REGULATION OF BONE SIALOPROTEIN AND OSTEOPONTIN MESSENGER-RNA EXPRESSION BY DEXAMETHASONE AND 1,25-DIHYDROXYVITAMIN-D-3 IN RAT BONE ORGAN-CULTURES

Bone sialoprotein (BSP) and osteopontin (OPN) are prominent components of the extracellular matrix of mineralized connective tissues that ha ve been implicated in the formation and remodelling of bone. Although these proteins have similar biochemical properties and are expressed b y bone cells during bone formation it has been suggested that they hav e different functions and that their expression is regulated independe ntly by hormones and cytokines. The precise role of these proteins has , however, yet to be firmly established. Since steroid hormones strong ly influence the formation of bone we have analyzed the effects of glu cocorticoids and 1,25 dihydroxyvitamin D-3 (1,25-(OH)(2)D-3) on the ex pression of BSP and OPN mRNAs in developing rat bone in vitro using in situ hybridization. In these studies it has been possible to identify the nature and spatial distribution of the cells that respond to thes e hormones by changes in sialoprotein expression. When cultured in the presence of the synthetic glucocorticoid, dexamethasone (dex), expres sion of BSP was increased >5-fold in osteoblastic cells of the primary spongiosa in the tibial growth plate, in the mandibular bone and in c alvarial bone. In addition, expression of BSP mRNA by hypertrophic car tilage cells in the was dramatically increased as were the number of r esponding cells indicating that glucocorticoids promote differentiatio n of hypertrophic cartilage cells as well as osteoblasts. Dexamethason e also stimulated a marked (>5-fold) increase in OPN expression by ost eoblasts and cells lining endosteal and periosteal bone surfaces. In c ontrast to dex 1,25-(OH)(2)D-3 suppressed BSP expression in osteoblast ic cells whereas OPN expression was strongly (>5-fold) stimulated in a ll three cultured bone tissues. Histological examination of the tissue s showed that cell viability was retained over the culture period. How ever, in the presence of 1,25-(OH)(2)D-3 considerable resorption of th e tissue was evident, with cement and reversal lines being prominent. The increased expression of BSP and OPN by dex is consistent with the stimulation of bone formation by glucocorticoids, whereas the differen tial effects of 1,25-(OH)(2)D-3 on BSP and OPN may reflect a stimulati on of bone remodelling.