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 critical determinant of its interaction with the glycoprotein (GP) Ib/V/IX complex. 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 aggregation required the presence of both soluble vWF and ristocetin. Ristocetin-induced CHO-GPIb alpha beta/IX cell aggregation was completely inhibited by the recombinant VCL fragment of vWF that contains the Al domain. Surprisingly, the substitution of heparin for ristocetin resulted in the formation of CHO-GPIb alpha beta/IX cell aggregates. Using monoclonal antibodies blocking vWF interaction with GPIb/V/IX or mocarhagin, a venom metalloproteinase that removes the amino-terminal fragment of GPIb alpha extending from aa 1 to 282, we demonstrated that both ristocetin- and heparin-induced aggregations involved an interaction between the Al domain of VWF and the GPIb alpha subunit of the GPIb/V/IX complex. The involvement of heparin in cell aggregation was also demonstrated after treatment of heparin with heparinase that abolished CHO-GPIb alpha beta/IX cell aggregation. These results indicated that heparin was able to induce vWF-dependent CHO-GPIb alpha beta/IX cell aggregation. In conclusion, we demonstrated that heparin is capable of positively modulating the vWF interaction with the GPIb/V/IX complex. (C) 1999 by The American Society of Hematology.