Abstract
Cellular effects of a Nedd8-activating enzyme (NAE) inhibitor, MLN4924, using the AlphaScreen format were explored. MLN4924 acts as a substrate-assisted inhibitor of NAE by forming a tight binding Nedd8-MLN4924 adduct. The inhibited enzyme can no longer transfer Nedd8 downstream to modify and activate the E3 cullin-RING ligases. This results in the stabilization of proteins regulated by the proteasome, leading to cell death. These studies monitored the endogenous cellular changes to NAE∼Nedd8 thioester, the formation of the Nedd8-MLN4924 adduct, and the reduction in the Cul1-Nedd8. Lysates derived from MLN4924-treated HCT116 cells showed that whereas the β-subunit of NAE remained constant, reductions of both NAE∼Nedd8 thioester and Cul1-Nedd8 levels occurred with a concomitant rise of the adduct. Moreover, the formation of the Nedd8-MLN4924 adduct was approximately stoichiometric with the concentration of NAEβ. Higher density 384-well cell-based assays illustrated the kinetics of enzyme inactivation across a wider range of MLN4924 concentrations, showing a rapid loss of NAE∼Nedd8 thioester and Cul1-Nedd8. The reduction of NAE∼Nedd8 thioester precedes the loss of Cul1-Nedd8 at twice the rate. Finally, these results clearly demonstrate the utility of the homogeneous assay for quantitative assessment of these endogenous cellular components in a 384-well plate in response to inhibition of NAE by MLN4924.