The expression of dentin sialophosphoprotein gene in bone

Dentin sialoprotein (DSP) and dentin phosphoprotein (DPP) are expressed as a single mRNA transcript coding for a large precursor protein termed dentin sialophosphoprotein (DSPP). DSP, DPP, and DSPP have been considered to be tooth-specific. To test for the expression of the dspp gene in bone, we performed Western immunoblots and reverse-transcription polymerase chain-reaction (RT-PCR). With Western immunoblots, we detected DSP in the Gdm/EDTA extracts of rat long bone, at a level of about 1/400 of that in dentin. Using RT-PCR, we detected DSPP mRNA in mouse calvaria. Similar to Western immunoblots, the results of RT-PCR indicated that the dspp gene is expressed at a lower level in bone than in dentin and odontoblasts. Analysis of the data shows that DSPP is not a tooth-specific protein, and that dramatically different regulatory mechanisms governing DSPP expression are involved in the bone and dentin.

Osteopontin posttranslational modifications, possibly phosphorylation, are required for in vitro bone resorption but not osteoclast adhesion

Osteopontin (OPN), a phosphorylated bone matrix glycoprotein, is an Arg-Gly-Asp (RGD)-containing protein that interacts with integrins and promotes in vitro attachment of a number of cell types, including osteoclasts. Gene knockout experiments support the idea that OPN is important in osteoclastic activity. We hypothesize that posttranslational modifications (PTMs) of OPN can influence its physiological function. Previous studies have suggested that phosphorylation of OPN and bone sialoprotein (BSP) is necessary for promoting osteoclast adhesion. However, no reports have explored the importance of phosphoserines and other PTMs in OPN-promoted bone resorption. To study this question, we determined the activities of different forms of OPN and BSP in three in vitro assays: attachment of osteoclasts; formation of actin rings; and bone resorption. For each assay, cells were incubated for 4-24 h, in the presence or absence of RGDS or RGES peptides, to test the involvement of integrin binding. In addition to OPN, activities of milk OPN (fully phosphorylated) and recombinant OPN (rOPN, no phosphate) were compared. We purified two forms of OPN (OPN-2 and OPN-5), which differ in the level of phosphorylation, and compared their activities. For comparison, the activities of BSP and recombinant BSP (rBSP) were determined. All forms of OPN, including rOPN, significantly increased attachment of tartrate-resistant acid phosphatase (TRAP)-positive osteoclasts. BSP and rBSP also promoted cell attachment. After 4 h of incubation, the proportion of cells with actin rings was increased with OPN, milk OPN, and BSP. In the presence of RGDS peptide, osteoclast retraction and the disruption of actin rings were observed, whereas no effect was seen with RGES. In the resorption assay, the number of pits and the total resorbed area per slice were increased in the presence of OPN, milk OPN, and BSP. As in other assays, the OPN enhancement of resorption was inhibited by RGDS, but not RGES, peptides. Significantly, rOPN and rBSP did not promote bone resorption. OPN-5 promoted resorption to a greater extent than OPN-2, and milk OPN significantly stimulated resorption to a greater extent than OPN. Our data suggest that: (1) the RGD sequence of OPN is essential in OPN-mediated cell attachment, actin ring formation, and bone resorption; and (2) some form of PTM, possibly phosphorylation, is necessary for in vitro osteoclastic bone resorption, but not for cell attachment and actin ring formation.

A comparative study of sialic acid-rich proteins in rat bone and dentin

Four sialic acid-rich (SA-rich) proteins found in bone and dentin, osteopontin (OPN), bone sialoprotein (BSP), bone acidic glycoprotein-75 (BAG-75), and dentin matrix protein 1 (DMP1), share some common features. We used SDS-PAGE and Western immunoblots to analyze and compare SA-rich proteins in bone and dentin extracts from rats with a single chromatographic procedure. OPN was detected in dentin extracts, with a relative level less than one-seventieth of that in bone. Both bone and dentin BSP demonstrated an extremely broad distribution pattern, probably due to a high degree of heterogeneity in post-translational modifications. BAG-75 in both bone and dentin was detected as an 83 kDa band, dramatically distinct from that of DMPI. Using a polyclonal antibody raised against a purified bone 57 kDa protein (a portion of DMPI), we detected 150 kDa protein bands in bone fraction; the same bands were recognized by antirecombinant rat DMPI antibody. Bands from dentin migrating at about 150 kDa in earlier fractions and progressing to 200 kDa in later fractions showed a clear immunoreactivity to the anti-57 kDa antibody. We conclude that the majority of DMPI in rat bone is processed into fragments, whereas that in dentin remains intact.

Bone matrix proteins in osteogenesis and remodelling in the neonatal rat mandible as studied by immunolocalization of osteopontin, bone sialoprotein, alpha 2HS-glycoprotein and alkaline phosphatase

The neonatal rat mandible was used as a model to study bone formation, mineralization, quiescence, and resorption, using immunolocalization and a variety of tissue-processing techniques. Monospecific antibodies for osteopontin (OPN), bone sialoprotein (BSP), alkaline phosphatase (AP) and alpha 2HS-glycoprotein (alpha 2HS-GP) were used on fixed paraffin-embedded tissue, fixed frozen tissue and unfixed frozen tissue. Immunostaining was correlated with mineral content by two procedures, the von Kossa and the morin techniques. Morin fluorescence was used with secondary immunostaining to provide a way of closely correlating bone matrix proteins and matrix mineralization. Co-immunolocalization procedures were used to compare the sites of bone proteins in the matrix. AP was found earliest during osteogenic cell differentiation, appearing in the preosteoblasts, followed by OPN and BSP, which first appeared in osteoblasts. alpha 2HS-GP expression was not observed in cells. The results provide clear evidence for the presence of OPN in osteoid, while BSP and alpha 2HS-GP were confined to the mineralized matrix. Immunostaining of bone proteins is highly technique-dependent: immunolocalization investigations required several methods of approach to ensure adequate demonstration of these proteins in cells and matrix. The results support the contention that osteopontin is multifunctional in bone metabolism, and that alpha 2HS-GP, though produced in the liver, is abundant in bone matrix and may also have a function in bone metabolism.

Differences in composition of cell-attachment sialoproteins between dentin and bone

Matrices of dentin and bone were compared with respect to the content of cell-attachment sialoproteins. The levels of two sialoproteins, osteopontin (OPN) and bone sialoprotein (BSP), were determined in dentin and bone by immunochemical procedures. Polyclonal antibodies against bovine BSP and an antibody against the amino-terminal decapeptide of rat OPN were used. The relative levels of OPN and BSP in dentin were less than one-tenth of the levels in bone. The differences between dentin and bone levels of OPN and BSP were thus larger than those for osteonectin or bone Gla protein in the two tissues. The scarcity of the cell-attachment proteins in dentin may reflect the metabolic inactivity of dentin.

Isolation, characterization and immunolocalization of a 53-kDal dentin sialoprotein (DSP)

We isolated a sialic-rich protein from rat dentin extracts and have named it dentin sialoprotein, DSP (formerly called 95K glycoprotein). DSP is rich in aspartic acid, glutamic acid, glycine and serine, but contains no cysteine or phosphate. The 30% carbohydrate content includes about 9% sialic acid and indicates that several N-glycosides and O-glycosides are present. Sedimentation equilibrium analysis gave a M(r) of 52,570. Based on this molecular weight we calculated that DSP contains about 350-amino acids and 75 monosaccharides. With automated Edman degradation the sequence of the first 8-amino acids was shown to be: Ile-Pro-Val-Pro-Gln-Leu-Val-Pro. The initial 3 residues of this sequence are identical to the first 3 in human osteopontin (OPN) and are closely similar to the Leu-Pro-Val sequences of OPN from other species, as well as at the beginning of bone acidic glycoprotein-75 (BAG-75). On Western immunoblots, purified polyclonal antibodies reacted only with DSP in dentin extracts and with none of the proteins from bone. Similarly, immunolocalization experiments showed the presence of DSP in dentin but not in enamel or alveolar bone. Along with immunohistochemical localization data reported elsewhere, these observations suggest that DSP may be an important marker for cells in the odontoblast lineage.

Identification of the rat bone 60K acidic glycoprotein as alpha 2HS-glycoprotein

Previous reports have described an Mr 60,000-64,000 glycoprotein present in guanidium chloride (GdmCl)/EDTA extracts of bovine and rat bone. We have purified this protein from the long bones of rats and have raised polyclonal antibodies to the purified protein. The 60K glycoprotein has amino acid and carbohydrate compositions that are similar to those reported for the 60-64K protein(s). Several lines of evidence indicate that the 60K bone glycoprotein is the rat homologue of human alpha 2HS-glycoprotein. First, immunochemical data demonstrated that the 60K bone glycoprotein was present in serum as well as in EDTA/GdmCl extracts of bone. Second, immunolocalization and metabolic labelling experiments showed that the 60K protein is synthesized in liver and not in bone cells, although it is sequestered in vascularized regions of bone matrix. Finally, the NH2-terminal sequence for the rat 60K bone glycoprotein was highly similar to that of the human alpha 2HS-glycoprotein A chain. A surprising finding was that small amounts of contaminating 60K/alpha 2HS-glycoprotein were found in several protein fractions purified by ion-exchange chromatography of bone EDTA/GdmCl extracts. Because this protein was found to be highly immunogenic, the presence of anti-60K antibodies in anti-sera prepared against purified bone proteins should be considered as a potential problem.

Regression of human tumors established in nude mice after continuous infusion of thymidine

A system for the continuous infusion of thymidine solutions in nude mice has been developed. High doses (0.5 to 1.0 ml/mouse/hr) of a 28.5-mg/ml thymidine solution (444 to 888 mg/kg/hr) can be administered continuously for 96 to 140 hr. The preliminary results indicate that it is possible to induce total tumor regression of human heterotransplants established in nude mice of one human teratocarcinoma, five different human melanomas, and one human lung carcinoma and to inhibit to a large degree the growth of two human breast carcinomas by multiple (two to eight) cycles of infusion. The life span of the thymidine-treated animals has been significantly increased compared to that of control animals.