Regulation of alphaVbeta3 and alphaVbeta5 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 alphavbeta3 and alphavbeta5 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 alphavbeta3 and alphavbeta5, 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 alphavbeta3 and alphavbeta5 integrins, which were increased after 2 days of Dex treatment, were decreased after 8 days. Similarly, total cellular alphav, beta3, and beta5 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 alphavbeta5, with stimulation observed during the second day of treatment, followed by inhibition during the 8th day of exposure. By contrast, the biosynthesis of alphavbeta3 was inhibited by Dex on day 1 and remained inhibited on day 8. Analysis of the mRNA indicated that alphav and beta5 levels were increased by Dex during early exposure (1-3 days), followed by inhibition after prolonged exposure (>/=7 days). By contrast, Dex decreased beta3 mRNA level at all the time points analyzed. Consistently, Dex decreased beta3 promoter activity after 1 day and persisted over 8-day period. By contrast, Dex stimulated beta5 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 alphav, beta3 and beta5 mRNA after an 8-day treatment. Thus, the regulation of alphavbeta3 was dependent on transcription and posttranscriptional events whereas the expression of alphavbeta5 was dependent mainly on posttranscriptional events after prolonged Dex treatment. In conclusion, Dex exhibited time-dependent regulation on the expression of alphavbeta3 and alphavbeta5 integrins in normal human osteoblastic cells. Short-term exposure to Dex increased the levels of alphavbeta3 and alphavbeta5 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.