Heparin modulates integrin function in human platelets

Purpose Heparin binds to human platelets and can cause activation and aggre gation, although the mechanisms are unknown. To determine how heparin alter s platelet function, we identified platelet-binding sites for heparin and m easured heparin's influence on the function of platelet integrin alpha (IIb )beta (3) (glycoprotein IIb/IIIa). Methods: Photoaffinity cross-linking and affinity chromatography experiment s were performed to identify platelet membrane proteins that bind heparin. Heparin's effect on fibrinogen binding to platelets was measured with a rad ioligand-binding assay. The translocation to the cytoskeleton of Rap2, a gu anosine triphosphate-binding protein, was measured from platelets aggregati ng in response to heparin and other agonists. Results: Cross-linking and affinity chromatographic experiments positively identified the integrin alpha (IIb)beta (3) as a heparin-binding site. Hepa rin aggregation was calcium dependent. Low concentrations of unfractionated porcine mucosal heparin (2-5 U/mL) significantly increased fibrinogen I 12 5 binding to activated platelets, whereas higher doses did not. Heparin-med iated platelet aggregation was completely blocked by GRGDS peptide (5 mmol/ L), a competitive inhibitor of fibrinogen binding, and was blocked by EDTA (2 mmol/L), which dissociates the functional integrin complex. Aggregation was associated with Rap2 translocation to the cytoskeleton, a sign of outsi de-in signaling. Conclusions Heparin binds to the alpha (IIb)beta (3) integrin in vitro and ex vivo, and heparin increases fibrinogen binding to the integrin. Heparin- mediated aggregation requires an intact integrin and ligand and leads to Ra p2 translocation to the cytoskeleton-an outside-in signal of ligand engagem ent. Heparin may directly modulate platelet integrin function, most likely through direct binding and modulation of integrin function.