Requirement of a bone morphogenetic protein for the maintenance and stimulation of osteoblast differentiation

The requirement of a bone morphogenetic protein for the maintenance and stimulation of an osteoblast phenotype was examined using mouse MC3T3-E1 cell cultures. Cells expressed BMP-4 mRNA, which correlated with the stimulation of the osteoblast phenotype. The addition of a BMP-4 specific antibody reduced bone nodules, suggesting that BMP-4 is required for the osteogenic activity of osteoblasts in an autocrine manner. Exogenously added BMP-7 gradually decreased the expression of BMP-4 with a concurrent stimulation of the osteoblast phenotype. Exogenous BMP-7 can therefore substitute for endogenously produced BMP-4 acting as a paracrine factor on osteoblasts. The addition of 17beta estradiol decreased BMP-4 expression but initiated synthesis of BMP-6 mRNA, an endocrine signal for osteoblasts, which also substituted for the lack of endogenous BMP-4, as evidenced by normal bone nodule formation. The addition of dexamethasone and parathyroid hormone did not affect the BMP-4 expression but induced transcripts for BMP-2 and BMP-3, respectively, suggesting that their effects on bone can be in part achieved via the BMP signaling. These experiments support the requirement of a BMP for osteoblast differentiation and function, demonstrating for the first time that a BMP can functionally substitute for another BMP in an autocrine/paracrine manner or mediate a response to an endocrine action on osteoblasts.