Interactions between microenvironment and cancer cells in two animal models of bone metastasis Running title: Bone microenvironment and cancer cells

The preferential proliferation of cancer cells in the bone microenvironment is poorly characterized. Expression pattern of bone marrow and other organs microenvironment in contact with osteolytic (Walker W256) and osteoblastic (MatLyLu MLL) metastases were investigated. Fisher and Copenhagen rats received respectively W256 and MLL cells injection. Bone and soft tissues were analyzed by immunochemistry for DKK1, cathepsin K, RANKL, MCSF, or IL6 expression. TRAcP+ cells were detected by a histoenzymatic technique. In bone, expressions of MCSF and DKK1 were shown in stromal cells of the bone marrow, in contact with metastatic foci of both tumors. Many stromal cells were found RANKL+ in the vicinity of the tumors. Cells expressing cathepsin K and multinucleated TRAcP+ cells were found in direct contact with trabeculae but also in bone marrow spaces near metastatic cells. In extraosseous tumors, cells in contact with malignant cells did not expressed DKK1, MCSF, cathepsin K, IL6. Some RANKL+ cells were found in the periphery of subcutaneous tumors but may represent Langerhans cells. Abnormal presence of TRAcP+ cells was never observed in the vicinity of malignant cells. Interaction between stromal and cancer cells induces the expression on the formers of characteristics leading to osteoclastogenesis only in the bone microenvironment.