The effects of bone morphogenetic protein-2, -4, and -6 on differentiation of rat osteoblast cells in vitro.

The effects of bone morphogenetic protein-2 (BMP-2), -4, and -6 were tested on the differentiation of rat osteoprogenitor cells using a bone nodule-forming assay system, and the kinetics of their actions were investigated by double labeling for alkaline phosphatase (ALP) and bromodeoxyuridine (BrdU) uptake in log phase cultures. All BMPs stimulated bone nodule formation, with an optimal concentration of 25 ng/ml resulting 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. No evidence of chondrocytic differentiation was found in any of the cultures. 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 early cultures. Nodule size and number of cells per nodule were increased 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, whereas BMP-6 increased the number of postmitotic ALP-negative cells. The results demonstrate that BMP-6, like other BMPs, can stimulate osteoblast differentiation independent of any chondrogenic effects and suggest that an early osteoprogenitor cell is an important target cell for the action of BMPs during bone induction. Overall, BMP-2 and -4 showed differences in potency in the assay systems used, but had qualitatively similar effects. In contrast, the qualitative differences found with BMP-6 suggest that BMP-6 may be acting principally on an early stage osteoprogenitor cell.