Interaction of coagulation factor VIII with members of the low-density lipoprotein receptor family follows common mechanism and involves consensus residues within the A2 binding site 484-509.

Coagulation factor VIII interacts with several members of the low-density lipoprotein receptor family including low-density lipoprotein receptor-related protein, low-density lipoprotein receptor, and very low-density lipoprotein receptor. The present study was aimed to compare the mechanisms of factor VIII interaction with low-density lipoprotein receptor-related protein, megalin, low-density lipoprotein receptor, and very low-density lipoprotein receptor in order to reveal a general mode of these interactions. Binding of plasma-derived factor VIII and its fragments to recombinant soluble ligand-binding domain of low-density lipoprotein receptor (sLDLR1-7) and purified megalin was studied in solid phase and surface plasmon resonance assays. Full-length factor VIII and its light chain bound to the receptors with similar affinities (KD = 260 +/- 9 and 156 +/- 4 nmol/l, respectively, for megalin and KD = 210 +/- 3 and 174 +/- 13 nmol/l, respectively, for sLDLR1-7). Von Willebrand factor inhibited factor VIII binding to both receptors. In contrast to the light chain, exposure of the high-affinity receptor-binding site within the heavy chain (KD = 22 +/- 4 nmol/l for megalin and 17 +/- 3 nmol/l for sLDLR1-7) required proteolytic cleavage by thrombin. This site was mapped to the A2 domain residues 484-509, based on the inhibitory effects of anti-A2 monoclonal antibody 413, and is shared by all four receptors. Using a panel of A2 mutants, we identified key amino acid residues- positively charged K466, R471, R489 and R490, and hydrophilic residues Y487 and S488- which form the frame of this 'consensus' binding site. We conclude that interaction of factor VIII with the members of the low-density lipoprotein receptor family follows the general mode, requires dissociation of factor VIII from von Willebrand factor, and is activation sensitive.

Identification of coagulation factor VIII A2 domain residues forming the binding epitope for low-density lipoprotein receptor-related protein.

Regulation of the coagulation factor VIII (fVIII) level in circulation involves a hepatic receptor low-density lipoprotein receptor-related protein (LRP). One of two major LRP binding sites in fVIII is located within the A2 domain (A2), likely exposed within the fVIII complex with von Willebrand factor and contributing to regulation of fVIII via LRP. This work aimed to identify A2 residues forming its LRP-binding site, previously shown to involve residues 484-509. Isolated A2 was subjected to alanine-scanning mutagenesis followed by expression of a set of mutants in a baculovirus system. In competition and surface plasmon resonance assays, affinities of A2 mutants K466A, R471A, R484A, S488A, R489A, R490A, H497A, and K499A for LRP were found to be decreased by 2-4-fold. This correlated with 1.3-1.5-fold decreases in the degree of LRP-mediated internalization of the mutants in cell culture. Combining these mutations into pairs led to cumulative effects, i.e., 7-13-fold decrease in affinity for LRP and 1.6-2.2-fold decrease in the degree of LRP-mediated internalization in cell culture. We conclude that the residues mentioned above play a key role in formation of the A2 binding epitope for LRP. Experiments in mice revealed an approximately 4.5 times shorter half-life for A2 in the circulation in comparison with that of fVIII. The half-lives of A2 mutant R471A/R484A or A2 co-injected with receptor-associated protein, a classical ligand of LRP, were prolonged by approximately 1.9 and approximately 3.5 times, respectively, compared to that of A2. This further confirms the importance of the mutated residues for interaction of A2 with LRP and suggests the existence of an LRP-dependent mechanism for removing A2 as a product of dissociation of activated fVIII from the circulation.