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

Abstract

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.