Osteoblast Creates a Non-permissive Niche for Myeloma Cells

Objectives: Bone lesions with defective osteoblast (OB) differentiation in multiple myeloma (MM) provide a “MM niche” suitable for MM expansion. However, we noticed that resumption of bone formation especially after treatment with proteasome inhibitors correlates well with suppression of MM tumor growth, which suggests that OB induction may disrupt the “MM niche” and create a milieu to avoid MM cells. In the present study, we therefore explored the precise role of OBs differentiated from bone marrow stromal cells (BMSCs) on MM cell growth.
Methods: MC3T3-E1 preosteoblastic cells were differentiated with rBMP2 in osteogenic media, and used as OBs. MM cell lines were cocultured with mature OBs or BMSCs, and viable cell number, cellcycle, and activation of various signaling pathways in MM cells were analyzed.
Results: When cocultured with mature OBs, all MM cell lines tested underwent apoptosis with Go/G1 cell cycle arrest over time, in sharp contrast to their growth expansion by BMSCs. Although BMSCs potently enhanced Pim-2 expression in MM cells, the mature OBs suppressed Pim-2 expression and NF-kB p65 phosphorylation in MM cells, thereby down-regulating their downstream pro-survival mediators, including IRF-4 and cMyc at day 1. OBs subsequently induced the phosphorylation of AMPK from around day 3 in MM cells. Addition of the AMPK inhibitor dorsomorphin mitigated the OB-induced MM cell death and cell cycle arrest, indicating a critical role of activation of the AMPK pathway.
Conclusions: These results collectively demonstrate emergence of anti-MM activity by bone-forming mature OBs, which may alter a “MM niche” in bone lesions to a non-permissive niche for MM growth. Screening of factors elaborated by mature OBs to inactivate NF-kB/Pim-2 pathway and activate the AMPK pathway in MM cells will help to therapeutically create an inducible non-permissive niche.