Functional osteoclast-like transformation of cultured human myeloma cell lines

Hyperactive osteoclastogenesis is a hallmark of multiple myeloma, a B cell neoplasia homing to bone marrow and resulting in multiple osteolytic lesions and skeleton devastation. We provide evidence that myeloma cells can themselves act as osteoclasts in vitro. By extending standard cultures of U-266 and MCC-2 myeloma cell lines, we found that subsets of adherent cells also expressed the osteoclast phenotype, including multinuclear morphology, cytoplasmic tartrate-resistant acid phosphatase, the calcitonin receptor and a specific osteoclast antigen. These subsets resorbed bone substrates by producing osteoclast enzymes as well as the characteristic redistribution of F-actin in their cytoskeleton, thus forming the sealing zone that is adopted by adherent osteoclasts to generate the acidified environment essential for bone resorption. Neither the phenotype nor the functional properties of osteoclasts were detected in parental non-adherent cells. In adherent cultures osteoclastogenesis was associated with deregulated expression of both receptor activator of nuclear transcription factor (NF)-kappaB (RANK) and its ligand RANK-L, which triggers cell maturation in osteoclast precursors. Resorption of bone substrates was prevented by a neutralising anti-RANK-L antibody. Our data indicate that osteoclast-like transformation of both U-266 and MCC-2 cellular models of human myeloma is dependent on RANK-L stimulation.