IGF-I MEDIATES THE STIMULATORY EFFECT OF HIGH-CALCIUM CONCENTRATION ON OSTEOBLASTIC CELL-PROLIFERATION

Since our recent study revealed that an increase in extracellular calcium ([Ca2+](e)) but not magnesium enormously stimulated DNA synthesis in osteoblastic MC3T3-E1 cells at the minimal and maximal effective concentration of 3 and 5 mM, respectively, the present study was performed to clarify how an increase in [Ca2+](e) caused a stimulation of DNA synthesis of these cells. Neither calcium channel blockers (verapamil, diltiazem, and nifedipine) and dantrolene, an inhibitor of Ca release from intracellular Ca pool, nor indomethacin, an inhibitor of prostaglandin synthesis, affected the high [Ca2+](e)-induced increase in DNA synthesis. DNA synthesis first increased after a 12-h exposure to 5 mM [Ca2+](e), and cycloheximide eliminated the stimulatory effect of high [Ca2+](e) on DNA synthesis, suggesting that this stimulatory effect of high [Ca2+](e) was dependent on new protein synthesis. There is recent evidence that MC3T3-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) blocked the high [Ca2+](e)-induced increase in DNA synthesis in a concentration-dependent manner. A neutralizing IGF-I antibody as well as IGF-I receptor antibody completely abolished DNA synthesis stimulated by high [Ca2+](e), indicating that IGF-I mediated the high [Ca2+](e)-induced effect. Furthermore, high [Ca2+](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 [Ca2+](e) stimulated DNA synthesis of osteoblasts through the mechanism of an increase in the production and secretion of IGF-I.