Polypeptide growth factor belong to a class of potent biologic mediator which regulate cell differentiation, proliferation, migration and metabolism. 1,25-dihydroxyvitamin D₃ decrease cell proliferation, and stimulate alkaline phosphatase activity which express in osteoblast during cell differentiation period. IGF-I is known to stimulate cell proliferation and differentiation too. 1,25-dihydroxyvitamin D₃ is known to increase IGF-I binding sites and IGF binding protein which inhibite the effect of IGF. The purpose of this study is to evaluate potential role of IGF-I as mediator that control the action of 1,25-dihydroxyvitamin D₃.

MC3T3-E1 cell were seeded 5×10⁵/ml at 100mm culture plate in α-MEM containing 10% fetal bovine serum. After 48 hour incubation period, medium were changed α-MEM containing 5% fetal bovine serum. After 24 hours, 10^-9M 1,25-dihydroxyvitamin D₃ added. Total mRNA was extracted at 0, 6, 24, 48, 72 hour. PR-PCR method was programed for the detection of IGF-I mRNA. In the both groups of 1,25-dihydroxy vitamin D₃ treated and control, alternative splicing form of IGF-I, IGF-IA and IGF-IB were expressed. In the 1,25-dihydroxyvitamin D₃ treated group, IGF-I mRNA expression was matained until 24 hour, there after expression was decresed.

MC3T3-E1 cell were seeded 2.5×10⁴/ml at 24well plate in α-MEM containing 10% fetal bovine serum. After 48 hour incubation period, medium were changed α-MEM containing 3% fetal bovine serum. After 24 hours, 10^-9M 1,25-dihydroxyvitamin D₃ and 10 ng/ml IGF-I were added separately or together. Cell were cultured for 1 and 3 days, 2µCi/ml [³H]-thymidine was added for the last 24h of culture of each days. [³H]-thymidine incorporation in to DNA was measured and expressed counter per minute(CPM).

DNA synthetic activity was significantly decreased by 1,25-dihydroxyvitamin D₃ both at 1 day and 3 day, and in the combination group of 1,25-dihydroxyvitamin D₃ and IGF-I, DNA synthetic activity was also decreased both at 1 day and 3 days. IGF-I did not affect the DNA synthetic activity compared to control group both at 1 day and 3 day.

From the above results, 1,25-dihydroxyvitamin D₃ was potent inhibitor of cell proliferaton in MC3T3-E1 cells. It assumed that the effect of 1,25-dihydroxyvitamin D₃ on osteoblast proliferation may be mediated in part by decreased level of IGF-I.