Heparin-binding epidermal growth factor cleavage mediates zinc-induced epidermal growth factor receptor phosphorylation

We have previously shown that exposure to zinc ions can activate epidermal growth factor (EGF) receptor (EGFR) signaling in murine fibroblasts and A431 cells through a mechanism involving Src kinase. While studying the effects of zinc ions in normal human bronchial epithelial cell, we uncovered evidence for an additional mechanism of Zn2+-induced EGFR activation. Exposure to Zn2+ induced phosphorylation of EGFR at tyrosine 1068, a major autophosphorylation site, in a dose- and time-dependent fashion. This effect of Zn2+ on EGFR was significantly blocked with an antibody against the ligand-binding domain of the receptor. Neutralizing antibodies against EGFR ligands revealed the involvement of heparin-binding EGF (HB-EGF) in Zn2+-induced EGFR phosphorylation. This observation was further supported by immunoblots showing elevated levels of HBEGF released by Zn2+-exposed cells. Zymography showed the existence of matrix metalloproteinase-3 in Zn2+-challenged cells. Incubation with a specific matrix metalloproteinase-3 inhibitor suppressed Zn2+-induced EGFR phosphorylation as well as HB-EGF release. Therefore, these data support an autocrine or paracrine mechanism whereby Zn2+ induces EGFR phosphorylation through the extracellular release of EGFR ligands, which may be mediated by metalloproteinases.