Abstract

Induction of apoptosis by the death ligand tumor necrosis factor-related apoptosis inducing-ligand (TRAIL) is a promising antitumor therapy. However, not all tumor cells are sensitive to TRAIL, highlighting the need for strategies to overcome TRAIL resistance. Inhibitor of Apoptosis (IAP) family member survivin is constitutively activated in various cancers and blocks apoptotic signaling. Recently we demonstrated that YM-155, a small molecule inhibitor, not only downregulates survivin in gliomas, but also Mcl-1, and upregulates anti-apoptotic Noxa levels. Because Mcl-1 and survivin are critical mediators of resistance to various anti-cancer therapies, we questioned whether YM-155 could sensitize resistant glioma cells to TRAIL. To address this hypothesis, we combined YM-155 with TRAIL and examined effects on cell survival and apoptotic signaling. TRAIL or YM-155 individually induced minimal killing in highly resistant U373 and LNZ308 cell lines, but combining TRAIL with YM-155 triggered a synergistic pro-apoptotic response, mediated through mitochondrial dysfunction via activation of caspases 8, 9, 7, 3, PARP and Bid. Apoptosis induced by combination treatments was blocked by caspase-8 and pan-caspase inhibitors. In addition, knockdown of Mcl-1 by RNA-interference also overcame apoptotic resistance to TRAIL. Conversely, silencing Noxa by RNA-interference reduced the combined effects of YM-155 and TRAIL on apoptosis. Mechanistically, these findings indicate that YM-155 plays a role in counteracting glioma cell resistance to TRAIL-induced apoptosis, by downregulating Mcl-1 and survivin and amplifying mitochondrial signaling through intrinsic and extrinsic apoptotic pathways. The significantly enhanced antitumor activity of the combination of YM-155 and TRAIL may have applications for therapy of malignant glioma.