Modulation of osteopontin post-translational state by 1, 25-(OH)2-vitamin D3. Dependence on Ca2+ influx

Osteopontin: An important protein in the formation of kidney stones

The incidence of kidney stones averages 10%, and the recurrence rate of kidney stones is approximately 10% at 1 year, 35% at 5 years, 50% at 10 years, and 75% at 20 years. However, there is currently a lack of good medicines for the prevention and treatment of kidney stones. Osteopontin (OPN) is an important protein in kidney stone formation, but its role is controversial, with some studies suggesting that it inhibits stone formation, while other studies suggest that it can promote stone formation. OPN is a highly phosphorylated protein, and with the deepening of research, there is growing evidence that it promotes stone formation, and the phosphorylated protein is believed to have adhesion effect, promote stone aggregation and nucleation. In addition, OPN is closely related to immune cell infiltration, such as OPN as a pro-inflammatory factor, which can activate mast cells (degranulate to release various inflammatory factors), macrophages (differentiated into M1 macrophages), and T cells (differentiated into T1 cells) etc., and these inflammatory cells play a role in kidney damage and stone formation. In short, OPN mainly exists in the phosphorylated form in kidney stones, plays an important role in the formation of stones, and may be an important target for drug therapy of kidney stones.

Phosphorylated Osteopontin Secreted from Cancer Cells Induces Cancer Cell Motility

Osteopontin (OPN) plays a pivotal role in cancer cell invasion and metastasis. Although OPN has a large number of phosphorylation sites, the functional significance of OPN phosphorylation in cancer cell motility remains unclear. In this study, we attempted to investigate whether phosphorylated OPN secreted from cancer cells affect cancer cell migration. Quantitative PCR and Western blot analyses revealed that MDA-MB435S, A549, and H460 cells highly expressed OPN, whereas the OPN expression levels in H358, MIAPaca-2, and Panc-1 cells were quite low or were not detected. Compared with the cancer cell lines with a low OPN expression, the high OPN-expressing cancer cell lines displayed a higher cell migration, and the cell migration was suppressed by the anti-OPN antibody. This was confirmed by the OPN overexpression in H358 cancer cells with a low endogenous OPN. Phos-tag ELISA showed that phosphorylated OPN was abundant in the cell culture media of A549 and H460 cells, but not in those of MDA-MB435S cells. Moreover, the A549 and H460 cell culture media, as well as the MDA-MB435S cell culture media with a kinase treatment increased cancer cell motility, both of which were abrogated by phosphatase treatment or anti-OPN antibodies. These results suggest that phosphorylated OPN secreted from cancer cells regulates cancer cell motility.

Nutritional Supplementation of Naturally Occurring Vitamin D to Improve Hemorrhagic Stroke Outcomes

Vitamin D deficiency, if left untreated, is associated with bone disorders, cardiovascular damage, and an increased risk of ischemic stroke. While there are various nutritional options for the natural intake of vitamin D, we hope to elucidate the potential mechanisms dietary vitamin D may play in hemorrhagic stroke pathology. This scoping review outlines findings from studies relevant to the biochemical activity of vitamin D, the impact of vitamin D deficiency on hemorrhagic stroke outcomes, and the potential benefit of nutritional vitamin D on hemorrhagic stroke outcomes. Here, we analyze the relevant factors that can lead to vitamin D deficiency, and subsequently, a higher risk of hemorrhagic stroke incidence with worsened subsequent outcomes. The neuroprotective mechanisms through which vitamin D works to attenuate hemorrhagic stroke onset and post-stroke outcomes have not yet been thoroughly examined. However, researchers have proposed several potential protective mechanisms, including reduction of blood brain barrier disturbance by inhibiting the production of reactive oxygen species, mitigation of inflammation through a reduction of levels of proinflammatory cytokines, and prevention of cerebral vasospasm and delayed cerebral ischemia following subarachnoid hemorrhage and intracerebral hemorrhage. While more research is needed and there are limitations to vitamin D supplementation, vitamin D as a whole may play a significant role in the dynamics of hemorrhagic stroke. Further research should focus on expanding our understanding of the neuroprotective capacity and mechanisms of vitamin D, as well as how vitamin D supplementation could serve as an effective course of treatment of hemorrhagic strokes.

Post-translational Modifications of SIBLING Proteins and Their Roles in Osteogenesis and Dentinogenesis

The extracellular matrix (ECM) of bone and dentin contains several non-collagenous proteins. One category of non-collagenous protein is termed the SIBLING (Small Integrin-Binding LIgand, N-linked Glycoprotein) family, that includes osteopontin (OPN), bone sialoprotein (BSP), dentin matrix protein 1 (DMP1), dentin sialophosphoprotein (DSPP), and matrix extracellular phosphoglycoprotein (MEPE). These polyanionic SIBLING proteins are believed to play key biological roles in the mineralization of bone and dentin. Although the specific mechanisms involved in controlling bone and dentin formation are still unknown, it is clear that some functions of the SIBLING family members are dependent on the nature and extent of post-translational modifications (PTMs), such as phosphorylation, glycosylation, and proteolytic processing, since these PTMs would have significant effects on their structure. OPN and BSP are present in the ECM of bone and dentin as full-length forms, whereas amino acid sequencing indicates that DMP1 and DSPP exist as proteolytically processed fragments that result from scission of X-Asp bonds. We hypothesized that the processing of DMP1 and DSPP is catalyzed by the PHEX enzyme, since this protein, an endopeptidase that is predominantly expressed in bone and tooth, has a strong preference for cleavage at the NH2-terminus of aspartyl residue. We envision that the proteolytic processing of DMP1 and DSPP may be an activation process that plays a significant, crucial role in osteogenesis and dentinogenesis, and that a failure in this processing would cause defective mineralization in bone and dentin, as observed in X-linked hypophosphatemic rickets.

Osteopontin: a leading candidate adhesion molecule for implantation in pigs and sheep

Osteopontin (OPN; also known as Secreted Phosphoprotein 1, SPP1) is a secreted extra-cellular matrix (ECM) protein that binds to a variety of cell surface integrins to stimulate cell-cell and cell-ECM adhesion and communication. It is generally accepted that OPN interacts with apically expressed integrin receptors on the uterine luminal epithelium (LE) and conceptus trophectoderm to attach the conceptus to the uterus for implantation. Research conducted with pigs and sheep has significantly advanced understanding of the role(s) of OPN during implantation through exploitation of the prolonged peri-implantation period of pregnancy when elongating conceptuses are free within the uterine lumen requiring extensive paracrine signaling between conceptus and endometrium. This is followed by a protracted and incremental attachment cascade of trophectoderm to uterine LE during implantation, and development of a true epitheliochorial or synepitheliochorial placenta exhibited by pigs and sheep, respectively. In pigs, implanting conceptuses secrete estrogens which induce the synthesis and secretion of OPN in adjacent uterine LE. OPN then binds to αvβ6 integrin receptors on trophectoderm, and the αvβ3 integrin receptors on uterine LE to bridge conceptus attachment to uterine LE for implantation. In sheep, implanting conceptuses secrete interferon tau that prolongs the lifespan of CL. Progesterone released by CL then induces OPN synthesis and secretion from the endometrial GE into the uterine lumen where OPN binds integrins expressed on trophectoderm (αvβ3) and uterine LE (identity of specific integrins unknown) to adhere the conceptus to the uterus for implantation. OPN binding to the αvβ3 integrin receptor on ovine trophectoderm cells induces in vitro focal adhesion assembly, a prerequisite for adhesion and migration of trophectoderm, through activation of: 1) P70S6K via crosstalk between FRAP1/MTOR and MAPK pathways; 2) MTOR, PI3K, MAPK3/MAPK1 (Erk1/2) and MAPK14 (p38) signaling to stimulate trohectoderm cell migration; and 3) focal adhesion assembly and myosin II motor activity to induce migration of trophectoderm cells. Further large in vivo focal adhesions assemble at the uterine-placental interface of both pigs and sheep and identify the involvement of sizable mechanical forces at this interface during discrete periods of trophoblast migration, attachment and placentation in both species.

Lipopolysaccharide-induced lung injury is independent of serum vitamin D concentration

Vitamin D deficiency is increasing in incidence around the world. Vitamin D, a fat-soluble vitamin, has documented effects on the innate and adaptive immune system, including macrophage and T regulatory (Treg) cell function. Since Treg cells are important in acute lung injury resolution, we hypothesized that vitamin D deficiency increases the severity of injury and delays injury resolution in lipopolysaccharide (LPS) induced acute lung injury. Vitamin D deficient mice were generated, using C57BL/6 mice, through diet modification and limited exposure to ultraviolet light. At 8 weeks of age, vitamin D deficient and sufficient mice received 2.5 g/kg of LPS or saline intratracheal. At 1 day, 3 days and 10 days, mice were anesthetized and lung elastance measured. Mice were euthanized and bronchoalveolar lavage fluid, lungs and serum were collected. Ex vivo neutrophil chemotaxis was evaluated, using neutrophils from vitamin D sufficient and deficient mice exposed to the chemoattractants, KC/CXCL1 and C5a, and to bronchoalveolar lavage fluid from LPS-exposed mice. We found no difference in the degree of lung injury. Leukocytes were mildly decreased in the bronchoalveolar fluid of vitamin D deficient mice at 1 day. Ex-vivo, neutrophils from vitamin D deficient mice showed impaired chemotaxis to KC but not to C5a. Vitamin D deficiency modestly impairs neutrophil chemotaxis; however, it does not affect lung injury or its resolution in an LPS model of acute lung injury.

Effect of Ca2+ ions on the adhesion and mechanical properties of adsorbed layers of human osteopontin

Using an atomic force microscope and a surface force apparatus, we measured the surface coverage, adhesion, and mechanical properties of layers of osteopontin (OPN), a phosphoprotein of the human bones, adsorbed on mica. OPN is believed to connect mineralized collagen fibrils of the bone in a matrix that dissipates energy, reducing the risk of fractures. Atomic force microscopy normal force measurements showed large adhesion and energy dissipation upon retraction of the tip, which were due to the breaking of the many OPN-OPN and OPN-mica bonds formed during tip-sample contact. The dissipated energy increased in the presence of Ca(2+) ions due to the formation of additional OPN-OPN and OPN-mica salt bridges between negative charges. The forces measured by surface force apparatus between two macroscopic mica surfaces were mainly repulsive and became hysteretic only in the presence of Ca(2+): adsorbed layers underwent an irreversible compaction during compression due to the formation of long-lived calcium salt bridges. This provides an energy storage mechanism, which is complementary to energy dissipation and may be equally relevant to bone recovery after yield. The prevalence of one mechanism or the other appears to depend on the confinement geometry, adsorption protocol, and loading-unloading rates.

Control of osteopontin signaling and function by post-translational phosphorylation and protein folding

Osteopontin (OPN) plays roles in a variety of cellular processes from bone resorption and extracellular matrix (ECM) remodeling to immune cell activation and inhibition of apoptosis. Because it binds receptors (integrins, CD44 variants) typically engaged by ECM molecules, OPN acts as a "soluble" ECM molecule. A persistent theme throughout the characterization of how OPN functions has been the importance of phosphorylation. The source of the OPN used in specific experiments and the location of modified sites is an increasingly important consideration for OPN research. We review briefly some of the ways OPN impacts on the biology of mammalian systems with an emphasis on the importance of serine phosphorylation in modulating its signaling ability. We describe experiments that support the hypothesis that differences in the post-translational phosphorylation of OPN expressed by different cell types regulate how it impacts on target cells. Analyses of OPN's potential secondary structure reveal a possible beta-sheet conformation that offers an interpretation of certain experimental observations, specifically the effect of thrombin cleavage; it is consistent with an interaction between the C-terminal region of the protein and the central integrin-binding RGD sequence.

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.

Cell type-specific post-translational modifications of mouse osteopontin are associated with different adhesive properties

Osteopontin (OPN) is a highly modified integrin-binding protein found in all body fluids. Expression of OPN is strongly correlated with poor prognosis in many different human cancers, suggesting an important but poorly understood role for this protein in tumorigenesis and metastasis. The protein exists in a number of different isoforms differing in the degree of post-translational modifications that are likely to exhibit different functional properties. This study examines for the first time the post-translational modifications of OPN from transformed cells and the effects of these modifications on cell biology. We have characterized the complete phosphorylation and glycosylation patterns of OPN expressed by murine ras-transformed fibroblasts (FbOPN) and differentiating osteoblasts (ObOPN) by a combination of mass spectrometric analyses and Edman degradation. Mass spectrometric analysis showed masses of 34.9 and 35.9 kDa for FbOPN and ObOPN, respectively. Enzymatic dephosphorylation, sequence, and mass analyses demonstrated that FbOPN contains approximately four phosphate groups distributed over 16 potential phosphorylation sites, whereas ObOPN contains approximately 21 phosphate groups distributed over 27 sites. Five residues are O-glycosylated in both isoforms. These residues are fully modified in FbOPN, whereas one site is partially glycosylated in ObOPN. Although both forms of OPN mediated robust integrin-mediated adhesion of mouse ras-transformed fibroblasts, the less phosphorylated FbOPN mediated binding of MDA-MD-435 human tumor cells almost 6-fold more than the heavy phosphorylated ObOPN. These results strongly support the hypothesis that the degree of phosphorylation of OPN produced by different cell types can regulate its function.

Expression Pattern of Osteopontin in Endometrial Carcinoma: Correlation With Expression of the Adhesion Molecule CEACAM1

Osteopontin (OPN) and CEACAM1 have diverse biological functions in the uterus and placenta throughout the estrous cycle and pregnancy and have been shown to interact with integrin beta3. OPN is a glycoprotein of the extracellular matrix, which has been shown to mediate cellular migration and invasion and to contribute to tumorigenesis in several types of cancers. Recently we showed the expression pattern of OPN in gestational trophoblastic tumors. CEACAM1 is an adhesion molecule of the carcinoembryonic antigen family that we have recently found to be expressed in endometrial cancer and that has been shown to be down-regulated in colorectal, prostate, and breast cancer. In this study, immunohistochemistry and immunofluorescence with specific antibodies were performed on a series of 20 normal endometrial samples, 17 endometrial hyperplasias, and 43 endometrial carcinomas (28 endometrioid, 10 serous, and 5 clear cell carcinomas) to investigate the expression pattern and cell-type specific localization of OPN and to correlate it with the expression of CEACAM1. In addition, Western blot was performed on normal human endometrium and endometrial neoplasia. Strong OPN expression with a consistent cytoplasmic localization in epithelial glandular cells was observed in the normal human endometrium in 80% of the samples of the proliferative and secretory phase (score 8-12). Similar results could be found in endometrial hyperplasias. Strong expression of OPN could be observed in 29 (67.4%) of the 43 analyzed endometrial carcinomas. Of the 43 analyzed tumors, 18 (41.8%) were in the high score (8-12) category with a strong OPN expression level; 11 of 43 (25.5%) showed a moderate score (4-7) category. In endometrioid carcinoma with increasing malignancy grade, increasing areas with low OPN expression level or complete loss of OPN expression could be observed. In contrast, serous tumors showed a strong OPN expression level. Similar results could be found in Western blot analysis. CEACAM1 showed similar results and could be found to be coexpressed with OPN in normal human endometrium and in endometrial neoplasia as we showed using immunofluorescence. In this study, the different expression patterns of OPN in endometrial tumors could additionally support the biological diversity of endometrioid and serous carcinomas together with other markers. We suggest that OPN might play a different role in the pathogenesis of endometrial cancer (possibly as a functional complex with CEACAM1) and could be relevant for invasive growth of such lesions.

Post-translationally modified residues of native human osteopontin are located in clusters: identification of 36 phosphorylation and five O-glycosylation sites and their biological implications

OPN (osteopontin) is an integrin-binding highly phosphorylated glycoprotein, recognized as a key molecule in a multitude of biological processes such as bone mineralization, cancer metastasis, cell-mediated immune response, inflammation and cell survival. A significant regulation of OPN function is mediated through PTM (post-translational modification). Using a combination of Edman degradation and MS analyses, we have characterized the complete phosphorylation and glycosylation pattern of native human OPN. A total of 36 phosphoresidues have been localized in the sequence of OPN. There are 29 phosphorylations (Ser8, Ser10, Ser11, Ser46, Ser47, Thr50, Ser60, Ser62, Ser65, Ser83, Ser86, Ser89, Ser92, Ser104, Ser110, Ser113, Thr169, Ser179, Ser208, Ser218, Ser238, Ser247, Ser254, Ser259, Ser264, Ser275, Ser287, Ser292 and Ser294) located in the target sequence of MGCK (mammary gland casein kinase) also known as the Golgi kinase (S/T-X-E/S(P)/D). Six phosphorylations (Ser101, Ser107, Ser175, Ser199, Ser212 and Ser251) are located in the target sequence of CKII (casein kinase II) [S-X-X-E/S(P)/D] and a single phosphorylation, Ser203, is not positioned in the motif of either MGCK or CKII. The 36 phosphoresidues represent the maximal degree of modification since variability at many sites was seen. Five threonine residues are O-glycosylated (Thr118, Thr122, Thr127, Thr131 and Thr136) and two potential sites for N-glycosylation (Asn63 and Asn90) are not occupied in human milk OPN. The phosphorylations are arranged in clusters of three to five phosphoresidues and the regions containing the glycosylations and the RGD (Arg-Gly-Asp) integrin-binding sequence are devoid of phosphorylations. Knowledge about the positions and nature of PTMs in OPN will allow a rational experimental design of functional studies aimed at understanding the structural and functional interdependences in diverse biological processes in which OPN is a key molecule.

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.

Comprehensive Identification of Post-translational Modifications of Rat Bone Osteopontin by Mass Spectrometry†

Osteopontin (OPN) is a highly modified protein that is found in many tissues and has been associated with a variety of physiological and pathological processes. Bone OPN is a potent inhibitor of hydroxyapatite crystal formation and stimulates bone resorption by osteoclasts; these activities, as well as others, are dependent upon phosphorylation of the protein. We have used mass spectrometry (MS) to perform a comprehensive analysis of the post-translational modification of OPN purified from rat bone. Matrix-assisted laser desorption time-of-flight (MALDI-TOF) MS showed masses of 37.6 and 36.8 kDa before and after enzymatic dephosphorylation, respectively, corresponding to a content of approximately 10.4 phosphate groups. Using proteolytic digestion and tandem MS, we localized 29 sites of phosphorylation: S10, S11, S46, S47, T50, S60, S62, S65, S146, T154, S160, S164, S167, S193, S196, S203, S220, S223, S232, S241, S245, S257, S262, S267, S278, S290, S295, S296, and S297. In addition, Y150 was shown to be sulfated and T107, T110, T116, and T121 are O-glycosylated. No glycan was detected at the potential N-glycosylation site. Other modifications, including deamidation, oxidation, and carbamylation, are also present. A 36-amino acid sequence from residues 67-102 could not be analyzed in detail, even after sialidase treatment, presumably because of the presence of a large number of acidic residues. In comparison to the previously characterized cow milk isoform, rat bone OPN is sulfated and has an additional site of glycosylation, many different sites of phosphorylation, and a lower overall phosphate content.

Phosphorylated osteopontin promotes migration of human choriocarcinoma cells via a p70 S6 kinase-dependent pathway

This study examined the role of osteopontin (OPN), a phosphorylated secreted glycoprotein, in the promotion of trophoblastic cell migration, an early event in the embryo implantation process. Three human choriocarcinoma cell lines, namely JAR, BeWo, and JEG-3, were treated with variants of OPN differing in the extent of phosphorylation following sequential dephosphorylation with tartrate-resistant acid phosphatase (TRAP), and their migratory response was measured. The highly phosphorylated human milk form of OPN (OPN-1) strongly triggered migration in all three cell lines, whereas the less phosphorylated variants, OPN-2a and OPN-2b, failed to stimulate migration. JAR cell migration in response to OPN-1 was accompanied by a rapid rearrangement of actin filaments to the cellular membrane. Using broad spectrum protein kinase profiling, we identified p70 S6 kinase as a major signal transduction pathway activated by OPN-1 during the migratory response in JAR cells. Activation was blocked completely by rapamycin and LY294002, thus demonstrating that OPN-1-stimulated migration occurs through mTOR and PI3K pathways, respectively. Conversely, PD98059 did not affect the activation of p70 S6 kinase by OPN-1, therefore, this response does not involve the Ras/ MAPK signaling cascade. Together, these data show that the highly phosphorylated human OPN-1 can stimulate trophoblastic cell migration and provides evidence for the involvement of the PI3K/mTOR/p70 S6 kinase pathway in the JAR cells response. Because both OPN and TRAP are expressed in the uterus during early pregnancy, it is conceivable that extracellular phosphatases such as TRAP may modify OPN charge state and thus modulate cell migration.

Inhibition of hydroxyapatite formation by osteopontin phosphopeptides

Osteopontin (OPN) is an acidic phosphoglycoprotein that is believed to function in the prevention of soft tissue calcification. In vitro studies have shown that OPN can inhibit the formation of hydroxyapatite (HA) and other biologically relevant crystal phases, and that this inhibitory activity requires phosphorylation of the protein; however, it is not known which phosphorylated residues are involved. We have synthesized peptides corresponding to four phosphoserine-containing sequences in rat OPN: OPN7-17, containing phosphoserines 10 and 11; OPN41-52, containing phosphoserines 46 and 47; OPN248-264, containing phosphoserines 250, 257 and 262; and OPN290-301, containing phosphoserines 295-297. The abilities of these peptides to inhibit de novo HA formation were determined using a constant-composition autotitration assay. All four OPN phosphopeptides caused a dose-dependent increase in nucleation lag time, but did not significantly affect subsequent formation of the crystals. However, OPN41-52 (inhibitory constant 73.5 min/microM) and OPN290-301 (72.2 min/microM) were approx. 4 times more potent inhibitors than OPN7-17 (19.7 min/microM) and OPN247-264 (16.3 min/microM). 'Scrambling' the amino acid sequence of OPN290-301 resulted in decreased potency (45.6 min/microM), whereas omission of the phosphate groups from this peptide caused a greater decrease (5.20 min/microM). These findings have identified phosphorylated sequences that are important for the ability of rat bone OPN to inhibit HA crystal formation, and suggest that negative-charge density is an important factor in this activity.

Integrins and extracellular matrix proteins at the maternal-fetal interface in domestic animals

Establishment of pregnancy in mammals requires coordinated conceptus-maternal interactions involving numerous hormones, growth factors and cytokines acting via specific receptors in the uterus. Uterine secretions play an important role in establishing synchrony between development of the conceptus and uterine receptivity, as well as in conceptus remodeling, adhesion, implantation and placentation in domestic species. Studies of non-invasive implantation in domestic livestock provide valuable opportunities to investigate fundamental processes of the initial events of apposition, attachment and adhesive interactions that are shared among species. In pigs and sheep, it appears that integrins play a dominant role in these fundamental processes via interactions with extracellular matrix molecules and other ligands to transduce cellular signals in uterine epithelial cells and conceptus trophectoderm. This review considers several of the potential integrin-binding ligands involved in the complex implantation adhesion cascade in pigs and sheep along with in vitro evidence for the transduction of cytoplasmic signals that may be required to sustain fetal and maternal contributions to the formation of the epitheliochorial placenta.

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.

The influence of culture conditions on extracellular matrix proteins synthesized by osteoblasts derived from rabbit bone marrow

The influence of culture conditions on the extracellular matrix (ECM) protein expressions of rabbit bone marrow stromal cells has been studied. The focus was on the effects of two kinds of sera, fetal calf serum (FCS) and Ultroser, on cells treated with dexamethasone. The induction of osteoblastic differentiation by dexamethasone addition is confirmed, particularly when cells are cultured in FCS. Bone marrow stromal cells produce alkaline phosphatase positive CFU-F and produce ECM with some mineralized nodules. Analysis by means of two-dimensional gel electrophoresis showed important changes in the composition of ECM proteins after dexamethasone treatment. Overexpression, underexpression, and new synthesized proteins were observed. The most significant modification was linked to the synthesis of four new proteins visible in the acidic area with a low molecular weight of around 17 kDa. These proteins did not correspond to those ECM proteins known to be induced by dexamethasone. Moreover, the effect of dexamethasone on osteoblastic differentiation induction appears very limited when cells are cultured in Ultroser compared to FCS. The protein pattern with Ultroser is different to that obtained with FCS. Cells cultured in Ultroser synthesized no new protein. The different behavior of cells according to the type of medium used is discussed in terms of the osteogenic factors present in the two different sera.

Multiple molecular mechanisms of 1 alpha,25(OH)2-vitamin D3 rapid modulation of three ion channel activities in osteoblasts

Rapid nongenomic responses to steroids include modulation of ion channel activities on the cell membrane of target cells, but little is known about the molecular mechanisms involved. In this paper we investigate the mechanisms underlying the combined action of the secosteroid hormone 1alpha,25-dihydroxyvitamin D3 [1alpha,25(OH)(2)D3] on three different ion channel types in rat osteoblasts, which include a voltage-gated L-type Ca(2+) channel, a mechanosensitive Cl(-) channel, and a stretch-activated cation (SA-Cat) channel. We found that physiological nanomolar concentrations of 1alpha,25(OH)(2)D3 rapidly modify the overall electrical activity of the membrane in ROS 17/2.8 cells. 1alpha,25(OH)(2)D3 increases the osteoblast L-type Ca(2+) channel activity at low depolarizing voltages in a fashion similar to the 1,4-dihydropyridine (DHP) agonist Bay K8644. At highly depolarizing potentials 1alpha,25(OH)(2)D3 potentiates volume-sensitive Cl(-) currents through mechanisms that may involve a putative membrane receptor. We show for the first time that 1alpha,25(OH)(2)D3 also increases inward currents through SA-Cat channels at positive membrane voltages in a dose-dependent manner. Contrary to our expectations, the stereoisomer 1beta,25(OH)(2)D3, which suppresses 1alpha,25(OH)(2)D3 activation of osteoblast Cl(-) currents, mimicked 1alpha,25(OH)(2)D3 agonist effects on Ca(2+) and SA-Cat channel activities. Cyclic AMP is involved in 1alpha,25(OH)(2)D3 effects on both Ca(2+) and SA-Cat channels, but not in Cl(-) channels. We conclude that 1alpha,25(OH)(2)D3 rapid effects on ion channel activities in ROS 17/2.8 cells occur through multiple mechanisms that, on the one hand, involve a possible direct interaction with the L-type Ca(2+) channel molecule and, on the other hand, molecular pathways that may include a putative membrane receptor.

Microarray detection of gene expression changes induced by 1,25(OH)(2)D(3) and a Ca(2+) influx-activating analog in osteoblastic ROS 17/2.8 cells

1,25-Dihydroxyvitamin D(3) (1,25(OH)(2)D(3)) treatment of osteoblastic ROS 17/2.8 cells initiates membrane-initiated rapid responses through activation of Ca(2+) influx and longer-term nuclear receptor-mediated changes in gene expression. Ca(2+) influx triggers a change in the phosphorylation state of the bone matrix protein, osteopontin (OPN), detectable at 3 h and prior to nuclear receptor-mediated events. This study aimed to determine if Ca(2+) influx induced by 1,25(OH)(2)D(3) would produce nuclear receptor-independent changes in gene expression. We employed a rat cDNA microarray strategy to screen the transcriptional changes at 3 h of treatment with 1,25(OH)(2)D(3) and with an analog of 1,25(OH)(2)D(3) (25(OH)-16ene-23yne-D(3) [AT]) that we previously showed to activate Ca(2+) influx without binding to the nuclear receptor. Arrays also were screened with cDNA from ROS 17/2.8 cells treated for 24 h, when nuclear receptor-mediated transcriptional events would occur. Rat gene filters (GeneFilter, Research Genetics) were hybridized with labeled cDNA probes from treatment groups. Among 5000 different clones on the array filters, we identified a family of genes which were altered 2-fold or greater following treatment with 1,25(OH)(2)D(3) or analog AT for 3 h. Cluster analysis also revealed genes whose expression was significantly up-regulated at 24 h, including OPN. Analysis of rapid changes in gene expression revealed changes affecting a diverse range of cellular pathways and functions, including protein kinases and phosphatases, Ca(2+) signaling, cell adhesion and secretion. These findings provide clear evidence of rapid changes in gene expression associated with Ca(2+) influx mediated by 1,25(OH)(2)D(3), and shed light on the nuclear-receptor independent signaling pathway affecting OPN phosphorylation.

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

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

Phosphorylated osteopontin peptides suppress crystallization by inhibiting the growth of calcium oxalate crystals

BACKGROUND

Osteopontin isolated from human urine [uropontin (uOPN)] is a potent inhibitor of calcium oxalate (CaOx) monohydrate (COM) crystallization. However, specific structural features responsible for its effects on CaOx crystallization were not previously known. The present studies were designed to define molecular features responsible for interactions of uOPN with COM crystals and the inhibition of crystallization.

METHODS

Peptides and phosphopeptides with sequences corresponding to potential crystal binding domains within the protein sequence of osteopontin were synthesized. Then the effects of these peptides on COM crystal growth and crystal aggregation were investigated and their secondary structures analyzed.

RESULTS

Growth of COM crystals was inhibited by approximately 50% at 1000 nmol/L concentrations by the two unmodified peptides with the closest clustering of aspartic acid residues. Growth was not inhibited by the other two unmodified peptides, with aspartic residues more evenly distributed within their sequences. Phosphorylation markedly increased inhibition of COM crystal growth, so that each of the four phosphorylated peptides inhibited growth by at least 50% at concentrations of < or =200 nmol/L. Phosphorylation of these peptides did not cause changes in secondary structure that would favor interaction with COM crystal surfaces.

CONCLUSIONS

These studies of synthetic peptides identify molecular features within the osteopontin molecule that contribute to the inhibition of one aspect of COM crystallization. The inhibition of crystal growth induced by phosphorylation appears to result from altered local patterns of charge density, since conformational changes favoring interaction with crystals were not caused by phosphorylation.

Bioactive analogs that simulate subsets of biological activities of 1alpha,25(OH)(2)D(3) in osteoblasts

1alpha,25-Dihydroxyvitamin D(3) [1alpha,25(OH)(2)D(3)] treatment of osteoblastic cells elicits a series of measurable responses that include both rapid, membrane-initiated effects and longer-term nuclear receptor-mediated effects. Structural analogs have been identified and characterized that selectively activate subsets of these pathways. Two analogs from over 35 that have been tested were chosen for this comparison because they activate non-overlapping response pathways, presumably representing either membrane-initiated or nuclear receptor-initiated activities. Compound AT [25(OH)-16ene-23yne-D(3)] lacks the 1-hydroxyl essential for interacting with the nuclear receptor, but triggers Ca(2+) influx through plasma membrane Ca(2+) channels, augments parathyroid hormone (PTH)-induced Ca(2+) signals, dephosphorylates the matrix protein osteopontin (OPN), and along with PTH stimulates release of calcium from calvaria in organ culture. Compound BT [1alpha,24(OH)(2)-22ene-24cyclopropyl-D(3)] does not elicit any of the rapid responses or enhance PTH-induced bone resorption, but binds to the nuclear receptor for 1alpha,25(OH)(2)D(3) and increases steady state mRNA levels of both OPN and osteocalcin over a 48 h period. Together, these two analogs recapitulate all of the known actions of 1alpha,25(OH)(2)D(3) on osteoblasts. Based on these findings, we conclude that Ca(2+) release from bone stimulated by 1alpha,25(OH)(2)D(3) and PTH is related to the rapid, membrane-initiated actions and is not likely to involve binding to the nuclear receptor for 1alpha,25(OH)(2)D(3). Longer term stimulation of bone formation by 1alpha,25(OH)(2)D(3), however, appears to involve solely the nuclear receptor-mediated effects. These findings support our model of 1alpha,25(OH)(2)D(3) as a coupling factor for bone resorption and formation during bone remodeling.

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.

1,25 dihydroxyvitamin D(3) activates sphingomyelin turnover in ROS17/2.8 osteosarcoma cells without sphingolipid-induced changes in cytosolic Ca(2+)

1,25-Dihydroxyvitamin D(3) [1,25(OH)(2)D(3)] initiates the hydrolysis of sphingomyelin in ROS 17/2.8 osteosarcoma cells with the resultant generation of cell-associated ceramide. Increases in ceramide levels were detectable at 15 min and maximal one hour after exposure of cells to 1,25(OH)(2)D(3). Neither 1,25(OH)(2)D(3) nor exogenous ceramide elicited a change in cytosolic free Ca(2+) ([Ca(2+)](i)). Transient elevations in [Ca(2+)](i) were observed when cells were exposed to exogenous sphingosine, but there was no detectable conversion of ceramide to sphingosine in 1, 25(OH)(2)D(3)-treated cells. Ceramide also did not stimulate Ca(2+) uptake across ROS 17/2.8 cell plasma membranes. Collectively, these results suggest that 1,25(OH)(2)D(3) activates sphingomyelin turnover in ROS 17/2.8 osteosarcoma cells but that the sphingolipid metabolite ceramide is not responsible for 1,25(OH)(2)D(3)-induced activation of plasma membrane Ca(2+) channels.

Ribozyme ablation demonstrates that the cardiac subtype of the voltage-sensitive calcium channel is the molecular transducer of 1, 25-dihydroxyvitamin D(3)-stimulated calcium influx in osteoblastic cells

1,25-Dihydroxyvitamin D(3) (1,25(OH)(2)D(3)) stimulates transmembrane influx of Ca(2+) through L-type voltage-sensitive Ca(2+) channels (VSCCs) in ROS 17/2.8 osteoblastic cells. Ca(2+) influx modulates osteoblastic activities including matrix deposition, hormone responsiveness, and Ca(2+)-dependent signaling. 1, 25(OH)(2)D(3) also regulates transcript levels encoding VSCCs. L-type VSCCs are multisubunit complexes composed of a central pore-forming alpha(1) subunit and four additional subunits. The alpha(1) subunit is encoded by one gene in a multimember family, defining tissue-specific subtypes. Osteoblasts synthesize two splice variants of the alpha(1C) cardiac VSCC subtype; however, the molecular identity of the 1,25(OH)(2)D(3)-regulated VSCC remained unknown. We created a ribozyme specifically cleaving alpha(1C) mRNA. To increase target ablation efficiency, the ribozyme was inserted into U1 small nuclear RNA (snRNA) by engineering the U1 snRNA expression cassette, conferring the ribozyme transcript with stabilizing stem-loops at both sides and the Sm binding site that facilitates localization into nucleoplasm. After transfection of ROS 17/2.8 cells with U1 ribozyme-encoding vector, stable clonal cells were selected in which the expression of alpha(1C) transcript and protein were strikingly reduced. Ca(2+) influx assays in ribozyme transfectants showed selective attenuation of depolarization and 1, 25(OH)(2)D(3)-regulated Ca(2+) responses. We conclude that the cardiac subtype of the L-type VSCC is the transducer of stimulated Ca(2+) influx in ROS 17/2.8 osteoblastic cells.

Calcium signals in prostate cancer cells: specific activation by bone-matrix proteins

Cancer of the prostate commonly metastasizes to bony sites where cells acquire an aggressive, rapidly proliferating, androgen-independent phenotype. The interaction between bone and prostate, thus, becomes a key factor in disease progression. Fluctuations in intracellular ionized Ca2+ [Ca2+]i are rapid, regulated signal transduction events often associated with cell proliferation. Hence, Ca2+ signals provide a convenient measure of early events in cancer cell growth. This study developed single cell fluorescent imaging techniques to visualize Ca2+ signals in Fura-2 loaded prostatic cancer cell lines of various metastatic phenotypes. Solubilized bone fractions containing extracellular matrix and associated proteins were tested for the ability to trigger Ca2+ signals in prostate cancer cell lines. Fractions representing the complete repertoire of non-collagenous proteins present in mineralized bone were tested. Results demonstrated that two bone fractions termed D3b- and D4a-triggered Ca2+ signals in prostate cancer cells derived from bone (PC-3), but not brain (DU-145) metastases of prostate cancer. Lymph-node derived LNCaP cells also did not produce a Ca2+ signal in response to addition of soluble bone matrix. No other bone fractions produced a Ca2+ signal in PC-3 cells. It is of interest that bone fractions D3b and D4a contain a number of non-collagenous matrix proteins including osteonectin (SPARC) and osteopontin (OPN), as well as prothrombin. Moreover, antibody LM609 that recognizes the alpha v beta 3 integrin, blocks the ability of OPN to trigger a Ca2+ transient in PC-3 cells. These studies support a conclusion that bone-matrix proteins play a role in the growth and progression of metastatic prostate cancer, and that prior growth in bone may be associated with development of a bone-matrix-responsive phenotype.

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.

Ovine osteopontin: II. Osteopontin and alpha(v)beta(3) integrin expression in the uterus and conceptus during the periimplantation period

Osteopontin (OPN) is an acidic 70-kDa glycoprotein that is cleaved by proteases to yield 45-kDa and 24-kDa fragments. The 70-kDa and 45-kDa proteins contain a Gly-Arg-Gly-Asp-Ser (GRGDS) sequence that binds to cell surface integrins (primarily alpha(v)beta(3) heterodimer) to promote cell-cell attachment and cell spreading. A 70-kDa acidic protein was previously detected by two-dimensional (2D) PAGE in Day 17 pregnant endometrial cytosolic extracts using Stainsall and identified as immunoreactive OPN using Western blotting. Three forms of immunoreactive OPN proteins (70, 45, and 24 kDa) were detected by 1D PAGE and Western blot analysis of endometrial extracts. OPN protein in endometrial extracts did not differ between cyclic and pregnant ewes. However, the amount of 45-kDa OPN increased in uterine flushings from pregnant ewes between Days 11 and 17. Immunoreactive OPN was localized to luminal and glandular epithelia of both cyclic and pregnant ewes, and to trophectoderm of Day 19 conceptuses. The alpha(v) and beta(3) integrins were detected on Day 19 endometrium and conceptuses by immunofluorescence. It was reported that OPN mRNA increases in the uterine glands of pregnant ewes and secretion of OPN protein into the uterine lumen increases during early pregnancy. The present results demonstrate accumulation of OPN protein on endometrial LE and conceptus trophectoderm. Therefore, it is hypothesized that progesterone and/or interferon-tau induce expression, secretion and/or proteolytic cleavage of OPN by uterine epithelium. Secreted OPN is then available as ligand for alpha(v)beta(3) integrin heterodimer on trophectoderm and uterus to 1) stimulate changes in morphology of conceptus trophectoderm and 2) induce adhesion between luminal epithelium and trophectoderm essential for implantation and placentation.