The Role of Adipocyte Lineage Cells in Myeloma Growth and Dissemination in Bone

Background: Multiple myeloma (MM) is hematologic malignancy of plasma cells that thrives in and progresses throughout the bone marrow microenvironment. The bone marrow is host to a variety of cell types, including bone marrow stromal cells and hematopoietic cells, as well as osteoblasts, osteoclasts and adipocytes. We and others have shown that MM cells not only alter the local bone microenvironment to support MM progression, but also modify distant bone sites through secretion of soluble factors before arrival of tumor cells. One critical alteration in bone is the shift of osteoblast progenitor cells from osteoblastogenesis towards adipogenesis. Whether and how these increased adipocyte lineage cells contribute to MM cells homing to and growth in bone are currently unknown. Both mature and pre-adipocytes have multiple endocrine functions such as cytokine and growth factor secretion. Thus, an increase in adipocyte lineage cells likely alters the bone microenvironment in favor of supporting MM. Here, we hypothesized that adipocytes and their precursors play an active role in MM progression that contributes to MM growth and dissemination throughout bone.

Methods: Our hypothesis was tested using a co-culture system in which 3T3-L1 mouse pre-adipocytes or mature adipocytes were separated by a porous membrane from 5TGM1-luc mouse MM cells. This system allowed cross-talk by secreted molecules but not through direct cell-cell contact. After three days of co-culture, MM cells were collected for (i) intravenous (I.V.) tail-vein injections into syngeneic C57Bl/KaLwRiJ mice or (ii) protein collection for Western blot analyses. For in vivo experiments, tumor progression was tracked by bioluminescent luciferase imaging and total tumor burden was evaluated by IgG2bκ (a soluble marker of 5TGM1-luc MM cells) levels in mouse serum. In addition, conditioned medium (CM) was collected from either pre-adipocytes or mature adipocytes for MM cell migration assays or for analyses of soluble factors in the CM by cytokine/chemokine array.

Results: I.V. injection of 5TGM1-luc MM cells into mice revealed that those previously co-cultured with pre-adipocytes more rapidly homed to bone and grew larger tumors compared to 5TGM1-luc MM cells cultured alone, whereas the MM cells cultured with mature adipocytes showed no significant increase in either bone homing or growth. Analysis of pre-adipocyte and mature adipocyte CM by cytokine/chemokine arrays demonstrated that pre-adipocytes secrete significantly more HGF, MCP-1, OPN and SDF-1α compared to mature adipocytes. Migration assays in which pre-adipocyte or mature adipocyte CM was used as a chemoattractant indicated that MM cells migrate significantly more towards both pre-adipocyte and mature adipocyte CM than fresh media. However, the pre-adipocyte CM exhibited significantly more chemoattraction than mature adipocyte CM. Furthermore, addition of an MCP-1 or SDF-1α neutralizing antibody to both pre-adipocyte CM and mature adipocyte CM resulted in significantly reduced migration of MM cells. However, pre-adipocyte CM required higher concentrations of antibodies than mature adipocyte CM, indicating a higher concentration of MCP-1 and SDF-1α in pre-adipocyte CM. MM cells also exhibited a significant dose-dependent migration towards recombinant pre-adipocyte factor-1 (Pref-1), a marker of pre-adipocytes that is down-regulated during adipogenesis. Finally, Western blots revealed that co-culture of MM cells with pre-adipocytes resulted in activation of β-catenin signaling, which is important for cell proliferation, survival and motility, in MM cells.

Conclusions: These data indicate that adipocyte lineage cells play active but differentiation-dependent roles in MM progression, likely via the secretion of soluble factors. Both pre-adipocytes and mature adipocytes directly attract MM cells by secreting chemoattractants such as MCP-1 and SDF-1α. Interestingly, our data identify pre-adipocytes, and not mature adipocytes, as the main driver of the aggressive bone phenotype of MM cells.  In sum, these data suggest that an increase in adipocyte lineage cells in the bone marrow at distant bone sites could feed back to MM cells and support MM dissemination to these distant bone sites. Studies to determine the intricacies of this novel role of pre-adipocytes in MM are currently ongoing.