IntroductionMLN4924 is a potent and selective small-molecule inhibitor of NEDD8-activating enzyme (NAE) that is currently in phase 1 clinical trials. 1-3 NAE plays an essential role in regulating the activity of a subset of ubiquitin E3 ligases, the cullin-RING ligases (CRLs), which are responsible for regulating destruction of many intracellular proteins. 4 NAE activates the small ubiquitin-like molecule NEDD8 as the first step in the neddylation cascade. 5 NAE hydrolyzes adenosine triphosphate (ATP) to adenylate NEDD8 at its C-terminus and transfers NEDD8 from the adenyl group to a specific cysteine within NAE. The activated NEDD8 is then transferred to the active-site cysteine of Ubc12 or UBE2F the E2s specific for the NEDD8 pathway. Finally, NEDD8 is conjugated on a conserved lysine near the C-terminal end of a cullin protein; this covalent modification is required for the cullin complex to recruit a ubiquitin-charged E2 protein facilitating polyubiquitination of proteins, targeting them for proteasomal degradation. Thus, NAE plays a key role in regulating the levels (and therefore the function) of a subset of proteins.Many of the proteins that are substrates for CRL-mediated polyubiquitination have key roles in cell-cycle progression and signal transduction, making NAE inhibition an attractive target for anticancer therapy. MLN4924 potently inhibits NAE in vitro, resulting in inhibition of CRL neddylation and an increase in levels of CRL substrate proteins (eg, Cdt-1, Nrf-2). 3 The primary mechanism of action of NAE inhibition in many cell types is induction of DNA rereplication because of blocking degradation of Cdt-1, a critical factor required for licensing origins of DNA replication. 3 Dysregulation of Cdt-1 activity leads to DNA rereplication. 6,7 For example, overexpression of Cdt-1 and Cdc6 induces DNA rereplication, activates DNA damage repair pathways, and induces cell death. 7 Induction of DNA rereplication by MLN4924 results in S-phase accumulation, DNA-damage responses, and cell death 3 (M.A.M., U.N., T.A.S., P. Veiby, P.G.S., B. Amidon, manuscript in preparation). Similar effects were observed in human tumor xenografts where MLN4924 inhibited NAE in vivo leading to tumor growth inhibition. 3 The nuclear factor-B (NF-B) signaling pathway plays a key role in many aspects of cancer initiation and progression through transcriptional control of genes involved in growth, angiogenesis, antiapoptosis, invasiveness, and metastasis. 8 Regulation of NF-B signaling occurs at many levels, one of which is through the regulation of protein turnover by the action of CRLs. Under normal conditions, NF-B transcription factors are maintained in an inactive state by binding to IB proteins. In canonical NF-B signaling, IB␣ binds to p50-p65, sequesters the transcription factors in the cytoplasm rendering them inactive. On stimulation of the IKK complex, IB␣ is phosphorylated at Ser32 and Ser36, resulting in its polyubiquitination and degradation, 9-11 thus resulting in nuclear accumulation of the complex and transcri...