IMMUNOGOLD LOCALIZATION OF BETA(1)-INTEGRIN IN BONE - EFFECT OF GLUCOCORTICOIDS AND INSULIN-LIKE GROWTH-FACTOR-I ON INTEGRINS AND OSTEOCYTEFORMATION

Insulin-like growth factor I (IGP-I) and high-dose glucocorticoids exe rt opposite effects on bone formation. Because integrins are involved in cell and matrix organization, the effect of glucocorticoids and IGF -I on integrins was investigated in bone. An immunogold transmission e lectron microscopic (TEM) method was developed and applied to an organ culture system of 20-day fetal rat parietal bones, which mineralize i n vitro. In parietal bone culture, 100 mM corticosterone treatment for 72 hr decreased calcification by 29%, disrupted osteoblast organizati on, and decreased the number of osteocytes, The quantity of osteoblast processes and the number of osteocytes per unit bone area were decrea sed by 48% and 56%, respectively, This effect was dose-dependent. The beta(1)-integrin subunit was localized equally to apical and basal ost eoblast surfaces by immunogold TEM. Compared to untreated control cult ures, corticosterone (100 nM) decreased beta(1) by one third. In contr ast, treatment with IGF-I for 72 hr increased calcification by 38%, ce ll processes by 71%, and osteocytes per unit area of bone by 107%, The number of gold particles localizing Pr on the osteoblast plasma membr ane doubled, almost entirely on the apical surface of the osteoblast. Glucocorticoids and IGF-I had no significant effect on beta(1) levels in osteocytes. In conclusion, glucocorticoids and IGF-I modulate integ rin levels on osteoblasts, and influence osteocyte formation and bone calcification. However, neither glucocorticoids nor IGP-I alter beta(1 )-integrin levels on osteocytes.