Fig. 1.

The epidermal growth factor receptor (EGFR) is restricted to the basal surface of the airway epithelial cells. This figure serves to highlight how the integrity of the apicoadherens junction can regulate epithelial cell signaling. Under conditions of low paracellular permeability created by the cytoskeleton and cell-cell contacts, there is segregation of EGF ligand and receptor, thereby inhibiting constant cellular signaling. In conditions of increased paracellular permeability, EGF can bind its receptor and activate its pathway, where phosphorylated ERK serves as readout of EGFR activation. Thus the epithelial barrier can modulate epithelial signaling pathways.