Compound C sensitizes Caki renal cancer cells to TRAIL-induced apoptosis through reactive oxygen species-mediated down-regulation of c-FLIP_L and Mcl-1

The tumor necrosis factor-related apoptosis-inducing ligand (TRAIL), either alone or in combination with other anticancer drugs, is considered as a new strategy for anticancer therapy. Compound C, a cell-permeable pyrrazolopyrimidine derivative, acts as a potent, selective, reversible ATP-competitive inhibitor of AMP-activated protein kinase (AMPK). In this study, we show that compound C sensitizes Caki human renal cancer cells, but not normal human skin fibroblast cells (HSF) and human mesangial cells, to TRAIL-mediated apoptosis. However, AMPK siRNA failed to affect TRAIL-mediated apoptosis in Caki cells and transduction of dominant negative AMPK rather attenuated TRAIL-induced apoptosis, indicating that the effect of compound C on sensitization of TRAIL-induced apoptosis is independent of AMPK activity. Interestingly, we found that down-regulation of c-FLIP_L and Mcl-1 contributes to compound C-enhanced TRAIL-induced apoptosis. Reduced expression of c-FLIP_L and Mcl-1 were caused by the decreased protein stability of c-FLIP_L and Mcl-1, but not by their transcriptional control, in compound C-treated cells. Pretreatment with N-acetyl-l-cysteine (NAC) significantly inhibited the cell death induced by the combined treatment with compound C and TRAIL as well as recovered the expression levels of c-FLIP_L and Mcl-1 down-regulated by the combinatory treatment with compound C plus TRAIL, suggesting that compound C-stimulated TRAIL-induced apoptosis appears to be dependent on the generation of reactive oxygen species for down-regulation of c-FLIP_L and Mcl-1. Taken together, the present study demonstrates that compound C enhances TRAIL-induced apoptosis in human renal cancer cells by ROS-mediated c-FLIP_L and Mcl-1 down-regulation.