Osteopontin (OPN), also known as bone /sialoprotein I (BSP-1 or BNSP), early T-lymphocyte activation (ETA-1), secreted phosphoprotein 1 (SPP1), 2ar and Rickettsia resistance (Ric), is a protein that in humans is encoded by the SPP1 gene (secreted phosphoprotein 1). The murine ortholog is Spp1. Osteopontin is a SIBLING (glycoprotein) that was first identified in 1986 in osteoblasts. The prefix osteo- 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. 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 (4q1322.1). 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 OPN is a highly negatively charged, heavily phosphorylated extracellular matrix protein that lacks an extensive secondary structure as an intrinsically disordered protein. It is composed of about 300 amino acids (297 in mouse; 314 in human) 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 (5' UTR). 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.
SPP1 structure corresponds to an osteopontin antibody
Isoforms Full-length OPN (OPN-FL) can be modified by thrombin cleavage, which exposes a cryptic sequence, SVVYGLR on the cleaved form of the protein known as OPN-R (Fig. 1). This thrombin-cleaved OPN (OPN-R) 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 (CPB) 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 (iOPN) 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.
Tissue distribution
Osteopontin is expressed in 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 (non-bone) cells in the inner ear, brain, kidney, deciduum, and placenta. Synthesis of osteopontin is stimulated by calcitriol (1,25-dihydroxy-vitamin D3).
Regulation Regulation of the osteopontin gene expression 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 (bone-forming cells) as well as osteoclasts (bone-resorbing cells). Runx2 (aka Cbfa1) and osterix (Osx) 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 (instances that stimulate kidney proximal tubule cells to produce calcitriol (1α,25-dihydroxyvitamin D3)) lead to increases in OPN transcription, translation and secretion. This is due to the presence of a high-specificity vitamin D response element (VDRE) in the OPN gene promoter. Osteopontin (OPN) expression is modulated by Schistosoma mansoni egg antigen. Schistosoma mansoni egg antigens directly stimulate the expression of the profibrogenic molecule osteopontin (OPN), and systemic OPN levels strongly correlate with disease severity, suggesting its use as a potential morbidity biomarker. Investigation into the impact of Praziquantel use on systemic OPN levels and on liver collagen deposition in chronic murine schistosomiasis revealed that Praziquantel treatment significantly reduced systemic OPN levels and liver collagen deposition, indicating that OPN could be a reliable tool for monitoring PZQ efficacy and fibrosis regression. Extracellular inorganic phosphate (ePi) 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 (e.g. tumour necrosis factor α [TNFα], infterleukin-1β [IL-1β]), angiotensin II, transforming growth factor β (TGFβ) and parathyroid hormone (PTH), although a detailed mechanistic understanding of these regulatory pathways are not yet known. Hyperglycemia and hypoxia are also known to increase OPN expression.
Function
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.
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