Involvement of ErbB2 in the signaling pathway leading to cell cycle progression from a truncated epidermal growth factor receptor lacking the C-terminal autophosphorylation sites

To investigate the mechanisms underlying the enhanced mitogenic activity of the truncated epidermal growth factor receptor (EGFR) lacking the C-terminal autophosphorylation sites (Δ973-EGFR), we studied the intracellular signaling pathways in NR6 cells expressing human wild type EGFR and Δ973-EGFR. Microinjection of dominant/negative p21^ras (N17) completely inhibited EGF-induced DNA synthesis in both cell types. EGF stimulated She phosphorylation as well as the formation of wild type EGFR-Shc complexes. In contrast, EGF stimulated She phosphorylation without formation of Δ973-EGFR-Shc complexes. Tyrosine-phosphorylated She formed complexes with Grb2·Sos, and microinjection of anti-She antibody and Shc-SH2 GST fusion protein inhibited EGF stimulation of DNA synthesis in both cell lines. EGF markedly increased ErbB2 tyrosine phosphorylation in wild type EGFR cells. In Δ973-EGFR cells, ErbB2 was tyrosine phosphorylated in the basal state and EGFR stimulated further phosphorylation of ErbB2. In addition to ErbB2, additional proteins were tyrosine phosphorylated in Δ973-EGFR cells, mostly in the molecular mass range of 120-170 kDa. Taken together with our findings indicating coupling of ErbB2 to She, these data suggest the importance of an alternative signaling pathway in Δ973-EGFR cells mediated by the formation of heterodimeric structures between the truncated EGFR and ErbB2, followed by coupling through She to Grb2-Sos and the p21^ras pathway, ultimately leading to mitogenesis.