LYMPHOID NEOPLASIA MLN 4924 , an NAE inhibitor , suppresses AKT and mTOR signaling via upregulation of REDD 1 in human myeloma cells

Posttranslational modifications of proteins such as phosphorylation, glycosylation, ubiquitination, and acetylation regulate protein structure, activity, localization, and stability. Aberrant protein modification profiles are related to disease and cancer pathogenesis. Among themost common posttranslationalmodifications of proteins, ubiquitination represents an emerging area of research interest due to its importance in so many aspects of cancer cell biology. Ubiquitination of proteins regulates cell cycle, differentiation, and apoptosis. Ubiquitination of target proteins is achieved via sequential enzymatic reactions mediated by E1 (ubiquitin activating enzyme),E2 (ubiquitin conjugating enzyme), andE3 ligases (ubiquitin ligases). Among the known E3 ligases, cullin-ring ligases (CRLs) represent the largest and most frequently used group. The activities of CRLs are regulated by neddylation of cullin family proteins within the complex through covalent attachment of NEDD8, a 9-kDa ubiquitinlike small molecule. Protein neddylation is mediated through an enzymatic cascade in a similar manner to the ubiquitin system,which is initiated by NEDD8 activating enzyme (NAE). Thus, E3 ligases are in part regulated by neddylation, suggesting that tight regulation of ubiquitination can be imposed by enzymes upstream of the proteasome. Among all the known neddylated proteins, the cullin protein family is the major group of substrates and acts as an essential component of CRLs.MLN4924 is a newly developed NAE inhibitor with high specificity for the target enzyme. Through its effects on protein neddylation, MLN4924 is known to have many effects including induction of DNA rereplication, apoptosis, autophagy, cell growth inhibition through p21-dependent senescence, and regulation of T-cell–mediated inflammatory response. However, from a plasma cell biology perspective, MLN4924 can selectively inhibit the turnover of specific proteins which are targeted by CRLs. The proteasome acts as the main lysosomal-independent protein degradation system within cells to regulate protein metabolism (turnover) which is critical in normal cell growth and survival as well as malignant cells. The phosphatidylinositol 3-kinase (PI3K)/ mammalian target of rapamycin (mTOR) signaling pathway integrates diverse signals to regulate cell growth, survival, metabolism, and autophagy. The interaction between protein metabolism and the PI3K/mTOR pathway is also an area of increasing interest amongmany cancers. In the current report, we describe the novel effects of NAE inhibition on myeloma cell survival alone or in combination with a focus on how neddylation can impact not only growth, but more directly impact cellular metabolism and proliferation through its effects on REDD1 (regulated in development and DNA damage responses 1) and thus on the PI3K/mTOR signaling pathway. These results suggest, in aggregate, that regulation of ubiquitination and proteasome function have broad implications for malignant plasma cell metabolism and survival.