Increased oxidative DNA damage in mammary tumor cells by continuous epidermal growth factor stimulation

Background: Growth factors can enhance the malignant potential of tumor cel ls, To examine the relationship between growth factors and tumor progressio n, we previously established a weakly malignant cell line, ER-1, We found t hat a 24-hour exposure of ER-1 cells to epidermal growth factor (EGF) induc ed malignant properties (tumor progression) that were reversible but that, after a 1-month exposure, these changes were irreversible. In this study, w e investigated the irreversible changes induced in ER-1 cells by a 1-month exposure to EGF and the possible involvement of oxidative stress. Methods: ER-1 cells were treated with EGF (100 ng/mL) for 1 month in the presence or absence of an antioxidant, N-acetylcysteine or selenium, and compared with untreated control ER-1 cells, We assessed tumor progression by measuring i ntracellular peroxide levels, 8-hydroxydeoxyguanosine (a marker for oxidati ve DNA damage) levels, in vitro invasiveness, and in vivo tumorigenicity an d metastatic ability. All statistical tests are two-sided. Results: After E R-1 cells were treated for 1 month with EGF, levels of intracellular peroxi de and 8-hydroxyguanosine in the DNA of treated cells were higher than thos e in the DNA of control tells, and treated ER-1 cells were more tumorigenic and metastatic in vivo and more invasive in vitro than untreated control c ells tall P <.001), Levels of 8-hydroxyguanosine in DNA increased as the le ngth of the EGF treatment increased (P <.001), However, when N-acetylcystei ne or selenium was added with EGF for 1 month, levels of intracellular pero xide and 8-hydroxyguanosine in DNA were comparable to those in control cell s (r = .795), Both tumori-genicity (P = .008) and metastatic ability (P <.0 01) decreased after addition of N-acetylcysteine or selenium, Conclusion: T he irreversible changes caused by continuous EGF stimulation of ER-1 cells result from increased oxidative damage in the DNA, which generates tumor ce lls with more malignant characteristics.