Cullin-RING ubiquitin ligase (CRL), with its founding member of 
SKP1-Cullins-F-box proteins (SCF) E3 ubiquitin ligase, is the largest family of 
E3 ligases, which requires cullin neddylation for its activation. Recently, an 
inhibitor of NEDD8 activating enzyme (NAE), MLN4924, was reported to block 
cullin neddylation and inactivate CRL/SCF E3, resulting in apoptosis induction 
and tumor suppression both in vitro and in vivo. We report here that apoptosis 
is not the sole mechanism by which MLN4924 suppresses tumor cell growth because 
apoptosis is moderately induced by the drug in some cancer cell lines and 
drug-induced growth suppression is only partially blocked by a pan-caspase 
inhibitor, z-VAD. MLN4924 treatment induces the characteristics of senescence 
phenotypes as evidenced by enlarged and flattened cellular morphology and 
positive staining of senescence-associated β-Gal. MLN4924-induced senescence is 
associated with cellular response to DNA damage, triggered by accumulation of 
DNA-licensing proteins CDT1 and ORC1, as a result of inactivation of CRL/SCF 
E3s. The senescence occurs in the manner independent of pRB/p16 and p53, but 
dependent on p21, a known substrate of CRL/SCF E3s and a mediator of senescence, 
which accumulates on CRL/SCF inactivation by MLN4924. Furthermore, 
MLN4924-induced senescence is irreversible and coupled with persistent 
accumulation of p21 and sustained activation of DNA damage response. Our study 
reveals a novel mechanism of MLN4924 action and showed that MLN4924 could be 
further developed as an effective anticancer agent by inducing apoptosis and 
irreversible senescence.