IGF-I mediates the stimulatory effect of high phosphate concentration on osteoblastic cell proliferation

Although high concentrations of inorganic phosphate (Pi) are known to have a distinct anabolic effect on bone structure and metabolism, the precise mechanism by which phosphate possesses anabolic effect on bone formation has not been elucidated. The present study was performed to examine the effects of an increase in extracellular Pi concentration (Pi](e)) on the proliferation of osteoblastic MC3T3-E1 cells. Increase in Pi](e)(2-4 mM) dose-dependently stimulated DNA synthesis. Indomethacin, an inhibitor of prostaglandin synthesis, did not affect high Pi](e)-induced DNA synthesis. DNA synthesis first increased affer a 3 h exposure to 4 mM Pi](e) and its stimulatory effect was observed in a time-dependent manner up to 24 h. On the other hand, DNA synthesis was significantly but partially blocked by cycloheximide, suggesting that this stimulatory effect of high Pi](e) was at least in part dependent on new protein synthesis. There is recent evidence that MG3T3-E1 cells constitutively produce and secrete insulin-like growth factor-I (IGF-I) and possess IGF-I receptors. IGF-I antiserum (1:10,000 to 1:100) significantly but partially blocked the stimulatory effect of Pi](e) (4 mM) on DNA synthesis in a concentration-dependent manner. A neutralizing IGF-I antibody as well as IGF-I receptor antibody also significantly but partially blocked DNA synthesis stimulated by high Pi](e) in a concentration-dependent manner, indicating that IGF-I at least in part mediated the high Pi](e)-induced effect. Actually, high Pi](e) significantly increased the secretion of immunoreactive IGF-I into the medium as well as the expression of IGF-I mRNA. Present findings indicate that an increase in Pi](e) stimulated DNA synthesis partly via an increase in IGF-I action.