Cellular FLICE-inhibitory protein regulates chemotherapy-induced apoptosis in breast cancer cells.

Combination treatment regimens that include topoisomerase-II-targeted drugs, such as doxorubicin, are widely used in the treatment of breast cancer. Previously, we showed that IFN-gamma and doxorubicin cotreatment synergistically induced apoptosis in MDA435 breast cancer cells in a signal transducer and activator of transcription 1-dependent manner. In this study, we found that this synergy was caspase-8 dependent. In addition, we found that IFN-gamma down-regulated the expression of the caspase-8 inhibitor cellular FLICE-inhibitory protein (c-FLIP). Furthermore, IFN-gamma down-regulated c-FLIP in a manner that was dependent on the transcription factors signal transducer and activator of transcription 1 and IFN regulatory factor-1. However, IFN-gamma had no effect on c-FLIP mRNA levels, indicating that c-FLIP was down-regulated at a posttranscriptional level following IFN-gamma treatment. Characterization of the functional significance of c-FLIP modulation by small interfering RNA gene silencing and stable overexpression studies revealed it to be a key regulator of IFN-gamma- and doxorubicin-induced apoptosis in MDA435 cells. Analysis of a panel of breast cancer cell lines indicated that c-FLIP was an important general determinant of doxorubicin- and IFN-gamma-induced apoptosis in breast cancer cells. Furthermore, c-FLIP gene silencing sensitized MDA435 cells to other chemotherapies, including etoposide, mitoxantrone, and SN-38. These results suggest that c-FLIP plays a pivotal role in modulating drug-induced apoptosis in breast cancer cells.