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 alpha v beta 3 and alpha v beta 5 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 alpha v beta 3 and alpha v beta 5, 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 alpha v beta 3 and alpha v beta 5 integrins, which were increased after 2 days of Dex treatment, were decreased after 8 days. Similarly, total cellular alpha v, beta 3, and beta 5 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 alpha v beta 5, with stimulation observed during the second day of treatment, followed by inhibition during the 8th day of exposure. By contrast, the biosynthesis of alpha v beta 3 was inhibited by Dex on day 1 and remained inhibited on day 8. Analysis of the mRNA indicated that alpha v and beta 5 levels were increased by Dex during early exposure (1-3 days), followed by inhibition after prolonged exposure (greater than or equal to 7 days). By contrast, Dex decreased beta 3 mRNA level at all the time points analyzed. Consistently, Dex decreased beta 3 promoter activity after 1 day and persisted over 8-day period. By contrast, Dex stimulated beta 5 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 alpha v, beta 3 and beta 5 mRNA after an 8-day treatment. Thus, the regulation of alpha v beta 3 was dependent on transcription and posttranscriptional events whereas the expression of alpha v beta 5 was dependent mainly on posttranscriptional events after prolonged Dex treatment. In conclusion, Dex exhibited time-dependent regulation on the expression of alpha v beta 3 and alpha v beta 5 integrins in normal human osteoblastic cells. Short-term exposure to Dex increased the levels of alpha v beta 3 and alpha v beta 5 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, J. Cell. Biochem. 77:265-276, 2000. (C) 2000 Wiley-Liss, Inc.