Osteopontin (OPN) is a secreted molecule in the SIBLING family of non-collagenous matricellular proteins. It is a 45-75 kDa acidic protein that is variably modified in a tissue specific manner by glycosylation, sulfation, phosphorylation, and transglutamination. OPN plays a major role in bone mineralization, kidney stone formation, cell adhesion and migration, and inflammation. It is upregulated in response to inflammation, cancer, and tissue damage. The central region of OPN contains RGD and non-RGD binding sites for multiple integrins. Adjacent to the RGD motif is the sequence SVVYGLR (SLAYGLR in mouse) which serves as a cryptic binding site for additional integrins: it is masked in full length OPN but is exposed following OPN cleavage by thrombin.1, 2, 3, 4, 5 The N-terminal fragment of OPN ending in the SVVYGLR motif is more avid than full length OPN at mediating cell adhesion and migration.1, 2, 3
OPN can also be cleaved by MMP-3, -7, -9, and -12 within the SVVYGLR motif and at sites closer to the C-terminus.6, 7 MMP-mediated cleavage of OPN at the first site has differential effects on binding to particular integrins.3, 6, 8 Cleavage at the more C-terminal MMP sites has not been shown to affect integrin binding, although complete digestion of OPN by MMP-9 releases a 5 kDa fragment which promotes CD44-dependent metastasis in hepatocellular carcinoma.7 It has not been determined if cleavage by thrombin or MMPs can be sequential or if digestion by one enzyme prevents further digestion by a second.
Recently, Kon et al. identified the proteoglycan Syndecan-4 as a novel OPN binding partner and regulator. These authors demonstrated that the heparan sulfate moiety of Syndecan-4 interacts with the heparin-binding domain (HBD) of OPN at the thrombin cleavage site but not at a second HBD near the C-terminus.9 Formation of the OPN/Syndecan-4 complex likely represses OPN proteolysis by obstructing the thrombin cleavage site.9 Syndecan-4 binding to OPN blocks the binding of integrin alpha 4 beta 1, which recognizes the cryptic SVVYGLR epitope and integrin alpha V beta 3, which recognizes the RGD motif.9
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|Cleavage of OPN by thrombin releases an N-terminal fragment with enhanced pro-inflammatory properties. Cleavage exposes the SVVYGLR epitope, enabling binding of additional integrins. Syndecan-4 inhibits inflammation by binding to OPN and preventing thrombin-mediated cleavage|
In a mouse model of hepatitis, concanavalin A-induced granuloma formation and hepatocyte cell death requires the secretion of OPN by resident CD1d-restricted natural killer T (NKT) cells.10, 11 These cells are responsible for the bulk of the OPN elevation seen in the liver and serum. The fraction of OPN that is cleaved by thrombin is critical for neutrophil migration and activation and for further stimulation of OPN release by NKT cells.11 Both NKT cells and neutrophils express integrin alpha 4 beta 1, which binds the cryptic epitope. Blocking antibodies to the SLAYGLR motif prevent OPN-induced hepatocyte necrosis and elevation of serum alanine aminotransferase, a physiological indicator of liver damage.11
Using the Concanavalin A-induced hepatitis model, Kon et al. showed that Syndecan-4 provides protection against OPN mediated toxicity. Compared to wild type mice, Syndecan-4 knockout mice sustained more extensive liver damage in terms of histology and serum liver enzyme levels.9 Circulating thrombin-cleaved OPN was elevated as previously described by Diao et al., and antibody neutralization of the N-terminal fragment improved the tissue and serological indicators of liver damage.9 The increased shedding of Syndecan-4 in this model may constitute a protective response, consistent with the observation that exogenous soluble Syndecan-4 blocked inflammatory cell infiltration into the liver and hepatocyte necrosis.9
The functions of OPN may be determined by the relative abundance of different forms and modifications. The ability of Syndecan-4 to block the generation of pro-inflammatory OPN fragments represents one of potentially many mechanisms to control the prevalence of these forms.
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