A novel functional motif of osteopontin for human lymphocyte migration and survival

Osteopontin in Vascular Disease

Inflammatory cytokines are necessary for an acute response to injury and the progressive healing process. However, when this acute response does not resolve and becomes chronic, the same proteins that once promoted healing then contribute to chronic inflammatory pathologies, such as atherosclerosis. OPN (Osteopontin) is a secreted matricellular cytokine that signals through integrin and CD44 receptors, is highly upregulated in acute and chronic inflammatory settings, and has been implicated in physiological and pathophysiologic processes. Evidence from the literature suggests that OPN may fit within the Goldilocks paradigm with respect to cardiovascular disease, where acute increases are protective, attenuate vascular calcification, and promote postischemic neovascularization. In contrast, chronic increases in OPN are clinically associated with an increased risk for a major adverse cardiovascular event, and OPN expression is a strong predictor of cardiovascular disease independent of traditional risk factors. With the recent finding that humans express multiple OPN isoforms as the result of alternative splicing and that these isoforms have distinct biologic functions, future studies are required to determine what OPN isoform(s) are expressed in the setting of vascular disease and what role each of these isoforms plays in vascular disease progression. This review aims to discuss our current understanding of the role(s) of OPN in vascular disease pathologies using evidence from in vitro, animal, and clinical studies. Where possible, we discuss what is known about OPN isoform expression and our understanding of OPN isoform contributions to cardiovascular disease pathologies.

The Phylogeny of Osteopontin-Analysis of the Protein Sequence

Osteopontin (OPN) is important for tissue remodeling, cellular immune responses, and calcium homeostasis in milk and urine. In pathophysiology, the biomolecule contributes to the progression of multiple cancers. Phylogenetic analysis of 202 osteopontin protein sequences identifies a core block of integrin-binding sites in the center of the protein, which is well conserved. Remarkably, the length of this block varies among species, resulting in differing distances between motifs within. The amino acid sequence SSEE is a candidate phosphorylation site. Two copies of it reside in the far N-terminus and are variably affected by alternative splicing in humans. Between those motifs, birds and reptiles have a histidine-rich domain, which is absent from other species. Just downstream from the thrombin cleavage site, the common motif (Q/I)(Y/S/V)(P/H/Y)D(A/V)(T/S)EED(L/E)(-/S)T has been hitherto unrecognized. While well preserved, it is yet without assigned function. The far C-terminus, although very different between Reptilia/Aves on the one hand and Mammals on the other, is highly conserved within each group of species, suggesting important functional roles that remain to be mapped. Taxonomic variations in the osteopontin sequence include a lack of about 20 amino acids in the downstream portion, a small unique sequence stretch C-terminally, a lack of six amino acids just upstream of the RGD motifs, and variable length insertions far C-terminally.

Overexpression of osteopontin promotes resistance to cisplatin treatment in HCC

Osteopontin (OPN) is a multi-functional cytokine involved in cell survival, migration and adhesion. Increasing evidence has elucidated its role in tumorigenesis, progression and metastasis. However, the role of OPN in chemoresistance of human hepatocellular carcinoma (HCC) has not yet been clarified. In the present study, we examined the expression of OPN in human HCC samples before and after cisplatin-treatment, the results showed that OPN was significantly increased in cisplatin-resistant specimens. We then studied the effect of cisplatin on OPN expression in HCC cells, after exposure to cisplatin, the expression of OPN in HCC cells was elevated compared to control cells. We also found that PI3K/AKT signaling pathway was also activated by cisplatin and this effect was induced by the OPN pathway. To study the effect of OPN on chemoresistance, HCC cells were treated with cisplatin along with OPN. Incubation with OPN enchanced the chemoresistance of HCC cells to cisplatin. In contrast, blockage of OPN pathway promoted the chemosensitivity of HCC cells to cisplatin. Our results suggest that OPN enhanced chemoresistance of cisplatin in HCC cells by activating PI3K/AKT signaling pathway, blocking the OPN pathway might be a novel way to overcome the disease.

Osteopontin induces vascular endothelial growth factor expression in articular cartilage through PI3K/AKT and ERK1/2 signaling

The expression of osteopontin (OPN) and vascular endothelial growth factor (VEGF) are associated with the severity of cartilage destruction in osteoarthritis. However, the biological connection between OPN and VEGF in osteoarthritis remains to be elucidated. The present study was performed to investigate the effect of OPN on VEGF expression in articular cartilage. Rat articular chondrocytes were isolated and cultured in monolayer conditions, and they were treated with OPN for different time periods (0, 2, 8, 12 or 24 h) and dosages (0, 0.1, 0.25 or 0.5 µM). VEGF expression was assessed by reverse transcription‑quantitative polymerase chain reaction and western blotting. The activation of the phosphoinositide 3‑kinase (PI3K)/AKT and extracellular signal‑regulated kinase (ERK)1/2 pathway was analyzed by detecting the expression of pPI3K, pAKT and pERK1/2. To inhibit the PI3K/AKT and ERK1/2 pathway, LY294002 and PD98059 were used, respectively or in combination. It was identified that OPN increased the expression of VEGF in a dose‑ and time‑dependent manner. The PI3K/AKT and ERK1/2 pathways were activated following OPN stimulation and the effect was concomitant with the upregulation of VEGF. Finally, the regulation of VEGF was inhibited by LY294002 and PD98059, and their combination exhibited a synergistic effect. In conclusion, these findings suggest that OPN may directly upregulate VEGF expression through the PI3K/AKT and ERK1/2 pathway. Further studies are required to reveal the mechanism of action of OPN on cartilage angiogenesis and cartilage destruction.

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.

Downregulation of osteopontin is associated with fluoxetine amelioration of monocrotaline-induced pulmonary inflammation and vascular remodelling

1. Osteopontin (OPN) has emerged as a key factor in inflammatory activation and cardiovascular remodelling. The aim of the present study was to investigate the involvement of OPN in fluoxetine amelioration of monocrotaline (MCT)-induced pulmonary inflammation and vascular remodelling in rats. 2. Wistar rats were divided into control, MCT and two fluoxetine-treated groups. Pulmonary arterial hypertension (PAH) was induced by a single injection of MCT (60 mg/kg, i.p.). Fluoxetine (2 and 10 mg/kg) was administered via the intragastric route once a day for 21 days. On Day 22, pulmonary haemodynamic measurements were undertaken, followed by ELISA, western blotting and immunohistochemistry. 3. Monocrotaline caused pulmonary inflammation and vascular remodelling and significantly enhanced OPN expression in the plasma, lungs and pulmonary arteries. Fluoxetine decreased pulmonary arterial pressure and ameliorated pulmonary inflammation and pulmonary vascular remodelling. At 10 mg/kg, fluoxetine significantly inhibited MCT-induced increases in the expression of serotonin transporter (SERT) and phosphorylated extracellular signal-regulated kinase 1/2 and downregulated the expression of OPN, macrophage inflammatory protein-1β and matrix metalloproteinase 2/tissue inhibitor of metalloproteinase 2. Although 2 mg/kg fluoxetine tended to ameliorate some MCT-induced changes in the lung, the differences did not always reach statistical significance. Linear regression analysis showed that there was a positive correlation between plasma OPN concentrations and mean pulmonary arterial pressure, as well as percentage medial wall thickness and percentage wall area in the pulmonary artery. 4. In conclusion, the amelioration by fluoxetine of MCT-induced pulmonary inflammation and vascular remodelling is associated with downregulation of OPN expression in rats.

Pre- and post-translational regulation of osteopontin in cancer

Osteopontin (OPN) is a matricellular protein that binds to a number of cell surface receptors including integrins and CD44. It is expressed in many tissues and secreted into body fluids including blood, milk and urine. OPN plays important physiological roles in bone remodeling, immune response and inflammation. It is also a tumour-associated protein, and elevated OPN levels are associated with tumour formation, progression and metastasis. Research has revealed a promising role for OPN as a cancer biomarker. OPN is subject to alternative splicing, as well as post-translational modifications such as phosphorylation, glycosylation and proteolytic cleavage. Functional differences have been revealed for different isoforms and post-translational modifications. The pattern of isoform expression and post-translational modification is cell-type specific and may influence the potential role of OPN in malignancy and as a cancer biomarker.

Osteopontin in macrophage function

The secreted phosphorylated protein osteopontin (OPN) is expressed in a variety of tissues and bodily fluids, and is associated with pathologies including tissue injury, infection, autoimmune disease and cancer. Macrophages are ubiquitous, heterogeneous cells that mediate aspects of cell and tissue damage in all these pathologies. Here, the role of OPN in macrophage function is reviewed. OPN is expressed in macrophage cells in multiple pathologies, and the regulation of its expression in these cells has been described in vitro. The protein has been implicated in multiple functions of macrophages, including cytokine expression, expression of inducible nitric oxide synthase, phagocytosis and migration. Indeed, the role of OPN in cells of the macrophage lineage might underlie its physiological role in many pathologies. However, there are numerous instances where the published literature is inconsistent, especially in terms of OPN function in vitro. Although the heterogeneity of OPN and its receptors, or of macrophages themselves, might underlie some of these inconsistencies, it is important to understand the role of OPN in macrophage biology in order to exploit its function therapeutically.

Regulatory effects of interferon-β on osteopontin and interleukin-17 expression in multiple sclerosis

Multiple sclerosis (MS) is a demyelinating disease characterized by autoimmune inflammation in the central nervous system. Despite over a decade of use of interferon-β (IFN-β) in the treatment of MS, its mechanisms of action are still not fully elucidated. New data now demonstrate that the 2 important proinflammatory cytokines involved in the pathogenesis of MS, osteopontin (OPN) and interleukin-17 (IL-17), are regulated by IFN-β. This review discusses the role of OPN and IL-17 in the development of MS and how the downregulation of the levels of OPN and interleukin-17 contributes to the therapeutic effects of IFN-β in MS.

Development and characterization of a novel anti-IgE monoclonal antibody

IgE is the central macromolecular mediator responsible for the progression of allergic reactions. Omalizumab (Xolair) is a humanized monoclonal anti-IgE antibody directed at the FcepsilonRI-binding domain of human IgE, which represents a novel therapeutic approach in the management of asthma. In this study, we developed a monoclonal antibody (7A5) against human IgE via hybridoma technique. Our data showed that 7A5 could inhibit free IgE molecules to bind to receptors without affecting IgE already bound to cellular receptors. Importantly, 7A5 was able to inhibit IgE-induced histamine release of basophilic leukemia cells. Next, the phage display peptide library technology was employed to select peptides binding to 7A5 and a striking peptide sequence motif was recovered, which is homologous to the sequence (391)KQR(393) within the Cepsilon3 domain of IgE-Fc, Our results further indicated that 7A5 specifically bound to the synthesized peptide "(388)KEEKQRN(394)" containing the (391)KQR(393) motif in IgE-Fc. The epitope of 7A5 was found to be spatially close to the FcepsilonRI-binding site, suggesting that 7A5 binding to IgE might block IgE binding to receptors via steric hindrance. The anti-IgE monoclonal antibody 7A5 may have the potential to be developed as a therapeutic agent for the treatment of allergic diseases.

The RGD domain of human osteopontin promotes tumor growth and metastasis through activation of survival pathways

BACKGROUND

Human osteopontin (OPN), a known tumor associated protein, exists in different isoforms, whose function is unclear. It also possesses a RGD domain, which has been implicated in diverse function. Here, we use genetic approaches to systematically investigate the function of the RGD domain in different OPN isoforms on tumor progression and metastasis for 2 different solid tumor models.

METHODOLOGY/PRINCIPAL FINDINGS

Using isoform-specific qRT-PCR, we found that OPN-A and B were the main isoforms overexpressed in evaluated human tumors, which included 4 soft tissue sarcomas, 24 lung and 30 head and neck carcinomas. Overexpression of either OPN-A or B in two different cell types promoted local tumor growth and lung metastasis in SCID mouse xenografts. However, expression of either isoform with the RGD domain either mutated or deleted decreased tumor growth and metastasis, and resulted in increased apoptosis by TUNEL staining. In vitro, whereas mutation of the RGD domain did not affect cell-cell adhesion, soft agar growth or cell migration, it increased apoptosis under hypoxia and serum starvation. This effect could be mitigated when the RGD mutant cells were treated with condition media containing WT OPN. Mechanistically, the RGD region of OPN inhibited apoptosis by inducing NF-kappaB activation and FAK phosphorylation. Inhibition of NF-kappaB (by siRNA to the p65 subunit) or FAK activation (by a inhibitor) significantly increased apoptosis under hypoxia in WT OPN cells, but not in RGD mutant cells.

CONCLUSION/SIGNIFICANCE

Unlike prior reports, our data suggest that the RGD domain of both OPN-A and B promote tumor growth and metastasis mainly by protecting cells against apoptosis under stressed conditions and not via migration or invasion. Future inhibitors directed against OPN should target multiple isoforms and should inhibit cell survival mechanisms that involve the RGD domain, FAK phosphorylation and NF-kappaB activation.

Osteopontin induces angiogenesis through activation of PI3K/AKT and ERK1/2 in endothelial cells

Angiogenesis is a key step in tumor growth and metastasis. The mechanism by which osteopontin (OPN) induces the angiogenesis of endothelial cells remains unclear. Here, we show that OPN confers cytoprotection through the activation of the PI3K/Akt pathway with subsequent upregulation of Bcl-xL and activation of nuclear factor-kappaB. OPN enhances the expression of vascular endothelial growth factor (VEGF) through the phosphorylation of AKT and extracellular signal-regulated kinase (ERK). In turn, OPN-induced VEGF activates PI3K/AKT and the ERK1/2 pathway as a positive feedback signal. Blocking the feedback signal by anti-VEGF antibody, PI3-kinase inhibitor or ERK inhibitor can partially inhibit the OPN-induced human umbilical vein endothelial cell (HUVEC) motility, proliferation and tube formation, while blocking the signal by anti-OPN or anti-alphavbeta3 antibody completely abrogates the biological effects of OPN on HUVECs. In addition, blood vessel formation is also investigated in vivo. The antiangiogenesis efficacy of anti-OPN antibody in vivo is more effective than that of anti-VEGF antibody, which only blocks the feedback signals. These data show that OPN enhances angiogenesis directly through PI3K/AKT- and ERK-mediated pathways with VEGF acting as a positive feedback signal. The results suggest that OPN might be a valuable target for developing novel antiangiogenesis therapy for treatment of cancer.

Plasma osteopontin modulates chronic restraint stress-induced thymus atrophy by regulating stress hormones: inhibition by an anti-osteopontin monoclonal antibody

Osteopontin (OPN) is a cytokine implicated in mediating responses to certain stressors, including mechanical, oxidative, and cellular stress. However, the involvement of OPN in responding to other physical and psychological stress is largely unexplored. Our previous research revealed that OPN is critical for hind limb-unloading induced lymphoid organ atrophy through modulation of corticosteroid production. In this study, we demonstrate that OPN(-/-) mice are resistant to chronic restraint stress (CRS)-induced lymphoid (largely thymus) organ atrophy; additionally, the stress-induced up-regulation of corticosterone production is significantly reduced in OPN(-/-) mice. Underlying this observation is the fact that normal adrenocorticotropic hormone levels are substantially reduced in the OPN(-/-) mice. Our data demonstrate both that injection of OPN into OPN-deficient mice enhances the CRS-induced lymphoid organ atrophy and that injection of a specific anti-OPN mAb (2C5) into wild-type mice ameliorates the CRS-induced organ atrophy; changes in corticosterone levels were also partially reversed. These studies reveal that circulating OPN plays a significant role in the regulation of the hypothalamus-pituitary-adrenal axis hormones and that it augments CRS-induced organ atrophy.

A functional motif QLYxxYP is essential for osteopontin induced T lymphocyte activation and migration

Osteopontin (OPN) plays an important role in regulating lymphocyte adhesion and cytokine production associated with inflammatory processes and autoimmune diseases. Here we developed and characterized a monoclonal antibody F8E11 specific for human OPN (hOPN). F8E11 could inhibit OPN-induced lymphocyte activation and migration. Epitope mapping showed that F8E11 could specifically recognize the peptide QLYxxYP. In addition, a synthesized mimetic peptide F8P (EEKQLYNKYPDA) could block the binding of F8E11 to hOPN and significantly inhibit the hOPN-induced lymphocyte migration. Moreover, mutations on the QLYxxYP motif of hOPN also markedly diminished its activity for lymphocyte activation and migration. The functioning assay indicated that this novel epitope is critically involved in the lymphocyte migration through activating MAPK/ERK/AP-1 pathway, which can be inhibited by the motif QLYxxYP blocking antibody, F8E11. These results suggest that this novel epitope of OPN may provide a potential therapeutic target for the treatment of T cell mediated-immune diseases.

Lymph node architecture collapse and consequent modulation of FOXO3a pathway on memory T- and B-cells during HIV infection

Lymph nodes (LNs) represent the principal site where antigen-specific memory T- and B-cell responses are primed and differentiated into memory and effector cells. During chronic viral infections such as HIV, these lymphoid tissues undergo substantial structural changes. These changes are mostly caused by an imbalanced cytokine milieu, hyper-immune activation and collagen deposition leading to fibrotic LNs. The structural integrity of the LNs is essential to prime and maintain memory responses. Because cellular signalling events both up- and down-stream of FOXO3a are critical to the generation and the maintenance of lymphocyte memory, this review will focus on the interplay between the deregulation of the immune system caused by the virus and its impact on FOXO3a.