Differential signaling and regulation of apical vs. basolateral EGFR in polarized epithelial cells

Overexpression of the epidermal growth factor receptors (EGFR) in polarized kidney epithelial cells caused them to appear in high numbers at bath the basolateral and apical cell surfaces. We utilized these cells to look for d ifferences in the regulation and signaling of apical vs. basolateral EGFR. Apical and basolateral EGFR were biologically active and mediated EGF-induc ed cell proliferation to similar degrees. Receptor downregulation and endoc ytosis were less efficient at the apical surface, resulting in prolonged EG F-induced tyrosine kinase activity at the apical cell membrane. Tyrosine ph osphorylation of EGFR substrates known to mediate cell proliferation, Src-h omologous and collagen protein (SHC), extracellularly regulated kinase 1 (E RK1), and ERK2 could be induced similarly by activation of apical or basola teral EGFR. Focal adhesion kinase was tyrosine phosphorylated more by basol ateral than by apical EGFR; however, beta-catenin was tyrosine phosphorylat ed to a much greater degree following the activation of mislocalized apical EGFR. Thus EGFR regulation and EGFR-mediated phosphorylation of certain su bstrates differ at the apical and basolateral cell membrane domains. This s uggests that EGFR mislocalization could result in abnormal signal transduct ion and aberrant cell behavior.