A metal-based inhibitor of NEDD8-activating enzyme

Abstract

A cyclometallated rhodium(III) complex [Rh(ppy)(2)(dppz)](+) (1) (where ppy=2-phenylpyridine and dppz=dipyrido[3,2-a:2',3'-c]phenazine dipyridophenazine) has been prepared and identified as an inhibitor of NEDD8-activating enzyme (NAE). The complex inhibited NAE activity in cell-free and cell-based assays, and suppressed the CRL-regulated substrate degradation and NF-κB activation in human cancer cells with potency comparable to known NAE inhibitor MLN4924. Molecular modeling analysis suggested that the overall binding mode of 1 within the binding pocket of the APPBP1/UBA3 heterodimer resembled that for MLN4924. Complex 1 is the first metal complex reported to suppress the NEDDylation pathway via inhibition of the NEDD8-activating enzyme.

Figure 1. Chemical structures of MLN4924, [Rh(ppy)₂(dppz)]⁺ (1) and Rh(III) analogues (2–4).

Figure 2. Complex 1 inhibits NAE activity in a dose-dependent manner.

Western blots show dose-dependent inhibition of a) Ubc-12-NEDD8 formation in a cell-free system and b) cellular Ubc12-NEDD8 levels by 1. MLN4924 was included for comparison.

Figure 3. Dose-dependent inhibition of IκBα degradation by 1.

Caco-2 cells were pre-incubated with indicated concentrations of 1 for 16 hours and then stimulated with 5 ng/ml of TNF-α at indicated time intervals. Whole cell lysates were analyzed by Western blot using anti-IκBα antibody. Densitometry estimates of IκBα levels normalized with GAPDH are shown under each lane. b) Caco-2 cells were treated with 1 or MLN4924 for 16 h. The cell lysates were immunoblotted to analyse the level of CRL substrate p27.

Figure 4. Complex 1 suppresses NAE-regulated NF-κB-dependent luciferase reporter gene expression in a dose-dependent manner.

Caco-2 cells were transfected with the NF-κB-dependent luciferase reporter p3EnhConA-Luc gene, treated with 1 for 16 hours, and then stimulated with 5 ng/ml of TNF-α for 3 hours. Luciferase expression was measured and normalized with β-galactosidase activity. Results are expressed as fold change compared to TNF-α stimulation alone and the errors bar show the standard derivation of triplicate results. MLN4924 was included for comparison.

Figure 5. Low-energy binding conformations of a) 1, b) MLN4924 and c) both 1 and ATP bound to NAE heterodimer generated by virtual ligand docking.

Proteins APPBP1 (blue), UBA3 (purple) and NEDD8 (yellow) are displayed in ribbon form. Small molecules are depicted as a ball-and-stick model showing showing carbon (yellow), hydrogen (grey), oxygen (red), nitrogen (blue), phosphorus (orange) and sulfur (green). Non-polar hydrogens were not shown.