Nitric oxide disrupts H2O2-dependent activation of nuclear factor κB -: Role in sensitization of human tumor cells to tumor necrosis factor-α-induced cytotoxicity

Tumor necrosis factor alpha (TNF-alpha) exerts its effect by two distinct-signaling pathways. It can trigger cytotoxicity in sensitive target cells. TNF-alpha can also promote nuclear factor kappaB (NF-kappaB) activity and regulate the expression of genes that interfere with apoptosis and thus conferring resistance to several apoptotic stimuli. We have observed that interferon-gamma (IFN-gamma) sensitizes human ovarian carcinoma cell lines to TNF-alpha -mediated apoptosis and further, IFN-gamma induces the expression of the inducible nitric-oxide synthase (iNOS) and the generation of nitric oxide (NO). This study, examines the role of NO in the sensitization of the ovarian carcinoma cell line AD10 to TNF-alpha -mediated cytotoxicity. Treatment of AD10 cells with the NOS inhibitor L-NMA. blocked the IFN-gamma -dependent sensitization whereas NO donors (S-nitroso-N-acetylpenicillamine) sensitized these cells to TNF-alpha cytotoxicity, Analysis of the activation status of NF-kappaB upon treatment with NO donors confirmed the inhibitory role of NO on both the NF-kappaB DNA-binding property and its activation. Moreover, the inhibition of NF-kappaB nuclear translocation by NO donors directly correlated with the intracellular concentration of H2O2 and was reversed by the addition of exogenous H2O2. These findings show that NO might interfere with TNF-alpha -dependent NF-kappaB activation by interacting with O-2(radical anion) and reducing the generation of H2O2, a potent NF-kappaB activator. Therefore, NO-mediated disruption of NF-kappaB activation results in the removal of anti-apoptotic/resistance signals and sensitizes tumor cells to cytotoxic cytokines like TNF-alpha.