THE EFFECTS OF BONE MORPHOGENETIC PROTEIN-2,PROTEIN-4, AND PROTEIN-6 ON DIFFERENTIATION OF RAT OSTEOBLAST CELLS IN-VITRO

The effects of bone morphogenetic protein-2 (BMP-2), -4, and -6 were t ested on the differentiation of rat osteoprogenitor cells using a bone nodule-forming assay system, and the kinetics of their actions were i nvestigated by double labeling for alkaline phosphatase (ALP) and brom odeoxyuridine (BrdU) uptake in log phase cultures. All BMPs stimulated bone nodule formation, with an optimal concentration of 25 ng/ml resu lting in nodule numbers of approximately 250% of controls using BMP-4 and -6. BMP-2 showed reduced potency compared to either BMP-4 or -6. N o evidence of chondrocytic differentiation was found in any of the cul tures. The effect of BMPs on nodule formation was seen after only 24 h of exposure to BMPs, but only affected nodule numbers when added to e arly cultures. Nodule size and number of cells per nodule were increas ed with BMP6 only.Continuous or 24-h exposure to BMP-2 or -4 increased the number of postmitotic ALP-positive cells in log phase cultures, w hereas BMP-6 increased the number of postmitotic ALP-negative cells. T he results demonstrate that BMP-6, like other BMPs, can stimulate oste oblast differentiation independent of any chondrogenic effects and sug gest that an early osteoprogenitor cell is an important target cell fo r the action of BMPs during bone induction. Overall, BMP-2 and -4 show ed differences in potency in the assay systems used, but had qualitati vely similar effects. In contrast, the qualitative differences found w ith BMP-6 suggest that BMP-6 may be acting principally on an early sta ge osteoprogenitor cell.