Chinese hamster ovary cells expressing α4β1 integrin stimulate osteoclast formation in vitro

It is reported that Chinese hamster ovary cells transfected with human alpha(4) cDNA (alpha(4)CHOs) and expressing functional alpha(4)beta(1) integrin developed bone metasasis in nude mice. To clarify the role of alpha(4)beta(1) integrin in bone metastasis, in terms of tumor-mediated bone destruction, we examined whether alpha(4)CHOs 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 hom bone marrow cells increased with the increasing number of alpha(4)CHOs cocultured, The effects of 1,25-dihydroxyvitamin D-3 (1,25(OH)(2)D-3) and prostaglandin E-2 (PGE(2)) on TRAP(+) MNC formation were enhanced in cocultures with alpha(4)CHOs. TRAP(+) MNCs induced by alpha(4)CHOs possessed calcitonin receptors and resorbed calcified tissues. In cocultures, alpha(4)CHOs 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 alpha(4)beta(1) integrin, TRAP(+) MNC formation was not stimulated in cocultures where direct contact between alpha(4)CHOs and bone marrow cells was inhibited by membrane filters. alpha(4)CHOs do not support TRAP(S) 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 alpha(4)CHOs, bone marrow cells, and cocultures of alpha(4)CHOs and bone marrow cells did not stimulate TRAP(+) MNC formation or enhance the effects of 1,25(OH)(2)D-3 and PGE, in bone marrow cultures. The concentrations of PGE, 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 alpha(4)CHOs. Anti-human cu, and anti-mouse VCAM-1 antibodies inhibited TRAP(+) MNC formation induced by alpha(4)CHOs. These results indicate that alpha(4)CHOs stimulated TRAP(+) MNC formation through direct cell-to-cell interaction between alpha(4)beta(1) 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.