The expression of metalloproteinase-2,-9, and-14 and of tissue inhibitors-1 and-2 is developmentally modulated during osteogenesis in vitro, the mature osteoblastic phenotype expressing metalloproteinase-14

During osteogenesis, in vitro, of tibial-derived rat osteoblasts (ROB) and derived clones, changes occur in the interactions of mature osteoblasts wit h the endogenous extracellular matrix (ECM) and these culminate in the form ation of tridimensional nodules, which become sites of mineral deposition. We investigated if these changes might be mediated by remodeling of ECM, an d we focused our sandy on the neutral metalloproteinases (MMPs), known agen ts of matrix remodeling, and on their tissue inhibitors (TIMPs). We report that during in vitro differentiation, osteoblasts express the secreted MMP- 2 and -9 and the membrane gelatinase MMP-14. These, along with the tissue i nhibitors TIMP-1 and -2, are developmentally regulated according to the mat uration stage of osteoblasts. Their levels change in a similar association with osteoblast phenotypic maturation in different populations of ROB, whic h take different times to complete osteogenesis in vitro. MMP-14 expression coincides in both cell populations with the mature osteoblastic phenotype and is localized in the cells forming nodules. MMP-2 and -9 are expressed d iffusely in the osteoblast population. Developmentally associated changes i n the activation of MMP-2 are detected, associated in their timing with the expression of MMP-14 in both populations of ROB, and MMP-14 activates pro- MMP-2 in vitro. Expression of messenger RNAs (mRNAs) for the three MMPs inc reases up to the time of nodule formation. At this stage, TIMP-1 mRNA level s are lowest. TIMP-2 mRNA decreases throughout osteogenesis. In situ hybrid ization in 7-day-old rat tibias shows the strongest expression of MMP-14 am ong osteogenic cells, in lining osteoblasts on the ne newly formed trabecul ae under the growth plate, and on the endosteal surface of cortical bone. O ur data support the concept that the developmentally regulated expression o f MMP-14 triggers localized proteolysis within the osteogenic population, c oncomitant in vitro to nodule formation.