Etoposide upregulates Bax-enhancing tumour necrosis factor-related apoptosis inducing ligand-mediated apoptosis in the human hepatocellular carcinoma cell line QGY-7703

Tumour necrosis factor-related apoptosis-inducing ligand (TRAIL) has attracted much attention because of its ability to kill tumour cells. In this study, we demonstrated that treatment of QGY-7703 cells with the combination of TRAIL and etoposide resulted in synergistic cytotoxic effects. In dissecting the mechanism underlying this synergistic effect, we found that treatment with etoposide alone resulted in the upregulation of Bax, while the level of truncated Bid (tBid) was unchanged. In contrast, while treatment with TRAIL alone significantly increased the level of tBid, the expression of Bax remained unaffected. The enhanced apoptosis was accompanied by an increased release of cytochrome c and second mitochondria-derived activator of caspase/direct IAP binding protein with low pI (DIABLO) from mitochondria, leading to the activation of cellular caspase-8, -9, -3 and -7, as well as poly ADP-ribose polymerase. This enhanced release of cytochrome c and second mitochondria-derived activator of caspase/DIABLO was inhibited by the general caspase inhibitor N -benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone. The RT-PCR and Western blotting results demonstrated that the levels of both mRNA and protein for death receptor-4, death receptor-5 and decoy receptor-2 remained unchanged in response to etoposide, indicating that the synergistic effect of TRAIL and etoposide is not a result of increasing the expression for TRAIL receptors, but rather is associated with amplification of the mitochondrial signal pathway.