Caspase-8 deficiency induces a switch from TLR3 induced apoptosis to lysosomal cell death in neuroblastoma cell lines
TLR3 converts in cancer cells from an inflammatory to a death receptor and TLR3-induced cell death activates the extrinsic apoptosis pathway. Here, we demonstrate that activation of TLR3 triggers a form lysosomal cell death. Following the combinational treatment of IFN-1/poly(I:C) of the Caspase-8 deficient neuroblastoma cell line SH-SY5Y, lysosomes enlarge and accumulate before cells display characteristic apoptotic morphologies. However, caspases are not involved in signalling from TLR3 to the lysosome as 25 M did not inhibit cell death. However, increasing zVAD concentrations to 50 M which is known to inhibit cathepsins, as well as a specific cathepsin B inhibitor reduced TLR3-induced lysosomal cell death. Thus lysosomal cathepsins have a role in cell death execution and overtake the role of caspase-8 in inducing the apoptotic caspase cascade. Further in the caspase-8 positive neuroblastoma cell line SK-N-AS, knockdown of caspase-8 induces a switch from TLR3-induced apoptosis to lysosomal cell death. Taken together our data suggest that lysosomal cell death represents a default death mechanism, when caspase-8 is absent.