Immunogold localization of beta 1-integrin in bone: effect of glucocorticoids and insulin-like growth factor I on integrins and osteocyte formation.
Insulin-like growth factor I (IGF-I) and high-dose glucocorticoids exert 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 electron microscopic (TEM) method was developed and applied to an organ culture system of 20-day fetal rat parietal bones, which mineralize in vitro. In parietal bone culture, 100 mM corticosterone treatment for 72 hr decreased calcification by 29%, disrupted osteoblast organization, and decreased the number of osteocytes. The quantity of osteoblast processes and the number of osteocytes per unit bone area were decreased by 48% and 56%, respectively. This effect was dose-dependent. The beta 1-integrin subunit was localized equally to apical and basal osteoblast surfaces by immunogold TEM. Compared to untreated control cultures, corticosterone (100 nM) decreased beta 1 by one third. In contrast, treatment with IGF-I for 72 hr increased calcification by 38%, cell processes by 71%, and osteocytes per unit area of bone by 107%. The number of gold particles localizing beta 1 on the osteoblast plasma membrane 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 integrin levels on osteoblasts, and influence osteocyte formation and bone calcification. However, neither glucocorticoids nor IGF-I alter beta 1-integrin levels on osteocytes.
Gohel
A R
AR
Department of Orthopaedics, University of Connecticut Health Center, Farmington, USA.
Hand
A R
AR
Gronowicz
G A
GA
D000818:Animals; D001842:Bone and Bones / Q000166:cytology* / Q000187:drug effects / Q000378:metabolism; D002118:Calcium / Q000378:metabolism*; D003345:Corticosterone / Q000494:pharmacology*; D004305:Dose-Response Relationship, Drug; D005333:Fetus; D007150:Immunohistochemistry / Q000379:methods; D007334:Insulin-Like Growth Factor I / Q000494:pharmacology*; D019012:Integrin beta1 / Q000032:analysis* / Q000096:biosynthesis*; D008854:Microscopy, Electron; D016253:Microscopy, Immunoelectron; D009924:Organ Culture Techniques; D010011:Osteocytes / Q000166:cytology* / Q000187:drug effects / Q000648:ultrastructure; D051381:Rats; D017207:Rats, Sprague-Dawley
D019012:Integrin beta1; D007334:Insulin-Like Growth Factor I; D002118:Calcium; D003345:Corticosterone