VON-WILLEBRAND-FACTOR BINDS TO NATIVE COLLAGEN-VI PRIMARILY VIA ITS A1 DOMAIN

Collagen VI is abundant in the arterial subendothelium. To investigate its mechanism of interaction with von Willebrand factor (vWF), collag en VI was isolated from human placenta and from the extracellular matr ix of the human lung fibroblast cell line MRC-5. Purified vWF bound to non-digested collagen VI with moderately high affinity (EC50 approxim ate to 5 nM) and could be inhibited by the Hirudo medicinalis collagen inhibitor calin. The anti-(human vWF A1 domain) monoclonal antibody ( AJvW-2), as well as aurin tricarboxylic acid (ATA), at concentrations that saturate the vWF Al domain, also inhibited this binding. In contr ast, the monoclonal anti-(human vWF A3 domain) antibody (82D6A3) inhib ited vWF binding to collagens I, III and IV, but had no effect on vWF binding to collagen VI. Likewise, vWF binding to collagen VI was not i nhibited by the recombinant vWF domain D4. Polyclonal anti-(collagen V I) antibodies, specifically neutralizing the binding of vWF to collage n VI, confirmed that in the intact endothelial cell extracellular matr ix, collagen VI was accessible for interaction with vWF. This binding was only marginally affected by 82D6A3 but was dose-dependently inhibi ted by AJvW-2, ATA and the Al domain analogue VCL (recombinant A1 doma in of vWF), with IC50 values comparable to those found for the inhibit ion of vWF binding to isolated collagen VI. The weak interaction of is olated human platelets with collagen VI was mediated via the platelet collagen receptor (GPIa/IIa) and was competitively inhibited by vWF bu t not by VCL, suggesting that vWF and GPIa/IIa bind to neighbouring bu t distinct sites on collagen VI. We conclude that vWF binds to collage n VI primarily via its A1 domain, which distinguishes it from the vWF A3 domain-mediated binding to fibrillar collagens.