Flexible structures of SIBLING proteins, bone sialoprotein, and osteopontin

Steroid regulation of cell specific secreted phosphoprotein 1 (osteopontin) expression in the pregnant porcine uterus

Secreted phosphoprotein 1 (SPP1, commonly referred to as osteopontin and formerly known as bone sialoprotein 1, early T-lymphocyte activation 1) is an extracellular matrix/adhesion molecule that is upregulated in the pregnant uterus of all mammals examined to date. This study focused on the pig, which has true epitheliochorial placentation and exhibits induction of SPP1 mRNA in luminal epithelium (LE) just before conceptus attachment and in glandular epithelium (GE) after Day 30 of pregnancy. The objective of this study was to determine steroid regulation of SPP1 mRNA and protein in porcine uterine epithelium. To examine the effect of estrogen, cyclic gilts were treated daily (Days 11-14) with 5 mg estradiol benzoate (i.m.) and hysterectomized on Day 15. To evaluate the long-term effect of pseudopregnancy, cyclic gilts were given daily injections (Days 11-15) with steroid as above and hysterectomized on Day 90. In situ hybridization showed high expression of SPP1 mRNA only in LE contiguous with apposing conceptus tissue on Day 15 of pregnancy. In contrast, estrogen injection resulted in moderate but uniform SPP1 mRNA in all LE of Day 15 nonpregnant gilts, with expression maintained through Day 90 of pseudopregnancy. SPP1 mRNA also localized to the GE of Day 90 pseudopregnant gilts, similar to expression in late gestation. Consistent with in situ hybridization results, SPP1 protein localized to the apical surface of LE in all estrogen-treated gilts and in the GE on Day 90 of pseudopregnancy. We conclude that, in pregnant pigs, SPP1 is induced by conceptus estrogen in uterine LE and is regulated in GE in a manner coincident with CL/placental progesterone production.

Expression of dentin sialophosphoprotein in human prostate cancer and its correlation with tumor aggressiveness

Recent studies have demonstrated that two SIBLING family members, bone sialoprotein (BSP) and osteopontin (OPN), are overexpressed in human prostate cancer. The expression of these proteins is associated with the acquisition of a metastatic phenotype by cancer cells and a poor prognosis for the patient. Dentin sialophosphoprotein (DSPP) shares several structural and genetic features with OPN and BSP. The presence of DSPP has been recently established in salivary glands, indicating that its expression is not restricted to mineralized tissues. However, its potential expression in human tumors has not been addressed yet. In this study, we sought to evaluate the expression of DSPP in human prostate cancer. Immunohistochemistry was performed on 69 prostate cancer specimens using LFMb-21 anti-DSPP monoclonal antibody. All of the prostate cancer lesions examined expressed detectable levels of DSPP, as compared with no or low level of expression in adjacent normal glands (p < 0.0001). High grade prostatic intraepithelial neoplasia (HGPIN) glands generally displayed DSPP expression levels that were similar to those found in neighboring cancer glands. DSPP expression was significantly associated with the pathological stage (p = 0.0087) and the Gleason score (p = 0.0176) of the tumors. Western Blot was performed on 5 representative prostate tumor extracts and 3 prostatic tumor cell lines (PC3, LNCaP and DU145). All tumor extracts and cell lines analyzed have been found to express DSPP. In addition, in situ hybridization was used to assess the presence of DSPP mRNA. DSPP was detected at the RNA level in both HGPIN and tumoral glands. This study shows for the first time that DSPP is ectopically expressed in human prostate cancer. The expression of this SIBLING protein strongly correlates with conventional histopathological prognostic indicators of prostate cancer progression.

Caprine uterine and placental osteopontin expression is distinct among epitheliochorial implanting species

Osteopontin (OPN) is the most highly up-regulated extracellular matrix/adhesion molecule in the uterus of humans and domestic animals as it becomes receptive to implantation. Studies in sheep and pigs have shown that OPN is a component of ovine and porcine histotroph characterized by a complex temporal and spatial pattern of uterine and conceptus expression involving immune, epithelial, and stromal cells. It is proposed that these expression events are orchestrated to contribute to conceptus attachment and placentation. However, differences in OPN expression between sheep and pigs have been detected that relate to differences in placentation. Therefore, this study examined OPN expression in the caprine uterus and conceptus to gain insight into mechanisms underlying OPN function(s) during pregnancy through comparative analysis of differences in placentation between pigs, sheep, and goats. Goats were hysterectomized (n = 5/day) on Days 5, 11, 13, 15, 17 or 19 of the estrous cycle, and Days 5, 11, 13, 15, 17, 19 or 25 of pregnancy. Slot-blot hybridization showed increases in endometrial OPN mRNA beginning on Day 17 of the estrous cycle and Day 19 of pregnancy. In situ hybridization localized OPN mRNA to endometrial glandular epithelium (GE), Day 25 myometrium, and cells scattered within the placenta hypothesized to be immune. Immunofluorescence microscopy detected OPN protein on the apical surface of endometrial lumenal epithelium (LE), in GE, and on conceptus (Tr). Western blot analysis detected primarily the native 70-kDa OPN protein in endometrial extracts from the estrous cycle and pregnancy, as well as in uterine flushings from pregnant goats. Co-induction of OPN and alpha-smooth muscle actin, but not desmin proteins, was observed in uterine stroma by Day 25 of pregnancy. OPN in cyclic GE, Day 25 myometrium, and desmin-negative endometrial stroma is unique and reflects subtle differences among superficial implanting species that correlate with the depth of Tr invasion.

Importance of phosphorylation for osteopontin regulation of biomineralization

Previous in vitro and in vivo studies demonstrated that osteopontin (OPN) is an inhibitor of the formation and growth of hydroxyapatite (HA) and other biominerals. The present study tests the hypotheses that the interaction of OPN with HA is determined by the extent of protein phosphorylation and that this interaction regulates the mineralization process. Bone OPN as previously reported inhibited HA formation and HA-seeded growth in a gelatin-gel system. A transglutaminase-linked OPN polymer had similar effects. Recombinant, nonphosphorylated OPN and chemically dephosphorylated OPN, had no effect on HA formation or growth in this system. In contrast, highly phosphorylated milk OPN (mOPN) promoted HA formation. The mOPN stabilized the conversion of amorphous calcium phosphate (a non-crystalline constituent of milk) to HA, whereas bone OPN had a lesser effect on this conversion. Mixtures of OPN and osteocalcin known to form a complex in vitro, unexpectedly promoted HA formation. To test the hypothesis that small alterations in protein conformation caused by phosphorylation account for the differences in the observed ability of OPN to interact with HA, the conformation of bone OPN and mOPN in the presence and absence of crystalline HA was determined by attenuated total reflection (ATR) infrared (IR) spectroscopy. Both proteins exhibited a predominantly random coil structure, which was unaffected by the addition of Ca(2+). Binding to HA did not alter the secondary structure of bone OPN, but induced a small increase of beta-sheet (few percent) in mOPN. These data taken together suggest that the phosphorylation of OPN is an important factor in regulating the OPN-mediated mineralization process.

A review of protein structure and gene organisation for proteins associated with mineralised tissue and calcium phosphate stabilisation encoded on human chromosome 4

Several proteins associated with mineralised tissue (teeth and bone) or involved in calcium phosphate stabilisation in the body fluids, milk and saliva have been mapped to the q arm of human chromosome 4. These include the dentine/bone proteins dentine sialophosphoprotein (DSPP), dentine matrix protein 1 (DMP1), bone sialoprotein (BSP), matrix extracellular phosphoglycoprotein, osteopontin (OPN), enamelin, ameloblastin, milk caseins, salivary statherin, and proline-rich proteins. The proposed function of those that are multiphosphorylated is: (i) the stabilisation of calcium phosphate in solution (e.g. casein, statherin) preventing spontaneous precipitation and seeded-crystal growth or (ii) promoting biomineralisation (e.g. the phosphophoryn domain of DSPP), where the protein described as a template macromolecule, is proposed to act as a nucleator/promoter of crystal growth. The genes of these proteins have been subjected to conserved chromosomal synteny during mammalian evolution. The multiphosphorylated proteins statherin, caseins, phosphophoryn, BSP and OPN have been characterised as intrinsically disordered. The codon usage patterns for the amino acid serine reveal a bias for AGC and AGT codons within the human genes dspp, dmp1 and bsp, mouse dspp and dmp1 but not significantly for statherin or caseins. This pattern was also observed in the gene encoding hen phosvitin that also contains stretches of multiphosphorylated serines and in the dmp1 gene sequences of mammalian, reptilian and avian classes. In conclusion, these intrinsically disordered multiphosphorylated proteins are the translation products of genes displaying examples of codon usage bias, internal repeats and conserved chromosomal synteny within the mammalian class.

Renal expression of SIBLING proteins and their partner matrix metalloproteinases (MMPs)

BACKGROUND

Three members of the small integrin-binding ligand N-linked glycoprotein (SIBLING) family of proteins have recently been shown to bind and activate specific promatrix metalloproteinases (MMPs) and to overcome the inhibition of tissue inhibitors of MMPs (TIMPs). Although usually associated with mineralized tissues, we have shown that the SIBLINGs and their MMP partners, when known, are coexpressed in salivary gland ductal cells. The present study examined the expression patterns of both the SIBLINGs and their MMP partners in adult kidney.

METHODS

The expression patterns of all five SIBLINGs known to date, and their MMP partners were determined in monkey kidney using immunohistochemistry and in situ hybridization techniques.

RESULTS

Bone sialoprotein (BSP) and its partner, MMP-2, were coexpressed in both the proximal and distal tubules. Osteopontin, as previously shown, was expressed in the distal tubules while its partner MMP-3 was expressed in both the proximal tubule and distal tubles. Dentin matrix protein-1 (DMP1) and MMP-9 were coexpressed throughout the nephron, including both parietal cells of Bowman's capsule and the thin limb of the loop of Henle. Dentin sialophosphoprotein (DSPP) and matrix extracellular phosphoglycoprotein (MEPE) were expressed in the proximal tubule and distal tubule, and proximal tubule, respectively.

CONCLUSION

In contrast to salivary gland in which all SIBLINGs and their MMP partners were coexpressed throughout the length of the ducts, these proteins were differentially expressed within the normal adult nephron. We hypothesize that the cells use the SIBLING/MMP pairs in the normal turnover of cell surface proteins and/or pericellular matrix proteins such as those in basement membranes.

Identification of the Type I Collagen-binding Domain of Bone Sialoprotein and Characterization of the Mechanism of Interaction

Bone sialoprotein (BSP) is an anionic phosphorylated glycoprotein that is expressed almost exclusively in mineralized tissues and has been shown to be a potent nucleator of hydroxyapatite formation. The binding of BSP to collagen is thought to be important for the initiation of bone mineralization and in the adhesion of bone cells to the mineralized matrix. Using a solid phase assay, we have investigated the interaction between BSP and collagen. Initial studies showed that raising the ionic strength, decreasing the pH below 7, or introducing divalent cations diminishes but does not abolish the binding of BSP to collagen, indicating that the interaction is only partly electrostatic in nature. Both bone-extracted and recombinant (r)BSP exhibited similar binding affinities, indicating that post-translational modifications are not critical for binding. To identify the collagen-binding domain, recombinant peptides of BSP were studied. Peptide rBSP-(1-100) binds to type I collagen with an affinity similar to that of full-length rBSP, whereas peptides containing the sequences 99-201 or 200-301 do not bind. Further studies showed that rBSP-(1-75) competitively inhibits the binding of rBSP-(1-100), whereas rBSP-(21-100) inhibits binding to a lesser extent, and rBSP-(43-100) does not inhibit binding. These results suggest that the collagen-binding site of rat BSP is within the sequence 21-42, with residues N-terminal of this region likely also involved. This site was confirmed by the demonstration of collagen-binding activity of a synthetic peptide corresponding to residues 19-46. The collagen-binding domain, which is highly conserved among species, is enriched in hydrophobic residues and lacks acidic residues. We conclude that residues 19-46 of BSP represent a novel collagen-binding site.

Osteopontin (Eta-1) is present in the rat basal ganglia

Osteopontin (OPN) is a secreted glycosylated phosphoprotein that is responsive to oxidative stress and inflammation and controls cytokine production, inducible nitric oxide synthase (iNOS) expression and apoptotic cell death. In this study, we demonstrate the presence of OPN in the rat basal ganglia. Using reverse transcriptase polymerase chain reaction (RT-PCR), OPN cDNA was found in the substantia nigra, and striatum. The presence of OPN mRNA was demonstrated in the same areas of the basal ganglia, using in situ hybridisation. OPN protein was found in the SN, using Western blotting and confirmed by immunohistochemistry. The protein was localised to neurones but not to microglia or astroglia. This is the first report of the presence of OPN in the basal ganglia where it may be involved in the maintenance of neuronal viability.

Dentin matrix protein 1 gene cis-regulation: use in osteocytes to characterize local responses to mechanical loading in vitro and in vivo

Dentin matrix protein 1 (DMP1) is highly expressed in osteocytes and is mechanically responsive. To study osteocyte-specific and mechanically regulated DMP1 gene expression, the transcriptional activity of three cis-regulatory regions was first examined in an osteoblast differentiation model in vitro using a green fluorescent protein (GFP) reporter. Expression of the -9624 to +1996 bp (10 kb) and -7892 to +4439 bp (8 kb) DMP1 cis-regulatory regions dramatically increased in areas of mineralized matrix, in dendritic, osteocyte-like cells. Mineralizing cultures expressing the 8-kb construct show dramatic GFP increases in response to loading in cells with a dendritic morphology. Transgenic mice expressing the 8-kb DMP1-GFP and -2433 to +4439 bp (2.5 kb) DMP1-LacZ were generated. Osteocyte-specific expression was found with the 8 kb but not with the 2.5 kb in postnatal animals. However, the 2.5 kb could support expression in rapidly forming osteoblasts and pre-osteocytes in the embryo. Primary calvarial osteoblast cultures demonstrated that the 2.5 kb supports weak expression in a subset of osteoblasts and pre-osteocytes, but not in mature osteocytes. However, the 8 kb supports robust expression in primary bone marrow cultures. Therefore the region -7892 to -2433 bp, termed a 5.5-kb "Osteocyte Enhancer Module," appears to be required for osteocyte specificity. Ulnae of mice with the 8-kb DMP1-GFP were subjected to mechanical loading where GFP expression increased selectively and locally in osteocytes, distal to the mid-shaft and near the surface of the bone. Thus, the 8-kb region of the DMP1 gene is a target for mechanotransduction in osteocytes, and its cis-regulatory activity may be correlated to local strain in bone.

Characterization of a rat model with site-specific bone metastasis induced by MDA-MB-231 breast cancer cells and its application to the effects of an antibody against bone sialoprotein

Metastasis into the skeleton is a serious complication of certain neoplastic diseases such as breast, prostate and lung cancer, but the reasons for this osteotropism are poorly understood. Our aim was to establish a physiologically relevant animal model that is characterized by osteolytic lesions confined to the hind leg of nude rats. For this purpose, we injected 1x10(5) MDA-MB-231 human breast cancer cells transfected with GFP into the superficial epigastric artery, which is an anastomosing vessel between the femoral and iliac arteries. As assessed with the aid of X-rays, computed tomography and immunohistochemisty, osteolytic lesions occurred exclusively in the femur, tibia and fibula of the animals. The tumor take rate was 93% in a series of 96 rats and the increase in lesion size was observed up to 110 days after tumor cell inoculation. When applying this animal model to the effects of an antibody against bone sialoprotein (BSP), a significantly reduced osteolytic lesion size was observed after preincubation of cells (2 hr, 600 microg/ml anti-BSP) prior to intra-arterial tumor cell injection resulting in 19 T/C% at day 60 after tumor implantation (p < 0.05). In addition, the osteolytic lesion size was also significantly reduced after s.c. treatment of the animals with the antibody (20 mg/kg anti-BSPx3 within 5 days after tumor implantation), resulting in 30 T/C% at day 60 after tumor cell implantation (p < 0.05). In conclusion, the novel rat model for site-specific osteolytic lesions provides in vivo evidence that preincubation of MDA-MB-231GFP cells and treatment of rats after tumor implantation with an antibody against BSP significantly reduces the size of lytic lesions in bone.

Osteopontin Distribution in the Canine Skeleton during Growth and Structural Maturation

The distribution of osteopontin (OPN) was studied immunohistochemically in cells and extracellular matrix in the humerus, scapula, and lumbar vertebrae of growing (age: 6 weeks, 12 weeks, 4.5 months) and adult dogs. OPN was expressed in hypertrophic chondrocytes of epiphyseal cartilage and in chondrocytes of the deep zone of mature articular cartilage, where extracellular matrix was also stained. OPN expression was strongest in 4.5-month-old puppies in cells of the osteoblastic lineage. It also varied with microlocation and was pronounced in areas prone to resorption due to modelling and remodelling activities. Osteoclasts were always strongly labelled with OPN. OPN deposition in extracellular bone matrix was detected particularly as a delineation of cartilage cores within secondary trabeculae and as a lining of the trabecular surfaces in resorption microlocations. The OPN distribution pattern is discussed here for each cell population with regard to its functional implications.

Osteopontin and related SIBLING glycoprotein genes are expressed by Sertoli cells during mouse testis development

Matrix proteins play important roles in tissue morphogenesis. We have studied the expression of genes encoding the related SIBLING glycoproteins osteopontin (OPN), bone sialoprotein (BSP), and dentin matrix protein (DMP) during the development of male and female gonads during mouse embryogenesis. Opn mRNA was expressed specifically by Sertoli cells of the developing testis cords, in the mesonephric tubules of both sexes, and, transiently, in the Müllerian ducts of both sexes, as determined by whole-mount and section in situ hybridization. OPN protein was detected in the cytoplasm of Sertoli cells and luminal cells of the mesonephric tubules, with small amounts associated with the plasma membrane of germ cells. We found no defects in developing testes of Opn-/- mice using a range of cell type-specific markers, suggesting that other SIBLING proteins may function in testis development. Dmp and Bsp mRNA was also expressed in the developing testis cords, supporting the view that all three SIBLING proteins may contribute to testis differentiation.

Surface plasmon resonance (SPR) confirms that MEPE binds to PHEX via the MEPE-ASARM motif: a model for impaired mineralization in X-linked rickets (HYP)

Matrix Extracellular Phospho-glycoprotEin (MEPE) and proteases are elevated and PHEX is defective in HYP. PHEX prevents proteolysis of MEPE and release of a protease-resistant MEPE-ASARM peptide, an inhibitor of mineralization (minhibin). Thus, in HYP, mutated PHEX may contribute to increased ASARM peptide release. Moreover, binding of MEPE by PHEX may regulate this process in normal subjects. The nature of the PHEX-MEPE nonproteolytic interaction(s) (direct or indirect) is/are unknown. Our aims were to determine (1) whether PHEX binds specifically to MEPE, (2) whether the binding involves the ASARM motif region, and (3) whether free ASARM peptide affects mineralization in vivo in mice. Protein interactions between MEPE and recombinant soluble PHEX (secPHEX) were measured using surface plasmon resonance (SPR). Briefly, secPHEX, MEPE, and control protein (IgG) were immobilized on a Biacore CM5 sensor chip, and SPR experiments were performed on a Biacore 3000 high-performance research system. Pure secPHEX was then injected at different concentrations, and interactions with immobilized proteins were measured. To determine MEPE sequences interacting with secPHEX, the inhibitory effects of MEPE-ASARM peptides (phosphorylated and nonphosphorylated), control peptides, and MEPE midregion RGD peptides on secPHEX binding to chip-immobilized MEPE were measured. ASARM peptide and etidronate-mediated mineralization inhibition in vivo and in vitro were determined by quenched calcein fluorescence in hind limbs and calvariae in mice and by histological Sanderson stain. A specific, dose-dependent and Zn-dependent protein interaction between secPHEX and immobilized MEPE occurs (EC50 of 553 nM). Synthetic MEPE PO4-ASARM peptide inhibits the PHEX-MEPE interaction (K(D(app)) = 15 uM and B(max/inhib) = 68%). In contrast, control and MEPE-RGD peptides had no effect. Subcutaneous administration of ASARM peptide resulted in marked quenching of fluorescence in calvariae and hind limbs relative to vehicle controls indicating impaired mineralization. Similar results were obtained with etidronate. Sanderson-stained calvariae also indicated a marked increase in unmineralized osteoid with ASARM peptide and etidronate groups. We conclude that PHEX and MEPE form a nonproteolytic protein interaction via the MEPE carboxy-terminal ASARM motif, and the ASARM peptide inhibits mineralization in vivo. The binding of MEPE and ASARM peptide by PHEX may explain why loss of functional osteoblast-expressed PHEX results in defective mineralization in HYP.

Phosphophoryn regulates the gene expression and differentiation of NIH3T3, MC3T3-E1, and human mesenchymal stem cells via the integrin/MAPK signaling pathway

Extracellular matrix proteins (ECMs) serve as both a structural support for cells and a dynamic biochemical network that directs cellular activities. ECM proteins such as those of the SIBLING family (small integrin-binding ligand glycoprotein) could possess inherent growth factor activity. In this study, we demonstrate that exon 5 of dentin matrix protein 3 (phosphophoryn (PP)), a non-collagenous dentin ECM protein and SIBLING protein family member, up-regulates osteoblast marker genes in primary human adult mesenchymal stem cells (hMSCs), a mouse osteoblastic cell line (MC3T3-E1), and a mouse fibroblastic cell line (NIH3T3). Quantitative real-time PCR technology was used to quantify gene expression levels of bone markers such as Runx2, Osx (Osterix), bone/liver/kidney Alp (alkaline phosphatase), Ocn (osteocalcin), and Bsp (bone sialoprotein) in response to recombinant PP and stably transfected PP. PP up-regulated Runx2, Osx, and Ocn gene expression. PP increased OCN protein production in hMSCs and MC3T3-E1. ALP activity and calcium deposition was increased by PP in hMSC. Furthermore, an alpha(v)beta(3) integrin-blocking antibody significantly inhibited recombinant PP-induced expression of Runx2 in hMSCs, suggesting that signaling by PP is mediated through the integrin pathway. PP was also shown to activate p38, ERK1/2, and JNK, three components of the MAPK pathway. These data demonstrate a novel signaling function for PP in cell differentiation beyond the hypothesized role of PP in biomineralization.

Dmp1-deficient mice display severe defects in cartilage formation responsible for a chondrodysplasia-like phenotype

Understanding the molecular mechanisms by which cartilage formation is regulated is essential toward understanding the physiology of both embryonic bone development and postnatal bone growth. Although much is known about growth factor signaling in cartilage formation, the regulatory role of noncollagenous matrix proteins in this process are still largely unknown. In the present studies, we present evidence for a critical role of DMP1 (dentin matrix protein 1) in postnatal chondrogenesis. The Dmp1 gene was originally identified from a rat incisor cDNA library and has been shown to play an important role in late stage dentinogenesis. Whereas no apparent abnormalities were observed in prenatal bone development, Dmp1-deficient (Dmp1(-/-)) mice unexpectedly develop a severe defect in cartilage formation during postnatal chondrogenesis. Vertebrae and long bones in Dmp1-deficient (Dmp1(-/-)) mice are shorter and wider with delayed and malformed secondary ossification centers and an irregular and highly expanded growth plate, results of both a highly expanded proliferation and a highly expanded hypertrophic zone creating a phenotype resembling dwarfism with chondrodysplasia. This phenotype appears to be due to increased cell proliferation in the proliferating zone and reduced apoptosis in the hypertrophic zone. In addition, blood vessel invasion is impaired in the epiphyses of Dmp1(-/-) mice. These findings show that DMP1 is essential for normal postnatal chondrogenesis and subsequent osteogenesis.

Functional regulation of T lymphocytes by modulatory extracellular matrix proteins

In addition to the major structural molecules, which are constitutively present in extracellular matrices, several proteins appear in the extracellular matrix only at specific stages in development or in association with specific pathological conditions. These proteins include thrombospondin-1 and -2, tenascin C, osteopontin, members of the cysteine-rich 61/connective tissue growth factor/nephroblastoma overexpressed family, and secreted protein acidic and rich in cysteine (osteonectin). These proteins play important roles in regulating cell fate during development and in the pathogenesis of several diseases in adult animals. We will review the interactions of T cells with this class of molecules and their resulting effects on T cell behavior. Receptors and signal transduction pathways that mediate the actions of matricellular proteins on T cells are beginning to be defined. Transgenic mice are providing new insights into the functions of these proteins in vivo and are yielding insights into the significance of their reported dysregulation in several human diseases.

Serum MEPE-ASARM-peptides are elevated in X-linked rickets (HYP): implications for phosphaturia and rickets

MEPE (Matrix Extracellular PhosphoglycoprotEin) expression is markedly elevated in X-linked-hypophosphatemic-rickets (HYP) and tumor-induced osteomalacia (TIO). In normal individuals, circulating serum-levels of MEPE are tightly correlated with serum-phosphorus, parathyroid hormone (PTH) and bone mineral density (BMD). Also, MEPE derived, C-terminal ASARM-peptides are candidate minhibins and/or phosphatonins. Our aims were to determine: 1. whether MEPE-ASARM-peptide(s) are abnormally elevated in HYP/hyp serum, and, 2. whether the ASARM-peptide(s) accumulate in hyp mice kidney renal-tubules. Using a specific competitive ELISA we measured a five fold increase (P=0.007) of serum ASARM-peptide(s) in human HYP patients (normal subjects 3.25 microM n=9; S.E.M.=0.51 and HYP-patients 15.74 microM, n=9; S.E.M.=3.32). A 6.23 fold increase (P=0.008) was measured in hyp male mice compared with their normal male siblings (normal-siblings, 3.73 muM, S.E.M.=0.57, n=3; and hyp-mice 23.4 microM, n=3, S.E.M.=4.01). Renal immuno-histological screening also revealed a dramatic increase of ASARM-peptides in regions anatomically consistent with the proximal convoluted tubules. This study demonstrates for the first time that markedly elevated serum levels of protease-resistant ASARM-peptide(s) occur in HYP/hyp and they accumulate in murine hyp kidneys. These peptides are thus likely responsible for the phosphaturia and defective mineralization in HYP/hyp and TIO.

Expression of SIBLINGs and their partner MMPs in salivary glands

Three members of the SIBLING family of integrin-binding phosphoglycoproteins (bone sialoprotein, BSP; osteopontin, OPN; and dentin matrix protein-1, DMP1) were recently shown to bind with high affinity (nM) and to activate 3 different matrix metalloproteinases (MMP-2, MMP-3, and MMP-9, respectively) in vitro. The current study was designed to document the possible biological relevance of the SIBLING-MMP activation pathway in vivo by showing that these 3 SIBLINGs and their known MMP partners are co-expressed in normal adult tissue. BSP, OPN, and DMP1 were invariably co-expressed with their partner MMPs in salivary glands of humans and mice. The 2 SIBLING proteins without known MMP partners, dentin sialophosphoprotein (DSPP) and matrix extracellular phosphoglycoprotein (MEPE), were also expressed in salivary glands. Expression of all SIBLINGs in this normal, non-mineralizing epithelial tissue suggests that they serve at least one function in vivo other than directly promoting matrix mineralization--a function we hypothesize involves local activation of MMPs.

Calcium oxalate crystal localization and osteopontin immunostaining in genetic hypercalciuric stone-forming rats

BACKGROUND

The inbred genetic hypercalciuric stone-forming (GHS) rats develop calcium phosphate (apatite) stones when fed a normal 1.2% calcium diet. The addition of 1% hydroxyproline to this diet does not alter the type of stone formed, while rats fed this diet with 3% hydroxyproline form mixed apatite and calcium oxalate stones and those with 5% hydroxyproline added form only calcium oxalate stones. The present study was designed to determine the localization of stone formation and if this solid phase resulted in pathologic changes to the kidneys.

METHODS

GHS rats were fed 15 g of the standard diet or the diet supplemented with 1%, 3%, or 5% hydroxyproline for 18 weeks. A separate group of Sprague-Dawley rats (the parental strain of the GHS rats), fed the standard diet for a similar duration, served as an additional control. At 18 weeks, all kidneys were perfusion-fixed for structural analysis, detection of crystal deposits using the Yasue silver substitution method, and osteopontin immunostaining.

RESULTS

There were no crystal deposits found in the kidneys of Sprague-Dawley rats. Crystal deposits were found in the kidneys of all GHS rats and this Yasue-stained material was detected only in the urinary space. No crystal deposits were noted within the cortical or medullary segments of the nephron and there was no evidence for tubular damage in any group. The only pathologic changes occurred in 3% and 5% hydroxyproline groups with the 5% group showing the most severe changes. In these rats, which form only calcium oxalate stones, focal sites along the urothelial lining of the papilla and fornix of the urinary space demonstrated a proliferative response characterized by increased density of urothelial cells that surrounded the crystal deposits. At the fornix, some crystals were lodged within the interstitium, deep to the proliferative urothelium. There was increased osteopontin immunostaining in the proliferating urothelium.

CONCLUSION

Thus in the GHS rat, the initial stone formation occurred solely in the urinary space. Tubular damage was not observed with either apatite or calcium oxalate stones. The apatite stones do not appear to cause any pathological change while those rats forming calcium oxalate stones have a proliferative response of the urothelium, with increased osteopontin immunostaining, around the crystal deposits in the fornix.

Downregulation of osteopontin and bone sialoprotein II is related to reduced colony formation and metastasis formation of MDA-MB-231 human breast cancer cells

Osteopontin (OPN), bone sialoprotein (BSPII), and osteonectin (ON) belong to a family of glycoproteins, which have been linked to cancer metastasis and progression. Here, we report on the selection of antisense oligonucleotides (ASOs), which are effective in reducing their protein levels. In human MDA-MB-231 breast cancer cells, the maximum inhibition of protein expression ranged from 84% (OPN) to 75% (BSPII) and 70% (ON). Erucylphospho-NNN-trimethylpropanolamine (ErPC3) was used as positive control and combination partner. Exposure to ErPC3 inhibited colony formation of MDA-MB-231 cells by 11% (10 microM), 45% (14 microM) and 78% (20 microM). The clonogenicity of breast cancer cells was reduced by 15%, 11%, 8% (5 microM), 39%, 19%, 14% (10 microM) and 46%, 39%, 21% (20 microM) in response to ASO-OPN-04, ASO-BSPII-06 and ASO-ON-03, respectively. Combination of ErPC3 with the ASOs caused additive combination effects. Pre-exposure to the ASOs, but not to the NSO, inhibited formation of osteolytic metastasis in three of four (ASO-OPN-04, P<0.03) and two of four (ASO-BSPII-06) nude rats, and reduced metastasis lesions significantly (T/C%=4.3 and 9.1, P=0.05, respectively). We conclude that downregulation of OPN and BSPII reduces colony formation of MDA-MB-231 cells and formation of osteolytic metastasis in nude rats.

The wrickkened pathways of FGF23, MEPE and PHEX

The last 350 years since the publication of the first medical monograph on rickets (old English term wrickken) (Glisson et al., 1651) have seen spectacular advances in our understanding of mineral-homeostasis. Seminal and exciting discoveries have revealed the roles of PTH, vitamin D, and calcitonin in regulating calcium and phosphate, and maintaining healthy teeth and skeleton. However, it is clear that the PTH/Vitamin D axis does not account for the entire picture, and a new bone-renal metabolic milieu has emerged, implicating a novel set of matrix proteins, hormones, and Zn-metallopeptidases. The primary defects in X-linked hypophosphatemic rickets (HYP) and autosomal-dominant hypophosphatemic rickets (ADHR) are now identified as inactivating mutations in a Zn-metalloendopeptidase (PHEX) and activating mutations in fibroblast-growth-factor-23 (FGF23), respectively. In oncogenic hypophosphatemic osteomalacia (OHO), several tumor-expressed proteins (MEPE, FGF23, and FRP-4) have emerged as candidate mediators of the bone-renal pathophysiology. This has stimulated the proposal of a global model that takes into account the remarkable similarities between the inherited diseases (HYP and ADHR) and the tumor-acquired disease OHO. In HYP, loss of PHEX function is proposed to result in an increase in uncleaved full-length FGF23 and/or inappropriate processing of MEPE. In ADHR, a mutation in FGF23 results in resistance to proteolysis by PHEX or other proteases and an increase in half-life of full-length phosphaturic FGF23. In OHO, over-expression of FGF23 and/or MEPE is proposed to result in abnormal renal-phosphate handling and mineralization. Although this model is attractive, many questions remain unanswered, suggesting a more complex picture. The following review will present a global hypothesis that attempts to explain the experimental and clinical observations in HYP, ADHR, and OHO, plus diverse mouse models that include the MEPE null mutant, HYP-PHEX transgenic mouse, and MEPE-PHEX double-null-mutant.

Expression of dentin matrix protein 1 (DMP1) during fracture healing

Dentin matrix protein 1 (DMP1) is one of the acidic phosphorylated extracellular matrix proteins called the SIBLING (small integrin-binding ligand, N-linked glycoproteins) family. Recent studies showed that DMP1 is expressed in the mineralized tissues and suggested that DMP1 is involved in the mineralization. We investigated the precise localization of DMP1 messenger RNA (mRNA) and protein during fracture healing. In situ hybridization demonstrated that DMP1 mRNA was strongly expressed in preosteocytes and osteocytes in the bony callus during intramembranous and endochondral ossification while DMP1 mRNA was not detected in osteoblasts and chondrocytes. During endochondral ossification, however, a low number of DMP1-expressing cells were identified in the cluster of hypertrophic chondrocytes. However, these DMP1-expressing cells were not hypertrophic and were likely to be osteoblast-lineage cells, which were embedded in the matrix of bone or cartilage, because type I collagen-expressing cells and invasion of capillary vessels were observed in the same area. Northern blot, in situ hybridization, and immunohistochemical analyses showed that DMP1 mRNA and protein expressions were increased until day 14 postfracture, when bony callus was formed, and then declined to a lower level during remodeling of the bony callus. Therefore, DMP1 is likely to play an important role in the mineralization of the bony callus.

Bone Sialoprotein, Matrix Metalloproteinase 2, and v 3 Integrin in Osteotropic Cancer Cell Invasion

BACKGROUND

Bone sialoprotein (BSP) interacts separately with both matrix metalloproteinase 2 (MMP-2) and integrin alpha(v)beta3 and is overexpressed in many metastatic tumors. Its role in tumor biology, however, remains unclear. We investigated whether BSP enhances cancer cell invasiveness by forming a trimolecular complex with MMP-2 and cell-surface integrin alpha(v)beta3.

METHODS

Invasiveness of breast, prostate, lung, and thyroid tumor cell lines was measured with a modified Boyden chamber assay. Binding and co-localization of BSP, MMP-2, and integrin alpha(v)beta3 were investigated with immunoprecipitation and in situ hybridization. All statistical tests were two-sided.

RESULTS

Treatment with BSP increased invasiveness of many breast, prostate, lung, and thyroid cancer cells through Matrigel in a dose-dependent manner. BSP at 50 nM increased the invasiveness of SW-579 thyroid cancer cells (95.2 units, 95% confidence interval [CI] = 90.4 to 100 units) by approximately 10-fold compared with that of untreated control SW-579 cells (9.1 units, 95% CI = 5.7 to 12.5 units) (P<.001). Addition of an inactive mutated BSP, in which BSP's integrin-binding RGD tripeptide was altered, or addition of integrin alpha(v)beta3-blocking antibodies resulted in invasiveness equivalent to that of untreated cells. Inhibiting cellular MMP-2 activity with chemical inhibitors or a specific antibody also blocked BSP-enhanced invasiveness. Osteopontin and dentin matrix protein 1, proteins related to BSP that also bind integrin alpha(v)beta3 and form complexes with other MMPs (but not MMP-2), did not enhance invasiveness. Immunoprecipitation showed that a complex containing BSP, integrin alpha(v)beta3, and MMP-2 formed in vitro. Addition of BSP increased the amount of MMP-2 bound by cells in an integrin-dependent fashion. Co-expression of BSP, integrin alpha(v)beta3, and MMP-2 in papillary thyroid carcinoma cells was shown by in situ hybridization.

CONCLUSION

Cancer cells appear to become more invasive when BSP forms a cell-surface trimolecular complex by linking MMP-2 to integrin alpha(v)beta3.

The role of osteopontin in lung disease

Osteopontin (Opn) is a multifunctional protein independently discovered by investigators from diverse scientific backgrounds and implicated in a broad array of pathological processes. Opn exists both intra- and extracellularly and in numerous pre- and post-translational isoforms. Structurally Opn resembles a matrix protein yet it has well-characterized cytokine like properties including the regulation of cellular migration and cell-mediated immunity. It has thus been classified as both a matricellular protein and a cytokine. Opn is among the most abundantly expressed proteins in a range of lung diseases and has been shown to regulate aspects of pulmonary granuloma formation, fibrosis, and malignancy. Future studies will explore the diagnostic and therapeutic potential of modulating the function of Opn in vivo.

Osteopontin enhances the cell proliferation induced by the epidermal growth factor in human prostate cancer cells

BACKGROUND

Susceptibility to extracellular matrix and growth factors has been demonstrated to play a critical role in the development of prostate cancer (PCa) metastases. The aim of this study was to elucidate some mechanisms by which stroma controls tumor progression.

METHODS

In our study we tested the growth ability of the LNCaP human prostatic cell line in steroid-free culture conditions in response to osteopontin (OPN), a non-collageneous matrix protein, localized in large amounts in the bone.

RESULTS

In the LNCaP cell model, OPN stimulates cell proliferation in serum-free medium and colony growth at high dilution but this effect is visible only in presence of epidermal growth factor (EGF). Proliferation induced by OPN is accompanied by a sustained activation of EGF receptor (EGFR) whose phosphorylation is detectable up to 12 hr after treatment in association with EGF. The colocalization of integrin beta1, a ligand of OPN, and of EGFR on the cellular membrane, suggests that the association of these cell surface receptors may be the principal mechanism involved in the long-term activation of the EGFR.

CONCLUSIONS

Our data describe a new possible mechanism involved in the establishment of bone metastases which may also account for the formation of androgen-independent cellular clones, frequently responsible of the clinical progression of PCa.

Deletion of Dentin Matrix Protein-1 Leads to a Partial Failure of Maturation of Predentin into Dentin, Hypomineralization, and Expanded Cavities of Pulp and Root Canal during Postnatal Tooth Development

The dentin matrix protein-1 (DMP-1) gene is identified in odontoblasts during both embryonic and postnatal development. In vitro study suggests that this noncollagen acidic phosphoprotein plays a role in mineralization. However, deletion of the Dmp-1 gene has little effect on tooth development during embryogenesis. To address the role of DMP-1 in tooth during postnatal development, we analyzed changes of dentinogenesis in Dmp-1 null mice from 3 days after birth to 1 year. Here we show that Dmp-1 null mice postnatally develop a profound tooth phenotype characterized by a partial failure of maturation of predentin into dentin, enlarged pulp chambers, increased width of predentin zone with reduced dentin wall, and hypomineralization. The tooth phenotype of these mice is strikingly similar to that in dentin sialophosphoprotein (Dspp) null mice and shares some features of the human disease dentinogenesis imperfecta III. We have also demonstrated that DSPP levels are reduced in Dmp-1 null mice, suggesting that DSPP is probably regulated by DMP-1 during dentinogenesis. Finally, we show the absence or delayed development of the third molar in Dmp-1 null mice, which is probably secondary to defects in Dmp-1 null bone. Taken together, these studies suggest that DMP-1 is essential for later dentinogenesis during postnatal development.

In Vitro Effects of Dentin Matrix Protein-1 on Hydroxyapatite Formation Provide Insights into in Vivo Functions

Dentin matrix protein-1 (DMP1) is a mineralized tissue matrix protein synthesized by osteoblasts, hypertrophic chondrocytes, and ameloblasts as well as odontoblasts. DMP1 is believed to have multiple in vivo functions, acting both as a signaling molecule and a regulator of biomineralization. Using a cell-free system in vitro, we evaluated the action of DMP1 in the regulation of hydroxylapatite (HA) formation and crystal growth. The non-phosphorylated recombinant protein acted as an HA nucleator, increasing the amount of mineral formed in a gelatin gel HA growth system relative to protein-free controls. The recombinant protein phosphorylated in vitro had no detectable effect on HA formation and growth. In contrast, phosphorylated bovine DMP1 expressed in marrow stromal cells with an adenovirus vector containing 29.7 phosphates/mol was an effective inhibitor of HA formation and growth. The native full-length protein appeared to be absent or present in only small amounts in the extracellular matrix of bones and teeth. However, two highly phosphorylated fragments representing the N- and C-terminal portions of DMP1 have been identified, apparently arising from proteolytic cleavage of four X-Asp bonds. The highly phosphorylated C-terminal 57-kDa fragment (containing 42 phosphates/mol), like the non-phosphorylated DMP1, was an HA nucleator. These data suggest that, in its native form, DMP1 inhibits mineralization, but when cleaved or dephosphorylated, it initiates mineralization. These in vitro data are consistent with the findings in the DMP1 knockout mouse.

Osteoblast-like cell adhesion to bone sialoprotein peptides

A number of studies have suggested that biomimetic peptides can be used in the design of a new generation of prosthetic implants to promote the successful biointegration of the implant materials. In the current study, the in vitro bioactivities of several peptides representing RGD (Arg-Gly-Asp)-containing sequences of bone sialoprotein (BSP) toward an osteoblast-like cell line (MC3T3-E1) were examined to provide insight into the molecular basis of BSP's interaction with bone cells. BSP residues 283-288, 281-290, 278-293 and 278-302 were coated on polystyrene surfaces in 96-well non-tissue (untreated) culture plates, and their osteoblast adhesive properties compared to intact BSP and fibronectin as positive controls. BSP peptides 278-302 and 278-293 were found to be the most potent in their adhesive activity, increasing the number of adherent cells to 350% of control levels at an added concentration of 1 microM. Since these two peptides were equivalent in potency, it is suggested that the region 294-302 beyond the RGD domain is not necessary for cell binding. In comparison, peptides 283-288 and 281-290 were only active at concentrations greater than 200 microM. 50-70% of the peptide-stimulated adhesion was inhibited by the pretreatment of cell suspensions with solution phase RGD, suggesting that a portion of the peptides' adhesive effects was specific and integrin-mediated, although other non-RGD flanking regions were probably also involved in the mechanism of adhesion. Importantly, a modified BSP peptide, in which an aspartic acid residue at position 288 of the RGD sequence was replaced by a glutamic acid residue to form RGE, was completely inactive as a cell adhesion stimulus at concentrations up to 200 microM. Thus, despite the potential role of non-RGD flanking regions, an intact RGD tripeptide was essential for all of the adhesive activity of the BSP peptides.

Three small integrin‐binding ligand N‐linked glycoproteins (SIBLINGs) bind and activate specific matrix metalloproteinases

Matrix metalloproteinases (MMPs) are critical for development, wound healing, and for the progression of cancer. It is generally accepted that MMPs are secreted in a latent form (proMMP) and are activated only upon removal of their inhibitory propeptides. This report shows that three members of the SIBLING (Small, Integrin-Binding LIgand, N-linked Glycoprotein) family can specifically bind (Kd approximately equal nM) and activate three different MMPs. Binding of SIBLING to their corresponding proMMPs is associated with structural changes as indicated by quenching of intrinsic tryptophan fluorescence, increased susceptibility to plasmin cleavage, and decreased inhibition by specific natural and synthetic inhibitors. Activation includes both making the proMMPs enzymatically active and the reactivation of the TIMP (tissue inhibitors of MMP) inhibited MMPs. Bone sialoprotein specifically binds proMMP-2 and active MMP-2, while osteopontin binds proMMP-3 and active MMP-3, and dentin matrix protein-1 binds proMMP-9 and active MMP-9. Both pro and active MMP-SIBLING complexes are disrupted by the abundant serum protein, complement Factor H, thereby probably limiting SIBLING-mediated activation to regions immediately adjacent to sites of secretion in vivo. These data suggest that the SIBLING family offers an alternative method of controlling the activity of at least three MMPs.

Molecular modulation of calcium oxalate crystallization by osteopontin and citrate

Calcium oxalate monohydrate (COM), which plays a functional role in plant physiology, is a source of chronic human disease, forming the major inorganic component of kidney stones. Understanding molecular mechanisms of biological control over COM crystallization is central to development of effective stone disease therapies and can help define general strategies for synthesizing biologically inspired materials. To date, research on COM modification by proteins and small molecules has not resolved the molecular-scale control mechanisms. Moreover, because proteins directing COM inhibition have been identified and sequenced, they provide a basis for general physiochemical investigations of biomineralization. Here, we report molecular-scale views of COM modulation by two urinary constituents, the protein osteopontin and citrate, a common therapeutic agent. Combining force microscopy with molecular modeling, we show that each controls growth habit and kinetics by pinning step motion on different faces through specific interactions in which both size and structure determine the effectiveness. Moreover, the results suggest potential for additive effects of simultaneous action by both modifiers to inhibit the overall growth of the crystal and demonstrate the utility of combining molecular imaging and modeling tools for understanding events underlying aberrant crystallization in disease.

Nanotexturing of titanium-based surfaces upregulates expression of bone sialoprotein and osteopontin by cultured osteogenic cells

Bone formation around implants is influenced by surface geometry. Since cell/matrix/substrate interactions associated with cell signaling occur in the nanoscale dimension, we have evaluated the influence of nanotexturing of titanium-based surfaces on the expression of matrix proteins by cultured osteogenic cells at initial time points. Cells were obtained by enzymatic digestion of newborn rat calvaria and grown on titanium and titanium alloy discs with nanotextured or machined surfaces, and on glass coverslips for periods of 6 h, 1 day, and 3 days, under standard culture conditions. Cultures were processed for single or dual immunolabeling with monoclonal and/or polyclonal antibodies against bone sialoprotein (BSP), fibronectin (FN), osteopontin (OPN), type-I pro-collagen, or tubulin, followed by corresponding fluorophore-conjugated secondary antibodies. Some samples were processed for scanning electron microscope analysis of morphology and immunogold labeling. After 6 h, nanotextured surfaces exhibited up to a nine-fold increase in the proportion of cells with peripheral OPN labeling. At day 3, the proportion of OPN and BSP labeled cells was higher, and the intensity of immunoreactivity dramatically increased. No significant differences were observed in the expression pattern and the proportion of cells immunoreactive for FN or type-I pro-collagen. Our results demonstrate that nanotexturing of titanium-based surfaces upregulates the early expression of BSP and OPN in osteogenic cell cultures.

MEPE has the properties of an osteoblastic phosphatonin and minhibin

Matrix extracellular phosphoglycoprotein (MEPE) is expressed exclusively in osteoblasts, osteocytes and odontoblasts with markedly elevated expression found in X-linked hypophosphatemic rickets (Hyp) osteoblasts and in oncogenic hypophosphatemic osteomalacia (OHO) tumors. Because these syndromes are associated with abnormalities in mineralization and renal phosphate excretion, we examined the effects of insect-expressed full-length human-MEPE (Hu-MEPE) on serum and urinary phosphate in vivo, (33)PO(4) uptake in renal proximal tubule cultures and mineralization of osteoblast cultures. Dose-dependent hypophosphatemia and hyperphosphaturia occurred in mice following intraperitoneal (IP) administration of Hu-MEPE (up to 400 microg kg(-1) 31 h(-1)), similar to mice given the phosphaturic hormone PTH (80 microg kg(-1) 31 h(-1)). Also the fractional excretion of phosphate (FEP) was stimulated by MEPE [65.0% (P < 0.001)] and PTH groups [53.3% (P < 0.001)] relative to the vehicle group [28.7% (SEM 3.97)]. In addition, Hu-MEPE significantly inhibited (33)PO(4) uptake in primary human proximal tubule renal cells (RPTEC) and a human renal cell line (Hu-CL8) in vitro (V(max) 53.4% inhibition; K(m) 27.4 ng/ml, and V(max) 9.1% inhibition; K(m) 23.8 ng/ml, respectively). Moreover, Hu-MEPE dose dependently (50-800 ng/ml) inhibited BMP2-mediated mineralization of a murine osteoblast cell line (2T3) in vitro. Inhibition of mineralization was localized to a small (2 kDa) cathepsin B released carboxy-terminal MEPE peptide (protease-resistant) containing the acidic serine-aspartate-rich motif (ASARM peptide). We conclude that MEPE promotes renal phosphate excretion and modulates mineralization.

mRNA expression and protein localization of dentin matrix protein 1 during dental root formation

Dentin matrix protein 1 (DMP1) is an acidic phosphoprotein. DMP1 was initially detected in dentin and later in other mineralized tissues including cementum and bone, but the DMP1 expression pattern in tooth is still controversial. To determine the precise localization of DMP1 messenger RNA (mRNA) and the protein in the tooth, we performed in situ hybridization and immunohistochemical analyses using rat molars and incisors during various stages of root formation. During root dentin formation of molars, DMP1 mRNA was detected in root odontoblasts in parallel with mineralization of the dentin. However, the level of DMP1 mRNA expression in root odontoblasts decreased near the coronal part and was absent in coronal odontoblasts. DMP1 protein was localized along dentinal tubules and their branches in mineralized root dentin, and the distribution of DMP1 shifted from the end of dentinal tubules to the base of the tubules as dentin formation progressed. During the formation of the acellular cementum, DMP1 mRNA was detected in cementoblasts lining the acellular cementum where its protein was localized. During the formation of the cellular cementum, DMP1 mRNA was detected in cementocytes embedded in the cellular cementum but not in cementoblasts, and its protein was localized in the pericellular cementum of cementocytes including their processes. During dentin formation of incisors, DMP1 mRNA was detected in odontoblasts on the cementum-related dentin, where its protein was localized along dentinal tubules near the mineralization front. The localization of DMP1 mRNA and protein in dentin and cementum was related to their mineralization, suggesting that one of the functions of DMP1 may be involved in the mineralization of dentin and cementum during root formation.

The phosphatonin pathway: New insights in phosphate homeostasis

Serum phosphate concentrations are maintained within a defined range by processes that regulate the intestinal absorption and renal excretion of inorganic phosphate. The hormones currently believed to influence these processes are parathyroid hormone (PTH) and the active metabolite of vitamin D, 1alpha,25-dihydroxyvitamin D (1alpha,25(OH)2D). A new class of phosphate-regulating factors, collectively known as the phosphatonins, have been shown to be associated with the hypophosphatemic diseases, tumor-induced osteomalacia (TIO), X-linked hypophosphatemic rickets (XLH), and autosomal-dominant hypophosphatemic rickets (ADHR). These factors, which include fibroblast growth factor 23 (FGF23) and secreted frizzled-related protein 4 (FRP4), decrease extracellular fluid phosphate concentrations by directly reducing renal phosphate reabsorption and by suppressing 1alpha,25(OH)2D formation through the inhibition of 25-hydroxyvitamin D 1alpha-hydroxylase. The role of these substances under normal or pathologic conditions is not yet clear. For example, it is unknown whether any of the phosphatonins are directly responsible for the decreased concentrations of 1alpha,25(OH)2D observed in chronic and end-stage kidney disease or whether they are induced in an attempt to correct the hyperphosphatemia seen in late stages of chronic renal failure. Future experiments should clarify their physiologic and pathologic roles in phosphate metabolism.

Osteopontin: roles in implantation and placentation

Osteopontin (OPN) is an acidic member of the small integrin-binding ligand N-linked glycoprotein (SIBLING) family of extracellular matrix proteins/cytokines that undergoes extensive posttranslational modification, including phosphorylation, glycosylation, and cleavage, yielding molecular mass variants ranging in size from 25 to 75 kDa. The result is a versatile protein(s) with multiple functions arising from its role as a mediator of cell-cell and cell-extracellular matrix (ECM) communication that encompass both normal and tumorigenic developmental processes, immunological responses during inflammation and wound healing, and biomineralization. Studies in primates, pigs, sheep, and rodents have revealed that OPN is a major constituent of the uterine-placental microenvironment with influence as 1) a component of histotroph required for adhesion and signal transduction at the uterine-placental interface throughout pregnancy, 2) a gene product expressed by uterine stroma contributing to a decidualization-like transformation that correlates with the degree of conceptus invasiveness, and 3) a product of resident uterine and placental immune cells that may regulate their behavior and cytokine production. This minireview summarizes information regarding uterine and placental expression of OPN that has accumulated over the past 15 yr, and we briefly describe structural/functional properties of this protein that are likely relevant to its role(s) during pregnancy. Comparative studies have offered insights into the potential hormonal/cytokine, cellular, and molecular mechanisms underlying OPN-mediated adhesion, remodeling, and cell-cell/cell-ECM communication within the uterus and placenta. OPN has the potential to profoundly impact pregnancy, and investigators are now challenged to focus on the mechanistic nature of the functions of this multifaceted and major component of the uterine-placental microenvironment.

Differential expression of osteopontin and bone sialoprotein in bone metastasis of breast and prostate carcinoma

Breast and prostate cancer often metastasise to the skeleton. Interestingly, the histopathological characteristics of the bone lesions that arise from these two cancer types differ. Breast tumours give rise to metastases in the skeleton with a mixed lytic/sclerotic pattern, whereas a predominantly sclerotic pattern is seen in metastases from prostate tumours. Osteopontin (OPN) and bone sialoprotein (BSP) are bone matrix proteins that have been implicated in the selective affinity of cancer cells for bone. In the present study, 21 patient cases with skeletal metastasis and their respective primary tumours (12 with breast cancer, 9 with prostate cancer) were investigated by immunohistochemistry in order to assess the level of OPN and BSP. Moderate to strong OPN expression was found in 42% of all breast tumours and in 56% of all prostate tumours. Significantly more breast cancer bone metastases exhibited high OPN expression, 83%, as compared with prostate tumour bone metastases, 11% (P = 0.0019). In contrast, moderate to strong BSP expression was found in 33% of breast tumours and in 89% of prostate tumours. In the bone lesions, only 33% of breast tumour metastases showed moderate/strong BSP expression compared to 100% of prostate tumour metastases (P = 0.0046). This divergent pattern of OPN/BSP expression could be an important determinant for the different characteristics of these two types of bone metastasis, i.e., lytic vs. sclerotic, consistent with the proposed role of OPN in differentiation and activation of osteoclasts and of BSP as a stimulator of bone mineralisation.

Evidence for the proteolytic processing of dentin matrix protein 1. Identification and characterization of processed fragments and cleavage sites

Full-length cDNA coding for dentin matrix protein 1 (DMP1) has been cloned and sequenced, but the corresponding complete protein has not been isolated. In searching for naturally occurring DMP1, we recently discovered that the extracellular matrix of bone contains fragments originating from DMP1. Shortened forms of DMP1, termed 37K and 57K fragments, were treated with alkaline phosphatase and then digested with trypsin. The resultant peptides were purified by a two-dimensional method: size exclusion followed by reversed-phase high performance liquid chromatography. Purified peptides were sequenced by Edman degradation and mass spectrometry, and the sequences compared with the DMP1 sequence predicted from cDNA. Extensive sequencing of tryptic peptides revealed that the 37K fragments originated from the NH2-terminal region, and the 57K fragments were from the COOH-terminal part of DMP1. Phosphate analysis indicated that the 37K fragments contained 12 phosphates, and the 57K fragments had 41. From 37K fragments, two peptides lacked a COOH-terminal lysine or arginine; instead they ended at Phe173 and Ser180 and were thus COOH termini of 37K fragments. Two peptides were from the NH2 termini of 57K fragments, starting at Asp218 and Asp222. These findings indicated that DMP1 is proteolytically cleaved at four bonds, Phe173-Asp174, Ser180-Asp181, Ser217-Asp218, and Gln221-Asp222, forming eight fragments. The uniformity of cleavages at the NH2-terminal peptide bonds of aspartyl residues suggests that a single proteinase is involved. Based on its reported specificity, we hypothesize that these scissions are catalyzed by PHEX protein. We envision that the proteolytic processing of DMP1 plays a crucial role during osteogenesis and dentinogenesis.

Intrinsically unstructured proteins evolve by repeat expansion

The proportion of the genome encoding intrinsically unstructured proteins increases with the complexity of organisms, which demands specific mechanism(s) for generating novel genetic material of this sort. Here it is suggested that one such mechanism is the expansion of internal repeat regions, i.e., coding micro- and minisatellites. An analysis of 126 known unstructured sequences shows the preponderance of repeats: the percentage of proteins with tandemly repeated short segments is much higher in this class (39%) than earlier reported for all Swiss-Prot (14%), yeast (18%) or human (28%) proteins. Furthermore, prime examples, such as salivary proline-rich proteins, titin, eukaryotic RNA polymerase II, the prion protein and several others, demonstrate that the repetitive segments carry fundamental function in these proteins. In addition, their repeat numbers show functionally significant interspecies variation and polymorphism, which underlines that these regions have been shaped by intense evolutionary activity. In all, the major point of this paper is that the genetic instability of repetitive regions combined with the structurally and functionally permissive nature of unstructured proteins has powered the extension and possible functional expansion of this newly recognized protein class.

Dentin matrix protein 1 is expressed in human lung cancer

We have previously shown that breast and prostate cancers express bone matrix proteins. DMP1 expression was evaluated in 59 human lung cancer samples at the protein and mRNA levels. It was detectable in 80% of the cases, suggesting a potential role for DMP1 in tumor progression and bone metastasis.

INTRODUCTION

Previously, we and others have shown that bone extracellular matrix proteins such as bone sialoprotein (BSP) and osteopontin (OPN) are expressed in various types of cancer that are characterized by a high affinity for bone including breast, prostate, and lung adenocarcinoma. Based on biochemical and genetic features, BSP, OPN, dentin matrix protein 1 (DMP1), and dentin sialophosphoprotein (DSPP) have been recently classified in a unique family named SIBLING (small integrin-binding ligand, N-linked glycoprotein). Therefore, we investigated whether DMP1 could also be detected in osteotropic cancers.

MATERIALS AND METHODS

We first used a cancer array for evaluating the relative abundance of DMP1 transcript in a broad spectrum of human cancer tissues. This screening showed that DMP1 was strongly detectable in lung tumors compared with normal corresponding tissue. In a second step, we used an immunophosphatase technique and a specific polyclonal antibody directed against DMP1 to examine the expression of DMP1 in 59 human non-small cell lung cancer samples, including 29 squamous carcinoma, 20 adenocarcinoma, and 10 bronchioloalveolar carcinoma. Student's t-test was used to determine the statistical significance of immunostaining scores between the lung cancer histological groups studied and between cancer and normal lung tissues.

RESULTS

Our results show that DMP1 is detectable in 90% of the adenocarcinoma and squamous carcinoma analyzed while 8 of 10 bronchioloalveolar specimens were negative. DMP1 immunostaining intensity and extent scores were significantly higher in adenocarcinoma (p = 0.0004) and squamous carcinoma (p < 0.0001) samples compared with adjacent normal lung tissue. In situ hybridization experiments confirmed that DMP1 mRNA is localized in lung cancer cells.

CONCLUSION

In this study, we show that a third SIBLING protein is ectopically expressed in lung cancer. The role of DMP1 in lung cancer is largely unknown. Further studies are required to determine the implication of this protein, next to its sisters SIBLING proteins, in tumor progression and bone metastasis development.

Osteopontin is synthesized by uterine glands and a 45-kDa cleavage fragment is localized at the uterine-placental interface throughout ovine pregnancy

Osteopontin (OPN) is a phosphorylated and glycosylated, secreted protein that is present in various epithelial cells and biological fluids. On freezing and thawing or treatment with proteases, the native 70-kDa protein gives rise to 45- and 24-kDa fragments. Secreted OPN functions as an extracellular matrix (ECM) protein that binds cell surface receptors to mediate cell-cell adhesion, cell-ECM communication, and cell migration. In sheep and humans, OPN is proposed to be a secretory product of uterine glandular epithelium (GE) that binds to uterine luminal epithelium (LE) and conceptus trophectoderm to mediate conceptus attachment, which is essential to maintain pregnancy through the peri-implantation period. Cell-cell adhesion, communication, and migration likely are important at the interface between uterus and placenta throughout pregnancy, but to our knowledge, endometrial and/or placental expression of OPN beyond the peri-implantation period has not been documented in sheep. Therefore, the present study determined temporal and spatial alterations in OPN mRNA and protein expression in the ovine uterus between Days 25 and 120 of pregnancy. The OPN mRNA in total ovine endometrium increased 30-fold between Days 40 and 80 of gestation. In situ hybridization and immunofluorescence analyses revealed that the predominant source of OPN mRNA and protein throughout pregnancy was the uterine GE. Interestingly, the 45-kDa form of OPN was detected exclusively, continuously, and abundantly along the apical surface of LE, on conceptus trophectoderm, and along the uterine-placental interface of both interplacentomal and placentomal regions through Day 120 of pregnancy. The 45-kDa OPN is a proteolytic cleavage fragment of the native 70-kDa OPN, and it is the most abundant form in uterine flushes during early pregnancy. The 45-kDa OPN is more stimulatory to cell attachment and cell migration than the native 70-kDa protein. Collectively, the present results support the hypothesis that ovine OPN is a component of histotroph secreted by the uterine GE that accumulates at the uterine-placental interface to influence maternal-fetal interactions throughout gestation in sheep.

Dentin sialoprotein isoforms: detection and characterization of a high molecular weight dentin sialoprotein

Dentin sialoprotein (DSP) is a glycoprotein accounting for 5-8% of the dentin non-collagenous proteins. The cDNA sequence predicts that rat DSP has 13 potential casein kinase phosphorylation sites and six potential N-linked glycosylation sites. However, its total phosphorylation level, as well as the nature and locations of the carbohydrate moieties, are unknown. Our findings in the present study show that rat DSP has 6.2 phosphates per molecule and that the majority of carbohydrates are attached to the protein through N-linked glycosylations. During our separation of dentin non-collagenous proteins with ion-exchange chromatography, we observed high molecular weight components eluting late in the salt gradient that were recognized by anti-DSP antibodies. We have purified these high molecular weight components using a monoclonal anti-DSP antibody affinity column. Data from amino acid analysis, phosphate level measurements and Edman degradation of tryptic peptides unequivocally proved that the very acidic, high molecular weight components are isoforms of DSP (designated HMW-DSP). Deglycosylation analysis indicates that the slower migration rate of HMW-DSP on SDS-PAGE results from its higher level of carbohydrate modifications.

Early expression of bone matrix proteins in osteogenic cell cultures

Osteogenic cells express some matrix proteins at early culture intervals. The aim of this study was to determine if, and in what proportion, cells used for plating contain bone sialoprotein (BSP) and osteopontin (OPN), two matrix proteins associated with initial events in bone formation. Their pattern of expression, as well as that of fibronectin (FN) and type I pro-collagen, was also examined at 6 hr and at 1 and 3 days. The cells were obtained by enzymatic digestion of newborn rat calvariae, and grown on glass coverslips. Cytocentrifuge preparations of isolated cells and coverslips were processed for single or dual immunolabeling with monoclonal and/or polyclonal primary antibodies, followed by fluorochrome-conjugated antibodies. The cell labeling was mainly associated with perinuclear elements. OPN was also distinctively found at peripheral cytoplasmic sites. About 31% of isolated cells were OPN-positive and 18% were BSP-positive. After 1 day, almost 50% of cells were immunoreactive for OPN and for type I pro-collagen, and still less than 20% reacted for BSP. Approximately 7% exhibited peripheral staining for OPN. Almost all cells were associated with extracellular FN. However, only 15% showed intracellular labeling. These results indicate that an important proportion of cells used for plating contain BSP and OPN, a situation that should be taken into consideration in experimental analyses of osteoblast activity in vitro.

Mechanical loading stimulates dentin matrix protein 1 (DMP1) expression in osteocytes in vivo

Dentin matrix protein 1 (DMP1) was originally postulated to be dentin specific. Further analysis showed that DMP1 is also expressed in mature cartilage and bone. In bone tissue, DMP1 is expressed predominantly in late osteoblasts and osteocytes. DMP1 belongs to the SIBLING (Small Integrin Binding Ligand N-linked Glycoprotein) family of cellular matrix proteins that also includes osteopontin, bone sialoprotein, dentin sialophosphoprotein, and others. In this study, we examined the effect of mechanical loading on expression of DMP1 mRNA and DMP1 protein in alveolar bone in the mouse tooth movement model by in situ hybridization and immunocytochemistry. The expression of DMP1 mRNA was determined quantitatively in mechanically loaded and control sites of dento-alveolar tissue at several time points from 6 h to 7 days after loading. The tooth movement model allows simultaneous evaluation of bone resorption and bone formation sites. Expression of DMP1 mRNA in osteocytes increased 2-fold as early as 6 h after treatment in both the bone formation and bone resorption sites. After 4 days, DMP1 expression in osteocytes increased to a maximum of 3.7-fold in the bone formation sites and 3.5-fold in the resorption sites. Osteoblasts responded in the opposite manner and showed a transient 45% decrease of DMP1 mRNA in bone formation sites and a constant decrease of DMP1 mRNA during the entire course of treatment in the bone resorption sites, with a peak inhibition of 67% at day 2. By immunocytochemistry using a C-terminal region peptide antibody to DMP1, we found that there was a transient decrease in immunoreactivity at 3 days after treatment on both the formation side and the resorption side compared with the matched contralateral control tissue. However by 7 days of loading, there was a dramatic increase in DMP1 protein immunoreactivity on both the formation side and the resorption side. These results represent changes in epitope availability using this antibody or true changes in protein levels. The observations imply that the DMP1 protein is undergoing dynamic changes in either synthesis or other protein/matrix interaction after mechanical loading of alveolar bone. The findings indicate that DMP1 is involved in the responses of osteocytes and osteoblasts to mechanical loading of bone. These results support the hypothesis that osteocytes alter their matrix microenvironment in response to mechanical loading.

MEPE/OF45, a new dentin/bone matrix protein and candidate gene for dentin diseases mapping to chromosome 4q21

Substantial progress has been made regarding the molecular etiology of the dentin diseases dentinogenesis imperfecta types II and III, and dentin dysplasia type II. Genetic linkage studies have identified the critical loci for these diseases on human chromosome 4q21. Located within an overlapping segment of these disease loci is a dentin/bone gene cluster that includes osteopontin, bone sialoprotein, dentin matrix protein 1, dentin sialophosphoprotein, and a new gene MEPE also known as OF45, renamed osteoregulin. In this paper, we report the location of MEPE/OF45 in relationship to the other members of the dentin/bone gene cluster as well as the genomic organization of the human gene. For the first time, MEPE/OF45 expression was shown in dental tissue, in particular odontoblasts, by reverse-transcription polymerase chain reaction (RT-PCR) amplification and characterization of a partial mouse cDNA. Our data provide the first evidence that MEPE/OF45 is expressed during odontogenesis and should be considered as a candidate gene for dentin structural diseases mapping to human chromosome 4q21.

Mineralized tissue and vertebrate evolution: the secretory calcium-binding phosphoprotein gene cluster

Gene duplication creates evolutionary novelties by using older tools in new ways. We have identified evidence that the genes for enamel matrix proteins (EMPs), milk caseins, and salivary proteins comprise a family descended from a common ancestor by tandem gene duplication. These genes remain linked, except for one EMP gene, amelogenin. These genes show common structural features and are expressed in ontogenetically similar tissues. Many of these genes encode secretory Ca-binding phosphoproteins, which regulate the Ca-phosphate concentration of the extracellular environment. By exploiting this fundamental property, these genes have subsequently diversified to serve specialized adaptive functions. Casein makes milk supersaturated with Ca-phosphate, which was critical to the successive mammalian divergence. The innovation of enamel led to mineralized feeding apparatus, which enabled active predation of early vertebrates. The EMP genes comprise a subfamily not identified previously. A set of genes for dentine and bone extracellular matrix proteins constitutes an additional cluster distal to the EMP gene cluster, with similar structural features to EMP genes. The duplication and diversification of the primordial genes for enameldentinebone extracellular matrix may have been important in core vertebrate feeding adaptations, the mineralized skeleton, the evolution of saliva, and, eventually, lactation. The order of duplication events may help delineate early events in mineralized skeletal formation, which is a major characteristic of vertebrates.

Delineation of the hydroxyapatite-nucleating domains of bone sialoprotein

Bone sialoprotein (BSP) is a highly modified, anionic phosphoprotein that is expressed almost exclusively in mineralizing connective tissues and has been shown to be a potent nucleator of hydroxyapatite (HA). Two polyglutamic acid (poly[E]) regions, predicted to be in an alpha-helical conformation and located in the amino-terminal half of the molecule, are believed to be responsible for this activity. Using a prokaryotic expression system, full-length rat BSP was expressed and tested for HA nucleating activity in a steady-state agarose gel system. The unmodified protein is less potent than native bone BSP, indicating a role for the post-translational modifications in HA nucleation. Site-directed mutagenesis of the poly[E] regions in full-length BSP was performed, replacing the poly[E] with either polyaspartic acid (poly[D]) or polyalanine (poly[A]) to examine role of charge and conformation, respectively, in HA nucleation. Replacement of single domains with either poly[A] or poly[D] did not alter nucleating activity nor did replacement of both domains with poly[D]. Replacement of both domains with poly[A], however, significantly decreased nucleating activity. In addition, two recombinant peptides, each encompassing one of the two poly[E] domains, were expressed and tested for nucleating activity. Whereas the peptide encompassing the second poly[E] domain was capable of nucleating HA, the first domain peptide showed no activity. The conformation of the wild-type and mutated proteins and peptides were studied by circular dichroism and small angle x-ray scattering, and no secondary structure was evident. These results demonstrate that a sequence of at least eight contiguous glutamic acid residues is required for the nucleation of HA by BSP and that this nucleating "site" is not alpha-helical in conformation.

A role for alphavbeta3 integrin during implantation in the rabbit model

The study of implantation has been facilitated by the identification of specific biomarkers that are associated with uterine receptivity. The alpha(v)beta(3) integrin is a cell surface adhesion receptor, whose expression has been shown to be elevated in the endometrium at the time of implantation in both humans and other mammalian species; however, the distribution of alpha(v)beta(3) in the rabbit model is unknown. The rabbit has been shown to be an excellent model for the study of implantation. As an obligate ovulator, the timing of pregnancy can be precisely established, and embryonic attachment occurs through specialized trophoblast-endometrial structures known as trophoblastic knobs. In the present study, the expression of alpha(v)beta(3) integrin subunit in the rabbit uterus was examined by reverse transcription-polymerase chain reaction (RT-PCR), immunohistochemistry, and in situ hybridization. Expression of the alpha(v)beta(3) integrin was examined in Day 6.5 embryos, flushed from pregnant does. Immunofluorescence demonstrated strong immunostaining on the rabbit blastocyst (Day 6.5). RT-PCR analyses showed higher levels of mRNA for beta(3) subunit at the implantation site, with reduced expression in nonimplantation sites and in nonpregnant adult and immature endometrium. Immunohistochemistry demonstrated little, if any, beta(3) immunoreactivity on the endometrial epithelium. In contrast, in situ hybridization showed expression of the beta(3) integrin subunit mRNA in the uterine myometrium and on the trophoblast. To further determine the functional significance of alpha(v)beta(3) integrin expression during implantation, pregnant female rabbits that underwent ventral laparotomy on the morning of Day 6 received intrauterine injection of the following into the right uterine horn: 1) the monoclonal alpha(v)beta(3) neutralizing antibody (LM609), 2) arg-gly-asp (RGD) hexapeptides (GRGDSP), 3) non-RGD hexapeptides (GRGESP), and 4) IgG isotype matched control antibody. The left horn served as a control and received only saline injections. A significant reduction in the number of implantation sites was observed in the horns receiving anti-alpha(v)beta(3) antibody (P < 0.001) and the RGD peptides (P = 0.03). In the rabbit, the alpha(v)beta(3) integrin is present on the embryo and trophoblast and appears to be involved in early embryo-maternal interaction.