Role of tumor necrosis factor-α and TRAIL in high-dose radiation-induced bystander signaling in lung adenocarcinoma

In the present study, ionizing radiation (IR)-induced bystander effects were investigated in two lung cancer cell lines. A549 cells were found to be more resistant to radiation-conditioned medium (RCM) obtained from A549 cells when compared with the H460 exposed to RCM procured from H460 cells. Significant release of tumor necrosis factor-α (TNF-α) was observed in A549 cells after IR/RCM exposure, and the survival was reversed with neutralizing antibody against TNF-α. In H460 cells, significant release of TNF-related apoptosis inducing ligand (TRAIL), but not TNF-α, was observed in response to IR, RCM exposure, or RCM + 2Gy, and neutralizing antibody against TRAIL diminished clonogenic inhibition. Mechanistically, TNF-α present in RCM of A549 was found to mediate nuclear factor-κB (NF-κB) translocation to nucleus, whereas the soluble TRAIL present in RCM of H460 cells mobilized the nuclear translocation of PAR-4(a proapoptotic protein). Analysis of IR-inducible early growth response-1 (EGR-1) function showed that EGR-1 was functional in A549 cells but not in H460 cells. A significant decrease in RCM-mediated apoptosis was observed in both A549 cells stably expressing small interfering RNA EGR-1 and EGR-1^-/- mouse embryonic fibroblast cells. Thus, the high-dose IR-induced bystander responses in A549 may be dependent on the EGR-1 function and its target gene TNF-α. These findings show that the reduced bystander response in A549 cells is due to activation of NF-κB signaling by TNF-α, whereas enhanced response to IR-induced bystander signaling in H460 cells was due to release of TRAIL associated with nuclear translocation of PAR-4.