ABSTRACT
BACKGROUND: Multiple trials have attempted to demonstrate the effective induction of cell death in TRAIL-resistant cancer cells, including using a combined treatment of recombinant TRAIL and various proteasome inhibitors. These studies have yielded limited success, as the mechanism of cell death is currently unidentified. Understanding this mechanism's driving forces may facilitate the induction of cell death in TRAIL-resistant cancer cells. METHODS: Three kinds of recombinant soluble TRAIL proteins were treated into TRAIL-resistant cells and TRAIL-susceptible cells, with or without bortezomib, to compare their respective abilities to induce cell death. Recombinant TRAIL was treated with bortezomib to investigate whether this combination treatment could induce tumor regression in a mouse syngeneic tumor model. To understand the mechanism of combined treatment-induced cell death, cells were analyzed by flow cytometry and the effects of various cell death inhibitors on cell death rates were examined. RESULTS: ILz:rhTRAIL, a recombinant human TRAIL containing isoleucine zipper hexamerization domain, showed the highest cell death inducing ability both in single treatment and in combination treatment with bortezomib. In both TRAIL-resistant and TRAIL-susceptible cells treated with the combination treatment, an increase in cell death rates was dependent upon both the dose of TRAIL and its intrinsic properties. When a syngeneic mouse tumor model was treated with the combination of ILz:rhTRAIL and bortezomib, significant tumor regression was seen as a result of the effective induction of cancer cell death. The combination treatment-induced cell death was both inhibited by TRAIL blocking antibody and caspase-dependent. However, it was not inhibited by various ER stress inhibitors and autophagy inhibitors. CONCLUSIONS: The combination treatment with ILz:rhTRAIL and bortezomib was able to induce cell death in both TRAIL-susceptible and TRAIL-resistant cancer cells through the intracellular TRAIL signaling pathway. The efficiency of cell death was dependent on the properties of TRAIL under the environment provided by bortezomib. The combination treatment-induced cell death was not regulated by bortezomib-induced ER stress response or by autophagy.
