Ribosome Profiling Reveals Translational Reprogramming via mTOR Activation in Omacetaxine-Resistant Multiple Myeloma.

Protein homeostasis is critical to the survival of multiple myeloma cells. Although this is targeted with proteasome inhibitors, mRNA translation inhibition has not entered trials. Recent work illustrates broad sensitivity of multiple myeloma cells to the translation inhibitor omacetaxine. We hypothesized that understanding how multiple myeloma becomes resistant will lead to the development of drug combinations to prevent or delay relapse. We generated omacetaxine resistance in H929 and MM1S multiple myeloma cell lines and compared them with parental lines. Resistant lines displayed decreased sensitivity to omacetaxine, with EC50 > 100 nmol/L, compared with parental sensitivity of 24 to 54 nmol/L. As omacetaxine inhibits protein synthesis, we performed both RNA sequencing and ribosome profiling to identify shared and unique regulatory strategies of resistance. Transcripts encoding translation factors and containing a terminal oligopyrimidine sequence in their 5' untranslated region were translationally upregulated in both resistant cell lines. The mTOR pathway promotes the translation of terminal oligopyrimidine motif-containing mRNAs. Indeed, mTOR inhibition with Torin 1 restored partial sensitivity to omacetaxine in both resistant cell lines. The combination was synergistic in omacetaxine-naïve multiple myeloma cell lines, and a combination effect was observed in vivo. Primary multiple myeloma cells from patient samples were also sensitive to the combination. These results provide a rational approach for omacetaxine-based combination therapy in patients with multiple myeloma, which have historically shown better responses to multiagent regimens. IMPLICATIONS: Through the use of ribosome profiling, our findings indicate mTOR inhibition as a novel combination therapy for partnering with the translation inhibitor omacetaxine in the treatment of multiple myeloma.