Pro-inflammatory chemokines CCL5, CXCL12, and CX3CL1 bind to and activate platelet integrin αIIbβ3 in an allosteric manner.

Abstract

Previous studies showed that pro-inflammatory chemokines CX3CL1 and CXCL12 bound to the allosteric binding site (site 2) of integrins and allosterically activated integrins, in addition to the classical ligand-binding site (site 1). We showed that CCL5 also bound to site 2, in addition to site 1, and activated soluble integrin v{beta}3. Platelet integrin IIb{beta}3, a fibrinogen receptor, is critical for hemostasis and thrombus formation and activation of IIb{beta}3 is a key event for thrombus formation. Activation of IIb{beta}3 is known to be mediated exclusively by inside-out signaling. We studied if IIb{beta}3 can be allosterically activated. We discovered that CCL5, CXCL12, and CX3CL1 are new ligands for IIb{beta}3. Notably they enhanced the binding of monovalent ligand to soluble IIb{beta}3 in 1 mM Ca2+ by binding to site 2. They activated cell-surface IIb{beta}3 on CHO cells quickly (half maximal response <1 min) and at low concentrations (1-10 ng/ml) compared to soluble IIb{beta}3, probably because chemokines bind to cell surface proteoglycans. Notably, activation of IIb{beta}3 by the chemokines was several times more potent than 1 mM Mn2+. Since CCL5 and CXCL12 are stored in platelet granules and rapidly transported to the surface upon platelet activation, we hypothesized that they are released from the granules and allosterically activate IIb{beta}3 by binding to site 2. Transmembrane CX3CL1 on activated endothelial cells likely mediates platelet-endothelial interaction by binding to and activating IIb{beta}3. Also, over-produced chemokines during inflammation may trigger IIb{beta}3 activation, which is a possible missing link between inflammation and thrombosis.