A Catalytic Role of Heparin within the Extracellular Matrix

Abstract

We investigated the mechanism by which heparin enhances the binding of vascular endothelial growth factor (VEGF) to the extracellular matrix protein fibronectin. In contrast to other systems, where heparin acts as a protein scaffold, we found that heparin functions catalytically to modulate VEGF binding site availability on fibronectin. By measuring the binding of VEGF and heparin to surface-immobilized fibronectin, we show that substoichiometric amounts of heparin exposed cryptic VEGF binding sites within fibronectin that remain available after heparin removal. Measurement of association and dissociation kinetics for heparin binding to fibronectin indicated that the interaction is rapid and transient. We localized the heparin-responsive element to the C-terminal 40-kDa Hep2 domain of fibronectin. A mathematical model of this catalytic process was constructed that supports a mechanism whereby the heparininduced conformational change in fibronectin is accompanied by release of heparin. Experiments with endothelial extracellular matrix suggest that this process may also occur within biological matrices. These results indicate a novel mechanism whereby heparin catalyzes the conversion of fibronectin to an open conformation by transiently interacting with fibronectin and progressively hopping from molecule to molecule.

PDGF-A interactions with fibronectin reveal a critical role for heparan sulfate in directed cell migration during Xenopus gastrulation

“…1E). These results are consistent with those observed with VEGF-A, which also bound to the C-terminal 40-kDa domain in a heparindependent manner (18). Taken together, these data indicate specific PDGF-AA-fibronectin binding in vitro.…”

“…These data indicate that heparin pretreatment led to the exposure or creation of nascent PDGF binding sites on fibronectin. This is similar to that observed previously for VEGF-A binding to fibronectin (14,18). PDGF and VEGF are members of the same protein structural family, and previous studies have noted that VEGF-A binding to fibronectin is enhanced at acidic pH.…”

“…1D). These data suggest that PDGF-AA can bind to fibronectin only after the binding sites on the fibronectin have been modified with heparin (14,18).…”

“…This binding was significantly enhanced by heparin (14,15,18), which acts by modifying the structure of fibronectin, potentially revealing previously masked VEGF binding sites that remain available to bind VEGF even after heparin is removed (14). We demonstrate that PDGF also directly binds to fibronectin, and that this interaction depends on pretreatment of the fibronectin with heparin.…”

Heparin-binding domain of fibrin(ogen) binds growth factors and promotes tissue repair when incorporated within a synthetic matrix

“…Thus, the presence of a heparin-binding domain within the GFs seems to be necessary, but not sufficient, to provide binding to Fg β15-66 (2) . We and others have shown enhanced binding of GFs to fibronectin in the presence of heparin, although heparin is not the linker between the two molecules (3,4,27). It has been suggested that heparin and heparan sulfate can change the conformation of fibronectin and increase its affinity for GFs (4,27); however, in the case of GF-Fg β15-66 (2) interaction, heparin did not increase GFbinding ability, but rather decreased it (Fig.…”