TIME-DEPENDENT EXPRESSION OF BONE SIALOPROTEIN FRAGMENTS IN OSTEOGENESIS INDUCED BY BONE MORPHOGENETIC PROTEIN

Bone sialoprotein is unique to bone and dentin, but its precise role i n these tissues is still unknown, although several hypotheses have bee n presented, We chose ectopic chondro- and osteogenesis induced by bon e morphogenetic protein (BMP) as a model system to examine the role of this protein. Partially purified bovine BMP obtained by a three-step chromatographic procedure contained all the active BMPs (natural BMP c ocktail). It was combined with insoluble bone matrix and subcutaneousl y implanted into rats, Expression of bone sialoprotein (BSP) in the im plants was followed by using a monoclonal antibody as previously repor ted, Immunostaining studies showed BSP in the osteoblasts lining the n ew bone surface at 5 weeks. Western blotting showed 53 and 30 kDa band s, instead of the 57 kDa band normally found in rat femur, These two f ragments were metabolically labeled with [H-3] proline. The total amou nt of the fragments rapidly increased after 3 weeks, and at 5 weeks wa s 3 times as high as that at 2 weeks and still increasing. This time-d ependent change was almost parallel to that of osteocalcin, The amount of bone estimated in terms of calcium content increased until 3 weeks and was remained at a plateau thereafter. Alkaline phosphatase activi ty was prominent only in the first 3 weeks. It was concluded that the 53 and 30 kDa BSP fragments might contribute to maintenance or remodel ing in BMP-induced ectopic bone formation.