Introduction: Osseous metastases from melanoma are relatively common (7% of cases), and occur most often in the axial skeleton. Bone destruction in skeletal metastases of solid tumours is due to stimulation of osteoclast formation and bone resorption. Osteoclasts are formed by the fusion of marrow-derived mononuclear phagocyte precursors which express RANK (receptor activator of nuclear factor κB) which interacts with RANKL expressed by osteoblasts/bone stromal cells in the presence of macrophage colony-stimulating factor (M-CSF). Osteoclast formation by a RANKL-independent, tumour necrosis factor α (TNFα)-induced mechanism has also been reported. Tumour-associated macrophages (TAMs) are present in both primary and secondary tumours and TAMs are known to be capable of osteoclast differentiation. Our aim in this study was to determine the role of TAMs and the humoral mechanisms of osteolysis associated with melanoma metastases.

Materials and Method: In this study we isolated TAMs from extraskeletal primary melanomas and lymph node metastases. TAMs were cultured for up to 21 days in the presence and absence of M-CSF and RANKL or TNF. In a separate experiment, conditioned medium was extracted from the melanoma cell line, SK-Mel-29, and cultured with human peripheral blood mononuclear cells in the presence of M-CSF.

Results: TAM-osteoclast differentiation, as evidenced by the expression of tartrate-resistant acid phosphatase, vitronectin receptor and lacunar resorption pit formation, occurred by both RANKL-dependent and RANKL-independent mechanisms. Osteoclast formation induced by RANKL-independent mechanism was not abolished by the addition of osteoprotegerin or RANK:Fc, decoy receptors for RANK. Conditioned medium from SK-Mel-29 could support osteoclast differentiation in the absence of RANKL. This effect was not abolished by antibodies to RANKL, TNFα, TGFβ, IL-8 or gp130.

Discussion: These results indicate that melanoma TAMs are capable of differentiation into osteoclasts and that both RANKL-dependent and RANKL-independent (TNFα) mechanisms are involved. Melanoma tumour cells also secrete a soluble factor that supports osteoclastogenesis.

Conclusion: Inhibitors of osteoclast formation targeting TAM-osteoclast differentiation and osteoclast activity and identification of the osteoclastogenic factor produced by melanoma cells may have a therapeutic potential in controlling tumour osteolysis.