The Translation Initiation Factor eIF4E Is Overexpressed in Multiple Myeloma and Is Critical for Myeloma Cell Proliferation and Survival

The translation initiation factor eIF4E is central to protein synthesis in general, and overexpression and/or activation of eIF4E is associated with a malignant phenotype by regulating oncogenic protein translation. Several previous publications indicate that aberrant control of protein synthesis contributes to lymphoma genesis but the exact role of protein translation in multiple myeloma (MM) is less clear. Therefore, understanding the mechanisms that control protein synthesis is an emerging new research area in MM with significant potential for developing innovative therapies. The goal of this study was to determine the role and regulation of eIF4E, as well as the effects of protein translation controlling drugs in MM.

By western blot analysis as well as RT-PCR we found that eIF4E protein and mRNA levels are significantly elevated (up to 20 fold) in MM cell lines (H929, RPMI-8226, MM.1S and OPM2) and primary myeloma cells compared to normal plasma cells. Silencing of eIF4E gene expression in RPMI-8226 MM cells by a stable and inducible shRNA system significantly decreased viability of myeloma cells (by ∼ 43%) but not of HEK 293 suggesting a higher dependency of MM cells to protein translation. Next we evaluated different drugs including pomalidomide, rapamycin, pp242, 4EGI-1 and ribavirin, that are known to inhibit protein synthesis for their effects on protein translation in MM. By m⁷GTP pull down assays we evaluated the effects of the different drugs on eIF4E expression and activity. Rapamycin blocked the phosphorylation of 4EBP1 and eIF4E release, and subsequently inhibited eIF4G binding. The compound 4EGI-1 decreased the interaction between eIF4E and eIF4G. Pomalidomide decreased eIF4E protein expression. All drugs inhibited MM cell DNA synthesis measured by ³H-Thymidine incorporation. Treatment with pomalidomide (10uM), rapamycin (40nM), pp242 (10uM), 4EGI1 (50uM) or ribavirin (50uM) for 48h significantly decreased (p<0.05) proliferation by 43–62% indicating that drugs controlling protein translation inhibit MM growth. We also found that all drugs decreased expression of eIF4E dependent targets such as cyclin D1 and c-myc.

Here we show that eIF4E, a key player in translational control, is highly expressed in MM cells and critical for MM growth and survival. Therefore our study helps to understand the function and regulatory mechanism of eIF4E in MM. Further the evaluation of drugs targeting protein translation provides the basis for the optimization of current MM treatment or to open up new strategies such as targeting protein translation in future MM therapy.

Lentzsch:Celgene Corp: Consultancy, Research Funding; Onyx: Consultancy; Genzyme: Consultancy; prIME Oncology: Honoraria; Imedex: Honoraria; Clinical Care Options: Honoraria.