Synergistic cytotoxicity through the activation of multiple apoptosis pathways in human glioma cells induced by combined treatment with ionizing radiation and tumor necrosis factor-related apoptosis-inducing ligand.

OBJECT: Malignant gliomas remain incurable despite modem multimodality treatments. Tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL), also known as Apo2L, a member of the TNF family, preferentially induces apoptosis in human tumor cells through its cognate death receptors DR4 or DR5, suggesting that it may serve as a potential therapeutic agent for intractable malignant gliomas. Here, the authors show that genotoxic ionizing radiation synergistically enhances TRAIL-induced cell death in human glioma cells expressing DR5.
METHODS: Combination treatment with soluble human TRAIL plus radiation induced robust cell death, while each of them singly led to only limited cytotoxicity. The combination resulted in cleavage and activation of the apoptotic initiator caspase-8 and the effector caspase-3 as well as cleavage of Bid and another initiator caspase-9, a downstream component of the apoptosome. Accordingly, it augmented the release of cytochrome c from the mitochondria into the cytosol, as well as apoptosis-inducing factor. Synergistic cell death was suppressed by TRAIL-neutralizing DR5-Fc, caspase inhibitors, expression of dominant-negative Fas-associated protein with death domain and CrmA, which selectively blocks caspase-8, and overexpression of Bcl-X(L). Finally, combination treatment had no influence on the viability of normal human astrocytes.
CONCLUSIONS: These results suggest that combination treatment with TRAIL and ionizing radiation kills human glioma cells through the activation of DR5-mediated death receptor pathways. This therapy involves direct activation of effector caspases as well as mitochondria-mediated pathways and provides a novel strategy in which TRAIL could be synergistically combined with DNA-damaging radiation.