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

Since our recent study revealed that an increase in extracellular calc ium ([Ca2+](e)) but not magnesium enormously stimulated DNA synthesis in osteoblastic MC3T3-E1 cells at the minimal and maximal effective co ncentration of 3 and 5 mM, respectively, the present study was perform ed 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 releas e from intracellular Ca pool, nor indomethacin, an inhibitor of prosta glandin synthesis, affected the high [Ca2+](e)-induced increase in DNA synthesis. DNA synthesis first increased after a 12-h exposure to 5 m M [Ca2+](e), and cycloheximide eliminated the stimulatory effect of hi gh [Ca2+](e) on DNA synthesis, suggesting that this stimulatory effect of high [Ca2+](e) was dependent on new protein synthesis. There is re cent evidence that MC3T3-E1 cells constitutively produce and secrete i nsulin-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 incr ease in DNA synthesis in a concentration-dependent manner. A neutraliz ing IGF-I antibody as well as IGF-I receptor antibody completely aboli shed 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 i ndicate that an increase in [Ca2+](e) stimulated DNA synthesis of oste oblasts through the mechanism of an increase in the production and sec retion of IGF-I.