Integrins αvβ3 and α4β1 act as co-receptors for fractalkine and the integrin-binding defective mutant of fractalkine is an antagonist of CX3CR1

摘要

The membrane-bound chemokine fractalkine (FKN, CX3CL1) on endothelial cells plays a role in leukocyte trafficking. The chemokine domain (FKN-CD) is sufficient for inducing FKN signaling (e.g., integrin activation) and FKN-CD binds to its receptor CX3CR1 on leukocytes. While previous studies suggest that FKN-CD does not directly bind to integrins, our docking simulation studies predicted that FKN-CD directly interacts with integrin αvβ3. Consistent with this prediction, we demonstrated that FKN-CD directly bound to αvβ3 and α4β1 at a very high affinity (KD= 3.0 ×10⁻¹⁰ M to αvβ3 in 1 mM Mn²⁺). Also membrane-bound FKN bound to integrins αvβ3 and α4β1, suggesting that the FKN-CD-integrin interaction is biologically relevant. The binding site for FKN-CD in αvβ3 was similar to those for other known αvβ3 ligands. wt FKN-CD induced co-precipitation of integrins and CX3CR1 in U937 cells, suggesting that FKN-CD induces ternary complex formation (CX3CR1, FKN-CD, and integrin). Based on the docking model, we generated an integrin-binding defective FKN-CD mutant (the K36E/R37E mutant). K36E/R37E was defective in ternary complex formation and integrin activation, while K36E/R37E still bound to CX3CR1. These results suggest that FKN-CD binding to CX3CR1 is not sufficient for FKN signaling, and that FKN-CD binding to integrins as co-receptors and resulting ternary complex formation is required for FKN signaling. Notably, excess K36E/R37E suppressed integrin activation induced by wt FKN-CD, and effectively suppressed leukocyte infiltration in thioglycollate-induced peritonitis. These findings suggest that K36E/R37E acts as a dominant-negative CX3CR1 antagonist and that FKN-CD/integrin interaction is a novel therapeutic target in inflammatory diseases.