RECEPTOR DIMERIZATION IS NOT A FACTOR IN THE SIGNALING ACTIVITY OF A TRANSFORMING VARIANT EPIDERMAL GROWTH-FACTOR RECEPTOR (EGFRVIII)

The type-III deletion variant of the epidermal growth factor receptor (EGFRvIII) is frequently found in glioblastomas and other malignant hu man tumours. Although EGFRvIII confers ligand-independent oncogenic tr ansformation of cell lines, the mechanism by which it promotes aberran t cellular proliferation is unknown. Using cell lines expressing compa rable numbers of either wild-type receptor (EGFRwt) or EGFRvIII, we co mpared several parameters of receptor activation: dimerization, tyrosi ne phosphorylation and activation of intracellular signalling proteins . Like activated EGFRwt, EGFRvIII was phosphorylated and bound constit utively to the Shc adapter protein. Indeed, EGFRvIII-associated Shc ha d a higher phosphotyrosine content than Shc associated with stimulated EGFRwt. EGFRwt dimerized in response to either EGF or transforming gr owth factor alpha. Higher cross-linker concentrations and incubation a t higher temperatures (37 degrees C) allowed detection of EGFRwt dimer s even in the absence of exogenous ligand. In contrast, EGFRvIII faile d to dimerize under any conditions studied. Moreover, neither mitogen- activated protein kinase nor phospholipase C gamma were phosphorylated in EGFRvIII-expressing cells. We conclude that the deletion of 267 am ino acids from the 621-amino-acid N-terminal domain of EGFR does not r esult simply in a constitutively activated receptor, but alters the sp ectrum of signalling cascades utilized. Furthermore the ligand-indepen dent transforming activity of EGFRvIII is independent of receptor dime rization.