MLN4924, An Investigational NAE Inhibitor, Suppresses AKT and mTOR Signaling Pathway Through up Regulating REDD1 in Human Myeloma Cells
Authors: Gu Yanyan , Kaufman Jonathan L. , Boise Lawrence H. , Lonial Sagar 
Published: October 2019

Abstract
Abstract 1870

Introduction:
The development and survival of normal plasma cells as well as multiple myeloma cells depend on an elaborately regulated ubiquitin proteasome system (UPS). Proteasome inhibitors such as bortezomib have proved to be highly active in the treatment of multiple myeloma. MLN4924, a newly developed investigational NEDD8 activating enzyme (NAE) inhibitor, exhibits promising anti-tumor effect through both clinical and laboratory observation. We sought to evaluate the individual signaling effects of MLN4924 in multiple myeloma, with the intent of further understanding the mechanism of action and identifying potential combinations.

Methods:
Human myeloma cell lines (MM.1S, MM.1R and U266) were treated with increasing concentrations of MLN4924 for 24, 48 and 72 hrs respectively or concurrently with bortezomib. Cell viability (MTT), apoptosis, western blot, RT-qPCR and siRNA assays were used to identify the cellular and molecular sequelae of MLN4924 treatment.

Results:
Single agent studies demonstrate that MLN4924 induces cytotoxicity in all three MM cell lines. Cytotoxicity is associated with increased apoptosis and suppression of AKT and mTOR signaling pathway, as detected by FACS and western blot. MLN4924 suppresses protein turnover of Cullin-ring ligases substrates leading to stabilization of specific proteins, such as p27, CDT1, NRF2. We find that REDD1, a substrate of CUL4 A–DDB 1–ROC1–β-TRCP ubiquitin ligase and negative regulator of mTOR pathway, increases in as early as 2 hours when treated with MLN4924. Knock-down of REDD1 using siRNA alleviates MLN4924 induced AKT, mTOR signaling suppression as well as the growth inhibition, which suggests that MLN4924 inhibits AKT, mTOR signaling through upregulating REDD1 protein leading to cytotoxicity.
Combining MLN4924 with bortezomib synergistically enhances the antitumor effect of MLN4924. Western blot confirms the development of significantly increased procaspase and PARP cleavage, as well as substantial down regulation of AKT and mTOR signaling. Neither IL-6 nor IGF-1 is able to abrogate these combination effects. When we silence REDD1 in the combination assay in MM.1R, we find less cell apoptosis and suppression of AKT, mTOR pathway, which further support that REDD1 is an important regulator for MLN4924 induced cytotoxicity in MMs.

Conclusion:
MLN4924 is a potent investigational antitumor compound suppressing mTOR signaling pathways in myeloma cell lines. Down-regulation of the mTOR signaling pathway is associated with increased expression of REDD1. Combination of MLN4924 with the proteasome inhibitor bortezomib induces synergistic apoptosis in MMs cell lines which can overcome the prosurvival effects of growth factor (IL-6 and IGF-1). These findings could positively impact clinical combination strategies.

Disclosures:
Kaufman: Millenium: Consultancy; Onyx Pharmaceuticals: Consultancy; Novartis: Consultancy; Keryx: Consultancy; Merck: Research Funding; Celgene: Research Funding. Lonial:Millennium Pharmaceuticals, Inc.: Consultancy; Celgene: Consultancy; Novartis: Consultancy; Bristol-Myers Squibb: Consultancy; Onyx: Consultancy; Merck: Consultancy.