Abstract.In this report we demonstrate an increase in the steady-state level of bone sialoprotein (BSP) mRNA in rat calvaria and a rat osteosarcoma cell line (ROS 17/2.8) after treatment with the synthetic glucocorticoid, dexamethasone. In contrast, 1.25-dthydroxyvitamin D3 reduced the amount of BSP mRNA in calvaria and inhibited the dexamethasone induction in ROS 17/2.8 cells. The increase in BSP mRNA is most likely due to an increase in the transcriptional rate. The stability of mRNA was unchanged after dexamethasone treatment with a half-life of r~5 h. Nuclear transcription experiments with nuclei isolated from ROS 17/2.8 cells showed an increased BSP mRNA synthesis in cells treated with dexamethasone.ONE sialoprotein (BSP), ~ which we previously called sialoprotein II, is an acidic glycoprotein associated with the mineral matrix in bone and teeth (2, 3). The protein contains ",,50% carbohydrate of which 15 % is sialic acid (3). Furthermore, some 30% of the serine residues are phosphorylated and contribute to the acidity of BSP. The amino acid sequence of BSP, deduced from eDNA, predicts a 34-kD protein core with predominantly glutamic acid and glycine residues, which constitute one third of all amino acid residues (10). The protein contains several clusters of up to 10 consecutive glutamic acid residues. These negatively charged domains, together with the sialic acid residues and phosphate groups, are presumably responsible for the strong interaction with hydroxyapatite.BSP promotes the attachment and spreading of rat osteosarcoma cells (ROS 17/12.8) as well as primary bovine chondrocytes in cell-binding experiments using plastic dishes coated with the protein (11,12). This cell binding is apparently mediated by an RGD-containing region in BSP, which is homologous to the cell-binding domain in vitronectin (10). The BSP receptor is an integrin indistinguishable from the vitronectin receptor on the surface of ROS 17/2.8 cells (11).BSP shares some structural features with another bone protein, osteopontin (OPN) (9). The OPN sequence contains a stretch of nine consecutive aspartic acid residues comparable to the glutamic acid clusters in BSP. OPN also contains a cell-binding RGD sequence which promotes attachment of cells in a similar way as BSP (9). The synthesis of OPN in bone cells is regulated by steroid hormones. It has been shown that 1,25-dihydroxyvitamin D3 (vit D3) increases the level of OPN mRNA, whereas the synthetic glucocorticoid, dexamethasone, reduces the steady-state level of OPN mRNA (15).It is well established that steroid hormones have pronounced influence on the metabolism of bone. Some of these 1. Abbre~ations used in this paper: BSP, bone siaioprotein; DRB, 5,6-dichloro-1-/~-ribofuranosyl benzimidazole; OPN, osteopontin; ROS, rat osteosarcoma; vit D3, 1,25-dihydroxyvitamin D3. effects may be exerted via effects on cells, possibly mediated via cell-binding proteins. The present study, therefore, was undertaken to determine the effects of different steroid hormones on the biosynthesis of two ...