TNFα Cooperates with IFN-γ to Repress Bcl-xL Expression to Sensitize Metastatic Colon Carcinoma Cells to TRAIL-mediated Apoptosis

Background: TNF-related apoptosis-inducing ligand (TRAIL) is an immune effector molecule that functions as a selective anti-tumor agent. However, tumor cells, especially metastatic tumor cells often exhibit a TRAIL-resistant phenotype, which is currently a major impediment in TRAIL therapy. The aim of this study is to investigate the synergistic effect of TNF alpha and IFN-gamma in sensitizing metastatic colon carcinoma cells to TRAIL-mediated apoptosis. Methodology/Principal Findings: The efficacy and underlying molecular mechanism of cooperation between TNF alpha and IFN-gamma in sensitizing metastatic colon carcinoma cells to TRAIL-mediated apoptosis were examined. The functional significance of TNF alpha- and IFN-gamma-producing T lymphocyte immunotherapy in combination with TRAIL therapy in suppression of colon carcinoma metastasis was determined in an experimental metastasis mouse model. We observed that TNF alpha or IFN-gamma alone exhibits minimal sensitization effects, but effectively sensitized metastatic colon carcinoma cells to TRAIL-induced apoptosis when used in combination. TNF alpha and IFN-gamma cooperate to repress Bcl-xL expression, whereas TNF alpha represses Survivin expression in the metastatic colon carcinoma cells. Silencing Bcl-xL expression significantly increased the metastatic colon carcinoma cell sensitivity to TRAIL-induced apoptosis. Conversely, overexpression of Bcl-xL significantly decreased the tumor cell sensitivity to TRAIL-induced apoptosis. Furthermore, TNF alpha and IFN-gamma also synergistically enhanced TRAIL-induced caspase-8 activation. TNF alpha and IFN-gamma was up-regulated in activated primary and tumor-specific T cells. TRAIL was expressed in tumor-infiltrating immune cells in vivo, and in tumor-specific cytotoxic T lymphocytes (CTL) ex vivo. Consequently, TRAIL therapy in combination with TNF alpha/IFN-gamma-producing CTL adoptive transfer immunotherapy effectively suppressed colon carcinoma metastasis in vivo. Conclusions/Significance: TNF alpha and IFN-gamma cooperate to overcome TRAIL resistance at least partially through enhancing caspase 8 activation and repressing Bcl-xL expression. Combined CTL immunotherapy and TRAIL therapy hold great promise for further development for the treatment of metastatic colorectal cancer.