Osteopontin
Osteopontin , also known as bone sialoprotein I, early T-lymphocyte activation, secreted phosphoprotein 1, 2ar and Rickettsia resistance, is a protein that in humans is encoded by the SPP1 gene. The murine ortholog is Spp1. Osteopontin is a SIBLING that was first identified in 1986 in osteoblasts.
The prefix indicates that the protein is expressed in bone, although it is also expressed in other tissues. The suffix -pontin is derived from "pons," the Latin word for bridge, and signifies osteopontin's role as a linking protein. Osteopontin is an extracellular structural protein and therefore an organic component of bone. Synonyms for this protein include sialoprotein I and 44K BPP.
The gene has 7 exons, spans 5 kilobases in length and in humans it is located on the long arm of chromosome 4 region 22. The protein is composed of ~300 amino acids residues and has ~30 carbohydrate residues attached, including 10 sialic acid residues, which are attached to the protein during post-translational modification in the Golgi apparatus. The protein is rich in acidic residues: 30-36% are either aspartic or glutamic acid.
Structure
General structure
OPN is a highly negatively charged, extracellular matrix protein that lacks an extensive secondary structure. It is composed of about 300 amino acids and is expressed as a 33-kDa nascent protein; there are also functionally important cleavage sites. OPN can go through posttranslational modifications, which increase its apparent molecular weight to about 44 kDa. The OPN gene is composed of 7 exons, 6 of which containing coding sequence. The first two exons contain the 5' untranslated region. Exons 2, 3, 4, 5, 6, and 7 code for 17, 13, 27, 14, 108 and 134 amino acids, respectively. All intron-exon boundaries are of the phase 0 type, thus alternative exon splicing maintains the reading frame of the OPN gene.Isoforms
Full-length OPN can be modified by thrombin cleavage, which exposes a cryptic sequence, SVVYGLR on the cleaved form of the protein known as OPN-R. This thrombin-cleaved OPN exposes an epitope for integrin receptors of α4β1, α9β1, and α9β4. These integrin receptors are present on a number of immune cells such as mast cells, neutrophils, and T cells. It is also expressed by monocytes and macrophages. Upon binding these receptors, cells use several signal transduction pathways to elicit immune responses in these cells. OPN-R can be further cleaved by Carboxypeptidase B by removal of C-terminal arginine and become OPN-L. The function of OPN-L is largely unknown.It appears an intracellular variant of OPN is involved in a number of cellular processes including migration, fusion and motility. Intracellular OPN is generated using an alternative translation start site on the same mRNA species used to generate the extracellular isoform. This alternative translation start site is downstream of the N-terminal endoplasmic reticulum-targeting signal sequence, thus allowing cytoplasmic translation of OPN.
Various human cancers, including breast cancer, have been observed to express splice variants of OPN. The cancer-specific splice variants are osteopontin-a, osteopontin-b, and osteopontin-c. Exon 5 is lacking from osteopontin-b, whereas osteopontin-c lacks exon 4. Osteopontin-c has been suggested to facilitate the anchorage-independent phenotype of some human breast cancer cells due to its inability to associate with the extracellular matrix.
Biosynthesis
Osteopontin is biosynthesized by a variety of tissue types including cardiac fibroblasts, preosteoblasts, osteoblasts, osteocytes, odontoblasts, some bone marrow cells, hypertrophic chondrocytes, dendritic cells, macrophages, smooth muscle, skeletal muscle myoblasts, endothelial cells, and extraosseous cells in the inner ear, brain, kidney, deciduum, and placenta. Synthesis of osteopontin is stimulated by calcitriol.Regulation
Regulation of the osteopontin gene is incompletely understood. Different cell types may differ in their regulatory mechanisms of the OPN gene. OPN expression in bone predominantly occurs by osteoblasts and osteocyctes as well as osteoclasts. Runx2 and osterix transcription factors are required for the expression of OPN Runx2 and Osx bind promoters of osteoblast-specific genes such as Col1α1, Bsp, and Opn and upregulate transcription.Hypocalcemia and hypophosphatemia lead to increases in OPN transcription, translation and secretion. This is due to the presence of a high-specificity vitamin D response element in the OPN gene promoter.
Extracellular inorganic phosphate has also been identified as a modulator of OPN expression.
Stimulation of OPN expression also occurs upon exposure of cells to pro-inflammatory cytokines, classical mediators of acute inflammation, angiotensin II, transforming growth factor β and parathyroid hormone, although a detailed mechanistic understanding of these regulatory pathways are not yet known. Hyperglycemia and hypoxia are also known to increase OPN expression.
Biological function
Role in biomineralization
OPN belongs to a family of secreted acidic proteins whose members have an abundance of negatively charged amino acids such as Asp and Glu. OPN also has a large number of consensus sequence sites for post-translational phosphorylation of Ser residues to form phosphoserine, providing additional negative charge. Contiguous stretches of high negative charge in OPN have been identified and named the polyAsp motif and the ASARM motif, with the latter sequence having multiple phosphorylation sites. This overall negative charge of OPN, along with its specific acidic motifs and the fact that OPN is an intrinsically disordered protein allowing for open and flexible structures, permit OPN to bind strongly to calcium atoms available at crystal surfaces in various biominerals. Such binding of OPN to various types of calcium-based biominerals ‒ such as calcium-phosphate mineral in bones and teeth, calcium-carbonate mineral in inner ear otoconia and avian eggshells, and calcium-oxalate mineral in kidney stones – acts as a mineralization inhibitor to regulate crystal growth.OPN is a substrate protein for a number of enzymes whose actions may modulate the mineralization-inhibiting function of OPN. PHEX is one such enzyme, which extensively degrades OPN, and whose inactivating gene mutations lead to altered processing of OPN such that inhibitory OPN cannot be degraded and accumulates in the bone extracellular matrix, likely contributing locally to the osteomalacia characteristic of XLH.
Along with its role in the regulation of normal mineralization within the extracellular matrices of bones and teeth, OPN is also upregulated at sites of pathologic, ectopic calcification – such as for example, in urolithiasis and vascular calcification ‒ presumably at least in part to inhibit debilitating mineralization in these soft tissues.
Role in bone remodeling
Osteopontin has been implicated as an important factor in bone remodeling. Specifically, research suggests it plays a role in anchoring osteoclasts to the mineral matrix of bones. The organic part of bone is about 20% of the dry weight, and counts in, other than osteopontin, collagen type I, osteocalcin, osteonectin, bone sialo protein, and alkaline phosphatase. Collagen type I counts for 90% of the protein mass. The inorganic part of bone is the mineral hydroxyapatite, Ca1062. Loss of this mineral may lead to osteoporosis, as the bone is depleted for calcium if this is not supplied in the diet.OPN serves to initiate the process by which osteoclasts develop their ruffled borders to begin bone resorption. It is also found in urine, where it inhibits kidney stone formation.
Role in immune functions
As discussed, OPN binds to several integrin receptors including α4β1, α9β1, and α9β4 expressed by leukocytes. These receptors have been well-established to function in cell adhesion, migration, and survival in these cells. Therefore, recent research efforts have focused on the role of OPN in mediating such responses.Osteopontin is expressed in a range of immune cells, including macrophages, neutrophils, dendritic cells, microglia and T and B cells, with varying kinetics. OPN is reported to act as an immune modulator in a variety of manners. Firstly, it has chemotactic properties, which promote cell recruitment to inflammatory sites. It also functions as an adhesion protein, involved in cell attachment and wound healing. In addition, OPN mediates cell activation and cytokine production, as well as promoting cell survival by regulating apoptosis. The following examples are found.
Role in Heart
OPN expression increases under a variety of conditions of the heart, and is associated with increased myocyte apoptosis and myocardial dysfunction.Chemotaxis
OPN plays an important role in neutrophil recruitment in alcoholic liver disease. OPN is important for the migration of neutrophil in vitro. In addition, OPN recruits inflammatory cells to arthritis joints in the collagen-induced arthritis model of rheumatoid arthritis. A recent in vitro study in 2008 has found that OPN plays a role in mast cell migration. Here OPN knock-out mast cells were cultured and they observed a decreased level of chemotaxis in these cells compared to wildtype mast cells. OPN was also found to act as a macrophage chemotactic factor. In this study, researchers looked at the accumulation of macrophages in the brain of rhesus monkeys and found that OPN prevented macrophages from leaving the accumulation site, indicating an increased level of chemotaxis.Cell activation
Activated T cells are promoted by IL-12 to differentiate towards the Th1 type, producing cytokines including IL-12 and IFNγ. OPN inhibits production of the Th2 cytokine IL-10, which leads to enhanced Th1 response. OPN influences cell-mediated immunity and has Th1 cytokine functions. It enhances B cell immunoglobulin production and proliferation. Recent studies in 2008 suggest that OPN also induces mast cell degranulation. The researchers here observed that IgE-mediated anaphylaxis was significantly reduced in OPN knock-out mice compared to wild-type mice. The role of OPN in activation of macrophages has also been implicated in a cancer study, when researchers discovered that OPN-producing tumors were able to induce macrophage activation compared to OPN-deficient tumors.Apoptosis
OPN is an important anti-apoptotic factor in many circumstances. OPN blocks the activation-induced cell death of macrophages and T cells as well as fibroblasts and endothelial cells exposed to harmful stimuli. OPN prevents non-programmed cell death in inflammatory colitis.Potential clinical application
The fact that OPN interacts with multiple cell surface receptors that are ubiquitously expressed makes it an active player in many physiological and pathological processes including wound healing, bone turnover, tumorigenesis, inflammation, ischemia, and immune responses. Manipulation of plasma OPN levels may be useful in the treatment of autoimmune diseases, cancer metastasis, bone mineralization diseases, osteoporosis, and some forms of stress.Role in autoimmune diseases
OPN has been implicated in pathogenesis of rheumatoid arthritis. For instance, researchers found that OPN-R, the thrombin-cleaved form of OPN, was elevated in the rheumatoid arthritis joint. However, the role of OPN in rheumatoid arthritis is still unclear. One group found that OPN knock-out mice were protected against arthritis. while others were not able to reproduce this observation. OPN has been found to play a role in other autoimmune diseases including autoimmune hepatitis, allergic airway disease, and multiple sclerosis.Role in cancers and inflammatory diseases
It has been shown that OPN drives IL-17 production; OPN is overexpressed in a variety of cancers, including lung cancer, breast cancer, colorectal cancer, stomach cancer, ovarian cancer, papillary thyroid carcinoma, melanoma and pleural mesothelioma; OPN contributes both glomerulonephritis and tubulointerstitial nephritis; and OPN is found in atheromatous plaques within arteries. Thus, manipulation of plasma OPN levels may be useful in the treatment of autoimmune diseases, cancer metastasis, osteoporosis and some forms of stress.Research has implicated osteopontin in excessive scar-forming and a gel has been developed to inhibit its effect.