Dual functions of the fibrin βN-domains in the VLDL receptor-dependent pathway of transendothelial migration of leukocytes.

Our previous studies revealed that fibrin interacts with the VLDL receptor (VLDLR) through a pair of its βN-domains and this interaction promotes transendothelial migration of leukocytes and, thereby, inflammation. In agreement, the NDSK-II fragment representing the central part of the fibrin molecule and containing these domains stimulates leukocyte transmigration. However, the recombinant (β15-66) ₂ fragment corresponding to a pair of the βN-domains inhibits NDSK-II-stimulated leukocyte transmigration. To explain this paradox, we hypothesized that fibrin βN-domains have dual function in fibrin-dependent inflammation, namely, their C-terminal regions containing the VLDLR-binding sites promote leukocyte transmigration while their N-terminal regions are responsible for inhibition of this process. To test this hypothesis and to further clarify the molecular mechanisms underlying fibrin-induced VLDLR-dependent pathway of leukocyte transmigration and its inhibition, we prepared the dimeric (β15-44) ₂ and (β40-66) ₂ fragments corresponding to the N- and C-terminal regions of the βN-domains and studied their effect on endothelial permeability and transendothelial migration of leukocytes. The results obtained revealed that (β40-66) ₂ bound to the VLDLR with high affinity and promoted endothelial permeability and leukocyte transmigration while (β15-44) ₂ did not interact with this receptor and had no effect on leukocyte transmigration, in agreement with our hypothesis. We also found that the first three N-terminal residues of the βN-domains play a critical role in the inhibitory properties of these domains. Further, the inhibitory properties of the βN-domains were expressed only upon their isolation from the fibrin molecule. The question of whether their inhibitory function may play a role in fibrin remains to be addressed.
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