Piperidine nitroxide Tempol enhances cisplatin-induced apoptosis in ovarian cancer cells

A nitroxide radical, Tempol (Tempol, TPL), is usually used as an antioxidative agent clinically, whereas the mechanism underlying its pro-oxidative effect has not been thoroughly investigated. The present study investigated the pro-oxidative effect of TPL on the inhibition of cellular proliferation and its role in enhancing the effect of anticancer drug cisplatin (DDP) on the induction of apoptosis in ovarian cancer cells. Cell viability and proliferation were evaluated by MTT assay. Cell apoptosis was analyzed by flow cytometry (FCM) following staining with Annexin V/propidium iodide. Western blot analysis was performed to determine the expression levels of anti-apoptotic protein B-cell lymphoma-2 (Bcl-2) and pro-apoptotic protein Bcl-2-associated X protein (Bax), and the Bcl-2:Bax expression ratio. Cellular reactive oxygen species (ROS) were labeled with dichlorofluorescin-diacetate and analyzed by FCM. The results revealed that cell viabilities of OVCAR3 and SKOV3 cells were decreased by TPL in dose-dependent manner at concentrations of 2 to 10 mM after 48 h incubation. The cell proliferation rates of OVCAR3 and SKOV3 cells were suppressed by TPL at lower toxic concentrations of 1.5 and 1 mM, respectively, compared with the control group. The MTT assay indicated that the combination therapy significantly inhibited the cell proliferation of OVCAR3 cells compared with treatment with DDP alone. FCM demonstrated that the combination treatment increased the proportion of early apoptotic cells in OVCAR3 cells compared with single DDP treatment. Western blot analysis revealed that the combination treatment markedly decreased the Bcl-2:Bax expression ratio compared with treatment with DDP alone. Detection of cellular ROS expression levels demonstrated that the combination therapy significantly increased cellular ROS generation compared with the DDP-only therapy. These data indicated that TPL increased the effect of DDP on inducing apoptosis in OVCAR3 cells.