Tissue specificity and developmental expression of rat osteopontin

Osteopontin is a 44 kd phosphoprotein abundant in bone matrix. We isolated a partial length cDNA for rat osteopontin and used it to examine its tissue specificity, its expression during bone development and its hormonal regulation. Osteopontin mRNA is most abundant in bone but is also found in considerable amounts in kidney. Osteopontin mRNA is regulated by the osteotropic hormones dexamethasone and 1,25(OH)2D3. Estimates of osteopontin mRNA levels indicate that the osteopontin gene is turned on relatively late in calvarial development.     

Fibroblast growth factor-2 regulates expression of osteopontin in periodontal ligament cells

Osteopontin is a protein found in the bone-related matrix and plays multiple regulatory roles in mineralizing and non-mineralizing tissue. In osteogenic cell-lines, the expression of osteopontin increases with the progression of differentiation, but both the expression and function of osteopontin vary with the cell type and its activation state. In this study, we examined the expression of osteopontin by clones established from mouse periodontal ligament, in response to inorganic phosphate and fibroblast growth factor (FGF)-2, which can induce periodontal tissue regeneration. The involvement of inorganic phosphate in the expression of osteopontin during the course of cell differentiation of a clone MPDL22 was confirmed by addition of foscarnet, an inorganic phosphate transport inhibitor. Although FGF-2 decreased the mRNA expression of almost every bone-related protein in MPDL22, FGF-2 upregulated the expression of osteopontin in MPDL22 at both mRNA and protein levels. Interestingly, FGF-2 enhanced the concentration of osteopontin in the culture supernatant of MPDL22, whereas inorganic phosphate did not. The FGF-2-induced osteopontin in the culture supernatant seems to be involved in cell survival activity. An immunohistochemical study showed that the FGF-2-induced osteopontin was mainly present in perinuclear matrices while the inorganic phosphate-induced osteopontin was associated with extracellular matrices in addition to perinuclear matrices. The present results indicated that FGF-2 induces unique expression of osteopontin, which may play a role different from the other bone-related proteins during the process of periodontal tissue regeneration by FGF-2.     

Osteopontin is expressed by adult human osteoarthritic chondrocytes: protein and mRNA analysis of normal and osteoarthritic cartilage.

Osteopontin, a sulfated phosphoprotein with cell binding and matrix binding properties, is expressed in a variety of tissues. In the embryonic growth plate, osteopontin expression was found in bone-forming cells and in hypertrophic chondrocytes. In this study, the expression of osteopontin was analyzed in normal and osteoarthritic human knee cartilage. Immunohistochemistry, using a monoclonal anti-osteopontin antibody was negative on normal cartilage. These results were confirmed in Western blot experiments, using partially purified extracts of normal knee cartilage. No osteopontin gene expression was observed in chondrocytes of adult healthy cartilage, however, in the subchondral bone plate, expression of osteopontin mRNA was detected in the osteoblasts. In cartilage from patients with osteoarthritis, osteopontin could be detected by immunohistochemistry, Western blot analysis, in situ hybridization, and Northern blot analysis. A qualitative analysis indicated that osteopontin protein deposition and mRNA expression increase with the severity of the osteoarthritic lesions and the disintegration of the cartilaginous matrix. Osteopontin expression in the cartilage was limited to the chondrocytes of the upper deep zone, showing cellular and territorial deposition. The strongest osteopontin detection was found in deep zone chondrocytes and in clusters of proliferating chondrocytes from samples with severe osteoarthritic lesions. These data show the expression of osteopontin in adult human osteoarthritic chondrocytes, suggesting that chondrocyte differentiation and the expression of differentiation markers in osteoarthritic cartilage resembles that of epiphyseal growth plate chondrocytes.     

Osteopontin: a versatile regulator of inflammation and biomineralization.

Osteopontin is a secreted glycoprotein with a multidomain structure and functions characteristic of a matricellular protein. Osteopontin interacts with cell surface receptors via arginine-glycine-aspartate (RGD)- and non-RGD containing adhesive domains, in addition to binding to components of the structural extracellular matrix. While normally expressed in bone and kidney, osteopontin levels are elevated during wound healing and inflammation in most tissues studied to date. Since 1986, over one thousand studies have been published on osteopontin, including recent experiments in osteopontin-deficient mice. These studies reveal osteopontin as a cell adhesive, signaling, migratory, and survival stimulus for various mesenchymal, epithelial, and inflammatory cells, in addition to being a potent regulator of osseous and ectopic calcification. Based on these reports, a general picture of osteopontin as an important regulator of inflammation and biomineralization is emerging. A common denominator in osteopontin function in these situations is its ability to regulate the function of macrophage and macrophage-derived cells (i.e. osteoclasts). While we have learned much about osteopontin and the processes it appears to regulate over the past decade, many questions regarding this important multifunctional protein remain unanswered and provide important directions for future studies.     

Altered bleomycin-induced lung fibrosis in osteopontin-deficient mice

Osteopontin is a multifunctional matricellular protein abundantly expressed during inflammation and repair. Osteopontin deficiency is associated with abnormal wound repair characterized by aberrant collagen fibrillogenesis in the heart and skin. Recent gene microarray studies found that osteopontin is abundantly expressed in both human and mouse lung fibrosis. Macrophages and T cells are known to be major sources of osteopontin. During lung fibrosis, however, osteopontin expression continues to increase when inflammation has receded, suggesting alternative sources of ostepontin during this response. In this study, we demonstrate immunoreactivity for osteopontin in lung epithelial and inflammatory cells in human usual interstitial pneumonitis and murine bleomycin-induced lung fibrosis. After treatment with bleomycin, osteopontin-null mice develop lung fibrosis characterized by dilated distal air spaces and reduced type I collagen expression compared with wild-type controls. There is also a significant decrease in levels of active transforming growth factor-beta(1) and matrix metalloproteinase-2 in osteopontin null mice. Type III collagen expression and total collagenase activity are similar in both groups. These results demonstrate that osteopontin expression is associated with important fibrogenic signals in the lung and that the epithelium may be an important source of osteopontin during lung fibrosis.     

Clonal dental pulp cells (RDP4-1, RPC-C2A) synthesize and secrete osteopontin (SPP1, 2ar).

Dental pulp cells play an important role in maintaining dental mineralized tissue throughout life. Supplementary mineralization such as reparative dentin and pulp stone frequently occurs after primary dentin formation. Dental pulp cells are thought to be closely associated with such mineralization. We found that clonal rat dental pulp cells, RDP4-1 and RPC-C2A, produce and secrete osteopontin, but do not synthesize phosphophoryn which is a major noncollagenous protein found in dentin. The dental pulp osteopontin was highly phosphorylated and identified by thrombin susceptibility and immunoprecipitation with osteopontin/2ar antibody. Osteopontin synthesis markedly increased by 12-O-tetradecanoylphorbol-13-acetate (TPA) as observed in many osteoblastic cells. This study indicates that these cells can produce osteopontin as a major phosphoprotein and suggests that the synthesis of osteopontin could be used as a characteristic marker of dental pulp cells.     

骨桥蛋白与糖尿病肾病关系的研究进展

骨桥蛋白（OPN）是一种具有细胞粘附和迁移功能的分泌型磷酸化糖蛋白。在肾脏中有广泛分布,研究发现其参与糖尿病肾病（DN）蛋白尿形成、DN的炎症反应及肾脏纤维化过程,抑制OPN可改善糖尿病肾脏病变。     

Ultrastructural localization of osteopontin immunoreactivity in phagolys osomes and secretory granules of cells in human intestine

A post-embedding ultrastructural immunogold method was used to detect osteopontin in human intestinal biopsies with special emphasis on secretory and phagocytic organelles. Osteopontin immunoreactivity was localized to phagolysosomes of macrophages, fibroblasts, absorptive epithelial cells of the small intestine and Paneth cells. The mucigen secretory granules and Golgi structures of mucous epithelial cells of the small intestinal epithelium contained osteopontin, but secretory granules of numerous other cells, including Paneth cells, did not. Extracellular and phagocytosed Tropheryma whippelii within macrophage phagolysosomes also bound osteopontin. These localizations are supportive of a role for osteopontin in phagocytic and some secretory cell functions in human intestine     

Osteopontin and skeletal muscle myoblasts: association with muscle regeneration and regulation of myoblast function in vitro

Osteopontin is a secreted glycoprotein expressed by many cell types including osteoblasts and lymphocytes; it is a constituent of the extracellular matrix (ECM) in bone, and a mitogen for lymphocytes. To investigate the role of osteopontin in muscle repair and development, expression of osteopontin by muscle cells in vivo and in vitro, and the effects of osteopontin on myoblast function in vitro were investigated. Osteopontin staining was weak in sections of muscle from normal mice, but associated with desmin-positive cells in areas of regeneration in muscles from mdx mice. In immunocytochemical, PCR and ELISA studies, cultured myoblasts were found to express osteopontin and secrete it into medium. Treatment of myoblast cultures with fibroblast growth factor-2, transforming growth factor beta1, interleukin-1beta or thrombin significantly increased osteopontin expression. Osteopontin-coated substrata promoted adhesion and fusion, but not proliferation or migration, of myoblasts. The effect of osteopontin on myoblast adhesion was RGD-dependent. In solution, osteopontin significantly increased proliferation and decreased fusion and migration of myoblasts. These results suggest that myoblasts are an important source of osteopontin in damaged muscle and that osteopontin released by myoblasts may assist in controlling both the myogenic and inflammatory processes during the early stages of muscle regeneration.     

Osteopontin, a multi-faceted molecule

Osteopontin (OPN) was initially isolated from bovine bone cortex, as a complex syalilated phospho-glyco-protein of around 60 kDa, with many postranslational modifications. It has been long considered a structural bone protein linking bone cells to the bone extracellular matrix (osteo : bone, pontin : bridge). It has been cloned for the first time in 1986. Since then, it was established that it is part of a protein family called SIBLINGs, which genes share common expression in bone and tooth, and encode among others a RGD motif. OPN is an intracellular as well as secreted protein, which binds to multiple organic or mineral ligands, like the integrin receptor alphaVbeta3, CD44, factor H and hydroxyapatite, depending on its final configuration (phosphorylation state). Pleiotropic functions of osteopontin have been demonstrated, and the osteopontin knock out phenotype in mice gave some new insight on the implication of the molecule in vivo. Osteopontin inhibits mineralization in bone and urine. Besides, it is a strong chemoattractive and proinflammatory molecule, implicated in tumors, like breast or prostate cancers, and in the defense against various infectious agents like tuberculosis, listeria or herpes. More recently, its key implication in TH1 mediated autoimmune diseases like multiple sclerosis and its animal model experimental autoimmune encephalomyelitis has been demonstrated. Osteopontin is a valuable therapeutic target in the animal model, and a biological tool correlating with clinical disease activity in humans. Structural, functional and pathological aspects of osteopontin are reviewed, as well as the osteopontin deficient phenotype in mouse.     

Clinical and Molecular Implications of Osteopontin in Heart Failure

The matricellular protein osteopontin modulates cell–matrix interactions during tissue injury and healing. A complex multidomain structure of osteopontin enables it not only to bind diverse cell receptors but also to interact with various partners, including other extracellular matrix proteins, cytokines, and growth factors. Numerous studies have implicated osteopontin in the development and progression of myocardial remodeling in diverse cardiac diseases. Osteopontin influences myocardial remodeling by regulating extracellular matrix production, the activity of matrix metalloproteinases and various growth factors, inflammatory cell recruitment, myofibroblast differentiation, cardiomyocyte apoptosis, and myocardial vascularization. The exploitation of osteopontin loss- and gain-of-function approaches in rodent models provided an opportunity for assessment of the cell- and disease-specific contribution of osteopontin to myocardial remodeling. In this review, we summarize the recent knowledge on osteopontin regulation and its impact on various cardiac diseases, as well as delineate complex disease- and cell-specific roles of osteopontin in cardiac pathologies. We also discuss the current progress of therapeutics targeting osteopontin that may facilitate the development of a novel strategy for heart failure treatment.     

Ultrastructural localization of osteopontin immunoreactivity in phagolys osomes and secretory granules of cells in human intestine

A post-embedding ultrastructural immunogold method was used to detect osteopontin in human intestinal biopsies with special emphasis on secretory and phagocytic organelles. Osteopontin immunoreactivity was localized to phagolysosomes of macrophages, fibroblasts, absorptive epithelial cells of the small intestine and Paneth cells. The mucigen secretory granules and Golgi structures of mucous epithelial cells of the small intestinal epithelium contained osteopontin, but secretory granules of numerous other cells, including Paneth cells, did not. Extracellular and phagocytosed Tropheryma whippelii within macrophage phagolysosomes also bound osteopontin. These localizations are supportive of a role for osteopontin in phagocytic and some secretory cell functions in human intestine This revised version was published online in November 2006 with corrections to the Cover Date.     

In situ hybridization of bone matrix proteins in undecalcified adult rat bone sections.

We have developed a method for in situ hybridization of adult bone tissue utilizing undecalcified sections and have used it to histologically examine the mRNA expression of non-collagenous bone matrix proteins such as osteocalcin (bone Gla protein, BGP), matrix Gla protein (MGP), and osteopontin in adult rats. Expression was compared with that in bone tissues of newborn rats. In the adult bone tissue, osteocalcin mRNA was strongly expressed in periosteal and endosteal cuboidal osteoblasts but not in primary spongiosa near the growth plate. Osteopontin mRNA was strongly expressed in cells present on the bone resorption surface, osteocytes, and hypertrophic chondrocytes, but not in cuboidal osteoblasts on the formation surface. Osteopontin and osteocalcin mRNAs were expressed independently and the distribution of cells expressing osteopontin mRNA corresponded with acid phosphatase-positive mononuclear cells and osteoclasts. Expression of MGP mRNA was noted only in hypertrophic chondrocytes. In newborn rat bone tissues, expression of osteocalcin mRNA was much weaker than in adult rat bone tissues. These results clearly indicate the differential expression of mRNAs of non-collagenous bone matrix proteins in adult rat bone tissues.     

Identification of osteopontin phosphorylation sites involved in bone remodeling and inhibition of pathological calcification

Osteopontin is a noncollagenous, phosphorylated extracellular glycoprotein, expressed in mineralized and nonmineralized tissues, organs and body fluids. The protein contains an RGD tripeptide cell-binding motif, and is subjected to a variety of posttranslational modifications that play important roles in its multiple biological functions, such as bone remodeling and inhibition of pathological calcification. In this study, we have expressed bovine osteopontin in a prokaryotic system and identified the seven amino acid residues phosphorylated in vitro by CKII.     

CD44 is not an adhesive receptor for osteopontin

Osteopontin is a secreted glycoprotein with adhesive and migratory functions. Cellular interactions with osteopontin are mediated through integrin receptors which recognize the RGD domain. Recently, CD44, a non-integrin, multifunctional adhesion molecule was identified as an osteopontin receptor. CD44 is a ubiquitous surface molecule that exists as a number of different isoforms, generated by alternative splicing. To analyze which forms of CD44 mediate binding to osteopontin, we used the standard form of CD44 as CD44-human immunoglobulin fusion proteins and several splice variants in enzyme-linked immunosorbant assays. Multiple preparations of osteopontin were used including native osteopontin derived from smooth muscle cells, human urinary osteopontin, full-length recombinant osteopontin, and two recombinant osteopontin fragments expected to be formed following thrombin cleavage. Our data show that although the CD44-hlg fusion proteins could interact with hyaluronic acid as expected, there was no interaction between CD44H, CD44E, CD44v3,v8-v10, or CD44v3 with osteopontin. These studies suggest that CD44-osteopontin interactions may not be common in vivo and may be limited to a specific CD44 isoform(s), and/or a particular modified form of osteopontin.     

Osteoclast migration on phosphorylated osteopontin is regulated by endogenous tartrate-resistant acid phosphatase

Osteopontin (OPN) is a multifunctional protein implicated in cellular adhesion and migration. Phosphorylation has emerged as a post-translational modification important for certain biological activities of OPN. This study demonstrates that adhesion of isolated neonatal rat osteoclasts in vitro was augmented on bovine milk osteopontin (bmOPN) with post-translational modifications (PTMs) compared to human Escherichia-coli-derived recombinant OPN (hrOPN) without PTMs. The difference in adhesiveness between these OPN variants was more pronounced at low coating concentrations (≤ 10 μg/ml). Both OPN forms adhered exclusively using a β₃-integrin. Partial (≤50%) dephosphorylation by tartrate-resistant acid phosphatase (TRAP) in vitro reduced osteoclast attachment to bmOPN to the same level as to hrOPN, demonstrating the importance of specific phosphorylations in OPN-dependent osteoclast adhesion.The involvement of PTMs of OPN in migration of primary rat and mouse osteoclasts was assessed on culture dishes coated with the different OPN forms and then overlaid with gold particles. Here, osteoclasts exhibited haptotactic migration on bmOPN but did not migrate on hrOPN. The presence of neutralizing antibodies to TRAP inhibited migration on bmOPN. Moreover, migration of osteoclasts isolated from TRAP-overexpressing transgenic mice was augmented on bmOPN, but not on hrOPN or type I collagen.These data collectively provide evidence in favor of a role for endogenous TRAP in regulating osteoclast migration on post-translationally modified OPN. In a tissue context, modulation of the phosphorylation level of OPN by extracellular phosphatases, e.g., TRAP, could regulate the extent of degradation such as depth and area at each bone resorption site by triggering osteoclast detachment and facilitate subsequent migration on the bone surface.     

Molecular basis of recognition of human osteopontin by 23C3, a potential therapeutic antibody for treatment of rheumatoid arthritis

Osteopontin plays an important role in the development and perpetuation of rheumatoid arthritis (RA). Antibodies targeting osteopontin have shown promising therapeutic benefits against this disease. We have previously reported a novel anti-RA monoclonal antibody, namely, 23C3, and shown it capable of alleviating the symptoms of RA in a murine collagen-induced arthritis model, restoring the cytokine production profile in joint tissues, and reducing T-cell recall responses to collagen type II. We describe here the crystal structure of 23C3 in complex with its epitope peptide. Analyses of the complex structure reveal the molecular mechanism of osteopontin recognition by 23C3. The peptide folds into two tandem β-turns, and two key residues of the peptide are identified to be critical for the recognition by 23C3: TrpP43 is deeply embedded into a hydrophobic pocket formed by AlaL34, TyrL36, LeuL46, TyrL49, PheL91, and MetH102 and therefore has extensive hydrophobic interactions with 23C3, while AspP47 has a network of hydrophilic interactions with residues ArgH50, ArgH52, SerH53, and AsnH56 of the antibody. Besides the complementarity-determining region loops, the framework region L2 of 23C3 is also shown to interact with the epitope peptide, which is not common in the antibody-antigen interactions and thus could be exploited in the engineering of 23C3. These results not only provide valuable information for further improvement of 23C3 such as chimerization or humanization for its therapeutic application, but also reveal the features of this specific epitope of osteopontin that may be useful for the development of new antibody drugs against RA.     

Diagnostic Value of Osteopontin in Ovarian Cancer: A Meta-Analysis and Systematic Review

AimsOsteopontin (OPN) plays an important role in many physiological and pathological processes (wound healing, inflammation, immune response, and tumorigenesis). This meta-analysis assessed the diagnostic value of osteopontin in ovarian cancer.ConclusionsOsteopontin could be a useful biomarker in diagnosis of ovarian cancer. Due to the design deficits of the eligible studies, a future study with a larger sample size and better design is needed to rigorously confirm the diagnostic potential of osteopontin in ovarian cancer.