Osteoblastogenesis and tumor growth in myeloma

Abstract

Myeloma is associated with suppression of osteoblastogenesis, consequentially resulting in increased osteoclast activity and induction of typical osteolytic bone disease. The molecular mechanisms by which myeloma cells suppress osteoblastogenesis and the consequences of increased osteoblast activity on myeloma cell growth have been partially delineated only recently. Reduced osteoblastogenesis is a consequence of abnormal properties and impaired osteogenic potential of osteoprogenitor cells from myeloma patients and is also the result of production of multiple osteoblastogenesis inhibitors by myeloma cells and by microenvironmental cells within the myelomatous bone. Nevertheless, novel osteoblast-activating agents (e.g. proteasome inhibitor bortezomib) are capable of inducing bone formation in myeloma animal models and clinically. These agents induce increased osteoblast activity, often coupled with a concomitant reduction in osteoclastogenesis, that is strongly associated with reduced myeloma tumor burden. In vitro, osteoblasts, in contrast to osteoclasts, attenuate the growth of myeloma cells from a large subset of patients; potential molecular mechanisms are discussed. These studies suggest that myeloma cells suppress osteoblastogenesis to their advantage and that increased osteoblast activity is a promising approach to treat myeloma bone disease and simultaneously control myeloma development and progression.

Figure 1

Interactions of myeloma cells with osteoblasts and their precursors. Soluble factors (e.g. DKK1) and dysregulated cell-surface molecules (e.g. ephrinB2/EphB4) are involved in MM-induced suppression of osteoblastogenesis and subsequent stimulation of osteoclastogenesis. Osteoblast precursors and osteoclasts provide sanctuary to and stimulate growth of myeloma cells through cell–cell contact interactions and production of growth factors. Osteoblast-activating agents (e.g. bortezomib, anti-DKK1) increase the number of bone-building osteoblasts, resulting in normalization of osteoclastogenesis through increased production of anti-osteoclastogenic factors (e.g. OPG). Bone-building osteoblasts attenuate myeloma cell growth by increasing levels of potential anti-myeloma and anti-angiogenic factors (e.g. certain SLRPs) in the MM bone marrow microenvironment.