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Keywords:

  • endocytosis;
  • factor VIII;
  • macrophages;
  • shear stress;
  • von Willebrand factor

Summary.  Background:  Low-density lipoprotein (LDL) receptor family members contribute to the cellular uptake of factor VIII. How von Willebrand factor fits into this endocytic pathway has remained poorly understood.

Objectives:  It has been suggested that macrophages contribute to the clearance of the factor VIII (FVIII)-von Willebrand factor (VWF) complex. We now assessed the mechanisms of uptake employing human monocyte-derived macrophages.

Methods:  A confocal microscopy study was employed to study the uptake by monocyte-derived macrophages of a functional green fluorescent FVIII-GFP derivative in the presence and absence of VWF.

Results:  The results revealed that FVIII-GFP is internalized by macrophages. We found that FVIII-GFP co-localizes with LDL receptor-related protein (LRP), and that the LRP antagonist Receptor Associated Protein (RAP) blocks the uptake of FVIII-GFP. However, FVIII-GFP was not detected in the macrophages in the presence of VWF, suggesting that the FVIII-VWF complex is not internalized by these cells at all. Apart from static conditions, we also investigated the effect of shear stress on the uptake of FVIII-GFP in presence of VWF. Immunofluorescence studies demonstrated that VWF does not block endocytosis of FVIII-GFP under flow conditions. Moreover, VWF itself was also internalized by the macrophages. Strikingly, in the presence of RAP, endocytosis of FVIII-GFP and VWF was inhibited.

Conclusion:  The results show that shear stress is required for macrophages to internalize both constituents of the FVIII-VWF complex.