Specific detection of EGF-EGFR complexes on the cell surface and their dependency on EGFR-directed TKI treatment

The epidermal growth factor receptor (EGFR) is among the most studied receptors with a well-established link to human cancer. However, despite its critical role in tissue development and homeostasis, as well as tumorigenesis, only little is known with respect to its ligand occupancy. This paucity of data is largely caused by the lack of reagents specifically detecting the ligand-bound receptor state. To address this limitation, we recently engineered the binder ActE_21, which exclusively recognizes EGF-bound EGFR. In the present study, we used ActE_21 to specifically analyze EGF-EGFR complexes on the surface of human cancer cells in the presence of different EGF concentrations. While we observed the expected ligand-induced internalization of EGFR upon incubation at 37 °C, considerable amounts of EGF-EGFR complexes remained detectable on the cell surface. This surface level was further increased upon treatment with various EGFR-directed tyrosine kinase inhibitors (TKIs) on A549 and SK-BR3 cells, but not on A431 cells. Together, we present a novel approach to specifically detect EGF-EGFR complexes and demonstrate that - at least in some cell lines - EGFR-blockade with TKIs results in the accumulation of EGF-bound EGFR on the cell surface.