TNF-related apoptosis-inducing ligand (TRAIL) is a tumor-selective cytokine with 
potential anticancer activity and is currently under clinical testing. Head and 
neck squamous cell carcinoma (HNSCC), like other cancer types, exhibits varied 
sensitivity to TRAIL. MLN4924 is a newly developed investigational small 
molecule inhibitor of NEDD8-activating enzyme with potent anticancer activity. 
This study reveals a novel function of MLN4924 in synergizing with TRAIL to 
induce apoptosis in HNSCC cells. MLN4924 alone effectively inhibited the growth 
of HNSCC cells and induced apoptosis. When combined with TRAIL, synergistic 
effects on decreasing the survival and inducing apoptosis of HNSCC cells 
occurred. MLN4924 decreased c-FLIP levels without modulating death receptor 4 
and death receptor 5 expression. Enforced expression of c-FLIP substantially 
attenuated MLN4924/TRAIL-induced apoptosis. Thus c-FLIP reduction plays an 
important role in mediating MLN4924/TRAIL-induced apoptosis. Moreover, MLN4924 
decreased c-FLIP stability, increased c-FLIP ubiquitination, and facilitated 
c-FLIP degradation, suggesting that MLN4924 decreases c-FLIP levels through 
promoting its degradation. MLN4924 activated c-jun-NH(2)-kinase (JNK) signaling, 
evidenced by increased levels of phospho-c-Jun in MLN4924-treated cells. 
Chemical inhibition of JNK activation not only prevented MLN4924-induced c-FLIP 
reduction, but also inhibited MLN4924/TRAIL-induced apoptosis, suggesting that 
JNK activation mediates c-FLIP downregulation and subsequent enhancement of 
TRAIL-induced apoptosis by MLN4924. Because knockdown of NEDD8 failed to 
activate JNK signaling and downregulate c-FLIP, it is likely that MLN4924 
reduces c-FLIP levels and enhances TRAIL-induced apoptosis independent of NEDD8 
inhibition.