FGF2 binds to the allosteric site (site 2) and activates αvβ3 integrin and FGF1 binds to site 2 but suppresses integrin activation by FGF2: a potential mechanism of anti-inflammatory action of FGF1

Abstract

FGF1 is known as an anti-inflammatory and has suppresses insulin resistance. Its homologue FGF2 is pro-inflammatory. Mechanism of FGF1 anti-inflammatory action and of FGF1 and pro-inflammatory action of FGF2 are unknown. Several inflammatory cytokines (e.g., CX3CL1, CCL5, and CXCL12, and CD40L) bind to the classical ligand (RGD)-binding site (site 1) of integrin αvβ3. In addition, they bind to the allosteric site (site 2) of αvβ3, which is distinct from site 1, and allosterically activate αvβ3. Site 2 is involved in inflammatory signals since inflammatory lipid mediator 5-hydroxycholesterol binds to site 2 and induces integrin activation and inflammatory signals (e.g., TNF and IL-6 secretion). We thus hypothesized that FGF1 and FGF2 bind to site 2 and affect activation status of integrins. Here we describe that FGF2 bound to site 2 and allosterically activated αvβ3 integrin. Point mutations in the site 2-binding interface of FGF2 suppressed this activation, indicating that FGF2 binding to site 2 is required for inducing integrin activation. In contrast, FGF1 bound to site 2 but did not activate αvβ3, and instead suppressed integrin activation induced by FGF2, indicating that FGF1 acts as an antagonist of site 2. These findings suggest that anti-inflammatory action of FGF1 is mediated by blocking site 2. FGF1 has potential as an anti-inflammatory agent, but is not appropriate for long-term use since it is potent mitogen. A non-mitogenic FGF1 mutant (R50E), which is defective in binding to site 1 of αvβ3, suppressed αvβ3 activation by FGF2 as effectively as WT FGF1. We propose that FGF1 R50E has therapeutic potential for inflammatory diseases.