GLUCOCORTICOIDS INHIBIT THE ATTACHMENT OF OSTEOBLASTS TO BONE EXTRACELLULAR-MATRIX PROTEINS AND DECREASE BETA(1)-INTEGRIN LEVELS

Prolonged glucocorticoid treatment causes osteoporosis in vivo and inh ibits bone formation in vitro. We have previously shown that glucocort icoids inhibit calcification and alter osteoblast organization in a mi neralizing bone organ culture system. In this study, the effect of glu cocorticoids on osteoblast adhesion to bone matrix proteins and integr in expression was examined in primary rat osteoblasts and a transforme d rat osteosarcoma-derived cell line ROS 17/2.8. After 24 h of treatme nt with corticosterone, these cells displayed a concentration-dependen t decrease in adhesion to type I collagen and fibronectin. Adhesion wa s significantly decreased as early as 4 h after glucocorticoid adminis tration. With 100 nM corticosterone treatment for 24 h, inhibition of the adhesion of ROS 17/2.8 cells and primary osteoblasts to fibronecti n was 75 +/- 10% and 50 +/- 8%, and inhibition of adhesion to collagen was 31 +/- 10% and 65 +/- 5%, respectively. This effect was specific for osteoblasts, because glucocorticoids did not change the adhesion o f fibroblasts. However, glucocorticoids did inhibit the adhesion of al l cell types to rat osteonectin. To determine whether the change in os teoblast attachment to collagen and fibronectin was due to an alterati on in integrin levels, the plasma membranes of these cells were labele d with [I-125]lactoperoxidase, solubilized, and immunoprecipitated wit h an antibody to beta(1). A 24-h treatment with 100 nM corticosterone caused 80 +/- 2% and 64 +/- 9% decreases in beta(1) levels in primary osteoblasts and ROS 17/2.8 cells, respectively. These results were con firmed with immunofluorescence microscopy, which showed a glucocortico id-induced decrease in beta(1) staining. Treatment of primary rat oste oblasts and ROS 17/2.8 cells for 72 h with corticosterone also decreas ed beta(1)-integrin messenger RNA levels in a dose-dependent manner. W e have demonstrated that the inhibition of integrin expression by gluc ocorticoids is involved in the decrease in osteoblast adhesion to bone extracellular matrix proteins. These data suggest that integrin modul ation may influence osteoblast function and bone formation and, thus, contribute to glucocorticoid-induced osteoporosis.