Targeting NEDD8-activated cullin-RING ligases for the treatment of cancer.

E3 ubiquitin ligases regulate many dynamic cellular processes important for cancer cell survival. Together with ubiquitin-activating enzyme (E1) and ubiquitin-conjugating enzymes (E2s), E3s catalyze the ubiquitination of numerous protein substrates that are subsequently targeted to the 26S proteasome for degradation. The clinical success of the proteasome inhibitor bortezomib has encouraged the evaluation of other components of the ubiquitin proteasome system for pharmaceutical intervention. Targeting specific E3s is particularly attractive because there is the potential to selectively block the degradation of certain cellular proteins and possibly avoid unwanted effects on other proteins. The cullin-RING ubiquitin E3 ligases (CRLs) represent the largest subfamily of E3s. The requirement that CRLs be activated by NEDD8 modification on the cullin protein offers an "achilles heel" for modulating this entire subfamily. NEDD8-activating enzyme (NAE) catalyzes the first step in the NEDD8 pathway and as such controls the activity of CRLs. In this article, we describe the role of the NEDD8 pathway in activating CRLs and discuss the preclinical findings with a first-in-class NAE inhibitor that is currently in phase I clinical trials for both solid tumor and hematological malignancies. In addition, we speculate where NAE inhibitors may find clinical utility.