Myeloma Cells Switch Osteoblastogenesis to Adipogenesis and Suppress Bone Formation

Bone destruction is a hallmark of myeloma, and has a severe impact on patients’ quality of life and survival. Unfortunately, current treatment only offers moderate palliative effects, and this disease remains incurable. The bone changes in myeloma patients results from increased osteoclast-mediated bone resorption and decreased osteoblast-mediated bone formation. In particular, new bone formation that usually occurs at sites of previously resorbed bones is deeply suppressed; as a result, areas of bone destruction rarely heal. Previous studies have shown that myeloma cells inhibit osteoblast differentiation from mesenchymal stem cells (MSCs), and the Wnt/b-catenin signaling pathway is suppressed via myeloma-produced Wnt antagonists such as dickkopf-1. However, the role of dickkopf-1 in myeloma-induced inhibition of bone formation remains controversial since myeloma cells alone do not produce sufficient dickkopf-1 to suppress osteoblast differentiation. In addition, the administration of an antibody against dickkopf-1 in myeloma patients failed to restore new bone formation, indicating there must be an additional mechanism for inhibition of osteoblast differentiation seen in myeloma. While MSCs can differentiate into mature osteoblasts, they are also capable of differentiating into adipocytes, which is a major cell type in marrow stroma. We observed that myeloma cells (cell lines and primary cells isolated from myeloma patients’ bone marrow) injected into human or mouse bone not only reduced osteoblast number, but also increased adipocyte number and activity in bone marrow. Similar observations were seen in the clinical setting where collections of adipocytes were found in the bone marrow of newly diagnosed, untreated myeloma patients. Patients with greater bone destruction had higher adipocyte numbers than those in patients with less bone destruction, indicating a relationship among myeloma cells, adipogenesis, and osteoblastogenesis. We hypothesized that inhibition of osteoblast differentiation is a consequence of myeloma-dependent alterations in the control of the MSCs’ fate into osteoblasts or into adipocytes. In our studies, we co-cultured MSCs with myeloma cells in a mixed medium (that contained both adipocyte and osteoblast media), and we observed co-culture with myeloma cells induced more adipocyte than osteoblast formation. Moreover, co-culture with myeloma cells enhanced adipocyte differentiation in vitro. Interestingly, separation of the cells by transwell inserts significantly reduced such effect. By analysis of the adhesion molecules in myeloma cells, we identified integrin α4β1 as a novel contributor in regulation of adipogenesis and osteoblastogenesis. Thus, our studies indicate that in the presence of myeloma cells, MSCs may be more prone to differentiate into adipocytes than into osteoblasts via α4β1. Our studies also suggest the development of new strategies to improve the care of myeloma patients with bone destruction by targeting α4β1 and its signaling pathways.