Differential processing of osteopontin transcripts in rat kidney- and osteoblast-derived cell lines

Serum Osteopontin as a Potential Marker for Metastasis and Prognosis in Primary Osteogenic Sarcoma: A Systematic Review

Osteosarcoma (OS), a primary malignant bone tumor, poses significant challenges in diagnosis and prognosis. It is a painful medical burden, and treating it is still a difficult issue. Osteopontin (OPN), a multifunctional extracellular matrix protein, has emerged as a promising biomarker in this context. This systematic review explores the role of OPN as a diagnostic and prognostic marker in OS, highlighting its potential in enhancing early detection, monitoring disease progression, and predicting patient outcomes. Various studies have demonstrated elevated levels of OPN in OS patients, correlating with tumor aggressiveness, metastatic potential, and poor prognosis. In addition, OPN's involvement in tumor microenvironment regulation and metastatic processes underscores its clinical relevance as a biomarker. For this systematic review, comprehensive literature searches were conducted in the PubMed databases for research published between the database's establishment and November 11, 2022. Out of the nine studies that were available for analysis, a higher level of OPN in primary osteogenic sarcoma patients indicates a poorer prognosis and higher incidence of metastasis. OS has not shown commensurable progress with concerns to treatment approches and survical outcomes. However, the discovery of a biological marker that can predict metastasis and severity will be a groundbreaking development for advancements in OS diagnosis and treatment. Therefore, understanding the intricate interplay between OPN and OS pathogenesis holds promise for improving patient management and developing targeted therapeutic strategies.

Endocrine regulation and metabolic mechanisms of osteopontin in the development and progression of osteosarcoma, metastasis and prognosis

Osteosarcoma is the most common type of malignant bone tumor, occurring in adolescents and patients over 60. It has a bimodal onset and a poor prognosis, and its development has not yet been fully explained. Osteopontin (OPN) is a high protein consisting of 314 amino acid residues with a negative charge and is involved in many biological activities. OPN is not only an essential part of the regulation of the nervous system and endocrine metabolism of skeletal cells. Still, it is also involved in several other important biological activities, such as the division, transformation, and proliferation of skeletal cells and their associated cells, such as bone tumor cells, including bone marrow mesenchymal stem cells, hematopoietic stem cells, osteoblasts, and osteoclasts. Osteoblasts and osteocytes. Recent studies have shown a strong correlation between OPN and the development and progression of many skeletal diseases, such as osteosarcoma and rheumatoid arthritis. This review aims to understand the mechanisms and advances in the role of OPN as a factor in the development, progression, metastasis, and prognosis of osteosarcoma in an attempt to provide a comprehensive summary of the mechanisms by which OPN regulates osteosarcoma progression and in the hope of contributing to the advancement of osteosarcoma research and clinical treatment.

Osteopontin as a biomarker for osteosarcoma therapy and prognosis

Osteosarcoma (OS) is the most common bone malignancy, and is particularly prevalent in children and adolescents. OS is an aggressive tumor with a tendency to metastasize and invade to para-carcinoma tissues. The primary treatment for this tumor is a combination of surgery and chemotherapy. However, the prognosis remains poor due to chemoresistance and early metastasis. Osteopontin (OPN), a multifunctional secreted protein, has emerged as an important potential biomarker for diagnosing and treating cancer. The overexpression of OPN has been found in numerous malignant tumors, including breast, lung, gastric and ovarian cancer, as well as melanoma. Recent studies have suggested that OPN may provide an important function in the diagnosis and treatment of OS. The present review summarizes current knowledge and progress in understanding the potential role of OPN as a biomarker in OS.

Human osteopontin splicing isoforms: known roles, potential clinical applications and activated signaling pathways

Human osteopontin is subject to alternative splicing, which generates three isoforms, termed OPNa, OPNb and OPNc. These variants show specific expression and roles in different cell contexts. We present an overview of current knowledge of the expression profile of human OPN splicing isoforms (OPN-SIs), their tissue-specific roles, and the pathways mediating their functional properties in different pathophysiological conditions. We also describe their putative application as biomarkers, and their potential use as therapeutic targets by using antibodies, oligonucleotides or siRNA molecules. This synthesis provides new clues for a better understanding of human OPN splice variants, their roles in normal and pathological conditions, and their possible clinical applications.

ERK1/2 and JNKs, but not p38 kinase, are involved in reactive oxygen species-mediated induction of osteopontin gene expression by angiotensin II and interleukin-1beta in adult rat cardiac fibroblasts

Osteopontin (OPN), also called cytokine Eta-1, expressed in the myocardium co-incident with heart failure plays an important role in post myocardial infarction (MI) remodeling by promoting collagen synthesis and accumulation. Angiotensin II (Ang II) and inflammatory cytokines are increased in the heart following MI. We studied the involvement of mitogen-activated protein kinases (ERK1/2, JNKs, p38 kinase) and reactive oxygen species (ROS) in Ang II- and cytokine-induced OPN gene expression in adult rat cardiac fibroblasts. Ang II alone increased OPN mRNA (3.3 +/- 0.3-folds; P < 0.05; n = 7), while interleukin-1beta (IL-1beta), tumor necrosis factor (TNF-alpha), and interferon-gamma (IFN-gamma) had no effect. A combination of Ang II with IL-1beta or TNF-alpha, not IFN-gamma, increased OPN mRNA more than Ang II alone. Nitric oxide donor, S-nitrosoacetylpenicillamine (SNAP), alone or in combination with Ang II had no effect. Diphenylene iodonium (DPI), inhibitor of NAD(P)H oxidase, and tiron, superoxide scavenger, inhibited Ang II- and Ang II+ IL-1beta-stimulated increases in OPN mRNA. Ang II activated ERK1/2 within 5 min of treatment, not JNKs. IL-1beta activated ERK1/2 and JNKs within 15 min of treatment. A combination of Ang II and IL-1beta activated ERK1/2 within 5 min of treatment. None of these stimuli activated p38 kinase. DPI almost completely inhibited Ang II + IL-1beta-stimulated activation of ERK1/2, while partially inhibiting JNKs. PD98059, ERK1/2 pathway inhibitor, and SP600125, JNKs inhibitor, partially inhibited Ang II + IL-1beta-stimulated increases in OPN mRNA. A combination of PD98059 and SP600125 almost completely inhibited Ang II + IL-1beta-stimulated increases in OPN mRNA. Thus, Ang II alone increases OPN expression, while IL-1beta and TNF-alpha act synergistically with Ang II to increase OPN mRNA possibly via NO independent mechanisms. The synergistic increase in OPN mRNA involves ROS-mediated activation of ERK1/2 and JNKs, not P38 kinase, pathways in cardiac fibroblasts.

Studies on osteogenic differentiation of rat bone marrow stromal cells cultured in type I collagen gel by RT-PCR analysis

The purpose of this investigation was to examine by reverse-transcriptase polymerase chain reaction analysis the osteogenic differentiation of twice-passaged Sprague-Dawley rat bone marrow stromal cells in type I collagen gel cultured for 3 weeks. Two culture media were used here, namely Dulbecco's modified Eagle (DME) medium supplemented with vitamin C [Dex (-)] and those with vitamin C, dexamethasone and beta-glycerophosphate [Dex (+)]. Culture with Dex (-) medium in collagen gel for 3 weeks brought about the well-developed cell network and middle-stage osteogenic phenotype expression characterized by mRNA for alkaline phosphatase, osteonectin and osteopontin while those for bone sialo protein and osteocalcin were not detected. On the contrary, culture with Dex (+) medium in collagen gel for 3 weeks lead to necrosis of the cells. These results indicate that culture in collagen gel with Dex (-) DME medium containing vitamin C was useful for three-dimensional culture and middle-stage osteogenic differentiation of twice-passaged bone marrow stromal cells. This study might contribute to tissue engineering therapy to fix bone and periodontal defects in the future.

Spatially and temporally different expression of osteonectin and osteopontin in the infarct zone of experimentally induced myocardial infarction in rats

Osteonectin and osteopontin, two secreted matricellular proteins, have a variety of functions that are exerted through interaction with matrix components. These proteins appear in response to tissue injury. To test our hypothesis that osteopontin and osteonectin are expressed with spatially and temporally different patterns in myocardial infarct tissue, we investigated osteonectin and osteopontin expression in experimentally induced myocardial infarction in rats, in comparison with Type I collagen expression. Northern blotting demonstrated that osteonectin mRNA did not markedly increase on Day 2 after the infarction, but it increased on Days 7 and 14 by 1.7+/-0.12- and 1.8+/-0.01-fold compared to that in preligation hearts. In contrast, osteopontin mRNA was increased on Day 1 (41.9+/-11.3-fold increase) and on Day 2 (58.3+/-7.6-fold increase), and then it declined on Days 7 and 14 (24.8+/-9.0- and 13.5+/-4.7-fold increase, respectively). In situ hybridization revealed that osteonectin mRNA signals were observed in fibroblasts, myofibroblasts and macrophages around infarct necrotic tissue on Days 7 and 14. Osteopontin mRNA signals were observed in macrophages in the infarct marginal zone on Day 2. Immunopositive staining for both osteonectin and osteopontin showed the same pattern as that obtained by in situ hybridization. The time course of osteonectin mRNA was almost parallel with that of Type I collagen mRNA, while that of osteopontin was not. These results demonstrated spatially and temporally different expression patterns of osteonectin and osteopontin in myocardial infarction and suggest that osteonectin appears to be involved in the pathological course in the late phase after infarction concomitantly with Type I collagen, while osteopontin may play a role in the early phase.

Establishment of conditionally immortalized rat retinal pericyte cell lines (TR-rPCT) and their application in a co-culture system using retinal capillary endothelial cell line (TR-iBRB2)

The purpose of this study was to establish and characterize a retinal pericyte cell line from retinal capillaries of transgenic rats harboring the temperature-sensitive simian virus 40 large T-antigen gene (tsA58 Tg rat), and to apply this to the co-culture with a retinal capillary endothelial cell line. The conditionally immortalized rat retinal pericyte cell lines (TR-rPCTs), which express a temperature-sensitive large T-antigen, were obtained from two tsA58 Tg rats. These cell lines had a multicellular nodule morphology and reacted positively with von Kossa staining, a marker of calcification. TR-rPCTs cells expressed mRNA of pericyte markers such as rat intercellular adhesion molecule-1, platelet-derived growth factor-receptor beta, angiopoietin-1, and osteopontin. Western blot analysis indicated that alpha-smooth muscle actin (alpha-SMA) was expressed in TR-rPCT3 and 4 cells. In contrast, alpha-SMA was induced by transforming growth factor-beta1 and its enhancement was reduced by basic fibroblast growth factor in TR-rPCT1 and 2 cells. When TR-rPCT1 cells were cultured with a rat retinal endothelial cell line (TR-iBRB2) in a contact co-culture system, the number of TR-iBRB2 cells were significantly reduced in comparison with that of a single culture of TR-iBRB2 cells, suggesting that physical contact between pericytes and retinal endothelial cells is important for the growth of retinal endothelial cells. In conclusion, conditionally immortalized retinal pericyte cell lines were established from tsA58 Tg rats. These cell lines exhibited the properties of retinal pericytes and can be applied in co-culture systems with a retinal capillary endothelial cell line.

Extracellular inorganic phosphate regulates Gibbon ape leukemia virus receptor-2/phosphate transporter mRNA expression in rat bone marrow stromal cells

In mammalian cells, several observations indicate not only that phosphate transport probably regulates local inorganic phosphate (Pi) concentration, but also that Pi affects normal cellular metabolism, which in turn regulates apoptosis and the process of mineralization. To elucidate how extracellular Pi regulates cellular functions of pre-osteoblastic cells, we investigated the expression of type III sodium (Na)-dependent Pi transporters in rat bone marrow stromal cells and ROB-C26 pre-osteoblastic cells. The mRNA expression level of gibbon ape leukemia virus receptor (Glvr)-2 was increased by the addition of Pi in rat bone marrow stromal cells, but not in ROB-C26 or normal rat kidney (NRK) cells. In contrast, the level of Glvr-1 mRNA was not altered by the addition of extracellular Pi in these cells. The induction of Glvr-2 mRNA by Pi was inhibited in the presence of cycloheximide (CHX). Moreover, mitogen-activated protein kinase (MEK) /extracellular-signal-regulated kinase (ERK) pathway inhibitors; U0126 (1.4-diamino-2, 3-dicyano-1, 4-bis [2-amino-phenylthio] butadiene) and PD98059 (2'-Amino-3'-methoxyflavone) inhibited inducible Glvr-2 mRNA expression, but p38 MEK inhibitor SB203580 [4-(4'-fluorophenyl)-2-(4'-methyl-sulfinylphenyl)-5-(4'pyridyl) imidazole] did not inhibit the induction of Glvr-2 mRNA expression, suggesting that extracellular Pi regulates de novo protein synthesis and MEK/ERK activity in rat bone marrow stromal cells, and through these, induction of Glvr-2 mRNA. Although Pi also induced osteopontin mRNA expression in rat bone marrow stromal cells but not in ROB-C26 and NRK cells, changes in cell viability with the addition of Pi were similar in both cell types. These data indicate that extracellular Pi regulates Glvr-2 mRNA expression, provide insights into possible mechanisms whereby Pi may regulate protein phosphorylation, and suggest a potential role for the Pi transporter in rat bone marrow stromal cells.

Analysis of osteopontin at the maternal-placental interface in pigs

Noninvasive, epitheliochorial placentation in the pig follows a prolonged preimplantation period characterized by migration, spacing and elongation of conceptuses, and secretion of estrogen for maternal recognition of pregnancy. Osteopontin (OPN) is an extracellular matrix protein that binds integrins to promote cell-cell attachment and communication. OPN appears to play a key role in conceptus implantation and maintenance of pregnancy in sheep; however, a role for OPN in the porcine uterus has not been established. Therefore, this study examined OPN expression and function in the porcine uterus and conceptus (embryo/fetus and associated extraembryonic membranes). Northern and slot blot hybridization detected an increase in endometrial OPN expression between Days 25 and 30, and levels remained elevated through Day 85 of pregnancy. In situ hybridization localized OPN mRNA to discrete regions of the uterine luminal epithelium (LE) on Day 15 of pregnancy and to the entire LE thereafter. Glandular epithelial (GE) expression of OPN mRNA was first detected on Day 35 of pregnancy and increased through Day 85. Both 70- and 45-kDa forms of OPN protein were detected in cyclic and pregnant endometrium by Western blotting. OPN protein was localized to the LE and GE by immunofluorescence; however, only the 70-kDa OPN was detected in uterine flushings. OPN protein was present along the entire uterine-placental interface after Day 30 of pregnancy. In addition, OPN mRNA and protein were localized to immune-like cells within the stratum compactum of the endometrium in both Day 9 cyclic and pregnant gilts. Incubation of OPN-coated microbeads with porcine trophectoderm and uterine luminal epithelial cells induced Arg-Gly-Asp (RGD)-dependent integrin activation and transmembrane accumulation of cytoskeletal molecules at the apical cell surface as assessed by immunofluorescence detection of talin or alpha-actinin as markers for focal adhesions. These results suggest that OPN, expressed by uterine epithelium and immune cells, may interact with receptors (i.e., integrins) on conceptus and uterus to promote conceptus development and signaling between these tissues as key contributors to attachment and placentation in the pig.

The osteopontin-CD44 survival signal involves activation of the phosphatidylinositol 3-kinase/Akt signaling pathway

We have recently demonstrated that the gene encoding the osteopontin (OPN) protein is activated both by interleukin-3 and granulocyte-macrophage colony-stimulating factor signaling pathways and that, through binding to the cell surface receptor CD44, OPN contributes to the survival activities of interleukin (IL)-3 and GM-CSF (Lin, Y.-H., Huang, C.-J., Chao, J.-R., Chen, S.-T., Lee, S.-F., Yen, J. J.-Y., and Yang-Yen, H.-F. (2000) Mol. Cell. Biol. 20, 2734-2742). In this report, we demonstrate that the CD44-binding domain of OPN involves a region containing amino acid residues from 121 to 140 and that both threonine and serine at positions 137 and 147, respectively, are essential for the survival stimulatory effect of OPN. Substitution of either residue with alanine results into a dominant negative mutant that overrides the survival effect of IL-3. Upon binding to the CD44 receptor, the wild-type OPN but not the inactive mutant induces activation of phosphatidylinositol 3-kinase and Akt. Last, we demonstrate that two waves of Akt activation are detected in IL-3-treated cells and that the survival promoting effect of OPN is mediated predominantly through the phosphatidylinositol 3-kinase/Akt signaling pathway. Together, our results suggest that a positive autoregulatory loop is involved in the survival pathway of IL-3.

Expression, roles, receptors, and regulation of osteopontin in the kidney

Osteopontin (OPN) is a secreted glycoprotein in both phosphorylated and non-phosphorylated forms. It contains an Arg-Gly-Asp cell-binding sequence and a thrombin-cleavage site. OPN is mainly present in the loop of Henle and distal nephrons in normal kidneys in animals and humans. After renal damage, OPN expression may be significantly up-regulated in all tubule segments and glomeruli. Studies utilizing OPN gene-deficient mice, antisense-treated or anti-OPN-treated animals have demonstrated that OPN promotes accumulation of macrophages, and may play a role in macrophage-mediated renal injury, but that the effect may be mild and short-lived. On the other hand, OPN has some renoprotective actions in renal injury, such as increasing tolerance to acute ischemia, inhibiting inducible nitric oxide synthase and suppressing nitric oxide synthesis, reducing cell peroxide levels and promoting the survival of cells exposed to hypoxia, decreasing cell apoptosis and participating in the regeneration of cells. In addition, OPN is associated with renal stones, but whether it acts as a promoter or inhibitor of stone formation is controversial. It has been demonstrated that OPN receptors include two families: integrin and CD44. The OPN integrin receptors include alpha(v)beta(3), alpha(v)beta(1), alpha(v)beta(5) and alpha(9)beta(1), and alpha(4)beta(1). In normal human kidneys, standard CD44 is expressed most dominantly. Different OPN functions are mediated via distinct receptors. Parathyroid hormone, vitamin D(3), calcium, phosphate and some cytokines increase OPN expression in vitro or in vivo, whereas female sex hormones and angiotensin-converting enzyme inhibitors or angiotensin II receptor antagonists decrease OPN expression in some renal damage states.

Regulation of angiotensin II-stimulated osteopontin expression in cardiac microvascular endothelial cells: role of p42/44 mitogen-activated protein kinase and reactive oxygen species

Using spontaneously hypertensive and aortic banded rats, we have shown that expression of myocardial osteopontin, an extracellular matrix protein, coincides with the development of heart failure and is inhibited by captopril, suggesting a role for angiotensin II (ANG II). This study tested whether ANG II induces osteopontin expression in adult rat ventricular myocytes and cardiac microvascular endothelial cells (CMEC), and if so, whether induction is mediated via activation of mitogen-activated protein kinases (p42/44 MAPK) and involves reactive oxygen species (ROS). ANG II (1 microM, 16 h) increased osteopontin expression (fold increase 3.3+/-0.34, n = 12, P < 0.01) in CMEC as measured by northern analysis, but not in ARVM. ANG II stimulated osteopontin expression in CMEC in a time- (within 4 h) and concentration-dependent manner, which was prevented by the AT1 receptor antagonist, losartan. ANG II elicited robust phosphorylation of p42/44 MAPK as measured using phospho-specific antibodies, and increased superoxide production as measured by cytochrome c reduction and lucigenin chemiluminescence assays. These effects were blocked by diphenylene iodonium (DPI), an inhibitor of the flavoprotein component of NAD(P)H oxidase. PD98059, an inhibitor of p42/44 MAPK pathway, and DPI each inhibited ANG II-stimulated osteopontin expression. Northern blot analysis showed basal expression of p22phox, a critical component of NADH/NADPH oxidase system, which was increased 40-60% by exposure to ANG II. These results suggest that p42/44 MAPK is a critical component of the ROS-sensitive signaling pathways activated by ANG II in CMEC and plays a key role in the regulation of osteopontin gene expression. Published 2001 Wiley-Liss, Inc.

Targeted inhibition of osteopontin expression in the mammary gland causes abnormal morphogenesis and lactation deficiency

Osteopontin (OPN) is a sialic acid-rich, adhesive, extracellular matrix (ECM) protein with Arg-Gly-Asp cell-binding sequence that interacts with several integrins, including alpha(v)beta(3). Since the ECM is a key regulator of mammary gland morphogenesis, and mammary epithelial cells express OPN at elevated levels, we sought to determine whether this protein plays a role in the postnatal mammary gland development. By generating transgenic mice that express OPN antisense-RNA (AS-OPN mice) in the mammary epithelia we achieved suppression of OPN production in this organ. The pregnant AS-OPN mice displayed a lack of mammary alveolar structures, a drastic reduction in the synthesis of beta-casein, whey acidic milk protein, and lactation deficiency. In agreement with these findings, we uncovered that a mammary cell line, NMuMG, which undergoes both structural and functional differentiation on ECM-coated plates, when transfected with an antisense OPN-cDNA construct, failed to undergo such differentiation. Furthermore, the results of gel-invasion assays demonstrated that these cells manifest elevated matrix metalloproteinase (MMP) activity when OPN expression is significantly reduced. The identity of this proteinase as MMP-2 is confirmed by Western blotting, zymography, and inhibition of its activity by a specific inhibitor, TIMP-2. Taken together, our results demonstrate, for the first time, an essential role of OPN in mammary gland differentiation and that the molecular mechanism(s) of its action, at least in part, involves down-regulation of MMP-2.

Expression of CD44 in kidney after acute ischemic injury in rats

De novo CD44 and ligand expression at wound margins accompanies cellular proliferation and migration that effect repair of injured mucosal and vascular endothelial tissues. To determine whether CD44 could play a role in recovery from acute ischemic renal injury, we characterized its renal expression and those of two of its ligands, hyaluronic acid and osteopontin. Although no expression is detectable in nonischemic kidneys, several mRNAs for CD44 are present within 1 day after injury. CD44 mRNA is expressed in proximal tubules undergoing repair. CD44 peptide is present in basal and lateral cell membranes. Hyaluronic acid is normally expressed in the interstitium of the renal papilla only. By 1 day postischemia, hyaluronic acid can be detected, in addition, in the interstitium surrounding regenerating tubules. Osteopontin, not normally expressed in the renal proximal tubule, is expressed in regenerating tubules by 3 days after induction of acute ischemic injury. Immunoreactive osteopontin peptide continues to be localized in those tubules still undergoing repair for as long as 7 days after the injury. Our data are consistent with a role for CD44-ligand interactions in the regenerating proximal tubule participating in the process of recovery after ischemic injury.

Differences in osteopontin up-regulation between proximal and distal tubules after renal ischemia/reperfusion

BACKGROUND

Osteopontin (OPN) is a highly acidic phosphoprotein containing an arginine-glycine-aspartic acid (RGD) cell adhesion motif. High OPN expression has been found in tissues with high cell turnover, and OPN up-regulation has been demonstrated in several models of renal injury, suggesting a possible role in tissue remodeling and repair. However, its exact function in the kidney remains unknown. In this study, the possible contribution of OPN to regeneration and repair in the kidney was explored by studying the time course and subcellular localization of OPN up-regulation after renal ischemia/reperfusion injury in different nephron segments and by investigating its relationship with tubular morphology.

METHODS

Rats that underwent 60 minutes of left renal ischemia and a right nephrectomy sacrificed at 10 different time points (from 1 hr to 10 days after reperfusion) were compared with uninephrectomized rats at each time point. In renal tissue sections immunostained for OPN, proximal (PTs) and distal tubules (DTs) in both the renal cortex and outer stripe of the outer medulla (OSOM) were scored for the degree of OPN expression and tubular morphology.

RESULTS

Kidneys of uninephrectomized rats showed no injury, and the localization and intensity of their OPN expression remained unaltered compared with normal rats. After ischemia/reperfusion, morphological damage was most severe in PTs of the OSOM, but all examined nephron segments showed a significant increase in OPN expression. The time course of OPN up-regulation was different in PTs and DTs. DTs in both cortex and OSOM rapidly increased their OPN expression, with a maximum at 24 hours after reperfusion followed by a slow decrease. In contrast, PTs showed a delayed increase in OPN staining, with a maximum after five to seven days, higher in the OSOM than in the cortex. In OSOM PTs, OPN expression was predominantly associated with morphological regeneration, whereas DTs showed a substantial OPN up-regulation without major morphological damage. PTs and DTs displayed a different subcellular OPN staining pattern: OPN staining in DTs was located to the apical side of the cell; PTs, however, presented a vesicular, perinuclear staining pattern.

CONCLUSIONS

Our study found a different pattern of OPN up-regulation after renal ischemia/reperfusion in PTs versus DTs, both with regard to time course and subcellular localization. DTs show an early and persistent increase in OPN staining in the absence of major morphological injury, whereas OPN staining in PTs is delayed and is mostly associated with morphological regeneration. PTs show a vesicular, perinuclear OPN staining pattern, whereas DTs show OPN staining at the apical cell side.

Functions of the extracellular matrix and matrix degrading proteases during tumor progression

Cell interactions with extracellular matrices are important to pathological changes that occur during cell transformation and tumorigenesis. Several extracellular matrix proteins including fibronectin, thrombospondin-1, laminin, SPARC, and osteopontin have been suggested to modulate tumor phenotype by affecting cell migration, survival, or angiogenesis. Likewise, proteases including the matrix metalloproteinases (MMPs) are understood to not only facilitate migration of cells by degradation of matrices, but also to affect tumor formation and growth. We have recently demonstrated an in vivo role for the RGD-containing protein, osteopontin, during tumor progression, and found evidence for distinct functions in the host versus the tumor cells. Because of the compartmentalization and temporal regulation of MMP expression, it is likely that MMPs may also function dually in host stroma and the tumor cell. In addition, an important function of proteases appears to be not only degradation, but also cleavage of matrix proteins to generate functionally distinct fragments based on receptor binding, biological activity, or regulation of growth factors.

Myocardial osteopontin expression coincides with the development of heart failure

To identify genes that are differentially expressed during the transition from compensated hypertrophy to failure, myocardial mRNA from spontaneously hypertensive rats (SHR) with heart failure (SHR-F) was compared with that from age-matched SHR with compensated hypertrophy (SHR-NF) and normotensive Wistar-Kyoto rats (WKY) by differential display reverse transcriptase-polymerase chain reaction. Characterization of a transcript differentially expressed in SHR-F yielded a cDNA with homology to the extracellular matrix protein osteopontin. Northern analysis showed low levels of osteopontin mRNA in left ventricular myocardium from WKY and SHR-NF but a markedly increased (approximately 10-fold) level in SHR-F. In myocardium from WKY and SHR-NF, in situ hybridization showed only scant osteopontin mRNA, primarily in arteriolar cells. In SHR-F, in situ hybridization revealed abundant expression of osteopontin mRNA, primarily in nonmyocytes in the interstitial and perivascular space. Similar findings for osteopontin protein were observed in the midwall region of myocardium from the SHR-F group. Consistent with the findings in SHR, osteopontin mRNA was minimally increased (approximately 1.9-fold) in left ventricular myocardium from nonfailing aortic-banded rats with pressure-overload hypertrophy but was markedly increased (approximately 8-fold) in banded rats with failure. Treatment with captopril starting before or after the onset of failure in the SHR reduced the increase in left ventricular osteopontin mRNA levels. Thus, osteopontin expression is markedly increased in the heart coincident with the development of heart failure. The source of osteopontin in SHR-F is primarily nonmyocytes, and its induction is inhibited by an angiotensin-converting enzyme inhibitor, suggesting a role for angiotensin II. Given the known biological activities of osteopontin, including cell adhesion and regulation of inducible nitric oxide synthase gene expression, these data suggest that it could play a role in the pathophysiology of heart failure.

Renal cell osteopontin production is stimulated by calcium oxalate monohydrate crystals

Specific anions in tubular fluid, including uropontin (UP), the urinary form of human osteopontin (OPN), block adhesion to renal tubular cells of the most common crystal in kidney stones, calcium oxalate monohydrate (COM). In this study, monkey renal epithelial cells (BSC-1 line) in monolayer culture constitutively secreted UP into the culture medium. COM crystals added to the medium avidly bound previously secreted UP, reducing its concentration by 46% one hour later. However, the net UP content of cultures after a 24-hour exposure to COM crystals was increased by 18%. Northern blotting showed that the constitutively expressed gene encoding human OPN was maximally stimulated in BSC-1 cells after exposure to COM crystals for 12 hours. Two other calcium-containing crystals, hydroxyapatite and brushite, did not alter OPN gene expression or protein production. OPN mRNA expression was enhanced in canine renal epithelial cells (MDCK line) after exposure to COM crystals for six hours, whereas the constitutive expression of murine OPN mRNA by 3T3 fibroblasts was unchanged. In vivo this glycoprotein could defend the cell against adhesion of crystals in tubular fluid, and/or promote renal interstitial fibrosis in subjects with heavy crystalluria.

Cell migration promoted by a potent GRGDS-containing thrombin-cleavage fragment of osteopontin

Osteopontin (OPN) is a secreted adhesive glycoprotein with a gly-arg-gly-asp-ser (GRGDS) cell binding domain. Several independent studies have suggested that OPN functions in tumor growth and metastasis, and one likely possibility is that OPN facilitates tumor invasion by promoting tumor cell migration. Consistent with this hypothesis, immobilized OPN promoted concentration-dependent tumor cell migration (i.e., haptotaxis) in modified Boyden chambers. In particular, cleavage of OPN by thrombin, which likely occurs in the tumor microenvironment, resulted in enhancement of OPNs haptotactic activity; and assays performed with purified preparations of the two individual OPN thrombin-cleavage fragments demonstrated that all detectable activity was associated with the GRGDS-containing fragment. In contrast to the activity of both OPN and its GRGDS-containing fragment in promoting haptotaxis, neither of these proteins in solution promoted chemotaxis, indicating that each must be immobilized to promote cell migration. In haptotaxis assays, antibody LM609 to integrin alpha v beta 3 blocked > 80% cell migration towards the GRGDS-containing OPN fragment, implicating alpha v beta 3 as its principal functional receptor. In comparison with equimolar quantities of other adhesive proteins, the GRGDS-containing OPN thrombin-cleavage fragment was not only > 2-fold more effective than intact OPN at promoting haptotaxis, but also > 8-fold and > 6-fold more effective than fibrinogen and vitronectin, respectively, indicating that this OPN fragment is highly active relative to other alpha v beta 3 ligands.

Osteopontin expression in platelet-derived growth factor-stimulated vascular smooth muscle cells and carotid artery after balloon angioplasty

Osteopontin (OPN), an arginine-glycine-aspartate (RGD)-containing adhesive glycoprotein, is constitutively expressed in rat aorta and carotid arteries and is markedly elevated in response to vascular injury. OPN is chemotactic for vascular smooth muscle cells (SMCs), suggesting a role in vascular remodeling. However, the mechanism for the regulation of OPN expression is poorly understood. In the present study, the effect of platelet-derived growth factor (PDGF) on OPN mRNA expression was investigated in cultured rat aortic SMCs (RASMCs). When RASMCs were stimulated with 1 nmol/L PDGF, a 2.4-fold increase in OPN mRNA expression was observed at 3 hours (P < .05) that peaked at 14 hours with a 6.7-fold increase (P < .001). This induction was blocked by a monoclonal anti PDGF antibody. Further studies revealed that OPN mRNA expression was induced by PDGF-AB or PDGF-BB but not by PDGF-AA, indicating that only the beta-type PDGF receptor mediates this response. Compared with basic fibroblast growth factor, epidermal growth factor, transforming growth factor-beta, and interleukin-1 beta, PDGF was the most potent factor studied to induce OPN mRNA expression in RASMCs. Immunohistochemical studies demonstrated the elevation of OPN protein in PDGF-stimulated RASMCs. The temporal expression of OPN mRNA after rat carotid artery balloon angioplasty as assessed by both reverse transcription-polymerase chain reaction and Northern blot analysis revealed a 1.5-fold increase at 6 hours (P < .01) that peaked at 1 and 3 days with a 3.1-fold increase (P < .001). Immunohistochemical studies of carotid artery after angioplasty localized OPN expression in the medical SMCs at 1 day, ie. at a time of significant platelet adherence to the injured vessel, and thereafter to the intimal lesion during neointimal formation. These data suggest that OPN expression in vascular SMCs is regulated by PDGF through the beta-type PDGF receptor in vitro, and possibly in vivo in situations that involve PDGF released from platelets or other cellular sources, such as blood vessels after angioplasty injury.

Glucocorticoids increase osteopontin expression in cardiac myocytes and microvascular endothelial cells. Role in regulation of inducible nitric oxide synthase

In heart muscle, the cytokine-inducible isoform of nitric oxide synthase (NOS2) is expressed in both cardiac myocytes and microvascular endothelial cells (CMEC). mRNA levels for both NOS2 and for osteopontin, a multifunctional extracellular matrix phosphoprotein containing and RGD integrin binding domain, are increased in cardiac muscle following intraperitoneal injection of adult rats with lipopolysaccharide. In vitro, interleukin-1 beta and interferon-gamma increased osteopontin mRNA levels in CMEC as well as NOS2 expression in both CMEC and cardiac myocytes. However, osteopontin mRNA levels in heart muscle in vivo, and in cardiac myocytes and CMEC in vitro, also are increased 10-30-fold by the synthetic glucocorticoid dexamethasone, an agent that suppresses cytokine induction of NOS2 in both cell types. The hexapeptide GRGDSP, which interrupts binding of RGD-containing proteins to cell surface integrins, increased NOS2 mRNA, while a synthetic osteopontin peptide analogue decreased NOS2 mRNA and protein levels in both cytokine-pretreated cardiac myocytes and CMEC cultures. Also, transfection with a full-length antisense-osteopontin cDNA in cytokine-pretreated CMEC decreased endogenous osteopontin mRNA and increased NOS2 mRNA levels. These results suggest that osteopontin could regulate the location and extent of NOS2 induction in the heart. Increased expression of osteopontin also may be one mechanism by which glucocorticoids suppress NOS2 activity in cardiac myocytes and microvascular endothelial cells.

Osteopontin expression detected in adult cochleae and inner ear fluids

Localization of protein epitopes and mRNA expression showed that there was a wide-spread distribution of osteopontin (OPN) within the membranous labyrinth of the adult mammalian cochleae. Immunoreaction product and mRNA were found within the stria vascularis, VIIIth cranial nerve, spiral ligament and limbus. Only specific cell types within these regions contained abundant OPN mRNA or protein, the main cell type being fibrocytes that populate the spiral limbus and spiral ligament. Epithelial cells that line the luminal surface of the stria vascularis (marginal cells) and neurons that compose the vestibular and auditory ganglia also showed high opn expression. The pattern of anti-OPN staining within membranous labyrinth was comparable to that observed in tissues such as gall bladder, breast and kidney. In those tissues, luminal epithelial cells, corresponding to the marginal cells of the stria vascularis, may be responsible for manufacturing and secreting OPN into the luminal fluids. consistent with those observations, we detected OPN epitopes in cochlear fluids withdrawn from the scalae media and tympani of the cochlea. We found that the protein species in cochlear fluid differed from those present in cerebrospinal fluid (CSF) suggesting that OPN exists in tissue-specific isoforms that may correspond to particular cellular functions.

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.

Osteopontin expression in cardiovascular diseases

Adhesive interactions are recognized requirements for cellular proliferation, migration and differentiation during normal morphogenesis as well as disease. By differential cloning, osteopontin was identified as an adhesive protein upregulated during vascular remodeling and neointima formation in both rat models and human vascular diseases including atherosclerosis and restenosis. In functional studies, purified osteopontin promoted adhesion, focal contact formation, and migration of vascular smooth muscle and endothelial cells. Utilizing neutralizing antibodies, three integrin-type receptors, alpha v beta 3, alpha v beta 1, and alpha v beta 5 were found to support cellular adhesion to osteopontin. In contrast, only cells containing the alpha v beta 3 integrin could migrate towards an osteopontin gradient, demonstrating for the first time that different functions of osteopontin are mediated via distinct receptors. These results suggest a model whereby osteopontin, via its integrin-type receptors, contributes to vascular remodeling during development and disease by facilitating smooth muscle migration and simultaneously promoting endothelial coverage of the affected area.

Identification of osteopontin (OPN) mRNA and protein in the rat testis and epididymis, and on sperm

We have identified a bone cell adhesion molecule, osteopontin, in the rat testis and epididymis by Northern analysis, RT-PCR, Western immunoblot analysis and immunocytochemistry. A polyclonal antibody raised against rat epididymal fluid proteins was used to detect fusion proteins produced by a testis lambda gt11 cDNA library. Sequence analysis of one of four positive cDNA clones, designated as pREP5, revealed identity with the rat osteopontin (OPN) cDNA. The partial cDNA clone pREP5 encompasses 64% of the 1,457 residues reported by Oldberg et al. (1986; Proc Natl Acad Sci USA 83:8819-8823). Immunoblot analysis with a monoclonal antibody against OPN detects the presence of immunoreactive polypeptides in rat testis homogenates as well as in epididymal fluid and sperm extracts. Immunocytochemical localization to the basal and adluminal region of the seminiferous tubule suggests that OPN could be a Sertoli cell product. Indeed, Northern blot analysis of testicular cell preparations demonstrated positive hybridization to Sertoli cell-enriched RNA, but not to RNA isolated from interstitial cell preparations or to isolated germ cell RNA preparations. OPN is also detected in the rat epididymis and on epididymal spermatozoa. This is the first report on the presence of OPN mRNA and protein in rat testis and epididymis and on the presence of OPN on the surface of epididymal spermatozoa. The characterization of this protein in other tissue suggests that OPN could play a role in testicular cell adhesion during spermatogenesis and/or epididymal maturation, although other potential functions in the male reproductive tract are discussed.