Extracellular matrix-associated bone morphogenetic proteins are essential for differentiation of murine osteoblastic cells in vitro

Osteoblastic differentiation is an essential part of bone formation that co mpensates resorbed bone matrix to maintain its structural integrity. Cells in an osteoblast lineage develop differentiated phenotypes during a long-te rm culture in vitro. However, intrinsic mechanisms whereby these cells diff erentiate into mature osteoblasts are yet unclear. Bone morphogenetic prote ins (BMPs) stimulate osteoblastic differentiation and bone formation. We de monstrate that mouse osteoblastic MC3T3-E1 cells constitutively expressed m essenger RNAs (mRNAs) for BMP-2 and BMP-4 and accumulated BMPs in collagen- rich extracellular matrices. BMPs associated with the extracellular matrice s were involved in the induction of osteoblastic differentiation of nonoste ogenic mesenchymal cells as well as cells in the osteoblast lineage. MC3T3- E1 cells constitutively expressed type LA and type II BMP receptors. When a kinase-deficient type IA BMP receptor was stably transfected to MC3T3-E1 c ells to obliterate BMP-2/4 signaling, these cells not only failed to respon d to exogenous BMP-2 but lost their capability of differentiation into oste oblasts that form mineralized nodules. These observations strongly suggest that endogenous BMP-2/4 accumulated in extracellular matrices are essential for the osteoblastic differentiation of cells in the osteoblast lineage. T herefore, the regulatory mechanism of BMP-2/4 actions in osteoblastic cells is a principal issue to be elucidated for better understanding of pathogen esis of bone losing diseases such as osteoporosis.