Abstract

We studied the effects of highly purified bone morphogenetic protein 2 and 3 (BMP-2 and -3) on growth plate chondrocytes and osteoblastic cells in vitro and compared to TGF-beta. A mixture of BMP-2 and 3 (BMPs) strongly stimulated DNA synthesis of chondrocytes in the presence of fibroblast growth factor (FGF). BMPs induced rapid maturation of chondrocytes at a growing stage: BMPs transformed the cells into rounded cells and induced marked accumulation of cartilage matrix; TGF-beta slightly reduced matrix accumulation and changed cell morphology into spindle-like in the presence of FGF. Moreover, exposure of chondrocytes to BMPs resulted in a dramatic increase of the putative approximately 80 kD PTH receptors expressed on the cell surface. In multilayered chondrocytes at the calcifying stage, BMPs stimulated alkaline phosphatase (ALPase) activity but TGF-beta inhibited it. In osteoblastic MC3T3-E1 cells, BMPs were found to be the most potent stimulator of ALPase activity thus far described: ALPase in the cells treated with approximately 100 ng/ml of BMPs reached 5- to 20-fold over the basal, whereas TGF-beta inhibited expression of ALPase activity in these cells. The stimulatory action of BMPs overrode the inhibition of ALPase activity by TGF-beta when the cells were incubated with TGF-beta and BMPs. BMPs also upregulated expression of the approximately 80 kD PTH receptor on the cells. These results suggest that BMPs have unique biologic activities in vitro that lead to growth and phenotypic expression of cells playing a critical role in endochondral bone formation.