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 inhibits bone formation in vitro. We have previously shown that glucocorticoids inhibit calcification and alter osteoblast organization in a mineralizing bone organ culture system. In this study, the effect of glucocorticoids on osteoblast adhesion to bone matrix proteins and integrin expression was examined in primary rat osteoblasts and a transformed rat osteosarcoma-derived cell line ROS 17/2.8. After 24 h of treatment with corticosterone, these cells displayed a concentration-dependent decrease in adhesion to type I collagen and fibronectin. Adhesion was significantly decreased as early as 4 h after glucocorticoid administration. With 100 nM corticosterone treatment for 24 h, inhibition of the adhesion of ROS 17/2.8 cells and primary osteoblasts to fibronectin 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 of fibroblasts. However, glucocorticoids did inhibit the adhesion of all cell types to rat osteonectin. To determine whether the change in osteoblast attachment to collagen and fibronectin was due to an alteration in integrin levels, the plasma membranes of these cells were labeled with [I-125]lactoperoxidase, solubilized, and immunoprecipitated with 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 confirmed with immunofluorescence microscopy, which showed a glucocorticoid-induced decrease in beta(1) staining. Treatment of primary rat osteoblasts and ROS 17/2.8 cells for 72 h with corticosterone also decreased beta(1)-integrin messenger RNA levels in a dose-dependent manner. We have demonstrated that the inhibition of integrin expression by glucocorticoids is involved in the decrease in osteoblast adhesion to bone extracellular matrix proteins. These data suggest that integrin modulation may influence osteoblast function and bone formation and, thus, contribute to glucocorticoid-induced osteoporosis.