Dysregulated Translational Control Is a Central Feature of MM Mediated By 14-3-3ε Interactome Promoting Multiple Myeloma Cell Growth and Viability

14-3-3 proteins are chaperone and scaffold proteins that exert a widespread influence on cellular processes through binding to serine/threonine-phosphorylated residues on target proteins, forcing conformational changes or influencing their interactions with other molecules. Altered 14-3-3 expression is associated with development and progression of cancer.

We therefore evaluated the status of all 14-3-3 isoforms in plasma cells disorders in publically available gene expression profiling (GEP) data. Using independent patient datasets, we observed a consistent higher expression of YWHAE (coding gene for the isoform 14-3-3ε) in MM and plasma cell leukemia (PCL) patients, while no consistent differences were observed with the other isoforms. Moreover, we also confirmed higher expression of YWHAE in our RNA-seq data from 420 newly-diagnosed MM patients, with relatively low expression in normal plasma cells. Finally, 14-3-3ε was also found to be constitutively expressed at protein level in primary patient MM cells and in a large panel of MM cell lines, with significantly lower expression in healthy donor B cells.

To evaluate if 14-3-3ε represents a functional dependency in MM, we performed genetic perturbation of YWHAE in a panel of MM cell lines. Depletion of YWHAE using 3 different shRNA inhibited cell proliferation and induced cell apoptosis across 5 different cell lines, independently of their genetic background. We next performed CRISPR-Cas9-mediated YWHAE knock out (KO) in H929 and JJN3 cells and observed a significant decrease in cell viability and a robust apoptotic response. H929 YWHAE KO cells infected with FLAG-YWHAE addback lentiviral construct completely rescued this phenotype, confirming that loss of YWHAE is responsible for the defective cell viability and apoptotic phenotype. These observations were corroborated by ectopic overexpression of YWHAE in H929 WT cells that significantly promoted MM cell viability.

To elucidate the underlying molecular mechanisms, proteins immunocomplexed co-precipitated with FLAG in H929 KO cells with 14-3-3ε-FLAG addback were analyzed by mass spectrometry. Protein analysis revealed interaction of 14-3-3ε with a large number of proteins, enriched in mTORC1, PI3K-AKT-mTOR and unfolded protein response (UPR) pathway-related genes. Among these, TSC2 and mTORC1 proteins were further studied. WB analysis confirmed interaction of 14-3-3ε with p-mTOR (S2448) and its upstream negative regulator p-TSC2 (S939), while mTORC1 downstream targets, p-p70 S6k and p-4E-BP1, did not interact with 14-3-3ε. WB analysis also revealed activation of TSC2 and consequent inhibition of mTORC1 (via decrease of p-mTOR S2448 levels) in YWHAE KD cells. YWHAE-FLAG addback reversed these effects. Additionally, GEP data in KD cells confirmed a significant impact on mTORC1 pathways. Importantly, YWHAE expression highly correlated (R> 0.8) with genes involved in the mTORC1 pathway, including PSMC4, COPS5, EIF2S2, in our RNA-seq dataset, demonstrating a clinical significance of 14-3-3ε and mTORC1 cooperation in the context of myeloma.

One of the most conserved functions of mTORC1 is to promote translation. We therefore assessed the impact of YWHAE on global translational efficiencies in MM cells, and observed significant impact on nascent protein synthesis by YWHAE modulation. 14-3-3ε KD induced 4EBP1 de-phosphorylation through inhibited mTORC1, and concomitantly induced EIF2α phosphorylation. Both effects inhibited translation initiation complex formation, mechanistically supporting a strong protein synthesis arrest. These data show the modulation of several hubs of the signaling apparatus controlling translation initiation in response to YWHAE modulation, ultimately producing a marked protein synthesis inhibition.

Deregulated translational control is a central feature of MM. Our findings highlight a unique function for YWHAE as promoter of MM cell survival through regulation of mTOR-dependent protein synthesis and apoptosis. Pharmacological inhibition of YWHAE/14-3-3ε is therefore a possibility to specifically target malignancies with deregulated translational control such as MM.