Selective synthesis of bone morphogenetic, proteins-1,-3,-4 and bone sialoprotein may be important for osteoinduction by Saos-2 cells

An ability to induce new bone formation at a required site would represent a considerable advance in bone repair and tissue engineering. It has been shown that the healing of critical-size bone defects in rats can be augmented by extracts of Saos-2 cells. These human ostcosarcoma cells uniquely contain a bone-inducing activity, whereas other human osteosarcoma cells, e.g., U-2 OS cells, cannot replicate the osteoinductive capacity. To understand the necessary components of the Saos-2 bone-inducing activity, this study compared osteoinductive Saos-2 cells with non-osteoinductive U-2 OS cells with respect to the synthesis of bone morphogenetic proteins (BMPs)- 1, -2, -3, -4, -5, -6, and -7 and the noncollagenous matrix proteins bone sialoprotein (BSP), osteonectin (ON), osteopontin (OPN), and osteocalcin (OC). The main differences were abundant synthesis of BMP-1/tolloid, BMP-3, -4, and BSP by Saos-2 cells, but absence or reduced synthesis in U-2 OS cells. BMP-2 and -7 were present in low amounts in both cell types, while BMP-5 and -6 were more abundant in U-2 OS cells, suggesting that these BMPs were of lesser importance for the osteoinductivity of Saos-2 cells. However, a relatively high expression of BMP-3 and -4, together with BMP-1/tolloid, may be important for the osteoinductive capacity of Saos-2 cells. The inability of U2-OS cells to induce bone, despite expressing most of the BMPs, may be due to an insufficiency of tolloid, BMP-3 or -4, BSP, and/or other unknown factors. A better understanding of the necessary components of the Saos-2 cell bone-inducing agent may, in future, lead to clinically useful Saos-2 cell products for bone repair and tissue engineering.