EphA2 Engages Git1 to Suppress Arf6 Activity Modulating Epithelial Cell-Cell Contacts

ADP-ribosylation factor (Arf) 6 activity is crucially involved in the regulation of E-cadherin-based cell-cell adhesions. Erythropoietin-producing hepatocellular carcinoma (Eph)-family receptors recognize ligands, namely, ephrins, anchored to the membrane of apposing cells, and they mediate cell-cell contact-dependent events. Here, we found that Arf6 activity is down-regulated in Madin-Darby canine kidney cells, which is dependent on cell density and calcium ion concentration, and we provide evidence of a novel signaling pathway by which ligand-activated EphA2 suppresses Arf6 activity. This EphA2-mediated suppression of Arf6 activity was linked to the induction of cell compaction and polarization, but it was independent of the down-regulation of extracellular signal-regulated kinase 1/2 kinase activity. We show that G protein-coupled receptor kinase-interacting protein (Git) 1 and noncatalytic region of tyrosine kinase (Nck) 1 are involved in this pathway, in which ligand-activated EphA2, via its phosphorylated Tyr594, binds to the Src homology 2 domain of Nck1, and then via its Src homology 3 domain binds to the synaptic localizing domain of Git1 to suppress Arf6 activity. We propose a positive feedback loop in which E-cadherin-based cell-cell contacts enhance EphA-ephrinA signaling, which in turn down-regulates Arf6 activity to enhance E-cadherin-based cell-cell contacts as well as the apical-basal polarization of epithelial cells.