Role of osteopontin in bone remodeling caused by mechanical stress

Changes in the number and proportion of osteopontin mRNA (Opn) expressing osteocytes and osteoclasts caused by the mechanical stress applied during experimental tooth movement were examined in the present study. Opn expression was detected in the osteocytes on the pressure side at the early stage, and gradually spread to those on the tension side and also to the osteoblasts and bone-lining cells in the alveolar bone. Only 3.3% of the osteocytes located on the pressure side expressed Opn in the interradicular septum of control rats; in contrast, the value was increased to 87.5% at 48 h after the initiation of tooth movement. These results indicate that these cells responded to mechanical stress loaded on the bone with expression of the osteopontin gene. Following the increased expression of Opn in these cells, a 17-fold greater number of osteoclasts compared with the control and numerous resorption pits were observed on the pressure side of the alveolar bone. Injection of arginine-glycine-aspartic acid-serine peptide but not that of arginine-glycine-glutamic acid-serine peptide strongly inhibited the increase in the number of osteoclasts. Furthermore, an in vitro migration assay demonstrated the chemotactic activity of osteopontin (OPN) on the precursor of osteoclasts. Our study strongly suggests that OPN is an important factor triggering bone remodeling caused by mechanical stress.