Reactive oxygen species (ROS) mediate SSL-induced Fyn kinase activation. (A) N-acetylcysteine (NAC) or (B) catalase attenuates SSL-induced Fyn kinase activity in HaCaT cells. NAC or catalase inhibits SSL-induced Fyn downstream signaling. HaCaT cells were treated with (C) NAC or (D) catalase at the indicated concentrations for 1 h and then irradiated with SSL (60 kJ/m2) and harvested. (E) Hydrogen peroxide (H2O2) stimulates Fyn kinase activity in HaCaT (a) and MEFs (b). HaCaT cells and MEFs were treated with H2O2 (200 µM) and harvested at the indicated time points. (F) Fyn downstream signaling is activated by H2O2 in Fyn+/+ but not Fyn−/− MEFs. Fyn+/+ and Fyn−/− MEFs were treated with H2O2 (200 µM) and harvested at the indicated time points. For A, B, and E, Fyn kinase activity was measured as described in Materials and Methods and for C, D, and F, Western blot analysis was performed using antibodies specific for the indicated proteins. For A, B, and E, the asterisks (**) indicate a significant (p < 0.01) difference compared to the untreated control and data are represented as means ± S.D. from triplicate experiments. For C, D and F, Western blots are representative of similar results from duplicate experiments.