NF-κb-dependent MnSOD expression protects adenocarcinoma cells from TNF-α-induced apoptosis

NF-κB is known to exert a cytoprotective action against TNF-α-induced apoptosis. To study the role of NF-κB in various TNF-α-treated epithelial cell lines, we generated stable transfectants overexpressing a mutated unresponsive form of the IκBα inhibitor (MT cells). As NF-κB prevented TNF-α-induced apoptosis in various epithelial cancer cell lines, we searched for NF-κB target gene products responsible for this difference of sensitivity. We observed an increased Bcl-X_L expression level in OVCAR-3 cells compared with OVCAR-3 cells expressing a mutated IκBα inhibitor (MT cells). Induction of the antioxidant enzyme MnSOD was detected only in TNF-α-treated OVCAR, MCF7A/Z and HCT116 cells but not in MT cells. Moreover, reactive oxygen species were involved in TNF-α-induced apoptosis, as various antioxidants partially protected these cells from apoptosis. At last, transfection of the MnSOD cDNA in MT cells, which do not express this protein after TNF-α stimulation, partially restored resistance to TNF-α-induced cell death, as observed by clonogenic assays. However, transfection of the Bcl-XL cDNA did not induce any protective effect. Therefore, MnSOD expression is induced by NF-κB in epithelial cancer cells in response to TNF-α, and is at least partially responsible for their resistance to TNF-α-induced apoptosis, presumably through the clearance of death-inducing ROS.