IRF9 is a key factor for eliciting the antiproliferative activity of IFN-alpha.

A number of tumors are still resistant to the antiproliferative activity of human interferon (IFN)-alpha. The Janus kinases/Signal Transducers and Activators of Transcription (JAK-STAT) pathway plays an important role in initial IFN signaling. To enhance the antiproliferative activity of IFN-alpha, it is important to elucidate which factors in the JAK-STAT pathway play a key role in eliciting this activity. In human ovarian adenocarcinoma OVCAR3 cells sensitive to both IFN-alpha and IFN-gamma, only IFN regulatory factor 9 (IRF9)-RNA interference (RNAi) completely inhibited the antiproliferative activity of IFN-alpha among the intracellular JAK-STAT pathway factors. Conversely, Stat1-RNAi did not inhibit the antiproliferative activity of IFN-alpha, whereas it partially inhibited that of IFN-gamma. As a cell death pathway, it is reported that tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) induces apoptosis through TRAIL-receptor (R) 1 and TRAIL-R2. In IFN-alpha-treated OVCAR3 cells, IRF9-RNAi inhibited transcription of TRAIL whereas Stat1-RNAi did not, suggesting that the transcription of TRAIL induced by IFN-alpha predominantly required IRF9. Furthermore, IFN-stimulated response element-like motifs of TRAIL bound to IFN-stimulated gene factor 3 (ISGF3) complex after IFN-alpha treatment. Subsequently, TRAIL-R2-RNAi inhibited both antiproliferative activities of IFN-alpha and TRAIL, suggesting that TRAIL-R2 mediated both IFN-alpha and TRAIL signals to elicit their antiproliferative activities. Finally, IRF9 overexpression facilitated IFN-alpha-induced apoptosis in T98G (human glioblastoma multiforme) cells, which were resistant to IFN-alpha. Thus, this study suggests that IRF9 is the key factor for eliciting the antiproliferative activity of IFN-alpha and TRAIL may be one of the potential mediators.