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 H(2)O(2) and was reversed by the addition of exogenous H(2)O(2). These findings show that NO might interfere with
TNF-alpha-dependent
NF-kappaB activation by interacting with O(2) and reducing the generation of H(2)O(2), 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.