Bone morphogenetic proteins (BMP-2 and BMP-3) promote growth and expression of the differentiated phenotype of rabbit chondrocytes and osteoblastic MC3T3-E1 cells in vitro

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-β. 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-β 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 ˜80 kD PTH receptors expressed on the cell surface. In multilayered chondrocytes at the calcifying stage, BMPs stimulated alkaline phosphatase (ALPase) activity but TGF-β 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 ˜100 ng/ml of BMPs reached 5- to 20-fold over the basal, whereas TGF-β inhibited expression of ALPase activity in these cells. The stimulatory action of BMPs overrode the inhibition of ALPase activity by TGF-β when the cells were incubated with TGF-β and BMPs. BMPs also upregulated expression of the ˜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.