Integrins were originally identified as receptors for extracellular matrix (ECM) and cell-surface molecules (e.g., VCAM-1 and ICAM-1). Later, we discovered that many soluble growth factors/cytokines bind to integrins and play a critical role in growth factor/cytokine signaling (growth factor-integrin crosstalk). We performed a virtual screening of protein data bank (PDB) using docking simulations with the integrin headpiece as a target. We showed that several growth factors (e.g., FGF1 and IGF1) induce a integrin-growth factor-cognate receptor ternary complex on the surface. Growth factor/cytokine mutants defective in integrin binding were defective in signaling functions and act as antagonists of growth factor signaling. Unexpectedly, several growth factor/cytokines activated integrins by binding to the allosteric site (site 2) in the integrin headpiece, which is distinct from the classical ligand (RGD)-binding site (site 1). Since 25-hydroxycholesterol, a major inflammatory mediator, binds to site 2, activates integrins, and induces inflammatory signaling (e.g., IL-6 and TNFα secretion), it has been proposed that site 2 is involved in inflammatory signaling. We showed that several inflammatory factors (CX3CL1, CXCL12, CCL5, sPLA2-IIA, and P-selectin) bind to site 2 and activate integrins. We propose that site 2 is involved in the pro-inflammatory action of these proteins and a potential therapeutic target. It has been well-established that platelet integrin αIIbβ3 is activated by signals from the inside of platelets induced by platelet agonists (inside-out signaling). In addition to the canonical inside-out signaling, we showed that αIIbβ3 can be allosterically activated by inflammatory cytokines/chemokines that are stored in platelet granules (e.g., CCL5, CXCL12) in the absence of inside-out signaling (e.g., soluble integrins in cell-free conditions). Thus, the allosteric activation may be involved in αIIbβ3 activation, platelet aggregation, and thrombosis. Inhibitory chemokine PF4 (CXCL4) binds to site 2 but did not activate integrins, Unexpectedly, we found that PF4/anti-PF4 complex was able to activate integrins, indicating that the anti-PF4 antibody changed the phenotype of PF4 from inhibitory to inflammatory. Since autoantibodies to PF4 are detected in vaccine-induced thrombocytopenic thrombosis (VIPP) and autoimmune diseases (e.g., SLE, and rheumatoid arthritis), we propose that this phenomenon is related to the pathogenesis of these diseases. P-selectin is known to bind exclusively to glycans (e.g., sLex) and involved in cell-cell interaction by binding to PSGL-1 (CD62P glycoprotein ligand-1). Unexpectedly, through docking simulation, we discovered that the P-selectin C-type lectin domain functions as an integrin ligand. It is interesting that no one has studied whether P-selectin binds to integrins in the last few decades. The integrin-binding site and glycan-binding site were close but distinct. Also, P-selectin lectin domain bound to site 2 and allosterically activated integrins.
Keywords: growth factor signaling; integrins; receptors.