Cryosupernatant regulates accumulation of unusually large vWF multimers from endothelial cells

In addition to the von Willebrand factor (vWF) multimers found in normal plasma, cultured human umbilical vein endothelial cells (HUVECs) synthesize and release unusually large vWF multimers (ULvWFM). ULvWFM are more effective than the largest plasma vWF forms in attaching to platelets and promoting platelet aggregation in the presence of elevated fluid shear forces (as in narrowed atherosclerotic vessels), and they may be involved in arterial thrombosis. ULvWFM are produced within HUVECs exposed for 48-60 h either to steady venous-like or pulsatile arterial-like wall shear stresses and are produced and released from HUVECs grown in serum-free as well as in serum (bovine or human)-containing media. An activity of 140,000-200,000 Da in the cryosupernatant fraction of both normal and severe von Willebrand's disease plasma, which is not obviously a protease, prevents specifically the accumulation of ULvWFM in the fluid above HUVEC monolayers but does not impair the release by HUVECs of ULvWFM in the retrograde direction into subendothelial collagen. The ULvWF regulatory activity in cryosupernatant may inhibit inappropriate platelet aggregation and thrombosis by preventing the accumulation of endothelial cell-derived ULvWFM in circulating blood.