Multiple myeloma cells promote migration of bone marrow mesenchymal stem cells by altering their translation initiation

The role of the bone marrow microenvironment in multiple myeloma pathogenesis and progression is well recognized. Indeed, we have shown that coculture of bone marrow mesenchymal stem cells from normal donors and multiple myeloma cells comodulated translation initiation. Here, we characterized the timeline of mesenchymal stem cells conditioning by multiple myeloma cells, the persistence of this effect, and the consequences on cell phenotype. Normal donor mesenchymal stem cells were cocultured with multiple myeloma cell lines (U266, ARP1) (multiple myeloma- conditioned mesenchymal stem cells) (1.5 h,12 h, 24 h, 48 h, and 3 d) and were assayed for translation initiation status (eukaryotic translation initiation factor 4E; eukaryotic translation initiation factor 4G; regulators: mechanistic target of rapamycin, MNK, 4EBP; targets: SMAD family 5, nuclear factor κB, cyclin D1, hypoxia inducible factor 1, c-Myc) (immunoblotting) and migration (scratch assay, inhibitors). Involvement of mitogenactivated protein kinases in mesenchymal stem cell conditioning and altered migration was also tested (immunoblotting, inhibitors). Multiple myeloma- conditioned mesenchymal stem cells were recultured alone (1-7 d) and were assayed for translation initiation (immunoblotting). Quantitative polymerase chain reaction of extracted ribonucleic acid was tested for micro- RNAs levels. Mitogen-activated protein kinases were activated within 1.5 h of coculture and were responsible for multiple myeloma-conditioned mesenchymal stem cell translation initiation status (an increase of > 200%, P < 0.05) and elevated migration (16 h, an increase of > 400%, P < 0.05). The bone marrow mesenchymal stem cells conditioned by multiple myeloma cells were reversible after only 1 d of multiple myeloma-conditioned mesenchymal stem cell culture alone. Decreased expression of microRNA-199b and microRNA-125a (an increase of <140%, P < 0.05) in multiple myeloma- conditioned mesenchymal stem cells supported elevated migration. The time- and proximity-dependent conditioning of normal donor mesenchymal stem cells in our model points to a dynamic interaction between multiple myeloma cells and the bone marrow niche, which causes profound changes in the nonmalignant bone marrow constituents. Future studies are warranted to identify clinically relevant means of blocking this crosstalk and improving multiple myeloma therapy.