Heparin modulates integrin function in human platelets

• Published in: Journal of vascular surgery Vol. 33; no. 3; pp. 587,2A - 594,2A
• Main Authors: Sobel, Michael, Fish, Wallace R., Toma, Naoto, Luo, Suge, Bird, Karyn, Mori, Keiji, Kusumoto, Shoichi, Blystone, Scott D., Suda, Yasuo
• Format: Journal Article
• Language: English
• Published: New York, NY Elsevier Inc 01.03.2001; Elsevier
• Subjects: Binding Sites; Biological and medical sciences; Blood Platelets - drug effects; Blood. Blood coagulation. Reticuloendothelial system; Cells, Cultured; Fibrinogen - metabolism; Heparin - pharmacology; Humans; Medical sciences; Pharmacology. Drug treatments; Platelet Activation - drug effects; Platelet Aggregation - drug effects; Platelet Glycoprotein GPIIb-IIIa Complex - metabolism; Receptors, Cell Surface - metabolism; Aggregation; Human; Glycoprotein IIbIIIa; Platelet; Integrin; Glycosaminoglycan; Anticoagulant; Heparin; Biological activity
• ISSN: 0741-5214; 1097-6809
• DOI: 10.1067/mva.2001.112696

Purpose: Heparin binds to human platelets and can cause activation and aggregation, although the mechanisms are unknown. To determine how heparin alters platelet function, we identified platelet-binding sites for heparin and measured heparin's influence on the function of platelet integrin αIIbβ3 (glycoprotein IIb/IIIa). Methods: Photoaffinity cross-linking and affinity chromatography experiments were performed to identify platelet membrane proteins that bind heparin. Heparin's effect on fibrinogen binding to platelets was measured with a radioligand-binding assay. The translocation to the cytoskeleton of Rap2, a guanosine triphosphate–binding protein, was measured from platelets aggregating in response to heparin and other agonists. Results: Cross-linking and affinity chromatographic experiments positively identified the integrin αIIbβ3 as a heparin-binding site. Heparin aggregation was calcium dependent. Low concentrations of unfractionated porcine mucosal heparin (2-5 U/mL) significantly increased fibrinogen I 125 binding to activated platelets, whereas higher doses did not. Heparin-mediated 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 outside-in signaling. Conclusions: Heparin binds to the αIIbβ3 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 Rap2 translocation to the cytoskeleton—an outside-in signal of ligand engagement. Heparin may directly modulate platelet integrin function, most likely through direct binding and modulation of integrin function. (J Vasc Surg 2001;33:587-94.)