Interferon γ (IFNγ) and tumor necrosis factor α synergism in ME-180 cervical cancer cell apoptosis and necrosis. IFN-γ inhibits cytoprotective NF-κB through STAT1/IRF-1 pathways

We investigated the molecular mechanism of the synergism between interferon γ (IFNγ) and tumor necrosis factor α (TNFα) documented in a variety of biological occasions such as tumor cell death and inflammatory responses. IFN-γ/TNFα synergistically induced apoptosis of ME-180 cervical cancer cells. IFNγ induced STAT1 phosphorylation and interferon regulatory factor 1 (IRF-1) expression. Transfection of phosphorylation-defective STAT1 inhibited IFN-γ/TNFα -induced apoptosis, whereas IRF-1 transfection induced susceptibility to TNFα. Dominant-negative IγBα transfection sensitized ME-180 cells to TNFα. IFNγ pretreatment attenuated TNFα- or p65-induced NF-κB reporter activity, whereas it did not inhibit p65 translocation or DNA binding of NF-κB. IRF-1 transfection alone inhibited TNFα-induced NF-κB activity, which was reversed by coactivator p300 overexpression. Caspases were activated by IFN-γ/TNFα combination; however, caspase inhibition did not abrogate IFNγ/TNFα-induced cell death. Instead, caspase inhibitors directed IFNγ/TNFα-treated ME-180 cells to undergo necrosis, as demonstrated by Hoechst 33258/propidium iodide staining and electron microscopy. Taken together, our results indicate that IFNγ and TNFα synergistically act to destroy ME-180 tumor cells by either apoptosis or necrosis, depending on caspase activation, and STAT1/IRF-1 pathways initiated by IFNγ play a critical role in IFNγ/TNFα synergism by inhibiting cytoprotective NF-κB. IFNγ/TNFα synergism appears to activate cell death machinery independently of caspase activation, and caspase activation seems to merely determine the mode of cell death.