Conflicting results concerning the ability of the epidermal growth factor (EGF) receptor to associate with and/or activate phosphatidylinositol (Ptdlns) 3-kinase have been published. Despite the ability of EGF to stimulate the production of Ptdlns 3-kinase products and to cause the appearance of PtdIns 3-kinase activity in antiphosphotyrosine immunoprecipitates in several cell lines, we did not detect EGF-stimulated Ptdlns 3-kinase activity in anti-EGF receptor immunoprecipitates. This result is consistent with the lack of a phosphorylated Tyr-X-X-Met motif, the p85 Src homology 2 (SH2) domain recognition sequence, in this receptor sequence. The EGF receptor homolog, ErbB2 protein, also lacks this motif. However, the ErbB3 protein has seven repeats of the Tyr-X-X-Met motif in the carboxy-terminal unique domain. Here we show that in A431 cells, which express both the EGF receptor and ErbB3, Ptdlns 3-kinase coprecipitates with the ErbB3 protein (pl80eR3) in response to EGF. p180B3 is also shown to be tyrosine phosphorylated in response to EGF. In contrast, a different mechanism for the activation of Ptdlns 3-kinase in response to EGF occurs in certain cells (PC12 and A549 cells). Thus, we show for the first time that ErbB3 can mediate EGF responses in cells expressing both ErbB3 and the EGF receptor.

c‐erbB‐3 is a member of the type I (EGF receptor‐related) family of growth factor receptors for which no ligand has been identified. To facilitate ligand stimulation we have constructed a chimeric receptor which possesses an activatable kinase and promotes the growth of NIH 3T3 fibroblasts. In this study we have shown that SHC and phosphatidylinositol 3′‐kinase bind to the activated EGF receptor/c‐erbB‐3 chimera. Whereas p85 is not phosphorylated to a significant extent, SHC appears to be a major substrate for phosphorylation on tyrosine. In contrast to EGF receptor and c‐erbB‐2, we were unable to detect binding of activated c‐erbB‐3 to GRB2. Using synthetic peptides corresponding to each of 13 potential phosphorylation sites on c‐erbB‐3, we have shown that tyrosine 1309 is responsible for SHC binding. Peptides containing the motif YXXM inhibit p85 association. By comparison with recently reported SHC binding sites on Middle T antigen and Trk we have identified a SHC binding motif, NPXY.

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Summary Abnormalities of the EGF receptor and/or the related ERBB2 receptor occur in a significant proportion of cases of human breast cancer and are important influences in the behaviour of this tumour type.In this report we demonstrate by nucleic acid analysis and immunohistochemistry that the recently recognised third member of this gene Costa et al., 1988). Overexpression of the ERBB2 protein is also a marker of poor prognosis for node-positive disease (Slamon et al., 1987) and probably also for node-negative disease (Perren, 1991). More exciting, however, is the accumulating evidence that the ERBB2 receptor protein has potential as a target for antibody-directed therapy (Shepard et al., 1991) and other approaches such as inhibition of receptor dimerisation and tyrosine kinase function (Gullick 1990b Thirty six biopsy samples of non-neoplastic breast tissue

A mutant epidermal growth factor receptor (⌬EGFR) containing a deletion of 267 amino acids from the extracellular domain is common in human glioblastomas. We have previously shown that the mutant receptor fails to bind EGF, is constitutively phosphorylated, and confers upon U87MG glioblastoma cells expressing it (U87MG.⌬EGFR), an increased ability to form tumors in mice. Here we demonstrate that the constitutively phosphorylated ⌬EGFR enhances growth of glioblastoma cells through increased activity of Ras: 1) there was an increase in the proportion of Ras present in the GTPbound form, and 2) introduction of neutralizing anti-Ras 259 antibodies into U87MG and U87MG.⌬EGFR cells by microinjection inhibited DNA synthesis to the same low level in both cell populations. We also show that the truncated EGF receptor constitutively associates with the adapter proteins Shc and Grb2 which are involved in the recruitment of Ras to activated receptors. Several derivatives of ⌬EGFR containing single, or multiple mutations at critical autophosphorylation sites were constructed and used to demonstrate that the major Shc binding site is Tyr-1148, and that Grb2 association occurs primarily through Tyr-1068. We conclude that the increased tumorigenic potential of glioblastoma cells expressing the truncated EGF receptor is due at least in part to Ras activation presumably involving the Shc and Grb2 adapter proteins.