Biosynthesis of bone proteins [SPP-1 (secreted phosphoprotein-1, osteopontin), BSP (bone sialoprotein) and SPARC (osteonectin)] in association with mineralized-tissue formation by fetal-rat calvarial cells in culture

Abstract

To determine the relationship between the expression of bone proteins and the formation of mineralized-tissue matrix, the biosynthesis of non-collagenous bone proteins was studied in cultures of fetal-rat calvarial cells, which form mineralized nodules of bone-like tissue in the presence of beta-glycerophosphate. The temporal pattern of protein synthesis in both mineralizing and non-mineralizing cultures was studied by metabolic labelling with [35S]methionine, 35SO4(2-) or 32PO4(3-) over a 5-day period. After a 24 h labelling period, the culture media were harvested and the cell layers extracted sequentially with aq. 0.5 M-NH3, followed by 4 M-guanidinium chloride (GdmCl), 0.5 M-EDTA and a second extraction with 4 M-GdmCl. Protein associated with collagenous bone matrix was analysed after digestion with bacterial collagenase. On the basis of [35S]methionine labelling, the major proteins extracted from the mineralizing matrix were secreted phosphoprotein-1 (SPP-1; osteopontin), bone sialoprotein (BSP) and a 14 kDa phosphoprotein. The presence of SPP-1 and BSP in the conditioned media of both mineralizing and non-mineralizing cultures and their incorporation into the mineralizing nodules indicated that these proteins associate with preformed mineral crystals. However, some BSP was also present in GdmCl extracts and, together with a 35 kDa sulphated protein, was released from a bacterial-collagenase digestion of the tissue residue in both non-mineralizing and mineralizing cultures. Two forms of sulphated SPP-1 were identified, a highly phosphorylated 44 kDa species being the predominant form in the mineralized matrix. The BSP was more highly sulphated but less phosphorylated than SPP-1. Bone SPARC (secreted protein, acid and rich in cysteine) protein (osteonectin) was present almost entirely in the conditioned media and did not incorporate 32PO4(3-) or 35SO4(2-). The SPP-1 and the 14 kDa protein were susceptible to thrombin digestion, the 44 kDa SPP-1 being specifically cleaved into 28 and 26 kDa fragments. The fragments were labelled uniformly with [35S]methionine, but the 28 kDa fragment incorporated more 35SO4(2-), but less 32PO4(3-), than the 26 kDa fragment. These studies demonstrate that SPP-1 and BSP are the major osteoblast-derived bone proteins to bind to the bone mineral. That BSP also binds to the collagenous bone matrix indicates a potential role for this protein in linking the hydroxyapatite with collagen.