Osteoblastic differentiation is enhanced by rapamycin in rat osteoblast-like osteosarcoma (ROS 17/2.8) cells.

The effects of three immunosuppressants (rapamycin, FK506 and cyclosporin A) on the proliferation and differentiation of rat osteoblasts-like osteosarcoma cell line, ROS 17/2.8 (ROS) cells were examined in vitro. All immunosuppressants showed a direct inhibition on the proliferation of ROS cells with different potencies. Growth inhibition by rapamycin was stronger than that by FK506 or cyclosporin A. Rapamycin caused a significant increase in alkaline phosphatase (ALP) activity and in the expression of osteopontin and osteocalcin mRNAs. FK506 caused a moderate increase in ALP activity and a decreased expression of osteopontin mRNA. Cyclosporin A caused a decrease in ALP activity and in the expression of type 1 alpha 1 collagen mRNA. Our study indicates that rapamycin directly acts on ROS cells and induces osteoblastic differentiation, however, the effect of FK506 and cyclosporin A is weak. Rapamycin significantly enhances the differentiation induced by 1,25(OH)2-vitaminD3.