FGF2 binds to the allosteric site (site 2) and activates integrin αIIbβ3 and FGF1 binds to site 2 but suppresses integrin activation by FGF2: A potential mechanism of anti-thrombotic action of FGF1

It has been believed that platelet integrin αIIbβ3 recognizes fibrinogen and several ECM proteins, and we recently showed that αIIbβ3 binds to several inflammatory cytokines (e.g., CCL5, and CXCL12), which are stored in platelet granules. These ligands bind to the classical ligand (RGD)-binding site (site 1) of integrin αIIbβ3. Also, they bind to the allosteric site (site 2) of αIIbβ3, which is distinct from site 1, and allosterically activate αIIbβ3. Site 2 is known to be involved in allosteric integrin activation and inflammatory signaling. FGF2 is also stored in platelet granules and known to be pro-thrombotic, but it is unclear if FGF2 binds to αIIbβ3. We studied if FGF2 and its homologue FGF1 bind to αIIbβ3 and induce allosteric activation. FGF1 (not stored in platelet granules) is known to be anti-thrombotic. Mechanism of FGF1’s anti-thrombotic action is unknown. Here we describe that FGF1 and FGF2 bound to site 1 of αIIbβ3, indicating that αIIbβ3 is a new receptor for FGF1/2. Notably, FGF2 bound to site 2 and allosterically activated αIIbβ3.

Point mutations in the site 2-binding interface of FGF2 suppressed this activation, indicating that FGF2 binding to site 2 is required for activation (FGF2 is an agonist to site 2). In contrast, FGF1 bound to site 2 but did not activate αIIbβ3, and instead suppressed integrin activation induced by FGF2, indicating that FGF1 acts as an antagonist of site 2. A non-mitogenic FGF1 mutant (R50E), which is defective in binding to site 1 of αvβ3, suppressed αIIbβ3 activation by FGF2 as effectively as WT FGF1. We propose that FGF1 R50E has therapeutic potential for anti-thrombosis.