Full length cDNA sequence of porcine secreted phosphoprotein-I (SPP-I, osteopontin)

Osteopontin - The stirring multifunctional regulatory factor in multisystem aging

Osteopontin (OPN) is a multifunctional noncollagenous matrix phosphoprotein that is expressed both intracellularly and extracellularly in various tissues. As a growth regulatory protein and proinflammatory immunochemokine, OPN is involved in the pathological processes of many diseases. Recent studies have found that OPN is widely involved in the aging processes of multiple organs and tissues, such as T-cell senescence, atherosclerosis, skeletal muscle regeneration, osteoporosis, neurodegenerative changes, hematopoietic stem cell reconstruction, and retinal aging. However, the regulatory roles and mechanisms of OPN in the aging process of different tissues are not uniform, and OPN even has diverse roles in different developmental stages of the same tissue, generating uncertainty for the future study and utilization of OPN. In this review, we will summarize the regulatory role and molecular mechanism of OPN in different tissues and cells, such as the musculoskeletal system, central nervous system, cardiovascular system, liver, and eye, during senescence. We believe that a better understanding of the mechanism of OPN in the aging process will help us develop targeted and comprehensive therapeutic strategies to fight the spread of age-related diseases.

Molecular characterization and expression analysis of osteopontin cDNA from lactating mammary gland in yak (Bos grunniens)

Osteopontin (OPN) is a secreted phosphorylated glycoprotein. It has an important role in mammary gland development and lactation, as well as, is thought to be a potential candidate gene for lactation traits. In the present work, we isolated and characterized a full-length open reading frame (ORF) of yak OPN cDNA from lactating mammary tissue, and examined its expression pattern in mammary gland during different stages of lactation, as well as, the recombinant OPN protein of yak was expressed successfully in E. coli. The sequencing results indicated that the isolated cDNA was 1132-bp in length containing a complete ORF of 837-bp. It encoded a precursor protein of yak OPN consisting of 278 amino acid with a signal peptide of 16 amino acids. Yak OPN has a predicted molecular mass of 29285.975 Da and an isoelectric point of 4.245. It had an identity of 65.50-99.16% in cDNA, identity of 52.06-98.56% and similarity of 65.40-98.56% in deduced amino acids with the corresponding sequences of cattle, buffalo, sheep, goat, pig, human, and rabbit. The phylogenetic analysis indicated that yak OPN had the closest evolutionary relationship with that of cattle, and next buffalo. In mammary gland, yak OPN was generally transcribed in a declining pattern from colostrum period to dry period with an apparent increase of OPN expression being present in the late period of lactation compared with peak period of lactation. Western blot analysis indicated that His-tagged yak OPN protein expressed in E. coli could be recognized not only by an anti-His-tag antibody but also by an anti-human OPN antibody. These results from the present work provided a foundation for further insight into the role of OPN gene in yak lactation.

Osteopontin and allergic disease: pathophysiology and implications for diagnostics and therapy

Osteopontin (OPN) is a phosphoglycoprotein that is expressed by various immune cells in a secreted and intracellular form. It has cytokine, chemotactic and cell signaling functions enhancing Th1 and Th17 immunity and protects against apoptosis. Recent studies found OPN to be modulatory in cell-mediated and immediate-type allergic diseases. In allergic asthma, OPN enhances sensitization but downmodulates Th2-driven IL-4-dominated inflammation. The finding that OPN expression is augmented during specific immunotherapy supports a Th2 suppressive effect of OPN. In Th1-driven delayed-type allergy, such as allergic contact dermatitis, OPN supports dendritic cell migration and IL-12 expression and is secreted by T effector cells and keratinocytes, augmenting Th1-mediated allergy and supporting disease chronification. There are numerous missing links as to how OPN variants modulate allergic inflammation through different OPN receptors. OPN research in allergy is an interesting, rapidly expanding field that has high potential for translational research.

Osteopontin: regulation in tumor metastasis

Osteopontin is a secreted phosphoprotein that has been implicated as an important mediator of tumor metastasis and has been investigated for use as a biomarker for advanced disease and as a potential therapeutic target in the regulation of cancer metastasis. The OPN DNA sequence is highly conserved and the protein contains several important functional domains including alpha(v)beta integrin and CD44 binding sites. High levels of OPN expression correlate with tumor invasion, progression or metastasis in multiple cancer. Studies demonstrate that osteopontin mediates the molecular mechanisms which determine metastatic spread, such as prevention of apoptosis, extracellular matrix proteolysis and remodeling, cell migration, evasion of host-immune cells and neovascularization. Transcriptional regulation of OPN is complex and involves multiple pathways, including AP-1, Myc, v-Src, Runx/CBF, TGF-B/BMPs/Smad/Hox, and Wnt/ss-catenin/APC/GSK-3ss/Tcf-4. The current state of knowledge of OPN biology suggests that it is an attractive target for therapeutic modulation of metastatic disease.

The role of osteopontin in tumor progression and metastasis in breast cancer

The use of cancer biomarkers to anticipate the outlines of disease has been an emerging issue, especially as cancer treatment has made such positive steps in the last few years. Progress in the development of consistent malignancy markers is imminent because advances in genomics and bioinformatics have allowed the examination of immense amounts of data. Osteopontin is a phosphorylated glycoprotein secreted by activated macrophages, leukocytes, and activated T lymphocytes, and is present in extracellular fluids, at sites of inflammation, and in the extracellular matrix of mineralized tissues. Several physiologic roles have been attributed to osteopontin, i.e., in inflammation and immune function, in mineralized tissues, in vascular tissue, and in kidney. Osteopontin interacts with a variety of cell surface receptors, including several integrins and CD44. Binding of osteopontin to these cell surface receptors stimulates cell adhesion, migration, and specific signaling functions. Overexpression of osteopontin has been found in a variety of cancers, including breast cancer, lung cancer, colorectal cancer, stomach cancer, ovarian cancer, and melanoma. Moreover, osteopontin is present in elevated levels in the blood and plasma of some patients with metastatic cancers. Therefore, suppression of the action of osteopontin may confer significant therapeutic activity, and several strategies for bringing about this suppression have been identified. This review looks at the recent advances in understanding the possible mechanisms by which osteopontin may contribute functionally to malignancy, particularly in breast cancer. Furthermore, the measurement of osteopontin in the blood or tumors of patients with cancer, as a way of providing valuable prognostic information, will be discussed based on emerging clinical data.

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.

The role of Osteopontin in tumor metastasis

Osteopontin (OPN) is a glyco-phosphoprotein that is expressed and secreted by numerous human cancers. OPN functions in cell adhesion, chemotaxis, macrophage-directed interleukin-10 (IL-10) suppression, stress-dependent angiogenesis, prevention of apoptosis, and anchorage-independent growth of tumor cells by regulating cell-matrix interactions and cellular signaling through binding with integrin and CD44 receptors. While constitutive expression of OPN exists in several cell types, induced expression has been detected in T-lymphocytes, epidermal cells, bone cells, macrophages, and tumor cells in remodeling processes such as inflammation, ischemia-reperfusion, bone resorption, and tumor progression. Recently, substantial evidence has linked OPN with the regulation of metastatic spread by tumor cells. However, the molecular mechanisms that define the role of OPN in tumor metastasis are incompletely understood. Transcriptional regulators that contribute to the induction of OPN expression have received significant attention as potential modulators of the OPN-mediated metastatic phenotype. The following review will discuss the molecular structure of OPN, the evidence for its functional role in tumor cell metastasis, the downstream signals that activate invasive mechanisms, and the recent reports concerning regulation of OPN transcription.

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.

Identification of dual alpha 4beta1 integrin binding sites within a 38 amino acid domain in the N-terminal thrombin fragment of human osteopontin

Previous work from our laboratory demonstrates that the alpha(4)beta(1) integrin is an adhesion receptor for OPN and that alpha(4)beta(1) binding site(s) are present in the N-terminal thrombin fragment of osteopontin (OPN) (Bayless, K. J., Meininger, G. A., Scholtz, J. M., and Davis, G. E. (1998) J. Cell Sci. 111, 1165-1174). The work presented here identifies two alpha(4)beta(1) binding sites within a recombinantly produced N-terminal thrombin fragment of human OPN. Initial experiments, using wild-type OPN containing an RGD sequence or an OPN-RGE mutant, showed identical alpha(4)beta(1)-dependent cell adhesive activity. A strategy to localize alpha(4)beta(1) binding sites within the thrombin fragment of osteopontin involved performing a series of truncation analyses. Removal of the last 39 amino acids (130) completely eliminated adhesion, indicating all binding activity was present within that portion of the molecule. Combined mutation and deletion analyses of this region revealed the involvement of dual alpha(4)beta(1) binding sites. Synthetic peptides for both regions in OPN, ELVTDFPTDLPAT (131) and SVVYGLR (162), were found to block alpha(4)beta(1)-dependent adhesion. The first peptide when coupled to Sepharose bound the alpha(4)beta(1) integrin directly whereas a mutated ELVTEFPTELPAT peptide showed a dramatically reduced ability to bind. These data collectively demonstrate that dual alpha(4)beta(1) integrin binding sites are present in a 38 amino acid domain within the N-terminal thrombin fragment of OPN.

Osteopontin

Osteopontin (OPN) is a highly phosphorylated sialoprotein that is a prominent component of the mineralized extracellular matrices of bones and teeth. OPN is characterized by the presence of a polyaspartic acid sequence and sites of Ser/Thr phosphorylation that mediate hydroxyapatite binding, and a highly conserved RGD motif that mediates cell attachment/signaling. Expression of OPN in a variety of tissues indicates a multiplicity of functions that involve one or more of these conserved motifs. While the lack of a clear phenotype in OPN "knockout" mice has not established a definitive role for OPN in any tissue, recent studies have provided some novel and intriguing insights into the versatility of this enigmatic protein in diverse biological events, including developmental processes, wound healing, immunological responses, tumorigenesis, bone resorption, and calcification. The ability of OPN to stimulate cell activity through multiple receptors linked to several interactive signaling pathways can account for much of the functional diversity. In this review, we discuss the structural features of OPN that relate to its function in the formation, remodeling, and maintenance of bones and teeth.

The role of osteopontin in the development of granulomatous lesions in lung

Osteopontin (OPN) has been shown to be expressed by cells in granulomas of various origins, but whether it plays a functional role in granuloma formation is not known. Here we used a cardiomyopathic hamster (TO2) model, to test the hypothesis that OPN contributes functionally to granuloma development. We immunized cardiomyopathic and normal hamsters by subcutaneous injection of bovine serum albumin in complete Freund's adjuvant, and assessed various tissues for both OPN RNA expression and granuloma formation. Cardiomyopathic hamsters expressed OPN, and formed granulomatous lesions, in heart tissue in both immunized and untreated animals. In addition, immunization induced expression of OPN in lung and lymph nodes of cardiomyopathic (but not normal) hamsters, and also induced granuloma formation in these organs. To test whether OPN expression could play a functional role in inducing granulomas, we produced an adenoviral vector containing the murine OPN gene, and introduced this vector intratracheally into the lungs of normal hamsters. The OPN-containing vector, but not the control vector, induced pulmonary granuloma formation. These studies provided direct in vivo evidence that OPN can contribute functionally to the formation of granulomatous lesions, and suggest that OPN expression may be a common factor involved in formation of granulomas of various origin.

Ovine osteopontin: I. Cloning and expression of messenger ribonucleic acid in the uterus during the periimplantation period

Trophoblast-derived interferon tau (IFNtau) acts on the endometrium to increase secretion of several proteins during the pregnancy recognition period in ruminants. One of these is a 70-kDa acidic protein that has not been identified. Our hypothesis was that the 70-kDa acidic protein is osteopontin (OPN). OPN is an acidic glycoprotein that fragments upon freezing and thawing or treatment with proteases including thrombin. OPN contains a Gly-Arg-Gly-Asp-Ser (GRGDS) sequence that binds to cell surface integrins to promote cell-cell attachment and cell spreading. Using antisera to recombinant human OPN, both 70-kDa and 45-kDa proteins were identified in uterine flushings from pregnant ewes by Western blotting. A clone containing the entire ovine OPN cDNA coding sequence was isolated by screening a Day 15 pregnant ovine endometrial cDNA library with a partial ovine OPN cDNA. In pregnant ewes, steady-state levels of OPN endometrial mRNA increased (P < 0. 01) after Day 17. In both cyclic and pregnant ewes, in situ hybridization analysis showed that OPN mRNA was localized on unidentified immune cells within the stratum compactum of the endometrium. In pregnant ewes, OPN mRNA was also expressed by the glandular epithelium. Results suggest that progesterone and/or IFNtau induce expression and secretion of OPN by uterine glands during the periimplantation period and that OPN may induce adhesion between luminal epithelium and trophectoderm to facilitate superficial implantation.

Ultrastructural localization of osteopontin immunoreactivity in phagolysosomes 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.

The staining of acidic proteins on polyacrylamide gels: enhanced sensitivity and stability of "Stains-all" staining in combination with silver nitrate

A number of acidic proteins, such as those found in bone and dentin, are poorly resolved on acrylamide gels using Coomassie blue or silver nitrate staining. The cationic dye Stains-all allows visualization and identification of these proteins due to their differential staining: highly acidic proteins stain blue and intact proteoglycans stain purple, whereas less acidic proteins stain pink. However, the use of Stains-all is limited due to relatively poor staining sensitivity and lack of stability to light. A procedure which addresses these deficiencies has been developed utilizing established protocols for Stains-all staining followed by silver nitrate incubation and development. In this way, phosphoproteins such as osteopontin, bone sialoprotein, dentin phosphophoryn, and other acidic glycoproteins are visualized at higher sensitivity (greater than fivefold) and staining stability than normally achieved with just Stains-all. The protocol stains a greater variety of proteins than a combined alcian blue/silver staining procedure previously described. Utilizing the Stains-all/silver protocol, porcine bone osteopontin, a protein not visualized by standard silver staining, can be observed in amounts as little as 0.25 ng on polyacrylamide gels. Furthermore, densitometric scans demonstrate that the staining intensity is proportional to osteopontin amount and can be used for quantification over a range from 0.25 to 50 ng.

Ultrastructural immunogold localization of osteopontin in human gastric mucosa

We performed ultrastructural immunogold localization of osteopontin in the mucosa of human stomach. This adhesive glycoprotein was present in mucous and chief cells of the epithelial layer and in macrophages in the lamina propria. Parietal and endocrine cells of the epithelial layer and mast cells and plasma cells in the lamina propria did not contain osteopontin, serving as internal negative controls. Subcellular localizations of osteopontin included secretory granules and synthetic organelles in mucous and chief cells and phagolysosomes in macrophages. Extracellular concentrations of osteopontin were present in the glycocalyx and in an electron-lucent band between epithelial surface cells and the gastric lumen. Paracellular edema between the epithelium of the same cells was devoid of osteopontin. Immunogold localization of pepsinogen II was done to identify cells with mixed granule populations and contents of multicompartmental secretory granules. These studies revealed mucous cell granules and chief cell granules, each containing compartmentalized storage products, which included osteopontin and mucigen in mucous cells and osteopontin and pepsinogen II in chief cells. Cytochemical controls for the immunogold localizations were negative. The subcellular distribution of osteopontin in human gastric mucosa suggests possible roles for this glycoprotein in barrier function, host defense, and/or secretion.

Posttranslational modifications of bovine osteopontin: identification of twenty-eight phosphorylation and three O-glycosylation sites

Osteopontin (OPN) is a multiphosphorylated glycoprotein found in bone and other normal and malignant tissues, as well as in the physiological fluids urine and milk. The present study demonstrates that bovine milk osteopontin is phosphorylated at 27 serine residues and 1 threonine residue. Phosphoamino acids were identified by a combination of amino acid analysis, sequence analysis of S-ethylcysteine-derivatized phosphopeptides, and mass spectrometric analysis. Twenty-five phosphoserines and one phosphothreonine were located in Ser/Thr-X-Glu/Ser(P)/Asp motifs, and two phosphoserines were found in the sequence Ser-X-X-Glu/Ser(P). These sequence motifs are identical with the recognition sequences of mammary gland casein kinase and casein kinase II, respectively. Examination of the phosphorylation pattern revealed that the phosphorylations were clustered in groups of approximately three spanned by unphosphorylated regions of 11-32 amino acids. This pattern is probably of importance in the multiple functions of OPN involving interaction with Ca2+ and inorganic calcium salts. Furthermore, three O-glycosylated threonines (Thr 115, Thr 124, and Thr 129) have been identified in a threonine- and proline-rich region of the protein. Three putative N-glycosylation sites (Asn 63, Asn 85, and Asn 193) are present in bovine osteopontin, but sequence and mass spectrometric analysis showed that none of these asparagines were glycosylated in bovine mammary gland osteopontin. Alignment analysis showed that the majority of the phosphorylation sites in bovine osteopontin as well as all three O-glycosylation sites were conserved in other mammalian sequences. This conservation of serines, even in otherwise less well-conserved regions of the protein, indicates that the phosphorylation of osteopontin at specific sites is essential for the function of the protein.

Regulation of osteopontin expression in osteoblasts

Osteopontin (OPN) is a prominent bone matrix protein that is synthesized by osteoblastic cells. To elucidate the function of OPN in bone we studied the regulated expression of the rat OPN protein during bone formation in vivo and in vitro. OPN mRNA is expressed by preosteoblastic cells early in bone formation, but the highest expression is observed in mature osteoblasts at sites of bone remodelling. A low-phosphorylated, 55-kDa form of OPN is produced by the preosteoblastic cells, whereas osteoblasts produce a highly phosphorylated, 44-kDa protein; the two forms of OPN corresponding to pp69 and pp62 in transformed rat cells. The synthesis of the 55-kDa OPN correlates with the formation of a 'cement' matrix that is synthesized prior to bone deposition, whereas the 44-kDa OPN synthesized by osteoblasts associates rapidly with hydroxyapatite, possibly regulating crystal growth, and may also provide a substratum for osteoclast attachment. Expression of OPN mRNA is upregulated by growth and differentiation factors (PDGF, EGF, TGF-beta and BMP-7/OP-1) and by mechanical stress, which promote bone formation, as well as by osteotropic hormones (retinoic acid and vitamin D3), which can promote bone resorption and remodelling. However, OPN mRNA is down-regulated by bisphosphonates, which abrogate bone resorption. Regulation of OPN expression is, therefore, consistent with a multiplicity of functions for OPN that involve specific structural motifs in both the synthesis and resorption of bone.

Expression of wild-type and mutated rabbit osteopontin in Escherichia coli, and their effects on adhesion and migration of P388D1 cells

Recombinant wild-type rabbit osteopontin (rOP) and the protein with an aspartate-to-glutamate transposition induced by a point mutation in the rabbit OP cDNA within the Gly-Arg-Gly-Asp-Ser (GRGDS) sequence were expressed in Escherichia coli and purified to homogeneity. P388D1 cells bound rOP in a saturable manner. rOP induced adhesion and haptotaxis of P388D1 cells, whereas mutated rabbit OP (rOPmut) did not. Anti-rOP IgG F(ab')2 and synthetic GRGDS peptide inhibited rOP-mediated adhesion and haptotaxis of P388D1 cells. Fibronectin (FN)-mediated adhesion of P388D1 cells was markedly inhibited in the presence of fluid-phase rOP. Adhesion of P388D1 cells to rOP was significantly inhibited by anti-[alpha-subunits of VLA4 (alpha 4) and VLA5 (alpha 5)] monoclonal antibodies (mAbs), but not by anti-[alpha-subunit of vitronectin (VN) receptor (alpha V) or Mac-1 (alpha M)] mAb. Adhesion of P388D1 cells to FN and VN was significantly inhibited by anti-alpha V mAb but not anti-alpha 4, -alpha 5 or -alpha M mAb. Haptotaxis of P388D1 cells to rOP was significantly inhibited by anti-alpha V mAb, but not by anti-alpha 4, -alpha 5 and alpha M mAbs, whereas that to FN showed no inhibition with all three mAbs. Haptotaxis of P388D1 cells to VN was significantly inhibited by anti-alpha 5 and -alpha V mAbs but not by anti-alpha 4 and -alpha M mAbs. Similar features of inhibition of adhesion and haptotaxis of P388D1 cells to human OP were observed by mAbs. rOP had no chemotactic effect on P388D1 cells. Significant polymorphonuclear leucocyte migration was observed 3-12 h after intradermal injection of rOP into rabbits.

The roles of autophosphorylation and phosphorylation in the life of osteopontin

Osteopontin is a secreted glycosylated phosphoprotein found in bone and other normal and malignant tissues. Osteopontin can be autophosphorylated on tyrosine residues and can also be phosphorylated on serine and threonine residues by several protein kinases. Autophosphorylation of osteopontin may generate sites for specific interactions with other proteins on the cell surface and/or within the extracellular matrix. These interactions of osteopontin are thought to be essential for bone mineralization and function. The polyaspartic acid motif of osteopontin, in combination with neighboring sequences that include serine residues phosphorylated by protein kinases, could fold and assemble into a molecular structure that participates in the mineralization of the bone matrix.

Cloning and characterization of the human osteopontin gene and its promoter

We isolated the human osteopontin (hOP) gene and the 5' upstream region, and analysed its exon-intron structure and potential regulatory sequences of the promoter region in comparison with those of the mouse and porcine gene. The coding sequence is split into 7 exons which are similar to those of the mouse gene, although the hOP gene is longer than the mouse gene. The difference in length is mainly due to variations in intron 3, which is approximately 2.7-fold longer than that of the mouse OP gene. The 5' upstream region of the hOP, which is highly conserved up to nucleotide -250, contains a number of potential cis regulatory consensus sequences. A series of sequentially 5'-deleted chimeric clones was tested for the ability to stimulate chloramphenicol acetyltransferase (CAT). Initial CAT analysis demonstrated that nucleotides at positions -474 to -270, -124 to -80, and -55 to -39 contained cis-acting enhancing sequences in a human monocyte cell line, SCC-3, although the -124 to -80 region was much more active than other regions. Deletion of the sequences between -474 and -270 localized this cis region to the sequence at positions -439 to -410, whereas the deletion between -124 to -80 localized the regions to -124 to -115, and -94 to -80. Gel-shift analysis using as probes synthesized double-stranded DNA corresponding to the 10 and 15 bp region at positions -124 to -115 and -94 to -80 respectively revealed that each probe formed a major band complexed with nuclear proteins prepared from SCC-3 cells.

Modulation of crystal formation by bone phosphoproteins: structural specificity of the osteopontin-mediated inhibition of hydroxyapatite formation

Osteopontin is a phosphorylated sialoprotein containing a conserved sequence of contiguous aspartic acid residues. This protein is expressed at high levels in mineralized tissues and has previously been shown to inhibit the in vitro formation of hydroxyapatite (HA). In the present study, protein modification and model compound studies have been used to identify the structural features of osteopontin that are responsible for its crystal-modulating properties. Using metastable calcium phosphate solutions buffered by autotitration, osteopontin caused half-maximal inhibition of HA formation at a concentration (IC50) of 0.06 microgram/ml. The hen egg yolk phosphoprotein phosvitin was a much weaker inhibitor, while dextran sulphate had no effect. The synthetic polypeptide poly(aspartic acid) was almost as effective an inhibitor of HA formation as osteopontin (IC50 0.11 microgram/ml), whereas poly(glutamic acid) was more than a thousand times less potent (IC50 155 micrograms/ml). In a steady-state agarose gel system, much higher polypeptide concentrations were required for inhibition of HA formation, but a similar relative order of inhibitory effectiveness was observed. Treatment of osteopontin with alkaline phosphatase removed 84% of the covalently bound phosphate and reduced its HA-inhibiting activity by more than 40-fold. Treatment with glycine ethyl ester in the presence of carbodi-imide modified 86% of the carboxylate groups in osteopontin and reduced its inhibitory activity by 6-fold. These findings indicate that osteopontin is a potent inhibitor of HA formation. This activity requires phosphate and carboxylate groups, possibly including the conserved sequence of contiguous aspartic acid residues. Osteopontin may act as an inhibitor of phase separation in physiological fluids of high supersaturation.

Characterization of the osteogenic stromal cell line MN7: identification of secreted MN7 proteins using two-dimensional polyacrylamide gel electrophoresis, western blotting, and microsequencing

Proteins secreted by the osteogenic stromal cell line MN7 were analyzed using two-dimensional polyacrylamide gel electrophoresis (PAGE), western blotting, immunodetection, and microsequencing. Trichloroacetic acid-precipitated proteins from the conditioned medium of MN7 cell cultures, harvested at different times of growth, were dissolved in denaturing and reducing sample buffer and separated in the first dimension according to isoelectric point and in the second dimension according to molecular weight. Protein patterns were visualized using silver staining. Among the 350 separated protein spots, we identified type I collagen, bone sialoprotein, osteonectin, and cathepsin B by western blotting and immunodetection using polyclonal antibodies. Osteocalcin could not be detected in the conditioned medium of MN7 cells. Furthermore, 15 MN7-specific protein spots were localized after comparison with two-dimensional PAGE patterns from the conditioned medium of the nonosteogenic stromal cell lines MM1 and MV1. Microsequencing of the internal peptides of five selected spots revealed three known proteins, namely the carboxyl-terminal propeptide of the alpha 2 chain of collagen type I, cathepsin L, and the tissue inhibitor of metalloproteinases-2, an 18 kilodalton peptide fragment from osteopontin that has not previously been described, and a novel glycosylated 85 kD protein with an average isoelectric point of 5.7. All identified proteins did not vary in presence between the different time points analyzed by two-dimensional PAGE. The use of two-dimensional PAGE to investigate the secreted proteins of MN7 cells will enable us to establish a complete protein data base of extracellular osteoblast-specific proteins. Furthermore, two-dimensional PAGE in combination with other techniques is a fast and accurate method for the identification of novel proteins that could function as markers in osteoblast differentiation and/or bone formation.

Adhesive properties of osteopontin: regulation by a naturally occurring thrombin-cleavage in close proximity to the GRGDS cell-binding domain

Osteopontin (OPN) is a secreted adhesive glycoprotein with a functional glycine-arginine-glycine-aspartate-serine (GRGDS) cell-binding domain. An interesting feature of OPN structure is the presence of a thrombin-cleavage site in close proximity to the GRGDS region. Cleavage of OPN by thrombin is likely to be of physiological importance, because cleavage of blood plasma OPN occurs naturally after activation of the blood coagulation pathway. To investigate functional consequences of OPN cleavage by thrombin, cell attachment and spreading assays were performed with uncleaved and cleaved forms of OPN. For all cell lines examined, thrombin-cleaved OPN promoted markedly greater cell attachment and spreading than uncleaved OPN. Cell attachment and spreading on thrombin-cleaved OPN was inhibited both by the soluble GRGDS peptides and an OPN-specific antibody raised to the GRGDS domain of OPN, thus implicating the GRGDS region in mediating the increased cell attachment and spreading observed on thrombin-cleaved OPN. Because the GRGDS sequence in OPN is only six residues from the thrombin-cleavage site, the data suggest that possibility that thrombin cleavage allows greater accessibility of the GRGDS domain to cell surface receptors. To investigate receptors that recognize uncleaved and thrombin-cleaved OPN, affinity chromatography was performed on placental extracts; the cell surface integrin alpha v beta 3 bound to columns constructed either with native or thrombin-cleaved OPN and was selectively eluted from each with soluble GRGDS peptide and EDTA. Moreover, adhesion assays performed in the presence of alpha v beta 3 blocking monoclonal antibody LM609 identified alpha v beta 3 as a major functional receptor for thrombin-cleaved OPN. Several lines of evidence suggest that cleavage of OPN by thrombin occurs in vivo, such as in tumors and at sites of tissue injury, and adhesion assay data presented here indicate that such cleavage is important in the regulation of OPN function.

Conserved phosphorylation of serines in the Ser-X-Glu/Ser(P) sequences of the vitamin K-dependent matrix Gla protein from shark, lamb, rat, cow, and human

The present studies demonstrate that matrix Gla protein (MGP), a 10-kDa vitamin K-dependent protein, is phosphorylated at 3 serine residues near its N-terminus. Phosphoserine was identified at residues 3, 6, and 9 of bovine, human, rat, and lamb MGP by N-terminal protein sequencing. All 3 modified serines are in tandemly repeated Ser-X-Glu sequences. Two of the serines phosphorylated in shark MGP, residues 2 and 5, also have glutamate residues in the n + 2 position in tandemly repeated Ser-X-Glu sequences, whereas the third, shark residue 3, would acquire an acidic phosphoserine in the n + 2 position upon phosphorylation of serine 5. The recognition motif found for MGP phosphorylation, Ser-X-Glu/Ser(P), has been seen previously in milk caseins, salivary proteins, and a number of regulatory peptides. A review of the literature has revealed an intriguing dichotomy in the extent of serine phosphorylation among secreted proteins that are phosphorylated at Ser-X-Glu/Ser(P) sequences. Those phosphoproteins secreted into milk or saliva are fully phosphorylated at each target serine, whereas phosphoproteins secreted into the extracellular environment of cells are partially phosphorylated at target serine residues, as we show here for MGP and others have shown for regulatory peptides and the insulin-like growth factor binding protein 1. We propose that the extent of serine phosphorylation regulates the activity of proteins secreted into the extracellular environment of cells, and that partial phosphorylation can therefore be explained by the need to ensure that the phosphoprotein be poised to gain or lose activity with regulated changes in phosphorylation status.(ABSTRACT TRUNCATED AT 250 WORDS)

Recombinant GST-human osteopontin fusion protein is functional in RGD-dependent cell adhesion

Osteopontin (OPN) is a secreted phosphoprotein expressed by many tumor cells, as well as a limited set of normal cells. Native OPN has been shown to support cell adhesion in an RGD-peptide-inhibitable fashion. Here we expressed human OPN in E. coli as a recombinant fusion protein with glutathione-S-transferase (GST). We report that the GST-OPN fusion protein has functional activity. PAP2 (ras-transformed, metastatic murine NIH 3T3) and MDA-MB-435 human mammary carcinoma cells bound to GST-OPN in an in vitro cell adhesion assay nearly as well as to native bovine OPN. Adhesion to the recombinant fusion protein was blocked by addition of GRGDS peptide, suggesting that the cells adhere to the recombinant and native OPN proteins by similar, integrin-mediated mechanisms. Adhesion to both sources of OPN also was inhibited by thrombin treatment of the protein. Thrombin cleaves GST from OPN in the fusion protein, and also cleaves internally in OPN, adjacent to the RGD sequence of the protein. Our results suggest that (a) thrombin cleavage of native OPN may be a natural regulator of OPN function, and (b) the majority of OPN cell binding activity is mediated by the RGD sequence in the protein backbone, with little or no requirement for post-translational modifications that occur in native OPN for adhesive function as measured here.

Osteopontin promotes vascular cell adhesion and spreading and is chemotactic for smooth muscle cells in vitro

Osteopontin is an Arg-Gly-Asp-containing acidic phosphoprotein recently shown to be upregulated in vascular smooth muscle during rat arterial neointima formation and in human atherosclerotic plaques. Functional studies showed that osteopontin promoted adhesion of both cultured aortic endothelial cells and aortic smooth muscle cells. Adhesion of vascular cells to osteopontin was dose dependent and half maximal when solutions containing 7 and 30 nmol/L osteopontin were used to coat wells for endothelial and smooth muscle cells, respectively. Smooth muscle cells adherent to osteopontin were spread after 60 minutes, whereas endothelial cells remained round, although flattened, at this time point but were spread at 90 minutes. Cell spreading on osteopontin was accompanied by the formation of focal adhesion plaques. A newly developed anti-osteopontin antibody completely inhibited adhesion of both cell types to osteopontin but not to fibronectin or vitronectin. In addition, the peptide GRGDSP blocked adhesion to osteopontin, suggesting that integrins mediate Arg-Gly-Asp-dependent adhesion. Indeed, an antibody against the alpha v beta 3 integrin neutralized adhesion of both endothelium and smooth muscle cells to osteopontin by approximately 50%, demonstrating that alpha v beta 3 is one osteopontin receptor on vascular cells. Osteopontin also promoted the migration of smooth muscle cells in a Boyden-type chamber, with half-maximal effects observed at 77 nmol/L osteopontin. Checkerboard analysis demonstrated that this stimulus was chemotactic in nature. Our findings suggest that osteopontin may be functionally important as an adhesive and chemotactic molecule for vascular cells, particularly when levels of osteopontin are dramatically increased, as is the case after arterial angioplasty and in atherosclerotic plaques.

beta-Glycerophosphate-induced mineralization of osteoid does not alter expression of extracellular matrix components in fetal rat calvarial cell cultures

When fetal rat calvarial cells are cultured in medium containing vitamin C, osteoid nodules develop after approximately 15 days of culture. Upon addition of an organic phosphate (beta-glycerophosphate, beta GP), these nodules mineralize. We have now used this system to explore the suggestion made by others that a negative feedback may exist between matrix mineralization on the one hand and the synthesis of alkaline phosphatase and bone matrix collagen on the other by analyzing the synthesis of these proteins and the levels of their mRNAs in mineralizing and nonmineralizing cultures. Our results indicate that in the osteoid nodule-bone nodule system, matrix mineralization did not affect the mRNA levels for osteopontin, type I collagen, bone sialoprotein, or osteocalcin. Synthesis of total protein and collagen and the osteocalcin content of culture media were also not different in the mineralizing and nonmineralizing cultures. However, alkaline phosphatase mRNA was increased in early mineralizing cultures and alkaline phosphatase activity in the cell layer was also increased in mineralizing cultures. Thus, the hypothesis that a direct negative feedback exists between mineralization and matrix protein synthesis is not supported by our experiments.

Expression and distribution of osteopontin in human tissues: widespread association with luminal epithelial surfaces

Osteopontin, a glycoprotein with a glycine-arginine-glycine-aspartate-serine (GRGDS) cell-binding domain, has been described in bone and is also known to be expressed in other organs, particularly kidney. The goal of the present work was to define the distribution of osteopontin synthesis and deposition in a wide variety of normal adult human tissues using a multifaceted approach that included immunohistochemistry, in situ hybridization, and Northern analysis. Immunohistochemical studies have revealed the unexpected finding that osteopontin is deposited as a prominent layer at the luminal surfaces of specific populations of epithelial cells of the gastrointestinal tract, gall bladder, pancreas, urinary and reproductive tracts, lung, breast, salivary glands, and sweat glands. Northern analyses identified gallbladder as a major site of osteopontin gene transcription comparable in magnitude with that of kidney, and immunoblotting identified osteopontin in bile. In situ hybridization localized osteopontin gene transcripts predominantly to the epithelium of a variety of organs as well as to ganglion cells of bowel wall. Osteopontin of epithelial cell origin, like bone-derived osteopontin, promoted GRGDS-dependent cell spreading in attachment assays. We postulate that osteopontin secreted by epithelium binds integrins on luminal surfaces. Collectively, these findings suggest an important role for osteopontin on many luminal epithelial surfaces communicating with the external environment.

Identification of osteopontin in isolated rabbit osteoclasts

Bone remodeling is a complex process coupling bone formation and resorption. Osteoblasts, the bone-forming cells, are known to produce various bone matrix proteins and cytokines; however, little is known about protein factors produced by osteoclasts or bone-resorbing cells. A method utilizing the high affinity of osteoclasts for tissue culture dishes was developed to isolate a large number of pure osteoclasts from rabbit long bones. A cDNA library was then constructed from these isolated osteoclasts, and differential cDNA screening was performed between osteoclasts and spleen cells. Two clones representing osteoclast-specific clones, named OC-1 and OC-2, were isolated. By Northern blot analysis, OC-1 was expressed in osteoclasts and in kidneys, whereas OC-2 was specific for osteoclasts. OC-1 was found to encode osteopontin from its nucleotide sequence, and therefore, osteopontin may have other functions for osteoclastic bone resorption besides osteoclast attachment to bone.

Characterization of the promoter region of the porcine opn (osteopontin, secreted phosphoprotein 1) gene. Identification of positive and negative regulatory elements and a 'silent' second promoter

Osteopontin (secreted phosphoprotein-1, Opn) is a phosphorylated glycoprotein expressed by transformed cells, macrophages, activated T-lymphocytes, specialized epithelial cells and bone cells that is characteristically enriched in milk and in the mineralized matrix of bone. The synthesis of Opn by bone cells is regulated by glucocorticoids and growth factors, which promote bone formation, and by the osteotropic hormone calcitriol (1,25-dihydroxycholecalciferol) and retinoic acid, which mediate bone resorption, indicating a bifunctional role for this protein in bone remodelling. To study the transcriptional regulation of the opn gene, two genomic clones (10 and 15 kb) encoding the opn gene were isolated from a porcine liver genomic library cloned into lambda phage. From the 15-kb clone a 4-kb EcoRI fragment containing the first two exons and 2.6 kb of the 5' flanking region of the opn gene was sequenced, and the transcriptional start site determined by primer extension analysis and S1 nuclease mapping. To identify the opn promoter, chimeric chloramphenicol acetyltransferase constructs were prepared using fragments from the first intron and the 5' flanking region of the opn gene. Transient transfection of porcine bone cells with these constructs showed strong promoter activity located within 74 bp upstream from the transcription initiation site. Within this region a TATA sequence, TTTAAA, was identified at positions -26 to -31. However, the highest transcription rate was observed in a construct extending 180 bp upstream that included a CCGCCC Sp1 binding sequence (-63 to -68), and an AP1 site (-74 to -80). Further upstream in the 5' flanking region and within the first intron of the opn, a number of consensus sequences could be identified. Chimeric constructs containing a GGGTCAtatGGTTCA direct repeat consensus sequence for a vitamin D3 response element located at nucleotides -2245 to -2259 responded to the addition of 0.1 microM calcitriol by a 2.5-fold stimulation of transcription, although a greater than 2-fold increase was also observed in shorter constructs -180 to -905 lacking such a consensus sequence. Promoter activity was also exhibited by a region containing a TTTAAA sequence in the first intron that corresponded to the putative promoter site reported for mouse opn in macrophages (Miyazaki, Y., Setoguchi, M., Yoshida, S., Higuchi, Y., Akizuki, S. & Yamamoto, S. (1990) J. Biol. Chem. 265, 14432-14438).(ABSTRACT TRUNCATED AT 400 WORDS)

Isolation and characterization of a cDNA for osteopontin-k: a kidney cell adhesion molecule with high homology to osteopontins

Screening of a bovine renal cDNA library with MAbs resulted in the isolation of a 1447 bp cDNA. This cDNA (pBk2.1) was sequenced and shown to contain an open reading frame with a putative protein of 261 amino acids, with a molecular weight of 29,573 (minute leader sequence) and a hydrophobic leader sequence of 16 amino acids. pBk2.1 was shown to share a high level of nucleic acid sequence homology over portions of its sequence to human, porcine, mouse, and rat osteopontins (40-60%). The peptide (osteopontin-k) had a potential glycosylation site (Asn-X-Ser/Thr), a GRGDS receptor binding region, a high level of asparagine residues, and a high abundance of acid amino acids characteristic of osteopontin-like cell adhesion molecules. The N-terminal amino acid region of pBk2.1 (the first 82 amino acids) and 42 amino acids at the C terminus had the highest level of homology with the osteopontins at 86%. The middle portion of the peptide had greatly reduced homology, ranging from 50% (amino acids 83-174) to 12% (amino acids 175-219). There were also deletions and additions of sequence in osteopontin-k that were not found in the other osteopontins. The homologies suggest that these proteins are highly related and may be derived from a common gene by alternative splicing. A 678 bp cRNA probe constructed from pBk2.1, containing a region with low homology to the osteopontins (amino acids 183-219 with less than 20% homology, plus amino acids 220-261 and untranslated sequence), was used in northern blots and RNAse protection assays.(ABSTRACT TRUNCATED AT 250 WORDS)

Acidic phosphoproteins from bone matrix: a structural rationalization of their role in biomineralization

Osteopontin, bone sialoprotein, and bone acidic glycoprotein-75 are three acidic phosphoproteins that are isolated from the mineralized phase of bone matrix, are synthesized by osteoblastic cells, and are generally restricted in their distribution to calcified tissues. Although each is a distinct gene product, these proteins share aspartic/glutamic acid contents of 30-36% and each contains multiple phosphoryl and sialyl groups. These properties, plus a strict relationship of acidic macromolecules with cell-controlled mineralization throughout nature, suggest functions in calcium binding and nucleation of calcium hydroxyapatite crystal formation. However, direct proof for such roles is still largely indirect in nature. The purpose of this review is to present two speculative hypotheses regarding acidic phosphoprotein function. The goal was to use new sequence information along with database comparisons to develop a structural rationalization of how these proteins may function in calcium handling by bone. For example, our analysis has identified a conserved polyacidic stretch in all three phosphoproteins which we propose mediates metal binding. Also, conserved motifs were identified that are analogous with those for casein kinase II phosphorylation sites and whose number correlates well with that of phosphoryl groups/protein. A two-state conformational model of calcium binding by bone matrix acidic phosphoproteins is described which incorporates these findings.

Inhibition of calcium oxalate crystal growth in vitro by uropontin: another member of the aspartic acid-rich protein superfamily

The majority of human urinary stones are primarily composed of calcium salts. Although normal urine is frequently supersaturated with respect to calcium oxalate, most humans do not form stones. Inhibitors are among the multiple factors that may influence the complex process of urinary stone formation. We have isolated an inhibitor of calcium oxalate crystal growth from human urine by monoclonal antibody immunoaffinity chromatography. The N-terminal amino acid sequence and acidic amino acid content of this aspartic acid-rich protein, uropontin, are similar to those of other pontin proteins from bone, plasma, breast milk, and cells. The inhibitory effect of uropontin on calcium oxalate crystal growth in vitro supports the concept that pontins may have a regulatory role. This function would be analogous to that of other members of the aspartic acid-rich protein superfamily, which stereospecifically regulate the mineralization fronts of calcium-containing crystals.

The cDNA cloning and RNA distribution of bovine osteopontin

We have isolated and sequenced the bovine cDNA (OPN) counterpart of osteopontin. The cDNA is 1356 nucleotides (nt) in length with an open reading frame of 834 nt, encoding a 278-amino acid (aa) protein. Cell-free transcription and translation of OPN RNA resulted in a major species of approx. 40 kDa in size, in agreement with the predicted size of the deduced aa sequence. Northern analysis of bovine OPN RNA indicated the presence of the message in mineralized, as well as soft tissues. A comparison of the deduced aa sequence among various species indicates both regions of similarity and divergence. One prominent region of dissimilarity in bovine OPN compared to all other species is a 22-aa gap which may represent a loss of a potential Ca(2+)-binding loop. Despite the variability among the species, several regions of conservation are apparent, including a hydrophobic leader sequence, a potential site for Asn-linked glycosylation, a stretch of polyaspartic acid residues, and the cell attachment Arg-Gly-Asp tripeptide. Whether bovine OPN enhances cell attachment is unknown. Furthermore, whether the loss of a potential Ca(2+)-binding loop alters the function of OPN would be interesting to determine.

Osteopontin, a substrate for transglutaminase and factor XIII activity

Osteopontin (OPN), an extracellular matrix cell adhesion protein, was found to serve as a substrate for the incorporation of radiolabelled putrescine mediated by a commercial preparation of guinea pig liver transglutaminase. Preliminary evidence also suggests that OPN serves as a substrate for the plasma transglutaminase, Factor XIIIa. While the protein substrates to which OPN is linked in vivo have not been identified, it is reasonable to speculate that this capacity of OPN may dictate its extracellular location and thereby affect its role in bone homeostasis, tumorigenesis, metastasis, resistance to bacterial infections or, perhaps, wound repair.

Differential regulation of the 55 and 44 kDa forms of secreted phosphoprotein 1 (SPP-1, osteopontin) in normal and transformed rat bone cells by osteotropic hormones, growth factors and a tumor promoter

Confluent cultures of rat bone cells synthesize several forms of secreted phosphoprotein 1 (SPP-1, osteopontin), the major phosphorylated forms of which migrate at 55 and 44 kDa on 15% cross-linked SDS-PAGE gels and correspond to the transformation-associated proteins pp 69 and pp 62. A clonal rat calvarial cell line (RCA 11), which expressed the highest level of SPP-1, produced only the 55 kDa form of the phosphorylated protein, whereas normal rat calvarial cells enriched in osteoblastic cells (RC IV cells) produced mostly the 55 kDa form, with small amounts of the 44 kDa form. In contrast, a 44 kDa form was the major [32PO4]-labelled SPP-1 synthesized by a rat osteocarcoma cell line (ROS 17/2.8 cells) with lesser amounts of the 55 kDa SPP-1. When [35S]methionine was used to measure protein synthesis, only the 55 kDa SPP-1 could be clearly detected in confluent cultures of each cell population, indicating that the 55 kDa SPP-1 is the prominent form of SPP-1 synthesized by each cell population. Following treatment of the normal rat bone cells for 24 h with osteotropic hormones (vit D3, PTH and RA), growth factors (PDGF, EGF, TGF-beta), a tumor promoter (TPA) and a plant lectin (Con A), the 55 kDa [35S]methionine labelled SPP-1 was increased 1.7-8.3-fold. Similar, but generally lower responses were observed in the clonal RCA 11 cell line, whereas the ROS 17/2.8 cells were more refractory, showing only a strong response to vit D3. In general, vit D3 produced the strongest stimulation in all populations with TGF-beta producing a good response in the non-transformed cells and RA in the RC IV cells. In contrast, PTH was inhibitory in both RCA 11 and ROS 17/2.8 cells. In most, but not all, cases the alteration in SPP-1 synthesis reflected similar changes in SPP-1 mRNA and in the intensity of the [32PO4]-labelled 55 kDa SPP-1. Collectively, these studies demonstrate that bone cells produce several forms of SPP-1 which are differentially regulated in normal and transformed cells through both transcriptional and posttranscriptional mechanisms.

Regulation of the expression of a secreted acidic protein rich in cysteine (SPARC) in human fibroblasts by transforming growth factor beta. Comparison of transcriptional and post-transcriptional control with fibronectin and type I collagen

Transforming growth factor beta (TGF-beta) and secreted protein acidic rich cysteine (SPARC) have been associated with the rapid remodeling of connective tissues that occurs in wound healing and developmental processes. To study the temporal and mechanistic aspects of TGF-beta-regulated extracellular-protein gene expression in human fibroblasts, confluent cells were pulse labeled for 30 min with [35S]methionine at various times following the single addition of 1.0 ng/ml TGF-beta. After a 4-h chase period, specific radiolabeled media proteins were isolated by either immunoprecipitation or affinity chromatography and quantitated. Stimulation of SPARC synthesis was first apparent 5 h after addition of TGF-beta, reached a maximum (3.5-fold increase) at 24 h and persisted for at least 96 h. A similar temporal response to TGF-beta was observed for the extracellular matrix proteins collagen and fibronectin. In contrast, TGF-beta induced a strong (greater than sixfold increase at 9 h after addition of TGF-beta), but transient stimulation of the synthesis of endothelial-type plasminogen activator inhibitor. Northern blot analysis showed that SPARC mRNA levels were increased by TGF-beta in parallel with increase in SPARC synthesis; a maximum 3.9-fold increase in SPARC mRNA being reached at 24 h. Similarly, the levels of both collagen and fibronectin mRNA were increased by TGF-beta treatment. In each case the stimulation of mRNA was blocked by the presence of the translation inhibitor, cycloheximide. Stability of SPARC mRNA (half-life of approximately 50 h) was not significantly altered by TGF-beta. In contrast, the stability of collagen and fibronectin mRNA were both increased in the presence of TGF-beta; the increased stability being pronounced in less dense cells. In addition to effects on stability, transcription of the collagen and fibronectin genes was increased 7 h after TGF-beta addition, but returned to control levels by 24 h. However, transcription of the SPARC gene was unaffected by TGF-beta at both time points and, together with the stability data, indicates that TGF-beta regulates SPARC expression via a nuclear post-transcriptional mechanism. Differential regulation of gene expression by TGF-beta in a precise temporal pattern via transcriptional and post-transcriptional pathways may be an important aspect of the response of fibroblast cells in a wound environment.

The murine gene encoding secreted phosphoprotein 1 (osteopontin): promoter structure, activity, and induction in vivo by estrogen and progesterone

Secreted phosphoprotein 1 (Spp-1) is a 41.5-kDa bone sialoprotein presumed to be important in the development and functioning of a number of mammalian organs and possibly also in the progression of malignancies. We report here the isolation of a phage lambda genomic clone of the murine Spp-1 gene containing the promoter and first six exons (4.6 kb of the 5.7-kb gene). We have found another exon located 5' to the 'exon 1' reported by Miyazaki et al. [J. Biol. Chem. 265 (1990) 14432-14438]. The DNA upstream from this 5' exon functions as a promoter in epidermal fibroblast and osteoblast-like cells, as demonstrated by transient transfection assays, S1 mapping of the transcription start point, and sequence analysis revealing TATA-like (TTTAAA) and CAAT (its inverse complement) boxes. A small region of the promoter (nt -253 to +79) was able to direct high-level expression of a fused cat reporter gene in JB6 mouse epidermal cells. The transient transfection assays indicated the presence of a positive transcription element between nt -543 and -253 and a negative transcription element between nt -777 and -543. Addition of the tumor promoter, 12-O-tetradecanoylphorbol-13-acetate (TPA), resulted in a 1.5-3-fold induction of transcription, depending on the promoter construct and the TPA concentration. The Spp-1 mRNA was localized in several tissues, consistent with previous reports, and to novel sites in ovary, and in the skin and ventral fatty tissue of pregnant and lactating mice. The induction of Spp-1 mRNA was partially mimicked by painting beta-estradiol or progesterone on the skin of nonpregnant females.

Comparison of two phosphoproteins in chicken bone and their similarities to the mammalian bone proteins, osteopontin and bone sialoprotein II

Two phosphorylated proteins of approximately 66 kDa and approximately 60 kDa mass with different DEAE-Sephacel elution patterns were isolated from chicken bone and were shown to be genetically distinct by both biochemical and immunological analysis. A tryptic peptide from the 60 kDa protein was identified that was similar to a sequence of the rat bone sialoprotein II. Both proteins showed RGD inhibited cell-attachment with the MG-63 osteosarcoma cell, and the approximately 66 kDa phosphoprotein appeared to promote cell adhesion better than human vitronectin. The two phosphoproteins appear to share functional and biochemical characteristics and to be homologous to the mammalian bone phosphoproteins, osteopontin and bone sialoprotein II.

cDNA cloning, mRNA distribution and heterogeneity, chromosomal location, and RFLP analysis of human osteopontin (OPN)

A human osteopontin (OP) cDNA was isolated from a library made from primary cultures of human bone cells. The distribution of osteopontin mRNA in human tissues was investigated by Northern analysis and showed that the human message was predominant in cultures of bone cells and in decidua cells isolated at 6-12 weeks of gestation. Immunohistochemical analysis confirmed that OP expression is high in decidua cells as well as in the endometrial glands of a non-pregnant secretory-phase human uterus. Two variants of the OP message were evident on the basis of DNA sequencing and polymerase chain reaction amplification of bone and decidua cell mRNA. The peptides potentially translated by the variant messages differ by the presence (OP1b) or absence (OP1a) of 14 amino acids at residue 58 of the molecule. The deduced human protein sequence shows a conservation between species in the position of the Arg-Gly-Asp (RGD) cell attachment site. Chromosomal mapping of the osteopontin gene (OPN) using human-rodent cell hybrids demonstrated a location on chromosome 4 in the human genome. In situ hybridization of metaphase chromosomes using radiolabeled OP1a as a probe indicated that the gene is located on a region of 4q that is near the centromere. A high-frequency restriction fragment length polymorphism was evident in the DNA from 29 unrelated individuals using the enzyme BglII. Analysis of total genomic DNA by digestion with several restriction enzymes, Southern blotting, and hybridization with the human osteopontin cDNA indicated that the gene is a single copy with an approximate length of 5.4-8.2 kb.