Chinese hamster ovary cells expressing α₄/β₁ integrin stimulate osteoclast formation in vitro

It is reported that Chinese hamster ovary cells transfected with human α₄ cDNA (α₄CHOs) and expressing functional α₄β₁ integrin developed bone metasasis in nude mice. To clarify the role of α₄β₁ integrin in bone metastasis, in terms of tumor-mediated bone destruction, we examined whether α₄CHOs stimulate osteoclast formation in cocultures with mouse bone marrow cells. The number of osteoclast-like cells identified as tartrate-resistant acid phosphatase positive multinucleated cells (TRAP(+) MNCs) formed from bone marrow cells increased with the increasing number of α₄CHOs cocultured. The effects of 1,25-dihydroxyvitamin D₃ (1,25(OH)₂D₃) and prostaglandin E₂ (PGE₂) on TRAP(+) MNC formation were enhanced in cocultures with α₄CHOs. TRAP(+) MNCs induced by α₄CHOs possessed calcitonin receptors and resorbed calcified tissues. In cocultures, α₄CHOs and bone marrow stromal cells were in contact with each other and bone marrow stromal cells expressed vascular cell adhesion molecule-1 (VCAM-1), which is one of the ligands for α₄β₁ integrin. TRAP(+) MNC formation was not stimulated in cocultures where direct contact between α₄CHOs and bone marrow cells was inhibited by membrane filters. α₄CHOs do not support TRAP(+) MNC formation in cocultures with spleen cells but do support TRAP(+) mononuclear cell and MNC formation from spleen cells in the presence of osteoblastic cells. Cultured media from α₄CHOs, bone marrow cells, and cocultures of α₄CHOs and bone marrow cells did not stimulate TRAP(+) MNC formation or enhance the effects of 1,25(OH)₂D₃ and PGE₂ in bone marrow cultures. The concentrations of PGE₂ and interleukin-6 (IL-6) in cultured media were not different between the cultures of bone marrow cells and the cocultures of bone marrow cells and α₄CHOs. Anti-human α₄ and anti-mouse VCAM-1 antibodies inhibited TRAP(+) MNC formation induced by α₄CHOs. These results indicate that α₄CHOs stimulated TRAP(+) MNC formation through direct cell-to-cell interaction between α₄β₁ and VCAM-1. It is suggested that in addition to various soluble factors regulating osteoclast formation, cell-to-cell interaction between tumor cells and bone marrow cells is important for inducing osteoclasts at the site of bone metastasis and leading to bone destruction.