Ga. Gronowicz et Mb. Mccarthy, GLUCOCORTICOIDS INHIBIT THE ATTACHMENT OF OSTEOBLASTS TO BONE EXTRACELLULAR-MATRIX PROTEINS AND DECREASE BETA(1)-INTEGRIN LEVELS, Endocrinology, 136(2), 1995, pp. 598-608
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.