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 (N17) completely inhibited EGF-induced DNA synthesis in both cell types. EGF stimulated Shc phosphorylation as well as the formation of wild type EGFR•Shc complexes. In contrast, EGF stimulated Shc phosphorylation without formation of Δ973-EGFR•Shc complexes. Tyrosine-phosphorylated Shc formed complexes with Grb2•Sos, and microinjection of anti-Shc 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 Shc, 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 Shc to Grb2•Sos and the p21 pathway, ultimately leading to mitogenesis.