MLN4924, a newly discovered small molecule inhibitor of NEDD8-activating enzyme 
(NAE), inactivates Cullin-RING E3 ubiquitin Ligases (CRLs) by blocking cullin 
neddylation. As a result, MLN4924 causes accumulation of several key substrates 
of CRLs and effectively suppresses tumor cell growth by inducing apoptosis and 
senescence. However, the role of MLN4924 in induction of autophagy and its 
biological significance are totally unknown. Here we showed that MLN4924 
effectively induces autophagy in both time- and dose-dependent manners in 
multiple human cancer lines, indicating a general phenomenon. Mechanistically, 
by inactivating CRLs, MLN4924 causes accumulation of DEPTOR and HIF1α. The siRNA 
knockdown and gene KO studies showed that DEPTOR and the HIF1-REDD1-TSC1 axis 
are responsible for MLN4924-induced autophagy via inhibiting mTORC1. 
Biologically, autophagy is a survival signal to tumor cells, and blockage of 
autophagy via siRNA knockdown, gene KO and small molecule inhibitor remarkably 
enhanced MLN4924-induced apoptosis. Our study reveals an uncharacterized 
mechanism of MLN4924 action and provides the proof-of-concept evidence for 
strategic drug combination of MLN4924 with an autophagy inhibitor for maximal 
killing of tumor cells via enhancing apoptosis.