Modulation by heparin of the interaction of the A1 domain of von Willebrand factor with glycoprotein Ib

The conformation of the Al domain of von Willebrand factor (VWF) is a criti cal determinant of its interaction with the glycoprotein (GP) Ib/V/IX compl ex. To better define the regulatory mechanisms of vWF Al domain binding to the GPIb/V/IX complex, we studied vWF-dependent aggregation properties of a cell line overexpressing the GPIb alpha, GPIb beta, and GPIX subunits (CHO -GPIb alpha beta/IX cells). We found that CHO-GPIb alpha beta/IX cell aggre gation required the presence of both soluble vWF and ristocetin. Ristocetin -induced CHO-GPIb alpha beta/IX cell aggregation was completely inhibited b y the recombinant VCL fragment of vWF that contains the Al domain. Surprisi ngly, the substitution of heparin for ristocetin resulted in the formation of CHO-GPIb alpha beta/IX cell aggregates. Using monoclonal antibodies bloc king vWF interaction with GPIb/V/IX or mocarhagin, a venom metalloproteinas e that removes the amino-terminal fragment of GPIb alpha extending from aa 1 to 282, we demonstrated that both ristocetin- and heparin-induced aggrega tions involved an interaction between the Al domain of VWF and the GPIb alp ha subunit of the GPIb/V/IX complex. The involvement of heparin in cell agg regation was also demonstrated after treatment of heparin with heparinase t hat abolished CHO-GPIb alpha beta/IX cell aggregation. These results indica ted that heparin was able to induce vWF-dependent CHO-GPIb alpha beta/IX ce ll aggregation. In conclusion, we demonstrated that heparin is capable of p ositively modulating the vWF interaction with the GPIb/V/IX complex. (C) 19 99 by The American Society of Hematology.