Regulation of αVβ3 and αVβ5 integrins by dexamethasone in normal human osteoblastic cells

Long-term administration of pharmacological doses of glucocorticoids inhibits bone formation and results in osteoporosis. Since integrin-mediated cell-matrix interactions are essential for osteoblast function, we hypothesized that the detrimental effect of glucocorticoids on bone derived, at least in part, from decreased integrin-matrix interactions. Because αvβ3 and αvβ5 integrins can interact with several bone matrix proteins, we analyzed the effects of dexamethasone (Dex) on the expression of these integrins in normal human osteoblastic cells. We found adhesion of these cells to osteopontin and vitronectin to be dependent on αvβ3 and αvβ5, respectively; this ligand specificity was not altered by Dex. The effects of Dex on the adhesion of human osteoblastic cells to osteopontin and vitronectin were biphasic with an increase after 2 days, followed by a decrease after 8 days of treatment. Consistently, surface αvβ3 and αvβ5 integrins, which were increased after 2 days of Dex treatment, were decreased after 8 days. Similarly, total cellular αv, β3, and β5 proteins, which were increased by Dex early in the culture, were diminished after 8 days. Metabolic labeling studies indicated that Dex exhibited biphasic regulation on the biosynthesis of αvβ5, with stimulation observed during the second day of treatment, followed by inhibition during the 8th day of exposure. By contrast, the biosynthesis of αvβ3 was inhibited by Dex on day 1 and remained inhibited on day 8. Analysis of the mRNA indicated that αv and β5 levels were increased by Dex during early exposure (1-3 days), followed by inhibition after prolonged exposure (≥7 days). By contrast, Dex decreased β3 mRNA level at all the time points analyzed. Consistently, Dex decreased β3 promoter activity after 1 day and persisted over 8-day period. By contrast, Dex stimulated β5 promoter activity after 1 or 2 days but had no effect after 8 days. To further evaluate mechanism(s) leading to the decreased integrin expression after prolonged Dex treatment, mRNA stability was analyzed. Dex was found to accelerate the degradation of αv, β3 and β5 mRNA after an 8-day treatment. Thus, the regulation of αvβ3 was dependent on transcription and posttranscriptional events whereas the expression of αvβ5 was dependent mainly on posttranscriptional events after prolonged Dex treatment. In conclusion, Dex exhibited time-dependent regulation on the expression of αvβ3 and αvβ5 integrins in normal human osteoblastic cells. Short-term exposure to Dex increased the levels of αvβ3 and αvβ5 on the surface and cell adhesion to osteopontin and vitronectin whereas long-term exposure to Dex decreased the expression of both integrins and inhibited the cell adhesion to matrix proteins. (C) 2000 Wiley-Liss, Inc.