Synthesis and processing of bone sialoproteins during de novo bone formation in vitro

Bone sialoprotein (BSP) and osteopontin (OPN) are sulphated and phosphoryla ted sialoglycoproteins that regulate the formation of hydroxyapatite crysta ls during de novo bone formation. To gain insights into the relationship be tween the synthesis and posttranslational modification of BSP and OPN and t he mineralization of bone, pulse-chase studies were conducted on cultures o f newly forming bone nodules produced by fetal rat calvarial cells in vitro . Cultures were pulse labelled with (SO4)-S-35, or with either (PO4)-P-32 o r [gamma-P-32]ATP to study intracellular and extracellular phosphorylation, respectively, and chased in isotope-free medium for various times up to 24 h. The presence of radiolabelled BSP and OPN was determined in the cells, in culture medium, and in various tissue compartments obtained by dissociat ive extraction with 4 M GuHCl (G1), 0.5 M EDTA (E), and again with 4 M GuHC l (G2) and a bacterial collagenase digestion of the demineralized collageno us tissue residue. With each isotope employed, radiolabelled BSP and OPN we re detected in the E extract within the 1-h chase period and increased in a mount with time. Similarly, (SO4)-S-35- and (PO4)-P-32-labelled BSP increas ed in the G2 extract, but OPN was not detected. In the G1 extract the (SO4) -S-35-labelled BSP decreased with chase time, whereas the (PO4)-P-32-labell ed BSP increased. No differences were evident in the profiles of BSP labell ed with (PO4)-P-32 or [gamma-P-32]ATP. In the absence of beta -glycerophosp hate, which is required for optimal mineralization of the bone nodules, (SO 4)-S-35-labelled BSP was increased in the medium and G1 extract and decreas ed in the E extract and G2 extract after 3 h. In addition to differences in the tissue compartmentalization of BSP and OPN, these studies indicate tha t (SO4)-S-35 is lost from BSP during mineralization and that isoforms of BS P exist with a selective affinity for the organic and mineral phases. Moreo ver, the additional phosphorylation of BSP and OPN catalyzed by ectokinase activity does not appear to alter the distribution of these sialoproteins.