title: Integrins αvβ3 and α4β1 act as coreceptors for fractalkine, and the integrin-binding defective mutant of fractalkine is an antagonist of CX3CR1.
abstract: 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. Whereas previous studies suggest that FKN-CD does not directly bind to integrins, our docking simulation studies predicted that FKN-CD directly interacts with integrin α(v)β(3). Consistent with this prediction, we demonstrated that FKN-CD directly bound to α(v)β(3) and α(4)β(1) at a very high affinity (K(D) of 3.0 × 10(-10) M to α(v)β(3) in 1 mM Mn(2+)). Also, membrane-bound FKN bound to integrins α(v)β(3) and α(4)β(1), suggesting that the FKN-CD/integrin interaction is biologically relevant. The binding site for FKN-CD in α(v)β(3) was similar to those for other known α(v)β(3) ligands. Wild-type FKN-CD induced coprecipitation 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, whereas 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 coreceptors and the resulting ternary complex formation are required for FKN signaling. Notably, excess K36E/R37E suppressed integrin activation induced by wild-type 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.
authors.affiliation: Department of Dermatology, University of California Davis School of Medicine, Sacramento, CA 95817, USA.
authors.forename: Masaaki
authors.initials: M
authors.lastname: Fujita
authors.forename: Yoko K
authors.initials: YK
authors.lastname: Takada
authors.forename: Yoshikazu
authors.initials: Y
authors.lastname: Takada
mesh_terms: D000818:Animals; D016466:CHO Cells; D000073883:CX3C Chemokine Receptor 1; D054428:Chemokine CX3CL1 / Q000235:genetics* / Q000378:metabolism*; D018925:Chemokines / Q000378:metabolism; D003198:Computer Simulation; D006224:Cricetinae; D003412:Cricetulus; D006801:Humans; D039041:Integrin alpha4beta1 / Q000378:metabolism*; D039302:Integrin alphaVbeta3 / Q000378:metabolism*; D020014:K562 Cells; D009154:Mutation / Q000235:genetics; D011485:Protein Binding / Q000235:genetics / Q000276:immunology; D017434:Protein Structure, Tertiary / Q000235:genetics; D018121:Receptors, Cytokine / Q000037:antagonists & inhibitors* / Q000378:metabolism*; D015728:Receptors, HIV / Q000037:antagonists & inhibitors* / Q000378:metabolism*; D015398:Signal Transduction / Q000235:genetics / Q000276:immunology; D020298:U937 Cells
chemical_list: D000073883:CX3C Chemokine Receptor 1; D054428:Chemokine CX3CL1; D018925:Chemokines; D039041:Integrin alpha4beta1; D039302:Integrin alphaVbeta3; D018121:Receptors, Cytokine; D015728:Receptors, HIV