Mechanism by Which mTOR Inhibitors Sesitize Multiple Myeloma Cells to Dexamethasone-Induced Apoptosis.

Previously we reported mTOR inhibitors such as rapamycin (RAPA) remarkably sensitize multiple myeloma (MM) cells to apoptosis induced by dexamethasone (DEX). In this study we investigated the molecular mechanisms behind this phenomenon. Firstly we excluded the involvement of post-translational modifications of the Related Adhesion Focal Tyrosine Kinase (RAFTK kinase), the stress-activated kinases jun kinase or p38 MAP kinase or the proapoptotic BCL-2 family member protein BAD in the sensitizing effect, since exposure to RAPA did not affect their phosphorylation status. Sensitization was not associated with a RAPA-mediated increase in glucocorticoid receptor reporter expression either. mTOR inhibitors are know to specifically paralyze cap-dependent translation through dephosphorylation of 4E-BP1, therefore we assessed whether the sensitizing effect is due to inhibition of cap-dependent translation. We transfected MM cells with a construct expressing a mutant 4E-BP1, which is resistant to phosphorylation induced by mTOR and acts as a constitutive translational repressor and found MM cells responded to DEX with significantly enhanced apoptosis, mimicking the effect of co-exposure to RAPA. These results indicate sensitization to DEX is mediated by inhibition of cap-dependent translation. A high throughput screening for translational efficiency of apoptosis-regulating transcripts identified several anti-apoptotic proteins whose translation was selectively inhibited by RAPA. Immunoblot assay confirmed a RAPA-induced down-regulated expression of XIAP-1, CIAP, HSP-27 and BAG 3, all which may play a role in the sensitization to apoptosis. Studies in a xenograft model demonstrated synergistic in vivo anti-myeloma effects when dexamethasone was combined with the mTOR inhibitor CCI-779. Synergistic effects were associated with a markedly enhanced degree of MM cell apoptosis in vivo. This study further supports the strategy of combining DEX with mTOR inhibitors in MM and identifies a potential mechanism by which the synergistic effect is achieved.