Interferon gamma (IFN gamma) and tumor necrosis factor alpha synergism in ME-180 cervical cancer cell apoptosis and necrosis IFN gamma inhibits cytoprotective NF-kappa B through STAT1/IRF-1 pathways

We investigated the molecular mechanism of the synergism between interferon gamma (IFN gamma) and tumor necrosis factor alpha (TNF alpha) documented i n a variety of biological occasions such as tumor cell death and inflammato ry responses. IFN gamma /TNF alpha synergistically induced apoptosis of ME- 180 cervical cancer cells. IFN gamma induced STAT1 phosphorylation and inte rferon regulatory factor 1 (IRF-1) expression. Transfection of phosphorylat ion-defective STAT1 inhibited IFN gamma /TNF alpha -induced apoptosis, wher eas IRF-1 transfection induced susceptibility to TNF alpha. Dominant-negati ve I kappaB alpha transfection sensitized ME-180 cells to TNF alpha. IFN ga mma pretreatment attenuated TNF alpha- or p65-induced NF-KB reporter activi ty, whereas it did not inhibit p65 translocation or DNA binding of NF-KB, I RF-1 transfection alone inhibited TNF alpha -induced NF-KB activity, which was reversed by coactivator p300 overexpression. Caspases were activated by IFN gamma /TNF alpha combination; however, caspase inhibition did not abro gate IFN gamma /TNF alpha -induced cell death. Instead, caspase inhibitors directed IFN gamma TNF alpha -treated ME-180 cells to undergo necrosis, as demonstrated by Hoechst 33258/propidium iodide staining and electron micros copy. Taken together, our results indicate that IFN gamma and TNFa synergis tically act to destroy ME-180 tumor cells by either apoptosis or necrosis, depending on caspase activation, and STAT1/IRF-1 pathways initiated by IFN gamma play a critical role in IFN gamma /TNF alpha synergism by inhibiting cytoprotective NF-KB, IFN gamma /TNF alpha synergism appears to activate ce ll death machinery independently of caspase activation, and caspase activat ion seems to merely determine the mode of cell death.