IFN-γ inhibition of TRAIL-induced IAP-2 upregulation, a possible mechanism of IFN-γ-enhanced TRAIL-induced apoptosis

Tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) is a type II transmembrane cytokine molecule of TNF family and a potent inducer of apoptosis. The anticancer activities of TNF family members are often modulated by interferon (IFN)-γ. Thus, we investigated whether IFN-γ enhances TRAIL-induced apoptosis. We exposed HeLa cells to IFN-γ for 12 h and then treated with recombinant TRAIL protein. No apoptosis was induced in cells pretreated with IFN-γ, and TRAIL induced 25% cell death after 3 h treatment. In HeLa cells pretreated with IFN-γ, TRAIL induced cell death to more than 70% at 3 h, indicating that IFN-γ pretreatment sensitized HeLa cells to TRAIL-induced apoptosis. We investigated molecules that might be regulated by IFN-γ pretreatment that would affect TRAIL-induced apoptosis. Western blotting analyses demonstrated that TRAIL treatment increased the level of IAP-2 protein and IFN-γ pretreatment inhibited the upregulation of IAP-2 protein by TRAIL protein. Our data indicate that TRAIL can signal to activate both apoptosis induction and antiapoptotic mechanism, at least, through IAP-2 simultaneously. IFN-γ or TRAIL treatment alone did not change expression of other pro- or antiapoptotic proteins such as DR4, DR5, FADD, Bax, IAP-1, XIAP, Bcl-2, and Bcl-XL. Our findings suggest that IFN-γ may sensitize HeLa cells to TRAIL-induced apoptosis by preventing TRAIL-induced IAP-2 upregulation, and IFN-γ may play a role in anticancer therapy of TRAIL protein through such mechanism.