Receptor-induced Mitochondria-operated Apoptotic Program Sensitizes Human Breast Tumor Cells to a Death

In this report, we have assessed the role of IFN-g as a sensitizing agent in apoptosis mediated by activation of death receptor CD95 in breast tumor cells. Treatment of the tumor cell lines MCF-7 and MDA-MB231 with IFN-g significantly facilitated apoptosis induced by CD95 receptor ligation at the plasma membrane, independently of p53 status. In contrast, IFN-g treatment did not enhance the apoptotic effect of the DNAdamaging drug, doxorubicin. Analysis of apoptosis regulators indicated that caspase-8 mRNA and protein levels were up-regulated in both of the cell lines after treatment with IFN-g. Furthermore, IFN-g sensitized MCF-7 and MDA-MB231 cells to CD95-mediated activation of caspase-8, induction of cytochrome c release from mitochondria, and processing of caspase-9. Release of cytochrome c, caspases activation, and apoptosis were prevented in MCF-7 cells overexpressing Bcl-2. Altogether these results indicate that IFN-g, maybe through the elevation of caspase-8 levels, sensitizes human breast tumor cells to a death receptor-mediated, mitochondria-operated pathway of apoptosis.