Identification of a Novel Pathway That Selectively Modulates Apoptosis of Breast Cancer Cells

Expression of the nuclear receptor interacting factor 3 (NRIF3) coregulator in a wide variety of breast cancer cells selectively leads to rapid caspase-2-dependent apoptotic cell death. A novel death domain (DDI) was mapped to a 30-amino acid region of NRIF3. Because the cytotoxicity of NRIF3 and DDI seems to be cell type-specific, these studies suggest that breast cancer cells contain a novel death switch that can be specifically modulated by NRIF3 or DID I. Using an MCF-7 cell cDNA library in a yeast two-hybrid screen, we cloned a factor that mediates apoptosis by DDI and refer to this factor as DDI-interacting factor-1 (DIF-1). DIF-I is a transcriptional repressor that mediates its effect through SirT1, and this repression is attenuated by the binding of NRIF3/DD1. DIF-I expression rescues breast cancer cells from NRIF3/DD1-induced apoptosis. Small interfering RNA (siRNA) knockdown of DIF-I selectively leads to apoptosis of breast cancer cells, further suggesting that DIF-1 plays a key role in NRIF3/DD1-mediated apoptosis. A protein kinase A inhibitor (H89) also elicits apoptosis of breast cancer cells but not of the other cell types examined, and DIF-I also protects these cells from H89-mediated apoptosis. In addition, H89 incubation results in a rapid increase in NRIF3 levels and siRNA knockdown of NRIF3 protects breast cancer cells from H89-mediated apoptosis. Our results indicate that DIF-1 plays a key role in breast cancer cell survival and further characterizing this pathway may provide important insights into developing novel therapies to selectively target breast cancer cells for apoptosis. [Cancer Res 2009;69(4):1375-82]