Osteopontin‐Deficiency Suppresses Growth of B16 Melanoma Cells Implanted in Bone and Osteoclastogenesis in Co‐Cultures

Tumor metastasis and invasion to bone is one of major medical issues in our modern societies. Osteopontin deficiency decreased tumor invasion in bone based on knockout mouse study. In bone, osteopontin is a positive factor to increase tumor invasion. Introduction: Osteopontin is an arginine‐glycine‐aspartate (RGD)‐containing protein and is recognized by integrin family members. Osteopontin promotes cell attachment to bone, where it is abundantly present. Because osteopontin levels were reported to be elevated in patients bearing highly metastatic tumors, this molecule has been implicated in the metastasis of tumors. However, the effect of osteopontin on the invasion of tumor cells in bone microenvironment has not been clear. The purpose of this paper is to elucidate the effect of host osteopontin on the behavior of tumor cells in bone. Materials and Methods: Bone marrow ablation was conducted in the femora of mice, and B16 melanoma cells were injected directly into the ablated bone marrow space of the osteopontin‐deficient and wildtype mice. Result: Invasion foci of B16 melanoma cells in the cortical bone was observed 7 weeks after tumor cell implantation. The number of the foci was 5‐fold less in osteopontin‐deficient mice compared with that in wildtype mice. In wildtype mice, trabecular bone formation was not observed in the ablated marrow space where tumor cells were injected. In contrast, significant levels of trabecular bone were observed in the marrow space of osteopontin‐deficient mice even after tumor cells were injected. To examine cellular mechanisms underlying these observations, co‐cultures of bone marrow cells and B16 cells were conducted. While the presence of B16 cells promoted TRACP+ cell development in wildtype bone marrow cells, such enhancement in TRACP+ cell formation by the co‐cultures with B16 cells was reduced in the case of bone marrow cells from osteopontin‐deficient mice. Conclusions: Osteopontin deficiency reduced the bone loss caused by tumor cell implantation into the bone marrow space.