Double-stranded RNA directly acts on fibroblast and myeloid lineages to induce necroptosis as in TNF-α. Here we investigated whether this type of cell death occurred in cancer cells in response to polyI:C and the pan-caspase inhibitor z-Val-Ala-Asp fluromethyl ketone (zVAD). We found the colon cancer cell line CT26 highly susceptible to necroptosis, judged by annexin V/propidium iodide. CT26 cells possess RNA sensors, TLR3 and MDA5, which are up-regulated by interferon (IFN)-inducing pathways and linked to receptor-interacting protein kinase (RIP) 1/3 activation via TICAM-1 or MAVS adaptor, respectively. Although exogenously-added polyI:C alone marginally induced necroptosis in CT26 cells, a combinational use of polyI:C and zVAD accomplished ~50% CT26 necroptosis in vitro without secondary effects of TNF-α or type I IFNs. CT26 necroptosis depended on the TLR3-TICAM-1-RIP3 axis in the tumor cells to produce reactive oxygen species, but not on MDA5, MAVS or the caspases/inflammasome activation. However, the RNA-derived necroptosis was barely reproduced in vivo CT26 implant Balb/c mouse models with administration of polyI:C + zVAD. Significant shrinkage of CT26 tumors was revealed only when polyI:C intraperitoneally and zVAD subcutaneously (s.c.) were injected to tumor-bearing mice with depletion of cytotoxic T lymphocyte and natural killer cells. The results were confirmed with immune-compromised mice with no lymphocytes. Although necroptosis-induced tumor growth retardation appears mechanistically complicated and dependent on the injection route of polyI:C and zVAD, anti-caspase reagent directed to tumor cells will make RNA adjuvant immunotherapy more effective by modulating formation of tumoricidal microenvironment and dendritic cell-inducing antitumor immune system.
- Received November 21, 2014.
- Revision received March 21, 2015.
- Accepted April 7, 2015.
- Copyright © 2015, American Association for Cancer Research.