confirmed that ER alpha plays a protective role in modulating the induction of DNA damage by E 2 in human breast cancer cells.

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