HEMOSTASIS, THROMBOSIS, AND VASCULAR BIOLOGY Modulation by Heparin of the Interaction of the A1 Domain of von Willebrand Factor With Glycoprotein Ib

The conformation of the A1 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 A1 domain binding to the GPIb/V/IX complex, we studied vWF-dependent aggregation properties of a cell line overexpressing the GPIba, GPIbb, and GPIX subunits (CHO-GPIbab/IX cells). We found that CHO-GPIbab/IX cell aggregation required the presence of both soluble vWF and ristocetin. Ristocetin-induced CHOGPIbab/IX cell aggregation was completely inhibited by the recombinant VCL fragment of vWF that contains the A1 domain. Surprisingly, the substitution of heparin for ristocetin resulted in the formation of CHO-GPIbab/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 GPIba extending from aa 1 to 282, we demonstrated that both ristocetinand heparin-induced aggregations involved an interaction between the A1 domain of vWF and the GPIba 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-GPIbab/IX cell aggregation. These results indicated that heparin was able to induce vWF-dependent CHO-GPIbab/IX cell aggregation. In conclusion, we demonstrated that heparin is capable of positively modulating the vWF interaction with the GPIb/ V/IX complex. r 1999 by The American Society of Hematology.