Translational control in malignant pleural mesothelioma

Protein synthesis is a cellular process finely regulated during growth and development and its deregulation can lead to cell apoptosis or disease. Translational control is rate-limiting in cancer growth and translation initiation step is emerging as an attractive therapeutic target. eIF6 is an antiassociation factor that regulates the availability of active 80S. Its activation is driven by the RACK1/PKCβ axis, in a mTORc1 independent manner. We previously described that eIF6 haploinsufficiency causes a striking survival in the Eμ-Myc mouse lymphoma model, with lifespans extendend up to 18 months. microRNAs have been shown to regulate a wide range of biological processes destabilizing messenger RNAs and by repressing the translation of these mRNAs. Involvement of microRNAs in repression of translation suggests that they might be associated with polysomes. Here we screen for 1) eIF6 expression in human cancers and 2) association of microRNAs with polysomes in Malignant Pleural Mesothelioma (MPM). We show that MPM tumors and a MPM cell line (REN cells) contain high levels of hyperphosphorylated eIF6. Enzastaurin is a PKC beta inhibitor used in clinical trials. We prove that Enzastaurin treatment decreases eIF6 phosphorylation rate, but not eIF6 protein stability. The growth of REN, in vivo, and metastasis are reduced by either Enzastaurin treatment or eIF6 shRNA. Molecular analysis reveals that eIF6 manipulation affects the metabolic status of malignant mesothelioma cells. Less glycolysis and less ATP content are evident in REN cells depleted for eIF6 or treated with Enzastaurin (Anti-Warburg effect). We propose that eIF6 is necessary for Malignant Mesothelioma growth, in vivo, and can be targeted by kinase inhibitors. Finally we found that the MPM miRNA signature was characterized also by differential miRNAs subcellular distribution. In particular, only some miRNAs were expressed in the polysomal pool with variability in miRNAs occupancy, indicating that some miRNAs can repress translation, while others cannot. Particularly, we evidenced that polysome-bound miRNAs present a correlation with the cell cycle pathway in REN cell, a MPM epithelioid cell line, suggesting that their polysomal localization could explain how these miRNAs may regulate cell cycle components translation.