Soluble, but not precursor EGF induces intracrine signaling of the EGFR

EGFR is a transmembrane receptor tyrosine kinase regulating growth and survival in epithelial tissues. Its ligand, the epidermal growth factor (EGF), is produced as a membrane-anchored precursor (preEGF) that is proteolytically cleaved to release soluble EGF (sEGF). EGFR overexpression can convert it from a physiological regulator into an oncogenic driver. Therapeutic strategies targeting EGFR include monoclonal antibodies (mAbs) and tyrosine kinase inhibitors (TKIs). Although mAbs such as cetuximab initially block EGFR activity, tumors often develop resistance. Recent findings indicate that sEGF can activate EGFR within intracellular vesicles, promoting intracrine signaling that sustains proliferation despite extracellular inhibition. Here, we investigated the mechanism of intracrine EGFR signaling in the Golgi apparatus using confocal microscopy, FRET and fluorescence correlation spectroscopy. sEGF, but not preEGF, bound and induced EGFR dimerization and phosphorylation. Erlotinib, a membrane permeable TKI, effectively blocked phosphorylation, whereas extracellular cetuximab did not. These findings imply sEGF-induced intracrine EGFR signaling in the Golgi. Our results may shed light on a potential resistance mechanism to antibody treatment.