Smac/DIABLO enhances the therapeutic potential of chemotherapeutic drugs and irradiation, and sensitizes TRAIL-resistant breast cancer cells

Breast cancer cells were overexpressed with Smac/DIABLO gene (full-length or Δ55 Smac/DIABLO) or treated with Smac/DIABLO peptide to enhance the apoptosis-inducing potential of chemotherapeutic drugs and irradiation, and sensitize TRAIL-resistant cells. Cell viability and apoptosis were measured by XTT assay and DAPI staining, respectively. Protein-protein interaction was determined by immunoprecipitation followed by the Western blot analysis.Overexpression of Smac/DIABLO gene (full-length or Δ55 Smac/DIABLO) or treatment with Smac/DIABLO peptide enhances apoptosis induced by paclitaxel, doxorubicin, etoposide, tamoxifen, and irradiation in breast cancer cells. Overexpression of Smac/DIABLO resulted in an increased interaction of Smac/DIABLO with IAPs, which correlated with an increase in caspase-3 activity and apoptosis. Furthermore, Smac/DIABLO sensitized TRAIL-resistant breast cancer cell lines to undergo apoptosis through caspase-3 activation. These data suggest that apoptotic events down-stream of mitochondria were intact in TRAIL-resistant cells since ectopic expression of Smac/DIABLO or pretreatment of cells with Smac/DIABLO peptide completely restored TRAIL sensitivity.The ability of Smac/DIABLO agonists to enhance the apoptosis-inducing potential of chemotherapeutic drugs and irradiation, and sensitize TRAIL-resistant tumor cells suggests that Smac/DIABLO may induce fundamental alterations in cell signaling pathways. Thus, Smac/DIABLO agonists can be used as promising new candidates for cancer treatment by potentiating cytotoxic therapies.The family of cysteine proteases known as caspases are the key components of apoptosis or programmed cell death [1]. TRAIL (TNF-related apoptosis-inducing ligand), a member of TNF family, uses caspase activation as a signaling mechanism leading to apoptosis via two distinct pathways, involving either ligation of death receptors at the cell surface in recruitment of certain procaspases or through the mitochondrial pathway w