Phosphorylation of ephrin‐B1 via the interaction with claudin following cell–cell contact formation

The interaction of the Eph family of receptor protein tyrosine kinase and its ligand ephrin family induces bidirectional signaling via the cell–cell contacts. Although most previous studies have focused on the function of Eph–ephrin pathways in the neural system and endothelial cells, this process also occurs in epithelial and cancer cells, of which the biological involvement is poorly understood. We show that ephrin‐B1 creates an in vivo complex with adjacent claudin1 or claudin4 via the extracellular domains of these proteins. The cytoplasmic domain of ephrin‐B1 was phosphorylated on tyrosine residues upon the formation of cell–cell contacts, possibly recognizing an intercellular adhesion of claudins. Phosphorylation of ephrin‐B1 induced by claudins was abolished by the treatment with 4‐amino‐5‐(4‐chlorophenyl)‐7–(t‐butyl)pyrazolo[3,4‐d]pyrimidine, an inhibitor of the Src family kinases. Moreover, overexpression of ephrin‐B1 triggered consequent change in the level of cell–cell adhesion depending on its phosphorylation. These results suggest that ephrin‐B1 mediated the cell–cell adhesion of epithelial and cancer cells via a novel Eph receptor‐independent mechanism.