Soluble Factors from Human Saos-2 Osteosarcoma Cells Induce Ectopic Bone Formation and Osteoblastic Differentiation of Cultured Mesenchymal Cells

Freeze-dried cells of the human osteosarcoma cell line Saos-2 have the capacity to induce bone formation. We studied the bone-inductive capacity of factors expressed by Saos-2 cells and secreted to a growth medium. When lyophilized Saos-2-conditioned medium was implanted in a mouse hamstring muscle pouch, new bone formation was observed. Unlike the radially symmetric cartilage usually seen in induced ossicles, an asymmetric morphology of the newly formed bone was observed, with hyaline cartilage on one side and fibrocartilage on the other side. The new bone area resembled histologically osteochondroma, a benign bone tumor, with flat chondrocyte-like cells arranged in rows, endochondral ossification and red bone marrow. Osteoinductivity was further confirmed in in vitro tests with mesenchymal cell lines. A serum-free ten-fold concentrated Saos-2 growth medium increased mineralization, measured as 45Ca incorporation, and also stimulated alkaline phosphatase activity in cultured rat myoblasts and mouse embryo cells. These in vitro results indicate that Saos-2 cells secrete factors, among which is BMP-2/4, which stimulate mesenchymal cell differentiation to osteoblastic lineage.